char of tagetes erecta (african marigold) flower as a potential electrode material for supercapacitors http://dx.doi.org/10.5599/jese.1472 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1472 open access : : issn 1847-9286 www.jese-online.org review treatment of oily wastewater by electrocoagulation technology: a general review (2018-2022) muhammad aiyd jasim1 and forat yasir aljaberi2, 1chemical engineering department, college of engineering, al-qadisiyah university, al-qadisiyah, iraq 2chemical engineering department, college of engineering, al-muthanna university, al-muthanna, iraq corresponding authorс: furat_yasir@yahoo.com received: may 12, 2022; accepted: october 26, 2022; published: october 29, 2022 abstract a huge amount of oily wastewater is discharged annually from several industries like petroleum and petrochemical factories. scientists and researchers are permanently concentrated on creating conventional technologies or identifying novel treatment options for oily wastewaters, since they need to be treated before being discharged into the soil and aquatic ecosystems. electrocoagulation technology (ect) is an electrochemical method employed to remove numerous pollutants from domestic and industrial wastewaters. this paper aims to review the recently published articles from 2018 to 2022 concerned with ect for oily wastewater remediation. based on the present review, it is obvious that ect is strongly dependent on the value of electric current or voltage applied to provide the required amounts of electro-coagulants for efficient remediation, reaction time duration for the generation of electro-coagulants and pollutants elimination, and electrode configuration such as shape, type of metal, and distance between electrodes. other operating parameters include solution ph (since some pollutants are removed based on their cationic or anionic nature), type of electrolyte which affects the electric conductivity and ohmic drop and stirring speed that may influence the contact among numerous ions throughout the ec reactor. the core findings show that the ect is highly effective, eco-friendly, and cost-effective in eliminating organic and inorganic pollutants from oily wastewater. keywords electrochemical treatment methods; hybrid systems; electrode configuration introduction oil is a vital economic resource for many nations since oil and its derivatives are used in many different sectors, including petrochemicals and oil refineries [1]. large amounts of oily wastewater are released during the oil production [2]. ecological degradation brought on by oily wastewater from various industrial operations, including oil drilling sites, petroleum refineries, and petrochemical plants, represents a serious threat to the ecosystem [3]. releasing untreated or http://dx.doi.org/10.5599/jese.1472 http://dx.doi.org/10.5599/jese.1472 http://www.jese-online.org/ mailto:furat_yasir@yahoo.com j. electrochem. sci. eng. 00(0) (2022) 000-000 treatment of oily wastewater by electrocoagulation 2 incompletely treated petroleum refinery wastewaters into the climate causes amphibian biological system contamination, unattractive ecological issues, costly wastewater remediation, and the deficiency of cropland and fish [4]. the ecologically sustainable administration of oil production regions and the economic growth of the petrochemical industry depends heavily on the effective treatment and reuse of oily wastewaters [5]. crude oil contains a wide range of constituents that can be classified as organic (hydrocarbons) or inorganic (biochemical oxygen demand (bod), total suspended solids (tss), chemical oxygen demand (cod), total organic carbon (toc), total dissolved solids (tds), turbidity (tur), ammonia, cyanide, and heavy metals) [6]. wastewaters vary in oil concentrations depending on the depth and position of the crude oil wells, with annual discharges of billions of cubic meters ranging from 1 to 40,000 ppm. as a result, the primary cause of oceanic ecological pollution is related to evacuated oily wastewaters [3]. in the extracted petroleum, oily wastewater levels range from 0.4 to 0.6 % [7]. figure 1 illustrates oily wastewater resources, forms, impacts and remediation benefits [8]. figure 1. general schematic for sources, impacts, physical forms and remediation benefits of oily wastewater oily wastewaters have a negative impact on humankind because they are difficult to degrade. consequently, the effluent sources should be treated utilizing distinct methodological techniques depending on the quantity and types of emulsified oily wastes [9]. cyclone separation, sorption, chemical precipitation, and electrochemical techniques are part of the accessible treatment strategies used for this objective [10]. the effectiveness of the applied strategies relies upon removal capacity, time expected to achieve the treatment interaction, optional pollution created, the building cost, and running/maintaining of the unit. subsequently, and because of the fast improvement of industries and human claims, some savvy treatment technique is necessary for oil removal from wastewater, and the reuse of freshwater [11]. due to their simplicity of automation and ease of use, electrochemical techniques such as electrooxidation, electrofloatation, and electro-fenton are frequently used to treat various wastewaters [12]. electrocoagulation (ec) has been established as an effective electrochemical treatment approach for enhancing the deep cleaning of oily wastewater. ec utilizes redox reactions that happen by passing an electric current across electrodes made usually from aluminum or iron. the quantity of metal cations, like al3+ or fe2+, is enhanced as a result of anode disintegration, whereas hydrogen gas (h2) and hydroxyl ions (oh-) are realized at the cathode [13,14]. various ions m. a. jasim and f. y. aljaberi j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1472 3 formed at both electrodes make electrocoagulants that eliminate contaminants through adsorption without using any chemical addition [15]. still, in 1904, elmore recommended using electrolysis for mineral extraction, while in 1906, dietrich obtained a patent for electrocoagulation (ec) that would clean ship bilge water of contaminants. up to now, the ec method has been established as an efficient, environmentally friendly, and cost-effective treatment for removing several poisons from wastewater. over the past three decades, ect has been successfully used as a water-treatment innovation to eliminate various pollutants. technological advancements that consume less energy and cleaner output have increased the interest in ec as a generated water remediation technique [16]. the current work aims to review a number of particular papers appearing in the literature from 2018 to 2022 and assess the efficacy of ect in treating actual oily wastewater. as a result, this study investigated ec mechanisms, operative factors that affect how well the ec process works, and its application to diverse petroleum effluents. this paper also provides an overview of the key findings from the publications investigated. electrochemical techniques electrochemical technology has been evolving in later years due to its highly efficient adaptability and ease of automation. electrochemical processes have high oxidation capacity, do not require adding extra oxidative agents, and are modular. due to these elements, electrochemical technology passed from theory to practice in practical engineering [17]. the unlimited evacuation of a few kinds of metal ions and organic and inorganic pollutants present in wastewater is regarded as effective and limited-cost when compared to ineffective modification techniques [18]. electrochemical technologies are most efficient in wastewater treatment due to their eco-friendliness and ability to remove many contaminants through reduction reactions, oxidation, and energy consumption [19]. electrochemical techniques are quite simple to use, have the characteristic of supplying electrons, which are immaculate, many-sided, and efficacious viable reagents for pollutant removal, and do not emit toxic byproducts. electrochemical mechanisms such as electrooxidation, electro-reduction, electro-flotation, electro-dialysis, and electrocoagulation have demonstrated their adequacy in pollutant expulsion from wastewaters. among these mechanisms, the electrochemical high-level oxidation processes have already received much interest. pollutants are oxidized electrochemically through either immediate or vicarious electrolysis. the last method employs electroactive types of the membrane to degrade organic pollutants [20]. a multitude of anodes and cathodes, primarily consisting of aluminum, iron, magnesium, and stainless steel, are often used in electrochemical ec processes. when a dc electric current is run through the apparatus, the anodes produce positively charged ions that act as coagulants in the solution. these anodes can be found in various shapes, including plates, balls, rods, and tubes. throughout the operation, cathodes should not corrode [21]. electrochemical techniques have several features, including reduced chemical prescribed dose and multilateral operation, as well as a shorter maintenance cost, and only electrons can be used for treatment rather than microorganisms and toxicants. furthermore, the sludge produced by this technology is easily settled down and dewaterable [22]. electrocoagulation technology (ect) because of its wide variety of utilizations and low cost, electrocoagulation technology (ect) is one of the most popular electrochemical technologies. the electrocoagulation operation can also http://dx.doi.org/10.5599/jese.1472 j. electrochem. sci. eng. 00(0) (2022) 000-000 treatment of oily wastewater by electrocoagulation 4 be used as a pre-processing unit to eliminate suspended solids as well as colloidal particles from water and work on improving wastewater biodegradability. in some research, electrocoagulation was designed to deal with highly concentrated organic wastewater [17]. ec is a powerful technique for better cleanup of oily wastewater via redox reactions caused by passing an electric current through electrodes. the anode is the electrode where oxidation occurs, while reduction occurs at the cathode. as schematically illustrated in figure 2, metal cations (mn+) that are launched because of the anode degeneracy are forming electro-coagulants (m(oh)n), while the hydrogen gas (h2) and hydroxyl ions (oh-) are developed at the cathode of the ec cell. figure 2. schematic representation of a basic ec cell in batch mode the ec process develops in 3 stages: a. oxidation reaction and development of electro-coagulants. b. portion emulsifiers and destabilization of contaminants. c. incorporating pollutants to shape loads that go with the flow or precipitate in line with the sort of pollution [1]. the ec utilizes electric current to break down metal electrodes and supply coagulants or destabilization flocs [23]. ec technique has many advantages, including simple device, shorter reaction time, neither chemical required, minimal energy usage and least electrode demand. moreover, ec can be performed as a pretreatment process or in hybrid systems with different treatment techniques along with adsorption and alternate ionic processes. regardless of the abundant benefits of the electrocoagulation process, the mechanism of oil elimination from oily wastewater is complex due to its physicochemical parameters. the electro-coagulants fashioned throughout the electrocoagulation process possess a bigger functionality than chemical coagulants for casting off contaminants from exceptional types of wastewaters. additionally, they are bigger than chemical coagulants, which results in the lowest solubility of products in a certain ph range, and as a result, this will enable an easier separation [6]. the ultralight substances will float to the solution surface by the buoyancy process, whereas heavier substances will settle at the bottom of the reactor [16]. main reactions of ec process the material used for the electrodes influences the coagulant, which immediately influences the separating efficiency. anode oxidation and cathode reduction occur when the electrode coagulation operation is initiated by the required voltage or current applied to a cell. the solution ph will m. a. jasim and f. y. aljaberi j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1472 5 increase while hydrogen gas is launched for the duration of the reduction process. the major reactions at the electrodes during the ec process are as follows [1, 24-26]: at the anode: m(s) ⇒ m +n + ne (1) 2h2o ⇒ o 2 + 4h+ + 4e (2) at the cathode: 2h2o + 2e⇒ h2(g) + 2oh (aq) (3) m +n(aq) + ne ⇒ m(s) (4) for example, the anodic reactions at the aluminium anode are al(s) ⇒ 2al+3(aq) + 6 e (5) al3+(aq) + 3oh⇒ al(oh)3 (6) while for the iron electrode, the following reactions occur [22,26,27]: fe(s) ⇒ fe2+(aq) + 2e (7) fe(s) ⇒ fe3+(aq)+ 3e (8) 4fe+2(aq)+ o2 + 4h+ ⇒ 4fe3+(aq) + 2h2o (9) fe3+(aq)+ 3h2o ⇒ fe(oh)3(s) + 3h+(aq) (10) electrode arrangement the electrode arrangement is an important factor in the ec process affecting the removal effectiveness. the electrodes in the ec cell may be placed in several configurations [28-32]. as shown in figure 3, electrodes may be set up as a monopolar or bipolar system. by connecting all anodes and cathodes in series (mp-s), the same current can flow through each pole. electrodes can also be connected to a parallel system (mp-p). in a bipolar system (bp-s), the external electrodes are connected in series to a source capable of producing both voltage and current. the negative side is obverse, and the other electrode side is polarized with a positive charge. according to research, the bipolar system is the most effective in removing various pollutants [33,34]. figure 3. arrangements of electrodes in ec cells: (a) monopolar electrodes in parallel connection; (b) monopolar electrodes in series connection; (c) bipolar electrodes in series connection http://dx.doi.org/10.5599/jese.1472 j. electrochem. sci. eng. 00(0) (2022) 000-000 treatment of oily wastewater by electrocoagulation 6 ect for oily wastewater treatment numerous scientists have used the electrocoagulation technique to deal with real and/or synthetic oily wastewater. several indicators of contaminated oily wastewater, such as cod, bod, toc, doc, oil content, etc., have been followed and minimized in oily wastewater using ect alone or in integrated systems. ect system the most important indicator of water pollution is cod, which has been taken into consideration by several studies. yang et al. [5] employed six parallel and sequential al-sheets as electrodes in a batch electrocoagulation reactor to investigate ec demulsification and oil elimination performance at various electric fields and both connection modes (monopolar and bipolar). bipolar ec, with an electric field intensity of 38 v/m, and monopolar ec of 180 v/m, were used at a solution ph of 7.0. cod and turbidity values were measured during the electrolysis process of 0 to 30 minutes. when the current density was 0.67 ma/cm2, the cod and turbidity removal rates under monopolar and bipolar ec were 77.9, 98.0, 66.9 and 89.1 %. this study proved that the monopolar connection mode is more effective for minimizing cod compared to the bipolar mode. moussavi et al. [21] used electrocoagulation to dispose of organics and suspended matter emanating from a petroleum refinery liquid waste using an al-rectangular plate as the anode positioned parallel to and 5 cm aside from the stainless-steel cathode. the optimum conditions were 120 min of treatment, ph 6, and a current density of 35 ma/cm2 to attain 94 % tss removal and 90 % cod removal. the core results of this work revealed that the configuration of electrodes affects the performance of the ec reactor. the same proof was stated by madhavan et al. [35] (2021), who developed a novel electrocoagulation device to treat the produced water using mild steel-parallel plates under switching electrode polarity applied to minimize electrode passivation. the experimental set-up for electrocoagulation research included filter-press cells for which the cell width can be changed using gaskets, frames, and a movable piston mechanism. the anode and cathode were made of flat, rectangular slabs of mild steel with 1 cm inter-electrode gap separating plates from one another. at the batch recirculation time (brt) of 15 min, the current density (cd) was 1.6 a/dm2, sc was 3 g/l, and switchover time (sot) was 1 min. it was shown that the majority of 99 % cod and 98 % oil and grease from a sample of produced water could be removed. oil removal of 88 % was reached using the brt operating parameters of 3 min, 2.14 a/ dm2 cd, 3 g/l sc, and 1.9 min sot at the lowest operating cost of 0.65 us$/kg cod. similar conclusions have been provided by [16,36-40], and other previous works are briefly listed in table 1. bajpai et al. [41] performed monopolar-parallel al/fe electrodes to optimize the most influential parameters on the cod removal efficiency from the pharmaceutical wastewater. they attained 86.70 % cod elimination at 16 ma/cm2, ph 8, and 40 min of the treatment time. this finding proved that the mode of connection of electrodes toward the electric power source has a high impact on the performance of the ec cell. the impact of the connection mode of the electrodes to the power supply has been investigated by [42-46], as shown in table 1. these works proved that the type of connection mode to the power supply is extremely important, affecting the performance of the ec reactor for eliminating cod and associated pollutants. aljaberi et al. 1 investigated the capacity of an innovative batch electrocoagulation reactor with concentric aluminum tubes of the total active area of 285 cm2 in a monopolar mode to remove 523.11 mg/l oil content and 168 ntu turbidity from real oily wastewater discharged from the moist oil unit. the electrolysis time ranged from 10 to 40 min. m. a. jasim and f. y. aljaberi j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1472 7 the current density was 1.77 to 7.07 ma/cm2, ph 6.5, 126 ms/cm conductivity, and 200 rpm stirring speed were studied as the operating variables. optimal parameter values were obtained at the current density of 5.675 ma/cm2, ph 6.5, and 40 min of electrolysis time. the maximum removal of oil content and turbidity were found as 85.982 and 84.439%, respectively. other studies on oily wastewater treatment were done by the same author [6,9,16], with details summarized in table 1. zhang et al. [36] used electrocoagulation as an effective technique for the pretreatment of shale gas extraction wastewater. a batch reactor with concentric iron slabs running in a monopolar mode was used, with fe electrodes consisting of 5 parallel slabs, each measuring 10 cm in width, 17 cm in height, and 0.2 cm in thickness. each plate having a surface area of 170 cm2 was spaced 2.0 cm apart. for each run, 1.45 l of wastewater sample was maintained in the ec reactor. the optimal processing conditions for turbidity, toc, and ca2+ elimination, were obtained. the electrolysis period varied from 0 to 20 minutes, and the conductivity was 14.48 ms/cm. the power source was 0 to 40 volts and 0 to 5 a. under the optimum conditions, the highest removal rates for turbidity, toc, and ca2+ were around 98.3, 78.5 and 56.5%, respectively. the following operating parameters were suggested for the efficient and cost-effective treatment of drilling wastewater by the ec process: 20 minutes for reaction time, 318 a/m2 for current density, and 4.4 for ph value [36]. ect in hybrid systems other studies have been performed to investigate the influence of using hybrid systems to minimize cod and associated indicators of contamination. lu et al. [17] integrated electrocoagulation (ec) and electrooxidation (eo) technologies to remove pollutants from industrial wastewater by using aluminum electrodes. they attained 81.62 and 50.92 % removal efficiencies of cod and ammonia nitrogen, respectively, at the optimal values of operating variables of 5 ma/cm2 cd, 60 min of contact time, and more acidic condition (ph 3) of solution. also, chanikya et al. [20] combined ec and eo to treat industrial wastewater containing cod and toc, using pt/ti mesh as the anode and acrylic sheet as the cathode. the best removal of cod and toc was 93.5 and 75 %, respectively, at ph 8.2, a current density of 10 ma/cm2, and a reaction time of 60 min. the integration efficiency of electrocoagulation and electro-fenton processes was studied by can-güven [23] in dye manufacturing wastewater treatment using monopolar-iron electrodes. the highest removal of cod was obtained at 21 ma/cm2, ph 7.3, and 25 min of the electrolysis time. ucevli and kaya [47] compared membrane filtration, chemical coagulation, electrocoagulation, and their hybrid systems for minimizing cod and bod from greywater treatment using al/fe electrodes. at the optimal values of 5 a/m2 and ph 7.5, the highest elimination was achieved after 30 min of the electrolysis time. changmai et al. [48] investigated the usage of electrocoagulation-microfiltration in a batch mode for the remedy of oily wastewater containing oil and grease at the side of metals such as na, cr, cu, pb, and ni. samples have been prehandled with the use of electrocoagulation with various running parameters, including current density (20–80 a/m2), electrode space (0.005 to 0.2 m), and initial ph (3.6 to 8.7). the presence of oil and grease was decreased from 35 to 10.2 mg/l in only 20 min. microfiltration was accomplished through the use of indigenously organized ceramic membranes to take away flocs generated by the electrocoagulation technique at three specific pressures of 98, 196 and 194 kpa. the electrocoagulation effluent was introduced to a filtration cell to be filtered using a ceramic membrane. the concentration of oil and grease was minimized from 35 to 10.2 mg/l. this combination of ec and other treatment technologies assists the treatment process of different types wastewater containing cod. the same conclusions were provided by [29,37,49-53], where ec is combined with anodic oxidation, http://dx.doi.org/10.5599/jese.1472 j. electrochem. sci. eng. 00(0) (2022) 000-000 treatment of oily wastewater by electrocoagulation 8 electrooxidation, electro-fenton, flotation, electrooxidation, ozonation, and peroxone, respectively, to eliminate organic and inorganic pollutants from wastewater. lia et al. [49] planned and built a unique incorporated system of electrocoagulation (ec)-carbon membrane coupling with electrochemical anodic oxidation (cm/eao) for the treatment of oily wastewater. electrocoagulation was taken as an essential unit, and the impacts of numerous boundaries, including current density, electrolyte focus, initial ph, feed oil content, and work time on the pretreatment performance, were examined and enhanced. after that, cm/eao was established as a supplementary unit for the additional deep purification of ec effluent. the outcomes demonstrated the excellent capability of the integrated system for wastewater oil removal. the ec unit could eliminate almost 50 % of cod and toc in the feed, and the accompanying cm/eao unit further debased the ec emanating, and cod and toc were diminished to 13 and 22 mg/l, separately. rectangular metal sheets of aluminum with a total active area of 10 cm2 were applied as electrodes concentrically. the electrolysis ranged from 0 to 60 min, the current density (0.5 to 4.5 ma/cm2), and neutral ph. the oil removal efficiency was 87.18 %. the most suitable values of the operational parameters were 2.0 ma/cm2 of current density, 0.5 g/l nacl, neutral ph, stirring speed of 250 rpm, and 60 min [49]. table 1 summarizes past studies published from 2018-2022 that discussed the application of electrocoagulation technology in the treatment of wastewater containing various contaminants, including organic and inorganic compounds and heavy metals. this table gives the reader important facts for each mentioned study, like the operating parameters and their optimal values for getting the best possible removal of different contaminants. table 1. literature summary of operational conditions for electrocoagulation removal of pollutants from wastewaters references pollutants current/ voltage reaction time, min anode metal anode shape cathode metal cathode shape optimum parameter values efficiency, % [1] oil content 1.77-7.07 ma/cm2 10-40 al concentric tubes al concentric tubes 5.68 ma/cm2 40 min, ph 6.5 85.98 [3] oil content 20-80 a/m2 20 al sheets al sheets 80 a/m2 20 min, ph 8.7 70.8 [5] cod 0.67 ma/cm2 30 al plates al plates 0.67 ma/cm2 30 min, ph 7 77.9 [16] toc 0.5-2.0 a 10-40 al concentric tubes al concentric tubes 1.606 a 40 min, ph 6.5 83.91 [17] cod 5 ma/cm2 60 al plates al plates 5 ma/cm2 60 min, ph 3 81.62 [20] cod, toc 10 ma/cm2 0-60 pt/ti mesh acrylic sheet fe acrylic sheet 10 ma/cm2 60 min, ph 8.2 cod: 93.5 toc: 75 [21] cod 35 ma/cm2 120 al rectangular plates stainless steel rectangular plates 35 ma/cm2 120 min, ph 6 90 [22] bod 0.08-0.77 a/dm2 0-60 fe plates al plates 0.77 ma/cm2 60 min, ph 8 75 [23] cod 5-41 ma/cm2 25 fe plates fe plates 21 ma/cm2 25 min ph 7.3, 50 °c 38.5 [35] cod, oil content 1-3 a/dm2 3-15 mild steel parallel plates mild steel parallel plates 2.14 a/dm2 3 min cod: 86.71 oil: 88 [36] toc 318–481 a/m2 20 fe plates fe plates 318 a/m2, 0 min, ph 4.4 78.5 [37] cod 14-56 ma/cm2 0-280 fe parallel sheets fe parallel sheets 56 ma cm2 280 min, ph 6.8 97 [38] cod 0.35-1.7 a 5-15 al/fe plates fe/al plates 1.05 a, 5 min ph 8.1 96 [39] cod 3-9 ma/cm2 0-180 al tubular stainless steel rotating tube 6 ma/cm2 40 min ph 7±0.7 70 [40] cod 0.05-0.3 a 0-30 fe plates fe plates 0.2 a, 5 min ph 4 60 [41] cod 4–20 ma/cm2 10–50 fe parallel sheets al parallel sheets 16 ma/cm2 40 min, ph 8 86.70 m. a. jasim and f. y. aljaberi j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1472 9 references pollutants current/ voltage reaction time, min anode metal anode shape cathode metal cathode shape optimum parameter values efficiency, % [42] cod 15-25 v 2-80 al plates al plates 17.6 v 43.8 min, ph 4 >99 [43] toc 1.39-6.95 a/m2 0-30 fe plates al plates 4.17 a/m2 15 min, ph 4.3 80.2 [44] cod 0.5-17 ma/cm2 30–180 al/fe plates al/fe plates 10 ma/cm2 120 min, ph 8.5 76 [45] cod al plates al plates 16 ma/cm2 14.2 min, ph 8 88.3 [46] cod 0.5-4 ma/cm2 0-60 al plates al plates 2 ma/cm2 60 min, ph 7 85.5 [47] cod 5-30 a/m2 30 al sheets fe sheets 5 a/m2, 30 min ph 7.5 82 [48] oil content 0.5-4.5 ma/cm2 60 fe rectangular sheets al rectangular sheets 2.0 ma/cm2 60 min neutral ph 87.18 [49] cod 2.5 a 0-280 al plates al plates 2.5 a, 140 min 90 [50] cod 2.57-15.43 ma/cm2 10-120 al plates al plates 10.29 ma/cm2 60 min, ph 7 85.01 [51] cod 0-60 fe plates fe plates 40 ma/cm2 60 min, ph 9 91.70 [52] cod al plates al plates 0.7 a, 45 min ph 7.35 70 [53] cod 6-12 a (3-40 al tubular al tubular 12 v, 30 min ph 7 27 [54] doc 5.55-14.8 ma/cm2 (0-20 fe plates fe plates 14.8 ma/m2 20 min, ph 8.3 74 [55] toc 20-100 ma 0-100 stainless steel plates graphite plates 60 ma, 100 min, ph 3 30.5 [56] cod 1.58-3.16 a 0-60 al plates al plates 3.16 a, 60 min ph 6.4 63 [57] cod 0-140 a/m2 60 al plates ti plates 115 a/m2 0 min , ph 7.5 75.33 [58] cod, bod 20-40 ma/cm2 10-60 fe or al plates stainlesssteel plates 30 ma/cm2 35 min ph 6.3 cod: 50.6 bod: 10 [59] cod 0.3-0.9 a 10-30 pt sheets carbonsheets 0.7 a, 30 min ph 10 69 disadvantages of electrocoagulation method the main disadvantages of the ec method and proposed solutions for improvement are summarized in table 2. table 2. main disadvantages and proposed solutions for improving ec method disadvantages proposed solutions the sacrificial anodes require to be replaced periodically new design(s) of electrode configuration could overcome this problem dissolved solids present in wastewater could limit the use of ec process new design(s) of electrode configuration could overcome this problem an oxide film generated on the cathode, which is impermeable that prevents the flow of current using of sodium chloride will be effective to solve this problem high cost required due to the use of electricity using of green technologies such as solar and wind power can solve this economic impact conclusions petroleum has predominantly harmful ecological effects on practically all life aspects due to its toxicity. oil pollution in the air and water could be hazardous and harmful to humans. several consequences on the soil ecological system are caused by the presence of organic (petroleum hydrocarbons) and inorganic substances, such as oil content, cod, bod, toc, turbidity, tds, and tss in water. their effects include changes in the physicochemical properties of soils and adverse effects on animals and plants. this study examines almost 50 most recent papers to provide the reader with an overview of http://dx.doi.org/10.5599/jese.1472 j. electrochem. sci. eng. 00(0) (2022) 000-000 treatment of oily wastewater by electrocoagulation 10 ec as an effective and straightforward technology and one of the most widely used strategies for removing contaminants from wastewater. it has been proven that electrochemical technologies for wastewater treatment are distinguished from other technologies by their common sense and minimum generation of current. according to the findings, electrochemical technologies attracted considerable interest among alternate strategies of byproduct treatment techniques. the process ec was operated under various parameters, including current density, voltage, initial pollutant concentration, temperature, ph, the distance between the electrodes, and their shape and arrangement. different types of electrolytic reactors have been employed, and some studies have compared the performance of the removal of pollutants using different electrode compositions. the current research reviews the ec mechanism, the possibility of using various electrodes designs in oil water treatment and wide ranges for operating variables according to the nature of the oil water, working boundaries impacting the ec process performance and its implementation in treating oily wastewater. this work likewise sums up the key findings from the publications investigated. the core findings of this paper proved that ec innovation is eco-accommodating, practical, and of high effectiveness in eliminating 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https://jhygiene.muq.ac.ir/search.php?sid=1&slc_lang=en&auth=karimi http://dx.doi.org/10.29252/archhygsci.9.1.48 https://creativecommons.org/licenses/by/4.0/) electrochemical study of doped lifepo4 as a cathode material for lithium-ion battery doi:10.5599/jese.234 1 j. electrochem. sci. eng. 6(1) (2016) 1-8; doi: 10.5599/jese.234 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical study of doped lifepo4 as a cathode material for lithium-ion battery andrey chekannikov* , ** , , svetlana novikova***, tatiana kulova**, alexander skundin**, andrey yaroslavtsev*** *skolkovo institute of science and technology, 100 novaya str., skolkovo, odintsovsky district, 143025 moscow region, russia **a.n. frumkin institute of physical chemistry and electrochemistry of the russian academy of sciences, 31-4 leninsky prospect, 119071 moscow, russia ***kurnakov institute of general and inorganic chemistry of the russian academy of sciences, 31 leninsky prospect, 11999 moscow, russia corresponding author: andrey.chekannikov@skolkovotech.ru; received: october 1, 2015; revised: december 17, 2015; accepted: february 4, 2016 abstract life1-xvxpo4/c (x= 0.01, 0.03, 0.05, 0.1) composites had been obtained by sol-gel method and characterized with the use of the xrd-analysis, sem and charge/discharge tests. the doping was shown to result in decrease of electrode polarization, and correspondingly in capacity increase at high c-rates. keywords cathode materials; lithium ion battery; lithium iron phosphate; vanadium doping introduction since the first report on olivine lithium iron phosphate (lfp) in 1997 [1] this substance became the most recent state-of-the-art material of positive electrodes of lithium-ion batteries. the theoretical specific capacity of lifepo4 is 170 ma h g -1 and electrodes based on lifepo4 demonstrate a flat voltage plateau of around 3.45 v (vs. li/li + ). rather low electronic conductivity (σ ≈ 10 -9 s cm -1 ) should be mentioned as one of the disadvantages of this material. however, electron transfer can be enhanced by a simple carbon coating of the lfp particles in situ during synthesis or ex situ by post-treatment. this leads to significant increase of the achievable specific capacity. different ways for further improvement of this active material have been extensively studied for the last few years: i) development of advanced nanostructured lfp-carbon composites; http://www.jese-online.org/ mailto:andrey.chekannikov@skolkovotech.ru j. electrochem. sci. eng. 6(1) (2016) 1-8 doped lifepo4 for lithium-ion battery 2 ii) replacement of carbon by conductive, electrochemically active polymers; iii) doping of lfp by the ions of transition metals and so on. in particular, lfp doping with vanadium has been suggested as a way for the increase in mobility and diffusion coefficient of li + ions due to lattice expansion and li–o interaction weakening [2]. the authors of [3] have studied the structure and properties of life0.9v0.1po4 and found that the cathode properties of doped counterpart, including reversible capacity, cycleability and rate capability are better than those of lifepo4. later the lengthening and weakening of li–o bond and improvement of the electrochemical performance especially under the high c rate were confirmed by the examples of life0.95v0.05po4 [4], life0.97v0.03po4 [5] and life0.99v0.01po4 [6]. intriguingly, even 0.32 wt.% v atoms substituted for some fe atoms can notably improve the cathode property. the authors of [7] have systematically studied a series of materials life1-xvxpo4 with 0 ≤ x ≤ 0.13. they found that in the whole concentration range, the chemical valence of fe 2+ remains invariant whereas the valence of vanadium evolves from +4 to +3. it was found also that the phase composition varies with the adding amount. the materials with 0 ≤ x < 0.08 are single-phase, and materials with x > 0.08 are two-phase, properties and electrochemical behavior of both kinds of materials being different. the electrical conductivity as well as the diffusion coefficient in single-phase region are enhanced with “x” increase. upon going into two-phase region, the both quantities plummet down. close results were reported in [8]. recently, the influences of adding vanadium to the structure evolution and electrochemical performance of lifepo4 are systematically investigated by in-situ x-ray powder diffraction and x-ray absorption near edge structure spectroscopy in [9]. the results indicate that the addition of a small amount of vanadium (less than at 1 %) significantly reduces the formation of non-crystalline (highly disordered) triphylite and remnant heterosite phases in the cathode of battery especially at the rate capability higher than 0.5c. the cycle stability of lifepo4 cathode with vanadium additive after 80 cycles retains higher by 14.9 % compared to that without vanadium additive. such an enhancement could be attributed to the ion diffusion kinetics being improved and inactive triphylite being reduced by the supervalent-vanadium additive in cathode during electrochemical redox cycles. in present work, lfp doped with vanadium was studied. the composition of samples studied is life1-xvxpo4/c, where x = 0.01, 0.03, 0.05, 0.1, therefore vanadium ions partially substituted for iron ions. experimental life1-xvxpo4/c (x = 0.01, 0.03, 0.05, 0.1.) composite materials coated with fine carbon layer were prepared by a sol–gel process described elsewhere [5,10]. the lino3, fe(no3)3·9h2o, nh4vo3, nh4h2po4 and sucrose were used as starting materials. the resulting precursor was finally calcined at 650 °c for 10 h in an argon atmosphere. carbon converted from sucrose acted as reducing and conducting agent and its amount was ∼5 wt. % in the final product according to the tga data. crystal structure was characterized by x-ray diffraction (xrd) with cukα radiation performed on a rigaku d/max 2200 diffractometer. xrd data were analyzed using rigaku application data processing software. microstructures of obtained materials were examined with the help of the scanning electron microscope carl zeiss nvision 40.юelectrode paste was prepared by thoroughly mixing 85 % life1-xvxpo4/c as an active material, 10 % conductive carbon black (timcal, belgium), and 5 % binder (polyvinylidene fluoride (aldrich) dissolved in anhydrous n-methyl-2-pyrrolidinone a. chekannikov et al. j. electrochem. sci. eng. 6(1) (2016) 1-8 doi:10.5599/jese.234 3 (aldrich, ≤50 ppm h2o)). the paste was applied to stainless steel gauze (current collector) as a 15 mg/cm 2 layer. the resultant electrode was pressed at 100 mpa and vacuum-dried at 120 °c for 8 h. electrochemical tests were performed in hermetically sealed three-electrode (life1-xvxpo4/c/li/li) cells. the area of the working electrode was 2.25 cm 2 , and that of the auxiliary (lithium) electrode was 5 cm 2 . the cells were assembled in a glove box under an argon atmosphere with a humidity of <10 ppm. a nonwoven polypropylene separator (npo ufim, russia) was placed between electrodes. 1 m lipf6 solution in a mixture of ethylene carbonate, diethyl carbonate and dimethyl carbonate (1:1:1) was used as electrolyte. the water content in the electrolyte, measured by a method of a fischer coulometric titration, was equal to 20 ppm. electrochemical cycling of the cells was performed at voltages range 2.5 4.3 v using a zru 50 ma-10 v charge–discharge system (buster, russia). certain experiments were carried out with using of more power device, specifically, zru 5 a-18 v (buster, russia). the tests were performed in galvanostatic mode at a currents densities of 20, 100, 200, 400, 800, 1600, 3200 ma/g. results and discussion xrd fig. 1 shows the xrd pattern for life1-xvxpo4 (x = 0.01, 0.03, 0.05, 0.1). all materials obtained have olivine structure and are indexed in the orthorhombic pnma space group. lifepo4 doping with v results in slight changes in unit cell sizes figure 1. xrd pattern of life1-xvxpo4/c when comparing life1-xvxpo4 with lifepo4, the a and c axes change is within the error, while b axis shrinks from 0.6007±0.0002 nm for lifepo4 to 0.5997±0.0002 nm for life0.9v0.1po4. that is in agreement with results reported in literature according to which a slight decrease in volume cell value is observed for v-doped lifepo4 [7,11,12]. at the same time, one can see that xrd pattern in te n si ty , a .u . j. electrochem. sci. eng. 6(1) (2016) 1-8 doped lifepo4 for lithium-ion battery 4 for life0.9v0.1po4/c does not differ from other patterns, and therefore there no evidences of twophase nature of this material. this result contradicts the data of [7]. it should be noted that no diffraction peaks from impurities or residual carbon were detected. this fact is no wonder because heat treatment at 650 o c resulted in carbonization with formation of non-crystalline matter. according to xrd data the mean size of x-ray coherent scattering regions for the investigated samples varies in the range 33 to 43 nm. sem electron microscopy data have shown that the average size of the lifepo4/c particles is consistent with the coherent scattering regions and amounts to ~40 nm. the aggregation of the particles was observed for v-doped samples. the particle size of life1-xvxpo4/c samples has a wide distribution range from 100 nm to more than 2 μm according to sem data (fig. 2). a b figure 2. sem microphotographs of life0.99v0.01po4/c at different magnifications (a,b). charge/discharge behavior the charge-discharge curves of various samples at current density 20 ma g -1 (what corresponded about c/8 rate) to a cutoff voltage between 2.5 and 4.3 v are shown in fig. 3a. one can see that the curves for all samples with x = 0.01, 0.03, and 0.05 coincide. the electrode with life0.9v0.1po4 demonstrates slightly less capacity. these results agree with data of [7] in spite of the fact that all materials were single phase. increase in current density results in increase of electrode polarization, and this increase depends on doping level: the higher doping level, the less polarization increase. this result also agrees with conductivity enhancement along with doping. fig. 3b shows the charge-discharge curves at current density 400 ma g -1 (what corresponded about 2.5 c rate). a half of difference between average potential of anodic and cathodic processes δe could present some generalized value of polarization. fig. 4a shows dependence of δe on current density j for all samples studied. the dependence of δe on j 1/2 is shown in fig. 4b. the latter plots are almost linear that is typical for the systems with distributed parameters, in which the polarization has diffusion and ohmic nature. surely, the date obtained in the present work do not allow discrimination of both factors (diffusion and ohmic). figs. 4a and 4b vividly show the effect of doping upon electrode polarization. a a. chekannikov et al. j. electrochem. sci. eng. 6(1) (2016) 1-8 doi:10.5599/jese.234 5 b figure 3. charge-discharge curves for life1-xvxpo4 (x= 0.01, 0.03, 0.05, 0.1): a current density 20 ma g -1 ; b current density 400 ma g -1 j. electrochem. sci. eng. 6(1) (2016) 1-8 doped lifepo4 for lithium-ion battery 6 fig. 5 presents the results of charge/discharge cycling with different current densities. it is worth noting that at rather low c rates the capacity of slightly doped samples exceeds that of heavy doped materials. at high c rates this ratio changes due to notable decrease in polarization for heavy doped samples. indeed, at current density 20 ma g -1 the discharge capacity of life1-xvxpo4 is equal to 146, 139, 132, and 128 mah g -1 for x = 0.01, 0.03, 0.05 and 0.1, corresponddingly. at 1600 mah g -1 discharge capacity of the same samples was 24, 28, 29 and 39 mah g -1 . a b figure 4. a electrode polarization vs. current density for doped lfp; b electrode polarization vs. square root of current density for doped lfp. the straight lines are drawn by minimal squares method a. chekannikov et al. j. electrochem. sci. eng. 6(1) (2016) 1-8 doi:10.5599/jese.234 7 figure 5. cycling performances of doped lfp cathodes measured with different rates in the voltage range of 2.5–4.3v conclusion lithium iron phosphate doped with vanadium (life1-xvxpo4 with x = 0.01, 0.03, 0.05, and 0.1) and covered by fine carbon was synthesized by a sol–gel method from lino3, fe(no3)3·9h2o, nh4vo3, nh4h2po4 and sucrose as starting materials. xrd investigation showed that materials obtained have olivine structure and are indexed in the orthorhombic pnma space group. lifepo4 doping with v results in slight changes in unit cell sizes. electron microscopy data have shown that the average size of the life1-xvxpo4/c primary particles amounts to ~40 nm. at rather low c rates the capacity of slightly doped samples amounts to 146 mah/g and exceeds that of heavy doped materials. at high c rates this ratio changes due to notable decrease in polarization for heavy doped samples. acknoweledgement: the present work is supported by the russian foundation for basic research (grant № нк 14-29-04068). references [1] a.k. padhi, k.s.nanjundaswamy, j.b.goodenough, j. electrochem. soc. 144 (1997) 1188‒1194. 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[10] d. v. safronov, s. a. novikova, t. l. kulova, a. m. skundin, and a. b. yaroslavtsev. inorganic materials 48 (2012) 513−519. [11] ming chen, leng-leng shao, hua-bin yang, tie-zhen ren, gaohui du, zhong-yong yuan, electrochim. acta 167 (2015) 278–286. [12] lu-lu zhang, gan liang, a. ignatov, m. c. croft, xiao-qin xiong, i-ming hung, yun-hui huang, xian-luo hu, wu-xing zhang, and yun-long peng. j. phys. chem. c 115 (2011) 13520–13527. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ application of the quadratic logistic differential equation for the rationalization of methanol electrooxidation dynamics doi:10.5599/jese.340 295 j. electrochem. sci. eng. 6(1) (2016) 295-301; doi: 10.5599/jese.340 open access : : issn 1847-9286 www.jese-online.org original scientific paper application of the quadratic logistic differential equation for the rationalization of methanol electrooxidation dynamics hossein heli, fereydoon gobal* nanomedicine and nanobiology research centre, shiraz university of medical sciences, shiraz, iran *department of chemistry, sharif university of technology, tehran, iran corresponding author: hheli7@yahoo.com, heli@sums.ac.ir, tel/fax: +98 71 36 12 22 25,6 received: september 22, 2016; revised: november 24, 2016; accepted: november 25, 2016 abstract the electrooxidation of methanol in both acidic and alkaline media on poly-crystalline platinum under the regime of cyclic voltammetry is analyzed by application of quadratic logistic equation. the current-charge curves in the anodic cycles fit the logistic differential equation reasonably well and are accounted on the basis of the non-linearity of the kinetics and the effect of positive feedback. in the reverse cycle however, no fit is observed, presumably due to the lack of correlation between the net faradaic current and the surface charge of adsorbates. keywords logistic differential equation; feedback; methanol electrooxidation; electrocatalysis; fuel cell introduction methanol is regarded as an attractive fuel in the fast-emerging fuel cell industry, but despite extensive electrocatalytic studies [1-4], there are major problems and kinetic limitations to its direct employment. ample information concerning the kinetics and mechanism and reactive surfaces, as well as stable bulk intermediates involved in the methanol electrooxidation reaction (meo) exist in the literature [5,6]. for further amplification of findings, studies on the non-linearity, multi-stability, and chaotic behavior of meo are essential. meo process is described very well by mathematical methods and modeling studies due to: (i) high quality data that either exist or can easily be obtained in a using relatively straight forward electrochemical measurements; (ii) the important factors, electrochemical or else, can easily be controlled and the responses of the system measured; (iii) multitude of correlations can be visualized in electrochemical measurements and in the present context, the current-charge (dq/dt vs. q) dependency seems to be most suitable. the quadratic http://www.jese-online.org/ mailto:hheli7@yahoo.com mailto:heli@sums.ac.ir j. electrochem. sci. eng. 6(4) (2016) 295-301 methanol electrooxidation via logistic equation 296 logistic differential equation (qle) [7-9] in the form of a difference for mapping [10,11] in the normalized form dx / dt = r x (1-x) (1) describes the effects of feedback, as well as the effect of parameter r on the value of normalized variable x (0 < x < 1). on the one hand, eq. (1) is associated with the intrinsic rate of the process under study. on the other hand, it describes non-linearity and chaos in the dynamics of the process in a simple way, by illustrating these phenomena through working with only one variable. for r > 3 the value of x progressively turns to a periodic motion and for 3.5699…< r <4 it behaves chaotically. eq. (1) can be alternatively written as: xi+1 = r xi (1-xi) (2) where consecutive values of x can be worked out on the basis of a set of initial values [12], while the term (1-x) surely guarantees non-linear effects. in the present study, a new approach is presented in an order to understand the methanol electrooxidation dynamics, both in acidic and alkaline solutions. this approach is based on the quadratic logistic equation that has been used extensively, as an example of simple equation with only one degree of freedom that illustrates occurrence of bifurcation and chaos in dynamical systems [9]. experimental materials and methods sodium hydroxide, sulfuric acid and methanol used in this work were of reagent grade of merck origin and used without further purifications. all solutions were prepared with distilled water. electrochemical experiments were carried out in a conventional three-electrode cell with platinum wire, exposing the surface area of 0.25 cm2, as the working electrode and its potential monitored against the saturated ag/agcl reference electrode. a large platinum sheet was used as the counter electrode. all electrochemical measurements were performed at room temperature. the cell was powered by a µ-autolab potentiostat/galvanostat run by computer through the gpes software. charges were calculated by integrating the area under voltammograms left after correction for the background. all potential values were further converted and reported relative to the normal hydrogen electrode. results and discussion figure 1 shows typical cyclic voltammograms (cv) representing meo on polycrystalline pt in 0.5 m sulfuric acid (a) and 0.5 m sodium hydroxide (b) solutions. in both systems, methanol concentration and potential sweep rates were 0.1 m and 20 mv s-1, respectively. the large peak observed in the anodic half cycle is believed [13,14] to be due to the oxidative stripping of protons from the adsorbed methanol molecules to form strongly adsorbed intermediates of cohads and coads. their subsequent reaction with ohads (or alternatively with oh-aq) at higher over-potentials yields the final oxidation products [3,15]. carbon dioxide or carbonates are the final products and the surface remains largely covered by ohads species at the end of an anodic half-cycle. desorption of this species in the reverse potential sweep creates vacant active sites and promotes continuation of methanol oxidation with a spurt of anodic current until the potential became so cathodic that neither oxidative stripping, nor the follow-up reactions can occur to any significant extent. higher h. heli and f. gobal. j. electrochem. sci. eng. 6(4) (2016) 295-301 doi:10.5599/jese.340 297 anodic currents met in the alkaline solution is due to abundance of available hydroxide ions. it has been suggested that formation of ohads from alkaline solutions is more kinetically favored over that from acidic media by energy roughly corresponding to that of water ionization [16,17]. surely, the higher currents attainable in alkaline media are off-set by formation of carbonates, which is undesirable, as far as the fuel cell industry is concerned [18]. figure 1. typical cyclic voltammograms representing meo on polycrystalline pt in 0.5 m sulfuric acid (a) and 0.5 m sodium hydroxide (b) solutions comprised 0.1 m methanol. potential sweep rates are 20 mv s-1. figure 2a illustrates the galvanostatic scan of meo on pt electrode. current sweep rate was 10 µa s-1. a hard transition from oscillatory to stationary behavior is seen at high current, while a soft supercritical hopf bifurcation is discerned at low current. the potential-time series of methanol electrooxidation on pt electrode with the anodic current step of 300 µa is represented in figure 2b. after approximately 3 s, a periodic pattern was observed, which was followed by an aperiodic oscillation as an indication of chaotic state. in the mechanistic front, it is believed that the current at low potentials is controlled by the oxidative decomposition of adsorbed methanol and formation of cohads and coads species [19-21]. it is conceivable [21] that three adjacent surface sites are initially required for the formation of cohads according to the following mechanism: ch3oh + pt pt-ch3ohads (3) pt-ch3ohads pt-ch2ohads + h+ + e (4a) pt-ch3ohads + ohpt-ch2ohads + h2o + e (4b) pt-ch2ohads + pt pt2=chohads + h+ + e (5a) pt-ch2ohads + pt + oh pt2=chohads + h2o + e (5b) pt2=chohads + pt  pt3≡cohads + h+ + e (6a) pt2=chohads + pt + ohpt3≡cohads + h2o + e (6b) j. electrochem. sci. eng. 6(4) (2016) 295-301 methanol electrooxidation via logistic equation 298 figure 2. (a) onset of sustained potential oscillation in the galvanostatic scan on polycrystalline pt in 0.5 m sulfuric acid comprised in 1 m methanol. current sweep rate is 10 µa s-1, (b) potential-time response of methanol electrooxidation on polycrystalline pt in 0.5 m sulfuric acid comprised 1 m methanol. current step is 300 µa. up to two of these sites, however, are subsequently liberated in the course of reactions yielding bridged or linearly bonded co species [14]: pt3≡cohads  pt2=coads + pt + h+ + e (7a) pt3≡cohads + oh pt2=coads + pt + h2o + e (7b) pt3≡cohads + pt-ohads  pt2=coads + 2pt + h2o (8a) pt3≡cohads + pt-ohads  pt-coads + 3pt + h2o (8b) regeneration of active sites through the processes involving oh species can just partly promote continuation of methanol adsorption and more importantly, it seems that this regeneration needs three adjacent sites with a c3v symmetry as a pre-requisite [22]. therefore, dissociative adsorption of methanol should follow a self-feeding mechanism, where surface diffusion of adsorbates accompanies flow of electrons into external circuit and subsequent regeneration of vacant active sites. surface diffusion of reaction intermediates has also been proposed in other research article [22], especially for interpretation of higher electrocatalytic activities of alloys compared to pure platinum. in this regard, although co is known to strongly adsorb, its surface mobility on platinum and some of its alloys has been reported [23-25]. indeed, this feedback is more clearly visualized through the “cleaning” of the surface according to the following reactions [13]: pt2=coads + pt-ohads  ptcoohads + 2pt (9) pt-coohads + pt-ohads  2pt + co2 + h2o (10) overall, the anodic oxidation of methanol on unmodified polycrystalline platinum seems to be coupling of the faradaic processes and surface transport of adsorbates. this is characteristically a non-linear and dissipative process, which must be influenced by feedback. non-linearity, dissipation and feedback in a dynamic system are clearly represented in the logistic map [26]. figure 3a and 3b present the current-charge (time derivative of charge vs. charge) dependencies obtained from the anodic half cycle voltammograms of figure 1. in order to make comparative plots like those in figure 3, it was necessary to normalize the logistic equation by setting r=4, and to normalize other quantities by dividing them by their maximum values. it has been observed that data was fitted the logistic differential equation quite well (with the correlation coefficient of 0.98), what indicates chaotic nature of the meo dynamics. chaotic switching in the oxidation of methanol carried out h. heli and f. gobal. j. electrochem. sci. eng. 6(4) (2016) 295-301 doi:10.5599/jese.340 299 under the regime of cyclic voltammetry has already been observed by schell and cai [27]. figures 4a and 4b represent the current-charge dependencies of the cathodic half cycle measurements and the related attempted logistic equations. no fit has been observed. apparently, in the cathodic half cycle the initial partial desorption of hydroxide ions surely promotes further dissociation and oxidation of methanol. however, in the further cathodic domains these two effects are no longer cooperative they are virtually independent and merely controlled by the potential itself with no feedback imposed by presence of one upon another. consequently, no fit to the logistic equation with one degree of freedom is expected. meo on polycrystalline platinum electrodes in both alkaline and acidic media seems to follow the non-linear electrocatalytic dynamics, which is characterized by regeneration of active sites in the course of an anodic potential sweep. this process complies with the logistic differential equation. it seems that the processes ensued in the cathodic half cycle of the potential sweep are not controlled by the feedback mechanisms and do not fit the logistic equation. figure 3. time derivative of charge (current) vs. charge dependencies of the anodic half cycle voltammograms of meo on polycrystalline pt, normalized with respect to corresponding maximum values and theoretical curves: (a) acidic solution, (b) alkaline solution figure 4. current-charge dependencies of the cathodic half cycle voltammograms of meo and the related logistic maps: (a) acid solution, (b) alkaline solution. conclusion the electro-oxidation of methanol on platinum was analyzed by quadratic logistic equation indicating domination of two different behaviors in the anodic and cathodic (reverse) half cycles. in the anodic sweep, positive feedback controlled the reaction kinetics, while, the reverse sweep did j. electrochem. sci. eng. 6(4) (2016) 295-301 methanol electrooxidation via logistic equation 300 not obey the logistic map. the results can be of importance in the analysis of the output of the hydrocarbon based fuel cells and the approach is expandable to other direct electrode processes. 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[27] m. schell, x. cai, electrochim. acta 38 (1993) 519-527. http://dx.doi.org/10.1016/00134686(93)85007-l 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.1021/ja0550475 http://dx.doi.org/10.1021/jp040729k http://dx.doi.org/10.1016/0013-4686(93)85007-l http://dx.doi.org/10.1016/0013-4686(93)85007-l http://creativecommons.org/licenses/by/4.0/ comparative voltammetric study and determination of carbamate pesticide residues in soil at carbon nanotubes paste electrodes doi: 10.5599/jese.2013.0041 19 j. electrochem. sci. eng. 4(1) (2014) 19-26; doi: 10.5599/jese.2013.0041 open access : : issn 1847-9286 www.jese-online.org original scientific paper comparative voltammetric study and determination of carbamate pesticide residues in soil at carbon nanotubes paste electrodes thommandru raveendranath babu, sarvareddy rajasekhar reddy, puchakayala sujana electroanalytical lab., department of chemistry, n.b.k.r. science and arts college, vidyanagar, nellore, ap, india corresponding authors: e-mail: sarvareddymadhavi@gmail.com received: october 22, 2013; revised: november 7, 2013; published: january 25, 2014 abstract in this investigation, the persistence of carbamate pesticides in soil samples was investigated. a simple and selective differential pulse adsorptive stripping voltammetry was selected for this investigation. carbon nanotubes paste electrodes were used as working electrodes for differential pulse adsorptive stripping voltammetry and cyclic voltammetry. a symmetric study of the various operational parameters that affect the stripping response was carried out by differential pulse voltammetry. peak currents were linear over the concentration range of 10 -5 to 10 -10 m with an accumulation potential of -0.6 v and a 70 s accumulation time with lower detection limits of 1.09x10 -7 m, 1.07×10 -7 m, 1.09×10 -7 m for chlorphropham, thiodicarb, aldicarb. the relative standard deviation (n=10) and correlation coefficient values were 1.15 %, 0.988; 1.13 %, 0.978; and 1.14 %, 0.987, respectively. universal buffer with ph range 2.0 6.0 was used as supporting electrolyte. the solutions with uniform concentration (10 -5 m) were used in all determinations. calculations were made by standard addition method. keywords thiodicarb; aldicarb; chlorpropham; differential pulse adsorptive stripping voltammetry; cyclic voltammetry; cntpe; soil samples introduction pesticides are extensively and indiscriminately used in modern agricultural practices, resulting in widespread distribution in the environment and posing serious health hazards to animals and human beings. besides inhalation from polluted environment, animals are also exposed to pesticides through the utilisation of treated feeds and fodders. thiodicarb (dimethyl n, n' –thiobis http://www.jese-online.org/ mailto:sarvareddymadhavi@gmail.com j. electrochem. sci. eng. 4(1) (2014) 19-26 cv study of carbamate pesticide in soil 20 (methyl imino) carbonyloxy bisethanimido thioate) is a new carbamate compound with a broad spectrum of activity that is being extensively used for crop protection. it is a class ii category compound (moderately toxic) as set forth by the united states environmental protection agency (usepa) and world health organization (who). various carbamate compounds have been reported to cause biochemical changes in different species of animals [1-5]. little information on the effect of thiodicarb on biochemical profiles is available in dogs and rats [6-8]. however, no detailed report is available regarding the effects of thiodicarb on various biochemical parameters and blood enzymes in animals. chlorpropham (c10h12clno2) chlorpropham is a plant growth regulator used for the pre-emergence control of grass weeds in alfalfa, lima and snap beans, blueberries, cane berries, carrots, cranberries, ladino clover, garlic, seed grass, onions, spinach, sugar beets, tomatoes, safflower, soybeans, gladioli and woody nursery stock. it is also used to inhibit potato sprouting and for sucker control in tobacco. parilla et al. [9] reported spe and hplc/dad methods to determine pesticide residues in water. richard [10] employed hplc method to determine carbamate residues using post-column hydrolysis electrochemical detection. aulakh et al. [11] reported solid phase microextraction hplc for the analysis of pesticides. tomomi et al. [12] developed a new analytical method for the determination of nine pesticide residues including chlorpropham in fruits and vegetables using esi-lc/ms/ms with direct sample injection into a short column. oosselton and snelling [13] reported the use of glc, hplc/dad and tlc for the determination of 51 common pesticides including chlorpropham. thiodicarb (c10h18n4o4s3) thiodicarb is a non-systemic carbamate insecticide whose acetyl cholinesterase activity is related to its main methomyl degradation product[14]. xu and li [15] determined thiodicarb by reverse-phase high performance liquid chromatography. aldicarb (c7h14n2o2s) aldicarb is a carbamate insecticide which is the active substance in the pesticide temik. it is effective against thrips, aphids, spider mites, lygus, fleahoppers, and leafminers, but is primarily used as a nematicide. waliszewski and szymczyński [16] reported a simple method for the gaschromatographic determination of aldicarb, aldicarb sulphoxide and aldicarb sulphone in soil and sugar beets. mora et al. [17] determined the presence of the nematicide aldicarb and its metabolites aldicarb sulphoxide and aldicarb sulphone in soils and potatoes by liquid chromatography with photodiode array detection. although there are reports in the literature for several methods of determinations of pesticides, there are few focused on electrochemical methods; hence, in this investigation, electrochemical determinations [18-20] were employed. experimental apparatus and electrodes the electrochemical measurements were carried out with metrohm model 101 potentiostat and galvanostat. the three-electrode system consisted of carbon nanotubes paste electrode as the working electrode, ag/agcl reference electrode and a platinum wire auxiliary electrode. the electrodes joined the cell through holes in its teflon cover. all of the potentials given in this work were measured with respect to this reference system. electrochemical experiments were carried out in a voltammetric cell at room temperature. a magnetic stirrer was used during the t. raveendranath babu at al. j. electrochem. sci. eng. 4(1) (2014) 19-26 doi: 10.5599/jese.2013.0041 21 accumulation step. the elico li-129 model glass calomel combined electrode was employed for measuring ph values. preparation of carbon nanotubes paste electrode the cntpe was prepared by mixing multiwall cnts powder (diameter 20-50 nm, either 1-5 mm or 5-20 mm lengths) and castrol oil in an agate mortar at a ratio of 50.0 % (w/w) each. a portion of the resulting paste was packed firmly into the cavity (0.8 mm diameter) of a teflon tube. the electrical contact was established via a copper wire [21]. reagents and solutions all reagents used were of analytical reagent grade. double distilled water was used throughout the analysis. in the present investigation, universal buffers in the ph range 2.0 to 6.0 were used as supporting electrolytes and were prepared using 0.2 m boric acid, 0.05 m citric acid and 0.1 m trisodium orthophosphate solutions. samples were obtained from rankem india, ltd. result and discussion all of the compounds exhibit well-defined voltammetric peaks at the same experimental conditions but the reduction electrode potentials are somewhat different; this is attributed to the difference in the nature of groups present in the compounds under investigation (scheme 1). although all of the compounds possess electron-donating nitrogen on one or both sides of carbonyl carbon, there are some differences in the environment of carbonyl carbon. scheme 1. structures of the pesticides investigated in this work in the case of chlorpropham, there is oxygen bonded with a propyl group on one side of the carbonyl carbon and on the other side nitrogen with chlorobenzene. because the aromatic ring is closer to the electroactive group, it will experience less negative charge and undergo reduction at somewhat lower electrode potentials when compared with the other two carbonyl groupcontaining pesticides. two electrons are involved in reduction of one carbonyl group into the hydroxyl group. in the case of thiodicarb, there are two carbonyl groups with the same environments; in the case of two carbonyl groups, there is oxygen bonded with electron-donating nitrogen on one side and nitrogen bonded with electronegative sulphur and electron-donating alkyl groups on the other side along with the other carbonyl group with the same environment. in the case of thiodicarb, however, there is electron-donating nitrogen, alkyl groups with positive inductive effect; their j. electrochem. sci. eng. 4(1) (2014) 19-26 cv study of carbamate pesticide in soil 22 impact on the electronic environment seems to be nil because of double bonds and electronegative groups. in the case of thiodicarb, there is a well-defined peak due to 4 electron reduction of two carbonyl groups. in the case of aldicarb, there is only one carbonyl group on one side with nitrogen, while there is electronegative oxygen bonded with nitrogen on the other side. because of the electro rich nitrogen being directly bonded with a carbonyl group, the environment around the electroactive species seems to be more negative and reduction will take place at greater negative potentials compared with the remaining two pesticides. two electron reductions will take place. figure 1 shows dp-adsv response for the samples (10 -5 m) under investigation over the ph range 2.0-6.0 at cntpe. the systematic studies of the various experimental and instrumental parameters that affect the voltammetric response were carried out in order to establish the optimum conditions. the ph of a solution is a critical factor affecting both the rate and equilibrium state of the reduction process, as well as the rate of the electrode reaction. the influence of ph on the voltammetric response was studied at cntpe of the 10 -5 m samples with ph between 2.0 and 6.0. the maximum peak currents were obtained with ph 4.0. voltammograms obtained for increasing values of the scan rate showed the existence of a linear dependence of the peak current intensity on the scan rate between 10 to 60 mv s. -1 the peak currents were directly proportional to the scan rate. the voltammetric behaviour of samples has been studied in the ph range from 2.0 to 6.0. a single well resolved peak was observed throughout the ph range and this single peak is attributed to the reduction of corresponding groups. all the compounds under investigation exhibit only one voltammetricpeak for each over the ph range 2.0 to 6.0. this wave / peak are attributed to the simultaneous reduction of carbonyl group. typical cyclic voltammograms are shown in fig. 2. no reduction peak is observed in basic medium (8  ph  12) for carbonyl groups due to the precipitation. the diffusion controlled nature of electrode process is evidenced from the linear plots of ip vs. v 1/2 (fig. 3). fig. 1.stripping voltammograms of a chlorpropham, b thiodicar and c – aldicarb at cntpe concentration: 10 -5 m l -1 , scan rate: 60 mv s -1 , ph 4.0 t. raveendranath babu at al. j. electrochem. sci. eng. 4(1) (2014) 19-26 doi: 10.5599/jese.2013.0041 23 fig. 2. cyclic voltammograms of a chlorpropham, b thiodicar and c aldicarb at cntpe, concentration: 10 -5 m l -1 , scan rate: 60 mv s -1 , ph 4.0 fig. 3. ip vs. v 1/2 plots of a chlorpropham, b thiodicarb, c aldicarb. concentration: 10 -5 m l -1 ; scan rate: 60 mv s -1 , ph 4.0 recovery experiments analysis based on the results obtained with differential pulse adsorptive stripping voltammetry and cyclic voltammetry at cntpe, differential pulse adsorptive stripping voltammetry and cyclic voltammetry have been used for the quantitative determination of samples using both calibration and standard addition methods. the investigated compounds were found to exhibit well resolved peaks at ph 4.0, and the sharp well resolved peak was chosen for quantitative studies. peak currents are linear over the concentration range of 10 -5 to 10 -10 m with lower detection limits of 1.09×10 -7 m for chlorpropham, 1.07×10 -7 m for thiodicarb, and 1.09×10 -7 m for aldicarb. the relative standard deviation and correlation coefficients were found to be 1.15 %, 0.988; 1.13 %, 0.978; and 1.14 %, 0.987, respectively, for 10 replicates. j. electrochem. sci. eng. 4(1) (2014) 19-26 cv study of carbamate pesticide in soil 24 determination of pesticide samples from their standard solutions to check the validity of the method, a standard solution (10 -5 m) was prepared in dimethyl formamide. 1 ml of the standard solution was transferred into a voltammetric cell and made up with 9 ml of supporting electrolyte (ph 4.0), before being deoxygenated with nitrogen gas for 10 min, and then subjected to voltammetry. after obtaining voltammograms, a small increment of the standard solution of samples was added to voltammetric cells and was deoxygenated for 10 min; voltammograms were recorded under similar conditions. in the same manner, 10 voltammograms were recorded for 10 standard additions. the optimum conditions for analytical determination were found to be at ph 4.0 and scan rate 60 mv s -1 . the average recovery obtained for the pesticide samples in soil samples ranged from 89.00 to 92.00 % for chlorpropham, from 97.50 to 99.33 % for thiodicarb and from 97.80 to 98.33 % for aldicarb for 10 replicates. the results are shown in table 1. table 1.recoveries of chlorpropham, thiodicarb, aldicarb in standard solution of 1.0×10 -5 m sample amount added, µg ml -1 amount found, µg ml -1 *recovery, % standard deviation chlorpropham 3.0 2.79 93.00 0.024 thiodicarb 3.0 2.98 99.33 0.034 aldicarb 3.0 2.95 98.33 0.028 *average of 10 replicates determination of pesticide samples in spiked soil samples the soil under investigation was spiked with known amounts of formulations and dried on filter paper at laboratory temperature. for extraction, 50 g of the dried soil was transferred into a 250 ml erlenmeyer flask. these samples and blanks were extracted 2-5 times by acetone. the extracts were then evaporated to dryness and the resulting residues were dissolved in dmf and transferred to 50 ml voltammetric flasks. this solution was filtered through whatman nylon membrane filter paper and voltammograms of the filtrates were recorded by following the previously mentioned procedure. the average recovery obtained for the sample in soil samples ranged from 90.00 to 93.00 % for chlorpropham (bud nip), from 93.50 to 95.66 % for thiodicarb (larvin) and from 92.70 to 95.66 % for aldicarb (aldicarb sulphone) for 10 replicates. the results are presented in table 2. table 2. recoveries of chlorpropham, thiodicarb, aldicarb (formulations) in spiked soil samples sample amount added, µg ml -1 amount found, µg ml -1 *recovery, % standard deviation bud nip 3.0 2.76 92.00 0.015 larvin 3.0 2.87 95.66 0.024 aldicarb sulphone 3.0 2.88 96.00 0.018 *average of 10 replicates conclusion in conclusion, the adopted method of differential pulse adsorptive stripping voltammetry is a less tedious and economically low consumption method; hence, this can be used satisfactorily for the determination of pesticide residues in soil. the obtained results also demonstrate the t. raveendranath babu at al. j. electrochem. sci. eng. 4(1) (2014) 19-26 doi: 10.5599/jese.2013.0041 25 suitability of the developed dp-adsv method for the determination of samples under investigation in soil samples. the electrochemical reduction mechanism of the carbonyl group in all three compounds was found to be irreversible. the nature of the electrode process for these compounds is found to be diffusion controlled and involves adsorption on the electrode surface without any kinetic complications. the variation of peak current with the ph of the supporting electrolyte influences the diffusion coefficient values. the slight variations in diffusion coefficient values with increasing ph may be attributed to a decrease in the availability of protons. the heterogeneous forward rate constant values obtained for the reduction of these three pesticides are found to decrease with an increase in the ph of the solution, as expected. from the comparison of the forward rate constant values of the three compounds, it can be seen that they reduce at different electrode potentials, which is attributed to the difference in the molecular environment of the samples under investigation. analytical procedures are described for the quantitative determination of these compounds using dp-adsv. in the present investigation, standard addition and calibration methods were utilised for the determination of these pesticides in soil samples. from the recoveries, it has been observed that the proposed method describes the successful application of an electroanalytical technique for the analysis of these compounds. it also demonstrates that dp-adsv at a carbon nanotubes paste electrode could conveniently be used for the quantitative determination of these pesticides in soil samples. the method shows a good reproducibility and high accuracy compared with spectrophotometric, spectrofluorimetric and chromatographic methods of analysis. references [1] m. jayapragasam, i. jasmine, v.,thenammai, r. kasthuri, madras agric. j. 68 (1981) 461465 [2] r. kiran, m. sharma, r. c. bansal pesticides 19 (1985) 42-43. [3] g. l. kennedy, j. appl. toxicol. 6 (1986) 423-429. [4] s. d. moregaonkar, b. b .deshpande, v. p vadlmudi, n. m. degloorkar, s. r. rajurkar, indian. vet. j. 70 (1993) 945-948. [5] satpal, s. k. jain, j. s. punia, toxicol. int .17 (2010) 30-32. [6] efsa scientific report, 55 (2005) 1-76 [7] h. b. knaak, b. w .wilson. acs symposium series, 273 (1985) 63-79. [8] n. n. hamada, rep. no. 210-216 from hazleton laboratories america inc., vienna, va to union carbide agricultural products company inc. research triangle park, north carolina, 1986 [9] p. parrilla, j. l. m. vidal, anal. lett. 30 (1997) 1719-1738. [10] r. t. krause, j. chromatogr. a 442(1988) 333-343. [11] j. s. aulakh; a. k. malik, v. kaur, p. schmitt-kopplin, crc cr. rev. anal. chem. 35 (2005) 7185. [12] t. goto, y. ito, s. yamada, h. matsumoto, h. oka and. h. nagase, anal. chim. acta 555 (2006) 225-232. [13] m. d. osselton, r. d. snelling j. chromatogr. a 368 (1986) 265-271. [14] g. hoizey, f. canas, l binet, m. l. kaltenbach, g. jeunehomme, m. h. bernard, d. lamiable, j. forensic sci. 53 (2008) 499-502. [15] g. xu, w. zheng, y. li, s. wang, j. zhang, y. yan., int. biodeter. biodegr. 62 (2008) 51–56. [16] s. m. waliszewski, g. a. szymczyński, fresen. j. anal. chem. 338 (1990) 75-76 [17] n. unceta, a. ugarte, a. sanchez, a. gómez-caballero, m. a .goicolea, r. j. barrio, j. chromatogr. a 1061 (2004) 211-216 j. electrochem. sci. eng. 4(1) (2014) 19-26 cv study of carbamate pesticide in soil 26 [18] s. rajasekharreddy, k. chandramohan and, ny. sreedhar, int. j. sci. eng. res, 2(10) (2011) 1-4. [19] s. rajasekhar reddy, t. raveendranath babu, b. sreenivasuluint, j. res. pharm. life sci. 1 (2013) 43-47. [20] s. rajasekhar reddy, t. raveendra nath babu, int. j. nanosci.12 (2013) 130058. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {electrocoagulation of textile wastewater containing a mixture of organic dyes by iron electrode} doi:10.5599/jese.366 103 j. electrochem. sci. eng. 7(2) (2017) 103-110; doi: 10.5599/jese.366 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrocoagulation of textile wastewater containing a mixture of organic dyes by iron electrode borislav n. malinovic1,, miomir g. pavlovic2 ,tijana djuricic1 1university of banja luka, faculty of technology, stepe stepanovića 73, 78000 banja luka, bosnia and herzegovina 2university of belgrade, ictm-ceh, njegoševa 12, belgrade, serbia corresponding author: borislav.malinovic@tf.unibl.org; tel.: +387-51-434-365; fax: +387-51-465-032 received: january 13, 2017; june: may 5, 2017; accepted: june 6, 2017 abstract this study focused on testing the efficacy of iron (fe) electrode in an electrochemical treatment (electrocoagulation) of wastewater containing a mixture of organic dyes. the mixture consists of the following azo dyes: acid black 194, acid black 107 and acid yellow 116. the present organic dyes are toxic, cause skin and eye irritation and are extremely dangerous to aquatic organisms. the study was conducted on a synthetic wastewater prepared in a laboratory electrochemical reactor. during the research, the impact of the current density, various concentrations of dye and supporting electrolyte, electrolysis duration and pulsed current regime were tracked. the results are shown through color removal efficiency, chemical oxygen demand (cod) removal efficiency, current efficiency, and specific energy consumption. at the initial concentration of dye (γ=200 mg/l) and concentration of supporting electrolyte (γnacl=1 g/l) the color removal efficiency of 80.64% was achieved for 420 seconds of treatment (ј=10 ma/cm2). at the initial concentration of dye (γ=50 mg/l) and γnacl= 8 g/l, the color removal efficiency of 96.01% was attained for 300 seconds of treatment (ј=10 ma/cm2). keywords organic dyes, electrochemical treatment; removal efficiency introduction the textile industry is one of the world's biggest polluters according to the amount of wastewater where the most problems are generated by the textile dyeing process. general characteristics of textile dyeing wastewaters are related to, a high content of organic matter, presence of heavy metals and high coloration. as the annual global production of synthetic dyes exceeded 900,000 tonnes in 2009, it is expected to be well over a million tonnes per annum at http://www.jese-online.org/ mailto:borislav.malinovic@tf.unibl.org j. electrochem. sci. eng. 7(2) (2017) 103-110 electrocoagulation of textile wastewater 104 present [1]. since nearly two-thirds (60 – 70 %) of all synthetic dyes are azo dyes [2], azo dyes are considered as the most commonly used dyestuff in the dye industry. during the dyeing processes in textile industry, up-to 50% of the used dyes may not be fixed to their fibre substrates and hence may be washed out to form highly coloured effluent streams [3]. according to yu and wen [4], many synthetic dyes belong to xenobiotic chemicals that are degraded with difficulty in nature. difficult degradation of synthetic dyes is the main reason why their removal from aqueous effluents from the textile industry has received a considerable environment research attention [4]. due to their complex structure and synthetic origin, azo dyes are difficult to be decolourized, what imposed an obligation for their removal from industrial effluents before being disposed into enviroment. many studies have shown that azo dyes contribute to mutagenic activity of ground and surface waters polluted by textile effluents [5,6]. also, discharge of azo dyes into surface water leads to aesthetic problems and obstructs light penetration and oxygen transfer into water bodies, affecting thus the aquatic life. thus, the removal of color from textile effluents has grown as a topic of major concern [7]. in this paper we used a mixture that is used for dyeing polyamide fibers. the mixture contained synthetic dyes that belong to azo dyes and heavy metals cobalt and chromium (co and cr iiicomplex). an extensive literature, reporting the characteristics and applications of most important conventional technologies developed for this purpose, including physico-chemical and chemical methods, advanced oxidation processes (aops), adsorption, microbiological treatments, enzymatic decomposition and electrochemical technologies, has already been published [8-13]. here, an increasingly present electrocoagulation (ec) treatment for removing color from wastewater is described. ec treatment implies formation of coagulants in situ by electrolytic dissolution of the anode made from either aluminum or iron. during the ec process, the anode leads to the formation of metal ions, while evolution of hydrogen gas at the cathode carries the flocculated particles to the surface of the water. the chemical reactions taking place at the anode are given as follows [14]: for iron anode: fe − 2e→ fe2+ (4) at alkaline conditions: fe2+ + 2oh− → fe(oh)2 (5) at acidic conditions: 4fe2+ + o2 + 2h2o → 4fe3+ + 4oh− (6) in addition, there is oxygen evolution reaction: 2h2o − 4e→ o2 + 4h+ (7) the reaction at the cathode is: 2h2o + 2e→ h2 + 2oh− (8) the ec treatment has received great attention in recent years due to its unique features, such as versatility, energy efficiency, automation and cost effectiveness [12,15]. alaton et al. [16] have proven the effectiveness of using ec as a method of removing color from wastewater of textile industry. according to their study, ec proved to be a very promising alternative treatment which is increasingly used in wastewater treatment. simulated wastewater polluted by acidic dyes has b. n. malinovic et al. j. electrochem. sci. eng. 7(2) (2017) 103-110 doi:10.5599/jese.366 105 already been treated by ec using aluminum and steel electrodes. the results showed that the cod removal efficiency, color removal efficiency and the production of sludge were affected by the current intensity, concentration of supporting electrolyte and the initial ph-value. ec with aluminum electrodes is found able to remove almost 90 % of color and reduce cod-value to 40 %, while with steel electrodes, cod value was reduced to about 50 % [16]. the phenomenon of electrode passivation has limited further application of ec treatment. application of pulsed current regime, also known as pulsed electrocoagulation pe (it uses the interactions of electrochemical technology and polarity reversal in an electrical field), brought a significant impact on the electrochemical processes though higher activity and efficiency during the process [17-20]. hence, the development of pe was explored in recent years [20]. experimental electrochemical batch reactor (fig. 1) of capacity 500 cm3 is made of polypropylene and has a possibility of constant mixing (500 rpm/min). the reactor contains two electrodes of the same dimensions (area), p=32.8 cm2, put at a distance, d=30 mm. electrodes were connected to a digital power source (atten, aps3005si; 30v, 5a). pulsed current regime is provided by a generator of variable periodic current (selekt-automatika, serbia). figure 1. schematic view of electrochemical reactor: 1 – source of electric power; 2 – anode; 3 – cathode; 4 – magnetic stir bar; 5 – electrochemical cell; 6 – magnetic stirrer electrode materials were metals of known compositions: iron (en10130-91; max. 0,08 % c, max. 0.12 % cr, max. 0.45 % mn, max. 0.60 % si) and stainless steel (en 1.4301/aisi 304; max. 0.07 % c, 18.1 % cr, 8.2 % ni). attention was put to the appropriate relations between electrode surface area (as) and volume (v) of the reactor. for used electrochemical reactor that relation, (as/v) / (m2/m3), is 16.4 m2/m3, which is in compliance with recommendations given by mameri et al. [21] for current density values of 10 to 200 a/cm2. all experiments were performed at an initial temperature of sample t=20°c. before each treatment, current density was set at the desirable value and electrodes were mechanically cleaned and washed with detergent and acetone in order to remove surface grease. electrode surfaces were additionally cleaned by emerging (5 min) in diluted (1:2) solution of hcl before each treatment. in present experiments, commercially available 99.5 % sodium chloride, nacl, 35 % hydrochloric acid, hcl, acetone, (ch3)2co (lachner, czech) and “ostalan black sr” dye (synthesia a.s., czech) were used. “ostalan black sr” is a mixture of synthetic dyes that belong to the group of azo dyes. according to safety data sheet (sds) [22], “ostalan black sr” consists of the following azo dyes: acid black 194 (the mixture containing 40-50 %), acid black 107 (mixture contains 40-50 %) and acid yellow 116 (mixture contains >1 %). all mentioned dyes are potentially harmful to humans and the environment and labeled as very toxic to living organisms and harmful to aquatic organisms [22]. j. electrochem. sci. eng. 7(2) (2017) 103-110 electrocoagulation of textile wastewater 106 cod was measured by the closed spectrophotometric method on cod reactor (hach, usa), colorimeter (cod checkitdirect, lovibond, germany) by standard cuvette (test tube mr, lovibond, germany), and the dye concentration was determined spectrophotometrically (max = 573nm) on uv-vis spectrophotometer (perkin elmer, lambda 25) according to standard methods [23]. measurement in the pulsed current regime is defined by the cathode current density (jk), time cathode deposition (tk), anodic current density (ja), and time of anodic dissolution (ta). the period of pulsed current waves, (t), is the sum of the time of cathode deposition (tk) and time of anodic dissolution (ta) [24]. figure 2. schematic view of pe [24] results and discussion results of the electrochemical color removal are showed through decrease of mass concentration, 𝛾/ (mg/l), and color removal efficiency, eu /% defined as 𝐸u / % = 𝛾𝑖−𝛾f 𝛾i 100 (9) where 𝛾i and 𝛾f are initial and final dye concentrations, mg/l. current efficiency, i, has been calculated according to the following equation (10):  i / % = (cod𝑖−codf)𝐹𝑉 8𝐼𝑡 100 (10) in eq. (10), codi and codf are initial and final chemical oxygen demand (cod) values, f is the faraday constant, v is the volume of solution (wastewater), 8 is constant, i is the current intensity and t is electrolysis duration. selection of electrodes (soluble or insoluble anode) depends primarily on the mechanism of electrolytic reaction and according to the literature data and previous research it plays the most important role [25]. in the case of ec, selection of electrode material is narrowed to the iron and aluminum electrodes, respectively. figure 3 shows the color removal efficiency at different current densities (1; 2.5; 5; 10 ma/cm2) for the initial dye concentration, γ0 = 200 mg/l, supporting electrolyte concentration, γnacl = 1 g/l, and electrolysis duration, t = 5 min. at the current density of j = 10 ma/cm2, eu = 67.75 %. change of current density to efficiency of color removal could be presented by the following equation. eu = 18.832 ln j + 25.085 (11) during the study, nacl has been used as the supporting electrolyte. figure 4 shows the effect of nacl concentration on color removal efficiency (γ0 = 200 mg/l). duration of electrolysis for all concentrations of the supporting electrolyte was t = 5 min at j =1 0 ma/cm2. figure 4 shows that b. n. malinovic et al. j. electrochem. sci. eng. 7(2) (2017) 103-110 doi:10.5599/jese.366 107 removal efficiency is the highest at γnacl = 8 g/l (eu = 96.01 %), and the lowest at γnacl=1 g/l (eu = 85.3 %). change of the supporting electrolyte concentration to efficiency of color removal could be presented by the equation (12). figure 3. effect of current density on color removal efficiency figure 4. effect of nacl concentration on color removal efficiency eu = 1.5164 γnacl + 83.931 (12) since there is a little difference in eu between γnacl = 1 g/l and γnacl = 8 g/l observed, γnacl= 1 g/l will be used in the further part of research. at the current density, j = 1 ma/cm2, electrolysis time duration, t = 5 min, and the concentration of supporting electrolyte γnacl = 1 g/l, the removal efficiency is increased by reducing the initial concentration of dye, which is shown in figure 5. removal efficiency is the highest at dye concentration, γ = 50 mg/l (eu = 85.3 %) and the lowest at γ = 400 mg/l (eu = 45.81 %.). effect of the initial concentration of dye to the removal efficiency could be presented by the equation (13). figure 5. efficiency of color removal for different initial concentrations of dye figure 6. effect of electrolysis duration on concentration decrease for different initial dye concentrations. eu = -0.1143 γdye + 91.323 (13) figure 6 shows the effect of electrolysis time on decrease of dye concentration for different initial dye concentrations (j = 10 ma/cm2, γnacl = 1 g/l). in figure 7, these results measured using the iron : iron (fe : fe) electrode pair are compared with the results obtained at same conditions (γ0 = 400 mg/l, j = 10 ma/cm2, γnacl = 1 g/l) using the stainless steel (ss) as the cathode material. data in figure 7 suggest that only slightly lower efficiency is achieved for the fe : ss vs. fe : fe j. electrochem. sci. eng. 7(2) (2017) 103-110 electrocoagulation of textile wastewater 108 electrode pair. it seems, however, that the whole process is easier to maintain using the ss cathode, what can be explained by much better anti-corrosion properties of ss than fe. figure 7. effect of application different electrode pairs figure 8. color removal efficiency with and without application of pe in the pulsed current regime (pe) the polarity of electrodes is reversed at a given time during the electrolysis (fig. 2). figure 8 shows differences between the efficiencies of color removal obtained by application pe and without pe in the same conditions (j = 5 ma/cm2, γnacl = 1 g/l, γ0 = 400 mg/l, tk = 42 s, ta = 42 s). it is evident in figure 8, that for longer treatment period, the removal efficiency is higher without applying pe. contrary to already given predictions [23], such behavior could be a sign of no significant passivation and "dirtying" of the electrodes. effect of electrolysis duration (85, 150 and 300 s) at a current density j = 5 ma/cm2 (γ0 = 400 mg/l, γnacl = 1 g/l) and j = 10 ma/cm2 (γ0 = 200 mg/l, γnacl = 1 g/l) on cod values is shown in figure 9. it is shown in figure 9 that in both cases cod-values are linearly depended on time. for t = 300 s, cod-value is reduced from the initial 385 mgo2/l to 224 mgo2/l (γ0 = 400 mg/l), and from the initial 125 mgo2/l to 56 mgo2/l (γ0 = 200 mg/l), what is in full agreement with the study alaton et al. [16]. figure 9. effect of electrolysis duration on cod value figure 10. removal efficiency vs. current efficiency changes of current efficiency on the color removal efficiency for different initial dye concentrations (100 and 200 mg /l) are shown in figure 10. measurements were made at the current density of j = 10 ma/cm2 and the concentration of supporting electrolyte γnacl = 1 g/l. it is obvious in figure 10 that current efficiency exceeds 100 % (119-389 %), which means that in addition to electrochemical reactions some chemical reactions may take place. as expected from previous researches [20,26], for higher color removal efficiency from wastewater, the current efficiency decreases. b. n. malinovic et al. j. electrochem. sci. eng. 7(2) (2017) 103-110 doi:10.5599/jese.366 109 figure 11. energy consumption as a function of color removal efficiency dependence of energy consumption on the color removal efficiency at different initial concentrations is shown in figure 11. at initial concentration, γ0=400 mg/l, energy consumption (per kilogram of dye removed from the waste water) at removal efficiency, eu = 54.76 %, is wsp=3.8021 kwh/kg dye (j = 10 ma/cm2, t = 600 s, γnacl = 1 g/l). conclusions this study shows that ec with fe anode is the efficient process for removing the mixture of azo dyes from waste water. the process is influenced mostly by concentration of supporting electrolyte (nacl), dye concentration, current density and reaction time. for 80.64 % color removal efficiency at initial dye concentration of solution γ0 = 200 mg/l (j = 10 ma/cm2, γnacl = 1 g/l), up to 3.64 kwh/kg dye of energy has been consumed. acknowledgements: this work was supported in part by the ministry of science and technology of the republic of srpska under project 19/6-020/961-171/14. references [1] f. m. d. chequer, j. p. f. angeli, e. r. a.ferraz, m. s. tsuboy, j. c. marcarini, m. s. mantovani, d. p. de oliviera, mutation research/genetic toxicology and environmental mutagenesis, 676(1-2) (2009) 83-86. 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[25] b. malinovic, m. g. pavlovic, j. mandic, glasnik hemičara, tehnologa i ekologa republike srpske (9) (2013) 21-27 (in serbian). [26] b. n. malinovic, m. g. pavlovic, n. halilovic, journal of environmental protection and ecology 16(4) (2015) 1273-1281. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://dyes.synthesia.eu/organics-dyes/textile-dyes/ostalan-black-sr http://dyes.synthesia.eu/organics-dyes/textile-dyes/ostalan-black-sr http://creativecommons.org/licenses/by/4.0/) morphology and cyclic voltammetry analysis of in situ polymerized polyaniline/graphene composites doi: 10.5599/jese.2013.0038 157 j. electrochem. sci. eng. 3(4) (2013) 157-166; doi: 10.5599/jese.2013.0038 open access : : issn 1847-9286 www.jese-online.org original scientific paper morphology and cyclic voltammetry analysis of in situ polymerized polyaniline/graphene composites devesh kumar mahla, subhendu bhandari, mostafizur rahaman*, dipak khastgir rubber technology centre, indian institute of technology kharagpur, kharagpur, india *presently at chemical engineering department, college of engineering sciences, king fahd university of petroleum and minerals (kfupm), dhahran, saudi arabia corresponding authors: e-mail: mrahaman1997@gmail.com; khasdi@rtc.iitkgp.ernet.in received: april 28, 2013; revised: august 10, 2013; published: novembar 09, 2013 abstract graphene (g) was synthesized from normal graphite powder via graphene oxide (go). graphene was also produced from finer particles of graphite subjected to ball milling to check the effect of particle size of graphene on the properties of composite. pani/graphene composites of different compositions were prepared by in situ polymerization of aniline to polyaniline in the presence of graphene powder. graphene, graphene oxide and pani/graphene composites were characterized by uv, ir, tem, and cyclic voltammetry. pani/graphene composites exhibit higher current in cyclic voltammetry study compared to either neat pani or neat graphene. the value of capacitance achieved for pani/graphene composites is found to depend on the size of graphene particles, finer the particle higher is the capacitance for the composites. however, the effect of composition on cv characteristics of composite is relatively less pronounced compared to the size of graphene sheets coated with pani. keywords polyaniline; graphene; conductive composites; cyclic voltammetry. introduction conductive polymer composites filled with different carbon fillers have been used in different electrical and electronic applications [1–7]. among the carboneous fillers, graphene based polymer composites have attracted much attention in the recent years [8–10]. graphene, oneatom-thick planar sheet of carbon atoms, is densely packed in a honeycomb crystal lattice of http://www.jese-online.org/ mailto:mrahaman1997@gmail.com mailto:khasdi@rtc.iitkgp.ernet.in j. electrochem. sci. eng. 3(4) (2013) 157-166 in situ polymerized polyaniline/graphene composites 158 graphite. graphene, at large scale, is produced by reducing graphene oxide by some reducing agent. graphene is having large aspect ratio (ratio of surface area to thickness) [11], high mechanical strength [12], high electrical conductivity [13], ability to disperse in polymers well [14], high thermal conductivity and stability [15]. it can be effectively exploited for many applications [16,17]. graphene oxide derived from graphite through oxidation has many polar groups on its surface like –c=o, –cooh, –oh, which give its properties like hydrophilicity, dispersibility, and compatibility with many polymers. among conductive polymers, polyaniline (pani) has attracted considerable attention because of its ease of preparation, low cost of monomer, good environmental stability, good conductivity control through doping as well as a good control of oxidation level. all these properties give pani the potential for wide applications [18–22]. it can effectively be used as opto-electronic sensors [23–26], conductive paints and adhesive [27–29]. pani has also good power and energy density, so it has been analyzed for the development of new supercapacitor materials [30–32]. generally, electrochemical super capacitors are of two types; type-1 is electrical double layer capacitor (edlc) due to charge separation at electrode/electrolyte interface, and type-2 is pseudocapacitor, where capacitance is due to both reversible faradic reaction and charge separation [33]. the latter category of capacitor materials can be further divided in to two different types of material based on their origin, for example i) metal oxide like ruo2 and ii) conducting polymer like pani. the energy and power density of power sources like batteries and capacitors can be shown by ragone plot. batteries have good energy density and edlc type capacitors have good power density whereas pseudocapacitors work on the principle of both batteries and capacitors, simultaneously. pani can act as a pseudocapacitor and it has the power to store energy by reversible chemical reactions, as observed in batteries, and also like edlc. in polyaniline/graphene composite, graphene not only increases the edlc capacity of pani but also helps in facilitating charge transfer and consequently enhances the utilization of pani as a capacitor material by increasing available surface area. the scrutiny of available literatures reveals that there exist some reports on pani/graphene composites. however, all these reports mainly deal with only one or at best two compositions of pani/graphene system. the effect of size of graphene particles on the properties of pani/graphene composites has not been extensively investigated. so the present study deals with synthesis and characterization of pani/graphene composites with different compositions. the preparation of pani/graphene composites with different size of graphene particles has also been reported. experimental details polyaniline was synthesized in aqueous medium as reported in the literature [34]. the graphite oxide was prepared by modified hummers method [35]. during the preparation of graphite oxide, 50 mg of graphite was mixed with 25 mg of nano3 and 2ml h2so4. after stirring of the mixture for 10 minute in ice bath, 150 mg of kmno4 was added to it with constant stirring for 15 minutes. then the resultant mixture was heated up to 98 °c and deionised water was added slowly with constant stirring. h2o2 was added and then the suspension was filtered followed by washing of filtrate with deionised water for 8 times to remove any water soluble residues. the filtrate so prepared is graphite oxide which was used in further synthesis. during the preparation of polyaniline/graphene composites the following steps have been adopted. in the first step, the d. k. mahla at al. j. electrochem. sci. eng. 3(4) (2013) 157-166 doi: 10.5599/jese.2013.0038 159 graphite oxide was added (as per the ratio desired, explained below) to deionised water. the mixture was ultrasonicated for one hour followed by addition of aniline to it, and the mixture was again ultrasonicated for half an hour. while ultrasonicated, layers in graphite oxide were delaminated to produce graphene oxide. this mixture of graphene oxide and aniline was then kept as such for a day at room temperature. in this mixed mass the requisite amount of 0.2 m solution of hcl was added at room temperature followed by requisite amount of 0.25 m solution of ammonium persulfate was added. continuous stirring was maintained during the reaction. after addition of ammonium persulfate the stirring was continued for 10 minutes and the conical flask was covered with lid and kept as such for 24 hours. the whole reaction was carried out between 5 10 °c with the help of ice bath. the polyaniline/graphene composite so prepared was filtered. to remove the unreacted reactant from the mixture and to dope pani adequately the filtrate was repeatedly washed with 2 m hcl solution. the so prepared composite was dried in vacuum oven at 50 °c temperature until the final weight became constant. the graphite oxide in polyaniline/graphene composites was varied in different ratio with its weight percent being 4, 8, 12 and 16 of combined weight of initially taken aniline and graphite oxide. the nomenclature of the so varied weight has been given as pg4, pg8, pg12 and pg16. pani/graphene composites were also prepared in a little different way. in this preparation the graphite powder as received was ball milled for 60 hours at 300 rpm to form finer particles of graphite and used for preparation of graphene oxide followed by preparation of pani/graphene composites. this was done to check the effect of size of graphene sheet on composite properties. the so prepared nano graphite powder was converted into graphite oxide by modified hummers method [19]. the procedure adopted for the preparation of polyaniline/nano-graphene composite is the same as described earlier. the weight per cent of nano-graphite oxide has also been varied in the same way as described above in the proportion of 4, 8, 12 and 16, and identified as png4, png8, png12 and png16. different polyaniline/graphene composites were characterized by different techniques like cyclic voltammetry (cv) and transmission electron microscopy (tem). the cv study was performed with different scan rates of 10 mv/s, 50 mv/s, 80 mv/s and 100 mv/s with electrolyte being 2 m h2so4 solution. the reference electrode used was ag/agcl and counter electrode being platinum wire. the working electrode used in the experiment was modified glassy carbon electrode. the polyaniline/graphene composite was mixed in the nafion solution of chloroform. the mixture was then put on the electrode with the help of glass rod. the cv was performed within the potential range of -0.2 to 1.0 v at room temperature. for tem analysis, the samples were used after ultrasonication for one hour in deionised water. ir and uv techniques were used for characterization, and sample of graphene and graphene oxide were ultrasonicated in acetone for half an hour before testing. this solution was spread over kbr pellet and ir was performed. for uv analysis the g and go samples were ultrasonicated in deionised water for 1 hour before test. results and discussion uv and ft-ir synthesized graphene oxide and graphene were characterized by uv and ft-ir spectroscopy. from uv spectra (figure 1), one can detect the π – π* transition at 260 nm (38461 cm -1 ) for the graphene while graphene oxide shows n – π* transition at 305 nm (32786 cm -1 ) and π – π* at 240 nm (41667 cm -1 ). from ir spectra (figure 2) for graphene a very few peaks can be detected as neat j. electrochem. sci. eng. 3(4) (2013) 157-166 in situ polymerized polyaniline/graphene composites 160 graphene contains very few organic groups. ft-ir peak in between 3500 to 3100 cm -1 is due to the absorbed moisture and the peak around 2200 cm -1 is due to atmospheric carbon dioxide often seen if the experiment is not done in nitrogen environment. there is a little hump around 1500 cm -1 because of c=c stretching in benzene ring of graphene. in the case of go, due to oxidation there are several absorption bands corresponding to large number of chemical groups attached to the graphene sheets. figure 1. uv spectra of graphene and graphene oxide. figure 2. ft-ir spectra of graphene and graphene oxide. d. k. mahla at al. j. electrochem. sci. eng. 3(4) (2013) 157-166 doi: 10.5599/jese.2013.0038 161 tem analysis in tem images the presence of single layer graphene (figure 3) and multi layer graphene sheets stacked together (figure 4) can be seen. from other tem images (figures 5 and 6) the formation of polyaniline particles (figure 5) and polyaniline coating on graphene sheets (figure 6) can also be detected. graphite to graphene was routed through formation of graphene oxide followed by ultrasonication when different degree of delamination of original graphite occured. if the extent of delamination is very less, the original layered structure of graphite is almost retained. all these studies reveal that successful preparation of graphene from graphite can be achieved by the method described in experimental section. figure 3. tem image of graphene sheet figure 4. tem image of few graphene sheets stacked together figure 5. tem image of polyaniline particles figure 6. tem image of polyaniline/graphene composite. cyclic voltammetry analysis the study of cv (current-voltage) for the polyaniline/graphene composite was carried out to understand the electrochemical behavior of these composites. figure 7 shows the cv of pani, g, go and pg4. the shapes of cv curves show the reversible charge-discharge behavior of composite electrode [36,37]. for pani the first anodic peak appears around 0.29 v, which is associated with the oxidation of leucoemeraldine to emeraldine [38–41]. the second anodic peak around 0.84 v is attributed to the emeraldine to pernigraniline transition (scheme 1). figure 7 also shows that with the addition of graphene the current flow increases and the peak also broadens as compared to j. electrochem. sci. eng. 3(4) (2013) 157-166 in situ polymerized polyaniline/graphene composites 162 neat pani and neat graphene as shown in the inset. the differences in qualitative and quantitative shape of cv curve for pani and polyaniline/graphene composites have been attributed to the dispersion of pani particles on graphene sheet which reduces the migration length of electrolyte ions during the charge-discharge process where graphene sheets provide an excellent path for the charge transfer [42]. hence, the graphene increases the efficacy of pani by providing a larger surface area and reducing the resistance to current flow. scheme 1 oxidation/reduction chemistry of polyaniline. figure 7. cyclic voltammograms of pg8, polyaniline, graphite oxide and graphene d. k. mahla at al. j. electrochem. sci. eng. 3(4) (2013) 157-166 doi: 10.5599/jese.2013.0038 163 the stability of the composite has been checked by repeating the cv plots under the same condition for the same sample for several cycles [43]. figure 8 shows the average particle size of graphite as received and the nano sized graphite with cumulative contribution in the total number of particles, %. the particle size analysis was done by fritz particle size analyzer. the average particle size of graphite is 800 µm while the particle size of nano graphite was beyond the range of analyzer. figure 9 shows that after 100, 200 and 300 cv cycles, at 0.08 v/s scan rate, the currentpotential graphs almost superimposes on each other. this reveals the good electrochemical stability of the composite which is essential when it is used as a capacitor. figure 8. particle size analysis of graphite and nano graphite figure 9. cyclic voltammogram of pg8 at 100 mv/s scan rate after 100, 200 and 300 cycles j. electrochem. sci. eng. 3(4) (2013) 157-166 in situ polymerized polyaniline/graphene composites 164 both composition of pani/graphene composite and size of graphene particles are expected to influence current-voltage characteristics of composites. figure 10 compares the current potential behavior of polyaniline/graphene composite with variation in graphene weight percent and size of graphene sheet. it shows that with the addition of graphene the peaks in current potential plots broaden in comparison to pani. this is due to availability of alternate as well as low energy paths for charge transfer in pani/graphene system through graphene. the presence of graphene in composite decreases the resistance of system hence there is an increase in the current and broadening of the peak. as shown in figure 10 the resistance in png4 is almost equal to that of pg8 composite. this shows that with smaller size of graphene sheet the available surface area is higher and hence it leads to better utilization of pani. hence, this infers that with larger surface area the small scale graphene better decreases the resistance of the system and increases the capacitance of composite material by better utilization of pani in composite. figure 11 shows that with increase in the graphene weight percent there is no much increase in the current flow relatively to the neat pani and in pani/graphene composite as shown in figure 7 [44]. it could also be seen that at lower scan rates (for example 0.01 v/s) different oxidation and reduction peaks could be distinguished but as the scan rate is increased, the peaks merged. this is due to the time constant effect in cv, since i=e/rs (e -t /cdrs), where e is the potential step, rs being the solution resistance, cd, the double layer capacitance and t is time [45-46]. here i decreases as the cdrs increases which happens at a low scan rate. as the scan rate is increased cdrs doesn’t give enough time to the current to decrease hence distinct peaks are not observed. figure 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[46] j. b. allen, r.f. larry, 2nd addition, electrochemical methods: fundamentals and applications, john wiley (2001). © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ abstract introduction experimental details results and discussion uv and ft-ir tem analysis cyclic voltammetry analysis conclusions acknowledgement references {a sensitive voltammetric sensor for specific recognition of vitamin c in human plasma based on mapbi3 perovskite nanorods:} http://dx.doi.org/10.5599/jese.1153 175 j. electrochem. sci. eng. 12(1) (2022) 175-183; http://dx.doi.org/10.5599/jese.1153 open access : : issn 1847-9286 www.jese-online.org original scientific paper a sensitive voltammetric sensor for specific recognition of vitamin c in human plasma based on mapbi3 perovskite nanorods gizem tiris1, yasamin khoshnavaz2, elif naz öven2, mohammad mehmandoust2,3, and nevin erk2,3,  1bezmialem vakif university, faculty of pharmacy, department of analytical chemistry, 34093 i̇stanbul, turkey 2ankara university, faculty of pharmacy, department of analytical chemistry, 06560 ankara, turkey 3sakarya university, biomaterials, energy, photocatalysis, enzyme technology, nano & advanced materials, additive manufacturing, environmental applications, and sustainability research & development group (bioenams r&d group), 54187 sakarya, turkey corresponding authors: mehmandoust@ankara.edu.tr; erk@pharmacy.ankara.edu.tr; tel.: +90-5526828057 received: october 31, 2021; accepted: january 19, 2021; published: january 28, 2022 abstract a novel and sensitive electrode was suggested for the rapid determination of ascorbic acid (aa) using a glassy carbon electrode (gce) modified with synthesized mapbi3 and l-cys (lcys/mapbi3/gce). determination of ascorbic acid as an important component of the human diet due to help in decreasing blood pressure and improving endothelial function is crucial. the synthesized mapbi3 was characterized by different methods, including transmission electron microscopy (tem), scanning electron microscopy (sem), energydispersive x-ray spectroscopy (edx), and x-ray diffraction (xrd). the fabricated electrode exhibited superior electrical conductivity and fast electron transfer kinetics. the results illustrated that the developed electrode had an outstanding electrocatalytic activity towards the oxidation of aa in 0.1 m britton–robinson buffer(b-r) as a supporting electrolyte. the modified electrode demonstrated a linear range in differential pulse voltammetry of 0.02–11.4 µm with a low detection limit of 8.0 nm for ascorbic acid. it can be stated that the proposed sensor can be successfully applied to the determination of ascorbic acid in human plasma samples. keywords voltammetry, ascorbic acid, mapbi3, cysteine, human plasma http://dx.doi.org/10.5599/jese.1153 http://dx.doi.org/10.5599/jese.1153 http://www.jese-online.org/ mailto:mehmandoust@ankara.edu.tr mailto:erk@pharmacy.ankara.edu.tr j. electrochem. sci. eng. 12(1) (2022) 175-183 sensor for recognition of vitamin c in human plasma 176 introduction ascorbic acid (aa), namely vitamin-c, is a six-carbon lactone produced by plants and some animal species but not by humans or other primates, and it must be obtained from food. vitamin c's biological action is critical for the skin's proper functioning. l-ascorbic acid is the more physiologically active of the two forms of vitamin c found in nature in fruits and vegetables such as oranges, broccoli, leafy greens, grapefruit, and peppers, and is the more useful, but the "d" form can be made via chemical synthesis but has no significant biological role. l-ascorbic acid protects against hydroxyl radicals, superoxide, and singlet oxygen, among other things. furthermore, it lowers the membrane-bound antioxidant -tocopherol. diabetes, coronary artery disease, hypertension, and chronic heart failure benefit from l-ascorbic acid's endothelium-dependent vasodilation. monitoring aa content should be regarded as an essential and relevant task for evaluating the quality of final food items, raw materials, and various other substances, considering the nutritional value and therapeutic aa characteristics. although a variety of analytical methods are utilized to determine aa, including spectroscopic [1], chromatographic[2,3], the electrochemical methods have been widely performed in recent years [48] because of their rapid response, high sensitivity, good stability, superior selectivity, and costeffectiveness [9-11]. however, enhancing electrochemical performance in real samples necessitates the development of well-defined chemical structures and porous nanomaterials [12-14]. recently, there is a big jump in fine-tuning the properties of nanomaterials and utilizing them in various applications [15-17]. in terms of materials, methylammonium lead iodide (ch3nh3pbi3 or mapbi3), a perovskite-like organic-inorganic metal halide, has emerged as a viable low-cost material for nextgeneration high-efficiency perovskite solar cells[18]. furthermore, the sensitivity of perovskite materials allows this disadvantage to be turned into an advantage [19]. as far as we know, this is the first sensor system using mapbi3 as a modifier that has appropriate results for measuring one of the main parts of the human diet. in biological systems, cysteine, a thiol-containing non-essential amino acid, plays a significant function. it's found in a lot of proteins and acts as a precursor for protein synthesis, and is also utilized as a modifier in the production of electrochemical sensors. in electrochemical investigations, cysteine and cysteine-containing materials adorned electrodes are particularly popular. in this study, l-cys were modified on mapbi3, and the resulting nanomaterial was utilized to modify the gce surface to fabricate the sensor. a voltammetric method was developed using l-cys@mapbi3/gce and utilized to determine aa in the human plasma samples. furthermore, our study exhibits a sensitive and selective sensor for aa determination with a lod value of 8.0 nm in a wider linear range of 0.02 to 11.4 µm. finally, the sensor's applicability is demonstrated by its application to human plasma, which yielded satisfactory recovery results. experimental materials: methylammonium iodide, lead iodide, oleic acid, oleylamine, l-cysteine were acquired from sigma aldrich co. (germany). a human plasma sample was also obtained from sera-flex inc (turkey). all chemicals were of analytical grade. apparatus xrd patterns were collected by rigaku smart lab xrd. scanning electron microscopy images and energy-dispersive x-ray spectroscopy were recorded by zeiss gemini at 3.00 kv. electrochemical methods such as cyclic voltammetry (cv), differential pulse voltammetry (dpv), and electrochemical g. tiris et al. j. electrochem. sci. eng. 12(1) (2022) 175-183 http://dx.doi.org/10.5599/jese.1153 177 impedance spectroscopy (eis) were carried out through pgstat128n (metrohm inc., switzerland). tem images were obtained using an fei tecnai g2 spirit microscope (oregon, usa) at 120 kv. the synthesis procedure of mapbi3 mapbi3 nanorods were synthesized using a facile ligand-assisted deposition approach. to begin with, 16.0 mg methylammonium iodide and 40.0 mmol lead iodide were dissolved in 2.0 ml dimethylformamide (dmf) and stirred for 24 h. following, 400.0 μl oleic acid and 50.0 μl oleylamine were introduced into the resultant dispersion and kept stirring for 4 h. moreover, 200.0 μl of the mixture solution was poured into 5.0 ml chloroform solution, stirring at the dark conditions over 5.0 h to obtain a dark brownish solution as mapbi3 nanorods were obtained [20]. preparation of fabricated electrode before modification, the bare electrode was polished with aluminum slurries (1.0 and 0.05 μm) and then washed by using hno3 (10 vol.%), ethanol, and distilled water, respectively. afterward, 8.0 µl of mapbi3 was dropped onto the gce and allowed to dry at room temperature. in the following, 5.0 µl of l-cys solution (2.0 mg ml-1) was dropped on the mapbi3 electrode surface and was left to dry at room temperature [21]. preparation of real sample for the preparation procedure for the human plasma as the real sample, firstly, 1.0 ml of human plasma was treated with 1.0 ml of acetonitrile to eliminate the proteins. after that, the solution was centrifuged over 20 minutes at 10 °c at 6000 rpm. before being put into the electrochemical cell, the samples were diluted to a certain concentration using b-r buffer at ph 7.0. [14]. results and discussion characterization figure 1 illustrates the x-ray diffraction patterns recorded in the 2 ranging from 5.0 to 90.0° for mapbi3. the typical peaks at 19.40, 24.47, 29.42 and 30.89° corresponded to (110), (202), (004), (220) lattice planes of the tetragonal phase perovskite, respectively [22-24]. moreover, as shown in fig. 1, the xrd pattern of the mapbi3 presents all characteristic peaks of mapbi3 with good tetragonal structures with good crystallinity [25,26]. 2 / o figure 1. xrd pattern of mapbi3 in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1153 j. electrochem. sci. eng. 12(1) (2022) 175-183 sensor for recognition of vitamin c in human plasma 178 sem and tem images of pristine mapbi3 are displayed in figure 2. as shown in figures 2a and 2b, the pristine mapbi3 nanorods have a size of 50.0-200.0 nm, relatively smooth surfaces, and high crystallinity. sem image (figure 2c) indicates the formation of highly crystalline, dense, and pinholefree mapbi3. elemental composition and percentage of as-synthesized mapbi3 were validated through edx analysis(figure 2d). it was clearly observed sharp peaks relative to pb, n, and i. a b c d figure 2. a and b tem and c sem images of mapbi3; d edx spectrum of mapbi3 electrochemical characterization of the l-cys@mapbi3/gce: the effect of the modification of electrode surface was studied via dpv techniques. figure 3 showed the dpv response of the bare electrode, mapbi3/gce, and l-cys@mapbi3/gce to a fixed concentration of aa (0.5 µm). obviously, it can be seen that the presence of the mapbi3 and l-cys increased the signal magnitude of the glassy carbon-based electrode to determine aa. in other words, such a described behavior can be related to the electrocatalytic activity of l-cys@mapbi3/gce for electro-oxidation of aa. g. tiris et al. j. electrochem. sci. eng. 12(1) (2022) 175-183 http://dx.doi.org/10.5599/jese.1153 179 figure 3. dpvs of bare electrode (a), mapbi3/gce (b), l-cys/mapbi3/gce in 0.1 m b-r buffer at ph 7.0 containing 0.5 µm aa. according to the obtained voltammograms in figure 4a, l-cys@mapbi3/gce illustrated the highest electrochemical activity compared to the bare electrode (5.63 to 10.2 µa). in addition, the values of δep were calculated as 107.4 mv for l-cys@mapbi3/gce and 298.3 mv for bare electrode, indicating the synergic effect between l-cys and mapbi3. eis experiments were also performed to extract impedance characteristics of the fabricated electrode in 0.1 m kcl containing 5.0 mm [fe(cn)6-3/-4] solution as a redox probe. the nyquist plots of the obtained data are shown in figure 4b. the charge transfer resistance magnitude at the respective electrodes was ascribed to the diameters of the depressed semicircles. as shown, the charge transfer resistance at l-cys@mapbi3/gce surface is decreased about 1.7-fold compared to the bare electrode (23.57 kω), indicating fast electron transfer on the electrode surface [27]. results confirmed that the presence of l-cys and mapbi3 increases the electrode surface's conductivity [28]. figure 4. (a); cvs and (b); eis of bare (a) and l-cys@mapbi3/gce (b) in in 0.1 m kcl containing 5.0 mm [fe(cn)6-3/-4]. ph and cv studies the effect of ph on the oxidation of 1.0 μm aa was depicted in figure 5a. the ph value of the b-r buffer was adjusted using 0.1 m hcl and naoh. the oxidation currents of aa were observed in the ph range between 2.0 to 8.0 in 0.1 m b-r buffer. the peak current enhanced when the ph http://dx.doi.org/10.5599/jese.1153 j. electrochem. sci. eng. 12(1) (2022) 175-183 sensor for recognition of vitamin c in human plasma 180 increased until ph 7.0, and after ph 7.0, the peak current decreased with a further increase of ph. finally, ph 7 was selected for further experiments. the diffusion-controlled mechanism at l-cys@mapbi3/gce was investigated using the linear relationship between the square root of the scan rates and current peak at different scan rates (10.0 to 400.0 mv s-1). the oxidation peak changed to positive potentials with increasing scan rates, as seen in figure 5b. as shown in figure 5c, the anodic peak currents enhanced linearly with the square root of scan rate, indicating that the redox reaction of the electrodes was a diffusion-controlled process [29]. therefore, the electrocatalytic behavior of the electrode was improved. as for an irreversible electrochemical reaction, the epa is determined by the following (eq. 1):         = − −        0 pa ln ln rt nf rt e e nf rtk nf (1) according to the linear relationship of epa against ln  (figure 5d), the value of n was observed to be approximately 1.0. figure 5. a the ipa vs. ph curve for electro-oxidation of aa at the surface of l-cys@mapbi3/gce; b cvs of l-cys@mapbi3/gce in the presence of 0.1 m kcl containing 1.0 µm aa; c the plot of ipa against ν1/2 relative to electro-oxidation of 1.0 μm aa at l-cys@mapbi3/gce; d the plot of epa vs. natural logarithm chronoamperometry study the chronoamperometric study was conducted using a developed electrode at 0.6 v in the presence of different concentrations of ascorbic acid (50.0 and 100 μm) in 0.1 b-r buffer (ph 7.0). g. tiris et al. j. electrochem. sci. eng. 12(1) (2022) 175-183 http://dx.doi.org/10.5599/jese.1153 181 by plotting i versus t-1/2 and cottrell’s equation (figure 6), the diffusion coefficient (d) was estimated as 8.99×10-7 cm2 s-1. figure 6. chronoamperograms obtained at l-cys@mapbi3/gce, (inset) cottrell's plot for the data from the chronoamperograms determination of ascorbic acid figure 7a demonstrated the dpv curves of various ascorbic acid concentrations at the lcys@mapbi3/gce in 0.1 m b-r at ph 7.0. the increase in the current values was linearly related to the aa concentration in the concentration range of 0.02 to 11.4 µm (figure 7b). the limit of detection (lod) was 8.0 nm aa, according to the definition of lod = 3sb/m [30]. cys@mapbi3/gce has a lower detection limit for aa compared with other modified electrodes (table 1). figure 7. a dpvs of l-cys@mapbi3/gce with increasing aa concentration; b the plot of the oxidation current as a function of aa concentration table. 1. comparison of different sensors for the determination of aa method modified linear range, µm lod, µm ref. dpv cl-tin/gce 50-1500 1.50 [31] amperometry gchs-cnpts/gce 10-3570 1.09 [32] amperometry toab/yd/mwcnt/gce 0.0187–1.85 0.18 [33] amperometry toab/ergo@yd/gce 1.33-1460 0.28 [34] dpv l-cys@mapbi3/gce 0.02-11.4 0.008 this work http://dx.doi.org/10.5599/jese.1153 j. electrochem. sci. eng. 12(1) (2022) 175-183 sensor for recognition of vitamin c in human plasma 182 reproducibility, repeatability the reproducibility, repeatability, and selectivity of l-cys@mapbi3/gce were investigated by recording dpv of 1.0 µm aa at ph 7.0. the relative standard deviations (rsd) of 2.3 and 1.2 % for five successive recorded signals (repeatability) and five independents (reproducibility) of l-cys@mapbi3/gce, respectively that confirmed outstanding repeatability and reproducibility for l-cys@mapbi3/gce as an electroanalytical sensor toward aa. determination of ascorbic acid in the human plasma sample the utilization of the developed sensor l-cys@mapbi3/gce in the real sample was also observed using the standard addition method in human plasma samples. the recovery of the spiked samples was obtained from 97.5 to 102.8 %. results show that the developed electrochemical sensor will be useful for diagnosing biological samples. conclusions in summary, a sensor based on l-cys@mapbi3 was fabricated on a glassy carbon electrode to determine ascorbic acid. the results offered a well-defined oxidation peak for oxidation of ascorbic at l-cys@mapbi3/gce, which were large enough to determine aa. moreover, the fabricated l-cys@mapbi3/gce exhibited acceptable results as a working electrode with a low detection limit, appropriate reproducibility, and repeatability. the fabricated sensor was successfully utilized to analyze aa in the real sample. the as-fabricated electrode could be an outstanding 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tramadol using modified screen-printed electrode sayed zia mohammadi1, maedeh jafari2, peyman mohammadzadeh jahani3,, sayed ali ahmadi4,, raana mashayekh5, 1department of chemistry, payame noor university, tehran, iran 2department of pediatrics, school of medicine, kerman university of medical sciences, kerman, iran 3student research committee, school of public health, bam university of medical sciences, bam, iran 4department of chemistry, kerman branch, islamic azad university, kerman, iran 5student research committee, bam university of medical sciences, bam, iran corresponding authors:  peyman1234@gmail.com, phone: (+98)34-31321345; ahmadi.iauk59@gmail.com received: october 17, 2021; accepted: november 1, 2021; published: december 2, 2021 abstract the detection of tramadol using a screen-printed electrode (spe) modified with la3+/zno nano-flowers and multi-walled carbon nanotubes (la3+/zno nfs-mwcnts/spe) is reported in this work. to examine electrochemical oxidation of tramadol, the modified electrode was implemented with the utilization of differential pulse voltammetry, chronoamperometry and cyclic voltammetry as diagnostic techniques. the proposed electrode displays favorable electrocatalytic behavior concerning tramadol oxidation with an approximately 330 mv potential shift to less positive potential. in the range of tramadol concentrations of 0.5 to 800.0 μm, differential pulse voltammetry displays the linear response. tramadol detection limit of 0.08 μm was achieved within optimized testing conditions for this sensor of simple construction. lastly, the fabricated sensor was utilized for the determination of tramadol in urine samples. keywords la3+/zno nano-flower; multi-walled carbon nanotubes; voltammetry. introduction drug analysis is vital for numerous uses, such as forensic science, quality control, and clinical applications. tramadol is a synthetic analgesic applied in the cure of mild to severe chronic or acute pain. together with other narcotics used to treat chronic, obstetric, and cancer pain control, as well as moderate surgical pain in adults and children, tramadol has been implemented since 1977. tramadol has similar characteristics with codeine with a difference in having a methyl substitute on the morphine structure phenolic moiety. tramadol restricts the reuptake of norepinephrine and http://dx.doi.org/10.5599/jese.1141 http://dx.doi.org/10.5599/jese.1141 http://www.jese-online.org/ mailto:peyman1234@gmail.com mailto:ahmadi.iauk59@gmail.com j. electrochem. sci. eng. 12(1) (2022) 127-135 determination of tramadol using modified spe 128 serotonin while expediting the release of these neurotransmitters. it is categorized as an opioid and binds to the brain receptors (opioid receptors). it is also a synthetic codeine analog and not categorized as a controlled substance. moreover, addicts may use it as an anti-addictive substance, although extreme tramadol dependence has been evident because of its impact on opioid receptors within the central nervous system [1,2]. there has been an increase in lethal tramadol poisoning in the past few years [3]. numerous analytical methods have been implemented to determine drugs in clinical and pharmaceutical preparations. such methods include high-performance liquid chromatography (hplc) combined with quadrupole electrospray ionization mass spectroscopy (q-tof-ms), uv-vis spectrophotometry coupled with chemometric analysis, ultrahigh performance liquid chromatography tandem mass spectrometry (uplc-ms/ms) [4-8]. compared to mentioned methods, electroanalytical methods are more advantageous in terms of higher sensitivity, lower equipment costs and simplicity. numerous electrochemical methods are mentioned in the literature for analyte determination, with and without the use of modified electrodes [9-14]. in designing authentic and portable electrochemical sensing platforms with improved performance, screen-printed electrodes (spes) are the most promising components. these components are extremely versatile, user-friendly, and cost-efficient analytical tools and are of a field-based size. for example, spes provide the possibility for sample volume reduction to a few microliters and may easily be configured for multiple analytes detection while avoiding contamination or memory effects. these devices have displayed competency for in situ monitoring of pollutants and real-time analysis to prevent pollution. in recent years, researchers have focused on designing and synthesizing nanomaterials for various applications due to their unique physical and chemical properties [15-19]. lately, advances in nanoscience and nanotechnology have aided the progression of spes fabrication. due to the specific characteristics of nanomaterials, spe modification with nanostructured materials has paved the way for improved electroanalytical performances compared to bare spes. when doped with carbon nanotubes, metal nanoparticles etc., spes have exhibited improved sensitivity and detection limit [20,21]. in chemistry, a nano-flower is the name given to a compound of certain elements resulting in compositions identical to flowers from a microscopic point of view. in a few cases, these compositions are referred to as nano-bouquets or nano-trees. within the scientific community, la3+/zno nano-flowers (la3+/zno nfs) are specific compounds that have attracted great attention because they can be implemented as catalysts and photocatalysts [22-24]. multi-walled carbon nanotubes (mwcnts) are widely used in electroanalytical chemistry owing to their unique onedimensional (1d) structural, mechanical and electronic properties [25,26]. in this paper, la3+/zno nfs and mwcnts modified spe is for the first time utilized as a sensitive, rapid, inexpensive and simple electrochemical sensor to determine tramadol. experimental chemicals and apparatus an autolab potentiostat/galvanostat (pgstat 302n, eco chemie, the netherlands) controlled using standard electrochemical system software was used for electrochemical experiments. a standard three-electrode cell was implemented at 25 ± 1 °c. an ag/agcl/kcl (3.0 m) in addition to a platinum wire and la3+/zno/spe were used as a reference, auxiliary and working electrode, respectively. to conduct ph measurements, a 710 metrohm ph meter was used. s. z. mohammadi et al. j. electrochem. sci. eng. 12(1) (2022) 127-135 http://dx.doi.org/10.5599/jese.1141 129 the remaining reagents were of analytical grade, including tramadol bought from merck, darmstadt, germany. phosphate buffer solutions (pbs) were prepared in the 2.0-2.9 ph range, using orthophosphoric acid and its salts. la3+-doped zno nanoflowers preparation each chemical applied to prepare nano-powder, i.e., ammonia (25 % nh3), zinc acetate (zn(ch3coo)2·2h2o), thiourea ((nh2)2cs), and lanthanum nitrate (la(no3)3·6h2o) were of analytical grades. all substances were dissolved in the deionized water. ammonia was a complexing agent during the process of nano-powder procurement. zno nanostructures were procured via dissolution of 0.46 mol of zinc acetate in 80 ml of the deionized water, 0.18 mol of thiourea in 80 ml of deionized water, 0.0046 mol of lanthanum nitrate in 80 ml of deionized water as well as via addition of 19.76 ml of ammonia to 80 ml of the deionized water. the amounts of thiourea, zinc acetate, and ammonia solutions have been kept fixed at a 1:1:1 ratio. afterward, zinc acetate solution was added to the reaction bath. then, thiourea and lanthanum nitrate solutions were added, and the mix was shaken for a few seconds. finally, ammonia was added slowly to the solution and the mixture was stirred for five minutes. next, the bath temperature was increased to 80 °c. the formed precipitate was put aside overnight and filtrated. afterward, ethanol was used to wash the precipitate. the resulting powder was dried at ambient temperature for a couple of days. electrode preparation la3+/zno nfs and mwcnts were used as a coating material for the bare spe. 1 mg of la3+/zno nfs and 1 mg of mwcnts were dispersed by ultrasonication to prepare a stock of la3+/zno nfsmwcnts dispersion in 1 ml water. then, 5 µl aliquot of the la3+/zno nfs-mwcnts/h2o suspension was cast on the carbon working electrode and left to evaporate at room temperature. the surface area of la3+/zno nfs-mwcnts/spe and bare spe were obtained by cyclic voltammetry (cv) using 1 mm k3fe(cn)6 at different scan rates. applying randles-ševčik formula [27] for la3+/zno nfs-mwcnts/spe, the electrode surface was calculated to 0.097 cm2 which was about 3.1 times greater than bare cpe. real samples preparation the preparation started by grinding five tramadol tablets (100 mg per tablet) obtained from tehran chemie pharmaceutical co, iran. then, 400 mg of the resulting powder was dissolved in 25 ml of water via ultrasonication. various volumes of the diluted solution were then poured in a 25 ml volumetric flask and diluted with phosphate buffer solution (pbs) ph 7.0 to the mark. the proposed method was used to analyze the tramadol content using the standard addition method. a refrigerator was used to store the urine samples. 10 milliliters of the specimen were taken or centrifuged for 15 minutes at 2000 rpm to prepare the experiment samples. a 0.45 µm filter was used to filter the supernatant and various resulting volumes were diluted using 25 ml volumetric flasks via pbs ph 7.0. various tramadol amounts were used to spike diluted urine samples. results and discussion electrochemical profile of the analyte on la3+/zno nfs-mwcnts/spe since the electrochemical behaviour of tramadol is ph-dependent (cf. scheme 1), the optimizing ph of the solution is necessary for obtaining the best results. http://dx.doi.org/10.5599/jese.1141 j. electrochem. sci. eng. 12(1) (2022) 127-135 determination of tramadol using modified spe 130 oh nh o oh n o oh n o + -h+ -e+. scheme 1. the probable electro-oxidation mechanism for tramadol at the surface of the modified electrode oxidation peak current values taken from measured dpvs in 0.1 m pbs containing 200.0 μm of tramadol in a dependence on solution ph are presented in figure 1, showing that the best results for tramadol electrooxidation at the modified electrode surface are obtained at ph 7. ph figure 1. plot of ip vs. ph taken from dpvs of la3+/zno nfs-mwcnts/spe in 0.1 m pbs containing 200.0 μm of tramadol at different ph(2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0). the cyclic voltammograms for la3+/zno nfs-mwcnts/spe (curve a) and unmodified spe (curve b) were used to acquire 400.0 μm tramadol as presented in figure 2. it is evident that at bare spe, the maximum oxidation of tramadol takes place at 890 mv, while at la3+/zno nfs-mwcnts/spe, it is attained at approximately 330 mv more negative value. figure 2. cyclic voltammograms of (a) la3+/zno nfs-mwcnts/spe and (b) bare spe in 0.1 m pbs (ph 7.0) in presence of 400.0 μm tramadol at the scan rate 50 mv s-1 scan rate impact the impact of potential scan rates on tramadol oxidation currents is presented in figure 3. it is clear that by increasing the scan rate, peak currents were also increased. moreover, it was proven that the oxidation process is diffusion-controlled according to the linear ip plots against ν1/2, shown in the inset of figure 3. i /  a s. z. mohammadi et al. j. electrochem. sci. eng. 12(1) (2022) 127-135 http://dx.doi.org/10.5599/jese.1141 131 figure 3. cyclic voltammograms of la3+/zno nfs-mwcnts/spe in 0.1 m pbs (ph 7.0) containing 500.0 μm tramadol at various scan rates (numbers 1-12 correspond to 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 and 200 mv s-1). inset: variation of anodic peak current vs. ν1/2 in this stage, tafel plot has been drawn from data obtained from the ascending part of current voltage curve recorded at the scan rate equal of 5 mv s-1 for epinine (figure 4). it is notable that this piece of voltammogram, which has been named the tafel region has been influenced by the electron transfer kinetics between the substrate (tramadol) and la3+/zno nfs-mwcnts/spe. the tafel slope of 0.1948 v has been observed in complete agreement with contribution of 1 electrons in the rate determining phase of the electrode [22] supposing the charge transfer coefficient, α = 0.7 for tramadol. figure 4. cyclic voltammogram (5 mv s−1) of electrode in 0.1 m pbs (ph 7.0) containing 500.0 µm tramadol. the inset shows the tafel plot derived from data denoted at raising part of cv chronoamperometric analyses figure 5 presents chronoaperometric analysis of various concentrations of tramadol samples using la3+/zno nfs-mwcnts/spe in pbs ph 7.0. y = 0.3902x + 0.6777 r2 = 0.9977 y = 0.1948x + 0.4196 r2 = 0.9971 log (i / a) http://dx.doi.org/10.5599/jese.1141 j. electrochem. sci. eng. 12(1) (2022) 127-135 determination of tramadol using modified spe 132 figure 5. chronoamperograms obtained at la3+/zno nfs-mwcnts/spe in 0.1 m pbs (ph 7.0) for different concentration of tramadol (1–4 correspond to 0.1, 0.4, 0.55, and 1.0 mm of tramadol). insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1–4; (b) plot of slopes of straight lines against tramadol concentration cottrell equation (1) for electroactive materials chronoamperometric analysis under mass transfer limited circumstances [27]: i =nfad1/2cbπ-1/2t-1/2 (1) where cb is the bulk concentration (mol cm−3) of an analyte, d is diffusion coefficient (cm2 s-1), n is the number of transferred electrons, a is the electrode surface area (cm2), and t is time (s). best fitted linear plots of i vs. t−1/2 for various tramadol concentrations are shown in figure 5a. figure 5b presents the resulting straight-line slopes vs. tramadol concentrations. the mean value of d was obtained as 9.2×10−6 cm2 s-1 from the cottrell equation and resulting slope for n = 1. calibration curve tramadol peak currents acquired using differential pulse voltammetry (dpv) a la3+/zno nfs-mwcnts/spe were utilized for quantitative analysis of tramadol residing in pbs. dpv has an adventage over other electroanalytical techniques regarding superior characteristics and enhanced sensitivity for analytical application. dpv analysis (step potential of 0.01 v and pulse amplitude of 0.025 v) performed for a range of tramadol solutions in 0.1 m pbs at la3+/zno nfs-mwcnts/spe is presented in figure 6. a linear relationship between tramadol concentration and peak current is obtained within the concentration range of 0.5-800.0 µm with 0.9992 correlation coefficient and detection limit (3σ) estimated as 0.08 µm. table 1. comparison of analytical results for determination of tramadol by dpv method at la3+/zno nfs-mwcnts/spe and some modified electrodes reported in the literature electrochemical sensor linear range limit of detection ref. carbon nanoparticles/glassy carbon electrode 101000 μm 1 μm [28] graphitic carbon nitride/fe3o4 nanocomposite/carbon paste electrode 0.2–14.0 and 14.0–120.0 μm 0.1 μm [29] poly(nile blue)/glassy carbon electrode 1.0×10–63.1×10–4 m 5.0×10–7 m [30] la3+/zno nfs-mwcnts/spe 0.5-800.0 μm 0.08 μm this work y = 10.4x + 1.92 r2 = 0.999 s. z. mohammadi et al. j. electrochem. sci. eng. 12(1) (2022) 127-135 http://dx.doi.org/10.5599/jese.1141 133 table 1 presents a comparison of la3+/zno nfs-mwcnts/spe analytical performance created in this work with other electrochemical sensors involved in tramadol analysis [28-30]. figure 6. dpvs of la3+/zno nfsmwcnts/spe in 0.1 m (ph 7.0) containing different concentrations of tramadol (1–12 correspond to 0.5, 1.0, 10.0, 20.0, 30.0, 40.0, 60.0, 100.0, 200.0, 400.0, 600.0 and 800.0 µm of tramadol). inset: plot of the peak current vs. tramadol concentration in the range of 0.5-800.0 µm real samples analysis the proposed method was implemented to determine tramadol in urine samples and tramadol tablets to examine the relevance of modified electrode application in tramadol determination in real samples. the standard addition method was implemented for analysis, and the results are presented in table 2. the ascertained tramadol recovery was favorable and reproducibility results were displayed according to the mean relative standard deviation (rsd). table 2. application of la3+/zno nfs-mwcnts/spe for determination of tramadol in tramadol tablet and urine samples (n=5) sample concentration of tramadol, µm recovery, % rsd, % spiked found tramadol tablet 0 6.0 3.5 2.0 7.9 98.7 2.7 3.0 8.2 102.5 2.4 4.0 9.9 99.0 1.9 5.0 11.1 100.9 2.3 urine 0 5.0 5.1 102.0 2.1 7.5 7.4 98.7 3.2 10.0 10.3 103.0 2.7 12.5 12.4 99.2 2.8 conclusion in summary, a rapid and simple method based on la3+/zno nfs-mwcnts/spe was designed for the electrochemical determination of tramadol. the proposed sensor displayed excellent electrocatalytic activity and high recognition capability toward the electrochemical detection of tramadol. this sensor demonstrated wide detection range (0.5 800.0 μm) and low limit of detection (80.0 y = 0.0052x + 0.8687 r2 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biological samples using 4-(2-hydroxy-phenylethamino)benzene-1,3-diol by extractive differential pulse polarography} doi:10.5599/jese.318 1 j. electrochem. sci. eng. 7(1) (2017) 1-9; doi: 10.5599/jese.318 open access : : issn 1847-9286 www.jese-online.org original scientific paper monitoring of nickel(ii) in biological samples using 4-(2-hydroxy-phenylethamino)benzene-1,3-diol by extractive differential pulse polarography dasari rekha, puthalapattu reddy prasad, neelam yughandhar sreedhar department of chemistry, sri venkateswara university, tirupati-517502, a.p, india corresponding author: sreedhar_ny@rediffmail.com; tel: +0877-2249666 received: june 15, 2016; revised: december 15, 2016; accepted: december 16, 2016 abstract a novel analytical reagent, 4-(2-hydroxyphenylethamino)benzene-1,3-diol was synthesized for determination of nickel(ii) in biological samples and plant materials by extractive differential pulse polarography. the analysis of nickel(ii) was carried out by extracting the nickel(ii)-(4-(2-hydroxyphenylethamino)benzene-1,3-diol) complex from chloroform prior to injecting into the instrument. the electrochemical behavior of nickel(ii) complex was studied under optimum parameters like effect of ph, pulse amplitude, scan rate and choice of solvent. the calibration curve was constructed in the concentration range of 0.05-42 µg ml-1 at ph 4.0 (acetate buffer) with a correlation coefficient of 0.992, respectively. interference of diverse ions was also investigated during the analysis of nickel(ii) to optimize the conditions. the accuracy and validity of the proposed method were checked by the analysis of certified reference materials which is distributed by the national institute of standard technology keywords monitoring, nickel(ii), extractive differential pulse polarography introduction nickel(ii) exposure into environment of more than 0.05 mg/kg has a toxic effect on human beings and animals [1,2]. nickel(ii) has a wide range of applications including but not limited to batteries, welding rods and wires, electronic equipment, pigment for paints, ceramics, consumer products, steel, magnets, and it can be the source of several diseases. considering these toxicological considerations, monitoring of nickel(ii) is of great importance in the field of the human health or environment and it is highly desirable to develop analytical methods at trace level. various analytical methods have been proposed for the monitoring of nickel(ii) in environmental matrices such as spectrophotometry [3,4], x-ray fluorescence spectrophotometry http://www.jese-online.org/ mailto:sreedhar_ny@rediffmail.com j. electrochem. sci. eng. 7(1) (2017) 1-9 monitoring of nickel(ii) 2 [5], atomic absorption spectrometry [6], inductively coupled plasma atomic emission spectrometry [7,8], inductively coupled plasma mass spectrometry [9], voltammetry [10,11] and chromatography [12]. most of these techniques, however, are either very expensive, suffer from many interferences or require highly skilled technicians. on the other hand, differential pulse polarography (dpp) is an old but a conventional approach in trace analysis. in addition, the concentration of nickel is generally at very low level in environmental samples and it is not possible to determine in a simple complex sample concentration below the quantification limit of the instrument used. hence, we have developed a simple, economic sensitive differential pulse polarography method, after extracting the metal complex from chloroform in biological samples and plant materials. the metal complex was formed by interaction of nickel(ii) with newly synthesized analytical reagent 4-(2hydroxyphenylethamino)benzene-1,3-diol at ordinary laboratory conditions. the various parameters have been evaluated. the developed method can be employed for efficient determination of nickel(ii) at ppm levels. the results of analysis obtained were compared with those obtained by the well knowing method with certified values of nickel(ii). the proposed method is found free from various interferences and successfully applied for determination of nickel (ii) in biological and plant materials. experimental apparatus an elico cl-362 model polarographic analyzer was used for differential pulse polarography (dpp) measurements and the threeelectrode system consisted of a hanging mercury drop working electrode, ag/agcl/kcl reference electrode and a platinum wire as the auxiliary electrode. the entire system was outfitted with the lx 300+ x-y recorder. cyclic voltammetric measurements were carried out by the metrohm 757 va computrace model afrde 4 potentiostat and msrx speed control unit supplied from pine instrument company (usa) coupled with digital electronics 200xy/t recorder. an elico li-129 model glass-calomel combined electrode (purchased from elico ltd, chennai, india) was employed for measuring ph values. all experiments were performed at 25+1 °c. reagents analytical reagent grade chemicals and doubly distilled water were used throughout the experiments. 2,4-dihydroxy acetophenone, 2-amino phenol, organic solvents, hcl, hno3, h2so4, hclo4, ch3chooh, c2h3nao2 were purchased from merck chemicals. a stock solution of nickel(ii) (0.01 m) was prepared by dissolving 2.377 g of nicl2 6h2o in double distilled water in 100 ml volumetric flask. the working standard solution was freshly prepared by diluting the stock solution with double distilled water at the time of analysis. a stock solution of 0.001 m solution of 4-2hpedb-1,3,d was prepared by dissolving an appropriate amount of the reagent in 100 ml of methanol. the working solution was prepared by its appropriate dilution with the same solvent. acetate buffer solutions of ph values ranging from 3.011.0 were prepared as appropriate ratios of 0.1 m acetic acid and 0.1 m sodium acetate (13.6 g/l). synthesis of 4-(2-hydroxyphenylethamino)benzene-1,3-diol (4-2-hpedb-,3,d) the equimolar ratio of 2,4-dihydroxy acetophenone (1.0 g) and 2-aminophenol (1.0 g) in methanol mixture was refluxed for three to four hours and the contents were cooled at room temperature creating an orange-red color precipitate. the resulting precipitate was filtered (whatman filter paper no. 44) and washed with 50 ml ethanol. the crude solid was recrystallized from aqueous ethanol and dried on cacl2, with a yield=97 % and m.p: 115-120 oc. infrared absorption spectrum gave the following peaks (cm-1, kbr pellet): 3375.3 (o-h), 2900 (c-h), 1601 (c=n) and 1465.9 and 1280 (c-o, o-h). the synthesis of analytical reagent and the metal complexation with the reagent are shown in the scheme 1. d. rekha et al. j. electrochem. sci. eng. 7(1) (2017) 1-9 doi:10.5599/jese.318 3 scheme 1. synthesis and complexation of 4-2-hpedb-1,3-d with nickel(ii) general procedure the aliquot of the working standard solution (0.001 m) containing 1100 µl (0.2377 g) of metal ion was taken into 25 ml volumetric flask what was followed by addition of the solution containing 5 ml of acetate buffer solution (ph 4.0) and 2 ml of reagent (soluble in methanol) into the mixture. the mixture was shaken with 5 ml portions of chloroform for 30 s and allowed to stand for 510 min. the organic layer was collected and transferred into the polarographic cell, diluted with 9 ml of acetate buffer and then deoxygenated with nitrogen gas for 10 min. after recording the polarogram, a small increment (0.2 ml) of standard solution was added to the cell, treated for one min and the polarogram was again recorded under similar conditions. in the present study, the greatest precision was obtained at ph 4.0 having the correlation factor of 0.992 and rsd 0.7 % with a drop time of 2 sec, pulse amplitude of 50 mv, scan rate of 12 mv s-1 and the applied potential of – 615.0 mv. sample collection tirupati, city located in the south of india was chosen as the study area for the sample analysis. it is a rapidly growing city which is widely known at the global level. the arrival of approximately 50,000-60,000 pilgrims per day to visit the lord balaji temple (tirumala) is mainly responsible for threatening the natural beauty of this holy town. at the same time, several small and medium scale industries are situated in the suburban fringes of tirupati town. for this reason, this holy pilgrim center has been chosen as the potential collection area for the present study. the biological and plant samples were collected from different locations of the study area in and around tirupati. all necessary and possible precautions were taken at various stages starting from sample containers, sample collection and storage, processing and analyzing the samples. oh oh och3 + nh2 oh oh oh n ch3 oh reflux 3 to 4h oh oh n ch3 oh ni(ii) oh oh n ch3 oh ni 2+ j. electrochem. sci. eng. 7(1) (2017) 1-9 monitoring of nickel(ii) 4 application the hair samples were washed with acetone two to three times in a beaker with continuous stirring and dried in an electric oven at 70 °c for four hours. 2.0 g of the sample was weighed and taken in a beaker. then, the (1:1) mixture of nitric acid and perchloric acid was added and the mixture was heated on a hot plate. the solution was evaporated to near dryness. the ash was taken up with 5 ml of hcl (35.5 %) and evaporated to dryness. the residue was taken up in 2 ml hcl (35.5 %) then filtered and made up to 25 ml with water. suitable volumes of these solutions were taken for determination of nickel(ii) as described above and the results are shown in table 1. table 1. determination of nickel(ii) in biological sample sample certified values reported method [18] r.s.d, % (n=5)a present method extractive-dpp r.s.d, % (n=5)a nies no.5 human hair: pb(6.0), cd(0.20), sb(0.07), zn(169), al(240), sc(0.05), fe(225), mg(208), hg(4.4), co(0.10), rb(0.19), k(34), mn(5.2), cu(16.3), ti(22), ca(728), cr(1.4), ba(2.2), se(1.4), na(26), sr(2.3). 1.80 1.90 0.12 1.96 0.65 amean values for five determinations analysis of plant material the freshly collected pisum sativum (hulls), mangifera indica, eucalyptus, azadirachta indica leaves (5.0 g each) samples were placed in a 250 ml beaker, and a solution of concentrated h2so4/hno3 1:1 (v/v) (10 ml) was added. this mixture was heated until the solution was clear. the solution was filtered (whatman filter paper no 44) off and concentrated in a porcelain bowl up to 5 ml, then cooled and diluted to 50 ml with deionized double-distilled water. the general procedure was then applied to 1 ml of this solution and the obtained results were presented in table 2. in addition, plant tissues like umbilicaria muhlenbergii has been examined in detail followed by the reference procedure [13]. the obtained results were compared with the reported method as shown in table 3. table 2. determination of nickel(ii) in plant material samplesa aas method [19] r.s.d, % (n=5)b present method r.s.d, % (n=5)b pisum sativum (hulls) 2.060 0.003 2.065 0.068 mangifera indica leaves 2.150 0.004 2.153 0.072 eucalyptus leaves 1.038 0.002 1.044 0.013 azadirachta indica leaves 1.481 0.005 1.489 0.015 a samples collected from acharya n. g. ranga agricultural college tirupati, a.p, india; b mean values for five determinations the accuracy and precision of the present method were validated by analyzing the certified reference materials (crm’s) which was distributed by the national institute of standard technology (nist) of rice flour (nies-crm-10a), wheat flour (arc/cl-wf), and rye bread flour (csrm-12-205). inter calibration was performed by using above materials and the analytical results obtained from the present method strongly agreed with the certified reference materials (crm’s) and the data were mentioned in table 5. d. rekha et al. j. electrochem. sci. eng. 7(1) (2017) 1-9 doi:10.5599/jese.318 5 table 3. determination of nickel(ii) in plant tissues (umbilicaria muhlenbergii) by differential pulse polarography. samples nickel(ii) content of zone, µg/g proposed method reported method(dpp) [13] set 1 12± 2 13± 3a set 2 14±6 12±5 a set 3 792±50 790±60 set 4 635±40 640±50 adpp+dmg analysis results and discussion the analytical reagent 4-2-hpedb-1,3,d is a ligand having chelating functional groups that form a complex with ni(ii) ions at ph 4.0 (acetate buffer). figure 1 shows the typical ftir spectrum of ni(ii)-(4-2-hpedb-1,3,d) complex. in the ftir spectra of the complex, a strong peak was observed in the region of 1610-1640 cm-1, which is ascribed to involvement of nitrogen donor atoms of azomethine (-c=n-) in coordination to the ni(ii) ions [14]. the band at 524 cm–1 observed for the ni(ii)-(4-2-hpedb-1,3,d) complex is assigned to ν(m–o). the stretching vibration of the free ligands ν(o-h), 3430-3464 cm–1 was not observed, suggesting deprotonation of hydroxyl groups and formation of ni-o band [15]. the bands observed for the complexes between 720–620 and 564-495 cm-1 were metal sensitive and are assigned to ν(m–o) and ν(m–n) [16], respectively. figure 1. ftir spectra of ni(ii)[4-2-hpedb-1,3-d] complex cyclic voltammetry is widely used for the characterization of electrochemically active systems. figure 2 shows the cyclic voltammogram of nickel(ii)-[4-2-hpedb-1,3,d] system at a scan rate of 12 mv s-1 in the acetate buffer (ph 4.0). in the forward scan, the cathodic peak potential (ecp) observed at 0.62 v corresponds to reduction of nickel(ii)-4-2-hpedb-1,3,d complex. the anodic peak potential (eap) is observed at -0.12 v and corresponds to oxidation of nickel(ii)-[4-2-hpedb1,3,d] complex generated in the anodic step. the anodic and cathodic peak potential difference (δe) is -0.50 v. the possible electrode mechanism can be described as follows: ni(ii) (solution) + 4-2-hpedb-1,3,d (hmde)  ni-(4-2-hpedb-1,3,d) (hmde) ni(ii)-[4-2-hpedb-1,3,d](hmde)+ e ni(i)-[4-2-hpedb-1,3,](hmde) ni(i)-[4-2-hpedb-1,3,d](hmde)  ni(ii) + e j. electrochem. sci. eng. 7(1) (2017) 1-9 monitoring of nickel(ii) 6 figure 2. cyclic voltammogram of (a) ni(ii) and (b) ni(ii)-[4-2-hpedb-1,3-d] complex at hmde, acetate buffer (ph 4.0), scan rate 50 mv s-1 differential pulse polarographic studies effect of ph, choice of solvent and scan rate the effect of ph is one of the important parameter for the investigation of nickel(ii) and [ni-(4-2-hpedb-1,3,d)] complex by differential pulse polarography [17]. the ph studied varied in the range of 2.0 to 10.0 for [ni-(4-2-hpedb-1,3,d)] complex. the maximum peak current was obtained with ph 4.0 at peak potential – 615 mv as shown in figure 3. when ph was increased from 2.0 to 10.0 at constant potential, the concentration of metal complex decreased. therefore, the acetate buffer of ph 4.0 was suitable for determination of nickel(ii) for better sensitivity and selectivity in further investigations. the extraction of the nickel(ii)-[4-2-hpedb-1,3,d] complex was tested in various organic solvents, such as toluene, benzene, n-butanol, dimethyl formaldehyde, carbon tetrachloride, cyclohexane and chloroform. among the various solvents studied, the chloroform was selected as the most suitable for extraction of nickel(ii)-[ 4-2-hpedb-1,3,d] complex. this was because of its maximum peak current and greater extraction ability, due to polarity of complex. in addition, the scan rate was also changed from 4 to 18 mv s-1. the peak current values increased with increasing scan rate up to 12 mv s-1 and after that the peak current decreased. therefore, the scan rate of 12 mv s-1 was selected for further studies. effect of foreign ions the selectivity of the proposed method was enhanced by the study of diverse ions for determination of nickel(ii) in biological samples, plant materials and the results are shown in table 4. foreign ions were added to the solution containing 10 µg ml-1 of nickel(ii) and measurements were taken under optimized conditions. many of diverse ions were tolerated up to the maximum level by using suitable masking agents with the error of ±2 % during the analysis of nickel(ii). though, some of the ions could be masked by addition of a sufficient amount of edta solution. low concentrations of as3+ at 40 °c was also used to reduce the chromium ions. in addition, ca2+, sr2+ interference was eliminated by prior extraction with ki/isobutyl methyl ketone. sodium, potassium, chlorides, nitrites, sulfates, bicarbonates, phosphates and other remaining ions do not interfere with determination of trace metals up to 50 µg ml-1. almost all the results were quantified in the presence of diverse ions to elevate the feasibility and selectivity of the proposed method at appropriate amounts of nickel(ii) solution, followed by the general procedure. d. rekha et al. j. electrochem. sci. eng. 7(1) (2017) 1-9 doi:10.5599/jese.318 7 table 4. tolerable limits of diverse ions in determination of nickel(ii) with error of ±2 % diverse ions tolerable limit, µg ml-1 k+, mg2+, no3 al3+, po43-, no2-, clo4-, so43-, ca2+, sr2+, b(iii), clo3-, bro3-, mn2+, fe3+, ce(iv), mo(vi), v(v), ti(iv), cr(vi), bi(iii), u(iv), cr(vi), ba2+ cu2+, pd2+, zn2+, cd2+, la3+, cr3+, cl-, zr(iv) pb2+, bi(iii), hg 2+, ag+, th(iv), sb3+, sn(iv), au3+, te(iv), se(iv), co2+ 60,000 40,000 30,000 20,000 3,000 1,000 500 250 100 calibration, detection limit and precision the calibration curve was constructed based on the general procedure under optimized conditions at the concentration range of 0.0542 µg ml-1 with the correlation coefficient of 0.992. the detection limit was found to be 0.15 µg ml-1 of the final solution by taking five individual replicates of nickel(ii) solution which gives the relative standard deviation of 0.7 %. typical calibration curve is shown in figure 3b that is drawn based on the s.d. (standard deviation) values. the method of quantification (moq) value, calculated based on the calibration curve was found to be 7.940 µg ml-1 for nickel(ii) in biological samples. figure 3. (left) differential pulse polarogram of nickel(ii), peak at – 615.0 mv, acetate buffer (ph 4.0), pulse amplitude 50 mv, scan rate 50 mv s-1; (right) calibration curve for nickel(ii) the present method was successfully applied for determination of nickel(ii) using the simple chloroform extraction procedure prior to dpp in biological samples, plant materials and the results are shown in tables 1, 2 and 3. the validity of the present method was checked by analyzing the crm’s and the results are shown in table 5. in addition, the obtained results are compared to several analytical reagents synthesized for the determination of nickel(ii) in various environmental samples, presented in table 6. the developed method shows good sensitivity in terms of solvent medium, techniques used for the determination of nickel(ii) ion, concentration range and detection limits. table 5. determination of nickel(ii) in certified reference material (crm’s) sample standard values present method rsd, % (n=5)b rice flour (nies-crm-10a) 0.19 0.187 1.33 wheat flour (arc/cl-wf) 0.153 0.155 0.62 rye bread flour (csrm-12-2-05) 0.234 0.230 0.8 a crm’s distributed by nist national institute of standard technology [nist (usa)] b mean values for five determinations j. electrochem. sci. eng. 7(1) (2017) 1-9 monitoring of nickel(ii) 8 table 6. comparison of reagents for the determination of nickel(ii) with other methods reagents solvent/medium techniques concentration range limit of detection ref. 1-pyridylazo-2-naphthol amberlite xad-resin icp-aes 0.10-275 µg l-1 0.20 μg ml-1 [8] hno3 surfactant triton x-100 icp-ms up to 2.0 µg l-1 0.11 µg l-1 [20] 1-phenyl (3-methyl-3-benzoyl)5 -pyrozolone sufactant triton x-100 gfaas up to 100 ng ml-1 0.12 ng ml-1 [21] 1-(2-pyridilazo)-2-naphthol faas 2.60 μg l−1 [22] 2,(2-quinolylazo) dimethyl dimethylaminoaniline(qadeaa) sodium dodecyl sulfonate (sds) spectrophotometry 0.01-0.4 μg ml-1 0.20 μg ml-1 [23] 5bromo-2hydroxyl -3-methoxybenzaldehyde-4-hydroxy benzoichydrazone dmf spectrophotometry 0.117 to 2.64 μg ml-1 0.016 μg ml-1 [24] 2-pyridylazo 2-resorcinol (par) acetate buffer hplc up to 50 µl [25] 2-(2-quinolylazo)-5dimethylaminoaniline qadmaa sodium dodecyl sulfonate (sds) spectrophotometry 0.01-0.6 μg ml-1 0.20 μg ml-1 [26] 4-(2-hydroxy phenyl ethaminodiol), benzene-1.3-diol acetate buffer extractivedpp 0.05-42 μg ml-1 0.15 μg ml-1 present work conclusions a simple, sensitive, rapid, and eco-friendly solvent extractive differential pulse polarography method was designed for determination of nickel(ii) in biological samples and plant materials. the preconcentration method following chloroform extraction procedure has been established. a new analytical reagent 4-2-hpedb-1,3,d has been synthesized, which can be able to form stable complexes with nickel (ii). the selective functional group of -c=nin the complex, gives a good response and is easily reduced at the electrode surface. the reagent also suppresses the interference of diverse ions such as cations, anions, and other salts by a judicial use of masking agents. references [1] j. j. scott-fordsmand, p. h. krogh, s. p. hopkin, ecotox. environ. safe. 43(1) (1999) 57-61. [2] k. s. kasprzak mutat. res. 533(1-2) (2003) 67-97. [3] e. y. hashem, m. s. abu-bakr, s. m. hussain, spectrochem. acta a 59 (2003) 761-769. [4] y. liu, x. chang. s. wang.y. guo, b. din, s. meng, talanta 64 (1) (2004)160-166. [5] z. augustynski, b. dziunikowski, j. meitin, j. radioanal. nucl. chem. 63 (2) (1981) 325-334. [6] j. l. manzoori, a. bavili-tabrizi, microchim. acta 141(3-4) (2003) 201-207. [7] e. vereda alonso, a. garcia de torres, j. m. cano pavon, mikrochim. acta 110(1-3) (1993) 41-45. [8] s. l. c. ferreira, c. f. de brito, a. f. dantas. n. m. lopo de araujo, a. c. s. costa, talanta 48(5) (1999) 1173-1177. [9] c. j. park, s. a. yim, j. anal. atom. spectrom. 14 (1999) 1061-1065. [10] m. korolczuk, talanta, 53(3) (2000) 679-686. [11] n. y. sreedhar, d. rekhs, anal. chem. ind. j. 8(2) (2009) 209-212. [12] x. yang. z. qiao. w. wei, s. yao, talanta 46(4) (1998) 697-702. [13] c. j . flora, e. nieboer. anal. chem. 52(7) (1980) 1013-1020. [14] l. larai, y. harek, a. reguig, m. m. mostafa, j. serb. chem. soc. 68(2) (2003) 85-95. [15] w. p. griffith, s. i. mostafa, polyhedron 11(23) (1992) 2997-3005. [16] t. priya devi, r. k. hemakumar singh, rasayan journal of chemistry 3(2) (2010) 266-270. [17] d. rekha, n. y. sreedhar, p. reddy prasad, global journal of science frontier research 10(2) (2010) 21-29. d. rekha et al. j. electrochem. sci. eng. 7(1) (2017) 1-9 doi:10.5599/jese.318 9 [18] a. a. almeida. x. jun, j. l. f. c. lima, talanta 50(2) (1999) 253-259. [19] a. praveen kumar, p. raveendra reddy, v. krishna reddy, j. autom. method. manag. 1 (2007) 1-6. [20] c. s. kira, a. m. sakuma, n. da cruz gouveia, journal of applied pharmaceutical science, 4(5) (2014) 39-45. [21] z. sun, p. liang, q. ding, j. cao, hazard. mater. b, 137(2) (2006) 943-946. [22] r. galbeiro, s. garcia, i. gaubeur, j. trace. elem. med. bio. 28(2), (2014) 160-165. [23] q. hu, g. yang, z. huang, j. yin. anal. sci. 19(10) (2003) 1449-1452. [24] b. saritha, t.s. reddy, iosr j. appl. chem. 7(3) (2014) 22-26. [25] h. cyftci. a. olcucu, t. cyftci, international journal of science and technology 2(2) (2007) 105-108. [26] q. hu, g. yang, z. huang, j. yin, b. kor. chem. soc. 25(4) (2004) 545-548. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) ruthenium redox equilibria: 2. thermodynamic analysis of disproportionation and comproportionation conditions doi:10.5599/jese.228 135 j. electrochem. sci. eng. 6(1) (2016) 135-143; doi: 10.5599/jese.228 open access : : issn 1847-9286 www.jese-online.org original scientific paper ruthenium redox equilibria 2. thermodynamic analysis of disproportionation and comproportionation conditions igor povar, oxana spinu institute of chemistry of the academy of sciences of moldova, 3 academiei str., md 2028, chisinau, moldova corresponding author: ipovar@yahoo.ca; tel.: +373 22 73 97 36; fax: +373 22 73 97 36 received: september 30, 2015; accepted: february 18, 2016 abstract the key property of frost diagram has been confirmed using thermodynamic and linear algebra methods. on the basis of the thermodynamic data, the areas of thermodynamic stability of ruthenium species of different valence states as a function of ph for each degree of oxidation have been determined. subsequently, based on the diagrams calculated for several values of ph, a narrow δph value is determined, in which the dismutation of appropriate form takes place. based on thermodynamic analysis, the exact value of the beginning of disproportionation (or comproportionation) is found. finally, the developed revised frost diagrams of ruthenium heterogeneous chemical and redox equilibria, as a function of ph and the total concentration of metal ion in solution, have been built. keywords disproportionation and comproportionation equilibria; revised frost diagram; reduced gibbs energy of half reaction of oxidation; ruthenium soluble and insoluble species. introduction predicting redox reactions is not necessarily a simple task. it involves a careful examination of the reaction experimental conditionsin particular, the influence of a number of such factors, as the medium acidity, the complexation and precipitation processes of the redox species is necessary to examine. predicting redox reactions with the aid of graphical means involves the knowledge of the predominance areas of the redox couples that participate in the equilibria. the superimposition of http://www.jese-online.org/ mailto:ipovar@yahoo.ca j. electrochem. sci. eng. 6(1) (2016) 135-143 ruthenium redox equilibria : 2. thermodynamic analysis 136 the predominance areas of the two different redox couples permits us to predict the evolution of the global redox reaction qualitatively. the frost diagrams (fd) are used to represent the disproportionation conditions of ions and allow judging the possibility along with the extent of disproportionation (comproportionation) of the valence states of the element. in this diagram, there is represented the dependence of the reduced standard energy of formation of ions 0 0 f f( ) ( ) /g i g i f   (where f is the faraday constant equal to 96485 c mol-1) on the degree of oxidation of an element (n) [1]. so, the gibbs energy change is expressed in ev/mol. fd, being simple to accomplish, characterizes clearly the disproportionation (dismutation) processes of ions in solution. the disproportionation occurs if the value 0 f ( )g i of the analyzed ion is situated above the straight line joining the points 0 f ( )g i of neighboring valence forms on the diagram. in the case of oxygen-containing species (ions, molecules), on the ordinate axis the standard gibbs free energy of formation of species ( 0 f ( )g i ) minus the gibbs energy of a number x of water molecules, equal to the number of oxygen atoms in the examined species ( 0 0 0 f f f 2( ) (h o)g i g x g    ), is placed. when the valence form contains more than one element, which is subject to redox transformations, the 0* f g values is calculated per one atom. the standard redox potential represents the negative slope of the straight line joining two points on the diagram, corresponding to any two valence forms, since the equality is valid: 0 0* f /e g n  here the n quantity coincides with the degree of oxidation of the ion of interest. the angle of inclination of the curve for the given pair of the valence forms characterizes the ability of interaction with the formation of the products with a lower gibbs energy. by means of this diagram it is easy to establish whether a particular ion is stable against disproportionation. therefore, the frost diagram is a convenient source of understanding complex redox chemistry of any element in its various oxidation states and sometimes under different reaction conditions like ph etc. details and the mechanics of constructing frost diagrams can be found in numerous basic chemistry books [1-17]. according to a rigorous line of argument, predictions based on frost diagrams are not more accurate than those given by only considering the standard potentials of couples. in other words, the predictions given by this strategy are only accurate when the redox species involved in the reaction are in their standard states. but these predictions may sometimes be inverted in other experimental conditions. because standard potentials involve ph 0, frost diagrams are implicitly drawn for ph 0. for other ph values, the apparent standard potentials must be used in order to build the diagrams. actually, they are often drawn for ph 0 and ph 14. hitherto, the frost diagrams can be used only under standard conditions, i.e. for semi-quantitative estimations. however, under non-standard, real conditions, a number of factors such as ph, concentrations of soluble forms, participation of each valence state of element in diverse auxiliary chemical reactions (precipitation, complex formation, hydrolysis etc.) can exert influence on the disproportionation-comproportionation processes. the frost diagrams, easily realizable, clearly represent the processes of ion dismutation. but fd drawn this way, suffer from a number of drawbacks, namely:  the diagrams can be only used under standard conditions. under real conditions a decisive influence on the possibility of occurring dismutation processes can have such factors as acidity, concentration of soluble species and the formation of solid phases;  in function of the solution ph, metal ions are subject to hydrolysis with the formation of mononuclear and polynuclear hydroxocomplexes. in the second case the degree of disproportionation depends on the concentration of metal ions in the solution [18]; )()( ~ 0 nfig f  i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 135-143 doi:10.5599/jese.228 137  conditions of dismutation reactions may be influenced by complex formation reactions. the goal of this paper is to propose a method to build frost diagrams under real, non-standard conditions (called here as modified frost diagrams, mfd), based on rigorous thermodynamic analysis of chemical equilibria in the system ru–h2o, taken into consideration all the factors mentioned above. proof of the theorem for necessary conditions of disproportionation processes has been also completed. theory and calculations proof of the theorem for necessary conditions of disproportionation processes the disproportionation occurs if the value 0 f ( )g i of the analyzed ion is situated above the straight line joining the points 0 f ( )g i of neighboring valence forms on the 0 f ( ) ( )g i f n  diagram. in the case of oxygen-containing species (ions, molecules), on the ordinate axis the standard gibbs free energy of formation of ion ( 0 f ( )g i ) minus the gibbs energy of a number x of water molecules, equal to the number of oxygen atoms in the examined species ( 0* 0 0 f f f 2(h o)g g x g     ) is placed. when the valence form contains more than one element, which is subject to redox transformations, the 0* f g values is calculated per one atom. the standard redox potential represents the negative slope of the straight line joining two points on the diagram, corresponding to any two valence forms, since the equality is valid: 0 0 /fe g n    here the n quantity coincides with the degree of oxidation of the ion of interest. the angle of inclination of the curve for the given pair of the valence forms characterizes the ability of interaction with the formation of the products with a lower gibbs energy. by means of this diagram is easy to establish whether a particular ion is stable against disproportionation. the disproportionation occurs when 0* f g of ion lies above the straight line joining the gibbs energies of two neighboring valence forms. this is explained by the fact that 0* f g of the products of disproportionation reaction corresponds to the point situated at the intersection of this line with the vertical passing through the point corresponding to this ion. in addition, the greater the gain in energy, the less stable the ion towards its disproportionation in solution. this property of diagram can be fundamented using thermodynamic and linear algebra methods. we present here a brief proof. we will examine the redox system formed from the ions of an element, which are in three valence states: a, b and c. let the following reaction of disproportionation takes place in this system: (c-a)m b+ = (c-b)m a+ + (b-a)m c+ (1) and 0 0 0 0 r f f f (c-b) (a)+(b-a) (c) (c-a) (b)<0g g g g       (2) the reaction (1) is characterized by the following sequence of the standard redox potentials: 0 0 0 a/b a/c b/c> >e e e in the case of reaction (1) occurring, the points a ( 0fg (a), a), b ( 0 f g (b), b) and c ( 0fg (c), c) on the diagram 0 f ( ) ( )g i f n  correspond to the subsequent valence states (fig. 1). j. electrochem. sci. eng. 6(1) (2016) 135-143 ruthenium redox equilibria : 2. thermodynamic analysis 138 figure 1. the frost diagram for the case analyzed in this paper. we will prove that if the point b ( 0 f g (b), b), corresponding to the intermediate valence, lies above the straight line ac joining the a and c points, which characterize respectively the forms with lower and larger valences, then the disproportionation reaction (1) occurs. if the straight line ac is drawn through the points a and c, then the point b is situated above the line. we will draw the line perpendicular to the axis n through the point b. we will denote the intersection point of the straight line perpendicular to straight line ac through x ( 0 f g (x), x). the equation of the straight line can be represented by the expression 0 0 0 0 f f f f ( ) ( ) ( ) ( )g x g a g c g a x b c b          (3) from where we get 0 0 0 f f f( ) ( ) ( ) ( ) ( ) ( )c x g a x a g c c a g x        (4) on the other hand, under the conditions needed for the reaction (1) to occur, from the inequality (2) it follows that 0 0 0 f f f( ) ( ) ( ) ( ) ( ) ( )c b g a b a g c c a g b        (5) therefore, if the b point is located right above the ac straight line, the spontaneous disproportionation of m b+ to m a+ and m c+ takes place. so, the proof is done! thermodynamic analysis of disproportionation comproportionation conditions of ru in different valence states fig. 2 shows the disproportionation of ru(ii) in ru(0) and ru(iii). the dotted line drawn through the points corresponding to ru(0) and ru(iii) is well below the corresponding 0 f g point of the ru(ii) ion. similarly, one deduces that species ru(v), ru(vi) and ru(vii) are unstable with respect to disproportionation in ru(iv) andru (vii). for example, at disproportionation of ru (vi), 0 f g comes down to the value corresponding to the cross point on the dotted line. therefore, the disproportionation of ru(vi) to ru(iv) and ru(viii) is accompanied by the 0 f g decrease and, as a result, it occurs spontaneously. higher is the energy gain, the more i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 135-143 doi:10.5599/jese.228 139 unstable is ion with respect to disproportionation in solution. but, as it was above-mentioned, fd drawn by this way, suffers from a number of weaknesses. the improved version of the frost diagrams under conditions, different from standard ones, mfd, enables to use the gibbs energy change of oxidation half reaction of metal to the corresponding valence state under real conditions instead of 0 f g . for example, in the case of ru(iv) the change in standard gibbs energy of half reaction: 2+ + 2 2ru+ 2h o= ru(oh) +2h + 4e (6) is equal to    0 0 2+ 0 0 2+r f 2 f 2 f 2ru(oh) 2 (h o) ru(oh)g g g g       (7) for the calculation of the change in standard gibbs energy of half reaction (6) under real conditions the equation of isotherm reaction should be applied: 0 0 + 4 r r rulog [h ]g g c    , where ln10rt f  , 0 ru c is the ruthenium total concentration in mixture. within the used approach of mfd, the slope of the line that connects two valence states, constitutes the formal (conditional) redox potential. in the case of formation of the solid phase, ruo2∙h2o(s) according to the equation of reaction + 2 2 2ru + 4h o = ruo × 2h o (s) + 4h + 4e , the equation of isotherm is 0 + 4 0 r r rlog[h ] 4 phg g g        . results and discussion a preliminary step in the construction of mfd is to determine the thermodynamic stability of valence states, in function on the ph solution and 0 ru c [19] (see table 1). table 1. calculation of the mfd for ru-h2o at different ph values ru(ii) ru 2+ 0 < ph < 14.0 ru(v) ru2o5(am) 0 < ph < 14.0 ru(iii) ru 3+ 0 < ph < 1.76 ru(vi) 24 ruo 0 < ph < 14.0 + 2 ru(oh) 1.76 < ph < 2.12 ru(vii) 4 ruo 0 < ph < 14.0 ru(oh)3(am) h2o 2.12 < ph < 14.0 ru(viii) 2 5h ruo 0 < ph < 11.53 ru(iv) 2+ 2 ru(oh) 0 < ph < 1.05 5 hruo 11.5 < ph < 14.0 4+ 4 12 ru (oh) 1.05 < ph < 3.93 ruo2∙h2o (s) 3.93 < ph < 14.0 then, the change in gibbs energy for the respective reactions is calculated. for example, at ph 13, we get 0 r1. ru(0) : 0 evg  j. electrochem. sci. eng. 6(1) (2016) 135-143 ruthenium redox equilibria : 2. thermodynamic analysis 140 2+ 0 r2. ru(ii) : ru = ru + 2e, 1.558 evg  0 0 r r rulog 1.321 evg g c     + 0 2 3 2 r3. ru(iii): ru + 4h o = ru(oh) × h o (am) + 3h + 3e, = 1.894 evg 0 r r 3 ph = -0.413 evg g     + 0 2 2 2 r4. ru(iv): ru + 4h o = ruo × 2h o (s)+ 4h + 4e, = 2.671 evg 0 r r 4 ph=-0.405 evg g     + 0 2 2 5 r5. ru(v): ru + 5/2h o = 1/2ru o (am) +5h + 5e, = 3.840 evg 0 r r 5 ph = -0.005 evg g     2+ 0 2 4 r6. ru(vi): ru + 4h o = ruo + 8h + 6e, δ =6.655evg 0 0 r r rulog 8 ph = 0.266 eg g c v     + 0 2 4 r7. ru(vii): ru + 4h o = ruo + 8h + 7e, = 7.241 evg 0 0 r r ru+ log 8 ph = 0.852 evg g c    + 0 2 5 r8. ru(viii): ru + 5h o = hruo + 9h + 8e, = 8.919 evg 0 0 r r rulog 9 ph = 1.761 evg g c      mfd for 0 4 ru 10c   and 10 -6 mol/l at different ph values are presented on figs. 2 and 3. in all the cases mfd suffer essential changes with ph variation. on this base the following conclusions can be made: 1. in acidic medium (ph = 0 and 1) stable valence states are: for ru(iii) (ru 3+ and + 2 ru(oh) ), ru(iv) ( 2+ 2 ru(oh) or 4+ 4 12 ru (oh) ) and ru(viii) (as 2 5 h ruo ). 2. ru(ii) disproportionates according to the equations: 2+ 3+ 3ru = ru + 2ru 2+ + + 2 23ru +4h o = ru+ 2ru(oh) + 4h . 3. ru(v) at ph = 0 and ph = 1 disproportionates according to the scheme + 2+ 2 5 2 2 2 52ru o (am) +h o+6h =3ru(oh) +h ruo (aq) at the same ph values, ru(vi) and ru(vii) are instable: 2+ 2+ 4 2 5 2 22ruo + 6h = h ruo + ru(oh) + h o (ph 1) 2+ 4+ 4 2 5 4 122ruo + 5h = h ruo + 1/4ru (oh) (ph 3) + 2+ 4 2 2 5 24ruo + h o+6h = 3h ruo + ru(oh) (ph 1) + 4+ 4 2 2 5 4 124ruo + 2h o+ 5h = 3h ruo + 1/4ru (oh) (ph 3) the diagrams δgr(n) can also be used in the presence of complexing agent: 2+ 2-n 2 2 nru(iv): ru(oh) + ncl = ru(oh) cl , n=0-5, 3+ 3-n nru(iii): ru + ncl = rucl , n=0-6, 2+ + ru(ii): ru +cl = rucl . i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 135-143 doi:10.5599/jese.228 141 in this case δgr is a function of 3 variables: 0 0 ru cl ,c c and ph. for the construction of diagrams δgr = f(n) at different 0 cl c , it is necessary to maintain constant other two parameters – ph and 0 ru c . the distribution of different species of ru(iv), ru(iii) and ru(ii) in function on log [cl ] (ph = 0, 0 -6 ru =10 mol/lc ) is the following: ru(iii) ru(iv) ru 3+ log[cl ] <-2.17 2+ 2 ru(oh) -log[cl ]<-1.39 2+ rucl --2.17<log[cl ]<-1.57 + 2ru(oh) cl -1.39<log[cl ]<-0.44 + 2rucl -1.57<log[cl ]<-0.54 22 4 ru(oh) cl -log[cl ]>-0.44 0 3rucl -0.54<log[cl ]<0.16 ru(ii) 4rucl 0.16<log[cl ]<0.30 2+ ru -log[cl ]<0.48 25 rucl 0.30<log[cl ]<0.40 + rucl -log[cl ]>0.48 36 rucl log[cl ]>0.40 the diagram δgr(n) for different 0 0 0 cl cl ru ( ,c c c thus 0 cl c = [cl ]) is shown in fig.4. figure 2. modified frost diagrams for the system ru-h2o, 0 -4 c 10 / ru mol l figure 3. change in gibbs energy of the half reaction of oxidation of metallic ru to respective valence state versus degree of oxidation (n) for the system ru–h2o, 0 4 c 10 /ru mol l   0 1 2 3 4 5 6 7 8 -1 0 1 2 3 4 5 6 7 8 ph = 15 ph = 10 ph = 7 ph = 3 ph = 1 ph = 0  g r, e v n 0 1 2 3 4 5 6 7 8 -1 0 1 2 3 4 5 6 7 8 ph = 13 ph = 10 ph = 7 ph = 8 ph = 1 ph = 0  g r, e v n j. electrochem. sci. eng. 6(1) (2016) 135-143 ruthenium redox equilibria : 2. thermodynamic analysis 142 figure 4. the diagram δgr(n) for the system ru–cl -h2o, 0 6 10 /ruc mol l   conclusions 1. the development of frost diagrams for the soluble and insoluble species of ruthenium has been performed. an essential point in the modified method is that instead of the standard gibbs energy of formation of ions 0 f ( )g i , the use of the gibbs energy change of the oxidation half reaction of element up to the respective valence state under non-standard conditions has been 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[19] i. povar, o. spînu, journal of electrochemical science and engineering (2016) doi: 10.5599/jese.226 © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {decolorization of industrial wastewater using electrochemical peroxidation process:} http://dx.doi.org/10.5599/jese.1017 373 j. electrochem. sci. eng. 12(2) (2022) 373-382; http://dx.doi.org/10.5599/jese.1017 open access : : issn 1847-9286 www.jese-online.org original scientific paper decolorization of industrial wastewater using electrochemical peroxidation process elin marlina1,, purwanto purwanto2 and sudarno sudarno3 1doctoral program of environmental science, school of postgraduate studies, universitas diponegoro, semarang, indonesia 2department of chemical engineering, faculty of engineering, universitas diponegoro, semarang, indonesia 3department of environmental engineering, faculty of engineering, universitas diponegoro, semarang, indonesia corresponding author: elin.marlina95@gmail.com received: june 8, 2021; accepted: december 3, 2021; published: december xx, 2021 abstract in this study, decolorization of wastewater samples taken from the paper industry is investigated using an electrochemical peroxidation process. the electrochemical peroxidation process is a part of electrochemical advanced oxidation processes, which is based on the fenton’s chemical reaction, provided by the addition of external h2o2 into the reaction cell. in this study, iron is used as anode and graphite as cathode placed at the fixed distance of 30 mm in a glass reaction cell. the cell was filled with the solution containing wastewater and sodium chloride as the supporting electrolyte. factors of the process such as ph, current intensity, hydrogen peroxide concentration, and time of treatment were studied. the results illustrate that all these parameters affect efficiencies of dye removal and chemical oxygen demand (cod) reduction. the maximal removal of wastewater contaminants was achieved under acid (ph 3) condition, with the applied current of 1 a and hydrogen peroxide concentration of 0.033 m. at these conditions, decolorization process efficiency reached 100 and 83 % of cod removal after 40 minutes of wastewater sample treatment. in addition, the electrical energy consumption for wastewater treatment by electrochemical peroxidation was calculated, showing an increase as the current intensity of the treatment process was increased. the obtained results suggest that the electrochemical peroxidation process can remove dye compounds and chemical oxygen demand (cod) from industrial wastewaters with high removal efficiency. keywords paper industry wastewater; electrochemical peroxidation; fenton’s reaction; decolorization efficiency; chemical oxygen demand http://dx.doi.org/10.5599/jese.1017 http://dx.doi.org/10.5599/jese.1017 http://www.jese-online.org/ mailto:elin.marlina95@gmail.com j. electrochem. sci. eng. 12(2) (2022) 373-382 decolorization of industrial wastewater 374 introduction dyes are widely used in various industries such as paper, textile, leather tanning, and printing industries, causing environmental pollution, especially water pollution. five million quintals of azo dyes are produced each year worldwide, which constitute half of the total dyes produced [1,2]. the paper industry is a type of industry that uses a lot of water and many active ingredients, including dyes [2]. therefore, besides some active compounds, the wastewater may contain different dyes. since dye wastewater becomes a problem for the environment, the industry is forced to carry out treatment procedure(s) that can overcome this problem [3–5]. in this context, the removal of active compounds measured as chemical oxygen demand (cod), and decolorization of wastewater, are considered crucial because many dyes and decomposition products are poisonous. elimination of colours in wastewaters, especially industrial wastewaters, is essential because colour could severely affect the water-living system. the electro-fenton’s processing is a part of electrochemical advanced oxidation processes (eaops) technology. the eaop process itself pertains to the advanced oxidation processes (aops) developed mostly over the last decade by using clean, efficient, and economical processing in removing pollutants in water [6–8]. on the other hand, eaops form a group of emerging technologies, where pollutant removal is based on the fenton’s chemical reaction. there are two types of processing, the first one is carried out with the addition of external h2o2, and the second involves internal regeneration of h2o2 [3]. the electrochemical peroxidation process is part of the first type, where a sacrificial iron or steel anode is used for electro-generation of fe2+ ions by anodic dissolution. h2o2 is externally added to the treated solution to degrade organic pollutants with hydroxyl radicals (•oh) generated by fenton's reaction [9–11]. the electrochemical peroxidation process has a similar mechanism to electrocoagulation, but better cod removal results were obtained with the addition of h2o2 [12–14]. several studies have reported that cod of coke wastewater can be removed up to 90 % by electrochemical peroxidation, whereas by electrocoagulation, up to 30 % was removed only [13]. during past decades, the electrochemical peroxidation process showed a promising perspective in treating several kinds of dyes that contaminated water, causing pollution. in this experimental study, the application of the electrochemical peroxidation process for the decolorization of paper industrial wastewater was explored. based on previous studies that showed successful decolorization by the fenton’s oxidative processes, in the present study, the opportunity of decolorization of paper industry wastewater has been investigated using the fenton’s oxidation processing. this study will explore the effects of various operating parameters, including the initial ph of the solution, applied current strength, the dosage of h2o2, and treatment time on decolorization and cod removal. energy consumption was also studied to determine the most efficient process conditions for paper industry wastewater treatment. positive results of this research should increase the knowledge of those responsible for wastewater treatment in the paper industry. experimental materials and chemicals paper wastewater samples were taken from the equalization tank effluent in the paper mill plant in kudus, district central java province, indonesia. the physicochemical characterization of these effluents showed cod of 240 mg/l, ph 6.8 and dark yellow colour. h2o2 (30 %, w/w), h2so4, and e. marlina et al. j. electrochem. sci. eng. 12(2) (2022) 373-382 http://dx.doi.org/10.5599/jese.1017 375 nacl were obtained from merck, germany. all chemicals were of analytical grade and directly used without purification process. the experiments were performed at room temperature, using the open single-cell glass reactor with dimensions of 12  10 12 cm (1.4 l) (figure 1). the reactor is equipped with two vertical plate electrodes, graphite as cathode and iron plate as anode with 376.2 cm2 of the total surface area (1090.3 cm). two electrodes were put at a distance of 3 cm and connected to a dc power supply (mds ps-305dm). a magnetic stirrer was used to homogenize the electrolyte solution. distilled water was used throughout this experiment. figure 1. glass reactor setup: dc power supply (1); magnetic stirrer (2); magnetic bar-stirrer (3); electrodes (4); solution (5) experimental procedures the electrodes were cleaned before the experiment by soaking in 0.5 m h2so4 solution for 15 minutes. one litre of wastewater solution was put into the reactor, together with 0.585 g of nacl (0.01 m) as the electrolytic support, and h2o2 was added externally. the batch experiments were carried out in a homogeneous solution. to decrease the ph value, 0.5 m h2so4 was added stepwise to reach the desired ph value. 15 ml of the treated solution were taken at regular intervals and filtered before further analysis. a water quality meter (trans instruments hp9000) was used to test solution ph values. cod samples were tested using a closed reflux titrimetric method based on sni-06-6989.2-2009 and colour tested using sni 6989.80:2011. a double-beam uv–vis spectrophotometer (shimadzu uv1700, japan) equipped with a 10 mm quartz cell was used to measure colour and cod concentration by determining absorbance at λ = 450–465 nm for colour and 600 nm for cod. the removal efficiency was determined by the following equation: o s o 100 c c ef c − = (1) where co and cs refer to initial dye concentration and dye concentration at time t, respectively. the electrical energy consumption for a liter of the solution was calculated by: e = vit (2) http://dx.doi.org/10.5599/jese.1017 j. electrochem. sci. eng. 12(2) (2022) 373-382 decolorization of industrial wastewater 376 here e is the energy consumption in j, v is the cell voltage in v, i is the current in a, and t is the reaction time in s [15]. results and discussion the electrochemical peroxidation is one kind of electro-fenton’s process, where the anode is used for electro-generation of fe2+ ions according to: fe → fe2+ + 2e(3) h2o2 is added from outside to degrade organic pollutants with hydroxyl radicals (•oh) created from the fenton’s reaction: fe2+ + h2o2→ fe3+ + oh+ •oh (4) fe3+ ions formed by fenton’s reaction (4) are continuously reduced at the cathode according to: fe3+ + e→ fe2+ (5) in this process, a part of fe3+ ions formed by the fenton’s reaction (4) precipitates as fe(oh)3 by the reaction, which depends on ph and the applied current value. these deposits can catalytically decompose h2o2 to o2 but also be an alternative for the removal of organic pollutants by coagulation [9]. effect of initial solution ph as pointed out in previous studies, the ph of the solution is one of the significant factors affecting the electrochemical work process [16–18]. ph value determines the speciation of iron in solution, and ph 3 was found as the optimum value for dye degradation by electro-fenton’s process. in acidic conditions, iron anode dissolves as fe2+ ions in water according to reaction (3), which will be the catalyst to produce •oh radicals with the added h2o2 according to reaction (4). at ph 3, iron ions (fe2+) and hydrogen peroxide will remain stable. therefore, the fenton’s reaction can occur perfectly under this condition [20,21]. as presented in figure 2, 100 % decolorization in acidic conditions (ph 3) was obtained after 60 min of treatment at 0.5 a, when the blue colour changed into clear watercolor. on the other hand, when ph was 6.8 (normal ph), 99 % decolorization was obtained only at the maximum electrolysis time of 120 min. figure 2. decolorization efficiency vs. treatment time at 0.5 a of wastewater samples containing 0.033 m h2o2 and 0.01 m nacl, at ph 3 and 6.8 e. marlina et al. j. electrochem. sci. eng. 12(2) (2022) 373-382 http://dx.doi.org/10.5599/jese.1017 377 cod levels were also tested at two ph values, and figure 3 presents the results of these experiments. by acidifying the solution, cod was removed up to 65 % in 120 min. cod removal started immediately with a decrease in cod value, reaching 100 mg/l after 20 min of treating (140 mg/l removed). after 20 min, cod removal did not increase significantly, which can be due to the ph increase of the solution to 5. previous research on optimal electrochemical peroxidation processes in acidic conditions showed that increased solution ph significantly inhibited cod removal [18,20]. the electrochemical peroxidation process removal decreases rapidly at higher ph values, especially at ph higher than 5 [19]. an increase of ph during the electrochemical peroxidation process leads to the domination of the electrocoagulation process due to the conversion of fe2+ and fe3+ to fe(oh)n [21]. in acidic solutions, ph increased significantly during cod removal. as seen in figure 3, cod removal slowed down after 20 min (ph 4.3 and removal efficiency 59 %). after 120 min, however, ph 9.21 and 62 % removal efficiency were reached. this reinforces the common statement of previous researchers that the best removal in the electrochemical peroxidation process is carried out in acidic conditions [21–23]. figure 3 cod concentration vs. treatment time at 0.5 a of wastewater samples containing 0.033 m h2o2 and 0.01 m nacl, at different ph effect of h2o2 as the main source of hydroxyl radicals, the initial concentration of h2o2 plays an important role in the electrochemical peroxidation process of oxidizing the pollutants. it has already been found that the removal efficiency increases with the increasing concentration of h2o2 in the solution [13,22,24–26]. as presented in figure 4, increasing the initial concentration of h2o2 in wastewater solution containing 0.585 g nacl, ph 3, improves colour removal. in the absence of h2o2, where only the electrocoagulation process is operative, the rate of colour removal after 10 min was 19 %, while after the addition of 0.0165 m h2o2, colour removal after 10 min increased to even 30 %. this is due to the presence of more oh• provided by fenton's reaction (4) in the reactor, which oxidized more organic compounds. the further increase of h2o2 concentration to 0.033 m and 0.0495 m improved decolorization after 10 min to 43% and 79%, respectively. note that for the highest concentration of 0.0495 m h2o2, full depolarization is http://dx.doi.org/10.5599/jese.1017 j. electrochem. sci. eng. 12(2) (2022) 373-382 decolorization of industrial wastewater 378 reached within 20 min. this refinement is related to the generation of more hydroxyl radical species in the presence of increasing amounts of hydrogen peroxide [27]. figure 4. decolorization efficiency vs. treatment time at 0.5 a of wastewater samples containing 0.01 m nacl, ph=3 and different concentrations of h2o2 the effect of h2o2 concentration on cod removal was evaluated at the constant current of 0.5 a and started with the solution of ph 3. the results are presented in figure 5, where it is seen that in the absence of h2o2, the rate of cod removal is 13 % since only electrocoagulation is effective in this case. it has already been revealed by previous researchers that the electrocoagulation process has not a significant effect on cod removal [28]. the mechanism of cod removal in the electrocoagulation process is going exclusively through the adsorption process by fe(oh)3. at h2o2 concentration of 0.0165 m, however, cod was reduced by 30 % in 20 min, and this is due to hydroxyl radicals produced in the electro peroxidation process caused by added h2o2 [9,29–31]. figure 5 indicates that increased concentration of h2o2 improves cod removal since efficiencies after 20 and 120 min were increased from 29.2 to 63.9 % for 0.0165 m h2o2, 33.3 to 65.3 % for 0.033 m and 40.2 to 69.4 % 0.0495 m h2o2. it is also seen in figure 5 that after 20 min, cod removal increased only slightly for all samples, which is due to the increasing ph value to 5 in 20 min, and 11.2 in 120 min. figure 5. cod removal vs. treatment time at 0.5 a of wastewater samples containing 0.01m nacl, ph 3, and different concentrations of h2o2 e. marlina et al. j. electrochem. sci. eng. 12(2) (2022) 373-382 http://dx.doi.org/10.5599/jese.1017 379 this suggests that uncontrolled ph conditions affect the process significantly. the performance of the electrochemical peroxidation process is optimal in acidic solutions, where generation of iron ions would occur and react by the classic fenton's reaction, developing oh• as efficient oxidizers of organic compounds [9]. effect of applied current the effect of applied current intensity on the electrochemical peroxidation process was also tested. the influence of different applied current intensities on colour and cod degradations was evaluated in 1 l of wastewater solution with 0.05 m nacl, ph 3 and 0.033 m h2o2. the obtained results are shown in figures 6 and 7. figure 6 shows that different processing results are obtained at different applied current intensities. generally, colour removal increased with increasing current strength. at 0.3 a, the results showed 10 % decolorization after 10 minutes, while 99 % degradation was observed after 120 minutes. at higher currents of 0.75 and 1 a, respectively, the colour removals reached 79 and 90 % after 10 minutes and 100 % after 120 minutes of treatment. better colour degradations observed at higher currents may be due to the fact that an increased amount of oxidized iron is generated from the anode at higher currents [32]. on the other side, the high current density is a trigger factor for the oxygen reduction process, which serves to regenerate hydrogen peroxide at the cathode [29,33–35]. the high currents cause an increase in the amount of oh• so that the degradation process is more reactive and responsive [25]. in addition to the increasing amount of oh• in solution, the use of high currents also causes the regeneration of iron ions, and the fenton process's efficiency also increases [36]. figure 6. decolorization efficiency vs. treatment time of wastewater samples containing 0.033 m h2o2 and 0.05m nacl, ph 3 at different current intensities figure 7 shows that a decrease in cod concentration with treatment time was observed at all current intensities. for the highest current of 1.0 a, there is a significant reduction of cod in 40 min, leaving the lowest cod concentration of 40 mg/l with a removal ratio of 83 %. at 0.3 a, the lowest removal efficiency was obtained, where the removal ratio reached only 54 and 61% in 120 min, with the remaining cod content 110 mg/l in 40 min. when applying the current intensity of 1 a, there is a decrease in the cod removal efficiency in the treatment period of 60 to 120 min. this is probably due to the increase in the amount of fe2+ ions released at the anode through the electrolysis time, http://dx.doi.org/10.5599/jese.1017 j. electrochem. sci. eng. 12(2) (2022) 373-382 decolorization of industrial wastewater 380 thereby reducing the efficiency electro-fenton’s process [18]. this study has similarities with previous studies [38–40, which showed that an excessive current or voltage would cause a decrease in cod removal. figure 7 cod concentration vs. treatment time of wastewater samples containing 0.033 m h2o2, 0.05m nacl, ph 3 at different current intensities on the other hand, high currents will increase energy consumption in the electrochemical process [37]. the energy consumption in the process was calculated by eq. (2), where electric voltages recorded after 120 min for current values between 0.3 and 1.0 a (figure 7), were 7.2, 10.5, 14 and 15 v, respectively. it is obvious from these values that rising currents caused rising voltage. according to eq. (2), energy consumption was calculated to be 15.6, 37.8, 75.6 and 108 kj. the linear correlation between current, voltage, and energy consumption has already been investigated, giving similar results [15]. conclusions in this study, a detailed exploration of the electrochemical peroxidation treatment of paper industrial wastewater is described. it was found that process factors such as ph, applied current, and concentration of added h2o2 significantly affect decolorization efficiency and cod removal from the paper wastewater solution. the following conclusions can be derived from the present study: • the electrochemical peroxidation process is facilitated in an acid condition. • colour and cod removal continuously increased as h2o2 was added to the process up to the concentration of 0.0495 m. • the current intensity influences colour and cod degradation in the electrochemical peroxidation process, where clear water was obtained for the current of 1 a in 20 min of treatment. • the electrochemical peroxidation process can be used as an efficient operational process to remove colour and cod from paper industrial wastewater. acknowledgement: authors thank deputy for strengthening research and development, ministry of research and technology / national research and innovation agency of the republic of indonesia for funding this research through pmdsu research grant 2020 contract: 647-02/un7.6.1/pp/2020. e. marlina et al. j. electrochem. sci. eng. 12(2) (2022) 373-382 http://dx.doi.org/10.5599/jese.1017 381 references [1] a. d. bokare, r. c. chikate, c. v. rode, k. m. paknikar, applied catalysis b 79(3) (2008) 270278. https://doi.org/10.1016/j.apcatb.2007.10.033 [2] g. thompson, j. swain, m. kay, c. f. forster, bioresource technology 77(3) (2001) 275-286. https://doi.org/10.1016/s0960-8524(00)00060-2 [3] e. brillas, i. sirés, m. a. oturan, chemical reviews 109(12) (2009) 6570–6631. https://doi.org/10.1021/cr900136g [4] c. a. martınez-huitle, e. brillas, applied catalysis b 87(3-4) (2009) 105-145. https://doi.org/10.1016/j.apcatb.2008.09.017 [5] n. klidi, f. proietto, f. vicari, a. galia, s. ammar, a. gadri, o. scialdone, journal of electroanalytical chemistry 841 (2019) 166-171. https://doi.org/10.1016/j.jelechem.2019.04.022 [6] n. oturan, m. a. oturan, in: electrochemical water and wastewater treatment, chap. 8 (2018) 193-221. https://doi.org/10.1016/b978-0-12-813160-2.00008-0 [7] s. loaiza-ambuludi, m. panizza, n. oturan, a. özcan, m. a. oturan, journal of electroanalytical chemistry 702 (2013) 31-36. https://doi.org/10.1016/j.jelechem. 2013.05.006 [8] t. m. do, j. y. byun, s. h. kim, catalysis today 295 (2017) 48-55. https://doi.org/10.1016/ j.cattod.2017.05.016 [9] h. lin, removal of organic pollutants from water by electro-fenton and electro-fenton like processes, ph.d. thesis, université paris-est, 2015. 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10.1016/j.seppur.2013.07.023 ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.seppur.2013.03.036 https://doi.org/10.1016/j.apcatb.2016.08.037 https://doi.org/10.1016/j.seppur.2014.05.007 https://doi.org/10.1016/j.dyepig.2005.09.027 https://doi.org/10.1016/j.dyepig.2005.09.027 https://doi.org/10.1016/j.chemosphere.2019.01.066 https://doi.org/10.1016/j.chemosphere.2019.01.066 https://doi.org/10.1016/j.jece.2018.08.022 http://dx.doi.org/10.1088/1742-6596/1524/1/012086 http://dx.doi.org/10.1088/1742-6596/1295/1/012045 https://doi.org/10.1016/j.cej.2011.12.010 https://doi.org/10.1016/j.jece.2019.103108 https://doi.org/10.1051/e3sconf/201912503003 https://doi.org/10.1051/e3sconf/201912503003 https://doi.org/10.1016/‌j.jelechem.‌2020.114163 https://doi.org/10.1016/‌j.jelechem.‌2020.114163 https://doi.org/10.1016/j.jhazmat.2008.06.067 https://doi.org/10.1016/j.jhazmat.2008.06.067 https://doi.org/10.1016/j.seppur.2013.07.023 https://doi.org/10.1016/j.seppur.2013.07.023 https://creativecommons.org/licenses/by/4.0/) @article{marlina2022, author = {marlina, elin and purwanto, purwanto and sudarno, sudarno}, journal = {journal of electrochemical science and engineering}, title = {{decolorization of industrial wastewater using electrochemical peroxidation process:}}, year = {2022}, issn = {1847-9286}, month = {dec}, number = {2}, pages = {373--382}, volume = {12}, abstract = {in this study, decolorization of wastewater samples taken from the paper industry is investigated using electrochemical peroxidation process. the electrochemical peroxidation process is a part of electrochemical advanced oxidation processes, which is based on the fenton's chemical reaction, provided by addition of external h2o2 into reaction cell. in this study, iron is used as anode and graphite as cathode put at the fixed distance of 30 mm in a glass reaction cell. the cell was filled with the solution containing wastewater and sodium chloride as the supporting electrolyte. factors of the process such as ph, current intensity, hydrogen peroxide concentration, and time of treatment were studied. the results illustrate that all these parameters affect efficiencies of dye removal and chemical oxygen demand (cod) reducing. the maximal removal of wastewater contaminants was achieved under acid (ph 3) condition, with the applied current of 1 a, and hydrogen peroxide concentration of 0.033 m. at these conditions, decolorization process efficiency reached 100 and 83 % of cod removal after 40 minutes of wastewater sample treatment. in addition, the electrical energy consumption for wastewater treatment by electrochemical peroxidation is calculated, showing increase as the current intensity of treatment process was increased. the obtained results suggest that electrochemical peroxidation process can be used for removing dye compounds and chemical oxygen demand (cod) from industrial wastewaters with high removal efficiency.}, doi = {10.5599/jese.1017}, file = {:d\:/onedrive/mendeley desktop/marlina, purwanto, sudarno 2022 decolorization of industrial wastewater using electrochemical peroxidation process.pdf:pdf;:jese_v12_no2_373-382.pdf:pdf}, keywords = {fenton's reaction, paper industry wastewater, chemical oxygen demand, decolori¬zation efficiency, electrochemical peroxidation}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1017}, } spectroscopic, voltammetry and molecular docking study of binding interaction of antipsychotic drug with bovine serum albumin doi:10.5599/jese.205 155 j. electrochem. sci. eng. 6(2) (2016) 155-164; doi: 10.5599/jese.205 open access : : issn 1847-9286 www.jese-online.org original scientific paper spectroscopic, voltammetry and molecular docking study of binding interaction of antipsychotic drug with bovine serum albumin mallappa mahanthappa, babu giriya gowda, jayant i. gowda*, raghavendran rengaswamy department of chemistry, maharani’s science college for women, bangalore-560 001, india *p.g. department of studies in chemistry, karnatak university, dharwad580 003, india corresponding author: babgowda@gmail.com, phone: +91-9900475412 received: july 22, 2015; accepted: february 4, 2016 abstract the interaction between perazine dimaleate (pdm) and bovine serum albumin (bsa) was investigated by voltammetry, fluorescence spectroscopy, uv–vis spectroscopy, molecular docking and viscometric methods. the study was carried out in acetate buffer solution of ph 7.2, which was prepared by using 0.1 m sodium acetate and adjusting ph using 0.1 m hydrochloric acid. the voltammetric study of pdm shows a pair of well redox peaks at 0.538 and 0.471 v (versus sce) on a gce in acetate buffer of ph 7.2 at 50 mv s -1 . after the addition of bsa into the pdm solution, the redox peak currents decreased gradually, and peak potentials shifted towards negative direction. the results of voltammetry, fluorescence quenching and uv–vis absorption spectra experiments indicated the formation bsa–pdm complex. the binding parameters like binding constant and binding free energy were determined from voltammetric data. the binding constant and binding energy was also determined from uv–vis and fluorescence spectroscopy with a value quite close to that obtained from cv. keywords bovine serum albumin; perazine dimaleate; cyclic voltammetry; spectroscopy; molecular docking; binding constant; binding energy introduction the drug–protein interaction is a fundamental issue in life process that helps us to understand the absorption, transport, metabolism and the target molecules of the drugs at the cellular level http://www.jese-online.org/ mailto:babgowda@gmail.com j. electrochem. sci. eng. 6(2) (2016) 155-164 binding interaction of antidepressant drug 156 [1-3]. serum albumins are the major soluble protein constituents of the circulatory system possessing many physiological functions of which the most important are serving as a depot and a transport protein for many endogenous and exogenous compounds such as drugs [4]. the interaction of drugs with serum albumin is great aspect and various types of application in research field in chemistry, life science and clinical medicine [5-8]. bovine serum albumin (bsa) has been widely investigated due to its structural homology with human serum albumin [9]. intercalation, groove binding, and electrostatic interactions are the three major binding modes of small molecules to bsa [10]. perazine dimaleate (pdm) is biologically active drug, chemically known as 10-[3-(4 methylpiperazine-1-yl)propyl]-10-phenothiazine·2maleicacid; perazine maleate; pernazinum (fig. 1). pdm is important class of tricyclic compounds belongs to the phenothiazine family drugs. generally, phenothiazine drugs are widely used as antiemetic, sedative and analgesic because they are able to reduce the side effects of chemotherapy and they they also have a role in inhibiting tumor cell proliferation. these drugs can also reduce anxiety, insomnia and other symptoms related to the psychological state of cancer patients [11]. fig. 1. chemical structure of perazine dimaleate (pdm) there has not any report about the study of pdm–bsa interaction based on the voltammetry behaviours at gce and more than ever on the change of spectroscopic characteristics. accordingly, in this work, investigations of the voltammetric behaviour of pdm upon addition of bsa were carried out. moreover, the changes in the uv-vis absorption and fluorescence emission when pdm binding to bsa were used to study the mode of such interaction. the results of voltammetry and spectroscopic techniques confirmed each other. the interaction study of pdm with bsa gives the information about the quenching mechanisms, binding constant, binding mode and number of binding sites. the advantage of this method is simple, rapid, accurate and inexpensive. materials and methods pdm sample was donated by global calcium ltd., hosur. stock solution, 1.0 mmol l -1 of pdm was prepared by dissolving an appropriate amount of the drug in doubly distilled water. bsa (sigma–aldrich) was used without further purification and its stock solution was prepared by dissolving an appropriate amount of bsa in doubly distilled water and stored at 4 °c. the concentration of bsa in stock solution was determined by uv absorption at 260 nm using a molar absorption coefficient ε260 = 66,000 l mol -1 cm -1 . purity of the bsa was checked by monitoring the ratio of the absorbance at 260 nm to that at 280 nm [12]. the supporting electrolyte used for 0.1 m acetate buffer solution of ph 7.2, which was prepared by using 0.1 m sodium acetate and ph was adjusted by using 0.1m acetic acid and all the experiments were conducted in this buffer m. mallappa et al. j. electrochem. sci. eng. 6(2) (2016) 155-164 doi:10.5599/jese.205 157 at 25 °c. all other reagents used were of analytical grade and doubly distilled water was used throughout the experiment. instrumentation voltammetric measurements were performed using a ch electrochemical analyzer. a conventional three-electrode cell was employed throughout the experiments, with glassy carbon electrode (3.0 mm diameter) as a working electrode, a saturated calomel electrode (sce) as a reference electrode, and a platinum electrode as a counter electrode. absorption spectra were measured on ellico uv-visible spectrophotometer. the electronic spectrum of a known concentration of the drug was obtained without bsa. the spectroscopic response of the same amount of the drug was then monitored by the addition of small aliquots of bsa solution. all of the samples were mixed properly and allowed to equilibrate for 5 min prior to every spectroscopic measurement. the viscosity measurements were carried out using ostwald viscometer. the ph measurements were made with scott gerate ph meter cg 804. an electronic thermostat water-bath was used for controlling the temperature. the steady-state fluorescence spectra were recorded using an f-2700 spectrofluorometer (japan). the fluorescence emission spectrum of pdm (excitation at 250 nm) was used to obtain the intensities of the peaks. good resolution of the bands was obtained at the slit width (ex. 10.0 nm; em.10.0 nm). the scan range used was from 300 to 600 nm. the molecular docking studies of bsa with compounds were achieved by using auto dock vina, developed at the scripps research institute (http://vina.scripps.edu) [13]. the input files for autodock vina were prepared with autodock tools (adt), which is a graphical user interface for autodock and autodock vina. the crystal structure of bovine serum albumin (3v03) was retrieved from protein data bank and the ligand binding site location was analyzed by q-site finder [14]. the co-crystallized ligand was removed. using adt the water molecules were removed from the protein and polar hydrogen were added followed by adding kollman charges. the grid box has been set according to the binding site on protein and saved as pdbqt format, which was required by autodock vina. the 2 dimensional structure of ligand were drawn by chem sketch and converted to pdb format, required by autodock tools by open babel [15]. the rotatable bonds were selected within the ligand using choose torsions option in adt and saved in pdbqt format the lamarckian genetic algorithm (lga), which is a novel and robust automated docking method available in autodock [16], was employed. results and discussion voltammetric studies of interaction of pdm with bsa the voltammetric behaviour of 1.5 x 10 -4 m pdm in the absence and presence of bsa on bare gce at 50mv s -1 is shown in fig. 2. the voltammogram without bsa featured a couple of well defined and stable redox peaks in the potential range of 0.0 – 1.0 v. the anodic and cathodic peaks were appeared at 0.538 and 0.471 v vs. sce with a formal potential (e 0 ) of 0.5045 v. the formal potential (e o ) in all voltammetric studies, predictable as the midpoint of reduction and oxidation potentials. by the addition of 5 30 µmol l -1 of bsa into 1.5×10 -4 m drug (fig. 2) the anodic peak potential was shifted towards negative direction and also peak current was decreased. the substantial decrease in peak current indicates the formation of pdm–bsa complex. http://vina.scripps.edu/ j. electrochem. sci. eng. 6(2) (2016) 155-164 binding interaction of antidepressant drug 158 e / v vs. sce fig. 2. cyclic voltammogram of 1.5 x 10 -4 m pdm in the absence of bsa and in presence of cbsa = 0, 5, 10, 15, 20, 25, 30 µmol l -1 (a to g) bsa in acetate buffer of ph-7.2 at 50 mv s -1 and inset of the plot of log (1/[bsa]) vs. log (i/ (i0 −i)). the mode of pdm–bsa interaction can be well-known from the variation in formal potential. in general the positive shift (anodic shift) in formal potential is caused by the intercalation of the drug with bsa [17], while negative shift is observed for the electrostatic interaction of the drug with bsa [18]. therefore, this is evident that negative peak potential shift (cathodic shift) in the cv behaviour of pdm by the addition of bsa is attributable to the electrostatic interaction of pdm with bsa. the cathodic peak potential shift further indicates that pdm is easier to oxidize in presence of bsa because its oxidized form is more strongly bound to bsa than its reduced form. in this system, both forms of the drug interact with bsa. based upon the decrease in peak current of pdm by the various concentration of bsa, the binding constant was calculated according to the following equation [19]: log (1/[bsa]) = log k + log (i/i0-i) where, k is the binding constant, i0 and i are the peak currents of the free pdm and pdm–bsa complex, respectively. the plot of log (1/[bsa]) vs. log (i/(i0 −i)) constructed (inset of fig. 2). from the linear fitting, the binding constant (k) can be estimated from the intercept. with this procedure, we obtained k = 4.591×10 4 l m -1 and the correlation coefficient was found as 0.99649. influence of scan rate the voltammetric behavior of 1.5×10 -4 m pdm in the absence and presence of bsa at bare gce is shown in fig. 3. the cvs of the 1.5×10 -4 m pdm on gce in 0.1 m acetate buffer solution at ph 7.2 shows pair of well defined and stable redox peaks in the potential range of 0.0 – 1.0 v. an increase in the scan rate, peak potential does not shift but peak currents of both anodic as well as cathodic increased linearly. the scan rate varies from 10 to 400 mv s −1 . at the same time, the cathodic and anodic peak currents increased linearly with the scan rate (fig. 3) and the redox peak currents increased linearly with the scan rate, the correlation coefficient was 0.9919 for cathodic peak and 0.9944 for anodic peak respectively. the redox process is diffusion-controlled as shown by ipeak versus υ 1/2 plot inset of fig. 3. this was further confirmed by the plot of log ipeak vs. log υ, which was linear within the same scan rate range and gave a slope of 0.5606 [figure as not shown]. this value is very close to the theoretical value of 0.5 reported by laviron for a diffusion-controlled i /  a -1.2 -0.9 -0.6 -0.3 0.0 0.3 log (1 / [bsa] / m lo g ( i/ (i -i 0 )) m. mallappa et al. j. electrochem. sci. eng. 6(2) (2016) 155-164 doi:10.5599/jese.205 159 electrode process [20]. this phenomenon suggested that the redox process was a diffusion controlled and the pdm was stable. . e / v vs. sce fig. 3 cyclic voltammograms of 1.5 x 10 -4 m pdm at various scan rates: 10, 25, 50, 100, 200, 400 mv s -1 respectively. inset, plot of ipeak versus υ 1/2 (red and black points are before addition of bsa and blue and green points are after addition of bsa) the redox peak of pdm is attributed to the one electron which involved reduction and oxidation of pdm to pdm free radical [21].   -e e pdm pdm    the electron transfer rate constant (ks) is calculated using laviron’s equations [21-23] as follows: e = e 0 – 2.3rt log (ν/αnf) e = e 0 – 2.3rt log (ν/(1-α)nf) log ks = log(1-α) + (1-α) log α log (rt/nfν) α(1-α)nf∆ep/2.303rt where α is the electron transfer coefficient, n is the number of electrons transferred (n = 1). r = 8.314 j mol −1 k −1 , t = 298 k and f = 96493 c/mol are gas, temperature and faraday constant, respectively and ks is the electron transfer rate constant which can be calculated according to the plot of δep versus ln ν. it is found to be 0.9890 s -1 in absence of bsa and 1.101 s -1 in presence of bsa. from the results of the above experiments, the formation of an electrochemically non-active complex of pdm with bsa resulted in the decrease of the free concentration of pdm in the reaction solution which caused the decrease of the peak currents. there is no appreciable difference in the value of k0 in presence and absence of bsa indicating that bsa did not alter the voltammetric kinetics of pdm. absorption spectral behaviour of pdm with bsa the uv-visible absorption titration is simple and more effective method for getting further clues about the mode of interaction and binding strength. figure. 4 show the electronic absorption spectra of 1.5×10 -4 m pdm in the absence and presence of different concentrations of bsa. the i /  a -3.0 -2.0 -1.0 0.0 1.0 i /  a ν 1/2 / (v s -1 ) 1/2 j. electrochem. sci. eng. 6(2) (2016) 155-164 binding interaction of antidepressant drug 160 absorbance of pdm was located around at 248 nm. the maximum absorption of the drug at this wavelength exhibited slight bathochromic shift and pronounce hyperchromic shifts by the incremental addition of bsa. the bathochromic shift is associated with the decrease in energy gap between the highest (homo) and lowest (lumo) molecular orbitals after interaction of pdm to bsa [24]. the compactness in the structure of either the drug alone and/or bsa after the formation of drug-bsa complex may result in hyperchromism. meanwhile, pdm molecule is inserted into the cavities of bsa and disrupted the original structures of bsa. this results which confirms the crystal structure of bsa had changed and there was a strong interaction between pdm and bsa. b wavelenght, nm c (1 / [bsa]) / m -1 figure. 4 uv-visible spectra of (a) pdm and bsa, (b) 1.5 x 10 -4 m pdm in the absence of bsa and the presence of [bsa] = 5.0, 10.0, 15.0, 20.0, 25.0 and 30.0 µmol bsa in acetate buffer of ph 7.2. (c) the plot of (a0 / (a-a0) versus 1 / [bsa]. based upon the increase in absorbance, the binding constant (kb) was calculated according to the following equation [24]. a0/(a-a0) = εg/(εh-g – εg) + εg/(εh-g εg) x 1/k[bsa] where, a0 and a are the absorbance of pdm in the absence and presence of bsa, εg and εh–g are the absorption coefficients of pdm and its complex with bsa, respectively. the slope to intercept ratio of the plot between a0/(aa0) vs. 1 / [bsa] yielded the binding constant, k = 6.6671×10 4 l m -1 , which is close to the value of k = 1.9824×10 4 l m -1 obtained from cv. the moderate binding constant indicative of electrostatic interaction and the gibbs energy 40000 80000 120000 160000 200000 10 15 20 25 30 a 0/a -a 0 1/[bsa] a b so rb a n ce , a .u . a 0 / ( a -a 0 ) m. mallappa et al. j. electrochem. sci. eng. 6(2) (2016) 155-164 doi:10.5599/jese.205 161 change (δg = -rt ln k) of approximately 24.039 kj mol -1 at 25 °c signifies the spontaneity of interaction of pdm with bsa. the type of quenching mechanism was determined using uv-visible absorption spectroscopy. quenching usually occurs either by static or dynamic mode. static quenching refers to fluorophore–quencher complex formation and the dynamic quenching refers to a process that the fluorophore and the quencher come into contact during the transient existence of the excited state. however, the formation of non-fluorescence ground-state complex induced the change in absorption spectrum of fluorophore. addition of the bsa to pdm leads to an increase in pdm absorption intensity without affecting the position of absorption band (fig. 4a). it showed the existence of static interaction between bsa and the complexes. spectrofluorimetric spectral behaviour of pdm-bsa complex the interaction of pdm with bsa was also examined by fluorescence titration. the fluorescence emission spectra of bsa in the presence and absence of various concentration of pdm is shown in fig. 5. the maximum emission of bsa is located at 342 nm (λex = 263 nm). the fluorescence emission maxima were gradually decreased with increase in concentration of pdm, indicating the quenching of fluorescence intensity of bsa upon binding to pdm. this can be attributed to the formation of pdm-bsa complex. the results suggest that microenvironment around the chromospheres of bsa is changed and it shows the strong interaction between pdm and bsa [25]. wavelenght, nm fig. 5. fluorescence spectra of 1.5×10 -4 m bsa in presence of cpdm = 0, 5, 10, 15, 20, 25, 30, 35 µmol l -1 . stern-volmer binding and quenching constant to determine the quenching mechanism between bsa and pdm (i.e. dynamic or static quenching), the fluorescence quenching data were analyzed using stern-volmer equation. f0 / f = 1+ kq τo [q] = 1 + ksv [q] where, f0 and f are the fluorescence intensities of bsa in absence and presence of pdm respectively, [q] is the concentration of bsa and kq is the quenching rate constant of the biomolecule, τo is the average excited-state life time and ksv is the stern-volmer quenching constant. 300 350 400 450 0 400 800 1200 1600 2000 2400 f lu o ro s c e n c e i n te n s it y wavelength in nm a i f lu o re sc e n ce i n te n si ty , a .u . j. electrochem. sci. eng. 6(2) (2016) 155-164 binding interaction of antidepressant drug 162 fig. 6 (a) plot of f0/f versus [q] and (b) plot of log [(f0-f)/f] versus log [q] the ksv is stern-volmer quenching constant, which was determined by linear regression of a plot of f0/f vs. [q] at 25 °c was constructed (fig. 6a) using the data from fluorescence titration and a linear fitting of the data yielded the binding constant 2.4158×10 4 l m -1 which is good agreement with result obtained from cv and uv/vis spectroscopy. the maximum rate constant of collisional quenching of various quenchers with biopolymers is about 2.0 x 10 10 l s -1 mol -1 [25]. the greater value for kq is obtained (ksv and kq values calculated were 1.964×10 4 l m -1 and 1.964×10 12 l m -1 s -1 ) (r 2 = 0.99523) respectively. as temperature increased, the decreasing trend of kq was in accordance with ksv’s dependency on temperature, which coincided with a static quenching mechanism. the binding of pdm to bsa was reduced as temperature increased, which suggests that the fluorescence quenching process may be mainly controlled by static quenching mechanism rather than dynamic. the binding constant and number of binding sites the binding constant and number of binding sites for pdm-bsa were determined by the following equation [26]: log[(f0-f)/f] = log kb + n log [q] where, kb is binding constant and n is number of binding sites in base pair units respectively. the values of n and kb can be determined from the slope and intercept of the double logarithm regression curve log [(log [(f0 f)/f] vs. log [q]) (fig. 6b). the binding constant value 5.43×104 l m -1 (r 2 = 0.99879) and values of n is found to be 1.2  1.0, indicating that there is one independent class of binding sites in bsa for pdm. the free energy change, δg 0 (at 27 °c) was evaluated from kb using the relationship δg 0 = -2.303rt log kb and its value was found to be -27.016 kj m -1 indicating the spontaneity of the reaction. molecular modeling studies molecular docking was performed to study the interaction between pdm and bsa to determine the preferred binding site and binding mode. the best confirmation was determined based on binding affinity and rmsd. the binding energy and rmsd was performed by auto dock vina. a three dimensional docked structure of pdm on bsa was shown in fig. 7. these structures were to study various inter molecular interaction and to determine the binding energy of the docked m. mallappa et al. j. electrochem. sci. eng. 6(2) (2016) 155-164 doi:10.5599/jese.205 163 complexes. the pdm were docked with receptor bsa using the parameters mentioned above. the energy of interaction for pdm-bsa complex was found to be -10.0 kcal mol -1 . fig. 7. molecular model of the surface and the active site of the complex of pdm (stick) and bsa (white surface) viscometer measurement viscometric technique is an effective tool in clarifying the mode of interaction of small molecules with bsa under the physiological condition. in general, intercalation (in-binding mode) causes an increase viscosity of bsa solution due to lengthening the bsa widened to accommodate the bonded molecule [27]. the reverse can be taken for the electrostatic interaction. the value of relative specific viscosity (η/η0) 1/3 vs. [bsa]/[pdm] were plotted (fig. 8). however, the relative viscosity of bsa dependence upon the concentration of pdm, the value of [bsa]/[pdm] is decreased. this behaviour indicates that the binding mode between bsa and pdm should be nonclassical intercalation or groove mode via hydrophobic interaction. fig. 8. plot of [bsa/pdm] vs. (η/η0) 1/3 conclusions in this work, the interaction of pdm with bsa was studied by voltammetric, spectroscopic and viscometric methods. in voltammetric studies, it was observed that the presence of bsa in a solution of perazine dimalaete reduces the equilibrium concentration of free perazine dimalaete and produces a voltammetric inactive complex. both electrostatic interactions and minor groove binding modes were deduced from the results of different methods applied to the system, j. electrochem. sci. eng. 6(2) (2016) 155-164 binding interaction of antidepressant drug 164 although groove binding seemed to be predominant. meanwhile, the experimental results indicate that the quenching mechanism of fluorescence of bsa by pdm is a static quenching procedure and the binding reaction is spontaneous. this work provides some significant information to clinical research about pdm and the theoretical basis for new drug designing. acknowledgement: the authors are grateful for the financial support provided by the university grants commission, new delhi, india (f. no. 42-308/2013 (sr) dated 28/03/2013). thanks are also due to global calcium ltd., hosur for supplying gift samples of pdm. thanks are also due to dr. siddalingeshwar, department of physics, msrit, bangalore for providing spectrofluorimetric instrumental facility references [1] e. la zaro, p. j. lowe, x. briand, b. faller, j. med. chem. 51 (2008) 2009-2017. 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[27] q. wang, x. wang, z. yu, x. yuan, k. jiao, int. j. electrochem. sci. 6 (2011) 5470 – 5481. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ oxygen source-oriented control of atmospheric pressure chemical vapor deposition of vo2 for capacitive applications doi:10.5599/jese.278 165 j. electrochem. sci. eng. 6(2) (2016) 165-173 ; doi: 10.5599/jese.278 open access : : issn 1847-9286 www.jese-online.org original scientific paper oxygen source-oriented control of atmospheric pressure chemical vapor deposition of vo2 for capacitive applications dimitra vernardou* , , antonia bei**, dimitris louloudakis* , ***, nikolaos katsarakis* , **** , *****, emmanouil koudoumas* , ***** *center of materials technology and photonics, school of engineering, technological educational institute of crete, 710 04 heraklion, crete, greece **department of mechanical engineering, school of engineering, technological educational institute of crete, 710 04 heraklion, crete, greece ***department of physics, university of crete 711 00 heraklion, crete, greece ****department of electrical engineering, school of engineering, technological educational institute of crete, 710 04 heraklion, crete, greece *****5institute of electronic structure & laser, foundation for research & technologyhellas, p.o. box 1527, vassilika vouton, 711 10 heraklion, crete, greece. corresponding author: dvernardou@staff.teicrete.gr received: march 18, 2016; revised: may 19, 2016; accepted: may 23, 2016 abstract vanadium dioxides of different crystalline orientation planes have successfully been fabricated by chemical vapor deposition at atmospheric pressure using propanol, ethanol and o2 gas as oxygen sources. the thick a-axis textured monoclinic vanadium dioxide obtained through propanol presented the best electrochemical response in terms of the highest specific discharge capacity of 459 mah g -1 with a capacitance retention of 97 % after 1000 scans under constant specific current of 2 a g -1 . keywords atmospheric pressure chemical vapor deposition; o2 source; vanadium dioxide; electrochemical properties. introduction vanadium dioxide (vo2) exists in more than 10 polymorphs, however, rutile, monoclinic (described as a slightly distorted rutile structure) and metastable phases have mainly attracted interest because of their interesting chemical and physical properties for catalytic and electrochemical applications [1-3]. the crystal structure of vo2 consists of sheets of edge-sharing http://www.jese-online.org/ mailto:dvernardou@staff.teicrete.gr j. electrochem. sci. eng. 6(2) (2016) 165-173 chemical vapor deposition of vo2 166 vo6 octahedra linked by corner sharing to adjacent sheets along the c-direction of the unit cell [4]. this corner-sharing structure strengthens the structural stability and the resistance to lattice shearing during cycling in lithium ion batteries [5]. in that context, vo2 was found to show better electrochemical performance compared with the well-known v2o5 [6]. to date, various vo2 forms have been synthesized such as nanocrystalline vo2(b) [4], nanothorn vo2(b) hollow microspheres [7], three dimensional hierarchical microflowers and microspheres [8,9] as well as vo2/reduced graphene oxide [10,11], vo2(b)/carbon nanotubes [12] and hydrogen treated vo2 [13] composite powder materials with sufficient capacitive characteristics. but, most of the above mentioned materials showed poor cycling stability after 500 cycles. hence, it is important to improve the long-term performance and high rate capability of vo2. due to the small compositional differences between numerous phases of vanadium oxides, vo2 preparation requires a stringent controlled process that provides desired oxygen stoichiometry and crystalline structure. in search of such a process, vo2 has been grown using sol-gel [14], pulsed laser deposition [15], sputtering [16] and chemical vapor deposition (cvd) [17-20]. among these techniques, cvd at atmospheric pressure (apcvd) is gaining attention because it does not require expensive vacuum systems, has fast deposition rates and can be easily integrated to floatglass production lines [21-24]. a range of precursors have been utilized including vanadium(iv) chloride (vcl4) [22], vanadyl(iv) acetylacetonate (vo(acac)2) [23-24], vanadium(v) oxytrichloride (vocl3) [25], vanadium(v) triisopropoxide oxide (vo(oc3h7)3 [26-28]. since highly oriented vo2 films exhibited significant variation in conductivity as thermochromic layers [29-31], we were inspired to investigate the orientation effect on the apcvd coatings for capacitive applications. in this paper, the apcvd of vo2 on sno2-precoated glass substrate was studied using vo(acac)2 and propanol, ethanol, o2 gas as oxygen sources from the perspective of the potential application of such coatings as cathodes. we have demonstrated the differences in the resulting coatings obtained from a structure-orienting role played by the oxygen sources utilized and possible explanation of this role is examined. experimental preparation of vo2 coatings the apcvd reactor utilized in this work was an in-house design as also reported previously [3,23,24]. the vanadium source was the vo(acac)2 (98 %, sigma-aldrich), which was placed in a bubbler at 200 o c, while the gas lines were kept at 220 o c to avoid any condensation or blocking. the carrier gas was n2 (99.9 %), which was passed through the reactor during all depositions. the n2 flow rate through the vanadium precursor bubbler was 1.4 l min -1 , for growth temperature and period of 500 o c and 7.5 min, respectively. additionally, the flow rate of propanol (99.5 %, sigmaaldrich), ethanol (≥ 99.5 %, sigma-aldrich) and o2 gas (99.9 %) was 0.8 l min -1 . finally, the total n2 flow rate was kept at 12 l min -1 in all cvd experiments. the substrates were commercial sno2-precoated glass (uniglass, greece), all of dimensions 2×2×0.4 cm. prior to deposition, they were all cleaned with h2o and detergent, rinsed thoroughly with h2o and deionised h2o, and allowed to dry in air. once the allotted time was complete, the reactor temperature was turned off and the substrate allowed cooling at 100 o c under an atmosphere of n2. then it was removed from the reactor, handled and stored in air. d. vernardou et al. j. electrochem. sci. eng. 6(2) (2016) 165-173 doi:10.5599/jese.278 167 structural and morphological characterization of coatings the structure of the coatings was examined in a siemens d5000 diffractometer for 2-theta = 15.00 – 60.00 o , step size 0.05 o and step time 5 min/ o . additionally, their morphology was evaluated in a jeol jsm-7000 microscope. in this case, coatings were over-coated with a thin film of gold prior to analysis to avoid charging. finally, the coating’s thickness was estimated using a profilometer a-step tencor. a step was done by etching the vanadium oxide coatings off the sno2-precoated glass substrate in 1:3, h2o2 (30 %):hcl. tin dioxide remained intact after this procedure and the thickness was obtained from the measured step height. electrochemical evaluation cyclic voltammetry experiments were performed using a three electrode electrochemical cell as reported previously [32-34] for a potential range of -1 v to +1 v, a scan rate of 10 mv s -1 and a number of scans up to 1000. in particular, pt, ag/agcl and vanadium oxides on sno2-precoated glass substrates were used as counter, reference and working electrodes, respectively. all measurements were carried out in 1 m lioh, which acted as electrolyte. the chronoamperometric measurements were done at -1 v and +1 v for a step of 200 s and a total period of 2000 s. additionally, the chronopotentiometric curves were obtained at a constant specific current of 2 a g -1 and a potential range of +0.1 v to +0.6 v. results and discussion presented here are investigations into apcvd vo2 coatings grown from vo(acac)2 and three oxygen sources: propanol, ethanol and o2 gas. it will be shown that a level of control can be exerted over orientation and morphology through the different sources. all coatings produced were stable in air for over six months and resistant to h2o, acetone and toluene. additionally, they passed the “scotch tape test”; a piece of sticky tape was placed on the coating and then removed without lifting off the coating. structure figure 1 presents the x-ray diffraction (xrd) patterns of the apcvd coatings using propanol, ethanol and o2 gas. as shown in the case of propanol and ethanol, one diffraction peak is observed at 18.21 o , which matches to monoclinic a-axis textured vo2 coatings [30,31,35]. this peak can be indexed to 100 plane showing the preferred orientation growth of these coatings. furthermore, the xrd pattern of the as-grown coatings using o2 gas show two peaks at 55.4 and 57.6 o with miller indices 022 and 220 due to monoclinic vo2 phase indicating that is a 022-oriented single phase [36-38]. finally, peaks at 26.5, 33.7, 37.1, 51.7 and 54.7 o with respective miller indices 110, 101, 211 and 220 (indicated with asterisk in figure 1) are due to sno2–precoated glass substrate [39]. the preferential orientation of the vo2 on sno2-precoated glass substrates at angles other than 27.8 o is not clearly understood. nevertheless, it seems to be more sensitive to o2 source during the growth rather than to glass substrate. the reaction mechanism for the formation of vo2 from vo(acac)2 in the presence of o2 gas has previously been studied in the literature [40]. the possible decomposition routes of vo(acac)2 species involve a simple intramolecular rearrangement of the vo(acac)2 precursor resulting in the release of two c3h4 molecules, followed by the decomposition of vo(ch3coo)2 to yield (ch3co)2o and vo2. on the other hand, in the case of alcohols, the vo2 deposits possibly act as heterogeneous catalytic sites for their oxidation to propanal (for propanol) and acetaldehyde (for ethanol). a similar behavior is also observed in the j. electrochem. sci. eng. 6(2) (2016) 165-173 chemical vapor deposition of vo2 168 presence of methanol [41,42]. the active oxygen required for their oxidation comes from the vo(acac)2 itself, since it can be regarded as a source of excess oxygen (there are 5 oxygen atoms to 1 v atom, while only 2 oxygen atoms are required for vo2 formation). other researchers have also attempted to control the crystalline orientation of vo2. gary et. al. reported the a-axis textured vo2 deposited on r-plane sapphire and suggested that the cause could be a stress developing on the interface between the substrate and the coating [30]. muraoka et. al. studied the epitaxial growth of vo2 001-oriented single phase on tio2 001 substrates and 110-oriented phase on tio2 110, respectively [29]. ngom et. al. indicated that the crystalline orientation of the vo2 thin films was drastically changed because of the formation of an interface layer between the vo2 and the soda lime glass [35]. chiu et. al. also attempted to grow vo2 on glass using a 5 nm zno buffer layer. in the case of the direct vo2 growth on amorphous glass, polycrystalline films formed, while only vo2 011 peaks located at 27.90 o were observed for the growth on zno [43]. figure 1. xrd of apcvd vanadium oxides at 500 o c for 0.8 l min -1 flow rate of propanol, ethanol and o2 gas. morphology figure 2 presents the field-emission scanning electron microscope (fe-sem) images of vanadium oxide coatings grown at 500 o c on sno2-precoated glass substrate for 0.8 l min -1 flow rate of propanol, ethanol and o2 gas. for propanol and ethanol, compact grains of nearly round shape are mainly observed with their sizes being 160 nm and 40 nm, respectively. it was observed that thinner coatings grown using ethanol (95 nm) were denser, while as thickness increased for propanol (120 nm), the surface appeared less dense with growth at specific sites, suggestive of a stranski-krastanov type of growth mechanism [44]. regarding the o2 gas, agglomeration of grains forming rod-like structures is shown with thickness of 80 nm. d. vernardou et al. j. electrochem. sci. eng. 6(2) (2016) 165-173 doi:10.5599/jese.278 169 figure 2. fe-sem of apcvd vanadium oxides at 500 o c for 0.8 l min -1 flow rate of propanol (a), ethanol (b) and o2 gas (c). electrochemical characteristics in order to study the effect of oxygen source on the electrochemical performance of the coatings, cyclic voltammetry curves were obtained as indicated in figure 3. the potential range was -1 v to +1 v at a scan rate of 10 mv s -1 . all curves are normalized to the mass of the working electrode. the mass was measured by a 5-digit analytical grade scale and found to be 0.00002 g, which was obtained by measuring the glass substrate before and after the growth. it can be observed that the as-grown vanadium oxide coatings using propanol present two anodic peaks at 0.05 v / +0.52 v and two cathodic peaks at -0.15 v / ≈+0.64 v (vs. ag/agcl), which are accompanied by color changes from green, blue to yellow and then yellow, blue to green. since, the electrochemical cell is made up of glass, we have observed these color changes during the measurements. one may then assume that v +5 ions are reduced to v +4 and v +3 , since two anodic peaks are observed. a similar explanation can be given for the oxidation peaks, i.e. v +3 ions oxidize into v +4 and v +5 . these color changes are attributed to li + intercalation and deintercalation [45]. on the other hand, the shape of the curve for the vanadium oxide coating using o2 gas is different indicating one anodic peak at +0.11 v and one cathodic peak at -0.34 v accompanied by color changes from green to yellow and vice versa. this may be due to the existence of different vo2 j. electrochem. sci. eng. 6(2) (2016) 165-173 chemical vapor deposition of vo2 170 orientation planes compared with the one observed for alcohols. furthermore, the specific current of the as-grown coatings using alcohols as oxygen source is the highest presenting an enhanced electrochemical activity. we then suggest that this is correlated to both the 100 plane and the increased thickness, which incorporate more active material for the insertion of li + . figure 3. cyclic voltammograms of the first scan for the apcvd vanadium oxide coatings for 0.8 l min -1 flow rate of propanol, ethanol and o2 gas and an electrode geometrical active area of 1 cm 2 . maximized cyclic voltammogram curve for the region of -0.2 v +1 v of the grown vanadium oxide coating using propanol as inset. chronoamperometry measurements were also performed to calculate the specific charge during li + intercalation / deintercalation. it is estimated by integration of excess current measured upon switching the bias potential with time [32] as shown in figure 4 for the as-grown vanadium oxide using 0.8 l min -1 of propanol. the amount of specific charge for propanol found to be 120 c g -1 , which is three times higher than that of o2 gas. figure 4. the chronoamperometric response of the first scan recorded at -1 v and +1 v for an interval of 200 s of the as-grown coatings at 500 o c for 0.8 l min -1 flow rate of propanol. d. vernardou et al. j. electrochem. sci. eng. 6(2) (2016) 165-173 doi:10.5599/jese.278 171 figure 5. intercalated and deintercalated specific charge as a function with oxygen source utilized. figure 6 presents the specific discharge capacities of the as-grown coatings at 500 o c for 7.5 in using 0.8 l min -1 flow rate of propanol, ethanol and o2 gas under a constant specific current of 2 a g -1 . the propanol’s curve indicates two plateaus at approximately 0.25 v and 0.5 v, which present the two-step li + intercalation process as also observed in cyclic voltammetry analysis. the specific discharge capacity was 459 mah g -1 with a capacitance retention of 97 % after 1000 scans (figure 6 inset) keeping the staircase shape, which is promising for lithium ion batteries. the specific discharge capacity was higher than the apcvd metastable [3] and 022-oriented monoclinic vo2 [24]. on the other hand, the ethanol and o2 gas samples lack of staircase-like shape probably due to the less defined phase transition associated with li + . this result may arise due to the largest thickness of the propanol sample, which facilitates larger number of li + within the vanadium oxide lattice. figure 6. the chronopotentiometric curves for the as-grown sample at 500 o c for 7.5 min using 0.8 l min -1 propanol, ethanol and o2 gas under a constant specific current of 2 a g -1 and potential ranging from 0.1 v to 0.6 v. the 1000 th scan of 0.8 l min -1 propanol is also included as inset. j. electrochem. sci. eng. 6(2) (2016) 165-173 chemical vapor deposition of vo2 172 conclusions vanadium dioxides of different crystalline orientation were grown by apcvd at 500 o c for 7.5 min using propanol, ethanol and o2 gas. the a-axis textured monoclinic was enhanced with propanol and ethanol, while the 022-oriented single phase vo2 was obtained with o2 gas. consequently, the samples with different orientations possessed different morphologies; a-axis textured coatings showed grains, while the 022-oriented phases presented agglomeration of grains forming rod-like structures. electrochemical analysis has shown that the a-axis textured monoclinic vo2 grown using propanol has several unique characteristics compared with the rest of the samples, which makes it promising electrode material for lithium ion batteries. these include the thickness, which facilitates li + access to a large volume of active material and the specific discharge capacity of 459 mah g -1 with capacitance retention of 97 % after 1000 scans under constant specific current of 2 a g -1 indicating both high rate performance and good stability. 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[48] y. wu, p. zhu, x. zhao, m. v. reddy, s. peng, b. v. r. chowdari, s. ramakrishna, j. mater. chem. a 1 (2013) 852-859. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ influence of reduction conditions of nio on its mechanical and electrical properties doi:10.5599/jese.220 113 j. electrochem. sci. eng. 6(1) (2016) 113-121; doi: 10.5599/jese.220 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of reduction conditions of nio on its mechanical and electrical properties yehor brodnikovskyi, bogdan vasyliv*, viktoriya podhurska*, mariusz andrzejczuk**, nikkia mcdonald***, oleksandr kyrpa, orest ostash*, oleksandr vasylyev, robert steinberger-wilckens***, malgorzata lewandowska** frantsevich institute for problems of materials science, 3 krzhyzhanovsky str., kyiv, 03680, ukraine *karpenko physico-mechanical institute, 5, naukova str., lviv, 79060, ukraine **warsaw university of technology, faculty of materials science and engineering, wołoska str., 141, 02-507 warsaw, poland ***centre for fuel cell and hydrogen research, school of chemical engineering, university of birmingham, birmingham, b15 2tt, united kingdom corresponding author: bregor@ukr.net; tel.: +380-44-424-0294; fax: +1-111-111-112 received: september 16, 2015; revised: december 10, 2015; accepted: february 10, 2016 abstract yttria stabilized zirconia with a nickel catalyst (ni-ysz) is the most developed, widely used cermet anode for manufacturing solid oxide fuel cells (sofcs). its electro-catalytic properties, mechanical durability and performance stability in hydrogen-rich environments makes it the state of the art fuel electrode for sofcs. during the reduction stage in initial sofc operation, the virgin anode material, a nio-ysz mixture, is reduced to ni-ysz. the volume decrease associated with the change from nio-ysz to ni-ysz creates voids and causes structural changes, which can influence the physical properties of the anode. in this work, the structural, mechanical and electrical properties of nio samples before and after reduction in pure h2 and a mixture of 5 vol. % h2-ar were studied. the nio to ni phase transformations that occur in the anode under reducing and reduction-oxidation (redox) cycling conditions and the impact on cell microstructure, strength and electrical conductivity have been examined. results show that the redox treatment of the nio samples influence on their properties controversially, due to structural transformation (formation of large amount of fine pores) of the reduced ni. it strengthened the treated samples yielding the highest mechanical strength values of 25.7 mpa, but from another side it is resulting in lowest electrical conductivity value of 1.9×10 5 s m -1 among all reduced samples. the results of this investigation shows that reduction conditions of nio is a powerful tool for influence on properties of the anode substrate. keywords nio, ni, redox, sofc anode http://www.jese-online.org/ mailto:bregor@ukr.net j. electrochem. sci. eng. 6(1) (2016) 113-121 mechanical and electrical properties of nio 114 introduction solid oxide fuel cells (sofcs) are energy conversion devices that convert chemical energy into electrical energy with higher conversion efficiencies and lower greenhouse gas emissions than conventional heat engines [1]. traditional materials used in the manufacture of sofcs include niysz cermet for the anode, ysz (8-10 mol. % yttria stabilized zirconia) for the electrolyte and lsmysz (lanthanum strontium manganite – ysz composite) for the cathode. these sofcs typically operate on h2 at temperatures of 700-850 °c. literature delivers vast information regarding sofc component requirements [1]. since this paper discusses research centered on anode material properties, anode component requirements will be the main focus. ni has proven to be the best material for use in sofc anodes due to its high catalytic activity for hydrogen oxidation, absence of undesirable chemical interactions with ysz and good electronic conductivity, making ni-ysz the anode material of choice [2]. for simplification of anode manufacturing, nio powder is typically used instead of ni metal as ni metal melts at temperatures commonly used for cell sintering. because nio does not form solid solutions with ysz, the nio-ysz composite is reduced to a ni-ysz cermet during cell operation [3]. the reduction kinetics of nio powders have been thoroughly studied [4-5], also it is known that the reduction of nio to ni is accompanied by volume and structural changes which influence the physical and mechanical properties of the anode [6-8]. it is clear that the transformation of nio to ni during reduction brings the changes into the anode structure and determines its final properties. to understand the impact of ni phase transformations during reduction on the creation of ni-metal networks within the anode composite the study of nio properties after reduction are required. moreover, normal sofc start-up/shut down procedures where fuel flow is interrupted and system temperature is reduced may result in oxidation and reduction of ni, which further greatly influences anode microstructure. this process, commonly referred to as redox cycling, is considered unfavorable by some researchers [8] due to drastic volume changes of the ni phase, cracking and reduced strength of anode. while other researchers show redox cycling improves anode electrical and mechanical properties [9], at least initially. there is insufficient data to determine whether redox cycling improves or impairs the physical properties (electrical conductivity, mechanical strength) of ni-ysz anodes. to develop a better understanding of the influence of nickel phase on structure formation and properties of sofc anode, it is important to study the microstructural changes of nio samples and their properties before and after reduction and redox cycling. this work continues the previous study [10] and represents more details concerning formation of nio structure under variable reduction conditions. the relationships between mechanical, electrical properties of treated nio and its structure were established. experimental in this study commercial nio powder (d50 = ~300 nm, donetsk chemical reagent plant, ukraine) was used. the nio powder was wet ball milled in alcohol using zro2 media balls for 24 h and left to dry in air. the powder was passed through a 100 mesh sieve to achieve homogeneity and a narrow particle size distribution. chromatographic thermal desorption (sorbtometr p2, n2 gas adsorbate, ar – gas carrier) and laser granulometry (sk laser micron sizer pro-7000) studies were carried out to determine powder surface area and agglomerate sizes. the size of the initial particles (crystallites) was estimated from tem images of the powder. reduction and redox cycling studies were performed on 25 mm diameter and 1.5-2 mm thick disc shaped samples that were pressed uniaxially at 20 mpa and then sintered at 1400 °c for 2 h in air using a vmk 1600 y. brodnikovskyi et al. j. electrochem. sci. eng. 6(1) (2016) 113-121 doi:10.5599/jese.220 115 linn high therm furnace (germany). to improve compaction of as-pressed samples, polyvinyl alcohol was used as a plasticizer. the porosity of the sintered nio samples was measured by means of the archimedes method. all nio samples were divided into three groups and their reduction was carried out under three different testing conditions. the first group was reduced by holding the samples in high purity hydrogen (99.99 vol. % h2) at 600 °c for 4h. the second group was reduced by holding the samples in a 5 vol. % h2-ar fuel mixture at 600 °c for 4h and the last group was exposed to 5 redox cycles. each redox cycle consisted of an initial 1h dwell at 600°c in the 5 vol. % h2-ar fuel mixture followed by a 1 h dwell at 600 °c in air and ending with a 1h hold at 600 °c in the 5 vol. % h2-ar fuel mixture [9]. 600 °c for the reduction of nio samples was chosen as temperatures above 600°c can promote not only the sintering of a reduced highly porous ni-metal layer but also decrease the kinetics of the reduction process due to mass transfer limitations [5]. it was shown that temperatures within the 550-600 °c range are most effective for the reduction of nio powders and that the reduction of nio compacts at 600 °c preserves the newly formed pores inside the ni-grains. resistance measurements of the reduced samples were obtained by employing the dc 4-pt probe lateral method in air at 25°c. mechanical strength tests for the non-reduced and reduced samples were performed by using the ring-on-ring biaxial bending method at room temperature [11]. micrographs of the nio grain shape and size were obtained using a tem jem100сxіі, the crosssectional microstructures of the reduced and non-reduced nio samples were compared using a superprobe 733 sem (jeol, japan), and a high-resolution nb5000 dual-beam system microscope (hitachi high technologies corporation, japan) was used for internal structure characterization. 3d image analysis of the treated nio was carried out using focused ion beam (fib-sem) microscopy. nio porosity and post-reduction volume phase changes were calculated via estimation of surface area of different nio phases contained within the sem sample images by means of gimp and imagej software. results and discussion powder characterization a summary of the characterized nio powder is presented in table 1. it can be seen that the powder has a low surface area of 3 m 2 g -1 and consists of big agglomerates with an average size of 12.9 µm. figure 1 represents the general view of the initial particles of the nio powder. nio samples made from this powder were highly porous with an average porosity of 40 ± 4 %. table 1. properties of nio powder. initial particles size, nm agglomerate size, µm specific surface area, m 2 g -1 100-150 12.9 3 figure 1. tem image of nio powder j. electrochem. sci. eng. 6(1) (2016) 113-121 mechanical and electrical properties of nio 116 microstructure the microstructure of the nio samples in the non-reduced state is presented in figure 2a. it can be seen that the sample consists of dense grains without visible defects along the boundaries and surfaces of the grains. the sem image in figure 2b shows a sample reduced in the 5-vol. % h2-ar mixture at 600 °c for 4 h which has fine pores along the grain boundaries and no visible changes in grain surfaces. it appears that the reduction of nio in the 5 vol. % h2-ar mixture mostly effected on its grain boundaries. it is well known that the areas nearby grain contacts has the higher surface energy comparing to the grain surfaces what, for example, initiate diffusion processes and neck growth between grains during sintering of the powders compacts [12]. thus, observed in figure 2b structural changes of grain contacts can be explained by the aspiration of the material (nio) to decrease its surface energy of the grain contacts via reduction of nio to ni. in other words, the grain boundary sites are more energy favored over grain surfaces for carrying out reduction processes. also, it is known that the reduction of nio in a hydrogen-containing medium begins from the nucleation of metallic clusters which then grow into crystallites at a near linear rate and this occurs at the interface between nio and reduced porous ni [4,5]. it means that the newly formed interface between nio and reduced ni is also more energy preferable site for further carrying out reduction processes comparing with the non-reduced nio surface. due to this, the primary reduction of the grain contacts results in further facilitation of the reduction front spreading in the direction from grains contacts to the grain cores due to already formed metallic ni. this conclusion is in a good agreement with the fib section of the reduced nio samples in the 5 vol% h2-ar mixture pictured in figure 3a, which reveals much reduced grain contacts compared with the grain surfaces. as can also be seen in figure 3a, as in figure 2b, the nio grains were only partly reduced the core remained unaffected. the sem micrograph obtained in bse mode (figure 3a), created a contrast in the imaging allowing a clear distinction between the ni and nio phases. the thickness of an outer ni-metal rim surrounding the nio grains is less than 400 nm. from figure 3a it is possible to estimate the specific area of the ni-metal rim relative to the total area of the grains. this estimation indicates that the ni-metal rim is 11 % of the whole nio volume. thus, only about 11 % of the nio phase was reduced to ni in a mixture of 5 vol. % h2-ar. the partial reduction of the nio samples can be explained by insufficient contact between h2 (reducing agent) and the nio surface due to the low content of h2 in the fuel and/or insufficient dwell time and/or too low operating temperature. these findings are in good agreement with the results of previous studies [13]. the detection of the reduction front nucleation and its spreading inside the grains of nio helps to better understand the kinetics of the nio reduction process. nowadays, in order to explain the kinetics of the reduction process of nio, the shrinking core model and the grain model employed by szekely et al. [14,15] are generally used. the shrinking core model assumes that during reduction, the solid nio particle consists of an un-reacted core (nio), shelled by a uniform layer of the reaction product (ni-metal rim). as the reaction proceeds, this layer thickens and the unreacted core shrinks. in the grain model, a particle is considered an agglomerate of the individual grains. the reduction of such particles occurs as the reduction of individual grains, proceeding non-uniformly and causing reduction fronts to appear at different sites [4]. in the case of highly porous nio samples, the presence of brachiate porous channels facilitates the reduction according to the grain model, following further clarification. firstly, the appearance of the nucleation of the reduction front is not as random as the grain model suggests [4]. the y. brodnikovskyi et al. j. electrochem. sci. eng. 6(1) (2016) 113-121 doi:10.5599/jese.220 117 reduction begins from specific sites with the highest surface energy such as grain contacts. it is possible to assume that the activity of these sites can be estimated via the dihedral angle between two given grains in the same way as for a sintering process [12]. secondly, the direction of the reduction front spreading from grain contacts to the grain cores is not observed as a gradual reduction of individual nio grains uniformly from grain surfaces to the cores as suggested by the shrinking core model. figure 2. typical sem images of fracture surfaces of nio samples before (a) and after reduction in different conditions: b – 5 vol. % h2-ar; c – pure h2; d – redox figure 3. sem images of nio sample after (a) reduction in mixture 5 % h2-ar at 600 °c; (b) reduction in pure h2 at 600 °c; (c) redox in mixture 5 vol. % h2-ar at 600 °c the full conversion of nio into ni was achieved during reduction of the nio samples in pure h2. this leads to a change in the grain surface as it becomes pitted with holes as seen it in fig. 2c. the observed pits are most likely caused by the volumetric change of nio to ni during reduction. research shows [7], that under reducing conditions, the physical dimensions (size/shape) of the initial nio particles essentially stay the same but the volume changes of nio-ni occur by means of j. electrochem. sci. eng. 6(1) (2016) 113-121 mechanical and electrical properties of nio 118 pore formation inside the particles. the fib sections of the nio samples reduced in pure h2 at 600 o c reveal a sponge-like ni structure with pore sizes in the range of 50 to 500 nm as presented in figure 3b. these results are also in agreement with previously reported findings [7,13]. correlating the specific area of the newly formed pores to the specific area of the grains could give an approximation of the apparent porosity of the post-reduced nio samples. so, the volume of the inner pores is 30% relatively to the volume of the grains. the initial porosity of the samples was 40 % and the volume of nio phase was 60%, accordingly. after reduction of nio to ni 30% of solid phase became the newly formed pores. this 30 % from the initial solid phase nio is 18 % of additional porosity to the initial porosity value 40 %. thus, the total porosity of nio sample after its reduction in pure h2 is about 58 %. the use of the 5 vol. % h2-ar fuel mixture for the reduction of the nio compacts was chosen to monitor the steps of the reduction process to better understand the kinetics of nio reduction. while the use of the 5 vol. % h2-ar gas mixture is not effective for full reduction of nio to ni-metal, it is very useful for studying the microstructural changes in nio during reduction. thus, this gas mixture was used to study the influence of redox treatment on the properties of nio samples. an initial view of the sem images of the redox treated samples pictured in figure 2d, show peculiar changes on the surface of the grains when compared against samples in the initial state and those reduced in the 5 vol. % h2-ar gas mixture as displayed in figures 2a and b respectively. the edges of the grains became smooth while their surfaces became corrugated without the presence of big pitting holes as observed for samples reduced in pure h2. more detailed analysis of the redox treated samples reveal the transformation of the grain’s surface structure as pictured in figure 3c. the formed ni-metal rim contained a lot of fine pores and resembled a sponge-like structure, while the cores of grains remain unreduced. the thickness of the ni-rim of the redox treated samples was thicker than what was observed in a single reduction in the 5 vol. % h2-ar gas mixture. from figure 3c the specific area of the ni-metal rim relatively to the total area of the grains was estimated. this estimation indicates that ni-metal rim is 19 % of total grain area. this is practically twice as much than what was seen during a single reduction treatment in the 5 vol. % h2-ar gas mixture. the appearance of a larger volume of reduced ni-layer in nio samples after redox treatment, which includes oxidation cycles, can be explained by the creation of a porous structure on the ni layer surface. this finding suggests that following each reduction cycle more volume of material is reduced due to an increase in the available sample area because of the presence of pores created during redox cycles. practically, the redox treatment actualizes the step by step reduction of nio increasing the thickness and volume of the reduced ni-layer. moreover, comparing the structures of nio samples pictured in figures 3a and 3c makes it possible to observe the dynamics of the propagation of the reduction front. it is noticeable that the thickness of the reduced ni-layer in a grains contact zone is thicker than the reduced ni-layer from the surface of the pore. this shows that the reduction of nio compacts occurs in regions where there is grain-to-grain contact and these sites are a priority zone for nio reduction. redox treatment has an even stronger influence on the formation of ni-to-ni intergranular contact than what has been observed during a single reduction in the 5 vol. % h2-ar gas mixture and pure h2. the difference in contacting between nio grains after redox treatment and a single reduction in h2 is clearly seen in the sem images of their surfaces pictured in figures 4a and 4b respectively. in the case of a single reduction, the angularity of the grains are visible much like what is observed for unreduced nio in figure 2a but after redox treatment, the grains have y. brodnikovskyi et al. j. electrochem. sci. eng. 6(1) (2016) 113-121 doi:10.5599/jese.220 119 smoother edges yielding a more rounded shape which serves to expand the contact area between neighboring grains as observed in figure 4b. figure 4. typical sem images of surfaces of nio samples after redox treatment (a) and after single reduction in pure h2 (b) mechanical strength the results of this investigation on the mechanical strength of nio samples before and after reduction in the aforementioned testing environments are summarized in table 2. it can be seen that the mechanical strength of the samples increase after reduction and redox treatment when compared against the initial state. table 2. properties of nio samples after reduction in different conditions [10]. state of nio samples strength, mpa electric conductivity, s m -1 initial state 13.3 – after reduction in 5 vol. % h2-ar 15.9 3.3·10 5 after reduction in pure h2 18.8 2.7·10 6 after redox treatment 25.7 1.9·10 5 an increase in the mechanical strength of the 5 vol% h2-ar reduced samples (15.9 mpa) when compared against samples in their initial non-reduced state (13.3 mpa) can only be related to the formation of ni-ni grain contacts what was discussed in paragraph 3.2. the presence of the ductile ni metal component in the grain contacts improved the strength of entire sample. when examining sample mechanical strength, it is clear that ni-ni contacting yields higher values than nio-nio contacting. despite the newly formed porosity (see paragraph 3.2), the strength (18.8 mpa) of the anode samples reduced in pure h2 was also relatively higher than that in the initial state as indicated in table 2. such behavior can be explained by full transformation of nio to ni-metal and as result appearance of plasticity in the samples. the strength of redox treated samples (25.7 mpa) is the highest compared with other samples. this obvious difference in strength can be explained by structural changes in the nio samples after redox treatment. the newly formed fine porosity was observed in the ni-rim of the redox treated samples as figure 3c shows. it is seen that reduced grain contacts are very porous too. these fine pores complicate the passage of cracks through grain contacts and thus make them stronger. the close examination of the microstructure of the fracture surfaces picture in figure 2 indicates the b a j. electrochem. sci. eng. 6(1) (2016) 113-121 mechanical and electrical properties of nio 120 cleavage facets only in redox treated samples (figure 2d). the sizes of these facets are proportional to the sizes of the grains of the sample. this implies that the sample fracture permeates through the bodies of the non-reduced nio grains. it is evident that formed porous nini grain contacts are stronger over the dense nio grain bodies (core). electrical conductivity depending on the selected reduction medium the nio samples were reduced to a varying degree. the mixture of ar with 5-vol. % h2 can reduced nio samples only partially. only about 11 % of nio was reduced to the ni-metal as it was introduced above in paragraph 3.2. the grain surfaces and grain contacts were reduced to ni-metal phase, but the grain cores left unreduced in nio phase. nio phase is a bad electronic conductor and limits the conductivity of the whole sample. thus, only the reduced ni-metal rim, a thin layer of the grain surfaces and grain contacts, can react as electronic conductor in this sample. due to this, nio samples reduced in the mixture 5 vol% h2-ar had relatively low electrical conductivity of 3.3·10 5 s cm -1 as listed in table 2. despite of newly formed porosity during reduction and a quite high value of total porosity of 58% these samples had the highest electrical conductivity 2.7·10 6 s cm -1 . this can be explained by the creation of the highest cross-sectional area of the conductor (ni-metal) as a result of full transformation of the ceramic nio sample into a ni-metal sample. for redox treatment, a mixture of 5 vol% h2-ar was used. only partial reduction of nio was observed in the treated samples. the reason is the same as in the case of single reduction of nio in a mixture 5 vol% h2-ar. as introduced in paragraph 3.2 above, redox treatment allowed reducing more amount of nio (about 19 %) when comparing with single reduction in a mixture – which was about 11 %. due to the limitation of electronic conduction (ni-metal phase) it was expected that these samples had much lower conductivity than the samples reduced in pure h2, but they represented the lowest electrical conductivity of 1.9·10 5 s m -1 as listed in table 2. in spite of the big difference in reduced ni amount (practically double), a 42 % decrease in the electrical conductivity from 3.3·10 5 to 1.9·10 5 s m -1 for the nio samples under redox treatment compared to the single gas mixture reduced samples was observed. this can be explained by newly formed fine porosity in the ni-metal layer as seen in figure 3c. newly formed fine porosity as nonelectronically conducting phase decreases the total conductivity of the ni-rim due to thinning of the cross-sectional area of the conductor (ni). in case of single reduction in the gas mixture the visible porosity in the ni-rim was not observed as figure 3a showed. thus, a thin and dense ni-rim formed after single reduction in gas mixture provides higher electrical conductivity compared with the thicker and highly porous ni-rim formed after redox treatment. conclusion results show that the redox treatment of the nio samples provides the formation of a large number of fine pores in the reduced ni-metal layer. these newly formed fine pores influence on the properties of the nio samples controversially. on one side, it strengthened intergranular contacts yielding the highest mechanical strength values of 25.7 mpa and, on the other side, resulted in the lowest electrical conductivity value of 1.9 . 10 5 s m -1 among all reduced samples. the mechanical strength of the nio samples after reduction in pure h2 and 5 vol. % h2-ar were 18.8 and 15.9 mpa respectively, while the strength of nio samples before reduction (initial state) was 13.3 mpa. the electrical conductivity of reduced nio in 5 vol. % h2-ar (3.3 · 10 5 s m -1 ) was y. brodnikovskyi et al. j. electrochem. sci. eng. 6(1) (2016) 113-121 doi:10.5599/jese.220 121 lower than the electrical conductivity of reduced nio in pure h2 (2.7 10 6 s m -1 ) due to incomplete reduction of nio. these results demonstrate the large influence the reducing conditions have on the physical properties of the ni-phase component of ni-based anodes and, thus, the performance of sofc cells. reducing conditions in start-up of virgin cells and conditioning by careful choice of operating conditions and/or planned initiating redox cycles can greatly predefine cell performance. it was also found that during reduction of the porous nio samples, the nucleation of the reduction front begins in the intergranular contact zone and then spreads from there to the grain core. it was shown that this model of reduction front nucleation and spreading is more preferable than the assumption of uniform reduction as proposed in the shrinking core model. these results support the application of grain model nio reduction kinetics for nio agglomerates and improves the understanding of the behavior of reduction fronts throughout sofc anodes. acknowledgments: the authors are grateful to the european fp7 nanomat-epc project ”deployment of societally beneficial nanoand material technologies in european partnership countries” # 608906, the national academy of science of ukraine, their projects “hydrogen for alternative energetic and advanced technologies application” and "sofc structural optimization based on consideration of interdiffusion at manufacturing and operation" for their respective support. references [1] n.q. minh, solid state ionics 174 (2004) 271 – 277. [2] a. atkinson, s. barnett, r. j. gorte, j. t. s. irvine, a.j. mcevoy, m. mogensen, s.c. singhal, j. vohs, nature 3 (2004) 17–27. [3] r.f. martins, m.c. brant, r.z. dominques, materials research bulletin 4 (2009) 451–456 [4] g. plascenciaa,t. utigard, chemical engineering science 64 (2009) 3879–3888. [5] t. a. utigard, m. wu, g. plascencia, t. marin, chemical engineering science 60 (2005) 2061 – 2068. [6] d. sarantaridis, a. atkinson, journal of fuel cells 3 (2007) 246–258. [7] d. waldbillig, a. wood, d.g. ivey, journal of power sources 145 (2005) 206–215. [8] m. ettler, h. timmermann, j. malzbender, a. weber, n.h. menzler, journal of power sources 195 (2010) 5452–5467. [9] o. p. ostash, b. d. vasyliv, v. podhurs’ka, o. d. vasyl’ev, e. m. brodnikovs’kyi, l. m. ushkalov, journal of materials science 46 (2011) 653–658. [10] v.ya. podhurs’ka, b.d. vasyliv, o.p. ostash o. d. vasyl’ev, e. m. brodnikovs’kyi, journal of materials science 49 (2014) 805 – 811. [11] m. radovic, e. lara-curzio, acta materialia journal 52 (2004) 5747–5756. [12] f.f. lange, journal of the european ceramic society 28 (2008) 1509-1516. [13] m. andrzejczuk, o. vasylyev, i. brodnikovskyi, v. podhurska, b. vasyliv, o. ostash, m. lewandowska, k.j. kurzydłowski, materials characterization 87 (2014) 159-165. [14] v. j.szekely, j. w.evans, h. y. soh, gas solid reactions, academic press, new york, usa, 1976, 372p. [15] j. szekely, j. w. evans, metallurgical transactions. 2 (1971) 1699-1710. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ a wse2@poly(3,4-ethylenedioxythiophene) nanocomposite based-electrochemical sensor for simultaneous detection of dopamine and uric acid http://dx.doi.org/10.5599/jese.1120 989 j. electrochem. sci. eng. 12(5) (2022) 989-1000; http://dx.doi.org/10.5599/jese.1120 open access : : issn 1847-9286 www.jese-online.org original scientific paper spray pyrolisis deposition and characterization of cd-tio2 thin film for photocatalytic and photovoltaic applications masilamani raja sekaran1,2, parasuraman kumaresan1,2,, subramanian nithiyanantham3,, vatakkaputhanmadom krishnaiyer subramanian4 and sundar kalpana5 1pg & research department of physics, thiru a. govindasamy govt. arts college, tindivanam 604002, india 2department of physics, research and development centre, bharathiar university, coimbatore 641046, india 3pg & research department of physics, (ultrasonics, ndt and bio-physics divisions), thiru, vi. kalyanasundaram govt. arts and science college, thiruvarur 610003, india 4pg & research department of chemistry, periyar govt. arts college, cuddalore 607001, india 5pg & research department of physics, amet university, chennai 603112, india corresponding author: s_nithu59@rediffmail.com, lokeshkumaresan@yahoo.com received: september 28, 2021; accepted: june 17, 2022; published: august 15, 2022 abstract in the present paper, an innovative approach to enhance the photocatalytic efficiency and energy of photovoltaics by modifying the surface morphology of a tio2 is demonstrated.the photovoltaic device provides sustainable power efficiency in tio2 (to) and cd-tio2 (cto) thin films grown through spray pyrolysis. the structural and optical properties of the prepared undoped and cd doped tio2 thin films were studied. the morphology and content of the produced samples were studied using scanning electron microscopy (sem with edax). a uv-vis spectrophotometer was used to record the optical absorption spectra of tio2 nanoparticles. xrd analysis showed that to and cto had anatase structure, and the average crystalline size was calculated as 132.0 nm.the photocatalytic efficiency of to and cto for degradation of rodhamine b (rhb) dye was examined. also, power-voltage (p-v) and photocurrent-voltage (i-v) output current intensity relations were discussed. keywords energy materials; doped tio2 thin film; photocatalytic degradation; photocurrent-voltage characteristics introduction among materials useful for either photocatalysis orphotovoltaic devices, tio2 has been immensely inquired due to its photochemical corrosion, strong optical absorption, nontoxicity, and http://dx.doi.org/10.5599/jese.1120 http://dx.doi.org/10.5599/jese.1120 http://www.jese-online.org/ mailto:s_nithu59@rediffmail.com mailto:lokeshkumaresan@yahoo.com j. electrochem. sci. eng. 12(5) (2022) 989-1000 spray pyrolisis deposition of cd-tio2 thin film 990 favorable band edge positions [1-3]. tio2 is a common n-type semiconductor with a direct bandgap with large chemical stability. tio2 is a biosafe and biocompatible material that can be used in a wide range of applications, including photovoltaics, photocatalysis, and optical, biomedical and other purposes. the photovoltaic and photocatalytic performances of nanocrystalline tio2 depend on several parameters, such as morphology, crystalline phase and single crystallinity [2,3]. therefore, intense research has usually focused on fabricating one-dimensional tio2 nanostructures having morphologies with size-related optical and electrical properties. tio2 properties depend greatly upon its existing phase, i.e. anatase, rutile or brookite. the anatase phase shows an indirect optical band gap of 3.2 ev, while the rutile phase has a direct band gap of 3.06 ev. it absorbs in the uv regime of solar radiation (less than 5 %), which significantly limits its widespread applications [4-9]. to expand light absorption to the visible region, doping with transition metals has usually been employed. among all transition metals used for tio2 doping, cd is the most favored due to large solid solubility [10,11]. photocatalysis is a valuable method for wastewater treatment, particularly in the textile industry. the textile industry wastewater produced in each of the processes like the sizing of textile fibers, bleaching, calendering, dyeing and finishing has high ph and temperature, which along with a large amount of oil and grease sodium substances create problems in the treatment and disposal [12]. azo dyes and metal-phthalocyanines are the most common synthetic colorants (60–70 %) used to dye natural textile fibers, which results in dyes and heavy metals found in the by-products. the degradation of azo dyes and removal of heavy metals were found insufficient by the application of conventional aerobic processes and other physical-chemical techniques usually used for the treatment of wastewater.the photocatalysisis is often applied with other advanced techniques [13,14], showing much better results. in the present work, we focus on titania with dopant cd, and titania free from dopant, for optical and other related properties [15-17]. experimental titanyl acetylacetonate (c10h14o5ti) and cadmium nitrate (cd(no3)2) were supplied by aladdin regent company. hydrochloric (hcl) ethanol solution was purchased from sino pharm chemical reagent co., ltd. the natural dye hibiscus rosiness flower extract (anthocyanins) and other materials were high purity. deionized water was used throughout the work. cd doped tio2 thin films were coated on microscopic glass substrates (fto) using spray pyrolysis. for making deposition, c10h14o5ti and cd(no3)2 were used as the titanium and dopant sources, respectively. in order to prepare tio2 thin film, the fto substrate was cleaned for 15 min with prepared solutions. while performing this experiment, the fto substrate was put firmly over the heating plate 24 cm from the sprayer. the sample holder's top surface was maintained at a constant temperature of about 80 oc, and the rotational speed was about 100 r/min. before making deposition, the substrate was pre-heated for a specific time, and then the solution was sprayed over the microscopic glass substrate having the dimension of 7525 mm2 at the temperature of 500 oc (ts=500 oc). finally, the grown film was harvested through a regular process. in the photocatalyst tests, 5 mg of nanocomposite of to and cto were dissolved in 100 ml of deionized water containing 0.025 m of to and cto, respectively. after mixing, the solution was maintained in a dark environment before being exposed to visible light. the structural characterization of the deposited film was confirmed by x-ray diffraction equipped with (shimadzu-6000) monochromatic cu-kα radiation , = 15.406 nm.the xrd patterns were m. raja sekaran et al. j. electrochem. sci. eng. 12(5) (2022) 989-1000 http://dx.doi.org/10.5599/jese.1120 991 recorded in 2 intervals from 10 to 90° with the steps of 0.05° at room temperature. the surface morphology was examined through hr-sem (jeol-jes-1600). elementary dispersive x-ray (edx) evaluation experiments were performed on an fei quanta feg 200 instrument with an edx analyzer facility at 25 °c. high-resolution transmission electron microscopy (hr-tem) images were taken using a jeol-jem-2010 uhr instrument operated at an acceleration voltage of 200 kv with a lattice image resolution of 0.14 nm. photoluminescence (pl) spectra at room temperature were recorded using a perkin-elmer ls 55 fluorescence spectrometer.uv spectral measurements were done using a hitachi-u-2001 spectrometer. the absorbance spectrum is observed by unity subtracting the transmission and reflectances, measured by a spectrophotometer (horiba, ihr320) equipped with an integrating sphere and commercial detectors. the photovoltaic properties of the material were characterized by recording the photocurrent-voltage (i-v) curve under the illumination of am1.5 (100mw/cm2). further, the photocatalytic phenomenon, uv light allow to expose on the deposited films. when the catalyst is lit with energy equivalent to or more than the band gap value, organic molecules are destroyed. after being exposed to uv radiation, the color of dye solutions degraded noticeably due to the presence of a catalyst. results and discussion hr-sem and edx analysis in order to understand the physical nature of the prepared surface, a morphological study of cto thin films was examined. morphologyof cd doped tio2 exhibits uniform and compact distribution of spherical grains. hr-sem image presented in figure 1(a) shows a nano spherical shaped structure and highly porous nature, as can be viewed clearly through the clusters formed during the growth. in figure 1(b), the edx spectrum shows the presence of cd and o in cto thin film that is free from any impurity. a b energy, kev figure 1. (a) sem image and (b) eds analysis of cd tio2 (cto) film hr-tem analysis hr-tem image presented in figure 2(a) shows a cross-sectional view of cd doped tio2 thin film. the film thickness is estimated to be around 20 nm, and a clear spherical shaped structure is shown in figure 2a. in figures 2b and 2c, typical average particle size range distribution of 24.2 to 138.2 nm was identified. in figure 2d,the 3d structure of the selected cross-section is highlighted. http://dx.doi.org/10.5599/jese.1120 j. electrochem. sci. eng. 12(5) (2022) 989-1000 spray pyrolisis deposition of cd-tio2 thin film 992 a b particle size, nm c particle size, nm d figure 2. (a) hr-tem micrograph, (b-c) average particle size (24.2 to 138.2 nm) and (d) image profile 3d structure of cto thin film structural characterization to recognize the crystal structure, an x-ray diffraction study was accomplished using xrd patterns of cto thin film shown in figure 3. it is revealed that cto thin film belongs to the hexagonal wurtzite structure having sharp peaks representing high crystallinity structure at 25.25, 34.62, 37.57, 48.5, 54.0, 63.0 and 69.12 and 75.0o characteristic for (101), (004), (200), (105) and (204) planes. the average crystalline size (d) of the prepared material was estimated through scherrer’s formula: d = k λ / β cos θ (1) where β is full width half maximum, θ is diffraction angle, λ is the wavelength of x-rays used, and k is the constant. the average crystalline size of cd doped tio2 particles was found to be 16 and 14 nm, respectively, confirming thus nano-sized particles. f re q u e n cy , % f re q u e n cy , % m. raja sekaran et al. j. electrochem. sci. eng. 12(5) (2022) 989-1000 http://dx.doi.org/10.5599/jese.1120 993 2 / o figure 3. xrd analysis of cto thin film optical properties the optical absorption properties of to and cto allow for analyzing the results of uv-visible spectroscopy, which are shown in figure 4.the optical absorption of to and cto thin films for small changes in absorption edge towards the maximum wavelength was observed. the increasing dispersion and gradual decreasing nature of extinction coefficient (k) is attributed to increasing doping concentration. with increasing impurities in the host material, the doped cto film shows a decrease in wavelength region due to high absorption radiation. the basic absorption edge was observed in both uv and visible regions by absorption in a range around 200-800 nm. r e fl e ct a n ce , % wavenumber, nm a b figure 4. optical study of (a) cto and (b) to thin films the optical energy band gap of both materials was calculated using the tauc plot. the extrapolation of this curve indicates the optical band gap value, i.e. suitability for photovoltaic in te n si ty ., a .u . http://dx.doi.org/10.5599/jese.1120 j. electrochem. sci. eng. 12(5) (2022) 989-1000 spray pyrolisis deposition of cd-tio2 thin film 994 applications [18]. the band gap efficiency of to and cto thin films is shown in figure 5. the band gaps of to and cto thin films were found to be 3.2 and 2.8 ev, respectively, revealing that the band gap decreases with cd content, leading to higher photo-electro catalytic efficiency. f ( r ) h   h / ev a b figure 5. optical study of band gap energy of (a) to and (b) cto thin films photoluminescence (pl) analysis figure 6 illustrates typical photoluminescence (pl) spectra of prepared to and cto thin films. pl spectra show emission peaks in the range 350–500 nm. however, no change in excitation and emission peak has been noted due to the increasing size of the quantum confinement effect. r e fl e ct a n ce , % wavenumber, nm a b figure 6. pl analysis of (a) to and (b) cto thin films this research shows that oxygen ions significantly modify synthesized cto thin films' optical characteristics. the luminous intensity of to and cto thin film nanoparticles was observed at max = 360 nm. the recombination of free carriers, charge trapping, and photogenerated electrons and holes in a semiconductor caused the photon energy band gap to decrease from 3.2 to 2.8 ev for doped thin film photons, indicating that visible luminescence is produced due to structural defects emissions at 375-490 nm appear blue-green band edge and oxygen vacancy (green emission m. raja sekaran et al. j. electrochem. sci. eng. 12(5) (2022) 989-1000 http://dx.doi.org/10.5599/jese.1120 995 at 500 to 540 nm). the results show that the lower reflectance of cto thin films improved photoelectro applications [19-21]. photocatalytic efficiency forrhb degradation with uv-light the manufactured to and cto thin films and rhodamine b (rhb) dye (0.1 mm) in an aqueous solution were subjected to uv light. organic compounds are degraded by light when the catalyst is illuminated with energy equal to, orhigherthan the band gap value. the color of dye solutions devalued notably under the existence of catalyst after its exposure to uv light. photocatalytic efficiency of to and cto thin films was found to be remarkable under uv light as shown in figure 7. time, min figure 7. photocatalytic efficiency of rhb dye degradation under uv-light irradiation:(a) dye + uv; (b) dye+cto+dark; (c) dye + to + uv; (d) dye + cto + uv such remarkable efficiency clearly indicates the transfer of photoexcited electrons from the conduction band of the semiconductors to the empty electron levels of the dye. the existence of metal prevents the recombination process in a semiconductor and thus intensifies photocatalytic reactions. a graph plotted in figure 7 represents the dye removal against the irradiation time to determine the optimal irradiation time for eliminating rhb dye molecules. it can be noted that the maximum removal efficiency of rhb occurs at 60 min, and after this time, rhb adsorption remains unchanged. therefore, 60 min irradiation time has been considered the ideal value for further studies. similarly, after 60 min irradiation time, the cto composite decolorized the dye solutions remarkably, exhibiting enhanced photocatalytic activity compared to clear to thin film. the photodegradation of dye with 60 min showed higher efficiency of doped cto thin film compared to undoped to thin film. the results indicate following content of degradation:dye + uv at 0-60 min (0, 1, 2, 4 and 5 %), dye + cd-tio2 at 0 to 60 min (0, 2, 3, 7 and 10 %), dye + tio2 + uv at 0 to 60 min (0, 9, 20, 33 and 40 %) and dye + cd-tio2+ uv in 0 to 60 min (0, 16, 41, 69 and 89 %). photocatalytic activity (reusability) to confirm the strength of the cto catalyst, five cycles of photocatalytic tests were carried out and the results are depicted in figure 8. the stability and reusability of to and cto thin films were investigated by repeating the cycle (1 to 5) of the degradation experiment. d a y d e g ra d a ti o n , % http://dx.doi.org/10.5599/jese.1120 j. electrochem. sci. eng. 12(5) (2022) 989-1000 spray pyrolisis deposition of cd-tio2 thin film 996 figure 8. stability and reusability of (◼) to and (◼) cto thin films figure 8 shows that the 1st to 3rdcycles resulted in different levels of degradation, whereas the 4th and 5th cycles resulted in almost equal degradation. during 1st-5th cycles, the undoped to exhibited 100, 92, 87, 80 and 80 % degradation, while for cd doped cto thin film, 100, 97, 92, 89 and 89 % degradation was recorded. based on the analysis of all parameters, it was concluded that cd doped tio2 thin films exhibit more effective dye degradation compared to to thin films. the first three cycles have shown almost no loss in degradation efficiency and obtained minute loss after the fourth cycle, verifying its steady nature [21]. photocatalytic efficiency (mechanism) the mechanism of photocatalytic degradation of rhb dye over cto is defined by eqs. (2) to (7): 1rhb dye0 + hν→1rhb dye1 (2) 1rhbdye1 + isc →3rhb dye1 (3) cd-tio2 (sc) + hν → e–(cb) + h+(vb) (4) oh+ h+→ •oh (5) •oh + 3rhb dye1→ leucorhb dye (6) leucorhb dye → product (7) where, sc is a semiconductor, cb is a conduction band, vb is a valance band and h+ is an electron hole, respectively. the main aim of this part of the study was to assess the catalyst activity for the degradation reaction of rhb dye. rhb forms an energized singlet state as it absorbs radiation of the preferred wavelength. in addition, it experiences interfacial surface complex (isc) to give its triplet state. during excitation, the transition of an electron in the cd-tio2 thin film semiconductor utilizes this energy. an electron can be absorbed from ohby a hole (h+) in the semiconductor valence band, resulting in hydroxyl radicals, •oh. rhb dye will be oxidized by this hydroxyl radical, which may eventually breakdown into products. the role of oh as an active oxidizing species in the degradation of rhb dye was confirmed, and the rate of degradation was significantly slowed in the presence of an oh scavenger [22,23]. the scheme of organic dye degradation at the tio2 surface under uv-light irradiation is shown in scheme 1. 0 20 40 60 80 100 1 2 3 4 5 d e g ra d a ti o n , % cycles no. m. raja sekaran et al. j. electrochem. sci. eng. 12(5) (2022) 989-1000 http://dx.doi.org/10.5599/jese.1120 997 scheme 1. mechanism of organic dye degradation at tio2 surface under uv-light irradiation i-v characterization the efficiency of conversion from solar energy to electrical energy for to and cto thin films in dye-sensitized solar cells (dssc) was measured from the short-circuit current (jsc) at open circuit photovoltage (voc). figure 9 shows current density–voltage curves for natural dyeing with rosab flower extract (anthocyanins) based dssc. the strong dye absorbance was identified at 516 nm for to and cto thin films. the solar power efficiency of to [jsc = 5.4 ma/cm2] thin film is less when it is compared to cto thin film [jsc = 6.3 ma/cm2] [24-26]. voltage, mv figure 9. i-v characterization of (__) to and (__) cto thin films on gce in 0.1 m kcl (dye-coated substrate) c u rr e n t d a n si ty , m a c m -2 http://dx.doi.org/10.5599/jese.1120 j. electrochem. sci. eng. 12(5) (2022) 989-1000 spray pyrolisis deposition of cd-tio2 thin film 998 p-v characterization with the same procedure, the solar power efficiency of to (jsc = 75.0 ma/cm2) thin film is less compared to cto thin film (jsc = 80.0 ma/cm2)(figure10). voltage, mv figure 10. p-v characterization of (__)to and (__) cto thin film on gce in 0.1 m kcl (dye coated substrate) conclusion in summary, tio2 (to) and cd-doped tio2 (cto) thin films were synthesized by spray pyrolysis technique, their photovoltaic properties and photocatalytic degradation activity were examined. the investigated samples have an anatase phase and a size of about 138.2 nm, which confirms formation of nanoparticles. the band gap value for tio2 (to) and cd doped tio2 (cto) thin films decreased from 3.2 to 2.8 ev. the surface morphology of films showed nano-spherical shaped structure, while crystalline nature was confirmed by edx analysis. photocatalytic efficiency of cto for rhb dyedegradation showed a higher value than the undoped thin to film. the stability and reusability of to and cto thin films were investigated by repeating degradation cycles. according to the findings, the addition of a blocking layer at the interface of fto and cd doped tio2 thin film layer enhanced the p-v and i-v output suitable for electrochemical industrial applications. acknowledgement:the authors are grateful to acknowledge the principal, tag govt arts college, tindivanam, for providing the necessary facility to complete this work. references [1] p. roy, s. berger, p. schmuki, angewandte chemie international edition 50(13) (2011) 2904-2939.https://doi.org/10.1002/anie.201001374 [2] m. xu, p. da, h. wu, d. zhao, g. zheng, nano letters 12(3) 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1007/s10854-015-3932-0 https://doi.org/10.3390/molecules26092461 https://creativecommons.org/licenses/by/4.0/) cyclic voltammetric study of tin hexacyanoferrate for aqueous battery applications doi:10.5599/jese.289 225 j. electrochem. sci. eng. 6(3) (2016) 225-234; doi: 10.5599/jese.289 open access : : issn 1847-9286 www.jese-online.org original scientific paper cyclic voltammetric study of tin hexacyanoferrate for aqueous battery applications denys gromadskyi, volodymyr chervoniuk, sviatoslav kirillov joint department of electrochemical energy systems, 38a vernadsky ave, 03680 kyiv, ukraine corresponding author: d.gromadskyi@gmail.com; tel.: +380 93 672 6654 received: april 28, 2016; revised: june 30, 2016; accepted: july 7, 2016 abstract a hybrid composite containing 65 mass % of tin hexacyanoferrate mixed with 35 mass % of carbon nanotubes has been synthesized and its electrochemical behavior as a negative electrode in alkali metal-ion batteries has been studied in 1 mol l-1 aqueous solution of sodium sulfate. the specific capacity of pure tin hexacyanoferrate is 58 mah g-1, whereas the specific capacity normalized per total electrode mass of the composite studied reaches 34 mah g-1. the estimated maximal specific power of an aqueous alkali-metal ion battery with a tin hexacyanoferrate electrode is ca. 3.6 kw kg-1 being comparable to characteristics of industrial electric double-layer capacitors. the maximal specific energy accumulated by this battery may reach 25.6 wh kg-1 at least three times exceeding the specific energy for supercapacitors. keywords aqueous alkali-metal-ion battery; tin hexacyanoferrate-based composite introduction nowadays, the list of electrochemical energy storage devices is quite wide, from electric doublelayer capacitors (the so-called supercapacitors) to various rechargeable batteries (lead-acid, lithiumion and others). their specific energy and power characteristics significantly vary depending on electrode materials, electrolyte compositions, etc. supercapacitors demonstrate very high specific power values (up to 6 kw kg-1 at 95 % efficiency) but their specific energy is immeasurably low compared to any commercially available battery (1–8 wh kg-1 vs. 26–170 wh kg-1) [1,2]. in recent years, alkali metal-ion batteries, supercapacitors and their hybrids with neutral aqueous electrolytes gain popularity due to their relatively low cost, fire safety and environmental friendliness as opposed to devices with nonaqueous organic solutions which decompose under http://www.jese-online.org/ mailto:d.gromadskyi@gmail.com j. electrochem. sci. eng. 6(3) (2016) 225-234 tin hexacyanoferrate for battery applications 226 applied voltage at the presence of a small amount of water impurity and, inter alia, may provoke the corrosion of metallic cell components [3–7]. as electrode materials for the aqueous rechargeable batteries, multivalent metal complexes (e.g. barium, cobalt, copper, iron, manganese, nickel, titanium and zinc hexacyanoferrates) [8–15] are commonly utilized. all these compounds having a perovskite-like structure similar to the prussian blue (axmy[fe(cn)6), where a is an alkali metal and m is a multivalent metal) are good hosts for alkali and alkaline earth ions [16]. their electrochemical properties arise from the process of alkali metal ion intercalation/deintercalation [17]: my[fe(cn)6] + xa+ + xe↔ axmy[fe(cn)6] (1) hexacyanoferrate anion is relatively low-toxic (ld50 oral-rat is 1600 mg kg-1) [18] because it does not tend to release free cyanide, and the total toxicity of a salt should be defined by the harmfulness of the multivalent metal. if the data on lethal doses (ld50, oral-rat) and prices for raw materials used for the synthesis of hexacyanoferrates (mainly nitrates and chlorides of reagent grade or p.a. quality) are compared, table 1, one can see that tin chloride has a low value of ld50 and is quite inexpensive [18,19]. this makes tin hexacyanoferrate (snhcf) an interesting candidate for electrochemical studies. its theoretical specific capacity qsp.(theor.) in the case of insertion/deinsertion of sodium ion, sn1.5[feiii(cn)6] + na+ + e↔ nasn1.5[feii(cn)6] (2) calculated as sp.(theor.) nf q m  (3) equals to 64.9 mah g-1. in eq. (3), n is the number of electrons taking part in the electrode halfreaction, f = 26801 mah mol-1 is the faraday constant, and m is the molecular mass of nasn1.5[fe(cn)6]. table 1 toxicity and cost of some multivalent metal chlorides and nitrates. chlorides bacl2 cocl2 cucl2 mncl2 nicl2 sncl2 zncl2 ld50, mg kg-1 118 80 584 1484 105 700 350 price, € kg-1 45 1272 174 199 904 148 122 nitrates ba(no3)2 co(no3)2 cu(no3)2 mn(no3)2 ni(no3)2 sn(no3)2 zn(no3)2 ld50, mg kg-1 355 691 794 300 1620 n/a 1190 price, € kg-1 122 458 300 100 1590 n/a 38 despite of the large number of publications referred to hexacyanoferrate-based batteries no research works exist directed towards the use of snhcf in energy storage applications, although it serves as a material for electrochemical sensors [20]. this paper aims at filling this gap, i.e. at exploring potentialities of a snhcf based composite as an electrode material for aqueous alkali-ion batteries. d. gromadskyi et al. j. electrochem. sci. eng. 6(3) (2016) 225-234 doi:10.5599/jese.289 227 experimental 1. composite synthesis a composite containing snhcf and multi-walled carbon nanotubes (cnts) as a conductive additive was synthesized via room-temperature precipitation technique according to the following equation, 3sncl2 + 2k3[fe(cn)6] = 2sn1.5[fe(cn)6] + 6kcl (4) flotubetm 9000 cnts from c-nano were treated in nitric-sulfuric acid mixture as proposed in ref. [21], dispersed in water, and then homogenized by a mechanical stirrer for 15 min. tin chloride dihydrate (reagent grade, reakhim) was added into the reaction vessel and mixed together for next 15 min. after that, the 0.3 mol l-1 solution of potassium hexacyanoferrate (reagent grade, reakhim) was added dropwise to avoid fast precipitation and coagulation of snhcf particles. a snhcf-cnt suspension was stirred for 1 h, filtered, washed with deionized water to neutral ph and dried at 80 °c for 12 h. it should be noted that the amount of cnts in the snhcf/cnt composite was varied from 20 to 80 mass %. 2. electrode preparation and characterization in order to prepare the electrodes a snhcf/cnt composite was homogenized in ethanol where polytetrafluoroethylene (60 mass % dispersion in water, sigma aldrich) as a binding agent was added in advance. the mass ratio of the snhcf/cnt composite and the polytetrafluoroethylene binder was 9 to 1. pre-electrode slurries were formed by manual mixing in a mortar and alcohol evaporation. ready-to-use electrodes were obtained after densification by rolling and drying at 80 °c for 12 h. their porosity analysis was performed at 77 k by means of quantachrome® autosorb station 3. 3. cell assembling and electrochemical testing electrochemical measurements were carried out in a flat three-electrode cell by means of cyclic voltammetric technique on a home-made potentiostat at room temperature. the working electrode was the snhcf/cnt composite with binding agent (mass 2 mg, geometric surface area 0.09 cm2). the counter electrode with the geometric surface area of 0.5 cm2 was made of a commercial activated carbon (kuraray yp50f). these electrodes were separated by two layers of a porous paper membrane (tf4030, nippon kodoshi), and a silver sulfate reference electrode fabricated as described in ref. [22] was located between them. its potential was +0.137 v vs. ag/agcl at 25 °c. after assembling, the cell was impregnated by an electrolyte, 1 mol l-1 na2so4 aqueous solution. cyclic voltammetric testing is suitable for determining key characteristics of any energy storage material, namely its operation potential range, reversibility of charge/discharge processes as well as specific capacity [23]. the latter was calculated as sp. 3.6 i u q vm    (5) where qsp. is the specific capacity (mah g-1), i is the current (a), δu is the operating potential range (v), v is the scan rate (v s-1) and m is the electrode mass. results and discussion the electrochemical behavior of snhcf-based electrodes depending on the cnt content is shown in fig. 1. cyclic voltammograms (cvs) demonstrate cathodic and anodic peaks characteristic of j. electrochem. sci. eng. 6(3) (2016) 225-234 tin hexacyanoferrate for battery applications 228 electrodes where reversible or quasi-reversible electrochemical reactions take place. adding cnts leads to changing the profile of cv making it typical for electrodes with electric double-layer capacitance. fig. 1. cvs of snhcf-based electrodes containing different amount of cnts recorded at 5 mv s-1. the heights of the peaks responsible for the intercalation/deintercalation of na+ ion into/from the snhcf structure are different. this means that respective specific capacities are also different and, moreover, non-linearly vary with the snhcf/cnt ratio. upon growing the amount of cnts the specific capacity increases rapidly reaching a maximal value (~45 mah g-1) at 35 mass % of cnts and then decreases. such behavior presumably reflects the contribution of cnts to the structure formation of the composites. cnts not only play the role of a conductive additive; they create a structured framework facilitating the access of ions to electroactive hexacyanoferrate species deposited in mesopores with pore radii greater than 12 nm. this is seen from fig. 2(a) where pore size distributions calculated according to the density functional theory (dft) is shown. besides, snhcf-based composites have a slightly higher volume v of micropores of about 1 nm radii r than cnts possibly due to oxidizing the cnt surface by potassium hexacyanoferrate. similar behavior is observable in fig. 2(b) for pore size distributions obtained by the barrett-joyner-halenda (bjh) method, which is less sensitive to microporosity being better suitable for mesoporous materials. fig. 2. (a) pore size distributions for snhcf electrode with 35 mass % cnt and pure cnt electrode calculated by means of (a) dft and (b) bjh techniques. as the snhcf-based material containing 35 mass % of cnts demonstrates the best electrochemical performance, all following experiments were conducted with it. it should be also mentioned that in performance estimates, some authors account only for the mass of the electroactive material d. gromadskyi et al. j. electrochem. sci. eng. 6(3) (2016) 225-234 doi:10.5599/jese.289 229 and ignore the masses of a conductive additive and a binder [24–26]. in this work, we prefer to operate with the mass of the electrode as a whole since just this value is of practical importance. the specific capacity of the electrode material without the contribution of cnts and polytetrafluoroethylene can be calculated via division by 0.585 in accord with the mass ratio of electrode components (see subsections 1 and 2 in experimental). this gives qsp. of about 77 mah g-1, while the specific capacity1 of other insoluble hexacyanoferrates in aqueous electrolytes varies from 36 to 59 mah g-1 for individual compounds [10,15,26]. the qsp. value obtained is 12 % higher than the theoretical one (64.9 mah g-1), probably due to the impact of unwanted faradaic side processes (e.g. the decomposition of electrode and/or electrolyte components). due to this fact in what follows we confined ourselves with a narrower potential range (-0.25...+0.50 v). the cv curves demonstrate a significant dependence on the scan rate, fig. 3(a), and respective peak heights linearly vary with the square root of the scan rate, fig. 3(b). this indicates the diffusion control of the na+ intercalation/deintercalation upon the charge/discharge of the snhcf-based electrode. an analysis of the cv for the snhcf-based electrode at the slowest scan rate (5 mv s-1) shown in fig. 4 gives the peak height ratio la/lc of almost unity characterizing the electrochemical process as a reversible one in spite of the deviation of the peak shift ua/uc from the theoretical value of 59 mv that may be caused by interactions between intercalated species as proposed by laviron [28]. fig. 3 (a) cvs of snhcf-based electrode recorded at different scan rates, and (b) dependence of anodic (filled circles) and cathodic (empty circles) peak current on the square root of scan rate for this electrode. diffusion coefficients d for na+ cation have been determined via the randles-ševčik equation [29] 5 3/2 1/2 1/2 p 2.69 10i n ad cv  (6) where ip is the anodic or cathodic peak current (a), n is the number of electrons taking part in the electrode half-reaction; a is the geometric surface area of the electrode (cm2), c is the concentration of na+ ions in the electrolyte solution (mol cm-3); v is the scan rate (v s-1). they are 2.1·10-6 cm2 s-1 and 2.6·10-6 cm2 s-1 for the intercalation and deintercalation processes, respectively. 1 in some articles authors consider insoluble hexacyanoferrates, which are typical battery-type materials, as supercapacitor-type ones. consequently, they characterize them by specific capacitances instead of specific capacities that is fundamentally flawed as described in ref. [27]. therefore, in order to compare our data with results obtained by other researchers we transfer the specific capacitance into the specific capacity through its multiplication by operating potential range and division by 3.6. j. electrochem. sci. eng. 6(3) (2016) 225-234 tin hexacyanoferrate for battery applications 230 fig. 4. cv of snhcf-based electrode recorded at 5 mv s-1 the d values obtained are significantly higher than diffusion coefficients in transition metal oxides commonly employed as electrodes in ‘traditional’ lithium-ion batteries, being rather comparable with effective diffusion coefficients obtained in supercapacitor electrodes produced from commercial activated carbons [30–33]. such behavior can be explained by an effect of cnts on the structure of the synthesized composite, where snhcf particles are located in 12 nm pores formed by a cnt network. these nanosized particles are better accessible to electrolyte, and all advantages of finely subdivided electrode materials [34] are applicable to them. the maximal specific capacity of the composite electrode (34 mah g-1) is observed at 5 mv s-1, fig. 5. using a 0.585 factor as described above, the capacity related to the mass of neat snhcf is determined as 58 mah·g-1 thus representing 89 % of the theoretical value. fig. 5. dependence of specific capacity of snhcf-based electrode on the scan rate another important requirement for energy storage devices in addition to high specific capacity is their charge/discharge cycling stability. according to data shown in fig. 6 the snhcf-based electrode demonstrates an excellent cycling behavior. no capacity decrease is observed after 500 charge/discharge cycles. furthermore, qsp. even increases a little (by ~5 %) at the first cycles, possibly due to deeper ion intercalation into the hexacyanoferrate structure (pretreatment), and becomes stable reaching the value of 23 mah g-1. this is confirmed by changes in the anodic and cathodic peak heights collected in table 2, which are leveling on growing the cycle number. the insignificant drift of the formal potential on cycling (10-5 v cycle-1) should also be considered as an additional verification of the good reversibility of the electrochemical system studied. furthermore, d. gromadskyi et al. j. electrochem. sci. eng. 6(3) (2016) 225-234 doi:10.5599/jese.289 231 it is asserted [35] that the constancy of the formal potential signifies right mass balancing of the negative and positive electrodes in the cell. fig. 6. changes in the specific capacity of snhcf-based electrode during voltammetric cycling at 50 mv s-1 table 2. cv parameters of snhcf-based electrode obtained from cyclic voltammograms given in fig. 6. cycle number ia, a g -1 ic, a g -1 ia/ic ua, v uc, v uf, v qsp., mah g -1 2 4.43 3.19 1.39 0.28 0.0024 0.141 22.0 250 3.68 3.80 0.97 0.29 -0.0050 0.143 23.3 500 3.70 3.80 0.97 0.30 -0.0075 0.146 23.3 using the data from fig. 5 recalculated into the specific energy esp. and power psp. by means of eqs. 7 and 8, one can build the so-called ragone plot (fig. 7), which helps to better understand the application field of the synthesized composite [36], esp. = qsp.u (7) sp. sp. e p   (8) fig. 7. ragone plot for pure snhcf (dashed line), snhcf-based electrode (stars), and a ‘virtual’ aqueous alkali metal-ion battery with snhcf-based electrode (an area within the red polygon) in comparison with industrial supercapacitors (triangles limited by a blue colored area) and various rechargeable batteries (squares limited by a pink colored area) [1,2]. in eq. (8), τ is the time of charging or discharging the electrode in h. j. electrochem. sci. eng. 6(3) (2016) 225-234 tin hexacyanoferrate for battery applications 232 in fig. 7 we also plot the data on specific power and energy recalculated per mass of the pure electroactive material as well as per the total mass of a battery with the snhcf-based composite as a negative electrode and additional cell elements (our approach to this calculation is given in footnote2). this clearly demonstrates how the characteristics are changing depending on the way of their presentation (material, electrode or device). as has been already mentioned, ‘the truest’ data are those normalized per electrode mass. despite of fundamental differences in the charge storage mechanism (intercalation/deintercalation instead of adsorption/desorption), the snhcf-based electrodes and hence energy accumulating devices with such electrodes demonstrate the specific power comparable with electric double-layer capacitors, and their specific energy values are close to lead-acid batteries. for instance, such composite electrodes can be charged up to half-capacity in a fast mode (by 7.5 seconds) with specific power of 6.1 kw kg-1, while in order to charge them fully (up to 25.6 wh kg-1), less than 3 min is needed. we expect that a battery pack with snhcf/cnt electrodes will be able to achieve the specific power and energy of (1.8–3.6) kw kg-1 and (12.8–25.6) wh kg-1, respectively. the formal potentials of some multivalent metal hexacyanoferrates taken from literature sources [40–50] are collected in fig. 8 where they are contingently divided into two groups representing negative and positive electrodes. the relatively low formal potential uf of the snhcf-based electrode determined as uf = (ua+ub)/2 (+0.143 v vs. ag/ag2so4 or ca. +0.28 v vs. ag/agcl) signifies an opportunity of employing the synthesized composite as a negative electrode (anode) for aqueous alkali metal-ion batteries. the best counter electrode for snhcf that allows for getting a maximal operating voltage for a battery is vhcf and inhcf, but the extra-high cost of vanadium and indium raw materials makes cuhcf preferable for large-scale manufacturing [19]. our further efforts will be directed towards both cathode and anode hcf materials. snhcf-based composites mixed with other multivalent metal hexacyanoferrates (e.g. fehcf and/or tihcf due to their low uf values) would be of great interest as such mixtures have been shown to have much higher specific capacity compared to the individual components [25,51]. fig. 8 formal potential of snhcf-based electrode compared with formal potentials of other hexacyanoferrates in the ag/agcl scale. 2 following to this approach the specific power of a single snhcf-based electrode should be doubled if compared to the specific power of a two-electrode cell, whereas the specific energy remains almost unchanged in the case if the cell is assembled in an anode-limited design and both (negatively and positively charged) electrodes have the same working potential ranges vs. a nominal zero point [37]. further, in our calculations, we account for the mass of the whole cell, since according to refs. 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[51] a. safavi, s. h. kazemi, h. kazami, electrochim. acta 56 (2011) 9191–9196. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {evaluation of paint systems on a36 steel through electro-chemical techniques: corrosion resistance of container tanks:} http://dx.doi.org/10.5599/jese.1130 529 j. electrochem. sci. eng. 12(3) (2022) 529-544; http://dx.doi.org/10.5599/jese.1130 open access : : issn 1847-9286 www.jese-online.org original scientific paper evaluation of paint systems on a36 steel through electrochemical techniques: corrosion resistance of container tanks hugo canahua loza1,, paul huanca zuñiga1, milagros minga adco1, carolyne vizcarra1, leslie canahua sosa1 and maría escudero2 1materials department of san agustín national university, calle santa catalina 117, arequipa 04001, perú 2department of materials engineering, degradation and durability national center for metallurgical research (cenim-csic), gregorio del amo 8, 28040 madrid, spain corresponding author: hcanahua@unsa.edu.pe; tel.: +51 959 436 185 received: october 3, 2021; accepted: march 11, 2022; published: april 5, 2022 abstract in this research work, the corrosion resistance of plate steel used in the construction of cylindrical container tanks of seawater was evaluated. these container tanks are usually used for fighting fires on the coast of the arequipa-peru region, where the shortage of drinking water is significant. the study was based on immersion tests of 2 × 2 cm square test plates in 3.5 wt.% nacl solution. five paint systems were studied, varying only in the primers: p1-860 (inorganic zinc silicate); p1-zc (epoxy-zinchromate); p1-850 (organic rich in zinc); p1-600 (reinforced inorganic zinc) and p1-sp1000 (high solids epoxy-amine). all systems consisted of a primer coat, an epoxy middle coat and a polyurethane topcoat. to characterize the behaviour of each system, the electrochemical impedance spectroscopy (eis) was mostly used. in addition, the scanning kelvin probe (skp) and scanning electrochemical microscopy (secm) were used as local techniques. the first three paint systems (p1860, p1-zc and p1-850) showed an invariable value of impedance modulus up to 3360 h of immersion in nacl. the last two paint systems (p1-600 and p1-sp1000) showed a decrease in impedance modulus by more than one order of magnitude. this research provides a clear contribution of results obtained by global electrochemical techniques such as eis, establishing excellent tools for monitoring the performance of organic anticorrosive coatings. keywords plate steel; seawater storage tank; corrosion resistance; epoxy coating; electrochemical impedance spectroscopy introduction nowadays, an economic, competitive and current alternative to protect steel from the aggressive environment, such as seawater, is constituted by organic and inorganic anticorrosive coatings. among the most important organic coatings applied for improving the corrosion resistance of steels http://dx.doi.org/10.5599/jese.1130 http://dx.doi.org/10.5599/jese.1130 http://www.jese-online.org/ mailto:hcanahua@unsa.edu.pe j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 530 are paints that contain an epoxy-type resin. paints are made up of two components that react to form a hard and inert coating. these two components (component a and component b) consist of an epoxy resin with pigments and extenders, in addition to a curing agent called a hardener. these paints are widely used as primary coatings on pipes, ships and any structure that is immersed in seawater due to their strong chemical and wear resistance, excellent adhesion, as well as impermeability and resistance to alkalis [1,2]. another system that is still used in this field is zinc chromate pigments. it has been shown that these pigments are good anodic inhibitors, as well as good cathodic inhibitors that hinder the reaction of oxygen reduction [3,4]. the important protective ability of zinc chromate pigments shows that they are very effective against metallic corrosion. however, the great disadvantage of cr6+ ions containing compounds is their toxic effects on human health, due to their carcinogenic effects, as well as contamination of the environment, which resulted in significant restrictions on their use [5]. recent research has shown that the modifying formula of paints by adding conductive polymers in low concentrations significantly increases the protective properties of the coating. in addition, they are an efficient option and are capable of replacing inorganic anticorrosive pigments that are generally used but can be detrimental to both health and the environment [6]. armelin et al. [6] studied the ability of conductive polymers to operate as an anticorrosive additive in marine paints, finding that a low concentration of conductive polymers allows better protection against corrosion to metallic substrates compared to unmodified paints. additionally, they indicated that excellent results are obtained when polythiophene (pth) derivatives are used as epoxy paint additives. in another study, armelin et al. [7] concluded that the negative and damaging effects generated by inorganic corrosion inhibitors on human health and the environment could be minimized by replacing them with a small concentration of ecological organic polymers. various studies on hybrid coatings have shown positive results on the effects of corrosion on steel surfaces. among these, we find the study by chen et al. [8], who prepared a hybrid siliconeepoxy and pre-hydrolyzed tetraethoxysilane (hteos) coating through the sol-gel process, with high solids content, showing good flexibility and better impact resistance. on the other hand, sanaei et al. [9] studied the corrosion inhibiting effect of a hybrid pigment composed of zn2+ cations and organic compounds. the incorporation of this hybrid pigment in the epoxy ester coating markedly improved its inhibition properties. it was also shown [10] that effective inhibition of corrosion for steel is achieved by the hybrid pigmentation based on zinc acetylacetonate/urtica dioica, which shows protection effectiveness against corrosion of 99.995 % after two months. inorganic coatings, such as inorganic zinc silicates and epoxy-amine coatings with a high content of solids, represent a step forward in the field of paint coatings for protection against atmospheric corrosion. these new isocyanate-free coatings exhibit low levels of volatile organic compounds (voc) due to the high solids content associated with their low viscosity, good heat and stability to uv radiation, and excellent chemical resistance [5]. based on all these facts, the objective of this study was to evaluate the effects of different paint systems on the corrosion resistance of plate steel used in the construction of cylindrical container tanks of seawater for fighting fires on the coast of the arequipa-peru region. experimental coatings, surface preparation and properties of coated steel the paint systems, which are used for commercial purposes in peru were supplied by chroma. prior to the application of paint coatings, the specimens were shot-blasted with shot steel abrasive h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 531 to sa2 grade according to sspc sp10, reaching an average roughness of 70 µm. the coatings were subsequently cured for 15 days under normal environmental conditions before being subjected to electrochemical impedance tests. the steel specimens were supplied by aceros arequipa s.a. with chemical and mechanical characteristics given in tables 1 and 2. table 1. chemical composition of a36 steel tested (source: aceros arequipa) maximum content, % quality astm* c mn p s si a36 0.25 0.8-1.20** 0.040 0.050 0.40 *quality calculated according to the procedure of the american society for testing and materials; **thickness less than ¾ inch. table 2. mechanical properties of a36 steel (source: aceros arequipa) quality astm standard yield limit, kg cm-2 tensile strength, kg cm-3 elongation, % structural a36 2.550 4.080-5.610 20 the basic characteristics of paint coatings used to study corrosion of a36 steel are shown in tables 3 and 4. likewise, photographs of a36 steel specimens with different coatings are shown in figure 1. table 3. proposed paints systems paint system primer dft, μm intermediate dft (epoxy), μm top coat dft (polyurethane), μm total dft, μm p1-a36 bare steel p1-860 57 (organic silicate zn) 170.60 29.12 257.30±5.08 p1-zc 55.29 (epoxy zynchromate) 166.96 43.94 266.19±9.91 p1-850 58.17 (organic zinc-rich) 168.15 30.74 257.05±10.41 p1-600 71.46 (reinforced inorganic zn) 173.91 24.39 269.75±6.60 p1-sp1000 epoxy amine 269.49±6.89 figure 1. photographs of a36 steel specimens with different coatings: (1a) p1-a36; (1b) p1-860; (1c) p1-zc; (1d) p1-850; (1e) p1-600; (1f) p1-sp1000 http://dx.doi.org/10.5599/jese.1130 j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 532 table 4. composition of coatings coating characteristics p1-a36 hot rolled with oxide coating p1-860 zinc primer based on inorganic ethyl silicate. contains metallic zinc powder (62 %), providing cathodic protection p1-zc epoxy with zinc chromate as an inhibitor pigment p1-600 coating of two components of zinc ethyl silicate inorganic, reinforced with glass flakes (80% solids). epoxy epoxy coating formulated with lamellar pigments (micaceous iron oxide) and magnesium silicate. based on a mixture of two epoxy resins. one is a solid in a low molecular weight solution and the other a liquid that reacts with a polyamidoamine with dimethylaminomethylphenol. polyurethane polyurethane coating based on the combination of a hydroxylated acrylic resin with an aliphatic polyisocyanate. the second coat of all systems is epoxy paint and the third coat is polyurethane. all systems that will be performed in marine environment consist of 3 coats. the study considers only different primers (the first coat). p1-sp1000 multipurpose epoxy-amine compound of 100% solids by volume adhesion tests the method used to evaluate the adhesion of all paint systems was the adhesion test, according to the standard d4541 type iii [11]. the test was carried out with a hydraulic push off adhesion tester (hate brand). cyanoacrylate glue was applied to bond two dollies per painted sample, allowing a curing time of 24 h before pulling. the dollies used were 20 mm in diameter, which gave a contact area of 3.14 cm2. in this type of test, the rupture can be adhesive, cohesive, a combination of both, or glue failure [12]. the ready-to-test specimens are shown in figure 2. figure 2. test panels with dollies glued in duplicate on each paint system measurement of corrosion potential with scanning kelvin probe (skp) the scanning kelvin probe (skp) is a variation of the atomic force microscope (afm) that makes it possible to map not only the surface morphology of solid samples but also their electrical potential, which is directly related to the corrosion potential [13]. to determine the surface activity (volta potential) of the specimens at time zero, skp shown in figure 3 was used through a 50 μm diameter tip of ni-cr alloy, scanning area of 500  500 μm at a velocity of 0.4 μm s-1 in an environment free of aggressive agents, maintaining a humidity of 85% and a temperature of 25 °c. the results will be presented in the form of a potential map, using that of a36 steel as a reference. h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 533 figure 3. scanning kelvin probe (cenim-spain) measurement of coating integrity with scanning electrochemical microscope (secm) among the methods used to map local microscopic processes, scanning electrochemical microscopy (secm) has the unique ability to recognize active/passive regions and enables the characterization of surfaces with resolution in the micrometer range or lower [14]. to determine the integrity of five paint systems studied, the samples were analyzed by scanning electrochemical microscopy (figure 4), using 1.1 mm of ferricyanide solution in 0.1 m kcl as the mediating electrolyte. a platinum microelectrode with a diameter of 25 µm, measuring an area of 500×500 µm and reducing the current limit value to 80 % was also used. the results will be presented on maps of the intensity of the current. figure 4. scanning electrochemical microscope (cenim-spain) electrochemical impedance spectroscopy (eis) tests the impedance measurements were carried out gradually until reaching 3696 h of exposure, using gamry potentiostat/galvanostat/zra reference 3000 equipment. it was carried out at a frequency scanning between 100,000 and 1 hz, with a disturbance amplitude of 10 mv. in the experimental set-up, a cell with three electrodes shown in figure 5 was used. graphite counterelectrode, a working electrode (the applied test tubes) and a saturated calomel electrode as a reference electrode were used in the working solution of 3.5 wt.% nacl. the samples were immersed for 30 min in the working solution before eis measurements, which was needed for stabilization of the open circuit potential. the data obtained by impedance measurements were simulated by means of an equivalent electrical circuit. the free software used for the simulations was gamry echem analyst version 6.04 http://dx.doi.org/10.5599/jese.1130 j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 534 and the circuit components were consequently related to the physicochemical phenomena that occurred in the system. figure 5. arrangement of the electrochemical cell connected to gamry 3000 equipment results and discussion results of adherence test according to the results of adhesion tests reported in table 5, it can be observed that all paint systems failed 100 % in the glue, with the exception of dolly 2 of the p1-sp1000 system, which failed 1 % by adhesion and 99 % glue. on the other hand, it is evident that the pressure at which the glue fails is above 8 mpa (1000 psi). table 5. summary of adhesion test results for five paint systems paint system n° of dolly thickness, μm pressure, mpa failure p1-860 1 254 10.34 100 % glue 2 254 8.27 100 % glue p1-zc 1 259.08 11.03 100 % glue 2 261.62 10.34 100 % glue p1-850 1 266.7 11.72 100 % glue 2 256.54 9.65 100 % glue p1-600 1 261.62 12.41 100 % glue 2 266.7 11.03 100 % glue p1-sp1000 1 261.62 15.17 100 % glue 2 261.62 17.24 1 % cohesion, 99 % glue characterization of surface activity scanning kelvin probe (skp) figure 6 is the comparative representation of potential maps of five paint systems obtained by means of the skp at zero time. it can be observed in figure 6 that p1-860 and p1-sp1000 systems are thermodynamically more stable since they present positive potentials throughout the studied area. this means that these paint systems are resistant to corrosion to a greater degree than a36 steel. at the other side, p1-zc, h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 535 p1-850 and p1-600 systems present zones of positive potential and zones of negative potential, indicating microscopic faults and possible zones of corrosion. figure 6. potential maps obtained by skp at zero time of five paint systems studied: (6a) p1-860; (6b) p1-zc; (6c) p1-850; (6d) p1-600; (6e) p1-sp1000 scanning electrochemical microscopy (secm) the maps of the intensity of current obtained by means of a scanning electrochemical microscope are presented in figures 7 and 8, where the electrochemical activity of the surface of paint systems can be observed in greater detail since here, a quantitative variable of corrosion is registered. a b figure 7. maps of current intensity obtained by secm of five paint systems studied: (a) p1-600a; (b) p1-600b the current intensity measured for p1-860 ranges between 0.8 and 4 na, showing on the map a greater homogeneity and protective character of the coating compared to the other samples. however, there are some small imperfections present, which over time may cause pores in the paint. in samples p1-zc, p1-850, p1-600, and p1-sp1000, the variation of the intensity of the current is similar (from 0 to 20 na). however, large areas of the studied section can be observed where the current is close to zero, indicating a high degree of metal protection. nevertheless, in some areas (about 10 %), an increase in the current intensity is observed, evidencing faults in the coating. http://dx.doi.org/10.5599/jese.1130 j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 536 a b c d e f g h figure 8. maps of current intensity obtained by secm of five paint systems studied: (a) p1-850a; (b) p1-850b; (c) p1-860a; (d) p1-860b; (e) p1-sp1000a; (f) p1-sp1000b (g) p1-zca; (h) p1-zcb h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 537 electrochemical impedance spectroscopy characterization p1-860 paint system the components of the system were provided by the company chroma-peru. the primer has a dry film zinc content of 86 % and is called cold galvanizing, as it protects the steel from corrosion through galvanic protection. the voc content is 367.3 g l-1. its formulation meets the requirements specified in the une 48293-2007 standard. the evolution of the impedance moduluszof this system over 3360 h of immersion in nacl solution is shown in figure 10 in the form of bode modulus plots (log zvs. log frequency). from figure 9 it can be concluded that the impedance modulus remained almost invariable over time. this is probably due to the fact that the fluid has not come into contact with the substrate. therefore, it can be concluded that p1-860 system presents an excellent barrier property, acting as an almost perfect condenser with capacitive behavior, and z of the order of 109 ω at the frequency of 1 hz [15]. in order to understand the behavior of this system, nyquist diagrams (zim vs. zreal) measured at zero time and 3360 h of contact with the fluid are presented in figure 10. f / hz figure 9. evolution of impedance modulus bode plots of p1-860 paint system with immersion time in 3.5 wt.% nacl zreal / g figure 10. nyquist diagrams for p1-860 paint system at 0 h and 3360 h of exposure in 3.5 wt.% nacl -z im / g  z  /  http://dx.doi.org/10.5599/jese.1130 j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 538 despite somewhat scattered data, figure 11 shows some differences in the capacitive behavior of the studied system. at 3360 h of contact with the fluid, a higher deviation from capacitive behavior can be observed, with the impedance values that decreased slightly, i.e. by less than an order of magnitude (g). p1-zc paint system the primer studied in this system is an epoxy-based paint with zinc chromate inhibitor provided by chroma perú, recommended to protect the steel as a base or first coat. the evolution of the impedance modulus of this system up to 3360 h of immersion in nacl solution is shown in figure 11. f / hz figure 11. evolution of the impedance modulus bode plot of p1-zc paint system with immersion time in 3.5 wt.% nacl from figure 12, it can be concluded that the impedance modulus remained almost invariable over time, suggesting, as for the previous sample (figure 12), that the fluid did not come into contact with the substrate. therefore, it can be concluded that the p1-zc system also presents an excellent barrier property, acting as an almost perfect condenser with capacitive behavior and z of the order of gω, at the frequency of 1 hz [16]. the nyquist diagrams are presented in figure 12 at zero time and at 3360 h of contact with the fluid. zreal / g figure 12. nyquist diagrams for p1-zc paint system at 0 h and 3360 h of exposure to 3.5 wt.% nacl z  /  -z im / g  h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 539 figure 12 shows differences in the impedance behavior of the studied system. at 3360 h of contact with the fluid, impedance became less capacitive and decreased slightly, although by less than an order of magnitude (gω). p1-850 paint system the p1-850 system is constituted of an organic primer rich in zinc with a high content of powder of zinc (85 % zinc in the dried film) and a voc content of 270 g l-1 in the epoxy base. the evolution of the impedance modulus of this system over 3360 h of immersion in nacl solution is shown in figure 13. f / hz figure 13. evolution of impedance modulus bode plots of p1-850 paint system with immersion time in 3.5 wt.% nacl from figure 13, it can be concluded that the impedance modulus remained almost invariable over time as for previous samples shown in figures 10 and 12. again, this happens probably because the fluid did not come into contact with the substrate. therefore, it can be concluded that the organic system rich in zinc/epoxy/polyurethane also has an excellent barrier property, acting as an almost perfect condenser with capacitive behavior and z of the order of gω at the frequency of 1 hz [16]. the nyquist diagrams are presented at zero time and at 3360 h of contact with the fluid in figure 14. zreal / g figure 14. nyquist diagrams for p1-850 paint system at 0 h and 3360 h of exposure to 3.5 wt.% nacl z  /  -z im / g  http://dx.doi.org/10.5599/jese.1130 j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 540 figure 15 shows highly dissipated data but also some differences in the impedance behavior of the system studied. at 3360 h of contact with the fluid, the response deviates from capacitive behavior and impedance decreases, although less than one order of magnitude (gω). p1-600 paint system this system has a primer that was created in peru four years ago. it constitutes a new generation of coatings called modified inorganic zinc, hybrid, or reinforced, equivalent to zinc ethyl silicate (in terms of performance), but with characteristics of application which are more environmentally friendly since its voc content is almost 20 % lower in emissions. in terms of performance, this has been increased due to the higher content of solids by volume (80 % approx.) in its formulation, which improved its performance per m2 of the coated surface compared to the inorganic zinc of the conventional formulation. the evolution of the impedance modulus of this system during 3360 h of immersion in nacl solution is shown in figure 15. from figure 15 it is concluded that the impedance modulus is gradually decreased with time, which is particularly evident at the lowest frequencies. after 3660 h, z decreased by almost two orders of magnitude at low frequencies, evidencing the gradual deterioration of the inorganic zinc coating. with the purpose of understanding the behavior of this system, the nyquist diagram is presented in figure 16 at zero time and at 3360 h of contact with the fluid. f / hz figure 15. evolution of impedance modulus bode plots of p1-600 paint system with immersion time in 3.5 wt.% nacl zreal / g figure 16. nyquist diagrams for p1-600 paint system at 0 h and 3360 h of exposure to 3.5 wt.% nacl in figure 16, significant differences are observed in the impedance behavior of the studied system. at 3360 h of contact with the fluid, the capacitive curve is strongly bent and impedance values are significantly decreased from g to m. after 3660 h the coating did not act as a barrier and showed some resistive contribution due to corrosion. in the equivalent circuit shown in figure 17, re denotes the uncompensated part of electrolyte resistance, while rpo, rct, cc and cdl represent the resistance of the coating to electrolyte penetration, the resistance of charge transfer at the substrate/electrolyte interface, coating capacitance and interfacial double-layer capacitance, respectively. figure 18 shows the degree of adjustment of measured and simulated data obtained with the proposed equivalent circuit in the form of a bode plot (log z and phase angle vs. log frequency). -z im / g  z  /  h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 541 zreal / m figure 17. nyquist diagram of the p1-600 paint system after 3360 h of exposure to 3.5 wt.% nacl and simulation results calculated using the associated equivalent circuit f/ hz figure 18. bode diagram of the p1-600 paint system after 3360 h of exposure to 3.5 wt.% nacl and simulation results calculated using equivalent circuit in fig. 16 p1-sp1000 paint system jet pox sp 1000 is a multipurpose epoxy-amine with excellent anticorrosive protection. it is a barrier-type coating with high solids content (98 %) and a minimum voc content of 5 to 12 g l-1. the evolution of the impedance modulus of this system over 3360 h of immersion in nacl solution is shown in figure 19. f / hz figure 19. evolution of impedance modulus bode plots of p1-sp1000 system with the immersion time in 3.5 wt.% nacl -z im / m  z  / m  p h a se a n g le , o z  /  http://dx.doi.org/10.5599/jese.1130 j. electrochem. sci. eng. 12(3) (2022) 529-544 corrosion resistance of container tanks 542 in figure 20 it can be observed that at low frequencies (1 hz), where the capacitive behavior of the coating is usually characterized, a decrease in impedance capacitive slope is observed over time, indicating an influence of some resistive contribution. for the purpose of a better understanding of the behavior of this system, the nyquist diagram is presented in figure 21 at zero time and 3360 h of contact with the fluid, which is more sensitive in highlighting the capacitive behavior of the system. zreal / g figure 20. nyquist diagrams for p1-sp1000 paint system at 0 h and 3360 h of exposure to 3.5 wt.% nacl for p1-sp1000 system after 3660 h of exposure, figure 20 shows a significant decrease of lower frequency impedance values at least by order of magnitude. also, the prominent deviation from pure capacitive behavior of the painting system studied suggests certain fails in corrosion protection. comparative results finally, the impedance modulus of all five tested systems at the end of tests (3360 h) are presented in figure 21, in order to select the system that would best protect a36 steel immersed in seawater. f / hz figure 21. impedance modulus bode plots of five paint systems studied after 3360 h of immersion in 3.5 wt.% nacl -z im / g  z  /  h. canahua et al. j. electrochem. sci. eng. 12(3) (2022) 529-544 http://dx.doi.org/10.5599/jese.1130 543 the lower performance of p1-600 system is probably due to the higher permeability of the coating and not its adhesion to the substrate since all the systems reported good adhesion to the metal. according to forms and impedance magnitudes at low frequencies shown in figure 21, it is observed that the paint systems p1-860, p1-zc and p1-850 retain their stability throughout 3360 h of immersion. paint systems p1-600 and p1-sp1000 systems, however, show certain deterioration of capacitive behavior and impedance more than one order of magnitude less than other paint systems. conclusions • regarding the adherence of organic coatings to metal, it is concluded that there is strong adhesion between paint systems and metal since in the majority of cases, the failure has occurred with the paint interface/glue at a pressure greater than 8 mpa. • from the potential maps obtained by means of the scanning kelvin probe at zero time, it can be concluded that p1-860 and p1-sp100 systems better protect the metallic substrate, while the other systems present possible micro corrosion defects. • from the current intensity maps obtained by scanning electrochemical microscopy, it can be concluded that p1-860 system presents greater homogeneity and protective character to the substrate, while the other systems present greater variability in the intensity of the current. • for p1-860, p1-zc and p1-850 paint systems, the modulus of impedance was high and invariable with respect to time up to 3360 h of immersion in 3.5 wt.% nacl, showing excellent protection of a36 steel. • the sp-1000 system reports a decrease in the impedance modulus by order of magnitude up to 3360 h of immersion in nacl. however, this paint system turns out to be the most environmentally friendly due to its lower voc. • the jet zinc p1-600 system exhibited a lower impedance modulus by three orders of magnitude compared to other systems, evidencing its lower ability to protect a36 steel in seawater. • the global electrochemical 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(2014) 693-699. https://doi.org/10.2355/isijinternational.54.693 [16] j. j. suay, m. t. rodríguez, k. a. razzaq, j. j. carpio, j. j. saura, progress in organic coatings 46 (2) (2003) 121-129. https://doi.org/10.1016/s0300-9440(02)00219-9 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.porgcoat.2019.105374 https://doi.org/10.1016/j.apsusc.2018.05.077 https://doi.org/10.1016/j.porgcoat.2019.105380 https://bit.ly/3ulc8ur https://doi.org/10.1016/j.porgcoat.2010.06.007 https://doi.org/10.1016/j.surfrep.2010.10.001 https://doi.org/10.1016/j.corsci.2014.04.035 https://doi.org/10.2355/isijinternational.54.693 https://doi.org/10.1016/s0300-9440(02)00219-9 https://creativecommons.org/licenses/by/4.0/) {electrosynthesis of gold nanocomposites based on a copolymer of 1-vinyl-1,2,4-triazol with crotonic acid:} http://dx.doi.org/10.5599/jese.1193 283 j. electrochem. sci. eng. 12(2) (2022) 283-291; http://dx.doi.org/10.5599/jese.1193 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrosynthesis of gold nanocomposites based on a copolymer of 1-vinyl-1,2,4-triazol with crotonic acid serzhik h. sargsyan1,, artur s. sargsyan2, тatevik s. sargsyan2, karina m. khizantsyan1, ida g. aghajanyan1 and karine s. margaryan2 1national polytechnic university of armenia, teryan 105, 0009 yerevan, armenia 2yerevan state medical university after mkhitar heratsi, koryun 2, 0025 yerevan, armenia corresponding author: artsar86@mail.ru received: november 28, 2021; accepted: january 11, 2022; published: january 25, 2022 abstract in this work, we have synthesized and discussed the results of electrosynthesis of metalpolymer nanocomposites of gold. nanocoatings were made on pure iron and steel electrodes by combining the process of electropolymerization of 1-vinyl-1.2.4-triazole with crotonic acid and cathodic deposition of gold. using uv, ir and atomic absorption spectroscopies, x-ray phase analysis, as well as thermogravimetric and elemental analyses, the structure and composition of the synthesized nanocomposites and nanocomposite coatings were studied. keywords electrolysis; nanocomposite film; electropolymerization; medicine; thermal stability introduction nanocomposite materials containing gold nanoparticles have unique properties and their development is promising for medicine, nanophotonics and catalysis [1-4]. electrosynthesis and study of the properties of functional polymers and on this basis obtained nanomaterials, is the most rapidly developing area of modern chemical science. as noted in [2-12], nanomaterials are used in medicine as antibacterial drugs, systems for targeted delivery of contrast agents and drugs, biosensors, and other biomedical purposes. for wider use in medicine, nanomaterials must have thromboresistance, hydrophilicity, biological activity, biocompatibility, etc., and also, due to the presence of functional groups they must have an ability to bind to various substances, including drugs. these properties are characteristic for copolymers of 1-vinyl-1.2.4-triazole (vt) with acrylic acid, which are poorly studied, but promising polymers for medical use [13-16]. materials based on nanoparticles of various metals are often used in medicine. they are used as coverings for wounds, various medical devices, surgical masks, etc. [17-20]. among metal nanoparticles, gold nanoparticles have the highest antibacterial and antiviral activity [20,21]. http://dx.doi.org/10.5599/jese.1193 http://dx.doi.org/10.5599/jese.1193 http://www.jese-online.org/ mailto:artsar86@mail.ru j. electrochem. sci. eng. 12(2) (2022) 283-291 electrosynthesis of gold nanocomposites 284 the synthesis of new functional thrombo-resistant, non-toxic polymeric materials with gold nanoparticles makes it possible to expand the area of their application, as well as to increase the range of materials used in pharmaceuticals when creating new dosage forms. the properties of these nanocomposites substantially depend on both the nature of the stabilizing polymer matrix and the conditions for the formation of nanoparticles. as a polymer matrix, vt polymers and copolymers can be used since they are non-toxic (ld50 > 3000 mg kg– 1) and have high film formation ability, solubility, and biocompatibility [22,23]. in [22,24], a possibility of electrochemical formation of metal-containing nanocomposites and nanocomposite coatings based on vinylazole copolymers has already been reported. the synthesis and research of nanostructured functional systems, including nanocatalytic ones, are the priority areas of modern science. due to their unusual physical and chemical properties, which differ from those of bulk metal, metal nanoparticles have a wide variety of potential applications in the field of catalysis, biomedicine, optics, electronics, etc. [25-35]. metal nanoparticles are thermodynamically unstable, and in a relatively pure individual form, they can be obtained only when fixed on a solid as immobile support. to obtain such particles on an electrically conductive carrier, the electrochemical method is widely used in various versions of its implementation [36]. nanocomposite materials containing gold nanoparticles have unique properties, and their use is promising for medicine [27-29]. polymer nanocomposites containing zero-valent gold are effective antimicrobial and antiviral agents. the high antibacterial activity of gold nanoparticles is due to its developed surface. in addition, nanoparticles are small enough and capable of penetrating cell membranes and affecting intracellular processes. in this work, we discuss the results of electrosynthesis of metal-polymer nanocomposites and their coatings on pure iron and steel electrodes when electrochemical (co) polymerization of solid solution with crotonic acid (ca) is combined with the cathodic discharge of metal. experimental the electrochemical initiation of polymerization was carried out in a glass electrolyzer without a diaphragm. electrosynthesis in galvanostatic and potentiostatic modes was carried out using a пи-50.1 potentiogalvanostat with a пр-8 programmer (russia). the thickness of films was determined by micrometric and magnetic methods. elemental analysis was performed on a flash ea 1112 series analyzer. the ir spectra of the polymers were recorded on specord m-80 and bruker vertex 70 spectrometers using fine powders pressed into tablets with kbr. absorption spectra were recorded on a perkin elmer lambda 35 uv/vis spectrophotometer. the metal content in the composites was determined by elemental and atomic absorption analysis on a perkin elmer analyst 200 spectrometer. the distribution of gold nanoparticles was established on a leo 906 e tem (germany). thermogravimetric analysis was performed on an mom derivatograph (hungary), the temperature rise rate was 5 °c min-1, while electrical conductivity was measured using a standard e6-13a teraohmmeter. as electrodes, we used pure armco iron with an iron content of 99.92 % and st-3 with an iron content of 97 %. 1-vinyl-1.2.4-triazole was obtained and purified by the method described in [37], and crotonic acid was purified by sublimation. general method of electrosynthesis of nanocomposites and coatings in a glass electrolytic cell with a capacity of 50 ml, electrolysis was carried out [e = -0.1 ... -1.2 v vs. аg/agcl, j = 1-15 ma cm-2] in aqueous or water-ethanol solutions (bi-distilled water and distilled s. h. sargsyan et al. j. electrochem. sci. eng. 12(2) (2022) 283-291 http://dx.doi.org/10.5599/jese.1193 285 ethanol) containing 0.5 1 mol l-1 of 1-vinyl-1.2.4-triazole, 0.5 1 mol l-1 of crotonic acid, 1.5 4 mmol l-1 haucl4, 0.02 0.05 % 4-tert-butylperoxy-4-oxobutanoic acid (tboba), and in some cases 0.05 to 0.07 wt.% chitosan. a pure iron or steel plate with an area of 1 2 cm2 was used as the working electrode (cathode), and a platinum or glassy carbon (su-12, su-20) plate with the same area was used as the anode. at high current densities j > 10 ma cm-2, the nanocomposite was deposited onto the electrolyzer floor. after the end of electropolymerization, the electrode package was removed, the cathode with the formed coating was separated, thoroughly washed with distilled water, and dried to the constant weight. the synthesized films were of lilac color, which confirms the presence of incorporated gold particles. results and discussion during the electrolysis of aqueous or water-ethanol solutions of vt and cа or their mixtures at various ratios in the presence of haucl4 and chitosan, nanocomposites and nanocomposite coatings with a gold content of 1-10 wt.% are formed only in the presence of a peroxide-type initiator, for example, tboba, at the potential of electroreduction of 0.6 1.2 v vs. аg/agcl. after drying, the formed nanocomposite coatings on the electrodes become insoluble in water and in commonly used organic solvents (dmso, dmfа, acetonitrile, etc.). the copolymer is crosslinked upon heating. in the electronic spectra of gold-containing nanocomposites, in contrast to aqueous solutions of the initial copolymers and haucl4, plasmon absorption bands appear with a maximum in the region of nanocomposite coatings containing gold. in figure 1, absorption bands appear with a maximum in the region of 517-521 nm, which is typical for systems with zero-valent gold. figure 1. electronic absorption spectra of vt-ca copolymer (1), gold 6.8 wt.% (2), gold 7.3 wt.% (3), gold 8.0 wt.% (4) the ir spectrum of gold-containing vt-ca copolymer shown in figure 2 contains bands corresponding to the frequencies of stretching bending vibrations of the triazole ring at 1503, 1434, 1138, 1005, 660 cm-1 (c-n, c = n), 1275 cm-1 (nn), 3106 cm-1 (ch), and a band at 1711 cm-1 related to stretching vibrations of the units of the carboxyl group. analysis of ir spectra shows that the formation of gold-containing polymer nanocomposites leads to insignificant changes in the chemical structure of the copolymer matrix. thus, the intensities of the absorption bands of the triazole ring, which can act as coordination centers of gold nanohttp://dx.doi.org/10.5599/jese.1193 j. electrochem. sci. eng. 12(2) (2022) 283-291 electrosynthesis of gold nanocomposites 286 particles, show a weak shift (by 3 4 cm-1) of one band at 1506 cm-1, which is characteristic of the stretching vibrations of the ring (c-n and c = n). figure 2. ir spectrum of gold nanocomposite based on vt-ca copolymer this shift may indicate the coordination interaction of the triazole ring with the surface atoms of metal nanoparticles. an intense band in the ir spectra of the copolymers at 1711 cm-1 indicates that the carboxyl group of ca in the copolymers is in unionized form. when passing from copolymers to nanocomposites with gold nanoparticles, it can be seen that the number of non-ionized carboxyl groups (–сооh) decreases significantly, and a new absorption band appears at 1577 cm-1, which is characteristic of stretching vibrations of the carboxylate anion (–cooh-). the gold content in nanocomposite films was 1-8 %, according to the data of elemental analysis and atomic absorption spectroscopy. the intrinsic viscosity of polymer nanocomposites, in contrast to the initial copolymers, increased by an average of 10 20 %, which can be explained by the presence of numerous bonds of polymer macromolecules with metal nanoparticles (table 1). table 1. nanocomposites based on vt-ca copolymers nanocomposite e / v au content, wt. % yield, % η / dl·g-1 λmax / nm nanoparticle sizes, nm 1 -0.60 6.8 73.4 1.52 521 2 10 2 -0.75 7.3 78.5 0.83 517 4 6 3 -0.90 8.0 80.1 0.19 517 2 4 as can be seen from the results, with an increase in the cathodic potential, the metal content in the nanocomposite increases while the size of the nanoparticles decreases. coordination occurs due to the nitrogen atoms of the heterocycle. as a result of this work, a nanocomposite of the following structure presented in scheme 1 is formed. the solubility of nanocomposites is mainly due to the intramolecular interaction of copolymer macromolecules with gold nanoparticles, due to which relatively loose polymer coils are formed rather unfolded in an aqueous solution since a small amount of nanoparticles participates in the coordination interaction. polymer coils represent a more compact conformational state of macromolecules of a polymer nanocomposite, in comparison with the initial copolymer, which arises due to partial crosslinking by nanosized particles. s. h. sargsyan et al. j. electrochem. sci. eng. 12(2) (2022) 283-291 http://dx.doi.org/10.5599/jese.1193 287 scheme 1. nanocomposite structure an increase in gold content over 8 wt.% leads first to a partial and then to a complete loss of solubility. this is due to the enhancement of intermolecular interaction and crosslinking of polymer macromolecules by metal nanoparticles under the influence of multiple cooperative forces. in this case, the intermolecular binding of macromolecules with the surface atoms of gold nanoparticles affects the hydration of the copolymer. the solubility of the copolymer is due to the formation of hydrogen bonds of the triazole ring and carboxyl groups [38,39] (hydrophilic hydration is characteristic of poly-1-vinyl-1.2.4-triazole). thus, with an increase in the content of gold nanoparticles, the hydration of the polymer nanocomposite decreases down to its complete loss (most of the triazole and carboxyl groups are involved in coordination interaction with gold nanoparticles and do not participate in the formation of hydrogen bonds with water molecules). the formation of organoinorganic nanocomposites, namely the presence of gold nanoparticles and an amorphous polymer phase, is confirmed by the results of x-ray phase analysis. the approximate sizes of metal nanoparticles are presented by the results of x-ray phase analysis shown in figure 3. figure 3. fragment of the diffractogram of gold nanocomposite in the region where the gold particle au (111) has d = 0.235 nm, a strongly broadened maximum is observed in the diffractogram of the composite, which confirms the presence of metallic gold in the nanosized state in the studied samples. the sizes of gold nanoparticles were calculated according to [40]. according to the results of transmission electron microscopy (figure 4), nanocomposite contains mostly elliptical gold nanoparticles, uniformly distributed in the copolymer matrix, having sizes of 2 10 nm. n n n cooh me haucl 4 n n n me cooh n m au n + 0 http://dx.doi.org/10.5599/jese.1193 j. electrochem. sci. eng. 12(2) (2022) 283-291 electrosynthesis of gold nanocomposites 288 figure 4. electron micrograph (left) and distribution diagram of gold nanoparticles by size (right) in vt-ca copolymer matrix the study of the thermal stability of polymer nanocomposites showed that the first stage of polymer matrix destruction is observed in the temperature range from 280 to 400 °c and is accompanied by a gradual weight loss down to 40 %, which refers to the elimination and oxidation of carboxyl and methyl groups (figure 5). figure 5. thermogravimetric curves: 1copolymer vt-ca, 2 gold-bearing nanocomposite the next stage of decomposition of the composite occurs in the temperature range 480-570 °c. the gold-containing nanocomposite is thermally more stable than the copolymer, since denser coils of the nanocomposite with the metal are formed. the electrical conductivity of nanocomposite polymer films based on vt-ca copolymers increases by three orders of magnitude, up to 8.910-10 7.210-9 s m-1, in comparison with the original copolymers. pure vt-ca copolymer has no electrical conductivity. the increase in electrical conductivity is apparently due to the contribution to the total electrical conductivity of individual local tunneling currents arising in these samples between electrically conducting metallic gold nanoparticles closely located in the dielectric polymer matrix. s. h. sargsyan et al. j. electrochem. sci. eng. 12(2) (2022) 283-291 http://dx.doi.org/10.5599/jese.1193 289 conclusion based on monomeric vt-ca systems, nanocomposites and nanocomposite coatings have been synthesized electrochemically. it was found that, depending on the electrode potential, the nanoparticle size changes from 2 to 10 nm, and the viscosity from 0.19 to 1.52 dl g-1. according to the transmission electron microscopy data, the gold nanoparticles are uniformly distributed over the polymer matrix. the composition and structure of the obtained nanocomposites have been investigated. the synthesized nanocomposites can be used in medicine as thromboresistant, biocompatible polymers. acknowledgements: this study was supported by the ministry of education, science, culture and sports ra, science committee (project no. 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croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1193 https://www.researchgate.net/scientific-contributions/nl-mazyar-77509433 https://www.researchgate.net/profile/vadim-annenkov-2 https://www.researchgate.net/scientific-contributions/va-kruglova-78589258 https://www.researchgate.net/scientific-contributions/s-m-ananev-77867214 https://www.researchgate.net/scientific-contributions/elena-n-danilovtseva-39022005 https://www.researchgate.net/scientific-contributions/a-v-rokhin-27851050 https://www.researchgate.net/profile/sergey-zinchenko-4 https://www.researchgate.net/profile/sergey-zinchenko-4 https://doi.org/10.1023/a:1009571908805 https://doi.org/10.1134/s1560090414020122 https://creativecommons.org/licenses/by/4.0/) {single microfluidic fuel cell with three fuels – formic acid, glucose and microbes: a comparative performance investigation:} http://dx.doi.org/10.5599/jese.1092 305 j. electrochem. sci. eng. 11(4) (2021) 305-316; http://dx.doi.org/10.5599/jese.1092 open access : : issn 1847-9286 www.jese-online.org original scientific paper single microfluidic fuel cell with three fuels – formic acid, glucose and microbes: a comparative performance investigation jayapiriya umarani sivakumar1,3,*, lanka tata rao2,3,*, prakash rewatkar1,3,*, haroon khan4, satish kumar dubey2,3, arshad javed2,3, gyu man kim4 and sanket goel1,3, 1department of electrical and electronics engineering, bits pilani, hyderabad campus, hyderabad, 500078, india 2department of mechanical engineering, bits pilani, hyderabad campus, hyderabad, 500078, india 3mems, microfluidics and nanoelectronics (mmne) lab, bits pilani, hyderabad campus, hyderabad, 500078, india 4school of mechanical engineering, kyungpook national university, daegu, 41566, south korea *equal contributions corresponding author: sgoel@hyderabad.bits-pilani.ac.in; tel.: + 91-40-66303686 received: august 19, 2021; accepted: september 30, 2021; published: october 5, 2021 abstract the development of microfluidic and nanofluidic devices is gaining remarkable attention due to the emphasis put on miniaturization of conventional energy conversion and storage processes. a microfluidic fuel cell can integrate flow of electrolytes, electrode-electrolyte interactions, and power generation in a microchannel. such microfluidic fuel cells can be categorized on the basis of electrolytes and catalysts used for power generation. in this work, for the first time, a single microfluidic fuel cell was harnessed by using different fuels like glucose, microbes and formic acid. herein, multi-walled carbon nanotubes (mwcnt) acted as electrode material, and performance investigations were carried out separately on the same microfluidic device for three different types of fuel cells (formic acid, microbial and enzymatic). the fabricated miniaturized microfluidic device was successfully used to harvest energy in microwatts from formic acid, microbes and glucose, without any metallic catalyst. the developed microfluidic fuel cells can maintain stable open-circuit voltage, which can be used for energizing various low-power portable devices or applications. keywords electrocatalysis; biocatalysis; chemical fuel; biofuel; portable devices. introduction the widespread interest in developing portable and miniaturized devices has led to the requirement of renewable and sustainable power sources for their operation, especially for critical http://dx.doi.org/10.5599/jese.1092 http://dx.doi.org/10.5599/jese.1092 http://www.jese-online.org/ mailto:sgoel@hyderabad.bits-pilani.ac.in j. electrochem. sci. eng. 11(4) (2021) 305-316 microfluidic fuel cell with three fuels 306 biomedical and biochemical applications [1]. a miniaturized fuel cell is one of the best alternatives, capable of generating stable electrical energy and electricity from various fuels [2]. depending on the application and power requirement, the catalysts in such fuel cells would be enzymes or microbes in powering biomedical implants or redox reagents for energizing micropower devices. in standard redox electrochemical fuel cells called chemical fuel cells (cfc), oxidation and reduction reactions occur on anode and cathode sides in the presence of fuel and oxidant, respectively [3]. fuel cells that use enzymes and microbes as biocatalysts for the production of energy are termed enzymatic biofuel cells (ebfc) and microbial fuel cells (mfc), respectively [4,5]. as both ebfc and mfc utilize biosamples as electrolytes, they can be jointly termed as biofuel cells (bfc), where extensively biocompatible materials are used as a substrate for energy harvesting. in ebfcs, the enzyme immobilized bio-electrodes catalyze fuel oxidation at the anode and the reduction of oxidant occurs at the cathode to produce power [6]. the power generation from ebfc can be utilized to power biomedical devices, like implantable devices or artificial organs, where fuel for the biofuel cell can be derived from physiological fluids [7]. similarly, mfcs are bio-electrochemical devices that harness energy from bacterial metabolism to clean energy. the generation of power has been accomplished by the electrons derived from natural biochemical redox reactions catalyzed by bacteria oxidant at anode and cathode side, respectively [8,9]. as a result, it was successfully developed as a miniaturized biosensor and power source by utilizing various fluid resources such as domestic and industrial wastewater, urine, and soiled water from sewage [10,11]. in cfcs, chemical conversion of energy happens in the presence of electrolytes [12]. the cfcs are redox flow fuel cells, where electrolytes are fed continuously. a wide variety of fuel selection, high energy density and faster reaction kinetics have made cfc a significant energy harvesting device [13]. the operational efficiency and utilization of these fuel cells could be further improvised by the realization of a microfluidic platform. microfluidic fuel cells (µfc) have great potential in portable applications [14,15], which has led to the micro-fabrication of energy conversion devices. the fluid flow in microscale devices is governed by low reynolds number, where viscous effects dominate over inertial forces. contrary to the conventional fuel cell, these µfc operate based on the concept of co-laminar flow of liquids at the microscale, eliminating the requirement of an additional cation exchange membrane or separator [16]. this membrane-free architecture separates the anolyte and catholyte through a virtual liquid-liquid interface using a laminar fluid flow process [17]. as a result, the total form-factor and size of the device are reduced, and other related major problems, such as corrosion, humidification, and fuel crossover, are abridged or eliminated [18]. to maintain the colaminar fluid flow, the fuels and electrolytes are fed by external sources such as syringe pumps or peristaltic pumps. the inexpensive fabrication method and utilization of low-cost materials make µfc to be potential fuel cell technology for various applications [19]. several techniques were used to fabricate portable miniaturized microfluidic devices, such as photolithography, soft-lithography [20], paper-based [21,22], xurography [23] and laser micromachining [24,25]. despite their uniqueness and benefits, the development and implementtation of microfluidic fuel cells have not been utilized for portable electronic applications until now. the time-consuming procedures such as design and optimization of device parameters, fabrication protocols, selection [26] and incorporation of advanced electrode materials [27,28] have hindered the microfluidic systems from being more applicable for real-time applications. this can be overcome by developing a single multifunctional microfluidic device capable of operating with a wide range of fuels. this will eventually lead to mass production of microdevices, making the processing time effective and commercially feasible. these aspects were the key challenges influencing the overall output performance. so far, j. umarani sivakumar j. electrochem. sci. eng. 11(4) (2021) 305-316 http://dx.doi.org/10.5599/jese.1092 307 several types of μfc have been successfully developed and reported [29–32] with description of device architecture for each type of fuel cell. in this work, a single microfluidic platform incorporated with multi-walled carbon nanotube (mwcnt) electrodes was demonstrated to function with three different types of fuel cells (chemical, microbial, and enzymatic). the electrodes were fabricated using drop-casting method, and carbon nanomaterial as the electrode material. the microchannel was developed using a soft-lithography technique with the required microchannel height and width. three different microfluidic fuel cells have been studied using different electrolytes, such as formic acid, microbes and glucose, on the same device with similar device parameters. in addition, various electrochemical characterizations were carried out to understand and analyze the electrocatalysis occurring during the energy harvesting process in terms of current density, power density and open circuit potential. experimental chemicals and materials glucose oxidase (gox) (100,000 units/g solid) from aspergillus niger, laccase (e.c. 1.10.3.2, 24 u/mg) from trametes versicolor and multi-walled carbon nanotubes (>8 % carboxylic acid functionalized) were procured from sigma-aldrich, india and used without any further modification. analytical grade reagents, such as polyethylenimine (pei), was procured from sigma-aldrich, india, and glutaraldehyde 40 %, formic acid 98 % (ch2o2), sulphuric acid, 98 % (h2so4) were procured from tci chemicals pvt. ltd., india. shewanella putrefaciens bacteria was procured from microbial type culture collection and gene bank, india and made into glycerol stocks for preservation. the media luriabertani (lb) broth, functionalized multi-walled carbon nanotube (mwcnt) and potassium ferricyanide were purchased from srl, india. polydimethylsiloxane (pdms) was obtained from dow corning as sylgard 184. double-distilled (18.2 mω cm) milli-q water was utilized to prepare all electrolytes. all electrochemical tests were conducted at room temperature conditions. design, preparation, and fabrication of electrodes the standard photolithography technique has been used to fabricate the electrodes on an indium tin oxide (ito) coated glass. the complete fabrication method has been adopted from the previously published research work [33]. the stepwise 3d schematic illustration of the electrode fabrication is displayed in figure 1. in brief, the required electrode design with dimensions (length 20 mm and width 1 mm) was created using photolithography techniques through a positive photoresist (az1512). thereafter, the standard 98 % hcl chemical etchant was utilized to etch the desired pattern. this method will help to establish the desired pattern of electrodes with a conduction gap of 1 mm between them (figure 1b). to enhance the surface-to-volume ratio and the morphological properties of the etched electrodes, mwcnt suspension (1 mg/ml in di water) was coated with the help of a pdms stencil (figure 1c). the detailed pdms stencil fabrication process using spin coating was adopted from the previously published research article [34]. using this stencil, three layers of mwcnt suspension were coated to achieve better step coverage and uniformity (figure 1d). after every coating, the ito substrate was dehydrated over the hot plate at 120 °c for 1 hour, facilitating the mwcnt working area of 0.2 cm2 (figure 1e). for mfc and cfc, the catalyst was used in the electrolytes which were flushed into the microfluidic device. in the case of ebfc, enzymes were immobilized on the mwcnt electrodes with the help of cross-linkers following the previously published work by our group [35]. http://dx.doi.org/10.5599/jese.1092 j. electrochem. sci. eng. 11(4) (2021) 305-316 microfluidic fuel cell with three fuels 308 table 1. detail parameters used for microfluidic fuel cells experimentations figure 1. stepwise 3d schematic illustration of etched ito patterned with mwcnt electrode (a) ito coated glass; (b) etched ito glass with a gap of 1 mm; (c) pdms stencil aligned with electrodes; (d) mwcnt pipetting for electrodes; (e) mwcnt electrodes fabrication and integration of microchannel the soft-lithography technique was used to fabricate the y-shaped microchannel using pdms polymer. prior to microchannel fabrication, a hydrogel mold was created by utilizing the previously reported protocol [36]. subsequently, the pdms microchannel fabrication has been done by using hydrogel mold with the conventional scientific protocol [15]. the detailed y-shaped pdms microchannel fabrication protocol process was adopted from the previously published research work [29]. the required inlets and outlets were created using a needle (blunt type) to stream the electrolytes. finally, the whole microfluidic fuel cell was realized by integrating the microchannel with mwcnt electrodes sealed in an acrylic case with screws to prevent leakage. detailed stepwise microfluidic fuel cell integration process is depicted in figure 2. electrochemical method to understand the electrochemical behavior of the fabricated mwcnt electrodes, various characterizations were carried out using an electrochemical workstation (biologic instruments). a three-electrode system consisting of mwcnt, platinum wire and ag/agcl (3m kcl) as working, counter and reference electrode, respectively. initially, cyclic voltammetry (cv) was conducted to find the redox behavior of the fabricated bioelectrode in the voltage range suitable for the working electrode and electrolytes. likewise, other electrochemical techniques, such as chronoamperemetry (ca) and linear sweep voltammetry (lsv), were performed to record the performance of the microfluidic fuel cell at different electrolytic conditions (chemical, microbial, and enzymatic). no. components microfluidic chemical fuel cell microfluidic microbial fuel cell microfluidic enzymatic fuel cell specifications 1 electrodes mwcnt 2 microchannel pdms 3 flow rate, ml/h 3, 6, 12, 18, 24 4 anolyte formic acid (0.2-3.5 m) shewanella putrefaciens (90 µl in 10 ml lb broth) glucose (10-70 mm) in pbs (0.1 m, ph 7) 5 catholyte sulphuric acid (0.2-3.5 m) potassium ferricyanide (50 mm) pbs (0.1 m, ph 5) j. umarani sivakumar j. electrochem. sci. eng. 11(4) (2021) 305-316 http://dx.doi.org/10.5599/jese.1092 309 figure 2. step-by-step fabrication of membraneless microfluidic fuel cell: (a) bottom support (acrylic); (b) pdms support for electrodes; (c) mwcnt electrodes; (d) microchannel with electrodes; (e) top support; (f) bolt and nut mechanism for preventing leakage; (g) complete fuel cell with inlets and outlet; (h) real time integrated device results and discussion microfluidic chemical fuel cell (mcfc) here, formic acid and atmospheric oxygen were used as fuel and oxidant, respectively. further, sulphuric acid was used as an electrolyte for ionic conductivity. the fuel and electrolyte were fed into the cell by a peristaltic pump with a flow rate (6 ml/hr) to form co-laminar flow. this co-laminar flow acts as a virtual membrane for the fuel cell at the liquid-liquid interface. electrolyte optimization prior to the performance study of the cell, electrochemical investigations were carried out using the electrochemical workstation. the electrochemical behaviour of anode and cathode (mwcnt electrodes) was studied with various fuel and electrolyte concentrations. the fuel and electrolyte concentrations were optimized by the linear sweep voltammetry (lsv) technique at standard scan rate of 50 mv/s. the detailed experimentation parameters are listed in table 1. the fuel and electrolyte optimization is given in figure 3. as shown in figure 3a, the fuel and electrolyte concentrations were varied in a uniform increment (0.2 m 3.5 m). correspondingly, electrochemical activity was observed in terms of current density (cd). as shown in figure 3a, the maximum cd was observed at 1 m of formic acid and 1.5 m of sulphuric acid. with a further increase in concentration, there was no increment observed in cd. this manifests that the electrons transfer kinetics has been in line with the electrochemical reaction. subsequently, the cyclic voltammetry (cv) technique was used to identify the behaviour of mwcnt electrodes with optimized fuel and electrolyte concentration solutions [37]. http://dx.doi.org/10.5599/jese.1092 j. electrochem. sci. eng. 11(4) (2021) 305-316 microfluidic fuel cell with three fuels 310 a b figure 3. the individual fuel and electrolyte solution optimization at various concentrations (m), (a) optimization; (b) cv responses at optimized concentrations the optimized fuel and electrolyte cv responses are depicted in figure 3b. the maximum cd hysteresis was observed at optimized fuel and electrolyte concentrations. as shown in figure 3b, no oxidation and reduction peaks were observed at electrodes with optimized fuel and electrolyte, meaning that no catalyst was present on mwcnt electrodes. from the literature [38], catalyst electrodes have shown more redox peaks compare to plain electrodes. therefore, the fabricated mcfc with plain mwcnt electrodes has drawn lower performance, but these electrodes are inexpensive. polarization performance after successful optimization of the electrode parameters, mcfc performance was investigated by chronoamperometry (ca) technique. here, 1 m of formic acid and 1.5 m of sulphuric acid were used as fuel and electrolyte, respectively. plain mwcnt were used as electrodes and atmospheric oxygen as the oxidant. the real-time experimentation setup of mcfc is depicted in figure 2h. to observe the mcfc performance, anode and cathode were connected to the electrochemical workstation with external sources. initially, a stable open circuit potential (ocp) of 510 mv was observed. subsequently, the performance of the fuel cell was investigated with stable ocp for a range of flow rates (3-24 ml h-1). it was found that at particular optimized flow rate (6 ml h-1), the co-laminar flow of mcfc is very stable, and the diffusion zone has been dominated over depletion zones. due to these reasons, mcfc provided the highest performance at 6 ml/h flow rate. the output polarization of mcfc was measured in terms of current density (cd) and power density (pd). the mcfc output is depicted in figure 4 in terms of voltage, cd, and power density (pd). as can be seen from figure 5, the optimized mcfc shows the maximum pd of 3.195 µw cm-2 at the maximum cd of 16.85 µa cm-2 at 6 ml h-1 of flow rate. microfluidic microbial fuel cell (mmfc) bacterial optical and inoculation study in the microbial fuel cell, the bacteria shewanella putrefaciens was used as a biocatalyst. 90 µl of this bacteria was inoculated in 10 ml of autoclaved in luria-bertani (lb) media and incubated for 8 hours at 37 °c. the inoculation study was accomplished at three time periods, such as 6, 8, and 14 hours at a fixed 37 ℃ followed by optical density measurement. amongst all, the 8-hour incubation time was ideally selected with an optical density (od at 620 nm) of 0.73. these optimized j. umarani sivakumar j. electrochem. sci. eng. 11(4) (2021) 305-316 http://dx.doi.org/10.5599/jese.1092 311 parameters were referred from the recently published research article [32]. the ideal 8-hour incubation time exhibited higher metabolic activity and significant replication rates compared to others. the detailed bacterial optical and incubation study are graphically plotted and elaborated in previous studies done by our group [32]. figure 4. polarization and power performance of mcfc at optimized fuel and electrolyte solutions with mwcnt electrodes (1 m of formic acid and 1.5 m of sulphuric acid), with standard deviation (n=3) figure 5. polarization and power performance of microfluidic microbial fuel cell with standard deviation (n=3) polarization study after the successful biofuel preparation, the ito-based mmfc was continuously tested by feeding shewanella putrefaciens as anolyte and 50 mm potassium ferricyanide in pbs (0.1m, ph 7) as catholyte at a flow rate of 6 ml/hour. during experimentation, initially the open circuit potential (ocp) was measured. after that, the polarization study was evaluated at the stable ocp. this study was carried out in a two-electrode configuration using potentiostat where the electrochemical ca technique was used to calculate the power performance. detailed comparative results of mmfc are summarized in table 2. as shown in figure 5, the developed mwcnt coated mmfc in the presence of inoculated s. putrefaciens delivered the maximum ocp of 0.195 v, with maximum power density (mpd) of 1 μw cm-2 and maximum current density (mcd) of 22.1 µa cm-2. likewise, the power performance of media (absence of s. putrefaciens) was observed in the same device to study the role of mwcnt material [39] and inoculated bacteria for an efficient electrochemical redox reaction. although the obtained output power is not too high, it can be improved after amendment in the design, electrochemistry, and catalyst loading [40]. microfluidic enzymatic fuel cell (mefc) bioelectrode characterization the optimum parameters for bioelectrodes and their biocatalytic activity were characterized using electrochemical techniques. the enzyme immobilization and crosslinking procedures were adopted from our previously published work to prepare bioelectrodes [20]. in brief, bioanode and biocathode were prepared by drop-casting 10 µl of glucose oxidase (4 mg ml-1) and laccase (4 mg ml-1) respectively, on the mwcnt electrode surface. a three-electrode system was used to study the electrocatalytic behavior of bioelectrodes. an electrode surface of 0.2 cm2 was considered, and all the reactions were carried out in pbs (0.1 m). initially, the glucose concentration was determined by the lsv technique. as summarized in table 1, various glucose concentrations in the range of 10-70 mm were considered http://dx.doi.org/10.5599/jese.1092 j. electrochem. sci. eng. 11(4) (2021) 305-316 microfluidic fuel cell with three fuels 312 and 50 mm was found to be the optimal concentration, as shown in figure 6a. the current density after 50 mm decreased and got saturated. with 50 mm fuel concentration, the bioanode was characterized in the presence and absence of glucose, which provided information about the oxidative behavior of the electrodes [41]. in the presence of glucose, a visible peak occurred at 0.3 v with a maximum current density of 0.6 ma cm-2. this corresponds to the oxidation of glucose [42] into gluconolactone (figure 6b), denoting the occurrence of efficient biocatalysis. similarly, biocathode was also studied using the cv technique. a glucose concentration, mm b c d figure 6. (a) optimization of glucose concentration; (b) bioanode characterization; (c) biocathode characterization; (d) optimization of flow rate the biocathodes were tested in the presence and absence of oxygen conditions. in the presence of oxygen, the maximum catalytic current density of 0.7 ma cm-2 was observed in figure 6c due to the cupric ions redox reaction happening during the enzymatic mechanism of laccase at a voltage of -0.6 v [43]. this confirms the efficient immobilization of enzyme and oxygen prominence during the cathodic reaction process [35]. device optimization and analysis the optimized fuel concentration was incorporated into the microfluidic device to study the polarization performance of the mefc. subsequently, the flow rate, at which the electrolyte should be flushed into the mefc was optimized. as it can be seen from figure 6d, a flow rate of 6 ml h-1 produced higher power density as better co-laminar flow was maintained in comparison to other j. umarani sivakumar j. electrochem. sci. eng. 11(4) (2021) 305-316 http://dx.doi.org/10.5599/jese.1092 313 flow rates because of which the power output was high [44]. this is due to the fact that at higher flow rates enzymes detachment could occur leading to lower biocatalysis [45]. following the optimization of flow rate and fuel concentration, the performance of the mefc was studied using chronoamperometric technique. the mefc delivered a maximum open-circuit voltage (ocv) of 325 mv. as shown in figure 7, the microfluidic fuel cell produced a maximum power output of 5.5 µw cm-2 with the highest current density of 68 µa cm-2. this power output is suitable for a mefc without using any mediator. figure 7. polarization and power density curves for optimized bioelectrodes and device parameters with standard deviation (n=3) table 2. summary of optimized fuel cell parameters and performance parameters/type of fuel cell mcfc mmfc mefc membraneless microfluidic device mcd, µa / cm2 16.85 22.1 68 mpd, µw / cm2 3.195 1.01 5.5 ocv, mv 510 195 325 conclusions this work demonstrates the successful development and implementation of a single microfluidic platform for three different types of fuel cells. the novelty of this work lies in the ability to utilize a single miniaturized platform with optimized cell dimensions for three types of major fuel cells. http://dx.doi.org/10.5599/jese.1092 j. electrochem. sci. eng. 11(4) (2021) 305-316 microfluidic fuel cell with three fuels 314 microfluidic device was fabricated using a soft-lithography process and the mwcnt electrodes were prepared by the simple drop-casting method. such fabricated microfluidic devices and electrodes are very inexpensive (less than $1), yet a single device can function as different fuel cells. with optimized device parameters, the microfluidic device was capable of operating at electrolyte conditions of three variant fuels and oxidants with stable open-circuit voltage. the complete summary of the operating parameters in the different 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ouyang, y. chen, microelectronic engineering 84(5-8) (2007) 1182-1185. https://doi.org/10.1016/j.mee.2007.01.175 [45] a. zebda, l. renaud, m. cretin, c. innocent, f. pichot, r. ferrigno, s. tingry, journal of power sources 193(2) (2009) 602-606. https://doi.org/10.1016/j.jpowsour.2009.04.066 ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.seta.2020.100811 https://doi.org/10.1109/tnb.2019.2896207 https://doi.org/10.1109/tnb.2018.2857406 https://doi.org/10.1016/j.mee.2007.01.175 https://doi.org/10.1016/j.jpowsour.2009.04.066 https://creativecommons.org/licenses/by/4.0/) electrochemical combustion of indigo at ternary oxide coated titanium anodes doi: 10.5599/jese.2014.0061 247 j. electrochem. sci. eng. 4(4) (2014) 247-258; doi: 10.5599/jese.2014.0061 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical combustion of indigo at ternary oxide coated titanium anodes maría i. león, zaira g. aguilar and josé l. nava departamento de ingeniería geomática e hidráulica, universidad de guanajuato, av. juárez 77, zona centro, c.p. 36000, guanajuato, guanajuato, mexico corresponding author: e-mail: jlnm@ugto.mx; tel.: +52-473-1020100 ext. 2289; fax: +52-473-1020100 ext. 2209 received: august 04, 2014; revised: august 22, 2014; published: december 6, 2014 abstract the film of iridium and tin dioxides doped with antimony (iro2-sno2–sb2o5) deposited on a ti substrate (mesh) obtained by pechini method was used for the formation of  oh radicals by water discharge. detection of  oh radicals was followed by the use of the n,n-dimethyl-p-nitrosoaniline (rno) as a spin trap. the electrode surface morphology and composition was characterized by sem-eds. the ternary oxide coating was used for the electrochemical combustion of indigo textile dye as a model organic compound in chloride medium. bulk electrolyses were then carried out at different volumetric flow rates under galvanostatic conditions using a filterpress flow cell. the galvanostatic tests using rno confirmed that ti/iro2-sno2-sb2o5 favor the hydroxyl radical formation at current densities between 5 and 7 ma cm 2 , while at current density of 10 ma cm 2 the oxygen evolution reaction occurs. the indigo was totally decolorized and mineralized via reactive oxygen species, such as (  oh, h2o2, o3 and active chlorine) formed in-situ at the ti/iro2-sno2-sb2o5 surface at volumetric flow rates between 0.10.4 l min -1 and at fixed current density of 7 ma cm -2 . the mineralization of indigo carried out at 0.2 l min -1 achieved values of 100 %, with current efficiencies of 80 % and energy consumption of 1.78 kwh m -3 . keywords dimensionally stable anodes; electrochemical degradation of organics; pechini method; textile effluents; indigo textile dye http://www.jese-online.org/ mailto:jlnm@ugto.mx j. electrochem. sci. eng. 4(4) (2014) 247-258 electrochemical combustion of indigo textile dye 248 248 introduction textile processing industries nowadays are widespread sectors in many countries. this industry is one of the most polluting industries in terms of the volume, color and complexity of its effluent discharge. textile effluents include dyes that have a complex chemical structure, which most of the time are disposed on municipal sewers or into surface waters. residual textile dyes tend to be transformed into toxic aromatic amines which cannot be degraded by sunlight and, once in the environment, they exhibit recalcitrant properties [1-3]. electrochemical incineration [4-10] is a technique that has been found adequate for the treatment of colored wastewaters. it is important to point out that several color degradation studies mention systems with platinum electrodes [7] and dimensionally stable anodes (dsa) [6,8], which have shown mineralization of 50-70 %. dogan and turkdemir [7] consider that mineralization of indigo dye on pt is induced by by-products of water and chloride discharge on the platinum surface; however, the indigo achieved mineralization of 60 %. similar results in the degradation of acid red 29 [11], reactive blue 19 [8], mediated by active chlorine (given by the mixture of chlorine (cl2), hypochlorous acid (hocl) and hypochlorite ion (ocl )), produced on dsa lead to mineralization of 56 % and 70 %, respectively. bdd electrodes exhibit a superior performance, since a large amount of hydroxyl radicals (  oh) are formed by water oxidation on the bdd surface [5-6,12-13], achieving 100 % efficiency in color removal and mineralization. the main problem encountered with bdd electrode is its high price limiting its industrial application. for the above it is necessary to develop a dsa of metal oxides as an alternative to oxidize recalcitrant organic matter similar to a bdd electrode, in other words to produce dsa(  oh) capable to oxidize recalcitrant organic matter. comninellis and coworkers have developed a dsa electrode of sno2–sb2o5 with an interlayer between supports (ti) of iro2 by the spray pyrolysis technique, capable to produce hydroxyl radicals physisorbed on dsa (eq. 1), by water discharge [14]. the interlayer of iro2 improves useful life of the electrode. these authors put on evidence that the physisorbed hydroxyl radical dsa(  oh) cause predominantly the complete combustion of organics (r), eq. (2); for example, these authors demonstrated that dsa(  oh) reacts with p-clorophenol leading to complete combustion. on such electrode, iro2 acts as a catalyst, sno2 acts as a dispersing agent and sb2o5 as a doping agent. such ternary electrodes are among the best electrocatalysts for o2 evolution, being able to produce physisorbed hydroxyl radicals on their surface from water discharge. the high catalytic activity of this ternary oxide electrode has been recently reported for the electrochemical oxidation of other organic compounds [15,16]. another paper by comninellis put on evidence the convenience of using ti/sno2 to oxidize phenol matter via  oh radicals adsorbed onto ti/sno2 [17]. however, the main problem encountered with the ti/sno2 anode is its low stability under anodic polarization, which is not the case of the sno2– sb2o5 coating having an iro2 interlayer between the ti substrate [18]. 2dsa h o dsa( oh) h 1e       (1) z 2r dsa( oh) co zh ze dsa        (2) in a previous paper carried out by our group a film of iridium and tin dioxides doped with antimony oxide (iro2-sno2–sb2o5) was deposited onto ti substrate mesh and plate by the pechini method [19]. the ternary oxide coating was used for the anodic decolorization of methyl orange (mo) azo dye via reactive oxygen species, such as (  oh, h2o2 and o3) formed in-situ from water m. i. león et al. j. electrochem. sci. eng. 4(4) (2014) 247-258 doi: 10.5599/jese.2014.0061 249 oxidation at the ti/iro2-sno2-sb2o5 surface. however, in that paper we did not follow the formation of  oh at dsa surface and the electrochemical combustion of organic matter. the indirect technique for the detection and identification of low concentration of  oh radicals formed by water discharge at the oxide anodes involves trapping of the  oh radical by an addition reaction (spin trap) to produce a more stable radical (spin adduct). a number of  oh radical spin traps are available in the literature but n,n-dimethyl-p-nitrosoaniline (rno) has demonstrated to be effective owing to the selective reaction of rno with  oh radicals, the high rate of the reaction with  oh radicals (k = 1.2×10 4 m 1 s 1 ) and the ease of application as one merely observes the bleaching of the sensitive absorption band at 440 nm [17, 20]. the goal of this manuscript is to prepare a film of iridium and tin dioxides doped with antimony (iro2-sno2–sb2o5) onto titanium mesh (expanded metal) to produce  oh radicals via water discharge for the electrochemical combustion of indigo textile dye (which resembles a denim laundry industrial wastewater). bulk electrolyses were then carried out at different mean linear flow velocities and at constant current density using a filterpress flow cell. the integral current efficiency and the energy consumption of electrolysis were estimated. the detection of  oh radicals formed by water discharge at the oxide anode using rno as spin trap was also examined. experimental indigo dye solution was 1 mm indigo textile dye (536 ppm cod) in 0.05 m nacl (which resembles a denim laundry industrial wastewater). the resulting solution exhibited a conductivity of 5.78 ms cm -1 , and a ph of 6.3 at 298 k. the solution was deoxygenated with nitrogen for about 10 minutes before each experiment. all the chemicals employed in this work were reactive grade. equipment a potentiostat-galvanostat model sp-150 coupled to a booster model vmp-3 (20v-10a) both from bio-logic tm with ec-lab ® software were used for the electrolysis experiments. the potentials were measured versus a saturated calomel reference electrode (sce), bio-logic model 002056re-2b. all electrode potentials shown in this work are presented with regard to a standard hydrogen electrode (she). cod analyses were performed using a dry-bath (lab line model 2008), and a genesys 20 spectrophotometer. chloride volumetric titrations were confirmed by potentiometric measurements using a silver wire and a sce, which was inserted in a glassy titration cell. the potential differences between silver wire and sce were detected by a high impedance multimeter (agilent-mo del-34401a). the colour removal was registered using a visible spectrophotometer (genesys 20). microelectrolysis experiments a 100-ml pyrex electrochemical cell, with a three electrode system and nitrogen inlet was used for the construction of the anodic polarization curves. the working electrode was mesh-(iro2-sno2–sb2o5) with 1 cm 2 geometric area exposed to the electrolyte. the potentials were measured vs. sce and the counter electrode was a glassy carbon. all the potential measurements shown in this work are presented with regard to standard hydrogen electrode (she). a divided cell made of two compartment quartz cells of 3 ml capacity each one for the indirect detection of  oh radicals was used. the anode was in the form of plate (1 cm 2 ) and the cathode was a vitreous carbon rod (1 cm 2 ). a home-made salt bridge to connect both semi-cells was employed; this was fabricated with vitreous pyrex tube of 2 mm diameter sealed with pt at the ends; this bridge was filled with phosphate buffer (ph 7.4). the quartz cell used as the anodic j. electrochem. sci. eng. 4(4) (2014) 247-258 electrochemical combustion of indigo textile dye 250 250 compartment was collocated into the uv-visible spectrophotometer (perkin elmer lambda 35) to follow the bleaching (in-situ) of the yellow color of rno during electrolysis. flow cell experiments the flow cell fm01-lc that includes the turbulence promoter type d was used; the detailed description of this cell is depicted elsewhere [21]. in this work the spacer was 0.55 cm thick. dsa anode was a mesh-(iro2-sno2–sb2o5), while platinum coated titanium flat sheet, was used as the cathode. dsa electrode was prepared by pechini method described below. the platinum coated titanium was provided by de nora. details on the fm01-lc cell characteristics are given in table 1. table 1. mesh-(ti/iro2-sno2-sb2o5) electrode dimensions, experimental details of the fm01-lc electrolyzer. electrode length, l 16 cm electrode height, b 4 cm electrode spacing, s 0.55 cm anode area, (ti/iro2-sno2-sb2o5) 112 cm 2 cathode área, (ti/platinized) 64 cm2 overall voidage,  (ti/iro2-sno2-sb2o5) 0.93 volumetric flow rate, from 0.1 to 0.4 l min-1 overall voidage is the ratio of the free space in the channel to overall channel volume. figure 1. electrical and flow circuit for the measurement of electrochemical incineration kinetics at fm01-lc electrolyzer. the undivided fm01-lc cell, with a single electrolyte compartment and the electrolyte flow circuit, is shown in figure 1. the electrolyte was contained in a 1 l polycarbonate reservoir. a magnetically coupled pump of 1/15 hp march mfg, model mdx-mt-3 was used; the flow rates were measured by a variable area glass rotameter from cole palmer, model f44500. the electrolyte + flow meter magnetic pump bdd anode cathode n 2 fm 01 l c reservoir potentiostat -galvanostat + flow meter magnetic pump bdd anode cathode n 2 fm 01 l c reservoir potentiostat -galvanostat m. i. león et al. j. electrochem. sci. eng. 4(4) (2014) 247-258 doi: 10.5599/jese.2014.0061 251 circuit was constructed from master flex tubing, c-flex 6424-16, of 0.5 inch diameter. the valves and the three way connectors were made of pvc. scanning electron microscopy surface characterization of the metallic coating was performed using a sem carl zeiss dsm 940a microscope. the energy of the primary electrons beam employed was 15 kev. methodology preparation of the dsa material a ternary oxide (iro2-sno2–sb2o5) film was deposited onto a ti plate and mesh to be used in the three electrode cell and in the flow cell (figure 1) by pechini method using appropriate molar ratios of the oxide components. the precursor polymer solution was a mixture of citric acid (ca) in ethylene glycol (eg) at 60-70 °c. after total dissolution of the ca, h2ircl6xh2o, sncl4 and sbcl3 were added to the mixture according to a molar composition of eg:ca:ir:sn:sb as 16:0.12:0.0296:0.0296:0.0004, maintaining the temperature at 60-70 °c for 30 min. this mixture was then applied with a brush to both sides of the pre-treated ti support. after the application of the coating, the electrode was heated at 100 o c for 5 min in a furnace in order to induce the polymerization of the precursor. this procedure was repeated eight times. after the final coating, the electrodes were maintained at 550 o c for 1 h in order to calcinate the polymer and form the ternary oxide (iro2-sno2–sb2o5); xrd analysis confirmed at such at temperature these oxide phases are obtained [22]. the temperature did not exceed 600 o c to avoid the formation of tio2 that markedly reduces the electrocatalytic properties of the ti/iro2-sno2–sb2o5 coating due to passivation [23]. microelectrolysis tests anodic polarization curves to determine the limits of potential and current density where the media is oxidized at ti/iro2-sno2–sb2o5 electrode were performed. these studies were carried out in the solution containing phosphate buffer (ph 7.4), and in the presence of 2×10 5 m rno in the same buffer at room temperature (298 k). anodic potential limit of 1.6 v vs. she was applied from open circuit potential (ocp) (0.82 v vs. she) using the linear sweep voltammetry technique at 50 mv s 1 . based on these polarization curves the detection of hydroxyl radicals was performed. detection of hydroxyl radicals in this paper rno was used as spin trap for the detection of low concentration of  oh radicals formed by water discharge at the ti/iro2-sno2-sb2o5 electrode and the bleaching of the yellow colour was measured during electrolysis [17]. rno traps the  oh radical by an addition reaction to produce a more stable radical (spin adduct), eq. (3) [17]. (3) it is important to mention that rno is electrochemical inactive at pt, sno2 and iro2 anodes [17,20]. a divided cell for the indirect detection of  oh radicals was used (see microelectrolysis experiments section). anodes screening tests were carried out in phosphate buffer (ph=7.4) containing 2×10 5 m rno. galvanostatic electrolyses at current densities of 5, 7 and 10 ma cm 2 applied to the ti/iro2-sno2-sb2o5 electrode were performed; at the same time the bleaching j. electrochem. sci. eng. 4(4) (2014) 247-258 electrochemical combustion of indigo textile dye 252 252 (in-situ) of the yellow color of rno during electrolysis was followed. the same tests were performed using pt plate (1 cm 2 ) as anode for which the surface  oh radical concentration is almost zero [17]. electrochemical incineration in the filter-press flow cell. electrochemical incinerations of indigo were carried out in the fm01-lc cell equipped with mesh-(ti/iro2-sno2-sb2o5) at current density of 7 ma cm -2 , value determined from microelectrolysis studies, at different volumetric flow rates between 0.10.4 l min. 1 . incineration evolution was estimated by cod analysis of samples taken at different times. the cod values were determined by closed reflux dichromate titration method [24]. it is important to mention that estimating residual organic matter by cod analysis allowed eliminating any interference from chloride species. for this method, an excess of hgso4 was added and ag2so4 in the digestion and catalyst solutions, respectively, with the purpose of eliminating possible interferences from chloride species during the estimation of the residual organic matter from cod analysis [12]. the chloride concentration was evaluated by volumetric titration using a 0.5 m agno3, confirmed by potentiometric measurements [12]. in addition, the color removal was determined by the decrease in absorbance at 639 nm, during electrolyzes. results and discussion characterization of dsa figure 2 presents typical scanning electron micrographs for freshly prepared electrode ti/iro2sno2-sb2o5. the surface morphology of the layer is characterized by the presence of crackers and plates. the presence of plates on the surface is probably due to the drastic heat treatment to which the sampled was submitted, that promoted the rapid exit of co2 gas originated from the decomposition of the organic polymer. edx analyses focused on several plate structures show heterogeneous atomic percentage ratio of sn and ir (between 1.6 to 2.74), indicating that sn segregates from other oxide to form a sn rich deposit. moreover, antimony was randomly detected along the electrode, showing that sb is not homogeneously distributed along the electrode surface owing to its low content. figure 2. sem images of ti/iro2-sno2-sb2o5. figure 3 shows typical linear sweep voltammetries obtained on ti/iro2-sno2-sb2o5 electrode in the solution containing phosphate buffer (ph 7.4), and in the presence of 2×10 5 m rno in the 20 mm m. i. león et al. j. electrochem. sci. eng. 4(4) (2014) 247-258 doi: 10.5599/jese.2014.0061 253 same buffer where no differences were detected. the fact that no changes were detected in both electrolytic solutions suggests the oxidation of water which is found in excess. tafel slope performed on ti/iro2-sno2-sb2o5 from these curves (see inset), gives value of 190 mv dec -1 , which is different to that reported for ti/iro2/sno2-sb2o5 and ti/pt/sno2-sb2o4, 120 and 204 mv dec -1 obtained at 298 k, respectively [12,25]; this difference is associated with the electrode composition and by the method of preparation. figure 3. typical linear sweep voltammetries on ti/iro2-sno2-sb2o5 anode. electrolyte: phosphate buffer (ph 7.4), and phosphate buffer + 2×10 5 m rno. the scan rate was 50 mv s 1 . the inset shows the tafel plot for j-e curves for phosphate buffer. a = 1 cm 2 . t = 298 k. figure 4. absorbance spectra of rno (2×10 5 m) in phosphate buffer (ph=7.4) obtained at 5 min intervals during galvanostatic electrolyses with ti/iro2-sno2-sb2o5 (a) and pt (b) anodes. a = 1 cm 2 . t=298 k. for screening tests of anodes we used rno as spin trap of  oh radicals. figure 4 shows the absorption spectrum of aqueous solution (2×10 5 m rno) in phosphate buffer at ph 7.4 during 0 4 8 12 16 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 j / m a c m -2 e / v vs. she phosphate buffer + rno phosphate buffer 1.1 1.14 1.18 1.22 1.26 1.3 -0.6 -0.4 -0.2 0 0.2 0.4 e / v v s . s h e log j / ma cm-2 j. electrochem. sci. eng. 4(4) (2014) 247-258 electrochemical combustion of indigo textile dye 254 254 galvanostatic electrolysis at 5, 7 and 10 ma cm 2 with ti/iro2-sno2-sb2o5 and pt electrode. with pt anode, there is no decrease in absorbance at 440 nm, at the three current densities, contrary to the ti/iro2-sno2-sb2o5 anode for which there is a rapid decrease in the absorbance at 5 and 7 ma cm 2 . these results show that there is accumulation of  oh radicals at the ti/iro2-sno2-sb2o5 electrode surface contrary to pt anode for which the surface  oh radical is almost zero. the fact that the ti/iro2-sno2-sb2o5 anode at 10 ma cm 2 behaves similar to that pt suggests that at such current density the accumulation of  oh radicals is zero and the oxygen evolution reaction starts to appear. therefore, according to the proposed reactions (eqs. (1) and (2)) [14,17] the ti/iro2sno2-sb2o5 will favor complete combustion of indigo textile dye at 5 and 7 ma cm 2 . electrochemical incineration of indigo textile dye in the fm01-lc using dsa electrode figures 5 (a) and (b) show the normalized color (detected at  = 639 nm) and cod results obtained from experiments performed at constant current density (7 ma cm -2 ) and variable volumetric flow rates. in these figures, the normalized color decreases faster than cod with the electrolysis time at different volumetric flow rates. cod kinetic was lower than that obtained for color decay owing to the slower combustion of by-products. however, color and cod depletion do not show marked improvement at the elevated volumetric flow rates. given that the presence of chloride ions (i.e., 0.05 m in this study) is relevant due to the possible formation of active chlorine by oxidation at ti/iro2-sno2-sb2o5, the chloride consumption at the end of the electrolysis was measured (figure 6), giving an average conversion between 15-40 %. this value did not show a marked dependence with hydrodynamics. this indicates that, despite the predominant role of ti/iro2-sno2-sb2o5 (  oh) as oxidant species, indigo and/or its byproducts can be simultaneously destroyed by other oxidants such as dissolved chlorine gas, hypochlorous acid (hclo) and hypochlorite ion (clo ), as well as chlorate and perchlorate ions formed upon electro-oxidation with ti/iro2-sno2-sb2o5 electrode. the complete combustion obtained here confirms that the  oh radical, in addition to the other oxidants, are responsible for the oxidation of indigo, which does not occur on platinum electrodes, where the oxidation of indigo in chloride medium achieved 60 % in terms of cod [7]. the results obtained here are in agreement with other articles carried out by our group, where we achieved the complete combustion of indigo mediated by oh  and active chlorine (produced on bdd in the same filter-press flow cell) [12,13]. the fact that hydrodynamics does not improve indigo oxidation and color removal may be associated with a complex mechanism of indigo degradation. hplc studies would be helpful in the identification of possible indigo oxidation by-products; however, these were beyond the scope of the present work. it is important to point out that all of the electrolyses presented herein were carried out in the undivided fm01-lc cell, for which reason the degradation of indigo may also involve reactions at the cathode (ti/pt). with the data obtained from cod for all of the electrolyses at their respective volumetric flow rates, integral current efficiency and energy consumption were analyzed as a function of percentage of indigo oxidation, for electrolyses performed at 7 ma cm -2 , figure 7 (a)-(b). the estimation of integral current efficiency and energy consumption were determined using equations (4) and (5) [12]: m. i. león et al. j. electrochem. sci. eng. 4(4) (2014) 247-258 doi: 10.5599/jese.2014.0061 255 figure 5. normalized color ( = 639 nm) (a) and cod (b) decay during the electrolyses of indigo on (ti/iro2-sno2-sb2o5) in the fm01-lc electrolyzer. electrolyte: 1 mm indigo in 0.05 m nacl; this composition resembles a denim laundry wastewater. a = 112 cm 2 , j = 7 ma cm -2 , t = 298 k. volumetric flow rates are shown in the figure. figure 6. normalized concentration of chloride versus volumetric flow rates evaluated at the end of the electrolyses similar to those from fig. 5(b). electrolyte: 1 mm indigo in 0.05 m nacl. a = 112 cm 2 , j = 7 ma cm -2 , t = 298 k. volumetric flow rates are shown in the figure. j. electrochem. sci. eng. 4(4) (2014) 247-258 electrochemical combustion of indigo textile dye 256 256 4 [ (0) ( )]fv cod cod t it    (4) cel c m 4 1 3.6 lfee v  (5) where f is the faraday constant, 96485 c mol -1 , v is the solution volume (cm -3 ), cod(0) and cod(t) are the chemical oxygen demand initially and at time (t) of the electrolysis, in mol cm -3 , i is the applied current, in a, t is the time of electrolysis (s), ecell is the cell potential in v, and vm is the molar volume in cm 3 mol -1 . the value of 3.6 is a correction factor which converts ec to units of kwh m -3 . figure 7(a) shows that current efficiency surpasses 100 % (theoretical value) at volumetric flow rates of 0.1 and 0.3 l min 1 , suggesting those indigo oxidation by-products and/or the processes taking place at the cathode enhance the degradation of indigo. a similar behavior was obtained in a previous communication carried out by our group [12], during indigo mineralization process in the same filter-press reactor. on the other hand, for the volumetric flow rates of 0.2 and 0.4 l min 1 , the current efficiencies were lower than that obtained for 0.1 and 0.3 l min 1 . it is important to remark that at the end of the electrolyses the current efficiency where 80 % for all volumetric flow rates studied, and there are no marked effects of the hydrodynamics on current efficiency in the set of electrolyses studied herein. the analysis of figure 7(b) shows that the energy consumption is not strongly influenced by hydrodynamics at 0.2-0.4 l min 1 . it is important to emphasize that the energy consumption is at least four times lower than those obtained in a previous paper, carried out by our group using the fm01-lc electrolyzer equipped with bdd electrodes in the same indigo solution [12]. this savings in energy consumption is due to the lower electrode polarization obtained using dsa (1.2 v) than the obtained on bdd (2.4 v), diminishing cell potential. figure 7. (a) integral current efficiency versus percentage of oxidized indigo in the fm01-lc electrolyzer, evaluated from the electrolyses similar to those from fig. 5(b). (b) energy consumption versus volumetric flow rate evaluated at 88 % of degradation from the electrolyses similar to those from fig. 5(b). m. i. león et al. j. electrochem. sci. eng. 4(4) (2014) 247-258 doi: 10.5599/jese.2014.0061 257 the study presented here indicates that, despite the predominant role of ti/iro2-sno2-sb2o5(  oh) as oxidant species, indigo and/or its by-products can be simultaneously destroyed by other oxidants such as dissolved chlorine gas, hypochlorous acid (hclo) and hypochlorite ion (clo ), as well as chlorate and perchlorate ions formed upon electro-oxidation with ti/iro2-sno2-sb2o5 electrode. conclusions the detection of  oh radicals formed by water discharge at ti/iro2-sno2-sb2o5 using n,n-dimethyl-p-nitrosoaniline (rno) as a spin trap showed that exits an accumulation of  oh radical at ti/iro2-sno2-sb2o5 surface. therefore, the ti/iro2-sno2-sb2o5 anode favors complete combustion of indigo by bulk electrolysis. the galvanostatic tests using rno as spin trap of  oh radicals confirmed that ti/iro2-sno2-sb2o5 will favor the hydroxyl radical formation at current densities between 5 and 7 ma cm 2 , while at current density of 10 ma cm 2 the oxygen evolution reaction occurs. electrolyses in a fm01-lc flow cell indicates, that despite the predominant role of ti/iro2-sno2-sb2o5 (  oh) as oxidant species, indigo and/or its by-products can be simultaneously destroyed by other oxidants such as dissolved chlorine gas, hypochlorous acid (hclo) and hypochlorite ion (clo ), as well as chlorate and perchlorate ions formed upon electro-oxidation with ti/iro2-sno2-sb2o5 electrode. the mineralization of indigo carried out at 0.2 l min 1 and 7 ma cm 2 achieved values of 100 %, with current efficiencies 80 %, and energy consumption of 1.78 kwh m -3 . the fm01-lc equipped with mesh-(ti/iro2-sno2-sb2o5) improves space-time yield, allowing better interaction between mesh-(ti/iro2-sno2-sb2o5)(  oh) and organics, a phenomenon that increases organic mineralization efficiency. in this manner, the complete mineralization of indigo with high current efficiency, obtained in this work is a notable improvement over those reported in the literature by using other dsa electrode. additionally, the performance of the fm01-lc electrolyzer equipped with mesh(ti/iro2-sno2-sb2o5) electrodes, demonstrate the convenience of using this electrochemical reactor as a pre-pilot cell for other water samples containing recalcitrant organic matter. acknowledgements: maría i. león and zaira g. aguilar thank conacyt for the given grant. authors are grateful to conacyt and concyteg for the economic support via the project fomix gto-2012-c04-195057. authors also acknowledge universidad de guanajuato for the financial support. references [1] y. wong, j. yu, water research 33 (1999) 3512-3520. 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[25] d. santos, a. lopes, m. j. pacheco, a. gomes, l. ciríaco, journal of the electrochemical society 161 (2014) h564–h572. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {graphene based sulfonated polyvinyl alcohol hydrogel nanocomposite for flexible supercapacitors} http://dx.doi.org/10.5599/jese.1031 197 j. electrochem. sci. eng. 11(3) (2021) 197-207; http://dx.doi.org/10.5599/jese.1031 open access : : issn 1847-9286 www.jese-online.org original scientific paper graphene based sulfonated polyvinyl alcohol hydrogel nanocomposite for flexible supercapacitors subhakaran singh rajaputra1,2, nagalakshmi pennada1,2, anjaneyulu yerramilli1,2, naga mahesh kummara1, 1centre for advanced energy studies, koneru lakshmaiah education foundation, vaddeswaram, guntur dist. 522 502, ap, india 2department of chemistry, koneru lakshmaiah education foundation, vaddeswaram, guntur dist. 522 502, ap, india *corresponding author: nagamaheshk@gmail.com received: june 22, 2021; revised: july 22, 2021; accepted: july 29, 2021; published: august 3, 2021 abstract graphene based sulfonated polyvinyl alcohol (pva) hydrogel was synthesized and its performance as nanocomposite gel polymer electrolyte was investigated for application in quasi solid-state flexible supercapacitors. hydrothermally reduced graphene (hrg) was synthesized through hydrothermal reduction of graphene oxide (go). sulfonated pva hydrogel (spva) was synthesized with predetermined quantities of hrg to obtain nanocomposite gel polymer electrolytes coded as spva-hrg-x (x = content (wt.%) of hrg). the amorphous nature of spva-hrg-x was determined using x-ray diffraction (xrd) technique. the electrochemical performance of spva-hrg-x was evaluated using techniques like cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical spectroscopy (eis) studies of a lab scale supercapacitor cell, fabricated using hydrothermally reduced carbon cloth (cchy) current collectors coated with hrg (hrg-cchy). in spva-hrg-0.5 electrolyte, hrg-cchy exhibited specific capacitance of 200 f g-1 at 1 a g-1 and specific energy of 6.1 wh kg-1 at specific power of 1 kw kg-1 and retained 93 % of its initial capacitance even after 5000 gcd cycles. the incorporation of spva with 0.5 wt.% of hrg-cchy can be attributed to the increase in amorphous nature of spva-hrg-0.5, which in-turn lowers its impedance. this contributed to the remarkable supercapacitive behaviour of hrg-cchy, demonstrating its potential as gel polymer electrolyte (gpe) for application in quasi solid-state flexible supercapacitors. keywords electrochemical double-layer capacitors; gel polymer electrolyte; carbon cloth; cyclic voltammetry; electrochemical impedance; specific capacitance; specific energy. http://dx.doi.org/10.5599/jese.1031 http://dx.doi.org/10.5599/jese.1031 http://www.jese-online.org/ mailto:nagamaheshk@gmail.com j. electrochem. sci. eng. 11(3) (2021) 197-207 polyvinyl alcohol hydrogel nanocomposite 198 introduction in recent years, there has been sharp increase in the development of flexible supercapacitors (fscs) for electronic devices as power sources, which have to be ultra-thin and flexible in-order to serve their purpose [1-4]. fscs with high specific energy, specific power and excellent cycle life are much desirable to supply the rising market of flexible and wearable electronic devices in the near future [5,6]. to increase the performance of supercapacitors, the usage of various types of carbons, mixed/binary metal oxide and sulfide-based electrode materials like nanochains [7], nanoflowers [8] and other nanostructures [9-11] has been widely reported. carbon based materials are most widely used electrode materials in supercapacitor application, due to their high surface area and capability of storing charge in the form of an electric-double layer [12,13]. currently, reduced graphene oxide (rgo), a graphene-based material with high ionic conductivity and surface area, has emerged as a potential electrode material in fscs and alternative to activated carbon [14]. the main components of a fsc are flexible electrodes, solid or quasi-solid-state electrolyte and a porous separator to prevent short-circuit [15]. the synergy between the electrolyte and the electrode material creates a significant impact on the properties of a supercapacitor, like charge-discharge capabilities, cyclic stability, energy storage in the form of charge, and power delivery [16]. the rate performance and specific power of a supercapacitor can be increased by increasing ionic conductivity of the electrolyte [17]. most of fsc assemblies use gel polymer electrolytes (gpes) to prevent leakage and packaging issues, as in the case of liquid electrolytes [18,19]. gpes contain a discontinuous phase of solvent entrapped inside a continuous phase of three-dimensional polymer network [20]. hydrogels of polyvinyl alcohol (pva) contain hydroxyl groups which contribute to its hydrophilic nature by absorbing large amounts of water, in-turn enhancing the conductivity of electrolyte ions [26] and establishing stable contact at the interface of electrode and electrolyte [21]. sulfonation of pva hydrogels using proton donor like sulfuric acid (h2so4) enhances their ionic conductivity [22,23]. in the past, extensive usage of pva-h2so4 (spva) as gpe in supercapacitors has been reported alongside electrode materials like activated carbon [24], carbon nanotubes (cnts) [25] and graphene [26-29]. incorporation of nanofillers and redox-active materials into gpes leads to rise in amorphous nature thereby improving ion conductivity [30, 31]. recently, redox-additives like na2moo4 [5], hydroquinone [32], alizarin red [33] and indigo carmine [34] have been incorporated into pva-h2so4 for application in supercapacitors. inorganic fillers like nano sio2 [35], nano tio2 [36], sb2o3 [37] and graphene oxide (go) [38] have been incorporated for improving performance in pva based gpes. several reports have mentioned the usage of go as a nanofiller in gpes in various electrochemical devices [39-41]. yang et al. [42] reported that incorporation of go into polyvinylidene difluoride (pvdf) based gpe enhances its ionic conductivity by forming 3d network structures in the polymer matrix facilitating the transport of ions. the current paper deals with incorporation of hydrothermally reduced graphene oxide (hrg) into spva hydrogels, performed for the first time to obtain nanocomposite gpes and evaluate their electrochemical performance for application in quasi solid state flexible supercapacitors. go was synthesized using a modified hummer’s method and reduced hydrothermally to obtain hrg. carbon cloth (cc) was modified using a hydrothermal method to obtain hydrothermally reduced carbon cloth (cchy) and use it as a flexible current collector. nanocomposite gpes were prepared by sulfonating pva hydrogel using h2so4, followed by addition of calculated amounts of hrg ranging from 0.1 to 1.0 wt.% to obtain hrg incorporated spva gpes, hereafter referred as spva-hrg-x (x = content (wt.%) of hrg). electrochemical performance of developed spva-hrg-x was evaluated s. s. rajaputra et al. j. electrochem. sci. eng. 11(3) (2021) 197-207 http://dx.doi.org/10.5599/jese.1031 199 using an in-house fabricated supercapacitor single cell with electrodes of hrg coated cchy (hrg-cchy). experimental materials graphite powder (particle size <20 µm) and sodium nitrate (nano3) were obtained from sigmaaldrich inc. and merck specialities pvt. ltd., india, respectively. carbon cloth was obtained from avcarb, usa. ethanol 99.9 % was procured from changshu hongsheng fine chemicals co. ltd., china. poly (vinyl alcohol) (pva) (m.w. ̴125,000), polyvinylidene difluoride (pvdf) (homopolymer powder, m.w. ̴320,000), n-methyl-2-pyrrolidone (nmp), sodium hydroxide (naoh) pellets, sulphuric acid (h2so4) 98 %, hydrogen peroxide (h2o2) (30 % w/v), hydrochloric acid (hcl) 35–38 %, and propan-2-ol (isopropanol/ipa) were purchased from s d fine-chem ltd., india. potassium permanganate (kmno4) was obtained from loba chemie pvt. ltd., india. whatman qualitative filter paper: grade 1 (circles, diameter 125 mm) purchased locally was used as a separator in two electrode studies. an in-house fabricated lab scale two electrode cell assembly made up of acrylic plates was used to carry out full-cell studies. always deionized (di) water was used in preparing various solutions. method synthesis of hrg hrg was synthesized following the procedure reported in our previous work [43]. firstly, a modified hummer’s method was followed for synthesizing go. in 50 ml of conc. h2so4, 1 g of nano3 was added and stirred for few minutes, followed by dispersing 1 g of graphite powder into it. the above dispersion was stirred at <5 oc in an ice bath for 4 h continuously, followed by slow addition of 6 g of kmno4. the above mixture was stirred uninterruptedly for 48 h at room temperature (rt). 92 ml of di water was slowly added to the above mixture and stirred for two more hours. later, 10 ml of 30 % h2o2 was added to the mixture leading to change in dispersion color from brown to yellow, indicating the formation of go. the collected go precipitate was washed with 1 m hcl and di water and centrifuged. the resultant precipitate was finally washed using ethanol and vacuum dried at 70 oc for 12 h. later, the sample was finely crushed to obtain go powder. 200 mg of go powder was dispersed in 200 ml through ultrasonication. the ph of the dispersion was maintained at 11 using naoh pellets. the dispersion was then transferred into a teflon lined hydrothermal reactor (300 ml) and autoclaved for 14 h at 180 oc. the precipitate collected after the reaction was washed several times using di water and dried under vacuum for 12 h at 60 oc. the dried sample was crushed to obtain a fine powder of hrg. preparation of cchy cchy was prepared following the procedure reported in our previous work [44]. commercially obtained carbon cloth (cc) (50 cm2) was oxidized via chemical route. an acidic mixture of 20 ml of h2so4 and 10 ml of hno3 was prepared into which a pristine cc was dropped and stirred at rt, followed by slow addition of 3 g of kmno4 and 100 ml of di water, and stirred for 3 h. later, 5-10 ml of 30 % h2o2 was added to the above mixture resulting in a clear solution with oxidized carbon cloth in it. the oxidized cc was washed with di water and transferred into a teflon lined hydrothermal reactor (300 ml), filled with di water and autoclaved for 14 h at 180 °c. later, the reduced cc was http://dx.doi.org/10.5599/jese.1031 j. electrochem. sci. eng. 11(3) (2021) 197-207 polyvinyl alcohol hydrogel nanocomposite 200 vacuum dried at 70 °c for 6 h. the hydrothermally reduced cc (cchy) thus obtained was used as a current collector in the following two electrode cell studies. preparation of spva to prepare spva, 270 μl of h2so4 was added to 4 ml of di water. then, 0.5 g of pva was added to this mixture, and stirred at 80 °c till all pva gets dissolved, resulting in a transparent viscous liquid. preparation of spva-hrg-x hrg based spva nanocomposite was prepared by adding predetermined quantities of hrg ranging from 0.1 to 1.0 wt. % to spva, separately. the hrg was dispersed in ipa using ultrasonication and added to the cooled spva and stirred continuously at 80 °c for 30 min, resulting in dark coloured spva-hrg-x. the obtained spva-hrg-x were coded as spva-hrg-0.1, spva-hrg0.2, spva-hrg-0.5 and spva-hrg-1.0 for spva incorporated with 0.1, 0.2, 0.5 and 1.0 wt. % of hrg, respectively. figure 1 shows the optical images of prepared spva and spva-hrg-x. figure 1. optical images of (a) spva; (b) spva-hrg-0.1; (c) spva-hrg-0.2; (d) spva-hrg-0.5; (e) spva-hrg-1.0 characterization studies x-ray diffraction (xrd) technique (rigaku miniflex 600) was used to analyse all gpes. electrochemical workstation (parstat pmc 2000a) was used to evaluate the electrochemical performance of prepared spva-hrg-x. hrg-cchy was prepared by coating hrg (1 mg cm-2) over cchy. ink of hrg was prepared by dispersing 1.8 mg of hrg in 100 μl of nmp along with 2 μl of 10 wt. % pvdf/nmp solution by ultrasonication. hrg ink was then deposited over flexible cchy current collectors, followed by drying under vacuum for 15 min at 120 oc. strands of hrg coated cchy (hrg-cchy) were placed on both sides of a gpe coated whatman filter paper and packed in between acrylic plates tightly to fabricate a cell. approximately, 100 μl of gpe was utilized during fabrication each cell. the prepared cell was then tested in two electrode configuration using cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical impedance spectroscopy (eis) techniques to evaluate the performance of spva-hrg-x. the specific capacitance (cs), specific energy (ed) and specific power (pd) of hrg-cchy in all gpes were calculated from gcd data, using the equations (1) to (3) [45]. s 2 i t c m v  =  (1) s. s. rajaputra et al. j. electrochem. sci. eng. 11(3) (2021) 197-207 http://dx.doi.org/10.5599/jese.1031 201 2 s d 8 c v e  = (2) d d e p t =  (3) where i represent constant discharge current, δt represents discharge time, δv represents discharge potential window and m represents the mass of the active material on one electrode. results and discussion xrd analysis figure 2 shows x-ray diffraction patterns of pure pva, spva and spva-hrg-x. the diffraction pattern of pure pva shows a characteristic semi-crystalline peak at 2 value of around 19.6 [46]. in the case of spva, h2so4 addition disturbs the semi-crystalline nature of pure pva, thereby increasing its amorphous nature [47]. from diffraction patterns of all spva-hrg-x it can be inferred that by increase in hrg concentration the intensity of peak around 2 value of 19.6 decreased, indicating an increase in amorphous nature of gpes [48]. the addition of hrg may contribute to the increase in amorphous nature of the spva-hrg-x [42], thereby enhancing the rate of penetration and conduction of ions [49]. figure 2. x-ray diffraction patterns of pure pva, spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spva-hrg-1.0 electrochemical studies cv studies the electrochemical performance of hrg-cchy was evaluated by executing cv studies for all five gpes at various scan rates, ranging from 10 to 100 mv s-1 , within potential window 0 to 1 v. the area under near rectangular cv curve is proportional to the double-layer capacitance of the electrode material [50]. figure 3a compares cv curves of hrg-cchy in all gpes at the scan rate of 50 mv s-1, with hrg-cchy showing comparatively superior double-layer capacitance in spva-hrg0.5 electrolyte. from cv curves of hrg-cchy in all gpes depicted in figure 3a, an increase of the area under near rectangular cv curve is observed by increasing hrg content in spva-hrg-x electrolyte up to 0.5 wt.%. the decrease in cv curve area of hrg-cchy in spva-hrg-1.0 compared http://dx.doi.org/10.5599/jese.1031 j. electrochem. sci. eng. 11(3) (2021) 197-207 polyvinyl alcohol hydrogel nanocomposite 202 to spva-hrg-0.5, indicates a decline in double-layer capacitance of hrg-cchy in spva-hrg-1.0 due to the excess concentration of hrg in spva-hrg-1.0, which could resist the flow of ions due to formation of agglomerates by restacking of hrg layers [45,48]. the cv curves of hrg-cchy in spvahrg-0.5 electrolyte at multiple scan rates ranging from 10 to 100 mv s-1 (figure 3b), indicate better rate capability and reversibility [51]. figure 3. (a) comparative cv curves of hrg-cchy in spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spva-hrg-1.0 at scan rate of 50 mv s-1; (b) cv curves of hrg-cchy in spva-hrg-0.5 at scan rates ranging from 10 to 100 mv s-1 gcd studies the charge-discharge behaviour of hrg-cchy in all five gpes was evaluated by performing gcd studies at various constant current densities ranging from 0.5 to 10 a g-1, within the potential window 0 to 1 v. figure 4a compares gcd curves of hrg-cchy in all gpes at 1 a g-1. it is obvious that hrgcchy shows better charging and discharging ability in spva-hrg-0.5 electrolyte, compared to the rest of the gpes, with an impressive specific capacitance of 200 f g-1 at 1 a g-1 with lowest ir drop of around 0.07 v. figure 4b shows gcd curves of hrg-cchy in spva-hrg-0.5 electrolyte at multiple constant current densities ranging from 0.5 to 10 a g-1. figure 4c represents the ir drop plot of hrg-cchy in all gpes at various constant current densities ranging from 0.5 to 10 a g-1, indicating the lower equivalent series resistance (esr) and superior conductivity of hrg-cchy when spva-hrg-0.5 was used as electrolyte [52]. figure 4d represents the ragone plot of hrg-cchy in all gpes, indicating superior specific energy to power ratio of hrg-cchy in spva-hrg-0.5, like 6.1 wh kg-1 of specific energy at specific power of 1 kw kg-1. in spva-hrg-1.0 electrolyte, hrg-cchy showed a sharp increase in ir drop and decrease in specific energy at current densities ≥7 a g-1, indicating an abrupt increase of esr of the cell. this distinct behavior of spva-hrg-1.0 electrolyte at higher current densities, could result from the excess of hrg in spva-hrg-1.0, which could lead to restacking of hrg layers forming agglomerates, in-turn resisting the flow of ions [39,42]. cyclic stability figure 5a represents the cyclic stability of hrg-cchy in all gpes for 5000 gcd cycles at 2 a g-1. even after 5000 gcd cycles, hrg-cchy retained 82, 86, 89, 93 and 87 % of their initial specific capacitances in spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spva-hrg-1.0, respecttively. figure 5b shows gcd curves of hrg-cchy in spva-hrg-0.5 before and after cycling for 5000 cycles, which clearly establishes the notable charge-discharge stability of hrg-cchy in s. s. rajaputra et al. j. electrochem. sci. eng. 11(3) (2021) 197-207 http://dx.doi.org/10.5599/jese.1031 203 spva-hrg-0.5. table 1 represents the comparison of supercapacitive performances of some carbonbased electrode materials in sulfonated pva based gpes. figure 4. (a) gcd curves of hrg-cchy in spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spvahrg-1.0 at 1 a g-1; (b) gcd curves of hrg-cchy in spva-hrg-0.5 at 0.5, 1, 2, 4, 5, 7 and 10 a g-1; (c) ir drop plot; (d) ragone plot of hrg-cchy in spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spva-hrg-1.0 figure 5. (a) cyclic stability of hrg-cchy in spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spva-hrg-1.0 at 2 a g-1 for 5000 gcd cycles; (b) gcd curves of hrg-cchy in spva-hrg-0.5 before and after cycling for 5000 gcd cycles at 2 a g-1 http://dx.doi.org/10.5599/jese.1031 j. electrochem. sci. eng. 11(3) (2021) 197-207 polyvinyl alcohol hydrogel nanocomposite 204 table 1. comparison of supercapacitive performances of carbon-based electrode materials in sulfonated pva based gpes electrolyte electrode material specific capacitance reference pva/h2so4 graphene 190 f g-1 at 0.5 a g-1 [28] pva/h2so4 rgo-pva composite film 184.6 f g-1 at 1 a g-1 [53] pva/h2so4 n-doped porous carbon 232 f g-1 at 0.5 a g-1 [54] pva/h2so4/h3bo3 poly carbon nanofibers 134 f g-1 at 1 a g-1 [23] pva/h2so4/na2moo4 mos2-ncnt 95.14 f g-1 at 1 ma [55] pva/h2so4/hrg-0.5 hrg 200 f g-1 at 1 a g-1 present work eis studies the eis studies were performed at ac amplitude of 5 mv and within the frequency range of 100 khz to 0.1 hz. figure 6a represents nyquist plots of hrg-cchy in all five gpes. for all spva-hrg-x, the imaginary impedance part in the low frequency region was near perpendicular to the real axis, indicating near ideal capacitive behaviour of the cell. from nyquist plots, it is evident that hrg-cchy has low impedance in spva-hrg-0.5 compared to other gpes [56]. the inset image of figure 6a exhibits the magnified image of nyquist plots, where spva-hrg-0.5 electrolyte showed lower electrolyte resistance compared to other gpes [57]. figure 6b represents the nyquist plot of hrg-cchy in spva-hrg-0.5 before and after 5000 gcd cycles. the size of the semicircle in the high frequency region of the nyquist plot indicates charge transfer resistance [58,59]. the inset image of figure 6b exhibits the magnified image of nyquist plots, where it can be inferred that diameter of the semi-circle of spva-hrg-0.5 increased after cycling 5000 gcd cycles. this confirms the increase of charge transfer resistance after cycling [57,58]. near-vertical line observed in the low frequency region of nyquist plot for spva-hrg-0.5 indicates superior capacitive behaviour compared to other gpes [60]. figure 6. (a) nyquist plots of hrg-cchy in spva, spva-hrg-0.1, spva-hrg-0.2, spva-hrg-0.5 and spva-hrg-1.0. inset image represents the magnified high frequency part of nyquist plots; (b) nyquist plots of hrg-cchy in spva-hrg-0.5 before and after cycling for 5000 gcd cycles at 2 a g-1. inset image represents the magnified high frequency part of nyquist plots conclusion spva-hrg-x were prepared by introducing hrg into spva and characterized using xrd technique. supercapacitor assembled by using hrg-cchy and gpes was characterized by cv, gcd s. s. rajaputra et al. j. electrochem. sci. eng. 11(3) (2021) 197-207 http://dx.doi.org/10.5599/jese.1031 205 and eis. as confirmed by xrd measurements, spva-hrg-x may have induced amorphous nature, improving thereby ionic conductivity and lowering impedance. hrg-cchy in spva-hrg-0.5 exhibited a specific capacitance of 200 f g-1 at 1 a g-1 with the lowest ir drop of around 0.07 v and an impressive specific energy of 6.1 wh kg-1 at the specific power of 1 kw 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https://doi.org/10.1002/adfm.201203556 https://doi.org/10.1007/s11581-021-04144-4 https://doi.org/10.1115/1.4051143 https://doi.org/10.1039/c3ta15046a https://jbesci.org/published/4.4.2.pdf https://doi.org/10.1016/j.jpowsour.2008.09.046 https://doi.org/10.1002/pen.23118 https://doi.org/10.1016/0032-3861(96)81134-7 https://doi.org/10.1016/j.electacta.2018.04.162 https://doi.org/10.1039/c4ra04927f https://doi.org/10.1002/adma.201305851 https://doi.org/10.1039/c7ta04920j https://doi.org/10.1002/slct.201801507 https://doi.org/10.1021/acsaem.0c02739 https://doi.org/10.1016/j.electacta.2011.06.039 https://doi.org/10.1039/c4ra05553e https://doi.org/10.5562/cca3452 https://doi.org/10.1039/c3ta10998d https://doi.org/10.1002/smll.201600841 https://creativecommons.org/licenses/by/4.0/) effect of heat treatment on structure and properties of multilayer zn-ni alloy coatings doi: 10.5599/jese.2013.0036 137 j. electrochem. sci. eng. 3(4) (2013) 137-149; doi: 10.5599/jese.2013.0036 open access : : issn 1847-9286 www.jese-online.org original scientific paper effect of heat treatment on structure and properties of multilayer zn-ni alloy coatings vaishaka r. rao, a. chitharanjan hegde and k. udaya bhat* electrochemistry research laboratory, department of chemistry, national institute of technology karnataka, surathkal, srinivasnagar-575025, india *department of metallurgy and materials engineering, national institute of technology karnataka, surathkal, srinivasnagar-575025, india corresponding author: e-mail: hegdeac@rediffmail.com, tel.: +91-9980360242 received: december 25, 2012; revised: april 12, 2013; published: novembar 09, 2013 abstract composition modulated multilayer alloy (cmma) coatings of zn-ni were electrodeposited galvanostatically on mild steel (ms) for enhanced corrosion protection using single bath technique. successive layers of zn-ni alloys, having alternately different composition were obtained in nanometer scale by making the cathode current to cycle between two values, called cyclic cathode current densities (cccd’s). the coatings configuration, in terms of compositions and thicknesses were optimized, and their corrosion performances were evaluated in 5 % nacl by electrochemical methods. the corrosion rates (cr)’s of multilayer alloy coatings were found to decrease drastically (35 times) with increase in number of layers (only up to 300 layers), compared to monolayer alloy deposited from the same bath. surface study was carried with sem, while xrd was used to determine metal lattice parameters, texture and phase composition of the coatings. the effect of heat treatment on surface morphology, thickness, hardness and corrosion behaviour of multilayer zn-ni alloy coatings were studied. the significant structural modification due to heat treatment is not accompanied by any decrease in corrosion rate. this effect is related to the formation of a less disordered lattice for multilayer zn-ni alloy coatings. keywords multilayer zn-ni alloy; corrosion study; heat treatment; sem; xrd. introduction an advanced coating technique, called composition modulated multilayer alloy (cmma) coating is gaining interest due to their improved properties, such as mechanical strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, enhanced corrosion http://www.jese-online.org/ mailto:hegdeac@rediffmail.com j. electrochem. sci. eng. 3(4) (2013) 137-149 heat treatment of multilayer zn-ni alloy coatings 138 and wear resistance, superior reflectance, soft magnetic properties, giant magnetoresistence and corrosion resistance not attainable in any of the metallurgical alloys [1]. the cmma materials basically consists of alternating layers of metals/alloys on micro/nanometer scale, deposited electrolytically by making the cathode to cycle between two current densities at definite time intervals. the coating with improved resistance to highly aggressive environmental condition is demanded for the extended safe service life of industrial objects. hence cmma coating is well studied while finding its wide spread industrial applications [2-4]. blum first introduced the electrodeposition of multilayered alloy on cu-ni, demonstrating the deposition of alternate cu and ni layers, tens of microns thick, from two different electrolytes [5]. the deposition using single bath technique (sbt) for the fabrication of modulated alloys was recorded by brenner [6]. tench and white proposed that the presence of ni in the deposit was responsible for improved corrosion resistance of fabricated cu/ni metal multilayers and also their mechanical properties [7]. the zn-ni alloy electrodeposition has attracted interest of scientific community because these alloys were found to be more corrosion resistant and thermally stable than pure zinc [8]. many authors have attempted to understand the characteristics of this deposition process [9-12]. the abrupt change in the composition of the alloys and in the current efficiency is observed during this complex codeposition at a given value of deposition potential/current density. multilayer coating by electrolytic method is the most promising approach for improving the corrosion resistance of zn-ni alloys, and is of distinct commercial interest [13-16]. the cmma coating with zn-fe alloy as alternate layers was tested to exhibit better corrosion protection than individual metals. the presence of high content of fe in zn-fe alloy was reasoned to be responsible for enhanced corrosion protection [17]. the thermal stability of the zn-ni alloy coatings is important for their general applications, especially where they are expected to perform at elevated temperature conditions, such as in some automotive applications. however, the conventional methods usually modify the physical properties of the metal being protected with the limitations, such as susceptibility to damage by heat, cost, and formation of oxide products. the real challenge to overcome these problem would be to develop a novel protection coating with an exceptional thermal stability with minimum changes to the physical properties of the protected metal. though there are many reports with regard to the development of cmma zn-ni coatings using different baths and additives, no work has been reported with regard to examine the thermal stability of those coatings [18-21]. hence the present paper reports the development of cmma zn-ni alloy coatings on mild steel (ms) from acid chloride bath using gelatin and glycerol as additives. the corrosion stability of cmma zn-ni coatings on heat treatment have been tested by subjecting the coated specimens to different temperatures. the cmma zn-ni coatings have been tested for their thickness, hardness, composition, surface morphology and corrosion stability before and after heat treatment, and results are discussed. experimental all electrodeposition were carried out from the same electrolytic bath prepared using analytical grade reagents and double distilled water. conventional hull cell method was used to examine the effect of current density (c.d.) and bath constituents [22]. the zn-ni alloy bath was prepared by adding known amount of gelatin in hot distilled water (insoluble in cold water) and glycerol as additives, to impart brightness to the coating. electroplating process was carried out in a stirred solution on a pre-cleaned ms panels, having 7.5 cm 2 active surface area, at 30 °c and ph 4.0. the zn anode was used with the same exposed area. the electroplating of both monolayer (monov. r. rao et al. j. electrochem. sci. eng. 3(4) (2013) 137-149 doi: 10.5599/jese.2013.0036 139 lithic) and cmma zn-ni coating was carried out using computer controlled dc power source, having output speeds of up to 160 microseconds per step voltage/current change (n6705a, agilent technologies) for 10 min (∼15 μm thickness), for comparison purpose. the coating thickness were determined by faraday’s law and verified by measuring in digital thickness meter (coatmeasure m&c, aa industries/yuyutsu instruments). the hardness of coatings was measured using digital micro hardness tester (clemex, model: mmt-x7). the electrodeposited ms plates were subjected to heat treatment by keeping in temperature controlled oven (technico ind. ltd., 3144) at temperature 100, 200, 300 and 400 °c for constant time duration of 60 minutes. the modulation in the composition of alternate layer was affected by pulsing the current periodically. the power pattern used for deposition of monolayer (direct current) and multilayer (pulsed current) coatings is shown in figure 1. the optimal composition and operating parameters of the bath is given in table 1. the corrosion behavior of the coatings were evaluated in 5 % nacl solution at ph 4.0 using potentiostat/galvanostat (acm instruments, gill ac series no-1480) at temperature 25 °c, using saturated calomel electrode (sce) as reference, and platinum as counter electrodes, respectively. the corrosion rates (cr) were measured by tafel extrapolation method at scan rate of 1 mv s -1 with start potential +250 mv to reverse potential -500 mv. the electrochemical impedance spectroscopy (eis) measurements were made in frequency range of 100 khz to 0.01 hz at ±10 mv perturbing voltage, to evaluate the barrier property of the coatings. the surface morphology and cross sectional view of the cmma coating were examined under scanning electron microscopy (sem, model jsm6380 la from jeol, japan). the variation in the phase structure of alloys in different layers were confirmed by x-ray diffraction (xrd) study, using cu kα (λ = 0.15405 nm) radiation, in continuous scan mode with a scan rate of 2°min -1 (xrd, jeol jdx-8p). conveniently, zn-ni cmma coatings are represented as: (zn-ni)1/2/n (where 1 and 2 indicate the first and second cathode current density (cccd’s) and ‘n’ represent the number of layers formed during total plating time. i.e. 10 min. figure 1. power pattern used for deposition of monolayer (direct current) and multilayer (square current pulse) coatings. results and discussion monolayer zn-ni alloy coatings the hull cell study confirmed that 3.0 a dm -2 is the optimal c.d. for deposition of monolayer zn-ni alloy from the bath (optimized) given in table 1. the zn-ni alloy coating developed at multilayer growth square current pulse monolayer growth direct current j. electrochem. sci. eng. 3(4) (2013) 137-149 heat treatment of multilayer zn-ni alloy coatings 140 3.0 a dm -2 with ~8.0 wt.% ni exhibited the least cr (14.46×10 -2 mm y -1 ) compared to other coatings at other c.d.’s as reported in table 2. table 1. bath composition and operating parameters of the optimized bath. bath ingredients concentration, g l -1 operating parameters zncl2 27.2 anode: pure zinc cathode: mild steel ph 4.0 temperature: 30 °c nicl2 x 6h2o 94.9 boric acid 27.7 nh4cl 100 gelatin 5.0 glycerol 2.5 from the experimental data given in table 2, it may be noted that the wt.% ni in the electrodeposited zn-ni alloy at all c.d. are less than that in the bath (64.5 % ni). hence it may be inferred that the proposed bath follow anomalous type of codeposition, characteristic of all zn-fe group metal alloys, explained by brenner [6]. hence, zn-ni alloy coating at 3.0 a dm -2 , represented as (zn-ni)3.0/mono has been taken as the optimal coating configuration for monolayer deposition from the proposed bath. table 2. corrosion data for monolayer zn-ni alloy coatings developed at different c.d.’s. j a dm -2 content of ni wt. % -e0 v vs. sce icorr µa cm -2 cr × 10 -2 mm y -1 1.0 2.62 1.019 17.62 23.42 2.0 4.05 1.086 13.87 18.44 3.0 7.95 1.105 10.88 14.46 4.0 8.07 1.162 12.53 16.66 optimization of cyclic cathode current densities (cccd’s) and number of layers in cmma coatings in the present work improving the corrosion resistance of monolayer zn-ni alloy by multilayer technique is guided by the following principles [23-25]: i. periodic change in current density (c.d.) allows the growth of coatings having periodic change in its chemical composition. ii. the corrosion resistance property of multilayer coatings or any functional property in general reaches its maximum value when thickness of the individual layers reaches optimal nanoscale. the amplitude of compositional modulation diminishes rapidly when layer thicknesses below certain limit (about 50 nm). nanoscale multilayer coatings with alternate layers of alloys of different composition generally show unique properties than bulk materials when each individual layer thickness is below ~100 nm. different layered combinations offer unique mechanical, optical, magnetic and electronic properties including improved good corrosion resistance. in all these cases, the properties of the multilayer coatings depend on two factors, namely the composition and thickness of the individual layers [26-28]. hence by proper setting up of composition (by selection of cyclic current density, cccd’s) and thickness (by fixing the time for each layer deposition) of the individual layer it is v. r. rao et al. j. electrochem. sci. eng. 3(4) (2013) 137-149 doi: 10.5599/jese.2013.0036 141 possible to optimize the coating configuration [29,30]. hence multilayer coatings have been accomplished under different combination of cccd’s and individual layer thickness. the experimental procedure for optimization of coating configuration is explained below. multilayer coatings having 10 layers (arbitrarily chosen) have been developed at different cccd’s, namely at 1.0/3.0 and 2.0/4.0 a dm -2 , using same binary alloy bath. the coatings at different sets of cccd’s were developed in order to try different possible modulations in composition of individual layers, and their corrosion behaviours were studied, and corrosion data are reported in table 3. table 3. corrosion rates of cmma zn-ni coatings having 10 layers at different cccd’s in comparison with (zn-ni)3.0/mono developed from same bath. cccd’s a dm -2 -e0 v vs. sce icorr. µa cm -2 cr × 10 -2 mm y -1 (zn-ni)1.0/3.0/10 1.085 8.416 11.18 (zn-ni)2.0/4.0/10 1.108 6.606 8.78 (zn-ni)3.0/mono 1.105 10.88 14.46 it may be noted that both (zn-ni)1.0/3.0/10 and (zn-ni)2.0/4.0/10 coatings exhibited less cr compared to (zn-ni)3.0/mono coating. hence, by choosing the above cccd’s, multilayer coatings with higher degree of layering, i.e. with 30, 60, 120, 300 and 600 layers have been developed by proper setting up of the power source. it may be noted the cr’s decreased drastically with an increase in number of layers in both sets of cccd’s. it further indicates that improved corrosion property is not the unique property of the layer composition; instead, the combined effect of composition modulation and thickness of individual layers, or the number of layers. it should be noted that cr decreased only up to 300 layers and then started increasing, i.e. 600 layers as shown in table 4. table 4. effect of layering on corrosion behavior of cmma (zn-ni)1.0/3.0 and (zn-ni)2.0/4.0 coating in comparison with monolayer (zn-ni)3.0/mono deposited from same bath at 303k coating configuration number of layers average thickness of each layer, nm e0 v vs. sce icorr µa cm -2 cr × 10 -2 mm y -1 (zn-ni)1.0/3.0 10 1500 1.085 8.416 11.18 30 750 1.037 7.125 9.47 60 250 1.056 4.531 6.02 120 125 1.067 2.357 3.13 300 50 1.041 1.319 1.75 600 25 1.050 3.577 4.75 (zn-ni)2.0/4.0 10 1500 1.108 6.606 8.78 30 750 1.085 4.989 6.63 60 250 1.050 2.125 2.82 120 125 1.007 1.056 1.40 300 50 1.135 0.31 0.41 600 25 1.036 1.975 2.61 (zn-ni)3.0/mono monolayer 15000 1.128 10.78 14.33 further, though decrease of cr was found in both sets of cccd’s, the cr corresponding to (zn-ni)2.0/4.0 at 300 layers is the least, it has been taken as the optimal configuration for peak j. electrochem. sci. eng. 3(4) (2013) 137-149 heat treatment of multilayer zn-ni alloy coatings 142 performance against corrosion, and is represented as (zn-ni)2.0/4.0/300. the cr found to increase at higher degree of layering, i.e. at 600 layers is due to shorter relaxation time for redistribution of metal ions at the diffusion layer, during plating. it should be noted that under optimal condition, the total thickness of cmma (zn-ni)2/4/300 coating was found to be ~15 µm, calculated from thickness tester (coatmeasure model m&c), verified by faraday law. then from the total thickness and number of layers allowed to form (300), it is predicted that the average thickness of each layer is 50 nm. thermal stability of the monolayer and multilayer coatings thickness and hardness of the coatings the thickness and hardness of coatings were found to be decreased on heat treatment in both (zn-ni)3.0/mono and (zn-ni)2.0/4.0/300 coatings. in the case of (zn-ni)2.0/4.0/300 coatings the decrease may be attributed to the structural changes, evidenced by xrd study. the decrease is more pronounced in case of monolayer when compared to the multilayer zn-ni alloy coatings. the thickness of the coatings is directly related to the high tensile residual internal stresses, which result from the presence of ni in the alloy [31]. the drop in the coating thickness with the temperature may be attributed to the iron enrichment caused by the formation of intermetallic zn/fe compounds due to the inter-diffusion at the coating/substrate interface. it was predicted that the iron enrichment in the interfacial region pushes nickel toward the surface [32]. the decrease of thickness and hardness with increase in temperature observed in case of (znni)2.0/4.0/300 is shown, in comparison with that of (zn-ni)3.0/mono coating in table 5. table 5. effect of temperature on thickness, hardness and corrosion behavior of cmma (zn-ni)2.0/4.0/300 coatings, in comparison with monolayer (zn-ni)3.0/mono deposited from same bath. cccd’s a dm -2 treated temperature, °c thickness, µm vicker hardness, v100 -ecorr v vs. sce icorr µa cm -2 cr×10 -2 mm y -1 (zn-ni)2.0/4.0/300 30 18.9 212 1.003 0.31 0.41 100 16.7 189 1.022 2.14 2.84 200 14.3 183 1.024 2.18 2.89 300 9.1 154 0.995 3.73 4.95 400 5.4 139 0.865 4.52 6.00 (zn-ni)3.0/mono 30 17.8 202 1.128 10.78 14.33 200 9.3 145 1.053 21.68 28.82 400 4.8 119 1.070 26.53 35.27 further, in the case of monolayer zn-ni coating the decrease of hardness may be attributed to the fact that the dislocation sources are active under the stress field (applied in the form of heat) [33]. the decrease in the residual stress is also reasoned to be responsible for decrease in hardness of the alloy coating with thermal treatment [34]. the cmma coatings provide stress free environment, because the attractive forces tend to bridge the gap between successive layers. however, with heat treatment the interaction with the substrate increased, and hence stress developed. corrosion study cyclic polarization study the corrosion resistance exhibited by zn-ni coatings (both monolayer and multilayer) can be better understood by tafel polarization method as shown in figure 2a, and corresponding v. r. rao et al. j. electrochem. sci. eng. 3(4) (2013) 137-149 doi: 10.5599/jese.2013.0036 143 corrosion parameters are reported in table 5. cyclic polarization study over a potential range of -1.25v to -0.5v was studied and is shown in figure 2b. the anodic current was made to move from negative to positive, and then reversed. cyclic polarization curves shows that there is no much significance of the corrosion product formation with regard to the corrosion protection of the alloy. the corrosion current value was found to be always higher than that of forward scanning, indicating the dissolution of oxide film had occurred in the forward scanning and selfrepairing occurred during the process of backward scanning. cmma zn-ni coating with optimal configuration, (zn-ni)2.0/4.0/300 showing the least cr was opted for examining the thermal stability of the coatings. the cr of the zn-ni cmma coating in 5% nacl solution, before and after heat treatment of 100 °c represented as (zn-ni)2.0/4.0/300 and (zn-ni)2.0/4.0/300/100 °c), was found to be, respectively about 35 times and 5 times, respectively, more corrosion resistive than conventional (zn-ni)3.0/mono alloy coating as shown in figure 2b. a b figure 2. polarization behavior of cmma (zn-ni)2.0/4.0/300 coatings: a) after treatment at different temperature b), cyclic polarization behavior of (zn-ni)3.0/mono, cmma (zn-ni)2.0/4.0/300 and cmma (zn-ni)2.0/4.0/300/100°c. thus the corrosion resistance of multilayered zn-ni alloy coating has also decreased with heat treatment. however, the decrease in cr’s were more significant in the case of multilayer zn-ni alloy coating compared to monolayer coating after heat treatment. it is due to the fact that the electroplated samples attained intrinsically different surfaces in terms of their electrochemical properties. however, increase in the cr’s is not in proportion of the structural changes observed, as will be discussed with xrd analysis. electrochemical impedance spectroscopy study the electrode process involved in double layer capacitance and corrosion behaviors can be better understood by electrochemical impedance spectroscopy (eis) method. the small amplitude signals from the test specimen are considered in eis method. the nyquist plot is the type of the plot in which the data is plotted as imaginary impedance, zimg vs. real impedance zreal with the provision to distinguish the contribution of polarization resistance (rp) verses solution resistance (rs) [35]. eis signals of zn-ni monolayer coating (at optimal c.d, i.e. 3.0 a dm -2 ) compared with the (zn-ni)2.0/4.0/300 coating before and after heat treatment (at different temperatures) is shown on j. electrochem. sci. eng. 3(4) (2013) 137-149 heat treatment of multilayer zn-ni alloy coatings 144 figure 3. progressive decrease of polarization resistance of cmma coatings with annealing temperature supports the reduced corrosion resistance, may be due to diffusion of layers. however, corrosion rate of monolayer coatings are more than multilayer coatings under all degree of layering. figure 3. electrochemical impedance response (real vs. imaginary reactance values) displayed by cmma (zn-ni)2.0/4.0/300 after heat treatment at 4 different set temperatures. it may be noted that all the coatings exhibits one capacitive loop. however, at low frequency limit the capacitive behaviour of the double layer tends to exhibit the inductive character, indicated by decrease of both zreal and zimg. the gradual decrease in the diameter of semicircle indicates that the polarization resistance, rp decreases with increase in temperature as shown in figure 3. the (zn-ni)2.0/4.0/300 coating showed maximum impedance, due to accumulation of corrosion products at the electrode surface acting as barrier. the negative value of imaginary impedance, zimg at lower frequencies observed in coatings treated for higher temperatures are attributed to the inductance behaviour caused by the change in corrosion potential at the interface [36,37]. however, the radii of both capacitive loop and inductive character decreased progressively with raise in temperature. surface morphology figure 4 displays the sem image for surface morphology of (zn-ni)2.0/4.0/300 alloy coatings after heat treatment at different temperatures. it may be noted that the surface non-homogeneity increased with an increase of treatment temperature. however, zn-ni alloy coatings are found to adhere onto the substrate as fine particles with compact arrangement. the increased compactness with the heat treatment was also predicted to be main reason for good appearance and high hardness [38]. the cross sectional view of cmma (zn-ni)2.0/4.0/10 coating before and after heat treatment (100 °c) is shown in figure 5. it may be noted that distinctly visible alloy layers (figure 5a) become v. r. rao et al. j. electrochem. sci. eng. 3(4) (2013) 137-149 doi: 10.5599/jese.2013.0036 145 fused (figure 5b) due to diffusion of layers upon heat treatment. thus thermal treatment of electrodeposited multilayer zn-ni alloy coatings leads to both structural and behavioral changes. figure 4. sem images of cmma (zn-ni)2.0/4.0/300 coatings after treatment at different temperatures: (a) 100°c (b) 200°c (c) 300°c and (d) 400°c. figure 5. sem images across the cross section of cmma (zn-ni)2.0/4.0/10 coatings before heat treatment (a), and after heat treatment at 200°c (b). xrd study the xrd patterns of (zn-ni)3.0/mono and (zn-ni)2.0/4.0/300 alloy coating after heat treatment, deposited from same bath is given in figure 6 and 7 respectively. variation in the surface morphology of the monolayer coatings after heat treatment is supported by the variation in xrd (a) (b) (d) (c) j. electrochem. sci. eng. 3(4) (2013) 137-149 heat treatment of multilayer zn-ni alloy coatings 146 peaks. it has been reported that the phases obtained by the zn-ni coatings up to 13 % nickel do not correspond to that reported on the thermodynamic phase diagram [39]. it may be noted that the reflection corresponding to zn(101), γ-phase (ni5zn21), zn(103) phases and ni3zn22(335) as well was observed in monolayer coating deposited at optimal c.d. i.e., 3.0 a dm -2 . however, upon heat treatment there was hardly any difference in the phases observed in the ((zn-ni)3.0/mono alloy coatings before and after heat treatment up to 200 °c. however, at temperatures higher than 200 °c additional phases were observed corresponding to the intermetallic zn/fe compound (figure 6). the formation of this phase occurs due to inter diffusion in the interface region between the zn-ni alloy and the steel substrate. zno phase formation at temperatures higher than 200 °c was attributed to the metal contact with the ambient oxygen. the x-ray diffraction line broadening at temperatures higher than 200 °c may be related to the increase in the corrosion current value, which is reported to be caused by the lattice strains [40]. figure 6. xrd patterns of the (zn-ni)3.0/mono coatings after heat treatment at different temperatures (200°c and 400°c) deposited from the same bath. the reflection corresponding to zn(101), γ-phase (ni5zn21) and ni3zn22(510) was highly suppressed in the case of (zn-ni)2.0/4.0/300 coating on heat treatment at 4 different set temperatures. however at temperatures higher than 300 °c, the coating has shown weak signal corresponding to zn(100), zn(102) and γ zn-ni phases, although zn(103) phase completely disappeared. the least cr exhibited by (zn-ni)2.0/4.0/300/100 was attributed to the ratio of the phase structure corresponding to zn(103) and weak signals corresponding to γ-phase of zn (411, 330), zn(101) and zn3ni22(006) phase. the comparison between the xrd signals of the zn-ni monolayer and cmma coating on post-heat treatment reveals the fact that the exhibition of lesser cr of the zn-ni cmma coating upon heat treatment to that of the zn-ni monolayer coating deposited at optimal c.d. i.e., 3.0 a dm -2 , was attributed to the diffusion of the layered structure and formation of different phase structure ratio as shown on figure 6 and figure 7. v. r. rao et al. j. electrochem. sci. eng. 3(4) (2013) 137-149 doi: 10.5599/jese.2013.0036 147 figure 7. xrd patterns of the cmma (zn-ni)2.0/4.0/300 coatings after heat treatment at different temperature (100°c, 200°c, 300°c and 400°c), deposited from the same bath. conclusions based on the experimental investigation on development and characterization of cmma zn-ni alloy coatings on mild steel following observations were made as conclusions: 1. the coating configuration in terms cccd’s and number of layers have been optimized for deposition of the most corrosion resistant coatings from acid chloride bath using glycerol and gelatin as additives. 2. the decrease in thickness and hardness of both monolayer and multilayer coatings due to heat treatment were due to the active dislocation sources and decrease in the residual stress. 3. progressive decrease of polarization resistance of cmma coatings with annealing temperature supports the reduced corrosion resistance, may be due to diffusion of layers. however, corrosion rate of monolayer coatings are more than multilayer coatings under all degree of layering. 4. the corrosion resistance of multilayer coatings increased only up to certain number of layers and then decreased due to interlayer diffusion. 5. thermal treatment of electrodeposited (both monolayer and multilayer) zn-ni alloy coatings led to significant structural change of the alloy, supported by sem and xrd study. 6. a small increase of corrosion rates of zn-ni alloy due to annealing is related to the formation of a more ordered lattice of the alloy. 7. however, the increase of corrosion rates, due to annealing is not in proportion of the structural changes of the alloy occurred. 8. the corrosion resistance of 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[40] r. ramanauskas, r. juskenas, a. kalinicenko, j. solid state electrochem. 8 (2004) 416-421. © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ abstract introduction experimental results and discussion monolayer zn-ni alloy coatings optimization of cyclic cathode current densities (cccd’s) and number of layers in cmma coatings thermal stability of the monolayer and multilayer coatings thickness and hardness of the coatings corrosion study cyclic polarization study electrochemical impedance spectroscopy study surface morphology xrd study conclusions acknowledgement references anodic oxidation of oxytetracycline: influence of the experimental conditions on the degradation rate and mechanism doi: 10.5599/jese.2014.0062 203 j. electrochem. sci. eng. 4(4) (2014) 203-213; doi: 10.5599/jese.2014.0062 open access: issn 1847-9286 www.jese-online.org original scientific paper anodic oxidation of oxytetracycline: influence of the experimental conditions on the degradation rate and mechanism annabel fernandes, catarina oliveira, maria j pacheco, lurdes ciríaco and ana lopes umtp and department of chemistry, university of beira interior, 6201-001 covilhã, portugal corresponding author: e-mail: annabelf@ubi.pt; tel.: +351-275-329-259; fax: +351-275-319-730 received: august 19, 2014; revised: september 1, 2014; published: december 6, 2014 abstract the anodic oxidation of oxytetracycline was performed with success using as anode a boron-doped diamond electrode. the experiments were conducted in batch mode, using two different electrochemical cells: an up-flow cell, with recirculation, that was used to evaluate the influence of recirculation flow rate; and a stirred cell, used to determine the influence of the applied current density. besides oxytetracyclin electrodegradation rate and mineralization extent, oxidation by-products were also assessed. both the flow rate and the applied current density have shown positive influence on the oxytetracycline oxidation rate. on the other hand, the mineralization degree presented the highest values at the lowest flow rate and the lowest current density tested. the main oxidation by-products detected were oxalic, oxamic and maleic acids. keywords tetracyclines; bdd; antibiotics; pharmaceutical compounds; electrochemical degradation introduction the increasing use of drugs has become a new environmental problem, which has aroused great concern in recent years. although these compounds are found in very low concentrations in the environment, there is still a lack of information about the long-term risks that the presence of a wide variety of drugs can bring to ecosystems and to human health. these drugs are continuously introduced into the environment due to their domestic, hospital and veterinary use, and its presence has been detected in wastewaters [1-4]. its potential biological activity associated http://www.jese-online.org/ mailto:annabelf@ubi.pt j. electrochem. sci. eng. 4(4) (2014) 203-213 anodic oxidation of oxytetracycline 204 with low removal during conventional wastewater treatment processes, can lead to adverse environmental effects, including the contamination of soil and water resources [5,6]. among these drugs, tetracyclines are one of the most widely used in the prophylaxis and therapy of human and animal infections and also at subtherapeutic levels in animal feed as growth promoters [7,8]. tetracyclines are considered bacteriostatic antibiotics, although they have also various non-antibiotic properties. they are characterized by a four ring structure with a carboxamide functional group and by several ionizable functional groups [8,9]. as a result of the waste disposal, the drug is transferred to different environmental compartments (water, sediment, soil) and can contaminate trophic network, transitioning into food and cause negative effects on natural resources, including the effects on microbial community structure and selection of strains with antibiotic resistance [10-12]. the presence of tetracyclines in several environmental matrices has been investigated and the evidence of their existence has been reported [8,13-15]. this is due to the fact that antibiotics are very resistant to biodegradation.. in recent years, there has been a growing awareness about pollution caused by pharmaceutical wastes, including antibiotics [16,17]. biological processes, the most economical for wastewater treatment, have been extensively studied, but they are ineffective in the removal of recalcitrant compounds with poor biodegradability [17,18]. some physical and physicochemical techniques, such as coagulation, flocculation, adsorption, ultrafiltration and reverse osmosis, have been successfully employed to remove recalcitrant pollutants. however, these conventional treatments simply transfer pollutants from one phase to another, resulting in secondary pollution [17,19]. new technologies based on the application of advanced oxidation processes have been reported in the treatment of effluents containing tetracycline [20-37]. tetracycline degradation efficiency obtained by photo-fenton processes in the treatment of wastewaters and surface waters was reported by bautitz and nogueira [21]. results showed that the photo-fenton process under solar radiation can be applied in the degradation of tetracycline present in surface water samples or even in more complex samples, such as effluent from sewage treatment plants. however, the processes that involve photolysis should only be applied to bleached effluents, since the color prevents the efficient propagation of radiation. li et al. [23] studied the effect of different ph values on oxytetracycline degradation by ozonation in aqueous solution and found that this technique had potential to be used as a partial step in combined treatment of pharmaceutical effluents containing high concentrations of oxytetracycline. the removal rates increased as a result of high decomposition rates, favored by ph increase. however, bioluminescence results indicate that after partial ozonation, byproducts of oxytetracycline have a higher toxicity than the parent compound. jiao et al. [22] and shaojun et al. [24] studied the degradation of tetracyclines by photolysis and reported high cod removals, of 80 %, but very low toc removal, about 14 %, which indicated the production of intermediate compounds. on their studies, it was also found that the toxicity of the treated effluent was higher than in the original effluent. photocatalysis studies applied to the treatment of waters with low loads of organic matter, such as rivers, ground water and drinking water, containing tetracyclines have been performed by several authors and high removals were obtained, indicating that this method is promising for this type of waters [20,35]. electrochemical processes have shown to be effective for the treatment of effluents containing refractory and toxic organic pollutants [38-41]. furthermore, they are an effective, versatile, easy and clean technology [39,42]. for all these reasons, this technology has been applied to remove tetracycline under different experimental conditions. removals above 90 % were achieved using a. fernandes et al. j. electrochem. sci. eng. 4(4) (2014) 203-213 doi: 10.5599/jese.2014.0062 205 ti/ruo2-iro2 anodes [25] and ti/iro2 anodes [29]. despite these good results in tetracyclines removal, no significant levels of mineralization were achieved using these active anodes. in the last decade, bdd anode, a non-active anode, is being widely used. it has several advantages, namely good chemical and electrochemical stability, extended lifetime and a high overpotential for water decomposition, being known for its ability to promote complete mineralization of a wide range of organic pollutants [39,43-47] due to the hydroxyl radicals formed from water decomposition on the electrode surface. brinzila et al. [30] reported an electrodegradation study of tetracycline on bdd anode where removals of 93 % and 87 % were obtained for cod and toc, respectively. these authors also studied the influence on the degradation rate of initial ph, applied current intensity and electrolyte added [37]. it was observed that an increase in current density leads to a decrease in the current efficiency of the process and the complete removal of tetracycline was much faster in the presence of chloride ions that promoted the complete degradation of this antibiotic in 30 min. the effect of different anode materials in both electrochemical oxidation and electro-fenton processes on the oxidation of tetracycline was investigated by oturan et al. [36]. they have reported that processes using bdd anode demonstrated superior oxidation/mineralization power. almost total mineralization (toc removal up to 98 %) of 100 mg l −1 of tetracycline solutions was achieved after 6 h treatment with bdd anode. considering the good results obtained with the bdd anode, in the present study it is proposed its use in the electrochemical degradation of oxytetracycline, an antibiotic from the tetracycline family, widely used in intensive animal husbandry to treat enteric and respiratory diseases. the influence of the hydrodynamics inside the electrochemical cell on the rate of electrodegradation and mineralization of oxytetracycline was studied and the oxidation by-products were also assessed, in order to establish the degradation mechanism. experimental oxytetracycline (otc) used in this study was purchased from sigma aldrich (purity 99 %), with the chemical formula c22h24n2o9.2h2o (table 1), and used without further purification. oxytetracycline degradation experiments were conducted in batch mode using two different electrochemical cells. the first set of assays was performed in an up-flow electrochemical cell, with recirculation, composed by a bdd anode with an area of 20 cm 2 and a stainless steel cathode with identical area, using 250 ml of solution. the recirculation of the solution was enabled by a centrifugal pump, pan world magnet, model: nh-30px, pan world co., ltd. tokyo, japan, which allowed the evaluation of different flow rates: 37, 75, 100 and 120 l h -1 . the applied current density was kept constant at 20 ma cm -2 . the second set of assays was conducted in batch mode, with stirring, in a cell containing a bdd anode, with an immersed area of 10 cm 2 , and a stainless steel cathode, with identical area. 200 ml of solution were used in each run. in order to study the oxidation mechanism and identify the by-products, assays were performed applying different current densities: 2.5, 5, 7.5, 10, 20 and 30 ma cm -2 . the experimental conditions used are summarized in table 1. for all the experiments performed, the initial oxytetracycline concentration was 100±10 mg l -1 . the assays were conducted at room temperature (25±2 °c), adding as support electrolyte anhydrous sodium sulfate (merck, 99.5 %), in a concentration of 5 g l -1 . a gw, lab dc, model gps-3030d (0–30 v, 0–3 a), was used as power supply. the assays were performed in duplicate, and the values presented for the parameters used to follow the assays are the mean values. j. electrochem. sci. eng. 4(4) (2014) 203-213 anodic oxidation of oxytetracycline 206 degradation tests were followed by total organic carbon (toc) and total nitrogen (tn), measured in a shimadzu toc-v cph analyzer combined with a tnm-1 unit, by chemical oxygen demand (cod), performed using closed reflux and titrimetric method, and by ammonia nitrogen (an), using a vapodest 20s distillation system from gerhardt, according to standard procedures [48]. uv–visible absorption spectra were also performed, with measurements made between 200 and 800 nm, using a shimatzu uv-1800 spectrophotometer. high performance liquid chromategraphy (hplc) was performed using a shimadzu 20a prominence hplc system equipped with a diode array detector spd-m20a, a column oven cto-20ac and a pump lc-20ad sp. for oxytetracycline determination a rp-18 reversed phase purospher star column (250 × 4 mm (i.d.), 5 µm) was used and the elution was performed isocratically with an oxalic acid aqueous solution (10 mm): acetonitrile, 70:30 (v/v), mixture at a flow rate of 1 ml min -1 and 30 ºc. the carboxylic acids determination was made by ion-exclusion chromatography using a biorad aminex hpx-87h column (300 × 7.8 mm (i.d.)) and the elution was performed isocratically with a sulfuric acid aqueous solution (4 mm) at a flow rate of 0.6 ml min -1 and 35 °c. the selected wavelength was 354 nm for oxytetracycline and 210 nm for carboxylic acids. the reagents used were hplc grade and supplied by sigma-aldrich. all the solutions for hplc were prepared with ultrapure water obtained with milli-q system. measurements of ph were carried out with a mettler-toledo phmeter. conductivity was determined using a conductivity meter mettler toledo (seveneasy s30k). table 1. otc chemical structure and experimental conditions used in the otc oxidation assays. otc chemical structure operating mode flow rate, l h -1 stirring speed, rpm [otc]0 / mg l -1 j / ma cm -2 oh ch3 oh ch3 ch3 n oh nh2 o o oh oh o oh batch with recirculation 37 100 20 75 100 120 batch with stirring 100 100 2.5 5.0 7.5 10 20 30 results and discussion figure 1 presents variation in time of cod, toc and otc concentration for the first set of assays performed in batch with recirculation conditions at different flow rates. initial cod and toc values are slightly different for the various experiments performed, since fresh solutions were prepared for each assay to avoid otc photodegradation, and the values presented are the mean values obtained for the different replicas. up to 4 h, there is a regular decay in time of cod and toc. after that, particularly at higher flow rates, there is a decrease in the organic load removal rate. however, after 8 h degradation, for the flow rates tested (37, 75, 100 and 120 l h -1 ) the remaining cods were 17, 12, 15 and 17 %, and the remaining tocs were 8, 9 11 and 9 %, respectively, meaning that the flow rates used almost didn’t interfere with the organic load removal. similar behavior was already observed by other authors [49,50]. on the other hand, if data for the first 4 hours assay is used to calculate the toc/cod ratios (insets of figure 1), different slopes can be a. fernandes et al. j. electrochem. sci. eng. 4(4) (2014) 203-213 doi: 10.5599/jese.2014.0062 207 obtained for the different flow rates, showing the influence of this parameter on the degradation mechanism. in fact, toc vs. cod slope decreases with the increase in flow rate, showing that the increase in flow rate has a negative impact on the otc mineralization degree at the earlier stages of the electrodegradation assay. this happens because the increase in flow rate decreases the diffusion layer width, favoring the counter diffusion of the reaction intermediate products, avoiding their complete mineralization. after 4 h assay, the toc vs. cod slope changes, since the products in solution are other than otc, as can be seen by the otc concentration determined by hplc. regarding the otc concentration decay (figure 1), the electrodegradation process kinetics is almost independent of the imposed flow rate. it presents a pseudo-first order kinetic and only the assay performed at the lowest flow rate shows a lower kinetic constant (1.18 h -1 ), probably due to some diffusional hindrance at low flux and to cathodic reactions that can contribute to the overall kinetic process (including cod and toc kinetic rates). figure 1. variation of cod, toc and [otc] with time for the electrodegradation assays performed in batch with recirculation mode for the different flow rates tested. insets: variation of toc with cod along time. the samples collected during the assays were also used to run uv-vis absorption spectra and results for the flow rates of 37 and 120 l h -1 are presented as the absorbance variation in time measured at 276 and 355 nm, the two major otc characteristic absorption bands (figure 2, a and b). similarly to cod and toc, the absorbance decay at both wavelengths increase with flow rate, being the decay at 355 nm faster than at 276 nm. this means that in the otc molecule the ring containing the n-groups (responsible for the absorbance at 276 nm) is not so easily opened as the other rings, or the intermediate products formed also absorb at this wavelength, thus contributing to increase the absorbance at 276 nm. j. electrochem. sci. eng. 4(4) (2014) 203-213 anodic oxidation of oxytetracycline 208 regarding nitrogen removal from solution (figure 2c), there is only a very slight decay in the total nitrogen amount and an increase followed by a decrease in the ammonium nitrogen concentration during the 8 h assays. this means that the organic nitrogen is slowly converted into ammonium and, only when the organic load is very small, ammonium is oxidized to nitrogen volatile species, being this conversion higher for higher flow rates, probably because cod and toc removal rates also increase with flow rate. figure 2. variation with time of absorbance, measured at (a) 276 and (b) 355 nm, (c) an and tn for the electrodegradation assays performed in batch mode with recirculation for two different flow rates tested: 37 and 120 l h -1 . the influence of the current density on the otc degradation rate was studied in a stirred batch system and results for the decays in cod, toc and absorbance, measured at 276 and 355 nm, are presented in figure 3. the absolute cod and toc removals increased with current density, mainly due to the increase in indirect oxidation promoted by the hydroxyl radicals, formed when the applied current exceeds the limiting current corresponding to the organic load of the solution. simultaneously, the formation of persulfate radicals can also happen, since sulfate was the chosen electrolyte. the absorbance variation measured at the otc characteristic absorption bands presents a behavior that seems dependent on the applied current density. for the lowest applied current, after 4 h assay there is a divergence between the absorbance curves at 276 and 355 nm, with an increase in the absorbance at 276 nm, meaning that products that absorb at this wavelength are being formed. these products must be resistant to oxidation, since the curve related with absorbance at 355 nm suffers a sudden decay that must be related with an increase in the degradation of the remaining otc, since at this applied current density the otc decay is slower, as will be discussed below. for higher applied current densities, there is a separation between the absorbance curves measured at the two characteristic wavelengths that increases with the applied current density, due to reasons already discussed. this fact must also be a consequence of the different mineralization degree for the different experimental conditions, as can be observed in figure 4. in fact, the mineralization degree decreases with applied current densities between 2.5 and 7.5 ma cm -2 , showing a smaller increase for higher current densities. this behavior must be related with a. fernandes et al. j. electrochem. sci. eng. 4(4) (2014) 203-213 doi: 10.5599/jese.2014.0062 209 hydroxyl radicals’ indirect oxidation and the diffusion hindrance promoted by the oxygen evolution at higher applied current densities. figure 3. cod and toc decays with time for the electrodegradation assays performed in batch with stirring mode for the different applied current densities tested. insets: relative absorbance decays in time, measured at 276 and 355 nm. regarding the electrodegradation kinetics (figure 5), the otc concentration decays show a dependence on the applied current density. as previously observed for the assays run in batch mode with recirculation, it presents a pseudo-first order kinetic, with the kinetic constant (figure 5, symmetric of the slopes of the adjusted equations) increasing with current density. j. electrochem. sci. eng. 4(4) (2014) 203-213 anodic oxidation of oxytetracycline 210 figure 4. toc vs cod variation for the electrodegradation assays performed in batch mode with stirring at different applied current densities. figure 5. variation of otc concentration with time for the electrodegradation assays performed in batch mode with stirring at different applied current intensities. in order to compare the values obtained in both operating systems used, the slope obtained for 20 ma cm -2 was converted to the same units as those in figure 1, giving 0.71 h -1 , showing that in the applied experimental conditions batch with stirring operating mode is less efficient than batch with recirculation mode, probably due to the importance of the otc diffusion to the reaction zone in the degradation process. the concentration of the main reaction intermediate products was also followed by hplc. besides the main carboxylic acids detected, oxalic, oxamic and maleic acids and vestiges of formic acid were also identified, particularly at higher current densities. in figure 6, the variation in time of the oxalic, oxamic and maleic acids is presented for the current densities of 2.5, 10 and 30 ma cm -2 , for the two first current densities between 0 and 8 h and for 30 ma cm -2 between 0 and 12 h. when otc is the main organic compound in solution, there is an increase in those intermediates concentration. after that, their concentration start to decrease and they are mineralized. the existence of only small dicarboxylic acids with conjugated double bonds points to a degradation mechanism characteristic of indirect oxidation, where the parent molecule is attacked in many different places, leaving unchanged double conjugated bond systems, less easily oxidized. a. fernandes et al. j. electrochem. sci. eng. 4(4) (2014) 203-213 doi: 10.5599/jese.2014.0062 211 figure 6. influence of the current density on the variation with time of the concentration of some of the intermediate products formed during the otc electrodegradation, performed in batch with stirring mode at different applied current intensities. conclusions in this study oxytetracycline was successfully degraded through anodic oxidation with a bdd anode. the investigation of the influence of the experimental conditions on the otc degradation allowed drawing the following conclusions: degradation mechanism occurred mainly through indirect oxidation. cod and toc removals increased with current density, being almost independent of the flow rates tested, meaning that the current density plays an important role in the otc oxidation rate, mainly due to the formation of hydroxyl and persulfate radicals that are the main species responsible for the indirect otc oxidation. the increase in flow rate has a negative impact on the otc mineralization degree. the organic nitrogen is slowly converted into ammonium and, only when the organic load is very small, ammonium is oxidized to nitrogen volatile species, being this conversion higher for higher flow rates. the electrodegradation process presents pseudo-first order kinetic and the kinetic constant increases with current density. for flow rates higher than 75 l h -1 , the process kinetics is almost j. electrochem. sci. eng. 4(4) (2014) 203-213 anodic oxidation of oxytetracycline 212 independent of the imposed flow rate. however, for lower flow rate the diffusion hindrance leads to lower otc removal rates. the main by-products detected were oxalic, oxamic and maleic acids, whose concentration increased while otc was the main organic compound in solution. after that, the 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sensor using iron-doped graphitic carbon nitride as transducer matrix: analysis of ciprofloxacin in blood samples:} http://dx.doi.org/10.5599/jese.1112 59 j. electrochem. sci. eng. 12(1) (2022) 59-70; http://dx.doi.org/10.5599/jese.1112 open access : : issn 1847-9286 original scientific paper development of ciprofloxacin sensor using iron-doped graphitic carbon nitride as transducer matrix: analysis of ciprofloxacin in blood samples hattna shivarudraiah vedhavathi1; ballur prasanna sanjay1; mahesh basavaraju2; beejaganahalli sangameshwara madhukar1 and ningappa kumara swamy1, 1department of chemistry, jss science and technology university, mysuru570006, karnataka, india 2department of chemistry, jss academy of technical education, bengaluru560060, karnataka, india corresponding author: kumaryagati@gmail.com; phone: +91-9741027970, fax: 0821-2548290 received: september 15, 2021; accepted: november 6, 2021; published: november 17, 2021 abstract in the present work, we have synthesized an iron-decorated graphitic carbon nitride (fe@g-c3n4) composite and employed it for electrochemical sensing of ciprofloxacin (cfx). the physicochemical characteristics of the fe@g-c3n4 composite were analyzed with x-ray diffraction (xrd), scanning electron microscopy (sem), and energy-dispersive x-ray diffraction (edx) spectroscopy methods. further, the pencil graphite electrode (pge) was modified with fe@g-c3n4 composite to get pge/fe@g-c3n4 electrode and characterized the resultant electrode by cyclic voltammetry (cv) and electrochemical impedance spectroscopy (eis). differential pulse voltammetry (dpv) was employed to determine the effect of concentration and interferents. the modified pge/fe@g-c3n4 electrode demonstrated the exceptional electrochemical performance for cfx identification and quantification with a lod of 5.4 nm, a wide linear range of 0.001-1.0 µm, and high sensitivity of 0.0018 µa mm-1 cm-2. besides, fe@g-c3n4 modified pge showed remarkable recovery results in qualitative analysis of cfx in human blood specimens. this research advocates that the fe@g-c3n4 composite acts as an excellent transducer material in the electrochemical sensing of cfx in blood and standard samples. further, the proposed strategy deduces that the pge/fe@g-c3n4 sensor can be a prospective candidate for the dynamic determination of cfx in blood serum and possibly ratified as an exceptional drug sensor for therapeutic purposes. keywords quinolone, drug sensor, electrochemical sensor, electrode matrix, differential pulse voltammetry introduction the quinolones are a class of broad spectrum of drugs due to their excellent activity against gramnegative pathogens. they are the choice for patients with intra-abdominal infections together with anti-anaerobic agents [1-3]. ciprofloxacin (cfx) continues to be the most efficient quinolone in http://dx.doi.org/10.5599/jese.1112 http://dx.doi.org/10.5599/jese.1112 mailto:kumaryagati@gmail.com j. electrochem. sci. eng. 12(1) (2022) 59-70 ciprofloxacin sensor 60 veterinary and human medication [4]. an immoderate dosage of cfx residues can purport significant antagonistic effects besides causing the ailments such as skin and respiratory infections, chronic bacterial prostatitis, and nosocomial pneumonia [5]. the european union has set the maximum residue level of ciprofloxacin in milk [6] to be 100 ng ml-1. the presence of cfx and other antimicrobials in the environment is a ground for attention, owing to the possible genesis of resistance to antibiotics. therefore, developing alternate sensitive and precise sensors to examine the antibiotics in the biological samples (blood) is a dynamic field of the probe [7]. to ascertain the cfx analytical models, like high-performance liquid chromatography, spectrophotometry, capillary electrophoresis, liquid chromatography-mass spectroscopy, spectrofluorimetry, immunoassays, chemiluminescence, impedance spectroscopy, and voltammetry, are performed. although these methods are sensitive, they are usually elaborate and time-consuming. moreover, continued specimen pretreatment is essential for the test. electrochemical methods (impedance spectroscopy, voltammetry) are methods of choice due to their low energy requirement, affordable costs, extraordinary sensitivity, expeditious, and user-friendliness [8-11]. many biomolecules, including active ingredients in pharmacological formulations and human body fluids, were analysed by electrochemical methodology [12–18]. diverse designs, procedures, and materials are engaged to promote the sensitivity and selectivity of an electrochemical sensor [15,16]. in the improving electrochemical sensor, nanomaterials were frequently used as transducer elements [19-27]. previously, carbon-based nanomaterials such as graphene, graphene oxide (go), multi-walled carbon nanotubes (mwcnts), reduced graphene oxide (rgo), graphene nanoribbons (gnrs), single-walled carbon nanotubes (swcnts), and carbon nanofibers began multiple favourable traits to drive direct electron transfer within the transducer and the electrode surface [28]. among them, g-c3n4 is one of the most promising materials attributable to its unusual two-dimensional graphene-like structure, non-toxicity, extraordinary chemical endurance, easy accesssibility, and photoresponsivity in the visible-light region [29]. concurrently, metallic nanoparticles (mnps) in conjunction with carbonaceous substances in various domains like sensors, batteries, drug delivery, medical, solar cells, etc. mnps possess unique attributes like charge surface-to-volume ratio, superior conductivity, and the capacity to function as an electron-conducting tunnel to magnify the direct electron transfer in an electrochemical sensor [30,31]. iron (fe) nanoparticles are acknowledged for their excellent electrocatalytic activity, low cost, non-toxicity, high stability, and fair conductivity, indicating that combining fe with g-c3n4 might improve the sensitivity of the intended sensor [31]. the combination of iron and g-c3n4 can potentially magnify the electrochemical sensing characteristics of the electrode surface. the decoration of fe on the g-c3n4 surface can demonstrate a stable sensing platform with good electron transfer characteristics, which ideally suit the construction of novel electrochemical sensing systems. an electrochemical sensor was described with this motivation by modifying a pencil graphite electrode (pge) with fe@g-c3n4 nanomaterials. this work demonstrates a new promising electroactive drug sensor for the qualitative and quantitative detection of cfx using g-c3n4 and fe@g-c3n4. the proposed sensor manifested an extensive linear detection range, high sensitivity, low detection limit, and selectivity concerning the detection of cfx. moreover, the sensor considers high accuracy, extensive shelf-life, and reproducibility, indicating that the fe@g-c3n4 is a proper matrix for the sensor fabrication. the realtime application of the advanced sensor is further validated with the analysis of cfx in human blood specimens. finally, a comprehensive comparison with earlier sensors highlighted the performance h. s. vedhavathi et al. j. electrochem. sci. eng. 12(1) (2022) 59-70 http://dx.doi.org/10.5599/jese.1112 61 of pge/fe@g-c3n4. therefore, it is anticipated that the drug sensor can be a useful tool for biomedical and diagnostic applications. experimental reagents and equipment analytical grade cfx was procured from sigma-aldrich (99 %), whereas urea (98 %), ferric (ii) chloride (98 %), acetone (99 %), ascorbic acid (aa) (99 %), uric acid (ua) (99 %) and glucose (99 %) were acquired from fischer scientific and used as such without further purification. the morphology and elemental composition of the developed sensor were described by scanning electron microscopy (sem), and energy dispersive x-ray analysis (edx), and the suggested g-c3n4 and fe@ gc3n4 samples were characterized by a powder x-ray diffractometer (proto axrd) to authenticate the physical traits like structure, crystallinity, lattice planes, etc. all the voltammetric readings (cv, eis, and dpv) are conducted utilizing a biologic science potentiostat (model sp-150) with the threeelectrode configuration. synthesis of fe@g-c3n4 the synthesis of g-c3n4 was carried out by urea pyrolysis (20 g) using a lid crucible (isotemp programmable muffle furnace 650-750 series, fisher scientific) in a muffle furnace at 550 °c for 3 h [31]. finally, the doping of fe on g-c3n4 nanosheets was made by mixing calculated quantities of ferric(ii) chloride and 0.6 g of g-c3n4 nanosheets in 50 ml of acetone with constant stirring. the resulting suspension was agitated for two hours at ambient temperature, centrifuged for 15 min at around 6000 rpm, and rinsed many times with acetone to eliminate aggregates. the color of the withered specimens slightly shifted from pale yellow to imperceptibly reddish-brown upon the doping of iron [29]. fabrication of cfx sensor with fe@g-c3n4 a cylindrically shaped pencil graphite rod with a diameter of 3 mm (surface area: 0.07068 cm2) was utilized as a working electrode. further, the electrode was modified by taking a tiny portion of the rod and polished along one face. a copper wire was fastened to secure the electrical contact flanked by the electrode and the potentiostat. to obtain a shiny surface, pge was polished using emery paper (80 and 300 grit) and consequently, with the electrode polishing solution containing alumina and silica using electrode polishing tool kit (pk-3 brand kit). then, the polished pge was sonicated and eventually washed with milli-q water and dried at room temperature to eliminate loosely bounded shreds. the prepared bare pge was further deposited with 3 µl of 5 mg ml-1 stock solution of fe@ g-c3n4 by drop-casting method and eventually, the electrode was dried at ambient temperature to get the working electrode pge/fe@g-c3n4. electrochemical studies the electrochemical investigations were performed in a three-electrode cell, where pge was used as a working electrode (surface area: 0.07068 cm2), saturated calomel electrode as the reference electrode, and platinum wire as the counter electrode. all electrochemical characteristics of bare and modified pges were performed in phosphate buffer solution (pbs) of ph 7.0. the charge transfer at the electrode/electrolyte interface of the altered electrode was studied in the frequency range of 100 khz to 0.1 hz using the eis technique, with 5 mm [fe(cn)6]3-/4as the electrochemical probe. the probe of eis had operated in 0.1m pbs (ph 7.0) constituting 1 mm [fe(cn)6]4-/3-, and at an amplitude of 5 mv and frequencies within 100 khz and 0.1hz range. http://dx.doi.org/10.5599/jese.1112 j. electrochem. sci. eng. 12(1) (2022) 59-70 ciprofloxacin sensor 62 preparation of sample for real analysis the serum samples collected from normal individuals (taking their inscribed consent) were refrigerated till examination. 5 ml of serum was treated with an equivalent volume of methanol as a serum desaturating and precipitating agent. the conduits were vortexed for 10 min and then centrifuged for 40 min at 5000 rpm to eliminate the protein residues. the supernatants were diluted up to 10 ml with the 0.1 m pbs buffer solution of ph 7.0. the standard addition method was employed for calculating the recoveries of the spiked cfx in human serum. the percentage recovery and detection precision were computed based on the known amount of spiked cfx (r) and empirical values (e) using equations 1 and 2. accuaracy, % 100 r e r − = (1) recovery, % 100 r e = (2) results and discussion x-ray diffraction and ft-ir studies figure 1a shows xrd patterns of the nanosheets of the integrated g-c3n4 and fe@g-c3n4. a strong diffraction peak at 27.3o demonstrates strong interlayer interactions of aromatic rings, indexed as the (002) planes for g-c3n4. the smaller diffraction signal at around 13.1o, listed as (100), is associated with the in-plane structural perpetual motif, i.e., the continuous tri-s-triazine structures [32]. besides, the depth of the (002) peak had substantially diminished and expanded for g-c3n4 nanosheets [33]. the resulting doping with fe evidenced no change of the crystal phase of g-c3n4. the location of diffraction peaks for nanosheets fe@g-c3n4 switched to a steadily higher angle for nanosheets of g-c3n4. the peak intensity diminished, and the diffraction peak width broadened for iron content, implying the presence of excess fe species caused the host-guest interactions and polymeric condensation inhibition. it is further evident that the iron is chemically coordinated to g-c3n4 via fe-n bonds [34]. . 2 / o wavenumber, cm-1 figure 1. a xrd data of g-c3n4 and fe@g-c3n4, and b ir spectrum of g-c3n4 and fe@g-c3n4 figure 1b shows the ft-ir spectra of the g-c3n4 and fe@g-c3n4 nanosheets. from figure 1b, the extended absorption band at nearly 3155 cm-1, assigned to the stretching vibrational modes of surplus n–h components connected with uncondensed amino groups [35,36], can be identified. the h. s. vedhavathi et al. j. electrochem. sci. eng. 12(1) (2022) 59-70 http://dx.doi.org/10.5599/jese.1112 63 peak at 1633 cm-1 is classified as vibrational stretching mode, whereas aromatic c-n stretching vibrations of heterocyclic rings matched with the bands at 1415, 1400, and 1234 cm-1 [26]. the specific particular peak at 807 cm-1 confirmed the s-triazine ring system [37]. further, it is evidenced from figure 1b that the intensity of the peaks reduced with an increase in fe content in the fe@g-c3n4, and the principal characteristic bands of g-c3n4 nanosheets change to smaller wavenumbers (redshift) intimates that the c-n and c=n bonds are weakened [29] sem and eds analysis the g-c3n4 and fe@g-c3n4 synthesis was demonstrated by sem and eds study. figures 2a and 2b denote the sem image of integrated g-c3n4 and fe@g-c3n4 nanocomposite. the micrographs were obtained at 3,000 magnifications by an expediting voltage of 5.0 kv led. the sheets-like morphology evidences a greater surface area for the catalytic reactions between target and transducer interface [38]. figure 2c shows the eds spectrum of the synthesized material, and it reveals elemental composition. the eds data illustrated the presence of carbon (c) 51.92 %, nitrogen (n) 31.24 %, oxygen (o) 2.96 %, and iron (fe) 13.88 % in the fe@g-c3n4 sample, indicating that fe-doped g-c3n4 has a pretty high rate of purity, and it comprises solely four elements. these essential considerations made on eds interpretation insinuate the purity of the substance. energy, kev figure 2. a sem image of g-c3n4, b sem image of fe@g-c3n4, (c) edx of fe@g-c3n4, (inset: elemental composition) electrochemical assessment of the modified pge the electrochemical response of the modified pge was monitored by cv using a 5 mm [fe(cn)6]3/4as an electrochemical mediator in 0.1 m pbs buffer (ph 7.0). the cv of the bare pge shows a in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1112 j. electrochem. sci. eng. 12(1) (2022) 59-70 ciprofloxacin sensor 64 well-defined quasi reversible oxidation and reduction peaks with a peak-to-peak separation (δep) of 201.6 mv (figure 3a, curve a). the peak current increased after the deposition of 3 µl fe@g-c3n4, which resulted in peak separation (δep) of 271.2 mv (figure 3a, curve b). the increase in current response and stability of the cv curve suggests the successful deposition of fe@g-c3n4 on the pge electrode surface. eis spectra of pge and pge/fe@g-c3n4 recorded in 1 mm [fe(cn)6]4-/3pbs solution figure 3b are displayed in the nyquist diagram. from the eis data, the charge transfer resistance (rct) could be calculated from the best fit of the randles electrical equivalent circuit. the rct for bare pge is 78.73 kω, and after the addition of fe@g-c3n4, rct is reduced to 19.41 kω on account of the higher rate of electron transfer between the redox probe and electrode surface. reduction in rct values on the deposition of fe@g-c3n4 is clearly in agreement with cv results, and it further confirms the successful electrode deposition. randles–sevcik equation (equation 3) was used to validate the improved catalytic response of modified pge in terms of increased active electrode surface area [39]. ip = (2.69 105)n3/2a0dr1/2v1/2 (3) where ip is the anodic peak current of pge/fe@g-c3n4 (ip = 1.38×10-4 a), and n is the number of electrons transferred in the redox reaction of cfx (n = 2). a / cm2 is the electroactive surface area to be determined, dr (cm2/s) is the solution diffusion coefficient (6×10-6 for [fe (cn)6]4-). c0 (mol/cm3) is the concentration of the reaction species in the electrolyte (10-6 for [fe (cn)6]4-), and ν (v/s) is the scan rate [40]. from this equation, the pge/fe@g-c3n4 active electrode surface area was calculated to 3.94 cm2, ensuring the high electrocatalytic surface area for the modified electrode [41]. figure 3. a cvs of 5 mm [fe(cn)6]3-/4solution obtained held at (a) bare pge, (b) pge/fe@g-c3n4(3 µl); b nyquist diagrams of eis data of (a) bare pge, (b) pge/fe@g-c3n4 (3 µl) effect of scan rate and ph figure 4a shows the effect of the scan rate on the modified pge/fe@g-c3n4 for various scan rates like from 25 up to 375 mv s-1 in 10 μm solution of cfx. the oxidation peak current increases with the scan rate. figure 4b shows the linear relationship between the current peak height and the square root of the scan rate with the regression coefficient of r2=0.9966. it is because the larger surface area facilitates faster electron transfer. further, the regression analysis of log ip versus log ν plot gives a relation of log ip=0.7601 log ν + + 1.7926; r2= 0.9973 with the slope close to 1. the value of slope of log ip versus log v confirms that the electrode process is dominantly diffusion-controlled [41]. h. s. vedhavathi et al. j. electrochem. sci. eng. 12(1) (2022) 59-70 http://dx.doi.org/10.5599/jese.1112 65 to know the effect of ph on the electrochemical properties of the proposed sensor, the electrochemical performance of the fe@g-c3n4 decorated pge electrode was investigated in different phs in the range 3 9) in the presence of 10 μm cfx in pbs buffer solution. from figure 4c, the current response of the sensor at different ph values reveals that the oxidation peak reached the maximum at ph 7.0. therefore, ph 7.0 was adopted for all further analyses. potential, v vs. sce v1/2 / mv s-1)1/2 figure 4. a cvs of pge/fe@g-c3n4 in 0.1 m pbs (ph 7) containing 10 μm cfx at different scan rates; b plot of anodic peak current vs. v-1/2 (c) effect of ph on the response of pge/fe@gc3n4 modified electrode electrochemical determination of cfx figure 5a shows cyclic voltammograms of pge/fe@g-c3n4 in the presence of different concentrations of cfx (1-100 µm) in nitrogen saturated pbs (ph 7.0) at a scan rate of 50 mv s-1. with the increase in the concentration of cfx, the oxidation peak current proportionately increases at the fe@g-c3n4 modified electrode. oxidation of cfx is the electrochemical reaction occurring at the electrode/electrolyte interface, as shown in scheme 1. the anodic peak current increased linearly from 1 to 100 µm of cfx with a correlation coefficient (r2) of 0.9675. hence, this proves the authenticity of the sensor performance. the dpv experiment was conducted in 0.1 pbs solution in the potential range -0.1 to 1.5 v at smaller concentrations of cfx. fig 5b illustrates the dpv current response from 1 to 1000 nm of cfx, and it is clear from fig 5b that there is a linear relationship between current and cfx concentration with a correlation coefficient of r2 = 0.9968. the data from dpv experiments were used to compute the analytical quantities of the sensor i.e., the limit of detection (lod), sensitivity, and quantification limit (loq) (equations 4, 5, and 6) [42]. c u rr e n t, m a http://dx.doi.org/10.5599/jese.1112 j. electrochem. sci. eng. 12(1) (2022) 59-70 ciprofloxacin sensor 66 potential, v vs. sce potential, v vs. sce figure 5. a cvs of pge/fe@g-c3n4 electrode at various cfx concentrations (1 to 100 µm) in 0.1m pbs at ph 7, inset: calibration plot of current vs cfx concentration; b dpvs of the pge/fe@g-c3n4 electrode in the presence of varying cfx concentrations (1 to 1000 nm) in 0.1 m pbs solution; inset: peak current vs cfx concentration scheme 1. the electrocatalytic interaction at the interface is represented using chemical equations lod = 3 / s (4) sensitivity = i / a (5) loq = 10 / s (6) here, σ represents the standard deviation of the blank, and s indicates the slope of the calibration plot (inset: figure 5b). the sensitivity, loq, linear range, and lod determined using the above experimental data are 0.0596 µa mm-1cm-2, 0.0018 µm, 0.001 to 1.0 µm, and 5.4 nm, respectively. the overall analytical performance of the suggested fe@g-c3n4 based sensor is in accordance with those already reported in the literature (table 1) [43-47]. the superior performance characteristics exhibited by the pge/fe@g-c3n4 sensor are explicitly related to the composite matrix's synergic effects [48]. this suggests that the as-developed sensor can be a promising tool in the analysis of cfx. table 1. comparison of different electrochemical sensors for the determination of cfx matrix linear range of detection of cfx, µm lod, nm references mgfe2o4-mwcnt/gce 0.1 1000 10 [43] mwcnt-gce 40 1000 6000 [44] tio2/pb/aunps/cmk3/nafion/ge 1 10 108 [45] β–cd/mwcnt/gc 10 80 5 [46] boron doped diamond electrode 0.15 2.11 50 [47] pge/fe@g-c3n4 0.001-1.0 5.4 proposed work effects of interferents the proposed sensor was subjected to radical scavenging experiments to affirm its selectivity. the dpv responses were recorded after the additions of 50 µm of some common interferents like aa, ua, h. s. vedhavathi et al. j. electrochem. sci. eng. 12(1) (2022) 59-70 http://dx.doi.org/10.5599/jese.1112 67 glucose, ca2+, and mg2+ to 10 µm cfx in 0.1 pbs buffer solution. the obtained dpv responses are exhibited in figure 6. figure 6 discloses no significant variation in the current peaks despite the residence of interferents, implying the selectivity and robustness of the sensor for field purposes. potential, v vs. sce figure 6. dpvs of pge/fe@g-c3n4 electrode in the presence of 10 µm cfx and 50 µm additions of interferents such as uric acid, ascorbic acid, glucose, ca2+, and mg2+ in pbs buffer (ph 7.0) repeatability, reproducibility, and stability these experiments are crucial to argue on the sensor’s practical applicability and reliability. the repeatability of the sensor performance was tested by measuring the cv response for 10 µm cfx in 0.1m pbs solution for twenty electro-analytical cycles between −0.1 to 1.5 v at a scan rate of 50 mv s-1. further, we examined the reproducibility of the pge/fe@g-c3n4 electrodes by preparing a set of five distinct electrodes using a method described in the experimental section. the current response of these electrodes was measured by cv in 0.1 m pbs comprising 10 µm cfx. the estimated magnitude of the current response under identical circumstances depicts comparable electrochemical properties for the sensor with a suitable shift in peak current, demonstrating an agreeable reproducibility as apparent from fig 7a. figure 7. a the peak current measured for 10 µm cfx in 0.1m pbs (ph) with five separately tailored pge/fe@g-c3n4 to symbolize the sensor’s repeatability; b stability of the sensor in the presence of 10 µm cfx the storage stability of the pge/fe@g-c3n4 electrode had been determined steadily for up to twenty days, and the results are shown in figure 7b. the current response for 10 µm cfx was http://dx.doi.org/10.5599/jese.1112 j. electrochem. sci. eng. 12(1) (2022) 59-70 ciprofloxacin sensor 68 monitored at regular intervals, and the developed sensor retained 100, 98.01, 97.13, 97.02, and 96 % of the initial current response after 0, 5, 10, 15, and 20 days of storage, respectively, suggesting that the developed sensor exhibits a high level of stability in the detection of cfx. real sample analysis the potency of the proposed sensor in practical applications was ascertained by analyzing cfx in human blood specimens. the actual samples were diluted with pbs in equal proportions, accompanied by spiking a known quantity of cfx to them. the responses were measured, and the observed percentage of the recovery is registered in table 2. as noted, the results attained are good, with insignificant errors and hence the developed sensor might be used for the determination of cfx in biological fluids. it ratifies the generated sensor for primary specimen analysis. the worthy administration characteristics buttressed by the pge/fe@g-c3n4 sensor are completely ascribed to the synergic effect of the composite matrix. table 2. detection of cfx using proposed fe@g-c3n4 sensor in blood samples sample amount of cfx spiked, nm amount of cfx found, nm recovery, % accuracy, % a 100 98.9 100.5 100.1 b 100 100.6 99.7 99.8 conclusion in the present work, we developed an electrochemical sensor using fe@g-c3n4 composite as a working electrode matrix. the physical, chemical and electrochemical investigations of the fe@g-c3n4 matrix confirmed the sensor's stability, conductivity, and electrocatalytic nature. it has the advantage of a low detection limit (5.4 nm) and a wide linear range (0.001-1.0 µm) in the detection of cfx. the offered sensor manifested an exceptional selectivity, sensitivity (0.0596 µa mm-1 cm-2) and reproducibility regarding the cfx determination. this sensor proved comparatively improved performance than various cfx sensors reported earlier in the literature. remarkably, the success had interlaced by the cost-effective matrix and optimized material usage of the sensor. the parameters including storage stability, reproducibility, and repeatability were studied. the pge/fe@g-c3n4 sensor may be an alternative to the reported sensors for detecting and quantifying 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https://doi.org/10.1016/j.matchemphys.2019.05.034 https://doi.org/‌10.1039/‌c5ay01619c https://doi.org/10.1016/j.chemo‌sphere.2021.‌132153 https://doi.org/10.1016/j.chemo‌sphere.2021.‌132153 https://doi.org/10.1039/c9ay01468c https://doi.org/10.2116/analsci.28.705 https://doi.org/10.1016/j.jelechem.2020.114574 https://doi.org/10.1080/‌10934529.‌2016.1258864 https://doi.org/10.1080/‌10934529.‌2016.1258864 https://doi.org/10.1016/j.sna.2018.07.049 https://creativecommons.org/licenses/by/4.0/) inhibition of carbon steel corrosion by 11-aminoundecanoic acid doi:10.5599/jese.242 157 j. electrochem. sci. eng. 5(3) (2015) 157-172; doi: 10.5599/jese.242 open access : : issn 1847-9286 www.jese-online.org original scientific paper inhibition of carbon steel corrosion by 11-aminoundecanoic acid saad ghareba, simon kwan*, sasha omanovic* department of chemical and petroleum engineering, al-mergib university, alkhums libya *department of chemical engineering, mcgill university, montreal, quebec, canada corresponding author: smghareba@elmergib.edu.ly received: october 18, 2015; revised: november 24, 2015; accepted: november 27, 2015 abstract the current study reports results on the investigation of the possibility of using 11aminoundecanoic acid (aa) as an inhibitor of general corrosion of carbon steel (cs) in hcl under a range of experimental conditions: inhibitor concentration, exposure time, electrolyte temperature and ph and cs surface roughness. it was found that aa acts as a mixed-type inhibitor, yielding maximum inhibition efficiency of 97 %. the adsorption of aa onto the cs surface was described by the langmuir adsorption isotherm. the corresponding apparent gibbs free energy of aa adsorption on cs at 295 k was calculated to be −30.2 kj mol –1 . the adsorption process was found to be driven by a positive change in entropy of the system. pm-irras measurements revealed that the adsorbed aa layer is amorphous, which can be attributed to the repulsion between the neighboring positively charged amine groups and a high heterogeneity of the cs surface. it was also found that the aa provides very good corrosion protection of cs of various surface roughness, and over a prolonged time. keywords corrosion inhibition; self-assembled monolayers; adsorption; acidic medium introduction acidization of a petroleum oil well is one of the important stimulation techniques for enhancing oil production [1]. in general, acid solutions are used in pickling processes, industrial acid cleaning, oil and gas well acidizing and for removal of rust, scale and corrosion products [2,3]. the most commonly-used acids are: hydrochloric acid (hcl), hydrofluoric acid (hf), acetic acid (ch3cooh), formic acid (hcooh), sulfamic acid (h2nso3h), and chloroacetic acid (clch2cooh). however, choice of the acid for a given situation depends on the underground reservoir characteristics and specific intention of the treatment [4]. in oil fields, hcl solution is recommended as the cheapest http://www.jese-online.org/ mailto:smghareba@elmergib.edu.ly j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 158 way to dissolve calcium carbonate scale inside the pipelines under most conditions [5]. however, hcl is a strong aggressive (corrosive) medium for oil and gas well equipment. thus, the effective way to reduce its corrosion attack on tubing and casing materials is to inject a suitable corrosion inhibitor which could be added to the acid solution during the acidization process [4,6-9]. this corrosion control technique is one of the most effective and economic methods for protection of metal corrosion in acidic media, common to the oil industry. corrosion inhibition is the most common way of mitigating the internal corrosion in industrial facilities, especially steel-made equipment. there are various types of inhibitors that tend to decrease the corrosion rate of steel and iron in acidic solutions. the use and development of new corrosion inhibitors have been continuously increasing over the years. however, due to environmental restrictions, certain bioorganic and naturally occurring molecules are the most obvious candidates for this type of corrosion inhibitors[10]. therefore, the biodegradability and nontoxicity degree of a corrosion inhibitor has also become one of the major selection requirements [11]. amino acids are example of these compounds. they are environmentally friendly [12], nontoxic, relatively very cheap, and easy to produce in purities greater than 99% [13]. therefore, the aim of the research presented in this work was to investigate the possibility of using 11-aminoundecanoic acid (aa), scheme 1, as a corrosion inhibitor for carbon steel in acidic media. aa was previously investigated as a corrosion inhibitor for steel, but in an alkaline solution containing chlorides[14]. in the current work, the corrosion efficiency of aa was investigated in hcl, as a function of inhibitor concentration, sample exposure time, electrolyte ph and temperature, and carbon steel surface roughness. 2. experimental 2.1. chemicals and solutions the corrosion inhibitor, 11-aminoundecanoic; nh2(ch2)10co2h (aa), 97% pure, was purchased from sigma-aldrich canada, ltd (product no. a82605) and was used as received without further purification. corrosive solutions (electrolytes) containing hydrochloric acid were prepared by diluting concentrated hcl (37%) to a required concentration using deionized water (18.2 mω cm resistivity). the inhibitor stock solution was prepared by dissolving the required amount of aa in the corrosive electrolyte. 2.2. electrochemical/corrosion cell and equipment a standard three-electrode electrochemical/corrosion cell was utilized in all electrochemical experiments. the counter electrode was a graphite electrode. the reference electrode was a saturated calomel electrode (sce). all potentials in this paper refer to the sce. the working electrode was prepared from a carbon steel (cs) rod (mcmaster-carr company product no. 8920k13), and sealed with epoxy resin to give a two-dimensional surface (0.713 cm 2 ) exposed to the electrolyte. all the surface-area-dependent values reported in this paper (current, impedance, resistance) are normalized with respect to the geometric surface area of the electrode. the chemical composition of the used cs is given in table 1. electrochemical measurements were performed using an autolab potentiostat/galvanostat/fra pgstat 30 controlled by fra2 and gpes v.4.9 software. chemical identification of the adsorbed aa layer was done by employing polarization modulation infrared absorption reflection spectrometry (pm-irras), using a bruker ftir spectrometer, tensor 27/pm50, equipped with an external polarization-modulation module and liquid nitrogen cooled mct detector. to s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 159 evaluate the sample surface morphology, scanning electron microscopy (feg-sem phillips xl30) was employed. the surface roughness of the carbon steel surface was measured using dektak3 st surface profile measuring system, contact profilometer (sloan technology). table 1. chemical composition of carbon steel used in this study. the data were provided by mcmaster-carr company. element fe c mn si p s content, wt. % balance 0.15–0.20 0.60–0.90 0.15–0.30 0–0.04 0–0.05 2.3. experimental methodology prior to each experiment, the working electrode surface was treated with different grit sand papers up to a 4000-grit, to give a mirror-like surface. for the surface roughness studies the cs samples were wet-abraded with different grit sand papers including 240, 320, 400, 600, 2400 and 4000 grits. after each abrasion step, the cs sample was thoroughly rinsed with deionized water. after this, the sample was degreased with ethanol in an ultrasonic bath (~5 min) and then rinsed with deionized water. the sample (electrode) was then immersed in the test electrolyte and equilibrated for 1 h at open-circuit potential (ocp), followed by a specific type of experiment. in electrochemical impedance spectroscopy (eis) measurements, in order to ensure complete characterization of the interface and surface processes, eis measurements were made at ocp in a wide frequency range, from 100 khz to 10 mhz, with the ac root-mean-square voltage amplitude of ±10 mv. tafel polarization measurements were performed after ocp measurements by polarizing the working electrode from -200 mv to +200 mv with respect to measured ocp, at a scan rate of 1 mv s –1 . weight loss measurements were carried out at 295 k according to the astm standard g 31-72 [15]. strip coupons of dimensions 2.5  2.5  0.3 cm, and a 0.6 cm diameter hole for mounting by a glass rod, were used. these coupons were kept immersed in the corrosive electrolyte in the absence and presence of different concentrations of aa for 6 h. before and after each experiment, the cs coupons were dried and weighed using an analytical balance (precision ±0.1 mg) and the mean weight loss and the corresponding standard deviation is reported in the paper. all measurements were carried out in oxygen-free solutions, which was achieved by continuous purging of the solution with argon gas (pre-purified grade purity) for at least 30 minutes prior the measurement, and also during the measurement. all the values reported in the paper represent mean values of at least three replicate experiments. 3. results and discussion 3.1. effect of inhibitor concentration 3.1.1. electrochemical impedance spectroscopy measurements electrochemical impedance spectroscopy (eis) technique was applied to investigate the electrode/electrolyte interface and (corrosion) processes that occur on the cs surface in the presence and absence of aa molecules in the electrolyte. fig. 1 shows an example of eis spectra in the form of a nyquist diagram, recorded on a cs electrode in 0.5 m hcl solution in the absence and presence of aa at different concentrations. the measurements were made after the stabilization of the electrode at ocp for 1 h, and at 295 k. fig. 1 shows that the diameter of the semicircle increases with the increase in inhibitor concentration in the electrolyte, indicating an increase in corrosion resistance of the material. j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 160 figure. 1. nyquist plot of cs recorded at different aa concentrations in 0.5 m hcl at 295 k. (+) 0 mm, (◊) 0.6 mm, () 1 mm, (□) 2 mm, (o) 4 mm and ()6 mm of aa. symbols are experimental data and solid lines represent the simulated (modeled) spectra in order to extract qualitative information, a nonlinear least-squares (nlls) fit analysis was used to model the spectra, employing the electrical equivalent circuits (eecs) presented in fig. 2. in these eecs, rel represents the ohmic resistance between the working and the reference electrode; r1 is the charge transfer resistance related to the corrosion reaction at ocp; cpe1 is the capacitance of the electric double-layer at the electrode/electrolyte interface; r2 can be prescribed to the pseudo-resistance of the surface-adsorbed aa layer (i.e. the combined resistance of the electro-active species diffusion and the electrolyte resistance in ‘pores’), while the element cpe2 is its pseudo-capacitance . thus, the corrosion resistance of cs in the presence of aa represents the r1 + r2 sum. in the absence of aa (control sample), the spectra was modeled using the one-time constant eec, fig. 2a, in which the cs corrosion resistance is represented by r1. cpe’s are often used to describe a non-ideal capacitive behaviour due to different factors such as surface roughness and heterogeneities or a non-uniform potential and current distribution[16]. the impedance of a cpe is given as zcpe = q -1 (jω) -n , where z is the cpe impedance (ω cm 2 ), q is a constant with units ω -1 cm -2 s n , n is a dimensionless constant in the range -1 ≤ n ≤1, j is the imaginary number j = (-1) 0.5 and ω is the angular frequency (ω = 2 π f, f being the frequency). an ideal capacitor behaviour yields n = 1, a resistor yields n = 0 and an inductor yields n = -1, while n = 0.5 represents the response of mass-transport processes. fig. 1 demonstrates that excellent agreement between the experimental data (symbols) and the model (lines) is obtained, justifying the use of the proposed eecs. the corrosion resistance values are presented in table 2 along with the corrosion inhibition efficiency of aa, , which was calculated by comparing the total resistance value, ri = r1 + r2 (ω cm 2 ), recorded at various concentrations of aa in the electrolyte, and the r0 value recorded in the absence of aa (control sample) [17]: 0 i i / % 1 100        r η r (1) ri and ηi values together with the corresponding standard deviation are listed in table 2, while fig. 3 illustrates the concentration dependence of the aa corrosion inhibition efficiency. with an ph 0.35 0 100 200 300 400 500 0 200 400 600 800 1000 z ' (ω cm 2 ) -z '' ( ω c m 2 ) -z " /  c m 2 z ’ /  cm 2 s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 161 increase in the inhibitor concentration in the bulk solution, the corrosion inhibition efficiency also increases, and levels off into a plateau, reaching a value of ca. 95 %. the trend in corrosion inhibition efficiency could be related to the surface coverage and different orientation of the inhibitor molecule on the surface. namely, with an increase in inhibitor concentration in the solution, its surface concentration also increases, but at low aa concentrations in the solution a highly disordered amorphous aa sub-monolayer is formed on the cs surface. this layer is still ‘porous’ and enables interaction of the corrosive electrolyte with the cs surface. however, with a further increase in the inhibitor concentration in the solution, the surface concentration, coverage and order of the aa monolayer increases, thus offering a tighter hydrophobic barrier (due to the presence of the long –ch2 chain) for the penetration of solvated aggressive ions to the underlying surface. figure 2. eec models used to fit eis data recorded on (a) a bare cs electrode (control sample), and (b) on a cs electrode immersed in an electrolyte containing aa c/ mm figure 3. corrosion inhibition efficiency of aa in protecting cs from general corrosion in 0.5 m hcl solution at 295 k. the data represent mean values obtained from eis measurements, performed at various concentrations of aa, while the error bars represent the corresponding standard deviation (note that at higher aa concentrations, the corresponding standard deviation is rather small) (a) (b) ph 0.35 0 20 40 60 80 100 0 1 2 3 4 5 6 7 c (mm) e ff ic ie n c y ( % ) e ff ic ie n cy , % j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 162 table 2 dependence of eec parameters on the concentration of aa in the electrolyte. the data were obtained by modeling the eis spectra recorded in 0.5 m hcl and at 295 k. the data also lists the corresponding corrosion inhibition efficiency values, ηi. quantity n represents sd = standard deviation. an average electrolyte resistance value was 4 ± 2  ci / mm 0 0.15 0.3 0.6 1 2 3 4 6 cpe1  10 6 / ω –1 cm –2 s n 652 645 175 111 39 43 102 29 19 ± sd 89 1 41 18 11 5 59 13 6 n1 0.83 0.8 0.81 0.81 0.83 0.82 0.74 0.78 0.81 ± sd 0.02 0 0 0 0 0 0.09 0.08 0.02 r1 / ω cm 2 50 70 107 167 311 407 578 722 840 ± sd 2 5 18 31 1 31 84 42 62 cpe2  10 6 / ω –1 cm –2 s n 423 167 160 94 91 71 79 82 ± sd 21 39 8 6 21 24 8 8 n2 0.82 0.85 0.82 0.83 0.82 0.88 0.83 0.82 ± sd 0 0 0.03 0.01 0.02 0.02 0.01 0.01 r2 / ω cm 2 3 3 9 57 35 37 58 82 ± sd 1 1 6 41 6 14 5 40 ri / ω cm 2 50 73 110 176 368 442 615 780 922 ± sd 2 5 18 37 42 26 92 39 103 i / % 0 30 53 71 86 89 92 93 95 ± sd 0 8 10 7 1 1 1 1 1 3.1.2. adsorption isotherm from the previous discussion it is obvious that the aa corrosion inhibition efficiency depends on its surface concentration. thus, the mechanism of interaction of aa with the cs surface is of adsorptive type. in order to describe adsorption of aa on the cs surface, several adsorption isotherms were tested. however, the best agreement was obtained using the langmuir adsorption isotherm [17,18]: ads max ads1 b γ c γ b c   (2) where c / mol cm –3 is the equilibrium concentration of aa in the bulk solution, γ / mol cm –2 is its amount adsorbed onto the surface (i.e. its surface concentration), γmax / mol cm –2 is the maximum value of γ, and the parameter bads (cm 3 mol –1 ) reflects the affinity of the inhibitor molecules towards surface adsorption sites. the surface coverage at a particular aa surface concentration could be expressed as  = γ / γmax. thus, eq.(2) can be rearranged to give [17,19-21]: ads 1c c b   (3) consequently, if the langmuir isotherm describes the inhibitor adsorption process, a plot of experimental data expressed as c /  vs. c should yield a straight line with a slope of one. assuming that the inhibition efficiency, , is proportional to the inhibitor surface coverage,  [22], the mean s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 163 corrosion inhibition efficiency values obtained using eis technique (fig. 3) are presented in fig. 4 in a form of the linearized langmuir isotherm (eq.(3)). the agreement between the langmuir isotherm and experimental data is very good (r 2 = 0.9992). the corresponding slope (1.01) is very close to the theoretical value of one. therefore, it could be deemed that the adsorption of aa on the cs surface indeed follows the langmuir isotherm. the intercept of the line in fig. 4 yielded bads = 3 .93 10 3 dm 3 mol –1 , and the corresponding apparent gibbs free energy of adsorption was then calculated from [18-20,22,23] ads ads solvent 1 exp g b c rt        (4) where r / j mol –1 k –1 is the gas constant, t / k is the temperature, and csolvent is the molar concentration of the solvent, which in this case, is water (ch2o = 55.5 mol dm –3 ). using this equation, the apparent gibbs free energy of adsorption of aa on the cs surface at 295 k was calculated to be –30.2 kj mol –1 (values of -31.5 and -33.7 kj mol -1 were obtained from tafel and weight-loss measurements presented later in the paper, respectively, yielding an average gibbs free energy of adsorption value of -32 ± 2 kj mol –1 ). this highly negative value demonstrates that the aa adsorption process is highly spontaneous [2,3,24,25]. c / mm figure 4. linearized form of the langmuir adsorption isotherm for adsorption of aa onto the cs surface at 295 k. the data was obtained from eis measurements we further investigated whether aa molecules are physicor chemisorbed on the cs surface, by performing pm-irras measurements (fig. 5). in these measurement, the cs sample was left in the aa-containing solution for three days, then removed from solution, rinsed very well, dried and finally scanned by pm-irras. the presence of the two peaks at 2853 cm –1 and 2926 cm –1 , figure 5, demonstrate the presence of aa on the surface, since these correspond to the symmetric and asymmetric c–h stretching vibrations of the –ch2 groups in the aa chain, respectively [26,27]. if aa molecules were only physisorbed on the cs surface, the thorough rinsing performed after pulling out the samples from the aa-containing solution would have removed the molecules and no ir absorption peaks would have been recorded. however, the wavenumber position of the – ch2 asymmetric peak in fig. 5 (2926 cm –1 ) indicates that the aa sam has a rather amorphous structure (a perfectly ordered sam would give 2918 cm –1 ) [28]. this could be attributed to the ph 0.35 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 c (mm) c / θ ( m m ) (c / ) / m m j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 164 repulsion between the neighboring positively charged amine groups and a high heterogeneity of the cs surface. wavenumber, cm -1 figure 5. pm-irras spectra of the aa sam on a cs substrate. the spectra were recorded at ocp after 3 days of immersing the cs sample in 0.5 m hcl containing 3 mm of aa at 295 k 3.1.3. tafel polarization measurements tafel polarization measurements were made to complement/verify the data obtained from eis measurements. fig. 6 shows tafel curves recorded on a cs electrode in 0.5 m hcl, in the absence of aa (control) and at various concentrations of aa in the electrolyte. under the experimental conditions performed, the cathodic branch represents the hydrogen evolution reaction, while the anodic branch represents the iron dissolution reaction. from fig. 6 it is clear that with an increase in concentration of aa in the bulk solution, both the cathodic and anodic current decreases. this indicates that aa acts as a mixed-type corrosion inhibitor. also, there is a slight shift in the corrosion potential to the negative direction which indicates a slightly stronger cathodic inhibition than anodic inhibition of the dissolution. the corresponding corrosion current densities (jcorr) were estimated by the extrapolation of the linear part of the cathodic and anodic curves to the corrosion potential, and presented in table 3 (mean value) together with the corresponding inhibition efficiency, which was calculated using the following equation [5,13,17,29,30]: corr,i i corr,0 / % 1 100         j η j (5) where jcorr,i / a cm –2 is the corrosion current density at a particular aa concentration, and jcorr,0 is the corrosion current density in the absence of aa in the solution. it can be seen from table 3 that the dependence of corrosion inhibition efficiency on the aa concentration is very similar to that obtained from eis measurements (table 2). consequently, these results can be explained in ph 0.35 -1 0 1 2 3 4 5 6 7 2700 2750 2800 2850 2900 2950 3000 3050 3100 wavenumber (cm -1 ) a b s o rb a n c e x 1 0 3 a b so rb a n ce , a .u . s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 165 the same way previously done in the eis section and, therefore; provide a good agreement between the two different techniques. e / v vs. sce figure 6. tafel plots of the cs recorded at various concentrations of aa in 0.5 m hcl at 295 k. (1) 0 mm, (2) 0.3 mm, (3) 1 mm, (4) 2 mm, (5) 4 mm, and (6) 6 mm aa. scan rate = 1 mv s –1 table 3. corrosion current density (jcorr,i) calculated form tafel measurements recorded on cs in 0.5 m hcl solution containing various concentrations of aa at 295 k. the data also lists the corresponding corrosion inhibition efficiency values, ηi. sd = standard deviation ci / mm 0 0.15 0.3 0.6 1 2 3 4 6 jcorr,i / a cm –2 528 350 221 111 42 41 24 24 25 ± sd 30 9 25 47 13 5 2 2 4 i / % 0 39 62 81 92 92 95 95 95 ± sd 0 2 2 7 3 1 1 1 2 3.1.4. weight-loss measurements weight loss measurements were carried out at 295 k in 0.5 m hcl solution, in the absence and presence of various concentrations of aa. table 4 reports the values of corrosion rate (cr) and the corresponding corrosion inhibition efficiency () at various concentrations of aa. the corrosion rate (cr, millimeters per year) was calculated from the following relation [15]: kw cr a t  (6) where t is the specimen immersion time (h), a is the surface area of the test specimen (cm 2 ), w is the weight loss (g),  is the density of the test specimen (g ) cm –3 , and k = 8.76 × 10 4 is the conversion constant that enables expressing cr in millimeters per year when t, a, w and  values are expressed in the quoted units. table 4 shows that with an increase in aa concentration in the bulk electrolyte, the corrosion rate decreases. in order to better quantify this, the corrosion inhibition efficiency was calculated from [31]: j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 166 i i 0 / % 1 100        w η w (7) where w0 and wi are the weight loss, g of the test specimen in the absence and presence of the inhibitor at a specified concentration (i), respectively. the i values in table 4 demonstrate a significant increase in inhibition efficiency with the aa concentration increase. it can be noticed from table 4 that the results are very similar to those obtained from eis and tafel measurements (tables 2 and 3), and could be explained in the same way. table 4 data obtained from weight-loss measurements of carbon steel immersed 6 h in 0.5 m hcl solution in the absence and presence of various concentrations of aa at 295 k. sd = standard deviation ci / mm 0 0.15 0.3 0.6 1 2 3 4 6 wi / mg cm –2 1.10 0.30 0.14 0.12 0.08 0.06 0.06 0.04 0.03 ±sd 0.16 0.02 0.02 0.02 0.01 0.01 0 0.01 0 cr / mm y –1 2.04 0.57 0.27 0.22 0.16 0.10 0.12 0.08 0.05 ±sd 0.30 0.04 0.04 0.04 0.01 0.02 0.01 0.02 0.01 i / % 0 72 87 89 92 94.9 94.5 96 97 ±sd 0 5 2 2 1 0.2 0.3 1 1 3.2. effect of time in order to investigate the aa adsorption kinetics, to determine the time needed for aa to yield the maximum inhibition efficiency at the given conditions, and to investigate the longer-term inhibitor efficiency, eis experiments and tafel experiments in the absence and presence of 3 mm of aa in 0.5 m hcl, ph 0.35, were made at selected time intervals, at ocp. the corrosion inhibition efficiency values were calculated and presented in fig. 7. time, h figure 7. dependence of the corrosion inhibition efficiency on the immersion time obtained from eis measurements recorded on cs in the absence and presence of 3 mm of aa in 0.5 m hcl at 295 k 0 20 40 60 80 100 0.5 1 1.5 2 3 24 72 time (h) e ff ic ie n c y ( % ) e ff ic ie n cy , % s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 167 the plot shows that an improvement in the corrosion inhibition efficiency is obtained with increasing immersion time. this can be, in the first approximation, associated with the kinetics of aa adsorption on the cs surface, and with the re-arrangement of the formed aa layer to achieve higher surface order, and thus better corrosion protection. a high inhibition efficiency was reached after 1 h, and remained high even after 3 days of testing (97 %). thus, it can be concluded that aa is a very efficient long-term action corrosion inhibitor for cs, under the experimental conditions investigated. 3.3. effect of surface roughness in order to study the influence of surface roughness on the corrosion inhibition efficiency, eis, tafel and weight-loss measurements were made on cs surfaces abraded with different sand paper grit numbers: 240, 320, 400, 600, 2400 and 4000 in the absence and presence of 3 mm of aa in 0.5 m hcl solution. the surface roughness of each fresh abraded sample were quantitatively evaluated using surface profilometery. a series of profile line-scans were performed and the corresponding average roughness values were calculated and presented in table 5. using the same procedure that was described earlier in the text, the corrosion inhibition efficiency values were calculated and presented in fig. 8. surface roughness, nm figure 8. dependence of corrosion inhibition efficiency on cs surface roughness obtained from eis, tafel and weight-loss measurements recorded on cs in the absence and presence of 3 mm of aa in 0.5 m hcl at 295 k it can be seen from fig. 8 that the inhibitor shows a similar effect and there is no significant difference in its effect at all different surfaces studied. this indicates a good affinity of aa molecules to adsorb on the cs surface of different roughness. the inhibition effect can be more clearly seen from fig. 9 which shows the cs surface topography for a freshly-abraded sample using 2400 sand paper and also after 3 days of immersion in the testing electrolyte in the absence and presence of aa. fig. 9b demonstrates that the cs surface is extensive and highly damaged. on the other hand, in the presence of 3 mm of aa in the solution (fig. 9c), the surface topography is similar to the fresh (non-corroded) sample (fig. 9a), evidencing that the cs surface corroded only negligibly in the presence of aa. similar observations were found when the surfaces were abraded using other sand papers. 0 20 40 60 80 100 50 53 68 93 135 386 surface roughness (nm) e ff ic ie n c y ( % ) e ff ic ie n cy , % j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 168 table 5 average surface roughness of cs samples prepared using emery paper of various grits sd = standard deviation grit number 240 320 400 600 2400 4000 surface roughness, nm 386 135 93 68 53 50 ±sd 16 2 2 1 1 1 (a) (b) (c) figure 9. sem images of cs surfaces prepared using 2400 grit paper. (a) fresh abraded sample, (b) after 3 days of immersion in the absence and (c) in the presence of 3 mm of aa in 0.5 m hcl solution at 295 k. all images have been taken at the same magnification, and the corresponding length bar is 10 μm 3.4. effect of temperature the effect of temperature on the inhibition effect of aa on carbon steel corrosion was studied over a wide temperature range, from 295 k to 323 k. in this study, eis measurements were carried out by using different aa concentrations from 0 mm to 3 mm aa. fig. 10a shows a set of eis spectra recorded at different temperatures in the absence of aa in the solution. with an increase in temperature, the diameter of the semicircle decreases, i.e. the charge-transfer resistance decreases, which is due to the increased corrosion rate. this is a typical behavior, since with an increase in temperature, the corrosion reaction kinetics also increases, and thus the corrosion rate. quite the same behaviour was observed in the presence of aa in the solution (fig. 10b). however, by comparing the charge-transfer values at a fixed temperature, it is obvious that aa acts as a good corrosion inhibitor in the whole temperature range. eis spectra at all investigated temperatures (example in fig. 10) were treated in the same way in the section 3.1.1 and the corrosion inhibition efficiency were evaluated at different concentration of aa, fig. 11. the data shows that the corrosion efficiency of the inhibitor remains almost constant in the whole temperature range, which indicates that the aa molecule is chemically stable in the investigated temperature range, and that the mechanism of its corrosion inhibition does not change. hence, aa seems to be a good inhibitor candidate even at higher temperatures. further, the eis data was treated in the same way as in fig. 3 and fig. 4 to find apparent gibbs free energy of adsorption at different temperatures, gads, which were calculated, and presented in fig. 12. subsequently, taking that: ads ads adsg h t s    (8) the enthalpy and entropy of adsorption was calculated to be hads = 36.6 kj mol –1 , and sads = 230 j mol –1 k –1 , respectively. the enthalpy value demonstrates that the adsorption of aa on the cs surface is endothermic. nevertheless, the relatively high negative gibbs free energy values (fig. 12) demonstrate that the overall adsorption process is highly spontaneous. therefore, the s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 169 major contribution to this spontaneity has to come from a positive gain in entropy. this gain seems to be the main contributor to the driving force for the adsorption of aa on cs. this can be explained with that the major contribution to the entropy gain in the system comes from the loss of the order of water molecules adsorbed on the cs surface upon aa adsorption. under the influence of the electric charge on the cs surface, the adsorbed water molecules are highly ordered on the electrode surface [32], but when displaced by aa molecules, this high degree of order significantly decreases due to the random orientation of the displaced molecules in the bulk solution. this results in an overall increase in the positional degree of freedom of the system, i.e. an entropy gain. on the other hand, adsorbed aa molecules lose positional degrees of freedom, but this effect is largely overcome by the positive entropy contribution coming from water. figure 10. nyquist impedance plots for carbon steel recorded at different temperatures () 295 k, (□) 308 k, and (o) 323 k (a) in the absence, and (b) presence of 3 mm of aa in 0.5 mm hcl. symbols are experimental data and solid lines represent the simulated (modeled) spectra (a) 0 5 10 15 20 25 30 0 10 20 30 40 50 60 z ' (ω cm 2 ) -z '' (ω c m 2 ) (b) 0 50 100 150 200 250 300 0 100 200 300 400 500 600 z ' (ω cm 2 ) -z '' (ω c m 2 ) -z " /  c m 2 -z " /  c m 2 z ’ /  cm 2 z ’ /  cm 2 j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 170 temperature, k figure 11. dependence of corrosion inhibition efficiency on the solution temperature. the data were obtained from eis measurements recorded on cs in the absence and presence of 3 mm of aa in 0.5 m hcl temperature, k figure 12. the dependence of apparent gibbs free energy of adsorption on temperature. the data were obtained from eis measurements recorded on cs at ocp and over a wide range of aa concentration in the bulk electrolyte 3.5. effect of ph the influence of ph on the cs corrosion inhibition efficiency by aa was also studied in hcl, in a ph range from 0.35 to 3.5, in the absence and presence of 3 mm of aa, using eis measurements. these measurements were performed at ocp, one hour after the stabilization of the cs electrode in the electrolyte at ocp. corrosion inhibition efficiency values were then calculated from these measurements, and are presented in fig. 13. the results show that aa offers high corrosion inhibition efficiency at all the investigated ph values. the corrosion inhibition efficiency of aa remains stable over the investigated ph region, which could be linked to the good solubility of aa in the electrolyte at all ph values. 0 20 40 60 80 100 295 303 308 313 318 323 temperature (k) e ff ic ie n c y ( % ) e ff ic ie n cy , %  g a d s / kj m o l-1 s. ghareba at al. j. electrochem. sci. eng. 5(3) (2015) 157-172 doi:10.5599/jese.242 171 figure 13. the inhibition efficiency of aa on cs general corrosion determined at various electrolyte ph values. the data were derived from eis measurements recorded in the absence and presence of 3 mm of aa in hcl electrolyte at 295 k 4. conclusions  eis, tafel and weight-loss measurements were used to evaluate the corrosion inhibition efficiency of aa. the corrosion inhibition efficiency increases with increasing the aa concentration. a very good agreement between results obtained by the three techniques was demonstrated.  tafel measurements revealed that aa inhibits both partial corrosion reactions, and can thus be considered to be a mixed-type inhibitor.  the adsorption of aa was described by the langmuir adsorption isotherm. the corresponding thermodynamic values showed that the aa adsorption is a spontaneous process, driven by an entropy increase.  kinetic measurements showed that high inhibition efficiency (ca. 84%) is achieved within 30 minutes of the immersion of the freshly polished cs surface in the presence of aa in the acid solution. complete equilibrium is achieved after ca. 1 h.  temperature-dependent measurements confirmed that aa maintains its high corrosion inhibition efficiency up to 323 k, which indicates that the adsorbed aa monolayer is chemically stable in the investigated temperature range.  the corrosion inhibition efficiency remains high with an increase in solution ph.  pm-irras measurements confirmed the chemical identity of the adsorbed aa layer. the formed layer is of an amorphous structure and seems to be chemisorbed.  the inhibitor shows a good protection of the cs of different surface roughness. acknowledgments: the authors express their sincere acknowledgment to the natural science and engineering research council of canada, to the ministry of higher education and al-mergib university, alkhums-libya, and to the summer undergraduate research program in the faculty of engineering at mcgill university for providing the support for this research. 0 20 40 60 80 100 0.35 1 1.5 2 2.5 3 3.5 ph e ff ic ie n c y ( % ) e ff ic ie n cy , % j. electrochem. sci. eng. 5(3) (2015) 157-172 inhibition of carbon steel corrosion 172 references [1] m. a. quraishi, d. jamal, corrosion 56 (2000) 156. 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[32] c. h. hamann, a. hamnett, w. vielstich in electrochemistry, wiley-vch, germany, 1998. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) 𝜗 http://creativecommons.org/licenses/by/4.0/ determination of kojic acid using carbon paste electrode modified by [fe(hl)2cl2] nano-complex and ionic liquid: http://dx.doi.org/10.5599/jese.1486 1179 j. electrochem. sci. eng. 12(6) (2022) 1179-1192; http://dx.doi.org/10.5599/jese.1486 open access : : issn 1847-9286 www.jese-online.org original scientific paper determination of kojic acid using carbon paste electrode modified by [fe(hl)2cl2] nano-complex and ionic liquid mahbubeh fazlia and niloufar akbarzadeh-t department of chemistry, university of sistan and baluchestan, p.o. box 98135-674, zahedan, iran corresponding authorс: n.akbarzadeh@chem.usb.ac.ir received: august 10, 2022; accepted: october 2, 2022; published: november 28, 2022 abstract a new mononuclear fe(ii) (1) complex with the formula [fe(hl)2cl2] (hl= n-(2-hydroxy-1-naphthylidene)-2-methyl aniline) was synthesized and characterized by fourier transform infrared spectroscopy (ft-ir), uv–vis and elemental analysis. the spectroscopy analyses revealed the two schiff base ligands via oxygen and nitrogen atoms and two chloride atoms create an octahedral geometry. the nano-size of [fe(hl)2cl2] complex (2) was synthesized by the sonochemical process. characterization of nano-complex (2) was carried out via x-ray powder diffraction (xrd), scanning electron microscopy (sem), uvvis, ft-ir spectroscopy. the nano-complex (2) average size synthesized via the sonochemical method was approximately 52 nm. in this work, a simple sensor based on a carbon paste electrode modified with [fe(hl)2cl2] nano-complex and the ionic liquid il (1-butyl3-methylimidazolium hexafluorophosphate) was developed ([fe(hl)2cl2] nano-complexil/cpe) for convenient and fast electrochemical detection of kojic acid. the modified electrode considerably improves voltammetric sensitivity toward kojic acid compared to the bare electrode. experimental conditions influencing the analytical performance of the modified electrode were optimized. under optimal conditions, the oxidation peak current was proportional to kojic acid concentration in the range from 0.3 to 237.0 μm with a detection limit of 0.09±0.001 μm. the [fe(hl)2cl2] nano-complex-il/cpe sensor was successfully applied for the highly sensitive determination of kojic acid in real samples with satisfactory results. keywords electrochemical sensor; voltammetry; real samples, mononuclear fe (ii) complex introduction kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone, is a metabolic product of several species of the economically valuable genus aspergillus, acetobacter, and penicillium [1,2]. kojic acid and some of its derivatives are used in cosmetic preparations to achieve a skin-lightening effect by inhibiting melanin formation. in food production, kojic acid has been used as an antioxidant, an additive for http://dx.doi.org/10.5599/jese.1486 http://dx.doi.org/10.5599/jese.1486 http://www.jese-online.org/ mailto:n.akbarzadeh@chem.usb.ac.ir j. electrochem. sci. eng. 12(6) (2022) 1179-1192 determination of kojic acid using carbon paste electrode 1180 its preservative actions against both chemical and microbial degradation, to preserve food color, and as an inhibitor of nitrosopyrrolidine formation in fried food [3-6]. in addition, kojic acid has also been used as an antibiotic, pesticide, and analytical chemical (in the determination of thorium and rare earth elements) [7]. although consumption of low levels of kojic acid does not concern safety, studies show that continuous overuse of kojic acid is carcinogenic and tumorigenic [8-10]. therefore, the development of a convenient, economical, rapid and sensitive method for the determination of trace amounts of kojic acid in different samples is highly desirable. various techniques have been developed for the quantitative determination of kojic acid, such as ion-pair liquid chromatography (iplc) [11], high-performance liquid chromatography (hplc) [12], fluorescent detection [13], and capillary electrophoresis [14]. unfortunately, these techniques require expensive instruments and skilled operators and are complicated procedures. electrochemical sensors, with their excellent ability for the determination of electroactive substances, have been suggested as powerful analytical tools in recent years [15-20]. electrochemical sensors are relatively cheap, portable, sensitive, and give rapid response [21-33]. however, the oxidation of kojic acid is kinetically sluggish, and a relatively high overpotential is required at the conventional electrodes [34,35]. in electrochemical analysis, the key component is electrode modification, which requires the selection of suitable material to improve the determination performance [36-43]. chemically modified electrodes (cmes) have attracted considerable interest over the past decades as researchers attempted to exert more direct control over the chemical nature of an electrode surface [44-55]. chemically modified carbon paste electrodes (cpes) are cheap, easy to make, and have a low background current [56-58]. ionic liquids and nanostructures have been used to modify cpes with improved conductivity, high mechanical stability, and fast electron transfer rates [59,60]. in recent years, nanostructured materials received wide attention owing to their unique structure, high specific surface area, high surface-to-volume ratio and excellent electronic conductivity [61-65]. nanomaterial-modified electrodes exhibit many favorable characteristics for electroanalysis, including fast response, high sensitivity and selectivity [66-70]. schiff bases are an important class of organic compounds [71]. this kind of ligand has significant importance in chemistry, especially in the development of schiff base complexes, because schiff base ligands are capable of forming stable complexes with metal ions [72]. recently, it has attracted more attention in metal complexes of unsymmetrical schiff bases ligands with nitrogen and oxygen atoms [73]. most of these unsymmetrical schiff bases ligands are obtained by the condensation of different types of primary amines with various ketones and aldehydes by direct syntheses [74]. schiff base ligands have biological activity and potential applications in many fields, such as oxidation catalysis, and electrochemistry [75]. facile and inexpensive synthesis, allied with the wide range of structural and electronic features of schiff bases and their coordination complexes, has increased interest in chemically modifying electrodes with these compounds. therefore, the resulting electrodes have been used as sensors and/or probes in a variety of fields. all this interest has culminated in the development of low-cost sensors. in particular, the many structural and electronic properties of schiff bases and their transition metal complexes allow them and the analyte to establish different interactions, which improves sensor selectivity and sensitivity [76-80]. in this work, schiff base ligand [hl= n-(2-hydroxy-1-naphthylidene)-2-methyl aniline)] and its iron(ii) complex (1) with (n-(2-hydoxynaphtalen)2-methyl aniline) schiff base ligand were synthesized. complex (1) was characterized using spectroscopic techniques such as ft-ir, uv-vis and elemental analysis. the crystal structures of [fe(hl)2cl2] (1) were determined by x-ray m. fazlia and n. akbarzadeh-t j. electrochem. sci. eng. 12(6) (2022) 1179-1192 http://dx.doi.org/10.5599/jese.1486 1181 crystallography. the schiff base iron nano-complex (2) was prepared by the sonochemical method. this synthesis was performed without the use of any surfactants. the nano-sized complex (2) was identified using xrd and sem techniques. also, we aimed to fabricate a highly sensitive nanostructured sensor based on ionic liquid il (1-butyl-3-methylimidazolium hexafluorophosphate) carbon pastes modified with [fe(hl)2cl2] nano-complex to investigate the electrooxidation of kojic acid. the electrocatalytic ability of [fe(hl)2cl2] nano-complex-il/cpe sensor was thoroughly investigated by cyclic voltammetry (cv), chronoamperometry, and differential pulse voltammetry (dpv). the analytical performance of the proposed sensor was assessed by quantitative evaluation of kojic acid in various real samples. experimental apparatus and chemicals 1h nmr spectra were accomplished with bruker avance3 3-300mhz spectrometer using dmsod6 as a solvent and at room temperature (298 k). fourier-transform infrared spectra were performed on a perkin-elmer ft-ir spectrophotometer model spectrum two with kbr discs in the range of 4000 to 400 cm-1. for elemental analyses of new compounds was utilized a costech ecs 4010 chns elemental analyzer. uv-vis spectra were recorded in the range of 200 to 800 nm on optizen view 2120uvplus spectrophotometer ver1-2. all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab consisting of a traditional three-electrode system: a bare or modified cpe as the working electrode, an ag/agcl as the reference electrode and a pt wire as a counter electrode. solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). phosphate buffer solution (pbs) was prepared with phosphoric acid and adjusted by naoh to the desired ph value. kojic acid and other chemicals used were analytical grade and were purchased from sigma aldrich and merck. synthesis of schiff base ligand [hl] the ligand (hl: n-(2-hydroxy-1-naphthylidene)-2-methyl aniline) was synthesized by adding a solution of 2-hydroxy-1-naphthaldehyde (0.17 g, 1 mmol) in 10 ml of ethanol to a solution of 2-methyl aniline (0.11 ml, 1mmol) in 10 ml of ethanol. then, the mixture was refluxed for 6 h. the resulting yellow solution was placed at room temperature. after 24 h, yellow crystals of hl were formed. the schiff base ligand was characterized by1h-nmr, ft-ir, elemental analysis and single-crystal x-ray diffraction [81]. mp: 154 °c; (yield: 88 %); (mw: 261.31); anal. calc. for [c18h15no]: c82.73; h6.12; n 5.79; found: c82.70; h6.09; n5.76. ft-ir (kbr, cm-1), 3421 ν (n-h), 1620 ν (c=n), 1161 ν (c-o), 1479 ν (c=c); 1h-nmr (300 mhz, dmso d6, ppm) 9.61 (1h, s, ch=n), 16.05 (1h, oh), 2.40(3h,ch3). synthesis of complex (1) the schiff base ligand (hl: n-(2-hydroxy-1-naphthylidene)-2-methyl aniline) (0.26 g, 1 mmol) was dissolved in 10 ml of methanol and to this yellow solution was added a solution of [fecl2.4h2o] (0.10 g, 0.5 mmol) in10 ml of methanol dropwise. the reaction mixture was refluxed for 6 h. the complex was precipitated, the precipitate was filtered and dried at room temperature. the complex was characterized by uv-vis, ftir, elemental analysis, and single-crystal x-ray diffraction scheme 1. mp: 220 °c; (yield: 85 %); (mw: 648.103); anal. calc. for [c36h30cl2fen2o2]:c66.58; h4.66; n4.31; found: c 66.60; h 4.74; n 4.28. ft-ir (kbr, cm-1), 3418 ν (o-h); 1596 ν (c=n); 1159 ν (c-o), 546 ν (fe-o); 443 ν (fe -n). http://dx.doi.org/10.5599/jese.1486 https://en.wikipedia.org/wiki/fourier-transform_infrared_spectroscopy https://en.wikipedia.org/wiki/fourier-transform_infrared_spectroscopy https://en.wikipedia.org/wiki/fourier-transform_infrared_spectroscopy https://en.wikipedia.org/wiki/fourier-transform_infrared_spectroscopy https://en.wikipedia.org/wiki/fourier-transform_infrared_spectroscopy https://en.wikipedia.org/wiki/fourier-transform_infrared_spectroscopy j. electrochem. sci. eng. 12(6) (2022) 1179-1192 determination of kojic acid using carbon paste electrode 1182 scheme 1. complex (1) synthesis of nano-complex (2) a solution of [fecl2.4h2o] (0.10 g, 0.5 mmol) in methanol (10 ml) was positioned in a high-density ultrasonic probe for 10 min. then to this solution, 10 ml of methanolic solution of schiff base ligand (0.26 g, 1 mmol) was added dropwise. the resultant solution was then irradiated for 60 min at 60 °c with a power of 100 w. the obtained brown precipitate was filtered and dried in air.mp: 230 °c; (yield: 87 %); (mw: 648.103); anal. calc. for [c36h30fecl2 n2o2]: c 66.58; h 4.66; n 4.31; found: c 66.62; h 4.70; n 4.30. ft-ir (kbr, cm-1): 3458 ν (o-h); 1596 ν (c=n); 1159 ν (c-o), 543 ν (fe-o); 444 ν (fe -n). preparation of [fe(hl)2cl2] nano-complex-il/cpe [fe(hl)2cl2] nano-complex-il/cpe was prepared by mixing of a certain level of ionic liquid and liquid paraffin, 0.1 g of [fe(hl)2cl2] nano-complex, and 0.9 g of graphite powder. then the mixture was mixed well for 60 min until a uniformly wetted paste was obtained. the paste was then packed into a glass tube. electrical contact was made by pushing a copper wire down the glass tube into the back of the mixture. when necessary, a new surface was obtained by pushing an excess paste out of the tube and polishing it on a weighing paper. results and discussion ft-ir spectra the ft-ir spectra of complex (1) and nano-complex (2) are shown in figure 1. wavenumber, cm-1 figure 1. the ft-ir spectra of a: complex (1) b: nano-complex (2) m. fazlia and n. akbarzadeh-t j. electrochem. sci. eng. 12(6) (2022) 1179-1192 http://dx.doi.org/10.5599/jese.1486 1183 the ft-ir spectrum of the free schiff base ligand [hl] exhibits a band in 1620 cm-1 due to ν (c=n) azomethine and a band at the 1161 cm-1 is assigned to the ν (c-o) phenolic group [81]. these bands have been shifted to lower frequencies for both complex (1) and nano-complex (2), at 1596 cm-1 and in the region of 1146 to 1147 cm-1 respectively, which indicates that both compounds are formed by the coordination of the nitrogen and oxygen atoms of [hl] to the metal ion. ft-ir spectra of both complex (1) and nano-complex (2) show weak bands at 546 and 543 cm-1 assigned to ν (feo) and show weak bands at 443 and 444 cm-1 attributed to ν (fe-n). both complexes (1) and nanocomplex (2) display bands at 3418 and 3458 cm-1 due to ν (o-h) of the phenolic group of [hl] [82]. uv-vis spectra uv-vis spectra of (1) and (2) in methanol solution contain different peaks related to the transition bands are shown in figure 2. the schiff base ligand display two bands in 315 and 360 nm attributed to π → π* and n → π*, respectively [35]. in the electronic spectra of both (1), (2), intra-ligand transations (π → π* and n → π*) were shifted to another wavelength in 230-250 and 295-310 nm, respectively. the electronic spectra of complex (1) and nano-complex (2) showed a d-d transition band in the region of 470 to 480 nm assigned to the transition 1a1g → 1t1g [82]. wavelenght, nm figure 2. uv-vis spectra of a: complex (1) b: nano-complex (2) in methanol solution. xrd and sem the xrd patterns of nano-sized fe(ii) complex (2) and standard powder fe(ii) complex (1) were obtained from single-crystal x-ray diffraction and are exhibited in figure 3. the xrd pattern shows the crystalline phase and the nature of the complex. by investigating the location and intensity of the peaks of both patterns, it can be deduced that the diffraction angle in both complexes obtained by different methods is the same [83]. this indicates that the nano-sized complex (2) has a single crystalline phase that this phase is similar to that obtained by single-crystal x-ray diffraction [84]. the width of the diffraction peaks shows the nanocrystal complex particles (2) are of nanometer scales [85]. the particle size of the nanocrystal complex (2) was calculated using the debye-scherrer equation. sem images of the nanocrystal complex (2) are shown in figure 4. the sem photos show the shape of the nano-particles and the surface morphology of the nanocrystal complex as well. the average size diameter obtained from the debye-scherrer equation of nanocrystal complex (2) was approximately 52 nm. wavelenght, nm http://dx.doi.org/10.5599/jese.1486 j. electrochem. sci. eng. 12(6) (2022) 1179-1192 determination of kojic acid using carbon paste electrode 1184 figure 3. the xrd patterns of a: standard powder fe(ii) complex (1) b: nano-sized fe(ii) complex (2) figure 4. sem images of the nanocrystal complex (2) electrochemical behavior of kojic acid at the surface of various electrodes to study the electrochemical behaviour of kojic acid, which is ph-dependent, it is necessary to obtain the optimized ph value to achieve accurate results. by performing the experiments using modified electrodes at various ph values ranging from 2.0 to 9.0, it was revealed that the best results for the electrooxidation of kojic acid occur at ph 7.0. scheme 2 demonstrates the electrooxidation process of kojic acid. scheme 2. electro-oxidation reaction of kojic acid the electrochemical behavior of kojic acid was investigated by linear sweep voltammetry (lsv). the linear sweep voltammograms obtained using the bare cpe and [fe(hl)2cl2] nano-complex-il/cpe in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm kojic acid are shown in figure 5. m. fazlia and n. akbarzadeh-t j. electrochem. sci. eng. 12(6) (2022) 1179-1192 http://dx.doi.org/10.5599/jese.1486 1185 figure 5. linear sweep voltammograms of (a) bare cpe and (b) [fe(hl)2cl2] nano-complexil/cpe in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm kojic acid at the scan rate 50 mv s-1 at the bare cpe, a weak oxidation peak current (ipa = 3.6 μa) could be seen at 0.93 v. in contrast, [fe(hl)2cl2] nano-complex-il/cpe exhibited an enhanced sharp anodic peak current (ipa = 15 μa) at much lower overpotential ep = 0.8 v. these results confirmed that the [fe(hl)2cl2] nano-complex and il improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of kojic acid. effect of scan rate on the determination of kojic acid at [fe(hl)2cl2] nano-complex-il/cpe the influence of the scan rate (v) on the peak currents (ipa) of kojic acid at [fe(hl)2cl2] nano-complex-il/cpe was investigated by lsv. figure 6 shows the voltammetric response of 100.0 μm kojic acid at [fe(hl)2cl2] nano-complex-il/cpe at different scan rates in the range of 10 to 400 mv/s. the oxidation peak current of kojic acid increases linearly with increasing scan rate. linear regression equation was obtained from the plot ipa and vs. v1/2 (square root of scan rate) for the oxidation process, which indicates that the reaction of kojic acid at [fe(hl)2cl2] nano-complex-il/cpe is diffusion controlled. figure 6. linear sweep voltammograms of [fe(hl)2cl2] nano-complex-il/cpe in 0.1 m pbs (ph 7.0) containing 100.0 μm kojic acid at various scan rates; 1-8 correspond to 10, 25, 50, 75, 100, 200, 300, and 400 mv s-1, respectively. inset: variation of anodic peak current vs. ν1/2 http://dx.doi.org/10.5599/jese.1486 j. electrochem. sci. eng. 12(6) (2022) 1179-1192 determination of kojic acid using carbon paste electrode 1186 to obtain some information on the rate-determining step, we drew a tafel plot (figure 7) using the data from the rising part of the current-voltage curve recorded at a low scan rate of 10 mv s-1 for 100.0 μm kojic acid. the linearity of the e versus log i plot implies the intervention of the kinetics of the electrode process. the slope of this plot can be used to estimate the number of electrons transferred in the rate-determining step. according to figure 7 inset, the tafel slope for the linear part of the plot was estimated to be equal to 0.208 v. the value of the tafel slope indicates that the oneelectron transfer process is the rate-limiting step, assuming a transfer coefficient (α) of about 0.72. figure 7. linear sweep voltammograms for 100.0 μm kojic acid with 10 mv s-1 scan rate. inset: the tafel plot derived from the rising part of the corresponding voltammogram chronoamperometric analysis the analysis of chronoamperometry for kojic acid samples was performed by use of [fe(hl)2cl2] nano-complex-il/cpe vs. ag/agcl/kcl (3.0 m) at 0.85 v. the chronoamperometric results of different concentrations of kojic acid in pbs (ph 7.0) are demonstrated in figure 8. the cottrell equation for the chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is as presented in eq. (1). i = nfad1/2cbπ-1/2t-1/2 (1) where d represents the diffusion coefficient (cm2 s-1), and cb is the applied bulk concentration (mol cm-3). experimental results of i vs. t-1/2 were plotted in figure 8a, with the best fits for different concentrations of kojic acid. the resulting slopes corresponding to straight lines in figure 8a were then plotted against the concentration of kojic acid (figure 8b). the mean value of d was determined to be 7.8×10-6 cm2/s according to the resulting slope and cottrell equation. m. fazlia and n. akbarzadeh-t j. electrochem. sci. eng. 12(6) (2022) 1179-1192 http://dx.doi.org/10.5599/jese.1486 1187 figure 8. chronoamperograms obtained at [fe(hl)2cl2] nano-complex-il/cpe in 0.1 m pbs (ph 7.0) for different concentrations of kojic acid; numbers 1-5 correspond to 0.1, 0.3, 0.6, 1.0, and 1.5 mm of kojic acid. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-5. (b) plot of the slope of the straight lines against kojic acid concentration calibration curve because dpv commonly has a higher sensitivity than the cv technology, the dpv technique was applied for the quantitative detection of kojic acid. figure 9 shows the differential pulse voltammograms of kojic acid at various concentrations using [fe(hl)2cl2] nano-complex-il/cpe (potential step = 0.01 v and pulse amplitude = 0.025 v). as seen, the oxidation peak currents of kojic acid enhance gradually by increasing its concentration. the oxidation peak currents (ipa) show a good linear relationship with the concentrations of kojic acid ranging from 0.3 m to 237.0 μm (figure 9 (inset)). also, the limit of detection (lod) was estimated to be 0.09±0.001 μm. in addition, table 1 shows that the [fe(hl)2cl2] nano-complex-il/cpe can compete with other sensors for the determination of kojic acid. table 1. linear range and lod obtained at the [fe(hl)2cl2] nano-complex-il/cpe for the determination of kojic acid compared with other sensors. electrochemical sensor method linear range, µm lod, µm ref. poly(glutamic acid)-modified glassy carbon electrode cyclic voltammetry 8.0 – 660 0.8 [5] polyvinylpyrrolidone (cross-linked) modified acetylene black paste electrode linear sweep voltammetry 1.0 100 0.5 [8] multi-walled carbon nanotubes modified screen-printed electrode differential pulse voltammetry 20.0 – 5000.0 16.0 [1] ni–fe layered double hydroxide modified glassy carbon electrode amperometric 1.0 4500.0 0.73 [10] [fe(hl)2cl2] nano-complex-il/cpe differential pulse voltammetry 0.3-237.0 0.09 this work http://dx.doi.org/10.5599/jese.1486 j. electrochem. sci. eng. 12(6) (2022) 1179-1192 determination of kojic acid using carbon paste electrode 1188 figure 9. dpvs of [fe(hl)2cl2] nano-complex-il/cpe in 0.1 m (ph 7.0) containing different concentrations of kojic acid. numbers 1–12 correspond to 0.3, 3.0, 7.0, 15.0, 25.0, 45.0, 65.0, 85.0, 110.0, 150.0, 200.0, and 237.0 µm of kojic acid. inset: plot of the electrocatalytic peak current as a function of kojic acid concentration in the range of 0.3-237.0 µm analysis of real samples the real samples for the analysis were prepared and quantified by the dpv method. the developed sensor was applied to detect kojic acid in real samples. the results are summarized in table 2. each measurement was repeated five times. the recovery and relative standard deviation (rsd) values confirmed that the [fe(hl)2cl2] nano-complex-il/cpe sensor has great potential for analytical application. table 2. the application of [fe(hl)2cl2] nano-complex-il/cpe for determination of kojic acid in real samples (n=5) sample c / µm recovery, % rsd, % spiked found edible oil 0 2.7 3.3 2.0 4.6 97.9 1.7 3.0 5.7 101.7 2.4 chilli sauce 0 4.5 1.9 1.0 5.7 103.6 3.6 2.0 6.3 97.0 2.1 conclusions we have reported the synthesis of a schiff base ligand and a new fe(ii) schiff base complex (1). the single crystal x-ray diffraction analysis of the complex showed that metal ions reacted with the ligand in a 1:2 molar ratio. in the formation of the complex, oxygen atoms of two ligands are coordinated to a metal ion, and with two chloride ions attached to the metal ion, octahedral geometry is formed around the metal ion. the xrd patterns indicated both (1) and (2) compounds prepared by different synthesis methods have the same crystal structure. we established a sensitive and fast electrochemical method to detect the kojic acid based on fe(hl)2cl2] nano-complex (2) and m. fazlia and n. akbarzadeh-t j. electrochem. sci. eng. 12(6) (2022) 1179-1192 http://dx.doi.org/10.5599/jese.1486 1189 ionic liquid-modified carbon paste electrode. the voltammetric investigation demonstrates that electrooxidation of kojic acid at the surface of [fe(hl)2cl2] nano-complex-il/cpe showed very distinct characteristics due to the presence of nanoparticles of [fe(hl)2cl2] complex and ionic liquid on the surface of the electrode. quantitative analysis performed by dpv showed 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https://doi.org/10.1016/j.saa.2014.04.176 https://doi.org/10.1007/s10854-021-06967-3 https://doi.org/10.1016/j.arabjc.2013.07.010 https://doi.org/10.1016/j.molstruc.2018.04.054 https://doi.org/10.1016/j.ultsonch.2010.01.011 https://doi.org/10.1016/j.molstruc.2016.03.060 https://creativecommons.org/licenses/by/4.0/) @article{fazlia2022, author = {fazlia, mahbubeh and akbarzadeh-t, niloufar}, journal = {journal of electrochemical science and engineering}, title = {{determination of kojic acid using carbon paste electrode modified by [fe(hl)2cl2] nano-complex and ionic liquid:}}, year = {2022}, issn = {1847-9286}, month = {nov}, number = {6}, pages = {1179--1192}, volume = {12}, abstract = {a new mononuclear fe(ii) (1) complex with the formula [fe(hl)2cl2] (hl= n-(2-hydroxy-1-naphthylidene)-2-methyl aniline) was synthesized and characterized by fourier transform infrared spectroscopy (ft-ir), uv–vis and elemental analysis. the spectroscopy analyses revealed the two schiff base ligands via oxygen and nitrogen atoms and two chloride atoms create an octahedral geometry. the nano-size of [fe(hl)2cl2] complex (2) was synthesized by the sonochemical process. characterization of nano-complex (2) was carried out via x-ray powder diffraction (xrd), scanning electron microscopy (sem), uv-vis, ft-ir spectroscopy. the nano-complex (2) average size synthesized via the sono­che­mical method was approximately 52 nm. in this work, a simple sensor based on a carbon paste electrode modified with [fe(hl)2cl2] nano-complex and the ionic liquid il (1-butyl-3-methylimidazolium hexafluorophosphate) was developed ([fe(hl)2cl2] nano-complex-il/cpe) for convenient and fast electrochemical detection of kojic acid. the modified electrode considerably improves voltammetric sensitivity toward kojic acid compared to the bare electrode. experimental conditions influencing the analytical performance of the modified electrode were optimized. under optimal conditions, the oxidation peak current was proportional to kojic acid concentration in the range from 0.3 to 237.0 $\mu$m with a detection limit of 0.09±0.001 $\mu$m. the [fe(hl)2cl2] nano-complex-il/cpe sensor was successfully applied for the highly sensitive determination of kojic acid in real samples with satisfactory results.}, doi = {10.5599/jese.1486}, file = {:d\:/onedrive/mendeley desktop/fazlia, akbarzadeh-t  2022 determination of kojic acid using carbon paste electrode modified by fe(hl)2cl2 nano-complex and ionic l.pdf:pdf;:11_jese_1486_1179-1192.docx:word_new;:www/jese_v12_no6_1179-1192.pdf:pdf}, keywords = {electrochemical sensor, mononuclear fe (ii) complex, real samples, voltammetry}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1486}, } edta as a corrosion inhibitor for al in 0.5 m hcl: doi:10.5599/jese.300 235 j. electrochem. sci. eng. 6(3) (2016) 235-251; doi: 10.5599/jese.300 open access : : issn 1847-9286 www.jese-online.org original scientific paper edta as a corrosion inhibitor for al in 0.5 m hcl: adsorption, thermodynamic and theoretical study rehab e. azooz chemistry department, faculty of science, jazan university, 2097 jazan, saudi arabia corresponding author: re_azooz@yahoo.com; tel.: +9966532324115 received: may 21, 2016; revised: june 18, 2016; accepted: july 4, 2016 abstract in this study; edta is used in very small amount (10-10 m) as an inhibitor for the al corrosion in 0.5 m hcl. thermodynamic and adsorption parameters are calculated. the result shows that, in this range of concentrations, edta is chemisorbed at the al surface, forming a stable complex with al and give inhibition efficiency up to 89 %. for more concentration, unstable complex is formed and acceleration of corrosion occurs. the adsorption fit well to langmuir, temkin isotherms and el-awady model. density functional theory (dft) is used to study the geometrical optimizations of edta. from the obtained optimized structure, the highest occupied molecular orbital (ehomo), the lowest unoccupied molecular orbital (elumo and their energy difference (δe), the total energy (te), electronegativity (χ), dipole moment (µ), global hardness (η), global softness (σ), electron affinity (a), ionization potential (i), the fraction of electrons transferred (∆n) and were determined using b3lyp/6-31g(d,p) basis set. keywords edta; inhibition efficiency; adsorption isotherms; thermodynamic parameters; theoretical parameters introduction the study of al corrosion is of great importance; various industrial operations depend mainly on al. most investigations on the corrosion of al have been carried out on. the development of corrosion inhibitor is a good branch based on a functional organic compound. the structure and function groups of used organic compounds are useful for obtaining a good inhibitors [1-4]. depending upon excellent conductivity (electrically and thermally) of al; application of al is varied and widespread. [5]. adsorption of inhibitor on the charged metal surface is the main process to inhibit corrosion, on this basis; multiple bond(s), an electron rich atom as, s, n or p or a ring is a http://www.jese-online.org/ mailto:re_azooz@yahoo.com j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 236 main centers for the adsorption processes. in aqueous media, inhibitors are used to prevent or reduce the corrosion of metals [6–11]. it was shown that, compounds containing n or/and o atoms exhibit a good inhibiting effect. a polyprotic acid, i.e. ethylenediaminetetraacetic acid (edta), with a lone pair of electrons in its amino groups and two carboxylic acid groups is used for complexation with the charged metal ions [6,12]. complexation occur between (free or π) -electrons from inhibitor and the vacant d-orbital of a metal through the formation of donor–acceptor surface [6,14-16]. in the last years, edta has been studied to protect metals from corrosion in different environments [16–19], it was found that, different parameter affects the inhibition effect of edta including, the ph value, temperature, concentration, and type of the metal. nahle [20] has found that the sn(ii)–edta complex increased the dissolution rate of sn in a basic medium. milošev et al. [16] have investigated the corrosion of stainless steel in physiological solutions, while, edta prevents the formation of a passive layer and increases the solubility of the metal. gadiyar et al. [22] have discovered that edta inhibits the corrosion of carbon steel. however, its inhibiting effect is imperfect. alhaji and reda [23] have stated that edta is effective in decreasing the corrosion rate of copper-nickel alloy in seawater contaminated with sulfur. s. zor et. al. [24] observe that, the corrosion of al is higher in 0.1 m nacl solution in higher concentration of edta, and become slower at 10-4m edta the molecular structure, electronic structure and reactivity of inhibitors are determined well by quantum chemical methods [25]. a powerful framework is provided by dft [25,26] that help in understanding a lot of chemical processes [27-31]. concepts as, electronegativity hardness or softness etc. are used to describe chemical reactivity [28], are appear naturally within dft. the local electron density/population displacements represented the inflow of a single electron is measured using fukui function [30] and is representing the relative local softness of the electron gas. in the present study the inhibition effect of edta for the corrosion of al in 0.5 m hcl has been done using both weight loss and electrochemical methods. the temperature effect and adsorption isotherms will be studied in details. also analyzing the inhibitive properties of edta using dft calculations will be done. experimental chemical and reagents al strips have a rectangular form (4.5×3.5×0.2 cm), with the composition 99.11 % of al, 0.019 % of zn, 0.036 % of cu, 0.001 % of mg, 0.834 % of si and, were mechanically polished using different grades of emery sheets, washed with acetone and distilled water and dried. edta disodium salt (analar grade) and hcl were obtained from fluka ag, switzerland. all solutions were prepared using freshly prepared bidistilled water. stock solution of edta was prepared, from which all used concentrations are prepared via dilution. methods weight loss measurements the al samples (coupons) were weighed before immersion in 250 ml beaker containing 50 ml of the respective prepared test solutions at room temperature and desired temperatures. the setups were exposed for a period of 100 min. corrosion reaction is quenched in concentrated hno3 r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 237 digressed in ch3coch3 washed under water, dried and weighed. a mean value triplicate experiments is reported in each case. the values of weight loss in the presence and the absence of edta is used to calculate efficiency at the end of definite intervals of time. temperatures effects the same procedure adopted where the temperature of the study was varied, in the range (303333 k), from at the end of each experiment. the specimens were taken out, washed both in running tap water and into distilled water. they were dried and their weights were recorded. the loss in weight was calculated. each experiment was duplicated to get good reproducibility. weight loss measurement was performed in 0.5 m hcl with and without the addition of edta in the range (6.4 – 10.07)×10-10 m. electrochemical methods all electrochemical experiments were recorded using a potentiostate/galvanostate (eg&g 326a, u.s.a). the potential was scanned at the scan rate 10 mv s-1. all experiments were repeated to ensure reproducibility. fresh solution was used for each experiment. the cell used is a three compartment home-made one, with a reference saturated calomel electrode (sce), an auxiliary (ptfoil) electrode and a working (al) electrode with 0.4 cm2 area exposed to corroded solution was used. adsorption isotherms the adsorption of inhibitor at a metal /solution interface is the main source of inhibition effect, accordingly, the isotherms of adsorption can be determined. in order to obtain the isotherm the fractional surface coverage values () as a function of inhibitor concentration must be obtained. the values of  can be easily determined from the weight loss measurements by the ratio; . . 100 i e   where ie is inhibition efficiency obtained by a weight loss method. so, it is necessary to determine empirically which isotherm fits best to the adsorption of inhibitor on the al surface. scanning electron microscopy (sem) after a period of 100 min, al coupons was removed from solution, rinsed with a double distilled water, dried and observed in a scanning electron microscope (jsm-t20 electron probe microanalyzer (jeol, tokyo, japan)) to examine the surface morphology. the following cases were examined, to understand the morphology of the al surface in the absence and presence of inhibitors, (i) aluminum coupon after polishing, (ii) aluminum coupon dipped in 0.5 m hcl for 100 min. at 303 k and (iii) aluminum coupon dipped in 0.5 m hcl containing 2.7×10-10 m of edta inhibitor 100 min quantum chemical calculations dft is used to obtain the complete geometrical optimizations of edta, with beck’s exchange functional along with nonlocal correlation functional (b3lyp) of lee–yang–parr [32–34] with 6-31g* basis set in gaussian 03 program package [35]. from the obtained optimized structure, serval quantum chemical parameters were calculated; ehomo, elumo, δegap, the dipole μ and te. j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 238 results and discussion the molecular structure of an organic compound used in the present study is given in scheme 1. scheme 1. structure of edta open circuit potential potential-time curves was recorded for 60 minutes of immersion of the al specimens in aqueous 0.5 m hcl solution without and with edta at required concentrations. as seen in figure 1. from figure 1, when al is immersed in the hcl solution eocp drop sharply, then began to increase to more positive value and reached a stationary value after 25 minutes of immersion. the aggressiveness of the corroded solution may cause the differences in eocp values at the beginning of al exposure, it was suggested that, adsorption of edta molecules on the al surface is the reason for the initial negative shift. time, min figure 1.potential time curves for al in 0.5 m hcl in absence and presence of different concentrations of (edta) at 303 k. the results have shown that the addition of edta molecules at the beginning shifts eocp to more negative values. and then become more positive with time, due to oxide film growth [36]. in particular, initial values are more negative than steady state values, also the dependence of the eocp on concentration suggests that, the inhibitor molecules are strong and rapidly adsorbed at the steady state potentials [36]. potentiodynamic polarization studies the cathodic and anodic polarization curves of al in 0.5 m hcl in the absence and presence of different edta concentrations at 303 k are shown in fig. 2. the electrochemical kinetic parameters e / v v s. s c e r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 239 (in the potential range ±50 mv from ecorr), namely, corrosion current (icorr), corrosion potential (ecorr), and tafel slopes, (c and a), have been determined simultaneously and are listed in table 1. data infer that, the addition of edta to the acid solutions increases both the anodic and cathodic overpotentials, decreases the corrosion current density, icorr, and shifts the ecorr to more positive values. this means that the presence of edta inhibits the partial anodic dissolution of al and also retards the partial cathodic reduction of hydrogen ion. e / v vs. sce figure 2. potentiodynamic polarization curves for al in 0.5m hcl at 303 k with scan rate of 10 mv s-1 with and without different concentrations of edta. these results reveal that edta acts as a mixed type inhibitor. the inhibition efficiency ie, at different inhibitor concentrations at 303 k for al electrode in 0.5 m hcl solution was calculated from the equation 1 [37-39]: 0 / % 1 100corr corr i ie i         (1) where, i°corr and icorr are corrosion current density for uninhibited and inhibited solutions respecttively. table 1. the electrochemical kinetic parameters (icorr, ecorr, c and a) and inhibition efficiency (ie) obtained from polarization curves of al electrode in 0.5m hcl at 303 k in the absence and the presence of edta. cedta / m icorr / ma cm -2 -ecorr / mv -c / mv dec -1 -a/ mv dec -1 ie / % blank 0.89 670 122 0.69 - 1.07 10-10 0.47 660 118 0.60 47.2 2.13 10-10 0.46 650 116 0.53 48.3 3.20 10-10 0.35 630 118 0.52 60.7 4.27 10-10 0.21 580 112 0.51 76.4 5.33 10-10 0.12 530 115 0.50 86.5 6.40 10-10 0.10 500 114 0.50 88.8 mass loss the mass losses of al in 0.5 m hcl solution, with and without different concentrations of the edta were recorded after 100 min. of immersion at different temperatures. the corrosion rates of al alloy were calculated using equation 2 [37]. 87.6 m cr atd   (2) lo g ( j / m a c m -2 ) j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 240 where δm is the mass lost (g), 87.6 is a constant, a is the surface area of the coupon (cm2), d is the density (g cm-3), t is the time of exposure (h). the calculated cr fits into the range (less than 0.50 mm year-1) at which the application is acceptable [39]. figure 3 (a and b) shows the variation in mass loss for al coupons in the absence and the presence of edta. the mass loss in the presence of inhibitor is much smaller than the blank solution. the significant difference shows reduce impact on the cr of al in 0.5 m hcl. both of the surface coverage () and the inhibition efficiency (ie) were calculated using mass loss data according to equations 3 and 4, respectively [38]. 1 inh blamk w w         (3) ie / % =  × 100 (4) where, wblank is the corrosion rate in the uninhibited environments. winh is the corrosion in the inhibited environment. the high inhibition efficiency as the inhibitor concentration increases could be understood to be due to the reduction in corrosion rate. thus, edta could be considered as an inhibitor of al in 0. 5 m hcl solution given the high level of inhibition efficiency. the inhibitor efficiency, increased with the inhibitor concentration. c / 10-10 m t / k figure 3. mass loss of al immersed for 100 min. in 50ml hcl in the presence or absence of edta at different temperatures and edta concentrations. figure 4 shows the inhibition efficiency in different concentration of the edta and it is seen that the ie increases linearly with the inhibitor concentration. figure 4. ie after 100 min. in 50 ml hcl at different [edta] at different temperatures 0 10 20 30 40 50 60 70 80 90 100 0 2e-10 4e-10 6e-10 8e-10 ie / % [edta] / m 303 k 313 k 323 k 333 k w ig h t lo ss , m g c m -2 w ig h t lo ss , m g c m -2 r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 241 adsorption studies inhibition efficiencies of al in 0.5 m hcl was increase with increasing additive concentrations of edta, this phenomenon can be explained on the basis of adsorption. adsorption of the inhibitor can explain the nature of al/edta interaction. in the acid solution, firstly inhibitor is adsorbed on the metal surface and cover certain area from corroded solution and decrease or prevent this area from dissolution, whereas corrosion reactions normally occurred on inhibitor-free areas. accordingly, the area covered with inhibitor species (), can follow as a function of inhibitor concentration and/or solution temperature. when  is tested as a function of the concentration (at constant temperature), the adsorption isotherm can be evaluated at the equilibrium condition. four adsorption isotherms were tested using data from both weight loss and electrochemical techniques; a. langmuir’s isotherm the dependence of  at the concentration of the inhibitor, was fitted to langmuir’s isotherm, assuming that, a fixed number of adsorption sites is present on al surface, each one of these sites holds only one adsorbed species. figure 5 shows linear plots of c/ versus c with r2 ≥ 0.90, the average correlation coefficient, which suggests that adsorption was fitted to langmuir’s isotherm as in equation 5 [37]. ads 1c c k   (5) where c is inhibitor concentration, kads adsorptive equilibrium constant representing the degree of adsorption (i.e. if kads having higher value, the inhibitor is strongly adsorbed on metal surfaces). as shown in table 2, the value of kads which was obtained from the reciprocal of the intercept of a langmuir plot lines, and r2 of all lines were near unity. this means that obtained results is fit well with langmuir isotherm. the higher values of kads indicating a strong interaction between edta and the al surface. it seemed, therefore, that electrostatic interaction (physisorption) between inhibitor molecules existing as cations should prevail over molecular interaction, and this often results in strong interactions (chemisorption). table 2. data obtained from figure 3 temperature, k 303 313 323 333 technique used* wt.-loss elec. wt.-loss elec. wt.-loss elec. wt.-loss elec. r2 0.99 0.93 0.99 0.96 0.98 0.97 0.98 0.97 δgoads, kj mol-1 -66.38 -66.38 -67.25 -70.95 -70.78 -70.76 -71.83 -75.16 * wt.-loss weight loss measurements; elec. electrochemical methods c / 10-10 m c / 10-10 m figure 5. plots of c/ versus c of langmuir’s adsorption isotherm for the corrosion of al in 0.5 m hcl at different temperatures. a: from weight loss technique and b: from electrochemical technique (c / / m (c / / m j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 242 the equilibrium constant of adsorption kads is related to the standard adsorption free energy (∆goads) by equation 6: 0 ads1 exp 55.5 ads g k rt        (6) where 55.5 is the concentration of water in the solution expressed in, r is the gas constant and t is the absolute temperature. from table 1, the average value of standard adsorption free energy (∆goads) > -40 kj mol-1. the negative value of ∆goads ensures spontaneity of the adsorption process and the stability of the adsorbed layer on metal surfaces. in general, the values of (∆goads up to -20 kj mol-1 are consistent with the electrostatic interaction between the charged molecules and the charged metal (physisorption), while those around -40 kj mol-1 or higher are associated with chemisorption as a result of sharing or transferring of electrons from organic molecules to metal surface to form a coordinate type of bond. in the present work, the calculated value of δg° in all studied temperatures in both techniques are > -40 kj mol-1 indicating that the adsorption mechanism of edta on al surfaces in 0.5 m hcl solution was typical of chemisorptions. b. temkin isotherm the nature of the interaction at metal/solution interface is studied by temkin isotherm. by assuming a uniform distribution of the adsorption energy that increases with the increase of the θ. temkin isotherm model are given by the equation (7a and 7b). exp (f,) = kadsc (7a) and it is rearranged = (1/f) log c + (1/f) log kads (7b) where kads is the equilibrium constant, c is the inhibitor concentration,  is the surface coverage, f is the interaction term parameter, a lateral attraction between the adsorbing molecules is assumed if f > 0, but if f < 0, there is a lateral repulsion. the plot of  versus log c, yields curve with linear correlation coefficient r2 ≥ 0.90, close to unity, in all cases. the obtained value of kads(average) ≈ 4.1×104 and ≈4.4×104 in case of weight loss and polarization techniques repetitively, f > 0 indicating a strong lateral attraction between the adsorbing molecules of edta and the surface of the al. figure 6. plots of c versus of temkin’s adsorption isotherm for the corrosion of al in 0.5 m hcl at different temperatures, a: from weight loss technique and b: from electrochemical technique r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 243 table 3. data obtained from figure 4 t / k 303 313 323 333 technique used* wt.-loss elec. wt.-loss elec. wt.-loss elec. wt.-loss elec. f 1.6 1.7 2.3 1.93 2.1 2.15 1.8 2.49 kads 3.9 x104 4.1 x104 5.1 x104 4.2 x104 4.1 x104 4.4 x104 3.3 x104 4.7 x104 r2 0.95 0.90 0.96 0.90 0.95 0.91 0.95 0.91 * wt.-loss weight loss measurements; elec. electrochemical methods c. flory-huggins isotherm the amount of the inhibitor molecules that could displace the water molecules from the metal surface is studied using flory-huggins isotherm, which is showed by equation. 8 log (/c) = log k + x log (1- ) (8) where x is the size parameter that measure the number of adsorbed water molecules replaced by a given inhibitor molecule. figure 7 shows the plot of log (/c) vs. log (1- ), linear relationships with r2 > 0.8 is obtained, and indicating flory-huggins isotherm was obeyed. the obtained (kads)avg = 1.5×104 and the calculated δgads > -34 kj/mol. the size parameter is approximately 1. log (1-) log (1-) figure 7. plots of log (1) versus log (/c) of flory huggin’s adsorption isotherm for the corrosion of al in 0.5 m hcl at different temperatures, a: from weight loss technique and b: from electrochemical technique table 4. data obtained from figure 5 t / k 303 313 323 333 technique used* wt.-loss elec. wt.-loss elec. wt.-loss elec. wt.-loss elec. x 0.58 0.47 1.45 0.71 1.15 1.1 0.76 1.4 kads 1.5 x104 1.5x104 1.8 x104 1.4x104 1.5 x104 1.5x104 1.3 x104 1.5 x104 r2 0.96 0.90 0.98 0.90 0.95 0.91 0.95 0.91 * wt.-loss weight loss measurements; elec. electrochemical methods d. thermodynamic-kinetic model the surface coverage values obtained from the gravimetric and polarization measurements were also fitted into the adsorption isotherm of the thermodynamic-kinetic model of el-awady et al. are represented in equation. 9 log log ' log 1 k c y c          (9) where c is the concentration of the exudates,  is the degree of surface coverage, kads is the equilibrium constant of adsorption process, and kads= k1/y. 1/y is the number of inhibitory molecules occupying one active site (or the number of water molecules replaced by one molecule of edta. curves fitting of the data in the thermodynamic-kinetic model is shown in fig. 8. this data gave straight lines, the values of 1/y and kads calculated from the el-awady et al. curve model is given in lo g ( ([ e d t a ] /  ) / m lo g ( ([ e d t a ] /  ) / m j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 244 table 4. the values of 1/y (average) obtained are more than unity in all cases, indicating that each molecule edta involved in the adsorption process is attached to more than one active site on the metal surface. log([edta] / m) log([edta] / m) figure 8. plots of log c versus log ( / 1-) of thermodynamic-kinetic model for the corrosion of al in 0.5 m hcl at different temperatures, a: from weight loss technique and b: from electrochemical technique table 5. data obtained from figure 5 t / k 303 313 323 333 technique used* wt.-loss elec. wt.-loss elec. wt.-loss elec. wt.-loss elec. y 1.31 1.32 0.77 1 0.86 0.84 1.05 0.72 1/y 0.77 0.76 1.31 1 1.22 1.18 0.92 1.39 kl 7.7 x1014 9.1x1014 4.1 x1012 3.7 x1013 8.9 x1012 7.9x 1012 5.3x 1013 2.1x1012 r2 0.95 0.90 0.94 0.90 0.94 0.92 0.93 0.91 * wt.-loss weight loss measurements; elec. electrochemical methods by rearrangement of gibbs-helmholtz equation we obtain equation 10, which is used to calculate the enthalpy of adsorption (δhads) δgads/t = (δhads/t) k (10) a plot between the variations of (δgads/t) and (1/t) gave a straight line whose slope is δhads as shown in figure 9. the entropy of adsorption δsads was calculated using the following thermosdynamic equation (equation 11): δsads = (δhads δgads) / t (11) where, data of δgads were taken from langmuir isotherm results (from its r2 value, it is the best fit model) the obtained date of the calculated δhads and δsads was tabulated in table 6. figure 9. gibbs-helmholtz rearranged relation between (δgads/t) and (1/t) lo g (  / (1 - )) lo g (  / (1 - )) r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 245 table 6. adsorption thermodynamic parameters obtained using langmuir isotherm t / k 303 313 323 333 technique used* wt.-loss elec. wt.-loss elec. wt.-loss elec. wt.-loss elec. -δgads / kj mol-1 66.38 66.38 67.25 70.95 70.78 70.76 71.83 75.16 -δhads / kj mol-1 6.02 0.02 6.02 0.02 6.02 0.02 6.02 0.02 δsads / j mol-1 k-1 199.2 196.1 195.6 203.7 200.4 196.1 197.6 202.7 * wt.-loss weight loss measurements; elec. electrochemical methods the negative sign of δhads indicated the exothermic process of adsorption of the inhibitor on aluminum surface in hcl. the positive value of δsads in the presence of inhibitor can be attributed to the increase in the solvent entropy and more positive desorption entropy. it is also interpreted that the increase of disorderness is due to more water molecules which can be desorbed from the metal surface by one inhibitor molecule. therefore, it is revealed that decrease in the enthalpy is the driving force for the adsorption of the inhibitor on the surface of aluminum [28,29].the calculated values of heat of adsorption and entropy of adsorption are listed in table (6). effect of temperature based on temperature effect, inhibitors may be classified into three groups: 1. inhibitors whose inhibition efficiency (ie) decreases with temperature increase. the value of the apparent activation energy ea, found is greater than that in the uninhibited solution; 2. inhibitors in whose ie does not change with temperature variation. the apparent activation energy ea, does not change with the presence or absence of inhibitors; 3. inhibitors in whose presence the ie increases with temperature increase while the value of ea for the process is smaller than that obtained in the uninhibited solution. thus, in examining the effect of temperature on the corrosion process in the presence of edta, the arrhenius equation (eq. 12) is helpful alog log 2.303 e cr a rt    (12) where cr is the corrosion rate, ea is the apparent activation energy, r is the molar gas constant, t is the absolute temperature, and a is the frequency factor. figure 10 represents the arrhenius plot as log cr vs. 1/t for al corrosion in 0.5 m hcl in free so inhibited solution, linear plots were obtained. the values of ea were obtained from the slope of the arrhenius plot and are presented in table 8. from the table, it is seen that ea increases in the presence of the inhibitors compared to the blank. the higher value of the activation energy of the process in an inhibitor’s presence when compared to that in its absence is attributed to its physisorption, while the opposite is the case with chemisorption. according to eyring relationships (eq. 13), both of s* and h* could be obtained, * * ln c rh h s r nt rt r      (13) where h is the planck’s constant (6.626176×10-34 j s), n is the avogadro’s number (6.02252×10-23 mol-1), r is the universal gas constant, ∆h is the enthalpy of activation and ∆s is the entropy of activation. the kinetic results were found to fit the arrhenius and eyring equation, where plots of 1/t vs. ln rc/t or 1/t vs. −ln(hrc/kbt) (kb is boltzman constant and equation the term r/n) resulted in good straight lines. the activation parameters ∆h* and ∆s* can be evaluated from the slopes and intercepts of the straight line. j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 246 figure 10. arrhenius plot as log cr vs. 1/t for al corrosion in 0.5 m hcl in the absence and presence of various concentrations of edta. data obtained from weight loss technique table 7. activation energy, ea for aluminum corrosion in the presence of edta in 0.5 m hcl. c / m -ea / kj mol-1 0 51.05 1.07 x10-10 56.01 2.13 x10-10 59.11 3.20 x10-10 58.55 4.27 x10-10 65.93 5.33 x10-10 70.02 6.40 x10-10 69.64 figure 11 shows eyring plot and all lines are straight from which ∆h and ∆s were evaluated and their values are put in table 8. the positive values of ∆h reflect the endothermic dissolution of al in the presence and absence of the inhibitor. the increase in δha with the increase in the concentration of the inhibitor for al corrosion reveals that, the decrease in al corrosion rate is mainly controlled by kinetic parameters of activation. the negative values of ∆s may reflect the association mechanism of corrosion, i.e., the decrease in disorder takes place on going from reactants to the activated state. t-1 / k-1 figure 11. eyring plot as log cr/t vs. 1/t for al corrosion in 0.5 m hcl in the absence and presence of various concentrations of edta. data obtained from weight loss technique r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 247 table 8. thermodynamic parameters, ∆h and ∆s (for aluminum corrosion in the presence of edta in 0.5 m hcl) c / m -∆s* / j mol-1 k-1 ∆h* / kj mol-1 0 100.49 21.02 1.07 10-10 88.52 23.18 2.13 10-10 80.07 24.52 3.20 10-10 84.13 24.28 4.27 10-10 62.26 27.48 5.33 10-10 50.85 29.26 6.40 10-10 54.26 29.10 sem sem analysis of al metal surface, the sem image of the aluminum specimen before and after immersing in 0.5m hcl for100 min in the absence and presence of inhibitor system are shown in figures 12 (a, b and c) repetitively. the sem micrographs of aluminum surface after polishing (fig. 8a) shows a smooth surface of the al with no corrosion products on its surface. the sem micrographs of the al surface immersed in 0.5 m hcl (fig. 12b) shows its roughness which indicate the corrosion of al in hcl. fig. 12c indicates that in the presence of 10-10 m of edta, the surface coverage increases, which in turn results in the formation of insoluble complex on the surface of the metal (edta/inhibitor complex) and the surface is covered by a thin layer of inhibitor which effectively control the dissolution of aluminum. figure 12. sem of al surface at 30 °c; a) after polishing, b) after immersion in 0.5 m hcl for 100 min., and c) the same as b but in the presence of 2.7x10-10 m edta j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 248 quantum chemical calculations the activity properties of an inhibitor is related to its geometry as well as the nature of its frontier molecular orbitals, fmo, namely, the homo and lumo. therefore, in this study, quantum chemical calculations were performed to investigate the relationship between molecular structure of this compound and their inhibition effect. the optimized molecular structure and the fmo density distribution of the studied molecule are shown in figs. 13 and 14, and the calculated quantum chemical parameters are given in table 9 . adsorption centers of the inhibitor molecules are predicted by fmo. these centers are responsible for the interaction with surface metal atoms [42,43]. it was reported that, inhibitors with high homo energy offering electrons to unoccupied d orbital of the metal. where, inhibitors with lower lumo energy accept electrons from metal surface, as the δeg decreased, the efficiency of inhibitor improved [44]. the dipole moment (μ) of edta is 5.0542 debye (1.69x10-29 c m), which is higher than that of h2o (μ = 6.20×10−30 c m = 1.856 debye). the high value of μ probably increases the adsorption between edta and al surface [45]. accordingly, the adsorption of edta from the aqueous solution can be regarded as a quasi-substitution process between the edta in the aqueous phase [edtasol] and water molecules at the electrode surface [h2oads]. analysis of fig. 13 shows that the distribution of two energies homo and lumo localized in the nitrogen and oxygen atoms, consequently this is the favorite sites for interaction with the metal surface. the total energy of the edta is equal to -691282.91 kcal mol-1. this result indicated that edta is favorably adsorbed through the active centers of adsorption. figure 13.optimized structure (a), total energy (b) frontier molecular orbital diagrams; humo (c) and lumo (d) of the edta by b3lyp/6-31g (d,p) the number of transferred electrons (δn) was also calculated according to eq. (14) [46,47] al edta al edta 2( ) n         (14) r. e. azooz j. electrochem. sci. eng. 6(3) (2016) 235-251 doi:10.5599/jese.300 249 where al and edta denote the absolute electronegativity of al and edta molecule, respecttively; al and edta denote the absolute hardness of al and edta molecule, respectively. these quantities are related to electron affinity (a) and ionization potential (i) 2 i a    and 2 i a    where, i and a are related in turn to ehomo and elumo i = -ehomo and a = -elumo values of ηedta and χedta were calculated by using the values of i and a obtained from quantum chemical calculation. the theoretical values of χal and ηal are 3.230 and 2.77 ev mol-1, respectively [46]. the fraction of electrons transferred from inhibitor to the iron molecule (δn) was calculated. according to other reports [46,47], value of δn showed inhibition effect resulted from electrons donation. also the softness is calculated depending upon the following relation: = 1/ in this study, the edta was the donators of electrons while the al surface was the acceptor. the edta was bound to the al surface, and thus formed inhibition adsorption layer against corrosion. the adsorption centers of edta are estimated by mulliken population analysis [48]. authors believe that, the heteroatom with more negatively charged, is adsorbed on the metal surface through the donor-acceptor type reaction [43]. figure 14. energy distribution of edta using b3lyp/6-31g (d,p) table 9. calculated quantum chemical data for edta by b3lyp/6-31g (d,p) t.e. / kcal mol-1 ehumo /ev elumo / ev ∆egap/ ev µ / debye i / ev a / ev  / ev η / ev σ / ev ∆n / ev -691282.91 -5.849 -0.613 5.236 5.0542 5.849 0.613 3.231 2.618 0.382 -9.28x10-5 the mulliken charge of edta was shown in table 10. it can be seen that the most favorable sites for the interaction with the al surface were the following atoms: n32, n24, n30, n13, o23, o31, o15 and o14. because these atoms have larger negative charge, that donate electron. this being the preferred zone for nucleophilic attack. for edta, the homo is localized over the nitrogen n and oxygen o atoms, consequently this is the favorite sites for interaction with the metal. j. electrochem. sci. eng. 6(3) (2016) 235-251 edta as al corrosion inhibitor 250 table 10. mulliken charge of edta by b3lyp/6-31g (d,p) atom charge atom charge atom charge 1 c 0.214984 22 c 0.325515 43 h 0.121164 2 c -0.086817 23 o -0.567231 44 h 0.133160 3 c -0.127361 24 n -0.721935 45 h 0.133125 4 c -0.144686 25 c 0.210742 46 h 0.140598 5 c -0.066095 26 c -0.010327 47 h 0.139131 6 c 0.015268 27 c -0.083902 48 h 0.140004 7 c 0.216462 28 c -0.148904 49 h 0.151805 8 c 0.040626 29 c -0.123161 50 h 0.116621 9 c -0.006822 30 c 0.326666 51 h 0.379970 10 c 0.000002 31 o -0.567208 52 h 0.337115 11 c -0.073095 32 n -0.732883 53 h 0.147454 12 c 0.028970 33 c 0.185485 54 h 0.162757 13 n -0.306615 34 c 0.001330 55 h 0.130880 14 o -0.413216 35 c -0.084377 56 h 0.379729 15 o -0.419654 36 c -0.146845 57 h 0.336862 16 n -0.302210 37 c -0.105158 58 h 0.154020 17 c 0.220038 38 c 0.388987 59 h 0.167164 18 c -0.031616 39 o -0.588086 60 h 0.137476 19 c 0.044560 40 n -0.863854 61 h 0.351188 20 c -0.021716 41 h 0.134813 62 h 0.302186 21 c -0.140200 42 h 0.120365 63 h 0.346753 conclusions 1. edta acts as inhibitors for aluminum corrosion in acidic medium at very low concentrations 10-10 m. 2. inhibition efficiency of edta increases with increase in concentration of the inhibitors, but decreases with increase in temperature. 3. the values of ∆goads are negative, which suggests that the inhibitors were strongly adsorbed on the al surface. the values obtained support the chemisorption adsorption mechanism. 4. edta is found to obey langmuir, temkin adsorption isotherm and kinetic-thermodynamic model of el-awady et al for both weight loss and polarization techniques, from the fit of experimental data. 5. thermodynamic parameters revealed that the adsorption process is spontaneous. 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[1]. they, after a brief review about the electrodeposition processes in these alloys, presented the investigation on the electrodeposition of cu-sb alloys from acid methanesulfonic electrolytes and the obtained coatings indicated the presence of optical heterogeneity. the observed phenomenon in its initial stages of structure formation is very similar to those observed in other electrodeposited systems like ag-sb, ag-in, in-co and etc. [2-8]. based on the same type of electrolyte the goal of the present investigation is, by changing drastically the metal content in the http://www.jese-online.org/ mailto:tsvetina@ipc.bas.bg j. electrochem. sci. eng. 6(1) (2016) 105-111 electrodeposition of copper-antimony alloys 106 electrolyte, to find out the conditions of electrolysis where the self-organization of the different phases is expressed in the form of higher-ordered structures not only waves, but also spirals and targets. nowadays, it has been shown that the structure formation in the electrodeposited alloys occurs as a result of a reaction-diffusion mechanism where the presence of intermetallic compounds in the alloy plays a key role [9]. the phase diagram of the cu-sb system shows the presence of a few intermetallic compounds stable at room temperature as well as several hightemperature phases [10]. the present study has also the task to find out the possibility to obtain unstressed (without cracks) coatings with pinkish-purple color. the establishment of wellfunctioning non-toxic electrolytes for colored tarnish resistive alloys of non-noble metals is a challenge in the modern electroplating [11,12]. aim of the present study was to establish the conditions of the electrolysis for the preparation of structured and unstressed purple-pink coatings on the alloy cu-sb, including their phase characterization. experimental the composition of the electrolytes for the deposition of the alloy coatings is presented in table1. distilled water and pro analisi grade reagents were used for the preparation of the electrolytes. the cv experiments were performed in a 100 cm 3 tri-electrode glass cell at room temperature. the working electrode (area 1 cm 2 ) was made from platinum and the two counter electrodes were made from platinum. ag/agcl reference electrode (еag/agcl= 0.197 v vs. nhe) was used. table 1. composition of the electrolyte components concentration, g dm -3 sb as k(sbo)c4h4o6 1/2h2o 27 cu as cuso4 5h2o 2.4 methanesulfonic acid 20 d(-)-c4h6o6 96 the experiments were performed by means of a computerized potentiostat/galvanostat princeton applied research model 273 using the software powersuite. the alloy coatings with thickness between 3 and 25 m were deposited onto brass substrates with an area of 2×1 cm in the glass cell. the preliminary preparation of the cathodes includes a standard procedure of electrochemical degreasing followed by pickling in a 20 % solution of sulphuric acid. two pt (ti) counter electrodes (about 4 cm 2 each) were used. the content of antimony (respectively copper) (as well as the thickness) in the coatings depending on the electrolysis conditions, was determined by x-ray fluorescence analysis (fischerscope x-ray xdal) in 3 points (in the bottom, middle and top of the sample, respectively). the electrode is situated vertically and at the ‘bottom’ means the closest point to the electrical contact. the sb or cu distribution on the surface of the coatings was examined by energy dispersive x-ray analysis (edx). the surface morphology of the coatings was investigated by scanning electron microscopy (sem) – jeol jsm 6390. the phase composition was characterized by x-ray diffraction (xrd) using a panalytical empyrean device equipped with a multichannel detector (pixel 3d) using cu-k𝛼 (45 kv, 40 ma) irradiation in the 2𝜃 range 20–115°, with a scan step of 0.01° for 20 s. v. s. kostov et al. j. electrochem. sci. eng. 6(1) (2016) 105-111 doi:10.5599/jese.235 107 results and discussion figure 1 presents the cyclic voltammetry curves of the electrolytes, containing both metals separately and together. the electrodeposition of antimony starts at about -0.5 v (curve 1) and the maximum of the reaction is characterized by the current peak at -0.8 v; the reduction of copper ions starts earlier (curve 2). the curve of the alloy electrodeposition follows the run of the copper curve at low potentials and after reaching the deposition potential of antimony runs similar to the antimony curve. the coatings of copper and antimony, deposited in the cathodic period are dissolved in the anodic region. only one dissolution peak (0.4 v for the antimony dissolution and 0.65 v – for the copper dissolution) could be seen in the electrolytes with single metals. the oxygen evolution reaction in the investigated region of the potentials is not reached. in the alloy electrolyte, containing both metals at the beginning of the process up to -0.4 v the curve almost repeat the run of the curve in the copper-containing electrolyte. the sharply increase of the current continues up to the -0.7 v. this potential corresponds to the maximum of the observed cathodic peak in the alloy electrolyte and most probably, belongs to the formation of some alloy phase. in the anodic period only one very broad anodic peak of the dissolution of all formed in the cathodic period phases of cu, sb and some intermetallides are observed. figure 1.cyclic voltammetry curves, obtained at 20 mv s -1 in the solution of: curve 1 75 g dm -3 k(sbo)c4h4o6×0.5 h2o + 96 g dm -3 d(-)-c4h6o6 + 20 ml dm -3 ch3so3h (alloy electrolyte without cu); curve 2 8 g dm -3 cu so4×5 h2o + 20 ml dm -3 ch3so3h + 96 g dm -3 d(-)-c4h6o6(alloy electrolyte without sb); curve 38 g dm -3 cu so4×5 h2o + 75 g dm -3 k(sbo)c4h4o6×0,5 h2o + 20 ml dm -3 ch3so3h + + 96 g dm -3 d(-)-c4h6o6(alloy electrolyte) figure 2 and 3 present the dependence of the content of the coatings and the deposition rate on the current densities. with the increasing of the current density up to 0.8 а dm -2 the content of sb in the coatings increases from 0 to 80 wt. % and reaches a plateau. at low current densities the coatings are pink, copper-like and at 0.3-0.4 а dm -2 some optical heterogeneity like darker spots and dots appear onto the cathodic surface. after 0.5 а dm -2 the coatings are lilac and at the highest current densities (after 1.3 a dm -2 ) they are shiny and stressed, with a lot of cracks. the deposition rate increases with increasing the current density up to 0.7 μm min -1 , which is a relatively high deposition rate at similar electrolysis conditions compared to other alloy systems [8]. j. electrochem. sci. eng. 6(1) (2016) 105-111 electrodeposition of copper-antimony alloys 108 figure 2. effect of the current density on the antimony percentage in the coatings. the composition of the electrolyte is presented in table 1. figure 3. effect of the current density on the deposition rate of the cu-sb coatings. the composition of the electrolyte is presented in table 1. the morphology of the coatings, containing more than 60 wt. % of sb is presented in the figure 6. the presence of aggregates (about 3 μm each) with needle-like crystallites (not bigger than 1 μm) could be clearly seen. moreover, there is a phase heterogeneity visible in the regime of back scattered electrons. reasons for this conclusion gave the images on the left side of spi electron microscopic image (figure 4) obtained in regime of back scattered electrons (bei), while the right part of the photograph shows an image, obtained in regime of secondary electrons (sei). the heterogeneity is closely connected with the presence of more than one phase in the coatings. the proof of the phase heterogeneity is presented below. figure 5 shows the x-ray diffraction patterns of samples with different sb-content. in the sample, containing 100 wt % cu (at 0.1 a dm -2 ) the reflexes of α-cu phase with a cubic lattice parameters a = b = c = 36.070 nm (ref. code 98-062-7117) together with the reflexes of the substrate of cu2.6zn1.4 (ref. code 98-062-9460) could be detected. v. s. kostov et al. j. electrochem. sci. eng. 6(1) (2016) 105-111 doi:10.5599/jese.235 109 figure 4. scanning – electron image of the pink-lilac coatings with content 64.9 wt. % sb, 35.1 wt. % cu and thickness 3.m. figure 5. x-ray diffractograms of cu-sb coatings with different antimony content, obtained in the electrolyte, presented in table 1.the existed phases are marked with numbers 1 cu; 2 sb; 3 – cu2.6zn1.4; 4 – cu2sb1; 5 – cu11sb3 with increasing the current density to 0.3 a dm 2 the content of sb in the coating increases to 38 wt. % and reflexes of the phase cu2sb whose lattice is tetragonal with parameters a = b = 4.002 and c = 61.040 nm (ref. code 98-041-2295) could be observed. with increasing the content of antimony in the coatings up to 64-78 wt. % the reflexes of the mentioned cu2sb phase and of a new-appearing cu11sb3 phase with orthorhombic lattice with parameters а=4.3240, b=19.0800, c=4.7240 (ref. code 98-010-3093) are detected. according to our knowledge the last mentioned phase is not presented in the literature as electrochemically obtained. in the paper of hrussanova and krastev only the presence of intermetallic cu2sb phase is reported [1]. obviously additional characterization and comparison of this electrochemically obtained phase with a metallurgically obtained phase is needed. this phase, according to the phase diagram should be the ξ-phase, with the region of homogeneity from 42 to 58 wt. %. at higher content of the sb most probably the η– phase of sb is also presented (figure 5). j. electrochem. sci. eng. 6(1) (2016) 105-111 electrodeposition of copper-antimony alloys 110 the next focus of this study was to find out the conditions of structure formation on the surface of the electrode. based on theexperience with other alloy systems [13], and performing a large number of experiments the electrolysis conditions obtaining coatings with a wide variety of structures – left and right handed spirals, targets, even two-armed spirals (see figure 6b – in themiddle of the optical image) were established. the distance between two fronts of the structures (~100 μm) is about one order of magnitude higher thanin the case of the ag-sb structures [3]. the spatio-temporal structures are for the first time observed in the coppercontaining alloy system. a b figure 6. optical images of the coating at different magnifications with spatio-temporal structures, obtained at: j = 0.4 a dm -2 , deposition time240 min, thickness -23.76 µm the results indicate that the spatio-temporal structuresare observed at a certain supersaturation and a specific adjustment of the ratio of the two metals in the electrolyte during the flow of a certain amount of electricity. the galvanostatic experiments on the reproducibility of the spiral pattern phenomena show that at certain cu and sb metal ratios in the electrolyte and at certain current densities the spiral structures can be observed in a fresh electrolyte without any preliminary electrolysis. as shown in previous studies [8] the spatio-temporal structures appear onto the electrode surface due to some instabilities. heterogeneous coatings are obtained as a result of random or ordered distribution of the different phases. so the phase composition of the patterned coating, v. s. kostov et al. j. electrochem. sci. eng. 6(1) (2016) 105-111 doi:10.5599/jese.235 111 shown in figure 6 is presented in figure 7. in this coating three phases sb, cu2sb, cu11sb3 are registered. the burnt parts of the surface at the ends of the electrodes are rich of antimony and the structures are composed from two phases of the alloy cu2sb and cu11sb3. figure 7. x-ray diffractogram of the sample, shown in figure 6. the registered phases are marked with numbers: 1 – sb; 2 – cu2.6zn1.4; 3 – cu2sb1; 4 cu11sb3. conclusions the possibility to obtain copper-antimony alloy with up to 80 wt % sb from methanesulfonic acid is shown. the deposition rate, morphology and the phase composition of the obtained coatings are established. the phenomena of formation of spatio-temporal structures in this alloy are described. most probably, the structures are formed by the phases cu2sb and cu11sb3. acknowledgments: the authors express their gratitude to bulgarian national science fund for the financial support of project т02-27/2014. references [1] a. hrussanova, i. krastev, a. zielonka, zaštita materijala 52 (2011) 145-151. [2] i. kristev, m. nikolova and i. nakada, electrochim. acta 34 (1989) 1219-1223. [3] i. kristev, m. nikolova, j. appl. electrochem. 16 (1986) 867-874. [4] i. kristev, m. nikolova, j. appl. electrochem. 16 (1986) 875-878. [5] n. dimitrova, t. dobrovolska, i. krastev, arch. metall. mater. 58 (2013) 255-260. [6] t. dobrovolska, v. d. jovic, b. m. jovic, i. krastev, j. electroanal. chem. 611 (2007) 232-240. [7] i. krastev,ts. dobrovolska, j. eng.proces. management intern. j 2 (2010) 99-105. [8] i. krastev, t. dobrovolska, j solid state electrochem.17 (2013) 481-488. [9] b. bozzini, d. lacitignola, i. sgura, j. solid state electrochem. 17 (2013) 467-479. [10] s. fürtauer, h. flandorfer, monatsh chem 143 (2012) 1275-1287. [11] u. e. klotz, gold. bull. 43 (2010) 4-10. [12] j. fischer-bühner, a. basso, m. poliero, gold. bull. 43 (2010) 11-20. [13] ts. dobrovolska, i. krastev, electrodeposition of silver-indium alloys, in electrolysis: theory, types and applications, nova science publishers inc, new york 2010,pp. 303-326. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ screen-printed carbon electrode/natural silica-ceria nanocomposite for electrochemical aptasensor application: http://dx.doi.org/10.5599/jese.1455 1225 j. electrochem. sci. eng. 12(6) (2022) 1225-1242; http://dx.doi.org/10.5599/jese.1455 open access : : issn 1847-9286 www.jese-online.org original scientific paper screen-printed carbon electrode/natural silica-ceria nanocomposite for electrochemical aptasensor application salma nur zakiyyah1, diana rakhmawaty eddy1, m. lutfi firdaus2, toto subroto3 and yeni wahyuni hartati1, 1department of chemistry, faculty of mathematics and sciences, universitas padjadjaran, jl. raya bandung-sumedang km. 21 jatinangor, sumedang 45363, indonesia 2graduate school of science education, bengkulu university, jl. w.r. supratman kandang limun, bengkulu 38371, indonesia 3bionformatics and biomolecular research center, universitas padjadjaran, jl. singaperbangsa 2, bandung 40132, indonesia corresponding author: yeni.w.hartati@unpad.ac.id received: july 7, 2022; accepted: september 25, 2022; published: october 6, 2022 abstract a nanocomposite of natural silica and ceria was synthesized to modify a screen-printed carbon electrode (spce) to develop an aptasensor to detect epithelial sodium channel (enac) protein in urine as a biomarker of hypertension. the method steps were the synthesis of natural silica-ceria nanocomposite using the hydrothermal method, obtaining of natural silica nanoparticles from the extraction of alkaline silica sand and ceria nanoparticles from cerium nitrate, modification of spce/natural silica-ceria, immobilization of aptamer through streptavidin-biotin, and detection of enac protein concentration. box-behnken’s design was employed to determine the optimal conditions of aptamer concentration (0.5 μg ml-1), streptavidin incubation time (30 min), and aptamer incubation time (1 hour), respectively. differential pulse voltammetry (dpv) characterization of the developed electrochemical aptasensor revealed that the [fe(cn)6]3-/4redox peak current increased from 3.190 to 9.073 μa, with detection and quantification limits of 0.113 and 0.343 ng ml-1, respectively. the method is proven as a simple and rapid method to monitor enac levels in urine samples. keywords silica-ceria; synthesis composite, aptamer; enac protein; hypertension biomarker; voltammetry introduction in the development of electrochemical-based biosensor methods, electrode modification is the most important step to immobilize bioreceptors onto the electrode surface [1]. in addition http://dx.doi.org/10.5599/jese.1455 http://dx.doi.org/10.5599/jese.1455 http://www.jese-online.org/ http://www.jese-online.org/ mailto:yeni.w.hartati@unpad.ac.id j. electrochem. sci. eng. 12(6) (2022) 1225-1242 spce/natural silica-ceria nanocomposite 1226 of improving biosensor performance, the basic functions of nanoparticles can be mainly classified as: 1) immobilization of biomolecules; 2) catalysis of electrochemical reactions; 3) enhancement of electron transfer; 4) labeling biomolecules and 5) acting as the reactant [2]. many types of nanomaterials have been reported to modify electrode surfaces, for example, a screen-printed carbon (spce) immunosensor was modified with aunp [3-8], and spce aptasensor was modified with ceria [3,9,10]. silica-based nanocomposites have been widely developed as electrode modifiers, such as silica-au nanocomposite for the detection of the enzyme glucose oxidase [11], c-creative protein [12], and arsenic detection [5]. silica-modified spce for sensors has already been reported [5,13,14] . it is worth noting that spce is an electrode that combines a carbon working electrode, ag/agcl reference electrode, and carbon counter electrode in one compact and easy-to-use design [15]. silica nanoparticles can be made into composites with metal nanoparticles [16]. silica nanoparticles used for dopamine detection in biosensors demonstrate excellent performance and low detection limits [17]. silica composites are used in electrochemical sensors because they have a very high specific surface area [5], while biologically modified silica composites in mono or multilayer configurations are commonly applied in electroanalysis, especially in the field of biosensors [18]. silica-ceria composite is an interesting material because silica can improve the thermal stability and texture of ceria. typically, silica-ceria composites are synthesized by the hydrothermal method [19] with silica obtained from various sources or precursors. natural sand is a commonly use source of silica which is mixed with oxides and other minerals and therefore, a separation is needed to obtain pure silica by the alkaline fusion method [20]. up to now, spce electrode modified with natural silica-ceria composite has never been carried out, and only synthetic silica was applied in biosensors to detect some inorganic compounds [21,22]. one of the uses of natural silica produced from natural sand in this study [20] is to stabilize ceria nanoparticles. ceria nanoparticles are transition metal oxide (ceo2) elements that have good potential in the development of biosensors with high electron mobility due to high oxygen storage capacity in its structure, redox transition ability, enhanced electron transfer, biocompatibility, and high conductivity. all these properties improve sensitivity, selectivity, stability, and long-term maintenance of biosensor bioactivity [23]. when ce(no3)36h2o is added to the silica dispersion, the positively charged ce3+ ions are attracted to the surrounding silica through electrostatic interactions, and precipitate on the surface of the silica in a weakly alkaline environment [24]. ceria from the silica-ceria composite strongly interacts with –nh2 groups to bind streptavidin and thionine for the matrix metalloproteinase-2 biosensor, and can form a bridge bond with the carboxyl functional group of the antibody even without the addition of other agents [3]. the amphipathic protein structure can non-specifically bind to the silica surface due to the curvature of the silica and the molecular weight, protein affinity constants, as well as the adsorption process. adsorption as a non-covalent electrostatic interaction model, with the layer by layer (lbl-esa, or lbl) electrostatic assembly method is used in bioconjugation with silica [25]. by comparing the effect of the size of the material on the electrode, damiati et al. [26] showed that it has a direct impact on the electrochemical response and sensitivity of the biosensor. it is known that smaller sized electrodes exhibit higher sensitivity, attributed to the better structural material properties due to the presence of the nearest neighbor. based on previous research, a biosensor can detect hypertension with the enac protein biomarker [27]. hypertension occurs when an individual experiences increase in blood pressure s. n. zakiyyah et al. j. electrochem. sci. eng. 12(6) (2022) 1225-1242 http://dx.doi.org/10.5599/jese.1455 1227 above normal and is associated with increased morbidity and mortality [28]. the epithelial sodium channel (enac) is a major regulator of salt and water reabsorption in many epithelial tissues, while abnormalities of enac function are directly associated with hypertension [29]. the enac controls the rate limiting step of sodium reabsorption in epithelial cells and is located on the apical membrane of cells in the distal nephron which is responsible for controlling na+ reabsorption. enac protein in urine is a biomarker of hypertension as enac expression promotes na+ retention, leading to an increase in blood pressure [30,31]. typically, enac protein is detected by enzyme-linked immunosorbent assay (elisa) [4,32], but this method is time consuming and expensive [27]. immunosensors can detect low levels of enac [3,31] since using antibodies as bioreceptors is highly selective and sensitive [33]. however, antibody based methods are relatively expensive for commercial purposes and therefore alternative methods have been developed, such as the determination of enac protein levels through its interaction with aptamers [34]. aptamers are single stranded rna or dna synthetic oligonucleotides selected through in vitro method known as systematic evolution of ligands by exponential enrichment (selex) which can bind to its target with high affinity and specificity due to its three-dimensional structure. the advantages of aptamers over antibodies are high stability, easily synthesis and modification, low immunogenicity, and possibility of recognition a wide variety of molecules [5]. thus aptasensors have high affinity, lower costs, and are easier to modify [35]. the use of streptavidin to bind to biotinylated aptamers has already been reported [8,36] . the biotinylated aptamer in this study was used from a previous study [37]. this paper reports the development of an aptasensor using a modified silica-ceria spce to detect enac protein as a biomarker of hypertension. experimental the materials used were biotinylated aptamer (biotin 5’cgg tga ggg tcg ggt cca gta ggc cta ctg ttg agt agt ggg ctc c -3’) (abcam), enac protein, bovine serum albumin (bsa), potassium ferricyanide k3[fe(cn)6] (sigma aldrich), streptavidin (promega), phosphate buffer saline (pbs) ph 7.4 (vwr), potassium chloride (kcl) (merck), sodium hydroxide (naoh) (merck), absolute ethanol (merck), methanol (merck), cerium nitrate (ce(no3)3.6h2o) (merck), sample urine (non-hypertention) and double distilled water (pt. ikapharmindo putramas). screen-printed carbon electrode (spce) (dwg no. 9601626, gsi technologies, usa) consisting of the carbon working electrode (we), carbon counter electrode (ce), and reference ag/agcl electrode (re), (zimmer and peacock), connected to a computer using software pstrace 5.8. fourier transform infra-red (ft-ir) spectrometer (thermo scientific), scanning electron microscopy (sem) (hitachi tm3000), particle size analyzer (psa) (horiba sz-100), and uv-vis spectrophotometer (thermo scientific g10s uv-vis). preparation of silica-ceria nanocomposites ceria nanoparticles were synthesized by first dissolving 2 g of cerium nitrate hexahydrate (ce(no3)36h2o) in 50 ml double distilled water before the dropwise addition of 25 ml (0.1 m) sodium hydroxide (naoh) solution with constant stirring for 30 minutes at room temperature. the pale yellowish white precipitate formed within 1 hour, and was collected by centrifugation at 8000 rpm for 20 min. the pellets were washed by double distilled water several times, dried at 70 °c for 2 hours, and cooled to room temperature before calcining at 240 °c overnight to obtain the nanoparticle ceria powder that was characterized by uv-vis spectroscopy, ft-ir, and psa [38]. http://dx.doi.org/10.5599/jese.1455 j. electrochem. sci. eng. 12(6) (2022) 1225-1242 spce/natural silica-ceria nanocomposite 1228 silica was obtained from in house synthesis as previously described [39]. the ceria-silica composite was prepared by mixing 4.5 ml of distilled water and 14.5 ml of absolute ethyl alcohol and stirring sonochemically for 15 minutes before addition of 0.018 g of ground silica. the solution was stirred sonochemically for 3 minutes and 0.1031 g of solid ceria was added and mixed for 45 minutes. immediately after mixing, 10 ml of double distilled water was added and stirred continuously for 20 minutes. the solution was centrifuged for 10 minutes at 6000 rpm and the silica-ceria composite was then dried at 110°c for 2 hours and calcined at 500 °c for 5 hours before redissolving in 50 ml methanol. the composites were made in various concentrations (0.007, 0.01, and 0.03 mm). the ceria-silica composite was characterized by uv-vis spectroscopy, ftir, and sem [40]. characterization of silica-ceria nanocomposite in this work, particle size analyzer (psa, horiba sz-100) was used to characterize the particle sizes, scanning electron microscopy (sem, hitachi tm3000) to characterize the surface morphology of electrodes, and uv-vis spectrometer (thermo scientific g10s uv-vis) to analyze the uv block properties of the samples. the stability of the surface functional groups was examined by ftir (thermo scientific), while differential pulse voltammetry (dpv, zimmer and peacock) was used for electrochemical measurements. preparation of spce spce electrodes were washed twice to remove impurities physically adsorbed on the electrode surface with double distilled water, then dried at room temperature and characterized using sem and dpv in 10 mm k3[fe(cn)6] solution in 0.1 m kcl as the supporting electrolyte [21]. modification of spce with silica-ceria nanocomposite the composite solution was dropped onto the surface of the spce and allowed to dry before rinsing with double distilled water and characterized using sem-eds and dpv performed as for bare spce [11]. immobilization of streptavidin on spce/silica-ceria ten microliters of 50 μg ml-1 streptavidin solution was dropped onto the surface of the spce/silica-ceria and incubated in the refrigerator. the obtained spce/silica-ceria/stv electrodes were rinsed and characterized using sem-eds and dpv as described above [11]. immobilization of biotinylated aptamer on spce/silica-ceria/stv ten microliters of 0.5 μg ml-1 aptamer-biotin solution was added to the dry spce/silicaceria/stv, and then measured by dpv as in previous stages [11]. determination of aptasensor response to enac protein fifteen microliters of 1 % bsa was added to the electrode for 20 minutes, then rinsed by double distilled water before 15 μl of enac solution was dropped onto the electrode and incubated for 20 minutes. then, dpv was performed in conditions described in previous sections. the schematic of this aptasensor preparation and testing is shown in figure 1. s. n. zakiyyah et al. j. electrochem. sci. eng. 12(6) (2022) 1225-1242 http://dx.doi.org/10.5599/jese.1455 1229 figure 1. preparation and testing of electrochemical aptasensor using a nanocomposite-streptavidinaptamer to detect enac protein optimization of parameters using the box-behnken method the parameters used were variations in the concentration of the enac aptamer (x1), the incubation time of the spce-nanocomposite (x2), and the incubation time of the spcenanocomposite-aptamer (x3). the factor is designed through 3 different levels, namely the lowest (-1), medium (0), and highest (+1) level, as shown in table 1. table 1. factors and levels of optimization analysis of experimental conditions factor level -1 0 +1 enac aptamer concentration (x1), mg ml-1 0.5 1 1.5 streptavidin incubation time (x2), min 30 60 90 enac aptamer incubation time (x3), h 1 2 16 calibration curve, detection limit, and quantification limit various concentrations of enac solutions (0.047, 0.094, 0.187, 0.375, 0.75, 1.5 and 3.0 ng ml-1) were measured by the electrochemical aptasensor with differential pulse voltammetry (dpv) method, in 0.1 m kcl containing [fe(cn)6]4−/3redox system. the potential range of -1.0 to +1.0 v and a scan rate of 0.008 v/s using the optimal conditions determined from the box-behnken method. a linear curve of the difference between average peak current values (∆i) vs. the concentration for each measurement was evaluated to determine limits of detection (lod) and quantification (loq), precision, and recovery values. determination of enac recovery in urine samples samples were prepared by the standard addition method [41,42], i.e. 20 μl urine sample was spiked with 2 μl of various concentrations of enac (1, 2, and 3 ng ml-1), and 2 μl of phosphate buffer saline (pbs) was used as a control. the electrochemical response of aptasensor was measured by dpv using a redox system of 10 mm k3[fe(cn)6] solution in 0.1 m kcl, in the potential range of -1.0 to +1.0 v at a rate of scan 0.008 v/s. http://dx.doi.org/10.5599/jese.1455 j. electrochem. sci. eng. 12(6) (2022) 1225-1242 spce/natural silica-ceria nanocomposite 1230 results and discussion characterization of ceria nps, silica-ceria composite, and spce characterization of the precipitated synthesized ceria, silica-ceria composite, and modified spce is shown in figures 2-4. figure 2a shows the uv-vis spectrophotometry results of ceria particles, showing a peak at 308 nm which is within the normal range of 250-400 nm [43]. this peak is produced by the transition of electrons from the 2p o level to the 4f ce level and by the 4f-5d electronic transition in ce3+ [44]. based on figure 2b, the silica nanoparticles showed a peak at 300 nm with an average particle size of 99.7 nm. the ceria-silica nanocomposite in figure 2c showed absorption peak at maximum wavelength of 337 nm, with a small peak at 231 nm. the maximum wavelength peak range for the ceria-silica composite is 250–400 nm [24] as reported by dalmis et al. [45], and vaja et al. [46], where a small shoulder observed is around 200–300 nm. the uv-vis absorbance of most of the ceria comes from the charge transfer transition from o 2p to ce 4f, and the spectrum shows two overlapping absorption peaks caused by the indirect and direct transitions. usually, the absorption edge energy is calculated assuming direct and indirect transitions [46]. figure 2. uv-vis characterization of (a) ceria with a maximum absorption wavelength at 308 nm, (b) ceria-silica nanocomposite with a maximum absorption wavelength at 337 nm and a small peak at 231 nm, (c) silica nanoparticles with a maximum absorption wavelength at 300 nm figure 3a of the particle size analyzer (psa) characterization shows that the ceria particles were 251.6 nm in size, classifying them as microparticles as they were >100 nm. figure 3b shows that the silica nanoparticles were 99.7 nm, i.e. nanoparticles. in this study, a composite modified electrode with composites that are materials consisting of two or more materials that have different properties with their constituent properties remaining in the resulting material [47]. the electrodes were modified with composites silica-ceria in 1:2 mole ratio as in a previous study [24]. silica particles have a smooth surface and are negatively charged in neutral or basic dispersions. when ce(no3)36h2o is added to the silica dispersion, the positively charged ce3+ ions are attracted to the vicinity of sio2 through electrostatic interactions and precipitate on the surface of sio2 in a weakly alkaline environment. figure 4a shows the ftir characterization of the silica-ceria nanocomposite, with peaks at 3440.9 cm-1 indicating oh stretching, 549.35 cm-1 (ce-o), 1621.51 cm-1 (oh bonding, 1095.3 cm-1 (si-o-si), and 491.28 cm-1 (si-o). figure 4b shows the peak at a frequency of 549.35 cm-1 confirming ce-o bond [44,48] . figure 4c shows a peak at 1097.973 cm-1 indicating si-o-si bonds and 421.24 cm-1 si-o bonds [39]. s. n. zakiyyah et al. j. electrochem. sci. eng. 12(6) (2022) 1225-1242 http://dx.doi.org/10.5599/jese.1455 1231 diameter, nm diameter. nm figure 3. psa characterization of (a) ceria particles with a size of 251.6 nm and (b) silica nanoparticles with a size of 99.7 nm figure 4. ftir spectra of (a) ceria-silica nanocomposite, (b) ceria particles, (c) silica nanoparticles characterization and modification of spce figure 5 shows the surface morphology of the spce modified by silica-cerium and biomolecules, taken with the scanning electron microscope (sem). figure 5a shows the morphology of the bare spce surface. in figure 5b, the surface of spce after modification by the silica-ceria nanocomposite looks different in morphology, suggesting that the composite is attached to the spce surface. after immobilization of streptavidin biomolecules, figure 5c shows that spce surface looks more homogeneous and denser than before. more homogeneous and denser surface morphology shown in figure 5d for spce-nanocomposite after addition of the aptamer also confirms that the electrode surface has been changed. some of chemical elements of scpe-ceria-silica nanocomposite and that modified by biomolecules, determined by the eds test shown in figure 5e and f, are listed in table 2. according to table 2, four elements are contained in the composite-modified spce, namely carbon (a component of the spce electrode), oxygen, silicon, and cerium (components of composite), indicating thus the successful modification of the spce electrode by ceria-silica composite. also, table 2 shows that spce-nanocomposite modified by streptavidin biomolecules contains carbon, oxygen, silicon, and cerium as well as nitrogen contained in the streptavidin protein, indicating the successful modification of the spce-nanocomposite electrode by streptavidin biomolecule. http://dx.doi.org/10.5599/jese.1455 j. electrochem. sci. eng. 12(6) (2022) 1225-1242 spce/natural silica-ceria nanocomposite 1232 figure 5. sem characterization of (a) bare spce; (b) spce-nanocomposite; (c) spce-nanocompositestreptavidin; (d) spce-nanocomposite-streptavidin-aptamer, and eds of (e) spce-nanocomposite surface; (f) spce-nanocomposite-streptavidin surface table 2. eds characterization the modified spce element spce-nanocomposite spce-nanocomposite-streptavidin content, wt. % carbon 8.699 37.601 oxygen 29.260 24.130 silicon 7.024 2.275 cerium 55.017 32.621 nitrogen 3.372 s. n. zakiyyah et al. j. electrochem. sci. eng. 12(6) (2022) 1225-1242 http://dx.doi.org/10.5599/jese.1455 1233 the modification of the spce with various concentrations of nanocomposite (0.007, 0.01 and 0.03 mm) was characterized by differential pulse voltammetry (dpv) in 0.1m kcl containing 10 mm k3[fe(cn)6], in the potential range of -1.0 to +1.0 v at a scan rate of 0.008 v/s. figure 6a shows that the highest fe(cn)63-/4redox peak current was obtained with 0.01 mm concentration, while at higher concentration of composite (0.03 mm), a decrease in the redox peak current occurs (table 3). silica-ceria composite material is semi-conductor [16,49] that can increase electron transfer at certain concentrations and conditions. in this experiment, the data obtained at the concentration of 0.01 mm has an optimum current of ferri-ferrocyanide reaction, while at higher concentration, the insulator properties increased. anyhow, figure 6b shows that modification of the electrode is needed to increase the sensitivity to the required biosensor sensing. figure 6b presents the characterization of bare spce electrode and spce electrodes modified with composites and biomolecules, showing firstly an increase in current due to the presence of composite at spce, and then a decrease in the current after the addition of biomolecules (table 3). the electrode is modified with biomolecules which function to make a specific bond in the biosensor analysis. biomolecule s are large and not conductive, thereby inhibiting electron transfer which results in a decrease in the redox peak current of k3[fe(cn)6]. as shown in figure 6b and table 3, a modification of the electrode is ultimately needed to increase the sensitivity to the biomarker. figure 6. dpvs of 10 mm k3[fe(cn)6] in 0.1m kcl at: (a) spce aptasensor for different composite concentrations a → c (0.007, 0.01, 0.03) mm and (b) a → g (spce-composite; spce bare; spcecomposite-stv; spce-composite-stv-apt; spce-composite-stv-apt-enac) the use of biomolecules in biosensors, especially electrochemical, is growing as they allow rapid and sensitive detection. in this study, a streptavidin biomolecules was added to the electrode surface to bind specifically to the enac aptamer [37] with a biotin tag. the interaction between the aptamer (apt) and the enac protein was analyzed using the biovia discovery studio visualizer, showing that the compound has eleven hydrogen bonds, one hydrophobic interaction, and six electrostatic interactions with four unfavorable interactions or electrostatic repulsion [37]. the interactions that occur are hydrogen bonds, van der waals forces, and are hydrophobic. the streptavidin-biotin interaction also binds like an antibody to an antigen via non-covalent interaction between the protein and ligand [2]. streptavidin has four identical subunits, each of which can bind biotin with high affinity and specificity, and non-covalent bond interactions occur when biotin binds to tryptophan in the subunit. the non-covalent bond formed between biotin and streptavidin has a higher binding http://dx.doi.org/10.5599/jese.1455 j. electrochem. sci. eng. 12(6) (2022) 1225-1242 spce/natural silica-ceria nanocomposite 1234 affinity than most antigen and antibody bonds and is of similar strength to a covalent bond. two amino acids play a role in avidin-biotin interactions, namely tryptophan and lysine [2]. table 3. dpv characterization the modified spce electrode modified peak current, μa spce bare 3.190 spce-composite 0.007 mm 3.063 spce-composite 0.01 mm 9.673 spce-composite 0.03 mm 6.544 spce-composite-stv 3.115 spce-composite-stv-apt 2.043 spce-composite-stv-apt-enac 1.229 streptavidin on the surface of the spce-nanocomposite will interact with ceria and protein adsorption can occur due to electrostatic interactions, hydrophobic interactions, and specific chemical interactions between proteins and nanoparticles, and is influenced by ph and concentration [50]. if the ph of the acid buffer creates a positive zeta potential due to the increased concentration of h+ ions, the ceria is dispersed with pbs ph 7.4, thus increasing adsorption of protein on ceria in the composite. ceria in the silica-ceria composite strongly interacts with –nh2 groups to bind streptavidin and thionine for the matrix metalloproteinase 2-biosensor, forming a bridge bond with the carboxyl functional groups of the antibody, even without the addition of other agents [3]. the amphipathic protein structure can bind to the silica surface non-specifically, due to the curvature of the silica and the molecular weight, protein affinity constants as well as the adsorption process. adsorption as a non-covalent electrostatic interaction model, with the layer by layer (lbl-esa, or lbl) electrostatic assembly method, was used in bioconjugation with silica [25]. characterization and performance of aptasensor the aptamer used in this study was a modeled enac specific ssdna aptamer designed using simrna (69 nt). the charge on the aptamer is negative because it has a phosphate group that interacts with positive amino acid residues (arginine, histidine, lysine). the interaction of aptamer with enac protein is electrostatic, with good structural complementarity of the aptamer enac and hydrogen bonding. the interaction between the aptamer and enac was measured by dpv based on electron transfer from the [fe(cn)6]3-/4redox system. figure 6b shows redox peak currents for streptavidin, interaction of the streptavidin-aptamer and interaction of the aptamer enac on the electrode surface. a significant decrease in the redox peak current value is seen when the enac protein is attached specifically to the aptamer (table 3). experimental optimization according to box-behnken method the aptamer concentration (x1), streptavidin incubation time (x2), and enac aptamer incubation time (x3) were evaluated via box-behnken experiment design. the experimental data resulted in the following regression equation: y = -1.379 + 0.597x1 + 0.0688x2 + 0.484x3 + 0.004x12 0.000463x22 0.02091x32 0.00927x1x2 0.0459 x1x3 0.000667x2x3 (1) based on equation (1), a factor with a negative value affects decreasing the response, while for a factor with a positive value it affects increasing the response in the experiment. anova s. n. zakiyyah et al. j. electrochem. sci. eng. 12(6) (2022) 1225-1242 http://dx.doi.org/10.5599/jese.1455 1235 analysis of the box-behnken experiment revealed the p-values of each factor as x1 = 0.0499; x2 = 0.5358; x3 = 0.0001. the p-value of 0.05 indicates that the factor is significant. in addition, the p-value of the less of fit (lof) obtained in this experiment is 0.2047 (>0.05) so it can be concluded that the resulting linear model is appropriate. lof indicates a deviation or inaccuracy to the linear model so that the tests carried out aim to detect whether the linear model is appropriate. the behnken box data revealed optimum conditions for enac aptamer concentration of 0.5 μg ml-1, 30 min incubation of the streptavidin protein, and 1-hour incubation of the enac aptamer. calibration curve, detection limit, and quantification limit figure 7a displays the characterization results of the enac aptasensor, showing that the current decreased with increasing enac concentration, since more biomolecules are blocked on the electrode surface, thereby reducing the current. the enac protein is a large biomolecule and is not electroactive, so at higher enac concentration, higher is inhibition of the [fe(cn)6]3-/4 electron transfer process. figure 7. dpvs of (a) aptasensor for enac concentration variation a → g (0.047, 0.094, 0.187, 0.375, 0.75, 1.5, 3) ng ml-1 and (b) aptasensor enac calibration curve the calibration curve in figure 7b shows the optimum experimental conditions to determine the detection limit of the aptasensor. the linear regression equation obtained was y = 0.1279x + + 0.7369, with r2 of 0.9966. based on the equation, y indicates the change in peak current and x for the analyte concentration. determination of lod and loq values in this work includes determining the smallest concentration of analyte that can be reliably measured by an analytical http://dx.doi.org/10.5599/jese.1455 j. electrochem. sci. eng. 12(6) (2022) 1225-1242 spce/natural silica-ceria nanocomposite 1236 procedure and will be used in real applications. the real application of this research is to detect analytes in a wide concentration range, so it is important to know the values of lod and loq concentrations [51]. the detection and quantification limits [52] were determined as 0.113 and 0.343 ng ml-1, respectively, with a precision of 98.320 %, accuracy of 99.485 % and recovery of 99.314 % at 0.75 ng ml-1 enac. aptasensor performance the determination of the recovery test was carried out to validate the standard addition method, with the addition of the standard to the sample for determining the concentration of non-hypertensive individual samples. spiking 0, 1, 2, and 3 ng ml-1 to the urine sample, and a standard addition curve was obtained as a voltammetric response by a linear regression equation. previous research using the elisa method showed that a patient of non-hypertensive, hypertensive with, and without a family history of hypertension, showed enac protein levels of 1.12, 2.7 and 4.0 ng ml-1, respectively [53]. in this study, the concentration of enac contained in the sample was 0.021 ng ml-1, what is less than for the hypertensive range. this suggests that using an aptasensor based on spce/silica-ceria/stv/aptamer is a rapid and easy method to detect enac in urine samples. the determined protein concentration of enac of 0.021 ng ml-1, indicates that the developed aptasensor can sensitively determine enac in urine samples, making this method to be an alternative method to the elisa. measurement of enac concentration and recoveries obtained are listed in table 4. the obtained recovery values in the range of 90-110 % verified the precision of the new method. table 4. the recovery of method spike sample concentraton, ng ml-1 recovery, % 0.0909 99.00 ± 0.007 0.1818 99.00 ± 0.001 0.2727 99.66 ± 0.002 the performance of the aptasensor regarding the repeatability of the signal on the calibration curve was carried out by a repeat test technique. signal repetition is precise under the repeated conditions carried out in this experiment, so that the replication data under the optimum parameter conditions resulted in rsd value of 0.003 (acceptable) [54]. the calibration curve in figure 7b shows the value of r2 = 0.9966 (acceptable)[55] and high signal stability with a narrow error bar. the selectivity of the aptamer to other proteins has been shown to be very good based on our previous study which was tested in silico. in that study, the performance of the aptasensor is performed using disposable electrodes [56]. the lod and loq of the developed aptasensor were determined as 0.113 and 0.343 ng ml-1, respectively. compared to previously reported aptasensor results (table 5), it appears that previously developed aptasensor by electrodeposition of silica had a lower lod [3]. however, the electrodeposition method requires a longer time than dropping of composite solution directly onto the electrode surface (direct assembly), and thus is faster for detection as a point of care and mass production. in addition, it is still possible to distinguish the level of enac in normal urine from those with hypertension because of sodium intake. s. n. zakiyyah et al. j. electrochem. sci. eng. 12(6) (2022) 1225-1242 http://dx.doi.org/10.5599/jese.1455 1237 table 5. comparison of the developed aptasensor performance to previously reported aptasensors conclusions spce-modified by natural silica-ceria composite exhibited a better ferro-ferricyanide current response than spce without modification. the fe(cn)6]3-/4-redox peak current showed an increase from 3.190 μa for bare spce to 9.073 μa for modified spce. this proves that the nanocomposite-modified spce electrode exhibits enhanced electron transfer which is generally beneficial for the detection process. the aptasensor method produces a quick diagnosis (point of care) when compared to existing methods such as elisa method. the advantages of silica from natural materials could contribute to future development of aptasensors and increase their use value. in this work, the electrodes modified by natural silica-ceria nanocomposite have proven to detect enac protein as a biomarker of hypertension. the obtained lod of 0.113 ng ml-1 suggests 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.bios.2017.01.025 https://doi.org/10.1016/j.foodchem.2018.06.101 https://doi.org/10.1007/s00604-018-3191-x https://doi.org/10.1007/s00604-018-3191-x https://doi.org/10.1016/j.bios.2017.04.014 https://doi.org/10.1007/s00216-017-0588-z https://doi.org/10.1039/c5ay01807b https://doi.org/10.1039/c1cc15303j https://doi.org/10.1016/j.bios.2011.01.041 https://doi.org/10.1016/j.bios.2017.06.026 https://doi.org/10.1007/s00604-017-2113-7 https://doi.org/10.1039/c3an01587d https://creativecommons.org/licenses/by/4.0/) @article{zakiyyah2022, author = {zakiyyah, salma nur and eddy, diana rakhmawaty and firdaus, m. lutfi and subroto, toto and hartati, yeni wahyuni}, journal = {journal of electrochemical science and engineering}, title = {{screen-printed carbon electrode/natural silica-ceria nanocomposite for electrochemical aptasensor application:}}, year = {2022}, issn = {1847-9286}, month = {oct}, number = {6}, pages = {1225--1242}, volume = {12}, abstract = {a nanocomposite of natural silica and ceria was synthesized to modify a screen-printed carbon electrode (spce) to develop an aptasensor to detect epithelial sodium channel (enac) protein in urine as a biomarker of hypertension. the method steps were the synthesis of natural silica-ceria nanocomposite using the hydrothermal method, obtaining of natural silica nanoparticles from the extraction of alkaline silica sand and ceria nanoparticles from cerium nitrate, modification of spce/natural silica-ceria, immobilization of aptamer through streptavidin-biotin, and detection of enac protein conc­entration. box-behnken's design was employed to determine the optimal con­ditions of aptamer concentration (0.5 $\mu$g ml-1), streptavidin incubation time (30 min), and aptamer incubation time (1 hour), respectively. differential pulse voltam­metry (dpv) characterization of the developed electrochemical aptasensor revealed that the [fe(cn)6]3-/4redox peak current increased from 3.190 to 9.073 $\mu$a, with detection and quantification limits of 0.113 and 0.343 ng ml-1, respectively. the method is proven as a simple and rapid method to monitor enac levels in urine samples.}, doi = {10.5599/jese.1455}, file = {:d\:/onedrive/mendeley desktop/zakiyyah et al. 2022 screen-printed carbon electrodenatural silica-ceria nanocomposite for electrochemical aptasensor application.pdf:pdf;:www/jese_v12_no6_1225-1242.pdf:pdf}, keywords = {enac protein, silica, aptamer, ceria, hypertension biomarker, synthesis composite, voltammetry}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1455}, } a multichannel frequency response analyser for impedance spectroscopy on power sources doi: 10.5599/jese.2013.0033 107 j. electrochem. sci. eng. 3(3) (2013) 107-114; doi: 10.5599/jese.2013.0033 open access : : issn 1847-9286 www.jese-online.org original scientific paper a multichannel frequency response analyser for impedance spectroscopy on power sources quentin meyer, simon barass, oliver curnick*, tobias reisch*, daniel j. l. brett electrochemical innovation laboratory, centre for co2 technology, department of chemical engineering, university college london, uk *intelligent energy ltd, loughborough, leicestershire, uk corresponding author: e-mail: d.brett@ucl.ac.uk; tel.: +44(0)207 679 3310; fax: +44(0)207 383 2348 received: december 19, 2012; published: june 12, 2013 abstract a low-cost multi-channel frequency response analyser (fra) has been developed based on a daq (data acquisition)/labview interface. the system has been tested for electric and electrochemical impedance measurements. this novel association of hardware and software demonstrated performance comparable to a commercial potentiostat / fra for passive electric circuits. the software has multichannel capabilities with minimal phase shift for 5 channels when operated below 3 khz. when applied in active (galvanostatic) mode in conjunction with a commercial electronic load (by discharging a lead acid battery at 1.5 a) the performance was fit for purpose, providing electrochemical information to characterize the performance of the power source. keywords labview, multi-channel impedance, electrical circuit, fuel cell, lead acid battery. introduction electrochemical impedance spectroscopy (eis) is a powerful diagnostic technique that has been the subject of significant technical development over the last fifty years [1–3]. it has proven to be particularly useful in the fields of electrochemistry, corrosion [4], primary and secondary batteries [5,6] and fuel cells [7–9]. advances in electronics and reduction in the cost of hardware has made the technique increasingly popular. however, eis is still mainly used as a high-end lab-based diagnostic that requires relatively expensive hardware. this has hindered its uptake as a diagnostic tool used in routine industrial analysis or for on-line measurements of electrochemical power systems. for applications that would benefit from multichannel input (on different cells in a fuel cell stack, for example) the cost and complexity multiplies. therefore, there is a need for a lowcost, modular approach for electrochemical impedance spectroscopy analysis. http://www.jese-online.org/ mailto:d.brett@ucl.ac.uk j. electrochem. sci. eng. 3(3) (2013) 107-114 frequency response analyser for impedance spectroscopy 108 alternative methods for performing multichannel eis analysis have been attempted previously using matlab gui builder [10] and a built-in 10 channels frequency analyser. single channel eis using commercially available daq (data acquisition) hardware and a labview interface has been attempted for fuel cell applications [11], but multichannel metrology has yet to be reported. this paper details the development of ucl-fra, a low-cost eis system based on the use of a commercially available daq interface and labview software (daq/labview interface). issues relating to the successful application of the technique are discussed, results of its operation in galvanostatic mode applied to a battery are presented and operation in multi-channel mode described. experimental software the software used for data collection and processing was developed using labview 2011 (national instruments) and was based on the use of vis (virtual instruments) in the sound and vibration toolkit library for generating and analyzing ac waveforms. the coding methodology required special consideration to account for low frequency detection, simultaneous sampling and multichannel responses. the multichannel response was achieved by outputting single discrete sine waves sequentially across the frequency range of choice, and measuring on 5 response channels simultaneously. the software is currently capable of 5 response channels, but can be modified to a higher number of channels to suit the application, provided that the performance of the hardware is adequate to ensure robust data collection. the processed signals are displayed as nyquist and bode plots in real time as the data is collected; the time for the software to process the signal does not impact on the rate at which the measurement can be made. an entire measurement from 10 khz to 0.1 hz with 10 points per decade, takes about 400 s, which is comparable to other commercial devices. the signal sampling frequency is limited by the hardware used. high frequency sampling improves the accuracy of the measurement when analyzing high frequency signals. however, when sampling low frequencies, where the minimum sampling time is the inverse of the signal frequency, the data buffer becomes saturated. it is also necessary to respect the nyquist-shannon principle for robust data sampling [12]; e.g. for a maximum bandwidth of 10 khz, the sampling rate cannot be smaller than 20 ks / s. in practice, to avoid aliasing, a clock speed 10 times greater than the maximum speed of the measurement was used for each channel (100 ks/s). therefore, the daq hardware suitable for this application needs at least 500 ks/s, for 5 response channels. hardware a usb data acquisition card (usb 6363, national instrument) was used for data collection and signal generation. the daq has 16 bit resolution, with 32 input channels (16 differential), 4 analog outputs, a sampling rate of 1 ms/s and an output rate of 2.86 ms / s. it was used in the ±5 v range (or lower), with a resolution of 153 μv (i.e. 10 v / 2 16 ) . all the response signals were recorded in differential mode. the ac perturbation was generated as a single sine wave, switching between discrete levels from high to low frequency. the ac+dc stimulus and response signals are both recorded by the daq and processed using a fast fourier transform (fft) to convert the signals from the time domain to the frequency domain. figure 1 shows the set-up used in this work. electrical impedance measurements on ‘dummy cells’ were performed in potentiostatic mode, where the stimulus voltage is imposed to the q. meyer et al. j. electrochem. sci. eng. 3(3) (2013) 107-114 doi: 10.5599/jese.2013.0033 109 system and the current response is recorded using the 22  series resistor as a shunt resistor. the electrochemical impedance measurements on the battery were performed in galvanostatic mode; the stimulus current is controlled using an analogue control port on the load unit and recorded using a current transducer. figure 1. electrical connection schematic for (a) electrical and (b) electrochemical impedance spectroscopy. black line represents the voltage stimulus from the daq; blue the voltage as a proxy for the current and red the actual voltage from the cell. measurement procedure for passive components in potentiostatic mode an electric circuit (dummy cell) was used for electrical impedance testing. this was composed of a 22 ω resistor r1 in series with an rc parallel combination (4.7 μf capacitor c1 and 200 ω resistor r2). this circuit was connected to the daq card to enable single and multiple channel response. the series resistor r1 is used as a shunt to derive the current through the circuit. figure 2. (a) electrical circuit comprising the ‘dummy cell’; (b) connections for measuring voltage across the shunt resistor in single and (c) multiple-input mode. ao: analog output. ai: analog input. j. electrochem. sci. eng. 3(3) (2013) 107-114 frequency response analyser for impedance spectroscopy 110 measurement procedure for electrochemical impedance in galvanostatic mode in order to trial the system on an electrochemical power source, a commercially available lead acid battery (6 v, 10 ah, yuasa) was tested. an electronic load (6060b agilent) with a bandwidth of 10 khz was used to control discharge rate. the load is equipped with an analogue input capable of remotely controlling the current that is passed [13], a 0-10 v signal from the daq can be converted into a 0-60 a current by the load unit. a current transducer, with a frequency bandwidth of 150 khz (its-ultrastab-60, lem) capable of zero flux detection was used to record the ac and dc current, and convert it to a voltage across a resistor. the voltage of the battery and the voltage across the burden resistor are then recorded at the same time; consequently the transfer function can be extracted, as described in figure 1(b). the load unit has a bandwidth of 10 khz, and therefore some attenuation could be expected for frequencies higher than 2 khz. for comparison with a commercially available high current potentiostat an iviumstat a11700 (10 v, 10 a, ivium, netherlands) was used. for both systems the ac perturbation was equivalent to 75 ma imposed across a frequency range of 10 khz – 1 hz. results and discussion assessment of daq hardware as a function generation and frequency response analyser. it is important to ensure that there are no artifacts or significant phase shifts introduced by the hardware/software combination. figure 3(a) shows that when the input and output of the daq are connected together there is agreement between the two in terms of the frequency generated and measured. figure 3(b) shows a bode plot of magnitude and phase. it can be seen that the magnitude of the signal is constant across the range and that there is only a small phase shift introduced above 1 khz (<0.03 degrees at frequencies up to 10 khz). figure 3. (a) frequency generated versus recorded frequency sweep, from 10 khz to 0.5 hz. (b) phase and magnitude for the impedance between two channels recording the same signal. electrical impedance: single channel mode the eis system was tested in single channel mode on the circuit shown in figure 2(b). figure 4 shows nyquist and bode plots comparing the response obtained from the ucl system, the commercial (ivium) frequency response analyzer and the model simulation based on the components used in the circuit. it can be seen that excellent agreement is obtained between each, over the entire frequency range (10 khz to 0.1 hz). this demonstrates that the system is suitable for performing eis on passive (non power source) systems. q. meyer et al. j. electrochem. sci. eng. 3(3) (2013) 107-114 doi: 10.5599/jese.2013.0033 111 figure 4. (a) nyquist and (b-c) bode plots for a single response channel using ucl-fra and commercial ivium systems compared with the modeled transfer function. frequency sweep between 10 khz and 0.1 hz, ac amplitude 0.1 v for a dc signal of 2 v with 10 frequencies per decade. electrical impedance in multi-channel mode operation in multi-channel mode imposes more of a challenge on the daq system. lower cost daq hardware uses a single clock that triggers each of the channels sequentially. a phase shift between each of the channels is therefore possible at high frequencies and care should be taken when introducing additional channels to the measurement. higher end hardware, with dedicated clocks on each channel, avoids this potential artefact. figure 5 shows the response from 5 channels on the daq system (z1 – z5) connected to the circuit described in figure 2(c). in the nyquist projection the system shows little difference between each channel across the full frequency range, so allowing an accurate measurement of the real resistances in the circuit. in the bode projection it can be seen that some phase difference is introduced at high frequency. however, below 3 khz this difference is less than 3 degrees. overall, these results suggest that this daq/labview interface system can be used for multichannel eis data acquisition over the frequency range of 3 khz to 0.1 hz. j. electrochem. sci. eng. 3(3) (2013) 107-114 frequency response analyser for impedance spectroscopy 112 figure 5. (a) nyquist plots and (b-c) bode plots showing the response from 5 channels, sampled concurrently, using the circuit described in figure 2(c) with ucl-fra. frequency sweep between 10 khz and 0.1 hz, ac amplitude 0.1 v for a dc signal of 2 v with 10 frequencies per decade. electrochemical impedance spectroscopy on power sources power sources are typically very low impedance devices and notoriously challenging for fras. a lead acid battery was tested (in discharge) to assess the performance of the system in active mode when incorporating an electronic load. figure 6 shows the nyquist and bode (phase and magnitude) responses for the ucl and commercial (ivium) systems. discharge from the battery was performed in galvanostatic mode at 1.5 a using the set-up of figure 1 (b). the measurement was achieved with a lowest frequency of 1 hz, to avoid discharging the battery too much due to the longer measurement time at low frequencies. q. meyer et al. j. electrochem. sci. eng. 3(3) (2013) 107-114 doi: 10.5599/jese.2013.0033 113 the nyquist plot shows that there is a difference (shift) in the real impedance between the daq/labview system and that from the ivium analyser. however, the form of the nyquist plot is very similar for each, exhibiting the shape of two depressed semi-circular arcs, representing two processes with different time constants (rc combinations). this is typical of an eis response for a battery of this kind [14]. the systematic difference in real resistance between the two (<10 m) is attributed to the different leads and connectors used for each system causing different ohmic resistance. the bode plots in figure 6(b-c) show that the two magnitudes are very similar across the range, although the one recorded with the ucl-fra exhibits a constant lower resistance associated with the different lead configuration. the two phases remain in close agreement up to a frequency of ~3 khz. above this frequency, the bandwidth limitation of the load starts to become apparent. figure 6. electrochemical impedance response for ucl-fra and commercial fra (ivium) in the form of: (a) nyquist, and bode, (b) phase and (c) magnitude for a lead acid battery being discharged at 1.5 a. frequency sweep between 10 khz and 1 hz, ac amplitude 75 ma for a dc signal of 1.5 a with 10 frequencies per decade. j. electrochem. sci. eng. 3(3) (2013) 107-114 frequency response analyser for impedance spectroscopy 114 conclusion a daq/labview interface has been developed capable of performing frequency response analysis for electrochemical impedance spectroscopy of low-impedance power sources. the system was tested using electrical circuits to demonstrate that multi-channel data acquisition is possible (here using 5 channels) without significant phase shift between the channels up to 3 khz. eis on a battery discharging at 1.5 a compared favorably with a commercial fra; the main difference being a systematic shift in real impedance and high frequency inductance, both of with can be attributed to differences in the cabling used for each arrangement. this method is attractive for applications where a low cost solution is required for measuring electrochemical power systems and a multichannel input is needed. for example, performing individual cell eis on fuel cell stacks or on-board diagnostics. acknowledgements: the authors would like to acknowledge university college london and intelligent energy ltd. for supporting the work of meyer, the epsrc supergen fuel cells programme (ep/g030995/1) and epsrc flexible fuel cell project (ep/g04483x/1) for supporting brett’s research. references [1] v. sense, m. cell, scribner associate-tutorial, 1-5. 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[14] h. blanke, o. bohlen, s.buller, j. power sources, 144 (2005) 418-425 © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {development and performance of hybrid coatings on aluminium alloy} doi:10.5599/jese.347 131 j. electrochem. sci. eng. 7(3) (2017) 131-138; doi: http://dx.doi.org/10.5599/jese.347 open access : : issn 1847-9286 www.jese-online.org original scientific paper development and performance of hybrid coatings on aluminium alloy makanjuola oki1,2,, adeolu adesoji adediran1, saheed olayinka1, oyeyemi ogunsola1 1department of mechanical engineering, college of science and engineering, landmark university, omu-aran, kwara state, nigeria 2department of mechanical engineering, college of science and engineering, landmark university, omu-aran, kwara state, nigeria corresponding author: e-mail: makanjuola.oki@lmu.edu.ng; tel.: +2347061135549 received: september 26, 2016; revised: january 26, 2017; accepted: july 21, 2017 abstract from gravimetric studies, hybrid nano-coatings, based on permanganate/fluoride/glycerol conversion coating solutions formed on aluminum alloy by immersion procedures developed rapidly at a rate which decreased with time of treatment and was about 16 mg in weight after a period of three minutes. the morphology of the coating during scanning electron microscopic (sem) examinations revealed randomly shaped coating materials with mud cracking patterns, characteristics of dried out coatings derived from gel-like materials. analyses of the coating using edx attachment in the sem showed that it was composed essentially of aluminum, oxygen and manganese compounds, probably hydrated. the corrosion resistance of the coating out-performed ‘bare’ aluminum alloy specimens exposed to natural environment and 1 m sodium chloride solution. the coating improved the paint adhesion characteristics of the substrate aluminum alloy. keywords adhesion, sem/edx, permanganate, glycerol, morphology, pitting introduction automobile customers’ demands for higher performance, more luxury and safety features signaled the development of lightweight and hence more energy efficient vehicles led to the introduction of aluminum and its alloy to achieve considerable weight reductions with no losses in strength and stiffness. however, the changes in choice of material and body structure presented significant challenges with respect to methods of joining and finishing of the automobiles. from a technical point of view, conversion coatings offer several advantages such as improved corrosion http://dx.doi.org/10.5599/jese.347 http://www.jese-online.org/ mailto:makanjuola.oki@lmu.edu.ng j. electrochem. sci. eng. 7(3) (2017) 131-138 hybrid coatings on aluminium alloy 132 resistance and adhesion of paint and/ adhesives to aluminum and its alloys used in the automotive industry. to attain a long service life under demanding conditions, pre-treatment of the aluminum and its alloys before any fabrication of parts is an extremely important factor. the successful use of chromate conversion coating in the aluminum finishing industry in the last five decades demonstrated that superior strength can be acquired, even when the coated structure is exposed to corrosive environments [1]. however, several protocols in the last two decades have limited the use of carcinogens like volatile organic compounds (vocs) and the chromates. in view of this shortcoming several authors [2-5] have researched into alternatives to chromates based on transition metal compounds without much success in terms of corrosion resistance superiority of the chromates. although there are claims in the literature that some composite precursor compounds such as permanganate/molybdate [6] conversion coating has superior corrosion resistance to chromate and molybdate conversion coatings separately, its application in the high technical end of the metal finishing industry may yet be in its infancy. other claimed success is the permanganate conversion coating developed on magnesium alloy [7]. the coating which was developed from an acidic bath is termed to be nearly crack-free and its corrosion resistance was markedly better than the untreated alloy. however, permanganate coatings have been described by hughes et al.[8], as a promising replacement for chromates in the aerospace industry. in their findings, the coating is generally about 50-70nm in thickness and it was said to be essentially of mno2 in its outermost regions with a composite of al-mn oxides predominantly at the metal/coating interphase. as the search for alternatives to chromates proceeds far and wide, other researchers are looking into sol-gel formulations such as aluminum-based sol-gel materials by oubaha et al. [9], and silanol-based nanocomposite by gonzalez et al [10] with much success recorded in terms of improved paint adhesion characteristics which compared favorably with the chromates. nonetheless, some of these formulations and processes have been patented [11] but their applications have been limited to the lower technical end of the metal finishing markets while in the high technical end as in the aerospace industry as well as military hardware, chromates are still preferentially employed [12]. in order to make the use of chromates more acceptable to the metal finishing industries, a none-rinse chromate formulation was developed. addition of organic compounds containing hydroxyl groups to chromate baths modified the surface features of the traditional coating where the usual mudcracking was obliterated and it became unnecessary to give a final water rinse to objects treated in such baths [13]. in addition, the coatings dried faster than the traditional chromate coating thus achieving reduction in cost of production. hence, from environmental safety and costs perspectives, the formation and process pushed the hybrid coating a notch higher than the traditional chromate coating and process in terms of lower carbon foot print. in view of the limited successes encountered in the search for viable alternatives to chromate, the current investigation, examined permanganate, containing mn, with variable valences similar to the chromate as a precursor conversion coating material of interest which may also act as a cathodic inhibitor at breached areas of the coating where the substrate may be exposed albeit, transiently. experimental spade like electrodes were made from sheet of aluminum alloy aa8000. these electrodes were etched in 10 % sodium hydroxide solution, rinsed in water prior to de-smutting in 50 % v/v nitric acid solution for three minutes each and then rinsed in water. electrodes, prepared in the afore mentioned manner, were immersed in a solution of 4 g/l kmno4, 1 g/l naf and 5 ml/l glycerol made up to 1 liter of water, at 30 oc for several periods of time ranging from 30 seconds to 600 seconds. m. oki et al. j. electrochem. sci. eng. 7(3) (2017) 131-138 doi:10.5599/jese.347 133 three separate specimens were treated in the conversion coating solution for each period of time. thereafter the mean changes in weights were recorded for each pretreatment time. the ph of the solution was 7.9 as obtained from jenway, model 3505 ph meter. all chemicals used were of laboratory or industrial grades. some specimens treated for various times in the manganate/fluoride/glycerol coating solution were examined in scanning electron microscope, phenom prox sem, model mve0224651193, operated at 15kev. the elemental compositions of the coatings were obtained from the edx attachment in the microscope. specimens treated for 180 seconds in the coating solutions were exposed in upright positions to the natural environment in the north central part of nigeria for 500 h with untreated aluminum alloy specimens exposed likewise. these were visually examined regularly. some other set of specimens were likewise treated in the coating solutions for 180 seconds. these were further coated with a nitrocellulose lacquer by immersing the spade like end of the pretreated specimens as near vertical as possible in 100 ml of the lacquer for 60 seconds and withdrawn as immersed, allowed to dry for 24 h prior to further examination. untreated aluminum and pretreated specimens as well as those over coated with lacquer were cross-scratched, using japanese industrial testing method [14] prior to exposure to near neutral 1 m nacl solution for 170 h. after the exposure period, transparent cellophane adhesive tapes were firmly applied on each of the specimens. the tapes were subsequently rapidly pulled from the substrates. the surfaces were examined to appraise the mode(s) of coating failure by optical microscopy and in the sem with analyses performed in the edx attachment of the sem. results and discussion coating development from visual examinations, the specimens immersed in the permanganate/fluoride/glycerol coating solution showed changes in color from the initial lustrous metallic appearance to progressively dark golden yellow colorations as time of immersion progressed. on surface value, this implied formation and development of coating on aluminum in the conversion coating solution. the formation and development of permanganate coating on aluminum probably followed “the substrate activation/coating materials deposition” model reported for chromate conversion coatings [15,16] which will involve the activation of aluminum by fluoride species in solution thus, -f 3+ -al al +3e (1) the 3 electrons generated during the activation of aluminum, an anodic reaction, are then taken up by permanganate species which are reduced to manganate species in the cathodic half of the redox reaction. another school of thought has it that though thinning of the oxide layer on aluminum does take place; a favorable anodic reaction is the reformation of aluminum oxide which allows for electron tunneling. these mn (iv) species, at their established equilibrium constant, are deposited as probably hydrated oxide/hydroxides and may be contaminated with permanganate in the coating solution and aluminum species generated in equation (1). these species may as well be adsorbed and/or occluded within the developing coating. 2h2o + mno4+ 3e→ mno2 + 4oh (2) figure 1 displays the coating weight versus time of immersion for aluminum specimens in permanganate conversion coating bath. the rate of coating growth was fast initially, however, the rate decreased with increase in immersion time. j. electrochem. sci. eng. 7(3) (2017) 131-138 hybrid coatings on aluminium alloy 134 figure 1. graph of coating weight against time of immersion in manganate/fluoride/glycerol solution at 30 oc this was envisaged as the initial coating materials deposited on the specimens will initially constitute barriers to further coating growth [17,18]. however, as espoused by [15,16,19] and others, further coating growth beyond the initial rapid deposition of coating materials will proceed at the metal coating interface with the creation of pathways within the coating. thus, coating solution species will come in contact with the substrate although at a reduced rate with continued growth of the coating. surface morphology and composition the surface morphology for the specimen treated for 180 seconds in the permanganate coating bath is displayed in fig. 2 and it portrays a typical surface morphology for specimens treated in the permanganate coating bath for various times. figure 2. sem micrograph of aluminium specimen treated for 180 seconds in manganate/fluoride/glycerol conversion coating bath at 30 oc 0.2 0.6 1.0 1.4 1.8 0 50 100 150 200 250 300 350 w e ig h t d if fe re n ce , m g time, s m. oki et al. j. electrochem. sci. eng. 7(3) (2017) 131-138 doi:10.5599/jese.347 135 the surface is comprised of variously shaped coating materials formed along with mud-cracking patterns derived from shrinkage stresses associated with drying out of gel-like materials. such randomly shaped and cracked surface features have been suggested to be anchor sites for subsequently applied organic coatings with attendant improvement in paint adhesion characteristics of conversion coated substrates [20]. however other researchers believed that in addition to these anchor sites, bonds formation between organic coatings and species in conversion coatings play significant roles in the improvement of paint adhesion properties of conversion coated substrates [21]. also, in an earlier study it was suggested that roughness at microscopic level imparted on conversion coated surfaces played added significant roles in adhesion improvement properties of conversion coated aluminum substrates [22]. the coating materials as revealed through analysis with the edx attachment in the sem are composed of mn, al and oxygen compounds, fig. 3, probably hydrated as the materials were derived from gel-like materials. this is in agreement with the findings of hughes et al. [8] who suggested two diffused layers of coatings with the interior being richer in al-mn hydrated oxides and an outer region rich in mno2. energy, kev figure 3. edx analysis of aluminium specimen treated for 180 seconds in manganate/fluoride/glycerol conversion coating at 30 oc aluminium may be derived from the coating as compounds formed from al3+ generated during activation by fspecies as well as from the substrate which is aluminum. oxygen and manganese will generally be derived from the coating materials as well as other contaminants such as k and s. corrosion and adhesion performance to all intents and purposes, all the specimens’ conversion coated were superior in performance to the ‘bare’ specimens on which incipient pits were resolvable with the naked eyes in addition to mounds of corrosion products which appeared whitish under the lacquer coatings. there were no obvious incidents of pitting on both the bare conversion coated and those with top coatings of lacquer after the exposure period of time. for specimens immersed in sodium chloride solution, ‘bare’ aluminum specimens with and without lacquer coatings showed severe pitting corrosion over the immersion period. a typical example is displayed in fig. 4, the scanning electron micrograph of the unscratched regions of ‘bare’ aluminum specimen with a top coating of lacquer. the micrograph portrays a relatively large pit at the top of the micrograph with corrosion products at the bottom of the micrograph. the corrosion products, likely to have migrated from within the pit, where active pitting corrosion occurred are composed of hydrated aluminum hydroxides and/or oxides which were initially gellike, developed cracked morphologies during drying out. the edx analysis in the region revealed the presence of al, o, cl and na which indicated the presence of aluminum products contaminated or in te n si ty , a .u . j. electrochem. sci. eng. 7(3) (2017) 131-138 hybrid coatings on aluminium alloy 136 otherwise with chlorides which are known pitting agents of aluminum and its alloys. on the other hand, the unscratched regions of the conversion coated specimen with a top coating of lacquer, displayed in fig. 5, did not show active pitted regions, however, a region at the top right corner of the micrograph, marked ‘+’ revealed disaggregated materials which may be a flawed region of the composite conversion/lacquer coating where pitting corrosion may have initiated but propagation was hindered in one manner or the other. figure 4. scanning electron micrograph of lacquer coated ‘bare’ aluminum immersed in 1 m nacl solution for 170 h at 30 oc. figure 5. scanning electron micrograph of conversion coated aluminum with a top coating of lacquer after immersion in 1 m nacl solution for 170 h at 30 oc after application of adhesion tests. edx spectra of spot analysis at this region, fig. 6, revealed the presence of aluminum, manganese, oxygen, potassium and chloride. mn, al, o are derived from the conversion coating whereas the presence of cl implied its corrosion activities at the flaw which led to production of disaggregated materials at the region. energy, kev figure 6. edx spot analysis of the region marked ‘+’ on the specimen conversion coated with a top coating of lacquer immersed in 1m nacl solution for 170 h at 30 oc after adhesion tests. another interesting feature appeared as light material at the top left corner of the micrograph in fig. 5, which from edx analysis is composed of al, mn, o and cl and may likewise be pitting corrosion products which had plugged the entrance to a pit. the presence of reducible mno4in manganate in te n si ty , a .u . m. oki et al. j. electrochem. sci. eng. 7(3) (2017) 131-138 doi:10.5599/jese.347 137 coatings had been confirmed by danilidis et al. [18]. such anions in the vicinity of active pitting activities can effectively pick up the electrons generated from the aluminum substrate to form mno2 mixed with aluminum compounds to give rise to such features on the specimen. from the foregoing analyses, it is obvious that the conversion coated specimens out-performed the ‘bare’ aluminum specimens in terms of corrosion resistance. as far as paint adhesion characteristics were concerned, paint delamination was not observed on the conversion coated specimen and the top coating of lacquer was not removed after the application of tape peeling adhesion tests. on the other hand, lacquer was easily removed from the ‘bare’ aluminum specimen. conclusions the hybrid manganate/fluoride/glycerol conversion coating on aluminum is composed of mn, al and o compounds and its surface morphology appeared rough on a microscopic scale. the corrosion resistance and paint adhesion characteristics of the conversion coating as well as the conversion coating/lacquer composite on aluminum substrate were superior to those of ‘bare’ aluminum from natural and accelerated corrosion tests in 1 m nacl solution. during the course of this investigation, the surface morphologies and elemental compositions of specimens treated in manganate coating solutions with and without glycerol additions were compared. the cracked surface morphologies were similar however, the yield of mn in the hybrid coating was higher than for the later. acknowledgements: the authors acknowledge department of mechanical engineering, covenant university, ota, ogun state, nigeria, for the use of sem/edx facilities references [1] o. lunder, chromate free pretreatment of aluminium for adhesive bonding, phd thesis, norges teknisk-naturvitenskapelige universitet, (2003) [2] s. joshi, b. l. treu, m. j. o’keefe, w.g. fahrenholtz, journal of the electrochemical society, 158 (2011). 88-93 [3] c. s. liang, z. f. lv, y.l. zhu, s.a. xu, surface coatings technology 249 (2014) 1-5 [4] h. r asemani, p. ahmadi, a. a sarabi, h. eivaz mohammadloo, progress in organic coatings 94 (2016) 18-27 [5] m. oki, journal of applied science and environmental management 11(2) (2007) 87-190 [6] w. guixiang, z. milin, w. applied surface science 258 (2012) 2648-2654 [7] s. y jian, y. r. chu, c. s. liu, corrosion science 93 (2015)301-309 [8] a. e. hughes, j. d. gorman, t. d. gorman, t. g.harvey, a. galassi, g. mcadam, corrosion science. 62 (9) 774-780 [9] m. oubaha, p. c. r. varma, b. duffy, z. m. gasem, s. j. hinder, advances in materials physics and chemistry.4 (2014) 75-84 [10] e. gonzalez, j. pavez, i. azocar, j. h. zagal, x. zhou, f. melo, g.e. thompson, m.a. páez, electrochimica acta 56 (2011) 7585-7595 [11] r. buchheit, u s. patent nov. 14, 7, 135,075 b2 (2006) [12] n. n. vovodin, v.n. balbyshev, m.s. donley, progress in organic coatings 52 (2005) 28-33 [13] m. oki, patent number, ng/p/2013/755, april (2014) [14] japanese industrial standard testing methods for paints jisk 5400, japanese standards association, tokyo, japan, (1990). [15] m. f. abd rabbo, j. a. richardson, g. c. wood, corrosion science 18(1) (1978) 117-123 [16] r. c. furneaux, g. e. thompson, g. c. wood, corrosion science 19(1) (1979) 63-71 [17] s. a. kulinich, m. farzaneh, x. w. du, inorganic materials 43(9) (2007) 956–963 j. electrochem. sci. eng. 7(3) (2017) 131-138 hybrid coatings on aluminium alloy 138 [18] i. danilidis, j. a. hunter, g. m. scamans, j. m.sykes, corrosion science 49(3) (2007) 1557– 1569 [19] k. asami, m. oki, g. e. thompson, g. c. wood, v ashworth, electrochimica acta 32(2) (1987) 337-343 [20] c. s. liang, z. f. lv, y. l. zhu, s. a. xu, surface and coatings technology 249 (2014) 1-5 [21] m. l. zheludkevich, i. miranda salvadob, m. g. s. ferreira, journal of materials chemistry 15 (2005) 5099-5111 [22] m. oki, materials science 2013, article id572379, doi: 10.115/2013/572379 ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {surfactant modified carbon nanotube paste electrode for the sensitive determination of mitoxantrone anticancer drug} doi:10.5599/jese.368 39 j. electrochem. sci. eng. 7(1) (2017) 39-49; doi: 10.5599/jese.368 open access : : issn 1847-9286 www.jese-online.org original scientific paper surfactant modified carbon nanotube paste electrode for the sensitive determination of mitoxantrone anticancer drug jamballi g. manjunatha department of chemistry, fmkmc college, madikeri, mangalore university constituent college, karnataka, india; e-mail: manju1853@gmail.com; tel.: +91-08272228334 received: january 20, 2017; revised: february16, 2017; accepted: march 7, 2017 abstract surfactant modified carbon nanotube paste electrode is prepared as electrochemical sensor with high sensitivity responding to mitoxantrone (mtx). electrochemical oxidation of mtx is investigated in buffered solution by cyclic voltammetry that is found very sensitive method for detection of mtx. it is shown that the sodium dodecyl sulfate modified carbon nanotube paste electrode (sdsmcntpe) gives enhanced current response for mtx compared to the bare carbon nanotube paste electrode (bcntpe). different parameters were tested to optimize the conditions for mtx determination. the effects of different surfactant and surfactant concentration, ph, scan rate, and concentration of mtx on the oxidation peak current values were determined. excellent results were obtained by cyclic voltammetry using sdsmcntpe, where two mtx oxidation peaks appeared around 370 and 600 mv vs. sce. detailed analysis of the second voltammetric peak showed the linear dependence on concentration between 2×10−7 and 7×10−6 m mtx with the slope of the corelation coefficient of 0.99271. lod and loq were determined as 3.5 ×10−8 m and 11×10−8 m, respectively. the sdsmcntpe showed very good reproducibility, high stability in its voltammetric response, high electrochemical sensitivity and low detection limit for mtx. keywords electrochemical sensors; mtx determination; carbon nanotubes; surfactant modified electrodes introduction mitoxantrone (mtx) is one of the important drugs that has extensively been used for the treatment of leukemia, breast cancer, non-small cell lung cancer and lymphoma and thereby its analytical determination is considered very valuable [1]. mtx is the anthraquinone derivative (scheme 1) and has usually been determined by means of spectrophotometry [2], chromategraphic methods [3,4], electrochemical assays [5,6], flow injection analysis [7,8] and radioimmunoassay [9,10]. http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 7(1) (2017) 39-49 determination of mitoxantrone anticancer drug 40 scheme 1. structure of mitoxantrone electrochemical determination of mtx is very rare [11] mainly due to the low sensitivity and relatively expensive equipment. after inventions of many new materials, a need has been expressed for developing of stable, simple and efficient materials as sensors for highly sensitive mtx determination by electrochemical methods. at present, there is the whole area of research occupied with developing of new materials and fabrication of new biosensors with controlled features on a nanometer scale. carbon nanotubes (cnts) with their very good electronic properties, great chemical resistivity and mechanical stability, have frequently been used for preparation of biosensors [12–14]. cnts behave electrically as a metal or semiconductor. the electronic properties suggest that cnts have the capability to promote charge-transfer reactions when used as an electrode [15]. the modification of traditional electrode materials with carbon nanotubes for use in analytical sensing is well evidenced and characterized by lower detection limits, increased sensitivities, reduced overpotentials and increased resistance to surface fouling. therefore, cnts have usually been applied as electrochemical sensors [16]. surfactants are amphiphilic molecules with hydrophilic and lipophilic properties, which through adsorption at the electrode surface are capable to change the electrode/solution interface and electrochemical process carried out there [17]. michaelis et al. [18] have prepared crystalline zno films by cathodic electrodeposition from aqueous solution in the presence of sodium laurylsulfate and obtained films of remarkably different morphologies. rusling et al. [19] immobilized hemoglobin on the electrode by incorporating it into surfactant film and realized direct electrochemistry of hemoglobin. chen and chzo [20] have found that simultaneous determination of dopamine and ascorbic acid can be made using didodecyldimethylammonium bromide filmmodified electrodes. our group has already reported [21–23] determination of several biomolecules and showed remarkably enhanced electrochemical responses of analytes in the presence of surfactants. surfactants also have potential applicability for suppressing electrode fouling from products of the electrochemical reaction [24]. due to their unique molecular structure, surfactants have been widely used in the area of electrochemistry [17,25] and for other purposes. it has already been shown that adsorption of surfactant aggregates on the electrode surface increases the peak current, changes the redox potential and charge transfer or diffusion coefficients. with the intention to propose an effective, quick and inexpensive method for determination of mtx, fabrication of various surfactant modified cnt paste electrodes by an easy immobilisation method is described in this paper. all prepared electrodes are tested for the presence of mtx using cycling voltammetry experiments. the electrode showing the highest sensitivity, the sodium dodecyl sulfate modified carbon paste electrode (sdsmcntpe), is analysed for all relevant details. j. g. manjunatha j. electrochem. sci. eng. 7(1) (2017) 39-49 doi:10.5599/jese.368 41 experimental reagents mtx was received from biovision inc, usa. spectroscopically pure multiwalled cnts were obtained from sigma aldrich. silicone oil and sodium dodecyl sulfate (sds) were purchased from nice chemicals, india. cetyltrimethylammonium bromide (ctab) was purchased from molychem, india, while triton x-100 (tx-100) was purchased from himedia, india. all chemicals were of analytical grade and used without further purification. stock solutions of 25×10-4 m mtx and 25×103 m sds, ctab and tx-100 were prepared by dissolving in double distilled water. the supporting electrolyte was the phosphate buffer solution, 0.2 m pbs, ph 7.0. pbss of other ph values (5.5-8.0) were prepared by mixing the corresponding volumes of standard 0.2 m na2hpo4 and 0.2 m nah2po4 solutions. apparatus voltammetric measurements were executed using a model-201 electrochemical analyzer and conventional three-electrode system. the following electrodes were used as the working electrodes: graphite (carbon) paste electrode (cpe), bare carbon nanotube paste electrode (bcntpe), sds modified carbon paste electrode (sdsmcpe), sds modified carbon nanotube paste electrode (sdsmcntpe), cetyltrimethylammonium bromide modified carbon nanotube electrode (ctabmcntpe) and tx-100 modified carbon nanotube electrode (tx-100mcntpe). the counter electrode was a platinum wire, while the saturated calomel electrode (sce) served as the reference electrode. all potential values are referred to the sce. preparation of electrochemical sensor electrodes the bcntpe was prepared by mixing cnts and silicone oil (60.0 % w/w cnts and 40.0 % w/w silicone oil) in a mortar [26]. the paste was then packed into the cavity (3 mm diameter) of a homemade electrode and smoothed out by a tissue paper. the cpe was prepared by grinding 70 % of graphite powder (particle size 150 mesh) and 30 % of silicone oil to produce a homogeneous carbon paste electrode. the paste was then filled into the cavity of a homemade electrode and smoothed out by a tissue paper. sdsmcpe, sdsmcntpe, ctabmcntpe and tx-100mcntpe were prepared by immobilizing 20 l of sds, ctab and tx-100 surfactants on the electrode surfaces for 5 min. electrochemical and microscopic measurements determination of mtx was carried out in a voltammetric cell with 0.2 m pbs, ph 7.0 supporting solution at room temperature. cyclic voltammograms and differential pulse voltammograms were usually recorded by changing potentials from 0 to 1000 mv and 0 to 750 mv, respectively, with potential scan rate,  =100 mv s-1. between two oxidation peaks appearing around 370 and 600 mv, the peak at 600 mv was chosen for detailed analysis in this study. prior each mtx measurement, the electrode surface was renewed. microscopic studies were made by using field emission scanning electron microscopy (fesem) and energy-dispersive x-ray spectroscopy (edx). results and discussion microscopic study of cnt, bcntpe and sdsmcntpe fig. 1 illustrates the fesem images of cnt (fig. 1a) bcntpe (fig. 1b) and sdsmcntpe (fig. 1c) surfaces. the surface of the bcntpe looks rougher than cnt, whereas in the case of the j. electrochem. sci. eng. 7(1) (2017) 39-49 determination of mitoxantrone anticancer drug 42 sdsmcntpe, the deposition of sds with some large groups is observed with fesem. it is well known that surfactants adsorb or cluster into supramolecular structures at the surface of the electrode, influencing thus electrochemical processes taking place. the bcntpe and sdsmcntpe structures were examined by edx measurement, giving spectra shown in fig. 2. edx spectrum of bcntpe indicates the presence of c, o, and si (fig. 2a), while for sdsmcntpe, presence of c, o, na, si and s elements is indicated in the spectrum (fig. 2b) suggesting thus successful modification of the electrode surface by sds. figure 1. fesem images of (a) cnt (b) bcntpe (c) sdsmcntpe a b energy, kev energy, kev figure 2. edx spectra of a -bcntpe and b sdsmcntpe optimization for the amount of sds surfactant in this part, the effect of amount of sds (2050 l) on the mtx oxidation peak current is studied by cyclic voltammetry experiments. the peak current increased gradually with the increase of sds amount at first and then tended to be stable for the amount realized by 20 l. this is probably due to the excessive compactness of the sds monolayer and non-negligible electrostatic interaction between adsorbed substrates. further increase of sds amount resulted in the decrease of peak current, what is presented in fig. 3. this effect may be due to the micellar effect of surfactant that causes abrupt change of oxidation current of mtx around the critical micelles concentration (cmc) of sds. therefore, 20 l sds was adopted as the optimum in this work. j. g. manjunatha j. electrochem. sci. eng. 7(1) (2017) 39-49 doi:10.5599/jese.368 43 a b figure 3. a cyclic voltammograms ( = 100 mv/s) of mtx (110-4 m) oxidation at dsmcntpe in 0.2m pbs, ph 7.0, for different amounts of sds (20 l50 l); b peak current values vs. amount of sds reproducibility and stability of the electrode. the reproducibility of the electrode response was measured by five times utilization of the same electrode for determination of mtx by cyclic voltammetry experiments under optimized conditions. for sdsmcntpe, the relative standard deviation (rsd) of five measurements was 4.93 %, which indicated good reproducibility in the electrochemical response for this electrode. the stability of the electrode was tested by measuring the current response of mtx during one month period. it was found that after 30 days, the sdsmcntpe maintained 94 % of its original activity, suggesting thus good stability of this sensor. electrochemical response of mtx at ctabmcntpe and tx-100mcntpe electrochemical responses of mtx oxidation at bcntpe and at the same electrode with very small amount of either tx-100 or ctab surfactants are compared in fig. 4. it is obvious that the voltammetric response is apparently improved in the presence of 20 µl of ctab (fig. 4a) and tx100 surfactants (fig. 4b) in immobilized forms. a b figure 4. cyclic voltammograms ( = 100 mv s-1) of mtx (1×10-4 m) oxidation in 0.2 m pbs, ph 7.0 at: a bcntpe (solid line) and ctabmcntpe (dashed line); b bcntpe (solid line) and tx-100mcntpe (dashed line) amount of sds, l c u rr e n t,  a j. electrochem. sci. eng. 7(1) (2017) 39-49 determination of mitoxantrone anticancer drug 44 when either anionic surfactant sds, cationic surfactant ctab or non-ionic surfactant tx-100 was applied, the peak current values increased, and the highest increase is observed for the sdsmcntpe. therefore, the sdsmcntpe has been selected for the further study. electrochemistry of mtx at sdsmcntpe fig. 5 exhibits cyclic voltammograms of mtx (10-4 m) electro-oxidation on sdsmcntpe in pbs (0.2 m, ph 7.0). no significant peak is observed for the blank solution, while after addition of mtx, the current enhancement and formation of the peak at 600 mv become completely evident. these results suggest that the sdsmcntpe increased electroactive surface area enhancing the electrocatalytic activity for mtx oxidation. figure 5. cyclic voltammograms ( = 100 mv s-1) of sdsmcntpe in 0.2 m pbs, ph 7.0: blank solution (curve a) and mtx (1×10-4 m) (curve b) electrochemical behavior of mtx at sdsmcntpe the redox nature of mtx (10-4 m) at bcntpe and sdsmcntpe in 0.2 m pbs, ph 7.0 is compared in fig. 6. at bcntpe, two redox peaks appeared at 310 and 490 mv. at sdsmcntpe, however, two redox peaks are observed at 370 and 600 mv, respectively. the peak currents at the sdsmcntpe increased by 61 % and 70 % compared to bcntpe, indicating the catalytic oxidation of mtx at the surfactant modified electrode. also, the peak potentials were shifted in a positive direction, i.e. from 310, 490 mv (bcntpe) to 370, 600 mv (sdsmcntpe). thus, presence of sds as a modifier facilitates the rate of the charge transfer and increases the over potential of mtx oxidation. the electrochemical responses of (110-4 m) mtx at sdsmcpe and sdsmcntpe were studied in 0.2 m pbs, ph 7 and the resulted cyclic voltammograms are displayed in fig. 7. the peak potential of mtx at the sdsmcpe were observed at 405 mv and 600 mv, while at the sdsmcntpe, the same peaks were observed at 370 mv and 600 mv. at the same time, peak current values at the sdsmcntpe are higher compared to those at the sdsmcpe. these results proved that the sdsmcntpe enhanced the sensitivity for mtx determination. it is possible that the bulky pore volume of carbon nanotubes provides a big specific area, leading to the increased peak current values. j. g. manjunatha j. electrochem. sci. eng. 7(1) (2017) 39-49 doi:10.5599/jese.368 45 figure 6. cyclic voltammograms ( = 100 mv s-1) of mtx (110-4 m) in 0.2 m pbs, ph 7.0 at bcntpe (solid line) and sdsmcntpe (dashed line) figure 7. cyclic voltammograms ( = 100 mv s-1) of mtx (110-4 m) in 0.2 m pbs, ph 7.0 at sdsmcpe and sdsmcntpe differential voltammetric study of mtx at sdsmcntpe differential voltammograms, dpvs, were recorded at 30 mv/s scan rate in the potential range from 0 to 750 mv for mtx (1×10-4 m) in 0.2 m pbs, ph 7.0 solution. for the bcntpe, fig. 8 shows a pair of mtx oxidation peaks at 264 and 470 mv, respectively. for the sdsmcntpe, however, peaks are located at 320 and 480 mv and showed strongly increased peak current values. figure 8. dpvs of bcntpe (solid line) and sdsmcntpe (dashed line) in 0.2 m pbs, ph 7.0 and mtx (110-4 m) j. electrochem. sci. eng. 7(1) (2017) 39-49 determination of mitoxantrone anticancer drug 46 influence of scan rate the scan rate dependence of the peak current values of mtx oxidation at the sdsmcntpe is shown in fig. 9. cyclic voltammograms of the sdsmcntpe in mtx (1×10-4 m), 0.2 m pbs, ph 7.0 measured at the scan rates between 100 and 500 mv s-1 are shown in fig. 9a. fig. 9b shows that the peak current values increased linearly with the scan rate. the linear regression equation was determined as ipa / a = 4.08 + 0.092  / mv s-1 (r = 0.99194) [27], what suggests that the adsorption controlled electrochemical oxidation reaction of mtx is taking place on this electrode. a b figure 9. a cyclic voltammograms of the sdsmcntpe in 0.2 m pbs, ph 7.0 and mtx (1×10-4 m) at various  = a) 100, b) 200, c) 300, d) 400 and e) 500 mvs-1 b plot of the peak current value as a function of  relationship between ph, peak potential and peak current values the effects of ph changes were investigated by measuring cvs of sdsmcntpe in solutions containing mtx and pbs of different ph in the range 5.5 – 8 and these results are shown in fig. 10a. as shown in fig 10a and fig. 10b, the topmost peak current value of mtx oxidation is obtained at ph 7.0, while at all other (lower and higher) ph, peak current values are lower. this, together with the fact that the physiological ph value is about 7 was the main reason why ph 7.0 is chosen as the supporting electrolyte in electrochemical determination of mtx. fig. 10c shows the relationship between the peak potential of mtx oxidation and ph of the solution. the linear regression equation for anodic peak potentials, epa, and ph could be depict as epa / v = 1038  63.6 ph (r =0.9973). the anodic peak potential of mtx is shifted negatively with the increase of the ph value, indicating reactions accompanied by proton transfer. the slopes of 63.6 mv/ph for mtx oxidation is nearby to the theoretical value of 59 mv/ph and pointed out toward two protons and two electrons involved in the oxidation process [28–30]. calibration plot and limit of detection for mtx fig. 11 depicts the calibration plot, i.e. the peak oxidation currents of cvs recorded at the sdsmcntpe in 0.2 m pbs, ph 7.0 for different concentrations of mtx in the range between 2×10-7 and 7×10-6 m. the oxidation peak current values increased linearly with increase of mtx concentration. the linear regression equation for the given range of concentration, c, is expressed as ipa / a = 5.92×10-5 + 3.06575 c (m) with the r= 0.99271.  / mv s-1 c u rr e n t,  a j. g. manjunatha j. electrochem. sci. eng. 7(1) (2017) 39-49 doi:10.5599/jese.368 47 a b c figure 10. a cyclic voltammograms ( = 100 mv s-1) of sdsmcntpe for mtx (1×10-4 m) in 0.2 m pbs, ph 5.5, 6, 6.5, 7, 7.5 and 8; b plot of anodic peak current vs. ph (5.5–8.0); c plot of peak potential, epa vs. ph (5.5–8.0) the detection limit (lod) was estimated by applying the standard formula of lod = 3sb/m [31], where m is the slope of the calibration curve and sb is the standard deviation of the peak currents of the blank solution (five replicates). the detection limit for determination of mtx at sdsmcntpe was evaluated as 3.510-8 and limit of quantification as 11×10−8. table 2 presents the linear range and detection limits for mtx determination at sdsmcntpe obtained in the present study and also for some other carbon modified electrodes taken from the literature. data in table 2 show that the linear range, as well as detection limit for mtx at sdsmcntpe is better than for other sensors [32,33]. in addition, the sensor described in the present work is less expensive and simpler to use than other ones. table 1. detection limits for determination of mtx at some modified carbon electrodes electrode detection limit, m method reference cs-dispersed graphene film coated/gce 2×10-10 dpv [40] dsdna/cpe 5.62 × 10-8 dpv [11] dna/gce 10-7 swv [41] sdsmcntpe 3.5 ×10-8 cv this work c u rr e n t,  a e p a / m v j. electrochem. sci. eng. 7(1) (2017) 39-49 determination of mitoxantrone anticancer drug 48 figure 11. calibration plot for determination of mtx at the sdsmcntpe in 0.2 m pbs, ph 7.0 analytical application suitability of here described cv method using the sdsmcntpe for determination of mtx in real samples was tested by analysis of mtx in a commercial injection. the injection was dissolved in 0.2 m pbs, ph 7.0 and the standard addition method was applied by adding the known concentration of mtx into the test solution. the recovery for determination of mtx was in the range of 99.2-111 % for three samples. the recovery and rsd were acceptable, expressing thus good accuracy of the prepared sensor. conclusions excellent sensors based on carbon nanotubes modified by different surfactants were prepared and tested for determination of mtx by cyclic voltammetry experiments. the prepared electrodes were characterized using cv, dpv, fesem and edx. the catalytic activity of sdsmcntpe electrode toward electrooxidation of mtx was investigated and compared with bcntpe, ctabmcntpe, tx100mcntpe and sdsmcpe. the results showed that the sdsmcntpe demonstrated excellent electrocatalytic activity for the oxidation of mtx. wide linear range, low detection limit, good repeatability and reproducibility, distant term stability and acceptable sensitivity and recovery values of the sdsmcntpe suggest that this electrode can be applied as a sensor for estimation of mtx in real samples. better properties of the sdsmcntpe were also observed in comparison with few other electrodes which have already been developed for electrochemical determination of mtx. the capability of other modified cnt electrodes for determination of mtx and possible analysis of other bioactive molecules will be the subject of further research. acknowledgement: we gratefully acknowledge the financial support (seed money for young scientist scheme) from the vgst, bangalore under research project. no. 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[33] a. m. o. brett, t. r. a. macedo, d. raimundo, m. h. marques, s. h. p. serrano, analytica chimica acta 385 (1999) 401408. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) 404 not found аnodic formation of nanoporous crystalline niobium oxide doi: 10.5599/jese.2014.0050 75 j. electrochem. sci. eng. 4(2) (2014) 75-83; doi: 10.5599/jese.2014.0050 open access: issn 1847-9286 www.jese-online.org original scientific paper аnodic formation of nanoporous crystalline niobium oxide leonid skatkov, larisa lyashok*, valeriy gomozov*, irina tokarevа*, boris вayrachniy* technical division, pcb “argo” ltd., 4/23 shaul ha-melekh str., 84797 beer sheva, israel *electrochemistry department, national technical university “khpi”, 21 frunze str., 61002 kharkov, ukraine corresponding author: e-mail: sf-lskatkov@bezeqint.net; tel.: +972 8 6482255 received: february 16, 2014; revised: april 6, 2014; published: may 13, 2014 abstract the research results of anodic deposition of crystalline niobium oxide are presented in this work. the factors that have an impact on crystalline phase nucleation and its primary growth are revealed. dependence of morphology and properties of nanoporous niobium oxide on modes of its formation is shown. keywords oxidation, anodic film, niobium, hf, crystallisation 1. introduction increasing attention is being paid nowadays to the creation and studying of material properties that have nanometer structures. anodic oxidation of valve metals (al, ti, nb, ta) is widely used for the formation of nanostructured oxide films. self-organisation of nanoporous structures during electrochemical processing is completely revealed during the formation of porous anodic oxides of aluminium and titanium. these are distinguished by a high degree of sequence in the arrangement of pores and the possibility to operate in a variation of surface morphologies and thickness of the oxide film [1 2]. anodic niobium oxide deposition in the fluoride-containing electrolytes differs significantly from the porous oxide in aluminium and titanium dioxide nanotubes. under certain conditions of anodisation, anodic oxide films (aof) nb2o5 are formed alongside the crystalline structure in the form of microcones [3,4]. this leads to the common use of similar layers in various devices and designs, for example, in gas sensors [5], catalysts [6], electric capacitors and electrochromic devices [7], as well as in thin-film lithium ion batteries [8], etc. http://www.jese-online.org/ mailto:sf-lskatkov@bezeqint.net j. electrochem. sci. eng. 4(2) (2014) 75-83 аnodic formation of niobium oxide 76 the researchers’ views on the mechanism of nucleation and the formation of crystalline structure of porous aof niobium are inconsistent. therefore, it is noted [9] that formation of the crystalline phase takes place under the influence of the internal tension that arises with a growth in oxide thickness. it is proposed in the work of oikawa et al. [10] that formation of niobium oxide microcones has to be connected with non-uniform chemical dissolution of the anodic film during anodisation. from our point of view, it is necessary to consider the chemical nature of niobium, which belongs to d-type elements, for understanding the nucleation and growth of the crystalline phase mechanism. when one electron has been released, it turns into an ion with empty external dlevels. as a result, the formed ion tries to achieve a stable electronic configuration, especially with oxygen and other non-metals. it is the existence of incomplete configurations of d-electrons that causes niobium to display a wide range of the valence states. the possibility of their existence is proved by thermodynamic calculations [11–13]; it is also shown that low valence oxides have to be on a metal/high valence oxide boundary. according to the authors [11–13], the transition zone cannot be considered as a certain plane-parallel layer between a metal and the high valence oxide. local inhomogeneity on the metal surface, the border of grains, dislocation, admixture atoms and other structural and chemical defects are considered to be the centres of increased surface energy. at these centres, oxygen diffusion into metal is simplified and the primary formation of the lowest valence oxides is possible exactly here. as was shown in [7], films generated in potassium nitrate melt instead of forming an entire niobium pentoxide layer (as had first been suspected), creating a «sandwich» of nb2o5, nbo2 and nbo phases (in the direction from oxide surface to niobium). during discussion of the research results in this article, it was taken into account [13] that, in contrast to thermal crystallisation, no transformation of an amorphous into crystalline film occurred at niobium anodising. the present work is aimed to identifying the factors defining primary formation of nanoporous crystalline in niobium oxide solutions containing the activator. 2. experiment 2.1. electrochemical investigations aof was formed in solutions of 1 m h2so4 with the addition of various hf concentrations (0.5 – 2 m). anodisation was carried out at room temperature in a volt-static mode and varying voltage in the range of 60 to 80 v. platinum was used as a cathode electrode. polarisation was carried out on potentiostat pi 50-1.1 with a scan rate of 10 -2 v/s in potentiodynamic mode [14]. the copperdecoration method was used for the research of synthesised aof conductivity. for this purpose, the cathode samples were polarised in a solution of 220 g/l cuso4 + 60 g/l h2so4. the copper plate worked as an anode, while niobium with the deposited oxide film functioned as a cathode. 2.2. sem and x-ray investigation composition of the synthesised coatings was defined using a scanning electronic microscope jsm-7001f with the x-ray energy-dispersive micro-analyser oxford inca pentafet-x3. morphology of the received coatings was studied by applying the scanning electronic microscopy (sem) method alongside the use of microscopes jsm-7001f, jsm-6390lv. the x-ray structure analysis of films was carried out using diffractometer dron-2 with cuka radiation. l. skatkov at al. j. electrochem. sci. eng. 4(2) (2014) 75-83 doi: 10.5599/jese.2014.0050 77 3. results and discussion initially, niobium surface structure was considered as defective and containing natural oxide film that inherited defects after preparatory operations. in this regard, it was assumed that along the niobium surface at potential imposing, distribution of the electric field is non-uniform and velocity of the process' electrochemical growth and oxide dissolution on various sites of the surface are not identical, and depend on the degree of its deficiency and the composition of liquid facing a electric double layer. in defective sites, the dissolution process has to proceed at a highspeed rate followed with formation of pore seeds when field density is small, and crystalline phase seeds when a strong field is available. for identification of the factors that have an impact on the generation of the crystalline phase at nb2o5 formation, we considered the received anodic polarisation curves (fig. 1). the size of the stationary potential of the niobium electrode in electrolyte with fluoride ions had a value that was more negative at fluoride concentration growth. therefore, activation of the surface takes place without imposing the electric current when fluoride is present. one maximum of current was observed on curves; this confirmed irreversibility of niobium oxide formation. current peak value increased with the growth of fluoride ion concentration in electrolyte. a sharp rise of anode current and system transition into a passive condition was connected to the formation of the oxide monolayer of the higher oxidation state at the interface with electrolyte. figure 1. polarisation curves оf nb in solutions: 1 1 м h2so4 + 1m нf; 2 1 м h2so4 + 0,5 m нf; 3 1 м h2so4 + 0,25 m нf; 4 1 м h2so4 +0,1m нf; 5 1 м h2so4. the higher niobium oxides were insulators with a wide band gap and therefore had bad electronic conductivity; the low oxides and hydroxides functioned like semiconductors. only stoichiometrical oxides acted as a barrier, with higher oxides being formed by means of full oxidation of the lowest oxides [12]. the results showed that processing of polarisation curves on a site of current rise (fig. 1) in semi-logarithmical coordinates transformed into a straight line in log (1 +ja/jlim) coordinates vs. δе; therefore, the process rate of monolayer formation of the higher oxide was determined by diffusion kinetics. j. electrochem. sci. eng. 4(2) (2014) 75-83 аnodic formation of niobium oxide 78 the multilayer niobium oxide formed when potentials were more positive than 1 v. if there was no activator in the solution (fig. 1, curve 5), current practically did not depend on potential; thus, the aof of the barrier type that had an amorphous structure was formed on niobium. the current grew in the system when concentration of the activator (fluoride ion) increased; thus, velocities of oxide formation and dissolution processes in active centres defined the geometry of porous amorphous anodic niobium oxide. being based on the analysis of theoretical regularities and experimental data on aof formation on niobium, the argument can be made that its ionisation proceeds on the solid-phase multi surface mechanism [12] by formation of niobium oxides of the lowest valence. оuter layers, which were enriched with more oxygen, were the oxides of highest valence. rate of niobium ionisation was controlled by the rate of mass transfer of oxygen-containing particles. the process of nanoporous oxide formation was defined by technological parameters of anodisation. results of the x-ray energy dispersive microanalyser (fig. 2) confirmed that the synthesised films corresponded to the stoichiometric composition of nb2o5 oxide, as concentration of elements (at. %) satisfied the ratio 2:5 in the received samples (table 1). figure 2. eds analysis of the niobium oxide film's surface. таble 1. analysis of the chemical composition of the niobium oxide film. element weight content, % atomic content, % oxygen 29.39 70.02 fluoride 0.65 1.31 niobium 69.96 28.67 totals 100.00 100.00 formation of crystalline oxide required the availability of the low oxides in a transitional layer on a metal/oxide boundary and a strong electric activity field. it is known that, in contrast to high oxides, in low oxides the link metal-oxide is mainly ionic, and it exists in a transitional layer in the form of the crystalline phase. it can be assumed that these oxides appear the elements that will form the future crystal grain. in paper [12], it is shown that crystallisation velocity grows with an increase in low valent cation concentrations of oxidised metal. l. skatkov at al. j. electrochem. sci. eng. 4(2) (2014) 75-83 doi: 10.5599/jese.2014.0050 79 as mentioned above, aof crystalline formation on niobium in electrolyte at room temperatures required an active and strong electric field, which was necessary for carrying out the process in a volt-static mode. crystals were formed from grains under an amorphous film in an upper metal surface layer. kinetics of crystalline phase formation in a volt-static mode was defined by the composition of electrolyte and temperature in the reaction zone. while the crystal grew under the film, without destroying it, gradual current decreases were applied at the volt-static mode of aof formation on niobium (figs. 3, 4). after separate crystals broke the amorphous oxide, the current increased as long as the area occupied with a crystal phase grew while crystallisation developed. crystals connected amongst themselves while they grew and then the current gradually began dropping. figure 3.current vs. time during anodisation of nb at 60 v in solution 1 m h2so4 with addition of hf: 1 2 m; 2 1 m; 3 0.5 m velocity of the crystalline phase formation and growth processes increased with the activator concentration jump (fig. 3). the authors of [12] offered to call the time from the start of anodisation to the current growth using the nb-nb2o5-electrolyte (moe) system, as the incubatory period was connected with nucleation and the growth of crystals under the amorphous film of nb2o5. after completion of the incubatory period in a chain of the moe system, rapid increase of the current was observed (figs. 3, 4), which can be explained due to contact of electrolyte with the crystal surface. while the crystallisation process was developing, the area occupied with the crystal phase increased and current growth in the moe system chain slowed down. growing crystals had the wrong polyhedron form (figs. 5, 6). the authors of [4] called similar formations microcones of niobium oxide. such crystalline formations consist of needle crystals that radiate from the crystallisation centre. wrongly formed crystals are connected with sectors that grow at various speeds. all microcones (figs. 5, 6) consisted of strongly furcated nanofibres of niobium oxide and created a well-developed oxide film surface. j. electrochem. sci. eng. 4(2) (2014) 75-83 аnodic formation of niobium oxide 80 figure 4. current vs. time during anodisation of nb in solution 1 m h2so4 + 1 m hfat: 1 80 v; 2 70 v; 3 60 v. а b figure 5. sem images of the surface (a) and cross-section (b) of anodic niobium oxide synthesised during 5 h at 60 v in solution1 мh2so4 + 0.5 mнf. it can be seen that crystalline oxide increased electric conductivity (fig.6, curve 3) in comparison with the amorphous porous structure of niobium oxide (fig. 7) and the solid barrier oxide using the decoration method. as stated above, anodic niobium oxides of the crystalline structure had to possess electronic conductivity, in contrast to amorphous films. as can be seen in fig. 6, allocation of copper on films of the amorphous structure practically did not occur in the range of 0.2 – 0.4 v. when cathode polarisation was 0.4 v and above, copper allocation on a sample with crystalline aof (fig. 6, curve 3) had a larger velocity. therefore, resistance on the interfaces of solution cuso4 – aof – niobium varied significantly for the received coatings. j / m a c m -2 l. skatkov at al. j. electrochem. sci. eng. 4(2) (2014) 75-83 doi: 10.5599/jese.2014.0050 81 a b figure 6. sem images of the surface of anodic niobium oxide synthesised in solution 1 м h 2 so 4 + 1 m нfat 60 v: a 3 h; b 5 h. figure 7. current vs. potential during deposition of copper on anodic niobium oxide synthesised in: 1 1 м h2so4, 60 v, 1 h; 2 1 м h2so4 + 0.25 m hf, 20 v, 1 h; 3 1 м h2so4 + 0.5 m hf, 60 v, 5 h. by means of the x-ray diffraction analysis method it was confirmed that niobium aof, having been synthesised during 1 h was x-ray amorphous (see fig. 8, curve 2); in the same electrolyte, the crystalline oxide was formed on long anodisation. on the received roentgenogram there were peaks that corresponded to crystalline nb2o5 (fig. 8, curve 1). j. electrochem. sci. eng. 4(2) (2014) 75-83 аnodic formation of niobium oxide 82 figure 8. xrd patterns from niobium aof synthesised in solution 1 м h2so4 + 0.5 mнf at 60 v during: 1 – 5 h; 2 1 h. 4. conclusions based on the research conducted it can therefore be assumed that aof formation on niobium proceeds on the solid-phase poly-surface mechanism by formation of low valence oxides, which are grains of the crystalline phase at formation of oxide monolayer. growth of poly-layer crystalline niobium oxide was observed to take place under the amorphous film of nb2o5. after completion of the incubatory period, growing crystals broke the barrier film and the crystallisation process developed on the entire surface of the sample. predominant formation of nanoporous crystalline oxide of niobium took place in a volt-static mode and at strong field activity in electrolyte containing the activator. the superficial crystalline structure had a nanoporous structure. its morphology depended on the fluoride ion concentration in the solution, voltage and anodisation time. references [1] s. minagar, c. c. berndt, j. wanga, e. ivanova, c. wen, acta biomater. 8 (2012) 2875–2888. [2] p. schmuki, nanostructure sci tech. 3 (2009) 435–466. [3] r. l. karlinsey, electrochem. com. 7 (2005) 1190–1194. [4] s. yanga, h. habazaki,t. fujii, y. aoki, p. skeldonb, g. e. thompson, electrochim. acta 56 (2011) 7446–7453. [5] r. a. rani, a. s. zoolfakara, j. z. oua, m. r. fieldb, m. austina, k. kalantar-zadeh, sens. actuators b 176 (2013) 149–156. [6] k. tanabe, catal. today 78 (2003) 65–77. l. skatkov at al. j. electrochem. sci. eng. 4(2) (2014) 75-83 doi: 10.5599/jese.2014.0050 83 [7] l. skatkov, v. gomozov, niobium: chemical properties, applications and environmental effects, nova science publ., new york ,usa, 2013, 123–136. [8] j. e. yoo, j. park, g. cha, j. choi, thin sol. films 531 (2013)583–587. [9] j. zhao, x. wang, r. xu, y. mi, y. li, electrochem. sol. state lett. 10 (2007) c31–c33. [10] y. oikawa, t. minami, h. mayama, k. tsujii, k. fushimi, y. aoki, p. skeldon, g. e. thompson, h. habazaki, acta mat. 57 (2009) 3941–3946. [11] r. collongues, la non-stoechiometrie, masson et cie, paris, france, 1971, p. 285. [12] l. odynetz, anodic oxide films, nauka, leningrad, ussr, 1990, p. 200. [13] ya. kolotyrkin, vestnik an ussr 7 (1977) 73–80. [14] v. gomozov v., l. skatkov, l. liashok, b. bayrachny, sov. j. appl. chem. 62 (1989)1284– 1287. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {electrochemical reversibility of me6fc/me6fc+pf6and me8fc/me8fc+pf6redox systems in acetonitrile:} http://dx.doi.org/10.5599/jese.1032 221 j. electrochem. sci. eng. 11(4) (2021) 221-225; http://dx.doi.org/10.5599/jese.1032 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical reversibility of me6fc/me6fc+pf6and me8fc/me8fc+pf6redox systems in acetonitrile nigar z. ibrahimova, gazanfar m. jafarov, dilgam b. tagiyev and iltifat u. lyatifov acad. m. nagiyev institute of catalysis and inorganic chemistry, national academy of sciences of azerbaijan, h. javid ave. 113, az 1143 baku, azerbaijan corresponding author: nigar-ibrahimova93@mail.ru received: june 23, 2021; accepted: august 3, 2021; published: august 12, 2021 abstract two new redox systems, sym. 1,2,4,1,2,4-hexamethylferrocene/cation sym. 1,2,4,1,2,4 -hexamethylferricinium and sym. octamethylferrocene/cation sym. octamethylferricinium (menfc/menfc+, n = 6, 8) were studied by the cyclic voltammetry method. the observed difference between potentials of anodic and cathodic peaks of 0.063 to 0.075 v, and its independence on the potential scan rate, the straight-line dependence of the current value of anodic (and cathodic) peak on square root of the potential scan rate, as well as shapes of the recorded cyclic voltammograms indicate that both redox systems in acetonitrile meet the most important requirement of iupac regarding internal reference redox systems (irrs) electrochemical reversibility of electron transfer reaction. the same method under identical conditions was used to study the effect of the number of methyl groups on the redox potential of menfc/menfc+ systems, n = 0, 6, 8, 10. it was shown that the successive displacement of half-wave potential in the series of fc/fc+ → me6fc/me6fc+ → me8fc/me8fc+ → → me10fc/me10fc+ towards negative potentials is attributed to the electron-donor property of methyl groups. the location of the redox potentials values of new systems [n=6 (111 mv), n=8 (23 mv)] between redox potentials of systems of n = 0 (431 mv) and n = 10 (-77 mv) means that the redox potential of the systems of menfc/menfc+(n = 6, 8) has an optimal position on the electrode potential scale, i.e. meets another of the iupac criteria for irrs. keywords internal reference redox systems; voltammetry; sym. hexamethylferrocene; sym. hexamethylferricinium; sym. octamethylferrocene; sym. octamethylferricinium. introduction ferrocene and its derivatives are, from the point of view of internal reference redox systems (irrs) in non-aqueous media, one of the most widely studied chemical materials over the past http://dx.doi.org/10.5599/jese.1032 http://dx.doi.org/10.5599/jese.1032 http://www.jese-online.org/ mailto:nigar-ibrahimova93@mail.ru j. electrochem. sci. eng. 11(4) (2021) 221-225 electrochemical reversibility of redox systems 222 10-15 years [1,2]. currently for this purpose, the ferrocene-ferricinium system is commonly used [1,3]. periodic reports on its instability [4,5], however, have initiated numerous researches to find, from the point of view of the iupac criteria [1,6], more promising systems consisting of methyl homologues of ferrocene and ferricinium cation [7,8]. an advantage of menfc/menfc+ (n = 6, 8, 10) systems in comparison with the fc/fc+ system arises from the fact that the potential of menfc/menfc+ (n = 6, 8, 10) systems depends to a lesser extent on the nature of the solvent than the potential of fc/fc+ system. this feature of menfc/menfc+ (n = 6, 8, 10) redox systems is due to electron-donor properties and spatial dimensions of methyl groups. on the one hand, electron-donor properties of methyl groups partially compensate for a positive charge on the iron atom. on the other hand, emergence of several me groups in cyclopentadienyl rings noticeably increases spatial dimensions (volume) of the polymethylcyclopentadienyl ring. both factors will weaken the interaction of solvent molecules both with the central iron atom and aromatic ring of the reagents (menfc and menfc+). therefore, the potential of the menfc/menfc+ (n = 6, 8, 10) redox systems is less affected by the solvent than the potential of the fc/fc+ system. among iupac criteria, the most important is the reversibility of the electron transfer reaction proceeding with the participation of the redox system (eq. 1): menfc  menfc+ + e(n = 6, 8) (1) in our previous work [9 the reversibility of the electron exchange in two redox systems, me6fc/me6fc+ and me8fc/me8fc+ was evidenced by the 1н nmr method. the purpose of this work is to define the electrochemical reversibility of the electron transfer reaction proceeding in the redox systems me6fc/me6fc+ and me8fc/me8fc+ by the method of cyclic voltammetry. experimental electrochemical study of redox system of menfc/menfc+ (n = 6, 8) was conducted in a threeelectrode cell using the potentiostat of iviumstat electrochemical interface. as the working electrode, a platinum wire was used, platinum plate served as the auxiliary electrode, while silver/silver chloride electrode (ag/agcl in acetonitrile, 0.1 m [nbu4][pf6]) was used as the reference electrode. the reference electrode was isolated from the cell solution with the luggin's capillary, filled with the buffer solution (hexafluorophosphate tetrabutylammonium ([nbu4][pf6]), 0.1 m). the concentration of the reagent (menfc) in the solution was 1 mm. as an electrolyte, hexafluorophosphate tetrabutylammonium ([nbu4][pf6]) (sigma-aldrich) was used. before using, the salt was dried during an hour at the temperature of 105 °c by a vacuum pump [3]. the cyclic voltammograms were recorded in the presence of 0.1 m [nbu4][pf6] in acetonitrile purified from oxygen and traces of water, in the argon atmosphere. temperature of the system was kept constant (0.1 k) by a thermostat. neutral complexes me6fc and me8fc were subliminated at temperature of 40 and 60 °c under the pressure of 0.01 mpa, and recrystallized twice from hexane solution. the salts menfc+pf6– (n = 6, 8) in the form of acetone solution passed through a column with an aluminum oxide with height of 3 4 cm and recrystallized twice from a mixture of acetone with hexane. results and discussion in figure 1, cyclic voltammograms recorded for the systems of me6fc/me6fc+ and me8fc/me8fc+, recorded at various scan rates of potential,  = 0.02, 0.05, 0.1, and 0.2 v s–1, are shown. n. z. ibrahimova et al. j. electrochem. sci. eng. 11(4) (2021) 221-225 http://dx.doi.org/10.5599/jese.1032 223 a b figure 1. cyclic voltammograms of redox systems me6fc/me6fc+ (a), and me8fc/me8fc+ (b) in electrolyte solution (supporting electrolyte: [nbu4][pf6] (0.1m), solvent: acetonitrile, concentration of me6fc, me8fc 1 mm, scan rate: 10.2 v s-1; 2 0.1 v s-1; 3 0.05 v s-1; 4 0.02 v s–1 the characteristics of cyclic voltammograms of the studied redox systems are described in table 1. table 1. the characteristics of cyclic voltammograms of the redox systems of me6fc/me6fc+ and me8fc/me8fc+  / v s–1 epa / v* epc / v ep / v e1/2 / v me6fc/me6fc+ 0.02 0.136 0.067 0.069 0.102 0.05 0.142 0.071 0.071 0.107 0.10 0.147 0.074 0.073 0.111 0.20 0.151 0.076 0.075 0.113 me8fc/me8fc+ 0.02 0.054 – 0.009 0.063 0.022 0.05 0.055 – 0.010 0.065 0.023 0.10 0.057 – 0.010 0.067 0.023 0.20 0.058 – 0.011 0.069 0.024 *potentials were calculated relatively to acetonitrile solution of silver chloride (ag/agcl) reference electrode (buffer solution – 0.1 м [nbu4][pf6]) here  is the scan rate of the potential, epa – anodic peak potential, epс – cathodic peak potential, ipa/ipc is the ratio of the anodic and cathodic peak current, ep is difference of potentials of anodic and cathodic peaks (epa epс), e1/2 is half-wave potential, (epa + epс)/2. consideration of parameters presented in table 1 and corresponding cyclic voltammograms allow distinguishing three general features of voltammograms that confirm reversibility of the redox reactions of me6fc/me6fc+ and me8fc/me8fc+: 1. the value of potential difference of the anodic and cathodic peaks (ep) is, regardless of the scan rate of the potential at 25 °c, in the range of 0.063 0.075v. 2. straight-line correlation of the current value of anodic (or cathodic) peak and square root of the scan rate of the potential shown in figure 2, supports the correspondence of both redox systems to the randles-ševčik equation. 3. the forms of cyclic voltammograms, as well as proximity of ratio of the anodic and cathodic peak current to the unit (0.92 1.15). above mentioned features of voltammograms allow us to make the conclusion that the heterogeneous reaction of the electron transfer in the redox systems of me6fc/me6fc+ and me8fc/me8fc+ is the one-electron reversible process. http://dx.doi.org/10.5599/jese.1032 j. electrochem. sci. eng. 11(4) (2021) 221-225 electrochemical reversibility of redox systems 224 a b figure 2. linear dependence of anodic peak current values and square root of potential scan rate for redox systems me6fc/me6fc+ (a) and me8fc/me8fc+ (b) the main point of this conclusion is that menfc/menfc+ (n = 6, 8) systems are quite stable in acetonitrile during electrochemical studies, whereas the data obtained from the cyclic voltammograms indicate the absence of the side chemical reactions in the system. therefore the systems of me6fc/me6fc+ and me8fc/me8fc+ showing the electrochemical reversibility in organic solvent (acetonitrile) can be used for the development of the reversible irrs in non-aqueous media. similar conclusion on stability of these two systems in other organic solvents and their mixtures with water was defined in the work on study of kinetics of the electron exchange reaction in homogeneous systems me6fc/me6fc+ and me8fc/me8fc+ by the method of 1н nmr [9. in table 2, the half-wave potential value (e1/2) of redox systems me6fc/me6fc+ and me8fc/me8fc+, along with the values e1/2 for the redox system of fc/fc+ and me10fc/me10fc+ are shown. the values of the half-wave potential for fc/fc+ and me10fc/me10fc+ [1,10 were obtained from our voltammetric studies conducted under identical conditions as for the systems of me6fc/me6fc+ and me8fc/me8fc+. table 2. the half-wave potential values for menfc/menfc+ (n=0,6,8,10) redox systems ( = 0.1 v s–1) redox system fc/fc+ me6fc/me6fc+ me8fc/me8fc+ me10fc/me10fc+ e1/2 / mv 431 111 23 – 77 it is seen from table 2 that half-wave potential values of me6fc/me6fc+ and me8fc/me8fc+ redox systems are in the scale of electrode potentials located approximately in the same region as e1/2 of fc/fc+ and me10fc/me10fc+ redox systems which were recommended as irrs in non-aqueous media [3]. the electrochemical reversibility of me6fc/me6fc+ and me8fc/me8fc+ redox systems in an organic solvent (acetonitrile), and the optimal range of location of their redox potentials indicate the necessity of the further study on establishing the correspondence of these systems (n = 6, 8) with remaining criteria for the reference electrode [1,6]. currently, we are continuing electrochemical studies of these two systems in various organic solvents, ionic liquids, and mixtures of organic solvent/water. it is also seen from table 2 that in the series of me6fc/me6fc+ → me8fc/me8fc+ → → me10fc/me10fc+, e1/2 of the subsequent redox system consistently shifts towards negative potentials. the shift for the system of me8fc/me8fc+ relatively to me6fc/me6fc+ is 87 mv, but for the system of me10fc/me10fc+ relatively to me8fc/me8fc+ is 101 mv. these suggest that appearing of two additional methyl groups in the composition of each reagent causes the shift of the half-wave potential for 90–100 mv in the direction of negative potential. the observed shifts of e1/2 in the series of fc/fc+ → me6fc/me6fc+ → me8fc/me8fc+ → → me10fc/me10fc+ should be attributed to the electron-donor property of the methyl groups which n. z. ibrahimova et al. j. electrochem. sci. eng. 11(4) (2021) 221-225 http://dx.doi.org/10.5599/jese.1032 225 facilitates one-electron oxidation of the corresponding neutral polymethylferrocene. indeed, a comparative study of photoelectron spectra of ferrocene and decamethylferrocene [11] showed that the ionization potential of d-electron of iron atom in decamethylferrocene is approximately 1.0 ev less than the corresponding ionization potential in the ferrocene molecule. moreover, as in the sequence me6fc/me6fc+ → me8fc/me8fc+→ me10fc/me10fc+, in the series of fc/fc+ → me2fc/me2fc+ → … → me10fc/me10fc+, the electron effect of methyl groups has an additive character: in case of decreasing ionization potential of d-electron of 1,1-dimethylferocene in comparison with ionization potential of ferrocene it is equal to 0.2 ev, then this value is 5 times higher for decamethylferrocene and it is 1 ev, as noted above. conclusion it was shown by cyclic voltammetry method that the electron transfer reaction of redox systems me6fc/me6fc+ and me8fc/me8fc+ in an organic solvent (acetonitrile) is a reversible one. the range of their redox potential is in the optimal region of electrode potential scale. this means that the systems of me6fc/me6fc+ and me8fc/me8fc+ meet two of seven criteria of iupac postulated for an irrs in non-aqueous media. therefore, further studies of these systems as internal reference redox systems are required. references [1] a. a. j. torriero, medicinal & analytical chemistry international journal 3(4) (2019) 1-8. https://doi. org/10.23880/macij-16000151 [2] w. e. geiger, organometallics 26(24) (2007) 5738-5765. https://doi.org/10.1021/om700558k [3] n. elgrishi, k. j. rountree, b. d. mccarthy, e. 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https://doi.org/10.1351/pac198456040461 https://doi.org/10.1351/pac198456040461 https://doi.org/10.1016/j.electacta.2014.06.005 https://doi.org/10.1016/j.electacta.2014.06.005 https://doi.org/10.1039/c2cp43177g https://10.0.127.225/2221-8688-2019-2-310-315 https://doi.org/10.1021/acs.jchemed.8b00021 https://doi.org/10.1021/acs.jchemed.8b00021 https://doi.org/10.1016/0368-2048(80)80058-2 https://doi.org/10.1016/0368-2048(80)80058-2 https://creativecommons.org/licenses/by/4.0/) {electrochemical sensor for determination of hydroxylamine using functionalized fe3o4 nanoparticles and graphene oxide modified screen-printed electrode:} http://dx.doi.org/10.5599/jese.1145 71 j. electrochem. sci. eng. 12(1) (2022) 71-79; http://dx.doi.org/10.5599/jese.1145 open access : : issn 1847-9286 original scientific paper electrochemical sensor for determination of hydroxylamine using functionalized fe3o4 nanoparticles and graphene oxide modified screen-printed electrode hamed tashakkorian1,2, behnaz aflatoonian3, peyman mohammadzadeh jahani4, and mohammad reza aflatoonian5, 1cellular and molecular biology research center (cmbrc), health research institute, babol university of medical sciences 2department of pharmacology, school of medicine, babol university of medical sciences, babol, iran 3research center of tropical and infectious diseases, kerman university of medical sciences, kerman, iran 4school of public health, bam university of medical sciences, bam, iran 5leishmaniasis research center, kerman university of medical sciences, kerman, iran corresponding author:  peyman1234@gmail.com; m.aflatoonian97@gmail.com received: october 19, 2021; accepted: november 2, 2021; published: november 17, 2021 abstract a simple strategy for determination of hydroxylamine based on fe3o4 nanoparticles functionnalized by [2-(4-((3-(trimethoxysilyl)propylthio)methyl)1-h1,2,3-triazol-1-yl)aceticacid] (fnps) and graphene oxide (go) modified screen-printed electrode (spe), denoted as (fe3o4 fnps/go/spe), is reported. the electrochemical behavior of hydroxylamine was investigated at fe3o4fnps/go/spe by cyclic voltammetry (cv), differential pulse voltammetry (dpv) and chronoamperometry (cha) techniques in phosphate buffer solution (ph 7.0). fe3o4 fnps/go/spe as a novel electrochemical sensor exhibited catalytic activity toward the oxidation of hydroxylamine. the potential of hydroxylamine oxidation was shifted to more negative potentials, and its oxidation peak current increased on the modified electrode, also indicating that under these conditions, the electrochemical process is irreversible. the electrocatalytic current of hydroxylamine showed a good relationship in the concentration range of 0.05–700.0 μm, with a detection limit of 10.0 nm. the proposed electrode was applied for the determination of hydroxylamine in water samples, too. keywords hydroxylamine; electroanalysis; voltammetry; modified electrode; introduction hydroxylamine is one of the important compounds in the chemical industry. hydroxylamine, a derivative of ammonia is one of the reducing agents widely used in industry and pharmacy. it is http://dx.doi.org/10.5599/jese.1145 http://dx.doi.org/10.5599/jese.1145 mailto:peyman1234@gmail.com mailto:m.aflatoonian97@gmail.com j. electrochem. sci. eng. 12(1) (2022) 71-79 determination of hydroxylamine 72 identified as a key intermediate in the nitrogen cycles and production of nitrous oxide [1]. hydroxylamine is a well-known mutagen, which induces highly specific mutations with the nucleic acid cytosine. modest levels of hydroxylamine can be toxic to humans, animals and even plants [2]. also, in the industry, it can be used as a raw material for the synthesis of pharmaceutical intermediates and final drug substances. therefore, from the industrial, environmental and health viewpoints, the development of a sensitive analytical method for the determination of hydroxylamine is very important [3-5]. due to the simple sample preparation, low-cost instrumentation, high sensitivity, selectivity, accuracy and precision of electrochemical methods, they have been used widely in biological and environmental analysis. however, many analytes exhibit irreversible oxidation requiring large overpotential at conventional electrodes [6-10]. therefore, the use of chemically modified electrodes for the analysis of metals and organics is a well-established practice [11-14]. a conductive substrate modified with electroactive thin films, monolayers, or thick coatings results in a chemically modified electrode. the modification of the conductive substrate has positive effects on (i) electron transfer kinetics, (ii) electrocatalytic activity due to the use of materials with large surface area, (iii) sensitivity of measurement in electroanalytical applications, (iv) selectivity toward specific molecules due to immobilized functional groups, and (v) extraction and accumulation of an analyte at the electrode surface [15,16]. researchers have recently focused on designing and synthesizing nano-materials for various applications due to their unique physical and chemical properties [17-21]. nano-materials as modifiers are used in various sensor and biosensor applications because they exhibit good electrocatalytic properties, high stability, high surface-to-volume ratio, and wide availability and provide fast electron transfer rates. metal nanoparticles, which exhibit very interesting physico-chemical properties, are among the materials most commonly used for the modification of electrodes. in recent years, magnetic nanoparticles of iron oxide (fe3o4) have attracted considerable interest due to their particular attributes, such as magnetic and optical properties, low toxicity, biocompatibility and easy preparation. many of the electrical and magnetic properties of metal nanoparticles are attributed to the electron transfer between fe2+ and fe3+ centers present in the oxide chemical structure [22,23]. graphene oxide (go) is a compound with different ratios of carbon, oxygen, and hydrogen, which is a derivative of graphene. the presence of various functional groups such as epoxy, hydroxy and carboxyl on the go surface affects the oxidation degree of graphene oxide [24]. the unique physicochemical properties such as large specific surface area and high conductivity suggest a great potential for go to provide new approaches and critical improvements in the field of electrochemistry [25]. according to the previous points, it is important to create suitable conditions for the analysis of hydroxylamine. in this study, we describe the application of functionalized fe3o4 nanoparticles as a nanostructure sensor for the voltammetric determination of hydroxylamine. eventually, the analytical performance of the suggested sensor for hydroxylamine determination in water samples is evaluated. experimental instrumentation an autolab pgstat 302n instrument (eco chemie, the netherlands) and a general-purpose electrochemical system software were used to perform and control the experiments. the screenprinted electrode (dropsens, drp-110, spain, working: 4 mm diameter) consists of three main parts: graphite counter electrode, silver pseudo-reference electrode, and graphite working electrode. ph h. tashakkorian et al. j. electrochem. sci. eng. 12(1) (2022) 71-79 http://dx.doi.org/10.5599/jese.1145 73 measurements were performed using a metrohm 710 ph-meter. field emission scanning electron microscopy (fesem) was recorded on a hitachi s4160 instrument (tokyo, japan). reagents analytical grade reagents were obtained from merck co. (darmstadt, germany). orthophosphoric acid buffer solutions (2.0 < ph < 12.0) were prepared using the acid and its salts (kh2po4, k2hpo4, and k3po4). all solutions were freshly prepared using double distilled water. synthesis of fe3o4 fnps the modified 2-(4-((3-(trimethoxysilyl) propylthio)methyl)1-h1,2,3-triazol-1-yl)aceticacid-fe3o4 nanoparticles (fe3o4 fnps) were prepared in the following four-step procedure: step 1 in a round-bottomed flask, propargyl bromide (0.11 ml, 1mmole) was added to the mixture of 15 ml acetone containing 3-mercaptopropyl(trimethoxy)silane (0.195 ml, 1.05 mmol) and potassium carbonate (0.167 g, 1.2 mmol). the reaction medium was kept for 20 hours in this condition at 70 °c while being stirred under n2. after the specified time, the reaction mixture was filtered, and the solvent was evaporated with a rotary evaporator. the product was obtained by two-phase ethylacetate/water) extraction procedure, and washed with 5 % nahco3 and brine solution, giving oily trimethoxy(3-(prop-2-yn-1-ylthio)propyl)silane. the oily product can be incorporated in the next step (step 2) without any further purification. analytical calculation for c9h18o3ssi gave: c: 46.15 %, h: 7.69 %, s: 13.67 %; elemental analysis found: c:46.46 %; h:7.25 %, s: 13.38 %. step 2 to the solution of chloroacetic acid (0.094 g, 1 mmol) in 15 ml dmf, sodium azide (0.072 g, 1.05 mmol) was added, and the mixture was stirred at 90 °c for 24 hours. then the solution was kept for the next step of the synthesis. step 3 this compound was synthesized based on click protocol and the preparation of triazole compounds discussed in our previous articles [26,27] thoroughly. one-pot synthesis of compound 3 was carried out by the addition of trimethoxy(3-(prop-2-yn-1-ylthio)propyl)silane (1) (1 mmol) to a freshly prepared 15 dmf solution of 2-azidoaceticacid. then, cuso4·5h2o (0.075 g, 0.3 mmol), sodium ascorbate (0.346 g , 1.5 mmol) and 2 ml of water were added to the reaction mixture. the mixture was stirred at room temperature for 30 hours. after the appropriate time, the mixture was poured into the water (30 ml) to remove the unwanted compounds. the product was extracted with 25 ml of ethyl acetate and washed with 1m ammonium hydroxide (15 ml) for removing the unreacted copper ions. the organic phase was washed for another time and dried over mgso4. after filtration, the solvent was removed under reduced pressure by heidolph rotary evaporator to obtain the oily product 3. analytical calculation for c11h21n3o5ssi gave: c: 39.40 %, h: 6.27 %, n: 12.54 %, s: 9.55 %; elemental analysis found: c: 39.76 %; h: 5.95 %, n: 12.34 %, s: 9.28 %. step 4 fecl3⋅6h2o (1.9 g) and fecl2⋅4h2o (0.67 g) were dissolved in 50 ml of deionized water under an argon atmosphere [28]. after half an hour, 3 ml of ammonium hydroxide solution (25 %) was added to the solution and the ph value was maintained around 11 with continuous stirring using a magnetic stirrer. after the addition of the mentioned volume of the ammonia solution, a deep black magnetite http://dx.doi.org/10.5599/jese.1145 j. electrochem. sci. eng. 12(1) (2022) 71-79 determination of hydroxylamine 74 precipitate was formed immediately. stirring was continued for another 1 hour under an argon atmosphere. the resulting precipitate was collected using a magnet and washed several times with deionized water to remove the excess amount of ammonia and until the ph of eluent water became neutral. the magnetite nanoparticles were kept in the dark glass for further applications. in a 50 ml round-bottomed flask, 1 g of magnetic nanoparticles (fe3o4) was added to the solution of the synthesized compound 3 in dmf (30 ml). the reaction mixture was sonicated for 10 minutes using an ultrasonic bath and heated at 110 °c for 24 hours under an argon atmosphere to reach the maximum dispersion. after the specified time, the functionalized magnetic nanoparticles were collected using a magnet and washed with dmf and acetone, respectively. the precipitate was kept in the dark glass and stored in a desiccator. the fesem image of magnetic nanoparticles is displayed in figure 1. 1m figure 1. fesem of fe3o4f nps derivative preparation of fe3o4 fnps/go/spe the bare screen-printed electrode was coated with functionalized fe3o4 nanoparticles (fe3o4 fnps) and go as follows: a stock solution of fe3o4 fnps and go in 1 ml aqueous solution was prepared by dispersing 1 mg fe3o4 fnps and 1 mg go with ultrasonication for 1 h. then, 5 µl aliquot of fe3o4 fnps-go/h2o suspension solution was cast on the carbon working electrodes and waited until the solvent was evaporated at room temperature. the surface area of fe3o4 fnps/go/spe and bare spe were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula [29] for fe3o4 fnps/go/spe, the electrode surface was 0.081 cm2 which was about 2.6 times greater than bare spe. result and discussion electrochemical profile of hydroxylamine on fe3o4 fnps/go/spe since the electrochemical behaviour of hydroxylamine is ph-dependent, the optimizing ph of the solution is necessary for obtaining the best results. hence, the evaluations were performed at different ph values ranging from 2.0–9.0. results presented in figure 2 showed that the best result during the electro-oxidation of hydroxylamine at the surface of the modified electrode is obtained at ph 7.0. h. tashakkorian et al. j. electrochem. sci. eng. 12(1) (2022) 71-79 http://dx.doi.org/10.5599/jese.1145 75 figure 3 illustrates the cyclic voltammograms of 200.0 μm hydroxylamine in 0.1 m pbs at fe3o4 fnps/go/spe (curve a) and unmodified spe (curve b). as it can be easily noticed, the maximum oxidation of hydroxylamine occurs at 860 mv in the case of fe3o4 fnps/go/spe, which is around 165 mv more negative than observed in the case of unmodified spe. ph figure 2. plot of ip vs. ph obtained from dpvs of fe3o4 fnps/go/spe in a solution containing 200.0 μm of hydroxylamine in 0.1 m pbs of different ph (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0) figure 3. cyclic voltammograms of (a) fe3o4 fnps/go/spe and (b) bare spe in 0.1 m pbs (ph 7.0) in the presence of 200.0 μm hydroxylamine at the scan rate 50 mvs-1 effect of scan rate figure 4 illustrates the effects of potential scan rates on the oxidation currents of hydroxylamine, indicating that increasing the scan rate increased the peak currents. also, based on the fact that the plot of ip against the square root of the potential scan rate (ν1/2) is linear (cf. inset of figure 4), it was concluded that the oxidation process of hydroxylamine at fe3o4 fnps/go/spe is diffusion controlled. further, the tafel curve of hydroxylamine was plotted using data from the rising sections of the current-voltage curves (i.e., tafel regions) obtained at 10 mv s−1 (figure 5). figure 4. cyclic voltammograms of fe3o4 fnps/go/spe in 0.1 m pbs (ph 7.0) containing 200.0 μm hydroxylamine at various scan rates. 1-9 correspond to 10, 25, 50, 75, 100, 200, 400, 600 and 800 mv s-1. inset: variation of anodic peak current vs. ν1/2 figure 5. cyclic voltammogram (10 mv s−1) of electrode in 0.1 m pbs (ph 7.0) containing 200.0 µm hydroxylamine. the points are data used for the tafel plot shown in the inset http://dx.doi.org/10.5599/jese.1145 j. electrochem. sci. eng. 12(1) (2022) 71-79 determination of hydroxylamine 76 the tafel region of the current-potential curve is influenced by the electron transfer kinetics of the electrode reactions. the results showed a tafel slope of 0.1771 v, indicating one-electron transfer as the rate-determining step for the electrode process [24] for charge transfer coefficient (α) of 0.67. chronoamperometric analysis the chronoamperometric analysis of hydroxylamine using fe3o4 fnps/go/spe was performed at 0.92 v vs. ag/agcl/kcl (3.0 m) and the results were obtained for different hydroxylamine samples in pbs (ph 7.0) are illustrated in figure 6. figure 6. chronoamperograms obtained at fe3o4 fnps/go/spe in 0.1 m pbs (ph 7.0) for different concentrations of hydroxylamine (1–5 correspond to 0.1, 0.3, 0.8, 1.2 and 2.0 mm). insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1–5; (b) the plot of the slope of the straight lines against hydroxylamine concentration for chronoamperometric analysis of electroactive materials under mass transfer limited conditions, the cottrell equation [29] can be applied. cottrell equation is defined as: i = nfad1/2cbπ-1/2t-1/2 where d is the diffusion coefficient, and cb is the bulk concentration (mol cm−3), a is the geometric area of the electrode, n is the number of electrons, f is the faraday number, and t is the time. experimental plots of i vs. t−1/2 were employed, with the best fits for different concentrations of hydroxylamine shown in figure 6a. the slopes of the resulting straight lines were then plotted vs. hydroxylamine concentration (figure 6b). from the resulting slope and cottrell equation, the mean value of d for hydroxylamine was found to be 6.86×10−6 cm2 s-1. calibration curve and limit of detection the peak currents obtained for hydroxylamine using fe3o4 fnps/go/spe were utilized for the quantitative analysis of hydroxylamine in water solutions. given the advantage of differential pulse voltammetry (dpv) in terms of improved sensitivity and better characteristics for analytical applications, the modified electrode was used as the working electrode in dpv analyses in a range of hydroxylamine h. tashakkorian et al. j. electrochem. sci. eng. 12(1) (2022) 71-79 http://dx.doi.org/10.5599/jese.1145 77 solutions in 0.1 m pbs. dpv (step potential=0.01 v and pulse amplitude = 0.025 v) results presented in figure 7 show a linear relationship between the peak currents and concentrations of hydroxylamine over the concentration range of 0.05-700.0 µm. the correlation coefficient of 0.9987 and detection limit (3σ) of 10.0 ± 0.02 nm was obtained. figure 7. dpvs of fe3o4 fnps/go/spe in 0.1 m (ph 7.0) containing different concentrations of hydroxylamine (1–13 correspond to 0.05, 0.25, 1.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0, 100.0, 300.0, 500.0 and 700.0 µm). inset: plot of oxidation peak current as a function of hydroxylamine concentration in the range of 0.05-700.0 µm analysis of real samples to assess the applicability of the modified electrode for the determination of hydroxylamine in real samples by using dpv, the described method was applied to the determination of hydroxylamine in tap, river and well water samples. the standard addition method was used for this analysis, and the results are given in table 1. the observed recovery of hydroxylamine was satisfactory, and the reproducibility of the results was demonstrated based on the mean relative standard deviation (rsd). table 1. the application of fe3o4 fnp/spe for determination of hydroxylamine in water samples (n=5) sample concentration, μm recovery, % rsd, % spiked, found, μm tap water 0 nd 5.0 4.9 98.0 2.4 10.0 10.2 102.0 3.3 15.0 15.5 103.3 2.7 20.0 19.8 99.0 2.1 river water 0 nd 7.5 7.7 102.7 1.9 12.5 12.4 99.2 2.8 17.5 17.8 101.7 2.4 22.5 23.2 103.1 3.2 well water 0 nd 10.0 9.8 98.0 3.4 20.0 19.8 99.0 1.8 30.0 30.7 102.3 2.7 40.0 39.5 98.7 2.4 http://dx.doi.org/10.5599/jese.1145 j. electrochem. sci. eng. 12(1) (2022) 71-79 determination of hydroxylamine 78 conclusion we have demonstrated the highly sensitive electrochemical determination of hydroxylamine using fe3o4 fnps and go modified spe. fe3o4 fnps/go/spe showed two times higher oxidation current with 165 mv less positive potential shift for hydroxylamine compared to bare spe. the current response was increased linearly while increasing the concentration of hydroxylamine from 0.05 to 700.0 μm and a detection limit was found to be 10.0 nm (s/n = 3). the practical application of the present modified electrode was demonstrated by measuring the concentration of hydroxylamine in water samples. references [1] s. liu, h. vereecken, n. brüggemann, geoderma 232 (2014) 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https://doi.org/10.10‌16/j.matchemphys.2020.122640 https://doi.org/10.10‌16/j.matchemphys.2020.122640 https://doi.org/10.26655/jmchemsci.2019.3.3 https://doi.org/10.1016/j.snb.2016.08.181 https://doi.org/10.1016/j.jelechem.2014.12.039 https://doi.org/10.1016/j.nanoen.2015.07.014 https://doi.org/10.1016/j.nanoen.2015.07.014 https://doi.org/10.1016/‌j.jelechem.2020.‌114352 https://doi.org/10.1080/10610278.2012.758366 https://doi.org/10.1080/10601325.2013.755859 https://doi.org/10.1155/2015/416012 https://creativecommons.org/licenses/by/4.0/) {construction of new sensors with copper and cobalt com¬ple¬xes anchored on organofunctionalized silica and their use for electrocatalytic oxidation of reagents of biological interest:} http://dx.doi.org/10.5599/jese.1100 317 j. electrochem. sci. eng. 12(2) (2022) 317-329; http://dx.doi.org/10.5599/jese.1100 open access : : issn 1847-9286 www.jese-online.org original scientific paper construction of new sensors with copper and cobalt complexes anchored on organofunctionalized silica and their use for electrocatalytic oxidation of reagents of biological interest marco a. g. benetti, denise i. soares, giovanna g. alves, eduardo d. a. pinto and angélica m. lazarin  departamento de química, universidade estadual de maringá, av. colombo, 5790, 87020-900 maringá, pr, brazil corresponding author: amlazarin2@uem.br; tel.: +55 4432633241; fax: +55 44 30311011 eceived: september 1, 2021; accepted: february 14, 2022; published: february 24, 2022 abstract this work describes the development and application of chemically modified carbon paste electrode (cmcpe) with metal ion complexes of transition metals co(ii) and cu(ii) anchored on functionalized silica (sf). the prepared cmcpe was used for the determination of dopamine (da) in the presence of pyridoxine (vb6) in commercially available pharmaceutical formulation, without any treatment of samples. the cmcpes were built by incorporating graphite and functionalized silica and complexes of metal ions (cu and co) anchored by nitrogen groups on the silica surface. the electrochemical analysis was performed by cyclic voltammetry, while a drug analyzed in the present study was revivan ampoules (da determination). keywords chemically modified carbon electrode; functionalized silica; metal complexes; dopamine, vitamin b6, cyclic voltammetry introduction in many studies detected in the literature associated with the preparing of chemically modified silica, particular regard has been put to modifying materials containing transition metal ions that create supported complexes on the silica surface. such materials are used to construct chemically modified electrodes (cme), which are usually linked with electroanalysis and eletrocatalysis [1-3]. due to the mechanical rigidity, chemical inertia and large surface area, chemically modified silica gel has also been used as a stationary phase for high-performance liquid chromatography [4], for immobilizing enzymes [5], catalysts [6] and as an adsorbent of metal ions in aqueous solution and not aqueous [7,8]. http://dx.doi.org/10.5599/jese.1100 http://dx.doi.org/10.5599/jese.1100 http://www.jese-online.org/ mailto:amlazarin2@uem.br j. electrochem. sci. eng. 12(2) (2022) 317-329 new sensors with copper and cobalt complexes 318 the present investigation describes the development and application of chemically modified carbon paste electrodes (cmcpe) with metal ion complexes of transition metals co(ii) and cu(ii) anchored on the functionalized silica, for the determination of dopamine (da) in the presence of pyridoxine (vb6) in commercially available pharmaceutical formulation, without any sample treatment. the cyclic voltammetry technique was used, and the drug analyzed in the present study is revivan drug ampoules (da determination). it must be added that vb6 plays an important role in neurotransmitters synthesis, such as da synthesis, and also participates in amino acid degradation reactions [9]. da is an important neurotransmitter in the central nervous system (cns) of mammals, having a concentration in extracellular fluid lower than that of ascorbic acid (aa) [10]. the voltammetric response with vitreous carbon electrode for dopamine, therefore, suffers the interference of ascorbic acid, which coexists in vivo in extracellular fluid of basic central nervous system, as anions in high concentration, and it has an oxidation potential close to dopamine (lower concentration) [11]. experimental materials all solutions were prepared using millipore milli-q water. all chemicals were of analytical reagent grade and used without further purification. silica gel (merck), 3-(2-aminoethyl)aminopropyl]trimethoxysilane (synth), ethanol (synth), m-aminobenzoic acid (sigma), hydrochloric acid (merck), sodium hydroxide (synth), ethylenediaminetetraacetic acid (synth), copper chloride (merck), and cobalt chloride (merck) were used for all preparations. graphite powder (1-2 mm particle size, aldrich) and mineral oil (aldrich) of high purity were used for the preparation of carbon paste. the supporting electrolyte used for all experiments was 0.10 mol dm-3 phosphate buffer solution (pbs). pyridoxine (merck) and dopamine (sigma) were used as analytes. aets modified agent coated on the silica gel surface silica gel was degassed at 423 k under vacuum (10-3 atm) for 8 hours. about 40 g of this material was mixed with 32.0 cm3 of [3-(2-aminoethyl)aminopropyl] trimethoxysilane dissolved in 350 cm3 of dry toluene. the mixture was kept under reflux, mechanical stirring and inert nitrogen atmosphere for 12 hours. the resulting silica was firstly washed and filtered with ethanol and ethyl ether, and then in a sohxlet extractor for 24 hours with ethanol, and finally dried at 353 k under vacuum (333 k). the prepared solid was named sf-aets. modification of sf-aets silica with maba in 10 g of sf-aets silica suspended in 50.0 cm3 of bidistilled ethanol, the reagent m-aminobenzoic acid (maba) was added. the mixture was refluxed under mechanical stirring for one hour, then filtered and washed with ethanol and anhydrous ethyl ether. the silica obtained was named sfaets/maba. characterization the nitrogen amount in sf-aets and sf-aets/maba samples was determined by nitrogen elemental analysis on perkin-elmer analyzer 2400 series h chns/o device. infrared spectra of samples were performed on perkin-elmer ftir spectrophotometer, model 1600, by using pressed kbr pellets in the 4000-400 cm-1 range with 4 cm-1 of resolution. thermogravimetric curves were recorded using a dupont model 1090 b apparatus, coupled to a model 951 thermobalance, by heating from room m. a. g. benetti et al. j. electrochem. sci. eng. 12(2) (2022) 317-329 http://dx.doi.org/10.5599/jese.1100 319 temperature to 1273 k at a heating rate of 0.16 k s-1 in argon flow of 1.67 cm3 s-1. the samples varied in weight from 15.0 to 30.0 mg. adsorption isotherms adsorption isotherms for cucl2 and cocl2 in ethanol solutions were determined with the use of the batch-wise method. a series of samples containing 100 mg of sf-aets/maba was shaken for 3 hours as previously established, in an orbital bath with variable concentrations of each metal halide at a constant temperature of 298  1 k. the concentration of metal ions in solution in equilibrium with solid phase was determined by direct titration with edta (0.010 mol dm-3) using murexide as an indicator. the amount of cations adsorbed, nf, was determined by applying the equation: nf = (na ns)/m, where m is the adsorbent mass and na and ns are initial and equilibrium amounts of metal in the solution phase in moles, respectively, nf is a measure of coverage degree in actual experimental conditions, defined according to: − = = m 2(sf-aets/maba) mcla s f m cn n n m m (1) in eq. (1), the summation extends over all species on the surface. the maximum number of moles of the adsorbed complex, nfmax, is equal to the maximum value of nf when the concentration of co(ii) and cu(ii) tends to infinity. adsorption of co(ii) and cu(ii) held in solution depends on the selectivity coefficients of complexes constituted with the ligand immobilized on the surface. the formation of equilibrium of the complex co(ii) and cu(ii) with sf-aets/maba was examined by a filippov model [12,13]. the reaction of a metal (m) complex formation at the surface can be expressed by the following equation: mxz + nr  mrnxz (2) where z is the charge of the metallic ion, x is an anion with unit charge and r is linking group, electrically neutral, and fixed on the support surface. in the equilibrium, the following relation is applied: n / (1 n) = nc (3) where the fraction of sf-aets/maba bonded to metal is n = ñ nf /n0 n0 is the quantity of maba immobilized on sf-aets (mol g-1), ñ is the number of ligands bonded to the metal, and n is the stability constant. substitution for n in eq. (3) results in the ending expression [14]: = + f n 1 ñ ñ n ny c n (4) considering that metallic ions cu(ii) and co(ii), when adsorbed on the surface (with a homogeneous covering ligands), form immobilized complexes with different ratios between them and are anchored at sf-aets/maba, the total concentration of immobilized ligand on the surface is equal to the sum of the concentrations of all species; n is the degree of functionalization, c is the concentration of the surface complex; ñ is the average number of supported ligands bound to a metallic ion, defined as: = z 2 z m z mrx mr x mr x f +2 + ... + mc c c ñ n (5) where m adsorbent mass and r is linking group. http://dx.doi.org/10.5599/jese.1100 j. electrochem. sci. eng. 12(2) (2022) 317-329 new sensors with copper and cobalt complexes 320 eq. (4) can be re-written as  = + f 1 n ñ ñ n n c n (6) where n is the selectivity coefficient, equal to =  n m 1m y . electrode preparation and electrochemical measurements modified cmcpe was prepared by mixing 30 mg of silica, 30 mg of graphite and a drop of mineral oil (2.010-2 cm3). the paste was placed into a cavity on the area of a platinum disk, fused to the end of a glass tube with 1 mm of inner diameter. this proportion was used because of a great response reached with a preliminary test after a detailed study of the paste composition. electrochemical measurements were carried out using cmcpe as the working electrode, ag/agcl (sat. kcl) as the reference electrode and a platinum wire as the counter electrode. electrochemical properties were explored by means of cyclic voltammetry with a par 273a (eg&g) potentiostatgalvanostat. all experiments were performed in 0.10 mol dm-3 pbs, under a pure argon atmosphere, and the ph value of electrolyte solutions was adjusted to 7.0 by adding hcl or naoh solutions. different supporting electrolytes were tested as well. results and discussion surface characterizations silica gel functionalized (sf) with [3-(2-aminoethyl)aminopropyl]trimethoxysilane group (aets) was obtained according to the following reaction: si(oh)3 + (ch3o)3si(ch2)3nh(ch2)2nh2 → sio3si(ch2)3nh(ch2)2nh2 + 3ch3oh (7) sf aets sf-aets for sf-aets silica, the amounts of supported groups were set by nitrogen analysis using the method described in the experimental part. the functionalization efficiency was determined for (silane) groups as 0.91±0.01 mmol g-1 in sf-aets based on the nitrogen content. further modification of sf-aets with m-aminobenzoic acid into sf-aets/maba was reached by the following reaction: sio3si(ch2)3nh(ch2)2nh2 + cooh nh2 sio3si(ch2)3nh(ch2)2nhoc nh2 (8) sf-aets maba sf-aets/maba subsequent coordination of metal ions at the surface of sf-aets/maba silica was obtained through reactions with metal chlorides, as shown below: sf/aets/maba + m(ii) → sf/aets/maba/m(ii) (9) figure 1 presents infrared spectra of sf, sf-aets and sf-aets/maba silicas. the assignment of vibrational frequencies to pure silica (sf) and functionalized silica (sf-aets) are listed in table 1. the appearance of new bands attributed to the stretch ch2, nh2, ch2n and sich2 pointed to the effective functionalization of silica gel with aets silylating agent on its surface. infrared spectra of sf-aets/maba/co(ii) are shown in figure 2. m. a. g. benetti et al. j. electrochem. sci. eng. 12(2) (2022) 317-329 http://dx.doi.org/10.5599/jese.1100 321 wavenumber, cm-1 figure 1. infrared spectra of (a) sf, (b) sf-aets and (c) sf-aets/maba table 1. vibrational frequencies assigned for sf and sf-aets silica sf / cm-1 sf-aets / cm-1 assignment 3400 (vs) 3400 (vs) oh -2940 (w) ch2 1992 (w) 1995 (w) * 1871 (w) 1870 (w) * 1637 (s) 1634 (s) oh --oh + δnh2 -1575 (m) nh2 --c=c ring -1480 (m) ch2n -1418 (m) sich2 * related to silica skeleton , vs = very strong band, m = mid band, s = short band, w = weak band figure 2. the infrared spectrum of sf-aets/maba/co(ii) the assignments for sf-aets/maba and sf-aets/maba/co(ii) silicas are shown in table 2. the comparison of infrared spectra in figures 1 and 2 shows that in all spectra, a strong and wide band 4000 3000 2000 1000 t ra n sm it a n c e , a . u . wavenumber, cm -1 t ra n sm it ta n ce , a .u . http://dx.doi.org/10.5599/jese.1100 j. electrochem. sci. eng. 12(2) (2022) 317-329 new sensors with copper and cobalt complexes 322 appears in the region of 3400 cm-1, attributed to oh stretches of water and silane groups, a band in the region of 1634 cm-1, which is characteristic of angular deformation of water [15], besides two bands close to 1900 cm-1 attributed to combinations of silica skeleton (tables 1 and 2). infrared spectra of sf-aets/maba silica with co(ii) ion anchored on its surface presented in figure 2 showed a similar behavior (table 2). the infrared spectra of the sf-aets/maba silica showed characteristic bands of amide bond (1626 + 1605 cm-1) (hoh + (c=o, band i of amide) and (1549 cm-1) (nh + cn, band ii of amide), of the band in (1386 cm-1) (cn of and so on primary aromatic amine), showing that the successive change in surface sf-aets silica actually occurred (table 2). table 2. vibrational frequencies assigned for sf-aets/maba and sf-aets/maba/co(ii). sf-aets/maba /cm-1 sf-aets/maba/co /cm-1 assignments 3400 (vs) 3400 (vs) oh 3343 (vw) 3253 (vw) nh2 3226 (vw) nh amide 3067 (vw) ch aromatic 2940 + 2893 (vw) 2940 (vw) νch2 2000 (vw) 2000 (vw) (*) 1869 (w) 1871 (w) (*) 1626 + 1605 (s) 1631 (s) hoh + c=o band i of the amide 1549 (vs) 1575(m)+1492 (vw) nh + cn band ii of the amide 1452 (w) 1458 (m) ch2-n 1407 (s) sich2 1386 (vs) 1387 (s) cn primary aromatic amine 1304 (vw) nh + ch band iii of secondary amide vs = very strong band, vw = very weak, m = mid band, s = short band, w = weak band thermogravimetric curves of pure silica (sf) and functionalized silica sf-aets were obtained as described in the experimental part and shown in figure 3. the sf and sf-aets silica showed mass loss of 9.7 and 12.9 %, respectively. in examining these compounds, the following processes were detected: i) water molecules are lost from room temperature at near 500 k, and ii) the functional groups are lost from 453 up to 813 k. temperature, k figure 3. thermogravimetric curves of (a) sf and (b) sf-aets silica the thermogravimetric curve of sf-aets/maba silica is shown in figure 4 and presented a percentage of mass loss of 15.7 % and the greatest mass loss occurring between 453 and 813 k. 0.0 5.0 10.0 15.0 w e ig h t lo ss , % m. a. g. benetti et al. j. electrochem. sci. eng. 12(2) (2022) 317-329 http://dx.doi.org/10.5599/jese.1100 323 temperature, oc figure 4. thermogravimetric curves (integral and differential) of sf-aets/maba silica adsorption of copper and cobalt the corresponding isotherms of metal ion adsorption from ethanol solutions of selected cations cu(ii) and co(ii), at sf-aets/maba were also investigated. early investigation showed that the original silica matrix does not adsorb these cations. the solid adsorption capacity of metal halide on sf-aets/maba depends on the nature of the complex formed on the surface and the affinity of any particular ligand attached to the metal. maximum adsorption capacity, nfmax, for cucl2 and cocl2, was 0.17 and 0.64 mmol g-1, respectively, indicating that cobalt binds more effectively than cooper to available basic centers. the average stability constant () and the average number of ligands bonded and coordinated to the metallic ion (ñ) were determined from the plot of 1/nf and 1/c. the average stability values of cucl2 and cocl2 in sf-aets/maba were 391 and 191 dm3 mol-1, respectively, and the average number of ligands (ñ) for sf-aets/maba was four to cooper and one to cobalt metallic ions. the values of calculated constants indicate the formation of stable complexes. electrochemical studies silica sf-aets/maba/cu(ii) voltammetry experiments using cmcpe modified with different silica materials were carried out. for the electrode modified with only sf-aets/maba, no current peaks were observed, while for cmcpe prepared with sf-aets/maba having complexed cooper (ii) attached, some peaks were obtained. they can be observed in figure 5, where stabilization of cmcpe/sf-aets/ maba/cu(ii) in 0.10 mol dm-3 pbs, ph 7.0 was followed by cyclic voltammograms recorded consecutively at 20 mv s-1 in the potential range of -1.0 to 1.0 v (vs. ag/agcl) at 298 k. in this potential range, a pair of peaks is observed in figure 5, with almost reversible characteristics, e1/2 = -0.11 v (where e1/2 = (epa + epc)/2 and epa and epc are anodic and cathodic peak potentials, respectively). these almost reversible peaks were assigned to the cu(ii)/cu(i) redox pair, while the irreversible anodic peak at epa= 0.90 v was assigned to the cu(iii)/cu(ii) redox pair. cyclic voltammograms of stabilized cmcpe/sf-aets/maba/cu(ii) electrode, recorded at different potential scan rates, are shown in figure 6. it is obvious that the value of the potential difference between anodic and cathodic peaks, ep (ep = epa epc), is increased at higher scan rates. this result reflects the kinetics of electron transfer on the electrode surface, which is not fast enough, as a consequence of having a matrix with considerable resistance. the graphs of anodic peak current as a function of the square root of scan rate and anodic peak current as a function of scanning rate (not shown here) presented linear behavior, which suggests a diffusion-controlled process [16,17]. 0 200 400 600 800 1000 100 96 92 88 84 80 w e ig t lo ss , % temperature, k -0.0012 -0.0010 -0.0008 -0.0006 -0.0004 -0.0002 0.0000 0.0002 d e riv a tiv e m a ss,/ m g o c -1 d e riva tiv e m a ss, m g oc -1 w e ig h t lo ss , % 0.0 4.0 8.0 12.0 16.0 http://dx.doi.org/10.5599/jese.1100 j. electrochem. sci. eng. 12(2) (2022) 317-329 new sensors with copper and cobalt complexes 324 since it was shown earlier that electroactive species strongly adhere to the matrix, the mechanism may be explained by the transport of the ion of supporting electrolyte from the electrode surface for charge compensation [18,19]. figure 5. cyclic voltammograms of cmcpe/sf-aets/maba/cu(ii) electrode stabilization in 0.10 mol dm-3 pbs, ph 7.0, at 20 mvs-1 and 298 k figure 6. cyclic voltammograms of cmcpe/sf-aets/maba/cu(ii) electrode in 0.10 mol dm-3 pbs ph 7.0, at 298 k, as a function of the scan rate (5, 10, 20, 30, 50 and 60 mvs-1) the nature of supporting electrolyte that was changed by varying the cation and anion (phosphate buffer, kcl, nh4cl, nano3 and nh4no3) did not produce any significant influence on e1/2. such results suggested that these supporting electrolytes are not interacting with the matrix surface and produce no significant change of the midpoint potential [20]. with cmcpe/sf-aets/maba/cu(ii) electrode, a preliminary study of oxidation of vitamin b6 was made by voltammetric measurements. figure 7 shows electrochemical responses of cmcpe/sfaets/maba/cu(ii) without (curve a) and with 9.810-4 mol dm-3 of vitamin b6 (curve b). it is obvious that the anodic oxidation potential of vb6 is around epa= 0.95 v, as shown in figure 7(b). figure 8 shows the electrochemical behavior of cmcpe/sf-aets/maba/ cu(ii) electrode in pbs containing 0.30 cm3 of revivan drug in the presence of vitamin b6 (cvb6 = 9.810-4 mol dm-3), and after successive additions of dopamine, in the concentration range of 9.610-5 to 2.510-4 mol dm-3. m. a. g. benetti et al. j. electrochem. sci. eng. 12(2) (2022) 317-329 http://dx.doi.org/10.5599/jese.1100 325 figure 7. electrochemical behavior of cmcpe/sf-aets/maba/cu(ii) electrode in 0.10 mol dm-3 pbs, ph 7.0, at 20 mv s-1and 298 k: (a) without, and (b) with 9.810-4 mol dm-3 vitamin b6 figure 8. dopamine determination in revivan by the standard multiple addition method on the cmcpe/sf-aets/maba/cu(ii) electrode in 0.10 mol dm-3 pbs, ph 7.0, at 20 mv s-1 and 298 k: (a) 0.30 cm3 of revivan in the presence of 9.8x10-4 mol dm-3 vb6 and after successive additions of dopamine (9.6x10-5 to 2.5x10-4 mol dm-3) the oxidation potential of dopamine is at epa= 0.39 v, and anodic peak current graph versus dopamine concentration (cda = 9.610-5 to 2.510-4 mol dm-3) allowed the quantification of dopamine present in the drug. a linear correlation was observed between ip and da concentration, with a detection limit of 9.410-5 mol dm-3. the determined value in mol dm-3 was 5.1×10-2, and after, this value was recalculated to g cm-3. the value was found equal to (4.9  0.3)×10-3 g cm-3 is in accordance with that supplied by the manufacturer. the reproducibility of the electrode response to da was also investigated by repetitive measurements. the results of 200 successive measurements showed a relative standard deviation of 1.03 %. thus, the modified cmcpe/sf-aets/maba/cu(ii) electrode was found to be a very stable and highly reproducible electrode, appropriate for da determination. silica sf-aets/maba/co(ii) first, the stabilization of cmcpe/sf-aets/maba/co(ii) electrode was followed by recording consecutive cvs in 0.10 mol dm-3 pbs, ph 7.0 at 298 k. cvs were carried out at 20 mv s-1 going from 1.0 to -0.70 v (vs. ag/agcl) (cathodic scanning), and presented in figure 9. an almost reversible peak with e1/2 = 0.84 v can be assigned to the co(iii)/co(ii) redox pair. after the stability of the cmcpe/sf-aets/maba/co(ii) electrode was attained, it was used the study of oxidation of dopamine in the presence of vitamin b6. up to now, the electrochemical oxidation of vitamin b6 has been mostly studied on carbon paste electrodes. http://dx.doi.org/10.5599/jese.1100 j. electrochem. sci. eng. 12(2) (2022) 317-329 new sensors with copper and cobalt complexes 326 figure 9. cyclic voltammograms of cmcpe/sf-aets/maba/co(ii) electrode stabilization in 0.10 mol dm-3 pbs , ph 7.0, at 20 mvs-1 and 298 k the reaction was supposed to involve the oxidation of pyridoxine (pyx) into pyridoxal derivative (pyo) [21]. the voltammetric response obtained with the cmcpe/sf-aets/maba/co(ii) after the successive additions of vitamin b6 in the concentration range 9.910-5 to 3.910-4 mol dm-3, however, did not present significant current variations (figure 10). for this reason, cmcpe/sfaets/maba/co(ii) cannot be used for the electrochemical determination of vitamin b6 in the drug. e / v figure 10. electrochemical behavior of the cmcpe/sf-aets/maba/co(ii)electrode in 0.10 mol dm3pbs, ph 7.0, at 20 mv s-1 and 298 k without and with added vitamin b6 in the concentration range of 9.9x10-5 to 3.9x10-4mol dm-3 figure 11 shows the electrochemical behavior of cmcpe/sf-aets/maba/co(ii) electrode in 0.10 mol dm-3 pbs, ph 7.0, after dopamine additions of 5.010-5 to 2.010-4 mol dm-3. figure 11. electrochemical behavior of the cmcpe/sf-aets/maba/co(ii)electrode in 0.10 mol dm-3 pbs, ph 7.0, at 20 mv s-1 and 298 k without and with added da in the concentration range of 5.010-5 to 2.010-4 mol dm-3 i /  a m. a. g. benetti et al. j. electrochem. sci. eng. 12(2) (2022) 317-329 http://dx.doi.org/10.5599/jese.1100 327 electrochemical behavior of cmcpe/sf-aets/maba/co(ii) electrode as a function of scanning rate in the presence of 2.010-4 mol dm-3 dopamine is presented in figure 12. figure 12. electrochemical behavior of cmcpe/sf-aets/maba/co(ii) electrode in 0.10 mol dm-3 pbs, ph 7.0, 298 k and 2.0×10-4 mol dm-3 da at different scan rates (2, 5, 10, 20, 30, 40, 50, 60, 70 mv s-1) the graph of anodic peak current for cmcpe/sf-aets/maba/co(ii) at the oxidation potential of dopamine (epa= 0.250 v) against 1/2 presented linear behavior with two slopes, one at high and the other at low scan rates [22]. the electroanalytical determination of dopamine in revivan ampoules was executed with this cmcpe/sf-aets/maba/co(ii) electrode by the method of multiple pattern addition. dopamine concentration was varied from 4.910-5 to 1.9310-4 mol dm-3 and the presence of b6 vitamin 4.510-5 mol dm-3 did not show significant variation (figure 13). a linear correlation is observed between peak current and dopamine concentration with a detection limit of 4.910-5 mol dm-3. the graph of anodic peak current versus dopamine concentration allowed the quantification of dopamine present in the drug. the determined value in mol dm-3 was 8.9×10-2, and after, this value was recalculated to g cm-3. the value was found equal to (4.8  0.3)×10-3 g cm-3, which coincides with that provided by the manufacturer (5 mg cm-3). figure 13. determination of dopamine in revivan by the multiple standard addition method at cmcpe/sf-aets /maba/co(ii) electrode in 0.1 mol dm-3 pbs, ph 7 at 20 mv s-1 and 298 k. (a) cyclic voltammogram in the presence of b6 vitamin 4.510-5 mol dm-3 (b) after addition of 0.50 cm3 of revivan and successive additions of dopamine in the concentration range of 4.910-5 to 1.9310-4 mol dm-3 the comparison of cmcpe/sf-aets/maba/co(ii) electrode response with that of glassy carbon electrode in the solution containing a mixture of vitamin b6 and dopamine is presented in figure 14. compared to glassy carbon, for the cmcpe modified with silica, there is no appearance of the http://dx.doi.org/10.5599/jese.1100 j. electrochem. sci. eng. 12(2) (2022) 317-329 new sensors with copper and cobalt complexes 328 oxidation peak of vitamin b6 in the studied potential range. also, the current catalysis for dopamine oxidation is clearly presented for cmcpe/sf-aets/maba/co(ii) electrode, showing much greater sensitivity than is observed for the glassy carbon electrode. figure 14. comparison of (a and b) cmcpe/sf-aets/maba/co(ii) electrode and (c) glassy carbon electrode responses in 0.10 mol dm-3 pbs, ph 7.0, and 4.510-5 mol dm-3 vb6 at 20 mv s-1, and 298 k after: (a) addition of 0.50 cm3 of revivan; (b and c) addition of 1.93x10-4 mol dm-3 of dopamine the performance of cmcpe/sf-aets/maba/co(ii) electrode over five months was followed by measurements of oxidation peak currents for dopamine in pbs on each consecutive day. this serves as an indicator of the stability of modified electrodes toward dopamine. the electrodes were used every day. the experimental results indicated that current responses showed a relative standard deviation of 2.0 %, suggesting that modified electrodes possess fine stability. the reproducibility of electrodes was investigated. repetitive measurements were performed in dopamine. the results of 200 successive measurements show a relative standard deviation of 1.03 %. thus, the modified electrodes are found very stable, showing great reproducibility. conclusions the enrichment of interfacial layer of silica gel by chemisorption, with interactions of covalent nature on its surface through the use of silylating agents with nucleophilic sites, favored the increased capacity of organofunctionalized silica to sequester metals, with complexes anchored on its surface. the analytical results obtained from the electrode containing cu(ii) and co(ii) complexes were very promising. the benefits to employ the suggested electrodes have been stated by our analytical processes, and its progress is easy. the electrodes did not present significant variation in response after five months of use, thus showing good chemical stability. these characteristics make this material very attractive for use as a sensor for dopamine determinations. acknowledgment: the authors are indebted josué a. da silva for manuscript revision. references [1] x. li, b. wang, y. cao, s. zhao, h. wang, x. feng, j. zhou, x. ma, chemical engineering 7(5) (2019) 4548-4563. https://doi.org/10.1021/acssuschemeng.8b05751 [2] s. k. parida, s. dash, s. patel, b. k. mishra, advances in colloid and interface science 121(1-3) (2006) 77-110. https://doi.org/10.1016/j.cis.2006.05.028 [3] k. kalcher, electroanalysis 2(6) (1990) 419-433. https://doi.org/10.1002/elan.1140020603 [4] a. m. faria, c. h. collins, i. c. s. f. jardim, journal of the brazilian chemical society 20(8) (2009) 1385-1398. https://doi.org/10.1590/s0103-50532009000800002 [5] h. maleki, a. t. portugal, l. durães, journal of non-crystalline solids 385 (2014) 55-74. http://dx.doi.org/10.1016/j.jnoncrysol.2013.10.017 https://doi.org/10.1021/acssuschemeng.8b05751 https://doi.org/10.1016/j.cis.2006.05.028 https://doi.org/10.1002/elan.1140020603 https://doi.org/10.1590/s0103-50532009000800002 http://dx.doi.org/10.1016/j.jnoncrysol.2013.10.017 m. a. g. benetti et al. j. electrochem. sci. eng. 12(2) (2022) 317-329 http://dx.doi.org/10.5599/jese.1100 329 [6] r. f. silva, w. l. vasconcelos, materials research 2(3) (1999) 197-200. https://doi.org/10.1590/s1516-14391999000300014 [7] a. mehdinia, s. shegeftib f. shemirani, journal of the brazilian chemical society 26(11) (2015) 249-2257. https://doi.org/10.5935/0103-5053.20150211 [8] m. g. vieira, g. souza, g. h. b. aristides, l. v. lopes, a. m. lazarin, international journal of sensors and sensor networks 5(2) (2017) 27-33. https://doi.org/10.11648/j.ijssn.20170502.12 [9] [9] m. parra, s. stahl, h. hellmann, cells 7(7) (2018) 84. https://doi.org/10.3390/cells7070084 [10] a. mobed, m. hasanzadeh, a. ahmadalipour, a. fakhari, analytical methods 12(4) (2020) 557-575. https://doi.org/10.1039/c9ay02390a [11] c. a. martínez-huitle, m. cerro-lopez, m. a. quiroz, materials research 12(4) (2009) 375384. https://doi.org/10.1590/s1516-14392009000400002 [12] a. p. filippov, theoretical and experimental chemistry 19 (1984) 427-433. https://doi.org/10.1007/bf00518093 [13] a. m. lazarin, b. b. cazula, materials chemistry and physics 186(15) (2017) 470-477. https://doi.org/10.1016/j.matchemphys.2016.11.021 [14] r. golbedaghi, f. khajavi, bulletin of the chemical society of ethiopia 28(1) (2014) 1-8. https://doi.org/10.4314/bcse.v28i1.1 [15] a. madejová, p. komadel, clays and clay minerals 49 (2001) 410-432. https://doi.org/10.1346/ccmn.2001.0490508 [16] a. j. bard, l. r. faulkner, electrochemical methods, fundamentals and applications, wiley & sons, new york, 1980, 218. 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2748-2753. http://dx.doi.org/10.1016/j.electacta.2010.12.060 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1100 https://doi.org/10.1590/s1516-14391999000300014 https://doi.org/10.5935/0103-5053.20150211 https://doi.org/10.11648/j.ijssn.20170502.12 https://doi.org/10.3390/cells7070084 https://doi.org/10.3390/cells7070084 https://doi.org/10.1039/c9ay02390a https://doi.org/10.1590/s1516-14392009000400002 https://doi.org/10.1007/bf00518093 https://doi.org/10.1016/j.matchemphys.2016.11.021 https://doi.org/10.4314/bcse.v28i1.1 https://doi.org/10.1346/ccmn.2001.0490508 https://doi.org/10.1346/ccmn.2001.0490508 https://doi.org/10.1016/j.mseb.2014.06.010 https://doi.org/10.1039/c3ay40769a https://doi.org/10.1021/acs.analchem.6b01800 https://doi.org/10.1590/s0103-50532007000800003 http://dx.doi.org/10.6180/jase.2002.5.4.03 http://dx.doi.org/10.1016/j.electacta.2010.12.060 https://creativecommons.org/licenses/by/4.0/) new potentiometric electrode based on ion pair complex for determination of tropicamide in pure and pharmaceutical formulations doi:10.5599/jese.321 277 j. electrochem. sci. eng. 6(4) (2016) 277-286; doi: 10.5599/jese.321 open access : : issn 1847-9286 www.jese-online.org original scientific paper new potentiometric electrode based on ion pair complex for determination of tropicamide in pure and pharmaceutical formulations mouhammed khateeb, basheer elias*, hazar alksair* department of basic sciences, faculty of mechanical and electrical engineering, university of albaath, homs, syria *department of chemistry, faculty of sciences, university of albaath homs, syria corresponding author: drkhateeb2010@gmail.com, mkhateeb74@hotmail.com received: june 20, 2016; revised: october 2, 2016; accepted: october 10, 2016 abstract construction and general performance of a novel modified carbon paste electrode (mcpe) for determination of tropicamide (tpc) in pure form and pharmaceutical formulations have been examined. tropicamide-tetraphenylborate (tpc–tpb) ion pair has been prepared and used as electroactive material. the best mcpe electrode was composed of 7 % ion-pair, 46.5 % dioctylphthalat and 46.5 % graphite powder. the electrode shows stable potentiometric response for tpc in the concentration range 0.3–221.0 µm at 25 °c and ph range of 2.0–8.0. the electrode exhibits near nernstian slope of 59.71±0.30 mv/decade and lower limit of detection of 0.09 µm with fast response time (less than 15 s). the selectivity of the electrode (tpc–tpb) was investigated with respect to some organic and inorganic cations. the mcpe was designed to have better mechanical resistance. the proposed method was successfully applied for determination of tpc in eye drop formulation. keywords tropicamide; sodium tetraphenylborate; ion pair complex; carbon paste electrode; pharmaceutical formulations introduction tropicamide (tpc), (r,s)-n-ethyl-3-hydroxy-2-phenyl-n-(pyrid-4-yl-methyl) propionamide (fig. 1), is an antimuscarinic agent with short duration of mydriatic and cycloplegic effect. tpc is used for refractive examinations and preservative for optimal eye tolerance and activity [1]. the action of tpc is more rapid in onset and wears off more rapidly than most other mydriatics and its use is generally much the same as those described for other mydriatics [2,3]. http://www.jese-online.org/ mailto:drkhateeb2010@gmail.com mailto:mkhateeb74@hotmail.com j. electrochem. sci. eng. 6(4) (2016) 277-286 determination of tropicamide 278 several methods have been reported for determination of tpc including spectrophotome try [4–8], high performance liquid chromatography (hplc) [9–12] and gas chromatography–mass spectrometry (gc–ms) [13]. in the united states pharmacopoeia (usp) [14] and british pharmacopoeia (bp) [15] the non-aqueous titration method for determination of tpc in raw materials and the extractive spectrophotometric method for pharmaceutical preparations have been described, respectively. in the literature survey, no potentiometric method has been reported yet for determination of tpc. therefore, the objective of the present study was to develop an accurate and validated potentiometric method for determination of tpc in raw material and pharmaceutical dosage forms, that can also be used in quality control laboratories. since their innovation by adams [16], carbon paste electrodes (cpes) have widely been used in many fields such as voltammetry, amperometry and potentiometry [17]. in comparison with ionselective electrodes based on polymeric membranes, chemically modified carbon paste electrodes (cmcpes) possess some advantages. in addition to lower limits of detection with respect to electrodes with an internal reference solution, cmcpes are ease for preparation and regeneration, have stable response and very low ohmic resistance [18]. n n oh o figure 1. chemical structure of tropicamide. experimental reagents and chemicals all chemicals used were of analytical grade. double distilled water was used throughout all experiments. working reference standard of tropicamide (tpc c17h20n2o2, 284.35 g/mole) was obtained from kunshan chemical and pharmaceutical co., ltd (india). its purity was found to be 99.5 % according to the compendial testing method. pharmaceutical preparation mydriamed eye drop was provided by medico company for pharmaceutical industries (homs, syria). sodium tetraphenylborate (na–tpb) was from merck, while dibutyl phthalate (dbp), dioctyl phthalate (dop) and paraffin oil (p.oil) were from bdh. graphite powder was obtained from aldrich. apparatus all potentiometric measurements were made at 25±1 °c using a sanwal potentiometer (ph meter, ion meter model of dt9201a) with combined ph electrode (model 250a, orion, usa) for ph measurements. saturated calomel electrode (sce) was used as the reference electrode. the electrochemical cell is represented as follows: hg, hg2cl2(s), kcl (sat.)║sample solution │carbon paste electrode│cu standard solution standard stock solution of 1.0×10-2 m tpc was prepared daily by dissolving the appropriate amount of drug in double distilled water. standard working solutions 1.0×10-3 and 1.0×10-4 m were freshly prepared by suitable dilutions of the stock solution with double distilled water. m. khateeb et al. j. electrochem. sci. eng. 6(4) (2016) 277-2860 doi:10.5599/jese.321 279 calibration graph different amounts of 1.0×10-4 or 1.0×10-3 m tpc were added to 50ml of double distilled water to achieve the concentration range from 0.3 to 221.0 µm. the measured potential was recorded. the calibration graph was constructed by plotting the potential value versus pctpc (–logarithm concentration of tpc). the amount of drug was obtained from the regression equation. analysis of eye drop 0.1 ml of the commercial eye drop was transferred into a 100 ml volumetric flask and diluted with double distilled water. the general procedure was then followed in the concentration range mentioned above. preparation of tropicamide-tetraphenylborate (tpc–tpb) ion-pair sensing element used in the carbon paste electrode is the ion-pair compound made by interaction of tpc and na–tpb. it was prepared by mixing 20 ml of 0.01 m acidic solution of tpc with 20 ml aqueous solution of 0.01 m na–tpb. the resulting solution was then filtered, the precipitate washed with double distilled water and dried at room temperature [19]. modified carbon paste electrodes (mcpes) mcpes were prepared by thoroughly mixing various amounts of ion pairing agents with carbon powder and plasticizer in the mortar, until homogenization of this mixture was achieved. the resulting paste was then packed firmly into the hole of the electrode body. electrical contact to the carbon paste was made with a copper wire. fresh surface was obtained by applying manual pressure to the carbon paste and polished on a filter paper to a shiny surface (fig.2). figure 2. schematic illustration of electrochemical cell with mcpe as an indicator electrode (ie) and sce as reference electrode (re). selectivity of the electrode potentiometric selectivity coefficient was evaluated using the matched potential method (mpm) [20]. according to the mpm, the activity of the analyte was increased from a = 9.90×10-5 m (reference solution) to’a =1.01×10-4 m and the corresponding change in potential (δe) is measured. then, 1.00×10-1 m solution of an interfering ion was added to a new 9.90×10-5 m analyte reference solution, until the same δe is recorded, determining thus the concentration of the added amount, b. the selectivity coefficient 𝐾a,b mpm for each interfering ion was calculated using the following equation: j. electrochem. sci. eng. 6(4) (2016) 277-286 determination of tropicamide 280 mpm a b a , a b ' k      results and discussion calibration graph and effect of ion pair (ip) percentage on electrode potential it is known that sensitivity and linearity of an electrode depend significantly on the amount of the ion–pair (ip) in the carbon paste composition. thus, influence of the tpc–tpb ip percentage in the carbon paste composition was investigated first. preliminary experiment showed that carbon paste electrode without ip modifier has no response towards the analyte. for this purpose, ten electrodes were prepared containing the ip modifier percentage from 1 to 10 % and the results are summarized in fig. 3 and table 1. increase of ip percentage in the paste is found to increase the electrode response and stability of potentiometric readings, as well as slopes (s) of linear parts of calibration graphs defined by equation e = f(pctpc), reaching s = 59.714 mv dec-1 at 7 % tpc–tpb (fig. 4). at percentages above 7 %, a decrease of slope and reduction of linearity range where e = f(pctpc) of the mcpe electrode are observed due to the kinetics of ip within the paste. figure 3. effect of ip percentage in the mcpe on the calibration graph of tpc figure 4. effect of ip percentage in mcpe on the slope (s) of the linear, e =f(pctpc), range of the calibration graph of tpc. -50 0 50 100 150 200 2345678 e / m v pctpc 1% 2% 3% 4% 5% 6% 7% 8% 9% 10% 0 10 20 30 40 50 60 70 0 2 4 6 8 10 s / m v d e c1 ip, % m. khateeb et al. j. electrochem. sci. eng. 6(4) (2016) 277-2860 doi:10.5599/jese.321 281 table 1. potentiometric data for mcpe with different ip percentage obtained from linear equations. r linear range, µm intercept s / mv dec-1 composition, % ip plasticizer graphite 0.9873 9.27-64.70 105.17 22.77 1 49.5 49.5 0.9810 1.25-33.40 136.41 25.83 2 49.0 49.0 0.9942 1.25-64.70 190.24 32.53 3 48.5 48.5 0.9868 0.62-64.70 215.31 38.43 4 48.0 48.0 0.9906 0.62-122.00 268.98 45.73 5 47.5 47.5 0.9912 0.62-64.70 366.42 55.53 6 47.0 47.0 0.9995 0.30-221.00 409.40 59.71 7 46.5 46.5 0.9904 1.25-64.70 344.94 53.49 8 46.0 46.0 0.9922 0.62-64.70 354.48 51.17 9 45.5 45.5 0.9971 0.62-33.40 286.89 46.22 10 45.0 45.0 response characteristics and statistical data the characteristics performance of the best electrode (shadowed in blue in table 1) was determined and the results are summarized in table 2. the best proposed mcpe electrode shows nearly nernstian response over the concentration range 0.3-221.0 μm. table 2. response characteristics of the best mcpe-tpc electrode. parameter tpc-tpb electrode ip, % 7 regression equation e = -59.7pctpc + 409.40 correlation coefficient (r) 0.9995 linear range, μm 0.3–221.0 lod, μm 0.09 loq, μm 0.300 response time, sec ≤15 life time, day 70 working ph range 2.0–8.0 the slope of calibration graph for the best mcpe is 59.7 mv/decade for tpc concentrations in the range of 0.3–221.0 µm, with standard deviation of ±0.3 mv after five replicate measurements. limit of detection (lod) was calculated from the intersection of two extrapolated segments of the calibration graph [21]. lod and limit of quantification (loq) were 0.09 and 0.3 µm, respectively. effect of plasticizer on the potential response in this study, three plasticizers, di-octylphthalate (dop), di-butylphthalate (dbp) and paraffin oil (p. oil) were used to examine possible optimization of the paste. contents of examined plasticizers were 46.5, 46.0 and 45.0 w %, contents of graphite powder were 46.5, 46.0 and 45.0 w % and contents of electroactive compound (tpc–tpb ip) were 7, 8 and 10 w %, respectively. the sum of percentages of all three components was always adjusted to 100 %. the results obtained showed that response performances of prepared pastes are rather different, depending on the kind of plasticizer, proportion of the plasticizer towards graphite and amount of electroactive compound (table 3). typical potential responses of electrodes constructed with three plasticizers are given in fig.5. as shown in fig. 5, the dop-graphite electrodes were superior to dbp-graphite and p.oilgraphite electrodes in both the response slope and linear concentration range. so, dop was selected as the plasticizer of the carbon paste. the best paste composition of the dop-graphite electrode was 46.5 % graphite, 46.5 % dop and 7 % ip. j. electrochem. sci. eng. 6(4) (2016) 277-286 determination of tropicamide 282 table 3. general characteristic of different plasticizers of mcpe-tpc electrode. composition of the plasticizer, % ip, % s / mv dec-1 linear range, µm lod, μm response time, s dop (46.5) 7 59.71 0.30 – 221.00 0.09 15 dbp (46.0) 8 52.18 0.62 – 64.70 0.31 20 p. oil (45.0) 10 44.88 2.48 – 122.00 0.89 30 figure 5. optimization of plasticizers with cpe compositions:(a) dop 46.5 %, graphite 46.5 %, ip 7.0 %, (b) dbp 46.0%, graphite 46.0 %, ip 8.0 %), (c) p. oil 45.0 %, graphite 45.0, ip 10.0 %. dynamic response time dynamic response time is the required time for the electrode to achieve values within ±1 mv of the final equilibrium potential after successive immersions in the sample solutions [22]. its calculation involved the variation and the recording of the tpc concentration in a series of solutions from 0.5 to 50.0 µm. the electrode was able to quickly reach its equilibrium response in the whole concentration range. this time for the mcpe was about 15 seconds in the concentrated solutions ≤221.00 μm (fig 6). figure 6. response time of mcpe for different concentrations of tpc. selectivity of the electrode influence of various basic substances on the response of mcpe-tpc electrode was investigated by measuring the potentiometric interference from different kinds of sugars, inorganic cations and certain alkaloids. selectivity coefficients were evaluated by the matched potential method (mpm). table 4 showed that the proposed mcpe is highly selective towards tpc. the electrode showed no response to a number of potentially interfering ions usually used in the manufacturing of pharmaceutical preparations, such as starch and lactose. the inorganic cations did not interfere due to differences in their mobilities and permiabilities as compared with tpc cation. -50 0 50 100 150 200 250 2345678 e / m v pctpc e(doph) e(dbph) e(p. oil) a b c 0 30 60 90 120 150 180 0 20 40 60 80 e / m v t / sec 50.0 µm 5.0 0.5 µm m. khateeb et al. j. electrochem. sci. eng. 6(4) (2016) 277-2860 doi:10.5599/jese.321 283 table 4. selectivity coefficients of some interfering ions interfering ion log kmpm interfering ion log kmpm na+ -3.51 ba2+ -2.51 k+ -3.38 fructose ca2+ -2.97 glucose mg2+ -3.60 sucrose nh4+ -2.43 maltose mn2+ -2.71 lactose zn2+ -2.43 starch effect of ph to examine the effect of ph on the mcpe-tpc electrode response, the potential was measured at specific concentration of the tpc solution (50.0, 5.0 and 0.5 µm) having ph value of 1.0 up to 14.0 (concentrated naoh or hcl solutions were employed for ph adjustments). the results showed that the potential remained constant despite ph change in the range of 2.0-8.0, indicating applicability of this electrode in the specified ph range (fig. 7). relatively prominent fluctuations in the potential vs. ph behavior took place below and above the formerly stated ph limits. in detail, the fluctuations above the ph value of 8.0 might be justified by removing the positive charge on the drug molecule. fluctuations below the ph value of 2.0 may be due to interference of hydronium ion. figure 7. the ph effect on potential response of the mcpe-tpc electrode. effect of temperature to investigate thermal stability of the electrode, the effect of temperature of tpc solution on the mcpe was studied at different temperature values (20-70 °c). the mcpe exhibited good nernstian behavior in the temperature range of (20-60 °c). the standard cell potentials (eocell) were determined at different temperatures and used to determine the isothermal temperature coefficient (de°/dt) of the cell with the aid of the following equation [23]: e°cell = e°cell,25 °c + (de°/dt)cell t plotting t vs. e°cell,25ºc produced the straight line (fig. 8). the slope of this line was taken as the isothermal coefficient of the cell which was found to be -1.96×10-3 v/°c. the values of isothermal coefficient of the electrode revealed relatively high thermal stability within the investigated temperature range. -20 0 20 40 60 80 100 120 140 160 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 e / m v ph 50.0 µm 5.0 µm 0.5 µm j. electrochem. sci. eng. 6(4) (2016) 277-286 determination of tropicamide 284 figure 8. variation of the e°cell with temperature for the mcpe-tpc electrode. potentiometric titration representative titration curve for determination of the investigated drug is shown in fig.9. this mcpe electrode can be used successfully as indicator electrode in potentiometric titrations of the investigated drug. it also indicates that 1:1 [tpc]:[tpb] is formed as seen from the curve. figure 9. typical potentiometric titration of 50 ml of 1×10-4m tpc with 1×10-2 m tpb using mcpe life-time study the mcpe electrode life time was estimated with the calibration graph, periodical tests with standard solutions (0.30-221.0 µm) and calculation of its response slope. for this estimation, the electrode was employed extensively (1 hour per day) for 80 days. as it can be seen from table 5, in the case of mcpe this time is 70 days which shows the long-term stability of this kind of electrode. in mcpes the surface of the electrode is renewable and can be used for longer time. table 5. life time of mcpe electrode time, day s / mv dec-1 linear range, µm time, day s / mv dec-1 linear range, µm 1 / 24 59.71 0.30-221.00 45 59.65 0.30-122.00 1 59.35 0.30-221.00 50 60.02 0.62-122.00 2 59.50 0.30-221.00 55 59.22 0.62-122.00 5 59.96 0.30-221.00 60 59.11 0.62-221.00 10 59.60 0.30-221.00 65 59.09 0.30-122.00 15 59.41 0.30-221.00 70 58.65 1.25-122.00 20 59.75 0.30-221.00 75 58.31 1.25-122.00 25 59.18 0.30-221.00 80 57.76 2.48-64.70 30 59.15 0.30-221.00 35 59.31 0.30-221.00 40 59.55 0.30-221.00 e°cell = -0.1959t + 413.46 r = 0.9894 400 402 404 406 408 410 0 20 40 60 80 e ° c e ll / m v t / °c 50 70 90 110 130 150 170 190 0,0 0,2 0,4 0,6 0,8 1,0 1,2 e / m v vtpb / ml m. khateeb et al. j. electrochem. sci. eng. 6(4) (2016) 277-2860 doi:10.5599/jese.321 285 accuracy and precision the precision and accuracy of the method were also evaluated. the standard deviation, relative standard deviation and recovery of different tpc amounts were determined and recorded in table 6. the accuracy of the method is indicated by excellent recovery (99.80-101.40 %) and precision is supported by low standard deviation. table 6. accuracy and precision for determination of tpc in pure form by the proposed method. ctpc, µm rsd, % recovery, % taken found ± sd* 0.5 0.499±0.01 2.00 99.80 5.0 5.07±0.08 1.58 101.40 50.0 50.10±0.58 1.16 100.20 100.0 99.80±0.82 0.82 99.80 *average of five replicates analytical application the application of the proposed method using calibration graph gives good results as shown in table 7. the results were compared with the official method [14] and shown that the mcpe has good efficiency as regard of sensitivity, index of retrieving and repetition. table 7. determination of tpc in eye drop by here proposed and official methods. formulation label claim recovery, % ± sda proposed method official method [14] mydriamed 0.5 % 5 mg/ml 99.60±0.10 tb= 0.45 fc=1.23 99.80±0.09 tb=0.25 1.0 % 10 mg/ml 100.90±0.11 tb=1.83 fc=1.21 100.60±0.10 tb=1.34 a average of five replicates; b tabulated t-value at 95 % confidence level is 2.776. c tabulated f-value at 95 % confidence level is 6.26 conclusion the proposed chemically modified carbon paste electrode (mcpe) demonstrated advanced performance with fast response time and long stability, and shows high sensitivity, reasonable selectivity, and applicability over wide concentration range without sample pretreatment. the proposed procedure can be used for the routine analysis of tpc in bulk and eye drop. the sample recoveries from all samples were in good agreement with their respective label claims. 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[24] n. l. teradal, s. n. prashanth, j. seetharamappa, j. electrochem. sci. eng. 2 (2012) 67-75. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ anticorrosive behaviour of lumefantrine hydrophobic layer on mild steel surface doi:10.5599/jese.279 175 j. electrochem. sci. eng. 6(2) (2016) 175-185; doi: 10.5599/jese.279 open access : : issn 1847-9286 www.jese-online.org original scientific paper anticorrosive behaviour of lumefantrine hydrophobic layer on mild steel surface pavithra m. krishnegowda, venkatesha t. venkatarangaiah , punith kumar m. krishnegowda*, anantha n. subba rao, shubha h. nataraj department of chemistry, kuvempu university, shankaraghatta-577451, shimoga, karnataka, india *department of materials engineering, indian institute of science, bangalore-560012, karnataka, india corresponding author: drtvvenkatesha@yahoo.co.uk; tel.: +91-9448855079 received: march 22, 2016; revised: may 17, 2016; accepted: may 18, 2016 abstract the surface modification of mild steel was achieved by chemical treatment in lumefantrine (lf) solution. the surface morphology and wettability of modified surface was analysed by 3d profilometer and contact angle goniometer. the corrosion inhibition performance of modified mild steel surface in 1.0 m hcl solution was investigated by potentiodynamic polarization and electrochemical impedance techniques.electrochemical measurements illustrate that the corrosion of mild steel in acidic chloride medium get substantially reduced by introducing lf film on its surface (94 % efficiency). quantum chemical parameters were evaluated by ab initio method and they confer appropriate theoretical support to the experimental findings. keywords acid solutions; mild steel; polarization; electrochemical impedance spectroscopy ;acid corrosion introduction the service life of metals can be improved by their surface modification using electroactive compounds [1]. organic molecules containing o, n, s and polar functional groups are the extensively used electro active compounds, owing to their strong adsorption tendency on the metal surface. these electro active compounds form a thin film or molecular layers on the metal surface. traditionally, hetero aromatic compounds have been used for mild steel (ms) protection [2-7]. however, these compounds are harmful to the environment due to their toxicity. therefore, it http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 6(2) (2016)1750-185 anticorrosive behaviour of lumefantrine on mild steel 176 becomes important to search for new, nontoxic and effective organic compounds to serve the same purpose. in this regard, drugs are the promising alternatives, referred as green corrosion inhibitors [8,9]. drug molecules have been used for the corrosion inhibition of ms in different corrosive media and remarkable results have been reported [10-14]. in the present work, the antimalarial drug ‘lumefantrine’ has been selected to protect the ms from corrosion in acidic chloride medium.the ms was chemically treated with chcl3 solution containing lumefantrine (lf) and the corrosion behaviour of modified mild steel (mms) surface was examined by electrochemical techniques. experimental materials and sample preparation the experiments were performed with ms specimens having composition 0.04 % c, 0.35 % mn, 0.022 % p, 0.036 % s, and the remainder being fe. the ms coupons of1 cm 2 area (exposed) with a 5 cm long stem isolated with araldite resin were used for electrochemical measurements. prior to each experiment, the metal samples were abraded with series of emery papers (grade no. 220, 660 and 1200) followed by thorough wash using millipore water, acetone and then dried in hot air. the aggressive solution of 1.0 m hcl was prepared from ar grade 37 % hcl and millipore water. the millipore water was obtained from elix 3 milli-pore system (resistivity greater than 18 mω cm at 25 o c). lumefantrine was obtained from ramdev chemicals india pvt. ltd., mumbai and its structure is as shown in figure 1. the different concentrations of chcl3 solutions of lf were prepared by dissolving desired amount of lf in chcl3. the ms coupons were dipped in 100 ml of chcl3 solutions of lf for about 1 hr to obtain mms. after surface modification, the metal samples were dried and stored in desiccator for further corrosion studies. the ms coupon modified in 3mm lf solution was used for surface analyses. figure 1. structure of lumefantrine contact angle measurement an oca 30 from dataphysics instruments gmbh, germany was used to measure the contact angles. the contact angle of water on the metal sample was measured by the sessile drop method. a commercial goniometer was used with the volumes of water droplets fixed at 2 μl and the measurements were performed at room temperature.images of the air/liquid/solid (a/l/s) systems were captured and processed using the 32-bit sca 20 software. p. m.krishnegowdaet al. j. electrochem. sci. eng. 6(2) (2016) 175-185 doi:10.5599/jese.279 177 infrared spectra an infrared reflection absorption spectroscopy (gx spectrometer, perkin elmer, u.s.a.) instrument outfitted with a liquid nitrogen cooled mercury cadmium telluride (mct) detector was used to take ir spectra. before starting the experiments the sample and detector chambers were purged with nitrogen gas. the reported ir spectra were obtained with reference to bare steel substrate over 1024 optimized scans at 4 cm −1 resolution by using polarized beam spectra of modified surface. the surface morphology the surface of ms samples was examined by zeta-20 true color 3d optical profiler (zeta instruments, ca, u.s.a.). after the experiment, the metal samples were kept in a vacuum desiccator and were mounted on sample holder under the objective of the optical profiler, and the 2d photos were taken from the 50× magnified surface. quantum chemical studies quantum chemical calculations were performed using standard hyperchem, release 8.0 software (hypercube, inc. gmbh austria, usa). geometrical structure of lf molecule was optimized by parametric (pm3) method. the optimized structure which has minimum energy value was used as the input structure for the geometrical optimization using ab initio method (hartree– fock method, 6-31g** basis set). the polak–rieberre algorithm which is fast and accurate was used for computation. the energy parameters in the form of root mean square gradient were kept at 0.1 kcal/å mol. 0.1 kcal/å mol. electrochemical measurements electrochemical measurements were conducted in a conventional glass cell using chi 660c electrochemical analyzer (ch instruments, austin, tx, usa). ms specimen (1 cm 2 ), a platinum electrode and ag/agcl electrode were used as working, auxiliary, and reference electrodes respectively. prior to each electrochemical measurement, a stabilization period of 30 min was allowed to establish a steady state open circuit potential (ocp). each experiment was carried out in triplicate and the average values of corrosion parameters are reported. the potentiodynamic polarization (pdp) measurements were carried out over a potential range of −200 mv to +200 mv at ocp with a scan rate of 0.5 mv s -1 . the corrosion kinetic parameters such as corrosion potential (ecorr), corrosion current density (icorr), and anodic (a), and cathodic (c) tafel slopes were evaluated by the software installed in the instrument. the percentage inhibition efficiency ηt / %was computed from icorr values using the following expression; o corr corr t o corr / % 100 i i i     (1) wherei o corrandicorr are the corrosion current densities without and with inhibitor, respectively. the electrochemical impedance spectroscopy (eis) measurements were carried at ocp in the frequency range 1 mhz to 100 khz with 5 mv sine wave as the excitation signal. impedance data were analyzed using zsimp-win 3.21 software. the inhibition efficiency ηz/ % was evaluated from charge transfer resistance (rct) values using the following equation j. electrochem. sci. eng. 6(2) (2016)1750-185 anticorrosive behaviour of lumefantrine on mild steel 178 o ct ct z ct / % 100 r r r     (2) wherer o ct and rct are the charge transfer resistances without and with inhibitor, respectively. results and discussion morphology and contact angle measurement the 3d profilometer images and contact angle of a water droplet (2 μl) on the naked ms surface and mms surface are presented in figure 2. in general, low contact angle represents the hydrophilic surface and high contact angle represents the hydrophobic surface [15-16]. in the present study, the contact angle of the naked ms is 77.15 ° (figure 2a) and that of lf covered ms is 92.9 ° (figure 2b). this indicate that the lf modified ms surface is more hydrophobic than the bare ms surface. moreover, the noticeable change in contact angle value on mms may be due to the formation of a well packed layer of lf molecules. the compact layer on mms may be attributable to the adsorption of differently oriented lf molecules on different sites of ms surface, which affords a barrier effect against the diffusion of chloride ions [17]. on the other hand, the surface of polished ms is comparatively rough and the smooth surface of mms may be due to the uniform coverage of lf molecules. thus, the surface morphology and contact angle measurements substantiates the formation of lf film on the ms surface. figure 2.profilometer images and optical micrographs of a water dropleton (a) the naked mild steeland (b) modified mild steel. ir spectral analysis the ir spectra were used to determine the presence of lf film on the modified ms surface and are depicted in figure 3.by comparing figure 3a and 3b,it can be observed that certain peaks have been disappeared completely and some have been shifted. figure 3a shows the ir spectrum of lf molecule. the characteristic bandat 3490 cm -1 and 1459 cm -1 wereattributed to stretching vibration of the o-hand aromatic c=c groups, 1151 cm –1 corresponds to c-n bond, 2864 cm -1 corresponds to aromatic and 2953 cm -1 corresponds to aliphatic c-h stretching. also the band at 874 cm -1 is attributed to c-cl stretching. the ir spectrum of lf film on mms is shown in figure 3b. in this spectrum, c-n stretching frequency is shifted to 1122 cm –1 , and aromatic c=c stretching frequency is shifted to 1409 cm -1 . furthermore, the o-h band at 3490 cm -1 gets disappeared in figure 3b. based on these observations, it can be concluded that the adsorption of lf molecule on ms surface takes place through c=c, o-h and c-n bonds and confirms the adsorption of lf molecules on the ms surface. p. m.krishnegowdaet al. j. electrochem. sci. eng. 6(2) (2016) 175-185 doi:10.5599/jese.279 179 figure 3. ir spectra of (a) lf powder and (b) lf film on the mild steel surface. quantum chemical study quantum chemical calculations will provide information about the molecular structure of lf molecule responsible for the formation of film on the ms surface.the quantum chemical calculations were carried out with the help of complete geometry optimization by ab initio method using 6-31g** basis set. the geometry of lf molecule was optimized by hartree–fock method and the optimized structure, distribution of highest occupied molecular orbitals (homo) and lowest unoccupied molecular orbitals lumo are shown in figure 4. the calculated quantum chemical parameters are presented in table 1. table1. quantum chemical parameters of lf quantum chemical parameters ehomo / ev ̶ 8.009 elumo / ev 1.525 ∆e / ev 9.534 µ / d 4.495 total energy, k cal mol -1 ̶ 1670734.09 the spatial orbitals are distributed randomly on o, n and phenyl rings present in the molecule. moreover, lf molecule possesshigh dipole moment (µ) and the low energy gap (∆e) values which indicate that electron transfer from lf to the surface takes place during the process of adsorption. conversely, the adsorption centers of organic molecules can be approximated by net atomic charges in the molecule. the net atomic charges of some atoms present in lf molecule are given in table 2. the regions of highest electron density on organic molecule are the most plausible sites of adsorption [18,19]. therefore, n, o and some c atoms are the active centers, which have the strongest ability to bond to the metal surface. thus, oxygen atom, nitrogen atom and π electrons in the phenyl ring are the main active sites of adsorption on ms surface. by reviewing these j. electrochem. sci. eng. 6(2) (2016)1750-185 anticorrosive behaviour of lumefantrine on mild steel 180 findings, it can be deduced that lf forms a compact film by the process of adsorption on the surface of ms. as a result, the above experimental findings of ir spectral analysis are in good agreement with the quantum chemical results. figure 4.(a) optimized structure, (b) distribution ofhomoand (c) distribution oflumo of lf molecule. p. m.krishnegowdaet al. j. electrochem. sci. eng. 6(2) (2016) 175-185 doi:10.5599/jese.279 181 table 2.list of atoms having highest electron density in lf molecule. atom charge c (2) -0.013 c (3) -0.026 c (5) -0.026 c (18) -0.025 cl (21) -0.116 cl (22) -0.116 cl (23) -0.116 o(25) -0.386 n (27) -0.301 c (29) -0.041 c (30) -0.055 c (31) -0.065 c (33) -0.041 c (34) -0.055 c (35) -0.065 potentiodynamicpolarisation measurements electrochemical analyses were performed to investigate the kinetics of electrode processes as well as the surface characteristics of the electrochemical system. the potentiodynamic polarization curves in 1m hcl for the naked ms and mms in different concentrations of lf solutions are given in figure 5. the electrochemical corrosion parameters such as corrosion potential (ecorr), corrosion current density (icorr) , anodic tafel slope (a), cathodic tafel slope (c) and inhibition efficiency (ηt/ %) are tabulated in table 3 . it is apparent from figure 5 that increasing the lf concentration reduces both the cathodic and anodic current densities and the ecorr get slightly shifted towards the anodic direction. moreover, the decrease in both the anodic and cathodic current densities enumerated that the lf film formed on the mms surface suppressed both the anodic and cathodic reactions [20]. this phenomenon may be due to the adsorption of the lf molecules, which in turn decreased the attack of chloride ions on the ms surface. table 3.polarization parameters in 1 m hcl for the bare mild steel and modified mild steel in different concentrations of lf solutions. c/mm ̶ ecorr/ mv ̶ c/ mv dec -1 a / mv dec -1 icorr/ µa cm -2 ηt/ % 0.00 449 106.46 84.38 257.5 0.50 376 157.28 64.71 43.67 83 0.75 370 148.77 71.03 30.89 88 1.00 373 142.07 63.38 17.41 93 3.00 365 154.30 60.75 14.58 94 5.00 364 143.97 56.27 19.15 92 j. electrochem. sci. eng. 6(2) (2016)1750-185 anticorrosive behaviour of lumefantrine on mild steel 182 figure 5. potentiodynamic polarization curves in 1 m hcl for the naked mild steel and mild steel modified inpresence of different concentrations of lf. it is apparent from the table 3 that with increasing concentration of lf, icorr values lowered to its minimum value at 3 mm and the ηt reached the maximum value 94 %. this implies that the film formed on the ms becomes denser with increase in lf concentration from 0.5 to 3 mm. but the ηt value decreases when the lf concentration is 5 mm. this may be due to the fact that the lf molecules get rearranged on the ms surface after the number of adsorbed molecules reaches a maximum value [21]. this makes easy for the acidic chloride ions to attack the ms surface through the interspaces. furthermore, the values of a and c change with increase in inhibitor concentration and it indicates that the film inhibits both the anodic and cathodic reactions. by reviewing all these results it can be considered that the lf layer acts as a good protective film. electrochemical impedance spectroscopy measurements the nyquist and bode plots of the bare ms and lf covered ms in 1m hcl are given in figure 6. for the naked ms, the impedance data were analyzed by the electrochemical equivalent circuit shown in the inset of figure 6a. in this circuit, rs represents the solution resistance, r1 is the charge transfer resistance corresponding to the corrosion reaction at the ms/solution interface, and cpe1 represents the constant phase element (cpe) as a substitute for the double-layer capacitance (cdl). the impedance of cpe is defined as zcpe = q -1 (j)-n (3) where q is the cpe constant, ω is the angular frequency, j 2 = −1 is the imaginary number and n is the cpe exponent which gives details about the degree of surface inhomogeneity resulting from surface roughness, inhibitor adsorption, porous layer formation etc. for the bare ms, the nyquist plot is the slightly depressed capacitive loop (figure 6a) which indicates the roughness and other inhomogeneity of the ms surface [22]. the nyquist and bode plots for mms in different concentration of lf solutions were fitted by the equivalent circuit shown in figure 7(a). rs, r1, and cpe1 have the same meaning of elements in figure 7a. in addition, r2 represents the membrane resistance, which reflects the protective property of the film and cpe2 is the capacitance of the film formed on ms [23-24]. the obtained impedance data are tabulated in table 4. p. m.krishnegowdaet al. j. electrochem. sci. eng. 6(2) (2016) 175-185 doi:10.5599/jese.279 183 figure 6. (a) nyquist plot and (b) bode plot of naked ms in 1 m hcl figure 7. eis results in 1 m hcl for lf covered mild steel modified in differentconcentration of lf solutions: (a) nyquist plots; (b) bode plots table 4.impedance parameters in 1 m hcl for the naked mild steel and mild steel modified in presence of different concentrations of lf solutions. c/ mm r1/ ω cm 2 q1/ μω -1 s n cm -2 n1 r2/ ω cm 2 q2/ μω -1 s n cm -2 n2 ηz/ % 0.00 64.14 56.26 0.856 --- 0.50 303.5 109.4 0.733 21.1 19.38 0.999 79 0.75 499.3 51.68 0.829 24.95 17.16 0.954 87 1.00 1136 34.93 0.730 98.02 64.51 0.921 94 3.00 1292 37.84 0.815 61.75 11.20 0.946 95 5.00 950.2 59.65 0.761 41.58 33.71 0.883 93 it is apparent from figure 7a that, the diameter of the capacitive loop increased to its maximum when the concentration of lf solution was 3 mm, implying that the compact film is formed on the surface of ms in this concentration. bode plot shows two overlapped phase maxima, indicating the model of two time constants and the z modulus increases with the increase of lf concentration within 3 mm, suggesting that a dense film formed on the ms exhibit better protection efficiency. it is evident that the diameter of the capacitive loop and the logarithm of z modulus decreases when the concentration of lf is 5 mm.the inhibition efficiency calculated from r1 also increases with increase in lf concentration, and reaches maximum (95 %) at 3 mm concentration. this may be due to the hydrophobic film of lf molecules on ms surface which inhibited the dissolution j. electrochem. sci. eng. 6(2) (2016)1750-185 anticorrosive behaviour of lumefantrine on mild steel 184 reactions of ms to a greater extent in 1 m hcl. the variation in r1 values imply that the corrosion reaction on the ms surface was inhibited by the absorbed inhibitor film. surface morphology morphologies of metal samples were analysed by the true color images obtained from 3d profilometer. the true color images of bare ms and mms immersed in 1m hcl is shown in figure 8. the surface of bare ms is completely damaged with large number of pits due to the attack of aggressive chloride ions. but mms retains smoother surface owing to the presence of lf film which inhibits the process of corrosion in acidic chloride medium. hence it can be concluded that, the film formed by the adsorption of lf molecules behaves as a barrier and retards ms corrosion in hcl media. figure 8.profilometer images of (a) the naked mild steel and (b) modified mild steel after immersion in 1 m hcl for 1 h. conclusion the contact angle and ir analyses indicate the presence of lf molecules on mild steel surface. the quantum chemical study reveals that the o, n and π electrons in the lf molecule are the main active sites for adsorption. the chemical treatment of ms for about 1 h leads to the formation of compact film of lf molecules and hampers acid corrosion. the lf film inhibits anodic and cathodic reactions of ms in 1 m hcl. the electrochemical studies and profilometer images confirmed the ms corrosion inhibition ability of lf molecules in acidic chloride solution. acknowledgement: the authors are grateful to the authorities of department of chemistry, kuvempu university, karnataka, india for providing lab facilities. authors also thank department of science and technology, new delhi, govt. of india [dst: project sanction no. 100/ifd/1924/20082009 dated 2.07.2008] for providing instrumental facilities. the authors are also gratified to university grant commission, new delhi, govt. of india [ugc grant ref. 41-231/2012(sr) dated 16.07.2012] for providing hyperchem software facility. references [1] t.t.qin, j. li, h.q. luo, m. li, n.b. li, corros. sci.53 (2011) 1072–1078. 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[24] m. lebrini, f. robert, c. roos, int. j. electrochem. sci.6(2011) 847–859. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ thermodynamic, adsorption and corrosion inhibitive behaviour of benzyl nicotinate on cold rolled steel in sulphuric acid solution doi:10.5599/jese.222 209 j. electrochem. sci. eng. 5(3) (2015) 209-220 doi: 10.5599/jese.222 open access : : issn 1847-9286 www.jese-online.org original scientific paper thermodynamic, adsorption and corrosion inhibitive behaviour of benzyl nicotinate on cold rolled steel in sulphuric acid solution mutugadahalli rangaswamy vinutha, thimmappa venkatarangaiah venkatesha department of chemistry, school of chemical science, jnana sahyadri campus, kuvempu university, shankaraghatta-577451, karanataka, india corresponding author: drtvvenkatesha@yahoo.co.uk received: september 24, 2015; revised: november 27, 2015; accepted: november 28, 2015 abstract the inhibition performance of benzyl nicotinate (bn) on corrosion of cold rolled steel (crs) in 0.5 m h2so4 solution has been investigated using weight loss, tafel polarization and electrochemical impedance spectroscopy (eis). the bn acts as mixed type inhibitor. the thermodynamic parameters indicate the comprehensive nature adsorption of bn on crs which obey langmuir isotherm. morphological investigation of corrosion inhibition is carried out using afm and optical microscopic studies support the formation of inhibitor film on crs. there is a good agreement between the values of weight loss measurements and electrochemical studies. keywords eis, inhibitor, thermodynamic properties, langmuir isotherm, afm, optical microscopy introduction the study on the rusting of steel received great deal of attention by academicians due to its application in different fields. the methods of various natures have been reported in scientific journals to control the rust [1]. the use of inhibitor is considered as the best and most commonly used practical method to control the corrosion of steel. in recent years different type organic compounds are used as inhibitors in liquid medium for steel corrosion and their role during the process of corrosion has been extensively studied. these inhibitors enter in to the metal–solution interface and replace water molecules [2-3]. the hetero atoms like oxygen, nitrogen, sulphur, and phosphorus, triple bonds, and aromatic rings of compounds enhances their population at the metalsolution interface making a sort of barrier between metal and solution. the inhibitor http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 5(2) (2015) 209-220 inhibitive behaviour of benzyl nicotinate on cold rolled steel 210 efficiency of heterocyclic organic compounds follows the sequence: oxygen < nitrogen < sulphur < phosphorus. the electronic characteristic of the inhibitor molecules, aromaticity, steric factor, molecular mass, presence of functional groups like –c=o,-n=n-,-oh,-coor, the chemical composition of the solution, the nature of the metal surface, the temperature and the potential at the metal–solution interface determine efficacy of the inhibitor. the primary requirement for adsorption of inhibitor depends on the existence of attractive forces between the inhibitor and the metal. the adsorption may be physisorption, chemisorption or a combination of both [4-9]. in the present study, the selected inhibitor was benzyl nicotinate (bn), which is pyridine heterocycle containing nitrogen and –coor group, where the r-group is a benzyl group. all these structural features of bn are enough to produce required attractive force responsible for its adsorption to metal surface. in the present study, inhibitive effect of bn on the corrosion of cold rolled steel (crs) in 0.5 m h2so4 solution was verified using weight loss, eis and tafel polarization method. the morphology of the crs surface was determined using atomic force microscopy (afm) and optical microscopy. experiment sample the experiments were performed on crs with the compositions of 0.14 % c, 0.4 % mn, 0.025 % p, and0.0008 % s, 0.025 % si, 0.003 % al and rest of fe. solution the test solution of 0.5 m h2so4 was prepared using double distilled millipore water. the different concentrations of bn 50, 250 and 500ppm were prepared in 0.5 m h2so4 solution. weight loss measurement the crs specimens of dimension 4 × 2 × 0.5 cm were used for weight loss measurements and were abraded with a series of emery paper from 220, 330, 400, 600 and 1200 grades. the specimens were washed with millipore water, rinsed in acetone and dried in air. the weight loss incurred by crs specimens in corrosion experiments were determined by weighing the samples before and after immersing in 100 cm 3 of 0.5m h2so4. the corrosion experiments were also conducted at temperature 303,313, 323, and 333 k. the experiments were performed in static open aerated condition. the corrosion rate (νcorr) of crs was determined using the following relation: corr m st    (1) where νcorr is the corrosion rate of mild steel (g cm −2 h −1 ), δm is the corrosion weight loss of mild steel (g), s is the surface area of mild steel specimen (cm 2 ), and t is the time of exposure. the percentage inhibition efficiency was calculated using the following relationship corr corr corr / % 100 o o ie       (2) where ν 0 corr and νcorr are the corrosion rates of crs in the absence and presence of bn respectively. m. r. vinutha et al. j j. electrochem. sci. eng. 5(2) (2015) 209-220 doi:10.5599/jese.222 211 electrochemical measurements the electrochemical measurements were conducted in a conventional three electrode glass cell using chi660d electrochemical analyser (usa made). the crs specimen of 1 cm 2 exposed area with 5 cm long stem isolated with araldite resin was used as working electrode and platinum wire, calomel electrode were used as counter and reference electrode respectively. prior to polarization and electrochemical impedance spectroscopic measurements (eis), a stabilization period of 30 min was allowed to establish a steady state open circuit potential (ocp). each experiment was carried out in triplicate and the average values were reported. eis measurements were done at ocp in the frequency range from 0.1hz to 10 khz with the amplitude of 5 mv. the inhibition efficiency of eis measurement were calculated using the following equation   o ct ct imp ct / % 100 r r ie r    (3) where rct and r 0 ct are charge transfer resistance of working electrode with and without inhibitor, respectively. polarization plots were obtained in potential range from -200 mv to -1000 mv with the scan rate of 10 mv/s at ocp. the corrosion parameters like corrosion current density (icorr), corrosion potential (ecorr), catodic (c) and anodic (a) tafel slopes were calculated. inhibition efficiency is calculated using the relation as follows,   o corr corr pol corr / % 100 o i i ie i    (4) where i 0 corr and icorr are the corrosion current density in the absence and in presence of inhibitor respectively. morphological studies the surface morphology of crs samples were investigated using both atomic force microscopy (afm bruker model) and optical microscopy using olympus cx31 microscope. results and discussions polarisation studies figure 1 shows the typical polarisation curve for crs generated at room temperature without and with different concentration of inhibitor bn. the corresponding electrochemical parameters such as corrosion potential (ecorr), corrosion current density (icorr ), cathodic (c) and anodic (a) tafel slopes and inhibition efficiency are given in table 1. from the table 1 it is indicated that there is decrease in the corrosion current density (icorr) value with increase of concentration of inhibitor, which suggested the adsorption of inhibitor molecule on surface of steel and thus acting as a barrier for charge and mass transfer reactions [10]. furthermore, with increase in concentration of inhibitor bn there is no significant shift in the values of either cathodic (c) or anodic (a) slope which infers the adsorption bn on crs controls both anodic metal dissolution and cathodic hydrogen evolution processes. moreover, the value of δecorr is within 5-30 mv [11], hence bn act as mixed inhibitor for crs in 0.5m h2so4. j. electrochem. sci. eng. 5(2) (2015) 209-220 inhibitive behaviour of benzyl nicotinate on cold rolled steel 212 table 1. electrochemical polarization parameters of crs at 0.5 m h2so4 solution in absence and presence of bn measured at 303 k concentration, ppm -ecorr / mv c / mv dec -1 a / mv dec -1 icorr / ma cm -2 ie / % 0 519 188.0 154.0 3.991 50 516 170.9 129.2 1.347 66 250 509 158.4 107.9 0.715 82 500 504 148.9 92.0 0.333 92 e / v vs. sce figure 1. tafel polarization response of various concentration of bn in 0.5 m h2so4 solution electrochemical impedance studies the eis findings of crs in 0.5 m h2so4 both in absence and in presence of different concentration of bn were (analysed using zsim win 3.21) presented in table 2. corrosion behaviour at the studied frequency ranges is represented in nyquist and bode plots as in figures 2, 3 and 4. the nyquist plots exhibit single slightly depressed semicircles and one time constant in bode plots, which suggests that metal dissolution is controlled by charge transfer process [12]. figure 2. nyquist plots for different concentration of bn in 0.5 m h2so4 solution. lo g ( i / a c m -2 ) m. r. vinutha et al. j j. electrochem. sci. eng. 5(2) (2015) 209-220 doi:10.5599/jese.222 213 the depression in nyquist semicircle is ascribed to some physical nature of surface i.e., inhomogeneity, roughness and active site of surface. the eis values were examined using a suitable equivalent circuit model shown in the figure 2, where rs is the solution resistance and rct is the charge transfer resistance, q is the constant phase element, which is used in place of pure capacitor, in order to explain the deviation of electrodes from ideal dielectric behaviour, which is related to surface inhomogeneity [13]. the impedance of constant phase element (zcpe ) is given by the following equation: zcpe = y0 -1 (iω) -n (5) where y0 is cpe constant, i 2 = -1, an imaginary number,  is angular frequency and n is cpe exponent which used to assess the surface inhomogeneity, resulting from surface roughness, inhibitor adsorption, porous layer formation, etc. [14]. the capacitance (cdl) of constant phase element is calculated using the following relation cdl= (qrct 1-n ) 1/n (6) from the nyquist plot, it is observed that with an increase in inhibitor concentration, there is an increase in semicircle diameter which corresponds to rct values. it accounts from the table that, there is decrease in the rct values and increase in cdl values with increase in concentration of inhibitor. this resulted from replacement of water molecule from the steel surface by the inhibitor molecule thereby reducing the active sites of corrosion. the same result is depicted in bode diagram, i.e., increment in phase angle with increase of inhibitor concentration is corresponding to decrease in surface inhomogeneity with inhibitor adsorption onto steel surface [15]. the above discussion it is concluded that bn works as efficient inhibitor for crs in 0.5 m h2so4. figure 3. bode phase angle plots for different concentration of bn in 0.5 m h2so4 solution table 2. electrochemical impedance parameters of crs at 0.5 m h2so4 solution in absence and presence of bn measured at 303 k. concentration, ppm rct / ω cm -2 q / s n ω -1 cm -2 n cdl / µf cm -2 ie / % 0 11.26 1.588x10 -4 0.86 57.2 50 46.2 2.296 x10 -4 0.76 52.6 76 250 76.16 1.655 x10 -4 0.76 41.8 85 500 136.8 0.853 x10 -4 0.85 38.9 92 j. electrochem. sci. eng. 5(2) (2015) 209-220 inhibitive behaviour of benzyl nicotinate on cold rolled steel 214 figure 4. bode phase plots for different concentration of bn in 0.5m h2so4 solution. weight loss experiment the effect of inhibitor concentration on corrosion rate at different temperature ranging between 303-333 k is as shown in the figure 5 and the corresponding values are tabulated in table 3. the corrosion rate decreases with increasing concentration of inhibitor due to the increase of surface coverage of inhibitor [16]. it is clear from the figure 5 that the corrosion rate decreases with the increase in the concentration of bn. the corrosion rate increases more rapidly in the absence and in the presence of 50 ppm of bn concentration, however, a small decrease in the corrosion rate values at 250 ppm and to very lesser extent in 500 ppm with the rise of temperature. the increase in corrosion rate and hence decreasing value of inhibition efficiency with rise of temperature is indicative of physisorption of bn molecule on crs. figure 5. variation of corrosion rate with concentration at different temperature m. r. vinutha et al. j j. electrochem. sci. eng. 5(2) (2015) 209-220 doi:10.5599/jese.222 215 adsorption isotherm and thermodynamic parameters studies corrosion inhibitor, bn, adsorbs on steel surface preventing its surface contact with the corrosive media and thus preventing steel dissolution. in order to gain deeper understanding about the mode of adsorption, the experimentally calculated data were verified with the different adsorption isotherm equations. the best fit was obtained with langmuir isotherm equation represented below: ads 1c c k   (7) where c is concentration of inhibitor,  is surface coverage and kads adsorption equilibrium constant. using the values of kads , we were able to calculate the thermodynamic parameters like δgads, δh and δs and which are depicted in the table 4. the langmuir isotherm equation is based on the assumption that the adsorption of organic molecules on metallic surface is monolayer without any chemical interaction between them. furthermore, the adsorption of the molecule on to metal surface depends upon the chemical structure and charge distribution of molecule and also depends on surface charge of metal [17]. we can classify the adsorption as physisorption if δgads < 20 kj, chemisorptions if δgads > 40 kj or comprehensive adsorption if δgads in between 20 – 40 kj. table 3. corrosion rate and the inhibition efficiencies at various temperatures at different concentration of bn in 0.5 m h2so4 solution as obtained from weight loss experiment. concentration, ppm temperature, k 303 313 323 333 νcorr / mg cm -2 h -1 ie / % νcorr / mg cm -2 h -1 ie / % νcorr / mg cm -2 h -1 ie / % νcorr / mg cm -2 h -1 ie / % 0 5.5625 12.8 15.34 19.68 50 1.525 72.58 4.425 65.29 7.31 52.32 13.63 31 250 0.775 86.07 2.175 82.94 2.95 80.76 4.663 76 500 0.5 91.01 1.225 90.39 1.575 89.73 2.500 88 figure 6. langmuir adsorption of bn on the surface of crs in 0.5 m h2so4 at various temperatures. j. electrochem. sci. eng. 5(2) (2015) 209-220 inhibitive behaviour of benzyl nicotinate on cold rolled steel 216 table 4. thermodynamic parameters for crs in 0.5 m h2so4 solution obtained from weight loss method. temperature, k r 2 kads / m -1 -δg / kj mol -1 -δh / kj mol -1 δs / j mol -1 k 303 0.999 53987 37.567 37.262 34.141 313 0.994 30951 37.360 323 0.999 21011 37.513 333 0.999 14022 37.555 the δgads values around -37kj implied that the adsorption of bn molecule on to crs is comprehensive in nature i.e., the nature is in between to that of physisorption and chemisorptions [18]. thus, δgads can be calculated according to the following equation: ads ads 1 exp 55.5 g k rt        (8) the value of δh and δs can be calculated using van’t-hoff equation expressed as follows: ads ads ads 1 ln 55.5 h s k rt r      (9) the graph of ln (kads) againt 1/t gives a straight line with a slope of (-δh/r) and intercept equal to [δs/r + ln(1/55.5)] and are represented in table 4. further, the negative values of δgads suggest spontaneous adsorption of inhibitor molecule on steel surface. the negative value of δh indicates adsorption is exothermic in nature and hence there is decrease in inhibition efficiency with increase in temperature. generally, exothermic process is attributed to either physisorption or chemisorption while endothermic process is for pure chemisorption. furthermore, if the value of δh < 41.86 kj/mol then it is physisorption, while for chemisorption δh values in and around 100 kj/mol [21-23]. for the present case, δhads = -37.26 kj is pure physisorption of inhibitor bn. the positive value of δs infers that adsorption of inhibitor onto metal surface is accompanied by increase in entropy. this thermodynamic value entropy is algebraic sum of decrease in entropy of solute and increase in entropy of solvent and hence the positive value of entropy corresponds to increase in solvent entropy [24]. figure 7. relation between ln kads against 1/t m. r. vinutha et al. j j. electrochem. sci. eng. 5(2) (2015) 209-220 doi:10.5599/jese.222 217 afm surface characterization studies in recent developments, afm emerged as powerful technique for characterising microstructures of materials. afm provides powerful means to study the surface morphology in the micro or nano ranges and for evaluating the influence of inhibitor on the corrosion process [19]. figure 9 shows three dimensional afm images of polished cold rolled steel (reference sample) (a), of crs specimens immersed in 0.5 m h2so4 solution (b) and of crs specimens immersed in 0.5 m h2so4 + 500 ppm of bn (c), respectively. it is apparent from the figure 8 that the inhibited surface is more uniform when compared to the surface immersed in 0.5 m h2so4 solution. figure 8. 3d afm images of polished cold rolled steel (a), crs specimens immersed in 0.5 m h2so4 (b) and in 0.5 m h2so4 + 500 ppm of bn (c). the afm analysis directly measures the surface roughness with the roughness parameters, peak height and texture and surface area of entire surface. average roughness (ra) is the arithmetic average of the absolute values of the surface height deviations measured from the mean plane. the root mean square roughness (rq) is the root mean square average of height deviations taken from the mean data plane and the (rmax) specifies the maximum vertical distance between the highest and lowest data points in the image i.e., it gives the maximum peak to valley height values [20]. the values so obtained are represented in a table 5 table 5. roughness parameters for crs as obtained from afm studies samples status average roughness ra / nm rms roughness rq / nm peak to valley height rmax / nm crs polished 34 50.5 379 crs immersed in 0.5m h2so4 168 208 1741 crs immersed in 0.5 m h2so4 + 500 ppm of bn 46.9 59.7 447 optical microscopic studies the optical microscopic images are taken from olympus cx31microscopy with motix camera provided with both trinacular and binocular lenses. the images were taken at 20x magnification with light illuminated at plane polarised and cross nicols (xn) condition. it is inferred from the images that the optical microscopic images as in figure 9a of corroded crs in 0.5 m h2so4 is j. electrochem. sci. eng. 5(2) (2015) 209-220 inhibitive behaviour of benzyl nicotinate on cold rolled steel 218 accompanied with cracks and small pits. whereas, crs immersed 0.5 m h2so4 + 500 ppm of inhibitor bn is protected and is free of cracks and pits as seen in figure 9b. figure 9. optical microscopic images of crs specimens immersed in 0.5 m h2so4 [a and a(xn)] and crs in 0.5 m h2so4 + 500 ppm of bn [b and b(xn)] mechanism of adsorption there are many models in the literature to explain the mechanism of adsorption of inhibitors. generally it is accepted that the adsorption of organic inhibitor molecules is often a displacement reaction involving removal of adsorbed water molecules from the metal surface i.e., org(sol) + xh2o(ads)↔ org(ads) + xh2o(sol) [25-28]. the organic inhibitor molecule get adsorbed to the steel surface through its heteroatom like nitrogen, oxygen, sulphur and phosphorus influencing the corrosion properties of steel. the inhibitor molecule adsorbs on to the corresponding active sites of steel and reduces either anodic or cathodic corrosion reactions. there are four primary modes of adsorption associated with inhibitor molecules at surfaces: electrostatic adsorption (physisorption), π-back bonding, chemisorption, and organometallic complex formation [25]. thus, the molecule bn can interact with crs in three possible ways as represented in figure 10. firstly, the neutral bn molecule undergoes chemical adsorption to crs by interaction of lone pair of electrons on nitrogen and oxygen atoms with the empty d-orbitals of iron (a). the second possibility involves the electrostatic interaction of protonated bn with the already adsorbed so4 ions through physisorption (b). the third mode is interaction of π-electrons of bn molecule with the steel surface through donor-acceptor interaction (c). thus, adsorption of bn forms a barrier layer on crs, thereby preventing its contact with the corrosive media and reduces the corrosion rate. m. r. vinutha et al. j j. electrochem. sci. eng. 5(2) (2015) 209-220 doi:10.5599/jese.222 219 figure 10. mechanistic representation of adsorption of bn on crs in 0.5 m h2so4 solution conclusion  benzyl nicotinate (bn) acts as an efficient inhibitor for the corrosion of mild steel in 0.5 m h2so4 solution.  inhibition efficiency increases with the inhibitor concentration but decrease with temperature. this shows that bn follows physisorption, obeying langmuir isotherm.  from the polarisation results it can be concluded that bn acts as mixed inhibitor which impedes both anodic and cathodic corrosion reactions.  the inhibition efficiency values found from weight loss method are in good agreement with electrochemical findings.  the morphology of crs samples were observed by optical microscopy and afm. microscopic images and the roughness parameters derived from afm are supported our experimental findings. acknowledgement: the authors are grateful to the university grant commission, new delhi, government of india for providing financial assistance and department of chemistry, kuvempu university for providing lab facilities. references [1] m. g .hosseini , m. ehteshamzadeh , t. shahrab, electrochim. acta 52 (2007) 3680–3685. 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[28] m .sahin, s. bilgic, h. yilmaz, appl.surf. sci. 195 (2002) 1–7. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ ruthenium redox equilibria: 3. pourbaix diagrams for the systems ru-h2o and ru-cl--h2o doi:10.5599/jese.229 145 j. electrochem. sci. eng. 6(1) (2016) 145-153; doi: 10.5599/jese.229 open access : : issn 1847-9286 www.jese-online.org original scientific paper ruthenium redox equilibria 3. pourbaix diagrams for the systems ru-h2o and ru-cl -h2o igor povar, oxana spinu institute of chemistry of the academy of sciences of moldova, 3 academiei str., md 2028, chisinau, moldova corresponding author: ipovar@yahoo.ca; tel.: +373 22 73 97 36; fax: +373 22 73 97 36 received: september 30, 2015; accepted: february 18, 2016 abstract on the basis of selected thermodynamic data, the standard electrode potentials of possible half reactions in the ru-h2o and ru-cl -h2o systems have been calculated. using the thermodynamic approach developed by the authors, the potential ph and potential pcl diagrams for the considered system have been built. keywords potential ph diagram; standard electrode potential; soluble and insoluble ruthenium species. introduction thermodynamic analysis is a valuable and powerful tool in predicting, comprehending, and rationalizing the stability relations in redox reaction systems. in order to create an integrated picture of the thermodynamic properties of compounds of the element in its different valence states in both aqueous and solid phases, the diagrams potential ph (or so-called pourbaix diagrams) are generally used [1-16]. pourbaix diagram is very important in predicting the thermodynamic equilibrium phases of an aqueous electrochemical system. these diagrams allow the graphical presentation of the thermodynamic properties of compounds of the given element based on the solution ph and the overall metal ion concentration in solution. the diagrams e(ph) are compact and contain a large quantity of information, which led to their wide application in various fields of science and technology, particularly in electrochemistry [1-9], hydrometallurgy [10-13] analytical chemistry [14,15], etc. in the presence of a small number of species in the redox system the construction of such diagrams does not present difficulties. the increase in the number of components and, in particular, the appearance of poly-nuclear species, calculating chemical and electrochemical equilibria becomes laborious. authors [16] proposed an original procedure of http://www.jese-online.org/ mailto:ipovar@yahoo.ca j. electrochem. sci. eng. 6(1) (2016) 145-153 ruthenium redox equilibria: 3. pourbaix diagrams 146 calculation. firstly, on the basis of the tabulated thermodynamic data, the thermodynamic stability areas of chemical species, depending on the solution ph for each valence state (degree of oxidation), are determined [17,18]. these areas are demarcated on diagrams by vertical lines. then, a system of independent electrochemical equations for electrode reactions between chemical species with varying degrees of oxidation, the predominance areas of which overlap, are drawn up. the electrode potentials of these reactions are linear functions of ph, which are depicted on diagrams. the potential ph (pourbaix) diagram constitutes an effective method of graphical representation of chemical and electrochemical equilibria, especially for systems under protolytic processes and contains as valence states oxides and hydroxides in the solid phase, protonated particles or hydroxocomplexes in solution. in addition to the "metal-water" system of a complexing agent that forms stable complexes with metal ions, the electrode potential often depends decisively on the ligand concentration cl 0 in solution. in this case, the pourbaix diagrams are less informative because of a large number of lines as cl 0 functions and then the more useful are diagrams representing the dependence of the potential on cl 0 or log cl 0 . the ligand usually is taken in large excess relative to the metal ion (cl 0 >> cm 0 ) and therefore cl 0 . in particular, the influence of a number of such factors, as the medium acidity, the complexation and precipitation processes of the redox species is necessary to examine. this article has done some work in order to extend the usefulness of the frost diagram. since some equilibria are also some functions of the metal ion concentration cl 0 and the solution ph, for the construction of the diagram e(log [l]) the conditions cl 0 = const and ph = const are assumed. in this paper, the following procedure of calculating e-ph diagram is proposed: 1. firstly, the predominance areas of different valence forms in function of ph, 0 m logc or 0 l log c are calculated; 2. on the basis of the diagrams δgr(n) [17-18], the thermodynamic stability of different valence forms toward disproportionation conditions is determined; 3. the system of electrochemical equations for electrode reactions between chemical species in different valence states, the predominance areas of which overlap, is composed; based on standard thermodynamic data of participating species in specific reactions, the electrode potential is calculated by the equation e 0 δgr 0 /nf, where δgr 0 is the value of the standard gibbs energy of electrode reaction; the electrode potentials of these processes are calculated as a function of 0 l c , respecting the conditions cm 0 = const and ph = const. the first two steps have been carried out in [17]. theoretical considerations the pourbaix diagram for the ru-h2o system is presented in [1], but it suffers from a number of drawbacks: a. it is carried out on the basis of outdated thermodynamic data and their interpretation may lead to erroneous conclusions; b. the formation of solid phases is not taken into account; c. it is calculated for on metal ion concentration only. this paper aims to remove such deficiencies and calculating diagrams e-ph [16] on the basis of selected thermodynamic data [19]. in [18] it was shown that for ruthenium the disproportionation reactions are characteristic, in particular for the ru(ii), ru(vi) and ru(vii). in this paper the frost diagrams are designed as a preliminary step for building e-ph diagrams. we will examine in detail the calculation of the e-ph diagram for cru 0 =10 -4 mol/l. in the range 0 < ph < 14 ruthenium for the degree of oxidations ru(ii), ru(v), ru(vi) and ru(vii) is represented i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 145-153 doi:10.5599/jese.229 147 by single species, ru 2+ , ru2o5 (s), ruo4 2, ruo4 correspondingly, while for the valence states ru(iii), ru(iv) and ru(viii), the hydrolysis is characteristic with formation of hydroxocomplexes. ru(ii) and ru(iv), within a wide range of ph and cru 0 , form also poorly soluble hydroxides. authors [17,18] determined the thermodynamic stability areas of the following species for consecutive degrees of oxidation: ru(viii) h2ruo5 0.00 < ph < 11.53 hru(oh)5 11.53 < ph < 14.00 ru(iv) ru(oh)2 2+ 0.00 < ph < 2.55 ru4(oh)12 4+ 2.55 < ph < 4.43 ruo2 ∙ h2o (s) 4.43 < ph < 14.00 ru(iii) ru 3+ 0.00 < ph < 1.76 ru(oh)2 + 1.76 < ph < 4.42 ru(oh)3 (s)∙h2o 4.42 < ph < 14.00 we will examine the calculation of respective values phd for cru 0 =10 -6 mol/l. from the δgr(n) diagram it follows that the reaction of disproportionation of ru(v) to ru(viii) and ru(iv) occurs between the ph values 1 and 3. this process is described by the equation: 2ru2o5 (s) + h2o + 6h + = 3ru(oh)2 2+ + h2ruo5 (1) the standard gibbs energy variation 0 r g is equal to gr 0 =gf 0 (h2ruo5) + 3gf 0 (ru(oh)2 2+ 2gf 0 (ru2o5 (s)) gf 0 (h2o) = 70.59 kj for reaction (1) the isotherm equation takes the form gr =gr 0 – 6rt ln[h + ] + 4rt ln cru 0 the phd value corresponds to the beginning of disproportionation, provided by the condition δgr = 0. wherein, ph(δgr = 0) = 1.94 (the point a on the diagram e-ph). in the same way, we determine phd for the following redox couples: 3ruo4 + ½ h2o + 3h + = ½ ru2o5 (s) + h2ruo5 , gr 0 = 136.0 kj gr = gr 0 + 3rt ln[h + ] + rt ln cru 0 , phd(gr=0) = 5.94 (point b) 2ruo4 + 3h + = ½ ru2o5 (s) + ruo4 2 + 3/2 h2o, gr 0 = 215.18 kj gr = gr 0 + 3rt ln[h + ] + rt ln cru 0 , phd(gr=0) = 10.57 (point c) ru2o5 (s) + 3h2o = ruo4 2 + ruo2 2h2o + h + , gr 0 = 158.97.18 kj gr = gr 0 + 2rt ln[h + ] + rt ln cru 0 , phd(gr=0) = 10.92 (point d). next, we will analyze the equilibria between chemical species in solution and solid phase. we will consider only the electrode potentials of redox couples, the predominance of areas of which overlap. so we get: ru(iii) – ru(0) ei 0 / v 1. ru 3+ +3e = ru 0.00 < ph < 1.76 0.599 2. ru(oh)2 + + 2h + + 3e = ru + 2h2o 1.76 < ph < 4.42 0.668 3. ru(oh)3 (s)×h2o + 3h + + 3e = ru + 4h2o 4.42 < ph < 14.0 0.631 j. electrochem. sci. eng. 6(1) (2016) 145-153 ruthenium redox equilibria: 3. pourbaix diagrams 148 ru(iv) – ru(iii) 4. ru(oh)2 2+ + 2h + + e = ru 3+ + 2h2o 0.0 < ph < 1.76 0.831 5. ru(oh)2 2+ + e = ru(oh)2 + 1.76 < ph < 2.55 0.612 6. ¼ ru4(oh)12 4+ + h + + e = ru(oh)2 + + h2o 2.55 < ph < 4.42 0.506 7. ¼ ru4(oh)12 4+ + h2o + e = ru(oh)3 (s)×h2o 4.42 < ph < 4.43 0.617 8. ruo2 (s)×2h2o + h + + e = ru(oh)3 (s)×h2o 4.43 < ph < 14.0 0.777 ru(v) – ru(iv) 9. ½ ru2o5 (s) + 3h + + e = ru(oh)2 2+ + ½ h2o 1.94 < ph < 2.55 1.222 10. ½ ru2o5 (s) + ½ h2o + 2h + + e = ¼ ru(oh)12 4+ 2.55 < ph < 4.43 1.328 11. ½ ru2o5 (s) + 3/2 h2o + h + + e = ruo2 (s) + 2 h2o 4.43 < ph < 10.92 1.168 ru(vi) – ru(v) 12. ruo4 2 + 3h + + e = ½ ru2o5 (s) + 3/2 h2o 10.57 < ph < 10.92 2.816 ru(vi) – ru(iv) 13. ruo4 2 + 4h + + 2e = ruo2 (s) + 2h2o 10.92 < ph < 14.00 1.992 ru(vii) – ru(v) 14. ruo4 + 3h + + 2e = ½ ru2o5 (s) + 3/2 h2o 5.94 < ph < 10.57 1.701 ru(vii) – ru(vi) 15. ruo4 + e = ruo4 2 10.57 < ph < 14.00 0.586 ru(viii) – ru(iv) 16. h2ruo5 + 6h + + 4e = ruoh2 2+ + h2o 0.00 < ph < 1.94 1.405 ru(viii) – ru(v) 17. h2ruo5 + 3h + + 3e = ½ ru2o5(s) + 5/2 h2o 1.94 < ph < 5.94 1.466 ru(viii) ru(vii) 18. h2ruo5 + e = ruo4 + h2o 5.94 < ph < 11.53 0.996 19. hruo5 + h + + e = ruo4 + h2o 11.53 < ph < 14.00 1.678 the following expressions for electrode potentials (within the respective ph ranges) correspond to these electrode processes: ru(iii) – ru(0) e1 = e1 0 + rt / 3f ln cru 0 0.00 < ph < 1.76 e2 = e2 0 + rt / 3f ln cru 0 + rt / 3f ln [h + ] 1.76 < ph < 4.42 e3 = e3 0 + rt / f ln [h + ] 4.42 < ph < 14.0 ru(iv) – ru(iii) e4 = e4 0 + 2 rt / f ln [h + ] 0.00 < ph < 1.76 e5 = e5 0 1.76 < ph < 2.55 e6 = e6 0 + rt / f ln [h + ] 3rt / 4f ln cru 0 – ¼ rt / ln 4 2.55 < ph < 4.42 e7 = e7 0 + rt / 4f ln cru 0 – ¼ rt / ln 4 4.42 < ph < 4.43 e8 = e8 0 + rt / f ln [h + ] 4.43 < ph < 14.0 ru(v) – ru(iv) e9 = e9 0 + 3 rt / f ln [h + ] 3rt / 4f ln cru 0 1.94 < ph < 2.55 e10 = e10 0 + 2rt / f ln [h + ] rt / 4f ln cru 0 – ¼ rt / ln 4 2.55 < ph < 4.43 e11 = e11 0 + rt / f ln [h + ] 4.43 < ph < 10.92 ru(vi) – ru(v) e12 = e12 0 + 3 rt / f ln [h + ] + rt / f ln cru 0 10.57 < ph < 10.92 ru(vi) – ru(iv) e13 = e13 0 + 2 rt / f ln [h + ] + rt / 2f ln cru 0 10.92 < ph < 14.00 ru(vii) – ru(v) e14 = e14 0 + 3 rt / 2f ln [h + ] + rt / 2f ln cru 0 5.94 < ph < 10.57 i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 145-153 doi:10.5599/jese.229 149 ru(vii)– ru(vi) e15 = e15 0 10.57 < ph < 14.00 ru(viii)-ru(iv) e16 = e16 0 + 3 rt / 2f ln [h + ] 0 < ph < 1.94 ru(viii) –ru(v) e17 = e17 0 + rt / f ln [h + ] + rt / 3f ln cru 0 1.94 < ph < 5.94 ru(viii)-ru(vii) e18 = e18 0 5.94 < ph < 11.53 e19 = e19 0 + rt / f ln [h + ] 11.53 < ph < 14.00 results and discussion from the selected thermodynamic data [19], the standard electrode potentials of possible halfreactions in the ru–cl -h2o system have been also calculated. the calculation results of redox equilibria and the areas of predominance of chemical species in the examined system are shown in the form of diagrams potential-log[cl ]. er 0 represents the standard electrode potential for respective redox couple, calculated in the basis of thermodynamic data [19] by the formula. er 0 = gr0. finally, the e-ph diagrams for cru 0 = 10 -4 and cru 0 = 10 -6 mol/l are shown in fig. 1 and 2. on their basis the following conclusions can be made: 1. with increasing of the total concentration of ruthenium: a. the areas of stability of ru(oh)2 + , ru(oh)2 2+ , ruo4 2, ruo4 significantly narrow; b. the thermodynamic stability areas of the solid phase ru(oh)3∙h2o(s), ruo2∙h2o(s) and ru2o5(s) increase; ru(ii) is thermodynamically unstable to dismutation in ru and ru(iii) within the entire range of ph and cru0 values. in [19] it is assumed that ru2+ does not participate in the disproportionation process due to the preponderance of the kinetics conditions on the thermodynamic inhibition. figure 1. the potential – ph diagrams for ruthenium compounds in the system ru-h2o, cru 0 = 10 -4 mol/l . 2. the most stable valence state of ruthenium is ru(iv). these results are in good agreement with existing experimental data [19]. 1 0 2 4 6 8 10 12 14 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 ru(oh) 2 + r u (o h ) 2 2 + ru 2 o 5(am) ru(oh) 3 . h 2 o (am) ru 4 (oh) 12 4+ ru 3+ ru ruo 2  2h 2 o (am) ruo 4 2ruo 4 hruo 5 -h 2 ruo 5 e , v ph 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 e / v j. electrochem. sci. eng. 6(1) (2016) 145-153 ruthenium redox equilibria: 3. pourbaix diagrams 150 figure 2. the potential – ph diagrams for ruthenium compounds in the system ru-h2o, cru 0 = 10 -6 mol/l . we will now examine the equilibrium between species in different valence states. along with the reaction equation, the calculated standard electrode potential e 0 is indicated: ru(iii) – ru(0) ei 0 / v 1. ru 3+ + 3e = ru log[cl ] < -2.17 0.599 2. rucl 2+ + 3e = ru + cl -2.17 < log[cl ] < -1.57 0.566 3. rucl2 + + 3e = ru + 2cl -1.57 < log[cl ] < -0.54 0.525 4. rucl3 0 + 3e = ru + 3cl -0.54 < log[cl ] < 0.15 0.514 5. rucl4 + 3e = ru + 4cl 0.15 < log[cl ] < 0.30 0.517 6. rucl5 2 + 3e = ru + 5cl 0.30 < log[cl ] < 0.40 0.523 7. rucl6 3 + 3e = ru + 6cl 0.40 < log[cl ] < 0.50 0.531 ru(iv) – ru(iii) 8. ru(oh)2 2+ + 2h + + e = ru 3+ + 2h2o log[cl ] < -2.17 0.821 9. ru(oh)2 2+ + 2h + + cl + e = rucl 2+ + 2h2o -2.17 < log[cl ] < -1.57 0.950 10. ru(oh)2 2+ + 2h + + 2cl + e = rucl + + 2h2o -1.57 < log[cl ] < -1.39 1.043 11. ru(oh)2cl + + 2h + + cl + e = rucl2 + + 2h2o -1.39 < log[cl ] < -0.54 0.961 12. ru(oh)2cl + + 2h + + 2cl + e = rucl3 + 2h2o -0.54 < log[cl ] < -0.44 0.993 13. ru(oh)2cl4 2+ + 2h + + e = rucl3 + cl + 2h2o -0.44 < log[cl ] < 0.15 0.915 14. ru(oh)2cl4 2+ + 2h + + e = rucl4 + 2h2o 0.15 < log[cl ] < 0.30 0.906 15. ru(oh)2cl4 2+ + 2h + + cl + e = rucl5 2 + 2h2o 0.30 < log[cl ] < 0.40 0.888 16. ru(oh)2cl4 2+ + 2h + + 2cl + e = rucl6 3 + 2h2o 0.40 < log[cl ] < 0.50 0.865 ru(viii) – ru(iv) 17. h2ruo5 + 6h + + 4e = ru(oh)2 2+ + 3h2o -2.50 < log[cl ] < -1.39 1.405 18. h2ruo5 + 6h + + cl + 4e = ru(oh)2cl + + 3h2o -1.39 < log[cl ] < -0.44 1.425 19. h2ruo5 + 6h + + 4cl + 4e = ru(oh)2cl4 2 + 3h2o -0.44 < log[cl ] < 0.50 1.445 1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 -0,2 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 ru(oh) 3 . h 2 o (am) ru ru(oh) 2 + ru 5+ ruo 4 2-ruo2 . 2h 2 o r u 4 (o h ) 12 4+ ru(oh) 2 2+ ru 2 o 5(am) ruo 4 hruo 5 -h 2 ruo 5 e , v ph 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 e / v i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 145-153 doi:10.5599/jese.229 151 finally, the electrode potential depending on the cl concentration is calculated: ru(iii) – ru(0) (= (rt / 3f) ln10) e1 = e1 0 +  logcru 0 log[cl ] < -2.17 e2 = e2 0 +  logcru 0  log[cl ] -2.17 < log[cl ] < -1.57 e3 = e3 0 +  logcru 0 2 log[cl ] -1.57 < log[cl ] < -0.54 e4 = e4 0 +  logcru 0 3 log[cl ] -0.54 < log[cl ] < 0.15 e5 = e5 0 +  logcru 0 4 log[cl ] 0.15 < log[cl ] < 0.30 e6 = e6 0 +  logcru 0 5 log[cl ] 0.30 < log[cl ] < 0.40 e7 = e7 0 +  logcru 0 6 log[cl ] 0.40 < log[cl ] < 0.50 ru(iv) – ru(iii) (= (rt / f) ln10) e8 = e8 0 + 2 log [h + ] log[cl ] < -2.17 e9 = e9 0 + 2 log [h + ] +  log[cl ] -2.17 < log[cl ] < -1.57 e10 = e10 0 + 2 log [h + ] + 2 log[cl ] -1.57 < log[cl ] < -1.39 e11 = e11 0 + 2 log [h + ] +  log[cl ] -1.39 < log[cl ] < -0.54 e12 = e12 0 + 2 log [h + ] + 2 log[cl ] -0.54 < log[cl ] < -0.44 e13 = e13 0 + 2 log [h + ]  log[cl ] -0.44 < log[cl ] < 0.15 e14 = e14 0 + 2 log [h + ] 0.15 < log[cl ] < 0.30 e15 = e15 0 + 2 log [h + ] +  log[cl ] 0.30 < log[cl ] < 0.40 e16 = e16 0 + 2 log [h + ] + 2 log[cl ] 0.40 < log[cl ] < 0.50 ru(viii) – ru(iv) (= (rt / 4f) ln10) e17 = e17 0 + 6 log [h + ] -2.50 < log[cl ] < -1.39 e18 = e18 0 + 6 log [h + ] +  log[cl ] -1.39 < log[cl ] < -0.44 e19 = e19 0 + 6 log [h + ] + 4 log[cl ] -0.44 < log[cl ] < -0.50 these functions along with predominance areas of the species in solution are diagrammatically shown in fig. 3 as the potential – log [l] diagram. compared with diagrams e ph, on the e (log [l]) diagram a considerable number of chemical species of ruthenium(iv) is outside of the thermodynamic stability area of water (the dotted line a). within the entire range of the [cl ] values, -2.20 < log[cl ] < 0.50, the valence states of ruthenium ru(ii), ru(v), ru(vi) and ru(vii) are unstable with respect to dismutation processes. j. electrochem. sci. eng. 6(1) (2016) 145-153 ruthenium redox equilibria: 3. pourbaix diagrams 152 figure 3. the potential log[l] diagram for the system ru-cl -h2o, ph 0, cru0 = 10 -6 mol/l. conclusions 1. on the basis of the thermodynamic data, the area of thermodynamic stability of ru chemical species as a function of ph (or pcl) for each degree of oxidation has been determined. 2. based on the gr = f(n)diagrams calculated for several values of ph, a narrow ph value is determined, in which the dismutation of appropriate form takes place. based on thermodynamic analysis, the exact phd value of the beginning of disproportionation (or comproportionation) is found. after that, the diagrams of heterogeneous chemical equilibria, developed by us earlier, as a function of ph and the total concentration of metal ion in solution, are built. 3. it is derived a system of electrochemical equations of electrode reactions between chemical species in different degrees of oxidation, the predominance areas of which are overlapped. finally, the dependencegr or e on ph (or pcl) is calculated for different redox pairs. depending on ph and pe, as well as the total concentration of inorganic ligands, the ru compounds may undergo various transformations to produce a whole range of chemical forms in solution. the potential—ph and potential-pcl diagrams of the ru-h2o and ru-cl -h2o systems have been constructed. the calculated pourbaix diagrams within our approach agree well with the previously reported experimental data. references: [1] m. pourbaix, atlas of electrochemical equilibria in aqueous solutions, nace cebelcor, huston, 1974, 644 p. [2] i. fishtik, environmental science & technology 40 (2006) 1902-1910. [3] a. minguzzi, f. r. f. fan, a. vertova, s. rondinini, a. bard, chemical science 3 (2012) 217229. [4] j. angus, b. lu, m. zappia, journal of appied electrochemistry (1987) 1-21. [5] m. pourbaix, lectures on electrochemical corrosion. springer science & business media, new york, usa, 2012, 336 p. [6] d. silverman, in uhlig's corrosion handbook, r.w. revie, john wiley & sons, new jercey, usa, 2011, pp. 1129-1166. 1 -2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4 2 lg [cl ] 3 -2,0 -1,6 -1,2 -0,8 -0,4 0,0 0,4 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 r u c l 6 3 r u c l 5 2 r u c l 2 + h 2 ruo 5 ru(oh) 2 cl 4 2ru(oh) 2 cl ru(oh) 2 2+ rucl 3rucl 2 + rucl 2+ r u 3 + ru e , v lg[cl ] 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 e / v lg ([cl ] / mol l -1 ) i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 145-153 doi:10.5599/jese.229 153 [7] s. yagi, h. nakanishi, t. ichitsubo, e. matsubara, journal of the electrochemical society 156 (2009) d321-d325. [8] g. abady, n. h. hilal, m. el-rabie, w.a. badawy, electrochimica acta 55 (2010) 6651-6658. [9] o. spinu fifth regional symposium on electrochemistry – south east europe (rsesee), thermodynamic approach for calculating potential ph diagrams in the system cr – natural waters, pravets, bulgaria, 2015, p. 152 [10] r. e. smallman, modern physical metallurgy, elsevier, amsterdam, the netherlands, 2013, p. 544 [11] t. nishimura, k. toshiaki, corrosion science 45 (2003) 1073-1084. [12] e. kim, o. kwadwo, hydrometallurgy 127 (2012) 178-186. [13] r. salhi, iranian journal of chemistry and chemical engineering 24 (2005) 29-39. [14] d. harris, quantitative chemical analysis, w.h. freeman and company, new york, usa, 2010, p. 719. [15] i. fishtik, environmental science & technology 40 (2006) 1902-1910. [16] i. fistic, i. povar, , i. vatamanu, t. spataru, bulletin of academy of sciences of the mssr, series of biological and chemical sciences 4 (1987) 53-58. [in russian] [17] i. povar, o. spînu, journal of electrochemical science and engineering (2016) doi: 10.5599/jese.226 [18] i. povar, o. spînu, journal of electrochemical science and engineering 2016) 10.5599/jese.228 [19] j. a. rard, chem. reviews 85 (1985) 1-39. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ corrosion behavior of ni content hypoeutectic al-si alloy doi:10.5599/jese.244 47 j. electrochem. sci. eng. 6(1) (2016) 47-55; doi: 10.5599/jese.244 open access : : issn 1847-9286 www.jese-online.org original scientific paper iron and manganese removal from drinking water daniela-elena pascu* ; , mihaela neagu (pascu)* , **, gina alina traistaru***, aurelia cristina nechifor*, alexandra raluca miron* *faculty of applied chemistry and materials sciences, politehnica university of bucharest, 1-7 gheorghe polizu street, bucharest, 011061, romania **s. c. hofigal s.a., analytical research department, 2 intr. serelor, bucharest, 042124, romania ***s. c. eneco consulting s.r.l, sos. pantelimon, no. 247, sector 2, bucharest, romania corresponding author: dpascu@yahoo.com; tel.: +40-21-402-39-27; fax: +40-21-402-39-34 received: october 19, 2015; accepted: november 23, 2015 abstract the purpose of the present study is to find a suitable method for removal of iron and manganese from ground water, considering both local economical and environmental aspects. ground water is a highly important source of drinking water in romania. ground water is naturally pure from bacteria at a 25 m depth or more. however, solved metals may occur and if the levels are too high, the water is not drinkable. different processes, such as electrochemical and combined electrochemical-adsorption methods have been applied to determine metals content in accordance to reports of national water agency from romania (anar). every water source contains dissolved or particulate compounds. the concentrations of these compounds can affect health, productivity, compliance requirements, or serviceability and cannot be economically removed by conventional filtration means. in this study, we made a comparison between the electrochemical and adsorption methods (using membranes). both methods have been used to evaluate the efficiency of iron and manganese removal at various times and temperatures. we used two membrane types: composite and cellulose, respectively. different approaches, including lowering the initial current density and increasing the initial ph were applied. reaction kinetics was achieved using mathematical models: jura and temkin. keywords ground water, solved metals, mathematical models, composite membranes, different methods introduction electrochemical treatment is an emerging technology used for the removal of organic and inorganic impurities from water and wastewater. electrochemically processes involve redox reactions, where oxidation and reduction reactions are separated in space or time [1,2]. usually, the electrochemically treatment of water is concerned with electron transfer at the http://www.jese-online.org/ mailto:dpascu@yahoo.com j. electrochem. sci. eng. 6(1) (2016) 47-55 iron and manganese removal 48 solution/electrode interface applying an external direct current in order to drive an electrochemical process [1,3]. electrocoagulation is an electrochemical result of destabilization agents (usually mn or fe ions) that neutralize the electrical charge of suspended pollutant. electrochemically generated metallic ions from these electrodes could undergo hydrolysis near the anode to produce a series of activated intermediates that are able to destabilize finely dispersed particles present in the water/wastewater to be treated. electrochemical treatment methods have a future as advanced technologies for additional treatment of potable water from domestically and remote areas. filter media (type, size and area), hydraulic and solids loading rate and backwashing regimes are all important aspects of filter design. autocatalytic removal of manganese can take place in a filter and could be critical for manganese removal. an investment in filter pilot testing could become significant. the sorption of metal ions from aqueous solution plays an important role in water pollution control and in recent years there has been considerable interest in the use of low cost adsorbents. many researchers have tried to exploit naturally, occurring materials as low-cost adsorbents, for removing of heavy metals. manganese and iron (especially the last) produce different problems that could be due to various causes [4]. many types of treatment are effective for the removal of iron and manganese from water, but not all methods are equally effective under any conditions. oxidation of dissolved iron particles in water, change the iron to white, then yellow and finally to red-brown solid particles (precipitates) that settle out from the water. iron that does not form particles large enough to settle out and that remains suspended (colloidal iron) leaves the water with a red tint. manganese is usually dissolved in water, although that some shallow wells contain colloidal manganese, leaving the water with a black tint. these sediments are responsible for the staining properties of water containing high concentrations of iron and manganese [4]. iron and manganese are common in groundwater supplies used by many small water systems. exceeding the suggested maximum contaminant levels (mcl) usually results in discolored water, laundry, and plumbing fixtures. this, in turn, results in consumer complaints and a general dissatisfaction with the water utility. there are secondary standards set for iron and manganese, but these are not health related and are not enforceable. anar establishedthe following limits (mcl): iron at 0.30 mg/l and manganese at 0.05 mg/l. the purpose of the present study is to find a suitable method for removal of iron and manganese from drinking water. experimental materials iron nitrate fe(no3)3 was used as a source of iron in form of fe (iii) and it was supplied by sigma aldrich. manganese (ii) nitrate mn(no3)2 was used as sources of manganese in form of mn (ii) and it was supplied by sigma aldrich. pure kallium chloride (kcl), purchased from merck, was used as electrolyte. distilled water was used throughout. analar sulfuric acid 98 % was purchased from chimexin. http://en.wikipedia.org/wiki/manganese http://en.wikipedia.org/wiki/nitrate d.-e. pascu at al. j. electrochem. sci. eng. 6(1) (2016) 47-55 doi:10.5599/jese.244 49 method aas for iron and manganese a laborator combined photo-electrochemical unit was used for the batch experiments. it consists of a cylindrical quartz photo reactor (1 l), with a coaxial and immersed medium pressure uv mercury lamp used as the uv emitter and light source (heraeus tq150, input energy of 150 w) emitting a polychromatic radiation in the range from 100 to 280 nm wavelength. the uv lamp was equipped with a cooling water jacket to maintain the temperature of the reaction of wastewater treatment at room temperature. the reaction vessel was filled with solution containing both iron and manganese. the electrochemical characterization of the solution was carried out by using dc power supply gw 3030 and two electrodes: graphite cathode and platinum anode. the measurements were performed in the temperature range: 288 k to 303 k, and the mixing was accomplished by using continuous magnetic stirrer. photo-electrochemical method was combined with electrocoagulation in the same unit. metal hydroxides generated during electrocoagulation were used to remove iron and manganesse from aqueous solution, and the effects of varying the current density and solution temperature of iron and manganesse adsorption characteristics were evaluated. the findings indicated that complete iron and manganesse removal could be achieved within reasonable removal efficiency and with relatively low electrical energy consumption [5,6]. the experimental data have been fitted with jura and temkin adsorption isotherm models to describe the electrocoagulation process. the adsorption of iron preferably fitted the jura adsorption isotherm and manganesse preferably fitted the temkin adsorption isotherm, and these suggested monolayer coverage of adsorbed molecules. definite amounts of kcl were added to improve the conductivity and ionic mobility through the electrolyte. low concentration of kcl (45 mg/l) was added to increase the conductivity and electric current beside its bactericide effect after electrolysis to chlorine. the solution was acidified to ph 3 by drops added of prepared dilute sulfuric acid 15 %. the effect of fe 2+ and mn 2+ concentrations revealed that the higher concentration of dissolved iron and manganese ions, the higher removal efficiency was obtained. the efficiency of the process was evaluated by measuring the metal removal from samples at the end of each experiment. samples were filtered with cellulose and composite membrane, before the measurement of metals, by applying atomic adsorption (carl zeiss jena aas). for all experiments there have been used a synthetic solutions of iron and manganesse with a concentration range between 1-12 ppm (fig. 1). time, min figure 1. iron and manganase concentratin evolution in time in the case photo-electrochemical combined with electrocoagulation method r e m o va l e ff ic ie n cy , % j. electrochem. sci. eng. 6(1) (2016) 47-55 iron and manganese removal 50 results and discussion the photo-electrochemical method combined with electrocoagulation method presents good results for removing iron ions from 12 ppm prepared synthetic solutions. as it could be seen from figure 2, the removal efficiency in the case of iron was about 46 %, while in the case of manganese it was 55 %. time, min figure 2. effect of different current densities on the removal efficiency of iron and manganese (c0 = 12 ppm, t = 298 k, ckcl = 45 mg/l) according to the applied electric current, the removal efficiency varied, while at low applied electric current, a low value of efficiency removal, % was obtained, but the removal efficiency in time was improved. the equilibrium of removal was achieved after 15 minute, from the processes' beginning. current dansity, ma figure 3. effect of current density on iron and manganese removal. the optimum current density and temperature have been establisheda at 3.2 ma cm −2 and 288 k, for iron and 3.4 ma cm −2 and 298 k for manganesse, respectively. another method to remove iron and manganese from drinking water was the adsorption method using two membranes (cellulose and composite respectively). both methods have been used to evaluate the efficiency of removing iron and manganese from waste waters at different times and temperatures [7,8]. r e m o v a l e ff ic ie n cy , % r e m o v a l e ff ic ie n cy , % d.-e. pascu at al. j. electrochem. sci. eng. 6(1) (2016) 47-55 doi:10.5599/jese.244 51 the experiments showed the feasibility of removing iron and manganese by adsorption and coprecipitation with aluminum hydroxides. photo-electrochemical method was combined with electrocoagulation in the same unit and were used for oxidation of soluble forms fe 2+ and mn 2+ to the insoluble forms fe 3+ and mn 4+ . the combined method revealed better efficiency compared with a single electrochemical method. the presence of both dissolved iron and manganese has the advantage of less resistivity of the waste water solution. low concentration of kcl (45 mg/l) was added to increase the conductivity and electric current. higher removal efficiency was obtained when fe 2+ and mn 2+ are presented in higher concentration (12 ppm). the study showed a more rapid oxidation of fe 2+ than of mn 2+ due to the lower oxidation potential of iron ion than of the manganese ion [9,10]. a time, min b time, min figure 4. iron and manganese removal efficiency evolution in time when using cellulose membrane (a) and composite membrane (b) composite membranes presented a better adsorption behavior for manganese ion (10.75 ppm) compared with the cellulose membranes (8.78 ppm). for iron ion the composite membranes' adsorption behavior (9.57 ppm) was better compared with the cellulose membranes (7.89 ppm) as they are presented in figure 4. the solution ph is an important parameter which controls the adsorption process. it influences the ionization of the adsorptive molecule and hence the adsorbent's surface charge. therefore, investigating the ph effect on the adsorption is essential in adsorption experiments. in this particular case, the solution ph can change the surface charge of the adsorbent as well as different iron and manganese ionic formsions. r e m o va l e ff ic ie n cy , % r e m o va l e ff ic ie n cy , % j. electrochem. sci. eng. 6(1) (2016) 47-55 iron and manganese removal 52 model temkin energy adsorption and ion interactions between aqueous solutions and membrane (adsorbentadsorbed) were studied by temkin-pzyhev model [10-13]. the temkin assumed that due to such interactions, the adsorption energy of all layer molecules decreases with surface coating. this pattern was obtained in view of adsorption phenomena and the interaction between adsorbed substance and was achieved by designing the chart functions: qe versus ln ce. the temkin isotherm considers the interaction between the aqueous solution and solid (composite or cellulose membrane) that contained the free energy adsorption as a function of coating the surface of the adsorbent material. the equation underlying the temkin model adsorption heat is: qe = b ln kt + b ln ce (1) the linearized form of the temkin’equation is: e t t t e ln ln rt rt q b k b c   (2) where t b is the temkin constant sorption heat, j/mg and the corresponding maximum energy constant connection between adsorbent and adsorbed is t k / (l/g). the isotherms of this model are shown in figure 5. ln (ce / ppm) ln (ce / ppm) figure 5.temkin adsorption isotherms of maganese and iron ions: cellulose membrane (a) and composite membrane (b) q e / ( m g /g ) q e / ( m g /g ) a b d.-e. pascu at al. j. electrochem. sci. eng. 6(1) (2016) 47-55 doi:10.5599/jese.244 53 b and kt are temkin equation parameters and respectively adequate with adsorption condescended and boundary constant adequate with maximum of boundary energy. amount of b is rt/bt and bt is based on temkin isotherm constant. by observing that we can see in figure 5, the maximum amount adsorption of kt for two ions: manganese and iron, these with be much better uptake by the composite membranes. taking into consideration the correlation coefficient for the surves presented in figures (5,6), we observe that there is an accessible competition between this model and harkins-jura model. harkins-jura adsorption isotherm the harkins-jura adsorption isotherm could be expressed as [14, 15]: e2 e 1 1 log b c q a a   (3) where: qe is the adsorbed ions amount at equilibrium (mg/g) and ce is the concentration at equilibrium for two ions (ppm). harkins-jura model is presented in figure 6, and from the diagram log ce against 2 e 1 / q , considering a as parameter slope and from intercept , we could compute b parameter. a log (ce / (mg/g)) b log (ce / (mg/g)) figure 6. harkins-jura adsorption isotherms of maganese and iron ions: cellulose membrane (a) and composite membrane (b) (1 / q e 2 ) / (m g /g )2 1.0 0.8 0.6 0.4 0.2 0.0 (1 / q e 2 ) / (m g /g )2 1.0 0.8 0.6 0.4 0.2 0.0 j. electrochem. sci. eng. 6(1) (2016) 47-55 iron and manganese removal 54 in this paper, the experimental adsorption data were tested applying the temkin and harkinsjura equations. it was observed that the correlation coefficient has a good correlation with experimental data. in figure 6 the specific amount capacity adsorption is in mg/g and equilibrium concentration in ppm. the validity of the harkins-jura solute adsorption isotherm to systems, it could be used for the determination of the specific surface area of solids (composite and cellulose membrane). all the plots contain two intersecting straight lines (for these two metals: iron and maganese) and according to the harkins-jura solute adsorption equation. the harkins-jura equation applies to these systems for the entire concentration range studied. the existence of two or more intersecting straight lines in the harkins-jura plot indicates that there are two, or many isotherms corresponding to each of these lines with different values for the constants a and b. as it could be seen from figure 6, in the case of jura isotherm representation there are two linear parts derivated from model equation. the difference between the surface tensions caused by the adsorbent material and aqueous solutions is a linear function of the molecule and therefore the area is indicated using the isotherm harkins-jura. it argues that the two straight lines correspond for two different orientations of the adsorbent (composite or cellulose membrane) in the process of adsorption and high slope corresponding to a plane in surface and the slope shows lower values corresponding to a vertical orientation, for the two metals adsorption. this point of view was supported by the research of soriaga et al. [13] work, where, using thin layer electrochemical techniques, it has been shown that admolecules assume a parallel orientation to the solid surface when adsorbed from solutions. therefore, we can see that a straight line with a higher slope in the harkins-jura model corresponds to a flat orientation adsorption process that changes from a vertical orientation as this initial concentrations becames greater. as consequence, by making this orienting change of the adsorption process, a new phase it would be obtained, which presents a new point of intersection between the lines of the model representation of the two ions by their adsorption through the two membranes. thus are obtained very good values for the adsorption capacity of the two membranes. the temkin and harkin-jura models are often used to describe the equilibrium sorption isotherms. conclusion ph is an important parameter influencing heavy metal adsorption from aqueous solutions. it has no influence on the adsorbent surface charge, the degree of ionization of material present in the solution, and also the dissociation of functional groups on the active sites of the adsorbent. the method applying the composite and cellulose membranes adsorption presented the best results compared to photo-electrochemical method for removing iron and manganese ions from drinking water. the adsorption membrane method has the advantage of simplicity in terms of installation compared to photo-electrochemical method. the disadvantage is the high cost of composite and cellulose membranes. considering the high consumption of electricity which is used for photo-electrochemical method, the adsorption membrane method has the advantage of low power consumption. d.-e. pascu at al. j. electrochem. sci. eng. 6(1) (2016) 47-55 doi:10.5599/jese.244 55 in addition, the adsorption kinetic studies showed that the electrocoagulation process was best described using the pseudo second-order kinetic model [16] at the various current densities and temperatures. acknowledgements: the work has been funded by the sectorial operational programme human resources development 2007-2013 of the ministry of european funds through the financial agreement posdru/159/1.5/s/132395, posdru/159/1.5/s/ 134398. faculty of applied chemistry and materials sciences, politehnica university of bucharest, support is also gratefully acknowledged. references [1] w .l. chou, c. t. wang, k. y. huang, t. c. liu, desalination 271 (2011) 55-61. [2] m. malakootian, n.yousefi, j. environ. health. sci. 6 (2009) 131-136. [3] e. totu, e. ruse, c. luca, rev. roumaine chim. 51(5)(2000) 331-336. [4] m. zaw, b. chiswell, water res. 33 (1999) 1900-1910. [5] h. d. doan, m. saidi, journal of hazardous materials 151 (2008) 306-315. [6] american water works association. 1998. water treatment plant design. third edition. [7] american water works association. 1990. water quality 2nd treatment. fourth edition. [8] metcalf, b. m. 1998. “pressure filtration for iron & manganese removal,” proceedings of the new england water works association conference and exhibition. marlborough, ma. [9] national research council. 1997. safe water from every tap: improving water service to small communities. national academy press. washington dc. [10] y. s. ho, g. mckay, trans icheme 76 (1998) 332-340. [11] m. h. jnr, a. i. spiff, acta chim. slov. 52 (2005) 174–181. [12] a. o. dada, a. p. olalekan, a. m. olatunya, o. dada, iosr journal of applied chemistry (iosr-jac) 3 (2012) 38-45. [13] gh. nechifor, d.-e. pascu, m. pascu (neagu), g. a. traistaru, p. c. albu u.p.b. sci. bull. 77 (2015) 63-72. [14] s. shanavas, a. s. kunju, h. varghese, c. y. panicker, oriental journal of chemistry 27(1) (2011) 245-252. [15] m. vadi, m. abbasi, m. zakeri, b. j. yazdi, j. phys. theor. chem. iau iran 7(2) (2010) 95-104. [16] e. totu, a. k. covington, e. segal, j. thermal. anal. 52 (1998) 383-391. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ electrochemical surface modification technique to impede mild steel corrosion using perfluorooctanoic acid doi:10.5599/jese.211 247 j. electrochem. sci. eng. 5(4) (2015) 247-259; doi: 10.5599/jese.211 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical surface modification technique to impede mild steel corrosion using perfluorooctanoic acid shubha h. nataraj, venkatesha t. venkatarangaiah, anantha n. subba rao department of studies in chemistry, school of chemical sciences, kuvempu university, shankaraghatta-577451, karnataka, india corresponding author: drtvvenkatesha@yahoo.co.uk, tel:+91-9448855079; fax: +91-08282-256255 received: august 17, 2015; revised: january 8, 2016; accepted: february 8, 2016 abstract the present work demonstrated that corrosion inhibition efficiency of electrochemically generated organic coat is remarkably more effective than self-assembled monolayer (sam) generated by dip coating technique. perfluorooctanoic acid (pfoa) is used to modify mild steel surface for effective protection. infrared reflection absorption spectroscopy and contact angle measurements substantiate the modification of mild steel surface and its effect on surface hydrophobicity. a comparison between electrochemical properties of pfoa sam generated by dip coat method (dc-pfoa) and pfoa coat generated by electrochemical method (ec-pfoa) is presented. electrochemical measurements reveal that the corrosion protection efficiency of ecpfoa (91 %) is much superior to dc-pfoa (28 %). keywords self-assembled monolayer; mild steel; contact angle; irras; corrosion; electrochemical coating. introduction mild steel is most frequently used engineering metallic material owing to its excellent mechanical material property, availability and relative low cost [1]. the corrosion of mild steel is a universal problem and poses major economical and safety threats. protective coatings [2] and inhibitors [3,4] are the most commonly used method of corrosion control. blocking anodic and cathodic sites on the surface is a smart approach to impede the corrosion process. this can be achieved by modifying the metal surface at molecular level using organic molecules to produce a protective layer to avoid the invasion of corrosive agents [5-7]. the self-assembled monolayer (sam) technique has gained a lot of attention in this regard, because it imparts hydrophobicity to the surface. the sam of organic molecule is generated by the http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 5(4) (2015) 247-259 surface modification to impede mild steel corrosion acid 248 immersion of substrate in a solution containing adsorbate molecule (dip coating). here, the molecular assembly takes place without the application of any external force i.e., pressure [8]. to generate sam, organic molecule should contain one hydrophilic head group and hydrophobic tail group, where the hydrophilic group attach to the surface and hydrophobic group extend towards the space. thus it forms barrier film over substrate and controls the diffusion of corrosive agents [9]. k. nozawa et. al. modified iron surfaces by alkanethiole monolayer and studied its corrosion property [10]. paul e. hintze et. al. studied corrosion behavior of modified aluminium surface by organosilane sam [11]. in an earlier paper we reported that sodium oleate sam protects mild steel from corrosion in saline water [12]. pfoa is a synthetic anionic surfactant with hydrophilic acid head group and hydrophobic as well as lipophobic fluorinated carbon chain. pfoa has excellent chemical stability, thermal stability, strong water repulsive and oil repulsive properties. hence, it is used in the leather stuffs as water proof repellents, as insulators in electric wires, in hydraulic liquids for aircrafts, furniture’s, aqueous film forming foam and textile industries [13]. loos et. al. suggest that the electrostatic interaction plays an important role in the adsorption of perfluorocarbon (pfc) on minerals [14]. wang et. al. studied the adsorption behavior of pfoa and reported that adsorption of pfoa decreases with increase in ph [15]. xiaodong gao et. al. studied molecular mechanism of pfoa adsorption at iron oxide surface [16]. we utilize this background knowledge to generate sam of pfoa on mild steel surface by dip coating (dc-pfoa) and in a new approach; it is intended to generate an organic coat of pfoa by electrochemical method (ec-pfoa) at room temperature. interestingly, the ec-pfoa showed remarkable enhancement in protection efficiency as compared to dc-pfoa. the probable reason for this enhancement in the corrosion protection efficiency of ec-pfoa has been discussed in this article. experimental materials and sample preparation mild steel (chemical composition-0.04 % c, 0.35 % mn, 0.022 % p, 0.036 % s, and remaining iron) coupons of dimension 1 cm 2 area were used as substrates for surface modification tests and for electrochemical studies. the pfoa (sigma aldrich) was used as received. the corrosive media 3.5 % nacl was prepared in milli-pore water (elix 3 milli-pore system). mild steel samples were abraded mechanically by using different grades of emery paper (220, 600, 1000, 2000, 2500) and further sonicated with acetone and alcohol for 15 min to remove all debris followed by sonication in milli-pore water for 20 min. these samples were flushed with a stream of dry nitrogen gas and were kept in an ultraviolet cleaning chamber (bioforce, nanosciences, u.s.a.) for 30 min to burn all carbonaceous contaminants. prior to modification process, unpolished samples (without cloth polishing) were washed with milli-pore water followed by acetone and dried. two different techniques were used to generate pfoa coating on mild steel; 1. dip coating (dc-pfoa); the steel samples were dipped in aqueous solution of 2 mm pfoa for 18 hours at room temperature. then taken out, rinsed and washed with ethanol followed by n-hexane and dried. 2. electrochemical coating (ec-pfoa); the steel sample was dipped in pfoa solution of different concentrations (1, 2, 4 and 6 mm aqueous solution) and anodized at 0.2 v vs. ag/agcl using chi660c electrochemical workstation (u.s.a.) for different time intervals s. h. nataraj et al. j. electrochem. sci. eng. 5(4) (2015) 247-259 doi:10.5599/jese.211 249 (5, 15, 30 and 60 min). the samples are taken out, rinsed and washed with ethanol followed by n-hexane and dried. these samples were then subjected to corrosion studies in 3.5 % nacl media. infrared reflection absorption spectroscopy (irras) analysis and contact angle measurement irra spectra were obtained at room temperature using gx spectrometer (perkin elmer, usa), equipped with a liquid nitrogen cooled mercury cadmium telluride (mct) detector. before starting the experiments the sample and detector chambers were purged with nitrogen gas to eliminate moisture content. all spectra were acquired at 4 cm -1 resolution and 1024 scans. a heating accessory (harrick scientific corporation, new york, usa) was used for taking the spectra with an incident angle of 75° from the surface normal. the spectral analysis was carried out by spectrum 3.02 version software (perkin-elmer, u.s.a.). goniometer (oca 30, data physics instruments) was used to measure the water contact angle on steel at room temperature. 2 μl of millipore water was used to measure the angle by sessile drop method. images of the air/liquid/solid (a/l/s) systems were captured and processed using the 32-bit sca 20 software. electrochemical studies electrochemical studies were carried out using chi660c electrochemical work station at room temperature. the steel samples of 1 cm 2 area, platinum wire and ag/agcl were used as working, counter and reference electrodes, respectively. the samples were immersed in 3.5 % nacl for 400 seconds prior to electrochemical measurements in order to establish open circuit potential (ocp). the electrochemical impedance spectroscopy (eis) measurements were carried out at ocp in the frequency range of 100 khz to 10 mhz with 5mv sinusoidal excitation signal. eis data were analyzed using zsimpwin-3.21 software. the polarization measurements were carried out at a scan rate of 1 mv s -1 . surface morphology the surface morphology of the bare steel, coated steel and corroded steel was investigated using fesem quanta 250 scanning electron microscopy (sem). the sem images of corroded samples were taken after immersion of coated sample in 3.5 % nacl for 6 h at room temperature. after the immersion, the samples were rinsed with distillated water and dried before imaging the samples by sem. secondary electron detector was used to obtain sem micrographs. elemental composition of the modified surface was performed by using energy dispersive x-ray spectroscopy (eds). results and discussion characterization of coating irras analysis the irras of adsorbed pfoa molecule on mild steel by dip coating method and electrochemical coating at room temperature are shown in figure 1. peak position and mode assignments are given in table 1. the presence of coo , c-c and c-f stretching modes in both the spectra confirms the adsorbed pfoa on steel surface [16]. the asymmetric stretch of deprotonated carboxylate head group [νas (coo )] is observed at 1683 cm -1 in both cases, whereas the νs (coo ) is shifted to slightly higher wavenumber (1437 cm -1 ) in ec-pfoa. for the ec-pfoa, there is a shift in adsorption band of νax (cf2) by 24 cm -1 compared to dc-pfoa in the direction of lower wavenumber. despite j. electrochem. sci. eng. 5(4) (2015) 247-259 surface modification to impede mild steel corrosion acid 250 the similarities between two spectra, the c-c stretching peaks are strong (1104, 1023 cm -1 ) for ecpfoa which can be attributed to the compactness of the film. figure 1. irra spectra of pfoa on steel by (a) dip coating method (b) electrochemical coating method. table 1. irras vibrational frequencies of pfoa deposited on mild steel substrate. wavenumber, cm -1 mode assignment dc-pfoa ec-pfoa νax(cf2) 1342 1366 νas(cf2) 1233 1239 νs(cf2) 1149 1146 νas (cf2) + νas (cf3) 1208 1205 νas(coo ) 1683 1683 νs(coo ) 1431 1437 ν(c-c) 1097 1104 ν(c-c) 1012 1023 s. h. nataraj et al. j. electrochem. sci. eng. 5(4) (2015) 247-259 doi:10.5599/jese.211 251 contact angle measurement contact angle measurements are the simple and effective technique to quantify surface wetting properties. the hydrophilic, hydrophobic and super hydrophobic character of the surface can be evaluated with the help of contact angle developed between three phases; liquid, solid and gas [17]. figure 2 shows the water contact angle (water) on modified steel surface by (a) dc-pfoa (b) ec-pfoa at room temperature. the water on dc-pfoa is 128° and for ec-pfoa is 135°, which illustrates hydrophobic nature of the modified surface. the acid head group of pfoa anchors to the metal surface by electrostatic interaction and the non-polar perfluorocarbon chain protrudes towards the space, stabilized by lateral van der waals force. the interaction between nonpolar tail group of pfoa and polar water molecule will be less, which leads higher contact angle [12,18]. the increase in the water contact angle for ec-pfoa by 7° can be attributed to the compactness of the film which is a result of electrochemically driven oriented growth. figure 2. water contact angle on (a) dc-pfoa (b) ec-pfoa at room temperature. the water for sodium oleate sam on mild steel was 113° [12], where the hydrocarbon chain has 18 carbon atoms. however, pfoa with only 8 carbon atoms exhibited higher contact angle. this may be due to the different structure of carbon chain. the hydrocarbon chain has zigzag c-c-c structure whereas perfluorocarbon has helical structure, which imparts rigidity to the molecule. this is because fluorine van der waals radii (0.147 nm) which is more than hydrogen (0.12 nm) leads to 12-14° twist of the f-c-f bond about the carbon backbone causes geometric and stearic hindrance [13]. electrochemical analysis electrochemical impedance analysis the eis helps to quantitatively determine the coating properties over a wide frequency range. moreover, it is a non-destructive and efficient tool to evaluate the corrosion behavior of organic coatings [19]. the impedance data were measured in 3.5 % nacl solution and represented in the form of nyquist plots. to analyze this acquired impedance data the electrical equivalent circuit (eec) model given in figure 3 has been used in all cases. it consists of solution resistance (rs), coating resistance (rcoat), charge transfer resistance (rct), coating capacitance (qcoat) and double j. electrochem. sci. eng. 5(4) (2015) 247-259 surface modification to impede mild steel corrosion acid 252 layer capacitance (qdl). the constant phase element (cpe) is replaced for the capacitor to fit the nyquist loops more precisely. the impedance of the cpe is defined as zjω = q −1 (jω) −n where, j is the imaginary unit, ω is the angular frequency (ω = 2πf, f is the frequency), q is the cpe constant and n is the cpe exponent (−1 ≤ n ≤ 1) where q and n are frequency independent parameters. cpe defines an ideal capacitor for n = 1, an ideal inductor for n = -1 and an ideal resistor for n =0 [20]. the decrease in the n value signifies that the non-uniform distribution of current arises from surface roughness and surface defects [21]. figure 3. electrical equivalent circuit used to fit the impedance measurements. figure 4 represents the nyquist plots recorded for coated sample (dc-pfoa, ec-pfoa) immersed in 3.5 % nacl solution at room temperature and the corrosion parameters calculated from the impedance measurement are tabulated in table 2. the figure 4a represents impedance measurement of bare and dc-pfoa immersed in 3.5 % nacl solution at room temperature. both (bare and dc-pfoa) the nyquist loops are similar with respect to their shape but they vary slightly in their size and this observation reveals that the same course of action taking place with different magnitude at the metal/electrolyte interface. each curve contains two semicircles one at higher and other at lower frequency end, indicating two relaxation processes is involved in the mechanism [22]. the electrochemical parameters tabulated in the table 2 clears that the double layer capacitance decreases for dc-pfoa compare to bare sample. the slight increase in charge transfer resistance for dc-pfoa as compared to bare steel, infers there is no significant corrosion protection. the charge transfer resistance is proportional to exposed area by following equation [23] ae = r 0 ct / rct where ae is the exposed area, r 0 ct is the charge transfer resistance for bare steel and rct is the charge transfer resistance for dc-pfoa. the calculated exposed area from above equation is 71 %, this is due to the formation of weak or discontinuous sam. the reason for insignificant protection from dc-pfoa can be explained as the following: the ph of aqueous pfoa solution is 4.5. the surface of steel has zero charge at ph 5.2 to 6.7, below this ph (<5.2) surface gets positive charge [24]. though the surface has positive charge below isoelectric point, at ph 4.5 it has feeble positive charge which doesn’t facilitate effective adsorption. hence, there is week adsorption between negatively charged acid head group of pfoa and the steel surface. hence, the sam thus generated is fragile and exhibits poor protection ability. the nyquist plots of ec-pfoa generated for 30 min (at various concentrations) immersed in 3.5 % nacl solution at room temperature are shown in figure 4b. two distorted capacitive loops s. h. nataraj et al. j. electrochem. sci. eng. 5(4) (2015) 247-259 doi:10.5599/jese.211 253 are observed in the nyquist plot for all concentration curves. with increase in concentration from 1 to 2 mm, the double layer capacitance values decreased from 52.26 to 15.06 µω -1 cm -2 s n and charge transfer resistance values increases from 3533 to 12520 ω cm 2 . during electrochemical coating 0.2 v potential was applied to the working electrode to increase the positive charge on the working electrode. the negatively charged head group of pfoa is thus directed to the steel surface due to electrostatic force of attraction. this forms a superior directional compact coating which gives excellent corrosion protection. the increase in double layer capacitance and decrease in the n2 value was observed for ec-pfoa generated in 4 mm and other higher concentration pfoa solution. this indicates inhomogeneous and uneven coating. figure 4. nyquist plots of (a) dc-pfoa (b) ec-pfoa at various concentrations (for 30 min) (c) ec-pfoa at different coating time (for 2mm pfoa solution), immersed in 3.5 % nacl solution at room temperature. the nyquist diagram of ec-pfoa generated in 2 mm solution (at different coating time) immersed in 3.5 % nacl solution at room temperature is represented in figure 4c. the curves seem to be similar with respect to their shape indicating the similarity in mechanistic electrochemical property of the films. however, it should be noted that they differ substantially in their size. the area enclosed by the loop indicates extent of corrosion resistance. with increasing time, the nyquist loops of ec-pfoa samples enclose larger area with higher charge transfer resistance. the charge transfer resistance is related to protection efficiency (pe) by the following equation [25]: pe / % = [(rct r 0 ct) / rct] ×100 j. electrochem. sci. eng. 5(4) (2015) 247-259 surface modification to impede mild steel corrosion acid 254 table 2. corrosion data obtained from eis technique. sample qcoat / µω -1 cm -2 s n n1 rcoat / ω cm 2 qdl / µω -1 cm -2 s n n2 rct / ω cm 2 error, % bare steel 249 0.8 7 376 0.8 01124 2.7 dc-pfoa 14 0.4 22 4 0.8 01575 2.7 ec-pfoa generated in various concentrations (at 30 min). 1 mm 145 0.6 566 52 0.8 03533 1.8 2 mm 15 0.7 1051 15 0.7 12520 4.2 4 mm 12 0.7 919 21 0.6 13094 1.2 6 mm 23 0.7 2481 36 0.6 10280 1.6 ec-pfoa generated in different coating time (at 2mm pfoa solution) without sam 93 0.8 29 72 0.8 02393 2.4 5 min 41 0.7 56 52 0.8 05015 4.3 15 min 36 0.7 712 12 0.8 07418 4.4 30 min 15 0.7 1051 15 0.7 12520 4.2 60 min 6 0.8 506 48 0.6 14860 2.2 figure 5. corrosion protection efficiency of ec-pfoa at different coating time in 3.5 % nacl solution at room temperature. where, r 0 ct and rct represent the charge-transfer resistance of bare and coated sample respectively. the protection efficiency of ec-pfoa for different coating time is shown in figure 5. the pe increases with increase in coating time. however, beyond 30 min there is no significant increase in pe. the double layer capacitance of ec-pfoa decreases up to 30 min and then increases (60 min). the n2 value also decreases for 60 min represents surface inhomogeneity. the charge transfer resistance for uncoated sample (bare steel kept in water without pfoa for 30 min) in 3.5 % nacl solution is 2393 ω cm 2 , but for coated sample (30 min immersion in pfoa solution) is 12520 ω cm 2 . this clearly indicates that the ec-pfoa blocks the active site and thus reduces the anodic reaction more efficiently than dc-pfoa due to strong electrostatic interaction between steel and pfoa molecule. potentiodynamic polarization analysis the tafel plots of coated sample (dc-pfoa, ec-pfoa) immersed in 3.5 % nacl solution at room temperature are displayed in figure 6. the values of the corrosion potential (ecorr), corrosion current density (icorr) and corrosion rate were tabulated in table 3. the figure 6a shows the s. h. nataraj et al. j. electrochem. sci. eng. 5(4) (2015) 247-259 doi:10.5599/jese.211 255 polarization behavior of bare steel and dc-pfoa immersed in 3.5 % nacl solution at room temperature. the icorr values for bare steel and dc-pfoa are 6.091 µa cm -2 and 5.558 µa cm -2 , respectively. there is no significant decrease in either corrosion current density value or corrosion rate values. this shows that the dc-pfoa has poor protection efficiency. figure 6. tafel plots of (a) dc-pfoa (b) ec-pfoa at various concentrations (for 30 min) (c) c-pfoa at different coating time (for 2 mm pfoa solution), immersed in 3.5 % nacl solution at room temperature. table 3. corrosion data obtained from tafel measurement. sample ecorr / v icorr / µa cm -2 corrosion rate, mils/yr bare steel -0.702 6.09 5.57 dc-pfoa -0.615 5.56 5.08 ec-pfoa generated in various concentrations (at 30 min). 1mm -0.632 4.77 4.36 2mm -0.434 1.50 1.37 4mm -0.488 1.85 1.69 6mm -0.506 2.19 2.00 ec-pfoa generated in different coating time (at 2 mm pfoa solution) without pfoa -0.551 5.50 5.03 5 min -0.510 3.32 3.04 15 min -0.518 3.02 2.76 30 min -0.434 1.50 1.37 60 min -0.536 5.46 4.99 j. electrochem. sci. eng. 5(4) (2015) 247-259 surface modification to impede mild steel corrosion acid 256 the tafel plots of ec-pfoa generated for 30 min (in different pfoa concentrations) immersed in 3.5 % nacl solution at room temperature are presented in figure 6b. the icorr and corrosion rate value of ec-pfoa decreases to a large extent from 1 to 2mm, then again increases with concentration. meanwhile, ecorr values for coating in 2 mm pfoa also shifted towards more positive direction compared to coatings at other concentrations. this indicates that the surface coverage was more effective in 2 mm concentration, thus decreases the dissolution rate of steel remarkably. the tafel plots of ec-pfoa generated in 2mm pfoa solution (at different coating time) immersed in 3.5 % nacl solution at room temperature are shown in figure 6c. the icorr and corrosion rate values decreases with increase in coating time and the lowest value observed for 30 min coating time. moreover, the ecorr value also shifted towards positive side prominently for 30 min coating time. this implies that the ec-pfoa generated for 30 min has significant effect on inhibiting the anodic dissolution. surface morphology scanning electron microscopic images were taken to establish the formation of protective layer on the mild steel surface. the sem images of bare steel, dc-pfoa and ec-pfoa before and after immersion in 3.5 % nacl for 6h at room temperature are depicted in figure 7. figure 7. sem micrographs of (a) unpolished bare steel (b) bare steel after corrosion (c) dcpfoa (d) dc-pfoa after corrosion (e) ec-pfoa (f) ec-pfoa after corrosion. s. h. nataraj et al. j. electrochem. sci. eng. 5(4) (2015) 247-259 doi:10.5599/jese.211 257 the significant corrosion debris can be seen on the surface of bare steel after immersion in corrosive media (figure 7a and 7b). pfoa molecules were dip coated giving smooth surface compared to bare steel surface which can be observed from figure 7c. figure 7d shows the surface morphology of dc-pfoa after corrosion. the pits formed on the surface imply the poor protection from dc-pfoa. interestingly the electrochemically coated surface turns out to be smoother one morphologically (figure 7e). however the ec-pfoa after immersion in corrosive medium (figure 7f) has only small notches with almost smooth surface indicate the remarkable protection. this advocates the significant corrosion protection ability of electrochemically generated pfoa. the protective film formed on the mild steel surface was analyzed using eds as shown in figure 8. figure 8a represents eds spectra of dc-pfoa. eds spectra confirms the presence of iron (base metal, 79.76 wt. %), carbon (11.55 wt. %), oxygen (3.06 wt. %) and fluorine (5.63 wt. %). figure 8b represents eds spectra of ec-pfoa. the spectra confirms the presence of iron (80.26 wt. %), carbon (11.09 wt. %), oxygen (3.12 wt. %) and fluorine (5.53 wt. %). the observed elemental composition from eds confirms that pfoa molecule precludes the corrosion of surface by blocking the flaws. figure 8. eds of (a) dc-pfoa (b) ec-pfoa http://www.sciencedirect.com/science/article/pii/s1110062114000634#f0105 j. electrochem. sci. eng. 5(4) (2015) 247-259 surface modification to impede mild steel corrosion acid 258 conclusions it is clearly evident from the above discussion that the electrochemical method of surface modification of mild steel is an effective way to protect the steel surface from the attack of corrosive agents. the electrochemical method provides a directional orientation to the organic film growth. the organic coat (ec-pfoa) thus generated is more compact and exhibits high hydrophobicity, which is evident from the well-defined irras peaks and higher water contact angle (water for dc-pfoa is 128°  water for ec-pfoa is 135°). the electrochemical studies provide the ground support for these observations. the rct for dc-pfoa coated in 2mm solution for 18 h is 1575 ω cm 2 but for 30 min coated ec-pfoa in 2mm solution for is 12520 ω cm 2 . the ec-pfoa exhibited remarkable protection efficiency (91 %) as compared to dc-pfoa (28 %) due to strong electrostatic interaction. acknowledgements: the authors are grateful to department of chemistry, kuvempu university, karnataka, india, for providing lab facilities. the authors also thank to prof. satish v. kailas, mechanical engineering department, indian institute of science, bangalore, karnataka, india, for providing instrumental facilities. we acknowledge the help given by mr. h. s. shamasundar, dr. m. k. punith kumar, dr. s. ranganatha, and mrs. m. k. pavithra in conducting different experiments. references [1] c. e. chuka, b. o. odio, j. l. chukwuneke, j. e. sinebe, international journal of scientific & technology 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[25] z. zhang, s. chen, y. li , s. li , l. wang, corros. sci. 51 (2009) 291-300. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {influence of supporting electrolyte on electrochemical formation of copper nanoparticles and their electrocatalytic properties} http://dx.doi.org/10.5599/jese.1077 253 j. electrochem. sci. eng. 12(2) (2022) 253-264; http://dx.doi.org/10.5599/jese.1077 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of supporting electrolyte on electrochemical formation of copper nanoparticles and their electrocatalytic properties noelia zurita and silvana g. garcía instituto de ingeniería electroquímica y corrosión (iniec), departamento de ingeniería química, universidad nacional del sur, av. alem 1253, (8000) bahía blanca, argentina corresponding authors:  sgarcia@criba.edu.ar; tel.: +54-9-291-4595100-3614 received: august 11, 2021; accepted: november 5, 2021; published: november 22, 2021 abstract comparative analysis of copper nanoparticles (cunps) obtained by electrodeposition on highly oriented pyrolytic graphite (hopg) substrates from different supporting electrolytes containing sulphate anions, was performed. voltammetric results indicated that cu electrodeposition follows a diffusion-controlled nucleation and crystal growth model for three solutions studied (na2so4, h2so4 and na2so4+h2so4). na2so4 solution was most effective because the copper reduction occurs at the most positive potential value, reaching the highest current density. analysis of potentiostatic current transients revealed that the process can be described predominantly by a model involving a 3d-progressive nucleation mechanism, which was corroborated by scanning electron microscopy (sem) analysis. sem images showed a high density of hemispherical shaped cu particles of different sizes (mostly between 80-150 nm), randomly distributed on the hopg surface for na2so4 electrolyte solution. in the presence of h2so4, the size dispersion decreased, resulting in particles with greater diameters (up to 339 nm). the electrolyte solution with na2so4+h2so4 revealed lower particle density with a considerable crystal size dispersion, where very small crystallites are prevailing. cyclic voltammetry was used to evaluate qualitatively the catalytic activity of cunps deposited from three electrolyte solutions towards the nitrate reduction reaction. an enhanced catalytic effect was obtained when copper particles were prepared from either na2so4 or h2so4 supporting electrolyte. keywords copper nanocrystals; electrodeposition; plating solution; highly oriented pyrolytic graphite (hopg); nitrate reduction introduction recent studies have reported that supported copper nanostructures, in addition to being suitable for a wide range of applications (nanoelectronics [1], biosensing [2,3], optoelectronics [4], etc.), show excellent results as electrocatalyst material for different reactions, in particular those of http://dx.doi.org/10.5599/jese.1077 http://dx.doi.org/10.5599/jese.1077 http://www.jese-online.org/ mailto:sgarcia@criba.edu.ar j. electrochem. sci. eng. 12(2) (2022) 253-264 electrochemical formation of cu nanoparticles 254 environmental interest [5-8]. nitrate ions are one of the chemical species that can be potentially dangerous for human health if their concentration in drinking water and food exceeds the upper limit allowed, as indicated by the world health association [9]. therefore, there is a high necessity for nitrate ions to be detected and quantified. the detrimental effect of nitrate ions comes mainly from nitrogen-based fertilizers contaminating drinking water supplies, and their removal or reduction has gained special attention. the ability of copper nanostructures as an electrocatalyst material for nitrate reduction has already been studied by several authors [10-12]. in particular, the influence of structural properties of nanoparticles has been analyzed on the reaction performance [13-15]. it has been demonstrated that the final properties of metal nanoparticles are strongly dependent on their surface morphology, which is essentially related to the selected preparation method. the electrodeposition is one of the most used preparation methods because it presents additional advantages. the main assets are simplicity and the possibility of working at room temperature and pressure. in addition, this method allows the kinetic process control and, consequently, the size of particles and the amount of formed deposit. as stated above, the morphological features of deposits have a significant influence on their electrocatalytic properties. it is well known that variations of the plating solution composition and ph can influence the deposit morphology. therefore, it is necessary to relate the operating conditions to the shape, size, and distribution of produced nanoparticles. previous works addressing the electrodeposition of copper nanoparticles on carbonaceous substrates, have analyzed the influence of some parameters of the electrochemical process on structural features of the deposit [16,17]. in particular, it was found that the ph and concentration of cuso4 solution have a remarkable effect on the texture and population density of deposited nanostructures the use of na2so4 supporting electrolyte. for the latter, an intriguing behavior was reported, which deserves a detailed study [18]. on the other hand, the effect of anions present in the electrolytic solution has been analyzed by several authors [19,20]. gonzález et al. [19] have investigated cu+ intermediate species formed during copper electrodeposition process and demonstrated that in the presence of no3ions, the value of the reduction peak current increases because this anion is catalytically reduced on cu nanoparticles freshly electrodeposited on a glassy carbon electrode. presence of so42anions in the plating solution does not interfere with the current measurement of the copper reduction, but they are adsorbed considerably, preventing the discharge of cu+ ions. for clo4− ions, adsorption is negligible; therefore, a reduction peak current higher than from so42-anions is evidenced. these studies were carried out at ph 3 with a higher cu2+ concentration than in the present study. bélanger et al. [20] analyzed the copper electrodeposition on highly oriented pyrolytic graphite (hopg) substrates using two different copper salts (cuso4 and cu(no3)2) in 1.8 m h2so4 aqueous solution and demonstrated that the deposition of copper in the presence of cuso4 induced the co-deposition of sulfate anions. in the present work, a comparative analysis of the initial stages of copper electrodeposition on hopg from different supporting electrolytes containing so42anions (na2so4, h2so4 and na2so4+ h2so4) has been carried out to increase the understanding of the nucleation and growth of copper particles and their electrocatalytic properties. the electrodeposition process is studied using conventional electrochemical techniques (cyclic voltammetry and chronoamperometry), and the copper deposits are characterized by scanning electron microscopy (sem) with energy-dispersive xray spectroscopy (edx). the catalytic effect for the reduction of nitrate anions in nano3 + na2so4 solution has been qualitatively evaluated by voltammetric measurements for cunps deposited from three different sulphate-containing media. n. zurita and s. g. garcía j. electrochem. sci. eng. 12(2) (2022) 253-264 http://dx.doi.org/10.5599/jese.1077 255 experimental the solutions used in this work for metal deposition were prepared with supra pure chemicals (e. merck, darmstadt, germany) and tri-distilled water. the supporting electrolytes added to 1 mm cuso4 solution were 0.1 m na2so4, 0.1 m h2so4 and 0.1 m na2so4 + 0.1 m h2so4, taking into account that the same anion was present in all three cases. the catalytic effect was studied in 0.1 m nano3 + + 0.1 m na2so4 solution. prior to each experiment, solutions were deaerated by nitrogen bubbling. cyclic voltammetric measurements and chronoamperometric studies were carried out in a conventional three-electrode electrochemical cell at a temperature of t = 298 k. hopg spi-2 grade substrates (spi supplies, usa) were used as working electrodes, which were prepared by cleaving its surface with an adhesive tape immediately prior to each experiment. the electrode was held within a teflon sheath with an exposed area of 0.216 cm2. the counter electrode was a pt sheet (1 cm2), and the reference electrode was a saturated calomel electrode (sce), (esce = 0.2415 v vs. she). all electrode potentials mentioned in this work are referred to this electrode. the experiments were performed with an eg&g princeton applied research model 273a potentiostat-galvanostat controlled by a microcomputer. cu nanoparticles prepared from different electrolytes by the potentiostatic step technique on hopg substrates were characterized by sem using a zeiss ma 10 microscope integrated with an edx analyzer. results and discussion cyclic voltammetry electrodeposition of cu on the hopg substrate was performed in the following plating solutions: a) 1 mm cuso4 + 0.1 m na2so4 (ph 4.66), b) 1 mm cuso4 + 0.1 m h2so4 (ph 0.95), and c) 1 mm cuso4 + 0.1 m na2so4 + 0.1 m h2so4 (ph 1.10). the process was initially examined qualitatively by cyclic voltammetry recorded within the potential range -0.7  e/v  0.3, at the scan rate (de/dt) of 10 mv s-1. as shown in figure 1, the voltammetric responses of hopg in three solutions exhibited similar features, including the hysteresis loop (the so-called nucleation loop) formed between cathodic and anodic sweep curves at negative potentials values, which is typical of a nucleation and growth process [21,22]. an extensive analysis of cyclic voltammograms at different scan rates confirming this conclusion was shown elsewhere [23]. during the potential scan towards the negative direction, a cathodic peak is observed, associated with the reduction of cu2+ ions to cu0. in the reverse scan, a single anodic peak is evidenced, consistent with the dissolution of cu clusters deposited previously. generally, a single cathodic peak is evidenced as a result of the cu2+ electroreduction process, and it is accepted that it occurs on a global stage via 2 electrons. however, it is well known that in an acid medium [24,25] or in the presence of complexing anions such as chloride [26], the electrodeposition of cu can be considered to occur in two consecutive stages with the formation of cu+ ions as intermediate species. these stages can be described by equations (1) and (2) cu2+ + e− = cu+ (1) cu+ + e− = cu0 (2) with the first reaction being the rate-determining step. in fact, in the case of the electrolyte containing only sulphuric acid, the broad cu2+ reduction peak could be considered to be formed by two waves, which could be associated with two steps of the process. http://dx.doi.org/10.5599/jese.1077 j. electrochem. sci. eng. 12(2) (2022) 253-264 electrochemical formation of cu nanoparticles 256 figure 1. cyclic voltammetry of hopg for 1 mm cuso4 in 0.1 m na2so4, 0.1 m h2so4 and 0.1 m na2so4 + 0.1 m h2so4.|de/dt|=10 mv s-1. inset: cyclic voltammograms of hopg in blank electrolyte solutions for comparison, cyclic voltammograms of hopg electrode immersed in blank solutions (without containing cu(ii) ions) shown in the inset of figure 1, indicate that the hydrogen evolution reaction occurs at more negative potential values and does not interfere with the cu electrodeposition process under the conditions considered in this study. when the supporting electrolyte is na2so4 (solution a), which is added to the plating bath primarily to increase electrolyte conductivity, the peak corresponding to the reduction of cu2+ ions, is occurring at a more positive potential value (epeak= -0.39 v) than in the other two cases, and reaching the maximum current density value of jpeak= -142.68 a cm-2. the presence of h2so4 in the plating solution (solution b) also favors the conductivity of the electrolyte, showing epeak and jpeak at -0.49 v and -112.68 a cm-2, respectively. according to the observed behavior, it could be inferred that part of so42ions can be adsorbed on the surface of hopg substrate, reducing thus the number of active sites for the formation of cu nuclei, in agreement with the literature reviewed [19,20]. moreover, hope and woods [27] have demonstrated by in-situ surface-enhanced raman scattering (sers) that sulphate adsorption occurs during metal electrodeposition. their spectro-electrochemical investigations revealed that sulphate ions were transient adsorbed species during cu electrodeposition process, which was not observed for copper surfaces in the absence of metal ions in solution. for the third solution studied (solution c), although the conductivity was higher than of other supporting electrolytes, the most negative epeak for cu deposition and the lowest jpeak values were obtained. the copper nucleation is retarded by this plating solution which provides more sulphate anions at the electrode interface, and consequently, it can be assumed that they could be adsorbed on the surface, blocking more sites for cu deposition. the values of epeak and jpeak for three studied electrolyte solutions are summarized in table 1. table 1. cathodic peak potentials and peak current densities for cu deposition from three supporting electrolytes containing 1 mm cuso4 supporting electrolyte 0.1 m na2so4 0.1 m h2so4 0.1 m na2so4 + 0.1 m h2so4 epeak / v -0.389 -0.494 -0.519 jpeak / µa cm-2 -142.68 -112.68 -87.88 -200 -100 0 100 200 300 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 j / μ a c m -2 e / v vs. sce series3 series1 series2 hopg/1 mm cuso4 + 0.1 m na2so4 hopg/1 mm cuso4 + 0.1 m h2so4 hopg/1 mm cuso4 + 0.1 m na2so4 + 0.1 m h2so4 -3500 -2500 -1500 -500 500 -1 -0.7 -0.4 -0.1 0.2 j / μ a c m -2 e / v vs. sce n. zurita and s. g. garcía j. electrochem. sci. eng. 12(2) (2022) 253-264 http://dx.doi.org/10.5599/jese.1077 257 chronoamperometry potentiostatic current transients were carried out to obtain information about the kinetics of copper electrodeposition on the carbonaceous substrate. representative j-t curves were performed applying simple potentiostatic pulses at selected potentials more negative than the cathodic peak potential (see figure 1) for three cases studied, and they are shown in figure 2 (inset graphics). these curves exhibit a typical shape of the nucleation process with 3d growth controlled by diffusion of electroactive species [28]. the potentiostatic current transients for three plating solutions showed an increase of current up to maximum, corresponding to the formation of cu crystallites through the nucleation and growth process, followed by a decreasing region, reflecting the transition to linear diffusion. these results revealed that the presence of sodium sulfate in the plating solution generates the highest current for nucleus formation, which was reached at shorter times. to analyze the type of nucleation, a commonly used criterion is the application of the theoretical model proposed by sharifker and hills [28]. by means of this model, two limiting nucleation mechanisms, instantaneous and progressive, can be distinguished based on the equations (3) and (4):      = − −          2 2 2 mm m 1.9542 1 exp 1.2564 j t t tj t (3)       = − −         2 2 2 2 mm m 1.2254 1 exp 2.3367 j t t tj t (4) where the parameters jm and tm are the maximum current density of the transient and the time at which the current maximum occurs, respectively. eq. (3) corresponds to the instantaneous nucleation where the nuclei are formed and grown on active sites of the carbonaceous substrate, all activated simultaneously. in contrast, eq. (4) corresponds to the progressive nucleation where the nuclei are formed on preferential sites and activated at different times during the electroreduction process. as shown in figure 2, representative potentiostatic current transient data were normalized and compared with both types of the described model. as stated above, the corresponding experimental transients are those included in the inset of the figures from which the type of nucleation is analyzed. figure 2a shows a very good correlation for the transient obtained at e= -0.42 v in solution a, with the theoretical curve for progressive nucleation. for solution b, shown in figure 2b, copper crystallites were generated at e= -0.50 v. cu nucleation begins in a progressive way at short times, passing to an intermediate state until reaching instantaneous nucleation in the last stage. these results are in agreement with those obtained by huang et al. [29]. they demonstrated that for copper deposition on hopg electrode from 1 mm cuso4 + 1 m h2so4 solution, the analysis of experimental transients according to the theoretical model proposed by sharifker and hills [28] showed a good correlation with the theoretical curve for progressive nucleation for a potential more negative than the reduction peak. at the last stage, nucleation transition from progressive to instantaneous was found at even more negative potential. when the experiments were performed using the solution c, i.e., cupric sulfate with sodium sulfate and sulfuric acid solution as supporting electrolyte (figure 2c), the nucleation mode follows predominately progressive nucleation, which is a behavior similar to that of the first electrolytic medium (0.1 m na2so4). in this case, the deposits were formed at e= -0.55 v. http://dx.doi.org/10.5599/jese.1077 j. electrochem. sci. eng. 12(2) (2022) 253-264 electrochemical formation of cu nanoparticles 258 figure 2. non-dimensional (j / jmax)2 vs. (t / tmax) plots of experimental current transients (shown in insets) for hopg in solution containing 1 mm cuso4 and: (a) 0.1 m na2so4; (b) 0.1 m h2so4; (c) 0.1 m na2so4 + 0.1 m h2so4 surface analysis direct information on the shape, size and density of cu nanoparticles can be acquired by scanning electron microscopy (sem) analysis. figure 3 exhibits hemispheric cu deposits generated by a simple potentiostatic pulse on hopg substrate in three analyzed solutions. figure 3. sem images of cunps electrodeposited on hopg from 1 mm cuso4 in: (a) 0.1 m na2so4, (b) 0.1 m h2so4 and (c) 0.1 m na2so4 + 0.1 m h2so4, and corresponding particle size distribution histograms 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 (j / j m a x )2 t / tmax instantaneous progressive e = -0.42 v -120 -80 -40 0 0 10 20 30 40 50 i / μ a t / s 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 (j / j m a x )2 t / tmax instantaneous progressive e = -0.55 v -30 -20 -10 0 0 10 20 30 40 50 i / μ a t / s 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 (j / j m a x )2 t / tmax instantaneous progressive e = -0.5 v -50 -40 -30 -20 -10 0 0 10 20 30 40 50 i / μ a t / s (a) (b) (c) e = -0.42 v e = -0.5 v e = -0.55 v n. zurita and s. g. garcía j. electrochem. sci. eng. 12(2) (2022) 253-264 http://dx.doi.org/10.5599/jese.1077 259 figure 3a reveals the formation of a considerable density of cu particles distributed on the hopg surface when na2so4 was used as the supporting electrolyte (solution a). these particles were electrodeposited at e=-0.42 v during 100 s. cu structures present different particle sizes (as shown in the corresponding histogram), consistent with progressive nucleation. a large number of small particles can be observed in figure 3a, with a minimum diameter of 16 nm, and large ones with a maximal diameter of 259.17 nm. figure 3b shows copper particles obtained by electrodeposition at -0.5 v during 120 s in the copper solution containing h2so4 (solution b). the particle size dispersion is slightly smaller, resulting in the formation of larger copper particles with a maximum diameter value around 339 nm and without the predominance of small particles. therefore, the particle size distribution tends to conform to the normal distribution, with crystals mostly between 180-210 nm in diameter. some of the cu nuclei formed in solution b are formed on hopg step edges acting as active sites for metal nucleation, and the presence of crystals forming dimers and trimers are also detected, as shown in figure 4. figure 4. sem image of cunps electrodeposited from 1 mm cuso4 + 0.1 m h2so4 showing deposits on hopg step edges and presence of single particles, dimers and trimers as seen in figure 3c, the number of metallic clusters generated at e= -0.55 v during 120 s in the combined electrolyte (solution c) decreases, and a wide range of particle sizes is observed with small particles of 37 nm and larger with a maximum diameter of 300 nm. these crystals grow on random active sites and also on some hopg step edges. generally, sem images confirmed the progressive nature of copper nucleation derived by the sharifker and hills dimensionless analysis for three solutions used. figure 5 exhibits sem images of cu crystallites formed on the hopg electrode with marked points where the edx analysis was performed. cu structures were generated on the surface substrate after applying a potential pulse at e= -0.5 v for 120 s in the solution containing h2so4. figure 5. sem micrograph of hopg substrate after cu deposition from 1 mm cuso4 + 0.1 m h2so4 solution at -0.5 v for 120 s and representative edx analysis applied on top of crystallite http://dx.doi.org/10.5599/jese.1077 j. electrochem. sci. eng. 12(2) (2022) 253-264 electrochemical formation of cu nanoparticles 260 only the elemental distribution for the most representative point is shown, confirming the presence of cu besides the signal corresponding to the c substrate (hopg). electrocatalysis the catalytic activity of cunps modified hopg substrates was evaluated qualitatively by cyclic voltammetry in 0.1 m nano3 + 0.1 m na2so4 solution. initially, cu nanoparticles were obtained on the hopg electrode after applying a single potentiostatic pulse to the electrode immersed in three electrolytes studied at e= -0.65 v for 400 s. potentiostatic pulse parameters, different from those applied for nucleation analysis, were chosen, i.e., a more negative pulse potential and a longer deposition time, to increase the copper-covered area on the hopg electrode surface, and therefore, to induce more noticeable nitrate anion reduction signal in the voltammetric analysis. the type of nucleation, in this case, was analyzed anew, as well as the surface characterization by sem of cunps modified hopg electrode. figure 6a show a representative sem image and the corresponding size distribution of cu deposits generated when na2so4 (solution a) was used as the supporting electrolyte. figure 6. sem images of cunps electrodeposited from 1 mm cuso4 in (a) 0.1 m na2so4, (b) 0.1 m h2so4 and (c) 0.1 m na2so4 + 0.1 m h2so4, with the corresponding particle size distribution histograms (left), and nondimensional (j / jmax)2 vs. (t / tmax) plots of experimental current transients (right) 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 (j / j m a x )2 t / tmax instantaneous progressive e = -0.65 ve = -0.65 v 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 (j / j m a x )2 t / tmax instantaneous progressive e = -0.65 ve = -0.65 v 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 (j / j m a x )2 t / tmax instantaneous progressive e = -0.65 ve = -0.65 v n. zurita and s. g. garcía j. electrochem. sci. eng. 12(2) (2022) 253-264 http://dx.doi.org/10.5599/jese.1077 261 crystals of different sizes with a predominance of very small particles of the order of 31-50 nm with maximum diameters of 352 nm, are observed. the covered area was 17 % of the electrode geometric area, calculated from the average of several low magnification sem images. the type of nucleation analyzed from sharifker and hills model leads to a progressive nucleation mode, the same as for e = 0.42 v. for the second solution containing h2so4 (solution b), figure 6b shows a representative sem image of cu-decorated hopg electrode revealing a great disparity in copper particle sizes as a result of the formation of many agglomerates that reached values up to 514 nm, but also a large number of crystallites between 30-50 nm. the covered area of 22 % was estimated. size diversity of metal deposits is consistent with a progressive nucleation and growth mechanism, which was corroborated throughout the analyzed time range by the theoretical model used. the same nucleation model was obtained previously for e=-0.50 v at short times. when the deposits were formed using the third solution (solution c), a large number of deposits with a diameter of approximately 104 nm is observed, some of them reaching a maximum size of 326 nm, with no predominance of particles with diameters less than 50 nm (figure 6c). in this case, the calculated covered area was 24 % and, unlike for the pulse at e= -0.55 v, the nucleation is predominantly instantaneous. electrocatalytic properties of cunps after this analysis, the electrocatalytic behavior of cunps modified hopg electrode was evaluated for the nitrate reduction reaction. figure 7 shows the voltammetric results for different cu structures generated from three plating electrolytes and recorded in the solution of 0.1 m nano3 + 0.1 m na2so4. cathodic current peaks observed in the range -0.40  e/v  -0.55, were attributed to the reduction reaction of nitrate ions, while the current increase evidenced at more negative potential values is related to the hydrogen evolution reaction. the voltammetric response for a solid copper electrode in the same solution containing nitrate ions is also shown in figure 7 for comparison purposes. figure 7. cyclic voltammograms of cunps electrodeposited on hopg electrode from three plating solutions and recorded in 0.1 m nano3 + 0.1 m na2so4. inset: cv of polycrystalline cu recorded in the same solution. |de/dt|=10 mv s-1 -50 -40 -30 -20 -10 0 -0.8 -0.6 -0.4 -0.2 0 i / μ a e / v vs. sce series4 hopg/cu nps (soln.a) hopg/cu nps (soln.b) hopg/cu nps (soln.c) -70 -60 -50 -40 -30 -20 -10 0 10 20 -0.7 -0.5 -0.3 -0.1 i / μ a e / v vs. sce http://dx.doi.org/10.5599/jese.1077 j. electrochem. sci. eng. 12(2) (2022) 253-264 electrochemical formation of cu nanoparticles 262 the hopg-modified electrodes showed voltammetric features similar to those of polycrystalline copper, i.e., a cathodic peak corresponding to nitrate reduction in the same potential region (-0.45 < e/v < 0.6), followed by an abrupt current increase due to hydrogen evolution. the hopg electrodes modified by copper crystals prepared from solutions containing h2so4 and na2so4 showed a more pronounced catalytic effect than the one observed for the solution containing both components. it can be inferred that this enhancement of the electrocatalytic effect, is due to the presence of a larger number of crystallites smaller than 50 nm, which favor adsorption of nitrate ions, despite the fact that a larger covered area was found for the third solution. this behavior is in accordance with the findings of other authors [30,31]. a. j. wain [30] reported that the electrocatalytic activity was increased with decreasing particle size in the range 5–50 nm, which was attributed to the exposure of different reactive facets of the crystallites. also, masel et al. [31] demonstrated that ag nanoparticles increased their electrocatalytic activity with decreasing particle size until a certain particle size (5 nm). they also indicated that this behavior could be caused by several reasons, such as variations with a particle size of the number of steps or kink sites of the crystallites, the electronic structure or work function of the particles, or the binding energy of the involved species. accordingly, the aforementioned assumption is not conclusive and related studies in this regard are in progress. table 2 summarizes the values of the peak potential and maximum cathodic current values for nitrate reduction reaction using cunps deposited on hopg surfaces from three supporting electrolyte solutions. the values corresponding to a freshly polished polycrystalline copper electrode were also included. table 2. cathodic peak potential and maximum cathodic current values of nitrate reduction at cunps deposited from different supporting electrolytes supporting electrolyte for cunps nitrate reduction epeak/ v imax / µa na2so4 -0.54 6.39 h2so4 -0.52 7.04 na2so4 + h2so4 -0.48 3.12 solid copper -0.56 29.50 conclusions formation of hopg supported copper nanoparticles (cunps) by electrodeposition from three different supporting electrolytes containing sulphate anions, and their potential application as electrocatalyst materials for the reduction reaction of nitrate ions were analyzed. cyclic voltammetric results indicated that for all supporting electrolyte solutions, the kinetics of the nucleation and growth of cunps on hopg electrodes follows predominately a progressive model, presenting a good correlation with sem images. the voltammetric response of cunps modified hopg electrode regarding nitrate reduction showed a better catalytic effect for cu nanoparticles generated from solutions containing either na2so4 or h2so4 species. acknowledgements: the authors wish to thank the universidad nacional del sur (argentina) for the financial 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https://doi.org/10.1021/jp310509z https://creativecommons.org/licenses/by/4.0/) {electrochemical paper-based biosensors for point-of-care diagnostics: detection methods and applications:} http://dx.doi.org/10.5599/jese.1104 399 j. electrochem. sci. eng. 12(3) (2022) 399-419; http://dx.doi.org/10.5599/jese.1104 open access : : issn 1847-9286 www.jese-online.org review electrochemical paper-based biosensors for point-of-care diagnostics: detection methods and applications nafiseh sahraei, mohammad mazloum-ardakani and farzaneh hoseynidokht department of chemistry, faculty of science, yazd university, yazd, 89195-741, iran corresponding author:  mazloum@yazd.ac.ir; phone: 00983518211670; fax: 00983518210644 received: september 7, 2021; accepted: march 26, 2022; published: april 11, 2022 abstract personalized health care (phc) includes personalized, preventive, predictive and participatory approaches that are significant in new diagnostics. this personal health care requires fast, accurate and minimally invasive diagnostic tools that make it possible to evaluate and monitor the process of disease by diagnosing specific disease biomarkers. point-of-care testing (poct) involves a wide range of diagnostic tools that meet this purpose. electrochemical paper-based devices (epads) have been introduced as simple, inexpensive, portable and disposable measurement devices to be used in many poct applications, especially in handling emergencies and outpatient as well as remote usages. electrochemical detection is a real quantitative detection method with better sensitivity, selectivity and detection limits than indirect measurement methods. in recent years, there has been a revolution in quantitative detections by poct, thanks to the benefits of electrochemical sensors and paper substrates. in this paper, recent developments in epads, focusing on the properties of paper, reasons for its use in the devices, techniques of device and electrode fabrications, and their application particularly in clinical diagnosis, are reviewed. keywords clinical analysis; electrochemical detection; electrochemical paper-based device, medical diagnosis; point-of-care (poc) introduction although mortality has declined worldwide over the past century, it has been accompanied by a dramatic shift from communicable to non-communicable diseases such as cancer and diabetes and cardiovascular, autoimmune, and respiratory diseases. more than 70 % (41 million) of deaths worldwide are due to non-communicable diseases [1]. also, based on the report of the world health organization, about one billion people worldwide do not receive health care services [2]. to minimize these deaths, the leading agencies, policymakers and especially the united nations have enacted a number of plans to improve public health [3]. among the measures taken to foster healthcare systems, one may refer to providing diagnostic kits, services thereafter, and affordable http://dx.doi.org/10.5599/jese.1104 http://dx.doi.org/10.5599/jese.1104 http://www.jese-online.org/ mailto:mazloum@yazd.ac.ir j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 400 health care [4]. so, primary diagnosis and personalized treatment in clinics play important roles in controlling diseases and enhancing the survival of patients, especially in retarded areas and in developing countries where patients have limited access to laboratory tests [5]. the need for the reduction of costs as well as accurate, rapid and minimally invasive diagnostic techniques has led to the advancement of point-of-care (poct) testing [6]. in this regard, certain attempts have been made, such as presenting the world health organization guidelines known as the assured guidelines, which postulate affordable, sensitive, specific, user-friendly, rapid and robust, equipment-free and deliverable services to end-users [7–9]. patient status tracking may also be performed with poc devices during recovery. for different sensors, especially healthcare sensors, electrochemical paper-based analytical devices (epads) can be useful. through these devices, electrochemical detection occurs with a paper substrate. among the many benefits of epads, the paper applied is low-cost, easily accessible, flexible, eco-friendly, biocompatible and lightweight. it also has a soft matrix with many capillaries that facilitate fluids' self-pumping when they come into direct contact. additionally, the surface of the paper can be modified with nanostructured materials to increase the detection sensitivity. electrochemical methods are increasingly utilized in the development and design of epads. this is owing to their advantages over indirect measurement techniques such as the colorimetric technique; they are capable of quantitative detection with better selectivity, sensitivity and detection limits. electrochemical detection is also advantageous in terms of sensor fabrication cost, power consumption and capacity for miniaturization. in combination with a paper substrate and a compact sensor, an electrochemical detector can serve as an accessible healthcare product to be typically used in loco analysis and poc evaluations [10]. among the benefits of miniaturization, one may refer to the reduction of the required sample volume, electrode surface saturation due to the high surface-tovolume ratio of the paper-fiber matrix detection region, and the portability of the device [11,12]. various electrochemical techniques such as potentiometry, voltammetry, conductometry, coulometry, polarography and amperometry can be used to detect electrochemical signals on epads. the use of paper substrates in electrochemical detections eliminates the disadvantage of the non-portability of potential acetate devices [11]. an example is one of the first and probably the most widely used poc electrochemical sensors for glucose detection [13–15]. this paper provides a review of some recently developed electrochemical, paper-based devices (epads) for the diagnosis of diseases. it also briefly accounts for electrode and device fabrication techniques and the use of paper in detecting devices. strategies for choosing a paper substrate a major advantage of using paper is the reduction of costs, particularly in developing countries that have limited resources. paper is amenable to different techniques due to its special features. the white color of the paper makes it suitable for use in colorimetric, fluorimetric, and chemiluminescent techniques, or any other assays that can be recognized by the naked eye [16,17] or in smartphone-assisted measurements [18–20]. the high availability and variety of paper, as well as its excellent mechanical properties like lightness, specific stiffness, flexibility and ease of use, are other reasons for using paper as a stable substrate for biosensors [21,22]. in addition, paper is biodegradable and quick for disposal (e.g., by incineration), which provides a good opportunity for creating disposable biological biosensors [23–25]. in addition, ease of transportation, wide availability and storage, portability and ease of patterning through the printing technology make it possible to develop an inexpensive and portable biosensing device for various applications [26–29]. n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 401 among the advantages of paper, some can be highlighted for electrochemical biosensing applications, on which we are focused in this article. for example, the intrinsic properties of paper, e.g., the fibrous and porous 3d structure of cellulose fibers, can lead to: a) higher absorption for effective storage (e.g., in antibody, aptamer or enzyme modifications) and delivery of samples which can lead to the increased sensitivity, faster response time and accuracy of time-dependent measurements [30–32]; b) elimination of air bubbles, which is beneficial in sensors especially in microfluidics [21]; c) large specific surface area to enhance the number of immobilized biomolecules [30,33,34]; d) elimination of a need for pumps and electrical power to transport fluids owing to its numerous capillaries [4,21,23,35,36]; e) better electrochemical detection by use of conductive inks easily deposited on paper [37] , and f) detection with just very low volumes of samples [37]. another notable benefit of paper to use in epad devices is that it helps to develop origami-like systems. for this purpose, various layers of paper are assembled by folding or stacking to achieve a controlled interaction via 3d structure [25]. obviously, the working electrode is printed on one side of the paper device, and the reference and counter electrodes are printed on the other side. the working electrode can, thus, be modified conveniently without influencing the other electrodes [38,39]. integrating origami into paper-based diagnostic sensors allows more flexibility and freedom in design and offers such advantages as specific and high sensitivity, fewer operation steps, simplicity of the device operations and, therefore, a decrease of the assay time. other advantages include good registration and repeatability, controlled test time, multistep processes, reduction of the total amount of the reagent (antibody, enzyme, anther substrates) required and, thus, the reduction of the cost, ability to interface with available hand-held devices (e.g., commercial glucometers), conduction of lateral flow assays, good flow control within 3d epads for multiplex assays [36]. therefore, signal amplification reactions are controlled, and target propagation problems are reduced by a lateral current in the channels of the flat paper system. thus, reagent incongruity is prevented [40], and an extremely homogeneous distribution encompasses all the surfaces of the paper reaction areas [40]. although, three-dimensional electrochemical paper-based analytical devices (3d-epads) possess these advantages, 2d-epads are still widely used [15,41,42]. although the paper makes great platform substrates for electrochemical paper-based devices, it has certain inherent limitations. for example, it cannot be used for quantifying analyses and programmable steps due to the continuous flow that occurs through inactive capillary wicking. of course, researchers have recently proposed an active paper-based microfluidic device to overcome this limitation [43]. thick and highly porous papers, such as filter papers, are also problematic. depending on the fabrication technique, the dielectric paste used to pattern hydrophobic walls cannot keep the capillary wicking of solutions from hydrophilic regions [44]. also, the epad canals formed inside the paper are open at the top and the bottom, increasing external risks such as pollution, leakage of fluids to any surface in contact with the canals and their evaporation. because the rate of solvent evaporation depends on the relative humidity, the capillary flow into the paper channels can be modified [45]. fortunately, different kinds of paper are available with various properties that can be selected according to the analytical requirements, the field of utilization, and the electrochemical paper-based device fabrication techniques [46]. filter, chromatographic and office papers are the most widely utilized substrates for epads. among them, whatman #1 paper is the most extensively used in laboratories, probably due to its availability, excellent wicking ability and absorption of more aqueous solution by a dry filter paper [34,47]. for more information, one can refer to the recent review research reported by desmet et al. with detailed information on the diffusion coefficient values of analytes with epads made of different types of paper, especially http://dx.doi.org/10.5599/jese.1104 j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 402 whatman paper, a comparison of the reported values in solution, the amount of water absorbency by the paper, and the electrochemical techniques used to determine these values [35]. figure 1 shows some examples of origami paper-based devices [25,48-50]. figure 1. examples of origami paper-based devices. a) comparison of one-layer epads and origami-epads [25] reprinted with permission copyright (2019), elsevier; b) schematic illustration of the origami-epads and preparation of epads [48] reprinted with permission copyright (2019), springer-nature; c) schematic illustration of design and details of an origami paper-based analytical device (opad) [49] reprinted with permission copyright (2019), elsevier; d) schematic design of device and size, and shape of the lab-on-paper device [50]; reprinted with permission copyright (2018), elsevier fabrication techniques fabrication of electrochemical paper-based devices since the production of the first paper-based device attributed to müller and clegg [47] and the production of paper-based microfluidics by the whitesides group, which was the first device for determination of glucose created through a photolithography technique [51], there have been a lot of developments in fabrication techniques and paper-based applications [52]. generally, the fundamental techniques of pads fabrication are photolithography [53], wax printing [54,55], inkjet [56], screen printing [57], laser treatment [58], and wet etching [59]. of course, the fabrication techniques of pads are similar to those of microfluidic pads; they are based on the creation of hydrophilic zones for electrochemical detection with hydrophobic borders on paper to confine the flow of fluid inside the desired position. in this regard, hydrophilic and hydrophobic zones can differ based on the volume of the needed solution. n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 403 in this section, the most common kinds of pad and electrode fabrication methods are introduced. according to the literature, wax printing is a very common technique. wax-based fabrication techniques are high-speed, inexpensive, facile and non-toxic. generally, the process involves two steps; first, wax is rubbed on the paper through a screen on it, and then the wax melts into the paper through a hot plate to form hydrophobic barriers to the paper [60,61]. martins et al. [10] produced the first wax-printed paper-based electrochemical device for the detection of an oxidative stress biomarker (3-nitrotyrosine, or 3-nt). to build a three-electrode system on paper, they used carbon and silver conductive inks. sun et al. [54] used an industrial solid-wax printer (xerox phaser 8560dn, japan) for the manufacture of a paper-based device. the technique of photolithography, which has been commonly used in the expansion of paper-based devices, is based on hydrophobization, followed by selective paper dehydrophobization. photolithography has a high resolution and good reproducibility, but it is disadvantageous due to the use of chemical solvents and expensive equipment [62,63]. kaur et al. [53] expanded a microfluidic paper-based cholesterol biosensor. they utilized a patterning filter paper (whatman≠1) to fabricate microfluidic channels by photolithography. a hydrophobic barrier was formed on the filter paper by su 8 photo-resistant microchannels with the dimensions of 1000 μm (wide) × 100 μm (thick). electrode fabrication techniques electrode production is the most important issue of epads that determines how successful and efficient the devices are. there are various techniques to fabricate electrodes on a paper bed, e.g., inkjet printing, screen printing, stencil printing, drawing with pencil/pen, sputtering and e-beam deposition [36,64–67] some of which will be explained here. the inkjet printing technique is easy to use, inexpensive and scalable for production. it also has a wide range of scientific and industrial applications, providing a carrier medium for a set of filler materials. lately, carbon nanotubes (cnt) types of ink, e.g., metal ink, have been applied in inkjet printing. due to requiring no prefabrication of patterns or masks, the technique makes rapid and low-cost printing possible [68]. two modes of printing are used in this technique, referred to as drop-on-demand printing and continuous inkjet printing. in continuous inkjet printing, a continuous stream of ink droplets is formed and then deflected by voltage plates. depending on the applied voltage, the deposition of droplets onto the paper varies via the gutter. in drop-on-demand inkjet printing, the injection of ink droplets occurs by pulses from a nozzle. thermal buckling, thermal resistors, acoustic waves and piezoelectric transducers can generate pulses [69,70]. the principles, applications and advancements of the inkjet printing technology have lately been reviewed by kholghi eshkalak et al. [71]. in a screen/stencilprinting system intended to obtain a desirable electrode on a paper substrate, the ink is pressed and diffused to the open regions of a screen/stencil. of course, hydrophobic zones on paper can be created using this technique. a benefit of the screen/stencil printing technique is reliable repeatability, although there may be low resolution, imprecision and high ink waste because of low contact pressure during production [66,68–73]. in one of the reviewed studies [64], the electrodes were created by screen printing, and the test zone was designed through solid wax printing. the biosensor could perform human antigen diagnosis (immunodeficiency virus p24) in serum with a very low detection limit. clinical analysis epads have been widely applied for a variety of purposes. extending extremely specific and accurate point-of-care (poc) devices for diagnosis, environmental monitoring, early clinical essays http://dx.doi.org/10.5599/jese.1104 j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 404 and treatment control are significant challenges in developing and developed countries [74,75]. reduction of the cost and the quantity of the sample, on-site diagnosis and reduction of patient anxiety are the advantages of poc analytical platforms. accordingly, interdisciplinary research can grow in this context [76,77]. the extremely large surface area of fiber-networks and the power-free fluid flow through a capillary force are the unique features of paper that have made it appropriate for multiple sensing applications, ranging from rather ordinary chemical sensing to specific detection with diverse diagnosis techniques [51,78]. in this regard, some examples of epad applications are mentioned here to indicate the ability of the device in various fields. cancer the unique technique of medical detection is the analysis of biomarkers [79,80]. there are many biomarkers to use for diagnosing human and animal diseases [81]. among them, cancer markers are widely analyzed to evaluate tumor outbreaks. it is of importance to diagnose and control diseases in their early stages [82,83]. regarding cancers, however, a delay in the diagnosis often leads to the loss of the best opportunity for treatment. therefore, it is essential to find ways of quick cancer marker sampling and analysis that are less expensive and easier to conduct as well [6,84]. thus, sensitive, quick and precise identification of cancer cells or carcinogenic biomarkers is a significant part of the studies on epad development. among various cancer tumors, the most malignant one posing the greatest risk to people is lung cancer. the identification of this cancer tumor in its early stages can improve the chance of survival [85]. in this case, the microfluidic paper-based electrochemical dna biosensor (µpedb) presented by tian et al. [84] can be of help. for the sensitive detection of epidermal growth factor receptor (egfr) mutations in patients with non-small cell lung cancer, microfluidic paper-based analytical devices (µpads) are introduced as an alternative to poct instruments. the attractive features of these modern devices for critical analysis are costeffectiveness, simplicity of use, compactness, disposability, and low reagent and sample consumption [86,87]. in this study, a paper zone was modified by aunp layer to create a large surface on a bare pwe and enhance its conductivity to serve as a working electrode, and then polypyrrole (ppy) was polymerized on the aunp-pwe surface. the electrical resistance of the electrode was found to be 0.8 μω, suggesting a wonderful electrical conductivity. next, through noncovalent interaction, the single-stranded dna was adsorbed onto the modified gold electrode surface with the ppy membrane. using the differential pulse voltammetry method, the interaction between methylene blue (mb) and h2o2 was catalyzed by horseradish peroxidase (hrp). this biosensor showed a very low limit of detection (lod) of 0.167 nm. because of the importance of diagnosing the disease, another research group designed an advanced wireless poct system for the detection of neuron-specific enolase. this system consisted of μpads, an electrochemical detector, and an android smartphone. moreover, for the diagnosis of small-cell lung cancer, neuron-specific enolase (nse) was of clinical significance. there was a high demand for the point-of-care diagnosis of nse. for the surface modification of μpads, amino-functional graphene, thionine and gold nanoparticles (nh2-g/thi/aunps) nanocomposites were used. the quality of the poct system was checked, and the results were stored in the eeprom memory; they could be demonstrated in real time using mobile bluetooth [88]. in another study, a gold nanoparticle electrode was used to construct an electrochemical immunosensor for the ultrasensitive diagnosis of the carcinoembryonic antigen (cea) [89]. the counter and the working electrode were printed using gold nanoparticle ink, while industrial silver ink was used to produce the reference electrode. the hydrophobic channels were also impregnated n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 405 on the paper substrates using wax printing. in this study, a mercapto-amine functionalized receptor was developed on a paper-based screen-printed gold (au) electrode (p-spge) for selective cea sensing and a self-assembled monolayer (sam) layer was used to covalently bind the active amine groups of the functionalized mercapto-amine receptors to the anti-cea carboxylic groups. differential pulse voltammetry (dpv) was used to quantitatively evaluate the cea levels, and the detection limit was found to be 0.33 ng ml−1. wang et al. [90] constructed a lab-on-paper system for the extremely sensitive estimation of mcf-7 cells using a polyhedral-aupd nanoparticle-based dual-mode cytosensor. regarding the low content of mcf-7 cells in biological samples, there would be a need to enhance detection sensitivity. therefore, nanoparticles with a wide surface area, superior catalytic activity and excellent conductivity were used for the decomposition of h2o2. in this research, a supersensitive cytosensor was designed based on ph-aupd nps and 3d-rgo/pwe modifier. also, aunps were grown on the surface of this modifier to increase the sample area conductivity. an au@3d-rgo/pwe was made on a suitable lab-on-paper (lpd). then, mcf-7 cells, and h1 and h2 as two different types of aptamers, were placed on the surface of that electrode (figure 2). as it emerged, mcf-7 cells could be better detected by the preloading of aptamer h2 on ph-aupd nps. figure 2. steps of the electrode fabrication and immobilization of a dual-mode cytosensor: (a) h1-sa / phaupd nps preparation process; (b) mcf-7 cells detection and the technique of signal detection [50] reprinted with permission copyright (2018), elsevier in this research, a novel electrochemical paper-based biosensor was also designed for the highly sensitive detection of microrna. for this purpose, the target chain substitution was combined with http://dx.doi.org/10.5599/jese.1104 j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 406 aunps and cu-mof (aunps@cu-mofstab) for their synergetic catalytic effect. all the chain displacement reactions and the electrical signal measurements were done on a compatibly designed origami electrochemical device (oecd). the aunps@cu-mof nanoparticles catalyzed the glucose oxidation, and a limit of detection of 0.35 fm was observed for mirna-155. for both healthy individuals and cancer patients, it is possible to detect mirna-155 in serum with this biosensor [50]. the simple manufacturing of epads has allowed the use of relatively sophisticated sensors that are also economically viable to develop. multiplexed or simultaneous identification of many targets can be carried out by any of these sensors, supplying more accurate information for therapeutic studies [29,91]. a multiplexed system mostly diminishes the amount of the sample volumes needed, the time of the analysis, and the overall expense of the analysis. in this context, sun et al. [54] constructed an advanced rotational paper-based analytical device (rpad) for the multiplexed identification of prostate-specific antigen (psa) and the cancer biomarkers of carcinoembryonic antigen (cea). they made the device by putting three paper disks together using an empty rivet. the on-off states of the paper-based vents were conveniently controlled by the rotation of the paper disks. while other vents are disposable, the humidity of these rotational vents can be taken so as to reuse them. they also have a fast response, which makes this system unique in the categories of such devices. the rotational electrochemiluminescence immunodevices proved to have detection limits of up to 0.07 ng ml-1 and 0.03 ng ml-1 for cea and psa, respectively. an epad was also designed to determine the cancer antigen 125 (ca125) through the screenprinting method. in this study, nanocomposites of (rgo/thi/aunps) were coated on an epad working electrode to immobilize and recognize the signal enhancement of ca125 antibody (anti-ca125). the immunoassay results showed a detection limit of 0.01 u ml−1. in addition, the immunesensor exhibited a good electrochemical performance and a potential for the poct of other tumor markers [33]. one of the important diagnostic methods in the field of sensors is paper-based electroanalytic strips. in this regard, cinti et al. [92] designed experimental comparisons of sensing breast cancer mutations by signal on and signal off paper-based electroanalytical strips. the signal on and signal off methods were tested in combination with paper-based electrodes. a single strand dna for h1047r (a3140g) missense mutation in exon 20 was used as the target in breast cancer. the two methods had limits of detection in the nm range and almost similar analytical results and binding constants measured at the nm level. there were, however, some differences in terms of miniaturization and expenses. while both techniques are promising, the signal off reflects optimum manufacturing and simplicity of use. in another paper, a mobile trap microfluidic paper-based electrochemical device (bio-mip-epads) was presented as a novel strategy for the clinical diagnosis of biomarkers [49]. first, a molecularly imprinted polymer (mip) was electro-synthesized on the standardized region of the instrument, and then it was added to a target analyte as a particular receptor. a non-imprinted polymer (nip–electro-synthesized polymer without the target analyte) was also constructed in the same manner as an mip in an identical area but separate from the system. these two identically structured but separate areas included three substrates of whatman chromatographic paper grade no. 1 patterned by wax printing. in another part of the study, a mobile trap was created to allow the continuous and quick reception and delivery of various liquids needed for the mip/nip syntheses and further electrochemical analysis. in this procedure, the carcinoembryonic antigen was used as a model target, and the cea detection limit was 0.32 ng/ml. kumar et al. [93] presented a nanostructured iron oxide (nfe2o3@pedot:pss) nanocomposite and an electrochemical paper-based cancer biosensor using poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) to detect cea by the amperometry technique. in addition, another research group [94] reported a n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 407 graphene-pedot:pss modified paper-based aptasensor for the diagnosis of this cancer biomarker by electrochemical impedance spectroscopy. compared to the expensive common electrodes (e.g., ito, gold and glass carbon), this paper electrode had a better electrochemical efficiency. table 1 presents a summary of surface modifiers and target biomarkers, as well as the analytical performance of paperbased biosensors for the detection of different cancer biomarkers. table 1. a summary of paper-based biosensors for cancer detection (from 2016 to 2020) electrode surface modifier biomarker sample type technique response range sensitivity detection limit ref. nh2-g/ thi / aunps cea serum dpv1 1-500 μg l‒1 10 ng l-1 [95] ppy/aunp egfr saliva eis2 0.5-500.0 nm 0.167 nm [84] mercapto-amine cea serum dpv1 1.0–100.0 ng ml-1 0.33 ng ml-1 [89] aunps@cu-mofs mirna serum dpv1 1.0 fm -10 nm 0.35 fm [90] au@3d-rgo mcf-7 serum dpv1 50–107 cells ml-1 -6.8 µa dec-1* 20 cells ml-1 [50] mwcnts cea psa serum serum eis2 eis2 0.1-100 ng ml-1 0.1-50 ng ml-1 0.07 ng ml-1 0.03 ng ml-1 [54] rgo/thi/aunps psa serum dpv1 0.05–200 ng ml-1 -2.0 µa dec-1* 10 pg ml-1 [96] rgo/thi/aunps ca125 serum dpv1 0.1–200 u ml-1 -0.37 a ml u-1 0.01 u ml-1 [33] go@chitosan cea serum dpv1 1.0–500.0 ng ml-1 19.3 µa dec-1 0.32 ng ml-1 [49] nfe2o3@pedot:pss cea serum cha3 4-25 ng ml-1 10.2 ma [93] ng/ thi/aunps pb/pedot/ aunps cea psa serum serum dpv1 dpv1 0.01-500 ng ml-1 0.05–500 ng ml−1 -2.8 µa dec-1* -2.8 µa dec-1* 2 pg ml-1 10 pg ml-1 [97] ag-rgo/cysa-au nps ca15.3 plasma cha3 15-125 u ml-1 [98] mos2/aunps/agnw microrna 141 and 121 serum eis2 0.1 fm [99] graphene-pedot:pss cea serum eis2 0.76-14 ng ml-1 0.45 ng ml-1 [94] 1dpv: differential pulse voltammetry; 2eis: electrochemical impedance spectroscopy; 3cha: chronoamperometry; *dec means log of the biomarker concentration neurotransmitters nervous system disorders (nsds) have been one of the main public health concerns over the past century [100]. the fast detection of these disorders is very important for more effective treatment and lower costs for patients [101]. the poct of neurotransmitters is of extreme importance in clinical studies and the fast detection of nsds. paper-based electrochemical biosensors, among the various types of poct platforms, have made enormous progress in the detection of neurotransmitters [102]. nantaphol et al. [103] reported a boron-doped diamond paste electrode (bddpe) coupled with μpads to create an electrochemical sensor with high efficiency. a mixture of boron-doped diamond (bdd) powder and mineral oil was used to prepare a bddpe. it was easily printed in different geometries of the electrode. using μpads, the performance of the bddpe was investigated through the analysis of heavy metals like cd and pb and biological samples like serotonin and norepinephrine. the bddpe proved to have a lower capacitive current and a wider potential window than conventional carbon paste electrodes (cpes). these findings indicate the ability of bddpes as poc sensors when coupled with μpads. also, for biochemical and neurochemical analyses, trouillon et al. [104] designed paper-based polymer electrodes. the electrodes were made by covering of a filter paper with a modified pedot: pss solution. it was found that, unlike planar electrodes, a paper electrode would show better resistance to neurotransmitter fouling as well as protein fouling. a noteworthy finding of the study was that long electrodes are more stable than short ones during the continuous oxidation of serotonin and dopamine. in another research work, casadio et al. [105] applied an electrochemical biosensor based on mips for noradrenaline thermal diagnosis in aqueous solutions. in order to generate modified screen-printed electrodes (spes) with mip, mip polymer particles were added to screen-printed inks, and they were mixed together. the performance of the mip-spes was evaluated through the heat-transfer method http://dx.doi.org/10.5599/jese.1104 j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 408 (htm), an easy and cheap detection method based on thermal resistance. among the advantages of polymer-based systems, one may refer to their high potential in pharmaceutical applications due to their features such as portability, simplicity, and low cost [106]. an electrode of this type has excellent flexibility to adapt to mip layers [107], and it can be targeted toward other disease markers. as a poc sensor, the system creates a good opportunity for pharmaceutical tests. cinti et al. [108] designed a paper-based electrochemical biosensor to detect nerve disorders. in this study, the paper provided an effective surface on which to place reagents (i.e., enzymes). the electrode was printed, and the environmental samples were measured for the recognition of paraoxon as a nerve agent simulant. this system made it possible for the simple and low-cost monitoring of a polluted site without a need for any chemicals or sample preparation, allowing for paraoxon detection down to 3 μg l-1. therefore, the fabrication of this device can be easily expanded for different types of user-friendly stand-alone biosensing platforms. viruses certain detection methods such as enzyme-linked immunosorbent assay (elisa) for igm antibodies [109], reverse transcriptase-polymerase chain reaction (rt-pcr) for rna [110] and electrochemical aptasensor using an aunp-modified screen-printed carbon electrode (spce) for the detection rbd protein s sars-cov-2 [111] are used to fight the spread of viruses. the implementation of these methods is time-consuming and costly. it also needs experienced staff in the laboratory. considering the drastic proliferation of emerging viruses, like the coronavirus (covid-19), which has killed many people around the world recently, there is an urgent need to establish affordable poc technologies for accurate, rapid, and sensitive screening of patients suspected of viral infections. in the meantime, epads provide amazing sensitivity and excellent selectivity and are easily miniaturized [112]. zhao et al. [113] constructed a portable μpad platform for the electrochemical multiplex detection of the antibodies of the hepatitis c and human immunodeficiency viruses in serum. this platform includes an electrochemical microfluidic paper-based immunosensor array and a multi-channel potentiostat and is capable of performing assays in eight samples at the same time within 20 min. the multiplexing function of this device enables it to generate multiple measurement data from a single run for the human immunodeficiency virus (hiv) and hepatitis c virus (hcv) markers. in addition, its wireless module can transfer the results to a remote telemedicine site. a unique combination of paper-based microfluidics and mobile devices makes the platform low-cost, portable, high-throughput, and userfriendly. the identification of hiv and hcv antibodies in the mouse serum with the detection limits of 300 and 750 pg ml-1 could be done by this biosensor. in another development, an economical and handmade paper-based device was designed for the electrochemical detection of the influenza virus [21]. the detection of this virus by means of this paper-based, label-free electrochemical immunosensor actually occurred for the first time. the paper was modified with a spray of hydrophobic silica nanoparticles and used in a stencil-printed electrode. this paper-based biosensor was highly hydrophobic. single-walled carbon nanotubes were used to modify the stencil-printed carbon electrode, and chitosan improved its sensitivity. through glutaraldehyde cross-linking, antibodies were also immobilized. the immunosensor displayed a good selectivity and a low limit of detection (113 pfu ml-1) for the h1n1 virus. this simple device was employed as a disposable and inexpensive biosensor to detect pathogenic microorganisms, especially in developing countries. in another study, an electrochemical biosensor was constructed based on paper-based peptide nucleic acid to determine human papillomaviruses (hpvs) [114]. an anthraquinone-labeled pyrrolidinyl peptide nucleic acid (acpcpna) probe (aq-pna) and a graphene-polyaniline (g-pani)n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 409 modified electrode were used to create this novel electrochemical biosensor. due to properties such as wide potential range, low cost and fast response time, carbon is appropriate to be used in epad dna biosensors. nevertheless, the limited relationship sensitivity of micro-scale electrodes, as a section of epads, is a significant impediment. to solve this problem, graphene, which has a large specific surface area and unique electrochemical properties, was utilized as a carbon-based nanomaterial to modify working electrodes through the inkjet printing technique. the aq-pna probe was also immobilized on the electrode surface through electrostatic attraction. the dna biosensor was then employed to find a synthetic 14-base oligonucleotide target with a sequence corresponding to hpv type 16 dna. square-wave voltammetry was used to measure the electrochemical signal response of the aq label before and after hybridization, as shown in figure 3. the detection limit of the biomarker was 2.3 nm under optimal conditions. figure 3. schematic display of: (a) electrode modification procedure, (b) immobilization and hybridization of the electrochemical paper-based dna biosensor device, and (c) electrochemical detection of the aq label utilizing square-wave voltammetry technique before and after hybridization [113] reprinted with permission copyright (2017), elsevier viral pathogens pose serious health threats around the world, but the common sensing methods are often insufficient and too slow to deal with those threats. to tackle the problem, an eis epad analytical device was designed by channon et al. [115] for the rapid detection of virus particles in minutes. the researchers applied easy patterning techniques to find the impediments on cellulosic paper to efficiently integrate it to functionalized au microwire electrodes. in this respect, dithiol modification could produce a strong base layer to cross-link to antibodies via carbodiimide coupling. any considerable change of impedance was considered as a sign of the virus particles captured onto the antibody-modified electrode. the proposed system and the intelligent electrode modification strategy can provide grounds to determine different intact viruses and other biological targets. the technique will ultimately allow the multiplexed poc diagnosis of viral infections in a rapid and sensitive manner. http://dx.doi.org/10.5599/jese.1104 j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 410 proteins there is an increasing need for the mass production of biosensors that are able to measure protein biomarkers rapidly and efficiently in both clinical and biological research. in this regard, boonkaew et al. [38] designed an origami paper-based electrochemical immunosensor to detect the c-reactive protein (crp) in a certified serum sample. a filter paper (whatman no. 1) and the wax printing method were used to manufacture the device. the adobe illustrator cs6 software was also employed to make an opad pattern. in this study, a screen-printed carbon electrode (spce) was modified with graphene to improve its sensitivity, and then gold nanoparticles were electrodeposited onto the g/spce, followed by the assembling of a monolayer of l-cysteine. finally, the anti-crp was adsorbed and immobilized on the modified electrode (figure 4). [fe(cn)6]3−/4− as a redox probe and sem and cyclic voltammetry were used to validate the modification of the electrode. this method was low-cost, disposable and portable. also, a label-free paper-based electrode was developed for the quantification and determination of the standard bovine serum albumin (bsa) protein [116]. the biosensor included a specific antibody, carbon nanotubes (cnts) and cellulose filtration paper. its limit of detection was found to be 2.89 ng/ml, which is similar to that in the typical elisa technique for bsa measurement. the electrochemical technique employed in that research takes approximately 10 minutes to perform, therefore greatly decreasing the time of analysis compared to the usual elisa method. in another research, gold nanoparticles were used to modify a screen-printed electrode (spe). for this purpose, poly (2-methacryloyloxyethyl phosphorylcholine) (pmpc-sh) with thiol-terminated material was self-assembled on the surface of the electrode. this paper-based electrochemical device was used for the diagnosis of the c-reactive protein (crp). the nonspecific adsorption of the protein was minimized on the pmpc-modified electrode. albumin, myoglobin and bilirubin proved not to interact with this system. it was used for crp identification in a licensed human serum. it is, indeed, a promising sensor for the electrochemical detection of crp using highly sensitive, inexpensive and disposable materials [48]. figure 4. a) schematic display of origami paper-based analytical device (opad) and its components; b) steps of electrode preparation and immobilization for detection of crp by an immunosensor [37] reprinted with permission copyright (2019), springer-nature n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 411 to see how protein immobilization occurs in screen-printed graphite layers, wróblewski et al. [117] investigated six different protein immobilization procedures, including physical adsorption, electrochemical carboxylic group generation, graphite functionalization with succinic anhydride, 1ethyl-3-(3-dimethylaminopropyl) carbodiimide activation, and graphite functionalization with 3(triethoxysilyl) propylsuccinic anhydride. the experiments showed that the best results would be obtained when graphite powder was functionalized before the preparation of the screen-printing paste. to verify the efficiency of the chemical functionalization process in the presence of functionalized groups, edx and xps analyses were conducted. generally, coating a big surface with a high yield is easy through the printing technique, and it is possible to use different substrates, hard ones such as ceramics or glass, as well as flexible ones such as polymer films, textiles or paper. clinical practices butyrylcholinesterase (bche) is localized in the muscles, brain and other tissues [118]. its biological role is unknown, but recent studies have shown that bche hydrolysis affects fat metabolism. it is, thus, called ‘hunger hormone’ [119]. this material has been used as a short-acting blocker of the acetylcholine receptor in anesthesia [120]. the first 3d paper-based printing device to measure bche in human serum samples was made by scordo et al. [121]. the screen printing and wax printing techniques were used to manufacture this paper-based sensor. to measure the bche activity, butyrylthiocholine was applied as an enzymatic substrate. a thiocholine by-product was also recognized by means of an epad biosensor modified with the prussian blue and carbon black nanocomposite. in another study, a ‘pop-up’ electrochemical paper-based analytical device (pop-up-epad) was designed by wang et al. [36] it was used for the analysis of beta-hydroxy-butyrate (bhb), which is a key biomarker of diabetic ketoacidosis. the device was a clever and advanced invention influenced by pop-up greeting cards as a (3d) pop-up system. it proved to have the ability to directly measure bhb in the blood. amor-gutiérrez et al. [122] constructed a multiplexed paper-based electrochemical device with inexpensive materials such as paper, multifunctional connector headers and carbon ink. to easily merge a sampling step, the paper-based electrochemical platform was combined with a glass fiber tape. it could perform eight simultaneous measurements. both tasks, i.e., sampling and simultaneous measuring, were designated to bioenzymatic glucose biosensors. they showed awesome reproducibility and dealt with a wide linear range of concentrations. in another study [123] for the determination of hemoglobin a1c (hba1c), a sensitive electrochemical nanobiosensor was developed. it was used to measure glucose concentration in people with diabetes. the nanobiosensor was made with a paper graphite sheet as an electrode and modified with a nanocomposite of rgo-gold. the nanocomposite increased the surface area and provided an appropriate substrate through the strong covalent bonding of a thiolated dna aptamer as a bioreceptor on the electrode surface. so far, many paper-based biosensors have been developed on the basis of the molecular recognition of single-strand dna (ssdna), antibodies (ab) and antigens coupled with enzymatic reaction. two label-free integrated µpads were developed by wang et al. [124] and ruecha et al. [125]. to immobilize antibodies, the devices were modified with multiwalled carbon nanotubes (mwcnts), thionine (thi), gold nanoparticles (aunps) and polyaniline (pani). the fabrication of the microfluidic channel of the paper-based sensors was done through a wax-printing technique. the modified electrodes were successfully applied to the sensitive, specific and poc diagnosis of two antibodies, 17β-estradiol (17β-e2) and interferon-gamma (ifn-γ), in http://dx.doi.org/10.5599/jese.1104 j. electrochem. sci. eng. 12(3) (2022) 399-419 electrochemical paper-based biosensors 412 human serum. it is to be noted that 17β-e2 plays an important role in regulating reproduction in human beings, while ifn-γ has a critical role in the diagnosis of tuberculosis susceptibility. in another study [12], the clinical biomarkers of uric acid (ua) and creatinine (cnn) were estimated simultaneously in urine samples with high recovery values. the device for this purpose was an epad which consisted of two spot sensors in the same working electrode. for the direct oxidation of ua, the surface of spot 1 was modified with quantum dots of graphene and cnn oxidation. also, the surface of spot 2 was modified with quantum dots of graphene, creatininase and ruthenium electrochemical mediator. the epad was entirely created by cutting a filter paper with a cheap domestic cutter printer. a microfluidic channel was made through the electrode by the production of a screen-printed electrode on a polyester film. these two biological biomarkers were simultaneously characterized sensitively and selectively through square-wave voltammetry. e. coli o157:h7 is one of the most important foodborne pathogenic bacteria, which can often lead to such diseases as bloody diarrhea, hemolytic uremic syndrome and even death [127]. the existing methods for the detection of e. coli o157:h7 mostly lack sufficient sensitivity and mainly have a low detection limit. burrs et al. [128] presented the first graphene paper functionalized with fractal platinum nanocauliflowers to detect e. coli o157:h7 and to perform the electrochemical biosensing of small molecules (e.g., glucose). the researchers illustrated the synthesis of platinum nanocauliflower-graphene hybrids on a nanocellulose paper to be used in poc biosensors. the platinum surface was functionalized with either arna aptamer through covalent linking or glucose oxidase through chitosan encapsulation. the response times were found to be 12 minutes for e. coli and 6 seconds for glucose, which were similar to those of commercial electrode sensors and silicon biochips. table 2 presents a summary of the analytical characteristics of various paper-based sensors for clinical detection. table 2. analytical characteristics of paper-based biosensors for clinical detection (from 2016 to 2020) electrode surface modifier analyte sample type technique response range sensitivity detection limit ref. aunps/cnts bisphenol a abs plastic toys lsv1 0.2 – 20.0 mg l-1 0.03 mg l [129] rgo-cunp/gce cfa urine dpv2 9.9 91.7 μm 367 nm [130] aunps glucose coke cv 0.5–10.0 mm 240 μa mm-1·cm-2 148 μm [55] graphite pencile ascorbic acid commercial tablet swv3 0.5-3.0 mm 0.47 μa m m-1 70 μmol l-1 [131] pb/cb gluthatione serum cha 1-10 mm 0.102 μa m m-1 60 μm [132] cnt metallocenes glucose 1,2-benzanthracene serum serum serum cv cv cv 1 ppt 1 ppt 10 ppt [133] carbon-ink nsaid tap water lsv1 0.1 5.0 μm 0.85 μa μm-1 70 nm [134] silica dexamethasone prednisolone herbal medicine herbal medicine dpv2 dpv2 10-500 mg ml-1 10-500 mg ml-1 19.3 µa dec-1 19.3 µa dec-1 3.59 mg ml-1 11.98 mg ml-1 [135] znonps/pedot:pss hydrazine water cha4 10-500 μm 0.14 μa μm−1 cm−2 5 μm [24] pet glucose sweat amperometric 0.0-1.9 mm 35.7 ma mm-1 cm-2 5 mm [136] silver nanostructure glucose serum cha4 3-3000 μm 4610 μa mm 1.1 μm [137] pt/nafion/gox/nafion glucose saliva potentiometric 316-3160 μm -93.2±1.8 mv dec-1* 120 μm [138] 1lsv: linear sweep voltammetry; 2dpv: differential pulse voltammetry; 3swv: square wave voltammetry; 4cha: chronoamperometry; *dec means log of the biomarker concentration conclusion the literature shows a recent enrichment of studies on epads in terms of quality and number. among all the available biosensing systems, paper-based devices have proved to be highly encouraging owing to their transportability, easy accessibility, low-cost fabrication, amenability for reagent integration, capillary flow properties, and easy patterning. besides, the natural polymeric materials used in them are eco-friendly and safe to discard. therefore, such biocompatible materials are used as solid electrochemical sensor substrates. despite their advantages, epads have some n. sahraei et al. j. electrochem. sci. eng. 12(3) (2022) 399-419 http://dx.doi.org/10.5599/jese.1104 413 limitations in terms of stability, lifetime, multiplexing capabilities and reproducibility. with regard to the unique properties of paper, the use of paper-based miniaturized sensors has widely expanded for point-of-care diagnosis in miniaturized settings. the miniaturization of devices is an issue that attracts increasing attention now that electronic measurements or spectrometric functions can be integrated in smartphones. it has been made easy to attach miniaturized printed electrodes to biomolecules, thus improving their analytical efficiency from the perspective of sensitivity and selectivity. accordingly, the number of analytical experiments conducted outside the laboratory has substantially grown in recent years. this advanced technology widely helps lowand middle-income countries. despite the tremendous achievements in this field, paper-based electrochemical biosensors require more research with a focus on a) achieving reproducible quantitative results, b) developing new methods for the fabrication and modification of electrode materials and paper substrates, c) developing advanced systems with enhanced functions but 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https://doi.org/10.1002/elan.202060221 https://creativecommons.org/licenses/by/4.0/) {electrocatalytic determination of levodopa in presence of cabergoline using carbon paste electrode modified with graphene quantum dots/2-chlorobenzoyl ferrocene/ionic liquid:} http://dx.doi.org/10.5599/jese.1133 81 j. electrochem. sci. eng. 12(1) (2021) 81-90; http://dx.doi.org/10.5599/jese.1133 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrocatalytic determination of levodopa in presence of cabergoline using carbon paste electrode modified with graphene quantum dots/2-chlorobenzoyl ferrocene/ionic liquid peyman mohammadzadeh jahani school of medicine, bam university of medical sciences, bam, iran corresponding author: peymanjahani1234@gmail.com received: october 8, 2021; accepted: november 3, 2021; published: november 17, 2021 abstract the electrochemical sensor was fabricated for the simultaneous determination of levodopa and cabergoline using carbon paste electrode (cpe) modified with graphene quantum dots (gqd), 2-chlorobenzoyl ferrocene (2cbf) and ionic liquid (il). then, the electrochemical behavior of levodopa alone and simultaneously with cabergoline at the surface of gqds/2cbf/il/cpe was investigated in phosphate buffer solution (pbs). under optimal pbs, ph=7 condition, oxidation peak current has been found proportional to levodopa concentration in the range between 0.07 μm and 500.0 μm, with the limit of detection (lod) of 0.02 μm (s/n=3). outputs showed that at gqds/2cbf/il/cpe surface, the levodopa and cabergoline oxidation peaks are separated by the potential difference of 200 mv. in addition, it was found that this modified electrode possesses acceptable sensitivity, selectivity, stability and repeatability. all these properties were sufficient to allow simultaneous detection of levodopa and cabergoline in real samples at the surface of gqds/2cbf/il/cpe. this was supported by the successful application of this electrochemical sensor electrode for the determination of levodopa and cabergoline in urine, serum, and cabergoline tablets. keywords electrochemical sensor; chemically modified electrode, levodopa, cabergoline. introduction there is a continuously growing interest in electrochemical sensors and bio-sensors for drugs in environmental, food, and agricultural analyses. this is probably a result of both, electrochemical behavior of biomolecules and advancement in electrochemical testing [1-4]. hence, merging the rapid, selective, sensitive, precise, affordable, as well as miniaturized electrochemistry-based sensing handhelds with biochemistry, proteomics, nanotechnology, molecular biology, and drug analyses resulted in developing electrochemical sensors [5-9]. http://dx.doi.org/10.5599/jese.1133 http://dx.doi.org/10.5599/jese.1133 http://www.jese-online.org/ mailto:peymanjahani1234@gmail.com j. electrochem. sci. eng. 12(1) (2021) 81-90 electrocatalytic determination of levodopa 82 it is a well-known fact that the carbon paste electrode (cpe) has a widespread utilization for electrochemical determinations of diverse biological and pharmaceutical species, which resulted from low residual currents and noise, simplified construction, wider cathodic and anodic potential range, fast surface renewal, and low cost. in addition, the cpe surface can be easily chemically modified via the addition of diverse materials to enhance the selectivity, quickness, and sensitivity of determinations [10-13]. generally, the chemical modification of an inert substrate electrode with mediators offered significant advantages in the development of electrochemical sensors [14]. redox-active sites shuttle electrons between the analyte solution and the substrate electrode, which is frequently in line with a considerable diminishing of the activation overpotential for the corresponding electrochemical reaction. other advantages of chemically modified electrodes over the unmodified substrate electrodes are their lower susceptibility to the surface fouling and formation of oxides at their surfaces. when choosing modifying materials, it is important to know that functional mediators should exhibit lower relative molar mass and possess reversible, rapid and regenerable reaction at low potentials. furthermore, they should be ph-independent, highly stable in oxidized and reduced forms, unreactive to oxygen, and nontoxic. beyond the most successful mediators, it was already shown that mediators based on ferrocene and its derivatives met most of thementioned criteria [14-16]. in recent years, researchers were also focused on designing and synthesizing nanomaterials for various applications due to their unique physical and chemical properties [17-21]. in this context, graphene quantum dots (gqds) have been introduced as small units of graphene with sizes smaller than 30 nm. gqds are zero-dimensional substances that combine both carbon dots (cds) and graphene features [22]. therefore, researchers considered gqds in diverse areas because of the confinement of quantum and edge impacts that cause specific electronic, optoelectronic, photoelectric, larger surfaces, and better conductive characteristics. with these unique features, gqds have been introduced as a useful material for electrochemical sensors. gqds contributed to larger surface areas in contact with the analyte. as the electroactive surface area is highly significant in electrochemistry, researchers predicted that modifying electrodes by gqds would increase electrochemical reaction rates [23,24]. room-temperature ionic liquids (ils) exhibit encouraging features for electrochemical utilizations, like high ionic conductivity and non-volatility [25,26]. due to high ionic conductivity, broader electrochemical window, and faster ion mobility, ils are frequently employed as electrolytes, binders, and solvents in modified electrode electrochemical preparations. according to the outputs, ils can enhance response sensitivity and simplify direct electron transfer of diverse electroactive compositions [27,28]. in this work, cpe has been modified with all three mentioned materials (ferrocene, gqds and il) to form an efficient sensor electrode for detecting and determining levodopa alone or in the presence of cabergoline. levodopa is one of the catecholamines with an alkylamine chain bond to a benzene ring with 2hydroxyl groups. levodopa that is chosen to treat parkinson’s disease is metabolized by one enzymatic reaction (dopa-decarboxylase) to dopamine and compensates for diminished dopamine in the brain [29]. actually, parkinson’s disease has been considered one of the progressive neurological disorders, which happens in the case of brain failure for the production of sufficient dopamine, which results in tremor, muscles rigidity or stiffness, slow movement (bradykinesia) as well as imbalance. however, it is not possible to directly administer levodopa because it cannot p. mohammadzadeh jahani j. electrochem. sci. eng. 12(1) (2021) 81-90 http://dx.doi.org/10.5599/jese.1133 83 permeate the blood-brain barrier. hence, levodopa that could be taken orally will be utilized for providing a resource of dopamine and for treating parkinson’s disease for relieving the symptoms in a majority of the patients at the early phases of the disease. consequently, researchers designed various analytical procedures to determine levodopa [30-32]. cabergoline is one of the ergot alkaloid derivatives for treating parkinson’s disease as the dopamine agonist. this drug is applied as the only agent for treating early parkinson’s disease and the adjunct to levodopa in the late phase of the disease. it was confirmed that using this drug delayed the initiation of the levodopa-induced motoric consequences and decreased total levodopa dose crucial for sufficient control over the symptoms of the disease. finally, its reverse impacts could be compared with the effects of other dopamine agonists like bromocriptine [33,34]. therefore, the combined treatment of cabergoline and levodopa is usually utilized to treat the disease. in general, the oral administration of cabergoline in a dosage >1 mg/day would be commonly prescribed to treat parkinson’s disease. moreover, researchers used radio-immunoassay and indicated that the level of cabergoline in plasma in the healthy participants receiving a single oral dosage equal to 0.6 mg had been ranged between 80 and 800 pg/ml [34]. the present research detected levodopa in the aqueous buffer solution by the developed gqds/2cbf/il/cpe sensor. also, analytical functions of the modified electrode for quantifying levodopa in the presence of cabergoline was assessed. ultimately, the developed electrochemical sensor has been utilized to determine cabergoline and levodopa in some real samples. experimental instruments and chemicals an auto-lab potentiostat/galvanostat (pgstat 302n, eco chemie, the netherlands) was utilized for electrochemical experimentations and monitored with general-purpose electrochemical system software. a traditional three-electrode cell was used at 25±1 °c. a platinum wire, a conventional ag/agcl/kcl (3.0 m kcl) electrode, and gqds/2cbf/il/cpe were employed as the auxiliary, reference, and the working electrodes, respectively. a metrohm 710 ph meter was used to measure ph. cabergoline, levodopa, and all remaining analytical grade reagents were obtained from merck (darmstadt, germany). finally, ortho-phosphoric acid and its salts (kh2po4, k2hpo4, k3po4) were utilized to prepare 0.1 m phosphate buffer solution (pbs) with ph ranging from 2.0 to 9.0. electrode preparation based on the research design, gqds/2cbf/il/cpe was prepared by dissolution of 0.01 g 2cbf in 3 ml diethyl ether. afterward, this solution was mixed into 0.1 g gqds and 0.89 g graphite powder composed of pestle and mortar. then, 0.6 ml of paraffin and 0.3 ml of il (n-hexyl-3methylimidazolium hexafluoro phosphate) were added to the mentioned mix and shaken for 15 minutes until a uniform paste was achieved. in the next stage, the paste was packed into a glass tube (ca. 3.4 mm i.d. and 10 cm long), with a copper wire located at the carbon paste to ensure electrical contact. afterward, a fresh surface was attained via impelling an excessive paste outside the tube and polishing with a weighing paper. it should be noted that in order to compare the materials, bare cpe was analyzed with gqds/cpe (without 2cbf and il), 2cbf/cpe (without gqds and il), and gqds/2cbf/cpe (without il), which were all procured similarly. the surface areas of gqds/2cbf/il/cpe and bare cpe were obtained by cvs of 1 mm k3fe(cn)6 in 0.1 m pbs, recorded at different scan rates. using the randles-ševčik formula [35] for http://dx.doi.org/10.5599/jese.1133 j. electrochem. sci. eng. 12(1) (2021) 81-90 electrocatalytic determination of levodopa 84 gqds/2cbf/il/cpe, the electrode surface was calculated as 0.35 cm2, about 3.9 times greater than bare cpe. preparation of real samples urine samples were stored in a refrigerator upon the collection. then, 10 ml of the specimens were centrifugated for 15 min at 2,000 rpm, and afterward, a 0.45 µm filter was applied to purify the supernatant. in the next step, various contents of the supernatant solution were transferred into 25 ml volumetric flasks and diluted to the marks with pbs (ph 7.0). the diluted urine specimens were spiked with various levels of cabergoline and levodopa. finally, the recommended process was used to analyze the amounts of cabergoline and levodopa, employing the standard addition technique. the serum samples were prepared similarly by centrifugation, filtration and dilution with pbs (ph 7.0). afterward, the diluted specimen of the serum sample has been injected with various contents of cabergoline and levodopa. next, the newly recommended procedure and standard addition technique were used to analyze cabergoline and levodopa. five cabergoline tablets were ground, and then 100 mg of the obtained powder was dissolved in 25 ml water through ultra-sonication. in the next stage, diverse contents of the above solution were transferred in the cell and diluted by pbs. the standard addition technique was used to determine the contents of cabergoline and levodopa in the tablets. results and discussion electrochemical features of levodopa at the surface of gqds/2cbf/il/cpe for studying the electrochemical oxidation behaviour of levodopa, which is ph-dependent reaction according to the oxidation mechanism presented in scheme 1, finding an optimal ph value would be of high importance for achieving acceptable outputs. scheme 1. electrochemical oxidation mechanism of levodopa at the surface of the modified electrode therefore, we used the modified electrode to run experiments using 100 m of levodopa in 0.1 m pbs of different ph values, ranging from 2.0 to 9.0. according to figure 1, showing oxidation peak current values of 100 m levodopa at gqds/2cbf/il/cpe in dependence on ph of the solution, the most acceptable output was observed for electro-oxidation of levodopa at ph of 7.0. figure 1. ip vs. ph curve obtained from dpvs of gqds/2cbf/il/cpe in solution containing 100.0 μm of levodopa in 0.1 m pbs of different ph (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0) p. mohammadzadeh jahani j. electrochem. sci. eng. 12(1) (2021) 81-90 http://dx.doi.org/10.5599/jese.1133 85 figure 2 represents cv responses for different electrodes in solutions with and without 100.0 µm of levodopa. cvs of bare cpe in the blank solution and solution containing 100.0 µm of levodopa are presented by curves a and b, respectively. cvs of gqds/2cbf/il/cpe in blank and levodopa solutions are presented as curves c and f, respectively. the rest two curves are cvs recorded for partially modified electrodes in levodopa solutions, i.e., gqds/cpe (curve d), and 2cbf/cpe (curve e). figure 2 demonstrates that in solutions containing levodopa, the potential of the anodic peak is approximately 820 mv for oxidizing levodopa at the bare cpe surface (curve b) and 645 mv at the surface of gqds/2cbf/il/cpe (curve f), what is a difference of 175 mv. e / mv vs. ag/agcl/kcl figure 2. cvs (10 mvs-1) of unmodified and modified cpes in 0.1m pbs (ph 7.0): (a) unmodified cpe without levodopa, and (b) with 100.0 µm levodopa; (c) gqds/2cbf/il/cpe without levodopa and (f) with 100.0 µm levodopa; (d) gqds/cpe and (e) 2cbf/cpe with 100.0 µm levodopa the highest oxidation currents were observed for the surface of 2cbf/cpe (curve e) and particularly for gqds/2cbf/il/cpe (curve f). the significant increase of the anodic peak current for gqds/2cbf/il/cpe compared to either gqds/cpe or 2cbf/cpe, implies the influence of ionic liquids (ils) present on cpe. il/cpe has some benefits like quick transfer of electrons, suitable antifouling traits, greater conductivity, and the catalytic nature of ils. therefore, il mass has been inserted into the carbon and paraffin oil that connect the granules. thus, il/cpe conductivity has been considerably improved, which is consistent with the electrochemistry outputs of the present study. finally, gqds on the il surface largely enlarged the electrochemical responses likely caused by the potential features of the gqds, such as the greater surface area, stronger chemical stability, and suitable electrical conductivity. impacts of the scan rate the effectiveness of the potential scan rate on the oxidation current of levodopa is presented in figure 3, showing linear sweep voltammograms of gqds/2cbf/il/cpe in 0.1 m pbs, ph 7.0 with 100.0 μm of levodopa. it is obvious from figure 3 that higher scan rates resulted in enhanced oxidation peak current values. additionally, it has been found that ip is linearly related to the square root of the potential scan rate (ν1/2), demonstrating that levodopa oxidation is the diffusion-controlled process (figure 3, inset a). in addition, the electrocatalytic mechanism (ec′) has been shown by the plot of the scan rate normalized current (ip/ν1/2) against the scan rate (figure 3, inset b). http://dx.doi.org/10.5599/jese.1133 j. electrochem. sci. eng. 12(1) (2021) 81-90 electrocatalytic determination of levodopa 86 e / mv vs. ag/agcl/kcl figure 3. linear sweep voltammograms of gqds/2cbf/il/cpe in 0.1 m pbs, ph 7.0 with 100.0 μm levodopa, at distinct scan rates (1-9 correspond to 5, 10, 20, 30, 40, 50, 60, 70, and 80 mv s-1). insets: (a) variation of anodic peak current with ν1/2; (b) variation of scan rate normalized currents (ip/ν1/2) with scan rate chronoamperometric measurements the analysis of chronoamperometry for levodopa specimens at gqds/2cbf/il/cpe was performed at 0.7 v. figure 4 displays chronoamperometric outputs of diverse concentrations of levodopa in pbs, ph 7.0. in addition, the cottrell equation was applied for the chronoamperometric analysis of electroactive moieties reaction based on the mass transfer restricted conditions [35]: i = nfad1/2cbπ-1/2t-1/2 (1) in eq. (1), d is diffusion coefficient (cm2 s-1), cb stands for the bulk concentration of analyte (mol cm−3), a is electrode surface area (cm2), n is the number of transferred electrons, and t is time. figure 4a shows experimental results of i vs. t−1/2, reflecting the best fit for distinct concentrations of levodopa. figure 4. chronoamperograms of gqds/2cbf/il/cpe in 0.1 m pbs, ph 7.0 for different levodopa concentrations (1-4 correspond to 0.1, 0.6, 1,5 and 3.0 mm). inset a: i plot vs. t-1/2 from chronoamperograms 1 to 4. inset b: slope plot of straight line vs. concentration of levodopa afterward, final slopes relative to the straight lines in figure 4a were drawn versus levodopa concentration (figure 4b). using the cottrell equation and resultant slopes, the mean value of d was calculated as 7.45×10-5cm2/s. calibration plot and detection limit considering the oxidation peak currents for different concentrations of levodopa with gqds/2cbf/il/cpe, levodopa can be quantitatively analysed in the water solution. the modified p. mohammadzadeh jahani j. electrochem. sci. eng. 12(1) (2021) 81-90 http://dx.doi.org/10.5599/jese.1133 87 electrode (gqds/2cbf/il/cpe) was used as a working electrode in the concentration range of levodopa from 0.07 to 500.0 µm m in 0.1 m pbs, ph 7.0, and differential pulse voltammetry (dpv) measurements were performed due to dpv merits like more reasonable sensitivity and more acceptable performance in analytical utilizations. the recorded dpvs (step potential=0.001 v, amplitude=0.02 v) are presented in figure 5, showing that peak currents are linearly related to levodopa concentration ranging from 0.07 to 500.0 µm, with the correlation coefficient equal to 0.9999. also, the detection limit, cm, of levodopa was obtained using the following equation: cm = 3sb / m (2) in equation (2), m is the slope of the calibration plot (0.1003 μa μm-1) and sb is the standard deviation of the blank response obtained from 20 replicate measurements of the blank solution. the detection limit was calculated as 0.02 μm. e / mv vs. ag/agcl/kcl figure 5. dpvs of gqds/2cbf/il/cpe in 0.1 m pbs, ph 7.0 with distinct concentrations of levodopa (1–9 correspond to 0.07, 5.0, 20.0, 50.0, 100.0, 200.0, 300.0, 400.0, and 500.0 μm). inset: peak current plot vs. levodopa concentration concurrent detection of levodopa and cabergoline no study has been reported on the use of cpe modified with gqds/2cbf/il for simultaneous detection of cabergoline and levodopa. in addition, because the electrochemical detection of levodopa in the presence of cabergoline at unmodified electrodes would have a drawback of interference with cabergoline due to the comparative oxidation capacity of both samples, we tried to separate two analyte peaks. this stage has been proceeded by concurrent changes in the analyte concentration and recording dpvs. it is shown in figure 6 that specific anodic peaks have been observed at 630 and 830 mv for oxidizing levodopa and cabergoline, respectively, which confirms the use of gqds/2cbf/il/cpe, by which it becomes possible to detect the analytes with no interference between them. stability of gqds/2cbf/il/cpe according to the research design, the stability of gqds/2cbf/il/cpe was tested via holding it in pbs ph 7.0 for 20 days. then, the cyclic voltammogram was registered in the presence of 50.0 µm levodopa after cycling the potential fifteen times at 50 mv s−1. the measured cv was then compared with that observed before submersion. according to the findings, the levodopa oxidation peak did not change the peak potential value, while peak current value showed a certain decrease (≤2.5 %) compared to the initial response, reflecting acceptable stability of gqds/2cbf/il/cpe. http://dx.doi.org/10.5599/jese.1133 j. electrochem. sci. eng. 12(1) (2021) 81-90 electrocatalytic determination of levodopa 88 e / mv vs. ag/agcl/kcl figure 6. dpvs of gqds/2cbf/il/cpe in 0.1 m pbs, ph 7.0, with various concentrations of levodopa and cabergoline (1–6 correspond to 0.0 + 8.0, 20.0 + 50.0, 50.0 + 125.0, 100.0 + 250.0, 200.0 + 500.0, and 400.0 + 900.0 µm of levdopa and cabergoline. inset a: ip plot vs. levodopa concentration. inset b: ip plot vs. cabergoline concentration interference study this study examined the possible effects of various materials as compounds that might have a potential interference with cabergoline and levodopa detection under optimized conditions with 50.0 µm levodopa at ph 7.0. it should be noted that potentially interfering materials have been selected from a group of materials usually observed with levodopa pharmaceuticals and/or biological fluids. the limit of tolerance has been described as the highest concentration of the interfering material, which gives rise to less than ±5 % error in detecting levodopa. as shown by the outputs, glucose, lactose, fructose, sucrose, ethanol, citric acid, methanol, fe3+, mg2+, fe2+, al3+, so42-, co32-, nh4+, f-, cl-, glycine, alanine, methionine, folic acid, phenylalanine, urea, and the saturated starch solution had no interference with cabergoline and levodopa detection. on the other side, ascorbic acid, dopamine, norepinephrine, and epinephrine with the same concentration showed some interference with cabergoline and levodopa detection. even though ascorbic acid had interfered, it might be omitted if required via the ascorbic oxidase enzyme that has an excellent selectivity to oxidizing the ascorbic acid. analysis of real samples for assessing the usability of the developed modified electrode for determination of levodopa and cabergoline in real samples, gqds/2cbf/il/cpe was applied for biological fluids and drugs, i.e., urine, serum and cabergoline tablets. consequently, a standard addition procedure was employed, and outputs are presented in table 1. as seen, reasonable recovery of levodopa and cabergoline, as well as reproducible outcomes were obtained based on the mean relative standard deviation (rsd). table 1. levodopa and cabergoline concentrations determined by gqds/2cbf/il/cpe in real samples (n=5). concentration, m recovery, % rsd, % sample spiked found levodopa cabergoline levodopa cabergoline levodopa cabergoline levodopa cabergoline urine 0 0 5.0 7.5 4.9 7.7 98.0 102.7 3.5 1.9 10.0 12.5 10.1 12.3 101.0 101.0 2.7 2.4 serum 0 0 6.0 10.0 6.1 9.8 101.7 98.0 2.3 3.1 12.0 15.0 11.7 15.5 97.5 103.3 2.9 1.7 cabergoline tablet 0 0 3.5 3.2 5.0 2.5 5.1 5.9 102.0 98.3 3.4 1.9 12.0 7.5 11.7 11.3 97.5 102.7 2.4 2.7 p. mohammadzadeh jahani j. electrochem. sci. eng. 12(1) (2021) 81-90 http://dx.doi.org/10.5599/jese.1133 89 conclusions an electrochemical sensor with high sensitivity, selectivity, and stability based on gqds/2cbf/il modified cpe was successfully fabricated. gqds/2cbf/il/cpe showed much better performance when compared with bare cpe, gqds/cpe and 2cbf/cpe. the developed gqds/2cbf/il/cpe showed excellent catalyzing effect for levodopa oxidation and found suitable for simultaneous determination of 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(https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.22034/chemm.2021.118446 https://doi.org/10.22034/chemm.2021.118446 https://doi.org/10.1016/j.matchemphys.2020.122640 https://doi.org/10.26655/jmchemsci.2019.3.3 https://doi.org/10.20964/2017.01.03 https://doi.org/10.1016/j.talanta.2015.12.016 https://doi.org/10.1016/j.elecom.2010.11.005 https://doi.org/10.30473/icc.2019.40174.1429 https://doi.org/10.1016/j.jelechem.2011.01.008 https://doi.org/10.1002/elan.200704042 https://doi.org/10.1016/j.foodchem.2015.04.009 https://doi.org/10.1016/j.aca.2008.01.006 https://doi.org/10.1016/j.aca.2008.01.006 https://doi.org/10.1016/j.jpba.2010.09.042 https://doi.org/10.1016/j.talanta.2007.03.023 https://doi.org/10.1016/j.talanta.2007.03.023 https://doi.org/10.1002/1531-8257(199909)14:5%3c725::aid-mds1003%3e3.0.co;2-l https://doi.org/10.1016/s1570-0232(03)00279-4 https://creativecommons.org/licenses/by/4.0/) electrocoagulation removal of anthraquinone dye alizarin red s from aqueous solution using aluminum electrodes: doi:10.5599/jese.290 199 j. electrochem. sci. eng. 6(2) (2016) 199-213; doi: 10.5599/jese.290 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrocoagulation removal of anthraquinone dye alizarin red s from aqueous solution using aluminum electrodes: kinetics, isothermal and thermodynamics studies abideen idowu adeogun* , ** , , ramesh babu balakrishnan** *department of chemistry, federal university of agriculture, abeokuta, nigeria **electrochemical pollution control division, csir central electrochemical research institute. karaikudi 630006, india corresponding author: abuaisha2k3@yahoo.com, tel: +23480306126987 and +918098772434 received: may 3, 2016; revised: may 22, 2016; accepted: may 23, 2016 abstract electrocoagulation (ec) was used for the removal of anthraquinone dye, alizarin red s (ars) from aqueous solution. the process was carried out in a batch electrochemical cell with al electrodes in a monopolar connection. the effects of some important parameters such as current density, ph, temperature and initial dye concentration, on the process were investigated. equilibrium was attained after 10 minutes at 30 °c. pseudo-first order, pseudo-second order, elovic, and avrami kinetic models were used to test the experimental data in order to elucidate the kinetics of the electrocoagulation process; pseudo-first order and avrami models best fitted the data. experimental data were analyzed using six isotherm models: langmuir, freudlinch, redlich–peterson, temkin, dubinin–radushkevich and sips isotherms and it was found that the data fitted well with dubinin–radushkevich and sips isotherm model. the study showed that the process depended on the current density, temperature, ph and initial dye concentration. the calculated thermodynamics parameters (∆g o , ∆h o and ∆s o ) indicated that the process is spontaneous and endothermic in nature. keywords batch; electrochemical cell; mnopolar connection; pseudo first order; equilibrium; spontaneous. introduction the advanced in technologies for industrialization and urbanization has substantially enhanced the degradation of the environment especially the aquatic environment, through the discharge of industrial wastewaters and domestic wastes [1,2]. the residual dyes from different sources such http://www.jese-online.org/ mailto:abuaisha2k3@yahoo.com j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 200 as: textile, paper and pulp, dye and dye intermediates, pharmaceutical, tannery, and kraft bleaching industries are considered as organic coloured pollutants [3–6]. these industries utilize large quantities of a number of dyes with residues leading to the large amount of coloured wastewaters, toxic and even carcinogenic, posing serious hazard to aquatic living organisms. most dyes used in industries are stable to light, heat and oxidation, they are not biologically degradable and are also resistant to aerobic digestion and even when they does, they produce toxic and hazardous products [7,8]. anthraquinone dye alizarin red s (ars) is available as sodium salt of 1,2-dihydroxy-9,10-anthraquinonesulfonic acid (fig. 1) and it is referred to as mordant red 3, c.i. no. 58005. alizarin, a natural dye obtained from madder (rubiatinctorum, l. rubiaceae) by sulphonation, is a watersoluble and widely used anthraquinone dye in textile and as a stain in clinical study of synovial fluid to assess basic calcium phosphate crystals [9]. ars is a durable pollutant when released to aquatic ecosystems. it cannot be completely degraded by general physicochemical and biological processes because of the complex structures of the aromatic rings that afford high physicochemical, thermal, and optical stability [7,10]. therefore, most treatments for such dye-laden effluents are largely inadequate; however, removal of this dye from industrial wastewaters is a crucial process, from both economic and environmental points of view [11]. figure 1. structure of alizarin red s (ars) electrochemical techniques which include electrooxidation, electrochemical reduction, electrocoagulation, electroflotation [12], have been developed for the treatment of organic pollutants in waste water with higher efficiency than any other biological, physical and chemical process [13,15]. electrocoagulation has been known for some time as a process capable of fractionating a number of organic substances in a rather efficient manner. the coagulants are generated in-situ by electrooxidation of the anode. the mostly used anode materials are iron and aluminum because of their availability and relatively low cost. electrocoagulation is accomplished in a three step processes as follows: (i) electrolytic reactions at surface of electrodes, (ii) formation of coagulants in aqueous phase and (iii) adsorption of soluble or colloidal pollutants onto coagulants and removal of them using sedimentation or flotation of flocs when hydrogen gas bubbles were produced at the cathode [16]. several methods have been used for the removal of ars from wastewaters, these include adsorption [17-19], electrochemical degradation [20-23] and electro-fenton process [24] using various electrodes. however, to the best of our knowledge, kinetics and isothermal studies have not been elucidated in any of these studies. in this study, ars was removed from aqueous solution in a mono-polar electrochemical cell using aluminum electrodes. the effect of current density, initial dye concentration, electrolyte concentration, ph and temperature were studied. adsorption kinetics of electrocoagulants was analyzed with pseudo-first order, pseudo-second order, elovic, and avrami kinetic models. the o o na+ so o o oh oh a. i. adeogun et al. j. electrochem. sci. eng. 6(2) (2016) 199-213 doi:10.5599/jese.290 201 diffusion mechanism was analyzed with intraparticulate diffusion model while the equilibrium adsorption behaviour was analyzed by fitting the equilibrium data with six isotherm models. thermodynamic parameters such as free energy (δg), enthalpy (δh) and entropy (δs) were also determined to understand the spontaneity of the electrocoagulation process. the following reactions were envisaged at the electrodes: anodic reaction: al  al 3+ + 2e (1) cathodic reaction: 3h2o + 3e  3oh+ 3/2h2 3h2o (2) overall reaction: al 3+ + 3oh  al(oh)3 (3) experimental dye solution preparation alizarin red s (sodium alizarin sulphonate (fig. 1), ci 58005, product no. – 13005) was a product of british drug house, poole, england. 1000 mg l -1 aqueous solution of ars was prepared with de-ionized water as the stock solution and was further diluted with de-ionized water to obtain the working standard solutions. the ph of the solution was adjusted when necessary with aliquots of 1.0 mol l -1 of hcl and naoh before the commencement of the experiment. the conductivity of the solution was maintained with nacl solution as the electrolyte. experimental apparatus and procedures figure 2 depicts the electrocoagulation cell which consists of a 0.6 l glass cell fitted with a polycarbonate cell cover with slots to introduce the electrodes, thermometer and electrolyte. the aluminum electrodes of dimensions 4.5×7×0.3 cm with inter electrode distance of 2 cm were fully immersed in the 0.5 l solution of the dye. a regulated direct current (dc) was supplied from a rectifier (0 – 2 a, 0 – 35 v; applab 7711 multi-output). the temperature of the electrolyte was controlled to the desired value with a variation of ±1 °c by adjusting the temperature knob on the ika rct basic magnetic hotplate stirrer and was allowed to equilibritate before the commencement of the experiment. figure 2. laboratory scale electrocoagulation cell analytical procedure the concentration of the dye in solution was estimated using spectrophotometer (uv-vis –nir varian 500 scan cary). to compare the dye obtained from the coagulation with the original ars j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 202 dye, the flock is filtered and the residue dried at 105 °c for 30 minutes, then the ftir of the two samples (ars and residue) were obtained using ftir spectrophotometer (tensor 27 bruker optik gmbh, germany). non-linear regression analysis method using a program written on micro math scientist software (salt lake city, utah) was used to obtain the least square fit for all the models. equilibrium studies the effects of current density, initial dye concentration, electrolyte concentration, ph and temperature on the electrocoagulation removal of ars were studied. sample solutions were withdrawn at intervals to determine the residual dye concentration by using uv-vis-nir spectrophotometer. the amount of dye coagulated at equilibrium, qe / mg g -1 , was calculated using equation 4 below: o e e ( )c c v q w   (4) where co / mg l -1 is the initial concentration and ce /mg l -1 is the concentration of the dye at equilibrium in the liquid-phase. v is the volume of the solution, l while w is the mass of the coagulant which can be estimated from faraday law according to the equation 5: mit w nf  (5) m is the molar mass (g mol -1 ) of the elements, i is the current (a), t is the electrocoagulation time in seconds, n is the number of electrons involved and f is faraday’s constant (96,485 c mol -1 ). the percentage dye removal as colour was estimated by: 0 e 0 ( ) colour removal, % 100 abs abs abs    (6) where abs0, is the blank absorbance and abse is the absorbance at equilibrium. adsorption isotherms the equilibrium data from this study were described with the six adsorption isotherm models. these are models by langmuir [25], freudlinch [26], temkin [27], dubinin and radushkevich [28], sip [29] and redlich and peterson [30]. the acceptability and suitability of the isotherm equation to the equilibrium data were based on the values of the correlation coefficients, r 2 estimated from linear regression of the least square fit statistic on micro math scientist software. electrocoagulation kinetics studies since the amount of coagulant can be estimated for a given time, the pollutant removal can be modelled using an adsorption phenomenon. the procedures for the kinetics studies were basically identical to those of equilibrium tests. the aqueous samples were taken at preset time intervals, and the concentrations of the dye were similarly determined. the amount of dye removed at time t, qt (mg g -1 ), was calculated using equation 7: o t( ) t c c v q w   (7) where co / mg l -1 is the initial concentration and ct / mg l -1 is the concentration of the dye at time t in the liquid-phase. v is the volume of the solution (l), and w is the mass of al(oh)3 calculated as stated in eqn. 5 above. in order to investigate the mechanisms of the adsorption process, pseudoa. i. adeogun et al. j. electrochem. sci. eng. 6(2) (2016) 199-213 doi:10.5599/jese.290 203 first order, pseudo-second order, avrami, and elovich models respectively were applied to describe the kinetics of adsorption of ars to al(oh)3 generated during the electrocoagulation process. since the diffusion mechanism cannot be obtained from the kinetic model, the intraparticulate diffusion model [8] was also tested. a model is adjudged best-fit and selected based on statistical parameters. statistical test for the kinetics data the acceptability and hence the best fit of the kinetic data were based on the square of the correlation coefficients r 2 and the percentage error function which measures the differences (sse, %) in the amount of the dye concentration coagulated at equilibrium predicted by the models, (qcal) and the actual, (i.e. qexp) measured experimentally. the validity of each model was determined by the sum of error squares (sse, %) given by: 2 exp cal exp (( ) ) sse, % 100 1 q q q n     (8) n is the number of data points. the higher is the value of r 2 and the lower the value of sse; the better fitted the data. thermodynamics of electrocoagulation process arrhenius equation is applied to estimate the activation energy of adsorption according to the relationship: aln ln e k a rt   (9) where k is the rate constant obtained from the kinetic model, ea is the arrhenius activation energy of adsorption, (kj/mol), a is the arrhenius factor, r is the universal gas constant (8.314 j mol -1 k -1 ) and t is the absolute temperature. the thermodynamics parameters i.e. ∆g◦, ∆h◦ and ∆s◦ were estimated using the following relation: o dlng rt k   (10) o o d ln s h k r rt     (11) the equilibrium constant, kd, is obtained from the value of qe/ce at different temperature equilibrium study. van’t hoff plot of ln kd against the reciprocal of temperature (1/t), should give a straight line with intercept as so/r and slope as ho/r. results and discussion batch equilibrium studies effect of current density the amount of coagulants generated is related to the time and current density [31]. current density determines the coagulant production rate, and adjusts the rate and size of the bubble production, and hence affects the growth of flocs [32,33]. the effect current density on the removal of ars, were carried at the current density varied between j of 7.15 to 35.77 a m -2 . figure 3 shows the plot of current density versus the percentage colour removal by the j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 204 electrocoagulation process. from the figure it is glaring that as the current density is increased the rate of colour removal also increased from 96 to 99.31 %. increasing current density results in a corresponding increase in the production of coagulant in the solution leading to high efficiency. the optimum current density of 35.77 a m -2 was used throughout the study. figure 3. effect of current density on colour removal ([ars] = 50 mg l -1 , t = 30 o c, ph 7.0 and [nacl] = 2 g l -1 ). effect of ph on electrocoagulation process ph is an important parameter influencing the performance of the ec process [34], it affects the chemistry of both the coagulants, dye molecules and that of electrochemical process in the solution. the effect ph on the removal of ars was studied at initial ph varied between 3 and 11. the ph was adjusted with 0.1 m naoh or 0.1 m hcl. the color removal percentages for dye solutions with various initial ph values were shown in fig. 4. figure 4. effect of initial ph on colour removal ([ars] = 50 mg l -1 , t = 30 o c, [nacl] = 2 g l -1 , j = 21.5 a m -2 ) the colour removal efficiency are optimum at the at ph range of 6.5 and 7.5 with about 97.5 % colour removal. the decrease in removal efficiency at more acidic and alkaline ph had been attributed to amphoteric behaviour of al(oh)3 which leads to soluble al 3+ cations (at acidic ph) and a. i. adeogun et al. j. electrochem. sci. eng. 6(2) (2016) 199-213 doi:10.5599/jese.290 205 formation of monomeric anions (at alkaline ph). these ions transform finally into solid al(oh)3 according to complex precipitation kinetics thereby affecting the removal efficiency [35,36]. effect of electrolyte concentration solution conductivity influences the current efficiency, cell voltage and consumption of electrical energy in electrolytic cells. the use of nacl to increase solution conductivity is also accompanied by the production of chloride ions that reduces the effects of other anions, such as bicarbonate and sulphate which may lead to the precipitation of ca 2+ leading to the high ohmic resistance of the electrochemical cell [17]. the tests of electrolyte concentration effects were performed in solutions containing a constant concentration of ars 50 mg l -1 , at current density of 21.5 a m -2 and ph 7, while the concentrations of nacl were varied between 1 to 5 g l -1 . figure 5 shows that colour removal efficiency percentage increases from 86 to 97.6 % as the electrolyte concentration rises up to 2 g l -1 . a further increase in electrolyte concentrations beyond these values does not significantly affect the removal efficiency of the dye from the solution. the results also suggest that high colour removal percentage with low cell voltages and low energy consumption can be obtained at nacl concentration of 2 g l -1 . figure. 5. effect of electrolyte concentrations on colour removal ([ars] = 50 mg l -1 , t = 30 o c, ph = 7, j = 21.5 a m -2 ) effect of initial dye concentrations the effect of initial dye concentration on the electrocoagulation removal of ars is shown in figure 6 for dye concentrations increasing from 25 to 100 mg l -1 at constant temperature of 30 °c, current density 21.5 a m -2 , ph 7.0 and electrolyte concentration maintained with 2 g l -1 nacl. the process showed rapid removal in the first 10 minutes for all the concentrations studied. the efficiency of the process increases from 138 to 604.8 mg g -1 as the initial concentration increases from 25 to 100 mg l -1 . as there is no significant difference in the amount coagulated after 20 minutes of the process, a steady-state approximation was assumed and a quasi-equilibrium situation was reached. the electrocoagulation curves were single, smooth, and continuous, leading to saturation. this is an indication of possible monolayer coverage on the surface of electrochemically generated coagulant [18,37]. j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 206 figure 6. effect of initial concentration on the electrocoagulation removal of ars (t = 30 o c, ph = 7, j = 21.5 a m -2 [nacl] = 2 g l -1 ) adsorption study adsorption isotherms the adsorption data obtained at different initial dye concentrations were fitted into six different isotherm models. the equations representing these models and the parameters are summarized in table 1, the details of which have been explained elsewhere [38]. table 1. isotherm models used for the study of the ec removal of ars isotherm name isotherm model parameters langmuir max e e e 1 q bc q bc   qmax and b freudlinch qe = kfc 1/n kf and n, temkin e t e t ln( ) rt q a c b  at and bt dubinin– radushkevich 2 e e s 2 1 ln 1 2 rt c q q exp e            qs and e redlich–peterson o e e g r e (1 ) q c q k c   qo, kr and g sip s s s s e e s e ( ) (1 ( ) ) q k c q k c     qs, ks and s the adsorption data fitted well with all the isotherms as shown in figure 7, however, dubinin -radushkevich and sip’s isotherms have the highest r 2 (table 2). the qm value of 785.31 mg g -1 obtained for the langmuir isotherm model when compared with other isotherm q values shows that the dubinin–radushkevich and sip’s isotherm values are lower than the langmuir isotherm (table 2). the 1/n value of > 1 obtained for freudlinch isotherm is also indication that the adsorption is favorable. a. i. adeogun et al. j. electrochem. sci. eng. 6(2) (2016) 199-213 doi:10.5599/jese.290 207 figure 7. least square plots of the (a) langmuir, (b) freundlich, (c) redlich–peterson, (d) temkin (e) dubinin–radushkevich and (f) sips isotherms for electrocoagulation of alizarin s dye at 30 °c. j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 208 table 2. isotherm parameters for electrocoagulation removal of alizarin s isotherms parameters values langmuir qmax / mg g -1 785.31 b / l mg -1 0.27 rl 0.055 r 2 0.996 freudlinch kf / mg g -1 (mg l -1 ) -1/n 188.85 n 0.53 r 2 0.992 tempkin at / l g -1 13.06 bt / j mol -1 g mg -1 2.12 r 2 0.997 dubinin–radushkevich qs / mg g -1 653.10 e / kj mol -1 5.65x10 -7 r 2 0.941 redlich–peterson q0 / mg g -1 4372.23 kr / (mg -1 g) 1/g 0.036 g 1.80 r 2 0.998 sips qs / mg g -1 524.63 ks / (mg -1 l) 1/β s 0.57 s 2.05 r 2 0.999 electrocoagulation kinetics the plots of four different kinetic models used to explain the adsorption data are shown in figure 8 (a d). the equations for these models are shown in table 3, details of which have explained elsewhere [38]. as shown in the figure, pseudo-first-order kinetic models fitted well with experimental data when compared with other models the r 2 value of the model also confirmed this (table 4). the rate constant from all the models increases with initial dye concentration up to 75 mg l -1 before decreasing at 100 mg l -1 . this shows that at higher initial concentration the electrostatic interaction decreases at the site, thereby lowering the adsorption rate. the behaviour of elovich constant shows that the process of adsorption is more than one mechanism. table 3: kinetic models for the ec process isotherm name isotherm model parameters pseudo first order 1( ) t e (1 ) k t q q e    qe and k1 pseudo secon order 2 e 2 t e 2 (1 ) q k t q q k t   qe and k2 elovich t 1 ln( )q t   α and β avramin avav t e (1 ) n k t q q e    kav, and nav intraparticulate model 0.5 t id i q k t c  kid and ci a. i. adeogun et al. j. electrochem. sci. eng. 6(2) (2016) 199-213 doi:10.5599/jese.290 209 figure 8. least square plots of (a) pseudo-first-order, (b) pseudo-second-order, (c) avramin and (d) elovich kinetic models for alizarin s dye electrocoagulation at 30 o c. table 4. comparison of kinetic model constant values for alizarin red s removal by electrocoagulation c0 / mg l -1 25.00 50.00 75.00 100.00 qe exp / mg g -1 74.52 130.75 313.80 369.92 first order qe cal / mg g -1 97.30 135.75 322.81 413.23 k1 / min -1 0.12 0.26 0.28 0.15 r 2 0.995 0.999 0.997 0.992 sse, % 6.11 0.76 0.57 2.34 second order qe cal / mg g -1 136.04 155.88 367.39 548.90 k2 10 -3 / g mg -1 min -1 0.7204 2.3817 1.1079 0.2444 r 2 0.993 0.998 0.998 0.989 sse, % 16.51 3.84 3.42 9.68 avramin qe cal / mg g -1 97.30 135.75 322.81 413.23 kav / min -1 0.35 0.51 0.53 0.39 nav 0.35 0.51 0.53 0.39 sse, % 6.11 0.76 0.57 2.34 r 2 0.995 0.999 0.999 0.992 elovich  / g mg -1 0.03 0.04 0.02 0.01 α / mg (g min) -1 23.52 267.45 840.16 129.21 r 2 0.993 0.997 0.998 0.989 sse, % 13.69 20.91 33.55 13.01 j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 210 adsorption mechanism the mechanism of adsorption was investigated by subjecting the data to intra-particulate diffusion model. the plots are shown in figure 9. the plots are not linear over the whole time range but rather they exhibit multi-linearity revealing the existence of two successive adsorption steps. the first stage is faster than the second, and it is attributed to the external surface adsorption referred to as the boundary layer diffusion. thereafter, the second linear part is attributed to the intraparticle diffusion stage; this stage is the rate controlling step. table 5 shows the intra-particle model constants for the electrocoagulation removal of ars dye. the kdi values were found to increase from first stage of adsorption toward the second stage. the increase in dye concentration results in an increase in the driving force thereby increasing the dye diffusion rate. figure 9. intraparticulate diffusion model plots for alizarin s dye electrocoagulation at 30 °c. table 5. intra-particle diffusion model for the electrocoagulation removal of alizarin s co / mg l -1 25 50 75 100 kd1 / mg g -1 min -0.5 22.34 41.72 100.53 110.72 kd2 / mg g -1 min -0.5 7.92 0.68 3.39 10.23 c1 -2.73 0.85 2.91 -14.00 c2 50.97 129.52 302.38 331.09 r1 2 0.980 0.999 0.999 0.979 r2 2 0.999 0.999 0.999 0.999 thermodynamic parameters figure 10 shows that the rate constants vary with temperature according to equation 9. the activation energy (3.55 kj mol -1 ) was obtained from the slope of the fitted equation. the free energy change, δg is obtained from equations 10 and 11 according to the van’t hoff linear plots of ln kd versus 1/t plot in figure 10 (a and b). the thermodynamic parameters are presented in table 6. it was found that the negative value of δg indicates the spontaneous nature of adsorption. positive value of enthalpy change indicates that the adsorption process is endothermic in nature, and the negative value of change in internal energy (δg) shows the spontaneous adsorption of ars on the coagulant. positive values of entropy change show the increased randomness of the solution interface during the adsorption process (table 6). a. i. adeogun et al. j. electrochem. sci. eng. 6(2) (2016) 199-213 doi:10.5599/jese.290 211 a b figure 10. thermodynamic plot (a) ln k2 and 1/t and (b) ln kd and 1/t for the removal of alizarin s dye by electrocoagulation. table 6. thermodynamic parameters for the removal of alizarin s t / k k δg / kj mol -1 ea / kj mol -1 δs / bj mol -1 δh / kj mol -1 r 2 303 1.0870 -5.301 3.55 9.083 2.549 0.981 308 1.0967 -5.346 318 1.1415 -5.437 323 1.1522 -5.483 ft-ir studies of the dye solution before and after electrocoagulation figure 11 presents the ft-ir spectrum of the dye solution before and after the process. before the electrocoagulation the spectrum shows several peaks; sharp and strong peak at 3440.8 cm -1 could be assigned to –oh stretching on the dye molecule while, that at 2821 cm -1 is due to –ch–. peaks at 1593 and 1350 cm -1 are due to the aromatic c=c stretching. after electrocoagulation, the extra structure noted such as that at 3840 cm −1 may be assigned to the (o–h) stretching vibration in the al(oh)3 structures. figure 12. ftir of the solution of alizarin red s dye before and after removal j. electrochem. sci. eng. 6(2) (2016) 199-213 removal of anthraquinone dye alizarin red s 212 conclusions this study revealed the feasibility of the use of electrocoagulation techniques for removing alizarin red s from its aqueous solution in a process. the process depends on numerous factors such as: current density, solution ph, temperature, initial dye concentration and contact time. the percentage removal of the dye increased with ph up to ph 7, also contact time and current density increase influence the removal positively. equilibrium data fitted very well in the langmuir isotherm equation, confirming the monolayer adsorption capacity of 785.31 mg g -1 at 303 k. the kinetics of the process is best explained using a pseudo first order kinetics model, with higher r 2 (table 4). intra-particle diffusion was not the sole rate controlling factor. the thermodynamics parameters obtained indicates that the process is spontaneous endothermic nature of the process. therefore, the present findings suggested a better performance of electrocoagulation with al electrode as an inexpensive method for the removal of ars aqueous solutions. acknowledgements: the financial support in the form of grants from csir, for twelve months twas-csir postdoctoral fellowship, fr number: 3240275035, awarded to abideen idowu adeogun that enables this work to be carried out. also he is thankful to the authority of the federal university of agriculture, abeokuta, nigeria for granting the study leave to honour the fellowship. references [1] g. crini, bioresources 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[38] a.i. adeogun, b.r. balakrishnan, applied nanoscience (2015) doi: 10.1007/s13204-0150484-9. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ understanding cold spray technology for hydroxyapatite deposition: review paper http://dx.doi.org/10.5599/jese.1424 41 j. electrochem. sci. eng. 13(1) (2023) 41-62; http://dx.doi.org/10.5599/jese.1424 open access : : issn 1847-9286 www.jese-online.org review paper understanding cold spray technology for hydroxyapatite deposition gaurav prashar1 and hitesh vasudev2, 1department of mechanical engineering, rayat bahra institute of engineering and nanotechnology, hoshiarpur, punjab, india 2division of research and development, lovely professional university, phagwara 144411, india corresponding author: hitesh.24804@lpu.co.in received: june 23, 2022; accepted: january 6, 2023; published: january 31, 2023 abstract the standard method for applying hydroxyapatite (hap) coatings to biomedical implants is plasma spraying. however, due to the high temperature of the plasma, these coatings frequently experience negative effects like evaporation, phase change, de-bonding, gas release, and residual stresses. this paper summarizes a revolutionary technique known as a cold spray (cs), which allows hap coatings to be applied at temperatures well below their melting point. cs has several advantages over conventional high-temperature technologies, and it seems to be approaching parity with other older methods. when applied using the cs approach, the hap coatings enhance bioactivity, increase corrosion resistance, and maintain the characteristics of calcium phosphate ceramics. this study aims to give a concise and comprehensive overview of hap-based materials, including substituted-hap and hap/polymer composites, and their applications in bone tissue engineering. to better understand the advantages of cs technology, a comparison of cs, high-velocity oxy-fuel (hvof), and plasma spray is given at the end. the perspective and difficulties were also highlighted. keywords cold spraying; high velocity oxygen fuel spraying; plasma spraying; hydroxyapatite coatings; 3d printing introduction aging is inevitable, as are its effects. the capacity of the human body to protect or mend tissue and cells is gravely impaired by toxins, genetic/hereditary anomalies, acute injuries, trauma, and illnesses. since ancient times, those interested in medicine have considered the above-mentioned harmful components and how they affect the body. they have put a lot of effort into developing methods for keeping them in check. thanks to evolutionary breakthroughs like thermal spray, science has advanced sufficiently to create methods for regaining function, correcting anomalies, and promoting healing in many human body parts. technologies using thermal spray provided http://dx.doi.org/10.5599/jese.1424 http://dx.doi.org/10.5599/jese.1424 http://www.jese-online.org/ mailto:hitesh.24804@lpu.co.in j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 42 versatility and adaptability in a range of biological applications. the various substrate and coating materials determined the technique that was used. applications for replacement and repair both benefit from these tactics [1,2]. bone is not consistently solid because it is made up of living bone cells arranged in a biomineral medium. this medium's surrounding intertwined cells are toughened to create bone. the majority of bone is composed of collagen fibres and an inorganic mineral in the form of small crystals [3]. about 30 % organic and 70 % inorganic components make up the biomineral medium of bone [4]. osteopontin and other bone matrix proteins, non-collagenous proteins, lipids, and proteoglycan molecules make up practically all of the remaining 10 % of this organic segment [5]. the bone matrix proteins' ability to adhere to tissue and maintain mechanical strength is crucial. hydroxyapatite (hap) (ca10(po4)6(oh)2) has been acknowledged for its good biocompatibility and efficiency in aiding biointegration for implants in soft tissue and osseous because of its similar composition to human bone [6,7]. figure 1 shows the hexagonal crystal structure of hap. the unit cell has 2 oh-, 10 ca2+ and 6 po43tetrahedra. the lattice parameters are as follows: a = b = 0.943 nm, c = 0.688 nm, and the a and b axes are at a 120° angle. calcium and phosphorus, which have an atomic ratio of 1.67, make up the majority of natural bones. it is, therefore, commonly used in prosthetic joints, bones, dental implants, and other implant goods. it currently holds a prominent position in the field of biomaterials and has a lot of development potential [8-10]. figure 1. hap crystal structure [11]. permissions under attribution 4.0 international (cc by 4.0) figure 2 depicts the four key cells involved in bone regeneration and structure, including osteogenic, osteoblastic, osteocyte, and osteoclastic cells (together known as the basic multicellular unit). figure 2. four important cells found in bone structure [12]. permissions under (cc by 4.0) g. prashar and h. vasudev j. electrochem. sci. eng. 13(1) (2023) 41-62 http://dx.doi.org/10.5599/jese.1424 43 bone turnover is governed by the equilibrium between osteoblast and osteoclast activity, ensuring that neither too much bone is made nor too much bone is degraded. these cells both create and break down the bone matrix, which is made up of osteoid and hap. metallic biomaterials have been utilized to replace biological implants since the nineteenth century. the mechanical properties of these metallic biomaterials, which satisfy the needs of human bone, have earned them the clinical success designation. when utilized as implants, the materials have significant downsides, such as releasing harmful and hazardous metal ions through wear and corrosion mechanisms after lengthy implantation. the bonding strength between human bone and metal implants is also believed to be limited due to the different components that make up both. thus, it is hoped that the problems with biocompatible metallic biomaterials will be resolved by the development of metallic biomaterials with hap coatings. the formation of hap coating on metallic biomaterials improved their ability for load-bearing and substrate-coating adhesion while also increasing their resistance to corrosion. hap possesses exceptional bond-binding capabilities (contrary to metals). coating implants with a layer of hap is one of the most common options due to its excellent biocompatibility and osteoconductive properties. coated implants benefit from higher mechanical qualities and increased biocompatibility thanks to the union of a hard surface and a ductile substrate. the two most often used compositions across a range of biomedical applications are tricalcium phosphate (tcp) and hap. the rate of hap dissolution is significantly less than that of tcp. because of its decreased dissolving rate, hap is a good option for covering metallic implants. ti-6al-4v is currently the most widely used metal alloy for orthopaedic applications due to its excellent properties [13]. the lower cytotoxicity associated with ti-6al-4v is due to vanadium ions leaching into the patient's bloodstream after an implant has been in place for a long time. vanadium ions can cause long-lasting disorders such as osteomalacia, alzheimer's disease and neuropathy. as a result, they require the application of hap coatings to alter the surface properties. there are further reasons to be concerned with hap since it limits the kinds of surface coating techniques that may be utilised to deposit it on ti-6al-4v for the therapeutic field as a result of its heat sensitivity and lack of plastic deformation. the development of new, commercially viable methods for generating hap coatings on ti-6al-4v is constantly being researched. several techniques have been used to successfully modify the metallic surface of ti-6al-4v alloy, including spraying it with a thin, continuous layer of hap [13]. the manufacture of hap coatings for commercial prosthetics using the plasma spraying method is well-established and permitted by fda rules [14,15]. in spite of this, using greater working temperatures and rapid cooling (10-7-10-8 k/s) during plasma spray can alter the phase composition, increasing the likelihood of implant failure [16]. the development of amorphous phases and impurity phases like (-ca3(po4)2, -ca3(po4)2, ca4p2o9, and cao) is mostly to blame for the greater dissolution rates in aqueous environments. numerous authors have reported that the production of calcium phosphate phases (non-apatite, /-tcp) as a result of hap disintegration at higher temperatures was observed between the 30.5 and 31.4° diffraction angles. furthermore, there are issues with hap coatings' poor mechanical characteristics, which restrict their clinical utility in orthopaedic applications [17]. these qualities include increased brittleness, wear, and less fracture toughness. the mechanical performance of hap coatings must be increased as a result without compromising biocompatibility. in table 1, the advantages and disadvantages of hap coatings applied using various techniques are highlighted and discussed indepth. it is also mentioned that wet deposition techniques are expensive and inappropriate because they cannot create thick, continuous hap coatings. the adverse effects of hap coatings made with http://dx.doi.org/10.5599/jese.1424 j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 44 plasma spraying technology are accelerated resorption while in use and weak interface adhesion to the substrate. table 1.various methods to deposit hap coatings on metal implants with their merits and demerits [18] methods thickness, µm demerits merits plasma spray 30-300 high temperatures encourage decomposition; quick cooling causes amorphous coatings; high temperatures hinder the simultaneous integration of biological agents; increased dilution rates; and poorly controlled physicochemical coating characteristics higher deposition rates result in improved biocompatibility, wear resistance, and corrosion resistance. in most cases, tensile adhesion strengths greater than 15 mpa are achievable cold spray 20-30 expensive process; no deposit of pure hap powder less porosity, lower processing temperatures, no phase disintegration after spraying, better biocompatibility, wear and corrosion resistance, green process, encourages bone fusing, crystalline coatings; high strength of adhesion magnetron sputtering 0.5-3 expensive method; lower deposition rates; fast cooling produces amorphous coatings; high temperatures inhibit the simultaneous integration of biological agents heat-sensitive substrates will also be coated, and flat substrates will have uniform coating thickness, high adhesion strength and purity, low porosity, and dense coatings sol-gel technique <1 some procedures call for the use of expensive raw materials and controlled environments thin coatings, lower processing temperatures, an affordable procedure, the ability to cover complicated forms, and the incorporation of biological molecules biomimetic process <30 process takes a long time; ph consistency and replenishment are required less processing temperatures, the ability to create bone-like apatite, the ability to cover complicated forms, and the incorporation of biological molecules pulsed laser deposition 0.05-5 high temperatures make the simultaneous integration of biological agents impossible. expensive procedure improved adhesive strength; crystalline phase coatings; porous, dense coatings hot isostatic pressing up-to 2 complex substrates can't be coated, coating demands high temperatures, thermal expansion mismatches, different elastic properties, is expensive, and encapsulating material removal/interaction; high temperatures prohibit biological agents from integrating at the same time a dense covering may be created ion beam deposition 0.05-1 high-priced, amorphous coatings greater adhesive strength and homogeneous thickness of the coating that is created micro-arc oxidation 3-20 every coating, with the exception of calcium orthophosphates, contains admixture phases simple, affordable, and eco-friendly coating technique that allows for covering of complicated geometries g. prashar and h. vasudev j. electrochem. sci. eng. 13(1) (2023) 41-62 http://dx.doi.org/10.5599/jese.1424 45 one alternative is to use post-heat treatments (ht) on such hap coatings to change amorphous hap into crystalline phases [19]. utilizing metallic porous-rough surfaces is another way to enhance mechanical properties through bone-in-growth interlocking [20]. a key factor in attaining long-term coating stability is coating dissolving behaviour. two factors primarily affect how quickly coatings dissolve: (i) inborn material characteristics like composition and crystallinity and (ii) environmental factors like media composition and ph. it is well known that ha crystallinity has a significant impact on the tendency of hap coatings to dissolve. subsequent hap phases like tricalcium phosphate, calcium oxide, tetracalcium phosphate, and amorphous calcium phosphate speed up the dissolution process [21,22]. the development of an amorphous calcium phosphate phase in the coating/substrate interface was found [23,24], which is detrimental to the coating's bond strength in in-vitro testing, even though high-velocity oxy-fuel (hvof) produced more crystalline hap coatings than plasma spray. hvof crystalline hap coatings were developed as an option to increase binding strength, and when exposed to simulated body fluids, they outperformed coatings without heat treatment [24]. on the other hand, osteoblastic differentiation was more pronounced in the presence of amorphous calcium phosphate. hap coatings with graded crystallinity seem to be the best way to strike a balance between the biological properties of the deposited coatings and the adhesion strength of the crystalline coatings [25]. to manage the composition of hap, low-temperature coating deposition techniques such as aerosol deposition [26,27] and nanoparticle deposition [28] were developed. on the other hand, these deposition techniques produced incredibly thin (nanometric) layers as a result of the reduced particle size. however, an additional ht of up to 400 °c was needed to prevent amorphous phases. the size of ha crystals increased from 16.2 to 29.3 nm under ht up to 400 °c, improving biological properties. on the other hand, ht beyond 400 °c led to a loss of biological features [18]. furthermore, different hap surface roughness was generated by altering the particle size distribution. so, cold spray (cs) was created as a substitute for hap coating deposition. cold spray (cs) cs, also known as cold-gas spraying, supersonic powder deposition, and kinetic spraying, is a relatively novel spraying technology. as new uncertainties develop and the inter-disciplinarity of research projects increases, it is difficult to forecast when or how the field of cs will reach its productivity plateau. due to cs's green attributes, manufacturing must shift toward a sustainable future, which indicates an increasing necessity for its use. there will probably be more uses for cs in a greener world (which is the future). due to the lack of fusion characteristics, the process has a smaller carbon footprint, and when other processes are put under pressure from high emissions, cs will fill a new market niche. a positive development is anticipated in the highly regulated biomedical field. however, as more process control and environmental compatibility will be required in the future, some spray methods are hitting their technological limits. as a result, we do anticipate increased interest in investigating process-specific certifications. figure 3 shows the indicative hype cycle curve modified for the emergence and development of cs technology. deformable metallic particles are fundamentally propelled toward a surface where they impact and form a coating after being fundamentally injected into a high-velocity stream travelling at up to 1200 m/s in an un-melted condition (often formed using a de-laval) [30]. as a result, cs is a suitable approach for spraying compounds that are delicate to oxygen and temperature, producing deposits with the same chemical composition as the input. on the other hand, metals flex plastically when they strike a surface and bind to it by kinetic compaction [29]. despite recent research addressing their deposition methods, which are http://dx.doi.org/10.5599/jese.1424 j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 46 greatly influenced by the feedstock's characteristics, ceramics are more challenging to spray [31-34]. because of this, cs makes it possible to spray customised coatings with the desired chemistry and microstructure, which is very helpful in the biomedical industry [35]. the cs keeps all the advantages of plasma spraying. however, pure hap powder lacks plastic deformation, which is a need for any material that will be cold-sprayed. cs is unable to spray pure hap powder on ti-6al-4v as a result. to solve the shortcomings of both cs and plasma spray techniques, biocomposite powders composed of a metal (often ti) and hap are employed in many studies. figure 3. indicative hype cycle of the cs technique. adapted form [29], permissions under (cc by 4.0) much research has been done on hap coatings sprayed using the plasma spray technique, and the majority of them are focused on the use and manufacture of hap [36-39]. this is according to a recent survey of the literature. both medical experts and material scientists are interested in the research of and application of hap-based materials. we think that reading review articles is a simple and effective way for new researchers to get a thorough understanding of hap right away. as a result, we make an effort to give a concise and comprehensive overview of hap-based materials, mg-doped hap, ag-doped hap, and hap/polymer composites in this study, as well as their applications in bone tissue engineering deposition via cs technology. cs technology process parameters the features of cs coatings are determined by the spraying process's settings. correlations between coating attributes and spraying parameters must be developed in order to produce coatings with the appropriate characteristics. the specific process by which the solid particles deform and connect during cs is still not entirely understood by researchers and industry experts. g. prashar and h. vasudev j. electrochem. sci. eng. 13(1) (2023) 41-62 http://dx.doi.org/10.5599/jese.1424 47 un-melted particles undergo significant plastic deformation when they collide with a surface, which in the case of ductile materials like metals, causes the production of jets known as adiabatic shear bands (asb). even recrystallized areas and elongated grains at particle interfaces where temperatures were higher, a result of adiabatic shearing, are present in the microstructures of metals that are deposited by cs, and these microstructures have been fairly compared to those of materials that have been explosively fused and powder-compacted. cs can deposit non-ductile materials like ceramics onto ductile substrates, where embedded particles may be present. for a material to deposit effectively onto a specific substrate, there is frequently a critical velocity (vc). the particle velocity must be higher than the critical velocity for effective deposition. but excessive speeds could cause the substrate's surface to erode. the cs gun parameters, such as the gas composition, preheat temperature, pressure, and nozzle shape, also affect the particle velocity. the vc is dependent on the intrinsic characteristics of the material being sprayed, i.e. the mechanical and physical characteristics like particle size, morphology, temperature, and substrate, as well as the density, melting point, and ultimate strength of the material. all of these factors must regularly be tuned for a successful deposition [40]. despite the efficient fabrication of metal, ceramic, and polymeric coatings onto a variety of substrates employing cs, ceramic coatings still pose a challenge due to their intrinsic brittleness. metal-ceramic feedstock powder combinations have been sprayed by cs, increasing coating properties such as wear and hardness [41]. the studies done on bioceramic coatings using the cs process will be covered in the following section. bioceramic coatings via cs particularly bioactive ceramic coatings emphasise their strong relationship to living tissues following implantation. when seeking fixation, bioactive fixation results in a stronger relationship than mechanical fixation. hap-coated prostheses offer superior options than cemented joints in terms of fixation and microparticle migration reduction [42]. but just now, inquiries are being made. hap biocoatings low temperature-related factors have previously been predominantly associated with the benefits of cs over conventional thermal spray techniques. because cs is defined by lower temperatures, undesirable evaporation processes, residual stresses, or phase transitions are prevented [14,43,44]. despite the fact that hap coatings have been found to promote rapid and increased attachment strength, the long-term durability of the fixation has been acknowledged to be a challenge in thermal spray procedures. cs is proposed as an alternative to obtaining hap coatings with higher density and controlled crystallinity. the hap cs approach differs from other low-temperature deposition techniques, such as sol-gel deposition, atomic layer deposition, solution deposition, and biomimetic deposition, in that it is an easy and inexpensive way to create coatings with precise microstructures at low temperatures. contrary to popular opinion, several techniques have been employed to blast an implant's metal surface with hap particles [45-49], and they have been made even more successful by addressing a shot-penning pathway [50]. in fact, the vacuum cold spray, also known as aerosol deposition, and nanoparticle deposition systems, which were established in the 1990s and 2000s, respectively, have been compared to the ceramics industry's cs. although the acceleration of sub-micrometer-sized particles is necessary for aerosol deposition, low vacuum conditions are necessary for supersonic flow control. on the other hand, bonding in a nanoparticle deposition system is assumed to be caused by the dissipation of the particles' kinetic energy. several http://dx.doi.org/10.5599/jese.1424 j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 48 plasticity traits have been observed, and it appears that employing sub-micrometeric feedstock particles is also essential [51-53]. dense hap coatings on ti have been produced using this method [54-55]. numerous numerical and virtual investigations have been carried out to establish the best cs of hap parameters. zhang et al. [56] looked at the factors impacting hap particle acceleration using the computational fluid dynamics programme fluent. the modelling results showed that the hap particle accelerates when its throat and exit diameters have expansion ratios within the optimal range of 1.5-4. additionally, hap particle velocity rises as hap particle size decreases until a minimum of 5 m, at which point it abruptly decelerates, with 5-20 m particle size being acceptable for cs spray. hap particle velocity increases as gas pressure rises, particularly from 0.2 mpa to 0.6 mpa. using the taguchi technique, singh [57] optimised the hap conditions in the cs. they discovered the effects of each element's percentage contribution on the exit particle velocity of the hap powder, which was as follows in descending order: gas type, particle size, gas entry pressure, particle temperature, and gas entry temperature are the order of importance. they also realised how those variables interacted could affect the result [58]. for instance, it was discovered that increasing gas pressure and the temperature of the feedstock particle increased the particle's velocity, whereas increasing the diameter of the hap particle caused the particle's velocity to decrease and had a greater effect than the respective effects of increasing gas pressure, the temperature of the gas, and the temperature of the particles. therefore, hap particle velocity is inversely related to particle size despite the increase in gas temperature and pressure. mg has been suggested as a replacement biomaterial due to its simple degradability and nearly comparable mechanical characteristics to bone. the benefits of using magnesium as an implant are further bolstered by its appealing biological properties [59,60]: a) mg metal is biodegradable by corrosion in body fluid; b) mg2+ is a vital element for the body; c) magnesium can help form new tissues; d) density, modulus of elasticity, and yield strength of magnesium are closer to the bone tissue. because of this, mg-based alloys offer excellent properties, but the biomedical implant industry hasn't yet used them effectively [61]. they have so far been studied for the production of cardiovascular stents, bone fixation materials, and porous scaffolds for bone healing. however, the biggest obstacle to their use in medicine is their corrosion behaviour. another issue mentioned is the production of hydrogen due to corrosion. when there is a considerable development of hydrogen gas, mg cannot be absorbed by the body, causing a balloon effect. as a result, developing coatings or performing surface treatments is crucial [62]. in order to reduce the pace of degradation, which might happen rather quickly, it is advantageous to cover the magnesium alloy with hap. hap contributes to accelerating the biodegradation of magnesium alloys. plasma spraying of mg alloys has not gotten much attention due to mg low melting temperature. additionally, when plasma is sprayed on hap, it may transform into various calcium phosphate phases. these modifications in chemistry and crystallinity commonly affect hap's distinctive bioactive properties as well as its adhesion to the implant [63-64]. cs has provided a solution to both difficulties [65]. in simulated physiological fluid, the rate of biodegradation of az51 coated with hap utilising cs coating technique was examined. the findings show that the coated alloy is bioactive and biodegradable, qualifying it for possible use as biodegradable orthopaedic implants. in a different work, hasniyati et al. [66] alter the cs technique to offer a novel method for coating hap onto an mg substrate at a low temperature. an xrd phase study showed that the hap coatings underwent no phase changes throughout processing. the stand-off distance, substrate surface roughness, substrate temperature, and the number of sprays are just a few of the cs process variables whose effects are examined in this article in relation to the features of the hap coating on the preheated mg substrate. the findings that were chosen for optimization only contained physical g. prashar and h. vasudev j. electrochem. sci. eng. 13(1) (2023) 41-62 http://dx.doi.org/10.5599/jese.1424 49 parameters, such as coating thickness, nano-hardness, and coating modulus. the response optimizer claims that at a standoff distance of 22 mm, a surface roughness of 649.2 grit, and a substrate heating temperature of 496 °c, good hap coatings with 46 m coating thickness, 436.5 mpa hardness, and 43.9 gpa coating modulus were attained. as a result, it is frequently utilised to improve bioactivities and corrosion resistance on mg surfaces. on poly(ether ether ketone) (peek) substrates, cs has developed pure hap coatings to give materials bioactivity while avoiding the weak characteristics of ceramic substrates and the stress shielding effect that frequently develops between bone and metallic materials [67]. incorporating hap into polymeric biomaterials is one of the best approaches to increase biocompatibility. however, to regulate or produce various calcium phosphate phases, hap ceramic coatings require either an expensive vacuum deposition method or an ht at a high temperature to encourage the crystallisation of the coating layer. when it comes to polymeric biomaterials, ht at a higher temperature causes polymers to distort, which eventually impairs the performance of the polymer and limits its usage as a biomaterial. furthermore, a vacuum deposition procedure at lower temperatures may deform polymer surfaces and injure polymer surfaces, which is not suitable. it also requires high production expenses to increase productivity. cs overcomes the limitations of a number of conventional coating processes and makes it possible to cover the surfaces of polymeric biomaterials while maintaining the intrinsic properties of both the polymer and the powder. cs also has low production costs and high productivity. hap, bioglass mixtures, bioglasses that have crystallised, and bioglasses containing cao, sio2, and p2o5 as main components are all claimed as bioactive coatings in this patent [67]. lee et al. study of the bioactivity of hap coatings on peek substrates by cs [68] revealed that these coatings were uniform and firmly adhered without distorting the substrate material. the hap-peek material might be applied in therapeutic settings in the upcoming years to hasten recuperation. hap-composite coatings hap has poor mechanical qualities and is fragile. due to the inherently brittle character of hap, particularly when applied to the common metallic prostheses, such as ti and ss, due to the inelastic deformation that results in failure fragmentation, direct deposition of a thick layer of hap by cs with good adhesion strength is still challenging. to better understand this behaviour, numerous investigations on the analysis of hap failure mechanisms during dynamic impacts are being conducted [69-70]. the application of metal-hap and polymer-hap composite powders is therefore actively pursued. hap can have better mechanical and bioactive qualities by adding more zn [71]. given that zn has a low melting temperature, the optimum method for depositing hap/zn composites on mg alloys is cs. when plasma is used to spray it, it will oxidise at a high temperature. cs hap [72-74], hap/ti [75,76], and hap/ta [77] composite coatings can improve the bioactivities of metals and stop hap breakdown. the results showed that compared to pure ti coating, cold-sprayed hap/ti composite coating had a larger corrosion current and lower corrosion resistance. however, a post-spray ht can greatly improve the corrosion resistance of the hap/ti composite coating. in addition, a post-spray ht treatment of up to three times boosted the mechanical properties of the 20 wt.% hap/ti composite coating (microhardness and ultimate shear strength). however, according to the published studies [75-77], interface cracks between ceramic and metal particles in hati and hap/ta are inevitable because of their significant mechanical and thermal expansion coefficient differences, which may be harmful to the substrates' ability to resist corrosion. http://dx.doi.org/10.5599/jese.1424 j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 50 a zn-hap/zn double-layer coating made of a zn underlayer and a hap/zn upper layer may be able to simultaneously achieve corrosion resistance and bioactivity. yao et al. cover the az91d alloy with a zn-hap/zn double-layer coating using the cs method. potentiodynamic polarization and eis experiments revealed that the cs zn-hap/zn double-layer coatings enhance the bioactivity and corrosion resistance of the mg alloy substrates [78]. other endeavours, such as hap-graphenenano-sheet (gn) and doping hap with silver, have been made to ease concerns regarding the long-term performance of hap-composite coatings [79-80]. it has been shown that adding gn is very suitable for load-bearing applications and demonstrates very acceptable biocompatibility. the gn-containing hap coatings significantly increased the osteoblast cells' adhesion and proliferation, most likely because the serum proteins fibronectin and other crucial components adsorb quickly. substituted hap coatings hap coatings are a very promising alternative to conventional calcium phosphate coatings. in addition to accelerating biomechanical fixation, the replaced hap coatings would be designed to help with a number of pathological problems like infection and osteoporosis. orthopaedic prostheses, dental implants, and macroporous scaffolds have all received replaced hap coatings in the past, expanding their uses for bone regeneration therapy. because of the variety of elements and ions discovered in the past 50 years that have therapeutic effects, the research of substituted hap coatings is currently a field of study that is constantly developing. the crystalline structure of hap also facilitates ion incorporation through substitutive and interstitial processes. these features have led to novel circumstances in which coatings not only hasten bone repair following early implantation but also actively treat illnesses. in conjunction with the recently discovered additively produced metallic scaffolds, substituted hap coatings are expanding the therapeutic applications of metallic implants from their typical substitutive functions towards bone regeneration goals. the recent combination of substituted hap and nanostructures has also opened up new prospects in coatings for orthopaedic applications [81-83]. the bioactive behaviour of stoichiometric hap can be improved by substituting the cationic and anionic sublattices. the anions like for ccan exhibit the same oxidation state as oh-, as can the cations like sr2+, ag+, cu2+, and others. in order to offer hap additional properties like osteoinduction or antibacterial action, ionic substitutions are used. hap can have several ionic substitutions that add utility by improving its composition or crystalline structure, for as, by making it antimicrobial or osteoinducing [84]. for instance, it's conceivable that co32in biological apatites will be substituted for po43 (type b) or oh(type a). the structures of substituted and unsubstituted hap are shown in figure 4. cationic substitutions in hap coatings we have already covered cationic mg-hap and zn-hap substitution coatings in the preceding section. other cationic replacements in hap coatings, such as copper, strontium, and silver, are covered in this section. finally, investigations on hap coatings replaced with co2+, na+, mn2+, and ce3+ were also discussed. the human body does not contain the metal element of silver, with the exception of inadvertent contamination (ag). ag+, a powerful antibacterial agent, is added to hap coatings to prevent infections of dental and orthopaedic implants. ag+ antibacterial impact is related to its capacity to bind to microbial dna, which stops bacterial reproduction. feng et al. [85] showed that the agsubstituted hap coatings on implants had good antibacterial characteristics. ag+ ions can also be used as a dependable bioactive delivery mechanism for the slow release of antibiotics by being g. prashar and h. vasudev j. electrochem. sci. eng. 13(1) (2023) 41-62 http://dx.doi.org/10.5599/jese.1424 51 included in hap coatings with micropores [86]. hap-ag/peek coatings were successfully applied to glass using cs by sanpo et al. [80] at room temperature. by using edx analysis, it was determined that the hap-ag/peek concentrations in the original powders and as-sprayed coatings were identical. according to the study, cs may deposit ceramic materials (hap-ag), nanophases, and composite powders (hap-ag/peek), all while conserving and eliciting coating functionality (bio) that is comparable to that of the starting material figure 4. structure of non-substituted and substituted hap [84]. permissions under (cc by 4.0) strontium is considered non-essential to human health. an average adult human carries 0.14 g of strontium. it is primarily found in the mineral phase of bones, especially in regions where bone turnover is more pronounced [87]. the main cause behind sr incorporation into caps was its inhibitory effect on bone resorption and augmentation of bone growth in osteoporotic patients. sr2+ for ca2+ substitution in caps has been proven in numerous studies to increase osteoblast activity and decrease osteoclast development [88-89]. sr2+ expands the hap unit cell and increases the cell volume because it has a larger ionic radius than ca2+ (112 vs. 99 pm). the sr-low hap wettability and strong sr concentration would provide good corrosion resistance for metallic substrates [90]. copper (cu), as the trace element, is necessary for most living organisms. an adult human weighing 70 kg contains about 0.15 g of copper. cu deficiency is a serious problem, especially in babies, and in cases of acute deficiency can result in anaemia, irregular bone formation, and fractures. however, toxicity brought on by too much cu can build up in the liver and brain. in persons with wilson syndrome, copper accumulation can lead to gradual deterioration of the liver and neurological tissue. cu2+ cations have been added to hap bone grafts in order to provide antibacterial and bactericidal action, angiogenic potential, and the capacity to boost the activity of osteoblastic cells [91-93]. a group of academics have proposed a technique for incorporation that substitutes cu2+ for ca2+ [94-95]. the advantages of cu-rich hap have been disputed because of this cation's cytotoxicity. when there is a large concentration of cu precursors, cuo can form, which seriously compromises cell viability. there aren't many investigations on cu-hap coatings because the majority of the prepared cu-hap has been purchased as powders. cu-hap coatings have been produced using plasma spraying, but they did not outperform hap that has not been substituted [96]. in addition to the aforementioned examples, substituted hap coatings containing co2+, na+, mn2+, and ce3+ are also described in a small number of studies. therefore, it is occasionally impossible to provide a detailed description and discussion. similar to co-hap coatings, there aren't http://dx.doi.org/10.5599/jese.1424 j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 52 many studies on them. co-hap coatings were recently produced by electrodeposition on ti22nb6zr alloy [97]. this study shows that adding co2+ improves corrosion resistance even though no biological effects have been studied. additionally, it has been discovered that coatings consisting of sodium-substituted hydroxyapatite (na-hap) offer better corrosion resistance. however, no inference could be made regarding the presence of na+ in the ceramic component because the crucial comparison with unsubstituted hap was not performed [98]. mn-hap coatings on stainless steel have most recently been applied by ananth et al. [99]. this bilayer coating improved the mechanical characteristics, the efficiency of metal ion leach-out, bioactivity, and biocompatibility as well as corrosion resistance. however, when paired with coatings other than calcium phosphates, mn2+ may not always have favourable benefits. there has recently been a rise in interest in hap replaced with rare earth elements. examples include the substitution of ce3+ or ce4+ in hap, which has proven to have antibacterial activity [100]. collagen and ce-hap have recently been applied to ti substrates as coatings. the resulting ce-hap coatings had bactericidal rates for e. coli and s. aureus of 92.61 and 73.59 %, respectively. anionic substitutions in hap coatings a 70 kg adult contains 2.6 g of the essential trace element fluor. despite the importance of biological role of f-, little is understood about its biochemical action. in the bones and teeth, where it isoelectronically replaces ohin hap, the majority of the fis present. fanions are also found in the intracellular compartment at much lower levels and in external fluids at very low concentrations (micromolar levels). fis incorporated into the hap structure, which results in a tougher structure and a slower rate of dissolution [101]. f-haps coatings have been made using a variety of methods, includeing slip coating and sol-gel [102,103]. whether fluoride has beneficial or harmful biological effects on osteoblast cells is a topic of intense discussion. the best bonding strength on ti substrates, the lowest rate of dissolution, and the most tolerable biological activity were, however, demonstrated by coatings with medium fconcentrations and ohsubstitutions in the range ca10(po4)6(oh)0.75-1f1.25-1. the carbonated (co32-)-hap coating is the second anionic substitution. the inclusion of co32in the hap structure contributes to maintaining consistent bone regeneration through cycles of dissolutioncrystallization. co32--hap coatings are more soluble than hap and are, therefore, likely to elicit a greater osteogenic response, despite the fact that the durability of the coating can be substantially damaged. another technique for enhancing the stability of c-hap coatings is the development of composites employing biocompatible polymers. comparison of cs hap coating with plasma and hvof spray hap coatings were applied to ti6al4v alloy substrates using a mix of thermal spray processes, including atmospheric plasma spray, hvof, and cs. surface properties such as topography, microstructure, phase composition, wettability, and crystallinity were evaluated to test cell response. for in-vitro tests, primary human osteoblasts were utilised. with the operating temperature of the thermal spray methods being lowered, the hap coatings showed an increase in hap crystallinity from 62.4 to 89 % and an increase in hydrophilicity from 32 to 0° [104]. the cs process has shown maximum crystallinity followed by aps and hvof process (cs > aps > hvof). in comparison to hvof and cs hap coatings, higher surface micro-features were apparent in plasma spray hap coatings. cells onto plasma spray hap coatings displayed speedier attachment by adopting osteoblastic morphology as opposed to the rounded cell shape seen on cs hap coatings at 1 day of cell growth. hvof hap coatings showed acceptable cell attachment despite the enlarged g. prashar and h. vasudev j. electrochem. sci. eng. 13(1) (2023) 41-62 http://dx.doi.org/10.5599/jese.1424 53 filopodia of cells on plasma spray hap coatings. the hap coatings with higher crystallinity, however, showed increased cell proliferation and differentiation after fourteen days of cell culture (hvof and cs techniques). it is thought that moderate surface wettability and surface microfeatures both encourage cell adherence. hap crystallinity and crystal size are hypothesised to have a substantial effect on cell proliferation and differentiation. table 2 lists the characteristics of the three coatings. table 2. properties of of aps, hvof and cs properties aps hvof cs thickness, μm 84.5 ± 6.1 68.6 ± 6.0 45 ± 20 microroughness, μm 5.8 ± 0.4 4.2 ± 0.4 12 ± 1 crystallinity, % 62.4 82 89 porosity, % 21-23 11-15 * poor adherence of hap plasma coatings is one of the main problems with coated prostheses, which could lead to failure and have catastrophic consequences for the patient. but tensile adhesion tests of cs ti-ha coatings exhibited greater values when compared to ps coatings and other thermal spray procedures with fda certification. some patents on the development of cs coatings the cold spraying of hap coatings was the subject of a straightforward patent filing in china [105]. for prosthetic joints and dental roots, "baked" 28-53 m hap powder is used during the treatment. the method allows for producing hap coatings with a high degree of crystallinity and strong biological stability while preventing pyrolysis of hap and minimising hydroxyl group loss. a method for spraying pacemakers, clips, bioresorbable stents, and orthopaedic support devices is described in a patent [106]. the method of mixed coatings deposition via cs route, from which the stents are edm-machined, is specifically covered in the document. the material porosity is specified as 0.2 % or less due to the cs deposition process. from biocompatible metals, ceramics, metal alloys, and polymers, cs processing is used to produce porous starting materials that can later be processed into porous medical devices like stents. the porous substrates and coatings have the potential to hold a medication or therapeutic ingredient [107]. conclusion and future trends the performance of the cs haps coating is the main topic of the current review. little oxidation or deterioration, as well as little microstructural change, occurs during the deposition of spray materials. this technique may be preferable to conventional thermal spray techniques because it may create dense coatings while maintaining the phase composition and feedstock material chemistry. studies on cs coatings are emerging in the orthopaedics field (for internal fixation systems and prostheses as well as for antibacterial applications). research on hap coatings is continually developing in order to produce implant surfaces that offer a balance between cell adhesion and minimum cytotoxicity, mechanical properties, and functionalization. despite being a fairly developed field, surface engineering advancements have the potential to help the biomedical industry overcome a number of obstacles. following is a summary of the key points: • it is necessary to establish correlations between the coating properties and the cs spraying parameters in order to produce hap coatings with the desired characteristics. particle velocity must be greater than the critical velocity for effective deposition. http://dx.doi.org/10.5599/jese.1424 j. electrochem. sci. eng. 13(1) (2023) 41-62 hydroxyapatite doping using plasma spray 54 • when compared to materials created using conventional production methods, cs produces porous, mechanically robust structures that are 40 % more powerful. • on samples of magnesium alloy, hap coatings have been produced utilising a quick and effective cs processing approach, and the structures are especially well suited for fabricating biomedical parts like replacement joints because of their tiny size and porosity. in order to reduce the pace of degradation, which might happen rather quickly, it is advantageous to cover the magnesium alloy with hap. hap contributes to accelerating the biodegradation of magnesium alloys. • by controlling the temperature and pressure of the cs coating conditions, the hap layer was uniformly coated on the peek implants surface. these coatings proved to be uniform and firmly adhering without distorting the substrate material in any way. • it is preferable to strengthen hap coatings with zn, ti, ta, ag, and gn or replaced hap coatings in order to enhance their mechanical qualities. the hap structure's ability to allow for different ionic replacements during cs gives it an advantage over currently available plasma-sprayed coatings. • the incorporation of ions with antibacterial properties to lower the risk of infection is a priority research field in the near future for the development of new coatings. • •3d printing is a further industry 4.0 transforming thrust area. it's also important to highlight the emergence of metallic implants produced via 3d printing. widespread clinical applications and mass production are still in their infancy because the majority of the existing investigations are still in the research stage. however, it is possible that 3d printing technology, which is being integrated with artificial intelligence, the internet, and big data, will become more common in the field of metals and medicinal equipment and eventually a crucial part of the digital economy and the production of medical devices. • to the best of the authors' knowledge, relatively 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processing porous substrate into medical device, us7514122b2, 2009. us7514122-b2, 2009 https://portal.unifiedpatents.com/patents/patent/us7514122-b2 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1111/ijac.12904 https://doi.org/10.1016/j.msec.2019.110306. https://patentimages.storage.googleapis.com/cc/8a/aa/bd755411df0fe3/cn101591777b.pdf https://patentimages.storage.googleapis.com/cc/8a/aa/bd755411df0fe3/cn101591777b.pdf https://patentimages.storage.googleapis.com/b3/10/dd/d63c1cc8515ff1/wo2013163747a1.pdf https://patentimages.storage.googleapis.com/b3/10/dd/d63c1cc8515ff1/wo2013163747a1.pdf https://portal.unifiedpatents.com/patents/patent/us-7514122-b2 https://portal.unifiedpatents.com/patents/patent/us-7514122-b2 https://creativecommons.org/licenses/by/4.0/) @article{prashar2023, author = {prashar, gaurav and vasudev, hitesh}, journal = {journal of electrochemical science and engineering}, title = {{understanding cold spray technology for hydroxyapatite deposition: review paper}}, year = {2023}, issn = {1847-9286}, month = {jan}, number = {1}, pages = {41--62}, volume = {13}, abstract = {the standard method for applying hydroxyapatite (hap) coatings to biomedical implants is plasma spraying. however, due to the high temperature of the plasma, these coatings frequently experience negative effects like evaporation, phase change, de-bonding, gas release, and residual stresses. this paper summarizes a revolutionary technique known as a cold spray (cs), which allows hap coatings to be applied at temperatures well below their melting point. cs has several advantages over conventional high-temperature technologies, and it seems to be approaching parity with other older methods. when applied using the cs approach, the hap coatings enhance bioactivity, increase corrosion resistance, and main­tain the characteristics of calcium phosphate ceramics. this study aims to give a concise and comprehensive overview of hap-based materials, including substituted-hap and hap/poly­mer composites, and their applications in bone tissue engineering. to better understand the advantages of cs technology, a comparison of cs, high-velocity oxy-fuel (hvof), and plasma spray is given at the end. the perspective and difficulties were also highlighted.}, doi = {10.5599/jese.1424}, file = {:d\:/onedrive/mendeley desktop/prashar, vasudev 2023 understanding cold spray technology for hydroxyapatite deposition review paper.pdf:pdf;:jese_v13_no1_41-62.pdf:pdf}, keywords = {cold spraying,high velocity oxygen fuel spraying,hydroxyapatite coatings,plasma spraying}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1424}, } {nickel contamination analysis at cost-effective silver printed paper-based electrodes based on carbon black dimethylglyoxime ink as electrode modifier:} http://dx.doi.org/10.5599/jese.1173 153 j. electrochem. sci. eng. 12(1) (2022) 153-164; http://dx.doi.org/10.5599/jese.1173 open access : : issn 1847-9286 www.jese-online.org original scientific paper nickel contamination analysis at cost-effective silver printed paper-based electrodes based on carbon black dimethylglyoxime ink as electrode modifier keagan pokpas1, nazeem jahed1,, petrone bezuidenhout2, suzanne smith2,3, kevin land2,3 and emmanuel iwuoha1 1sensorlab, department of chemistry, university of the western cape, robert sobukwe road, bellville, 7530, south africa 2materials science and manufacturing, council for scientific and industrial research (csir), meiring naude road, brummeria, pretoria 0001, south africa 3department of electrical, electronic, and computer engineering, university of pretoria, hatfield, 0028 south africa corresponding author: njahed@uwc.ac.za; tel: 021-959 3053 received: november 14, 2021; accepted: december 31, 2021; published: january 14, 2022 abstract electrochemical detection of metal cations at paper-based sensors has been suggested as an attractive alternative to current spectroscopic and chromatographic detection techniques due to the ease of fabrication, disposable nature, and low cost. herein, a novel carbon black (cb), dimethylglyoxime (dmg) ink is designed as an electrode modifier in conjunction with 3-electrode inkjet-printed paper substrates for use in the adsorptive stripping voltammetric electroanalysis of nickel cations in water samples. the developed method provides a novel, low-cost, rapid, and portable adsorptive stripping detection approach towards metal analysis in the absence of the commonly used toxic metallic films. the study demonstrated a novel approach to nickel detection at paper-based sensors and builds on previous work in the field of paper-based metal analysis by limiting the use of toxic metal films. the device sensitivity is improved by increasing the active surface area, electron transfer kinetics, and catalytic effects associated with non-conductive dimethylglyoxime films through cb nanoparticles for the first time and confirmed by electroanalysis. the first use of the cb-dmg ink allows for the selective preconcentration of analyte at the electrode surface without the use of toxic mercury or bismuth metallic films. compared to similarly reported paper-based sensors, improved limits of detection (48 µg l-1), selectivity, and intermetallic interferences were achieved. the method was applied to the detection of nickel in water samples well below world health organization (who) standards. keywords http://dx.doi.org/10.5599/jese.1173 http://dx.doi.org/10.5599/jese.1173 http://www.jese-online.org/ mailto:njahed@uwc.ac.za j. electrochem. sci. eng. 12(1) (2022) 153-164 carbon black-dmg paper electrodes 154 carbon black; dimethylglyoxime; nickel; stripping voltammetry introduction remediation of metal pollution and its quantitative analysis remains a major area of focus for analytical chemists worldwide with considerable funding and time dedicated to novel and innovative means for improving current methods. heavy metal pollution continues to garner significant attention due to its hazardous effects on the environment and organisms owing to its nonbiodegradable and toxic nature [1]. current spectroscopic and chromatographic techniques for monitoring this pollution are limited by their high costs, cumbersome equipment, and skilled operation [2]. therefore, much attention has been placed on low-cost, rapid, and portable means of analysis. electrochemical detection of metal ions is one such technique that offers highly selective, repeatable, and sensitive quantitative analysis [3]. to date, a range of metal pollutants has been determined by electroanalysis including heavy, platinum group, and transition metals. nickel determination has been studied following several approaches, however, adsorptive stripping voltammetry remains the most effective technique utilized [4]. the method relies on the reduction of the metallic species from a stable metal-centered coordination complex formed with inorganic chelating agents or biological materials [4,5]. several metallic films have been suggested to aid in the accumulation of the metal analyte at the electrode surface [6-8], but work has been done to improve the detection capability when a metallic film is not employed. carbon nanomaterials have been extensively studied as electrode modifiers in electrochemical sensor applications. graphene [9,10] carbon nanotubes [11,12], carbon dots [13,14], fullerenes [15], carbon onions, and other nano-sized carbon structures have been effective in improving the sensitivity of sensors owing to their high active surface, large surface-to-volume ratio, improved electron transfer properties, increased stability, and wide active potential window. these nanoscale carbon sensors have been utilized in a diverse array of applications, however, the cost of synthesis, tedious preparation methods, and lack of availability hinder their use. to combat this, carbon black (cb) nanoparticles have been proposed as electrode modifiers for cannabis seed oil, antihypertensive drugs, phenolic compounds, dna, flavonoids, and metal analysis. carbon black offers comparable results to other carbon nanostructured sensors[16-23]. it is a commonly available carbon source with elemental carbon arranged in aggregates of fine particles from 3 100 nm. the nanomaterial is often employed as thin film or electrode coating in conjunction with other nanoparticles and active materials. recently, few studies have proposed the use of cb-based sensors to enhance the sensitivity of electrochemical sensors for stripping voltammetric analysis [24-28]. however, few studies have been conducted for metal analysis. to date, no work has been reported on the use of carbon black in conjunction with chelating agents to form composite materials to improve detection sensitivity and selectivity in adsorptive stripping voltammetric detection of metal ions. paper-based sensor use has steadily gained traction as an analytical technique in recent times due to the increased availability, ease of modification, capillary flow, and biodegradable nature [29]. many standard analytical systems have been adapted to paper-based methods including colorimetric and electrochemical detection. the excellent sorption behaviour of the paper-based devices makes it an attractive option for these analysis systems creating lateral flow and origami devices particularly for use in resource-limited settings on the african continent where access to costly instrumentation and skilled laboratories is lacking. due to this access to proper medical care is delayed causing severe illnesses. hydrophobic paper substrates have also been studied by k. pokpas et al. j. electrochem. sci. eng. 12(1) (2022) 153-164 http://dx.doi.org/10.5599/jese.1173 155 several laboratories to replace costlier carbon, metallic and ceramic substrates through coating via printing techniques. while screen-printing is the simplest and most employed method, inkjet printing has been suggested as a suitable alternative [30-32]. owing to these benefits, electrochemical sensors [33-38] and energy devices [39-43] have recently been reported based on inkjet printing. inkjet-printed paper electrodes have previously been studied by our research group where a graphene, gold nanoparticle composite was studied in the presence of a mercury film for nickel detection. to date, a number of works have reported the use of paper-based sensors for stripping voltammetric analysis of metal ions. here, screen-printed electrodes, paper disks, and microfluidic devices have been studied. however, only the work by pokpas et al. has shown the applicability of paper-based devices to be used in adsorptive stripping voltammetry for metal ions. this study demonstrated the first use of nanostructured carbon materials for use in adsorptive detection mechanisms for metal analysis. the low sensitivity of paper-based sensors remains the biggest drawback of the technique to date [7,44]. as a result, metallic films like mercury are still readily employed. further work to improve the sensitivity for detection towards metal cations is required before commercial use of these is implemented and particularly to eliminate the use of commonly used amalgam and alloy formation. in addition, no work has been reported to utilize carbon black inks over costlier nanomaterials to enhance the sensitivity of paper-based electrodes. in this work, a disposable inkjet-printed paper electrode is fabricated for the rapid, cost-effective, and portable detection of nickel, metal cations in water samples as an electrochemical sensor. the detection sensitivity and selectivity of the low-cost silver printed paper electrodes were improved through modification with carbon black dimethylglyoxime (cb-dmg) paste as electrode modifier in the absence of a metallic film. the cb-dmg allows for effective preconcentration of the electrode surface with nickel cations in the form of ni-dmg complexes while maintaining excellent electron transfer properties required for electrochemical detection. the study builds on previous work by our research group for use of inkjet-printed paper-electrodes in adsorptive stripping voltammetric detection of metal ions. furthermore, the study showcases an attractive alternative to the use of toxic mercury films previously studied. the developed, point-of-care sensor was applied to the detection of metal ions in tap water samples below the world health organization (who) limits of detection of 0.1 mg l-1 and demonstrates for the first time a carbon black ink combined with a chelating agent, dimethylglyoxime as electrode modifier. experimental apparatus a fujifilm dimatix dmp-3281 materials deposition printer was employed for all inkjet printing following designcad templates onto penguin photo paper. all electrochemical voltammetric experiments were performed with a metrohm autolab pgstat101 instrument, in combination with the nova 1.11 software, and controlled by a personal computer. all electrochemical experiments were performed in one compartment 20 ml voltammetric cells at room temperature unless stated otherwise. chemicals and reagents all chemicals used in the study were of analytical reagent grade. ultra-pure distilled water (millipore) was used to prepare all solutions. harima nps-jl-nano-silver ink, for low-temperature curing (harima chemicals group) and carbon conductive paste for screen printing (gwent group), was used without modification. nafion perfluorinated resin solution 5 wt.% in lower aliphatic http://dx.doi.org/10.5599/jese.1173 j. electrochem. sci. eng. 12(1) (2022) 153-164 carbon black-dmg paper electrodes 156 alcohols and water, 2,3-butanedione-dioxime (dimethylglyoxime) and nickel(ii) standard stock solutions (1 g l-1, atomic absorption standard solution) were obtained from sigma-aldrich and diluted as required. carbon black was purchased from alfa aesar. ammonia/ammonium chloride (nh3/nh4cl) buffer solution (0.1 m, ph 9.3), prepared by mixing appropriate quantities of ammonia (nh3) and ammonium chloride (nh4cl) was used as supporting electrolyte for all studies. a metrohm 827 ph lab ph meter was calibrated using ph 4 and 7 calibration buffer solutions and then used to verify the ph of the prepared nh3/nh4cl buffer solution. design and fabrication of carbon-coated agnp inkjet-printed electrodes a similar procedure described in our previous work was utilized for inkjet-printed electrode preparation. generally, a piezoelectric materials printer was used to carry out all inkjet printing of harima nps-jl-nano-silver ink onto hydrophobic photographic paper. preparation of nafion, dimethylglyoxime, carbon black, silver printed paper-based electrodes (ndmg-cb-ag-ppe) appropriate quantities of carbon black (0.8 g) and dimethylglyoxime (0.2 g) were mixed in a nafion solution (0.2 wt. %, in etoh) to prepare the nafion-dimethylglyoxime-carbon black (n-dmg-cb) ink. the viscosity of the prepared ink was adjusted using etoh if needed. a variety of cb: dmg ratios were prepared as needed. consecutive, 5 µl aliquots of as-prepared n-dmg-cb ink were drop-cast and screen printed onto the working electrode to prepare the nafion/dimethylglyoxime/carbon black/silver printed paper electrodes (n-dmg-cb-ag-ppes). n-dmg-cb-ag-ppes were dried at 85 oc for 1 hour. procedure for the sw-adcsv analysis of ni2+ the n-dmg-cb-ag-ppe was used to analyze a 10 ml sample of 0.1 m ammonia buffer (ph 9.4) ni2+ containing 2 mm dmg and 10 mg l-1 hg3+ respectively. the electrochemical detection was carried out by cathodic stripping voltammetry between – 0.7 v and -1.35 v with a 120 s accumulation time at -0.7 v. f = 5 hz and amplitude = 10 mv. results and discusion ni2+ detection at the cb-dmg-ag-pppe silver printed working electrodes have shown low sensitivity towards [ni(dmgh2)] complex detection and a narrow active potential window, as discussed in previous sections. carbon inks have been shown to significantly improve the active potential window and have shown a clear affinity towards the desired application. carbon black ink, formed from the nanoparticle form of carbon was proposed as an ultrasensitive carbon coating material to further improve the electrode sensitivity of inkjet-printed paper-based sensors in the absence of an electroplated metallic film. figure 1 represents typical square-wave adsorptive cathodic stripping voltammograms (sw-adcsv) for 300 µg l-1 ni2+ at the carbon black silver printed paper electrode (cb-ag-pppe) and carbon black/dimethylglyoxime silver printed paper electrode (cb-dmg-ag-pppe) respectively. the cb-ag-pppe shows no cathodic stripping peaks in the active potential window between – 0.7 and – 1.3 v in 0.1 m nh3/nh4cl buffer (ph 9.4) with deposition potential (0.7 v), deposition time (120 s), frequency (5 hz), amplitude (0.1 v) and voltage step (0.005 v). in contrast, the cb-dmg-ag-pppe shows a sharp, well-resolved stripping peak at -1.14 v for ni2+ reduction from the formed [ni(dmgh2)] adsorption complex at the electrode surface. immobilization of the chelating agent at the electrode surface in conjunction with the high active surface area and conductive carbon nanoparticles provides sensitive k. pokpas et al. j. electrochem. sci. eng. 12(1) (2022) 153-164 http://dx.doi.org/10.5599/jese.1173 157 and selective detection of ni2+ cations. the overall electrolytic reaction may be described in the twostep process below (equations 1 and 2): ni2+ + 2 dmgh2 +2 oh→[ni(dmgh2)] + 2 h2o pre-concentration step (1) [ni(dmgh2)] + 10 e+ 10 h+ → ni2+ + 2 dhab stripping/reduction step (2) figure 1. sw-adcsv of 300 µg l-1 ni2+, obtained at the (a) cb-ag-pppe and (b) cb-dmg-ag-pppe. supporting electrolyte (0.1 nh3/nh4cl buffer, ph 9.4), deposition time (90 s), deposition potential (0.7 v), rotation speed (1000 rpm), frequency (5 hz), amplitude (0.1 v) influence of chelating agent concentration in cb-dmg ink the detection of ni2+ is highly dependent on the formation of the [ni(dmgh2)] and its subsequent adsorption onto the working electrode surface. a sensitive technique was established by immobilizing the chelating agent at the electrode surface, simplifying the stripping voltammetric mechanism. here, its application at printed paper electrodes was achieved. optimization of the dimethylglyoxime loading within the prepared carbon black ink (cb:dmg) is illustrated in figure 2 below. the cb:dmg ratios were varied between 1:0, 1:4, 1:2, and 1:1, and its effect on the stripping voltammetric peak currents were investigated towards 300 µg l-1 ni2+ detection by adcsv. the absence of a chelating agent (cb:dmg, 1:0) prevents adequate pre-concentration of ni2+ at the electrode surface and no cathodic stripping peak is observed, as previously stated. figure 2. influence of dmg loading within carbon black inks on the stripping peak current of 300 µg l-1 ni2+. the addition of a dmg chelating agent at the working electrode facilitates the adsorption of metal cations onto the electrode surface and the subsequent formation of metallo-chelate http://dx.doi.org/10.5599/jese.1173 j. electrochem. sci. eng. 12(1) (2022) 153-164 carbon black-dmg paper electrodes 158 complexes. ni2+, present in larger concentrations can now be reduced and a stripping peak current achieved. the inclusion of more non-conductive dmg inhibits electron flow and a decrease in the measured peak current is seen. a 1:4 weight ratio of cb:dmg yielded the optimum stripping peak currents and was therefore selected for all further experiments. morphological characterization of cb-dmg-ag-pppe the morphological characterization of the prepared electrodes was studied by scanning electron microscopy. figure 3 shows the high-resolution electron microscope (hrsem) images of (a) unmodified inkjet printed agnp electrode, (b) carbon black modified ag-electrode and (c) dimethylglyoxime, carbon black ag-electrode. the hrsem image of the unmodified agnp electrode shows a uniform film of spherical ag nanoparticles deposited on the photographic paper substrate. an even coating with no distinct defects in the printing process is observed. figure 3b demonstrates ‘bushy’ aggregates of carbon black particles deposited on the silver we surface shown by spherical particles. carbon black nanoparticles with an average particle size of ~ 80 nm are observed. adequate surface coverage of the printed paper electrode surface is observed with a somewhat uniform distribution. upon inclusion of dimethylglyoxime chelating agent in a 1:4 ratio, crystalline deposits can be seen within the carbon black film with an uneven surface distribution as indicated in figure 3c and confirm the formation of the cb-dmg composited electrode. figure 3: high resolution scanning electron microscope images of (a) inkjet printed agnps, (b) cb-agnp and (c) dmg-cb-agnp working electrodes on photographic paper at 1 kx magnification. cb-dmg-ag-pppe instrumental parameter optimization the square wave instrumental parameters affecting the analytical response of the cb-dmg-agpppe; namely deposition potential, deposition time, frequency, and amplitude were optimized and illustrated in figure 4. the influence of deposition potential on the stripping response of ni2+ at the cb-dmg-ag-pppe was interrogated in the potential range between 0.0 and – 1.0 v (figure 4a). variations in the accumulation potential towards the established reduction potential of ni2+ into ni0 (0 v to -0.8), showed a gradual decrease in recorded peak currents achieved from ni2+ detection. reduction of the metal cations present in solution converts metal cations to their neutral state lowering the overall concentrations of cations available for reduction in the cathodic sweep. a further move to – 1.0 v, sharply decreases the stripping peak currents. eacc of -0.75 v was selected. the effect of accumulation time on the ni2+ stripping response was studied between 30 and 300 s. figure 4b, shows a rapid increase in the ni2+ peak current with increasing accumulation time between 0 and 180 s confirming the increase in adsorption of the ni2+ on the cb-dmg-ag-pppe surface. saturation of the electrode surface takes place at accumulation times greater than 180 s k. pokpas et al. j. electrochem. sci. eng. 12(1) (2022) 153-164 http://dx.doi.org/10.5599/jese.1173 159 and results in a gradual stabilization in stripping peak current. a deposition time of 120 s was selected for all analyses. figure 4c shows the dependence of peak currents on the square wave frequency over the 0 to 30 hz range. maximum stripping voltammetric peak currents are achieved at low-frequency values indicating longer analysis times are required to improve the resolution and sensitivity of the paper-based sensors. a steady decline in reduction peak currents between 5 and 30 hz can be seen due to slower electron transfer processes through the dmg film. a frequency of 5 hz was selected for the square-wave waveform. the influence of amplitude on the stripping peak current of ni2+ was studied between 0 and 20 mv and shown in figure 4d. a steady increase in recorded stripping cathodic peak currents is observed. figure 4. the influence of (a) accumulation potential (eacc), (b) accumulation time (tacc), (c) frequency, and (d) amplitude on ni2+ peak currents at cb-dmg-ag-pppe. sample composition: 300 µg l-1 ni, 0.1 m nh3/nh4cl buffer quantitative analytical performance of the cb-dmg-ag-pppe the favorable adsorptive stripping voltammetric performance of the cb-dmg-ag-pppe is demonstrated in figure 5. square-wave voltammograms and the calculated calibration plot recorded over 50 – 500 µg l-1 ni2+ ions, at optimum conditions were performed in deaerated samples. the voltammograms were recorded between -0.9 and -1.3 v. voltammograms show a single, broad, reversible cathodic stripping peak at  -1.18 v. peak currents, credited to the reduction of ni2+ cations from the [ni(dmgh)2] complex increase with increasing ni2+ concentration. positive peak potential shifts are indicated at higher metal ion concentrations. the amount of energy to convert http://dx.doi.org/10.5599/jese.1173 j. electrochem. sci. eng. 12(1) (2022) 153-164 carbon black-dmg paper electrodes 160 the cation into its neutral state is therefore decreased. figure 5, inset represents the noted calibration plot obtained from the corresponding voltammograms. a constant, linear increase in peak currents is observed between 0 and 350 µg l-1 [ni(dmgh)2] concentration. at ni2+ concentrations greater than 350 µg l-1 saturation of the electrode surface with adsorbed metalcomplex is experienced and a deviation from linear concentration is observed. a plateau is evident in the calibration curve. a dynamic linear range between 50 and 350 µg l-1 [ni(dmgh)2] concentration is established with a correlation coefficient of 0.989 indicating good linearity in the recorded region at an accumulation time of 90 s. table 1 is a summary of the analytical parameters obtained from the calibration curve. figure 5. sw-adcsvs and corresponding calibration plot of individual analysis of ni2+ obtained at cb-dmg-ccag-ppe over a 50 – 500 µg l-1 range. supporting electrolyte 0.1 nh3/nh4cl buffer, ph 9.4, deposition time 90 s, deposition potential 0.7 v, rotation speed 1000 rpm, frequency 5 hz, amplitude 0.01 v table 1. a summary of recorded analytical data for the cb-dmg-ag-ppe over the 50 – 500 µg l-1 range analytical parameter analysis of ni2+ sensitivity, µa l µg-1 4.19 10-8 correlation coefficient 0.989 detection limit ± sd, µg l-1 48.01 ± 12.24 limit of quantification ± sd, µg l-1 144.03 ± 51.92 analysis of tap water samples at the cb-dmg-ag-pppe the carbon black, dimethylglyoxime paper-based sensor (cb-dmg-ag-pppe) was applied to the quantitative analysis of ni2+ ions in tap water samples collected in our laboratory. quantitative analysis of tap water was performed using a simple standard addition technique whereby successive additions of known sample concentrations were added to the sample under analysis to determine its concentration by extrapolation methods. the recorded voltammogram and standard addition calibration curve are shown in figure 6. k. pokpas et al. j. electrochem. sci. eng. 12(1) (2022) 153-164 http://dx.doi.org/10.5599/jese.1173 161 figure 6. analysis of tap water (ph 9.4) spiked with 150 µg l-1 of the metal ion in the presence of 2 mm dmg and 10 mg l-1 hg3+. (a) sw-adsv and (b) calibration plot. supporting electrolyte: 0.1 m nh3/nh4cl buffer (ph 9.4). swv instrumental parameters: eacc = -0.7 v, tacc = 120 s, f = 5 hz and amplitude= 10 mv. a linear increase in peak current was observed with increasing “spike” concentration with good regression of 0.986. the extrapolated x-intercept was used to determine the concentration of ni2+ cations in the unknown sample. a summary of the reported values for ni2+ concentrations found in both the spiked and un-spiked samples are shown in table 2, below. the samples were analysed before (original) and after (found) known concentrations (added) of ni2+ were added. it was found that the ni2+ ion concentration in tap water samples could not be determined as it was below the sensor limits of detection or was not present in the sample at all. spiked samples allowed for accurate recovery studies in the new sample matrix when studied within the linear dynamic range found previously. good recovery percentages were determined for ni2+ detection in both test (buffer) and real tap water samples respectively with an error below 5 %. table 2. recorded recovery percentages in both test and water samples ni2+ sample cni / µg l-1 recovery, % original1 added2 found3 test sample n/d 150 148.15 98.77 ± 7.41 real water sample n/d 150 155.49 103.66 ± 5.34 1original – determined concentration of sample prior spiking with ni2+ 2added – concentration of ni2+ spiked into the sample 3found determined concentration of sample after spiking with ni2+ a summary of the work performed at dmg modified electrodes is shown in table 3, below. limits of detection ranging from 0.5 to 48 µg l-1 were observed. solid electrodes like glassy carbon and screen-printed electrodes provided the lowest limits of detection owing to the high conductivity of the solid substrates even at shorter analysis times. dmg modified carbon paste electrodes were also studied in select works. previous work done in paper-based sensors by our research group is also shown. paper-based electrochemical cells used in conjunction with spces demonstrated slightly lower sensitivity due to the low sample volumes required for analysis, limiting the number of available ions for detection. as expected, the two works performed in paper-based sensors showed low sensitivity owing to the lack of electron transport in the cellulose matrix. the ergo-aunp-cc-ag-pppe in conjunction showed improved lod because of the mercury film. http://dx.doi.org/10.5599/jese.1173 j. electrochem. sci. eng. 12(1) (2022) 153-164 carbon black-dmg paper electrodes 162 table 3. a summary of recent work performed at dmg modified electrodes metal ions substrate technique accumulation time, s dynamic linear range, µg l-1 detection limit, µg l-1 ref. ni2+ dmg-cpe dpadsv 120 80 600 27 [2] ni2+ dmg-n-spe dpadsv 120 60 500 30 [3] ni2+ with co2+ & zn2+ ngr-dmg-gce swadcsv 120 2 20 1.5 [4] ni2+ dmg-hg-µppec swadcsv 90 15 90 6.27 [5] ni2+ ergo-aunp-cc-ag-pppe swadcsv 120 50 – 500 32.19 [6] ni2 dmg-spce dpadsv 120 1.7 150 0.5 [7] ni2 dmg-spce swadsv 60 7.6 200 2.3 [8] ni2 dmg-cpe dpadsv 120 50 340 27 [9] ni2+ cb-dmg-ag-pppe swadcsv 120 50 350 48.01 this work conclusion this study describes a simple inkjet printing process to produce patterned three-electrode systems based on silver nanoparticles (agnps) deposited on commercial photographic paper without the need for stencils or templates. the use of nanoparticles in the electrode fabrication provided highly conductive and reproducible electrodes with uniform morphology. the fabricated ag-ppe showed good conductivity in the low ohm range. further, modification of the ag-ppe with a nafion carbon black dimethylglyoxime film demonstrated improved sensitivity and provided an accurate and simple quantitative analytical approach towards the detection of ni2+ in drinking water samples by adsorptive cathodic stripping voltammetry (adcsv). this was the first reported work on a carbon black dimethylglyoxime 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https://doi.org/10.1039/d0ra04786d https://doi.org/10.1002/admt.202000226 https://doi.org/10.1007/s12678-019-00516-7 https://doi.org/10.1007/s12678-019-00516-7 https://creativecommons.org/licenses/by/4.0/) routes to enhanced performance of electrolytic hydrogen evolution reaction over the carbon-encapsulated transition http://dx.doi.org/10.5599/jese.1446 947 j. electrochem. sci. eng. 12(5) (2022) 947-974; http://dx.doi.org/10.5599/jese.1446 open access : : issn 1847-9286 www.jese-online.org review paper routes to enhanced performance of electrolytic hydrogen evolution reaction over the carbon-encapsulated transition metal alloys haruna adamu1,2,3, and mohammad qamar1,4 1interdisciplinary research center for hydrogen and energy storage, king fahd university of petroleum and minerals, dhahran, 31261, saudi arabia 2department of environmental management technology, abubakar tafawa balewa university, yalwa campus, 740272, bauchi, nigeria 3department of chemistry, abubakar tafawa balewa university, yalwa campus, 740272, bauchi, nigeria 4k. a. care energy research & innovation center, king fahd university of petroleum and minerals, dhahran, 31261, saudi arabia corresponding author:  hadamu2@atbu.edu.ng received: july 7, 2022; accepted: august 2, 2022; published: august 14, 2022 abstract a substantial and steady decrease in the energy cost produced from renewable sources has revived interest in hydrogen production through water electrolysis. deployment of electrolysis for h2 production is now closer to reality than ever before. yet, several challenges associated with production cost, infrastructure, safety, storage, and so forth remain to be addressed. one of the overriding challenges is the production cost caused by a platinum electrode. to overcome such limitations, developing low-cost and stable electrocatalysts very close to the same electrode activity as platinum (pt) metal is crucial to solving the efficiency issue in the process. therefore, this review is in the direction of designing binary and ternary alloys of transition metal-based electrocatalysts anchored on carbon and focuses more on routes to enhance the performance of the hydrogen evolution reaction (her). the strategic routes to reduce overpotential and enhance electrocatalysts performance are discussed thoroughly in the light of her mechanism and its derived descriptor. keywords electrocatalysis; climate change; sustainable and clean energy; water splitting introduction considering the foreseeable undesired energy crisis, which conceivably is caused by dominating energy insecurity due to ever-growing demand tied with associated environmental problems, the development of clean, renewable and sustainable energy sources has become one of the major http://dx.doi.org/10.5599/jese.1446 http://dx.doi.org/10.5599/jese.1446 http://www.jese-online.org/ mailto:hadamu2@atbu.edu.ng j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 948 directions taken today by the global communities to relief the crisis even before it comes and meet the future energy requirements. in the rhetoric of sustainable development and resource utilization mechanisms, which is based on scientific principles to create a balance between developmental requirements and the environment, energy has become a thing of necessity and one of the pivotal issues in today's modern society for economic and social development [1,2]. it has been reported that almost 80 % of the energy supply of the world economy comes from conventional fossil energy sources such as petroleum, associated and non-associated natural gas, and coal, which are nonrenewable and are persistently depleting if not affecting the quality of the natural environmental scenery [3]. the continual expansion of energy demand globally due to population explosion and economic growth can expose humankind globally to a myriad of serious energy and environmental crisis if not careful handle beforehand with global rhymes song of energy-mix, not necessarily only the issue of climate change and turbulence caused by the ever-increasing levels of co2 in the atmosphere. to deal with such energy crisis and environmental problems, demanding interest in clean and sustainable energy sources has become a global motive for seeking alternatives to fossil fuels. although nature provides renewable energy sources, including solar, wind and biomass, for electricity generation, cost, need for sophisticated technology, and low-efficiency output compromises the feasibility. in addition, irregular electricity supplies depending on the weather and the time-of-day control by variation in regional and/or seasonal dynamics limit many of their benefits [4]. in addition, such energy sources also often suffer from intermittent availability. it is this limitation that incubates the idea of converting the energy supply into a chemical fuel source as an ideal solution to the myriad problems of energy and is of high environmental benefit, as the energy in such form can be stored and preserved in chemical bond as well as transported at the desired time for subsequent utilization [5-7]. this constitutes a research topic, a primary driving force behind numerous advancements in energy conversion and storage systems. in energy conversion and storage, hydrogen production by water oxidation via electrolysis has gotten much attention in the last few decades [8]. although solar and wind power can generate electricity on a massive scale, an alternative pathway for energy storage technologies is required in order to meet and ease the energy demands from the vast continuum of solar and wind energy reserves. accordingly, the possibility of converting solar energy into hydrogen tackles one of the major drawbacks of electricity generation from renewable energy sources, such as solar. hence, hydrogen generation by electricity-driven water oxidation process has emerged as a promising approach for converting huge amounts of stored energy in renewable energy sources to clean fuel known as hydrogen fuel (h2). hydrogen, as a sustainable energy carrier, not only has high efficiency in energy conversion and storage, but it also emits no pollutants because its combustion process produces only water as a by-product, thereby limiting unwanted releases into the environment and eventually can sustain earth’s hospitality. within the realm of common efforts in the ongoing science and needed areas of research to combat climate change and the other important issues of co2 levels in the atmosphere, decarbonization of global energy sources remains an urgent need while simultaneously fulfilling energy needs for global development. to understand the future production and storage of zerocarbon energy by switching to low and ultimately no-carbon generation options, the history of past transitions can help to understand how the entire world moves towards climate-neutral energy transition where there are clearly visible changes and more significant and weighty ones are still to come. the bell-ringing weather statistics of the growing levels of co2 in the earth’s atmosphere (figure 1a) causing a rise in average global temperatures coupled with projections of these data under different scenarios by environmentalists, geologists, and climatologists have led to suggested h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 949 paths of action [9]. as a result, a dominant trend in the change of energy source transition dynamics is the pursuance of different approaches in energy decarbonization from the high-carbon energy source to zero-carbon energy option in the form of a clean fuel-hydrogen (figure 1b). the impetus for this change comes from the deep impacts human societies have had on the earth’s ecological environment during the past decades and the forecasts about what will happen in the future if stay without transformative action within the next decades. accordingly, more and more countries are seeking ways toward zero emissions in the energy sector, which is the central focus that pulls the attention of the scientific communities in today’s energy research ─ the need for decarbonization in the global energy landscape. as a result, the development of water oxidation through the electrochemical splitting process using electrolytic cells for hydrogen production from renewable sources and fuel cells for efficient hydrogen fuel conversion and usage for electric power has become a global motive for a future sustainable energy package (figure 2). specifically, this technological advancement is paving the road to resolving many of the previously discussed and shown conflicting issues between energy and the environment. it is evident that tremendous progress has been made in the field of electrolytic water splitting cells [1,2,10-15] and fuel cells [16-21] and thus, providing promises and hopes for a sustainable energy transition to a carbon-neutral operative regime. a b figure 1. (a) co2 emission from fossil fuel combustion worldwide. reproduced with the authors’ permission from ref. [22]. copyright of world carbon budget, 2017, (b) a diagram depicting the evolution and transition of fuels in terms of h:c ratio. reproduced with permission from ref. [23]. copyright of springer open, 2021 among many aspects of the progress made in addressing the worsening energy crisis and longterm environmental pollution, electrocatalysis has an impact and thus can play a crucial role in disabling or breaking the kinetic energy barrier limiting the efficiency of the electrochemical reactions of water oxidation and combustion that evolve oxygen and hydrogen as well as water during the splitting process and fuel cell energy consumption, respectively (figure 2). hence, the role of carbon-supported transition metal alloyed materials in enhancing the performance of the electrocatalytic water splitting process for hydrogen generation and their prospects is the focal point of this review. however, the primary focus is centrally vested on the routes to enhanced performance in electrolytic hydrogen evolution reactions (her). http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 950 figure 2. display of a dual cell functioning as an electrolytic water oxidation cell for hydrogen generation from renewable solar energy and fuel cell for conversion of hydrogen to electricity, an illustration of sustainable energy generation and the role of electrocatalysis. reproduced with permission from ref. [23]. copyright of springer open, 2021 in recent times, intensive research interest has been dedicated to hydrogen production from water electrolysis, and as a result, its evolutionary process and growth of research in her have been extensively reviewed and documented in the literature [4,12,24]. however, despite the in-depth progress made in the field, several reviews largely focused on comprehensive overviews of the mechanisms of the reactions in both acidic and alkaline media. on the other hand, the progress overview relative to routes to enhance the performance of electrolytic hydrogen reactions upon the low-cost noble metal-free electrocatalysts, particularly 3d-transition metal alloys integrated with conductive carbon supports to enhance long-term activity and durability for her at low energy consumption, has not been thoroughly reviewed. thus, this review has chosen to do so because alloying of transition metals (emphasis is mainly on metals, not of their other forms such as oxides or sulphides, phosphides, nitrides, carbides, borides, etc.) has been found to be an effective route for enhancing performance in terms of the activity of electrocatalysts for hydrogen evolution reaction [1,11,24], but suffered instability and other morphological deficiencies caused by aggregation during synthesis. these limitations have opened windows for integrating conductive carbon supports with transition metal alloyed electrocatalysts. thus, there remain numerous avenues to discuss detailed routes to enhanced performance in terms of electrocatalysts structure and morphology, and synergistic effects between various components composition. because these are variant factors that facilitate the adsorption/desorption ability towards the key reaction intermediates or regulate charge transfer during water electrocatalysis [11]. this includes multifunctional active sites and improved electrical conductivity, porosity, and surface area architectural design to overcome diffusion and mass transport of ions and produced gases and their relationship with her activity and stability in both acidic and/or alkaline mediums [11]. these are design routes targeted toward one fundamental aim to reduce energy consumption or overpotential. in particular, identification of the key contribution of surface adsorption/binding free energies of the carbon-supported transition metal alloyed electrocatalysts for the reaction intermediates in enhancing the overall water splitting process has been reportedly achieved [12] but is dispersed and characterized by heterogeneity. thus, guidelines or routes for designing electrocatalysts towards achieving that have not been fully established and therefore largely lacking. this implies that an obvious gap must be bridged between the two h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 951 disconnects. therefore, more efforts are required to be devoted to this point to establish the inherent trends in the electrocatalytic ability of carbon-supported transition metal alloyed electrocatalysts in her processes. these are what constitute the focus of the present review. the idea of the focus stems from the fact that, in addition to the electronic conductivity of noble metal-free nanoparticles, carbon matrix serves as a conducting medium that quickens the electron and charge transfer. besides, carbon enhances hydrogen binding/adsorption and provides a protective layer that enhances phase stability and prevents the aggregation of noble metal-free nanoparticles [15]. moreover, the flexibility of carbon-containing electrocatalysts offer the feasibility to manipulate material structural design and electronic conductivity modulations via (i) constructing unique architectural surfaces that expose a large density of surface active sites; (ii) integrating the noble metal-free nanoparticles with conducting carbon supports accelerates charge transfer and mobility of electrons and ions, thereby limiting the kinetic reaction barriers of the electrochemical process; (iii) building nanostructured architecture of the noble metal-free nanoparticles over high-conducting carbon supports to tuning electronic structure and optimize the thermodynamic hydrogen adsorption/desorption on the surfaces of electrocatalysts; (iv) capping the surface of carbon matrix with different surface dopants or functional groups not only disable the spontaneous surface oxidation of noble metal-free nanoparticles but also results in increase in charge carrier density of the target nanocomposite which leads to increase in electrode-electrolyte interaction and enhance surface charge capacitance of the prepared material; (v) building architecture of the target electrode with an enormous surface area and varied hole sizes (porosity that controls diffusion) on which the hydrogen evolution reaction occurs seamlessly, as large bubbles of hydrogen escape easily through the big holes in the carbon matrix [2,15]. the structural architecture of carbon-based electrocatalyst nanocomposite prevents wetting of electrode surface— a common problem that makes electrodes less efficient. also, opportunities to further manipulate carbon-supported noble metal-free alloyed nanoparticles remain open for more exploration. the introduction of hetero-species rich with lone pair of electrons into bulk carbon matrix in order to enhance electrocatalyst performance with multifunctional surface sites/groups of such as n, and/or −nh2 that will play important roles in electron-transfer reactions. this, in effect, offers further enhancement of carbon-based electrocatalyst performance activity due to reduced o-containing functionality and increased n-containing terminal nucleophilic sites instead of electrophiles. in this review, the discussion begins with a description of hydrogen production system cleanliness the involvement of no carbon in the natural cycle of electrochemical production of hydrogen energy. this follows by focusing largely on modifications with respect to manipulation of structural design and electronic conductivity modulations that have been carried out to enhance the performance of noble metal-free alloyed materials supported on a carbon framework. electrochemical hydrogen generation from water the zero-carbon energy system now, the renewed interest in hydrogen has come from moving toward decarbonizing global energy systems in the possible nearest future. the increasing demand for hydrogen energy is associated with its zero-carbon potential, which is an important thought in the global energy transition and is a key complement to the electricity supply chain [25]. electrolysis of water makes hydrogen without co2 emission, and therefore, such technological practice is a solution to co2 and climate change crisis, particularly when the process is powered by renewable energy sources (figure 3). this is the only non-fossil fuel means of hydrogen production, which has the potential to play a large role in supporting the journey to zero-carbon energy generation systems. http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 952 figure 3. clean and renewable cyclic production and consumption of hydrogen from renewable sources, which involves generation from water splitting process using solar energy, storage, transportation, distribution and combustion without co2 emission. reproduced with permission from ref. [26]. copyright of royal society, 2010 the atmospheric environment hosts numerous cycles of compounds, including oxygen, nitrogen, water, and the climate change agent-the co2. the decarbonization potential of hydrogen lies in its offer as clean energy that produces only water as a combustion product, while the combustion byproduct(s) of any other kind of fuel is water, co2 and/or nitrogenous oxides, which are cumulatively injurious to the climatic ecosystem. hence, hydrogen is environmentally wholesome with zero net challenge of causing pollution and also considered renewable energy only if it is produced via energy supply derived from a renewable source, e.g., solar radiation. however, the production of hydrogen through the electrolysis of water is not only an uphill reaction, as proven by the positive value of δg (gibbs free energy), but also limited by a significant kinetic barrier [9,23]. in fact, it is a thermodynamically unfavourable reaction, as the reaction is accompanied by δg = 237.2 kj mol−1 and a theoretical potential of 1.23 v [27,28], which requires additional voltage to proceed against standing obstacles to this promising energy production technology. therefore, the electrochemical reaction process needs highly active and durably stable electrocatalysts, which can play a dominant role in lowering the reaction process barrier (figure 4a). indeed, electrocatalysts are the heartmaterials for the conversion process, as they are necessary to speedily drive the production of h2 at the cathode electrode through the hydrogen evolution reaction. the benchmarked performance indexes of an electrocatalyst for the electrochemical water splitting process are based on several key important parameters for activity, stability, and efficiency [27], as presented in figure 4. the electrocatalyst activity evaluation is characterized by overpotential, tafel slope, and exchange current density (figure 4b). on the other hand, stability is evaluated by reflective changes of the overpotential or current over time (figure 4c), whereas efficiency is generally adjudged by the faradaic efficiency and turnover frequency in terms of observable experimental results against theoretically predictable data (figure 4d). h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 953 figure 4. (a) display of the role of electrocatalyst in lowering the activation energy barrier; (b–d) graphical presentations of the activity performance evaluation parameters of an electrocatalyst, including, (b) activity performance displayed in terms of overpotential, tafel slope, and exchange current density, (c) stability shown in terms of currentand potential-time curves, and (d) efficiency illustrated in terms of faradaic efficiency and turnover frequency. reproduced with permission from ref. [23]. copyright of springer open, 2021 fundamentals of electrocatalytic reactions of water splitting process electrolysis of water is nowadays considered an essential and clean way to produce hydrogen, which aims to address the global energy crisis and long-term energy-causing environmental pollution, given the fact that hydrogen could be believed to be an everlasting and promising energy resource owing to global water volume estimated to be around 1.4×109 km3 and the process can be easily integrated with renewable energy sources such as solar [29,30]. thus, water is widely accepted as the most interesting source of sustainable hydrogen production [30]. the overall electrochemical water splitting process can be simply presented as in equation (1): h2o → h2 (g) + ½ o2 (g) (1) the electrolytic reaction appears simple, but this production method of hydrogen through electrochemical reactions taking place between two electrodes is more complicated than the described simple reaction. the reaction process involves multiple reaction steps. the multiplicity of the process is described by electrons being captured or released by electrolytic ions at the electrode’s surface, resulting in a multiphasic gas-liquid-solid transition occurring within the overall process. during the multiphasic switch, water is continually split into hydrogen and oxygen (o2) through two crucial multi-proton/electron combined half-cell reactions-the cathodic hydrogen evolution reaction (her) and anodic oxygen evolution reaction (oer). therefore, in the quest to achieve an efficient water oxidation/splitting process, a clear and thorough understanding of her mechanisms in different ph environments is crucial. this is also an important part considered in designing efficient and effective electrocatalysts. also, it is undoubtedly an important factor that determines the easiness of http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 954 future large-scale application of the technology to satisfy the global clean energy demand and free the environment from pollution liabilities caused by conventional hydrocarbon energy sources. hydrogen evolution reaction (her) it has been known that the electrochemical reaction activities responsible for the cathodic evolution of hydrogen during the electrolytic water splitting process are accomplished by a two-electron transfer process [27,31], which is a multistep reaction that proceeds through two possible mechanisms that are highly ph-dependent (figure 5) [32]. from the figure 5 illustration, the electrochemical her proceeds either via the reduction of protons governed by acidic conditions or h2o species in an alkaline medium to generate molecules of h2 upon the surface of the cathodic electrode, which the reaction is preferably required to be driven with a minimum external energy supply [33,34]. figure 5. mechanistic reaction pathways of the hydrogen evolution at the electrocatalyst surfaces in acidic and alkaline reaction media. reproduced with permission from ref. [27]. copyright of american chemical society, 2021 for convenience and easy comprehension, figure 5 means that her involves the transfer of electrons in a stepwise process that proceeds through successive events of reaction taking place on the surface of cathode. the sequential reaction steps involve the two most common surface her reaction mechanisms, namely volmer-tafel and volmer-heyrovsky, where the former occurs in an acidic reaction environment while the latter in alkaline (figure 5). both mechanisms involve adsorption and desorption processes. the latter process leads to the release of h2 molecules from the surface of the cathode electrode via chemical and electrochemical desorption in the volmertafel and volmer-heyrovsky reaction routes, respectively. of all the reaction routes, the sequential reaction steps can be summarized coupled with the surface participation of the active site of the cathode electrode catalyst as follows: (1) volmer reaction route h3o+ + m + e─ → m―h* + h2o (acidic reaction medium) (2) h2o + m + e─ → m―h٭ + oh─ (alkaline reaction medium) (3) it is obvious that an electrochemical hydrogen adsorption mechanism dominates the volmer reaction pathway, where h+ and h2o molecule react with an electron over the cathodic electrode surface labelled as m and initiates an active h٭ intermediate in acidic and alkaline medium, respectively. (2) heyrovsky reaction route m―h٭ + h+ + e─ → m + h2 (acidic reaction medium) (4) m―h٭ + h2o + e─ → m + oh─ + h2 (alkaline reaction medium) (5) h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 955 the heyrovsky mechanism is a product-yielding reaction, where an electrochemical desorption process leads to the production of h2 molecules after an active h ٭ intermediate chemically combines with h+ in an acidic reaction medium or h2o in an alkaline electrolytic solution and electron (e-). (3) tafel reaction route 2m―h 2 → ٭m + h2 (acidic or alkaline reaction medium) (6) however, in the above reaction route, the adsorbed active h٭ intermediates on the surface of the cathode electrode catalyst combine, leading to the release of h2 molecules through the chemical desorption process. on the elucidation of the above reaction mechanisms, the volmer reaction is basically an activation route for the formation of active h٭ intermediate on the surface of cathode catalyst, while production of h2 during the electrochemical process of splitting h2o molecules is concluded through the heyrovsky and tafel reaction mechanisms. therefore, irrespective of whichever her follows, the volmer-heyrovsky or volmer-tafel reaction route, the key factor involved in the her is the adsorbed active h٭ intermediate that occurs on the surface of the cathode electrode catalyst. for this reason, gibbs free energy of active h٭ intermediate (∆gh*) has been made to serve as a parametric value for evaluating cathode electrode catalyst over her activity performance. for example, a too positive value of ∆gh* makes heyrovsky or tafel reaction process to become so sluggish due to strong adsorption of active h٭ intermediate on the surface of cathode electrode catalyst, which of course, will be responsible for slow desorption process causing to require additional voltage for the reaction to proceed. on the other hand, if the value is strongly negative, the volmer reaction route will be affected owing to the weak interaction between the active h ٭ intermediate and the surface of the cathode electrode catalyst. this could halt the proceeding of the heyrovsky and tafel reaction processes and eventually lead to the cessation of hydrogen evolution during the electrochemical reactive process. therefore, a balanced hydrogen adsorption/desorption behaviour of the cathode electrode catalyst is needed to circumvent the sluggish her rate (figure 6a). in addition, neither too strong nor too weak adsorption of hydrogen over cathode electrode catalyst is energetically and kinetically favourable for hydrogen evolution reaction (figure 6b and c) [35]. this implies that for her to be achieved efficiently, rapid hydrogen (reactant) supply and quick active h ٭ intermediate (product) release are needed to be met simultaneously and satisfactorily. to achieve that, it requires both strong hydrogen adsorption and strong hydrogen desorption behaviours on the surface of the cathode electrode catalyst. therefore, this information is important in the selection of cathode electrode catalyst for enhanced hydrogen evolution electrocatalysis. for instance, corresponding adsorption energy between metal electrocatalysts and h atom has been measured by density functional theory (dft), from which a volcano scale relationship has been drawn to tactically display the adsorption behaviour of each metal electrocatalyst surface towards hydrogen (h) (figure 6d) [36,37]. this is in addition to the earlier illustration of figure 6a-c showing different hydrogen/desorption behaviours over the solid surface of electrocatalysts. from the volcano curve, the noble-metal electrocatalysts, like pt-based electrocatalysts, have an adsorption free energy of hydrogen (∆gh*) value close to zero making them the referenced performing electrocatalysts for her process. let us now analyse the situation conclusively. for the cases of her, the electrochemical reaction of her is recognized by three thermodynamic parameters that are fundamental in the water splitting electrocatalysis, namely proton affinity (pa) of electrocatalyst controlled by pka of the surface active site, electron affinity (ea) of the formed species of intermediate for the production of hydrogen governed by the ph of the reaction medium, and the ph (or gh+) itself of the reaction medium. http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 956 figure 6. (a) graphical diagram of balanced hydrogen adsorption and desorption mode of behaviours, (b and c) too strong and too weak hydrogen adsorptions, respectively. the dark yellow ball represents a hydrogen atom. reproduced with permission from ref. [35]. copyright of nature publishing group, 2019, (d) volcano scaling curve for the hydrogen evolution reaction (her) on the metallic electrodes in an acidic reaction medium. reproduced with permission from ref. [36,37]. copyright of american chemical society, 2010 it is because the electrochemical reaction proceeds through direct protonation and electronation of the surface active site of the electrocatalyst under the influence of the ph of the reaction medium. therefore, h+ and eaffinity at the operational ph of the electrochemical reaction medium is a key issue in the electrocatalysis of her. in this case, there are two situations of required consideration: (i) a surface active site could have a favourable proton affinity at the working ph (pka > ph); (ii) an unfavourable proton affinity at the working ph (pka < ph); and pka = ph (corresponds to an optimum condition with approximately zero thermodynamic overpotential) [38]. it has been reported that an optimum electrocatalyst with approximately zero thermodynamic overpotential can only be found if pka = ph and defined by ea = 0 (figure 7a) [38]. however, the best sub-optimum electrocatalyst activity performance will always proceed under the condition of the other two cases [38,39]. on the other hand, since ea is difficult to be accessed and quantified in the solution of electrochemical reaction, the same reaction possibility analysis as operationalized above can be performed with ph as the activity performance ‘‘descriptor’’ instead of ea. in this case, three possibilities of ea influence on the activity performance of electrocatalyst are considered: (i) ea < 0; (ii) ea = 0; and (iii) ea> 0. in a similar h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 957 analysis as above, an optimum activity performance of electrocatalyst in her is observed only if ea = 0 and pa = gh+ or ph = pka (figure 7b). the two other scenarios lead to sub-optimum activity performance of electrocatalyst in her accompanied with a price of overpotential burden thermodynamically [38]. overall, in all situations, it is clearly shown that an electrocatalyst operates best with optimum activity performance during electrochemical reaction for hydrogen evolution when ph comes very close to pka of the surface active site of the electrocatalyst and/or of the intermediates. it, therefore, means that the electrochemical process of her is sensitive to the electrocatalysts surface structure. figure 7. (a) the thermodynamic volcano plot for the mechanism of electrochemical reaction of he where pka = ph, or pa = gh+, (b) the thermodynamic volcano plot with gh+ = ─2.303 rt ph as the reaction descriptor for the mechanisms representing protonation and electronation in the hydrogen evolution reaction. reproduced with permission from ref. [38]. copyright of the royal society of chemistry, 2013 integration of transition metal alloys and conductive carbon nanomaterials for her process transition metal alloys several studies in the past several years produced a significant number of transition metal compounds, including their alloys, which are primarily designed and developed as efficient her electrocatalysts. because that transition metals possess several unique physical and chemical properties with flexibility of fine-tuning such properties into desired structures electronically and crystal form as well as the creation of defect sites, for example, for the adsorption of intermediates, oxygen vacancy/excess and excellent charged ions transport ability. in addition, these materials are mated into alloys to contain enough surface active sites for an excellent electrocatalytic activity for her in the case of the water splitting process. it is with such uniqueness; that transition metal alloys are considered ideal model materials for major electrocatalytic studies with the prime objective to establish a correlation between their physico-chemical properties and the corresponding electrocatalytic activities in the her process. in this context, the discussion here will dwell on the structure-properties-electrocatalytic activity relationship in terms of reactants and intermediates surface interactions for her during the cause of hydrogen production by the electrochemical water splitting process because structure provides a framework for arrangement, rearrangement and strategic placement of key surface active sites. based on the influence of different reaction media, the reaction processes for the her proceed as presented in the several reaction equations shown above, where surface active sites and intermediates in any case of the commonly known water-splitting reaction pathways are appropriately characterized. however, whether the water splitting process is conducted in acidic or alkaline media, hydrogen adsorption is the fundamental phase in the her. accordingly, the gibbs free energy of hydrogen adsorption stands as an excellent characteristic feature that describes the http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 958 activity performance of an electrocatalyst for the her and is enough to serve as an indicator for evaluation. for a case in point, pt-based electrocatalyst with the best ∆gh* (approaching zero) still remains the performance model with the highest activity for the her. when the value of ∆gh* is too negative, it means that the adsorption/binding strength between the electrocatalyst and the adsorbed h is too strong, which makes it not easy for the hads to be dislodged from the surface of the electrocatalyst and that eventually leads to slowing down the kinetics of tafel or heyrovsky mechanistic steps. on the contrary, a too positive ∆gh* value markedly results in weak interaction between hads and surface of the electrocatalyst, which is a hindrance to the volmer step in the overall electrochemical h2o oxidation process. therefore, the design of efficient her electrocatalysts must be directed to optimize the ∆gh* value. to this effect, it is important to direct discussion on the adsorption behaviour of carbon-supported transition metal alloyed materials (tmas/c) used in the overall electrochemical water splitting reactions towards their mode of interaction with the h* intermediates for the her process. as shown in figure 6d, apart from the typical noble metals, a few transition metals such as co, cu, fe, and ni are the closest to noble metals in the volcano curve and thus have low ∆gh* values compared to the other metal electrocatalysts used for her in the electrochemical water splitting process. at the same time, their surface ability to release molecules of h2 based on the sabatier principle has been regarded as a broad-spectrum type of highly efficient her electrocatalysts [35]. as the positions of the earlier mentioned transition metals are close to the apex of the her activity volcano curve, it can certainly be understood why their family materials, apart from the noble metals, are the most commonly chosen electrodes for electrolysis in the acidic medium in the early research and practices of electrochemical processes. therefore, apart from the noble metals, transition metals have been accepted as the most reliable electrocatalysts for her in the water splitting process, which was earlier explained by the volcano curve. however, despite occupying positions nearby the summit of the volcano curve, transition metals intrinsically always suffer severe corrosion both in acidic and alkaline electrolytes [39-41]. this is the distressing characteristic in the view of using electrodes in scalable applications that require long-lasting working period. in addition, among the earlier mentioned transition metals, some are comparatively characterized by low activity due to low conductivity [41]. all these, to some extent, limit the utilization of transition metals as candidates for her in the water splitting process. so, the development of noble metal-free electrocatalysts, as substitutes for noble metals, with acceptable electrochemical performance, low cost and long-lasting durability was quite challenging [42-44]. however, to overcome such challenges, the electrochemical performance and material stability are synergistically boosted through alloying of the transition metals [45,46]. based on the volcano curve, designing multi-constituent alloys of transition metals constructed with optimized surface chemistry is an ideal way to develop bior multifunctional electrocatalysts with optimized ∆gh* for enhanced water splitting process. this involves intermixing of properties of two or more different metals together to create alloys with a different surface affinity for hydrogen intermediates. specifically, it is inferred that transition metal alloying is expected to tune and alter the d-band electron filling, fermi level, and interatomic spacing [47], which could modify the electronic structure of the alloyed materials and induce optimum adsorption/binding strength over the surface of the materials for the intermediates. this is caused by lattice mismatch due to the creation of twin structures in alloyed materials and is responsible for the surface strain that controls the adsorption strength of intermediates [47]. overall, the alloying approach allowed the development of hybrid structures of transition metalbased materials with a synergetic effect, which has been achieved with acceptable and enhanced her h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 959 activity in the electrocatalytic water splitting process [48]. alloying of transition metals is a strategy that weakens the adsorption strength of h ٭ intermediates due to a strain caused by shifting the dband centres down to the fermi level. in addition, the interface between two or three blended transition metals could result in the formed alloyed metals retaining the suitable hydrogen (h) adsorption energies, leading to an optimum gibbs free energy of the h* intermediates’ state and thus granting a superior her in the overall water splitting technological process. this could be possible, for example, as transition metal atoms hold a large number of unpaired d-band electrons and unfilled dorbitals. therefore, according to brewer―engel theory, the unpaired d-band electrons are susceptible to initiating chemisorption bonds with hydrogen atoms [49], which could easily facilitate her in the water splitting process. thus, alloying binary or ternary composition of transition metals offers more possibilities for improving the electrocatalytic performance of their electrocatalysts through tuning metals’ composition and proportions [50,51]. on the other hand, bare alloyed transition metal-based electrocatalysts suffer from a shortage of low conductivity due to aggregation normally caused by a lack of self-supporting platform, thus causing low surface area that results in poor mass transport and stability in acidic and alkaline electrolytic solutions [52-64]. these are some of the challenges, which to date, have greatly obstructed the actual and full utilization of unsupported transition metal alloyed electrocatalysts for broad applications, sustainable and large-scale production of hydrogen through the electrolytic water splitting process. to address these challenges with one way out, the addition of highly conductive materials such as carbon has become necessary. transition metal alloys―carbon-encapsulated transition metal alloyed electrocatalysts transition as generally known, conductive carbon supports such as graphite, carbon nanotubes, and graphene possess excellent stability and conductivity. it has been reported that integrating carbon nanomaterials with transition metal into hybrids or alloys addressed the above-mentioned challenges encountered in the utilization of bare transition metal alloyed materials in electrocatalysis [65-69]. in addition, it has been identified that the mode of adsorption of integrated forms of transition metal alloys and carbon supports is directly linked to their electronic structure [70]. therefore, supporting transition metal alloys on the conductive carbon phase openly tunes to extend the surface utilization and stability of the alloyed phase and enhance the electrical conductivity. this is closely related to the creation of cooperation resulting from interfacial interaction between carbon and metal alloyed particle components. as shown in figure 8, the carbon phase provides a conductive network that enhances electrical connectivity between the alloyed metal phase and the electrode, which could grant full utilization of the alloyed electrocatalyst for the her process. also, in the hybrid, carbon phase enhances surface active sites exposition and acceleration of transport of charged carriers and intermediates, as well as electron transfer that always occurs at the and across hetero-interface of transition metal alloyed particles and carbon phase. thus, the electronic structural hybridization between the carbon phase and transition metal alloyed particles can regulate their adsorption behaviour towards her reactants and intermediates, which can greatly enhance the electrochemical activity of the water splitting process for hydrogen production. in comparison with the corresponding counterparts with a single metal part or not alloyed, carbonsupported transition metal, alloyed electrocatalysts are more promising owing to their demonstration of highly impressing activity for the her in the water splitting process compared to single or unalloyed systems (see table 1). the promotion in their activity resulted from the promoted synergy between carbon nanostructure and the transition metal alloyed nanoparticles [72-74]. http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 960 figure 8. imagery modelling of the role of carbon in the carbon-supported transition metal alloyed electrocatalyst for hydrogen evolution and oxygen evolution reactions. adapted with permission from ref. [71]. copyright of wiley-vch, 2017 hybridization between the transition metal alloyed atomic orbitals and carbon nanostructure creates a unique interface that exposes and augments surface area and active sites. this promotes adsorption and transportation of reactants and intermediates for her processes and stimulates high electrocatalytic water splitting performance [75-77]. it is also inferred that the insertion of one 3-d metal into another in the alloyed material is able to modify their d-band centres, which can facilitate modulation of the adsorption of h* intermediate and which, at the same time, is able to downshift the activation barrier of the bond cleavage that easily facilitates the release of h2 molecules 75,77]. on the material protection functionality, the integration of transition metal alloys with carbon supports reduces aggregation and oxidation of the active components of the formed material through supporting and coating, respectively, by the carbon supports. as a result, the material's activity lifespan is boosted with superior, long-lasting stability [78-88]. this was further confirmed with the alloys of coni and feco supported on gro designed for the her process. the unique structure of the carbon in the materials promoted electron transfer, mass transport ability and stability of the material [83,88]. in addition, it was also reported that carbon support restricted aggregation and oxidation of mo nanoparticles, and consequently, the electrocatalyst exhibited a low overpotential and excellent long-lasting stability with a negligible current density loss at 20 ma cm─2 within 10 hours [78]. it is, therefore, worthwhile to show a series of electrocatalytic performances of carbonsupported transition metal alloyed materials relative to material design for water oxidation electrocatalysis on the scale of descriptors of gibbs free energy of adsorption for h* (∆gh*) intermediates for her in the overall water splitting technological process. table 1. comparison of her activity performance between the carbon-supported single transition metal and carbon-supported alloys of transition metals electrocatalyst η10 / mv tafel slope, mv dec-1 electrolyte reference feco@n-g shells 211 77 1.0 m koh [88] fe@n-cnts 590 114 0.1 m h2so4 [89] co@n-cnts 320 78 0.1 m h2so4 [89] feco@n-cnts 310 72 0.1 m h2so4 [89] feni@n-c 260 112 0.1 m koh [90] feco@n-c 330 125 0.1 m koh [90] nicofe@c 256 102 1.0 m koh [91] nimnfemo 290 not stated 1.0 m koh [92] n-cnts = nitrogen-doped carbon nanotubes; n-c = nitrogen-doped porous carbon support; c = carbon support, n-g = n-doped graphene layer h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 961 designing efficient her electrocatalysts based on ∆gh* as a descriptor of activity performance recently, carbon-supported transition metal alloyed materials have been commonly embraced and considered to fully explore their potential as active electrocatalysts for her and to optimize ∆gh* for high intrinsic electrocatalytic activity. the design of carbon-supported multi-constituent alloys of transition metals with improved surface chemistry suitable for her is an ideal but, at the same time, quite interesting challenge. conventionally, observed enhancement in her electrocatalytic performance is commonly assessed by experimental characterization and electrochemical studies. in addition to the two mentioned performance indicators, thermodynamic dft calculations can easily reveal the origin of an observed her enhancement for particular carbon-supported multi-metal engineered alloys. for example, dft calculations revealed that mo2c@np/prgo showed a favourable ∆gh*, which was advantageous and helpful for the adsorption and desorption of hydrogen, and thus, the observed low overpotential of the electrocatalyst compared to pt/c was directly related to the optimum adsorption behaviour of the material [76]. in another example, which convincingly evidenced the role of ∆gh* as a descriptor for enhanced her activity, three different transition metal-based electrocatalysts (fe, co, and feco alloy) supported on carbon nanotube (cnt) were prepared and tested with the feco alloy supported on cnt exhibited superior performance [86]. from this study, it was evidently deduced with dft that the reduced adsorption free energy of hydrogen was responsible for the observed improvement of her electrocatalyst performance. in a similar work comprised of coni alloy encapsulated in an ultrathin graphene shell with three different hierarchical layers of 1-3 separately, dft calculation indicated that the observed excellent activity was ascribed to the increased electronegativity of the graphene shell induced by penetrated electrons from the coni core, which in effect decreased the h adsorption free energy of the graphene surface and favoured key hydrogen adsorption for the her [83]. more interestingly, particularly with regard to material surface engineering, the group also found that graphene thickness in the design of the electrocatalyst had a great influence on the her activity and increasing the graphene support thickness to more than three layers could have been different materials with an unintended barrier of electron transfer ability from the coni core to the surface of the graphene shell, thereby reducing the her activity (figures 9a and b). in addition, calculated electronic structure also revealed that the stabilization of the h* intermediates on the surface of the electrocatalysts might also originate from the increased closeness of cloud of electron density between graphene shell and coni cluster, which perhaps was a part of electrochemical events that remoted the her activity (figure 9c). this implies that the thinner the graphene shell, the higher the performance of her activity. in addition, the result further suggests that apart from the regulation of the thickness of graphene shell, adjustment of the chemical composition of transition metal alloy core constituents was also an important factor in boosting the her activity of the metal alloy/carbon hybrid made of the graphene-encapsulated nanostructure, which is all guided by the descriptor of gibbs free energy of adsorption of hydrogen (∆gh*). still, on the utilization of ∆gh* for gaining an in-depth understanding of the electrocatalytic processes of her, dft calculations were carried out to get a further understanding of the nature of the electrocatalytic process, which can serve as an avenue for subsequent improvement in designing the carbon-supported transition metal alloyed electrocatalysts for her process. the electrochemical chain of events that presumably occurred during the her is summarized in a three-state diagram, involving initiation of h+, followed by propagation of an intermediate h* and termination with ½h2 as the final product (figure 9d). on the analysis of the obtained result, it was found that the adsorption h* over the surface of coni alloy was too strong while too weak on the n-doped graphene, which led to low her activity in both cases. interestingly, the http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 962 value of ∆gh* for the coni@c alloy was effectively modified by the encapsulation effect of the graphene shell to the core coni alloy, which resulted in a high her activity (figure 9d). therefore, a good electrocatalyst can be prepared with the guide of adsorption free energy for h, ∆gh*, through which the surface of an electrocatalyst can be engineered and modulated to achieve a moderate surface adsorption free energy for h that can compromise the barriers of the adsorption and desorption steps in the overall her [84,85. the importance and need for ∆gh* in the current design trend of electrocatalysts were shown and represented by a volcano curve (figure 9e). on the analysis of the structural configuration of the graphene shell encapsulated alloy electrocatalyst (coni@c), it was found that electronic potential on the side much closer to the enclosed alloy cluster was 0.3 ev lower than the other sides, and was the side with higher h+ affinity of the graphene shells (figure 9f). this demonstrates that the carbon shell not only modified the gibbs free energy of hydrogen adsorption for the electrocatalyst but also offered better conductivity that facilitated charge transfer, which is also a key factor that enhances her activity [86,87]. it also defines that synergistic effects between the core metallic component of the alloy and graphene shell occurred. the carbon layers effectively protected the core metal alloy nanoparticles and prevented direct exposure of the core alloyed constituents by shielding contact between metal atoms of the alloy and electrolytic solution. this further enhanced the corrosion resistance of the electrocatalyst and improved long-lasting stability. figure 9. (a) imagery modelling of coni alloy encapsulated in 3-layer graphene; (b) a plot of ∆∆gh* versus electric potential as a function of the number of graphene layers represented in red and blue lines, respectively where ∆∆g = ∆g (without metal) ─ ∆g (with metal); (c) imagery models showing variation in electron density with respect to different (1-3) layers of graphene. the charge variation (∆ρ) is defined as the difference in the electron density with and without the coni cluster. the red and blue regions of intensity represent increase and decrease of electron density, respectively; (d) gibbs free energy (∆g) profile of the hydrogen evolution reaction over the surface of various electrocatalysts; (e) a plot volcano curve of the polarized current (і0) versus ∆gh* for coni cluster, coni@c, and an n-doped graphene shell (ncarbon); (f) the electronic potential of coni@c, the vacuum level was set to zero; (g) the free energy of h adsorption ∆gh* on the pure and n-doped (1-3 nitrogen atoms per shell) graphene shells with and without an enclosed coni cluster, of which carbon, cobalt, and nickel are represented with grey, red, and green colours, respectively. reproduced with permission from ref. [83]. copyright of wiley-cvh, 2015 from the different features of the material, the increased number of doped nitrogen atoms from 0-3 per shell also contributed to decreasing ∆gh* from 1.3 to 0.1 ev for graphene shell without an enclosed coni cluster, while an additional decrease in the value of ∆gh* was observed with the material comprised of coni cluster enclosed in graphene shell. this implies that doped-nitrogen atoms and enclosed transition metal alloyed nanoparticles participated synergistically in the h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 963 promotion of adsorption of hydrogen over the graphene shells (figure 9g). the effect of decreasing ∆gh* by the doped nitrogen in the carbon-supported transition metal alloyed material may be associated with its nucleophilic nature that facilitated its affinity to bind with h+ via facile adsorption. with an in-depth understanding of previous studies, another research group crafted two different transition metal alloys encapsulated in graphene layers. the carbon-supported metal alloy hybrids comprised of a binary mixture of fe and co (feco alloys) and a ternary mixture of fe, co, and ni (feconi alloys) encapsulated in graphene layers separately (figure 10a) [88]. in response to the electrochemical reaction of hydrogen production, the dft calculations revealed that compositional and proportional control of the metal alloy constituents were the key factors responsible for changing the electron transfer ability from the alloy core constituents to the graphene shell. such also enhanced the optimization of hydrogen adsorption over the metal alloys/carbon hybrid electrocatalysts, as well as tuned the her performance (figures 10b and c). in short, the dft calculations indicate that the design of an active electrocatalyst does not only rely on the normal choice of components of an electrocatalyst but also careful tuning of their rightful proportion is needed before a highly enhanced electrocatalytic performance can be achieved. this was observed by the enhancement of the activity performance by the reduced content of ni in the alloy [87]. therefore, the intrinsically low electrical conductivity, the sluggishness of the reaction kinetics, limited surface adsorption sites for h, and inappropriate h-adsorption/binding energy of most of the transition metals that are barriers to achieving high electrocatalytic properties [93-95] can be overcome through alloying with tunable composition protected by conductive carbon support. figure 10. (a) feconi alloy encapsulation into graphene shell, (b) illustration of the adsorption and desorption of h* and h2 over optimized structure of the n-doped graphene-encapsulated feconi alloys, of which yellow, pink, and green spheres represent fe, co, and ni, respectively, (c) graphical diagram of calculated ∆gh* of different model materials, reproduced with the permission from ref. [88]. copyright of american chemical society, 2017 the lack of intrinsic stability of transition metal alloys in both acidic and alkaline electrolytic media has been the foremost obstacle to achieving a remarkable her performance with these materials. http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 964 therefore, coupling the alloys with a highly protective and conductive substrate is an important way to enhance their electrocatalytic activity. several studies have demonstrated that transition metal alloys integrated with conductive substrates were quite protected and, simultaneously, boosted the overall her activity of the hybrid electrocatalyst by enhancing electronic conductivity and regulated ∆gh* value [96,97]. for example, among the carbon-based materials, carbon nanotubes (cnts) have shown great potential as carbon supports for molybdenum-based materials owing to their high conductivity and stability [98-107]. the hybridization of transition metal alloys with cnts is a promising strategy for improving the conductivity of the alloys couple with the promotion of her electrocatalytic activity. the group designed highly efficient her electrocatalysts comprised of fe, co, and feco alloys encapsulated in n-doped cnts. the electrocatalysts exhibited high activity and longterm stability. as was earlier demonstrated in the explanation of the observed her activity of graphene-transition metal alloys, dft calculations were also helpful in understanding the origin of the her activity of the cnts encapsulated transition metal alloys [83-85]. the deduction is usually derived from the volcano curve of the relationship between ∆gh* and measured current, from which an electrocatalyst with moderate surface adsorption free energy ∆gh* close to zero was suggested to be a good material candidate for the her process [85]. initially, the adsorption free energy of hydrogen (∆gh*) on the carbon surface of pristine cnt used to encapsulate the transition metal alloy was calculated and the value was found to be 1.29 ev. interestingly, after the insertion of the enclosed fe4 cluster, the ∆gh* on the carbon support decreased greatly from 1.29 to 0.3 ev. in the same way, the same her electrocatalytic activity performance descriptor was also calculated for the outer surfaces of cnts contained of co4 and fe2co2 clusters encapsulated separately, and their values were 0.18 and 0.11 ev for their encapsulated forms of co@cnts and feco@cnts, respectively, which are found to be even lower than that on the fe@cnts. this implies that the insertion of co in the formulation of the electrocatalyst was quite an advantage that enhanced her activity efficiently, which agreed with the experimental results. in addition, the value of ∆gh* was further drastically decreased to 0.05 ev upon the introduction of the n atom in the carbon lattice of fe@cnts. this clearly specifies that when it comes to the design and formulation of carbon-encapsulated transition metal alloys using cnts, nitrogen doping can significantly promote the hydrogen adsorption on cnts through synergetic effect between the two components of the electrocatalyst. it is of significant importance to make analysis of the results above. beginning with the pristine cnt, the calculated value of 1.29 ev signifies that hydrogen adsorption on the pristine cnts was thermodynamically unfavourable due to the inert cnt walls, which is perhaps caused by low delocalization influx of electrons due to thickness. because it was previously shown that the electronic structure of cnts was modified close to the fermi level by the clustered ions of fe due to charge transfer from the fe cluster to nearby carbon atoms [106]. likewise, in the same study, the band centre of the occupied states of the c―h bond on the fe@cnts was reportedly shifted to a lower energy regime compared to the pristine cnts (figure 11a). this means that a small energy barrier was achieved with the introduction of fe clusters, corresponding to the fast adsorption and desorption kinetics of h atom and h2 molecule, respectively, over the carbon surface in the fe@cnts electrocatalyst. the analysis of the electronic structure of the electrocatalyst revealed that the stabilization of h* intermediates in the her arose from the carbon-enhanced charge density near the fe cluster, as well as the n-dopants [89]. for this reason, on the reaction mechanism on the carbon surfaces of cnts, dft calculation was also used to explain the reaction mechanism that occurred on the carbon surface of the three different materials comprised of h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 965 cnts tested for her. in view of that, the free energy profiles, together with the intrinsic reaction coordinate (irc), are presented in figures 11b and c. figure 11. (a) illustration showing differences in projected density of states (dos) of h (1s) when adsorbed on different surfaces of the bare cnts, fe@cnts, and fe@ncnts and in the diagram, the dashed lines represent the center of the occupied band, (b) the free energy profiles of tafel and heyrovsky routes for the fe@cnts electrocatalyst (c) the free energy profiles of the heyrovsky reaction mechanistic route for the bare cnts, fe@cnts, and fe@ncnts, (d) an imagery representation of the occurred hydrogen evolution reaction process over the surface of fe@ncnts, in which the gray, yellow, blue, red, and white balls represent c, fe, n, o, and h, respectively, (e) tafel plots for feco@ncnts, feco@ncnts-nh, and 40 % pt/c, respectively. reproduced with permission from ref. [89]. copyright of royal society of chemistry, 2014 http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 966 as presented in figure 11b, it is demonstrated that the activation barrier of fe@cnts for the heyrovsky reaction step is 1.02 ev, which was much lower than that of the tafel reaction step (2.44 ev). so, because of its low energy barrier, the predominant route of the observed her in the operated electrocatalytic system was mainly through the volmer-heyrovsky reaction mechanism, which was quite consistent with the experimentally measured tafel slopes, as shown in figure 11e. besides, from the presentation of the adsorption free energy profiles of the heyrovsky route for pristine cnts, fe@cnts, and fe@ncnts (figure 11c), it is also very clear that her is hardly to occur on the pristine cnts except at a very high overpotential, which is not economical in terms of energy consumption. on the contrary, the her can easily occur over the surface of carbon in the fe@cnts and fe@ncnts due to thermodynamically favourable surface adsorption of h atoms and stabilized formation of h* intermediates, and thereafter, the imagery illustration of her over the surface of fe@ncnts was schemed as presented in figure 11d. however, it is noteworthy to mention that sometimes the correlation of poor or outstanding her activity with the thickness of the carbon shell is confusing. as indicated above, the thickness of the carbon shell was a problem in the cnts considering electrical conductivity. still, in the case of the carbon-encapsulated wox anchored on carbon support labelled as wox@c/c, it was the opposite, as the activity performance of the electrocatalyst was closer to pt-like electrocatalytic behaviour for her with a super low η60 of 36 mv (η60 represents the overpotential achieved at a current density of 60 ma cm⁻²) and an ultra-small tafel slope of 19 mv dec-1 (figure 12a) [108]. figure 12. (a) illustration of polarization plots with ir compensation of pt/c and wox@c/c, (b) the computed gibbs free energy of h* values for the adsorption sites on the wox@c/c model, the sole wox, and graphene, as well as the reported value for pt16 as a benchmark. reproduced with permission from ref. [108], copyright from john wiley and sons, 2018 the remarkable her activity of wox@c/c was primarily connected to the thickness of the carbon shell used in the electrocatalyst, which offered a good electrical conductivity and, as a result, accelerated the charge transfer and modified the gibbs ∆gh* value to the best possible ground level (figure 12b) [108]. from this, the h adsorption free energy was the key descriptor used in theory to predict the performance of their activity, and therefore, appropriate surface engineering of the transition metal alloyed nanoparticles combined with carbon nanostructures is required to be the desirable approach to balance the h adsorption free energy through a modulated interface. because the theoretical prediction is mainly guided by the volcano curve from which a good electrocatalyst is expected to have a moderate h adsorption free energy close to 0 ev, expectantly resulting in an optimum her activity due to a low h adsorption energy barrier. fundamentally, in an attempt to design rationally and exploit the electrochemical potential of carbon-supported transition metal alloyed materials for the her process, aiming to replace the noble-metal-based electrocatalysts, h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 967 emphasis is quite needed to be placed on promising strategies that regulate surface adsorption free energy of hydrogen (∆gh*) through structural modulation and surface engineering of electronic structures of integrated carbon supports and transition metal alloy nanoparticles. this quite indispensable before low energy consuming (a low overpotential) electrocatalytic water splitting process is to be achieved. strategies for optimizing h* adsorption/desorption over carbon-supported transition metal alloyed electrocatalysts in summary, the strategies that provide procedural guidelines for rational design of efficient electrocatalysts for her through the employment of free energy of h adsorption including (i) increasing density of reaction surface active sites by increase of specific surface area that impedes aggregation of alloy nanoparticles, thereby enhances both h2o and h adsorptions for fast evolution of h2 molecules, as presented in a model below (figure 13); (ii) modulation of electronic structure through increase of electronegativity within the carbon-supported transition metal alloyed matrix; (iii) infusion of hetero-atom that possesses lone pair of electrons into the design to optimize adsorption strength of h* on the carbon-supported transition metal alloyed electrocatalyst and also by implycation decreases water dissociation barrier; (iv) increase conductivity and electron/mass transfer ability by intimate interface between carbon support and the core-metal alloyed nanoparticles via thickness modulation of carbon-support shell and metal proportion of the metal alloy constituents, which also shortens distance of diffusion to reactants and intermediates. the density of the metals in the alloy can be controlled by tuning the metal content neither too high nor too low, aiming to optimize the interface between the two phases of the carbon support and core metal alloys in the electrocatalysts sphere. in addition, designing of heterostructure is quite a vital strategy for creating active interfaces for her electrocatalysis. the strategy creates new electrocatalytic sites by modifying the interfacial electronic structure as well as improving the kinetics of interfacial charge transfer by shortening the distance between the components of the formed electrocatalyst through heterostructural engineering [109,110]. accordingly, these stipulated strategies guided by hydrogen adsorption free energy (∆gh*) are enough to serve as guidelines for the rational design of highperformance carbon-supported transition metal alloyed electrocatalyst for her. generally, highquality formation of heterostructure unavoidably induces electronic structure reconfiguration owing to differences in their fermi level energies, which are obviously shown to affect the h* adsorption/binding energy and in effect optimizes the h* adsorption behaviour of the surface active sites of the formed electrocatalyst and remarkably improved the her electrocatalytic performance [109]. figure 13. schematic illustration of the her process over carbon-encapsulated transition metal alloyed electrocatalyst http://dx.doi.org/10.5599/jese.1446 j. electrochem. sci. eng. 12(5) (2022) 947-974 electrolytic hydrogen evolution reaction 968 summary and outlook the review summarized and discussed the progress of alloys of transition metals dispersed on carbon supports for electrocatalytic hydrogen evolution reaction. the reaction mechanisms of her were thoroughly discussed. on the basis of the majority of research findings, it seems reasonable to infer that the integration of one transition metal with another anchored on conductive carbon support offers great potential to (i) enhance the activity through offering routes to modification of d-band centres by insertion of the 3-d of one metal into another in the alloyed system that facilitates modulation of the surface adsorption/binding free energies of the reaction intermediates; (ii) modify the interface that exposes and augments surface area and active sites by hybridization between metal alloyed atomic orbitals and carbon nanostructure that promotes adsorption by downshifting the activation barrier, mass transport of reactants and intermediates, as well as transfer of electrons; (iii) improve the electronic state of metals d-orbital close to fermi level, and (iv) enhance dispersion of core metal nanoparticles and long-lasting stability through supporting and coating effect of the carbon supports. some compositions, feco and feni clusters supported carbon, for instance, exhibited promising electrocatalytic activity for the her process (table 1). however, to further enhance their performance to comparable levels of the benchmark pt/c electrocatalyst, more efforts need to be directed at (i) exploration of vital insights on the basis of activity descriptors need to be complemented with reaction kinetics data to thoroughly investigate the her mechanistic route under realistic conditions with aim to lead to the rational design of the highly efficient earlier mentioned electrocatalysts; (ii) in order not to recede the electrocatalytic performances of the mentioned electrocatalysts, increase mechanical robustness, long-lasting durability and reliability of carbon supports by glazing with antioxidant could make improvement of oxidation resistance of carbon supports under high anodic overpotentials; (iii) tailoring the shape to expose optimum facet of the metal alloy nanoparticles of the electrocatalysts that can regulate activity by providing different atomic arrangements and electronic structure to affect adsorption/binding and activation energies of reactants and intermediates at the their surfaces. overall, in pursuance of further optimization of the carbon-supported transition metal alloyed electrocatalysts with desired catalytic behaviour for the her process greater efforts are required. i. although significant progress in gainful insight into the detailed mechanisms of the electrocatalytic process of her has been made in the past decade, direct in situ observation on the routes of the identifiable mechanisms through which her follows during the electrolysis of water based on the structural design and transformation of carbon-supported transition metal alloyed electrocatalysts is still lacking. therefore, integration of in situ characterization couple with theoretical modelling in an advanced approach to gain insight into that would significantly help in the rational design of the electronic structure of electrocatalysts with specificity to a particular mechanistic route towards achieving the desired reaction product(s) with fast kinetics that translates to a very low overpotential. ii. to achieve an overall water splitting process with low consumption of energy (low overpotential), understanding the working mechanism is essential. therefore, ex situ characterization techniques to probe the active sites and substantiate the distinction between the volmer-heyrovsky and volmer-tafel reaction mechanisms in the course of her are highly are and be of great if they could be made realizable in the future. this is essential in the logical development of experimental strategies required to target the most favourable mechanism among all the possibilities in her processes. h. adamu and m. qamar j. electrochem. sci. eng. 12(5) (2022) 947-974 http://dx.doi.org/10.5599/jese.1446 969 iii. at the moment, dft calculations are used for the rational design of electrocatalysts based on the interpretations of the classical theories derived from the activity of the noble metal electrocatalysts. this may not be adequate enough to reflect the real electrocatalytic operational conditions and thus, investigation under realistic conditions needs to be accompanied. iv. it has been commonly shown that the surface adsorption free energies of the reaction intermediates of her, i.e., ∆gh*, are connected and correlated with the electrocatalytic activity of the process, which obeys the volcano relationship. however, the correlation between the inherent characteristics of the surface active sites of electrocatalysts that govern the adsorption/binding strength of the intermediates still remains indescribable. in addition, modulating the adsorption/binding energy of reaction intermediates towards desirable electrocatalytic activity is unrealistic experimentally. this is supported by the fact that direct measurements of the adsorption energy of h* intermediates is impracticable under the operational conditions of water electrocatalysis. hence, there is a need in the future to make it practical to easily engineer the surface active sites of carbon-supported transition metal alloyed electrocatalysts to optimize the ∆gh* based on the relationships between the intrinsic activity and adsorption/binding energy values of the adsorbates. this is a prime target for achieving low consumption of energy (low overpotential). v. because the adsorption/binding energies of the reaction intermediates of the process cannot be determined experimentally, the use of ∆gh* is inconvenient for the fast screening of electrocatalysts. therefore, they lack predictive guidance for the design of new carbon-encapsulated transition metal alloyed electrocatalysts rationally. this has now become a key issue in the electrocatalytic water splitting process and therefore, it is essential in the future to identify and determine the underlying electronic structure of active sites on the surface of the electrocatalysts. thus, can be easily fine-tuned and manipulated experimentally to directly adjust the adsorption/binding energies of the 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https://creativecommons.org/licenses/by/4.0/) preparation of nio-graphene oxide nanosensor for adsorptive stripping voltammetric determination of dinoterbon in food samples doi:10.5599/jese.307 253 j. electrochem. sci. eng. 6(3) (2016) 253-263; doi: 10.5599/jese.307 open access : : issn 1847-9286 www.jese-online.org original scientific paper preparation of nio-graphene oxide nanosensor for adsorptive stripping voltammetric determination of dinoterbon in food samples kasaram roja, puthalapattu reddy prasad, punyasamudram sandhya*, neelam yugandhar sreedhar electroanalytical lab, department of chemistry, sri venkateswara university, tirupati-517 502, a. p., india *department of chemistry, sri padmavati mahila visvavidyalayam, tirupati-517 502, a. p., india corresponding author: sreedhar_ny@rediffmail.com; tel.: +91-877-2249666 (303) received: may 29, 2016; revised: august 28, 2016; accepted: september 9, 2016 abstract graphene oxide (go) modified nio electrochemical nanosensor was developed for the determination of the dinoterbon in food samples using adsorptive stripping voltammetry. the modified nanosensor characterized by tem, xrd, cyclic and adsorptive stripping voltammetry. dinoterbon pesticide exhibited a single well-defined cathodic peak at ph 4.0 at britton–robinson buffer (-810.0 mv). the voltammetric characterization of the pesticide residues is evaluated and the parameter such as the effect of ph, scan rate, pulse amplitude, deposition potential and deposition time were optimized. the current–concentration plot obtained using this peak was straight-lined over the range from 0.05 to 50.00 µg ml-1 with limit of detection (lod) 0.028 µg ml-1. the proposed method was efficiently applied to the determination of dinoterbon in food samples. the mean recoveries of the pesticide 97.40 to 99.88 % with a relative standard deviation of 0.114 % in food samples respectively. keywords nanosensor, pesticide, voltammetry introduction wide-range usage of pesticides in agriculture, which leads to accumulation of pesticide residues in soil, water and food, has imposed a serious risk to human health and the environment worldwide [1]. carbamates are one of the major classes of synthetic pesticides and due to their broad biological activity, these compounds are used on a large scale around the world [2]. the most commonly used http://www.jese-online.org/ mailto:sreedhar_ny@rediffmail.com j. electrochem. sci. eng. 6(3) (2016) 253-263 determination of dinoterbon using nio-go 254 pesticide with acaricides and fungicides activity is dinoterbon (2-tert-butyl-4,6-dinitrophenyl ethyl carbonate), see fig. 1. it has been used extensively by farmers on major crops and other field crops [3]. however, it is used to prevent the larvae of the pests from growing up. dinoterbon, like all fungicide pesticides exhibits toxicity to humans, including carcinogenicity, reproductive, developmental toxicity, neurotoxicity and acute toxicity [4]. no2 o2n o c o ch2 ch3 c ch3 ch3 h3c o figure 1. chemical structure of dinoterbon graphene oxide a two-dimensional sheet of sp2-bonded carbon atoms arranged in a honeycomb lattice, has attracted increasing attention since it was first isolated from three-dimensional graphite by mechanical exfoliation [5]. due to its extraordinary thermal, mechanical, and electrical properties, graphene is usually considered as a competitive candidate for next generation electronic application devices such as super capacitors [6,7], batteries [8], fuel cells [9], solar cells [10], sensors [11], biosensors [12], energy storage [13] and catalysts [14]. however, many researchers have reported that the pure graphene actually exhibit unsatisfactory electrical conductivity because of the inevitable aggregation [15]. on the contrary, functionalized graphene sheets are easier to disperse in organic solvents, which can improve the dispersion and the homogeneity of the graphene with in aqueous solutions and yield novel types of electrically conductive nanocomposites [16-18]. also, some of the useful and unique properties of graphene can only be realized after it is functionalized with organic groups such as hydroxyl, carboxyl and amino groups [19]. however, nanostructural metal oxide semiconductors possess a high surface area, nontoxicity, good biocompatibility, catalytic activity and chemical stability. among the metal oxide semiconductors, nickel oxide (nio), a p-type semiconductor with a wide band gap of 4.0 ev at 300 k, has been investigated for various applications such as solar cells, electrochemistry nanosensors [20,21]. electrochemical nanosensors based voltammetric techniques have become extremely useful for the monitoring of pesticides [22,23]. in the present paper authors developed a novel nio/go electrochemical sensor prior to determination of dinoterbon pesticide residues at low concentration levels in food samples. the developed nio-go/gce nanocomposite characterized by xrd, tem, cyclic voltammetry and adsorptive stripping voltammetry (adsv). under the optimized operational conditions, the developed electrochemical nanosensor showed a specific and excellent performance with a good sensitivity, selectivity and wide dynamic range toward the quantification of dinoterbon. the results implicate the applicability of nio/go nanocomposite for rapid, sensitive and selective analysis of dinoterbon. k. roja et al. j. electrochem. sci. eng. 6(3) (2016) 253-263 doi:10.5599/jese.307 255 experimental instrumentation and reagents the electrochemical measurements were carried out in autolab, three electrode systems consisting of modified glassy carbon electrode as a working electrode, ag/agcl (salt kcl) as a reference electrode and a platinum wire as an auxiliary electrode. transmission electron microscopy (tem) micrographs were performed a jeol jem 200cx operating at 200 kv. all reagents used were of analytical reagent grade. ultrapure water was used throughout the experiment. the technical grade samples of dinoterbon fungicide in the form of 50 % wet-table powders were obtained from bayer india ltd. india. synthesis of nickel oxide nanocomposite the nio prepared from nino36h2o precursor by drop wise addition of 0.1 mol l-1, koh to a 0.1 m nino36h2o solution was kept vigorously stirred until the ph becomes 10.0. the precursor was filtered and rinsed with ultrapure water for twice and with ethanol once. wet cake obtained was dried in oven at 100 °c overnight and was heated at 400 °c for 4 h to form black nio nanoparticles. fabrication of the nio-go modified gce before modification, the bare gce (diameter 3 mm) was prudently polished to a mirror-like surface with 0.3, 0.05 m alumina slurries in sequence, then sonicated in ethanol and distilled water for 3 min respectively and dried with nitrogen. the 15 µl of nio/go suspension, prepared by simply mixing of nio and go suspensions with an appropriate volume ratio (v/v), was then dropped onto the clean electrode. then the electrode was dried under room temperature, making the go/gce, nio/gce, go and nio-go modified gce. recommended analytical procedure for the determination of dinoterbon an aliquot of working standard solution containing 5.0 µg ml-1 of dinoterbon pesticide is taken into 25 ml volumetric flask. to this 5 ml of britton-robinson buffer of (ph 4.0) added and transferred into electrolytic cell and diluted with 9.0 ml of supporting electrolyte and then deoxygenated with nitrogen gas for 5 min. the pesticide residue was treated with electrochemical nio-go nanocomposite on the surface of glassy carbon electrode. the dinoterbon pesticides were determined by cyclic and adsv mode. electrolysis was done at +0.8 to -1.20 v vs. sce, accumulation time 80 s, pulse amplitude of 25 mv, scan rate 20 mv s-1 and ph 4.0. the maximum voltammetric peaks appearing for sample is at -0.810 v for dinoterbon. figure 2. tem images of go (a), nio (b) and nio/go (c) modified glassy carbon electrode. j. electrochem. sci. eng. 6(3) (2016) 253-263 determination of dinoterbon using nio-go 256 results and discussion the structure of the go fabricated nio nanocomposite was examined by tem and xrd. the morphological structure of the resulting nio/go was investigated by tem (fig. 2). the tem image of single layer graphene oxide as shown in fig. 2a. fig. 2b, shows the tem image of many nio nanoparticles with sphere-like morphology are homogeneously dispersed. the many nio nanoparticles with sphere-like morphology are homogeneously anchored on the surface of the go sheets as shown in fig 2c. the revealing sphere-like nio nanoparticles with an average size of 28.6 nm. the xrd was used to investigate the phase structure of the resulting hybrids (fig. 3). fig. 3a shows the x-ray diffraction spectrum of nio samples. the xrd patterns exhibit five prominent peaks at 2= 37.4°, 43.3°, 63.5°, 75.4° and 79.6° can be readily indexed as (111), (200), (220), (311) and (222) crystal planes of the bulk nio, respectively. fig. 3b shows the x-ray diffraction spectrum of go fabricated nio nanocomposite. the xrd pattern of the nio/go hybrid exhibits crystalline nio diffraction peaks, which are in good agreement with the standard nio (jcpds no. 04-0835). the asprepared go displays a characteristic (002) peak at 25.4 which is in good agreement with previous reports [24]. by applying the scherer formula to the xrd peaks, the average crystal size was calculated to be 22.4 nm for the nickel crystalline particles. this result indicates that the intermediate products was completely converted to nio, which can be indexed as disordered stacked graphitic sheets. this finding indicates that go is similar to previous reports [25]. figure 3. xrd patterns of nio and go-nio nanocomposite voltammetric response of dinoterbon at various electrodes the electrochemical performance of various electrodes was first investigated by cyclic voltammetry. fig. 4, showed the cyclic voltammetry of the bare gce, go/gce, nio/gce and nio-go/gce in 0.1 m ph 4.0 br buffer solution in the presence of 5.0 µg ml-1 dinoterbon. there was no obvious peak observed at bare gce. however, a cathodic peak response at −0.810 v was observed at the nio/gce and go/gce, respectively. according to the currently accepted mechanism [24,25], the 10 20 30 40 50 60 70 80 in te n s it y , a . u . 2 nio go/nio k. roja et al. j. electrochem. sci. eng. 6(3) (2016) 253-263 doi:10.5599/jese.307 257 reduction peak should be attributed to a four-electron transfer reduction of the each nitro group (no2) to give the hydroxylamine derivative. it can be reoxidized to the nitroso compound at a more positive potential. under subsequent cycling, the nitroso group was reversibly reduced to respective hydroxylamine. consequently, the redox behaviors of dinoterbon at the nio-go electrode is shown in scheme. 1. ch3 ch3 ch3 o o o ch3 n + o o n + o o rate determining step 4h + , 8 e ch3 ch3 ch3 o o o ch3 hohn hohn scheme 1. reduction mechanism of dinoterbon figure 4. typical cyclic voltammogram of dinoterbon for an accumulation time of 80 sec at gce (a); go/gce (b); nio/gce (c); nio-go/gce(d); rest time: 10 s; scan rate: 20 mvs-1; concentration: 5.0 µg ml-1; ph: 4.0 (br buffer); pulse amplitude: 25 mv. the nio-go/gce strong cathodic peak current response at -0.810 v, which was more prominent than those obtained at the bare gce, nio/gce and go/gce. furthermore, one can see that the incorporation of nio into go nanocomposite possesses more prominent peaks, indicating that the use of go can significantly enhance the electron transfer between nio-go and the gc electrode. the normalized signal response of the different electrodes to 5.0 µg ml-1 dinoterbon were calculated. the dinoterbon response at the nio-go/gce is more than the normalized response at the nio/gce and go/gce. these showed the nio/go/gce gave the highest normalized signal response. the enhanced performance of the nio-go/gce nanocomposite can be attributed to the excellent affinity of dinoterbon with nio and the enhanced electron transfer, which could amplify the interaction between dinoterbon and nio due to the formation of entangled nio-go structure. the difference in the electrochemical behaviour of dinoterbon at bare gce, go/gce and niogo/gce were also evaluated by adsv in 0.1 mol l-1 br buffer ph 4.0, containing 20 µg ml-1 dinoterbon, at scan rate of 20 mv s‒1 (fig. 5). j. electrochem. sci. eng. 6(3) (2016) 253-263 determination of dinoterbon using nio-go 258 figure 5. adsorptive stripping voltammogram of dinoterbon at bare gce (a), gce/go (b), gce/nio (c), niogo/gc electrode(d); ph 4.0 (br buffer) accumulation time: 80 s.; stirring rate: 1500 rpm; scan rate:20 mvs-1; pulse amplitude: 25 mv. no characteristic peak related to the dinoterbon reduction was observed for the bare glassy carbon electrode which indicates gce does not exhibit electrocatalytic activity for dinoterbon. however, when the determination was performed at the go/gce, nio/gce and nio-go/gce electrodes, there was a peak for dinoterbon at -0.713 and -0.688 v, respectively. these peaks indicate that both go/gce and nio-go/gce electrodes exhibit electrocatalytic activity and can identify the fungicide dinoterbon. however, the electrode modified with the hybrid material, niogo/gce, showed a well electrocatalytic response with higher catalytic current and the potential was less positive compared to the other modified electrodes. it can be explained due to the go physically incorporated into the nio nanocomposite were oriented in such a way that their extremities were more susceptible to reacting with the fungicide dinoterbon. figure 6. effect of ph on dinoterbon at nio-gce/gce; accumulation time: 80 s.; rest time: 10 s., stirring rate: 1500 rpm; scan rate: 20 mvs-1; concentration: 5.0 µg ml-1; pulse amplitude: 25 mv. figure 7. effect of accumulation time on the adsv response of dinoterbon at nio-go/gce; rest time:10 s; stirring rate: 1500 rpm; scan rate: 20 mvs-1; concentration: 5.0 µg ml-1; ph: 4.0 (br buffer); pulse amplitude: 25mv. c u rr e n t d e n si ty , μ a c m -2 c u rr e n t d e n si ty , μ a c m -2 c u rr e n t d e n si ty , μ a c m -2 k. roja et al. j. electrochem. sci. eng. 6(3) (2016) 253-263 doi:10.5599/jese.307 259 optimization parameters for dinoterbon detection at nio-go/gce the peaks of adsv for dinoterbon (20 µg ml-1) with nio-go/gce were compared in different supporting electrolytes, namely, 0.1 mol l-1 britton–robinson buffer, 0.2 mol l-1 sodium acetate– acetic acid buffer, 0.1 mol l-1 phosphate buffer, carbonate buffer, and borate buffer solution. the highest peak current was obtained with 0.1 mol l-1 britton–robinson buffer as the electrolyte. thus, 0.1 mol l-1 britton–robinson buffer was chosen as the analytical medium, in which the peak shape was well defined. as shown in fig. 6, the effects of ph on the adsv peak current of dinoterbon (20 µg ml-1) with nio-go/gce was also studied in 0.1 m britton–robinson buffer (ph 2.0–10.0). the maximum current appeared at ph 4.0 for the determination of dinoterbon. in the following experiment, ph 4.0 was selected. further investigation of electroactivities using the various ratios of nio to go was preliminarily studied to optimise the catalytic performance of nio-go/gce toward dinoterbon. the mass ratio of nio:go was varied from 1:0.25 to 1:0.5, 1:1, 1:2, and 1:4; the highest peak current was obtained at a mass ratio of 1:2. the catalytic activity of the as-prepared nio-go hybrid for dinoterbon reduction was under the synergistic effect of the primary component (nio) and the support component (go). the combination of go with nio is an ideal strategy of improving the catalytic performance of nio by enhancing its charge transfer efficiency. however, excessive go in the composite can also decrease the loading level of nio. thus, the mass ratio of nio-go has an important influence on the catalytic oxidation reactions of dinoterbon. the optimum catalytic performance was achieved from nio-go with a mass ratio of 1:2. the excellent adsorption capacity of go, (2) the larger effective surface area of nio-go/gce, and (3) the synergistic catalytic effect of go, and nio toward dinoterbon, as detailed above. the adsv peak current of dinoterbon increases with accumulation time increasing from 0 to 80 s, as shown in fig. 7. but when it exceeds 80 s the peak current remains almost constant for a 5.0 µg ml-1 dinoterbon solution, meaning that an accumulation/or extraction equilibrium is achieved at the electrode/solution interface. the influence of accumulation potential is examined from +0.3 to −0.5 v. the results showed that the peak current of dinoterbon is almost independent of accumulation potential. this is due to the neutral nature of dinoterbon under this condition. thus, an accumulation is performed under open-circuit. the effect of pulse height variation on the peak current of dinoterbon voltammograms was studied in the range of 15–100 mv. the obtained results showed that increasing the pulse heights up to 25 mv will cause an increase in peak current. pulse heights more than 25 mv cause broadening of the dinoterbon voltammogram and so decreasing the peak current intensity of the analyte. so the optimum pulse height value was selected to be 25 mv. table 1. tolerance limits of matrix substances for determination of dinoterbon by the adsv method substances tolerance limit, µg ml-1 substances tolerance limit, µg ml-1 na+, k+, nh4 + 1200 cu2+, cr6+ 60 ca2+, mg2+ 1000 phenols and nitrophenols 50 cl-, no3 -, hco3 800 po4 -, so4 2750 al3+, zn2+, fe(ii) 400 interference study prior to the application of the developed method on food samples it was vital to investigate the effect of some of the interfering ions on the recovery percentage of dinoterbon. the adsv j. electrochem. sci. eng. 6(3) (2016) 253-263 determination of dinoterbon using nio-go 260 determination of dinoterbon was tested in the presence of spiked known amounts of interfering ions and molecules. the tolerance limit was defined as the amount of the foreign substance causing a change of ±5 % in the peak current intensity reading. the tolerable limits of interfering substance are given in table 1. the results showed that most of the investigated substances do not interfere in the adsv determination of dinoterbon in food samples determination of dinoterbon pesticide in food samples the food samples namely, onion (alium sepa), cauliflower (brassica olera. var. botrytis), lady’s finger (abelmoschus esculentus), cucumber (cucumis sativus), sweet potato (ipomoea) and tomato (lycopersicum esculentum) were collected from the tirupati local market, a.p, india. afterwards, they were taken in small mesh and dried in an oven at 90 °c to constant weight. in order to digest the samples, 1.0 g of food sample was digested with 10 ml concentrated hno3 (65 %) and 3.0 ml h2o2 (30 %) in microwave system, then again evaporated to near dryness. after evaporation, 10 ml of deionized water was added and the sample was mixed. the resulting mixture was filtered through filter paper. the filtrate was diluted to 25 ml with deionized water. all the samples were stored in polyethylene bottles. for sample analysis spiked with dinoterbon standards at 10.0, 50.0, 100 µg ml1. then 10 µg ml-1 of the digested sample solution was dissolved in 25 μl br buffer solution at ph 4.0 for determination. samples were also analyzed by gc methods. the recovery rates in food samples exhibited a range of 97.40 to 99.88 % (average of five determinations) with less than 1.72 % of rsd and precision data are reported in table 2. figure 8. adsv of the nio-go/gce by (a) 0.2, (b) 0.4, (c) 0.8, (d) 1.0 (e) 2.0 (f) 4.0, (g) 8.0, (h) 16, (i) 32 (j) 50 µg ml-1 dinoterbon; accumulation time of 80 s, stirring rate:1500 rpm; scan rate: 20 mvs-1; ph: 4.0 (br buffer); pulse amplitude:25 mv. adsorptive stripping voltammetric quantification of dinoterbon due to the good sensitivity of adsorptive stripping voltammetry was applied for further electrochemical detection of dinoterbon under the optimized operating conditions. fig. 8, displayed the adsv of the nio-go/gce in 0.1 mol l-1 ph 4.0 br buffer solution at a potential range from −0.4 v to 0.2 v in the presence of various dinoterbon concentrations. well-defined peaks, proportional to the concentration of the corresponding dinoterbon, were observed in plots. the corresponding calibration plot was presented in fig. 9, indicating that the response was gradually saturated at a c u rr e n t d e n si ty , μ a c m -2 k. roja et al. j. electrochem. sci. eng. 6(3) (2016) 253-263 doi:10.5599/jese.307 261 higher dinoterbon concentration. under the optimal experimental conditions, nio–go/gce was used to detect dinoterbon by adsv. it can be seen from the figure that well-defined adsv responses from adsorbed dinoterbon were observed and increased gradually with the increase of the dinoterbon concentration. the good linear relationship between oxidation current and log c dinoterbon was obtained from 0.05 µg ml-1 to 50.0 µg ml-1 with the regression equation of i / µa = 0.4742x+0.1283 (r = 0.9992). a lod of 0.0283 µg ml-1 was calculated according to the formula lod = 3 σ/s, where σ is the mean of standard deviation of five measurements taken from the signal obtained from the blank, s is the slope of the calibration curve, and the number 3 comes from the required ~98 % level of confidence in the difference between the observed signal and the blank response. concentration, ppm figure 9. calibration curve on the adsv response of dinoterbon at nio-go/gce; 5.0 µg ml-1; accumulation time of 80 s, stirring rate: 1500 rpm; scan rate: 20 mv s-1; ph: 4.0 (br buffer); pulse amplitude: 25 mv. table 2. adsv determination of pesticide in food samples (no. of determinations =5) name of the food samples amount added, µg ml-1 amount found, µg ml-1 recovery, % r.s.d* onion 5.0 4.98 99.60 0.11 10.0 9.92 99.20 0.08 25.0 24.96 99.84 0.16 cauliflower 5.0 4.87 97.40 0.02 10.0 9.94 99.40 0.12 25.0 24.82 99.28 0.14 cucumber 5.0 4.88 97.60 0.22 10.0 9.94 99.40 0.04 25.0 24.96 99.84 0.16 sweet potato 5.0 4.91 98.20 0.12 10.0 9.89 98.90 0.05 25.0 24.97 99.88 0.20 tomato 5.0 4.99 99.80 0.16 10.0 9.93 99.30 0.08 25.0 24.89 99.56 0.06 *r.s.d: relative standard deviation the inter-assay precision was estimated at six different nio-go a nanocomposite modified electrodes for the determinations in 0.1 mol l-1 br buffer (ph = 4.0) containing 5.0 µg l-1 dinoterbon. c u rr e n t d e n si ty , μ a c m -2 j. electrochem. sci. eng. 6(3) (2016) 253-263 determination of dinoterbon using nio-go 262 similarly, the intra-assay precision was evaluated by assaying one working electrode for five replicate determinations under unvarying conditions. the relative standard deviation values of inter-assay and intra-assay were found to be 6.8% and 4.6%, respectively, indicating acceptable precision and reproducibility. in addition, the developed electrochemical metal oxide nanosensor (nio-go) was very stable at room temperature. no obvious decrease in the electrochemical response was observed in the 5 days and over ~90% of the initial response remained after four weeks, indicating that is acceptable stability. conclusions in this paper, combining the advantageous characteristics of dinoterbon at nio/gce, go/gce and the nio-go/gce nanocomposite have been prepared. the nio-go/gce nanocomposite with excellent electrocatlytic and between nio and go modified electrode. it’s improved the absorptivity and charge transfer properties on the surface of nanocomposite and improve the stability. the constructed nio-go/gce sensor exhibited many advantages such as low applied potential, good fabrication reproducibility, acceptable stability, fast response and low detection limit. the niogo/gce sensor has potential application in monitoring of dinoterbon in food samples. references [1] r. c. gupta, toxicol. mechan. methods, 14 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open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {electrochemical nanobiosensors perspectives for covid 19 pandemic:} http://dx.doi.org/10.5599/jese.1116 25 j. electrochem. sci. eng. 12(1) (2022) 25-35; http://dx.doi.org/10.5599/jese.1116 open access : : issn 1847-9286 www.jese-online.org review electrochemical nanobiosensors perspectives for covid 19 pandemic rajshri kundlik satvekar department of technology, shivaji university, kolhapur 416004, maharashtra, india corresponding authors: rajshrinaik5@gmail.com; tel.: +91 8625065414 received: september 20, 2021; accepted: november 2, 2021; published: november 23, 2021 abstract early, rapid and ultrasensitive diagnosis of covid-19 to facilitate high-throughput analysis without a high degree of technical expertise or sophisticated equipment is necessary to expand covid-19 testing capability. leveraging interdisciplinary proficiency in analytical chemistry, biomedical instrumentation, molecular biology, microfluidics, and nanotechnology, considerable advances have been made to develop a novel diagnostic tool that assures superior key performances for covid-19 diagnosis. this review summarizes the nano-enabled systems such as electrochemical nanobiosensor for sarscov-2 virus detection and emphasizes promising diagnostic techniques to extensively facilitate the diagnostic practices during the covid-19 pandemic. currently, three main diagnostic methods have been widely used in the covid-19 pandemic: nucleic acid (na)based testing, computed tomography (ct), and serological testing. na-based detection of sars-cov-2 such as reverse transcription polymerase chain reaction has become the gold standard for covid-19 diagnosis. this review congregates significant contributions in the electrochemical nanobiosensor research area, which is helpful for further nanobiosensor development. although many efforts were taken to detect the sars-cov-2, the covid 19 diagnosis still relies on expensive prolonged analysis. a rapid and reliable alternative is the utilization of a low-cost nanobiosensor for sars-cov-2 detection that can rapidly diagnose the disease even in asymptomatic conditions with high reliability and sensitivity. keywords nanomaterial; bio analytical method, rt pcr, portable biosensor. list of abbreviations aiv: avian influenza virus assured: affordable, sensitive, specific, user friendly, robust and rapid, equipment-free, and deliverable to end-users aunp-lf: gold nanoparticle-based lateral-flow cdc: center for disease control and prevention http://dx.doi.org/10.5599/jese.1116 http://dx.doi.org/10.5599/jese.1116 http://www.jese-online.org/ mailto:rajshrinaik5@gmail.com j. electrochem. sci. eng. 12(1) (2022) 25-35 nanobiosensors perspectives for covid 19 pandemic 26 chikv: chikungunya virus covid-19: coronavirus disease 2019 crispr: clustered regularly interspaced short palindromic repeats ct: computed tomography denv3: dengue fet: field-effect transistor hiv: human immunodeficiency virus hpv-16: human papillomavirus hrp: horseradish peroxidase ev 71: human enterovirus 71 jev: japanese encephalitis virus lfa: lateral flow assay lspr: localized surface plasmon resonance mab: monoclonal antibody mers-cov: middle east respiratory syndrome coronavirus na: nucleic acid n gene: nucleocapsid protein gene nps: nanoparticles pcr: polymerase chain reaction pems: piezoelectric microcantilever sensors poct: point-of-care testing qcm: quartz crystal microbalance rabv: rabies virus rbd: receptor-binding domain rdrp gene: rna-dependent rna polymerase gene rt pcr: reverse transcription–polymerase chain reaction sars-cov-2: severe acute respiratory syndrome coronavirus 2 sers: surface-enhanced raman scattering spce: screen-printed carbon electrode voc: volatile organic compounds we: working electrode who: world health organization introduction nanotechnology can improve the whole healthcare process; starting from diagnosis to treatment, prognosis, and discovery of drugs, and follow-up monitoring [1-4]. nano-enabled systems provide two major advantages for diagnosis: rapid testing and early detection [5-10]. nanobiosensor have shown tremendous outcomes in literature in the past 20 years with a large number of research articles entitled early detection, reliable, accurate, disposable, ultrasensitive and affordable biosensors for diagnosis of various diseases [11-14]. however, in this covid 19 pandemic scenario, none of the nanobiosensor is commercially available in the market. considering various technological challenges one can find out the opportunity for the development of nanobiosensor for pandemic diseases. accurate, rapid, and early diagnosis of covid-19 is vital for treatment and control. to overcome these shortcomings, the electrochemical biosensors should be considered, which rely on their relative simplicity and inexpensive technique to perform rapid measurements in miniaturized portable systems. electrochemical biosensors with a volumetric transducer exhibit great potential for detecting viruses and are extensively used because of their rapid response, sensitivity, simplicity, miniaturization, cost-effectiveness, and portability [15]. world health organization (who) advised the international community to carry out massive diagnostic testing to fight transmission of the virus and decrease the number of undetected cases because diagnosis is also valuable to help researchers gain knowledge of the epidemiology of the disease. furthermore, diagnosis plays a crucial role in making appropriate decisions on treatment r. k. satvekar j. electrochem. sci. eng. 12(1) (2022) 25-35 http://dx.doi.org/10.5599/jese.1116 27 and isolation of infected people, in that way, slowing or stopping the spread of the covid 19. nucleic acid tests can rapidly and sensitively identify the virus in suspected covid 19 patients; however, large amount of genetic variations, as well as mismatches of primers, probes, and target sequences, may result in reduced detection performance and false-negative results [16]. serological surveys can assist in the investigation of an ongoing outbreak and retrospective assessment. antibody detection of igm and igg antibodies in vivo is a supplement to molecular diagnostic methods. however, a lacking in the construction of diagnostic sensing techniques having a quick response to the threat and starting early diagnosis has an important impact globally. the increasing severity of the pandemic situation could be related to a lack of effective point-of-care testing (poct) assays for rapid and accurate identification of sars-cov-2-infected patients. in this view, here we review the current developed diagnostic techniques in response to the covid-19 pandemic and compare the different types through their pros and cons, such as nucleic acid detection tests (pcr and crispr), serological tests. the purpose of this review is to discuss the different techniques used for detection of viruses, critically scrutinize the various diagnostic platforms and summarize the studies on electrochemical nanobiosensors development in this covid 19 scenario. morphology and overview of sars-cov-2: sars-cov-2 includes in the genus β-coronavirus, consisting of a crown-like envelope, and positive-sense single-stranded rna (þssrna). it is a circular-shaped virus (60-140 nm) comprised of the nucleocapsid phosphoprotein (n) on the surface and the genome þssrna to form a helical nucleocapsid. the largest structural (s protein) protein of sars-cov-2 makes the distinct spikes on the virus's surface, as shown in figure 1. sp protein facilitates viral entry into host cells by using its receptor-binding domain (rbd) region to bind host cell receptors. figure 1: sars-cov-2 genomic organization and virus structure the rbd region of the sp is, therefore, the main target for the detection of sars-cov-2. an antisp antibody assay can be used to screen serum containing high titers of sars-cov-2 neutralizing antibodies targeting the sp [17]. the nanopore-based direct rna sequencing approach has revealed that n rna is the most abundantly expressed transcript in sars-cov-2 infected cells, followed by s, 7a, 3a, 8, m, e, 6, and 7b. this suggests that the n gene is one of the simplest targets for highsensitivity detection of sars-cov-2 infection. thus, collectively, the sars-cov-2 virus particle, rna, http://dx.doi.org/10.5599/jese.1116 j. electrochem. sci. eng. 12(1) (2022) 25-35 nanobiosensors perspectives for covid 19 pandemic 28 np antigen, and human antibodies targeting the sp and therefore the np used to accurately screen covid-19 patients [18]. biosensors for covid-19 are mostly designed on the surface nucleoproteins, which binds to the host ace-2 receptor and the internal genetic material [19]. results and discussion detection of sars-cov-2 virus nucleic acid detection via rt-pcr: polymerase chain reaction (pcr) based tests are widely used to detect viruses in human diseases in clinical laboratories and tests consist of nucleic acid extraction followed by purification from the human sample using authorized extraction methods. virus-specific pcr tests required generating primers and probes exclusive to sars-cov-2, excluding other closely related coronaviruses [20]. for the detection process of the sars-cov-2, primarily, the upstream oligonucleotides of the envelope gene (e gene) are screened, followed by the confirmation of the nucleocapsid gene (n gene) using the rt-pcr approach. center for disease control and prevention (cdc), suggests nucleic acid testing is the main approach for covid-19 diagnostic [21]. the rt-pcr technique is considered the gold standard for diagnosing viral agents. in rt-pcr, the rna is reverse transcribed into cdna and followed by amplification using the primer sets and detected by specific probes. there are identified three regions on the viral genome with conserved sequences: rna-dependent rna polymerase gene (rdrp gene), envelope protein gene (e gene), and nucleocapsid protein gene (n gene). rdrp and e genes have great analytical sensitivity for detection as compared to the n gene. therefore, it is believed that the development of a two-target platform with a universal primer (all the coronaviruses strands) and a specific sars-cov-2 primer would give more accurate identification [22]. false negative results can occur for several reasons, including inappropriate specimen type, suboptimal specimen collection, early testing, low analytical sensitivity, low viral load, and inconsistency in viral shedding [23]. an analysis of 103 genomic data was done by tan et al. and demonstrated that these virus strains underwent a total of 149 point mutations (ancestral states for 43 synonymous, 83 non-synonymous, and two stop-gain mutations) [24]. if a mutation is situated in the probe or primer binding site, the sensitivity and accuracy of existing rt-pcr detection kits will be affected. rt-pcr still has many disadvantages; one of them is having false-negative results needing complementary tests like ct scans. rt-pcr reagents and test kits are not affordable, and hospitals with rt-pcr infrastructures are located in centralized cities. last but not least, rt-pcr diagnosis depends on the presence of the sars-cov-2 in the sample, which means that it will not identify asymptomatic patients who recovered from the infection, and the prevention measures could not be applied. clustered regularly interspaced short palindromic repeats (crispr) the clustered regularly interspaced short palindromic repeats (crispr)/cas equipment has recently been tailored as a poc tool for the rapid detection of nucleic acids (dna or rna). crispr is a genome programming system; it is programmed to cleave specific sequences in the dna/rna target oligonucleotides having specific genetic code to edit rna and dna at precise locations where the results can be easily observed by combination with a lateral-flow strip. broughton et al. have developed a rapid, easy-to-implement and precise crispr cas12-based lateral flow assay to detect sars-cov-2 from respiratory swab rna extracts, called sars-co dna endonuclease-targeted crispr trans reporter [25]. r. k. satvekar j. electrochem. sci. eng. 12(1) (2022) 25-35 http://dx.doi.org/10.5599/jese.1116 29 computed tomography scans different imaging techniques such as chest x-rays, lung ultrasounds, and pulmonary computed tomography are significant tools in the early diagnosis of pneumonia associated with covid-19 patients. the procedure for ct scan is non-invasive and consists of x-ray images of different sections of the chest area. an expert radiologist examines the images to identify any abnormal characteristics related to the disease, which are diverse and depend on the infection stage. it has been found that during the early onset of the infection (first 2 days), 56 % of the cases presented similar ct results, while after 10 days, the lung's implication was observed. usually observed characteristics include lung consolidation and bilateral/peripheral opacities. ct scans showed a higher sensitivity (86–98%) in addition to fewer false-negative results compared to rt-pcr [26, 27]. however, the disadvantage of ct scans is that many characteristics observed for sars-cov-2 are overlying with other viral pneumonia, and accordingly, the specificity is extremely low (25 %). the major drawbacks of ct scans are the high price, the need for highly experienced staff, and are not specific to sars-cov-2. ct scan is not suitable for asymptomatic, pre-symptomatic patients and some mild symptomatic individuals without pneumonia. antibody-based tests the serological antibody test is carried out for symptomatic patients. diagnosis of covid-19 related igm and igg tends to signify a recent and sometimes previous virus exposure to sars-cov2, respectively. sars-cov-2 specific protein utilization for immunoassays is ideal for minimizing possible false-positive test results ensuing by cross-reactivity with other viruses. antibody-based testing is carried out for the detection of sars-cov-2 specific igm and igg in whole blood, plasma, or serum [28]. in another study, in patients diagnosed with covid-19 shows the presence of iga antibodies targeting the s protein from day 6 to 8 and up to 42 days, with a peak value at 20 to 22 days [29]. serologic tests are not recommended for rapid diagnosis of the infection because the timing of the appearance of igm and igg is variable and may be delayed. nano-enabled methods nanobiosensors have improved sensitivity, stability as required by analysis as well as enhanced stability and specificity of the detection system. noteworthy, nanoparticles can produce a synergy effect between conductivity, catalytic activity, and biocompatibility to enhance signal transduction [30]. the specific properties of nanomaterials for the construction of biosensors can be summarized as follows: i) higher surface area allowing the immobilization of a larger density of biomolecules, ii) lower diffusion limitations for the analyte to reach immobilized biomolecules, iii) direct electron transfer between the modified electrode and active site of the enzyme making biosensors more selective, iv) higher current density and/or analysis at lower overpotentials, iv) enhanced loading of secondary biorecognition elements. nanomaterials have great potential to improve the performance of nano-enabled methods due to their high surface area promoting electron transfer reactions, electrical conductivity, good chemical stability, and mechanical robustness [31,32]. lateral flow assay (lfa) the lfa is user-friendly, inexpensive, and simply mass-produced in an endeavor to manage the covid-19 pandemic. encouragingly, the detection sensitivity of the lfa has been improved with the use of novel nanomaterials as immune labels such as quantum dots, gold nanoparticles, and magnetic nanoparticles. lfas that use ultra-bright fluorescence nanomaterials with longer fluorescence lifetimes can significantly reduce background noise and enhance lfa detection http://dx.doi.org/10.5599/jese.1116 j. electrochem. sci. eng. 12(1) (2022) 25-35 nanobiosensors perspectives for covid 19 pandemic 30 sensitivity using the time-resolved analysis technique. layqah et al. developed an immunosensor based on carbon electrodes modified with aunps, a highly selective, single step, sensitive, and accurate method [33]. in another study, a colloidal gold nanoparticle-based lateral-flow (aunp-lf) assay was developed to achieve rapid diagnosis and on-site detection of the igm antibody against the sars-cov-2 virus through the indirect immune-chromatography method [34]. potential portable nanobiosensors several new nanobiosensing prototypes have been developed recently for the diagnosis of covid-19, such as electrochemical biosensors, colorimetric biosensors, localized surface plasmon resonance (lspr), surface-enhanced raman scattering (sers), quartz crystal microbalance (qcm), and piezoelectric microcantilever sensors (pems), etc. [35]. in most recent work, qiu et al. [36] developed a dual functional plasmonic sensor with high sensitivity, rapidity, and reliable diagnostic capability for the sars-cov-2 virus detection. in other studies, the authors used a field-effect transistor (fet) biosensor and genosensor to detect sars-cov-2 from covid 19 patient samples [37,38]. the recent article presents a non-invasive intelligent nanomaterial-based hybrid sensor array with multiplexed capabilities for detecting and monitoring of covid-19-specific volatile organic compounds (voc) mixtures from exhaled breath [39]. electrochemical nanobiosensors for virus detection electrochemical nanobiosensors generally depend on the enzymatic catalysis reaction between the immobilized biorecognition element and the targeted analyte that produces electrons and affects the electrical properties of the solution. moreover, depending on the incorporated biorecognition element, different electrochemical biosensors are constructed, such as immunosensors, enzymatic biosensors, and dna biosensors [40]. the main benefits of electrochemical biosensors are easy construction, simple instrumentation, high sensitivity, costeffectiveness and, the possibility of portability and miniaturization, high sensitivity, and relatively low costs [41]. a recent work [42] has established a miniaturized label-free electrochemical impedance spectroscopy-based detection of biomarkers with metallic nanoparticles (nps), electrochemically engineered nano-dendroids, and graphene oxide nanocomposites. these nanomaterials were deposited over the screen-printed carbon electrode (spce) and were incorporated antibodies against the specific biomarker. it detects the infection precisely, owing to its robustness and high analytical performance. another important feature of such nanobiosensors is data accusation in requisites of electronic signals, linked with smartphone-based analytical systems. this system offers a semi-automated user interface that can be used by an untrained person without technical knowledge. through inbuilt tailored hardware and sensing software, sensing systems can be developed within a smartphone to miniaturize the system to be carried out to any location and operated by any semi-trained people. it can provide a commercial substitute for expensive standalone technologies [43]. in recent years, electrochemical biosensors such as immunosensor and dna based nanobiosensors have shown great accomplishment in medical diagnosis due to their distinctive properties and user-friendly platform for the detection of pathogenic viruses. the main steps involved in the construction of the genosensor include immobilization of a single strand of dna probe on the working electrode, which is surface-functionalized with nanomaterials; hybridization with a complementary strand (target) and electrochemical detection. the target dna detection depends on measuring changes of electric signals such as current, impedance, and potential across the three electrodes, which are originated by hybridization of the target dna in the sample and the r. k. satvekar j. electrochem. sci. eng. 12(1) (2022) 25-35 http://dx.doi.org/10.5599/jese.1116 31 dna probe on the we and the related chemical reactions [44]. the surface of the genosensor is important in the performance of the nucleic acid-based biosensors for that various nanomaterials have been utilized due to properties such as strong affinity toward bioreceptor probes with reactive groups such as thiols leading to high sensitivity and low limits of detection. singhal and coworkers proposed a novel paper-based dna biosensor utilizing fe3o4@au nanocubes for the detection of chikv [45]. the electrochemical biosensors can yield a dramatic detection limit for sensing a single strain of dna/rna molecules [46]. over an era of decades, viruses such as dengue, avian influenza virus (aiv), hepatitis virus, zika, chikungunya virus (chikv), rabies virus (rabv), human nor virus, japanese encephalitis virus (jev), hiv, and coronavirus [47-51] cause occurrence of infectious diseases. the performance of different detection methods is compared in table 2. table 1. different nanobiosensors for detection of viruses the multiplexing method offers immense advantages such as rapid speed and lowering costs for screening multiple viruses simultaneously [60]. pocts technologies are generally optimized in specific laboratory conditions, and through the affordable, sensitive, specific, user friendly, robust and rapid, equipment-free, and deliverable to end-users (“assured”) criteria [60] proposed by who can be applied effectively to an individual for diagnosis of infectious diseases. in a recent study, a multiplexed, wireless portable electrochemical platform for ultra-rapid detection the sars-cov-2 which detects viral antigen nucleocapsid protein, igm and igg antibodies, as well as the inflammatory biomarker c-reactive protein, based on mass-producible laser engraved graphene electrodes. this sars-cov-2 rapidplex platform successfully evaluated the applicability with covid19-positive and negative blood and saliva samples [61]. the detection methods targeting antibodies are inappropriate for screening early and asymptomatic cases as most patients had an antibody response at about ten days after onset of symptoms. on the other hand, antibody detection methods can be combined with quantitative rtpcr to significantly improve the sensitivity and specificity of diagnosis and boost vaccine research [62]. a low level of limit-of-detection is critical to change the diagnostic window of opportunity toward the initial infection process, to detect newly infected individuals. the test can be based on immunoassays, using antibodies to detect a specific antigen produced by the body's immune system or polymerase chain reactions (rt-pcr) to detect a viral genome sequence [64]. the rtpcr test no. analyte/virus limit of detection linear range recognition element ref. electrochemical impedance spectroscopy-based biosensor 1 jev 2.60 ng ml-1 0.1–20.0 ng ml-1 antigen [52] 2 norovirus 1.7 copies ml-1 0 105 copies ml-1 virus [53] 3 chicken guinea virus 8 ng ml-1 0.025 -1 μg ml-1 chikv nsp3 [54] electrochemical immunosensor 4 mers-cov 1.0 pg ml-1 0.001 100 ng ml-1 spike protein s1 [33] 5 ev71 0.01 ng ml-11 0.01 -1.0 ng ml-1 hrp and ev71 mab [55] 6 zikv 10 pm 0.01 – 1.00 nm specific envelop protein antibody [56] dna based electrochemical biosensors 7 hpv-16 2.3 nm 10-200 nm aq-pna dna probe [57] 8 ebola (dna) 4.7 nm 0 5 nm biotinylated target strand dna [58] 9 dengue (denv3) 9.55 × 10-12 m 10-12 10-6 m specific dna probe [59] http://dx.doi.org/10.5599/jese.1116 j. electrochem. sci. eng. 12(1) (2022) 25-35 nanobiosensors perspectives for covid 19 pandemic 32 becomes the method of choice owing to its sensitivity and specificity, and it is accomplished of detecting a single copy of the virus, resulting in a decrease in the diagnostic window compared to immunoassays. moreover, the rt-pcr technique has various limitations in their respective applications, as shown in table 2. table 2. limitations of pcr test for covid 19 no. pcr tests are prone to the following limiting factors [64]: (i) sampling error: nasopharyngeal swab is suitably performed to take mucus from the ventilator system; however, this may give rise to false negatives as the optimal sampling moment is still ambiguous. (ii) sample preparation: including cell lyses and nucleic acid purification for pcr analysis is required, and the number of extraction kits is a limiting factor for ramping up covid-19 testing. (iii) specialized handling and transportation: viral genome may be denatured during transportation, also bringing about false negatives (iv) quality of reagents: the quality of reagents used by different pcr kit manufacturers may also affect consistency among results. (v) lack of sensitivity standard pcr methods: it may be leading to false-negative results in covid-19 patients with unapparent clinical symptoms. (vi) false-positive outcomes: for recovered patients, even weeks after full recovery, pcr tests can report false positive outcomes due to the presence of genetic material of the dead sars-cov-2. (vii) low flexibility of pcr: special primers and probes for each target are necessary, which confines pcr’s flexibility of scaling up for other nucleic acids in simple and rapid manner. (viii) mutations sars-cov-2 rna: since sars-cov-2 rna is likely to undergo mutations, require special primers and probes the same target. conclusions diagnosis of covid 19 using nanobiosensor system is a frontier research area that has to deal with much unmet disputes required to develop and commercialize the nanobiosensors. in some particular cases, the validation with real samples in clinical scenarios strengthens nanobiosensors suitability. considering the urgent need for fast and reliable detection of covid-19 the nano-enabled biosensor can play a crucial role as it will reduce the time to detect, be inexpensive, and, the integration of biosensor with the microfluidic system. with the continuing progress in nanotechnology tools and increasing research on the nano-scale phenomena, one may look forward to further achievements in the development of nanobiosensors for the diagnosis of infectious diseases. references [1] h. karimi-maleh, f. karimi, l. fu, a. l. sanatie, m. alizadeh, c. 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hajializadeh department of natural resources, sirjan branch, islamic azad university, sirjan, iran corresponding author: hajalizadeh.813@gmail.com received: december 13, 2021; accepted: december19, 2021; published: february 9, 2022 abstract this paper presents a sensitive simultaneous detection procedure for sudan i and bisphenol a based on the multiwalled carbon nanotubes (mwcnts)/co3o4 nanocomposite modified screen-printed graphite electrode (spge). this mwcnts/co3o4 nanocomposite was prepared by the hydrothermal technique, and characterized by fourier transform infrared (ft-ir) spectroscopy, field emission scanning electron microscopy (fe-sem) and x-ray diffraction (xrd). the electrochemical properties of mwcnts/co3o4 nanocomposite modified spge were analyzed by cyclic voltammetry (cv), differential pulse voltammetry (dpv), chronoamperometry (cha), as well as linear sweep voltammetry (lsv). from the electrochemical results, a synergy between mwcnts and co3o4 nanoparticles (nps) was detected as improved interfacial electron transfer, which was accompanied by a greater catalytic function for electrochemical oxidation of sudan i. based on the optimized condition, mwcnts/co3o4/spge exhibited the linear dynamic ranging between 0.05 and 600.0 μm detection of sudan i with a limit of detection (lod) 0.02 μm. also, the as-prepared electrode was assessed for simultaneous detection of sudan i and bisphenol a. in the course of electrooxidation processes of these analytes, two complete peaks at 380 and 520 mv were observed on the modified electrode. at the end, utility of this new electrochemical sensor was performed to determine sudan i and bisphenol a in some real samples with good accuracy and precision. keywords electroanalysis; voltammetry; azo dyes; food analysis; modified electrodes; spge; mwcnts/co3o4 introduction sudan i (1-phenylazo-2-naphtol) is a synthetic azo dye commonly employed as one of the coloring materials in industrial products like shoes, textile, oil, plastic, printing ink, cosmetics, as well as floor polish [1]. moreover, sudan i is also proposed as food additive material, sauces, chili powder, readymade meals, and chutneys because of its bright red colour, affordability, and coloring fastness. http://dx.doi.org/10.5599/jese.1211 http://dx.doi.org/10.5599/jese.1211 http://www.jese-online.org/ mailto:hajalizadeh.813@gmail.com j. electrochem. sci. eng. 12(1) (2022) 185-197 sensor based on mwcnts/co3o4/spge 186 however, in the case of ingestion of the materials consisting of sudan i by human bodies, carcinogenic amines would be produced following the metabolism process [2]. hence, european union (eu) and food standards agency have strongly forbidden the utilization of sudan i as an additive in foodstuff because of its higher teratogenetic and carcinogenic nature. despite the legal prohibitions, sudan i circulates in the food markets due to temptations for massive profits [3]. thus, the determination of sudan i residues in the food products would be necessary to preclude health risks to humans. experts in the field have introduced bisphenol a (4,4ʹ-(propane-2,2-diyl)diphenol) (bpa) as one of the key monomeric compounds applied in fabricating polycarbonate and epoxy in plastic outputs. they have also been employed for making diverse plastic products like water and milk bottles, food containers, bottles for infant feeding, dental fillings, bottle tops, table-ware, food cans, and other storage vessels [4-6]. moreover, bpa can migrate from plastic products into the environment and food chain resulting in considerable humans' exposure [7]. additionally, bpa has been proposed as one of the endocrine-disrupting compounds associated with several sorts of health issues like cancer progression and reproduction issues [8,9]. considering public health, it would be of high significance to develop a valid analytical procedure to determine bpa. several analytical methods such as gas chromatography-mass spectrometry (gc–ms) [10], highperformance liquid chromatography (hplc) [11,12], fluorescence [13], capillary electrophoresis [14], surface-enhanced raman scattering spectroscopy [15], and chemiluminescence [16,17] have been developed for detecting bisphenol a and sudan i. these techniques showed good outputs, although, they might be laborious and demand numerous raw materials, professional workforce, and costly instruments. in this regard, researchers have also studied electrochemical sensing as one of the very attractive fields in analytical chemistry [18-22]. electrochemical methods are broadly employed for the analysis of biological, environmental, food, pharmaceutical and industrial compounds due to merits such as fast responses, higher sensitivity, lower costs, portability, and ease of operation [23-29]. it is notable that as an electrochemical sensing platform, the screen-printed electrode (spe) has been largely attracted because of merits like easier application, portability, and affordability [30,31]. hence, the screen-printed technology significantly contributes to the transition from the conventional unwieldy electrochemical cells to the portable miniaturized electrodes, satisfying the on-site analysis requirements [32,33]. in the electrochemical approach, using a proper electrode is of utmost importance. in this regard, developing the modified substances for improving the electrochemical response of the electrodes would be a challenging task [34-40]. in recent years, researchers have focused on the designing and synthesizing nanomaterials for various applications due to their unique physical and chemical properties [41-43]. investigations demonstrated that electrodes modified by catalytically active nanomaterials might enhance efficient mass transfer, achieve more acceptable control, and enhance the active specific surface area in a local micro-environment, improving the sensitivity and selectivity of electrochemical sensors [44-49]. experts in the field have largely employed the carbon nanotubes (cnts) in the fabrication of electrochemical sensors because of the respective merits like higher carrier mobility, a larger surfaceto-volume ratio, good conductivity, and flexibility. cnts are specific carbon materials with cylindrical curled graphitic sheets [50]. numerous investigations have shown that constructing the hybrid catalysts via immobilization of metal and metal oxide nanoparticles (nps) on carbon nanotubes can improve their electrochemical responses [51,52]. furthermore, nanostructured transition metal a. hajializadeh j. electrochem. sci. eng. 12(1) (2022) 185-197 http://dx.doi.org/10.5599/jese.1211 187 oxides contribute significantly to the improvement of sensitivity and stability of sensors. it should be mentioned that among the family of transition metal oxides, nanostructures of cobalt oxide (co3o4) exhibited specific electrocatalytic activity for detecting different compounds [53-55]. the possible cause would be higher activity and selectivity of the metal oxide catalysts resulting from the differences in oxygen defects, oxygen absorbed in various states of cobalt in co3o4 (a mixed valance state of co(ii) and co(iii)), as well as oxygen holes [56]. in the present report, mwcnts/co3o4 nanocomposite was prepared and employed for the modification of spge. the modified spge was then utilized to simultaneously detect sudan i and bisphenol a. experimental chemicals and instrumentations according to the research design, each chemical was of the analytical reagent grade without additional purification. co(ch3co2)2 4h2o, sodium hydroxide, ethanol, nh3⋅h2o (25 to 28 wt.%), and mwcnts-cooh (purity >95 %) were supplied from aldrich. sudan i, bisphenol a and other chemicals were supplied from the merck company (darmstadt, germany). phosphate buffer solutions (pbs) were prepared from h3po4 and the respective salts (kh2po4, k2hpo4 and k3po4) (merck). food samples such as ketchup sauce, tomato paste and chili powder were purchased from a local store in kerman, iran. nano-pure (18 mω⋅cm) water from a milli-pore milliq system (bedford) was applied to prepare all solutions. electrochemical experiments were recorded using an auto-lab potentiostat/galvanostat (pgstat-302n, eco chemie, the netherlands). electrochemical tests were carried out using spge (dropsens, drp-110, spain). electrochemical cells consisted of a three-electrode arrangement with graphite as a counter electrode and graphite in 4 mm diameter as a working electrode. in addition, a silver pseudo-reference electrode was used to complete the circuit. ph values were measured using a digital ph meter (metrohm 710). fourier transform infrared (ft-ir) spectra of the synthesized nanocomposite were recorded using a spectrometer (shimadzu corporation, kyoto, japan) between 4000 and 400 cm-1, applying kbr pellets. xrd patterns were examined with the xrd device model x'pert pro, the netherlands, while edx and fe-sem images were obtained using mira3tescan-xmu. synthesis of the mwcnts/co3o4 nanocomposite a hydrothermal approach was used for preparing mwcnts decorated with co3o4 nps. firstly, 20 mg of mwcnts-cooh were dispersed into 12.5 ml of ethanol, and 0.25 g co(ch3co2)2 .4h2o was added to the mixture for sonication during 40 min. then, 2.5 ml of aqueous nh3 were dropped into the solution and vigorously stirred. the homogeneous slurry was taken to a teflon-lined stainless steel autoclave, sealed, and heated at 150 °c for 3 h. after cooling the solution to the room temperature, water and ethanol were used to wash the products and achieved mwcnts/co3o4 nanocomposite followed by the drying process at 60 °c for 8 hours. modification of spge the construction of mwcnts/co3o4 nanocomposite over the spge surface was accomplished in this way: 1 mg of mwcnts/co3o4 nanocomposite was suspended in 1 ml of the distilled water for forming the suspension, which was sonicated for 20 min to disperse the nanocomposite. finally, 5 μl aliquot of the suspension was pipetted over the surface of an spge and dried at the ambient temperature. http://dx.doi.org/10.5599/jese.1211 j. electrochem. sci. eng. 12(1) (2022) 185-197 sensor based on mwcnts/co3o4/spge 188 the surface areas of mwcnts/co3o4/spge and bare spe were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using randles-ševčik formula for mwcnts/co3o4/spge, the electrode surface was determined to be 0.08 cm2, about 2.5 times greater than bare spge. preparation of real samples in this step, 20 ml ethanol were used for mixing ketchup sauce, tomato paste, as well as chili powder, and the obtained mixtures were filtrated after 20 min of ultra-sonication, with 100 ml volumetric flasks employed for collecting the liquid phase. then, treatment was performed for three times and ethanol was applied for diluting the filtrated sample to the intended volumes. the tap water gathered from the laboratory was analyzed in twelve hours. before the analysis process, 0.22 μm cellulose acetate membrane was used to filter the water. results and discussion characterization of mwcnts/co3o4 nanocomposite the ftir spectrum of mwcnts/co3o4 nanocomposite is shown in figure 1. for mwcnts/co3o4 nanocomposite, two sharp bands are found at 660 and 562 cm-1, indicating presence of co2+ and co3+ in the spinal co-o stretching vibration, wherein co2+ is associated with the tetrahedral coordinate and co3+ to the octahedral coordinate. the spectra bands at 2921 cm-1 to 2854 cm-1 are attributed to the asymmetric or symmetric stretching vibrations of c-h in ch2 and –ch3 group in mwcnts. the peak at 1641 cm-1 (aromatic c=c) can be ascribed to the stretching vibration c=c of sp2 hypridization in cnt back-bone. the band at 3442 cm-1 is attributed to o–h stretching vibration. ft-ir spectrum confirmed the decoration of mwcnts with co3o4 nanoparticles. figure 1. ft-ir spectrum of mwcnts/co3o4 nanocomposite the crystal structure of mwcnts/co3o4 nanocomposite was identified by xrd. the xrd peaks of the treated mwcnts/co3o4 and mwcnts are shown in figure 2. as can be seen, mwcnts patterns have two key diffraction peaks at 2=25.9° and 43.1°, which belong to the (002) and (100) planes of graphite. diffraction peaks at 2  values of 19.08, 31.3, 36.9, 38.6, 44.9, 55.7, 59.4 and 65.3o° observed for mwcnt/co3o4 nanocomposite correspond to several planes (111) (220) (311) (222) (400) (422) (511) as well as (440) of co3o4. these characteristic peaks belong to the cubic phase of co3o4 (jcpds, pdf, file no. 00-042-1467). furthermore, the peaks have higher intensity reflecting acceptable crystallinity of the samples. however, we did not find any impurity, representing the a. hajializadeh j. electrochem. sci. eng. 12(1) (2022) 185-197 http://dx.doi.org/10.5599/jese.1211 189 substantial contribution of the mwcnt/co3o4 nanocomposite. finally, debye-sheerer equation was used for calculating crystallite size (d) of co3o4 nps: d = kλ /  cos  (1) here, λ represents the wavelength,  the peak width at half maximum, and  diffraction angle. dimension of co3o4 nanoparticles is found to 23.8 nm. figure 2. xrd patterns of mwcnts and mwcnts/co3o4 nanocomposite morphology of mwcnts/co3o4 nanocomposite was investigated using fe-sem. figure 3 shows the fe-sem image of mwcnts/co3o4 nanocomposite, where the presence of co3o4 nanoparticles coated on the surface of mwcnts could be evidenced. figure 3. fe-sem image of mwcnts/co3o4 nanocomposite http://dx.doi.org/10.5599/jese.1211 j. electrochem. sci. eng. 12(1) (2022) 185-197 sensor based on mwcnts/co3o4/spge 190 electrocatalytic oxidation of sudan i at mwcnts/co3o4/spge for ensuring the best response of mwcnts/co3o4/spge in detecting sudan i, the influence of the solution ph on the reaction was evaluated by recording voltammetric behavior of mwcnts/co3o4/spge toward 100.0 μm of sudan i in ph range between 2.0 and 9.0 at the scan rate 50 mv/s. it is shown in figure 4 that the maximum oxidation current of sudan i was obtained at ph 7.0. hence, ph 7.0 was chosen for all further measurements in 0.1 m pbs. figure 4. plot of ip vs. ph obtained from dpvs of mwcnts/co3o4/spge in a solution containing 100.0 μm of sudan i in 0.1 m pbs of different ph (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0) in the next step, the potential application of mwcnts/co3o4/spge for electrooxidation and determination of sudan i was examined by cyclic voltammetry (cv). figure 5 depicts the cv response of oxidation of 200.0 μm of sudan i on (a) bare spge, and (b) mwcnts/co3o4/spge in 0.1 m pbs, ph=7.0 at the scan rate 50 mvs-1. as seen in figure 5, oxidation potential decreased and the peak current height increased for electroxidation of sudan i at mwcnts/co3o4/spge as compared to bare spge. these are probably caused by increasing the rate of the electron transfer process at the modified spge electrode, due to the higher surface area, better conductivity of mwcnts, and electrocatalytic activity of co3o4 nanoparticles. figure 5. cv response of 200.0 μm sudan i in 0.1 m pbs of ph 7.0 at (a) bare spge and (b) mwcnts/co3o4/spge effect of potential scan rate on electrochemical response of sudan i at mwcnts/co3o4/spge for obtaining information on the kinetics of electrode reactions, the linear sweep voltammetry (lsv) at several scan rates from 10 to 400 mv s-1 in 0.1 m pbs, ph 7.0 was applied, and the results a. hajializadeh j. electrochem. sci. eng. 12(1) (2022) 185-197 http://dx.doi.org/10.5599/jese.1211 191 are illustrated in figure 6. at higher scan rates, a gradual increase of the oxidation peak current can be observed. obtained voltammograms are irreversible, as were already seen in cv responses shown in figure 5. also, figure 6 shows that the oxidation peak potential is shifted slightly towards positive potentials with increase of the scan rate. according to the linear plot of the oxidation peak current versus the square root of the scan rate, shown in the inset of figure 6, the linear regression eqation was derived. this result indicates that the oxidation process of sudan i at mwcnts/co3o4/spge electrode is under diffusion control. figure 6. lsv curves of 50.0 μm of sudan i in 0.1 m pbs, ph 7.0, at mwcnts/co3o4/spge and different scan rates (a-g refer to 10, 25, 50, 100, 200, 300, and 400 mv s-1). inset: plot of the oxidation peak current vs. square root of the scan rate to define the electron transfer coefficient (α) between sudan i and mwcnts/co3o4/spge electrode, tafel diagram (e vs. log i) was drawn in the inset of figure 7, using data of the ascending section of the voltammogram registered at 10 mvs-1 for 50.0 μm of sudan i (figure 7). the calculated slope from the linear plot was equal to 0.084 v-1. from the slope, α value was estimated to 0.3. figure 7. lsv response (10 mvs-1) of 50.0 μm of sudan i in 0.1 m pbs, ph 7.0 at mwcnts/co3o4/spge. inset: tafel plot derived from data of the rising part of voltammogram http://dx.doi.org/10.5599/jese.1211 j. electrochem. sci. eng. 12(1) (2022) 185-197 sensor based on mwcnts/co3o4/spge 192 cha studies to measure the diffusion coefficient of sudan i, chronoamperometry was performed by applying the potential step from 0.0 to 0.43 v.. figure 8 shows the single-step chronoamperograms recorded for mwcnts/co3o4/spge in the presence of several concentrations of sudan i. as expected, when sudan i concentration is increased, the anodic current also increases. the experimental plots of current vs. t-1/2 for different concentrations of sudan i are shown in figure 8a. the slopes of the straight lines versus the concentration of sudan i is shown in figure 8b. diffusion coefficient was calculated from the slope of the straight line and it equals to 2.0×10-5 cm2 s-1. figure 8. chronoamperograms obtained for mwcnts/co3o4/spge in 0.1 m pbs, ph 7.0 at different concentrations of sudan i (a–f relate to 0.1, 0.4, 0.6, 0.9, 1.4 and 2.0 mm). inset a: plot of i vs. t-1/2 obtained from chronoamperograms a to f. inset b: plot of straight line slope vs. concentration of sudan i. detection of sudan i at mwcnts/co3o4/spge by dpv technique differential pulse voltammetry (dpv) technique was employed for determining sudan i concentration due to its higher sensitivity as well as accuracy leading to lower limit of detection (lod) values. figure 9 represents differential pulse voltammograms observed at different concentrations of sudan i in 0.1 m pbs, ph 7.0 (step potential = 0.01 v and pulse amplitude = 0.025 v). obviously, oxidation peak current increased with the increase of sudan i concentration and the inset of figure 9 demonstrates the linearity of current peak height and concentration of sudan i. the linear regression has an equation ipa = 0.0817 csudan i + 1.1311 (r2 = 0.9997). the sensitivity of mwcnts/co3o4/spge is 0.0817 μa μm-1. the detection limit, cm, of sudan i was obtained using the equation (2): cm = 3sb / m (2) in the above equation, m is the slope of the calibration plot (0.817 µa µm−1) and sb is the standard deviation of the blank response obtained from 20 replicate measurements of the blank solution. the detection limit of sudan i was calculated as 0.02 μm. a. hajializadeh j. electrochem. sci. eng. 12(1) (2022) 185-197 http://dx.doi.org/10.5599/jese.1211 193 figure 9. dpv responses of different concentrations of sudan i at mwcnts/co3o4/spge in 0.1 m pbs, ph 7.0 (a-l refer to 0.05, 0.5, 5.0, 15.0, 30.0, 70.0, 100.0, 200.0, 300.0, 400.0, 500.0 and 600.0 µm). inset: calibration curve of dpv peak current against concentration of sudan i simultaneous detection of sudan i and bisphenol a at mwcnts/co3o4/spge a simultaneous test of sudan i and bisphenol a was carried out by dpv (step potential = 0.01 v and pulse amplitude = 0.025 v) in 0.1 m pbs, ph 7.0. in fig. 10, two distinctive oxidation peaks of sudan i and bisphenol a could be discerned. the peak currents increased linearly with the increase of analyte concentrations without any interference (figures 10a and 10b). therefore, a possible simultaneous assay of sudan i and bisphenol a could be made with mwcnts/co3o4/spge sensor. figure 10. dpvs of mwcnts/co3o4/spge in 0.1 m pbs, ph 7.0 containing different concentrations of sudan i and bisphenol a (a-f refer to mixed solutions of 0.5 + 0.5, 30.0 + 35.0, 100.0 + 125.0, 200.0 + 225.0, 400.0 + 475.0, and 600.0 + 700.0 μm of sudan i and bisphenol a, respectively). inset a: plot of peak current as a function of sudan i concentration. inset b: plot of peak current as a function of bisphenol a concentration http://dx.doi.org/10.5599/jese.1211 j. electrochem. sci. eng. 12(1) (2022) 185-197 sensor based on mwcnts/co3o4/spge 194 analysis of the real samples to investigate the possibility of using mwcnts/co3o4/spge for the simultaneous detection of sudan i and bisphenol a in the real samples, electrochemical analysis were carried out in tomato paste, ketchup sauce, tap water and chili powder according to the standard addition approach. table 1 reports the outputs and achieved recovery percentages of 97.6 to 103.6 % for these samples. these observed outputs suggest that mwcnts/co3o4/spge has acceptable practical viability for simultaneous detection of sudan i and bisphenol a. table 1. determining of sudan i and bisphenol a in food samples (n = 5) sample concentration spiked, μm concentration found, μm recovery,% rsd, % sudan i bisphenol a sudan i bisphenol a sudan i bisphenol a sudan i bisphenol a ketchup sauce 0 0 5.0 5.5 4.9 5.7 98.0 103.6 2.3 3.5 7.0 7.5 7.2 7.4 102.9 98.7 2.7 2.1 tomato pste 0 0 1.3 2.7 4.0 4.5 4.1 4.4 102.5 97.8 3.0 1.9 6.0 6.5 5.9 6.6 98.3 101.5 2.2 3.5 chilli powder 0 0 6.0 6.5 5.9 6.7 98.3 103.1 2.4 2.1 8.0 8.5 8.1 8.3 101.2 97.6 2.8 1.6 conclusion in this work, mwcnts/co3o4 nanocomposite modified spge was prepared and introduced to simultaneously detect sudan i and bisphenol a. this modified electrode exhibited good electrocatalytic activity for the oxidation of sudan i in 0.1 m pbs, ph 7.0 solution. the mwcnts/co3o4/spge linearly 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(2016) 45-53. https://doi.org/10.1016/j.bios.2016.04.037 https://doi.org/10.3390/s16111863 https://doi.org/10.3390/s16111863 https://dx.doi.org/10.26655/jmchemsci.2020.4.3 https://doi.org/10.1080/03067319.2019.1655558 https://doi.org/10.1080/03067319.2019.1655558 https://dx.doi.org/10.22034/chemm.2018.63835 https://dx.doi.org/10.26655/jmchemsci.2021.3.2 https://doi.org/10.1039/d0tb00569j https://dx.doi.org/10.22034/jaoc.2021.289344.1023 a. hajializadeh j. electrochem. sci. eng. 12(1) (2022) 185-197 http://dx.doi.org/10.5599/jese.1211 197 [53] a. e. vilian, b. dinesh, m. rethinasabapathy, s. k. hwang, c. s. jin, y. s. huh, y. k. han, journal of materials chemistry a 6 (2018) 14367-14379. https://doi.org/10.1016/j.bios.2016.04.037 [54] s. buratti, b. brunetti, s. mannino, talanta 76 (2008) 454-457. https://doi.org/10.1016/j.ta lanta.2008.03.031 [55] y. xia, h. dai, h. jiang, l. zhang, catalysis communications 11 (2010) 1171-1175. https://doi.org/10.1016/j.catcom.2010.07.005 [56] n. r. radwan, m. s. el-shall, h. m. hassan, applied catalysis a: general 331 (2007) 8-18. https://doi.org/10.1016/j.apcata.2007.07.005 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1211 https://doi.org/10.1016/j.bios.2016.04.037 https://doi.org/10.1016/j.talanta.2008.03.031 https://doi.org/10.1016/j.talanta.2008.03.031 https://creativecommons.org/licenses/by/4.0/) enhanced electrocoagulation: new approaches to improve the electrochemical process doi: 10.5599/jese.2014.0060 285 j. electrochem. sci. eng. 4(4) (2014) 285-296; doi: 10.5599/jese.2014.0060 open access : : issn 1847-9286 www.jese-online.org review enhanced electrocoagulation: new approaches to improve the electrochemical process carlos e. barrera-díaz, gabriela roa-morales, patricia balderas hernández, carmen maría fernandez-marchante* and manuel andrés rodrigo* centro conjunto de investigación en química sustentable uaem – unam, carretera tolucaatlacomulco, km 14.5, unidad el rosedal, c.p. 50200, toluca, estado de méxico, méxico *departartment of chemical engineering. faculty of chemical sciences & technologies, enrique costa building, campus universitario s/n 13071 ciudad real, spain corresponding author: e-mail: cbd0044@yahoo.com received: august 14, 2014; published: december 6, 2014 abstract electrocoagulation is a promising technology for the removal of colloids from different types of wastewater and it has also demonstrated good efficiencies for the breaking-up of emulsions. it consists of the dissolution of aluminum or iron anodes, promoting the formation of coagulant reagents in wastewater that helps to coagulate pollutants and the formation of bubbles that favors the mixing (electroflocculation) and the removal of suspended solids by flotation (electroflotation). during the recent years, the combination of this technology with other treatment technologies has become a hot topic looking for a synergistic improvement in the efficiencies. this work aims to review some of the more recent works regarding this topic, in particular the combination of electrocoagulation with ozonation, adsorption and ultrasound irradiation. keywords electrocoagulation; ozonation; adsorption; ultrasound irradiation; pulse application introduction electrochemical treatment techniques have attracted a great deal of attention because of their versatility and environmental compatibility. electrochemical reactions take place at the anode and the cathode of an electrolytic cell when an external direct current voltage is applied. in fact, the main reagent is the electron, which is a “clean reagent”[1,2] and this fact helps to explain the lower production of wastes associated to these technologies. applications studied in the recent years range from the oxidation of organic pollutants contained in wastewater to the electroremediation of soils. http://www.jese-online.org/ mailto:cbd0044@yahoo.com j. electrochem. sci. eng. 4(4) (2014) 285-296 enhanced electrocoagulation 286 286 electrochemical methods have also been used as coagulation processes to remove color and cloudiness from turbid industrial wastewater. in this application, the electrochemical process generated numerous flocculates, achieving high efficiency in clearing the wastewater[3,4]. the term electrocoagulation involves the in situ generation of coagulants by electrolytic oxidation of an appropriate sacrificial anode (iron or aluminum), which causes the dissolution of electrode plates into the effluent. metal ions, at an appropriate ph, can form wide range of coagulated species and metal hydroxides that destabilize and aggregate particles or precipitate and adsorb the dissolved contaminants. main stages involved in the electrocoagulation process using aluminum anodes have been previously identified [5,6]. the anodic process involves the oxidative dissolution of aluminum into aqueous solution as reaction (1) indicates as well as the oxidative dissociation of water as reaction (2) shows. al → al 3+ + 3e − (1) 2h2o → o2(g) + 4h + + 4e − (2) in the case of iron or steel anodes, it is not iron (iii) but iron (ii) the main product of the electrochemical process (eq.3) [7]. then, oxygen is known to be involved for further fe 2+ oxidation into fe 3+ (eq. 4) fe(s) → fe 2+ + 2e − (3) 4 fe 2+ + 4h + + o2(g) → 4fe 3+ + 2h2o (4) once dissolved iron and aluminum, can participate in many chemical reactions (eqs. 5-10). in fact, speciation of iron and aluminum during electrocoagulation is very complex [8,9] and the description of the interactions between pollutants and the coagulant species is one of the most relevant topics nowadays in this field [10-13]. m(oh)4 + h +  m(oh)3 + h2o (5) m(oh)3 + h +  m(oh)2 + + h2o (6) m(oh)2 + + h +  m(oh) +2 + h2o (7) m(oh) +2 + h +  m +3 + h2o (8) m(oh)3(s)  m 3+ + 3oh (9) it is interesting that in electrocoagulation papers little attention has been paid on cathodic reactions. regardless of whether iron or aluminum is used, the main reaction that is reported is water reduction (eq. 10). 2h2o + 2e − → h2(g) + 2oh − (aq) (10) however, this reaction has three important implications on the electrocoagulation technology: a. provides hydroxyl ions which then react in bulk solution with iron or aluminum cations to form insoluble species and other coagulants (eqs. 5 to 9); b. hydrogen gas is produced increasing turbulence. this process contributes in the destabilization of colloidal particles leading to flocculation (so-called electro-flocculation process), and c. contribution to electroflocculation which is a simple process that floats pollutants (or other substances) by their adhesion onto tiny the bubbles formed by the hydrogen evolution [14] (so-called electroflotation process) c. e. barrera-díaz et al. j. electrochem. sci. eng. 4(4) (2014) 285-296 doi: 10.5599/jese.2014.0060 287 as a consequence of this complex interaction, the electrochemical cell combines several processes at the same time in the same reactor and this becomes a significant advantage of this type of processes as compared with conventional coagulation treatments. in particular, from the economical point of view they compare favorably with coagulation processes [15-17] in many applications. as for coagulation processes, electrocoagulation highly depends on the wastewater ph and it becomes a critical parameter in the performance of this technology. this parameter determines the speciation of aluminum and iron and hence the primary coagulation mechanisms occurring in the electrocoagulation cell. in fact, ph is one of the key differences between coagulation and electrocoagulation as conventional coagulation acidifies the treated wastewater due to the acidic properties of the typical coagulants dosed (iron chloride, aluminum sulfate, etc.), which are known to behave as lewis acid. these properties make necessary the neutralization of wastewater after the coagulation treatment and this process implies an undesired increase in the salinity of the effluent. on the other hand, electrocoagulation typically buffers the ph during the treatment in values within the range 8-9, which should be a proper value even for direct discharge and no further neutralization is required [18]. recent studies shows many promising applications of electrocoagulation in the treatment of lowland surface water [19], water [6,20,21], metal plating wastes [22], other types of industrial wastewater [5,23-30], urban wastewater [31-33] and even in disinfection [34,35]. in fact, it is one of the most promising environmental technologies based on electrochemical engineering [36,37]. electrochemical methods offer two main advantages over traditional chemical treatment: less coagulant ion is required and less sludge is formed [19,22,31]. in the recent years, the potential of this technology is tried to be even further increased by the synergistic combination with other treatment technologies. the objective of the present manuscript is to review the potential of electrocoagulation for the treatment of industrial effluents coupling it with four types of processes:  electrocoagulation-ozone processes  electrocoagulationadsorption processes  electrocoagulation-ultrasound processes  electrocoagulation-pulses processes 2. electrocoagulation-ozone processes ozonation implies the use of ozone in the treatment of wastewater. ozone is a strong oxidant that oxidizes organic pollutants via two pathways: direct oxidation with ozone molecules and/or the generation of free-radical intermediates, such as the •oh radical, which is a powerful, effective, and non-selective oxidizing agent [38]. the ozonation process has the advantage of being able to be applied when the flow rate and/or composition of the effluents are fluctuating. however, the high cost of equipment and maintenance, as well as energy required to supply the process, constitutes some of the disadvantages. moreover, ozonation process requires the transfer of ozone molecules from gas phase to liquid phase, where the attack on the organic molecules occurs. therefore, mass transfer limitations are also a relevant factor to be considered in the oxidation process involving ozone. in many cases, the ozone consumption rate per unit of volume can be so high that mass transfer is the limiting step, reducing the process efficiency and increasing the operating costs [39]. in addition, the ozonation performance is affected by the presence of organic matter, suspended solids, carbonate, bicarbonate and chlorine ions and also j. electrochem. sci. eng. 4(4) (2014) 285-296 enhanced electrocoagulation 288 288 by ph and temperature [40]. some studies using real industrial wastewater have pointed out that ozone by itself does not achieve high levels of pollutant removal [41]. in particular, the oxidation of wastewater from molasses fermentation with ozone results in an effective color removal but is less effective in removing organic matter [42]. similar results were obtained when ozone was used to treat textile wastewater, where ozone treatment proves to be very effective for complete color removal but provides only partial reduction of the chemical oxygen demand (cod) [43]. also, previous research on ozone-coupled methods indicates that the ozonation of anaerobically pretreated wastes enhances significantly the organic removal in comparison to the ozonation of unpretreated wastes, and substrate conversions in the range of 40–67 % are obtained [44]. this behavior in the reduction of cod can be ascribed to the initial ph value of wastewater, where the decomposition of ozone in water to form hydroxyl radicals occurs through the following mechanism [45], where hydroxide ions initiate the reaction (eqs. 11-16): o3 + oh − → o2 + ho2 − (11) o3 + ho2 − → ho2 . + o3 .− (12) ho2 . → h+ + o2 .− (13) o2 .− + o3 → o2 + o3 .− (14) o3 .− + h+ → ho3 . (15) ho3 . → oh. + o2 (16) according to reactions (11) and (12) the initiation of ozone decomposition can be artificially accelerated by increasing the ph value. side reaction (eq. 17) is a fast process and plays an important role in waters with low dissolved organic carbon and alkalinity [46] since it can reduce the oxidative capacity of the system: oh. + o3 → ho2 . + o2 (17) regarding the combined process, table 1 summarizes the main papers found in the literature. typically, the iron provided by the electrochemical reactor is not enough to remove all the pollutants present in aqueous solution. thus, the ozone contributes importantly to improve the pollutant removal. initially, the ozone contribution in the integrated process increases the oxidation of pollutants that are dissolved in the solution and that cannot be eliminated via electrocoagulation. an advantage of supplying ozone into the reactor is that it promotes the mixing between the reactants and also maximizes the organics oxidation, that results in the decreasing of cod and color [28,47,48]. furthermore, the ozone provides good mixing thought the reactor which improves the mass transfer. the ozone action also contributes to reduce the amount of sludge produced. in addition to processes coming from the combination of the effects of the single treatment technologies, the combined process involves an increased hydroxide radical production because fe 2+ catalyzes ozone decomposition to generate hydroxyl radicals (eqs. 18-20) in the well-known fenton process. this process helps to explain the synergistic effect of the combination of both technologies and the resulting high efficiencies. fe 2+ + o3  (feo) 2+ + o2 (18) feo 2+ + h2o  fe 3+ + ho· + oh (19) feo 2+ + fe 2+ +2h +  2fe 3+ + h2o (20) c. e. barrera-díaz et al. j. electrochem. sci. eng. 4(4) (2014) 285-296 doi: 10.5599/jese.2014.0060 289 table 1. pollutant removal using coupled electrocoagulation ozone processes wastewater process conditions poll. removal ref. c.i. reactive yellow 84 ozone flow rate 20 ml min -1 , iron electrodes; current density 15 ma cm -2 85 % toc 100 % color [49] reactive blue 19 ozone flow rate 20 ml min -1 , iron electrodes; current density 10 ma cm -2 80 % toc 96 % color [50] reactive black 5 ozone flow rate 20 ml min -1 , iron electrodes; current density 10 ma cm -2 60 % cod 94 % color [51] distillery effluent ozone flow rate 15 l min −1 ; initial ph 6 iron electrodes; current density 3 adm −2 83 % cod 100 % color [52] industrial wastewater ozone flow rate 23 l min −1 ; initial ph 7 iron electrodes; current density 26 ma cm −2 63 % cod 90 % turbidity [42] red mx-5b ozone flow rate 0.5 l min −1 ; initial ph 6.1 iron electrodes current density of 1.5 ma cm −2 100 % color [53] boat pressure washing wastewater iron and aluminium electrodes current density 17 ma cm −2 88.46 %,toc 76.28 % cod [54] acid orange 6 azo dye ozone concentration 36 mg l −1 ; initial ph 4.5 iron electrodes current density 88.6 ma cm −2 50 % toc 80 % color [55] main challenge for this technology is the scale-up. most of the studies are at the lab-scale or the bench-scale and typically efficiencies can be greatly improved if a proper scale up is carried out. the design of the reactor seems to be a critical point because it fixes the flow patterns and hence the interaction of the species formed by electrocoagulation with ozone. another challenge for this technology is the production of ozone by simultaneous anodic oxidation, taking advantage of the possibilities of electrochemical technology to produce oxidants[56]. a good possibility could be the use of cells equipped with bipolar cells such as the recently proposed by llanos et al. [35] 3. combined electrocoagulationadsorption processes adsorption is a very well-known water and wastewater treatment process, which is gaining prominence as a means of reducing metal ion and organic concentrations in industrial effluxents [57]. the biosorbents derived from dead biomass, are considered the cheapest and most abundant environmentally friendly option [58,59]. nowadays, the development of inexpensive adsorbents for the treatment of wastewater is an important area in the environmental sciences [60,61]. the use of an electrochemical treatment in combination with adsorption as a pre-treatment step to enhance adsorption capability of biosorbents has been assessed in many cases. however, the applications must be carefully evaluated, because technical incompatibilities may arise. this combined technology demonstrates a very good efficiency in the removal of many different pollutants as it is shown in table 2. the filtering capacity of the sorbent bed is an efficient treatment to remove the suspended solids produced by the electrocoagulation process while simultaneously it helps to remove all soluble pollutants that were not effectively trapped by the flocs. most of the studies select aluminum instead of iron as anode because aluminum coagulants promotes neutralization coagulation processes instead of enmeshment into growing precipitates which helps avoiding operational problems in the filtering system. j. electrochem. sci. eng. 4(4) (2014) 285-296 enhanced electrocoagulation 290 290 table 2. pollutant removal using coupled electrocoagulation-adsorption processes wastewater process conditions pollutant removal ref. cr(vi) al electrodes, sorbent red onion skin, ph 3-6 97 % cr [62] cardboard paper mill effluents al electrodes, current density 4.41 ma cm -2 sorbent granular activated carbon, ph 5.3 99 % cod [63] marine blue erionyl mr al electrodes, sorbent granular activated carbon, ph 6.0 100 % dye [64] reactive black 5 current density 277 a m -2 sorbent granular activated carbon, ph 7 100 % dye, 100 % cod, 100% toxicity [65] cr(vi) al-fe, current density 26.7 ma cm -2 sorbent granular activated carbon 99 % cr(vi) [66] indigo carmine al electrodes sorbent granular activated carbon 99 % colorant [67] nakdong river al electrodes, al-fiber filter 65 % toc [20] industrial wastewater al electrodes, current density 45.45 a m −2 sorbent ectodermis of opuntia, ph 8 84 % cod, 78 %, bod5, 97 % color, 98 % turbidity, 99 % fecal coliforms [68] thus, the coupling of electrochemical and adsorption processes might prove a judicious choice for treating industrial wastewater with mixtures of different types of pollutants including both organic and inorganic pollutants. this technology has also been studied in systems in which an adsorbent bed used for the fast removal of pollution from wastewater is continuously regenerated using electrolysis [69,70]. efficiencies obtained are high enough to consider this technology as a promising choice in the treatment of many effluents polluted with organic species. most studied found in the literature are carried out at the lab or bench-scale. as for the combination of electrocoagulation with ozone, it is expected that with a proper scale-up, which develop an efficient cell from the view point of the filtering, adsorption and electrochemical processes, efficiencies obtained would be even higher. 4. combined electrocoagulation-ultrasound processes the treatment of wastewater in an electrolytic cell by ultrasound irradiation is expected to improve significantly the kinetics and the effectiveness of the electrode processes taking place in the cell [71-73]. a number of favorable impacts of using ultrasound in electrocoagulation are the following:  destruction of the compact layer formed at the electrode surfaces by the products of electrode reactions.  decrease in the thickness of the diffuse part of the electrical double layer created at the electrode surface.  direct activation of the ions in the reaction zone at the electrodes by ultra-sound waves.  activation of the electrode surfaces by means of generation of defects in the crystal lattices of the electrodes.  local augmentation of the temperature at the electrode surfaces as a result of friction between the liquid and the surfaces. however, the ultrasound used may cause a few negative effects directly related to the purification process, such as the following:  destruction of a part of the obtained colloidal hydroxides by the action of the acoustic waves. this means a diminution of the solid phase that takes part in the adsorption process and a diminution of the removed contaminations respectively. c. e. barrera-díaz et al. j. electrochem. sci. eng. 4(4) (2014) 285-296 doi: 10.5599/jese.2014.0060 291  destruction of a part of the formed adsorption layer at the surface of the colloidal particles and possible return of the adsorbed ions to the liquid phase.  disorganization of the migration processes in the medium by the ultra-sonic waves. main studies found in the literature regarding this combined technology are shown in table 3. table 3. pollutant removal using coupled electrocoagulationus irradiation processes pollutant process conditions pollutant removal, % ref. cl so4 -2 treatment time 60 min; ultrasonic low-frequency electrocoagulation fe, current density 40 ma cm −2 media cl500 ppm ph 3.8; media so4 -2 500 ppm ph 2.8; mine water very important removals with an increase in the amount of sludge at 25hz. [71] cr(vi) ultrasonic and sludge obtained for electrocoagulation fe of pharmaceutical wastewater ; ec: conditions: rpm = 150, ph = 7.0 and sludge = 10 g l -1 .; sono-ec conditions: frequency 30 khz, ph = 7.0 and time = 100 min. 100 % of removal at 275 min when used 200 mg l -1 of cr(vi); 100 % of removal at 190 min when used 200 mg l -1 of cr(vi) [72] cu(ii) current 1.0 a. electrolysis 8 h in the sonicated field, voltage and temperature were constantly increased, in order to maintain the same thermal conditions for non-sonicated solutions. temperature was adjusted to match those during the sonicated process. electrolysis 100 and 200 mg l -1 removed 55 an 63 % of cu(ii) increasing concentration of cu the removal was of 93 %; sonoelectrolysis 100 and 200 mg l -1 removed 94.6 an 95.5 % of cu(ii) increaseing concentration of cu the removal [73] non-ionic surfactants (sa) current density 0.5–2.5 a dm -2 , treatment time 5–40 min, ultrasonic power density 0.5–3.0 w cm -2 . frequency 22 ± 1 khz. treatment time: 10 min 68 % of as only ce ec with us 90 % [74] car-washing wastewater i=1.2 a ph= 6 treatment time 20 min cod 68.77 % and turbidity 96.27 % [75] main results of these studies show that the combined process promotes the flocculation through vigorous mixing and the oxidation through the formation of radicals that contribute to the enhancement of the efficiency of electrocoagulation processes by chemical polishing of the surface of the flocs and by the oxidation of soluble pollutants in the bulk. this fact helps to explain the high efficiencies reached. in this case, performance of iron electrodes is better than that of aluminum electrodes. this fact can be explained in terms of the enhanced performance of the enmeshment of the pollutant into growing metal hydroxide flocs which is much more important for iron than for aluminum coagulants. hence, sono-electrocoagulation treatment has demonstrated superior performances in treatment of industrial effluents than single electrocoagulation. however, and as it was described for the other two previous technologies, scale-up should be considered as a major challenge. 5. electrocoagulation-pulsed processes pulsed electrocoagulation technology is a novel method for wastewater treatment. it uses the interactions of electrochemical technology and polarity reversal in an electrical field to induce dipole formation in nonpolar particles in the wastewater, thus enabling the formation of microaggregates of insoluble substances. the aggregates formed are further assisted in forming macro-aggregates. charge neutralization of ions or charged materials also takes place in the electrochemical reactor, turning them into insoluble, suspended substances in the wastewater. the neutralization process enhances the efficiency of removing electrical conductivity [76]. in table 4, there are some examples of using j. electrochem. sci. eng. 4(4) (2014) 285-296 enhanced electrocoagulation 292 292 electrocoagulation pulsed treatments, which show that has been done with wastewater of different origins. as it can be observed, it has proven effective in the treatment of urban wastewater and different types of industrial wastes. table 4. pollutant removal using coupled electrocoagulation-pulsed processes wastewater process conditions pollutant removal ref. higher cr(vi) concentrations cr(vi) initial concentrations (501000 mg l -1 ) electrical energy consumption (eec) range: 4-58 kwh m -3 wastewater, current density (cd): 56–222 a m -2 , operating time: 20–110 min, ph 3–9 (phoptimum 5), voltage: 15–25 v. 99 % [47] synthetic solutions containing mercury(ii) hg (ii) 2×10 −5 m,distance between the electrodes was 3 cm, current density ranging from 2.5 to 3.1 a dm −2 ; charge loading 9.3315.55 f m −3 , iron and aluminum electrodes, 3 7. 99.9 %. with iron, 15 min of electrolysis was sufficient to reach the highest removal; aluminum required 25 min for the same result. [77] solutions of a dye dianix yellow cc (dy) and procion yellow (py) range ph (4-8), current density (40-120 a m -2 ) frequency (200-900 hz -1 ), operation time (100 min) 99 % [78] industrial and municipal wastewater pilot plant of electrochemical treatment system (0.3 m 3 h −1 ). ti/ruo2–tio2 anode was larger than with a platinum anode the removal of t-n, t-p, nh4–n and cod was approximately 90 % [79] berberine hydrochloride (bh) wastewater fe electrodes and al electrodes. the optimal conditions of reaction time of 3.5 h, pulse duty cycle of 0.3, pulse frequency of 1.0 khz, current density of 19.44 ma cm -2 , and electrode distance of 2.0 cm 90.1 % bh and 62.6 % cod [80] dye wastewater fe electrodes and 1000 mg l -1 dye solution in a 15 mins electrolyzing time 99.62% of color removal and 91.15% of cod [81] old corrugated containerboard (occ)based paper mill wastewater current density of 0 to 240 a m -1 , a hydraulic retention time of 8 to 16 min and a coagulant (anionic polyacrylamide) dosage of 0 to 30 mg l -1 electrical conductivity: 47.7 %; suspended. solids: 99.3 %;cod: 75 % [76] cooking oil (1800 mg/l, scour (1000 mg/l) and sodium sulfate (1g/l) al electrodes, dimensions of 50×110×2 mm; ac power (smd 30) passivation of al electrodes is not observed [82] electroplating wastewater having a ph of 4, voltage 2.5 v, hydraulic retention time of 15 minutes, current density of 25 a m -2 99.5 % [83] oil wastewater electrode distance of 3.3 cm, ph of 4, current density of 49.38 macm -2 , reaction time of 15 min and pole switching time of 10 s. 96.21% [84] conclusions electrocoagulation has demonstrated to be a promising technology in the removal of pollutants from different types of wastewater. however its combination with other technologies can help to increase efficiency due to synergistic effects such as those derived from the formation of radicals in the ozonation (by interaction of ozone with iron (ii)) or in the us irradiation. results depend on the particular application (technology combined and type of wastewater) and should be evaluated carefully. scale-up is the major challenge of this technology for the next years, although the very positive results obtained at the lab and bench scales make these studies very promising. c. e. barrera-díaz et al. j. electrochem. sci. eng. 4(4) (2014) 285-296 doi: 10.5599/jese.2014.0060 293 acknowledgements: the authors wish to acknowledge the support given by the uaem trough the project 3409/2013 m and financial support from conacyt in project 153828 is greatly appreciated. references [1] g. roa-morales, e. campos-medina, j. aguilera-cotero, b. bilyeu, c. barrera-díaz, separation and purification technology 54 (2007) 124-129. 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[84] y.-f. xiang, z.-m. xie, y. zou, journal of chongqing university, 9(1) (2010) 41-46. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ preparation and characterization selective electrode for determination of copper ion(ii) doi:10.5599/jese.236 237 j. electrochem. sci. eng. 5(4) (2015) 237-246; doi: 10.5599/jese.236 open access : : issn 1847-9286 www.jese-online.org original scientific paper preparation and characterization of selective electrode for determination of copper ion(ii) salwa fares rassi department of chemistry, faculty of science, al-baath university, homs, syria corresponding author: rassi.salwa@gmail.com; tel.: +963-966-243-153 received: october 4, 2015; revised: january 2, 2016; accepted: january 4, 2016 abstract a chemically modified carbon paste electrode with diphenyl carbazide the potentiometric determination of cu(ii) is demonstrated. the electrode exhibits linear response to cu(ii) over a wide concentration range (9.2×10 −7 5.0×10 −1 ) with nernstian slope of 30±0.15 mv per decade. it has a response time of about 40 s and can be used for a period of two months with good reproducibility. the detection limit of this electrode was 7.0×10 −7 m. the proposed electrode shows a very good selectivity for cu(ii) over a wide variety of metal ions. this chemically modified carbon paste electrode was successfully used for the determination of cu(ii) in various water samples solution and pharmaceutical formulation. keywords diphenyl carbazide; copper; potentiometric; method validation introduction a few years ago chemically modified carbon paste electrodes (cmcpes) were applied to analytical determinations. copper is an essential element and is also toxic. copper is one of the important and essential nutrients for human health as well as the growth of animals and plants [1-7]. copper is required for normal metabolic processes. copper combines with certain proteins to produce enzymes that act as catalysts to help a number of body functions. copper helps provide energy required by biochemical reactions. although copper is an essential micronutrient and is required by the body in very small amounts, excess copper in the human body can cause stomach and intestinal distress such as nausea, vomiting, diarrhea, and stomach cramps. the lowest level at which these adverse http://www.jese-online.org/ mailto:rassi.salwa@gmail.com j. electrochem. sci. eng. 5(4) (2015) 237-243 selective electrode for cu(ii) deternination 238 effects occur has not been well defined. copper is also a commonly occurring element in natural waters. several analytical techniques have been monitored for the determination of copper(ii) in different matrices. the common methods for the detection of copper (ii) and other metals include liquid chromatography [8], electrophoresis [9,10], electrochemical [11-13], spectrophotometry [14-18], solid-phase extraction coupled with atomic absorption spectroscopy [19], flame atomic absorption spectrometry [20] atomic emission spectroscopy [21], inductively coupled plasma mass spectrometry [22], and flow injection analysis [23]. copper is an industrially important metal and it is used in coin making, wire making, medicine, alloys, fashioning metal products, transportation industry and thermal conductance. however, even if these methods present high sensitivity and selectivity, they are expensive, time-consuming and cannot be used for in situ analyses. therefore, there is a growing interest in developing electrochemical methods for the detection of copper ions in different matrices. the purpose of the present work is to obtain a better selective modified electrode based on diphenyl carbazide (dphcz) for cu(ii) ion. it can be blend with graphite and paraffin oil to make a very highly reproducible copper selective electrode. in this work we are introducing a very simple and inexpensive (potentiometric) method for determination of copper(ii) in wide concentration range and in presence of variety of metal ions with minimum number of interferences. experimental reagents all reagents used were analytically pure grade and doubly distilled water was used throughout. pure graphite powder (merck), paraffin oil (fluka) were used for the preparation of carbon paste electrode. dphcz (merck), tetrahydrofuran (thf) (fluka). all other solutions used in interference studies were prepared from analytical grade nitrate salts from analytical grade nitrate salts (all from merck company). apparatus potentiometric and ph measurements was carried out using a digital shott gerate ph meter (consort c 830, belgium) with combined glass ph electrode. a water bath shaker (grant instruments, cambridge ltd, england) was used to control the temperature of the test solutions. a saturated calomel electrode (sec) was used as the external reference (mettler, switzerland). the electrochemical system may be represented as follows: carbon paste electrode/test solution/saturated calomel electrode . ft-ir 4100 (fourier transform infrared spectrometer) jasco using kbr disk in the range 4000 400 cm −1 , nernst glower, deuterium try glycine sulfate (dtgs), 0.1 cm −1 . preparation of solutions solutions were prepared from a stock solution of 0.50 m copper(ii), prepared from a sufficient quantity of copper(ii) nitrate in triple distilled deionized water (0.01 µs) and titrated with a standard solution of edta using murexide as indicator. the solution was placed in the dark and protected against light. the working solutions were prepared daily by suitable dilution of stock solution potassium nitrate (1 m) solution was prepared and used as supporting electrolyte, to maintain constant ionic strength. all other solutions used in interference studies were prepared by s. f. rassi j. electrochem. sci. eng. 5(4) (2015) 237-246 doi:10.5599/jese.236 239 dissolving the appropriate amount of the compounds, and the diluted solutions from these were prepared by subsequent dilutions of the stock solutions. preparation of complex (cu -dphcz ) complex (cu-dphcz) was prepared by mixing equal volumes of 10 –2 m solution of cu 2+ with methanolic solution of dphcz with stirring, then we evaporate methanol gradually to obtain a precipitate. ir data (figure 1) of cu -dphcz is shown in table 1. wavenumber, cm -1 figure 1. ir spectrum of dphcz (black light spectrum), cudphcz (black dark spectrum) table 1. ir data for dphcz and cu-dphcz complex wavenumber, cm -1 cu –dphcz dphcz --3363-3270 n-h 1384 1281 c-n 1637 1666 c=o 1500-1600 1500-1600 aromatic preparation of chemically modified carbon paste cmcpe and construction of cu electrode. pure graphite powder (2.375 g) and 0.75 g of cu-dphcz was mixed in a 25 ml beaker and 5–10 ml of thf was added and shaked by sonication for 15–20 min. the mixture was left for a long time to evaporate thf. after complete evaporation of the thf 0.75 g of liquid paraffin was added to the mixture and then mixed well to form a uniform paste. electrode bodies were made from a disposable 1 ml polyethylene syringes the tip of which had been cut off with a razor blade. these bodies were filled with approximately 0.3 ml of cmcpe. smooth surfaces were obtained by applying manual pressure to the piston while holding the electrode surface against a smooth solid support. a fresh electrode surface was obtained by squeezing out a small amount of paste, scrapping off the excess against a conventional paper and polishing the electrode on a smooth paper to obtain a shiny appearance. the electrical connection was made with a copper wire. electrode potential was measured against the sce as the reference electrode. selectivity of sensors potentiometric selectivity coefficient, mpm a b k , of an ion-selective electrode (ise) was commonly used as quantitative expression of the ability of the electrode to respond primarily to the analyze ion in the presence of interfering ions. the effect of the presence of some different species on the t ra n sm it ta n ce , % j. electrochem. sci. eng. 5(4) (2015) 237-243 selective electrode for cu(ii) deternination 240 response of cu 2+ electrode was investigated, and the selectivity coefficient, mpm a b k , of the proposed electrode was calculated in the presence of related organic and inorganic substances using matched potential method (mpm) [24-25]. the selectivity coefficient which was measured by matched potential method was calculated according to the following equation: mpm a a a b b ' a a k a   (1) where a′a is the known activity of primary ion, aa is the fixed activity of primary ion, and ab is the activity of interfering ions. general procedure the performance of the electrode prepared was investigated by measuring emf values of 9.2×10 −7 5×10 −1 m of cu 2+ . the electrode was calibrated by added volumes of 50 mm stock solution of cu 2+ successively in 50 ml of water to generate a total concentration ranging from 9.2×10 −7 5×10 −1 m cu 2+ , followed by immersing the cu 2+ electrode, together with a calomel reference electrode in the solution. the potential reading was recorded after stabilization, and the e.m.f was plotted as a function of the logarithm of the cu 2+ concentration. the concentration graph was used for subsequent determinations of unknown cu 2+ concentrations. potentiometric determination of cu copper was determined potentionmetrically by the direct and standard addition methods [26-27]. in this method the proposed electrode (cu-dphcz) was immersed into a sample of 15 ml with an unknown concentration of a cu 2+ solution, and the equilibrium potential of eu was recorded. then 1 ml of 50 mm of standard cu 2+ was added into the testing solution and the equilibrium potential of es was obtained. from the potential change, ∆e = eu es, we could determine the concentration of the testing sample using the equation: s s x / x s x( ) 10 e s c v c v v v     (2) where cx and vx are concentration and volume of an unknown sample, cs and vs are concentration and volume of the standard, respectively. s is the slope of the calibration graph (slope of the electrode response), and ∆e / mv is the difference between e.m.f. after and before addition of the standard solution. results and discussion optimization of the amount of modifier in the electrode. for this purpose seven electrodes were prepared. the amounts of carbon powder and paraffin oil were constant in each electrode. the proportions of modifier in these seven electrodes were, 5.0, 7.5, 10.5, 12.5, 15.0, 17.5, and 20.0 % of weight percentage of ionophore. the resulting nernstian slopes and correlation coefficients are shown in table 2. these results show that by increasing the percentages of modifier up to 12.5 % the slopes decrease sharply, but within 15-20 % the slopes are becoming more nernstian and slopes of the electrode are only slightly changed. electrodes with less than 15.0 % of modifier show super nernstian slopes, since in these compositions electrodes behave mostly as a pure carbon paste electrode [28]. by using more than 25.0 % of the modifier non linearity in the in the electrode response was observed. since electrode s. f. rassi j. electrochem. sci. eng. 5(4) (2015) 237-246 doi:10.5599/jese.236 241 with 15 % of ionophore has a good slope and the amount of the ionophore was less than the other nernstian electrode this percentage was chosen as the optimum amount for the copper electrode. table 2. the paste compositions and the electrode characteristics of copper(ii) sensor composition, % w/w slope, mv decade1 detection limit, m electrodes ionophore graphite paraffin i 5.0 47.50 47.50 52.15 5.39×10 −5 ii 7.5 46.25 46.25 51.02 8.12×10 −6 iii 10.5 44.75 44.75 46.22 4.03×10 −6 iv 12.5 43.75 43.75 37.91 9.23×10 −7 v 15.0 42.50 42.50 30.15 7.00×10 −7 vi 17.5 41.25 41.25 29.94 6.61×10 −7 vii 20.0 40.00 40.00 29.57 5.35×10 −7 effect of ionic strength on the response of the electrode the effect of ionic strength (0.1–0.5 m kno3) on the calibration curve of copper electrodes was investigated. the electrode response is slightly changed within the 0.1– 0.5 m kno3 electrolyte solution. however, we choose 0.3 m as an optimum value, since in this ionic strength the linear range was wider than for the other concentrations response of electrode to various cations the potential responses of various chemically modified cmes based on dphcz are shown in figure 2. the copper selective electrode exhibited linear response to the logarithm of the activity of cu 2+ within the concentration range of 9.2×10 −7 5×10 −1 m of cu(no3)2 with nernstian slope of 30±0.15 mv per decade and correlation coefficient of 0.996. figure 2. schematic diagram of electrode response to various cations -10 20 50 80 110 140 170 200 230 036 e / m v -log (acu 2+ / m) j. electrochem. sci. eng. 5(4) (2015) 237-243 selective electrode for cu(ii) deternination 242 optimization of ph the behavior of the copper electrode in relation to the variation of ph (1–12) was studied. the composition of the electrode and the ionic strength of the solution were kept constant during all experiments. the results showed that the electrode response is more nernstian at lower phs (≤ 6). since at higher phs (≥ 6) copper will precipitate as copper hydroxide, the electrode has a better characteristic response (ph 4 offered a better slope and a wider linear range than in the other ph values), therefore, we chose ph 4 as an optimum condition for this electrode (figure 3). figure 3. effect of the ph on the response of the electrode calibration graphs using the optimized composition and conditions described above, the potentiometric response of the electrode was studied based on the cu concentration in the range of 5.0×10 −7 5.5×10 −1 m. the calibration curves for the electrodes containing 15 % of ionophore gave an excellent linear response from 9.2×10 −7 5.0×10 −1 m, as shown in fig. 4. the results given in table 3 show the characteristics performance of the electrode. figure 4. calibration graph of cu electrode 20 70 120 170 220 0 5 10 15 e / m v ph y = -30.15x + 248.2 r² = 0.9963 0 50 100 150 200 250 036 e / m v -log (acu 2+ / m) 1×10 -2 m 1×10 -3 m 1×10 -4 m 1×10 -5 m s. f. rassi j. electrochem. sci. eng. 5(4) (2015) 237-246 doi:10.5599/jese.236 243 table 3. response characteristics of electrodes 15 % -ionophore electrode paraffin oil plasticizer parameter 30.15 slope, mv decade -1 0.996 correlation cofficient 9.2×10 −7 -5.0×10 −1 linearity range, m 7.00×10 −7 lower detection limit, m t ≤40 response time, s 2-6 working ph range 25 temperature, °c 61 life time, day response characteristics of modified and unmodified carbon paste electrodes the unmodified electrode shows no response under the optimum condition. the response time of the modified electrode is measured according to iupac recommendation. the response time in variation of concentration from 1×10 −5 to 1×10 −1 m cu +2 is shown in figure 5; the measured response time was 40 s. figure 5. response time of electrode lifetime the electrode lifetime was investigated by performing the calibration curve and the periodic testing of standard solutions (9.0×10 -7 2.5×10 -1 m) and calculating the response slope. it was observed that the electrode exhibits a good stability in terms of slope in the linear domain of concentration and the electrode can be used continuously for about 61 days without considerable decrease in its slope value. selectivity of electrode the influence of some inorganic cations such as of li + , na + , k + , ca 2+ , zn 2+ , ba 2+ , mn 2+ , mg 2+ , ni 2+ , nh 4+ , pb 2+ , co 2+ , fe 3+ , al 3+ and cr +3 on the electrode response was investigated. the selectivity of the electrode was measured by applying the matched potential method (mpm). according to this 30 80 130 180 230 280 0 50 100 150 200 250 300 350 e / m v t / s 1×10 —1 m 1×10 -4 m 1×10 -5 m j. electrochem. sci. eng. 5(4) (2015) 237-243 selective electrode for cu(ii) deternination 244 method, the activity of cu 2+ was increased from aa = 1 mm (reference solution) to a′a = 1.1 mm, and the changes in potential (∆e) corresponding to this increase were measured. then a solution of an interfering ion of concentration ab is added to a new 1 mm reference solution until the same potential change (∆e) was recorded. the selectivity factor, mpm a b k , for each interference was calculated using equation (1). the results are given in the table 4. results revealed reasonable selectivity for cu +2 in presence of many related substances. the selectivity coefficient obtained by this method showed that there were no significant interferences from the cations; this reflected a very high selectivity of the investigated electrode towards cu. table 4. selectivity coefficients for the cu-dphcz responsive electrode interferent k mpm a b interferent k mpm a b na 1+ hg 2+ k 1+ 1.12×10 -7 zn 2+ 1.12×10 -4 ag 1+ pb 2+ 9.74×10 -5 nh4 + cs 2+ 3.37×10 -7 mg 2+ 1.01×10 -7 al 3+ 2.51×10 -7 ca 2+ 1.17×10 -5 cr 3+ 1.85×10 -7 ba 2+ 6.25×10 -7 fe 3+ 3.35×10 -6 ni 2+ 6.30×10 -6 cd 2+ 1.33×10 -4 co 2+ 1.00×10 -5 mn 2+ 3.33×10 -6 electrode renewal and its reproducibility the electrode surface should be renewed when the copper solution is changed from higher to lower concentration to remove residual copper adsorbed on the surface of carbon paste electrode. this process improves reproducibility which was tested by using four similar carbon paste electrodes at the same time to determine 5×10 −4 m cu 2+ . as results (148.1, 147.5, 148.4, 146.9 and 147.5 mv) illustrate all electrodes show similar potential response with a standard deviation of 0.84. to evaluate the reproducibility of emf response of the cell assembly alternating measurements were performed in 5×10 −5 and 5×10 −2 m of copper solution. the procedure was repeated five times and the results are given in (table 5). table 5. alternating measurements in 5×10 -5 m and 1×10 -2 m copper solutions c / m e1 / mv e2 / mv e3 / mv e4 / mv e5 / mv sd, mv rsd, % 5×10 -5 117.9 117.2 117.6 117.5 116.8 0.42 0.36 5×10 -4 148.1 147.5 148.4 146.9 147.5 0.58 0.40 5×10 -3 178.2 177.3 178.3 176.4 176.8 0.84 0.47 5×10 -2 208.4 209.2 210.2 210.9 207.4 1.39 0.67 validity of the proposed method the accuracy and precision of the proposed methods were carried out by five determinations at three different concentrations using both direct and standard addition methods. the precision and accuracy of the method expressed as percentage relative standard deviation as precision, and percentage of deviation of the measured concentration (recovery, %) as accuracy. the results obtained are within the acceptance range. average recovery of 100.80-101.91 %, percentage relative standard deviation, rsd = 1.58-0.44 %. table 6 shows the values of rsd and r for different concentrations of the cu +2 determined from the calibration curves and by using standard addition s. f. rassi j. electrochem. sci. eng. 5(4) (2015) 237-246 doi:10.5599/jese.236 245 methods. the accuracy and precision show that the electrode has a good repeatability and reproducibility. the proposed electrode was found to be selective for the estimation of cu 2+. table 6. accuracy and precision for the determination of cu 2+ using the proposed electrode in pure solution direct mothed standard-addition method taken amount of cu, m ccu / m sd, m rsd, % r, % ccu / m sd, m r, % rsd, % 2.00×10 -6 2.03×10 -6 3.21×10 -8 1.58 101.91 2.03×10 -6 1.92×10 -8 101.90 0.94 2.00×10 -4 2.01×10 -4 2.16×10 -6 1.08 100.52 2.01×10 -4 1.64×10 -6 100.90 0.81 2.00×10 -2 2.01×10 -2 1.36×10 -5 0.68 100.52 2.01×10 -2 8.94×10 -5 100.80 0.44 average of five determinations determination of copper ions in various water samples the proposed cu-electrode was found to work well under laboratory conditions. it was successfully applied to the determination of copper ions in tap water and well water using the direct and standard additions methods. the analysis of samples does not required pretreatment for poteniometric determination using the present electrode. it has been also used for the potentiometric determination of copper(ii) in pharmaceutical preparations (new bonacare, daily vit) by applying two methods. the obtained average recovery, relative standard deviation and confidence limit values are summarized in tables 7 and 8, which reflect the high accuracy and precision of the electrode table 7. determination of copper (ii) in various water samples sample direct standard addition ccu2+ / mol l -1 cl, mol / l a rsd, % ccu2+ / mol l1 cl, mol/l rsd, % tap water 1.10×10 -6 1.10×10 -6 ±1.43×10 -8 1.04 9.23×10 -7 9.23×10 -7 ±3.13 ×10 -8 2.72 well water 2.11×10 -5 2.11×10 -5 ±2.59×10 -7 0.98 1.99×10 -5 1.99×10 -5 ±1.90 ×10 -7 0.76 well water 1.57×10 -6 1.57×10 -6 ±1.90×10 -8 0.96 1.48×10 -6 1.48×10 -6 ±1.24 ×10 -8 0.67 a five independent analyses. cl confidence limit; t = 2, for four degree of freedom and 95 % confidence limit). table 8. determination of copper (ii) in different pharmaceutical formulations. formulation label claim proposed electrode potentiometry direct standard addition detected cu 2+ , % ± sd a rsd, % detected cu 2+ , % ± sd a rsd, % new bonacare b 60 mg / tablet 100.940.83 1.27 99.84 0.10 0.15 daily vit c 2 mg / tablet 99.730.01 0.50 100.820.02 1.02 a five independent analyses; b supplied by aphamia, syria; c supplied by biomedpharma products, syria. conclusions the proposed electrode based on cu-dphcz as the electroactive compound can be used as an interesting alternative analytical tool for the determination of copper(ii) in pure solutions, pharmaceutical preparations and water samples. the electrode showed a nernstian slope of 30.0±0.15 mv decade -1 , a wide concentration range from 9.2×10 -7 to 5.0×10 -1 mol l -1 , a low detection limit of 7.00×10 -7 mol l -1 and a short response time (≤ 40 s) over the ph range 2.0-6.0. the proposed potentiometric method offers the advantages of simplicity, accuracy and applicability to turbid and sample solutions j. electrochem. sci. eng. 5(4) (2015) 237-243 selective electrode for cu(ii) deternination 246 acknowledgements: i gratefully acknowledge the support of this work by the al-baath university research council references [1] a. kumar, m. f. hussain, m. satake, b. k. puri, journal of the chinese chemical society 31(1) (1984) 55-62. 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[27] m. arvand, m. vejdani, m. moghimi, desalination 225 (2008) 176-184. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.e-b-i.net/ebi/contaminants/copper.html http://www.ly-copper.com/e-tyhj.htm http://www.ianr.unl.edu/pubs/water/g1360.htm http://creativecommons.org/licenses/by/4.0/ {experimental study and mathematical modeling of the corrosion inhibition of mild steel with an organic compound in 1 m hcl:} http://dx.doi.org/10.5599/jese.1050 227 j. electrochem. sci. eng. 11(4) (2021) 227-239; http://dx.doi.org/10.5599/jese.1050 open access : : issn 1847-9286 www.jese-online.org original scientific paper experimental study and mathematical modeling of the corrosion inhibition of mild steel with an organic compound in 1 m hcl wafia boukhedena1,3,, and samir deghboudj2,3 1department of science materials, larbi tebessi university, 12002 tebessa, algeria 2department of mechanics, larbi tebessi university, 12002 tebessa, algeria 3mines laboratory, larbi tebessi university, 12002 tebessa, algeria corresponding author: wafia.boukhedena@univ-tebessa.dz; tel.: +213 7 71 64 25 62 received: july 10, 2021; accepted: august 5, 2021; published: august 20, 2021 abstract in this paper, a synthesized organic compound from the family of ketene dithioacetal was studied as corrosion inhibitor for mild steel in 1.0 m hydrochloric acid by gravimetric measurements. the aim of this work is to study the effect of inhibitor concentration and temperature on the corrosion resistance, and to compare the experimental results with those obtained by mathematical models. the structural properties are characterized using the scanning electron microscopy technique. it has been found that the inhibition efficiency increases with increasing inhibitor concentration. the adsorption of studied compound on mild steel surface follows langmuir’s isotherm. taking into account the influence of inhibitor concentration and temperature on the corrosion inhibition efficiency, obtained data were analyzed by two mathematical models based on linear and quadratic regression. the obtained experimental results are in a good agreement with those predicted by the quadratic regression models. keywords ketene dithioacetal; hydrochloric acid; gravimetric measurements; linear regression; quadratic regression. introduction the use of organic inhibitors for the control of corrosion of metals and alloys is of practical importance for many industrial applications, where acid solutions are commonly used in several processes. among these, hydrochloric acid is one of the most widely used for pickling, and chemical and electrochemical etching of some metals and alloys [1]. because of acid aggressiveness, the use of corrosion inhibitors is considered as the most effective method for the protection of many metals and alloys against acid attack [2-7]. inhibitors are also employed to reduce the dissolution rate of metals. http://dx.doi.org/10.5599/jese.1050 http://dx.doi.org/10.5599/jese.1050 http://www.jese-online.org/ mailto:wafia.boukhedena@univ-tebessa.dz j. electrochem. sci. eng. 11(4) (2021) 227-239 corrosion inhibition with an organic compound 228 many authors have reported various types of organic inhibitors used as corrosion inhibitors for steel in hydrochloric acid solution [8-15]. these investigations revealed that organic compounds are acting as inhibitors due to heteroatoms such as nitrogen, sulfur, oxygen and phosphorus, which owing to their free electron pairs are capable to form coordinate covalent bonds with metals. in addition, π electrons in triple or conjugated double bond also exhibit good inhibitive properties [16-18]. the purpose of this work is to investigate the inhibitory action of the organic compound 1,3-dithianes, substituted with two electroactive groups a1 and a2: 3-(1,3-dithian-2-ylidene) pentane-2,4 dione (pddy). the behavior of mild steel in acidic medium in the absence and presence of the inhibitor at different concentrations (5×10-6 10-3 m) and at various temperatures (20-60 oc) was explored. the chemical structure of pddy is shown in figure 1. the assessment of corrosion behavior is carried out using weight loss measurements and scanning electron micrograph (sem) imaging at 20 °c. in the second part of this work, two mathematical models, based on linear and quadratic regression, are suggested to investigate the effect of concentration and temperature upon inhibition efficiency of pddy. c s s c a1 a2 figure 1. chemical structure of 3-(1, 3-dithian-2-ylidene) pentane-2,4 dione compound (pddy) experimental preparation of samples the composition of mild steel used in this study is: c 0.09 wt.%; si 0.05 wt.%; mn 0.13 wt.%; s 0.24 wt.%; p 0.24 wt.% and fe balance. the mild steel specimens were cut into 1 × 1 × 1 cm pieces for the mass measurements. the specimens were abraded with a series of sic papers (grades 320, 400, 500, 800, 1000, 1200 and 2000), washed with distilled water, degreased with acetone, and dried with a cold air stream at room temperature before use in the experiments. electrolytic solution the aqueous electrolyte solution (1 m hcl) was prepared by dilution of analytical reagent grade, 37 % w/w hcl (merck) with bi-distilled water. the measurements were carried out in 1 m hcl in the absence and presence of the inhibitor within the concentration range 5×10-6 to 1×10-3 m. preparation of inhibitor the inhibitor 3-(1,3-dithian-2-ylidene) pentane-2,4-dione was synthesized using k2co3 (21 g, 0.15 mol) and active methylene compound, 5.2 ml (0.05 mol) in 25 ml of dmf. the mixture was stirred magnetically. 4.5 ml (0.075 mol) of carbon disulfide was then added in just one time at room temperature. the stirring was maintained for 10 min before dropwise addition of reagent dielectrophile, 7.2 ml (0.06 mol) 1,3-dibromopropane, during 20 min. after seven hours of stirring at room temperature, 250 ml of ice water was added to the reaction mixture. the formed precipitate was filtered, dried and then purified by recrystallization from ethanol and used directly in the experiments in the concentration range (5×10-6, 10-5, 5×10-5, 10-4, 5×10-4 and 10-3 ) mol l-1 [19]. the studied compound was recovered in the form of orange crystals, exhibiting the following w. boukhedena and s. deghboudj j. electrochem. sci. eng. 11(4) (2021) 227-239 http://dx.doi.org/10.5599/jese.1050 229 characteristics: molar mass, m = 216 g/mol, yield 83 %, m.p. =104 °c. ft-ir (ft-ir spectra of pddy in its solid state, 1/ : 1630 cm-1 (c=o), 1725 cm-1 (h3c-c=o), 1173-1234 cm-1 (c-s-c), 1415 cm-1 (c=c). 1h nmr ((cdcl3), 250 mhz)):  = 2.32 ppm (s, 6h, 2ch3),  = 2.25 ppm (m, 2h, ch2),  = 2.95 ppm (t, 4h, 2ch2). the mass spectroscopy analysis revealed that the inhibitor produced fragment ions m/z 217. scanning electron microscopy (sem) the morphological structures of mild steel surfaces before and after total immersion for 3 h in a corrosive solution (1 m hcl) with and without addition of 10−3 m of pddy at 20 °c, were determined by the scanning electron microscope, model jeol jsm-6360 lv. gravimetric measurements gravimetric experiments were performed according to the standard methods [20-22]. experiments were conducted under total immersion of mild steel specimens in the stagnant and aerated conditions, using 250 ml capacity beakers. after being weighed accurately with high sensitivity balance, the specimens were immersed in 100 ml of 1 m hcl with and without various concentrations (5×10-6 10-3 m) of pddy at various temperatures (20, 30, 40, 50 and 60 °c) in aerated conditions. after 3 hours of immersion, the specimens were taken out, rinsed thoroughly with distilled water, dried and weighed accurately again. the average of three replicates was used to further process the data. the average weight loss ∆w was calculated using the equation (1): ∆w= w1 – w2 (1) where, w1 and w2 are respectively the average weight of specimens before and after immersion. the corrosion rate (cr) in mg cm-2 h-1, the surface coverage () and inhibition efficiency (iew), obtained from gravimetric experiments were computed using the equations (2)-(4) [20,23,24]: cr w st  = (2) 0 i 0 cr -cr cr  = (3) 0 i 0 cr -cr 100iew cr = (4) where ∆w is the average weight loss, s is the total surface area of the specimen and t is the immersion time. cr0 and cri are corrosion rates in the absence and presence of various concentrations of pddy, respectively. corrosion rates and inhibitor efficiencies were evaluated and computed at different operating conditions. results and discussion mass loss measurements corrosion of mild steel in 1 m hcl in the absence and presence of various concentrations (5×10-6, 10-5, 5×10-5, 10-4, 5×10-4 and 10-3 m) of pddy was studied by weight loss experiments at various temperatures (20, 30, 40, 50 and 60 °c). the corrosion rate (cr) in mg cm-2 h-1 and values of inhibition efficiency, obtained by 30 test runs of weight loss measurement after 3 hours of immersion in 1 m hcl solution, are summarized in table 1. it was found that addition of pddy inhibits corrosion of mild steel at all concentrations used in this study. as presented in table 1, it is clear http://dx.doi.org/10.5599/jese.1050 j. electrochem. sci. eng. 11(4) (2021) 227-239 corrosion inhibition with an organic compound 230 that corrosion rate increased with temperature and decreased with inhibitor concentration. many similar results were obtained by other researchers, who already found that corrosion rate and inhibition efficiency depend on the concentration of the inhibitor and temperature of the medium [10,11]. table 1. corrosion parameters obtained from weight loss measurements of mild steel after 3 h of immersion in 1 m hcl solution in the absence and presence of different concentrations of pddy at various temperatures c / m cr, mg cm-2 h-1 iew, % t / °c 20 30 40 50 60 20 30 40 50 60 blank 0.897 1.983 4.751 9.774 13.65 ----- 5×10-6 0.361 0.807 2.419 6.271 9.510 59.755 59.304 49.084 35.840 30.330 1×10-5 0.277 0.681 2.087 5.016 8.947 69.119 65.658 56.072 48.680 34.454 5×10-5 0.194 0.552 1.649 4.354 8.161 78.372 72.163 65.291 55.453 40.212 1×10-4 0.166 0.393 1.337 3.755 6.860 81.494 80.181 71.858 61.582 49.744 5×10-4 0.074 0.336 0.925 2.598 4.944 91.750 83.056 80.530 73.419 63.780 1×10-3 0.062 0.187 0.776 1.943 4.165 93.088 90.570 83.667 80.121 69.487 inhibition efficiencies are for different concentrations of ppdy at different temperatures presented in figure 2, where it is clearly seen that iew reached the maximum value of 93.09 % for 10-3 m ppdy at 20 °c. it is well known that adsorption of inhibitor at steel surface is primarily responsible for the reduction of metal dissolution process in corrosive media [23]. the adsorption is enhanced by the presence of hetero atoms with lone pairs of electrons of the molecule inhibitors that facilitate the electrostatic adsorption on the steel surface by forming stable insoluble films. data in table 1 show that without inhibitor, the corrosion rate is as high as 0.897 (mg cm-2 h-1), while in the presence of 10-3 m of inhibitor, cr value is reduced to 0.062 (mg cm-2 h-1). from these measurements, we can conclude that pddy is an effective inhibitor for mild steel in 1 m hcl solution. 20 30 40 50 60 0 10 20 30 40 50 60 70 80 90 100 110 t / °c cpddy/ 10 4 m 10 m 5 m 1 m 0.5 m 0.1 m 0.05 m ie w , % figure 2. variation of inhibition efficiency with temperature after 3 hours of mild steel exposure in 1m hcl and various concentrations of pddy adsorption isotherm corrosion inhibition process is characterized by adsorption of the inhibitor on the metal surface. adsorption of inhibitor is a displacement reaction in which the adsorbed water molecule is removed from the metal surface according to [13,14]: w. boukhedena and s. deghboudj j. electrochem. sci. eng. 11(4) (2021) 227-239 http://dx.doi.org/10.5599/jese.1050 231 org(sol) + nh2o(ads) ↔ org(ads) + nh2o(sol) where n is the number of water molecules replaced by one organic molecule and h2o(ads) is the water molecule on the metal surface. org(sol) and org(ads) are organic molecules in the aqueous solution and adsorbed on the metal surface, respectively. for the effective adsorption of an inhibitor on the metal surface, the interaction force between metal and inhibitor must be greater than the interaction force of metal and water molecule [15]. therefore, it is of great importance to find the appropriate adsorption isotherm that fits the experimental results. the experimental data have been tested with several adsorption isotherms including langmuir, temkin, frumkin and freundlich [16,17]. in this work, langmuir adsorption isotherm was found as the best description of adsorption behavior of the studied compound. an expression of the langmuir isotherm is given by [24,25]: ads 1c c k = + (5) where c is inhibitor concentration, kads is equilibrium adsorption constant and  is fractional surface coverage. according to eq. (5), the plot of (c / ) versus c should yield straight line with nearly unit slope. best results for langmuir adsorption isotherm for pddy on mild steel surface are presented in figure 3. it is clear from figure 3, however, that slopes of the straight lines are slightly greater than unity (cf. slope data referred in figure 3). therefore, it could be concluded that each pddy unit occupies more than one adsorption site on the mild steel surface. 0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016 c  -1 / m o l l -1 c / mol l-1 20 °c r²= 0.9998; slope= 1.07 30 °c r²= 0.9983; slope= 1.11 40 °c r²= 0.9996; slope= 1.19 50 °c r²= 0.9981; slope= 1.24 60 °c r²= 0.9977; slope= 1.42 figure 3. langmuir adsorption plots for mild steel in 1 m hcl containing different concentrations of pddy at various temperatures to account for this phenomenon, a modified langmuir adsorption isotherm could be applied, which is given by [7,19,20]: ads c n nc k = + (6) in eq. (6), n represents the slope of the respective line. the values of kads obtained from the values of slopes at different temperatures (figure 3) were computed and gathered in the second column of table 2. the increased values of kads for pddy reflect the increasing adsorption capability due to structural formation on the metal surface [26,27]. kads is related to the standard gibbs free energy of adsorption (∆g°ads) by the following equation δgads0 = rt ln (55.5 kads) (7) c  -1 / m o l l1 http://dx.doi.org/10.5599/jese.1050 j. electrochem. sci. eng. 11(4) (2021) 227-239 corrosion inhibition with an organic compound 232 where r is the universal gas constant (8.134 j k-1 mol-1), t is temperature and 55.5 is molar concentration of water in the solution. the thermodynamic parameters derived from langmuir isotherms are listed in table 2. the values of ∆g°ads for pddy are all negative, what indicates the stability and spontaneity of adsorption of the inhibitor on the metal surface. the effectiveness of corrosion inhibition increases with increasing of negative ∆g°ads values. adsorption enthalpy and entropy ∆h°ads and ∆s°ads are determined graphically from the following equation δg°ads = δh°ads δs°ads (8) table 2. thermodynamic parameters of adsorption of pddy on mild steel in 1 m hcl at various temperatures it is well known that values of ∆g°ads less negative than -20 kj / mol are associated with the physical adsorption, characterized by an electrostatic interaction between the charged molecule and the charged metal. ∆g°ads values around -40 kj / mol or higher, are associated with the chemical adsorption where the sharing or transfer of organic molecules charge with the metal surface occurs [20-24]. the decrease observed for kads (table 2), and also for iew (figure 3) with increasing temperature, suggests that the pddy molecules are physically adsorbed on the metal surface which favors their desorption processes. the value of ∆h°ads provides further information about the mechanism of corrosion inhibition. the negative value of ∆h°ads indicates that adsorption process is exothermic [3,7,23]. an exothermic adsorption process signifies chemical, physical or a mixture of both [24-27], whereas the endothermic process is attributed to chemisorption [28,29]. results obtained from thermodynamic calculations for adsorption are in good agreement with the values of inhibition efficiency obtained from the weight loss. thermodynamic calculations for corrosion reaction the stability of a corrosion inhibitor in an aggressive medium at some required operating temperature is very important for its practical applications. in this paper, the corrosion of mild steel in 1 m hcl was studied in the temperature range of 20-60 °c, in the absence and presence of different concentrations of pddy, after 3 h of immersion time. temperature dependence of corrosion rate (cr) is described by arrhenius equation: a cr e rtae − = (9) where a is the pre-exponential factor, ea is activation energy for metal dissolution (corrosion) reaction, t is absolute temperature, and r is universal gas constant. the dependence of logarithm of the corrosion rate (ln cr) on the reciprocal of the absolute temperature (1 / t) for mild steel in 1 m hcl is presented figure 4. the corrosion rate of mild steel in acidic solution increases with rise of the temperature, regardless to the presence of inhibitor in the corrosive solution. in the case of uninhibited solution, the increase in corrosion rate was significant compared to inhibited cases involving a decrease in inhibition efficiency with increasing temperature up to 60 °c. the decrease in inhibition efficiency reveals that the film formed on the t / °c kads / 10-4 l mol-1 ∆g°ads / kj mol-1 ∆h°ads / kj mol-1 ∆s°ads / kj mol-1 k-1 20 0.1374 -38.604 -24.36 0.049 30 8.935 -38.837 40 9.580 -40.300 50 5.424 -40.060 60 3.968 -40.434 w. boukhedena and s. deghboudj j. electrochem. sci. eng. 11(4) (2021) 227-239 http://dx.doi.org/10.5599/jese.1050 233 metal surface is less protective at higher temperatures, since the desorption rate of the inhibitor is greater at higher temperatures [30]. thermodynamic parameters of the corrosion reaction, namely activation energy ea, entropy (∆sa) and enthalpy (∆ha) were calculated using arrhenius equation (9) and transition state theory equation [19]: a a cr e s h r rt rt a e nh  − = (10) in eq. (10), n is avogadro's number (6.022×1023 mol-1), and h is planck's constant (6.63×10-34 j s). eq. (10) is plotted in figure 5 as ln (cr / t) against (1 / t). 0.0030 0.0031 0.0032 0.0033 0.0034 -3 -2 -1 0 1 2 3 cpddy / 10 4 m 0 m 10 m 5 m 1 m 0.5 m 0.1 m 0.05 m ln ( c r / m g c m -2 h -1 ) (1 / t) / k-1 figure 4. plots for mild steel corrosion rates in 1 m hcl in absence and presence of different concentrations of pddy 0.0030 0.0031 0.0032 0.0033 0.0034 -9.0 -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 (1 / t) / k-1 ln ( (c r / t ) / m g c m -2 h -1 k -1 ) cpddy / 10 4 m 0 m 10 m 5 m 1 m 0.5 m 0.1 m 0.05 m figure 5. transition state plots for mild steel corrosion rates in 1 m hcl in absence and presence of different concentrations of pddy the values of ea and a, obtained from the slopes (-ea / r) and intercepts (ln a) of the lines in figure 4, are displayed in table 3, together with values of δha and δsa deduced from the slopes (-δha / r) and intercepts ln (r / nh) +δsa / r of the lines in figure 5. table 3. thermodynamic activation parameters of mild steel dissolution in 1 m hcl with and without various concentrations of pddy c / 104 m ea / kj mol-1 δha / kj mol-1 δsa / j mol-1 k-1 ln a blank 57.334 54.739 -58.37 23.483 0.05 69.877 67.283 -23.7 27.655 0.10 72.741 70.147 -16.0 28.584 0.50 77.604 75.009 -2.11 30.248 1.00 78.797 76.202 -0.0025 30.504 5.00 85.139 82.545 16.93 32.541 10.00 87.480 84.885 22.09 33.161 examination of table 3 shows that the activation energy values in the presence of pddy ranged from 69.877 to 87.480 kj mol−1, revealing that the activation energy for metal dissolution increased in the presence of inhibitor. also, ea values in the presence of inhibitor are all higher than in absence of inhibitor, which indicates physical adsorption (electrostatic interaction). it has already been reported in the literature that inhibitors for which the activation energy in the inhibited solution is greater than that of the blank solution, ea(inh) > ea, are adsorbed on the substrate by electrostatic http://dx.doi.org/10.5599/jese.1050 j. electrochem. sci. eng. 11(4) (2021) 227-239 corrosion inhibition with an organic compound 234 bonds (physisorption). at the other side, inhibitors where ea(inh) < ea, adsorb on the metal surface through strong bonds (chemisorption) [31,32]. the activation energy rises with increasing inhibitor concentration, suggesting strong adsorption of inhibitor molecules at the metal surface [33-37]. the values of δha and ea are nearly the same and are higher in the presence of the inhibitor, indicating that the energy barrier of the corrosion reaction increased in the presence of inhibitor without changing the mechanism of dissolution [19]. the negative sign of the activation entropy values either in absence, or presence of the inhibitor may be explained by activated molecules in a higher order state than that at the initial stage [38,39]. at higher inhibitor concentrations (5×10-4 and 10−3 m), positive values of the entropy of activation δsa were observed in the media. this indicates that the system passes from a more ordered state to a more random arrangement [40]. morphological characterization the scanning electron micrographs (sem) of the mild steel surface in the absence and presence of 10-3 m of pddy are presented in figure 6. comparison of figures 6a and 6b shows that the surface of the sample is heavily damaged and severely corroded after 3 hours of immersion in 1 m hcl. damages appear uniform with some lines resulting from polishing made before the testing. in the presence of pddy, however, figure 6c shows that the external morphology appears softer, indicating a protected surface. these images suggest that protection comes from the formation of pddy layer on the mild steel surface that prevents the attack of acids. a b c figure 6. sem micrographs of mild steel surface: before corrosion (a); after immersion in 1 m hcl solution for 3 h at 20 °c (b); after immersion in 1 m hcl containing 10-3 m of pddy for 3 h at 20 °c (c) mathematical modeling mathematical regression analysis can be employed as a powerful tool for data representation [19]. the least squares method is the standard statistical approach in the regression analysis, used to find w. boukhedena and s. deghboudj j. electrochem. sci. eng. 11(4) (2021) 227-239 http://dx.doi.org/10.5599/jese.1050 235 the optimum fit for a set of data points by minimizing the sum of the squares of the residuals of points from the plotted curve. in this study, two mathematical models were applied to illustrate the inhibition efficiency of pddy for mild steel in 1.0 m hcl. the proposed mathematical models are classified into two groups, linear and quadratic models. experimental data obtained in this work and presented above were used to construct the models that describe the relationship between the inhibition efficiency iew as a response, and temperature t and inhibitor concentration c as factors. both models take into account the individual effect of each variable and the interaction between them. linear model a linear mathematical model was used as a statistical tool aiming to predict the effect of the variables xi and yi which are respectively the temperature t and the inhibitor concentration to the function zi which is the inhibitor efficiency iew. the proposed model is given by the equation (11): z = a1x + a2y + a3xy + a4 +  (11) where  is the error term. the optimal model parameters ai within the meaning of the least-squares method are these that minimize the quantity: ( ) n 2 1 2 3 4 1 2 3 4 i i=1 , , , ( )s a a a a a x a y a xy a z= + + + + − (12) in this expression, n is the number of experiments, ai are constants representing the model parameters. the minimum of this expression is found when the partial derivatives (∂s/∂ai) are equal to zero: i 0 s a  =  i= 1, 2, 3, 4 (13) this leads to a system of equations. parameters of the model were estimated based on the least square method. the data were analyzed using computer program matlab that performs these calculations. since x = t, y = c and z= iew, the final model equation is: iew = -0.865 t + 17324.609 c + 312.357 tc + 92.125 (14) based on the proposed linear model, the predicted values of inhibition efficiency as a function of temperature and inhibitor concentration are computed and listed in the first part of table 4. quadratic model the second proposed quadratic model equation was obtained by representing the inhibition efficiency iew by the response function z which can be expressed by the following equation: z = a1x² + a2y² + a3x + a4y + a5xy + a6+  (15) where,  is the error term, x the temperature, °c and y the inhibitor concentration (mol l-1). similarly, the optimal model parameters ai within the meaning of the least-squares method are these which minimize the quantity: ( ) 52 25 6 6 n 2 1 2 3 4 1 2 3 4 i i=1 , ,, , , ( )a a ys a a a a a x a y a x a a xy a z+= + + + + + − (16) the minimum of this expression is found using the eq. (13). using matlab software, parameters a1-a6 of the model were estimated and the final model equation is defined as: http://dx.doi.org/10.5599/jese.1050 j. electrochem. sci. eng. 11(4) (2021) 227-239 corrosion inhibition with an organic compound 236 iew = -0.01143 t² 5.19673×107 c² + 0.13615 t + 80173.4333 c + 0.01667 tc + 69.6171 (17) as for the linear model, the predicted values of inhibition efficiency were computed using the quadratic regression model and gathered in the second part of table 4. for analysis of the accuracy of the predicted results obtained with linear and quadratic regression models, we estimated the coefficient of determination known as r-squared (or r2), using the equation below ( ) ( ) n 2 i mi i=1 n 2 i i=1 ² 1 z z r z z − = − −   (18) in this relationship zi is experimental inhibition efficiency, zmi predicted inhibition efficiency and z̅ the average. the determination coefficient was found equal to 0.802 for the linear model and 0.902 for quadratic regression model, respectively. in comparison with experimentally determined iew given in table 1, data in table 4 show similar iew values at all temperatures and concentrations of pddy. figures 7 and 8 show the relationship between the values of the inhibitor efficiency obtained from the experimental work and those predicted by linear and quadratic regression models. it is obvious that almost 80 and 90 % of the data obtained from the linear and quadratic regression models are located on the line of equality, which means that the experimental and predicted values of inhibitory efficacy are close. table 4. inhibition efficiencies values computed by linear and quadratic regression models for different inhibitor concentrations and temperatures c / 104 m iew, % linear regression quadratic regression t / °c 20 30 40 50 60 20 30 40 50 60 0.05 74.944 66.310 57.676 49.042 40.408 68.168 63.814 57.185 48.249 37.038 0.01 75.062 66.443 57.825 49.207 40.588 68.565 64.211 57.572 48.646 37.435 0.50 76.005 67.511 59.018 50.524 42.031 71.647 67.293 60.654 51.728 40.517 1.00 77.183 68.846 60.509 52.171 43.834 75.266 70.912 64.273 55.347 44.136 5.00 86.612 79.524 72.436 65.348 58.261 94.863 90.510 83.870 74.945 63.734 10.00 98.398 92.872 87.346 81.820 76.294 95.975 91.621 84.982 76.057 64.845 30 40 50 60 70 80 90 100 40 50 60 70 80 90 100 r²= 0.8294 p re d ic te d e i w , % experimental eiw, % figure 7. fitting curves of predicted against experimental inhibition efficiency obtained by the linear regression model 30 40 50 60 70 80 90 100 30 40 50 60 70 80 90 100 r²= 0.90259 p re d ic te d ie w , % experimental ie w , % figure 8. fitting curves of predicted against experimental inhibition efficiency obtained by the quadratic regression model w. boukhedena and s. deghboudj j. electrochem. sci. eng. 11(4) (2021) 227-239 http://dx.doi.org/10.5599/jese.1050 237 this is quite true because the determination coefficients were r²= 0.829 for linear model and r² = 0.920 for quadratic regression model. according to [19], when r2 is < 0.30 the relationship is weak, for r2 = 0.50 and 0.70, the relationship is important, while for r2 > 0.90, the relationship is powerful. based on correlation coefficients, the present results indicate a strong relationship between experimental and mathematical data, indicating the concordance between the experimental and the predicted results. as shown in figure 9, examination of data provided by linear and quadratic regression models and presented as surface plots, reveals that the addition of pddy at different concentrations decreases the corrosion rate of mild steel. the inhibition efficiency increases with increasing inhibitor concentration (red zone) and decreases with increasing temperature (blue zone). this can be explained by a regression of adsorption induced by temperature rise. at concentration 10-3 m and temperature 20 °c, pddy exhibits maximum inhibition efficiency for both models: 98.398 % (linear model) and 95.975 % (quadratic regression). a comparison with the experimentally found value of inhibition efficiency of 93.088 % shows more reliability of the quadratic regression model than the linear model. a b figure 9. surface plot for inhibition efficiency, temperature and inhibitor concentration based on: a) linear regression model; b) quadratic regression model conclusion in this paper, the inhibition effect and adsorption behavior of the organic compound 3-(1,3-dithian-2-ylidene) pentane-2,4 dione (pddy) on mild steel in 1 m hcl medium were examined, using the weight loss method, morphological characterization and mathematical modeling. the following conclusions are drawn: • the inhibition efficiency of pddy increases with increasing inhibitor concentration in the range of 5×10-6 to 10-3 m, and reaches the maximum value of 93.088 % in the presence of 10-3 m of pddy. the thermodynamic study showed that adsorption of this inhibitor on the mild steel surface is spontaneous and follows the langmuir adsorption isotherm model. the negative value of the gibbs free energy of adsorption (δgads) is indicative for a strong interaction between inhibitor molecules and the surface of mild steel. • the evolution of the corrosion rate of mild steel in the corrosive solution alone (1 m hcl) shows a regular and rapid growth, confirming an increasing metallic dissolution with increasing temperature. the inhibitory efficiency of pddy decreases, while corrosion rate increases with temperature in the range 20 to 60 oc for all inhibitor concentrations used. this behavior illustrates physisorption of pddy molecules on mild steel surface. http://dx.doi.org/10.5599/jese.1050 j. electrochem. sci. eng. 11(4) (2021) 227-239 corrosion inhibition with an organic compound 238 • formation of a protective layer on mild steel surface by the inhibitor observed by sem, confirmed high performance of the inhibitive effect of pddy. • linear and quadratic mathematical regression models are found suitable to represent the experimental data with high correlation coefficients. experimentally determined inhibition efficiency of 93.088 % is found closer to the result predicted by the quadratic regression model. the predicted result also confirm that inhibition efficiency is influenced by temperature, inhibitor concentration and their combined effect. acknowledgements: the authors like to thank the algerian general direction of research (dgrsdt) for their support. references [1] l. afia, r. salghi, a. zarrouk, h. zarrok, e. bazzi, b. hammouti, m. zougagh, transactions of the indian institute of metals 66(1) (2013) 43-49. http://doi.org/10.1007/s12666-012-0168-z [2] s. issaadi, t. douadi, a. zouaoui, s. chafaa, m. a. khan, g. bouet, corrosion science 53(4) (2011) 1484-1488. http://doi.org/10.1016/j.corsci.2011.01.022 [3] h. jafari, i. danaee, h. eskandari, m. rashvandavei, industrial & engineering chemistry research 52(20) (2013) 6617-6632. http://doi.org/10.1021/ie400066x [4] o. ghasemi, i. danaee, g. r. rashed, m. rashvandavei, m. h. maddahy, journal of materials engineering and performance 22(4) (2013) 1054-1063. http://doi.org/10.1007/s11665-0120348-3 [5] z. moallem, i. danaee, h. eskandari, transactions of the indian institute of metals 67(6) (2014) 817-825. http://doi.org/10.1007/s12666-014-0403-x [6] a. döner, r. solmaz, m. özcan, g. kardaş, corrosion science 53(9) (2011) 2902-2913. http://doi.org/10.1016/j.corsci.2011.05.027 [7] a. fiala, w. boukhedena, s. lemallem, h. b. ladouani, h. allal, journal of bioand tribocorrosion 5(2) (2019) 42. http://doi.org/10.1007/s40735-019-0237-5 [8] d. daoud, t. douadi, h. hamani, s. chafaa, m. al-noaimi, corrosion science 94 (2015) 21-37. http://doi.org/10.1016/j.corsci.2015.01.025 [9] e. e. abd el aal, s. abd el wanees, a. farouk, s. m. abd el haleem, corrosion science 68 (2013) 14-24. http://doi.org/10.1016/j.corsci.2012.03.021 [10] h. gerengi, i. uygur, m. solomon, m. yildiz, h. goksu, sustainable chemistry and pharmacy 4 (2016) 57-66. http://doi.org/10.1016/j.sc2016.10.003 [11] m. mobin, i. ahmad, m. basik, m. murmu, p. banerjee, sustainable chemistry and pharmacy 18 (2020) 100337. https://doi.org/10.1016/j.scp.2016.10.003 [12] a. k. singh, s. k. shukla, m. singh, m. a. quraishi, materials chemistry and physics 129(1-2) (2011) 68-76. http://doi.org/10.1016/j.matchemphys.2011.03.054 [13] s. cheng, s. chen, t. liu, x. chang, y. yin, materials letters 61(14-15) (2007) 3276-3280. http://doi.org/10.1016/j.matlet.2006.11.102 [14] m. hazwan hussin, m. jain kassim, materials chemistry and physics 125(3) (2011) 461-468. http://doi.org/10.1016/j.matchemphys.2010.10.032 [15] e. ghali, v. s. sastri, m. elboujdaini, corrosion prevention and protection: practical solutions, john wiley & sons, chichester, england, 2007. p. 579. http://doi.org/10.1002/9780470024546 [16] s. manimegalai, p. manjula, journal of materials and environmental science 6(6) (2015) 16291637. 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https://www.sciencedirect.com/science/article/abs/pii/s0010938x09002856#! https://www.sciencedirect.com/science/article/abs/pii/s0010938x09002856#! https://www.sciencedirect.com/science/article/abs/pii/s0010938x09002856#! https://www.sciencedirect.com/science/article/abs/pii/s0010938x09002856#! http://doi.org/10.1016/j.corsci.2009.06.024 https://creativecommons.org/licenses/by/4.0/) {organic and inorganic compounds as corrosion inhibitors to reduce galvanic effect for the hybrid structure aa2024-cfpr:} http://dx.doi.org/10.5599/jese.1126 343 j. electrochem. sci. eng. 12(2) (2022) 343-358; http://dx.doi.org/10.5599/jese.1126 open access : : issn 1847-9286 www.jese-online.org original scientific paper organic and inorganic compounds as corrosion inhibitors to reduce galvanic effect for the hybrid structure aa2024-cfpr roy lopez-sesenes1,, jose gonzalo gonzalez-rodriguez2, josé gerardo vera-dimas1, rene guardian-tapia2 and luis cisneros-villalobos1 1universidad autónoma del estado de morelos, facultad de ciencias químicas e ingeniería, av. universidad 1001 col. chamilpa,cp. 62209, cuernavaca morelos, méxico 2universidad autónoma del estado de morelos, ciicap, av. universidad 1001 col. chamilpa,cp. 62209, cuernavaca morelos, méxico corresponding author: rlopez@uaem.mx received: september 29, 2021; accepted: october 30, 2021; published: november 13, 2021 abstract the effect of the galvanic corrosion process taking place between aluminium alloy (aa2024 -t3) and carbon fiber reinforced plastic (cfrp) immersed in 0.05 m nacl was studied using organic and inorganic compounds as corrosion inhibitors. electrochemical approaches such as electrochemical noise analysis (ena) and electrochemical impedance spectroscopy (eis) were carried out to evaluate efficiencies of 1,2,4-triazole (c2h3n3) and cerium nitrate hexahydrate (ce(no3)3·6h2o) as corrosion inhibitors. the highest efficiency was reached for ce(no3)3 6h2o, with some improvement observed by adding c2h3n3 in a mixed inhibitor solution. the noise resistance (rn) and polarization resistance (rp) values calculated from ena and eis data showed almost identical behavior with different magnitudes but similar trends. adsorption isotherm models estimated with fractional surface coverage () parameter were fitted better to langmuir model for c2h3n3 and temkin model for ce(no3)3·6h2o. the calculated values of gibbs free energy suggested physisorption and chemisorption as spontaneous interactions between a metal surface and both inhibitors. energy-dispersive x-ray spectroscopy (eds) was carried out before and after immersing aa2024-t3 in the electrolyte, identifying rich zones in copper with cerium deposited over it and confirming the presence of rare-earth oxide deposition and oxide film products. the eds analysis for cfrp revealed the deposition of ce and al particles over its surface after immersion in the electrolyte, especially in the areas rich in carbon. keywords electrochemical methods, rare earths, adsorption isotherm, gibbs free energy, synergistic effect, power spectral density http://dx.doi.org/10.5599/jese.1126 http://dx.doi.org/10.5599/jese.1126 http://www.jese-online.org/ mailto:rlopez@uaem.mx j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 344 introduction hybrid structures have received great interest from the aerospace industry, especially in cases where a single material can not satisfy structural demands [1]. materials with a high strength-todensity ratio, such as carbon fiber reinforced plastic (cfrp) and aluminum alloys, are promising candidates for modern aircraft structures since when both are combined, the mechanical properties of a material are improved [2]. nevertheless, aluminum alloys such as aa2024-t3 in contact with other nobler materials (cfrp) tend to accelerate their electrochemically-driven degradation since they are more active in the galvanic series, showing an anodic behavior [3, 4]. for example, when cfrp is coupled to aa2024-t3 in nacl solution, the structural integrity of this last material is compromised since cfrp has an open circuit potential (ocp) around +0.28 mv (nobler) [5] than aa2024-t3 having ocp around -500 mv (more active) [6]. furthermore, aa2024-t3 contains impurities that stimulate the formation of galvanic microcells between the matrix and its intermetallic phases, thus increasing its susceptibility to the degradation processes [7]. a lot of efforts have been focused on analyzing mechanical properties of the hybrid structure aa2024-t3-cfrp, such as tensile and compressive properties, shear strength and damping behavior, and on establishing conditions under which this hybrid material could work without risk of damage during its operating life [8-10]. based on the latter, payan et al. did some experiments to find an adequate method for the analysis of the corrosion mechanism of aluminum matrix composite reinforced with graphite fibers, giving quantitative information on the morphology and kinetics of corrosion [11]. sherif et al. investigated different aluminum-graphite composites, showing the graphite concentration effect in the matrix, and observing an increment in the corrosion rate with reduction of the polarization resistance [12]. it is certainly important to find a way to reduce the effect of the corrosion process that is taking place in these hybrid materials, with the aim to reduce the damage caused by the oxidetion/reduction process. an effective way to resolve it is the utilization of corrosion inhibitors. so far, only a few studies have focused on using inhibitors or their combination for corrosion protection of aa2024-cfrp hybrid structure. wang et al. have studied the galvanic corrosion resistance of carbon fiber metal laminates (carall) with aa2024-t3 and cfrp and proposed a surface treatment technique combining anodizing in sulfuric acid to prevent galvanic corrosion [13]. these authors also showed that the corrosion rate decreased in the presence of the coating over the surface. the goal of the present work was to reduce the effect of the galvanic corrosion process present in the hybrid structure aa2024-t3-cfrp with corrosion inhibitors. a systematic study of organic and inorganic inhibitors was conducted using a set of complementary electrochemical analyses. experimental materials and chemicals the nominal composition of aa2024-t3 alloy is listed in table 1. several samples were cut to platelets with an exposed area of 2 cm2 with 0.2 cm thickness. platelets were ground with emery cloth of 300, 600 and 1000 grit size, degreased with ethanol, washed with distilled water, and dried with dry air. the carbon fiber composite was obtained from a fully cured carbon fiber composite sheet of 0.2 cm thickness. samples with a nominal area of 2 cm2 were cut. the carbon fiber composite specimen was prepared to expose one individual region, removing impurities with 320 grade emery paper to improve its conductivity. r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 345 table 1. composition of aa2024-t3 aluminum alloy content, wt.% cu cr fe mg mn si ti zn other al 3.8-4.9 0.1 0.5 1.2-1.8 0.3-0.9 0.5 0.15 0.25 0.15 balance distilled water with a resistivity of 18 mω cm was used for rinsing and solution preparation. the test solution was naturally aerated at 25 °c. all electrochemical tests were done in 0.05 m nacl with stirring at room temperature using a gill ac computer-controlled potentiostat. high-grade reagents ce(no3)3·6h2o and c2h3n3 received from sigma-aldrich with chemical structures shown in figure 1 were used as corrosion inhibitors at three concentrations (0.5, 2 and 10 mm) with the aim to observe their protective efficiency against the corrosion process at the metal surface. figure 1. chemical structures of compounds used as corrosion inhibitors: a) c2h3n3 and b) ce(no3)3·6h2o electrochemical techniques electrochemical noise analysis (ena) ena technique was carried out recording 1024 points per second each hour for 24 h. data were recorded simultaneously using a silver-silver chloride (ag/agcl) reference electrode (re) and aa204t3 and cfrp in the galvanic couple as working electrode one (we1) and working electrode two (we2), respectively. the exposed area of each we was 0.5 cm2. the galvanic couple was closed by a switch before each test, connected via a zero-resistance ammeter (zra). electrochemical impedance spectroscopy (eis) eis measurements were conducted in a conventional cell of three electrodes, in the following arrangement: ag/agcl electrode was used as re, a platinum wire as an auxiliary electrode (ae), and aa2024-t3 and cfrp in galvanic couple with an exposed area of 0.5 cm2 as working electrodes (we1 or we2). eis was performed at the open circuit potential value with a sinusoidal perturbation of 10 mv rms (root mean square) amplitude at room temperature, in a frequency range from 30 khz to 0.01 hz. the electrochemical cell used for the experiment setup is shown in figure 2. figure 2. electrochemical corrosion cell for ena and eis measurements http://dx.doi.org/10.5599/jese.1126 j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 346 surface characterization of corroded surfaces micrographs of corroded aa2024-t3 and cfrp samples in solutions of 0.05 m nacl with and without inhibitor, extracted after 24 h of immersion, were examined with a tescan vega3 sb scanning electronic microscope (sem) with an edx analyzer. results and discussion electrochemical noise analysis (ena) inhibition effect of 1,2,4-tr. figure 3 and 4 shows the current and potential electrochemical noise time series recorded for aa2024-t3-cfrp galvanic couple in 0.05 m nacl without and with different concentrations (0.5 to 10 mm) of c2h3n3. the electrochemical current noise (ecn) and the electrochemical potential noise (epn) plots measured after 1 h (figure 3) showed a decrement in their fluctuations when the inhibitor was added into the solution. the single exception is seen for the concentration of 0.5 mm, which showed an increment in the current fluctuation from 10-5 to 510-5 ma cm-2, increasing its frequency domain. the latter is attributed to the formation of a protective film on intermetallic sites, especially where there is copper contained in the sample, forming cu-c2h3n3. moreover, copper reacts with cl ions present in the solution to form cuclcomplexes with c2h3n3 [14], minimizing the oxygen reduction processes at intermetallic particles. figure 4 shows the current and potential time series recorded after 24 h, where it is evident that the test without inhibitor showed an increment in current fluctuations. for the test with 2 mm c2h3n3 and 10 mm c2h3n3, a diminishing in current fluctuations is appreciable. no change is evident for the test at 0.5 mm c2h3n3. 0 200 400 600 800 1000 -6.0x10 -5 -3.0x10 -5 0.0 3.0x10 -5 6.0x10 -5 9.0x10 -5 0 200 400 600 800 1000 -0.18 -0.12 -0.06 0.00 0.06 0.12 0.18 0 200 400 600 800 1000 -8.0x10 -4 -6.0x10 -4 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 8.0x10 -4 0 200 400 600 800 1000 -3 -2 -1 0 1 2 3 4 0 200 400 600 800 1000 -8.0x10 -4 -6.0x10 -4 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 8.0x10 -4 0 200 400 600 800 1000 -0.18 -0.12 -0.06 0.00 0.06 0.12 0.18 0 200 400 600 800 1000 -8.0x10 -4 -6.0x10 -4 -4.0x10 -4 -2.0x10 -4 0.0 2.0x10 -4 4.0x10 -4 6.0x10 -4 8.0x10 -4 0 200 400 600 800 1000 -0.18 -0.12 -0.06 0.00 0.06 0.12 0.18 j / m a c m -2 0 mm e / v 0 mm j / m a c m -2 0.5 mm e / v 0.5 mm j / m a c m -2 2 mm e / v 2mm j / m a c m -2 10 mm time,stime,s e / v 10 mm figure 3. time series of ecn (left) and epn (right) for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl without and with 0.5-10 mm c2h3n3 at the beginning of the test (1 h) r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 347 aa2024-t3 is highly susceptible to pitting corrosion due to the existing intermetallic particles [15]. when cfrp is kept in contact with aa2024-t3, these species cause increased electrons flow between cathodic and anodic areas. through the electrochemical noise analysis done, the noise resistance (rn) was calculated by the ratio of standard deviations of the current and potential (i/p) measured after 24 h and shown in figure 5. the highest value of rn of 8.27 kω cm2 was obtained at 10 mm of c2h3n3 which remains almost constant until the end of the test. it is obvious that with addition and further increment of the inhibitor concentration in the solution, a decrement in the current fluctuation of the time series was recorded at the beginning of the test as shown in figure 3, and kept almost constant until the end of the test (figure 4). these suggest the almost constant corrosion resistance of aa2024t3-cfpr in a galvanic couple (figure 5). 0 200 400 600 800 1000 -1.2x10 -4 -6.0x10 -5 0.0 6.0x10 -5 1.2x10 -4 1.8x10 -4 0 200 400 600 800 1000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0 200 400 600 800 1000 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 0 200 400 600 800 1000 -4 -3 -2 -1 0 1 2 3 4 5 0 200 400 600 800 1000 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 0 200 400 600 800 1000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0 200 400 600 800 1000 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 0 200 400 600 800 1000 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 j / m a c m -2 0 mm e / v 0 mm j / m a c m -2 0.5 mm e / v 5 mm j / m a c m -2 2 mm e / v 2 mm j / m a c m -2 time, s 10 mm e / v time, s 10 mm figure 4. time series of ecn (left) and epn (right) for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl without and with 0.5-10 mm c2h3n3 at the end of the test (24 h) 0 5 10 15 20 25 10 3 10 4 r n /  cm 2 time, h 0 mm 0.5 mm 2 mm 10 mm figure 5. time behavior of rn for aa2024t3-cfrp in galvanic couple immersed in 0.05 m nacl at different concentrations of c2h3n3 http://dx.doi.org/10.5599/jese.1126 j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 348 the single exception in figure 5 is observed at 0.5 mm c2h3n3, where rn tends to decrease due to the accelerated degradation of the surface, which forms a passive film susceptible to metastable pitting. the power spectral density (psd) plots represent the potential and current fluctuations of ena over time as functions of frequency, allowing to determine energy changes in the system and the stability of a passive film formed over the metal surface. when an increment in current is observed, the mass transport increases too, and when the potential increases, corrosion over the metal surface increases also. psd plots for aa2024-t3-cfpr with and without c2h3n3 are shown in figure 6. psd plots for the current at the beginning and the end of the test (figure 6a and 6c) did not show significant changes. at both testing times, the current density increased with the addition of 0.5 mm c2h3n3 into the solution, which accelerates the mass transport phenomena from the bulk to the metallic surface and increases the exposed area due to the formation of porosities at the surface [16]. moreover, psd plots for the potential (figure 6b and 6d) showed increments at 0.5 mm c2h3n3, indicating rising corrosion over the metallic surface. for all the remainder concentrations, all is kept constant until the end of the test. 10 -3 10 -2 10 -1 10 -17 10 -15 10 -13 10 -11 10 -9 10 -3 10 -2 10 -1 10 -11 10 -9 10 -7 10 -5 10 -3 10 -3 10 -2 10 -1 10 -17 10 -15 10 -13 10 -11 10 -9 10 -3 10 -2 10 -1 10 -11 10 -9 10 -7 10 -5 10 -3 p s d j / m a c m -2 0 mm 0.5 mm 2 mm 10 mm a) p s d e / m v 0 mm 0.5 mm 2 mm 10 mm b) p s d j / m a c m -2 f / hz 0 mm 0.5 mm 2 mm 10 mm c) p s d e / m v f / hz 0 mm 0.5 mm 2 mm 10 mm d) figure 6. psd plots of current (left) and potential (right) for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl without and with 0.5-10 mm c2h3n3 at the beginning (a and b) and the end (c and d) of the test inhibition effect of cerium nitrate time series of current and potential noise fluctuations for aa2024-t3-cfrp immersed in 0.05 m nacl with and without ce(no3)3·6h2o as corrosion inhibitor are shown in figure 7. at the beginning of the test (1 h), the addition of ce(no3)3·6h2o generated an increment in the current and potential fluctuations (figure 7), accelerating the corrosion process at the metal surface, and forming a passive layer of corrosion products. at the end of the measurement (24 h), this layer promotes a magnitude decrement of current fluctuations from 10-4 to 10-6 ma/cm2, which was lower than the test without inhibitor (figure 8). r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 349 0 200 400 600 800 1000 -1.0x10 -4 -5.0x10 -5 0.0 5.0x10 -5 1.0x10 -4 0 200 400 600 800 1000 -0.15 -0.10 -0.05 0.00 0.05 0.10 0.15 0 200 400 600 800 1000 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 0 200 400 600 800 1000 -8 -6 -4 -2 0 2 4 6 0 200 400 600 800 1000 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 0 200 400 600 800 1000 -8 -6 -4 -2 0 2 4 6 0 200 400 600 800 1000 -1.0x10 -3 -5.0x10 -4 0.0 5.0x10 -4 1.0x10 -3 0 200 400 600 800 1000 -8 -6 -4 -2 0 2 4 6 j / m a c m -2 0 mm e / v 0 mm j / m a c m -2 0.5 mm e / v 0.5 mm j / m a c m -2 2 mm e / v 2 mm j / m a c m -2 time, s 10 mm e / v time, s 10 mm figure 7. time series of ecn (left) and epn (right) for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl without and with 0.5-10 mm ce(no3)3·6h2o at the beginning of the test (1 h) the potential time series showed a decrement in intensity fluctuations from the beginning to the end of the test and also sudden potential drops and recovery transients with high amplitude and high frequency, typical of localized corrosion [17]. in the presence of the inhibitor, rn values presented in figure 9 showed a clear trend of increase with time of exposure and significant increment with the addition of ce(no3)3·6h2o into the solution. the highest value of rn was obtained for 0.5 mm ce(no3)3·6h2o, reaching 29.9×104 ω cm2, which is at least one order of magnitude higher than without inhibitor (2.77 kω cm2). during the first hour, psd for the current density showed an increment in the current density due to the mass transport phenomena (figure 10. a), while psd for the potential showed an increment correlated with the corrosion magnitude (figure 10. 10b). at the end of the test, psd values for the current density and potential (figure 10. 10c and 10d) dropped slightly, suggesting improvement of the corrosion resistance of aa2024-t3-cfrp due to the formation of a passive film with high susceptibility to pitting corrosion. based on the fluctuations observed in figures 3, 4, 7 and 8 for the time series for the current and potential, it was possible to determine that mixed corrosion (uniform type of corrosion combined with localized corrosion) is present over the metal surface of aa2024. the psd for both inhibitors at the http://dx.doi.org/10.5599/jese.1126 j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 350 beginning of the test had an increment in the current density and potential, the first one is due to a rise in the mass transport and the second one suggests an increment in the corrosion process. 0 200 400 600 800 1000 -2.0x10 -4 -1.5x10 -4 -1.0x10 -4 -5.0x10 -5 0.0 5.0x10 -5 1.0x10 -4 1.5x10 -4 2.0x10 -4 0 200 400 600 800 1000 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0 200 400 600 800 1000 -4.0x10 -5 -2.0x10 -5 0.0 2.0x10 -5 4.0x10 -5 0 200 400 600 800 1000 -1.0 -0.5 0.0 0.5 1.0 0 200 400 600 800 1000 -4.0x10 -5 -2.0x10 -5 0.0 2.0x10 -5 4.0x10 -5 0 200 400 600 800 1000 -1.0 -0.5 0.0 0.5 1.0 0 200 400 600 800 1000 -4.0x10 -5 -2.0x10 -5 0.0 2.0x10 -5 4.0x10 -5 0 200 400 600 800 1000 -1.0 -0.5 0.0 0.5 1.0 j / m a c m -2 0 mm e / v 0 mm j / m a c m -2 0.5 mm e / v 0.5 mm j / m a c m -2 2 mm e / v 2 mm j / m a c m -2 time, s 10 mm e / v time, s 10 mm figure 8. time series of ecn (left) and epn (right) for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl without and with 0.5-10 mm ce(no3)3··6h2o at the end of the test (24 h) 0 5 10 15 20 25 10 3 10 4 r n /  c m 2 time, h 0 mm 0.5 mm 2 mm 10 mm figure 9. time behavior of rn for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl with different concentrations of ce(no3)3·6h2o r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 351 10 -3 10 -2 10 -1 10 -17 10 -15 10 -13 10 -11 10 -9 10 -3 10 -2 10 -1 10 -11 10 -9 10 -7 10 -5 10 -3 10 -3 10 -2 10 -1 10 -17 10 -15 10 -13 10 -11 10 -9 10 -3 10 -2 10 -1 10 -11 10 -9 10 -7 10 -5 10 -3 p s d j / m a c m -2 0 mm 0.5 mm 2 mm 10 mm a) p s d e / m v 0 mm 0.5 mm 2 mm 10 mm b) p s d j / m a c m -2 f / hz 0 mm 0.5 mm 2 mm 10 mm c) p s d e / m v f / hz 0 mm 0.5 mm 2 mm 10 mm d) figure 10. psd of current (left) and potential (right) for aa2024-cfrp in galvanic couple immersed in 0.05 m nacl without and with 0.5-10 mm ce(no3)3·6h2o at the beginning (a and b) and the end (c and d) of test eis measurements figure 11. shows nyquist and bode's plots recorded for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl with and without c2h3n3. nyquist plots in figure 11a showed the formation of a depressed semicircle at high to middle frequencies for all tests, which is usually attributed to the charge transfer phenomena. in addition, from middle to low frequencies, a second semicircle was recorded for each test, except at 0.5 mm c2h3n3, which can be associated with the corrosion process at the metal surface. for solution without inhibitor and 0.5 mm c2h3n3, an inductive response is indicated at the lowest frequencies, usually related to intermediate adsorption/desorption and/or pitting corrosion. 0 1 2 3 0 1 2 3 0 0.5mm 2mm 10mm -z im g  k  c m 2 zreal /k cm 2 a) 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 2 10 3 0 0.5mm 2mm 10mm f / hz im p e d a n ce ,  c m 2 b) 20 0 -20 -40 -60 -80 p h a se a n g le , ° figure 11. nyquist (a) and bode (b) plots for aa2024-t3-cfrp galvanic couple immersed for 24 h in 0.05 m nacl without and with different concentrations of c2h3n3 http://dx.doi.org/10.5599/jese.1126 j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 352 the impedance modulus values of bode plots shown in figure 11b increased gradually when c2h3n3 was added into the solution, reaching the highest value at 10 mm c2h3n3. each phase angle plot of the galvanic couple presented in figure 11b exhibits one well-defined time constant about 2 hz, and one barely seen at about 1000 hz. the high-frequency time constant can be associated with the presence of a native oxide layer over the metallic surface [18], while this at about 2 hz to the intermediate layer of corrosion products, including a thin layer of inhibitor molecules adsorbed over the surface. at frequencies lower than 0.1 hz, another time constant is present. phenomena at the lowest frequencies are usually ascribed to the corrosion process at the substrate. figure 12. shows eis spectra recorded for aa2024-t3-cfrp in galvanic couple exposed to 0.05 m nacl without and with different concentrations of ce(no3)3·6h2o. it can be noticed that when ce(no3)3.6h2o was added into the solution, an increment in the impedance of the system occurs, particularly in the range of low frequencies, reaching its maximum value at 2mm ce(no3)3·6h2o. in the phase angle spectra, it is possible to observe the formation of two-time constants at middle and low frequencies (figure 12b). as was previously explained, the first-time constant could be related to the formation of a passive layer of corrosion products over the metal surface, including a thin layer of inhibitor over it. the low frequency related constant phase angle and clear inductive response at 0.5 mm of ce(no3)3·6h2o could be attributed to the corrosion process, which evidently changes with the concentration of inhibitor used for each test. several studies have already been done to establish the mechanism of corrosion in the presence of ce(no3)3·6h2o as an inhibitor. in this way, it has been found that ce(no3)3·6h2o acts as a cathodic inhibitor, blocking the occupied zones by intermetallic particles [19], preferentially in areas rich in copper, which has more cathodic potential with respect to the matrix [20]. 0 1 2 3 4 0 1 2 3 4 0 0.5mm 2mm 10mm -z im g  k  c m 2 zreal  k cm 2 a) 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 2 10 3 0 0.5mm 2mm 10mm f / hz im p e d a n ce ,  c m 2 b) 20 0 -20 -40 -60 -80 p h a se a n g le , ° figure 12. nyquist (a) and bode (b) plots for aa2024-t3-cfrp galvanic couple immersed for 24 h in 0.05 m nacl without and with different concentrations of ce(no3)3·6h2o a mix of the optimal concentrations of c2h3n3 (10 mm) and ce(no3)3.6h2o (2 mm) was carried out to observe the effect of both inhibitors in a possible synergistic combination. the corresponding eis spectra are presented in figure 13. a significant increment of the semicircle diameter (figure 13a) and impedance magnitude (figure 13b) can be observed, which were both higher than for two inhibitors looking separately. this improvement of protection ability can be attributed to the fact that ce(no3)3·6h2o is added preferentially to intermetallic sites (cathodic zones) through a hydroxide film formed over them, whereas c2h3n3 is adsorbed over the aluminum matrix due to its polar groups, combined with double bonds in its structure, offering more stability to the film adsorbed over the metal surface. based on the latter, it seems that each inhibitor enhanced the r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 353 other's inhibition efficiency, reducing microgalvanic effects that promote localized attacks around the matrix, delaying the cathodic and anodic reactions. 0 2 4 6 8 10 0 2 4 6 8 10 0 10mm_c 2 h 3 n 3 + 2mm (ceno 3 ) 3 .6h 2 0 ) -z im g / k  c m 2 zreal / k cm 2 a) 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 0 10 1 10 2 10 3 10 4 10 5 0 10mm_c 2 h 3 n 3 + 2mm (ceno 3 ) 3 .6h 2 0 ) f / hz im p e d a n ce ,  c m 2 b) 0 -20 -40 -60 -80 p h a s e a n g le , ° figure 13. nyquist (a) and bode (b) plots for aa2024-t3-cfrp galvanic couple immersed for 24 h in 0.05 m nacl without and with 10 mm of c2h3n3 + 2mm ce(no3)3·6h2o electrical equivalent circuit to simulate eis data measured for aa2024-t3-cfrp in galvanic couple and evaluate polarization resistance (rp) values, two electrical equivalent circuits were used for modeling (figure 14). rs is ascribed to the uncompensated solution resistance, while cpedl and rct are the constant phase element of double layer and charge transfer resistance, respectively. cpeinh+oxy is the constant phase element related to the film formed by corrosion products and adsorbed inhibitor, whereas rinh+oxy is ascribed to the corresponding surface film resistance. a) b) figure 14. electrical equivalent circuits used to fit eis data for aa2024-t3-cfrp in galvanic couple immersed in 0.05 m nacl in a) presence and b) absence of inhibitor table 2 summarizes rp and ie values in solutions with and without c2h3n3, ce(no3)3·6h2o and their optimal concentrations mixture. all impedance parameter values, including rct and rinh+oxy, were obtained by fitting an electrical equivalent circuit in figure 14 to measured impedance spectra (figures 11-13). rp was calculated as the sum of rct + rinh+oxy [21]. table 2. eis-based polarization resistance (rp) and inhibition efficiency of c2h3n3, ce(no3)3·6h2o and their optimal concentrations mixture for aa2024-cfrp in galvanic couple immersed in 0.05 m nacl. inhibitor c2h3n3 ce(no3)3·6h2o c2h3n3+ce(no3)3·6h2o cinh / mm rp / ω cm2 ie, % rp / ω cm2 ie, % rp / ω cm2 ie, % 0 9.90 102 -9.90102 -9.90 102 - 0.5 1.12103 11 2.54103 61 -- 2 1.97103 49 5.28103 81 8.36103 88 10 5.05103 80 3.39103 70 the inhibition efficiency of the inhibitor was calculated using eq (2), http://dx.doi.org/10.5599/jese.1126 j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 354 p/inh p/o p/inh ie= 100 r r r − (1) where rp/inh and rp/0 are polarization resistance (rp) with and without inhibitor in the electrolyte. the repeatability and reproducibility of rp values at optimal concentrations for both inhibitors were determined by the standard deviation obtained after repeating the tests three times for each inhibitor. at 10 mm c2h3n3, rp was calculated as 5.05 kω cm2 with the standard deviation of ±47 ω cm2, while for 2 mm ce(no3)3·6h2o, rp value of 5.28 kω cm2 was estimated with the standard deviation of ±699 ω cm2. figures 15 and 16 compare rn and rp values obtained from ena and eis measurements, presenting them in dependence on concentrations of two inhibitors, showing seemingly close behavior between two resistances. 0 2 4 6 8 10 10 2 10 3 10 4 10 5 r /  c m 2 c inh / mm r n r p figure 15. rn and rp values in dependence on the concentration c2h3n3 inhibitor 0 2 4 6 8 10 10 2 10 3 10 4 10 5 r /  c m 2 c inh / mm r n r p figure 16. rn and rp values in dependence on the concentration of ce(no3)3·6h2o inhibitor characterization of inhibitor adsorption rn and rp values can be used to estimate the gibbs free energy (δg°ads), which is an indicator of the type of molecular interaction between the inhibitor and metal surface through the establishment adsorption isotherm model. for this, it is necessary to estimate the fractional surface coverage (𝜃) values, which would give an idea about the surface covered by an inhibitor. the values of 𝜃 were estimated by equation (2), and the results are listed in table 3. p,n/inh p,n/0 p,n/inh r r r  − = (2) table 3. fractional surface coverage (𝜃) for different concentrations of two corrosion inhibitors at aa2024t3-cfpr in a galvanic couple, calculated by rn and rp values obtained by en and eis analysis cinh / mm 𝜃 c2h3n3 ce (no3)3.6h2o calculated by rn rp rn rp 0.5 0.37 0.12 0.84 0.61 2 0.17 0.50 0.72 0.81 10 0.65 0.80 0.65 0.71 langmuir and temkin isotherms are defined by the equations (3) and (4),   = − inh 1 kc (3) r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 355  = + inh log log k g c (4) where k is equilibrium adsorption constant, 𝑔 is molecular interaction parameter, and cinh is the concentration of the inhibitor. by determining k value, gibbs free energy of adsorption can be calculated by equation (5): goads = -rt ln(55.5k) (5) where r is gas constant, t is the temperature in kelvin grades, and 55.5 is the molar concentration of water in solution. the interactions between the surface of aa2024-t3-cfrp in galvanic couple with either c2h3n3 or ce(no3)3·6h2o as corrosion inhibitors are best described by langmuir isotherm in the first case and temkin isotherm in the second case. the best-fitted results using models described by eqns. (3) and (4) are shown in figures 17 and 18, respectively. the langmuir model (figure 17) assumes that there are sites with the capability to physically or chemically hold one molecule over the surface. all sites are equivalent and there are no interactions between molecules. the temkin model (figure 18) establishes a heterogeneous surface divided into zones, some of them without molecular interactions, establishing an attraction or repulsion of the inhibitor over the metal surface by zones. the values of δg°ads were calculated using rn and rp values reported in table 3. applying the eq. (5), values of δg°ads of c2h3n3 adsorption were calculated as -35 kj/mol based on rn, and -37 kj/mol based on rp. for ce(no3)3·6h2o, δg°ads of -26.78 kj/mol based on rn, and -26.89 kj/mol based on rp were calculated. these δg°ads values suggest physisorption and chemisorption of both inhibitors over the metal surface, since values below -20 kj/mol are indicative of physisorption, while values higher than -40 kj/mol suggest chemisorption. the values between -20 and -40 kj/mol as obtained here, are indicative of a combination of both physisorption and chemisorption. in addition, negative values of δg°ads are indicative of spontaneous adsorption. the estimated values of gibbs free energy establish the exothermic adsorption process, in agreement with langmuir and temkin isotherms for c2h3n3 and ce(no3)3·6h2o, respectively. negative values of δg°ads values suggest that chemisorption and physisorption proceed at the metallic surface as spontaneous processes. 0 2 4 6 8 10 0 1 2 3 4 5 langmuir r p r 2 = 0.99   ( 1 - ) c inh / mm langmuir r n r 2 = 0.90 0 2 4 6 8 10 -1.2 -0.8 -0.4 0.0 0.4 c inh / mm temkin r p r 2 = 0.94 lo g (  c in h -1 / m m -1 ) temkin r n r 2 = 0.90 figure 17. langmuir isotherm for adsorption of c2h3n3 inhibitor figure 18. temkin isotherm for adsorption of ce(no3)3·6h2o inhibitor surface analysis the scanning electron microscopy (sem) allowed obtaining images of aa2024-t3 and cfrp surfaces. figure 19 shows the surface of aa2024-t3 before and after its exposure for 24 h in the aggressive media containing ce(no3)3·6h2o. eds analysis was also carried out before and after http://dx.doi.org/10.5599/jese.1126 j. electrochem. sci. eng. 12(2) (2022) 343-358 corrosion inhibitors for aa2024-cfpr 356 immersion of aa2024-t3 into 0.05 m nacl (figure 19). corrosion products can be identified in places with cu particles, as demonstrated in previous research [23]. ce(no3)3·6h2o was preferentially deposited over cathodic regions. eds analysis confirms the presence of rare-earth oxide deposition, as well as the formation of oxide film products. figure 19. sem micrographs and eds analysis for aluminum alloy aa2024-t3 before a) and after b) exposure for 24 h in 0.05 m nacl with ce(no3)3·6h2o as a corrosion inhibitor figure 20 shows sem micrographs taken for cfrp surface before and after it was immersed in the electrolyte with ce(no3)3·6h2o as a corrosion inhibitor. the eds analysis revealed the deposition of ce(no3)3·6h2o and aluminum particles on its surface after it is immersed in the electrolyte, especially in the areas rich in carbon. figure 20. sem micrographs and eds analysis for cfrp before a) and after b) exposition for 24 h to 0.05 m nacl with ce(no3)3·6h2o as a corrosion inhibitor r. lopez-sesenes j. electrochem. sci. eng. 12(2) (2022) 343-358 http://dx.doi.org/10.5599/jese.1126 357 conclusion the inhibition effect against the galvanic corrosion effect between the aluminum alloy 2024-t3 and cfrp immersed in 0.05 m nacl was evaluated at different concentrations (0.5, 2, 10 mm) of inorganic (ce(no3)3·6h2o) and organic (c2h3n3) compounds. the polarization resistance (rp) measured using eis for aa2024-cfrp galvanic couple increased slightly in the presence of c2h3n3, reaching its maximum efficiency at 10 mm with 80 % of inhibition efficiency. ce(no3)3·6h2o showed the highest polarization resistance at a lower concentration of 2 mm, with 81 % inhibition efficiency. for a mix of optimal concentrations of c2h3n3 and ce(no3)3·6h2o, rp of 8.36 kω cm2 ± 243 ω cm2 was measured, suggesting a synergistic effect of these two inhibitors in enhancing corrosion protection. similar behavior was observed for the noise resistance (rn) obtained using ena, where ce(no3)3·6h2o showed higher resistance than c2h3n3. free energy of adsorption δg°ads was calculated using rn and rp values obtained by ena and eis analysis, respectively. δg°ads values of 35 kj/mol for rn and -37 kj/mol for rp were calculated for c2h3n3, while for ce(no3)3·6h2o, 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http://dx.doi.org/10.1016/j.surfcoat.2004.05.012 http://dx.doi.org/10.1016/j.surfcoat.2004.05.012 https://www.researchgate.net/publication/268002945 https://www.researchgate.net/publication/268002945 https://doi.org/10.1016/j.porgcoat.2011.02.013 https://doi.org/10.1016/j.electacta.2005.04.021 https://creativecommons.org/licenses/by/4.0/) 404 not found ion-exchange membranes for blue energy generation: a short overview focused on nanocomposite http://dx.doi.org/10.5599/jese.1447 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1447 open access : : issn 1847-9286 www.jese-online.org review ion-exchange membranes for blue energy generation: a short overview focused on nanocomposite jin gi hong1, and tae-won park2, 1department of civil engineering and construction engineering management, california state university, long beach, ca, 90840, united states 2department of earth science education, chonnam national university, gwangju, south korea corresponding author: jingi.hong@csulb.edu; tel.: +1-562-985-2440; fax: +1-562-985-2380 park2760@jnu.ac.kr; tel.: +82-62-530-2511; fax: +82-62-530-2510 received: july 7, 2022; accepted: july 28, 2022; published: july 31, 2022 abstract blue energy can be harvested from salinity gradients between saline water and freshwater by reverse electrodialysis (red). red as a conversion technique to generate blue energy has received increasing attention in recent decades. as part of the red system, ion exchange membranes (iems) are key elements to the success of future blue energy generation. however, its suboptimal performance often limits the applications and stagnates the development of the technology. the key properties of iems include ion exchange capacity, permselectivity, and electrical resistance. the enhancement of such physical and electrochemical properties is crucial for studying energy production with acceptable output efficiency on a commercial scale. recently, many studies have tried blending nanotechnology into the membrane fabrication process. hybridizing inorganic nanomaterials with an organic polymeric material showed the great potential of improving electrical conductivity and permselectivity, as well as other membrane characteristics for power performance. in this short review, recent developments on the iem synthesis in association with potential nanomaterials are reviewed and raising issues regarding the application and commercialization of red-based energy production are discussed. keywords salinity gradient energy; electrochemical properties; membrane fabrication, reverse electrodialysis introduction clean and renewable energy is increasingly important to sustain the global energy supply and lessen the escalating environmental deterioration issues. salinity gradient is one of the emerging energy sources that has yet to be tapped. its utilization is widely considered promising due to its abundance in various forms of different salinity levels. the technology of extracting electrical energy http://dx.doi.org/10.5599/jese.1447 http://dx.doi.org/10.5599/jese.1447 http://www.jese-online.org/ mailto:jingi.hong@csulb.edu mailto:park2760@jnu.ac.kr j. electrochem. sci. eng. 00(0) (2022) 000-000 ion-exchange membranes for blue energy generation 2 from water salinities is often named blue energy, or salinity gradient energy (sge). the blue energy has received attention as zero-emitting technology that could potentially supplement or replace the current global energy resource. such technology not only can produce electricity from salinity gradients, but it also can provide the additional benefits of energy storage and wastewater treatment. the chemical energy available in seawater-freshwater gradients is significant, mixing one cubic meter of freshwater (the volume of six oil barrels) into the ocean releases energy equivalent to the same volume of water falling from nearly 1,000 feet. tapping these waters where rivers and secondary effluent pour into the ocean could yield nearly 2 tw of power production globally, roughly half of current global electricity production [1-4]. reverse electrodialysis (red) is one method that utilizes ion exchange membranes (iems) to harvest sge. red generates electricity when solutions of different salinities are separated by multiple membranes that allow only ions (e.g., sodium and chloride) to pass through them. ionic diffusion in water allows the transport of ions that are selectively determined by corresponding iems. a continuous ion flux using alternating series of cation exchange membranes (cems) and anion exchange membranes (aems) can be maintained inside the red stack. the flow of ions through the membranes to even out the salinity gradients is then converted to electric current on the surface of two terminal electrodes via redox reactions. therefore, electrons travel through the closed electrical circuit and generate power for an energy-consuming load (figure 1). figure 1. schematic of red stack. the arrows indicate fluid and ion transport through the iems; cem= cation exchange membrane, aem= anion exchange membrane compared to other conventional energy technologies, the red power generation using salinity gradient has a great potential to play due to its technical and economic superiority. since natural salinity gradients are renewable and located near land, red also avoids the problems of intermittent electricity generation and high offshore installation costs facing many other ocean energy technologies. like many other membrane-based systems, the role of membranes is of considerable importance in the development of the red operation. nowadays, the iems are widely used in various applications ranging from water treatment to industrial separation, to power generation, particularly in electrodialysis (ed), electrodialysis reversal (edr), and fuel cell processes. however, each application addresses different physical and electrochemical requirements as their goals and j. g. hong and t.-w. park j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1447 3 compositions vary. considering the nature of the electrochemical system, the performance of iems used in red is significantly affected by electrical resistance, permselectivity, and biofouling tendency [5-8]. optimizing any of these properties in existing membrane materials or through the development of improved membranes would improve red system performance. despite the importance of iem characteristics and performance, most previous studies on red have focused on system design and optimization. the lack of suitable iems for red, which entails low ionic resistance and high permselectivity has become a major challenge toward the commercialization of the technology [9,10]. recently, some efforts have been made to develop red-customized iems on a lab scale for enhancing the physical and electrochemical properties. reinforcing the physical structure of existing membrane material is one good way to decrease the electrical resistance in high salinity electrolyte conditions. the performance of iems deteriorates when they are in contact with high-salinity electrolytes in part due to shrinkage caused by osmotic effects. to address this problem, many studies are focused on reinforcing structural formation with nanomaterials while maintaining conductive characteristic. nanocomposite structural combination has received increasing attention due to their synergistic impact, which mainly stems from the properties of both inorganic and organic components. nanocomposite structure of the iems specifically designed for red application was first proposed by hong and chen in 2014 [11]. the concept of composite structures created by introducing inorganic nanomaterials into an organic polymer matrix was demonstrated to be highly competent for red application, as it allows carrying extra ion-exchangeable functional groups by revamping the membrane structure. inorganic nanoparticles have unique electronic, magnetic, and optical properties that can be incorporated into the polymer matrix. these small particles are considered fillers and are widely used in various forms and applications. its high specific surface increases the interface between the inorganic particles and polymer molecules and makes the particle more easily coated on surfaces. moreover, its ability to conduct proton at lower activation energy allows the nanocomposite membranes more attractive to lessen the electrical resistance as well [12]. also, when the small particles are furnished with the ionic charged group, the polymeric matrix increases the affinity to adsorb the inorganic materials, and thus, the composite maintains the homogeneity characteristics to a certain extent. sustaining homogeneity of the iems is crucial for the transport of counter ions, which may affect the area resistance and charge density of the membrane. it is often critical to have polymeric materials and filler particles functionalized via sulfonation or quaternization, thus the membranes avoid the tendency of having distinct regions of charged and uncharged in the composite structure. moreover, if there are ionic charged groups on the surface of small nanoparticles, membrane hydrophilicity and surface charge density also increase, which is preferable in the red application. these unique features of inorganic nanoparticles are favorable as it combines with organic polymers, deriving synergies for their physical and electrochemical characteristics. such synergy enables enhanced power performance in blue energy red cells. this short review focuses on the overview of iem preparation, emerging nanomaterials, and nanocomposite structures for potential applications in red. considering the importance of membranes in red-sge, the incorporation of nanotechnology in membrane preparation offers broader opportunity to promote blue energy from the laboratory bench to practical application at a greater scale. preparation of nanocomposite iems cation exchange composite membranes as a key component of the red power system, the dependency on good iems has assumed considerable importance. iems should contain respective charged groups attached to their http://dx.doi.org/10.5599/jese.1447 j. electrochem. sci. eng. 00(0) (2022) 000-000 ion-exchange membranes for blue energy generation 4 backbone; thus, the membrane selectively allows the migration of a specific groups of ionic species. nanocomposite cems generally consist of the polymeric matrix and inorganic nanoparticles (i.e., nanofiller), and are often tailored to enhance the membrane performance. different combinations of composite materials with various ranges of functionalization may be tested via different methods. the negatively charged functional groups execute the selectivity of cems. in many cases, the main functional groups for cems are based on sulfonic acid. these ion exchange groups are introduced into polymeric membranes by reaction with sulfonating reagents such as chlorosulfonic acid, sulfuric acid, dioxane-so3 complex, etc. specifically, sulfonic groups are introduced into the aromatic rings to provide negatively charged ion groups in the form of –rso3-. fundamental understanding of preparing of preparing organic-inorganic hybrid nanocomposite membrane systems pursue the same idea with various forming materials in diversified ways. anion exchange composite membranes the aems function is based on cationic moieties combined with a polymer matrix. these positively charged ligands are introduced to dissolved polymer, forming a casting solution that later attracts anions. a preformed polymer can also be modified chemically to bring required cationic moieties. for ion-exchangeable ligands, most commercially available aems have quaternary ammonium groups. thus, the common and physically stable way to modify polymeric films for the preparation of aem usually involves two processes: chloromethylation and quaternary amination. chloromethylation reaction is an electrophilic substitution process of aromatic polymer for allowing ionic characteristics. it is rendered by copolymerization of chloromethylstyrene with other vinyl or divinyl monomers (e.g. sodium p-styrene sulfonate). the aim of this process is to get the polymeric backbone ready for the quaternary ammonium function group to be inserted during the following quaternization reaction. an ordinary method for chloromethylation procedure involves the large excess of chloromethyl methyl ether (cmme) or bis-chloromethyl ether (bcme) reagent, which provide good conversion and yields [13]. however, these toxic agents are known as carcinogens and often result in poor control of chloromethylation positioning and quantity. as an alternative, nbromosuccinimide (nbs) or para-formaldehyde were proposed in some reported studies as much safer and more controllable options [13-17]. general approach to the synthesis of nanocomposite iems one simple method to combine inorganic precursor (monomer and oligomer) and polymer matrix is blending (figure 2). blending can be performed if no strong chemical interaction exists between the inorganic and organic building blocks. the inorganic moiety is embedded into the organic polymer under mixing in common solvents or melts at specific temperatures. the physical interaction of the organic polymer with entrapped discrete inorganic nanoparticles often provides easy hybridization of multiple components and good flexibility that many types of polymers and inorganic nanoparticles can be employed to form nanocomposite cems. for a conductive nanocomposite iem, an appreciable strength of ion selectivity is always desired, and such characteristics can be enhanced when embedded nanoparticles are evenly distributed in the polymer matrix. however, nanoparticles tend to aggregate easily, often causing performance deterioration. therefore, proper mixing and optimized dosage of nanoparticles are needed. inorganic nanoparticles and organic polymer can be blended either by melting or in a liquid state. melt blending is common but often found more vulnerable to unwanted agglomeration, leading to the uneven distribution of nanoparticles in the polymer matrix and alteration of membrane properties and morphology. liquid-state blending, on the other hand, lessens the constraints of such j. g. hong and t.-w. park j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1447 5 challenges to a certain extent and thus receives more attention as a better preparation technique in nanocomposite synthesis [18]. figure 2. simplified experimental procedure of nanocomposite iem preparation via blending method. doping (or pore-filling) is another technique, making inorganic nanoparticles or precursors infiltrate the polymeric membrane. the pore and void spaces of the polymeric membranes are swelled in solvent to increase the space for nanoparticles to be well positioned and dispersed. the nanocomposite is then cured with heat or chemical grafting to promote the proper bonding of particles with the polymer matrix. however, this method often results in a concentration gradient of the particles throughout the pore depth of the membrane: a greater amount of particle deposit near the surface and less in the deeper region. this is due to the nature of diffusion that alters the access for fillers in the narrow pore space of polymer membrane. such inhomogeneity of particle distribution may also cause unwanted leaching of particles from the matrix. the use of metal alkoxides in the preparation of composite helps to avoid the problem as they often increase the diffusivity within the composite. the method of sol-gel reaction has long been used for the preparation of organic-inorganic hybrid materials. the fundamental concept of the sol-gel process was established considerably earlier but has gained wider attention and has been studied more extensively since the late 1970s. the alkoxides of mainly silicon, titanium, aluminum, zirconium and boron and a variety of polymeric media are condensed to form a gel. the sol-gel approach provides enhanced bonding and properties created from organic and inorganic components. the improved dispersion of inorganic nanoparticles in the polymeric matrix allows better network between inorganic nanoparticles and organic polymer than blending method does. the use of a catalyst also determines the structure and morphology of the hybrid composites, which is often recommended for silicon-based metal alkoxides [19,20]. another method that is often adopted for the enhancement of monovalent-ion selectivity with antifouling potential is called the layer-by-layer (lbl) self-assembly technique [21-25]. the technique of layered polymeric multicomposites was first introduced in 1997 by decher to prepare http://dx.doi.org/10.5599/jese.1447 j. electrochem. sci. eng. 00(0) (2022) 000-000 ion-exchange membranes for blue energy generation 6 hybrid systems [24]. the lbl self-assembly can be established by creating an electrostatic association between oppositely charged layers of polyelectrolytes on the membrane surface. these multilayers constitute greater repulsion between multivalent anions and negative surface charge, which enhances selectivity for monovalent ions through the membrane. this preparation approach is particularly helpful for red membrane development because the negative impact of multivalent ions on sge-red power generation is not negligible as it results in a significant decrease in power output. red power performance test the power performance of the iems can be demonstrated in a red electrochemical cell as depicted in figure 1. a typical red stack uses multiple membrane couples, consisting of an aem and a cem stacked alternately between cathode and anode (figure 3). an additional cem is used as a shielding membrane at the end of the stack next to the electrode to close the end compartment. the stack with membranes may contain spacers to form compartments for the feeding solutions to pass through. in a laboratory setting, high concentration (hc) stream often mimics artificial seawater using 0.5 m nacl solution and low concentration (lc) stream with 0.017 m nacl for artificial river water [3,11,26]. however, the feeding streams for red application can potentially be ranged from brine, seawater, river water, and wastewater to reclaimed water [1,27,28]. the feed streams (hc and lc) can be fed into the red stack using peristaltic pumps at desired flow rates. during operation, an electrode rinsing solution is also pumped to the electrode compartments and circulate the system for facilitating the electron transfer in the stack. performance evaluation of the red stack can be measured with an external potentiostat. typically, the gross power density is the term used as an outcome and is estimated from the product of the maximum measured values of voltage and electrical current and divided by the total membrane area applied in the system. lastly, the power output values are further corrected by subtracting the power values obtained in a blank test with only one cem in the stack. figure 3. exploded view of the membrane configuration in a red stack j. g. hong and t.-w. park j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1447 7 potential iem materials for sge-red performance structural composition for red iems the preparation and fabrication of iems are well developed and studied for various fields of applications. there have been studies on utilizing diverse monomers and inorganic materials in various preparation methods. the most critical aims are focused on developing high-quality membranes using materials that are commonly available, cost-competitive, easy to handle, and environmentally friendly via simple fabrication methods. however, it has often been challenging to compromise adequate materials and methods with a fine performance, especially under unique circumstances such as red. the membrane performance has a direct correlation with associated properties of iem and thus relative to its structural representation of charged functional groups. the physicochemical characteristics of the membrane and material durability are of common importance in many ion exchange applications. many physicochemical properties dominate the ion mobility within the membrane matrix. swelling degree, permselectivity, ion exchange capacity (iec), and ionic conductivity (or resistance) are the crucial iem characteristics, which are often determined empirically (figure 4). in red, some of these physical and electrochemical characteristics regulate the performance more vigorously than the others and is largely affected by their active domain of charged polymer matrix. the cems contain anionic charged groups such as sulfonic acid, phosphoryl, phosphonic acid, carboxylic acid, monosulfate ester groups, mono-, and diphosphate ester groups, hydroxylic groups of phenol groups, perfluoro tertiary alcohol groups, sulfonamide groups and other groups that provide a negative fixed charge in aqueous or organic solvent solutions to selectively allow the passage for cations in the membrane structure. the aems, on the other hand, are amino groups, quaternary ammonium groups, tertiary sulfonium groups, and quaternary phosphonium groups that provide positive fixed charge groups to allow selective permeation of anions through the membrane. the membrane composition can further be customized by altering its microstructure, which classifies it into heterogeneous and homogeneous. the charged domain in the matrix of homogeneous iems is uniformly distributed, but in a heterogenous structure, finely powdered ion exchange resins are blended with an uncharged thermoplastic polymer, which separates domains of charged regions in the matrix. homogeneous iems are usually prepared by either copolymerization of monomers or made from polymeric films or solutions, which then be functionalized by proper methods (e.g., chloromethylation-amination, sulfonation, grafting functional monomers) for active ion mobility. heterogeneous iems are typically synthesized by blending two or more materials (e.g. ion exchange resin, nanoparticles, nanofiber) with uncharged (or charged) polymer. different fabrication strategies using various materials are proposed in the literature to optimize the properties specifically for red application. microstructural tailoring is the common and crucial approach for enhanced membrane performance and depends on material composition and functionalization. the homogeneity and heterogeneity of the membranes thus carry different abilities and strengths, which directly affect their performance. for example, the electrochemical properties may become slightly inferior for heterogeneous membranes, although with greater mechanical strength. however, there also have been increasing efforts in combining appropriate materials to induce synergistic effects of each component, resulting in enhanced ion exchange processes. various materials and preparation methods for the synthesis of red iem have been studied over the past decades. the quality and performance of salinity gradient energy generation via the red membrane system have improved to a greater extent in recent years. such progress is attributed to the membrane optimization primarily focused on obtaining preferred electrochemical characteristics for red, leading to a more desirable power density level. http://dx.doi.org/10.5599/jese.1447 j. electrochem. sci. eng. 00(0) (2022) 000-000 ion-exchange membranes for blue energy generation 8 figure 4. common setup (four-electrode system) for membrane resistance measurement recent advances in nanocomposite for red applications there have been many studies focusing on composite membrane materials for water treatment, fuel cell, and electrodialysis applications (table 1), but in recent years, it has gained more attention, particularly in the field of red application. inorganic nanoparticles offer enhanced charged species region for polymer matrix, thus facilitating more suitable counterion navigating paths. introducing filler materials to the polymeric membrane may also derive advantageous features of various nanoparticles, which coordinate to highlight certain membrane characteristics. however, as the portion of nanoparticle filling increases, the heterogeneity structure becomes more prominent, and particles often tend to agglomerate. this leads to uneven particle distribution in the polymer matrix and thus disturbs the ionic path and weakens selectivity during the exchange process. hosseini et al. reported the work on polyvinylchloride (pvc)-based heterogeneous cems, blended with graphene oxide nanoplates (gons) and resin by solution casting technique [29]. thus, the presence of -cooh (gons) and -so3h (resin) functional groups was allowed in the prepared membranes, providing additional conducting regions with higher charge density. the loading amount of gons was controlled to observe the effect on ion transport characteristics of membranes. the results revealed that the iec was enhanced to 1.4 meq/g as the concentration of gons increased to 0.5 wt.% in the membrane solution. low water content was also maintained while more gons were loaded. however, further dosage (0.5-2.0 wt.%) plunged some of the transport properties (e.g. membrane potential, selectivity, transport number), possibly caused by disturbed flow channel and resin functional groups at a high dose of gons concentration. hong et al. utilized the powder of fe3o4 nanoparticles mixed with poly (2,6-dimethyl-1,4-phenylene oxide) (ppo) polymer for the synthesis of cem [30]. in this work, both nanoparticle and polymer were functionalized with anionic charged groups without blending the composite structure with resin particles. this membrane was compared with both commercial and other custom-made membranes and further tested for red performance. in this study, those ppo-based nanocomposite membranes with the lowest area resistance performed the highest power outputs among other membrane combinations investigated. hong et al. j. g. hong and t.-w. park j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1447 9 also concluded that there might be less dependency of permselectivity and other related properties on the sge performance. specifically, there appear to be greater links between area resistance and red power production than permselectivity. this is owing to low consistency in the pattern portrayed by permselectivity and other related ones compared to membrane resistance. jashni et al. tested the efficacy of nanocomposite, fe3o4/pvp, embedded in pvc-based heterogeneous cem via casting solution technique [31]. the membranes fabricated in various blend ratios of fe3o4/pvp combination also exhibited some notable degree of membrane characteristics. for example, the areal electrical resistance and permselectivity enhanced significantly with the one at 2.0 wt.% fe3o4/pvp (i.e. resistance of 5.8 ω cm2 and permselectivity of >90 %). this result proved that the composite, fe3o4/pvp, also has a good potential in ion exchange performance as a competent candidate over other composite combinations. zehra et al. also fabricated composite cem by using various mass compositions of pvc and tin aluminium molybdophosphate (tamp) [32]. four combinations with different mass ratios of binder (pvc) and exchanger (tamp) were tested and the ratio of 1:3 (polymer to inorganic filler) was found to be most reliable formation for obtaining fine ion transport properties (e.g. iec, membrane potential) as well as enhanced thermal and structural stabilities. this work also confirms the good potentiality of iem material selection for red application. table 1. recent composite applications inorganic material organic material thickness, µm swelling degree, % iec, meq/g r / ω cm2 permselectivity, % applications ref. fe2o3 ppo1 100 20-26 0.87-1.4 0.87-2.05 59.5-87.7 salinity gradient red [11,30] fe2o3 nafion 22.9-40.7 fuel cell [33] go2 speek3 24 38 0.75-1.11 fuel cell [34] sulfonated go speek 20-40 10 37 0.75-1.65 fuel cell [34] zwitterionic go pbi4 50 10.2-35.2 0.27-1.12 fuel cell [35] go nafion 20.9-29.9 0.92-1.02 high proton conductivity [36] go spes5 180 12.12-15.19 1.27-1.40 87.0-96.0 fuel cell and ed [37] go pvc6 8.5-13 1.15-1.40 5.5-7.0 87.0-89.0 water deionization [29] zro2 nafion 120 0.13-0.15 electrolyzer application [38] zno pvc 70 10-50 15-18 85.0-92.0 electrodialysis [39] sio2 fumasep fap 60 20-22 1.07-1.16 0.7-1.088 flow batteries [40] sio2 pes 60 9.7-14.3 0.74-1.1 66.94-100 electrodialysis [41] fe3o4/pvp pvc 11-14.7 14.7-5.8 77-90 electrodialysis [31] tamp7 pvc 900 10-12.5 0.57-0.83 separation process [32] al2o3 pva8 200-400 0.2-0.4 fuel cell [42] ceo2 nafion 25 17-22 fuel cell [43] fe2nio4 pvc 80-100 17-23 1.5-1.6 9.1-12.8 79-84 cem characterization [44] mwcnt9 pva 100, 120 38.2-284.0 0.7-2.25 fuel cell [45] 1poly(2,6-dimethyl-1,4-phenyleneoxide); 2graphene oxide; 3sulfonated polyetheretherketone; 4polybenzimidazole; 5sulfonated polyethersulfone; 6polyvinyl chloride; 7tin aluminium molybdophosphate; 8polyvinyl alcohol; 9multi-walled carbon nanotube many reported works on nanocomposite iem investigated the optimum loading amount of nanomaterial to enhance the key membrane characteristics that determine the red power performance. however, the optimum dosage may vary depending on the type of binding material, filler particle, and http://dx.doi.org/10.5599/jese.1447 j. electrochem. sci. eng. 00(0) (2022) 000-000 ion-exchange membranes for blue energy generation 10 synthesis condition. therefore, optimizing the membrane preparation and performance remains a great challenge for adopting nanocomposite membranes in red power generation. several studies of composite iem preparation have also been conducted mainly by surface modifications. for the first time, gao et al. investigated the monovalent anion selectivity and antifouling potential using layer-by-layer deposition with negatively charged poly(styrenesulfonate) and positively charged pei polyelectrolytes based on standard grade conventional aem [21]. as a result, the modified membrane with 7.5 bilayers of poly(styrenesulfonate) and pei (cjma-2-7.5) showed comparable monovalent selectivity to commercial acs membranes and exhibited around 30-38 % improved antifouling potential over unmodified and commercial membranes. in addition, the power performance of a red system equipped with prepared membranes achieved up to 17% improvement in power density. tufa et al., on the other hand, chemically modified commercial cem (fuji cemt1, the netherlands) to investigate the key electrochemical properties (e.g., monovalent cation selectivity, resistance, iec) using polypyrrole (ppy)/chitosan (cs) composites [46]. by controlling ppy concentrations (0.025-1 m) and polymerization time (0-8 h), the modified membrane with low ppy content (<0.05 m) exhibited better monovalent (i.e. na+) selectivity compared to the pristine membrane and is possibly due to slightly higher iec characteristics of those modified ones. as a result, this work also demonstrated the great potential of composite iem for red application. the modified membranes achieved more than 42 % power density improvement over pristine commercial membranes in red operation. conclusions although the advantages of sge-red harvesting over other conventional energy technologies are well recognized, there is still a great effort needed to facilitate large-scale electrical energy systems on the number of factors in the red system. as part of such efforts, developing a new membrane and understanding the property optimization are the most crucial routes. in particular, the red iem studies should aim toward a performance enhancement, which entails proper fabrication methods using materials that are easy to handle and cost-competitive. considering the large-scale (or pilot scale level) operation with newly developed iems, including nanocomposites, the economic aspects of membrane price and affordability and validation of power performance in a natural saline environment are still of great concern. the use of inorganic nanomaterial for the synthesis of the red membrane is in an early stage of development and only a limited group of nanomaterials have been investigated for application in red. developing proper nanocomposite membranes usually deal with the amount of nanoparticle dosage for maximizing the property performance and the optimum loading varies depending on the type of materials used. further studies are expected to demonstrate more variety of material combinations and to optimize the composition ratios in forming nanocomposite iems. thus, exploring the new and innovative red iem preparation and demonstrating the researched iems at pilot and commercial settings under dynamic operating conditions should be entailed to overcome the existing challenges and expedite the nanocomposite application on a practical red scale. some materials have already been characterized and tested in other related fields, such as electrodialysis and fuel cell. however, there are different property requirements depending on the applications, which implies the need for unique tailoring to specific conditions and solutions for the red process. there are a few exemplary materials (e.g. go, fe3o4, tamp) that showed great potential with significant improvement in ion transport efficiency. also, some innovative fabrication methods have been introduced recently to advance the power performance in red. if the cost of j. g. hong and t.-w. park j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1447 11 membrane material and fabrication methods are compromised to ensure better performance, large-scale sge-red implementation can 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http://dx.doi.org/10.5599/jese.1447 https://doi.org/10.1016/j.electacta.2013.04.155 https://doi.org/10.1039/b925357b https://doi.org/10.1016/j.jpowsour.2009.10.091 https://doi.org/10.1016/j.memsci.2012.07.014 https://doi.org/10.1016/j.desal.2011.08.057 https://doi.org/10.1016/j.memsci.2011.07.010 https://doi.org/10.1016/j.cej.2019.122461 https://creativecommons.org/licenses/by/4.0/) {pouteria sapota as green co2-corrosion inhibition of carbon steel:} http://dx.doi.org/10.5599/jese.1196 383 j. electrochem. sci. eng. 12(2) (2022) 383-398; http://dx.doi.org/10.5599/jese.1196 open access : : issn 1847-9286 www.jese-online.org original scientific paper pouteria sapota as green co2-corrosion inhibition of carbon steel guillermo salinas-solano1, jesus porcayo-calderon2, ana karen larios-galvez1 and jose gonzalo gonzalez-rodriguez1, 1universidad autonoma del estado de morelos, centro de investigación en ingeniería y ciencias aplicadas, av. universidad 1001, 62209-cuernavaca, morelos, mexico 2universidad del estado de sonora, departamento de ingenieria quimica y metalurgica, hermosillo, sonora 83000, mexico corresponding author: ggonzalez@uaem.mx; tel: 52777 3297084 received: october 21, 2021; accepted: january 21, 2022; published: january 31, 2022 abstract imidazoline obtained from the essential oil contained in pouteria sapota seed was tested as an environmentally-friendly corrosion inhibitor of 1018 carbon steel in a co2 saturated 3.5 % nacl solution using electrochemical techniques. this imidazoline contains fatty acids with long hydrophobic chains, with 52.73 % of unsaturated (oleic and linolenic acids) and 40 % of saturated (palmitic and myristic acids) compounds. polarization curves revealed that this inhibitor is a highly efficient mixed-type of inhibitor with the inhibitor efficiency of 99.9 % reached at 25 ppm. also, the lowest pitting potential value was observed at 25 ppm of inhibitor, making the carbon steel highly susceptible to the pitting type of corrosion. corrosion current density value decreased by nearly four orders of magnitude, and a passive film formation was induced for inhibitor concentrations higher than 5 ppm. accordingly, polarization resistance values were increased from 100  cm2 up to about 106  cm2 at 25 ppm of inhibitor. the inhibitor forms a protective film of corrosion products adsorbed on the metal surface in a very strong chemical way, following a langmuir type of adsorption isotherm. this was supported by electrochemical impedance spectra that showed two relaxation processes ascribed to electrode interface and film regions. in agreement with polarization resistance data, the total electrode resistance determined by interfacial charge transfer and film resistance increased up to 8.2  105  cm2 in the presence of 25 ppm of inhibitor. sem images additionally showed that type of corrosion was fully changed from uniform to a localized type when 25 ppm of inhibitor was added into the solution. keywords acid corrosion; naturally occurring inhibitor; electrochemical impedance http://dx.doi.org/10.5599/jese.1196 http://dx.doi.org/10.5599/jese.1196 http://www.jese-online.org/ mailto:ggonzalez@uaem.mx j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 384 introduction the presence of water in the oil and gas field, together with high content of co2, acetic acid, and h2s, has increased the corrosion rate of involved metals [1,2]. one of such involved metals is carbon steel, which is extensively used in the oil industry as the most common material for tubing. one of the most frequently used methods to mitigate co2-corrosion is by applying organic inhibitors, where the presence of heteroatoms including phosphorus, nitrogen, sulfur and carbon protects steel surface by forming a barrier of corrosion products. in such a way, metals are protected from the action of the environment [3-10]. however, due to serious environmental concerns, a lot of research concerning less toxic, cheaper, and more environmentally friendly compounds has been undertaken in the last years [11-13]. thus, allium cepa [11], expired drugs [14-18], thymbra capitate [15], amino surfactants and imidazolines obtained from palm, avocado and rice bran oil [13,19,20] were already evaluated as corrosion inhibitors for iron and steel in different media. abdallah et al. [12] evaluated natural nutmeg oil as a green inhibitor for carbon steel in 1.0 m hcl solution, finding that inhibitor efficiency increased with its concentration, reaching a maximum value of 94 % at 500 ppm. according to langmuir adsorption isotherm, this inhibitor acted as a mixed type of inhibitor and was adsorbed on the steel. similarly, carmona-hernandez et al. [13] studied imidazole obtained from palm oil as a corrosion inhibitor for uns s41425 type supermartensitic stainless steel in an h2s-containing environment. the authors found that at inhibitor concentrations between 0 and 100 ppm, the maximum inhibitor efficiency was obtained at 25 ppm, decreasing with a further increase of inhibitor concentration. imidazole was physically adsorbed onto the steel according to langmuir adsorption isotherm, behaving as a mixed-type of inhibitor. in another work, abdallah et al. [16] evaluated curcumin, parsley and cassia bark extracts as green inhibitors for carbon steel corrosion in 0.5 m sulfuric acid. they found that inhibition efficiency increased with inhibitor concentration, while inhibition efficiency of each particular inhibitor decreased in the following order: cassia bark extract > parsley extract > curcumin extract. imidazolines obtained from avocado and rice bran oil were obtained and evaluated as co2-corrosion inhibitors for x-52 steel [19,20]. both inhibitors contained fatty acids and were evaluated in concentrations of 0, 5, 10, 25, 50 and 100 ppm. it was found that in both cases, the maximum inhibition efficiency was reached at 25 ppm and decreased with a further increase of inhibitor concentration. mamey sapote [pouteria sapota] is a very popular fruit growing in mexico and central america [21]. it is a fruit tree that has the potential to be cultivated in 15 of 32 states, i.e., over almost 50 % of the mexican territory. its production is concentrated in the south, southeast, west and some central states with an area of 1,618 hectares, with a total production of 20,120 tons of fruit, and a national average yield of 12.4 tons ha-1 [22]. it is a cheap fruit, less than u.s. $1000 per ton, although currently, oil extraction is somehow expensive. antioxidant activity of pouteria sapota pulp has already been assessed, finding different phenolic acids, flavonoids, and carotenoids, and the inclusion of such fruit has been recommended in daily diets [23]. nevertheless, such studies have considered only the flesh and little information exists regarding the seed or bone, although some information suggests the existence of fatty acids [24]. recently, different researchers evaluated fatty acids contained in palm, avocado and rice bran oil, similar to antioxidants reported for pouteria sapota, as green corrosion inhibitors for steel in co2 and h2scontaining environments [13,19,20]. thus, the goal of this research is to evaluate use of the oil contained in the pouteria sapota seed to obtain an imidazoline as corrosion inhibitor for 1018 carbon steel in a co2-saturated nacl solution. g. salinas-solano et al. j. electrochem. sci. eng. 12(2) (2022) 383-398 http://dx.doi.org/10.5599/jese.1196 385 experimental procedure materials and sample preparation the material used in this research work includes bars of 1018 carbon steel having a chemical composition given in table 1. specimens were grounded with 1200 grade emerging paper, rinsed with distilled water, acetone and blown with hot air. table 1. chemical composition of 1018 carbon steel element content, wt.% fe balance c 0.14 mn 0.60 s <0.05 p <0.05 inhibitor synthesis imidazoline from pouteria sapota seed oil was obtained by the soxhlet method to extract the crude oil using hexane as a solvent. afterward, a bleaching process was carried out on the crude oil using tonsil and activated carbon in concentrations of 2 and 0.5 %, respectively. this process was performed at 95 °c under constant stirring during 20 min. the oil obtained was filtered under a suction method and mixed with water at 95 °c. finally, the obtained emulsion was centrifuged to obtain the semi-refined oil. this semi-refined oil was the precursor solution in the synthesis of the imidazoline-based inhibitor. the inhibitor synthesis process was carried out in two stages, as illustrated in figure 1. figure 1. synthesis of the imidazoline-based inhibitor from semi-refined pouteria sapota oil: (1) n-(2-hydroxyethyl) amino ethylamine; (2) pouteria sapota oil; (3) n-[2−[(2-hydroxyethyl) amino]ethyl]−amide, fatty amide, and (4) fatty-imidazoline derivative from the raw pouteria sapota oil, where r=alkyl chain of pouteria sapota oil fatty acid at first, amidation was carried out by hydroxyethyl ethylene diamine during 2.5 hours at 140 °c. this reaction was monitored by the thin layer chromatography technique (tlc) using heptane-ethyl acetate (sigma aldrich) (9:1) as eluent. the reaction products were removed by washing and filtering and analysed by the fourier transform infrared spectroscopy (ftir) technique. the second synthesis step consisted of performing the imidazoline cyclization reaction. this was carried out once purified fatty-amide was placed at vacuum (79.58 kpa) and heated at 160 °c for 16 hours using (8.5:1.5:5) dichloromethane (sigma aldrich), methanol (baker), ammonium hydroxide (baker) as eluent while the reaction products were analysed using ftir. the obtained inhibitor was stored inside a glass beaker http://dx.doi.org/10.5599/jese.1196 j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 386 at room temperature. gas chromatography was used in order to know the type of fatty acids found in the pouteria sapota seed oil as described elsewhere [25]. the oil extracted from pouteria sapota seed was analyzed with an ftir spectrophotometer from bruker. as a corrosive environment, a co2saturated 3.5 % nacl heated at 50 °c was used according to the nace standard method nace tm 1772005, widely used in the literature [13,19,20,25]. before starting with corrosion tests, the solution was bubbled with co2 gas during 2 hours and the gas bubbling continued during testing. inhibitor concentrations were chosen to be 0-100 ppm, as were also used in previous works [13,19,20]. these concentrations have also been recommended by manufacturers for commercial imidazolines. tests were carried out under stagnant conditions as a first inhibitor screening in order to know the optimum inhibitor concentration. electrochemical tests electrochemical measurements were conducted in a conventional glass cell using a potentiostat from acm instruments. rods of 1018 carbon steel were encapsulated in polymeric resin with an exposed working area of 0.5 cm2. as auxiliary and reference electrodes, graphite cylindrical bars and a saturated calomel electrode (sce) were used. tests were performed at the temperature of 50 °c. before starting the experiments, 30 minutes was given for the open circuit potential value (ocp) to reach a steady-state value. potentiodynamic polarization curves were obtained by applying a cathodic potential 800 mv more negative than the free corrosion potential (ecorr) and then the sweeping started in an anodic direction at the scan rate of 1 mv s-1, ending at a potential of 800 mv more anodic than ecorr. tafel extrapolation was used to calculate the corrosion current density values, jcorr. electrochemical impedance spectroscopy (eis) measurements were performed at the ecorr by applying an alternating potential of ± 10 mv in the frequency range between 10 khz and 0.04 hz. in order to know the change in the corrosion behavior with time, linear polarization resistance (lpr) measurements were carried out. for this, specimens were polarized ±15 mv every hour during 24 hours. all experiments were performed three times. the surface of specimens used for lpr experiments was analyzed by the scanning electronic microscope (sem) leo vp 1450, whereas chemical analysis of corroded specimens was performed with an x-ray energy dispersive spectrometer (eds) attached to it. results and discussion inhibitor characterization palmitic acid is the major fatty acid contained in the pouteria sapota oil with 25.5 wt.%, followed by 19.1 % of oleic acid, 16.1 % of myristic acid and 14.1 % of linoleic acid. in table 2, the full profile of fatty acids contained in the pouteria sapota oil is shown. chemical structures of palmitic and oleic acids are given in figure 2. table 2. fatty acid composition of pouteria sapota oil fatty acid structure type of fatty acid content of fatty acid, wt. % palmitic acid c16:0 saturated 24.5 myristic acid c14:0 saturated 16.1 oleic acid c18:1 n-9 cis unsaturated 19.1 linoleic acid c18:2 n-6 cis unsaturated 14.1 linolenic acid c18:3 n-3 cis unsaturated 2.4 arachidic acid c20:0 unsaturated 0.6 g. salinas-solano et al. j. electrochem. sci. eng. 12(2) (2022) 383-398 http://dx.doi.org/10.5599/jese.1196 387 figure 2. chemical structures of oleic and palmitic acids ir spectra of pouteria sapota oil and synthesized inhibitor are shown in figure 3. the observed c=o stretching signal at 1743 cm-1 corresponds to the ester vibration, whereas at 1642 cm-1 was assigned to the fatty amide. the inhibitor response presents a signal observed at 1720 cm-1 which was assigned to the ester group. observed peaks at 1560 and 1470 cm-1 were assigned to c=c and c=n stretches in the ring of imidazole, whereas another signal observed at 1080 cm-1 to c-h stretch. the group n-h of imidazole at 3150 cm-1 is due to the aromaticity of the ring. on the other hand, the peak observed at 1630 cm-1 corresponds to the c=o stretch of the amide. in addition, observed peaks at 1280 and 1200 cm-1 correspond to the stretching of c-n and c-o groups, respectively. finally, observed signals at 2920 and 2852 cm-1 were assigned to the methyl and methylene groups, respectively. wavenumber, cm-1 figure 3. ftir spectra of pouteria sapota oil and synthesized inhibitor open circuit potential the time changes of ocp values for 1018 carbon steel exposed to the co2-saturated 3.5 % nacl solution, without and with different inhibitor concentrations, are shown in figure 4. time, h figure 4. variation of ocp value with time for 1018 carbon steel in co2-saturated 3.5 % nacl solution containing different concentrations of inhibitor at 50°c t ra n sm it ta n ce , % o c p , m v http://dx.doi.org/10.5599/jese.1196 j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 388 figure 4 shows that the ocp value is rapidly shifted towards the noble direction for the blank, uninhibited solution, reaching a steady-state value after 2 or 3 hours of exposure to the environment. this shift can be due to the protection of steel by the formation of iron carbonate film on its surface [26-29]. when the lowest inhibitor concentration (5 ppm) was introduced into the solution, the ocp value became more negative, reaching a steady-state value within 2-3 hours of testing. at higher inhibitor doses, i.e., at 10 and 25 ppm, the ocp value became more negative. with higher doses of inhibitor, however, the ocp value shifted to a noble direction, reaching the noblest value with the addition of 100 ppm. a shift of the ocp value towards more negative values means that the metal is getting corroded, dissolving any protective film formed on the metal surface. on the contrary, if the ocp value moves toward the noble direction, it indicates that the metal has been protected by the formation of a film made from corrosion products [30]. polarization curves the effect of inhibitor concentrations on the polarization curve of 1018 carbon steel in the co2saturated 3.5% nacl solution is shown in figure 5. j / ma cm-2 figure 5. effect of inhibitor concentrations on polarization curve of 1018 carbon steel in co2saturated 3.5 % nacl solution at 50°c figure 5 shows that in the uninhibited solution, there is no evidence of the existence of any passive layer on the steel surface. similar behavior is observed when 5 ppm of inhibitor were added into the system, although jcorr value decreased from a value of 2.0  10-1 ma/cm2 down to 10-2 ma/cm2 as shown in table 3. in the absence of inhibitor and in the co2 environment, iron carbonate film, which is not protective, is developed on the steel surface, which is why the passive layer was not formed. with the addition of low doses of inhibitor, inhibitor reacts with the released iron ions, forming a layer of corrosion products that acts as a barrier between the metal and the environment. this barrier makes their contact more difficult, which results in a decrease of jcorr value. as the inhibitor doses increased up to 25 ppm, a significant reduction in the jcorr value can be observed, reaching its lowest value of 3.0  10-5 ma/cm2. after a further increase of the inhibitor concentration, however, jcorr increased again. generally, a decrease of jcorr value with the addition of inhibitor is due to its adsorption on the steel surface and the formation of a passive layer. as the inhibitor concentration increases, the number of its molecules covering the steel surface increases too, and due to the existence of the same electric charge, there is an electrostatic repulsion among p o te n ti a l, m v g. salinas-solano et al. j. electrochem. sci. eng. 12(2) (2022) 383-398 http://dx.doi.org/10.5599/jese.1196 389 inhibitor molecules. this results in the inhibitor desorption from the steel surface, leaving the unprotected metal at the places of desorption. salinas-solano et al. [19] reported the lowest jcorr value of 2.34  10-4 ma/cm2 whereas cruz-zabalegui et al. [20] reported the value of 6.0  10-3 ma/cm2 for carbon steel in co2-saturated 3.5 % nacl solution, by using amino surfactants and imidazolines obtained from avocado and rice bran oil, respectively. both reported jcorr values are, however, much higher than the minimal value of jcorr = 3.010-5 ma/cm2 reported in this study (table 3). polarization curves for inhibitor concentrations higher than 5 ppm in figure 5 displayed the presence of a passive layer. the values of jcorr were used to calculate the inhibitor efficiency values (ie) as follows: − = corr corr/inh corr 100 j j ie j (1) where the corrosion current density values obtained in the absence and presence of the inhibitor are represented by jcorr and jcorr/inh, respectively. calculated ie values are listed in table 3. table 3. electrochemical parameters obtained from polarization curves cinh / ppm ecorr / mv jcorr / ma cm-2 epit / mv a / mv dec-1 c / mv dec-1 ie / %  0 -240 2.0  10-1 50 245 5 -300 3.0  10-2 60 200 95.0 0.95 10 -850 3.0  10-4 -255 65 380 99.8 0.99 25 -700 3.0  10-5 -290 70 400 99.9 0.99 50 -830 2.0  10-5 -230 75 450 99.9 0.99 100 -680 6.0  10-4 -110 80 300 99.7 0.99 data in table 3 show an increase in the inhibitor efficiency value with an increase in its concentration up to 25 ppm, and a decrease for higher concentrations. the fractional surface coverage by the inhibitor (), which was calculated by dividing the inhibitor efficiency value by 100, behaves in the same fashion as the inhibitor efficiency. the highest efficiency value of 99 % was obtained with the addition of 25 ppm, which is similar to the reported by salinas-solano et al. [19] but higher than 93 % reported by cruz-zabalegui et al. [20]. zheng et al. [31] used a mercaptopropionic acid-modified oleic imidazoline as a highly efficient corrosion inhibitor for carbon steel in co2-saturated formation water and obtained ie value of 95 % with the addition of 20 ppm. at the same time, jcorr value was decreased for 2 orders of magnitude. sotelo-mazon et al. [32] obtained ie of 99 % with imidazoline synthesized from wasted avocado oil for carbon steel in co2-saturated 3.5 % nacl solution, and jcorr value was reduced for 2 orders of magnitude, from 0.1 to 0.001 ma cm-2. okafor et al. [33] obtained an inhibitor efficiency of 97 % with the addition of 200 ppm of a rosin amide imidazoline for n80 carbon steel in co2saturated simulated formation water, with the reduction of jcorr value from 0.1 down to 0.005 ma cm-2. therefore, the present results, which show that for a relatively low inhibitor concentration of 25 ppm, high inhibitor efficiency of 99 % is attained and jcorr value is reduced for even four orders of magnitude, are very encouraging. all these justify the use of pouteria sapota, not only because it is a green inhibitor but also because it is obtained from a waste agro-industrial product that is a very abundant product in our country that can be utilized in this way. a possible effect of pitting corrosion, which is a localized type of corrosion characteristic for breaking of passive film on stainless steel in solutions containing chloride ions, is further explored. the effect of the inhibitor concentration on the pitting potential (epit) value can be seen in table 2. since no passive film formation at 0 and 5 ppm was observed, there is no value for epit at these inhibitor concentrations. however, it can be seen that at the inhibitor concentration of 10 ppm, epit http://dx.doi.org/10.5599/jese.1196 j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 390 value of -255 mv is observed, and it became more active at 25 ppm, showing epit of -290 v. as the inhibitor concentration increased further, epit attained more positive values of -230 and -110 mv at inhibitor concentrations of 50 and 100 ppm, respectively. data in table 2 clearly shows that anodic tafel slopes remained virtually unaffected by the addition of the inhibitor, unlike the cathodic one, which was greatly affected, indicating that this inhibitor behaves as a mixed-type of inhibitor with a predominant cathodic effect. this way, it can be said that anodic reactions such as iron dissolution remained practically unaffected by the addition of the inhibitor, whereas cathodic reactions such as oxygen reduction and hydrogen evolution (he) were severely affected. this indicates that this inhibitor blocked the sites where protons can be adsorbed on the metal surface for the he reaction [34,35]. it has already been established that the adsorption of an organic compound such as an inhibitor on a metal surface depends, among other things, upon its chemical structure and chemical compositions of substrate and electrolyte [36]. at the steel/electrolyte interface, an organic compound can be absorbed by: i) electrostatic interaction between metal and inhibitor, ii) interaction between the inhibitor uncharged electron pairs and metal surface, and iii) interaction of π electrons of inhibitor with the metal [25]. in order to get more information about the way of interaction between an inhibitor and metal, it is necessary to know the adsorption isotherm. in figure 6, different adsorption isotherm models, i.e., langmuir, temkin and frumkin, are tested with data obtained for different concentrations of pouteria sapota oil as an inhibitor for 1018 carbon steel in co2 saturated 3.5 % nacl. it is obvious from figure 6 that the best fit to predicted linear dependences, measured by the correlation factor (r2), were obtained for the langmuir type of adsorption isotherm. for this isotherm, this factor was 0.99, whereas, for temkin and frumkin isotherms, it had values of 0.70 and 0.79, respectively. this factor is a measure of the correlation between experimental and predicted data, and closer to the unit, the better is the correlation. cinh / mmol l-1   figure 6. a) langmuir, b) frumkin and c) temkin isotherm plots for 1018 carbon steel in co2saturated 3.5% nacl solution containing different concentrations of inhibitor at 50 °c lo g ( (c in h  ) (1 - ) -1 / m m o l l1 c in h  / m m o l l1 lo g ( c in h  / m m o l l1 r2 = 0.60 r2 = 0.99 r2 = 0.79 g. salinas-solano et al. j. electrochem. sci. eng. 12(2) (2022) 383-398 http://dx.doi.org/10.5599/jese.1196 391 langmuir isotherm assumes that the interaction between molecules and a surface can be described by a simple equilibrium, with the equilibrium constant kads. it does not take the interaction between adsorbed molecules into account, nor modification of the surface by the adsorption. langmuir adsorption isotherm is a function between the fractional surface coverage by the inhibitor () and its concentration (cinh) according to equation (2):  = +inh inh ads 1c c k (2) the adsorption equilibrium constant kads has a relationship with the standard free energy of adsorption (g0ads) through the following equation (3): g0ads = -rt ln(106 kads) (3) where r is the universal gas constant, and t is the absolute temperature. using data from figure 6a, the calculated value for g0ads was −43.34 kj mol-1, indicating strong adsorption on the metal surface due to the sharing of charges between the molecules and/or the formation of coordinated type bonds (chemisorption). one of the explanations for the high inhibitory effect of imidazoline is the presence of nitrogen atoms located in the structural ring, as well as the high number of carbon atoms in the molecule, which will act as a barrier against electrolyte. also, double bonds in the compound chemical structure have been reported as part of this high inhibitory efficiency [36]. compounds found by the gas chromatography analysis detected the presence of palmitic (c16:0), myristic (c14:0), oleic (c18:1 n-9 cis), linoleic (c18:2 n-6 cis) and linolenic (c18:3 n-3 cis) acids in proportions of 24.5, 16.1, 19.1, 14.1 and 2.4 wt.% [36]. these compounds meet two characteristics that make an organic compound a very efficient inhibitor, i.e., long chains of hydrocarbons and the presence of double bonds. linear polarization resistance (lpr) measurements data given in figure 7 represent the variation on the polarization resistance (rp) value with the inhibitor concentration in the co2 environment. time, h figure 7. effect of inhibitor concentration on rp value for 1018 carbon steel in co2-saturated 3.5 % nacl solution containing different concentrations of inhibitor at 50 °c for uninhibited solution, rp value remained practically constant throughout the testing time, exhibiting a relatively low value, close to 100  cm2. a remarkable increase of rp value was observed as soon as the inhibitor was added into the solution, which is believed to be due to the adsorption of the inhibitor onto the steel surface. as long as the metal surface area covered by the inhibitor increases, the value for rp increases too. in the same way, as the inhibitor concentration increases, an increase of rp value was also observed, but only up to the inhibitor concentration of 25 ppm. the r p /  c m 2 http://dx.doi.org/10.5599/jese.1196 j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 392 obtained rp values were up to four orders of magnitude higher than that obtained in the uninhibited solution. with a further increase of the inhibitor concentration, rp values decreased, which can be due to the electrostatic repulsion between inhibitor molecules, when they are too many and very close to each other that a desorption process occurs. it was already shown [36] that the tested inhibitor is not a pure compound because it contains fatty acids (table 2), where the main are oleic and palmitic acids with chemical structures given in fig. 3, although the presence of myristic and linoleic acids is important also. it is not very clear which one of these compounds is responsible for corrosion inhibition, but it is very likely that it is due to a synergistic effect of different compounds. the presence of n atoms in the imidazole group makes these fatty acids highly susceptible to protonation [37]. however, as established above, organic compound structure affects its performance, and according to jovancicevic et al. [38], it was found that the longer the alkyl group's hydrocarbons chain is, the compound is a more efficient inhibitor. alternatively, the presence of double bonds into the inhibitor structure improved the inhibitor adsorption onto the steel surface [38]. thus, the presence of compounds with long hydrophobic chains and double bonds, in addition to the presence of n atoms susceptible to protonation, makes this inhibitor highly efficient. eis measurements in order to elucidate the corrosion mechanism for 1018 carbon steel in co2-saturated 3.5 % nacl solution in the absence and presence of inhibitor, some eis measurements at the open circuit potential were performed. figure 8 shows the results presented in both nyquist and bode formats. a b zre / k cm2 f / hz c f / hz figure 8. effect of inhibitor concentration on: a) nyquist and b) and c) bode plots of 1018 carbon steel in co2saturated 3.5 % nacl solution containing different concentrations of inhibitor at 50 °c z re / k  c m 2 z im / k  c m 2  / o g. salinas-solano et al. j. electrochem. sci. eng. 12(2) (2022) 383-398 http://dx.doi.org/10.5599/jese.1196 393 nyquist data display two capacitive semicircles, one at high to medium frequencies and the other at the lowest frequencies (figure 8a). the resulting semicircles seem to be imperfect due to the heterogeneity and roughness of the sample surface. the first, high-frequency semicircle, is related to the formation of corrosion products, whereas the second, lower frequency semicircle, is related to the electrochemical double-layer and charge transfer reaction due to corrosion. the shapes of nyquist and bode plots did not change with the addition of inhibitor, indicating that the corrosion mechanism is not affected by inhibitor addition. the semicircle diameters, however, increased with an increase of inhibitor concentration, reaching the highest values at 25 ppm of inhibitor. a further increase of the inhibitor concentration above 25 ppm caused a decrease of semicircle diameters. on the other hand, bode plots in the modulus format (figure 8b) showed that at the lowest frequencies, impedance modulus is the lowest for the solution in the absence of an inhibitor. impedance modulus increased with the inhibitor concentration, attaining more than four orders higher magnitude with the addition of 25 ppm of inhibitor. a further increase in the inhibitor concentration caused a decrease in the low-frequency impedance modulus value. the modulus impedance value at the lowest frequency takes into account all resistance contributions such as electrolyte or solution resistance (rs), charge transfer resistance through double electrochemical layer (rct) and the resistance of the film formed by the corrosion products (rf). since the sum of the last two, i.e., rct + rf is defined as the polarization resistance value (rp), we can see that the variation of the lowest frequency impedance modulus value observed in figure 8b behaves in the same way as rp in figure 7. at higher frequencies, impedance modulus data in figure 8b show sloping lines characteristic of capacitive impedance responses, where higher impedance values imply lower capacitance values. it is seen in figure 8b that capacitive impedances increased (capacitance decreased) with the addition of inhibitor up to 25 ppm, and decreased (capacitance increased) for higher inhibitor concentrations. it has already been known that two different electric interface values, such as electrical resistance and capacitance, are inversely proportional [37-43], and thus, an increase in the resistance value decreases the capacitance value. this proves the ability of the studied extract to reduce the aggressive action of the acid medium. according to the helmholtz model, the reduction in the capacitance value indicates an increase of the double layer thickness, which can be referred to as the development of a compact protective film on the metal surface by the inhibitor adsorption [44-46]. the phase angle bode plots (figure 8c) show the presence of two peaks, one at higher and the other at lower frequencies, defining two separate relaxations (rc) time constants characteristic for interfacial and film regions. these suggest two phenomena are happening and that the corrosion process is not only under interfacial charge transfer control. surface analysis sem micrographs of corroded steel surfaces in co2-saturated 3.5 % nacl solution in the absence and presence of inhibitor are shown in figure 9. for corroded steel surface in the absence of inhibitor, figure 9a presents a high area that corroded uniformly, just as predicted by polarization and rp results. similar is observed in figure 9b, presenting the image of steel surface when 5 ppm of inhibitor were added to the solution, although the corroded surface area is somewhat smaller in the presence of the inhibitor. as the amount of inhibitor was increased further, the corrosion type shifted from uniform to a localized type of corrosion such as pitting corrosion, which is clearly seen in figure 9c. the surface with the lowest damaged area by corrosion was found for steel corroded in the presence of 25 ppm of inhibitor, shown in figure 9d. here, the number of pits is the lowest, as predicted by polarization curves and rp measurements shown in figures 3 and 6, respectively. http://dx.doi.org/10.5599/jese.1196 j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 394 figure 9. sem micrographs of 1018 carbon steel corroded in co2-saturated 3.5% nacl solution at 50 °c containing following concentrations of inhibitor: a) 0; b) 5; c) 10; d) 25; e) 50; f) 100 ppm eds microchemical analysis performed on the corrosion products layer is presented in figure 10. for specimens corroded in the absence of inhibitor, figure 10a shows only chemical elements present in the steel and the corrosive environment such as fe, c and o. for specimens corroded in the presence of the inhibitor (figure 10b-d), fe, c and o were also present, but the amounts of c and o were higher than found in specimens corroded in the absence of inhibitor since imidazoline contains both chemical elements. elements present in the abrasives, such as al from al2o3 particles and sic in the abrading paper, were also present. thus, the addition of the inhibitor decreased not only the corrosion rate but also the type of corrosion, changing it from the uniform type in the absence of inhibitor to a localized type of corrosion when the inhibitor is added. this is expected since for inhibitor concentrations higher than 5 ppm a passive layer is formed on the top of the steel according to polarization curves shown in figure 5, and this passive layer can be disrupted, giving place to localized type of corrosion such as pitting. g. salinas-solano et al. j. electrochem. sci. eng. 12(2) (2022) 383-398 http://dx.doi.org/10.5599/jese.1196 395 figure 10. eds microchemical analysis of corrosion products layer formed on 1018 carbon steel corroded in co2-saturated 3.5% nacl solution at 50 °c, containing following concentrations of inhibitor: a) 0; b) 5; c) 10; d) 25 ppm corrosion inhibition mechanism as mentioned above, the obtained imidazoline is chemically adsorbed on the steel surface due to the sharing of charges between molecules and/or the formation of coordinated type bonds. imidazoline contains a hydrophilic, imidazole group, and a hydrophobic alkyl group given by fatty acids, which present saturated and unsaturated structures. the high inhibitory effect of imidazoline compounds has been related to the structure of the inhibitor due to the nitrogen atoms located in the structural ring, to the long chain of hydrocarbons acting as a barrier against water and chlorides, and finally, to unsaturated double bonds present in the inhibitor which is chemically adsorbed on the surface of fe (which form a very stable protective film). to give a better explanation about the corrosion inhibition process in the presence of the antioxidant constituents of the extract on the steel surface, a schematic diagram of the adsorption is shown in figure 11. http://dx.doi.org/10.5599/jese.1196 j. electrochem. sci. eng. 12(2) (2022) 383-398 pouteria sapota as co2-corrosion inhibitior 396 figure 11. chemisorption process between antioxidant constituents in pouteria sapota extract. unshared pairs of electrons are in pink color, heteroatoms in red, and π bond orbitals are the dotted line compounds contained in the pouteria sapota extract have a strong electron-donating and chelating capacity with steel, giving the extract the ability to inhibit metal corrosion. this chemisorption is preferable by an electronic interaction between unshared pairs of electrons from heteroatoms and π bond orbitals from the molecules and the benzene rings with the d orbitals on the metallic surface until the formation of a very adherent barrier of molecules. conclusions an imidazoline obtained from fatty acids contained in pouteria sapota seed oil has been evaluated as an inhibitor for co2 corrosion of 1018 carbon steel in 3.5 % nacl at 50 c. it was found that the main fatty acids are palmitic, oleic, myristic and linoleic acid. the obtained imidazoline was proved to be an excellent mixed-type of inhibitor, which affected both cathodic and anodic reactions but with a stronger effect on the cathodic one. the inhibitor is chemically adsorbed onto the steel surface according to the langmuir type of adsorption isotherm. inhibitor efficiency increased with its concentration up to 25 ppm, but after this critical concentration, its efficiency decreased. polarization curves indicated that icorr value decreased more than four orders of magnitude when 25 ppm of inhibitor were added due to the formation of a passive layer onto the steel surface. this is supported by eis data that showed two separate relaxation processes due to interfacial and film regions. the type of corrosion was affected by the 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iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.corsci.2018.02.038 https://doi.org/10.1016/j.corsci.2018.02.038 https://doi.org/10.1016/j.electacta.2017.02.114 https://doi.org/10.1016/j.electacta.2017.09.138 https://doi.org/10.1016/j.corsci.2005.06.017 https://doi.org/10.1080/19440049.2020.1794055 https://doi.org/10.1016/j.corsci.2021.109930 https://doi.org/10.1080/17518253.2019.1629698 https://doi.org/10.1021/ie1024112 https://doi.org/10.1016/j.porgcoat.2011.06.001 https://doi.org/10.1016/j.electacta.2017.02.114 https://doi.org/10.1016/j.electacta.2017.09.138 https://doi.org/10.1016/j.corsci.2005.06.017 https://doi.org/10.5006/1.3284006 https://doi.org/10.5006/1.3284006 https://doi.org/10.1080/19440049.2020.1794055 https://doi.org/10.1016/j.corsci.2021.109930 https://doi.org/10.1080/17518253.2019.1629698 https://doi.org/10.1021/ie1024112 https://doi.org/10.1016/j.porgcoat.2011.06.001 https://doi.org/10.1016/j.rinp.2018.01.008 https://doi.org/10.1021/‌acs.jpcc.9b03994 https://doi.org/10.1021/‌acs.jpcc.9b03994 https://doi.org/10.1016/j.porgcoat.2019.105254 https://creativecommons.org/licenses/by/4.0/) {inhibitory action of vernonia amygdalina extract (vae) on the corrosion of carbon steel in acidic medium} doi:10.5599/jese.353 145 j. electrochem. sci. eng. 7(3) (2017) 145-152; doi: http://dx.doi.org/10.5599/jese.353 open access : : issn 1847-9286 www.jese-online.org original scientific paper inhibitory action of vernonia amygdalina extract (vae) on the corrosion of carbon steel in acidic medium olamide olawale1,, adeolu adesoji adediran2, segun isaac talabi3, geraldine chika nwokocha1, alewo opuada ameh4 1department of chemical engineering, college of science and engineering, landmark university, omu-aran, kwara state, nigeria 2department of mechanical engineering, college of science and engineering, landmark university, omu-aran, kwara state, nigeria 3department of materials and metallurgical engineering, university of ilorin, ilorin, nigeria 4chemical engineering department, ahmadu bello university, zaria, nigeria.; corresponding authors e-mail: lamstock2@yahoo.com; olawale.olamide@lmu.edu.ng received: november 2, 2016; revised: august 8, 2017; accepted: august 9, 2017 abstract this study investigates the inhibition potential of vernonia amygdalina stem extract (vae) on the corrosion of mild carbon steel immersed in 1 m hcl solution containing various concentrations of the inhibitor. the corrosion rate and inhibition efficiency were evaluated using mass loss and adsorption techniques. the results revealed that vae efficiently inhibits the corrosion rate of mild carbon steel immersed in the acidic medium. the concentration of 0.6 g/l provides the highest inhibition efficiency of 78.1 %. the adsorption process was consistent with the physisorption mechanism and obeyed the langmuir isotherm. the functional groups responsible for inhibition were identified using fourier-transformed infrared (ftir) spectra. the surface morphology of the corroded samples examined with scanning electron microscopy (sem) revealed the presence of protective oxide layer. inhibition efficiency was found dependet on the presence of oxalate, phytate, tannins, saponins and flavonoids in the vae. it has been concluded that vae is safe, cheap and efficient corrosion inhibitor for mild carbon steel. keywords phytochemical; vernonia amygdalina stem extract; mild steel degradation; inhibition efficiency; hydrochloric acid introduction corrosion is gradual deterioration of materials generated by their chemical interaction with the environment [1]. the corrosion of metal and alloy structures can be disastrous leading to economic http://dx.doi.org/10.5599/jese.353 http://www.jese-online.org/ mailto:lamstock2@yahoo.com mailto:olawale.olamide@lmu.edu.ng j. electrochem. sci. eng. 7(3) (2017) 145-152 inhibitory action of vae on the corrosion 146 loss in terms of repair, replacement, safety and environmental pollution. mild steel has gain wide industrial application because of its excellent mechanical properties and low cost. this material, however, is prone to corrosion when exposed to the action of either bases or acids in the industries. the use of inhibitors is one of the most practical methods for protecting materials against corrosion, especially in acid descaling bathes, and preventing not only the metal dissolution but also acid consumption. inhibitors protect the metals by effectively adsorbing on their surfaces and blocking the active sites for metal dissolution and hydrogen evolution, respectively. in such a way, metal corrosion in aggressive environments becomes hindered [2]. the environmental toxicity of some inhibitors has led researchers to investigate the use of green and agro based materials as the alternatives to inorganic substances. these organic materials are biodegradable, cheaper and ecofriendly since they do not contain heavy metals or other toxic compounds [3]. they function via adsorption of the molecules on the metal surface creating a barrier to corrosion attack. green inhibitors from many plants have been investigated and reported by several authors as materials having the potential to reduce metal dissolution in various aggressive acids. such plants include jatropha curcas [4-6], tithonia diversifolia [7], carica papaya [8], isertia coccinea [9], telfaria occidentalis [10], rice husk [11], nicotiana tabacum [12], water hyacinth [13], moringa oleifera [14], curcuma longa [15], azadirachta indica [16], phyllanthus amarus [17] and bitter leaf [18]. it has been reported that the extract from these plants contained tannin and saponins that contribute to the inhibitory action via physical adsorption mechanism [19]. the protonated species are often adsorbed at the cathodic sites of the metal surface and retard hydrogen evolution reaction [20]. this was probably responsible for the pronounced cathodic inhibiting effect of g. africana observed at the ambient temperature (30 °c). plants leaves, especially bitter leaf (bf) has been reported to contain chlorophylls and carotenoids, phenols, alkaloids and terpenoids which are constituents of most organic inhibitors [21-23]. omotosho and ajayi [3] used the gasometric technique and reported the inhibitive effect of bitter leaf extract on environmentally assisted cracking of al-alloy in 2.0 m hydrochloric (hcl) at the ambient temperature. additionally, ajayi et al [24] investigated the deterioration pattern and corrosion rate of the mild steel in 2 m h2so4 at 333 k and related it with the concentration of bf extract. vernonia amygdalina (va) (botanical name of bf) is a common plant largely available in all geopolitical zones of nigeria, especially the south-western part of nigeria. hence, the use of bf as corrosion inhibitors (ci) constitutes one way to mitigate corrosion rate, protects metal surfaces against corrosion and preserves industrial facilities [25]. the present work reports the inhibitory action of vae (extract from va stem) on the mild steel corrosion in 1 m hydrochloric acid. it is noteworthy, that chloride ions present in the media under investigation have the tendency to be specifically adsorbed on metal surfaces. they facilitate adsorption of protonated inhibitor species by forming intermediate bridges between the metal surfaces and the inhibitor [26]. experimental technique preparation of mild steel the composition of the mild steel used in this experiment is presented in table 1. the 0.2 cm thick steel was mechanically cut into coupons of dimensions 5.0 × 4.0 cm. the surface treatment was done by degreasing the coupons using absolute ethanol, followed by drying in acetone. the samples so prepared were stored in the desiccator. the experiments were carried out under non-stirred and aerated conditions. o. olawale et al. j. electrochem. sci. eng. 7(3) (2017) 145-152 doi:10.5599/jese.353 147 table 1. composition of mild steel element content, %wt. c 0.2 p 0.024 si 0.003 mn 0.35 fe 99.4 preparation of plant extract 4 kg of fresh stems (vernonia amygdalina) was sun dried for 28 days to completely remove the moisture content. stems were then grounded into powder and sieved. 20 g of the powdered material was soaked in 700 ml ethanol solution and kept inside a desiccator to prevent evaporation for a period of 24 h and filtered. finally, the filtrate was heated at 80 °c for 25 minutes inside a rotary evaporator in order to remove the ethanol. ftir (shimadzu, model ir affinity, is ftir) was used to determine the functional groups responsible for the inhibition. corrosion rate and inhibition efficiency measurement for the corrosion experiment, the mild steel coupons were totally immersed in 250 ml capacity beakers containing 100 ml of 1m hcl solution. the initial weight of the mild steel coupons was taken before immersion in the acid solution with and without the addition of inhibitor. to achieve total coverage, the coupons were suspended in the beaker with the aid of rod and hook. the quantities of inhibitor ranging from 0.2-0.8 g/l were added to the corrosion medium. the experiment was conducted at ambient temperature during 360 h (15 days). the weight of the specimens after immersion was taken every 72 h (3 days) using electronic weighing balance (hx 302t with 0.01 g accuracy). prior to weight measurements, the samples were cleaned with distilled water and dried with acetone to remove the corrosion products. the experiment was carried out in duplicate to ensure reproducibility. the corrosion rate (cr) and inhibition efficiency (ie) were calculated using equations (1) and (2) 1 2cr w ww at at    (1) 1 2 1 cr cr / % 100 cr ie        (2) where w1 is the mild steel weight before immersion, w2 is the mild steel weight after immersion, a is the surface area of the coupon in cm2, t is the period of immersion in h, ie / % is inhibition efficiency, cr1 is corrosion rate of mild steel coupons in the absence of inhibitors and cr2 is the corrosion rate of mild steel coupons in the presence of concentration of inhibitors. surface examination the surface features of the mild steel coupons were examined with sem before and after exposure to hcl solution for 360 h (15 days) in the presence and absence of inhibitor, respectively. results and discussion phytochemical screening and inhibitive ability of vae the phytochemical screening of vae indicates compounds such as tannins, saponins and alkaloids [27]. the weight percents of these compounds were reported by ayeni and co-workers [24]. the assessment of the inhibitive property of vae was investigated through the weight loss technique. j. electrochem. sci. eng. 7(3) (2017) 145-152 inhibitory action of vae on the corrosion 148 figure 1 shows the corrosion rate (g/cm2 h) of mild steel in 1m hcl in the presence and absence of various concentrations of inhibitor with exposure time (days). the corrosion rate was found to decrease with increasing concentration of vae in the corrosion medium. the improved resistance to corrosion is due to adsorption of the constituents of the extract on the surface of the mild steel [28]. moreover, a significant retardation in the rate of corrosion can be observed after 288 h (12 days) and the effect is more pronounced with the addition 0.8 g/l of vae. the corrosion rate diminished due to the increased thickness of the protective oxide layer. direct relationship between inhibition efficiency and exposure time varies for different vae concentrations as shown in figure 2. the inhibitor efficiency increases with increased exposure time. the highest efficiency of 78.12 % was achieved in the corrosion medium containing 0.6 g l-1 vae after 12 days of exposure. irrespective of the amount of vae in the acidic medium, its efficiency began to decline after 6 days. figure 1. variation of the corrosion rate with exposure time figure 2. variation of inhibition efficiency with exposure time adsorption isotherms adsorption is usually modeled by langmuir, frumkin, freundlich, flory-huggins, temkin or bockris-swinkles adsorption isotherms [30]. here, equation (3) was used to calculate degree of surface coverage () for the different concentrations of vae. in this study, langmuir model was adopted to determine the adsorption isotherm according to equation (4). ie = × 100 (3) 1 ads c c k   (4) in equation (4), c is inhibitor concentration and kads is equilibrium constant of the inhibitor adsorption. to determine kads, the value of c/ was plotted against c giving the linear line with the root mean square value of 0.998 (figure 3). according to the equation (4), the inverse of the intercept gives the equilibrium adsorption constant, kads. the inhibition process may take place through either chemical adsorption or physical adsorption of the inhibitor’s molecules [15]. to ascertain this and according to other researchers’ reports [31-32], kads can be related to the adsorption free energy (∆gads) using the equation (5): 0.00 0.05 0.10 0.15 0.20 0.25 3 6 9 12 15 c o rr o si o n r a te , g /c m 2 h exposure time, days blank 0.2 g/l 0.4 g/l 0.6 g/l 0.8 g/l 0 20 40 60 80 100 3 6 9 12 15 in h ib it io n e ff ic ie n cy , % exposure time, days 0.2g/l 0.4g/l 0.6g/l 0.8g/l o. olawale et al. j. electrochem. sci. eng. 7(3) (2017) 145-152 doi:10.5599/jese.353 149 ads ads 1 exp 55.5 g k rt             (5) in the equation (5), 55.5 is water concentration, ∆gads is free energy of adsorption, r is the universal gas constant and t is temperature. figure 3. langmuir adsorption plot for mild steel in 1 m hcl containing different concentrations of vae at 30 oc table 2 shows the langmuir adsorption parameters obtained for the vae at 30 °c table 2. some parameters from langmuir isotherm methods temperature ∆gads/ kj mol-1 slope kads / l g-1 r2 weight loss 30 oc -18.349 0.257 26.247 0.998 r2= correlation coefficient free energy of adsorption, ∆gads, obtained from the equation (5) shows a negative value suggesting that adsorption process of the vae was spontaneous. according to awe et al. [19], if ∆gads was around 20 kj/mol or less, the adsorption is consistent with physisorption, while for ∆gads around 40 kj/mol or higher, the adsorption is consistent with chemisorption mechanism. hence, it seems that vae adsorption process in the present corrosive medium is physisorption. this is in full agreement with observations of other authors [18,28,30]. the strength of the adsorption bond depends upon the electron density of the functional group donor atom and the polarizability of the group [33]. the replacement of h atom attached to the c in the ring by a substituent group (nh2, oh, nh, no2, cho, or cooh) improves inhibition. ft-ir spectra shown in figures 4 and 5 are plots of percentage of intensity (relative abundance of each functional group) and the frequency values given in cm-1, depicting the surface coverage of each functional group in the inhibition process. thus, the peak at 2113 cm-1 in figure 5, which has already been attributed to hydroxyl (oh) group [34], confirms the presence of large amount of saponin compound in vae. the inhibitive ability of inhibitors depends greatly on their phytochemical constituents. compounds such as alkaloids, saponins, tannins and flavonoids have been reported to have inhibitory property [27,29]. for example, the presence of tannins may lead to the formation of passivating layer of tannates on the carbon steel surfaces. in addition, oh groups around the molecules can form strong complexes which block the generated micro anodes formed in the contact with corrosion medium [29]. j. electrochem. sci. eng. 7(3) (2017) 145-152 inhibitory action of vae on the corrosion 150 figure 4. ft-ir spectra of mild steel immersed in 1 m hcl without vae inhibitor figure 5. ft-ir spectra of mild steel immersed in 1 m hcl containing 0.8 g/l of vae inhibitor surface morphology examination to justify the observations, the surface morphology of the corroded samples in the presence and absence of vae was examined. figure 6 shows that the mild steel sample immersed in 0.1 m hcl without inhibitor corroded more than the mild steel sample exposed to degradation in the presence of vae inhibitor. figure 6. sem images of mild steel coupon in 1 m hcl solution after 360 h of total immersion in absence (a) and presence of 0.8 g/l vae inhibitor (b) 20 µm 20 µm o. olawale et al. j. electrochem. sci. eng. 7(3) (2017) 145-152 doi:10.5599/jese.353 151 the white patches on the surface of the samples represent the protective oxide. typically, its formation was partly dependent on the inhibitor chemical composition and molecular structure, and the substrate charged surface (i.e. the oxide affinity for the metal surface) [35]. conclusion in agreement with typical characteristic of all organic inhibitors, vae shows an outstanding ability to provide improved corrosion resistance of mild steel in acidic medium (hcl). the inhibitive ability of vae depends on protective oxide layer formed at the mild steel surface by adsorption mechanism due to the presence of tannin and saponin. the adsorption parameters obeyed the langmuir model. the value of adsorption free energy implies a spontaneous process consistent with physical adsorption mechanism. consequently, vae can be recommended for use in industrial processes to reduce metal dissolution in acidic media. moreover, being a cheap, non-toxic and eco-friendly compound, vae can easily substitute most of the inorganic synthetic inhibitors. references [1] p. neha, a. shruti, s. pallav, chinese journal of engineering. doi.org/10.1155/2013/748186 [2] s. bhat, v.k. sharma, s. thomas, p.f. anto, s.k. singh, material performance 50 (2011) 50-53. 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[35] a. singh, s. eno, e. e. ebenso, m. a. quraishi, international journal of corrosion (2012) 1-20, doi:10.1155/2012/897430 ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) electrochemical behaviour of pes ionomer and pt-free catalyst for pemfcs doi: 10.5599/jese.2013.0035 115 j. electrochem. sci. eng. 3(3) (2013) 115-123; doi: 10.5599/jese.2013.0035 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behaviour of pes ionomer and pt-free catalyst for pemfcs stefania giordano, mariangela longhi, leonardo formaro, hermes farina and giuseppe di silvestro dipartimento di chimica, università degli studi di milano, via golgi, 19, 20133, milano, italy corresponding author: e-mail: stefania.giordano@unimi.it; tel.: +39-50314226; fax: +39-50314300 received: january 5, 2013; revised: april 10, 2013; published: june 12, 2013 abstract proton exchange membrane fuel cells (pemfcs) represent promising technologies to the world economy, with many applications and low environmental impact. a most important aspect concerning their widespread implementation is the cost of polymeric membranes, typically perfluorinated membranes and platinum-based catalytic electrode materials, all of which are necessary to promote electrode reactions, thus increasing fuel cell energy efficiency. in this work, we present some data about non-fluorinated polyetheresulphone (pes) membranes and pt-free catalysts, as possible substitutes of the above materials. their electrochemical behaviour in oxygen reduction reaction in acidic media are investigated and compared with available reference materials. keywords orr, pt-free catalyst, pes, pemfc. introduction polymer electrolyte membrane fuel cells (pemfcs) have achieved significant progress over the past few decades. they are considered one of the most promising fuel cell technologies for both stationary and mobile applications owing to their high energy efficiency, convenient operation, and environmentally friendly characteristics. the main objective in fuel cell technologies is to develop low cost, high-performance and durable materials [1-6]. at present, platinum is the best cathode catalyst for oxygen reduction reaction (orr) in pemfcs; however, because of the scarcity and cost of the metal, there is a strong effort to find alternative metals or alloys with similar activity [7-10]. promising advances have been made with new composites (non-precious metals/heteroatomic polymers), pyrolysed metal porphyrins (cobalt or iron porphyrins viewed as the most promising precursors) or bio-inspired materials. as they are capable of combining high oxygen-reduction activity with good performance, these materials appear viable alternative catalysts for orr [11-17]. in pemfc, catalysts are commonly supported on proton conductive http://www.jese-online.org/ mailto:stefania.giordano@unimi.it j. electrochem. sci. eng. 3(3) (2013) 115-123 pes ionomer and pt-free catalyst for pemfcs 116 membranes. the most common material of this kind is dupont’s nafion®, a perfluorinated sulphonic acid polymer with many noteworthy features: good conductivity up to 80°c, which decreases beyond 80°c by dehydration; however, this also has a high cost. extensive research has been done to produce cheaper membranes to replace nafion®. new electrolyte membranes obtained by grafting with styrene and sulphonic acid look promising and further development is underway to improve their performance [18-20]. considering the above mentioned critical aspects, in this work we are looking for new materials for pemfcs applications, i.e. non-fluorinated polymeric membranes and platinum-free catalysts. as possible nafion substitutes, we synthesised non-fluorinated polyetheresulphones (pes) with different degrees of sulphonation. polymers were obtained starting from different ratios of sulphonated and non-sulphonated co-monomers. varying the monomers ratio it was possible to obtain polymers with different values of ion exchange capacity, swelling, hydrodynamic volume and rigidity. pes membranes have been characterised by thermogravimetric analysis (tga) and differential scanning calorimetry (dsc) in order to study thermal stability and water retention. on the side of pt-free catalysts, the one presented below was synthesised by a sugar pyrolysis route in the presence of a nitrogen precursor and a non-precious transition metal. thiamine was chosen as the precursor because, besides nitrogen, it also contains sulphur that may be a useful carbondoping element [21]. iron was used as the transition metal. to increase catalyst surface area and to control porosity in the mesoporosity range, a hard template method was adopted using a silica gel in water [22]. catalyst surface area and porosity were determined by bet and bjh theory, respectively. the electrochemical behaviour of the pes membrane and thiamine catalyst in oxygen reduction (orr) conditions were investigated and compared to reference materials, i.e. nafion 1100 and ec-20 catalyst (20 % pt dispersed on vulcan xc72r). an orr study was carried on in acidic media by cyclic voltammetry using a rotating disk electrode (rde). experimental materials glucose, thiamine, fe(ii) acetate, glacial acetic acid, sodium hydroxide, silica (silica gel 60 hr), hcl (37 %), h2so4 (95-97 %), ethanol, n-methylpyrrolidone (nmp), dimethyl acetamide (dmac), and nafion® 1100 (5 wt. % suspension) were purchased from aldrich and used as received. hclo4 and k2co3, dimethyl sulphoxide (dmso) were obtained from fluka and carlo erba, respectively. high purity water from a milliq system (millipore) was used. nitrogen and oxygen (5.5 and 5 nines respectively) were purchased from sapio. catalyst synthesis the pt-based catalyst ec-20 was used as received. home-made catalyst carbon was obtained by the following procedure. thiamine was added to a nearly saturated glucose-in-water solution (1.68 mol l -1 ) in a 1:10 molar ratio. thiamine dissolution was aided by equimolar glacial acetic acid. thereafter, acetate iron salt was added (0.96 wt. % on total amount of non-water reactants). ten millilitres of the solution was stirred with 4.3 g of silica in order to form a gel. then, the suspension was loaded in a quartz reactor, degassed with nitrogen and inserted in a preheated vertical oven at 600°c for one hour to carbonise the precursors as fast as possible. silica was removed in 3 m boiling sodium hydroxide followed by repeated carbon washing/filtering. products were dried in nitrogen (100°c, 24 hours) and finely ground. materials were heat-activated in a second step at 900 °c (ramping at 6°c min -1 , three hours standing) under constant nitrogen flow. s. giordano et al. j. electrochem. sci. eng. 3(3) (2013) 115-123 doi: 10.5599/jese.2013.0035 117 catalyst specific surface area and porosity surface area and porosity were determined using a tristar® ii 3020 apparatus (micromeritics instrument corporation system). before measurement, samples were outgassed at 150 °c for 4 hours under nitrogen in a flowprep 060 degas system. surface area was determined by low temperature bet (brunauer, emmett and teller) n2 adsorption. pore size and pore size distribution were calculated by the barrett, joyner, halenda (bjh) method. polyetheresulphones (pes) synthesis pes copolymers were synthesised by a co-polycondensation method in nmp solution at high temperatures in the presence of k2co3. different ratios of sulphonated and non-sulphonated comonomers were used. pes membranes were prepared by casting from dmac solutions. pes thermal analysis thermal stability and pes water retention were determined by tga (using a perkin elmer tga 400 system, t = 25-900 °c, 20 °c min -1 ) and dsc (using a mettler toledo dsc 1 system, t = 25-400°c, 20 °c min –1 ). data were recorded in n2. similar data were recorded for nafion 1100. electrochemical characterization before measurement, all glassware was first washed in hcl (37 %), then h2so4 (95-97 %) and finally rinsed with milliq water. the working electrode was a rotating disk electrode (rde-edi 101 radiometer) with a glassy carbon tip (cross-sectional area of 0.07 cm 2 ) embedded in a teflon sheath. before use, the carbon tip was gently cleaned with soft sandpaper, polished with diamond powder (aldrich) and finally degreased with ethanol. the thin film rotating disk electrode method (tf-rde) was used to immobilise small catalyst amounts onto the polished carbon tip. to improve reproducibility the electrode was initially conditioned by cycling in o2 saturated solution for 50 min (10 mv s -1 without electrode rotation) in the potential range e = –0.275/+0.800 v vs. ag/agcl. measurements were thereafter recorded in a range of rotation rates (300–1600 rpm) at 5 mv s -1 . when performing measurements on pt-containing-materials, a pt counter-electrode was used (0.6 cm 2 ); a glassy carbon rod (5.5 cm 2 ) was used instead for pt-free materials. i/e recordings were obtained by means of an amel 7050 potentiostat in a standard three-electrode electrochemical cell with an ag/agcl external reference electrode in 3 m nacl. all potentials are reported on the normal hydrogen electrode scale (nhe). measurements were carried out in 0.1m hclo4 solution at room temperature. the electrode preparation consisted of two successive deposition steps: the first with a catalyst suspension in water (ink) and the second with an ionomer solution as catalyst binder. in the first step, 10 mg of catalyst was dispersed in 1 ml water and sonicated for 30 min; 7 µl of this mixture was pipetted onto the electrode tip and dried in a bottom-up position under a tungsten lamp. in the second step a few µl of an ionomer solution (see details below) were deposited onto the catalyst layer. this operation was found to be critical, especially in the case of the homemade pes ionomer which, besides being insoluble in water, was also found to be less effective in “gluing” the catalyst layer onto the rde tip. because of solubility, organic solvents had to be used for pes; these were also good wetting agents for the rde teflon sheath. this feature is unfortunate because it may hamper the accurate determination of the ionomer mass per catalyst surface area, depending on the more or less effective wetting by ionomer solutions of the teflon sheath that surrounds the graphite tip. therefore, different ionomer deposition procedures were adopted depending on the ionomer used. for nafion, deposition was straightforward because water is the j. electrochem. sci. eng. 3(3) (2013) 115-123 pes ionomer and pt-free catalyst for pemfcs 118 nafion suspension solvent and does not wet teflon. for pes, several solvents were tested (dmac, nmp, dmso). among them nmp and dmso were found to be suitable; for dmac a compromise had to be obtained by mixing dmac with water in various ratios. detailed ionomer deposition conditions were as follows: for nafion, 7 l of a nafion solution in water (3.5 10 -4 g ml -1 ) was deposited on the electrode tip giving rise to a final nafion loading of 3.51 10 -5 g cm -2 ; for pes, 4 l of a pes solution (10 -2 g ml -1 ) was used with a final pes electrode loading of 5.7 10 -4 g cm -2 . the adopted pes solvent and solvent composition will be specified when reporting electrochemical results. results and discussion catalyst surface area and porosity after heat-activation at 900°c, the catalyst surface area was about 800 m 2 g -1 , which is lower than that after the first heat treatment at t = 600°c. fractional pore volume distributions were found to be essentially independent of heating temperature. the pore size distribution shows a maximum (70 %) at about 40-50 nm; however, there was a sizeable presence of micropores (10 %). the high percentage of 40 nm mesopores is presumably useful to orr behaviour, providing easy diffusional access to the reactive catalyst surface and to the nafion ionomer units [23, 24]. pes and nafion thermal analysis tga: to increase the pes water storing capability, the virgin polymer was conditioned in hcl and hclo4 (compare the red and green curves in figure 1). it can be noted that the two acidic conditionings give indistinguishable results. figure 1 also reports the pes behaviour without acid conditioning (light blue curve). as seen in the figure, acidic conditioning definitely decreases the polymer water retention. figure 2 reports a comparison between pes and nafion 1100, with both of them hcl conditioned (red and blue curve, respectively). the most notable feature is the greater pes mass loss in the respect of nafion up to the crossing point of the curves, at t  480 °c. at higher temperatures, the nafion mass losses become much greater than the pes ones. in a further aspect, the pes mass loss up to t  100-120 °c, which is the upper project temperature for pemfcs operation, is greater than for nafion. at high temperatures, nafion behaviour can be interpreted by references to literature data. according to wilkie et al. [25], almeida and kawano [26, 27] fluorocarbon polymers exhibit high thermal stability and decompose by a first step (290–400 °c) that may be associated with a polymer desulphonation process, then by a second step (400–470 °c) related to side-chain decomposition and, finally, by a third one (470–560 °c), due to the ptfe backbone decomposition. in comparison, figure 2 shows similar thermal degradation on pes after the completion of water removal at t = 200-220 °c. at present, no details are available on pes decomposition processes. dsc: figure 3 shows some features of the virgin pes (see the black curve) and after acidic conditioning (the red curve). in the black curve, a rather weak endothermic maximum extends over a wide temperature range (t  50-170 °c, δh = -120 j g -1 ), followed at higher t by a second sharp endothermal peak that, by comparison with a pure dmac sample, is due to the removal of residual reaction solvent (dmac). the sample behaviour is strongly affected by acidic conditioning (red curve). as the most relevant feature, a stronger endothermic process appears in the abovementioned temperature range (t  50-170°c, with a sharp maximum at t = 110 °c, δh = -480 j g -1 ). by comparison with tga results in figure 1, the process is due to water removal. by the integration of figure 3 curves, the water amount released from the acid conditioned samples is s. giordano et al. j. electrochem. sci. eng. 3(3) (2013) 115-123 doi: 10.5599/jese.2013.0035 119 approximately four times greater than that from the unconditioned ones. at higher temperatures, the water removal is followed by a second, more diffuse endothermic process ending at t  200°c. in comparison, the nafion behaviour (see figure 4) is characterised by a sequence of comparatively weaker endothermal (δh = -120 j g -1 ) processes with a first maximum at t  100°c and a second, barely distinguishable one at t  180°c. by reference again to the tga results in figure 1, both of these processes are attributed to water removal. overall, from a pemfc operative viewpoint, the water amount retained in pes is favourably greater than in nafion and is, moreover, more thermally stable. figure 1. tga curves of different pes acidic treatment recorded at 20 °c min -1 from 25 to 900 °c in n2 atmosphere. (a) light blue curve is relative to nonconditioned pes; (b) red curve for hcl conditioned; (c) green curve for hclo4 conditioned. figure 2. tga curves obtained in the same conditions reported in fig. 1. (a) red curve is relative to pes membrane conditioned in hcl; (b) blue curve is relative to nafion membrane conditioned in hcl. figure 3. dsc curves recorded from 25 to 400 °c, heating rate of 20 °c min -1 in n2 atmosphere. (a) virgin pes is the black curve; (b) pes hcl conditioned is the red one. figure 4. dsc curves recorded in the same condition of figure 3. membranes are hcl conditioned: (a) nafion is the green curve; (b) pes is the red one. electrochemical characterisation the following results report net oxygen reduction currents, corrected for background residual currents recorded in n2. results from different working electrodes (catalysts, ionomers) are normalised with respect to the geometrical electrode surface area. nafion 1100 and ec-20 were used as external references to evaluate the homemade materials, pes and pt-free catalyst. relevant reference data in o2 saturated solution are reported in figure 5 for many rde rotation rates. an extended region of limiting current is present and depends on the j. electrochem. sci. eng. 3(3) (2013) 115-123 pes ionomer and pt-free catalyst for pemfcs 120 rde rotation rate. the onset potential is about 0.9 v vs. nhe, which is very near to the value of massive platinum [28]. by comparison, figure 6 reports orr data for ec-20 catalyst with a pes ionomer layer. for the steeper current increase with decreasing electrode potential, orr results obtained with nafion are clearly better than pes ones. this difference may be due to the greater loading of pes than nafion (see the experimental), or to a greater nafion acidity/conductivity in respect of pes. despite many attempts, we were unable to decrease the pes amounts due to increasing mechanical instability of the catalyst layer. as a further main feature, pes causes an apparent slope decrease of j/e plots in the mixed control region (e  0.9-0.7 v), and lower limiting current density values at more negative potentials. figure 7 reports koutecky-levich plots for the used nafion and pes ionomer onto the used pt catalyst (ec-20). to avoid graphical crowding, the figure is only based on results in the limiting current region (e = 0.535 v vs. nhe). figure 5. voltammetric data for ec-20 with nafion binder. rde rotation rates are shown in the figure label. (0.1 m hclo4; v = 5 mv s -1 ; t = 25 °c). figure 6. voltammetric data obtained for ec-20 with pes binder (dmso only). rde rotation rates are shown in the figure label. (0.1 m hclo4; v = 5 mv s -1 ; t = 25 °c). nafion data are interpolated by a straight line with an almost exactly zero intercept at the axes origin and a slope from which, also taking into account numerical uncertainties, the number of stoichiometric exchanged electrons, n, ranges from 3.6-3.8. by comparison, pes data give rise to a straight line with a similar slope, which is in accordance with a comparable kinetic hindrance from diffusion in solution. from this slope, n is 3.8; however, there is a greater intercept that, as mentioned above, might be due to the greater pes amount necessary. figure 8 shows the tafel plots of ec-20/nafion and ec-20/pes electrodes obtained from koutecky-levich plots for many potentials of the investigated range. for nafion results, a first, extended tafel region, with a slope of ca. -60 mv dec -1 , begins at somewhat more cathodic potentials than the orr onset and ends at e 0.83 v. a second, much shorter, tafel section with a higher slope (ca. -120 mv dec -1 ) can be specified, although with some difficulty, at more cathodic potentials. these features are in accordance with data from the literature [27-30]. the pes behaviour is characterised by a curved tafel plot whose slope continuously increases with increasing over-potential. figure 9 shows results for ec-20 electrodes with a pes binder layer deposited using various solvents. as mentioned in the experimental section, many attempts were necessary to balance a good pes solubility in a given solvent with low affinity of the resulting solution for the rde teflon sheath. as shown in the figure, results outline a single response in which the behaviour of each s. giordano et al. j. electrochem. sci. eng. 3(3) (2013) 115-123 doi: 10.5599/jese.2013.0035 121 electrode closely superimposes on the others. this uniformity in behaviour presumably outlines a range of similar interactions between the pes ionomer, solvent and catalyst. figure 7. koutecky-levich plots for ec-20 catalyst orr currents at 0.535 v vs. nhe obtained (a) from figure 5 for nafion (blue line); (b) from figure 6 for pes membrane (red one). figure 8. tafel plots for ec-20 catalyst obtained (a) from figure 5 for nafion (blue curve); (b) from figure 6 for pes membrane (red one). figure 9. voltammetric data obtained for ec-20 with different pes/solvent deposition: (a) yellow curve, dmac/water 1:2 vol; (b) blue, nmp; (c) red, dmso. (0.1 m hclo4; v = 5 mv s -1 ; t = 25 °c; ω = 1600 rpm) orr results for the thiamine sample catalyst with a nafion binder layer are reported in figure 10 for various rde rotation rates. although not exciting, this behaviour is acceptable overall, with a noteworthy and unfavourable shift of the curves towards more negative potentials in respect of the external reference ec-20. figure 11 reports the behaviour of the thiamine sample with a pes, instead of nafion, binder. a dramatic worsening in behaviour is observed in comparison to figure 10. this becomes more evident by comparison with a similar result for the pes/ec-20 electrode assembly (the relevant curve of figure 11 is taken from figure 6). in the last figure, pes in itself or the pes/solvent mixture used behaves as a real “killer” for the homemade catalyst, while still affording an acceptable response when used on ec-20, even though less satisfactory than nafion. this brings to light a complexity of the many interactions that may occur among separate components necessarily involved in building up a final, well behaving pemfcs electrode catalytic assembly. j. electrochem. sci. eng. 3(3) (2013) 115-123 pes ionomer and pt-free catalyst for pemfcs 122 figure 10. voltammetric data obtained for thiamine catalyst with nafion. rde rotation rates are shown in the figure label. (0.1 m hclo4; v= 5 mv s -1 ; t = 25 °c). figure 11. voltammetric data obtained for pes membrane (in dmso only) for (a) ec-20 (red curve) and (b) thiamine catalyst (black curve). (0.1 m hclo4; v = 5 mv s -1 ; t = 25 °c; ω = 1600 rpm). conclusions the present homemade catalyst, synthesised by the pyrolysis of sugar thiamine mixtures, is characterised by unsatisfactory orr features. it is, however, an element of the catalyst family that is currently under investigation as a substitute for pt. similarly, the reported pes sample belongs to a polymer class that is being investigated for favourable water retention and thermal stability. improvements may likely concern ionic conductivity in pemfc operational conditions. it can be also mentioned that the present homemade catalyst and ionomer afford acceptable results when separately tested with one or another reference material. their orr response fails completely when used together. this shows that, in view of real pemfc applications, many mixed interactions have to be taken into account and optimised in detail. acknowledgements: financial support from cariplo foundation (project 2010-0588 “non fluorinated polymeric 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electrochem. 24 (1994) 863-869 [30] n.m. markovic, h.a. gasteiger, p.n. ross, j. phys. chem. 99 (1995) 3411-3415 [31] d. van der vliet, d. strmcnik, c. wang, v. stamenkovic, n.m. markovic, m.t.m. koper, j. electroanal. chem. 647 (2010) 29-34 © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ decolorization of reactive violet 5 dye in textile wastewater by electrocoagulation doi:10.5599/jese.223 67 j. electrochem. sci. eng. 6(1) (2016) 67-75; doi: 10.5599/jese.223 open access : : issn 1847-9286 www.jese-online.org original scientific paper decolorization of reactive violet 5 dye in textile wastewater by electrocoagulation borislav n. malinovic , miomir g. pavlovic* university of banja luka, faculty of technology, stepe stepanovica 73, 78000 banja luka, bosnia and herzegovina *university of east sarajevo, faculty of technology, karakaj bb, 75400 zvornik, bosnia and herzegovina corresponding author: borislav.malinovic@unibl.rs; tel.: +387-51-434-357; fax: +387-51-465-032 received: september 29, 2015; revised: march 8, 2016; accepted: april 5, 2016 abstract the textile dyeing industry consumes large quantities of water and produces large volumes of wastewater from different steps in the dyeing and finishing processes. wastewater from printing and dyeing units is often rich in color, containing residues of reactive dyes and chemicals, such as complex components. this study investigates the decolorization of synthetic dye wastewater containing textile dye reactive violet 5 (rv5) by electrocoagulation. a laboratory batch reactor was used to investigate the effect of various operating parameters using aluminium (al), iron (fe) and stainless steel (ss) anode. the effect of dye concentration, current density, supporting electrolyte, supporting electrolyte concentration, electrolysis duration, and material of anode of the systems were evaluated. color removal efficiency was 22, 91.5 and 99.8 % in 15 minutes using al, fe and ss anode, respectively (j = 10 ma/cm 2 , cnacl = 0.171 m). keywords electrochemical removal; reactive dye, rv5 introduction production processes of the textile industry require a huge quantity of water and chemicals, which causes emergence of a huge quantity of wastewater containing acids, bases, dissolved solids, toxic substances and different dyes that present even in small concentrations have to be removed. traditional methods for the treatment of textile industry wastewater represent a combination of biological, physical and chemical methods [1,2]. biological treatment of dyeing wastewater is cheaper than other methods, but less efficient in decolorization due to toxicity of http://www.jese-online.org/ mailto:borislav.malinovic@unibl.rs j. electrochem. sci. eng. 6(1) (2016) 67-75 decolorization of violet 5 dye by electrocoagulation 68 wastewater and requires the system of aeration. although the dyestuff and colored substances in wastewater can be effectively destroyed by an advanced chemical oxidation such as uv/h2o2, o3 [3,4] and adsorption using activated carbon [5,6], the costs of these methods are relatively high for an economically sustainable treatment of the textile industry wastewater. recently we could note an increased interest in the development of electrochemical methods for removal of toxic organic substances present in wastewater [7-12]. such methods were also successfully tested in different wastewater containing dye pollutants [13-17]. nowadays, electrochemical technologies for wastewater processing have reached such a state that they are not only comparable with other technologies in terms of cost, but also are more efficient and more compact. in some situations, electrochemical technologies may be an indispensable step in treating wastewaters containing refractory pollutants [8]. electrochemical treatment is actually a combination of many processes like anodic oxidation, electrocoagulation and electroflocculation, occurring in the electrochemical reactor. the reactor consists of electrodes (cathodes and anodes), which may be sacrificial or non-sacrificial. iron and aluminum electrodes are sacrificial electrodes. the use of electrodes depends on electrochemical reactions taking place in the reactor. the desired electrodes for electrocoagulation and electroflocculation are so-called sacrificial electrodes. non-sacrificial electrodes are appropriate for anodic oxidation. the aim of this study is to examine electrochemical removal of the textile azo dye reactive violet 5 (c20h16n3o15s4 × 3na) in aqueous solution using aluminium (al), iron (fe) and stainless steel (ss) as electrode material. reactive violet 5 (rv5) is a synthetic organic dye whose structural formula is presented in figure 1. this dye is used as a dye for coloring cotton, wool, silk and polyamide textiles. it is also used in antifreeze. figure 1. structural formula of reactive violet 5 dye many microorganisms are capable of azo dyes decolorization, including gram-positive and gram-negative bacteria [15-17] and algae [18,19]. a bacterial consortium rvm 11.1 was selected on the basis of a rapid reactive violet 5 dye decolorization. the consortium exhibited 94 % decolorization ability within 37 h under a wide ph range from 6.5 to 8.5 and temperature ranging from 25 to 40 °c. the bacterial consortium was able to grow and decolorize rv5 under static conditions in the presence of glucose and yeast extract and also showed an ability to decolorize in the presence of starch in place of glucose. maximum decolorization efficiency was observed at 200 ppm (mg/l) concentration of rv5 [17]. bacterial mixed culture sb4 proved to be proficient in complete decolorization of azo dye reactive violet 5r, which was developed through the culture enrichment technique. sb4 grows well in the minimal medium containing a low amount of glucose and yeast extract (ye) (1 g/l) and the dye of 200 mg/l concentration gets decolorized for rv5 within 18 h under static conditions. mixed culture sb4 decolorizes a wide range of azo dyes and the maximum rate of decolorization was observed at 37 °c and ph 7.0 [20]. bioprocess for biodegradation and bioremediation of reactive violet 5r (rv5) manufacturing industry's wastewater in the laboratory scale was examined in the designed “down flow fixed film” b. n. malinovic et al. j. electrochem. sci. eng. 6(1) (2016) 67-75 doi:10.5599/jese.223 69 bioreactor (dffr), packed with furnace charcoal as a support material. during the batch and continuous operation of dffr, more than 95 % degradation, 88 % cod reduction and 99 % copper remediation was obtained in less than 8 h of contact time. the continuous mode treatment enables degradation of more than 2500 mg dye in only 1 h of the contact time. the addition of 0.25 % peptone enhanced biodegradation rate by more than three times [21]. efficiency of the photocatalytic degradation of rv5 by using titanium dioxide was 90 % after 20 min of irradiation and reached nearly 100 % after 80 min under the condition of ph 4 and temperature of 25 °c. decolorization rate typically followed the first-order reaction, and increased significantly with increasing the amount of photocatalyst, ph, as well as light intensity [22]. experimental commercially available reactive dye, tecofix brilliant violet vs-5r (reactive violet 5), from textilcolor ag, switzerland, was used in experimental studies. distilled water was used to prepare the desired concentration of dyestuff solution, and the electrode material used was aluminium (en 1050; 99.5 %), steel (c.0147; max. 0.08 % c, max. 0.12 % cr, max. 0.45 % mn, max. 0.60 % si) and stainless steel (en 1.4301/aisi 304). electrochemical reactor was made of polypropylene (height 95 mm, diameter 71 mm), with the volume of 250 cm 3 , at the constant stirring speed (400 rpm) and with the electrodes of the same size placed in it. both electrodes are made of metal plates (the total effective electrode area was 22.2 cm 2 ) and the spacing between electrodes was 30 mm. the electrodes were connected to a digital dc power supply (atten, aps3005si; 30v, 5a) with potentiostatic or galvanostatic operational options. all experiments were performed at the constant temperature of 25 °c with 200 cm 3 volume of the wastewater solutions. the current density was adjusted to a desired value and the treatment was started. at the end of electrochemical treatment, the solution was filtered and then analyzed. before each run, electrodes were washed with 1,1,2-trichloroethene to remove surface grease, and the impurities on the electrode surfaces were removed by dipping for 5 min in a diluted solution of hcl or naoh. total dissolved solids (tds), ph-value and conductivity were determined according to the standard methods for examination of water and wastewater [22]. the ph, conductivity and tds were measured by a multi meter (consort c861). the amount of dye rv5 in aqueous solution was measured by using uv-vis spectrophotometer (perkin elmer, lambda 25), and the ir spectrum was measured by ftir (bruker, tensor 27). figure 2 and 3 presents adsorption and ir spectra of rv5. figure 2. adsorption spectra of rv5 j. electrochem. sci. eng. 6(1) (2016) 67-75 decolorization of violet 5 dye by electrocoagulation 70 figure 3. ir spectra of rv5 results and discussion the aqueous solution of rv5 dye, with the mass concentration rv5=100 mg/l, was used for the experimental study of electrochemical removal of organic dyes. the results of electrochemical treatment are expressed through color removal efficiency ed expressed as a percentage and calculated by the following formula: 100%/ i fi d    e (1) where i and f are initial and final dye concentrations expressed in mg/l. decolorization efficiency (ed) was examined in terms of current density, supporting electrolyte, supporting electrolyte concentration, electrolysis time and electrode material in order to determine the optimum operating conditions for maximum decolorization efficiency of the reactive dye. the values of ph, conductivity and tds of samples are presented in table 1. table 1. ph values, conductivity and tds of samples (uzx) samples ph , s cm -1 tds, mg l -1 uz1 (100 mg/l rv5) 5.8 123 73 uz2 (100 mg/l rv5; 0.5 g/l nacl) 5.8 1084 645 uz3 (100 mg/l rv5; 1.0 g/l nacl) 5.8 2030 1210 uz4 (100 mg/l rv5; 1.5 g/l nacl) 5.8 2990 1790 uz5 (100 mg/l rv5; 1.0 g/l na2co3) 11.2 1767 1056 uz6 (100 mg/l rv5; 1.0 g/l na2so4) 5.8 1537 916 figure 4 presents decolorization efficiency depending on the material of electrodes (anode:cathode) with the presence of the supporting electrolyte (nacl) of the concentration nacl = 1.0 g/l. figure 4 presents the effect of choice of electrode material under the same conditions of the treatment, where we notice an advantage of steel (fe:ss) for decolorization efficiency (>99 %) as b. n. malinovic et al. j. electrochem. sci. eng. 6(1) (2016) 67-75 doi:10.5599/jese.223 71 anode compared with stainless steel (ss:ss) and aluminium (al:ss). in all cases cathode is made of stainless steel. figure 4. effect of electrode material on the efficiency of colour removal (j = 10 ma/cm 2 , nacl = 1 g/l) different anode material causes different reaction mechanisms (reaction order). in the case of iron and stainless steel it is a first-order reaction, and the reaction at the anode of aluminum is a zero-order (figure 5.). figure 5. effect of concentration (rv5) on time (j = 10 ma/cm 2 , nacl = 1 g/l) the effect of the supporting electrolyte under the same conditions of the treatment is presented in figure 6 where we notice a great advantage of nacl compared with na2co3 and na2so4 that have almost identical results. j. electrochem. sci. eng. 6(1) (2016) 67-75 decolorization of violet 5 dye by electrocoagulation 72 figure 6. effect of supporting electrolyte on the efficiency of colour removal (j = 10 ma/cm 2 , s.e. = 1 g/l) decolorization efficiency without the presence of a supporting electrolyte, due to small conductivity and impossibility to apply the currents of higher density, in the above described reactor, is very low (10.5 % in 15 min.), while the efficiency significantly increases (91.5 % for j = 10 a/cm 2 , t = 15 min.) with the increased concentration of the supporting electrolyte (nacl) up to 1 g/l (0,171 mol/l), which represents an optimal concentration of the supporting electrolyte. values of the concentration above 1 g/l nacl insignificantly increase decolorization efficiency (figure 7.). figure 7. effect of concentration of nacl on the efficiency of colour removal (j = 10 ma/cm 2 ) figure 8 presents increase of decolorization efficiency in 30 minutes of electrolysis depending on the concentration of the supporting electrolyte and time of electrolysis (j = 15 a/cm 2 ). maximum efficiency of 95.8 % was realized in the conditions of j = 15 a/cm 2 , nacl = 1.5 g/l, and 95.7 % in the conditions of j = 15 a/cm 2 , nacl = 1.0 g/l in 30 minutes of electrolysis. b. n. malinovic et al. j. electrochem. sci. eng. 6(1) (2016) 67-75 doi:10.5599/jese.223 73 figure 8. effect of concentration of nacl on the efficiency of colour removal (j = 15 ma/cm 2 ) the decolorization efficiency depends on the current density as can be seen the figure 9. maximal efficiency of 97 % was reached with the current density j = 25 a/cm 2 in 15 minutes of electrolysis (nacl = 1.5 g/l). lower values of current density j = 10 a/cm 2 (72 92.5 %), j = 15 a/cm 2 (89.5 91.5 %) and j = 20 a/cm 2 (92 %) also reached satisfactory values of efficiency. we can conclude from the mentioned values that the optimal current density is j = 10 a/cm 2 with the presence of the supporting electrolyte of the minimal concentration nacl = 1.0 g/l. figure 9. effect of current density (10-25 a/cm 2 ) on the efficiency of colour removal (nacl = 1.5 g/l) the time of electrolysis depends on the desired decolorization efficiency. the electrolysis lasting for 5 minutes can achieve the efficiency of up to 89 %, and further increase of time insignificantly increases efficiency for bigger applied current densities, while for smaller current j. electrochem. sci. eng. 6(1) (2016) 67-75 decolorization of violet 5 dye by electrocoagulation 74 densities (j = 10 a/cm 2 ) time significantly affects decolorization efficiency. figure 10 presents adsorption spectra of samples before nad after treatment. figure 10. adsorption spectra of sample uz3 (rv5 = 100 mg/l ,nacl = 1.0 g/l, j = 10 a/cm 2 ) and samples ex after treatment (2,5, 5 and 15 min.) with fe:ss electrodes conclusions electrochemical treatment is one of the most effective techniques to remove color and organic pollutants from wastewater. decolorization efficiency by this electrochemical process was influenced by current density, supporting electrolyte, supporting 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[26] american public health association (apha), standard methods for examination of water and wastewater, 17 th ed., washington, dc, 1992. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {influence of operating temperature on the activation efficiency of li-ion cells with xli2mno3-(1-x)limn0.5ni0.5o2 electrodes:} https://dx.doi.org/10.5599/jese.1458 767 j. electrochem. sci. eng. 12(4) (2022) 767-776; https://dx.doi.org/10.5599/jese.1458 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of operating temperature on the activation efficiency of li-ion cells with xli2mno3-(1-x)limn0.5ni0.5o2 electrodes renny nazario-naveda1,, segundo rojas-flores2, moises gallozzo-cardenas3, luisa juárez-cortijo4 and luis angelats-silva5 1universidad autónoma del perú, lima, perú 2universidad señor de sipán, chiclayo, perú 3universidad césar vallejo, trujillo, perú 4universidad privada del norte, trujillo, perú 5universidad privada antenor orrego, trujillo, perú corresponding author: scored731@gmail.com; tel.: +51-951-344-316 received: july 15, 2022; accepted: september 3, 2022; published: september 12, 2022 abstract in this study, the effect of operating temperature at 55 °c on xli2mno3-(1-x)limn0.5ni0.5o2 electrodes during the charge/discharge process at different current densities was investigated. x-ray diffraction (xrd) and scanning electron microscopy (sem) were used for structural and morphological analysis of the fabricated cathode materials, while charge-discharge curves and differential capacity were used to study the electrochemical behavior. results confirm the formation of the structures with two phases associated with the components of the layered material. it was found that at 55 °c, a capacity higher than 357 mah g-1 could be achieved at a voltage of 2.5-4.8 v vs. li/li+, which was larger than the capacity achieved at room temperature. at 55 °c, a change in valence could be observed during charging and discharging due to the change in the position of the peaks associated with mn and ni, highlighting cathodic material with x = 0.5 as the material that retains the layered structure at this temperature. this work confirms the good performance of electrodes made with this material at elevated temperatures and gives a better understanding of its electrochemical behavior. keywords lithium-ion battery; cathode material; li-rich mn-ni oxide; operating temperature; specific capacity introduction in the 21st century, energy and the environment are fundamental issues for human survival and social development [1]. several industrial applications require rechargeable batteries working at elevated temperatures, such as the power supply of electric vehicles [2]. for such particular https://dx.doi.org/10.5599/jese.1458 https://dx.doi.org/10.5599/jese.1458 http://www.jese-online.org/ mailto:scored731@gmail.com j. electrochem. sci. eng. 12(4) (2022) 767-776 activation efficiency of li-ion cells 768 applications, batteries are exposed to an operating temperature range of 20 to 50 °c and should be able to undergo only moderate degradations at those temperatures. rechargeable lithium-ion batteries are good candidates for such applications [3-5]. many combinations of positive and negative electrodes have already been tested, graphite or li4ti5o12 materials for the negative electrode, and limn2o4, licoo2 or lifepo4 materials for the positive electrode [6,7]. however, in recent years there has been a growing interest in the layered xli2mno3–(1-x)limo2 (m = mn, ni) (lmo) oxides as positive electrode materials for lithium-ion batteries due to their good electrochemical properties and better thermal stability in the charged state [8,9]. these good properties are mainly related to the presence of ni at a divalent state and to a large amount of stable mn4+ cations playing a stabilizing role of the structure, especially in the charged state of batteries. also, the excess of li can be easily incorporated into the layered structure through the integration of li2mno3 in limo2, substituting transition metals m [10]. commercial lithium-ion batteries manufactured today are well known for their good performances at room temperature and temperatures below 50 °c, whereas a strong capacity fading is reported for storage and/or cycling at elevated temperatures of 50-90 °c [11]. several factors might limit the calendar life of the lithium-ion battery at these temperatures. it can be due to the electrode active material instability caused by the dissolution of transition metal in the electrolyte. this dissolution process does not involve changes in the bulk material but implies a degradation of the first surface layers of a positive electrode, which reduces surface diffusion pathways and leads to an increase in charge-transfer resistance [12,13]. changes in temperature directly affect the discharge performance and discharge capacity of a lithium-ion battery [13,14]. when the temperature increases, the internal resistance of the battery decreases, the electrochemical reaction rate slows up, the internal polarization resistance decreases rapidly, and the discharge capacity and discharge platform increase. high temperatures can accelerate the migration rate of lithium ions [14]. among all environmental factors, the temperature has the greatest impact on the charge and discharge performance of lithiumion batteries. currently, many investigations are oriented to the study of the performance of cells operated at higher temperatures. bodenes et al. [15] have found a good performance of lini1-x-ymnxcoyo2 cells when cycled at 85 °c compared to room temperature and attributed it to the formation and thickness of the passivation layer at the positive electrode surface. idemoto et al. [5] using 0.4li2mno3-0.6limn1/3ni1/3co1/3o2 electrodes operating at 60 °c, found a capacity higher than 280 mah g-1 [5]. also, lee et al. [16], using li[ni0.5co0.2mn0.3]o2 and li[ni0.6co0.2mn0.2], found a significant thermal stabilizing effect of vinylene carbonate as ni content increased, due to the formation of a thermally stable layer on the electrode surface. temperature is considered to be an important indicator that affects the capacity of lithium-ion batteries. therefore, it is of great significance to study the relationship between the capacity and operating temperature of lithium-ion batteries. that is why in this work, the aim was to evaluate the influence of elevated temperature operating conditions (55°c) on cells composed of lmo as the positive electrode and to compare the obtained results with those previously measured at 25 °c [17]. in addition, structural and morphological characterizations of prepared cathode materials were made by x-ray diffraction (xrd) and scanning electron microscopy (sem). r. nazario-naveda et al. j. electrochem. sci. eng. 12(4) (2022) 767-776 https://dx.doi.org/10.5599/jese.1458 769 experimental synthesis of xli2mno3-(1-x)lini0.5mn0.5o2 cathode material synthesis of the cathodic material was carried out following the co-precipitation method shown by nazario-naveda et al. [17]. 0.2 m of nickel (ii) sulphate hexahydrate [niso46h2o] (sigma-aldrich 99 %) and 0.2 m of manganese (ii) sulphate monohydrate [mnso4h2o] (sigma-aldrich 98 %) were separately dissolved in distilled water, and then mixed through 10 minutes at constant stirring. separately a solution of 1 m of sodium bicarbonate [nahco3] (sigma-aldrich 99 %) in distilled water was prepared. the first solution was slowly added drop by drop into the second one at constant magnetic stirring and a temperature of 60 °c. the mixed solution was kept at a ph of 8.5 in order to get the precipitated ni(1-x)/2mn(1+x)/2co3, and ph was controlled with drop-by-drop addition of ammonium hydroxide solution [nh4oh] (sigma aldrich 28-30 %). the obtained mixture was left for 12 hours at the same temperature and with constant magnetic stirring in order to get a complete reaction. after that, the mixture was filtered and washed three times with distilled water. the resulting wet powder was dried for 12 hours at 100 °c and then grounded with a mortar and pestle to get nickel manganese carbonate final precursor powder. the powder was mixed with a stoichiometric amount of lithium carbonate [li2co3] (strem chemicals 99.999 %) and grounded using a mortar and pestle. after intense grinding, the mixture was annealed at 950 °c for 12 hours, and for this purpose a 15.24 cm w x 15.24 cm d x 15.24 cm h kerr 666 furnace was used. after 12 hours, the calcined powder was quenched at room temperature to keep the oxidation states of transition metals at 4+ and 2+ for mn and ni, respectively. the calcined powder was ground again to obtain the xli2mno3-(1-x)lini0.5mn0.5o2 final powder. the quantities of niso46h2o and mnso4h2o were varied stoichiometrically depending on the x value of xli2mno3-(1-x)lini0.5mn0.5o2 final cathode material powder. electrochemical coin cell fabrication coin type cells (cr2032) were assembled inside the argon-filled mbraun glove box. cathode material was prepared using 80 wt.% of xli2mno3-(1-x)lini0.5mn0.5o2 powder, 10 wt.% of carbon black and 10 wt.% of polyvinylidene fluoride. a slurry of the mixed powders was made by using n-methyl-2-pyrrolidinone. the aluminum foil (alfa aesar 99.999 %) with 0.025 mm of thickness was covered by a slurry, and the coated aluminum foil was put in a furnace at 100 °c for 12 hours and cut in small circles. for the anode, li foil with 0.75 mm of thickness (sigma aldrich 99.9 %) was used, and the separator was a polypropylene membrane (celgard 2500) with 25 µm of thickness. lipf6 dissolved in ethylene carbonate (ec) and dimethyl carbonate (dmc) in 1:2 volume ratio was used as the electrolyte. characterization techniques x-ray diffraction (xrd) patterns of the cathode material powder were obtained using siemens d5000 x-ray diffractometer with cu kα radiation (0.15405 nm in the 2 angle range from 15 to 75° with a step of 0.02°. morphological properties of the material were studied using a scanning electron microscope (sem), and the analysis was conducted with jeol 7600 fesem system interfaced to a thermo-electron system microanalysis systems. samples were adhered to the top of the sample holder with double-sided conductive tape, and for better resolution, au coating was applied. the electrochemical measurements were evaluated at a 0.05, 0.10 and 0.15 c current rates between 2.5 4.7 v (1 c = 300 ma g-1) using a gamry instruments g/pc14 potentiostat system. the c-rate was calculated using the weight of the active material and the capacity of the electrode. https://dx.doi.org/10.5599/jese.1458 j. electrochem. sci. eng. 12(4) (2022) 767-776 activation efficiency of li-ion cells 770 results and discussion x-ray diffraction patterns of xli2mno3-(1-x)lini0.5mn0.5o2 (x = 0.3, 0.5 and 0.7) synthesized materials are presented in figure 1(a), which shows typical peaks in good agreement with the r3m space group related to the rhombohedral type structure for limo2 [18]. the low-intensity peaks between 20 and 30° show characteristic peaks of the monoclinic phase with space group c2/m associated with layered composite material li2mno3 [19,20]. xrd patterns confirm the superlattice phase corresponding to li2mno3, and it becomes more notorious when the fraction of li2mno3 layered material is incremented in xli2mno3–(1-x)limo2. the presence of small peaks (110), (020), (-111) and (021) is related to the existence of superstructure caused by the reordering of li and mn in the main structure, where lithium ions enter into the transition metal layers positions as expected [19,20,27]. furthermore, the presence of (006) and (012) peaks confirm the successful formation of a well crystalline layered structure with no spinal structure using the co-precipitation synthesis method for all values of x [21]. 2 / ° 2 / ° figure 1. x-ray diffraction patterns of xli2mno3–(1-x)lini0.5mn0.5o2 scanning electron microscopy (sem) was used to verify the morphology and topology of the synthesized material. micrographs of synthesized xli2mno3–(1-x)lini0.5mn0.5o2 (x = 0.3, 0.5 and 0.7) final powders are showed in figure 2. the primary particles are micrometric in size and concentrated in irregularly shaped agglomerations. the size of these agglomerations increases with the increase of x, and for x = 0.7, particles exceed a micrometer in length. micrographs also show free space between agglomerations giving a high material porosity, which is good for the ions and electrons' mobility [22-24]. relatively smaller primary particle agglomerates allow better charge/discharge capacity due to their higher trap density which shortens lithium diffusion [25]. eds spectra shown in figure 3 confirm the presence of respective elements in the composite materials, where intensity represents the proportions of the materials used to make the precursor materials. it is also observed that with increasing x, ni decreases and mn increases according to the stoichiometric proportions. also, the presence of oxygen is increased considerably due to the predominance of the li2mno3 layer. r. nazario-naveda et al. j. electrochem. sci. eng. 12(4) (2022) 767-776 https://dx.doi.org/10.5599/jese.1458 771 a b c figure 2. sem micrographs of cathode material powders: (a) 0.3li2mno3–0.7lini0.5mn0.5o2, (b) 0.5li2mno3– 0.5lini0.5mn0.5o2, and (c) 0.7li2mno3–0.3lini0.5mn0.5o2 figure 3. eds spectra of xli2mno3-(1-x)lini0.5mn0.5o2 cathode material powders https://dx.doi.org/10.5599/jese.1458 j. electrochem. sci. eng. 12(4) (2022) 767-776 activation efficiency of li-ion cells 772 the electrochemical behavior of the composite at 25 °c (room temperature) and 55 °c are presented in figures 4 and 5. previous investigations showed that at elevated temperatures, li2mno3 activation is more effective than at room temperature, showing capacity values over their theoretical ones [11]. this may be due to more activation of li ions in composite and less irreversible capacity, but the causes of this behavior are still under study. in this work, the coin cell was tested at different current densities at 55 °c after 20 cycles, i.e., after the first activation and when capacity is almost constant. a capacity, mah g-1 b c capacity, mah g-1 capacity, mah g-1 figure 4. charge-discharge profiles during 20th cycle at different c-rate of the lmo/li coin cells: (a) 0.3li2mno3–0.7lini0.5mn0.5o2, (b) 0.5li2mno3–0.5lini0.5mn0.5o2 and (c) 0.7li2mno3–0.3lini0.5mn0.5o2 cycled between 2 and 4.8 v at 25 and 55 °c. data at 25 °c are taken and adapted from ref. [17] results of charge-discharge for 0.3li2mno3–0.7lini0.5mn0.5o2 cathode material coin cells operating at different current densities are shown in figure 4a. at room temperature, the capacity is reduced from 150 mah g-1 (0.05c) to 120 mah g-1 (0.10c) and 100 mah g-1 (0.15c). there is a plateau voltage between 4 4.5 v corresponding to the li extraction from the limo2 component and the li extraction from the li2mno3 component starts from 4.5 v. at 55 °c, a longer activation plateau is observed. activation is done after 4.2 v and capacities due to the limo2 component are higher than those at room temperature. for 0.05 c, a capacity of 200 mah g-1 could be reached, which is higher than the theoretical value of 196 mah g-1. figure 4a also shows a spinel phase assigned plateau below 3.25 v at the discharge process of 0.3li2mno3-0.7lini0.5mn0.5o2 electrode. this change from layered to spinel phase transformation is responsible for reducing capacities at r. nazario-naveda et al. j. electrochem. sci. eng. 12(4) (2022) 767-776 https://dx.doi.org/10.5599/jese.1458 773 high cycle numbers due to the migration of transition metal ions to li sites without high disarrange of the main layered structure, which is one of the limitations of the prolonged operation of this material at high temperatures [26]. a b c figure 5. differential capacity plots during 20th cycle at different c-rate of the lmo/li coin cells (a) 0.3li2mno3–0.7lini0.5mn0.5o2, (b) 0.5li2mno3–0.5lini0.5mn0.5o2 and (c) 0.7li2mno3–0.3lini0.5mn0.5o2 cycled between 2 and 4.8 v at 25 and 55 °c. data at 25 °c are taken and adapted from ref. [17] figure 4b shows 0.5li2mno3–0.5lini0.5mn0.5o2 cathode material results. the two activation steps of this material are observed at room temperature, a first step (2-4.5 v) related to the li extraction from limo2 and a second step (4.5 v) due to the activation of the li2mno3 component. the achieved capacities were 210 mah g-1 (0.05 c), 190 mah g-1 (0.10 c), and 170 mah g-1 (0.15 c), which are significantly below theoretical values, revealing that not all li2mno3 material could be activated at this current density [18]. on the other hand, at 55 °c, a new li2mno3 activation is shown after 4.5 v, and capacity increased from 266 to 357 mah g-1, contributing thus with 91 mah g-1. this means that at room temperature, all li2mno3 component is not activated, the temperature increase contributes to its activation, which doesn't occur at higher current density. discharge capacity raised to 330 mah g-1, which is higher than found by other authors [15,16,26]. figure 4c shows results for x = 0.7 cathode materials. at room temperature, differential capacity values are very low, indicating an inefficient charge transfer process between the material and the electrolyte, possibly due to the size of the conglomerates that form the material [27]. the achieved capacities were 140 mah g-1 (0.05 c), 64 mah g-1 (0.10 c), and 57 mah g-1 (0.15 c). at 55 °c, an https://dx.doi.org/10.5599/jese.1458 j. electrochem. sci. eng. 12(4) (2022) 767-776 activation efficiency of li-ion cells 774 activation related to li extraction of the li2mno3 layer is observed, and a maximum capacity of 200 mah g-1 is reached at 0.05 c, despite no observed changes to the spinel structure as observed for x = 0.3. li2mno3 shows to be electrochemical active at high temperatures, possibly due to low impedance, making easy li+ intercalation [5]. figure 5 shows differential capacity plots of (a) 0.3li2mno3-0.7lini0.5mn0.5o2, (b) 0.5li2mno30.5lini0.5mn0.5o2 and (c) 0.7li2mno3-0.3lini0.5mn0.5o2 cathode materials in coin cells operating at different c-rates of 0.05, 0.10 and 0.15 c. differential capacities at room temperature show peaks which decrease in their intensities with an increase of the current density, and also, a degradation of the voltage when the current density increases [28,29]. these reductions in electrochemical performance could be explained considering that at high current densities, the amount of lithium insertion/extraction is affected by the structure of the interface, not allowing li ions to leave their sites, resulting in low capacities [30]. for example, for x = 0.5 (figure 5b), the mn oxidation from m3+ to mn4+ at 3.4 v is only seen at 0.05 c in the charging process, but not at 0.15 c. at 55 °c, figure 5 shows the main redox peaks of ni and mn for electrode material. according to the research results, a change in valence during charging and discharging based on a shift in the position of peaks associated with mn and ni can be observed at high temperatures. for x = 0.3 (figure 5a) oxidation peaks appear at 3.35 v and 3.8 v for mn and ni oxidation respectively. ni reduction peak appears at 3.7 v at 0.05 c and is slightly shifted to 3.64 v. on the other hand, the mn reduction peak appears at 3.17 v at 0.05 c but quickly decreases to 2.95 v at 0.15 c, showing the same layered to spinel phase transformation tendency [31]. for x = 0.5 sharp activation peak appears at 4.54 v. standard oxidation peaks for ni and mn appear at 3.8 and 3.28 v, respectively. ni reduction peak at 3.63 v, and mn reduction peak goes from 3.25 v at 0.05 c to 3.11 v at 0.15 c, conserving the layered-layered structure, suggesting a relatively more stable and efficient sei film would form at 55 °c [16]. figure 5c shows peaks of oxidation and reduction related to ni and mn reactions appearing for x = 0.7 but shifted to the higher voltage in the charging process and to the lower in the discharge process, indicating the formation of spinel structure [32,33]. in all cases, differential capacity values are very low, indicating an inefficient charge transfer process between the material and the electrolyte, possibly due to the size of the conglomerates that form the material [34]. conclusions in this work xli2mno3-(1-x)lini0.5mn0.5o2 layered composite materials for x = 0.3, 0.5 and 0.7 were synthesized by carbonate assisted co-precipitation method. x-ray diffraction patterns confirmed the formation of two phases of layered crystal structure. cr2032 coin cells were successfully assembled in ar filled glove box in order to test the electrochemical behavior of the synthesized electrode materials at 25 and 55 °c. electrochemical results showed that at 55 °c, a change in valence could be observed during charging and discharging due to the change in the position of the peaks associated with mn and ni on differential capacity plots. the cells with the cathodic material with x = 0.5 presented greater stability, achieving the capacity of more than 357 mah g-1 at a voltage of 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https://doi.org/10.1039/c5ee03573b https://doi.org/10.1021/acsami.9b21271 https://doi.org/10.3390/en15010060 https://doi.org/10.3390/en15010060 https://creativecommons.org/licenses/by/4.0/) @article{nazario-naveda2022, author = {nazario-naveda, renny and rojas-flores, segundo and gallozzo-cardenas, moises and ju{\'{a}}rez-cortijo, luisa and angelats-silva, luis}, journal = {journal of electrochemical science and engineering}, title = {{influence of operating temperature on the activation efficiency of li-ion cells with xli2mno3-(1-x)limn0.5ni0.5o2 electrodes:}}, year = {2022}, issn = {1847-9286}, month = {sep}, number = {4}, pages = {767--776}, volume = {12}, abstract = {in this study, the effect of operating temperature at 55 °c on xli2mno3-(1-x)limn0.5ni0.5o2 electrodes during the charge/discharge process at different current densities was investigated. x-ray diffraction (xrd) and scanning electron microscopy (sem) were used for structural and morphological analysis of the fabricated cathode materials, while charge-discharge curves and differential capacity were used to study the electrochemical behavior. results confirm the formation of the structures with two phases associated with the components of the layered material. it was found that at 55 °c, a capacity higher than 357 mah g-1 could be achieved at a voltage of 2.5-4.8 v vs. li/li+, which was larger than the capacity achieved at room temperature. at 55 °c, a change in valence could be observed during charging and discharging due to the change in the position of the peaks associated with mn and ni, highlighting cathodic material with x = 0.5 as the material that retains the layered structure at this temperature. this work confirms the good performance of electrodes made with this material at elevated temperatures and gives a better understanding of its electrochemical behavior.}, doi = {10.5599/jese.1458}, file = {:d\:/onedrive/mendeley desktop/nazario-naveda et al. 2022 influence of operating temperature on the activation efficiency of li-ion cells with xli2mno3-(1-x)limn0.pdf:pdf;:12_jese_1458.pdf:pdf}, keywords = {li, lithium, ni oxide, cathode material, ion battery, operating temperature, rich mn, specific capacity}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1458}, } influence of operating parameters on electrocoagulation of c.i. disperse yellow 3 doi: 10.5599/jese. 2014.0065 271 j. electrochem. sci. eng. 4(4) (2014) 271-283; doi: 10.5599/jese.2014.0065 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of operating parameters on electrocoagulation of c.i. disperse yellow 3 djamel ghernaout* , ** , , abdulaziz ibraheem al-ghonamy**, mohamed wahib naceur*, noureddine ait messaoudene** and mohamed aichouni** *department of chemical engineering, university of blida, po box 270, blida 09000, algeria **binladin research chair on quality and productivity improvement in the construction industry; college of engineering, university of hail, po box 2440, ha’il 81441, saudi arabia corresponding author: e-mail: djamel_andalus@yahoo.fr tel.: +213-025-433-631; fax: +213-025-433-631 received: july 24, 2014; revised: august 14, 2014; published: december 6, 2014 abstract this work deals with the electrocoagulation (ec) process for an organic dye removal. the chosen organic dye is c.i. disperse yellow 3 (dy) which is used in textile industry. experiments were performed in batch mode using al electrodes and for comparison purposes fe electrodes. the experimental set-up was composed of 1 l beaker, two identical electrodes which are separated 2 cm from each other. the main operating parameters influencing ec process were examined such as ph, supporting electrolyte concentration cnacl, current density i, and dy concentration. high performance ec process was shown during 45 min for 200 mg/l dye concentration at i = 350 a m -2 (applied voltage 12 v) and cnacl = 1 g l -1 reaching 98 % for phs 3 and 10 and 99 % for ph 6. after 10 min, dy was also efficiently removed (86 %) showing that ec process may be conveniently applied for textile industry wastewater treatment. ec using fe electrodes exhibited slightly lower performance comparing ec using al electrodes. keywords: dye removal; aluminium; iron; current density; mechanism. introduction the main problem of access to safe drinking water is continuous pollution of water resources by agriculture, urban waste and industry. in countries where water resources are relatively limited, treated wastewater reuse in agriculture has become an urgent necessity. the textile industry consumes considerable amounts of water in the dyeing and finishing. effluents containing dyes http://www.jese-online.org/ mailto:djamel_andalus@yahoo.fr j. electrochem. sci. eng. 4(4) (2014) 271-283 electrocoagulation of c.i. disperse yellow 3 272 272 can be toxic to the environment [1-4]. in addition, their presence in aquatic systems, even at low concentrations, is very visible. it reduces the penetration of light and has a detrimental effect on photosynthesis [5-7]. therefore, the remediation of water contaminated by these chemicals is necessary both to protect the environment and for future reuse [8-12]. therefore, several biological, physical and chemical methods are used for the treatment of industrial effluents with different efficeincies [1315]. electrochemical technologies, such as electrocoagulation technique (ec), seem to be well adapted to the textile industry wastewaters treatment [16-22]. this work is devoted to the study of the ec process for bleaching synthetic water containing an azo dye, c.i. disperse yellow 3 (dy), used in the algerian textile industry and the assessment of its performance versus certain operating parameters. experimental experimental set-up the ec tests were performed using an experimental set-up shown in figure 1. in a 1000 ml beaker, filled with 500 ml synthetic dye solution (distilled water + dy + nacl), two al (or fe in some experiments) 4 × 20 cm electrodes were immersed (active surface s = 4 × 10.5 = 42 cm 2 ). the anode is connected to the positive pole and the cathode to the negative pole of the direct current power supply. the interelectrode distance is fixed at 2 cm. when the electric current is applied, the magnetic stirrer is started at an average velocity agitation. figure 1. photo of the ec experimental set-up. electrodes cleaning before experiments, the fe electrodes were prepared to avoid the presence of any impurity as follows: (1) polishing with abrasive paper; (2) rinsing with distilled water; (3) degreasing by means of a solution composed of: naoh (25 g), na2co3 (25 g), k2co3 (25 g) and distilled water (q.s.p. 1,000 ml); (4) rinsing with distilled water; (5) pickling in a solution of sulphuric acid h2so4 at 20 % for 20 min at room temperature; and again (6) rinsing with distilled water. for al electrodes: (1) rinse with distilled water and polish using abrasive paper, (2) clean in hydrochloric acid solution (hcl at 20 %) during 10 min, and (4) rinse with distilled water. d. ghernaout et al. j. electrochem. sci. eng. 4(4) (2014) 271-283 doi: 10.5599/jese.2014.0065 273 prepared solutions to prepare a solution of 200 mg l -1 dye, 0.2 g of the latter was poured into a 1 l flask and distilled water was added during stirring for better solubilisation. the initial ph was varied using a solution of 0.1 m hcl (acidic conditions) or naoh (alkaline medium). the solution conductivity was increased by sodium chloride addition. all chemicals used were of analytical grade. methods once the ec test ends, the treated solutions were left to settle for 30 min in order to sediment the flocs formed. after decantation, and using a pipette, 25 ml of the solution were carefully collected for analysis. the analyses done before and after treatment were as follows: ph, conductivity and ultraviolet (uv) absorbance (shimadzu 1601, dual beam with 1 cm quartz vessel). the best uv absorbance long wave was found at 346 nm (uv346). the dy removal was calculated using the relation (1): i f f / % 100 ab ab r ab    (1) where abi and abf were initial and final uv absorbances, respectively. all the tests were conducted at ambient temperature (20 °c). results and discussion the aim of this work was to perform bleaching ec tests on dye synthetic solutions (distilled water+dye+nacl) using ec process and evaluate its performance based on certain key parameters. influencing parameters on ec process common remarks during ec tests, some common observations were: aluminium dissolution according to reaction (2): al → al 3+ + e (2) production of h2 gas bubbles at the cathode according to reaction (3): 2h2o + 2e → 2oh + h2(g) (3) production of o2 gas bubbles at the anode according to reaction (4): 2h2o → 4h + + o2 + 4e (4) flocs formation and their fixation on the h2(g) bubbles during their ascension to the solution surface as a white foam (figure 2a and b). indeed, anode dissolution generates coagulant species which destabilise the dye molecules forming flocs. figure 2. foam formation: (a) face view, (b) top view. (c) initial and final state of a 200 mg l -1 dy solution treated by ec during 1 h: from wright to left, initial solution, treated solution at 12, 8 and 4 v, respectively. j. electrochem. sci. eng. 4(4) (2014) 271-283 electrocoagulation of c.i. disperse yellow 3 274 274 ec time the ec efficiency is strongly influenced by the time residence in the electrochemical device. to study its effect, the ec period was varied from 5 to 75 min and the other parameters were kept constant. the results obtained are illustrated in figure 3. the h2 and o2 release and flocs formation increased over time and the foam became thicker. from 30 min, the flocs settled and the solution became clearer. as seen in figure 3, the dye removal efficiency increased with electrolysis time until 45 min. after this time, ec performance decreased. moreover, the good ec efficiencies were reached between 15 and 45 min. the removal efficiency was directly dependent upon the metal concentration in solution [2325]. the positive metallic species were produced by the al anode neutralising the negative charges on the polluting molecules [26-30]. when the electrolysis duration was increased, the cationic species as well as metal hydroxide (al(oh)3(s)) concentrations increased [30-32]. consequently, the pollutant removal increased [33-36]. figure 3. dye removal as a function of ec time (ph 6.5; cnacl = 1 g l -1 ; cdy = 20 mg l -1 ; d = 2 cm). electric current density the electric current density is the most important parameter of the electrochemical process [37]. the electric current density effect on the dye removal was studied. the current intensities were 120, 250 and 350 ma corresponding to the applied voltages of 4, 8 and 12 v, respectively. the dye concentration was fixed at 20 and 200 mg/l. the other parameters were maintained constant (ph 6.5; cnacl = 1 g/l; d = 2 cm). the obtained results are shown in figure 4. for i = 120 ma (i = 29 a/m 2 ), the produced gas bubbles were small and the formed froth was thin. for i = 250 ma (i = 60 a/m 2 ), the gas emanation was medium and the formed froth became important. for i = 350 ma (i = 83 a/m 2 ), flocs settling became significant, the gas emanation became intense and the solution was transformed clear. as seen in figure 4, the electric current had a great effect on the dye removal especially for the first ten minutes. after 20 min, the electric current had a small effect. this is explained by the fact that the negative charge on the organic dye is neutralised after the al 3+ action on the dye molecules. d. ghernaout et al. j. electrochem. sci. eng. 4(4) (2014) 271-283 doi: 10.5599/jese.2014.0065 275 figure 4. effect of the electric current i on the ec efficiency for dy removal (ph 6.5, d = 2 cm, cnacl = 1 g l -1 ) (a): cdy = 20 mg l -1 ; (b): cdy = 200 mg l -1 ; (c): cdy = 200 mg.l -1 ; tec = 5 min initial ph the solution ph is an important factor influencing the ec performance [37]. this is due to the fact that ph determinates the metallic ions speciation, the chemical state of other species in the j. electrochem. sci. eng. 4(4) (2014) 271-283 electrocoagulation of c.i. disperse yellow 3 276 276 solution, and the formed products solubility. in order to examine the ph effect, the solution ph was adjusted to the values from 3 to 12 maintaining other parameters constant: u = 12 v (i = 83 a m -2 ), cnacl = 1 g l -1 , d = 2 cm, tec = 30 min). the obtained results are shown in figure 5. figure 5. effect of the ph on the ec efficiency for dy removal (cdy = 200 mg l -1 , u = 12 v (i = 83 a m -2 ); tec = 30 min; d = 2 cm). for the acidic medium, the solution colour became orange after the addition of hcl. from the cathode, there is an intense emanation of h2 bubbles. from the anode, there is an important formation of o2 bubbles. the foam and sediment formed are denser at the anode. for the alkaline medium, there was formation of white sediment at the bottom of the beaker, and its volume increased with time in comparison with the acidic medium. as seen in figure 5, we noted that the dye removal was well performed between ph 3 and 10. several researchers found that the best removal efficiency with aluminum electrodes was reached in the ph range between 3 and 9 [19,34-36]. in figure 6, we chose four ph values: 3, 4, 6.5 and 10 with other parameters fixed in order to illustrate the ph effect. we also followed the change in ph as a function of time; figure 7 shows the obtained results. the medium ph changed during the ec process. this change depended on the type of electrode material and the initial ph of the treated solution. we note from figure 7 an increase in ph in the case of solutions with ph < 7. this increase was probably due to the release of h2 from the cathode and the formation of oh according to the reaction (3). moreover, a decrease in ph in the case of solutions with ph > 7 was also noticed. this decrease was affected to the hydroxyl (oh ) consumption according to reaction (5): al(oh)3(aq) + oh (aq) → al(oh)4 (aq) (5) initial conductivity conductivity promotes the performance of electrochemical processes [26]. we chose nacl as a supporting electrolyte. in order to determine its effect on the efficiency of bleaching of the synthetic solutions, we varied the concentration (0.25, 0.5, 1 and 1.5 g l -1 ) while keeping the other parameters constant. the results are shown in figure 8. we have observed that (1) the gas production becomes higher with the increase in salt concentration, (2) the conductivity decreases during ec treatment and, (3) the formation of a small deposit on the anode. d. ghernaout et al. j. electrochem. sci. eng. 4(4) (2014) 271-283 doi: 10.5599/jese.2014.0065 277 figure 6. dy removal as a function of ph (u = 12 v (i = 83 a m -2 ), tec = 30 min; cnacl = 1 g l -1 ; d = 2 cm (a) cdy = 20 mg l -1 ; (b) cdy = 40 mg l -1 ; (c) cdy = 200 mg l -1 j. electrochem. sci. eng. 4(4) (2014) 271-283 electrocoagulation of c.i. disperse yellow 3 278 278 figure 9 shows the solution conductivity as a function of time during ec process. we note that the conductivity decreased over time and the difference in the changes in ph was different due to the hcl and naoh added during the ph adjustment before ec treatment. figure 7. evolution of ph during ec treatment (same conditions as for figure 6). (a) cdy = 20 mg l -1 ; (b) cdy = 40 mg l -1 ; (c) cdy = 200 mgl -1 figure 8. effect of the nacl concentration on the ec efficiency u = 12 v (i = 83 a m -2 ); tec = 30 min; d = 2 cm; cdy = 200 mg l -1 0 2 4 6 8 10 12 0 20 40 60 80 p h tec / min (a) 0 2 4 6 8 10 12 0 20 40 60 80 p h tec / min (b) 0 2 4 6 8 10 12 0 20 40 60 80 p h tec / min (c) 0 20 40 60 80 100 120 0 0.25 0.5 0.75 1 1.25 1.5 r / % cnacl / g l -1 d. ghernaout et al. j. electrochem. sci. eng. 4(4) (2014) 271-283 doi: 10.5599/jese.2014.0065 279 figure 9. solution conductivity as a function of time during ec process (cnacl = 1 g l -1 ; cdy = 200 mg l -1 ; d = 2 cm; u = 12 v (i = 83 a -2 ) inter-electrode distance we varied the distance between the electrodes d = 0.8; 1; 1.5 and 2 cm while fixing the other factors. the results are shown in figure 10. when increasing the inter-electrode distance, the ec efficiency also increased. this can be explained as follows: for d = 2 cm, there would be more probabilities to generate global flocs that are able to adsorb more dye molecules. figure 10. effect of the inter-electrode distance on the ec performance (cnacl = 1 g l -1 , cdy = 200 mg l -1 , ph = 6.5, u = 12 v (i = 83 a m -2 ). dy concentration the aim of this part is to determine whether the ec method was applicable to solutions with a range of concentrations from 20 to 500 mg l -1 . the solutions were electrolysed, for a treatment time of 45 min, at a fixed voltage u = 12 v (i = 83 a m -2 ) and an inter-electrode distance of 2 cm. the results obtained are shown in fig. 11. figure 11 shows that the ec method is effective in the range of selected concentrations. a yield of 97 % is reached at a dye concentration of 200 mg l -1 . the results obtained can be justified by j. electrochem. sci. eng. 4(4) (2014) 271-283 electrocoagulation of c.i. disperse yellow 3 280 280 the increased probability of contact with the dye molecules to aluminum hydroxide al(oh)3 to form flocs of large sizes; thereby facilitating their separation by their attachment to the bubbles of released gases at the electrodes. figure 11. ec performance as a function of dy concentration cnacl = 1 g l -1 ; ph 6.5; d = 2 cm, u = 12 v (i = 83 am -2 ) ec using fe electrodes to check if dy can be removed by ec using iron electrodes, some tests using the al optimum conditions are performed. the results obtained are compared with those obtained with aluminum electrodes (figure 12). figure 12. ec using al and fe electrodes: cdy = 200 mg l -1 ; cnacl = 1 g l -1 ; ph 6.5; d = 2 cm, u = 12 v (i = 83 a/m 2 ) during ec treatment using fe electrodes, (1) clouds of green flocs came from the anode surface and settled to the beaker bottom and, (2) on the solution surface, two layers of foam were observed: the first a red-brown color, and below, the second green. we find that the rate of reduction of the dye increased with time until a yield of 96.28 % in the case of iron electrodes (figure 12). comparing the test results with aluminum electrodes (r = 98.96 %), we can say that iron is also effective in removing dy. 91 92 93 94 95 96 97 98 0 100 200 300 400 500 600 r / % cdy / mg l -1 d. ghernaout et al. j. electrochem. sci. eng. 4(4) (2014) 271-283 doi: 10.5599/jese.2014.0065 281 the reactions involved in the ec using iron electrodes are as follows: the iron, after oxidation in the electrolytic system, produces iron hydroxide fe(oh)n(s), with n = 2 or 3; and two mechanisms have been proposed [6,12,14,15,24,38]:  mechanism 1 (green coloration, fe(oh)2(s)): fe(s) → fe 2+ (aq) + 2e (6) fe 2+ (aq) + 2oh (aq) → fe(oh)2(s) (7)  mechanism 2 (brown coloration, fe(oh)3(s)): fe 2+ (aq) → fe 3+ (aq) + 1e (8) fe 3+ (aq) + 3oh (aq) → fe(oh)3(s) (9) the species fe(oh)n(s) formed (by the two mechanisms) remained in the aqueous phase in the form of gelatinous suspension which can then remove the pollutants from the water (figure 13), either by complexation, or by electrostatic attraction, followed by coagulation and flotation or sedimentation [24,25,27,28]. figure 13. predominance-zone diagrams for (a) fe(ii) and (b) fe(iii) chemical species in aqueous solution. the straight lines represent the solubility equilibrium for insoluble fe(oh)2 and fe(oh)3, respectively, and the dotted lines represent the predominance limits between soluble chemical species. (c) diagram of solubility of al(iii) species as a function of ph [27,28]. j. electrochem. sci. eng. 4(4) (2014) 271-283 electrocoagulation of c.i. disperse yellow 3 282 282 conclusions highly performant ec process is shown in dye removal during 45 min for 200 mg/l dye concentration at i = 350 a/m 2 (applied voltage 12 v) and cnacl = 1 g/l reaching 98 % for ph 3 and 10 and 99 % for ph 6. for 10 min, dy is also efficiently removed (86 %) showing that ec process may be well convenient for textile industry wastewater treatment. ec using fe electrodes is slightly less performant than ec using al electrodes. acknowledgements: the present research work was undertaken by the binladin research chair on quality and productivity improvement in the construction industry funded by the saudi binladin constructions group; this is gratefully acknowledged. the opinions and conclusions presented in this paper are those of the authors and do not necessarily reflect the views of the sponsoring organisation. references [1] a. r. amani-ghadim, s. aber, a. olad, h. ashassi-sorkhabi, chemical engineering and processing 64 (2013) 68-78. 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[38] c. barrera-díaz, b. bilyeu, g. roa, l. bernal-martinez, separation and purification reviews 40 (2011) 1-24. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {electrochemical sensing platform for simultaneous detection of 6-mercaptopurine and 6-thioguanine using rgo-cu2o/fe2o3 modified screen-printed graphite electrode:} http://dx.doi.org/10.5599/jese.1101 47 j. electrochem. sci. eng. 12(1) (2022) 47-57; http://dx.doi.org/10.5599/jese.1101 open access : : issn 1847-9286 original scientific paper electrochemical sensing platform for simultaneous detection of 6-mercaptopurine and 6-thioguanine using rgo-cu2o/fe2o3 modified screen-printed graphite electrode fatemeh irannezhad, jamileh seyed-yazdi and hoda hekmatara department of physics, faculty of science, vali-e-asr university of rafsanjan, rafsanjan, iran corresponding author: j.seyedyazdi@gmail.com received: september 3, 2021; accepted: october 14, 2021; published: november 14, 2021 abstract a sensitive electrochemical sensor was developed using reduced graphene oxide rgo-cu2o/ /fe2o3 nanocomposite for 6-mercaptopurine detection based on a facile fabrication method. the surface morphology and structural composition of this nanocomposite were evaluated by x-ray diffraction (xrd), field emission scanning electron microscopy (fe-sem), and fourier transform infrared (ft-ir) spectroscopy. the screen-printed graphite electrode (spge) modified with rgo-cu2o/fe2o3 nanocomposite (rgo-cu2o/fe2o3/spge) indicated excellent electrochemical properties to detect 6-mercaptopurine. the linear dynamic range was estimated between 0.05 and 400.0 μm for 6-mercaptopurine detection, with a limit of detection of 0.03 μm. also, rgo-cu2o/fe2o3/spge sensor showed good activity for simultaneous detection of 6-mercaptopurine and 6-thioguanine. in the coexistence system of 6-mercaptopurine and 6thioguanine, two clear and well-isolated voltammetric peaks were obtained by differential pulse voltammetry (dpv). additionally, the proposed sensor was examined for applicability by determining 6-mercaptopurine and 6-thioguanine in real samples, and the recovery in the range of 97.5-103.0 % was obtained. keywords electrochemical sensor; reduced graphene oxide; copper(i) oxide; iron(iii) oxide introduction 6-mercaptopurine (6-mp), a sulfur analog of adenine, was introduced in early 1950 and subsequently applied as a chemotherapy drug to treat childhood and adulthood leukemias [1,2]. now, it has usually been prescribed as an anti-inflammatory and immunosuppressive agent to manage medical conditions like inflammatory bowel disease (ibd), rheumatoid arthritis, polycythemia, chorioadenoma and choriocarcinoma [3,4]. nevertheless, some disadvantages limited the use of cytotoxic antitumor 6-mp, such as hepatotoxicity and bone marrow toxicity [5]. 6-mp cannot detect the difference between healthy and cancer cells [6], highlighting the necessity for monitoring doses of this agent in various biological and pharmaceutical media. http://dx.doi.org/10.5599/jese.1101 http://dx.doi.org/10.5599/jese.1101 mailto:j.seyedyazdi@gmail.com j. electrochem. sci. eng. 12(1) (2022) 47-57 rgo-cu2o/fe2o3 modified spge as sensor 48 6-thioguanine (6-tg) or thiopurine antimetabolite, as an analog of purine nucleosides, was first recognized as a health-promoting agent in the treatment of neoplastic conditions. this cytotoxic agent is extensively applied to treat disorders like acute leukemia, auto-immune disease, and inflammatory bowel disease [7,8]. nevertheless, high doses of 6-tg are toxic and can lead to serious side effects, such as bone marrow depression, causes myelosuppression, gastrointestinal complications, and liver problems [6,9]. accordingly, there is a need to develop a facile and sensitive approach to control the 6-tg concentration in various biological and pharmaceutical media. the administration of 6-mp and 6-tg in treating various cancers such as acute lymphoblastic leukemia at an appropriate and effective dose is of great importance in analytical measurements for both biological and pharmaceutical preparations. hence, numerous methods have been employed in this regard so far, including high-performance liquid chromatography [10,11], chemiluminescence [12], colorimetric assays [13], surface-enhanced raman scattering [14], liquid chromatography-tandem mass spectrometry [15,16], and localized surface plasmon resonance [17] for the determination of 6-mp and 6-tg. it should be noted that these techniques, in addition to excellent selectivity and sensitivity, have some disadvantages such as expensiveness, the need for pretreatment, and complicated analysis. among these, electrochemical analytical techniques have attracted special attention due to their unique properties like simplicity, rapidity, field-based portability, and cost-effectiveness [18-24]. screen printing is now a facile and easy technique that can be applied in the fabrication and modification of electrochemical sensors [25-27]. this method has been used by researchers to construct stable, reproducible, and disposable electrodes and devices. among these, a screen-printed electrode (spe) is a promising candidate for real sample analysis. spes have some benefits for produced electrochemical sensors which can perform in situ analysis because of unique features, including cost-effectiveness, high availability, linear output, miniaturized morphology, low power, easy to use at ambient temperature, rapid response, and ability to connect to portable devices. nevertheless, there are some shortcomings for electrochemical sensitivity, such as huge overpotential, slow electrochemical behavior in detecting different analytes, which might be eliminated using approaches like electrode surface modification to improve interfacial electron transfer in the electroanalysis [28-33]. the electrochemical sensors can benefit from such nano-materials due to cost-effectiveness, large specific surface area, variable morphologies, excellent electrocatalytic properties, and the ability to enhance electron transfer at low overpotentials [34-40]. the given work aimed to produce rgo-cu2o/fe2o3 nanocomposite for electrochemical detection of 6-mp, explore the synergistic effects of fe2o3, cu2o, and rgo nano-particles, and evaluate sensitivity, the limit of detection, and linear dynamic range of electrocatalyst performance for 6-mp detection. moreover, this sensor was examined for co-detection of 6-mp and 6-tg. the applicability of the proposed sensor was examined to determine 6-mp and 6-tg in biological and pharmaceutical media. experimental materials and equipment all chemicals of analytical grade utilized in the current work belong to merck and sigma-aldrich. metrohm electrochemical equipment was used to record all electrochemical experiments. dropsens spge (drp-110, spain) was utilized to carry out the electrochemical tests. the experiments were performed by a three-electrode cell system consisting of 4 mm graphite as a working electrode, a silver pseudo-reference electrode, and graphite used as an auxiliary electrode. f. irannezhad et al. j. electrochem. sci. eng. 12(1) (2022) 47-57 http://dx.doi.org/10.5599/jese.1101 49 in addition, ortho-phosphoric acid as well as the respective salts (kh2po4, k2hpo4, k3po4) with ph ranging between 2.0 and 9.0, have been utilized to procure buffer solution. a digital ph meter (metrohm 710) was recruited to measure all ph values. construction of rgo-cu2o/fe2o3 nanocomposite as explained elsewhere, a modified hummers’ method was used to construct graphene oxide (go) by graphite powder [41,42]. first, 50 mg go was dispersed in 20 ml ethanol in an ultrasonic bath. then, iron(iii) nitrate nonahydrate and copper(ii) nitrate trihydrate with a ratio of 3:7 were added to the mixture and stirred until it turned into a uniform suspension. subsequently, using sodium hydroxide (6 m) solution, ph was adjusted to 11, and a brown viscous suspension was obtained. the mixture was transferred into a stainless-steel autoclave and then held in an oven at a temperature of 240 °c for 26 hours, followed by centrifuging and washing thoroughly with distilled water and ethanol to set a neutral ph value. at last, the resulting product was dried at 50 oc for 16 hours. construction of rgo-cu2o/fe2o3/spge the unmodified spge was coated with rgo-cu2o/fe2o3 nanocomposite. rgo-cu2o/fe2o3 nanocomposite stock solution was obtained by adding rgo-cu2o/fe2o3 nanocomposite (1 mg) in aqueous solution (1 ml) under ultrasonication for an hour, followed by casting 4 μl aliquot of rgo-cu2o/fe2o3 solution on the working electrode, and finally air-drying for 30 min at room temperature. the surface areas of rgo-cu2o/fe2o3/spge and bare spge were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula [43] for rgo-cu2o/fe2o3/spge, the electrode surface was found 0.103 cm2 which was about 3.2 times greater than bare cpe. results and discussion determination of rgo-cu2o/fe2o3 nanocomposite characteristics the xrd spectrum taken from as-fabricated rgo-cu2o/fe2o3 nanocomposite is shown in figure 1. figure 1. xrd spectrum analysis of synthesized rgo-cu2o/fe2o3 nanocomposite diffraction peaks of α-fe2o3 at 29.4, 32.3, 36.5, 40.2, 48.0, 55.5, 56.5 and 64.8° correspond respectively to the crystalline planes (220), (104), (110), (113), (024), (116), (018), (214), and (030). diffraction peaks of cu2o at 27.2, 35.6, 42.2, 61.3, 73.1 and 77.5° correspond respectively to the crystalline planes (110), (111), (200), (220), (311), and (222). http://dx.doi.org/10.5599/jese.1101 j. electrochem. sci. eng. 12(1) (2022) 47-57 rgo-cu2o/fe2o3 modified spge as sensor 50 figure 2 shows the fe-sem image of the prepared rgo-cu2o/fe2o3 nanocomposite. it clearly shows cu2o nanoribbons, α-fe2o3 nanoparticles, and graphene oxide layers that formed rgo-cu2o/fe2o3 nanocomposite. figure 3 shows ft-ir spectrum analysis of the rgo-cu2o/fe2o3 nanocomposite. formation of the nanocomposite, i.e., the presence of fe-o and cu-o bonds, is confirmed by peaks at about 833 cm−1. the peaks at about 1465 and 1588 cm−1 are related to c=o and c=c bonds of graphene oxide, respectively, and the bond of 3415 cm-1 to o-h stretching vibration due to adsorbed water molecules. figure 2. fe-sem image of synthesized rgo-cu2o/fe2o3 nanocomposite figure 3. ft-ir spectrum analysis of synthesized rgo-cu2o/fe2o3 nanocomposite electrochemical behavior of 6-mercaptopurine at rgo-cu2o/fe2o3/spge the supporting electrolyte ph showed a significant effect on the electrocatalysis of 6-mp on the surface of rgo-cu2o/fe2o3/spge (scheme 1). the impact of ph on 6-mp (100.0 μm) detection in pbs on the modified electrode surface was evaluated at different ph values of 2.0-9.0. the maximum peak current response of 6-mp was found at the ph value of about 7.0. hence, we selected ph 7.0 as the optimal experimental condition for experiments. the electrochemical behavior of 6-mp in pbs at ph 7.0 was studied by cyclic voltammetry (cv) at bare spge, and spge modified by rgo-cu2o/fe2o3, and results are shown in figure 4. as seen in figure 4 for unmodified spge (curve a), a lower peak current of 6-mp oxidation is observed than for modified spge (curve b). f. irannezhad et al. j. electrochem. sci. eng. 12(1) (2022) 47-57 http://dx.doi.org/10.5599/jese.1101 51 scheme 1. electrochemical oxidation mechanism of 6-mp at the surface of the modified electrode this means a lower sensitivity for 6-mp for bare than for modified spge. the voltammogram of 6-mp detection on both electrodes displayed an irreversible oxidation peak, with the elevation of the peak current and reduction of the anodic peak potential value of 6-mp observed for rgocu2o/fe2o3/spge. the optimal electrocatalytic impact for 6-mp oxidation was reported on the rgocu2o/fe2o3/spge surface. figure 4. cv responses of 100.0 μm 6-mercaptopurine at (a) bare spge and (b) rgo-cu2o/fe2o3/spge in pbs (0.1 m, ph 7.0) scan rate influence figure 5 displays the linear sweep voltammetry (lsv) behavior of 100.0 μm 6-mp in pbs of ph 7.0 at variable scan rates (v) on the surface of rgo-cu2o/fe2o3/spge. figure 5. lsv curves of 100.0 μm 6-mp in pbs (0.1 m, ph 7.0) at different scan rates (10 300 mv/s) on the surface of rgo-cu2o/fe2o3/spge (a-g stand for 10, 25, 50, 75, 100.0, 200.0, and 300.0 mv/s). inset: plot of 6-mp oxidation peak current versus square root of scan rate http://dx.doi.org/10.5599/jese.1101 j. electrochem. sci. eng. 12(1) (2022) 47-57 rgo-cu2o/fe2o3 modified spge as sensor 52 there is a gradual elevation in the peak current value of 6-mp oxidation and a positive shift of oxidation peak potential by rising  from 10 to 300 mv/s. according to figure 5 (inset), the anodic peak current (ipa) of 6-mp is proportional to the square root of scan rate (v1/2), and the regression equation (1) ipa = 1.7054 v1/2 + 0.3634 (r2 = 0.9993) (1) hence, a diffusion-controlled electrochemical reaction of 6-mp at rgo-cu2o/fe2o3/spge can be assumed. to provide data about the rate determining step, a tafel plot was drawn based on data of rising sector related to a current-voltage curve at low scan rate (10 mv/s) for 100.0 μm 6-mp (figure 6, inset). the linearity of e versus log i plot reveals the intervention of electrode process kinetics. in accordance with the slope of this plot, the number of transferred electrons for the rate-determining step can be calculated. the inset in figure 6 displays the tafel slope of 0.1133 v for the linear sector. the tafel slope value means the rate-limiting step related to one-electron transfer, with a transfer coefficient (α) of 0.48. figure 6. lsv response for 100 μm 6-mp in pbs (0.1 m, ph 7.0) at the scan rate of 10 mv/s on rgo-cu2o/fe2o3/spge; inset: tafel plot of the rising sector of the related voltammogram chronoamperometric analysis chronoamperometric measurements for 6-mp detection on rgo-cu2o/fe2o3/spge surface shown in figure 7 were performed at the potential of 0.65 v and variable 6-mp concentrations in pbs (0.1 m, ph 7.0). the cottrell equation defines the electrochemical reaction current at the limited condition of mass transport for 6-mp as electroactive material having a certain diffusion coefficient (d) value [43]. figure 7a displays the experimental plots of i versus t−1/2 with the optimal fits at various 6-mp concentrations. figure 7b shows the slopes of achieved straight lines versus 6-mp concentration. according to the obtained slope and cottrell equation, the mean d value of 6-mp was calculated as 7.0 10−5 cm2/s. dpv analysis of 6-mp on rgo-cu2o/fe2o3/spge surface differential pulse voltammetry (dpv) is a versatile technique for the determination of 6-mp because of its higher sensitivity, and the obtained voltammograms measured for step potential of 0.01 v and pulse amplitude of 0.025 v are shown in figure 8. figure 8 shows that with increasing concentration of 6-mp from 0.05 μm – 400.0 μm, ipa values increased with slight shifts of oxidation potential values. f. irannezhad et al. j. electrochem. sci. eng. 12(1) (2022) 47-57 http://dx.doi.org/10.5599/jese.1101 53 figure 7. chronoamperograms obtained for rgo-cu2o/fe2o3/spge in pbs (0.1 m, ph 7.0) at variable 6-mp concentrations (a–e: 0.1, 0.3, 0.7, 1.8 and 3.0 mm of 6-mp); inset a: plot of i versus t-1/2 based on chronoamperograms (a – e); inset b: slope plot of straight line versus 6-mp concentration figure 8. dpv responses of 6-mp on rgocu2o/fe2o3/spge at different concentrations of 6-mp in pbs (0.1 m, ph 7.0) (a-m refer to 0.05, 2.5, 7.5, 15.0, 30.0, 70.0, 100.0, 150.0, 200.0, 250.0, 300.0, 350.0 and 400.0 µm); inset: calibration curve of dpv peak current against concentration of 6-mp the plot of ipa versus 6-mp concentration is drawn in the inset of figure 8, showing a nearly straight line with excellent linearity. the linear regression equation is defined as: ipa = 0.1065 c6-mp + + 1.0948 (r2 = 0.9977). the equation of 3sbl/m was selected to calculate the limit of detection (lod), where m stands for the slope of the standard plot and sbl for a standard deviation for linearity of blank solution anodic peak current after five determinations. the lod was estimated as 0.02 μm. the analytical performance of the present electrochemical method is in table 1 compared with those obtained by some other relevant methods for the electrochemical detection of 6-mp. table 1. comparison of linear range and detection limits for 6-mp reported for different electrochemical sensors electrochemical sensor method linear range, μm lod, μm ref. multiwall carbon nanotubes paste electrode/carbon paste electrode square wave voltammetry (swv) 0.5–900 0.1 [44] cobalt salophen complex/carbon nanotube-paste electrode dpv 1–100 0.1 [45] boron-doped diamond electrode dpv 1-450 0.51 [46] rgo-cu2o/fe2o3/spge dpv 0.05-400.0 0.03 this work http://dx.doi.org/10.5599/jese.1101 j. electrochem. sci. eng. 12(1) (2022) 47-57 rgo-cu2o/fe2o3 modified spge as sensor 54 simultaneous detection of 6-mp and 6-tg on rgo-cu2o/fe2o3/spge dpvs for simultaneous detection of 6-mp and 6-tg using rgo-cu2o/fe2o3/spge are presented in figure 9. two oxidation peaks appear at 0.6 v and 0.75 v for 6-mp and 6-tg, respectively. there is a linear elevation in the peak current intensity for both analytes with the simultaneous enhancement in their contents. figure 9 (insets a and b) presents the related standard curves for 6mp and 6-tg. the sensitivity for 6-mp in the simultaneous detection was the same as the single one. based on these findings, the applicability of simultaneously measuring concentrations of both 6-mp and 6-tg is evident on the rgo-cu2o/fe2o3/spge surface. figure 9. dpvs of rgo-cu2o/fe2o3/spge in pbs (0.1 m, ph 7.0) at variable 6-mp and 6-tg concentrations (a-f refer to mixed solutions of 2.5 + 3.0, 15.0 + 35.0, 70.0 + 125.0, 150.0 + 250.0, 250.0 + 425.0, and 350.0 + 550.0 μm of 6-mp and 6-tg); inset a: plot of peak currents versus 6-mp concentrations; inset b: plot of peak currents versus 6-tg concentrations real sample analysis the applicability of the prepared rgo-cu2o/fe2o3/spge sensor was explored via detection of 6-mp and 6-tg present in urine specimens, 6-mp tablets, and 6-tg tablets. table 2 displays the recovery rates of 97.5 to 103.3 % for tested samples. these findings showed the efficiency of the proposed sensor for the detection of 6-mp and 6-tg in real specimens. table 2. determination of 6-mp and 6-tg in real specimens (n=5). sample c / μm recovery, % rsd, % spiked found 6-mp 6-tg 6-mp 6-tg 6-mp 6-tg 6-mp 6-tg urine 0 0 5.0 6.0 4.9 6.2 98.0 103.3 2.8 2.9 7.5 8.0 7.6 7.8 101.3 97.5 3.1 1.7 6-mercaptopurine tablet 0 0 7.0 3.5 1.0 5.5 7.8 5.6 97.5 101.8 2.7 3.0 2.0 6.5 9.3 6.4 103.3 98.5 2.1 2.4 6-thioguanine tablet 0 0 7.5 2.9 4.5 1.0 4.6 8.3 102.2 97.6 1.7 2.4 8.5 2.0 8.6 9.6 96.5 101.1 2.9 3.2 f. irannezhad et al. j. electrochem. sci. eng. 12(1) (2022) 47-57 http://dx.doi.org/10.5599/jese.1101 55 conclusion in this work, rgo-cu2o/fe2o3 nanocomposite was prepared by a facile method and applied for construction of rgo-cu2o/fe2o3/spge sensor for electrochemical detection of 6-mercaptopurine. the proposed sensor is easy to use without complicated preparations prior to 6-mercaptopurine detection. the developed sensor showed a linear electrochemical response to 6-mercaptopurine ranging from 0.05 to 400.0 μm, providing the limit of detection of 0.03 μm. also, the oxidation peaks of 6-mercaptopurine and 6-thioguanine can be entirely separated with a clear peak potential difference of 0.15 v, which allowed simultaneous determination of two drugs. finally, the applicability of the as-fabricated rgo-cu2o/fe2o3/spge sensor was verified by detections of 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http://dx.doi.org/10.5599/jese.1498 open access : : issn 1847-9286 www.jese-online.orghttp://www.jese-online.org/ review artificial intelligence in use of zro2 material in biomedical science jashanpreet singh1,, simranjit singh2 and amit verma1,3 1university center for research & development, chandigarh university, mohali 140413, india 2school of computer science engineering and technology, bennett university, greater noida 201310, india 3department of computer science, chandigarh university, mohali 140413, india corresponding authorс: ijashanpreet@gmail.com received: may 30, 2022; accepted: october 13, 2022; published: october 21, 2022 abstract the rapidly growing discipline of artificial intelligence (ai) seeks to develop software and computers that can do tasks that have historically required the intelligence of people. machine learning (ml) is a subfield of ai that makes use of algorithms to "learn" from data's innate statistical patterns and structures to extrapolate information that is otherwise hidden. a growing emphasis on cosmetic dentistry has coincided with zro2‘s rise to prominence as a result of its improved biocompatibility, visually pleasant look, strong oxidation resistance, better mechanical properties, and lack of documented allergic responses. advances in the field of ai and ml have led to novel applications of zro2 in dental devices for biological objectives. artificial intelligence (ai) technologies have attracted a lot of attention in zro2-related research and therapeutic applications due to their ability to analyze data and discover connections between seemingly unrelated events. specifically, their incorporation into zirconia is largely responsible for this. zirconia's versatility in the scientific community means that how ai is used in the area varies with the specific directions in which zirconia is utilized. therefore, this article primarily focuses on the use of ai in the biomedical use of zro2 in dentistry. keywords biomedical engineering; artificial intelligence; machine learning; zirconia introduction the field of dentistry makes extensive use of digital technology, which plays an important part in a variety of processes and activities, including clinical treatment, laboratory operations, student teaching, administration, and dentistry research [1]. clinical therapy including the use of digitally performing cad/cam, shade analysis, smile design, impressions, and virtual communication are all examples of how digitization has played a part in clinical treatment [1,2]. the term "artificial intelligence" (ai) was invented in the 1950s to describe a technology that is currently undergoing http://dx.doi.org/10.5599/jese.1498 http://dx.doi.org/10.5599/jese.1498 http://www.jese-online.org/ http://www.jese-online.org/ mailto:ijashanpreet@gmail.com j. electrochem. sci. eng. 00(0) (2022) 000-000 zro2 material in biomedical science 2 fast development based on computer technology [3]. artificial intelligence (ai) i.e. a subfield of computer science enables computers or intelligent software to carry out activities that would normally need human intellect. the development of artificial intelligence has allowed for the creation of contemporary robots that are capable of learning from their past mistakes, adapting to new needs, and performing duties that are analogous to those performed by people [4]. the use of ai technology may be seen in many facets of human civilization, such as dentistry and medicine, and it is becoming increasingly widespread in both of these fields (figure 1). implants should be corrosion-resistant. most of the materials degrade due to corrosion and wear processes [5-18]. zirconia is a type of high-tech ceramic that has been utilized in many biomedical applications ever since the 1960s [19]. zirconia has received a lot of interest in the field of dentistry since it has great biocompatibility, is visually beautiful, has high corrosion resistance, has strong mechanical qualities, and there have been no recorded adverse responses to it [20-22]. in zro2-based research and biomedical applications over the past few decades, ai techniques have garnered a great response because they are associated with data analysis and provide regression/correlation between complicated phenomena. this is large since these techniques can be applied in clinical applications. therefore, to study the uses of zro2, dentists require a complete grasp of ai in zro2based research. in this study, we provide a summary of current advancements and issues about ai approaches used in zro2-based dental applications. figure 1. ai applications in the field of dentistry science use of ai in dentistry applications and industry machine learning (ml) is now expanding at a very quick rate. ml may teach itself and progress on its own by analyzing various data sets, followed by compiling previous knowledge and techniques [23,24]. the advent of ai-ml has not only opened up new potential but also presented new obstacles in the field of dentistry, medicine, and other medical specialties. a precise diagnosis serves as the foundation for an effective treatment plan in several subspecialties of dentistry, including maxillofacial surgery, orthodontics, and prosthodontics, amongst others [25]. because it can identify ai in dentistry patient medical information decision making environment information scheduling of surgery or treatment machine vision imaging/data collection treatment planning machine learning natural language processing speech safety big data approach j. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1498 3 links between operational records as well as patterns in large data, machine learning makes it easier to diagnose and anticipate illnesses as well as assess the efficacy of different treatments [26-28]. the dentistry industry is seeing significant advancements in ai application technology as a result of the rise of data computation as well as the acquisition and analysis of a large number of clinical patient data sets [2,19]. the investigators offered a comprehensive review of the most current data available. the investigators provided a complete and up-to-date summary of the most recent facts about the diagnostic and diagnostic imaging of ai dentistry. it is vital for dentists and dental surgeons to comprehend artificial intelligence (ai), learn it, and become an expert in it to stay updated with the latest technology of medicine and implement it clinically. for example, hung et al. conducted an in-depth review of the research that has been done on the clinical applications of ai in the fields of dentistry and maxillofacial radiology [29]. an artificial neural network (ann) was built by kositbowornchai et al. [29] to fix a vertical fracture in a tooth. to assist orthodontists in determining the treatment plan, jung et al. developed neural network ml models through the use of a backpropagation algorithm. these models were used to diagnose extractions [29]. li et al. [30] used a neural network prediction technique to obtain the medical records of a new patient and characterized the 24 different inputs which included demographic data, cephalometric data, dental data, and soft tissue data which were retrieved, as illustrated in figure 2. because the extraction probability (0.955) was greater than 0.692, they concluded that this was an extraction instance, and the information was then sent to the other two networks. the results that are produced by the other networks include the probabilities of a variety of extraction patterns and anchoring patterns. the physician investigated each of these potential courses of therapy, considered a number of other factors, and in the end came up with an efficient treatment strategy. at this moment in time, ai is implemented in many dentistry applications like oral disease diagnosis and oral monitoring. however, in dental clinics and hospitals, ai-implemented applications like appointments and medical advice are more advanced technologies. in the future, ai-based dentistry applications possible can be oral surgery, cosmetic dentistry, radiography analysis, oral healthcare, etc. [31]. for example, li et al. employed ai algorithms on pixel semantic segmentation of patient images to identify gum inflammation [32]. this was accomplished with the assistance of a deep neural network. the findings point to the possibility that this method, which utilizes mobile applications, might be appropriate for dental self-examination. in addition, ai and ml play an increasingly significant role in the classroom, in scientific research, in the management of oral health, and the treatment of oral diseases. there is no question that ml can be of assistance to the dentist and offer a great deal of ease. it is not safe to believe that ml can perform at the same level as humans. however, the goal of using ai-ml in dental science is not to obsolete dentists, but rather to help them make more accurate clinical diagnoses and treatment recommendations. this is the case even though the goal of the implementation of ai-ml in dentistry is to replace dentists. a brand new era of ai is going to dawn as a direct result of the rapid advancement of technology. the progression of artificial intelligence and machine learning has resulted in the development of unique ways the use zro2 in dental devices for biomedical purposes. the implementation of ai in the field of zro2 science shifts depending on the direction in which zro2 is applied. in the current day, ai-ml technologies have transitioned from being a concept of the future to practical use in everyday life. researchers from a wide variety of professions had in-depth conversations about the effect that it had on society, the economy, the healthcare system, and politics. additionally falls under this category in the field of dentistry. it is widely held that ai will play a crucial role in advancing dentistry and contributing to its future growth. http://dx.doi.org/10.5599/jese.1498 j. electrochem. sci. eng. 00(0) (2022) 000-000 zro2 material in biomedical science 4 figure 2. an example of the clinical applications of the ann [30] {creative commons attribution 4.0 international license} preparation of artificial tooth ai technology a complex process is followed during the preparation of zro2 restorations. the job that a dentist does daily includes preparing teeth for crowns and bridges. even though the dentist has years of expertise, the job is nevertheless difficult. the most difficult part of the process is figuring out how to preserve as much of the natural tooth as possible while yet leaving enough room for restoration. tooth preparation normally utilizes mechatronics engineering. the use of a robotic arm as a tool to aid dentists in the process of tooth preparation is an intriguing and astute suggestion. a dental drill was the first invention by simon et al. [33] which was the first electromechanical system. during the process of tooth preparation, the robotic arm may assist the dentist in operating the instrument more accurately and smoothly. the mechatronic technology lessens the likelihood of iatrogenic oral injuries and may decrease the number of handshakes that are necessary due to weariness. using this mechatronic system resulted in a 53 % increase in positional accuracy. the mechatronic system improved the accuracy by giving support and stability while the dentist was working with dental drills. the general agreement and goal of the global medical community are to go in the direction of precision medicine. as a result, the great precision, dexterity, and speed of the robot may eventually surpass the limitations of manual operation, therefore improving the effectiveness and precision of j. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1498 5 clinical operation [34]. yuan et al. [35] developed a robotic tooth preparation system to increase the quality, accuracy, and clinical effectiveness of the procedure. this was done to avoid the drawbacks of the constraints that conventional manual procedures provide. laserbot is a micro-robotic system that was developed by wang et al. [36] which was effective in tooth finishing by utilizing the laser beam. li et al. [37] developed a robotic manipulator system with a smaller and softer bracer for dentistry applications. this system was fitted with a tendon sheath transmission mechanism. this particular robot's electric-motor actuators don't have to be in close proximity to the manipulator. this system provides tool interchangeability and can be completely modified to meet the requirements of any dental operation. as a result, it has the potential to be used in a variety of contexts, such as the treatment of crowns and the elimination of caries. many other systems were developed to improve the precision of dentistry treatments as compared to conventional treatment [38,39]. ai in digital impression ai is also helpful in obtaining colorimetry and 3d impressions of the teeth and tissues [40]. this further helps in designing the restorations through readable 3d data. the dental prosthesis was produced using a process known as computer-aided manufacturing. at the moment, more recent research makes use of tooth preparation robots with a respectable level of intelligence and precision. these robots have become the direction that the development of digital dental prostheses is heading. in the realm of dentistry, high-precision restorations may be crafted with the use of cad/cam-based technology (figure 3). in addition, inlays, crowns, bridges, and inlays are designed and manufactured with the help of ai-based technologies [41]. figure 3. digital impression of teeth using itero scan [42] {creative commons attribution 4.0 international license} these systems take the place of the conventional approach to the creation of restorations, which has the potential to minimize the amount of time spent on production and the number of mistakes that may occur. dentists have greater standards than ever before for the ease with which their practices may be carried out. patients are becoming more particular about the aesthetics of their dental care, and they are hoping that their visits to the dental clinic would take less time. the cad/cam technology that is used in contemporary dentistry has become an essential component in the production of zirconia restorations. additionally, the digital intraoral impressions technology that http://dx.doi.org/10.5599/jese.1498 j. electrochem. sci. eng. 00(0) (2022) 000-000 zro2 material in biomedical science 6 is used in dentistry is regarded as an effective impression procedure [43,44]. a study was conducted by gao et al. [45] to examine the precision of digital scanning. they identified that digital impression scanning was far better than conventional intraoral and cast scanning. oh et al. [46] also suggested that scanning an impression is the most effective method for developing a digital dental model. ai in digital media digital technologies particularly digital information and communication, are finding more and more use in the dentistry industry [41]. computers play a vital role in dental practices [47,48]. a large-scale online survey was carried out by parmar et al. [49] to investigate the perspectives of patients and dentists regarding the utilization of social media platforms (specifically facebook) and their activities conducted online in the present day. they researched to investigate the prospects and possible hurdles associated with the adoption of social media techniques by dentists. they investigated the beliefs, ideas, and activities associated with using social media from the points of view of both patients and dentists. according to the findings, the level of contact and involvement of patients may be raised with a greater level of social media activity on the part of the dentist. they can contact their dentists more conveniently and efficiently via the use of social media platforms for their dental treatment. in the same vein, dentists may use digital media to connect with patients as well as cad/cam tools to fabricate restorations [49]. this made the clinical job of the dentist easier and more efficient. in the future, ai-ml will digitalize the numerous phases throughout the process of aesthetic treatment by assessing through digital smile design [50]. ai in dentistry labs additionally, ai-adapting dentistry labs can learn from the experiences of millions of patients in order to create more effective designs for prostheses inside design software used for restoration [51]. an ai system may be provided with a data set for picture training. within this system, one network can concentrate on creating a newer image. however, at the same time, another network attempts to determine which photos are false and which are genuine. with the use of this technology, restorations may be crafted to look exactly like the patient's native anatomy. library-based systems tend to create more intricate anatomical structures comparable to surrounding dentitions that wear down with usage. this is especially true for elderly individuals. the design that was produced by the gan program successfully matches the patterns and detail that occur as a result of wearing dentures. use of ai in zro2 biomedical applications in dentistry zro2 in dentistry researchers and dentists are currently focusing on producing an aesthetically pleasing restorative material that does not include any metals because of rising concerns about cytotoxicity and allergic reactions associated with certain metals [52,53]. in restorative dentistry, zro2 may be used in a variety of applications, including implants, abutments, posts, cores, crowns (both complete and partial), bridges, inlays and onlays, and veneers [54,55]. zirconia has been shown in previous clinical investigations to have an abrasive impact on dentition, which results in excessive wearing in the structure of the tooth [56-58]. in 2018, pjetursson et al. [59] conducted a comprehensive study to explore the survival rates of zro2 and metallic-ceramic crowns as well as the rates of technical, biological, and cosmetic complications associated with these crowns. single crown implants made of zirconia showed a 97.6 % survival rate after 5 years (95 % confidence interval: 94.3-99.0), and it exhibited a similar frequency of biological difficulties while having fewer cosmetic issues. j. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1498 7 in addition, by studying the evolution of modern dental zro2 ceramics, zhang et al. aimed to make zirconia materials more transparent without compromising their strength [60]. zro2 might also be employed as an option for titanium implants even though it is a non-metallic biomaterial. additionally, zro2 exhibits high fracture toughness and flexural strength as compared to many other ceramics materials [61,62]. according to the findings of hashim and colleagues, the survival rates of 1and 2-piece zro2 implants after one year of function were found as 0.92 (0.95 confidence interval: 87-95) [63]. it has been found that monolithic zirconia with no veneer possesses greater fracture resistance than traditional zro2, and it is anticipated that this will lead to a decrease in the frequency of porcelain fracture in the region of the posterior teeth [64]. shen et al. performed a retrospective clinical analysis on the monolithic zro2 single crowns and tried to learn more about the performance of monolithic zro2 prostheses that are held in place by implants [65]. they took panoramic radiographs at various times throughout the therapy and the follow-up visit to research the marginal bone level (mbl). during the healing phase, patients whose implants were covered by monolithic zro2 saw mbl changes of 0.25 mm, whereas those whose implants were covered by conventional ceramics saw mbl changes of 0.43 mm. there are no statistically significant differences between the monolithic zirconia and metal-ceramic groups (p > 0.5), suggesting that both groups have similar rates of peri-implant bone resorption. zro2 crown implant material namely monolithic zro2 crowns (mzcs) mounted on the back of patients' mouths was included in a recent retrospective study by lerner et al. [66]. they checked the mzcs' chromatic integration, survival, and success rates. their research created the customized zro2 abutment in cad software, after which they obtained the initial visual imprint of the patient's mouth with the help of the cs 3600xr intraoral scanner. notably, the scientist employed a fully digital procedure to create the zirconia crown, automating the process of creating margin lines with ai. notably, the scientist employed a fully digital procedure to create the zirconia crown, automating the process of creating margin lines with ai. as a result, they were able to effectively produce mzcs that were subsequently cemented on bespoke hybrid abutments. according to the findings of the study, the success rate and survival rate of mzcs produced by an all-digital process were, respectively, 99.0 and 91.3 % after three years. prediction of the longevity of zro2 restorations dental restorations have a limited lifespan, and this lifespan is heavily influenced by the material that was used to create them [67]. zhang et al. [60] presented an overview of the several generations of commercial dental zirconia and a synopsis of each generation's mechanical and compositional characteristics. the first-generation 3y-tzps had to bend strengths more than 1 gpa in flexure. the sintered al2o3 content of these first-generation 3y-tzps was 0.25 weight percent. the next step is monolithic zro2, which was created to take into complete consideration the aesthetics and mechanical properties of zirconia. developing a partially stabilized zro2 with higher yttria contents, such as 4ypsz (4 mol.% ) or 5y-psz (5 mol.%), which produces a more non-birefringent c-phase, achieves this goal. this decreases the opacity of the material. the development of transparent zro2 resulted in several benefits, including improved mechanical qualities, increased decreased wear, less tooth preparation, and increased strength on antagonistic surfaces [68,69]. because of this, there are a wide variety of zirconia materials on the market, each with its distinct brand name and set of technical parameters, from which patients and dentists may pick. however, therapies for patients might change http://dx.doi.org/10.5599/jese.1498 j. electrochem. sci. eng. 00(0) (2022) 000-000 zro2 material in biomedical science 8 depending on the characteristics of the restorative material that is used. they have a hard time deciding which material will serve them the best and endure the longest. fortunately, ai is playing an important part in resolving this issue. for instance, aliaga et al. [67] gathered data from dr. vera's restorative therapy notes, graphs, and radiological data. they next used artificial intelligence (ai) to analyze the data gathered to find the best material and subsequently advance the creation of teeth restoration. in addition, ai might be utilized to estimate how long cad/cam crowns will last in the patient's mouth. case-based reasoning (cbr), a method developed by aliaga et al. [67], can model and predict how long dental restorations will survive. in a separate investigation that was carried out by yamaguchi et al. [68], an ai-based convolution neural network (cnn) was utilized to develop the cad/cam crowns. data was procured from 24 instances in total, of which half had debonding problems with their crowns. additionally, they acquired 8,640 2d images of the 3d models created from virtual teeth. according to the findings, artificial intelligence technology, namely the cnn approach, demonstrates improved performance in forecasting the likelihood of debonding in cad/cam crowns. matching of zro2 colors patients place more importance on the cosmetic qualities of their restorations, in addition to the zirconia material's reputation for durability and capacity for functional recovery. aesthetic dentistry places a significant emphasis on the processes of color matching and shade reproduction [69]. recently, a variety of zirconia ceramics, each with its distinct optical characteristics, have been available for purchase in the marketplace. when trying to match the color of the restoration to the patient's natural teeth, it can be challenging for both the patient and the dentist to select the proper configuration, appropriate material, and precise shade. a back-propagation neural network, also known as a bpnn, has previously been put to use in the dental clinic for computer color matching [70]. however, bpnn has some drawbacks, including low accuracy and instability. to improve the accuracy of the matching, the initial weight and thresholds in the bpnn, li et al. [70] used a genetic algorithm (ga). the findings of the experiment show that the suggested strategy plays a significant part in enhancing the consistency and accuracy of color matching when choosing repair materials. additionally, ai was utilized to forecast the shade of the teeth that would result from the bleaching treatment. the clinical decision support system that was created by thanathornwong et al. [71] used an ai-based regression model. results demonstrated that this approach was capable of accurately predicting the color shift by making use of colorimetric variables. zro2 abutment zro2 abutments are advised alternatively to metal abutments since they produce superior outcomes in terms of aesthetics. after five years, fixed implant single crowns with zirconia abutments had a 99.3 % success rate in the posterior locations, which did not show a statistically significant difference when compared to titanium abutments, which had a success rate of 99.57 % (p = 0.26). the research was conducted by vechiato-filho et al. [72] and was based on a systematic evaluation and analysis. in most cases, the bespoke abutment begins with the use of computeraided design (cad), which is followed by milling and zirconia sintered production [73]. during the extraoral cementation procedure, there is tolerance between the zro2 abutment and the boding foundation. this can lead to cementing mistakes [74]. even though they are extremely minor, these inaccuracies can lead to positioning issues for monolithic zro2 restorations when they are delivered to patients in the form of bespoke abutments and temporary restorations [75]. fortunately, the j. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1498 9 aforementioned challenges may be conquered with the help of ai, which has decreased the number of mistakes and the prosthetic therapy cost [76,77]. biomedical applications of zro2 additionally to its use in therapeutic applications, artificial intelligence has found widespread usage in zirconia-related research, being the subject of several studies [78]. hydroxyapatite (hap)/zro2-based composites were also used in biomedical applications. hap is a bioactive material used in metallic implants [12,13]. hap coated by plasma spraying is used in many dental and orthopedic prostheses [13]. arif et al. [79] developed an ann model to wear the performance of al (element) hybrid composites that were reinforced with nano zro2 (0-9 %). the use of ai was successful in studying the impact of several control parameters on hybrid composite wear behavior. the advancement of robotics, automated systems, and ai-integrated devices will be greatly aided by the creation of artificial muscle shortly. because of its substantial free surface area and fewer grain boundaries, zirconia shape-memory ceramics have the potential to dramatically improve shape-memory characteristics by an additional 8 %. du et al. [80] created highly aligned shape-memory zro2-based yarns and springs using ai as a consequence. these materials have the potential to be employed as artificial muscles at very high temperatures. in addition, zro2 is an essential transition metal-oxide that plays a significant role in the development of high-performance computer systems. the authors the behler-parrinello neural network (bpnn) may be employed in the molecular dynamics simulation of the o2 vacancy diffusion since its accuracy is similar to simulations [81] based on density functional theory (dft) [82]. conclusion and future perspective in conclusion, zro2 has received a great deal of attention in the field of dentistry since it is highly biocompatible, has appealing aesthetics, is very resistant to corrosion, and does not cause allergic responses. the use of technology that utilizes artificial intelligence is hastening the transition from one period to the next in the field of dentistry. the progression of artificial intelligence and machine learning has resulted in the development of unique ways the use zirconia in dental devices for biomedical purposes. as a result, having a solid comprehension of the principles behind ai technology and applications will be advantageous in the years to come. we have high hopes that all aspects of dentistry will soon be able to make full use of ai in their respective disciplines. references [1] t. joda, m. ferrari, g. o. gallucci, j.g. wittneben, u. brägger, digital technology in fixed implant prosthodontics, periodontol 2000 73 (2017) 178-192. https://doi.org/10.1111/prd.12164 [2] p. jain, m. gupta, digitization in dentistry: clinical applications, springer nature, cham, switzerland, 2021. https://doi.org/10.1007/978-3-030-65169-5 [3] j. m. helm, a. m. swiergosz, h. s. haeberle, j. m. karnuta, j. l. schaffer, v. e. krebs, a. i. spitzer, p. n. ramkumar, machine learning and artificial intelligence: definitions, applications, and future directions, current reviews in musculoskeletal medicine 13 (2020) 69-76. https://doi.org/10.1007/s12178-020-09600-8 [4] s. deshmukh, artificial intelligence in dentistry, journal of international clinical dental research organisation 10 (2018) 47-48. https://doi.org/10.4103/jicdro.jicdro_17_18. 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functional theory parameterised neural network model of zirconia, molecular simulation 44 (2018) 623-630. https://doi.org/10.1080/08927022.2017.1420185 [82] j. singh, s. singh, materials science & engineering b a review on machine learning aspect in physics and mechanics of glasses, materials science and enginering b 284 (2022) 115858. https://doi.org/10.1016/j.mseb.2022.115858 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1498 https://doi.org/10.1080/08927022.2017.1420185 https://doi.org/10.1016/j.mseb.2022.115858 https://creativecommons.org/licenses/by/4.0/) ellipsometric measurement of thickness of tin oxides grown by voltammetry in phosphate solution of ph 8.7 doi:10.5599/jese.326 303 j. electrochem. sci. eng. 6(4) (2016) 303-314; doi: 10.5599/jese.326 open access : : issn 1847-9286 www.jese-online.org original scientific paper ellipsometric measurement of thickness of tin oxides grown by voltammetry in phosphate solution of ph 8.7 tiago brandão costa*,, tania maria cavalcanti nogueira*,**, ladário da silva*,*** *programa de pós-graduação em engenharia metalúrgica (ppgem), escola de engenharia industrial metalúrgica de volta redonda (eeimvr), universidade federal fluminense (uff), 27255125 volta redonda, rj, brazil **departamento de metalurgia, eeimvr, uff, 27255-125 volta redonda, rj, brazil ***departamento de física, instituto de ciências exatas (icex), uff, 27213-145 volta redonda, rj, brazil corresponding author: tiagobrandao@id.uff.br, phone: +55-2421073731 received: july 5, 2016; revised: november 25, 2016; accepted: november 28, 2016 abstract the voltammetry induced growth of tin oxides on tin in the buffer solution of 0.18 mol l-1 na2h2po4 and 0.18 mol l-1 kh2po4 (ph 8.7) has been studied. ex-situ ellipsometric measurements were made in an order to determine thicknesses of the grown oxides. from these results the film volume per charge unit, vf, was calculated for different charge densities of the film. this parameter was used to calculate the variable ionic resistivity of the film, ρf, considered by the ohmic model for the case of voltammetric growth of oxides on metals having a previously existing continuous film. tin oxide films grown at 2 mv s-1 showed to be less dense for values of charge density below 50 c m-2, having vf near 5.7x10-10 m3 c-1. for higher values of charge density, tin oxide films become denser, having vf near 0.5x10-10 m3 c-1. the calculated values of the variable ionic resistivity of the film during voltammetric growth showed that ρf passes through a minimum (justifying the maximum in current densities). this behavior was also found by other authors in the cases of zn, nb, ni and galvanized steel sheets. keywords ellipsometry, tin oxide, ohmic model, voltammetry, variable ionic resistivity introduction active metals cannot be obtained without an initial thin film covering the surface, as have already been observed in the cases of cd, fe, ni, pb, sn, zn, valve metals, etc. for this reason, the studies of oxide growth on these metals are found very important. the growth of passivating films on metals http://www.jese-online.org/ mailto:tiagobrandao@id.uff.br j. electrochem. sci. eng. 6(4) (2016) 303-314 measurement of thickness of tin oxides grown 304 under voltammetric conditions gives rise to peaks or polarographic waves in current/potential plots. this phenomenon is related to the active/passive transition, or growth of a continuous film on the metal surface. some electrochemical processes, such as electrodeposition of metallic ions [13] or growth of oxide on metals [48], can be considered as processes presenting ohmic behaviour. the ohmic model developed by d’alkaine [4] describes the relation between the current density and the film overpotential during the growth of passivating film under voltammetric conditions. this model considers that the relation between the electrode potential e and overpotential at the metal/film/solution interface is given by: e = m/f + f + ef (1) in eq. (1) ηm/f is the overpotential at the metal/film interface, ηf is the overpotential across the film and ef is the flade potential. the overpotential at the film/solution interface is considered constant, i.e. independent of the current density, j. the relation between current density (j) and overpotential at the metal/film interface (ηm/f) is given by the general expression:     0m/f a m/f c m/fexp expj j nf nf        (2) in eq. (2), 0 m/fj is the exchange current density at zero m/f, f is f/rt, αa and αc are anodic and cathodic transfer coefficients, while n is charge of the metal ion in the film. if vf and qf are respectively the volume per charge unit and the charge density related to the growing film, then the film thickness, ℓ, is given by: ffqv (3) the qf value can be calculated as:   e e qej v qqq i 0a0voltf d 1 (4) in eq. (4) q0 is the charge density related to the amount of film initially present at the beginning of the voltammetric growth and qvolt is the charge density related to the amount of film which has grown on the metal surface during the voltammetric experiment. ei is the initial potential, e is the potential attained and ja is anodic component of the current density, taking into account eq. (2). for peak or plateau, transient conditions during voltammetric growth of a film on a metal surface, the following equation should be valid for any model: pf, p pf, q j v  (5) in eq. (5) ηf.p is the overpotential across the film at the voltammetric peak, v is the sweep rate, jp is the current density at the peak and qf,p is the peak or plateau charge density. in the case of the ohmic model, the relation between j and ηf, even at high fields, is given as: f = vf f qf j (6) in eq. (6), ρf is the average ionic specific resistivity of the film. the ohmic model takes into consideration that at the beginning of the scanning, the current density increases as a consequence of reduction of ionic resistivity of the film due to injection of defects in the film. beyond the peak potential, it is possible to observe reduction of the current t. b. costa et al. j. electrochem. sci. eng. 6(4) (2016) 303-314 doi:10.5599/jese.326 305 density, what is explained by ageing of the oxide film. this phenomenon turns the film more resistive to the passage of the current due to recombination of defects and to structural rearrangement [4]. this model, however, contains an intrinsic difficulty related to impossibility to determine ρf and vf separately, using only electrochemical measurements. despite this fact, many authors [48] applied the ohmic model, considering the volume per charge unit (vf) as the constant equal to: nf m v f (7) in eq. (7) m is the molar mass of the film, δ is density of the film, n is the number of electrons and f is the faraday’s constant. the real values of vf during the voltammetric growth could only be determined by independent measurements of the film thickness. the aim of the present work is to study the voltammetric growth of tin oxides in a solution of phosphate (ph 8.7) applying the ohmic model. this solution was chosen in view of stability of the passive film on tin at this ph, avoiding thus loss by chemical dissolution [9]. at the same time, there are some early studies concerning the passivating species under voltammetric growing conditions in the ph range 8–9 [10–14]. the thickness of the oxides grown by voltammetry will be determined by ex-situ ellipsometric measurements in an order to verify the intrinsic difficulty explained above. the ex-situ condition was first examined as a preliminary study in order to verify stability of the film in the atmosphere. experimental electrochemical measurements the working electrode was made of a tin disc (99.999 % of purity) with the circular area of 50 μm2. before the experiments, the electrode surface was polished with 600-emery paper. the electrochemical experiments were performed using the eg&g princeton applied research model 273a potentiostat. reagents pa and purified water (millipore q system) were used. the electrolyte solution was 0.18 mol l-1 na2h2po4 + 0.18 mol l-1 kh2po4, ph 8.7. the experiments were carried out in a conventional three-compartment electrolysis cell, using a platinum wire as the counter and hg/hg2cl2/kcl (1 m) as the reference electrode, respectively. all current and charge densities are given in terms of the geometric surface area of the analyzed samples. anodic voltammetries were carried out at sweep rates of 2, 5, 10, 20, 70, 100, 200, 300 and 400 mv s-1, always using the same sample. for this purpose, before each voltammetry experiment, the previously grown oxide film was reduced at constant cathodic potential equal to -1.2 v during 600 s. after this treatment, the obtained voltammograms were reproducible, indicating that surface roughness of samples was also reproducible. ellipsometric measurements ellipsometry [15] is a powerful and non-destructive technique, which allows one to access optical and dielectric properties of various materials in different forms (solids, liquids, and gases), as well as thicknesses of thin films [1617]. it can also be used to monitor and analyze both in-situ [1819] and ex-situ [20] experiments. ellipsometry considers the change in the polarization state of incident light upon reflection from the studied material. initially it accesses the ellipsometric parameters  and . these parameters are connected by the fundamental equation of ellipsometry [16]: j. electrochem. sci. eng. 6(4) (2016) 303-314 measurement of thickness of tin oxides grown 306 ip j j s tan( ) ( , , ) j r e f n k d r    (8) in eq. (8), rp and rs stand for the complex fresnel reflection coefficients for p and s polarized light beams, respectively.  is the amplitude ratio for the reflected and polarized components (p and s), while  represents the phase difference between reflected p and s polarizations, created by reflection from the material. nj, kj, dj are, respectively, the refractive index, the extinction coefficient and thickness of the jth layer of the material. the ex-situ ellipsometric measurements were made using a semilab spectroscopic ellipsometer, model sopra ges 5e, equipped with a xe lamp, over the spectrum range of 195 – 1,000 nm. the investigation of the thickness of oxide films grown on the substrate of tin has been performed in air and at ambient temperature using the incident angle of 75°. as an indirect technique, it is necessary to model the material structure with its film layers in an order to obtain each layer film thickness. the thicknesses of the oxide films were obtained by analyzing the measured ellipsometric spectra through the drude and gauss model [16]. results and discussion as was explained before, in an order to obtain reproducibility, the voltammetric experiments were always carried out using the same sample. before each voltammetry measurement, the oxide grown in the previous experiment was reduced at the constant cathodic potential during 600 s. this cathodic treatment was sufficient to reproduce the initial roughness of the surface, which was verified by reproducible voltammograms obtained at the same sweep rate and shown in fig. 1. fig. 1. first and second cycle at 50 mv s-1 in the phosphate solution ph 8.7 the anodic voltammograms for tin oxide growth at different sweep rates are shown in fig. 2. from these results, the overpotential values in the film at the peak condition (ηf,p) were calculated using the eq. (5). the value of q0 in eq. (4) was first considered equal to 0.0 c m-2. the curve j vs. potential at the metal/film interface (em/f), also shown in this figure, is obtained after correction of the ohmic drop through the film at the peak potential (ep ηf,p). t. b. costa et al. j. electrochem. sci. eng. 6(4) (2016) 303-314 doi:10.5599/jese.326 307 fig. 2. voltammetric growths of tin oxide film, together with the plot of the calculated j vs. (ep – ηf,p) relation at the metal/film interface, considering q0 = 0.0 c m-2. the tafel plot of the curve j vs. em/f in fig. 2 is represented in fig. 3. fig. 3. tafel plot at the tin/oxide interface. in order to provide the best straight line region of the tafel plot, the value of q0 was taken equal to 2.5 c m-2. fig. 4 presents the obtained result. from these results, the obtained tafel slope (ba) is equal to 41.995 mv dec-1. by using eq. (9) [21] rt nf b m/f a   (9) the transfer coefficients at the metal/film interface has been calculated as αm/f = 1.07. ln ( j p / a m 2 ) j. electrochem. sci. eng. 6(4) (2016) 303-314 measurement of thickness of tin oxides grown 308 fig. 4. tafel plot at the tin/oxide interface, considering q0 = 2.5 c m-2. fig. 5 presents the corrected curve j vs. em/f, which was obtained from the tafel plot of fig. 4. the anodic voltammograms are also shown in the same figure. fig. 5. voltammetric growths of tin oxide film, together with the plot of the calculated j vs. (e – ηf) relation at the metal/film interface, considering q0 = 2.5 c m-2 and figure 4. these results turn possible to determine the overpotential at the film (ηf) for any growing condition of the film beyond the peak condition. this can be done by calculating the difference of the potential in the voltammogram at any sweep rate and the potential in the curve j vs. em/f for a given current density. this is shown in fig. 5 for the voltammogram at 400 mv s-1. thus, by calculating, ηf values for each potential of each voltammetry experiment, it becomes possible to determine the ionic specific resistivity of the film (ρf) and its variation with e and v using eq. (6). ln ( j p / a m 2 ) t. b. costa et al. j. electrochem. sci. eng. 6(4) (2016) 303-314 doi:10.5599/jese.326 309 as was explained in the introduction of this work, the real values of vf during the voltammetric growth could only be determined by an independent way of measuring the thickness of the film according to the following equation: f f q a v   (10) in eq. (10), ℓ is thickness and a is surface area the working electrode. the thicknesses of oxides films grown at 2 mv s-1 were measured by ex-situ ellipsometry. this condition was chosen because using this sweep rate, higher thicknesses can be achieved. fig. 6 presents the final potential (ef) of each voltammetry experiment performed at 2 mv s-1. fig. 6. voltammetric growth of tin oxide films at 2 mv s-1, together with indication of the final potential, ef, and corresponding film charge density values. as pointed out before, spectroscopic ellipsometer was used to obtain thin oxide film thicknesses for various charge densities. the proposed oxide layer structure is presented in fig. 7. this oxide layer structure can be optically modelled using the both known refractive index (n) and extinction coefficient (k) of an available library (nk files) or dispersion relations for each of its layer constituents. using the drude and gauss dispersion relations [16] for the film, the excellent fit was obtained between the used model and experimental data. sno2 sno sn substrate fig. 7. model for the tin oxide layer structure. fig. 8 exhibits typical results of fittings for measured and modelled values of tan  and cos  for charge densities equal to 5.9 c m-2 and 215.0 c m-2. j. electrochem. sci. eng. 6(4) (2016) 303-314 measurement of thickness of tin oxides grown 310 fig.8. typical plots of measured (open circles) and modelled (blue line) of tan () and cos () for tin oxide films and charge densities equal to: (a) 5.9 c m-2 and (b) 215.0 c m-2 . it is clear from the graphs in fig. 8 that excellent fits (r2  0.99) were obtained, what make us confident to report the obtained thicknesses. the average thicknesses obtained considering eight measures at each charge density, are shown in table 1. except thicknesses, ℓ, in table 1, one can also see values of the final potential, ef, charge density of the film, qf, and statistical results (r2, as well as standard deviations of the average thickness values, ). data in table 1 clearly show that for greater ef and qf, greater average film thickness is obtained. table 1: final potentials, charge densities, thicknesses, average thicknesses and statistical results of calculations ef / v qf / c m-2 ℓ / nm r2 average thicknesses, nm  -0.850 5.9 3.8 3.8 3.5 3.5 3.0 3.1 3.0 3.3 0.994 3.37 0.307 0.810 26.6 4.3 3.8 4.6 4.5 3.9 4.0 3.8 3.7 0.990 4.10 0.323 0.796 51.2 4.3 4.5 4.6 4.1 3.9 4.3 4.1 4.5 0.975 4.29 0.226 0.730 129.2 9.4 9.0 9.3 8.9 9.4 9.0 9.3 9.1 0.981 9.20 0.185 0.300 215.0 10.7 11.6 10.3 11.3 10.3 11.0 12.2 10.8 0.995 11.00 0.612 0.100 268.1 13.7 12.5 13.2 12.9 12.4 11.2 11.8 11.5 0.987 12.40 0.806 in the preliminary experiment, a sample was left in the atmosphere during 24 h in order to measure the stabilized oxide film thickness and the resulting value was 2.0 nm. in table 1, however, one can notice that the lowest average thickness for the film grown in conditions of voltammetric (bb) t. b. costa et al. j. electrochem. sci. eng. 6(4) (2016) 303-314 doi:10.5599/jese.326 311 experiments was 3.37 nm. this suggests that here obtained results are significantly influenced by the voltammetric way of film growth. using the experimentally found values of average thicknesses, eq. (10) was applied and vf was determined for the values of charge density of voltammetry grown films at 2 mv s-1. the results are shown in table 2 and fig. 9. table 2: film volume per charge unit obtained from the thicknesses measurements of tin oxide films for different charge densities. qf / c m-2 ℓ / nm vf / 10-10 m3 c-1 5.9 3.37 5.70 26.6 4.10 1.54 51.2 4.29 0.84 129.2 9.20 0.71 215.0 11.00 0.50 268.1 12.40 0.46 fig. 9. film volume per charge unit determined for different values of charge densities. (●) experimental vf, (---) vf = 1.08×10-10 m3 c-1 (considering sno) and (─) vf =0.568×10-10 m3 c-1 (considering sno2) data in table 2 and fig. 9 show that the values of vf significantly decrease until 50 c m-2 of charge density of the film is achieved. this suggested that the film is less dense for lower values of charge density and becomes denser as the thickness increases [22]. besides that, the value of vf for charge density of 26.6 c m-2 is close to the value of vf calculated by eq. (7), considering a film of sno (1.08×10-10 m3 c-1). for higher values of charge density, the value of vf turns close to the value of vf calculated considering a film of sno2 (0.568×10-10 m3 c-1). these results suggest that changes in the composition of the film are taking place as the potential turns more anodic. in fact, duc and tissot [23], studying the oxidation of tin in neutral phosphate solution, suggested that sn(oh)2 or sno and sn(oh)4 or sno2 are present on the electrode surface at the beginning of polarization. at more anodic potentials only stannic species are formed. electrochemical behavior of tin has also been studied in aqueous alkaline solutions [10–11,14,24–29]. the authors considered that primary passivation occurs due to formation of sn(oh)2 or sno film by a dissolution-precipitation j. electrochem. sci. eng. 6(4) (2016) 303-314 measurement of thickness of tin oxides grown 312 mechanism. the final passivation is characterized by a continuous film of sn(oh)4 [10]. then, more stables species like sno2 or sno2.h2o can be formed from dehydration of this last film [9]. some authors have suggested a duplex film [28], consisting of 5sno 2h2o and sno2.h2o [29]. all these results emphasized importance of ellipsometric measurements in order to determine the thickness of the films. in fact, the exact composition of the film cannot be determined electrochemically. by introducing experimentally determined vf values into eq. (6) the values of ρf can be calculated. fig. 10 illustrates the results obtained using the values of charge density in table 2 for the case of the voltammetry experiment at 2 mv s-1. fig. 10. ionic specific resistivity vs. charge density of the film for sweep rate equal to 2 mv s-1. as can be observed in fig. 10, ρf passes through a minimum (justifying the maximum in current densities). this behavior was also found by other authors for zn, nb, ni and galvanized steel sheets [48]. according to the theory, this happens because the passage of current in the film of initial thickness (q0) generates injection of specific defects, what resulted in decrease of ρf (inversely proportional to concentration of defects) [48]. increase of recombining specific defects (interstitial and cationic vacancies) in the film ends up by generating the recombination reaction (interstitial cation + cationic vacancy → cationic net), and making ρf to increase again [48]. conclusions the ex-situ ellipsometric measurements of the thickness of tin oxides film grown by voltammetry experiments at 2 mv s-1 in the studied solution showed that the film is less dense for values of charge density below 50 c m-2 having vf near 5.7×10-10 m3 c-1. for higher values of charge density, the film becomes denser having vf near 0.5×10-10 m3 c-1. from the experimentally determined values of vf, the values of the variable ionic resistivity of the film during the voltammetric growth, ρf, were calculated. the behavior of ρf versus the charge density of the film was like that found by other authors in the cases of zn, nb, ni and galvanized steel sheets. acknowledgements: the author (tiago brandão costa) is grateful to capes for his doctor fellowship. the authors are grateful to programa de pós-graduação em engenharia metalúrgica (ppgem), escola de engenharia industrial metalúrgica de volta redonda (eeimvr), universidade federal t. b. costa et al. j. electrochem. sci. eng. 6(4) (2016) 303-314 doi:10.5599/jese.326 313 fluminense (uff) for the opportunity of realizing this work, making the laboratory of electrochemical available. notation a electrode surface area, m2 ba tafel slope e potential, v ef flade potential, v em/f potential at the metal/film interface, v f faraday’s constant, c mol-1 j current density, a m-2 jp current density at the voltammetric peak, a m-2 0 /m fj exchange current density at zero m/f, a m -2 ℓ thickness of oxide layer, m m molar mass of the film, kg n number of electrons qf charge density of the film, c m-2 q0 charge density related to the amount of film initially present, at the beginning of the voltammetric growth on the metal surface, c m-2 qvolt charge density related to the amount of film which has grown during voltammetric experiment, c cm-2 qf,p peak or plateau charge density, c cm-2 r gas constant, 8.314 j/mol k se spectroscopic ellipsometry t temperature, k vf volume per charge unit, m3 c-1 αa anodic transfer coefficient αc cathodic transfer coefficient δ density of the film, kg m-3 ηm/f overpotential at the metal/film interface ηf.p overpotential across the film at the voltammetric peak ηf overpotential across the film ν sweep rate, v s-1 ρf ionic specific resistivity of the film, ω m references [1] a. s. gliozzi, a. l. alexe-ionescu, g. barbero, phys. lett. a. 379 (2015) 2657-2660. [2] a. l. alexe-ionescu , g. barbero , s. bianco, g. cicero, c.f. pirri, j. electroanal. chem. 669 (2012) 21-27. [3] g. barbero, p. batallioto, a. m. figueiredo neto, j. appl. phys. 101 (2007) 054102(1)054102(5). [4] c. v. d’alkaine, p. c. tulio, m. a. c. berton, electrochim. acta 49 (2004) 1989-1997. [5] c. v. d’alkaine, l. m. n. souza, f. c. nart, corr. sci. 34 (1993) 129-149. [6] c. v. d’alkaine, m. n. boucherit, j. electrochem. soc. 10 (1997) 3331-3336. [7] c. v. d’alkaine, m. a. santanna, j. electroanal. chem. 457 (1998) 13-21. [8] t. b. costa, c. v. d’alkaine, t. m. c. nogueira, 67th abm international congress, voltammetric growth of zno on galvanized steel sheets containing sb or pb, rio de janeiro, rio de janeiro, 2012, p. 3162. j. electrochem. sci. eng. 6(4) (2016) 303-314 measurement of thickness of tin oxides grown 314 [9] m. pourbaix, atlas of electrochemical equilibria in aqueous solution, pergamon, oxford, 1966. [10] s. d. kapusta, n. hackerman, electrochim. acta 25 (1980) 1625-1639. [11] m. metikos-hukovic, a. resetic, v. gvozdic, electrochim. acta 40 (1995) 1777-1779. [12] t. hurlen, electrochem. acta 39 (1994) 1359-1364. [13] s. a. m. refaey, electrochim. acta 41 (1996) 2545-2549. [14] v. brunetti, m. l. teijelo, j. electroanal. chem. 613 (2008) 9-15. [15] h. fujiwara, spectroscopic ellipsometry: principles and applications. john wiley & sons ltd, tokyo, japan, 2003. [16] d. e. aspnes, thin solid films 571(3) (2014) 334-344. [17] k. vedam, thin solid films 313–314 (1998) 1 9. [18] l. arsov, i. mickova, j. electrochem. sci. eng. 5(4) (2015) 221-230. [19] i. arsova, lj. arsov, n. hebestreit, a. anders, w. plieth, j. solid state electrochem. 11 (2007) 209-214. [20] l. f. n. guedes et al. j. solid state electrochem 20 (2016) 2517-2523. [21] a. j. bard and l. r. faulkner, electrochemical methods fundamentals and applications, john wiley & sons, new york, united states, 2001. [22] k. wasa, m. kitabatake, h. adachi, thin films materials technology: sputtering of compound materials, william andrew, new york, united states, 2004, p.28. [23] h. do duc, p. tissot, corros. sci. 19 (1979) 179-190. [24] c. a. gervasi, p. e. alvarez, corros. sci. 47 (2005) 69 -78. [25] s. d. kapusta, n. hackerman, electrochim. acta 25 (1980) 949-955. [26] s. d. kapusta, n. hackerman, electrochim. acta 25 (1980) 1001-1006. [27] s. d. kapusta, n. hackerman, electrochim. acta 25 (1982) 1886-1889. [28] m. metikos-hukovic, m. seruga, f. ferina, ber. bunsenges, phys. chem. 96 (1992) 799-805. [29] a. ammar, s. darwish, m. w. khalil, s. el-taher, electrochim. acta 33 (1988) 231-238. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.sciencedirect.com/science/journal/00406090/571/supp/p3 http://creativecommons.org/licenses/by/4.0/ synthesis of cuinse2 thin films from electrodeposited cu11in9 precursors by two-step annealing doi: 10.5599/jese.2013.0042 27 j. electrochem. sci. eng. 4(1) (2014) 27-35; doi: 10.5599/jese.2014.0042 open access : : issn 1847-9286 www.jese-online.org original scientific paper synthesis of cuinse2 thin films from electrodeposited cu11in9 precursors by two-step annealing tsung-wei chang, shao-yu hu, wen-hsi lee department of electrical engineering, national cheng kung university, tainan 701, taiwan, r.o.c corresponding authors: e-mail: leewen@mail.ncku.edu.tw received: february 12, 2013; revised: november 22, 2013; published: january 25, 2014 abstract in this study, copper indium selenide (cis) films were synthesized from electrodeposited cu-in-se precursors by two-step annealing. the agglomeration phenomenon of the electrodeposited in layer usually occurred on the cu surface. a thermal process was adopted to turn cu-in precursors into uniform cu11in9 binary compounds. after deposition of the se layer, annealing was employed to form chalcopyrite cis. however, synthesis of cis from cu11in9 requires sufficient thermal energy. annealing temperature and time were investigated to grow high quality cis film. various electrodeposition conditions were investigated to achieve the proper atomic ratio of cis. the properties of the cis films were characterized by scanning electron microscopy (sem), x-ray diffraction (xrd), and raman spectra. keywords cuinse2, cis, annealing, electrodeposition introduction the solar cell has emerged as a very important non-conventional energy source. copper indium selenide (cuinse2) is a i–iii–vi group semiconductor compound offering good possibilities for thinfilm photovoltaic (pv) applications because it has a energy gap of 1.02 ev [1–5]. electrochemical deposition is a low cost method of producing thin cis films because it has several advantages for large-area non-vacuum thin film production and little material waste. however, the crystallinity of cis film grown by single-step electrodeposition is inferior because its growing temperature is much lower than that of the physical vapour deposition (pvd) method. the grains were small and loose. the cis film can also be synthesized from co-sputtered cu11in9-se precursors by the thermal annealing process [6,7]. large cis grains can be grown due to the gas–liquid reaction during the annealing process [8]. the cu-in alloys are usually co-sputtered by pvd. the pvd technology is http://www.jese-online.org/ mailto:leewen@mail.ncku.edu.tw j. electrochem. sci. eng. 4(1) (2014) 27-35 synthesis of cuinse2 thin films from cu11in9 28 excellent for good quality film growth but difficult to scale up because of the high manufacturing costs. in this study, cu-in precursors were prepared by multi-step electrodeposition. however, the agglomeration phenomenon of the electrodeposited in layer usually occurred on the cu surface. the surface was non-uniform and discontinuous. an annealing process was adopted to transform cu-in to cu11in9 compound and create a uniform surface structure. after deposition of the se layer on the annealed cu11in9, another annealing process is required to synthesize the cis structure. various annealing temperatures and times were adopted to investigate the proper annealing conditions and the mechanism of cis synthesis. the deposition conditions were adjusted to achieve a better atomic ratio. the properties of the cis films were characterized by scanning electron microscopy (sem), x-ray diffraction (xrd), and raman spectra. experiment the aqueous solution for the cu deposition contained 0.75 m cuso4, 4 mm h2so4, and 0.5 mm hcl. the aqueous solution for the se deposition contained 17 mm h2seo3 and 0.5 mm hcl. the aqueous solution for the in deposition contained 50 mm incl3 and 30 mm hcl. the electrodepositions were carried out with autolab pgstat302, a conventional threeelectrode potentiostat, and the deposition conditions listed in table 1. a thin slice of 99.99% pure pt electrode measuring 1 × 4 cm was employed as the counter electrode, and an ag/agcl electrode served as the reference electrode. glass substrates with sputtered mo film were used as the working electrodes. the electrodeposition area was a square measuring 1 x 1 cm. the substrates were cleaned by ultrasonication in acetone, 99.5 % pure ethanol, and water before sputtering and electrodeposition. a magnetic stirrer was used for the stirring procedure. the rotation speed of the magnetic stirrer was set at 50 rpm. table 1. electrodeposition and annealing condition of samples a f. cu in se tannealing / °c  = 5 min) i / ma : / s  / s (i = 5 ma) i / ma / s sample a 60 : 20 + 20 : 10 350 4 1450 550 sample b 60 : 20 + 20 : 10 350 4 1450 600 sample c 60 : 20 + 20 : 10 350 4 1450 650 sample d 70 : 20 + 20 : 10 350 4 1450 650 sample e 60 : 10 + 30 : 10 375 4 1450 650 sample f 50 : 10 + 20 : 10 350 4 1450 650 the in deposition was carried out after the two-step growth of cu. the cu-in layers were treated by rapid thermal annealing (rta) at 500 °c for 5 min. the se layer was deposited on the cu-in layer after annealing. the cis film was synthesized by annealing the cu-se and in-se precursors. the deposition conditions and rta listed in table 1 were found to achieve better proportions and structures. the surface morphology and chemical composition of the films were characterized by sem (philips xl-40feg) and eds, respectively. the raman spectra were produced with a backscattering configuration at room temperature with unpolarized light using a dilor xy 800 spectrometer and ts-w chang at al. j. electrochem. sci. eng. 4(1) (2014) 27-35 doi: 10.5599/jese.2014.0042 29 an ar laser with a 514.5 nm wavelength as the light source. the phase composition and the crystallographic structure were analysed by xrd using a bruker d8 sss multipurpose thin-film x-ray diffractometer. results and discussion in this study, cuinse2 thin films were synthesized from electrodeposited cu, in, and se thin-film precursors. however, the electrodeposition of the in layer on the copper surface with a current of 60 ma in 20 s + 20 ma in 10 s induced serious agglomeration phenomenon. figure 1 shows the sem image of the deposited in on the cu layer. it was deposited with a current density of 3-6 ma cm -2 . the agglomeration phenomenon made it difficult for the in layer to cover the whole surface. the grains of in were separated. deposition with a high current density can lead to a better distribution of in. however, it also leads to large over potential, which would cause bubbles and a rough structure. a suitable value of 5 ma was employed for the deposition of in. after the deposition of se, the cu-in-se precursor was annealed by rta to form the cis structure. the xrd results in figure 2 show that the cis structure can be synthesized at 450-550 °c. however, the sem images in figure 3 show that the film has a rough surface and a non-uniform grain size. the cis structure came from the non-uniform in layer and the miscellaneous precursor type. the precursors not only contain cu, in and se, but also some binary compounds, including cu11in9, cuxse and in2se3, which were produced in the electrodeposition and annealing process. different precursors have different reactions and temperature requirements to form cis, and other reactions lead to a uniform structure [9]. fig. 1. sem images of deposited in on cu surface with current densities (a) 3 ma cm -2 (b) 4 ma cm -2 (c) 5 ma cm -2 (d) 6 ma cm -2 for 200s. j. electrochem. sci. eng. 4(1) (2014) 27-35 synthesis of cuinse2 thin films from cu11in9 30 2 / ° fig. 2. xrd of annealed cis film synthesized from cu-in-se precursor with temperature. fig. 3. sem images of annealed cis film synthesized from bilayer electrodeposition of cu-in-se precursors with temperature (a) 250 °c, (b) 350 °c, (c) 450 °c, (d) 550 °c. in te n si ty , a .u . ts-w chang at al. j. electrochem. sci. eng. 4(1) (2014) 27-35 doi: 10.5599/jese.2014.0042 31 in order to create a smooth in layer distribution and uniform cis synthesis, a thermal pretreatment at 500 °c for 5 min was employed to improve the topography of the cu-in layer. figure 4 shows the sem image of the cu-in thin film after annealing at 500 °c for 5 min. the film re-grows continuously and covers the whole cu surface. figure 5 shows the xrd pattern of the annealed cu-in precursor. most of the precursors were turned to cu11in9 and small amounts to cuinse2. fig. 4. sem images of electrodeposited in layer on cu surface (a) as-deposited film (b) annealed film. 2 / ° fig.5. xrd pattern of annealed cu/in precursors. the se layer was deposited on the annealed cu-in surface at 4 ma cm -2 current density for 1200 s. all of the precursors were recrystallized by rta at 600 °c for 3 min and observed by sem and eds. figure 6(a) shows the microstructure of the annealed cis film at 600 °c for 3 min. the grain size of the annealed cis film synthesized from cu11in9-se precursors is much larger than that of cis film annealed by co-electrodeposition. during the thermal annealing, the cu-in became liquid phase and reacted with the gas phase se. cis grain growing and diffusion were easier in the gas-liquid reaction. however, many voids were observed in the sem image. this was because cis synthesis from the reaction of cu11in9-se precursors requires a higher annealing temperature. in te n si ty , a .u . j. electrochem. sci. eng. 4(1) (2014) 27-35 synthesis of cuinse2 thin films from cu11in9 32 figure 6(b) shows the microstructure of the cis film with an annealing temperature of 630 °c for 3 min. a higher synthesis temperature certainly reduces the voids in the film. however, the eds results shown in table 2 indicate that the composition of the film is not optimum for cis. se gas would easily dissipate during annealing. in samples a, b, and c listed in table 1, the deposition times of se was increased to 1450 s and the annealing temperature was adjusted to find the proper value. the eds results shown in table 2 indicate that the atomic ratio of se increases to an appropriate value of 50 %. fig. 6. sem images of cis film after annealing at (a) 600 °c, (b) 630°c for 3min. table 2. the eds analysis of atomic percent of cis film after increasing the deposition time for 1200 s and 1450 s of se at. % of cu at. % of in at. % of se cu/in ratio se deposition time, s 29.1 24.7 46.2 1.17 1200 26.6 23.2 50.2 1.14 1450 figure 7 shows the sem images of samples a, b, and c. it is observed that increasing the annealing temperature can increase the grain size of the cis film. the voids on the surface were also reduced with the higher temperature. a higher annealing temperature could provide sufficient energy for the cis film to diffuse and react more completely. the deposition condition was adjusted in samples d, e, and f to achieve a proper cu-in ratio. the eds results are shown in table 3. the cu-in ratio of sample f achieved nearly 1:1. fig. 7. sem images of cis film with the se layer deposited on the annealed cu/in surface at 4 ma cm -2 current density for 1200 s after annealing at (a) 600℃ (b) 630°c for 3min. ts-w chang at al. j. electrochem. sci. eng. 4(1) (2014) 27-35 doi: 10.5599/jese.2014.0042 33 table 3. the eds analysis of atomic percent in samples d, e, and f. at. % of cu at. % of in at. % of se cu/in ratio sample d 25.6 24.3 50.1 1.05 sample e 25.4 22.8 51.8 1.11 sample f 24.9 24.5 50.6 1.01 figure 8 shows the sem images of sample f after annealing at 630 °c for 5 min and 10 min. the grain of cis film became large and dense, and the voids almost disappear in sample f. the crosssection images of sample f are shown in figure 9. large dense grains could be clearly observed and the thickness of the cis approached 2 μm. fig. 8. sem images of sample f, annealed at 630 ℃ for (a) 5 min (b) 10 min. fig. 9. sem images of cross-section of sample f, annealed at 630 °c for (a) 5 min, (b) 10 min. figure 10 shows the xrd patterns of the cis films with various annealing conditions. the main (112) peak confirmed the existence of chalcopyrite cis. the (112) main peaks of the cis films annealed at 550 °c for 5 min and at 600 °c for 5 min contained some small peaks of impure phase. however, the peaks were too close to be differentiated clearly by xrd, but the raman analysis found cu1-xsex or inxse. a raman spectrum was employed to analyse the film composition. figure 11 shows the raman spectra of the cis films with various annealing conditions. cu2se was found in the samples with the lower annealing temperature or shorter annealing time. this is because cu2se was formed before the synthesis of cis. if the annealing does not provide sufficient energy or reaction time, the precursors cannot completely transform to cis. figure 12 shows the xrd patterns of the cis film with the lower annealing temperature. cu-se and in-se were found at 300 °c and cis (112) was found at 350 °c. this indicates that the precursors would turn into cu-se and in-se binary compounds before the synthesis of the chalcopyrite cis [10]. j. electrochem. sci. eng. 4(1) (2014) 27-35 synthesis of cuinse2 thin films from cu11in9 34 2 / ° 2 / ° fig.10. xrd patterns of cis films with conditions. wavenumber, cm -1 fig.11. raman spectra of cis films obtained with conditions. wavenumber, cm -1 fig.12. xrd patterns of cis films after annealing at low temperature 250℃~350℃. in te n si ty , a .u . in te n si ty , a .u . in te n si ty , a .u . in te n si ty , a .u . ts-w chang at al. j. electrochem. sci. eng. 4(1) (2014) 27-35 doi: 10.5599/jese.2014.0042 35 however, synthesized cis from co-sputtered cu11in9 can produce high quality film with large grains. in this study, synthesis of the cis from the electrodeposited cu-in precursors was investigated. a thermal process was adopted to eliminate the agglomeration phenomenon of electrodeposited in and to form cu11in9 compound. the electrodeposition conditions of cu, in and se, were adjusted to achieve the preferred atomic proportion. however, the annealing temperature of the synthesized cis from cu11in9 is critical. the xrd patterns and raman spectra show that the residue of the cu2se compound is due to an incomplete reaction at lower annealing temperatures. large dense grains could be grown at 650 °c for 5 min. finally, we produced a high quality cis film with large grains from a cheap method of electrodeposition of cu-in precursors. conclusions electrodeposition is a cheap and efficient method of producing cis film. the crystallinity of the co-electrodeposited film is inferior because of the low growing temperature. synthesizing cis from co-sputtered cu11in9 can produce high quality film with large grains. in this study, synthesizing cis from electrodeposited cu-in precursors was investigated. a thermal process was adopted to eliminate the agglomeration phenomenon of electrodeposited in to form cu11in9 compound. the electrodeposition conditions of cu, in and se, were adjusted to achieve the preferred atomic proportion. however, the annealing temperature of synthesized cis from cu11in9 is critical. the xrd patterns and raman spectra show that the cu2se compound residue is due to an incomplete reaction at lower annealing temperatures. large dense grains could be grown at 650 °c for 5 min. references [1] k. siemer, j. klaer, i. luck, j. bruns, r. klenk, d. bräunig, sol. energy mater. sol. cells 67 (2001) 159. [2] j. klaer, i. luck, a. boden, r. klenk, i. gavilanes perez, r. scheer, thin solid films 432 (2003) 534. [3] d. lincot, j. f. guillemoles, s. taunier, d. guimard, j. sicx-kurdi, a. chaumont, o. roussel, o. ramdani, c. hubert, j. p. fauvarque, sol. energy 77 (2004) 725. [4] r. n. bhattacharya, j. f. hiltner, w. batchelor, m. a. contreras, r. n. noufi, j. r. sites, thin solid films 361 (2000) 396. [5] k. singh, r. tanveer, sol. energy mater. sol. cells 36 (1995) 409. [6] f.o. adurodija, j. song, s.d. kim, s.h. kwon, s.k. kim, s.h. yoon and b.t. ahn, thin solid films 338 (1999) 13. [7] f. adurodija, j. song, s. k. kim, k. h. kang and k. h. yoon, journal of the korean physical society 32 (1998) 87. [8] t. l. chu, shirley s. chu, j. yue, solid-state science and technology 131 (1984) 2182-2185. [9] o. volobujeva, m. altosaar, j. raudojaa, e .mellikov, m. grossberg, l. kaupmees, p. barvinschi, solar energy materials & solar cells 93 (2009) 11. [10] s. d. kim and h. j. kim, journal of the korean physical society 35 (1999) 403. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {deposition efficiency in the preparation of ozone-producing nickel and antimony doped tin oxide anodes} doi:10.5599/jese.374 51 j. electrochem. sci. eng. 7(1) (2017) 51-64; doi: 10.5599/jese.374 open access : : issn 1847-9286 www.jese-online.org original scientific paper deposition efficiency in the preparation of ozone-producing nickel and antimony doped tin oxide anodes staffan sandin1, alicia cheritat1, joakim bäckström2, ann cornell1, 1applied electrochemistry, school of chemical science and engineering, kth royal institute of technology, stockholm, sweden 2department of natural sciences, mid sweden university, sundsvall, sweden corresponding author: amco@kth.se received: february 8 2017; revised: march 7, 2017; accepted: march 13, 2017 abstract the influence of precursor salts in the synthesis of nickel and antimony doped tin oxide (nato) electrodes using thermal decomposition from dissolved chloride salts was investigated. the salts investigated were sncl45h2o, sncl22h2o, sbcl3 and nicl26h2o. it was shown that the use of sncl45h20 in the preparation process leads to a tin loss of more than 85 %. the loss of sb can be as high as 90 % while no indications of ni loss was observed. as a consequence, the concentration of ni in the nato coating will be much higher than in the precursor solution. this high and uncontrolled loss of precursors during the preparation process will lead to an unpredictable composition in the nato coating and will have negative economic and environmental effects. it was found that using sncl22h20 instead of sncl45h2o can reduce the tin loss to less than 50 %. this tin loss occurs at higher temperatures than when using sncl45h2o where the tin loss occurs from 56 – 147 °c causing the composition to change both during the drying (80 – 110 °c) and calcination (460 -550 °c) steps of the preparation process. electrodes coated with nato based on the two different tin salts were investigated for morphology, composition, structure, and ozone electrocatalytic properties. keywords nato, ato, tin chloride precursor, thermal decomposition, tga, deposition, efficiency, dopant enrichment, ozone electrocatalysis introduction the formation of ozone by electrolysis (reaction (1)) has been a topic of research for decades and many different anode materials such as pbo2 and boron doped diamond (bdd) have been investigated. a major problem to overcome is a low current efficiency for the desired ozone http://www.jese-online.org/ mailto:amco@kth.se j. electrochem. sci. eng. 7(1) (2017) 51-64 ozone-producing doped tin oxide anodes 52 formation as o2 evolution is the dominating anode reaction in aqueous media. current efficiencies of up to 50 % [1,2] have been reached, but then requiring either low operational temperatures (pbo2) [2,3], very high cell voltage (bdd) [4], or both [1]. this affects the overall efficiency resulting in a high-energy consumption, 65 84 kwh/kg o3 and 130 kwh/kg o3 for pbo2 [5,6] and bdd [1], respectively, to be compared with the energy consumption of the commercially common cold corona discharge (ccd) process of 8 29 kwh/kg o3 [7]. 3h2o → o3 +6h+ + 6eeo = 1.51 v vs. she (1) in 2004 a research group at the university of hong kong presented a new electrode coating for electrochemical ozone formation [8]. the ceramic coating was an antimony doped tin oxide (ato) deposited on a titanium substrate showing very promising results for the formation of ozone (15 % current efficiency). in a later publication [9] the same group presented the discovery of trace amounts of nickel in their ato coatings. on this basis, they continued the work on the nickel and antimony doped tin oxide (nato) electrode [9]. an intriguing aspect of the material was that a very small concentration of nickel added to the mixed oxide yielded an electrode that made it possible to produce ozone at low anode potentials (2.2 v vs. ag/agcl) at room temperature and at relatively high current efficiencies (∼30 %) while the current efficiency was almost 0 % in the absence of nickel. since then, several groups have studied the material and current efficiencies of up to 54 % at potentials below 3 v vs. ag/agcl have been reported [7,10-16]. the minimum energy demand found so far is 18 kwh/kg o3 [7] which, to our knowledge, is the lowest value reported for the electrochemical formation of ozone and is moreover low enough to compete with the ccd process. in addition, unlike ccd, it has the advantages that ozone produced is directly dissolved in water, that no nitrogen oxides are formed and no refined gas (o2) is needed. the technique used for preparation of the mixed oxide has been the same in most of the publications on nato electrodes. the metal salts sncl45h2o, sbcl3 and nicl26h2o were dissolved in alcohol to the molar ratios of 1000:(16-20):(2-6) (sn:sb:ni) [7,9-13,15]. this precursor solution is then coated on a pre-treated titanium substrate (foil or mesh) by brush, dip or spray coating. the coated substrate is dried for 10-15 minutes at 100-110 °c followed by calcination for 10-30 minutes at 460-550 °c. this is repeated until a satisfactory loading of the oxide layer is achieved (7 -20 layers) followed by a final calcination for a longer period (∼1 hour). the current efficiencies reported for the electrodes prepared in this fashion ranges from 24 to 54 %. the molar ratios mentioned are optimized for the generation of ozone (maximizing current efficiency) [7,9,15]. the variation in current efficiencies in the above-mentioned studies can depend on a number of parameters such as number of applied layers [11,14], electrolyte type [9,11,17] and concentration [9,11], current densities and potentials [9,14], cell design [12], or calcination temperatures [14,18]. in the present study, we have focused on the importance of the sn, sb, and ni precursor salts. the tin precursor used in all of the previously mentioned nato studies is the volatile sncl45h2o. the melting point for the hydrous salt is 56 °c [19] and the boiling point for the anhydrous salt is 114 °c [19]. this low boiling temperature will lead to loss of tin during the preparation process, as described briefly by comninellis et al. [20] in a study concerning the problems in dsa® coating deposition by thermal decomposition. they reported the deposition efficiency for this salt (ηeff = moles of tin after heating / moles of tin added) to be 21 % when dissolved in the commonly used solvent ethanol. the boiling temperature of sncl4 is very close to the drying temperature chosen in the studies previously described and it is therefore likely that tin is lost during both drying and calcination steps of the electrode preparation. as a consequence of this loss, the s. sandin et al. j. electrochem. sci. eng. 7(1) (2017) 51-64 doi:10.5599/jese.374 53 composition of the oxide coating will differ from the composition in the precursor solution. since the tin loss occurs in both heating steps, the composition in the electrode coating and consequently its properties will be very sensitive to the choice of temperatures and duration of the two heating steps. this most probably leads to difficulties in controlling the resulting composition in the oxide coating which will have a negative effect on the reproducibility of the samples. the properties of antimony doped tin oxide anodes prepared in this fashion have indeed been shown to be very sensitive to the choice of temperature and duration during both drying and calcination [14,21,22]. as mentioned earlier, the optimized compositions (and maximum current efficiencies) determined for the nato electrode coatings varies among studies. although very few of the reviewed articles concerning the nato electrodes have presented any data on sample reproducibility, christensen et al. [7] reported a variation of as much as 30 % in current efficiency within the same batch of electrodes. one contributing reason for this is most probably the volatile nature of the tetravalent salt, leading to an uncontrolled loss of tin during the electrode preparation and consequently an enrichment of dopants. the actual composition of these doped tin oxide coatings has proven difficult to determine [9,13,14,18,23] using xps (x-ray photoelectron spectroscopy) or edx (energy-dispersive x-ray analysis), probably due to low nickel concentrations and the overlap of the tin and antimony spectral lines. some studies have however reported measured compositions of their oxide coatings: cheng et al. [8] determined the sn:sb ratio of their coating to 7:1 using icp-ms (inductively coupled plasma -mass spectrometry), far from the composition in the prepared precursor solution of 65:1. this was later discussed to be a consequence of surface enrichment of sb [9, 14]. other studies have also reported an enrichment of either sb or ni, or both [24-26]. most authors correlate this to surface enrichment. to our knowledge, the loss of precursor salts has not been discussed as a possible reason for the high dopant concentration. in addition to the difficulties in predicting the composition of the final coating, the high loss of tin also means that the coating process has to be repeated more times than would be necessary if a less volatile precursor salt were to be used. this results in a preparation process that is inefficient from a practical and economical, as well as an environmental perspective. a preparation process that can be made in such a way that the resulting anode coating composition can be controlled and reproduced is of utmost importance for the continued work on the understanding of the detailed reaction mechanisms on the nato surface, as well as for industrial implementations. the problems described above with the loss of tin during the preparation process have, to our knowledge, not yet been considered in the published works concerning the nato electrodes. one reason for this could be the difficulty in determining the composition in the oxide layer as discussed above. to understand and mitigate the issue of the evaporating tin source, the preparation process was investigated using both the common tetravalent tin precursor sncl45h2o and an alternative precursor salt, the divalent sncl22h2o. the divalent salt has been reported to have a deposition efficiency of 57 % when dissolved in ethanol [20], and the boiling point for the anhydrous salt (623 °c) [19] is well separated from the preparation temperatures, so loss of tin in the form of sncl2 will be avoided. the heating process was examined using tgdsc (coupled thermogravimetry and differential scanning calorimetry), and electrodes prepared using both precursor salts were investigated regarding morphology and composition. the ozone formation capabilities of the electrodes were also briefly tested. j. electrochem. sci. eng. 7(1) (2017) 51-64 ozone-producing doped tin oxide anodes 54 experimental the oxide formation process (i.e. heating of salts and solutions) was characterized using coupled tg-dsc (netzsch sta 449 f3), from room temperature (22-25 °c) to 600 °c at a heating rate of 5 k/min in dry air at atmospheric pressure. the structure and morphology of the coatings was examined using xrd (x-ray diffraction, bruker d2 phaser) and sem (scanning electron microscopy, hitachi s-4800). the composition of the coatings was analysed using xrf (x-ray fluorescence, philips pw2400). the electrodes were prepared as follows. grade 2 (commercially pure) titanium discs, 0.5 mm in thickness and 59 mm in diameter, were first cleaned in an ultrasonic bath followed by rinsing in ethanol, pickling in 1 % hf for 2 minutes, and followed by another ultrasonic cleaning and ethanol rinsing. the clean substrates were then dried at 80 °c for 60 minutes. the precursor solutions were prepared by mixing solutions of the individual salts, sncl45h2o (pro analysi, sigma-aldrich), sncl22h2o (pro analysi, merck), sbcl3 (acs reagent, sigma-aldrich), and nicl2 6h2o (pro analysi, merck) to appropriate compositions with a tin concentration of 1 m. ethanol (analytical grade 99.5 %, solveco) or n-propanol (acs reagent ≥99.5 %, sigma-aldrich) were used as solvents and hcl (37 %, merck) was added to keep the salts in solution (when n-propanol was used as solvent). the precursor solutions were coated on the titanium discs using either spin coating (1500 rpm for 30 s) or drip coating. the coated substrates were then dried at 80 °c for 10 minutes followed by calcination at 500 °c for 10 minutes. this coating process was repeated 5 or 7 times followed by a final calcination at 500 °c for 60 minutes. analysis of ozone selectivity on the nato electrodes was made at room temperature in galvanostatic experiments in an undivided batch cell from which electrolyte (0.1 m h2so4) samples were taken for concentration determination by uv-vis absorption spectroscopy (expedeon versastat) in a 1 cm path length quartz glass cell. as the gas phase was not analysed for ozone, the calculated current efficiencies were probably underestimated. a more specific description of the electrode preparation process and chemicals and instruments used can be found in the supplementary information. results and discussion first the decomposition processes of the pure precursor salts sncl45h2o, sncl22h2o, sbcl3, and nicl26h2o were studied using tg-dsc. after that, electrode coatings of mixed sn, sb, and ni oxides using the two different tin precursors were prepared and characterized regarding structure, morphology, and composition. finally, the electrodes were briefly tested for ozone generation. details of some of the results can, as referred to in the text, be found in the supplementary information. tg-dsc of non-dissolved salts sncl45h2o as can be seen in figure 1, the mass of the tetravalent tin salt rapidly starts decreasing almost as soon as the heating is initiated and goes along three distinct steps as the temperature increases. the first large mass loss is initiated by the melting of the hydrous salt at just above 50 °c, seen as a sharp endothermic peak (a) in the dsc data plot. that this is indeed the melting of the salt is supported by the absence of any peaks at this temperature in the differentiated tg (dtg) data. this temperature is also well in agreement with the literature melting point value of 56 °c [19]. the second peak (b) in the dsc data covers the entire temperature range (room temperature -106 °c) of the first mass loss step where 49 % of the initial mass is lost. s. sandin et al. j. electrochem. sci. eng. 7(1) (2017) 51-64 doi:10.5599/jese.374 55 figure 1. tg (upper) and dsc (lower) data plotted with the differentiated tg data (dtg) from heating of sncl45h2o the second mass loss step then continues up to approximately 147 °c during which another 41.5 % of the mass is lost (double peak c in dsc data). this is followed by a slow mass loss up to approximately 410 °c where 4.5 % of the initial mass is lost. the mass stabilized at 5.2 % of the initial mass, yielding a deposition efficiency (ηeff) of 12 % assuming that the end product is pure sno2. anhydrous sncl4 has a melting point at −33 °c and a boiling point at 114 °c [19]. this means that the vapor pressure above the molten salt will be very high as soon as it loses its water of crystallization. the first mass loss region (106 °c) therefore most probably corresponds to the evaporation of sncl4 and of h2o. however, the total loss of tin corresponds to a 65 % mass loss assuming sncl4 as the only evaporating tin specie and the mass loss in the first step is 49 %. there is therefore most likely a competing reaction to the evaporation processes forming a more stable intermediary of tin that reacts in several steps in the second and third mass loss region before finally ending up in the tin oxide state. the loss of chlorine as cl2 or some oxychlorine is also occurring in the process. tin oxychlorines could be the intermediates forming and evaporating during the heating, as discussed by others [27,28] for the heating of the divalent tin salt sncl22h2o. sncl22h2o as seen in figure 2, the heating characteristics of sncl22h2o are very different from those of the tetravalent salt. the first peak (a) in the dsc data corresponds to both the loss of water (seen also as mass loss in the tg data), as well as the melting of the hydrated salt at approximately 40 °c. the melting temperature agrees well with its literature value of 37 °c [19]. the second peak in the dsc data (b) is also accompanied by a loss in mass, and since the anhydrous salt does not melt until 247 °c [19] (peak c in dsc), this loss in mass is attributed to further evaporation of water. after the melting of the anhydrous salt there is an exothermic change in the dsc signal (d) followed by a large mass loss (e), seen both in the dsc and tg data. the loss in mass ends abruptly with an exothermic peak in the dsc signal (f). the total amount of mass lost before the melting of the anhydrous salt was approximately 12 %, corresponding to about 75 % of the total initial water content (16 %) in the hydrous salt. reactions of this melted salt results in the exothermic peak (d). the products are volatile and thus evaporates causing an endothermic increase in the dsc data (e) and a corresponding mass loss in the tg data. the evaporation is ended by an exothermic reaction (f), j. electrochem. sci. eng. 7(1) (2017) 51-64 ozone-producing doped tin oxide anodes 56 most probably the formation of tin oxide. tin oxychlorine species have been proposed to form and evaporate during the oxide formation process [27,28]. assuming sno2 as the only product, the deposition efficiency (ηeff) was 23 % which is almost double that of the tetrachloride salt. a very important difference between the two tin salts is the absence of tin loss at low temperatures (300 °c) for sncl22h2o while all tin loss for sncl45h2o occurs in the temperature range 50 -150 °c. figure 2. tg (upper) and dsc (lower) data plotted with the differentiated mass loss (dtg) from heating of sncl22h2o sbcl3 the tg-dsc data of the antimony salt is presented in figure 3. the first peak with an onset temperature at slightly above 72 °c appearing only in the dsc data is the melting of the salt, well in agreement with its literature value of 73.4 °c [19]. as the second dsc peak has a corresponding peak in the tg/dtg signal, and this loss in mass is initiated by the melting of the salt, this is most probably the evaporation of sbcl3. the abrupt end of the mass loss, and close to zero residual mass (0.7 %), indicates that close to all sbcl3 evaporated before reaching its boiling point of 220 °c [19]. this is an issue that should also be investigated to improve the stability and predictability of the electrode preparation process which, to our knowledge, has not been addressed in earlier studies. figure 3. tg (upper) and dsc (lower) data plotted with the differentiated mass loss (dtg) from heating of pure sbcl3 salt s. sandin et al. j. electrochem. sci. eng. 7(1) (2017) 51-64 doi:10.5599/jese.374 57 nicl26h2o in the tg-dsc data of nicl26h2o (figure 4) all peaks up to 400 °c are present both in the dsc (endothermic) as well as the tg data, indicating that evaporation is occurring. the total mass loss up to this temperature was slightly above 47 %, which is well in agreement with the total water content of the salt (45.5 %). this therefore means that the crystal water of the hydrated nickel salt evaporates in steps over the temperature range 24 -450 °c. the remaining sample after the loss of water is the solid and anhydrous nicl2 (melting point: 1031 °c) [19]. as the temperature is increased beyond 450 °c mass loss is again seen and a stable residual mass has not been obtained when reaching the end temperature of 600 °c. the reactions occurring in the higher temperature range are most probably the formation of nickel oxide and the evaporation of chlorine. as the reactions were not completed within the temperature range of the experimental run, it is not possible to give any estimation of the deposition efficiency of this salt. however, judging from the data obtained and the high melting temperature of nicl2, it is concluded that no nickel will evaporate during the electrode preparation process. nickel salt dissolved in ethanol and n-propanol, respectively, was also heated, both with similar results (see supplementary information). figure 4. tg (upper) and dsc (lower) data plotted with the differentiated mass loss (dtg) from heating of pure nicl2 6h2o salt tg-dsc of dissolved salts sncl45h2o and sncl22h2o figures 5 and 6 show the heating of the dissolved tin salts in ethanol. data for the non-dissolved salts from the previous section have been included for comparison. because of the lower amount of salt the signal is weaker for the dissolved salts but the sncl45h2o dissolved in ethanol mostly follows the same reaction path as the non-dissolved salt (with the addition of the evaporation of solvent). the deposition efficiency was similar in both cases (10 and 12 %, respectively) and the only clear difference is a lower temperature at which the weight curve stabilizes for the dissolved salt (approximately 275 versus 410 °c). the heating of sncl22h2o in ethanol, however, shows a large difference in deposition efficiency compared to that of the non-dissolved salt (23 and 54 %, respectively). it can also be seen that the decomposition process differs somewhat in the two cases of non-dissolved and dissolved sncl22h2o. the last large mass loss seen for the non-dissolved salt at 320 438 °c is not present for the dissolved salt. it seems like the oxide is formed at a lower temperature for the dissolved salt (approximately 310-320 versus 438 °c), considering the j. electrochem. sci. eng. 7(1) (2017) 51-64 ozone-producing doped tin oxide anodes 58 exothermic peak showing in the dsc data and the stabilization of the mass. a very small mass loss (0.4 %) occurs after this temperature, this could be the result of the decomposition of remaining organic compounds from the solvent. solutions of the tin precursors in ethanol and n-propanol with added antimony and nickel salts showed no significant difference from the ethanol solutions without the dopant salts (see supplementary information). figure 5. heating of 1 m sncl45h2o dissolved in ethanol shown together with the data from fig. 1 figure 6. heating of 1 m sncl22h2o dissolved in ethanol shown together with the data from fig. 2 tg-data of sbcl3 dissolved in n-propanol is given together with data of the non-dissolved salt in figure 7. the residual mass from heating of the ethanol and n-propanol solutions of sbcl3 were 3.1 and 1.2 % of the initial mass, respectively. these are much higher yields compared to the nondissolved salt considering that the mass of antimony salt added was only ∼25 % of the initial mass including the solvent. a possible reason for the increase in yield of the antimony salt is that it reacts with the solvent to form a more stable antimony specie, or with the very small amount of water present in the solvents to form antimony hydroxide. as a support for the latter, precipitations have been observed upon mixing antimony salt solutions with solutions of the hydrated nickel and tin salts. the s. sandin et al. j. electrochem. sci. eng. 7(1) (2017) 51-64 doi:10.5599/jese.374 59 deposition efficiency of antimony (dissolved or not) is however still very low (10 %), which should be considered when used for electrode preparation. figure 7. tg-data from heating of sbcl3 as salt and dissolved in etoh and n-propanol to a concentration of 1 m heating on titanium substrates as the sample volume and size of the sample container can have an effect on the deposition process, trials where a titanium substrate was coated by dripping a known volume of a sn:sb:ni salt solution followed by drying and calcination were made. in this test n-butanol (acs reagent ≥99.4%, sigma-aldrich) which was not tried in the tg-dsc analysis was included. the drying was made at 80 °c for 10 minutes and the calcination at 500 °c for 10 minutes. these trials (presented in table 1) show similar results to those of of the tg-dsc trials discussed above with the additional indication that n-butanol might increase the deposition efficiency when using sncl45h2o as tin source. what should also be noted is that the normalized standard deviations for the tetravalent based coatings are higher than those of the divalent based coatings. these variations could be an effect of the evaporation of sncl4 at low temperatures (56 -147 °c, figures 1 and 5), probably resulting in loss of tin in both the drying and the calcination step of the heating process. table 1. deposition efficiency average (ηeff,avg) and normalized standard deviations (σ / ηeff,avg) of three samples per salt and solvent type from heating of sn, sb, and ni solutions on titanium substrates. composition in solution (1000:32:4 molar ratios of sn:sb:ni, sn concentration 1 m). solvent sncl22h2o sncl45h2o ηeff,avg / % σ / ηeff,avg average ηeff,avg / % σ / ηeff,avg ethanol 48.2 5.6 12.4 16.7 n-propanol 48.0 5.8 8.1 13.2 n-butanol 45.4 4.6 21.0 15.3 electrode preparation figure 8 shows the increase in loading for the spin coating of titanium substrates with the nato precursor solutions (drying and calcination after each coated layer) based on the tetravalent and divalent tin salts. the linear fits in figure 8 yield a 3.3 times faster mass increase for the sncl22h2o based coatings than for the sncl45h2o based ones. this agrees well with the higher deposition efficiency seen for the divalent tin salt. j. electrochem. sci. eng. 7(1) (2017) 51-64 ozone-producing doped tin oxide anodes 60 figure 8. increase in loading per layer for sncl45h2o, or sncl22h2o based electrode coatings with the precursor composition 1000:16:2 (molar ratios of sn:sb:ni) in etoh (average of three samples, all spin coated) surface characterization xrd in figure 9, xrd data from sncl22h2o and sncl45h2o based electrodes, both made by spin coating using the precursor composition 1000:16:2 and ethanol as solvent, is presented. both samples exhibit the rutile tetragonal crystal structure for sno2 (literature data for sno2 is seen in the bottom of the figure). no reflections other than those of sno2, ti, and tio2 were seen. it is very clear from the xrd data that the coating made from the sncl22h2o solution is thicker than the one made using the sncl45h2o solution as the sno2 peaks have higher, while the titanium peaks have lower intensity. figure 9. xrd data from measurement on sn:sb:ni oxide coatings on titanium substrates. identifed sno2 rutile peaks are indexed sem in figure 10 sem micrographs of the two different electrodes presented in figure 8 and 9 are shown. it can be seen that the surface of the dichloride based electrode is less cracked and smoother than the one prepared using the tetrachloride salt as tin source. s. sandin et al. j. electrochem. sci. eng. 7(1) (2017) 51-64 doi:10.5599/jese.374 61 figure 10. sem micrographs of a) sncl45h2o based and b) sncl22h2o based electrode coatings the difference in surface morphology between the two can be a consequence of several processes. however, the boiling and evaporation of tin tetrachloride probably has a large influence since the calcination is performed almost 400 ◦c above its boiling temperature. xrf xrf measurements were made on the nato electrodes prepared by spin coating also presented in figures 8, 9, and 10. although sb could not be determined (probably due to overlapping characteristic x-ray lines), the relative amount of ni in the coatings was determined by xrf to a 3.7 times higher concentration in the electrode coating prepared using sncl45h2o than in the coating prepared using sncl22h2o. this agrees with the results presented above from the tg-dsc measurements on the dissolved tin salts where a higher tin loss was observed for the tetravalent salt compared to the divalent salt, meaning that a dopant that does not evaporate will be enriched more when sncl45h2o is used as a precursor compared to when using sncl22h2o. electrochemical formation of ozone a brief comparison of the ozone formation on sncl22h2o and sncl45h2o based electrodes (2 cm in diameter) was made by galvanostatic electrolysis at different current densities in a batch cell (13 ml). after each step in current density, the electrolyte (0.1 m h2so4) was exchanged. samples were taken after 2 minutes of electrolysis at each current step and was directly analysed for aqueous ozone concentration using uv-vis. results from optical absorption measurements on the samples are presented in figure 11. figure 11. absorption spectra of samples taken during galvanostatic electrolysis using a) an sncl45h2o based electrode, and b) an sncl22h2o based electrode j. electrochem. sci. eng. 7(1) (2017) 51-64 ozone-producing doped tin oxide anodes 62 the tetrachloride based electrode was prepared using a precursor solution with the composition 1000:16:2 in sn:sb:ni and was coated by drip coating as the spin coating yields very thin oxide layers when using the tetrachloride salt. as the results presented above indicates a higher dopant concentration in the tetrachloride based electrodes due to tin evaporation, the concentration of the dichloride based electrode precursor solution was increased 4 times to compensate for this, i.e. in order to obtain similar compositions in the two electrode types. the dichloride based electrode was therefore prepared using a precursor solution with the composition 1000:64:8 and was coated using spin coating. using the molar absorption coefficient of 3000 l mol-1 cm-1 [29], and the current passed during the time of the experiment, the maximum ozone current efficiencies determined for the two electrodes were both approximately 14 % at 25 and 30 ma cm-2 for the tetrachloride and dichloride based electrode, respectively. at current densities higher than these, the current efficiency decreased for both electrodes. it should be noted that these figures were calculated only based on the aqueous ozone, and also that the precursor compositions were not optimized for ozone current efficiency. the electrochemical properties of these electrodes were not the main focus of this study and will be further investigated in future studies. importance of preparation route the comparison between the two precursors salts sncl45h2o and sncl22h2o was made with the goal of shedding some light on the problems of using a volatile precursor salt under the preparation conditions previously described rather than to find a replacement salt. even though the divalent tin salt is more suitable than the tetravalent one under the preparation conditions normally used, it is not an ideal precursor salt as tin loss does occur during the preparation process, causing enrichment of dopants. three possible ways to overcome the low deposition efficiency could be to change the precursor salts, change the solvent, or to use an alternative synthesis technique such as a hydrothermal process. a few studies have used water as a solvent instead of alcohol which possibly leads to the formation of non-volatile intermediary species. a higher precursor concentration of dopants would then be necessary in order to end up at similar compositions as when the dopants are enriched due to loss of tin. indeed, higher dopant levels in the precursor solution (1000:53:11 molar ratios of sn:sb:ni) were used by yang et al. [17] with water as solvent, however no reason for this change is given in the paper. in a later publication [26] they used ethanol as solvent but kept the high dopant levels in the precursor solution. this resulted in a very low current efficiency for ozone formation of 1 %, whereas 9.3 % was reached when water was used as solvent. we believe that this may be related to too high dopant concentrations in the coating when ethanol was used as solvent. christensen et al. [18] recently published a study on ozone forming nato electrodes where they used water as solvent and by refluxing and autoclaving formed the precursor for their electrode coating. wang et al. [16] used ammonia and oxalic acid to prepare their precursor, probably forming oxalates [30,31]. these salts should not evaporate [32-34] and thereby the problem of enrichment due to loss of precursors during heating is probably avoided. conclusions the commonly used preparation procedure for nato electrodes leads to substantial losses of tin and antimony through evaporation during the drying and heating steps. the nickel salt is not as volatile and will therefore, relatively speaking, be enriched in the electrode coating. because of the s. sandin et al. j. electrochem. sci. eng. 7(1) (2017) 51-64 doi:10.5599/jese.374 63 evaporation, it is very difficult to control the coating composition and this is a problem from both an environmental as well as an economic perspective. also, as it has proven difficult to accurately analyse the coating composition, this complicates fundamental studies of the reaction mechanisms on these very interesting electrocatalysts. exchanging the frequently used tin precursor sncl45h2o to the divalent sncl22h2o resulted in a significantly better deposition efficiency, 5.5 times higher according to tg-data. the divalent salt will not evaporate in the drying step, but still tin will be lost in the following heating. further studies for a higher deposition efficiency are needed in order to obtain a preparation procedure where the composition in the resulting oxide coating can be properly controlled. it is also important to find methods for accurate analysis of the composition in the oxide coating, both for total concentrations as well as for how the composition varies with coating depth. acknowledgments: the authors would like to thank the formas research council for financial support and lina norberg samuelsson for her help with the tg-dsc measurements. references [1] k. arihara, c. terashima, and a. fujishima, journal of the electrochemical society, 154 (2007) e71–e75. 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[34] d. zhan, c. cong, k. diakite, y. tao, and k. zhang, thermochimica acta 430 (2005) 101–105. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {modelling the effect of anode particle radius and anode reaction rate constant on capacity fading of li-ion batteries:} http://dx.doi.org/10.5599/jese.1147 359 j. electrochem. sci. eng. 12(2) (2022) 359-372; http://dx.doi.org/10.5599/jese.1147 open access : : issn 1847-9286 www.jese-online.org original scientific paper modelling the effect of anode particle radius and anode reaction rate constant on capacity fading of li-ion batteries vikalp jha and balaji krishnamurthy department of chemical engineering, bits pilani, hyderabad 500078, india corresponding author: balaji@hyderabad.bits-pilani.ac.in received: october 21, 2021; accepted: november 26, 2021; published: december 6, 2021 abstract this paper investigates the effect of anode particle radius and anode reaction rate constant on the capacity fading of lithium-ion batteries. it is observed through simulation results that capacity fade will be lower when the anode particle size is smaller. simulation results also show that when reaction rate constant is highest, the capacity loss is the lowest of lithiumion battery. the potential drop across the sei layer (solid electrolyte interphase) is studied as a function of the anode particle radius and anode reaction rate constant. modelling results are compared with experimental data and found to compare well. keywords sei; potential drop; side reaction; discharge introduction side reactions can cause various adverse effects leading to capacity fading in lithium-ion batteries. the aging of li-ion batteries usually occurs due to various parameters and electrochemical reactions, and capacity loss varies between all stages during a charge-discharge load cycle, depending on various parameters such as cell voltage, electrolyte concentration, temperature, and cell current. this work shows the model for aging and capacity loss in the anode of a li-ion battery, where the formation of a thin film of solid-electrolyte-interface (sei) shows an adverse capacity loss of cyclable lithium. capacity fading in a lithium-ion battery has been studied under various load conditions. haran et al. [1] studied the effect of various temperatures during various cycles in the capacity fading of 18650 li-ion cells. it is observed that with an increase in temperature of li-ion batteries, capacity fading is increased. it is observed that at temperatures higher than 55 °c, the cell ceases to operate after 500 cycles due to ongoing sei film formation over the anode surface. han et al. [2] studied the cycle life of commercial li-ion batteries with lto anodes in electric vehicles. the author also found that at 55 °c, the capacity fading in the battery is more than lower operating temperature lithium-ion battery. liaw et al. [3] studied the correlation of arrhenius behavior in power and capacity loss with cell impedance and heat generation at different temperatures and state of charge http://dx.doi.org/10.5599/jese.1147 http://dx.doi.org/10.5599/jese.1147 http://www.jese-online.org/ mailto:balaji@hyderabad.bits-pilani.ac.in j. electrochem. sci. eng. 12(2) (2022) 359-372 capacity fading of li-ion batteries 360 in 18650 cylindrical li-ion cells. it is observed that degradation in power and capacity fade seems to relate to impedance increase in the cell with the activation energy of cell at different temperatures. ramadesigan et al. [4] studied the effect of the solid-phase diffusion coefficient and side reaction rate constant in the anode, cathode, and electrolyte as a function of cycling with various reformulated models. colclasure et al. [5] studied various detailed chemistries and transport for sei films on li-ion batteries with various states of charge (soc) at different cycles. the author states that sei film grows with time according to net production rate from heterogeneous chemistry on sei film surface because electric-potential and concentration profiles in the sei layer are functions of the intercalation fraction [5]. pinson and bazant [6] also studied the formation of sei layer in rechargeable batteries with capacity loss, aging, and lifetime prediction in li-ion batteries. various models are studied at different temperatures and c-rates to study sei layer formation and capacity fading of li-ion batteries. the authors postulate that capacity fading depends on time, not on the number of cycles. the temperature dependence of the diffusivity of the limiting reacting species through sei can be modelled using an arrhenius dependence. ziv et al. [7] examined electrochemical performance and capacity loss of half and full li-ion batteries with several cathode materials experimentally. the authors stated that the loss of lithium ions due to side reactions is the main reason for the capacity fading of li-ion batteries. liu et al. [8] studied a thermal-electrochemical model for sei formation in li-ion batteries during load cycles. the authors state that the growth of sei film is very sensitive to the diffusion process and side reaction rate. it is also found that sei film grows at a higher rate during charging than during the discharge cycle. guo et al. [9] also studied the capacity fading of li-ion batteries with different experiments. the authors stated that capacity fading occurred due to several reasons, including discharge rate, number of cycles, and battery type. ramesh et al. [10,11] developed a mathematical model to study capacity loss in li-ion batteries due to temperature, formation, and dissolution rate constants of the sei layer. the author also developed an empirical model to study capacity fading in li-ion batteries under different temperatures. xu et al. [12] also studied electrode side reactions, capacity loss and mechanical degradation of li-ion batteries through experimental observations. the author states that during load cycles for higher reaction rates, columbic efficiency is lower, but capacity fading is also lower. shirazi et al. [13] studied the effect of composite electrode’s particle size effect on electrochemical and heat generation of li-ion batteries. the author states that for smaller particle size, the thermal characteristics of the battery is improved in comparison to larger particle size [13]. singhvi et al. [14] developed a mathematical model to observe the effect of acid attack on capacity fading in li-ion batteries. the author considers sei formation due to the transport and reaction of solvent species. cheng et al. [15] developed a mechanism for capacity loss of 18650 cylindrical li-ion battery cells. the author postulates that the capacity loss of li-ion batteries can be explained by continuous sei layer formation over the surface of anode and side reactions. side reaction products deposit on a separator and reduce its porosity, leading to capacity fading. tomaszewska et al. [16] reviewed various research on fast charging of li-ion batteries. it was observed that for fast charging, ratelimiting processes are beneficial to reduce battery degradation and increase in cycle life. meanwhile, li plating, the structure of li deposits, and temperature distribution during cycling lead to the degradation of li-ion batteries. gantenbein et al. [17] studied the capacity loss of li-ion batteries over different soc ranges. the author states that capacity fading originates from active electrodes and active lithium loss. lee et al. [18] also studied the loss of cyclable li on the performance degradation of li-ion batteries. the author stated that the discharge behavior of the jha and krishnamurthy j. electrochem. sci. eng. 12(2) (2022) 359-372 http://dx.doi.org/10.5599/jese.1147 361 cell had a strong dependence on discharge c-rate and loss of cyclable lithium. khaleghi rahiman et al. [19] developed a mathematical model to study cell life with various parameters. the author studied capacity loss and sei formation in li-ion batteries at different temperatures at different socs. the author postulates that cathode side reactions are accelerated at higher socs and temperatures. in our model, we compare the effect of anode particle radius and anode reaction rate constant on the capacity fading of a lithium-ion battery. model development a 1d model of a li-ion battery interface is created, as shown in figure 1. the components of a liion battery are the negative electrode, positive electrode, and separator. graphite electrode (lixc6) mcmb is used for negative electrode material, nca electrode (lini0.8co0.15al0.05o2) is used for positive electrode material and lipf6 (3:7 in ec: emc) is used as a liquid electrolyte. figure 1. schematic of the 1d electrochemical model of li-ion battery model equations the model equations analyze the current equilibrium in the electrolyte and electrodes, the mass balance for the lithium and electrolyte in li-ion batteries. the li-ion battery physics at interface analyses five dependent variables: a) s the electric potential, b) e the electrolyte potential, c) ∆sei the potential losses due to solid-electrolyte interface (sei), d) cli the concentration of lithium in the electrode particles e) ce the electrolyte salt concentration. the domain equations in the electrolyte are the conservation of current and the mass balance for the salt according to the following [20]: ( ) e,eff sum e e e e e 2 ln 1 1 ln ln rt f i q t c f c    +      + = −  + + +         (1) ( )e sume e e e e ec i qd c r t t f   +  +  + −  = −     (2) where e denotes the electrolyte conductivity, f is the activity coefficient for the salt, t+ is the transport number for li+, isum is the sum of all electrochemical current sources, and qe denotes an arbitrary electrolyte current source. in the mass balance for the salt, e denotes the electrolyte volume fraction, de is the electrolyte salt diffusivity, and re the total li+ source term in the electrolyte. in the electrode, the current density, is is defined as http://dx.doi.org/10.5599/jese.1147 j. electrochem. sci. eng. 12(2) (2022) 359-372 capacity fading of li-ion batteries 362 is = -ss (3) where s is electrical conductivity. the domain equation for the electrode is the conservation of current expressed as is = -isum + qs (4) where qs is an arbitrary current source term. the electrochemical reactions in the physics interface are assumed to be insertion reactions occurring at the surface of small solid spherical particles of radius rp in the electrodes. the insertion reaction is described as: during charging, at anode xli+ + xe+ graphite → lixc6 (5) at cathode lixni0.8co0.15al0.05o2 → xli+ + xe+ ni0.8co0.15al0.05o2 (6) during discharging, at anode lixc6 →xli+ + xe+ graphite (7) at cathode xli+ + xe+ ni0.8co0.15al0.05o2 → lixni0.8co0.15al0.05o2 (8) an important parameter for lithium insertion electrodes is the state-of-charge variable for the solid particles, denoted soc. this is defined as li li,max soc c c = (9) the equilibrium potentials e0 of lithium insertion electrode reactions are typically functions of soc. the electrode reaction occurs on the particle surface and lithium diffuses to and from the surface in the particles. the mass balance of li in the particles is described as li s li( ) c d c t  =   (10) where cli is the concentration of li in the electrode. this equation is solved locally by this physics interface in a 1d pseudo dimension, with the solid phase concentrations at the nodal points for the element discretization of the particle as the independent variable. the gradient is calculated in cartesian, cylindrical, or spherical coordinates, depending on if the particles are assumed to be best described as flakes, rods or spheres, respectively. the boundary conditions are as follows: li = 0 0 r c r  =  (11) li s lir p p r r r r c d r = =  − = −  (12) where rli denotes the molar flux of lithium at the particle surface caused by the electrochemical insertion reactions. in the porous electrodes, isum denotes the sum of all charge transfers current density contributions according to: isum = σaviloc (13) where, av denotes the specific surface area at any node of the lithium-ion battery interface. active specific surface area (m2/m3) defines the area of an electrode-electrolyte interface that is catalytically active for porous electrode reactions. equation 13 describing the total current source jha and krishnamurthy j. electrochem. sci. eng. 12(2) (2022) 359-372 http://dx.doi.org/10.5599/jese.1147 363 in the domain is a function of active specific surface area and local current in the electrode. the source term in the mass balance is calculated from: li loc l v l,src i r a r nf  = − + (14) where rl.src is an additional reaction source that contributes to the total species source. at the surface of the solid particles, the following equation is applied: li locv li shape s p ia r s nf r   = − (15) where n is the number of electrons and sshape (normally equal to 1) is a scaling factor accounting for differences between the surface area (av) used to calculate the volumetric current density and the surface area of the particles in the solid lithium diffusion model. sshape is 1 for cartesian, 2 for cylindrical, 3 for spherical coordinates and 𝜐li is the stoichiometric coefficient. a resistive film (also called solid-electrolyte interface, sei) might form on the solid particles resulting in additional potential losses in the electrodes. to model a film resistance, an extra solution variable for the potential variation over the film is introduced in the physics interface. the governing equation is then according to sei = rseiisum (16) where rsei denotes generalized film resistance, which can be expressed by: 0 sei sei r    +  = (17) where, 0 is initial film thickness, ∆ is film thickness change and sei is film conductivity. the activation overpotentials,  for all electrode reactions in the electrode then receives an extra potential contribution, which yields  = s e sei eeq (18) where, eeq is the equilibrium potential of a cell. the battery cell capacity, qcell,0 is equal to the sum of the charge of cyclable species in the positive and negative electrodes and additional porous electrode material if present in the model [20]. qcell,0 = qcycle,pos + qcycle,neg + qcycl,addm (19) butler-volmer equation is used to calculate the local current density in the electrode. a c loc 0 exp exp f f i i rt rt     −    = −          (20) a c a e 0 c a li,max li li e,eff ( ) c c i fk k c c c c       = −      (21) where a and c are the anode and cathode transfer coefficient and ka and kc are reaction rate constant for anode and cathode. numerical methods 1d model of li-ion battery consists of 3 geometric regions for analysis: negative electrode, separator and a positive electrode. for numerical analysis of the computational domain, 1d meshing is done for 49,59 and 95 mesh elements. the 1d mathematical model is developed for transient analysis of our computational domain in comsol 5.3a on viable concerns of li-ion battery, electrochemical parameters, species transport and current distribution, consisting of the principal http://dx.doi.org/10.5599/jese.1147 j. electrochem. sci. eng. 12(2) (2022) 359-372 capacity fading of li-ion batteries 364 model assumptions and equations with different initial conditions, boundary conditions and numerical solver strategies for solution. results and discussion capacity fading of li ion battery is studied with the effect of various parameters. a summary of the list of parameters used for simulation is shown in table 1. table 1. list of parameters description value particle radius, µm 0.5, 1, 2, 2.5 reaction rate coefficient, pmol m-3 s-1 200 , 20 , 2 initial capacity, c m-2 55761 1c discharge current, a 15.767 thickness of negative electrode, µm 55 thickness of separator, µm 30 thickness of positive electrode, µm 55 cell temperature, ℃ 45 maximum cell voltage, v 4.1 minimum cell voltage, v 2.5 initial electrolyte salt concentration, mol m-3 1200 constant current (charge and discharge), a 15.767, -15.767 sei layer conductivity, s m-1 5×10-6 initial sei layer thickness, nm 1 number of cycles 2000 the battery cycling consists of 3 various stages of charging and discharging, as shown in figure 2: figure 2. charge-discharge load cycle • charging at a constant current rate of 1 c until the cell potential reaches 4.1 v. • charging at a constant voltage of 4.1 v. • discharging at constant current discharge rate at 1 c until the cell potential reaches the minimum voltage of 2.5 v. jha and krishnamurthy j. electrochem. sci. eng. 12(2) (2022) 359-372 http://dx.doi.org/10.5599/jese.1147 365 effect of particle radius on capacity fading in lithium-ion batteries research on anode particle radius on capacity fading in lithium-ion batteries has been done previously. rai [21] postulated that batteries with smaller anode particle sizes generate better capacity. the authors postulated that smaller particles(graphite) allow quicker lithium-ion intercalation and deintercalation due to the short distances for lithium-ion transport within the particles. there is no agreed-upon consensus for optimal particle size in lithium-ion batteries though particles less than 150 nm are mainly used. wu [22] investigated the effect of silicon particle size in the micrometer range when used as a lithium-ion battery anode. the authors have found out in their study that particle size of 3μm shows better outcomes with respect to the 20 μm particle size with an initial capacity of 800 mah/g and retention of 600 mah/g after 50 cycles. buqa [23] investigated three different graphite particle sizes (6, 15 and 44 µm) and showed that smaller particles could achieve better capacity retention. several authors like drezen [24] and fey [25] have postulated that smaller particles improve capacity retention. mei has [26] postulated that energy and power density increase with smaller particle sizes due to lower overpotential. mei [26] has also postulated that smaller particle size increases the surface area for reaction. our focus was to study the effect of the anode particle radius on the capacity fading in lithium-ion batteries taking into consideration lithium losses during cycling. figure 3 shows the capacity fading of a lithium-ion battery with cycling for various anode particle radii. the model assumes zero lithium loss during the process of cycling. four different anode particle radii (0.5, 1, 2 and 2.5 m) were considered for analysis. it is seen that the least capacity fading (high relative capacity) is seen for an anode particle radius of 0.5 m. relative capacity is defined as the capacity of the battery at any point of time divided by the initial capacity of the battery. it can be seen that as the anode particle radius increases from 0.5 to 2.5 µm, the relative capacity decreases over 2000 cycles. figure 3. capacity fading with cycling for various anode particle radius with zero lithium loss figure 4 shows the capacity fading in a lithium-ion battery cycling for four different particle radii (0.5, 1, 2 and 2.5 m) with 10 percent lithium loss during cycling. during charge-discharge cycling, there is more lithium loss during initial cycles. a comparison of figures 3 and 4 shows that the capacity loss is seen to be less without cyclable lithium loss compared to 10 % initial lithium loss as the number of cycles increases. this is clearly shown in figure 5. it is seen from figure 5 that there is less capacity loss of around 3 % when we go from zero percent lithium loss to 10 percent lithium loss during cycling. http://dx.doi.org/10.5599/jese.1147 j. electrochem. sci. eng. 12(2) (2022) 359-372 capacity fading of li-ion batteries 366 figure 4. capacity fading with cycling for various anode particle radius with lithium loss figure 5. comparison of relative capacity with and without lithium loss figures 6 and 7 show the capacity loss in the battery as a function of the anode reaction rate constant. figure 6. capacity loss with cycling for various anode reaction rate constant without lithium loss the anode reaction rate constant indicates the intercalation/deintercalation reaction rate constant. figure 6 shows that when the intercalation/deintercalation reaction rate constant is the jha and krishnamurthy j. electrochem. sci. eng. 12(2) (2022) 359-372 http://dx.doi.org/10.5599/jese.1147 367 highest, the capacity loss is the lowest. with increasing intercalation/deintercalation reaction rate, the rate of lithium transport increases, effectively increasing the capacity of the battery. while figure 6 shows the capacity loss when there is no initial cyclable lithium loss during cycling, figure 7 shows the capacity loss when there is 10 % initial lithium loss during cycling. figure 8 shows the comparison of the capacity losses when there are 0 and 10 % lithium losses during cycling. the figure shows that when the initial lithium loss during cycling increases from zero percent to 10 percent, there is a 4 % differential in the capacity loss due to side reactions. figure 7. capacity loss with cycling for various anode reaction rate constants with 10 % li loss during cycling figure 8. comparison of capacity loss for 0 % li loss and 10 % li loss effect of anode radius on lithium-ion concentration at the anode/sei interphase figure 9 shows the concentration of lithium ions at the anode/sei interphase as a function of anode particle radius (4 different particle radii are shown in the figure). it is seen that the highest concentration of lithium ions at the anode/sei interphase occurs at the smallest particle radius. smaller anode particles allow lithium ions to intercalate and deintercalated quickly due to the short diffusion path for lithium ion transport within the particles. this leads to a higher concentration of lithium ions at the anode/sei interphase. figure 10 shows the concentration of lithium ions at the anode/sei interphase as a function of the reaction rate constant for lithium intercalation. with the increasing rate constant of deintercalation, the concentration of lithium ions at the anode/sei interphase is seen to increase. http://dx.doi.org/10.5599/jese.1147 j. electrochem. sci. eng. 12(2) (2022) 359-372 capacity fading of li-ion batteries 368 figure 9. the concentration of lithium ions at the anode/sei interphase as a function of anode particle radius figure 10. the concentration of lithium ions at the anode/sei interphase as a function of the anode reaction rate constant figure 11 shows the concentration of lithium ions at the anode/sei interphase varying with anode radius in the first and the 2000th cycle. figure 11. the concentration of lithium ions at the anode/sei interphase in the first and 2000th cycle jha and krishnamurthy j. electrochem. sci. eng. 12(2) (2022) 359-372 http://dx.doi.org/10.5599/jese.1147 369 during the charging cycle, lithium from the cathode moves to the anode and hence the concentration of lithium ions at the anode/sei interphase increases. the lithium ions move from the anode to the cathode during the discharging cycle. hence, the concentration of lithium ions at the anode/sei interphase is seen to go from maximum to zero. as the battery cycles, lithium ions are lost in the intercalation deintercalation process. hence, the concentration of lithium ions at the anode/sei interphase is lower in the 2000th cycle than in the 1st cycle. the potential drop across the sei layer as a function of anode particle radius figure 12 shows the effect of anode particle radius on the potential drop across the sei layer. the figure analyses the effect of four different particles sizes on the potential drop across the sei layer. the least potential drop across the sei layer occurs when the anode particle size is the smallest. as explained earlier, smaller anode particle sizes lead to higher intercalation deintercalation rates leading to higher current densities. given a constant power output, this indicates a lower potential drop across the cell and hence a lower potential drop across the sei layer. figure 12. potential drop over the sei film with cycle number for various anode particle radius figure 13 shows the effect of the anode reaction rate constant on the potential drop across the sei layer. figure 13. potential drop over the sei layer as a function of anode reaction rate constant http://dx.doi.org/10.5599/jese.1147 j. electrochem. sci. eng. 12(2) (2022) 359-372 capacity fading of li-ion batteries 370 the graph shows that the potential drop across the sei layer increases with decreasing rate constant. the anode reaction rate constant indicates the rate of intercalation deintercalation of lithium ions in the anode particles. when the anode reaction rate constant is lower, the intercalation/deintercalation of lithium ions in the anode is reduced, giving rise to a lower current density. given a constant power output, this indicates an increased potential drop across the cell and, hence, a potential drop across the sei layer. this is shown in figure 13. figure 14 shows the comparison of modelling predictions with experimental data [3,11]. modeling predictions are found to compare well with experimental data. the model comparisons are made for 1 c discharge at 45 oc operating conditions for the lithium-ion battery cell. the parameters used for data fitting are shown in table 1. figure 14. comparison of modelling predictions with experimental data of capacity fading percentage at 1c discharge rate and 45 °c [3,11] conclusion a 1-dimensional mathematical model is developed to study the effect of anode particle radius and anode reaction rate constant on capacity fading of a li-ion battery. simulation results predict that for the smallest anode particle radius of 0.5 m, capacity fading is less in comparison to 2.5 m. smaller anode particle radii lead to faster lithium intercalation/deintercalation rates leading to higher current densities and lesser capacity fade. smaller anode particle radii also lead to increasing anode surface area for reaction. the anode reaction rate constants are also found to play a major role in the capacity fading of lithium-ion batteries. it is found that the higher the anode reaction rate constant, the lesser is the capacity fade in the battery. model results are compared with experimental data and found to compare well. nomenclature s the electric potential at electrode e electrolyte potential ∆sei the potential losses due to sei layer cli concentration of lithium in the electrode particles ce electrolyte salt concentration e electrolyte conductivity f activity coefficient for the salt jha and krishnamurthy j. electrochem. sci. eng. 12(2) (2022) 359-372 http://dx.doi.org/10.5599/jese.1147 371 t+ transport number for li+ isum sum of all electrochemical current sources qe, qs arbitrary electrolyte and electrode current source e electrolyte volume fraction de electrolyte salt diffusivity, re total li+ source term in the electrolyte is current density in electrode s electrical conductivity of electrode rp particle radius cli,max total concentration of reaction sites, ds salt diffusivity at electrode rli molar flux of lithium at the particle surface av specific surface rl,src additional reaction sources that contributes to total species source rsei film resistance  0 film thickness ∆ film thickness change sei film conductivity η activation over potential eeq equilibrium potential of cell qcell,0 battery cell capacity a , c anode and cathode transfer coefficient ka , kc reaction rate constant for anode and cathode acknowledgement: the authors would like to acknowledge bits pilani, hyderabad and council for 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[1-3]. the basics of all of these procedures are as follows: assume an electrolysis cell with a metal (me = al or fe) anode, and some neutral aqueous solution with direct current, dc, flowing through it. on the cathode hydrogen evolution, on the anode metal dissolution proceeds, the metal dissolution yields me z+ (al 3+ or fe 2+ or fe 3+ ) ions. these cations induce decontamination by two different ways: http://www.jese-online.org/ mailto:pajkossy.tamas@ttk.mta.hu j. electrochem. sci. eng. 6(1) (2016) 57-65 electrocoagulation for water cleaning 58 (1) the ions of high positive charge, in accord with the schulze-hardy rule [4], cause the coagulation of the contaminant colloids of negative surface charge. decontamination in this case called "discharge coagulation" means that the colloids coalesce and finally are separated from the liquid phase. (2) in neutral aqueous solutions the hydrolysis of the me z+ proceeds immediately yielding me(oh)z. the growth of me(oh)z particles starts as polynuclear complexes, continues as growth of colloids, which finally merge yielding flocs. during the growth, certain components of the solutions, like the contaminating particles, substances can be incorporated in the flocs, thereby the flocs finally comprise of the metal-hydroxide and the contamination. decontamination effect is a consequence of incorporation, which is much more pronounced at the initial stage of the hydroxide formation. in other words, the “nascent” me(oh)z colloids are the active particles rather than the big flocs. this mechanism of decontamination is called "sweep coagulation". depending on their specific density, the merged particles are separated from the liquid in two ways: either precipitate, or form a float or a scum (note that even the relatively heavy flocs may float if are attached to hydrogen bubbles evolved on the cathode). the difference between the two basic separation techniques of the electrocoagulation: electroflocculation and electroflotation, is ultimately based on the specific density of the merged particles (sink or swim). here we note that me z+ ions just as the me(oh)z colloids can be generated by simply dissolving me-salts in water, the main advantage of the electrochemical generation of the coagulant ion (over that of dissolving the salts) is that electrochemically the "nascent" colloid is formed and introduced in the bulk of the solution in a much more controlled way. from the technical viewpoint a further important advantage is the simple unit design without external solution dosing. for a comparison, see, e.g. ref. [5] or chapters 3-5 of [3]. various sorts of electrocoagulation as water cleaning, or waste-water processing industrial procedures have been introduced since the beginning of the past century. however, due to big energy costs and to various technical problems (e.g. inhibition of electrodes' dissolution) the procedure could not penetrate in industrial waterand waste-water processing up till recently. in the recent years a number of enterprises have appeared and are selling electrocoagulation process units, typically with 1-10 m 3 /h capacity. also the number of the technical papers have been increasing in the past twenty years. most of these are on experiments on removal of some, usually organic contaminants from some wastewater. the list of the substances which can be or at least were attempted to be removed is fairly long; for a listing of the systems see ref. [6]. many of the systems are oily emulsions associated with petrol industry [7-13] or with vegetable oils [14-18] or with mechanical industries [19-21]. nevertheless, removal of inorganic compounds (e.g. heavy metals from groundwater or from effluents of various metal industries) has also been implemented. it must be noted that the efficacy of the wastewater cleaning through coagulationflocculation process is generally low in the removal of the dissolved organic components. our motivation – just as the subject of the present paper – is associated with a project of ic product co. (hungary) aimed at the development of a mobile, 1 m 3 /h capacity electrocoagulation waste-water cleaning unit. for the development, basic design parameters data were needed mostly on the materialand energy balances. to this end, we performed small-scale experiments with oil-in-water type emulsions, whose organic content was in the 1 g/l order-of-magnitude. the work is aimed at to answer the following three main questions: 1. how much metal is to be dissolved electrochemically to coagulate a certain amount of organics? e. fekete at al. j. electrochem. sci. eng. 6(1) (2016) 57-65 doi:10.5599/jese.218 59 2. what is the efficiency of the organic material removal and how much energy is needed to remove the organic material? 3. what are the important aspects of electrochemical cell design? in what follows we show demonstration experiments, carried out with a commercially available oil-in-water emulsion used as a cooling lubricant for cutting and drilling, providing the guidelines to answer the above questions. we focus mainly on the behaviour of al electrodes; in some cases, for comparison, the results of analogue experiments with iron electrodes are also shown. we note that the technical issues associated with settling of the flocs, filtering the sludge, removing the float is just as difficult as the electrochemistry involved. these problems, however, are not discussed here. experimental we used a non-thermostatted rectangular-channel flow-through cell, shown in fig. 1, containing vertically placed parallel-plate electrodes (height 10 cm, width 5 cm, interelectrode distance, w, 0.2 cm). the electrolyte was pumped through in upward direction by a peristaltic pump providing a flow rate in the 1 16 l/h, ~0.3-4 ml/s range. a b c figure 1. the flow through cell used in the experiments (a). by changing the assembly of the electrode block, it could be used with 2 monopolar electrodes (b) and with maximum 6 electrodes comprising 5 serially connected cells with bipolar electrodes (c). the current cell voltage measurements have been performed in galvanostat mode by an ef 427e high-power potentiostat, the voltage and/or current data were collected by dvms via rs-232 interfacing. the typical precision of the measured voltages and current is 1 mv and 0.1 %, respectively. the al electrodes were made of technical alumínum alloy plates (almgsi0.5 alloy of standard en aw 6060). in some cases, for comparison fe (carbon steel with 0.2 % mn and 0.2 % si content of standard 10130) electrodes were also used. in some cases the metal content of the effluent after acidifying the samples was determined by inductively coupled plasma spectrometry (icp). j. electrochem. sci. eng. 6(1) (2016) 57-65 electrocoagulation for water cleaning 60 solutions: the oil emulsion is a commercially available oil product of mol co. (hungary) with trade name emolin 400, major components of which are emulsified mineral oil (60-65 w/w %), surfactants (11-23 w/w %) and organic corrosion inhibitors (15-20 w/w %). its 2-5 % aqueous dilution is widely used in factories, machine shops as a cutting & drilling cooling lubricant. we note that the results obtained with this oil emulsion are rather similar to those with other emulsions like diluted bovine milk, or oil-in-water emulsions of olive oil. the typical dilution in our experiments was 1 g/l, for which the total organic content, measured by the regular chemical oxygen demand method (cod, determined by digesting the appropriately diluted sample by potassium bichromate and titrating back the residual bicromate [22]) was found to be around 2000 mg cod/l. the solutions of the metal dissolution experiments have been made with pure chemicals (na2so4 and nacl) from reanal or fluka, in deionized water produced by an elga purewater system. results when the white-opaque oil-water emulsion is electrolyzed between al or fe electrodes, first the metal hydroxide changes the appearance of the emulsion. provided that a sufficient charge has passed through the cell, after a certain time typically within a couple of hours the flocs are settled, the liquid above the flocs gets become clear, as shown in fig. 2. 300 c/l 360 c/l 420 c/l 540 c/l 420 c/l 480 c/l 600 c/l 720 c/l figure 2. emolin-water emulsion (0.1 % for emolin) after electrolysis and 1 day settling. the charge/volume data are under the photos. 1st and 2nd row of photos: electrolysis with alal and fe-fe electrodes, respectively. the cleaning effect can be quantified by measuring turbidity of the liquid after the settling. when turbidity is low and the dissolved components are colorless, optical absorption in the visible range is also a good measure of suspended particles. hence, the amount of coagulant needed to achieve complete or at least the maximum possible coagulation can be easily measured by measuring the optical absorption spectra of the clear part of the liquid as function of the charge passed through the electrodes. this is shown in fig. 3a. three features are important: first, there exists a threshold value of charge (in what follows, named as coagulation charge) above which the absorbance (just as turbidity) is negligible, i.e. suspended particles disappear from the liquid; second, coagulation charge is almost the same for al as for fe electrodes and third, the coagulation charge is proportional to the oil concentration, as shown in fig. 3b. e. fekete at al. j. electrochem. sci. eng. 6(1) (2016) 57-65 doi:10.5599/jese.218 61 figure 3. (a) absorbance of the clear part of the liquid as function of charge passed between al-al electrodes (red symbols) and between fe electrodes (blue symbols) for emolin-water emulsions of concentrations as indicated. (b) charge needed for the effective coagulation. based on the "calibration line" on fig.3b, provided that the oil concentration, c, is known, we can calculate the amount of charge needed to coagulate the oil content of unit volume of the liquid as q / c = 4000 × c / %. charge and dissolved metal’s moles, m, is connected by faraday’s law as m= q / (nef), where f= 96500 c/mol, and ne is the effective charge number, which can be determined by measuring charge, and the metal content of the liquid (flocs, sludge, scum, float, and clear liquid altogether) by icp. the ne ≡ q / mf effective charge number has been calculated from a number of dissolution experiments; its value is ≈2 both for al and fe, because of different reasons: a. the ne ≈ 2 for al is an indication that al is dissolved on both electrodes. on the cathode hydrogen evolution, i.e. strongly reducing conditions keep the al surface oxide-free, hence the al – being an “electronegative” metal – chemically splits water according to the reaction: 2 3 2 2al 6h o 2al(oh) 3h   . the fact that al dissolves also cathodically, is well-known in corrosion science[23] and has also recognized in electrocoagulation studies [24,25]. b. iron dissolves in fe 2+ form – this is clearly seen as the solution becomes dark green in the vicinity of the anode. however, the dark green flocs slowly (typically in 10 minutes 1 hour time, depending on the conditions) get the pale brown colour of the fe 3+ , just as seen in fig.2, 2 nd row. the effectivity of the cleaning procedure can be determined by chemical analysis of the sludge and of the cleaned liquid: the total organic content of the liquid, approximated and measured as cod, decreases to 10-20 % that of the original emulsion containing approximately this amount of dissolved organics (in what follows we consider the average, 15 %). with these data, the mass balance can be estimated, as follows: assume an oily waste water of 0.1 %≈1 g/l organic content. to remove 85 % (≈850 mg) of the organics, ≈400 c is needed; with this charge ≈400/(2*96500) 27 g= 56 mg al or ≈400/(2*96500) 56 g = 116 mg fe is dissolved. the energy consumption of an electrocoagulation cell is much less well-defined. to illustrate this, lets perform the current – cell potential (polarization curve) measurement between the two al electrodes, in a solution, whose conductivity, κ, is similar to that of some tap water, ≈500 ms/cm or somewhat larger. to avoid passivation of these metals, the electrolyte should contain j. electrochem. sci. eng. 6(1) (2016) 57-65 electrocoagulation for water cleaning 62 some chloride – the minimum concentration was reported to be 60 ppm chloride (=1.7 mm) [26]. to be safely above this low limit, the polarization curves have been measured in 20 mm nacl solution (κ ≈2.3 ms/cm at 25 o c); such a curve is shown in fig. 4. first, in the range of ±1 v the curve has a clockwise hysteresis, clearly indicating passivation-depassivation. in this cell voltage range – or with smaller currents than 5 ma – al 3+ ions thicken the oxide layer on the electrode rather than enter the solution. hence the cell voltage must be larger than approximately 1 v; the use of 10 ma/cm 2 current density seems to be reasonable. out of the ±1 v range, u ≈ 1 v +ir. the straight lines starting at +1 v and -1 v are in the ir drop-controlled voltage ranges. the reciprocal of the slopes of the lines =1 / 3.07 ms cm 2 = 325  cm 2 , are larger by a factor of almost 4 than the area-normalized resistance of the solution between the electrodes ar = wκ = 87  cm 2 . this difference indicates that the electrode surfaces are covered by some layer of significant resistance. in fact, white layers can be seen on the electrodes after the experiments; their appearance depends on the experiment's conditions. for example, if high dc current flows along with fast streaming of the electrolyte for a long time, then the layer is practically missing; the resistance is that of the solution. this is why we specify the lower limit of the cell voltage as u = 1 v + ir = 1 v + jw/κ for the cases of surface-layer-free, "clean" electrodes where the ir drop is due to solution resistance only. in real systems, the ir terms may be somewhat larger. figure 4. polarization curve measured galvanostatically (0.66 (ma/cm 2 )/s scan rate) in 20 mm nacl solution in a flow cell of thickness 2 mm, between two al electrodes. flow rate 1 l/h. with these considerations, energy consumption is estimated as follows: employing typical values, j = 10 ma/cm 2 , κ = 2 k cm (usual value for tap water), w = 0.5 cm, and clean electrodes we get an inacceptably high value of u = 11 v. (note that in this case almost the full amount of electricity heats the cell.) since in practical cases the interelectrode distance cannot be decreased in order to avoid blocking of the cell; with smaller currents the electrodes are prone to get passivated; the only possibility to decrease cell voltage, salt addition to the liquid is inavoidable. finally, the energy consumption is calculated with the assumption that the cell voltage can be lowered to 5 v. while passing 400 c charge, 5v × 400 c = 2000 j≈0.55 wh energy is consumed. e. fekete at al. j. electrochem. sci. eng. 6(1) (2016) 57-65 doi:10.5599/jese.218 63 mass balance and energy consumption on a practical scale, based on the above data: for removing approximately 1 kg oily contaminant from a waste water of about 1 m 3 , we need 56/0.85 = 66 g al and 0.55 kwh electric energy for performing the electrocoagulation. for these data of mass of al and electric energy, two comments are needed. first, the aluminium plates used as electrodes are dissolved somewhat unevenly. we found that (on an average) about 90 % of their mass can be utilized. second, we found that in some cases certain coatings are formed on the cathode (a sticky organic or limescale) which can be removed by changing the polarity for a while. prompted by some studies whose authors advocate for the use of alternating current, ac, rather than dc [27,28] we now demonstrate the worse performance of the ac method with the following experiment: the current of the flow-through cell with al electrodes was set to a value at which the al concentration of the effluent was 2.5 mm. then, dc of alternating polarity was applied polaritychanges in every t seconds (5 ms ≤ t ≤ 50 s). the al content of the effluent was measured by icp. the al content decreases with decreasing t, as shown in fig. 5, clearly pointing to that electrocoagulation should be done with dc. polarity changes should be done rarely, only if needed. as a matter of curiousity: alternating the polarity blocks fe electrodes much more than al ones. figure 5. dependence of metal dissolution rate on frequency of the current of alternating polarity. abscissa: time between polarity changes (half of period time); ordinate: concentration of dissolved metal (al or fe) in the effluent. with dc electrolysis, the concentration is 2.5 mm. discussion on the whole, the obtained data of cleaning efficiency (mainly cod) are in agreement with those in papers on similar studies [18-20]. thus, we can specify the design parameters for the waste water cleaning unit of 1 m 3 /h capacity, with assuming a 1 kg/m 3 = 1 g/l oily contamination, as follows: 1. the unit will remove 85 % of the contamination (in terms of cod). provided that cod is proportional to concentration of the organics, the rest is the dissolved organics of 0.15 g/l, with cod 300 mg/l . j. electrochem. sci. eng. 6(1) (2016) 57-65 electrocoagulation for water cleaning 64 we note that this residual contamination level might imply that a secondary purification step of activated carbon adsorption is required. however, this water quality generally fits to reuse purposes in industrial applications. 2. to remove 1 kg organics, approximately 66 g al is to be dissolved. for this, (66/27) × 2 × 96500 = 0.47 × 10 6 c charge is needed in one hour, i.e. ≈131 c/s. provided that we use 10 ma/cm 2 current density, the area of anodes and cathodes is ≈131 a / (0.01 a / cm 2 ) ≈ ≈ 1.3 m 2 . 3. as it has been found in the context of fig.4, the cell voltage is u ≈ 1 v+ir =1 v + jw/κ; with 0.5 cm interelectrode distance the cell voltage and hence energy consumption are inacceptably high if the conductivity of the waste water is like that of tap water. for the increase of conductivity, salt, nacl or salt plus for balancing ph, hcl should be added to the waste water up till it reaches the maximum emission value of waste water, 2 ms/cm. this requires less than 1 kg nacl (per 1 m 3 waste water). then u ≈ 1 v + 10 ma/cm 2 × 0.5 cm / (2 ms/cm) = 3.5 v. we note that from energy consumption point of view the continuous conductivity monitoring and adjusting system plays a central role. 4. points 2+3: the dissolution of 66 g al requires 0.47 × 10 6 c × 3.5 v = 1.65 mj = 0.45 kwh electric energy. the overall electrode area of 1.3 m 2 may be divided to many small sections to yield an ensemble of parallely and serially connected cells, thus the power supply can be set up employing parallely operating e.g. 24 v modules. although there is a number of other important issues (e.g. ph regulation, flow control, separation of flocs and float, detection of electrodes' blockage) which are out of our present scope, the main electrochemical points of electrocoagulation have all been presented here. using also these guidelines, which were formulated based on our small-scale experiments, a couple of prototypes of electrocoagulation waste water cleaning units have been built; their properties are close to those predicted. conclusions the subject of this paper is a couple of small-scale experiments by which design parameters of larger-scale electrocoagulation units could be established. as a result of these experiments with oil-in-water type emulsions with about 1 kg/m 3 organic content, we conclude that 80-90% of the organic content can be removed on the expense of dissolution of al of less than one-tenth of mass of the removed organics plus about 0.5-1 kwh electric energy per kg of removed organics. acknowledgements: the project of code kmr_12-1-2012-0386 was funded by the national research, technology and innovation office of the hungarian government. references [1] m. vepsäläinen: electrocoagulation in the treatment of industrial waters and wastewaters, thesis, vtt technical research centre of finland, 2012 [2] m. y. a. mollah, r. schennach, j. r. parga, and d. l. cocke, journal of hazardous materials, 84 (2001) 29-41. 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {electrochemical treatment of dye wastewater using nickel foam electrode} http://dx.doi.org/10.5599/jese.1011 209 j. electrochem. sci. eng. 11(3) (2021) 209-215; http://dx.doi.org/10.5599/jese.1011 open access: issn 1847-9286 www.jese-online.org original scientific paper electrochemical treatment of dye wastewater using nickel foam electrode kaavya muthumanickam, ramanujam saravanathamizhan department of chemical engineering, a.c. tech anna university, chennai-600025, india corresponding author:thamizhan79@rediffmail.com; tel +91 44 22359237 received: may 28, 2021; revised: july 5, 2021; accepted: july 10, 2021; published: july 16, 2021 abstract removal of dye from wastewater has been investigated using the electrocoagulation method. batch experiment has been conducted to remove the color from synthetically prepared acid red 87 dye wastewater. stainless steel and nickel foam sheets are used as cathode and anode, respectively. the effect of some operating parameters, such as current density, initial dye concentration and supporting electrolyte concentration, on color removal has been studied. it can be observed from the present investigations that the nickel foam electrode effectively removes color from the wastewater. nickel hydroxyl species formed during the operation and also, nickel(ii) hydroxide flocs formed in a subsequent stage, trap colloidal precipitates and make solid-liquid separation easier during the flotation stage. these stages of electrocoagulation must be optimized to design an economically feasible process. keywords acid red 87, color removal, electrocoagulation, kinetics, wastewater introduction wastewaters generated from various process industries contain various types of pollutants. these pollutants must be treated, and only clean water must be disposed to the environment. industries such as textile, paper and pulp, as well as leather and pharmaceutical industries, are generating large amounts of pollutants. distinct pollutant treatment techniques have already been adopted for the wastewater treatment, namely physical, chemical and biological techniques. pollutants coming out from the textile industry contain organics and colours, and treatment of these pollutants is mandatory before letting them to the environment [1]. in this aspect, the cost-effective treatment techniques are ultimately required to treat a wide range of wastewater pollutants in a diverse range of conditions. compared with conventional treatment techniques, electrocoagulation method of pollutant removal provides a robust and http://dx.doi.org/10.5599/jese.1011 http://dx.doi.org/10.5599/jese.1011 http://www.jese-online.org/ mailto:thamizhan79@rediffmail.com j. electrochem. sci. eng. 11(3) (2021) 209-215 dye wastewater treatment by ni foam electrode 210 efficient treatment method. electrocoagulation is usually used to treat textile wastewater [2], dairy wastewater [3], paper industry effluent [4], printing wastewater [5], metal finishing [6] and plating wastewater [7], pharmaceutical wastewater [8], etc. in the electrocoagulation process a sacrificial metal anode is used for the electrochemical reaction that provides active metal cations for the coagulation process. the advantage of electrocoagulation is that coagulants are generated in situ, i.e. they are not added externally to the wastewater. electrodes such as aluminium and iron are widely used for the electrocoagulation process [9]. electrocoagulation of textile wastewater was studied using al-fe anode by ghanbari et al. [10]. the authors reported that 98 % color removal efficiency was achieved using this combined anode. safwat [5] performed the electrocoagulation process for the treatment of real printing wastewater using two different, zinc and titanium electrodes. with the electrode distance of 4 cm, the author observed 47 and 41 % chemical oxygen demand (cod) removal using titanium and zinc electrode, respectively. devlin et al. [11] studied the influents from municipal wastewater treatment plant, which were electrochemically treated with sacrificial aluminum, iron, and magnesium electrodes. it was noticed that aluminum and iron electrodes remove 68 % cod, while magnesium electrodes, removed 49 % cod. electrocoagulation of metal fining wastewater was studied by ilhan et al. [12], where the authors reported the importance of ph adjustment after the electrocoagulation process using iron electrodes, which are found 10 % more effective than aluminum electrodes. nickel has been also used as a sacrificial anode for the wastewater treatment. karikaningsih et al. [13] performed electrocoagulation using metallic ni foam anode for the removal of boron present in wastewater. the authors reported that removal efficiency was maximized at ph 8-9, and decreased as ph increased beyond this range. nickel foam electrode has also been used for electrooxidation of wastewater. ni-foam coated with iron–chitosan solution was designed as a new cathode for the electro-fenton treatment of colored effluents [14]. the authors concluded that new cathode is a suitable alternative for the treatment of colored wastewater by continuous electro-fenton treatment. similar studies have been also reported [15,16], showing that ni-foam cathode is suitable for the treatment of colored wastewater by continuous electro-fenton treatment. nickel foam is a three-dimensional electrode, with many advantages including good electrical conductance and high specific surface area. in literature data, however, very few authors have used ni-foam for electrocoagulation process. hence in the present study, we are focused to remove acid red 87 dye from the simulated textile wastewater using nickel foam electrode. experimental chemicals acid red 87 dye and other chemicals were purchased from srl pvt ltd., chennai and used without any purification. the dye, also called eosin yellow has the molecular formula of c20h6br4na2o5. known amount of acid red 87 dye was dissolved in distilled water to prepare the synthetic wastewater. concentration of nacl was varied by adding required amount of salt into prepared dye solution. ph of the solution was adjusted by dropwise adding either alkali or acid (1 m) solution. electrochemical experiments the electrochemical setup consisted of 100 ml capacity cell, filled with the prepared synthetic wastewater solution and two immersed electrodes. the schematic setup of the experiment is shown in figure 1. the electrolytic solution was stirred throughout with the help of magnetic stirrer during the process. batch experiments were conducted using a digital dc regulated power supply, having m. kaavya and r. saravanathamizhan j. electrochem. sci. eng. 11(3) (2021) 209-215 http://dx.doi.org/10.5599/jese.1011 211 the range 0-30 v and 0-5 a. stainless steel sheet was used as cathode and nickel foam as anode. the electrodes having total active electrode area of 14 cm2 were placed vertically into solution, parallel to each other at a distance of 1 cm, and connected to a dc power supply. figure 1. scheme of experimental setup a metallic foam used as a sacrificial electrode does not only catalytically alter the energy barrier to the electron transfer to form metal hydroxides, but also provides a large surface area where the redox reaction proceeds. therefore, nickel foam is an attractive material owing to its high specific capacitance, well-defined electrochemical activity and high stability. in the electrocoagulation process, the main reactions occurring at the electrodes involving ni anode are: anode: ni (s) →ni2+(aq) + 2e(1) cathode: 2h2o + 2e-→h2(g) + 2oh-(aq) (2) ni2+(aq) + 2oh-(aq)→ni(oh)2(s) (3) overall reaction: ni(s) + 2 h2o→ni(oh)2(s) +h2(g) (4) ni is oxidized at anode by reaction (1), water is reduced at cathode by reaction (2), while dissolved ni2+ ions form ni-hydroxide species by reaction (3). the overall reaction (4) suggests a removal mechanism by formation of ni(oh)2 of high adsorption properties for dye bonding and subsequent coagulation and precipitation. depending upon ph of an aqueous medium, different electrolytic forms can be generated like nioh, ni(oh)2, ni(oh)-3, ni(oh)24, and ni4(oh)4, suggesting that ph of the electrolyte governs precipitation. at higher ph values, nio, niooh, or ni(oh)3can be also formed at specific overpotentials. the formed ni(oh)2(s)/niooh appear as sweep flocs with large surface area, which favor adsorption and thus removal of pollutants. nickel oxide was proven to be a good adsorbent for the treatment of organic and inorganic wastewaters. experimental procedure the experiments were performed at room temperature and constant stirring speed of 250 rpm. in each run, 100 ml of synthetic wastewater dye solution was placed into the electrolytic cell. the current density was adjusted to a desired value for the experiment. at the end of the run, the solution was filtered, and the filtrate was centrifuged at 200 rpm. before each run, organic impurities on electrode surfaces were removed by washing with acetone, while the surface oxide layer was removed by dipping for 5 min in dilute hydrochloric acid. at the end of the experiment, the electrodes were washed thoroughly with water to remove any solid residues on the surfaces, http://dx.doi.org/10.5599/jese.1011 j. electrochem. sci. eng. 11(3) (2021) 209-215 dye wastewater treatment by ni foam electrode 212 dried, and reweighed. operating parameters such as initial dye concentration, supporting electrolyte concentration and current density were varied to study the color removal of dye wastewater at initial ph value of 7 and distance between electrodes of 1 cm in all experiments. analytical method samples of dye solution were analyzed using a uv-spectrophotometer at max (518 nm of acid red 87). the percentage of color removal was calculated according to [17]: initial absorbance final absorbance removal = 100 initial absorbance (5) initial and final absorbances in equation (5), are absorbances measured at max initially and after treatment time (t). result and discussion kinetic of decolorization the rate of decolorization reaction in a batch reactor can be described as the first order kinetics [17]: = 0 ln c kt c (6) where c0 and c are initial dye concentration and dye concentration after reaction time t, while k is the reaction rate constant. from the linear plot -ln(c/c0) vs. t, the rate constant can be determined from its slope. the rate constants of dye wastewater decolorization for different dye concentrations are given in table 1, where it can be noticed that the rate constant decreases from 0.1232 to 0.0171 min-1 with increasing dye concentration. table 1. decolorization kinetics of dye wastewater at different concentrations of dye initial dye concentration, mg l-1 k / min-1 100 0.1232 300 0.0332 500 0.0171 effect of supporting electrolyte concentration the effect of the concentration of supporting electrolyte (nacl) on dye removal is shown in figure 2. figure 2. effect of supporting electrolyte (nacl) concentration (1-9 g l-1) on dye removal. initial dye concentration: 100 mg l-1; current density: 12 ma cm-2; ph 7.0 m. kaavya and r. saravanathamizhan j. electrochem. sci. eng. 11(3) (2021) 209-215 http://dx.doi.org/10.5599/jese.1011 213 the supporting electrolyte nacl concentrations were varied from 1 to 9 g l-1 and the color removal was observed. it can be noticed from figure 2 that color removal efficiency increased not only with the reaction time, but with increase in the concentration of nacl too. this is probably due to the generation of hypochlorite ions (ocl-) formed together with hypochlorous acid (hocl) by electrolysis of near-neutral aqueous nacl solutions. presence of oclions in wastewater enhances the formation of nickel hydroxide. increase in the quantity of the supporting electrolyte results in producing of larger amount of sludge during the electrocoagulation process [18]. the sludge generated during the experiment was minimal and safely removed in each experiment and disposed. effect of current density current density is important operating parameter in batch electrocoagulation process, since it is the only parameter that can control the removal process directly. electrode distance and ph of the solution were fixed for the present experiment and the current density is varied. current density directly determines both coagulant dosage and bubble generation rates, and for higher current density, the shorter treatment time is required for dye removal. this is ascribed to the fact that at high current density, the dissolution of nickel anode increases, resulting in a greater amount of ni hydroxide for the removal of dyes pollutants. the effect of current density on color removal is shown in figure 3. current density was varied from 3 to 12 ma cm-2, and color removal was observed. it is noticed from figure 3 that color removal increases with increase in the current density. the decolorization efficiency increases rapidly up to 92 % at the current density of 12 ma cm-2, and then it remains almost constant for higher current densities [19]. the dissolved amount of anode is fairly linearly dependent on the current density. high current density increases the coagulant generation with lesser size which provides larger surface area between the coagulant and pollutants. increase in the current density decreases the treatment time of the dye solution. figure 3. effect of current density (3-12 ma cm-2) on dye removal. initial dye concentration: 100 mg l-1; nacl concentration: 9 g l-1; ph 7.0 effect of initial dye concentration it is seen in figure 4 that decolorization efficiency falls from 92 to 67 % when the dye concentration was increased from 100 to 500 mg l-1, suggesting that the removal efficiency is inversely proportional to dye concentration. the final ph of solution and the mass of dissolved nickel foam anodes were nearly independent, while the cell voltage increased slightly with increasing dye concentration. despite small increase of the cell voltage at a constant current density, the energy consumption shows a strong inverse relationship with dye concentration. when the decolorization rate is inversely proportional to the concentration of dye solution, absorbance of the sample is directly proportional to the concentration of the dye solution. http://dx.doi.org/10.5599/jese.1011 j. electrochem. sci. eng. 11(3) (2021) 209-215 dye wastewater treatment by ni foam electrode 214 figure 4. effect of initial dye concentration (100-500 mg l-1) on dye removal. current density: 12 ma cm-2; nacl concentration: 9 g l-1; ph 7.0 conclusion removal of acid red 87dye from synthetic wastewater samples by electrocoagulation method was conducted using nickel foam electrode. batch experiments were performed using nickel foam as anode and stainless steel as cathode. influence of experimental operating parameters such as initial dye concentration, supporting electrolyte (nacl) concentration and current density on dye removal efficiency was studied, keeping initial ph of solution and distance between electrodes constants. the removal efficiency was found proportional to the concentration of supporting electrolyte and current density, but inversely proportional to the initial dye concentration. up to 92 % dye removal was observed after 40 min of treatment time for the optimum operating conditions (current density: 12 ma cm-2, initial dye concentration: 100 mg l-1, nacl concentration: 9 g l-1, initial ph: 7.0; distance between electrodes: 1 cm). kinetics of the dye removal was found as 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of water process engineering 41 (2021) 102040. https://doi.org/10.1016/j.jwpe.2021.102040 ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1011 https://doi.org/10.1016/j.jhazmat.2018.10.017 https://doi.org/10.1007/s11783-019-1152-1 https://doi.org/10.1016/j.chemosphere.2016.09.091 https://doi.org/10.1016/j.psep.2015.04.011 https://doi.org/10.1002/jctb.4626 https://doi.org/10.1016/j.jhazmat.2012.10.024 https://doi.org/10.1186/s42834-019-0043-2 http://dx.doi.org/10.13171/mjc10102001201163mh https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://www.sciencedirect.com/science/article/abs/pii/s2214714421001276#! https://doi.org/10.1016/j.jwpe.2021.102040 https://creativecommons.org/licenses/by/4.0/) ellipsometric study of passive and anodic oxide films formed on ti and nb electrodes doi:10.5599/jese.245 221 j. electrochem. sci. eng. 5(4) (2015) 221-230; doi: 10.5599/jese.245 open access : : issn 1847-9286 www.jese-online.org original scientific paper ellipsometric study of passive and anodic oxide films formed on ti and nb electrodes ljubomir arsov, irena mickova faculty of technology and metallurgy, university ss. cyril and methodius, rudjer boskovic 16, 1000 skopje, republic of macedonia corresponding author: arsov@tmf.ukim.edu.mk; tel. +389 2 3088 43; fax: + 389 2 3 065 389 received: october 20, 2015; accepted: november 26, 2015 abstract electrochemical formation of passive films and active/passive transition on ti and nb metal surfaces in various concentrations of h2so4 and koh solutions was investigated using potentiostatic and cyclic voltammetry methods. by simultaneous electrochemical and in-situ ellipsometric measurements the coefficients of film thickness growth of passive films in the potential region from -1.5 v to 4 v were determined. results indicate the strong influence of the concentration and electrolyte nature to the active/passive transitions and stability of passive films. the influence of cathodic pre-treatment on the passive films dissolution and appearance of the reactivation peaks during the reverse potential cycling were shown. by multiple cycle sequences in which the final anodic potential was gradually enlarged, the barrier properties of passive films on investigated electrodes were confirmed. the electrochemical and ellipsometric data showed that the passive films formed on nb electrode are more resistant that passive films formed on ti electrode, especially in higher concentrations of investigated aggressive solutions. keywords potentiostatic method, cyclic voltammetry, film thickness growth, stability and reactivation of passive films introduction over the past 50 years there has been a growing interest for the use of titanium and niobium, as well as theirs alloys, in various branches of the chemical and mechanical industry as construction materials, specifically in transport and stocking of aggressive fluids. they have been also used in electro-synthesis, solar energy conversion bio-medical and air-space applications. research on the electrochemical behavior of ti and nb have covered the wide range of topics as http://www.jese-online.org/ mailto:arsov@tmf.ukim.edu.mk j. electrochem. sci. eng. 5(4) (2015) 221-230 passive and anodic oxide films on ti & nb 222 active-passive transition [1-2], formation of anodic oxide films [3-4], structure and chemical composition, breakdown processes [5-6], semi-conducting and optical properties [7-8] etc. titanium and niobium belong to the group of valve metals because they develop spontaneously the stable passivating layers either when contacting air and/or water solutions. these layers in reality are natural oxide films, which are always present on ti and nb surfaces and explain their high corrosion resistance. in order to increase the corrosion resistance of ti and nb, the natural oxide films can be controllably thickened by anodic oxidation in appropriate electrolytes or by thermal oxidation in atmospheric conditions [9,10]. the anodic oxide film formation on ti and nb electrodes have been studies in some acid and alkaline electrolytes, using ellipsometry and other electrochemical and optical techniques, but generally for potentials/voltages higher than the trans-passive region [11,12]. although the studies of metal passivity exist for almost 200 years, in literature data can be found very rarely the use of elliposmetric methods for determining the film thickness only in passive region during the cyclic voltammetry cv measurements. a few information’s can be found in ref. [13] about the formation of passive films on ti electrode in 0.1 m h2so4 depending of sweep rate and in one of our recent papers concerning the thickness growth and dissolution of electrochemical passive film on nb in 10 m koh [14]. the purpose of present investigations is to study the active passive transition of ti and nb electrodes, simultaneously with in-situ ellipsometric measurements. in that way is expected to get valuable results about the film thickness growth for each applied potential during the potentiostatic and cv scans measurements in forward and reverse potential directions. the comparative studies of ti and nb in acidic and alkaline solutions are carried out. experimental electrodes. massive cylindrical ti and nb rods (alfa aesar, johnson matthey company), with purity 99.95 % and 99.8 % respectively and with 12.7 mm dia., were cut in disc species with length of 10 mm which were served as the working electrodes. on one basis of the disc a shout cooper wire was employed as the electrical contact. the discs were fitted into glass tubing of appropriate internal diameters by an epoxy resin (struers), leaving the other basis with surface area of 1.27 cm 2 to contact the solution. before each experiment the electrode surfaces were mechanically polished using metallographic emery paper 600 and then electro-polished to the mirror brightness in the baths: (i) for ti electrodes containing: 60 ml perchloric acid + 540 ml methyl alcohol + 350 ml ethylene glycol mono butyl ether. the electro polishing conditions are given in ref. [15], (ii) for nb electrodes containing: 170 ml nitric acid + 50 ml. hydro fluoric acid + 510 ml methyl alcohol + 5 g citric acid at voltage of 15.2 v. after electro-polishing the electrodes were rinsed with re-distilled water, ultrasonically cleaned in ethanol and finally dried using argon gas under pressure. the counter electrode was a pt grid with large surface area. the reference electrode was a saturated calomel electrode (sce) in acidic aqueous solutions and mercury oxide electrode in alkaline aqueous solution. the potentials in this work are presented against using reference electrode. to avoid contamination, the reference electrodes were connected to the working electrode through the bridges with a luggin capillary filled with the test solutions. after each experiment the working electrodes were mechanically re-polished and prepared for the next measurements using the procedures of electro-polishing as described above. optical electrolytic cell. a three compartment optical electrolytic cell with an inlet and outlet for bubbling inert gas was adopted for electrochemical and ellipsometric in-situ measurements. experiments were done in a pyrex vessel with two optical quartz widows fixed at an angle of 70 o . lj. arsov at al. j. electrochem. sci. eng. 5(4) (2015) 221-230 doi:10.5599/jese.245 223 the specimen surfaces were mounted vertically, with a rigid clamping system having adjustments for rotation, tilt and central positioning in the cell. details of the design of this optical electrolytic cell are given elsewhere [16]. prior to each experiment, when the electrode was immersed in the electrolyte solution, the electrolyte in the cell was de-aerated by flowing argon gas through a fritted bubbler for at least 30 min. before the run. the gas flow was disconnected during the run. solutions. aqueous solutions merck p.a. with concentrations of 1 m h2so4, 3 m h2so4, 1 m koh and 6 m koh was prepared in re-distilled and de-ionized water. after each experiment the electrolyte in the cell was exchanged in order to avoid eventual build-up of soluble ti or nb species. apparatus. the electrochemical measurements were carried out potentiostatically and potentio-dynamically using heka model 488 potentiostat/galvanostat connected to a personal computer. the ellipsometric measurements were performed with a rudolph research type 43603-200 thin-film ellipsometer at a wavelength of 546.1 nm and an incident angle of 70 o results and discussion electrochemical passivity of ti and nb has been mainly studied using galvanostatic, potentiostatic, potentiodynamic and eis techniques [17-20]. in this paper we used potentiostatic and potentiodynamic methods combined with ellipsometry. ellipsometry, also known as reflection polarimetry, is a very precise method for determining the optical constants, thickness and nature of reflecting surface, especially the metal surfaces with and without existence the thin films on them. this method derives its name from the measurements of the elliptically polarized light results from optical reflection from the investigate surface [21]. hence, ellipsometry finds applicability in a wide variety of fields such as physical, chemical and micro-electronic engineering, corrosion, electrochemical and chemical formation of passive layers on metals, oxides, polymer films semiconductors, biology, medicine etc. the main strength of this technique lies in its capability to allow in-situ measurements and simultaneous determination of many optical parameters necessary to quantify the investigated system. the new generation of ellipsometers allows determination the film thicknesses of from 0.1 nm to 30 m. in ellipsometric measurements the experimentally measured parameters δ and ψ are related to the physical properties of the system by the use of fresnel’s equation / tg exp( δ)p sr r i     (1) in eq. 1, tg  represents relative amplitude attenuation, δ is relative phase change, i is imaginary number, rp and rs represent the fresnel reflection coefficients for electromagnetic wave polarized parallel and perpendicular to the plane of incidence, respectively. for a three component system, the ellipsometric parameters δ and  are complex function of the following parameters. m f stg exp( δ) (n , n , n , , , )i d         (2) in eq. 2,  mn is complex refractive index of medium,  fn complex index of film,  sn complex index of metal substrate, d film thickness, wavelength of incidence electromagnetic wave and angle of incidence. to minimize the unknown parameters in fresnel’s equation the refractive index of medium (electrolytic solutions) could be determine with abbe’s refractometer, refractive index of metal substrate by some separate methods, for example elipsometrical measurements in j. electrochem. sci. eng. 5(4) (2015) 221-230 passive and anodic oxide films on ti & nb 224 vacuum of evaporated metal on microscope glasses. the other parameters as: wavelength , was given in dependence of used laser and angle of incidence was adjusted to the ellipsometer. by method of mathematical iteration, using computer program, the film thickness and complex index of refraction are determined. in our case the experimentally measured parameters δ and  are fitted with theoretically calculated curves for δ and  for increasing direction of film thickness for in advance given step of film growth. passive film on titanium electrode after electropolishing, the ti electrodes were immersed in h2so4 solutions with various concentrations and open circuit potential, ocp, was recorded during the establishment of steady state potential, i.e. when the variations of potentials with time are negligible. the potentiostatic measurements were initiated from steady state potential with successive increasing the potential on the same electrode up to begin the evolution of oxygen in trans-passive region. then, on the same electrode the potential was also successively changed in the reverse, cathodic direction up to value of starting potential. the variation of currents with time during establishment of steady state conditions were recorded for each successive change of potential. the steady state currents were then taken in construction of potentiostatic curves. fig. 1 (a) illustrates the profile of one typical potentiostaic curve in the potential region from 0.75 to 3.5 v. in the first forward scan an active dissolution region exists and the anodic current increases exponentially with potential up to point a, where the anodic current peak is formed. in this region two processes take place: metal dissolution and in the same time growth of anodic oxide film. however, the metal dissolution is prevalent process. in the second region the anodic current decreases toward a low value corresponding to the beginning of the passivation. the passive film formation and its growth with imposed potential are followed with long current plateau whose values are near to zero. the third region corresponds to trans-passive region, point c, where anodic current begin again to increase. in the first forward scan simultaneously with potentiostatic current-voltage, the ellipsometric measurements were also performed during imposed potentials. from ellipsometrically measured parameters  and  the film thicknesses were calculated, presented in fig. 1 (b). for calculation the film thicknesses the input data in equation (1) and (2) were: for 3m h2so4  mn = 1.364, for virgin ti substrate  sn = 2.94(1-1.217 i), for wavelength of incident light  = 546.1 nm and for angle of incidence  = 70 o , whereas the unknown values of  fn and d were calculated by fitting the theoretical data to the experimentally measure ones. the fitting procedure was performed by a special prepared computer program in which  fn was searched for prior given values of the thicknesses in increments’ of 1 nm, in an increasing direction. the number of theoretical   points on the theoretical fitted curves depend of the thickness increments’ given from our side. taking into account that our ellipsomteric measurements were performed in one relatively short potential region where the final film thickness at the end of the passive region is relatively low, the large dispersion of experimental points from the theoretically fitted curves is expected. during the computation only one theoretical function t = f(t) was searched from the family of theoretical functions which had minimal distance to the experimental measured points  in determination the thicknesses of passive films the theoretical values of ( t) were taken in consideration whereas experimental dispersion of measure points ( ) were neglec ted. details of computer program and fitting procedure are given in ref. [22]. lj. arsov at al. j. electrochem. sci. eng. 5(4) (2015) 221-230 doi:10.5599/jese.245 225 a 1 2 -50 0 50 100 150 200 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 e / v vs. sce i /  a a b c b 0 2 4 6 8 10 12 14 -1 -0.5 0 0.5 1 1.5 2 2.5 3 3.5 4 e / v vs. sce d / n m fig. 1. (a) potentiostatic i – e curve obtained on electropolished ti surface in 3 m h2so4, (b) simultaneous determination the thickness of passive film by ellipsometry as it can see from fig. 1 (b) in the forward cycle the film thickness increases almost linearly and there is no noticeable dispersion of points which present growth of film thickness. from the slope of forward curve the coefficient of film thickness growth is determined, i.e.  = 2.51 nm/v. in the reverse potential scan, with decrease of imposed potential in cathodic direction up to potential of -0.25 v, the long current plateau with current values also near to zero are observed. in this potential region ellipsometric measurements show that the thickness of the already formed passive film has constant value and there is no indication for its dissolution and thinning. at potential of -0.25 v begins reactivation process with sharp increase of anodic currents indicating electrochemical dissolution of the formed passive film. the reactivation peak already covers the activation. in the potential region between -0.25 v and -0.75 v, corresponding to the reactivation processes, the passive film was subject of continuously thinning and at the final potential of -0.75 v the film thickness is minimized and already has the same value as at the starting potential of the first forward scan. this indicates that the formed passive film is completely dissolved. j. electrochem. sci. eng. 5(4) (2015) 221-230 passive and anodic oxide films on ti & nb 226 passive film on niobium electrode in the similar way as in case of ti, the nb electrode was electropolished and then immersed in h2so4 solutions with various concentrations and the ocp was recorded during the establishment of steady state potential. the potentiodynamic measurements were initiated at potential of -0.75 v where a considerable cathodic current is observed and ended at 1 v (sce), fig. 2(a) a 1 2 -0.9 -0.6 -0.3 0.0 0. 0.6 0.9 1.2 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 i / m a e / v vs. sce 1 2 b 1 2 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 1.2 e / v vs. sce first cycle second cycle d / n m fig. 2. (a) voltammograms of nb recorded in 10 m h2so4, 1 – first cycle, 2 – second cycle; (b) simultaneous determination the thickness of passive film by ellipsometry in the first forward scan the cathodic current approaches zero value and when it get anodic values the process of metal dissolution and in the same time formation of anodic oxide films occur, as in case of titanium electrode. the active-passive transition exists in all investigated concentrations, but with increasing the concentration of h2so4 this process is more pronounced. during the reverse scan the reactivation processes or reduction of the already formed passive films were no recorded, even in high concentration of h2so4 up to 10 m. this was the main reason why our measurements started at more cathodic potential where the noticeable catholic current appears in the beginning of the first forward scan and in each other next subsequent scans. it was expected that the natural or later formed passive films will be reduced at second scan during the lj. arsov at al. j. electrochem. sci. eng. 5(4) (2015) 221-230 doi:10.5599/jese.245 227 cathodic polarization of electrode. but the passive film formed in first forward scan cannot be cathodically reduced in the reverse scan. in the second cycle a constant current close to zero, for both, forward and reverse scans were recorded. the voltammograms in the next subsequent scans were almost identical to the second one. it is evident that after the first cycle, the nb electrode remained passive over the whole investigated potential region and the formed passive film blocked all possible redox reaction at the nb/passive film/electrolyte interface. compeering the cv curves between nb and ti electrode it can conclude that in h2so4 solution the nb is corrosion more resistant than ti electrode. during the cv scans simultaneous ellipsometric measurements were also performed, as in casa of titanium electrode. from ellipsometric parameters  and  the film thicknesses were calculated fig 2 (b). in the beginning of the measurements, when cathodic current pass through the electrochemical cell, small decrease of the film thickness is observed, probably due to the partly dissolution of natural oxide film. with apparition of anodic current begin growth of anodic oxide film which is linear function up to the final anodic potential of 1 volt. from the slope of this linear curve the coefficient of film thickness grow of  = 2.58 nm/v was determined. in the first reverse scan the thicknesses of the film have constant values up to the starting potential of -0.75 v. in the second and subsequent scans the thickness of the film has almost the same constant values, as in the end of first forward and reverse scan. the dispersion of the measured points in the second scans could be observed, probably for small dissolutions of the passive film and its reparation during the potential scan, or by existing of some side reactions. a 0.2 0.4 0.6 -0.9 -0.6 -0.3 0.0 0.3 0.6 0.9 43 2 1 e / v vs hg/hgo i / m a e / v vs. hghgo b d / n m 6 5 4 3 2 1 0 -0.9 -0.6 -0.3 0 0.3 0.6 0.9 e / v vs. hg/hgo 1 2 3 4 1 b 2 3 4 5 6 7 8 fig. 3. (a) multiple cycle sequences at increasing final anodic voltage of nb recorded in 1m koh; 1, 2, 3, and 4 represent number of cycle, (b) simulated film thickness growth in each subsequent cycle i / m a j. electrochem. sci. eng. 5(4) (2015) 221-230 passive and anodic oxide films on ti & nb 228 fig. 3 (a) shows sequences of cv measurements where the final anodic potential on nb in 1 m koh is gradually enlarged in each next cycle. all subsequent presented voltammograms started from the same cathodic potential of -0.9 v and also finished to the same catholic potential of -0.9 v. the difference in measurements is only to the increasing of final anodic potential. the progress of film thickness growth with anodic potential may take place only if the actual potential exceeds the maximum value attained in the previous cycle. the shapes of the sequences recorded in fig. 3 (a) represent just the copy segments of the corresponding voltammogram recorded from the starting potential at -0.9 v to final potential at 0.9 v at once, without sequences. the simulated film thickness growth that should be obtained by ellipsometric measurements are presented in fig.3 (b). -15 -10 -5 0 5 10 15 20 -2 -1 0 1 2 3 4 1, 2 e / v vs hg/hgo i / m a a d e / v vs. hghgo 1 2 -5 0 5 10 15 20 25 -2 -1 0 1 2 3 4 e / v vs. hg/hgo a b c d d / n m fig. 4. (a) voltammograms of nb recorded in 6 m koh, a – initial current at potential of -1.5 v, d – final current at potential of -1.5 v, 1 – first cycle, 2 – second cycle; (b) simultaneous determination the thickness of passive film by ellipsometry, ainitial potential, b – final anodic potential, c – beginning of reactivation process, d – final potential at -1.5 v after the first cycle, d-a –cathodic dissolution at -1.5 v during 15 min of the rest film thickness after its partly dissolution in the reverse scan. i / m a lj. arsov at al. j. electrochem. sci. eng. 5(4) (2015) 221-230 doi:10.5599/jese.245 229 for each sequence, film thickness growth in the first forward scan and has constant value in the reverse scans. for each next sequence, in the first forward scan the film thickness has constant value up to the end of previous cycle and then continues to grow up to the final anodic potential. it is evident that by gradually increasing the final anodic potential the passive film grows by additional building of the new film on the already existing. the experiments in fig. 3, confirm the barrier properties of anodic oxide film build on the nb surface. the voltammograms recorded on nb at the higher concentration of 6 m koh are shown in fig. 4 (a). the starting cathodic potential was at -1.5 v and final anodic potential at 4 v. in the first forward scan at the initial potential of -1.5 v, point a, a small cathodic current occurs and then in the anodic direction active/passive transition is recorded. in the reverse scan, at potential of about -0.5 v, begin reactivation process which partly covers the activation region. this suggested that with appearance of reactivation peak only part of the formed passive film is dissolved. at the end of the first reverse cycle, at cathodic potential of -1.5 v, point d, the noticeable cathodic current flows through the electrochemical cell. if we keep for example 15 min. the electrode at this potential and then initiate the second cycle, the voltammogram of the second cycle will be completely the same as in the first cycle. this indicates that after the first cycle, with keeping of some period of time the nb electrode at -1.5 v, the previously formed passive film in the first cycle will be completely dissolved. the simultaneous ellipsometric measurements of the film thickness growth, presented in fig. 4 (b), showed continuous increasing of film thickness in the first forward scan and constant thickness values in the first reverse scan up to -0.5 v where begin reactivation process. in the region of reactivation peak the film thickness partly decrease and did not reaches the initial value at -1.5 v, as before starting the cv measurements. during the keeping the electrode at potential of -1.5 v, the film dissolution continue and finally it get the same value as in the beginning of the first forward scan. conclusions from potentiostatic and potentiodynamic measurments of ti and nb electrode in h2so4 and koh solutions, simultaneous with ellipsometric in-situ measurements, the following conclusions can be drawn: (i) comparative studies of mechanically polished and electropolished metal surfaces have shown that the dispersion of experimental points from the theoretically fitted curves is smaller for electropolished metal surfaces. it should be noted that during the fine mechanical polishing with diamond spray, smooth surfaces with mirror brightness were obtained. however, mechanical polishing leaves numerous micro-surface scratches and defects with random distributions which cause areas of differing electrical potentials due to surface stress. this is the main reason why in this work we used electropolished metal surfaces instead of fine mechanical polished. (ii) the potentiostatic and potentiodynamic curves of ti and nb recorded in all investigation concentrations of h2so4 and koh solutions showed active passive transition with formation of passive films which are stable in lower concentration of investigated solution. (iii) for all investigated concentration of h2so4 and koh solutions, the passive film thickness growth on ti and nb electrodes are linear function with applied potential, up to final anodic potential in the forward scan. (iv) for ti electrode in 3m h2so4, the passive films formed in the forward potentials were completely dissolved in the reverse potentials, where reactivation process was recorded. (v) for nb, even in 10 m h2so4, the high stability of the passive films was recorded. in the reverse potential scan, no reactivation peak and dissolution of passive films was observed. (vi) in 6 m koh the passive film on nb electrode was partially dissolved with apparition of the reactivation peak j. electrochem. sci. eng. 5(4) (2015) 221-230 passive and anodic oxide films on ti & nb 230 whose current intensity is about half intensity of the activation peak. with keeping the electrode at potential of -1.5 v the rest of the film was completely dissolved. (vii) finally it can conclude that nb electrode is more resistance in h2so4 and koh solutions than the ti electrode. references [1] i. mickova, a. prusi, t. grchev, lj. arsov, portugaliae electrochimica acta 24 (2006) 377-385 [2] m. a. m. ibrahim, d. pongkao, journal of solid state electrochemistry 6 (2002) 341-350 [3] a. efremova, lj. arsov, electrochimica acta 37 (1992) 2099-2100 [4] r. torresi, f. nart, electrochimica acta 33 (1988) 1015-1018 [5] lj. arsov, c. korman, w. plieth journal of raman spectroscopy 22 (1991) 573-575 [6] j. w. schultze, m. m. lohrengel, electrochimica acta 45 (2002) 2499-2513 [7] k. heusler, m. schultze, electrochimica acta 20 (1975) 237-244 [8] a. prusi, lj. arsov, corrosion science 33 (1992) 153-164 [9] i. mickova, international review of chemical engineering 2 (2010) 692-701 [10] w. l. lee, g. olive, d. l. pulfrey l. young, journal of the electrochemical society 117 (1970) 1172-1176 [11] a. prusi, lj. arsov, b. haran, b. popov, journal of the electrochemical society 149 (2002) b491-b498 [12] i. arsova, lj. arsov, n. hebestreit, a. anders, w. plieth, journal of solid state electrochemistry 11 (2007) 209-214 [13] t. ohtsuka, n. nomura, corrosion science 39 (1997) 1253-1263 [14] i. mickova, a. prusi, t. grchev, lj. arsov, croatica chimica acta 79 (2006) 527-532 [15] lj. arsov, m. froelicher, m. froment, a. hugot le-goff, comptes rendus de l'académie des sciences, paris series c – électrochemie 279 (1974) 485-488 [16] lj. arsov, electrochimica acta 30 (1995) 1645-1657 [17] i. a. amar, s. darwish, m.w. khalil, materialwissenschaft und werkstofftechnik, 12 (1981) 309-315 [18] s. l. assis, i. costa, materials researchs 10 (2007) 293-296 [19] m. t. woldemedhina, d. raabea, a. w. hassel, electrochimica acta 82 (2012) 324-332 [20] m. k. han, j. y. kim, m. j. hwang, h. j. song, y. j. park, materials 8 (2015) 5986-6003 [21] lj. arsov, m. ramasubramanian, b. popov, ellipsometry, chapter in book methods in materials research 1 (2001) 8b.5.1-8b.5.10, john wiley & sons inc. [22] a. efremova, lj. arsov, journal of physics france 2 (1992) 1353-1361 © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ impedance aspect of charge storage at graphite and glassy carbon electrodes in potassium hexacyanoferrate (ii) redox active electrolyte doi:10.5599/jese.230 37 j. electrochem. sci. eng. 6(1) (2016) 37-45; doi: 10.5599/jese.230 open access : : issn 1847-9286 www.jese-online.org original scientific paper impedance aspect of charge storage at graphite and glassy carbon electrodes in potassium hexacyanoferrate (ii) redox active electrolyte katja magdić, višnja horvat-radošević, krešimir kvastek rudjer bošković institute, bijenička c. 54, 10000 zagreb, croatia corresponding author: : kmagdic@irb.hr; tel.: +385-01-456-1152 received: september 30, 2015; accepted: february 10, 2016 abstract different types of charge storage mechanisms at unmodified graphite vs. glassy carbon electrodes in acid sulphate supporting solution containing potassium hexacyanoferrate (ii) redox active electrolyte, have been revealed by electrochemical impedance spectroscopy and supported by cyclic voltammetry experiments. reversible charge transfer of fe(cn)6 3-/4 redox reaction detected by assessment of cvs of glassy carbon electrode, is in impedance spectra indicated by presence of bulk diffusion impedance and constant double-layer/pseudocapacitive electrode impedance compared to that measured in the pure supporting electrolyte. some surface retention of redox species detected by assessment of cvs of graphite electrode is in impedance spectra indicated by diffusion impedance coupled in this case by diminishing of double-layer/pseudocapacitive impedance compared to that measured in the pure supporting electrolyte. this phenomenon is ascribed to contribution of additional pseudocapacitive impedance generated by redox reaction of species confined at the electrode surface. keywords graphite; glassy carbon; ferri/ferrocyanide redox couple; charge storage; electrochemical impedance spectroscopy introduction except in electroanalytical and (bio)sensing fields where serve as standard redox probes for testing electrode activities 1,2, presence of a redox active couple in a supporting electrolyte solution has also been considered valuable in the field of electrochemical capacitors, ec, and supercapacitors, sc, 37. within this context of research, high redox reaction rate coupled with high electrode capacitances accomplished mostly by high surface-area carbon electrodes has been http://www.jese-online.org/ mailto:kmagdic@irb.hr j. electrochem. sci. eng. 6(1) (2016) 37-45 charge storage at graphite and glassy carbon electrodes 38 considered advantageous for ec/sc devices with improved charge storage properties. the main goal with using redox electrolyte is to increase the total charge capacity, qt, of the system, by adding the charge realised by redox reaction, qredox, to already existing electrode doublelayer/pseudocapacitive charge, qelectrode, according to the scheme given below. scheme 1. scheme of redox couple reaction at an electrode along with some other redox systems that have already been tested for such a purpose 8,9, the ferro/ferricyanide, fe(cn)6 3/4 , redox couple was found useful due to high redox reaction rate and adequate standard redox potential 5,10,11. potassium hexacyanoferrate (ii) and (iii) are water soluble compounds, where negatively charged fe(cn)6 3/4 complex ions undergo one electron transfer oxidation/reduction interfacial reaction, requiring 9.63  10 4 c per mol of reactants: fe ii (cn)6 4  fe iii (cn)6 3 + e  r-1 for a long time, the redox reaction r-1 has been considered a typical outer-sphere electron transfer reaction 1214, what is inherently associated with fast electron transfer reaction controlled by mass transport of redox active species toward/from the electrode surface. in dependence not only on kind, concentration and ph of the supporting electrolyte, 1517, but also on chemistry, morphology and electronic properties of a carbon electrodes, charge transfer can be suppressed, resulting in decreased rate of r-1 that becomes additionally controlled by electrolyte and/or electrode surface/bulk properties 1,17,18. inherent complication with applying fe(cn)6 3/4 redox couple is relative instability of fe iii (cn)6 3 complex ions, particularly in acid solutions 19. fe iii (cn)6 3 complex ions can also be destroyed in the course of electrochemical reaction r-1 by forming the ferri-ferricyanide complex and insoluble ferri-ferrocyanide, fe4 iii fe ii (cn)63 (prussian blue, pb) after subsequent reduction. it has already been shown that pb can be adsorbed at electrode surfaces spontaneously, at open circuit, or during potentiodynamic sweeps 20,21. at favourable conditions, stable active layer of pb can be formed, showing prominent sensing, electrocatalytic and/or pseudocapacitive properties 1921. to avoid formation of pb, in majority of already published papers on electrochemical characterization of either separate high surface-porous carbon/fe(cn)6 3/4 electrodes, or electrode assemblies into an ec/sc device containing fe(cn)6 3/4 redox couple, experiments were performed in ph neutral supporting electrolyte solutions, using mostly cyclic voltammetry, cv and galvanostatic charging discharging electrochemical techniques. for some types of high surfaceporous carbon/fe(cn)6 3/4 electrodes, voltage separations of redox peaks, ratios of anodic and cathodic peak currents and their dependences on scan potential rates showed the characteristics k. magdić et al. j. electrochem. sci. eng. 6(1) (2016) 37-45 doi:10.5599/jese.230 39 of reversible or quasi-reversible redox reaction kinetics 11,2225. these results, supported by discharge voltage plateaus in galvanostatic experiments 11,25, suggest that these types of carbon electrodes store charges as hybrid electrodes, exhibiting a combined effect of already high surface electrode double-layer capacitance and additional faradaic redox reaction 3,7. for some other types of high surface-porous carbon/fe(cn)6 3/4 electrodes, however, different voltammetric responses have been indicated by higher, sharper and narrower redox peaks and no discharge voltage plateaus in galvanostatic experiments 10,26. explanation was given in the terms of either surface adsorption of redox ions 7,10 or retention of some amount of charges at the electrode surface or near-surface region by thin-layer electrochemistry, tle 26. tle is usually accomplished with highly porous materials 26,27 and its electrochemical response has been found equivalent to adsorption of reaction species at the electrode surface 26. these results suggest that these types of carbon electrodes store redox charges at the electrode surface, exhibiting enhanced electrode capacitance through a pseudocapacitive effect 3,7,28. although distinguishing between only diffusion and adsorption/tle controlled redox reaction kinetics can easily be performed by electrochemical impedance spectroscopy, eis, technique 28, eis has been applied sporadically in this field, giving mostly superficial information 9,11,15,25. herein, in addition to ordinary cv experiments, the results of measurements and analysis of impedance spectra measured at graphite and glassy-carbon electrodes in the supporting electrolyte containing fe(cn)6 3/4 redox couple will be presented. in spite of possible complications due to pb formation, 0.5 mol dm 3 h2so4 was chosen for the supporting electrolyte due to its enormous effect on enhancement of specific capacitance/pseudocapacitance values of carbon electrodes 1,3,30. experimental all experiments were performed in conventional three-electrode cell, filled with 0.5 mol dm 3 h2so4 (ph 0.6) supporting electrolyte. balanced quantities of k4fe(cn)6  3h2o (potassium hexacyanoferrate (ii)) (sigma-aldrich inc.) were added directly into the cell to give the final concentration of 510 3 mol dm 3 . electrochemical measurements were performed in absence and presence of always freshly prepared k4fe(cn)6/h2so4 solution. high purity (18×10 6  cm) water was used for all solution preparations. graphite, g, (goodfellow, uk) and glassy carbon, gc, (eg&g, usa) electrodes, sealed in glass tubes and exposing 0.07 cm 2 and 0.03 cm 2 of respective geometrical surfaces and put vertically in the cell served as the working electrodes, we. high surface platinum spiral in a separate compartment and saturated calomel electrode, sce, (radiometer, denmark) equipped with a haber-luggin cappilary and the pt wire pseudo-reference electrode [31] were used as the counter and reference electrodes, respectively. all potentials are reported with respect to the sce. prior measurements, we surfaces were hand polished, sonicated in ultrapure water and slightly preconditioned in 0.5 mol dm 3 h2so4, by applying 10 potential cycles between 0.30 and 1.60 vsce and scan rate of 10 mv s 1 . the pre-condition procedure always ends at 0.30 vsce where the we was left for at least 15 min prior further experiments [32,33]. impedance spectroscopy, eis, and cyclic voltammetry, cv, experiments were carried out in nonstirred electrolyte solutions, at ambient temperature of 252 c and oxygen free atmosphere maintained by blowing nitrogen through electrolyte solution prior measurements. cv experiments were employed by the 1287 eci under zcorr software (scribner assoc. inc.) control in monitoring the current-voltage profiles performed in two potential scans between 0.20 and 1.05 vsce and j. electrochem. sci. eng. 6(1) (2016) 37-45 charge storage at graphite and glassy carbon electrodes 40 different scan rates (5  250 mv s 1 ). eis data were measured with 1260 fra and 1287 eci (solartron, uk) under zplot software (scribner assoc. inc.) control. sine wave ac signal of 10 mv amplitude, frequency, f, ranged between 10 5 hz and 0.02 hz and 5  10 frequency points measured per decade were applied at different bias potentials. contributions of experimental artefacts were corrected as before [33]. results and discussion cyclic voltammograms of gc and g electrodes in h2so4/k4fe(cn)6 redox electrolyte representative cvs of glassy carbon, gc and graphite, g electrodes in 0.5 mol dm 3 h2so4 containing 5  10 3 mol dm 3 k4fe(cn)6, recorded between 0.20 and 1.05 vsce for the scan rates, , changed between 250 and 5 mv s 1 are presented in figure 1a. figure 1. a) cvs (at denoted ) and b) log jpa vs. log  curves of gc and g electrodes in 0.5 mol dm 3 h2so4 / 510 3 mol dm 3 k4fe(cn)6 electrolyte solution well shaped j  e curves with a pair of prominent peaks, epa and epc, situated at about 0.40 vsce, are presented for both carbon electrodes in figure 1a. relatively positive value of peak potential, (epa + epc)/2, defining the formal redox potential, e 0 , of the fe(cn)6 3 redox couple in the given electrolyte solution, can be related to the electrolyte of higher ionic strength 16,34. the shapes of peaks of two carbon electrodes are rather different, showing sharper shapes and higher peak current densities values (jpa and jpc) for g electrode, as opposed to wider peaks and lower jpa and jpc b) a) k. magdić et al. j. electrochem. sci. eng. 6(1) (2016) 37-45 doi:10.5599/jese.230 41 values for gc electrode. peak separation values, epp = (epa  epc), usually utilized to assess electron transfer activities 1,35 are 70 mv (at all ) for gc electrode and between 36 and 45 mv (dependent on ) for g electrode, respectively. slopes of the power law relationship between log of peak current density, jpa, and log  are shown in figure 1b. the values of slopes were calculated as 0.49 for gc and 0.65 for g electrode, respectively. note that epp  57 mv for one electron transfer and slope = (dlog jpa/dlog ) = 0.5 have already been theoretically predicted for ideally reversible redox reaction with mass transport (diffusion) controlled kinetics, while epp = 0 and (dlog jpa/dlog) = 1 are predicted for fully confined redox species resulting in pseudocapacitance. a mixed process is predicted for all slopes (dlog jpa/dlog) found between 0.5 and 1 3,35. consequently, the results presented in figure 1 indicate almost reversible, diffusion controlled kinetics of fe(cn)6 3/4 redox reaction at gc electrode and a mixed process including some retention of redox active species at the g electrode surface, respectively. cvs similar to that of gc electrode presented in figure 1a, denoting reversible or quasy-reversible reaction kinetics, have already been reported not only for slightly modified gc electrode 36, but also for micro-bundles of multi-walled carbon nanotubes 22, porous diamond 23, porous boron-doped diamond/carbon nanotubes 24, graphene paper 11 and reduced graphene oxide sheets electrodes 25. at the other side, cvs similar to that of g electrode presented in figure 1b, indicating some retention of electroactive species at the electrode surface were already observed for activated carbon 10 and vertical arrays of macroporous single-walled carbon nanotube electrodes 26. impedance spectra of gc and g electrodes in h2so4/k4fe(cn)6 redox electrolyte impedance spectra of glassy carbon, gc, and graphite, g, electrodes measured in 0.5 mol dm 3 h2so4/5  10 3 mol dm 3 k4fe(cn)6 electrolyte solution are presented in figure 2. bode (log z and  vs. log ) plots are presented for e = 0.30, 0.40 and 0.50 vsce (cf. figure 1a). z is impedance magnitude,  is phase angle while  = 2f is angular frequency. as is seen in figure 2, bode plots of both carbon electrodes are of similar values and also of similar frequency responses at the same potential values. at high to medium frequencies, both impedance spectra show resistive (log z slopes  0 and   0) to capacitive responses ((log z slopes  1 and   90), what is typical for electrolyte resistance and double-layer capacitive impedance responses 29. at lower frequencies, however, the shapes of bode plots in figure 2 becomes typical for semi-infinite diffusion controlled faradaic reaction (log z slopes  0.5 and   45) 29, what for both electrodes become the most prominent at e = e 0 = 0.40 vsce (cf. figure 1a). the shifts of capacitive impedance lines in log z and  curves in bode plots in figure 2, are evident for both electrodes, but are more prominent for g vs. gc electrode. these shifts indicate changes in double-layer capacitive impedance values at different potentials, what has already been noticed for various types of carbon electrodes 32,33,37,38. potential dependence of double-layer capacitance values of carbon electrodes is commonly observed phenomenon being usually ascribed to potential dependent pseudocapacitance, generated by fast redox reaction(s) of oxygen containing species formed inherently and more profoundly in acid solutions and/or strongly electrochemically modified carbon surfaces 1,3,12,32,33,39. for here explored gc and g electrodes, potential dependences of capacitive impedances were recorded in the pure supporting electrolyte (0.5 mol dm 3 h2so4) and in the form of bode plots presented in figure 3. in agreement with previous reports 32,33,37, bode plots of gc electrode in figure 3 changed with potential, showing lower capacitive impedances (higher capacitance values) at 0.30 and 0.40 vsce than 0.50 vsce. j. electrochem. sci. eng. 6(1) (2016) 37-45 charge storage at graphite and glassy carbon electrodes 42 figure 2. bode plots measured at denoted potential values of gc and g electrodes in 0.5 mol dm 3 h2so4 / 5  10 3 mol dm 3 k4fe(cn)6.electrolyte solution figure 3. bode plots measured at denoted potential values of gc and g electrodes in 0.5 mol dm 3 h2so4 electrolyte solution k. magdić et al. j. electrochem. sci. eng. 6(1) (2016) 37-45 doi:10.5599/jese.230 43 this has to be ascribed to the fast faradaic reaction of redox active surface carbon oxygen containing species identified as hydroquinone/quinone redox couple with e 0 = 0.35 vsce in acid sulphate electrolyte 3,12,32,33. for g electrode, however, impedance spectra are not fully capacitive (cf. change of slope of capacitive impedance line and decrease of  values at the lowest frequencies) what can be ascribed to different morphologies and inherent porosity of slightly modified g vs. slightly modified gc electrode surface 40. anyhow, capacitive impedances that are dominant at medium to low frequencies in impedance spectra of g electrode in figure 3, are almost unchanged at given potentials, suggesting that capacitive shifts of bode plots presented in figure 2 should be ascribed to presence of the fe(cn)6 3/4 redox couple in electrolyte solution. in figure 4, bode plots measured at e = 0.40 vsce in either pure supporting electrolyte solution or solution containing fe(cn)6 3/4 redox couple can be compared for each electrode. figure 4 bode plots of gc and g electrodes measuredat e = 0.40 vsce in 0.5 mol dm 3 h2so4 and 0.5 mol dm 3 h2so4 / 5  10 3 mol dm 3 k4fe(cn)6.electrolyte solutions. whereas for the gc electrode, bode plots in figure 4 do not show differences between capacitive impedance responses measured in absence and presence of fe(cn)6 3/4 redox couple (cf. perfect matches of two capacitive lines and  plots denoted by arrows in figure 4), for g electrode, however, there is a significant decrease of capacitive impedance (increase of capacitance) of the spectrum measured in the presence of redox couple, compared to that measured in pure supporting electrolyte (denoted by shaded area in figure 4). increase of capacitive impedance in the supporting electrolyte containing redox couple suggest some interaction of fe(cn)6 3/4 ions with the g electrode, that in turn has not been observed for the gc electrode surface. charge storage on gc and g electrodes in h2so4/k4fe(cn)6 redox electrolyte the results and discussion on cvs shown in figure 1 and impedance spectra shown in figs. 24 pointed to different charge storage mechanisms of r-1 at gc and g electrodes, respectively. the j. electrochem. sci. eng. 6(1) (2016) 37-45 charge storage at graphite and glassy carbon electrodes 44 obtained result for gc electrode suggest that r-1 is proceeding as a reversible, bulk diffusion controlled faradaic reaction with the electrode surface serving only as a source/sink of electrons. for this type of reaction, the total charge, qt (cf. scheme 1) is stored by a combined effect of already high electrode double-layer capacitance/pseudocapacitance and additional faradaic redox reaction realising the charge, qredox, stored by the amount of reactants present in the bulk of electrolyte solution 3,7,28. for g electrode, however, the obtained results suggest that r-1 is proceeding partly as a reversible, bulk diffusion controlled faradaic reaction and partly by surface confined species generating a pseudocapacitive effect. irrespectively to the origin of confinement that can be adsorption, tle or pb formation, the rate of r-1 is affected by the amount of surface confined species. for this type of reaction, the realised qredox is stored partly on the electrode surface and partly in the bulk of electrolyte solution, what has generally been considered more beneficial from the ec/scspecific power point of view 3,7,28. conclusions the present study has demonstrated the efficient application of electrochemical impedance spectroscopy technique in evaluation of the charge storage mechanisms of the fe(cn)6 3/4 redox reaction on unmodified gc and g electrodes. even qualitative analysis of impedance bode plots measured at, or near formal redox potential of fe(cn)6 3/4 redox couple in acid sulphate electrolyte solution pointed to some differences between impedance responses of two electrodes. whereas the bulk diffusion impedance, indicating almost reversible redox reaction has been detected in impedance spectra of both electrodes, significant diminishing of capacitive impedance, compared to that measured in pure supporting electrolyte has been detected for the g electrode only. differences in capacitive contributions in impedance spectra pointed directly to different charge storage mechanisms at two electrodes. almost constant capacitance of gc electrode in impedance spectra measured in either pure supporting electrolyte or in electrolyte containing fe(cn)6 3/4 redox couple does not indicate any interactions between electrode surface and electroactive species in electrolyte solution. contrary to that, significant increase of electrode capacitance of g electrode capacitance of g electrode in the electrolyte containing redox couple points to high 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[40] k. magdić, electrochemical impedance spectroscopy in characterization of unmodified and electrochemically modified carbon electrodes, ph.d.thesis, university of zagreb, 2014. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ etoricoxib selective sensor based on uracil-5,6-diamino-2-thio hydrochloride as neutral carrier for potentiometric analysis in pharmaceutical preparations doi:10.5599/jese.284 187 j. electrochem. sci. eng. 6(2) (2016) 187-198; doi: 10.5599/jese.284 open access : : issn 1847-9286 www.jese-online.org original scientific paper etoricoxib selective sensor based on uracil-5,6-diamino-2-thio hydrochloride as neutral carrier for potentiometric analysis in pharmaceutical preparations salwa fares rassi department of chemistry, faculty of science, al-baath university, homs, syria corresponding author: rassi.salwa@gmail.com; tel.: +963-966-243-153 received: april 4, 2016; revised: may 28, 2016; accepted: may 29, 2016 abstract a construction and electrochemical behavior of novel potentiometric membrane sensor responsive to the etoricoxib was described. the sensor was based on the ion-pair complex of etoricoxib (et) with uracil-5,6-diamino-2-thio hydrochloride (udth) (etudth) as exchange sites in a pvc matrix with different plasticizers dioctyl phthalate (doph) (electrode a), dibutyl phthalate (dbph) (electrode b), and tri-n-butyl phosphate (tbp) (electrode c). the electrodes exhibited near-nernstian response for et-udth over the concentration range 0.051-40.042 mm. the electrode offered significant advantages including long lifetime (about 2 months), excellent stability and reproducibility, good response time (10-25 s), and wide ph working range (ph 5-12). selectivity coefficients of et related to a number of interfering cations and some organic compounds were investigated, and there were negligible interference caused by most of the investigated species. the direct determination of 0.5-10 mm of et showed an average recovery of 99.03-101.75 % and a mean relative standard deviation 0.40-1.88. the results were obtained by determination of et in tablets using the proposed electrodes which were comparable favorably with those obtained by spectrophotometric method. keywords etoricoxib, uracil-5,6-diamino-2-thio hydrochloride, pvc membrane, potentiometric method validation. introduction etoricoxib (5-chloro-6'-methyl-3-[4-(methylsulfonyl)phenyl]-2,3'-bipyridine) represents a second-generation of cox-2 inhibitors that has been developed for the treatment of many inflammatory diseases such as rheumatoid arthritis, osteoarthritis, pain relief and acute gout, http://www.jese-online.org/ mailto:salwa@gmail.com j. electrochem. sci. eng. 6(2) (2016) 187-198 etoricoxib selective sensor 188 causing fewer gastrointestinal complications than conventional non-steroidal anti-inlammatory drugs (nsaids) [1-2]. the potential use of this cox-2 inhibitor among the general public is high due to their improved safety profile over traditional nsaids. traditional nsaids such as aspirin and ibuprofen, inhibit both the cox-1 and cox-2 enzymes [3]. the cox-1 enzyme, which is expressed in essentially all tissues, has been found to play a role in the homeostasis of the gastrointestinal tract and kidneys, while the cox-2 enzyme is absent unless induced by an inflammatory event. consequently, side effects of chronic use of traditional nsaids include gastrointestinal ulceration and bleeding [4]. etoricoxib, a highly selective cox-2 inhibitor, was developed in order to address the safety issues associated with traditional nsaids, while providing pain relief from inflammatory illness such as osteoand rheumatoid arthritis [5]. in order to provide quality control over the manufacture of any active pharmaceutical ingredient, it is essential to develop highly selective analytical techniques. therapeutic importance of etoricoxib has prompted the development of many methods for its assay. the drug is available in tablet dosage form and is not yet official in any of the pharmacopoeias. several methods have been reported for the analysis of etoricoxib in pharmaceutical dosage form as well as in the biological fluids and tissues, such as spectrophotometric methods [6-8], chromatographic methods hplc [9-12], high-performance thin-layer chromatography [13], lc/ms spectrophotometry [14-18] and rp-hplc method [6,19,20] for the estimation of etoricoxib. experimental 1. reagents all reagents used were of analytically pure grade and doubly distilled water was used throughout. high-molecular-weight poly(vinyl chloride) (pvc) was obtained from sabic. co., dioctyl phthalate 98.9 % (doph), tri-n-butyl phosphate 97 % (tbp), and di-n-butyl phthalate 99 % (dbph) were obtained from bdh. co., england. tetrahydrofuran (thf) was obtained from merck. pure-grade etoricoxib (99.95 %) (c18h15 cl n2o2s, 358.1 g mole -1 ) was obtained from aarti drugs limited (indian). uracil-5,6-diamino-2-thio hydrochloride (udth) (c4h7n4oscl, 194.5 g mole -1 ) was synthesized (figure 1) and identified in laboratory by spectroscopic means. the h-nmr and ir data (figure 2) of udth are shown in table 1. nh2 nh2 s n n nh2hs oh (1) na, etoh (2) aq. hoac n nhs nh2 no oh n nhs nh2 nh2 oh .hcl aq. hoac nano2 (2) hcl (1) na2so4 co2et ch2 nc figure 1. synthesis of udth. 2. apparatus potentiometric and ph measurements was carried out using a digital shott gerate ph meter (consort c 830, belgium) with combined glass ph electrode. a water bath shaker (grant instruments, cambridge ltd, england) was used to control the temperature of the test solutions. a saturated calomel electrode (sce) was used as the external reference (mettler, switzerland) while an ag/agcl electrode was used as an internal reference. the electrochemical system may be represented as follows: ag/agcl | inner solution | pvc membrane | test solution || kcl salt bridge || hg/hgcl2(sat.). s. f. rassi j. electrochem. sci. eng. 6(2) (2015) 187-198 doi:10.5599/jese.284 189 ft-ir 4100 (fourier transform infrared spectrometer) jasco using kbr disk in the range 4000 400 cm −1 , nernst glower, deuterium try glycine sulfate (dtgs), 0.1 cm −1 . 3. preparation of solutions standard et solutions (0.05-35 mm) were prepared in doubly distilled water. udth solution (10 mm) was prepared by dissolving appropriate amount of the compound in the methanol. et and udth stock solutions were stored in dark bottle at refrigerator. stock solutions of 1 m for each of licl, nacl, kcl, nh4cl, cacl2, mgcl2, bacl2, zncl2, mnso4, ni(no3)2, co(no3)2, cu(no3)2, pb(no3)2, fecl3, alcl3, crcl3, glucose, fructose, lactose, starch, micro crystalline cellulose, carboxymethyl cellulose, polyethylene glycol, titanium dioxide, and polysorbate-80 were prepared by dissolving the appropriate amount of the compounds, and the diluted solutions from these were prepared by subsequent dilutions of the stock solutions. figure 2. ir spectrum of etoricoxib (black spectrum), udth (red spectrum), and et+udthcomplex (blue spectrum) 4. sample preparation etoricoxib tablets were supplied by razi pharmaceutical industries (aleppo, syria). each tablet was labeled to contain etoricoxib 120, 90, 60 mg/tab. the homogenized powder was prepared from ten accurately weighed et tablets. an appropriate amount of this powder was dissolved in methanol and doubly distilled water. the mixture was then filtered and made up to the mark in a 100 ml volumetric flask. different volumes of the stock solution were taken and subjected to the direct and standard addition methods. 5. preparation of ion-pair compound ion-pair (et + udth ) was prepared by mixing equal volumes of 10 –2 m methanolic solution of et with methanolic solution of udth with stirring. methanol was then gradually evaporated to obtain a precipitate. 1 h-nmr and ir data (figure 2) of et + udth are shown in table 1. 6. construction of etoricoxib membrane electrodes the electrodes were constructed according to the method described by craggs et al. [8]. membrane composition was studied by varying the percentages (w/w) of the ion-pair complex, pvc and doph (electrode a), dbph (electrode b), or tbp (electrode c) as plasticizing solvent mediators. the optimum composition that exhibited perfect performance characteristics was j. electrochem. sci. eng. 6(2) (2016) 187-198 etoricoxib selective sensor 190 reached. the membranes were prepared by dissolving the required amounts of ion pair complex, pvc and doph, dbph or tbp in thf, and then the homogeneous mixture was poured into glass petri dishes (8 cm diameter), covered with a glass plate, and allowed to evaporate overnight at room temperature. the thickness of obtained membrane was about 0.15 mm. membranes (12 mm diameter) were cut out and then adhered to the polished end of a plastic cap attached to a glass tube using pvc-thf paste. the electrodes bodies were filled with a 1×10 -1 m kcl solution and 1×10 -3 m et as the inner electrolyte, and then ag/agcl was inserted in it as internal reference electrode. electrode potential was measured against the sce as the reference electrode. before use, the electrode membranes were conditioned by immersion in 25 mm of et solution for 3 h. table 1. h-nmr and ir data for udth and complex between udth and et. wavenumber, cm -1 δ / ppm et + udth udth udth et + udth --2479 -sh 12.48 ---sh 3440 3460 -oh 11.55 11.53 -oh 3250-3325 3328-3390 -nh2 6.27 4.28 -nh3 + 1186 --n-s 8.00 7.74 -nh2 2.76 -ch3 3.25 -ch3 7.88-8.57 9 h aromatic 7 selectivity of sensors potentiometric selectivity coefficient, pota,bk , of an ion-selective electrode (ise) was commonly used as quantitative expression of the ability of the electrode to respond primarily to the analyte in the presence of interfering ions. the effect of the presence of some different species on the response of et electrodes was investigated, and the selectivity coefficient pota,bk of the proposed electrodes was calculated in the presence of related organic and inorganic substances using matched potential method (mpm) [9-10]. the selectivity coefficient, pota,bk , which was measured by matched potential method was calculated according to the following equation: mpm a a a,b b ( ' ) a a k a   (1) where a′a is the known activity of primary ion, aa is the fixed activity of primary ion, and ab is the activity of interfering ions. 8. general procedures the performance of the three electrodes prepared was investigated by measuring e.m.f. values of 0.05-45 mm of et. the electrodes were calibrated by added volumes of 50 mm stock solution of et successively in 50 ml of water to generate a total concentration ranging from 0.05 to 45 mm et, followed by immersing the et-electrode, together with a sce in the solution. the potential reading was recorded after stabilization, and the e.m.f was plotted as a function of the logarithm of the et concentration. the concentration graph was used for subsequent determinations of unknown et concentrations. the e.m.f. measurements with the polymeric membrane electrodes were carried out with the cell assembly shown schematically in the section 2.2. s. f. rassi j. electrochem. sci. eng. 6(2) (2015) 187-198 doi:10.5599/jese.284 191 9. potentiometric determination of et et was determined potentionmetrically by the direct and standard addition methods [11-12]. in this method the proposed electrodes (a,b) (et-udth) were immersed into a sample of 15 ml with an unknown concentration of a et solution, and the equilibrium potential, eu, was recorded. then 1 ml of 50 mm of standard et was added into the testing solution and the equilibrium potential, es, was obtained. from the potential change, ∆e = eu es, we could determine the concentration of the testing sample using the equation: s s x / x s x( ) 10 e s c v c v v v      (2) where cx and vx are concentration and volume of an unknown sample, cs and vs are concentration and volume of the standard, respectively. s is the slope of the calibration graph (slope of the electrode response), and ∆e is the difference in (mv) between e.m.f. after and before addition of the standard solution. standard addition method was applied for determining et in commercial preparations. results and discussion 1. optimization of the membrane composition the effect of the amount of ion pair in the membrane phase on the potentiometric response was investigated. the data shown in table 2 clearly indicate that the electrode (v) with 5 wt% of et-udth ion pair has performance characteristics (slope 57.00 mv decade -1 , at 25 °c, usable concentration range, 0.031-40.042 mm et), and response time 25 s. for all construted electrodes, the percentage of ion-pair ranging from 1 to 8 % was found to offer better slopes and correlation coefficients. the results obtained with ion-pair for the three plasticizers are summarized in table 2. the electrodes a and b exhibit comparable linear ranges and the lowest detection limit. table 2. optimization of the membrane ingredients. composition, % (w/w) slope, mv decade1 detection limit, mm membrane ion-pair complex pvc dbph i 1 49.5 49.5 48.62 0.39 ii 2 49.0 49.0 50.24 0.31 iii 3 48.5 48.5 53.14 0.25 iv 4 48.0 48.0 55.69 0.19 v 5 47.5 47.5 57.00 0.13 vi 6 47.0 47.0 54.06 0.25 vii 7 46.5 46.5 52.09 0.20 viii 8 46.0 46.0 49.51 0.34 2. effect of the internal filling solution the concentration of the internal solution of et in the electrode was changed from 10 to 0.01 mm and the potential response of the electrode was measured. it was found that variation of the concentration of the internal solution did not cause any significant difference in the potential response of the electrode. et concentration of 1 mm as internal solution was quite appropriate for proper functioning of the electrode. j. electrochem. sci. eng. 6(2) (2016) 187-198 etoricoxib selective sensor 192 3. effect of plasticizer type on the characteristic performance of the sensor three plasticizers, doph, dbph and tbp were evaluated. as shown in table 3, the best performances, in terms of slopes, linear range and detection limit obtained had the following order: dbp (electrode b) > doph (electrode a) > tbph (electrode c). the working characteristics for the electrodes were assessed on the basis of their calibration curves. the tbp which had a low viscosity (3.11 cst), led to leaching of the complex from the membrane. all further studies were conducted using dbph and doph as plasticizers. table 3. effect of the nature of an ion-pair and a plasticizer on characteristics of the electrodes. c b a electrode 7 5 5 ion-pair complex, wt % tbp dbph doph plasticizer 41.23 57.00 56.26 slope, mv decade -1 0.653-33.140 0.031-40.042 0.051-39.810 linear range, mm 0.9699 0.9999 0.9997 correlation coefficient 0.324 0.023 0.042 detection limit, mm 4. effect of soaking freshly prepared electrodes must be soaked to activate the surface of the membrane to form an infinitesimally thin layer for ion-exchange process to occur [13]. this preconditioning process required different times, depending on the diffusion of ions and their equilibrium at the electrode test solution interface. fast establishment of equilibrium was certainly a condition for a fast potential response. thus, the performance characteristic of the et electrode was investigated as a function of the soaking time. for this purpose the electrode was soaked in a 50 mm solution of et, udth and water at room temperature. the optimum soaking time and the optimum solution was found to be 3 h for 50 mm solution of et. during this period sensors were washed with water after each application and kept dry in air at room temperature when off-use. the results indicate that for a time of 3 h of soaking the slope remained constant at about 57.00 mv decade -1 , at 25 °c. e, mv vs. t, min plots (fig. 3) were obtained after the electrode was soaked continuously in 50 mm et, water and udth for 30-630 min. 5. effect of ph the influence of the ph on the potentiometric responses of membrane electrodes was investigated by following the potential variation over the ph range of 1-14. the electrode response for different et concentration was tested at various ph values. the ph of the solution was adjusted by adding small volumes of hydrochloric acid or sodium hydroxide to the test solution. as can be seen from figure 4, membrane electrode exhibited a negligible potential change within the ph range of 5-12. at lower phs (ph˂6) there was a decrease in potential which may be due to the interference of hydronium ion and the penetration of h3o + into the membrane surface, or a gradual increase of the protonated species [14-15]. 6. effect of the temperature of the test solution the effect of the temperature of the test solution on the potential response of the membrane was tested by following the slopes variation in the temperature range 20-65 °c (fig. 5). the results show that within the temperature range investigated the electrodes respond practically to the et concentration with slopes from 54.23 to 57.30 mv decade -1 and usable concentration range of about 0.031-40.042 mm. s. f. rassi j. electrochem. sci. eng. 6(2) (2015) 187-198 doi:10.5599/jese.284 193 figure 3. effect of stock solution on response electrode figure 4. effect of the ph on the response of the electrode figure 5. effect of the temperature of the test solution on the potential response of the membrane. 0.0 20.0 40.0 60.0 80.0 0 200 400 600 e / m v t / min water udth etx etx-udth5%se,cetx=5×10 -3 -80 -40 0 40 80 120 0 2 4 6 8 10 12 e / m v ph 2×10-2m 5×10-3m 5×10-4m 5×10-5m etx-udth5% se y = -0.124x + 61.224 r² = 0.9569 52 54 56 58 20 30 40 50 60 70 s lo p etx-udth5%se j. electrochem. sci. eng. 6(2) (2016) 187-198 etoricoxib selective sensor 194 7. calibration graphs using the optimized membrane composition and conditions described above, the potentiometric response of the electrode was studied based on the et concentration in the range of 0.0150 mm. the calibration curves for the electrodes a and b containing dop or dbp as plasticizer gave an excellent linear response from 0.031 to 40.042 mm, as shown in figure 6. the results given in table 3 show the characteristics performance of the membrane electrodes. the least squares equation obtained from the calibration data was as follows: e/ mv = s × log([et,m]) + intercept where e is the potential and s the slope of the electrodes. figure 6. calibration graph of et membrane electrode. table 4. response characteristics of membrane electrodes electrode number a b plasticizer doph dbph parameter slope, mv/decade -1 56.26 57.00 correlation cofficient 0.9996 0.9997 linearity range, mm 0.051-39.810 0.031-40.042 lower detection limit, mm 0.042 0.023 response time, s t ≤35 t ≤25 working ph range 5.0-12.0 5.0-12.0 temperature, °c 25 25 life time, days 44 55 8. response time the time required for the electrodes to reach steady potential values, after immersing the electrode in different concentration namely from 0.05, 0.5, 5 and 25 mm of et solution was studied. the average time was found to be short, ranging from 10 s for concentrations 25 mm and 5 mm, to about 25 s for 0.05 mm solution. two electrodes gave the same range of response times. these values indicated the high stability of the electrodes during the measurements. a typical plot for response time is shown in fig. 7 for the electrode based on dbp as the plasticizer. y = -57.00x + 193.57 r² = 0.9997 -70 -35 0 35 70 105 140 1.002.003.004.005.00 e / m v -log(aetx / m) dbph lod etx-udth y = -56.26x + 190.75 r² = 0.9996 -60 -30 0 30 60 90 120 1.002.003.004.005.00 e / m v -log(aetx / m) doph etx-udth s. f. rassi j. electrochem. sci. eng. 6(2) (2015) 187-198 doi:10.5599/jese.284 195 figure 7. plot the response time of dbp electrode using 0.05, 0.5, 5, 25 mm etoricoxib. 9. lifetime the electrode lifetime was investigated by performing the calibration curve and the periodic testing of standard solutions (0.031-40.042 mm of et) and calculating the response slope. it was observed that the electrode b exhibited good stability in terms of slope in the linear domain of concentration and the electrodes could be used continuously for about 55 days without considerable decrease in its slope value. electrode a exhibited good stability in terms of slopes of 56.20 to 53.4 mv decade -1 in the linear domain of concentration from 0.051 to 39.810 mm. this electrode could be used continuously for about 44 days. but two changes were observed, firstly, a slight gradual decrease in the slope (from 57.00 to 55.23 mv decade -1 ) was found, and secondly an increase in the detection limit (from 0.023 to 0.2 mm) was noted. however, the electrode with dbp as plasticizers could be used for about 55 days without any considerable decrease in the slope value. 10. selectivity of electrode the influence of some inorganic cations such as of li + , na + , k + , ca 2+, zn 2+ , ba 2+ , mn 2+ , mg 2+ , ni 2+ , nh4 + , cu 2+ pb 2+ , co 2+ , fe 3+ , al 3+ , cr 3+ , and some organic compound like sugars (glucose, fructose) and excipients on the electrode response was investigated. the selectivity of the electrode was measured by applying the matched potential method (mpm). according to this method, the activity of et was increased from aa = 1 mm (reference solution) to a′a=1.1 mm, and the changes in potential (∆e) corresponding to this increase were measured. then a solution of an interfering ion of concentration ab is added to a new 1 mm reference solution until the same potential change (∆e) was recorded. the selectivity factor, pota,bk , for each interference was calculated using equation (1). the results are given in table 5. results revealed reasonable selectivity for et in presence of many related substances. the selectivity coefficient obtained by this method showed that there were no significant interferences from the cations, this reflected a very high selectivity of the investigated electrode towards et. -60 -15 30 75 120 0 50 100 150 200 e / m v t / sec 5×10-5 m 5×10-4 m 5×10-3 m 2.5×10-5 m etx-udth5% se j. electrochem. sci. eng. 6(2) (2016) 187-198 etoricoxib selective sensor 196 table 5. selectivity coefficients for of the et-udth responsive electrode. mpm a,bk interferent mpm a,bk interferent 9.9×10 -5 2.0×10 -5 2.5×10 -5 4.3×10 -5 8.0×10 -4 1.2×10 -5 6.2×10 -6 4.8×10 -5 8.6×10 -6 glucose fructose lactose starch microcrestaline cellulose carboxymetyle cellulose polyethylene glycol titanium dioxide polysorbate 80 1.00×10 -4 1.25×10 -4 9.09×10 -5 5.56×10 -5 6.25×10 -5 6.45×10 -5 1.10×10 -5 7.14×10 -5 5.13×10 -5 5.81×10 -5 7.69×10 -5 9.52×10 -5 7.81×10 -5 6.76×10 -5 1.22×10 -5 li +1 k +1 na +1 nh4 + mg +2 mn +2 ca +2 ba +2 ni +2 cu +2 zn +2 pb +2 cd +2 cr +3 fe +3 even though, the inorganic cations have different ionic sizes, mobilities and permeabilities they did not interfere with et. the selectivity of the electrode towards neutral sugars was evaluated. the tolerance was considered as the concentration imparting a ± 0.2 mv drift in the potential reading. the results indicated that glucose, fructose, lactose and starch did not interfere. the experiments showed no interference with respect to et response for electrodes a and b. 11. analytical application the et membrane electrodes were used for the determination of et in pharmaceutical preparations using both direct and standard-addition methods. the direct method is the simplest for obtaining quantitative results. a calibration graph was constructed and concentration of the unknown was calculated from the linear equation of the calibration curve. direct determination of et in tablets was carried out using the developed membrane electrodes. the results are summarized in table 6 and 7. table 6. accuracy and precision for the determination of et using the proposed pvc membrane sensors in pure solution electrode taken, mm direct mothed standard-addition method found, mm rsd, % r / % ase found, mm rsd, % r / % ase a 0.5 0.50 1.88 100.00 0.005 0.49 2.13 98.00 0.007 2.5 2.49 1.80 99.60 0.026 2.44 1.97 97.60 0.034 5 4.95 1.45 99.00 0.042 5.08 1.74 101.60 0.063 10 9.95 0.86 99.50 0.050 9.94 1.56 99.40 0.110 b 0.5 0.49 1.52 98.00 0.004 0.51 1.46 102.00 0.005 2.5 2.54 1.24 101.70 0.018 2.53 1.32 101.20 0.024 5 4.98 0.81 99.60 0.023 4.98 1.01 99.60 0.036 10 10.06 0.62 100.60 0.035 10.19 0.91 101.90 0.068 average of four determinations the content of drug in its formulation had good agreement with the declared amount. the standard-addition method was applied by adding a small portion (1 ml) of a 50 mm standard et solution to 15 ml of various formulation drug concentrations (60, 90, 120) mg et/tablet, (0.167, s. f. rassi j. electrochem. sci. eng. 6(2) (2015) 187-198 doi:10.5599/jese.284 197 0.251, 0.335) mm. the change in the potential reading (at a constant temperature of 25 °c) was recorded after each addition, and was used to calculate the concentration of et by the equation (2). thus, the determination of the concentration depended mainly on ∆e, hence, to obtain a noticeable ∆e we needed to prepare a higher concentration of the et standard. results of the standard-addition method are given in table 7. the determination of et in tablet was carried out using the proposed electrode. the results were compared to those obtained using the spectrophotometric method [16]. the determination of et in its pharmaceutical formulations etoxia gave an average recovery of (100.06-100.75). mean values were obtained with a student’s tand f-tests at 95 % confidence limits for four degrees of freedom are shown in table 7. the data reveal that results compare favorably with those obtained by spectrophotometric. the results showed comparable accuracy (t-test) and precision (f-test). table 7. determination of et in its pharmaceutical preparation using the proposed electrode. sample nominal value potentiometry spectrophotometry mget/tablet direct standard addition etoxia 60 r / %±sd a 100.75±0.58 100.47±0.61 99.73±0.31 t-value b 1.85 1.72 1.830 f-value b 3.50 3.87 etoxia 90 r / %±sd a 100.29±0.49 100.43±0.54 100.08±0.33 t-value b 1.32 2.15 0.57 f-value b 2.20 1.86 etoxia 120 r / %±sd a 100.06±0.23 100.46±0.55 100.46±0.49 t-value b 1.89 0.58 2.05 f-value b 1.26 4.53 a five independent analyses. b theoretical values for tand f-values at four degree of freedom and 95% confidence limit are (t=2.776) and (f=6.26). conclusions in conclusion, the developed pvc membrane sensors described in this work offer a simple, accurate, selective, and specific tool for quantitative determination of et in some pharmaceutical formulations. the membrane sensor et-udth based on dbph, seem to be better than et-udth based on doph with respect to calibration, slop, and accuracy. the statistical evaluations of the proposed method in comparison with spectrophotometric method indicate that the method is accurate and precise. the proposed analytical method is proved to be simple and rapid, with good accuracy. references [1] d. riendeau., m.d. 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[19] r. chandra, a. sanghi, d. kumar, k.k. hindwan, british biomedical bulletin 2 (4) (2014) 706714. [20] s. gholve, o. bhusnure, o. mathew1and j. sangshet, int. j. pharm. bio. sci. 7(2) (2016) 246– 253. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ some progress in voltammetric methods to detect malachite green in real samples using carbon electrodes http://dx.doi.org/10.5599/jese.1480 437 j. electrochem. sci. eng. 13(3) (2023) 437-449; http://dx.doi.org/10.5599/jese.1480 open access : : issn 1847-9286 www.jese-online.org review some progress in voltammetric methods to detect malachite green in real samples using carbon electrodes madhusudan dasnur nanjappa1 and gururaj kudur jayaprakash2, 1department of chemistry, institute of science, banaras hindu university, varanasi 221 005, uttar pradesh, india 2department of chemistry, nitte meenakshi institute of technology, bangalore, karnataka, 560064, india corresponding author: rajguru97@gmail.com; tel.: +91-953-876-2343 received: august 5, 2022; accepted: may 31, 2023; published: june 2, 2023 abstract carbon electrode materials have generated considerable research interest in recent years due to their ease of use, higher charge transfer kinetics, and cost-effectiveness. malachite green (mg) is an organic compound with metallic-looking powdered green crystals, which got its name from the color of malachite. mg can be easily converted to leuco-malachite green, a colorless form (lmg). because both mg forms are dangerous for human health, detecting them in the environment is important. many researchers across the globe worked on mg detection using various techniques, and this article provides brief information on their results. in this review article, some specific information about electrochemical detection techniques, which are frequently employed for mg determination, is discussed. this review highlights some advances in voltammetric methods using carbon-based electrodes such as glassy carbon, carbon paste, pencil graphite, and their chemically modified forms in various configurations that can be used for the electrochemical detection of mg. some of the future scopes in using these advanced, carbon-based electrodes in mg determination are also discussed. keywords toxic organic dye; electrochemical sensors; modified carbon electrodes; voltammetry techniques, real samples introduction malachite green (mg) is a well-known organic dye that is a derivative of triphenyl-methane, with the chemical formula c23h25n2cl, color index of 42000, and the iupac denomination as 4-[(4-dimethylaminophenyl)-phenyl-methyl]-n, n-dimethylaniline (figure 1) [1]. mg is highly soluble in water and ethanol and develops a blue-green color solution. the three phenyl rings in the mg structure provide significant hydrophobic properties, making it a fascinating http://dx.doi.org/10.5599/jese.1480 http://dx.doi.org/10.5599/jese.1480 http://www.jese-online.org/ mailto:rajguru97@gmail.com j. electrochem. sci. eng. 13(3) (2023) 437-449 detection of malachite green in real samples using ce 438 probe for electrochemical research [2]. it is a common fabric dye and fades slowly by the action of strong acids and bases. the base causes the color to fade because it converts the dye back to the alcoholic color base, which is colorless, but in the presence of an acid, the h+ ions coordinate with the lone pairs of electrons of the amino groups. this reduces resonance in the molecule and leads to fading [3]. figure 1. chemical structures of (a) malachite green and (b) leuco-malachite green since mg is cheap and efficient against major protozoal and fungal species, it is widely employed as a biocide in aquaculture [4]. mg is used as an antiseptic in african aquaculture against fungal and parasitic infections. it is also used in dying jute, leather, paper, polymers, and as a food ingredient. due to its water solubility, the untreated discharge of industrial effluents into water resources results in the contamination of water bodies and subsequent absorption of the contaminant by various fish species. in the dyeing process, around 10-15 % of all colors are lost to wastewater [5]. among the synthetic dye pollution, mg has been classified as one of the most harmful since longterm exposures to its high concentrations can result in carcinogenesis, chromosomal fractures, mutagenesis, teratogenicity, and pulmonary toxicity, among others. researchers discovered that mg, and particularly lmg, may stay in edible fish tissues for long periods. eating contaminated fish can cause major difficulties for human health, such as a detrimental influence on the reproductive and immunological systems. mg is even designated as a class ii health hazard [6]. since leftover mg in aquaculture water can severely pollute water resources, several nations and regions have enacted legislation and regulations limiting or outright prohibiting the use of mg in aquaculture. because of the restricted light penetration, mg may substantially influence the photosynthetic job in aquatic life [7]. it may also be harmful to species due to aromatics and metals, chlorides, and other chemicals. therefore, it is preferable to break down the dye into a non-toxic form before releasing it into the water supply to reduce contamination. these pollutants cause high biochemical oxygen demand (bod), high chemical oxygen demand (cod), toxicity, foul odor, and, most importantly, wastewater color. because dyes in water are extremely visible, even in very low quantities, the color of these pollutants may be identified [8]. this impact is undesirable because the color inhibits photosynthetic activity within the environment by blocking sunlight access to aquatic vegetation and wildlife. a chemical like this has lately been linked to an increased risk of cancer. malachite green is a cytotoxic chemical for mammalian cells that also works as a liver tumor promoter. because of all these detrimental effects of mg, the proper way of its detection, determination, and degradation must undoubtedly be figured out. mg may be determined using a variety of analytical techniques, including liquid chromatographytandem mass spectrometry (lc-ms/ms), surface-enhanced raman spectroscopy (sers), immunelogy, spectrophotometric methods, and fluorescent approaches. however, these procedures are troublesome, time-consuming, and expensive apparatus is required. additionally, these techniques m. d. nanjappa and g. k. jayaprakash j. electrochem. sci. eng. 13(3) (2023) 437-449 http://dx.doi.org/10.5599/jese.1480 439 offer lesser sensitivity with a higher limit of detection levels and are less suitable for the standard analysis of mg. electrochemical techniques, on the other hand, are more practical approaches that reveal some significant applications for the evaluation of biologically active substances, pharmaceuticals, food, water, soil, and some other biological samples. this is because of their quick rate analysis, straightforward working procedures, high sensibility, flexibility, and selectivity, as well as their low cost, exceptional response, and ease of handling in the available laboratory settings [9,10]. in the context of electrochemical techniques, the effectiveness of chemically changed electrodes looks to be a useful tool for enhancing low-level analysis through successfully transformed materials. due to their high reproducibility, propensity for modification by numerous modifiers, ability to execute steady and well-resolved voltammograms, and low cost and simple preparation processes, carbonbased materials have been widely used in the voltammetric analysis for sensitive and selective identification of bioactive molecules [11]. the primary objective of this review paper is to provide a comprehensive overview of the requirements for detecting malachite green (mg) and to highlight recent advancements in voltammetric technology utilizing modified electrodes. by considering crucial factors and simulating the performance of each approach, this article intends to showcase the research trends in mg detection using different types of carbon electrode materials. detection techniques for mg in standard and real samples malachite green (mg) has garnered significant attention among researchers due to numerous issues associated with its use. various analytical methods have been developed to detect and quantify mg in response. these methods include high-performance liquid chromatography (hplc), mass spectrometry (ms), immunological approaches such as enzyme-linked immunosorbent assays (elisa), raman spectroscopy (particularly surface-enhanced raman spectroscopy or sers), and spectrophotometry. hplc is a widely employed chromatographic technique that utilizes a high-pressure infusion system to separate components in the mobile phase. it offers high accuracy and can analyze samples with different polarities or variable volumes of mixed buffer [12]. hplc techniques such as hplc-uv and hplc-dad exhibit detection limits ranging from 0.1 μg kg-1 to 1.7 μg kg-1. mass spectrometry plays a significant role in mg detection, with atmospheric pressure chemical ionization (apci) and extractive electrospray ionization (esi) being commonly used interfaces for ms analysis. mild ionization methods in apci and esi generate low-energy ions for analysis [13]. mass spectrometry methods have demonstrated detection limits of 0.005 μg kg-1 to 2.5 μg kg-1. immunological approaches, such as elisa, rely on the specific antigen-antibody response for mg detection. elisa methods utilize a color response between an enzyme and a substrate to quantify mg levels in a sample. elisa offers high sensitivity, specificity, throughput, and cost-effectiveness, with detection limits ranging from 0.06 ng ml-1 to 4.8 ng ml-1. raman spectroscopy, a technique that monitors molecule vibrations to reflect molecular structure, has gained popularity. sers, a variant of raman spectroscopy, amplifies the raman signal of molecules adsorbed on a metal substrate by 106-1014 times. sers has been successfully employed for mg detection, often combined with other approaches such as cloud point extraction [14]. the detection limits of sers range from 0.5 μg l-1 to 40.0 μg l-1. spectrophotometric methods, particularly uv/visible spectrophotometry, offer simplicity and costeffectiveness. mg can be directly measured by the absorbance at 610-630 nm using a uv/visible spectrophotometer. however, direct absorbance methods are susceptible to interferences from http://dx.doi.org/10.5599/jese.1480 j. electrochem. sci. eng. 13(3) (2023) 437-449 detection of malachite green in real samples using ce 440 chemicals in complex mixtures. to overcome this, rigorous clean-up processes like solid phase extraction (spe) and cloud point extraction are often required. additionally, fluorescent probes, sensors, and quantum dots (qds) in fluorescence resonance energy transfer (fret) devices have gained popularity for mg detection, with detection limits ranging from 0.28 ng ml-1 to 2.9 ng ml-1. while these methods have shown promise, they also have some drawbacks. one challenge is the inability to reuse active substrates, leading to increased production costs. specific methods may encounter difficulties with preor post-column transformations, interferences, expensive equipment, low mass resolution, or interference from structural mimics. researchers are actively working to address these limitations and enhance mg detection and determination methods' accuracy, sensitivity, and efficiency [15-17]. electrochemical detection of mg in standard and real samples the electrochemical detection method is based on the electrochemical properties of a substance in solution [18]. it is a typical green analytical approach since the instrumental setup is simple and costeffective, and the application requires minimal energy. mg is generally electroactive on carbon paste, gold, and glassy carbon electrodes (gce). the use of naked (bare) electrodes, however, is unsuitable because other chemicals quickly poison electrode surfaces, lowering their specificity and sensitivity. the approach based on chemically altered electrodes is preferred because of their enhanced selectivity and sensitivity. modified electrodes containing suitable catalysts, however, improve the dynamic range in the quantitative analysis and speed up the response for mg detection [19]. detection of mg using glassy carbon electrode (gce) zhu et al. [20] investigated the electrochemical behavior of mg and leucomalachite green (lmg) using cyclic voltammetry with a glassy carbon electrode. they found that mg could be detected at ph 7.4 without interference from lmg. the authors also employed differential pulse voltammetry (dpv) for calibration, with a linear calibration plot ranging from 0.2 µm to 1.2 µm of mg. additionally, adsorptive stripping voltammetry (adsv) was used for mg detection in aquaculture water. interference studies showed that various metal and acidic ions did not affect the measurement of mg. this electrochemical method proved effective for detecting mg in ambient water samples, utilizing an unmodified glassy carbon electrode [20]. muresan et al. [21] modified glassy carbon electrode (gc/ceo2/nafion) for electrochemical detection of mg. the electrode was modified by drop-casting ceo2 nanoparticles (nps) and nafion. ceo2 nps improved catalytic activity, surface area, and electron transport, while nafion enhanced electrode stability. square wave anodic stripping voltammetry (swasw) showed high sensitivity, resolving power, and increased oxidation current with the gc/ceo2/nafion electrode. analytical characteristics included sensitivity (0.28±0.01 a/m), detection limit (1.025 µm), and linear range (1 to 10 µm). the electrode exhibited strong selectivity against mg and high short-term stability after 50 scans [21]. a rapid, accurate, and easy electrochemical method for the detection of mg was developed by yi et al. [22]. they created a glassy carbon electrode (gce) coated with a multi-wall carbon nanotube-hexadecyl hydrogen phosphate (mwnt-dhp) film. the mwnt-dhp film demonstrated remarkable facilitation of electron transfer, resulting in a significant shift in potential and enhanced mg oxidation peak current in cyclic voltammograms. the catalytic effects of the mwnt film on mg oxidation were also observed. the mwnt-dhp film-modified gce exhibited a substantial increase in the oxidation peak current of mg due to the large surface area, adsorptive characteristics, and moderate electric properties of mwnt. the method demonstrated excellent performance with a wide m. d. nanjappa and g. k. jayaprakash j. electrochem. sci. eng. 13(3) (2023) 437-449 http://dx.doi.org/10.5599/jese.1480 441 linear range, high sensitivity, low detection limit (6.0 nmol l-1), outstanding repeatability, and longterm stability. it showed high selectivity for mg detection, as demonstrated by minimal interference from various foreign species. the method was successfully applied to the analysis of mg in fish samples, highlighting its potential for real-time sample analysis [22]. using three various types of ordered mesoporous silica, including sba-15, mcm-41, and mcm41-nh2, sacara et al. [23] developed innovative glassy carbon electrodes modified with silica for the electrochemical detection of mg. to attain good stability, more nafion coating was put on the electrode. following previous findings, the anodic oxidation of mg involves the ejection of an integral unit of the central carbon attached to a phenyl group, followed by the intramolecular coupling of two phenyl fragments, which results in the formation of the oxidized form of n,n,n',n'tetramethylbenzidine, which is 1,1'-biphenyl-4.4'-diamine. for the examination of the effectiveness of innovative silica-modified glassy carbon electrodes towards the mg (10-6-10-4 m) in 0.1 m phosphate buffer of ph 3, the scientists utilized the swasw technique. according to the swasv measurements, the oxidation peak of mg at the gc/sba-15/nafion electrode is shown at 0.85 v vs. ag/agcl/kcl (figure 2a), where peak height varies with mg concentration. the authors looked at how the electrochemical reaction of mg at modified electrodes was affected by the concentration of silica suspension, quantity of nafion solution, and ph of the electrolyte. according to the results of the investigation, the presence of any form of silica at the gc electrode surface enhanced the electrochemical response compared to the gce, which had not been changed. this was attributable to the adsorption of mg on the silica surface. due to the affinity between mg and the functional group -nh2, mcm-41-nh2 performed better than the other two types of silica. the authors observed two oxidation peaks, one at 0.4 v and the other at 0.85 v vs ag/agcl/kclsat, corresponding to the gc/mcm-41-nh2/nafion modified electrode. this shows that there are two one-electron stages in the oxidation process. e / v vs. ag/agcl (kclsat) figure 2. swasv curves at gc/sba-15/nafion electrode. reproduced under terms of the cc-by license [23] copyright [2017], wiley-vch verlag gmbh & co. kgaa, weinheim during the electrochemical detection of mg, the gc/mcm-41-nh2/nafion modified electrode demonstrated an outstanding detection limit of 0.36 µm with a sensitivity of 0.164±0.003 a/m and linear range of 1-6 µm. i /  a http://dx.doi.org/10.5599/jese.1480 j. electrochem. sci. eng. 13(3) (2023) 437-449 detection of malachite green in real samples using ce 442 the unique silica-modified glassy carbon electrodes demonstrated high stability for up to five days, less time than previous electrodes, and great repeatability with five consecutive experiments using the same electrode. new glassy carbon electrodes with silica modifications were discovered to have good mg detection selectivity. however, the impact of potential foreign species, such as salts and contaminants, was not considered. the authors employed a commercial product (promedivet, romania) often used as a biocide in aquaria for ornamental fish to examine the implementation of the novel gc/mcm-41-nh2/nafion electrode in practical sample analysis. it was discovered that a new electrode effectively found the mg [23]. glassy carbon electrodes were modified using multi-walled carbon nanotubes and cetylpyridinium bromide (gc/mwcnts/cpb), according to zhao et al. [24]. it was discovered that adsorption of the cpb surfactant caused a substantial rise in the anodic peak current of mg. utilizing cyclic voltammetry, the electrochemical behavior of mg (210-6 m) at a modified gc electrode making gc/mwcnts/cpb was observed in 0.1 m phosphate buffer solution of ph 7.0. it was shown in figure 3 (curve-b) that mg in pbs solution showed a minor anodic peak at 0.42 v vs. saturated calomel electrode (sce). the peak at 0.42 v emerged with the addition of cpb, and a considerable anodic peak appeared at 0.62 v vs. sce as a result of the oxidation of mg (curve-c). it was hypothesized that the capacity of cpb to adsorb on the electrode surface would impair mg accumulation by impeding mg ability to transmit electrons. it brought about the gradual oxidation process and raised the concentration of mg used in the electrochemical reaction. curve-d revealed that cpb does not generate any peak in the predicted potential range. it was determined that increasing the concentration of cpb surfactant up to a certain point enhanced the peak current of mg, but additional increases in the concentration of cpb might result in a drop in the peak current. e / v figure 3. cyclic voltammograms of mwcnts/gce electrode in solution containing (a) phosphate buffer solution, (b) phosphate buffer solution+mg, (c) phosphate buffer solution+cetylpyridinium bromide (surfactant)+mg, (d) phosphate buffer solution+cetylpyridinium bromide. potential is referred to the sce reference. reproduced under terms of the cc-by license [24] copyright [2009], electrochemical science group the gc/mwcnts/cpb modified electrode showed an outstanding detection limit of 910-10 m with good repeatability and stability, and the measured anodic peak current was proportional to the concentration of mg. the usefulness of gc/mwcnts/cpb-modified electrodes for the detection of mg in the experimental sample analysis was examined by the authors. the modified electrode demonstrated excellent sensing capabilities toward mg in the sample of pond water. in addition, researchers have looked at how insoluble cationic gemini surfactant affected the surface of glassy i /  a m. d. nanjappa and g. k. jayaprakash j. electrochem. sci. eng. 13(3) (2023) 437-449 http://dx.doi.org/10.5599/jese.1480 443 carbon electrodes modified with multi-walled carbon nanotubes in the absence of cpb surfactant. in this case, enhanced electrode sensitivity toward the mg was demonstrated, although not as with cpb. it is still difficult to create electrodes for sensing applications with insoluble surfactants [24]. a novel multilayer-modified glassy carbon electrode with graphene quantum dots and gold nanoparticles was created by hou et al. [25]. gold nanoparticles and graphene quantum dots (gqds) have typical diameters of 10 nm and 200 nm, respectively. according to the report, certain gqds were aggregated, whereas the au nanoparticles were evenly dispersed. utilizing cyclic voltammetry, the authors examined the electrochemical behavior of multilayer modified electrodes made of gce/(gqds/au). it was noted that both monolayer and multilayer modified electrodes for gce/(gqds/au) showed a rise in peak current. this is attributed to the strong catalytic activity of the modified electrode. additionally, the peak current augmentation was greater at the multilayermodified gce/(gqds/au) electrodes. using cyclic voltammetry, the authors calculated the electrochemical behavior of mg. in the presence of mg, gce/(gqds/au) multilayer-modified electrodes showed high oxidation and reduction peaks. however, no redox peak was seen at the gce/(gqds/au) multilayer modified electrodes in the absence of mg, indicating that gqds and au nanoparticles were naturally electro-inactive. the use of gce/gqds/au multilayer modified electrodes for the detection of mg in the fish samples was estimated by the authors. for the real sample analysis, it displayed good detection capability. with a linear range of 4.010-7 to 1.010-5 mol l-1 and a detection limit of 1.010-7 mol l-1, the gce/gqds/au multilayers modified electrode demonstrated high sensitivity. the obtained lod, however, was higher than the detection limit of 1×10-7 nm from graphene oxide-modified gce [26]. additionally, this electrode showed excellent repeatability and stability for up to 10 days. furthermore, it showed excellent selectivity for the mg by ignoring the effects of other foreign species such as ascorbic acid, uric acid, dopamine, caffeine, vitamin e, xanthine, hypoxanthine, leucomalachite green, and na+, ca2+, mg2+, fe3+, al3+, zn2+, cu2+, cl-, so42-, po43-, and no3[25]. table 1 displays literature data on the performance of the unmodified and modified glassy carbon electrode and some important parameters resulting from the electrochemical analysis. table 1. analytical results for electrochemical detection of malachite green in real samples using glassy carbon electrode (gce) electrode materials lod, nm linear range real samples ref. gce 4.379×104 43.79 437.9 µg/l aquaculture water [20] ceo2 and nafion-modified gce 1025 1 x 10-6 1.0 x 10-5 mol/l fish [21] mwnt/dhp-modified gce 6 5.0 × 10-8 8.0 × 10-6 mol/l carp [22] silica/nafion-modified gce 360 1 x 10-6 6.0 x 10-6 mol/l water [23] mwnt-coated gce 0.9 1 × 10-9 5 × 10-6 m pond water [24] gqds/au nps-modified gce 100 4.0 × 10-7 1.0 × 10-5 mol/l salmon [25] graphene oxide (go)-modified gce 10-7 1.0 × 10-16 1.0 × 10-12 mol/l water [26] detection of mg using carbon paste electrode (cpe) huang and colleagues developed a sodium dodecyl benzene sulfonate-modified carbon paste electrode (cpe/sdbs), which was then used to establish a novel, sensitive, and uncomplicated voltammetric method for measurements of mg, which was based on the enhancement effect of surfactant [27]. the ability of the surfactant sdbs to bind to the hydrophobic c-h chains on the surface of the carbon paste electrode (cpe) served as the foundation for this electrode development. positively charged mg will interact with the negatively charged head group of sdbs via electrostatic interactions during the adsorption process, causing mg to adsorb at the cpe surface http://dx.doi.org/10.5599/jese.1480 j. electrochem. sci. eng. 13(3) (2023) 437-449 detection of malachite green in real samples using ce 444 and significantly increase its electrode surface concentration. it was discovered that sdbs effectively boosts the oxidation peak current and thus the sensitivity of mg measurements too. cyclic voltammetry was used to analyze the electrochemical response of mg at the cpe/sdbs modified electrode, and the results showed that the surfactant sdbs had an enhancing impact on the electrochemical determination of mg. with a detection limit of 4.010-9 mol l-1 and a linear concentration range between 8.0×10-9 to 5.0×10-7 mol l-1, the cpe/sdbs modified electrode demonstrated excellent sensitivity for the detection of mg. additional interference analysis revealed that the presence of interfering substances including ca2+, mg2+, fe3+, al3+, ni2+, zn2+, mn2+, ascorbic acid, uric acid, dopamine, caffeine, vitamin e, xanthine, hypoxanthine, and leucomalachite green did not affect the detection process. to test the sample analysis feasibility, this novel electrode was used to detect mg. it was highlighted that the proposed approach has tremendous promise for real sample analysis with good accuracy [27]. the voltammetric reaction of mg at the conductive carbon black paste electrode (ccbpe) was examined by zhang et al. [28]. their study revealed that paste electrodes constructed of conductive carbon black have good signal-to-noise ratios and more appealing voltammetric analytical capabilities. in addition, the surface topography of ccbpe was recorded using the sem method. the authors concluded that the surface structure of the carbon paste is very important for sensing applications. with a linear range of 10-510 nm, the ccbpe demonstrated a good detection limit of 6 nm. the selectivity of ccbpe for mg analysis was found impressive. studies on interference have shown that non-native species such as inorganic ions (na+, k+, no3-, and po4-) do not affect the method used to determine mg. ascorbic acid, dopamine, methylene blue, and sodium dodecyl sulfate are a few examples of organic compounds that showed some competition to the mg during adsorption. by using the fishing water samples, it was calculated how much mg might be detected in practical samples. the aforementioned findings demonstrated that due to their simplicity, cost-effectiveness, and quick reaction in the measurement of mg, conductive carbon black paste electrodes are promising agents [28]. a very sensitive electrochemical approach for mg detection was developed by ye et al. [29]. the innovative self-assembly modified carbon paste electrode based on ethylenediamine and graphene oxide made the electrochemical sensor. the improved covalent connection between the carboxyl group of the carbon paste electrode and the amino group of ethylenediamine served as the design concept for the cpe/en/go modified electrode. the electrode showed exceptional stability and a long lifespan because of the graphene oxide covering. additionally, graphene oxide efficiently promotes the redox reaction of mg at the electrode surface, which increases in current intensity. using cyclic voltammetry, the electrochemical sensing capabilities of cpe/en/go modified electrodes were proven. a redesigned cpe/en/go electrode showed a higher peak current. because go has strong electrical conductivity, this work showed that attaching these two layers (en/go) to the surface of the cpe might improve the sensing capability for the detection of mg. with the use of electrochemical impedance spectroscopy, the authors utilized [fe(cn)6]4-/3as a probe to examine the characteristics of the electron transfer resistance at modified electrode. the outstanding interface quality of the cpe/en/go modified electrodes was demonstrated by the eis research. the electrochemical behavior of mg at cpe/en/go modified electrode was investigated using cyclic voltammetry in a ph 7.5 britton-robinson buffer solution. from the standpoint of conductivity and accumulation, this investigation aided in estimating the cooperative impact of go and en on the cpe surface. with a detection limit of 510-9 mol l-1 and a good linear concentration range of 8.010-9 mol l-1 to 8.010-7 mol l-1, the cpe/en/go modified electrode demonstrated outstanding sensitivity. m. d. nanjappa and g. k. jayaprakash j. electrochem. sci. eng. 13(3) (2023) 437-449 http://dx.doi.org/10.5599/jese.1480 445 furthermore, it demonstrated outstanding consistency and stability for up to 7 days. however, the modified electrode showed extremely strong selectivity for the mg, although the interference investigation was performed with common interfering agents such as ca2+, mg2+, zn2+, mn2+, fe3+, cu2+, uric acid, xanthine, and hypoxanthine. the use of cpe/en/go modified electrodes in mg determination in lake water used for the cultivation of ornamental fishes was studied. the sensitivity of the modified electrode demonstrated in the study of lake water pointed to its use in practical sample analysis [29]. cpe/cu-btc modified electrode was created by li et al. for the accurate detection of mg [30]. copper nitrate and 1,3,5-benzenetricarboxylic acid were used as starting materials to produce copper-based metal-organic frameworks. cu-btc materials provide several benefits, including a larger active surface, increased adsorption capacity, increased catalytic mg oxidation activity, and decreased charge-transfer resistance. cu-btc compounds were found to have a surface made up of uniform and porous nanoparticles. k3[fe(cn)6 was used as a probe in cyclic voltammetry to determine the electrochemical sensing capabilities of the cpe/cu-btc modified electrode. the peak current on the surface of the cpe/cu-btc modified electrode increased significantly. it was also found that the reduction peak potential moved positively while the oxidation peak potential shifted negatively on the cpe/cu-btc modified electrode. by using linear sweep voltammetry (lsv) at various ph levels, it was possible to demonstrate the electrochemical oxidation capability of mg at cpe/cu-btc. according to obtained results, the oxidation peak potential changed negatively with rising ph levels, indicating that the ratio of protons to electrons transferred during mg oxidation is the same. additionally, the oxidation peak currents on cpe/cu-btc modified electrodes were progressively raised with a rise of ph value in the range of 5.77.0, and then gradually lowered in the range of 7.0-8.0. these findings demonstrated the capacity of the cpe/cu-btc modified electrode to improve the oxidation signals of mg. with a detection limit of 0.67 nm, the cpe/cu-btc modified electrode showed remarkable linearity throughout the potential range of 2-500 nm. the selectivity of the modified electrode was examined concerning potential interferences such as glucose, oxalic acid, ascorbic acid, citrate, thiourea, histidine, dopamine, uric acid, or hypoxanthine. aquaculture drugs (furazolidone, nitrofurazone, oxytetracycline, and chloramphenicol) were also considered. for the mg, the electrode demonstrated excellent detection accuracy. to detect mg, a modified cpe/cu-btc electrode was tested in a water sample taken from fishing pools. in the real sample analysis, the electrode performed satisfactorily [30]. a very affordable and straightforward carbon paste electrode for the detection of malachite green was described by dasgupta et al. [31]. carbon paste electrodes provide excellent electroanalytical performance, are simple to manufacture, and are extremely sensitive. in three distinct ph 6 buffer solutions of acetate buffer, phosphate buffer, and citrate buffer in the presence of mg, the researchers looked at how cpe responded. according to the findings, cpe responded most strongly to the phosphate buffer solution. furthermore, cyclic voltammetry was used in ph 6 phosphate buffer solution to quantify the performance of cpe toward the detection of mg. no oxidation peak was observed in the absence of mg, but when mg was added, a distinct oxidation peak was observed. this proved that the cpe sensor was quite efficient in picking up mg. for up to 21 days, the carbon paste electrode showed excellent repeatability, sensitivity, and stability. it demonstrated two distinct linearity ranges for the detection of malachite green, namely 10-75 μm and 75-300 μm [31]. http://dx.doi.org/10.5599/jese.1480 j. electrochem. sci. eng. 13(3) (2023) 437-449 detection of malachite green in real samples using ce 446 table 2 represents collected literature data on the results obtained from the electrochemical analysis of mg using a carbon paste electrode. table 2. analytical results for electrochemical detection of malachite green in real samples by carbon paste electrode (cpe) electrode material lod, nm linear range, mol/l real samples ref. cpe 4 8.0×10-9 5.0×10-7 fish samples [27] conductive carbon black paste electrode (ccbpe) 6 10-8 5.1×10-7 lake water [28] ethylenediamine and gomodified cpe 5 8.0×10-9 8.0×10-7 fish samples [29] cu-btc/ cpe 0.67 2.0×10-9 5.0×10-7 water and fish samples [30] cpe 10-5 7.5×10-5 & 7.5×10-5 3×10-4 [31] hexadecyl pyridinium bromide modified cpe (hdpb/abpe) 2.0×10-8 4.0×10-5 [32] butyrylcholinesterase (buche) & polypyrrole (ppy)-modified cpe 250 0.00025 -0.01* fish samples [33] *ppm detection of mg using pencil graphite electrode (pge) an electrochemical sensor was created by sanjay and colleagues as a tool for malachite green analysis [34]. the electrochemical sensor was created by altering pencil graphite electrodes with a two-dimensional graphene nanoribbons-cob composite (gnr-cob). the key benefit of adopting 2d gnr-cob composite as an electrode material was the improvement of electron mobility which supports quick and accurate detection of mg. according to the authors, graphene nanoribbons (gnr) have a greater diffraction peak at 2 = 25.2 and 2d cob has a wide diffraction pattern at 2 = 42.99. cob produces a two-dimensional sheet-like structure that wrinkles around ribbons. these two values confirm the formation of the gnr-cob nanocomposite. the electrochemical behavior of the pge/gnr-cob modified electrode was also investigated using electrochemical impedance spectroscopy. the strong conducting nature of the gnr and the increased surface area of the electrode were demonstrated by a reduction in charge transfer resistance for pge/gnr-cob modified electrodes. it was discovered that the pge/gnr-cob modified electrode showed higher sensitivity of 1.714 μa μm−1cm−2, a lower detection limit of 1.92 nm, and a linear range of 25-350 nm. the pge/gnr-cob modified electrode can detect the mg in the presence of many interferents with excellent repeatability and long-term stability for up to 25 days. conclusions electrochemical analytical procedures can be used as quantitative methods in practical applications for malachite green detection. furthermore, it was discovered that gc electrode materials are the most widely utilized materials for electrochemical detection of mg and take part in the essential task of constructing high-performance electrodes. however, a few flaws should not be neglected. many documented electrochemical methods, for example, are unsuitable for the examination of biological materials because they are consistently prone to influence from the matrix with many variables. future perspectives concerning electrochemistry several 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https://doi.org/10.1016/j.chemosphere.2021.132153 https://creativecommons.org/licenses/by/4.0/) cerium oxide as conversion coating for the corrosion protection of aluminum doi: 10.5599/jese.2013.0037 151 j. electrochem. sci. eng. 3(4) (2013) 151-156; doi: 10.5599/jese.2013.0037 open access : : issn 1847-9286 www.jese-online.org original scientific paper cerium oxide as conversion coating for the corrosion protection of aluminum jelena gulicovski, jelena bajat*, vesna mišković-stanković*, bojan jokić*, vladimir panić**, slobodan milonjić vinča institute of nuclear sciences, university of belgrade, p. o. box 522, 11000 belgrade, serbia *faculty of technology and metallurgy, university of belgrade, karnegijeva 4, 11120 belgrade, serbia **ictm – center of electrochemistry, university of belgrade, njegoševa 12, 11000 belgrade, serbia corresponding author: e-mail: rocenj@vinca.rs; tel.: +381-11-3408-865; fax: +381-11-3408-224 received: august 9, 2013; published: novembar 09, 2013 abstract ceo2 coatings were formed on the aluminum after al surface preparation, by dripping the ceria sol, previously prepared by forced hydrolysis of ce(no3)4. the anticorrosive properties of ceria coatings were investigated by the electrochemical impedance spectroscopy (eis) during the exposure to 0.03% nacl. the morphology of the coatings was examined by the scanning electron microscopy (sem). eis data indicated considerably larger corrosion resistance of ceo2-coated aluminum than for bare al. the corrosion processes on al below ceo2 coating are subjected to more pronounced diffusion limitations in comparison to the processes below passive aluminum oxide film, as the consequence of the formation of highly compact protective coating. the results show that the deposition of ceria coatings is an effective way to improve corrosion resistance for aluminum. keywords ceria, coatings, corrosion, microstructure. introduction it is well known that chromate-based chemical conversions were used for corrosion protection of aluminum for decades. on the other hand, because of toxic and carcinogenic properties of chromium, this kind of coatings should be soon withdrawn from all industrial processes [1,2]. http://www.jese-online.org/ mailto:rocenj@vinca.rs j. electrochem. sci. eng. 3(4) (2013) 151-156 cerium oxide in corrosion protection of al 152 many researchers tried to find alternative for chromate conversions; it is believed that rare earth elements (cerium, lanthanum, neodymium and yttrium) could effectively inhibit the corrosion of aluminum. hinton et al. [3,4] introduced the application of lanthanide compounds for this purpose. the reported results showed that these barrier coatings simply prevent contact between aluminum surface and corrosive species. the best degree of corrosion inhibition is achieved with cerium [5-8]. based on these findings, the aim of this study was to examine efficacy of ceria based coating for corrosion protection of aluminum, by means of the scanning electron microscopy (sem) and the electrochemical impedance spectroscopy (eis). experimental materials the aluminum panels, 20 mm18 mm1 mm in size, were used as substrates. the surface of specimens was mechanically polished by 800, 1200, 2400 and 4000 grit emery papers. to remove the surface contamination the panels were degreased in alkaline solution containing: naoh, na3po412h2o, na2sio3 and ethoxylate of nonylphenol with nine molecules of ethylene oxide, during 1 min at 70 c. finally, the panels were rinsed by distilled water and dried in air at room temperature. for the investigation in this study ceria sol which was obtained by forced hydrolysis of 1 n ce(no3)4 aqueous solution was used. the ceria sol was synthesized by gulicovski et al. [9] according to the reported procedure. the main characteristics of the synthesized ceria sol are: ph value of synthesized sol, 0.66, average particle size, 71 nm, and solid phase content, 0.51 mass%. in order to obtain the stable and pure ceria dispersion, nitrates were removed from the prepared sol by ultrafiltration. the final ph of the sol was around 4. the ceo2 coatings were formed on the aluminum panels by dripping method. the ceria sol was applied in the quantity of 230 μl. between the two applications, dispersing medium was evaporated at 35 c. methods the corrosion behavior of the ceria-coated aluminum was determined by the electrochemical impedance spectroscopy (eis). eis data were recorded at the open circuit potential using a gamry, reference 600 potentiostatgalvanostatzra. to examine the surface morphology of the coatings, scanning electron microscope (sem) joel, model jsm-5800, at 20 kv was used. results and discussion surface morphology of bare al and al/ceo2 figure 1 shows typical sem images of the al and al/ceo2 specimens before and after prolonged exposure to 0.03% nacl solution. an uniform pattern of the al surface can be seen after mechanical polishing and degreasing (figs. 1a and 1b). a highly porous film consisted of uniform m-sized grains is observed at the al surface after immersion in nacl solution (figs. 1c and 1d), i.e., native homogeneous surface has been destroyed and a new layer of corrosion products was produced. j. gulicovski at al. j. electrochem. sci. eng. 3(4) (2013) 151-156 doi: 10.5599/jese.2013.0036 153 figure 1. typical scanning electron micrographs of al sample at two different magnifications: before (a and b) and after exposure to 0.03% nacl (c and d). typical surface morphology of a cerium oxide coating deposited onto al panel is shown in fig. 2. narrow cracks and sparing pits are visible (figs. 2a and 2b); however, no signs of the peeling were detected. the cracks are typical for rather thick conversion film and it is the result of the stress induced in the film during the drying process. sem micrographs of the film surface after immersion in 0.03% nacl solution (figs. 2c and 2d) revealed presence of agglomerates of corrosion products (fig. 2c). the agglomerates appear to lay over compact granular structure (fig. 2d), uniformly consisted of m-sized fluffy grains. corrosion behavior the corrosion behavior and stability of prepared al/ceo2 sample were investigated by eis after different times of the exposure to 0.03 % nacl. the data registered after 24 h of exposure are shown in fig. 3 as complex plane and bode phase shift spectra and compared to the data of al reference sample. the complex plane spectra are consisted of at least three semicircles. the semicircle at lowest frequencies (below 0.4 hz) is more pronounced for al/ceo2, whereas it is only weakly indicated in the spectrum of al (below 0.3 hz). the peaks corresponding to two the high frequency semicircles appear clearly in the phase shift spectra, whereas low frequency semicircle can be observed only as a shoulder around 0.1 hz for al/ceo2. however, phase shifts at low frequencies for both al/ceo2 and al are similar, around 20. the semicircles are larger for al/ceo2, especially two of them appearing at lower frequencies. this indicates good corrosion protection of the applied ceria coating, since this kind of the spectra, usually observed for the corrosion of aluminum, are assigned to the passive al-oxide film (high frequency data) and corrosion-related charge transfer processes beneath (low frequency data) [10-12]. j. electrochem. sci. eng. 3(4) (2013) 151-156 cerium oxide in corrosion protection of al 154 figure 2. typical scanning electron macrographs of al/ceo2 sample at two different magnifications: before (a and b) and after exposure to 0.03% nacl (c and d). figure 3. complex plane and bode plots of al and al/ceo2 sample registered at open circuit potential after 24 h exposure to 0.03 % nacl. inset: high frequency parts of the complex plane spectra. j. gulicovski at al. j. electrochem. sci. eng. 3(4) (2013) 151-156 doi: 10.5599/jese.2013.0036 155 in order to examine the effect of exposure time on the characteristic features of the eis spectra, they were recorded at prolonged exposure times. typical complex plain and bode plots of eis data are presented in fig. 4 by those registered after 312 h of exposure. figure 4. complex plane and bode plots of al and al/ceo2 sample registered at open circuit potential after 312 h exposure to 0.03 % nacl. inset: high frequency parts of the complex plane spectra. as fig. 4 shows, prolonged exposure to the corrosive environment does not considerably change the characteristic features of the spectra. the comparison of figs. 3 and 4 reveals that the spectra for both al and al/ceo2 samples at high frequencies are poorly affected by the prolongation of the exposure, which indicates that 24 h of exposure is sufficiently long to allow complete formation of passive oxide layer. however, it appears that corrosion processes on bare al are not hindered since its spectra from figs. 3 and 4 at lower frequencies, related to the charge transfer, are similar. it follows that corrosion products formed after 24 h are not incorporated into the passive film with impedance response at high frequencies. these observations, except those related to the high frequency semicircles and passive aluminum-oxide interlayer does not hold for al/ceo2 sample. it is to be noted from figs. 3 and 4 that the semicircles for al/ceo2 sample in low frequency domain grow considerably upon the increase in exposure time, which can be considered as a result of increased corrosion resistance with respect to bare al. at lowest frequencies (below 0.04 hz), the semicircle in fig. 4 is followed by a straight line assignable to the diffusion controlled corrosion processes. indeed, the corresponding bode plot reaches the values around 40, which is quite close to the theoretical value of 45. well pronounced diffusion limitations are due to the presence of a compact protective film induced by ceo2. j. electrochem. sci. eng. 3(4) (2013) 151-156 cerium oxide in corrosion protection of al 156 conclusions with an appropriate surface pre-treatment, ceria was formed on the aluminum panels by dripping method. these substrates were characterized by sem and eis methods in order to study their possible applications as protective barriers in corrosive environment. sem analysis confirmed formation of the oxide film on al and al/ceo2 substrates after immersion in 0.03 % nacl solution. the film is consisted of al2o3 agglomerates with compact granular structure that improve corrosion resistance of the ceria coatings. the results achieved by eis measurements show that the semicircles obtained for al/ceo2 sample in low frequency domain grow considerably upon the increase in exposure in nacl, which can be considered as a result of increased corrosion resistance with respect to bare al. it can be concluded that deposition of ceria coating is an effective way to improve corrosion resistance for aluminum. acknowledgement: this work was financially supported by the ministry of education, science and technological development of the republic of serbia (project number: 45012) references [1] s.h. abdel, surface coatings tech. 200 (2006) 3786-3792. 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[12] f.h. cao, z. zhang, j.f. li, y.l. cheng, j.q. zhang, c.n. cao, mater. corros. 55 (2004) 18-23. © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ abstract introduction experimental materials methods results and discussion surface morphology of bare al and al/ceo2 corrosion behavior conclusions acknowledgement references brief review on carbon derivatives based ternary metal oxide composite electrode materials for lithium-ion batteries http://dx.doi.org/10.5599/jese.1470 s1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1470 open access : : issn 1847-9286 www.jese-online.org review paper brief review on carbon derivatives based ternary metal oxide composite electrode materials for lithium-ion batteries veerabhadrachar pavitra1,2, isha soni3, beekanahalli mokshanatha praveen1 and ganganagappa nagaraju2, 1department of nanotechnology, college of engineering and technology, srinivas university, mukka, mangaluru-574146, karnataka, india 2energy materials research laboratory, department of chemistry, siddaganga institute of technology (affiliated to vtu, belagavi), tumakuru-572103, india 3laboratory of quantum electrochemistry school of advanced chemical sciences, shoolini university, bajhol, solan, himachal pradesh 173229, india corresponding author: bm.praveen@yahoo.co.in received: july 26, 2022; accepted: november 14, 2022; published: december 8 2022 abstract revolutionized lithium-ion batteries (lib) have taken a very important role in our day today life by powering all sorts of electric devices. the selection of electrode materials is very important, which impacts the electrochemical performance of libs. advancements in the electrode materials and synthesis procedure greatly influence the electrochemical performance. this review discusses the carbon derivatives based ternary composite as electrode materials. a detailed explanation of the ternary electrode materials synthesis and spectroscopic, microscopic and electrochemical analysis of libs has been carried out in this study. ternary composites are composed of highly conducting carbon derivatives, which are incorporated with sno2/zno/moo3/siox and additionally any one metal oxide. carbon derivatives-based ternary metal oxide composites can exhibit enhanced electrochemical results based on their heterostructures. the availability of more active sites contributes the reversible topotactic reactions during the charging-discharging process due to the porosity and other unique structures of different dimensions of the electrode materials. concepts and strategies can extend the focus on developing the ternary metal oxides for high-performance libs. keywords nanoparticles; sno2; zno; moo3; siox, ternary composite; libs introduction the development of high energy storage materials is a global challenge to combat the rise in fuel prices and environmental problems brought on by the rapid depletion of non-renewable and intermittent renewable energy sources like wind, solar, geothermal biomass, tidal and wind [1,2]. http://dx.doi.org/10.5599/jese.1470 http://dx.doi.org/10.5599/jese.1470 http://www.jese-online.org/ mailto:bm.praveen@yahoo.co.in j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 2 electrochemical energy storage systems exhibit zero emission, which is necessary to fulfill the large energy demands worldwide and to drive electric gadgets and electric vehicles (evs) [3]. large-scale sustainable and portable electrical devices' main consideration is lithium-ion batteries (libs) because of their high charge storage capacity, power and energy densities, and low self-discharge rate [4]. the operating principle of libs is based on the movement of li+ ions between the electrodes. during discharge, lithium ions move from the anode to the cathode and the reverse mechanism occurs during the charging process, i.e. li+ ions move from the cathode to the anode. electrons move through an external circuit. since the intercalation of lithium takes place on both electrodes, electrode materials play a vital role in improving electrochemical performance for facilitating topotactic activity [5]. developing different electrode materials for libs has gained a lot of attention. a schematic representation of the li-ion battery (libs) is shown in figure 1. figure 1. schematic representation of the li-ion battery operation metal oxides (mos) have received increased attention and are being exploited in energy storage applications due to their simple synthesis procedures and potential to deliver superior electrochemical performance. graphite exhibits a lower gravimetric capacity, i.e. 372 mah g-1, than metallic lithium because it cannot increase the li content beyond lic6 [6]. additionally, graphite operates at a decreased voltage compared to metallic lithium anode. on the other hand, numerous structural geometries with electronic structures highlight the metallic, semiconductor, or insulator properties of mos electrode materials. mos are employed in a wide range of applications in the technological realm. due to the size, surface area, and density of their particles, oxides at the nanoscale size exhibit special and distinctive physical and chemical properties. particle size affects how lattice symmetry and cell characteristics are changed. low surface free energy is related to thermodynamic stability and is attained by metal oxides in the nanoscale size. numerous mo nanoparticles (nps) have been thoroughly researched in terms of their costand abundance-benefits as hosts for ion insertion [7-9]. in the carbon derivatives-based ternary metal oxide (cdtmos), electron mobility will be greatly enhanced through the entire surface. synergistic effects among the materials help to overcome the individual mos drawbacks and impacts in increasing the overall electrochemical performance. functionalization of the carbonaceous materials with the mo nps greatly enhances the electrochemical performance of libs. cdtmos also act as a buffer matrix to reduce the capacity fading issues that occur through volume expansion. besides, materials undergo less aggregation and enhance ionic conductivity [10-14]. usually, single metal oxides suffer from large volume expansion, which leads to poor electrochemical performance. literature suggested that the designing of electrodes with more than one a n o d e c a t h o d e electrolyte charge discharge eeli+ li+ li+ li+ li+ li+ separator -+ v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 3 metal oxide and carbon materials could enhance the electrochemical activity. li+ insertion-extraction takes place in both the metal oxides. it enhances the kinetics of redox reactions and electrical conductivity [15]. usually, sno2 and zno undergo alloy reactions with li metal and form lixsnoy/ licuo phases, or directly, they may undergo a partial conversion reaction. some papers mentioned that sno2 undergoes irreversible conversion and zno undergoes reversible conversion reactions [16]. materials that undergo conversions and alloying-dealloying reaction mechanisms deliver superior electrochemical performance than those that involve only one reaction [17]. carbon materials act as buffer matrices and help to avoid volume expansion, while mo composites protect the active materials from aggregation. binary composites are more remarkable than ternary composites. here in this review, we will provide a comprehensive review of carbon derivatives-based ternary metal oxide compounds for libs. it includes a detailed discussion of the electrode materials synthesis procedure and the spectroscopic as well as microscopic analysis of cdtmos. among the various available metal oxides, we have considered mainly carbon derivatives incorporated to sno2, zno, moo3, siox and other mos synthesis procedure, morphologies, and their electrochemical performance for libs from various literature. synthesis of carbon derivatives based ternary metal oxide (cdtmos) composites synthesis of nanomaterials is broadly categorized into top-down and bottom-up approaches, figure 2. the top-down method uses chemical, mechanical, or other energy sources for the synthesis. electron beam lithography [18], aerosol spray [19], gas-phase condensation [20], and ball milling [21] are some of the techniques that fall under the top-down approach. the bottom-up synthesis strategy of nanomaterials derived from atoms or molecules includes hydrothermal [22], co-precipitation [23], ultrasonication [24], sol-gel [25], electrodeposition [26] and combustion [27] methods. figure 2. advantages and disadvantages of top-down and bottom-up approaches the bottom-up approach is preferable for electrode materials preparation because it can achieve uniform chemical composition and defect-free nanostructures. the bottom-up methods are costeffective, less time-consuming, and simple procedures for material preparation. to achieve the superior electrochemical performance of libs, electrode materials of high surface area and unique morphology are very important. varying the different experimental parameters, such as operating temperature and duration, while synthesizing the electrode materials, it is possible to achieve the desired particles' size, shape and structure. various unique structures of different dimensions of 2d and 3d will become more conductive and facilitates ion transportation with a large surface area by creating more active sites [28]. top-down huge scale production is possible; large substrate is used to deposit; no compound purification is necessary sample distribution is around 10 to 100 nm; costly equipment's for the preparation of nps; difficult to achieve the desired parameters bottom-up accomplished different nps of nanotubes, nanowires and so on easily; can control desired parameters; can achieve particles size of 1 to 20 nm; less expensive strategy huge scale creation is troublesome; sample purification is required http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 4 carbonaceous materials cdtmos can be prepared by in-situ wet chemical bottom-up routes or through ex-situ. i.e. mechanical mixing of the prepared individual metal oxide compounds [29]. carbon derivatives can exhibit splendid electrochemical activity because of their higher conductivity and surface area [30]. by combining different metal oxides, it is possible to overcome the individual metal oxide drawbacks due to synergistic effects. each individual metal oxide has a different role by possessing the multiple oxidation states in the composite, which makes the feasible topotactic redox reactions in the libs easier. this way, they greatly enhance the rate performance, cyclic stability and storage capacity of libs because they have very good conductivity and high surface area of carbonaceous materials. this review discusses the ternary composites composed of two mos and a carbon derivative. carbon derivatives include go (graphene oxide), rgo (reduced graphene oxide)/ graphene, cnt (carbon nano tubes), swcnt (single-walled carbon nano tubes), mwcnt (multi-walled carbon nano tubes) [31-35]. the impressive adsorption capacity of metal ions can be accomplished by the high surface area of go, this acts as a potential sink of electrons by providing great electron mobility. the hydrophilic and dispersible nature of go can be varied by the availability of vast number of epoxides and oh functional groups. hummer’s method is the standard procedure for the preparation of go using sulfuric acid and potassium permanganate. some sulfur impurities are often found when the preparation is performed with organosulfate groups. due to this demerit, research was extended to add some modifications. in the modified hummer’s method [36], double bonds of go are unlikely to persist in strong oxidizing conditions. graphene/rgo provide a huge surface area of 2600 m2/g compared to go, i.e., 890 m2 g-1 [37]. therefore, rgo facilitates a very good topotactic activity of li ions during redox reactions. rgo is a 2d monoatomic layer of go, which is composed of hexagonal structures of sp2 hybridized carbon atoms. the rgo can be obtained by exfoliating and reducing the go of 0.142 nm interlayer distance. it acts as a matrix for enabling the electrons and ions to migrate into the active sites. it could greatly increase the energy density, relieve the strain, and avoid particle agglomeration during redox reactions in libs. rgo also has high thermal conductivity and is stable for longer periods [38,39]. there are several methods to prepare rgo. green extracts are good reducing agents can be used for the reduction of go. commonly used inorganic reducing agents are ascorbic acid, oxalic acid, sodium borohydride, etc [40]. carbon nanotubes (cnts) provide higher surface area, high conductivity, provides good ion transport channels and can restrain the π-π interaction of graphene. therefore, cnts act as graphene modifiers. cnts and graphene have almost comparable conductivities, but cnts undergo more controlled diffusion of li ions than graphene during charging-discharging process. therefore, cnts provide superior ionic transport channels [41]. cnts greatly alter the surface characteristics of electrode materials compared to graphene and avoid the agglomeration of the active materials during the topotactic activity of libs. this enables higher initial discharge capacity and cyclic rate performance in the libs. cnt is also a good additive for the electrode material due to its excellent electrical conductivity, large surface area and high aspect ratio [42]. cnts are prepared by many methods, such as chemical vapor deposition (cvd) [43], arc discharge [44], laser ablation [45], etc. despite the many crucial advantages, such as increasing electronic conductivity and reducing pulverization by adding carbon derivatives, there will be a compromise between capacity and cyclic life because carbon is hardly active and low density of the carbon additives, results a poor volumetric and gravimetric energy densities. the battery electrode's faradaic contribution will also be diminished. therefore, it is desirable to tune the carbon additives content in the final compound [46]. v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 5 different synthesizing methods sonochemical method the sonochemical method is very effective for obtaining the uniform distribution of the particles through ultrasonic radiation. in this method, the precursor's solution is sonicated to carry out the ultrasonication. during the reaction, the atoms diffuse into the bubbles by ultrasonic radiation of 20 khz-10 mhz and undergo mechanical agitation in liquid solution. nucleation and diffusion of the molecules, as in terms cause the breakage of chemical bonds, takes place in the sonicator [47]. potle et al. [48] sonochemically prepared the ternary composite of rgo-zno-tio2. they began by combining go, titanium isopropoxide, and zinc acetate in naoh solution and sonicated for 60 min. the reaction mixture was washed and dried to obtain the final ternary composite rgo-zno-tio2. raj et al. [49] prepared nzno–mn2o3@rgo composite by the sonochemical method using manganese oxide (mn2o3) and nitrogen-doped zinc oxide (nzno). equimolar ratios of the precursors were homogeneously ultrasonicated for 30 min and then stirred for 1 h vigorously. they were finally calcined at 750 °c for 3 h. asgar et al. [50] fabricated a hybrid nanocomposite of zno-cuo-rgo through the sonochemical method. go suspension was dissolved in zn (no3)2 and cu (ii) sulfate precursors by maintaining ph 11 of the mixture by adding 1 m koh. the final particles were heat treated at 200 °c in the air for 2 h. t. shinde et al. [51] prepared graphenece-tio2 and graphene-fe-tio2 nanocomposites through a sonochemical route using titanium isopropoxide, cerium nitrate, naoh and go in sonicator and calcined at 300 °c for 3 h for graphenece-tio2 preparation and titanium isopropoxide, ferric nitrate, naoh and go sonicated for 30 min calcined at 300 °c for 3 h to obtain graphene-fe-tio2 ternary composite. table 1. explains the cdtmos composites prepared sonochemically. table 1. sonochemical synthesis of ternary composites no ternary composites reaction time, min advantages ref. 1 rgo-zno-tio2 60 application of ultrasound impacts the uniform dispersion of zno and tio2 nps on graphene sheets [48] 2 nznomn2o3@rgo 30 high acoustic cavitation significantly reduces the attraction force between individual particles, leading to the formation uniform sized particles [49] 3 zno-cuo-rgo 60 prevention of aggregation [50] 4 rgo-ce-tio2 60 high surface area [51] 5 rgo-fe-tio2 30 uniform dispersion of particles [51] combustion method the combustion method is a single-step technique and an easy procedure for the preparation of metal oxide nps. this method is cost-effective, less time-consuming and helps with the creation of porosity in the structure. usually, the combustion method prefers the bulk-scale production of nps. the combustion method involves generating the nps by applying the appropriate temperature to the specific quantity of oxidizer and fuel for the exothermic reactions. nowadays, the preparation of metal oxides using green fuels is also trending [52]. kumar et al. [53] synthesized go-cufe2o4-zno ternary nanocomposite by solution combustion method. sonicated go, copper nitrate (0.0991 g), iron (iii) nitrate (0.02751 g) and zinc nitrate (0.0048 g) solution was kept in an electric jacket at a temperature of 300 °c to remove solvents until the combustion process and calcined to 350 °c for 12 h to obtain the final product. rotte et al. [54] synthesized mgo and nio decorated graphene composite through rudimentary combustion of ball-milled precursors. in this, mg metal turnings (1 g), nio (0.5 g) powder samples were mixed using ball milling with a 1:5 ratio. the mixed blend of nio and mg was combusted http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 6 in the presence of dry ice (solid co2). rai et al. [55] have synthesized mgfe2o4/graphene ternary nanocomposite by the auto-combustion method. in the synthesis, the precursors of magnesium nitrate hexahydrate and iron(iii) nitrate nonahydrate were of 1:2 ratio and urea as fuel along with the reduced graphene nanosheets in hot plate magnetic stirrer at 350 °c. they were finally annealed at 600 °c for 5 h in an n2 atmosphere for the formation of mgfe2o4/graphene nanocomposite. kang et al. [56] synthesized cobalt-nickel oxides with cnts (co/ni/cnt) nanocomposite through one step solution combustion method using co (no3)2·6h2o, ni (no3)2·6h2o, citric acid and cnts. table 2. explains the cdtmos composites prepared via the combustion method. table 2. combustion synthesis of ternary composites no ternary composites operating conditions advantages ref. 1 go-cufe2o4-zno 350 °c, 12 h nanoball interlinked structure over go sheet [53] 2 mgonio / graphene dry ice (solid co2) 12 h at 100 °c wavy few-layered graphene [54] 3 mg fe2o4 / graphene 600 °c for 5 h in n2 atmosphere strong interfacial interaction between mg fe2o4 nps and rgo [55] 4 co-ni oxides / cnts 300 °c, 30 min scalable [56] hydrothermal route the hydrothermal method has been considered for preparing the desired shape and sized nps. teflon-lined air-tight autoclave acts as a hydrothermal bomb to maintain the pressure. the hydrothermal method offers a simple, low-cost procedure and is conducted with easily available raw materials. in the solvothermal method, different solvents with different physical and chemical characteristics can affect the solubility, reactivity, and diffusion behavior of the reactants; in particular, the solvent's polarity and coordinating capability can affect the final product's shape and crystallization nature. the ionic liquid is employed as a co-solvent with water or an organic solvent in the iono-thermal process, which also involves heat treatment [57,58]. botsa et al. [59] synthesized sno2-fe2o3-rgo ternary composite through the hydrothermal method. firstly, tin chloride (2 mmol), naoh (0.1 mmol) was transferred into an autoclave at 190 °c for 22 h. 0.01 m of iron nitrate, 50 ml of ethanol, 0.5 g of sno2 powder were and with an appropriate amount of go placed on a hot plate at 70 °c for 10 h for the ternary composite preparation and finally calcined at 300 °c for 6 h. m. amarnath et al. [60] have prepared rgo/mn3o4/v2o5 ternary composite by hydrothermal route. in the beginning, mn3o4 and v2o5 were prepared by hydrothermal route. preparation of rgo/mn3o4 begins with the preparation of go, then ultrasonically mixed with hydrazine hydrate (10 ml) to reduce go to rgo solution and kmno4 (0.1 m), and all together transferred into an autoclave (100 ml) at 180 °c for 24 h. for v2o5 preparation, navo3 (1 g) and kmno4 were used for ultrasonication. then both solutions were transferred into an autoclave at 180 °c for 24 h. chung et al. [61] prepared cuo/ruo2/mwcnts hydrothermally using cupric acetate monohydrate and ruthenium chloride hydrate in n-methylpyrrolidone as a surfactant, sonicated mwcnt and naoh were transferred to autoclave under 180 °c for 12 h. kumar et al. [62] synthesized ceo2-sno2/rgo by low-temperature hydrothermal method. sonicated go, ce(so4)24h2o (0.08 m) and sncl2·2h2o (0.02 m), naoh (2 m) were transferred into the autoclave at 150 °c for 2 h. wu et al. [63] fabricated the sno2-fe2o3/swcnts nanocomposite through the hydrothermal method using sncl4·h2o (0.2 mmol), fe(no3)39h2o (0.25 mmol) and sonicated swcnts in phthalic acid (0.2 mmol) was added to the hydrothermal bomb for the reaction to occur at 180 °c for 48 h. huang et al. [64] prepared sno2/nio/graphene ternary composite via hydrothermal route using nickel nitrate (0.005 mol), sodium dodecyl sulfate (0.05 mol), tin chloride (0.01 mol) with go in ethanol transferred into autoclave maintained at 200 °c for 18 h and calcined at 600 °c for 2 h. v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 7 zhao et al. [65] developed sno2-cuo/graphene composite synthesized via a hydrothermal route. cuo/graphene nanocomposite prepared using copper acetate, cetyltrimethylammonium bromide (ctab) as a cationic surfactant, and ultrasonicated graphene nanosheets in a hydrothermal bomb at 120-150 °c for 12 h. obtained cuo/graphene, tin chloride, naoh, nacl, again transferred into an autoclave for the hydrothermal reaction at 80 °c for 24 h. xiaoli et al. [66] worked on the core-shell structure of a metal-organic framework for the hybrid composite zno/znco2o4/c synthesized through a hydrothermal route using co(acac)2, zn (no3)26h2o, 1,4-benzenedicarboxylic acid (h2bdc) precursors, for hydrothermal treatment at 150 °c for 12 h. finally heated at 400 °c for 1 h in the nitrogen atmosphere and again heated to 600 °c for 1 h in the air for the hybrid nanosphere formation. yao et al. [67] prepared comoo4 nps/ rgo by hydrothermal route using prepared go nanosheets, co(no3)2·6h2o (4 mmol) and h2moo4 (4 mmol), nh3·h2o were hydrothermally treated at 180 °c for 12 h. 3d architecture acts as an excellent scaffold to host 3-5 nm comoo4. lee et al. [68] synthesized zn1.67mn1.33o4/graphene prepared using zinc acetate dihydrate (2 mmol), manganese acetate tetrahydrate (1.59 mmol), urea (20 mmol), ammonium bicarbonate (60 mmol), 100‐ml ethylene glycol and isopropanol (ipa) in an autoclave for the solvothermal process. the resulting product was calcined at 500 °c for 2 h to achieve the final ternary compound. sebastian et al. [69] have prepared go-fe/zno ternary compound via the hydrothermal method to prepare the honeycomb sort of zno particles and sonochemical route for the ternary composite. they added the macrocyclic fe complex [fe(c10h20n8)(h2o)2](bf4)2] solution dropwise to the prepared go solution and allowed it to stir for 24 h. 3d honeycomb structures of zno prepared using zinc nitrate hexahydrate, hmt (hexamethylenetetramine) and trisodium citrate dihydrate. after tuning the morphological structure and intensely sonicating at 50 °c for 2 h in a high-intensity ultrasonic reactor, they achieved a homogeneous ternary composite. guofeng et al. [70] group developed rgo/fe2o3/sno2 ternary nanocomposite via in-situ coprecipitation and hydrothermal method. combination of fecl36h2o and go mixtures was sonicated for the precipitation formation and then transferred into an autoclave at 120 °c for 4 h to reduce go. then sncl2·h2o was introduced under stirring and annealed at 400 °c for 1 h. table 3 explains the list of cdtmos composites prepared hydrothermally. table 3. hydrothermal synthesis of ternary composites no ternary composites operating conditions advantages ref. 1 sno2-fe2o3-rgo 190 °c for 22 h high porosity, surface area [59] 2 rgo/mn3o4/v2o5 180 °c for 24 h, large surface area due to mn3o4 spheres, v2o5 rod nanostructures [60] 3 cuo/ruo2/mwcnts 180 °c for 12 h high crystallinity, narrow particles distribution [61] 4 ceo2–sno2/rgo 150 °c for 2h spherical particles, [62] 5 sno2-fe2o3/swcnts 180 °c for 48 h high surface area, interconnected electron pathway [63] 6 sno2/nio/rgo 200 °c for 18 h unique hybrid nanostructures increased active sites [64] 7 sno2-cuo/rgo 120-150 °c for 12 h synergistic effect among the nps [65] 8 zno/znco2o4/c 150 °c for 12 h porous core-shell, synergistic effect abundant active sites [66] 9 comoo4/ rgo 180 °c for 12 h 3d architecture, as an excellent scaffold to host 3-5 nm comoo4 particles [67] 10 zn1.67mn1.33o4/graphene 200 °c for 24 h porous structure of micro spheres particles tends to have a large surface area [68] http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 8 sol-gel method sol-gel method of synthesizing achieves nanosized particles with the optimized conditions to control the particles' growth. the sol-gel method is a cost-effective and eco-friendly method. the sol-gel method greatly impacts the engineering surface morphology. establishing the inorganic network through colloidal suspension (sol) and gelation to obtain a gel network takes place [71,72]. kose et al. [73] produced zno/ sno2/ mwcnt composite through sol-gel synthesized sno2/zno (shell) composite on mwcnt (core) in 3d bucky papers network by spin coating to fabricate the free-standing sno2/zno/ mwcnt ternary composite. precursor of zn(ch3coo)22h2o was used to prepare zno sol by ethanol solvent and chelating with glycerin. sno2 sol synthesis using sncl22h2o precursor through chloride removing route with chloride precipitation nh3 solution. mwcnt buckypaper substrates were consecutively coated with the synthesized zno and sno2 sols by spin coating and finally calcined at 400 °c for 2 h in ar atmosphere. kose et al. [74] developed the architecture of zno-sno2-rgo via a sol-gel route with the precursors sncl2·2h2o with zn (ch3coo)22h2o, glycerin acts as a gelating agent to produce the homogeneous sol. finally, the product was calcined at 500 °c for 2 hours, providing free-standing flexibility due to a synergistic effect. figure 3 explains the advantages and disadvantages of different wet-chemical routes figure 3. advantages and disadvantages of different wet-chemical routes spectroscopic analysis of carbon derivatives based ternary metal oxide (cdtmos) composites this section discusses the characterization of cdtmos composites. the primary analysis for the confirmation of elemental presence in the prepared material has been achieved by advanced analyzing tools such as x-ray diffractometer (xrd), raman spectroscopy and fourier transform infrared spectroscopy (ftir). xrd is the main spectroscopic technique for the preliminary confirmation of the material and identifies the phase structure and crystallinity of cdtmos. research papers revealed that the xrd failure to locate rgo peaks might be due to low intensity or the complete reduction of go, so further raman studies can confirm the presence of carbonaceous materials [75]. therefore, hereby we discussed the xrd and raman spectroscopic techniques for the chemical and structural characteristics of cdtmos. x-ray diffractometer (xrd) in this spectroscopic analysis, we discuss the xrd patterns of cdtmos, mainly focused on carbonaceous materials. xrd 2 values of carbonaceous materials and phase structures of cdtmos sonochemical pros ▪ develops reaction rate ▪ conducts experiment in high energies ▪ pressures in a brief time frame ▪ reduced reaction steps cons ▪ extending the issues ▪ not sufficient energy ▪ less yield hydrothermal pros ▪ can synthesise the nps near melting points ▪ particle coarsening, higher quality ▪ agglomeration can be avoided due to low temperature synthesis cons ▪ high cost of equipment ▪ product crystallinity is poor sol-gel pros ▪ ability to develop thin coatings to confirm the adhesion particles ▪ synthesizes high purity products cons ▪ simple, economical and efficient method combustion pros ▪ easy way of preparation. more yield, low cost, ▪ power and time saving method, no special equipment is required cons ▪ less efficient, difficult to achieve optimal fuel/oxidiser ratio v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 9 composites are tabulated in table 4. in mgo/nio/graphene ternary composite, the carbon diffracttion peak located at 26.4° indexed as (002) plane of the hexagonal phase. the crystalline constituents are not clear from the xrd analysis. therefore, the saed pattern of tem confirms the presence of multi-layered graphene [54]. in co3o4/ceo2/cnts composite, the cnt peak is at 26° for the (002) plane [76]. in the sno2-fe2o3-rgo ternary composite, the 2 diffracted peaks centered at 11 and 43° for (001) and (002) go planes. fe2o3 exhibits the presence of a two-phase composite. compared to individual compounds, ternary composite peaks were shifted slightly at lower positions, the intensity of the diffracted peak decreased and broadened the peak width was based on the go addition [59]. the intensity of the rgo/tio2/zno ternary composite’s diffracted peaks was decreased and the width of the peak broadened when the amount of go increased in the composite, which signifies the interaction of rgo in the tio2/zno composite. for graphite, a sharp peak occurred around 11.63° representing the (001) plane and a peak at 26° representing the (002) plane. in rgo formation, the characteristic peaks of go disappeared and a new peak formed at 23.61°, indicating a reduction of go to rgo. the corresponding lattice spacing value of 0.760 nm reduces to 0.336 nm, which confirms the oxidation [77]. table 4. structure and 2 theta information of ternary composites no. ternary composites carbon derivatives, 2 / ° (hkl) structures ref. 1 rgo-tio2-zno g -26.5 (002) go11.63 (001) rgo23.61 zno-hexagonal phase, wurtzitestructure, graphite-hexagonal [77] 2 mgo-nio-graphene c26.4 (002) mgo-cubic, nio-cubic, graphene-hexagonal [54] 3 co3o4/ceo2/cnts cnt -26 (002) co3o4 -cubic [76] 4 sno2–fe2o3-rgo go -11 (002), 43 (100) sno2-tetragonal rutile [59] 5 rgo/mn3o4/v2o5 go10 (001) mn3o4-orthorhombic, v2o5-orthorhombic [60] 6 ceo2–sno2/rgo g -26.4, go11.4 (001) ceo2-cubic, sno2-tetragonal [62] 7 zno-sno2-rgo rgo26 (002) zno-hexagonal, sno2-cassiterite [74] 8 comoo4/ rgo rgo -26 (002) comoo4-monoclinic [67] 9 cnt@fe@sio2 cnt26.4 (002) fe-cubic [78] 10 ag/tio2/cnt no peak for cnt tio2-anatase phase [79] 11 sio/ni/graphite graphite26.2 (002) ni-face centered cubic, graphite-hexagonal [80] 12 cu2o-cuo-rgo rgo25 (002) [82] 13 cuo-zno/rgo go10.5 (001), rgo -23.72 (002) cuo-monoclinic zno-wurtzite [83] 14 nio-zno/rgo rgo -26 (002) [29] 15 wo3-zno@rgo wo3-monoclinic, zno-hexagonal wurtzite [84] 16 zno-rgo/ruo2 zno-hexagonal wurtzite, ruo2-rutile [85] 17 cnt/sio2/moo3 cnt -25.9 (002), 43.3 (110) [86] the conversion of graphite to go is indicated by the characteristic peak located at 10° with the 0.87 nm d-spacing value in the rgo/mn3o4/v2o5 ternary composite. the characteristic peaks were shifted due to composite formation [60]. in the ceo2–sno2/rgo ternary composite, the diffracted peak shifted to a lower angle side at 11.4° compared to the graphite peak located at 26.4°. this confirms the oxidation of go and reduction of go by the 22.4° for the (002) plane [62]. in the zno-sno2-rgo ternary composite, broadened rgo peak located at 26° confirms the reduction of go http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 10 represents the (002) plane [74]. in comoo4/ rgo ternary composite, the formed broad peak attributed to the (002) plane of the rgo, but in the xrd of the ternary composite, rgo disappeared, indicating the rgo flakes were well separated by the comoo4 nps. the intensity of the ternary composite peaks was stronger by increasing the comoo4 addition. the characteristic peak of the rgo did not appear in the ternary composite. this could be due to lower intensity and less quantity of go. raman is the best characterization technique to confirm rgo [67]. in the cnt@-fe@-sio2 ternary composite, the cnt characteristic peak at 26.4° for the (002) plane was quite diminished in the xrd of the composite [78]. in ag/tio2/cnt ternary composite, no diffraction peaks were noticeable in the composite xrd plot due to the shielding of the mwcnt peak at 26.1° by the anatase peak of tio2 located at 25.3°. because of the low amount of mwcnt presented in the composite, mwcnt exhibited low intensity compared to tio2 [79]. in sio/ni/graphite ternary composite, a sharp peak located at 26.2° was assigned to the hexagonal (002) plane of graphite. the initial amorphous nature of the sio/ni gained crystallinity upon the addition of graphite, which displayed sharp diffraction peaks [80]. xrd pattern of cnts@tio2/coo ternary composite is shown in figure 4. rutile braggs positions were labeled in the xrd pattern, which corresponds to the higher temperature required for the cnts formation by spray pyrolysis [81]. in the cu2o-cuo-rgo ternary composite, (002) plane represents the rgo nanosheets. diffracted peaks of two copper phases (cuo and cu2o) were present in the xrd pattern [82]. cuo-zno/rgo ternary composite films showed a strong characteristic peak at 10.5° attributed to the (001) plane. this peak disappeared for rgo and formed one broad peak at 23.72° for the (002) plane. ternary composite peaks have represented low intensities due to the low contents of the precursors [83]. in nio-zno/rgo composite, diffracted peaks explained the nio, and zno characteristic peaks and a broad peak centered at 26° explains the graphene stock disorder [29]. 2 / ° figure 4. xrd pattern of cnts@tio2/coo ternary composite (mdpi open access journal [81]) in wo3-zno@rgo, peaks corresponding to wo3, and zno appeared as per the standard jcpds card numbers, but no peak for rgo represents unstacked rgo sheets. the intensity of the characteristic peaks depends on the precursor content ratio. especially, no characteristic diffraction peaks for the v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 11 distinct go are seen, which may be caused by the low quantity and low diffraction intensity of go or by the full elimination of go by the synthesis process during the formation of zno [84]. in the case of zno-rgo/ruo2, no go peaks due to the low intensity or complete reduction of go [85]. in cnt/sio2/moo3 composite, 25.9 and 43.3° diffraction peaks correspond to the (002) and (110) planes of carbon materials. moo3 sharp peaks represented the highly crystalline and broadened sio2 peaks for the amorphous nature of the ternary composite [86]. we can understand that bulge/broad peaks explain the amorphous nature. the addition of carbonaceous materials into the metal oxide matrix could decrease the size of the particles and make it possible to achieve uniform particles. the interplanar spacing value will be increased upon the incorporation of carbonaceous materials. the characteristic peak of graphite increased with increasing graphite content in the composites. the more active phases of the materials more likely it is to accommodate volume changes during redox reactions of the libs. but active phases can also contribute to the feasible topotactic activity. raman spectroscopy raman spectroscopy is a non-invasive, near-surface non-destructive technique. its analysis provides information about a material's composition or properties. to identify and quantify the functional moieties and covalency of the attached carbon frame has been carried out through the raman analysis [87]. below, we discussed the raman analysis of the carbonaceous materials in the ternary composites. in co3o4/ceo2/cnt ternary composite, two dominant raman peaks at 1332 and 1588 cm-1 represent the d and g bands. d band signifies the defective structure or disordered carbon. the g band signifies the tangential modes of graphitic layers. the degree of graphitization is measured by the ratio of d-band intensity to g-band intensity (id/ig) and is used to estimate the defect density. the ratio (id/ig) of the composite material was 1.49, which explains cnts were highly disordered, and the introduction of metal oxides doesn’t change the cnt’s degree of the disorder [76]. raman modes of the disordered d band located at 1335 cm-1 indicated the defects in sp2 hybridized carbon and g band at 1587 cm-1 indicated the vibration mode of sp2 bonded carbon. the d and g band values were in accordance with the formation cnts [81]. in the rgo/tio2/zno ternary composite, go is represented by 1356 and 1586 cm-1 raman characteristic modes represented as d and g bands. the ratio of id and ig is 0.99. the ratio value increased to 1.21 in the case of rgo explains the decrease in the average size of sp2 domains and increases the defects in rgo ternary composite. the relation between the integrated intensities of disorderedinduced raman bands (id/ig) with the different crystallite sizes (la) is explained by the equation (1): la = 2.410-10 λ14 (id / ig)-1 (1) where λ1 represents the excitation wavelength (514.5 nm), la represents the sp2 domain size. as per the calculated domain sizes of go (16.98 nm) and rgo/tio2/zno (13.89 nm), size was reduced in the ternary composite compared to go. the primary cause of this size reduction is the reduction of surface epoxy groups, which has resulted in the conversion of go to rgo. therefore, the increase in the id/ig ratio's intensity is due to the reduction in the sp2 domains' average size [77]. in the rgo-zno-tio2 ternary nanocomposite, the d and g bands correspond to the k-point photons of a1g symmetry and the e2g phonon resulting from sp2 c atoms, corresponding to 1339 and 1594 cm-1 raman modes. go shows the prominent raman modes at 1356 and 1586 cm-1, termed as d and g bands, also ascribed as modes of k point modes of the a1g symmetry involving phonons near boundary and scattering of first order e2g phonon of carbon atoms [88]. in the mgo-nio/ graphenaceous ternary compound, the presence of d mode demonstrated the disorder. these sp2-bonded carbon nanostructured materials http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 12 exhibited the g band, which provides information about in-plane vibrations. in mgo-nio decorated graphenaceous materials d, g, g*, 2d, d+g and 2d’ bands representative of typical multi-walled carbon nanotubes [54]. in zno‐sno2‐rgo raman spectrums, the g band (around 1590 cm-1) arises from sp2 c atoms and the d band (around 1350 cm-1) originates mainly from sp3. carbon atoms are based on defects found at the end of graphite and graphene layers. the id/ig ratios for mo/rgo composites were greater than that of rgo, which suggests that structural change was increased [74]. in ag/tio2/cnt ternary composite, the intensity of the characteristic modes was decreased compared to tio2 modes, which may be due to the addition of ag nps. two prominent raman modes occurred at 1360 and 1579 cm-1, for the d and g bands of mwcnt [79]. in sio/ni/graphite ternary composite, two characteristic bands at 1356 and 1583 cm-1 are in good agreement with the typical raman modes for the d and g bands of graphite, respectively [80]. in the cu2o-cuo-rgo ternary composite, d-band (1346.9 cm-1) and g bands (1583.5 cm-1) represented the rgo nanosheets. the intensity ratio of the ternary composite was higher than the pure rgo, which signifies more defects in the composite [82]. in co3o4-zno/rgo composite, the two main intrinsic raman peaks, in that d band centered at 1358 cm-1 aroused due to the breathing mode of k-point photons, and another one due to a1g symmetry and due to the first-order scattering of e2g phonon of sp2 carbon atom formed g band centered at 1584 cm-1 [89]. in cnt/sio2/moo3 ternary composite, there were two unique peaks: the peak at 1597 cm-1 was associated with the c=c backbone stretching (g band), and the peak at 1336 cm-1 was associated with the d band. the ratio of id/ig is slightly greater than 1, but if it is less than 1, as in the case of cnt, more defects and lower crystallinity in the ternary composite than in cnt can be found [86]. raman spectroscopic information revealed that the carbon-related prominent raman modes are designated as d-band and g-band, representing the structural defects disorder and in-plane vibrations of sp2-bonded carbon atoms. the presence of these bands confirms the interfacial interaction between the mos and carbon materials. table 5 explains the peak positions and id/ig ratio information of ternary composites. table 5. peak positions and id/ig ratio information of ternary composites no. ternary composites raman peak positions id/ig ratio ref. 1 rgo-tio2-zno d-1356, g-1586 go 0.99, ternary 1.21 [77] 2 co3o4/ceo2/cnt d 1332, g 1588 ternary 1.48 [76] 3 zno-sno2-rgo go 1.1263, rgo 1.286, ternary 1.411 [74] 4 ag/tio2/cnt d-1350, g-1579 [79] 5 sio/ni/graphite 1356, g-1583 [80] 6 cu2o-cuo-rgo d-1346.9, g-1583.5 rgo 1.16, ternary 1.38 [82] 7 cnt/sio2/moo3 g-1597, d-1336 cnt <1, ternary >1 [86] 8 co3o4-zno/rgo d-1358, g-1584 go 0.83, ternary 0.92 [89] 9 cnts@tio2/coo d-1335, g-1587 [81] microscopic analysis of carbon derivatives based ternary metal oxide (cdtmos) composites versatile topographical information of the cdtmos composites was examined with scanning electron microscopy (sem), field-emission scanning electron microscopy (fesem) and transmission electron microscopy (tem). morphology information includes surface features, shape, size and structure of the compounds. sem provides digital image resolution as low as 15 nm and instructive data about the characterization of microstructure, including morphology, roughness, and boundaries [90]. fesem provides morphological images of higher magnifications [91]. tem can produce a much higher resolution of quality images than sem and saed (selected area electron diffraction) v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 13 for crystallinity confirmation. morphological variations of structures can be performed during the synthesis through the usage of surfactants, reducing agents, protecting agents (pva in dmf) and optimizing the experimental parameters such as experiment duration, temperature and so on [92]. the nanowires are typically tens to hundreds of microns long, randomly orientated, and have either curved or straight morphologies. the aspect ratio of nanowires is high, and their diameter is around 10 nm. long aspect ratio particles are considered nanowires [93]. the ternary nanocomposite of rgo/moo3@c morphology explored by carbon-coated uniformly straight α-moo3 nanowires of 6-8 µm length and diameter of 100-150 nm along with the intertwined rgo nanosheets as per fesem. tem revealed that the graphene sheets were homogeneously wrapped on moo3@c nanowires [94]. there are many more nanowires sort of structures available as battery materials for libs, including co3o4 nanowires [95], co3o4/nio/c [164] and so on. nanorods are 1-d nanostructures and synthesis is more flexible. nanorods typically range in length from 10 to 120 nm. metal oxide nanorods are effectively created by converting nanoparticles at a comparatively low temperature of 80 °c [96]. examples of nanorods include the co-ni-based ternary molybdate nanorods [97], co3o4 nanorods [98], α-moo3@ mno2 core-shell nanorods [99]. figure 5 demonstrates the highly-magnified image of the cnts@tio2/coo nanotubes. the figure explains the effective connection of cnts with the tio2/coo nanotubes and displays a web-like structure [100]. figure 5. sem image of cnt@tio2/coo composite (open access mdpi journal [100]) two-dimensional (2d) inorganic nanosheets have attained great attention of study because of their unusual characteristics, such as a high degree of anisotropic morphology and increased surface area with numerous surface-active sites, and distinctive electronic structures [101]. cu2o/cuo/rgo ternary composite was explored in the 3d hierarchical structure, in that cuo displayed a sheet-like structure of average 500 nm width. in this, cu2o/cuo was wrapped by rgo layers [102]. in rgo/sno2/au nanocomposite, the introduction of rgo relieved the aggregation of sno2 nanosheets. therefore, sno2 formed homogeneously on rgo with the au nps as per the fesem [103]. in niosno2/rgo, as per sem and tem results, nio-sno2 particles are distributed on graphene nanosheets with different magnifications. nio-sno2 of different particle sizes exhibited no fixed dimensions and rgo exhibit ed plate-like crumpled featurees [104]. sno2/moo3/c ternary composite was composed of sno2-moo3 and graphite nanosheets. tem revealed that sno2 and moo3 nps are between 5-10 nm in size and equally wrapped in plate sort of graphite [105] sem http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 14 moo3 exhibits a nanobelt structure and are completely dispersed in the rgo layer in the ternary composite of moo3/fe2o3/rgo, according to the fesem. tem revealed that the width of nanobelts ranges between 50 and 100 nm, and the length ranges from several hundreds of nanometers to several tens of micrometers [106]. cuo-zno/rgo ternary composite displayed unique architecture consisting of cuo 1d nanochains and zno nanoseeds of average length 674 nm, and average width 287 nm. highly magnified rectangular seeds are of average size of 20.4 nm. zno nanoseeds dispersed on the rgo sheets and interlinked with cuo nanochains [83]. nanoflakes exhibit porous structures created by a group of a large number of nps. nio-zno/rgo composite composed of many thin curly flakes and few discrete particles covered with graphene sheets indicated uniform distribution. loosely packed nio-zno formed on the high surface rgo sheets [29]. examples of nanoflakes sort of structures are sno2@ mno2 nanoflakes [107]. nanospheres possess uniform structures, consisting of spherical particles with diameters between 10 and 200 nm, and exhibit various new, improved size-dependent features, which exhibit a conventional core-shell structure compared to bulk spheres nanospheres/microspheres. in v2o5/rgo/cnt ternary composite, tem revealed that the v2o5 microspheres of 1 µm were embedded in rgo/cnt matrix. composite described the penetration of cnt into v2o5 assembly, representing the homogeneous distributions of v, o and c [108]. quasi-nanospheres of sno2/tio2/c, spherical hollow structure of sno2-tio2-c, micro hollow spheres of zno/znfe2o4 /ndoped c, hollow hybrid nanospheres of sno2@c@mno2, and sno2/tio2 spheres are good examples [109,110]. ternary co3o4-zno/rgo composed of cubic-shaped co3o4 particles and hexagonal disks-shaped zno particles were anchored on the rgo sheets as per fesem and tem [89]. cnt/sio2/moo3 composite contains cactus-shaped structure composed of a large number of cnts in the form of cactus leaves, which are 0.5 to 2 µm in length and 3 to 30 nm in diameter. in this, sio2 and moo3 were present on the surface of cactus according to sem and tem [86]. co3o4 performs the nanocage sort of structure [112], flower type of structure can be noticeable in co3o4 [111]. wo3-zno@rgo ternary composite displayed stacked nanopetals sort of structure, consisting of different sized wo3 and zno nanorods. the increased magnified image revealed that the wo3 consists of agglomerated nanocuboid of average dimensions 165/1533/73 nm3. fesem explains the two varied morphologies of spherical-shaped nanorods with infrequent nanowires for zno. the average diameter of nanowires and nanorods were estimated as 54 and 118 nm, respectively [84]. figure 6 represents the sem images (a and b) of ternary fe2o3/tio2/c fibers with different magnifications. after the application of carbon, the composite showed a smooth surface fiber morphology [100]. figure 6. sem images of α-fe2o3tio2/c fibers with different magnifications (a) (b) v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 15 unique structures promote feasible electron transportation by reversible redox reactions during the lithium-ion intercalation-deintercalation process. volume expansion can be reduceable effectively by the feasible redox reactions. mesoporous and microporosity structures contributed to the increase in the surface area. carbonaceous materials provide fast electron transfer activity and excellent conductivity. the nanosheet structure of the composite helps to avail the large spacing to transfer the li ions efficiently. electrochemical analysis of cdtmos the electrochemical performance of the libs will be measured by the discharge-charge capacity, rate capability for different c-rates or current density and cyclic stability. discharge-charge curves of a battery cell are measured with respect to the specific voltage and current. the c-rate refers to how fast the battery is being charged or discharged. lifetime and performance of a battery are drastically affected by the high c-rates. battery cycling tests are carried out for the analysis of long-term stability [113]. decomposition of the electrolyte or degradation of the electrode materials within the coin cell might reduce battery performance. incomplete or slow rection kinetics of the redox reactions during the discharge-charge cycles leads to volume expansion of the cell, which might be detrimental to cyclic stability [114]. graphite anode will react with the electrolyte and aggregate the lithium, reducing the battery performance and causing safety-related issues such as thermal runaway, exacerbating low-temperature operations. electron and lithium-ion transportation is hampered by the weak conductivity of the electrode materials [115]. in recent years, extensive research has been conducted to develop novel composite materials with unique nanostructures. novel architectures act as potential electrodes for improved electrochemical performance. different combinations of the electrode materials could elucidate the battery materials' degradation during the charging-discharging process. various synthesis methods are developed to improve the electrochemical performance. nano-scaled particles of porous structures reduce the li+ diffusion length enabling the feasible li-ion insertion-extraction mechanism [116]. first cycle coulombic efficiency or fce usually refers to the ratio of the discharge capacity after the full charge and the charging capacity. fce will be significantly different from its subsequent cycles. the intercalated lithium ions do not leave the electrode completely during subsequent cycles. when the cell is discharged for the first time, impeded lithiation reaction takes place called sei film before the li ions insertion. those lithium ions cannot be returned during subsequent charging cycles. the coulombic efficiency of the battery is affected by the electrolyte decomposition, material aging, ambient temperature, and different charge-discharge current rates [117]. in this section, we discussed the electrochemical performance of selective promising metal oxides such as sno2, zno, moo3, siox and others. among the mentioned, one mo is fixed, and the second compound is any other metal oxide, along with any carbon derivative, to form ternary composites. figure 7 shows the schematic representation of carbon derivatives based ternary metal oxides. electrochemical performance of sno2 and carbon derivatives based ternary composites tin oxide (sno2) is propitious electrode material that serves as an anode in the libs because of its high theoretical capacity of 790 mah g-1, safety, and stability. sno2 alone suffers from structural integrity, intrinsic conductivity, rate capability and volume expansion [39]. the general electrochemical rection mechanism of sno2 electrode for libs is presented in equations (2) and (3) [40]. sno2 + 4li+ + 4e→ sn + 2li2o (2) sn + xli+ + xe lixsn (3) http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 16 figure 7. schematic representation of carbon derivatives based ternary metal oxides here, in this case, reaction (1) represents the conversion reaction. it is irreversible and responsible for the initial capacity fading. equation (2) represents the alloying-dealloying reaction, which is reversible and contributes dominant capacity for the libs [40]. many sno2-based ternary composites are explored in the literature. ternary composites of two mos electrode materials along with the carbon derivatives undergo a synergistic effect. the synergistic effect plays an important role in the ternary composite by adding up each compound’s advantages. porous carbon materials act as a buffer matrix to overcome the volume expansion. however, ternary composites are less exposed compared to binary composites. xia et al. [70] fabricated sno2/fe2o3/rgo anode through homogeneous precipitation method. as per tem analysis, fe2o3 nps surrounded sno2 nps. sno2 particles prevented the agglomeration of fe2o3 nps. feasible ions transportation and stress relieving took place during charge-discharge process. this novel composite initially exhibited an 1179 mah g-1 discharge capacity at a current density of 400 ma g1 and maintained a stable 700 mah g-1 capacity. in another work, the same composition, sno2-fe2o3/rgo was designed by zhu et al. [118] through a facile wet-chemical solvothermal approach. they maintained the weight ratios (sno2: fe2o3: rgo is 11:1:13) in the ternary composite. composite exhibited the controlled phase ratio, which depicted the higher specific capacity of 958 mah g-1 at 0.5 c (395 ma g-1) current density even after 100 cycles and high rate capability of 530 mah g-1 at 5 c (3950 ma g-1). significant discharge capacity at higher current density was attributed to the addition of a small quantity of amorphous fe2o3. the combination of fe3o4 with sno2 and rgo ternary composite prepared by wang et al. [119] via the hydrothermal method, fe3o4 and sno2 nps uniformly loaded on rgo nanosheets without aggregation prepared. the ternary composite of sno2/fe3o4/rgo displayed higher capacity than its binary counterparts due to the benefits of the synergistic effect between sno2 and fe3o4 by achieving superior cyclic stability. smaller crystallite size facilitated li-ion transportation and reduced the diffusion path. this composite exhibited a higher reversible capacity of 947 mah g-1 at 200 ma g-1 current density in the initial cycle, maintaining a capacity of 831 mah g-1 after 200 cycles. this composite experienced faster capacity fading by increasing the current density. sno2 mo (metal oxide) carbon based materials (c/go/rgo/cnt) cdtmos zno mo (metal oxide) carbon based materials (c/go/rgo/cnt) cdtmos moo3 mo (metal oxide) carbon based materials (c/go/rgo/cnt) cdtmos siox mo (metal oxide) carbon based materials (c/go/rgo/cnt) cdtmos v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 17 along with swcnt, the same sno2-fe2o3 was developed by wu et al. [120] through the hydrothermal method. in this case, sno2 and fe2o3 nps homogeneously settled on the surface of swcnt. sno2-fe2o3/swcnt ternary composite exhibited the specific capacity of 692 mah g-1 after 50 cycles at 200 ma g-1 current density and at a higher current density of 2000 ma g-1 electrode was capable of delivering 656 mah g-1. the good electronic conductivity and flexible mechanical strength of the swcnt are the main reasons for the improved rate capability and reduced volume expansion. joshi et al. [121] have engineered the flexible and freestanding fe2o3-snox-carbon nanofiber composite by electrospinning method. fe2o3-snox-c nanofiber of weight ratio (fe:sn/3:1) exhibited 756 mah g-1 after 55 cycles and 540 mah g-1 specific capacity at a higher current density of 1000 ma g-1 due to accommodating the volume changes of fe and sn by carbon nanofiber. hilal et al. [122] prepared sno2/zno/mwcnt composite by sol-gel and spin coating method. this anode maintained a high capacity of 487 mah g-1 after 100 cycles. the authors explained the advantages of individual compounds and the impact of high surface area, mesoporous nature, and electronic properties. kose et al. [74] have developed the architecture of sno2-zno-rgo via a solgel wet chemical route. they also confirmed that the ternary composite delivered higher performance than the binary electrode. sno2-zno material was designed between rgo layers. superior properties of each compound contributed to enhancing electrochemical activity attributed to the synergistic effect, which delivered 731 mah g-1 specific capacity after 100 cycles at 0.2 c-rate and displayed good rate performance. ternary sno2-moo3-c nanosheet structure was prepared using hydrothermal and dry ball milling by feng et al. [105]. sno2/moo3 nps were encapsulated in plate graphite to exhibit the nanosheet structure. moo3 additive protected sn from aggregation and nano-sized composite provided more active sites. sno2-moo3-c electrode exhibited the outstanding discharge capacity of 1338.3 mah g-1 at 200 ma g-1 after 300 cycles and stable rate performance carried out at a current density of 5000 ma g-1 delivered 715.08 mah g-1. one more important notable thing is they used lamina sort of graphite, which alleviated the volume expansion and reduced the li-ion transportation distance, therefore displaying remarkable electrochemical performance. huang et al. [64] prepared a graphene-supported porous sno2/nio ternary composite via a hydrothermal route. a special structure increased the electronic conductivity and buffered the volume expansion. this composite exhibited the 1280 mah g-1 initial discharge capacity at 300 ma g-1 current density and maintained 410.74 mah g-1 after 50 cycles, also achieving 99.4 % of coulombic efficiency. hydrothermally synthesized sno2-tio2@graphene ternary composite was prepared by jiao et al. [124]. unique structures of sno2 and tio2 were grown on graphene. it exhibited 1276 mah g-1 discharge capacity even after 200 cycles at 200 ma g-1 current density and produced 611 mah g-1 capacity at high-rate of 2000 ma g-1. the comparative study was performed with sno2, go, and sno2-tio2@graphe [125-128]. quasi-nanospheres of sno2/tio2/c were developed by d. bao et al. [129] through the hydrothermal method. in this case, sno2 nps sandwiched between tio2 quasi nanospheres and carbon coating presented a sphere structure in between the gaps. more spaces can effectively accommodate li+ ions and reduce the aggregation of the particles. composite exhibited 895.3 mah g-1 specific capacity after 70 cycles at 100 ma g-1 current density and maintained a 347.3 mah g-1 reversible capacity at 3000 ma g-1 higher current density due to the improved reaction kinetics. zhao et al. [65] developed sno2-cuo/graphene synthesized via a hydrothermal route composed of cuo nanorods uniformly loaded on graphene nanosheets. it maintained 584 mah g-1 at 0.1 c after 30 cycles. http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 18 agubra et al. [130] developed sno2/nio/c composite nanofiber by force spinning followed by a subsequent thermal treatment method called carbonization. this electrode directly acted as a working electrode without a current collector, binder and conducting additives. in this study, they compared sno2/nio/c and sn/c composites and confirmed that the sno2/nio/c composite delivered higher performance than sn/c. sno2/nio/c exhibited 677 mah g-1 specific capacity even after 100 cycles at 100 ma g-1 current density. yukun et al. [131] synthesized sno2-co3o4-c composite through the hydrothermal method followed by two-step ball milling by uniformly anchored sno2 and co3o4 nps on the graphite nanosheets. this composite exhibited a stable capacity of 842 mah g-1 even after 300 cycles at 0.2 a g-1 and retained a discharge capacity of 596.1 mah g-1 after 980 cycles at 1 a g-1. wang et al. [132] synthesized the ternary composite of sno2@c@mno2 through the reflux method. the structure is composed of the coating of mno2 nanosheet-based shell on sno2@c hollow nanospheres. the ternary composite sno2@c@mno2 exhibited a discharge capacity of 644.5 mah g-1 after 200 cycles at 100 ma g-1 current density and maintained 434.2 mah g-1 specific capacity at a current density of 1000 ma g-1. the better electrochemical performance was attributed to the rational design of hierarchical nanostructures. li et al. [133] developed sno2@mno2@graphite through facile ballmilling followed by a hydrothermal method. the graphite nanosheets contributed to good conductivity and minimized the volume expansion. the sno2@mno2@graphite exhibited a specific capacity of 1048.5 mah g-1, a superior rate capability of 522.2 mah g-1 at a current density of 5.0 a g-1 and maintained a stable long-life cyclic performance of 814.8 mah g-1 at 1.0 a g-1 even after 1000 cycles. along with the sno2 electrode material, moo3 and tio2 are good substituting metal oxides for achieving the improved electrochemical performance of libs. lamina sort of graphite incorporation enhanced the rate capability by reducing the volume expansion. unique structures of sno2-fe2o3 imposed on rgo sheets displayed high discharge capacity. sno2-mno2-graphite, sno2-moo3-c and sno2-tio2@rgo ternary composites delivered superior cyclic stability among the cited literature in table 6. table 6. electrochemical performance of the cdtmos associated with sno2 no. ternary composites synthesis methods initial discharge capacity, mah g-1; current density, ma g-1 or c-rate specific capacity, mah g-1 / cycles; current density, ma g-1 or c-rate rate capability, mah g-1, current density, ma g-1 or c-rate ref. 1 sno2-fe2o3/rgo precipitation 1179; 400 700/100; 400 139, 5 c [70] 2 sno2-fe2o3/rgo solvothermal 1509; 395/ 0.5 c 958/100; 0.5 c 530; 3950 [118] 3 sno2/fe3o4/rgo hydrothermal 947; 200 831/200; 200 50; 5000 [119] 4 sno2fe2o3/swcnts hydrothermal 1541; 200 692/50; 200 656; 2000 [120] 5 sno2-fe2ox-carbon electrospinning 1053; 100 756/55,100 540; 1000 [121] 6 sno2/zno/mwcnt sol-gel, spin coating 1287; 0.2 c 487/100; 0.2 c [122] 7 sno2-zno-rgo sol-gel 1702; 0.2 c 731/100, 0.2 c 150; 1c [74] 8 sno2-moo3-c hydrothermal, ball milling 2057.5; 200 1338.3/300; 200 715.08, 5000 [105] 9 sno2/nio @rgo hydrothermal 1280; 300 410.74/50, 300 [64] 10 sno2-tio2@rgo hydrothermal 2170; 200 1276/200; 200 611; 2000 [128] 11 sno2/tio2/c hydrothermal 1508.1; 200 895.3/70; 100 347.3, 3000 [129] 12 sno2-cuo-g hydrothermal 2490; 0.1 c 584/30; 0.1 c [65] 13 sno2/nio/c forcespinning and thermal treatment 1885; 100 677/100; 100 [130] 14 sno2-co3o4-c hydrothermal, ball milling 842/300; 200 596.1; 1000 [131] 15 sno2@mno2@c redox 1378; 100 644.5/200; 100 434.2; 1000 [132] 16 sno2@mno2@gaphite ball milling, hydrothermal 814.8/1000; 1000 522.2; 5000 [133] v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 19 electrochemical performance of zno and carbon derivatives based ternary composites zno is an electrode material with a promising theoretical capacity of 978 mah g-1 towards libs. zno has a band gap of 3.37 ev along with exciting properties such as good electron mobility, photoelectric response, exciton binding energy of 60 mev, chemical and thermal stability towards many applications [134]. the general electrochemical mechanism of zno electrode for libs is presented in equations (4) and (5). zno + 2li+ + 2e zn + li2o (4) zn + li+ + e lizn (5) zno undergoes a conversion reaction (equation 1) and, in addition, more li+ ions can be captured by alloying-dealloying reaction (equation 2). the materials which undergo both conversion and alloying-dealloying mechanisms are capable of delivering higher discharge capacity compared to those involving either one of the reaction mechanisms. both reactions are reversible [135]. besides, zno has several limitations, such as capacity fading issues upon cycling, low-rate capability and slow reaction kinetics [136]. there is a large volume change during discharge-charge cycling and accompanied by aggregation. moreover, a thin layer forms at the first cycle due to the volume variation of zno. to achieve high specific capacity and structural stability, highly conductive electrode materials are necessary [137]. different hierarchical architectures of nano-scaled particles could greatly increase electrochemical performance. binary and ternary composites provide more contact area for easy transportation of ions and decrease the degradation during the cycling process. carbon derivatives alleviate the volume expansion issues [138]. zno-based cdtmos electrode materials could extend the lifetime of the battery by shortening the li ions transportation path. ma et al. [29] fabricated the nio-zno/rgo composite by annealing and ultrasonic agitation. nio -zno nanoflakes were homogeneously dispersed on rgo sheets. this anode exhibited 1017 mah g-1 / / 200 cycles at 100 ma g-1, and a higher current density of 2000 ma g-1 delivered 185 mah g-1. this composite is a very good example of achieving higher rate performance by shortening the diffusion of li+ during redox reactions. in another work on the same composite by huang et al. [139], a ternary composite of zno-nio 3d flower-like mesoporous structure on graphene was developed by the hydrothermal method. this ternary composite demonstrated a higher discharge capacity of 452.7 mah g-1 at a current density of 300 ma g-1 after 50 cycles and maintained the coulombic efficiency of 99 %. this unique structure improved the li storage space for feasible transportation and increased the conductivity. he et al. [140] developed mesoporous foldable zno/geox/c ternary composite nanofibers with the proper distribution of geox and zno. due to the high surface area (532.56 m-2g-1) and mesoporous structure, carbon nanofibers exhibited the electrochemical discharge capacity of 1000 mah g-1 at a current density of 0.2 a g-1. xiaoli et al. [66] worked on the core-shell structure of a metal organic framework for the hybrid anode composite zno/znco2o4/c material synthesized through a hydrothermal route. abundant active sites created by the core-shell structure for the electrolyte penetration increase the contact area of electrode-electrolyte interfaces. this ternary composite exhibited 669 mah g-1 specific capacity at 0.5 ma g-1 current density after 250 cycles and maintained a high discharge capacity of 715 mah g-1 at 1.6 a g-1 current density. ma et al. [141] fabricated the hierarchical hollow structure of zno/znfe2o4/n-doped c micro polyhedrons through the self-sacrificial template method followed by calcination. n-doped carbon matrix increased the conductivity and the unique hollow structure exhibited a large discharge http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 20 capacity of 1000 mah g-1/100 cycles at 200 ma g-1 current density, conserving a very high capacity of 620 mah g-1 after 1000 cycles. zhao et al. [142] analyzed the composite zno/tio2/c of nanofibers structures prepared by electrospinning. this composite displayed the highest specific capacity of 912 mah g-1 at a current density of 100 ma g-1 even after 500 cycles and maintained a stable reversible capacity 294 mah g-1 at 1000 ma g-1. equal molar percentages of zn/ti (1:1) attributed to the synergistic effect of c and interface among zno and tio2. usually, core-shell structures of the metal-organic framework are composed of organic components with inorganic moieties bonded with covalent or other interactions capable of creating a novel huge porous structure with a very large surface area. nitrogen, sulfur or phosphor-doped carbon compounds increased the conductivity further, increase the li storage sites, and improve electrode/electrolyte wettability compared to undoped carbon compounds. table 7. electrochemical performance of the cdtmos associated with zno no. ternary composites synthesis methods initial discharge capacity, mah g-1; current density, ma g-1 specific capacity, mah g-1 / cycles; current density, ma g-1 rate capability, mah g-1; current density, ma g-1 ref. 1 zno-nio/rgo annealing and ultrasonication 1393; 100 1017/200; 100 185; 2000 [29] 2 zno-nio @rgo hydrothermal 1205; 300 452.7/50; 300 [139] 2 zno/geox/c electrospinning 464/500; 1000 [140] 3 zno/znco2o4/c hydrothermal 1278; 0.5 669/250; 0.5 715; 1600 [66] 4 zno/znfe2o4/n-doped c template method 1801; 100 1000/100; 200 620; 2000 [141] 5 zno/tio2/c electrospinning 931; 100 912/500; 100 294; 1000 [142] as per the listed ternary composites in table 7, the metal-organic framework of zno/znfe2o4/n-doped c hollow structure exhibited remarkable cyclic capacity and rate capability due to hierarchical hollow structure, a synergistic effect between the two active components and n-doped carbon matrix. electrochemical performance of moo3 and carbon derivatives based ternary composites molybdenum is a transition refractory metal and a promising battery electrode material due to low thermal expansion, high melting point, and good electrical and thermal conductivity. therefore it can sustain higher temperatures and possess wear resistance. compared to orthorhombic α moo3 and monoclinic β-moo3 polymorphs, metastable h-moo3 is more stable. h-moo3 is composed of an anisotropic zigzag pattern. the general electrochemical reaction mechanism of moo3 for libs is presented in equations (6) and (7). moo3 + xli+ + xe→ lixmoo3 (6) lixmoo3 + (6-x)li+ + (6-x)e mo + 3li2o (7) equations explain the intercalation and de-intercalation of li-ions into the moo3 matrix. the oxidation process of intercalating li-ions into layered moo3 takes place and the reverse mechanism occurs during the de-intercalation process. initially, a thin layer called solid electrolyte interface (sei) takes place and later forms bulk lixmoo3 to form mo metal and li2o composites by conversion mechanism [143,144]. zeng et al. [86] have fabricated the cnt/sio2/moo3 composite of cactus-like structure synthesized through in-situ carbonization of the self-assembly method. cnt/sio2/moo3 ternary composite delivered a specific capacity of 700 mah g-1/500 cycles at 1000 ma g-1. here higher capacity is attributed to the alleviation of strain by the unique structure contributed by each component. v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 21 deng et al. [145] anchored sno2-moo3 nps to cnts through hydrothermal and ball-grinding methods. sno2-moo3/cnt exhibited a specific capacity of 1372.2 mah g-1/280 cycles at 200 ma g-1. the sno2-moo3-cnt ternary composite displayed excellent cyclic capacity and rate capability. cao et al. [146] have decorated the ultrasmall wo2 nps for the carbon-coated moo3 nanorods by hydrothermal method. wo2 provided a high conductivity of 2.2 mω cm at room temperatures. moo3/wo2@c composite delivered a reversible capacity of 815 mah g-1/100 cycles at 0.05 c and an outstanding cyclic capacity of 80 mah g-1 after 5000 cycles at 1 c-rate. teng et al. [147] fabricated moo3-nio/graphene through a one-pot synthesis method. in this paper, moo3 nanosheets and nio nps, anchored homogeneously on the graphene layers, contributed to achieving a short diffusion path and reducing the volume expansion. moo3-nio/graphene exhibited a higher discharge capacity of 946.9 mah g-1/ 180 cycles at 1000 ma g-1. table 8. electrochemical performance of the cdtmos associated with moo3 no. ternary composites synthesis methods initial discharge capacity, mah g-1; current density, ma g-1 or c-rate specific capacity, mah g-1 / cycles; current density, ma g-1 or c-rate rate capability, mah g-1; current density, ma g-1 or c-rate ref. 1 moo3/sio2/cnt carbonization 1090; 100 450/150; 100 320; 5000 [86] 2 moo3-sno2-cnt hydrothermal and ball milling 1372.2/280; 200 743.6; 5000 [145] 3 moo3/wo2@c hydrothermal 869; 0.05 c 815/100; 0.05 c-rate 80; 1 c-rate [146] 4 moo3-nio/graphene one-pot method 1164/50; 100 946.9; 1000 [147] the highest remarkable cyclic capacity and rate capability have been exhibited by the moo3nio/graphene ternary composite impacted by low volume expansion, fast rection rate and short diffusion path; among the listed ternary composites based on moo3 (table 8). electrochemical performance of siox and carbon derivatives based ternary composites silicon (siox) is the active component for storing the high capacity towards libs. si is primarily desirable for ev applications because of its high density and theoretical volumetric capacity higher than graphite. si anodes exhibited a higher theoretical capacity of 3579 mah g-1. si might show four separate phases, li12si7, li7si3, li13si4, and li22si5, at high temperature (415 °c), compared to the si lithiation process which has two transition stages at room temperature. siox + 2xli+ + 2xe→ xli2o + si (8) siox + xli+ + xe→ 0.25xli4sio4 + (10.25x)si (9) siox + 0.4xli+ + 0.4xe→ 0.25xli2si2o5 + (1 0.4x)si (10) in the first lithiation process (equations 8, 9 and 10), a high concentration of li atoms is accumulated in the lixsi/si. importantly, only the amorphous si-li phase could be formed as per the concern of equilibrium phases, but it is kinetically hindered [148,149]. despite the concerted efforts, the inherent drawbacks of silicon for usage as an anode remain unaddressed for commercial implementation. first, silicon is a bad conductor of electricity. a low-cost carbon matrix solution could easily fix this. this strategy has been proved in several articles. this way helps to achieve electrical conductivity. reducing the silicon size to nanoscale shortens lithium-ion diffusion distance. gu et al. [150] reported siox@sno2@c ternary composite of microspheres by hydrothermal approach. this unique architecture exhibited a 796 mah g-1 specific capacity at 1000 ma g-1 after 300 cycles and achieved a high capacity of 515 mah g-1 at the higher current density of 4000 ma g-1. hu et al. [151] fabricated fe3o4@sio2@rgo ternary composite anode material by chemical etching. this composite displayed a 514 mah g-1 specific capacity at the higher current density of http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 22 5000 ma g-1 even after 1000 cycles. porous sio2 shell and rgo nanosheets enable the pseudocapacitance for obtaining fast discharge-charge cycles. wang et al. [80] prepared promising anode material composed of sio/ni/graphite through twostep ball-milling method. this ternary hierarchical composite fixes its volume expansion issue by increasing the utilization efficiency of sio. it exhibited a higher initial discharge capacity of 1331.5 mah g-1 and maintained a 522 mah g-1 stable capacity even after 50 cycles at a higher current rate of 1000 ma g-1. tan et al. [152] fabricated siox@tio2/c fibers composite by electrospinning method. this combination provided mechanical stability to enhance the conductivity. it exhibited a stable charge capacity of 855 mah g-1 at 100 ma g-1 current density even after 100 cycles and maintained 640.4 mah g-1 specific capacity at 1000 ma g-1 even after 100 cycles. jiang et al. [153] developed tio2/sio2/c film through an electrospinning approach. it exhibited a 380 mah g-1 specific capacity at 200 ma g-1 current density after 700 cycles. this combination of current collectors also lowers the electrode's weight and cost. table 9. electrochemical performance of the cdtmos associated with siox no. ternary composites synthesis methods initial discharge capacity, mah g-1; current density, ma g-1 specific capacity, mah g-1 / cycles; current density, ma g-1 rate capability, mah g-1; current density, ma g-1 ref. 1 siox@sno2@c hydrothermal 2186, 100 796/300, 1000 515, 4000 [150] 2 fe3o4@sio2@rgo chemical -itching 1630, 100 901/150, 100 514/1000, 5000 [151] 3 sio/ni/graphite ball milling 1332, 100 522/50, 1000 [80] 4 sio @tio2/c electrospinning 1125, 100 855/100, 100 640.4/200, 1000 [152] 5 tio2/sio2/c electrospinning 380.1/700, 200 115.5, 8000 [153] superior cyclic capacity and rate capability were exhibited by the fe3o4@sio2@rgo and siox@sno2@c ternary composite by increasing the conductivity and tuning the structures of ternary composites among the cited ternary composites based on siox (table 9). electrochemical performance of other mos and carbon derivatives based ternary composites other metal oxides of cobalt, titanium, nickel, copper, iron, manganese and other metal oxidesoriented graphene derivatives incorporated ternary composites are discussed in this section. the flexible matrix can be made with manganese oxide mn3o4. it is a good candidate for the accommodation of the volume change, low cost and can exhibit 900 mah g-1 specific capacity. mno2 introduces a high theoretical capacity of 1233 mah g-1 [154]. co3o4 exhibits a high theoretical capacity of 890 mah g-1 but also possesses a few disadvantages, including toxicity, high cost, and high operating voltage (2.2 to 2.4 v vs. li+/li) [155]. nio is a low-cost, abundant, environmentally benign electrode material with a theoretical capacity of 718 mah g-1 [156]. tio2 and fe2o3 electrode materials are popular due to their low cost and high discharge-charge capacities [157]. titanium oxide stores the li efficiently through insertion reaction and displays different crystal structures, including rutile, anatase, tio2 (bronze) and brookite [158]. cuo can store the high reversible capacity of 716 mah g-1, structural collapsing due to single metal oxide [159]. these single metal oxides suffer from large volume changes during redox reactions. combining with other metal oxides and carbon derivatives helps overcome the pulverization issues. here we discussed the few electrode materials for cdtmos. zuniga et al. [160] developed α-fe2o3/tio2/carbon composite fibers anode materials through centrifugal spinning and thermal processing. the α-fe2o3/tio2/c composite fibers exhibited a 340 mah g-1 after 100 cycles at a current density of 100 ma g-1 compared to tio2/c and fe2o3/c composite electrodes due to the high diffusion kinetics and structural stability of tio2 and fe2o3. v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 23 kaprans et al. [161] have electrophoretically deposited α-fe2o3/tio2/rgo. formed films were between 2-6 µm thick. they have performed comparative study between α-fe2o3/tio2/rgo and tio2/rgo and composites. proved that the composite α-fe2o3/tio2/rgo displayed the specific discharge capacity of 790 mah g-1 after 150 cycles at the current density of 100 ma g-1, coulombic efficiency of 66 % and exhibited 390 mah g-1specific capacity at 500 ma g-1 compared to other compounds. wang et al. [162] fabricated a co3o4/tio2/carbon composite using natural cellulose substrate through a sol-gel route to form a thin tio2 gel layer coating and hydrothermal method for the uniform deposition of co3o4 particles. this hierarchical nanostructure exhibited a porous structure with a high surface area, providing enough ions transportation path. the inner carbon nanofiber acts as a buffer layer, helps to relieve the strain during volume expansion and improves the efficiency of electrons movement. this composite delivered the initial discharge capacity of 1239 mah g-1 at a current density of 100 ma g-1 after 200 cycles, displayed a discharge capacity of 764 mah g-1and at a higher rate of 1 a g-1 exhibited a 348 mah g-1. zhang et al. [163] have designed coo/cuo/rgo ternary nanocomposites synthesized through a facile cost-effective method. anode material, the coo/cuo/rgo ternary composite, delivered a stable capacity of 1364.6 mah g-1 at a current density of 0.2 a g-1/ 100 cycles. it retained 423.5 mah g-1 even after 100 cycles at an increased rate capacity of 2000 ma g-1. wu et al. [164] designed co3o4/nio/c core/shell nanowire arrays by hydrothermal synthesis, chemical bath deposition and annealing carbonation methods. porous co3o4 acts as a core backbone on the developed nio nanoflakes. ternary composite exhibited 1050 mah g-1discharge capacity after 50 cycles at 0.5 c and maintained 769 mah g-1 specific capacity at the higher rate of 2 c compared to individual co3o4 and nio metal oxides due to the lowering of the polarization. table 10. electrochemical performance of the cdtmos associated with other metal oxides no . ternary composites synthesis methods initial discharge capacity, mah g-1; current density, ma g-1 or c-rate specific capacity, mah g-1 / cycles; current density, ma g-1 or c-rate rate capability, ah g-1; current density, ma g-1 or c-rate ref. 1 αfe2o3/tio2/c spinning and thermal processing 1832 340/100; 100 200, 500 [160] 2 αfe2o3/tio2/rgo electrophoretically deposited 765; 50 790/150; 100 390, 500 [161] 3 co3o4/tio2/c sol-gel and hydrothermal 1239; 100 764/200; 100 348, 1000 [162] 4 coo/cuo/rgo facile method 1732.4; 200 1364.6/100; 200 423.5, 2000 [163] 5 co3o4/nio/c hydrothermal, chemical bath deposition and carbonization 1426; 0.5 c 1053/50; 0.5 c 769, 2 c [164] according to table 10, the ternary composite of co3o4/nio/c displayed higher cyclic capacity as well as rate capability ascribed due to the core/shell structure of nanowire arrays. this novel architecture succeeded in lowering the polarization of the electrode. capacity fading mechanism in ternary composites li-ion battery capacity decreases gradually during cycling. capacity fading occurs due to many different reaction mechanisms. the reasons behind the capacity fading mechanism in the available literature towards libs are mainly due to: 1. the pulverization of the electrode materials happens due to increasing the internal resistance during the cycling process, which causes the barrier to the flow of charge carriers by current collectors. current collectors undergo passive film formation and corrosion. 2. irreversible redox reactions forming the solid electrolyte interface, self-discharge, and creating the passive layer on the electrodes. http://dx.doi.org/10.5599/jese.1470 j. electrochem. sci. eng. 00(0) (2022) 000-000 carbon based composite materials for li-ion batteries 24 3. many reaction mechanisms and the formation of unnecessary side reactions and undesirable phase changes. 4. decomposition of the electrolyte due to the increasing li content. overcharging also induces li deposition on the electrode. balamurugan et al. [165] reported the capacity fading mechanism nife2o4/sio2 aerogel anode prepared via the sol-gel method. they loaded a small amount of li2o to increase electrical conductivity. the initial discharge capacity of 930 mah g-1 gradually faded to 370 mah g-1 at the 50th cycle of discharging. they have explored the capacity fading mechanism by ex-situ x-ray photoelectron spectroscopy (xps) technique to identify the changing in the oxidation states during dischargeing/charging process. capacity fading began from the accumulation of irreversible lixsioy. it causes the loss of active li. cell resistance was increased during the 1st cycle to the 10th discharge cycle. ziv et al. [166] have investigated the reasons for capacity fading in li-ion cells. they have examined the cathode materials such as limn0.8fe0.2po4 (lmfp), lini0.5mn1.5o4 (lmno), li[lixniycozmn1−x−y−z]o2, li-rich layered oxides (hc-mnc) and li-rich layered oxides (hc-mnc). analysis of dissembled electrodes revealed that the main cause of capacity fading in li-ion battery full cells is the loss of active lithium ions due to parasitic side reactions. kim et al. [167] reported the fading mechanism of linico0.1mn0.1o2 in li-ion cells. in this work, they tested the pouch cell after 1, 100, 200 and 300 cycles. 16.3 % of the capacity was faded and the loss of the lithium source was confirmed by xrd. mechanical failure was confirmed by fe-sem. conclusion and future perspectives we provided a brief review of sno2, zno, moo3, siox and other important metal oxides with carbon derivatives based ternary composites for the lithium-ion battery application. we started with a discussion of various synthesis methods for synthesizing metal oxides, carbonaceous materials, and their combinations. carbonaceous materials include carbon, graphite, graphene oxide (go), reduced graphene oxide (rgo), and carbon nanotubes (cnts). they have gained attention for their enhanced electrochemical performance by incorporating metal oxides. we reviewed the importance of various synthesis methods of electrode materials by tailoring the experimental conditions, such as operating duration and temperature and choosing additional agents for the surface modification to explore the different shapes and sizes. we have discussed the reaction mechanisms of sno2, zno, moo3, and siox metal oxides during the intercalation-deintercalation of li-ion. then we discussed the sno2, zno, moo3, siox and carbon derivatives oriented ternary composites synthesis methodology, morphology, and electrochemical performance towards libs. ternary composites alleviate many drawbacks, such as initial capacity fading and volume expansion, with the synergistic effect of taking out the individual compound's advantages and enhancement of rate performance by carbonaceous additives. surveying the sno2-based ternary electrode materials revealed that the many drawbacks, including pulverization, and capacity fading of the individual sno2, can be greatly alleviated by the preparation of composite materials, especially with carbonaceous materials. composite materials incorporating carbonaceous materials tend to increase the conductivity of electrode materials and tailor the surface morphology, which can greatly impact the cyclic stability and rate capability. zno is good electrode material with few drawbacks, including capacity fading through slow reaction kinetics resulting in poor rate capability. carbon derivatives incorporated zno-based ternary composites resolved the individual drawbacks of zno by the synergistic effects between the compounds. correlation among the individual metal oxides and carbon derivatives plays a premier v. pavitra et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1470 25 role in improving the overall performance electrochemically. zno-based ternary composites provided a large interaction area, more reaction area, short diffusion kinetics and, reduced volume expansion, different synthesis methods to explore the various structures of zno-based ternary composites. in the reaction mechanism, reversible alloying-dealloying reactions have benefitted more in increasing the coulombic efficiency compared to the conversion mechanism. moo3-based composite ternary electrode materials have become potential electrode materials for libs due to the capability of storing a large amount of li. as per the literature, h-moo3 has a more beneficial polymorph version. remarkable electrochemical performance could be achievable by combining the other mos and carbonaceous materials. siox is a very good alloying anode material for high energy density libs. high concentrations of li atoms reversibly can incorporate with siox. siox anode fails to provide good cycle life. composite materials of silicon can become good candidates for libs. siox with fe3o4 and rgo ternary composite exhibited superior cyclic and rate performance. developing promising novel electrode materials by engineering their structure and combining them with carbonaceous materials could achieve superior electrochemical performance for libs. 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issn 1847-9286 www.jese-online.org review research developments in carbon materials based sensors for determination of hormones girish tigari, jamballi g. manjunatha, hareesha nagarajappa, nambudumada s. prinith department of chemistry, fmkmc college madikeri, mangalore university constituent college, karnataka, india corresponding author: manju1853@gmail.com; tel: +91 082 7222 833 received: august 20, 2021; accepted: september 27, 2021; published: october 8, 2021 abstract various carbon-based sensors (graphene, carbon nanotubes, graphite, pencil graphite, glassy carbon, etc.) have distinctive behavior and a broad range of importance for identifying sex hormones like estriol, estradiol, estrone, progesterone, and testosterone. the current review emphasizes voltammetric, amperometric, and electrochemical impedance spectroscopic methods for detecting some of these hormones. the existence, structural aspects, nature, and biological importance of each hormone were analyzed in detail and their analysis with different electroanalytical methods was considered. unique methodologies and innovations of electrochemical sensors for hormones based on carbon materials modified by different agents were examined. in this review, the interaction among various sensor materials and analytes in different supporting electrolyte media is premeditated. the most important significances of the electroanalytical methodologies were discussed based on sensor selectivity, sensitivity, stability, the limit of detection, repeatability, and reproducibility. keywords electroanalysis; estriol; estradiol; estrone; progesterone; testosterone. introduction the recent new materials based on carbon have fascinated the researchers of various fields of science and technology. thus, different carbon-based materials have already been developed as sensing platforms for various analytes using voltammetry techniques. the choice of analytes for voltammetric analysis are based on their electroactive nature and biological significance. hormones belong to cell-signaling molecules in multicellular organisms that transmit information between organs and tissues. they play a substantial involvement in controlling the functions and mechanisms of the living organisms and life activities like respiration, lactation, digestion, excretion, reproduction, sleep, growth development, sensory stimulation, and emotions [1-3]. hence, http://dx.doi.org/10.5599/jese.1094 http://dx.doi.org/10.5599/jese.1094 http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 12(1) (2022) 3-23 carbon based sensors for hormones 4 investigations of hormones are imperative. variety of analytical methods for hormone analysis were already reported, such as bioassay [4], immunoassay [5], high-performance liquid chromatography [6], capillary electrophoresis [7], spectrophotometry [8], etc. all these techniques give accurate results, but they require expensive instruments with complex methodologies, what considerably restricts their application [9-12]. electroanalytical methods, which are suitable for recognizing hormones observed in the human blood serum, urine, and plasma samples could overcome this problem. in addition, electroanalytical methods hold merits like rapid response time, easy operation, sensitivity, precision, accuracy, etc. [13-17]. as hormones have a significant role in human biochemistry and metabolism, assessing hormones is becoming a trending research. rapid monitoring of hormone levels is essential as they lead to major health issues [18,19]. so, the estimation of hormones became the focus of a broad scientific investigation. rapid testing of hormonal concentration in biological matrices is important. in this review paper, we address to some new advancements in the fabrication of electrodes for the analysis of hormones. carbon and its different forms occupy a special place in electroanalysis due to their unique properties like high mechanical and thermal stability, low resistance for electrons transfer, wide potential window, low price, and eco-friendliness. carbon and its derivatives such as graphite (3d/sp2), glassy carbon (3d/sp2-sp3), diamond (3d/sp3), carbon nanotubes (1d/sp2), graphene (2d/sp2), pyrolytic graphite (2d/sp2), carbon dots (0d/sp3), fullerenes (0d/sp2), amorphous carbon, pencil graphite, carbon black, carbon fibers, etc. were often used in electrochemical device assemblies. these carbon materials are abundant and low-cost but significantly improve the current/voltage characteristics in electrochemical studies because these materials possess very high surface area, low ohmic resistance, high mechanical stability, and biocompatibility [20-41]. carbonbased electrode materials have a broad spectrum of real-time applications for detection and estimation of molecules and ions. presently there are various reports of carbon-based electrochemical devices in investigations of vitamins [42-46], hormones [47-49], drugs [50-52], dyes [53,54], metal ions [55,56], pesticides [57,58], phenolic compounds [59,60], neurotransmitters [6163] etc., and even pathogens like viruses [64] and bacteria [65]. also, carbon-based materials have great application in the research of kinetics of electrochemical reactions and electronic states [66], supercapacitors, batteries [67], aerospace applications [68], different types of sensors [69], drugdelivery platforms, anticancer therapy, photothermal and photodynamic therapies, radiation treatment, bimolecular absorption [70] etc. this review focuses on the analytical performance of electrochemical sensors based on carbon materials for estriol, estrone, estradiol, progesterone, and testosterone hormones. abbreviations cv: cyclic voltammetry dpv: differential pulse voltammetry swv: square wave voltammetry lsv: linear sweep voltammetry gce: glassy carbon electrode swadsv: square wave adsorptive stripping voltammetry cpe: carbon paste electrode eis: electrochemical impedance spectroscopy lod: limit of detection loq: limit of quantification er: estriol g. tigari et al. j. electrochem. sci. eng. 12(1) (2022) 3-23 http://dx.doi.org/10.5599/jese.1094 5 ed: estradiol en: estrone pn: progesterone tn: testosterone gce: glassy carbon electrode spce: screen-printed carbon electrode bdde: boron doped diamond electrode co-poly (met): cobalt-poly (methionine) rgo-gnps-ps: reduced graphene oxide-gold nanoparticles-potato starch lac/rgo/sb2o5: reduced graphene oxide doped with sb2o5 film and with immobilized laccase enzyme cch/wge: carbamylcholine modified paraffin-impregnated graphite electrode pt/mwcnts: pt nanoclusters/multi-wall carbon nanotubes cnb-agnp: carbon black nanoballs/ silver nanoparticles fe3o4nps: ferrimagnetic nanoparticles rgo/agnps: reduced graphene oxide/ silver nanoparticles, sdsmcntpe: sodium dodecyl sulphate modified carbon nanotube paste electrode oxl -9mgpe: octoxynol-9 modified graphite paste electrode, rgo/agnws/agnps: reduced graphene oxide (rgo)/silver nanowires/silver nanoparticles rgo sbnps: reduced graphene oxide/antimony nanoparticles fe3o4 nps-bmi.pf6: magnetite nanoparticles/ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate cnts/pvi/ ito: carbon nanotubes/poly (vinylimidazole)/ indium titanium oxide vs2/aunps: vanadium disulfide nanoflowers and au nanoparticles rgo/cutthp: reduced graphene oxide and metal porphyrin complex cu-bdc/cpe: 1,4-benzenedicarboxylic and copper framework, mwcnt-nafion: multi-walled carbon nanotubes and nafion fetpypz: iron tetrapyridinoporphyrazine cupc-p6lc-nafion/spef: screen-printed sensor modified with cupc, printex 6l carbon and nafion film dhp: dihexadecylphosphate erg/aunp/ito: electro-reduced graphene and gold nanoparticle on indium tin oxide bpids: 1-butyl-3-[3-(n-pyrrole) propyl] imidazolium tetrafluoroborate cuo: copper (ii) oxide mwnt gnp/pge: multi-walled carbon nanotube gold nanoparticles modified graphite electrode ergo: electrochemically reduced graphene oxide fscpe: fused silica modified carbon paste electrode ctab nafion: cetyltrimethylammonium bromide/nafion mwnt-[bmim]pf6/gce: multi-walled carbon nanotubes/1-butyl-3-methylimidazolium hexafluorophosphate gce/nife2o4-mc: nife2o4 metal oxide/mesoporous carbon gox/aunp/cus: glucose oxidase/gold nanoparticles/copper sulfide nanoparticles a-ucppynt: carboxylate polypyrrole nanotubes aunps/cos: gold nanoparticles/cobalt sulfide nanoparticles lsge: laser-scribed graphene electrode mwnt-cr: multi-walled carbon nanotubes/congo red functionalized fe3o4np-bmi pf6: fe3o4 nanoparticles / ionic liquid 1-butyl-3-methylimidazolium hexafluoro-phosphate http://dx.doi.org/10.5599/jese.1094 j. electrochem. sci. eng. 12(1) (2022) 3-23 carbon based sensors for hormones 6 cch/wge: carbamylcholine modified paraffin-impregnated graphite electrode mip: molecularly imprinted polymers mab: monoclonal antibody bife: bismuth film-plated electrode, anti-prog-aucollantiprogesterone/gold nanoparticles-modified graphite-teflon -graphite-teflon: composite electrode fe3o4@gqds/f-cnt: fe3o4/graphene quantum dots/ functionalized carbon nanotubes pedot/zro2-nps: poly (3, 4ethylenedioxythiophene)/ zirconium oxide nanoparticles bsa/aptamer/gqds bovine serum albumin/ aptamer/graphene quantum nio-aunfs/f-mwcnts: dots/ functionalized multiwalled carbon nanotubes aunp/ambi: gold nanoparticles/ 5-amino-2-mercaptobenzimidazole hrp-p4-(p4)-anti-p4peroxidase labelled progesterone/anti-human progesterone capture -protein-g-mbs: antibody/protein g functionalized-magnetic microbeads hrp-p4-(p4)-cab peroxidase-labeled progesterone/ anti-human-lh-biotin capture -protein g-mbs: antibody/ protein g functionalized-magnetic microbeads go-imz: imidazole-functionalized graphene oxide acn: acetonitrile rigg/mab/: anti-progesterone monoclonal antibody /rabbit anti-sheep igg, mn(iii)-sb: mn(iii) schiff base film coox: cobalt oxide swnt modified eppge: single-wall carbon nanotubes modified edge plane pyrolytic graphite electrode anti-testosterone anti-testosterone-gold nanoparticles-carbon nanotubes-teflon -nau/mwcnts/teflon: composite electrodes mbs/abtes: protein a-functionalized magnetic beads/anti-testosterone ncd/silicon wafer/mip: nano-crystalline diamond/ silicon wafer/ molecularly imprinted polymer sa/bsa/binb16: streptavidin/ bovine serum albumin/ biotinylated nanobod electroanalysis of estriol at carbon based sensors estriol or oestriol (er) is a sort of female sex hormones (estrogens) that belong to steroids, secreted by the placenta [71]. er is the most dominant sex hormone in females. its release level is enriched during pregnancy periods [72,73]. therefore, er is essential for women's reproductive and sexual characters. its abnormal levels in the body leads to heart disorders, osteoporosis, hyperandrogenism, cancer, and urogenital diseases [74,75]. the central issue of er hormone is its chemical stability resisting sewage management which might lead to serious health risks to aquatic organisms [76,77]. therefore, it is necessary to develop fast and accurate clinical and environmental diagnostic methods. there are several literature reports on the electrochemical estimation of er at carbon-based electrodes. the probable er oxidation reaction at carbon-based electrode [90] is drawn in figure 1. in this regard, hareesha and manjunatha [78] examined the er by sodium dodecyl sulfate and electropolymerized xanthacridinum carbon nanotube and graphite composite paste electrode through different voltammetric methods in buffer solutions (ph 7.0). the studied concentration of er varied from 2.0 to 200.0 µm and 10.0 to 70.0 µm with the lod values of 0.29 and 0.19 µm, respectively. in addition, the stability of electrodes was studied by cycling 30 cv cycles with 95.5 % current retention. reproducibility was obtained for five measurements with rsd 3.42 % and repeatability for five measurements was obtained with an rsd of 2.57 %. for analytical application, the water samples were analyzed by standard addition method with 98.0-99.0 % recovery. g. tigari et al. j. electrochem. sci. eng. 12(1) (2022) 3-23 http://dx.doi.org/10.5599/jese.1094 7 figure 1. oxidation of er at carbon incorporated electrode manjunatha [79] has studied electrooxidation of er at poly(glycine) modified cpe in phosphate buffer, ph (6.0), using dpv and cv techniques. the suitable concentration range for er analysis was from 2.0 to 100 µm, with the lod of 8.7×10−7 m and good recovery in injection samples. in addition, stability was examined after 15 days for er sensing with 84 % current retention. furthermore, good reproducibility towards er detection was observed for five measurements with rsd of 4.75 %. charithra and manjunatha [80] have examined er on l-proline electropolymerized cpe using cv technique in 0.1 m phosphate buffer (ph 6.5). the linear range for er analysis was from 6×10-6 6×10-5 m with lod of 2.2×10-7 m and loq of 7.6×10-7 m. moreover, the method was stable with 92.87 % current retention even after 30 cv cycles of er sensing, and reproducible responses for er were observed with rsd of 4.75 %. also, simultaneous separation of er, folic acid, and ascorbic acid was achieved with the proposed methodology. in addition, ochiai et al. [81] effectively fabricated the spe surface modified with carbon nanotubes as an electrochemical sensor for the amperometric estimation of er in medicinal products. the authors defined a convenient concentration range from 1.0 to 1000 μmol l−1 with lod and loq of 0.53 and 1.77 μmol l−1, respectively. for comparison, the spectrophotometry method was also applied and the obtained results are found to be in agreement with 95 % confidence level. the results of some other already reported works [82-97] are tabulated in table 1. the tabulated er sensors yield good lod with real-time applications in pharmaceutical, biological and environmental samples. comparatively, xinet al. [85] observed the smallest lod of 0.00693 µm in clinical and water sample applications. however, the carbon fibers, pencil graphite, activated carbon-based electrodes are still to be explored for er sensing. table 1. analytical properties of some carbon-based sensors for er determination using various electrochemical techniques and real samples electrode technique er linear range, µm* lod, µm analytical application ref. co-poly(met)/gce dpv 0.596 9.90 0.034 pharmaceutical tablets and human urine [82] rgo-gnps-ps/gce lsv 1.5-22 0.48 water and urine samples [83] gce/lac/rgo/sb2o5 chronoamperometry 0.025 1.03 0.011 human urine [84] n-mwcnt/gonrs amperometry 0.34 69.35 0.00693 clinical and water samples [85] bdde swv 0.2 20.0 0.17 pharmaceutical and urine sample [86] http://dx.doi.org/10.5599/jese.1094 j. electrochem. sci. eng. 12(1) (2022) 3-23 carbon based sensors for hormones 8 electrode technique er linear range, µm* lod, µm analytical application ref. mwcnts/pt/gc swv 1.0 75.0 0.62 blood serum [87] cnb-agnp/gc dpv 0.2 3.0 0.16 creek water [88] cpe/fe3o4nps swv 3.0 110.0 2.6 pharmaceutical sample and artificial urine [89] rgo/agnps/gce dpv 0.1 3.0 0.021 tap water synthetic urine [90] ni-gce cv 5 100 0.1 [91] oxl -9mgpe cv 40 120 1.46 [92] rgo/agnws/ agnps/spce dpv 1 90 0.58 synthetic urine [93] gce/rgo sbnps dpv 0.2 1.4 0.0005 natural water [94] sdsmcntpe cv 6.0 20 and 25 150 0.32 et injection [95] fe3o4 npsbmi.pf6/cpe swv 1.0 10.0 0.3 pharmaceutical samples [96] pvi/cnts/ito dpv 2.0 15 0.090 serum [97] *linear concentration range of estriol detection electroanalytical estimation of estradiol estradiol/oestradiol (c18h24o2) is a steroid and the most important female hormone. it participates in the process of oestrous and is involved in menstrual reproductive cycles. estradiol (ed) is important for the growth of secondary women sexual characteristics such as the female-associated pattern of fat distribution, and widening of hips and the breasts. ed is also very much important in the protection of reproductive muscles such as the adulthood vagina during puberty, uterus, mammary glands, and pregnancy [98,99]. however, ed level in males is lower as compared to females. ed also originated in most crustaceans, vertebrates, fish and insects [100,101]. ed causes serious problems in premature puberty of children and also makes a risk in ovarian and breast cancer in women [102,103]. a deficiency of ed causes diseases such as heart diseases or menopausal symptoms and osteoporosis [104,105]. due to this risk factor in humans, the determination of ed is necessary. several literature reports are available for the detection of ed using voltammetric techniques, which are based on the possible electrooxidation reaction of ed [107] shown in figure 2. figure 2. possible electrooxidation of ed at carbon-based electrode chen and his research group [106] have analyzed ed by fe3o4-doped nanoporous carbon (fe3o4nc), which was made through the carbonization of fe-porous coordination polymer (fe-pcp). fe3o4https://en.wikipedia.org/wiki/steroid_hormone https://en.wikipedia.org/wiki/female https://en.wikipedia.org/wiki/sex_hormone https://en.wikipedia.org/wiki/estrous_cycle https://en.wikipedia.org/wiki/secondary_sexual_characteristic https://en.wikipedia.org/wiki/gynoid_fat_distribution https://en.wikipedia.org/wiki/gynoid_fat_distribution https://en.wikipedia.org/wiki/widening_of_the_hips https://en.wikipedia.org/wiki/breast https://en.wikipedia.org/wiki/reproductive_organ https://en.wikipedia.org/wiki/adulthood https://en.wikipedia.org/wiki/vagina https://en.wikipedia.org/wiki/puberty https://en.wikipedia.org/wiki/uterus https://en.wikipedia.org/wiki/mammary_gland https://en.wikipedia.org/wiki/pregnancy https://en.wikipedia.org/wiki/crustacean https://en.wikipedia.org/wiki/vertebrate https://en.wikipedia.org/wiki/fish https://en.wikipedia.org/wiki/insect g. tigari et al. j. electrochem. sci. eng. 12(1) (2022) 3-23 http://dx.doi.org/10.5599/jese.1094 9 nc was characterized with a scanning electron microscope (sem), powder x-ray diffraction (pxrd), x-ray photoelectron spectroscopy (xps), and raman spectroscopy. it was fabricated into an electrochemical sensor for the detection of ed in toner using britton-robinson buffer (ph 8.69) and concentrations range from 0.01 to 20 μm/l, with a detection limit of 4.9 nm/l were obtained. additionally, simultaneous determination of diethylstilbestrol and ed was performed and achieved diethylstilbestrol and ed 91.2-110 % recovery in toner samples. in addition, masikini et al. [107] studied the detection of ed in environmental samples (tap water and wastewater) by electrooxidation on multi-walled carbon nanotubes (mwcnt) and gold nanoparticles (aunp) modified glassy carbon electrode (gce) in 0.1 m phosphate buffer solution of ph 7.0, and obtained result of a dynamic range up to 20 % moll-1 and the value of lod was estimated to be 7.0×10-8 moll-1. additionally, the sensor retained 79 % of its initial response even after five days after storing at 4° c, and good reproducibility was observed for ed detection with rsd of 1.7 %. zaid et al. [108] successfully analysed ed by a sensitive impedimetric aptasensor based on a screenprinted electrode /carbon nanodots/76-mer aptamer. the prepared electrode was characterized by uv-visible absorption spectra, fluorescence spectra, and transmission electron microscopy. the detection limit of 0.5×10−12 m in a concentration range from 1.0×10−7 to 1.0×10−12 m was achieved, using eis. moreover, a stable and selective sensor was applied for determination of ed in river water samples with recovery rates of 92.3 and 101.2 %. yuan et al. [109] analysed ed by electrochemical sensor based on molecularly imprinted membranes at the platinum nanoparticles-modified electrode in phosphate buffer solution ph (6.86). under optimized conditions, dpv was applied for the determination of ed in the concentration range from 3.0×10−8 5.0×10−5 mol l−1 (r = 0.996) with the assessed lod of 1.6×10−8 mol l−1. acceptable stability was observed even after ten days with 97 % of 17-estradiol response retention. also, good reproducibility and repeatability of ed response were observed with rsd 2.9 and 2.3 %, respectively. the proposed method was utilized for ed detection in hospital wastewater with 93.9 % recovery. zhang et al. [110] determined the ed by a glassy carbon electrode (gce) which was modified by gold nanoparticles (aunps) and molecular imprinted polymer (mip) in phosphate buffer solution (pbs) (0.01 m, ph 6.97). the produced sensor was characterized by cv and eis. the proposed sensor exhibits a linear range from 1.0×10−12 to 1.0×10−7 mg/ml, and lod of 1.28×10−12 mg/ml under calibrated conditions. the stable and reproducible sensor was applied for ed detection in milk samples with a recovery of 84.7 102.9 %. analytical results from some other reported works [111134] are tabulated in table 2. comparatively, erg/aunp/ito [121] showed a low lod of 0.1×10−15 m with practical application in water and pharmaceutical samples. finding of new modifiers with good sensitivity is still a hot topic. table 2. analytical properties of some carbon-based sensors for ed determination using various electrochemical techniques and real samples electrode materials method ed linear range, m* lod, m real sample ref. gce dpv 4×10−5 1×10−3 1.21×10−5 tablet and serum [111] aptamer/aunps/vs2/gce dpv 1.0×10−11 1.0×10−8 1.0×10−12 human urine [112] rgo/cutthp/gce dpv 1×10−7 1.0×10−6 5.3×10−9 river water sample [113] cu-bdc/cpe dpv 5.0×10−9 6.5×10−7 3.8×10−9 water samples [114] mwcnt-nafion/gce swv 2.5×10−7 1.0×10−5 1.0×10−8 ---[115] fetpypz/cpe ----4.5×10−5 4.5×10−4 13.0×10−6 injection [116] cupc-p6lc-nafion/spef dpv 8.0×10−8 7.3×10−6 5.0×10−9 environmental and synthetic urine samples [117] rgo-dhp/gce lsv 4.0×10−7 2×10−5 7.7×10−8 synthetic urine [118] http://dx.doi.org/10.5599/jese.1094 j. electrochem. sci. eng. 12(1) (2022) 3-23 carbon based sensors for hormones 10 electrode materials method ed linear range, m* lod, m real sample ref. poly(l-serine)/gce lsv 0.1×10−6 30×10−6 20×10−8 human blood serum [119] al2o3/gce lsv 4.0×10−7 4×10−5 8.0×10−8 --[120] erg/aunp/ito cv 1×10−3 0.1×10−12 0.1×10−15 --[121] cnt ni(cyclam)-gc swv 5.0×10−7 4×10−5 60×10−9 human serum [122] bpids/gce dpv 1.0×10−7 1.0×10−5 5.0×10−8 [123] cuo/cpe swv 60×10−9 800×10−9 21×10−9 urine and buttermilk samples [124] mwnt gnp/pge lsv 7.0 10-8 4.2 10-5 1×10−8 blood serums [125] ergo/gce swv 1.0×10−15 -2.3×10−10 0.5×10−15 wastewater and pharmaceutical samples [126] fscpe dpv 0.1 ×10−6and 15.0 ×10−6 2.3×10−8 milk and pharmaceutical samples [127] ctab nafion/gce lsv 2.5×10−8 to 1.5×10−6 1.0×10−9 blood serum [128] mwnt-[bmim]pf6/gce 1.0×10−8 to 1.0×10−6 1.0×10−6 7.5× 10−6 5.0×10−9 rabbit blood serum and environmental water [129] gce/nife2o4-mc swv 20.0×10−9-56.6×10−8 6.88×10−9 tablet samples [130] gox/aptamer/aunps/ /gox/aunps/cus/gce dpv 5.0×10−13 to 5.0×10−9 6.0×10−14 urine samples [131] fe3o4@au-gsh/mips/gce dpv 0.025× 10−6 10× 10−6 2.76×10−9 pharmacological product [132] cdna/aptamer/aunps/ /cos/gce dpv 1.0×10−12 to 1.0×10−9 7.0×10−13 urine samples [133] lsge dpv 1.0×10−13 to 1.0×10−9 6.31×10−14 milk samples [134] *linear concentration range of estradiol detection electrochemical sensing of estrone estrone (e1) is also known as 3-hydroxyestra-1,3,5(10)-trien-17-one/oestrone. it is a minor women's sex hormone that plays a substantial role in female sexual growth and functions. estrone (en) is produced from gonads, adrenal androgens, and adipose tissue. it is used in menopausal hormone therapy and prostate cancer control, and is excreted through urine and feces. en is an endocrinedisrupting chemical that affects endocrine systems and can lead to carcinogenic effects, birth faults, and other growth issues [135-140]. so, its detection is important for human health and environmental concern. possible electrochemical oxidation of en at carbon electrode [144] is shown in figure 3. figure 3. oxidation of en at carbon incorporated electrode in this perspective, okina et al. [141] described a sensor for en, based on a glassy carbon electrode incorporated mwnts, functionalized with carboxylic groups. the reported sensor detects g. tigari et al. j. electrochem. sci. eng. 12(1) (2022) 3-23 http://dx.doi.org/10.5599/jese.1094 11 en at 0.59 v as an irreversible electrode process. swv results showed 2.5 times higher current of en oxidation reaction as compared to bare glassy carbon electrode. the proposed electrode offers a lower lod of 0.117 and loq of 0.392 μm l−1 with a sensitivity of 0.1521 μa/m l−1. moreover, the sensor electrode showed a good recovery (91 %) in seawater samples. in addition, chai et al. [142] described sequential ion-exchange and in-situ chemical reduction strategy synthesis of au nanoparticles ornamented bimetallic metal-organic framework, and its sensing application towards endocrine-disrupting chemical en. the prepared sensor material exhibits high sensing performance towords en. the authors showed a low lod of 12.3 nm in a linear range of concentrations from 0.05 5 μm with a sensitivity of 101.3 a m−1 cm−2. the results of some other literature reports [143-147] are listed in table 3. it is obvious that in comparison to other sensors, mip/cpe [147] showed significantly lower lod of 1.18×10−12 m with real-time application in pregnant mare urine. the carbon materials like pencil graphite, carbon dots, carbon fibers, fullerenes, pyrolytic graphite, etc., are yet to be explored. table 3. analytical properties of some carbon-based sensors for en determination using various electrochemical techniques and real samples sensor method en linear range, m/l* lod, m/l real sample ref. mwnt-cr/gce lsv 5.0×10−8 2.0×10−5 5.0×10−9 tablets [143] cpe/fe3o4np-bmi.pf6 swv 4.0× 10−6 9.0×10−6 & 9.0× 10−6 100.0× 10−6 4.7× 10−7 pork meat [144] cch/wge swv 0.3× 10−6 30.0× 10−6 0.10× 10−6 blood serum [145] cpe in presence ctab -9.0×10-8 8.0×10-6 4.0 x 10-8 tablets [146] mip/cpe cv 4.0×10−12 6.0×10−9 1.18×10−12 pregnant mare urine [147] *linear concentration range of estrone detection electrochemical determination of progesterone at carbon-based electrodes progesterone (pn) is an unsaturated α,β-ketone hormone derived from cholesterol, which is shaped by 21 carbon hydrophobic steroids framed by the corpus luteum in the ovary during pregnancy [148-150]. pn hormone has a vital contribution to pregnancy maintenance, synthesis of sex hormones, cognitive development, monthly menstrual cycle, growth of breast, etc. the imbalance of pn can cause severe problems in the body. it forms infertility and abnormality of the reproductive system. in humans, it causes the secretion of gonadotropin-releasing hormone (gnrh), which may give rise to a decline of released testosterone and affect male behavior. therefore, it is necessary to determine pn in mammals for clinical diagnosis [151,152]. there are several literature reports on the voltammetric determination of pn using carbon-based electrode. possible reduction process of pn at carbon-based electrode [157] is schematically presented in figure 4. in this perspective, naderi and jalali [153] have successfully determined pn at glassy carbon electrode modified with mwcnt, au nanoparticles, and poly-l-serine. it was characterized by fesem, energy dispersive x-ray spectroscopy (eds), cv, and eis techniques. the modified electrode showed improved current response as compared to the bare electrode by lowering overpotential. under optimized conditions, sensor exhibits lower lod of 0.2 nm (0.063 ng ml−1) in a concentration range of 0.001 2.0 μm (0.31 to 636 ng ml−1) using pbs buffer 0.1 m with ph (7.4). a reproducible and stable sensor was utilized for pn in pharmaceutical and blood serum samples. esmaeili et al. [154] have successfully examined determination of pn using a gadolinium(iii) tungstate nanoparticles modified carbon paste electrode in 0.1 m britton robinson buffer (brb) solution at ph 11.5, using fast fourier transformed swv technique. under optimized conditions, the sensor detects pn in the concentration domain of 0.1 to 1.0 μm (31.45-314.47 ng ml−1) with http://dx.doi.org/10.5599/jese.1094 j. electrochem. sci. eng. 12(1) (2022) 3-23 carbon based sensors for hormones 12 acceptable sensitivity of 485.64 a m−1 and lod of 50 nm (15.72 ng ml−1). the stable and selective sensor was used for estimation of pn in human blood plasma with a recovery of 103.7 %. figure 4. reduction of pn at carbon-based electrode analytical results of some other sensors from already reported works [155-169] are tabulated in table 4. reported sensors provide lower lod values with practical applicability in complicated matrices such as serum, urine, and milk samples. comparatively, gce/mn(iii)-sb [169] offers lod of 0.00000314 ng ml−1. table 4. analytical properties of some carbon-based sensors for pn determination using various electrochemical techniques and real samples electrode technique pn linear range, ng/ml* lod, ng/ml real sample ref. microfluidic immunosensor amperometry 0.5 12.5 0.2 serum [155] mab spces amperometry 1.56 15.75 0.315 cow milk [156] ex situ bife swadsv 125 2485 56.61 pharmaceutic products [157] anti prog aucoll graphite teflon electrode amperometry 0 30 0.84 milk [158] fe3o4@gqds/f-cnt/gce dpv 3.15 945 0.63 serum and commercial ampoules [159] pedot/zro2-nps/gce dpv 0.314 1886.8 0.102 human blood serum & pharmaceutical products [160] bsa/aptamer/gqds nioaunfs/f-mwcnts/spce dpv 0.00314 314.46 0.00058 [161] aunp/ambi/rgo/spce swv 0.28 to 8490.57 88.0 [162] hrp-p4-(p4)-anti-p4protein-g-mbs/spce amperometry 0.02 100 0.005 saliva [163] hrp-p4-(p4)-cab-protein gmbs/spce amperometry 0.01 1000 0.0017 saliva [164] go-imz/gce swv 69.184402.52 20.13 pharmaceutical samples [165] acn /gce cv and swv 1257.86-314465.41 157.23 [166] rigg/mab/screen printed carbon electrodes cv 0-5 --cow's milk [167] sn-modified gc dpv 1572.3 25156.8 35.74 pharmaceutical commercial samples [168] gce/mn(iii)-sb cv 3.1445 10-6 31.446 10-6 3.1445 10-6 milk [169] *linear concentration range of progesterone detection g. tigari et al. j. electrochem. sci. eng. 12(1) (2022) 3-23 http://dx.doi.org/10.5599/jese.1094 13 electrochemical determination of testosterone testosterone/7β-hydroxy-4-androsten-3-one pleiotropic hormone plays a substantial role in human health. it is predominantly produced in men from testes and adrenalin glands, and has a vital role in the growth of the testes and prostate. it is also involved in the development of muscles, bones and stimulates sexual desire. abnormal testosterone (tn) concentration in the body may lead to hypogonadism, prostate cancer, metabolic syndrome, depression, obesity, anxiety, cardiovascular diseases, memory loss, hair loss, loss of muscle mass, etc. tn has also been abused for enhanced sports performance, but the world anti-doping agency (wada) prohibited its use. so, it is important to have a fast and accurate analytical method for tn detection [170-178]. possible mechanism of electroreduction reaction of tn [179] at carbon electrode is shown in figure 5. in this regard, levent et al. [179] reported a bismuth-film coated glassy carbon electrode for the determination of tn in britton-robinson buffer, ph 5.0, containing 3 mmol l−1cetyltrimethylammonium bromide. tn showed an irreversible, adsorption-controlled reduction peak at the electrode. the authors obtained lod of 0.3 nm. the electrode responses were reproduced and repeated with rsd not exceeding 5 %. finally, a sensor was utilized for the recognition of tn in medicinal and bio-samples. bulut et al. [180] introduced a sensor based on poly(benzenediamine-bis[(2-ethylhexyl) oxy]benzodithiophene)/testosterone antibodies via glutaraldehyde/screen-printed carbon electrode for the determination of tn. the surface morphology of the modified electrode was studied with atomic force microscopy. the modifications in the exterior topography due to tn binding were inspected through electrochemical techniques. the amperometric studies were conducted to measure tn in the range of 10 500 ng/ml with lod of 17 ng/ml. finally, a sensor was utilized for the analysis of tn in synthetic urine (recovery 103.4 ± 1.0 and 98.0 ± 5.3 %) and serum samples (113.8 ± 1.1 and 105.6 ± 2.2 %). additionally, repeatability of electrode response towards tn was studied by 10 measurements, showing rsd of 0.433 %. figure 5. possible mechanism of tn electroreduction at carbon electrode liu et al. [181] developed a sensor for tn based on nanosized molecularly imprinted polymer (mip) film that was electrochemically grafted on graphene oxide sheets modified electrode. measurement of tn was performed using eis technique in the range 1.0 fm to 1.0 μm with lod of http://dx.doi.org/10.5599/jese.1094 j. electrochem. sci. eng. 12(1) (2022) 3-23 carbon based sensors for hormones 14 0.4 fm. moreover, stability was good with 93.4 % retention of the initial response of electrode towards tn even after 30-day storage at room temperature. also good reproducibility and repeatability were observed with rsd not exceeding 5 %. practical application of sensor was carried in human serum samples with recovery 98.6 to 104.2 %, and rsd not exceeding 5 %. data in table 5. [182-190] shows the comparison between different electrochemical sensors for tn. results given by liu et al. [181] for molecularly imprinted polymer /electrochemically grafted on graphene-oxide sheets modified electrode provide the lowest lod of 0.4 fm. table 5. analytical properties of some carbon-based sensors for tn determination using various electrochemical techniques and real samples electrode technique tn linear range, m* lod, m real sample ref. gce/coox cv 0.33 2.00 × 10-6 1.6×10-7 --[182] surfactant/glassy carbon swv 10 – 70 × 10-9 1.18×10-9 human urine [183] swnt modified eppge swv 5 – 1000 × 10-9 2.8×10−9 female urine [184] rgo/gce dpv 2.0 – 210.0 × 10-9 0.1×10-9 human plasma & urine [185] anti-testosteronenau/mwcnts/teflon amperometry 0.4 and 34.67 × 10-9 0.29×10-9 human serum [186] spce/mbs/abtes amperometry 0.0174 – 173.35 × 10-9 0.0059×10-9 human serum [187] ncd/silicon wafer/mip eis 0.5 – 20×10-9 0.5×10-9 human urine &saliva [188] gce/sa/bsa/binb16 eis 0.1734 – 17.336×10-9 0.156×10-9 human serum [189] cuo/ceo2/gce electrochemical (i v) approach 0.01 – 0.01 × 10-3 9.30×10-12 human, mouse, and rabbit serum [190] *linear concentration range of testosterone detection future perspectives in the last decade, many carbon-based sensors and biosensors for the determination of hormones have been probed. however, the simultaneous determination of sex hormones seems to be a challenge manifested in a significantly lower number of literature reports. hormones like pregnenolone, allopregnanedione, allopregnanolone, 17α-hydroxy pregnenolone, 17α-hydroxyprogesterone, dehydroepiandrosterone, androstenedione, androstanedione, androsterone, androstenediol, dihydrotestosterone, androstanediol, 2-hydroxyestrone, 16α-hydroxyestrone, 2-hydroxyestradiol, and estetrol are still waiting to be analyzed electrochemically. carbon materials like activated carbon, carbon derived from biomass, carbon fibers, pencil graphite electrode, pin-based electrodes, fullerenes, carbon nanohornes, graphyne, carbon nanomaterials, etc. could be possibly utilized to achieve this goal. breakthrough in analytical methods with new materials is always important in the field of analytical science and technology. conclusions electroanalytical methods for biomolecular diagnosis have increased widely in recent years, especially for hormone determinations. electrochemical techniques are used for the detection of various hormones because of their excellent response, easy instrumentation, minimal sample pretreatment criteria, rapid and satisfactory sensitivity, and low cost. recent developments include use of graphene, graphite, carbon paste, carbon nanotubes (multi-walled, single-walled, etc.), glassy carbon electrodes, and other combinations, mostly due to their high effective surface area and distinct electrochemical properties. also, these materials substantially improve analytical signals, decrease overpotentials of hormone oxidation or reduction, and solve peak overlapping problems g. tigari et al. j. electrochem. sci. eng. 12(1) (2022) 3-23 http://dx.doi.org/10.5599/jese.1094 15 in complex samples. the characteristics of the sensor constituents and functionalized groups and their interfaces with analyte molecules greatly stimulates degree of electrooxidation/reduction, ph, pka of the background electrolyte, and peak voltage and current. the interface and degree of electrooxidation/reduction depend extremely on the following aspects: alteration of the sensor assembly, pre-treatment of the electrode, exterior functionalities, ph, background electrolyte, and presence of other molecules. for determining the electrooxidation or reduction rate and electrode efficiency, the communication (reaction) between the respective type of analyte (targeted molecule) present at some ph value (range) and the oxygen functionalities and other groups on the sensor are found to be vital. references [1] s. hiller-sturmhöfel, a. bartke, alcohol health research world 22(3) (1998) 153-164. 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materials in electronics 32 (2021) 5259-5273. https://doi.org/10.1007/s10854-021-05257-2 ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1094 https://doi.org/10.1016/j.aca.2009.10.035 https://doi.org/10.1016/j.msec.2015.12.005 https://doi.org/10.1002/elan.201000419 https://doi.org/10.1016/j.bios.2010.07.060 https://doi.org/10.1016/j.bios.2018.07.032 https://doi.org/10.1021/acsami.6b04658 https://doi.org/10.1007/s10854-021-05257-2 https://creativecommons.org/licenses/by/4.0/) {synthesis and antioxidant activity of six novel n-ferrocenyl-methyl-n-(nitrophenyl)and -n-(cyanophenyl)-acetamides: cyclic voltammetry and molecular docking studies:} http://dx.doi.org/10.5599/jese.1162 293 j. electrochem. sci. eng. 12(2) (2022) 293-304; http://dx.doi.org/10.5599/jese.1162 open access : : issn 1847-9286 www.jese-online.org original scientific paper synthesis and antioxidant activity of six novel n-ferrocenylmethyl-n-(nitrophenyl)and -n-(cyanophenyl)-acetamides: cyclic voltammetry and molecular docking studies abdellatif kedadra, touhami lanez, elhafnaoui lanez, hadia hemmami and meriem henni vtrs laboratory, department of chemistry, faculty of sciences, university of el oued b.p.789, 39000, el oued, algeria corresponding author: touhami-lanez@univ-eloued.dz; received: november 8, 2021; accepted: december 28, 2021; published: january 25, 2022 abstract cyclic voltammetry (cv) assays were performed to measure superoxide anion radical (o2-) scavenging activities of six novel n-ferrocenylmethyl-n-(nitrophenyl)-acetamides and n-ferrocenylmethyl-n-(cyanophenyl)acetamides (fma1-fma6), followed by molecular docking simulations and in silico toxicity prediction. the obtained values of ic50 from cv assays indicated that all studied compounds showed promising scavenging activity against o2 radicals, with the compounds fma1, fma3, and fma4 possessing the most significant potency. a molecular docking study revealed that all compounds interact with amino acid residues of glutathione reductase via hydrogen bonding and hydrophobic interactions. the compound fma4 was the most inactive compound against the glutathione reductase enzyme having the highest inhibitory concentration of 2.61 µm and the lowest docking score of -31.85 kj/mol. toxicity studies demonstrated that among six studied compounds, fma4, fma5, and fma6 are predicted to be nontoxic. keywords ferrocene derivatives; superoxide anion radical; binding free energy; toxicity study introduction recently, ferrocene derivatives have been widely studied because of their versatile potential applications in many research fields such as medicinal chemistry [1–7], material sciences [8], and diagnostic applications fields [9]. n-ferrocenylmethylamines and their derivatives, in particular, n-ferrocenylmethyl-n-(nitrophenyl)and -n-(cyanophenyl)aniline, have shown important biological activities due to their promising biological potential as anticancer properties [10,11], antimicrobial agents [12], antioxidants [13,14], and anti-proliferative agents against mcf-7 human breast cancer cell lines [15]. http://dx.doi.org/10.5599/jese.1162 http://dx.doi.org/10.5599/jese.1162 http://www.jese-online.org/ mailto:touhami-lanez@univ-eloued.dz j. electrochem. sci. eng. 12(2) (2022) 293-304 synthesis and activity of six novel acetamides 294 most of ferrocenylmethylaniline derivatives scavenge superoxide free radicals, and this reaction is useful in the inhibition of cancer growth [16]. the superoxide radical scavenging activities of ferrocenylmethylanilines are mainly due to the presence of functionalized aniline in the ferrocene moiety that could donate protons to the superoxide anion radical to form the corresponding radical species. the mechanism might be described by a first proton transfer (eq. (1)), followed by the electron transfer (eq. (2)). with a second proton transfer (eq. (3)) the overall reaction is a two-electron reduction of oxygen (eq. (4)). in our experimental conditions with ferrocene derivatives, we did not observe the increase of oxygen current reduction, which is expected in the hypothesis of a proton donating mechanism [17]. o2+ fch → ho2 +fc (1) ho2 + o2→ ho2+ o2 (2) ho2+ fch → h2o2 +fc (3) o2+ 2e +fch → h2o2 + 2fc (4) glutathione, a tripeptide protein naturally produced by the body, is made up of three amino acids: glutamic acid, cysteine, and glycine. it plays an essential antioxidant intracellular role [18], since it is involved in the elimination of reactive oxygen species and acts as a scavenger for various oxygen radicals. the enzyme glutathione reductase (gr), also called glutathione-disulfide reductase, reduces the oxidized form of glutathione disulfide (gssg) to the reduced glutathione form (gsh). elevated levels of gssg/gsh ratio lead to intracellular signal transduction, elimination of free radicals and reactive oxygen species, and the preservation of intracellular redox status [19]. thus, inhibition of glutathione reductase results in a decrease in gsh, an increase in gssg, and consequently a high gssg/gsh ratio. studying the glutathione reductase inhibition by potentially antioxidant compounds could serve for choosing antioxidants candidates. a good antioxidant candidate should inhibit glutathione reductase enzyme less. in this work, we describe the synthesis and the scavenging activity against o2of six novel n-ferrocenylmethyl-n-(nitrophenyl)and n-ferrocenylmethyl-n-(cyanophenyl)acetamides using cyclic voltammetry assays. further, the compounds were scrutinized through toxicity study and molecular docking to predict the median lethal dose (ld50) and the toxicity class (tc) to afford an insight into the inhibition and binding partialities of the most potent compounds with glutathione reductase. experimental chemicals all starting materials and solvents used for the synthesis of n-ferrocenylmethyl-n-(nitrophenyl) and n-ferrocenylmethyl-n-(cyanophenyl)acetamides were of analytical grade and obtained from different commercial sources and used as received. tetrabutylammonium tetra-fluoroborate (bu4nbf4) (electrochemical grade 99 %) was from sigma-aldrich. oxygen gas (research-grade 99.99 %) was supplied by linde gaz algérie. materials and methods cv experiments were run on a pgz301 potentiostat (radiometer analytical sas, france) connected to an electrochemical cell having a volume of 15 ml and equipped with three electrodes: a glassy carbon working electrode of an area equal to 0.013 cm2, a platinum wire auxiliary electrode, and a hg/hg2cl2 reference electrode. the reaction medium was saturated with high-purity commercial oxygen for 15 min before each experiment. 1h nmr spectra were obtained on a bruker a. kedadra et al. j. electrochem. sci. eng. 12(2) (2022) 293-304 http://dx.doi.org/10.5599/jese.1162 295 avance dpx 300 mhz spectrometer. all 1h nmr spectra are reported in ppm relative to the central line of the singlet for cdcl3 at 7.28 ppm. structure optimization was run using density functional theory implemented in gaussian 09 package [20]. all calculations were carried out with the unrestricted becker’s three-parameter hybrid exchange functional [20] combined with lee-yang-parr nonlocal correlation function, abbreviated as b3lyp [22–24] with combined basic sets, lanl2dz [25–27] for optimizing iron atom and 6311g+(d) for the rest of atoms [28–30]. the in silico toxicity study was performed using the protox-ii web server [31]. molecular docking simulations were performed using autodock 4.2 docking software [32,33], executed on a pentium 2.7 ghz and 8 gb ram microcomputer mb memory with windows 8 operating system. synthesis n-ferrocenylmethyl-n-(nitrophenyl)and n-ferrocenylmethyl-n-(cyanophenyl)acetamides coded as fma1-fma6 were synthesized by coupling the well-known quaternary salt (ferrocenylmethyl)trimethyl-ammonium iodide [34] with the corresponding substituted anilines, following our previously reported procedure [35,36]. the obtained products were then acetylated using acetic anhydride and their molecular structures are shown in figure 1. the antioxidant activities of the synthesized compounds against superoxide anion radicals were measured using cv assays. fma1, x = 2-no2; fma2, x = 3-no2; fma3, x = 4-no2; fma4, x = 2-cn; fma5, x = 3-cn; fma6, x = 4-cn figure 1. chemical structures of n-ferrocenylmethyl-n-(nitrophenyl)acetamide and n-ferrocenylmethyl-n-(cyanophenyl)acetamide general procedure for the synthesis of compounds (fma1-fma6) the corresponding n-ferrocenylmethyl-n-(nitrophenyl)aniline (500 mg, 1.49 mmol) or n-ferrocenylmethyl-n-(cyanophenyl)aniline (500 mg, 1.58 mmol) was dissolved in acetic anhydride 160 ml (1.7 mmol) and the reaction mixture was heated at 65 °c for 30 minutes under an atmosphere of nitrogen. then it was allowed to cool to room temperature and poured on 140 ml 0.1 m of an aqueous solution of sodium carbonate and extracted three times with dichloromethane. the combined extracts were evaporated and the obtained residue was recrystallized from a mixture of methanol/water (30/70) to yield a red/orange yellowish solid. results and discussion antioxidant activity assays against superoxide anion radicals superoxide anion radicals (o2-) scavenging activity assays were used to measure the antioxidant activity of n-ferrocenylmethyl-n-(nitrophenyl)and n-ferrocenylmethyl-n-(cyanophenyl)-acetamides. the o2was electrochemically generated in situ by one-electron reduction of commercial molecular oxygen dissolved in dmf containing 0.1 m tetrabutylammonium tetra-fluoroborate (bu4nbf4) as a supporting electrolyte. increased concentrations of each studied compound were then added to the electrochemical cell containing a solution of the generated o2-, and the cyclic voltammograms were recorded after each addition of the test compounds, in the potential window http://dx.doi.org/10.5599/jese.1162 j. electrochem. sci. eng. 12(2) (2022) 293-304 synthesis and activity of six novel acetamides 296 from −0.0 to −1.6 v at the scan rate of 0.1 v s-1. obtained voltammograms of oxygen-saturated dmf containing 0.1 m of bu4nbf4 in the absence and presence of gradually increasing concentrations of the compounds fma1-fma6 in the same solvent are shown in figure 2. -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 0 5 10 15 20 25 (i 0 -i )/ i 0 c / mm 00 1.04 mm 1.54 mm 2.03 mm 2.52 mm 2.99 mm 3.06 mm 3.43 mm 3.88 mm fma1 i /   c m ² e / v y = 5.247x 2.3685 r 2 = 0.975 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -600 -400 -200 0 200 400 0 1 2 3 4 5 6 7 8 4 6 8 10 12 14 (i 0 -i )/ i 0 c / mm 00 0.785 mm 1.555 mm 2.310 mm 3.051 mm 3.777 mm 5.876 mm 7.211 mm r 2 = 0.976 y = 1.4529x + 3.5928 fma2 i /   c m ² e / v -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -800 -600 -400 -200 0 200 400 0 1 2 3 4 5 -2 0 2 4 6 8 10 12 14 16 (i 0 -i )/ i 0 c / mm 00 0.52 mm 1.04 mm 1.54 mm 2.52 mm 2.99 mm 3.46 mm 3.92 mm 4.37 mm 4.81 mm fma3 i /   c m ² e / v y = 6.416x + 0.4686 r2 = 0.954 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -600 -400 -200 0 200 400 0 1 2 3 4 5 -2 0 2 4 6 8 10 12 14 16 (i 0 -i )/ i 0 c / mm 00 0.55 mm 1.09 mm 1.63 mm 2.66 mm 3.16 mm 3.56 mm 4.14 mm 4.61 mm 5.08 mm fma4 i /   c m ² e / v y = 3.482x 2.7505 r 2 = 0.954 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -600 -400 -200 0 200 400 0 1 2 3 4 5 6 0 2 4 6 8 10 12 14 (i 0 -i )/ i 0 c / mm 00 0.47 mm 1.38 mm 1.82 mm 2.26 mm 3.51 mm 3.92 mm 4.31 mm fma5 i /   c m ² e / v r 2 = 0.878 y = 1.5336x + 0.342 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -600 -400 -200 0 200 400 0 2 4 6 8 10 12 0 5 10 15 20 25 (i 0 -i )/ i 0 c / mm 00 0.59 mm 1.75 mm 2.86 mm 3.93 mm 4.96 mm 5.46 mm fma6 i /  c m ² e / v y = 2.2389x + 0.514 r 2 = 0.987 figure 2. cyclic voltammograms of oxygen-saturated dmf in the absence and presence of gradually increased concentrations of fma1-fma6. inset plots of [(i0 – i) / i0 vs. c] all voltammograms showed a decrease in the anodic peak current density of the o2-/o2 redox couple upon addition of gradually increasing concentration of test compounds, and this decrease was used for the calculation of the half-maximal inhibitory concentration (ic50). the inhibition of o2was calculated using the following equation [17,37,38] o2 scavenging activity = 0 0 100 i i i − (5) a. kedadra et al. j. electrochem. sci. eng. 12(2) (2022) 293-304 http://dx.doi.org/10.5599/jese.1162 297 where i0 and i are the anodic peak current densities of the superoxide anion radical in the absence and presence of the test sample, respectively. the half-maximal inhibitory concentration (ic50) values were obtained from the plot of o2-scavenging activity against different compound concentrations (inset plots of figure 2). the antioxidant activity has been expressed as ic50. the ic50 value was defined as the concentration of the compound that inhibits the formation of o2by 50 %. obtained values of ic50 shown in figure 3 indicate that all n-ferrocenylmethyl-n-(nitrophenyl) and n-ferrocenylmethyl-n-(cyanophenyl)acetamides showed promising scavenging activity against o2radicals, with the activity of compounds fma1, fma3, and fma4 almost comparable to that of the standard α-tocopherol (7.058 mm) used as a positive control. figure 3. ic50 values of n-ferrocenylmethyl-n-(nitrophenyl)and nferrocenylmethyl-n-(cyanophenyl)acetamides (fma1-fma6) and αtocopherol as a positive control molecular docking study a molecular docking study was carried out to afford an insight into the inhibition and binding parameters of n-ferrocenylmethyl-n-(nitrophenyl)and n-ferrocenylmethyl-n-(cyanophenyl)-acetamides with the enzyme glutathione reductase. the enzyme glutathione reductase (gr), also called glutathione-disulfide reductase, reduces glutathione disulfide (gssg) to glutathione (gsh), which is involved in the elimination of reactive oxygen species and acts as a scavenger for various oxygen radicals. glutathione exists in reduced (gsh) and oxidized (gssg) forms, the reaction symbolized by the equation (6): 2ghs + nadp+  gssg + nadph + h+ (6) inhibition of glutathione reductase results in a decrease in reduced glutathione (gsh) and an increase in glutathione disulfide (gssg) using nicotinamide adenine dinucleotide phosphate (nadph), particularly from the pentose phosphate pathway in bacteria, plants, and animals to regenerate glutathione, a molecule essential for resistance against oxidative stress and the preservation of intracellular ph. so, studying glutathione reductase inhibition could serve as a good means for the selection of antioxidants candidates. a good antioxidant candidate should reduce the inhibition of the glutathione reductase enzyme. rigid receptor and flexible ligand molecular docking models were carried out to study the inhibition of glutathione reductase by n-ferrocenylmethyl-n-(nitrophenyl)and n-ferrocenylmethyl-n-(cyanophenyl)acetamides, and to understand how strong the interactions are between them. the crystal 3d structure of the glutathione reductase target involved in this study was retrieved from the online data bank, rcsb pdb (https://www.rcsb.org/pdb, id: 1xan) [40], figure 4. ic 5 0 / m m http://dx.doi.org/10.5599/jese.1162 https://www.rcsb.org/pdb j. electrochem. sci. eng. 12(2) (2022) 293-304 synthesis and activity of six novel acetamides 298 figure 4. ucsf chimera ribbons chemical structure view of human glutathione reductase in complex with a xanthene inhibitor (id: 1xan) receptor preparation the receptor was first imported into the autodocktools interface. missing atoms were inserted in incomplete residues, alternate conformations were deleted, all water molecules and ligands were removed, and polar hydrogen atoms and charges were added to the receptor structure. receptor-ligand docking in silico molecular docking simulations studies were executed by using the autodock 4.2 software [32,33]. lamarckian genetic algorithms were utilized with the grid box size set at 60×60×60 å in the x, y, and z directions and the coordinates were fixed at x = 69.46, y = -17.32, and z = 55.62. all docking experiments consisted of 50 docking runs while the other parameters were left to their default values. the best conformation was selected with the lower docking energy for further docking analysis. the visualization of the interaction was generated with the plip webserver (protein-ligand interaction profiler) [41,42]. results from the molecular docking suggest that hydrogen bonding, hydrophobic forces, and πcation interactions are involved in the binding process. figure 5 illustrates the interactions of compounds fma1, fma2, fma3, fma4, fma5, and fma6 with the nearby residues in the active site of glutathione reductase. interacting residues and their corresponding bond types and length are summarised in table 1. table 1. interaction types between ligands fma1, fma2, fma3, fma4, fma5, fma6 and glutathione reductase molecule bond type amino acid (number of bonds/interactions) distance, ǻ fma1 h-bonds thr339 (2) 2.25, 2.00 hydrophobic interactions ile198(2), leu338, pro340, thr369, phe372 3.89, 3.95, 3.27, 3.79, 3.41, 3.47 π-stacking interactions tyr197 5.25 fma2 h-bonds thr339 (2) 2.62, 1.95 hydrophobic interactions thr369(2), tyr197, ile198, leu338, phe372 3.57, 3.55, 3.34, 3.62, 3.04, 3.45 fma3 h-bonds thr339 1.94 hydrophobic interactions tyr197, leu338, pro340, thr369, phe372 3.88, 3.21, 3.81, 3.41, 3.35 π-stacking interactions tyr197 5.42 fma4 h-bonds tyr197, thr339 (2) 3.10, 2.65, 2.01 hydrophobic interactions thr57, tyr197, ile198, leu338, thr369, phe372 3.82, 3.71, 3.85, 3.25, 3.47, 3.30 π-stacking interactions tyr197 5.25 fma5 h-bonds ser177, thr339 (2) 3.04, 3.29, 2.96 hydrophobic interactions thr57, tyr197, ile198, leu338, thr369, phe372 3.58, 3.49, 3.86, 3.10, 3.51, 3.37 fma6 h-bonds thr339 (2) 2.55, 1.93 hydrophobic interactions thr57, tyr197, leu338, thr369, phe372 3.79, 3.56, 3.16, 3.44, 3.37 a. kedadra et al. j. electrochem. sci. eng. 12(2) (2022) 293-304 http://dx.doi.org/10.5599/jese.1162 299 fma1-gr fma2-gr fma3-gr fma4-gr fma5-gr fma6-gr figure 5. best docking poses for glutathione reductase interacting with fma1, fma2, fma3, fma4, fma5, and fma6 illustrating h-bonds, hydrophobic, and -cation interactions it can be seen from this table that besides hydrophobic interactions, compounds fma4 and fma5 formed with the gr three hydrogen bonds. compounds fma1, fma2, and fma6, however, reacted via two hydrogen bonds, while the compound fma3 interacts only via one hydrogen bond beside one -stacking interaction. obtained binding free energy and inhibitory concentration from molecular docking study for the compounds fma1-fma6 are summarised in table 2. http://dx.doi.org/10.5599/jese.1162 j. electrochem. sci. eng. 12(2) (2022) 293-304 synthesis and activity of six novel acetamides 300 table 2. binding free energies and inhibitory concentration obtained from molecular docking study adduct fma1-gr fma2-gr fma3-gr fma4-gr fma5-gr fma6-gr δg / kj mol-1 -34.90 -34.82 -33.36 -31.85 -33.94 -33.56 ic50 / µm 0.76 0.79 1.42 2.61 1.11 1.31 in silico toxicity study in silico toxicity study aims to help in optimizing compounds regarding their toxicity proprieties. the study could offer an important improvement to the awareness of the full perspective of virtual screening for the identification of target compounds with negligible or no toxicity, which may open a path for the selection of novel nontoxic ferrocenylmethylaniline derivatives with high antioxidant activity. in silico toxicity study of the compounds fma1-fma6 was performed using the protox-ii web server [31]. it aims to predict hepatotoxicity (dili), carcinogenicity (carcino), immunotoxicity (immuno), mutagenicity (mutagen), cytotoxicity (cyto), median lethal dose (ld50), and toxicity class (tc). according to in silico toxicity profiles presented in table 3, the toxicity class of all compounds was detected to be equal to 3. fma4, fma5, and fma6 were predicted to be nontoxic. fma1, fma2, and fma3 were predicted to be toxic in mutagenicity. table 3. in silico toxicity profiles of the studied compounds molecule dili carcino immuno mutagen cyto ld50 / mg kg-1 tc fma1 inactive active inactive active inactive 237 3 fma2 inactive active inactive active inactive 237 3 fma3 inactive active inactive active inactive 237 3 fma4 inactive inactive inactive inactive inactive 256 3 fma5 inactive inactive inactive inactive inactive 237 3 fma6 inactive inactive inactive inactive inactive 256 3 based on the obtained results from the antioxidant study, molecular docking simulations and toxicity prediction, the compounds fma2, fma5, and fma6 cannot be accepted as antioxidant candidates because they have the lowest antioxidant activity (figure 1), the highest binding affinities, and the highest inhibitory activities towards the enzyme glutathione reductase (table 2). furthermore, the compound fma1 possesses the highest antioxidant activity (figure 1), but it cannot be the best antioxidant candidate because it has the highest binding affinity towards the enzyme glutathione reductase and also possesses the lowest inhibitory concentration of 0.76 µm that is necessary to reduce the rate of glutathione reductase enzyme reaction by 50 %. the compounds fma4 and fma3 possess the highest antioxidant activities, the lowest binding free energy of -31.85 and -33.36 kj/mol, and the highest inhibitory concentration of 2.61 and 1.42 µm against glutathione reductase enzyme reaction, respectively, these values indicate weak binding affinity towards the enzyme glutathione reductase compared to the other compounds. thus, the compounds fma4 and fma3 are weaker inhibitors of glutathione reductase. however, although compound fma3 has higher antioxidant activity than fma4, it cannot be a good antioxidant candidate because it is predicted as mutagenic (table 3). finally, based on what is cited above, the compound fma4 can be chosen as the best antioxidant candidate. detailed procedure for the synthesis of compounds (fma1-fma6) nferrocenylmethyl-n-(2-nitrophenyl)acetamide (fma1) nferrocenylmethyl-n-(2-nitrophenyl)acetamide (275 mg, 49 %) was obtained, as described above, m.p. 148 °c, orange solid. a. kedadra et al. j. electrochem. sci. eng. 12(2) (2022) 293-304 http://dx.doi.org/10.5599/jese.1162 301 ir (kbr:) ν = 3381 cm-1 (c-h), 1658 cm-1 (c=o), 1610 cm-1 (c=c), 1506 and 1572 cm-1 (c-no2) uv-vis: λmax (ch3cn) = 230 and 278 nm (π→π*, fc, ar), 431 nm (n→π*, no2, co), cv (ν = 100 mv s-1, ch3cn): ipa = 26.02 µa cm-2, ipc = -25.09 µa cm-2, ipa/ ipc = 1.04, epa = 508 mv vs. sce, epc = 445 mv vs. sce, δe = 63 mv, e1/2 = 476.5 mv vs. sce, nmr 1h (300 mhz, cdcl3): δ = 1.91 ppm (3h, s, h1), 4.16 ppm (2h, m, h2), 4.23 ppm (2h, m, h3), 4.30 ppm (5h, s, h4), 4.64 ppm (2h, s, h5); 6.70 ppm (1h, dd, j = 11.03 hz, h9), 6.92 ppm (1h, d, j = 11.37 hz, h8), 7.49 ppm (1h, m, j = 22.18 hz, h7), 8.24 ppm (1h, dd, j = 13.49 hz, h6). nferrocenylmethyl-n-(3-nitrophenyl)acetamide (fma2) nferrocenylmethyl-n-(3-nitrophenyl)acetamide (478 mg, 85 %) was obtained, as described above, m.p. 132 °c, orange leaflet. ir (kbr:) ν = 3058 cm-1 (c-h), 1656 cm-1 (c=o), 1612 cm-1 (c=c) 1350 and 1531 cm-1 (c-no2), uv-vis: λmax (ch3cn) ) = 232 and 325 nm (π→π*, fc, ar), 423 nm (n→π*, no2, co), cv (100 mv s-1, ch3cn): ipa =8.06 µa cm-2, ipc = -8.18 µa cm-2, ipa/ ipc = 0.98, epa = 534 mv vs. sce, epc = 468 mv vs. sce, δe = 66 mv, e1/2 = 501 mv vs. sce, nmr 1h (300 mhz, cdcl3): δ = 1.87 ppm (3h, s, h1), 4.00 ppm (2h, m, h2), 4.07 ppm (2h, m, h3), 4.10 ppm (5h, s, h4), 4.67 ppm (2h, s, h5), 7.29 ppm (1h, t, j = 6.28hz, h6), 7.53 ppm (1h, t, j = 8.94 hz, h7), 7.90 ppm (1h, s, h9), 8.18 ppm (1h, d, j = 7.76 hz h8). nferrocenylmethyl-n-(4-nitrophenyl)acetamide (fma3) nferrocenylmethyl-n-(4-nitrophenyl)acetamide (461 mg, 82 %) was obtained, as described above, m.p. 158°c, red needles. ir (kbr:) ν = 3075 cm-1 (c-h), 1659 cm-1 (c=o), 1602 cm-1 (c=c), 1593 cm-1 (c-no2), uv-vis: λmax (ch3cn) ) = 232 nm (π→π*, fc, ar), λ = 380 nm (n→π*, no2, co), cv (100 mv s-1, ch3cn): ipa = 9.62 µa cm-2, ipc = -9.73 µa cm-2, ipa/ ipc = 0.99, epa = 494 mv vs. sce, epc = 431 mv vs. sce, δe = 63 mv, e1/2 = 462.5 mv vs. sce, nmr 1h (300 mhz, cdcl3): δ = 1.87 ppm (3h, s, h1), 4.01 ppm (2h, m, h2), 4.07 ppm (2h, m, h3), 4.12 ppm (5h, s, h4), 4.69 ppm (2h, s, h5), 7.17 ppm (2h, d, j = 8.92hz, h6), 8.22 ppm (2h, d, j = 8.94hz, h7). n-ferrocenylmethyl-n-(2-cyanophenyl)acetamide (fma4) n-ferrocenylmethyl-n-(2-cyanophenyl)acetamide (fma4) (311 mg, 55 %) was obtained, as described above, m.p. 146°c, orange reddish solid. ir (kbr:) ν = 3304 cm-1 and 3081 cm-1 (c-h), 2210.6 cm-1 (cn), 1602.3 cm-1 (c=o), uv-vis: λmax (ch3cn) ) = 337 nm (π→π*, fc, ar), λ = 444 nm (n→π*, co), cv (100 mv s-1, ch3cn): ipa = 100.61 µa cm-2, ipc = -102.13, ipa/ ipc = 1.015, epa = 525 mv vs. sce, epc = 421 mv vs. sce, δe = 99 mv, e1/2 = 475 mv vs. sce, nmr 1h (300 mhz, cdcl3): δ = 2.13 ppm (3h, s, h1), 4.02 ppm (2h, m, h2), 4.20 ppm (2h, m, h3), 4.22 ppm (5h, s, h4), 4.26 ppm (2h, s, h5), 7.12-7.19 ppm (3h, m, j = 8.92hz, h7,h8,h9), 7.55 ppm (2h, d, j = 18.95hz, h6). n-ferrocenylmethyl-n-(3-cyanophenyl)acetamide (fma5) n-ferrocenylmethyl-n-(3-cyanophenyl)acetamide (fma4) (345 mg, 61 %) was obtained, as described above, m.p. 162°c, orange reddish solid. ir (kbr:) ν = 3081 cm-1 (c-h), 2214.2 cm-1 (cn), 1636.1 cm-1 (c=o), uv-vis: λmax (ch3cn) ) = 433.5 nm (n→π*, co), cv (100 mv s-1, ch3cn): ipa = 10.71, µa cm-2, ipc = -11.09 µa cm-2, ipa/ ipc = 1.03, epa = 598 mv vs. sce, epc = 463 mv vs. sce, δe = 99 mv, e1/2 = 531 mv vs. sce, nmr 1h (300 mhz, cdcl3): δ = 1.77 ppm (3h, s, h1), 3.99 ppm (2h, m, h2 ), 4.07 ppm (2h, m, h3), 4.10 ppm (5h, s, h4), 4.63 ppm (2h, s, h5),7.21 ppm (1h, d, j = 7.74hz, h6), 7.29 ppm (1h, s, h7), 7.48 ppm (1h, t, j = 7.75hz, h9), 7.61 ppm (1h, d, j = 7.74 hz, h8). http://dx.doi.org/10.5599/jese.1162 j. electrochem. sci. eng. 12(2) (2022) 293-304 synthesis and activity of six novel acetamides 302 n-ferrocenylmethyl-n-(4-cyanophenyl)acetamide (fma6) n-ferrocenylmethyl-n-(4-cyanophenyl)acetamide (fma4) (385 mg, 68 %) was obtained, as described above, m.p. 155°c, yellow solid. ir (kbr:) ν = 3081 cm-1 (c-h), 2214.2 cm-1 (cn), 1636.1 cm-1 (c=o), uv-vis: λmax (ch3cn) ) = 265.5 nm (π→π*, fc, ar), cv (100 mv s-1, ch3cn): ipa = 18.61, µa cm-2, ipc = 18.76 µa cm-2, ipa/ ipc =1, epa = 488 mv vs. sce, epc = 426 mv vs. sce, δe = 62 mv, e1/2 = 457 mv vs. sce, nmr 1h (300 mhz, cdcl3): δ = 1.82 ppm (3h, s, h1), 4.01 ppm (2h, s, h2), 4.08 ppm (2h, m, h3), 4.12 ppm (5h, s, h4), 4.66 ppm (2h, s, h5), 7.13 ppm (2h, d, j = 8.39 hz, h6),7.67 ppm (2h, d, j = 8.36 hz, h7). the 1h nmr spectra of all the synthesized compounds presented in figure 6 reveal one downfield singlet at  = 1.77-2.13 ppm which is ascribed to methyl protons. 8 7 6 5 4 3 2 1  / ppm fma1 8 7 6 5 4 3 2 1  / ppm fma2  / ppm  / ppm 8 7 6 5 4 3 2 1  / ppm fma3 8 7 6 5 4 3 2 1  / ppm fma4  / ppm  / ppm 8 7 6 5 4 3 2 1  / ppm fma5 8 7 6 5 4 3 2 1  / ppm fma6  / ppm  / ppm figure 6. 1h nmr spectra of fma1, fma2, fma3, fma4, fma5, and fma6 compounds the and -protons of the substituted ring of ferrocene c5h4 appeared as a multiplet at 3.994.16 and 4.07-4.23 ppm respectively, the unsubstituted protons of the c5h5 ring of ferrocene appeared as a singlet at 4.10 to 4.30 ppm. a singlet appeared at  = 4.26-4.69 ppm was due to methylene protons, this downfield shift of the methylene protons was observed due to electronegativity of the nitrogen atom. the aromatic protons appeared in the range of  = 6.70-8.24 ppm. a. kedadra et al. j. electrochem. sci. eng. 12(2) (2022) 293-304 http://dx.doi.org/10.5599/jese.1162 303 conclusions in this work, in vitro and in silico studies have been carried out to evaluate the scavenging activity against o2and the antioxidant activity of six novel n-ferrocenylmethyl-n-(nitrophenyl)acetamides and n-ferrocenylmethyl-n-(cyanophenyl)acetamide using cyclic voltammetry assays. the obtained values of ic50 indicated that all derivatives showed promising scavenging activity against o2-, with the compounds fma1, fma3, and fma4 possessing the most significant potency. a molecular docking study and an in silico toxicity prediction revealed that compound fma4 is the most inactive compound against glutathione reductase enzyme, having an inhibitory concentration of 2.61 µm and a docking score of -31.85 kj mol-1, which make the best good antioxidant candidate. the obtained in vitro and in silico results correspond with one another and give room for the design of novel antioxidant ferrocenylmethylaniline derivatives with less activity against glutathione reductase. the in silico toxicity study allowed us to predict the toxicity, the median lethal dose (ld50), and the toxicity class (tc) of the studied 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sudan i as an azo dye in food samples using a fe3o4-zif-67/ionic liquid modified carbon paste electrode mahboobeh shahsavari and iran sheikhshoaie department of chemistry, faculty of science, shahid bahonar university of kerman, kerman 76175-133, iran corresponding author: i_shoaie@yahoo.com received: december 24, 2021; accepted: december 31, 2021; published: january 19, 2022 abstract the present study developed a facile and fast electrochemical approach to sensitively analyze sudan i using fe3o4-zif-67 nanocomposite plus ionic liquid (il). the carbon paste electrode (cpe) modified with fe3o4-zif-67/il exhibited an excellent electrochemical sensing performance to sudan i. compared with the unmodified cpe, fe3o4-zif-67/ilcpe could significantly increase the peak current of sudan i oxidation and decrease the oxidation overpotentials. under the best experimental conditions, the sensor using differential pulse voltammetry (dpv) technique responded to sudan i linearly (0.5 560 μm) with a low limit of detection (lod) of 0.1 μm. additionally, the applicability and effectiveness of our proposed method in sensing sudan i present in food samples was confirmed by acceptable recovery rate (96.0 103.6 %). keywords electrochemical method; fe3o4-zif-67 nanocomposite; ionic liquid; introduction a group of sudan azo dye (including types i, ii, iii and iv), known as important compounds, are synthetic industrial dyes that their carcinogenicity has been proven by iarc (international agency for research on cancer) [1]. they are extensively applied as additives in chemistry preparations in the structure of fuels, oils, solvents, textile, coloring waxes, shoe polish, cosmetic products, and in the formulations of medical instruments and drugs owing to unique properties like high stability and cost-effectiveness [2]. although most countries have issued restrictions on the use of these azo compounds as coloring additives, they are still present in the ingredients of some foods such as red peppers, sausages, relishes and condiments [3]. 1-phenylazo-2-naphthol, or sudan i, were able to react with a certain dna sequence under in vitro conditions. therefore, this substance is toxic and damaging to genetic material [4]. thus, a majority of countries enacted strict laws regarding the http://dx.doi.org/10.5599/jese.1217 http://dx.doi.org/10.5599/jese.1217 http://www.jese-online.org/ mailto:i_shoaie@yahoo.com j. electrochem. sci. eng. 12(1) (2022) 165-173 analysis of sudan i as an azo dye 166 permissible use of sudan i in food and beverages [5]. so, it is vital to detect the presence of sudan i in various food products. accordingly, many techniques have been employed so far for sudan i detection, including high performing liquid chromatography [6], surface-enhanced raman scattering method [7], fluorescence spectroscopy [8], enzyme-linked immunesorbent assay [9], capillary electrophoresis [10], and electrochemical methods [11]. further attention has been recently attracted towards electrochemical methods owing to some unique properties such as rapid response, simple protocols, cost-effectiveness, real-time determination in situ condition and excellent sensitivity [12]. consistent with recent evidence, adams (1958) introduced carbon paste electrode (cpe) in electroanalysis [13] and to date, has been a facile tool for quantitative analysis of diverse compounds. cpes have advantages such as easy preparation, low cost, the provision of repeatable signals [14], low background current, prolonged stability, wide polarization range in cathodic and anodic sides [15]. with these features in mind, special attention has been paid to the fabrication of inexpensive, facile and sensitive electrochemical sensors by cpes for electroanalysis. in this analytical method, electrode surface modification has been proposed using effective modifiers to enhance the efficiency of electrochemical measurements [16]. the electrode surface modification may play a role in enhancing the sensitivity of the electrochemical sensor with a low limit of detection (lod). the chemical modification increases electron transfer, resulting in enhanced electrocatalytic properties at the electrode surface and reduced surface fouling [17]. some effective modifiers include conductive polymers, nanomaterials, metal oxides, metals, composite materials and ionic liquids [18]. fe3o4 nanoparticles are one of the most researched forms of iron oxides due to their attractive characteristics, including low toxicity, biocompatibility, super magnetism, and high surface area [19]. moreover, this magnetic material is economical, easy to synthesize and more effective. these unique properties have enabled fe3o4 nanoparticles to serve as an ideal candidate material in the electrochemical sensor [20-22]. zeolitic imidazolate frameworks (zifs) belong to metal-organic frameworks (mofs) and are appropriate porous materials containing inorganic metal nodes interconnected with ligands of imidazolate bridging or imidazole [23]. zifs possess common features of mofs, and also have unique thermochemical stability [23]. therefore, they are considered as proper options for gas adsorption [24], catalysis [25] and separation [26]. they also show very large surface area and excellent porosity, thus making them promising candidates for fabricating potent electrochemical sensors [27]. hence, the incorporation of zif-67 crystals and fe3o4 nanoparticles can be useful to achieve potential applications. ionic liquids (ils) are salts in a liquid state at ambient temperatures and commonly consist of many organic cations and organic/inorganic anions. they have been extensively applied in electrochemistry because of excellent physicochemical properties like fairly great ionic conductivity, poor vapor pressure, admirable chemical stability, acceptable biocompatibility stable and electrochemical windows [28,29]. the present research aimed to introduce a facile and sensitive electrochemical approach to detect sudan i using fe3o4-zif-67 nanocomposite and ionic liquid. experimental chemicals and instrumentations electrochemical experiments were recorded using a pgstat-302n autolab potentiostat/galvanostat (eco chemie, the netherlands). the control of all experiments was carried out by a general m. shahsavaria and i. sheikhshoaie j. electrochem. sci. eng. 12(1) (2022) 165-173 http://dx.doi.org/10.5599/jese.1217 167 purpose electrochemical system (gpes) software. electrochemical experiments were performed by a three-electrode system containing modified or unmodified cpe as working electrode, ag/agcl (3m kcl) as a reference electrode, and platinum wire as a counter (auxiliary) electrode. all ph values were measured by a digital metrohm 710 ph meter. all materials with analytical grade applied throughout this work were supplied from aldrich and merck. the fe3o4-zif-67 nanocomposite was synthesized according to a simple method as reported in our recent work [30]. the field emission scanning electron microscopy (fe-sem) of fe3o4-zif-67 nanocomposite is shown in figure 1. figure 1. the fe-sem image of fe3o4-zif-67 nanocomposite preparation of fe3o4-zif-67/ilcpe for the preparation of fe3o4-zif-67/ilcpe, fe3o4-zif-67 nanocomposite was grounded with graphite powder with the ratio 10:90 (wt.%), followed by mixing with paraffin oil and il (1-butyl-3methylimidazolium hexafluorophosphate) (90:10 vol.%) in a mortar and pestle for 30 min till a homogenous paste was obtained. the modified electrode was made by pressing the obtained modified paste into the bottom of a glass tube with the help of mechanical force. then, electrical contact was made by placing a copper wire at the end of the syringe. if the electrode reaction tended to decrease, the electrode surface was polished on soft paper to give a smooth aspect before use. the surface area of the fe3o4-zif-67/ilcpe and the bare cpe were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula for fe3o4-zif-67/ilcpe, the electrode surface was found 0.288 cm2, about 3.2 times greater than bare cpe. result and discussion the electrochemical response of sudan i on the fe3o4-zif-67/ilcpe surface the supporting electrolyte ph significantly affects the sudan i electrocatalysis on fe3o4-zif-67/ilcpe surface. the ph influence on sudan i detection in the presence of pbs on the modified electrode http://dx.doi.org/10.5599/jese.1217 j. electrochem. sci. eng. 12(1) (2022) 165-173 analysis of sudan i as an azo dye 168 surface was explored at different ph values (2.0-9.0). the maximum peak current of sudan i oxidation was found at the ph value of about 7.0; accordingly, this value was selected to be the optimal experimental condition for the experiments. figure 2 shows the application of the cyclic voltammetry (cv) method to evaluate the electrochemical behavior of 200.0 μm sudan i at different electrodes (unmodified cpe, and fe3o4zif-67/ilcpe) in pbs (0.1 m, ph 7.0) at the scan rate of 50 mv/s. based on the results, there was an oxidation peak on the surfaces of electrodes, but no reduction peak, highlighting an irreversible electrochemical response of sudan i on the electrodes. a relatively wide and weak peak current (ipa) of sudan i oxidation was found on the unmodified cpe (at 590 mv with 5.5 μa), which reveals that the electrochemical oxidation does not happen spontaneously due to high activation overpotential. the sudan i ipa on fe3o4-zif-67/ilcpe, when compared with unmodified cpe, displayed further elevation to 13.1 μa, meaning an increase up to 2.4 times that on the unmodified cpe. in addition, sudan i oxidation occurred at a lower potential than unmodified cpe. e / mv vs. ag/agcl/kcl figure 2. cv response of 200.0 μm sudan i at (a) unmodified cpe, (b) fe3o4-zif-67/ilcpe in pbs (0.1 m, ph 7.0) effect of scan rate on the results figure 3 displayed the linear sweep voltammetry (lsv) behaviors of 100.0 μm sudan i in pbs at the ph value of 7.0 at variable scan rates on the surface of fe3o4-zif-67/ilcpe. based on the results, there was a gradual elevation in the peak current of sudan i oxidation and a positive shift of peak potential of oxidation by rising the scan rates to 10 from 400 mv/s. according to figure 3 (inset), the sudan i anodic peak current (ipa) fitted to the square root of scan rate (v1/2). hence this study showed that the electrode reaction was a diffusion-controlled process. to provide data about the rate determining step, a tafel plot was drawn based on data of rising sector related to the current-voltage curve at low scan rate (10 mv/s) for 100.0 μm sudan i (figure 4, inset). the linearity of e versus log i plot reveals the intervention of electrode process kinetics. in accordance with the slope of this plot, the number of transferred electrons can be calculated in the rate-determining step. the inset in figure 4 shows the tafel slope of 0.2312 v for the linear sector. the tafel slope value means the rate-limiting step related to one-electron transfer, with a transfer coefficient α of 0.74. m. shahsavaria and i. sheikhshoaie j. electrochem. sci. eng. 12(1) (2022) 165-173 http://dx.doi.org/10.5599/jese.1217 169 e / mv vs. ag/agcl/kcl figure 3. lsv curves of 100.0 μm sudan i in pbs (0.1 m, ph 7.0) at different scan rates (10-400 mv/s) on fe3o4-zif-67/ilcpe (a-g refers to 10, 30, 70, 100, 200.0, 300.0, and 400.0 mv/s). inset: plot of scan rate square root versus sudan i oxidation peak current e / mv vs. ag/agcl/kcl figure 4. lsv response for 100.0 μm of sudan i at the scan rate of 10 mv/s; inset: tafel plot of rising sector or related voltammogram chronoamperometric analysis chronoamperometric records for sudan i detection on the fe3o4-zif-67/ilcpe surface were performed using a working electrode with the potential of 0.58 v at variable sudan i in pbs (0.1 m, ph 7.0), as shown in figure 5. the cottrell equation describes the electrochemical reaction current at the limited condition of mass transport for sudan i (as an electroactive material) with a certain d value [31]. figure 5a shows the experimental plots of i versus t−1/2 with the optimal fits at variable sudan i concentrations. then, we plotted the slopes of obtained straight lines versus sudan i concentration, as shown in figure 5b. based on the achieved slope and cottrell equation, the mean d value was estimated at 1.5×10-6 cm2/s. y = 1.026x 0.8761 r2 = 0.9982 y = 0.2314x – 0.3488 r2 = 0.998 http://dx.doi.org/10.5599/jese.1217 j. electrochem. sci. eng. 12(1) (2022) 165-173 analysis of sudan i as an azo dye 170 figure 5. the chronoamperograms obtained on fe3o4-zif-67/ilcpe in pbs (0.1 m, ph 7.0) at different sudan i concentrations; note) a–f : 0.1, 0.3, 0.6, 0.8, 1.4, and 2.0 mm of sudan i. inset a) plot of i versus t-1/2 based on chronoamperograms (a – f). inset b) slope plot of straight line versus sudan i concentration dpv analysis of sudan i on the fe3o4-zif-67/ilcpe surface dpv is a versatile technique for sudan i detection because of its higher sensitivity and the obtained voltammograms for sudan i detection is shown in figure 6 (step potential = 0.01 v and pulse amplitude = 0.025 v). e / mv vs. ag/agcl/kcl figure 6. dpv response of sudan i at fe3o4-zif-67/ilcpe in the concentration range 0.5 560.0 μm in pbs (0.1 m, ph 7.0); a-k refers to 0.5, 2.5, 10.0, 30.0, 70.0, 100.0, 200.0, 300.0, 400.0, 500.0, and 560.0 µm; inset: the calibration curve of dpv peaks against concentration of sudan i figure 6 shows that with increasing concentration of sudan i from 0.5 to 560.0 μm the ipa is elevated with a slight shift in oxidation potential. the ipa plot versus sudan i concentration was drawn (figure 6, inset), the results of which display a nearly straight line with admirable linearity having linear regression equation of ipa = 0.0608 csudan i + 0.7525 (r2 = 0.9999). the equation of 3sbl/m was considered to compute the lod, where m stands for the slope of the standard plot and sbl for a y = 12.192x – 4.4337 r2 = 0.998 y =0.0608x – 0.7525 r2 = 0.9999 m. shahsavaria and i. sheikhshoaie j. electrochem. sci. eng. 12(1) (2022) 165-173 http://dx.doi.org/10.5599/jese.1217 171 standard deviation for linearity of blank solution anodic peak current after five determinations. the lod is found to be 0.1 μm. table 1 presents a comparison of fe3o4-zif-67/ilcpe analytical performance created in this work with other electrochemical sensors involved in sudan i analysis. table 2. comparison of the determination of sudan i at fe3o4-zif-67/ilcpe and modified electrodes reported in the literature electrochemical sensor method linear range, μm limit of detection, μm ref. ag nanoparticles decorated graphene oxide/glassy carbon electrode amperometry 3.90×10−3-3.19×10−2 11.4×10−4 [32] cuo nanoparticles-decorated 3d n-doped porous carbon/glassy carbon electrode dpv 2.0×10-3-0.1 8.4×10-4 [33] gemini surfactant–ionic liquid–multiwalled carbon nanotube/glassy carbon electrode linear sweep voltammerey 5.0×10-5-2.0×10-3 3.0×10-5 [34] bi2wo6 nanosheets/glassy carbon electrode dpv 0.02-114.6 0.002 [35] fe3o4-zif-67/ilcpe dpv 0.5 – 560.0 0.1 this work real sample analysis the applicability of the sensor developed in the present study was tested by detecting sudan i present in real food specimens, including chili powder and ketchup sauce. table 2 shows the recovery rates of 96.0 to 103.6 % for spiked specimens. these results indicated the effectiveness of the proposed sensor in the detection of sudan i in real specimens. table 2. sudan i detection in real food specimens using fe3o4-zif-67/ilcpe (n=5) sample c / m recovery, % rsd, % spiked found chili powder 0 nd 5.0 4.9 98.0 3.2 7.0 7.1 101.4 1.7 9.0 8.8 97.8 2.9 11.0 11.4 103.6 2.3 ketchup sauce 0 nd 5.0 5.1 102.0 1.8 7.5 7.4 98.7 3.5 10.0 9.6 96.0 2.2 12.5 12.6 100.8 2.6 conclusion in this work, the fe3o4-zif-67 nanocomposite was synthesized via a simple method and characterized by various microscopic and spectroscopic methods. this fe3o4-zif-67/ilcpe 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[32] e. prabakaran, k. pandian, food chemistry 166 (2015) 198-205. https://doi.org/10.1016/j. foodchem.2014.05.143 [33] q. ye, x. chen, j. yang, d. wu, j. ma, y. kong, food chemistry 287 (2019) 375-381. https://doi.org/10.1016/j.foodchem.2019.02.108 [34] z. mo, y. zhang, f. zhao, f. xiao, g. guo, b. zeng, food chemistry 121 (2010) 233-237. https://doi.org/10.1016/j.foodchem.2009.11.077 [35] v. vinothkumar, a. sangili, s. m. chen, t. w. chen, m. abinaya, v. sethupathi, international journal of electrochemical science 15 (2020) 2414-2429. https://doi.org/10.20964/2020.03.08 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1217 https://doi.org/10.1039/c3ee40507a https://doi.org/10.1039/c3ee40507a https://doi.org/10.1002/cphc.200301017 https://doi.org/10.1016/j.electacta.2016.02.044 https://doi.org/10.1007/s11244-021-01471-8 https://doi.org/10.1016/j.foodchem.2019.02.108 https://doi.org/10.1016/j.foodchem.2009.11.077 https://doi.org/10.20964/2020.03.08 https://creativecommons.org/licenses/by/4.0/) electrochemical treatment of trypan blue synthetic wastewater and its degradation pathway doi: 10.5599/jese.2013.0039 167 j. electrochem. sci. eng. 3(4) (2013) 167-184; doi: 10.5599/jese.0039 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical treatment of trypan blue synthetic wastewater and its degradation pathway anantha n. subba rao and venkatesha t. venkatarangaiah department of p.g. studies and research in chemistry, school of chemical sciences, kuvempu university, shankaraghatta-577451, shimoga, karnataka, india corresponding authors: e-mail: drtvvenkatesha@yahoo.co.uk, tel.: +91-9448855079; fax: +91-08282256255. received: october 17, 2013; revised: october 28, 2013; published novembar 09, 2013 abstract the trypan blue (tb) dye synthetic wastewater was treated in presence of chloride ions by electrochemical method. the effect of current density, ph, initial concentration of dye and supporting electrolyte on color and cod removal were investigated. the uv-vis absorption intensity, chemical oxygen demand (cod), cyclic voltammetry (cv), fourier transform infrared spectroscopy (ft-ir), gas chromatography – mass spectrometry (gc-ms) analysis were conducted to investigate the kinetics and degradation pathway of tb dye. keywords cataract, teratogenic, carcinogenic, niagara blue, diamine blue, decolorization, cod removal, active anode, oxidants, indirect oxidation. introduction dyes find extensive applications in industries like textile, leather, pulp and paper, printing, photographs [1]. the wastewater from these industries is therefore loaded with the dyes. it is aesthetically unbearable and can turn out to be fatal if this wastewater is allowed to combine with the natural water resources. the presence of dyes or organic molecules increases the biological oxygen demand (bod), chemical oxygen demand (cod) and total organic carbon (toc) content of natural water, which especially poses challenges to the survival of aquatic life besides causing serious health hazards leading to the imbalance in ecology [2-4]. chemical, physical, biological, and advanced oxidation processes are in practice to treat the wastewater. the electrochemical method is efficient, less time consuming and it does not require the addition of any chemical reagent. in this method, the organic pollutant is oxidized either by http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 168 direct electron exchange with the anode (direct oxidation) or by the electrochemically generated oxidants (indirect oxidation). direct oxidation is possible if the organic pollutant is oxidizable within the overpotential of oxygen (oer) and hydrogen evolution reactions. indirect oxidation is brought about by the oxidants generated during electrolysis, such as hydroxyl radical ( • oh). hydroxyl radical ( • oh, h + /h2o) is a powerful oxidizing agent with standard reduction potential of 2.7 v vs. standard hydrogen electrode. active chlorine species (oxidants) like cl2, cl3 , clo2 , clo3 , hocl, clo are generated in presence of chloride electrolyte and these species can oxidize the organics into simple biodegradable molecules [5,6]. the electrochemical degradation of methyl violet 2b, acridine, eosin yellow [7], alphazurine dye [8], reactive blue 81, reactive red 2 [9], reactive blue-4, reactive orange-16 [10], reactive red 120 [11], reactive brilliant red k-2bp [12], methyl green [13], methyl red [14] and other dyes has been successfully achieved in presence of suitable electrolyte and operating conditions. furthermore, the results from these works infer that the extent of degradation of an organic pollutant not only depends on the operating conditions employed, but also on the structural, chemical and electrochemical properties of the pollutant itself [7]. the nature of the electrolyte and its concentration, electrode material, ph, current density, wastewater flow rate (stirring rate) and the mechanism of oxidation also significantly influence the extent of degradation of organics. complete incineration of organics with high efficiency can be achieved by electrochemical method under optimum conditions. the molecules o-tolidine, benzidine and o-dianisidine are carcinogenic in nature [16,17]. trypan blue (tb) (congo blue 3b / niagara blue 3b / diamine blue 14) is an o-tolidine based diazo dye, well known for its ability to selectively color the dead cells blue. it is also used in the dyeing of textiles, leather and paper [18]. table 1: properties and structure of trapan blue properties chemical structure molecular formula: c34h24n6na4o14s4 molecular weight: 960.8 color index number: 23850 λmax / nm = 590 [19] tb dye (table i) emerged as one of the most frequently used staining agent in cataract, as well as other anterior segment surgeries in the late 1990s [20]. however, there are reports which unveil the complications in cataract surgery with the usage of tb. c lüke et al. [21] reported the toxic effects of tb after a short duration of retinal exposure and recommended minimizing the usage of tb in intraocular applications [20,22]. tb carries associated risk at higher concentrations and longer duration of exposure [20-22]. long term presence of 0.2 % tb can cause considerable damage to the retina [23]. international agency for research on cancer provides sufficient evidences for the carcinogenic and teratogenic effects of tb in animals and possibly in humans [24,25]. a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 169 the photocatalytic degradation of tb is reported in the literature [15,26] but there are no work solely focused on the degradation of tb by electrochemical method [15]. in the present work, the electrochemical oxidation of tb with pt as anode material has been investigated and probable degradation pathway of tb has been proposed. materials and chemicals chemicals commercially available trypan blue (tb) dye was (purchased from himedia, mumbai, india) used for the studies. analytical grade nacl, na2so4, na2co3 and nano3 and their aqueous solutions were (himedia laboratories pvt. ltd, bangalore, india) used as electrolytes. ph of each electrolyte was adjusted by h2so4 and naoh. the dil. hno3 was used for the pretreatment of electrodes (himedia laboratories pvt. ltd, bangalore, india). experimental a single compartment glass container was used as electrolytic cell (6.5 cm diameter x 10 cm height, covered with teflon® lid provided with gaps to insert the electrodes). two pt foils (dimension 0.5 cm × 0.5 cm, thickness 0.018 cm, supplied by systronics india ltd. bangalore, india) of surface area, 0.54 cm 2 were used as anode and cathode which were separated by 0.5 cm in the electrolytic cell. the potentials of working electrode (anode) were recorded relative to saturated calomel electrode (sce). a dc source with galvanostatic mode (model ps-618 chemilink systems, navi mumbai, india) was used for power supply. the synthetic wastewater of tb dye was prepared with ultrapure water obtained from elix 3 milli-pore system (resistivity, > 18 mω cm at 25 °c). a magnetic stirrer (550 rotations per minute) was used for the agitation of wastewater during electrolysis. electrolysis the solution with the composition of 50 mg l -1 tb dye solution and nacl, 0.2 % was prepared in millipore water and was subjected to electrolysis under galvanostatic condition for 60 minutes. the pt electrodes were dipped in dilute hno3, sonicated for 1 minute and thoroughly washed with millipore water. all experiments were carried out at ambient temperature. to monitor the progress of the degradation process, electrolyzed samples were collected at appropriate time intervals and subjected to analysis. the effect of current density, ph, nature of electrolyte and its concentration on cod and color removal efficiency was examined. the optimum condition for the maximum color and cod removal efficiency was determined. analysis cyclic voltammetry (cv) the cyclic voltammetric measurements were performed at room temperature with conventional three electrode system connected to software controlled electrochemical work station (ch instruments 660c, usa). the working electrode was a pt disk and counter electrode was a pt foil. the working electrode surface was polished with 0.05 μm alumina, washed with methanol, dipped in 10 % hno3, sonicated for 2 min and then washed thoroughly with millipore water before use. the open circuit potential was recorded in 0.2 % nacl solution and the potential window from +1.1 v to -1.1 v was selected for cv scans. cv for blank solution (0.2 % nacl) and tb dye solution (50 mg l -1 + nacl 0.2 %) were recorded at the scan rate of 100 mv s -1 . j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 170 spectroscopic and chemical analysis the extent of degradation was monitored by evaluating color and cod removal. uv-vis absorbance spectra of samples were recorded by ocean optics uv-vis hr 4000 spectrophotometer (uv-vis-nir light source, dt-mini-2-gs and jaz detector). the percentage color removal was calculated by using the relation (1) with reference to the absorption intensity at λmax of 590 nm. [ ] ( ) where abs0 and abst are the absorbance at 590 nm at time 0 and t minutes of electrolysis respectively. the cod (g(o2) l -1 ) estimation was carried out by open reflux method and its values used to monitor the progress in degradation. the percentage cod removal was calculated using the relation (2). [ ] ( ) where cod0 and codt are the cod at time 0 and t minutes of electrolysis respectively. the percentage average current efficiency (ace) was calculated using the following relation (3) [6]. [ ] ( ) where f is the faraday’s constant (96,485 c mol -1 ), cod in (g o2 l -1 ), i is the current (a), v is the volume (l) of electrolyte, t – time (s), 8 – gram equivalent mass of oxygen (g equiv -1 ). to determine the degradation pathway, the test samples at selected time intervals (15 and 40 min) were subjected to freeze drying using lyophilizer (delvac, india) at -50 ± 5 °c under reduced pressure of 0.12 mbar. freeze drying eliminates the chances of decomposition of intermediates and only the volatile compounds are removed. the solid residue thus obtained was subjected to uv-vis spectroscopy, fourier transform infrared spectroscopy ft-ir (alpha, brucker) and gas chromatography mass spectrometry (gc-ms) analyses. the solid residue was finely mixed with kbr, pressed into a disk and subjected to ft-ir analysis. the ft-ir spectrums recorded are the average 16 scans from wave number 4000 cm -1 to 500 cm -1 . gc-ms analysis was carried out in order to detect the end products of tb degradation. the samples were analyzed using gc-ms system (trace gc ultra and trace dsq thermo-electron corporation, austin, tx, usa) with a hp 5ms, 5 % phenyl methyl silox column (30 m × 0.25 mm, film thickness 0.25 µm). a temperature gradient program at 10° c min -1 was applied between 40 °c and 250 °c. results and discussion cyclic voltammetry studies the cyclic voltammograms are shown in fig. 1. a peak at -0.23 v was observed during cathodic sweep of blank solution. the hydrogen evolution begins approximately at -0.80 v. the voltammogram obtained for the dye solution showed the same pattern as the blank and no new peak was observed during the anodic sweep. this indicates that the tb dye does not undergo direct electrochemical reaction on pt in the potential range from -1.1 to +1.1 v. the decrease in anodic peak (at -0.80 v vs. sce) current can be attributed to the adsorption of dye on the surface of working electrode. a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 171 figure 1. cv of 0.2 % nacl and 50 mg l -1 tb + 0.2 % nacl solutions; scan rate 100 mv s -1 effect of supporting electrolyte to determine the effect of different supporting electrolytes, preliminary investigations on tb degradation were carried out separately in presence of chloride (cl ), sulfate (so4 2), nitrate (no3 ) and carbonate (co3 2) ions. in all the cases 34.2 mm solution of electrolyte was was used. on electrolysis of the solution for 60 min, showed a 100 %, 42 %, 27 %, 15 % color removal in presence of cl , co3 2, no3 , so4 2 ions respectively. similar trend was observed by fornazari et al. [27] in the electrochemical degradation of phenol-formaldehyde mixture [27,28]. the presence of cl ions is not only essential for the color removal but also its in situ generated oxidants cleave the tb bonds. therefore, nacl was selected as the supporting electrolyte for the present studies. fig. 2a and fig. 2b depict the effect of different electrolytes on absorption intensity and color removal of dye. the pt is an active anode and during electrolysis the generation of hydroxyl radical is limited [6]. in presence of co3 2, no3 , so4 2 ions, the oxidation of tb is possible only either by the hydroxyl radicals generated on pt anode or by direct electron transfer with the anode. but pt anode does not show any electrochemical activity for the oxidation of tb (fig. 1). on pt, in presence of oxygen and above 0.8 v there exists pt and pto mixture [29]. slight decolorization of tb in presence co3 2, no3 , so4 2 ions is perhaps due to oxidation by the hydroxyl radicals generated on pt and by pto/pt couple. below are the possible reactions (4) – (7) on the surface of the pt anode [19,30]. pt + h2o  pt-ohads + h + + e (4) pt-ohads  pto + h + + e (5) xpto + raq  rox + xpt (6) 2pto  2pt + o2 (7) j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 172 decoloration is the clear evidence for the cleavage of the chromophoric (–n=n-) group of tb dye. since decolorization was very slow in presence of co3 2, no3 , so4 2 electrolytes, hence the cod removal cannot be more than color removal and is very small. therefore cod was not measured in these cases. figure 2a. the variation of absorption with electrolysis time. conditions; 0.2% nacl + 50 mg l -1 tb, current density – 93 ma cm2 , λmax = 590 nm figure 2b. effect of supporting electrolytes on color removal decoloration is the clear evidence for the cleavage of the chromophoric (–n=n-) group of tb dye. since decolorization was very slow in presence of co3 2, no3 , so4 2 electrolytes, hence the a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 173 cod removal cannot be more than color removal and is very small. therefore cod was not measured in these cases. effect of nacl concentration the effect of nacl concentration on the rate of color and cod removal was investigated with nacl concentration ranging from 0.05 % to 3.0 %. fig. 3a and 3b, show the effect of nacl on color and cod removal respectively. figure 3a. effect of nacl concentration on color removal figure 3b. effect of nacl concentration on cod removal (electrolysis time 60 min) j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 174 the amount of oxidants on the anode surface increases with the applied current density. but, at the given current density, the amount of cl ions is the limiting factor. if cl ion concentration in the solution is less, the current supplied would be lost in oxygen evolution, which is otherwise could be used to generate active chlorine species. the optimum cl ion concentration can facilitate high current efficiency and thus less energy consumption. at higher concentrations of cl ions, the applied current density is efficiently utilized to generate more oxidants [6]. at the same time, the concentration of cl ions should not exceed the concentration level commonly found in any real wastewater sample. so, the effect of cl ion concentration was tested within 0.3 % nacl. complete decolorization of tb solution occurred in 30 min of electrolysis with all concentrations of nacl from 0.1 % to 0.3 %, except with 0.05 % nacl, in which 98 % color removal was achieved. the rate of decoloration increased with increase in cl ion concentration. concomitant cod removal was observed with decolorization. the cod removal after 60 min electrolysis increased largely with nacl concentration from 0.05 % to 0.2 %. after this concentration, only slight improvement in the cod removal was noticed. therefore, 0.2 % nacl was chosen as the optimum concentration for further studies. effect of current density the efficiency and energy consumption for the degradation process is dependent on the applied current density and electrolysis time. higher current density always leads to high energy consumption and low current efficiency. the degradation of tb in the present case takes place by indirect oxidation brought about by the oxidants generated during electrolysis. higher current density enhances the rate of electrode reactions and hence rate of generation of oxidants also elevates. a range of current density from 37 to 111 ma cm -2 was applied to determine its effect on color and cod removal. the rate of color and cod removal increased with applied current density (fig. 4a and 4b). the cod removal increased as current density increased from 37 to 55 ma cm -2 , beyond which slow and steady raise in cod was noticed. in contrast, the current efficiency decreased with increse in applied current density. figure 4a. effect of current density on color removal a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 175 figure 4b. effect of current density on cod removal; conditions 0.2 % nacl, current density93 ma cm -2 , dye – 50 mg l -1 ; electrolysis time – 60 min the color removal was 100 % in all cases and the cod removal was 70 % to 95 % with 25 % to 11 % current efficiency. this implies that with increase in current density, though the production of oxidants increased, the undesired hydrogen and oxygen evolution reactions also increased alongside and consumed major part of the supplied current. a charge consumption of 3.60 kc l -1 was recorded for 92 % of cod removal under the current density of 93 ma cm -2 (60 min) whereas, 2.88 kc l -1 charge was consumed for the same cod removal under the current density of 37 ma cm -2 (120 min). this indicates that higher cod removal can be achieved with less charge (and hence energy) consumption, but requires longer time duration. effect of ph ph influences the chemical form of the oxidant present in the electrolyte. the electrolysis was carried out at four different ph. the electrochemical reaction in presence of chloride ions can be represented as in reactions (8) – (11); 2cl → cl2(aq) + 2e (standard potential of cl2 (e o = 1.40 v vs. she) (8) cl2(aq) + h2o → hocl + h + + cl (e o of hocl = 1.49 v vs. she) (9) hocl ↔ h + + ocl (e o of clo = 0.89 v vs. she) (10) tb + ocl → intermediates → co2 + h2o + cl (11) the ph variation influences the equilibrium in reaction (10) and the hocl predominates in the ph range 3-8 but above ph 8, clo is the major candidate [26,31]. the oxidation by hocl and cl2(aq) [5,6] is therefore expected to be more favorable and faster at near neutral ph. furthermore, reactions in reaction (12) and (13) are expected to take place on the active anode such as pt. the mox(hocl) mediates the oxidation unselectively and hence higher cod removal efficiency was noticed [26]. j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 176 mox(•oh) + cl → mox(hocl) + e (12) mox(hocl) + tb → intermediates → mox + co2 + h2o + cl + h + (13) the effect of ph on color and cod removal with electrolysis time is shown in figs. 5a and 5c, respectively. fig. 5b illustrates the slow decrease in the absorption intensity of the peak in the visible region with increase in ph (11.8). it is evident that color and cod removal is above 90 % in ph 3.1 and 6.2. as the ph increased to 9.0, the rate of color and cod removal reduced to 82 % and became 67 % in ph 11.8. figure 5a. effect of ph on color removal figure 5b. variation in the intensity of peak at 590 nm in ph 11.8 a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 177 figure 5c. effect of ph on cod removal; conditions cnacl, 0.2 % (w/v), current density – 93 ma cm -2 , [tb] – 50 mg l -1 , electrolysis time – 60 min complete decoloration was achieved in all ph except with ph 11.8, for which 88 % color removal was achieved. the difference in cod removal in ph 3.1 and ph 6.2 was only 3 % and in both the cases it was above 90 %. but cod removal decreased above ph 9.0. hence, for further studies, ph 6.2 was selected. effect of initial dye concentration at the optimized operating conditions of electrolysis process, only fixed amount of oxidants are generated at the anode. the energy would be simply lost if the generation of oxidants is more than that required to oxidize the available organics in the wastewater. sufficient concentration of organics should be available for the efficient utilization of generated oxidants. the tb solutions of different concentrations ranging from 25 to 100 mg l -1 were prepared and electrolysed under optimized operating conditions (93 ma cm -2 , ph 6.3 and 0.2 % nacl concentration). the color and cod removal decreased gradually with increase in initial concentration of tb. fig. 6 shows the trend of color and cod removal with time for different initial concentrations of tb dye. a 100 % decolorization was achieved within 30 min for all initial concentrations of tb. but 100, 91, 75 and 67 % cod removal was attained with 25, 50, 75 and 100 mg l -1 initial tb concentrations respectively after 60 min electrolysis. at initial concentration of 25 mg l -1 , the in situ generated oxidants were efficienty oxidized the tb completely. but, the dye concentration was less than the amount of oxidants available in the electrolysis cell (dye concentration is the limiting factor). when the tb dye concentration was increased to 100 mg l -1 , the amount of oxidants was less than the equivalent value, not sufficient to degrade the entire amount of tb (oxidant concentration is the limiting factor). under the currrent density 93 ma cm -2 and 60 min of electrolysis, the cod removal was 92 % for the initial concentraiton of 50 mg l -1 chosen as the optimum condition. j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 178 figure 6. effect of initial concentration of tb on cod and color removal. kinetics of tb degradation during the electrolysis, the concentration of tb in synthetic wastewater decreased exponentially for all applied current densities but the rate of oxidation increased with increase in current density, indicating that the degradation of tb followed the first order kinetics [32]. if we consider that the oxidants generated on electrolysis spontaneously react with tb, then from the steady state approximation, the concentration of oxidants (ox) can be estimated [32] and the rate of oxidation of tb can be expressed as in eq. (13) and (14); table 2. degradation kinetics data parameters j / ma cm -2 * initial concentration of tb, mg l -1 # 37 55 74 93 111 25 50 75 100 kapp / min -1 0.022 0.032 0.037 0.044 0.053 0.058 0.043 0.026 0.021 regression 0.867 0.978 0.967 0.974 0.993 0.974 0.986 0.92 0.841 * tb concentration 50 mgl -1 ; # current density 93 ma cm -2 [ ] [ ][ ] (13) [ ] (14) on integrating the above equation; [ ] [ ] (15) where [ox] is the concentration of oxidants, kapp is the apparent pseudo first-order rate constant ([ox] is considered as very high), cod0 and codt are the cod at time 0 and t minutes of electrolysis respectively. the slope gives kapp which are tabulated in table ii. the first order rate constant steadily increased with increase in applied current density and decreases with increase in initial a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 179 concentration of tb dye (fig. 7a and 7b). the regression value indicates the deviation from the first order reaction rate equation for current density 37 ma cm -2 and tb concentration 100 mg l -1 can be attributed to the fact that at lower applied current density and high initial concentration of tb, the active concentration of oxidants is the limiting factor. moreover, the area of the anode used in all these experiments was only 0.57 cm 2 . figure 7a. plot of ln (c0/ct) vs. time for different applied current densities figure 7b. plot of ln(c0/ct) vs time for different initial dye concentrations the oxidants generated on the surface of such a small area of pt anode under the current density of 37 ma cm -2 might not be so high to satisfy the steady state approximation for first order j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 180 kinetics. similarly, oxidants generated under the current density of 93 ma cm -2 and 100 mg l -1 initial concentration of tb dye, a similar deviation was observed. spectroscopic analysis uv-vis spectroscopic analysis uv-vis spectrum of tb dye showed three major peaks at 235, 318 and 590 nm. the peak at 590 nm corresponds to the chromophoric azo group. on electrolysis (50 mgl -1 dye, 93 macm -2 ), the intense blue color of the solution turned in to wine red in the first 10 min, then transformed slowly into pale yellow color, which persisted till the end. the intensity of the peak at 590 nm rapidly decreased to zero absorbance on electrolysis, which can be ascribed to the cleavage of azo group. figure 8a. uv-visible absorbance of tb at different time intervals of electrolysis figure 8b. absorbance of residue obtained by the evaporation of electrolyzed tb solution a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 181 for the first 10 min of electrolysis, the peak at 585 nm showed blue shift and gradually reached zero absorbance. the blue shift can be attributed to the detachment of auxochromes from the parent dye structure during electrolysis. a new peak at 407 nm was observed, which can be ascribed to the formation of a fragment from the parent dye. this peak diminishes very slowly. the pale yellow color is due to the absorption at 412 nm, (absorption color – violet (412 nm), perceived color – yellow) which reduced slowly with time (fig. 8a). peak at 318 nm reduced with time, reached minimum value and again increased with a blue shift at 288 nm. this peak can be assigned to the low molecular weight oxidized fragments with conjugated π bonds in which nπ* and ππ* transitions are allowed (-c=o, -c=c-, -c=n-, -(co)nh-). similar effect was observed by hanene et al. [34] in the degradation of amido black (ab) on boron doped diamond electrode. the electrolyzed dye solution was evaporated at lower temperature with reduced pressure. uv-vis spectrum of the solid thus obtained was taken by dissolving it in water. interestingly, the peak at 288 nm was absent. this observation validates our suspicion that compounds leading to this peak were low molecular volatile compounds (fig. 8b). after 40 min electrolysis, the uv-vis spectrum of the residue obtained by evaporation does not show any peak, except a peak in 200 to 220 nm range. this is due to the small amount of aromatic compounds resistant to indirect oxidation remained in the solution, which also accounts for the residual cod after 60 min. ft-ir analysis the ft-ir spectrum of tb dye (fig. 9a) exhibits a characteristic band at 1579 cm -1 corresponding to azo group [34, 35]. the bands at 1492 cm -1 and 1423 cm -1 correspond to c=c aromatic ring stretching in combination with c=n [34] and methyl bending respectively. the keto form of the tb is dominant (table i) [36]. the peak at 1617 cm -1 is due to α,β-unsaturated c=o stretch. the peaks at 1339, 1188 and 1128 cm -1 are due to aromatic amine c-n and -so3 (s=o) stretch [35]. the peak obtained at 1042 cm -1 is due to c-o. the broad doublet peak at 674 cm -1 and 644 cm -1 indicates the aromaticity. in fig. 9b and 9c, the peaks at 1579, 1492, 1423 cm -1 and 1339, 1128, 1042, 674 cm -1 disappeared completely indicating the breakage of corresponding groups. figure 9. ft-ir spectra of a) tb dye b) residue obtained by evaporation of sample electrolyzed for 15 min c) electrolyzed for 40 min j. electrochem. sci. eng. 3(4) (2013) 167-184 trypan blue electrochemical oxidation 182 the peak at 1229 cm -1 disappeared in fig. 9c, indicating that group corresponding to this frequency is susceptible to oxidation by the electrochemically generated oxidants [37]. the peak at 1119 cm -1 shifted to 1108 cm -1 . while peak 674 cm -1 disappeared completely, 644 cm -1 shifted to 621 cm -1 . in fig. 9c the peak at 1637 cm -1 is due to the carbonyl stretch of carboxylate salts, 1380 cm -1 is due to c-n of aliphatic amines. this suggests the formation of carboxylic acids and amines. 1008 cm -1 is devoted to c-o stretch and peak at 966 cm -1 is due to the formation of clo3 and ocl during electrolysis [34,35]. these changes in the ft-ir spectrum are the clear evidence for the degradation of tb into simpler molecules, like aromatic and aliphatic amines and carboxylic acids. identification of intermediate products and degradation pathway the intermediates of tb degradation were identified by gas chromatography mass spectrometry (gc-ms). on the basis of the intermediates and end products as depicted by gc-ms, a possible degradation pathway is proposed in fig. 10. gc-ms revealed that the parent tb molecule was completely disintegrated into simpler molecules. figure 10. a possible degradation pathway of tb a. n. subba rao at al. j. electrochem. sci. eng. 3(4) (2013) 167-184 doi: 10.5599/jese.2013.0039 183 decomposition of tb was initiated with the cleavage of azo bonds resulting in the formation of three different intermediates 1 (3,3'-dimethyl-1,1'-bi(cyclohexa-2,5-dien-1-ylidene)-4,4'-diimine), 2 (5-amino-3,4-dihydroxynaphthalene-2,7-disulfonic acid) and 3 (2,5,7-trihydroxy-3-[(4-hydroxy-2methylphenyl)diazenyl]naphthalene-1,4-dione). intermediate 3 resulted from the cleavage of one of the azo bonds in the parent molecule was found in very small concentration. the absorption at 412 nm in uv-vis spectroscopy can be attributed to this intermediate. the sulfonic group in intermediate 2 renders it water soluble. a foamy layer was formed on the surface of the electrolytic bath solution during electrolysis gradually disappeared with time. this can be attributed to the formation of intermediates 1 and 3 at the beginning of electrolysis. further oxidation of intermediates 2 and 3 produced aromatic acids and phenols, 5 (naphthalene1,2,3,6,8-pentol), 6 (3,5-dihydroxybenzene-1,2-dicarboxylic acid), 9 (benzene-1,2,4-triol). aliphatic acids were generated on the oxidative ring opening of 4, 6 and 9, which decomposed into co2 to accomplish the degradation process. conclusion the tb dye synthetic wastewater was successfully treated in presence of cl with pt foil as anode material. the removal of tb was dependent on the applied current density, ph, type of supporting electrolyte and concentration of tb dye and nacl. the degradation of tb was achieved only in presence of nacl as supporting electrolyte suggesting that the active chlorine species (cl2, clo , hocl) generated during the electrolysis played crucial role in the oxidation of tb. the rate of removal of tb increased with increase in current density and nacl concentration and decreased with rise in initial concentration of tb and ph. under the optimum conditions (current density – 93 ma cm -2 , ph 6.2, nacl concentration 0.2 %, initial tb concentration – 50 mgl -1 ) maximum cod and color removal achieved after 60 min electrolysis was 92 % and 100 % respectively. the first order rate constant for cod removal at these conditions was 0.044 min -1 . the uv-vis spectroscopy, ft-ir and gc-ms analysis confirmed the complete degradation of tb into simple aliphatic carboxylic acid and co2 suggesting that the treatment of wastewater containing tb dye as pollutant can be successfully accomplished by electrochemical method. acknowledgement: we thank department of chemistry, kuvempu university, shankaraghatta and ugc, new delhi for their financial support. references [1] h. zollinger, synthesis, properties of organic dyes and pigments. in: color chemistry. new york, usa: vch publishers; (1987) pp-92-102 [2] m. b. ibrahim, n. poonam, s. datel, m. roger, bioresour technol. 58 (1996) 217-227 [3] e. forgacs, t. cserháti, g. oros, environ inter. 30 (2004) 953971 [4] w. przystaś, e. zabłocka-godlewska, e. grabińska-sota, water, air, soil 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(2012) 5033-5041 [34] h. z. zhao, y. sun, l. n. xu, j. r. ni, chemosphere 78 (2010) 46-51 [35] s. raghu, c. w. lee, s. chellammal, s. palanichamy, c. a. basha, j. hazard. mater. 171 (2009) 748-754 [36] j. p. graham, m. a. rauf, s.hisaindee, m. nawaz, j. molecul. struc. 1040 (2013) 1-8 [37] s. s. vaghela, a. d. jethva, b. b. mehta, s. p. dave, s. adimurthy, g. ramachandraiah, environ. sci. technol, 39 (2005) 2848-2855 © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://toxnet.nlm.nih.gov/cgi-bin/sis/search/f?./temp/~s8y7ri:1 http://toxnet.nlm.nih.gov/cgi-bin/sis/search/f?./temp/~s8y7ri:1 http://www.springer.com/978-1-84800-935-6 http://creativecommons.org/licenses/by/3.0/ abstract introduction materials and chemicals chemicals experimental electrolysis analysis cyclic voltammetry (cv) spectroscopic and chemical analysis results and 􀀘iscussion cyclic voltammetry studies effect of supporting electrolyte effect of nacl concentration effect of current density effect of ph effect of initial dye concentration kinetics of tb degradation spectroscopic analysis uv-vis spectroscopic analysis ft-ir analysis identification of intermediate products and degradation pathway conclusion acknowledgement references simultaneous electrochemical determination of acetaminophen and metoclopramide at electrochemically pre-treated disposable graphite pencil electrode doi:10.5599/jese.308 265 j. electrochem. sci. eng. 6(3) (2016) 265-276; doi: 10.5599/jese.308 open access : : issn 1847-9286 www.jese-online.org original scientific paper simultaneous electrochemical determination of acetaminophen and metoclopramide at electrochemically pre-treated disposable graphite pencil electrode shreekant m. patil, vijay p. pattar, sharanappa t. nandibewoor p. g. department of studies in chemistry, karnatak university, dharwad 580003, india corresponding author: stnandibewoor@yahoo.com; tel.: +91 836 2215286; fax: +91 836 2747884 received: may 30, 2016; revised: september 22, 2016; accepted: september 26, 2016 abstract a sensitive and economic voltammetric method was developed for the simultaneous determination of acetaminophen (amp) and metoclopramide (mcp) using pre-treated graphite pencil electrode (ptgpe). compared to a graphite pencil electrode, the pretreated electrode showed an apparent shift of the oxidation potentials in the positive direction and a notable enhancement in the current responses for both amp and mcp. cyclic voltammetry (cv) was used to study the voltammetric behavior of the drugs, while differential pulse voltammetry (dpv) was used to determine amp and mcp simultaneously. the dependence of the current on scan rate, ph and concentration was investigated to boost the experimental conditions for simultaneous determination. the calibration curves were obtained over the range of 0.1×10-7 to 1.1×10-7 m, the concentration of each of both the drugs was varied by keeping the other constant, and achieved lower detection limit of 3.25 nm for amp and 1.16 nm for mcp. the developed method was found to be selective and rapid for the simultaneous determination of amp and mcp. the proposed method was applied simultaneously in real samples and pharmaceutical samples, with satisfactory results. keywords acetaminophen, metoclopramide, voltammetry, graphite pencil electrode, analytical applications introduction the development of a sensitive, simple, reliable and rapid method for the determination of analyte is of great importance. the pre-treated graphite pencil electrode (ptgpe) has been used as a biosensor in modern electroanalytical field due to its high mechanical rigidity, highly economical, good electrochemical reactivity, ease of modification, renewal of electrode and low background current [1,2]. ptgpe has good application in analysis of drugs, detection of traces of metal ions and http://www.jese-online.org/ mailto:stnandibewoor@yahoo.com j. electrochem. sci. eng. 6(3) (2016) 265-274 determination of acetaminophen & metoclopramide 266 neurotransmitters. metoclopramide (4-amino-5-chloro-n-[2-(diethylamino)ethyl]-2-methoxybenzamide) (mcp) as shown in scheme 1 has wide range of clinical applications in different fields such as gastroenterology, gynaecology, surgery, radiology and cardiology. it shows antiemetic and prokinetic properties in disorders of decreased gastrointestinal motility and it is a dopamine receptor antagonist which also plays very important role as active ingredient of many pharmaceutical formations related with the modification of digestive behavior. mcp hydrochloride is commonly used in prevention and relief of nausea and vomiting [3-4] but mainly used in combination with chemotherapy, where drugs such as cisplatin, and some cytotoxic agents, are highly emetic [5]. scheme 1. chemical structure of amp and mcp acetaminophen (amp) (paracetamol, n-acetyl-p-aminophenol) (scheme 1) is a well-known drug which has extensive applications in pharma industries. it is a non-steroidal, antipyretic and antiinflammatory drug [6]. it is the preferred alternative to aspirin, specifically for patients who cannot tolerate aspirin [7] and its use is one of the common causes of poisoning worldwide [8] and analgesic compound that has high therapeutic value. it is also used as a precursor in penicillin and as stabilizing agent for hydrogen peroxide, photographic chemical, etc. [9]. at the recommended dosage, there are no side effects. however, overdoses of acetaminophen cause liver and kidney damage [10] and may lead to death. it is suspected that a metabolite of acetaminophen is the actual hepatotoxic agent [11]. acetaminophen/metoclopramide hydrochloride is an oral fixed dose combination containing the analgesic acetaminophen and the antiemetic metoclopramide hydrochloride. the combination is used to treat the migraine symptoms, both to relieve headache and to treat associated nausea and vomiting (the antiemetic). in addition to its direct antiemetic effect, mcp also stimulates gastric emptying (prokinetic), which is often delayed during migraine attacks, and accelerates the absorption of amp. the combination of amp and mcp has shown to enhance the analgesia relief when used to treat the arthritis pain. as mcp is always given with amp in any formulations, it is important to determine both the components simultaneously. allthough there are different methods of anlysis of amp [12] and mcp [13] separately mentioned in the literature, only a few classical methods are available [14-18] for the simultaneous determination of these componds. in the present study, the electrochemical method, a very sensitive and selective method of analysis using newly functionalised ptgpe has been used in the determination of amp and mcp simultaneously. the proposed method was applied in real samples and pharmaceuticals. experimental aspects reagents and chemicals the pencil-lead rods (hb 5 cm length and 0.5 mm in diameter) were purchased from a local stationary. amp and mcp were purchased from sigma aldrich, india. stock solutions of amp and o h n o n cl h2n s. m. patil et al. j. electrochem. sci. eng. 6(3) (2016) 265-274 doi:10.5599/jese.308 267 mcp (1.0 mm) were freshly prepared in millipore water. the phosphate buffer solutions (pbs) from ph 3.0 to 11.2 were prepared according to the method of christian and purdy [19]. other reagents used were of analytical grade. all other solutions were prepared with millipore water. instrumentation and analytical procedures the voltammetric experiments were carried out on a chi 630d electrochemical analysing system (ch instruments inc., usa). the voltammetric experiments were carried out in a 10 ml singlecompartment of three electrode glass cell with the reference electrode as silver electrode (ag/agcl), platinum as counter electrode and ptgpe as working electrode. ph measurements were carried with elico li120 ph meter (elico ltd., india). all experiments were performed at an ambient temperature of 298± 0.2k. the area of the electrode was obtained by the cyclic voltametric method using 1.0 mm k3fe(cn)6 at different scan rates. for a reversible process, the following randles-sevcik formula was used [20]. ip = 0.4463(f3/rt)1/2n3/2a0d01/2c0ν1/2 (1) where ip refers to the anodic peak current, a0 is the surface area of the electrode, n is number of electrons transferred, ν is the scan rate, d0 is the diffusion coefficient and c0 is the concentration of k3fe(cn)6. for 1.0 mm k3fe(cn)6 in 0.1m kcl electrolyte, t = 298 k, r= 8.314 j k-1mol-1, f = 96,480 coulombs mol-1, n=1, and d0=7.6×10-6cm2s-1;then from the slope of the plot of ip versus ν1/2, relation, the surface area was calculated. in our experiment the slope was 4.136x10-5a (vs-1)1/2 and the area of electrode was calculated to be 0.1311 cm2 which is three times more than that of the gpe. measurement procedure stock solutions of 1 mm of amp and mcp were prepared by dissolving the desired amount in millipore water. voltammograms were then recorded using voltammetric analyzer under the optimized parameters. the parameters for dpv were, amplitude: 0.05 v; initial potential: 0.0 v; final potential: 1.4 v; increase in potential: 0.004 v; pulse width: 0.06s; sample width: quiet time: 2 s; sensitivity: 1×10-6 a v-1. pre-treatment of electrode the electrochemical treatment of gpe was performed in different supporting electrolytes by potential cycling between -2.0 v and +2.0 v with a scan rate of 50 mv s-1 for six scans. the investigated supporting electrolytes were each of 0.1 m h3bo3, nano3, hclo4, h2so4, h3po4, hcl, liclo4, and na2co3. the results showed that the gpe electrodes pretreated in 0.1 m hcl was the most selective and sensitive towards a amp and mcp. so 0.1 m hcl was chosen for pre-treatment of electrodes. the prepared electrodes (ptgpe) were stored at room temperature in desiccators. preparation of real and pharmaceutical samples the urine samples were collected from healthy humans and were diluted 100-fold with the phosphate buffer solution before analysis. quantitative determination was performed by adding a standard solution of amp and mcp to the detection system with the urine sample. metpar® tablets (cipla, india) containing 500 mg of acetaminophen and 5 mg of metoclopramide were purchased from local pharmacy and were ground to a homogeneous fine powder in a mortar separately. a suitable amount of this powder was weighed and was treated with millipore water for 20 minutes. the mixture was then filtered and solutions obtained from the filtration were diluted j. electrochem. sci. eng. 6(3) (2016) 265-274 determination of acetaminophen & metoclopramide 268 to 100 ml with millipore water. aliquots of this solution were analyzed within the calibration conditions. results and discussion cyclic voltammetry electrochemical response of a solution having homogeneous mixture of 1.0 µm of each amp and mcp was estimated by cyclic voltammetry at 10 mv s-1 under optimized parameters using ptgpe. anodic peaks for the oxidation of amp and mcp were observed at 0.693 v and 1.123 v, respectively, as shown in fig. 1. reduction peak is absent in the reverse sweep for both compounds which clearly indicates the irreversibility of the electrode reaction. the peak intensities for ptgpe are better compared to the peak intensities obtained by using bare gpe. potential, v fig. 1. comparison of electrochemical behavior of mixture of 1mm of amp and 1mm of mcp at (a) bare gpe, (b) ptgpe, with scan rate 10 mv s-1 at ph 4.2. effect of ph the electro-oxidation of mixture of 1.0 µm each of amp and mcp was studied over the ph range of 3.0–8.0 in phosphate buffer solution by differential pulse voltammetry which is as shown in fig. 2a. the ph of solution influenced the peak current [21-23]. the ph dependence of the peak potential when dpv was used is shown in fig. 2b (a and b). with an increase in ph of the solution, peak potential shifted to less positive values and was found to obey the following equations: ep = -0.0293 ph + 0.7214; r = 0.9821 for amp ep = -0.0297 ph + 1.1483; r = 0.9615 for mcp the best result with respect to sensitivity accompanied with sharper response and well separated peaks was obtained with ph = 4.2 (fig. 2c), and hence it was selected for further experiments. a c u rr e n t, ! 0 -5 a s. m. patil et al. j. electrochem. sci. eng. 6(3) (2016) 265-274 doi:10.5599/jese.308 269 potential, v b ph ph c ph ph fig. 2. a dependence of ph on the oxidation of mixture of amp and mcp at ph (i) 3.0, (ii) 4.2, (iii) 5.0, (iv) 6.0, (v) 7.0, (vi) 8.0, (vii) 9.0, (viii) 10.4, (ix) 11.2; b variation of peak potential with ph for (a) 1.0 µm mcp and (b) 1.0 µm amp; c variation of peak current with ph for (a) 1.0 µm mcp and (b)1.0 µm amp effect of scan rate the effect of scan rate on the voltammetric oxidation of a mixture of amp and mcp was examined by cyclic voltammetry between 10 to 190 mv s-1 (fig. 3a). a linear relationship was observed between log ip and log v, (fig. 3b) corresponding to the following equations: a c u rr e n t, 1 0 -7 a e p / v e p / v i p / 1 0 -5 a i p / 1 0 -5 a j. electrochem. sci. eng. 6(3) (2016) 265-274 determination of acetaminophen & metoclopramide 270 potential, v b log (v / v s-1) log (v / v s-1) c log (v / v s-1) log (v / v s-1) fig. 3. a effect of scan rate on the electro-oxidation of a mixture of amp and mcp. (i) 10, (ii) 30, (iii) 50, (iv) 70, (v) 90, (vi) 110, (vii) 130, (viii) 150, (ix) 170, (x) 190 mv s-1; b observed dependence of peak current on the square root of scan rate for (a) mcp and (b) amp; c plot of variation of peak potential with logarithm of scan rate for (a) mcp and (b) amp. log ip = 0.6508 log v + 1.4111, r = 0.9942 for amp and log ip = 0.3989 log v + 1.4981, r = 0.9796 for mcp the slopes of 0.65 v s-1 and 0.39 v s-1 were in the neighborhood of the theoretically expected value of 0.5 v s-1 for a purely diffusion controlled process [24] which, in turn, further confirms that the electro-oxidation of amp and mcp were diffusion controlled. with an increase in scan rate, the peak potential shifted to a more positive value and a linear relationship was observed in the range 0.01 to 0.19 v s-1 as shown in fig. 3c (a) and (b). the corresponding equations are expressed as: ep = 0.0887 log v + 0.8629, r = 0.9746 for amp and c u rr e n t, 1 0 -7 a lo g ( i p / 1 0 -4 a ) lo g ( i p / 1 0 -4 a ) e p / v e p / v s. m. patil et al. j. electrochem. sci. eng. 6(3) (2016) 265-274 doi:10.5599/jese.308 271 ep = 0.0550 log v + 1.1962, r = 0.9858 for mcp for an irreversible electrode process, according to laviron [25], ep is defined by the following equation (2).                 0 0 p 2.303 2.303 log log rt rtk rt e e v nf nf nf (2) where n is the number of electrons transferred, v is the scan rate, α is the transfer coefficient, k0 is the standard heterogeneous rate constant of the reaction and e’ is the formal redox potential. other symbols have their usual meanings. thus the value of αn was easily calculated from the slope, and k0 was calculated from the intercept of ep vs. log v. e0 was obtained from the intercept of ep vs. v plot by extrapolating to the vertical axis at v = 0 [24] taking t = 298 k, r = 8.314 j k-1 mol-1 and f = 96 480 c mol-1, the transfer coefficient () and number of electrons transferred (n) were calculated for both the drugs. the α value was calculated according to the bard and faulkner [26] equation (3).    p p/2 47.7 mv e e (3) where ep/2 is the potential when the current is at half the peak value. from this, the value of  was calculated to be 0.29 for amp and 0.47 for mcp. further, the number of electrons (n) transferred in the electro-oxidation of amp and mcp were calculated to be 2.1≈2 and 2.28≈2.0, respectively. plausible mechanism based on the experimental results, the number of electrons transferred (n) for both the drugs were calculated to be two. hence the probable electrooxidation mechanisms for both the drugs are proposed as given in scheme 2, which are based on earlier works [27,28]. i ii scheme 2. proposed mechanisms for oxidation of (i) amp and (ii) mcp hn oh c o ch3 n o c o ch3 + 2e + 2h+ nh2 r meo cl 2 nh nh meo cl r + 2e + 2h+ cl r meo nh nh meo cl r cl r meo nh2 nh meo cl cl r meo + r+ + ohh2o r = conh(ch2)2n(c2h5)2 j. electrochem. sci. eng. 6(3) (2016) 265-274 determination of acetaminophen & metoclopramide 272 analytical application to obtain an analytical curve for the sensor, quantitative analyses of the amp and mcp concentrations were performed by dpv [29] at a ptgpe under the optimized experimental conditions. the oxidation of mcp occurs at high positive potential and there can be a chance of overlapping of mcp potential with the increasing concentration of drugs. oxidation peak current was found to increase and no change in the peak current or potential of another drug indicates that these two drugs do not interact with each other. the peak current versus concentration of drug plots show a good linearity for amp and mcp in certain concentration ranges as depicted in fig. 4a and b. peak current values were obtained by subtracting the background current of pbs and average of three replicate measurements were used to plot calibration curves. linear regression equations for both the drugs arising from calibration plots are represented as: ip = 0.212 c + 0.344, r = 0.9703 for amp ip = 0.928 c + 1.257, r=0.9767 for mcp a potential, v b potential, v fig. 4. a differential pulse voltammograms for increasing concentration of mcp at ptgpe (i) 1, (ii) 3, (iii) 5, (iv) 7, (v) 9, (vi) 11, (vii) 13, 10-7m with amp concentration 1×10-6m.; b differential pulse voltammograms for increasing concentration of amp at ptgpe (i) 1, (ii) 3, (iii) 7, (iv) 9, (v) 11, (vi) 13, 10-7m with mcp concentration 1x10-7m. the limit of detection (lod) and limit of quantification (loq) were calculated by using the formulae lod = 3s/m and loq = 10s/m, where s is the standard deviation of peak current and m is c u rr e n t, 1 0 -7 a c u rr e n t, 1 0 -7 a s. m. patil et al. j. electrochem. sci. eng. 6(3) (2016) 265-274 doi:10.5599/jese.308 273 the slope of calibration curves. lods were found to be 3.25×10-9 m and 1.16×10-9 m for amp and mcp respectively and the loqs were calculated as 10.84×10-9 m and 3.88×10-9 m, respectively. since, it is important to calculate validation parameters for any analytical method, the calibration characteristics obtained for mcp and amp are given in table 1. table 1. the calibration characteristics for metoclopramide and acetaminophen at ptgpe. mcp amp linearity range, 10-7 m 0.1-1.1 0.1-1.1 slope of the calibration plot 0.928 0.212 intercept 1.275 0.344 correlation coefficient (r) 0.9767 0.9703 rsd of slope, % 1.21 0.82 rsd of intercept, % 0.12 0.65 number of data points 5 5 lod, nm 1.16 3.25 loq, nm 3.88 10.84 repeatability of peak current, % 0.79 1.14 repeatability of peak potential, % 0.51 0.35 reproducibility of peak current, % 0.92 0.71 reproducibility of peak potential, % 0.24 0.43 the precision of the method was calculated by repeating five experiments on the same day in standard conditions (repeatability) and over two days from the different standard solutions (reproducibility). for these studies 1.0×10-6 m of each of amp and mcp standard solutions were used. from rsd values of peak potential and peak current between day reproducibility were similar to that of within a day if the temperature was kept almost unchanged which shows the excellent stability and reproducibility of ptgpe. effect of excipients the effect of some common excipients used commonly in pharmaceutical preparations was examined. the tolerance limit was defined as the maximum concentration of the interfering substance that caused less than 5 % error for the determination of amp and mcp. the effects of these excipients on the voltammetric response were obtained by analyzing sample solutions containing a fixed amount of amp and mcp (1.0×10-6 m) spiked with various amounts of each excipient under the same experimental conditions. the experimental results showed that a hundred-fold excess of gum acacia, citric acid, dextrose, glucose, lactose, tartaric acid and sucrose did not interfere with the voltammetric signal of amp and mcp. hence, these compounds need not be extracted from these tablet additives prior to their determination in tablets. detection of amp and mcp in human urine samples the developed dpv method for the amp and mcp determination was applied to human urine samples. the recoveries from urine were measured by spiking drug free urine with known amounts of amp and mcp. the urine samples were diluted 100 times with the pbs before analysis without further pretreatment. a quantitative analysis was carried out by adding the standard solutions of amp and mcp into the detection system of urine samples. the peak current increased linearly in height. the calibration plot was used for the determination of spiked amp and mcp in urine samples. the results of four urine samples obtained are listed in table 2. thus, satisfactory j. electrochem. sci. eng. 6(3) (2016) 265-274 determination of acetaminophen & metoclopramide 274 recoveries of the analytes from the real samples were in a good agreement with the concentration ranges studied and the real ranges encountered in the urine samples when treated with drug, make the developed method applicable in clinical analysis. table 2. results of analysis of metoclopramide and acetaminophen in spiked urine samples a -average of five determinations determination of amp and mcp in pharmaceutical samples this method was applied for the determination of amp and mcp in tablets. as shown in table 3, the content of amp and mcp was calculated to be 495.1 mg/tablet (the nominal content is 500 mg/tablet) and 9.7 mg/tablet (the nominal content is 10 mg/tablet) respectively. to validate and obtain the accuracy and precision of the developed method, recovery studies were performed at different drug concentrations by the standard addition method. for the present study, known quantities of amp and mcp were mixed separately with definite amounts of pre-analyzed formulations and mixtures were analyzed as before. table 3 shows the measurement results, and the average recovery was found to be 99.02 % and 97.0 % for amp and mcp respectively. the f and student t tests were also calculated with confidential level of 95 % and are shown in table 3. table 3. results of analysis of tablet containing both the analytes (acetaminophen 500 mg and metoclopramide 10 mg) in commercially available tablet samples by dpv acetaminophen metoclopramide labeled claim, mg 500.00 10.00 quantity found, mg 495.10 9.70 recoverya, % 99.02 97.00 rsda, % 1.92 2.31 tvalue at 95% confidence level 0.16 0.21 f-value at 95% confidence level 1.47 1.76 a -average of five determinations conclusion the pre-treated pencil graphite electrode was applied successfully as a sensor for fast, accurate and simultaneous determination of amp and mcp in some pharmaceutical samples. the present method is a good alternative for the analytical determination of amp and mcp simultaneously, because it is simple, sensitive, accurate, fast, and inexpensive. the results were successfully applied mcp added, 10-8 m quantity found(a), 10-8 m average recovery, % rsda, % metoclopramide 10 9.7 97.0 1.91 15 14.5 96.6 2.20 20 19.2 96.0 2.12 25 24.6 98.2 2.66 30 29.6 98.8 1.59 35 34.2 97.4 2.21 acetaminophen 10 9.5 95.0 1.83 15 14.9 99.5 1.24 20 20.1 100.5 2.62 25 24.4 97.6 2.31 30 29.1 98.8 1.02 35 34.1 96.4 1.61 s. m. patil et al. j. electrochem. sci. eng. 6(3) (2016) 265-274 doi:10.5599/jese.308 275 in urine samples and pharmaceuticals. furthermore, the present method could possibly be employed for pharmacokinetic studies and also in clinical and quality control laboratories. acknowledgement: the author thanks the ugc, new delhi for the award of ugc-bsr faculty fellowship to dr. s. t. n. references [1] j. i. gowda, s.t.nandibewoor, anal. methods 6 (2014) 5147-5154. 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[29] n. l. teradal, s. n. prashanth, j. seetharamappa, j. electrochem. sci. eng. 2 (2012) 67-75. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {solvent dielectric effect on electrochemical properties of 3,4-propylenedioxythiophene:} http://dx.doi.org/10.5599/jese.1035 263 j. electrochem. sci. eng. 11(4) (2021) 263-277; http://dx.doi.org/10.5599/jese.1035 open access : : issn 1847-9286 www.jese-online.org original scientific paper solvent dielectric effect on electrochemical properties of 3,4-propylenedioxythiophene keziban huner1,2, abdulkadir sezai sarac1,3, 1department of chemistry, istanbul technical university,maslak, 34469 istanbul, turkey 2department of chemistry, yildiz technical university, esenler, 34220 istanbul, turkey 3polymer science and technology, istanbul technical university, maslak, 34469 istanbul, turkey corresponding author: sarac@itu.edu.tr received: june 28, 2021; accepted: september 13, 2021; published: october 1, 2021 abstract the present study is focused on the electrochemical properties of poly(3,4-propylenedioxythiophene) (poly(prodot)), electrocoated on the single carbon-fiber microelectrode (scfme) in different electrolytic media, with different solvent dielectric constants (35.9, 41.7, 47.5, 53.3, 59.1 and 64.9). the highest deposition charge density of 24.49 mc cm-2 and the highest specific capacitance of 23.17 mf cm-2 were obtained for poly(prodot) synthesized in a medium with the lowest solvent dielectric constant ( = 35.9). electrochemical impedance spectroscopy (eis) results of poly(prodot) coated scfme measured at open circuit potential showed continuously increased impedance magnitudes as ε was increased from 35.9 to 59.1. for all films, almost capacitive impedance responses at lower frequencies at least were obtained. the highest capacitance was observed for the polymer film synthesized in the medium of  = 35.9. the impedance of this film was also measured in different solvent mixtures with different dielectric constants at open circuit potential. keywords carbon fiber; surface modification; electropolymerization; polythiophene derivate; electrochemical impedance spectra. introduction today, electrochromic, optical and thermoelectric properties of conductive polymers used in many fields continue to be investigated [1-5]. the conductive polymer materials with enhanced properties can be produced by electrochemical methods [6-8]. electrochemical properties of electrochemically synthesized conductive polymers can be investigated by cyclic voltammetry (cv) and electrochemical impedance spectroscopy (eis) [9-12]. electrochemical impedance spectroscopy can be used both for volume and interfacial region studies and is associated with time constants ranging from minutes to microseconds [13]. http://dx.doi.org/10.5599/jese.1035 http://dx.doi.org/10.5599/jese.1035 http://www.jese-online.org/ mailto:sarac@itu.edu.tr j. electrochem. sci. eng. 11(4) (2021) 263-277 electrochemical properties of poly(prodot) 264 in conducting polymers of importance for biomedical devices, low interfacial impedance and enhanced charge storage capacity are generally considered essential. it was already shown that electroplating with poly(3,4-alkylenedioxythiophene) reduced electrical impedance in biomedical devices and produced films with sufficient softness and large surface area [14-16]. due to these film properties, there is a significant reduction of the electrical impedance of biomedical devices in the biologically relevant frequency range (around 1000 hz), and just this frequency corresponds to the typical pulse width of a neural signal (around 1–2 ms) [17]. therefore, poly(3,4-alkylenedioxythiophene) and its derivatives are promising electronic materials used in organic bioelectronics for biosensing [18-21]. carbon-based electrodes electrochemically coated with poly(prodot) exhibit enhanced capacitive behavior and are promising materials for supercapacitor applications [22-24]. micron size carbon fibers having low resistivity, high surface area, and small crystallites formed in the stacking direction during electropolymerization can improve the interface properties between polymer and carbon fiber. the approach of using a single carbon-fiber rather than a fiber bundle has the advantage of precise characterization of the coated film by electrochemical techniques. conductive polymers electrocoated on cfmes were previously studied and reported in detail by sarac and coworkers [25-27]. ftir-atr, afm and sem characterizations of poly(prodot) and poly(prodot-con-phenylsulfonyl pyrrole) films synthesized by electropolymerization in 0.1 m naclo4/mecn were previously studied by the same group, and some characterization details are reported in [28]. in this study, polymer of 3,4-propylenedioxythiophene (prodot), a derivative of polythiophene (pth) [29-31] was electro-coated specifically on a single carbon-fiber microelectrode (scfme) in different electrolytic solutions having different solvent dielectric constants. the effect of the dielectric constant of the solvent on electrochemical properties of poly(prodot) coated sfcme was investigated in detail by cv and eis techniques. experimental chemicals 3,4-propylenedioxythiophene (prodot), propylene carbonate (pc), and tetrabutylammonium hexafluorophosphate (tbapf6) were purchased from aldrich. acetonitrile (mecn) and propylene carbonate (pc) were obtained from riedel de haën. carbon fibers of aksa acrylic co. were used in all experiments. all chemicals were of high purity. electrochemical experiments electrochemical polymerization was performed potentiodynamically by cv technique using a potentiostat 2263 electrochemical analyser (princeton applied research, usa) interfaced to a pc and controlled by powersuit software package in one-compartment three-electrode cell. the electrochemical behavior of polymer samples prepared in different solutions was examined by the cv technique with the same three-electrode system. scfme with an area of 2.19×10-3 cm2, a platinum wire, and a silver wire were used as working, counter and reference electrodes, respectively. ag wire used as the reference electrode was checked and calibrated by ferrocene (0.1 m). eis measurements of poly(prodot) coated sfcme in different dielectric media were performed using potentiostat 2263 electrochemical analyser (princeton applied research, usa) within a frequency range between 100 khz and 0.01 hz and ac signal amplitude of 10 mv, at applied potentials of -0.2, 0.0, and 0.2 v and open circuit potential (ocp) condition. electrochemical impedance software (powersine) was used to carry out impedance measurements of polymer coated scfmes in monomer-free electrolytic solution with the same three-electrode system. k. huner and a. s. sarac j. electrochem. sci. eng. 11(4) (2021) 263-277 http://dx.doi.org/10.5599/jese.1035 265 impedance data were analyzed using zsimpwin (version 3.1) ac impedance data analysis software (princeton applied research). results and discussion electropolymerization poly(prodot) was obtained from 2 mm prodot monomer solution by performing 20 cv cycles at the scan rate of 50 mv/s in the potential range of -0.5-1.5 v, and the recorded graphical results were compared. in all experiments, 0.1 m tbapf6 was used as the supporting electrolyte, while acetonitrile (mecn), propylene carbonate (pc), and their mixtures were used as solvents. equation (1) was used to calculate dielectric constant values of solvent mixtures: m = 1x1 + 2x2 (1) the dielectric constant of solvent 1 is 1, the volume fraction of the same solvent is x1, 2 is the dielectric constant of solvent 2, and the volume fraction of solvent 2 is x2 [32]. dielectric constants of the solvents and their mixtures at room temperature are given in table 1. table 1. dielectric constants of solvents and their mixtures (at 293.15 k). solvent solvent ratio (v / v) dielectric constant mecn 1 35.9 mecn-pc 4:1 41.7 mecn-pc 3:2 47.5 mecn-pc 2:3 53.3 mecn-pc 1:4 59.1 pc 1 64.9 electropolymerization of prodot in electrolytic solutions with different solvent dielectric constants is shown in figure 1. as the number of cycles increased, the oxidation-reduction current values increased, mostly for the coating obtained at  = 35.9 (figure 1a). coatings obtained at solvent dielectric constants 41.7, 47.5 and 64.9 changed to a much lesser extent (figure 1b-d), while these obtained at the solvent dielectric constants of 53.3 and 59.1 did not change at all (not shown). as the solvent dielectric constant increased from 35.9 to 59.1, the maximum oxidation current value decreased from 3.1 to 0.003 a and then increased to 0.2 a in the electrolytic solution with a solvent dielectric constant of 64.9. the inner graphs in figure 1 show the oxidation-reduction current values versus scanning rate in electrolytic solutions where coatings were made but without monomer. accordingly, the current values increased as the scan rate increased from 50 to 200 mv s-1, except in electrolytic solutions with solvent dielectric constant of 53.3 and 59.1. it can be said that the system is diffusion-controlled [27,33,34] since the current values of the electrocoating obtained in electrolytic solutions with solvent dielectric constants of 35.9, 41.7, 47.5 and 64.9 increase with the increase of the scan rate. the linear dependence of the peak current with the square root of scan rate occurs with electrodes dependent on the diffusion limited redox reaction, while for reaction molecules that adsorb on the electrode surface, peak current will be linearly dependent on the scan rate [35]. the graphs of anodic and cathodic currents against the scan rate and square root of the scan rate for  = 35.9 are given in figure 2. the linear dependence of the peak current with the scan rate for poly(prodot) coated scfme in monomer-free solution indicates that redox reaction is primarily dependent on the adsorption of molecules. note that for the diffusion limited redox reaction, the linear dependence of the peak current on the square root of scan rate was predicted [33,34]. http://dx.doi.org/10.5599/jese.1035 j. electrochem. sci. eng. 11(4) (2021) 263-277 electrochemical properties of poly(prodot) 266 -0.5 0.0 0.5 1.0 1.5 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 c u rr e n t, m a potential, v -0.5 0.0 0.5 1.0 1.5 -0.008 -0.006 -0.004 -0.002 0.000 0.002 0.004 0.006 0.008 0.010 0.012 c u rr e n t, m a potential, v 50 mv/s 75 mv/s 100 mv/s 150 mv/s 200 mv/s a) -0.5 0.0 0.5 1.0 1.5 -0.002 0.000 0.002 0.004 0.006 0.008 -0.5 0.0 0.5 1.0 1.5 -0.004 -0.002 0.000 0.002 0.004 0.006 c u r r e n t, m a potential, v 50 mv/s 75 mv/s 100 mv/s 150 mv/s 200 mv/s c u rr e n t, m a potential, v b) -0.5 0.0 0.5 1.0 1.5 -0.0010 -0.0005 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 0.0035 -0.5 0.0 0.5 1.0 1.5 -0.003 -0.002 -0.001 0.000 0.001 0.002 0.003 50 mv/s 75 mv/s 100 mv/s 150 mv/s 200 mv/s potential, v c u rr e n t, m a c u rr e n t, m a potential, v c) -0.5 0.0 0.5 1.0 1.5 -0.0005 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 c u rr e n t, m a potential, v -0.5 0.0 0.5 1.0 1.5 -0.0004 -0.0003 -0.0002 -0.0001 0.0000 0.0001 0.0002 0.0003 0.0004 0.0005 c u rr e n t, m a potential, v 50 mv/s 100 mv/s 200 mv/s d) figure 1. cyclic voltammograms of polymerization (20 cycles, 50 mv/s) at scfme from 2 mm prodot monomer in 0.1 m tbapf6 and solvent dielectric constant: a) 35.9; b) 41.7; c) 47.5; d) 64.9. inset graphs: cvs of poly(prodot) in monomer-free 0.1 m tbapf6 at different scan rates (50 200 mv/s). surface area of we 2.19×10-3 cm2 0.00 0.05 0.10 0.15 0.20 -0.008 -0.004 0.000 0.004 0.008 0.012 c u rr e n t, m a v/vs -1 ia (anodic current) ic (cathodic current) a) 0.0 0.1 0.2 0.3 0.4 0.5 -0.008 -0.004 0.000 0.004 0.008 c u rr e n t, m a v1/2/v1/2s-1/2 b) ia (anodic current) ic (cathodic current) figure 2. plot of anodic (ia) and corresponding cathodic (ic) peak current vs. a) scan rate; b) square root of scan rate for  = 35.9 (inset of fig. 1a) the graph of anodic and cathodic peak potential values against the scan rate is given in figure 3. with increasing scan rate, the anodic peak potential is shifted positively, while the cathodic peak potential is shifted negatively. an increase of the scan rate can limit the time interval of the electrochemical process, which results in a shift towards more positive potentials (for anodic peaks) and negative potentials (for cathodic peaks). the shift is mainly due to the delay of the electrok. huner and a. s. sarac j. electrochem. sci. eng. 11(4) (2021) 263-277 http://dx.doi.org/10.5599/jese.1035 267 chemical reaction due to the shortness of time compared to a lower scan rate that allows more time for the reaction to occur (figure 3). figure 3. anodic (ea) and cathodic (ec) peak potential values vs. scan rate for  = 35.9 (inset of fig. 1a) figure 4 presents schematic steps of the electrochemical polymerization of prodot on the scfme. electron transfer from prodot monomer to scfme working electrode due to oxidation is the first step of electropolymerization. in the next step, coupling reactions with the formed prodot radical cations, followed by a dehydrogenation step can be realized. by coupling radical cations, oligomers are formed, and further oxidation of these oligomeric and polymeric radical cations can interact with electrolyte anions (pf6-), depending on the solvent medium dielectricity. solvent with a high dielectric constant is easily polarized. polarization allows countercharges to be placed around an ion, which results in coulombic interactions between solvent and radical cations of poly(prodot), and reduces the polymerization ability of oligomers of prodot during the radical cation coupling (figure 4). therefore, the deposition charge density of polymer (α thickness) was found to decrease by increasing the dielectric constant of the medium. solvents influence the solubility, stability and reaction rates, and choosing the appropriate solvent allows thermodynamic and kinetic control over a chemical reaction. stabilization of the intermediates may occur through different non-covalent interactions with the solvent i.e., h-bonding, dipole-dipole interactions, van der waals interactions, etc. stabilization of radical cations (prodot radical cation and/or oligomeric radical cation) is larger in polar solvents than in less polar solvents. polar solvent one with a high dielectric constant will stabilize radical cations by forming interactions and reducing the reactivity of these intermediates with each other (figure 4). the deposition charge density of the polymer electrogrowth (q) and specific capacitance (csp) in monomer free solution were calculated from cv measurements according to equations (2) and (3): d 2 i v q va =  (2) sp d q c v = (3) where idv is the integrated area under the cv curve, ν is the scan rate, v s−1; v / v is the potential window in cv measurements and a / cm2 is the electrode surface area. 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.36 0.38 0.40 0.42 0.44 ea ec  / vs -1 p o te n ti a l, v -0.29 -0.28 -0.27 p o te n tia l,v http://dx.doi.org/10.5599/jese.1035 https://en.wikipedia.org/wiki/thermodynamic_versus_kinetic_reaction_control https://en.wikipedia.org/wiki/intermolecular_force https://en.wikipedia.org/wiki/hydrogen_bond https://en.wikipedia.org/wiki/van_der_waals_force j. electrochem. sci. eng. 11(4) (2021) 263-277 electrochemical properties of poly(prodot) 268 a s oo e 2 2 s oo + s o o + s oo s o o h h + + s oo s o o -2h + oo s oo s o o s oo s s s s oo o o o o n s oo s o o s oo s o o + + ch3c n   c    pf6pf6  a b ) ) b s oo e 2 2 s oo + s o o + s oo s o o h h + + s oo s o o -2h + oo s oo s o o s oo s s s s oo o o o o n s oo s o o s oo s o o + + ch3c n   c    pf6pf6  a b ) ) figure 4. a) tentative mechanism of poly(prodot) electrodeposition onto scfme and doping by anion of electrolyte (pf6-); b) change of dielectric media from low to high polarity the graph of the calculated deposition charge density of prodot and specific capacitance (eqs. 2 and 3) versus dielectric constant of the solvent, obtained from cvs of poly(prodot) in monomerfree solution is shown in figure 5. 36 40 44 48 52 56 60 64 68 0 4 8 12 16 20 24 deposition monomer free dielectric constant d e p o s it io n c h a rg e d e n s it y , m c c m -2 0 5 10 15 20 s p e c ific c a p a c ita n c e , m f c m -2 figure 5. deposition charge density of electrogrowth (50 mv/s, 20 cycles) of prodot (left axis), and specific capacitance (right axis) vs. dielectric constant of solvent obtained from cv (200 mv/s) of poly(prodot) in monomer-free 0.1 m tbapf6. surface area of we  2.19×10-3 cm2 k. huner and a. s. sarac j. electrochem. sci. eng. 11(4) (2021) 263-277 http://dx.doi.org/10.5599/jese.1035 269 as the solvent dielectric constant increased, the deposition charge density in the system decreased. accordingly, the highest deposition charge density value was obtained from the coating realized in the media with a dielectric constant of 35.9. the deposition charge density of film electrogrowth can be generally correlated with its thickness which becomes lower at higher dielectric constants. according to figure 5, specific capacitance values of these films show a similar trend with the exception of  = 41.7. the highest deposition charge density of 24.49 mc cm-2, and the highest specific capacitance of 23.17 mf cm-2, were obtained for poly(prodot) synthesized in the solvent with the lowest dielectric constant ( = 35.9). electrochemical impedance spectroscopy impedance (z) defined by eq. (4) is the angular frequency () dependent resistance of a system to the alternating current (ac) flow caused by the application of frequency-dependent potential. in the limit of zero frequency, impedance becomes frequency independent resistance against the electrical current, or dc resistance [36]. ( ) ( ) e z i   = (4) solution resistance is an important factor in electrochemical impedance measurements of an electrochemical cell. in a three-electrode system, the uncompensated solution resistance is measured between the working and the reference electrode [37]. eis results are usually presented by plotting the measured impedance against the frequency (f = /2). impedance spectrum presentation is the frequency (f) dependence of two real quantities (absolute impedance value |z| and phase angle ). in the spectrum set as bode plot, log |z|and φ are drawn against log . in nyquist plot, the imaginary part of impedance (zim) is put against the real part of impedance (zre). for capacitive systems, complex capacitance, c = (iz)-1, is the most valuable presentation, where total capacitance can be evaluated directly from a spectrum as cre for  → 0. the impedance spectrum is generally studied with equivalent circuit modelling (ecm) containing different electric elements. each element in ecm and its connection with other elements point to some specific physical event. there are three basic elements used in ecm [38-40]. the first is a resistor that appears in the bode plot as a plateau having zero phase angle, a capacitor that appears as a straight line with a slope of -1 and phase angle of -90, and element related to the diffusion of reaction species, showing a straight line with a slope of -0.5 and phase angle of -45. in the nyquist plot, resistance appears as a point at zre, and capacitance as a vertical line (angle of 90), where capacity value can be calculated from zim value at certain  as c = (zim)-1. the element related to diffusion (warburg element) shows a straight line having a slope of 45. eis results of poly(prodot) coated scfmes measured in monomer-free electrolytic solutions under open-circuit potentials are given in figure 6. for each poly(prodot) coated scfme, eis was taken in the same electrolytic solution from which the coating was made but under monomer-free conditions. according to the eis results in figure 6a, for higher dielectric constants (53.3 and 59.1), more or less inclined capacitive lines are generally observed, with impedance much higher than these measured for low dielectric media (35.9, 41.7, 47.5). impedance for  = 64.9 lies somewhere between these two groups. also, for higher  (53.3 and 59.1), it seems that nyquist plots are started to bend with a significant increase of the real resistance at the lowest frequencies, tending to form a semicircle. for all films, bode plots in figure 6b show resistive impedance at higher frequencies and almost capacitive impedance responses at lower frequencies. two groups of impedance spectra can be generally distinguished, a group with lower impedances ( = 35.9, 41.7 and 47.5) and consequently http://dx.doi.org/10.5599/jese.1035 j. electrochem. sci. eng. 11(4) (2021) 263-277 electrochemical properties of poly(prodot) 270 higher capacitances and a group with higher impedances ( = 53.3 and 59.1) and hence lower capacitances. the impedance spectrum for  = 64.9 lies somewhere between these two groups. bode phase plots in figure 6d clearly show that almost pure capacitive impedance response, characterized by phase angle near -90, are obtained for  = 35.9, and  = 53.3 and 59.1. the other films, including that for  = 64.9, show more complex behavior, with clear resistive contribution(s) making peaks in phase angle responses at higher frequencies and almost capacitive impedance responses at lower frequencies. anyhow, it is clear that going from films formed at higher dielectric constants ( = 53.3 and 59.1), there is a continuous change from almost pure capacitive response showing lower capacitance, via complex impedance response involving some resistance contribution(s) ( = 41.7, 47.5, 64.9), to pure capacitive impedance response showing higher capacitance ( = 35.9) (figure 6c). 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 0 500 1000 1500 2000 2500 0 2000 4000 6000 8000 10000 12000 14000 z im / k  zre / k dielectric constant 35.9 41.7 47.5 64.9 synthesis and eis at dielectric constant: 35.9 41.7 47.5 53.3 59.1 64.9 z im / m  zre / m a) 0.01 hz 0 .1 6 h z 0 .0 5 h z 10-3 10-1 101 103 105 0 200 400 600 800 1000 1200 1400 10-3 10-2 10-1 100 101 102 103 104 105 0 1000 2000 3000 4000 5000 6000 35.9 41.7 47.5 frequency, hz iz i / k  iz i / m  frequency, hz synthesis and eis at dielectric constant: 35.9 41.7 47.5 53.3 59.1 64.9 b) 0.000 0.004 0.008 0.012 0.016 0.000 0.002 0.004 0.006 0.008 0.010 0.012 0.014 0.016 -c im / m f cre / mf dielectric constant 35.9 41.7 47.5 53.3 59.1 64.9 c) 10-3 10-1 101 103 105 0 20 40 60 80 100 p h a s e a n g le , o frequency, hz synthesis and eis at dielectric constant: 35.9 41.7 47.5 53.3 59.1 64.9 d) figure 6. eis results of poly(prodot) synthesized in solvents with different dielectric constants and measured in monomer-free 0.1 m tbapf6 under open circuit potential: a) nyquist; b) bode magnitude; c) complex capacitance; d) bode phase plots eis results of poly(prodot) coated scfme, measured in monomer-free electrolytic solutions at different applied potentials (-0.2, 0.0 and 0.2 v), are given in figure 7. for each poly(prodot) coated scfme, eis was taken in the same electrolytic solution from which the coating was made but without a monomer. generally, films formed in solutions of higher dielectric constant ( = 53.3 and 59.1) showed the highest and almost capacitive impedances, independent of the potential of measurements (figure 7a-b). for other films at all three potential values, much lower impedances are generally observed. phase angle bode plots in figure 7c show that almost pure capacitive impedance - / o k. huner and a. s. sarac j. electrochem. sci. eng. 11(4) (2021) 263-277 http://dx.doi.org/10.5599/jese.1035 271 responses are observed for the films ( = 35.9, 41.7 and 47.5), all measured at 0.2 v. this might be related to the fact that 0.2 v is closest to the potential of redox peak of the faradic/doping process recorded by cvs shown in figure 1. other films measured at either 0.0 or -0.2 v, showed more complex behavior involving some clear resistance contribution(s), which is less prominent for films of lower ε and the most prominent for the film  = 64.9 at both 0 and -0.2 v. generally, higher capacitances are obtained for polymer films formed in lower dielectric media than those from higher dielectric solvent media. in addition to the ordinary double-layer capacitance formed at interfacial regions of polymer films, high capacitance of conducting polymers originates from the film's active layer, where fast redox reaction with low charge transfer/transport resistances results in pseudocapacitance. 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 z im / m  zre / m synthesis and eis at dielectric constantpotential applied (v) (for eis) 35.9-0 35.9-0.2 35.9-(-0.2) 41.7-0 41.7-0.2 41.7-(-0.2) 47.1-0 47.1-0.2 47.1-(-0.2) 53.3-0 53.3-0.2 53.3-(-0.2) 59.1-0 59.1-0.2 64.9-0 64.9-0.2 64.9-(-0.2) a) 126.9 hz 0.01 hz 0.03 hz 10-3 10-2 10-1 100 101 102 103 104 105 0 200 400 600 800 1000 1200 1400 synthesis and eis at dielectric constantpotential applied (v) (for eis) 35.9-0 35.9-0.2 35.9-(-0.2) 41.7-0 41.7-0.2 41.7-(-0.2) 47.5-0 47.5-0.2 47.5-(-0.2) 53.3-0 53.3-0.2 53.3-(-0.2) 59.1-0 59.1-0.2 64.9-0 64.9-0.2 64.9-(-0.2) iz i / m  frequency, hz b) 10-3 10-2 10-1 100 101 102 103 104 105 0 20 40 60 80 100 p h a s e a n g le , o frequency, hz synthesis and eis at dielectric constantpotential applied (v) (for eis) 35.9-0 35.9-0.2 35.9-(-0.2) 41.7-0 41.7-0.2 41.7-(-0.2) 47.5-0 47.5-0.2 47.5-(-0.2) 53.3-0 53.3-0.2 53.3-(-0.2) 59.1-0 59.1-0.2 64.9-0 64.9-0.2 64.9-(-0.2) c) figure 7. eis results of poly(prodot) synthesized in solvent mixtures with different dielectric constants and measured in monomer-free 0.1 m tbapf6 at -0.2, 0.0 and 0.2 v: a) nyquist; b) bode magnitude; c) bode phase plots eis results of poly(prodot) synthesized on scfme in 0.1m tbapf6/mecn,  = 35.9) measured in different dielectric media under monomer free conditions at open circuit potential presented in figure 8. bode plots (figure 8a and 8d), nyquist plots (figure 8b) and complex capacitance spectra (figure 8c) indicate that the polymer coating on the scfme is very stable since all measured spectra are rather similar for different dielectric solvent media. - / o http://dx.doi.org/10.5599/jese.1035 j. electrochem. sci. eng. 11(4) (2021) 263-277 electrochemical properties of poly(prodot) 272 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 20 40 60 80 0 200 400 600 800 1000 1200 108.26 hz 0.03 hz 0.01 hz zre / k z im / k  synthesis (at 35.9) and eis at dielectric constant: 35.9 41.7 47.5 53.3 59.1 64.9 zre / k z im / k  b) 0.03 hz 108.26 hz 0.01 hz 0.000 0.004 0.008 0.012 0.016 0.000 0.004 0.008 0.012 0.016 c) synthesis (at 35.9) and eis at dielectric constant: 35.9 41.7 47.5 53.3 59.1 64.9 c im / m f cre / mf 0.001 0.1 10 1000 100000 0 20 40 60 80 100 p h a s e a n g le , o frequency, hz synthesis (at 35.9) and eis at dielectric constant: 35.9 41.7 47.5 53.3 59.1 64.9 d) figure 8. eis results of poly(prodot) synthesized in the solvent with  = 35.9 and measured in solvent mixtures with different dielectric constants at open circuit potential: a) bode magnitude; b) nyquist; c) complex capacitance; d) bode phase plots; e) comparison of measured (fig. 8b and 8d) for  = 35.9, and calculated (according to ecm in fig. 8a) bode phase and magnitude plots equivalent circuit modeling poly(prodot) were synthesized by electrochemical polymerization in electrolytic solutions containing different solvent mixtures that have different dielectric constants. electrochemical parameters of poly(prodot) electrocoated on scfme were evaluated with zsimpwin program and complex nonlinear least squares (cnls) analysis, using the presumed equivalent circuit model. if χ2 (chi-squared) value obtained from the comparison between measured and calculated impedance spectrum is around 10-4, it means that the circuit fits the measured results. the value of 2 is expressed as the sum of squares of the residuals. 0,01 0,1 1 10 100 1000 10000 100000 0 10 20 30 40 50 60 70 80 90 100 10 100 1000 p h a s e a n g le , 0 log (f /hz) bode phase measured bode phase calculated bode magnitude measured bode magnitude calculated iz i / k  z  / k  - / o e) - / o k. huner and a. s. sarac j. electrochem. sci. eng. 11(4) (2021) 263-277 http://dx.doi.org/10.5599/jese.1035 273 the model (ecm) drawn within figure 8a was chosen for the electrical equivalent circuit modelling procedure. it consists of three resistances (rs, r1, and r2), two cpe elements (q1 and q2) and a pure capacitor (c). the constant phase element (cpe) is used instead of ideal capacitors (c) to compensate for the porosity, roughness, and inhomogeneity of the electrode surface in the system [41]. rs should be ascribed to the pore, and uncompensated electrolyte resistances, a combination (q1 and r1) presents interfacial impedance, characterized by the double-layer capacitance (cdl) and charge-transfer resistance (rct) in the first parallel combination [42]. the second parallel combination (q2 and r2 + c) is related to the active interior of polymer film, where r2 comprises ion transfer/transport resistances within the film of pseudocapacitance (c). r3 was put into ecm in figure 8a to account for not ideal capacitive impedance response at the lowest frequencies. ecm was well fitted to the experimental data for poly(prodot), as is already shown in figure 8e by drawing measured and calculated impedance spectra for the film formed in the solvent with  = 39.5 and measured at the open circuit potential. table 2 summarizes impedance parameter values obtained after fitting of ecm from figure 8a to impedance spectra of poly(prodot) obtained at some  and measured in monomer-free solution at 0.0 v (figure 7). table 2. values of elements calculated by ecm in fig. 8a fitted to eis results of poly(prodot) presented in fig. 7 for 0.0 v. surface area of we = 2.19×10-3 cm2  rs / ω cm2 q1 / s sn cm-2 n1 r1 / ω cm2 q2 / s sn cm-2 n2 r2 / ω cm2 c / f cm-2 r3 / kω cm2 35.9 17.03 38.0 0.78 26.92 2610 0.96 26.25 0.01035 65.2 41.7 15.96 11.3 0.89 113 2930 0.97 4.49×104 -- 47.5 20.64 11.2 0.85 136.8 1150 0.96 1.45×105 -- 59.1 23.76 4.65 0.97 3.73×105 ----- data in table 2 show well resolved parameters for poly(prodot) formed at  = 35.9. for higher , however, a reduced number of parameters was obtained, which is obviously due to higher resistance, primarily r2 values. for polymer film formed at low , r1 and r2 resistances are relatively low, allowing a pseudocapacitance (c) of about 10 mf cm-2 to be achieved. r1 and r2 values for poly(prodot) are significantly increased for higher , but at the same time, a decrease is observed for q values. decrease of double layer capacitance with the increase of  of medium is due to doping of polymer cationic sites (figure 4). r1 and r2 increase under higher dielectric constants, where the deposition charge densities decrease (figure 5). this indicates that less thickness and less conductive films are obtained at a higher dielectric constant. dielectric constant () is a measure of a substance's ability to insulate charges from each other. by considering solvent polarity, higher ε means higher polarity and greater ability to stabilize charges. increasing the polarity of the solvent increases the solvation of the anion of electrolyte (pf6-). there will be an increase in dipole-dipole interactions between the solvent and dopant pf6-. as the polarity of the solvent increases, the mobility of pf6ions is reduced, reducing the polymer doping, which results in less conductive material than that formed at low dielectric conditions. when prepared in favourable conditions of low solution dielectric constant, poly(prodot) shows low resistance and high capacitance values (figure 9). due to these properties, poly(prodot) can be used in biosensing applications because it has sufficient electronic conductivity and is also biocompatible and stable. enzymes can be immobilized on conductive polymer coated electrodes in several ways, such as physical adsorption [43,44]. http://dx.doi.org/10.5599/jese.1035 j. electrochem. sci. eng. 11(4) (2021) 263-277 electrochemical properties of poly(prodot) 274 figure 9. q1, q2 and r1 vs. dielectric constant data of poly(prodot) taken from table 2 conducting polymers based on thiophene are used as a transducer in lactose biosensor [45], glucose biosensor [46], vitamin c biosensor [47], and hydrogen peroxide biosensor[48]. the prodot can be used as a new material for neural interfaces [49,50], for flexible supercapacitors [51,52], capacitive sensors, and charge storage electrodes. conclusions in this study, 3,4-propylenedioxythiophene (prodot) was successfully electro-coated on scfme in different electrolytic solutions and solvent mixtures having different dielectric constants. the effect of dielectric properties of solvents on poly(prodot) coating on sfcme was investigated by cv and eis techniques in detail. as the solvent dielectric constant increased, the deposition charge density in the system decreased. accordingly, the highest deposition charge density value was obtained from the coating made in the media with the lowest dielectric constant of 35.9. the deposition charge density of electrogrowth and specific capacitance values versus dielectric constant of solvent shows a similar trend, indicating both processes of electrogrowth and that in monomer free conditions are in line. the highest deposition charge density and specific capacitance were obtained from poly(prodot) synthesized at the lowest solvent dielectric constant medium ( = 35.9). all resistance values (charge transfer/transport) are lower, while capacitances (double layer and film) are higher for poly(prodot) films formed in solutions of lower dielectric constant. an opposite trend with high resistance and low capacitances is observed for films formed in solutions of higher dielectric constant where deposition charge densities were decreased. increasing the polarity of the solvent increases the solvation of the anion of electrolyte (pf6-). in these conditions, increased dipole-dipole interaction between the solvent and dopant pf6will reduce the mobility of pf6ions and doping of polymer, resulting in a less conductive material compared to low dielectric conditions. this study may help researchers pave the way toward more efficient strategies to optimize structural properties of poly(prodot) in different application fields, i.e., flexible electronics, sensors, and organic photovoltaics. acknowledgement: the authors acknowledge the suggestions and inputs provided by dr. baran sarac. 35 40 45 50 55 60 0 10 20 30 40 0 100 200 300 400 1000 1500 2000 2500 3000 q 1 /  s s n c m -2 dielectric constant q1 r1 q2 r 1 / k  c m 2 q 2 / 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1035 https://doi.org/10.1007/s10118-012-1167-6 https://doi.org/10.1039/c0py00077a https://doi.org/10.3389/fneng.2014.00015 https://doi.org/10.3389/fmats.2018.00083 https://doi.org/10.1002/macp.201800355 https://creativecommons.org/licenses/by/4.0/) pre-post electron transfer regioselectivity at glycine modified graphene electrode interface for voltammetric sensing applications http://dx.doi.org/10.5599/jese.1120 1001 j. electrochem. sci. eng. 12(5) (2022) 1001-1008; http://dx.doi.org/10.5599/jese.1438 open access : : issn 1847-9286 www.jese-online.org original scientific paper pre/post electron transfer regioselectivity at glycine modified graphene electrode interface for voltammetric sensing applications gururaj kudur jayaprakash1,2,, roberto flores-moreno3, bahaddurghatta eshwaraswamy kumara swamy4, kaustubha mohanty5, pravesh dhiman6 1laboratory of quantum electrochemistry, school of advanced chemical sciences, shoolini university, bajhol, himachal pradesh, 173229, india 2department of chemistry, nitte meenakshi institute of technology, bangalore, karnataka, 560064, india 3departamento de química, universidad guadalajara,blvd. marcelino garcía barragán 1421, guadalajara, jalisco, c.p. 44430, méxico 4department of p.g. studies and research in industrial chemistry, kuvempu university, shankaraghatta -577451, shimoga, karnataka, india 5department of chemical engineering, indian institute of technology guwahati, guwahati, 781039, india 6 medical oncology cell, department of radiotherapy and oncology, igmc, shimla, 171001, himachal pradesh, india corresponding author: rajguru97@gmail.com; tel.: +91-953-876-2343 received: july 1, 2022; accepted: july 31, 2022; published: august 23, 2022 abstract in the last few years, glycine (gl) showed good experimental evidence as an electron transfer (et) mediator at the carbon (in particular graphene (gr)) interface. however, et properties of gl modified gr interface are still not known completely. these can be achieved using density functional theory-based models. modelling of modified carbon electrode interfaces is essential in electroanalytical chemistry to get insights into their electronic and redox properties. here we have modelled glycine modified graphene interface to find out its interfacial redox et properties. conc eptual density functional theory concepts like frontier molecular orbital (fmo) theory and analytical fukui functions were utilized to predict the et sites on the modified graphene surface. it is shown that at the glycine-modified graphene interface, amine groups act as additional oxidation sites and carboxylic acid groups as additional reduction sites. therefore, glycine acts as an et mediator at the graphene-based electrode interface. the obtained results are well supported by previously published experimental reports. keywords redox reaction, density functional theory; frontier molecular orbitals (fmo); fukui analysis http://dx.doi.org/10.5599/jese.1120 http://dx.doi.org/10.5599/jese.1438 http://www.jese-online.org/ mailto:rajguru97@gmail.com j. electrochem. sci. eng. 12(5) (2022) 1001-1008 pre-post electron transfer regioselectivity 1002 introduction over the past few decades, the carbon paste electrode (cpe) has become a rising star in the electrochemical community, where it is used as a working electrode in voltammetric sensing technologies due to various reasons like easy fabrication, huge redox potential window, and low resistance with minor background current [1,2]. in voltammetry, the charge transfer interaction between the working electrode surface and the analyte is crucial for sensing applications. therefore, the electroanalytical community engineers are using cpe surfaces with various mediators like metal derivatives [3,4], electrodeposited amino acids [5-7], or surfactants [8]. polymer-modified electrodes have recently attracted a lot of attention due to biocompatibility and a broad range of applications in electrochemistry. because of their excellent selectivity, sensitivity, and uniformity in electrochemical deposition, strong adhesion to the electrode surface, and chemical durability, polymer-modified electrodes have several advantages in the detection of analytes. amino acidbased monomers can also be immobilized on the cpe surface using electrodeposition [9-12]. glycine (gl) is an important non-essential amino acid that is essential for humans for the production of collagen and creatine and also for the protection of cells from free radicals. gl also acts as an inhibitory neurotransmitter and helps in processing sensory information. owing to its biocompatibility and easiness of fabrication, gl has been utilized as an electrode modifier and has already been applied for sensing various electroactive compounds like catechol (cc) [9], hydroquinone (hq) [9], dopamine (da) [10], ascorbic acid (aa) [10], uric acid (ua) [11], glucose (glu) [12,13], guanine (gn) [11], adenine (an) [11], and indigo carmine (ic) [14]. gilbert et al. [10] deposited gl on cpe using cyclic voltammetry, and the prepared gl-modified cpe (gl-mcpe) was later applied for the simultaneous detection of da and aa, showing that gl-mcpe is able to detect da in the presence of aa. harish et al. [9] showed that gl-mcpe could detect cc in the presence of hq, and detection limits for cc and hq were determined as 0.16 and 0.20 µm, respectively. manjunatha [14] showed that glmcpe can detect ic in injection samples with a recovery range of 98-105 %. nowadays, preparing composites (gl + graphene (gr)) is gaining attraction because of possible synergistic contributions, which could enable a wide range of prospective applications. unfortunately, gl-gr composite research is still in its infancy, and only a few reports are available in the literature on this area. thus he et al. [11] constructed gl-gr composite glassy carbon electrode for simultaneous detection of da, ua, gn, and aa. the gl-gr composite showed excellent electrocatalytic activity for sensing all analytes in real samples of urine and fish sperm. the authors concluded that gl-gr composite increases electrode sensitivity, stability, and reliability. gl is the simplest known amino acid comprising methylene, carboxylic acid, and an amine group. it will be interesting to know which group acts as an electrocatalyst during redox reactions. understanding gl interaction and its electron transfer (et) regioselectivity with gr will be very helpful in further improving the catalytic activity of this combination for sensing applications. quantum chemistry can be used to better understand electrochemical reactions on the electrode surface. in particular, conceptual density functional theory is more useful for understanding electron transfer regioselectivity of modified carbon surfaces [15,16]. at the atomic scale, quantum chemistry approaches like density functional theory (dft) can be utilized to calculate the surface properties of electrodes. as a result, electroanalytical findings may be supported and explained using first principles and dft [8]. earlier, we used the dft-based models to understand the effect of defects on the graphene surface [16] and regioselectivity at the lysine [5], cetyl pyridinium bromide [8], and tx-100 [3]. ambrusi et al. [17] used the dft model to understand the carbon dot surface interaction with silver g. k. jayaprakash et al. j. electrochem. sci. eng. 12(5) (2022) 1001-1008 http://dx.doi.org/10.5599/jese.1120 1003 nanoparticles and found that carbon dots with the cooh group is beneficial for silver nanoparticle interactions. pineda-urbina et al. [18] utilized the carbon surface model to understand the 2-mercaptobenzothiazole-modified carbon paste electrode interface. their theoretical results are useful to understand the selectivity of the modified surface for metal ions (cu2+ > pb2+ > cd2+) sensing. saghravanian et al. [19] utilized the dft to understand metronidazole non-covalent interaction with swcnt. the authors calculated the binding energy between metronidazole (with different orientations) and swcnt. the minimum binding energy is obtained when metronidazole interacts with swcnt by its –ch2-ch2-oh moiety. thus the earlier theoretical studies suggested that dft-based modelling is helpful in understanding the electrode interface at a molecular level. in the current article, we have studied the binding interaction between graphene and gl, and by using frontier molecular orbitals (fmo) with analytical fukui interpretation, the electron transfer regioselectivity of graphene gl complexes is differentiated. the obtained theoretical results are found in good agreement with earlier experimental interpretations. theoretical methods geometries of gl, gr, and glgr complexes are drawn using the sinapsis tool [20]. frontier molecular orbitals (fmo) and analytical fukui plots are also plotted using sinapsis [20]. entire geometry optimization of all models and analytical fukui calculations were performed using auxiliary functions employed density functional theory (dft) in vogue of the demon2k program [21] with the vwn [22] correlation functional with dzvp [23] basis sets. single point energies were calculated using pbe [24,25] correlation functionals with tzvp [23] basis sets. auxiliary functions were automatically generated as previously described in the literature [26–29]. graphene models were optimized as described in our previous work [16]. the graphene model consists of 96 carbon atoms and 24 hydrogen atoms. the geometries of the graphene model were optimized without freezing hydrogen positions. results and discussion electrochemical behaviour of potassium ferrocyanide at gl-mcpe (earlier experimental status) the electrochemical behaviour of potassium ferrocyanide (k4[fe(cn)6]) is commonly used as a standard redox probe to compare the bare carbon paste and modified carbon paste electrodes. several experimental reports have been published to compare the k4[fe(cn)6 et activity of bare carbon paste electrode (bcpe) and gl-modified carbon paste electrode (gl-mcpe). raril and manjunatha [30] have compared the active surface area of bcpe and gl-mcpe. they found that glmcpe has a higher active surface area when compared to the bcpe. the performance of the cpe is dependent on the substrate and binder ratios, grinding time, and homogenous quality of the surface. therefore, different electrodes will give different δep values. in table 1, the redox et behaviour of different gl-mcpe prepared by different authors is compared. it is seen from table 1 that δep values of gl-mcpe are lower than the bcpe. therefore, at the gl-mcpe surface, gl acts like an electron transfer mediator and electrocatalyst. table 1. redox et behavior of gl-mcpe. reference δep bcpe / v δep gl-mcpe / v scan rate, v s -1 7 0.056 0.048 0.050 27 0.248 0.148 0.100 6 0.101 0.093 0.100 http://dx.doi.org/10.5599/jese.1120 j. electrochem. sci. eng. 12(5) (2022) 1001-1008 pre-post electron transfer regioselectivity 1004 quantum modeling of glgr complexes. density functional theory-based first principle calculations are helpful in predicting the interaction between gr and gl. usually, several monomers (here gl) may deposit on the cpe surface as monomers, dimers, or polymers. for modelling purposes with the limited computational facility, we have placed a single gl (monomer) on the gr substrate as prescribed in previous literature [5]. gl comprises methylene, carboxylic acid, and an amino group. it is interesting to know which group has favourable interaction with the graphene surface. all possible modes of gl interaction with graphene are shown in figure 1. glgr complex formed by the interaction of the carboxylic acid group has minimum energy as shown in table 2. therefore, gl will horizontally interact with the gr surface as shown in figure 1(a), and this model is considered for further analysis. gl interaction with gr surface by amine group (figure 1(b)) and carboxylic group (figure 1(c)) are not considered for further studies. glgra; gl interacting with gr in a horizontal way glgrb; gl interacting with gr with an amine group glgrc; gl interacting with gr with a carboxylic acid group figure 1. quantum chemical models of glgr. gray: carbon, blue: nitrogen, red: oxygen, white: hydrogen table 2. energies of glgr complex model energy, ev glgr-a 0.000 glgr-b 0.124 glgr-c 0.065 fmo and analytical fukui analysis of gl fmo theory was introduced by k. fukui et al. [31-33], and it provides an intriguing method for predicting et reactivity based on the locations of the highest occupied molecular orbital (homo) and lowest unoccupied molecular orbital (lumo). during redox electrochemical processes, homo undergoes oxidation and lumo undergoes reduction. for identifying electron transfer sites, simulations based on the fukui function can be used in chemical and electrochemical applications. fukui function is defined by the following equation [34-36]: g. k. jayaprakash et al. j. electrochem. sci. eng. 12(5) (2022) 1001-1008 http://dx.doi.org/10.5599/jese.1120 1005 / ( ) ( ) ( ) v r f n  + −   =    r r (1) here, ρ(r) is the electron density, n denotes the number of electrons in the system, and the + and signs denote electron addition and removal, respectively. in electrochemistry, the fukui function is commonly employed to explain redox reaction processes [3-8,35]. however, analytical fukui functions based on auxiliary density perturbation theory (adpt) have advantages like applicability to larger systems with reduced human errors [36]. it will also eliminate artifacts of the numerical method [36]. therefore, we are using adpt-based analytical fukui functions in the current work. fmo and analytical fukui computational results of glycine are shown in figure 2. the homo of glycine (figure 2(a)) is situated in the amine group and lumo is positioned in the carboxylic acid group (figure 2(b)) results were further supported by simulation utilizing the fukui function in this study. f−(r) in figure 2(c) and f+(r) in figure 2(d) show the results of simulations using the fukui equation, which consistently demonstrates that the glycine oxidation site is situated at the amine group and the glycine reduction sites are placed on the carboxylic acid group. to say it in another way, the amine group of glycine will be easily oxidized, whereas the carboxylic acid group will be reduced easily. homo [iso=0.02; grid=0.2] lumo [iso=0.05; grid=0.2] f−(r) [iso=0.03; grid=0.2] f+(r) [iso=0.03; grid=0.2] figure 2. fmo and analytical fukui analysis of glycine fmo and analytical fukui analysis of glgr fmo and analytical fukui computational results are shown in figures 3 and 4, respectively. the highest occupied molecular orbital (homo) of glgr is positioned on the terminal carbon atoms and amine group (figure 3(a)), whereas the lowest occupied molecular orbital (lumo) is placed on the terminal carbon atoms and a carboxylic acid group of glycine (figure 3(b)). the results of this study show that the amine group in the glycine molecule is involved in oxidation, whereas the carboxylic acid group is involved in reduction. http://dx.doi.org/10.5599/jese.1120 j. electrochem. sci. eng. 12(5) (2022) 1001-1008 pre-post electron transfer regioselectivity 1006 as can be seen in figure 4(a), the amine group is more probable for nucleophilic reactions (loss of electrons). in gl amino group has the n atom, which is comparatively less electronegative than the o atoms of the carboxylic acid group. therefore, amine group of gl acts as an oxidation site on the electrode interface. similarly, as can be seen in figure 4(b), the carboxylic acid group is more probable for electrophilic reactions (gain of electrons). carboxylic acid has two o atoms with comparatively higher electronegative than c and n. therefore, the carboxylic group of gl acts as a reduction site on the electrode interface. homo [iso=0.01; grid=0.2] lumo [iso=0.03; grid=0.2] figure 3. fmo of glycine graphene complex f−(r) [iso=0.03; grid=0.2] f+(r) [iso=0.03; grid=0.2] figure 4. analytical fukui analysis of glycine graphene complex g. k. jayaprakash et al. j. electrochem. sci. eng. 12(5) (2022) 1001-1008 http://dx.doi.org/10.5599/jese.1120 1007 conclusion previous reports proved that gl acts as an electrocatalyst on the gr surface and improves its et activity. the et properties of the gl-modified carbon surface can be understood using dft-based quantum chemical models. till now, glgr et properties were not verified by theoretical methods, but in the present work, fmo and analytical fukui functions are utilized to observe et sites at the glgr interface. the obtained results propose that on the gl-modified graphene electrodes, the amine group increases oxidation et sites, and the carboxylic acid group increases reduction sites. as a result, voltammetric signals will also increase, increasing the sensing ability. thus, current work will give theoretical evidence to explain the catalytic activity of gl on the carbon surface. acknowledgements: this research was funded by himachal pradesh council for science, technology and environment (himcoste) sanction number stc/f(8)-2(r&d 20-21)-461 and serb-tare tar/2021/000197. all authors are very thankful to the editor for giving constructive suggestions to improve the quality of the article. references [1] i. švancara, k. vytřas, k. kalcher, a. walcarius, j. wang, electroanalysis 21(1) (2009) 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https://doi.org/10.1139/v92-079@cjc-uc-0101 https://doi.org/10.1103/physrevlett.77.3865 https://doi.org/10.1103/physrevlett.80.891 https://doi.org/10.1103/physrevlett.80.891 https://doi.org/10.1063/1.2842103 https://doi.org/10.1063/1.4971292 https://doi.org/10.1007/s00214-021-02755-8 https://doi.org/10.1063/1.2431643 http://abechem.ir/no.%203-2018/2018,%2010(3),%20372-382.pdf https://doi.org/10.1063/1.1740412 https://doi.org/10.1063/1.1700523 https://www.science.org/doi/abs/10.1126/‌scie‌nce.218.4574.747 https://www.science.org/doi/abs/10.1126/‌scie‌nce.218.4574.747 https://doi.org/10.1039/c8nj03679a https://doi.org/10.1021/ja00326a036 https://doi.org/10.1063/1.3036926 https://creativecommons.org/licenses/by/4.0/) {mechanical and microstructural characterization of yttria-stabilized zirconia (y2o3/zro2; ysz) nanoparticles reinforced wc-10co-4cr coated turbine steel:} http://dx.doi.org/10.5599/jese.1190 651 j. electrochem. sci. eng. 12(4) (2022) 651-666; http://dx.doi.org/10.5599/jese.1190 open access : : issn 1847-9286 www.jese-online.org original scientific paper mechanical and microstructural characterization of yttriastabilized zirconia (y2o3/zro2; ysz) nanoparticles reinforced wc-10co-4cr coated turbine steel rajinder kumar1, deepak bhandari1 and khushdeep goyal2,  1mechanical engineering, yadavindra department of engineering, punjabi university guru kashi campus, talwandi sabo, punjab, india 2department of mechanical engineering, punjabi university patiala, punjab, india corresponding author:  rkrajinder16@gmail.com received: november 26, 2022; accepted: february 12, 2022; published: february 24, 2022 abstract the aim of this paper is to investigate the wc-10co-4cr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (y2o3/zro2; ysz) nanoparticles deposited on the ca6nm turbine steel by using the high-velocity oxy-fuel (hvof) thermal spraying technique. in the hvof technique, the hot jet of the semi-solid particles strikes against the workpiece and creates a layer of coating of varying thickness on the substrate material. the coatings fabricated with hvof were analyzed by scanning electron microscope (sem) / energy-dispersive x-ray spectroscopy (eds). the phase identification of a crystalline material was made with the x-ray diffraction (xrd) technique. the mechanical properties in terms of porosity, surface roughness and microhardness of the nanocomposite coatings were also evaluated. the sem/eds analysis showed that dense and homogeneous coatings were developed by the reinforcement of ysz nanoparticles. the peaks of xrd graphs of wc-10co-4cr coating reinforced with 5 and 10 % of ysz nanoparticles revealed that the wc was present as a major phase and w2c, co3w3c, co, co6w6c, co6w and y2o3/zro2 nanoparticles were observed as a minor phase. the porosity level decreased up to 42 and 56 % by the addition 5 and 10 % of ysz nanoparticles as compared with conventional wc-10co4cr coating. the surface roughness values for wc-10co-4cr conventional coating, 95 % (wc-10co-4cr) + 5 % ysz and 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated samples were found to be 5.03, 4.89 and 4.28 respectively. the nanocomposite coatings reinforced with 10 % ysz nanoparticles exhibited the highest microhardness value (1278 hv). the wc-10co-4cr coatings reinforced with 10 % of ysz nanoparticles resulted in low porosity, low surface roughness and high microhardness. during the coating process, the nanoparticles of ysz flow into the pores and are dispersed in the gaps between the micrometric wc particles and provide a better shield to the http://dx.doi.org/10.5599/jese.1190 http://dx.doi.org/10.5599/jese.1190 http://www.jese-online.org/ mailto:rkrajinder16@gmail.com j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 652 substrate material. the wc-10co-4cr with 10 % of ysz nanoparticles showed better results in terms of mechanical and microstructural properties during the investigation. keywords high velocity oxy fuel (hvof); mechanical properties; microhardness; nanocomposite coating; porosity introduction hydroelectric power plants have a significant contribution to power generation. half of the hydroelectric power resources lie only in the asian countries [1]. india lies at the seventh position in hydroelectric power production in the world. due to the increase of sediment content in the water, the problem of erosion becomes more critical during the rainy season and sometimes may cause the major shutdown of these hydroelectric power stations, which causes huge financial loss to the hydropower industry [2]. major components of the hydro turbines, which are badly eroded, are draft tube, facing plate, runner inlet and outlet, shaft seal, guide vanes, nozzle, spear and spiral casing. traditional steels used in the manufacturing of hydro turbine components are not able to overcome the problems that occurred due to erosion in the hydroelectric power stations. but different types of surface coatings can be deposited to enhance the life of the materials used in the fabrication of hydro turbine components [3]. many authors also highlighted that the problem of slurry erosion of hydro turbine components cannot be eliminated, but it can be reduced by using coated components for the hydro turbine. many researchers compared the conventional coatings with nanostructured coatings and lots of improvements were observed in mechanical and microstructural properties of as-sprayed material as like increase in the microhardness, lower the value of porosity, surface roughness, erosion rate etc. different types of thermal spray coatings are used daily to resist the erosion wear in various industrial applications such as hydropower plants, thermal power plants and mineral industry. choy highlighted the utilization of thermal spray processes along with the versatility to spray an extensive variety of materials like composites and ceramics [4]. hvof spray coatings are used very vastly due to their better mechanical strength, corrosion resistance, erosion resistance, higher value of microhardness, low porosity and improved surface properties of the substrate materials [5,6]. by the application of coatings, the erosion resistance increased and the life of the turbine components was also enhanced [6,7]. to date, many researchers have shown the better mechanical and microstructural properties of the various types of coating deposited by hvof technique. from the literature survey, it was observed that the tungsten-based wc-10co-4cr coatings are mostly used to resist slurry erosion. lee et al. have observed that the mostly used harder phases are al, ni, cr and fe, while wc, si and al2o3 are mainly used as harder binders. the significant improvement was observed in the mechanical properties and the erosion resistance of the target material by applying wc-based coatings [8]. many researchers like santa et al. and singh et al. applied wc and cr as the harder materials to control the surface properties of the materials [9,10]. lee et al. examined the surface properties of wc–10co–4cr coated stainless steel by using the hvof technique. the results reveal that stainless steel's wear resistance and bond strength were increased after coating with powders of various particle sizes [8]. maiti et al. studied the effect of wc-based hvof coatings on the performance of stainless steel 304 by using different compositions of wc-based coatings [11]. results reveal that the hardness of wc–9co–5cr was improved with the addition of 20% wc powder. hong et al. observed the surface properties of ni–cr coated 13cr4ni steel using the hvof technique. r. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 651-666 http://dx.doi.org/10.5599/jese.1190 653 it was found that the microhardness of substrate material increased due to the presence of coating composition of different elements [12]. by the addition of chromium, wc-co-cr coatings improve the wear resistance, erosion and corrosion as compared to the wc-co coatings [13]. investigations showed that the erosion resistance of wc-10co-4cr coatings deposited with the hvof technique increased up to 50% more than substrate stainless steel. the results of wc-based coatings mostly depend on the coating technique [14,15]. further, the composition of feedstock powders also plays a major role. chromium carbide coatings enhance the erosion resistance of hydro turbine components [16]. cermets coatings can be deposited by hvof, which is a promising technique due to its characteristics [17,18]. the selection of the coating mainly depends on the application; however, the scope of thermal spray coatings has increased vastly in recent years [19,20]. murthy et al. studied hvof sprayed carbide coatings that have better erosion and wear resistance as compared to d gun spray coatings [21]. the studies of various researchers revealed that the adhesion strength, fracture strength and hardness of the coatings are mainly responsible for the resistance to erosion. murthy et al. deposited cr3c2-20nicr and wc-10co-4cr on a mild steel material by using the hvof coating technique [22]. results revealed that the wc-based coating has a better wear resistance in comparison to the cr c-based coating due to the high value of hardness of the wc and better bond strength of co cr. additionally, the hvof coated carbide has excellent wear resistance as compared to other spraying techniques [23]. goyal et al. analyzed the effect of erodent particles on astm a743 turbine steel coated with cr3c2-nicr. they observed that coated steel exhibited brittle behavior, but uncoated steel showed ductility [24]. kitamura et al. studied that y2o3 (yttrium oxide) based coatings performed better than al2o3 (aluminum oxide) based coatings against the erosion resistance [25]. the life span of the various components of the hydro turbine can be increased by depositing nanocomposite coatings on the ca6nm turbine steel material. the major advantage of the hvof thermal spray technique is that the coatings have low porosity, low surface roughness and high microhardness. there is wide scope of yttria-stabilized zirconia (y2o3/zro2; ysz) nanoparticles reinforced wc-10co-4cr coatings as no more literature exists in this field. in the present research work, the wc-10co4cr reinforced with 5 and 10 % ysz nanoparticles were deposited on the ca6nm turbine steel by the hvof technique. the microstructure, different phases of the coatings, porosity, surface roughness, microhardness were investigated using sem/eds, x-ray diffraction (xrd), image analyzer software, surface roughness tester, vickers microhardness tester. materials and methods substrate material ca6nm turbine steel material suitable for hydro turbine components was selected for the present investigation. the substrate material (ca6nm turbine steel) was procured from the atul precision cast, rajkot, gujarat. the chemical composition of substrate material is reported in table 1. the samples with dimensions (35×25×5 mm) were prepared from the substrate material to deposit the coatings. table 1. chemical composition of substrate material (ca6nm turbine steel) element c mn cr ni p si mo s cu al fe content, wt. % 0.018 0.58 12.02 3.63 0.035 0.53 0.76 0.006 0.34 0.005 rest http://dx.doi.org/10.5599/jese.1190 j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 654 coating powders the conventional wc-10co-4cr coating powder and wc-10co-4cr reinforced with varying percentages of ysz nanoparticles were used for the present investigation. table 2 shows the coating powders of different coating compositions. to prepare coating powders of different compositions, wc-10co-4cr was reinforced with 5 and 10 % of ysz nanoparticles and rolled continuously for four hours at 200 rpm using retsch-planetary pm-100 low-energy ball mill. the nanocomposite coating powders were prepared to apply on the ca6nm turbine steel to improve the performance and life of turbine components. in the mixture of nanocomposite coating powders, the nanoparticles of ysz were uniformly dispersed. the presence of blended elements was observed during sem/eds analysis. table 2. composition of coating powders deposited on ca6nm turbine steel samples no. coating powder content, % base powder (wc-10co-4cr) reinforcement, ysz (y2o3/zro2 nano powder) 1 100 0 2 95 5 3 90 10 coating technique the samples of the substrate material were polished using sic papers of different grit sizes. then an abrasive blasting machine was used to shoot blast the samples to obtain better bonding between the samples and the coatings. different nanocomposite feedstock powders were applied on the samples of the substrate material with the hvof spray coating technique to obtain better bonding with the base material. the different process parameters of hvof spraying selected for the fabrication of nanocomposite coating are shown in table 3. after deposition of the nanocomposite coatings, the coatings were cooled with the help of air supplied by air jet. table 3. process parameters of hvof used for the fabrication of nanocomposite coating process parameter spray distance, mm particle size, mm flow rate, l min-1 powder feed rate, gmin-1 pressure, kpa air oxygen fuel (lpg) fuel air oxygen value 138 15 640 260 75 30 6.2 10 5 characterization after the deposition of coatings with the hvof technique, the samples were cut into the required size (10×10×5 mm) to perform the sem analysis. sem analysis was carried out on jeol (jsm-6510lv) microscope. sem allows to analyze the grain morphology of the coating powders and coated surface after the deposition of coatings. the sem provides detailed information about surface characteristics at high magnifications that produce high-resolution images of the coating powders, uncoated, conventionally coated and nanocomposite coated surfaces. energy dispersive x-ray spectroscopy, also known as eds analysis, were performed to obtain detailed information about the elemental composition of all samples. x-ray diffraction (xrd) was used for the identification of the crystalline and grains forms known as phases of mixtures present in coating powders, uncoated, conventionally coated and nanocomposite coated surfaces. to investigate the various phases, present in all types of samples, xrd analysis was carried out using panalytical x’pert pro x-ray diffractometer with the copper target (0.15419 nm, 40 kv and 45 ma). phase identification was made with high score plus software. r. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 651-666 http://dx.doi.org/10.5599/jese.1190 655 table 4. mechanical properties of uncoated and coated samples s. no. material/coating combination porosity, % average microhardness, hv average surface roughness, µm 1 uncoated ca6nm steel 3.402 319 1.59 2 wc-10co-4cr 1.942 1088 4.82 3 95 % (wc-10co-4cr) + 5 % ysz 1.131 1257 4.13 4 90 % (wc-10co-4cr) +10 % ysz 0.853 1278 3.45 to observe the porosity of the substrate material, wc-10co-4cr conventionally coated sample and wc-10co-4cr + ysz nanocomposite coated samples were cut by wire edm on either side of the sample for porosity analysis. measurements of the porosity of the different samples were done with an image analyser, having software of the envision 3.0 series (chennai metco private limited, chennai, india). the surface roughness of the uncoated and coated samples was measured with the help of a surface roughness tester (surftest sj310, mitutoyo). the microhardness of the substrate material, wc-10co-4cr coating and wc-10co-4cr + ysz nanocomposite coatings was measured at a load of 2.942 n using a digital micro vickers hardness tester (shv-1000, chennai metco private limited, chennai). results and discussion sem/eds and xrd analysis figure 1 shows the surface morphologies of the wc-10co-4cr conventional powder, ysz nanopowder, wc-10co-4cr + 5 % ysz and wc-10co-4cr + 10 % ysz nanocomposite powder. the wc-10co-4cr powder is spheroidal in shape and the surface of the powder particles is porous. as seen with high magnification that wc grains are blocky in shape. the density of the ysz nanopowder is higher than that of the wc-10co-4cr conventional powder. figure 1. sem images of (a)wc-10co-4cr conventional powder (b) ysz nanopowder (c) 95 % (wc-10co-4cr) + 5 % ysz nanocomposite powder (d) 90 % (wc-10co-4cr) + 10 % ysz nanocomposite powder http://dx.doi.org/10.5599/jese.1190 j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 656 the sem/eds micrographs of the wc-10co-4cr coating powder, as shown in figure 2, depicted that most of the particles are of spherical shape. it also highlights w is present as a major phase, while co, cr and c as a minor phase. after mixing the 5 and 10% of ysz nanoparticles in wc-10co-4cr coating powder using a low-speed ball milling, the sem/eds micrographs were taken, as shown in figure 3 and figure 4. energy, kev figure 2. sem/eds micrograph of wc-10co-4cr conventional coating powder energy, kev figure 3. sem/eds micrograph of 95 % (wc-10co-4cr) + 5% ysz nanocomposite coating powder r. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 651-666 http://dx.doi.org/10.5599/jese.1190 657 energy, kev figure 4.sem/eds micrograph of 90% (wc-10co-4cr) + 10 % ysz nanocomposite coating powder these figures clearly show the uniformly dispersed nanoparticles in conventional coating powders. they also exhibited that w, c, o, co and zr are present as a major phase while cr and y are observed as a minor phase. the ysz nanoparticles have formed a uniform layer on the outer surface of wc-10co-4cr coating powder. the microstructure of hvof coated samples was also obtained by sem/eds surface morphology analysis. figure 5 shows the surface morphology and composition of the substrate material. energy, kev figure 5. sem/eds micrograph of the ca6nm steel substrate http://dx.doi.org/10.5599/jese.1190 j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 658 the wc-10co-4cr coated samples have a uniform, dense structure, as shown in figure 6. some un-burnt particles were seen on the coated surface. the figure also revealed that the cr, c, o and ni are present in more percentage. figure 7 and figure 8 showed the sem images of wc-10co-4cr + 5 % ysz and wc-10co-4cr + 10 % ysz nanocomposite coatings. the dense and uniform layer of coating was obtained by the reinforcement of nanoparticles of ysz. the cr, o and c are observed as a major phase. energy, kev figure 6. sem/eds micrograph of wc-10co-4cr coated sample energy, kev figure 7. sem/eds micrograph of 95 % (wc-10co-4cr) + 5 % ysz nanocomposite coated sample r. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 651-666 http://dx.doi.org/10.5599/jese.1190 659 energy, kev figure 8. sem/eds micrograph of 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated sample the oxide formation was reduced as the nanoparticles of ysz filled the porous gaps. it was also analyzed by sem/eds that the nanoparticles of ysz are well dispersed in wc-10co-4cr coating powders, which are responsible for decreasing the porosity and surface roughness of the nanocomposite coatings. the xrd tests were performed on the substrate material, traditional coating of wc-10co-4cr and nanocomposite coatings of wc-10co-4cr with 5 and 10 % of ysz nanoparticles. the xrd pattern for wc-10co-4cr coated surface, as shown in figure 9(a), presented the peaks corresponding to wc, w2c, co3w3c, and co phases. wc was identified as the major phase present in the coating. further w2c, co3w3c phases were also present due to the decarburization of wc particles. co was also identified from the xrd diffraction pattern. it is in good agreement with the observations made by other researchers [26,27]. the transformation of wc to w2c on the surface of wc particles occurred due to the direct oxidation of solid wc [28,29]. figures 9(b) and 9(c) represent the results of the xrd analysis of wc-10co-4cr coatings reinforced with 5 and 10 % of ysz nanoparticles. the xrd pattern in the figures reveals the presence of wc, w2c, co6w6c, co6w and y2o3/zro2crystalline phases. the wc was observed as the major phase followed by co6w6c. similar trends have been reported by the other authors [30-32]. the existence of co6w is owing to the reaction of wc particles slightly melted in the bond phase co–cr during solid-state sintering. the diffraction peaks of the co phase disappear owing to the formation of the amorphous phase by rapid cooling. the peaks of the co6w6c phase are observed in both nanocomposite coatings because of the dissolution of wc in the cobalt matrix. the xrd peaks corresponding to y2o3/zro2 indicate that the ysz nanoparticles are dispersed in the gaps between the micrometric wc particles or partially agglomerated in the bonding phase. the xrd patterns depicted that the hvof method can effectively limit the generation of decarburization phases because of the high velocity and relatively low temperature of the flame. the increase in the formation of non-crystalline amorphous phases occurs due to very fast cooling during the spraying process [33]. stewart et al. demonstrated that the presence of different phases and their proportion http://dx.doi.org/10.5599/jese.1190 j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 660 mostly depends on the process conditions when depositing the coating powder on the base material [34]. a b c figure 9. xrd images of (a) wc-10co-4cr coated sample (b) 95 % (wc-10co-4cr) +5 % ysz nanocomposite coated sample(c) 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated sample figure 10(a) shows the microstructure of the cross-section of uncoated ca6nm steel. pores were observed in the substrate material, resulting in low microhardness and high porosity. figure 10(b) shows the small voids and pores in the cross-sectional microstructure of the wc-10co-4cr coated sample. dense grains were observed in the cross-sectional microstructure of nanocomposite coating of wc10co-4cr + 5 % ysz and wc-10co-4cr + 10 % ysz as presented in figure 10(c) and figure 10(d). the material spreads uniformly on the surface and forms a more homogeneous layer as the ysz nanoparticles fill the pores in the coatings. minor imperfections were observed in the cross-sectional microstructure of nanocomposite coating of wc-10co-4cr + 10% ysz, as shown in figure 10(d). figure 11(a) shows the morphological characteristics of the substrate material. dense grains were observed in the substrate material, due to which material spreads uniformly on the surface and forms the more homogeneous layer. some imperfections and protrusions in the wc-10co-4cr coated surface as shown in figure 11(b). goyal et al. also observed that protrusions were formed due to improper melting of ceramic particles before sticking with base metal during the hvof coating process [35]. the microstructure of nanocomposite coating of wc-10co-4cr + 5 % ysz showed delamination, unmelted nanoparticles of y2o3/zro2, small pores and a layer with lamellar structure, containing a discrete oxide film in their outline, which is the common feature of hvof coatings as presented in figure 11(c). r. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 651-666 http://dx.doi.org/10.5599/jese.1190 661 figure 10. microstructure of the cross-section of (a) uncoated ca6nm steel; (b) wc-10co-4cr coated sample (c) 95 % (wc-10co-4cr) + 5 % ysz nanocomposite coated sample; (d) 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated sample figure 11. sem images of (a) uncoated ca6nm steel (b) wc-10co-4cr coated sample (c) 95 % (wc-10co-4cr) + 5 % ysz nanocomposite coated sample (d) 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated sample these features are appeared due to the overlapping of high-speed melt and semi-fused particles deposited on the base material [36]. as the ysz nanoparticles fill the pores in the coatings, only minor imperfections were observed in the microstructure of nanocomposite coating of wc-10co4cr + 10 % ysz as shown in figure 11(d). these types of coatings have dense grains, which create a more homogeneous layer because material spreads more uniformly over the surface. also, this type of surface morphology is desirable for the hvof coating process, as it allows the better flow of the http://dx.doi.org/10.5599/jese.1190 j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 662 coating powder in the spray gun during the deposition of the coating [37-39]. thus, wc-10co-4cr + 10 % ysz nanocomposite coating performed better than the other coatings mentioned. the mixing of coating powders improved the microstructure of the coating, attributed to the decrease in process temperature during hvof coating [40]. ramesh et al. and rao et al. studied that lower porosity and higher microhardness of the coating were exhibited due to the dense laminar structure of the coating along with higher cohesive strength [41-43]. porosity and surface roughness analysis the porosity of the coatings was measured at the as polished cross-section of the samples. the apparent porosity values for the substrate material, wc-10co-4cr conventional coating, 95 % (wc-10co-4cr) + 5 % ysz and 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coatings were observed as 3.040, 1.942, 1.131 and 0.853 %. table 4 shows that the porosity decreases with an increase in the percentage of nanoparticles of ysz in the conventional coating powder. the porosity of the nanocomposite coatings was decreased as the nanoparticles of ysz filled the pores and interlocked the grains of the coating matrix. the maximum decrease of the porosity level 0.853 % was observed for 90% (wc-10co-4cr) + 10 % ysz nanocomposite coating. the surface roughness values for uncoated and coated samples are also given in table 4. improvements were observed by the addition of the nanoparticles of ysz in the wc-10co-4cr coating powder. the surface roughness values for wc-10co-4cr conventional coating, 95 % (wc-10co-4cr) + 5 % ysz and 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated samples were found to be 4.82, 4.13 and 3.45 respectively. better surface characteristics were observed for 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coating as compared to conventional wc-10co-4cr coating as the surface roughness was decreased by the addition of ysz nanoparticles. the lower value of the porosity for 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coating is the main reason for decreasing the surface roughness. porosity is one of the major coating characteristics that greatly affect the coating properties. manjunatha et al. studied that carbon nanotubes reinforced hvof coating resulted in low porosity as the carbon nanoparticles fill the voids in the coatings and improve the bond strength of the coatings [44]. goyal and goyal studied that the coatings of cr3c2–20nicr reinforced with carbon nanotubes exhibited a decrease in the porosity of the coating. carbon nanotubes interlocked the particles of cr3c2–20nicr and provided resistance to the surface against eroding particles [45]. figure12. microhardness profile for substrate material and coated samples r. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 651-666 http://dx.doi.org/10.5599/jese.1190 663 vickers micro-hardness the variation of microhardness along the cross-section of coated substrate material is shown in figure 7. there is a substantial increase in the microhardness from the substrate to the coating. the average microhardness values of substrate material, wc-10co-4cr conventional coating, 95 % (wc-10co-4cr) + 5 % ysz and 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coatings were found to be in the range of 320 hv, 1088 hv, 1257 hv, and 1278 hv respectively as reported in table 4. the reinforcement of ysz nanoparticles in the coatings helps to reduce the voids and pores between the coating matrix, thereby decreasing porosity and increasing the microhardness of the nanocomposite coatings. the increase in slurry erosion resistance of coated materials can be attributed to the increase in the microhardness of the coatings [42,35]. singh et al. analysed the erosion tribo performance of wc-10co-4cr and wc-10co-4cr + 2 % y2o3 deposited on pump impeller steel by hvof. results showed that the reinforcement of 2 % y2o3 in wc-10co-4cr has increased the erosion wear resistance as the hardness of the substrate material increases from 1130 hv to 1190 hv [46]. the presence of different phases in the sem images is mainly responsible for the variation in microhardness values. with the addition of 5% and 10 % of the ysz nanoparticles in the wc-10co-4cr, the surface roughness and porosity were reduced significantly. the porosity decreased up to 42 and 56 % and the surface roughness was reduced up to 15 and 28 % by the reinforcement of 5 and 10 % of the ysz nanoparticles, respectively, compared to conventional wc-10co-4cr coating. nanoparticles of ysz interlocked the wc-10co-4cr grains and filled the pores in the nanocomposite coating matrix, which resulted in a decrease in surface roughness and porosity. the decrease in porosity with the reinforcement of nanoparticles in the conventional coatings has also been observed by various researchers [47-51]. reduction in the surface roughness and porosity resulted in the increase in the microhardness of the wc-10co-4cr nanocomposite coatings as compared to conventional coating. indentation resistance was enhanced due to the reinforcement of nanoparticles of ysz in the wc-10co-4cr. enhanced melting due to the higher thermal conductivity of ysz nanoparticles resulted in better adhesion with the substrate material and increased microhardness of the wc-10co-4cr nanocomposite coatings. therefore, based on the result and discussion, it was reported that the wc-10co-4cr + ysz nanocomposite coatings are able to enhance the microstructural and mechanical properties of turbine steel. conclusion the experimental analysis conducted on the nanocomposite coating concluded as given below: the sem/eds analysis revealed that the reinforcement of ysz nanoparticles developed dense and homogeneous coatings. it was also analyzed by sem/eds that the nanoparticles of ysz is well dispersed in wc-10co-4cr coating powders, which are responsible for decreasing the porosity and surface roughness of the nanocomposite coatings. the xrd peaks of wc-10co-4cr coatings reinforced with 5 and 10 % of ysz nanoparticles revealed that the wc was present as a major phase and w2c, co6w6c, co6w and y2o3/zro2 nanoparticles were observed as a minor phase. the xrd peaks corresponding to y2o3/zro2 indicate that the ysz nanoparticles are dispersed in the gaps between the micrometric wc particles. for the coated samples, the porosity values were found as 1.942, 1.131 and 0.853 % and the porosity decreased up to 42 and 56 % by the addition of ysz nanoparticles compared with conventional wc-10co-4cr coating. the maximum decrease of 0.853 % of porosity was http://dx.doi.org/10.5599/jese.1190 j. electrochem. sci. eng. 12(4) (2022) 651-666 characterization of yttria-stabilized zirconia 664 obtained for the nanocomposite coating reinforced with 10% ysz nanoparticles. the surface roughness values for wc-10co-4cr conventional coating, 95 % (wc-10co-4cr) + 5 % ysz and 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coatings were found to be 4.82, 4.13 and 3.45 respectively. the surface roughness was improved up to 15 and 28 % by reinforcing 5 and 10 % of the ysz nanoparticles. the average microhardness for the coated samples was found to be 1088 hv, 1257hv, and 1278 hv, respectively. the nanocomposite coatings reinforced with 10 % ysz nanoparticles exhibited the highest microhardness value (1278 hv) and low value of porosity and surface roughness. the ysz 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@article{kumar2022, author = {kumar, rajinder and bhandari, deepak and goyal, khushdeep}, journal = {journal of electrochemical science and engineering}, title = {{mechanical and microstructural characterization of yttria-stabilized zirconia (y2o3/zro2; ysz) nanoparticles reinforced wc-10co-4cr coated turbine steel:}}, year = {2022}, issn = {1847-9286}, month = {feb}, number = {4}, pages = {651--666}, volume = {12}, abstract = {the aim of this paper is to investigate the wc-10co-4cr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (y2o3/zro2; ysz) nanoparticles deposited on the ca6nm turbine steel by using the high-velocity oxy-fuel (hvof) thermal spraying technique. in the hvof technique, the hot jet of the semi-solid particles strikes against the workpiece and creates a layer of coating of varying thickness on the substrate material. the coatings fabricated with hvof were analyzed by scanning electron microscope (sem) / energy-dispersive x-ray spectroscopy (eds). the phase identification of a crystalline material was made with the x-ray diffraction (xrd) technique. the mecha­nical properties in terms of porosity, surface roughness and microhardness of the nanocomposite coatings were also evaluated. the sem/eds analysis showed that dense and homogeneous coatings were developed by the reinforcement of ysz nanoparticles. the peaks of xrd graphs of wc-10co-4cr coating reinforced with 5 and 10 % of ysz nanoparticles revealed that the wc was present as a major phase and w2c, co3w3c, co, co6w6c, co6w and y2o3/zro2 nanoparticles were observed as a minor phase. the porosity level decreased up to 42 and 56 % by the addition 5 and 10 % of ysz nanoparticles as compared with conventional wc-10co-4cr coating. the surface roughness values for wc-10co-4cr conventional coating, 95 % (wc-10co-4cr) + 5 % ysz and 90 % (wc-10co-4cr) + 10 % ysz nanocomposite coated samples were found to be 5.03, 4.89 and 4.28 respectively. the nanocomposite coatings reinforced with 10 % ysz nanoparticles exhibited the highest microhardness value (1278 hv). the wc-10co-4cr coatings reinforced with 10 % of ysz nanoparticles resulted in low porosity, low surface roughness and high microhardness. during the coating process, the nanoparticles of ysz flow into the pores and are dispersed in the gaps between the micrometric wc particles and provide a better shield to the substrate material. the wc-10co-4cr with 10 % of ysz nanoparticles showed better results in terms of mecha­nical and microstructural properties during the investigation.}, doi = {10.5599/jese.1190}, file = {:d\:/onedrive/mendeley desktop/kumar, bhandari, goyal 2022 mechanical and microstructural characterization of yttria-stabilized zirconia (y2o3zro2 ysz) nanoparticl.pdf:pdf;:06_jese_1190.pdf:pdf}, keywords = {high velocity oxy fuel (hvof), mechanical properties, microhardness, nanocomposite coating, porosity}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1190}, } {construction of a simple and selective electrochemical sensor based on nafion/tio2 for the voltammetric determination of olopatadine:} http://dx.doi.org/10.5599/jese.1117 91 j. electrochem. sci. eng. 12(1) (2022) 91-103; http://dx.doi.org/10.5599/jese.1117 open access : : issn 1847-9286 www.jese-online.org original scientific paper construction of a simple and selective electrochemical sensor based on nafion/tio2 for the voltammetric determination of olopatadine mohammad mehmandoust1,2, , amirhossein mehmandoust3 and nevin erk1,2, 1department of analytical chemistry, faculty of pharmacy, ankara university, ankara, turkey 2sakarya university, biomaterials, energy, photocatalysis, enzyme technology, nano & advanced materials, additive manufacturing, environmental applications, and sustainability research & development group (bioenams r&d group), 54187 sakarya, turkey 3department of chemistry, faculty of science, urmia university, 1177, urmia, iran corresponding authors: mehmandoust@ankara.edu.tr and erk@pharmacy.ankara.edu.tr received: october 21, 2021; accepted: october 14, 2021; published: november 20, 2021 abstract a selective and facile voltammetric method based on titanium dioxide nanoparticles and nafion (nafion/tio2 nps) on the screen-printed electrode (spe) was proposed for olopatadine determination. followed by the synthesis of tio2 nanoparticles, various methods, including high-resolution transmission electron microscopy (hr-tem), ultraviolet-visible spectroscopy (uv-vis), energy-dispersive x-ray (edx) raman spectrum, and electrochemical impedance spectroscopy (eis) were utilized to characterize the nanomaterials. nafion/tio2 on the screen-printed electrode (nfn/tio2/spe) was used to determine olopatadine in concentration ranges of 0.01 to 0.07 and 0.07 to 14.6 µm with a limit of quantification as low as 7.0 nm, via differential pulse voltammetry technique. the nfn/tio2/spe offered a high-performance ability to determine olopatadine in the eye drop sample with satisfactory recovery data of 98.2–99.0 %. also, the developed electrode showed good reproducibility, repeatability, and high selectivity features. the obtained results indicate that nfn/tio2/spe could be utilized as an appropriate candidate for electrochemical olopatadine sensing. keywords monitoring; screen-printed electrode, differential pulse voltammetry. introduction olopatadine (olp) is a tricyclic drug that impedes mast cell mediator liberating and possesses histamine h1-receptor antagonist activity. olopatadine hydrochloride is utilized to treat allergic http://dx.doi.org/10.5599/jese.1117 http://dx.doi.org/10.5599/jese.1117 http://www.jese-online.org/ mailto:mehmandoust@ankara.edu.tr mailto:erk@pharmacy.ankara.edu.tr j. electrochem. sci. eng. 12(1) (2022) 91-103 nafion/tio2 sensor for determination of olopatadine 92 conjunctivitis. in vivo and in vitro, a relatively specific histamine h1 antagonist reduces type 1 immediate hypersensitivity, including histamine-induced effects on human conjunctival epithelial cells [1]. the physiologic effect of olopatadine is utilizing decreased histamine release. the iupac name of olopatadine is 2-[(11z)-11-[3-(dimethylamino) propylidene]-6h-benzo[c][1]benzoxepin-2-yl]acetic acid and it is shown in scheme. 1. scheme. 1. the chemical structure of olopatadine currently, different analytical methods have been proposed to determine olopatadine, including uv–vis spectrophotometry [2-4] and high-performance liquid chromatography (hplc) [5, 6]. however, it is worth noting that the current electrode systems, such as screen-printed electrodes (spe), present significant advantages such as simple operation and portability, compared to the widespread electrode systems such as glassy carbon electrodes [7]. therefore, it is desirable to fabricate a portable electrochemical sensor to determine antihistamine agents in real samples. in this regard, fabricating an electrode with a novel nanomaterial is essential for electrochemical applications engineering a multi-functional sensing material[8-16]. nanomaterials showed many advantages in different branches of science [17-24]. in recent years, nanostructured materials, including metal oxides nanoparticles, have been used to fabricate the modified electrodes for biological, pharmaceutical, energy and environmental purposes [25-33]. among the various nanomaterials, titanium dioxide (tio2) has outstanding properties such as appropriate sensitivity and stability for its strong affinity to a phosphate group, photocatalytic effect, porosity, steadiness, and high specific surface areas [34,35]. tio2 nanoparticles have also been applied in wide potential windows, improving the electrode's stability, increasing the electrode response's repeatability [36,37]. combining metallic characteristics' electronic conductivity and electroactivity with nafion's (nfn) high ionic conductivity and ion-exchange capacity opens up many possibilities for nanoparticle design in nfn modified electrodes with unique features and uses [38]. this study employed the spe electrodes and the promising development of a susceptible electrochemical sensor based on titanium dioxide nanoparticles and nafion. it would be expected that the combination of tio2 with nafion could provide excellent electrochemical sensing performance for olopatadine. titanium dioxide nanoparticles were used to prepare additional improvements because their stated physicochemical features include strong biocompatibility, solid adsorptive ability, large surface area, thermal stability, non-toxicity, and electrical/electrochemical capabilities. the synergic effect of nafion and tio2 remarkably enhanced the electrocatalytic activity towards the olp and consequently allowed us to reach its detection limit up to 7.0 nm. moreover, nfn/tio2/spe exhibited a promising analytical performance in the wide linear concentration ranges of 0.01 to 0.07 and 0.07 to 14.66 µm towards the olp under optimized experimental conditions. the developed electrochemical sensing platform was successfully utilized to determine the amount of olp in real samples with satisfactory recovery. m. mehmandoust et al. j. electrochem. sci. eng. 12(1) (2022) 91-103 http://dx.doi.org/10.5599/jese.1117 93 experimental chemical and reagents all materials were of analytical purity and have not been subjected to a further purification process. c3h6o (acetone), c2h5oh (ethanol), naoh (sodium hydroxide) were supplied from merck (darmstadt, germany). l-ascorbic acid (99.0 %), l-cysteine (97.0 %), l-arginine (98.0 %), dopamine hydrochloride (99.0 %), glucose (99.5 %), uric acid (99.0 %), potassium hexaferrocyanide(ii) (k4fe(cn)6) (99.5 %), potassium hexacyanoferrate(iii) (k3fe(cn)6) (99.5 %) were obtained from sigma-aldrich (germany). olopatadine and eye drop (patanol®) were acquired from novartis turkey inc. deionized (di) water was utilized in the experiments. the stock solution of 1.0 mm olp was prepared via di water. apparatus energy-dispersive x-ray analysis of the sample was conducted using a quanta feg 450 field emission scanning electron microscope. the sample's morphology was characterized by transmission electron microscopy (fei talos f200s) operated at 200 kv. raman spectrum of tio2 nps was observed from renishaw invia with a 785 nm solid-state diode laser at 1.0 mw power and 10 s acquisition times. the diffused reflectance (drs) analysis was conducted by a shimadzu uv-2401 uv–vis spectrophotometer. synthesis of tio2 nanoparticles tio2 nanoparticles were synthesized by the microwave-assisted hydrothermal method (mwht). as comparing the conventional hydrothermal method (ht), the mwht also has several benefits as follows; i) shorter reaction time and ii) complete reaction at lower temperatures. thus, the microwave reaction formation helps shorten the reaction time and form reactions at lower temperatures. the microwave process may also facilitate tio2 crystals to be developed, and microwave radiation can accelerate the tio2 nucleation. moreover, low temperatures and power for microwave-assisted hydrothermal preparation may observe many morphological structures mentioned above. the tefloncovered autoclave microwave-assisted hydrothermal vessel received 5.0 ml titanium isopropoxide, 5.0 ml 1.0 m naoh, and 60.0 ml ultrapure water. a cem mars 5 microwave digestion machine was employed to prepare the reaction solution, forcefully agitated at room temperature for 30 minutes. microwave-assisted hydrothermal synthesis was performed at 100 oc for 30 minutes with a microwave output of 380 w. white tio2 nps were washed with ultrapure water and ethanol after centrifugation at 5000 rpm for 15 minutes. subsequently, the powder was dried overnight at 70 oc [39]. fabrication of nfn/tio2/spe firstly, the as-synthesized tio2 nanoparticles (0.5 mg) and 1.0 % nafion were homogeneously dispersed in 1.0 ml milli-q water by ultrasonication until a uniform tio2 dispersion was gained. following this, a 10.0 µl of nfn/tio2 dispersion was coated onto the previously cleaned spe surface by drop-casting method and dried at room temperature. nfn/tio2/spe was mildly rinsed with milli-q water to remove the non-attached composite materials on the surface of spe. moreover, to compare the electrochemical performance of the prepared composite materials, the spe surfaces were also modified with tio2 as precursor material. results and discussion characterization of tio2 fe-sem, hr-tem, and raman spectroscopy techniques were implemented to characterize tio2 nanoparticles synthesized via the microwave hydrothermal method, and the obtained results were http://dx.doi.org/10.5599/jese.1117 j. electrochem. sci. eng. 12(1) (2022) 91-103 nafion/tio2 sensor for determination of olopatadine 94 exhibited in fig. 1. in fig. 1, the diffused reflectance spectrum of tio2 nps was depicted. tio2 nps' drs spectra reflected a 380-800 nm light spectrum. at a wavelength of 383 nm, tio2 nps exhibited a drs signal. a transfer of electrons from the o 2p orbital to the ti 3d orbital was presented by this high peak [40]. a kubelka-munk function was used to determine the bandgap values of this sample, and the kubelka-munk graph of produced tio2 nps was illustrated in fig 1. inset. tio2 nps were of an estimated bandgap of 3.14 ev. anatase tio2 has a theoretical bandgap of 3.2 ev [41]. figure 1. drs and hν / ev of synthesized tio2 nps the tem images of tio2 nps were depicted in fig. 2a and 2b. aggregates with a nanoparticle diameter of 15 nm were detected in the tio2 sample. the d-spacing of the (101) and (110) planes of anatase structure corresponded to the lattice spacing of 0.318 and 0.325 nm, respectively. a b c d fig. 2. tem images (a, b), raman spectra (c), and edx spectra (d) of tio2 (f r h  )2 / ( e v / m )2 m. mehmandoust et al. j. electrochem. sci. eng. 12(1) (2022) 91-103 http://dx.doi.org/10.5599/jese.1117 95 the raman spectra of tio2 nps are illustrated in fig. 2c. the spectrum presented the five distinctive peaks of anatase tio2. the following peaks appeared: eg = 145, 197, 394, 518, and 639 cm−1 [42]. the crystalline size of the material was likewise impacted by the raman peak broadenings. the crystalline size of the material might be estimated using the raman low frequency eg mode peaks (143, 155 cm-1). the tio2 nps had a full-width hall maximum of 16 cm-1. according to the raman spectra, these predicted crystalline size estimates were likewise in excellent agreement with those computed according to debye-scherrer. the existence of elements was verified by energy-dispersive x-ray spectroscopy (fig. 2d). electrochemical behavior of olopatadine at nfn/tio2/spe the electrochemical performance of the modified sensing platforms was assessed by cyclic voltammetry (cv) in the presence of 0.1 m britton-robinson (b-r) buffer at ph 4.0 containing 100.0 µm olp at a potential scan rate of 50.0 mv s-1. as can be seen in fig. 3, a well-defined oxidation signal of olp at 1.0v was achieved at bare spe (blue line) and modified spe surfaces with tio2 (yellow line) and nfn/tio2 (red line). the corresponding voltammetric curves represented by cv demonstrated that the modification of spe surfaces by nfn/tio2 remarkably enhanced the sensitivity towards the olp with an excellent electrocatalytic activity compared to that of bare spe due to their larger surface areas. the corresponding oxidation current for olp at nfn/tio2/spe was 2.7 µa, which was 2.2 times greater than that of unmodified spe. this significant enhancement in the voltammetric response of olp with a negative oxidation potential shift that occurred at nfn/tio2/spe could be assigned to the synergistic effect of nfn and metal oxides. fig. 3. cvs of nfn/tio2/spe (blue line), tio2/spe (red line), and bare spe (black line) electrode in ph 4.0 br buffer ata scan rate of 50.0 mv s-1. the ability of electron transfer rate of the nfn/tio2/spe electrochemical sensors was evaluated by eis using [fe(cn)6]3−/4− solution as a redox probe. they were obtained at different electrodes of the bare electrode, tio2/spe, and nfn/tio2/spe in the presence of 5.0 mm [fe(cn)6]3−/4− in 0.1 m kcl. by comparing different electrodes, the charge transfer resistance (rct) values were calculated to be 7.4, 5.8 and 2.2 kω for bare electrode, tio2/spe, and nfn/tio2/spe, respectively (fig. 4). the results confirmed that the synergistic effect of tio2 and nfn in the modified electrode improved due to their properties, such as the increase in the conductivity of the electrode and the electron transfer. according to fig. 4, the electrochemical reaction occurred more efficiently on nfn/tio2/spe thanks to the least charge transfer resistance and the special electron transfer rate [32]. http://dx.doi.org/10.5599/jese.1117 j. electrochem. sci. eng. 12(1) (2022) 91-103 nafion/tio2 sensor for determination of olopatadine 96 fig. 4. eis responses at the bare electrode, tio2/spe, and nfn/tio2/spe in [fe(cn)6]3−/4− as redox probe containing 0.1 m kcl optimization of experimental conditions investigation of ph and scan rate the impact of ph upon the voltammetric response of olp was investigated at nfn/tio2/spe using a b-r buffer system in the varying ph range from 2.0 to 7.0. as depicted in fig. 5a, the oxidation peak current of olp increased gradually from 2.0 to 4.0 and then attained its maximum current value with a good peak shape at ph 4.0. a b c d fig. 5. (a) impact of varying ph values on the peak current of olp; (b) cvs of 100.0 µm olp on nfn/tio2/spe in ph 4.0 b-r buffer at varying scan rates (10.0, 25.0, 50.0,100.0, 150.0, 200.0, 300.0, 400.0 and 500.0 mvs-1); (c) the plot of the logipa vs. log v obtained at the surface of nfn/tio2/spe; (d) the plot of epa vs. natural logarithm of scan rate for olp m. mehmandoust et al. j. electrochem. sci. eng. 12(1) (2022) 91-103 http://dx.doi.org/10.5599/jese.1117 97 above ph >4.0, the corresponding peak current of olp started to decrease continually until ph 7.0. therefore, the best ph condition for olp was selected as ph 4.0 for further analytical measurements. the linear curve equation that plotted the potentials against phs was ep =−0.0381ph+ 1.1405 (r2 = 0.993). the slope attained for the principal oxidation peak in the first scan was 38.1 mv/ph, close to the theoretical value of 59 mv/ph. these results demonstrated that the transferred electron and proton numbers were equal[43]. the scan rate step is an essential parameter for determining the electrode reaction of the analyte at the working electrode. fig. 5b exhibited the scan rate effect on the voltammetric response of olp at nfn/tio2/spe in 0.1 m b-r buffer (ph 4.0) at different scan rates ranging from 10.0 to 500.0 mv s-1.the plot of the logarithm of the oxidation peak current (log ipa ) against the logarithm of the scan rate (log v) was found to be linear with the following regression equation: log ipa = 0.6171 log v –0.7381 (r2=0.9913), indicating a diffusion-controlled electrode reaction with a contribution of the adsorption (fig. 5c). likewise, it exhibited that the oxidation peak potential (epa) of olp shifted towards the positive direction with the scan rate's enhancement, which recommended the expected result for an irreversible electrode reaction. the number of electrons transferred in the oxidation step of olp can be calculated by laviron's equation from the slope of epa vs. ln v plot. the relationship between epa vs. ln v was expressed as: epa=0.0398 ln v + 0.8727 with a correlation coefficient of 0.9921. the number of electrons in the electro-oxidation of olp was estimated to be ~1.25, demonstrating that the oxidetion reaction of olp at nfn/tio2/spe is of a single electron and proton transfer process. as a result, using ph scan and scan rate studies, a possible oxidation mechanism for olp was illustrated in scheme.2. the amount of olp absorbed on the surface of the nfn/tio2/spe electrode was observed by the following equation: 2 2 p 4 4 nfqv n f v i rt rt  = = (1) where n presents the number of electrons transferred, f is the faraday's constant (96485 c mol-1), a represents the area of the electrode,  is the surface coverage of adsorbed substance, q exhibits the quantity of charge consumed, and v is scan rate. using the relationship of ip with ʋ and integrating the peak area for olp oxidation, the values of τwere calculated to be 20 µmol cm-2. scheme.2 possible electro-oxidation mechanism of olp at nfn/tio2/spe surface effect of accumulation potential and time accumulation time and potential are essential aspects that significantly impact sensitivity in analyses. for an adsorption-controlled electrode reaction, optimizing pre-concentration steps of the target analyte on the electrode surface is an essential factor in enhancing the sensitivity of the proposed electroanalytical method. for this purpose, the effect of accumulation potential (eacc) and http://dx.doi.org/10.5599/jese.1117 j. electrochem. sci. eng. 12(1) (2022) 91-103 nafion/tio2 sensor for determination of olopatadine 98 time (tacc) on the oxidation responses of olp was investigated by dpv at nfn/tio2/spe. for eacc optimization, the oxidation peak current of olp was measured by applying various eacc in the range of 0.2 to 0.9 v under a constant tacc of 30 s (fig. 6a). the maximum peak current for olp was achieved at the eacc of 0.50 v. in addition, the oxidation peak current of olp at nfn/tio2/spe enhanced piecemeal with increasing of tacc from 15 to 60 s under fixed eacc of 0.50 v and reached its maximum value at tacc of 60 s (fig. 6b). for longer tacc values, the peak currents for olp have almost remained stable due to the saturation of nfn/tio2/spe with target molecules. hence, eacc value of 0.50 v and tacc value of 60.0 s were determined as the optimal accumulation step parameters for further analytical measurements. a b fig. 6. (a) effect of tacc upon the peak current with 0.5 v of eacc, (b) effect of eacc upon the peak current with 60 s of tacc for olp in b-r (ph 4.0) on nfn/tio2/spe analytical performance differential pulse voltammetry (dpv) for olp was implemented to enhance the sensitivity of the proposed electrochemical sensing platform. dpv investigated the analytical performance of developed nfn/tio2/spe towards the different concentrations of olp under optimized experimental conditions (such as ph of the supporting electrolyte, scan rate, accumulation potential, and time). fig. 7 illustrated the dp voltammograms and the corresponding calibration curve by plotting the oxidation peak current against the increasing olp concentration (colp). two linear calibration ranges from 0.01 to 0.07 and 0.07 to 14.66 µm for olp were obtained at the surface of nfn/tio2/spe with the following regression equations: ipa = 3.101 colp + 0.255 (r2=0.9924) and ipa = 0.211colp + 0.5597 (r² = 0.9982). the sensitivity (slope) of the second linear segment for olp decreased due to the kinetic limitations. the limit of detection (lod) value for olp was calculated as 7.0 nm (3.3sb/m = 3) [24]. here, sb represents the standard deviation of the peak current (n = 10) of 0.01 µm olopatadine, and m is the slope of calibration plots. this improved sensing performance of nfn/tio2/spe for olp could be attributed to the synergic effect of nfn and metal oxide nanoparticles, which provided an enhanced conductivity, fast electron transfer, and large surface area. the analytical performance of the proposed sensing platform was compared with the similar developed electrochemical sensing platforms for the determination of olp. the obtained analytical parameters were compared to similar reported analytical methods, which have been utilized to sense olp (table 1). the analytical performance of nfn/tio2/spe, which was of wide linearity and low lod value, exhibited to be much more appropriate than other comparative analytical methods towards the detection of olp. the results suggested that nfn/tio2/spe was of comparable analytical performance (linearity and lod) towards the olp (table 2). m. mehmandoust et al. j. electrochem. sci. eng. 12(1) (2022) 91-103 http://dx.doi.org/10.5599/jese.1117 99 table. 1. a collation between diverse methods to detect olp by the developed methods method materials linear range, µm lod, µm ref. potentiometric olp–pm-cpe 3×10-5 ×10-2 1.39×10-5 [44] potentiometric poly/onoe 10-5 10-2 5.0×10-6 [45] cv hmde 10-8 4.0×10-7 5.7×10-9 [46] dpv nfn/tio2/spe 10-8 14.66×10-6 7.01×10-8 this work a b fig. 7. (a) dpvs of nfn/tio2/spe in 0.1 m br buffer at ph 4.0 containing different concentrations of olp, (b) plots of ip vs. olp concentrations table. 2. the analytical parameters were obtained by electrochemical determination of olp at nfn/tio2/spe in 0.1 m b-r(ph 4.0) parameters nfn/tio2/spe measured potential, mv 500 measured time, s 60 linear working range, µm 0.01-14.63 slope, µa/µm 0.2188 r2 0.9914 intercept 0.4829 lod, nm 10 loq, nm 33.4 interference test study the detection capability of the proposed sensing platform towards the olp in the presence of various potential interfering agents, which have commonly been found in biological samples, was assessed by dpv under optimized experimental conditions. the results exhibited that a 100-fold excess of biological compounds (ascorbic acid, dopamine, glucose, and uric acid) and amino acids (l-cysteine and l-arginine) did not show no or negligible interference effect in the determination of olp (table 3). table 3. influence of various interfering agents on olp (1.0 µm) at nfn/tio2/spe (n = 4), cinterf agents : colp = 100 : 1 interfering agents rsd, % ascorbic acid 1.16 dopamine 0.32 glucose 0.52 uric acid 0.91 l-cysteine 1.3 l-arginine 2.41 http://dx.doi.org/10.5599/jese.1117 j. electrochem. sci. eng. 12(1) (2022) 91-103 nafion/tio2 sensor for determination of olopatadine 100 the corresponding relative errors for olp were lower than ±2 %, which was correlated with the tolerance limit defined in the selectivity measurements, indicating that nfn/tio2/spe has a promising selectivity for the determination of olp. repeatability, reproducibility to prove the repeatability of the developed electrochemical sensor, the five replicate dpv measurements for olp were conducted using the same nfn/tio2/spe in ph 4.0 b-r buffer containing 1.0 µm olp under optimized accumulation conditions. similarly, the nfn/tio2/spe reproducibility was evaluated by monitoring the 1.0 olp solution in ph 4.0 b-r using five independently prepared electrochemical sensors. the corresponding relative standard deviation (rsd) values for repeatability and reproducibility tests were found as 0.73 and 2.50 %, respectively, confirming that nfn/tio2/spe has satisfactory repeatability and reproducibility towards the detection of olp. real sample analysis in real sample analysis, the eye drop sample with a spiked value of olopatadine was employed for the nfn/tio2/spe capability. the obtained data by the standard addition method were tabulated in table 4. good recovery data 98.2 and 99.0 % confirmed the powerful ability of nfn/tio2/spe in determining olopatadine in the real samples. table 4. the results data relative to analysis of olopatadine in the real sample. sample c / µm rsd, % recovery, % added found eye drop 2.0 1.98 3.49 99.0 4.0 3.93 4.21 98.2 conclusions in this paper, a new method for fabricating a selective and straightforward electrochemical sensor based on nfn/tio2 was proposed for future electrochemical sensing applications to determine the trace level of olp in the eye drop sample. the modification of nfn/tio2 on spe dramatically enhanced the electrocatalytic activity towards olp oxidation due to its large surface area, improved electron transfer kinetics, and high adsorption ability. nfn/tio2/spe illustrated a highly desirable analytical performance at the concentration ranges of 0.01 to 0.07 and 0.07 to 14.63 µm with a meager detection limit of 7.0 nm. the developed portable sensing platform also presents several advantages, such as high reproducibility, repeatability, and appropriate selectivity with an rsd of less than 5 %. the feasibility of the proposed sensing platform was successfully tested in an eye drop sample with adequate accuracy, precision results, and recovery of 98.2-99.0 %. it can be speculated that the proposed novel nfn/tio2/spe-based electrochemical sensing platform could be utilized as an alternative analytical approach with a high potential to determine antiviral agents such as olp 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(2015) 285-294. https://www.researchgate. net/publication/339102987_carbon_paste_ion-selective_electrodes_for_the_determinati on_of_olopatadine_hydrochloride_in_bulk_and_pharmaceutical_dosage_forms/link/5e3d6d 88299bf1cdb9163a01/download [45] m. m. sebaiy, m. a. elmosallamy, m. m. elhenawee, m. k. alshuwaili, , microchemical journal 147 (2019) 170-175. https://doi.org/10.1016/j.microc.2019.03.030 [46] n. sreedhar, a. sreenivasulu, m. s. kumar, m. nagaraju, journal of pharmaceutical sciences and research 3(8) (2012) 2517-2521. http://dx.doi.org/10.13040/ ijpsr.0975-8232.3(8).2517-21 ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1117 https://doi.org/10.1007/s11694-019-00353-8 https://doi.org/10.1007/s11694-019-00353-8 https://doi.org/10.1016/j.microc.2019.03.030 http://dx.doi.org/10.13040/ijpsr.0975-8232.3(8).2517-21 http://dx.doi.org/10.13040/ijpsr.0975-8232.3(8).2517-21 https://creativecommons.org/licenses/by/4.0/) https://www.researchgate. net/publication/339102987_carbon_paste_ion-selective_electrodes_for_the_determinati on_of_olopatadine_hydrochloride_in_bulk_and_pharmaceutical_dosage_forms/link/5e3d6d 88299bf1cdb9163a01/download https://www.researchgate. net/publication/339102987_carbon_paste_ion-selective_electrodes_for_the_determinati on_of_olopatadine_hydrochloride_in_bulk_and_pharmaceutical_dosage_forms/link/5e3d6d 88299bf1cdb9163a01/download {a review of the electrochemical corrosion of metals in choline chloride based deep eutectic solvents:} http://dx.doi.org/10.5599/jese.1135 237 j. electrochem. sci. eng. 12(2) (2022) 237-252; http://dx.doi.org/10.5599/jese.1135 open access : : issn 1847-9286 review paper a review of the electrochemical corrosion of metals in choline chloride based deep eutectic solvents mihael bučko and jelena b. bajat1 university of defence, military academy, 33 pavla jurišića šturma st, 11000 belgrade, serbia 1faculty of technology and metallurgy, university of belgrade, karnegijeva 4, 11120 belgrade, serbia corresponding author:  mbucko@tmf.bg.ac.rs; tel.: +381-11-3603-464; fax: +381-11-3603-065 received: october 19 2021; accepted: november 3, 2021; published: november 13, 2021 abstract deep eutectic solvents (dess) are a class of mixtures with melting points notably lower than those of their raw constituent components. these liquids have found a tremendously wide spectrum of applications in the last two decades of their research, so their contact and interaction with technical metals and alloys are inevitable. therefore, the corrosivity of dess towards metals is an extremely important topic. this review summarizes research efforts collected in the last two decades related to the corrosion rate of various metals in different dess. since the dess are mainly composed of organic raw compounds, and by their physicochemical properties they may be regarded as a separate class of ionic liquids, the literature data about dess corrosivity has been compared to the data related to the corrosivity of various organic solvents and ionic liquids as well. all the results gained until now show significantly low corrosivity of dess. this observation is discussed in relation to the chemical composition of dess. the absence of the oxidizing agents, the inhibitory action of organic ions and molecules, high viscosity and low electrical conductivity have been recognized as the main factors contributing to the low metal corrosion rate in dess. keywords ionic liquids; ethaline; reline; glyceline; hydrogen bond donor contents 1. introduction 2. chemical composition of deep eutectic solvents 3. protic character od dess 4. high concentration of ligands in dess 5. electrochemical tests and measured corrosion rates in dess 6. effect of water content 7. conclusion http://dx.doi.org/10.5599/jese.1135 http://dx.doi.org/10.5599/jese.1135 mailto:mbucko@tmf.bg.ac.rs j. electrochem. sci. eng. 12(2) (2022) 237-252 electrochemical corrosion in choline chloride 238 1. introduction deep eutectic solvents (dess) refer to mixtures of two or three solid or liquid compounds in a eutectic composition, where an unusually deep melting point depression is observed [1,2]. the preparation of dess usually consists of simple mixing of the two components for several hours at a slightly elevated temperature until the homogeneous liquid is obtained (figure 1). the main characteristic of dess is that they are in a liquid state at room or slightly elevated temperatures [3]. although they may not be considered ionic liquids, the dess share many physical characteristics, such as relatively high viscosity and density, low electrolytic conductivity compared to aqueous solutions, low volatility and vapour pressure, and high thermal stability. these attributes make dess good candidates for the replacement of traditional volatile organic solvents in many industries [4]. in addition, the dess have some beneficial features compared to conventional ionic liquids: they are usually composed of inexpensive, widely available compounds that are biodegradable and nontoxic [5]. figure 1. preparation of ethaline: dried choline chloride and ethylene glycol are mixed in a controlled atmosphere (ar‐filled glovebox), heated to 80 °c for 2 hours, then cooled to room temperature. improper heating can lead to the formation of choline chloride crystals (i.e., precipitation from solution) [6]. reprinted with permission from ref. [6], copyright (2019), american chemical society during the last two decades, dess have been involved in various applications, the most important being: dissolution of metal oxides and salts in metallurgy [7]; electrolytes in metal and alloy electroplating [8,9]; liquid-liquid extraction [10]; metal extraction [11]; gas solubility and capture [12]; electrolytes in batteries [13] or solar cells [14]; biocatalysis [15]; extraction and preparation of biodiesel [16]; biomass processing [17]; biomolecular structure stabilization [18]; genomics [19]; pharmaceutical and medical applications [20]; nanomaterials synthesis [21]. the dess have been particularly recognized as a convenient way to extend the range of coating/substrate combinations that may be produced by the electrodeposition process, in comparison to the existing electroplating processes in water-based baths [22-24]. for example, the dess may be an alternative for the electroplating of metals having electroreduction potential more negative to the potential of water decomposition, such as ti, al, and w [22,25,26]. furthermore, the electroplating in dess may afford the replacement of the electroplating systems known to be toxic and carcinogenic, such as cr, ni and co [9,23,25,27]. figure 2 illustrates an example of the surfacesensitive ni electrodeposition at pt(111) single crystal surface, from choline chloride + urea des (reline) [9]. the number of pure metal and alloy coatings that have already been successfully obtained by electrodeposition in dess is significant, so here only the most important examples are mentioned, including corrosion-resistant coatings like cu [28-31] and zn-alloys [32], magnetic alloys like sm–co [33,34], semiconducting alloys like cugase2 [35], electrocatalytic surface alloys like pt– co [36], etc. m. bučko and j. bajat j. electrochem. sci. eng. 12(2) (2022) 237-252 http://dx.doi.org/10.5599/jese.1135 239 figure 2. representation of ni(ii) electrodeposition in des on pt(111) and afm image (22 μm2) of ni clusters [9] reprinted with permission from ref. [9] copyright (2018), american chemical society to apply deep eutectic solvents in any large-scale process, it is important to gain knowledge about their corrosivity and interaction with different materials, particularly technical metals and alloys. this article first addresses the important features of the dess chemical composition: their protic/aprotic character and the presence of strong complexing agents. it further recaps the data related to the metal corrosion rates collected in various dess and by different measuring methods. finally, it summarizes the most relevant factors responsible for a generally low corrosion rate in dess. 2. chemical composition of deep eutectic solvents a mixture of any two compounds that exhibits a deep melting temperature decrease at the eutectic ratio of the two components may be regarded as a des. up to now, various dess have been prepared by combining a quaternary ammonium salt, a metal salt or a metal salt hydrate, and a hydrogen bond donor (hbd), usually an organic molecule such as an amide, carboxylic acid, or polyol [2]. table 1 explains the classification of dess into five types according to their components [4,22,37]: table 1. five types of deep eutectic solvents metal salt metal salt hydrate organic molecule as hbd quaternary ammonium salt type 1 type 2 type 3 metal salt hydrate type 4 organic molecule as hydrogen bond acceptor (hba) type 5 it is estimated [38] that there may be 106–108 potential des formulations as various binary combinations of ammonium salts, metal salts, and organic molecules. since the majority of dess contain an organic compound as one of the constituents, to predict the corrosivity of a particular des, it is advisable to first analyse the metal corrosion in a particular organic, since the literature about this topic is usually vastly available [39]. interestingly, up to now, only the metal and alloy corrosion in the type 3 dess has been studied, and particularly in dess containing choline chloride as a quaternary ammonium salt, and as a result, this review focuses primarily on the metal corrosion in type 3 dess. http://dx.doi.org/10.5599/jese.1135 j. electrochem. sci. eng. 12(2) (2022) 237-252 electrochemical corrosion in choline chloride 240 3. protic character of dess metals with the corrosion potential more negative than the equilibrium potential of hydrogen electrode corrode with the electroreduction of hydrogen proton. the presence of solvated hydrogen proton in protic organic media thus induces corrosion of electronegative metals [39]. it should be held in mind that the most frequently used hbds in type 3 dess (alcohols, amides and carboxylic acids) are protic organic compounds, and so the presence of free hydrogen ions may be expected in these dess. the ph value of a des depends on the ability of the des’s cation, anion, and hbd to act as proton acceptors and proton donors [40,41]. if ha is a protic hydrogen bond donor, then the protonation reaction in des is ha + y ⇌ hy+ +a− (1) where y represents the des constituent [40,41]. metal corrosion may occur due to the partial cathodic reaction of the protonated species 2hy++ 2e−→ 2y+ h2 (2) but also, as it is often the case in organic solutions, due to the direct hydrogen evolution from the non-dissociated proton donor [42]: 2ha+ 2e−→ 2a−+ h2 (3) the experimentally determined ph values of a few representative choline chloride based type 3 dess are listed in table 2. the ph value of chcl based dess is significantly influenced by the type of hbd. the acidity decreases in the following order: dess with carboxylic acids (citric, glycolic, lactic, malic, malonic, oxalic acid)>polyols (ethylene glycol, glycerol) > sugars(fructose, glucose) > amines and amides (urea, ethanolamine, diethanolamine) [41]. for all choline chloride based dess, a linear decrease of ph value was observed with the increase in temperature, and as concerning the water influence, the increase in water content decreases the ph for the majority of dess. however, there are dess where the opposite was observed, for instance in chcl-citric acid mixture [41]. table 2. experimentally measured ph values of several dess des ph value reference 1 chcl : 1 oxalic acid 1.32 [43] 1 chcl : 1 malonic acid 2.39 [43] 1 chcl : 1 citric acid : 3 h2o 0.63−0.67 [44] 1 chcl : 2 ethylene glycol 4.77 5.93 (ph indicator) 6.89 (ph glass electrode) [45] [43] [43] 1 chcl : 2 glycerol 7.54 7.48 [46] [43] 1 chcl : 2 urea 10.39 10.07 [46] [47] the fast metal corrosion due to the rapid hydrogen evolution reaction in acidic dess was demonstrated by abbott et al. [48], where it was shown that the corrosion rate (expressed in μm per year) for mild steel in oxaline was two orders of magnitude higher in comparison to reline and ethaline, whereas it was one order of magnitude higher for ni. interestingly, the corrosion rate of al was low and very similar in both acidic and ph neutral dess, probably due to the al passivation by oxalate anion [48]. high acidity of an organic acid containing dess is beneficial for the dissolution m. bučko and j. bajat j. electrochem. sci. eng. 12(2) (2022) 237-252 http://dx.doi.org/10.5599/jese.1135 241 of various metal oxides in industrial and recycling processes because the hydrogen protons act as oxygen acceptors and break the metal–oxide bonds [49]. 4. high concentration of ligands in dess choline chloride based dess contain 5 mol dm-3 chloride anions, and these species are well known to be detrimental for metal corrosion in two ways. firstly, the chloride ions cause the metal oxide film rupture and pitting corrosion. according to the kinetic model of pitting corrosion, the pit initiation starts with the adsorption of chloride ions on the metal oxide surface and their penetration through the oxide film and propagates with the localized dissolution of metal at the metal/oxide interface [50,51]. secondly, chloride is a ligand that forms complex salts with metal ions, increasing the solubility of metal ions and metal compounds, preventing metal passivation, and displacing the redox electrode potential of the metal to the negative side, making its anodic dissolution easier [52]. as a matter of fact, since the 1990s the halide salts have been used as ligands to promote metal solubility in organic solvents [53]. the catalytic role of chloride anion in the anodic partial reaction of a metal corrosion process is well known for aqueous media, for example, in the cases of copper and steel corrosion [54,55]. it is assumed that the first step in the metal anodic dissolution mechanism is one of the following reactions: cu + cl−⇌cucl + e− (4) cucl + cl−⇌cucl2(5) cu + 2cl−⇌cucl2+ 2e− (6) fe + cl−⇌fecl + e− (7) fecl ⇌fe+ + cl− (8) the identical participation of cl– is also very likely present in the metal dissolution process in dess [28,56]. furthermore, by identifying feocl in the corrosion products at steel immersed in ethaline and reline, kityk et al. [56] concluded that the chloride ion present in large concentrations even changes the mechanism and accelerates steel corrosion in des, in comparison to water solution. particularly, cl– ions enable the formation of feocl, an intermediate compound that facilitates the formation of γ-feooh [56]. due to the high cl– concentration in dess, the majority of metal ions are solvated in dess in the form of various chloro-complexes. the metal speciation was studied in des mixtures of choline chloride with several hbds (urea, ethylene glycol, propylene glycol, and 1,3-propanediol), by dissolving various metal salts, namely sulphates, nitrates, oxides, thiocyanates, and perchlorates [57]. it was found that m+ ions form [mcl2]–and [mcl3]2−species and/or their mixtures, depending on the hbd present, for instance [cucl2]− and [cucl3]2−, [agcl2]−and [agcl3]2−, [aucl3]2−, etc. this speciation is consistent with the chemistry in aqueous solutions with high chloride concentrations [58]. as concerning m(ii) salts, in diol-based dess all m2+ ions, except ni2+, form tetrachloro complexes [mcl4]2−, like for example, [fecl4]2−, [zncl4]2−, [ptcl4]2−, etc. interestingly, chloride was not found in the ni2+coordination shell, but rather only the [ni(hbd)3]2+ cationic complexes were detected [57]. contrary to the diol based dess, in urea-based des, the complex ion composition depends on the metal: the late transition metals form tetrachloro complex anions, but early transition metals like fe, mn and cr, form salts with hbd as a ligand, where hbd may be water or urea, i.e., [mn(hbd)6]2+, [fe(hbd)5]2+ and [cocl3(hbd)][57]. http://dx.doi.org/10.5599/jese.1135 j. electrochem. sci. eng. 12(2) (2022) 237-252 electrochemical corrosion in choline chloride 242 among m(iii) salts, the experiments with cr3+ in the same study [57] showed that in ethylene glycol-based des, chromium forms complex salt with mixed ligands [cr(h2o)2cl4]2−, and in ureabased des, the first shell consists only of hbd, i.e. [cr(hbd)6]3+. dissimilar metal speciation in dess compared to water is responsible for the difference in the electrochemical series, i.e., the series of equilibrium electrode potentials for redox reactions of various metal/metal ion couples in different media. electrode potentials for some redox couples in ethaline were reported [59] and compared to the analogous values in an aqueous, metal chloride containing medium. it was apparent that some equilibrium electrode potentials were positive, while others were more negative compared to the values measured in water. for the redox couples with more negative potential in ethaline, the redox equilibrium in ethaline is shifted to the species with a higher oxidation state compared to the equilibrium in water, and the reverse is true for the couples above the line. it was noticeable that for metals that form strong chloro-complex ions (au, pd, ag), there was a significant negative deviation of the equilibrium redox potential in ethaline. an interesting case was observed for cu: the equilibrium potential for the redox couple cu+/cu is surprisingly negative in ethaline. however, the potential of the cu2+/cu+ couple is very positive, pointing to the fact that in ethaline, the cu+ species is stable along with cu and cu2+ [59]. interesting investigations of cu corrosion in ethaline show that cu dissolution is accelerated when cu2+ species are present in the medium due to the comproportionation reaction (eq . 9). cucl42− + cu → 2cucl2− (9) this reaction is well known in chloride-containing aqueous media, and it seems that the analogous mechanism may be applied for the case of ethaline [28,29]. the consequence of this reaction is observed in the electrodeposition process with cu soluble anode, where the so-called “anomalous dissolution” occurs, i.e., the mass loss of cu anode is much higher than anticipated based on the amount of charge passed for a single electron transfer. in other words, in case cu2+ is present in ethaline, the anodic dissolution processes occurring at the cu anode to form cucl2− are coupled with the cathodic process involving the reduction of cucl42− [28,29]. 5. electrochemical tests and measured corrosion rates in dess the corrosion rate of various metals in choline chloride-based dess has been analysed by electrochemical methods in majority of the previous studies, and the reported data are summarized in table 3. for nearly ph neutral dess, i.e., with the exception of acidic dess, generally, it may be concluded that all the sources report significantly low corrosion rates, ranging from around 1 µa cm-2 to extremely low values of the order of na cm-2. for example, the comparison of the surface morphology of al alloy samples exposed to the air and to reline at 60 °c for 19 days (figure 3) showed that all the samples were shiny. the brownish deposits at the samples immersed in reline, were not corrosion products or signs of pitting, but rather only reline deposits not removed by the washing process [60]. except for the results reported in [63], the dess were reported as of very low corrosivity to various metals, in practically all of the conducted studies available in the literature, in spite of containing strong ligands in high concentration. the cu dissolution in [63] was promoted by the rotation of the working electrode and the ultrasonic agitation to enhance metal leaching in ethaline. therefore, it is understandable that the corrosion rate in such working conditions is significantly higher than the values measured in stationary conditions. m. bučko and j. bajat j. electrochem. sci. eng. 12(2) (2022) 237-252 http://dx.doi.org/10.5599/jese.1135 243 figure 3. visual inspection of the alloy samples after 19 days: (a,c) aa2024-t6, aa6065-t6 in the air; (b,d) aa2024-t6, aa6065-t6 in reline, respectively [60] reprinted with permission from ref. [60] copyright (2020), elsevier very low corrosivity of dess had been ascribed to the absence of an oxidizing species in dess, high viscosity and low electrical conductivity of dess, the formation of a protective layer at the metal surface immersed in des, or the inhibiting action of the constituting compounds in dess [48,56,60-62]. the chemical corrosion of a metal in a corrosive medium implies the reaction of metal with species x and the formation of corrosion product: m + x → mx (10) in analogy, electrochemical corrosion implies the transfer of electrons from a metal surface to the corrosion agent, i.e., to the species in a corrosive medium capable of metal oxidation: m → mn+ + ne(11) ne+ x → xn(12) the well-known corrosion agents in aqueous media are water molecules, oxygen molecules, and hydrogen ions. understandably, if these species are present as impurities in non-aqueous media, they also may act as agents inducing metal corrosion. yet, in case that the liquid medium does not contain any compound that can act as a corrosion agent for metal, the medium remains chemically inert and the metal corrosion does not occur. it has been stated [52,56,60] that apart from oxygen and water present as impurities, the phneutral choline chloride based dess do not contain any other species capable of metal oxidation. to support this statement, it is useful to tackle the oxidizing power of the organic compounds representing the most often used constituents in dess. http://dx.doi.org/10.5599/jese.1135 j. electrochem. sci. eng. 12(2) (2022) 237-252 electrochemical corrosion in choline chloride 244 table 3. corrosion rate data for several dess. metal des jcorr / µa cm-2 testing method, reference steel reline 0.87 at 25 °c 1.72 at 80 °c tafel plots, [56] reline 10-3 at 25 °c 0.02 at 75 °c tafel plots, [61] ethaline 1.18 at 25 °c 1.78 at 80 °c tafel plots, [56] ethaline 0.2 at 25 °c 3.9 at 75 °c tafel plots, [61] glyceline 6 10-3 at 25 °c 0.52 at 75 °c tafel plots, [61] chcl-malonic acid 5 at 25 °c 187 at 75 °c tafel plots, [61] copper reline 3.58 at 25 °c 30.45 at 75 °c tafel plots, [61] reline 340 at 80 °c tafel plots, [62] ethaline 17.44 at 25 °c 166.53 at 75 °c tafel plots, [61] ethaline 460 at 50 °c 8700 at 50 °c 27540 at 50 °c tafel plots, rde 5000 rpm [63] tafel plots, ultrasonic agitation (us) [63] tafel plots, rde 5000 rpm + us [63] glyceline 6.03 at 25 °c 31.22 at 75 °c tafel plots, [61] chcl-malonic acid 7.43 at 25 °c 333.17 at 75 °c tafel plots, [61] stainless steel 316 reline 8  10-3 at 25 °c 0.013 at 75 °c tafel plots, [61] ethaline 0.014 at 25 °c 0.23 at 75 °c tafel plots, [61] glyceline 8  10-3 at 25 °c 0.11 at 75 °c tafel plots, [61] chcl-malonic acid 0.4 at 25 °c 22.7 at 75 °c tafel plots, [61] aluminum alloys aa2024 and aa6065 reline 0.4 at 60 °c inductively coupled plasma optical emission spectroscopy, [60] extremely low corrosion rates for mild steel, ni and al in reline, ethaline, and glyceline: 1.9 to 5.02 μm year-1 tafel plots, [48] very slow reaction of az31b mg alloy in ethaline, at temperatures up to 85 °c [64] cholinium cation, being the representative of quaternary ammonium cations, is extremely stable toward the electroreduction, even at very negative electrode potentials [65]. pure ethylene glycol (a constituent of ethaline) is almost inert even to reactive metals like magnesium [66]. as concerning urea (a constituent of reline), although the molten urea is a very versatile solvent that dissolves the majority of inorganic chemicals [67], the data on its corrosivity are very scarce. yet, it is known from the research in urea production plants that the pure molten urea is not corrosive to stainless steels [68] and in addition, it has been used as a supporting electrolyte with a wide electrochemical window [69]. this brief summary shows that, indeed, the pure organics used for preparing the most common dess, are not capable of metal oxidation. m. bučko and j. bajat j. electrochem. sci. eng. 12(2) (2022) 237-252 http://dx.doi.org/10.5599/jese.1135 245 the second important reason for the low corrosivity of choline chloride-based dess is probably the corrosion inhibiting nature of the raw compounds. like most organics, the choline cation and the common hydrogen bond donors should be easily adsorbed onto a metal surface, which is the main prerequisite for the inhibition of the corrosion process by blocking the anodic and/or cathodic sites. consequently, the literature on the application of quaternary ammonium ions [70], choline based salts [71], choline based ionic liquids [72], urea [73], ethylene glycol [66, 74], alcohols in general [75], etc., as corrosion inhibitors in aqueous media, is extensive. in [60], very low corrosion of aa2024-t6 and aa6065-t6 alloys in reline was ascribed to the adsorption of choline cation or urea molecules on the metal surface, where one or both of these species constitute the first liquid layer in contact with al2o3 passive film, thus separating and protecting it from the cl– anions. it was well documented for various dess (choline chloride with ethylene glycol, 1,2ethanediol, 1,2-propanediol, 1,3-propanediol, urea or thiourea) that at negative and open circuit electrode potential, the choline cations and hbd molecules occupy the first layer of adsorbate at the metal surface, whereas at a positive potential, the cl– anion is adsorbed [76, 77]. the mechanism of reline adsorption at various 2d nanomaterials was studied using molecular simulation methods [78]. independently on the material used as a substrate, the number density profiles showed that the first adsorbed layer mainly consisted of urea molecules, while the number of clions was significantly low, as presented in figure 4 [78]. figure 4. number of molecules, n, in the first adsorbed layer for reline adsorbed on graphene, boron nitride, silicene, germanene, and molybdenum disulfide [78] reprinted with permission from ref. [78] copyright (2020), american chemical society finally, as is the case in other organic media, an important factor of the metal corrosion rate in des, may be the des high viscosity or low electrical conductivity. for example, in the case of zn corrosion in hcl-containing representatives of alcohols, ketones, esters, ethers, aromatics and chlorinated hydrocarbons, the viscosity was singled out as the decisive factor in the corrosion rate since it plays an important part in the transport of the oxidation agent and corrosion products [79]. on the contrary, in the same investigation and same media, for other metals under study (fe, al and stainless steel), the electrolytic conductivity of the solvents showed the most notable influence on the corrosion rate. it was concluded that the conductivity of around 10 ms cm-1 represents a threshold value below which the corrosion process is considerably retarded, because the corrosion mechanism changes from electrolytic to non-electrolytic. the industrial application of organic media with conductivity lower than the listed threshold value would enable the employment of construction materials that are lower in price without fear of corrosion [79]. http://dx.doi.org/10.5599/jese.1135 j. electrochem. sci. eng. 12(2) (2022) 237-252 electrochemical corrosion in choline chloride 246 it is very important to draw the parallel between the described findings in organic media and the case of dess, bearing in mind the high viscosity and low electrical conductivity of the majority of dess, as illustrated in table 4. for instance, the high reline viscosity and low diffusivity of species in this liquid were assumed to be an important factor in al-oxide passive film's stability at the al-alloys in reline. namely, the passive layer at the aa2024-t6 and aa6065 alloy surfaces remained intact even after 35 days of exposure to reline at 60 °c [60]. it was concluded that the pitting corrosion, even if it occurs at some point, will be terminated due to the significant mass transport limitation in a viscous medium because the diffusion of metal cations within the pit is necessary for a stable pit formation. although the conductivity of dess with common hbds listed in the table is notably higher than the conductivity of molecular solvents like ethylene glycol and glycerol, it is still well below the critical value of 10 ms cm-1. table 4. viscosity and electrical conductivity of several dess, some ionic liquids, and some molecular solvents at 298 k solvent viscosity, cp conductivity, ms cm-1 reference chcl-malonic acid 721 0.55 [4] chcl-urea des 632 0.75 [4] chcl-glycerol 376 1.05 [4] chcl-ethylene glycol des 36 7.61 [4] c4mimbf4 ionic liquid 115 3.5 [4] glycerol 967 5 10-5 [80] ethylene glycol 16.1 1.4 10-4 [81] 6. effect of water content since choline chloride and typical hbds in dess (amides, alcohols, polyols, carboxylic acids, etc.) are very hygroscopic substances, a highly hydrophilic behaviour of most choline chloride-based dess is expected [82]. as an illustration, reline contains 2500 ppm water even after a drying process conducted in a vacuum oven at 353 k over 24 h, and it can absorb atmospheric moisture up to a water concentration of 40 wt.% [83]. similarly, the as-prepared ethaline contained 2.4 wt.% water even after thorough drying of precursor materials, and after two weeks in the open air, the water content reached 14.3 wt.% [82]. in general, water in des decreases its kinematic viscosity (figure 5), increases its electrical conductivity, and notably narrows its electrochemical window [82,84], and all these factors contribute to the increase in the corrosion processes rate. consequently, it was shown in [85] that the corrosion of mild steel in ethaline and reline is significantly enhanced even with the addition of only 10 wt.% of water. very similar observations may be found in numerous researches of the water effect on the corrosivity of ionic liquids. when the corrosion behaviour of carbon steel, austenitic stainless steel, nickelbased alloy, copper, brass and almg3 alloy was examined in seven ionic liquids with different chemical structures, the addition of only 10 % of water increased the corrosivity of all ionic liquids significantly [87]. the increased corrosion rate with water addition of up to 8 wt.% was also measured for the case of mg alloys in 1-butyl-3-methylimidazolium trifluoromethyl sulfonate ionic liquid [88]. the water effect on the metal corrosion rate in ionic liquids and deep eutectic solvents was ascribed to the increased solubility of oxygen as a cathodic species in the corrosion mechanism, higher diffusivity of oxygen, and the easier removal of corrosion products [85,89]. moreover, it is known that m. bučko and j. bajat j. electrochem. sci. eng. 12(2) (2022) 237-252 http://dx.doi.org/10.5599/jese.1135 247 the presence of water in an ionic liquid can cause anion hydrolysis, increasing electrolyte aggressiveness [89]. this should also be held in mind when a particular des is selected for technical applications. figure 5. experimental values of viscosity vs. water mole fraction for the choline chloride+dlmalic acid des, where symbols refer to experimental data points at several temperatures: 298.15 k (♦); 303.15 k (○); 308.15 k (●); 313.15 k (δ); 318.15 k (▲); 323.15k (□); 328.15 k (■); 338.15 k (+); 348.15 k (⌂); 358.15 k (◄) and 363.15 k (×) [86] reprinted with permission from ref. [86] copyright (2017), serbian chemical society according to the previous knowledge, it may be stated that the appropriate des handling, storage, and water removal strategies should be developed to minimize the water content in des in commercial applications. finally, when the future application of these types of electrolytes at an industrial scale is taken into account, it should be held in mind that even if the corrosion rates of various metals in dess are low, future studies should focus on the long exposure to the dess, which is characteristics of the real-life exploitation. only a few of the cited studies in this review report the results from the prolonged contact of metals with dess. in [56], the behaviour of the mild steel was monitored for 30 days of exposure to ethaline and reline, but it was observed that the most significant changes on the sample surface occurred on the first day of immersion. in the other example, the corrosion of al alloys in reline was monitored for 36 days and it was concluded that the resistance of the passive layer steadily increased over the immersion time [60]. 7. conclusions in spite of the fact that the term “deep eutectic solvent” may be attributed to as many as 106–108 various mixtures, the corrosion problem has been raised for a remarkably low number of dess until now. a comprehensive literature data is nowadays available exclusively for traditional dess containing choline chloride as their component, so these sources were the focus of the current review. one of the first and still actual applications of dess has been the dissolution of various metal salts in electroplating, anodic polishing, and metal extraction/recycling by electrolysis due to the very strong solvating power of dess. however, as this review clearly shows for various metals and dess, when it comes to the dissolution of metals or alloys in their reduced (m0) state, the solvating power of dess shows to be very low, resulting in strikingly low corrosion rates. http://dx.doi.org/10.5599/jese.1135 j. electrochem. sci. eng. 12(2) (2022) 237-252 electrochemical corrosion in choline chloride 248 this article recognizes the most distinguishing causative factors for the low des corrosivity, and these are the absence of the oxidizing agents (apart from hydrogen ion, oxygen and water), the inhibiting role of organic des components, and physical properties that do not allow the rapid corrosion process to occur or enhance the metal passivation process. yet, concerning the water impurity influence on their corrosivity, dess are no different from the majority of other organic solvents or ionic liquids: the increase in water content notably increases the des aggressiveness. if corrosion problems are to be avoided, strategies to minimize the water content in des should be applied. acknowledgement: this research was financed by the ministry of education, science and technological development of the republic of serbia (contract no. 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australian journal of chemistry 60 (2007) 35-42. https://doi.org/10.1071/ch06305 [89] s. noori, m. v. diamanti, m. p. pedeferri, a. brenna, m. ormellese, materials and corrosion 69 (2018) 1658-1668. https://doi.org/10.1002/maco.201810215 ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.2298/jsc170316054v https://doi.org/10.1039/b419320m https://doi.org/10.1071/ch06305 https://doi.org/10.1002/maco.201810215 https://creativecommons.org/licenses/by/4.0/) editorial i j. electrochem. sci. eng. 6(1) (2016) i-ii open access : : issn 1847-9286 www.jese-online.org editorial this special issue of the journal of electrochemical science and engineering presents papers form the fifth regional symposium on electrochemistry – south east europe (rse-see5) held in pravetz, bulgaria, june 7-11, 2015. following the 20 th century traditions of the strong regional electrochemical schools and their excellent networking, the four successful symposium meetings in croatia (red island, 2008), serbia (belgrade, 2010), romania (bucharest, 2012) and slovenia (ljubljana, 2013) have built a strong and solid base for the fulfillment of the 5th edition mission to contribute to the excellence in electrochemical science as part of the european research area. rse-see5 crossed regional borders, creating a common platform for scientists and experts from regional and international research and industrial institutions who brought their extensive knowledge, novel ideas, and boundless enthusiasm to the conference venue, presenting 77 talks (6 plenary, 17 keynote) and 88 posters. the fifth edition of the symposium will be remembered with a scientific committee represented by members from 17 countries of the south east europe region with the special “brokerage” session aiming to initialise common projects, with the decision for accepting the journal of electrochemical science and engineering as the official proceedings journal of rse-see. this action ensured the submission of papers to become open for all authors, independent of the form of presentation – oral or poster. however, the most memorable symposium event was the official establishment of the association of south-east european electrochemists (asee), aiming to contribute to the advancement of the fundamental and applied research in the field of electrochemistry and to enhance the regional scientific communication and cooperation. we would like to acknowledge the financial support of the international society of electrochemistry, the bulgarian ministry of education and science, the bulgarian academy of sciences, the institute of electrochemistry and energy systems at the bulgarian academy of sciences, the organisation for the prohibition of chemical weapons, dropsence and palmsence which ensured 15 awards for young scientists covering their conference registration fees. we wish to express our gratitude to the members of the scientific and organising committees for their unrelenting dedication and support and the commendable decision to incorporate the national event sofia electrochemical days into the symposium. http://www.jese-online.org/ j. electrochem. sci. eng. 6(1) (2016) i-ii ii we would like to sincerely thank the authors for their contribution, the reviewers for their commitment and professional work and the jese-editorial office for their invaluable assistance in the proceedings publication. we wish the hungarian electrochemists community successful organisation of the rse-see 6 in hungary, where we shall meet again in 2017. guest editors: co-chairs of rse-see 5: daria vladikova daria vladikova gergana raikova zdravko stoynov methanol oxidation at platinized copper particles prepared by galvanic replacement doi:10.5599/jese.239 17 j. electrochem. sci. eng. 6(1) (2016) 17-28; doi: 10.5599/jese.239 open access : : issn 1847-9286 www.jese-online.org original scientific paper methanol oxidation at platinized copper particles prepared by galvanic replacement ioanna mintsouli*, jenia georgieva**, athanasios papaderakis*, stephan armyanov**, eugenia valova**, volodymyr khomenko***, stella balomenou****, dimitrios tsiplakides* , ****, sotiris sotiropoulos* *department of chemistry, aristotle university of thessaloniki, thessaloniki 54124, greece **rostislaw kaischew institute of physical chemistry, bulgarian academy of sciences, sofia 1113, bulgaria ***kiev national university of technologies and design, department for electrochemical power engineering & chemistry, kiev 01011, ukraine ****chemical process & energy resources institute, centre for research and technology hellas, 570 01, thermi, thessaloniki, greece corresponding author: eczss@chem.auth.gr; tel.: +30-2310-997-742; fax: +30-2310-997-709 received: october 14, 2015; accepted november 23, 2015 abstract bimetallic pt-cu particles have been prepared by galvanic replacement of cu precursor nanoparticles, upon the treatment of the latter with a chloro-platinate acidic solution. the resulting particles, typically a few tens of nm large, were supported on high surface area carbon (vulcan® xc–72r, cabot) and tested as electrodes. surface electrochemistry in deaerated acid solutions was similar to that of pure pt, indicating the existence of a pt shell (hence the particles are denoted as pt(cu)). pt(cu)/c supported catalysts exhibit superior carbon monoxide and methanol oxidation activity with respect to their pt/c analogues when compared on a per electroactive surface area basis, due to the modification of pt activity by cu residing in the particle core. however, as a result of large particle size and agglomeration phenomena, pt(cu)/c are still inferior to pt/c when compared on a mass specific activity basis. keywords transmetalation; electrocatalysts; platinum introduction the need for reducing the cost and at the same time maintaining or improving the performance of catalysts used in fuel cells and electrolyzers has led to intensive research into non-precious mehttp://www.jese-online.org/ mailto:eczss@chem.auth.gr j. electrochem. sci. eng. 6(1) (2016) 17-28 methanol oxidation at platinized copper 18 tal alternatives or multi-metallic noble-metal-based systems. all common preparation methods for the latter involve a reduction step, either using a reducing agent in solution phase or in a reducing atmosphere (e.g. hydrogen) at elevated temperatures [1-5]. a high surface area support (usually carbon) is either present in the catalyst synthesis mixture or added during a subsequent step. since the appearance of the first papers on the subject [6-12], galvanic replacement has been established during the last decade as an alternative method for preparing multi-metallic noblemetal-based electrode materials. the principle of the method lies on the fact that when a nonnoble metal (e.g. pb, cu, fe, co, ni, mo, bi) is immersed in a solution containing ions of a noble metal (e.g. pt, au, pd, ir, ru) then, due to a difference in standard potentials, the latter is deposited in metallic form replacing layers of the former (which is oxidized/released in ionic form in the solution). an example of the process for cu and pt, resulting in a pt-rich shell and a cucontaining core (denoted as pt(cu)), is given by reaction (1) below: 2 cu + ptcl6 2  pt + 6 cl + 2 cu 2+ (1) among advantages of the technique one can mention its room temperature conditions, the use of dilute solutions of the precious metal, as well as the potential formation of a noble-metal-rich shell / bimetallic-core structure. the latter can reduce the amount of the precious metal in the catalyst and also lead to beneficial electronic effects of the second metal on its catalytic activity. there have been three main research groups in the area. adzic and co-workers have put emphasis on the replacement of cu upd monolayers by precious metals and the catalysis of oxygen reduction (see for example [8-10,13-19]); kokkinidis, sotiropoulos and co-workers have used the replacement of surface layers of transition metal deposits by pt, mostly for methanol oxidation catalysis and with a recent emphasis on catalyst preparation on carbon and semiconductor oxide high surface area supports [11-12,20-33]; finally, podlovchenko and co-workers have focused on the replacement of cu or mo layers by pt or pd and their use as methanol and formic acid oxidation catalysts [34-39]. methanol oxidation in acid is the fuel reaction in direct methanol fuel cells (see for exam ple [40-41]) which are envisaged as candidates for portable/micro-fuel cells. to improve tolerance towards co intermediate poisoning, multi-metallic systems based on pt have long been explored. foreign metals adsorbed as ad-atoms or alloyed with pt have been extensively studied (see for example [42-46]). we have tested the catalytic activity of pt(pb) [20] and pt(cu), pt(ni), pt(co) [26] core-shell catalytic layers on glassy carbon electrodes and have found a pronounced activity and stability of the pt(cu) formulations towards methanol oxidation. a rather limited number of papers of pt(cu) particles prepared by chemical methods on c particle supports can be found in the literature [29-33,47-52], which include those by our group [29-33]. in all but a few of these works [48,49,52] galvanic replacement occurred after the cu precursor was cast on the c support or in the presence of the latter. as discussed in [29], the presence of the conducting c particles provides an alternative location for pt deposition (to that of the surface of cu): electrons released from cu dissolving in the solution can travel through c to nearby locations whereby pt ions can be reduced and deposited as metallic pt. in that case the catalyst comprises of a mixture of pt(cu) and pure pt particles, moderating the beneficial effect of cu on pt catalytic activity. however, if the pt(cu) particles were first to be synthesized and then mixed with/ancored on the c particles one would ensure the bimetallic character of the catalyst. the aim of this work has been to prepare pt(cu) bimetallic particles by the galvanic replacement technique and then use them as supported catalysts for methanol oxidation. specific i. mintsouli at al. j. electrochem. sci. eng. 6(1) (2016) 17-28 doi:10.5599/jese.239 19 objectives have been: i. the preparation of cu nanoparticles and their conversion into platinized bimetallic particles; ii. the microscopic, spectroscopic and crysrallographic characterization of the pt(cu) and pt(cu)/c materials; iii. the electrochemical characterization of the deposits by means of pt surface electrochemistry in acid, co oxidation and methanol oxidation. experimental a. preparation of cu and pt(cu) nanoparticles and pt(cu)/c modified electrodes the cu powder was prepared by reduction in a hydrogen-containing recirculate gas stream. the required amount of cu(no3)2·3h2ο was dissolved in a 1:1 water-ethanol mixture and heated at 80 ο c for 1 h 30 min under stirring until complete solvent evaporation. the resulting solid was crushed in a mortar and then placed in a crucible and thermally treated for 60 min at 300 ο c in an ar atmosphere for cuo formation. in our experiments metallic copper powder was obtained by the reaction of cuo at 250 ο c under a 8 % h2/92 % ar mixture (gas flow 20 cm 3 /min) for 60 min. finally, the powder was left to cool to room temperature under ar. in a typical process, 0.05 g of freshly prepared cu particles were added in 25 ml of a 5 x 10 -3 μ k2ptcl6 + 0.1 m hcl dearated solution and were sonicated for 45 min in an ultrasound bath so that spontaneous partial replacement of cu by pt could take place: 2 cu(cu) + ptcl 6 2  pt (cu)+2 cu 2+ + 6 cl  (2) ((cu) denotes copper in the bulk/core of the particles). finally, the suspension is filtered to obtain ca 0.022 g of a black powder product (pt(cu)). note that the filtrate has turned light blue (indicative of the release of cu ions according to (2)) from the originally light yellow colour of the suspension (due to k2ptcl6). 0.0173 g of the pt(cu) powder were mixed with 0.0692 g of carbon powder (vulcan® xc–72r cabot) so that the mixture was 20 % w/w in the catalyst. the pt(cu)/c mixture was dispersed in 1 ml ethanol and was ultrasonicated for 5 min and then left to dry. for the electrochemical testing of the catalyst, 3 mg of the thus prepared pt(cu)/c material (or of commercial 20% w/w pt/c catalyst, etek) were suspended in 0.5 ml methanol together with drops of a nafion  (protonic form) 5 % w/w solution in a mixture of low aliphatic alcohols and 45% water (aldrich), amounting to 2.5 mg of nafion® polymer. the as prepared suspension was homogenized in an ultrasonic bath and a given quantity of the slurry (containing the desired catalyst + support weight in the 0.8-1 mg range) was placed in a drop-wise manner on a flat glassy carbon electrode (gc, bas inc.) over a total area of 0.385 cm 2 and left to dry. the resulting pt loading in the pt(cu)/c electrode whose results are presented here (based on the eds analysis of the catalyst , see also below) was 0.20 mgpt cm -2 ; that of the 20 % w/w pt/c commercial catalyst was 0.43 mgpt cm -2 . b. microscopic, spectroscopic and crystallographic characterisation of materials scanning electron microscopy (sem) was carried out using a jeol jsm-5510 microscope and elemental analysis of the coatings was performed by the accompanying eds (edax) system. the morphology of the catalysts was investigated with high resolution transmission electron microscopy (tem) using a jeol jem 2010 microscope coupled with oxford inca x-ray eds. x-ray diffraction (xrd) deposit characterisation was performed with the help of a rigakou miniflex diffractometer. j. electrochem. sci. eng. 6(1) (2016) 17-28 methanol oxidation at platinized copper 20 c. electrochemical experiments voltammetry was carried out with an autolab 30 (ecochimie) system in a three-electrode cell. a saturated calomel electrode (sce) was used as the reference electrode and a pt foil as the counter electrode. the catalytic electrode was first scanned in a deaerated 0.1 m hclo4 solution between the hydrogen and oxygen evolution potential limits at 500 mv s −1 until a steady state voltammetric picture was recorded (typically after 20-30 cycles, whereby anodic currents due to uncovered cu dissolution vanished). next, the electrode was scanned again in a fresh deaerated 0.1 m hclo4 solution at 10 mv s −1 , for a steady state voltammogram (typical of pt) to be attained (typically after 2–3 cycles). following that, the solution was saturated with pure co gas (>99.99 % purity; air liquide) and the electrode was polarized at +0.10 v vs. sce for 5 min (for co adsorption to take place) and then scanned to more positive potentials at 50 mv s −1 (for co oxidative removal). finally, the electrode was scanned in a 0.5 meoh + 0.1 m hclo4 solution at 5 mv s -1 to study the methanol oxidation reaction. d. chemicals cu(no3)2·3h2ο from sigma-aldrich (acs reagent) was used in the precursor solutions for cu particle preparation. h2ptcl6 hexahydrate from sigma-aldrich (acs reagent, 37.50 % as pt) was employed for the pt exchange solution. meoh was from riedel (chromasolv  , for hplc, gradient grade, 99.9 %). hclo4 from riedel, (puriss p.a., acs reagent, 70 %) was added both in the galvanic replacement solution and as the supporting electrolyte in meoh oxidation experiments. results and discussion a. microscopic, spectroscopic and crystallographic characterisation of cu, pt(cu) and pt(cu)/c particles figure 1 shows the xrd diffractogram of the freshly prepared cu nanoparticles. it can be seen that these are made of metallic cu, as expected for a material annealed in a reductive atmosphere (see experimental). the average crystallite size could be estimated as 43 nm, using scherrer’s formula for the sharp 2 cu(111) = 43.4501 degrees peak. it should be noted that, despite prolonged ultrasonic treatment of cu particle suspensions in ethanol, these were heavily aggregated and no individual particles or even nano-sized aggregates could be identified by tem microscopy. figure 2 is a sem micrograph of the filter cake of as prepared pt(cu) material, prepared by the partial galvanic replacement of cu by pt. the as prepared material is particulate and consists of aggregates smaller than 1 μm. eds analysis gave a ptcuo atomic ratio of 22708, indicating a cu-rich material (with some cu oxides also formed during galvanic replacement or/and prolonged exposure to atmosphere). figure 3 presents the xrd difractogram of the pt(cu) catalyst and the wide peaks correspond to small crystallites, estimated to have an average size of ca 4 nm, using scherrer’s formula. the shrinkage of crystallites from 43 to 4 nm as one passes from cu to pt(cu) is in line with cu dissolving as pt deposits on its surface according to the galvanic replacement mechanism. the shift of the pt peaks to lower  values is indicative of alloy formation between pt and a metal with a smaller lattice constant such as cu. using vegard’s law, the atomic composition of the alloy is estimated as ptcu = 7327. comparing this composition with that of the eds results (2270) it follows that a large part of cu remains non-alloyed, most likely as very small pockets of amorphous material within the pt(cu) particles; the degree of alloying can thus estimated not to exceed 37% and expected to be restricted to the outer part of the particles. i. mintsouli at al. j. electrochem. sci. eng. 6(1) (2016) 17-28 doi:10.5599/jese.239 21 1 2 / ° 10 20 30 40 50 60 70 80 90 0 10 4 2.5x10 3 in te n si ty , s1 figure 1. xrd diffractogram( of cu particles prepared by hydrothermal treatment of cu(ch3coo)2 followed by high temperature reduction in a hydrogen atmosphere (see experimental) figure 2. sem micrographs of the filter cake of pt(cu) particles, prepared by hydrothermal treatment of cu(ch3coo)2 followed by high temperature reduction in a hydrogen atmosphere (see experimental) figure 4 shows tem micrographs of a pt(cu) large aggregate (a) and of detached smaller aggregates of the material (b), following dispersion of the pt(cu) filtrate of figure 2 in ethanol (with magnetic stirring). the smaller particles are spherical in shape with dimensions in the 20-30 nm range and they seem to consist of even smaller particles; the atomic composition of each individual particle is found by eds to be ptcu=3070, in line with what was found for the material as a whole (see above). j. electrochem. sci. eng. 6(1) (2016) 17-28 methanol oxidation at platinized copper 22 2 / ° 10 20 30 40 50 60 70 80 90 0 10 4 2.5x10 3 in te n si ty , s1 figure 3. xrd diffractogram of pt(cu) particles prepared by galvanic replacement of cu particles’ surface upon immersion of the latter in a solution containing pt in ionic form (see experimental) figure 4. tem micrographs of pt(cu) particles prepared by galvanic replacement of cu particles’ surface upon immersion of the latter in a solution containing pt in ionic form (see experimental); (a) micrograph of a large aggregate, (b) micrograph of individual particles figure 5 shows tem micrographs of the pt(cu) particles supported on /mixed with c particles by means of ultrasonic treatment in ethanol. in figure 5(a) the spherical carbon support particles are seen, having a fairly even size distribution of ca. 30 nm. figure 5(b) reveals the scatter of small (less than 20 nm in diameter) pt(cu) particles (seen as dark spots) in the pt(cu)/c system, whereas figure 5(c) depicts in more detail an individual such particle. the observed morphology of individual particles is in line with the expected deposition of pt clusters on top of a cu rich core. figure 5(d) shows the pt nanoparticles of a commercial pt/c catalyst made of very small nanoparticles (2-3 nm) with limited aggregation phenomena. i. mintsouli at al. j. electrochem. sci. eng. 6(1) (2016) 17-28 doi:10.5599/jese.239 23 50 nm (d) figure 5. tem micrographs of pt(cu) particles supported on carbon particles (pt(cu)/c) (see experimental); (a), (b), (c) micrographs at different magnifications as indicated by the scale bars. for comparison, a micrograph for commercial 20% w/w pt/c catalyst (etek) is shown in (d) b. electrochemical characterisation of pt(cu)/c in acid figure 6 presents the stabilized picture (see experimental) of cyclic voltammograms of the pt(cu)/c and pt/c electrodes, obtained at 50 mvs -1 in acid. both electrodes show the typical hydrogen adsorption/desorption and oxide formation/stripping features of high surface areacarbon-supported pt. this is macroscopic evidence that the outer surface of the pt(cu) catalyst is made of a pure, protective pt shell. figure 6. cyclic voltammograms (50 mv s -1 potential scan rate) of pt(cu)/c and pt/c electrodes in deaerated 0.1 m hclo4 j. electrochem. sci. eng. 6(1) (2016) 17-28 methanol oxidation at platinized copper 24 two further points can be made: first, the hydrogen peak area (between +0.1 and -0.3 v) is smaller for the pt(cu) electrode, as expected for a low specific area due to larger particles and aggregation phenomena (figure 5); second, the ill-defined hydrogen peak area (typical of carbonsupported pt nanoparticles) does not allow for a reliable estimate of the catalyst electroactive area (from the charge corresponding to the area under the cathodic or anodic part of the curve in that range, associated with adsorption and desorption of a h monolayer). in order to obtain a more accurate estimate of the catalyst electroactive area we therefore resort to the oxidative removal of a pre-adsorbed co monolayer during the positive-going potential scan of the voltammograms shown in figure 7 (see also experimental). the anodic charge between ca +0.4 and +0.7 v corresponds to the oxidation of a full co monolayer and is associated with a charge density of 420 μc cm -2 of pt [53]. the pt electroactive area (per electrode substrate geometric area) of the catalyst layers can thus been calculated as 5.19 cm 2 cm -2 for pt(cu)/c and 77.92 cm 2 cm -2 for pt/c. the mass specific electroactive areas in the prepared coatings can then be estimated (based on the pt loadings-see experimental) as 2.6 and 18.1 m 2 g -1 respectively. (such low values can be interpreted by extensive agglomeration taking place in coatings with relatively large catalyst loadings; note that ca 1mg of catalyst and support were loaded over a 0.385 cm 2 surface, resulting in a ca 2.5 mg cm2 of catalyst+support total loading). figure 7. cyclic voltammograms (50 mv s -1 potential scan rate) of pt(cu)/c and pt/c electrodes in a deaerated 0.1 m hclo4, following co pre-adsorption from a co-saturated solution the most interesting feature of the voltammograms of figure 7 is the ca 150 mv negative shift of the co oxidation peak in the case of the pt(cu) catalyst, proving its better catalytic activity for co oxidation. note that this is higher than the shift obtained at other pt(cu) systems prepared by galvanic replacement where c was present during the replacement step (and thus some of pt was deposited directly on c and remained without strong pt-cu interactions) [29,30]. figure 8 presents the positive-going voltammogram corresponding to methanol oxidation, with the current density, je, reported per pt electroactive surface area as obtained from the results of figure 7. it can be seen that the intrinsic activity (corrected for mere surface area effects) of the pt(cu) catalyst is superior to that of plain pt catalysts, highlighting the beneficial effect of pt-cu i. mintsouli at al. j. electrochem. sci. eng. 6(1) (2016) 17-28 doi:10.5599/jese.239 25 interactions on methanol oxidation, presumably because of its ability to remove the co poisonous intermediate, in line with the findings of the oxidative co removal discussed above. the current density obtained is the highest among similar catalysts [29,30]. figure 8. voltammograms (5 mv s -1 potential scan rate; positive going scan) of pt(cu)/c and pt/c electrodes in 0.5 m meoh+ 0.1 m hclo4 solutions; current densities, je, are per electroactive surface area in figure 9 the positive-going voltammograms for methanol oxidation are shown again, where the current density, jm, is now reported per mass of pt. in this case, the commercial catalyst appears to be better since the effect of very low surface area of pt(cu) (see large particles and agglomerates in figure 5) overrides its higher intrinsic catalytic activity. figure 9. voltammograms (5 mv s -1 potential scan rate; positive going scan) of pt(cu)/c and pt/c electrodes in 0.5 m meoh+0.1 m hclo4 solutions; current densities, jm, are per mass of pt conclusions i. bimetallic pt-cu catalysts have been prepared by partial galvanic replacement of cu nanoparticles synthesized by a precipitation/reduction method. the catalyst was then supported on a high surface area c powder. j. electrochem. sci. eng. 6(1) (2016) 17-28 methanol oxidation at platinized copper 26 ii. following electrochemical treatment in acid the voltammetric picture of the pt(cu)/c electrode was similar to that of a pure pt/c one, indicating the formation of a protective outer pt shell over a cu-containing core (pt(cu)). iii. the pt(cu)/c electrodes have superior intrinsic catalytic activity towards co oxidation and methanol oxidation in acid. iv. due to the formation of large pt(cu) particles and extensive agglomeration, the bimetallic catalyst exhibited a low electroactive surface area and hence a moderate mass specific activity. v. future research should be directed towards the preparation of smaller pt(cu) bimetallic nanoparticles (possibly by the synthesis of smaller cu particle precursors) so that not only the intrinsic catalytic activity is improved (due to pt-cu interactions) but also the overall, mass specific activity increases significantly (due to a higher surface area). acknowledgements: this research has been co-financed by the european union (european social fund – esf) and greek national funds through the operational program "education and lifelong learning" of the national strategic reference framework (nsrf) research funding program: heracleitus ii. investing in knowledge society through the european 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e. duarte, int. j. hydrogen energy 39 (16) (2014) 8667 [52] y. -x. wang, h. -j. zhou, p. -c. sun, t. -h. chen, j. power sources 245 (2014) 663 [53] r. w. lindström, k. kortsdottir, m. wesselmark, a. oyarce, c. lagergren, g. lindbergh, j. electrochem. soc. 157 (12) (2010) b1795 © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {electrochemical studies of lateral flow assay test results for procalcitonin detection:} http://dx.doi.org/10.5599/jese.1127 265 j. electrochem. sci. eng. 12(2) (2022) 265-274; http://dx.doi.org/10.5599/jese.1127 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical studies of lateral flow assay test results for procalcitonin detection yachana gupta, kalpana, aditya sharma ghrera applied science department, the northcap university, gurugram, india corresponding author:  adityasghrera@gmail.com received: september 30, 2021; accepted: november 22, 2021; published: november 28, 2021 abstract in this study, the lateral flow assay (lfa) has been developed for the detection of bacterial infection (bi) by specific biomarker procalcitonin (pct), without a need for complicated instrumentations and technical expertise. for the development of the assay, gold nanoparticles (aunp) and their conjugates with antibodies specific to the model antigen pct are assessed. polyclonal antibody (pab) labelled with gold nanoparticles (aunp) to obtain the aunp-pab complex and the specific monoclonal antibody (mab) have been dropped at the test zone. this complex is placed over the conjugate line of the lfa strip. in the absence of pct or the presence of other biomarkers, the test line remained colourless, which revealed the specificity of assay towards pct among a pool of various analytes. herein, observations have been made through two different platforms for quantitative and qualitative analysis for the detection of pct biomarker. the qualitative analysis has been performed on the basis of appearance red color in the test band, while for quantitative analysis, a novel approach has been adopted. herein, the nitrocellulose membrane (paper strip) is cut out from the lfa strip and used for electrochemical studies under similar solution conditions. different paper strips presented different cyclic voltammograms (cv) that could be correlated to varying pct concentrations captured at the test line of the paper strip. the qualitative detection limit for pct using this lfa was determined to be 2 ng ml-1 and the quantitative detection limit was 1 ng ml-1. the electrochemical response studies of the paper strip by cv technique revealed the sensitivity value of 0.695 a ml ng-1. keywords cyclic voltammetry, lateral flow assay; gold nanoparticles, polyclonal antibody, monoclonal antibody; biomarker introduction globally, bacterial infections (bi) are one of the primary reasons for illness and death, and they are becoming an increasingly serious problem due to the regular growth of bacteria. according to who report, antibiotic-resistant bacteria pose a risk to public health. therefore, for bi's diagnosis and http://dx.doi.org/10.5599/jese.1127 http://dx.doi.org/10.5599/jese.1127 http://www.jese-online.org/ mailto:adityasghrera@gmail.com j. electrochem. sci. eng. 12(2) (2022) 265-274 lateral flow assay test for procalcitonin detection 266 immediate response, some on-site detection technique is required [1]. for the clinical utility of bi specific biomarkers, numerous immunoassays like time-resolved fluoro-immunoassay (trfia) [2,3], chemiluminescence immunoassays (clia) [4], immunochromatographic tests (ict), and enzyme-linked immunosorbent assays (elisa) [5] have been presented in a wide variety of research applications for diagnosis purposes. nevertheless, the lateral flow immunoassay (lfa) remains an ideal technique for the point of care (poc) testing of bi and any other pathological changes inside the human body [6]. lfa technique allows even unskilled users to perform difficult tests at the point of need in a cost-effective manner and without the need for any additional equipment, which makes it popular among users. as a labeling agent, gold nanoparticles (aunp) [7] remain of interest to date for lfa development, owing to their enriched intensity, optical stability, biocompatibility, and easy surface modification properties [8,9]. in several already reported studies [10,11], the lfa technique was used to detect bi-specific biomarker procalcitonin (pct) by aunp as a label. for the diagnosis of bi or sepsis, pct acts as a forerunner of the hormone calcitonin and can differentiate between bacterial and viral infections [10,12]. in serum, the value of pct is < 0.1 ng ml-1, which rises with the severity of the infection. during bi in healthy adults, pct is < 0.05 μg ml-1, if the systemic infection is unlikely. in the case of localized infection, pct increases from 0.05 to <0.5 μg ml-1, while in the conditions like major trauma, recent surgery, or severe cardiogenic shock, pct rises further from 0.5 to <2 μg ml-1 [13-15]. some reported works have developed lfa strips using spherical and popcorn-like aunp to detect varying pct concentrations on multiple test lines [16-18]. in the present work, procalcitonin (pct) has been selected for bi diagnosis as a model antigen taken from the literature [19]. regarding the new approach to pct detection, the results of the test line of lfa strips were examined by qualitative and quantitative determinations. test line qualitative results were obtained with the lfa technique followed by the sandwich assay. on the other hand, the portion of the nitrocellulose (nc) membrane, where the antigen-antibody complex is already formed in the test area of the lfa test strips, was subsequently used to confirm the electrochemical quantification. the effectiveness of electrochemical quantification was dependent on the presence of electron transfer agents at the electrode surface [20]. electrochemical studies were carried out for the reliability of the test results and signal intensity [21-23]. signal detection of redox species curving in the lfa-modified strip indicates the possibility of developing a onestep analysis format with electrochemical quantification. in numerous published papers, it has been shown that as the concentration of analyte increases, the redox peak current of an electron transfer agent decreases due to the slowing of electron transmission between the paper strip and electrolytic solution. these results confirmed the accuracy of the currently presented cyclic voltammetry (cv) studies. from observing cv curves, it was found that the pattern of oxidation currents reduces with increasing concentration and proved that the responses are concentration-dependent [24-27]. here, the lfa strip test results are validated by the quantitative and qualitative determination of pct. the purpose of this study is to detect pct on a paper strip using lfa and electrochemical cv technique. this new paper strip is a convenient method for quantifying pct with high sensitivity and low detection limit. this study showed that electrochemical detection is independent of aunp-antibody conjugation and works normally when colorimetric detection suggests a false positive test, whereas lfa can respond to aunpantibody conjugation and antigen-antibody interaction. experimental reagents and instruments chloroauric acid, tri-sodium citrate dihydrate, bovine albumin serum (bsa), disodium hydrogen phosphate, and monobasic sodium phosphate were purchased from cdh chemicals. sodium y. gupta et al. j. electrochem. sci. eng. 12(2) (2022) 265-274 http://dx.doi.org/10.5599/jese.1127 267 borohydride and tween-20 were purchased from thomas baker (chemicals), sucrose, tris (hydroxymethyl) methylamine, sodium chloride and sodium azide from fisher scientific, while mab and pab were purchased from mybiosource.com. glass fiber sample pad (24×260 mm), polyester conjugation pad (70×260 mm), nc membrane (50×260 mm) and absorption pad (21×260 mm) were purchased from advanced micro devices (mdi) membrane. the electrochemical characterization has been conducted on an autolab pgstat204 potentiostat/galvanostat (eco chemie, the netherlands) using a three-electrode system with the paper strip as working electrode, ag/agcl as reference electrode and platinum foil as a counter electrode, in-0.1 m kcl containing 10 mm [fe(cn)6 ]3−/4−. preparation of aunp-antibody conjugate aunp were synthesized by the citrate method. for the synthesis, 50 ml of the aqueous solution of (2.5 mm) hydrogen tetrachloroaurate(iii) trihydrate (haucl4· xh2o) was prepared and boiled at about 65 °c, and after that 1ml of 5 % aqueous solution of trisodium citrate (tsc; 1 ml) [28-31] was mixed dropwise. the solution was stirred continuously for 30 minutes, resulting in a color variation from pale yellow/colorless to purple and finally to ruby red (figure 1). for the conjugation of aunp with pab, a purified concentration of 0.2 mg ml-1 pab was diluted in pbs buffer (100 mm, ph 7.4). potassium carbonate is used to maintain ph to 6.7 with a solution of colloidal gold and diluted antibodies. the diluted solution was incubated for 30 minutes at 25 °c followed by the addition of 0.25 % blocking agent bsa and then continued to stir for twenty minutes. subsequently defining the sanctified concentration of binding pab, the aunp-pab was prepared. for the prediction of conjugation of aunp-pab, the absorption peak was monitored at about 523-526 nm by ultravioletvisible (uv-vis) spectroscopy. about 15-20 minutes, the entire solution was centrifuged at 8000 rpm. after centrifugation, the unconjugated antibody supernatant was gradually removed. pallets were resuspended in buffers 10 mmol pbs, 0.5 % bsa, 5 % sucrose, and 0.1 % freshly prepared solution of sodium azide and stored at 4 °c for future use. a b c d figure 1. the appearance of the color of aunp: (a) 25 mm of haucl4·xh2o solution; (b ),(c) after adding tsc and continued to stir; (d) ruby red color of prepared aunp assembling of lfa strip components the complete assembly parameters of the lfa test are described in figure 2. the nc membrane has an adhesive plastic backing plate. the conjugate pad (1.40.5 cm2) was pasted in a position overlapping the nc (2.40.5 cm2) membrane by 5 mm. an absorbent pad (2.40.5 cm2) was overlapped on the other side of the membrane by 5 mm. the sample pad (1.30.5 cm2) was further http://dx.doi.org/10.5599/jese.1127 j. electrochem. sci. eng. 12(2) (2022) 265-274 lateral flow assay test for procalcitonin detection 268 pasted over the conjugate pad with overlapping of 5 mm length. to confirm the proper placement and operation of the components, the liquid sample was dripped from the sample pad to the absorbent pad. for the preparation of the test line, 0.2 mg ml-1 mab antibody containing pbs was dropped by a micropipette on the nc membrane and dried for 24 h at room temperature. once fully assembled, lfa strips were stored in a dry place. figure 2. schematic illustration of lfa test strip pre-treatment of pads the glass fiber sample pad strips were pretreated with an aqueous solution containing 1 % sucrose, 1 % bsa, 0.05 % sodium azide, 20 mol l-1 sodium borate, and 0.05 % tween-20 for 60 min. the strips were dried at 45 °c and stored in a dry state for further use. the polyester fiber conjugation pad was immersed in a solution containing 5 % sucrose and 0.05 % sodium azide in water for 90 minutes and then dried for 120 minutes at 50 °c. the conjugation pad was dipped in 5 ml of preprepared conjugate (pab-aunp) solution for 24 h. the conjugation pad was then dried at 37 °c and stored for further use. the nc membrane was pretreated by 1 % tween-20 and 1 % bsa for 1 h and dried at 38 °c for 60 minutes and stored in the dry state for further use. results and discussion in this study, we discussed the qualitative and quantitative determination of the bi-specific biomarker pct because pct is promising and very sensitive to increased bi levels and is often reported to be more significant than other biomarkers. pct ranges from 2 to 10 ng ml-1 in sepsis, whereas pct above 10 ng ml-1 indicates a state of septic shock. however, pct concentration of 2 to 0.15 ng/ml indicates uncertainty in the presence of bi. aunp is used as a labelling agent and pctspecific mab was dropped at the test line by a micropipette. when the complex (aunp-pab/pct) moves through the nc membrane and reaches the test line containing mab, a sandwich complex (aunp-pab /mab/pct) is formed. the formation of this complex can be visualized by appearing a red-colored band on the test line due to the presence of aunp in the sandwich complex. the strength of the red color band indicates pct concentration which has further been quantified using colorimetric and electrochemical techniques. characterization of aunp and pab-aunp conjugate uv-vis spectroscopy, transmission electron microscopy (tem), and scanning electron microscopy (sem) were used for the spectroscopic and morphological characterization of aunp and its conjugates. uv-vis spectroscopy can be used to determine the stability of nanoparticle solution over time. when y. gupta et al. j. electrochem. sci. eng. 12(2) (2022) 265-274 http://dx.doi.org/10.5599/jese.1127 269 aunp are exposed, they appear in a red solution, but when the particles are conjugated, the solution color turns blue/purple and may turn into a clear solution with a black precipitate. figure 3 shows the uv-visible absorption spectra of aunp and its conjugate with pab. in this graph, the red color curve shows bare aunp, and a black colour curve is indicates for aunp-pab conjugation. bare aunp peak was recorded at 524 nm, which approves the existence of sphere-shaped aunp with remarkable colloidal constancy and consistency in size and composition. the peak of aunp-pab was recorded at 530 nm, which explains that aunp-pab conjugation is taking place. here, the shift of the peak to the right indicates a longer wavelength with increasing aunp [32]. wavelength, nm figure 3. uv-vis absorption spectra of aunp and aunp-pab for tem studies, carbon-coated copper grids were used. the prepared aunp solution (10 μl) was dropped on-grid and dried for 24 h. experimental data was collected at 200.0 kv and 50000× magnification. the tem images reveal that synthesized aunps are well separated and spherical (figure 4a), indicating their stable dispersion in the solution condition. furthermore, the synthesized particles are monodispersed, having a size range from 15.4 -25.6 nm (figure 4b). a b figure 4. tem images of aunp for the preparation of sem samples, the drop-casting method was applied. by this method, polyvinyl alcohol and aunp (pva-aunp) film were prepared. in this method, the ito plate surface (1.0×1.0 cm2) a b so rb a n ce , a .u . http://dx.doi.org/10.5599/jese.1127 j. electrochem. sci. eng. 12(2) (2022) 265-274 lateral flow assay test for procalcitonin detection 270 was ultrasonically cleaned with an aqueous solution of ammonia and ethyl alcohol. the mixed solution (1ml of colloid gold and 5 % pva) was dropped on the bare surface of ito. then the strips were dried at 140 °c for 1.5 h to make pva-aunp film [33]. after that, the annealing process of pva-aunp film was done and kept about 4-5 h at different temperatures (400-600 °c). eventually, the strengthened film was cooled at room temperature in the air. the sem image of the sample has been demonstrated in figure 5. experimental data was collected at 20.00 kv and x 1000 magnification. figure 5. sem image of aunp deposited on ito glass after annealing at 400 °c optimization of standard sample assay for the lfa reaction, a sandwich analysis was performed to detect the pct signal. for this purpose, standard solutions of pct with concentrations of 1, 2, 10, 20, 30, 40, 50, and 60 ng ml-1 were prepared from the stock solution of 0.1 mg ml-1 solution in pbs solution (ph 7.4). the sample pad and conjugate pad were pretreated with buffers and used to develop antigen-antibody interaction. the aunp-pab complex was applied to the conjugation pad. samples containing pct antigen with different standard concentrations were dropped to the sample pad to initiate the reaction. the sample antigen interacts with the pab-aunp complex and forms a labeled antigen-antibody complex as it starts flowing through the sample pad. this complex moves across a porous membrane where specific mab is already present in the test zone, forming a sandwich complex. the test line was drawn on the nc membrane 2.1 cm away from the conjugated pad. when pct (100 μl) was dropped at sample pad at concentrations of 10 to 60 ng ml-1, a strong red band appeared at the test line (figure 6d-i). a b c d e f g h i figure 6. illustration of lfa strips test results of different concentrations of pct: (a)  1 ng ml-1 and (b) 1 ng ml-1 are showing negative test results; (c) 2 ng ml-1 is showing weak positive bands; (d-i) 10 to 60 ng ml-1 are showing strong positive test results y. gupta et al. j. electrochem. sci. eng. 12(2) (2022) 265-274 http://dx.doi.org/10.5599/jese.1127 271 at the concentration of 2 ng ml-1, a weak red band appeared at the test line (figure 6c), and thus, this concentration has been considered as the qualitative limit of detection of the test assay. however, at 1 ng ml-1 (figure 6b) and lower concentrations (figure 6a) no bands appeared at the test line, and thus, 1 ng ml-1 has been considered as a negative test result. electrochemical characterization of lfa strip and response studies cyclic voltammetry (cv) studies were conducted for electrochemical detection of a range of pct concentrations using an autolab potentiostat/galvanostat instrument with a three-electrode setup. platinum wire was used as the counter electrode, ag/agcl as the reference electrode, and the portion of nc membrane cut from the modified lfa test strip (pab-aunp/mab/pct) was used as the working electrode (paper strip). for electrochemical analysis, 0.1 m kcl containing 10 mm [fe(cn)6 ]3−/4− solution was used as a redox probe [19,35]. in the present work, cvs were performed with the paper strip as a working electrode which exhibits low conductivity. thus, to enhance electron transmission between the working electrode and electrolyte and to record oxidation/reduction currents in cv curves, the redox probe [fe(cn)6 ]3−/4− was added as a marker to investigate changes in electrode behaviour after loading different concentrations of pct on the paper strip [34,36,37]. to obtain voltammetric signals, portions of the nc (2.4×0.5 cm2) membrane of the strips which have an antigen-antibody complex, were cut (2 cm) and immersed in a redox probe solution (0.1 m kcl containing 10 mm [fe(cn)6 ]3−/4−. cvs were performed between -0.4 and 0.8 v at the scan rate of 50 mv s-1. figure 7a shows decreasing oxidation/reduction currents in recorded cvs for every increase in the concentration of pct analyte on the paper strip. in an electrochemical investigation through cv, redox currents produced as a result of electron transfer from an electrolyte to a working electrode or vice versa were monitored. the intensity of produced redox currents depends upon the electroactive species present in the electrolytic solution. with the help of this mechanism, redox currents are observed to identify the pct analyte on the paper strip. oxidation and reduction current peaks are not visible in recorded cvs in our work as the paper strip is a poor conductor and does not facilitate charge transfer. thus, a characteristic cyclic voltammogram is not obtained even after the application of redox agents. nevertheless, the recorded cvs of the paper strips in the presence of redox agents in the solution are in conformity with work published by various research groups [38,39]. thus, to study variation in oxidation current of the strip with respect to varying pct concentrations, the oxidation cv signal has been recorded at the fixed potential (0.3v). when the concentration of 1 ng ml-1 pct paper strip was investigated by cv, the oxidation current was 60.03 μa, and when the concentration of the analyte was increased to 2 ng ml-1, a decrease of oxidation current to 53.29 μa was observed. on further increasing of the analyte concentration from 10, 20, 30, 40, 50 and 60 ng ml-1, a continuous decrease of oxidation current was observed as 46.96, 37.21, 29.52, 23.49, 21.13 and 17.46 μa, respectively. this can be attributed to higher loading of pct on the paper strip surface with increasing concentration, which hinders electron transmission between the redox probe and the electrode, and so, the current density is decreased. using cv current responses, the calibration curve is obtained and presented in figure 7b as a function of measured oxidation current at 0.3 v and pct concentration on the paper strip in the range of 1 to 60 ng ml-1. linear proportionality of this plot indicates that magnitude of current changes linearly with the change in analyte concentration, while the negative slope of this plot indicates that oxidation current decreases with the increase of analyte on the paper strip. the linear proportionality of the current value and pct concentration in the range from 1 to 60 ng ml-1 corresponds to the equation (1), showing the regression coefficient value of 0.97 and sensitivity value of 0.695 a ml ng-1. http://dx.doi.org/10.5599/jese.1127 j. electrochem. sci. eng. 12(2) (2022) 265-274 lateral flow assay test for procalcitonin detection 272 i = 0.695 [cpct] + 54.646 (1) a b figure 7. (a) cv recorded for modified lfa strip as a function of pct concentrations (1 60 ng ml-1) in 0.1 m kcl containing 10 mm [fe(cn)6 ]3−/4−; (b) calibration plot of oxidation current at 0.3 v as a function of pct concentration conclusions the present paper successfully developed a portable and inexpensive detection test kit for the quantitative and qualitative detection of pct analyte through lfa and electrochemical cv techniques. here, aunp performed excellent specificity and color appearance at the test line. the color intensity on the test zone was directly proportional to the concentration of the pct analyte. lod of the lfa technique of 2 ng ml-1 was found qualitatively and 1 ng ml-1 quantitatively. by electrochemical cv response studies, a linear relation between oxidation current and pct in the concentration range of 1 to 60 ng ml-1 was obtained, with the sensitivity of the modified lfa strip of 0.695 a ml ng-1. this approach proves that for electrochemical detection of pct, there 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1038/s41598-018-22976-5 http://dx.doi.org/10.1088/2043-6262/3/4/045017 http://dx.doi.org/10.1088/2043-6262/3/4/045017 https://doi.org/10.1080/17458080.2018.1520399 https://doi.org/10.1021/acsanm.9b01057 https://doi.org/10.1016/j.tsf.2010.08.071 https://doi.org/10.1038/micronano.2015.14 https://doi.org/10.1038/micronano.2015.14 https://doi.org/10.1088/1742-6596/704/1/012010 https://www.ncbi.nlm.nih.gov/pubmed/?term=shriver-lake%20lc%5bauthor%5d&cauthor=true&cauthor_uid=29364153 https://www.ncbi.nlm.nih.gov/pubmed/?term=zabetakis%20d%5bauthor%5d&cauthor=true&cauthor_uid=29364153 https://www.ncbi.nlm.nih.gov/pubmed/?term=dressick%20wj%5bauthor%5d&cauthor=true&cauthor_uid=29364153 https://www.ncbi.nlm.nih.gov/pubmed/?term=stenger%20da%5bauthor%5d&cauthor=true&cauthor_uid=29364153 https://dx.doi.org/10.3390%2fs18020328 https://doi.org/10.1016/j.snb.2016.08.087 https://creativecommons.org/licenses/by/4.0/) studying the electrochemical deposition process of molybdenum from aqueous solution of molybdate ions doi:10.5599/jese.219 231 j. electrochem. sci. eng. 5(4) (2015) 231-235; doi: 10.5599/jese.219 open access : : issn 1847-9286 www.jese-online.org short communication studying the electrochemical deposition process of molybdenum from aqueous solution of molybdate ions samira fikret cafarova, akif shikhan aliyev, mahmoud elrouby* , , nsh soltanova, dilgam babir tagiyev institute of catalysis and inorganic chemistry, national academy of sciences of azerbaijan, az 1143 baku, azerbaijan *chemistry department, faculty of science, sohag university, 82524 shag, egypt corresponding author: dr_mahmoudelrouby@hotmail.com; tel.: +2-011-43909358; fax: +2-093-4601159 received: september 14, 2015; revised: december 13, 2015; accepted: december 17, 2015 abstract in this study, the tracing of the electroreduction process of molybdate ions in aqueous media at different conditions is achieved for obtaining molybdenum metal in a simple and easy way. the kinetics and the mechanism of the electroreduction of molybdate ions are studied using cathodic polarization technique. it is observed that, the speed of the electroreduction process depends on the speed of the change of the cathodic potential sweep as well as on the temperature of the electrodeposition bath. moreover, it is observed that, at the potential range from the stationary potential (est = 0.25 v) to −0.7 v, the electroreduction of molybdate ions occurs into two steps. moreover it is observed that, after −0.7 v the electroreduction process of molybdate is accompanying with evolution of hydrogen. keywords molybdate in aqueous media; electrochemical behavior; platinum substrate; cathodic polarization introduction molybdenum and its compounds are used in a wide range of different fields of industry. molybdenum sulfide occupies a good and important position among the molybdenum compounds. mos2 has a good crystalline structure and other physical properties such as high greasy character in high temperature and oxidation environment, stability, semi-conductivity (eg=1.7 ev) and diamagnetic properties, these features make it a significantly candidate in many industrial applications and scientific research works, such as “oily substances” and effective photoelectrodes [1]. mos2 in its crystalline structure is observed “in sliding” shapes [2], this makes it http://www.jese-online.org/ mailto:dr_mahmoudelrouby@hotmail.com j. electrochem. sci. eng. 5(4) (2015) 231-235 mo deposition from aqueous solution of molybdate ions 232 suitable for the using as a good lubricant material. also, mose2 has been found to be highly stable against photo-corrosion due to the d-d transitions and has a band gap of about 1.4 ev. thus, it possesses a high potential towards photoelectric applications [3]. the mos2 can be synthesized by various methods depending on the application area. therefore, to get mos2 thin layer it is preferred to use the electrochemical method [1]. in this method by using inexpensive equipments one can get thin layers with bigger surface area and the easiness to manage the synthesizing process. the synthesis of mos2 thin layers as photoelectrode material using the electrochemical method in different solutions were investigated [4-5]. it should be noted that, the kinetics and mechanism of this process, the structure of the film, optical properties were not thoroughly investigated. for investigation the process of electrochemical synthesis of mos2 thin film from aqueous solutions, at first it should be clarified kinetics and mechanism of the deposition process of the combination of the two components (mo and s) from aqueous solutions on the cathode. the kinetics of the mechanism of the electrodeposition process of sulphid from sulphite and thiosulphate were thoroughly investigated [6-8]. the electroreduction process of molybdenum ions to molybdenum on the cathode from different solutions were investigated by a number of researchers and the obtained results are different depending on the composition of the solution and the terms of the electrolysis process [9-12]. there is no sufficient information in the previous works about the kinetics and the mechanism of the electroreduction and the electrodeposition processes of the molybdate ions to molybdenum metal. in this regard, in this article the kinetics and the mechanism of the electrochemical reduction process of sodium molybdate from aqueous solution has been studied and investigated. experimental all of the electrochemical experiments were carried out in a three electrodes electrochemical cell. this electrochemical cell allows the adding sample from the electrolyte, adjusting the desired temperature of the electrolyte and passing a variety of gases through the electrolyte. for adjusting the temperature in the cell and for mixing the solutions, the utu-4 universal “ultra thermostat” and alpha magnetic mixer were used. silver/silver chloride (ag/agcl/kcl) and platinum plaques were used as reference and auxiliary electrodes, respectively. before starting the experiment, the working electrode should be washed in a solution containing 1:1 h2so4 and h2o2, then with the deionized water. cathodic polarization curves were recorded with "iviumstat electrochemical interface" potentiostat equipped with a computer. pure and grade chemicals for analysis were used, as brand sodium molybdate. the deionized water was used for preparing the solutions and electrolytes without further purification. results and discussion the kinetics and the mechanism of the electroreduction of molybdate ions were studied and traced using the cathodic polarization method. figure 1 shows the cathodic polarization curves of 0.25 m na2moo4 on platinum electrode at different speed rates of potential; 1-5, 2-10, 3-20, 4-40, 5-60 and 6-80 mv/s at t = 295 k. as can be seen from figure 1 the changing and the increasing of the potential sweep rate from 5 mv/s to 80 mv/s in the direction of the cathodic process leads to an increase of the cathodic current peak height. moreover, at −0.072 v the cathodic process increases about 2.93 times, at −0.7 v 1.91 times and at −0.8 v 1.72 times. s. f. cafarova et al. j. electrochem. sci. eng. 5(4) (2015) 231-235 doi:10.5599/jese.219 233 figure 1. cathodic polarization curves of 0.25 m na2moo4 on platinum electrode at different speed rates of potential: 1 5, 2 10, 3 20, 4 40, 5 – 60 and 6 80 mv/sec at t=295 k the obtained data in figure 2 shows the dependence of the peak current density, ip, on the square root of the speed potential change. this dependence allows assigning the effective nature of the rate determining step in the electro-precipitation process of the metal. as seems from figure 2 this dependence is linear and confirms that, the cathodic process is diffusion in nature at the peak of current density. figure 2.dependence of the peak current density ip on the square root of speed change of potential ν 1/2 . the composition of solution like as figure 1. for more information about the nature of cathodic polarization process, we need to investigate the reaction dependence on the temperature. thus, the dependence of the speed of electroreduction of molybdate ions on temperature has been performed. for detecting the nature of cathodic polarization process, the areas around the electroreduction peaks in polarization curves at each temperature should be taken into account [13]. investigation of the dependence of the electroreduction process of molybdate ions at the cathode on temperature illustrates that, the reduction process occurs rapidly while temperature increases. as shown from figure 3, by increasing the temperature from 295 to 343 k at the j. electrochem. sci. eng. 5(4) (2015) 231-235 mo deposition from aqueous solution of molybdate ions 234 potential of −0.2 v, the cathodic peak current (eletcroreduction process) increases 2.48 times and increases about 1.37 times at −0.62 v potential in the cathodic process. figure 3. cathodic polarization curves on platinum electrode of electrolyte coposition 0.25 m na2moo4. at different temperatures: 1 295; 2 303; 3 313; 4 323; 5 – 343 k the activation energy of the cathodic process was calculated based on the results given in figure 3. for this purpose the equation logik = const – aef / 2.303 rt is used [14]. where, ik is the current density, aef the effective activation energy, r gas constant and t temperature in kelvin. the activation energy was calculated from the coordination between logik and 1/t, then from the slope angle of straight lines aef can be determined. as shown from fig. 4, the dependence of the rate of the cathodic process at different potentials on the inverse of temperature is linear in character. this also confirms the diffusion control of the electroreduction process. figure 4. dependence of logarithmic values of the cathodic current density on temperature inverse, the electrolyte consists of −0.25 m na2moo4. at different selected values of potentials vs. ag/agcl/kclsat. 1 0.2; 2 0.4; 3 0.5; 4 0.6; 5 0.7 and 6 0.8 v the values of the activation energy calculated from figure 4 relatively decrease when cathodic potential increases, where at −0.2v, the aef = 37.28 kcal and at −0.7 v aef = 36.99 kcal at t = 295 k s. f. cafarova et al. j. electrochem. sci. eng. 5(4) (2015) 231-235 doi:10.5599/jese.219 235 but at t = 343 k, the activation energy decreased from 44.86 to 40.36 kcal. from these results, it can be concluded that the rate determining step of the cathodic process is mainly under diffusion control in character. the mechanism of the electroreduction route of molybdate can be deduced from the previous data as follows: moo4 2+2h2o+2e =moo2+4o at first peak moo2+2h2o+2e =mo 2+ +4oh at second peak mo 2+ + 2e = mo  after the second peak accompanying with hydrogen evolution. conclusions after the tracing of the electroreduction process of molybdate ions in aqueous media at different conditions, the kinetics and the mechanism of the electroreduction were detected using cathodic polarization. it is concluded that, the speed of electroreduction process of molybdate ions at the discussed conditions is affected the speed of the change of the cathodic potential sweep as well as the temperature of electrodeposition bath. moreover, the mechanism of the route of the electroreduction process of molybdate is detected and proposed. from the stationary potential (est = 0.25 v) to −0.7 v the reduction of molybdate ions occurs at two steps and after −0.7 v the electroreduction process of molybdate is accompanying with evolution of hydrogen. references [1] a. sh. aliyev, m. elrouby, s. f. cafarova, mater. sci. semicond. process. 32 (2015) 31-39. 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[in russian] © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ ruthenium redox equilibria: 1. thermodynamic stability of ru(iii) and ru(iv) hydroxides doi:10.5599/jese.226 123 j. electrochem. sci. eng. 6(1) (2016) 123-133; doi: 10.5599/jese.226 open access : : issn 1847-9286 www.jese-online.org original scientific paper ruthenium redox equilibria 1. thermodynamic stability of ru(iii) and ru(iv) hydroxides igor povar, oxana spinu institute of chemistry of the academy of sciences of moldova, 3 academiei str., md 2028, chisinau, moldova corresponding author: ipovar@yahoo.ca; tel.: +373 22 73 97 36; fax: +373 22 73 97 36 received: september 30, 2015; accepted: february 18, 2016 abstract on the basis of the selected thermodynamic data for ru(iii) and ru(iv) compounds in addition to original thermodynamic and graphical approach used in this paper, the thermodynamic stability areas of sparingly soluble hydroxides as well as the repartition of their soluble and insoluble chemical species towards the solution ph and initial concentrations of ruthenium in heterogeneous mixture solid phase–saturated solution have been investigated. by means of the δg–ph diagrams, the areas of thermodynamic stability of ru(iii) and ru(iv) hydroxides have been established for a number of analytical concentrations in heterogeneous mixtures. the diagrams of heterogeneous and homogeneous chemical equilibria have been used for graphical representation of complex equilibria in aqueous solutions containing ru(iii) and ru(iv). the obtained results, based on the thermodynamic analysis and graphic design of the calculated data in the form of the diagrams of heterogeneous chemical equilibria, are in good agreement with the available experimental data. keywords heterogeneous chemical equilibria; sparingly soluble hydroxides; soluble and insoluble ruthenium chemical species; thermodynamic analysis introduction chemistry of ruthenium is characterized by some specific properties, such as the existence of various valences from 0 to 8, the relatively easy formation of stable polynuclear compounds, occurrence of various disproportionation and comproportionation processes etc. widespread application of ruthenium and its compounds in various practical purposes serves as an impulsion for the development of the ruthenium applied analytical chemistry. currently, selective fast and http://www.jese-online.org/ mailto:ipovar@yahoo.ca j. electrochem. sci. eng. 6(1) (2016) 123-133 ruthenium redox equilibria: 1. thermodynamic stability 124 efficient methods for determination of ruthenium were developed [1-5]. the choice of the appropriate method requires knowledge of the forms of existence of ruthenium in solution and the composition of its complex compounds. depending on the solution ph, metal ion concentrations in solution, presence of oxidants and reductants as well as their concentration, ruthenium can exist in the form of several complexes, each exhibiting its own catalytic and voltammetric activities [6,7]. within research of the reduction – oxidation mechanism, it is necessary to know if some coexisting species are reduced simultaneously or, conversely, the process has a “stepwise” character and one species reduces gradually [8]. the knowledge of the forms of metal ion in solution facilitates the choice of optimal sorption and extraction conditions. therefore, the use of suitable analytical methods and procedures of ruthenium research involves a priori knowledge of its forms of existence for the interpretation of mechanisms of predominant reactions of certain species for selecting a suitable procedure. it is known that the assessment of solubility (s) based on the value of the solubility product can lead to large misinterpretation. this is explained by the fact that the precipitate (solid phase) components in solution are subjected to different reactions of hydrolysis, complex formation etc. generally, these secondary processes contribute to increasing the precipitate solubility [9-15]. a particularly strong influence on the precipitate solubility (s) exercises the solution ph value. determination of the dependence of s on ph requires extensive calculations, since this method involves solving systems of nonlinear equations for mass balance (mb). however, from the solubility diagrams one cannot draw conclusions on the thermodynamic stability of the solid phase when the precipitate solubility is relatively high. authors [9-15] proposed a strict criterion for assessing the solid phase stability, based on the gibbs energy change value of the precipitation dissolution process of solid phases. in general, gibbs energy variation calculations for these systems were examined in [9]. the paper presents a thermodynamic approach for the complex chemical equilibria investigation of two-phase systems containing ru(iii) and ru(iv) hydroxides. this approach utilizes thermodynamic relationships combined with original mass balance constraints, where the solid phases are explicitly expressed. the factors influencing the distribution and concentrations of various soluble ruthenium species were taken into account. the new type of diagrams, based on thermodynamics, graphical and computerized methods, which quantitatively describe the distribution of soluble and insoluble, monomeric and polymeric ruthenium species in a large range of ph values was used. the developed thermodynamic approach is based on: 1. analysis of thermodynamic stability areas for the solid phase; 2. determination of the molar fractions of chemical species in heterogeneous systems precipitate-solution based on the mb equations of the method of residual concentrations (mrc). theory and calculations thermodynamic stability areas of sparingly soluble ru(iii) and ru(iv) hydroxides in the present paper the relations, obtained in [10], will be applied to determine the thermodynamic stability of hydroxides ru(iii) and ru(iv) under real conditions, different from standard ones. we will expose the quintessence of this method on the example of the equilibrium of ru(iii) hydroxide with aqueous solution: i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 123-133 doi:10.5599/jese.226 125 + 3+ 3 2 2 s0 ru(oh) ×h o+3h =ru +4h o, lg 1.64k  (1) where ks is the equilibrium constant of reaction (1). the variation of the gibbs energy of reaction (1) under standard conditions is equal to 0 0 3+ 0 0 r f f 2 f 3 2 s(ru ) 4 (h o) (ru(oh) ×h o (s)) lng g g g rt k        (2) where 0 f ( )g i is standard gibbs energy of formation of species i. the selected values for 0 f ( )g i [16] are shown in table 1. table 1. the values of standard gibbs energy of formation of ruthenium chemical species at 298.15 k chemical species 0 f ( )g i / kj mol -1 chemical species 0 f ( )g i / kj mol -1 2 5 h ruo (aq) -391.2 4+ 4 12 hru (oh) (aq) -1877 5hruo (aq) -325.4 3+ ru (aq) 173.4 4ruo (aq) -250.1 2+ ru(oh) (aq) -51.0 24 ruo (aq) -306.6 + 2ru(oh) (aq) -280.9 2 2 ruo ×2h o (am) -691.0 8+ 4 4ru (oh) (aq) -193.5 3 2 ru(oh) ×h o (am) -766.0 2+ru (aq) 150.3 2 5 ru o (aq) -445 2 h o (aq) -237.19 2+ 2ru(oh) (am) -221.8 oh (aq) -157.33 but the 0rg value cannot serve as a characteristic of thermodynamic stability of ru(iii) hydroxide under real conditions. in this case, the equilibrium is described by the equation of isothermal reaction that for the scheme (1) takes the form: 0 + 3 r r ru (s) (s) ln( /[h ] )g g rt c    (3) where 0 ru c denotes the concentration of ruthenium in heterogeneous mixture. it is easy to see that the gibbs energy change of reaction (1) depends strongly on the ph value. at the same time the equation (3) can also be applied for the determination of thermodynamic stability of the ruthenium(iii) hydroxide as the ru 3+ ion, depending on the ph and the initial concentration of the ruthenium, 0 ru c , is subjected to complex chemical transformations in solution: 3+ 2+ + 2 10 3+ + + 2 2 20 3+ 8+ + 2 4 4 44 1. ru +h o=ru(oh) +h , lg 2.24 2. ru +2h o=ru(oh) +2h , lg 3.52 3. 4ru +4h o=ru (oh) +4h , lg 10.80 k k k       (4) (the hydrolysis constants were calculated from 0 r g values using the equation 0 ij ijlng rt k   ). obviously, the δg calculation requires accounting for all equilibria (4). a rigorous thermodynamic analysis shows that the change in gibbs energy of reaction (1) in real conditions is described by the equation [13]: 0 0 3 r r ru ru (s) ln ln( / [ ] )g g rt rt c h       . (5) here αi represents the hydrolysis coefficient of reactions (4) [13], defined as: j. electrochem. sci. eng. 6(1) (2016) 123-133 ruthenium redox equilibria: 1. thermodynamic stability 126 + -1 + -2 3+ + -3 ru 10 20 441 [h ] [h ] 4 [ru ][h ]k k k     (6) in relation (6) [ru 3+ ] is the equilibrium concentration of the ruthenium(iii) ion, which is calculated for the fixed values of ph and 0 ru c from the mb equation: 0 3+ 2+ + 8+ ru 2 4 4 3+ 3+ + -1 3+ + -2 3+ 4 + -4 3+ 10 20 44 ru =[ru ]+[r(oh) ]+[ru(oh) ]+4[ru (oh) ] [ru ]+ [ru ][h ] [ru [h ] 4 [ru ] [h ] =[ru ]× c k k k      (7) the changes in equation (7) are made by using the consequence of mass action law: 3+ i + -j i j ij[ru (oh) ] [ru ] [h ]k in the case of ruthenium(iv) hydroxide similar relationships are obtained. we present here only the basic equations: + 2+ + 2 2 2 2 s ruo ×2h o+2h =ru(oh) +2h o , lg 0.91k  (8) 0 0 + 2 r r ru ru (s) ln( /[h ] ) lng g rt c rt      (9) 0 r slng rt k   where αru is coefficient of hydrolysis reactions of 2+ 2 ru(oh) : 2+ 4+ + 2 2 4 12 44 4ru(oh) +4h o=ru (oh) +4h , lg 7.19k  calculated by the equation: 2+ 4 + -4 ru 44 2=1+4k [ru(oh) ] [h ] (10) the equilibrium concentration of 2+ 2 ru(oh) ion is determined for respective values of ph and 0 ru c from mb conditions: 0 2+ 4+ 2+ 2+ + -4 2+ ru 2 4 12 2 44 2 2=[ru(oh) +4[ru (oh) ]=[ru(oh) ](1+4k [ru(oh) [h ] =[ru(oh) ] ruc  within this method, when δgr < 0 the solid phase is thermodynamically unstable towards dissolution according the schemes (1) and (8) and, vice-versa, for the values δgr > 0 the formation of solid phase takes place. results of calculation of the δgr dependence on ph for 0 6 ru 10c   and 10 -4 mol l -1 for both the hydroxides are presented graphically in figs. 1 and 2. from these graphs it is observed that in acidic solutions ru(iii) and ru(iv) hydroxides are thermodynamically unstable in regard to dissolution. from the 0 ru r (ph) c g , diagrams the ph of beginning of precipitation (ph0) corresponds to the condition δgr = 0. in the case of ruthenium(iii) hydroxide, ru(oh)3∙h2o(s), one obtains for 0 ru c = 10 -4 mol l -1 , ph 2.24 and for 0 ru c = 10 -6 mol l -1 , ph 4.42 (table 2). therefore, by increasing 0 ru c in mixtures, the ph0 value shifts to region of acidic solutions. for ruthenium(iv) hydroxide, ruo2 ∙ h2o(s), from the δgr(ph) diagram one can see that ph0 is 3.93 for 0 ru c = 10 -4 mol l -1 , ph0 is 3.93 for 0 ru c = 10 -6 mol l -1 (see also table 2). i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 123-133 doi:10.5599/jese.226 127 table 2. areas of thermodynamic stability of solid phases, δgr > 0 of the ru(iii) and ru(iv) hydroxides in the system ru n+ -h2o the precipitate composition area of thermodynamic stability of solid phase 0 4 ru 10c   mol l -1 0 -6 ru =10c mol l -1 ru(oh)3 ∙ h2o (s) 2.42 < ph < 14.0 4.42 < ph < 14.0 ruo2 ∙ h2o (s) 3.93 < ph < 14.0 4.43 < ph < 14.0 figure 1. the dependence of the gibbs energy change on solution ph for the precipitationdissolution process of ru(iii) hydroxide. 0 ru c : 1 – 10-4 mol l-1; 2 10-6 mol l-1 figure 2. the curves δgr(s)(ph) of the dissolution-precipitation process of ru(iv) hydroxide, 0 ru c : 1 – 10 -4 mol l -1 ; 2 10 -6 mol l -1 j. electrochem. sci. eng. 6(1) (2016) 123-133 ruthenium redox equilibria: 1. thermodynamic stability 128 repartition of ru(iii) and ru(iv) soluble and insoluble chemical species towards the solution ph and initial concentrations of ruthenium in heterogeneous mixture solid phase–saturated solution in formulating mb conditions for the precipitate components in heterogeneous mixture we have taken into consideration the quantity of each component in the solid and liquid phases (residual quantity). knowing the residual concentration of i-th ion ( r i c ) in solution, its quantity in precipitate in a unit volume is easily calculated as the difference between the analytical concentration in the mixture ( 0 i c ) and that in solution. therefore, in terms of molar concentrations, 0 r i i i c c c   where δci denotes the quantity of i ion in precipitate (moles) in 1 l of solution. if this quantity is recalculated related to the mixture volume (vam), then 0 r i i i m m m   where 0 i i ,m m and r i m denote the quantity of ion (in moles) in precipitate, in mixture and in volume of liquid phase, respectively. for several systems it was established [17], that this approximation can be applied for solutions up to 0.5 mol l -1 . initially, a set of possible reactions [18] in the system ru(iv)–h2o is taken into account: + 2+ 2 2 2 2 2+ + -2 s 2 ruo ×h o (s) +2h =ru(oh) +2h o, =[ru(oh) ][h ]k (11) 2+ 4+ + 2 2 4 12 4+ + 4 2+ 4 4 ,4 4 12 2 4ru(oh) +4h o=ru (oh) +4h , [ru (oh) ][h ] /[ru(oh) ]k  (12) the mb equation for ru(iv) is written as: 0 r 2+ 4+ ru ru ru ru 2 4 12+[ru(oh) ]+4[ru (oh) ]c c c c     (13) taking into consideration equation (12), the expression for r ru c has the form: r 2+ 4+ 4 + -4 2+ ru 2 4,4 4 12 2=[ru(oh) ]+4 [ru (oh) ] [h ] =[ru(oh) ] ruc k  here αi is the coefficient of hydrolysis reactions for ruthenium: 2+ 4 + ru 4,4 2=1+4 [ru(oh) ] [h ]k from equation (11) it follows 2+ + 2 2[ru(oh) ] [h ]sk substituting this expression in (13), one gets: 0 + 2 4 + 4 ru ru s 4 ,4 s[h ] 4 [h ]c c k k k    (14) results and discussion from obtained relations, δcru for the set of values ( 0 ru c , [h + ] = 10 -ph ), can be easily determined. further, the concentrations of the chemical species in solution are calculated. then the molar fractions of chemical species in solution are computed. finally, the molar fractions of chemical species in heterogeneous mixture, in function of ph for constant values of 0 ru c are determined, using the equations: 0 2+ 0 4+ 0 s ru ru 10 2 ru 44 4 12 ru sum 10 44 / ; [ru(oh) ]/ ; 4[ru (oh) ]/ ;c c c c           (15) i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 123-133 doi:10.5599/jese.226 129 the subscript index (sum) symbolizes the sum of the all soluble species fractions containing ru(iv). it is easily to observe that sum s 1   . the molar fraction of the metal ion from precipitate is defined also as the degree of precipitation [12,13]. for heterogeneous equilibria, the molar fractions of chemical species depend on the mixture initial composition, in this case of 0 ru c , even in the absence of polynuclear complexes, thus 0 i ru( , ph).f c  consequently, the diagrams of repartition in heterogeneous systems (drhs) can be plotted in coordinates 0 ru i =co ( , ph) c nst  or 0 i ru ph=const( , )c . it should be mentioned that the relationships derived above is valid only within the thermodynamic stability area of the precipitate (∆gr < 0) [9]. analogically, drhs are calculated for the system ru(iii)–h2o, where the following equilibria are possible [11]: + 3+ 3 2 2 s ru(oh) ×h o (s) +3h ru +4h o, lg 1.64k  (16) 3+ 2+ + 2 10 ru +h o=ru(oh) +h , lg 2.24k   (17) 3+ + + 2 2 20 ru +2h o=ru(oh) +2h , lg 3.52k   (18) 3+ 8+ + 2 4 4 44 4ru +4h o=ru (oh) +4h , lg 10.80k   (19) here, the particular equilibrium constants represent: 0 3+ + -3 s =[ru ][h ]k (20) 2+ + 3+ 10 =[ru(oh) ][h ]/[ru ]k (21) + + 2 3+ 20 2=[ru(oh) ][h ] /[ru ]k (22) 8+ + 4 3+ 4 44 4 4=[ru (oh) [h ] /[ru ]k (23) the mb equations for this system are following: 0 r ru ru ruc c c   (24) r 3+ 2+ + 8+ ru 2 4 4 3+ + -1 + -2 3+ 3 + -4 10 20 44 =[ru ]+[ru(oh) ]+[ru(oh) ]+4[ru (oh) ] [ru ](1+ [h ] + [h ] +4 [ru ] [h ] c k k k   (25) the equilibrium concentration of ru 3+ ion is calculated using the relation: 3+ + 3 s [ru ] [h ]k (26) from equations (24) and (25) one obtains the expression for calculating δcru, then the equilibrium concentrations of the free ru(iii) ion and its hydroxocomplexes are determined. finally, on the diagram of repartition the γij(ph) functions are plotted for fixed 0 ru c values: 0 3+ 0 2+ 0 s0 ru ru 00 10 ru + 0 8+ 0 20 2 ru 44 4 4 ru 00 10 20 44 / ; [ru ]/ ; =[ru(oh) ]/ =[ru(oh) ]/ ; =4[ru (oh) ] / ru sum c c c c c c                  therefore, the procedure of drhs construction includes several stages: j. electrochem. sci. eng. 6(1) (2016) 123-133 ruthenium redox equilibria: 1. thermodynamic stability 130 1. the determination of the thermodynamic stability of solid phases (δcru > 0) in function of solution ph or 0 ru c . 2. calculation of the molar fractions of all species γi in the solid phase and solution within the ph (or 0 ru c ) range, established at the first stage. 3. for the image integrity, outside this range, in the case of homogeneous solution, the molar fractions of chemical species are calculated by typical equations for plotting the diagrams of distribution in solutions. usually, in the γi(ph) diagrams the ph values vary between 0 and 14. by the developed diagrams, the precipitate quantity (in moles or grams), in 1 l of solution for certain ph and 0 ru c values, is easily determined. for example, from fig. 3(b) it results that for ph 3.95 and 0 ru c =10 -4 mol l -1 the degree of precipitation s0 = 0.5, whence it follows that δcru=s0 0 ru c =5·10 -5 mol l -1 or quantity of precipitate p (in g l -1 ) prum(ruo2·2h2o)=5·10 -5 mol l 1 ·169 g mol -1 = =8.45·10 -3 g l -1 . thus, under these conditions in one liter of solution 8.45 mg of ruthenium(iv) hydroxide is deposited. a b figure 3. repartition for the system ru(oh)3(s)·h2o – saturated aqueous solution, 0 cru : a) 10 -4 ; b) 10 -6 mol l -1 i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 123-133 doi:10.5599/jese.226 131 in fig. 3 (a,b) and 4 (a,b) there are shown drhs for both systems for two values of 0 ru c : 10 -4 and 10 -6 mol l -1 . from these diagrams, the ph regions of predominance of different ru(iii) and ru(iv) species are depicted in table 3. a b figure 4. repartition for the system ruo2·2h2o(s) – h2o ; 0 cru : a) 10 -4 , b) 10 -6 mol l -1 j. electrochem. sci. eng. 6(1) (2016) 123-133 ruthenium redox equilibria: 1. thermodynamic stability 132 table 3. ph regions of predominance of chemical species in a ru(oh)3(s)·h2o – h2o and b ruo2·2h2o(s) – h2o system system chemical species ph regions of predominance 0 ru c = 10 -4 mol l -1 0 ru c = 10 -6 mol l -1 a ru 3+ 0.00 < ph < 1.76 0.00 < ph < 1.76 + 2 ru(oh) 1.76 < ph < 2.42 1.76 < ph < 4.42 ru(oh)3∙h2o (s) 2.42< ph < 14.0 4.42 < ph < 14.0 b 2+ 2 ru(oh) 0.00 < ph < 1.05 0.00 < ph < 2.55 4+ 4 12 ru (oh) 1.05 < ph < 3.93 2.55 < ph < 4.43 ruo2∙h2o (s) 3.93 < ph < 14.0 4.43 < ph < 14.0 conclusions from the obtained results the following conclusions can be drawn: 1. by means of the diagrams δg–ph, the areas of thermodynamic stability of ru(iii) and ru(iv) hydroxides have been established for a number of analytical concentrations in heterogeneous mixtures. the diagrams of heterogeneous and homogeneous chemical equilibria have been used for graphical representation of complex equilibria in aqueous solutions containing ru(iii) and ru(iv). 2. the ruthenium (iii) polynuclear hydroxocomplex 8+ 4 4ru (oh) (aq) for the considered values is not formed. 3. at increasing 0 ru c from 10 -6 to 10 -4 mol l -1 , the region of predominance of the ruthenium(iv) polynuclear complex 4+ 4 12 ru (oh) shifts to the acidic solutions and is increased by one ph unit. 4. the ruthenium(iii) monohydroxide + 2 ru(oh) is formed in insignificant amounts under analyzed conditions. 5. the thermodynamic stability area of solid phases increases with growing the initial concentration of ruthenium in mixture. references [1] n. radhey, a. srivastava and s. prasad, spectrochimica acta part a: molecular and biomolecular spectroscopy 69 (2008) 193-197. 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[english translation] i. povar et al. j. electrochem. sci. eng. 6(1) (2016) 123-133 doi:10.5599/jese.226 133 [13] i. povar, russian journal of inorganic chemistry 42 (1997) 607-612. [english translation] [14] i. povar et al., chemistry journal of moldova 6 (2011) 57-61. [15] i. povar, o. spinu, solvent extraction and ion exchange 33 (2015) 196-209. [16] j. rard, chemical reviews 85 (1985) 1-39. [17] e. beresnev, the method of residual concentrations, nauka, moscow, russia, 1992. [in russian] [18] i. povar, o. spinu, fifth regional symposium on electrochemistry – south east europe (rse see), book of abstracts, pravets, bulgaria, 2015, p. 24. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {comparative study between synthetic and dairy wastewaters in single chamber microbial fuel cell for power generation:} http://dx.doi.org/10.5599/jese.1030 279 j. electrochem. sci. eng. 11(4) (2021) 279-289; http://dx.doi.org/10.5599/jese.1030 open access : : issn 1847-9286 www.jese-online.org original scientific paper comparative study between synthetic and dairy wastewaters in single chamber microbial fuel cell for power generation payel choudhury1,, ria majumdar2, tarun kanti bandyopadhyaya3 1department of bioengineering, national institute of technology agartala, agartala-799046, india 2department of civil engineering, national institute of technology agartala, agartala-799046, india 3department of chemical engineering, national institute of technology agartala, agartala-799046, india corresponding author: payell.moon12@gmail.com received: july 3, 2021; accepted: august 12, 2021; published: august 24, 2021 abstract to investigate the performance of microbial fuel cell (mfc) with a single-chamber membrane, pseudomonas aeruginosa is used as a biocatalyst for various synthetic wastewaters rich in carbohydrate and is compared with real dairy wastewater in this experiment. therefore, the choice of appropriate carbon, nitrogen, nacl, inoculum content, temperature, and ph process parameters used for preparing synthetic wastewater was agreed upon by one-variable-at-a time approach. maximum level of voltage generation attained from the synthetic wastewater was 485 mv when supplemented with 1.5 % of lactose as a source of carbon, 0.3 % of ammonium chloride as a decent nitrogen source, 0.03 % of nacl, inoculum concentration of 3 %, the temperature at 37 oc and ph 7. on the other hand, the maximum voltage attained with real dairy wastewater was 561 mv with high chemical oxygen demand (cod) value of 801 mg l-1. the maximum power density obtained from dairy wastewater was 73.54 mw m-2. high voltage achieved for mfc operating with real dairy wastewater suggests that it can be used not only for the industrial application to generate more renewable power, but also for the wastewater treatment carried out at the same time. keywords green technology; microbial fuel cell; voltage generation; synthetic wastewater; dairy wastewater. introduction in current years, an expanding emphasis is put on inexhaustible energy sources, which are substantially biodegradable, can't be depleted, and preserve the valuable conventional power http://dx.doi.org/10.5599/jese.1030 http://dx.doi.org/10.5599/jese.1030 http://www.jese-online.org/ mailto:payell.moon12@gmail.com j. electrochem. sci. eng. 11(4) (2021) 279-289 wastewaters in microbial fuel cell 280 sources like coal, oil, and petrol [1]. in previous couple of decades, however, people had consistently used non-renewable energy sources and utilized them for power generation [2]. as they belong to the conventional sources of energy, they are exhaustible [3] and are getting depleted at an alarming rate. thus, a significant number of petroleum derivatives may never be accessible in only a couple of years. henceforth some additional energy sources for the nourishment of the present advancement have to be discovered [4]. among them, microbial fuel cell technology is gaining more attention for industrial applications and attaining more power at a large scale [5]. the utility of the energy from microbial fuel cells (mfc) is significant to keep attention on their production [6]. therefore, the performance of mfc can be increased by changing the physicochemical parameters and these days, research with mfc is also achieving more attention [7]. there is no common medium for the generation of voltage with microorganisms [8], because specific physicochemical parameters and nourishment are necessary for growth and voltage generation by microorganisms [9]. thus, perfect process optimization is vital for the reduction of handling costs. the present paper describes preparation of the synthetic wastewater media for voltage generation. as per previous reports, there is no sole information on the synthetic wastewater preparation under dissimilar types of process parameters and a comparison with real wastewater from the dairy industry was never performed. the experiments were carried out with synthetic wastewater and dairy wastewater as a substrate to check the performance of mfc for advanced renewable power generation [10]. the experiments conducted for power generation from wastewater contribute to green technology by reducing the extent of carbon dioxide and other gases which cause global warming [11]. preparing a synthetic wastewater media for voltage generation needs a proper assortment of carbon sources, nitrogen sources, and salts at first. then, nutritional necessity can be enhanced by the statistical methods. one-variable-at-a time methodology might be beneficial for defining essential inhibitory or stimulation variables before conducting statistical methods. at first, optimizing process parameters for the maximum voltage generation from the synthetic wastewater in a single chamber mfc is carried out, and the maximum output voltage with the optimized values is reported in this paper. secondly, the same single-chamber mfc operating with real dairy wastewater is carried out, and the maximum voltage obtained is also reported. finally, the maximum voltage outputs from both the synthetic and dairy wastewater are compared. methods chemicals and reagents luria-bertani broth (himedia, india), glucose, fructose, lactose, maltose, sucrose, starch (himedia, india), sodium chloride (merck, india), ammonium chloride, soyabean meal, sodium nitrate, ammonium sulphate, potassium nitrate (himedia, india), potassium dihydrogen phosphate (himedia, india), and di-potassium hydrogen phosphate anhydrous (himedia, india), were acquired for the experiment. other chemicals required in this experiment were of analytical grade [12,13]. microorganism the pseudomonas aeruginosa, which is an exoelectrogenic bacteria was taken from the bioengineering lab of nit agartala, india. luria-bertani broth and agar (lba) media were used as maintenance media for sub-culturing and ph of media was kept at 7.4 before sterilization. after sterilization, lba media was transferred to the plate and waited for 30 minutes until it became solidified. pseudomonas aeruginosa were then transferred to solidified media and incubated at p. choudhury et al. j. electrochem. sci. eng. 11(4) (2021) 279-289 http://dx.doi.org/10.5599/jese.1030 281 37 °c for 24 hours. the synthetic wastewater was prepared in 200 ml conical flask holding lactose (10 g l-1), ammonium chloride (0.20 g l-1), sodium chloride (0.30 g l-1), dipotassium phosphate (1.26 g l-1), potassium dihydrogen phosphate (0.42 g l-1) and trace metals solution (1 ml) [14,15]. 2 % new culture of pseudomonas aeruginosa optical density at 550 nm ≈ 0.2 was inoculated in mfc on the synthetic media at ph 7.4. individually, various carbon and nitrogen sources were used in the experiments. the dairy wastewater was collected from the local area and further diluted during experiments. the mfc was examined in batch mode each time for 360 hours (15 days). mfc arrangement and operating processes the investigation was run in 300 ml single-chamber mfc of working volume 200 ml, and cylindrical reactor made from acrylic resin. the anode and the cathode were carbon-cloth with a surface area of 50 cm2 having cloth thickness 0.37 mm and a diameter of 50 mm. carbon cloth cathode was loaded with 0.5 mg cm-2 of pt. nafion 117 was used as the membrane. the pipe length was 110 mm with an outer diameter of 70 mm and 60 mm. the membrane showed only ionic conductivity of about 0.078 s cm-1. the electrodes were kept at a gap of 175 microns, and membrane was placed between electrodes. the electrodes were attached by stainless-steel wires and digital multimeter. the single-chamber mfc and its components are presented in figure 1. figure 1. single-chamber mfc components and setup effects of various process parameters the carbon source, nitrogen source, inoculum content, temperature, and ph were considered as process factors for the acquirement of maximum power generation from the preparing synthetic wastewater. one-variable-at-a time approach was applied to select the values of main factors. results and discussion choice of appropriate carbon and nitrogen sources for preparing synthetic wastewater the lactose was replaced by other carbon sources viz. fructose, glucose, maltose, sucrose, and starch on the base carbon content of 1 % w/v [16]. the maximum voltage was obtained in the presence of lactose as a simple carbon source, which gives a reading of 410 mv as open-circuit voltage at digital multimeter and is maximum yield when compared with other selected sources of carbon (figure 2). the voltages obtained by other carbon sources is presented in figure 2, showing less encouragement on the voltage generation than lactose [17]. the suppressive effect on such an http://dx.doi.org/10.5599/jese.1030 j. electrochem. sci. eng. 11(4) (2021) 279-289 wastewaters in microbial fuel cell 282 event was witnessed when other carbon sources were examined [18]. catabolite suppression may be the utmost probable cause for this oppressive outcome [19,20]. carbon source (1 % w/v carbon content) figure 2. effect of different carbon sources on voltage generation it was formerly proven that a catabolite control protein was responsible for this controlling mechanism that transduced signal for enzyme synthesis suppression [21]. in the medium, organic and inorganic nitrogen sources were used, such as ammonium chloride, soybean meal, sodium nitrate, ammonium sulfate, and potassium nitrate based on nitrogen content at 0.2 % w/v [22,23]. maximum voltage was attained in the presence of ammonium chloride as a simple nitrogen source, producing 415 mv, thus considered maximum yield in assessment among other nitrogen sources as presented in figure 3. nitrogen source (1 % w/v nitrogen content) figure 3. effect of different nitrogen sources on voltage generation the selected nitrogen sources presented in figure 3 significantly affect to voltage generation when matched with ammonium chloride [24]. it is also witnessed that voltage production progresses in the presence of lactose and ammonium chloride. the sources of nitrogen other than ammonium chloride displayed a comparative drop in voltage production [25]. the effect of carbon, nitrogen, nacl, inoculum content, temperature and ph on voltage generation the assessment of voltage generation at dissimilar levels of carbon source (lactose) in the synthetic wastewater media as components is presented in figure 4. the outcomes display that the voltage generation is improved with increase of lactose concentration from 0.5 to 1.5 %. hence, the upper level of lactose harms voltage generation, as presented in figure 4. the voltage upsurges with a rise in ammonium chloride concentration in the media up to 0.3 %. however, above concentration of 0.3 %, voltage generation is declined (figure 5). p. choudhury et al. j. electrochem. sci. eng. 11(4) (2021) 279-289 http://dx.doi.org/10.5599/jese.1030 283 figure 4. effect of carbon content (lactose) on voltage generation figure 5. effect of nitrogen content (ammonium chloride) on voltage generation voltage generation was improved by adding sodium chloride media from 0.02 to 0.04 %. the voltage generation at 0.03 % of nacl was observed to achieve the highest voltage, while at higher sodium chloride content, the generated voltage declined (figure 6). moreover, sodium chloride acts as an inducer for voltage generation. it was shown previously that microbes utilize sodium-driven solute carriage systems for their existence and adjustment in high ph environments [26-28]. hence sodium ions are compulsory for bioenergetics and metabolic courses of bacterium such as ph homeostasis and atp synthesis [29]. after the effective media optimization, the remaining physical and chemical process parameters were improved considering the optimized medium. the assessment of voltage generated by the impact of dissimilar levels of main physical and chemical factors is presented in figures 7, 8 and 9. voltage generation upswung with rise of inoculum content from 1 to 3 %, and from the maximum at 3 % of inoculum concentration, the value of voltage started to decrease (figure 7). figure 6. effect of sodium chloride content on voltage generation figure 7. effect of inoculum content on voltage generation. this may occur due to the restriction of other medium components, which cause a decrease in voltage generation [30]. on the other side, in the case of temperature, it was observed that voltage increases with an increase of temperature from 30 to 37 °c (figure 8). the obtained results specify that the voltage generation is stimulated by temperature. the voltage generation decreases at lower temperature, what is due to the inactivation of enzymes and ribosomes which are directly or indirectly responsible for the growth of the cell [31]. at the same time, it means http://dx.doi.org/10.5599/jese.1030 j. electrochem. sci. eng. 11(4) (2021) 279-289 wastewaters in microbial fuel cell 284 that at temperatures higher than 37 °c, the volatility of membranes can change, which in turn alters the carriage movement of compounds that are soluble [32]. thus, the highest voltage generation was obtained at 37 °c. voltage generation was progressively improved from ph 6 to ph 7, but beyond ph 7, radical reduction of protease production was obtained and presented in figure 9. figure 8. effect of temperature on voltage generation. figure 9. effect of ph on voltage generation thus, the optimum ph value for the generation of voltage is 7. the inactivation of the enzyme at higher ph is straight or incidentally responsible for the cell growth, which in turn reduces the voltage [33]. thus, the catalytic action of these enzymes is governed by ph of the medium. hence, the alteration in the media ph is mainly responsible for the variation in the rate of reaction [34]. voltage generation with synthetic wastewater the maximum voltage generation of 485 mv was obtained with synthetic wastewater having lactose of 1.5 %, ammonium chloride of 0.3 %, 0.03 % of nacl, inoculum content of 3 %, the temperature at 37 °c and ph 7 after 360 hours of experiment in the single-chamber mfc (figure 10). a b time, h figure 10. (a) maximum voltage reached in batch operation of mfc with synthetic wastewater; (b) voltage vs. time in batch operation of mfc with synthetic wastewater voltage generation with real dairy wastewater since real dairy wastewater comprises several organic matters with high chemical oxygen demand (cod) value of 8010 mg l-1, the probable influence was conveyed for open circuit voltage p. choudhury et al. j. electrochem. sci. eng. 11(4) (2021) 279-289 http://dx.doi.org/10.5599/jese.1030 285 (ocv) observation by running a batch process for 360 hours (15 days) [35]. therefore a full batch operation was carried out with real dairy wastewater to understand the practicality of ocv generation in single-chamber mfc [36,37]. the value of cod of real dairy wastewater was adjusted at 10 mg l-1 and ph to 7, and mfc was operated at the temperature of 37 °c for 360 hours (15 days) [38]. according to figure 11 (a), maximum ocv of 561 mv was reached using real dairy wastewater. it is also marked from figure 11(b), that ocv is progressively increasing during 48 hours of operation, reached 573 v at 120 hours, and remained constant for 168 hours. after that, ocv is gradually declining by duration of the process [39]. former studies showed that natural dairy wastewater is considered as the most efficient substrate for mfc to produce renewable energy [40]. a b time, h figure 11. (a) maximum voltage reached in batch operation of mfc with real dairy wastewater; (b) voltage vs. time in batch operation of mfc with real dairy wastewater from the above experimental output, we can say that wastewater from the dairy industry contains a higher amount of lactose related to synthetic wastewater, as the output voltage is higher with real dairy wastewater. so, scientists and scholars need to put more attention towards the application of wastewater from organic matter to attain products with added value [41]. in the single-chamber mfc reactor, both energy production and wastewater treatment processes were provided from single section fuel cells without the need of any mediator, as exoelectrogenic bacteria pseudomonas aeruginosa were used [42]. the main challenge in the fuel cell is to reduce the internal resistance of the system. as in an mfc, there are no rotating parts, so the main contribution of the total resistance in mfc is the internal resistance caused by the substrate. thus, if the internal resistance of the system was reduced, more electrons from the organic substrate would be transferred and more power produced [43]. therefore, understanding the origins of internal resistance in mfc seems to be important. in the following experiment with dairy wastewater, the external resistance was varied from 50 to 15000 ω, and the gained voltages were plotted vs. current density to achieve the polarization curve. at the same graph, the power density obtained from the product of current density and voltage was plotted against current density to achieve the power density curve shown in figure 12. remarkably, three different phases phase-i, phase-ii, and phase iii are evident in the polarization curve [44]. it is clear from phase i of the polarization curve that rapidly reduced voltage is due to the transfer of charges initially, which contributes to the system resistance. this internal resistance can be minimized by increasing the surface area of the anode, using a mediator in the substrate for http://dx.doi.org/10.5599/jese.1030 j. electrochem. sci. eng. 11(4) (2021) 279-289 wastewaters in microbial fuel cell 286 better transmission of electrons, and increasing of temperature [45]. secondly, in phase-ii, the constant drop of voltage is observed due to the solvent resistance, resistance present in the material used for connecting electrodes, and resistance caused by the membrane. all these resistances contribute to the increase in internal resistance, which can be reduced using a buffer in the system, low resistance wires, and highly conductive proton exchange membrane [46]. at last, the phase-iii causes the sudden reduction of voltage at high current density due to activation of biochemical reaction, the energy requirement for metabolism during bacterial growth, and restriction of mass transport inside mfc. in this case, the internal resistance can be limited by a suitable membrane [47]. therefore, from phase-ii of the polarization graph, the developed internal resistance of mfc can be measured. in the above experiment with dairy wastewater, 760 ω of internal resistance is obtained during batch operation of mfc. figure 12. polarization and power density graphs as shown in figure 12, the power density curve firstly increases with the increase of current density up to 73.54 mw m-2, and then decreases with the additional increase of current density. christwardana et al. [48] reported that the maximum power density of seawater, lake water, and tap water is about 21.92, 4.69, and 11.79 mw m-2, respectively, which is all much less than our experimental power density of 73.54 mw m-2 obtained from dairy wastewater. the maximum power density was attained at 0.364 v, with the maximum current density of 295 ma m-2. arulmani et al. [50] reported the maximum current density of 185.23 ± 15.10 (p1) and 291.23 ± 7.50 ma m-2 (p2) with bio-slurry, which is less, compared to our current density. the optimum level of cell voltage can be anticipated from the junction point where the polarization and power density curve intersect, i.e. 0.539 v. the present investigation clearly showed that comparatively higher power density and current density can be attained using real dairy wastewater [49]. conclusion the conducted experiments showed that the mfc batch process is an active procedure for the workable generation of sustainable power by exoelectrogenic bacteria (pseudomonas aeruginosa). the batch process examined for 360 hours (15 days) with one-variable-at-a-time optimized synthetic wastewater media containing lactose of 1.5 %, ammonium chloride of 0.3 %, 0.03 % of nacl, inoculum concentration of 3 %, temperature at 37 °c and ph 7, generated ocv of 485 mv. secondly, the experiment was carried with real dairy wastewater containing organic compounds, and the batch process of mcf generated ocv of 561 mv. the maximum power density and current density p. choudhury et al. j. electrochem. sci. eng. 11(4) (2021) 279-289 http://dx.doi.org/10.5599/jese.1030 287 obtained from dairy wastewater were 73.54 mw m-2 and 295 ma m-2. this experiment pointed to the workable methodology for removing environmental pollutants using bioreactor (mfc), which thus provides an added value to its ordinary power generation. acknowledgments: all authors expressed gratefulness to the honorable director, nit agartala, for endless support and encouragement for creating a research environment at institute. payel choudhury acknowledges csir, government of india, for research fellowship with ack no. 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1030 https://doi.org/10.1016/j.procbio.2021.01.003 https://doi.org/10.1016/j.electacta.2020.137388 https://doi.org/10.1016/j.ijhydene.2021.06.034 https://doi.org/10.1007/s11356-021-14940-0 http://5.10.230.12/index.php/anjs/article/view/2391 https://doi.org/10.1088/1755-1315/765/1/012060 https://doi.org/10.1088/1755-1315/765/1/012060 https://doi.org/10.1063/5.0045395 https://doi.org/10.1016/j.rser.2021.111261 https://doi.org/10.1016/j.procbio.2021.04.015 https://creativecommons.org/licenses/by/4.0/) {electrochromism in tungsten oxide thin films prepared by chemical bath deposition} doi:10.5599/jese.357 27 j. electrochem. sci. eng. 7(1) (2017) 27-37; doi: 10.5599/jese.357 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochromism in tungsten oxide thin films prepared by chemical bath deposition julijana velevska1,, nace stojanov1, margareta pecovska-gjorgjevich1, metodija najdoski2 1institute of physics, faculty of natural sciences and mathematics, university sts cyril and methodius, arhimedova 3, 1000 skopje, republic of macedonia 2institute of chemistry, faculty of natural sciences and mathematics, university sts cyril and methodius, arhimedova 5, 1000 skopje, republic of macedonia corresponding author: julev@pmf.ukim.mk;tel.: +389 2 3 249 860 received: november14, 2016; revised: february14, 2017; accepted: february 15, 2017 abstract tungsten oxide (wo3) thin films were prepared by a simple, economical, chemical bath deposition method onto fluorine doped tin oxide (fto) coated glass substrates. the electrochemical properties of the films were characterized by cyclic voltammetry. the obtained films exhibited electrochromism, changing color from initially colorless to deep blue, and back to colorless. visible transmittance spectra of (wo3) films were recorded insitu in their both, bleached and colored states. from those spectra, absorption coefficient () and the optical energy gaps were evaluated. the dependence of the optical density on the charge density was examined and the coloration efficiency () was calculated to be 22.11cm2c-1. the response times of the coloring and bleaching to an abrupt potential change from -2.5 v to +2.5 v and reverse, were found to be 9.3 and 1.2 s respectively. the maximum light intensity modulation ability of the films, when the am1.5 spectrum is taken as an input, was calculated to be about 50 %. keywords optical materials; cyclic voltammetry; response time; solar light modulation introduction electrochromism is a unique property of the material to change reversibly its optical properties when it is electrochemically reduced or oxidized [1]. electrochromic materials exhibit color change between the clear transparent state and a darkened colored state, or between two colored states. at the same time, there are materials that exhibit multiple colored states and are described as http://www.jese-online.org/ mailto:julev@pmf.ukim.mk j. electrochem. sci. eng. 7(1) (2017) 27-37 tungsten oxide thin films 28 polyelectrochromic. the classification of the electrochromic materials is related to the potential at which the coloration process occurs. materials with cathodic coloration exhibit coloration at negative potential, i.e. they darken upon reduction (charge insertion). anodic materials, on the other hand, exhibit coloration at positive potential, i.e. they darken upon oxidation (charge extraction). when the electrochromic material is integrated in device, it could modulate the reflectance/transmittance of the incident illumination [2]. electrochromic materials are currently attracting much interest in academia and industry for both their spectroelectrochemical and commercial applications [3]. electrochromism is known to exist in many types of materials, both organic and inorganic. common inorganic electrochromic materials are transition metal oxides and metal hexacyanometallates, while viologens, phthalocyanines, conducting polymers and metallopolymers are common organic and polymer electrochromic materials. transition metal oxides have attractive technological importance for electrochromic applications because they show considerable variation in stoichiometry, and can be quite easily deposited in a form of thin film which is appropriate for device manufacturing. among them, tungsten oxide (wo3) is of intense interest and has been extensively investigated due to its appreciable electrochromic properties in the visible and infrared region. it exhibits large optical modulation, good durability, low power consumption, less stress for the viewer’s eyes, and relatively low price [4]. eectrochromic wo3 thin films have been prepared by a large number of techniques, such as thermal evaporation, electrodeposition, spray pyrolysis, chemical vapor deposition, electron beam evaporation, magnetron sputtering, sol-gel methods [5 16] etc. among these techniques, chemical bath deposition methods have many advantages: they do not require sophisticated expensive equipment, various substrates including metals, semiconductors or insulators can be used, the starting chemicals are commonly available and cheap, and the preparation parameters are easily controlled [17 21]. these methods have benefit of being easily realizable from the point of view of industrialization, especially on large area devices, with the required electrochromic properties [22, 23]. the electrochromic properties of wo3 thin films, like the transmittance modulation (t), coloration efficiency (), switching time (), and cyclic durability, strongly depends on their structural, morphological and compositional characteristics [1, 2], which, in turn, depends directly on the deposition method and deposition conditions. the target of this research is to investigate electrochromic properties of wo3 thin films prepared by a simple chemical bath deposition method [24] and their possible application for solar light modulation. experimental tungsten oxide films were deposited onto fluorine doped tin oxide (fto) coated glass substrates commercially available with transparency of about 80 % for visible light, and sheet resistance of about 10 – 20 ω□-1. before the deposition, the substrates were immersed in acetone and ethanol to be degreased in an ultrasonic bath, and then rinsed in deionized water and dried in air. wo3 films were deposited from a chemical bath with optimized composition and process conditions. the bath solution was prepared by dissolving 1.65 g na2wo4·2h2o in 90 ml deionized water. the substrates were immersed in the beaker filled with deposition solution, and vertically supported against its walls. then, the whole system was heated slowly, up to 95c with continuous stirring. the preparation of the bath solution and the deposition of the thin films have been explained in details in ref. [24]. the thickness of the films depends on the deposition time. in this work the deposition time was 20 min, and the thickness of the films was 150 nm. j.velevskaet al. j. electrochem. sci. eng. 7(1) (2017) 27-37 doi:10.5599/jese.357 29 the electrochemical properties of wo3 films were characterized by cyclic voltammetry measurements performed using micro autolab ii equipment (eco-chemie, utrecht, netherlands) in one compartment three electrodes electrochemical cell with wo3 film as working, platinum wire as counter, and saturated calomel electrode (sce) as a reference electrode. the cycling was carried out in 1 moldm-3kcl aqueous solution as an electrolyte. the voltage scan rate was 10 mv s-1, and the film working area was 1 cm2. electrochromic investigations were performed in situ in an electrochromic device (ecd) consisted of: home-built glass cell (4 ×2.5×4 cm), wo3 film deposited on fto substrate as working electrode (we), blank fto substrate as counter electrode (ce), and 1 moldm-3kcl aqueous solution as an electrolyte. the distance between the electrodes was about 1.5 cm, the volume of the electrolyte was about 20 ml, and the active surface area of the electrodes was approximately 6 cm2. the optical transmittance spectra were recorded by using varian cary 50 scan uv-visible spectrophotometer in the wavelength range from 300 to 900 nm, in both, the completely colored and bleached states of the film. an electrochromic cell with two clean fto substrates filled with electrolyte was measured as 100 % background. coloration and bleaching of the film were performed with -2.5 v and +2.5 v respectively. spectra were recorded 3 min after the voltage was applied. in order to obtain intermediate states of coloration, the film was also colored with coloration potentials of -1.5 v and – 2 v. the visible transmission spectra were used for evaluation of the optical band gaps eg of the wo3. for that purpose, the absorption coefficient () was evaluated from the transmittance data (t) and the film thickness (t), using the equation [25]: tt 1 ln 1  (1) the optical band gaps of the film were evaluated from the absorption coefficient by fitting the data to the relation [26 28]:  ngehah   (2) where, a is a constant, h is the energy of the incident photon, eg is the optical energy gap, and n is a number which determines the type of electron transition causing the absorption. the value of n is 1/2 for direct allowed, 3/2 for direct forbidden, 2 for indirect allowed, and 3 for indirect forbidden transitions. the coloration efficiency () of the wo3 was calculated from the optical density change (od) at a wavelength of 700 nm, and the charge density (q/s) during coloration after the films were fully bleached:   q tts   cblog (3) where tb and tc are the transmittance of the fully bleached and colored states respectively, q is the injected charge, determined by the applied current and the time of its application, and s is the active area of the electrochromic film. the time needed for the electrochromic film to reach some fraction (usually above 70 %) of its maximum colored or bleached state (response time, ) was examined as a change in the transmittance at 700 nm due to abrupt voltage change between +2.5 v and -2.5 v. for an electrochromic material to be practically successful, it must have the ability to switch between its bleached and colored states frequently, whilst maintaining other important features j. electrochem. sci. eng. 7(1) (2017) 27-37 tungsten oxide thin films 30 consistently. cycle life is defined as the number of cycles completed before the material fails, and measures material stability. the cycling behavior of chemically deposited wo3 films during electrochromic switching was directly observed spectroscopically by the in situ measurements of the transmittance at 700 nm of the fully bleached and colored states of the film after some number of cycles. the cycling was performed by alternatively applying potential of 2.5 v. the ability to switch between two states (bleached and colored) in a relatively short response time makes the tungsten oxide films a possible candidate for transmittance modulation device. taking the solar irradiance spectrum am1.5 for a normal incident illumination on tungsten oxide based electrochromic device (ecd = glass/fto/wo3/electrolyte/fto/glass) and the absorption coefficient spectra of the wo3 film in its bleached and colored states, the output integral of the spectral intensity and the integral of the spectral modulation could be calculated [29]. results and discussion the wo3 films investigated in this work exhibited good electrochromic behavior. they could be repeatedly colored and bleached by alternative application of a negative and positive potential respectively, versus a counter electrode. wo3 is cathodically coloring material which means that it possesses a reduced colored state. it is transparent in oxidized state (positive potential), and has a deep blue color in reduced state (negative potential). the x-ray diffraction (xrd) analysis showed that the films were crystalline [24]. figure 1. cyclic voltammetric curves (six cycles) of chemically deposited wo3thin film. arrows indicate direction of the potential scan. the electrochromic behavior of the films was examined by cycling voltammetry. the cyclic voltammetric (cv) curves were obtained by sweeping the potential in the range of 0.8 v to 0 v vs. sce at scanning rate of a 10 mv s-1. in fig. 1 are presented six cv curves of the wo3 thin film. as can be seen all cv curves (except the first one) have a same shape, which means that the films exhibited good stability. the cv curves also showed an increase of the cathodic current density to -0.86 ma cm-2 at -0.8 v, due to the reduction process occurring in the film and its switching to the blue color, whereas the anodic peak is observed at around 0.541 v due to the oxidation process and bleaching of the film. the coloring process is followed by reduction of wvi ions and double injection of potassium ions and electrons, and the bleaching process is followed by oxidation of wv ions and double extraction of potassium ions and electrons. the following equation can express the coloring/bleaching process: j.velevskaet al. j. electrochem. sci. eng. 7(1) (2017) 27-37 doi:10.5599/jese.357 31   (blue)wokxexkttransparenwo 3x 3   (4) it was observed [30] that during the coloration process, the xrd peaks change in the position and in the intensities indicating a structural transition associated with intercalation of ions. when the ions were deintercalated by applying reverse potential, the crystalline structure reverts to the initial lattice. however, this phenomenon remains to be studied in our future research. the optical transmittance of the films was recorded to understand the type of electron transition and it was used to evaluate optical energy gap. by analyzing the optical transmission spectra, it is also possible to determine whether the optically induced transition is direct or indirect, and allowed or forbidden. the optical transmission spectra of the wo3 film in the wavelength range from 350 to 900 nm in both, bleached and colored states, taken in situ, are presented in fig. 2. one can see significant transmittance difference (more than 60%) that occurs in the red region of the visible spectrum with tendency to continue in the near infrared (nir) region. figure 2. in-situ visible transmittance spectra of chemically deposited wo3 thin film in the bleached and colored states. fig. 3 shows the optical transmittance of the ecd constructed by using chemically deposited wo3 film in its bleached and colored states for an applied bleaching potential of +2.5 v and coloring potentials of -1.5 v, -2 v, and -2.5 v. the recorded transmission spectra for the device in the bleached state for positive potentials lower than +2.5 v are indistinguishable from the spectrum of the device bleached at +2.5 v, and hence are not shown. also, no difference was observed when negative potentials higher than -1.5 v were applied. the film started to change its color at -1.5 v, and one can see that the spectra recorded at -1.5 v, -2 v and -2.5 v are distinguishable from each other, and from the spectrum of the bleached state of the film. no significant difference was observed between the spectra recorded at coloration potentials of -2.5 and -3 v, which means that the film is wholly reduced at -2.5 v. actual photographs of the device in the bleached (+2.5 v) and colored (-1.5 v, -2 v, and -2.5 v) states are presented in fig. 4. from these photographs one could clearly notice the different shades of blue obtained by different coloring potentials. j. electrochem. sci. eng. 7(1) (2017) 27-37 tungsten oxide thin films 32 figure 3. in-situ visible transmittance spectra of chemically deposited wo3 thin film bleached at +2.5 v and colored -1.5 v, -2 v, and -2.5v. a b c d figure 4. photographs of ecd a: bleached at +2.5 v, b: colored at -1.5 v,c: -2 v and d:-2.5 v. the optical energy gaps of the films were evaluated utilizing the transmittance data and the equations (1) and (2). the plots of (h)2 versus h for the chemically deposited wo3 thin film in both, bleached and colored states, are shown in fig. 5. the film showed a better fit for n = 0.5 which shows the direct electron transition mechanism in both states (bleached and colored) of chemically deposited wo3 films. the energy gaps were calculated from the linear parts in fig. 3 as intercepts with the photon energy axis. the evaluated band gaps for the film in its bleached and colored states were 3.38 ev and 3.32 ev respectively. these values are in good agreement with the reported values on tungsten oxide thin films [31]. figure 5. the plots of (h)2 vs. hfor wo3 thin films prepared by chemical bath deposition. j.velevskaet al. j. electrochem. sci. eng. 7(1) (2017) 27-37 doi:10.5599/jese.357 33 to investigate in more detail the optoelectrochemical properties of the chemically deposited wo3 thin films, the optical density change (od) was plotted against the charge density change (q/s), and displayed in fig. 6. the coloration efficiency  at 700 nm was extracted as the slope of the line fitted to the linear region of the curve. the calculated  value was found to be 22.11 cm2 c-1. the  value obtained in this work is higher when compared to the values obtained for electrodeposited and sol-gel coated wo3 thin films [32-35], but lower compared to the values obtained for the sputtered wo3 [36]. figure 6. optical density variationwith respect to the charge density measured at 700 nm in order to investigate the transition response time between coloration and bleaching, the transmittance was measured in-situ through the ecd. the applied potential was switched between 2.5 v (transparent state) and 2.5 v (blue state). fig. 7 shows the dynamic coloration/bleaching characteristics of the ecd, recorded at the wavelength of 700 nm. the coloration and bleaching times (c and b), defined as time required for achieving 70 % of the total transmission change [37, 38] was found to be 9.3 s and 1.2 s respectively, which means that the coloring kinetics is slower than the bleaching one. the faster bleaching time is due to the good conductivity of the tungsten bronze (kxwo3) and the conductor (kxwo3) to semiconductor (wo3) transition. on the other hand, the slower coloration time is due to the higher resistance during wo3 to kxwo3 transition [39]. figure 7. switching time characteristics (at 700 nm) between the colored and bleached states for ecd, measured at ±2.5 v. charge density, c cm-2 j. electrochem. sci. eng. 7(1) (2017) 27-37 tungsten oxide thin films 34 fig. 8 shows the transmittance at 700 nm of the ecd in the bleached and colored states up to 10000 color-bleach cycles. as can be seen from fig. 8, the transmittance shows insignificant variations which means that the electrochemically deposited wo3 films are stable and could be electrochemically switched for 104 cycles without serious deterioration. unfortunately, after the 104 cycles the optical transmittance change rapidly decreased, so we could say that the device durability is up to 104 cycles of bleaching and coloring. figure 8. in-situ transmittance (at 700 nm) of chemically deposited wo3 thin film in the bleached and colored states vs. number of cycles. finally, the irradiance of the solar spectrum am 1.5 [40] and the absorption coefficient spectra (calculated from the transmittance spectra) of the chemically deposited wo3 films in their bleached and colored states (fig. 9), were taken as input parameters. the output spectral intensities transmitted across the wo3 films were calculated and presented in fig. 10. the results of the numerical integration for the spectral intensity within the visible region (350 – 900 nm) are presented in table 1. table 1. integral transmitted intensity from 350 to 900 nm(it) through the wo3 films in their bleached and colored states. state it/ w m-2 bleached 506 colored 253 the relative change of the integrated intensity (the visible transmitted intensity and the light modulation) could be calculated by the equation:      bleached coloredbleached modulation t tt i ii   (5) using the results from the table 1 and the equation (5), the integrated intensity modulation of about 50 % was achieved, which is considerable value that gives the opportunity for implementation of the chemically deposited wo3 films in electrochromic devices such as electrochromic windows. j.velevskaet al. j. electrochem. sci. eng. 7(1) (2017) 27-37 doi:10.5599/jese.357 35 figure 9. absorption coefficient spectra of the chemically deposited wo3 film in the bleached and colored states. figure 10. spectral intensity of the transmitted am 1.5 solar irradiance spectrum through the wo3 film in bleached and colored states conclusions tungsten oxide thin films investigated in this work were deposited onto fto coated glass substrates by chemical bath deposition method. the method is simple, economical, and has benefit of being easily realizable from the point of view of industrialization, especially on large area devices, with the required electrochromic properties. the obtained films exhibited good electrochromic properties. they were stable and exhibited excellent reversibility, with color changed from originally colorless into deep blue when negative potential was applied, and back to colorless when the potential was reversed. transmittance difference of more than 60 % was achieved in the red region of the visible spectrum. also, by controlling the coloring potential, intermediate states of coloration were achieved. optical energy gaps were evaluated from the transmittance measurements for the both, bleached and colored states of the films, assuming a direct semiconductor transition mechanism. the coloration efficiency (at 700 nm) was found to be 22.11 cm2c-1, the 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tables for reference solar irradiances: direct normal and hemispherical on 370 tilted surface (2012) ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {sensitive electrochemical detection of bisphenol a at screen-printed graphite electrode modified with nitrogen-doped graphene sheets:} http://dx.doi.org/10.5599/jese.1103 37 j. electrochem. sci. eng. 12(1) (2022) 37-45; http://dx.doi.org/10.5599/jese.1103 open access : : issn 1847-9286 www.jese-online.org original scientific paper sensitive electrochemical detection of bisphenol a at screenprinted graphite electrode modified with nitrogen-doped graphene sheets sakineh esfandiari baghbamidi department of chemistry, bandar abbas branch, islamic azad university, bandar abbas, iran corresponding author: esfandiari.576@yahoo.com; tel: +987633665500 received: september 3, 2021; accepted: september 24, 2021; published: october 21, 2021 abstract a novel voltammetric sensor was developed by modifying screen-printed graphite electrode (spge) with nitrogen doped graphene sheets (n-gss) to detect bisphenol a. the electrochemical results exhibited that n-gss / modified spge has high sensing performance towards the oxidation of bisphenol a. excellent results were obtained for bisphenol a detection in the linear range from 0.08 to 300.0 µm with a sensitivity of 0.1626 µa µm-1 and limit of detection of 0.02 µm. also, the fabricated n-gss/spge sensor showed good stability. the as-prepared sensor was tested towards the detection of bisphenol a in real samples. the measured results established the great sensing ability of n-gss/spge for bisphenol a with high selectivity and good stability in real samples. keywords voltammetric sensor; high sensitivity; lengthy stability; reproducibility. introduction bisphenol a is a compound widely used in the synthesis of polycarbonate plastics and epoxy resins. it can be used in medical materials, thermal receipts, and packagings such as canned beverage containers, nursing bottles, and children's toys. thus, trace amounts of bisphenol a can migrate into food and beverage packing of products and consequently are absorbed by humans [1–4]. besides, bisphenol a is also one of the endocrine-disrupting compounds, which can affect the reproduction of aquatic organisms and cause a different of unfavorable health problems to human beings [5]. because of its severe threat to the environment and human health, developing a more convenient and specific method of higher sensitivity is crucial to detect bisphenol a rapidly. at present, the detection of bisphenol a relies on spectrophotometry [6], chromatography [7], and immunoassay [8,9], which are expensive and time-consuming techniques. by contrast, electrochemical methods are considered an effective method for quantifying analytes due to their high sensitivity, simplicity, and low cost [10-16]. http://dx.doi.org/10.5599/jese.1103 http://dx.doi.org/10.5599/jese.1103 http://www.jese-online.org/ mailto:esfandiari.576@yahoo.com j. electrochem. sci. eng. 12(1) (2022) 37-45 detection of bisphenol a at spg electrode 38 electrochemical sensors based on screen-printed electrodes (spes) have gained increasing interest as analytical tools for electro-analysis. spes provide great advantages that make these kinds of sensors have important characteristics of ideal sensors: ease of use, low-cost, and portability [17,18]. so, the screen-printed technology has significantly contributed to the transition from traditional unwieldy electrochemical cells to miniaturized and portable electrodes that meet the needs for on-site analysis [19,20]. different materials and methods can be used to modify the surface of the working electrode to enhance the electro-activity and sensitivity of the modified electrode [21-26]. the sensing materials based on nano-materials are usually used due to their high electrical conductivity, high thermal conductivity, and unique chemical and mechanical properties [27-33]. graphene, a carbon monolayer packed into a 2d honeycomb lattice, has spurred extensive interest in the area of electrochemical sensors because of its superior electronic conductivity, low cost, and chemical and mechanical stability [34]. the capacity of graphene is greatly dependent on its specific surface area, pore structure, doping, and interlayer distance. various strategies have been developed, such as the design of graphene foam [35], assembly with the assistance of templates [36], the recombination of graphene and carbon nanotube [37], and introduction of heteroatoms into graphene lattice [38]. among them, nitrogen doping has been an effective way to improve the sensing properties of graphene for electrochemical sensors [39-41]. as is well known, n-doping would further promote the electrochemical performance of graphene electrodes, which could be attributed to more active sites induced by doping, and the increased specific surface area. in this study, the electrochemical detection of bisphenol a using n-gss modified spge is reported. the electrochemical and electrocatalytic properties of n-gss modified spge toward bisphenol a are studied. finally, accurate detection of bisphenol a in real samples is demonstrated using n-gss/spge as an electrochemical sensor. experimental chemicals and instruments the chemicals and reagents were of analytical grade without any further purification. all chemicals were obtained from sigma-aldrich. in addition, ortho-phosphoric acid and the respective salts (kh2po4, k2hpo4, k3po4) with a ph ranging between 2.0 and 9.0 have been utilized to procure buffer solution. electrochemical measurements were carried out on a pgstat-302n autolab potentiostat/galvanostat (eco chemie, the netherlands). all the measurements were done using spes system (dropsens, drp-110, spain) with a three-electrode cell system consisting of graphite as a working electrode, a silver pseudo-reference electrode and graphite used as an auxiliary electrode. a digital ph-meter (metrohm 710) was employed for measuring ph values. synthesis of n-gss n-gss were synthesized via hydrothermal treatment method with graphene oxide (go) as the base material and urea as the reducing and doping agent. briefly, exfoliated go (50 mg) was dispersed through ultrasonication in 50 ml of redistilled water, and the ph of the solution was adjusted to 10 by adding nh3·h2o (30 %). next, urea (3.0 g) was added and ultrasonicated for three hours. then, the mixture was sealed in a polytetrafluoroethylene (teflon)-lined autoclave and maintained at 180 oc for 12 h. after being cooled down to room temperature, the finally obtained hydrothermal product of n-gss was collected and washed several times with double distilled water and its ph was adjusted to neutral. finally, the n-gss were freeze-dried. a typical sem image of the synthesized n-gss is shown in figure 1. s. e. baghbamidi j. electrochem. sci. eng. 12(1) (2022) 37-45 http://dx.doi.org/10.5599/jese.1103 39 figure 1. fe-sem image of synthesized n-gss electrode preparation firstly, 1 mg of n-gss was put into 1.0 ml of redistilled water to form a homogeneous solution through ultrasonic treatment. the n-gss/spge was obtained by adding 4 µl of the dispersed solution on the spge surface directly and finally dried at the ambient temperature. the surface areas of n-gss/spge and bare spge were obtained by cyclic voltammetry (cv) using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula [42], the electrode surface area of n-gss/spge was calculated as 0.116 cm2, which was about 3.7 times greater than the surface area of bare spge. results and discussion electrocatalytic oxidation of bisphenol a at n-gss/spge the electrochemical behavior of bisphenol a is strongly determined by the ph value of the aqueous electrolyte solution (scheme 1). hence, to ensure the optimal performance of n-gss/spge in the determination of bisphenol a, the influence of ph was investigated through the cyclic voltammetry behavior of n-gss/spge toward 100.0 μm bisphenol a in the ph range between 2.0 and 9.0 at the scan rate of 50 mv/s. scheme 1. electrochemical oxidation mechanism of bisphenol a at the surface of the modified electrode the oxidation peak current value of bisphenol a showed a tendency to rise to ph 7 and fall beyond it (figure 2). since the maximum oxidation peak current value of bisphenol a was achieved at ph value 7.0, it was chosen as the optimal ph value for subsequent measurements in 0.1 m pbs. http://dx.doi.org/10.5599/jese.1103 j. electrochem. sci. eng. 12(1) (2022) 37-45 detection of bisphenol a at spg electrode 40 figure 2. plot of ip vs. ph obtained from cvs of n-gss/spge in a solution containing 100.0 μm of bisphenol a in 0.1 pbs of different ph (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0). the electrochemical properties of the spge before and after modification with n-gss were investigated by cv. cv curves recorded at the scan rate of 50 mv s-1 are shown in figure 3. it can be seen in figure 3 that the oxidation current response of bisphenol a at n-gss/spge was significantly higher than at spge. in addition, the anodic peak potential of bisphenol a at the surface of the ngss/spge is shifted to a negative value relative to the bare electrode. the reason could be that the ngss provided a large specific surface area of high electrical conductivity, which accelerated the electron transfer rate of electroactive compounds at the interface between the electrode and solution. figure 3. cv response (50 mv s-1) of 100.0 μm of bisphenol a in 0.1 m pbs of ph 7.0 at: a bare spge and b n-gss/spge effect of scan rate on the response of bisphenol a at n-gss/spge the effect of the scan rate on the oxidation peak current values of 70.0 μm bisphenol a was investigated using linear sweep voltammetry (lsv) (figure 4a). various scanning rates have been tested (10, 25, 50, 75, 100, 200 and 400 mv s−1). the results show that the oxidation peak current increased upon increasing the scan rate. moreover, the peak potential is slightly shifted to more positive potentials. the results show that at higher scan rates, a kinetic limitation between bisphenol a and modified electrode surface can occur. according to figure 4b, the anodic peak current (ipa) of bisphenol a is proportional to the square root of scan rate (1/2), suggesting that the reaction of bisphenol a at n-gss/spge is controlled by diffusion. s. e. baghbamidi j. electrochem. sci. eng. 12(1) (2022) 37-45 http://dx.doi.org/10.5599/jese.1103 41 figure 4. (a) lsv curves at different scan rates for 70.0 μm bisphenol a in 0.1 m pbs (ph 7.0) at of n-gss/spge (a-g refer to 10, 25, 75, 100, 200.0, and 400.0 mv s-1); (b) plot of the oxidation peak current of bisphenol a vs. square root of scan rate. to define the electron transfer coefficient (α) between bisphenol a and n-gss/spge electrode, a tafel diagram (e vs. log i) was plotted (figure 5b), using the activation region (rising part) of the voltammogram recorded at 10 mv s-1 for 70.0 μm bisphenol a (figure 5a). the slope from the linear plot was estimated to be 0.0898 v-1. the slope value is equal to nα (1-α)f/2.3rt. from this equation, the α value can be estimated to be 0.34 (assuming nα = 1). e / mv log (i / ma) figure 5. (a) lsv response at 10 mvs-1 scan rate for 70.0 μm bisphenol a in 0.1 m pbs (ph 7.0) at ngss/spge; and the (b) tafel plot derived from the rising part of voltammogram cha studies according to the previous section, a diffusion-controlled process dominated during the electrooxidation of bisphenol a at the surface of n-gss/spge. therefore, single-step chronoamperograms were recorded by setting the applied potential step at 0.5 v to measure the diffusion coefficient of bisphenol a on the surface of the modified electrode. figure 6a shows the single-step chronoamperograms recorded for n-gss/spge in the presence of different concentrations of bisphenol a. it can be seen that increase in the concentration of bisphenol a was accompanied by an increase of the anodic current. the experimental plots of current versus t-1/2 with the best fits for different concentrations of bisphenol a are shown in figure 6b. the slopes of the resulting straight lines were then plotted versus the concentration of bisphenol a (figure 6c). using the cottrell equation, the diffusion coefficient of bisphenol a was calculated, and it equals 2.7×10-5 cm2 s-1 http://dx.doi.org/10.5599/jese.1103 j. electrochem. sci. eng. 12(1) (2022) 37-45 detection of bisphenol a at spg electrode 42 i = nfad1/2cbπ-1/2t-1/2 (1) in eq. (1), i is current (a), d is the diffusion coefficient (cm2 s-1), cb is the bulk concentration of analyte (mol cm-3), a is the surface area of the electrode (cm2), f is faraday’s constant, t is the time (s), and n is the number of electrons transferred. figure 6. (a) chronoamperograms of n-gss/spge in 0.1 m pbs, ph 7.0 for different concentrations of bisphenol a: (a–e refer to 0.1, 0.25, 0.4, 0.9, and 1.5 mm) (b) i versus t-1/2 obtained from chronoamperograms a to f. (c) slope plot vs. concentration of bisphenol a detection of bisphenol a at n-gss/spge by differential pulse voltammetry (dpv) technique to evaluate the linear range and limit of detection of the designed electrochemical sensor, the determination of bisphenol a at n-gss/spge in different concentrations of standard solutions was performed by dpv performed with step potential of 0.01 v, and pulse amplitude = 0.025 v. dpv responses are shown in figure 7a linear relation was observed between the current signal and the bisphenol a concentration in the range of 0.08–300.0 μm. figure 7. (a) dpv response of various concentrations of bisphenol a in 0.1 m pbs of ph 7.0 at n-gss/spge (a-h refers to 0.08, 1.0, 10.0, 30.0, 70.0, 100.0, 200.0, and 300.0 µm); (b) calibration curve of dpv oxidation current peaks against concentration of bisphenol a the calibration graph was defined according to the following equation: ipa = 0.1626 cbisphenol a + 1.0948 with a correlation coefficient (r2) of 0.9998 (figure 7b). also, the detection limit, cm, of bisphenol a was obtained using the following equation: cm = 3sb / m (2) s. e. baghbamidi j. electrochem. sci. eng. 12(1) (2022) 37-45 http://dx.doi.org/10.5599/jese.1103 43 in the above equation, m is the slope of the calibration plot (0.1626 μa μm-1) and sb is the standard deviation of the blank response obtained from 14 replicate measurements of the blank solution. the detection limit achieved by the designed electrochemical sensor was 0.02 μm. reproducibility and stability of n-gss/spge in this section, the reproducibility of the modified electrode was examined using four different sensors (n-gss/spge) that were fabricated in the same condition containing 50.0 µm bisphenol a by cv. the relative standard deviation (rsd) value for this compound was found to be 2.9 %. this rsd value for analysis of bisphenol a reflects that n-gss/spge displayed good reproducibility properties. for checking n-gss/spge sensor stability, the sensor was kept within pbs, ph equal to 7.0 for 15 days. then, cv was recorded in the solution consisting of 50.0 µm of bisphenol a and compared with cv observed before immersing. the oxidation peak of bisphenol a did not change much and compared to the earlier response, there was less than 2.9 % reduction in signal, reflecting acceptable stability of n-gss/spge sensor. application for real sample analysis the applicability of the fabricated n-gss/spge sensor was explored via detection of bisphenol a present in tomato paste, chili sauce and water bottles. the results are shown in table 1, indicating that recoveries for bisphenol a were in the range from 97.3 to 104.6 %, and relative standard deviations were less than 3.5 %. this demonstrates the efficiency of the proposed sensor in the detection of bisphenol a in real samples. table 1. determination of bisphenol a in real samples using n-gss/spge (n=5) sample concentrations of bisphenol a, μm recovery, % rsd, % spiked founded tomato paste 0.0 1.5 3.5 4.0 5.4 98.2 1.7 6.0 6.8 104.6 2.4 chilli sauce 0.0 5.0 5.1 102.0 2.3 7.5 7.3 97.3 2.9 water bottle 0.0 5.0 4.9 98.0 3.4 7.0 7.2 102.9 2.1 conclusion a sensitive electrochemical sensor for the detection of bisphenol a was fabricated by utilizing ngss as a sensing platform. the n-gss exhibited improved electrochemical performance when dropcasted onto the spge, forming n-gss/spge sensor. dpv response of ngss/spge increased linearly with bisphenol a concentration in the range of 0.08 μm to 300.0 μm (r2= 0.9998) with a low detection limit of 0.02 μm and high sensitivity of 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electrochem. sci. eng. 11(4) (2021) 241-245; http://dx.doi.org/10.5599/jese.1069 open access : : issn 1847-9286 www.jese-online.org original scientific paper new complex dimensionless variable in cyclic staircase voltammetry on the rotating disk electrode* milivoj lovrić, divkovićeva 13, zagreb 10090, croatia corresponding author: milivojlovric13@gmail.com received: july 26, 2021; accepted: september 5, 2021; published: september 9, 2021 abstract cyclic staircase voltammograms of a simple, reversible oxidation on the rotating disk electrode is analysed by the digital simulation. it is demonstrated that the peak currents and potentials depend on the single dimensionless variable that considers nonlinear relationship between peak currents and the potential increment. the reverse, cathodic branch of voltammograms depends on this variable differently than the anodic one. keywords reversible oxidation; sigmoidal dependence; digital simulation. introduction in the linear scan voltammetry on the rotating disk electrode the maximum current depends on the product of steady state diffusion layer thickness and the square root of scan rate [1]. however, in the staircase voltammetry on this electrode, the relationship between peak current and the above mentioned product depends on the potential increment [2]. this is because in the later technique the peak current does not depend linearly on the square root of the formal scan rate e /  (where e is the potential increment and  is the step duration), but on the ratio e0.446 /  0.5 [3]. for this reason, a new complex variable for the staircase voltammetry on the rotating disk electrode is proposed here. model a simple, fast and reversible electrode reaction is investigated: red ↔ ox+ + e (1) both components of this redox couple are solution soluble and only red is initially present in the solution. on the rotating disk electrode, the mas transport can be described by the following differential equations and boundary conditions [1,4]: *dedicated to the memory of dr. šebojka komorsky-lovrić. http://dx.doi.org/10.5599/jese.1069 http://dx.doi.org/10.5599/jese.1069 http://www.jese-online.org/ mailto:milivojlovric13@gmail.com j. electrochem. sci. eng. 11(4) (2021) 241-245 variable in cyclic staircase voltammetry on rde 242 cr / t = d2cr / x2 vcr / x (2) co / t = d2co / x2 vco / x (3) v = -0.5103/2v-1/2x2 (4) t = 0, x ≥ 0: cr = cr*, co = 0 (5) t > 0, x →: cr → cr*, co → 0 (6) x = 0: co,x=0 =cr,x=0 exp(f(e-e0) / rt (7) d(cr/x)x=0 = i / fs (8) d(co/x)x=0 = -d(cr/x)x=0 (9) the meanings of all symbols are reported in table 1. table 1. list of parameters symbol meaning co concentration of product cr concentration of reactant cr* bulk concentration of reactant d diffusion coefficient  diffusion layer thickness e potential increment t time increment x space increment e potential e0 standard potential f faraday constant i current  kinematic viscosity  rotation rate r gas constant s electrode surface area t temperature  step duration v flow rate of solution the differential equations (2) and (3) are solved by the finite difference method [1]. a cyclic staircase voltammetry is applied and the currents flowing at the end of each potential step are calculated. the following fixed parameters are used: d = 10-5 cm2/s, v =10-2 cm2/s, t = 10-5 s and dt / x2 = 0.4. the results are reported as the relationship between the dimensionless current  and electrode potential:  = iss / fsdcr* (10) ss = 1.61d1/3v1/6-1/2 (11) results and discussion cyclic staircase voltammograms on the rotating disk electrode depend on the formal scan rate and the electrode reaction rate. two examples are shown in figure 1. the anodic and cathodic branches of the response are either separated and characterized by the maximum and minimum, m. lovrić j. electrochem. sci. eng. 11(4) (2021) 241-245 http://dx.doi.org/10.5599/jese.1069 243 respectively, or overlapped and resembling polarographic wave. this transformation is achieved either by the decreasing scan rate or by the increasing rotation rate. e e0 / v figure 1. cyclic staircase voltammograms of the reaction (1) on the rotating disk electrode;  =10-3 s,  = 40  rad / s and e = 10-3 (1) and 10-5 (2) v in the analog linear scan voltammetry, the response depends on the complex, dimensionless variable  = f(de/dt)ss2 / drt [1], but in the digital staircase voltammetry this variable cannot be used because the same formal scan rate e /  can be obtained by combining various e and  pairs. figures 2 and 3 show two examples of many in which responses corresponding to the same 𝜎 value are not identical. e e0 / v figure 2. csv on rde;  = 10-3 s,  = 40  (1) and 4 (2)  rad s-1 and e = 10-3 (1) and 10-4 (2) v e e0 / v figure 3. csv on rde;  = 40  rad s-1,  = 10-3 s and 5×10-4 (2) s and e = 10-3 (1) and 5×10-4 (2) v -0.1 0 0.1 0.2 0.3 0 0.5 1  (e e0) / v 1 2 -0.1 0 0.1 0.2 0.3 0 0.5 1  (e e0) / v 12 -0.1 0 0.1 0.2 0.3 0 0.5 1  (e e0) / v 12    http://dx.doi.org/10.5599/jese.1069 j. electrochem. sci. eng. 11(4) (2021) 241-245 variable in cyclic staircase voltammetry on rde 244 in figure 2, the experimental conditions are similar only in , but the variable  = 2.8335 is common by the coincidence. still, the second anodic peak current is 3.75 % higher than the first one and the difference between the peak potentials is 3 mv. the curves in figure 3 have common 𝜔 and the formal scan rate, but different e and . again, the difference in anodic peak currents is 1.56 % of the smaller one. three different peak currents for the same 1/2 can be ascribed to different e values, in the agreement with our calculations that the peak currents on the stationary planar electrode depend on e0.446 [3]. for this reason, a new dimensionless variable s = (fe / rt)0.892ss2/d is proposed here. an advantage of this variable is that various pairs of e and 𝜏 values cannot give the same s value. the curve 1 in figures 2 and 3 corresponds to s1/2 = 3.09, while the curves 2 in these figures correspond to s1/2 = 3.29 and 3.15, respectively. by the variation of  e and  the relationship between anodic and cathodic peak currents and potentials and the square root of the variable s is calculated and presented in figure 4. a s1/2 b s1/2 figure 4. dependence of peak currents (a) and peak potentials (b) on the square root of the complex dimensionless variables s = (fe / rt)0.892 ss2 / d the peak currents, both anodic and cathodic, vanish if s < 1.8. under this condition, the response acquires the form of polarographic wave, and its dimensionless limiting current is equal to 1. if s1/2 > 4, the anodic peak currents tend to the asymptote p = 0.378  + . this means that the real peak current is not entirely independent of the rotation rate: ip = fscr* d1/2[0.378 (fe/rt)0.446-1/2 + + 0.060 d1/2/ss]. the anodic peak potentials tend to 0.033 v vs. e0 ifs1/2 > 6. as the peak currents gradually disappear below s1/2 = 3, the peak potentials increase to 0.090 v. the cathodic and anodic peak potentials are symmetrical versus e0. the cathodic peak currents depend on s1/2 as sigmoidal 2 4 6 8 10 12 14 -2 0 2 4 p s1/2 2 4 6 8 10 12 14 -0.05 0 0.05 0.1 ep / v s1/2  p e p / v m. lovrić j. electrochem. sci. eng. 11(4) (2021) 241-245 http://dx.doi.org/10.5599/jese.1069 245 function. within the interval 3 < s1/2 < 6 the slope p / s1/2 is equal to -0.36 and then the peak currents tend to the asymptote p = -0.284 s1/2 + 0.160 if s1/2 >9. the values of parameters e and 𝜏 used in these calculations are rather high with the purpose to obtain pronounced effects, while in the experiments they can be as much as one hundred times lower. however, the limit of the ratio (fe / rt)0.892 /  as e and  tend to zero is infinite, which means that the described effect does not disappear at very low e and  values. nonlinear relationship between the peak current and the potential increment can be described by the function 0.446 / (1 + 0.375(nfe / rt)0.52 [5]. this function and the exponential one fit the results of simulation equally well [3]. these results are relevant for the application of the rotating disk electrode in the investigation of electrochemical mechanisms [6-8]. conclusions nowadays the cyclic staircase voltammetry is frequently used electroanalytical technique [9]. our calculations explain the difference between this method and the analog linear scan voltammetry when applied to the rotating disk electrodes. a sigmoidal dependence of the peak currents in the reverse branch of cyclic voltammogram on the newly proposed dimensionless variable  is discovered. data availability all relevant data are available on demand. references [1] j. strutwolf, w. w. schoeller, electroanalysis 8(11) (1996) 1034-1039. https://doi.org/10.1002/elan.1140081111 [2] m. lovrić, š. komorsky-lovrić, to chemistry journal 1(3) (2018) 370-375. 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https://creativecommons.org/licenses/by/4.0/) effect of supplied co-co2 in the presence of carbon doi: 10.5599/jese.145 67 j. electrochem. sci. eng. 5(1) (2015) 67-77; doi: 10.5599/jese.145 open access : : issn 1847-9286 www.jese-online.org short communications effect of supplied co-co2 in the presence of carbon lisa deleebeeck and kent kammer hansen department of energy conversion and storage, danish technical university (dtu), risø campus, frederiksborgvej 399, po box 49, dk-4000 roskilde, denmark corresponding author: ldel@dtu.dk; tel.: +45-46 77 58 35; scientific enquiries: kkha@dtu.dk received: november 11, 2014; revised: january 28, 2015 published: march 15, 2015 abstract the effect of varying the co-co2 and co2-n2 ratios was investigated in the presence of coal in a specially designed 3-electrode setup, used to simulate the anode compartment in a hybrid direct carbon fuel cell (hdcfc). the hdcfc consists of a hybrid between a molten carbonate and a solid oxide fuel cell (sofc). it was shown that the cell performance improved with increased co2 content in the co2-n2 mixture, due to the formation of co from the inverse boudouard reaction. the same was seen for co/co2 gas mixtures in the presence of coal, in contrast to co-fueled sofcs. keywords hybrid direct carbon fuel cell (hdcfc); dcfc; 3-electrode; half-cell; cyclic voltammetry (cv); coal; carbon monoxide introduction direct carbon fuel cells (dcfcs) include different types of fuel cells, which may be differentiated by their electrolytes: including molten hydroxide, molten carbonate and solid oxide [1,2]. solid oxide fuel cell (sofc)-type dcfcs are identical to gas-fueled (e.g. h2, co) sofcs, with the addition of carbon loaded into the anode chamber, either in direct contact with the electrolyte, typically yttria-stabilized zirconia (ysz), or with an anode layer (e.g. ni-ysz). carbon sources may include carbon black, graphite, coals or various types of biomass [3]. due to the solid nature of the fuel and the electrolyte, the electrochemically active contact area is relatively limited in sofc-type dcfcs. to partially overcome this limitation, molten slurries containing dispersed carbon are introduced into the anode chamber. molten materials include metals [4] or alkali carbonates (e.g., (li-k)2co3), where the latter is termed a hybrid direct carbon fuel cell (hdcfc) as it is a hybrid between molten carbonate (mcfc) and sofcs [2]. http://www.jese-online.org/ mailto:ldel@dtu.dk mailto:kkha@dtu.dk j. electrochem. sci. eng. 5(1) (2015) 67-77 co-co2 in the presence of carbon 68 68 several reports in the literature have previously dealt with gas-fueled sofc performance at various co:co2 ratios, focusing primarily on different anode/fuel-electrode materials, including niysz [5,6], gadolina-doped ceria (gdc) [7], la1-xsrxcr1-ymnyo3±δ (lsmc) and pt-(ce, y, zr)o2-δ [8]. when operating in fuel[9] or electrolyzer-cell [5] modes, the deposition of carbon is undesirable in these systems, as this leads to decreased electrochemical activity, and destruction of ni-ysz electrodes due to carbon dusting of ni. in the case of dcfcs, carbon is purposefully loaded into the anode chamber to serve as a source of fuel. in this investigation, we explore the effects of variable co:co2 ratio on fuel cell performance of a ni-ysz anode in the presence, and absence (co-fueled sofc), of carbon fuel (treated anthracite and bituminous coals, sofc-type dcfc) and mixed carbon-carbonate (hdcfc). although such a combination of fuels (solid carbon and co(g)) is unlikely in technological applications, these investigations aid in the understanding of the chemical reactions occurring in the vicinity of the anode within the carbon-molten carbonate slurry of a hdcfc. experimental three types of coal were acquired from incar (spain): milled and demineralised bituminous (coal i) [10], carbonized, milled and demineralised anthracite (coal ii) and milled bituminous coal (coal iii). coals were combined, as received, with (62-38 wt% li-k)2co3 (co3) (4:1 wt% c:co3) and ball milled dry for 3 h. mixed coal-carbonate, carbonate and b-ii coal alone were loaded into a zirconia tube sealed onto a 3-electrode (3e) ysz pellets developed at dtu-risø [11,12], equipped with a au mesh at the ni-ysz working electrode (we, 7.4 mm diameter), and pt counter (ce) and reference electrodes (re), as described in deleebeeck and hansen [13], and shown schematically in figure 1. figure 1. three-electrode half-cell electrochemical setup, illustrated as cross-sections, with (a) carbon and carbonate (hdcfc configuration) and (b) carbon (sofc-type dcfc configuration) loaded at the we, as well as (c) without solid carbon fuel (gaseous-fueled sofc configuration). where 1 = zirconia tube, 2 = we current collector (au), 3 = ni-ysz we layer, 4 = alumina sealant, 5 = re (pt), 6 = alumina tube/guide rod, 7 = ysz pellet, 8 = ce (pt), and 9 = ce current collector (pt). cells were heated to 800 °c in n2 (180 °c/h), and held at 800 °c for 30 min. experiments were carried out between 700 and 800 °c in mixed n2-co2 and co-co2 environments (2.3 l/h total flow rate). chronopotentiometry was performed to determine open circuit potential (ocp) by holding l. deleebeeck et al. j. electrochem. sci. eng. 5(1) (2015) 67-77 doi: 10.5599/jese.145 69 current fixed at 0 ma for 5 min. ocp was determined by combining potential measured during chronopotentiometry (galvanostatic measurement) with po2 sensor data acquired simultaneously, such that ocp values are reported as absolute values vs. pt/air. cyclic voltammetry (cv) was acquired between ± 500 mv vs. internal re (pt/co-co2, termed ‘potential difference’, δe) at 10 mv/s (3 replicate cycles). from cv measurements, current, reported in ma, was measured at a fixed overpotential (η) of 500 mv vs. ocp (pt/air). the natural logarithm of currents measured at fixed overpotential was plotted versus inverse temperature in kelvin according to the arrhenius equation, allowing the calculation of activation energies ea / ev. electrochemical impedance spectroscopy (eis) was acquired between 85,000 and 0.1 hz (6 pts/decade) with an ac perturbation of 30 mv. nyquist plots of eis data were fitted using a model circuit consisting of a series resistance (rs) in series with two resistor-constant phase element units (rq, in parallel). polarization resistance (rp) was calculated from the sum of the two resistors in series. results and discussion temperature typical cv data acquired in the presence of carbonate (li-k)2co3, coal iii_co3 and coal iii (only), i.e., without carbonate present, at 800 °c in 96-4 vol% n2-co2 is shown in figure 2(a). these cvs illustrate the various types of direct carbon/coal fuel cell (dcfc) performance investigation, including a sofc-type dcfc [1] with only carbon (coal iii) present at the we (figure 1(b)), and a hdcfc [2] with mixed carbon-carbonate (coal iii_co3) present (figure 1(a)) [14-16]. the cv acquired in molten carbonate 62-38 wt% (li-k)2co3 represents the potential cycling of the ni-ysz anode in a molten carbonate environment, and is included to illustrate the difference versus a hdcfc [17]. as seen in figure 2(a), sofc-type dcfc performance was relatively limited due the finite contact between solid carbon and the we. hdcfc performance was greater due to the extended contact between the carbon fuel and electrolyte, as expanded by the molten carbonate medium. further, the cv shape of carbonate and mixed carbon-carbonate are seen to differ significantly. with the exception of ni-ysz in molten carbonate, the chemistry in this investigation is expected to be dominated by two processes at the we: the boudouard equilibrium (reaction 1) and the electrochemical oxidation of co by o 2 (reaction 2) supplied through the ysz electrolyte or co3 2 (reaction 3) [18]. additionally, reactions occurring at the we include, amongst others, the decomposition/formation of carbonate (reaction 4), while co2 reduction (reverse of reaction 2) proceeds at the ce and re [19]. c(s) + co2 ↔ 2co (1) co + o 2 → co2 + 2e (2) co + co3 2 → 2co2 + 2e (3) co3 2 ↔ o 2 + co2 (4) current was measured at a fixed overpotential (η) of 500 mv (vs. ocp), as illustrated in figure 2(a) for coal iii_co3, between 700 and 800 °c in 96-4 n2-co2, and are plotted according to the arrhenius equation in figure 2(b). sofc-type dcfc (coal iii (only)) showed activation energy of 2.73 ev, while hdcfc (coal iii_co3) showed a significantly lower ea (1.27 ev) under identical conditions, illustrating that addition of molten carbonate significantly facilitated (lower ea) the oxidation of carbon under these conditions. the presence of the ni-ysz anode was not found to j. electrochem. sci. eng. 5(1) (2015) 67-77 co-co2 in the presence of carbon 70 70 significantly influence the ea values determined for carbon black in the hdcfc configuration, however, the magnitude of the measured current was significantly reduced [19]. figure 2. (a) cv acquired at 800 °c in 96-4 vol % n2-co2 (6 l/h total flow), plotted as a function of potential for carbonate (li-k)2co3 (∙ ∙), coal iii + co3 (coal iii_co3, -) and coal iii (only) (----) loaded at the we of a 3e half-cell. (b) arrhenius plot of coal iii_co3 (closed symbols) in 96-4 vol% n2-co2 (◊, acting as a hdcfc) and 50-50 vol% co-co2 (□), and coal iii (only) (open symbols) in 96-4 vol% n2-co2 (◊, acting as an sofc-type dcfc) and 50-50 vol% co-co2 (□). (b) l. deleebeeck et al. j. electrochem. sci. eng. 5(1) (2015) 67-77 doi: 10.5599/jese.145 71 activation energies were calculated for both systems (coal iii_co3 and coal iii (only)) in 50-50 vol% co-co2, giving similar values (0.68 and 0.72 ev, respectively), suggesting that reaction(s) were dominated by the oxidation of co in both cases. these experiments [13,18,19] do not allow the distinction between oxidants (co3 2 and o 2), such that the primary electrochemical reaction(s) may include reactions 2, 3 or both. these activation energy values are slightly lower than those determined on patterned ni (on ysz) electrodes (0.85-1.42 ev) [20], but are within the range calculated (0.43-1.64 ev), depending on the rate determining step (rds), for co oxidation on niysz [21]. previous side-by-side comparisons of co-fueled and carbon-loaded sofcs have included dry co vs. fe-loaded activated carbon over a cu/ceo2 anode between 700 and 850 °c [22] and over a ag/gdc anode ((la,sr)(ga,mg)o3-δ electrolyte) at 850 °c [23], coconut carbon-charcoal over a ag-impregnated ni-ysz anode under various sweep gases (he, 8.6 vol% co2-he, and 7.4 vol% co-he) at 750 °c [24], and 92-8 and 50-50 vol% co-co2 vs. lignite coal over a ni-ysz/ni-gdc anode between 750 and 850 °c [25]. previous studies [22-25] have been carried out in full-cell configurations. variable co2 content for each sample, temperature was fixed and content of co2 varied in mixed n2-co2 and co-co2 environments. typical results, including ocp and currents measured at fixed overpotential, in n2-co2 are shown for coal i_co3 (770 °c), coal ii_co3 (770 °c) and coal iii_co3 (755 °c) in figures 3-5. in all cases, ocp was seen to decrease as content of co2 increased, as expected, due to the inverse relationship between electrochemical potential (e(v)) and the partial pressure of co2 (pco2) in the nernst equation (equation 5) for the oxidation of co (reactions 2-3). current measured at fixed overpotential was seen to increase (coal i_co3 and coal iii_co3) or remain essentially constant (coal ii_co3) as content of co2 increased in mixed n2-co2, as we have shown previously for carbon black [13]. electrochemical performance, expressed as current at fixed overpotential, varied being coal samples according to source coal characteristics, with bituminous coal (coals i and iii) with lower degree of crystallinity (less graphitic nature) and higher oxygen to carbon content ratios generally showing higher activity. single-atmosphere half-cell performance as a function of carbon fuel characteristics is discussed in more detail in [19]. as more co2 was supplied to the carbon bed, the inverse boudouard reaction (reaction 1) produced co, which was subsequently electrochemically oxidized, generating current under load (e > ocp). activation (increased current under load) as content of co2 increased was also seen in eis data, where rp decreased (activity increased) with content of co2 in n2-co2. figure 3(d) further illustrates that rs was largely unaffected by variation in content of co2. 2 0 co co 2 prt e e ln f p           (5) in equation 5, e 0 and e are the standard and measured potentials (mv vs. pt/air), r the ideal gas constant, f is faraday’s constant, t is temperature in kelvin, and pco and pco2 are the partial pressures of co and co2, respectively, where pco + pco2 = 1 atm. it should be noted that ocp is a function of pco and pco2 in the pore volume of the ni-ysz we, which may differ significantly from the introduced gaseous atmosphere [15, 16]. especially pco will vary as a function of experimental variables, including its relative rates of formation through the boudouard reaction (reaction 1) and consumption through electrochemical oxidation (reactions 2 and 3). the boudouard reaction will depend on temperature, pco2, and the reactivity of carbon. while solid carbon is typically j. electrochem. sci. eng. 5(1) (2015) 67-77 co-co2 in the presence of carbon 72 72 ascribed an activity of one, which is assumed to remain unchanged with time, carbon reactivity will depend on its characteristics: degree of crystallinity/graphitic nature, surface area, pore volume, concentration and type of surface groups and hetero-atoms, presence of catalytic/inhibiting impurities, etc. [19]. figure 3. milled and demineralised bituminous coal (coal i_co3) at 770 °c. (a) ocp and (b) current measured at η = 500 mv in n2-co2 (◊) and co-co2 (□). (c) typical nyquist plot acquired at 770 °c in 50-50 vol% n2-co2, where points represent acquired data, curves represent the fit of the model circuit (rs-rq-rq), and frequencies are given in hz. (d) rs (open symbols) and rp (closed symbols) as a function of content of co2 in n2-co2. in (a), (b) and (d), points represent acquired data, while lines are included only as visual aids. figures 3-5 illustrate how ocp and current measured at fixed overpotential vary as a function of content of co2 in co-co2. in all cases, ocp is seen to decrease with increasing content of co2, as in mixed n2-co2 environments. decreasing ocp values with increasing co2 in mixed co-co2 is consistent with behavior previously reported in the literature for ni-ysz [6,9,26] and gdc anodes [7]. as content of co2, % content of co2, % content of co2, % l. deleebeeck et al. j. electrochem. sci. eng. 5(1) (2015) 67-77 doi: 10.5599/jese.145 73 seen in figures 3 and 5, ocp values under identical experimental conditions, such as under 100 % co2 may not necessarily coincide, this arises due to changing reactivity of the carbon bed as a function of time [19], such that local pco/pco2 are non-identical despite the same gaseous atmosphere being introduced. as shown in figure 3, current under load was seen to decrease with increasing content of co2 in co-co2 for coal i_co3. as supplied co was assumed to be the primary electrochemical reactant, decreased content of co in the supplied gas (co-co2) was expected to result in decreased activity (current at fixed overpotential), with performance dramatically reduced in the absence of supplied co (i.e. 100 % co2) [26]. however, activity in the presence of coal i_co3 remained essentially constant at > 50 % co2 in mixed co-co2. this suggested that the inclusion of co2 to the reaction gas (supplied co) allowed for the inverse boudouard reaction (reaction 1) to supply co produced inside the reaction chamber, in the coal i carbon bed. supply of internally generated co kept electrochemical activity from decreasing as externally supplied content of co was decreased. a similar effect was observed for gas-fueled sofcs h2-co mixtures, where for ph2 > 0.5 atm (ph2 + pco = 1 atm) the power density at 800 °c was seen to decrease only slightly, relative to a dry h2 (ph2 = 1 atm) system, with the inclusion of co. this was suggested to result from the in-situ (near the electrode/electrolyte interface) generation of h2 through the water-gas shift reaction between generated h2o and co (h2o + co ↔ h2 + co2) [26]. figure 4. carbonized milled and demineralised anthracite (coal ii_co3) at 770 °c. (a) ocp and (b) current measured at η = 500 mv (overpotential vs. ocp) in n2-co2 (◊) and co-co2 (□). content of co2, % content of co2, % j. electrochem. sci. eng. 5(1) (2015) 67-77 co-co2 in the presence of carbon 74 74 for coal ii_co3 at 770 °c (figure 4) and coal iii_co3 at 755 °c (figure 5), the current at fixed overpotential is seen to increase as content of co2 increases in mixed co-co2. this suggested that even when external co is supplied (mixed co-co2 environment), the enhancement from internally generated co, illustrated in mixed n2-co2 as content of co2 was increased, may be observed. this illustrates the rapid kinetics of the inverse boudouard reaction (reaction 1), which is strongly favored at temperatures > 750 °c [1]. figure 5. milled bituminous coal (coal iii _co3) at 755 °c. (a) ocp and (b) current measured at η = 500 mv (overpotential vs. ocp) in n2-co2 (◊) and co-co2 (□). the increasing cell performance (increased current measured at fixed overpotential) with increasing content of co2 in co-co2 was found to not be limited to hdcfcs. figure 6 illustrates the same effect in the absence of alkali carbonates, for coal iii (only), at 770 °c in an sofc-type dcfc configuration (figure 1(b)). further, in the absence of carbon, this effect is not seen. when content of co2 in co-co2 was increased for (li-k)2co3 alone, in the absence of a solid carbon source, current under load was seen to decrease gradually. in the absence of co (100 % co2), current decreased sharply due to fuel starvation conditions. in the absence of both carbon and carbonate, a ni-ysz we (co-fueled sofc [11], figure 1(c)) supplied with progressively lower quantities of co (increasing content of co2 in mixed co-co2) shows a decrease in activity, as expected [6,26,27]. content of co2, % content of co2, % l. deleebeeck et al. j. electrochem. sci. eng. 5(1) (2015) 67-77 doi: 10.5599/jese.145 75 interestingly, supplying carbon containing systems with 100 % co2 produces lower ocp values, but higher current at fixed overpotential values compared to supplying predominantly co (96-4 vol% co-co2) (e.g. coal iii (only), figure 6). figure 6. current measured at η = 500 mv in co-co2 at 770 °c for a ni-ysz we (×), and with milled bituminous coal (coal iii (only), ■) and (li-k)2co3 (∆) at the we. conclusions single-atmosphere 3-electrode electrochemical tests were carried out between 700 and 800 °c in mixed n2-co2 and co-co2 for a ni-ysz we in the presence of molten alkali carbonate (li-k)2co3, treated coal (coal iii), and mixed coal-carbonate, including bituminous and anthracite-type coals. treated coal (coal iii (only)) was tested in a sofc-type dcfc and mixed coal-carbonates were tested in hdcfc configurations. activation energies determined in mixed n2-co2 revealed the greater ease of reaction in a mixed carbon-carbonate system. half-cell performance was shown, as current measured at fixed overpotential (mv vs. ocp), to increase as content of co2 was increased in mixed n2-co2 and co-co2, as supplied co2 allowed the generation of co according to the inverse boudouard reaction, which was subsequently electrochemically oxidized. in the absence of carbon, with and without (co-fueled sofc) carbonates, cell activity was seen to decrease as content of co2 in mixed co-co2 was increased, i.e., as the supply of co was decreased. acknowledgements: this work was funded in part by the european commission research fund for coal and steel, as the efficient conversion of coal to electricity – direct coal fuel cells project, in collaboration with the university of st. andrews, university of western macedonia, and the spanish instituto nacional del carbón (incar). additional funding was supplied by the department of energy conversion and storage at the danish technical university 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and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://link.springer.com/article/10.1007/s10008-012-1866-5 http://www.sciencedirect.com/science/article/pii/s0925838814009840 http://www.sciencedirect.com/science/article/pii/s0378775312007513 http://dx.doi.org/10.1016/j.ijhydene.2014.05.039 http://jes.ecsdl.org/content/150/7/a942.short http://www.sciencedirect.com/science/article/pii/s0378775310018513 http://creativecommons.org/licenses/by/4.0/ {new modified mesoporous silica nanoparticles with bimetallic ni-zr for electroanalytical detection of dopamine:} http://dx.doi.org/10.5599/jese.1200 463 j. electrochem. sci. eng. 12(3) (2022) 463-474; http://dx.doi.org/10.5599/jese.1200 open access : : issn 1847-9286 www.jese-online.org original scientific paper new modified mesoporous silica nanoparticles with bimetallic ni-zr for electroanalytical detection of dopamine mohamad rafizie aiman mohamed roduan1, mohamad idris saidin1,, siti munirah sidik1, jaafar abdullah2, illyas md isa1, norhayati hashim1, mohamad syahrizal ahmad1, siti nur akmar mohd yazid1, anwar ul-hamid3 and aireen aina bahari4 1department of chemistry, faculty of science and mathematics, universiti pendidikan sultan idris, 35900 tanjong malim, perak, malaysia 2department of chemistry, faculty of science, universiti putra malaysia, 43400 seri kembangan, selangor, malaysia 3materials characterization laboratory, centre for engineering research, research institute, king fahd university of petroleum and minerals, dhahran 31261, saudi arabia 4department of english language and literature, faculty of languages and communication, universiti pendidikan sultan idris, 35900 tanjong malim, perak, malaysia corresponding author: idris.saidin@fsmt.upsi.edu.my; tel.: +6015 4879 7958 received: december 3, 2021; accepted: march 29, 2022; published: april 4, 2022 abstract in this research, bimetallic nickel-zirconia supported on mesoporous nanoparticles (nizr/msn) were successfully synthesized by a simple in situ electrolysis method. ni-zr/msn were well-characterized by fourier transform infrared spectroscopy (ftir), x-ray diffraction (xrd), x-ray photoelectron spectroscopy (xps), brunauer-emmett-teller (bet) analyzer, field emission scanning electron microscopy (fesem), and transmission electron microscopy (tem). ni-zr/msn were then cast onto a modified glassy carbon electrode (ni-zr/msn/gce) as dopamine (da) sensor. under optimal conditions, the sensor showed a linear concentration relationship in the range of 0.3 µm–0.1 mm with a limit of detection of 0.13 µm. the relative standard deviation for 0.1 mm da solution was 2.1 % (n = 5). the presence of excess catechol, saccharose, glycine, lactose, uric acid, and cr3+, fe2+ and na+ as interferents was negligible, except for uric acid in 10-fold excess. the analytical recovery of the sensor was successfully demonstrated by the determination of da in da-containing medicine and wastewater samples. the results presented herein provide new perspectives on ni-zr/msn as a potential nanomaterial in the development of da sensors. keywords porous material; nickel-zirconia; silica nanoparticles; modified gc electrode; electrochemical sensor; 3,4-dihydroxyphenethylamine http://dx.doi.org/10.5599/jese.1200 http://dx.doi.org/10.5599/jese.1200 http://www.jese-online.org/ mailto:idris.saidin@fsmt.upsi.edu.my j. electrochem. sci. eng. 12(3) (2022) 463-474 silica nanoparticles for detection of dopamine 464 introduction dopamine (da), which is an important chemical found in human bodies, helps cells to transmit impulses to the receptor other than functioning as a catecholamine neurotransmitter [1]. da helps in regulating the central nervous system and the cardiovascular system, where an abnormal da level can cause vital problems in the mammalian body [2–4]. abnormal da level often contributes to schizophrenia, parkinson's disease, attention deficit hyperactivity disorder, euphoria, and alzheimer's and huntington's diseases [1–7]. individuals suffering from mental illness are more prone to coronavirus (covid-19) when they have difficulties comprehending protective measures such as frequent handwashing, social distancing, and isolation than normal persons [8]. research conducted on 50,750 covid-19 patients in france by mohan et al. [9], found that 1.6 % had schizophrenia. the data revealed an alarming increase in the lifetime prevalence of schizophrenia in the west [9]. hence, a rapid, accurate and simple method to detect da levels is demanded. formerly, chromatography [10], fluorometry [11], chemiluminescence analysis [12], colorimetry [13], and electrochemistry [14-16] were employed to detect da. however, various researchers proved that only electrochemical techniques have simple operation and fast response, being at the same time eco-friendly, costeffective, mobile, highly sensitive and highly selective [17,18]. in electrochemical analysis, the electrode material acts as the main factor influencing the performance of the sensor. due to the special traits that certain sensors have, the traits will help enhance the electrochemical analysis for the detection of the desired element [19]. various types of electrochemical sensors have been developed to detect da, such as reduced graphene oxide (rgo) [20], gold nanoparticles/polyaniline-modified gspes (aunps@pani/gspes) [21], calcium stannate-graphitic carbon nitride nanohybrid material (cso-gcn) [22], poly(3,4-ethylenedioxythiophene) modified laser scribed graphene (pedot-lsg) [23], single-walled carbon nanotubes arraymodified glassy carbon electrode (swcnts array-gce) [24], reduced graphene oxide with manganic manganous oxide (rgo-mn3o4) [25] and polymerization-modified carbon paste electrodes [26-29]. however, these sensors have several drawbacks, such as narrow working concentration ranges and low sensitivity and selectivity. mesoporous silica nanoparticles (msn) are one of the essential elements in sensor modification, as msn has a large surface area easily doped with metals, equally sized pores and ordered structure [30]. although various attempts have been made in the development of electrochemical sensors using msn, there is a lack of studies on the use of msn modified by a bimetal as an electrochemical sensor for da detection. therefore, in this research, a newly synthesized nickel-zirconia doped with mesoporous silica nanoparticles (ni-zr/msn) was successfully fabricated as a modified gce sensor for da detection. the performance of the sensor could provide a new perspective on the modified msn as a potential nanomaterial in the field of electrochemical sensors. experimental materials and reagents all chemicals used in the synthesis of ni-zr/msn and electrochemical sensing experiments were of analytical grade (merck and sigma-aldrich) and used without further purification. ethanol, methanol, n,n-dimethylformamide, cetyltrimethylammonium bromide, ethylene glycol, ammonium hydroxide, tetraethyl orthosilicate, 3-aminopropyl triethoxysilane and disodium phosphate (na2hpo4) were purchased from merck. monosodium phosphate (nah2po4), catechol, glycine, uric acid, lactose, saccharose, chromium (iii) chloride (crcl3), iron (ii) sulfate (feso4), sodium chloride m. ra. a. mohamed roduan et al. j. electrochem. sci. eng. 12(3) (2022) 463-474 http://dx.doi.org/10.5599/jese.1200 465 (nacl), and dopamine hydrochloride were purchased from sigma-aldrich. the distilled deionized water from easypure lf, barnstead, was used to wash synthesized ni-zr/msn and prepare all analytical solutions. the supporting electrolyte was 0.1 m phosphate buffer solution (pbs) prepared by mixing stock solutions of na2hpo4 and nah2po4. apparatus cyclic voltammetry and differential pulse voltammetry were performed using a potentiostat interface 1010b (gamry, usa). three-electrode system consisting of ag/agcl (3m kcl) reference electrode mf-2052 with (bioanalytical system, usa), counter electrode of platinum wire, and nizr/msn gce as the working electrode was used in all electrochemical measurements. the ph of a solution was determined using an orion 720a glass electrode (mass., usa). the characteristics of ni-zr/msn were studied using a fourier transform infrared (ftir) spectrophotometer model cary 630 (agilent, usa), an x-ray diffraction (xrd) instrument model rigaku 600 (miniflex, japan), an xray photoelectron spectroscopy (xps) instrument model phi quantera ii (ulvac-phi, japan), energy-dispersive x-ray spectroscopy (edx) model su 8030 uhr (hitachi, japan) and a brunaueremmett-teller (bet) analyzer model tristar ii plus (micromeritics, usa). the xrd data were analyzed using rigaku smartlab studio ii software. electrochemical impedance spectroscopy (eis) measurements were carried out using a potentiostat/galvanostat model ref 3000 (gamry, usa). eis measurements were performed in the frequency range of 1.0 mhz to 1.0 hz with 5.0 mv of alternating signal amplitude. finally, the surface morphology of ni-zr/msn was characterized by a fieldemission scanning electron microscope (fesem) model su8030 (hitachi, japan) and a transmission electron microscope (tem) model jem 2100f (joel, japan). synthesis of ni-zr/msn co-condensation and sol-gel methods were implemented to synthesize msn. a mixture of cetyltrimethylammonium bromide, ethylene glycol, and ammonium hydroxide solutions was vigorously stirred for 30 min at 50 °c. then, 1.2 mmol tetraethyl orthosilicate and 1 mmol 3-aminopropyl triethoxysilane were added to the homogenous mixture. this solution was stirred for another 2 h at 80 °c and dried at 110 °c overnight. a white powder of msn was collected after being dried and calcined at 550 °c for 3 h to remove impurities. preparation of ni-zr/msn was done by in situ electrolysis method where the platinum (pt, nilaco) and zirconia/nickel plates were used as the anode and the cathode, respectively. 30 ml of n,ndimethylformamide (dmf) solution was added to tetraethylammonium perchloride (teap), naphthalene, and 1.5 g of msn. electrolysis was then performed under continuous stirring at the constant current density of 480 ma cm−2 and 0 °c under a nitrogen atmosphere. the ni-zr/msn collected at the cathode was heated at 85 °c before drying overnight at 110 °c. finally, the sample was calcined at 550 °c for 3 h. preparation of ni-zr/msn/gce modified electrode the surface of gce was first polished using an alumina slurry (0.05 µm) sequentially, washed ultrasonically in ethanol and deionized water before modification with ni-zr/msn. the ni-zr/msn (2.5, 5.0, and 7.0 mg) was mixed in 10 ml dmf solution and ultrasonicated for 30 min and 2.5 µl of the suspension was drop-casted on the gce surface. the electrode was allowed to dry at room temperature. as a result, ni-zr/msn modified gce known as ni-zr/msn/gce was obtained. similar procedures were applied for the preparation of unmodified gce (bare gce) but without the addition of ni-zr/msn. http://dx.doi.org/10.5599/jese.1200 j. electrochem. sci. eng. 12(3) (2022) 463-474 silica nanoparticles for detection of dopamine 466 results and discussion characterization of ni-zr/msn figure 1(a) shows the ftir spectrum of ni-zr/msn recorded in the range of 4000 to 400 cm−1. the absorption band at 1056 cm−1 was assigned to the asymmetric and symmetric stretching vibrations of si-o-si. the addition of bimetallic elements (e.g., ni and zr) has possibly formed interaction with si-o-si group due to desilication [31]. as shown in figure 1(b), four diffraction peaks representing nio were observed at 37.3 (003), 43.3 (012), 62.9 (104), and 75.4° (015). a small diffraction peak was also observed at 79.4° (440), attributing to the characteristic peak of ni4zro phase and possibly indicating that existence of this phase facilitates the formation of ni-zr alloy. a b c figure 1. (a) ftir spectrum, (b) xrd patterns and (c) xps spectrum of ni-zr/msn next, xps wide scan analysis, as shown in figure 1(c), the sample surface consists of ni, zr, c, si, and o with binding energies of ni2p3, zr3d, c1s, si2p, and o1s at 854, 182, 285, 103, and 532 ev, respectively. in the ni2p3 region, the spectra were fitted with a single deconvoluted peak at 855 ev, which could be assigned to metallic ni(oh)2 (ni2+) species. the zr3d spectra exhibited peaks at 182 and 183 ev for the presence of zr metallic zro2 (zr2+) and zroh (zr+) deconvolution, respectively. for the si2p spectra, only a single deconvolution occurred (i.e., sio2 (si2+)), which exhibited a peak at 103 ev. thus, the results confirm that ni and zr have been successfully loaded into the msn. this result implies a d-electron transfer between ni and zr in the ni-zr/msn, resulting in the enrichment of electrons on elemental ni [32]. figure 2 shows the morphology of ni-zr/msn as characterized by scanning electron microscopy (sem). uniformed size distribution of ni-zr/msn could be observed in the range of 25.9–68.8 nm. m. ra. a. mohamed roduan et al. j. electrochem. sci. eng. 12(3) (2022) 463-474 http://dx.doi.org/10.5599/jese.1200 467 additionally, the sem image also reveals new trigonal bipyramidal particles corresponding to a possible interaction between ni and zr, which was later confirmed by edx mapping analysis (figure 3). the analysis confirmed that ni and zr are the main components in this trigonal bipyramidal structure. figure 2. sem images of ni-zr/msn figure 3. edx-mapping of ni-zr/msn the tem image (figure 4) shows that ni and zr were doped in the ordered porous framework of spherical msn, protecting themselves from aggregation. then, the selected area electron diffraction (saed) (figure 4, inset) spot pattern shows two characteristic diffraction rings, indicating the crystalline nature of ni-zr/msn. the porosity of ni-zr/msn was identified through the bet surface area analysis. prior to the analysis, ni-zr/msn was degassed at 300 °c for 1 h to obtain more accurate results on the pore size and bet surface area. from the analysis, the langmuir surface area was 594.43 m2 g–1 and the micropore volume was 0.03559 cm3 g–1, which indicates a good pore size of ni-zr/msn, which allows bimetallic elements to be doped into it as previously reported [33]. http://dx.doi.org/10.5599/jese.1200 j. electrochem. sci. eng. 12(3) (2022) 463-474 silica nanoparticles for detection of dopamine 468 figure 4. tem image and saed pattern (inset) of ni-zr/msn electrochemical behavior of ni-zr/msn/gce the electrochemical behavior of ni-zr/msn/gce was evaluated by cv and eis analyses. the cv voltammogram of ni-zr/msn/gce in the absence and presence of 0.1mm da in 0.1 m pbs (ph 7.0) is shown in figure 5. as seen, no redox peak current is observed in the absence of da (curve a), but a significant redox peak current is observed in the presence of da (curve b). this means that the sensor shows a sensitive response in the presence of da. the comparison of cv response between ni-zr/msn/gce and bare gce in 0.1 mm da and 0.1 m pbs (ph 7.0) as a supporting electrolyte was also evaluated. as illustrated in figure 5, the cv peak current at ni-zr/msn/gce (curve b) indicated higher current reading than bare gce (curve c) due to the increase of electron transfer rate and conductivity on the electrode surface. the anodic peak current (ipa) and cathodic peak current (ipc) of bare gce were 2.649 and 2.637 µa, respectively. meanwhile, the redox peak current of nizr/msn/gce increased to 5.789 µa for ipa and 6.166 µa for ipc. the peak-to-peak separation (∆ep) of ni-zr/msn/gce also decreases to 60.00 mv compared to bare gce, which has ∆ep of 240.01 mv. therefore, the results showed that ni-zr/msn/gce has good electrocatalytic activity and good selectivity, hence improving the electron transfer rate and electrochemical response [34]. figure 5. cv voltammograms of ni-zr/msn/gce in the absence (curve a) and in presence (curve b) of 0.1 mm da in 0.1 m pbs (ph 7.0). curve c is cv voltammogram of bare gce. scan rate is 100 mv s−1 cv analysis was also conducted to study the reaction kinetics of ni-zr/msn/gce. the curves of cv responses towards da at different scan rates are plotted in figure 6a. plots of anodic and cathodic peak currents versus the scan rate as illustrated in figure 6b, showing straight lines with linear equations defined as follows: ipa (µa) = 0.0297  (mv s–1) + 6.2519 (r2 = 0.9685) and ipc (µa) = -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -0.000006 -0.000004 -0.000002 0.000000 0.000002 0.000004 0.000006 0.000008 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -6 -4 -2 0 2 4 6 8 c u r r e n t/ m a e / v (vs. ag/agcl) a b c e / v vs. ag/agcl a b c c u r r e n t, m a m. ra. a. mohamed roduan et al. j. electrochem. sci. eng. 12(3) (2022) 463-474 http://dx.doi.org/10.5599/jese.1200 469 -0.031  (mv s–1) – 5.2176 (r2 = 0.9396). it could be observed that the increasing scan rate from 90 to 400 mv s–1 resulted in the gradual potential shift of the oxidation peak toward positive values. these results suggest that the oxidation reaction of da at ni-zr/msn/gce is predominantly an adsorption-controlled process. e / v vs. ag/agcl  / mv s-1 figure 6. cv voltammograms of 0.1 mm da in 0.1 m pbs (ph 7.0) on ni-zr/msn/gce at (a) various scan rates (90-400 mv/s) and (b) corresponding plots of anodic and cathodic peak currents vs. scan rate the performance of ni-zr/msn/gce at different amounts of ni-zr/msn was also evaluated in order to determine the optimized amount of ni-zr/msn drop-casted on the gce surface. figure 7(a) shows that 0.625 µg of ni-zr/msn achieved a higher current response; hence, ni-zr/msn increased the charge transfer kinetics, thus enhancing the performance of ni-zr/msn/gce. nevertheless, further addition of ni-zr/msn (1.25 and 1.75 µg) reduced the current response, which might be the result of oversaturated modifier loading at the electrode surface. thus, 0.625 µg of ni-zr/msn was utilized as a modifier in the subsequent analysis. the interfacial properties of bare gce and ni-zr/msn/gce were evaluated through the eis nyquist plots shown in figure 7(b). figure 7. (a) cv voltammograms of 0.1 mm da in 0.1 m pbs (ph 7.0) on ni-zr/msn/gce at different quantities of ni-zr/msn; (b) nyquist plots of bare gce (curve a) and ni-zr/msn/gce (0.625 µg) (curve b). inset (top-right): enlarged nyquist plot of ni-zr/msn/gce. frequency range: 1.0 mhz to 1.0 hz. inset (bottom-right): randles equivalent circuit model used to fit impedance data http://dx.doi.org/10.5599/jese.1200 j. electrochem. sci. eng. 12(3) (2022) 463-474 silica nanoparticles for detection of dopamine 470 generally, a diameter of semicircle impedance response at higher frequencies represents the charge transfer resistance, while a linear part at lower frequencies represents the diffusion process [35]. as illustrated in figure 7b, the nyquist plot of the bare gce exhibits a larger semicircle (curve a) compared to ni-zr/msn/gce (curve b), indicating lower charge transfer resistance in the interfacial region of ni-zr/msn/gce. by fitting the randles equivalent electrical circuit (figure 7b, inset), charge transfer resistance (rct) values for bare gce and ni-zr/msn/gce were 106.4 and 0.613 k. additionally, the electron transfer apparent rate constant (kapp) values [36] calculated for bare gce and ni-zr/msn/gce were 2.50×10−8 and 4.34×10−6 cm s−1. the high kapp and low rct values for ni-zr/msn/gce indicate a fast electron transfer process facilitated by ni-zr/msn [37]. the eis results correlate with the finding of cv studies. effect of ph the performance of ni-zr/msn/gce in 0.1 mm da in 0.1 m pbs at different ph values was investigated in the range of ph 6.0–8.0 using the differential pulse voltammetry (dpv) method. the graph of ipa versus ph is shown in figure 8. the oxidation current for da reached the most optimized state at ph 7.0, which is a neutral buffer state. there was a slight improvement from ph 7.6 to 8.0, but it was still inferior to the current reading for ph 7.0. the decreased peak current beyond ph 7.0 could be attributed to the deprotonation of da, and at ph below 7.0, the decrease could be due to protonation [38]. thus, the pbs ph of 7.0 was chosen as an optimized ph for further measurements of the study. figure 8. effects of various ph on dpv peak current of 0.1 mm da in 0.1 m pbs at ni-zr/msn/gce. frequency: 100 hz; step increment: 3.0 mv; pulse size: 120 mv electrochemical performance for dopamine detection under optimal analysis conditions, the analytical curve of da was constructed from the dpv quantification carried out at different da concentrations. from figure 9(a), the peak voltammogram for da increased with the increase of da concentration in the range from 0.3 µm to 0.1 mm. the linear regression equation ipa (µa) = 28.073 da + 0.0506 with r2 of 0.9929 was obtained within this concentration range (figure 9(b)). the limit of detection of 0.13 µm was estimated using the formula of 3 sb/m, where sb is the standard deviation obtained from five measurements of the blank signal and m is the slope of the linear calibration curve. it seems that the results obtained in this work are more worthy than those reported earlier (table 1). the reproducibility of ni-zr/msn/gce was evaluated by five replicate measurements of 0.1 mm da, and ni-zr/msn/gce showed good reproducibility where the relative standard deviation of 2.1 % (n = 5) was obtained. 6.0 6.5 7.0 7.5 8.0 2.0 2.4 2.8 3.2 3.6 4.0 c u r r e n t, m a ph m. ra. a. mohamed roduan et al. j. electrochem. sci. eng. 12(3) (2022) 463-474 http://dx.doi.org/10.5599/jese.1200 471 figure 9. (a) dpv voltammograms and (b) corresponding calibration plot of 0.3 µm 0.1 mm da in 0.1 m pbs (ph 7) at ni-zr/msn/gce table 1. comparison of ni-zr/msn/gce with other reported da sensors electrode technique lod, µm linear range, µm ref. rgo-cb-cts/gce swv 0.20 3.2 32 [20] aunps@pani/gspes dpv 0.86 1 100 [21] cso-gcn/gce dpv 29 100 800 [22] pedot-lsg dpv 0.33 1 150 [23] swcnts array-gce swv 0.82 10 200 [24] ge/rgo-mn3o4/nafion-au amperometric 0.25 1 1450 [25] poly (adenine) film-cpe cv 0.67 20 190 [28] ni-zr/msn/gce dpv 0.13 0.3 100 this work interference study the possible interference for da detection was investigated by adding some organic compounds and inorganic ions that may coexist with da in real samples. the potential interferents used were catechol, saccharose, glycine, lactose, uric acid, and cr3+, fe2+, and na+ ions. under optimal conditions, dpv measurements were performed for da concentration established at 0.1 mm, while concentrations of interferents were kept at 0.1 and 1.0 mm in 0.1 m pbs (ph 7). as shown in table 2, ni-zr/msn/gce demonstrated good anti-interference behavior, except for 10-fold excess of uric acid (the criterion for interference was a ±20 % error in the peak height of da). table 2. interference of potential interferents at concentrations of 0.1 mm and 1.0 mm (ph 7) interferers interference, % da content = 0.1 mm pbs content = 1.0 mm catechol 5.98 12.59 saccharose 8.68 12.4 glycine 1.82 7.28 lactose 2.75 3.83 uric acid 10.02 25.6 na+ 1.1 4.3 fe2+ 2.31 10.63 cr3+ 2.01 12.19 recovery analysis in an attempt to test the applicability of ni-zr/msn/gce, a recovery study of the da-controlled medicine and wastewater samples were performed by the standard addition method. 1 ml of 0.01 http://dx.doi.org/10.5599/jese.1200 j. electrochem. sci. eng. 12(3) (2022) 463-474 silica nanoparticles for detection of dopamine 472 mm da-containing medicine was diluted with 0.1 m pbs and a known amount of da solution was added after the original concentration of da in the prepared sample was detected. next, wastewater samples from two different locations were collected and the residue was filtered. 10 ml of filtered wastewater samples were diluted with 0.1 m pbs and a known amount of da solution was added. table 3 shows satisfactory recovery percentages in the range between 96.0 and 107.6 %, indicating that ni-zr/msn/gce is capable for the quantification of da in real samples. table 3. detection of da in da-containing medicine and wastewater samples (n= 3) using ni-zr/msn/gce sample da content, µm recovery, % original added found da-containing medicine 10 10 20.0 ± 0.5 100.0 10 30 40.3 ± 0.2 103.1 wastewater 1 0 10 9.6 ± 0.2 96.0 0 30 29.1 ± 0.3 97.0 wastewater 2 0 10 10.3 ± 0.2 103.0 0 30 32.3 ± 0.5 107.6 conclusions the modified mesoporous silica nanoparticles (msn) with bimetallic nickel-zirconia have been successfully synthesized using electrodeposition technique. characterization by ft-ir, xrd, xps, bet, tem and fesem surface techniques confirmed the formation of a highly porous system with a uniform particle size distribution of 25.9 and 68.8 nm, and the presence of ni, zr, si and o in the prepared nizr/msn nanomaterial. different amounts of ni-zr/msn were deposited on gce and the prepared nizr/msn/gce was employed as an electrochemical sensor for da determination. the cv and eis studies of the sensor showed an excellent response and increased charge transfer rate of da oxidation at the electrode-solution interface. the sensor displayed an excellent response in the working da concentration range of 0.3 µm–0.1 mm, with a limit of detection of 0.13 µm with good reproducibility. the sensor also demonstrated good recovery for the analysis of da in da containing medicine and wastewater samples. this current work will help the scientific community to understand and realize a new perspective of modified msn in the field of electroanalytical chemistry. acknowledgment: the authors would like to thank the ministry 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.powtec.2020.07.114 https://doi.org/10.1021/acsami.1c04523 https://doi.org/10.3390/catal8050203 https://doi.org/10.3390/catal8050203 https://doi.org/10.1016/j.snb.2017.06.160 https://doi.org/10.1016/j.aca.2018.03.036 https://doi.org/10.1002/0471716243.ch2 https://doi.org/10.20964/2019.11.46 https://doi.org/10.1016/j.jelechem.2015.08.010 https://creativecommons.org/licenses/by/4.0/) inhibition of mild steel corrosion using jatropha curcas leaf extract doi: 10.5599/jese.2014.0046 67 j. electrochem. sci. eng. 4(2) (2014) 67-74; doi: 10.5599/jese.2014.0046 open access : : issn 1847-9286 www.jese-online.org original scientific paper inhibition of mild steel corrosion using jatropha curcas leaf extract olorunfemi michael ajayi, jamiu kolawole odusote*, raheem abolore yahya* department of mechanical engineering, university of ilorin, ilorin, nigeria *department of materials and metallurgical engineering, university of ilorin, ilorin, nigeria corresponding author: e-mail: ajayimichael2012@gmail.com; tel.: +2348035231400 received: august 13, 2013; revised: february 19, 2014; published: may 13, 2014 abstract jatropha curcas leaf was investigated as a green inhibitor on the degradation of mild steel in 4 m hcl and 4 m h2so4 aqueous solutions using gasometric technique. mild steel coupons of dimension 2 × 1.5 cm were immersed in test solutions of uninhibited acid and also those with extract concentrations of 4 ml, 6 ml, 8 ml and 10 ml at 30 o c, for up to 30 minutes. the results showed that as the concentration of the extract increases, there was reduction in the corrosion rate. as the extract concentration increased from 4 ml to 10 ml at 30 minutes exposure, the volume of hydrogen gas evolved decreased from 19.1 cm 3 to 11.2 cm 3 in h2so4 medium, while it reduced to 5 cm 3 from 9 cm 3 in hcl medium. also, the metal surface-phytoconstituent interaction mechanism showed that 6 minutes is the best exposure time for the adsorption of the extract in both acidic media. the jatropha curcas leaf extract was adsorbed on the mild steel surface to inhibit corrosion, while the experimental data obtained at 30 minutes exposure in both acidic media were well fitted with the langmuir adsorption isotherm. hence, jatropha curcas leaf extract is a good and safe inhibitor in both acidic solutions. keywords gasometric, inhibitor, adsorption, mild steel, langmuir isotherm introduction mild steel is a material commonly used in industries due to its low cost, availability and excellent mechanical properties [1]. however, the major drawback to its application is corrosion attack, which usually leads to structures degradation, equipment shutdown, loss of machines efficiency, and loss of valuable products, to mention but few [2]. the average corrosion cost has been reported to be about 3.5-4.5 % of the gross national product of most industrialized nations [3]. http://www.jese-online.org/ mailto:ajayimichael2012@gmail.com j. electrochem. sci. eng. 4(2) (2014) 67-74 jatropha curcas leaf extract as corrosion inhibitor 68 corrosion can be prevented in several ways but the use of inhibitors is one of the most acceptable practices. the use of synthetic inhibitors has been seriously discouraged due to its high cost, non-biodegradability and harmfulness. hence, naturally occurring compounds from plants origin have been a subject of interest for researchers because of their abundant availability, cost effectiveness and environmentally friendly [4]. several studies have been carried out on the use of these naturally occurring compounds as corrosion inhibitors for metals in different media [5-19]. jatropha curcas (jc) is a perennial, multi-purpose and drought resistant plant that belongs to the family of euphorbiaceous jc is also a tropical plant that can be grown in low to high rainfall regions [20], on both fertile and even in less fertile soil. jatropha oil, obtained by crushing the seeds is used as biodiesel fuel. the plant is planted by farmers all over the world, because it is not browsed by animals. non-toxic variety of jatropha could be a potential source of oil for human consumption and the seed cake can be a protein source for humans as well as for livestock [21]. another potential application of the leaves as corrosion inhibitor for mild steel in acidic media is established in this study. experimental procedure the chemical composition of the mild steel specimen used for this experiment in wt % is 0.17 % c, 0.21 % si, 0.55 % mn, 0.02 % p, 0.02 % s, 0.18 % cu, 0.01 % ni, 0.02 % sn and 98.81 % fe. specimens were press cut into pieces with dimension of 1.5 × 2 cm coupons. the specimens were polished using linn major struer-italy (model no. 224732) with emery papers 140/0304 – 140/0308 grades. subsequently, they were degreased in ethanol, dried in acetone and stored in desiccators. the solutions of hcl and h2so4 were prepared by using double distilled water. the fresh leaf of jatropha curcas (jc) plant was taken, washed under running water, cut into pieces, air dried and then grounded well and sieves into powdery form. then, 10 g each of the powdery leaf was put into flat bottom flask containing 200 cm 3 of 4 m hcl and h2so4 aqueous solutions. this concentration was used in order to fasten the rate of reaction between the metal surface and the acidic extract of the inhibitor within the period of the experiment. the resulting solutions were refluxed for 2 hours and left overnight before it was carefully filtered. the stock solution was prepared from the filtrate and into the desired concentrations. in this study, extract amount of 4-10 ml correspond to 0.2 g dm -3 , 0.3 g dm -3 , 0.4 g dm -3 and 0.5 g dm -3 , respectively. the gasometric assembly used for the measurement of hydrogen evolution was as reported by aisha et al. [22]. a reaction vessel was connected to a burette through a delivery tube. the 4 m hcl solution was introduced into the mylius cell, and the initial volume of air in the burette was recorded. then, mild steel coupon was dropped into the hcl solution, and the mylius cell was quickly closed. the volume of hydrogen gas evolved from the corrosion reaction was monitored by the volume change in the level of water in the burette. the change in volume was recorded every 120 seconds for up to 30 minutes. similar procedure was repeated with the inhibitor. the same experimental procedure was followed for 4 m h2so4 solution. the inhibition efficiency and the degree of surface coverage were determined using equations 1 and 2 [21]: inhibition efficiency (i.e.), % = h0 h1 h0 100 v v v   (1) surface coverage  = h0 h1 h0 v v v  (2) o. m. ajayi at al. j. electrochem. sci. eng. 4(2) (2014) 67-74 doi: 10.5599/jese.2014.0046 69 where vh0 is the volume of h2 gas evolved without inhibitor and vh1 is the volume of h2 gas evolved with inhibitor. results and discussion figure 1 shows the variation of volume of hydrogen gas evolved with time for the corrosion of mild steel in various concentrations of the inhibitor in hcl aqueous solution. as shown in the figure, the hydrogen gas was not evolved in the first 8 minutes due to slow rate of corrosion reaction at the initial stage resulting from the inability of the acidic extract to quickly penetrate the metal surface. above this exposure time, the volume of hydrogen gas evolved increased with increasing period of exposure, but decreases with increasing concentration of acidic extract of jatropha curcas leaf. the volume of hydrogen gas evolved at 30 minutes was 21.8 cm 3 for the blank solution, while that of 4 ml, 6 ml, 8 ml and 10 ml concentrations of jatropha curcas leaf extract are 9.0, 8.0, 6.8 and 5.0 cm 3 , respectively. this shows that oxide film developed faster on the surface of mild steel coupon with higher inhibitor concentration, and thus reduces the corrosion rate. the blank system having no inhibitor gave the highest hydrogen gas evolution and is far apart when compared to when varying concentration of the extract of jatropha curcas leaf was added. this may be due to the absence of inhibitor that will prevent acidic solution from reaching the metal surface [22]. presence of oxide film causes the rate of hydrogen gas evolution to decrease (i.e. decrease in the rate of corrosion) [22]. aisha et al. [21] also opined that increase in hydrogen evolution gas in the blank system may be due to direct reaction between the acid and the metal, since there is no adsorption layer to inhibit the reaction. hence, the rate of hydrogen gas evolution, that is, the corrosion rate will be faster in the blank solution as compared with the inhibited. ulaeto et al. [11] found that the leaf and root extracts of eichornia crassipe effectively inhibited the corrosion of mild steel in 5 m hcl, and that the extracts performed better at higher concentration. figure 1. variation of volume of h2 evolved with time of mild steel coupons for different volumes of jc extract in 4 m hcl solution the variation of inhibition efficiency against time of immersion with varying concentration of the inhibitor in 4 m hcl aqueous solution is shown in figure 2. the results show that from 0 to j. electrochem. sci. eng. 4(2) (2014) 67-74 jatropha curcas leaf extract as corrosion inhibitor 70 4 minutes, the inhibition efficiency was 0 %, corresponding to the latency period [23]. the corrosion rate was faster at the initial stage above 4 minutes, resulting in higher inhibition efficiency. however, at 6 minutes there was a re-ordering of the inhibition efficiencies from highest to the least value in descending order of the inhibitor concentration i.e. (10 ml < 8 ml < 6 ml < 4 ml) at all the exposure times. this revealed that there is an adsorption of the constituents of the jatropha curcas leaves extract on the surface of mild steel with 10 ml concentration of the inhibitor having the highest inhibition efficiency. the adsorption of the constituents resulted in the steady rate of corrosion (fig. 2) due to the formation of oxide film separating the metal surface from the corrosive medium. aisha et al. [21] investigated the use of plectranthus tenuifloros (sahara) plant as safe and green inhibitor of mild steel corrosion in acidic solutions and observed that as the concentration of the extract increases, the inhibition efficiency increases. this was reported to be due to the adsorption layer formed on the surface of mild steel which inhibits the rate of corrosion. it was reported by kuznetsov [23] that the longer the latency period, the higher the inhibition efficiency. figure 2. variation of inhibition efficiency with the time of immersion in 4 m hcl. figure 3 shows the variation of the volume of hydrogen gas evolved with time of exposure in sulphuric acid solution. the results revealed that the corrosion rate of mild steel as indicated by the amount of h2 gas evolved decreased in the presence of jatropha curcas leaf extract when compared to the control. the volume of hydrogen gas for blank solution was the highest as compared to those with different concentrations of jatropha curcas leaf extract. this infers that the jc leaf extract in the solution had a retarding effect on the corrosion of mild steel in h2so4. thus, the degree of inhibition can be said to be governed by the amount of jc extract present. the 10 ml concentration of the inhibitor was able to reduce the rate of hydrogen gas evolution further due to the formation of more adsorption layer on the surface of mild steel sample. the trend agrees with the result of eddy et al. [10] during the determination of the inhibition efficiency of ethanol extract of phyllanthus amarus on corrosion of mild steel in h2so4 solution. they reported that the volume of hydrogen decreased as the concentrations of phyllanthus amarus increased and the highest concentration of 0.5 g/l gave the least value of hydrogen gas evolution. o. m. ajayi at al. j. electrochem. sci. eng. 4(2) (2014) 67-74 doi: 10.5599/jese.2014.0046 71 figure 3. variation of volume of h2 evolved with time of mild steel in coupons for different volumes of jc extract in 4 m h2so4. figure 4 shows the variation of percentage inhibition efficiency with time of immersion in sulphuric acid solution. the results obtained revealed that the inhibition efficiency increases as the concentration of jatropha curcas leaf extract increases, which follow similar trends with results in hcl medium. similar observations were made when gravimetric method was used [24]. also, from 6 minutes there was a re-ordering of the inhibition efficiencies from the highest to the least value in descending order of the inhibitor (10 ml < 8 ml < 6 ml < 4 ml) for the time of immersion. the corrosion inhibition of the plant extract on the surface of mild steel may be due to presence of the phytochemical constituents such as 1.610 mg/l alkaloid, 0.672 mg/l flavonoid, 0.412 mg/l saponins, 0.124 mg/l tannins and 0.465 mg/l phenol in the extract [24]. from the plot, it can be seen that the inhibition efficiencies of varying concentrations begin to reduce after reaching the maximum at the initial or transient stage up to 6 minutes. this stage was preceded by the latency period [23]. this reduction may be due to the faster rate of corrosion resulting from breakaway of the oxide film formed from extract inhibitor adsorption into the metal surface. however, above 20 minutes exposure at all the inhibitor concentrations, the efficiencies become relatively steady due to the protective nature of the barrier film separating the metal surface from the acidic medium. figure 4. variation of inhibition efficiency with the time of immersion in 4 m h2so4. j. electrochem. sci. eng. 4(2) (2014) 67-74 jatropha curcas leaf extract as corrosion inhibitor 72 adsorption isotherm adsorption isotherms are very important in knowing the mechanism of inhibition of corrosion reaction of metals. the most frequently used adsorption isotherms are frumkin, temkin, freundlich, flory huggins and langmuir isotherms. however, only langmuir isotherms is reported in the present study, while other adsorption methods were evaluated and reported elsewhere [26,27]. langmuir gives an expression for the concentration to the degree of surface coverage ( ) according to equation 3 [28]:   ads/ 1 /c k c (3) figures 5 and 6 represent the langmuir isotherm plots of jatropha curcas leaves extract in both hcl and and h2so4 aqueous solutions, respectively, showing the variation of c/ against c at 30 minutes exposure. the plots showed that langmuir adsorption isotherm model is appropriate for the determination of the adsorption mechanism of the extract of jatropha curcas leaves in both acidic media, since the points were well fitted linearly (as indicated by the values of coefficient of correlation, r 2 , as given in table 1) at a fixed slope of 1 according to equation 3. the equilibrium constant of adsorption isotherm, kads, of the jatropha curcas leaf extract in both hcl and h2so4 media were obtained from the intercept and the results are presented in table 1. however, due to the complexity of the compounds in the extracts of leaves of j. curcas [29], it is not possible to determine the exact molecular weight of the inhibitor and hence the concentration in mol dm -3 . as a result, values of the standard free energy of adsorption (δgads) in both media could not be calculated [30]. table 1. calculated values of langmuir adsorption isotherm parameters of jatropha curcas extract in hcl and h2so4 aqueous solution at 30 minutes plant extract concentration on intercept point, g dm -3 slope kads / g dm -3 r 2 jc in 4m hcl 0.177 1.000 5.65 0.999 jc in 4m h2so4 0.206 1.000 4.85 0.995 c / g dm -3 figure 5. langmuir isotherm for the adsorption of the extract of jatropha leaves on the surface of mild steel in 4 m hcl at 30 minutes exposure. (c / ) / g d m -3 o. m. ajayi at al. j. electrochem. sci. eng. 4(2) (2014) 67-74 doi: 10.5599/jese.2014.0046 73 c / g dm -3 figure 6. langmuir isotherm for the adsorption of the extract of jatropha leaves on the surface of mild steel in 4 m h2so4 at 30 minutes exposure. plant extract contains organic compounds having polar atoms or groups which are adsorbed on the metal surface. obot and obi-egbedi [13] reported that compounds interact by mutual repulsion or attraction when ipomoea involcrata plant extract was used as an inhibitor. this may be advocated as the reason for the slight departure of the slope values from unity as explained by obot and obi-egbedi [13]. although, in this study, the slope is fixed at 1 prior to linear fitting but few points were still slightly deviated from the straight line, which may be due to mutual repulsion or attraction of the polar atoms or groups as observed by obot and obi-egbedi [13]. in addition, the adsorption of the jatropha curcas leaves extract on the mild steel surface may not involve the interaction of the adsorbate molecules with one another. according to nnanna et al. [7], it was assumed that there was no interaction between the adsorbate molecules in the derivation of langmuir isotherm. the adsorption was also assumed to be monolayer because the sites on the metal surface were taken to be energetically identical and uniformly distributed [8]. however, the adsorption process may be assumed to be due to an electrostatic interaction between the polar atoms/ions on the metal surface and the adsorbate molecules [7,29]. conclusions  the leaf extract of jatropha curcas acts as a good and efficient inhibitor for corrosion of mild steel in hcl and h2so4 solutions.  inhibition efficiencies of the jatropha curcas leaf extract in hcl medium were higher than those in h2so4 environment. after 30 minutes exposure with extract concentration of 10 ml, the efficiency is 77.1 % in hcl medium while 71.3 % was obtained in h2so4 medium.  the inhibition of the corrosion of mild steel by acid extract of jc is due to the phytochemical constituents in the plant extract.  the experimental data obtained at 30 minutes exposure in both hcl and h2so4 solutions with jatropha curcas leaf extract were well fitted with the langmuir adsorption isotherm indicating that the langmuir adsorption model is applicable in the corrosion inhibition mechanism.  further work will be carried out using other techniques with micrographs from sem to show the effect of temperature and/or ph on the corrosion efficiency of jatropha curcas leaf extract on mild steel and other materials. 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[29] e. e. ebenso, n. o. eddy and a. o. odiongenyi, afr. j. pure applied chem. 2 (2008) 107-115. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://www.cctechnologies.com/ http://www.dovebiotech.com/ http://creativecommons.org/licenses/by/3.0/ on the stability of platinum-composite electrocatalysts prepared with different substrate materials doi:10.5599/jese.269 29 j. electrochem. sci. eng. 6(1) (2016) 29-35; doi: 10.5599/jese.269 open access : : issn 1847-9286 www.jese-online.org original scientific paper on the stability of platinum-composite electrocatalysts prepared with different substrate materials * milica g. košević, gavrilo m. šekularac, vladimir v. panić institute of chemistry, technology and metallurgy, department of electrochemistry, university of belgrade, belgrade, serbia corresponding author: panic@ihtm.bg.ac.rs received: february 1, 2016; accepted: february 4, 2016 abstract cyclic voltammetry (cv) measurements were conducted and analyzed for a preliminary estimation of the stability of composite electrocatalysts based on pt. the changes in cv currents of platinum nanoparticles supported on tio2 were compared to the changes of those supported on commercial carbon. tio2 was synthesized by sol-gel method and pt was deposited from pt colloidal dispersion synthesized by microwave-assisted polyol process. it was found that pt component in both pt/tio2 and pt/c behaves similarly with respect to stability and activity during the cycling. the loss in activity with cycling was linear and strongly depended on sweep rate, i.e., the relative loss is higher at lower sweep rates. the steady state activities for both electrocatalysts were reached at the level of 65 % of initial activity and required more than 100 voltammetric cycles. keywords catalytic activity, metal colloids, pt supported on tio2, sweep rate dependent stability introduction fuel cells-related investigations are nowadays in expansion, due to their promising application as alternative energy sources [1,2]. the main focus of current research activities are directed toward fuel cell reliability and durability [1]. durability of polymer electrolyte membrane (pem) fuel cell is considerably influenced on the electrocatalytic stability of electrode materials. hence, a proper selection of nanoarchitecture and composition of membrane electrode assembly (mea), * some parts of this work have been presented at 5 th regional symposium on electrochemistry – south east europe (rsesee 5) and awarded as one of the best posters presented. http://www.jese-online.org/ mailto:panic@ihtm.bg.ac.rs j. electrochem. sci. eng. 6(1) (2016) 29-35 stability of pt-composite electrocatalysts materials 30 and particularly the electrode material, is of extreme importance. electrode material is required to be stable, economically suitable and nonpolluting [3]. platinum nanoparticles supported on carbonaceous substrates (pt/c) are widely envisaged as electrocatalysts in fuel cells [4], but carbon suffers from some disadvantages, such as low chemical inertness and modest potential window of stability [5]. hence, the development of a support alternative to carbon could be of high importance. tio2 appears to be suitable replacement, because this oxide is of good mechanical and chemical resistance toward acidic and oxidative environments [6]. pt nanoparticles could be deposited on tio2 by various methods, e.g., hydrothermal treatment [7,8], photo-assisted reduction [9] and underpotential deposition [10-12]. tio2 can influence catalytic activity of noble metal, e.g., platinum, due to hypo-d-electron configuration, which can interact with similar configuration of the noble metal [13]. there are studies showing better mea stability and activity of pt nanoparticles supported on mesoporous tio2 in comparison to commercial carbon as a support [14,15]. however, there are rather opposite findings for the influence of tio2 to pt electrocatalysis; there are papers reporting the improvement for oxygen reduction kinetics [16,17] and the activity in h2 evolution [18], but some results show that tio2-supported pt suffers from significantly lower activity in h2 and o2 reactions than pt supported on carbon [19]. hence, it could be of interest to take into consideration new approaches in composite catalyst synthesis and more detailed analysis of electrocatalytic properties of synthesized materials, able to mutually produce new findings and benefits for pt catalysts supported on materials other than carbon of rather modest properties. the aim of the present work was to synthesize tio2 as a supporting material for pt and to estimate stability and activity of prepared composite at a glance, and to compare these properties with pt supported on commercial carbon. experimental tio2 synthesis. tio2 was synthesized by forced hydrolysis of ticl3. ticl3 was added dropwise into the boiling 0.7 mol dm -3 hcl solution. during 90 min of boiling under reflux, tio2 was formed as a fine white precipitate. obtained precipitate was centrifuged and washed with water, dried and thermally treated at 400 c for 3 h in air, to remove residual chlorides [20]. pt colloid synthesis. pt colloid was synthesized by standard polyol process [21]. the mixture of ethylen glycol, which serves as reducing agent for ptcl6 2 and stabilizing agent for produced pt particles, and h2ptcl6 was stirred for 15 min. 0.1 m naoh was added to increase ph to 12 and this mixture was placed in microwave at 70 w for 1 min. pt deposition onto tio2/c support from the colloidal dispersion. pt was deposited onto tio2, as well as on commercial carbon black (vulcan xc72r, c) by following procedure. 20 mg of obtained tio2 powder (or c) was ultrasonically dispersed in 20 ml h2o for 1 h and transferred into 150 ml of 2 m h2so4. the obtained suspension was stirred for 15 min before pt colloidal dispersion was added. the stirring was continued for additional 3 h. upon filtration and rising with water, the obtained pt/tio2 (or pt/c) composite was thermally treated at 160 o c in n2 atmosphere. the composite suspension for the preparation of thin layer electrode was formed by ultrasonic treatment (40 khz, 70 w) of 3 mg of pt/tio2 (pt/c) in 1 ml h2o for 1 h. the suspension was pipetted onto glassy carbon electrode (a = 0.196 cm 2 ) and room-dried to form 0.31 mg cm -2 composite thin layer. electrochemical measurements were conducted in three-electrode cell using biologic sp-200 potentiostat. saturated calomel electrode (sce) was used as a reference electrode and platinum m.g. košević et al. j. electrochem. sci. eng. 6(1) (2016) 29-35 doi:10.5599/jese.269 31 mesh as a counter electrode in 0.1 m hclo4 electrolyte. all potentials in the paper are given in sce scale. before measurements, the cell was deaerated by bubbling the nitrogen for 15 min. stability of the samples was analyzed as a function of cycling and sweep rate. loss of activity was quantified by the relative change of voltammetric charge (q / %) in a given cycle with respect to initial one, which has been spent for hydrogen adsorption/desorption processes [22]. the charge was averaged from the data of the well-developed first peak in cathodic direction and its anodic counterpart. the particle size characterization of the tio2 was performed by dynamic light scattering (dls) on zetasizer ver. 6.20 instrument malvern instruments ltd., england. the sample for dls analysis was prepared to mimic the pt-free mixture for composite synthesis. results and discussion figures 1 and 2 show cv curves for pt/tio2 and pt/c, respectively, registered in 0.1 m hclo4 at different sweep rates. the cv currents for both pt/tio2 and pt/c decrease during cycling in a different degree, indicating the continuous loss of the activity. the loss appears differently pronounced in different potential regions and at different sweep rates. in case of pt/tio2 (fig. 1), the cv response related to hydrogen adsorption/desorption and oxide formation is better resolved at higher sweep rate (fig. 1b), which indicates more defined structure of pt particles more easily accessible to the electrolyte (those located at the outer parts of the composite catalyst layer). the loss of activity upon cycling is more pronounced for pt/tio2 than for pt/c (fig. 2), although in both cases the substantial loss takes place during the first 50-80 cycles (figs. 1a and 2, respectively). in addition, the decrease in cv currents of pt/tio2 is visible in whole region between potential cv limits (with the smallest decrease related to narrow double region), whereas the oxide formation (up to 0.6 v) and reduction in pt/c appears almost insensitive to the cycling (fig. 2). similarly, the oxide reduction region (down to 0.6 v) for pt/tio2 negligibly depends on cycling at higher sweep rate (fig. 1b). if the charge spent for hydrogen adsorption/desorption is taken as a measure of pt activity (see experimental), the overall loss of the activity in fig. 1a is 35 %, which is quite larger than the loss found in fig. 1b 20 %. this indicates also the different structure of outer-layer pt particles in comparison to those located in most inner, loose parts of composite layer. the corresponding loss for pt/c is a bit lower (fig. 2), 12 %, although the number of cycles spent is considerably lower. hence, it could be stated that pt/tio2 and pt/c are of similar characteristics upon cycling at higher sweep rates, or that outer-layer pt in pt/tio2 reaches the finely tuned structure upon ca. 150 cycles. although the two composite catalysts are prepared by the same procedure, the cv currents of pt/tio2 are considerably lower than those of pt/c. the pt loading was projected to 20 mass %. according to the pt/c cv response, the pt nanoparticles diameter, calculated on the basis of standard procedure [21], is around 7 nm. in order to check pt loading in pt/tio2 composite, spectrophotometric measurements were employed. in this procedure, composite was dissolved in the aqua regia and obtained solutions were analyzed on the uv-vis spectrophotometer and compared to the standards. it was found that pt loading is almost the same as in the pt/c composite, i.e. 19 mass %. however, the calculation of pt content from pt/tio2 cv response returns the value of ca. 3 mass %, with the assumption that pt particles of similar size are formed in pt/tio2 and pt/c due to identical preparation procedure. it follows that considerable amount of pt in pt/tio2 is not involved in cv response. this could be due to semiconductive nature of tio2. j. electrochem. sci. eng. 6(1) (2016) 29-35 stability of pt-composite electrocatalysts materials 32 namely, if there are some distinct heaps of pt particles on tio2 surface, there will not be conductive pathways toward external circuit as it is in the case carbon support. (a) (b) figure 1. characteristic cyclic voltammograms of pt supported on tio2 at sweep rates of 50 (a) and 200 mv s -1 (b), registered during continuous cycling in deaerated 0.1 m hclo4. figure 2. characteristic cyclic voltammograms of pt supported on c, registered during continuous cycling in deaerated 0.1 m hclo4 at sweep rate of 100 mv s -1 . in order to check possible morphological relationships between pt and tio2, particle size distribution (psd) of the tio2 powder suspended in the medium for pt deposition was analyzed by dynamic light scattering (dls). figure 3 shows the registered psd averaged on ten successive runs expressed as distributions by intensity and volume. dls registers the particles of ca. 400 nm and agglomerates of ca. 2.5-3 µm. the material appears mainly concentrated in agglomerates since the distribution by volume is considerably larger. on the other hand, the number of particles and m.g. košević et al. j. electrochem. sci. eng. 6(1) (2016) 29-35 doi:10.5599/jese.269 33 agglomerates are comparable since the distribution by intensity is similar (10 and 8 %, respectively). fig. 3. particle size distribution by dynamic light scattering of tio2 solid phase in the medium for pt deposition. if 400 nm-sized tio2 particles would be considered as a support to host 7 nm-sized pt particles (both ideally spherical), the monolayer of pt particles could produce around 30 mass % of pt, fairly above projected value. this indicates that the size of tio2 and pt particles are optimal for the production of composite catalyst with desired pt content. on the other hand, 3 µm-sized tio2 agglomerates are able to accommodate 4.5 mas. % of pt in full particle monolayer. it could be that pronounced agglomeration as seen in fig. 3 causes the pt hosted by 400 nm-sized tio2 to be trapped within agglomerates and hence not available for cv response. consequently, those pt particles on the surface of agglomerates are able in a high degree to create connection pathways toward gc substrate, and produce a cv response corresponding to few mass % as obtained from fig. 1. loss of activity, quantified by the voltammetric charge related to hydrogen adsorption/desorption, can be additionally analyzed as a function of a cycling and applied sweep rate. the relative changes in charge, i.e., catalyst activity, is presented in fig. 4. pt/tio2 loses 10-13 % of initial activity in first 40 cycles, which appears only slightly dependent on sweep rate. the initial decrease of ca. 8 % is registered at 50 mv s -1 , whereas additional 20 cycles at 200 mv s -1 produces further loss of ca. 5 %. this slowing down of the loss by the increase in sweep rate indicate that related transformations of pt particles are sweep rate-dependent. it could be that the transformations are of more pronounced reversibility at higher sweep rates (e.g., reverse coarsening) [22] or that pt particles from the inner part of a layer reach the final state of transformations much easier than the particles from the outer part of a layer. these effects could cause also the differences in cv responses from fig. 1a and b, in which the hydrogen adsorption/desorption region is less pronounced at lower (fig. 1a) than at higher sweep rate (fig. 1b). j. electrochem. sci. eng. 6(1) (2016) 29-35 stability of pt-composite electrocatalysts materials 34 figure 4. the relative changes in voltammetric charge of hydrogen adsorption/desorption for pt/tio2 and pt/c during the cycling. in order to check the validity of the differences in charge at different sweep rate, the cv responses at 50 and 200 mv s -1 are compared after 60th and 150th cycle. indeed, the loss of the activity at 50 mv s -1 is more than twice of that registered at 200 mv s -1 after 60 cycles. as the pt reaches the stable transformation state during next 60-90 cycles at 200 mv s -1 , this difference becomes considerably less pronounced (the losses of activity at 50 and 200 mv s -1 are c.a. 35 and 28 %, respectively). in addition, this feature is checked also for pt/c after 80 cycles; the result is similar and even more pronouncedthan for pt/tio2: around 12 % loss at 100 mv s -1 and 35 % at 50 mv s -1 . these findings are in fair accordance to the suppositions of sweep rate-sensitive reversed transformations and their rate distribution throughout composite layer. the reported results indicate that pt component in both pt/tio2 and pt/c behaves similarly with respect to stability and activity during the cycling. the loss of activity is linear upon cycling and appears strongly dependent on sweep rate. the steady state transformations of pt is reached at the level of 65 % of activity with respect to initial state and require more than 100 voltammetric cycles. this behavior appears related to sweep rate-dependent reverse transformations, probably coarsening, and distribution of such transformations through the composite layer in a way that pt particles from inner part of a layer are transformed much easier and faster. conclusion electrocatalytic activity and stability of pt supported on tio2 and c were examined and compared on the basis of routine cyclic voltammetry measurements at various sweep rates. platinum was synthesized by polyol process and deposited on sol-gel synthesized tio2 and commercial carbon. the loss of activity of pt particles was quantified by the voltammetric charge related to hydrogen adsorption/desorption. the results showed that the pt particles behave in similar manner for two different supporting materials. steady state activities were reached after 100 cycles, when activity decreased by 35 % with the respect to the initial activity. the loss of m.g. košević et al. j. electrochem. sci. eng. 6(1) (2016) 29-35 doi:10.5599/jese.269 35 activity is strongly dependent on sweep rate and tends to be higher at lower sweep rates. this is probably due to less reversible pt particles transformations at lower sweep rates, including agglomeration and coarsening. acknowledgement: this work was financially supported by the ministry of education, science and technological development of the republic of serbia. the authors thank sanja stevanović and dušan tripković of the institute of chemistry, technology and metallurgy, university of belgrade, for a fruitful assistance in collecting the data related to the cyclic voltammetry behavior of pt/c catalyst. literature: [1] y. shao, g. yin, y. i. gao, journal of power sources 171 (2007) 558–566 [2] a. b. stambouli, e. traversa, renewable and sustainable energy reviews 6 (2002) 297–306 [3] s. hadži jordanov, p. paunović, o. popovski, a. dimitrov, d. slavkov, bulletin of the chemists and technologists of macedonia 23 (2004) 101–112 [4] s. zhang, x-zi yuan, j. n. c. hin, h. wang, k. a. friedrich, m. schulze, journal of power sources 194 (2009) 588-600 [5] d. s. kim, e. f. a. zeid, y.t. kim, electrochimica acta 55 (2010) 3628–3633 [6] z. liu, j. zhang, b. han, j. du, t. mu, y. wang, z. sun, microporous and mesoporous materials 81 (2005) 169–174 [7] j. yu, l. qi, m. jaroniec, journal of physical chemistry c 114 (2010) 13118–13125 [8] s. c. colindres, j. r. v. garcía, j. a. t. antonio, c. a. chavez, journal of alloys and compounds 483 (2009) 406–409 [9] h. schulz, l. mädler, r. strobel, r. jossen, s. e. pratsinis, t. johannessen, journal of materials research 20 (2005) 2568–2577 [10] s. gan, y. liang, d. r. baer, m. r. sievers, g. s. herman, c. h. f. peden, journal of physical chemistry b 105 (2001) 2412–2416 [11] b. sun, a. v. vorontsov, p. g. smirniotis, langmuir 19 (2003) 3151–3156 [12] j.-m. herrmann, j. disdier, p. pichat, the journal of physical chemistry 90 (1986) 6028– 6034 [13] n. rajalakshmi, n. lakshmi, k.s. dhathathreyan, international journal of hydrogen energy 33 (2008) 7521–7526 [14] y. p. g. chua, g. t. k. k. gunasooriya, m. saeys, e. g. seebauer, journal of catalysis 311 (2014) 306 –313 [15] g. p. lópez, r. r. lópez, t. viveros, catalysis today 220–222 (2014) 61–65 [16] a. bauer, k. lee, c.j. song, y.s. xie, j.j. zhang, r. hui, journal of power sources 195 (2010) 3105– 3110 [17] b. hammer, j.k. norskov, advances in catalysis 45 (2000) 71–129 [18] q. du, j. wu, h. yang, acs catalysis 4 (2014) 144–151 [19] h. zhao, y. wang, q. tang, l. wang, h. zhang, c. quan, tao q, international journal of hydrogen energy 39 (2014) 9621-9627 [20] j. croy, s. mostafa, j. liu, y. sohn, b. r. cuenya, catalysis letters 118 (2007) 1–7. [21] x. x. wang, z. h. tan, m. zeng, j. n. wang, scentific reports 4 (2014) 4437 [22] a. pozio, m. de francesco, a. cemmi, f. cardellini, l. giorgi, journal of power sources 105 (2002) 13–19 [23] m. s. bootharaju, v. m. burlakov, t. m. d. besong, c. p. joshi, l. g. abdulhalim, d. m. black, r. l. whetten, a. goriely, o. m. bakr, chemistry of materails 27 (2015) 4289–4297 © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.ncbi.nlm.nih.gov/pubmed/?term=wang%20xx%5bauth%5d http://www.ncbi.nlm.nih.gov/pubmed/?term=tan%20zh%5bauth%5d http://www.ncbi.nlm.nih.gov/pubmed/?term=zeng%20m%5bauth%5d http://www.ncbi.nlm.nih.gov/pubmed/?term=wang%20jn%5bauth%5d http://creativecommons.org/licenses/by/4.0/ {voltammetric growth of tin oxides in borate solution of ph 8.7} doi:10.5599/jese.377 65 j. electrochem. sci. eng. 7(2) (2017) 65-76; doi: 10.5599/jese.377 open access : : issn 1847-9286 www.jese-online.org original scientific paper voltammetric growth of tin oxides in borate solution of ph 8.7 tiago brandão costa1,, tania maria cavalcanti nogueira1,2, ladário da silva1,3 1programa de pós-graduação em engenharia metalúrgica (ppgem), escola de engenharia industrial metalúrgica de volta redonda (eeimvr), universidade federal fluminense (uff), 27255-125 volta redonda, rj, brazil 2departamento de engenharia metalúrgica, eeimvr, (uff), 27255-125 volta redonda, rj, brazil 3departamento de física, instituto de ciências exatas (icex), (uff), 27213-145 volta redonda, rj, brazil corresponding author: tiagobrandao@id.uff.br, phone: +55-2421073731 received: february 22, 2017; revised: may 22, 2017; accepted: may 23, 2017 abstract voltammetry has been employed to study the growth of tin oxides in buffer solution of 0.3 mol l-1 h3bo3 + 0.15 mol l-1 na2b4o7·10h2o (ph 8.7). voltammetric data were compared with the results of tin in a phosphate solution of ph 8.7, presented in the previous work, in order to study the influence of these anions on the growth of tin oxides. the thicknesses of grown oxides were determined using ex-situ ellipsometric technique and the volume per charge unity of the film, vf, was calculated for different charge densities of the film. the results showed that less dense films were obtained at higher sweep rates. tin oxide films grown in phosphate solution at 2 mv s-1 were denser than those grown in borate solution at the same sweep rate. the kinetic parameters, determined applying the ohmic model, showed that there are no significant differences between the kinetics at the metal/film interface of tin in borate and tin in phosphate solutions. despite these facts, the ionic specific resistivity for oxide film growth in borate solution were significantly higher than in phosphate solution. this result indicates that incorporation of anions occurs during the growth of the films. keywords ellipsometry; tin passivation; ohmic model; voltammetry; variable ionic resistivity introduction many studies concerning the composition of tin oxides films grown by voltammetry in borate and phosphate solutions in a ph range of 5 to 9 are found in literature. the results showed that the composition of tin passivating film, in terms of sn(ii) and sn(iv) oxides or hydroxides, depends on the growing anodic potential and different anions in solution. however, these studies do not give information about properties of tin oxides such as density and ionic resistivity [1 -11]. http://www.jese-online.org/ mailto:tiagobrandao@id.uff.br j. electrochem. sci. eng. 7(2) (2017) 65-76 growth of tin oxides 66 in our recent publication [12] we applied the ohmic model to describe the induced growth of tin oxides by voltammetry in phosphate buffer solution ph 8.7. the thicknesses of the grown oxides were measured by ex-situ ellipsometry to determine the film volume per charge unit, vf, for different charge densities of the film. this parameter makes it possible to calculate the variable ionic resistivity of the film, ρf, during the voltammetric oxide growth, described by the ohmic model. tin oxide films grown at 2 mv s-1 become denser for higher values of charge density, near 50 c m-2, having vf of 0.5×10-10 m3 c-1. at the same time, the representation of ρf vs. charge density of the film presents a minimum. this behavior was also found by other authors in the cases of zn, nb, ni and galvanized steel sheets [12-17]. the ohmic model developed by d’alkaine [12-17] presented in earlier work, describes the relation between the current density and the film overpotential during the growth of passivating film under voltammetric conditions. the following equation should be valid for peak or plateau transient conditions during voltammetric growth of a film: pf, p pf, q j v  (1) in eq. (1), ηf.p is the overpotential across the film at the voltammetric peak, v is the sweep rate, jp is the current density at the peak and qf,p is the peak or plateau charge density. the qf,p values can be determined considering that :   e ei qej v qqq 0a0voltf d 1 (2) in eq. (2), q0 is the charge density related to the amount of film initially present at the beginning of the voltammetric growth and qvolt is the charge density related to the amount of film which has grown on the metal surface during the voltammetric experiment. ei is the initial potential, e is the potential attained and ja is anodic component of the current density. the thickness of the growing film, ℓ, is given by: ffqv (3) in eq. (3), vf and qf are respectively the volume per charge unit and the charge density related to the growing film. the relation between the current density, j, and ηf, even at high fields, is given by: jqv ff f f    (4) in eq. (4), ρf is the average ionic specific resistivity of the film. in order to calculate the values of ρf using eq. (4), many authors [12-17] applied the ohmic model, considering the volume per charge unit (vf) as the constant equal to: nf m v f (5) in eq. (5), m is the molar mass of the film, δ is density of the film, n is the number of electrons and f is the faraday’s constant. in fact, the real values of vf during the voltammetric growth could only be determined by independent measurements of the film thickness. a lot of techniques are available to access such information. generally, it can be measured [19] or calculated [20]. ellipsometry is an interesting t. b. costa et al. j. electrochem. sci. eng. 7(2) (2017) 65-76 doi:10.5599/jese.377 67 technique to be considered for the thickness measurement [21]. ellipsometry is an indirect technique, which allows one to access layer thickness in ex-situ [12,22], and in-situ [19,20,23] experiments, which is the main interest in voltammetric measurements. in an in-situ experiment it can also monitor layer growth [19,20]. as ellipsometry is an indirect technique, it is important to match the measured quantities, i.e., tan and cos, with their counterparts in the adopted model [21,24]. the measured quantities are functions of the various layers and substrate refraction indexes and extinction coefficients [21]. the aim of the present work is to study the voltammetric growth of tin oxides in borate solutions (ph 8.7) applying the ohmic model. the thickness of the oxides grown by voltammetry will be determined by ex-situ ellipsometric measurements in order to determine experimental values of vf. the results will be compared with those obtained in previous work, in the case of tin in phosphate solution ph 8.7 [12], to verify the influence of these anions in the voltammetric growth of tin oxides. experimental electrochemical measurements the working electrode was made of a tin disc (pine, 99.99 % purity) with circular area of 0.5 cm2. before the experiments, the electrode surface was polished with 600-emery paper. the electrochemical experiments were performed using the eg&g princeton applied research model 273a potentiostat. solution of 0.3 mol l-1 h3bo3 + 0.15 mol l-1 na2b4o7·10h2o (ph 8.7) was prepared from ar chemicals (merck) and purified water (millipore q system). the experiments were carried out in a conventional three-compartment electrolysis cell, using a platinum wire as the counter and hg/hg2cl2/kcl (1 m) as the reference electrode, respectively. the solution was kept at room temperature (22 °c). all current and charge densities are given in terms of the geometric surface area of the analyzed samples. anodic voltammetries were carried out at sweep rates of 2, 5, 10, 20, 70, 100, 150, 200, 250 and 300 mv s-1 always on the same surface. before each voltammetry experiment, the previously grown oxide film was reduced at constant cathodic potential equal to -1.2 v during 600 s. after this treatment, the obtained voltammograms were reproducible, indicating that surface roughness of samples was recovered. ellipsometric measurements the ex-situ ellipsometric measurements were made using a semilab spectroscopic ellipsometer, model sopra ges 5e, equipped with a xe lamp, over the spectrum range of 195 – 1,000 nm. the measurements were made in air and at room temperature (roughly 22 °c) with incident angle of 75°. the thicknesses of the oxide films were obtained by analyzing the measured ellipsometric spectra through the drude and gauss model [16]. results and discussion figure 1 presents the voltammogram for tin in the borate solution. a cathodic peak (c1) is related to the reduction of the tin oxide. in the anodic scan, two anodic peaks (a1 and a2) appear related to the tin oxides growth. at near 1.5 v the oxygen evolution process begins. in view of this result, the potential of – 1.2 v was chosen to reduce the previously grown oxide film before each voltammetric experiment, as explained in the experimental section. j. electrochem. sci. eng. 7(2) (2017) 65-76 growth of tin oxides 68 figure 1. potentiodynamic curve for tin in solution borate, ph 8.7, at sweep rate 100 mv s-1. figure 2 presents the anodic voltammograms for tin oxide growth at different sweep rates. the overpotential values in the film at peak condition (ηf,p) were calculated using the eq. (1). the values of qvolt and jp were determined from the voltammograms. the value of q0 in eq. (2) was first considered to be equal to 0.0 c m-2. the curve j vs. potential at the metal/film interface (em/f), also shown in this figure, is obtained after correction of the ohmic drop through the film at the peak potential (ep ηf,p). figure 2. voltammetric growth of tin oxide film in borate solution at different scan rates, together with the plot of the calculated j vs. (ep – ηf,p ) relation at the metal/film interface, considering q0 = 0.0 c m-2 (■)ep – ηf,p the tafel plot of the curve j vs. em/f in figure 2 is represented in figure 3. in figure 4, the value of q0 was taken equal to 0.8 c m-2, in order to provide the best straight line region of the tafel plot giving a tafel slope (ba) equal to 41.14 mv dec-1. by using eq. (6) [25]: t. b. costa et al. j. electrochem. sci. eng. 7(2) (2017) 65-76 doi:10.5599/jese.377 69 rt nf b m/f a   (6) figure 3. tafel plot at tin/oxide interface in borate solution. in eq. (6), αm/f is the transfer coefficient, n is the number of electrons, f is faraday’s constant, r is gas constant and t is temperature. the product (αm/f n ) at the metal/film interface turned to be 1.05. figure 4. tafel plot at tin/oxide interface in borate solution, considering q0 = 0.8 c m-2. the value of 𝑗m/f 0 , obtained as shown in figure 4, is equal to 0.60 a m-2. these values should be compared with 1.07 and 1.0 a m-2, respectively, obtained in the case of phosphate solution [12]. these results suggest that there are no significant differences between the kinetics in the two studied metal /film interfaces. from the tafel plot of figure 4 the corrected curve j vs. em/f, was obtained. figure 5 illustrates these results. the anodic voltammograms are also shown in the same figure. figure 6 present the variation of the peak charge density up to the peak potential (qf,p) with v. j. electrochem. sci. eng. 7(2) (2017) 65-76 growth of tin oxides 70 figure 5. voltammetric growths of tin oxide film in borate solution at different scan rates, together with the plot of the calculated j vs. (e – ηf,p) relation at the metal/film interface, considering q0 = 0.8 c m-2 and figure 4. (■)ep – ηf,p figure 6. peak charge density up to the peak potential (qf,p) vs. the sweep rate. this result shows that the charge required for film growth up to peak potential becomes constant for sweep rates higher than 0.10 v s-1. according to d’alkaine [13] this means that the parameters related to the film (aging phenomenon) become independent of the growing conditions for sweep rates higher than 0.10 v s-1. in view of this fact, the variable ionic resistivities were studied for 2 and 100 mv s-1. figure 5 can be used to determine the overpotential at the film (ηf) for any growing condition of the film beyond the peak condition, by calculating the difference of the potential in the voltammogram at any sweep rate and the potential in the curve j vs. em/f for a given current density. this is shown in figure 5 for the voltammograms at 250 and 300 mv s-1. t. b. costa et al. j. electrochem. sci. eng. 7(2) (2017) 65-76 doi:10.5599/jese.377 71 by calculating ηf values for each potential, it is possible to determine the ionic specific resistivity of the film (ρf) and its variation with e and v, for each voltammetric curve using eq. (4). the real values of vf during the voltammetric growth can be determined by an independent way of measuring the thickness of the film according to the following equation: f f q a v   (7) in eq. (7), ℓ is thickness and a is surface area the working electrode. in the present work, the thicknesses of oxides films grown at 2 and 100 mv s-1 were measured by ex-situ ellipsometry. figure 7 and figure 8 present the final potential (ef) of each voltammetry experiment performed at 2 and 100 mv s-1, respectively. figure 7. voltammetric growth of tin oxide films in borate solution, at 2 mv s-1, together with indication of the final potential, ef, and corresponding film charge density values. as pointed out before, spectroscopic ellipsometer was used to obtain thicknesses of tin oxide films grown at distinct values of charge density. measured and modelled values of tan  and cos  vs. wavelength presented good fitting parameters with for r2  0.97. the average thicknesses obtained for charge densities grown in borate solution, at 2 and at 100 mv s-1 are shown in table 1 and table 2, respectively. in table 1 and table 2, the values of the final potential, ef, charge density of the film, qf, r2, standard deviations of the average thickness values and standard deviations (). data in table 1 clearly show that for greater ef and qf, greater average film thickness is obtained. table 1. final potentials at 2 mv s-1, charge densities, thicknesses, average thicknesses and statistical results of calculations ef / v qf / c m-2 ℓ / nm r2 average thicknesses, nm  -0.80 5.5 3.1 3.3 3.5 3.3 3.3 4.2 3.4 3.3 0.971 3.43 0.311 0.60 7.8 5.0 5.0 4.2 5.0 4.4 5.1 5.0 4.4 0.992 4.76 0.338 0.30 35.8 5.6 5.6 5.8 5.8 5.8 5.5 5.7 5.9 0.991 5.71 0.127 j. electrochem. sci. eng. 7(2) (2017) 65-76 growth of tin oxides 72 figure 8. voltammetric growth of tin oxide films in borate solution, at 100 mv s-1, together with indication of the final potential, ef, and corresponding film charge density values. table 2. final potentials at 100 mv s-1, charge densities, thicknesses, average thicknesses and statistical results of calculations ef / v qf / c m-2 ℓ / nm r2 average thicknesses, nm  -0.75 4.4 4.4 4.3 4.8 4.2 4.8 4.0 4.2 4.3 0.992 4.35 0.274 0.60 10.4 6.4 7.3 6.6 7.3 6.6 7.3 6.3 7.5 0.983 6.90 0.455 0.30 23.2 7.1 7.6 7.5 7.3 7.3 7.5 7.3 7.5 0.980 7.40 0.154 0.75 53.8 9.0 8.6 9.4 8.7 9.0 8.1 9.4 8.1 0.988 8.79 0.478 figure 9 presents the average thicknesses obtained for charge densities grown at 2 and 100 mv s-1. the results for tin in phosphate solution at 2 mv s-1 are also presented in this same figure. it is possible to observe that the values of thickness of tin oxides grown at 100 mv s-1 in borate solution are higher than those grown at 2 mv s-1. this fact shows that less dense films are obtained at higher sweep rates. in comparison with the results of tin in phosphate solution at 2 mv s-1, it is interesting to notice that the films grown in this last solution are denser. using the experimentally found values of average thicknesses, eq. (7) was applied and vf was determined for the values of charge density of voltammetry grown films at 2 and 100 mv s-1. the results are shown in table 3. table 3. film volume per charge unit obtained from the thicknesses measurements of tin oxide films for different charge densities and sweep rates. 2 mv s-1 100 mv s-1 qf / c m-2 ℓ / nm vf / 10-10 m3 c-1 qf / c m-2 ℓ / nm vf / 10-10 m3 c-1 5.5 3.43 6.24 4.4 4.35 9.89 7.8 4.76 6.10 10.4 6.90 6.63 35.8 5.71 1.59 23.2 7.40 3.19 53.8 8.79 1.63 figure 10 presents the values of vf vs. charge density of the film obtained in borate solution, together with the values of vf obtained in phosphate solution [12] and the values of vf calculated by eq. (7), considering sno and sno2. t. b. costa et al. j. electrochem. sci. eng. 7(2) (2017) 65-76 doi:10.5599/jese.377 73 figure 9. thickness vs. charge density of the films grown in borate solution at (■) 2, (●) 100 mv s-1 and in phosphate solution at (▲) 2 mv s-1 figure 10. film volume per charge unit for different values of charge density. (■) experimental vf for tin oxide grown at 2 mv s-1 in borate solution, (●) experimental vf for tin oxide grown at 100 mv s-1 in borate solution, (▲)experimental vf for tin oxide grown at 2 mv s-1 in phosphate solution, (---) vf = 1.08×10-10 m3 c-1 (considering sno) and (─) vf =0.568×10-10 m3 c-1 (considering sno2). data in table 3 and figure 10 show that, for borate and phosphate [12] solutions, the values of vf significantly decrease until 50 c m-2 of charge density of the film is achieved. this result shows that the film is less dense for lower values of charge density and becomes denser as the thickness increases. this behavior suggests that changes in the composition of the film are taking place as the potential turns more anodic, approximating the values of vf considering sno and sno2. the influence of the final potential in changes in composition of the film, in terms of sn(ii) and sn(iv), was already mentioned by other authors [1–11]. by introducing experimentally determined vf values into eq. (4) the values of ρf can be calculated. figure 11 illustrates the results obtained using the values of charge density in table 3 for the case of the voltammetry experiment at 2 and 100 mv s-1 in borate solution. as can be observed in the case of sweep rate 2 mv s-1, ρf passes through a minimum (justifying the maximum in current densities). this behavior was also found in previous work [1] and in voltammetric j. electrochem. sci. eng. 7(2) (2017) 65-76 growth of tin oxides 74 oxide film growth of zn, nb, ni and galvanized steel sheets [1317]. according to the theory this happens because the passage of current in the film of initial thickness (q0) generates injection of specific defects, what resulted in decrease of ρf (inversely proportional to concentration of defects) [1317]. figure 11. ionic specific resistivity vs. charge density of the film for sweep rate equal to 2 mv s-1 (■) and 100 mv s-1(●) in borate solution. increase of recombining specific defects (interstitial and cationic vacancies) in the film ends up by generating the recombination reaction (interstitial cation + cationic vacancy → cationic net), and making ρf to increase again [1317]. in the case of the voltammetry at 100 mv s-1 the values of ρf are lower than the values of ρf in the voltammetry at 2 mv s-1. this can be explained considering that at higher sweep rates the injection of defects occurs faster than their recombination. it is also interesting to point out that this phenomenon generates less dense films, as can be observed in figure 9. figure 12 presents the values represented in figure 11 together with the values of ρf vs. charge density of the film found in previous work in the case of tin in phosphate solution for sweep rate equal to 2 mv s-1. figure 12. ionic specific resistivity vs. charge density of the film for sweep rate equal to 2 mv s-1, for tin in borate solution (○) and for tin in phosphate solution (●). t. b. costa et al. j. electrochem. sci. eng. 7(2) (2017) 65-76 doi:10.5599/jese.377 75 it is interesting to notice that the ionic specific resistivities for oxide film grown in borate solution are significantly higher than in phosphate solution. this result indicates that incorporation of anions probably occurs in the films during their growth leading to changes in ionic specific resistivities. considering eq. (4), it is possible to verify that this difference is a consequence of higher ηf and lower j values in the voltammetry in borate solution. differences in film densities and vf, in the two studied solutions, do not significantly determine the different values of ionic specific resistivities. conclusions the application of the ohmic model and ellipsometric measurements in the voltammetric growth of tin oxides in borate solution ph 8.7 turned possible to verify that: • there are no significantly differences between the kinetics in the metal/film interface of tin in borate and tin in phosphate solutions. this conclusion was supported by the obtained values of 𝑗m/f 0 and α in both cases. • the values of vf significantly decrease until 50 c m-2 of charge density of the film is achieved approximating the values of vf considering sno and sno2. this result shows that the film is less dense for lower values of charge density and becomes denser as the thickness increases. • the ionic specific resistivities for oxide film grown in borate solution are significantly higher than in phosphate solution at 2 mv s-1. this result indicates that incorporation of anions probably occurs in the films during their growth, which leads to changes in ionic specific resistiveties, contributing to different values of ηf and consequently different voltammograms. acknowledgements: the author (tiago brandão costa) is grateful to capes for his doctor fellowship. the authors are grateful to programa de pós-graduação em engenharia metalúrgica (ppgem), escola de engenharia industrial metalúrgica de volta redonda (eeimvr), universidade federal fluminense (uff) for the opportunity of realizing this work, making the laboratory of electrochemical available. notation a electrode surface area, m2 ba tafel slope e potential, v ef flade potential, v em/f potential at the metal/film interface, v f faraday’s constant, c mol-1 j current density, a m-2 jp current density at the voltammetric peak, a m-2 0 m/fj exchange current density at zero m/f, a m -2 ℓ thickness of oxide layer, m m molar mass of the film, kg n number of electrons qf charge density of the film, c m-2 q0 charge density related to the amount of film initially present, at the beginning of the voltammetric growth on the metal surface, c m-2 qvolt charge density related to the amount of film which has grown during voltammetric experiment, c cm-2 qf,p peak or plateau charge density, c cm-2 r gas constant, 8.314 j/mol k se spectroscopic ellipsometry t temperature, k vf volume per charge unit, m3 c-1 αa anodic transfer coefficient j. electrochem. sci. eng. 7(2) (2017) 65-76 growth of tin oxides 76 αc cathodic transfer coefficient δ density of the film, kg m-3 ηm/f overpotential at the metal/film interface ηf.p overpotential across the film at the voltammetric peak ηf overpotential across the film ν sweep rate, v s-1 ρf ionic specific resistivity of the film, m references [1] h. do duc, p. tissot, corrosion science 19 (1979) 179-190. [2] s. d. kapusta, n. hackerman, electrochimica acta 25 (1980) 1625-1639. [3] s. d. kapusta, n. hackerman, electrochimica acta 25 (1980) 949-955. [4] s. d. kapusta, n. hackerman, electrochimica acta 25 (1980) 1001-1006. [5] s. d. kapusta, n. hackerman, electrochimica acta 25 (1982) 1886-1889. [6] a. ammar, s. darwish, m. w. khalil, s. el-taher, electrochimica acta 33 (1988) 231-238. [7] m. metikos-hukovic, m. seruga, f. ferina, ber. bunsenges phys. chem 96 (1992) 799-805. [8] m. metikos-hukovic, a. resetic, v. gvozdic, electrochimica acta 40 (1995) 1777-1779. [9] c. a. gervasi, p. e. alvarez, corrosion science 47 (2005) 69 -78. [10] v. brunetti, m. l. teijelo, journal of electroanalytical chemistry 613 (2008) 9-15. [11] n. a. al-mobarak, chemistry and technology of fuels and oils 48 (2012) 321-330. [12] t. b. costa, t. m. c. nogueira, l. silva, journal of electrochemical science and engineering. 6 (2016) 303-314. [13] c. v. d’alkaine, p. c. tulio, m. a. c. berton, electrochimica acta 49 (2004) 1989-1997. [14] c. v. d’alkaine, l. m. n. souza, f. c. nart, corrosion science 34 (1993) 129-149. [15] c. v. d’alkaine, m. n. boucherit, journal of the electrochemical society 10 (1997) 33313336. [16] c. v. d’alkaine, m. a. santanna, journal of electroanalytical chemistry 457 (1998) 13-21. [17] t. b. costa, c. v. d’alkaine, t. m. c. nogueira, 67th abm international congress, rio de janeiro, brazil, 2012, p. 3162. [18] r. díaz, i. díez-pérez et al, journal of brazilian chemical society 14 (2003) 523-529. [19] m. hernández ubeda, m.a.pérez et al, journal of the electrochemical society 152(1) (2005) a37-a41. [20] z. szklarska-smialowska, w. kozlowski, journal of the electrochemical society 131 (1984) 499-505. [21] h. fujiwara, spectroscopic ellipsometry: principles and applications. john wiley & sons ltd, tokyo, japan, 2003. [22] l. f. n. guedes et al, journal of solid state electrochemistry 20 (2016) 2517-2523. [23] w. kozlowski, j. flis, corrosion science 32 (1991) 861-875. [24] d. e. aspnes, thin solid films 571(3) (2014) 334-344. [25] a. j. bard and l. r. faulkner, electrochemical methods fundamentals and applications, john wiley & sons, new york, united states, 2001. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.sciencedirect.com/science/journal/00406090/571/supp/p3 http://creativecommons.org/licenses/by/4.0/) {p(o-chlorophenol–co-o-hydroxyphenol): kinetic formation studies and ph-sensor application} doi:10.5599/jese.330 11 j. electrochem. sci. eng. 7(1) (2017) 11-26; doi: 10.5599/jese.330 open access : : issn 1847-9286 www.jese-online.org original scientific paper p(o-chlorophenol–co-o-hydroxyphenol): kinetic formation studies and ph-sensor application said m. sayyah1, sayed s. abd-elrehim2, rehab e. azooz2,3,, fatma mohamed1 1polymer research laboratory, chemistry department, faculty of science, beni-suef university 62514, beni-suef city, egypt 2chemistry department, faculty of science, ain shams university, 11566 abbassia, cairo, egypt 3present address: chemistry department, faculty of science, jazan university, 2097 jazan, saudi arabia corresponding author: e_azooz@yahoo.com; tel.: +9966532324115 received: july 23, 2016; revised: december 29, 2016; accepted: january 17, 2017 abstract electrochemical copolymerization of o-chlorophenol (o-clph) with o-hydroxyphenol (o-hoph) was conducted in aqueous h2so4 using cyclic voltammetry technique at the pt electrode. the reaction rate was found to be of the second order in the monomer concentration and first order in the acid concentration. the activation energy, enthalpy, and entropy for the copolymerization were found to be 20.20 kj mol-1, 19.24 kj mol-1 and -281.47 j k-1 mol-1, respectively. the obtained copolymer films show smooth feature with amorphous nature. copolymer films adhere pt electrode very well and show less reactivity in the h2so4 medium. the ph sensitivity of the poly(oclph-co-hoph)-modified electrode has been investigated potentiometrically using different polymer thicknesses. the potentiometric responses to ph change of the poly(oclph-co-hoph)-modified electrode appeared reversible and linear in the range from ph 2-11 with a maximum sub-nernstian potentiometric response slope of 40.7 mv/ph (30 °c). the slope became close to 56.2 mv/ph in the range from ph 4 to 9 at (30 °c). the poly(oclph-co-hoph)-modified electrode readily responded to ph change but was not stable with time. keywords pt-anode; poly(o-chlorophenol-co-o-hydroxyphenol); electrocopolymerization; kinetic study; activation parameters; potentiometric ph sensor introduction phenolic compounds (phs) are a class of pollutants which, when absorbed through the skin and/or mucous membranes, can cause damage to numerous organs of living bodies, like the lungs, liver, http://www.jese-online.org/ mailto:e_azooz@yahoo.com j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 12 kidney, and genitourinary tract [1]. phs are widely used in wood preservatives, textiles, herbicides and pesticides, and released into the ground and surface water. the identification and quantification of these compounds represent an important issue in the environmental monitoring. ohydroxyphenol (o-hoph) and o-chlorophenol (o-clph) are two phenolic compounds, which are present as contaminants in medical, food and environmental matrices. different methods were used for identification and clearing of them. some of these methods are time-consuming, of low sensitivity with complicated pretreatments and expensive. electrochemical methods, however, provide an easy and fast alternative for the analysis of such materials [2,3]. electropolymerization is a good approach for preparing polymer-modified electrodes by adjusting the electrochemical parameters that control film thickness, permeation, and charge transport characteristics. in the field of electropolymerization, two topics are of great importance: the first is the study of the electropolymerization kinetics [4–9], which provides information about the nature of the reactions taking place at the electrode surface and the chemical structure of the polymer film, while the second is the potential use of polymer modified electrodes as sensors for qualitative and quantitative analyses of hazardous and biologically active compounds [4–6, 9-11]. there have already been a few reports on the kinetics and mechanism of the electrochemical polymerization [12-18]. sayyah et. al. [19] studied the kinetics of homopolymerization of o-hoph and o-clph by cyclic voltammetry (cv) in aqueous h2so4 solution at the pt anode. these authors used different tools for characterization of the obtained films. the obtained films with a smooth lamellar surface were characteristic for poly(o-chlorophenol), p(o-clph), and a smooth feature with uniform thickness for poly(o-hydroxyphenol), p(o-hoph). in addition, they discussed the possible applications of p(o-clph) in the field of sensors and p(o-hoph) in the field of dye removal from water. previously, we showed that a block copolymer film was deposited by polymerizing o-clph with ohoph using cv methods [20]. the copolymer formation was directly affected by monomer and acid concentrations and temperature. the copolymer consisted of higher fraction of o-hoph than o-clph units suggesting that o-hoph is much more reactive than o-clph. determination of ph is one of the most common measuring procedure in a wide variety of industries and industrial applications [21]. a glass ph electrode, as one of the traditional ph electrodes, has excellent electrode performances with respect to response slope straightforward operation, selectivity, and long-term stability. however, the electrode with the internal liquid system exhibits some drawbacks, such as mechanical fragility, high cost, and limited miniaturization for use in clinical or biological applications [22, 23]. much interest in the development of ph sensors based on polymers has been created [24]. the positive charges in the oxidized state of the polymer are compensated by counter-ions from the solution, giving ion-exchange properties of the polymer films. therefore, polymers are applicable in potentiometric ph sensors [25]. the conducting polymer-based ph sensors, however, have suffered from long response time, non-reproducibility and salt effect [26]. the aim of the present work is to study the kinetics of electrooxidation of the binary mixture of o-hoph and o-clph in the aqueous acid solution. the reaction orders with respect to the electrolyte and monomer will be determined from electrochemical data and the prepared films/electrode will be tested as a potentiometric ph sensor. s. m. sayyah at al. j. electrochem. sci. eng. 7(1) (2017) 11-26 doi:10.5599/jese.330 13 experimental materials o-clph (+97 %, hopkin & williams, dagenham, essex, uk), sulfuric acid, k2hpo4, potassium hydrogen phthalate (khp) , borax, naoh, hydrochloric acid, nahco3 (merck, darmstadt, germany) and o-hoph (99 %, aldrich, germany). all chemicals were of analytical pure grade and used as received. all solutions were prepared by using freshly double-distilled water. instrumentation electrochemical experiments were performed using a potentiostat/galvanostat wenking pgs 95. the i-e curves were recorded by the computer software from the same company (model ect). all experiments were conducted at a given temperature (± 0.5 c) with the help of circular water thermostat. electrochemical polymerization was carried out in a home-made three-electrode singlecompartment cell with platinum plates as working and counter electrodes and a saturated calomel electrode (sce) as the reference electrode. the control temperature (± 0.5 °c) was achieved by means of a jacketed cell through which water was circulated from a polytemp thermostat. all electrochemical reactions were carried out at 25 °c unless otherwise specified in the text. the areas of the working and counter electrodes were 0.5 cm2 each. the oxidation potentials were measured by linear sweeping voltammetry as the anodic peak values (ip(ii) a). the obtained copolymer was peeled off from pt electrode and washed with h2so4 (0.6 m), then washed with bi-distilled water and finally heated to 95 °c for 3 h and stored for further characterization. characterizion by uv–vis spectrum was achieved using shimadzu uv spectrophotometer (m160 pc) at room temperature in the range 200–700 nm. dimethylformamide (dmf) was used as a solvent and reference (200 μl of the solution was diluted to 3 ml in a quartz cuvette of 1.0 cm optical path). ir-spectra was recorded using a shimadzu ftir-340 jasco spectrophotometer (japan) from 400 to 4,000 cm-1 with a spectral resolution of 4 cm-1 using kbr tablets (merck, darmstadt, germany, 99 %) with 1:100 sample: kbr proportion. tga was performed using a shimadzu dt-30 thermal analyzer (shimadzu, kyoto, japan) the weight loss was measured from ambient temperature up to 600 25 c, at the rate of 20 c min-1 and nitrogen flow rate of 50 ml min-1 to determine the degradation rate of the copolymer. sem is performed by a jxa-840a electron probe microanalyzer (jeol, tokyo, japan). xrd was done using (philips 1976 model 1390, netherlands) x-ray tube: cu; current: 30 ma; preset time: 10 s; scan speed: 8◦min-1; voltage: 40 kv. the ph measurements were carried out using a phs-3b ph meter and a commercial sce. a series of buffer solutions were used for solutions of various phs. the buffers were adjusted by adding diluted hcl or naoh solution to the solution of kh phthalate, kh2 phthalate, borax and/or nahco3. results and discussion electrochemical copolymerization the first cycle cyclic voltammogram (cv) recorded during the copolymerization a mixture of oclph and o-hoph with a molar feed ratio kept at 1:1 in the potential range from 0 to 1.7 v vs. sce in the absence and presence of monomers is represented in figure 1. from this figure, the adsorption peak of h2 gas on pt surface was observed at -0.3 v vs. sce [19] in absence or presence of monomers, peak (i), whiile the oxidation peak of comonomers to yield copolymer, peak (ii) was developed at j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 14 0.70 vs. sce. by increasing number of cycles ip(i) was not changed while ip(ii) decreased significantly and disappeared after 20 cycles. figure 1: cyclic voltamigrams of pt electrode in aqueous solution containing 0.6 m h2so4 at 303 k using 25 mv s-1 in absence and presence of 0.05 m comonomer (1:1 molar ratio) , a first cycle and b repetative cycling up to 20 cycles. the mechanism of polyphenol formation was proposed elsewhere [20,27-30]. similarly, we could propose that a removal of an electron from hydroxyl groups (o-atoms) to form free radicals adsorbed on the pt electrode surface occurs at 0.70 vs. sce. adsorbed radicals react with each other or with a comonomer molecules via head-to-tail (c-o)-coupling to form dimeric radical what is followed by formation of the copolymer film. on reversing the potential scan, the anodic current is very small indicating the presence of a copolymer film well adhered to the pt surface [30,31]. the absence of any cathodic peaks indicates totally irreversible system as a result of the strong adhesion of the copolymer film to pt surface. insulating property of the copolymer film, no electroactive species, and/or the nature and kind of oxide species formed on the pt surface during the electrooxidation is the reason of observed irreversibility [32,33]. cvs recorded for o-clph polymerization showed one oxidation peak at 0.86 v and one reduction peak at 0.25 v vs. sce) while cvs for o-hoph polymerization showed one oxidation peak at 0.62 v and one reduction peak at 0.20 v vs. sce, respectively [19,20]. as seen in figure 1, the obtained cvs for their mixtures differ, while no reduction peaks confirmed the copolymer formation. by increasing the number of cycles repetitively (i.e. see figure 1b), a fouling of pt electrode is seen as in other poly phenols [30,33,34]. the reason of this phenomenon may be due to the formation of insulating and good adhered films on pt surface which causeed a decrease of the peak current densities ip(ii)) with repetitive cycling [35]. of the formation process does not shift with increasing number of cycles, indicating that the oxidation reactions were independent of the copolymer thickness [31,36,37]. effect of the scan rate the influence of the scan rate on the anodic current density ip(ii) of the pt electrode was studied in the range from 15 to 40 v vs. sce for 0.05 m co-monomer (1:1 molar ratio) in an aqueous solution containing 0.6 m h2so4 at 303 k. it is obvious from figure 2 that almost linear relationship was observed for the dependence of ip(ii) on both scan rate (v) and the square root of the scan rate (v0.5). this behavior may be explained as the mixed diffusion-kinetically controlled process [20]. according to randless [38] and sevick [39] (relation between ipii and (v 0.5 is linear see fig 2b), the average diffusion coefficient (ddiff) is calculated to be 6.53 10-7 m2s-1. the values of ddiff are seen to be i p / m a c m -2 s. m. sayyah at al. j. electrochem. sci. eng. 7(1) (2017) 11-26 doi:10.5599/jese.330 15 constant over the range of sweep rates, which again shows that the oxidation process is diffusioncontrolled [33,40-43]. figure 2. relation between ipii and a scan rate, b square root of scan rate for pt electrode and 0.05 m comonomer (1:1 molar ratio) in aqueous solution containing 0.6 m h2so4 at 303 k. increasing the temperature by 35 °c, the ddiff increased by a factor of 6. the walden product (the product of diffusion coefficient and the viscosity, d × η) was constant over the temperature range studied suggesting that decrease of viscosity at higher temperature caused the observed increase of ddiff. similarly to the arrhenius law, the following equation allows the activation energy of diffusion, ediff, to be calculated using a linear plot of ln ddiff versus t-1: rt e edd d i ff a    depending on the above equation and data obtained from figure 3 the linearity of relation >0.95 confirm good stimulation of arrhenius relation and eadiff was found to be 38.25 kj mol-1 at 25 mv s1. figure 3. plot of ln d versus t-1. kinetics of the electropolymerization process the electropolymerization kinetics was evaluated using deoxygenated aqueous solution containing monomer in the concentration range between 0.03 and 0.08 m and h2so4 concentration in the range between 0.2 and 0.6 m at 303 k. i p / m a c m -2 i p / m a c m -2 j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 16 assuming that the polymerization follows the following equation: pem  where m is the monomer, e is the electrolyte and p is the polymer, then the kinetic equation can be formulated as follows: ba mek t w r ][][ d ][d p  where rp is the polymerization rate, which represents the polymer weight, w, per unit time and cm2 of the electrode surface area, a and b are the reaction orders with respect to the electrolyte and monomer, respectively, and k is the specific rate constant of the electropolymerization process. the electrochemical study of the copolymer formation is used instead of gravimetric study since the polymer yield on pt electrode is so small to be used in kinetic studies. the value of the anodic current density ip(ii) is proportional to the electropolymerization rate (rp) at given concentrations of the monomer, acid, and electrolyte, and then we can replace the electropolymerization rate by the anodic current density [31,44-46]: ba meki t w r ][][ d ][d pp  or expressed in logarithmic form as: ]log[]log[loglog p mbeaki  for the electropolymerization of the comonomer in aqueous solutions, the kinetic equation can be represented by: ba comonomersohkr ][][ 42p  comonomer mixture and sulfuric acid concentrations were varied keeping one of them constant to evaluate their respective reaction orders. the comonomer concentration varied from 0.04 to 0.08 mol l-1 (1:1 molar ratio) at a constant 0.6 mol l-1 h2so4 concentration. then, the h2so4 concentration varied from 0.2 to 0.6 mol l-1 at a constant comonomer concentration of 0.08 mol l-1 (1:1 molar ratio). figure 4a presents the polymerization i-e plots corresponding to the copolymer formation from a constant 0.6 mol l-1 h2so4 concentration and varying monomer concentrations (with 1:1 molar ratio). the log ip(ii) versus log ccomonomer relation is presented in figure 4b. the slope of the linear relation was found to be 1.80 which means that the polymerization reaction is of the second order with respect to the comonomer concentration. figure 5a shows the effect of the change of h2so4 concentration on the polymerization ip(ii) at a constant concentration of comonomer 0.08 mol-1 using (1:1 molar ratio). the plot of log ip(ii) against log ch2so4 is presented in figure 5b. the linear relation has a slope of 0.80 which means that the polymerization reaction is of the first order with respect to h2so4 concentration. based on the above data, the kinetic equation of the copolymerization process is defined as: 8.180.0 42p ][][ comonomersohkr  from the above equation, the order of the copolymerization reaction is first order with respect to h2so4 and second order with respect to comonomer concentration. s. m. sayyah at al. j. electrochem. sci. eng. 7(1) (2017) 11-26 doi:10.5599/jese.330 17 figure 4. ai-e curves for the effect of comonomer concentration on the electropolymerization of ohp and ocp from solution containing 0.6m h2so4 at 303k with scan rate 25 mv s-1 on pt electrode; b double logarithmic plot of jp(ii) versus comonomer concentrations). figure 5. a i-e curves for the effect of acid concentration on the electropolymerization of ohp and ocp (1:1molar ratio) from solution containing 0.08m comonomer at 303 k with scan rate 25 mv s-1 on pt electrode; b double logarithmic plot of ip(ii) versus acid concentrations effect of temperature figure 6a shows the potentiodynamic polarization curves as a function of the solution temperature in the range between 288 and 323 k. i-e curves were recorded in the solution of 0.6 m h2so4 and 0.09 m monomer concentration using scan rate of 25 mv s-1. from figure 6a it appears that an increase of the reaction temperature resulted in a progressive increase of the charge included in the anodic peaks. in general, the rate of polymerization increases with the increase of the reaction temperature. the arrhenius-like plot exhibits a linear relationship between the logarithm of the rate constant of the electropolymerization reaction (iog ip(ii)) and inverse absolute temperature (t-1), rt e ai alnln p . according to the arrhenius equation, the apparent activation energy (ea) and pre-exponential factor (a) can be determined from the slope and intercept of the plot respectively [47]. i p / m a c m -2 lo g ( i p / m a c m -2 ) lo g ( i p / m a c m -2 ) j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 18 figure 6. a i-e curves for the effect of temperature on the electropolymerization of ohp and ocp (1:1 molar ratio) from solution containing 0.09 m comonomer and 0.6 m h2so4 at 303 k with scan rate 25 mv s-1 on pt electrode; b arrhenius plot of the electropolymerization of ohp and ocp (1:1 molar ratio); c eyring equation plot of the electropolymerization of a binary mixture of ohp and ocp (1:1 molar ratio). at temperatures higher than 323 k the log ip(ii) deviates slightly from the straight line (i.e. the value is lower than expected) owing probably to the excessive diffusion of radicals away from the vicinity of the electrode. such diffusion is evidenced by the color change in the electrolyte solution at higher temperatures. the plot of log ip(ii) versus t-1 yields a straight line (figure 6b), indicating that the copolymerization also obeys the arrhenius equation. the apparent activation energy was calculated using arrhenius equation and found to be 20.2 kj mol-1. other activation parameters, i.e. activation enthalpy (∆h*) and activation entropy (∆s*) were determined from the slope and intercept of the plot of log (ip(ii) t-1) against t-1, respectively, according to the following equation [48]: i p / m a c m -2 lo g ( i p / m a c m -2 ) lo g ( i p t -1 / m a c m -2 k -1 ) s. m. sayyah at al. j. electrochem. sci. eng. 7(1) (2017) 11-26 doi:10.5599/jese.330 19 h k r s rt h t i lnln ** p      where r is the gas constant, k is boltzmann’s constant, and h is planck’s constant. the change in δh* and δs* of activation for the electropolymerization reaction can be calculated from the eyring equation plot at different temperatures (figure 6c). we obtained a linear relationship with a slope of -δh*/2.303 r and intercept of {log (k/h) + δs*/2.303 r}. from the slope and intercept the values of δh* and δs* were found to be 19.24 kj mol-1 and -281.47 kj mol-1, respectively. the negative ∆s* value suggests the formation of ordering activated complexes. there are several possible reasons for the negative ∆s* which are consistent with the proposed mechanism [49]. first, the radicals generated during the polymerization are much more solvated in the media in comparison with the corresponding neutral reactants. second, the polymer chain growth involves bimolecular reactions between the radicals and the incoming monomers. both factors may have contributed to the observed entropy decrease in the activated complexes. electroactivity of the pt-copolymer film the obtained polymer film prepared on pt electrode at the optimum conditions (0.08 m comonomer and 0.6 m h2so4 at 303 k using 25 mv s-1) was transferred to another cell with only 0.6m h2so4 and cycled from 600 to +2000 mv, vs. sce (figure 7). figure7 shows that both oxidation peak (developed at +700 mv vs. sce) and the h2 peak (developed at -300 mv vs. sce) disappeared completely. figure 7. electroactivity behavior of copolymer coated pt electrode in 0.6 m h2so4 with different cycle numbers. scan rate=25 mv s-1. the lack of oxidation peaks confirms the insulating properties and the lack of active species inside the copolymer film. the appearance of a new broad reduction peak can be explained as follows: at the absence of other cations (comonomers cations) h+ will accumulate on the copolymer/platinum surface forming local charge densities which are considered as a new copolymer form (copolymer h). during the reduction process, the formed copolymer h will reduce to copolymer and h+ via a chemical process. this phenomenon is the basis for the construction of polymeric ph sensors. by repetitive cycling, the current of cathodic peak decreases as a result of decreasing the copolymer amount on the pt surface. for p(o-clph) and p(o-hoph), adsorption peak of h2 is observed and not affected by repeating cycles, no oxidation peaks were formed and one broad reduction peak is developed at -300 and -200 mv vs. sce respectively [19]. j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 20 uv-vis and ir spectroscopy the uv-visible spectrum of p(o-hoph -co-o-clph) shows that the ππ* transition ((e2 band) of the benzene ring and the -band (a1g to b2u)) appear at 257, 307 and 317 nm where the π-polaron transition (conjugation of the aromatic polymeric chain) appears at 485 nm [20,45,50]. this band doesn't appear at both homopolymers [19] which confirm the copolymer formation. the assignments of the absorption bands of the obtained infrared spectra (kbr pellets) of copolymer sample is recorded and represented in table 1. table 1. ir wavenumbers and thier assignments for the prepared copolymer sample. assignments wavenumber, cm-1 electronic transition from the valence band to the conduction band 3700 – 1800 o-h group (phenolic) streching vibration 3320 and 3420 aromatic ring c–h stretching vibration 3044 and 3105 out-ofplane bending of =c–h bonds of an aromatic ring 830 and 860 c=c stretching vibration bands 1500 and 1665 c–o–c stretching frequency of phenyl ether [51] 1230 dopant so4-2 incorporation [20] 1180 stretching vibration of the c-cl bond 912 thermogravimetric analysis of the prepared sample the presence of water molecules in the repeated unit is confirmed by the tga. the tga steps of the prepared p(o-clph-co-o-hoph) are shown in figure 8. from the figure 8 the loss of (water and doped anions) in the temperature ranging between 25 and 220 °c can be observed. at higher temperatures the polymer begins to decompose. it is suggested that p(o-clph -co-o-hoph ) is thermally stable until 220 °c where it starts to decompose resulting in only 27 % polymer weight remaining at 600 °c. however, the remaining amount of p(o-hoph) at 600 °c is 65 % and for p(oclph) 70 % [19] which confirms the copolymer formation. figure 8. tga curve for p(o-clph-co-o-hoph) surface morphological studies based on data obtained from xrd and sem techniques, we previously [19] showed that in a case of o-clph, a homogeneous, smooth, brown and well-adhering p(o-clph) films were electrodeposited m a ss lo ss , % 0 50 100 s. m. sayyah at al. j. electrochem. sci. eng. 7(1) (2017) 11-26 doi:10.5599/jese.330 21 on pt electrode surface with a lamellar surface feature of the amorphous nature. in a case of ohoph, a smooth, black and well-adhering p(o-hoph) films were electrodeposited on the pt-surface with uniform thickness and amorphous structure. in the case of the binary mixture, a homogeneous, smooth, black and well-adhering p(o-clph -coo-hoph ) films were electrodeposited on the ptelectrode surface. the xrdpattern shows that the prepared copolymer was amorphous as shown in figure 9a. the surface morphology of the copolymer obtained at the optimum conditions was examined by sem that shows smooth and amorphous in nature as seen in figure 9b. figure 9. a xrd patterns of p(o-clph-co-o-hoph); b sem image of p(o-clph-co-o-hoph) the solubility of the obtained copolymer and the two homopolymers were examined in different solvents. both p(o-hoph) and p(o-clph) were found to be soluble well in dmso, dmf and thf, while their copolymer is soluble in dmf, and hf but not soluble in dmso. this confirmed the copolymer formation. potentiometric study of p(o-clph-co-o-hoph)pt modified electrode the ph measurements were carried out using a phs-3b ph meter and a commercial sce. a series of buffer solutions was used for various ph solutions which were adjusted by adding diluted hcl or naoh solution to the solution of kh phthalate, kh2 phthalate, borax and/or nahco3. figure 10 shows that p(o-clph-co-o-hoph)-pt modified electrodes gave linear responses over the ph range from 2 to 12, with the slope values ranging from 32.5 to 56.7 mv/ph, and a linear correlation coefficients (r2) with a value of > 0.93 as summarized in table 2. table 2. slopes and r2 values obtained from figure 10 no of cycles ph range 2-12 -slope, mv ph-1 r2 3 56.29 0.99 5 48.80 0.99 10 44.90 0.97 15 29.71 0.99 figure 10 and table 2 show that the best slope value was obtained at a moderate thickness (no of cycles 3) while the smallest deviation of the straight line is found at ph < 4 and also at ph > 9. this in te n si ty , a .u . j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 22 suggests that the p(o-clph -co-o-hoph)/pt modified electrode may not be an effective ph sensor for more basic or more acidic solutions. figure 10. p(o-clph-co-o-hoph) response at different ph values, the prepared copolymer films after a 3 cycles, b 5 cycles, c 10 cycles and d 15 cycles figure 11 shows the responses of p(o-clph-co-o-hoph)-pt modified electrode at different ph values (from 5 to 9). the best slopes were improved i.e. 56.19 mv ph-1 with r2=0.98 for the film prepared after 3 cycles. figure 11. the responses of p(o-clph-co-o-hoph)-pt modified electrode having different thicknesses at ph range (4-9). s. m. sayyah at al. j. electrochem. sci. eng. 7(1) (2017) 11-26 doi:10.5599/jese.330 23 electrode stability potentiometric response of the p(o-clph-co-o-hoph)-pt modified electrode was examined over a period of 9 days in order to study the stability of electrode. as seen in figure 12 and table 3, a significant decrease in the response slope is observed for the p(o-clph-co-o-hoph)-pt modified electrode. also, the calibration curves of the p(o-clph-co-o-hoph)-pt modified electrode lost their linearity and were compared to the fresh electrode (1st day) even after repeatedly using it for 9 days. figure 12. p(o-clph-co-o-hoph) response from the first to the ninth day table 3. slopes and r2 values obtained from figs 12 time, day -slope, mv ph-1 r2 1 58.7 0.97 2 54.5 0.97 3 51.5 0.97 4 49.9 0.97 5 37.9 0.95 6 40.9 0.91 7 40.5 0.91 8 31.3 0.94 9 32.5 0.93 response mechanism we have shown that the potentiometric responses to ph changes of the different modified electrodes are linear in the range 2-12. these responses must be mainly attributed to the copolymer films rather than the platinum substrate. possible explanation is the affinity of the numerous hydroxyl groups and cl atoms to the protons in solutions. the binding of h+ onto copolymer creates local charge density excess at the electrode surface. surface reactions seemed to take place on the copolymer film, essentially protonation and deprotonation of superficial oh groups of the polymers as symbolically described by:   copolymerhhcopolymer . when the equilibrium is attained at the copolymer/solution interface, we can write the equilibrium expression of the surface reaction and the equilibrium potential e as: j. electrochem. sci. eng. 7(1) (2017) 11-26 poly(oclph-co-hoph) ph-sensor application 24 ]][h[copolymer ]h[copolymer   k ][hln ][copolymer ]h[copolymer ln '      f rt e f rt ee according to this mechanism of the reaction, we expect the slope of 59 mv ph-1 at 25 °c at all ph values. but our electrodes showed lower response slope. the presence of anionic and cationic species in the vicinity of the modified electrode surface such as (k+, na+, cl-, etc.) in the buffer solutions which have an effect on the modified electrode response as mentioned in the literature [52–54]. conclusion p(o-clph-co-o-hoph) films are prepared by the electropolymerization of a mixture of o-clph and o-hoph (with 1:1 molar ratio) on pt substrates from an aqueous medium. the process is fast and economic. the electropolymerization reaction was found to be first order with respect to the h2so4 concentration, but of the second order for the comonomer concentration. the prepared p(o-clphco-o-hoph ) films are stable up to 220 °c and exhibit amorphous and smooth structure with uniform distribution on the pt substrate. ir and uv-vis spectroscopy confirm the copolymer formation. the calculated activation energy, enthalpy, and entropy for the copolymerization were found to be 20.20 kj mol-1, 19.24 kj mol-1 and -281.47 j k-1 mol-1, respectively. the diffusion coefficient d is 6.53 10-7 m2 s-1 in the scan rate of 15 up to 40 mv s-1 at 25 °c and increased with increasing temperature where eadiff was calculated to be 38.25 kj mol-1 at 25 mv s-1. the potentiometric sensitivity to ph change of the p(o-clph-co-o-hoph)-modified electrode exhibits a response slope of 52.16 mv/ph (30 °c), a linearity range from ph 4 to 9 and the response time less than 10 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[54] a. michalska, k. maksymiuk, a. hulanicki, j electroanal chem 392(1/2) (1995) 63-68. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ fe3o4/go nanocomposite modified glassy carbon electrode as a novel voltammetric sensor for determination of bisphenol a: http://dx.doi.org/10.5599/jese.1482 1205 j. electrochem. sci. eng. 12(6) (2022) 1205-1214; http://dx.doi.org/10.5599/jese.1482 open access : : issn 1847-9286 www.jese-online.org original scientific paper fe3o4/go nanocomposite modified glassy carbon electrode as a novel voltammetric sensor for determination of bisphenol a fariba beigmoradi1 and hadi beitollahi2, 1department of chemistry, graduate university of advanced technology, kerman, iran 2environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran corresponding author: h.beitollahi@yahoo.com; tel: +98 3426226613; fax: +98 3426226617 received: august 3, 2022; accepted: august 23, 2022; published: september 14, 2022 abstract a new voltammetric sensor is proposed for the determination of bisphenol a, using a glassy carbon electrode (gce) modified with fe3o4/graphene oxide (go) nanocomposite. the modification of the electrode surface was performed by dispersion drop-casting. the electrochemical behavior of bisphenol a was evaluated by cyclic voltammetry (cv). the oxidation peak was observed during the anodic potential scan at potentials of 0.45 v. higher anodic peak currents (ipa) were observed at fe3o4/go/gce modified electrode than at bare gce. the electrochemical determination by differential pulse voltammetry (dpv) revealed a linear response in the concentration range of 1.0×10-7 to 5.0×10-5 m, with a detection limit of 9.0×10-8 m. the proposed method was successfully applied using water samples, with good recoveries. keywords modified electrode, water samples; differential pulse voltammetry; high repeatability introduction bisphenol a (bpa) is a polyphenol used in the production of polycarbonates and epoxy resins, and then it can be found in plastic packaging and canning coatings [1]. because of its large-scale use and physicochemical properties, there is a risk of contamination of food or water by bisphenol migration from the packages. bisphenol a shows estrogenic effects for humans, and it may increase the probability of developing prostate cancer and decrease fertility in animals, among other diseases [2]. as an emerging pollutant, threshold limits have not been established by environmental regulations. still, bisphenol a was included as a future substance in the “list of 33 priority substances” in annex ii of the 2008/105/ec. moreover, the european community has established a maximum daily intake of 0.05 mg kg-1 of body weight [3-5]. because of these health concerns, bisphenol a has been determined in urine, river, sea, and tap waters [6-8]. thus, the development of a sensitive and selective method for the detection of bisphenol a is very important for both human health and environmental protection. methods that have been used to monitor these http://dx.doi.org/10.5599/jese.1482 http://dx.doi.org/10.5599/jese.1482 http://www.jese-online.org/ mailto:h.beitollahi@yahoo.com j. electrochem. sci. eng. 12(6) (2022) 1205-1214 modified gce for determination of bisphenol a 1206 analytes include electrochemical sensors, mass spectrometry (gc/ms) and high-performance liquid chromatography (hplc) with detection by fluorescence or spectrophotometry (uv), or mass spectrometry (lc/ms or lc/ms/ms) [9-14]. electrochemical methods have the advantages of being simpler and more sensitive, offering excellent repeatability and short analysis times, and are less expensive compared to traditional methods [15-31]. glassy carbon (gc) is a type of non-graphitic carbon formed by the pyrolysis of certain polymeric precursors. the microstructure of gc is composed of discrete fragments of curved carbon planes, like imperfect fullerene-related nanoparticles. gc synthesized at high temperatures above 2000 °c exhibits a network of stacked graphite-like ribbon molecules. entities of polyhedral graphite crystals were also detected in commercial glassy carbon, as used in our studies [32]. the network of randomly and tangled carbon planes forms a dense carbon structure. due to the low reactivity, high hardness, impermeability and good electrical conductivity of gc, it is commonly employed as an electrode material for electroanalysis [33,34]. due to the assumed chemical inertness, it is also often used as a substrate to cast powder catalysts in order to evaluate their catalytic performance in electrochemical reactions. however, the response signal of conventional sensors toward analytes detection is quite weak, making it very difficult to achieve accurate measurements of analytes. therefore, in order to improve the response signal of the analyte determination, electrochemical sensors are modified with various materials [35-47]. nanomaterials have unique physicochemical properties, such as a large surface area to mass ratio, ultra-small size and high reactivity, which differ from bulk materials with the same composition. the unique property or desirable characteristics of nanomaterials decide its potential application in many fields [48-64]. owing to its superior electronic, thermal and mechanical properties as well as chemical stability, graphene, as a well-defined, two-dimensional honeycomb structure of carbon materials, has been attracting more and more attention from both experimental and theoretical scientific communities [65,66]. graphene and graphene oxide (go) are potential nanoscale building blocks for new hybrid materials because of their layered structure and special surface properties [67-69]. it is believed that the composites of metal oxides and graphene would have better functionalities and performances in their applications. among those hybrid materials, the ones with the magnetic nanoparticles (e.g., fe3o4, fe2o3) are an important class of materials due to their numerous applications in various technological fields [70]. magnetite (fe3o4) nanoparticles (nps) have attracted worldwide research attention not only because of their unique sizeand morphologydependent physical and chemical properties but also for their potential applications in many fields, including magnetic storage, biosensors, communication materials, magnetic resonance imaging and as materials for microwave absorbing and shielding research [71-73]. according to the previous points, it is important to create suitable conditions for the analysis of bisphenol a in real samples. in this study, we describe the application of fe3o4/go nanocomposite as a nanostructure sensor for the voltammetric determination of bisphenol a. the proposed sensor showed a good electrocatalytic effect on bisphenol a. the modified electrode shows advantages in terms of reproducibility and sensitivity. also, we evaluate the analytical performance of the suggested sensor for bisphenol a determination in water samples. f. beigmoradi and h. beitollahi j. electrochem. sci. eng. 12(6) (2022) 1205-1214 http://dx.doi.org/10.5599/jese.1482 1207 experimental chemicals and apparatus the electrochemical measurements were performed with an autolab potentiostat/galvanostat (pgstat 302n, eco chemie, the netherlands). the experimental conditions were controlled with general purpose electrochemical system (gpes) software. a conventional three electrodes cell was used at 25 ± 1 °c. an ag/agcl/kcl (3.0 m) electrode, a platinum wire, and the fe3o4/go/gce were used as the reference, auxiliary and working electrodes, respectively. a metrohm 710 ph meter was employed for ph measurements. bisphenol a and all other reagents were analytical grade and were purchased from merck (darmstadt, germany). phosphate buffer solution (pbs) was prepared with phosphoric acid and adjusted by naoh to the desired ph value in the range of 2.0–9.0. preparation of the modified electrode the bare glassy carbon electrode was coated with fe3o4/go nanocomposite according to the following simple procedure. 1 mg fe3o4/go nanocomposite was dispersed in 1 ml aqueous solution within 45 min ultrasonication. then, 5 µl of the prepared suspension was dropped on the surface of carbon working electrodes. it remains at room temperature until it becomes dry. the surface areas of the fe3o4/go/gce and the unmodified electrode were obtained by cv using 1 mm k3fe(cn)6 at various scan rates. using randles-ševčik formula [74], in fe3o4/go/gce, the electrode surface was found 0.13 cm2 which was approximately 4.1 times greater than the unmodified electrode. results and discussion electrochemical profile of the bisphenol a on the fe3o4/go/gce to study the electrochemical behaviour of bisphenol a which is ph-dependent, it is necessary to obtain the optimized ph value to achieve accurate results. by performing the experiments using modified electrodes at various ph values ranging from 2.0–9.0, it was revealed that the best results for electrooxidation of bisphenol a occur at ph 7.0. scheme 1 demonstrates the electrooxidation process of bisphenol a. scheme 1. elector-oxidation reaction of bisphenol a the obtained cyclic voltammograms in the presence of 30.0 μm bisphenol a using the fe3o4/go/gce (trace a) and unmodified gce (trace b) are shown in figure 1. according to cv results, the oxidation current peak of bisphenol a on the fe3o4/go/gce appears at 450 mv, which is about 150 mv more negative compared with unmodified gce. http://dx.doi.org/10.5599/jese.1482 j. electrochem. sci. eng. 12(6) (2022) 1205-1214 modified gce for determination of bisphenol a 1208 figure 1. cyclic voltammograms of (a) fe3o4/go/gce and (b) unmodified gce in 0.1 m pbs (ph 7.0) in the presence of 30.0 μm bisphenol a at the scan rate 50 mvs-1 effect of scan rate on the results increasing scan rate leads to enhanced oxidation peak current according to the obtained results from the study of the effect of potential scan rates on the oxidation currents of bisphenol a, figure 2. in addition, there is a linear relationship between ip and the square root of the potential scan rate (ν1/2), demonstrating the diffusion control of analyte oxidation. figure 2. lsvs of fe3o4/go/gce in 0.1 m pbs (ph 7.0) containing 15.0 μm bisphenol a at various scan rates; numbers 1-8 correspond to 10, 20, 40, 60, 80, 100, 200 and 300 mv s-1. inset: variation of ipa vs. ν 1/2 f. beigmoradi and h. beitollahi j. electrochem. sci. eng. 12(6) (2022) 1205-1214 http://dx.doi.org/10.5599/jese.1482 1209 chronoamperometric analysis the chronoamperometry analysis for bisphenol a samples was performed using of fe3o4/go/gce vs. ag/agcl/kcl (3.0 m) at 0.5 v. the results of different concentrations of bisphenol a sample in pbs (ph 7.0) are demonstrated in figure 3. figure 3. chronoamperograms obtained at fe3o4/go/gce in 0.1 m pbs (ph 7.0) for different concentrations of bisphenol a. the numbers 1-4 correspond to 0.1, 0.5, 1.5 and 3.0 mm. (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-4. (b) plot of the slope of the straight lines against bisphenol a concentration according to the cotrell equation (equation 1) [68], experimental results of i vs. t−1/2 were plotted in figure 3a, with the best fits for different concentrations of bisphenol a. the resulting slopes corresponding to straight lines in figure 3a were then plotted against the concentration of bisphenol a (figure 3b). the mean value of d was determined to be 4.6 × 10−5 cm2 s-1. i =nfad1/2cbπ-1/2t-1/2 (1) calibration curves based on the resulting peak currents of bisphenol a using fe3o4/go/gce, the quantitative analysis of bisphenol a was done in water solutions. the modified electrode (fe3o4/go/gce) as a working electrode in the range of bisphenol a concentration in 0.1 m pbs was used in dpv due to the advantages of dpv, including the improved sensitivity and better performance in analytical applications (figure 4). according to the results, a linear relationship exists between the peak currents and concentrations of bisphenol a within the concentration range of 0.1-50.0 µm with a correlation coefficient of 0.999. the limit of detection, cm, of bisphenol a was calculated using equation (2): cm=3sb/m (2) where, m is the slope of the calibration plot (0.1508 μa μm-1) and sb is the standard deviation of the blank response obtained from 15 replicate measurements of the blank solution. the detection limit for determination of bisphenol a using this method 0.09 μm was obtained. http://dx.doi.org/10.5599/jese.1482 j. electrochem. sci. eng. 12(6) (2022) 1205-1214 modified gce for determination of bisphenol a 1210 figure 4. dpvs of fe3o4/go/gce in 0.1 m pbs (ph 7.0) containing different concentrations of bisphenol a. numbers 1–9 correspond to 0.1, 0.5, 1.0, 5.0, 10.0, 20.0, 30.0, 40.0 and 50.0 μm. inset: plot of i vs. bisphenol a concentrations. in all cases the scan rate was 50 mv s-1. step potential 0.01 v and pulse amplitude 0.025 v also, table 1 shows a comparison of the analytical features of merit of the proposed electrochemical method with a different reported modifier for the determination of bisphenol a [75-79]. table 1. comparison of the efficiency of different modifiers used in the detection of bisphenol a modifier lod, nm ldr, μm ref. multiwalled carbon nanotube and gold nanoparticle 4.0 0.01-0.7 [75] reduced graphene oxide-silver/poly-l-lysine nanocomposites 540 1.0-80.0 [76] graphene oxide and cuprous oxide nanocomposite 53 0.1-80.0 [77] reduced graphene oxide-multi-walled carbon nanotubes 1 0.005-20.0 [78] aupd nanoparticles-loaded graphene nanosheets 8.0 nm 0.5-10.0 [79] fe3o4/go nanocomposite 90 0.1-50.0 this work analysis of real samples the applicability of this modified electrode in the determination of real samples was assessed through the determination of bisphenol a in water samples using the described method. in order to perform this analysis, the standard addition method was employed and the results are listed in table 2. table 2. application of fe3o4/go/gce for determination of bisphenol a in water samples (n = 5) sample cbisphenol a / m recovery,% rsd, % spiked found well water 0 7.5 7.6 101.3 1.7 12.5 12.4 99.2 2.2 17.5 17.3 98.9 3.1 22.5 23.1 102.7 2.9 river water 0 10.0 9.9 99.0 2.5 15.0 15.3 102.0 3.1 20.0 19.5 97.5 3.2 25.0 25.9 103.6 1.9 f. beigmoradi and h. beitollahi j. electrochem. sci. eng. 12(6) (2022) 1205-1214 http://dx.doi.org/10.5599/jese.1482 1211 accordingly, the results of bisphenol a recovery are satisfactory and the reproducibility of the results is proved by the mean relative standard deviation (rsd). conclusions the fe3o4/go/gce sensor showed excellent performance in the determination of bisphenol a, offering an alternative analytical technique that is reliable, effective, and inexpensive. modifying the electrode surface with fe3o4/go nanocomposite resulted in the combination of a large surface area and high conductivity, providing high catalytic activity, highly reproducible and repeatable 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open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.22034/pcbr.2021.270178.1177 https://doi.org/10.3390/nano12030491 https://doi.org/10.15171%2fijb.1436 https://doi.org/10.1142/9789814287005_0002 https://doi.org/10.22034/chemm.2020.111201 https://doi.org/10.1016/j.bios.2017.02.038 https://doi.org/10.22034/chemm.2021.118260 https://doi.org/10.1021/acs.iecr.2c00370 https://doi.org/10.33945/sami/chemm.2020.4.12 https://doi.org/10.3390/scipharm85040036 https://doi.org/10.5599/jese.1101 https://doi.org/10.22034/chemm.2020.108149 https://doi.org/10.1016/j.snb.2017.06.160 https://doi.org/10.1016/j.jelechem.2017.10.022 https://doi.org/10.1016/j.talanta.2017.03.042 https://doi.org/10.1016/s1872-2040(17)61014-4 https://doi.org/10.1016/j.talanta.2017.01.049 https://creativecommons.org/licenses/by/4.0/) @article{beigmoradi2022, author = {beigmoradi, fariba and beitollahi, hadi}, journal = {journal of electrochemical science and engineering}, title = {{fe3o4/go nanocomposite modified glassy carbon electrode as a novel voltammetric sensor for determination of bisphenol a:}}, year = {2022}, issn = {1847-9286}, month = {sep}, number = {6}, pages = {1205--1214}, volume = {12}, abstract = {a new voltammetric sensor is proposed for the determination of bisphenol a, using a glassy carbon electrode (gce) modified with fe3o4/graphene oxide (go) nanocomposite. the modification of the electrode surface was performed by dispersion drop-casting. the electro­chemical behavior of bisphenol a was evaluated by cyclic voltammetry (cv). the oxidation peak was observed during the anodic potential scan at potentials of 0.45 v. higher anodic peak currents (ipa) were observed at fe3o4/go/gce modified electrode than at bare gce. the elec­trochemical determination by differential pulse voltammetry (dpv) revealed a linear response in the concentration range of 1.0×10-7 to 5.0×10-5 m, with a detection limit of 9.0×10-8 m. the proposed method was successfully applied using water samples, with good recoveries.}, doi = {10.5599/jese.1482}, file = {:d\:/onedrive/mendeley desktop/beigmoradi, beitollahi 2022 fe3o4go nanocomposite modified glassy carbon electrode as a novel voltammetric sensor for determination.pdf:pdf;:www/jese_v12_no6_1205-1214.pdf:pdf}, keywords = {differential pulse voltammetry, high repeatability, modified electrode, water samples}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1482}, } {electrodeposited palladium as efficient electrocatalyst for hydrazine and methanol electro-oxidation and detection:} http://dx.doi.org/10.5599/jese.1166 275 j. electrochem. sci. eng. 12(2) (2022) 275-282; http://dx.doi.org/10.5599/jese.1166 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrodeposited palladium as efficient electrocatalyst for hydrazine and methanol electrooxidation and detection jelena d. lović university of belgrade-institute of chemistry, technology and metallurgy, department of electrochemistry, njegoševa 12, 11000 belgrade, republic of serbia corresponding author: jelena.lovic@ihtm.bg.ac.rs tel.: +381-11-337-0389; fax: +381-11-337-0389 received: november 10, 2021; accepted: december10, 2021; published: december 17, 2021 abstract electrodeposited palladium was used as an electrocatalyst for electrochemical oxidation of hydrazine and methanol and the development of a sensitive platform for their detection. the electrochemical behavior of the electrode was evaluated by cyclic voltammetry (cv), while electroanalytical properties were determined by differential pulse voltammetry (dpv). the electrodeposited pd catalyst exhibited good electrocatalytic activity towards oxidation of hydrazine in neutral solution and methanol oxidation in alkaline solution. under optimized dpv conditions, the electrodeposited pd electrode shows good sensing capability for hydrazine and methanol detection. keywords sensitivity; differential pulse voltammetry; electroanalytical properties introduction the interest in fuel cells has been growing over the past few decades, particularly for low temperature fuel cells, since they can be used in portable electronic devices, electric vehicles, or stationary power supplies. direct methanol fuel cells (dmfcs) are promising power sources, and as such, they demand highly efficient electrocatalysts for methanol electrooxidation. it was shown that pd is an appropriate replacement for pt because of its lower cost, higher tolerance against co-like species, and better stability [1]. in addition, the choice of potential fuels is varied, and among them, hydrazine is of interest due to its large theoretical energy density, ease of storage and transport, and faster overall oxidation kinetics compared to the carbon-containing compounds [2]. it has been known that organic compounds, such as formaldehyde, isopropanol, methanol, hydrazine, are volatile and harmful to human health. to detect these compounds, many research groups investigate and have already established sensors with rapid response rates, excellent stability, and selectivity. nanostructures could be promising and effective electrodes for sensing applications and the detection of various chemicals. electrodeposition is one of the most efficient methods for the http://dx.doi.org/10.5599/jese.1166 http://dx.doi.org/10.5599/jese.1166 http://www.jese-online.org/ mailto:jelena.lovic@ihtm.bg.ac.rs j. electrochem. sci. eng. 12(2) (2022) 275-282 palladium as efficient electrocatalyst 276 preparation of metals or their alloys as nanostructures with appropriate composition and morphology. this rapid and facile technique provides nucleation and growth of metal nanoparticles with different sizes and shapes depending on applied potential, current, time, or concentration of the solution. in recent years the electrochemical detection of hydrazine or methanol using pd nanostructures has been reported in the literature. the highly dispersed and ultrafine carbon-supported pd nanoparticles (pd nps) catalyst was synthesized by the nabh4 reduction method in the presence of ethylenediaminetetramethylene phosphonic acid (edtmp) [3]. this pd/c catalyst exhibited a significant electrocatalytic performance for hydrazine oxidation in 0.05 m h2so4. rastogi et al. investigated pd nps immobilized on organic-inorganic hybrid nanocomposite material coated on a gc electrode as electrocatalyst in the oxidation of hydrazine and its amperometric sensor [4]. using pulsed electrodeposition technique, pd species were deposited on the multi-walled carbon nanotubes dispersed in nafion membrane to catalyze the electrooxidation of hydrazine and investigate their electroanalytical properties [5]. zhang et al. reported the preparation of pd nps loaded on cobalt nanoparticles wrapped in nitrogen-doped carbon nanotubes for hydrazine determination [6]. considering electrochemical methanol sensors, research is directed toward developing electrode materials that can electrochemically oxidize methanol at low potentials with high selectivity, providing high sensitivity. tao et al. proposed an electrochemical methanol sensor based on palladium-nickel nanoparticles dispersed onto the surface of the silicon nanowires [7]. this composite electrode shows good electrooxidation capability for methanol, and because of that, it can be used for its detection. electrochemical sensor for the determination of methanol-based on pd nps supported on santa barbara amorphous-15prnhetnh2 (pdnps@sba-15-pren) is synthetized as well [8]. the aim of this paper is to prepare the electrode applying the electrochemical deposition of pd without any additives in a simple one-step procedure and to examine its electrocatalytic activity and sensitivity in hydrazine and methanol oxidation. the sensing parameters such as sensitivity and detection limits are determined by means of differential pulse voltammetry (dpv) measurements. experimental all electrochemical measurements were performed using volta lab pgz 402 (radiometer analytical, lyon, france). a conventional three-electrode system was employed, including a gold rotating disk electrode (au) electrode (geometric surface area 0.196 cm2) as the working electrode, pt wire as the counter electrode, and a saturated calomel electrode (sce) as the reference electrode. a mirrorlike polished au electrode was prepared as previously stated [9]. the electrochemical deposition was performed without any additives in a simple one-step procedure at a potential of -0.16 v within 100 s and 1000 rpm at room temperature from deaerated 0.1 m nacl solution containing 30 mm pdcl2 as was previously described [10]. in dpv measurements, the accumulation of the hydrazine at the working electrode was carried out for 0.2 s at -400 mv and 0.2 s at -600 mv for methanol. after that, the potential was scanned at a scan rate of 0.125 mv s-1, pulse amplitude of 50 mv, f = 5 hz and step value 2 mv. all reagents were of analytical grade and purchased from sigma-aldrich. all solutions were prepared using nanopure water supplied by a milli-q system (millipore®) with resistivity ≥18.2 mω cm. electrochemical measurements were performed under n2 atmosphere at ambient temperature. results and discussion electrochemical oxidation of hydrazine a complete characterization of prepared pd catalyst involving structural, morphological, and surface studies, was previously reported [10]. according to sem measurements, a simple j. d. lović j. electrochem. sci. eng. 12(2) (2022) 275-282 http://dx.doi.org/10.5599/jese.1166 277 electrochemical deposition process without surfactants created the morphology of nanorods with a diameter of ∼5 nm and an aspect ratio of ∼8. a clear rod shape was resolved by tem image. it is well known that the electrode surface area and reactant concentration affect the current. in the presented work, one electrodeposited pd electrocatalyst was examined and it is reasonable to assume that the electrochemically active surface area is the same in all experiments. if the other parameters such as temperature and the viscosity of the solution remain constant, the current is proportional to the reactant concentration. because of that, the results were given per geometric surface area or just as current intensity. the electrocatalytic activity of electrodeposited pd in the oxidation of 0.5 mm hydrazine was investigated in 0.1 m k2so4 (ph 7) at 100 mv s-1 as is presented in fig 1a. neutral solution was selected as a blank electrolyte since it has been shown that the electrooxidation of hydrazine is more favored under neutral conditions regarding alkaline medium [11,12]. one anodic peak for hydrazine oxidation at 80 mv in the forward scan can be noticed. the absence of a current peak in the backward scan implies that the electrooxidation of hydrazine is an irreversible process that occurs during the positive potential scan. it has been shown that for the oxidation of hydrazine at platinum electrodes, surface oxides are substantial to electrocatalysis [13]. to establish the necessity of surface oxides, voltammetry was conducted in the absence of hydrazine at the pd electrode. figure 1b shows the typical response feature of the pd surface. the peak in the range of -700 to -400 mv is for the oxidation of the adsorbed and absorbed hydrogen. a reversible redox feature in the potential region -200 – 100 mv is ascribed to the influence of ions in a sulphate electrolyte [14] and possibly also to the effect of the metal loading [15]. as is illustrated in figure 1b the formation of pd oxides starts at e > 400 mv, which is a higher overpotential than noticed in hydrazine oxidation. so, the effect of surface palladium oxide on electrocatalytic activity of pd catalyst should be neglected during the hydrazine electrooxidation. e / mv vs. sce e / mv vs. sce figure 1. cv of electrodeposited pd in 0.1 m k2so4 in the presence (a) and absence (b) of 0.5 m n2h4. voltammetric response, forward direction, of pd electrode towards electrochemical oxidation of hydrazine was investigated as a function of scan rate (fig. 2). with an increasing scan rate, a positive shift in the oxidation peak potential along with enhancement in catalytic current density was noticed. the plot of current density vs. square root of scan rate (fig. 3a) showed a linear -600 -400 -200 0 200 400 0 200 400 -800 -600 -400 -200 0 200 400 600 -400 -300 -200 -100 0 100 200 300 a) pd / 0.1 m k 2 so 4 + 0.5 m n 2 h 4 v = 100 mv s -1 e / mv (sce) j /  a c m -2 j /  a c m -2 e / mv (sce) b) pd / 0.1 m k 2 so 4 v = 50 mv s -1 http://dx.doi.org/10.5599/jese.1166 j. electrochem. sci. eng. 12(2) (2022) 275-282 palladium as efficient electrocatalyst 278 dependency, demonstrating that hydrazine electrooxidation on pd electrode is a diffusioncontrolled process which is in accordance with the reported results [3,16]. e / mv vs. sce figure 2. the forward direction of cvs obtained on pd electrode towards 0.5 mm n2h4 in 0.1 m k2so4 at a scan rate from 5 to 160 mv s-1 a b v0.5 / (mv s-1)0.5 log (v / mv s-1) figure 3. (a) plot of peak current vs. square root of scan rate. (b) plot of peak potential from the log of scan rate. data collected from fig. 2 the oxidation potential simultaneously shifts to positive values with a linear correlation between the peak potential and the logarithm of scan rate, as is presented in fig. 3b. bearing in mind that dependency, the tafel slope was considered using the equation (1) for the irreversible diffusioncontrolled process [17]: ep = b/2 log v + const. (1) the slope of the dependency ep – log v is b/2, and it is found to be 62 mv in this work, thus b = 2 × 62 = 124 mv. the tafel slope is b = 59/(1-α)n (α is the transfer coefficient; n the number of electrons involved in the rate-determining). it is well known that the first electron transfer of hydrazine oxidation is rate-determining [4,18]. assuming the number of electrons is 1, the transfer coefficient is estimated as 0.48, illustrating that the rate-limiting step is a one-electron transfer 2 4 6 8 10 12 0.00 0.05 0.10 0.5 1.0 1.5 2.0 2.5 3.0 0 20 40 60 80 100 120 v 0.5 / (mv s -1 ) 0.5 i p /  a log (v / mv s -1 ) e p / m v j. d. lović j. electrochem. sci. eng. 12(2) (2022) 275-282 http://dx.doi.org/10.5599/jese.1166 279 process. the mechanism of hydrazine oxidation was studied electrochemically at the pd electrode in solutions of different phs (ph 2-11), presented in detail [19-21]. it has been shown that the mechanism of hydrazine oxidation depended significantly on the electrolyte solution and the nature of the electrodes [2,19]. hydrazine is oxidized through the 4eprocess with the final product n2 in an aqueous solution [14,18]. the first intermediate of hydrazine oxidation in aqueous solutions has been identified as the radical ion n2h4+ [22] or radical di-cation n2h52+ [19]. the formation of n2h4+ ion was usually regarded as the rate-determining step, while other proposed intermediates are not stable [23]. electrochemical oxidation of methanol figure 4 shows the voltammetric response of investigated pd catalyst in 1 m naoh containing 1 mm methanol at 50 mv s-1. electrodeposited pd showed two anodic peaks for methanol oxidation at -250 mv in the forward scan and at -430 mv in the backward scan. the peak observed in the forward scan was ascribed to the oxidation of chemisorbed species coming from methanol adsorption on the surface of the catalyst in the presence of adsorbed ohspecies. the peak in the reverse scan might be caused by oxidation of freshly chemisorbed species or carbonaceous species, which are not completely oxidized in the forward scan. oh formation ability of a catalyst is in relation to alcohol oxidation activity. it is well documented in the literature that the kinetics of the alcohol oxidation reaction is enhanced by increasing the concentration of ohions in the solution, thus causing the higher coverage of the electrode surface by ohspecies [24,25]. so, unlike hydrazine, methanol oxidation is determined by the degree of chemisorbed species originating from methanol and ohad coverage. kinetic study of methanol electrooxidation reaction on pd-based catalysts is well described in the literature [25,26]. e / mv vs. sce figure 4. cv of electrodeposited pd in 1 m naoh in the presence of 1 mm ch3oh at a scan rate 50 mv s-1 electroanalytic determination of hydrazine and methanol to check the analytical performance of the electrodeposited pd electrode in the electrochemical determination of hydrazine and methanol, dpv and square wave voltammetry (swv) were tested. since the dpv showed improved peak current compared to swv the first-mentioned method was used and optimized to detect these molecules. dpv response of electrodeposited pd catalyst towards different concentrations of hydrazine was presented in figure 5a. with increasing hydrazine -800 -600 -400 -200 0 200 -50 0 50 100 pd / 1 m naoh + 1 mm ch 3 oh v = 50 mv s -1 j /  a c m -2 e / mv (sce) http://dx.doi.org/10.5599/jese.1166 j. electrochem. sci. eng. 12(2) (2022) 275-282 palladium as efficient electrocatalyst 280 concentration, the current of hydrazine oxidation also increases. according to fig. 5b, the calibration plot of current vs. hydrazine concentration gives a linear dependence in the concentration range of 0.1-2 mm hydrazine. an excellent correlation coefficient (r = 0.999) was obtained as well. e / mv vs. sce c / mm figure 5. (a) dpvs obtained on the electrodeposited pd electrode in the presence of increasing hydrazine concentrations. (b) dependency of peak current on hydrazine concentration figure 6a shows the results of dpv obtained on the electrodeposited pd electrode in the presence of an increasing concentration of methanol. alkaline media were chosen because of their higher activity and poisoning resistance regarding acidic or neutral media. it can be noticed that the addition of methanol leads to a rapid and well-resolved increase of the peak currents at 300 mv. ep slightly shifts (15 mv) towards more positive potentials, indicating that electrodeposited pd shows satisfactory poison resistance from electrochemically formed intermediates. e / mv vs. sce c / mm figure 6. (a) dpvs obtained on the electrodeposited pd electrode in the presence of increasing methanol concentration. (b) dependency of peak current from concentration the constructed linear dependency was used to evaluate several electroanalytical parameters. thus, sensitivity was obtained from the slope of the calibration curve. the other electroanalytical parameters are the limit of detection (lod) and the limit of quantification (loq). lod demonstrates the lowest compound concentration which can be detected, while loq demonstrates the lowest concentration of compound, which can be quantitatively determined. both lod and loq were -400 -300 -200 -100 0 100 200 300 400 0 2 4 6 8 0.0 0.5 1.0 1.5 2.0 3 4 5 6 a) pd / 0.1 m k 2 so 4 e / mv (sce) 0.1 mm n 2 h 4 0.2 mm n 2 h 4 0.5 mm n 2 h 4 1 mm n 2 h 4 1.5 mm n 2 h 4 2 mm n 2 h 4 blank solution i /  a b) c / mm i p /  a y = 3.1 + 1.3 x r = 0.999 -600 -500 -400 -300 -200 -100 0 1 2 3 4 5 6 7 8 0 1 2 3 3 4 5 6 7 8 pd / 1 m naoha) c / mm i p /  a blank solution 0.1 mm ch 3 oh 0.2 mm ch 3 oh 0.5 mm ch 3 oh 1 mm ch 3 oh 1.5 mm ch 3 oh 2 mm ch 3 oh 2.5 mm ch 3 oh i /  a e / mv (sce) b) y = 3.7 + 1.5 x r = 0.997 i /  a j. d. lović j. electrochem. sci. eng. 12(2) (2022) 275-282 http://dx.doi.org/10.5599/jese.1166 281 calculated according to the equations k’ sd / b, where k’ = 3 for lod and 10 for loq, sd is the standard deviation of the intercept and b is the slope of the calibration curve [27]. the obtained electroanalytical parameters are given in table 1. it should be noted that the comparison of the electrochemical result is not unambiguous, as the electrochemical parameter is dependent on more than one experimental variable. table 1. determination of investigated compounds by dpv compound concentration linear range, mm sensitivity, µa mm-1 lod, mm loq, mm r methanol 0.1-2.5 1.5 0.21 0.71 0.997 hydrazine 0.1-2 1.3 0.09 0.3 0.999 the sensing capability of pd in hydrazine detection is comparable to that established on pd species deposited on the multi-walled carbon nanotubes dispersed in the nafion membrane [5]. nevertheless, the obtained lod and loq values for both investigated compounds are lower than the reported data obtained on pd nanostructures [5,8,18,28], indicating the unavoidable improvement of the electrochemical characteristic. following this strategy, the synthesis of pd nanostructures of diverse shapes has been extensively used. additionally, a mixture of metals forming alloys or core-shell structures can be found in literature as an effective way of improving electrocatalytic activity. enhancing the electrochemical activity and sensitivity towards hydrazine and methanol oxidation will be a matter of future studies. conclusions electrochemical deposition of pd was performed without any additives in a simple one-step procedure. electrodeposited pd was then used as an electrocatalyst for the electrochemical oxidation of hydrazine and methanol. the electrochemical behavior of the pd electrode reveals that the effect of surface palladium oxide should be neglected in the course of hydrazine 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the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.electacta.2010.04.013 https://doi.org/10.1016/j.ab.2018.01.033 http://dx.doi.org/10.1016/j.electacta.2014.06.048 http://dx.doi.org/10.1021/ja8063765 https://doi.org/10.1016/‌j.microc.‌2019.104004 https://doi.org/10.1016/‌j.microc.‌2019.104004 http://dx.doi.org/10.1039/c8ta06219f http://dx.doi.org/10.1039/c0cp02261f http://dx.doi.org/10.1016/j.jelechem.2015.01.009 https://doi.org/10.1016/j.ijhydene.2018.07.137 http://dx.doi.org/10.2116/analsci.21.1317 https://doi.org/10.1016/s0022-0728(70)80060-2 http://dx.doi.org/10.1016/j.snb.2007.03.044 http://dx.doi.org/10.1016/j.snb.2007.03.044 https://doi.org/10.1016/j.electacta.2021.138655 https://doi.org/10.1016/j.electacta.2021.138655 https://doi.org/10.1016/j.jelechem.2019.113661 https://doi.org/10.1021/acs.jpcc.5b10156 https://doi.org/10.1021/j100682a023 https://doi.org/10.1021/j100682a023 https://doi.org/10.1016/0022-0728(93)87056-2 https://doi.org/10.1016/j.elecom.2007.06.036 https://doi.org/10.1016/j.electacta.2015.10.151 https://doi.org/10.22052/jns.2019.03.015 https://doi.org/‌10.1039/c2ay25111f https://doi.org/‌10.1039/c2ay25111f https://doi.org/10.1002/elan.201500453 https://creativecommons.org/licenses/by/4.0/) {a 3-dimensional mathematical model to study effects of geometrical parameters on performance of solid oxide fuel cell:} http://dx.doi.org/10.5599/jese.1097 291 j. electrochem. sci. eng. 11(4) (2021) 291-304; http://dx.doi.org/10.5599/jese.1097 open access : : issn 1847-9286 www.jese-online.org original scientific paper three-dimensional mathematical model to study effects of geometrical parameters on performance of solid oxide fuel cell vikalp jha, vikranth kumar surasani and balaji krishnamurthy department of chemical engineering, bits pilani, hyderabad 500078, india corresponding author: balaji.krishb1@gmail.com received: august 31, 2021; accepted: september 12, 2021; published: september 25, 2021 abstract a 3d mathematical model is developed to study effects of various geometrical parameters such as cathode to anode thickness ratio, rib width, and channel width under various flow conditions, on the performance of solid oxide fuel cell (sofc). these parameters represent the cathode supported configuration of the solid oxide fuel cell. it is observed from simulation results that performance of sofc fuel cell is increased at higher cathode to anode thickness. simulation results also showed that for different volumetric flow rates, the current density and fuel cell performance decrease as rib width increases, what is due to higher contact resistance. it is also shown that by increasing the channel width, the fuel cell performance was increased due to increase in the reaction surface area. simulation results are compared and validated with literature experimental data, showing well agreement. keywords current density; channel width; cathode; cathode to anode thickness. introduction in past several years, solid oxide fuel cells (sofcs) of planar configurations were established as some of the most efficient energy conversion devices, convenient for various industrial applications. various researchers have worked on different configurations (tubular, planar, …) of solid oxide fuel cells serving for conversion of chemical energy to electrical energy. yakabe et al. [1] studied a 3-dimensional mathematical model of a planar sofc to observe the effect of heat transfer and chemical reactions in fuel cells of co-flow parallel and counter flow configurations. the authors postulated that co-flow configurations are better in comparison to counter flow configuration of planar sofc owing to lower internal stresses. sun et al. [2] studied the experimental fabrication of anode supported single chamber sofc for various electrolyte and cathode structures. the authors postulated that the fuel cell performance is better for lower electrolyte thickness and with increment in fuel (methane and hydrogen) and oxidizer ratio. http://dx.doi.org/10.5599/jese.1097 http://dx.doi.org/10.5599/jese.1097 http://www.jese-online.org/ mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 292 chinda [3] studied a micro scale model of sofc in order to study its performance. the author postulated that an increase of the electrode surface area improves the performance of the fuel cell. shichuan et al. [4] studied a 3d numerical model of anode and cathode supported sofc stack to observe effects of cell design on the fuel cell stack performance. the authors postulated that for the optimal rib width, cathode supported fuel cell is more efficient in comparison to anode supported fuel cell. zaccaria et al. [5,6] studied a 1d transient model to simulate a co-flow parallel anode supported sofc to study the effect of model characteristics on fuel cell degradation. the authors postulated that the current density, fuel utilization and temperature at inlet are reduced with time, whereas at cell outlet these parameters increase with time. these authors also studied the fuel cell performance for ohmic, activation and diffusion losses on degradation of sofc. cunio et al. [7] developed a fuel cell degradation model for sofc and a gas turbine hybrid system. the authors postulated that the fuel cell life was increased for hybrid systems in comparison to stand alone configurations. giosue et al. [8] developed a computational fluid dynamics model to analyse the thermal effect, and flow rate of a sofc system in the fuselage of a hybrid electric mini unmanned aerial vehicle. khan et al. [9,10] studied experimentally the effect of applied current density on anode supported tubular sofc. the authors postulated that with increasing time, cell voltage and power density decrease at higher current densities, whereas degradation rate increases with time. shen et al. [11] developed a 3d mathematical model to analyse fuel cell performance in channel flow with consideration on obstacles to see thermal and chemical reaction effect on sofc. the authors postulated that the peak temperature of fuel cell with obstacles is lower than fuel cell without obstacles, and maximum current density is observed at the cathode of fuel cell. bianco et al. [12] studied transient degradation of material in the interconnect between anode and cathode in sofc. dwivedi [13] studied the effect of various materials used for anodes, cathodes and electrolytes of sofc. the author postulated that among various materials used for efficient electrolytes, yttriastabilized zirconia (ysz) is the most efficient. zhou at al. [14] studied the effect of diffusion of co and co2 between the electrodes in a direct carbon solid oxide fuel cell. the authors postulated that the fuel cell performance decreases with increase of a distance between electrodes. min et al. [15] developed a 1d model for thermodynamic analysis of sofc stack and its operating conditions. the authors postulated that an increase in current density is suitable for more power density and thermal energy utilization. wang et al. [16] studied a 3d finite element-based model to study effect of inhomogeneous oxidation on mechanical degradation of anode supported sofc where inhomogeneous oxidation induces a large stress gradient in anode. hussain et al. [17] studied various electrochemical properties in a 3-dimensional model of sofc. the authors showed that with an increase of cell temperature, the overall performance of sofc increases due to enhanced electrochemical reaction rate and lower concentration loss. the authors also stated that with reduced anode thickness and electrolyte thickness, the fuel cell performance increases as result of reduced ohmic loss and concentration loss, respectively. in this study, a 3-dimensional mathematical model is developed to study the effect of various geometrical parameters such as cathode to anode thickness ratio, and the effect of flow conditions at different rib to channel width ratios on the performance of solid oxide fuel cell (sofc). to the best of our knowledge, these have never been studied before. model development model assumptions a) all fuel cell reactions are considered under steady state. v. jha et al. j. electrochem. sci. eng. 11(4) (2021) 291-304 http://dx.doi.org/10.5599/jese.1097 293 b) laminar flow is considered (reynolds number = 25, based on channel dimensions). c) water formation is assumed in vapour form under high operating temperature. d) all reacted vapour and gases are considered as ideal gases. governing equations at anode: h2 + o2→ h2o + 2e(1) at cathode: 0.5o2 + 2e→ o2(2) in the channels, gas flow is governed by the navier-stokes equations. the mass conservation equation is given by: for anode: a( ) 0u = (3) for cathode: c( ) 0u = (4) momentum equation for anode (channel flow): a a i,j i,j( )u u pi = − − + (5) i,j a a a i,j 2 ( ) ( ) 3 t u u u i   =  +  −    (6) for cathode (channel flow): c c i,j i,j( )u u pi = − − + (7) i,j c c c i,j 2 ( ) ( ) 3 t u u u i   =  +  −    (8) in eqns. (3-8), ρ is the density, ua and uc is the inlet velocity vector at anode and cathode, p is the pressure, τi,j is the viscous stress tensor, ii,j is identity tensor and µ is the dynamic viscosity. secondary current distribution the electrochemical reactions at the electrodes, and their kinetics responsible for activation over-potential are described by butler-volmer equation[17,18]. 0r 0 exp exp ff i i rt rt     −−    = −          (9) where i is the current density, i0 is the exchange current density, and r and 0 are transfer coefficients, t is the operating temperature, f and r are the faraday and the gas constants, and  is the overpotential. when the overpotential is high as for the cathode, butler-volmer equation can be simplified to the tafel equation [17,18] 0 ln i a i    =     (10) where a is tafel slope. charge transport in the electrode and electrolyte is based on ohm’s law, described by: l li q = (11) http://dx.doi.org/10.5599/jese.1097 j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 294 l l li  = −  (12) s si q = (13) s s si  = −  (14) in eqn. (11), il is the current density, ql is a source term, l is the conductivity and l is the potential in electrolyte. in eqn, (12), is the current density qs is a source term, s is the conductivity and s is the potential in electrode. the concentrations of hydrogen and oxygen at the electrode-membrane interface can be determined from henry’s law equation of the forms expressed in the following two equations [17,18]: 2 h h h h x p c k = (15) 2 o o o o x p c k = (16) where, xh and xo are mass fraction of hydrogen and oxygen respectively. kh and ko are henry’s constants and ph, po pressure for hydrogen and oxygen in fuel cell channel. brinkman equations (anode) in porous media of the catalyst and diffusion layers, the navier-stokes equation is changed into the brinkman equations and chemical species transport in ideal gas mixtures is described by the maxwell-stefan equation [17,18]. ( ) ( )( ) ( ) 1 ma a i,j a a a i,j f a a2 p p p p p 1 1 2 1 + + + 3 t a q u u p i u u u i u u f           −  =  −  +  −  +             (17) a m( )u q = (18) brinkman equations (cathode) ( ) ( )( ) ( ) -1 mc c c i,j c c c i,j f c c2 p p p p p 1 1 2 1 = + + + + + 3 t q u u p i u u u i u u f                             (19) c m( )u q = (20) where εp is gas diffusion layer porosity, pc and pa are pressure at cathode and anode, uc and ua is inlet velocity vector at cathode and anode respectively,  is permeability of porous media, ii,j is identity tensor,  is density and qm is mass source. boundary condition wall –no slip a c 0u u= = (21) cathode inlet – pressure inflow ( )( ) ( )c i,j c c c i,j 0 p p 1 2 1 3 tt n p i u u u i n p      − +  +  −  = −     (22) anode inlet – pressure inflow ( )( ) ( )a i,j a a a i,j 0 p p 1 2 1 3 tt n p i u u u i n p      − +  +  −  = −     (23) where p0 is inlet pressure. v. jha et al. j. electrochem. sci. eng. 11(4) (2021) 291-304 http://dx.doi.org/10.5599/jese.1097 295 transport of concentrated species for anode and cathode the equation is given by i i i( )j u r  +  = (24) where ji is flux density, u is velocity vector,  is density and i is mass fraction. anode and cathode outlet boundary condition: pressure (p0 = 0). numerical methods a three-dimensional computational domain of a single unit of solid oxide fuel cell (sofc) is shown in figure 1. in our optimization, various cathode to anode electrode thickness, rib width and channel width are considered for analysis. to study the effect of all ratios individually, other flow channel parameters were considered constant. rib width is considered equal on both sides of channel. in our analysis, three different ratios of cathode to anode electrode thickness (1, 1.5, 2), and channel to rib width (0.5, 0.75, 1) are considered to observe fuel cell performance at flow rates of 0.0002 and 0.0003 mm3/s. at the fixed flow rate, rib width is analysed for higher values of 1 mm, 1.25 mm and 1.5 mm. structural hexahedral elements are used for meshing of computational flow domain. mesh distribution across computational domain is shown in figure 2. figure 1. schematic of sofc figure 2. mesh distribution across computational domain of sofc our computational domain consists of 7424 hexahedral mesh elements, 3456 boundary elements and 572 edge elements. average skewness quality of our mesh elements is 1. maximum element size of 1.6 mm and minimum element size of 0.288 mm is used for meshing. mesh elements of our computational domain are adaptive with respect to various cathode to anode thickness ratios and channel width to channel depth ratios. http://dx.doi.org/10.5599/jese.1097 j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 296 figures 3(a), 3(b) and 3(c) show mesh distribution across cross section of three different cathode to anode thickness ratios (1, 1.5, 2) configurations. a b c figure 3. mesh distribution across cross section of sofc configurations with cathode to anode thickness ratio: (a) 1; (b) 1.5; (c) 2 grid independence test is done for 6800, 7424, 8756 and 10512 number of mesh elements. for various mesh sizes, the fuel cell performance variance is negligible as shown in table 1. a mathematical model is developed to analyse our computational domain in comsol software version 5.3a on performance of solid oxide fuel cell, fluid dynamics, species transport and current distribution. table 1. grid independence test for current density at 101325 pa pressure and 1073 k temperature number of mesh elements cathode to anode thickness ratio 1 1.5 2 current density, a m-2 6800 4848.0 5109.8 5232.8 7424 4848.3 5110.6 5234.1 8656 4847.7 5110.3 5233.7 10512 4847.1 5110.2 5233.8 results cathode to anode thickness ratio solid oxide fuel cell performance is studied under various operating conditions and parameters. figure 4 shows the current density variation for different sofc configurations with respect to cathode to anode thickness ratio. it is observed from figure 4 that as the cathode to anode thickness ratio increased from 1 to 2, the current density increases by 8.5 %, from 4800 to 5250 a/m2. increasing the cathode thickness gives rise to several outcomes. the reactive active sites (ras) for the evolution of oxygen ions increases with increasing the cathode thickness. the cathode thickness should be optimized in such a way that there is sufficient ras for the cathode reaction, but the reactant gas should be able to diffuse through to the reaction sites. increasing the cathode thickness also increases the ohmic resistance across the cell. thus, optimizing of the cathode thickness is very critical to sofc performance. the generation of oxygen ions on the cathode is the driving force for the anode reaction as per equation 2. figure 4 indicates that increasing the cathode to anode thickness ratio from 1 to 1.5 causes an increase in the performance of the sofc, while increasing the cathode to anode thickness ratio from 1.5 to 2 causes a very limited increase in the performance of the sofc. figure 5 shows that as cathode to anode thickness ratio increased from 1 to 2, the average cell power increases by 13.3 %, from 1280 to 1450 w/m2. however, it is seen that the increase in power density when the thickness ratio is increased from 1.5 to 2 is lesser than when the thickness ratio is increased from 1 to v. jha et al. j. electrochem. sci. eng. 11(4) (2021) 291-304 http://dx.doi.org/10.5599/jese.1097 297 1.5. this indicates that increasing the thickness ratio beyond a certain value does not improve sofc performance. it is highly possible that with increasing cathode thickness, ras increases, increasing the generation of oxygen ions which drive the power density of sofc. however, the increasing thickness of the cathode film increases the ohmic resistance of the cell, causing power density to decrease. the optimum value of cathode thickness needs to be identified so that the maximum power density of the sofc can be obtained. sun et al. [2] postulated that for the optimum performance of sofc, the cathode to anode area should be close to 1. our simulation results indicate that the cathode/anode thickness of 1.5 would be ideal for the optimum performance of sofc. figures 6 and 7 show the fuel utilization and hydrogen mole fraction variation across sofc channel length. it is observed from these figures that as cathode to anode thickness ratio increases, reactive area increases, and more hydrogen fuel is consumed per unit length of sofc channel. figures 6 and 7 show that increasing cathode to anode thickness ratio from 1 to 2 leads to 3% increment in hydrogen fuel consumption. figure 8 shows oxygen mole fraction along the sofc channel length. it is observed that with increase in cathode to anode thickness ratio, fuel oxidation increases, and more oxygen is consumed with increase in the reaction rate. sun et al. [2] have postulated that cathode to anode thickness ratio should be close to 1 for efficient performance of the fuel cell. although in our simulation we further increased cathode to anode thickness ratio from 1 to 2, which increases cathode reaction area, more oxygen ion formation for driving force to anode reaction as defined earlier and leads to increase in more current and power density. figure 4. polarization curve of sofc for various cathode to anode thickness ratios figure 5. cell power and current density variation of sofc for various cathode to anode thickness ratios http://dx.doi.org/10.5599/jese.1097 j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 298 figure 6. fuel utilization per channel length of sofc for various cathode to anode thickness ratios figure 7. hydrogen mole fraction of sofc for various cathode to anode thickness ratios figure 8. oxygen mole fraction of sofc for various cathode to anode thickness ratios rib width the sofc performance with various channel and rib widths is also studied. the ribs which separate and define flow channels make a direct contact with electrodes. to optimize the performance, there must be a trade-off between the rib and channel sizes. on one hand, wider ribs and ribs covering bigger fraction of the cell area may reduce the interface resistance to current flow by increasing the electrode interconnect contact area with electrodes and reducing the current path through the electrode material [19]. such ribs will give better conduction of the electrical current and reduce ohmic losses. however, chemical species do not diffuse very well under wide ribs. v. jha et al. j. electrochem. sci. eng. 11(4) (2021) 291-304 http://dx.doi.org/10.5599/jese.1097 299 narrow ribs are required to facilitate more uniform distribution of the reactive gases across the area of the electrode surface and promote electrochemical performance. it is very important to understand this trade-off between rib dimensions and performance of the sofc. figure 9 shows polarization curve of sofc for various rib widths. it is observed from this figure that as rib width of sofc increases from 1 to 1.25 mm, the current density decreases by 10.11 %, from 4450 to 4000 a/m2. further increasing of rib width to 1.5 mm leads to further reduction of current density by 10%, from 4000 to 3600 a/m2. increase of rib width will also lead to larger contact area resistance which will lead to decrease in the fuel cell performance. when the rib width is higher, due to more contact resistance, gas concentration is not uniform across fuel cell and lower compared to the narrow rib width. hence, lower rib width is better than higher rib width configuration. figure 10 shows the variation of cell power and current density for different rib widths. it shows that as the rib width increases from 1 to 1.5 mm, peak cell power decreases by 13.27 %, from 1311 to 1137 w/m2. figures 11 and 12 show hydrogen fuel consumption across channel length for various rib widths. these figures show that with increase of rib width from 1 to 1.5 mm, hydrogen fuel consumption is decreased from 18 to 6 %. figure 13 shows oxygen mole fraction for various rib widths along flow channel length. it shows that with an increase in rib width, oxygen consumption is reduced from 23.80 to 9.5 %. figure 9. polarization curve of sofc for various rib widths figure 10. cell power and current density variation of sofc for various rib widths http://dx.doi.org/10.5599/jese.1097 j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 300 figure 11. fuel utilization per channel length of sofc for various rib widths figure 12. hydrogen mole fraction of sofc for various rib widths figure 13. oxygen mole fraction of sofc for various rib widths flow rate to rib width ratio solid oxide fuel cell performance decreases as rib width were increased from 1 to 1.5 mm. for further analysis, smaller rib width (0.5 mm, 0.75 mm, 1mm) is considered at 0.0002 and 0.003 mm3/s volumetric flow rates. figure 14 shows the polarization curve for different flow rate to rib width ratios. it is observed from figure 14 that as the rib width was decreased from 1 to 0.5 mm, the current density increased. also, the current density increased with the increase of the volumetric flow rate for the same rib width. lower rib widths with higher flow rates seem to give the maximum v. jha et al. j. electrochem. sci. eng. 11(4) (2021) 291-304 http://dx.doi.org/10.5599/jese.1097 301 current density. similarly, figure 15 shows the average cell power along with current density variation for various flow rate to rib width ratios. for higher flow rate to rib width ratio, the current density, as well as average cell power increased. with increase of the flow rate, more hydrogen fuel is consumed during reaction, what leads to higher reaction rate and increase of the current density. figure 14. polarization curve of sofc for various flow rate to rib width ratios figure 15. cell power and current density variation of sofc for various flow rate to rib width ratios flow rate to channel width ratio sofc fuel cell performance is studied for various flow rate to channel width ratios, and polarization curves are presented in figure 16. from figure 16 it is observed that as the channel width was increased from 0.5 to 1 mm, the current density increased. increasing the volumetric flow rate is also seen to increase the current density. figure 16. polarization curve of sofc for various flow rate to channel width ratios http://dx.doi.org/10.5599/jese.1097 j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 302 further increase in channel width from 1 to 1.5 mm also increased the fuel cell performance but with less increment in current density values. similarly, figure 17 shows average cell power along with current density variation for various flow rate to channel width ratios. for higher flow rate to channel width ratio, current density, as well as average cell power increased. with an increase in channel width, the reaction area increases what leads to transport of more fuel for reaction driving the reaction rate. figure 17. cell power and current density variation of sofc for various flow rate to channel width ratios figure 18 shows the comparison between simulation results and experimental data. sun et al. [2] studied experimental modelling of single chamber solid oxide fuel cell at various temperatures and electrode particles. in figure 18, simulation results are compared with the experimental polarization graph at 873 k temperature, showing well agreement. figure 18. comparison of simulation with experimental data 2 at 873 k temperature conclusion a 3d mathematical model is developed to study the effect of various geometrical parameters such as cathode to anode thickness ratio, rib width, and channel width at different flow rates, on the performance of 3-dimensional model of solid oxide fuel cell (sofc). simulation results show that performance of sofc fuel cell is increased at higher cathode to anode thickness. from simulation results, it is also observed that as the rib width increases, due to increase of the contact area resistance, the fuel cell performance and average cell power decrease. however, with increase in v. jha et al. j. electrochem. sci. eng. 11(4) (2021) 291-304 http://dx.doi.org/10.5599/jese.1097 303 channel width, the current density is increased due to increase of the reaction area and consequently, the average cell power is also increased. simulation results indicate that the cathode supported sofc show better performance as the cathode to anode thickness ratio was increased. it is also observed that flow rate plays major role in the fuel cell performance. it is seen that with increase in the volumetric flow rate at various rib and channel widths, the performance of sofc fuel cell increases. higher cathode to anode thickness ratio, smaller rib widths, larger channel widths and increasing volumetric flow rate are found to increase the performance of the fuel cell. model results are compared with experimental data taken from the literature and found to compare well. list of parameters [17,18] xo = 0.21 inlet oxygen mole fraction (cathode) xh = 1 inlet hydrogen mole fraction (anode) wr = 1 mm rib width; wch = 1 mm channel width; hc = 1 mm channel depth hl = 0.1 mm electrolyte thickness va = 0 v anode voltage; vc = 1 v cathode voltage ha and hc = 0.1, 0.15, 0.2 mm anode and cathode electrode thickness t = 1073 k cell temperature l = 5 s m-1 electrolyte conductivity; s = 1000 s m-1 electrode conductivity pa and pc = 101325 pa reference pressure for anode and cathode µa and µc = 3 × 10 -5 pa s anode and cathode viscosity l = 16 mm cell length a and c = 1×10-10 m2 anode and cathode permeability f = 96847 c mol-1 faraday constant; r = 8.314 j mol-1 k-1 l = 0.3 -electrolyte phase volume fraction p = 0.4 -gas diffusion layer porosity io,a = 0.1 a m-2 exchange current density(anode) io,c =0.01 a m-2 -exchange current density (cathode) 2 2o n d = 1.9235×10-4 m2 s-1 o2-n2 binary diffusion coefficient 2 2o h o d = 2.451×10-4 m2 s-1 o2-h2o binary diffusion coefficient 2 2n h o d = 2.4477×10-4 m2 s-1 n2-h2o binary diffusion coefficient 2 2h h o d = 8.5871×10-4 m2 s-1 h2-h2o binary diffusion coefficient data availability statement some or all data, models, or code that support the findings of this study are available from the corresponding author upon reasonable request. acknowledgement: the authors would like to acknowledge bits pilani, hyderabad and council for scientific and industrial research, csir grant no: [22/0784/19/emr ii] which helped us in publishing this article. references [1] h. yakabe, t. ogiwara, m. hishinuma, i. yasuda, journal of power sources 102(1-2) (2001) 144154. https://doi.org/10.1016/s0378-7753(01)00792-3 [2] l. p. sun, m. rieu, j. p. viricelle, c. pijolat, h. zhao, international journal of hydrogen energy 39(2) (2014) 1014-1022. https://doi.org/10.1016/j.ijhydene.2013.10.117 [3] p. chinda, energy procedia 34 (2013) 243-261. https://doi.org/10.1016/j.egypro.2013.06.753 http://dx.doi.org/10.5599/jese.1097 https://doi.org/10.1016/s0378-7753(01)00792-3 https://doi.org/10.1016/j.ijhydene.2013.10.117 https://doi.org/10.1016/j.egypro.2013.06.753 j. electrochem. sci. eng. 11(4) (2021) 291-304 performance of solid oxide fuel cell 304 [4] s. su, x. gao, q. zhang, w. kong, d. chen, international journal of electrochemical science 10 (2015) 2487-2503. 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https://doi.org/10.1016/j.cap.2020.02.018 https://doi.org/10.1016‌/j.jpowsour.2008.04.054 https://doi.org/10.1016‌/j.jpowsour.2008.04.054 https://creativecommons.org/licenses/by/4.0/) {the optimization of an electrochemical aptasensor to detect rbd protein s sars-cov-2 as a biomarker of covid-19 using screen-printed carbon electrode/aunp:} http://dx.doi.org/10.5599/jese.1206 219 j. electrochem. sci. eng. 21(1) (2022) 219-235; http://dx.doi.org/10.5599/jese.1206 open access : : issn 1847-9286 www.jese-online.org original scientific paper the optimization of an electrochemical aptasensor to detect rbd protein s sars-cov-2 as a biomarker of covid-19 using screen-printed carbon electrode/aunp arum kurnia sari1, yeni wahyuni hartati1,2,, shabarni gaffar1,2, isa anshori3, darmawan hidayat4, hesti lina wiraswati5 1department of chemistry, faculty of mathematics and natural sciences, universitas padjadjaran, indonesia 2moleculer biotechnology and bioinformatics research center, universitas padjadjaran, indonesia 3lab-on-chip group, biomedical engineering department, bandung institute of technology, indonesia 4department of electrical engineering, faculty of mathematics and natural sciences, universitas padjadjaran, indonesia 5department of parasitology, faculty of medicine, universitas padjadjaran, indonesia corresponding author:  yeni.w.hartati@unpad.ac.id; tel.: +628122132349 received: december 7, 2021; accepted: february 8, 2022; published: february 21, 2022 abstract severe acute respiratory syndrome coronavirus 2 (sars-cov-2) is the virus identified as the cause of the coronavirus disease 2019 (covid-19) outbreak. the gold standard for detecting this virus is polymerase chain reaction (pcr). the electrochemical biosensor method can be an alternative method for detecting several biomolecules, such as viruses, because it is proven to have several advantages, including portability, good sensitivity, high specificity, fast response, and ease of use. this study aims to optimize an electrochemical aptasensor using an aunp-modified screen-printed carbon electrode (spce) with an aptamer to detect rbd protein s sars-cov-2. aptasensors with the streptavidin-biotin system were immobilized on the spce/aunp surface via covalent bonding with linkers to 3-mercaptopropionic acid (mpa) and electrochemically characterized by the [fe(cn)6]3-/4 redox system using differential pulse voltammetry. the results showed that the immobilized aptamer on the spce/aunp electrode surface experienced a decrease in current from 11.388 to 4.623 µa. the experimental conditions were optimized using the box-behnken experimental design for the three factors that affect the current response. the results of the optimization of the three parameters are the concentration of aptamer, incubation time of aptamer, and incubation time of rbd protein s sars-cov-2, each of which is 0.5 µg/ml, 40 minutes, and 60 minutes, respectively. the rbd protein s sars-cov-2 with various concentrations was tested on an electrochemical aptasensor to determine the detection http://dx.doi.org/10.5599/jese.1206 http://dx.doi.org/10.5599/jese.1206 http://www.jese-online.org/ mailto:yeni.w.hartati@unpad.ac.id j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 220 limit and quantification limit, and the respective results were 2.63 and 7.97 ng/ml. the electrochemical aptasensor that has been developed in this study can be applied to detect rbd protein s sars-cov-2 as a covid-19 biomarker in a simple, practical, and sensitive way. keywords box-behnken design, 3-mercaptopropionic acid, differential pulse voltammetry, portability, fast response introduction sars-cov-2 is a positive single-strand rna virus with a size of 29.9 kb, which is responsible for the covid-19 disease. based on the phylogenetic analysis, sars-cov-2 is included in the betacoronavirus of the subgenus sarbecovirus. sars-cov-2 has four structural proteins, namely spike protein (s), membrane protein (m), an envelope protein (e), and nucleocapsid protein (n), which are integrated into the phospholipid bilayer [1,2]. the s protein consists of two functional subunits, namely the s1 subunit, which is responsible for attaching the virus to receptors on the host cell surface via the receptorbinding domain (rbd), and the s2 subunit, which is responsible for the fusion of the viral membrane with the host cell to facilitate the entry of the virus into the host cell [3,4]. as a result, the rbd protein s sars-cov-2 has emerged as a prime target for diagnosis, treatment, and vaccination [5]. currently, there are several types of methods used to detect sars-cov-2, such as molecular tests based on detection of viral rna, antigen tests based on detection of viral proteins, and antibody tests based on detection of specific antibodies against viral proteins. reverse transcription polymerase chain reaction (rt-pcr) is the gold standard method for the detection of sars-cov-2 virus rna. the main limitations of this method are the expensive equipment requirements and the need for highly qualified experts. the molecular technique necessitates a lengthy sample processing time, as well as complex and expensive facilities. lateral flow immunoassays (lfias) provide a faster response but have lower sensitivity [6]. meanwhile, detection methods targeting antibodies based on enzyme-linked immunosorbent assay (elisa) are not suitable for early diagnosis because most patients have antibody responses around 7 to 21 days after infection [7,8]. the electrochemical biosensor has been widely used to detect a variety of biomolecules, such as viruses [9]. biosensors have been shown to have many benefits, including portability, ease of use, quick response, high sensitivity, and high specificity [10,11]. antibodies, aptamers, and nucleic acids, among some other bioreceptors, can be well immobilized on the electrode surface for detection purposes [9]. electrochemical biosensor methods have also been reported to be used for the detection of sars-cov-2 by using various bioreceptors, such as antibodies [12–15], ssdna [16,17], antigens [18,19], and aptamers [20,21]. the aptamer is a single-stranded dna or rna nucleic acid molecule selected through an in vitro method known as systematic evolution of ligands by exponential enrichment (selex) [22], which can bind to various targets with high selectivity and specificity, such as metal ions, small molecules, proteins, amino acids, peptides, microorganisms, cells, viruses, and other nucleic acids [23–25]. compared with antibodies, aptamers have several advantages, namely smaller size, thermal stability, high affinity, excellent sensitivity, easy synthesis, lower toxicity, and easy chemical modification [22]. biosensors that utilize aptamers as identification elements are referred to as aptasensors. an aptamer for sars-cov-2 has been reported by song et al. [5] of the selex method that binds to the rbd protein s sars-cov-2. the rbd protein s sars-cov-2 and the aptamer cov2-rbd form hydrogen bonds with the amino acids of the rbd protein s sars-cov-2 [5]. a. k. sari et al. j. electrochem. sci. eng. 21(1) (2022) 219-235 http://dx.doi.org/10.5599/jese.1206 221 aptamer immobilization technique is another critical step in the development of electrochemical aptasensors because it has a significant impact on the overall performance of an aptasensor. immobilization is used to attach or conjugate a bioreceptor to a transducer. the most commonly used aptamer immobilization technique is physical adsorption and covalent bonding, such as selfassembled monolayers (sam) with thiol-based interactions, streptavidin-biotin interactions, and surface activation with edc/nhs [26–28]. abrego-martinez et al. [21] report an aptamer-based biosensor developed using a screen-printed carbon electrode for the detection of sars-cov-2. aptamer immobilization on screen-printed carbon electrode modified gold nanoparticles (aunps) through self-assembled monolayer (sam), a thiol-based interaction formed spontaneously by incubation at 4 °c for 8 hours. idili et al. [21] also reported an electrochemical aptamer-based (eab) sensor capable of quantifying the sars-cov-2 spike (s) protein using a standard three-electrode cell with a platinum counter electrode and an ag/agcl (3 m kcl) reference electrode. aptamer immobilization through self-assembled monolayer (sam), namely thiol-based interaction on the surface of gold electrodes. despite some complexity in the procedure, the aptamer immobilization via streptavidin-biotin interaction has been chosen based on the known advantages of the methodology. these include the need for very low amounts of biotinylated aptamer, the fact that it is less affected by changes in buffer concentration, ph, denaturants, and high temperatures, as well as the decrease of non-specific adsorption and improvement of the signal-noise ratio. however, it is important to mention that other options could have been evaluated to immobilize the aptamer onto the gold electrode surface, such as via an au-s bond [27]. in addition, liebana and drago. [29] study on bioconjugation and stabilization of biomolecules in biosensors stated that the characteristics of immobilization methods based on bioaffinity such as streptavidin-biotin have advantages such as good orientation, high specificity, high selectivity, high functionality, and well-controlled [29]. the streptavidin-biotin interaction is used in the aptamer immobilization technique, which is based on the specific affinity of streptavidin and biotin. streptavidin immobilization on the electrode surface can be achieved through the mpa linker (3-mercaptopropionic acid). mpa is an organosulfur compound that can bind to gold through au-s bonds. the other terminal end of mpa is the carboxylate group, so the carboxyl group can chemically bind to other groups [30]. most analytical methods face the problem of time and high reagent consumption, which can be caused by the procedure for determining the optimal conditions of a study that requires many experiments with sufficient numbers. this is especially detrimental in terms of cost, materials, and time. an approach with a chemometric method can be used to solve this problem. this method assesses the significant factors that will affect the response [31,32]. amongst the tools for the analysis of chemometrics, the box-behnken is able to actualize optimization of processes through the implementation of fewer test sets. moreover, box-behnken allows the determination of interactive effects and the measurement of the effect level of parameters [33]. in the biosensor, the incubation time and the stability of the biomolecules on the electrode surface were the most important parameters. the optimum incubation time determines the time required to bond completely to the electrode surface. when the incubation time is lower, it will result in an imperfect bond. meanwhile, if the incubation time is too long. this will cause the bonds to become saturated [34]. so, using the differential pulse voltammetry, the effect of bioreceptor incubation time, bioreceptor concentration, and target analyte incubation time on this electrochemical aptasensor current response was determined electrochemically with k3[fe(cn)6] redox system. http://dx.doi.org/10.5599/jese.1206 j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 222 in this study, an electrochemical aptasensor method was developed to detect the rbd protein s sars-cov-2 as a biomarker of covid-19 disease. the aptamer will then bind to the rbd protein s sars-cov-2, which can be detected electrochemically with the k3[fe(cn)6] redox system using the differential pulse voltammetry. furthermore, the optimum experimental conditions were determined and a standard rbd protein s sars-cov-2 test was carried out with various concentrations to determine the analytical parameters. according to the literature we read, aptasensors using spce modified gold nanoparticles with an aptamer immobilization system based on the interaction between streptavidin and biotinylated aptamers, and the optimization of the experiment are the first to be reported. the developed electrochemical aptasensor was tested on saliva samples and also for selectivity against interferences in saliva samples. experimental materials the materials used in this study were 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (edc) (sigma aldrich, singapore), 3-mercaptopropionic acid (mpa) (sigma aldrich, singapore), demineralized water (pt ikapharmindo putramas, indonesia), biotinylated aptamer (biotin 5'cag cac cga cct tgt gct ttg gga gtg ctg gtc caa ggg cgt taa tgg aca-3') (bioneer, korea), chloroauric acid (haucl4.3h2o) (synthesized by the chemical analysis and separation laboratory december 28, 2018, indonesia), ethanolamine (merck, germany), potassium ferricyanide k3[fe(cn)6] (sigma aldrich, singapore), potassium chloride (kcl) (merck, germany), rbd protein s sars-cov-2 solution (genscript, usa), trisodium citrate (na3c6h5o7.2h2o) (merck, germany), nhydroxysuccinimide (nhs) (sigma aldrich, singapore), phosphate-buffered saline (pbs) ph 7.4 (merck, germany), and streptavidin (promega, usa). tools the spce (gsi technologies, usa) was used as a working, auxiliary, and ag/agcl as a reference electrode for the electrochemical transducer. the electrochemistry measurements were conducted using a zimmer & peacock potentiostat connected to a computer using pstrace 5.8 software (zeamer & peacock, uk). a scanning electron microscope (sem) (hitachi tm3000, japan) was used for electrode surface morphology analysis. uv-vis spectrophotometer (thermo scientific, us) and particle size analyzer (psa) (horiba sz-100) were used for the characterization of compounds. preparation of colloidal gold nanoparticles (aunp) the gold nanoparticles (aunp) colloids were prepared by adding 10 ml of 0.75 mm chloroauric acid, stirring and heating on a magnetic stirrer. after boiling, add as much as 1.7 ml of 1 % trisodium citrate and stir while heated until the solution color changes to wine red. then the prepared aunp colloid was stored in a brown glass bottle at 4 oc. after that, the colloidal gold nanoparticles formed were characterized using a uv-vis spectrophotometer and a particle size analyzer (psa) [35,36]. spce modification with aunp the overall schematic of the electrochemical aptasensor method is shown in figure 1. the spce surface was rinsed with demineralized water and dried at room temperature. then, 25 μl of colloidal aunp solution was dropped onto the surface of the spce and incubated at room temperature for 24 hours. the spce/aunp was rinsed with demineralized water and dried at room temperature before being electrochemically characterized by differential pulse voltammetry over a a. k. sari et al. j. electrochem. sci. eng. 21(1) (2022) 219-235 http://dx.doi.org/10.5599/jese.1206 223 potential range of -1.0 to 1.0 v at a scan rate of 0.008 v/s, estep 0.004 v with an epulse of 0.025 v and tpulse of 0.05 s. spce before and after modification were also characterized using sem [37]. figure 1. schematic of an electrochemical aptasensor for rbd protein s sars-cov-2 detection fabrication of the electrochemical aptasensor the spce/aunp was incubated with 0.01 m mpa for 20 minutes at 25 °c. after that, the spce/ /aunp/mpa were rinsed with ethanol. then 0.1 m edc solution and 0.1 m nhs solution (1:1 v/v) were incubated for 60 minutes at 25 °c, and rinsed with demineralized water. the streptavidin solution was incubated overnight at 4 °c on the surface of the spce/aunp/mpa/edc:nhs, then rinsed with pbs solution ph 7.4 0.01 m. ethanolamine was dropped onto the surface of the spce/aunp/ /mpa/edc:nhs/streptavidin for 20 minutes at 25 °c, then rinsed with demineralized water. furthermore, the 0.5 µg/ml biotinylated aptamer was immobilized on the spce/aunp surface using a streptavidin-biotin system for 40 minutes at 25 °c, then rinsed with pbs solution ph 7.4 0.01 m. after the aptamer was successfully immobilized on the surface of the spce/aunp electrode with the streptavidin-biotin system, the non-specific binding site on the electrode surface was incubated using 1 % bsa solution for 15 minutes at 25 °c, and then rinsed with pbs solution ph 7.4 0.01 m. after that, a solution of rbd protein s sars-cov-2 with a certain concentration was dropped on the electrode and incubated for 60 minutes at 25 °c. then, using a redox system of 10 mm k3[fe(cn)6] solution in 0.1 m kcl, differential pulse voltammetry was performed over a potential range of -1.0 to 1.0 v at a scan rate of 0.008 v/s, estep 0.004 v with an epulse of 0.025 v and tpulse of 0.05 s [22]. determination of optimum conditions from parameters affecting experiments factors such as aptamer concentration (x1), aptamer incubation time (x2), and incubation time of rbd protein s sars-cov-2 (x3) were selected as factors to be optimized in the experiment [22,38]. each factor is designed at three different levels, the lowest (-1), medium (0), and highest (+1) levels, as shown in table 1. the response of the measurement results from the experiment was then processed and determined the optimum value of each of these factors using the box-behnken experimental design with minitab18 software. http://dx.doi.org/10.5599/jese.1206 j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 224 table 1. optimization factors affecting the experimental conditions factor level -1 0 +1 aptamer concentration, µg/ml 0.5 1.0 1.5 aptamer incubation time, h 0.67 2 16 rbd protein s sars-cov-2 incubation time, min 30 45 60 results and discussion characterization of colloidal gold nanoparticles (aunp) the turkevich method was first performed in 1951 and is one of the most commonly used methods for the synthesis of gold nanoparticles. it is based on the reduction of the precursor tetrachloroauric acid (haucl4) with trisodium citrate in a boiling water solution. figure 2a shows the results of uv-vis spectrophotometry of gold nanoparticles with a peak at 521 nm. however, figure 2b shows the results of the particle size analyzer (psa) characterization that the gold nanoparticles formed are 38.6 nm in size. the oxidation number of gold includes au+1 (aurous), au+3 (auric/aurat), and the one that is not oxidized is au0. au0 is the final condition required for a nanoparticle. therefore, the basic principle of this method involves the reduction of au3+ to au0 in the presence of a reducing agent such as trisodium citrate. the precursor chosen is chloroauric acid (haucl4), where gold is in the oxidation state au+3 [39–41], while trisodium citrate functions as a stabilizing and reducing agent. trisodium citrate electrostatically stabilizes gold nanoparticles. because of the repulsion between the negative charges on the surface, the negative charge of the citrate ion adsorbed on the surface of the gold nanoparticles can prevent aggregation between nanoparticles [42,43]. a b figure 2. characterization of gold nanoparticles. (a) the results of the characterization of gold nanoparticles using a uv-vis spectrophotometer. the maximum absorption wavelength of gold nanoparticles is at 521 nm; (b) characterization results of particle size analyzer (psa) of gold nanoparticles with a size of 38.3 nm the use of nanoparticles is one of the methods used in the development of a biosensor. the function of nanoparticles in biosensors is to increase the immobilization of biomolecules, catalyze electrochemical reactions, label molecules, and enhance electron transfer. several types of nanoparticles are used in biosensor applications, one of which is gold nanoparticles. gold nanoparticles have a suitable function for increasing the sensitivity and selectivity of biosensors because of their a. k. sari et al. j. electrochem. sci. eng. 21(1) (2022) 219-235 http://dx.doi.org/10.5599/jese.1206 225 relatively simple biocompatibility, optical, electronic, production, and modification properties [40], and also exhibit a high surface area ratio and excellent conductivity [44]. the characterization of gold nanoparticles aims to determine the distribution of particle size and morphology because these two parameters are very important in the characterization of a nanoparticle. the uv-vis spectrophotometer is one of the important instruments used for the characterization of synthesized nanoparticles. the interaction of gold nanoparticles with light can be determined by the size, morphology, shape, and chemical environment of the synthesized nanoparticles. the resonance resulting from the electron oscillation of gold nanoparticles and incident light waves provides surface plasmon resonance of gold nanoparticles. the surface plasmon resonance for gold nanoparticles is in the wavelength range of 500-600 nm, depending on the particle size [45]. the aunp colloid produced in this study has an absorption peak at a maximum wavelength of 521 nm, which indicates that the size of the gold nanoparticles produced is quite good. figure 2a shows the absorption of gold nanoparticles at 521 nm. gold nanoparticles were assessed using a particle size analyzer (psa) and the dynamic light scattering (dls) technique, which shows the average diameter of hydrodynamic particles in a liquid solution. figure 2b shows the dls analysis of gold nanoparticles with an average size of 38.3 nm. characterization of electrochemical aptasensor one of the working electrodes often used is the screen printed-carbon electrode (spce). spce has many advantages, such as high efficiency, ease of carrying and use, fast analysis, and small sample size, so it is very prospective for sensor development [46]. the use of gold nanoparticles in spce can increase the electroactive surface area and increase electron transfer rate between the electrode and the analyte. the modified electrodes provide fast, accurate measurements, as well as high sensitivity and selectivity [47]. spce before and after modification with gold nanoparticles was characterized using differential pulse voltammetry in 10 mm k3[fe(cn)6] solution in 0.1 m kcl. the redox system of k3[fe(cn)6] solution at bare spce (curve a) and spce/aunp (curve b) showed an increase in peak current response after spce was modified with gold nanoparticles, as shown in figure 3a. this is due to an increase in the conductivity of gold nanoparticles in spce/aunp by increasing electron transfer between the electrode and analyte, compared to bare spce. to test the electrode changes before and after the modification process, a k3[fe(cn)6] solution was used as a mediator. the reduction and oxidation reactions that occur in k3[fe(cn)6] are as shown in equation (1) [48]. [fe(cn)6]3+ e [fe(cn)6]4(1) as a bioreceptor in this research, the dna aptamer developed by song et al. [5], named cov2-rbd-1c, which can recognize the rbd protein s sars-cov-2, was used in this study because of the binding interaction between the rbd protein s sars-cov-2 and the aptamer involved, previously characterized by molecular dynamics (md) techniques. more specifically, the aptamer can interact with the rbd protein s sars-cov-2 through hydrogen bond formation, the cov2-rbd1c aptamer forming hydrogen bonds with threonin500, glutamine506, and asparagine437 of the rbd protein s sars-cov-2. moreover, the aptamer has been selected in the working buffer under physiological conditions, therefore, it can support the measurement of protein s in biological fluids. as for the approximate dissociation constant (kd) of the aptamer, it is 5.8 ± 0.8 nm, which is comparable to commercially available antibodies developed to bind to protein s. the selected http://dx.doi.org/10.5599/jese.1206 j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 226 aptamers are ideal recognition probes for rbd protein s sars-cov-2 because of their high affinity, small size, and ease of modification and use [5]. a b c d figure 3. (a) differential pulse voltammogram characterized using a redox system solution of k3[fe(cn)6] solution 10 mm in 0.1 m kcl; spce characterization using sem (b) bare spce, (c) spce/aunp, and (d) spce/aunp/streptavidin/aptamer aptamer immobilization on the spce/aunp surface was carried out with the streptavidin-biotin system. as for the immobilization of streptavidin on the surface of spce/aunp, it is based on direct covalent bonds on the surface of the transducer that functions on the gold surface through the alkanethiol group, namely mpa (3-mercaptopropionic acid). mpa is an organosulfur compound that can bind to gold through au-s bonds. the other terminal end of mpa is the carboxylate group, so the carboxyl group can chemically bind to other groups. the strong affinity for sulfur atoms to the surface of gold due to the interaction of sulfur-gold atoms is 188.28 j/mol, so it can form stable covalent bonds [30]. the modified carboxylate group on the spce/aunp surface was then activated with a solution of edc:nhs to form a reactive succinamide ester. this reactive group can bind to the primary amine group of streptavidin covalently on the surface of spce/aunp. after the addition of streptavidin, spce/aunp/mpa/edc:nhs/streptavidin were formed. the addition of ethanolamine is needed to avoid non-specific bonding on the electrode surface that has been immobilized by streptavidin. the cooh group on the electrode surface that is not bound to streptavidin will be blocked with ethanolamine so that the immobilized streptavidin on the surface of the aptasensor has a good orientation. after that, the biotinylated aptamer was incubated on the surface of the electrode and then rinsed thoroughly with pbs solution ph 7,4 to remove the free aptamer. the biotinylated aptamer binds to streptavidin by non-covalent interaction. to determine whether the biotinylated aptamer was successfully formed on the surface of the electrode, differential pulse voltammetry was used with a redox system of k3[fe(cn)6] solution 10 mm a. k. sari et al. j. electrochem. sci. eng. 21(1) (2022) 219-235 http://dx.doi.org/10.5599/jese.1206 227 in 0.1 m kcl over a potential range of -1.0 to 1.0 v at a scan rate of 0.008 v/s, estep 0.004 v with an epulse of 0.025 v and tpulse of 0.05 s. in figure 3a shows the highest peak current increase occurred when spce was modified with gold nanoparticles (curve b), then decreased gradually after the addition of mpa (curve c), the addition of mpa/edc:nhs (curve d), the addition of mpa/ /edc:nhs/streptavidin (curve e), and the addition of mpa/ edc:nhs/ streptavidin/aptamer (curve f). the current decreases for the first time when mpa (curve c) is added to the surface of the spce/aunp electrode, forming spce/aunp/mpa because mpa has been immobilized on the surface of spce/aunp through covalent bonds, allowing electron transfer between electroactive species in solution k3[fe(cn)6] and the electrode is blocked due to the density present on the electrode surface after chemisorption between mpa and gold nanoparticle. the current decrease occurred again when the addition of edc:nhs (curve d) activated the mpa carboxylate group by forming a reactive succinamide ester on the surface of spce/aunp to form spce/aunp/mpa/ /edc:nhs. edc:nhs was also used as a facilitator to covalently mobilize streptavidin on the surface of spce/aunp containing carboxylic groups. the decrease in current indicates that the immobilization of these compounds was successful, as the transfer of electrons between electroactive species solution k3[fe(cn)6] and the electrode is blocked as the electroactive species solution k3[fe(cn)6] moves away from the electrode surface. the addition of streptavidin then also causes a decrease in the current (curve e). because streptavidin is a nonelectroactive macromolecule, it can prevent electron transfer from k3[fe(cn)6] solution. at the time of immobilization of the biotinylated aptamer (curve f) there was also a decrease in current because the electrode surface was getting denser. this indicates that the immobilization was successful, as the flow of solution k3[fe(cn)6] will be lower, causing the [fe(cn)6]3-/4-species to be further away from the electrode surface due to electron transfer being hampered by a decrease in the conductivity of the aunp-modified electrode. a morphology analysis with sem was carried out to determine the success of the modification of the working electrode, which could be seen from its morphology. figure 3b, c, and d show the bare spce, spce/aunp, and spce/aunp/streptavidin/aptamer surfaces. figure 3c shows that the spce surface is smoother and more homogeneous than after being modified with aunp (figure 3c). this indicates that the modification process has been successfully carried out. figure 3d, on the other hand, displays a rougher electrode surface, showing that streptavidin and aptamer have been successfully immobilized on the electrode surface. the biotinylated aptamer that had been immobilized was incubated with a 1 % bsa solution to cover the active site on the surface of the electrode. the bsa molecule will fill the small gaps in the empty areas on the electrodes. the goal is that the measurement does not produce a current that can interfere with the analysis results of the analyte. to detect rbd protein s sars-cov-2, the protein standard solution in pbs ph 7.4 was incubated on the electrode surface. after that, the electrodes were rinsed with pbs solution ph 7.4 to eliminate molecules that did not bind to the biotinylated aptamer, such as rbd protein s sars-cov-2. to determine whether rbd protein s sars-cov-2 has been immobilized on the electrode surface, differential pulse voltammetry is used to observe the redox system of k3[fe(cn)6] solution over a potential range of -1.0 to 1.0 v at a scan rate of 0.008 v/s, estep 0.004 v with an epulse of 0.025 v and tpulse of 0.05 s. the attachment of rbd protein s to sars-cov-2 to the biotinylated aptamer causes a decrease in the peak of the k3[fe(cn)6] current, as shown in figure 3a (curve g). this is because the rbd protein s sars-cov-2 is a large biomolecule that is not electroactive, so it can block the electron transfer process on the surface of the electrode. the measured current response will be inversely proportional to the number of non-electroactive biomolecules involved. because the more http://dx.doi.org/10.5599/jese.1206 j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 228 rbd protein s sars-cov-2 is attached to the surface of the electrode, the more it blocks the electron transfer process from the redox system of k3[fe(cn)6] solution, the higher the concentration of rbd protein s sars-cov-2, the lower the current response. on the other hand, the lower the concentration of rbd protein s sars-cov-2, the higher the current response. the optimal experimental conditions three factors, such as aptamer concentration (x1), aptamer incubation time (x2), and incubation time of rbd protein s sars-cov-2 (x3), were selected as factors to be optimized in the experiment. the effect of aptamer concentration, aptamer incubation time, and rbd protein s sars-cov-2 incubation time on the current response of the aptasensor was tested using differential pulse voltammetry with a redox system of 10 mm k3[fe(cn)6] solution in 0.1 m kcl over a potential range of -1.0 to 1.0 v at a scan rate of 0.008 v/s, estep 0.004 v with an epulse of 0.025 v and tpulse of 0.05 s. in spce/aunp/mpa/edc:nhs/streptavidin, aptamer was dripped with various concentrations of 0.5, 1.0, and 1.5 µg/ml and was incubated with time variations of 0.67, 2, and 16 hours at 25 oc. while rbd protein s sars-cov-2 incubated with time variations of 30, 45, and 60 minutes at 25 oc. the optimum value of each factor was determined using the box-behnken experimental design with the minitab 18 program. the relationship between the response and the factors was determined through a sequence of experiments to obtain the optimum response results, and each factor was designed through 3 levels, namely, the lowest level (-1), medium level (0), and the highest level (+1), as observed in table 1. experiments using 3 factors with 3 different levels were carried out 15 times, so that there were 15 trials. the response of the measurement results from the suggested experiment is then processed with the minitab 18 program so that it can predict the maximum current value. from the experimental data obtained, the following regression equation (2) was found: y = 3.00 + 3.40x1 + 0.237x2 0.1598x3 2.265x1x1 0.01172 x2x2 -0.002109 x3x3 0.0112 x1x2 + 0.0214 x1x3 + 0.000891 x2x3 (2) as shown in eq. (2), if the response coefficient was positive, the presence of these factors would increase the current response. a negative coefficient indicated that the presence of these factors would decrease the current response. from the analysis of the current response obtained from the differential pulse voltammetry, we obtained the anova results presented in table 2, which explain the variability of the data. the p-value data for each factor was obtained from the anova. a model that is in accordance with the linear model is indicated by a p-value of less than 0.05, which implies that a single variable showed a linear effect. the interaction effect is shown from a combination of two variables, and to determine the significance of each variable, the p-value was used. the independent variable, which had no significant effect, is indicated by a p-value of more than 0.05. in addition, based on the anova results from the box-behnken experimental design, table 2 shows the p-value of each factor, namely x1 = 0.189; x2 = 0.001; and x3 = 0.874. the factor is considered significant if the p-value < 0.05. it can be seen that the incubation time factor of the aptamer has a significant effect on the experimental results. response optimization helps you identify the combination of variable settings that jointly optimize a single response or a set of responses. this is useful when you need to evaluate the impact of multiple variables on a response. response optimization is most effective when interpreted in conjunction with relevant subject matter expertise, including background information, theoretical principles, and knowledge obtained through observation or previous experimentation. when optimizing responses, you must specify whether your goal is to minimize, maximize, or meet a target response. in this research, we use minimize, so the response when the optimum response is low. the performance of all design a. k. sari et al. j. electrochem. sci. eng. 21(1) (2022) 219-235 http://dx.doi.org/10.5599/jese.1206 229 and response variables is shown in figure 4, where the best optimization approaches an overall optimum operating condition of aptamer concentration at 0.5 µg/ml, aptamer incubation time at 40 minutes (0.67 hours), and rbd protein s sars-cov-2 incubation time at 60 minutes. thus, when the experimental conditions proposed by minitab are applied to our experimentation, the values of the responses obtained are almost equal to the values obtained by calculating. tabel 2. analysis of variance source df adj ss adj ms f-value p-value model 9 4.88120 0.54236 11.43 0.008 linear 3 2.65810 0.88603 18.67 0.004 x1 1 0.10954 0.10954 2,1 0.189 x2 1 2.54723 2.54723 5.68 0.001 x3 1 0.00133 0.00133 0.03 0.874 square 3 1.93177 0.64392 13.57 0.008 x1x1 1 1.18355 1.18355 24.94 0.004 x2x2 1 0.13378 0.13378 2.82 0.154 x3x3 1 0.83103 0.83103 17.51 0.009 2-way interaction 3 0.16966 0.05655 1.19 0.402 x1x2 1 0.00996 0.00996 0.21 0.666 x1x3 1 0.10336 0.10336 2.18 0.200 x2x3 1 0.05634 0.05634 1.19 0.326 error 5 0.23726 0.04745 lack-of-fit 3 0.05122 0.01707 0.18 0.900 pure error 2 0.18603 0.09302 total 14 5.11845 figure 4. response optimization of the optimal experimental conditions electrochemical aptasensor performance after knowing the optimum conditions of several parameters that affect the experiment, a calibration curve is made and the detection limit of the aptasensor is calculated using variations in the concentration of rbd protein s sars-cov-2. variations in the concentration of rbd protein s sars-cov-2 used are: 50; 40; 30; 20; 10 ng/ml. the aptasensor was characterized electrochemically by a redox system of k3[fe(cn)6] solution using differential pulse voltammetry over a potential range of -1.0 to 1.0 v at a scan rate of 0.008 v/s, estep 0.004 v with an epulse of 0.025 v and tpulse of 0.05 s. the electrochemical response was measured using differential pulse voltammetry, as shown in figure 5a. the voltammogram shows that the higher the concentration of rbd protein s sars-cov-2, the lower the current response. this is because the rbd protein s sars-cov-2 is a large biomolecule http://dx.doi.org/10.5599/jese.1206 j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 230 that is not electroactive, so the more rbd protein s sars-cov-2 binds to the aptamer, the electrode surface becomes denser and this causes the electron transfer process on the electrode surface to be hindered. a b figure 5. (a) differential pulse voltammogram determining the concentration of rbd protein s sars-cov-2 (10; 20; 30; 40; 50 ng/ml) using a redox system of 10 mm k3[fe(cn)6] solution in 0.1 m kcl; (b) electrochemical aptasensor calibration curve for detection of rbd protein s sars-cov-2 then the resulting current response is plotted into a calibration curve for various concentrations of rbd protein s sars-cov-2 (50; 40; 30; 20; 10 ng/ml) to create a calibration curve as shown in figure 5b, so that a linear regression equation is obtained with y = 0.0634x – 0.4449 with r2 = 0.9983. the detection limit obtained is 2.63 ng/ml and the quantification limit is 7.97 ng/ml. the detection limit obtained in this study is quite low, as can be seen in table 3. the detection limit in this study is higher when compared to abrego-martinez et al. [21] study of an aptamer-based biosensor developed on a screen-printed carbon electrode modified with gold nanoparticles for sars-cov-2 detection. table 3. electrochemical biosensor studies to detect sars-cov-2 method limit of detection, ng/ml ref. two-dimensional (2d) metal–organic framework (mof)-based photoelectrochemical (pec) aptasensor for sars-cov 2 spike glycoprotein (s protein) detection 72 [49] electrochemical aptasensor to detect sars-cov-2 protein s rbd as a biomarker of covid-19 disease using a screen printed carbon electrode/aunp 2.63 this research electrochemical aptasensor using screen printed carbon electrode/aunp for sars-cov-2 spike (s) protein detection [20] electrochemical aptasensor using screen printed carbon electrode/aunp targeting the receptor-binding domain (rbd) in the spike protein (s-protein) of the sars-cov-2 0.066 [21] electrochemical dual-aptamer biosensor based on the metal-organic frameworks mil-53(al) decorated with au@pt nanoparticles and enzymes to determine sars-cov-2 nucleocapsid protein (2019-ncov-np) via co-catalysis of the nanomaterials, horseradish peroxidase (hrp) and g-quadruplex dnazyme 0.00833 [50] the aptasensor relies on an aptamer targeting the receptor-binding domain (rbd) in the spike protein (s-protein) of the sars-cov-2. but the detection limit in this study is lower when compared a. k. sari et al. j. electrochem. sci. eng. 21(1) (2022) 219-235 http://dx.doi.org/10.5599/jese.1206 231 to the jiang et al. [49] study about a two-dimensional (2d) metal–organic framework (mof)-based photoelectrochemical (pec) aptasensor with high sensitivity and stability for sars-cov-2 spike glycoprotein (s protein) detection. this proves that the electrochemical aptasensor method using spce modified gold nanoparticles developed in this study can be used for the detection of covid-19 disease using the rbd protein s sars-cov-2 as a biomarker. selectivity and stability of the aptasensor a method's selectivity refers to its ability to test just particular analytes in the presence of other components in the sample matrix [51–53]. although the presence of aptamers as bioreceptors provides sensitivity to target analytes, the aptasensors to be developed must also have good interference rejection characteristics in order to be useful in real sample analysis. saliva has an important role in diagnostics as it provides a convenient and cost-effective point of care technology (poct) platform for rapid detection [54]. liv. [55] investigated the effect of several enzymes, compounds, and ions found in saliva on electrochemical immunosensors used to detect sars-cov-2 antibodies, including α-amilase, lipase, na+, k+, ca2+, mg2+, h2po4-, hpo42-, urea, hco3-, and nh3 [55]. a b figure 6. (a) na+, k+, ca2+, and mg2+ ion selectivity of the aptasensor; (b) differential pulse voltammogram response obtained with 30 ng/ml rbd protein s sars-cov-2 to evaluate the stability of the aptasensor after 1 month of storage in an oven at 50˚c using a redox system of 10 mm k3[fe(cn)6] solution in 0.1 m kcl selectivity was determined for na+, k+, ca2+, and mg2+ ions. the measured current peak response was compared with the rbd protein s sars-cov-2 peak response at a concentration of 100 ng/ml. figure 6a shows the current responses of na+, k+, ca2+, and mg2+ ions, which are not much different from the aptamer as a negative control. this indicates that there is no binding between the aptamer and na+, k+, ca2+, and mg2+ ions. other treatments were studied to see the effect of interference in determining the percentage of selectivity. as shown in figure 6a, the addition of an interference that is a mixture of na+, k+, ca2+, and mg2+ ions in the rbd protein s sars-cov-2 showed a measurable response peak of 3,389 µa, which was not significantly different from the peak response for the rbd protein s sars-cov-2, which was 3,280 µa. this means that the aptamer activity is very good at recognizing the rbd protein s sars-cov-2 as the target analyte. the presence of interference did not affect the activity of the aptamer against rbd protein s sars-cov-2. aptamer selectively recognized only rbd protein s sars-cov-2 as the target analyte in a matrix containing na+, k+, ca2+, and mg2+ ions, which neither interfered with nor bound to the aptamer, and obtained a selectivity value of 91.36 %. http://dx.doi.org/10.5599/jese.1206 j. electrochem. sci. eng. 21(1) (2022) 219-235 aptasensor to detect rbd protein s sars-cov-2 232 the screen-printed carbon electrodes that are used in this aptasensor are disposable devices specially designed to work with microvolumes of sample. ideal for quality control or research purposes, and also for teaching electrochemistry. biosensors are sensitive to aging, which is defined as a decline in signal over time. biosensor aging is defined as a loss of signal at a specific concentration of the measured analyte. time, handling or manner of usage, and temperature all have a role in aging. shelf-life studies are frequently carried out, but they are rarely published. for single-use disposable sensors, such investigations are sufficient, but they reveal little about aging characteristics for long-term usage [56]. the term "stability" refers to the variance in detection signals during the course of long-term storage [57]. to assess the stability of the aptasensor, the current response to 30 ng/ml rbd protein s sars-cov-2 was recorded after 1 month of storage in an oven at 50 oc. the differential pulse voltammogram response shown in figure 6b revealed that the response increased after periodic measurements of 1 day (curve a; 3.503 µa), 15 days (curve b; 3.532 µa), and 30 days (curve c; 3.565 µa), representing a loss of sensory activity, but not significantly different. these findings suggest that the aptasensor is relatively stable, which could indicate the suggested platform's potential utility for quick sars-cov-2 screening. real sample analysis the aptasensor that has been developed was then tested on saliva samples by standard addition with the rbd protein s sars-cov-2 standard. saliva samples were taken from the same negative individual, then dissolved in buffer solution. each saliva sample was diluted with 100 ng/ml of rbd protein s sars-cov-2, and then tested on the aptasensor. the resulting peak current response is entered into the equation y = 0.0448x + 0.3395. from the calculation results, the recovery for biological samples in the form of saliva is 99.90 %. the results revealed that the aptasensor could detect rbd protein s sars-cov-2 in real samples without any preparation or preprocessing, and that it had a lot of potential as a reliable instrument for detecting sars-cov-2 virus in real biological materials. conclusions during the covid-19 pandemic, the development of highly sensitive and rapid biosensing devices has become increasingly important. in this paper, a label-free electrochemical aptasensor for detection of rbd protein s sars-cov-2 has been presented as a potential approach for covid-19 diagnosis. the disposable spce were aunp-modified, and streptavidin was used to immobilize the biotinylated aptamer, which serves as a bioreceptor in the electrochemical aptasensor developed to detect the rbd protein s sars-cov-2. we present the novelty of using an aptamer immobilization technique at the electrode surface with an aptamer immobilization technique based on the streptavidin-biotin interaction, with specific affinity between streptavidin and biotin, through the mpa (3mercaptopropionic acid) linker. the aptasensor demonstrated excellent sensing performance in terms of sensitivity, accuracy, and fast response with the time of analysis (60 min), easiness of use, and the requirement of 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electrochemical and spectrophotometric methods simona patriche, elena georgiana lupu, andreea cârâc*, rodica mihaela dinică, geta cârâc department of chemistry, physics and environment, faculty of sciences and environment, “dunărea de jos” university of galati, 111 domneasca street, 800201 galati, romania *department of fundamental science, faculty of pharmacy, “carol davila” university of medicine and pharmacy of bucharest, 6 train vuia street, 020956 bucharest, romania corresponding author: geta.carac@ugal.ro; tel.: +40 745 358 371; fax: + 40 236 46 13 53 received: september 30, 2015; accepted: february 10, 2016 abstract the interaction between pyridinium ligands derived from 4,4’-bipyridine (n,n’-bis(p-bromophenacyl)-4,4’-bipyridinium dibromide – lr) and (n,n’-bis(p-bromophenacyl)-1,2-bis (4-pyridyl) ethane dibromide – lm) with lipase enzyme was evaluated. the stability of the pyridinium ligands, having an essential role in biological systems, in 0.1 m kno3 as supporting electrolyte is influenced by the lipase concentration added. the ph and conductometry measurements in aqueous solution suggest a rapid ionic exchange process. the behavior of pyridinium ligands in the presence of lipase is investigated by cyclic voltammetry and uv/vis spectroscopy, which indicated bindings and changes from the interaction between them. the voltammograms recorded on the glassy carbon electrode showed a more intense electronic transfer for the lr interaction with lipase compared to lm, which is due to the absence of mobile ethylene groups from lr structure. keywords enzyme; pyridine; cyclic voltammetry; morphology; physicochemical properties introduction pyridinium ligands are very interesting compounds with many applications and they have significant antimicrobial properties, being involved in the inhibition of microorganism growth (bacteria and fungus) [1,2]. also, the compounds are used as electronic transporters, biological http://www.jese-online.org/ mailto:geta.carac@ugal.ro j. electrochem. sci. eng. 6(1) (2016) 91-104 tinteractions between lipase and pyridinium 92 redox indicators [3], model systems in photosynthesis [4], cardiovascular, hypotensive [5] and neuromuscular agents [6], catalysts [7], acylation agents and ionic liquids [8]. pyridinium ligands have an essential role in biological systems and could be involved in cycloaddition reactions with different dipolarophiles (ethyl propiolate), in order to obtain the indolizine core using enzymes as biocatalysts. dipolarophiles such as ethyl propiolate are important precursors used to obtain indolizines through cycloaddition reactions of quaternary pyridinium ligands with activated alkynes.phenacyl bromide is also an important precursor involved in the biocatalytic process with lipase [9]. n-heterocyclic quaternary ligands were designed as precursors for fluorescent indolizine synthesis [10]. lipases are biocatalysts with a broad application in various industries, such as chemical [11-12], pharmaceutical [13,14], cosmetics [15] or agrochemical sectors [16,17]. they have a significant capacity to catalyze the conversion of various compounds (enzymatic substrate) to different products. these enzymes belong to the hydrolases group (hydrolytic enzymes), having the ability of acting at the interface between the aqueous and organic phase [18,19]. contrary to other hydrolytic enzymes that act invariably on monomolecular substrates, lipases exhibit a growth in activity at the water-lipid interface [20]. commercial lipases could also be used in biocatalytic reactions to obtain indolizines [9]. due to their eco-friendly and recyclable properties, lipases are involved in the synthesis of tricyanovinylated compounds [21] and in the design of mesoporous materials [22], green polymers [23] and bioelectrodes used to detect triglycerides in human serum [24]. there are many methods, both analytical and electrochemical, to detect lipase activity. selective review about lipase activity were recently reported in the literature [19,25]. nevertheless, there are limited studies involving the interaction of lipase with the pyridinium salts. the aim of our study is to detect whether the interaction between lipase and pyridinium ligands generates redox properties, in order to predict a possible biocatalytic mechanism. as such, the lipase interaction of various concentrations with pyridinium ligands derived from 4,4 ’ -bipyridine was investigated by cyclic voltammetry and spectrophotometryic method. the pyridinium ligands studied are viologens [26,28] with interesting spectrophotometric and electrochemical properties [27]. it is the first time when the interaction of such compounds and lipase is investigated using electrochemical methods, and the biocatalytic properties derived from these interactions could be further used to understand their use in biocatalysis leading to the indolizine ring. experimental the synthesis of two pyridinium ligands was performed by reacting the heterocycles 4,4’-pyridyl and 1,2-bis(4-pyridyl)ethane with phenacyl bromide (as precursor) according to the method already reported in the literature [26,28]. the synthesized ligands are n,n’-bis(p-bromophenacyl)4,4’-bipyridinium dibromide (rigid ligand, lr) and n,n’-bis(p-bromophenacyl)-1,2-bis(4-pyridyl)ethane dibromide (mobile ligand, lm). all chemical reagents were obtained from commercial sources of analytical grade (merck) and used without further purification. solutions of 0.1 mm of each pyridinium ligand, dissolved in 0.1 m kno3 as electrolyte support were prepared. aqueous solutions were prepared with double deionized water having a conductivity of 1.6 µs cm -1 (milli-q millipore losheim france). variable concentrations of lipase (0.05, 0.25 and 0.50 mg ml -1 ) were added to the pyridinium ligands solutions. commercial lipase enzyme s. patriche et al. j. electrochem. sci. eng. 6(1) (2016) 91-104 doi:10.5599/jese.232 93 (candida antartica) was stored at -5 °c. data were collected from fresh-made ligand solutions (lower acid ph) and during a certain period of time (1-14 days). the stability of the aqueous soluteons with and without lipase at room temperature (20±1 °c) was evaluated by physicochemical measurements. the values of ph and conductivity were measured with a consort c862 multiparameter analyzer. the spectrophotometric analysis from 200 to 800 nm using quartz cuvettes was performed. uv-visible absorption spectra were recorded by a uv-vis t90+ spectrophotometer (varian, australia) with 1 cm path length. the redox properties of the interaction between lipase and ligands were investigated by cyclic voltammetry. the measurements were performed using the bio-logic sp50 equipment with a carbon electrode immersed in the ligand solutions with and without enzyme. the voltammetric curves were recorded to show the electrochemical responses of the reaction system in the potential range from a negative direction of e = -1.0 to 1.0 v vs. ag/agcl, at various scan rates between 0.50 – 0.02 v s -1 . an electrochemical cell of 10 ml capacity with three electrodes was used (carbon working electrode 1.6 mm 2 , ag/agclsat. reference electrode (eag/agcl sat. = 0.197 v vs. ehn), pt wire counter electrode). all measurements were performed at 20±1 °c without deoxygenating the solutions. however, to evaluate all the changes before an electrochemical measurement, several solutions were deoxygenated by bubbling with highly purified nitrogen for 5 minutes. the enzyme interaction with the ligand solutions was also investigated at 40 °c. the free redox potential (open circuit potential – ocp) and cyclic voltammetry (cv) measurements were repeated three times to mark the significant changes that might appear in the solutions. the interaction of the enzyme with the precursor (phenacyl bromide) and the dipolarophile ethyl propiolate was also electrochemically investigated. the morphology of the lipase after interaction with ligands was characterized by scanning electron microscopy (sem) using quanta 200 equipment. after filtering the solutions, the lipase was dried in air at room temperature and placed on carbon-coated copper grid to perform the sem analysis and energy dispersive x-ray spectroscopy (edx). results and discussion the stability of pyridinium ligands in the absence and the presence of lipase the lipase enzyme interaction with the rigid ligand (lr) and mobile ligand (lm) through the evaluation of physico-chemical properties (ph, conductivity and spectrophotometric measurements) was investigated during a certain period of time (1-14 days) at room temperature. lr (0.1 mm) has shown a ph of 6.5 in the fresh-made aqueous solution and remained stable after 2 days as over 14 days. by adding a small amount of lipase (0.05 mg ml -1 ), no essential modification in the ph of lr electrolyte (0.1 m kno3) was observed after 2 days from the initial contact with the enzyme. with the increase of the lipase amount added in the solution, the ph decreased slowly in time. after 7 days the system with 0.5 mg ml -1 lipase showed a decrease with one unit of ph compared to lr without enzyme (figure 1). usually, after 14 days the ph of all solutions turned to the initial ph as an effect of the potential equilibrium reached in solutions. lm, initially characterized also by a weak acidic ph, remained stable after 2 days with an increase of 0.4 ph units over 14 days. at the same time, the presence of lipase induced significant changes in the ph of the lm solution compared to lr, mainly at a higher amount of enzyme added (0.5 mg ml -1 ), a neutral to weak alkaline ph being recorded (figure 1). as the time passed, an optimum operating ph of lipase (neutral ph) was achieved and an increase of oh ions was observed and the enzyme became more active at ph more than 7.4, as was reported in the literature [29]. j. electrochem. sci. eng. 6(1) (2016) 91-104 tinteractions between lipase and pyridinium 94 figure 1. time evolution of ph of the solutions containing the rigid and mobile ligand (lr and lm) with and without lipase aqueous solutions of the ligands (0.1 mm) electrolyte without lipase presented conductivities of 12-13 ms cm -1 , which confirms an intense dissociation process of zwitterion structures according to reference [30]. in all systems, a constant decrease of conductivity in time was recorded with 2 ms cm -1 after the first and second day, and after that it remained almost constant (figure 2). however, a more significant variation of conductivity was obtained in the case of lm in the presence of lipase. thus, for 0.05 and 0.25 mg ml -1 of lipase after 7 days, a decrease to half of conductivity was reached. the fact that the dissociation of ions decreases suggests a binding between the enzyme and pyridinium ligands according to reference [31]. figure 2. time evolution of conductivity of the solutions containing the rigid and mobile ligand (lr and lm) with and without lipase temperature effect on pyridinium ligands in the absence and presence of lipase an enzymatic reaction is affected by temperature and many studies showed that the optimum activity of the enzyme occurs at a temperature between 35 – 40 °c [22,32]. in the case of lipase, the optimal temperature was reported at 3740 °c [33]. the ligands in 0.1 m kno3 electrolyte in contact with different lipase amounts were analysed at 40 °c (keeping the temperature constant) without stirring and emulsifying agent, to observe changes which occur in the lipase interaction. with the temperature increase, for both lr and lm, changes of ph were recorded, showing an augmentation of the obtained values (figure 3). lr showed the decrease of ph in the presence of enzyme. on the other hand, the ph of lm increased to a more alkaline one upon the interaction with lipase, varying between 6.8 (0.05 mg ml -1 lower enzyme concentration) to 8.2 (0.50 mg ml -1 higher enzyme concentration). s. patriche et al. j. electrochem. sci. eng. 6(1) (2016) 91-104 doi:10.5599/jese.232 95 the conductivity was drastically reduced at 40 °c, being situated in this case in the μs cm -1 range compared to the systems’ electrolyte, which at 20 °c was 10 3 times lower. these results indicate the existence of an interaction between pyridinium ligands and lipase. the ionic dissociation of the ligands electrolyte solutions without enzyme indicates a difference of approx. 200 μs cm -1 more for lm compared to lr. in the presence of lipase different behaviour of the dissociation process was observed. lr from fresh-made solution without lipase indicated a conductivity reduced in half (114 μs cm -1 ) in contact with 0.05 mg ml -1 lipase and a slight increase of up to 135 μs cm -1 for 0.50 mg ml -1 lipase (figure 3). at the same time, lm showed in the presence of 0.05 mg ml -1 lipase a decrease of conductivity of 145 μs cm -1 from fresh electrolyte without lipase (343 μs cm -1 ) and a slight increase of up to 317 μs cm -1 for more lipase added. therefore, an inhibition of ionic dissociations occurred by raising the temperature of both ligands in the absence and presence of lipase, depending on the enzyme amount. figure 3. the effect of temperature at 40 °c in the evolution of ph and conductivity of the pyridinium ligands in the presence of lipase uv-vis spectrophotometric studies of the interaction with lipase uv-vis spectra in the scanning range of 200-700 nm for all aqueous solutions with and without lipase were recorded. the absorption peak for lr and lm is at 264 nm (uv), the maximum wavelength (λmax) being caused by the π → π* electron transition of benzene ring, which is in accordance with references [1,26]. the absorbance indicates a shift for both ligands aqueous solutions when in contact with lipase. the obvious variation of the uv-vis data is caused by the influence of ligands’ structure and lipase complex structure, as well as the interaction between them. the highest absorbance was obtained for lr compared to lm in the presence of lipase, obtaining an interaction between them. the absorbance of lipase and ligands, respectively is not equal to the sum of the absorbance, indicating that lipase could interact with the ligands at room temperature according to reference [34]. figure 4a shows the variations of the absorbance from uv-vis spectra of ligand solutions at room temperature when different quantities of lipase were added. when the enzyme was added, the absorption peak of pyridinium ligands decreased without alteration of the maximum absorption wavelength. the interaction of lipase with the ligands’ structure has induced the diminution of the absorbance, more obvious for lm than lr, so the mobile ligand is more favourable for these interactions. the higher lipase concentration generated the lower absorbance, which suggests that a binding reaction had taken place between the enzyme and ligands. the lowest absorbance was attained for the lipase concentration of 0.5 mg ml -1 , as a consequence of the diminution of the major 6.5 7.0 7.5 8.0 (a) 0 0.500.250.050 0.500.250.05 ligands + (lipase content, mg·ml -1 ) lm lr lm lm lm lr lr lr p h 0 50 100 150 200 250 300 350 40 0 c (b) 0.50 lm 0.050 lr 0.250.050 ligands + (lipase content, mg·ml -1 ) lm lm 0.250.50 lr lr lm lr c o n d u c ti v it y ,  s ·c m -1 j. electrochem. sci. eng. 6(1) (2016) 91-104 tinteractions between lipase and pyridinium 96 component in the ligands electrolyte [35]. the same downward trend of the absorbance was maintained in the interaction of the lipase for both pyridinium ligands in time over 14 days (figure 4a). figure 4. time evolution of the absorbance of pyridinium ligands with and without lipase and at room temperature (a) and the effect of temperature of 40 ° c (b) the lipase interaction with ligands was also evaluated at 40 °c by the absorbance peak evolution from the uv-vis spectra of fresh-made aqueous solutions (figure 4b). in both systems a decrease of absorbance was observed, as an effect of the enzyme activity increase at that temperature, compared to the behaviour at room temperature. with the increase of the lipase concentration, a hypochromic effect of the absorption peak was observed. the absorbance decrease showed an intensive lipase binding interaction with the ligands molecule, more clearly for lm, having an ethylene group in its structure [35,36]. electrochemical studies the ligands electrolyte with and without lipase, initial and after 1, 2, 7 and 14 days respectively, kept at constant temperature (20 °c) were analyzed by electrochemical measurements. ocp measurements of the lr electrolyte without enzyme showed a potential ranging from 0.034 v to 0.045 v vs. ag/agcl to 2000 s and for lm between 0.052 0.056 v vs. ag/agcl, as effect of the zwitterionic ligands’ structure (results not shown). the influence of the ionic interaction between lr and lipase in the aq. electrolyte on carbon electrode was observed by sifting from the beginning of the ocp values (results not shown). in time, by adding more lipase, the ocp values increase (to the positive region) as an effect of the formation of an electro-active complex with changes of the electrochemical parameters. smaller lipase concentration has influence on the ocp values recorded, so, for 0.25 mg ml -1 lipase δe was 12 mv and respectively for 0.05 mg ml -1 lipase the δe was 5 mv (results not shown). the increase of ocp values until 2000 s up to δe of 30 mv for 0.50 mg ml -1 lipase confirms a rapid initiation of the enzyme’s activity and an electronic exchange mechanism. the same trend is observed on ocp values of lr in the deoxygenated aq. electrolyte in presence of lipase; δe increase with 10 mv for all analysed lipase concentrations (results not shown). in case of lm which has an ethylenic group, the interaction with lipase is indicated by the begging a decrease (to the negative region) of ocp values with a δe of 20 mv in comparison with lm without enzyme. a shift to the positive and negative region of ocp values is an indication of an active surface of ligands. in time, essential modifications of ocp values depending on the lipase added were not recorded (results not shown). s. patriche et al. j. electrochem. sci. eng. 6(1) (2016) 91-104 doi:10.5599/jese.232 97 cyclic voltammetry measurements were performed as a useful electroanalytical method to characterize the reduction ability and electrochemical behaviour of pyridinium ligands in the lipase biocatalyzed cycloaddition [37,38]. oxidation processes (anodic reactions) manifest themselves in positive current peaks, and reduction processes (cathodic reactions) in negative peaks and these are useful in understanding the mechanism of the reaction [39]. the cyclic voltammetric curves were recorded to show the electrochemical responses in the potential range between e = ±1 v vs. ag/agcl. the lipase content in the ligands` electrolyte has substantial effects on the electrochemical properties as voltammetric response. effect of the lipase concentration figure 5 shows the cyclic voltammograms of the ligands` electrolyte in the absence and presence of different amounts of lipase. in the absence of enzyme an anodic current peak ia of 0.24 μa (at +0.7 v for 0.1 v s -1 ) for lr is recorded in comparison with lm which presents lower anodic current, of 0.14 μa (at +0.7 v). an explanation is that lr is more electrochemical active compared to lm, because of the structural differences, lm having a mobile ethylenic group in its structure [9,26,27]. figure 5. cvs recorded of lr and lm electrolyte in the presence of different concentrations of lipase. ewe / v vs. (ag/agcl), 0.5 v s -1 for lr and 0.1 v s -1 for lm the anodic current peak increases with 0.25 μa and a new current peak appeared for lr electrolyte in the presence of 0.05 mg ml -1 lipase added, in comparison with lr without enzyme. when more lipase was added to the lr electrolyte, a relatively distinct anodic current peak (peak a) appears at a potential of 0.25 v vs. ag/agcl. the increase with 1.50 μa was observed when 0.5 mg ml -1 lipase was added and the lr molecule readily undergoes electrooxidation. when the concentration of enzyme was gradually increased from 0.05 to 0.50 mg ml -1 there was a j. electrochem. sci. eng. 6(1) (2016) 91-104 tinteractions between lipase and pyridinium 98 gradual increase in the current peak response and this response was finally saturated for a 0.25 mg ml -1 of lipase (ia = 1.76 μa) and remained almost constant (figure 5). the interaction of lm with lipase has shown a constant reduced anodic current peak between 0.8 μa to 1.5 μa over the applied potential without any distinct peak. both ligands present a reductive peak, around at -0.45 v vs. ag/agcl for lr and around at -0.7 v for lm, which indicated that the electrochemical behaviour on carbon electrode is reversible. anyway, the reductive peak current of lm is reduced in half compared to lr (e.g. at scan rate of 0.5 v s -1 ic = -14.5 μa for lr and respectively ic = -8.5 μa for lm is shown). on the lipase adding, the reductive current peak of lr decreased without the shift of the potential. at the same time the interaction of lm with lipase has shown a decrease of the reductive peak and a slightly shift of the potential to a more positive direction (from -0.7 to -0.5 v) because is not consistent with competetive adsorption. the concentration dependence on the peak current shows a sensitive linear correlation when different amounts of lipase were added (0.05 to 0.50 mg ml -1 ). the increase of the oxidation current in the presence of lipase is attributed to the weak formation of the lr cation on carbon electrode. the results indicate that a binding reaction has occurred in the solutions and the electrode process was reversible. the anodic peak current of ligands did not disappear completely with the increase of the concentration of lipase, which was not the character of competitive adsorption. the reason for the decrease of the reductive peak current after the interaction of the ligand with lipase may be the competitive adsorption between the lr and lipase on the carbon electrode, or the formation of electrochemical active complex with changes of electrochemical parameters. in the case of lm, the formation of electro-inactive complex without a significant change of the electrochemical parameters may be considered. the competitive adsorption could have limitations, also the uv-vis absorption spectrophotometric results proving an existing interaction between ligands and lipase, by the decrease of the absorbance of the ligands in the presence of lipase and no changes in its absorption wavelength (figure 4). the enzyme is more active in the presence of oxygen according to references [40,41]. lr, in the presence of the lipase and in the absence of oxygen (by introducing the sample into inert nitrogen atmosphere before the cv recording) presents a diminished of the anodic peak current, depending on the scan rate of the potential applied (results not shown). in the presence of oxygen, an evident anodic peak was observed when 0.25 mg ml -1 lipase was added, which disappeared in deoxygenated lr electrolyte, confirming an inhibition of the enzyme activity (figure 6). figure 6. cvs recorded of lr in 0.1 m kno3 and in the presence of 0.25 mg ml -1 lipase with and without oxygen; e / v vs. ag/agcl, 0.5 v s -1 s. patriche et al. j. electrochem. sci. eng. 6(1) (2016) 91-104 doi:10.5599/jese.232 99 effect of the scan rate cvs were recorded at various scan rates to know variation with the lipase added which inform what type of electrochemical process is occurring at the electrode surface. the lipase interaction at room temperature with the pyridinium ligands is intensively affected by the scan rate of the potential applied. cvs have shown change of waves for both ligands when the scan rate was changed from 0.02 v s -1 to 0.5 v s -1 , but the discussions are made from 0.2 v s -1 . figure 7 shows cvs of the pyridinium ligands in the absence and in the presence of 0.25 mg ml -1 lipase (ph 7.0) at different scan rates. an increase of ia is obtained for lr from 2.77 μa (at 0.2 v s -1 ) to 6.3 μa (at 0.5 v s -1 ) and also a slight shift of the potential from e1 of 0.22 v to e2 of 0.28 v vs. ag/agcl. in the same time, lm does not indicate an evident anodic peak observed in the oxidation region but a slight increase of ia with the scan rate of the potential applied was observed. the current peak increased with the increase of the scan rate and the relationship of the current oxidative peak against the scan rate in the range of 0.02 0.5 v s -1 was plotted (results not shown). figure 7. cvs recorded of the lr and lm in the presence of 0.25 mg ml -1 of lipase at different scan rate, e / v vs. ag/agcl figure 8 shows cvs of both ligands in the presence of 0.50 mg ml -1 enzyme (ph 7.0) at the scan rate of 0.5 v s -1 . a distinct anodic peak is obtained as an effect of an intensive oxidation-reduction process, more evident for lr (at +0.25 v) than lm. the electrochemical behaviour of ligands is different, lipase showing a positively catalytic effect on lr in comparison with lm. this catalytic activity of lipase is mainly due to the absence of the ethylenic bridge from the structure of lr. the enzymatic activity of lipase is dependent on the substrate structure. the ia value for lr is higher than ia of lm (δia= 45 μa), which could be explained by a more rapid electron transfer process for lr, as a result of the favourable its structural arrangement, comparative with lm. the presence of the mobile ethylenic group marks the changes in the electrochemical performances of the lm. j. electrochem. sci. eng. 6(1) (2016) 91-104 tinteractions between lipase and pyridinium 100 figure 8. cvs recorded of the lr and lm in the presence of 0.50 mg ml -1 of lipase; e / v vs. ag/agcl, 0.5 v mv s -1 no reduction wave was observed in the presence of phenacyl bromide, the precursor of pyridinium ligands, and respectively on the ethyl propiolate (synthon in cycloaddition reaction) in presence or absence of lipase. the lipase interaction with the precursors is not observed (figure 9). cvs recorded only the effect of the diffusion process on the carbon electrode. these results demonstrated that the two pyridinium ligands with different structures than the precursor phenacyl bromide have shown an electro-oxidation behaviour and an interaction with lipase was observed (figures 5-8). figure 9. cvs recorded of phenacyl bromide in the absence and in presence of lipase 0.25 mg ml -1 (1, respectively 2); ethyl propiolate in the absence and in presence of lipase 0.25 mg ml -1 (3, respectively 4); e / v vs. ag/agcl, 0.5 v s -1 structural characterization the lipase was analyzed before and after interaction with the ligands to observe the morphology and structural changes of enzyme. the lipase (white powder) became violet-red after 1 day in contact with lr and weak yellow after the contact with lm. the recuperated lipase from the contact with ligands after the cv measurements was filtered and dried in air at room temperature. this result has shown that the competitive absorption between ligand molecule and lipase can exist. the sem images and elemental analysis (edx) was performed when lipase was placed on carbon-coated-copper. sem images show a modification in the structure of lipase before of the experiment and after the interaction with the ligands, in the presence or in the absence of oxygen (figure 10). s. patriche et al. j. electrochem. sci. eng. 6(1) (2016) 91-104 doi:10.5599/jese.232 101 the particle size of lipase (figure 10a) changes significantly, being reduced in comparison with the particle size after the interaction of enzyme with the lr electrolyte, when the nitrogen was purged before the electrochemical analysis (figure 10c). anyway, the chemical analysis indicated almost same concentration of the enzyme, carbon 95.89 wt % versus 95.44 wt % and respectively oxygen 4.11 wt % versus 4.56 wt %, so enzyme was present in both systems. the reduction of carbon at 88.2 wt % and oxygen at 1.5 wt %, boron (10.18 wt %), also na and mg in small content with a strengthening role in the cell is evident a result of a contact of lipase with lr in the presence of oxygen (figure 10b). the results have also shown an interaction of lipase with ligands with the formation of an enzymatic complex. figure 10. sem images and edx analysis of l enzyme (a); lipase interaction with lr in the presence of oxygen (b) and without oxygen (c) suggested mechanism the voltammetric method is used for the investigation of the interaction of the ligands with lipase [42-44]. the decrease of the reductive current’s peak of the reaction solution when lipase was added suggests the decrease of free ligands concentration (figures 5-8). based on the decrease of current’s peak when the enzyme increasing, the electrochemical method could estimate the determination of lipase or different kinds of proteins according to references [42-44]. the specific adsorption of ligand on quasireversible reduction wave at -0.4 v vs. ag/agcl is associated with the one electron reduction of the pure ligand. the oxidation mechanism of lr with small lipase amount could proceed in a successive steps, as the two anodic peaks, one of lower j. electrochem. sci. eng. 6(1) (2016) 91-104 tinteractions between lipase and pyridinium 102 intensity, could be an explanation of a secondary product, process controlled by diffusion. the formation of a single ligand-lipase complex was proposed. in the acidic solution, at ph 6.5 7.0 the lipase are positively charged, while the ligands species are zwitterion structures and an electrochemical quasireversible process is provided. initially the ligands, possibly negatively charged, are electrostatically attracted to lipase. according to literature [41,42] the composition and the equilibrium constant could be calculated based on the changes of peak current. our results show that both ligands follow different mechanisms their structures. the differences of electrochemical behaviour could be attributed to the structural differences between the two pyridinium ligands investigated. the chemical reaction is proposed to take place following a protonation ece mechanism [45]. the proposed mechanism of lr is the reducing in the protonated form at lower ph in two electronic steps. in acidic media, lr was deprotonated on the radical cation formed after the first one electron transfer. firstly, it is the reduction of ligands, noted as lr (lr / lr +) and the second step is the role of electron carrier of pyridinium ligand (lr + / lr .+ ) [27]. the radical intermediate subsequently undergoes ah to the formation of a new radical on the ligand lr .+ . lm having the ethylene group follows sequence ece in acidic media with the second step that from the mobile ethylene group and the deprotonation is not fast realized. our study and suggested mechanism is useful to understand the steps of the cycloaddition reactions mechanism in which the studied compounds (pyridinium ligands) could participate as synthons [9]. conclusions the interaction between lipase and two pyridinium ligands derived from 4,4 ’ -bipyridine in 0.1 m kno3 electrolyte from initial contact and during a certain period of time has been demonstrated. the ph and conductivity measurements also ocp sustain a rapid ionic exchange between ligands and lipase. the stability of the ligands is influenced by the lipase content. the decrease of the absorbance from uv/vis spectra of the both ligands in aq. electrolyte and in the presence of lipase confirms binding interactions have occurred in the reaction media. the temperature is an important factor of this interaction and an inhibition of lipase activity on ligands structure is confirmed at 40 °c. the lipase content has substantial effects on redox properties of the electron transfer between pyridinium ligands and 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[44] f. hasan, a. a. shah, a. hameed, biotechnology advances 27(6) (2009) 782-798. [45] v. t. kumar, r. l. burke, analytical chemistry 65(18) (1993) 2428-2436. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://pubs.rsc.org/en/results?searchtext=author%3amadalina%20tudorache http://pubs.rsc.org/en/results?searchtext=author%3aandreea%20nae http://pubs.rsc.org/en/results?searchtext=author%3asimona%20coman http://pubs.rsc.org/en/results?searchtext=author%3avasile%20i.%20parvulescu http://pubs.rsc.org/en/journals/journal/ra http://creativecommons.org/licenses/by/4.0/ {molecular interaction of natural dye based on zea mays and bixa orellana with nanocrystalline tio2 in dye sensitized solar cells} http://dx.doi.org/10.5599/jese.1014 179 j. electrochem. sci. eng. 11(3) (2021) 179-195; http://dx.doi.org/10.5599/jese.1014 open access : : issn 1847-9286 www.jese-online.org original scientific paper molecular interaction of natural dye based on zea mays and bixa orellana with nanocrystalline tio2 in dye sensitized solar cells arnold huamán1,2, karim salazar3 and maría quintana1, 1universidad nacional de ingeniería, av. tupac amaru 210, rímac 15333, lima, perú 2universidad tecnológica del perú, av. arequipa 265, cercado de lima 15046, lima, perú 3universidad nacional agraria la molina, av. la molina s/n, la molina 15024, lima, perú *corresponding author: mariavnac@yahoo.com received: june 4, 2021; revised: july 13, 2021; accepted: july 15, 2021; published: july 21, 2021 abstract this work studies the interaction between natural dyes obtained from peruvian zea mays and bixa orellana seeds and nanostructured titanium dioxide, in order to evaluate their function as sensitizers into solar cell devices. the effective attachment of dyes to the tio2 layer is corroborated by the comparison of uv-visible absorption and ft-ir spectra of the extracted dye solutions and sensitized tio2 electrodes. the principal compounds from the seed extraction of zea mays and bixa orellana are cyanidin-3-glucoside (c3g) and bixin respectively, which were analyzed in an isolated dye/cluster tio2 system by molecular dynamic simulation. the results showed that chemisorption is carried out through a consecutive deprotonation process, and ti-o bond formation by the monodentate oh and cooh anchoring groups, for c3g and bixin respectively. finally, we tested the effect of the dye tio2 interaction on the charge transfer by the comparison of the current-voltage curves and incident photon-to-current conversion efficiency (ipce) of the cells. we found that dye agglomeration in films with bixa orellana and high charge recombination of films with zea mays are critical points to be solved. for this reason, we propose the pretreatment of the tio2 film before sensitization with bixa orellana and analyze the effects of ph in zea mays solution, in order to obtain better device efficiencies. keywords molecular dynamic simulation; nanostructured titanium dioxide; plant colorants; sensitizer chemisorption. introduction the third-generation of photovoltaic cells employs mainly organic molecules nanocrystalline materials. the high band gap semiconductor compound tio2 is extended to the visible spectrum http://dx.doi.org/10.5599/jese.1014 http://dx.doi.org/10.5599/jese.1014 http://www.jese-online.org/ mailto:mariavnac@yahoo.com j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 180 absorption by the addition of dyes (known as sensitizers). the dye-sensitized solar cells (dssc) are solar energy conversion devices that since their appearance, have attracted considerable attention due to their low production cost and the environmental friendliness. this type of solar cell bases its operation on the regeneration of dye molecules by a redox system based from electrolyte, which is regenerated by the coming electrons from an external load [1]. the heart of the system consists of a mesoporous oxide layer composed by nanometric particles attached with dye molecules, which are responsible for activating the charge transfer. the sensitizing dye plays a very important role in the process of solar conversion into electrical energy. numerous metal and organic complexes have been synthesized as sensitizers, the most efficient being those based on the ruthenium complexes [2,3]. however, from the environmental point of view, the presence of this heavy metal in synthesized dyes is undesirable and besides, the methods of their preparation are still complicated and expensive. in this sense, the dyes obtained from natural dyes appear to be good alternatives due to their nontoxicity, low-cost production and complete biodegradation. natural dyes, including plant colorants such as anthocyanins [4-12], betalains [13-16], chlorophyll [17-21] and carotenoids [22-24], have already been targeted in many studies. for this particular work, two natural products from peru have been used. the first is zea mays, known as purple maize. this product is rich in anthocyanins, which give it the characteristic purple color. the main compound of the group of anthocyanins is cyanidin-3-glucoside (c3g) and, to a lesser extent, pelargonidin-3-glucoside, peonidin-3-glucoside and cyanidin-3-(6"-malonylglucoside) [25]. the second is bixa orellana, also called achiote. it is a tropical shrub native to the southwestern amazon, which has an inedible red fruit with about 50 seeds. a dark red extract is obtained from achiote seeds, which is widely used as coloring and flavoring. the pericarp of seeds contains a high concentration of carotenoids, which make up to 80 % of the carotenoid 9'-cis-bixin or bixin. the remaining 20 % includes transand cis-norbixin [26]. the chemical structures of the main components of these dyes are shown in figure 1. a b figure 1. chemical structure and optimized geometry of cyanidin-3-glucoside (a) and apocarotenoids cis-bixin (b) a. huamán et al. j. electrochem. sci. eng. 11(3) (2021) 179-195 http://dx.doi.org/10.5599/jese.1014 181 commonly, natural sensitizers present lower efficiency due to their limited light absorption capability and weak bonding with semi-conductor network [27]. for this reason, it is necessary to understand the interaction of the plant-based dye with tio2 cluster in order to develop more efficient materials to improve the efficiency of natural dsscs (figure 2). in this article we analyze the sensitization mechanism of natural dyes obtained from zea mays and bixa orellana. a combined theoretical and experimental study is carried out to achieve this purpose. the interaction between the main components of both dyes with a tio2 cluster is analyzed using molecular dynamics simulation (mds). ftir and uv/vis spectroscopy methods, in addition to verifying the sensitization and identifying the functional anchoring groups of the dyes, provide an experimental corroboration of the theoretical mechanisms proposed by mds. the most favorable characteristics for the sensitization of each particular dye revealed by this study, allow us to propose additional procedures for the optimization of dye preparation. figure 2. architecture of dye-sensitized solar cell from this study experimental extraction of natural dyes the purple corn was first unshelled and dehydrated at 80 c overnight. then the crown of the corn was scratched to obtain floccules from the surface layer, which were crushed with a mortar to obtain a fine powder. the obtained product was diluted in water in 1:500 ratio by weight. after one hour of heating at 80 c, the solution was filtered and centrifuged at 3000 rpm by 30 min. the powder obtained from achiote seeds was diluted in acetone in 1:100 ratio by weight. after one hour of vigorous agitation, the solution was filtered. the concentration of anthocyanins contained in the purple corn solution was calculated with the method of wrolstad et.al. [28], obtaining 14.65 µm. the concentration of bixin contained in the achiote solution was approximately 0.26 µm, and was calculated using lambert's law [29]. preparation of solar cells sensitized with dye glasses coated with fluorine-doped tin dioxide (fto) with sheet resistance of 15 /sq (1.1 mm) were used, which were previously washed and rinsed in an ethanolic solution of 0.1 m hcl. a paste of titanium dioxide nanoparticles dsl 30nr-d provided by the solaronix company was deposited on http://dx.doi.org/10.5599/jese.1014 j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 182 the conductive substrate using doctor-blade (tape casting) method in a circular area delimited by 3 mm radius. the film was consolidated by a heat treatment at 450 °c for 45 min. figure 3 shows sem images of one of the films prepared with this method. films were made up of particles whose sizes range from approximately 20 to 50 nm. the average thickness of the films was 8 µm. a b figure 3. sem images of the front (a) and side (b) view of tio2 film prepared with doctor blade method the experimental xrd pattern in figure 4 agrees with the jcpds card nº 21-1272 corresponding to anatase tio2, while the other peaks correspond to the fto film. the electrodes were immersed in the corresponding solutions containing the natural colorant in a period of 18 hours. on the other hand, a chloroplatinic acid solution was prepared to obtain platinum films by heat treatment at 450 °c for 15 min. the solar cells were assembled into redox electrolyte, constituted by tetra-n-butylamonium iodide (tbai) 0.6 m and 0.05 m iodine (i2) in acetonitrile [30] put between the sensitized conductive glass/fto electrode and the platinum counter electrode. 2 / o figure 4. xrd pattern of tio2 film on fto prepared with doctor blade method characterization natural colorants solutions and sensitized tio2 films were characterized using uv-visible usb4000 spectrophotometer from ocean optics. the fourier-transform infrared (ftir) spectra were obtained by a nicolet is10 team from thermo fisher. the current-voltage curves of the solar cells were tested under a standard light source, which consists from a white light emitting diode (led). the light intensity was 1 sun equivalent (1000 w m-2 am1.5g). incident photon-to-current conversion efficiency (ipce) was measured by an ipce station consisting mainly of 175 w xenon lamp source, monochromator cm110 (spectral products), si calibrated detector and a digital acquisition board labjack u6. a. huamán et al. j. electrochem. sci. eng. 11(3) (2021) 179-195 http://dx.doi.org/10.5599/jese.1014 183 computational methods molecular dynamics simulations of the complexes tio2 and isolated dyes were carried out with an initial separation of 5.0 å between them. the tio2 cluster corresponds to anatase configuration and consists of two-unit cells, resulting in a small cluster of 12 atoms. the initial structures of the complexes were equilibrated during 8 ns simulation time, while the individual component was equilibrated during 4 ns. the force field used is reax ff for all the molecular dynamics simulations [31]. it allows the study of reactive environments, and its algorithms is based on the bond order calculations. during the calculation, the charge equilibration is performed in all the molecular system. the used ensemble is nvt at 300 k, which allows to maintain the system at the constant temperature at environmental condition with mass conservancy. the energy of the molecular system is calculated solving the potential and kinetic energy. the calculation of the binding energy of the dye/tio2 complex is performed by the following operation, the total energy of the complex minus the total energy of each componen., taking into consideration the average of the total energy of the molecular system each 1000 fs. results ftir characterization of natural dyes the ft-ir spectra of natural dyes can provide information about the functional groups from the plant extraction compounds, which allows the identification of the most abundant dyes from each extraction and its interaction with tio2 np film. that is why we compare the measurements taken on dye samples before and after the sensitization. the ir analysis of the zea mays extraction in powder is shown in figure 5a. it has common vibration band of functional groups present in cyanidin-3-glucoside (c3g) dye such as oh functional groups (3271 cm-1), c-h stretching vibrations modes of benzenic ring (2915 cm-1), carbonyl group (1718 cm-1), c=c aromatic ring stretching vibrations (1604 cm-1), and ester linkage (1048 cm-1) [32]. the detected functional groups confirm the presence of anthocyanins in the zea mays (zm) sample, which is correlated to the cyanidin-3-glucoside (c3g) structure. a b figure 5. ft-ir spectrum of dye present of zea mays extraction in powder (a) and its comparison with ft-ir spectrum of dye on tio2 (b) after sensitization onto tio2, we observe in figure 5b that the peak corresponding to free oh bonds (3600 cm-1) in the powder sample [33] disappear, passing to form a single band corresponding to -h bonds that include inter and intramolecular interactions [34]. another interesting peak http://dx.doi.org/10.5599/jese.1014 j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 184 corresponds to the c-h stretching from benzene, which becomes weak and shift to the red (2910 cm-1) after sensibilization. these chemical bond vibrations could be affected by the interaction between the molecules themselves or with the tio2 cluster. however, the most effective evidence of chemical interaction between the dye and the electrode is the presence of ti-o at 559 cm-1 [35]. ft-ir spectra of dried bixa orellana powder and the vibrational modes shown in figure 6a suggests the presence of apocarotenoid cis-bixin (bixin) compound because of the designation of characteristic peaks: c-h stretch due to the methyl and methylene group at 2922 and 2851 cm-1, carboxylic acid group at 1713 cm-1, alkene c=c stretch at 1642 cm-1, the asymmetric bending vibrations of the methyl -ch3 group at 1377 cm-1, stretching vibration of c-o at 1286 cm-1, and the symmetric and asymmetric c-o-c ether groups at 1248 and 1151 cm-1 [36]. figure 6b shows a decrease in the peak corresponding to c=o stretching of the carboxylic acid group (1713 cm-1). typically, this peak disappears after dye electrode interaction through the carboxylic group [37]. however, this band does not disappear completely, suggesting that presence of molecules on the surface are stacked and create a shielding effect. a broadening of the band assigned to the c=c alkene bond (1642 cm-1) is observed, which suggests the presence of macroaggregates [38], which would confirm the aforementioned effect. another interesting peak corresponds to the methyl group, which is shifted to the red (from 1371 to 1365 cm-1), what could be due to interaction through this group at the interface. in addition, a small peak appears after the dye sensibilization corresponding to ti-o bond formation at 503 cm-1 [35]. a b figure 6. ft-ir spectrum of dye present in bixa orellana extraction in powder (a) and its comparison with ft-ir spectrum of dye on tio2 (b) uv-visible characterization of natural dyes in order to assure the dye/tio2 interaction, we run uv-vis spectrometry for the extracted natural dyes when they are in solution and after being absorbed on tio2. in figure 7, the absorption spectrum of the zea mays solution shows a maximum peak at approximately 520 nm within the typical broad absorption peak of anthocyanins, which is the main compound of this dye [39]. while the absorption band of the adsorbed dye onto semiconductor film is broadened, the corresponding maximum is red shifted with respect to their solution [40]. the visible absorption band can shift to a lower energy due to the complexing with metal atoms [41]. the broadening of the absorption band is also an indication of a charge transfer interaction between the sensitizing dye and tio2 surface [42]. a. huamán et al. j. electrochem. sci. eng. 11(3) (2021) 179-195 http://dx.doi.org/10.5599/jese.1014 185 wavelength, nm figure 7. uv-visible absorption spectra of the extraction from zea mays in water solution (dotted line) and adsorbed on tio2 (solid line) figure 8 shows three absorption peaks for the bixa orellana solution spectrum located at approximately 435, 460 and 490 nm, coinciding very well with the bixin spectrum which is the main compound within the group of carotenoids that make up this dye [43]. once the dye is adhered on the tio2 surface, the absorption spectrum gets wider. in addition, the intensity of maximum absorption peaks increases, which shows the resulting strong interaction between bixin and tio2 surface [44]. it shows blue-shifted peaks in the spectra of bixin absorbed in tio2, which has been attributed to the aggregate formation of dyes and/or the disarraying of dyes on tio2 surface [45]. wavelength, nm figure 8. uv-visible absorption spectra of the extract from bixa orellana in ethanol solution (dotted line) and adsorbed on tio2 (solid line) molecular dynamic simulations of the complexes tio2 and isolated dyes the carbonyl and hydroxyl groups represent the linking groups for the attachment of dye molecules to tio2 [46]. in order to evaluate how dye interacts with tio2 electrode, molecular dynamics simulations of the complexes using reax ff were performed. we build the initial configuration of the complex dye/anatase tio2 cluster (n=12), considering a monomeric dye adsorbent on http://dx.doi.org/10.5599/jese.1014 j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 186 cluster. in the initial complex system (figure 9), c3g is located over the cluster by the cyanidin side at the distance of 5 å. in the initial complex system (fig. 9), c3g is located over the cluster by the cyanidin side at the distance of 5 å. figure 9 shows the progression of the dynamic simulation of the complex tio2-c3g through snapshots at relevant events. it shows that the first hydrogen lost from the oh group of c2’ glucoside part occurs at 5 ps, and this proton passivates tio2 over the surface oxygen. a b c figure 9. initial structure of the complex c3g/tio2 cluster: side view (a), back view (b) and bottom-up view (c). color code: oxygen from tio2 cluster (magenta), titanium (green); oxygen from dye (red), carbon (grey) and hydrogen (cyan) figure 10 shows the progression of the dynamic simulation of the complex tio2-c3g through snapshots at relevant events. it shows that the first hydrogen lost from the oh group of c2’ glucoside part occurs at 5 ps, and this proton passivates tio2 over the surface oxygen. at 21 ps, an intramolecular hydrogen bond between the oxygen from oh group of c3’cyanidin and hydrogen from c2’ glucoside leads the proton transfer. at 23.6 ps, the oxygen from oh at c4’ of cyanidin is close to titanium atom at 1.93 å forming a bond. at 27.5 ps, the second hydrogen from oh at c6’ of the glucoside is lost and forms a part of tio2 surface. the hydrogen, which forms the hydrogen bond between the cyanidin and glucose, is finally transferred to the glucose side at 29 ps. at 545.5 ps, c6’ of the glucoside part is close to oxygen from tio2 at 1.67 å, and then at 859.5 ps, the glucoside get stability with tio2 by hydrogen bonds. finally, the constant hydrogen transfer between the glucose (c2’) and cyanidin (c3’) is stabilized when the oh at c3’ cyanidin is formed after the hydrogen lost at continues c4’ at 1.8 ns. the third hydrogen lost comes from the oh at c4’ glucose at 3.2 ns. at 7 ns the complex gets stabilized after the dye lost three hydrogens and forming one ti-o and two hydrogen bonds. figure 11 shows the initial configuration of the complex bixin-tio2 cluster that maintains a separation of 5.0 å, and the dye is located over tio2 by cooh side. figure 12 shows the progression of the dynamic simulation of tio2-bixin through snapshots at certain events, such as passivation process of the semiconductor with hydrogens and the formation of ti-o bond. h bonds at 1.0 ps between hydrogen atoms at c8 and ch3 19 with oxygen atoms from tio2 cluster. at 2.0 ps, a new hydrogen bond is formed between the hydrogen from cooh and the oxygen from tio2. a hydrogen lost from cooh occurs at 3.0 ps. at 4 ps the ti-o bond is formed and the chemisorption between dye and tio2 cluster becomes stronger (1.8 å). the second hydrogen lost occurs from the hydrogen ch3 19 to tio2 cluster at 15.7 ps, and subsequently the hydrogen from the remaining ch3 19 forms a hydrogen bond with tio2 at 15.8 ps. a. huamán et al. j. electrochem. sci. eng. 11(3) (2021) 179-195 http://dx.doi.org/10.5599/jese.1014 187 5 ps 21 ps 23.6 ps 27.5 ps 545.5 ps 859.5 ps 1.8 ns 3.2 ns 8 ns figure 10. snapshots of the dynamic molecular simulation of c3g and tio2. color code: oxygen from tio2 cluster (magenta), titanium (green); oxygen from dye (red), carbon (grey), and hydrogen (cyan) a b figure 11. initial structure of the complex bixin/tio2 cluster: front view (a) and bottom-up view (b). color code: oxygen from tio2 cluster (magenta), titanium (green); oxygen from dye (red), carbon (grey) and hydrogen (cyan) the dye migrates on the surface, and new hydrogen bond is formed between the hydrogen from ch3 20’ and tio2 cluster at 685 ps. the hydrogen atom from c11’ is lost and form a bind to the oxygen from tio2 at 1.918 ns, forming an alkyne at c11’. then, the dye wraps up the tio2 cluster at 1.944 ns increasing the contact area between them. the self-folding of the dye molecule on tio2 surface occurs at 3.632 ns, and this event stabilizes bixin on the surface better. figure 13 shows the summary of the principal events that occurred in the dynamic simulation during 8 ns for both complexes (c3g/bixin and tio2). the corresponding numbering of c3g and bixin molecules are presenting in order to identify the progression of the trajectory of atoms. http://dx.doi.org/10.5599/jese.1014 j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 188 1.0 ps 2.0 ps 3.0 ps 4.0 ps 15.7 ps 15.8 ps 685 ps 1.918 ns 1.944 ns 3.632 ns figure 12. snapshots of dynamic simulation of bixin and tio2. color code: from dye carbon (grey), oxygen (red) and hydrogen (cyan); from tio2 cluster oxygen (purple) and titanium (green) a figure 13. numeration and summary of dynamic molecular simulation of cyaniding-3-glucoside (c3g) (a) and 9’-cis-bixin (bixin) (b) b a. huamán et al. j. electrochem. sci. eng. 11(3) (2021) 179-195 http://dx.doi.org/10.5599/jese.1014 189 binding energy of the dye/tio2 and dye dimer complexes we calculate the binding energy of the dye (bixin and c3g) with a small cluster of tio2 in order to understand the effect of the dye/electrode interaction on the photocurrent of dssc device. because we suspect that there is an aggregation event during the bixin/tio2 interaction, which causes uv/vis blue shift of bixin in solution compared to bixin adsorbed on tio2, we calculated the binding energy of both dye dimers. in order to get these results, we optimize each isolated element of the complexes. figure 14 shows the stable configuration of the small anatase tio2 cluster, which occurs when all titanium atoms are 4-coordinated. it also shows that for the isolated bixin, the stable configuration is almost linear. in other words, bixin does not prefer to fold over it. isolated c3g configuration remains almost the same in isolation and with tio2 interaction. a b c figure 14. optimized n=12 tio2 cluster (a), bixin (b), and c3g (c) molecules figure 15 shows the optimized dyes dimers after 8 ns molecular dynamics simulation at 300 k. it shows for c3g dimer that benzene rings overlap and get closer (3.5 å), while hydrogen bonds among oh groups of the glucoside part of c3g dyes stabilize the rest of the complex. c3g dimer gets stable when the dipole moments rearrange according to the edge-to-edge. on the other hand, the bixin dimer distance between aliphatic site is 3.2 å and the dipoles rearrange according to edge-to-face motif. a b figure 15. optimized structures of dye dimers: c3g (a) and bixin (b) molecules table 1 shows the biding energy values of the dye to tio2 electrode and dye dimer. bixin shows a better affinity to tio2 (-78.12 kj mol-1) than c3g. however, dyes prefer to associate by themselves (aggregation). table 1. binding energy of complexes dye/tio2 cluster and dye dimmers complexes binding energy, kj mol-1 c3g-tio2 cluster -0.79 c3g dimer -144.14 bixin-tio2 cluster -78.12 bixin dimer -150.29 http://dx.doi.org/10.5599/jese.1014 j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 190 photoelectric response of sensitized solar cells with natural dyes the photocurrent voltage plots of dssc devices are shown in figure 16. the parameters of dsscs sensitized with natural dyes measured under am 1.5 solar light (1000 w m-2), are summarized in table 2. the results show that the best power conversion efficiency (pce) was achieved by cell manufactured with bixa orellana extract due to the higher short-circuit current density (jsc) in comparison with zea mays sensitized solar cell. table 2. characteristic parameters of cells manufactured with zea mays and bixa orellana cell jsc / ma cm -2 voc / v ff η / % zea mays (c3g) 0.889 ± 0.032 0.590 ± 0.006 0.580 ± 0.057 0.304 ± 0.004 bixa orellana (bixin) 1.263 ± 0.106 0.610 ± 0.049 0.599 ± 0.026 0.462 ± 0.025 table 3 shows the characteristic parameters of solar cells manufactured with other natural products based on anthocyanins and carotenoids. it is observed that the cells manufactured with zea mays and bixa orellana present better efficiencies than the other previously reported dyes. table 3. characteristic parameters of cells manufactured with other natural products containing anthocyanin and carotenoid plant source structural class jsc / ma cm -2 voc / v ff pce, % reference rosa xanthina anthocyanin 0.637 0.492 0.52 na [40] tradescantia zebrina anthocyanin 0.630 0.350 0.55 0.23 [47] begonia anthocyanin 0.630 0.537 0.72 0.24 [48] ixora macrothyrsa anthocyanin 1.310 0.400 0.57 0.30 [6] capsicum carotenoid 0.225 0.412 0.63 n.a. [40] gardenia blue carotenoid 0.530 0.440 0.69 0.16 [49] allamanda cathartic carotenoid 0.878 0.405 0.54 0.40 [50] n.a. not applicable figure 17 shows the incident-photon-to-current conversion efficiencies (ipce) spectra obtained with the natural dyes. the ipce corresponds to the number of electrons, measured as photocurrent in the external circuit, divided by the monochromatic photon flux that strikes the cell. the results are in good agreement with the short-circuit current (jsc) values obtained from the current densityvoltage (j v) curves. further, the maximum peaks of the ipce, 6.58 % at 530 nm for zea mays and 12.42 % at 510 nm for bixa orellana, are located within the corresponding regions of maximum absorption observed in figures 7 and 8. figure 16. curves of current density as a function of voltage for cells manufactured with natural dyes wavelength, nm figure 17. incident-photon-to-current conversion efficiencies (ipce) absorption spectra of dssc utilizing dye extracted from purple corn and achiote a. huamán et al. j. electrochem. sci. eng. 11(3) (2021) 179-195 http://dx.doi.org/10.5599/jese.1014 191 discussion the evidence of interaction between dye molecules and tio2 found in optical characterizations were confirmed with the final results of mds in the following aspects. in the case of c3g, the deprotonation of the oh group and the formation of hydrogen bonds promote the chemisorption event, and create the close inter molecule interaction corresponding to -h bonds as observed in ir spectra. intra molecular -h bond was observed in the repeated interaction between the c2’glycoside and c3’ hydroxyl by means of proton exchange in mds. the c-h of the glycosidic part forms a hydrogen bond, while that of the benzene ring weakens after the torsion in c5'. all these interactions cause the c-h bond to become less energetic, obtaining a red shift. evidence of ti-o bonding was found in ir and uv-visible spectra. according to the structure of c3g, the ideal anchor with tio2 would be through the two hydroxyl groups of the molecule benzene-diol [40], following the bidentate mechanism of typical ruthenium-based molecules [51]. however, mds confirm a bond through hydroxyl groups of c4’ and disregard that it can occur through the other branches of cyanidin. this agrees with the optimized structures of anthocyanin-tio2 adsorption complexes of marcano [52] that were also formed with a monodentate bond. in the case of bixin, the peaks in ir and uv-visible spectra of sensitized films showed a strong tendency to agglomeration. this was corroborated with the highest binding energies calculated for the bixin dimers. although both dyes have a high affinity to form aggregates, the optimized structures of the bixin dimer are organized in such a way that generate π-π interactions between its chains. this type of aggregation in carotenoids was previously observed by tay-agbozo et al.[53], and it is stronger than van der waals bonds present in c3g dimer. the evidence of ti-o bonding was also found in the ir spectrum, and it was confirmed by mds that it occurs through the carboxylic group. mds also shows a greater affinity to the point of completely folding over the cluster of tio2. in this configuration, hydrogen bonds could be observed in the -ch3 groups that caused the red shift in the corresponding peak in ir spectrum. the broadening of uv-vis absorbance spectra of dyes after sensitization is the experimental evidence of electronic transfer through the formed ti-o bonds. the passivation of tio2 surface due to the loss of protons from the molecules is a point in favor since through hydrogenization the photocatalytic activity of tio2 can be improved to provide more free electrons [54]. in this sense, it is possible than greater affinity of bixin leads to having a large number of molecules available for electron transfer and to a greater passivation. however, the formation of aggregates on the surface of tio2 implies the appearance of intermolecular processes which decrease the efficiency of electron injection from excited states [55]. despite this, the photocurrent obtained by the cell manufactured with bixa orellana (1.263 ma cm-2) was higher than that obtained with zea mays (0.889 ma/cm2). likewise, the incident photon to current conversion efficiency was also higher in the case of bixa orellana. therefore, we can affirm that the conduction of free electrons through the film sensitized with bixin is more efficient compared to c3g. the electronic transport through the film sensitized with zea mays and bixa orellana into the solar cell system was previously studied in detail by huamán et al. [56]. the work concluded that better photocurrent achieved by the bixa orellana, despite its the tendency to aggregation, can be explained by a higher resistance to recombination in the sensitized film. recombination in solar cells is explained by the presence of a trap state distribution due to the surface modification. in energy terms, a higher density of trap states implies a lower fermi level of the film [57]. the greater distribution of c3g trap states is reflected in the lower voc obtained from the characteristic j-v curves. to avoid a decrease in the fermi level of the film, the effect of passivation during http://dx.doi.org/10.5599/jese.1014 j. electrochem. sci. eng. 11(3) (2021) 179-195 interaction of natural dye to nanocrystalline tio2 192 chemisorption must be controlled and this can be achieved with an increase of ph. in their work on optimizing dye-sensitized solar cells with anthocyanins chien et al. [58] found that efficiency increased with ph. indeed, low ph can suppress deprotonation causing poor anthocyanin binding. this would lead to a low injection of electrons in the tio2 conduction band and increase the probability of charge recombination. h-aggregation models at the tio2/dye interface, where blue-shifted absorption spectra are observed, have been widely reported in the literature [59-62]. however, the intermolecular interaction of bixin aggregates with tio2 surface has not yet been modeled. for now, our results show that this type of aggregation is present and is an important factor that limits the efficiency of the solar cell. marotta et al. studied the effect of aggregation of dye d5 [61]. this molecule, like bixin, has a central chain and a carboxylic branch for bonding with tio2, and their maximum absorption peaks are remarkably close. they found that pretreatment with hydrochloric acid on the tio2 film induces the stabilization of charge transfer from the excited state and attenuates intermolecular interactions. in this sense, it is highly recommended to carry out this treatment before the bixin chemisorption process, in order to mitigate the aggregation effects and achieve better efficiencies. conclusions molecular dynamic simulation provided important information about the sensitization mechanisms of tio2 by natural dyes, which were validated by uv-visible and ft-ir characterizations. the results indicated that the sensitization of c3g was a monodentate anchor through c4’ hydroxyl groups of the benzene-diol of the molecule. the interaction caused hydrogen bonds and the passivation of tio2 with protons from the glycoside part of the molecule. on the other hand, bixin showed greater affinity with tio2 folding over it and forming a bond through the carboxylic group of the molecule. bixin also showed a tendency to dimer formation due to π-π interactions between molecules, facilitating the aggregation on the surface, what was verified by the optical characterizations. the events analyzed during the interaction of dyes and tio2 allow a better overview of the sensitized films through which the current is produced in the solar cell. the affinity of bixin with the surface of tio2 allows the more effective formation of ti-o bonds and thus generates free electrons for conduction. detrimentally, however, aggregation causes less efficient injection of electrons. c3g does not have a strong tendency to aggregate but presents a higher trap states density for free electrons, increasing the charge recombination and decreasing the generated photocurrent. therefore, in order to obtain better efficiencies in the future, we recommend carrying out a study of the effects of ph in tio2 sensitization process with the colorant extracted from zea mays, while in the case of bixa orellana, the treatment of tio2 film with hydrochloric acid could be recommended. in sum, the state of the tio2 surface is a critical factor for adequate chemisorption of 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https://doi.org/10.1038/srep35893 https://creativecommons.org/licenses/by/4.0/) mathematical modeling and reliability analysis of a 3d li-ion battery doi: 10.5599/jese.2013.0040 1 j. electrochem. sci. eng. 4(1) (2014) 1-17; doi: 10.5599/jese.2013.0040 open access : : issn 1847-9286 www.jese-online.org original scientific paper mathematical modeling and reliability analysis of a 3d li-ion battery richard hong peng liang, tangsheng zou, karthik somasundaram*, wei tong*, erik birgersson** raffles science institute, raffles institution, one raffles institution lane, singapore 575954 *department of mechanical engineering, national university of singapore, singapore 117576 **department of chemical and biomolecular engineering, national university of singapore, singapore 117576 corresponding authors: e-mail: vivekarthik81@yahoo.co.in; tel.: +65-6516 4657; fax: +65-6779 1936 received: july 24, 2013; revised: october 24, 2013; published: january 25, 2014 abstract the three-dimensional (3d) li-ion battery presents an effective solution to issues affecting its two-dimensional counterparts, as it is able to attain high energy capacities for the same areal footprint without sacrificing power density. a 3d battery has key structural features extending in and fully utilizing 3d space, allowing it to achieve greater reliability and longevity. this study applies an electrochemical-thermal coupled model to a checkerboard array of alternating positive and negative electrodes in a 3d architecture with either square or circular electrodes. the mathematical model comprises the transient conservation of charge, species, and energy together with electroneutrality, constitutive relations and relevant initial and boundary conditions. a reliability analysis carried out to simulate malfunctioning of either a positive or negative electrode reveals that although there are deviations in electrochemical and thermal behavior for electrodes adjacent to the malfunctioning electrode as compared to that in a fully-functioning array, there is little effect on electrodes further away, demonstrating the redundancy that a 3d electrode array provides. the results demonstrate that implementation of 3d batteries allow it to reliably and safely deliver power even if a component malfunctions, a strong advantage over conventional 2d batteries. keywords 3d batteries, li-ion battery, mathematical model, reliability analysis, thermal model http://www.jese-online.org/ mailto:vivekarthik81@yahoo.co.in j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 2 introduction the demand for small scale, high power density sources has increased with the advent of miniaturized electronic devices such as micro-electromechanical systems (mems), micro-robots, micro-sensors and implantable medical devices. the lithium-ion battery is considered as a viable energy storage system that can cater to many of these applications, as it is able to attain the high energy densities required. conventional batteries with planar cathode and anode layers arranged in a parallel-plate configuration with a separator in between are known as two-dimensional (2d) cells, in which transport of li-ions between the electrodes is one-dimensional (1d) in nature. this 2d cell currently used in commercial applications still face several constraints, especially regarding power limitations [1–3] and reliability which affect millions of industrial and small-scale consumers [4,5]. as a result, three-dimensional (3d) architectures have been developed for the lithium-ion battery [1,3,6] partly to ameliorate some of these concerns and further harness its potential as a key energy solution for the future. this nascent concept describes cells with key structural features extending in and fully utilizing 3d space. as shown in fig. 1a, a 3d cell typically consists of anodes and cathodes which have active surface areas exposed in three dimensions in closely-spaced arrays in a 2d plane. with this, we have to reconsider the phenomena of mass and charge transport, electronic and ionic conductivity and electron-transfer kinetics in the form of 3d batteries. the 3d cell promises many benefits: it can attain enhanced energy capacity without compromising on power density, while maintaining the same areal footprint [6–8]; it enables us to take advantage of more extensive interactions between the active materials [1,9,10], allowing us to adopt a design that improves reliability in the event that an active component ceases to function. the flow of energy and current in the conventional parallel-plate design essentially stops when an intermediate component fails. on the other hand, even if one of the electrodes in a 3d design fails, the battery can potentially continue to operate (albeit with reduced capacity and performance) as repeating units of electrodes arranged in a tessellation can provide redundancy. experiments have been able to produce working precursors to fully functional 3d batteries through the use of a variety of electrochemical deposition techniques. for instance, c-mems (carbon-microelectromechanical systems), [11–13] a solution for miniaturization, uses photolithography to implement photoresist arrays on a sio2 surface, followed by pyrolysis at high temperatures in an oxygen-free environment. changing conditions under which these steps are carried out allows one to vary design shapes, and also mechanical and transport properties [2,9,14,15]. lithographic techniques vapour deposition techniques are being used to prepare independent arrays of the electrodes [1,2,16–21]. recently, interdigitated li-ion microbatteries are prepared using 3d printing techniques [22]. however, computational studies and mathematical modelling have yet to be entirely developed for the 3d li-ion cell, as they either do not completely solve for coupled electrochemistry, transport phenomena and heat generation, or consider only electrochemical phenomena for optimization studies. hart et al. [6] modelled and estimated current densities and potentials for arrays of electrodes with different geometries, while zadin et al. [23–25] focused on simulating the ionic transport mechanisms in liquid and polymer electrolytes inside a 3d microbattery assuming non-porous solid electrodes to show how cell geometry can give rise to qualitatively non-uniform current densities and thus suboptimal surface utilization. however, neither model considers the electrochemical activity or thermal behaviour inside the electrodes. on the other hand, detailed r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 3 mathematical models have been formulated to predict transient local electrochemical and thermal changes in a li-ion cell in a rectilinear geometry [26–29] and a spiral-wound geometry [30,31], both essentially 2d-collapsible geometries. by virtue of the lack of a detailed resolution for modelling and simulation of coupled electrochemistry, transport phenomena and heat generation for the 3d li-ion cell, the aim of this paper is twofold: first, to employ a thermal-electrochemical model for studying the behaviour of a 3d liquid electrolyte li-ion cell applicable to various geometries; second, to apply this model to a planar tessellated electrode geometry and conduct a reliability analysis to demonstrate the redundancy and longevity that a 3d battery can attain. our mathematical model will investigate the transient conservation of charges, species and energy; it couples the electrochemical and thermal behaviour through the heat generation arising from reversible, irreversible and ohmic heating as well as through the temperature-dependent transport and electrochemical parameters. mathematical formulation the 3d battery has the advantage of having a larger areal energy capacity than the conventional 2d design, but also has a disadvantage of having non-uniform current density. this would lead to the poor utilization of the electrode materials, resulting in lower cell efficiency, non-uniform heat dissipation etc. studies have shown that a checkerboard cathode/anode array configuration, where each electrode is surrounded by four nearest neighbour opposite electrodes, provides a more uniform current output around every electrode compared to that of other arrays. current uniformity in this 3d design would render it more useful in a wider variety of applications [6]. as a result, we have selected a 3d battery that consists of positive and negative electrodes arranged in square planar tessellation as shown in fig. 1 for our study. each electrode is adjacently bounded by four electrodes of opposite sign. current collectors plates are present at either end of the electrodes; one for the anode and another for the cathode. for the individual electrodes, we consider two shapes of different extremes circles and squares. square electrodes provide a cleaner tessellation in the array, shorter average distances between electrodes and a higher packing efficiency. circular electrodes are however much more feasible to implement and representative of real-life manufacturing processes. hence, we consider both the square and circular electrode arrays in this paper, and compare the differences in performance between these two extremes. in order to ensure that the comparison between both square and circular arrays is fair, a few restrictions have been imposed. the minimum distance between each electrode (wse), as well as the thickness of both electrodes (wpe, wne) i.e. the diameter of the circular cross-section or the edge length of the square cross-section, is kept constant in both arrays. the dimensions are provided in tables 1 and 2. the electrochemical and thermal behaviour of a three-dimensional (3d) li-ion cell consisting of a graphite negative electrode (ne) and a manganese oxide spinel positive electrode (pe) as shown in fig. 1 is studied. the electrodes and the spaces between the electrodes are filled with an electrolyte solution (el) of lipf6 salt in 1:2 ethylene carbonate: dimethyl carbonate solvent. the materials considered here are the same as that used for a conventional cell that is commercially available with the assumption that these can be utilized to fabricate 3d batteries using the conventional techniques. as depicted in fig. 1, there are two main scales involved in the modeling of a li-ion cell: the macroand the micro-scale. in short, the transport of ions and electrons in the cell between the j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 4 electrodes is referred as the transport at the macroscale, which includes species transport in the liquid electrolyte, electronic charge conduction in the solid phase and ionic charge conduction in the liquid electrolyte; and the diffusion of ions in the active material present in the electrodes is referred to as transport at the microscale, which includes diffusion of lithium in the active material of the porous electrodes. figure 1. (a) schematics of (a) 3d li-ion battery, (b) section aa showing the various functional layers in the battery with the roman numerals indicating the interfaces of these layers and the boundaries, (c) agglomerate structure in the negative electrode (positive electrode also exhibits similar structure), (d) diffusion of lithium in active material in the electrodes on the microscale, (e) top view of the battery (xz-plane) with square cross-section, and (f) circular cross-section r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 5 table 1. physical properties and design parameters of battery components parameter unit cc (-) ne el pe cc (+) ref. 0 lc mol m -3 2.0 × 10 3 27 cp j kg -1 k -1 3.8 × 10 2 7.0 × 10 2 7.0 × 10 2 7.0 × 10 2 8.7 × 10 2 34,37 sc 0 mol m -3 1.5 × 10 4 3.9 × 10 3 27 max sc mol m -3 2.6 × 10 4 2.3 × 10 4 27 dl m 2 s -1 7.5 × 10 -11 27 ds m 2 s -1 3.9 × 10 -14 1.0 × 10 -13 27 ea,di kj mol -1 10 34 ea,ds kj mol -1 4 20 34 la,σ e kj mol-1 20 34 hi m 10 × 10⁻⁶ 5 × 10⁻⁴ 5 × 10⁻⁴ 10 × 10⁻⁶ k w m -1 k -1 3.8 × 10 2 0.05 × 10 2 0.01 × 10 2 0.05 × 10 2 2.0 × 10 2 34,37 k0 mol 2.5 m -0.5 s -1 2 × 10⁻ 11 2 × 10⁻ 11 rs m 12.5 × 10⁻ 6 8.5 × 10⁻ 6 27 wi m 10 × 10⁻⁵ 10 × 10⁻⁵ a, c 0.5 0.5 27 p 0.14 0.19 27 l 0.36 0.44 27 f 0.03 0.07 27 0 iθ 0.56 0.17 27  kg m -3 9.0 × 10 3 1.9 × 10 3 1.2 × 10 3 4.1 × 10 3 2.7 × 10 3 27,34,37 s s m -1 6.0 × 10 7 1.0 × 10 2 3.8 3.8 × 10 7 34 table 2. other model parameters parameter unit value h m 5.7 × 10⁻⁴ ht w m -2 k -1 5 iapp a m -2 7 × 10² (circular); 9 × 10² (square) l m 6.6 × 10⁻⁴ ta, tref k 298.15 w m 6.6 × 10⁻⁴ wse m 5.2 × 10⁻⁵ the model is based on the porous-electrode theory developed by newman and tiedemann [32,33] and embodies the following main assumptions: 1. isotropic material properties; 2. uniform distribution of active materials of the same size in the electrodes; 3. the active material is assumed to be spherical; i.e., we only need to consider the radial direction at the microscale; 4. side reactions are assumed negligible. the mathematical formulation consists of the conservation equations of species and charge, together with conservation of energy at macroscale [27,34]; the diffusion length or the polynomial approximation approach is employed for the conservation of lithium inside the active material at microscale. for the sake of brevity, the governing equations, initial conditions, and constitutive relations are provided in tables in the appendix and the details can be found in our earlier work [31]. j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 6 the physical properties and design adjustable parameters pertaining to the geometry studied in this work are given in tables 1 and 2. the current density is prescribed at the positive current collector at the boundary i (see fig. 1b for placement of roman numerals) and also newton’s law of cooling is specified here. at the interface between the current collector/electrode or the current collecting tab/current collector, continuity of energy flux and solid-phase current is specified; insulation is specified for the ionic flux and current. at the current collector/electrolyte, insulation is defined for the solid phase current and continuity for the energy flux. at the boundaries iv and v, there is no flow of ions/electrons as well as energy and hence insulation is specified here as well for solid phase current and energy. at the electrode/electrolyte interfaces, continuity of energy flux and ionic flux as well as ionic current is defined and since there is no flow of electrons across the interface, insulation for solid phase current is defined. the current is collected from the negative current collector at the boundary vii or otherwise this end is grounded and also newton’s law of cooling is specified here. numerics the commercial finite-element solver, comsol multiphysics 3.5a [35], was employed to solve the 3d model. linear elements were implemented for all dependent variables s, l, cl, surf sc , t and avg sc the direct solver umfpack was chosen as linear solver with a relative convergence tolerance of 10 −4 , and solutions for all models were tested for mesh independence. all computations were carried out on a workstation with dual-core processors (2.33 ghz) and a total of 64 gb random access memory (ram). charge and discharge currents, iapp, were applied at the respective boundaries with a smoothed heaviside function. for both 3d arrays, there were 3×10 4 elements and 1.6×10 5 degrees of freedom (dof) which required around 45 gb of memory for solution under 5 c discharge with a solution time of around 1 hr. the reliability analysis was carried out by targeting one of the electrodes – either positive or negative – as marked in our computational cell in fig. 1f. we assume that there will be no exchange current in the malfunctioning electrode as no reaction occurs; to reflect this, the transfer current per unit volume, j, is set to zero in the simulation for that electrode alone. results and discussion in this section, we begin by studying the behaviour during discharge of a 3d lithium-ion battery with tessellated electrode geometry, for both the square and circular electrode arrays. a reliability analysis is then carried out for the circular electrode array to study the behaviour of the battery when one of the electrodes in the array malfunctions. only the discharge process will be illustrated as charging exhibits a similar behaviour. standard discharge behaviour discharge curves first, we shall explore the global behaviour of the 3d battery in terms of potential during discharge for different c-rates, as shown in fig. 2. the drop in cell voltage during the discharge in both the square and circular electrode arrays is similar. both arrays exhibit a gentle and constant decrease in potential for most of the discharge period except for the sharp drop towards the end of discharge, similar to that of the 2d cell. under all discharge rates, there is a sudden drop in the r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 7 potential from the initial value of 4.2 v during the initial period of discharge that corresponds to ohmic losses. at 1 c-rate, there is a gradual drop in the potential from 3.9 v to 3 v until the end of discharge at 3600 s when a sudden drop in the potential is seen as in fig. 2. while a similar trend is observed for discharge at 2 c-rate, the array experiences a much faster drop in potential at 5 c-rate due to a higher discharge current density. figure 2. battery voltage during discharge at various discharge rates for square (continuous) and circular (dashed) electrodes because electrodes in a 3d array are arranged in a parallel configuration, an array containing a large number of electrodes will have a greater capacity compared to a single pair of electrodes as compared to the 2d cell. having more electrodes in an array would increase the total current delivered but keeping the potential unchanged. the array capacity can also be increased by elongating the electrodes into the plane, instead of increasing the capacity by making the electrodes thicker as in 2d batteries. as such, power density is not sacrificed for an increase in the array capacity in 3d batteries since the distance between electrodes remains the same. on the other hand, array capacity is gained at the expense of power density in 2d cells as an increase in capacity is attained by increasing the thickness of electrodes. electrochemical behaviour during discharge, lithium ions deintercalate from the active material in the negative electrode and enter the electrolyte; the reverse process happens in the positive electrode. hence, the concentration of lithium ions in the electrolyte increases in the negative electrode and decreases in the positive electrode, as depicted in fig. 3. the time constant for diffusion is around 100 s ( 2 eff i lw / d ) after which the concentration profile reaches a pseudo-steady state. the lithium-ion concentration reaches a maximum of 2100 mol m⁻³ at 1 c-rate and 2500 mol m⁻³ at 5 c-rate in the cell, as well as a minimum of 1900 mol m⁻³ at 1 c-rate and 1700 mol m⁻³ at 5 c-rate. these maximum and minimum concentration values are found in the negative and positive corner electrodes respectively, since the corner electrodes are only surrounded by two nearest neighbours: a negative corner electrode has fewer adjacent positive electrodes to consume fewer j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 8 lithium ions, while a positive corner electrode has fewer adjacent negative electrodes to produce lithium ions. figure 3. variation of concentration of lithium ions in the electrolyte along the xz-plane at y=2.8 × 10⁻⁴ m at various times during discharge at 1 c and 5 c-rates for the circular electrodes moreover, there is variation in the lithium ion concentration in the electrolyte along the height of the electrodes, as illustrated in fig. 4a. both arrays display similar behaviour, with the square electrode array attaining a slightly higher concentration of 2105 mol m⁻³ in a negative corner electrode compared to 2090 mol m⁻³ in the equivalent circular electrode. extreme values are observed in the areas nearer the current collector: negative electrodes have the highest lithium ion concentrations near the negative current collector as consumption of lithium ions by the positive electrode is lowest near the negative current collector; positive electrodes have the lowest lithium ion concentrations near the positive current collector as production of lithium ions in the negative electrode is lowest near the positive current collector. figure 4a. concentration profile of lithium ions in the electrolyte at the end of discharge at 1 c-rate along the xy-plane at z = 10⁻⁴ m (a, c) and at z = 2.5 × 10⁻⁴ m (b, d) for circular and square electrodes respectively r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 9 figure 4b. liquid phase potential profile in the electrolyte at the end of discharge at 1 c-rate along the xyplane at z=10⁻⁴ m (a, c) and at z=2.5 × 10⁻⁴ m (b, d) for circular and square electrodes respectively the unequal distribution of lithium ions also results in a variation in the liquid phase potential in the cell, as can be seen in fig. 4b. again, behaviour of both arrays are similar, with a maximum potential drop of 12×10⁻³ v in the circular electrode array compared to 13×10⁻³ v in the square electrode array. this potential drop, which can be attributed to ohmic losses and the concentration overpotential, gradually accentuates as the concentration gradient steepens during discharge. extreme values of potential drop are observed nearest to the current collectors, because of similar reasons that cause extreme values in lithium ion concentration. first, we shall explore the global behaviour of the 3d battery in terms of potential during discharge for discharge curves. heat generation and thermal behaviour the temperature of both circular and square arrays increases over time during discharge due to heat generation, as shown in fig. 5. for the square electrode array, the temperature increases by 12k, 20 k and 30 k above the ambient temperature for discharge rates of 1 c, 2 c and 5 c, respectively. figure 5. average battery temperature of the battery during discharge at various discharge rates for square (continuous) and circular (dashed) electrodes j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 10 on the other hand, for the circular electrode array, the temperature increase is smaller, being 5 k, 8 k and 11 k for same discharge rates. such a difference in array temperature over time is due to the higher volume of the square electrodes as compared to the circular ones leading to more heat generation in the square electrodes than the circular ones. a comparison of the contribution from various layers towards heat generation is provided in figs. 6a-b. the negative electrode is the highest contributor, amounting to 60 % in both discharge rates, due to its lower ionic conductivity and less porous nature. the positive electrode is the second highest contributor. heat generation in the current collector and electrolyte is purely by ohmic heating and remains almost constant throughout the discharge. a b figure 6. time history of total heat generation and heat generation in various layers during discharge for square (continuous) and circular (dashed) electrodes at a – 1 c-rate, and b – 5 c-rate reliability analysis due to the similarity in the behaviour of both the circular and square electrode arrays in the reliability analysis, we will only consider the circular electrode array. the behaviour of neighbouring electrodes is studied when either a target anode or a cathode malfunctions/fails. given the extreme discharge conditions of 5 c discharge for lithium-ion batteries, we shall discuss the results for 5 c discharge rates as a worst-case scenario. discharge curves the variation of cell voltage with time during discharge at 5 c-rate under both perturbed cases is presented in fig. 7. as expected, there is a decrease in the discharge time compared to the normal case due to the loss in the energy capacity of the battery because of the malfunctioning of the electrodes. when the positive electrode malfunctions, there is a decrease of 60 s in the discharge time corresponding to a 10 % decrease in the energy capacity. similarly, for the malfunctioning negative electrode, there is a decrease of 110 s in the discharge time corresponding to a 17 % decrease in the energy capacity. the negative electrode has higher theoretical capacity than the positive electrode (defined as ci in the constitutive relations) and hence the capacity of the battery is reduced more when the negative electrode malfunctions compared to the positive electrode malfunctioning. thus, unlike the conventional parallel-plate design, the 3d array is still able to safely generate power when an electrode malfunctions, though with a lower energy capacity. r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 11 figure 7. battery voltage during discharge at 5 c-rate under normal (continuous) and perturbed cases of malfunctioning either positive (dashed) or negative (dotted) electrodes as marked in fig. 1 electrochemical behaviour when a positive electrode malfunctions, the concentration distribution of lithium ions in the rest of the array away from the perturbed cell remains largely similar throughout high discharge rates relative to that in a non-perturbed array, as shown by comparing figs. 3b and 8a. however, the concentration of lithium ions in the electrolyte in the malfunctioning positive electrode increases continuously from the initial value of 2×10³ mol m⁻³ to 2.3×10³ mol m⁻³ at the end of discharge, compared to a decrease to 1.8×10³ mol m⁻³ on average in the electrolyte in the other positive electrodes. because there is an inflow of ions from the adjacent negative electrodes that act as source of lithium ions, but no reaction in the active material taking place to consume them, lithium ions accumulate within the electrolyte in the malfunctioning positive electrode. this effect is concentrated locally in and around the malfunctioning electrode as shown in fig. 8a and is insignificant towards the other electrodes in the array. figure 8. variation of concentration of lithium ions in the electrolyte along the xz-plane at y=2.8 × 10⁻⁴ m at various times during discharge at 5 c-rate under malfunctioning (a) positive and (b) negative electrodes of circular cross-section when a negative electrode malfunctions, the concentration distribution of lithium ions in the rest of the array also remains largely similar throughout high discharge rates relative to that in a non-perturbed array, as shown by comparing figs. 3b and 8b. however, the concentration of j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 12 lithium ions in the electrolyte in the malfunctioning negative electrode decreases continuously from the initial value of 2 x 10 3 mol m -3 to 1.8 x 10 3 mol m -3 at the end of discharge, compared to an increase to 2.3 x 10 3 mol m -3 on average in the other negative electrodes. as the adjacent positive electrodes act as sink for lithium ions due to the reduction reaction, a concentration gradient develops between these electrodes and the malfunctioning electrode. lithium ions diffuse into the positive electrodes from the electrolyte and the malfunctioning negative electrode does not give out the lithium ions due to the absence of reaction in the active material, resulting in the depletion of the ions within the electrolyte there. this effect is also concentrated locally in and around the malfunctioning electrode as shown in fig. 8b and is insignificant towards the rest of the array. the cessation of function of any electrode would directly impact the electrochemical behaviour on neighbouring electrodes to a significant degree, due to diffusion of lithium ions in and out of the malfunctioning electrode which otherwise would not occur in a non-perturbed array. however, this effect is negligible for electrodes further away during both the reaction phase and the diffusion phase, due to the presence of many other functioning electrodes in an otherwise intact array. heat generation and thermal behaviour the rise in average temperature of the array in the perturbed cases in similar to that in the standard discharge, in that the average temperature increases steadily at first before becoming more gradual towards the end of discharge. however, there are some minor differences. when a positive electrode malfunctions, the increase in temperature is smaller by 0 2 k compared to the normal case during a 5 c discharge, and only 0 0.2 k during 1 c discharge rates, due to the decrease in the number of heat sources. on the other hand, when an anode malfunctions, the increase in temperature is smaller by 0 1 k compared to the normal case during a 5 c discharge, and 0 0.2 k during 1 c discharge, due to the decreased heat generation in the negative electrodes which is highest contributor to heat generation as seen before. differences in thermal behavior are negligible for electrodes further away from the malfunctioning electrode. conclusions this paper presents a thermal-electrochemical coupled model for next-generation 3d li-ion batteries applied to two different electrode geometries square and circular. a reliability analysis was also conducted to analyse the effect of a single malfunctioning electrode on the rest of the array. in summary, the performance of the 3d cell during discharge under normal conditions was similar for both the square and circular electrode arrays; potential, thermal behaviour and electrochemical behaviour also did not show marked differences at any discharge rates under the selected design parameters. furthermore, the cell capacity can be increased by simply adding more electrodes in the plane of the array or increasing the height of the electrodes, without compromising on power density unlike in the conventional 2d design. the performance of the 3d electrode array during perturbed conditions, in terms of the changes in the potential and the concentration distribution, was only significant in and around the malfunctioning electrode. a maximum change of around 17 % in the energy capacity and 10 % in lithium ion concentration in the electrolyte in and around the malfunctioning electrode under a 5 c discharge was seen. deviation in electrochemical behaviour is negligible more than one cell away from the malfunctioning electrode. the model can also easily be extended to account for various types of 3d designs and conditions. r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 13 this design demonstrates a clear advantage in terms of reliability over the 2d battery, as the 3d array does not stop functioning even if one of the electrodes fail, unlike the conventional 2d parallel-plate design. this allows power to be continuously delivered in a safe manner until the battery is eventually replaced, as there is only minor deviation in thermal behaviour of the cell. as the array behaviour is likely to be significantly affected only when many electrodes malfunction, actual implementation of 3d batteries with full-size arrays is viable. with good performance due to the redundancy provided by the array, the 3d cell will be able to deliver reliability benefits which are crucial in many modern applications. further, the model can be extended to study the transport in solid polymer electrolytes as well. also, the model can be employed to study the behaviour of the battery when there is short-circuiting of the electrodes which seems to be a common problem in microbatteries. nomenclature as specific surface area for the faradaic reaction per unit volume, m 2 /m 3 cs concentration of lithium in active material in the electrodes, mol m -3 cl electrolyte concentration, mol m -3 cp specific heat capacity, j kg -1 k -1 avg sc average concentration of li in the active material, mol m -3 surf sc surface concentration of li in the active material, mol m -3 dl diffusion coefficient of electrolyte, m 2 s -1 ds diffusion coefficient of li in the active material in the electrodes, m 2 s -1 ea activation energy for a variable, kj mol -1 f faraday’s constant, 96487 c mol -1 h height of the battery, m hi height of the functional layers in the battery, m ht heat transfer coefficient, w m -2 k -1 iapp applied current density, a m -2 i0 exchange current density, a m -2 il liquid phase current density, a m -2 is solid phase current density, a m -2 if faradaic transfer current density, a m -2 j local charge transfer current per unit volume, a m -3 k thermal conductivity, w m -1 k -1 k0 reaction rate constant, mol 2.5 m -0.5 s -1 l length of the battery, m ls diffusion length, m nl species (lithium ion) flux, mol m -2 s -1 n normal vector q volumetric heat generation, w m -3 q conductive heat flux, w m -2 r gas constant, j mol -1 k -1 rs radius of active material, m r radial coordinate t time, s t 0 + transference number of cation t temperature, k ta, t0 ambient and initial temperature, k tref reference temperature, 298.15 k u ref open circuit potential of the electrode, v vi volume of the electrode i, m 3 w width of the battery, m wi thickness of the layer i, m greek a, c anodic/cathodic transfer coefficient l volume fraction of the electrolyte in the electrodes  f volume fraction of the conductive filler additive in the electrodes p volume fraction of the polymer in the electrodes  overpotential, v  bruggeman constant (= 1.5)  density, kg m -3 l ionic conductivity of electrolyte, s m -1 s electronic conductivity of solid matrix, s m -1 l liquid phase potential, v s solid phase potential, v  local state of charge of the electrodes subscripts cc current collector ne negative electrode pe positive electrode el electrolyte l liquid/ electrolyte superscripts 0 initial values eff effective values max maximum values j. electrochem. sci. eng. 4(1) (2014) 1-17 modeling and reliability analysis of a 3d li-ion battery 14 references [1] j. w. long, b. dunn, d. r. rolison, h. s. white, chem. rev. 104 (2004) 4463–4492 [2] m. beidaghi, c. wang, microand nanotechnology sensors, systems, and applications ii. edited by george, thomas; saif islam, m.; dutta, achyut k. proceedings of the spie, volume 7679, 2010 [3] m. roberts, p. johns, j. owen, d. brandell, k. edstrom, g. e. enany, c. guery, d. golodnitsky, m. lacey, c. lecoeur, h. mazor, e. 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table b.1 constitutive relations j =    s fa i (ne, pe) 0 (el, cc) fi =                a c 0 α ηf α ηf i rt rt exp exp 0i = surf surf 0 l s s sfk c c c c max( ) η = eff s l ref,φ φ u , = i i ne, pe sa = l f p s ε ε ε r 3(1 ) q =       eff ref, eff eff l s s l l l l u rtσ jη jt σ φ σ φ t c φ t f i 2 2 0 + 2 + + ( ) + ( ) + (1 ) (ln ) , i = ne, pe θ θne pe, = surf s s c cmax c =i l f p thv ε ε ε ρc , i i(1 ) i = ne,pe eff ref,u i =   ref, ref, ref u u t t t i i + ( ) , i = ne, pe eff sσ = s l f pσ ε ε ε(1 ) eff lσ =  l lσ ε effki = l l lk ε k εi (1 ) + ,i = ne, pe eff pρc i( ) = i ( ) (1 ) + ( ) , i = ne, pep l p l lρc ε ρc ε eff ld =  l ld ε lσ = 3 l c    l l lc c c -14 4 -10 -7 2 -4 -2-1.172 10 + 1.3605 10 5.2245 10 + 6.7461 10 + 1.0793 10 tθ( ) = ,            a,θ ref s l l ref e t d ,d ,σ r t t 1 1 θ( )exp θ = r. hong peng liang at al. j. electrochem. sci. eng. 4(1) (2014) 1-17 doi: 10.5599/jese.2013.0040 17 sl = sr 5 sφ 0 = ref, ref, u θ u θ 0 0 pe pe ne ne( ) ( ) lφ 0 = ref, u θ 0 ne ne( ) celle = s sφ φi vii   ref, u t ne = θ θ θ θ 2ne ne ne ne 344.1347exp(-32.9633 + 8.3167) 0.852 + 0.3622 + 0.2698 1 + 749.0756exp(-34.7909 + 8.8871)   ref, u t pe =       θ θ θ θ θ θ θ θ 2 pe2 3 pe pe pe pe pe pe pe 0.5169 -4.1453 + 8.1471 26.0645 + 12.766 + 4.3127exp(0.5715 ) 0.1842exp 0.0462 +1.2816sin(-4.9916 ) 0.0904sin(-20.9669 12.5788) + 0.0313sin(31.7663 22.4295) ref, u ne =    θ θne ne-0.16 + 1.32exp -3 + 10exp -2000 ref, u pe =                  θ θ θ θ pe 0.4924 pe 8 pe pe 1 4.1983 + 0.0565tanh -14.5546 + 8.6094 0.0275 1.9011 0.9984 -0.1571exp -0.0474 + 0.8102exp -40 0.1339 appendix c table c.1 boundary conditions i s app t ai h t t= , = ( ) n i n q ii      s s l l + + -ii ii ii ii = , = , = = 0n i n i n q n q n i n n iii   s + -iii iii = 0, =n i n q n q iv, v   s l= 0 (iv), = 0 (v), = 0n i n i n q vi       s l l l l + + + -vi vi vi vi vi vi = 0, = , = , =n i n i n i n q n q n n n n vii s t aφ h t t= 0, = ( )n q table c.2 initial conditions   surf avg s s sc c c 0 l lc c 0=    s s φ φ0 0 (ne, cc(-)) = (pe, cc(+)) l lφ φ 0= (ne,pe,el) t t0= © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {editor's note} doi:10.5599/jese.548 111 j. electrochem. sci. eng. 8(2) (2018) 111; doi: http://dx.doi.org/10.5599/jese.548 open access : : issn 1847-9286 www.jese-online.org editor’s note this special issue is the second part of the proceedings of the 6th regional symposium on electrochemistry – south east europe (rse–see 6), held in balatonkenese, hungary, from 11−15 june 2017 (part 1 has been published in j. electrochem. sci. eng. 8(1) (2018)). these are some of the best papers presented at the rse-see 6 meeting and reflect the broad interests of the electrochemical community, including both new techniques and application areas. since its start in 2008, the series of rse-see symposia provides a stimulating international forum for electrochemistry researchers to share their current research results. following this tradition, more than 110 participants from 24 countries presented papers in balatonkenese and discussed recent progress in electrochemistry and electrochemical engineering. i would like to thank the authors for their contribution, the reviewers for their professional work and the jese-editorial office for their assistance in the proceedings publication. also, i would like to acknowledge the financial and other support of the hungarian academy of sciences, the international society of electrochemistry, the furukawa electric group, jászplasztik, ametek, zahner, metrohm, palmsens, ivium, and paks nuclear power plant. the next 7th regional symposium on electrochemistry – south east europe will be organized jointly with the 8th kurt schwabe symposium in croatia in the late spring of 2019. gyözö g. láng, guest editor ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.548 http://www.jese-online.org/ http://creativecommons.org/licenses/by/4.0/) determination of nevirapine in the presence of cucurbit(7)uril with a gold electrode doi: 10.5599/jese.2014.0043 1 j. electrochem. sci. eng. 4(1) (2014) 37-44; doi: 10.5599/jese.2014.0043 open access : : issn 1847-9286 www.jese-online.org original scientific paper determination of nevirapine in the presence of cucurbit(7)uril with a gold electrode ana m. esteva, elías blanco* ,, juan j. piña, abel i. balbin, carmen quintana*, pedro hernández* departamento de química analítica, facultad de química, universidad de la habana, la habana 10400, cuba *departamento de química analítica y análisis instrumental, facultad de ciencias, universidad autónoma de madrid, cantoblanco 28049, madrid, spain corresponding author: e-mail: elias.blanco@uam.es; tel.: +34-91-497-4172; fax: +34-91-497-4931 received: july 24, 2013; revised: november 21, 2013; published: january 25, 2014 abstract the electrochemical oxidation of nevirapine, an anti-hiv drug, at a gold electrode was studied by voltammetric techniques. nevirapine showed a signal that interfered with a working electrode wave. this interference was solved by the use of cucurbit(7)uril allowing nevirapine to be determined in tablets (80.4 % recovery, presence of stavudine and lamivudine) and urine (98.4 %). keywords antiretroviral, voltammetry, tablets, urine introduction nevirapine (nev, figure 1) is a non-nucleoside reverse transcriptase inhibitor (nnrti) of hiv-1 that causes acquired immunodeficiency syndrome (aids). the drug directly bounds to and blocks the activities of rna and dna polymerases, both dependent, which caused breakdown of the enzyme catalytic site. nev activity was not competitive with the reverse transcriptase enzyme or with nucleoside triphosphates. reverse transcriptase (rt) of hiv-2 and dna polymerases of eukaryotic cells (eg. human dna polymerases alpha, beta, gamma and sigma) were not inhibited by nevirapine. the in-vitro antiviral activity was determined in peripheral blood mononuclear cells (pbmc), monocyte-derived macrophages and a lymphoblastoid cell line. the values of the 50 % inhibitory concentrations (ic50) were in the range of 10 to 100 µm against laboratory and clinical isolates of hiv-1. in cell cultures, nevirapine demonstrated additive to synergistic action against http://www.jese-online.org/ mailto:elias.blanco@uam.es j. electrochem. sci. eng. 4(1) (2014) 37-44 determination of nevirapine with a gold electrode 38 hiv-1 in combination regimens with zidovudine, didanosine, stavudine, lamivudine, saquinavir and indinavir [1]. figure 1. nevirapine; 11-cyclopropyl–5,11–dihydro–4–methyl–6h di pyrido[3,2–b:2′,3′-e]-[1,4]diazepin–6–one. different analytical techniques were used to detect nev, including high performance liquid chromatography (hplc) [2], matrix-assisted laser desorption/ionization-time of flight mass spectrometry (maldi-tof) [3], and capillary electrophoresis [4]. these techniques require expensive equipment, costly reagents for sample preparation and analysis and quite some time. electroanalytical methods are an accurate and cheap alternative which offer very low detection limits for electroactive molecules. different drugs were determined by this technique achieving very low detection limits [5]. some articles have recently been published about the electroanalytical determination of nev by means of different working electrodes [6-8]. the family of compounds of cucurbit(n)urils (cb(n)) are polymeric macrocycles obtained by the condensation reaction of glicoluryl and formaldehyde in acid conditions and have n units of glicoluryl bridged by methylene groups. they bind molecules by hydrophobic and ion-dipole interactions (but not exclusively) due to the cavity portals delineated by a rim of carbonylic oxygens. the hydrophobic cavity allowed the inclusion of different molecules depending on the cb(n) homologue and the size of the guest [9,10]. we developed a method for the analysis of nev using cb(7) and gold electrode. low detection limits were obtained. the method was applied to biological fluids (urine) and a pharmaceutical formulation (which also contained lamivudine and stavudine) and it was demonstrated that the methodology had fewer steps than other ones. experimental reagents nev was provided by the center for state control of drugs (cecmed-cuba). aqueous solutions of the analyte were prepared at a 2 mg ml -1 concentration in acid medium (ph < 3). diluted solutions were prepared in supporting electrolyte just before use. cb(7) was supplied by sigmaaldrich chemical co. all reagents were of analytical grade (> 98 %) and were provided by scharlau. ultrapure water was produced by a milli-ro and milli-q system (millipore, waters). solutions of these compounds were stored at 4 °c and protected from light. britton-robinson buffer solutions (br, mixture of boric, acetic and phosphoric acids) were used as supporting electrolytes, prepared at a final concentration of 0.04 m and the buffer ph was adjusted with 0.1 m naoh. nev was determined in a tablet sample whose nominal content was 250 mg per tablet together with 40 mg of stavudine and 150 mg of lamivudine. a tablet was dissolved in methanol and filtered a. m. esteva at al. j. electrochem. sci. eng. 4(1) (2014) 37-44 doi: 10.5599/jese.2014.0043 39 through a cellulose membrane of 0.45 µm pore size to get a 2.5 mg ml -1 nev solution which was stored at 4 °c. apparatus electrochemical measurements were performed by means of a μautolab iii potentiostat made by eco-chemie in a three electrodes cell: an au working electrode (2.01 mm 2 geometric area) provided by bas, a coiled platinum wire as counter electrode and an ag/agcl (3 m kcl) reference electrode (all potentials in this paper were referred against it). the ph was controlled by means of a methrom 827 ph meter with combined glass and an ag/agcl/ (3 m kcl) electrode. procedure activation and regeneration of the gold electrode surface was carried out by successive scanning in 0.1 m sulphuric acid between 0.0 v and 1.5 v at 100 mv s -1 by cyclic voltammetry (cv). an ultrasonic bath was used to clean the electrode surface when required and prior to the described activation procedure. differential pulse voltammetry (dpv) was the chosen technique for the analyte determination in solutions of a nev:cb(7) ratio of 1:2, the measurements started at 0.4 v and the chosen scan rate and pulse amplitude were 25 mv s -1 and 25 mv, respectively. results and discussion our studies were performed with nev at a concentration of 100 µm (26.6 µg ml -1 ) by cv at gold electrode in 0.04 m br buffer at ph 2. the analyte showed irreversible redox behaviour and a reduction wave was observed at 0.66 v. as it can be seen in figure 2, a broad and intense signal at 1.14 v was seen in the anodic scan (green line) but close to the gold oxidation wave at 1.3 v (black line). it shifted to potential values lower than 1.1 v between ph 2 and 6 until disappearance at higher ph. above that ph, nev was not electroactive. figure 2. cyclic voltammograms of nev at 100 mv s -1 in 0.04 m br buffer at ph 2. black line, supporting electrolyte; red line, 20 µm cb(7); green line, 100 µm nev; blue line, 10 µm nev; cyan line, 10 µm nev and 20 µm cb(7). j. electrochem. sci. eng. 4(1) (2014) 37-44 determination of nevirapine with a gold electrode 40 an increase in the sweep rate (vb) between 10 and 400 mv s -1 entailed a variation of the intensity and potential of the signals when a 266 µg ml -1 analyte solution in 0.04 m br at ph 2 was analysed. when the logarithm of the anodic peak current was represented versus the logarithm of the scan rate in figure 3 (black points), a straight line was obtained with a 0.53 slope value (close to 0.5) so the oxidation could happen after diffusion of the analyte to the electrode surface. however, it was showed in figure 2 (green line) that nev was oxidized by means of at least two processes and its shape was not a diffusion-like one so the 0.53 slope value was a chance. the dependence between the logarithm of the cathodic peak current of the nev oxidation product and the logarithm of the scan rate was studied (figure 3, red points) and a slope close to 1 was found so the reduction of that product could be concomitant with an adsorption process. nevertheless, that wave was overlapped with the gold oxide reduction wave and both processes were connected. figure 3. effect of the change of the scan rate on the oxidation (black points and line) and reduction peak currents (red points and line). as before stated, the nev oxidation wave was at a potential very close to the gold oxides formation one so the measurements analysis could be complicated or even impossible if the analyte concentrations of the sample solution were low, as in the case of a 10 µm nev (blue line, figure 2). when cb(7) was added to solutions of this low nev concentration (10 µm nev and 20 µm cb(7), cyan line, figure 2), the anodic signals were more separated, the nev wave was narrower and a huge increase in the peak current was observed when they were compared to the signal of solutions of the same analyte concentration and no added cb(7) (blue line, figure 2). voltammetric measurements of blank solutions of cb(7) did not show any signal but the same waves observed when the cell just contained supporting electrolyte, as it can be seen in figure 2, red and black lines, respectively. if a 1:2 nev:cb(7) molar ratio was kept constant, the separation of nev and gold waves in the anodic scan observed at ph 2 in figure 2 (cyan line) continued up to neutral ph. in these conditions, the nev cathodic signal was not observed at ph higher than 2. the peak potential (ep) depended on the medium ph so it can be concluded that the anodic reaction was coupled to an acid-base one. in this case, this dependence followed a straight line whose equation was a. m. esteva at al. j. electrochem. sci. eng. 4(1) (2014) 37-44 doi: 10.5599/jese.2014.0043 41 ep / v = 1.19 0.021 ph (r 2 = 0.998). as the line slope value (dep/dph) was close to 0.029 v per ph unit, the number of exchanged protons was the half of the number of electrons according to the nernst equation. the stoichiometric nev:cb(7) ratio was studied by cv in 0.04 m br buffer ph 2, at a constant cb(7) concentration and changing nev concentration, and vice versa. current and potential values were plotted and the slope change depending on the nev:cb(7) ratio was indicative of successive formation of nev-cb(7) complexes of 1:2 stoichiometry. as it is shown in figure 4, the effect of the concentration (0.3-1.6 μg ml -1 ) on the signal was studied at a 1:2 nev:cb(7) ratio in 0.04 m br ph 2 by dpv. therefore, what it was done was to augment the analyte concentration but also the macrocycle one in the measured solutions. the peak current and the concentration were directly proportional up to 1 µg ml -1 , data points that were fitted to ip / µa = -0.038 + 4.300c / µg ml -1 , r 2 =0.999. for higher concentrations the analytical signal was relatively constant probably due to surface saturation. figure 4. effect of the concentration of nev on the dpv measurements, at a constant nev:cb(7) ratio of 1:2 in 0.04 m br ph 2 (see text). the voltammograms of the nev concentrations 0.267, 0.534, 0.801, 1.07 µg ml -1 are shown. the inset graph gives the peak current vs. nev concentration. cv measurements of approximately 200 µg ml -1 stavudine and lamivudine solutions in 0.04 m br were obtained at different ph. dpv measurements of these two interferences at ph 6 are shown in figure 5 and it can be seen that the lamivudine reduction signal was at -0.2 v (red line) and the stavudine one was at -0.1 v (black line). they both were well defined when cb(6) or cb(7) were present in the solution. the signal could be a product of the possible formation of inclusion complexes. these compounds did not show oxidation signals so did not interfere in nev determination by dpv when they were in the analysed sample. j. electrochem. sci. eng. 4(1) (2014) 37-44 determination of nevirapine with a gold electrode 42 figure 5. dpv measurements of 4 µm stavudine and lamivudine and with cb(7) (1:2) in ph 6 br buffer as electrolyte. black line, stavudine; red line, lamivudine; green line, electrolyte. determination in tablet a crushed tablet was left in contact with methanol for 24 hours, the suspension was filtered, the resulting solution was transferred to a 100 ml volumetric flask and the volume was completed with methanol. it contained 2.5 mg ml -1 of nev and the working solutions were prepared from this one. voltammograms of sample solutions were recorded and, as it can be seen in figure 6 and as previously shown, the gold oxidation and nev waves were overlapped if no cb(7) had been added but they were separated if the macrocycle was present (1:2 nev:cb(7) ratio). the results obtained by means of the standard addition method showed that the content of nev/tablet was 80.4% of the nominal one (n = 4) in the presence of stavudine and lamivudine. figure 6. dpv measurements of solutions of the pharmaceutical sample in 0.04 m br buffer ph 2. black line, no cb(7) in solution; red line, with cb(7) (ration 1:2 nev:cb(7)). a. m. esteva at al. j. electrochem. sci. eng. 4(1) (2014) 37-44 doi: 10.5599/jese.2014.0043 43 determination in urine a previous treatment of liquid-liquid extraction was necessary given the complexity and characteristics of the urine sample. 2 ml of urine of a healthy individuals were spiked with nev to reach a final concentration of 26 µg ml -1 and were subjected to liquid-liquid extraction with 10 ml of diethyl ether. after shaking, the liquid was left for 3 min, the aqueous phase was discarded and the organic one was evaporated. 10 ml of 0.04 m br buffer ph 2 were used to dissolve the residue, the solution was introduced in the electrochemical cell and dpv measurements of increasing concentrations of nev were performed to analyse the sample, voltammograms that are shown in figure 7. a recovery of 98.4 % (n = 3) was obtained for the spiked urine with 26 µg ml -1 nev in presence of cb(7). figure 7. dpv measurements of nev doped urine with successive additions of the drug in presence of cb(7), maintaining the 1:2 ratio. black line, 0 µg ml -1 nev added; red line, 5.32 µg ml -1 ; green line, 10.6 µg ml -1 . conclusions an electroanalytical method was developed for the analysis of nev in pharmaceutical formulations in the presence of stavudine and lamivudine and in urine by means of a gold electrode and dpv. nev was electroactive between ph 2 and 6 and cv measurements showed that nev oxidation wave was very close to the gold oxides formation one but if cb(7) was added to a nev solution, both waves were separated and an increase in the analyte peak current was observed. measurements in 0.04 m br buffer ph 2 were performed to get the stoichiometry of the nev-cb(7) complex behind this electrochemical behaviour and it was found that one nev molecule interacted with two cb(7) molecules, ratio which was kept constant in every calibration or analyzed sample. the calibration of the response was performed and found the equation ip / µa = -0.038 + 4.300c / µg ml -1 , r 2 =0.999. the analysis of tablets gave an 80.4 % recovery (n = 4) just dissolving the sample in methanol. 2 ml of urine were doped with nev at a concentration of 26 µg ml -1 and were subjected to liquid-liquid extraction due to the complex matrix and a 98.4 % recovery (n = 3) was found. j. electrochem. sci. eng. 4(1) (2014) 37-44 determination of nevirapine with a gold electrode 44 acknowledgements: authors thank to spanish agency for international development cooperation (aecid, a/030784/10) and comunidad de madrid (s2009/ppq-1642, avansens). references [1] d. burch, martindale the complete drug reference, pharmaceutical press , london, united kingdom, 2006 [2] v. kabra, v. agrahari, c. karthikeyan, p. trivedi, tropical journal of pharmaceutical research 8 (2009) 79-86 [3] s. notari, c. mancone, t. alonzi, m. tripodi, p. narciso, p. ascenzi, journal of chromatography b-analytical technologies in the biomedical and life sciences 863 (2008) 249-257 [4] r. sekar, s. azhaguvel, chromatographia 67 (2008) 389-398 [5] b. dogan-topal, s. a. sibel, b. uslu, the open chemical and biomedical methods journal 3 (2010) 56-73 [6] a. a. castro, r. q. aucelio, n. a. rey, e. m. migueland, p. a. m. farias, combinatorial chemistry & high throughput screening 14 (2011) 22-27 [7] n. l. teradal, s. n. prashanth, j. seetharamappa, j. electrochem. sci. eng. 2 (2012) 67-75 [8] f. f. zhang, l. li, l. q. luo, y. p. ding, x. liu, j. appl. electrochem. 43 (2013) 263-269 [9] j. lagona, p. mukhopadhyay, s. chakrabarti, l. isaacs, angew. chem. int. ed. 44 (2005) 4844-4870 [10] e. masson, x. x. ling, r. joseph, l. kyeremeh-mensah, x.y. lu, rsc advances 2 (2012) 12131247 © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {evaluation of strontium substituted lanthanum manganite-based solid oxide fuel cell cathodes using cone-shaped electrodes and electrochemical impedance spectroscopy} doi:10.5599/jese.503 255 j. electrochem. sci. eng. 8(3) (2018) 255-260; doi: http://dx.doi.org/10.5599/jese.503 open access : : issn 1847-9286 www.jese-online.org original scientific paper evaluation of strontium substituted lanthanum manganite -based solid oxide fuel cell cathodes using cone-shaped electrodes and electrochemical impedance spectroscopy kent kammer hansen department of energy conversion and storage, technical university of denmark, dk-4000 roskilde, denmark e-mail: kkha@dtu.dk received: february 3, 2018; revised: february 27, 2018; accepted: march 19, 2018 abstract five la1-xsrxmno3+−based perovskites (x = 0, 0.05, 0.15, 0.25 and 0.50) were synthesized and investigated by powder xrd, dilatometry and electrochemical impedance spectroscopy measurements and cone-shaped electrode techniques. the thermal expansion coefficient increased with increasing strontium content. it was shown that the total polarization resistance was the lowest for the intermediate compound, la0.95sr0.05mno3+. two arcs were found in the impedance spectra. these arcs were attributed to two one-electron processes. the results indicate that either mn(iii) is the catalytically active species or that the redox capacity is important for the activity of the compounds towards the reduction of oxygen in a solid oxide fuel cell. at higher temperatures, the oxide ionic conductivity may also play a role. keywords lsm; eis; oxygen; sofc; cathode introduction a solid oxide fuel cell (sofc) is a high-temperature unit, which converts chemical energy directly into electricity and heat. the choice for a cathode is a lsm and ysz (lsm = strontium-substituted lanthanum manganite and ysz = yttria-stabilized zirconia) composite cathode [1]. many studies of the activity of manganite-based perovskite electrodes towards the electrochemical reduction of oxygen have been undertaken in the literature, see i.e [2] for a review. there is insufficient agreement in the literature. hammouche et al. [3] found that the activity towards the reduction of oxygen increased with an increasing amount of strontium, at least until the strontium content reached 50 %, using point electrodes at 960 °c. this finding was later confirmed by hansen et al. [4] at a somewhat lower temperature of 500 °c. in contrast, yasamoto et al. [5] found that the activity of lsm10 was much http://dx.doi.org/10.5599/jese.503 http://www.jese-online.org/ mailto:kkha@dtu.dk j. electrochem. sci. eng. 8(3) (2018) 255-260 evaluation of lsm-based sofc cathodes 256 higher than the activity of lsm40, using porous planer electrodes. in a later publication, hansen [6] showed that at temperatures of 400 °c or less, a less clear dependence on the composition existed. this study employed a combined cone-shaped electrode and electrochemical impedance spectroscopy (eis) measurements of lsmx (la1-xsrxmno3+)-based perovskite sofc cathodes. the use of cone-shaped electrodes was first suggested by fabry and kleitz [7]. cone-shaped electrodes are a useful technique when comparing different electrode materials [3,8-20]. there are several reasons for this. when using porous electrodes, the performance depends strongly on the microstructure. also, the electrode is often mixed with electrolyte, itinerating the true properties of the electrode material. in addition to this, the electrode and electrode is sintered together during processing, leading to reaction between the electrode and the electrolyte. for cone-shaped electrodes the dependence on microstructure is less, it is not mixed with electrolyte and the electrode and electrolyte is sintered separately. in addition to the electrochemical properties, the thermal expansion coefficient was determined. in this study we for the first time report the detailed correlation between impedance data and the composition of lsm. experimental the synthesis of the lsm-based perovskites was performed using a glycine-nitrate process [21]. in short, aqueous solutions of the metal-nitrates were mixed in the appropriate ratio in a beaker, before glycine was added. the mixtures were then heated on a hot plate until combustion. the resulting powders were then transferred to alumina crucibles, and the powders were calcined at 1100 °c/12 h in air in a box furnace. the metal-nitrates used were the following: la(no3)3·6h2o (alfa aesar, 99.9 %), sr(no3)2 (alfa aesar, 99 %), and mn(no3)2·4h2o (alfa aesar, 99.98 %). the phase purity of the lsm perovskites was checked using powder x-ray diffraction using a stoe theta-theta diffractometer equipped with cukα radiation. the diffractograms were recorded in the interval 2  20 to 80°. the cone-shaped electrodes were made by pressing 7-8 g of the powder in a die with a diameter of 10 mm. the resulting cylinders were then sintered at 1250 °c for 12 h in air in a box furnace. finally, the sintered cylinders were machined into cone-shaped electrodes. the electrochemical measurements were conducted as follows. the cone-shaped electrodes were equilibrated at temperature for 24 h before the eis measurements. the measurements were performed at temperatures of 800, 700 and 600 °c in the given order. for the eis measurements, a solartron 1260 in standalone mode was used. the frequency ranged from 1 mhz to 0.05 hz (or to 0.01 hz at 600 °c) with five points measured per decade. an amplitude of 24 mv was used throughout. a pellet of ce0.9gd0.1o1.95 was used as the electrolyte. the pellet was polished with a 1 m diamond paste. the contact areas of the cone-shaped electrodes were calculated using newman’s formula [22]: s 1 4 r r = (1) where r is the radius of the contact point,  is the specific conductivity of the electrolyte and rs is the intercept at high frequency in the impedance plot. the eis data were fitting using a program by boukamp [23]. in general, the eis data could be fitted using two (rq)’s in series with a series resistance. q is a constant phase element with the admittance: y = y0(j/0)n (2) where y0 and n are constants found from the fitting and  is the angular frequency. at first, the n-values were allowed to vary freely. then, the average of the n-values was estimated, and the fitting was repeated with a constant n. k. k. hansen j. electrochem. sci. eng. 8(3) (2018) 255-260 doi:10.5599/jese.503 257 results the results from powder xrd are shown in table 1. all the manganite-based perovskite compounds are phase pure and belong to the hexagonal crystal system. an example of an impedance spectrum is depicted in figure 1. the electrochemical behavior of the manganite depends strongly on the amount of strontium, and the most active perovskite is the intermediate compound with x in la1-xsrxmno3+d equal to 0.05, as seen in figure 2. the thermal expansion coefficient (tec) is given in table 2. the tec increases with increasing strontium content. in table 3, the results from the fitting can be found. it is seen that the magnitude of the arcs is strongly dependent on the composition of the manganite. table 4 gives the impedance of the individual arcs at 600 °c. the low frequency is the most dominant for all the compositions. in table 5 the near equivalent capacities for two arcs at 600 °c are listed. the high frequency arc is seen to vary a lot with composition, whereas the low frequency arc is almost constant with composition. the last table, table 6, gives the contribution from the two arcs at 800 °c. as is the case at 600 °c the low frequency arc has the highest values. figure 1. an example of an eis spectrum for lsm5 at 600 °c. the dots are the measured data, and the solid line is the fitted data. figure 2. total asr values as a function of x in the lsmx compounds at 600 °c. the total asr is lowest for the intermediate compound lsm5 table 1. unit cell parameters of the manganites. all the manganites belong to the hexagonal crystal system. lsm0 lsm5 lsm15 lsm25 lsm50 a / å 5.5197 5.5172 5.5128 5.4862 5.4440 c / å 13.354 13.342 13.358 13.569 13.424 table 2. tec values recorded from room temperature to 1000 oc lsm0 lsm5 lsm15 lsm25 lsm50 tec, 10-6 k-1 9.53 11.87 12.97 13.68 14.38 table 3. total asr values at the three different temperatures covered asr;  cm2 t / oc lsm0 lsm5 lsm15 lsm25 lsm50 600 150.3 132.0 142.9 232.9 333.3 700 48.5 16.1 29.0 37.1 37.8 800 14.0 1.06 6.45 5.71 4.83 table 4. asr values of the individual arcs at 600 °c. r1 is high frequency arc and r2 is low frequency arc asr;  cm2 lsm0 lsm5 lsm15 lsm25 lsm50 r1 2.66 6.78 6.24 13.95 64.26 r2 148.7 125.2 136.7 218.9 222.2 j. electrochem. sci. eng. 8(3) (2018) 255-260 evaluation of lsm-based sofc cathodes 258 table 5. near-equivalent capacitances for the individual arcs per unit area at 600 °c. c1 is high frequency arc and c2 is low frequency arc c / f cm -2 lsm0 lsm5 lsm15 lsm25 lsm50 c1 12.6 2500 26.7 350 17.5 c2 400 1000 300 600 80 table 6. asr values of the individual arcs at 800 °c. r1 is high frequency arc and r2 is low frequency arc asr;  cm2 lsm0 lsm5 lsm15 lsm25 lsm50 r1 0.13 0.22 0.25 0.09 0.48 r2 13.9 2.26 6.40 5.54 4.28 discussion the xrd results agree with the literature, that is, all the manganites belongs to the hexagonal crystal system [24]. that the tec increases with sr content is expected as sr(ii) has approximately 2/3 of the bonding energy of la(iii). the magnitude of the asr is very high on the cone-shaped electrodes compared to that of the porous composite electrodes of the same materials [8,25]. this finding is partly because the coneshaped electrodes not are composite electrodes, and partly due to the interface between the electrode-electrolyte, which not is optimized in the case of the cone-shaped electrodes. the geometrical configuration of the cone-shaped electrode limits the number of contact points at the electrode/electrolyte interface. additionally, the cone-shaped electrode is dense and therefore has a low surface area. for mixed conducting perovskite-based electrodes a three-step mechanism has been proposed [26]. this mechanism includes a slow redox reaction at the surface of the electrodes, the diffuseion of oxide anions through the bulk of the electrodes and the transport of oxide anions across the electrode-electrolyte interface. at 600 oc the oxide ionic conductivity of the lsm-based perovskites is very low, and they are not considered to be mixed conductors [27,28]. however, a simple mechanism has been suggested for pure electronic conductors that can explain the two-arc impedance spectrum. the mechanism has been suggested by van hessel et al. [29,30] for a gold electrode and by divisek et al. for lsm30 [31]. the mechanism is given below using kröger-vink notation. 2 ad ½o (g)+ad o→ (4) ad ado +e' o→ (5) •• x ad o oo +v +e' o +ad→ (6) where •• o v is a vacancy in the electrolyte, x o o is an oxide anion in the electrolyte and ad is an adsorption site on the surface of the electrode. it is seen that this mechanism includes two oneelectron transfer reactions. if one arc is the result of the transport of oxide anions across the electrode-electrolyte interface, then the double layer capacity of this arc should be in the range of 10-6 to 10-4 f cm-2[27]. the near-equivalent capacitance of a constant phase element in parallel with a resistance can be calculated from the equation [32]: (1 )/ 1/ ω 0 n n n c r y − = (3) where r is the resistance and y0 and n are constants. all three values are found from the fitting. as seen from table 4, the values of the near-equivalent capacity for the high-frequency arc is almost within the values predicted for a double layer capacity, suggesting that the high-frequency arc is due to the transport of oxide anions across the electrode-electrolyte interface. the magnitude of k. k. hansen j. electrochem. sci. eng. 8(3) (2018) 255-260 doi:10.5599/jese.503 259 the high-frequency arc is seen to increase with increasing strontium content. this strong dependence on composition indicates that the catalytic properties of the electrode material are also important for the magnitude of the high-frequency arc. this finding again indicates that the highfrequency arc not only is due to a simple transport of oxide anions across the electrode-electrolyte interface, as suggested by siebert et al. [26], but also includes the transference of electrons, as predicted by the mechanism suggested by van hessel et al. [29,30] and divisek et al. [31]. the low-frequency arc is then, accordingly to the mechanism, due to the transference of one electron from the electrode to an adsorbed oxygen atom. this step can be described as a slow redox reaction at the surface of the electrode. the values found in table 4 are in good agreement with the capacitance found for a surface reaction. the amount of mn(iii) and the redox capacity decrease with increasing strontium content, and the activity of the manganite-based cathodes decreases with increasing strontium content. this result indicates that one of these two parameters is important for the activity of the manganites towards the electrochemical reduction of oxygen. the two one-electron processes can explain why the magnitude of both arcs depends on the strontium content. the greater activity of lsm5 compared with that of lsm0 indicates that the amount of oxide ion vacancies is important for the activity of the manganites towards the electrochemical reduction of oxygen. the trend found in this study for the manganites at 600 °c follows the trend found for the ferrites [33]. this finding also indicates that mn(iii) is the catalytically active species in manganites, just as fe(iii) is the catalytically active species in ferrites [33]. at 800 °c the picture changes, as it is seen that the activity towards the reduction of oxygen increases with increasing strontium content, except for lsm5. at 800 °c, the oxide ionic conductivity of the manganites is higher than the conductivity at 600 °c, and this may explain why a change in the order is observed at 800 °c, as the ionic conductivity of the manganites increase with increasing strontium content [27]. however, it must be said that the differences in the total asr among lsm15, lsm25 and lsm50 are very small. the results at 600 °c presented in this work confirm the findings by yasamoto et al. [4]. however, the results reported by hammouche et al. [2] were obtained at higher temperature (960 °c), and it is seen that the values reported in this study at 800 °c follow the trend reported by hammouche et al. [2] except for lsm5, which not was studied by hammouche et al. [2]. some discrepancies with earlier studies are observed. these discrepancies can be due to the sluggishness of the reactions at low temperatures compared to that at a high temperature and the different pretreatments of the electrodes before recording the impedance spectroscopy or cyclic voltammetry measurements. additionally, the eis was conducted at ocv, whereas the cyclic voltammetry data were recorded far from the ocv. conclusion the activity of lsm-based perovskite cathodes are determined mainly by the amount of mn(iii) or perhaps the redox capacity. the amount of oxide ion vacancies might also be important. the intermediate compound lsm5 has the highest activity towards the reduction of oxygen in an sofc. the reduction of oxygen is suggested to take place via two one-electron processes. acknowledgments: our colleagues at the department of energy conversion and storage are thanked for fruitful discussions. the financial support from energinet.dk through pso-r&d-project no. 2006-1-6493 is gratefully acknowledged. j. electrochem. sci. eng. 8(3) (2018) 255-260 evaluation of lsm-based sofc cathodes 260 references [1] n. q. minh, t. takahashi, science and technology of ceramic fuel cells, elsevier science b.v. 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[33] k. kammer hansen, m. mogensen, ecs transactions, 13 (2008) 153-160 ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) development of cnt reinforced al2o3-tio2 coatings for boiler tubes to improve hot corrosion resistance https://dx.doi.org/10.5599/jese.1291 937 j. electrochem. sci. eng. 12(5) (2022) 937-945; https://dx.doi.org/10.5599/jese.1291 open access : : issn 1847-9286 www.jese-online.org original scientific paper development of cnt reinforced al2o3-tio2 coatings for boiler tubes to improve hot corrosion resistance rakesh goyal1, and khushdeep goyal2 1chitkara university institute of engineering & technology, chitkara university, punjab, india 2mechanical engineering department, punjabi university, patiala, india corresponding author: rakesh.goyal@chitkara.edu.in received: february 9, 2022; accepted: july 25, 2022; published: august 31, 2022 abstract this study examines the hot corrosion behaviour of plasma-coated t12 steel for 10 cycles of 100 h each in an industrial boiler. the coating contains cnt (carbon nanotubes) reinforced aluminatitania powders. the substrates were exposed to the boiler at 750 °c. a thermogravimetric examination was conducted to evaluate the kinetics of corrosion. corroded samples were examined with scanning electron microscopy and x-ray diffraction analysis after the end of the corrosion cycle. this research study concludes that cnt-reinforced coatings provide better corrosion resistance than conventional alumina coatings in the boiler environment. keywords reinforced coatings; composites; ceramic matrix; carbon nanotubes; plasma spraying; t12 steel corrosion introduction hot corrosion is an unwanted phenomenon in boilers of thermal power plants due to its high operating temperatures [1,2]. with the depletion of fossil fuels, the use of low-grade fuels is increasing in thermal power plants across the globe. the low-grade fuels contain impurities like na, s, v and k elements, which react with oxygen at high temperatures of boilers, forming corrosive compounds such as na2so4 or v2o5. these compounds are responsible for the initiation of corrosion in boiler components [3-6], which is accelerated at higher temperatures [2-3]. thermal spray coatings have been used by various researchers to improve the corrosion resistance of boiler components. among various available thermal spraying techniques, the plasma spraying technique has been used to deposit ceramic materials such as al2o3, cr2o3, tio2 and other similar materials on the boiler components [5,7]. among various ceramic materials, alumina (al2o3) is known for its high hardness and corrosion resistance even at higher temperatures like 1100 °c [8]. cnts (carbon nanotubes) are nanofibers invented in 1991 with exceptional properties. these properties make cnts a suitable reinforcement material for composites [9-10]. thermal spray coatings are produced by melting powder particles, which solidify on the substrate. these layers of coatings https://dx.doi.org/10.5599/jese.1291 https://dx.doi.org/10.5599/jese.1291 http://www.jese-online.org/ mailto:rakesh.goyal@chitkara.edu.in j. electrochem. sci. eng. 12(5) (2022) 937-945 cnt reinforced al2o3-tio2 coatings for boiler tubes 938 contain porosities and through holes, corrosive elements can attack the substrate and induce corrosion. therefore, in the recent past, researchers have developed composite coatings by mixing cnts in conventional powders and tried to decrease the porosity of developed coatings [11]. therefore, there is a scope to develop new composite coatings with low porosities, which in turn can enhance the corrosion resistance of boiler components. in this experimental work cnts reinforced alumina-titania coatings have been developed and corrosion resistance of these newly developed coatings has been evaluated in a boiler environment. experimental test material and coating characterization grade t12 steel used in boiler tubes is selected as substrate material. this material was procured from the thermal power plant of north india. the specimens with 20×15×5 mm dimensions were prepared by cutting on an electrical discharge machine. the size of samples was cross-checked with a digital vernier caliper. the samples were polished with emery papers. the chemical composition of the steel checked at the cidco-idfc, chandigarh, is mentioned in table 1. table 1. chemical composition of t12 steel asme-code content, wt.% c mn si s p cr mo fe t-12 nominal 0.05-0.15 0.3-0.6 0.5-1.00 0.025 0.025 0.8-1.25 0.44-0.65 balance actual 0.11 0.48 0.61 0.021 0.0214 0.941 0.27 balance a number of composite coatings were prepared by blending al2o3, tio2 (size 40±10 µm) with 1.5 and 3 wt.% cnts in a laboratory ball mill. the plasma spraying method was used to deposit the coatings on the substrates. the chemical composition of coating powders is given in table 2. table 2. chemical compositions and designations of coating powders coating designation composite powder coating 1 0cat 87.0 wt.% al2o3 + 13.0 wt.% tio2 coating 2 1.5cat 85.5 wt.% al2o3 + 13.0 wt.% tio2 + 1.5 wt. % cnt coating 3 3cat 84.0 wt.% al2o3 + 13.0 wt.% tio2 + 3.0 wt. % cnt the sem image and xrd of 84 wt.% al2o3 13 wt.% tio2 3 wt.% cnt are shown in figures 1 and 2 to validate the authenticity of the powder used. the alumina and tio2 particles with the dispersed carbon nanotubes can be seen in the sem image, as shown in figure 1. the peaks of al2o3, tio2, and carbon can be clearly observed in the xrd analysis of the 3cat powder shown in figure 2. optimized standard process parameters were applied in the spraying process. these deposition parameters are listed in table 3. table 3. process parameters of plasma spraying plasma spraying parameters value arc current, a 550 voltage, v 50 pressure of ar, kpa 400 pressure of h2, kpa 60 spray length, mm 90 to 110 rpm of hopper 1.5 pressure of powder gas, kpa 300 r. goyal and k. goyal j. electrochem. sci. eng. 12(5) (2022) 937-945 https://dx.doi.org/10.5599/jese.1291 939 figure 1. sem image of 3cat powder 0 20 40 60 80 100 120 0 200 400 600 800 1000 1200    c c c c ccarbon     tio 3               al 2 o 3 in te n s it y . a .u . 2 /  2 / ° figure 2. xrd peaks 3cat powder the thickness of the coating was found in the range of 250±5 µm, measured out from backscattered electron cross-sectional images, and shown in figure 3 [12]. figure 3. cross-section image of as-sprayed coating https://dx.doi.org/10.5599/jese.1291 j. electrochem. sci. eng. 12(5) (2022) 937-945 cnt reinforced al2o3-tio2 coatings for boiler tubes 940 the porosity of al2o3-tio2 coated specimen measured is more than 3 %, and it was reduced below 2.5 % on adding cnts in al2o3-tio2 powder. eds analysis shown in figure 4 shows uniformly dispersed cnts in al2o3-tio2 matrix in 3cat coated steel. figure 4. fe-sem and eds for 3cat coated t12 boiler steel hot corrosion analyses in the actual industrial environment all substrates, bare as well as coated, were exposed in the actual boiler at the height of 31 m from its base, where the average temperature was around 750±10 °c. the samples were hung with a nichrome wire. the samples were weighed after every 100 hours of exposure for 10 cycles. this includes 1000 hours of heating inside the boiler at 750±10 °c. degradation behaviour was evaluated utilizing cumulative weight gain and thickness loss. corroded samples were analysed by xrd, sem and eds. results visual examination all specimens were checked visually after each cycle of 100 hours of heating. after the completion of 10 cycles, the macrographs of all specimens were taken and shown in figure 5 (a-d). the uncoated sample showed more degradation due to corrosion at high temperatures. ash was stuck onto the sample after the completion of 1st cycle. the deposition of ash on the sample continued till the last cycle. figure 5. macrographs of (a) bare steel, (b) 0cat coating, (c) 1.5cat coating, (d) 3cat coating r. goyal and k. goyal j. electrochem. sci. eng. 12(5) (2022) 937-945 https://dx.doi.org/10.5599/jese.1291 941 this ash will be cleared off the sample by acetone washing. following the completion of each cycle, the specimen weight was recorded. for the rest of the samples, a little spalling was observed in 0cat coated steel in the 2nd cycle. for 1.5cat and 3cat coated samples, no spalling was observed after 1000 hours of cyclic study, and only ash deposition was observed. change in weight and loss of thickness information figure 6 shows the cumulative weight gain/unit area for all tested specimens following 1000 hours of cyclic testing. the overall weight gain, gain in weight caused by the formation of oxide scale and loss in weight is caused by scale spalling, for the bare t12 steel came out to be 147.51 and 102.21 mg/cm2 for 0cat coated t12 steel, and 51.24 and 24.58 mg/cm2 for 1.5cat and 3cat coated t12 steel, respectively. the graph represents that cnt reinforcement has reduced the weight gain in coated steels. bare t12 0cat 1.5cat 3cat 0 20 40 60 80 100 120 140 w e ig h t g a in / a re a , m g c m -2 type of coating figure 6. cumulative weight gain per unit area for bare steel, 0cat, 1.5cat and 3cat coatings for a more accurate prediction of corrosion rate, the values are analyzed in mm year-1. the corrosion rate measured in terms of loss of thickness is shown in table 4. table 4. thickness loss coating designation thickness loss, mm uncoated bare t12 steel 0.8354 coating 1 0cat 0.3599 coating 2 1.5cat 0.0963 coating 3 3cat 0.0612 the corrosion rate for bare steel is 288.13 mpy. further, the corrosion rates for 0cat, 1.5cat coated, and 3cat coated steel were recorded as 124.11, 33.21 and 21.12 mm year-1, respectively, and are shown in figure 7. bare t12 0cat 1.5cat 3cat 0 50 100 150 200 250 300 c o rr o s io n r a te , m il s y e a r1 type of coating 288.13 124.11 33.21 21.12 figure 7. corrosion rate for bare t12 steel, 0cat, 1.5cat and 3cat coatings https://dx.doi.org/10.5599/jese.1291 j. electrochem. sci. eng. 12(5) (2022) 937-945 cnt reinforced al2o3-tio2 coatings for boiler tubes 942 x-ray diffraction analysis x-ray analysis for tested t12 samples after exposure is given in figure 8. bare steel has oxides of fe, and ash sio2 (figure 5a). 0cat coated sample showed al2o3 and tio2 phase along with na2o and sio2 (figure 5b). for 1.5cat, and 3cat coated t12 steels, al2o3 and tio2 were the major phases and c, na2o, and sio2 were the minor xrd phases (figure 5 c-d). 10 20 30 40 50 60 70 80 90 100 110 0 500 1000 1500 2000 2500 3000 s  s           s s s s s s s s s s s s s        − fe 2 o 3 c c c c       b b c c c             b bs ccc ccarbon          b b  sss        in te n s it y , a . u . 2 / o  b na 2 o  al 2 o 3 s sio 2  tio 3 (a) (b) (c) (d) 2 / ° figure 8. xrd pattern for the (a) bare, (b) 0cat coated, (c) 1.5cat coated, (d) 3cat coated t12 steel after exposure fe-sem and eds analysis this analysis for all samples after industrial exposure is shown in figure 9. the surface micrograph of the uncoated sample indicated severe loss due to spalling and corrosion in figure 9a. there are whitish and greyish regions. erosion of the surface was also observed. eds analysis at points 1 and 2 indicates that si, al, and o are present along with fe. the presence of ash may be the reason for this. hence the formation of fe2o3 and sio2 may be predicted. the micrograph of the 0cat coated specimen (figure 9b) shows that al, ti, si, v and o elements are present at points 1 and 2. this predicts the formation of aluminum and titanium oxide layer and the ash. higher amounts of al and ti were observed in the micrographs of 1.5cat and 3cat coated t12 steels (figure 9c-d). carbon (c) in a minor percentage is also visible due to the cnt presence in the coating matrix at both points. discussion a very thin scale that seems to be peeled off was observed for bare steel. the sample was eroded and spalled continuously throughout the experimentation. there was an ash deposition as the experimentation was done within the boiler. therefore, spalling and erosion were a reason for the higher corrosion rate for bare steel. the coal used in thermal plants has more than 30 % ash. this ash contains k2o and na2o and reacts with boilers flue gases (so2, so3 and o2). the authors [13-16] reported that this content of ash was rendered because of coal burning. deposition of such low melting-point elements initiates hot corrosion on the boiler tube surface [17]. navo3 is a deposit detected on boiler tubes with a low melting point and causes corrosion of boiler steel [18]. further, k2so4, and na2so4 present in flue gases and ash react with fe2o3 and so3 and form trisulfide [19], and these sulphates cause corrosion in boiler steels [20]. the scale is spalled and restored on the surface of the boiler steel tube. r. goyal and k. goyal j. electrochem. sci. eng. 12(5) (2022) 937-945 https://dx.doi.org/10.5599/jese.1291 943 figure 9. fe-sem and eds of (a) bare steel, (b) 0cat coating, (c) 1.5cat coating, (d) 3cat coating xrd analysis depicts fe2o3 and sio2 as the main phases in the case of bare t12 steel. the same has been reported by various researchers [21]. the corrosion rate of coated t12 steel is much lesser in comparison to bare steel. the dense coating of alumina-titania present on t12 steel may be the reason for the increase in the degradation resistance. the same has been reported by authors [22-23]. cnt-reinforced alumina-titania coating was efficient in controlling degradation even after exposure to the boiler environment. 3cat coated sample has shown maximum resistance to degradation. this may be due to the diminishing in porosity of the coating caused by cnt reinforcement into the main coating matrix. the coating with lesser porosity has more resistance to corrosion, as observed by the author [22]. xrd graphs showed the peaks for tio2, al2o3 and sio2 along with c in the scale, which has already been noticed by other authors [13]. the sio2 phase may be because of ash particles present on the surface of a specimen. cnts are responsible for reduced porosity and improved adhesion, which further decrease the corrosion rate [7,9]. a number of researchers [15,16] have reported similar findings in their studies on carbon nanotubes. https://dx.doi.org/10.5599/jese.1291 j. electrochem. sci. eng. 12(5) (2022) 937-945 cnt reinforced al2o3-tio2 coatings for boiler tubes 944 conclusions • the bare t12 steel showed more corrosion after the cyclic testing. • coatings proved to help enhance the resistance to corrosion in the actual boiler environment. • the better performance of 3cat coated steel than 0cat and 1.5cat 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://dx.doi.org/10.5599/jese.1291 https://doi.org/10.1016/j.surfcoat.2014.10.046 https://doi.org/10.1016/j.actamat.2007.10.038 https://doi.org/10.1080/09603409.2020.1810922 https://doi.org/10.1016/s0921-5093(00)01382-4 https://doi.org/10.1016/j.surfcoat.2004.07.094 https://creativecommons.org/licenses/by/4.0/) wear behaviour and microstructural characteristics of cold sprayed nickel-alumina coatings on boiler steel http://dx.doi.org/10.5599/jese.1270 841 j. electrochem. sci. eng. 12(5) (2022) 841-849; http://dx.doi.org/10.5599/jese.1270 open access : : issn 1847-9286 www.jese-online.org original scientific paper wear behaviour and microstructural characteristics of cold sprayed nickel-alumina coatings on boiler steel deepak dhand1,2,, parlad kumar1 and jasmaninder singh grewal2 1department of mechanical engineering, punjabi university, patiala 147002, india 2department of mechanical and production engineering, guru nanak dev engineering college, ludhiana 141006, india corresponding author: dmechd@gmail.com received: january 31, 2022; accepted: march 15, 2022; published: may 6, 2022 abstract there is an excessive material loss in steel components due to sliding wear in different industrial applications. the ss 316 steel is extensively used in the power generation industry for boilers, induction fans, ducts, etc., and counter-high sliding wear. the studies have shown that protective coatings deposited by thermal spray methods successfully control the wear and enhance the service life of steels. in this work, the nickel-alumina coating was deposited on ss 316 by cold spray technique to understand the effectiveness of coatings in resisting wear. the wear behavior of coatings was analyzed by conducting the wear test on a pin-on-disc apparatus at different loads, i.e., 30, 40 and 50 n keeping the speed constant. the wear trends and variation in friction coefficient due to wear were observed. the mechanical and microstructural characterization was done by fe-sem/eds and xrd techniques. the coatings were found effective in resisting the wear on ss 316 steel. the results indicate that the wear rate of coatings increased with an increase in normal load. keywords cermet coatings; cold spray; sliding wear; pin-on-disk testing; tribology; morphology introduction the ss 316 steel grade is extensively used in pipelines, transfer ducts, shafts, bearings, and other dynamic and static structures of the power plant, petroleum, refinery, and automotive industry [1–3]. it undergoes mild to severe wear effects, both under room and high-temperature conditions. in order to protect it from degradation and enhance its service life, various surface modification techniques such as weld overlays, heat treatments, physical vapor deposition (pvd), chemical vapor deposition (cvd), electroless plating, thermal spray deposition, etc., were found useful [4–6]. the thermal spray deposition techniques such as electric arc spray, plasma spray, detonation spray, high-velocity oxygen fuel (hvof) spray, and cold spray were found highly effective in resisting wear [7]. the thermal spray methods use chemical or electrical energy to melt the feedstock and spray it on the http://dx.doi.org/10.5599/jese.1270 http://dx.doi.org/10.5599/jese.1270 http://www.jese-online.org/ mailto:dmechd@gmail.com j. electrochem. sci. eng. 12(5) (2022) 841-849 cold sprayed nickel-alumina coatings 842 metal surface at high velocities ranging between 800-1200 m/s. after striking the metal surface, the molten droplets spread out and solidify to provide the protective layer on the steel surface [8]. the recent addition to the thermal spray family is the cold spray (cs) technique. it is one of the advanced methods which can overcome the problems associated with conventional techniques. in this technique, the coating powder is not melted or partially melted and is sprayed at room temperature under controlled conditions on the metal surface. the main advantage of using this technique is that negligible or no oxidation of the feedstock particles takes place during the spray of protective coatings [9,10]. secondly, due to high spray velocity, i.e., above 800 m/s, the coatings exhibited high density, deposition rate, and lamellar microstructure. it has also proved its compatibility with a wide variety of feedstock materials, i.e., metals, alloys, and ceramics, in the pure or composite form. koivuluoto et al. [11] used the cold spray technique to understand and compare the microstructural and mechanical properties of cu, ni, and zn-based coatings. gao et al. [12] studied the influence of powder morphology on the deposition of coatings by the cold spray technique. the researchers revealed that the cs technique exhibited high deposition efficiency when loosely bound cermet feedstock was used. therefore, this technique is widely investigated by researchers [13,14]. the nickel metal is known for its stability, corrosion, and wear resistance in room and hightemperature applications. khan et. al. [15] studied the sliding wear behavior of nickel-based superalloys on nickel-chromium-based cermet coatings by using pin-on-disc apparatus. it was found that the coating helped in resisting the wear. similar types of research were also done by other researchers [16,17]. researchers have also explored its tribological behavior in the form of cermet. the studies have shown that cermet coatings helped to enhance the wear resistance performance of coatings. the hard ceramics used in protective coatings are usually carbides or oxides of tungsten, chromium, or aluminum. the metal matrix composites exhibit high strength and improved wear resistance over the base metals. previous studies have shown that the method of deposition plays a key role in governing the wear resistance behavior of metal-matrix composite coatings [18–21]. however, most of the studies reported the metal matrix composites based on tungsten-carbide or chromium-carbide, while the limited literature is available related to nickel-based cermet coatings. in the present investigation, the cold spray technique is used to deposit the nickel-alumina cermet coating on ss 316. this article will help the researchers to understand the tribological behavior and mechanisms of cermet coatings deposited by the cold spray technique. the focus of this work is to study the microstructural characteristics and analyze the wear behavior of the coatings in increasing load conditions at room temperature during testing on the pin-on-disc test rig. the coated and worn-out surfaces are analyzed by using the fe-sem (field emission scanning electron microscope) and xrd (x-ray diffraction) techniques. experimental deposition of coating the ss 316 steel was used for the coating deposition. the chemical composition of (stainless steel) ss 316 is shown in table 1. the pin samples having size  6 30 mm were prepared from the bar. before applying coatings, the samples were polished with emery papers of grades 180, 400, 800, and 1200 and grit blasted. the electrolytically prepared nickel-alumina (ni-al2o3) powder (k-32) was supplied and sprayed by m/s mecpl, jodhpur, with the cold spray technique using standard spray parameters. the cold spray equipment used the air as a process gas at the pressure of 65 pa. the feedstock powder was fed at the rate of 30 g min-1 through the spray gun having a temperature range of 200-600 °c, keeping the spray distance of 10-12 mm from the substrate. d. dhand et al. j. electrochem. sci. eng. 12(5) (2022) 841-849 http://dx.doi.org/10.5599/jese.1270 843 table 1. the chemical composition of ss 316. element c cr ni mn si cu mo p s fe content, wt.% 0.7 17.0 11.0 1.12 0.7 0.1 2.0 0.02 0.03 balance characterization the surface morphology of coating powder and coatings was studied with fe-sem (jeol7610f) and the elemental composition was found by edax attached with it. the phases developed during the deposition of the coatings were determined after analyzing the patterns of the x-ray diffraction (xrd) diffractometer with bruker d8 using cu radiations. the porosity of coated samples was measured with the optical microscope, xjl-17. the surface roughness of coatings was determined with mitutoyo, a surface roughness tester, in terms of ra values. the average micro-hardness coated samples were determined using the vickers micro-hardness tester as per astm standards. the average values were calculated after taking an average of 10 readings at different points. before analyzing the microstructural characteristics, the samples were polished and cleaned properly with acetone to remove any unwanted particles present on the coated surface. wear testing the pin-shaped samples were tested for wear test on a pin-on-disk test rig (ducom) to determine their wear and friction behavior. the cs samples were slid against the hardened en31 steel disc on the apparatus at room temperature and dry conditions. the counter steel disc used in the apparatus (en31 grade) had a hardness value of 550 hv. a track radius of 60 mm was selected. the pin was fixed into a steady pin holder and pressed against the counter disc with a normal load of magnitude 30, 40, and 50 n, individually keeping the rotating speed of the counter disc constant at 1 m/s. the wear rate and coefficient were measured online with the interface data analyzing software for every test cycle. the software evaluated wear in pin samples in terms of loss of pin length (in micrometers) with the increase in sliding distance. the wear rate of every worn-out sample was calculated by measuring the area of the wear track, which was an average of five measurements, and then the wear volume was calculated. the schematic view indicates various elements of the pin-on-disc apparatus, as shown in figure 1. figure 1. schematic view of various elements of the pin-on-disc tribometer results and discussion microstructural characterization of powder and coatings the electrolytically prepared nickel-alumina powder used in the deposition of coatings was provided by the manufacturers of the cold spray equipment. the customized powder was prepared http://dx.doi.org/10.5599/jese.1270 j. electrochem. sci. eng. 12(5) (2022) 841-849 cold sprayed nickel-alumina coatings 844 specially to use in the low-pressure cold spray equipment. as the sem image in figure 2 represents, the particle morphology of the powder is highly irregular. the alumina particles have rigid, brittle, and blocky structures, whereas the nickel particles have globular and fibrous structures. the particles of both constituents were randomly distributed in an undefined pattern. the average size range of alumina particles was found in 25-30 µm, whereas for nickel particles, it was between 35-60 µm. figure 2. sem image of nickel-alumina powder during the visual inspection of the as-sprayed coatings, it was found to have a smooth, hard, and lamellar structure in grey color. on observing the same samples with sem shown in figure 3, it was found that the coating has a thick splat microstructure. the nickel-alumina particles were completely fused into each other. the presence of small pores was also noticed without any indication of a major crack on the surface of coatings. however, the porosity of the coatings was in the range of 2-3 %. in figure 3, the irregular bright particles were revealed as nickel elements, whereas the alumina particles were revealed as dark shaded elements. the average value of coating thickness was found at 629 µm and the average micro-hardness of coatings was 171 hv. the surface roughness of the coatings was found at 6.62 µm (ra value). figure 3. sem image and eds analysis of the coated sample the cross-sectional microstructure of the cold sprayed nickel-alumina coating on steel substrate in figure 4 represents the dense and layered morphology. the coating was found intact with the substrate surface throughout the length of the sample. the presence of voids was also noticed due to the formation of splats of coating material during the deposition process. d. dhand et al. j. electrochem. sci. eng. 12(5) (2022) 841-849 http://dx.doi.org/10.5599/jese.1270 845 figure 4. cross-sectional sem image of the coated sample figure 5 represents the xrd pattern of the as-spray coated sample. the xrd graph reveals the amorphous behavior of coating. the analysis of the xrd pattern indicated the presence of major phases consisting of ni, ni3al, and al2o3. the nickel-aluminide (ni3al) formed during the process possesses good stability at high temperatures. the weak phases consisting of nio and nial were found present in a negligible amount in the coating microstructure. 2 / o figure 5. xrd pattern of the as-sprayed sample. wear trends of coatings figure 6a-6c shows the typical variation of wear of nickel-alumina coated pin sample with the increase in the sliding distance during testing on pin-on-disc apparatus. as indicated in figure 6a, the loss of pin length due to wear raised for approx. 400 m of sliding distance and then fall suddenly. this wear behavior of coatings was due to pulling out of coating particles at the initial testing stage at 30 n normal load. it was found that during the whole test cycle, negligible loss of length of sample was recorded. for the same test cycle, the variation of the coefficient of friction (cof) is represented in figure 6d. it shows that the cof reflected minimal disturbance throughout the cycle representing the increasing trend starting from 0.29 (at 11 m of sd) value to 0.51 (at 5399 m of sd). similar trends were also revealed by several researchers [21–23]. similarly, as shown in figure 6b, when a normal load of 40 n is applied, the loss of pin length represented the abrupt changes during the initial 700 http://dx.doi.org/10.5599/jese.1270 j. electrochem. sci. eng. 12(5) (2022) 841-849 cold sprayed nickel-alumina coatings 846 m of sliding distance. after 700 m, the wear behaviour of the sample represented a regular pattern of material removal. a similar trend can be observed with the cof plot given in figure 6e. it is also found from the plot that the range of cof decreased as compared to that which is represented during testing at 30 n load. figure 6c shows the loss of length in the coated sample at 50 n of normal load. the wear trend represented by the plot is very uncertain, up to 3700 m of sliding distance. above this, there was an abrupt rise in length loss of pin sample and showed highly increasingdecreasing trends. these variations in wear trends can also be further co-related with the trends of cof shown in figure 6f. the reason behind this wear behaviour and cof is discussed in detail with sem images in the next section. a b c d e f figure 6. wear trend of pin sample at a normal load of (a) 30 n, (b) 40 n, (c) 50 n and the variation of cof at a normal load of (d) 30 n, (e) 40 n, (f) 50 n the wear trend shown by the as-spray coated pin samples and the variation of cof during each test cycle at different normal loads are summarized and compared with the bare steel in figures 7a and 7b, respectively. d. dhand et al. j. electrochem. sci. eng. 12(5) (2022) 841-849 http://dx.doi.org/10.5599/jese.1270 847 as discussed above, the wear rate of as-sprayed coated samples was almost negligible at 30 n load. with the increase in normal load, i.e., at 40 n and 50 n, the wear rate of the coatings increase, but these trends follow the opposite direction for bare steel. on the other hand, the average value of cof was found highest at 30 n load, both for coated and bare steels. as the graph in figure 7b represents, with the increase in normal load values, the cof between the rotating counter plate and the tip of the pin sample decreased. a b figure 7. (a) wear rate vs. load and (b) coefficient of friction (cof) vs. load analysis of worn-out surfaces the sem image in figure 8a represents the surface morphology of the worn-out sample tested at 30 n of normal load and 1 m/s sliding speed. as indicated in figure 8a, during the initial stage of the low loading condition, the coating particles were pulled out after sliding to a certain distance by the coated tip of the pin sample. initially, these particles cause three-body abrasion between the rotating counter plate and the coated surface of the pin sample exposed to the counter plate. this rise in wear was also justified by figures 6a and 6d. after this, some of these pull-out particles get adjusted in the pores of coatings, while others draw wear tracks on the surface of the coating under test. this stabilizes the varying trends in wear and cof graphs, represented at the tails of the plot (above 4000 m). figure 8b indicates the ploughing by the wear particles at moderate loads, i.e., 40 n. the sem image also exhibits the partial delamination of the coating layer along with particle removal. the represented image, however, shows a relatively smooth and less uneven surface compared with the surface morphology represented by the worn-out sample at 30 n. on relating the sem image shown in figure 8b with the wear trends in figure 6b, represented as sharp declining trends at certain intervals. this wear behavior of coatings was due to the peeling off wear mechanism of the coated layer after sliding through a certain distance against the rotating disc. the sem image shown in figure 8c indicates the presence of wear debris and peeling off the coatings when the normal load is increased to 50 n during the wear testing. the delamination of coatings was found to be a dominant wear mechanism along with the presence of wear debris at high loads. due to this wear behavior, a high wear rate and low cof were observed in figures 6c and 6f. moreover, the formation of uneven oxide and carbide films on the coating surface during wear at high load was also observed in the sem image. the elemental composition of one of the films is represented in the eds analysis shown in figure 8c. these films were brittle in nature which later got converted into wear debris and found responsible for ploughing and micro-cracking the coating layers. http://dx.doi.org/10.5599/jese.1270 j. electrochem. sci. eng. 12(5) (2022) 841-849 cold sprayed nickel-alumina coatings 848 a b c figure 8. sem images of the worn-out surface of the as-sprayed sample at (a) 30 n, (b) 40 n, (c) 50 n, along with eds of the image conclusions the cold sprayed nickel-alumina coatings successfully resisted the sliding wear at low loads, but its performance decreased with the increase in load. this was due to the bulk removal of coating material at high normal loads. the coefficient of friction value between the rotating counter plate and the stationary tip of the coated sample decreased with the increase in normal load due to flattened coated layers at high loads. the delamination of coatings, micro-cracks, formation of uneven films of oxide-carbides, and wear debris were dominant wear mechanisms at high normal loads. whereas wear due to pulled-out coating particles, formations of pits, and adhesion of wear particles with the coated surface were the major wear mechanisms at low loading conditions, as observed with the fe-sem technique. acknowledgements: the authors want to express their gratitude to the punjabi university, patiala for providing us the opportunity to work on this topic. we also acknowledge the principal and teqipiii cell of guru nanak dev engineering college, ludhiana for helping in research. references [1] s. kumar, m. kumar, a. handa, engineering failure analysis 94 (2018) 379-395. http://doi.org/10.1016/j.engfailanal.2018.08.004 [2] a. v. levy, wear 138(1–2) (1990) 111-123. http://doi.org/10.1016/0043-1648(90)90171-6 [3] e. raask, wear 13(4–5) (1969) 301-315. 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: : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behaviour of selenite ions in tartaric electrolytes vusala asim majidzade1,, akif shikhan aliyev1, parvin haydar guliyev2, yasin nagi babayev2, mahmoud elrouby3, dilgam babir tagiyev1 1nas of azerbaijan, institute of catalysis and inorganic chemistry named after m. nagiyev, baku, azerbaijan 2nakhchivan state university, nakhchivan, azerbaijan 3chemistry department, faculty of science, sohag university, sohag, egypt corresponding authors e-mail: vuska_80@mail.ru, phone: +99450 640 02 25 received: december 12, 2017; revised: january 22, 2018; accepted: january 24, 2018 abstract in recent years, a great interest has been emerged on electrochemical preparation of semiconductor films based on selenium. therefore, a study of electrochemical reduction of selenium could be very important. in this work, the kinetics and mechanism of the electrochemical reduction of selenite ions on the pt cathode have been studied in the electrolyte containing selenious and tartaric acids. the study shows that electroreduction of selenite ions from tartaric electrolytes proceeds in two stages. the effect of various factors on the cathodic reduction of selenium ions has also been studied. the effective activation energy was calculated using the polarization curves of the temperature dependence of the electroreduction process of selenite ions. it was established that the process of electroreduction under investigation is proceeding by mixed kinetics, at first by concentration and then by electrochemical polarization. keywords selenite ions; tartaric acid; electrochemical reduction; polarization; cyclic voltammetry introduction progress of the modern science and technology is closely related to formation of traditional semiconductor materials, as well as formation and exploration of new promising semiconductors what become one of the urgent problem facing scientists. different physical properties of semiconductor materials create wide opportunities for their application in modern electronics and electrical engineering [1−3]. they are used in modern computers, light diodes, solar cells, lasers, and photodetectors. as is already known, selenium element is one of the important semiconductors with wide scope of applications. for example, selenium is important technological and biological http://dx.doi.org/10.5599/jese.490 http://www.jese-online.org/ mailto:vuska_80@mail.ru j. electrochem. sci. eng. 8(3) (2018) 197-204 selenite ions in tartaric electrolytes 198 element used in industry and medicine. in non-ferrous metallurgy, processing of raw material containing selenium as pulp and paper products is one of the selenium related industrial area. selenium is available in the form of various chemical compounds in many natural materials such as organic fertilizers, water, living organisms and plants. the biological properties of selenium are ambiguous. at the one side it is vitally important strong adjuvant, antioxidant anti-carcinogenic agent, but at the other side it is very toxic material. the nature of crystal structure and chemical bonds for selenium are rather complex. in spite of the presence of many active defects in the crystal structure, it always has a conductivity of p-type [1]. effective properties of selenium make it to be a good candidate for photo elements, light diodes, luminescent materials, ionizing radiation detectors, optical filters and sensors, what all open wide opportunities for its usage in industry. selenium is widely used in production of photo-elements due to the optimal value of the band gap (eg = 1.83 ev), direct optical transitions and high absorption coefficient in the visible region of the spectrum [1]. elemental selenium is also included in the composition of binary and ternary semiconductors, where sb2se3 is one of the most important binary semiconductor compounds. in order to get thin films and coatings of sb2se3 by electrochemical method, the processes of electrodeposition of sb and se should be firstly studied separately. kinetics and the mechanism of the electroreduction process of antimony on the cathode and the effects of various factors on the deposition process have already been studied [4]. these results, together with the results from a study of the electroreduction of selenite ions on the cathode, would ensure easily obtaining of sb2se3 semiconductor films. electroreduction of selenite ions on the cathode has frequently been studied by various researchers [5–10]. on the contrary, the deposition of thin films of selenium by photo electrochemical methods has been rarely studied. the results showed that selenium coatings obtained during illumination have higher quality than selenium coatings obtained in the dark [5,6]. the electrochemical behavior of selenide ions has already been studied in different ways on different electrodes such as mercury (hg), graphite, platinum and other electrodes [7–10]. synthesis of cuinse2 from electrodeposited cu-in precursors was investigated in [11]. a thermal process was adopted to turn cu-in precursors into uniform cu11in9 binary compounds. the conditions for electrodeposition of cu, in and se were adjusted to achieve the preferred atomic proportions. however, the annealing temperature of synthesized cu-in-se films from cu11in9 was found critical, since the xrd patterns and raman spectra showed that the cu2se compound residue is due to an incomplete reaction at lower annealing temperatures. large dense grains could be grown at 650 °c for 5 min. since the kinetics and mechanism of an electrochemical reduction process should be thoroughly investigated to develop a technology of obtaining a high-quality semiconductor of desired properties based on elemental selenium and its compounds, the present contribution is devoted to a study of the electroreduction process of selenite ions in selenious and tartrate electrolytes on the platinum electrode. experimental deposition of selenite ions on the cathode by the electrochemical method was carried out as follows. an electrolyte solution was prepared by dissolving a proper amount of tartaric acid (c4h6o6) in the distilled water, and then an appropriate amount of selenious acid (h2seo3) was dissolved in the prepared tartaric acid solution. the composition of the obtained electrolyte consisted of 0.1 mol/l h2seo3 + 0.007 mol/l c4h6o6. after that, polarization curves were taken using a v. a. majidzade et al. j. electrochem. sci. eng. 8(3) (2018) 197-204 doi:10.5599/jese.490 199 computerized potentiostat of iviumstat electrochemical interface. all electrochemical experiments were carried out in a three-electrode electrochemical cell. pt with a surface area of 2 mm2 was used as the working electrode and the pt sheet of surface area 2 cm2 as the counter electrode, respectively. silver/silver chloride (ag/agcl) was used as the reference electrode. the temperature of the electrolyte solution during electrolysis was controlled via utu 4 universal ultrathermostats. results and discussion the electroreduction of selenite ions on the pt cathode has already been studied in alkaline and acidic solutions [12,13], where acidic electrolytes have been found to be more characteristic for electroreduction of selenite ions. in this regard, we have used the tartaric acid as an organic additive electrolyte since this acid has two advantages; firstly, it is an effective solvent and the h2seo3 easily dissolved in this solution and secondly, it has a great effect on the brightness and smoothing of electrodeposits. it is known that selenious acid is a dibasic acid and when dissociated, it forms the hseo3− (1) and seo32− (2) anions in solution [14]: h2seo3 ↔ h+ + hseo3− ka = 2.88×10-3 (1) hseo3−↔ h+ + seo32− ka = 9.55×10-9 (2) as seen from the dissociation constant, ka, values, it is possible to ignore the amount of seo32− ions in relation to hseo3− ions. therefore, h2seo3 molecules and hseo3− ions are involved in the process of decomposition of the selenious acid. these processes can be described by the following reactions [14,15]: hseo3− + 5h+ + 4e− → se + 3h2o e0 = 0.778 v vs. rhe (3) h2seo3 + 4h+ + 4e−→ se + 3h2o e0 = 0.740 v vs. rhe (4) hseo3− + 6h+ + 6e− → hse−+ 3h2o e0 = 0.349 v vs. rhe (5) two cathodic peaks (a and b) and one anodic peak (c) are noticed in the cyclic voltammetry polarization curves shown in figure 1. figure 1. cyclic voltammetry polarization curves of the pt electrode. t = 296 k, ν = 0.02 v s-1. electrolyte composition: (1) 0.007 m c4h6o6, ph 2.16; (2) 0.1 m h2seo3 + 0.007 m c4h6o6, ph 1.5; (3) 0.1 m h2seo3 + h2o, ph 1.6. peak a starts from 0.56 v to 0.43 v and attains the peak current value of −4.4×10-5 a, corresponding to the electroreduction of selenite ions by the reaction (3). the surface of an j. electrochem. sci. eng. 8(3) (2018) 197-204 selenite ions in tartaric electrolytes 200 electrode is covered with selenium at the value of a peak current and this process continues until the potential value of 0.33 v was reached. note that h2seo3 molecules and hseo3− ions are more stable at ph 1.3-1.5 of the solution. over time, the concentration of elemental selenium on the surface of the electrode increases and after that, the second cathodic peak b is formed as the result of the reaction (4). formation of the peak starts at +0.34 v and reaches the maximum value at 0.22 v at which the current value is equal to −7.8×10-5 a. after 0.13 v, the process of conversion of elemental selenium into hse− ions takes place according to the reaction (5). peak c shows the oxidation process of the elemental selenium formed during the electroreduction of selenite ions by reactions 3 and 4 on the pt electrode. the impact of a number of factors on the electroreduction process of selenite ions has also been investigated. the electroreduction process at different ph values were carried out in the range of 2.2–1.5, corresponding to the selenious acid concentration of 0.005-0.1 mol/l. based on the measurements polarization curves, the ep f(ph) and ln ip f(ph) dependences were constructed and shown in figure 2. a b figure 2. ph dependences of: a peak potential value of peaks a and b and b logarithmic peak current values of peaks a and peak b. the dependences of the peak potential value on ph for the peaks (a) and (b) at the concentration interval of 0.005-0.1 mol/l of selenious acid are presented in the figure 2a. as can be seen from the figure 2a, at 0.1-0.03 mol/l concentration range (ph 1.5-1.85), the potential in the curve a shifts towards the less positive side with the increase in ph, reaches a maximum at 0.025 mol/l (ph 2.06) and then again reduces sharply. for the second peak b, the potential is the most positive for 0.1 mol/l (ph 1.5) and gradually moves towards more negative values with increase of ph, reaching a minimum at 0.03 mol/l (ph 1.85). thus, from the ep-ph dependence shown in fig. 2a, it is obvious that the potential value moves in a positive direction with increase in the concentration of the selenious acid and the corresponding decrease of the ph value. however, in the figure 2b showing the ln ip ph curves, the current values of current increase with decrease of the ph value or increase of the concentration of the selenious acid. this means that the current spent on the electroreduction process reaches a maximum value at ph > 2. selenite ion concentration also affects the quality of gold-and-red-colored selenium films deposited on the surface of the electrode. v. a. majidzade et al. j. electrochem. sci. eng. 8(3) (2018) 197-204 doi:10.5599/jese.490 201 figure 3 shows the effect of concentration in the selenious acid in the range of 0.005–0.1 m on the rate of the electroreduction process. it is observed that the rate of the electroreduction process increases by increasing the concentration of selenite ions. figure 3. effect of the concentration of selenite ion on reduction process on pt electrode: (1) 0.005; (2) 0.025; (3) 0.03; (4) 0.05; (5) 0.08; (6) 0.1 m. electrolyte composition: 0.007 m c4h6o6, t = 296 k, ν = 0.02 v s-1. figure 4. effect of scan rate on electroreduction of selenite ions on pt electrode: (1) 0.005; (2) 0.01; (3) 0.03; (4) 0.05; (5) 0.08; (6) 0.12; (7) 0.2 v s-1. electrolyte composition: 0.1 m h2seo3 + 0.007 m c4h6o6, t= 296 k, ph 1.5 one of the factors affecting the reduction process of selenite ions is the scan rate of potential changes. the effect of the scan rate on the reduction process of selenite ions was studied in the range of 0.005-0.2 v s-1 and the cyclic polarization curves are shown in figure 4. from polarization curves given in figure 4, it appears that the scan rate has a significant impact on the reduction process. the current height of peaks formed during the electroreduction process is directly proportional to the value of the scan rate. thus, the peak current value at the scan rate of 0.005 v s-1 is equal to −1.52×10-5 a for the first peak a and −1.32×10-5 a for the second peak b, respectively. at the scan rate of 0.2 v s-1, however, the peak current values are higher and equal to −5.04×10-4 a for the first peak a and −1.0×10-3 a for the second peak b, respectively. the effect of temperature on the rate of electroreduction process on the pt cathode has been also studied in the 296-348 k range and the results are shown in figure 5. figure 5. effect of temperature on electroreduction of selenite ion on pt electrode: (1) 296; (2) 308; (3) 318; (4) 328; (5) 338; (6) 348 k. electrolyte composition: 0.1 m h2seo3 + 0.007 m c4h6o6, ν = 0.02 v s -1, ph 1.5. j. electrochem. sci. eng. 8(3) (2018) 197-204 selenite ions in tartaric electrolytes 202 from the cyclic polarization curves shown in figure 5, it appears that by increasing the temperature, the reduction potential of selenite ions gradually moves in the positive direction. at the same time, the peak of the anodic current shifts in the negative direction with increase of temperature up to 348 k with the simultaneous increase of current values. these suggest that acceleration of the anodic process takes place with an increase of the temperature. based on the polarization curves of the temperature effect shown in figure 5, the log ik 1/t dependences were derived and presented in figure 6. based on tg α values obtained from the slopes of linear graphs, the effective activation energy, aef, were calculated. the calculated aef e dependence is shown in figure 7. figure 6. log ik 1/t dependence constructed from fig. 5 at different selected cathodic potentials figure 7. dependence of the calculated activation energy on the selected cathodic potentials the obtained results show that, the polarization in the reduction process of selenite ions according to the first peak is of concentration nature, and the one of the second peak is of electrochemical nature. this may be explained by the fact that, the first reduction process proceeding by eq. (3) takes place depending on the transport rate of the selenite ions to the electrode surface, and the reduction of the h2seo3 proceeding by eq. (4) is based on the electrochemical reaction. the effect of temperature has also a significant impact on the adhesion of selenium film deposited on the surface of the electrode as a result on the uploading selenite ions on the cathode. as the temperature rises the collapse of deposited films from surface is observed. v. a. majidzade et al. j. electrochem. sci. eng. 8(3) (2018) 197-204 doi:10.5599/jese.490 203 by performing several experiments in this regard, it has been found that the surface films having good adhesion are obtained within the temperature range of 296 318k. the impact of the electrode material on the electroreduction process of selenite ions has been investigated by comparing the results obtained using pt and pt/se surfaces and presented in figure 8. from the cyclic polarization curves shown in figure 8, it appears that the cathodic currents are higher for the reduction process on the pt than the pt/se electrode. this is due to the fact that selenium is a semiconductor. one can assume that the entry of se ions into the cage of pt/se takes place easier than on pt. from the anodic branch of the polarization curve shown in figure 8, it is seen that the oxidation of selenium on the pt electrode occurs at lower current values than on the pt/se electrode. this is due to the fact that before the reduction of the selenite ions on the pt/se electrode there is already a selenium layer present on the electrode surface and dissolution occurs at higher current values than for the pt electrode. figure 8. effect of the electrode material on electroreduction of selenite ions on: 1) pt/se and 2) pt electrodes. electrolyte composition: 0.1 m h2seo3 + 0.007 m c4h6o6, ν=0.02 v s -1, t = 296k, ph 1.5. conclusions the cathodic reduction process of selenite ions from selenious and tartaric acid electrolyte on the pt electrode has been studied using the cyclic polarization curves. influence of temperature, concentration, ph, scan rate of potential and other factors to the reduction process have been studied. the results showed that the electroreduction of h2seo3 and hseo3− forming se and hse−occurs at a potential of 0.56 to -0.45 v. the kinetics and mechanism of electrochemical reduction of selenite ions on the pt cathode have been studied. it was found that the reduction process is proceeding by mixed kinetics, firstly by concentration, and then by electrochemical polarization. the experimental results showed that the optimal reduction process occurs at 0.1 mol/l h2seo3 + 0.007 mol/l c4h6o6 containing electrolyte, t = 296 k, ν = 0.02 v s-1 and at ph 1.5. references [1] i. a. sluchinskaya fundamentals of materials science and semiconductor technology, mir, moscow, in russia, 2002, p. 376 (in russian). [2] a. a. kovalev, s. p.zhvaviy, g. l. zykov, physics and engineering of semiconductors 39(11) (2005) 1345-1349. [3] а. sh. aliyev, sh. о. eminov, т. sh. soltanova, v. а. mejidzadeh, d. а. kuliyev, h. d. jalilova, d. b. tagiyev, chemical problems 2 (2016) 139-145 (in azerbaijan). j. electrochem. sci. eng. 8(3) (2018) 197-204 selenite ions in tartaric electrolytes 204 [4] v. a. majidzade, p. h. guliyev, a. sh. aliyev, m. elrouby, d.b. tagiyev, journal of molecular structure 1136 (2017) 7-13. [5] m. b. dergacheva, k. a. maeva, n. n. gudeleva, k. a. urazov, v. p. grigoryev news of the national academy of sciences; a series of chemistry and technology 5 (2012) 54-61. [6] m. b. dergacheva, n. v. penkova, k. a. urazov, k. a. maeva, bulletin of kazakh national university named after al-farabi chemistry series 3(63) (2011) 101-104. [7] m. b. dergacheva, n. v. penkova, k. a. urazov, k. a. maeva, actual achievements of european science, vii international scientific and practical conference, bulgaria, 2011, p. 6. [8] m. b. dergacheva, k. a. leontiev, n. n. gudeleva, g. m. husurova, k. a. urazov, proceedings of the national academy of sciences of the republic of kazakhstan chemistry and technology series 1(409) (2015) 76-84. [9] y. n. zhiharev, d. i. elantsev, m. a.kuznetsov, herald of tyumen state university 2 (2003) 226-230. [10] a. b. baeshov, g. s. serazitdinova, s. a. baeshova, proceedings of the national academy of sciences of the republic of kazakhstan, series chemistry 1 (2006) 24-26. [11] t.-w. chang, s.-y. hu, w.-h. lee, journal of electrochemical science and engineering 4 (1) (2014) 2735 [12] a. sh. aliyev, m. n. mamedov, z. f. gyulahmedova, azerbaijan journal of chemistry 1 (2007) 72-77. [13] v. a. majidzade, european conference on innovations in technical and natural sciences, 10th international scientific conference, vienna, austria, 2016, p.164. [14] d. dobos, electrochemical data, verlag akadémiai kiadó, budapest, 1975, p. 353. [15] handbook of electrochemistry, a. a. sukhotin, ed., chemistry, leningrad, 1981, p. 488 (in russian). ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {laser induced selective electroless copper plating on polyurethane using edta-cu as active material} doi:10.5599/jese.564 331 j. electrochem. sci. eng. 8(4) (2018) 331-339; doi: http://dx.doi.org/10.5599/jese.564 open access: issn 1847-9286 www.jese-online.org original scientific paper laser induced selective electroless copper plating on polyurethane using edta-cu as active material binggong yan, xiaodi huang, xuan song, lei kang, qihe le, jiang kaiyong fujian key laboratory of special energy manufacturing, xiamen key laboratory of digital vision measurement, huaqiao university, xiamen 361021, china corresponding authors e-mail: jiangky@hqu.edu.cn received: june27, 2018; revised: august 10, 2018; accepted: august 11, 2018 abstract using edta-cu as the active material and polyurethane as the matrix, flexible cathodes were fabricated by laser-induced electroless copper plating process (lpkf-lds) and characterized by sem, x-ray energy spectrum and auger electron spectroscopy. flexible cathodes prepared from edta-cu and polyurethane showed good selectivity in copper plating process. composition and particle morphology of edta-cu, laser power, scanning speed, laser wavelength, laser spot size, pulse frequency etc. are the main factors that affect the fineness of electroless copper plating. by adjusting these parameters, the fineness of the copper plating was improved. x-ray energy spectrum and auger electron spectroscopy results showed that after the laser scanning, both cu0 and cu+ appeared in the scanning area, revealing thus the mechanism of electroless copper plating for polyurethane-edta-cu flexible cathodes. keywords edta-cu, polyurethane, lpkf-lds, flexible cathode introduction laser-induced electroless copper plating process (lpkf-lds) is an innovative method for printing circuit on a polymer and its use in fabrication of a flexible cathode for electrochemical machining (fcem) has also been expected [1]. electrochemical machining using fcem can make preparation of textures on freeform surfaces easier than a traditional process and it effectively reduces time and money costs due to fcem reusability [2,3]. the main problem of fcem is the fineness of the electroless copper plating, which is associated with properties of active material and the size of laser heat affected zone. therefore, choosing of suitable active materials, reducing their particle size, increasing the uniformity of particles and minimizing the laser heat affected zone, are the main ways to improve the fineness of fcem. in recent years, a variety of materials has been used as active http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:jiangky@hqu.edu.cn j. electrochem. sci. eng. 8(4) (2018) 331-339 electroless copper plating on polyurethane 332 materials to prepare fcems, such as palladium oxide [4-7], copper aluminate [8] and carbon nanotubes [9-12]. these materials, however, showed different processing problems that limited development of fcems. for example, the cost of palladium is too high to be suitable for industrial production, the manufacturing process of copper aluminate is complicated, and multi-wall carbon nanotubes related flexible cathodes are brittle and fragile. therefore, new active materials need to be developed to resolve these problems. based on the special copper based composite material, our group has already carried out some systemic studies on fcem [13-15]. owing to poor dispersion and easy agglomeration of the particles, however, the selectivity of electroless copper plating and the flexibility of cathode were found still unsatisfactory. considering the present researching progress on fcems, here, we propose application of edtacu related flexible cathode for fcem. edta-cu is a chelated complex formed between ethylene diamine tetraacetic acid (edta) and copper. edta is a representative material of chelating agents and can form stable water-soluble complexes with alkali metals, rare earth elements and transition metals. also, it is commonly used in the sewage treatment and can be applied to titrate nickel, copper, etc. edta-cu is a low cost and easy to be prepared stable metal complex. in this paper, edta-cu is used as the active material and polyurethane as the matrix for flexible cathodes that were fabricated and characterized. experimental preparation of edta-cu considering the molar ratio of cu:edta = 1:1, 2.67 g copper acetate monohydrate, (ch3coo)2cuh2o (99 %, aladdin) and 3.9 g ethylene diamine tetraacetic acid, edta (99 %, aladdin) were dissolved in 100 ml deionized water. the solution was refluxed at 100 °c for 2 hours using magnetic stirrer (zncl-b, gongyi yihua instrument co., ltd.). the resultant was poured into ethanol and suction filtered for three times using a glass filter to remove ch3cooh. the obtained powder was dissolved in deionized water and sprayed by a closed spray dryer (yc-501, shanghai yage instrument equipment co., ltd.). fabrication of flexible cathode edta-cu powder was dried in a vacuum oven at 70 oc for 2 hours, and then mixed according to the mass ratio of polyurethane : edta-cu = 85 : 15. the resultant mixture was stirred thoroughly with a solder paste stirrer (zb500s), after which an antifoaming agent (3 wt.%, w-160, zhongwan co., ltd.) was added and vibrated by ultrasonic curing [16] for 24 hours. finally, the obtained modified polyurethane solution was smoothly poured onto the glass panel to get the polyurethane sheet with a thickness of 1 mm. laser activation a normal laser marking machine with a wavelength of 1064 nm, a picosecond laser marking machine with a wavelength of 1064 nm and a uv laser marking machine with a wavelength of 355 nm were used to scan the edta-cu modified polyurethane. the parameters of laser scanning process used in the experiments were as follows: pulse repetition frequency 1000 khz, scanning speed 0.1 m/s, spot mode tem00. electroless copper plating 1.6 g copper sulfate was fully dissolved in 25 ml of deionized water and complexing agents (1.4 g c4o6h4kna and 2.5 g edta-2na) were added to the copper sulfate solution to form a complex of b. yan et al. j. electrochem. sci. eng. 8(4) (2018) 331-339 doi:10.5599/jese.564 333 copper ions. after that, a sodium hydroxide solution (1.4 g sodium hydroxide, 25 ml deionized water) was added to the copper sulfate solution. finally, the stabilizer (0.8 mg 2,2'-bipyridyl and 7 mg k4fe(cn)6·3h2o), reductant (1.2 ml hcho) and deionized water were added to obtain 100 ml of electroless copper plating solution. the laser activated modified polyurethane substrate was sonicated in 10 % sodium hydroxide solution to clean the oil, rinsed with deionized water, and then kept in an oven at 80 °c for 2 hours. the edta-cu modified polyurethane sheet was immersed in the electroless copper plating solution at 80 °c for 1 hour and then rinsed with deionized water. characterization surface morphologies of plated coatings were observed using a built-in spectrometer benchtop scanning electron microscope (phenom). adhesion between plated copper and polyurethane was tested by the scotch tape that was produced by 3m according to astm-d3359-09. the resistivity of the plated copper was measured by 4-probe method using sx1934(sz-82). the surface of the substrate before and after the laser activation was characterized by xps (thermo escalab 250xi). before characterization, the surface of the sample was etched with an ar particles beam having an etching depth of 5 nm to eliminate the effect of surface contamination on the characterization results. xps photoelectron spectroscopy of the scanned area and auger electron energy spectrum xaes were used to characterize the valence of copper on the surface of the substrate. electrochemical machining electrochemical machining was carried out using edta-cu modified polyurethane as cathode, 5 % sodium nitrate solution (with deionized water as solvent) as electrolyte and galvanized iron sheet as anode. processing parameters were as follows: voltage 15 v, pulse frequency 5 khz and duty ratio 50 %. results and discussion morphology of edta-cu during the preparing process of edta-cu, ch3cooh is generated from the reaction between edta and (ch3coo)2cu·h2o, playing a negative role in the electroless copper plating process. figure 1a shows the surface morphology of the coating with ch3cooh remaining at the surface. many voids can be seen and the edge of the plated copper line that is not clear. the main reason for this phenomenon could be that ch3cooh dissolved in the copper plating solution resulted in many voids on the top surface of the polyurethane, and copper particles are deposited around the voids by microporous adsorption. in order to eliminate the influence of ch3cooh, as-prepared edta-cu was dissolved in the ethanol and suction filtered for several times using a sand core filter device. figure 1b shows the morphology of the coating after removing ch3cooh, where the quality of the coating is found significantly improved. however, the continuity of copper lines is still not good. poor continuity observed in figures 1a and 1b resulted from irregular particle morphology and uneven sizes. in order to obtain edta-cu with uniform particles, the suction filtered edta-cu powder was dispersed in deionized water and spray dried. the morphology of spray-dried edta-cu particles is shown in figure 2a, where it can be seen that spray-dried edta-cu particles are nearly spherical in shape and uniform in size. j. electrochem. sci. eng. 8(4) (2018) 331-339 electroless copper plating on polyurethane 334 figure 1. surface morphology of flexible cathode with active materials: a) as-prepared edta-cu, b) suction filtered edta-cu figure 2. sem images of spray-dried edta-cu: a) particle morphology, b) electroless plated copper line using the spray-dried edta-cu powder as an active ingredient and polyurethane as a matrix, electroless copper plating was conducted and the result is presented in figure 2b. it can be seen that only the laser-scanned area is plated with copper, and the copper line shows clear edge and good continuity, i.e., exhibits good copper plating selectivity. influence of laser laser activation is the key step in lpkf-lds, where power, pulse frequency, wavelength, spot size, etc. play important roles in controlling the plating fineness. in order to get a rule, three different types of lasers were used to scan edta-cu modified polyurethane: 1) normal laser marking machine with a wavelength of 1064 nm (figs. 1a, 1b and 2)), 2) picosecond laser marking machine with a wavelength of 1064 nm (figs. 3a, 4a and 4b) and 3) ultraviolet (uv) laser marking machine with a wavelength of 355 nm (fig. 3b). it can be seen from figure 3a that the copper line prepared by a picosecond laser is dense and the edge is clear, what resulted from the short heat conducting time of picosecond laser [17]. however, due to the big spot diameter, the width of the plated copper line is still unideal. figure 3b presents the surface morphology of flexible cathode activated by an uv laser. due to the short wavelength of uv laser (355 nm), the laser absorption of modified polyurethane is higher than for b. yan et al. j. electrochem. sci. eng. 8(4) (2018) 331-339 doi:10.5599/jese.564 335 1064 nm laser, so although the uv laser power was only 1 w, the polyurethane-edta-cu was more effectively etched. figure 3. sem images of flexible cathode scanned by different lasers: a) picosecond laser (1064 nm), b) uv laser (355 nm) after electroless copper plating, compact and clear copper lines were obtained, showing no mutual influence between copper lines and the line width of about 50 μm. specific resistivity was found equal to 210-3  m what is five orders of magnitude higher than for pure cu. considering bubbles which cannot be fully eliminated by the antifoaming agent, this high resistivity could result from low continuity and compactness of the plated copper layer. in order to further improve the surface fineness of electroless copper plating, a small amount of multi-wall carbon nanotubes was added into polyurethane-edta-cu with the mass ratio of edta-cu to carbon nanotubes 10:1. as shown in figures 4a and 4b, the copper lines were denser and clearer than for pure edta-cu samples. figure 4. sem images of flexible cathodes with edta-cu and carbon nanotubes with diameters: a) 300 μm, b) 40 μm the line width is only 40 μm, while specific resistivity was determined as 1.4  10-3  m. during the laser scanning, polyurethane was removed and carbon nanotubes were exposed to air. chemical bond between edta and copper ion was broken down and consequently, during the electroless copper plating process, both carbon nanotubes and copper ions act as copper plating seeds, leading to improvement of the surface fineness. j. electrochem. sci. eng. 8(4) (2018) 331-339 electroless copper plating on polyurethane 336 electroless copper plating mechanism of edta-cu in order to reveal the copper plating mechanism of the flexible cathode with edta-cu, laser activation (scanning) areas were characterized by xps and xaes. the results are shown in figures 5 and 6. binding energy, ev figure 5. xps results of the flexible cathode of edta-cu modified polyurethane curve a) in figure 5 shows that no copper was detected before laser scanning, indicating that the surface of the edta-cu modified polyurethane exhibited non-metallic material properties. obvious cu2p peak can be identified in figure 5 curve b) which indicates that copper appeared after laser scanning. during the laser scanning, the polyurethane on the top surface was removed and the chelating bond between copper and edta was broken down. as a consequence, cu particles were exposed and exist in a free form which can play as catalytic sites in the electroless copper plating process [18]. as shown in figure 6 a), two main peaks can be identified in the laser scanned area, cu 2p3/2 (933.29 ev) and 2p1/2 (953.21 ev). binding energy, ev binding energy, ev figure 6. a) xps spectrum of cu2p, b) xaes spectrum of culmm in te n si ty , a .u . in te n si ty , a .u . in te n si ty , a .u . b. yan et al. j. electrochem. sci. eng. 8(4) (2018) 331-339 doi:10.5599/jese.564 337 no obvious vibrational peak between two peaks indicates that there was no divalent copper in the activation area. the shapes of cu0 and cu+ peaks are similar to each other and it is difficult to distinguish between them. to identify the specific valence of copper, the laser scanned area was further characterized by xaes [19]. figure 6b) shows the xaes spectrum of culmm (be=572.28 ev) and kinetic energy as much as 913.8 ev can be obtained according to ke = 1486.6 – be(culmm). the modified auger parameter can be obtained according to:  = be(cu2p2/3) + ke = 1847.09 ev. referring to auger spectral database, it can be inferred that in the laser scanned area, the copper exists in a form of cu+ [20]. therefore, it can be confirmed that chemical bonds between edta and cu were broken down by the laser, and the resultant cu+ acted as a catalytic core in the electroless copper plating. adhesion measuring figure 7 shows adhesion measuring results of electroless plated copper at different substrates. figure 7. adhesion measuring results of: a) as-prepared edta-cu, b) suction filtered edta-cu, c) spray-dried edta-cu, d) edta-cu with carbon nanotubes according to the norm astm-d3359-09, adhesion can be divided into six levels. for level 5b, the edges of the cuts are completely smooth and none of the squares of the lattice is detached. for level 4b, small flakes of the coating are detached at intersections and less than 5 % of the area is affected. for level 3b, small flakes of the coating are detached along edges and at intersections of cuts and the affected area is 5 to 15 % of the lattice. for level 2b, the coating has flaked along the edges and on parts of the squares and the affected area is 15 to 35 % of the lattice. for level 1b, the coating has flaked along the edges of cuts in large ribbons and whole squares have detached. the affected area is 35 to 65 % of the lattice. for level 0b, flaking and detachment is worse than for level 1b. through comparing with the standard card, the adhesion strength of the samples can be classified and rated. carbon nanotubes doped edta-cu (fig. 7d) and spray-dried (fig. 7c) edta-cu performed a level 4b, while samples that used suction filtered edta-cu (fig. 7b) and as-prepared edta-cu (fig. 7a) can be separately identified as level 3b and 2b, respectively. after removing ch3cooh and spray drying, spherical edta-cu particles can be evenly distributed into polyurethane and thus performed good adhesion. results of electrochemical machining figure 8 shows the electrochemical machining results using the flexible polyurethane-edta-cu as cathode. as shown in figure 8, the flexible cathode containing edta-cu can etch clear and neat lines in 5% sodium nitrate electrolyte on zn-coated steel. but the width of the etched line is still not ideal, the processing parameters and the flow channel of electrochemical machining need to be further optimized. j. electrochem. sci. eng. 8(4) (2018) 331-339 electroless copper plating on polyurethane 338 figure 8. electrochemical machining sample based on flexible cathode of edta-cu and polyurethane conclusions using edta-cu as the active material and polyurethane as the matrix, flexible cathodes were prepared by the laser-induced electroless copper plating process. lasers with different wavelength and frequency were used to scan the edta-cu modified polyurethane, and the polyurethane-edtacu showed good selectivity in electroless copper plating process. the valence state of copper was investigated and the influence of composition and particle morphology of active material was analyzed. experimental results showed that after being scanned by laser, copper is present in the form of cu+ and played a catalytic role in the metallization process. presence of ch3cooh, however, played a negative role in the electroless copper plating process. dissolved in the plating solution, ch3cooh initiated formation of many microscopic voids on the surface, what reduced the selectivity of electroless copper plating. the particle morphology of edta-cu was found important through experiments with spray-dried spherical edta-cu particles that were uniformly dispersed in the matrix, improving thus the quality of the plated copper significantly. properties of the laser also played a significant role on the coating quality. the picosecond laser can produce a dense copper coating, uv laser can be efficiently absorbed by the polyurethane-edta-cu, and the plated copper lines were clear and uniform. multi-wall carbon nanotubes can also improve the coating fineness, while keeping the flexibility of the material. in summary, flexible cathodes from edta-cu and polyurethane showed good selectivity for copper plating. due to additional properties such as low cost and easy preparation, edta-cu is a good choice for flexible cathode electrochemical machining (fcem). acknowledgments: supported by national natural science foundation of china (51475174), natural science foundation of fujian province (2017j01089), scientific research funds of huaqiao university (z16y0020). references 1. m. hüske, j. kickelhain, j. müller, g. eßer, proc. lane 8 (2001) 266-270. 2. yang li, bin liu, engineering plastics application 7 (2013) 100-104. 3. zhen-fa xing, printed circuit information 21 (2003) 37-39. 4. k. c. yung, c. chen, c. p. 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the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {optical and electrochemical detection of a verotoxigenic e. coli gene using dnazyme-labeled stem-loops} doi:10.5599/jese.471 193 j. electrochem. sci. eng. 7(4) (2017) 193-200; doi: http://dx.doi.org/10.5599/jese.471 open access : : issn 1847-9286 www.jese-online.org original scientific paper optical and electrochemical detection of a verotoxigenic e. coli gene using dnazyme-labeled stem-loops gloria longinotti1, gabriel ybarra1,, javier montserrat2,3 1u.t. nanomateriales, inti-procesos superficiales, instituto nacional de tecnología industrial (inti), av. gral. paz 5445, b1650wab san martín, argentina 2universidad nacional de general sarmiento, j. m. gutiérrez 1150, b1613gsx, los polvorines, argentina 3consejo nacional de investigaciones científicas y técnicas (conicet), argentina corresponding authors e-mail: e-mail: gabriel@inti.gov.ar; tel./fax: +51-11-4724-6333 received: november 1, 2017; revised: november 28, 2017; accepted: november 30, 2017 abstract the activity of a peroxidase-mimicking dnazyme was optimized to be used as a catalytic label in a stem-loop genosensor construction for quantifying the gene sequence shiga-like toxin i of verotoxigenic e. coli. experimental conditions such as ph, buffer composition, potassium ion concentration, and hemin-to-oligonucleotides ratio, were analyzed to maximize optical and electrochemical responses using microvolumes. different stem-loop constructions were evaluated to obtain the optimum response against the target concentration. linear ranges of 0.05-0.5 µm and limits of detection of 174 nm and 144 nm were estimated for the optical and electrochemical measurements, respectively. selectivity was proved by assaying other verotoxigenic, enterotoxigenic and enteroinvasive sequences. the results show that, if a combination of small-volume electrochemical cells and low-cost untreated screen-printed electrodes with a relatively high geometric area is used, electrochemical measurements present similar sensitivity and limits of detection to the more usual optical ones, allowing the development of lowcost electrochemical biosensors based on the use of soluble dnazymes as labels. keywords biosensors; genosensors; dnazyme peroxidase; verotoxigenic e. coli introduction peroxidase-mimicking dnazymes are complexes formed by oligonucleotides containing guanine quadruplexes and hemin as prosthetic group, which catalyze the decomposition of hydrogen peroxide with the consequent oxidation of a substrate. since their first reports by breaker [1] and joyce [2] in the late 1990s, the possibility of using dnazymes as artificial enzymatic labels in http://dx.doi.org/10.5599/jese.471 http://www.jese-online.org/ mailto:gabriel@inti.gov.ar j. electrochem. sci. eng. 7(4) (2017) 193-200 detection of a verotoxigenic e. coli gene 194 bioassays and biosensors has become apparent. dnazymes have some advantages when compared to enzymes. dnazymes are usually smaller than the corresponding enzymes, have a higher chemical stability, can be easily prepared using solid phase synthesis and can also be chemically modified. moreover, their structures can be modulated for the adoption of active or inactive states. these versatile characteristics allow the combination, in a single dna molecule, of both a recognition element and a catalytic label, as is the case of a peroxidase dnazyme [3] combined with another dna strand which acts as a biorecognition element [4]. although this analytical strategy is attractive for the development of biosensors, its practical use can be hampered by a low sensitivity due to the lower catalytic activity of peroxidase dnazymes when compared to peroxidase proteins. the use of biorecognition-activated peroxidase-mimicking dnazymes can be an especially attractive choice for the detection of complementary dna sequences; for instance, those associated to bacterial toxins. in this regard, a considerable amount of work has been carried out for the detection of specific sequences of verotoxigenic e. coli (vtec) or shiga-toxigenic e. coli, including o157:h7 and other non-o157 serogroups, which produce verotoxins (verocytotoxins) that result in human diseases [5]. these toxins produce profound cytopathic effects in vero cells. clinical symptoms may include bloody diarrhea and hemorrhagic colitis, along with complications associated with hemolytic-uremic syndrome (hus), acute and chronic kidney disease, thrombotic thrombocytopenic purpura (ttp), neurological sequelae and death. optical detection has been the preferred means of transduction for the development of dnazyme-based biosensors, usually with the use of abts, a reducing agent which becomes greenblue upon oxidation [4,6-9]. on the other hand, electrochemical transduction has been chosen when dna strands were immobilized onto electroactive surfaces [10-12]. from the biosensors construction viewpoint, electrochemical detection shows a higher degree of electronic integration and a simplified design resulting from the fact that an electric signal is the natural result of electrochemical transduction [13]. therefore, much effort has been devoted to developing optimized interactions between dna strands and the electrode surface [14,15]. although the use of soluble dna structures avoids this issue, very few works have been published involving biosensors in which the catalytic activity of soluble dnazymes is detected electrochemically [16]. based on strategies similar to those reported previously [4-11,17], the aim of the present work was to prove the hypothesis that, by employing small-volume electrochemical cells in combination with inexpensive untreated printed electrodes, a sensitive detection of specific sequences of verotoxigenic e. coli could be carried out using dnazymes as soluble electrochemical labels, yielding similar results to those obtained with the more usual optical detection. to that end, the conditions for the optimum response of a peroxidase dnazyme were firstly optimized (ph, buffer composition, potassium ion concentration and hemin-to-oligonucleotides ratio) for electrochemical response. secondly, different stem-loop oligonucleotides structures (recognition element and peroxidase dnazyme) were designed and assayed for verotoxigenic e. coli sequences in order determine the best construction. finally, response ranges, limits of detection and selectivity were studied for a stem-loop structure. experimental reagents and preparations the sequences of dna oligomers used are shown in tables 1 and 2. the sequence showed in italic forms the dnazyme known as dz5t3 upon combination with hemin. in order to prepare the oligomer-hemin complex, the dna oligomer stock solution was heated at 95 °c for 5 minutes and g. longinotti et al. j. electrochem. sci. eng. 7(4) (2017) 193-200 doi:10.5599/jese.471 195 then diluted to a final concentration of 10 µm with tkt buffer. the hemin stock solution in dmso was diluted to a final concentration of 20 µm with the same buffer. finally, hemin was added to the oligomer solution with a hemin-to-dna oligonucleotide ratio of 2:1. the mixture was allowed to stay at room temperature for 30 minutes. table 1. dna oligomers used in the construction of the dz5t3 dnazyme and stem-loop sequences against shiga-like toxin i genes of verotoxigenic. underlined: complementary sequences that form the stem. bold: dz5t3 sequence. italic: target recognition element against shiga-like toxin i genes of verotoxigenic e. coli sequence (5’→ 3’) dz5t3 tttgggtagggcgggttggg stem loop i tttgggtagggcgggttgggttttttagaacgcccactgagatcatccagtgttgtcccaaccc stem loop ii tacccaaaagaacgcccactgagatcatccagtgttgtttttgggtagggcgggttggg stem loop iii cccaacccagaacgcccactgagatcatccagtgttgtttttgggtagggcgggttggg stem loop iv ccgccctaagaacgcccactgagatcatccagtgttgtttttgggtagggcgggttggg for the binding assay between the stem-loop and the target, the stock solutions of the dna sequences were heated at 95 °c for 5 min and then diluted to a final concentration of 5 µm with tkt buffer. subsequently, different volumes of target sequence solutions were mixed with a volume of stem-loop solution in different tubes and brought to the final volume with tkt buffer. the mixture was heated at 57 °c for 30 minutes. finally, hemin was added and allowed to complex with dna with stirring for 30 minutes at room temperature with a hemin-to-dna ratio of 2:1. for the specificity assay, a similar procedure was followed employing the stxi sequence and five different oligonucleotides of others pathogenic e. coli (table 2). table 2. pathogenic e. coli sequences [18] sequence (5’→ 3’) shiga-like toxin i genes of verotoxigenic (stxi) acaacactggatgatctcagtgggcgttct shiga-like toxin ii genes of verotoxigenic (stxii) gcgttctgttcgcgccgtgaatgaag heat-labile toxin genes of enterotoxigenic (it) agcggcgcaacatttcaggtcgaagtcc heat-stable toxin genes of enterotoxigenic (st) atcagaaaatatgaacaacacattttactgctgtgaac adherence factor gene of enteropathogenic (eaf) cggcgctggtgatttcgggttcgtca invasiveness plasmid of enteroinvasive (ial) cttatgttcaaggaaataattgttggcctccttctc optical and electrochemical measurements uv-vis spectra were acquired with a nanodrop 2000 thermo scientific spectrophotometer. disposable acrylic cartridges with eight electrochemical cells were used for the electrochemical measurements [19,20]. each cell with a volume of 40 µl contained three screen printed electrodes: a carbon working electrode with an area of 1.6 mm2, a carbon counter electrode of approx. 10 mm2, and one ag│agcl reference electrode. a teq4 potentiostat was used. results and discussion 3.1 optical and electrochemical determination of the catalytic activity of dz5t3-hemin dnazyme peroxidase-mimicking dnazymes catalyze the reduction of hydrogen peroxide by abts, which is in turn oxidized to abts+•. the rate of this reaction can be followed by measuring the absorbance at 420 nm, the absorbance maximum of abts+• [21]. however, from the viewpoint of biosensors j. electrochem. sci. eng. 7(4) (2017) 193-200 detection of a verotoxigenic e. coli gene 196 design, it is more convenient to perform a single measurement at a given time rather than to determine the reaction rate as a series of measurements. a higher sensitivity and a lower limit of detection could be obtained if the accumulated concentration of formed abts+• was determined after 3 minutes of reaction. as reported earlier, the catalytic activity of peroxidase-mimicking dnazyme depends on several factors, such as the concentration of potassium ions, buffer composition, ph, and the ratio between the concentration of hemin and oligonucleotides [3]. in order to obtain a high sensitivity when dnazymes are used as labels, the optimum conditions for the catalysis with dnazymes must be established. under the experimental conditions used, the concentration of potassium ions did not show a significant influence on the kinetics, although a minimum concentration of 1.3 mm was found to be necessary. however, the hemin-to-oligonucleotide concentrations ratio and, especially, ph and buffer composition showed a strong influence on the reaction kinetics. the optimum values for the highest catalytic activity were found to be a ph of around 7.5 (fig. 1a) and a ratio of hemin and oligonucleotides was found to be between 1.5 and 2.5 (fig. 1b). fig. 1. (a) dependence of the catalytic activity of 1 µm dnazyme (full) and 2 µm hemin (empty) on ph, determined by the absorbance measured at 420 nm developed by the oxidation of 50 µm abts after 3 min of reaction with 1 mm h2o2, oligonucleotide 0.31 µm, hemin 0.62 µm, abts 50 µm in buffer 25 mm: phthalate (), acetate (▲), phosphate (■), and tris-hcl (●); (b) dependence of the catalytic activity with hemin:oligonucleotide ratio, with 1 µm dnazyme, 0.5-3 µm hemin, determined by the absorbance measured at 420 nm developed by the oxidation of 50 µm abts after 3 min of reaction with 0.5 mm h2o2. electrochemical methods can also be used to determine the concentration of abts+•. if a potential step is applied in conditions so that the limit current for the reduction of abts+• is attained, the limit current follows the well-known cottrell equation [25]. current transients (shown in fig. 2a) followed a linear relationship between the limit current and the inverse of the square root of time, as expected from the cottrell equation (inset in fig. 2a). as can be seen in fig. 2b, after a period of catalytic decomposition of hydrogen peroxide (in this case, 3 min), the current measured at a given time after applying a potential step (20 s) was also directly proportional to the concentration of dz5t3 dnazyme. therefore, soluble dnazymes can also be used as labels whose activity can be determined by chronoamperometry. g. longinotti et al. j. electrochem. sci. eng. 7(4) (2017) 193-200 doi:10.5599/jese.471 197 fig. 2. (a) current transients for different concentrations of dnazyme, from bottom to top: 0-2.5 µm. inset: plots of i vs. tm-1/2; (b) current measured at 20 s with a potentiostatic step after 5 min of reaction for different concentrations of dnazyme (●) and hemin (○). optical and electrochemical determination of e. coli o157 verotoxigenic gene using dnazyme-labelled stem-loops four different dna sequences (table 1) differing in their complementary sequences were designed in order to analytically determine the presence of an e. coli o157 gene. these constructions included the dz5t3 sequence, a fragment which is complementary to a specific region of an e. coli verotoxigenic gen [18] and another moiety that allows the complete dna sequence to be folded into a stem-loop structure. fig. 3 shows the schematic representation for the detection of a target dna strand with a stem-loop containing a complementary sequence and a dz5t3 sequence which adopts an active dnazyme conformation after hybridization with the target. fig. 3. schematic representation of bioanalysis with a stem-loop for the recognition of a target dna sequence upon activation of a dnazyme. black: complementary sequences to part of dz5t3 sequence. green: complementary sequences to shiga-like toxin i genes of verotoxigenic. pink: dz5t3 sequence. blue: shiga-like toxin i genes of verotoxigenic sequences the activity of the four dna stem-loops designed were spectrophotometrically tested (fig. 4). in all cases, the sequence complementary to the target was located in the loop region and the stem length was 8 mer. the first construction (stem-loop i, table 1) was designed with the dz5t3 sequence located in the 5’ end, while the other constructions were prepared using the dz5t3 located in the 3’ end of the stem-loop. another difference between the designs of the stem-loops was the j. electrochem. sci. eng. 7(4) (2017) 193-200 detection of a verotoxigenic e. coli gene 198 moiety that blocked the dz5t3 avoiding the active conformation. in the case of stem-loop i, the last 8 bases of the 3' end of dz5t3 were blocked. in all other three cases (stem-loops ii-iv), different zones of the dz5t3 sequence were explored. fig. 4. absorbance at 420 nm after 3 min of reaction for 1 µm hemin (grey) and stem-loops i, ii, iii and iv 0.5 µm + 1 µm hemin, in the absence (black) and the presence of 0.5 µm target (strips). a poor signal-to-noise ratio was obtained for stem-loop i. this low response could be related to an inappropriate stem-loop design. zhang et al. [22] have reported that modifications at the 3' end of sequence corresponding to the peroxidase dnazyme produced a decrease in the catalytic activity. it was assumed that, by adding bases at the 3' end of the dz5t3 oligonucleotide, the ability of the oligonucleotide to bind hemin decreases. fig. 5. variation of absorbance (●) and limit current density at 20 s (о) after 3 min of reaction for a stemloop concentration of 0.5 µm, 1 µm hemine, 20 mm kcl, 0.2 m nacl, 0.05 % triton x-100, 3.4 mm abts 0.5 mm h2o2. for stem-loops ii-iv, a significant increase in the absorbance with the concentration of the target was observed. these three constructions differ in the region where the dz5t3 sequence was blocked (near the 5’ end, the intermediate region, or near the 3’ end). no significant differences in the results were obtained with designs ii and iii. the results for stem-loop iv showed a higher sensitivity, and a maximum value of absorbance was obtained when stem-loop and target were used in a one-to-one ratio. due to these results, stem-loop iv was chosen for successive measurements. fig. 5 shows the absorbance at 420 nm and the density current obtained at a fixed time (20 s) produced by abts+• as a function of different concentrations of the target complementary sequence. https://www.google.com.ar/search?espv=2&biw=1218&bih=702&q=stoichiometric&spell=1&sa=x&ved=0ahukewjtt4a63ijtahvcqzakhc_qcq8qvwuifsga g. longinotti et al. j. electrochem. sci. eng. 7(4) (2017) 193-200 doi:10.5599/jese.471 199 it can be seen that the concentration of abts+• increases almost linearly for low values of target concentration and reaches a plateau for values higher than 1 µm. it can also be noted that even in the absence of the target sequence, a certain amount of abts+• is formed, which can be assumed to be the consequence of the formation of a certain amount of dnazyme. from the data shown in fig. 5, it can be concluded that approximately 13 % of the total possible concentration of available dnazyme structure was formed in the absence of the target sequence. on the other hand, when an excess of target was used, the formation of the dnazyme structure reached about 80 % of the expected maximum value. for stem-loop iv, limits of detection of 174 nm and 144 nm were estimated for the optical and electrochemical measurements, respectively. selectivity against shiga-like toxin i verotoxigenic gene in order to evaluate the stem-loop iv selectivity for shiga-like toxin i verotoxigenic gene (stxi, table 2), an activity assay employing different sequences for other e. coli toxins was carried out. fig. 6 shows the density current response for stem-loop iv (1 µm) in the presence of different targets at the same concentration. target sequences different from stxi showed responses similar to stem-loop iv in the absence of the stxi target, thus indicating the selectivity of the construction. fig. 6. current density measured for the different e. coli pathogenic sequences (see table 2) employing stem-loop iv. conclusions the catalytic activity of a peroxidase-mimicking dnazyme depends on factors such as ph, buffer composition, potassium ions concentration, and the hemin-to-oligonucleotides ratio. for the dz5t3 dnazyme, it was found that the buffer composition is as important as ph itself, and the highest catalytic activity was obtained when a tris buffer of ph 7.5 was used. additionally, a minimum amount of potassium ions of 1.3 mm and a minimum hemin-to-oligonucleotides ratio of 2:1 were found to be necessary to achieve a high catalytic activity. dz5t3 dnazyme was used as a label, combined in a stem-loop with a dna sequence which recognizes a specific sequence of shiga-like toxin i of verotoxigenic e. coli. it was demonstrated that the catalytic activity obtained was specific and proportional to the amount of the target concentration. the dz5t3 dnazyme catalytic activity was followed optically and electrochemically. it is worth noting that soluble dna species were used throughout this work, avoiding problems usually found with immobilized dna strands, related to undesired dna-surface interactions. the combination of electrochemical cells with relatively small volume and low-cost screen printed working electrodes with a relatively high geometric area allowed obtaining amperometric measurements with sensitivities and limits of detection j. electrochem. sci. eng. 7(4) (2017) 193-200 detection of a verotoxigenic e. coli gene 200 comparable to optical methods. to our knowledge, this is the first report of a dnazyme-based electrochemical bioanalysis with these features, which opens the way to the development of costeffective portable devices for the diagnosis of a verotoxigenic e. coli infection using of dnazymes as labels. references [1] r. r. breaker, nature biotechnology 16 (1997) 427–431. 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[24] a. j. bard, l. r. faulkner, electrochemical methods, fundamentals and applications, john wiley & sons, u.s.a., 2001. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) sucrose fatty esters from underutilized seed oil of terminalia catappa as potential steel corrosion inhibitor in acidic medium doi:10.5599/jese.316 287 j. electrochem. sci. eng. 6(4) (2016) 287-294; doi: 10.5599/jese.316 open access : : issn 1847-9286 www.jese-online.org original scientific paper sucrose fatty esters from underutilized seed oil of terminalia catappa as potential steel corrosion inhibitor in acidic medium adewale adewuyi, omolade rahman bello*, rotimi a. oderinde* department of chemical sciences, faculty of natural sciences, redeemer’s university, ede, osun state, nigeria *industrial unit, department of chemistry, university of ibadan, ibadan, oyo state, nigeria corresponding author: walexy62@yahoo.com; phone: +2348035826679 received: june 7, 2016; revised: november 17, 2016; accepted: november 18, 2016 abstract corrosion of metals is a common problem which requires definite attention. in response to this, the oil was extracted from the seed of terminalia catappa and used to synthesize sucrose fatty esters via simple reaction mechanism which was considered eco-friendly and sustainable. the corrosion inhibition capacity of sucrose fatty esters for mild steel in 1 m hcl was studied using the weight loss method. it was shown that sucrose fatty ester inhibited corrosion process of mild steel and obeyed langmuir isotherm. corrosion rate and inhibition efficiency of sucrose fatty esters were found to reduce with increase of immersion time. the study presented sucrose fatty ester as a promising inhibitor of mild steel corrosion in acidic medium. keywords adsorption; corrosion; adsorption isotherm; mild steel; fatty esters introduction corrosion is usually taken as breaking down or wearing of materials, mostly metals, through chemical reactions. the most known form of corrosion is rusting, which takes place when iron combines with oxygen and water. mild steels are known to contain carbon alloyed mainly with copper, chromium, phosphorus, nickel, etc. [1]. generally, the mild steel shows some resistance to atmospheric corrosion which is attributed to its ability to develop a protective layer. it has already been shown that development of this layer depends on nature, amount and type of alloying elements in the steel and also on exposure, or corrosion initiator conditions [25]. corrosion is the major problem encountered in the industries and areas involving surface equipment and processing facilities. this is seen mainly as leaks in tanks, casings, tubing, pipelines, and other equipment [6,7]. the problem of corrosion affects operations and equipment http://www.jese-online.org/ mailto:walexy62@yahoo.com j. electrochem. sci. eng. 6(4) (2016) 286-294 terminalia catappa as steel corrosion inhibitor 288 maintenance, which may lead to recurrent partial and even total process shutdown [8]. use of highly concentrated acids for some industrial unit operations makes the environment corrosive to mild steel, carbon and low-alloy steels. some of these acids include hydrochloric, hydrofluoric, acetic, or formic acids which are injected into the system during the acidizing stimulation process [9]. this has raised the need or search for a way to control or minimize corrosion. although several efforts have been made in past, it is still almost impossible to fully prevent corrosion [10]. it is possible, however, to control the process in order to minimize the havoc it brings. some control of corrosion has been achieved by use of several organic and organometallic compounds [11,12]. even in very small amounts added to a corrosive environment, such compounds are expected to be effective and efficient in decreasing corrosion rates. in some cases, it has been noticed that these compounds are specific in their actions, being efficient only towards particular metals under specific environmental conditions. in other cases, aside being efficient, they were found toxic to the environment, human and animals [13,14]. inhibition of corrosion is very important phenomenon. without inhibition, corrosion rate can be extremely high and may go out of control, particularly at increasing temperatures. due to this, it is important to develop inhibitors that will be effective, environmentally friendly, affordable and of industrial relevance, what would also be important for processes involving acid pickling, industrial cleaning, and acid descaling, where corrosion conditions are usually milder. underutilized seed oils are resources which are readily available, cheap, environmentally friendly and affordable. they can be used as starting materials for production of useful products that can serve as corrosion inhibitors [15]. terminalia catappa oil is an example of underutilized seed oil in nigeria that can serve this purpose. presently, the oil has no specific use and is usually being discarded as waste. finding useful application for this oil will go a long way in making the environment cleaner. as has been reported by matos et al. [16], the oil contains mainly palmitic (35.96 %), oleic (31.48 %) and linoleic (28.93 %) fatty acids. presence of these fatty acids and level of unsaturation represent this oil useful for corrosion inhibitor production. the present work reports the synthesis of sucrose fatty ester from terminalia catappa oil and examination of its ability to inhibit corrosion of mild steel in acidic condition. the effect of time and concentration of sucrose fatty acid ester on corrosion rate will also be examined. experimental materials the seeds of terminalia catappa were collected from the botanical garden, university of ibadan, oyo state, nigeria. the seeds were air dried and subsequently ground in a laboratory mill. dimethyl sulphoxide, hydrochloric acid and sucrose were purchased from merck, darmstadt, germany. further chemicals and all solvents used in this study were of analytical grade and were purchased from vwr international gmbh, darmstadt. the dried seeds of terminalia catappa were later extracted with n-hexane for 10 h using a soxhlet extractor [17]. synthesis of sucrose fatty ester from terminalia catappa oil for the synthesis of the sucrose fatty ester, 25 g of sucrose was transferred into a round bottom flask which previously contained dimethyl sulfoxide (55 ml). the mixture was heated and gently swirled to ensure complete dissolution of the sucrose in the solvent. terminalia catappa oil (65 ml) was added to the resulting solution and followed by addition of k2co3 (3 g). the flask with a stopper having thermometer inserted was placed on a heating mantle and allowed reaction to a. adewuyi et al. j. electrochem. sci. eng. 6(4) (2016) 287-294 doi:10.5599/jese.316 proceed for 9 h at an average temperature of 110 °c with occasional swirling. a thick brownish product was formed at the end of the reaction. at the end of reaction, the excess dimethyl sulfoxide was removed by distillation and the unreacted sucrose was removed by dissolving it in water. the residue of sucrose fatty ester was filtered, weighed and taken for ftir analysis. corrosion study in this study, the weight loss procedure was used for determination of corrosion rates. the mild steel sheets were obtained from the local market in ibadan, oyo state, nigeria. the sheets were degreased in ethanol, dried in acetone, and stored before their use in corrosion studies. the sheets were cut into 4×3 cm size. the corrosion process was initiated by hcl, while the sucrose fatty ester was used as the inhibitor. the effect of both, initiator and inhibitor, on the corrosion rate was examined by varying the concentration of sucrose fatty ester (1 – 3 mg/l) and hcl (1 – 8 m). 1 m hcl solution was used as the control solution, while other solutions containing sucrose fatty ester were used as test solutions. corrosion study was conducted under total immersion of mild steel sheets in 100 ml capacity beakers containing 50 ml test solution at room temperature (298 k). mild steel sheets were weighed and separately suspended in different beakers containing solutions. the sheets were retrieved at 1 h interval progressively for 24 h, washed thoroughly in 20 % naoh solution containing 200 g/l of zinc dust, rinsed severally in deionized water, cleaned, dried in acetone, and re-weighed. the weight loss (in grammes) was taken as difference in the weight of sheets before and after immersion in test solution [18]. the experiments were carried out in duplicate. results and discussion synthesis of sucrose fatty ester from terminalia catappa oil the oil of terminalia catappa has a light yellowish color and is liquid at room temperature. ftir spectra of terminalia catappa seed oil and that of the sucrose fatty ester showed some important absorption bands. for both, the oil and sucrose fatty ester, peaks were noticed at 1747 cm-1 and 1733.33 cm-1, respectively, what were attributed to c=o stretching of ester functional groups. also, the bands at around 1164.62 cm-1 were ascribed to c─o vibrational frequencies in both oil and sucrose fatty ester. signals that appeared at 2920.04 cm-1 in the oil and at 2931.42 cm-1 in the sucrose fatty esters, were taken to be due to c-h stretching of alkanes. peaks at around 3011 cm-1 were considered to be due to c=c-h vibrational frequency of alkene. broad and prominent peak at 3384.00 cm-1 was found only in the sucrose fatty ester and attributed to the o─h stretching; this peak suggested formation of the sucrose fatty ester. corrosion study weight loss during corrosion was calculated as difference in weights at an interval of 1 h. the relationship between weight loss of mild steel in 1 m hcl at different immersion times is presented in figure 1. the weight loss of mild steel was found to increase with increase in time. it was observed that mild steel in the blank solution (solution without inhibitor) shows faster loss in weight than in the solution containing inhibitor. this observation indicates that presence of sucrose fatty esters in the solution is able to slow down the process of corrosion. j. electrochem. sci. eng. 6(4) (2016) 286-294 terminalia catappa as steel corrosion inhibitor 290 figure 1. relationship between weight loss of mild steel and immersion time in 1 m hcl the observed retardation of corrosion may be due to deposition or adsorption of sucrose fatty esters at the mild steel surface [18,19]. figure 2 also revealed that the process of corrosion of mild steel increases as the strength of the acidic solution increases. this suggests that presence of positively charged hydrogen ions in solution plays a role in attacking the surface of the mild steel for corrosion. similar observation has been previously reported by adewuyi et al. [15]. figure 2. effect of hcl concentration on weight loss of mild steel in presence (inhibitor) and absence (blank) of sucrose fatty esters corrosion rate was determined using the following expression: w r at   (1) in eq. (1), r (g cm-2 h-1) is the corrosion rate, ∆w is the average weight loss after immersion, a is the surface area of mild steel and t is the total time (24 h) of immersion. inhibition efficiency (ew / %) for the process was determined as follows: t w 0 0 / % 100 r e r r         (2) in eq. (2), rt and r0 are corrosion rates of mild steel in solutions with and without inhibitor, respectively. the plots of corrosion rates against immersion time are presented in figure 3 which revealed that with increase of immersion time, the rate of corrosion becomes reduced in both solutions, i.e. blank and solution with inhibitor. 0 0,02 0,04 0,06 0,08 0,1 0 1 2 3 4 5 6 7 8 9 immersion time, h w e ig h t lo ss , g 0 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0 2 4 6 8 10 concentration of hcl, m w lo ss , g a. adewuyi et al. j. electrochem. sci. eng. 6(4) (2016) 287-294 doi:10.5599/jese.316 figure 3. corrosion rate versus immersion time for mild steel corrosion in presence and absence of sucrose fatty esters corrosion is obviously slower in the sucrose fatty ester containing solution, what could be due to the covering of mild steel surface by this inhibitor. for the blank solution, however, this observation may be due to corrosion caused reduction of available surface area during time of immersion. inhibition efficiency of sucrose fatty esters against mild steel corrosion was also found to reduce with time as shown in figure 4. it has already been reported that inhibitive mechanism of most organic compounds is based on their adsorption at corroding metal surfaces [2023]. therefore, desorption of inhibitor from the surface of mild steel as immersion time increases can possibly explain the results given in figure 4. figure 4. relationship between inhibition efficiency and immersion time for mild steel corrosion table 1. comparison of the concentration of hcl, weight loss, rate and inhibition efficiency during the corrosion of mild steel concentration of sfe, g/l weight loss, g concentration rate, (g/cm2)/h inhibition efficiency, % 1.00 0.018 0.0015 34.80 2.00 0.022 0.0018 33.30 4.00 0.031 0.0026 27.80 6.00 0.048 0.0040 20.00 8.00 0.067 0.0056 15.10 corrosion rate and weight loss were found to increase, while inhibition efficiency was found to decrease as the concentration of hcl increased. it could be inferred from the results in table 1 that the corrosion initiator’s solution strength was influential in promoting corrosion. it seems that as 0 0,0005 0,001 0,0015 0,002 0,0025 0 1 2 3 4 5 6 7 8 9 immersion time, h r a te , (g /c m 2 )/ h 0 5 10 15 20 25 30 35 40 0 2 4 6 8 10 immersion time, h in h ib it o n e ff ic ie n cy , % j. electrochem. sci. eng. 6(4) (2016) 286-294 terminalia catappa as steel corrosion inhibitor 292 the strength of hcl increased, there were more species in solution to corrosively attack the metal surface, increasing thus corrosion rate. reduction of inhibition efficiency may be due to the fact that as concentration of hcl increased, inhibitor molecules have more contending species at the metal surface and so they are less effective in adsorption/deposition. the effect of concentration of inhibitor on the corrosion rate, inhibitor efficiency and weight loss is presented in table 2. data in table 2 show that inhibition efficiency increased, while corrosion rate is reduced with concentration of sucrose fatty ester. as has already been mentioned above, the observed reduction of corrosion rate may be due to the ability of inhibitor to cover or adsorb at the metal surface. table 2. comparison of the concentration of sucrose fatty ester, weight loss, corrosion rate and inhibition efficiency during corrosion of mild steel concentration of sfe, g/l weight loss, g concentration rate, (g/cm2)/h inhibition efficiency, % 1.00 0.0280 0.00183 20.43 1.50 0.0220 0.00173 24.78 2.00 0.0190 0.00166 27.83 2.50 0.0193 0.00161 30.00 3.00 0.0180 0.00150 34.78 sfe = sucrose fatty esters adsorption isotherm interaction between the metal surface and inhibitor was subjected to adsorption isotherm. the surface coverage () was derived from the expression: t 0 0 r r r         (3) in eq. (3), rt and r0 are corrosion rates of mild steel with and without inhibitor, respectively. the correlation between  and amount of inhibitor in the corroding medium was determined by langmuir adsorption isotherm which can be expressed as: ads 1c c k   (4) in eq. (4), c is the inhibitor concentration, θ is the surface coverage and kads is the equilibrium constant of the inhibitor adsorption process. this model operates on the assumption that adsorption takes place only at specific homogenous sites of metal surface and that adsorption forms a monolayer [22]. as shown in figure 5, the plot of c/ versus c gave straight lines (r2 = 0.968), suggesting thus the monolayer adsorption of sucrose fatty esters at the surface of the mild steel. figure 5. langmuir adsorption plot for mild steel in 1 m hcl containing sucrose fatty esters 0 0,5 1 1,5 2 2,5 3 3,5 0 0,5 1 1,5 2 2,5 3 3,5 (c /θ ) / (m g /l ) concentration, mg/l a. adewuyi et al. j. electrochem. sci. eng. 6(4) (2016) 287-294 doi:10.5599/jese.316 the essential characteristic of langmuir isotherm was calculated as follows: ads 1 1 k k c   (5) in eq. (5), kr is the equilibrium parameter, kads is the langmuir constant and c is the inhibitor concentration. the parameter kr indicates the shape of the isotherm accordingly. it has been reported that when kr > 1, adsorption is unfavourable; when kr = 1, adsorption is linear; when kr = 0, adsorption is irreversible and when 0 < kr< 1, adsorption is favourable [2426]. here, the calculated kr values were < 1, confirming that the adsorption process was favourable and reversible. this also supports the findings that inhibition efficiency is reduced with time which may have been due to the possibility of desorption taking place at the surface of the metal over a period of time. conclusion sucrose fatty ester was synthesized from the seed oil of terminalia catappa and evaluated for corrosion inhibition capacity against mild steel in acidic medium using hcl. the synthesis of the sucrose fatty ester was achieved using simple reaction mechanism which was analyzed using ftir. the sucrose fatty esters inhibited corrosion of mild steel in 1 m hcl with inhibition efficiency increasing as the concentration of sucrose fatty esters increased and decreasing with increase in time. inhibition of corrosion is going via adsorption of sucrose fatty ester on the surface of the metal which obeyed langmuir isotherm. the study presents sucrose fatty ester from terminalia catappa as the potential corrosion inhibitor of mild steel. acknowledgement: the authors are most grateful to the department of chemistry, university of ibadan, ibadan, oyo state and the department of chemical sciences, redeemer’s university, ede, osun state for supplying chemicals, equipment and laboratory space for this work. references [1] t. murata, weathering steel, in: r.w. revie (ed.), uhlig’s corrosion handbook, j. wiley &sons, new york, 2000, 569–580 [2] m. morcillo, b. chico, i. díaz, h. cano, d .de la fuente, corros. sci. 77 (2013) 6–24. 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[26] a. o. dada, a. p. olalekan, a. m. olatunya, o. dada, j. appl. chem. 3 (2012) 38-45. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ synthesis and electrochemical performance of hierarchical sb2s3 nanorod-bundles for lithium-ion batteries doi: 10.5599/jese.2014.0045 45 j. electrochem. sci. eng. 4(2) (2014) 45-53; doi: 10.5599/jese.2014.0045 open access : : issn 1847-9286 www.jese-online.org original scientific paper synthesis and electrochemical performance of hierarchical sb2s3 nanorod-bundles for lithium-ion batteries xiaozhong zhou, shuhua hua, lianhua bai and dong yu key laboratory of eco-environment-related polymer materials of ministry of education, key laboratory of polymer materials of gansu province, college of chemistry and chemical engineering, northwest normal university, lanzhou 730070, gansu province, p. r. china corresponding author: e-mail: zxz20004@163.com; tel.: +086 0931-7972663; fax: +086 0931-7972663 received: november 24, 2013; revised: february 20, 2014; published: may 13, 2014 abstract uniform hierarchical sb2s3 nanorod-bundles were synthesised successfully by l-cysteine hydrochloride-assisted solvothermal treatment, and were then characterised by x-ray diffraction, field emission scanning electron microscopy, and high-resolution transmission electron microscopy, respectively. the electrochemical performance of the synthesised sb2s3 nanorod-bundles was investigated by cyclic voltammetry and galvanostatic charge−discharge technique, respectively. this material was found to exhibit a high initial charge specific capacity of 803 ma h g -1 at a rate of 100 ma g -1 , a good cyclability of 614 ma h g -1 at a rate of 100 ma g -1 after 30 cycles, and a good rate capability of 400 ma h g -1 at a rate of 500 ma g -1 when evaluated as an electrode candidate material for lithium-ion batteries. keywords sb2s3 nanorod-bundles, electrochemical performance, lithium-ion batteries introduction lithium-ion batteries (libs) are currently the most advanced rechargeable batteries for powering portable electronic devices such as laptop computers and cellular phones in view of their high energy density and benign design flexibility [1]. nowadays, graphitic materials are extensively spread as commercial anode materials because of their low operating potential close to that of li + /li and a good structural stability during cycling [2]. unfortunately, the small theoretical specific capacity of the graphite anode (li1/6c, 372 ma h g -1 ) restricts its future applications for powering electric vehicles (evs). as a result, a lot of research efforts have been made to investigate various alternative anode materials with improved performance over the last decade [3]. among alterhttp://www.jese-online.org/ mailto:zxz20004@163.com j. electrochem. sci. eng. 4(2) (2014) 45-53 hierarchical sb2s3 nanorod-bundles for li-ion batteries 46 native anode materials, metal sulfides have attracted particular attention because of their unique structures and high specific capacity [4,5]. as a typical metal sulfide, sb2s3 is an important v-vi semiconductor. owing to their prominent optical, photoelectronic and electrochemical properties, sb2s3 nanomaterials can be potentially applied in photosensors [6], near-infrared optical devices [7], photoelectronic devices [8,9], lithium-ion batteries [10,11], etc. zheng et al. synthesised sb2s3 nanostructures with various dimensional nanostructures by a hydrothermal method and found that the reversible capacity of column-like superstructures, nanorods, and sheaf-like superstructures sb2s3 electrodes are all around 700 ma h g -1 [10]. recently, biomolecule-assisted synthetic routes have become a promising strategy in the preparation of various nanostructured materials because they are green chemistry approaches without toxic reagents and solvents and have obtained unique structures [12]. as an available biomolecule, l-cysteine has attracted considerable attention because of its special structure, which contains multifunctional groups (–nh2, –sh, and –coo–) [13]. according to previous reports, l-cysteine can form a polymeric network structure under solution-phase reaction because its multifunctional groups can help to form interactions between l-cysteine molecules [14]. besides, the presence of l-cysteine is critical to the formation of the final product, which showed excellent cycle stability with a high specific capacity and outstanding rate capability when used as a material for the anodes in lithium-ion batteries [15]. therefore, it would be interesting to develop l-cysteine-assisted methods to prepare metal sulfide-based composites with excellent properties. in this paper, we introduce a simple process for the fabrication of hierarchical sb2s3 nanorodbundles on the basis of a hydrothermal method assisted by l-cysteine hydrochloride (l-cys·hcl). in this method, l-cys·hcl can serve as a template, structure-directing agent and environmentally friendly sulfur source, while sbcl3 serves as the antimony source. in addition, the formation mechanism of the sb2s3 nanorod-bundles is also discussed, and it has been found that the hierarchical sb2s3 nanorod-bundles exhibited high reversible capacity with good cyclic stability and high-rate capability. experimental synthesis all chemicals were of analytical grade and were adopted without further purification. a typical solvothermal experiment for synthesising sb2s3 material was conducted as follows. at first, 0.03 mol of l-cysteine hydrochloride with the formula hsch2ch(nh2)cooh·hcl and 0.01 mol of sbcl3 were successively dissolved in 50 ml of anhydrous ethanol under magnetically stirring for 0.5 h in air. next, the above-prepared solution was transferred to a 100 ml stainless steel teflon-lined autoclave, followed by being sealed and then heated inside a conventional oven at 180 °c for 8 h. at last, after the autoclave was naturally cooled to room temperature, a black precipitate was collected by centrifugation and was then washed thoroughly successively with anhydrous ethanol and deionised water. the collected precipitate was dried in vacuum at 80 °c overnight for later uses. material characterisations the structure, composition, and morphology of the as-prepared sb2s3 material were characterised by powder x-ray diffraction (xrd, rigaku d/max 2400, operating with cu kα radix. zhou at al. j. electrochem. sci. eng. 4(2) (2014) 45-53 doi: 10.5599/jese.2014.0045 47 ation of λ = 0.15416 nm), field emission scanning electron microscopy (fesem, jeol jsm-6701f, operating at 5 kv), and transmission electron microscopy (tem, fei tecnai tf20, operating at 200 kv), respectively. electrochemical measurements to evaluate the electrochemical performance of the as-prepared sb2s3 material, galvanostatic charge-discharge (gscd) and cyclic voltammetry (cv) techniques were employed. the gscd technique was performed in the voltage range of 0.001-2.5 v vs. li + /li on a battery testing system (land ct2001a, wuhan jinnuo electronics co., ltd., china) at room temperatures. the cv technique was performed in the voltage range of 0.001-2.5 v vs. li + /li at a scan rate of 0.2 mv·s -1 on an electrochemical workstation (autolab pgstat128n, metrohm, switzerland) at room temperatures. the testing electrodes were prepared by coating a copper foil substrate with a slurry comprising 80 wt. % active material of sb2s3, 10 wt. % conducting additive of carbon black, and 10 wt. % binder of polyvinylidene fluoride (pvdf). this composition of the active material, the conducting additive, and the binder was found to be optimal for sb2s3, the electrodes were cut into discs with a diameter of 10 mm, and then were assembled in a cr2032-type coin cell with a lithium foil as the counter electrode, a celgard 2400 polypropylene foil as the separator, and a liquid solution of 1m lipf6 in ethylene carbonate (ec)/ethylmethyl carbonate (emc)/dimethyl carbonate (dmc) (1:1:1 by volume, shenzhen capchem technology co., ltd., china) as the electrolyte in an ar-filled glove box. for clarification, discharging here refers to intercalation of li into sb2s3, whereas charging here refers to deintercalation of li from sb2s3. results and discussion fig. 1 gives the xrd patterns of the as-prepared sb2s3 and the standard peaks of sb2s3 (jcpds no. 42-1393). we can see that all of the obtained diffraction peaks could be well indexed to the orthorhombic sb2s3 phase, and no obvious peaks of other crystalline phases were detected. this means that a well-crystallised form of sb2s3 phase has been produced by our one-pot solvothermal treatment. figure 1. xrd patterns of the as-prepared sb2s3 material and standard peaks of sb2s3 (jcpds no. 42-1393) j. electrochem. sci. eng. 4(2) (2014) 45-53 hierarchical sb2s3 nanorod-bundles for li-ion batteries 48 the morphology and structure of the as-prepared sb2s3 material characterised by fesem and tem are presented in fig. 2. it shows that the as-prepared sb2s3 samples consist of cylindrical rods with a diameter of about 100 nm put into bundles. fig. 2b shows the tem and the relevant selected-area electron diffraction (saed) patterns, which are composed of regular sharp diffraction spots characteristic of a single crystal of sb2s3. this clearly implies that the nanorod grows preferentially along the [010] direction in a single crystalline form. figure 2. morphology and structure of the as-prepared sb2s3 material characterised by a fesem and b tem based on the previous results, it is reasonable to conclude that uniform hierarchical sb2s3 nanorod-bundles are synthesised successfully by an l-cys·hcl-assisted solvothermal treatment. lcys·hcl contains some multifunctional groups (-sh, -nh2 and -coo ) [16,17], which can be used for the conjugation of metallic ions or other functional groups [18]. when heated, l-cys·hcl can release h2s, which acts a sulfide source as well as a reducing agent, resulting in the formation of metal sulfide nanoparticles. the proposed growth process for the formation of hierarchical sb2s3 nanorod-bundles is similar to the cos nanowires [13], as shown in fig. 3. and the reaction routes for the synthesis of sb2s3 by l-cys·hcl could be expressed as follows [12,19]: oh sh clnh cocoohchchchohchch hcl)coohch(nhhsch 2242232322  (1) ohcocoochsossbcocoohchchchshbs 23 2 4322232 3   (2) the electrochemical properties of the as-prepared sb2s3 material are first evaluated by a cyclic voltammetry (cv) test. fig. 4 presents the cv curves of sb2s3 sample for the initial three cycles in the voltage range of 2.5 to 0.001 v at a scanning rate of 0.2 mv s −1 . for the first cycle, there are two broad reduction peaks at around 1.0 and 0.4 v, respectively. the peak around 1.0 v can be attributed to the lithiation decomposition of the pristine sb2s3 nanorods directly to fresh sb and li2s. the peak around 0.4 v corresponds to the formation of li3sb. corresponding to the reduction peak, the oxidation peak at approximately 1.1 v was observed, which can be attributed to the dealloying process of li3sb. besides, three small and broad oxidation peaks at approximately 1.4 v, 1.9 v and 2.1 v may originate from the reconstruction of fresh sb2s3 [10,11,20] and the transition of li2s into s, respectively, just as already found in cosbs [21] and sulfide-graphene composite anodes [22]. x. zhou at al. j. electrochem. sci. eng. 4(2) (2014) 45-53 doi: 10.5599/jese.2014.0045 49 figure 3. the proposed growth process for formation of hierarchical sb2s3 nanorod-bundles figure 4. cv curves of the as-prepared sb2s3 material in the first three cycles at a scanning rate of 0.2 mv s−1 from the second cycle on, we can see that the reduction peaks are different than that during the first cycle, which indicates a different electrochemical reduction mechanism. as shown in fig. 4, the reduction and oxidation peaks at approximately 0.8 and 1.1 v originate from the alloying and dealloying process of fresh sb metallic particles, respectively. the others are attributed to the decomposition/reconstruction of fresh sb2s3 [10,11,20] and the electrochemical conversion between s and li2s [21,22]. apparently, all the reduction peaks have their corresponding counterparts of oxidation peaks; this result suggests that the nanosized fresh sb2s3 phase could be reversibly reconstructed and re-decomposed during repeated charging and discharging processes. the potential profiles of the as-prepared sb2s3 sample in cycles 1, 2, 5, 10, 30 and 49 are shown in fig. 5. we can see that the discharging potential plateaus in the first cycle differ evidently from the following cycles, implying a different lithiation mechanism in the first cycle. this is in good agreement with the above cv analysis. the first discharging and charging specific capacities are 1166 ma h g -1 and 803 ma h g -1 , respectively, and the initial capacity loss could come from the irreversible loss of lithium ions due to the formation of a solid electrolyte interphase (sei) layer [23], and the partial irreversibility of the electrochemical reaction (3) [20]. the initial coulombic efficiency is 68.9 %, which is higher than that (50 %) of alloying-dealloying mechanism. j. electrochem. sci. eng. 4(2) (2014) 45-53 hierarchical sb2s3 nanorod-bundles for li-ion batteries 50 figure 5. potential profiles of the as-prepared sb2s3 sample in cycles 1, 2, 5, 10, 30 and 49 at a current density of 100 ma g -1 the cycle performance of the as-prepared sb2s3 material at a current density of 100 ma g -1 is shown in fig. 6. we can see that the sb2s3 material displays a reversible capacity of 614 ma h g -1 even in the 30 th cycle, the capacity retention after the 30 th cycle is approximately 74.9 % with respect to the first charging specific capacity. during the cycling, the discharging specific capacity is slightly larger than the previous charging specific capacity, indicating that the electrode material becomes gradually activated, and this is beneficial to the capacity retention. these excellent electrochemical characteristics might be attributed to its special structure and smaller size of the nanorods. the sheaf-like sb2s3 superstructures are composed of nanorods with a relatively smaller diameter; this special structure favors both the diffusion of the lithium ion and the electrolyte and sb2s3. fig. 6. cycle performance of the as-prepared sb2s3 material at a current density of 100 ma·g -1 . x. zhou at al. j. electrochem. sci. eng. 4(2) (2014) 45-53 doi: 10.5599/jese.2014.0045 51 the entire reaction in relation to sb2s3 can be expressed as the following reaction [10,11,20-22]. sli3sb2li6sbs 232   （3） sbli2li6sb2 3  （4） slisli2   （5） the reversible specific capacity of the sb2s3 is 473 ma h g -1 if we only consider the reversible reaction (3). however, if we consider both the reversible reactions (3) and (4), the reversible specific capacity of the sb2s3 can be achieved as high as 946 ma h g -1 . in our work, the first reversible specific capacity of the sb2s3 sample is as high as 803 ma h g -1 , and 68.9 % can be achieved for the initial coulombic efficiency. this suggests that our as-prepared sb2s3 undergoes both the conversion reaction mechanism and alloying-dealloying lithiation mechanism, which contributes to the nanometer-sized effect. according to the nanometer-sized effect, the nanosized electrode material could have reactive activity. as a result, the reduction and oxidation of metal antimony could be observed, and the electrode exhibited larger capacity and better cycling performance than powder electrode [24]. the rate capability of the as-prepared sb2s3 material is reflected in fig. 7. we can see that the sb2s3 material displays a good rate capability of 400 ma h g -1 at a current density of 500 ma g -1 . but when the charge/discharge current density changes from 1000 to 50 ma g -1 , the specific capacities of the sb2s3 materials cannot return to the last values; the major reason for capacity fading is that li2s formed during the first discharge reaction and li2sx (x>1, lithium polysulfide) are known to dissolve in the electrolyte [25]. a promising route to circumvent this drawback is by producing composite carbon materials whereby the carbon network provides good conductivity, prevents li2s and li2sx from dissolving in the electrolyte and buffers the large volume changes induced by charging [26-28]. fig. 7. potential profiles of the as-prepared sb2s3 material at varying current densities j. electrochem. sci. eng. 4(2) (2014) 45-53 hierarchical sb2s3 nanorod-bundles for li-ion batteries 52 conclusions in summary, uniform hierarchical sb2s3 nanorod-bundles are synthesised successfully by a lcysteine hydrochloride-assisted solvothermal treatment. the presence of l-cysteine is critical to the formation of the sb2s3 material, which is found to display a high lithiation and delithiation specific capacity of 1166 and 803 ma h g -1 at a current density of 100 ma g -1 , a good cyclability of 614 ma h g -1 at a current density of 100 ma g -1 after 30 cycles, and a superior rate capability of 400 ma h g -1 at a current density of 500 ma g -1 when evaluated as an electrode candidate material for lithium-ion batteries. this good lithium storage performance can be ascribed to the nanosized structure. in addition, the preparative method could be a universal green chemistry approach to the synthesis of other metal sulfides. acknowledgements: financial supports from the specialized research fund for the doctoral program of higher education of china under grant no. 20116203120005 and the natural science foundation of gansu province under grant no. 1107rjza147 are acknowledged. references [1] j. m. tarascon and m. armand, nature 414 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[28] k.-j. huang, l. wang, j. li and y.-m. liu, sens. actuators, b 178 (2013) 671-677. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {numerical modelling of buried pipelines under dc stray current corrosion} http://dx.doi.org/10.5599/jese.567 125 j. electrochem. sci. eng. 9(2) (2019) 125-134; http://dx.doi.org/10.5599/jese.567 open access: issn 1847-9286 www.jese-online.org original scientific paper numerical modelling of buried pipelines under dc stray current corrosion yaping zhanga,, qiong fenga,d, xue hana, lianqing yua,, chi-man lawrence wub,, siu-pang ngb, xiao tangc acollege of science, china university of petroleum (east china), qingdao, 266580, p. r. china bdepartment of materials science and engineering, city university of hong kong, hong kong, sar, p.r. china ccollege of mechanical and electrical engineering, china university of petroleum (east china), qingdao, 266580, p. r. china dsemiconductor lighting technology research and development center, institute of semiconductors, chinese academy of sciences, beijing, 100083, p. r. china corresponding authors: e-mail zhangyp@upc.edu.cn; iyy2000@163.com; lawrence.wu@cityu.edu.hk received: july 7, 2018; revised: october 29, 2018; accepted: november 5, 2018 abstract corrosion of buried pipelines caused by stray currents is becoming a serious industrial and environmental problem. it is therefore necessary to study corrosion mechanisms of buried pipelines under dc stray currents in order to propose effective anti-corrosion measures. since measurement of the potential is one of important ways to identify stray current intensity, the comsol multiphysics software was used to simulate stray current corrosion dynamics of buried pipelines. it was also used to calculate the distribution and intensity changes of electrolyte potential in the cathodic protected system by solving laplace’s threedimensional equation. the obtained results showed that increased applied voltage leads to more positive shift of a pipeline potential, resulting in acceleration of stray current corrosion. on the contrary, increased soil resistivity can retard the corrosion process. the protected pipeline with a sacrificial anode suffers less corrosion interference than unprotected pipeline. two crossed arrangement of pipelines makes no difference in corrosion of protected pipeline, but affects greatly on unprotected pipeline. keywords stray current corrosion; numerical modeling; buried pipelines introduction stray currents arising from railway systems can induce corrosion of buried pipeline structures and result in severe damage [1]. basically, there are four types of corrosion [2], involving general http://dx.doi.org/10.5599/jese.567 http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:zhangyp@upc.edu.cn mailto:iyy2000@163.com mailto:lawrence.wu@cityu.edu.hk j. electrochem. sci. eng. 9(2) (2019) 125-134 buried pipelines under dc stray current corrosion 126 corrosion (chemical in nature), concentration cell corrosion (caused by differences in the electrolyte concentration), galvanic corrosion (caused by different metals), and stray current corrosion (caused by external electrical sources). among all corrosion types, just the stray current corrosion (scc) is considered as the most serious one. scc is caused by the flow of stray currents through pipelines, which usually occurs on their external surfaces. the consequences of scc have been manifested as severe localized pitting and pin holes formed on metal surfaces at the place where stray currents leave the pipeline surface [3,4]. a routine way to mitigate the scc of pipelines is to install sacrificial anodes or apply a current to inappropriate bedding for protected structures. both sacrificial anodes and applied current belong to the cathodic protection procedure. cathodic protection is benefit for charges transport from the metal pipeline to the anode [5]. corrosion easily happens because part of a pipeline in the soil is anodic and the other part in the air is cathodic. therefore, the goal of cathodic protection is to apply a direct current to the steel and provide a sacrificial anode [6]. with the development of technology, the numerical modeling becomes very convenient and accurate method for dealing with scc problems. there are three kinds of numerical analysis for cathodic protection systems, including finite difference method, boundary element method and finitude method. among these methods, the boundary element is suited to off-shore structures as a method capable to infer the distribution of potential and current densities along the metallic structure/electrolyte interface [7]. at the other side, the finitude method is well suited to cathodic protected pipelines. comsol multiphysics is the simulation software based on the finitude method for corrosion numerical calculations. in this paper, the comsol multiphysics software is used to evaluate the effect of stray current on the protected/unprotected pipelines resulting from a nearby cathodic protection system. theory governing equation the boundary and initial conditions were set during the simulation, of the protected pipeline and anode ball constituting a whole [8]. the anode ball acts as an anode and produces the anodic oxidation reaction which defines the anode boundary condition. the protected pipeline acts as a cathode for oxygen reduction which defines the cathode boundary condition. the unprotected pipeline can self-corrode in the soil environment, where anode and cathode reactions occur simultaneously on the pipeline surface. the electrode boundary conditions should be set in two ways. the initial value of electrolyte potential is set to -0.90 v, and the electrolyte potential is set zero at infinity. the principles of cathodic protection of underground structures have already been discussed in detail [9]. the soil is treated as a homogeneous medium with a uniform conductivity. the potential (φ)distribution is governed by the laplace’s equation 2 0  = (1) and the current density (i) is related to the electric field by the ohm’s law as follows = −  i (2) in eq. (2),  is electrical conductivity of the soil. the boundary conditions are defined as 0  = / m (3) yaping zhang et al. j. electrochem. sci. eng. 9(2) (2019) 125-13434 http://dx.doi.org/10.5599/jese.567 127 where m is the number of electrons transferred directly. eq. (3) is defined for the region between the insulation layer of unprotected pipeline without any defect and the soil-air surface. ia = exp (a / a) (4) ic = i0,c exp (-c / c) (5) in boundary conditions defined by eqs. (4) and (5), ia, ic, 0,ai , 0,ci are anodic and cathodic current and exchange current densities, while a, c and a, c are corresponding tafel slopes and surface overpotentials, respectively. the boundary conditions defined by eqs. (4) and (5) represent the electrode surface kinetics which strictly follows the tafel kinetics. reaction chemistry three kinds of electrochemical reactions happen on the steel interface, including iron oxidation, oxygen reduction, and hydrogen evolution [10]. (1) hydrogen evolution reaction anode reaction: 2fe→2fe2++4e cathode reaction: 4h-+4e-→4h2 4h2o+4e-→2h2+4oh (2) oxygen reduction reaction anode reaction: 2fe→2fe2++4e cathode reaction: o2+4h++4e-→2h2o o2+2h2o+4e-→4oh when stray currents induce pipeline corrosion, the faraday’ law should be conformed [11] = nc j m k nf (6) in eq. (6) kc is the amount of metal corrosion, jn is the normal current density, m is atomic mass of the metal, f is the faraday’ constant, and n is the number of electrons required for the reaction. for a given metal and reaction, parameters m, f, and n are constant, and so kc  .jn modeling configuration in order to ensure the accuracy of simulation and save computing resources, according to the length and diameter of actual pipelines, we setup a 1:100 scale model to build up the simulation model (schematically shown in figure 1). the length, width and height of the scale model were set as 500, 800, 500 mm, respectively, and the length of pipelines was set proportional to its diameter. the distance of sacrificial anode to the pipeline was set proportional to the actual project. the pipelines are evenly coated with epoxy resin. epoxy resin coating on pipeline surface should be strictly in accordance with sht3022-2011 code for corrosion prevention design of petrochemical equipment and pipeline coatings. models were built up to simulate the corrosion of the protected/unprotected structure due to stray currents. two study cases with different arrangements of pipelines were investigated, involving parallel and crossed arrangements of protected and unprotected pipelines under different interference conditions. geometrical, physical and electrochemical model parameters are listed in tables 1 and 2. a tank was filled with the soil having measured electrical resistivity of 50.0 ω·m, which is consistent with the reference [12]. the protected cathode and unprotected structure were placed in the tank at a height of 20 cm from the bottom. two break points were set at both ends of the pipeline. the copper plates were used to insure the homogeneous potential differences, and a constant direct voltage was applied to the copper plates. the solution side potential of the http://dx.doi.org/10.5599/jese.567 j. electrochem. sci. eng. 9(2) (2019) 125-134 buried pipelines under dc stray current corrosion 128 protected/unprotected structure was scanned with a saturated copper sulfate reference electrode (cse). stray current was picked upon the unprotected pipeline and the protected pipeline was connected with a sacrificial anode. physical fields are added to the geometric model, and the secondary current distribution model (corrosion process satisfy the ohm's law and activation loss during charge transfer) in the corrosion module is selected. the anode and cathode reactions in the corrosion process satisfied the dilute material transfer model in the electrochemical module. as shown in figure 2, the sacrificial anode has more positive potential, and so, electric field lines are emitted from it and terminate at the cathodically polarized structure having more negative potential. the current in the structure flows towards the anode, and regions where currents enter will be at a more negative potential, protecting thus the structure from corrosion. sacrificial anode protection is equivalent to a direct current flowing to the pipeline compulsively, resulting in negative potential shift and reducing the corrosion rate of the structure. fig. 1. schematic view of models: (a) parallel pipelines; (b) crossed pipelines table 1. specification of physical model parameters quantity value protected pipeline length 400 mm unprotected pipeline length 400 mm protected pipeline diameter 4 mm unprotected pipeline diameter 4 mm anode diameter 2 mm soil resistivity (if not specified) 50.0 ω·m anode current density (if not specified) 100 a/m² yaping zhang et al. j. electrochem. sci. eng. 9(2) (2019) 125-13434 http://dx.doi.org/10.5599/jese.567 129 table 2. electrochemical model parameters parameter name value description f 96485 c/mol faraday constant eeq_fe -0.76 v equilibrium potential of fe-fe2+ i0_fe 7.1×10-5 a/m2 exchange current density of fe-fe2+ 𝛽fe 0.41 mv/decade ion reduction tafel slope eeq_h2o -1.03 v equilibrium potential of h2o-h2 i0_h2o 0.11 a/m2 exchange current density of h2o-h2 𝛽h2o 0.15 mv/decade h2o reduction tafel slope eeq_o2 0.189 v equilibrium potential of o2 and oh i0_o2 7.7×10-7 a/m2 exchange current density of o2 and oh 𝛽o2 -0.18 mv/decade oxygen reduction tafel slope gap 5×10-4 m gap between coating and steel surface phi0 -0.9 v measuring potential of body phase fig. 2. scheme of pipeline protection by sacrificial anode results and discussions parallel pipelines figure 3 shows the calculated potential distributions for parallel pipelines in a cathodic protection system, with a voltage of 10 v applied between the anode and the protected cathode, which is consistent with literature data [8,9] and actual project construction. different colors represent potential magnitudes, where red and blue mean positive and negative values, respectively. as shown in figure 3, the potential at the position of 0 ~ 20 cm is much higher than at other parts of the tank along the pipeline direction. figure 3(b) shows the potential contour profile surrounding the parallel pipelines in detail. as shown in figure 4, currents flow into the cathodically protected pipeline and return to the anode through the unprotected pipeline where corrosion occurs due to the dissolution reaction of the anode. because there is potential gradient surrounding the anode, anode will interfere with the current flow path, that is, protected pipeline can accept current near the anode and then leading to current flowing along the pipeline. that is why the protected pipeline has more negative potential than the unprotected pipeline. http://dx.doi.org/10.5599/jese.567 j. electrochem. sci. eng. 9(2) (2019) 125-134 buried pipelines under dc stray current corrosion 130 fig. 3. calculated potential distributions for parallel pipelines: (a) profile of equipotential surfaces; (b) chart of equipotential lines fig. 4. current flow direction for parallel pipelines yaping zhang et al. j. electrochem. sci. eng. 9(2) (2019) 125-13434 http://dx.doi.org/10.5599/jese.567 131 a b c d fig. 5. potential profiles along the parallel pipeline axial length: protected pipeline at (a) different voltage and (c) different soil resistivity; unprotected pipeline at (b) different voltage and (d) different soil resistivity dependences of potential profiles on positions of protected/unprotected pipeline for different voltages are shown in figure 5. the potential of the unprotected pipeline is more positive than the protected pipeline, which suggests that the unprotected pipeline will be corroded firstly. at the voltage of 10 v, the potential of the protected pipeline ranges from -0.30 v to 0.17 v. for the unprotected pipeline at 10 v, however, the potential values are higher, and ranged between 0.00 v and 1.00 v. according to the calculated data, it is proved that the sacrificial anode protection is effective. with voltage increase, the potentials show a positive trend of changes. according to figure 5(a), at the position of 10 cm, the potential rises from -0.20 v to 0.20 v when the applied voltage increases from 5 v to 20 v. likewise, as shown in figure 5(c) and (d), the potential profile shifts positively when the soil resistivity decreases, what can accelerate charge transfer and facilitate the process of corrosion. the calculated potential distributions for crossed pipelines at a voltage of 10 v are shown in figure 6. the protected pipeline is situated along y-axis. it can be seen that the potential of the pipeline parallel to y-axis decreases progressively along the positive y-axis and the color changes from red to blue. potential versus position profiles for different voltages and soil resistivities of protected/unprotected pipelines are shown in figure 7. for protected pipelines, the trend of potential change in figures 7(a) and (c) is similar to figures 5(a) and (c) and shows monotonous potential decrease. 0 5 10 15 20 25 30 35 40 -0.4 -0.2 0.0 0.2 0.4 p o te n ti a l, v v s . c s e position, cm 5 v 10 v 15 v 20 v 0 5 10 15 20 25 30 35 40 0.0 0.5 1.0 1.5 p o te n ti a l, v v s . c s e position, cm 10 v 15 v 20 v 25 v 0 5 10 15 20 25 30 35 40 -0.4 -0.3 -0.2 -0.1 0.0 p o te n ti a l, v v s . c s e position,cm 70.63 m 52.54 m 30.94 m 0 5 10 15 20 25 30 35 40 0.2 0.4 0.6 0.8 1.0 p o te n ti a l, v v s . c s e position, cm 70.63  m 52.45  m 30.94  m http://dx.doi.org/10.5599/jese.567 j. electrochem. sci. eng. 9(2) (2019) 125-134 buried pipelines under dc stray current corrosion 132 crossed pipelines fig. 6. calculated potential distributions for crossed pipelines a b c d fig. 7. potential profiles along the crossed pipeline axial length: protected pipeline at (a) different voltage and (c) different soil resistivity; unprotected pipeline at (b) different voltage and (d) different soil resistivity the stray current transfers from one broken side of protected pipeline to another broken side where corrosion occurs seriously. potential distributions for unprotected pipelines shown in figures 0 5 10 15 20 25 30 35 40 -0.45 -0.40 -0.35 -0.30 -0.25 -0.20 -0.15 p o te n ti a l, v v s . c s e position, cm 10 v 15 v 20 v 25 v 0 5 10 15 20 25 30 35 40 -0.2 0.0 0.2 0.4 0.6 0.8 p o te n ti a l, v v s . c s e ) position, cm 10v 15v 20v 25v 0 5 10 15 20 25 30 35 40 -0.4 -0.3 -0.2 -0.1 p o te n ti a l, v v s . c s e position, cm 70.63  m 52.45  m 30.94  m 0 5 10 15 20 25 30 35 40 0.2 0.4 0.6 0.8 1.0 p o te n ti a l, v v s . c s e position, cm 70.63  m 52.45  m 30.94  m yaping zhang et al. j. electrochem. sci. eng. 9(2) (2019) 125-13434 http://dx.doi.org/10.5599/jese.567 133 7(b) and (d) are, however, significantly different from that shown in figures 5(b) and (d). both potential profiles exhibit a sharp downturn with the change of position, reaching a minimum value in the middle of pipeline and then begin to increase. simply speaking, the potential distribution is presented as a letter “v”. in other words, the potential on the unprotected pipeline is symmetrical perpendicular to the protected pipeline. as shown in figure 7(a) and (b), in the middle position at 20 cm, the potential of the unprotected pipeline reaches -0.20 v, what is more positive than -0.42 v attained for the protected pipeline. thus, the unprotected pipeline will suffer from severely concentrated corrosion. the corrosion reaches the maximum in the middle zone where the current flows out from the unprotected pipeline. as shown in the scheme drawn in figure 8, the current flows out from the anode, enters the unprotected pipeline at the remote areas and then escapes from the crossed point of two pipelines. since the potential of the unprotected pipeline is generally positive, a minimal potential value can be obtained at the position of current flowing out. fig. 8. current direction for crossed pipelines conclusions a mathematical model was developed to solve laplace’s three-dimensional equation with nonlinear boundary conditions for a cathodic protection system, and applied for considering the interference of dc stray current corrosion. two different arrangements of protected and unprotected pipelines (parallel and crossed) were studied, and potential distributions of buried pipelines were for both arrangements obtained by simulations. it was shown that the type of protected/unprotected pipelines arrangements have a great impact on the potential distribution of the unprotected pipeline. the results also showed that both applied voltage and soil resistivity are crucial factors impacting stray current corrosion. again, the sacrificial anode protection in a cathodic protection system was certified as an effective way for corrosion prevention. acknowledgements: the work described in this article was supported by grants from the national natural science foundation of china (no.21476262), technology project of qingdao 14-2-4-108-jch. the authors also acknowledge the associated support service by associate professor xiao tang’s research group. references [1] l. bertolini, m. carsana, corrosion science 49 (2007) 1056-1068. 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[12] md. a. salam, q. m. rahman, s. p. ang, f. wen, journal of modern power system and clean energy, 5 (2017) 290-297. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) an improved method of water electrolysis – effect of complexing agent doi:10.5599/jese.296 215 j. electrochem. sci. eng. 6(3) (2016) 215-223; doi: 10.5599/jese.296 open access : : issn 1847-9286 www.jese-online.org original scientific paper an improved method of water electrolysis – effect of complexing agent seetharaman swaminathan*,**, balaji rengarajan*, ramya krishnan*,, kaveripatnam samban dhathathreyan*, manickam velan** *centre for fuel cell technology, international advanced research centre for powder metallurgy and new materials( arci), ii floor, iit-m research park, no.6 kanagam road, taramani, chennai – 600 113, india **department of chemical engineering, a.c. college of technology, anna university, chennai-600 025, india corresponding author: ramya.k.krishnan@gmail.com,fax: +91-44-66632702 received: may 17, 2016; revised: july 1, 2016; accepted: july4, 2016 abstract the present work investigates the efficiency of an alkaline water electrolysis process in the presence of a complexing agent like citric acid (ca) when added directly into the electrolyte during the electrolytic process. high surface area nickel electrodes prepared by electrodeposition technique were used as the electrode to evaluate the efficiency of the oxygen evolution reaction (oer) by the polarization measurements and cyclic voltammetry. the quantity of the complexing agent ca in the electrolyte was varied from 0-1 wt. %. an increase in the current density of about 25 % resulted at a temperature of 30 °c in the presence of 0.2 wt. % of ca at 1.0 v vs. hg/hgo. ca was found to improve performance by forming a complex with the alloy electrode and by formation of the high surface area catalyst for efficient oer. keywords citric acid; alkaline water electrolysis; oxygen evolution reaction; complexing agent introduction clean energy is considered as the solution to world’s increasing energy demand and to concerns regarding pollution and contamination. hydrogen is important energy alternatives as its combustion product is only water. water electrolysis is the best technology for producing hydrogen and oxygen with no resulting greenhouse gas emissions [1-3]. the energy required for production of hydrogen by electrolytic method is very high (4.5-5 kwh n m-3 of h2) in most industrial electrolyzers. alkaline http://www.jese-online.org/ mailto:ramya.k.krishnan@gmail.com j. electrochem. sci. eng. 6(3) (2016) 215-223 complexing agent in water electrolysis 216 water electrolysis (awe) offers the advantages of the use of non-noble metal catalysts, ease of manufacturing and scalability. therefore, research on suitable catalyst materials and methods for modifying the electrodes to improve efficiency of water electrolysis techniques are always being carried out. efficiency in electrolysis can be improved by reducing the cell voltage through the use of catalysts with increased surface area or by changing the nature of the overpotential of the reaction. in awe anodic overpotential is a major factor that limits the efficiency and hence is researched frequently [4-6]. ni based electrodes, being less corrosive in alkaline solution, offer low cost solution despite its higher overpotential in alkaline solutions when compared to costly ruo2 and iro2 based catalyst used in acidic media [7]. further, it has been found that incorporation of zinc and sulphur in the nickel electrode increases surface area and catalytic activity, lowers overvoltage and helps in increasing the current density by removal of the gas bubbles from the electrode surface [8]. citric acid (ca) is a chelating agent often used for depositing metals at a controlled rate in electroplating and electroless plating operations. it is also used to buffer the ph of the plating bath. the effects of additives in the electroless nickel deposition process have been studied by electrochemical analysis [9,10]. it has been shown that the additives significantly improved the deposition rate and helped in forming finer grain structure during the plating process. the importance of additives such as boric acid, citric acid and ascorbic acids in influencing the electrodeposition of nickel alloys was described by kieling [11]. it was shown that complexing agents influence the kinetics of powder electrodeposition, as well as the morphology of the ni powders. according to the conclusion of the authors finer powders were produced in the presence of citric acid than those obtained with oxalic acid. a study on organic additive, tri sodium citrate as a stabilizing agent reveals that they can be used for structure related factors to control the nucleation, growth and alignment of crystal phases [12,13]. nikolić et al. [14] have shown that alkaline electrolytic hydrogen production can be made efficient (~15 %) by the addition of activating compounds of tungsten and cobalt in the ionic and complex form into the electrolyte (6 m koh) during electrolytic process. additions of ionic activators were found to increase surface area, porosity and performance due to the synergistic effect of the two different metals on the surface of the electrode. the effect of ionic activators and complexes in alkaline water electrolysis has been emphasized by improved performance in the presence of molybdate, chromate and cobalt based compounds [15,16]. the ability of citric acid to form complexes with ni is very well established. it has been used to decrease the precipitation of ni as hydroxides thus stabilizing it in the electrolyte. it has been used in both the electroless and electrodeposition of ni and its alloys [17,18]. it was hence decided to study the effect of this stabilizer during the electrolysis of water by adding it to the electrolyte, i.e. to study whether the ni citrate complex formed will act similar to the ionic activator and improve the efficiency of the electrolytic process. in the present system the activator is formed in situ from the electrode and the complexing agent in the solution compared to those in the literature where the metal complexes are externally added. a systematic study on oer has been carried out by varying the concentration of citric acid in the electrolyte between 0 and 1 % by weight. experimental 1. materials nickel sulphate (merck), nickel chloride (merck), zinc sulphate (merck), ammonium nickel sulfate (merck), sodium thiocyanate (fisher), potassium hydroxide (merck), citric acid (merck) and other s. seetharaman et al. j. electrochem. sci. eng. 6(3) (2016) 215-223 doi:10.5599/jese.296 217 chemicals were all used as received. ni mesh (procured from champion manufacturing co, india with 0.10×0.32×0.20 mm nominal thickness, long diagonals of the diamond and strand width respectively) of area 9.0 cm2 size were used for preparation of the electrodes. 2. electrode fabrication the surface of nickel mesh was electrochemically cleaned before electroplating using the bath solution containing nickel chloride (240 g l-1) and conc. hcl (125 ml l-1). a plating current density of 50 ma cm-2 was used for cleaning. the plating bath consisting of nickel (ii) sulphate (70 g l-1), ammonium nickel sulfate (40 g l-1), zinc sulphate (35 g l-1) and sodium thiocyanate (12 g l-1) were used for plating. electrodes were prepared by the pulse electrodeposition technique as mentioned in our previous paper with a ph of 5-6, a current density of 40 ma cm-2 and duty cycle of 40 % under the influence of ultrasonic vibration [19]. once the electrodeposition was concluded, the electrodes were treated with 28 g l-1 of koh to leach out excess zinc to give a porous electrode having a high surface area. 3. cell frame-up electrochemical measurements were carried out using a solartron analytical potentiostat/galvanostat 1400 cell test system in a small undivided beaker cell (volume 50 cm3). as prepared nickel alloy electrode (working electrode), large area platinum gauze (counter electrode) and hg/hgo reference electrode in 1 m naoh were used for the study. the area of the electrode was 9 cm2 and 30 % koh solution to which ca between 0 and 1 wt. % has been added was used as the electrolyte. figure 1 shows the cell set up used for testing of the electrodes of the present study. figure 1. three electrode cell set up used for electrochemical characterization 4. electrode characterization the surface morphology of the electrodes was studied using hitachi su1500 scanning electron microscopy (sem). energy dispersive analysis of x-ray (edax) using hitachi 4300 was used to study the composition of metals on the electrode. rigakuminiflex x-ray diffraction (xrd) with cukα j. electrochem. sci. eng. 6(3) (2016) 215-223 complexing agent in water electrolysis 218 (λ0.154 nm) radiation was used for x-ray characterization of the electrodeposits. perkin-elmer system was used to obtain the fourier transform infrared spectra (ft-ir). results and discussion 1. ft-ir analysis figure 2 shows ft-ir spectra of the nickel electrode with and without the addition of stabilizer. the bands due carboxyl groups of citric acid are present between 1800 and 1300 cm-1. the band at 1721 cm-1 has been ascribed to free coogroup. the band at 1630 cm-1 has been ascribed to the carboxylic group forming intramolecular hydrogen bonds. the band at 1425 cm-1 may be assigned to the bidentate carboxylate anion. for nickel alloy electrode in citric acid spectra, the band around 3450 cm-1 was very broad, due to hydration by water molecule indicating the electrolytes ability to wet the electrode. in spectra without citric acid due to less adsorbed water molecule this peak was small. the bands around 1150 and 650 cm-1 represent the stretching vibrations of the nickel sulphate used for electroplating of nickel [12]. these studies confirm the formation of nickel citrate complex on dipping the electrode in the electrolyte containing ca. literature studies have shown that upon immersion of a nickel electrode into the solution of aqueous alkali, a film of ni(ii) oxide species (αni(oh)2) is formed spontaneously [20]. this on ageing dehydrates and gets converted into ni(oh)2. the electroprecipitation of this oxide forms ni plaques and may lead to decrease in performance. on addition of citric acid into the electrolyte ni forms citrate complexes that may help in cleaning the surface of the oxide layer. the complexes formed may improve the wetting ability of the electrolyte by decreasing the surface tension of the electrodes. 500 1000 1500 2000 2500 3000 3500 25 50 75 100 t ra n s m it ta n c e , % wavenumber, cm -1 after electrolysis as prepared coo coo coo (sym) c=c -c-c(ch 2 -coo ) oh c-c (asym) figure 2. ft-ir spectra of electrolytic surface: (a) before and (b) after citric acid addition 2. effect of variation of ca figure 3 shows the polarization curves with the variation in the amount of ca at 30 °c. a current density of 0.40 acm-2 was achieved with 0.1 wt % addition of ca compared to 0.32 a cm-2 achieved without the addition of ca at 1.0 v vs. hg/hgo reference electrode. the maximum current density of 0.47 a cm-2 was obtained on the addition of 0.2 % ca at 1.0 v. further increase in citric acid led to decrease in current density to 0.37 a cm-2 at 0.3 % ca addition. in alkaline solution without ca, ni(oh)2 precipitates on the surface and gets absorbed on the electrode, passivates the electrode decreasing the kinetics of the reaction. on the addition of ca, formation of ni(oh)2 on the surface s. seetharaman et al. j. electrochem. sci. eng. 6(3) (2016) 215-223 doi:10.5599/jese.296 219 decreases and a citrate complex of ni is formed resulting in increase of current density. an increase in ca concentration leads to a strong citrate adsorption on the surface reducing the catalytic activity. ni citrate particles present on the surface of the electrode may also act as nucleation sites for in situ deposition of metal thereby increasing the activity. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.0 0.1 0.2 0.3 0.4 0.5 c u rr e n t d e n s it y , a c m -2 potential, v vs. hg/hgo without ca 0.1 % ca 0.2 % ca 0.3 % ca 0.5 % ca 1.0% ca figure 3. polarizations curve for alkaline water electrolysis with the different wt.% of ca at 30 °c. 3. cyclic voltammetry studies cyclic voltammograms obtained for the nickel alloy electrodes when scanned between 0 to 0.8 v vs. nhe at a scan rate of 5 mv s-1 in 30 % koh solution are represented in fig. 4. the oxide formation on surface of the nickel electrode is represented by the anodic current between the potentials of +0.37 and +0.58 v vs. hg/hgo. the peak at potential of +0.16 v vs. hg/hgo represents the oxide layer reduction upon current reversal. the ni(ii)ni(iii) oxide transformations as represented by peaks acts as electrocatalyst for the oer[13,21]. of all the additions, 0.2 wt. % of ca in 30 % potassium hydroxide solution showed the highest area. -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 c u rr e n t d e n s it y , a c m -2 voltage, v 0 % ca 0.1 % ca 0.2 % ca 0.3 % ca 0.5 % ca 1 % ca figure 4. cyclic voltammograms of nickel alloy electrode with different wt.% of ca. the addition of 0.2 wt. % of ca enabled the current to increase more sharply when the potential reaches ~0.52 v. however, upon increasing the citric acid concentration above a 0.2 wt. % a further j. electrochem. sci. eng. 6(3) (2016) 215-223 complexing agent in water electrolysis 220 increase was not seen and the curve almost truncated. ca may reduce the surface tension of the electrolyte so the bubbles formed due to evolution of oxygen gas can leave the system easily. at higher concentration of ca, a strong absorption on the surface leads to decrease in catalytic sites and decrease in activity. 4. effect of temperature figure 5a shows the steady state polarization curves of ni-zn-s alloy electrode at different temperatures on the addition of citric acid (0.2 wt. %) in alkaline water electrolysis. as can be seen from the polarization curves, performance increases with increase in temperature. a high current density of 0.78 a cm-2 was achieved at 1.0 v and 80 °c. with the introduction of citrate in the system, the overall efficiency increases and overvoltage also decreases for oer. the improved performance may be explained on the basis of an increase in the conductivity of the electrolyte at higher temperatures and also due to increase in catalytic activity with the increase in temperature. figure 5b gives the long term stability of the cell, studied by holding the cell continuously at a potential of 1.0 v vs. hg/hgo electrode and monitoring the current at the temperature of 30 °c. the current was constant indicating the stability of the cell with added citric acid. (a) (b) 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 c u rr e n t d e n s it y , a c m -2 potential, v vs. hg/hgo 30 o c 60 o c 80 o c 0 10 20 30 0.3 0.4 0.5 0.6 c u rr e n t d e n s it y , a c m -2 time, h figure 5.effect of temperature on the addition of ca (0.2 wt. %) in alkaline water electrolysis (a) and stability test with addition of ca (0.2 %) at 1.0 v vs. hg/hgo at 30 °c (b) 5. sem analysis scanning electron microscopy (sem) of the nickel anode was performed before and after (24 h of continuous operation) the electrolytic process with the addition of the ca in the electrolyte. the typical sem images are presented in fig. 6. the morphology of deposits before electrolysis are shown in figure 6 (a) and (b). in these micrographs homogenous globular particles are seen. the surface of the electrodes is also divided by coarser cracks and channels. the cracks are likely to be filled with the electrolyte when dipped in the solution. this increases the surface area of the catalyst in contact with the electrolyte and leads to increase in current density. further, efficiency may also be increased due to convective replenishment of the electrolyte due to gas evolution. the edax data of the nickel alloy electrodes prepared has a composition of nickel (55 %), zinc (30 %) and sulphur (15 %). the morphology of deposits after addition of the ca during the electrolytic process is shown in fig. 6 (c) and (d). the surface of the electrodes is characterized by the presence of flakes covering the electrodeposited layers. s. seetharaman et al. j. electrochem. sci. eng. 6(3) (2016) 215-223 doi:10.5599/jese.296 221 figure 6. sem images of nickel alloy electrodes: (a) and (b) after electrolysis with koh, (c) and (d) after electrolysis with koh and ca the deposits appear to be interspersed on the surface of the electrode and make the surface appear rough. thus, it is evident that the addition of ca in the electrolyte has resulted in the deposition of ni metal particle on the surface during the electrolytic process. thus, an increase in surface area and an increase in possible active centers are revealed in the sem micrographs of the sample after electrolysis. the citric acid complexing agent thus acts as a bridge for the transfer of electrolyte on the surface of the electrode and the in situ metal deposits with their high surface area and catalytic activity improve the performance of the electrode for oer. these results compare well with the powder deposits of fe-ni observed by use of citric acid by lačnjavec et al. [22] in their studies on electrodeposition of fe-ni from citrate electrolytes. j. electrochem. sci. eng. 6(3) (2016) 215-223 complexing agent in water electrolysis 222 6. xrd analysis figure 7 shows the xrd patterns of electrodeposited nickel electrodes before and after (24 h of continuous operation) electrolysis. the x-ray diffraction pattern of the before electrolysis sample has a broad peak around 44o corresponding to (111) position of ni. due to the high concentration of nickel, the intensity of these peaks is high and hence only these are seen. the zn and s present are detected only by the edax analysis. in the after electrolysis curve the intensity of the nickel peak is very high and the peak is also very sharp. the average particle size was calculated using debyescherrer relation. the particle size of samples after and before electrolysis was found to be between 11 and 16 nm respectively. this may be explained by the deposition of the in situ ni particles from the ni citrate complex formed. such in situ deposited particles may be responsible for the increased electrochemical activity shown in the oer reaction. it shows that the size of ni deposit particles can be modified by the addition of citric acid. similar observations of metal deposition have been reported from the solution by nikolić et al.[14]. 10 20 30 40 50 60 70 2/ o (2 0 0 ) ( 1 1 1 ) (b) (2 0 0 ) (1 1 1 ) in te n s it y , a .u (a) figure 7. xrd patterns of nickel surface: (a) after electrolysis; (b) before electrolysis the addition of 0.2 wt. % citric acid to the standard electrolyte as a complexing agent thus decreases the energy consumption during electrolysis of water. the addition resulted in the in situ deposition of ni on the surface (as shown in sem figures). the improved performance suggests that an extremely active surface center is created to improve the catalytic activity of the electrode. further the formation of the nickel citrate complex may improve the wettability of the electrode and may act as a bridge for ion transfer between the electrolyte and the metal surface [23]. conclusions the results presented help in concluding that a deposit formed in-situ during the electrolytic process by using a complexing agent such as ca in the electrolytic solution is extremely active. an increase in the current density of about 25 % resulted at a temperature of 30 °c in the presence of 0.2 wt. % of ca at 1.0 v vs. hg/hgo. the ca distributed throughout the nickel matrix as a complex created more organized transfer channels to produce high surface area electrodes for alkaline water electrolysis. sem and xrd figures obtained after electrolysis confirm the deposition of the metal. this form of deposit is very active and improves the catalytic activity of the electrode. improvement may also be explained due to increase in wettability and the bridging ability of the complexing agent to transfer ions between the electrolyte and metal surfaces. thus, through simple processing, high s. seetharaman et al. j. electrochem. sci. eng. 6(3) (2016) 215-223 doi:10.5599/jese.296 223 current density and good efficiency can be obtained. we believe this type of complexing agent deserves further investigation and is a good candidate for use in alkaline water electrolysis. acknowledgement: the authors would like to thank director, arci for his constant encouragement and support and department of science and technology (dst), government of india for financial assistance. s. seetharaman thanks arci for the senior arci fellowship under which this work was carried out. references [1] e. rasten, g. hagen, r. tunold, electrochim. acta 48 (2003) 3945-3952 [2] p. millet, d. dragoe, s. grigoriev, v. fateev, c. etievant, int. j. hydrogen energ. 34 (2009) 4974-4982 [3] s. p. s. badwal, s. giddey, f. t. ciacchi, ionics 12(2006)7-14 [4] j. panek, a. budniok, surf. interface anal.40 (2008) 237-241 [5] a. a. kamnev, b. b. ezhov, v. rusanov, v. angelov, surf. interface anal. 19(1992) 577-580 [6] p. w. t. lu, s. srinivasan, j. electrochem. soc. 125 (1978) 1416-1422 [7] g. zhang, y. qiu, x. yang, x. x. li, h. pan, desalin. water treat. 56 (2015) 905-911 [8] g. sheela, m. pushpavanam, s. pushpavanam, int. j. hydrogen energ. 27 (2002) 627-633 [9] m. s. sadovnikova, v. m. belikov, russ. chem. rev. 47 (1978) 199-212 [10] b. h. chen, l. hong, y. ma, t. m. ko, ind. eng. chem. res. 41 (2002) 2668-2678 [11] v. c. kieling, surf. coat. tech. 96 (1997) 135–139 [12] q. mao, l. zhang, d. huang , d. wang, y. huang, h. xu, h. cao, z. mao, surf. interface anal. 43 (2011) 903-912 [13] i. g. casella, m. gatta, j. electroanal. chem. 534 (2002) 31–38 [14] v. m. nikolić, g. s. tasić, a. d. maksić, d. p.šaponjić, s. m. miulović, m. p. m. kaninshi, int. j. hydrogen energ. 35 (2010) 12369-12373 [15] m. p. m. kaninski, a. d. maksić, d. l. stojić, s. c. miljanić, j. power sour. 131 (2004)107-111 [16] s. m. miulović, s. l. maslovera, m. m.šeović, b. b. raduk, m. p. m. kaninski, int. j. hydrogen energ. 37 (2012) 16770-71675 [17] e. gomez, s. pane, e. valles, electrochim. acta 51(2005) 146-153 [18] a. afshat, a. g. dolati, m. ghorbani, mater. chem. phys. 77 (2003) 352-358 [19] s. seetharaman, r. balaji, k. ramya, k. s. dhathathreyan, m. velan, ionics 20 (2014) 713720 [20] a. seghiover, j. chevalet, a. barhoun, f. lantelme, j. electroanal. chem. 442(1998)113-123. [21] m. c. biesinger, b. p. payne, l. w. m. lau, a. gerson, r. s. c. smart, surf. interface anal. 41 (2009) 324-332 [22] u. lačnjevac, b. m. jović, v. d. jović, electrochim. acta 55 (2009) 535-543 [23] o. gyliene, j. aikaite, o. nivinskiene, j. hazard. mater. 109 (2004) 105-111. 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ electrochemical studies on zirconium phosphoborate based heterogeneous membranes doi: 10.5599/jese.2013.0048 55 j. electrochem. sci. eng. 4(2) (2014) 55-65; doi: 10.5599/jese.2014.0048 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical studies on zirconium phosphoborate based heterogeneous membranes sandeep kaushal, pritpal singh* and susheel k. mittal** ,  department of applied sciences and humanities, punjab technical university, jalandhar, india *sri guru granth sahib world university, fatehgarh sahib (pb), india **school of chemistry and biochemistry, thapar university, patiala 147004, india corresponding author: e-mail: smittal@thapar.edu; fax: +91-175-2364498 received: september 30, 2013; revised: march 18, 2014; published: may 13, 2014 abstract electrode potential measurements have been applied to study electrical characteristics like transport numbers, permselectivity & fixed charged density of zirconium phosphoborate ion exchange membranes. the potential measurements were made across the cation exchange membrane maintained at 27±0.1 °c, using halide and nitrate salts of alkali and alkaline earth metals as electrolytes. the membrane potentials, transport numbers and permselectivity values increase with increase in average concentration from 0.0055 m to 0.0495 m for 1:1 and 1:2 electrolytes. with the increase in concentration of the electrolyte, the number of counter ions interacting with the membrane surface increases leading to enhanced donnan exclusion responsible for the increase of transport numbers. fixed charge density of the membrane (x) for 1:2 electrolytes is higher in magnitude than for 1:1 electrolytes indicating that the cation exchange is taking place as hydrated species. this hypothesis is supported by higher transport numbers for alkaline earth metal ions than alkali metal ions throughout the range of concentration. keywords electrochemical studies, ion-exchange membrane, transport numbers, alkali metal ions, zrpb introduction inorganic ion-exchangers are stable towards chemical degradation and are more selective for metal ions over their organic analogs [1]. the ion-exchange membranes have diverse applications, from desalting of brackish water to treating industrial effluents as well as processing biological effluents. due to the development of new ion-exchange membranes with better selectivities, lower electrical resistance and improved electrochemical and chemical properties, the ion http://www.jese-online.org/ mailto:smittal@thapar.edu j. electrochem. sci. eng. 4(2) (2014) 55-65 zr phosphoborate based heterogeneous membranes 56 exchange membranes find applications in food, drugs, chemical processes and biotechnology industries [2,3]. the great interest in the ion exchange membranes is mainly due to their exceptional electrochemical properties. the ion exchange membranes combine the ability to act as a separation wall between the two solutions, with chemical and electrochemical properties of ion exchanger. the most important of these are the pronounced difference in permeability for counter ions, co-ions and neutral molecules and their high electrical conductivity. when in contact with electrolyte solutions of low or moderate concentrations, the membrane contains a large number of counter ions but relatively few co-ions, due to donnan exclusion. the membrane is perm selective for counter ions. the perm selectivity is reflected not only in differences in permeability, but also in electrical potential difference which arises between the two solutions (membrane potential). the effect of membrane potential and adsorption on the permeability of ions, electrical conductance, ion exchange capacity and perm selectivity behavior on diffusion phenomena in a large number of inorganic membranes was studied by malik et. al. [4-9]. gnusin et. al. [10] investigated the concentration dependence of a wide range of transport characteristics of inorganic ion exchange membranes. to assess the suitability of any membrane for any specific application, two vital parameters i.e. the transport characteristics and its structural properties must be evaluated. the behavior of ion exchange membranes (iems) depends on the physiochemical properties, in addition to the nature of electrolyte solutions used. it is also beneficial to predict the behavior of prepared membranes on the basis of their structural properties. few reports are available on investigation of the effect of nature of electrolyte solutions’ on iems properties despite the fact that they have significant influences [11-16]. in this paper, electrode potential measurements have been used to study electrical characteristics like transport numbers, perm selectivity and fixed charge density across zirconium phosphoborate based heterogeneous ion exchange membranes. halide and nitrate salts of alkali and alkaline earth metals were used as electrolytes. the effect of electrolyte concentration on transport numbers and perm selectivity on the ion exchange characteristics of zirconium phosphoborate membrane has also been studied. the membrane behaved as cation selective under ambient experimental conditions. the counter ion transport number, membrane perm selectivity and concentration of fixed ionic sites in the zirconium phosphoborate membrane are estimated by membrane potential data. fixed ionic concentration data have been analyzed in the light of non-thermodynamic principle for its correlation with membrane structure and permeability behavior. the proposed membrane system has been used to study the selective behavior for the alkali/ alkaline earth metal ions. experimental reagents zirconyl oxychloride (loba chemie, india), boric acid (s.d. fine chem., india) and phosphoric acid (s.d. fine chem., india) were used for synthesis. all other chemicals used were of a.r. grade. standard solutions were prepared by direct weighing of ar grade reagents using double distilled water. synthesis of zirconium phosphoborate the exchanger was prepared by adding zirconyl oxychloride (0.1 m) solution to a continuously stirred mixture of boric acid solution (0.1 m) and phosphoric acid solution (0.1 m) at 60 °c, in the s. kaushal at al. j. electrochem. sci. eng. 4(2) (2014) 45-65 doi: 10.5599/jese.2014.0048 57 volume ratio 2:1:1. the gel produced was allowed in round bottomed flask to stand overnight. then the gel was repeatedly washed with distilled water to remove chlorides from the mother liquor. the absence of chlorides in the mother liquor was tested with agno3 solution. after the gel became free from chlorides, it was filtered through whatmann no.1 filter paper using buchner funnel and suction pump. the gel was transferred from buchner funnel to petri dish. the precipitates in the petri dish were dried in an air oven at 40 °c. when the gel dried completely, distilled water was added. small granules of the ion-exchanger were formed with cracking sound. determination of ion-exchange capacity ion-exchange capacity was determined by column operation. exchanger in the h + -form was placed in a column containing a glass wool support. sodium nitrate solution (1.0 m) was used as an eluent and about 400 ml of it was passed at a rate of 8-10 drops per minute through the ion exchanger column containing 1 g of exchanger. h + eluted from the column was determined titrimetrically against standard solution of sodium hydroxide. preparation of membrane a desired quantity of the finely ground zirconium(iv) phosphoborate (zrpb) was added to epoxy resin in the ratio zrpb : epoxy resin = 80 : 20 (w/w) with constant stirring till a homogeneous slurry was obtained. this slurry was spread between the folds of a filter paper and dried in air to get the membrane of 1 mm thickness. the dried membrane was dipped in distilled water to remove the filter paper. the sheet of the membrane thus obtained was cut with a sharp knife into circular discs of about 18 mm diameter. the membrane discs with good surface qualities were selected for further investigations electrode assembly the membrane was pasted on one end of the electrode assembly using araldite. the membrane was fixed from backside to other part of the electrode assembly. the electrode chambers were filled with 1.0 m solution of each of the electrolytes such as lithium chloride, sodium chloride, potassium chloride, magnesium chloride and barium chloride for 16 hours, to convert it into appropriate ionic form. after equilibration, the electrode assembly was washed with demineralized water (dmw). the membrane was kept immersed in dmw for 2 hours to remove the excess of electrolyte solution. when not in use, the electrode chambers were filled with demineralized water. the electrode assembly was kept immersed in water thermostat maintained at 27 ± 0.1 o c. membrane potential measurements were made using saturated calomel electrodes as reference electrodes. ag/agcl(s), cl ||solution c1|membrane|solution c2|| cl , agcl (s)|ag potential measurements were made for different concentrations of the same electrolyte on two sides of the membrane in such a way that the concentration ratio c2/c1 = 10. the potential difference developed across the membrane was measured against ag/agcl reference electrode on either side of membrane, using a digital potentiometer. the electrode chambers were rinsed with electrolyte solution of next higher concentration and then filled with the same solution. the membrane was allowed to equilibrate for 2 hours and the new potential difference was then noted. the membrane potentials across zirconium phosphoborate membrane were determined using some 1:1 electrolytes such as lithium chloride, sodium chloride, potassium chloride, sodium j. electrochem. sci. eng. 4(2) (2014) 55-65 zr phosphoborate based heterogeneous membranes 58 nitrate, lithium nitrate and potassium nitrate and some 1:2 electrolytes such as magnesium chloride, calcium chloride, strontium chloride, barium chloride, magnesium nitrate, calcium nitrate strontium nitrate and barium nitrate, in overall concentration range of 0.001 m to 0.1 m. the membrane potential measurements were reproducible to ± 0.1 mv. results and discussion membrane potentials a membrane separating electrolyte solutions of unequal concentrations exhibits a difference in electrical potential due to unequal ionic mobility and is equal to liquid junction potential in a non selective membrane. in an ideally selective membrane, according to the tms theory [17], the membrane potential values are related to the activities of the electrolyte on the two sides of membrane, a1 and a2:   2 1 2.303 2 1 logm art e t nf a    (1) where em is membrane potential across the membrane. the maximum electrical potential is given by: 2 m t= 0 max 1 [( ) ] ln art e nf a  (2) provided the solutions are dilute. em changes with change in mean concentration of the electrolyte. from table 1 and 2, it is evident that higher membrane potentials are observed at higher concentrations of electrolytes and membrane potential increases with increase in concentration of the electrolyte. for 1:1 halide electrolytes, membrane potentials are in order li + > k + > na + table 1: membrane potential values of 1:1 electrolytes for zirconium phosphoborate concentration of electrolyte, m membrane potential, v licl nacl kcl lino3 nano3 kno3 0.001-0.01 0.02 0.012 0.02 0.01 0.026 0.016 0.002-0.02 0.03 0.016 0.02 0.016 0.026 0.018 0.003-0.03 0.04 0.017 0.02 0.018 0.027 0.022 0.004-0.04 0.04 0.019 0.02 0.019 0.031 0.023 0.005-0.05 0.05 0.02 0.02 0.02 0.033 0.025 0.006-0.06 0.05 0.02 0.03 0.021 0.035 0.028 0.007-0.07 0.05 0.021 0.02 0.024 0.036 0.027 0.008-0.08 0.05 0.021 0.02 0.025 0.036 0.028 0.009-0.09 0.05 0.022 0.02 0.025 0.037 0.029 0.01-0.1 0.05 0.026 0.01 0.025 0.038 0.028 this order of the membrane potentials prevails in the concentration range 0.01 to 0.06 m, while for 1:1 nitrates, membrane potentials are in the order: k + > na + > li + s. kaushal at al. j. electrochem. sci. eng. 4(2) (2014) 45-65 doi: 10.5599/jese.2014.0048 59 table 2. membrane potential values of 1:2 electrolytes for zirconium phosphoborate concentration of electrolyte, m membrane potential, v mgcl2 cacl2 srcl2 bacl2 mg(no3)2 ca(no3)2 sr(no3)2 ba(no3)2 0.001-0.01 0.021 0.025 0.017 0.034 0.019 0.020 0.0165 0.019 0.002-0.02 0.028 0.032 0.026 0.041 0.027 0.026 0.027 0.026 0.003-0.03 0.033 0.035 0.031 0.042 0.032 0.031 0.032 0.030 0.004-0.04 0.036 0.038 0.034 0.044 0.035 0.034 0.035 0.031 0.005-0.05 0.038 0.040 0.036 0.045 0.038 0.035 0.036 0.032 0.006-0.06 0.040 0.041 0.038 0.046 0.040 0.036 0.037 0.033 0.007-0.07 0.041 0.042 0.039 0.046 0.042 0.037 0.038 0.034 0.008-0.08 0.042 0.043 0.040 0.047 0.042 0.038 0.040 0.034 0.009-0.09 0.042 0.043 0.40 0.047 0.042 0.038 0.040 0.035 0.01-0.1 0.043 0.042 0.038 0.048 0.041 0.039 0.039 0.036 the ionic perm selectivity of membrane is also expressed quantitatively, based on migration of counter ions through cation exchange membrane [11,16,18-26]: )1()(   tttp s where _ t  refers to the value of transport number in the membrane and t+ is the transport number of counter ions in solution [27]. ion exchange capacity of zrpb the ion exchange capacity of zirconium phosphoborate was determined for some monovalent and bivalent cations like na + , k + , mg 2+ and ca 2+ cations and is given in table 3. table 3. ion exchange capacity of zirconium phosphoborate for some alkali and alkaline earth metals sr. no. metal ion salt solution used ion-exchange capacity, eq kg -1 1 na + nacl 0.29 2 k + kcl 0.58 3 ca 2+ cacl2 0.52 4 mg 2+ mgcl2 0.32 the ion exchange capacity of zrpb has been found in the order k + > na + and ca 2+ > mg 2+ . these results suggested that the ion exchange capacity decreases as the radii of hydrated metal ions increase. the magnitude of membrane potential depends on: 1. adsorption of anions of diffusing electrolyte on membrane surface 2. exchangeability of cations 3. size of cations 4. diffusion potential across the membrane higher membrane potential for lithium is because of its small size and high positive field around it, hence, it establishes higher potential across the charged membrane. higher the potential, higher is the permeability [28] as observed in our experiments in the concentration range (0.01 to j. electrochem. sci. eng. 4(2) (2014) 55-65 zr phosphoborate based heterogeneous membranes 60 0.06 m). the trend in the membrane potential and hence greatest permselectivity for lithium ions is because ion mass transfer coefficient through the ion-exchange membrane increases as a function of ion size [29]. ionic radii and hydrated ionic radii smaller the size of the alkali metal less is the ionic radius. as the size increases from top to bottom, ionic radii increase. the extent of hydration depends upon the size of the ion. smaller the size of the ion, more highly it is hydrated and greater is its hydrated ionic radius and less is its ionic mobility (conductance). the radii and mobility of alkali metal ions are given in table 4. table 4. values of ionic radii, hydration numbers, hydrated ionic radii and ionic mobility of alkali metal ions metal ions li + na + k + rb + cs + ionic radius, pm 76 102 138 152 167 hydration number 25.3 16.6 10.5 10.0 9.9 hydrated radius, pm 340 276 232 228 228 ionic mobility 33.5 43.5 64.5 67.5 68 transport numbers the transport number in exchanger phase is calculated from the slope of equation (1). the transport numbers and perm selectivity values of 1:1 halides and nitrates, and the transport numbers of 1:2 halides and nitrates are given in the table 5 and 6, respectively. table 5. transport numbers and perm selectivity values at mean concentration for 1:1 electrolytes mean concentration of electrolyte, m transport no. ( t  ) perm selectivity (ps) metal halide electrolyte metal nitrate electrolyte metal halide electrolyte metal nitrate electrolyte licl nacl kcl lino3 nano3 kno3 licl nacl kcl lino3 nano3 kno3 0.0055 1.08 0.53 0.75 0.45 1.15 0.8 1.12 0.23 0.51 0.179 1.247 0.609 0.011 1.44 0.71 0.67 0.74 1.17 0.85 1.65 0.52 0.35 0.613 1.280 0.706 0.0165 1.66 0.75 0.89 0.81 1.21 0.92 1.98 0.59 0.71 0.717 1.344 0.843 0.022 1.85 0.86 0.93 0.85 1.4 0.94 2.26 0.77 0.86 0.777 1.656 0.882 0.0275 2.02 0.89 0.97 0.92 1.47 0.97 2.51 0.82 0.94 0.881 1.771 0.941 0.033 2.06 0.91 0.99 0.94 1.55 1.03 2.56 0.85 0.98 0.911 1.899 1.059 0.0385 2.09 0.93 0.98 1.1 1.6 1.01 2.6 0.89 0.96 1.147 1.979 1.02 0.044 2.14 0.95 1.01 1.1 1.62 1.04 2.67 0.91 1.02 1.147 2.01 1.078 0.0495 2.16 0.96 1.02 1.12 1.66 1.05 2.7 0.93 1.04 1.176 2.075 1.098 0.055 2.22 1.2 0.39 1.13 1.71 1.03 2.78 0.32 0.19 1.190 2.145 1.059 it is observed that the transport numbers increase with increase in concentration of the electrolytes. this may be due to the fact that with increase in average concentration of the electrolyte, the number of counter ions interacting with the membrane surface increase leading to enhanced donnan exclusion responsible for increase of transport numbers. the obtained results are in contrast with the donnan equilibrium theory. the transport numbers tend to stabilize up to a mean concentration of 0.045 m. thereafter, the values of transport numbers and permselectivity s. kaushal at al. j. electrochem. sci. eng. 4(2) (2014) 45-65 doi: 10.5599/jese.2014.0048 61 tend to stay constant. this is due to concentration polarization phenomenon at high concentration resulting in increased co-ion percolation and hence resisting further increase in transport number of cations. table 6. transport numbers at mean concentration for 1:2 electrolytes mean concentration of electrolyte, m transport no. t  mgcl2 cacl2 srcl2 bacl2 mg(no3)2 ca(no3)2 sr(no3)2 ba(no3)2 0.0055 1.43 1.60 1.25 2.01 1.34 1.39 1.25 1.34 0.011 1.74 1.92 1.65 2.32 1.70 1.65 1.65 1.65 0.0165 1.96 2.05 1.87 2.37 1.90 1.87 1.87 1.83 0.022 2.09 2.2 2.0 2.45 2.0 2.0 2.00 1.87 0.0275 2.18 2.27 2.09 2.50 2.18 2.05 2.09 1.92 0.033 2.27 2.31 2.18 2.54 2.27 2.09 2.18 1.96 0.0385 2.31 2.36 2.23 2.54 2.35 2.14 2.23 2.0 0.044 2.36 2.40 2.27 2.59 2.35 2.18 2.27 2.0 0.0495 2.36 2.40 2.27 2.59 2.35 2.18 2.27 2.05 0.055 2.35 2.36 2.18 2.56 2.31 2.16 2.18 2.02 fixed charge density the electrical character of a membrane is expressed in terms of fixed charge density. this fixed charged density of zirconium phosphoborate membrane for 1:1 electrolytes has been evaluated by using kobatake’s equation [30] and is given in table 7. table 7. values of parameters α,  and x for zirconium phosphoborate membrane electrolyte α  ̅ licl 0.80 2.04 0.0075 nacl 0.56 4.79 0.0078 kcl 0.60 3.83 0.0047 lino3 0.56 4.42 0.0072 nano3 0.68 2.55 0.0024 kno3 0.62 4.34 0.0011 mgcl2 0.67 0.34 1.51 cacl2 0.69 0.23 2.22 srcl2 0.70 0.26 1.94 bacl2 0.75 0.60 0.61 mg(no3)2 0.57 6.22 .0036 ca(no3)2 0.57 1.15 0.49 ba(no3)2 0.62 0.76 0.67 sr(no3)2 0.61 0.13 4.69 when negatively charged membrane separates solutions of electrolyte of different concentrations, the membrane potential is given by: 22 m 1 1 1 1 ln 1 2 ln c xcrt e f c c x                   (3) j. electrochem. sci. eng. 4(2) (2014) 55-65 zr phosphoborate based heterogeneous membranes 62 where u u v    and 1 kfx u    em = membrane potential difference, u and v = molar mobilities of cation and anion, respectively, k = a constant depending on the solution viscosity, x = fixed charged density, f = faraday constant. in order to evaluate ,  and x , kobatake has derived two useful limiting forms, (a) and (b) of equation (3) a) when c2 << equation (3) may be written as: σ 2 m 1 1 1 ln 1 2 c e x                      (3a) where, σ me = absolute value of membrane potential given by m m fe e rt   and 2 1 c c   from the plot of σ me vs. c2 in low concentration region, a straight line with an intercept equal to 1/ ln δ is obtained (figure 1). thus can be calculated. b) at fixed δ, inverse of apparent transport number 1/ +app t for a coion species in a negatively charged membrane, varies linearly with the inverse of concentration c2 at higher electrolyte concentrations, where 1/ +appt is defined as +app(1 2 )lnme t    substituting for σ me in equation (3) and expanding resultant for expression for 1/ +appt in power of 1/c2 gives 2 +app 2 1 1 (1 2 )( 1) 1 2(1 ) ln x t c                    (3b) figure 1. variation of membrane potential with the concentration of electrolyte s. kaushal at al. j. electrochem. sci. eng. 4(2) (2014) 45-65 doi: 10.5599/jese.2014.0048 63 from equation (3b), it is clear that a plot of 1/ +appt vs. 1/c2 (figure 2) at fixed δ value should be a straight line with an intercept equal to 1/(1- , from where  can be calculated. figure 2. variation of apparent transport number with reciprocal of concentration for determining the fixed charge density x in dilute concentration range, the value of slope determined from the plot of σ me vs. c2 plot was equated with the slope of equation (3a). α and  being known earlier, x can be calculated. slope = 1 1 1 1 2 x             (4) apparent transport number 1/ +app t indicates transport number of a metal ion in the exchanger whereas the transport number of the same metal ion in solution phase is represented by t+. as cited in the literature [31], transport numbers of alkali metal ions in an aqueous system generally increase sharply with initial increase in concentration in low concentration range and then become constant at higher concentration. this is true for all alkali metal ions including li + , na + and k + . as shown in figure 2, the apparent transport numbers for na + and k + do not change at all, whereas for li + , an appreciable linear increase in trend is observed, which indicates that the ion exchanger matrix is selective for some metal ions. the selective behavior of the ion exchange membrane for li + ions may be either due to steric or electronic reasons. this can’t be due to steric reasons because hydrated radii of all the alkali metal ions are almost of the same size. li + in dehydrated form has large charge to radius ratio as compared to that of na + and k + . the different trends of transport number in solution and in membrane phase confirm that the membrane is highly selective for li + over na + and k + . this property of the membrane can also be generalized for transition metal ions as well because metal ions of lanthanide series are also different from one another due to their electronic properties while their ionic radii (steric factor) do not change much. j. electrochem. sci. eng. 4(2) (2014) 55-65 zr phosphoborate based heterogeneous membranes 64 it is observed that +appt decreases with increase in mean concentration of the lithium chloride electrolyte. these membrane permeate interactions indicate crystalline morphology [32], being more in amorphous and less in crystalline membranes. the low values indicate very low degree of crystallinity of exchanger material. the observed values (table 7) of fixed charge density x are much lower than those expected from the fixed charge concentration of the exchanger. it indicates that larger part of internal fixed charge remains inactive. it may be due to the reason that active fixed charges in these membranes are essentially those of external surface of grain. as observed in table 7, the magnitude of x values for alkali metal nitrates is lower than for alkali metal chlorides. hence, the trend in fixed charge density observed for halide and nitrate salts indicates that donnan exclusion is more applicable for halide salts than for nitrate salts conclusions the present investigation shows that ion exchange capacity of the ion exchanger decreases with the increase in the radii of hydrated metal ions. the transport numbers increase with increase in average concentration of the electrolyte due to enhanced donnan exclusion at low electrolyte concentration. the transport numbers decrease at high concentration due to concentration polarization phenomenon. hence, zirconium phosphoborate membrane shows better characteristics at lower concentrations (up to 0.045 m) beyond which no appreciable change in activity of the membrane is noticed and remains almost constant. the proposed ion exchange membrane behaves much more selectively for alkaline earth metal ions than alkali metal ions as observed from their respective fixed charge density values. acknowledgement: sk and pps gratefully acknowledge punjab technical university (ptu), jalandhar for permission to work on the project. skm is thankful to director, thapar university, patiala for the support. references [1] k. g. varshney, u. gupta, bull. chem. soc. 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[32] r. k. nagarale, g. s. gohil, v. k. shahi, r. rangrajan, colloids surf. a 251 (2004) 133-140. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ electrochemical treatment of leachates from sanitary landfills doi: 10.5599/jese.2013.0034 125 j. electrochem. sci. eng. 3(3) (2013) 125-135; doi: 10.5599/jese.2013.0034 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical treatment of leachates from sanitary landfills annabel fernandes, edite catalão, lurdes ciríaco, maria j pacheco and ana lopes umtp and department of chemistry, university of beira interior, 6201-001 covilhã, portugal corresponding author: e-mail: e-mail:annabelf@ubi.pt; tel.: +351275329259; fax: +351275319730 received: december 19, 2012; published: june 12, 2013 abstract the electrochemical treatment of leachate samples from a portuguese intermunicipal sanitary landfill was carried out using anodic oxidation. the treatment was performed in a pilot plant that possesses an electrochemical cell, with boron-doped diamond electrodes, working in batch mode with recirculation. the influence of the applied current density and the flow rate on the performance of the electrochemical oxidation was investigated. current density was decreased by steps, during the degradation, in order to study this effect on the efficiency of the process. for the assays run at equal flow rate and initial current intensity, chemical oxygen demand (cod) removal seems to depend mainly on the charge passed and the variation of the current density during the anodic oxidation process can reduce the energetic costs. an increase in the recirculation flow rate leads to an increase in the organic load removal rate and a consequent decrease in the energetic costs, but it decreases the nitrogen removal rate. also, the bias between dissolved organic carbon and cod removals increases with flow rate, indicating that an increase in recirculation flow rate decreases the mineralization index. keywords landfill leachate treatment; bdd; anodic oxidation. introduction leachate generation is an inevitable consequence of the deposition of solid wastes in sanitary landfills. it is the result of rainwater percolation through wastes, that extracts and brings with it several pollutant materials dissolved and in suspension [1]. sanitary landfill leachate composition is very complex and depends mainly on the type of solid wastes that are deposited, the climatic conditions and the age of the sanitary landfill [2]. inadequate leachate management involves considerable risks, particularly contamination of water resources, at the surface and groundwater, and soils [1]. a common treatment for sanitary landfill leachates comprises biological reactors with nitrification/denitrification steps, followed by membrane technologies. however, due to variability http://www.jese-online.org/ mailto:annabelf@ubi.pt j. electrochem. sci. eng. 3(3) (2013) 125-135 electrochemical treatment of leachates 126 in the quality and quantity of leachate throughout the life span of the treatment plant, these conventional treatments become ineffective. thus, it is necessary to implement technologies that can be adjusted to the in situ needs [3]. electrochemical technologies have shown high efficiency in the elimination of persistent pollutants and several studies have described the application of electrochemical methods in wastewater treatment [4-10]. a promising electrochemical method that can be used in wastewater treatment is the anodic oxidation. despite several different materials are being used as anodes in the oxidation of persistent pollutants, the best results are obtained with boron-doped diamond (bdd) anodes, due to their unique chemical, electrochemical and structural stabilities that allow their use at high potentials, where most organic pollutants can be oxidized [11-13]. there are already several reports describing the application of electrochemical oxidation with bdd anodes for the treatment of landfill leachates [14-24]. cabeza and co-workers [14,15] reported the application of electrochemical oxidation process, using a bdd anode, to treat raw leachates and biologically and physicochemically pre-treated leachates from a municipal landfill site. experimental results showed very high chemical oxygen demand (cod) and ammonia removals, although ammonia removal was slower than that of cod. they also observed that when additional chloride ions were provided, the treatment efficiency increased. anglada and co-workers [17,20,21] also studied the effect of the applied current density and of the initial concentration of chloride ions, as well as the influence of other operating conditions, such as treatment time and initial ph, in the electrochemical oxidation of landfill leachates, using a bdd anode. they have shown that when high current densities are applied, a change in the mechanism of the organic matter oxidation occurs and that organic matter and ammonia oxidation are highly influenced by the applied current density [17]. also, they have reported that the concentration of chloride has an effect on the oxidation of ammonia and that chloride ions compete with organic matter to be oxidized at the anode. it was found that some chlorinated organic compounds are formed as a result of organic matter oxidation and their concentration increase continuously with treatment time [21]. acidic conditions were found to favour the formation of haloacetonitriles and haloketons. a kinetic modelling of the electrochemical removal of ammonium and cod from landfill leachates was proposed in literature [23-24]. authors found that the use of bdd anodes promotes the generation of hydroxyl radicals, while the high content of chloride induces the simultaneous formation of free chlorine, responsible for the ammonium indirect oxidation and for the formation of undesirable products such as chloramines, chlorate and perchlorate. chlorine evolution is enhanced at lower cod concentrations. during this process, ammonium removal leads to the formation of nitrogen gas and nitrate as the main oxidation products. in this work, the influence of the raw leachate dilution on the electrochemical degradation of a biologically pre-treated leachate from a sanitary landfill, using a bdd anode, was assessed and it has shown that mineralization of the organic matter improves with the dilution of the leachate [22]. however, an increase in the dilution greatly increases the energy consumption. the aim of this work was to study the influence of flow rate and applied current density, carried out with multiple step electro-oxidation, on the performance of the electrochemical oxidation of raw leachate from a sanitary landfill. the energy consumption in the different experimental conditions tested was also assessed. a. fernandes et al. j. electrochem. sci. eng. 3(3) (2013) 125-135 doi: 10.5599/jese.2013.0034 127 experimental the leachate samples used in this study were collected at a portuguese intermunicipal sanitary landfill site, in the equalization tank, before any kind of treatment. samples characterization is presented in table 1. experiments were conducted in a semi-pilot plant operating in batch mode with recirculation, at room temperature and natural ph, without adding background electrolyte. a bdd diacell 100 electrochemical cell, with an electrode area of 70 cm 2 , and a diacell-ps1500 power supply, with automatic polarity reversal, were used. in all assays, automatic polarity reversal occurred every minute. different current densities, between 50 and 200 ma cm -2 , and different flow rates, between 100 and 950 l h -1 , were tested in sample volumes of 5, 10 or 15 l. during the degradation process, current density was kept constant or decreased by steps, in order to study this effect on the efficiency of the process. potential differences between anode and cathode were registered throughout the experiments in order to determine energetic consumptions. all assays were performed in duplicate. degradation tests were followed by chemical oxygen demand (cod), dissolved organic carbon (doc), total nitrogen (tn), total kjeldahl nitrogen (tkn) and ammonia nitrogen (an). table 1. physicochemical characteristics of the raw leachate. parameter medium value sd* cod, g l -1 8.9  0.8 bod5, g l -1 ** 1.3  0.3 bod5/cod 0.15 0.05 doc, g l -1 3.5  0.4 tn, g l -1 2.8  0.2 tkn, g l -1 2.4  0.2 an, g l -1 2.2  0.3 cchloride / g l -1 4.5  0.3 csuspended solids / g l -1 0.7  0.1 cdissolved solids / g l -1 16.6  0.1 ph 8.3  0.2 conductivity, ms cm -1 29.1  1.0 *sd standard deviation; **bod5 – biochemical oxygen demand cod determinations were made using the closed reflux and titrimetric method [25]. doc and tn were measured in a shimadzu toc-v csh analyser. before doc and tn determinations, samples were filtrated through 1.2 µm glass microfiber filters. tkn and an were determined according to standard procedures using a kjeldatherm block-digestion-system and a vapodest 20s distillation system, both from gerhardt [25]. results and discussion the effect of the applied current density on the rate of electrochemical oxidation was studied by performing the electrodegradation assays at three different current intensities, 4, 7 and 14 a, at a flow rate of 360 l h -1 , and using leachate volumes of 15, 5 and 5 l, respectively. figure 1 presents j. electrochem. sci. eng. 3(3) (2013) 125-135 electrochemical treatment of leachates 128 the results of the normalized cod variation with time and with specific charge passed for these electrodegradation assays. specific charge was calculated as it/v, in c l -1 , where i is the current intensity, in a, t is the time, in s, and v is the leachate volume, in l. figure 1. (a) normalized cod variation with time for the electrodegradation assays performed at different current intensities, at a flow rate of 360 l h -1 . (b) normalized cod variation with specific charge for the electrodegradation assays performed at different current intensities, at a flow rate of 360 l h -1 . error bars refer to the standard deviation of the cod mean values. it can be observed (fig. 1a) that, for the assays performed with equal leachate volume cod removal rate increases with current density, which points to electrolysis operating under charge transfer control. in fact, for a single-compartment electrolytic reactor similar to the one used in this work, operating at flow rates of 200 and 600 l h -1 , mass transport coefficients, km, of 1.39x10 -5 and 1.5x10 -5 m s -1 , for 200 l h -1 , and 2.2x10 -5 m s -1 , for 600 l h -1 , are presented in literature [26-28]. with these km values from literature, limiting currents between 10.4 and 16.5 a were obtained, a. fernandes et al. j. electrochem. sci. eng. 3(3) (2013) 125-135 doi: 10.5599/jese.2013.0034 129 showing that at least two of the assays presented in fig. 1 started at current limited control conditions. according to the model previously proposed in the literature for electrolysis under current limited control [29], i.e., at maximum current efficiency, the trend of cod during electrochemical oxidation can be predicted by eq. (1), where f is the faraday constant, 96485 c mol -1 , and v is the volume of the samples treated, in m 3 . thus, theoretical slopes of cod vs. time, i/4fv, can be calculated for each of the assayed current intensities.   0 i cod cod 4 t t fv   (1) the comparison between these theoretical slopes and the experimental curves (fig. 1a), for equal recirculation volume, shows that the discrepancy between experimental data and predicted slopes slightly decreases with current density. this can be explained if one assumes that the degradation process happens also by indirect oxidation. the increase in the leachate recirculation volume also seems to contribute to an increase in the efficiency of the process, due to the lower ratio electrode area/treated volume. in fact, when the volume is increased, keeping the same anodic area, the quantity of the compounds that are more easily degraded and that behave ideally augments. thus, their concentration is kept higher for longer times when the recirculation volume is increased. the effect of applied current on the trend of the cod with the specific charge consumed during the treatment (fig. 1b) is less pronounced than the effect on the variation of cod vs. time. for equal leachate volume, an increase in current density leads to a more efficient use of the electric charge, since the experimental curve for 14 a is closer to the theoretical prediction. however, since higher current densities imply higher potentials, although the electric charge is more efficient the energetic consumption can be higher. figure 1b also shows that an increase in the leachate recirculation volume approaches the experimental results to the theoretical prediction. to try to improve the current efficiency, assays were performed with successive decreases in current density, by steps, during the oxidation process, at different recirculation flow rates. in figure 2 it can be observed the variation of normalized cod with specific charge for the assays run at constant (14 a) and variable current density (5 h at 14 a + 5 h at 7 a + 10 h at 4 a), at a flow rate of 360 l h -1 . cod removal seems to depend only on the charge passed. variation of normalized cod with time (fig. 2, inset) shows that during the first five hours, where the applied current density was equal, no difference can be seen in the cod removal rate. but, when the applied current density decreases, in the assays with steps, a decrease in the cod removal rate can be observed. the influence of the recirculation flow rate in the electrochemical oxidation performance of the assays that were run with current density decreased by steps during the experiment was also studied. figure 3 shows the normalized cod variation with the specific charge consumed for the assays performed with three or four current density steps at different recirculation flow rates: three steps, 5 h at 14 a + 5 h at 7 a + 10 h at 4 a, flow rates of 160 and 360 l h -1 , leachate volume 5 l; four steps, 4 h at 14 a + 4 h at 10.5 a + 4 h at 7 a + 4 h at 4 a, flow rates of 100 and 950 l h -1 , leachate volume 10 l. j. electrochem. sci. eng. 3(3) (2013) 125-135 electrochemical treatment of leachates 130 figure 2. normalized cod variation with specific charge passed and with time (inset) for the electrodegradation assays performed at constant and variable current intensity, at a flow rate of 360 l h 1 , with a leachate volume of 5 l. error bars refer to the standard deviation of the cod mean values. the theoretical curves at these conditions are also presented. a slight variation in the trend of the cod depletion was observed, pointing to better removals at higher recirculation flow rates. the same behaviour is observed when normalized cod variation with electrolysis time is plotted (fig. 3, insets). the discrepancy between experimental values and theoretical curves, after the first step of the assays, indicates a high loss in current efficiency that increases when current density is further decreased during the steps process. comparing the discrepancy between experimental values and theoretical curves for three and four steps, it can be concluded that charge efficiency is higher when four steps are applied, although this fact must be also related with the higher recirculation volume of leachate used in the four steps experiments. nitrogen removal was also assessed. in figure 4 are plotted the normalized variation with time of ammonia nitrogen and of total nitrogen. both parameters present similar behaviour of that described for cod in these assays, i.e., a decrease in the applied current density, in the steps assays, leads to a decrease in the nitrogen removal rates. it can be seen that, for the experiments performed at 14 a, an increase in the removal rate is observed after eight hours assay. this fact is consistent with previous reports from other authors [23], which indicate that while bdd anodes promotes the generation of hydroxyl radicals, the high content of chloride induces the simultaneous formation of free chlorine, causing indirect oxidation of ammonium. in fact, this leachate presents high chloride concentration (4.5 g l -1 ), thus enhancing the chlorine evolution at lower cod concentrations, justifying the increase in the nitrogen removal rate when cod levels are lower. in contrast to what was observed with cod removal, nitrogen removal is higher when the recirculation flow rate is lower. in fact, at higher flowrates cod oxidation is favoured, chlorine a. fernandes et al. j. electrochem. sci. eng. 3(3) (2013) 125-135 doi: 10.5599/jese.2013.0034 131 evolution, that is a competitive reaction, is delayed as a consequence and thus it influences and slows down the rate of ammonium removal. in order to analyse the energy consumption, the specific energy consumptions, esp, in w h/ g cod removed were calculated, by means of eq. (2): cod sp ui t e v    (2) where u is the cell voltage, in v, resulting from the applied current intensity i, in a, t is the duration of the electrolysis, in h, v is in m 3 and cod is the removed cod, in g m -3 , during t. figure 3. normalized cod variation with specific charge and with time (inset) for electrodegradation assays performed with (a) three current density steps, with a leachate volume of 5 l and with (b) four current density steps, with a leachate volume of 10 l. error bars refer to the standard deviation of cod mean values. j. electrochem. sci. eng. 3(3) (2013) 125-135 electrochemical treatment of leachates 132 figure 4. (a) normalized ammonia nitrogen variation with time for the electrodegradation assays performed at constant current density and at three and four current density steps. (b) normalized total nitrogen variation with time for the electrodegradation assays performed at constant current density and at three and four current density steps. figure 5 reports the specific energy consumption as a function of the time for the different assays performed. the specific energy consumption seems to increase with current density (fig. 5a), which is a consequence of the increase in potential when the current density is increased. when constant current density was imposed (fig. 5a), there is an increase in the energy consumption during the first part of the assay, followed by a decrease. this behavior must be due to the different types of compounds that are present and that are not degraded simultaneously, being first degraded those that are present in higher concentration and, among them, those who have higher diffusion coefficients. the introduction of steps, although leads to a overall decrease in the energetic consumption, did not present the expected results in terms of specific energy consumption, since it leads to more irregular consumptions rather than lower consumptions a. fernandes et al. j. electrochem. sci. eng. 3(3) (2013) 125-135 doi: 10.5599/jese.2013.0034 133 (fig. 5b). for these assays, an increase in the recirculation flow rate seems to slightly decrease the esp (fig. 5c and 5d). on the other hand, the increase in the leachate volume being recirculated really decreases the specific energy consumption, since the values in the yy’ axis are much lower in fig. 5d (10 l) than in fig. 5c (5 l). figure 5. evolution of specific energy consumption with time for (a) electrodegradation assays performed at different current densities, at a flow rate of 360 l h -1 (b) electrodegradation assays performed at constant and variable current density, at a flow rate of 360 l h -1 (c) electrodegradation assays performed with three current density steps at different recirculation flow rates, with a leachate volume of 5 l (d) electrodegradation assays performed with four current density steps at different recirculation flow rates, with a leachate volume of 10 l. the removals in cod, doc, tn, tkn and an for all assays performed with current density decreased by steps, as well as the medium specific energy consumption, are presented in table 2. this table includes also the results obtained in the assay performed at constant current intensity of 14 a and 360 l h -1 recirculation flow rate, in order to allow comparison between assays performed with and without reduction in the current intensity during the assay. the apparent discrepancy between absolute and percentage values presented in table 2 is due to the variation of the experimental determinations of those parameters for the different assays, due to the complexity and heterogeneity of the leachate suspension. data reported confirm the previous analysis, showing that for both multiple step designs, with 3 or 4 current density steps, and for a wide range of recirculation flow rate, from 100 to 950 l h -1 , an increase in the recirculation flow rate increases cod removal rate and decreases nitrogen removal rate (tn, tkn and an). also, it can be seen that doc removals are always lower than cod removals and these differences increase with flow rate, indicating that a decrease in the flow rate increases the mineralization index. regarding the energy j. electrochem. sci. eng. 3(3) (2013) 125-135 electrochemical treatment of leachates 134 consumption, an increase in the recirculation flow rate leads to a decrease in the medium energy consumption, mainly because cod removal rate increases with recirculation flow rate. table 2. cod, doc, tn, tkn and an removals and medium specific energy consumption for assays performed with one, three and four current density steps at different recirculation flow rates. parameter experimental conditions 16 h (14 a) v = 5 l; t = 16 h 5 h (14 a) + 5 h (7 a) + 10 h (4 a) v = 5 l; t = 20 h 4 h (14 a) + 4 h (10.5 a) + 4 h (7 a) + 4 h (4 a) v = 10 l; t = 16 h 360 l h -1 160 l h -1 360 l h -1 100 l h -1 950 l h -1 cod removal g l -1 % 5.42 69 3.11 41 4.09 50 2.56 25 2.57 34 doc removal g l -1 % 1.35 44 0.50 19 0.94 30 0.61 15 0.18 6 tn removal g l -1 % 1.23 48 1.04 39 0.93 35 0.59 19 0.36 15 tkn removal g l -1 % 1.72 72 1.11 53 1.06 45 0.83 35 0.33 15 an removal g l -1 % 1.66 80 0.99 60 0.95 45 0.78 32 0.24 14 spe / kw h (kg cod) -1 90.1 106.0 80.9 55.7 49.5 conclusions the anodic oxidation was used to treat leachate from an intermunicipal sanitary landfill and the following conclusions can be drawn:  organic load removal rate increases with applied current density. this happens mainly because, due to the high organic load content, the electrochemical processes are under current control most of the assay period.  an increase in the recirculation flow rate leads to an increase in the organic load removal rate. however, it decreases the nitrogen removal.  by reducing the current density along the anodic oxidation process it is possible to reduce energetic costs. similar results can be obtained by increasing the recirculation flow rate.  doc removals are always lower than cod removals and these differences increase with flow rate. thus, a decrease in flow rate seems to increase the mineralization index. thus, although huge variations can be found in the composition of leachates from sanitary landfills, the anodic oxidation, performed with a bdd anode, can be an alternative/complement to treat this kind of wastewaters. also, the variation found in the medium specific energy consumption shows that it is possible to optimize the process in order to reduce energy costs. acknowledgements: financial support from feder, programa operacional factores de competitividade – compete, and fct, for the projects ptdc/aacamb/103112/2008 and pestoe/ctm/ui0195/2011 of the mt&p unit and for a. fernandes grant to sfrh/bd/81368/2011. references [1] t. eggen, m. moeder, a. arukwe, science of the total environment 408 (2010) 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this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://www.sciencedirect.com/science/journal/13882481 http://www.sciencedirect.com/science/journal/13882481 http://creativecommons.org/licenses/by/3.0/ {sensitive determination of uric acid at layered zinc hydroxide-sodium dodecyl sulphate-propoxur nanocomposite:} http://dx.doi.org/10.5599/jese.1237 331 j. electrochem. sci. eng. 12(2) (2022) 331-341; http://dx.doi.org/10.5599/jese.1237 open access : : issn 1847-9286 www.jese-online.org original scientific paper sensitive determination of uric acid at layered zinc hydroxidesodium dodecyl sulphate-propoxur nanocomposite mohamad hafiz ahmad tajudin1,2, mohamad syahrizal ahmad2,3, , illyas md isa2,3, norhayati hashim2,3, anwar ul-hamid4, mohamad idris saidin2,3, rahadian zainul5 and suyanta m. si6 1faculty of applied sciences, universiti teknologi mara, perak branch, tapah campus, tapah road, 35400 perak, malaysia 2department of chemistry, faculty of science and mathematics, universiti pendidikan sultan idris, 35900 tanjong malim, perak, malaysia 3nanotechnology research centre, faculty of science and mathematics, universiti pendidikan sultan idris, 35900 tanjong malim, perak, malaysia 4center for engineering research, research institute, king fahd university of petroleum & minerals, dhahran 31261, saudi arabia 5department of chemistry, faculty of mathematics and natural science, universitas negeri padang, west sumatera 25171, indonesia 6department of chemistry education, faculty of mathematics and natural science, yogyakarta state university, indonesia corresponding author:  syahrizal@fsmt.upsi.edu.my; tel.: +60-196266485; fax: +60-54506000 received: january 15, 2022; accepted: february 15, 2022; published: february 26, 2022 abstract an electrochemical chemical sensor for the determination of uric acid (ua) with high sensitivity and a wide working range was fabricated using the layered zinc hydroxide-sodium dodecyl sulphate-propoxur (lzh-sds-pro) nanocomposite, modified with multiwall carbon nanotubes (mwcnt). the introduction of lzh-sds-pro as a conducting matrix has enhanced the conductivity of mwcnt. the morphology of lzh-sds-pro/mwcnt was characterized by transmission electron microscopy (tem) and scanning electron microscopy (sem), while electrochemical behavior of ua and k3[fe(cn)6] at lzh-sds-pro/mwcnt paste electrode was studied by square wave and cyclic voltammetry, respectively. under the optimized experimental conditions, the electrode established linear plot for ua concentrations 7.0 mol l-1 to 0.7 mmol l-1 (r2 = 0.9920) and lod was calculated to be 4.28 µmol l-1 (s/n = 3). the fabricated lzh-sds-pro/mwcnt electrode was successsfully applied to urine samples, exhibiting excellent stability and reproducibility, which made it worthwhile for analytical applications. keywords electrochemical sensor; pharmaceutical sensor; modified mwcnt; layered metal hydroxide; functional nanocomposite; square wave voltammetry http://dx.doi.org/10.5599/jese.1237 http://dx.doi.org/10.5599/jese.1237 http://www.jese-online.org/ mailto:syahrizal@fsmt.upsi.edu.my j. electrochem. sci. eng. 12(2) (2022) 331-341 determination of uric acid 332 introduction 2,6,8-trihydroxypurine, also known as uric acid (ua), is a fundamental electroactive molecule resulting from the metabolism of endogenous purine, which occurs inside the human body [1,2]. biological responses such as inflammation, vasoconstriction, oxidative stress and endothelial dysfunction can be stimulated by ua. identifying diseases such as gout, hyperuricemia, kidney stone, type-2 diabetes, renal impairment, and lesch-nyan syndrome can be done by quantifying the concentration level of ua in blood and urine [3,4]. various high precision, accurate and robust methods have been developed for analytical purposes, such as chromatography, fluorescence, electrophoresis, chemiluminescence and spectrophotometry, but all these methods are energy-consuming, high-cost, time-consuming and need complex operating processes [5-11]. since ua is highly electrochemically active compound, alternative electrochemical methods were developed and characterized by cost-effectiveness, fastness, simplicity and portability for quantitative analysis. different functional materials have been introduced in developing ua sensors to improve the electron transfer of ua at electrodes, such as co(ii)-based zeolitic imidazolate framework [12], platinum nanoparticles (ptnps) [13,14], nano resin [15] and ferrocene derivative and core-shell magnetic nanoparticles [16]. since the first discovery of carbon nanotubes (cnt) in 1991, various materials have been incorporated into them and attracted widespread attention in the field of electroanalysis [17]. the unique electrochemical properties of cnt in terms of large surface area, excellent electron transfer, fine structure and light-weight make them be good electrodes for various applications [18-20]. in recent studies, multi-walled carbon nanotubes (mwcnt) were more preferable than single-walled cnt (swcnt) due to rapid electron transfer for different reactions and better conductivity [21]. besides, metal layered hydroxides have also gained much attention in carbon paste electrode (cpe) fabrication, owing to their remarkable capability of anion exchange and also excellent physicochemical properties in terms of low toxicity, chemical inertness and high surface area [22]. in our previous work, abilities of layered zinc hydroxide-ferulate (lzh-f), layered zinc hydroxide-l-phenylalanate (lzh-lp) and layered zinc hydroxide-sodium dodecyl sulphate-isoprocarb (lzh-sds-iso) applied as modifiers with mwcnt were demonstrated for the determination of hydroquinone (hq) [23], acetaminophen (pcm) [24] and dopamine (dop) [25], respectively. propoxur or 2-isopropoxyphenyl-n-methylcarbamate, was introduced to the market in 1959 and widely used in the pest control industry. intercalation of propoxur with layered zinc hydroxide produces a nanocomposite material that has good electron transfer ability and a large surface-tovolume ratio. it has also been reported that this material shows low toxicity, high thermal stability, biocompatibility and the potential for controlled release [26,27]. based on the listed advantages, in this work, we are presenting for the first time lzh-sds-pro material as a mediator used to increase the electrocatalytic activity of the redox reaction important for the determination of ua. experimental chemicals and reagents the uric acid stock solution was prepared by dissolving the appropriate amount of ua in 0.1 mol l1 naoh solution. potassium acetate, chloride salts (sigma-aldrich, usa), paraffin oil, copper(ii) sulphate, potassium nitrate, potassium iodide, phosphate buffer solution (pbs) (k2hso4 and kh2so4) (merck, germany), barium chloride, glucose, fructose, sucrose and mwcnt (timesnano, china) were of analytical grade and used as received. ultra-pure water was used during the work. m. h. ahmad tajudin et al. j. electrochem. sci. eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 333 apparatus fesem model su8020 uhr (hitachi, japan) and fetem model jem2100f (jeol, japan) were used for the characterization of surface morphologies of lzh-sds-pro/mwcnt and mwcnt. potentiostat/ galvanostat model ref 3000 (usa) was used for electrochemical impedance spectroscopy (eis) measurements, while cyclic voltammetry (cv) and square wave voltammetry (swv) were conducted using potentiostat series-g750 (usa). three-electrode system was used in this study, where lzh-sdspro/mwcnt, a platinum wire and ag/agcl electrode mf-2052 (bioanalytical syst, usa) with fiber junction, acted as the working electrode, counter electrode and reference electrode, respectively. preparation of lzh-sds-pro nanocomposite lzh-sds was prepared by the addition of 40 ml of 0.5 m of zn(no3)2·6h2o and 1.0 m naoh into a solution of 40 ml of 0.25 m sds. the ph value was adjusted to 6.5. then, the slurry was centrifugated and dried in an oven at 70 °c. intercalation of propoxur into the interlayer of lzh-sds was done by an ion-exchange method. 0.5 g of lzh-sds was dissociated in 0.001 m propoxur solution and kept under a magnetic stirrer for 3 hours. the slurry was then aged 24 hours in an oil bath at 70 °c. after that, the slurry was centrifuged and the white solid was dried in an oven [26]. electrode preparation lzh-sds-pro (5 mg), mwcnt (100 mg) and paraffin oil (3 drops) were mixed using mortar and pestle. the homogenized mixture was firmly packed into teflon tubing (i.d. 2.0 mm and 3 cm long). to establish the electrical contact, one of the ends of the paste was connected to the copper wire, and the other end was smoothened using soft paper. the non-modified mwcnt electrode was prepared with the same method but without lzh-sds-pro added. measurement procedure ua solutions present at desired concentrations in 0.1 mol l-1 pbs (ph 6.4) as the supporting electrolyte were used throughout the work to perform voltammetry unless otherwise stated. the ua solutions were deoxygenated with n2 before measurements for about 15 minutes. swv experiments of ua determination were performed between 100 mv to 500 mv, with a frequency of 150 hz, pulse height of 60.0 mv and step increment of 6.0 mv. cvs taken between -300 to 800 mv at the scan rate of 100 mv s-1 were applied for electrochemical characterization of the fabricated electrode with k3[fe(cn)6] as a redox probe. for eis measurements, the frequency range of 10 khz to 0.1 hz and amplitude of the alternating voltage of 5.0 mv were used. all experiments were conducted at the ambient temperature of 25 ± 1 oc. results and discussion surface morphology the sem image of lzh-sds-pro shown in figure 1, resembles nanoflower-like particles with a thickness of approximately 1.5 – 3.0 µm. to confirm the formation of lzh-sds-pro nanocomposite, eds analysis was carried out. different areas were focused during the eds measurement and the conforming peaks were observed. the lzh-sds-pro can be seen in the synthesized composite nanostructure in the eds spectrum. in the spectrum a, the quantity of c, zn and o (measured in wt.%) were 40.1, 29.4 and 22.6, respectively, while in spectrum b, the values were 50.3, 29.5 and 17.0 % for c, zn and o, respectively. http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(2) (2022) 331-341 determination of uric acid 334 figure 1. sem image and eds spectrum of (a) lzh-sds-pro and (b) lzh-sds-pro/mwcnt tem analysis was performed to further investigate the morphology of the lzh-sds-pro composite. figure 2 shows tem images of the composite nanostructure at a low magnification where sharp edges and smooth surface of lzh-sds-pro (a) and carbon nanotubes strings (b) were observed, confirming the results from sem images. figure 2. tem image of (a) lzh-sds-pro and (b) lzh-sds-pro/mwcnt electrochemical response of k3[fe(cn)6] at lzh-sds-pro/mwcnt paste electrode figure 3 shows cv voltammograms of 4.0 mmol l-1 k3[fe(cn)6] contained in 0.1 mol l-1 kcl at the lzh-sds-pro/mwcnt and unmodified mwcnt paste electrodes. figure 3. cyclic voltammograms of (a) non-modified mwcnt and (b) lzh-sds-pro/mwcnt for 4.0 mmol l-1 k3[fe(cn)6] in 0.1 mol l-1 kcl, at scan rate 100 mv s-1 a b a b m. h. ahmad tajudin et al. j. electrochem. sci. eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 335 the lzh-sds-pro/mwcnt paste electrode showed redox peak current at ipa = 8.743 µa, ipc = 7.618 µa, and peak-to-peak separation (∆ep) = 71.6 mv. meanwhile, redox peak current of the non-modified mwcnt paste electrode was ipa = 5.854 µa, ipc = 5.843 µa, and ∆ep = 92.7 mv. it is evident from these findings that the introduction of lzh-sds-pro as an mwcnt modifier is responsible for improving electron transfer rate, electroactive surface area and the conductivity performance of the modified electrode. the investigation of interfacial redox reaction kinetics of [fe(cn)6]3-/4redox probe at lzh-sds-pro/ /mwcnt paste electrode was done by using the eis method. the charge transfer resistance, rct values were estimated as diameters of semicircles appearing in high-frequency regions of nyquist plots, where the diffusion process is represented by straight-line plots at lower frequencies [28]. in the inset of figure 4, the randles equivalent circuit used for fitting measured impedance spectra is presented. rct value for bare mwcnt paste electrode was 1.325 kω, while rct for lzh-sds-pro/ /mwcnt was 0.245 kω. it is clearly observed that lzh-sds-pro/mwcnt displayed lower charge transfer resistance within the interfacial layer, as suggested by more than five times smaller diameter of the semicircle. z’ /  figure 4. nyquist plots recorded in the solution of 4.0 mmol l-1 k3[fe(cn)6] in 0.1 mol l-1 kcl using (a) non-modified mwcnt, and (b) lzh-sds-pro/mwcnt paste electrode. inset: randles equivalent electrical circuit used for data fitting the apparent rate constant, kapp of the electron transfer on unmodified mwcnt and lzh-sds -pro/mwcnt paste electrode was 1.17×10-5 and 6.38×10-5 cm s-1, respectively, which was calculated using the eq. (1): kapp = rt/f2 rct c (1) where t represents temperature, r is the gas constant, c is the concentration of k3[fe(cn)6] and f is faraday’s constant. as a consequence of the high specific area and high conductivity, lzh-sds-pro/mwcnt electrode effectively promotes the electron transfer process. the effect of scan rate () change on the redox peak currents of 4.0 mmol l-1 k3[fe(cn)6] contained in 0.1 mol l-1 kcl at lzh-sds-pro/mwcnt paste electrode was also studied. as can be observed in figure 5a, anodic and cathodic peak currents were progressively increased with increasing scan rate from 10 to 300 mv s-1, while their ep values shifted positively and negatively, respectively, suggesting kinetic limitation in the reaction [29]. in addition, there is also a straightline relationship between peak current and scan rate () with the linear regression equations, ipa = 0.0635 + 3.9461 (r2 = 0.9910) and ipc = -0.0636 3.599 (r2 = 0.9903) as shown in figure 5b. -z  /  http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(2) (2022) 331-341 determination of uric acid 336 besides that, the graph of peak currents versus square root of scan rate (1/2) was plotted in figure 5c, showing a good linear relationship with the following linear equations: ipa = 1.26791/2 1.1281 and ipc = -1.25661/2 + 1.394. the correlation coefficients obtained were 0.9913 and 0.9927, respecttively. these results revealed that the redox reaction of k3[fe(cn)6] on lzh-sds-pro/mwcnt paste electrode is reversible, i.e. diffusion-controlled [30]. figure 5. (a) cyclic voltammograms in the solution of 4.0 mmol l-1 k3[fe(cn)6] in 0.1 mol l-1 kcl at scan rates of 10, 20, 40, 70, 100, 150, 200 and 300 mv s-1; (b) plot of peak currents versus scan rate; (c) plot of peak currents versus square root of scan rate electrochemistry of ua on lzh-sds-pro/mwcnt paste electrode the square wave voltammetry (swv) measurements were carried out to compare electroanalytical performance of the non-modified mwcnt and lzh-sds-pro/mwcnt paste electrodes. as illustrated in figure 6, the peak current of ua oxidation at the non-modified mwcnt is observed at 5.613 µa. figure 6. sw voltammograms in the solution of 0.1 mmol l-1 ua in 0.1 mol l-1 pbs at ph 6.4 using: (a) non-modified mwcnt and (b) lzh-sds-pro/mwcnt m. h. ahmad tajudin et al. j. electrochem. sci. eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 337 the oxidation peak current of lzh-sds-pro/mwcnt, however, is dramatically improved by the factor of 3 to 18.16 µa. except for the effect of increased surface area, this might be due to the excellent electrical conductivity of lzh-sds-pro/mwcnt that can act as an effective electrons promoter during the electrochemical reaction. hence, the addition of lzh-sds-pro into mwcnt has enhanced the electrode performance for the detection of ua. scheme 1 illustrates the proposed mechanism for the oxidation reaction of ua at the lzh-sds -pro/mwcnt paste electrode. by ua oxidation, imine alcohol is produced from ua by donating two protons and electrons, while two moles of propoxur at the electrode surface accept those protons and electrons to produce o-isopropoxyphenol [31,32]. hn n h n h h n o o o hn n n h n o o o hn n n h h n o o o -2h+, -2eh2o oh o h n o o +h+, +eoh o 2 scheme 1. probable mechanism of oxidation reaction of ua at lzh-sds-pro/mwcnt paste electrode the effect of ph figure 7 shows how ph values between 6.0 and 8.0 of 0.1 mol l-1 pbs affect the oxidation peak currents of 0.1 mmol l-1 ua at lzh-sds-pro/mwcnt paste electrode since pbs was optimized at the ph scale of 6.2 to 8.0 [33]. the oxidation peak current of ua increased with increasing the ph value from 6.0, reached a maximum point at ph 6.4, and then decreased with further ph increasing. therefore, throughout the work for ua determination, the optimum ph was set at 6.4. it is clearly seen from figure 7 that the peak potential shifted negatively with ph increase, proving thus the involvement of protons in the oxidation of ua. figure 7. plot of oxidation peak current (i) and oxidation potential (e) vs. ph of 0.1 mol l-1 pbs containing 0.1 mmol l-1 ua at lzh-sds-pro/mwcnt paste electrode http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(2) (2022) 331-341 determination of uric acid 338 the relationship between ph and peak potential (e) of ua can be expressed as: e = -50.226 ph + + 643.83 (r2 = 0.9921), suggesting the equal number of electrons and protons involved in the electrochemical oxidation of ua at lzh-sds-pro/mwcnt. this conclusion is based on the obtained slope of 50.226 mv ph-1, close to the nernst value of 59 mv ph-1 [34]. calibration curve and limit of detection in order to study the relationship between the concentration of ua and oxidation peak current (i) on the lzh-sds-pro/mwcnt paste electrode, a series of ua solutions was prepared, containing 7.0 µmol l-1 to 0.7 mmol l-1. as shown in figure 8a, i increased linearly as ua concentrations were increased. the plot of i vs. [ua] showed a linear relationship with the following linear regression equation: i = 0.058 cua – 1.417 (r2 = 0.9920) and the limit of detection (lod) was found to be 4.28 µmol l-1. lod was determined using eq. (2): lod = 3σ / m (2) where m = slope of the calibration curve and σ = relative standard deviation of its intercept. figure 8. (a) linear plot of i vs. cua and (b) swvs at different concentrations (7, 10, 30, 70, 100, 300 and 700 µm) of ua in 0.1 mol l-1 pbs (ph 6.4) the high sensitivity of the lzh-sds-pro/mwcnt paste electrode compared to those obtained for several other electrodes by different electroanalytical methods is presented in table 1. the obtained results proved that the suggested sensor can be used for the determination of ua in environmental and even biological analytes. table 1. comparison of analytical properties of different fabricated electrodes for the determination of ua electrode materials method linear range of concentration, µmol l-1 lod, µmol l-1 ref. dmf / spce dpv 100.0 – 500.0 0.19 [35] poly(isoniazid) / cpe cv 10.0 – 1000.0 1.173 [36] pedot / gce cv 6.0 – 100.0 7.0 [37] mwcnt-pedot / gce dpv 10.0 – 250.0 10.0 [38] gf / nico2o4 swv 10.0 – 26.0 0.2 [39] mwcnt / gce swv 10.0 – 200.0 1.0 [40] lzh-sds-pro / mwcnt / cpe swv 7.0 – 700.0 4.28 this work selectivity, stability, reproducibility and repeatability of lzh-sds-pro/mwcnt paste electrode the selectivity of the lzh-sds-pro/mwcnt paste electrode was tested by evaluating differrences in the oxidation peak current value of 0.1 mmol l-1 ua in 0.1 mol l-1 pbs (ph 6.4) in the presence of possible coexisting interfering species such as d-glucose, l-fructose, and na+, mg2+, ca2+, cl-, so42 m. h. ahmad tajudin et al. j. electrochem. sci. eng. 12(2) (2022) 331-341 http://dx.doi.org/10.5599/jese.1237 339 and no3ions. as illustrated in figure 9, lzh-sds-pro/mwcnt paste electrode has the antiinterference ability in the presence of 10and 50fold higher concentrations of interfering species with the relative error of less than ± 10 %. the stability of the fabricated paste electrode was recorded towards 0.1 mmol l-1 ua in 0.1 mol l-1 pbs (ph 6.4) within 14 days and the results were retained about 90 % from the initial response, indicating high stability of electrode over a long period. the reproducibility of the fabricated paste electrode was conducted using five individual electrodes prepared by the same procedure. the relative standard deviation (rsd) of these electrodes was 3.19 %. moreover, the rsd value of 4.73 % obtained after 10 successive measurements with similarly fabricated electrodes suggested that lzh-sds-pro/mwcnt paste electrode can be used repeatedly for the determination of ua. figure 9. interference analysis of 0.1 mmol l-1 ua at lzh-sds-pro/mwcnt paste electrode real samples analysis the validity of the results obtained using the lzh-sds-pro/mwcnt paste electrode was studied by determining the concentration of ua in the urine sample via the standard addition method. the urine sample was directly diluted 30 times in 0.1 mol l-1 pbs (ph 6.4) without pre-treatment. then, known concentrations of ua were spiked into the urine sample to study electrode recoveries. as a result, lzh-sds-pro/mwcnt paste electrode exhibited good recoveries, as summarized in table 2. table 2. determination of ua in urine sample using lzh-sds-pro/mwcnt paste electrode (n = 3) sample cua / µmol l-1 recovery, % rsd, % determined spiked found urine 140.3 100 243.8 101.5 3.81 200 337.3 99.1 2.94 300 445.7 101.2 3.72 conclusions in this experiment, a simple, highly sensitive, and cost-effective sensing material was proposed for the determination of ua with low lod. these beneficial sensing electrode properties were realized through a combination of unique properties of lzh-sds-pro nanomaterial and mwcnt. http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(2) (2022) 331-341 determination of uric acid 340 the prepared nanocomposite electrode exhibited significant electrocatalytic activity toward ua oxidation with satisfactory results of selectivity, stability and reproducibility, suggesting that lzhsds-pro/mwcnt paste electrode is an attractive candidate for practical applications. acknowledment: the authors would like to extend their gratitude to the research management and innovation 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open access : : issn 1847-9286 www.jese-online.org original scientific paper analysis of sulfamethoxazole by square wave voltammetry using new carbon paste electrode izabel c. eleotério1, marco a. balbino1, josé f. de andrade1, bruno ferreira1, adelir a. saczk2, leonardo l. okumura3, antonio carlos f. batista4, marcelo f. de oliveira1, 1departamento de química, faculdade de filosofia, ciências e letras de ribeirão preto, usp, 14040-901, ribeirão preto, sp, brazil 2departamento de química, universidade federal de lavras, 37200–000, lavras, mg, brasil 3departamento de química, universidade federal de viçosa, 36570–000, viçosa, mg, brasil 4universidade federal de uberlândia campus do pontal, 38304-402, ituiutaba mg, brazil corresponding author e-mail: marcelex@ffclrp.usp.br; tel. +55-16-3315-9150 received: february 17, 2018; revised: april 20, 2018; accepted: april 20, 2018 abstract in this work a new model of carbon paste electrode was employed to determine sulfamethoxazole (smx), an antibiotic used to treat infections in human and veterinary medicine, by the square wave voltammetric modality (swv). more specifically, the electrochemical behavior of smx was investigated by cyclic voltammetry (cv), and the quantitative analysis of smx was provided by swv. the analytical curve was obtained with a linear correlation coefficient (r) of 0.985 and standard deviation (sd) of 0.005 μa. limits of detection and quantification were found as 2.3×10-6 and 7.7×10-6 mol l-1, respectively. according to the obtained results, the new carbon paste prototype electrode can successfully be employed in this kind of electroanalytical applications. keywords carbon paste electrode; electroanalysis of pharmaceutical compounds; electrochemical sensors; voltammetric analysis introduction concerning a drug analysis, electroanalytical methods offer several advantages: they are versatile, fast, sensitive, inexpensive, and environmentally friendly due to the limited use of chemicals [1]. recently developed electrochemical devices efficiently monitor pollutants through direct or indirect reactions between the contaminant and the electrode surface, what makes them potentially applicable in situ [2-4]. the large-scale use of this technology relies on the scientific knowledge http://www.jese-online.org/ mailto:marcelex@ffclrp.usp.br http://dx.doi.org/10.5599/jese.583 j. electrochem. sci. eng. 8(4) (2018) 281-289 analysis of sulfamethoxazole 282 and nowadays, scientists have investigated many electrodes for this purpose [5]. gold electrode, glassy carbon electrode, platinum disc electrode, graphite electrode, chemically modified electrodes, carbon nanotube and carbon paste electrodes (cpes) are some of the examples already reported in the literature [1,5]. ralph norman adams introduced cpe in 1958 [6-9]. this electrode consists of a mixture of carbon powder and a non-electroactive liquid binder and offers a broad potential window, low residual current (background), unique surface characteristics, low cost, and versatile preparation [6-16]. for this reason, cpe has been widely employed to determine the sulfamethoxazole (smx) by the voltammetric analysis. smx is chemically known as 4-amino-n-(5-methylisoxazol-3-yl)-benzene sulfonamide and its chemical structure is shown in figure 1. smx constitutes a sulfonamide that helps to treat infections in human and veterinarian medicine [17,18]. obviously large consumption of this antibiotic agent, however, can lead to environmental and public health problems. figure 1. chemical structure of sulfamethoxazole. several methods for sulfamethoxazole determination have already been described in the literature, including chromatographic methods coupled with different detectors 18,22,24, capillary electrophoresis [25,26], spectrophotometry [27,28], and electroanalytical methods [29-39]. it is possible to observe in the literature, however, that non-renewable surface devices for cpe application were applied. conventional cpes must usually be refilled with the carbon paste, increasing thus the consumption of material and time analysis. in this context, we developed a new model of cpe and applied it for the sulfamethoxazole analysis using the square-wave voltammetry. experimental materials and reagents a stock solution of 0.005 mol l-1 smx (sigma aldrich) was prepared in the methanol (merck). the following electrolyte solutions were used in this research: 0.1 mol l-1 potassium chloride (synth), 0.1 mol l-1 sodium nitrate containing 0.01 mol l-1 potassium ferricyanide, 0.1 mol l-1 sodium perchlorate; 0.05 mol l-1 sulfuric acid (merck)/methanol in 70:30 % (v/v) ratio and 0.04 mol l-1 britton– robinson (br) buffer, ph 2.18, prepared by mixing 0.04 mol l-1 boric acid (carlo erba), acetic acid and orthophosforic acid (merck). mixtures of graphite powder (particle size = 19.2-168.5 µm, analítica) and mineral oil (nujol, união química) were used to prepare the carbon paste [14,16]. carbon paste electrode construction the carbon paste electrode was prepared by mixing 2.25 g of graphite powder with 0.75 g of mineral oil. this mixture was homogenized by magnetic stirring in a 25 ml beaker containing 10 ml of chloroform (merck). the paste was obtained after evaporation of the solvent. the carbon paste was packed into a versatile electrode body fabricated in our laboratory. it consisted of a glass cylindrical tube in the form of a syringe (o.d. 7 mm, i.d. 3 mm) and contained a platinum rod to establish the electric contact (figure 2). nh2 s o o nh n o ch3 i. c. eleotério et al. j. electrochem. sci. eng. 8(4) (2018) 281-289 doi:10.5599/jese.583 283 1 electric contact (platinum) 2 poston (teflon) 3 cylindrical glass tube 4 carbon paste 5 electric contact (stainless steel) figure 2. carbon paste electrode developed in our research group. instrumentation the voltammetric experiments were conducted using a potentiostat model μautolab iii (eco chemie) connected to a personal computer. the experiments were carried out in a three-electrode system consisting of a carbon paste working electrode, platinum spiral wire auxiliary electrode and ag/agcl, 3.0 mol l-1 kcl, reference electrode, all arranged in a 5-ml electrochemical cell (figure 3). the solutions were deoxygenated with nitrogen for 15 min prior measurements. figure 3. electrode arrangement in electrochemical cell. voltammetric analysis the voltammetric behavior of the carbon paste electrode was initially investigated by cv measurements performed in different solutions, such as 0.1 mol l-1 kcl, 0.1 mol l-1 nano3 containing 0.01 mol l-1 k3fe(cn)6, 0.04 mol l-1 brbuffer (ph 2.18) and 0.1 mol l-1 naclo4. cv analysis was carried out at potentials ranging from 0.1 v up to 1.8 v and scan rates from 10 to 100 mv s-1. subsequently, the quantitative analysis of smx was conducted by swv with previously optimized experimental parameters (frequency and pulse amplitude) and a constant step potential of 50 mv. potential window ranged from 0.1 to 1.8 v vs. ag/agcl and 0.04 mol l-1 brbuffer (ph 2.18) was used as the supporting electrolyte. the voltammetric parameters investigated for the smx assay were: frequencies of 8, 12, 18, 20, and 24 hz and pulse amplitude ranging from 10 to 100 mv. smx was j. electrochem. sci. eng. 8(4) (2018) 281-289 analysis of sulfamethoxazole 284 analyzed at different concentrations by the standard addition method, i.e. the analytical curve was constructed by adding aliquots of the smx stock solution to the electrochemical cell. a linear curve was achieved for smx concentrations ranging from 7.9 to 24.0 µmol l-1. results and discussion electrochemical properties of cpe in different electrolyte solutions the cpe was initially tested in various aqueous systems, in order to provide information about its basic response and stability in different electrolytes. figure 4(a-d) illustrates the cyclic voltammograms of cpe measured at different scan rates in 0.1 mol l-1 kcl, 0.1 mol l-1 nano3 containing 0.01 mol l-1 k3fe(cn)6, 0.01 mol l-1 naclo4, and 0.04 mol l-1 br–buffer (ph 2.18). among almost all presented cvs suggesting stability of cpe with not specific response in the specific electrolyte and given potential region, figure 4b presents a typical cyclic voltammogram for the k3fe(cn)64-/3redox couple. this particular reaction usually served as the test for the redox activity of an electrode. peaks at 0.36 v (epa) and 0.19 v (epc) suggest reversible behavior of redox couple at the cpe in the potential range from 0.1 v to 0.7 v vs. ag/agcl, with scan rate between 10 to 100 mv s-1 [12]. e / v vs. ag/agcl e / v vs. ag/agcl e / v vs. ag/agcl e / v vs. ag/agcl figure 4. cyclic voltammograms of the cpe in (a) 0.1 mol l-1 kcl (100 mv s-1), (b) 0.1 mol l-1 nano3 + 0.01 mol l-1 k3fe(cn)6, (c) 0.04 mol l-1 brbuffer (ph 2.18) (100 mv s-1) and (d) 0.1 mol l-1 naclo4, at denoted scan rates and potential range from 0.1 to 1.8 v vs. ag/agcl. i /  a i /  a i /  a i /  a i. c. eleotério et al. j. electrochem. sci. eng. 8(4) (2018) 281-289 doi:10.5599/jese.583 285 literature reveals that the supporting electrolyte plays an essential role in the voltammetric signal of smx [29,40-42]. besides, the sulfa drugs have two dissociation constants (pka), which in the case of the smx correspond to the amino functional group with pka value of 1.8 and amide functional group with a pka value of 5.6 [29,33,37,40,44]. therefore the brbuffer solution (0.04 mol l-1 in acetic, phosphoric and boric acids) (ph 2.18), with cv shown in figure 4c was chosen as the experimental medium in the voltammetric studies of smx. figure 5(a-b) displays representative cvs of 0.005 mol l-1 smx together with the corresponding background currents recorded for the proposed cpe and a commercial glassy carbon electrode, respectively. figure 5a shows cv profiles of cpe in a blank solution of 0.04 mol l-1 brbuffer (ph 2.18) and in the same solution containing 0.005 mol l-1 smx, whereas figure 5b shows cv profiles of glassy carbon in a blank solution of 0.05 mol l-1 s acid/methanol 70:3 (v/v) (ph 1.38) and in the same solution containing 0.005 mol l-1 smx. an irreversible two-electron oxidation voltammetric peak appeared in both cases when 0.005 mol l-1 smx was present in the solution [29,40]. also, much lower background current that was obtained for the cpe as compared with the solid glassy carbon, suggests that the proposed cpe electrode could be more sensitive for smx oxidation [6-16]. the higher background current observed for the glassy carbon electrode in figure 5b stems from the oxygen evolution [43]. e / v vs. ag/agcl e / v vs. ag/agcl figure 5. cyclic voltammograms at 100 mv s-1 of 5 mmol l-1 smx for (a) cpe in 0.04 mol l-1 brbuffer (ph 2.18), potential range = 0.1 v to 1.8 v vs. ag/agcl and (b) glassy carbon electrode in 50 mmol l-1 sulfuric acid/methanol 70:3 (v/v) (ph 1.38), potential range = 0.5 v to 1.5 v vs. ag/agc). cpe electrochemical response toward smx the optimized experimental parameters that pointed out the best results for smx determination using swv technique were obtained by variations of pulse amplitude and frequency. the pulse amplitude was varied in the range of 10–100 mv, at the constant frequency of 12 hz. in this case, the optimized result was defined as the parameter value that produced increase of the peak current without shifting the peak potential or making any significant increase in the peak width. hence, 100 mv was chosen as the square-wave pulse amplitude value. afterwards, the effect of frequency was evaluated in the range 8–24 hz, keeping constant the pulse amplitude of 100 mv. the best result was achieved at f = 12 hz. according to previous literature investigations, sulfonamide oxidation results in a formation of the corresponding iminobenzoquinone intermediate, shown as peak (1) in figure 6. the swv i /  a i /  a j. electrochem. sci. eng. 8(4) (2018) 281-289 analysis of sulfamethoxazole 286 responses presented in figure 6 showed that the oxidation current peaks increased with increase of the frequency. hence the frequency of 12 hz was chosen for further analysis because of the best resolution of the voltammetric peak (figure 6, peak (2)). e / v vs. ag/agcl figure 6. effect of the frequency parameter on the swv response of the cpe in 0.04 mol l-1 br–buffer (ph 2.18) containing 24.0 µmol l-1 smx: (a) 12 hz, (b) 18 hz, (c) 20 hz, (d) 24 hz. potential range = 0.5 to 1.6 v vs. ag/acl, pulse amplitude = 100 mv, scan increment = 2 mv. according to the literature, the smx electrochemical oxidation occurs at the primary amino groups (-nh2) [29,40]. figure 7 illustrates the mechanism of smx oxidation that as a two-electron and ph dependent reaction, possibly takes place in an acid medium. figure 7. smx oxidation at carbon paste electrode in acid medium the electrochemical behavior of smx in different concentrations was assessed by successive additions of this drug in concentrations ranging from 7.9 to 24.0 µmol l-1 to the electrochemical cell. as seen in figure 8(a), the anodic peak current at 1.07 v vs. ag/agcl (irreversible oxidation peak) increased upon rising of the smx concentration. the analytical curve drawn in figure 8(b) shows a linear correlation coefficient r = 0.985 with a standard deviation sd = 0.005 μa. the corresponding linear equation was adjusted as ipa = 0.24 μa + 6.5×103 μa /mol l-1 [smx]. the limit of detection calculated according to the criterion 3sd/m ratio, where m is the slope of the analytical curve, gave 2.3×10-6 mol l-1, while the limit of quantification based on the criterion of 10sd/m ratio, was adjusted as 7.7×10-6 mol l-1. n + h h h s o o nh o n ch3 -2e , -2h onh + h2so3 + n o ch3 n + h h h i /  a i. c. eleotério et al. j. electrochem. sci. eng. 8(4) (2018) 281-289 doi:10.5599/jese.583 287 figure 8. (a) influence of smx concentration in 0.04 mol l-1 britton–robinson buffer (ph 2.18) solution on the voltammetric response of cpe: (a) 24.0 µmol l-1, (b) 21.9 µmol l-1, (c) 20.1 µmol l-1, (d) 18.0 µmol l-1, (e) 15.9 µmol l-1, (f) 14.0 µmol l-1, (g) 12.1 µmol l-1, (h) 10.0 µmol l-1, and (i) 7.9 µmol l-1. potential range = 0.5 to 1.6 v vs. (ag/acl), frequency = 12 hz, pulse amplitude = 100 mv, scan increment = 2 mv. (b) analytical curve of the peak current, µa vs. smx concentration, µmol l-). conclusions the novel and efficient support for the carbon paste substrate is developed allowing determination of sulfamethoxazole at the mol l-1 level. the developed cpe showed an excellent stability in different electrolyte media and excellent voltammetric response for the k3fe(cn)64-/3 redox couple probe. oxidation of smx at the cpe occurring at about 1.07 v vs. ag/agcl was found to be an irreversible 2-electron and ph dependent process. an electrocatalytic effect was observed in comparison with glassy carbon electrode. another peak occurring at about 0.49 v vs. ag/agcl was observed when frequency values higher than 12 hz were applied and ascribed to the formation of the corresponding iminobenzoquinone intermediate. the developed cpe is an inexpensive and versatile electrode, having high potential for application as a transducer in a device serving for determination of sulfamethoxazole. acknowledgments: the authors acknowledge the financial support of fapesp (processes 2011/10216-5 and 2016/23825-3) and capes (edital pro-forenses 25/2014). the authors also acknowledge dr. cynthia m. c. p. manso for editing and revising the text. references [1] s. a. ozkan, j.-m. kauffmann, p. zuman, electroanalysis in biomedical and pharmaceutical sciences, springer-verlag berlin heidelberg, 2015. 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[44] r. jain, p. padmaja, s. gupta, canadian journal of chemistry 75 (1997) 567-574. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {modeling and synthesis of carbon-coated limnpo4 cathode material: experimental investigation and optimization using response surface methodology:} http://dx.doi.org/10.5599/jese.1191 305 j. electrochem. sci. eng. 12(2) (2022) 305-316; http://dx.doi.org/10.5599/jese.1191 open access : : issn 1847-9286 www.jese-online.org original scientific paper modeling and synthesis of carbon-coated limnpo4 cathode material: experimental investigation and optimization using response surface methodology redouan el-khalfaouy1,2, , khadija khallouk2, alae elabed3, abdellah addaou2, ali laajeb2 and ahmed lahsini2 1laboratory of natural substances, pharmacology, environment, modeling, health and quality of life, polydisciplinary faculty of taza, sidi mohamed ben abdellah university, b.p. 1223,taza, morocco 2materials, processes, catalysis and environment laboratory, high school of technology, sidi mohamed ben abdellah university, bp 2427, fez, morocco 3microbial biotechnology laboratory, faculty of science and technology, sidi mohammed ben abdellah university, bp. 2202, fez, morocco corresponding author: redouan.elkhalfaouy@usmba.ac.ma received: november 21, 2021; accepted: january 17, 2022; published: january 25, 2022 abstract nanostructured limnpo4 cathode materials for lithium-ion batteries (libs) have been successfully prepared by a modified solvothermal method under controlled conditions. polyethylene glycol (peg-10000) was used as a solvent to optimize the particle size/morphology and as a carbon conductive matrix. in order to investigate the effect of synthesis parameters such as concentration of peg-10000, reaction time and reaction temperature on the limnpo4 phase purity, response surface methodology was carried out to find variations in purity results across the composition. the purity of all materials was checked using highscore software by comparing the matched lines score to ones of reference data. as a result, it has been found that the pure phospho-olivine material limnpo4 can be synthesized using the following optimum conditions: peg concentration = 0.1 mol l-1, reaction time = 180 min, and reaction temperature = 250 °c. the as-prepared limnpo4 under optimum conditions delivered an initial discharge capacity of 128.8 mah g-1 at 0.05 c-rate. the present work provides insights and suggestions for optimizing synthesis conditions of this material, which has been considered the next promising cathode candidate for highenergy lithium-ion batteries. keywords response surface methodology; olivine structure; solvothermal synthesis; peg-10000; lithium-ion batteries http://dx.doi.org/10.5599/jese.1191 http://dx.doi.org/10.5599/jese.1191 http://www.jese-online.org/ mailto:redouan.elkhalfaouy@usmba.ac.ma j. electrochem. sci. eng. 12(2) (2022) 305-316 synthesis of carbon-coated limnpo4 cathode 306 introduction rechargeable lithium-ion batteries (libs) with high-energy, high power density, durability, and lightweight have become the most requested energy source in order to meet future society's needs in many renewable energy storage systems, starting from laptops, cell phones to electric vehicles. with the increasing demand for higher capacity and improved safety, many efforts have been made to further develop the next generation of libs with high volumetric/gravimetric energy density. most commercial libs are currently based on licoo2 layered structure as a cathode material. therefore, one of the main challenges is to replace the commercialized layered structure cathode (which exhibits a theoretical specific capacity of 274 mah g-1) with other promising and efficient cathode materials. limpo4 (m = fe, mn, co, ni) olivine-based high-performance cathodes are the recommended alternative cathode materials to replace traditional ones (licoo2) due to their low cost, non-toxicity, high thermal and cyclic stability, and environmental impact [1–5]. compared to the first commercialized cathode, which is lifepo4, limnpo4 is considered as the most promising cathode material in the next generation of lithium-ion batteries due to the high theoretical energy density (701 wh/kg), which is higher than that of lifepo4 (586 wh kg-1)[6,7]. moreover, the low voltage (4.1 v vs li/li+) of limnpo4, which is positioned within the stable window of the most commercialized electrolytes, makes it the best candidate material compared to licopo4 and linipo4, which have higher potentials, being respectively 4.8 and 5.1 v vs. li/li+ [8–10]. however, limnpo4 exhibits significantly lower electrochemical performances than lifepo4 because of two important drawbacks that limit its electrochemical activity, including low electronic conductivity (˂10-10 s/cm) being even lower than that of lfp (10-9 s cm-1), and low lithium-ion diffusion rate ≈10-16 cm2 s-1 [11,12]. furthermore, the anisotropic distortion of the jahn-teller lattice in the mn3+ sites and the interface strain during phase transitions between the lithiated and delithiated phases (limnpo4-mnpo4) cause a significant volume change (≈8.9 %) compared to lifepo4-fepo4 (≈7 %) [13,14]. recently, many attempts have been reported to overcome these limitations [15–17]. the results confirmed that particle size reduction could strongly increase the lithium-ion diffusion during the charge/discharge process [17–19]. the same behavior has been reported by the surface carbon coating [20–22], and the partial substitution of transition elements [23–26]. the synthesis process was also considered a direct approach to achieving desired performances. for this reason, several methods have been applied to prepare limnpo4 with high purity, such as spray-pyrolysis [22,27], sol-gel method [28,29], hydrothermal synthesis [30–33], precipitation method [34,35] and solution combustion process [36,37]. among all these methods, some selected ones offer more advantages such as morphology control, better homogeneity, submicron-sized particles, and larger specific surface area with increased electrochemical performances [38,39]. the solvothermal technique has significant assets compared to other methods such as simplicity to handle, short reaction time, moderate reaction temperature, good crystallinity and high purity [40,41]. the process is widely used for preparing various micro and nanostructured materials such as cathodes/anodes, oxides, semiconductors, ceramics, etc. however, morphology and particle size are difficult to control since they are determined by many factors such as precursor types, additives or surfactants, ph, reaction time/temperature, and physico-chemical properties of the used solvent. polyethylene glycol (peg) is an organic solvent that can be easily adsorbed on the crystal's surface by hydrogen bonding, consequently influencing nucleation and crystallite growth. r. el-khalfaouy et al. j. electrochem. sci. eng. 12(2) (2022) 305-316 http://dx.doi.org/10.5599/jese.1191 307 based on these advantages of peg, we report in this work the synthesis of limnpo4 cathode material under solvothermal conditions, using the peg-10000 as a solvent to optimize particle size/morphology and as a carbon-coated source. to the best of our knowledge and after a thorough literature review, no study is presented on optimizing the synthesis parameters of limnpo4 using the response surface method (rsm). figure 1 is a schematic representation of limnpo4 synthesis and analysis performed in this work. figure 1. schematic figure for limnpo4 synthesis and analysis experimental materials preparation all chemical precursors are of analytical grade and used without any further purification. the cathode limnpo4 was prepared via facile solvothermal reaction using the following raw precursors; li3po4, mnso4.h2o (99 %, sigma aldrich) and peg-10000 (flakes, sigma aldrich). firstly, li3po4 intermediate compound was prepared by mixing li2co3 (99 %, honeywell fluka) with (nh4)2hpo4 (99 %, merck) and citric acid (2m) (99.5 %, merck) in appropriate amounts under magnetic stirring and heat at 90 °c for 60 min. the resulted product was filtered, washed with deionized water (dw) and dried overnight. then, mnso4.h2o, li3po4, and peg-10000 (with different concentrations: 0.00, 0.05 and 0.1 m) solvent were mixed under vigorous stirring for 60 min. the suspension was transferred into a 100 ml stainless steel autoclave followed by thermal treatment at different temperatures, i.e., 150, 200 and 250 °c for a certain reaction time ranging from 60 to 180 min. the autoclave was then taken out of the furnace and cooled down to room temperature. the obtained products were washed with distilled water several times, collected by filtration, and finally dried at 80 °c overnight. surface carbon coating of limnpo4@c was activated by sintering the as-prepared products at 700 °c for 6 hours under argon atmosphere with a heating rate of 5 °c min-1. experimental design and statistical analysis the box-behnken design was used for the response methodology to examine the relationship between one or more dependent response variables and a set of quantitative experimental factors precursors mixing and stirring for 1 h solvothermal treatment under controlled conditions peg-10000 li3po4 mnso4.h2o limnpo4 phase purity reaction time reaction temperature peg concentration the suspension transfer into a 100 ml teflon-lined stainless steel autoclave -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 purty a: concentration c : t im e 0 20 20 40 60 80 100 5 factor coding: actual design points 0 100 x1 = a: concentration x2 = c: time actual factor b: temperature = 0 %purity t im e (m in ) peg concentration (mol/l) -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 -50 0 50 100 150 p u r t y a: concentration c: time 3d surface factor coding: actual design points: above surface below surface 0 100 x1 = a: concentration x2 = c: time actual factor b: temperature = 0 factor coding: actual design points: above surface below surface 0 100 x1 = a: concentration x2 = c: time actual factor b: temperature = 0 % p u r it y (a) (b) p u r it y , % http://dx.doi.org/10.5599/jese.1191 j. electrochem. sci. eng. 12(2) (2022) 305-316 synthesis of carbon-coated limnpo4 cathode 308 (independent variables). a mathematical model, followed by the second polynomial equation, was developed to describe the relationship between the predicted response variable (matching lines score (purity) of the synthesized limnpo4) and the independent variables of solvothermal synthesis conditions. it was described by eq. (1)     = = =  = = + + +  4 3 3 3 3 2 limnpo 0 i i ii i ij i j i 1 i 1 i 1 i j 1 y x x x x (1) where ylimnpo4 is the predicted response variable, xi, xj (1 ≤ i, j ≤ 3; i ≠ j) represent the coded independent variables (solvothermal conditions), 0 is the intercept coefficient,  i are linear terms,  ii are squared terms, and  ij are interaction terms. this study used this design to determine the effect of three factors (peg concentration, solvothermal reaction time and temperature) on limnpo4 phase purity. the ranges and levels of the experimental parameters are depicted in table 1. the design-expert12 software was used to analyze the results of all experiments. table 1. experimental ranges and levels of independent variables variables symbol level -1 0 1 peg concentration, mol l-1 x1 0 0.05 0.1 reaction temperature, °c x2 150 200 250 reaction time, min x3 60 120 180 structural, morphological and electrochemical characterization crystalline structure and phase purity of all products were analyzed and evaluated by x-ray diffraction using diffractometer panalytical's x'pert pro, with cu kα radiation ( = 1.5418 å). the surface morphology and the chemical compositions were observed with a scanning electron microscope (fei quanta 200) equipped with eds for microanalysis of the surface. the electrochemical tests were performed at room temperature in the potential range between 2.5 and 4.5 v using battery test systems (basytec gmbh, germany). all experiments were conducted using coin-type cells (cr2032) assembled according to our previous work [25]. phase and morphology of the intermediate compound li3po4 all detectable peaks of the as prepared li3po4 are shown in figure 2, where the peaks are indexed as li3po4 according to the standard data pdf # 071-1528. figure 2. xrd patterns of asprepared li3po4 intermediate compound 10 20 30 40 50 60 70 80 90 100 in te n s it y ( a .u .) 2theta[deg] li 3 po 4 01-071-1528 2 theta, o in te n si ty , a .u . r. el-khalfaouy et al. j. electrochem. sci. eng. 12(2) (2022) 305-316 http://dx.doi.org/10.5599/jese.1191 309 based on the matching lines score, no impurity-related peaks could be detected, indicating a high level of purity of the as-prepared li3po4 material. the prepared sample has an orthorhombic crystal structure with a pmn21 space group. figure 3 shows sem images of the li3po4 product at different magnifications, which suggest that the product is of an irregular nanoplate-like structure. the present results are in good agreement with the literature [33,42]. the microstructure of li3po4 was studied by energy dispersive spectroscopy (eds) to obtain the elemental composition. the collected eds results shown in figure 3, confirm the presence of only p and o atoms with a high amount of carbon (from the sample holder and citric acid), without the appearance of any other element. figure 3. sem images and eds spectrum of the as-prepared li3po4 results and discussion effect of operating conditions on limnpo4 phase purity the design matrix composed of 17 experiences, along with their experimental and predicted responses, are shown in table 2. table 2. experimental design matrix proposed for limnpo4 phase purity run x1 x2 x3 matching lines score with reference data #01-074-0375, % experimental predicted 1 0 0 0 75.00 74.60 2 0 0 0 73.00 74.60 3 -1 1 0 12.00 5.75 4 0 0 0 75.00 74.60 5 -1 0 1 10.00 11.75 6 1 1 0 99.00 93.25 7 1 -1 0 91.00 97.25 8 0 0 0 74.00 74.60 9 0 0 0 76.00 74.60 10 0 1 1 50.00 44.50 11 1 0 -1 77.00 65.25 12 0 1 -1 30.00 32.50 13 0 -1 -1 11.00 16.50 14 -1 -1 0 6.00 11.75 15 1 0 1 100.00 99.25 16 -1 0 -1 3.00 8.25 17 0 -1 1 88.00 70.50 0.0 200.0 400.0 600.0 800.0 1.0k element wt.% at.% c 66.37 73.28 o 30.75 25.49 p 2.88 1.23 total 100.00 100.00 li 3 po 4 p o c energy, ev 0.0 200.0 400.0 600.0 800.0 1.0k elem wt % at % c k 66.37 73.28 o k 30.75 25.49 p k 2.88 1.23 total 100.00 100.00 li 3 po 4 p o c ev http://dx.doi.org/10.5599/jese.1191 j. electrochem. sci. eng. 12(2) (2022) 305-316 synthesis of carbon-coated limnpo4 cathode 310 the results show good agreement between experimental and predicted responses. the matched lines score with reference data#01-074-0375 (purity) of limnpo4 was found to range from 3 to 100 %. based on the results presented in table 2, the coefficients of the developed model in eq. (1) are estimated using multiple regression analysis technique. the polynomial model for the phase purity of limnpo4 is represented by eq. (2): ylimnpo4 = 74.60 + 43.25x1 – 2.50 x2 + 16.50x3 + 0.50x1x2 + 6.50x1x3 – – 10.50x2x3 – 9.30x12 – 13.30x22 – 20.30x32 (2) the fit quality of the limnpo4 purity model was attested with an analysis of variance (anova) [43]. generally, the suitability of the model is confirmed by higher fisher’s value (f-value) with probability (p-value) as low as possible (p<0.05)[44]. table 3 shows the analysis of variance (f-test) and the p-value for this experiment. the p-value of this model is about 0.0002, which indicates that the model was suitable for use in this experiment. table 3. analysis of variance (anova) for the fitted quadric polynomial model for optimization of limnpo4 phase purity source degree of freedom sum of squares mean square f-value p-value model 3 17192.50 5730.83 14.61 0.0002 residual 4 5.20 1.30 corrected total sum of squares 17 72621.00 4271.82 r2 = 0.93 adjusted r2= 0.90 the calculated f-value for the regression is higher than 14, much higher than the value from fisher tables (f3,4 = 6.69, for a 95 % confidence level), confirming that the model is well fitted to the experimental data [45,46]. the determination coefficient (r2) quantitatively evaluates the correlation between the experimental data and the predicted responses [47]. with r2 = 0.93, we conclude that the predicted values match the experimental values perfectly. the adjusted r2 ≈ 0.90 is very close to the corresponding r2 value, which confirms that the model is highly significant [48]. the regression coefficients of eq. (2) and the corresponding p-values are presented in table 4. from this result, we can conclude that the linear effect of peg concentration (x1) and reaction time (x3) are the principal determining factors for the response on limnpo4 phase purity. table 4. estimated regression coefficients and corresponding p-values obtained during box-behnken design for limnpo4 material purity: parameter term estimate regression coefficient standard error f-value p-value 0 intercept 74.60 6.26 12.05 0.0017  1 x1 43.25 4.91 77.50 < 0.0001  2 x2 -2.50 4.91 0.2589 0.6265  3 x3 16.50 4.91 11.28 0.0121  11 x1x1 -9.30 6.77 1.89 0.2120  12 x1x2 0.5000 6.95 0.0052 0.9446  22 x2x2 -13.30 6.77 3.86 0.0903  13 x1x3 -10.50 6.95 2.28 0.1745  23 x2x3 6.50 6.95 0.8752 0.3807  33 x3x3 -20.30 6.77 8.99 0.0200 r. el-khalfaouy et al. j. electrochem. sci. eng. 12(2) (2022) 305-316 http://dx.doi.org/10.5599/jese.1191 311 the response surface plot as a function of peg concentration (x1) and reaction time (x3) is presented in figure 4(a). x1x3 was chosen as the interaction key, which exhibits a low p = 0.1745 compared to others that are not significant (since they exhibit a p-value higher than 0.1) [49,50]. figure 4. 3d response surface (a) and contour plot (b) of limnpo4 phase purity for different coded values of x1 (peg concentration) and x3 (reaction time) the combined effects of the two factors are positive and statistically significant, as also revealed by the contour lines presented in figure 4(b). the optimum conditions for maximum limnpo4 phase purity are as follows: cpeg = 0.1 mol l-1, t = 250 °c and  = 180 min. the synthesized material limnpo4 under optimum conditions was characterized by x-ray diffracttion to confirm the phase purity. figure 5 shows xrd results of the pure sample before and after calcination. it is clearly seen that the two patterns are very similar, with a difference in the peaks intensity which is much higher for the calcined sample. it is also observed that thermal treatment has not a remarkable effect on the formation process of the limnpo4 phase and does not change the purity of the material, which indicates that the reaction has been done in the autoclave under solvothermal/optimum conditions. on the other hand, the main objective of calcination is the conversion of peg layer adhered on the surface of the particles to the carbon layer, which promotes a higher electronic conductivity and consequently an improvement of the electrochemical performances. figure 5. xrd patterns of pristine and calcined limnpo4 material synthesized under optimum conditions -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 purty a: concentration c : t im e 0 20 20 40 60 80 100 5 factor coding: actual design points 0 100 x1 = a: concentration x2 = c: time actual factor b: temperature = 0 %purity t im e (m in ) peg concentration (mol/l) -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 -50 0 50 100 150 p u r t y a: concentration c: time 3d surface factor coding: actual design points: above surface below surface 0 100 x1 = a: concentration x2 = c: time actual factor b: temperature = 0 factor coding: actual design points: above surface below surface 0 100 x1 = a: concentration x2 = c: time actual factor b: temperature = 0 % p u ri ty (a) (b) purity, % x1 x 3 p u r it y , % position [°2theta] (copper (cu)) 20 30 40 50 60 counts 0 1000 0 1000 2000 0 10000 calcined limnpo4 pristine limnpo4 ref: 01-074-0375 2 theta, o in te n si ty , a .u . calcined limnpo4 pristine limnpo4 ref data: 01-074-0375 http://dx.doi.org/10.5599/jese.1191 j. electrochem. sci. eng. 12(2) (2022) 305-316 synthesis of carbon-coated limnpo4 cathode 312 the obtained results also confirm that a pure phospho-olivine structure limnpo4 can be generated with a peg-10000 concentration of 0.1 mol l-1, a reaction temperature of 250 °c and a reaction time of 180 min. this pure phase was indexed as limnpo4 crystal structure according to the standard data #01-074-0375, crystallizes in the orthorhombic system with the pmnb space group. during the thermal treatment process, most materials are generally subjected to some changes in the crystal structure, i.e., crystallite size and microstrain (such as crystal lattice defects, stacking errors, displacement, etc. [51]). in order to verify these two parameters, both samples before and after calcination were examined by the williamson hall (w-h) method as explained previously [35,36]. the w-h curves for all samples are displayed in figure 6. figure 6. williamson-hall plots of pristine and calcined limnpo4 obtained under optimum conditions (struct.b means structural broadening) according to these results, we can state that the crystallites size after calcination is about 68 ± 19 nm, which is strictly lower than that of the pristine material (150 ± 90 nm). this difference could be due to the thermal process that leads to the coalescence of the polyethylene glycol particles remaining adhered to the limnpo4 material surface during the synthesis steps, leading to the formation of smaller, well-carbonated nanocrystallites. the lowest microstrain value of about 0.1 ± 5 % was observed for the calcined sample, while the highest strain value of 0.3 ± 1 % was detected for the pristine one. it can be noticed that crystal lattice defects can be reduced using an optimized peg-10000 concentration, which can act as a protective matrix during the synthesis process due to the viscous property of this solvent. figure 7 shows the corresponding sem images of the obtained products, pristine limnpo4 and calcined limnpo4@c materials. the surface morphology of the pristine sample seems like particles embedded in a polyethylene glycol matrix. however, the calcined sample image shows irregular secondary particles, with degradation of peg matrix formed during synthesis steps, which confirms the transformation of peg particles still adhered on the limnpo4 material surface to a thin carbon layer. r. el-khalfaouy et al. j. electrochem. sci. eng. 12(2) (2022) 305-316 http://dx.doi.org/10.5599/jese.1191 313 figure 7. sem images of the synthesized pristine and calcined limnpo4 material under optimum conditions electrochemical performance of calcined limnpo4@ccathode material the charge-discharge behavior of the calcined limnpo4@c obtained under optimum conditions was studied using the “galvanostatic charging–discharging” method in the potential range of 2.5 to 4.5 v. as seen in figure 8, the charge-discharge curves of the 1st, 2nd and 3rd cycles exhibit clear charge/discharge plateaus around 4.25 and 4.05 v, which is in agreement with the electrochemical de-lithiation/lithiation process, respectively [52]. the initial charge-discharge specific capacities were 164.8 and 128.8 mah g-1 at 0.05 c-rate, respectively, which can be mainly attributed to the nanostructured crystallite size with the reduced microstrain that promotes good intercalation/disintercalation of lithium ions within limnpo4@c material structure [36,53]. our findings are in good agreement with some previous works, where it was confirmed that limnpo4 olivine structure without impurity could generate improved electrochemical performances [54]. however, the initial coulombic efficiency of about 78.2 % is mainly affected by unavoidable passivation phenomena of the electrolyte and the active electrode materials [55]. the as-prepared material under optimum synthesis conditions will be subjected to a wide range of electrochemical characterization in order to fully explain the different reaction mechanisms during the charge-discharge process. figure 8. charge–discharge profiles of prepared limnpo4@c material at 0.05 crate 0 20 40 60 80 100 120 140 160 2.0 2.5 3.0 3.5 4.0 4.5 v o lt a g e , v v s . l i specific capacity, mah/g 1st cycle 2nd cycle 3rd cycle http://dx.doi.org/10.5599/jese.1191 j. electrochem. sci. eng. 12(2) (2022) 305-316 synthesis of carbon-coated limnpo4 cathode 314 conclusions during this study, the intermediate compound li3po4 was firstly synthesized by a simple precipitation method. thereafter, the main material limnpo4 was prepared by solvothermal reaction under controlled conditions. the objective of this research was the optimization of solvothermal synthesis parameters using response surface methodology based on box-behnken design. three independent variables were considered in this study, which are the concentration of solvent (peg), reaction time and reaction temperature. the rsm optimization of operating conditions for the preparation of the pure limnpo4 phase was applied. analysis of variance (anova) confirmed that the proposed regression model is in good agreement with the experimental data, providing a high determination and adjusted determination coefficients.the obtained results confirmed that the optimum conditions for maximum limnpo4 phase purity are: cpeg = 0.1 mol l-1, t = 250 °c and  = 180 min. the material synthesized under optimum conditions was subjected to supplementary characterization techniques 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https://doi.org/10.1021/nl1007085 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.ijhydene.2019.05.129 https://doi.org/10.1016/j.matpr.2017.11.259 https://doi.org/10.1016/j.jpowsour.2007.06.203 https://doi.org/10.1016/j.jallcom.2007.09.118 https://doi.org/10.1016/j.jssc.2011.08.042 https://doi.org/10.1039/c2jm34193j https://doi.org/10.1016/j.electacta.2014.12.012 https://doi.org/10.1080/00224065.2007.11917695 https://doi.org/10.1002/ceat.201300328 https://doi.org/10.1016/j.apcatb.2011.01.003 https://doi.org/10.1021/ie070990y https://doi.org/10.1016/j.seppur.2013.08.001 https://doi.org/10.1016/j.apsusc.2013.05.098 https://doi.org/10.1007/s11356-013-1578-0 https://doi.org/10.1016/j.chemosphere.2012.01.036 https://doi.org/10.1016/j.jpowsour.2015.09.103 https://doi.org/10.1039/c5ta01218j https://doi.org/10.1002/ppsc.202100203 https://doi.org/10.1155/2014/768912 https://doi.org/10.1021/nl1007085 https://doi.org/10.1021/nl1007085 https://creativecommons.org/licenses/by/4.0/) benzimidazole-modified polyaniline micro-shells for electrochemical detection of cadmium in aqueous solution http://dx.doi.org/10.5599/jese.1440 275 j. electrochem. sci. eng. 13(2) (2023) 275-286; http://dx.doi.org/10.5599/jese.1440 open access : : issn 1847-9286 www.jese-online.org original scientific paper benzimidazole-modified polyaniline micro-shells for electrochemical detection of cadmium in aqueous solution eldhose v. varghese1,2, bejoy thomas1, carsten schwandt3, praveen c. ramamurthy4 and alex joseph1, 1department of chemistry, newman college, thodupuzha 685585, india 2department of medicinal and applied chemistry, kaohsiung medical university, kaohsiung 807, taiwan 3department of materials science and metallurgy, university of cambridge, cambridge cb3 0fs, united kingdom 4department of materials engineering, indian institute of science, bangalore 560012, india corresponding author: alex.chiramattel@gmail.com received: july 1, 2022; accepted: december 19, 2022; published: february 24, 2023 abstract benzimidazole-functionalized polyaniline (bmpani) was synthesized by interfacial polymerization technique and used for electrochemical sensing of cadmium ions in an aqueous solution. the material was characterized for its structural and morphological features using fourier-transform infrared spectroscopy (ftir), x-ray diffractometry (xrd), and scanning electron microscopy (sem). the bmpani has a micro-shell structure produced from the selfassembly of the monomer units in solution before the polymerization reaction. the material was trialed for cadmium ion sensing using a bmpani-modified carbon paste electrode (bmpani-cpe). electrochemical techniques, i.e., cyclic voltammetry (cv) and differential pulse anodic stripping voltammetry (dpasv), were performed to assess the sensing characteristics of the material. various electrode preparation parameters, i.e., deposition potential, ph of deposition solution, and thickness of the active layer, were optimized to achieve the highest level of sensitivity. the selectivity towards cadmium ions, interference from other ions, as well as stability and reusability of the bmpani-cpe, were also examined and found to be satisfactory. keywords polyaniline; electrochemical biosensor; cyclic voltammetry; differential pulse anodic stripping voltammetry; sensor stability; sensor reusability introduction the agency for toxic substances and disease registry (atsdr) and environmental protection agency (epa) positioned cadmium at the top seventh in the latest hazardous substances priority list http://dx.doi.org/10.5599/jese.1440 http://dx.doi.org/10.5599/jese.1440 http://www.jese-online.org/ mailto:alex.chiramattel@gmail.com j. electrochem. sci. eng. 13(2) (2023) 275-286 detection of cadmium in aqueous solution 276 (atsdr-2022), with 1317 against arsenic with 1675 total points. cadmium is detrimental and poses potential threats to human health due to its known and suspected toxicity, causing issues such as cancer and organ system toxicity. the latter regards the skeletal, respiratory, cardiovascular, urinary and reproductive systems, as well as the central and peripheral nervous systems [1]. for instance, it affects the mitochondrial respiratory chain, which plays an essential role in maintaining energy homeostasis through oxidative phosphorylation (oxphos), generating energy in the form of adenosine triphosphate (atp) which is the energy mandate for life systems [2]. the maximum permissible concentration of cd(ii) ions in drinking water asserted by the world health organization (who) is 3 µg/l [3]. several reliable techniques are used for the detection of cd(ii) ions, including atomic absorption spectrometry (aas) [4], flame atomic absorption spectrometry (faas) [5,6], inductively coupled plasma mass spectrometry (icp-ms) [7], inductively coupled plasma optical emission spectrometry (icp-oes) [8], and fluorescence spectrometry (fs) [9]. the prerequisites for these methods are tedious and corrosive sample preparation protocols in addition to the requirement of highly experienced personnel and expensive instruments, all of which limit their use. an alternative to the aforementioned methods is electrochemical sensors, employing measuring techniques such as cyclic voltammetry (cv), differential pulse anodic stripping voltammetry (dpasv) and chronoamperometry. dpasv is considered to be one of the most sensitive electrochemical techniques in electroanalysis. the electrodes used are usually based on carbon paste and modified with various active materials like polymers, inorganic compounds or composites. conducting polymers are considered one of the best choices to modify electrodes because they can often be synthesized in nanostructured forms possessing large surface area, high porosity, fast electron mobility, and possibilities for surface modifications to tune the electrochemical properties [10,11]. the sensitivity can be enhanced further by chemical modification through covalently-bound chelating functional groups on the polymer backbone [12,13] or by physical modification mediated through van der waals forces. to mention specific examples, polyaniline with backbones modified with either imidazole [14] or iminodiacetic acid [15] was reported for heavy metal ion sensing with improved sensitivity compared to the unmodified form. in another study by mustafin et al. [16], polyaniline modified with alkenyl side chains was used for resistive sensing of humidity. all said above shows that chemical modification is a preferred strategy to modify the transduction properties of polyaniline. the present work reports an electrochemical cd2+ ion sensor based on poly(2-(1h-benzimidazol -2-yl) aniline) with enhanced sensitivity. the polymer was synthesized by polymerizing 2-(1h-benzimidazol-2-yl) aniline monomer using the interfacial polymerization technique. the resultant polymer micro-shells (bmpani) were then used to modify a carbon paste electrode (cpe), and this modified electrode was trialed for the quantitative determination of cd2+ ions in aqueous media. the electrochemical techniques used to investigate the interaction of cd2+ ions with the electrode surface were cv and dpasv. the potential of cd deposition, ph of deposition solution, and thickness of the active layer were all optimized to ensure a high sensing efficacy. selectivity and interferences, as well as stability and reusability of sensor bmpani-cpe, were also investigated. experimental materials and methods all chemicals and solvents used for the synthesis of poly(2-(1h-benzimidazol-2-yl) aniline) were of analytical grade and used without any further purification unless specified. cadmium nitrate, lead nitrate, sodium acetate and acetic acid were obtained from merck india pvt. ltd. zinc nitrate, nickel nitrate and cobalt nitrate were acquired from spectrochem india ltd. ammonium peroxodisulfate (aps), e. v. varghese et al. j. electrochem. sci. eng. 13(2) (2023) 275-286 http://dx.doi.org/10.5599/jese.1440 277 sulfuric acid, hydrochloric acid, nitric acid and chloroform were obtained from nice chemicals pvt. ltd. india. oxalic acid and phosphoric acid were acquired from sd fine chemicals ltd. india. 2-(1h-benzimidazol-2-yl) aniline was purchased from astatech inc. usa. chloroform was double distilled over 4 å molecular sieve and preserved under an inert atmosphere prior to use. deionized (d.i.) water was used throughout. the chemical structure of the synthesized polymer was analyzed by ftir using a thermo nicolet is5 spectrometer in the atr mode with 64 scans in the range of 400 to 4000 cm-1. the crystalline properties of the polymer were studied by xrd using a rigaku mini flex 600 diffractometer in the 2 range of 5 to 80 degrees. its morphology was analyzed by sem using a jeol jsm-6490la field emission scanning electron microscope. all electrochemical studies were performed with a metrohm autolab pgstat302n workstation. a three-electrode configuration was used, comprising an inhouse-prepared chemically modified cpe as the working electrode, a saturated calomel electrode (sce) from ch instruments inc. as the reference electrode, and a platinum wire as the counter electrode. interfacial polymerization of 2-(1h-benzimidazol-2-yl) aniline (bmpani) the polymerization of 2-(1h-benzimidazol-2-yl) aniline was carried out by interfacial polymerization, following a procedure reported previously for the synthesis of poly(anthranilic acid) [17-19]. briefly, two solutions consisting of (a) 0.43 g of ammonium peroxodisulfate in 100 ml 0.25 m h2so4 and (b) 0.36 g of 2-(1h-benzimidazol-2-yl) aniline in 100 ml chloroform were prepared separately. then, the former solution was added to the latter dropwise without mixing the solutions. the interface between the two solutions served as the site where the polymerization reaction was initiated, as thereafter indicated by the appearance of a reddish-brown coloration. the reaction was left for 24 h in order to ensure complete polymerization, and the polymer formed gradually filled up the entire aqueous phase. the black precipitate was filtered out, washed with d.i. water, and finally dried in a vacuum oven at 50 °c for 24 h. the yield was 0.124 g, corresponding to 36 %. preparation of bmpani-modified carbon paste electrode (bmpani-cpe) the carbon paste required for cpe was prepared by mixing graphite powder and paraffin oil in a 7:3 ratio in a ball mill for 72 h. the carbon paste thus prepared was then packed in a glass tube of 4 mm inner diameter and 60 mm length. a thin silver wire was inserted into the paste and served as the electrical contact. the exposed carbon paste was polished on plain printer paper to provide a smooth surface. uniform dispersion of the polymer in water was achieved by stirring 1 mg of poly (2-(1h-benzimidazol-2-yl) aniline) in 1 ml of d.i. water for 24 h followed by sonication for 20 min. the polymermodified carbon paste electrode (bmpani-cpe) was then made by drop casting the desired volume of this dispersion onto the cpe surface and subsequent drying in air. results and discussion characterization of the reaction product a schematic representation of the interfacial polymerization reaction of 2-(1h-benzimidazol-2 -yl) aniline is shown in scheme 1. the polymerization occurs through the aniline moiety, forming a 1,4-polymerized polyaniline backbone that is substituted with benzimidazole at its ortho position (bmpani). the purpose of the benzimidazole side chain is to serve as the metal-coordinating ligand in the envisioned sensing application. http://dx.doi.org/10.5599/jese.1440 j. electrochem. sci. eng. 13(2) (2023) 275-286 detection of cadmium in aqueous solution 278 scheme 1. illustration of the interfacial polymerization reaction for the production of 1,4-polymerized polyaniline the structures of bmpani and pani were determined using ftir spectroscopy. figure 1 shows the ftir spectrum of the synthesized bmpani along with that of pani synthesized under similar conditions as reference material. in the case of pani (curve a), the main peaks observed at 1502 and 1560 cm-1 correspond to the characteristic c=c stretching vibrations of the quinoid and benzenoid rings of polyaniline, respectively, and the peak at 1289 cm-1 is due to the typical c–n stretching vibration of the polyaniline backbone [20]. the band around 3230 cm-1 results from the n–h stretching vibration, and the weak band at 795 cm-1 is from the c–h out-of-plane bending vibration of the aromatic ring [20]. in the case of bmpani (curve b), the spectrum shows the characteristic features of pani, indicating that the bmpani is based on the polyaniline backbone. the benzenoid and quinoid characteristic c=c stretching vibrations are now observed at 1506 and 1570 cm-1, respectively, and the typical c–n stretching vibration of the polyaniline backbone now appears at 1302 cm-1. the peak at 1670 cm-1 corresponds to the characteristic c=n stretching vibration of benzimidazole [21], and the peaks at 1459 and 1220 cm-1 are due to the c=c [22] and c–n [23] stretching vibrations, respectively, of benzimidazole. altogether, these findings prove the presence of the benzimidazole entity as the side chain in the synthesized polymer. 500 1000 1500 2000 2500 3000 3500 4000 40 50 60 70 80 90 100 b t ra n s m it ta n c e , % wavenumber, cm -1 3230 3205 1560 1570 1506 1502 1459 1302 1289 1220758 795 1670 a 3365 figure 1. fourier-transform infrared spectra of: a – pani and b – bmpani the crystallinity of bmpani and pani was investigated by xrd, and the patterns for both are shown in figure 2. broad absorption bands, as opposed to sharp peaks, are observed for both materials, indicating their amorphous nature. in the case of pani, these bands are centered around 2 = 13.4, 19.5 and 26.2° and correspond to the (011), (020) and (200) crystal planes, respectively [24]. bmpani exhibits these three bands equally, with the angles shifted slightly to 2 = 13.8, e. v. varghese et al. j. electrochem. sci. eng. 13(2) (2023) 275-286 http://dx.doi.org/10.5599/jese.1440 279 18.8 and 26.2°, corresponding to the above three planes. the shift in band positions is ascribed to the presence of the benzimidazole group, intervening sterically between the polyaniline backbone and altering the interlayer spacings. 15 30 45 60 75 0 100 200 300 400 500 600 in te n s it y 2   a b 2 / ° 2 / ° figure 2. xrd patterns of: a – pani and b – bmpani the measured diffraction angles and the calculated d-spacings of bmpani and pani are tabulated in table 1, showing that bmpani and pani are similar in properties regarding their backbones and amorphous nature. table 1. angles of diffraction and corresponding d-spacings for bmpani and pani pani 2 / ° 13.4 19.5 26.2 d spacing, nm 0.66 0.46 0.34 bmpani 2 / ° 13.8 18.8 26.2 d spacing, nm 0.64 0.47 0.34 the morphology of bmpani was analyzed by sem, and the images recorded are shown in figure 3. it is observed that bmpani has a unique microstructure of spherical shells with hollow spaces inside. the formation of this distinct morphological feature is explained on the basis of the selfassembly of the monomer at the interface before polymerization in such a way that it forms hollow micelles that are then converted into the spherical shells of the resultant polymer. the average diameter of the shells is measured to be in the range of 1 to 5 µm. figure 3. sem images of bmpani at two different magnifications http://dx.doi.org/10.5599/jese.1440 j. electrochem. sci. eng. 13(2) (2023) 275-286 detection of cadmium in aqueous solution 280 electrochemical detection of cd2+ with bmpani-cpe a series of electrochemical studies were performed in order to validate the efficacy of bmpanicpe for the detection of cadmium ions in aqueous media. cyclic voltammetric studies were carried out to understand the interaction of the bmpani-cpe with cd2+ ions. the cvs recorded for the bmpani-cpe and, for comparison, the bare cpe, both in the absence and presence of cd2+ ions, are shown in figure 4. the experimental conditions were an aqueous acetate buffer of ph 5, a potential scan window from 0.5 to -1.8 v, and a scan rate of 50 mv/s. it is observed that, in the plain buffer solution, both the cpe and the bmpani-cpe display no characteristic features. in contrast, in the presence of 10 mm cd2+ ions in the buffer solution, both the cpe and the bmpani-cpe exhibit a characteristic redox peak couple. the presence of two peaks in these voltammograms results from a reduction of cd2+ ions to cd metal in the cathodic direction and oxidation of cd back to cd2+ in the anodic direction. for the cpe, the reduction and oxidation peaks are observed at -1.21 and -0.97 v, respectively, and for the bmpani-cpe, they are seen at -1.16 and -0.89 v, respectively. -2.0 -1.5 -1.0 -0.5 0.0 0.5 c u rr e n t, m a potential, v cpe bmpani-cpe cpe with cd 2+ bmpani-cpe with cd 2+ -0.4 -0.3 -0.2 -0.1 0.0 0.1 figure 4. cyclic voltammograms showing the interaction of bmpani-cpe and cpe with cd2+ ions (cd2+ ion concentration is 10 mm and potential is vs. sce) it is further observed that the bmpani-cpe has an improved signal intensity compared to the bare cpe. this indicates an enhanced interaction of the bmpani-cpe with cd2+ ions, which can readily be attributed to the benzimidazole functional group on the bmpani chelating the ion. overall, the cv results suggest that the bmpani-cpe can sense cadmium ions in an aqueous solution. stripping voltammetric studies were carried out for the quantitative determination of cd2+ ions in an acetate buffer solution. the dpasv analyses were done in two steps. in the first step, the preconcentration of cd2+ ions from the solution onto the electrode surface was induced by applying an appropriate cathodic deposition potential. in this step, cd2+ is coordinated by the bmpani functional group and then reduced to cd on the cpe surface, as represented in equation (1). cd2+ → [cd-bmpani]2+ → cdsorbed (1) in the second step, the stripping of the cd analyte from the electrode surface back into the solution was effected by sweeping the potential in the anodic direction from −1.2 to 0 v. in this step, the reactions leading to the deposition are reversed, as represented in equation (2). cdsorbed → cd2+ (2) in each measurement, the anodic stripping current peak, due to the oxidation of the deposited cd, was observed at around -1.0 v during the potential sweep. e. v. varghese et al. j. electrochem. sci. eng. 13(2) (2023) 275-286 http://dx.doi.org/10.5599/jese.1440 281 in order to achieve maximum sensitivity of bmpani-cpe for sensing cd2+ ions, the dpasv electrode parameters were optimized by trial-and-error experiments. this included the potential of cd deposition, ph of the deposition solution, and thickness of the polymer layer coated onto the electrode surface, all as summarized in figures 5a to 5c. the cd2+ ion concentration was kept at 1 µm in all cases. first, the effect of the cd deposition potential on the anodic stripping current was analyzed by varying the deposition potential from -1.4 to -0.8 v (figure 5a). from the stripping currents in the figure, it is clear that the amount of cd2+ ions deposited onto the electrode surface firstly increases on lowering the deposition potential from -1.4 to -1.2 v and then decreases on further lowering the potential. therefore, the deposition potential of -1.2 v was selected as the optimum one in further analysis. second, the effect of the ph of the deposition solution on the anodic stripping current was investigated by changing the ph of the acetate buffer from 3.5 to 5.5 (figure 5b). it is seen that the stripping current increases steadily with the increase of ph, attains a maximum at ph 5, and declines thereafter. this phenomenon is ascribed to the ph-dependent coordination ability of the bmpani [25], which then affects the subsequent electrochemical reduction of the cd2+ ions and, thereby, the cd stripping current. acetate buffer of ph 5 was hence chosen for further analysis. finally, the impact of the thickness of the active polymer layer coated onto the cpe surface was assessed by varying the volume of the drop-casting solution from 5 to 40 µl (figure 5c). here, the stripping current increases with the increase in layer thickness corresponding to solution volumes from 5 to 20 µl, and then decreases with a further increase in layer thickness for solution volumes from 20 to 40 µl. the optimum thickness of polymer coated onto the electrode surface was therefore taken as that produced by 20 µl of deposition solution. -1.4 -1.3 -1.2 -1.1 -1.0 -0.9 -0.8 c u r r e n t , a deposition potential, v a 0.0 1.0 2.0 3.0 4.0 5.0 3.5 4.0 4.5 5.0 5.5 c u r r e n t,  a ph b 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0 5 10 15 20 25 30 35 40 45 c u rr e n t,  a volume, µl c 0.5 1.0 1.5 2.0 2.5 3.0 3.5 r 2 = 0.9750 r 2 = 0.9855 c u rr e n t,  a a b c d 0 8 16 24 32 0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2 concentration,  volume, l concentration, m figure 5. stripping current as a function of: a – deposition potential; b – ph of deposition solution; c – thickness of polymer layer (cd2+ ion concentration is 1 m); d – stripping current as a function of cd2+ ion concentration in the form of two linear correlations table 2 lists the optimized electrode preparation parameters obtained from the dpasv analysis. http://dx.doi.org/10.5599/jese.1440 j. electrochem. sci. eng. 13(2) (2023) 275-286 detection of cadmium in aqueous solution 282 table 2. optimized electrode preparation parameters for dpasv studies with cd2+ ions (cd2+ ion concentration is 1 m) parameter optimized value deposition potential, v -1.2 ph of deposition solution 5 volume of deposition solution controlling thickness of polymer layer coated onto cpe, l 20 the pre-set default instrumental parameters used in the individual dpasv experiments were a scan rate of 50 mv/s, pulse amplitude of 100 mv, and pulse period of 40 ms. applying the above-optimized electrode preparation parameters and the pre-set dpasv instrumental parameters, a graph was constructed to plot the recorded anodic stripping currents as a function of the cd2+ ion concentration in the range from 1 µm to 0.1 nm, as shown in figure 5d. it is evident that the anodic current increases with the cd2+ ion concentration. more specifically, this occurs in two linear regions, a first linear regime between the concentrations from 1 to 15 nm with r2 value of 0.9855 and a second linear regime over the concentrations from 15 to 300 nm with r2 value of 0.9750. consequently, with knowledge of these calibration lines, bmpani-cpe has the potential to be utilized for the determination of cd2+ ions within the specified concentration range with a high degree of accuracy. the limit of detection (lod) for cd2+ ion sensing was calculated using the equation lod = 3sa/b, where sa is the standard deviation of the lowest concentration measured, and b is the slope of the linear regression line [26]. for the regions ab and bc, as marked in figure 5d, the lod values are calculated as 0.139 and 25.8 nm, respectively. selectivity, stability, and reusability of bmpani-cpe selectivity, stability, and reusability are three important qualifying parameters for the commercial application of any sensor. selectivity determines how the electrode behaves in the presence of interfering species, stability defines the shelf-life of the electrode prior to use, and reusability specifies the number of times a single electrode can be used for the same analysis. the selectivity of the bmpani-cpe sensor towards cd2+ ions was tested by recording dpasv signals in aqueous solutions of 100 nm concentration of cd2+ ions in the presence of 0.1 equivalent concentrations of potentially interfering ions, i.e., zn2+, pb2+, ni2+, co2+, so42-, cl-, no3-, c2o42and po43-, under the experimental conditions previously optimized. figure 6 represents the effects of these ions on the anodic stripping current. it is found that 0.1 equivalents of zn2+, pb2+, ni2+, co2+ and so42ions have no marked effect on the cd2+ ion sensing, whereas cl-, no3-, c2o42and po43ions cause noticeable interferences and thus affect the quantitative analysis of cd2+ ions. cland no3ions decrease the anodic stripping current, while c2o42and po43ions increase the stripping current, indicating opposing impacts of these ions on the sensing mechanism. altogether, the results prove that the bmpani-cpe possesses a good selectivity towards cd2+ ions in the presence of zn2+, pb2+, ni2+, co2+ and so42ions and should be useable in practical applications where these ions occur in small amounts. in order to test the selectivity towards cd2+ ions in the presence of higher concentrations of ni2+, zn2+, co2+, pb2+ and so42ions, dpasv studies were carried out in the presence of both 1 and 10 equivalent concentrations of these ions. figure 7 presents the results. it is seen that 1 equivalent of ni2+, zn2+ and so42ions and 10 equivalents of zn2+ and so42ions do not interfere with the quantitative analysis of cd2+ ions. therefore, the proposed sensor should be useable for the determination of cd2+ ions even in the presence of enhanced concentrations of these ions. only 10 equivalents of ni2+ ions and 1 and 10 equivalents of co2+ and pb2+ ions cause visible interferences. e. v. varghese et al. j. electrochem. sci. eng. 13(2) (2023) 275-286 http://dx.doi.org/10.5599/jese.1440 283 c d + p o 4 c d + c 2 o 4 c d + n o 3 c d + c l c d + s o 4 c d + c o c d + n i c d + p b c d + z n c u rr e n t,  a interfering species c d 0.0 1.0 2.0 3.0 4.0 figure 6. effect of 0.1 equivalent concentrations of interfering ions on the determination of cd2+ ions (100 nm) under optimized experimental conditions c d c d + n i c d + z n c d + c o c d + p b c u rr e n t,  a interfering species 1 equivalent 10 equivalent c d + s o 4 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 figure 7. effect of 1 and 10 equivalent concentrations of interfering ions on the determination of cd2+ ions (100 nm) under optimized experimental conditions the long-term stability, or shelf-life, of the bmpani-cpe sensor was assessed by recording the dpasv currents for a number of sensors after storage periods between 1 and 15 days. cd2+ ion solutions of concentration 100 nm were used under the optimized experimental conditions, and sensors were kept in a closed vessel prior to analysis. the sensor responses are shown in figure 8a, in which each current value is the average of three different sensors. the results indicate that more than 80 % of the initial electrode response is retained up to a storage time of 11 days. this demonstrates that the bmpani-cpe sensor has an acceptable shelf-life for applications with a time gap between sensor preparation and sensor utilization. 2 4 6 8 10 12 14 c u rr e n t,  a number of day a 0.0 1.0 2.0 3.0 4.0 5.0 0 2 4 6 8 10 12 c u rr e n t,  a number of measurements b 0.0 1.5 3.0 4.5 6.0 number of days number of measurements figure 8. a – stability over time of bmpani-cpe for cd2+ ion sensing (100 nm); b – reusability of bmpani-cpe for cd2+ ion sensing (1 m) http://dx.doi.org/10.5599/jese.1440 j. electrochem. sci. eng. 13(2) (2023) 275-286 detection of cadmium in aqueous solution 284 the reusability of the bmpani-cpe sensor was evaluated in a series of successive dpasv analyses of the same sensor in a cd2+ ion solution of concentration 1 m. each measurement was followed by leaching in 0.1 m hno3 solution to ensure the complete removal of cd from the electrode surface, as confirmed by dpasv analysis in pure acetate buffer solution. the sensor responses are shown in figure 8b, with each current reading being the average from three different sensors. the results indicate that more than 85 % of the initial electrode response is retained in up to 7 successive measurements. this highlights the excellent reusability of the bmpani-cpe sensor, which is hugely beneficial for its practical use. conclusions polyaniline modified with a side chain of benzimidazole (bmpani) has been synthesized by interfacial polymerization and characterized with regard to its structure and morphology. the synthesized bmpani contains the benzimidazole entity as an ortho substituent on its polyaniline backbone and has a unique microstructure of spherical shells resulting from the applied polymerization technique. the synthesized bmpani has been used to modify the surface of a carbon paste electrode (cpe), and the bmpani-cpe obtained has been employed as the working electrode for cadmium ion sensing in aqueous media. the electrode preparation parameters have been optimized for maximum sensor sensitivity, and the parameters empirically arrived at are deposition potential of -1.2 v vs. sce, ph 5 for the deposition solution, and a volume of 20 µl for the deposition solution that controls the thickness of the polymer layer coated onto the cpe. sensor measurements using dpasv have provided a calibration plot exhibiting two linear regions, one in the range from 1 to 15 nm and the other from 15 to 300 nm, with the corresponding lods of 0.139 and 25.8 nm, respectively. the bmpani-cpe has been found to possess a good selectivity towards cd2+ ions in the presence of 0.1 equivalents of zn2+, pb2+, ni2+, co2+ and so42ions. it also displays excellent stability, reusability, and 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gupta, methods for the determination of limit of detection and limit of quantitation of the analytical methods, chronicles of young scientists 2 (2011) 21-25. https://doi.org/10.4103/2229-5186.79345 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://dx.doi.org/10.5012/bkcs.2012.33.6.1919 https://doi.org/10.1007/s10965-013-0142-4 https://doi.org/10.1016/s0079-6700(97)00030-0 https://doi.org/10.1016/s0079-6700(97)00030-0 http://dx.doi.org/10.15254/h.j.d.med.7.2015.141 https://dx.doi.org/10.7897/2230-8407.050469 http://dx.doi.org/10.13005/ojc/340152 https://doi.org/10.1016/s0043-1354(00)00127-5 https://doi.org/10.4103/2229-5186.79345 https://creativecommons.org/licenses/by/4.0/) characterization of an hrp-aox-polyaniline-graphite composite biosensor doi: 10.5599/jese.2014.0057 165 j. electrochem. sci. eng. 4(4) (2014) 165-175; doi: 10.5599/jese.2014.0057 open access: issn 1847-9286 www.jese-online.org original scientific paper characterization of an hrp-aox-polyaniline-graphite composite biosensor ana carolina o. santana, erica f. southgate, joão paulo b. g. mendes*, jo dweck, eliana mosse alhadeff and ninoska isabel bojorge ramirez**, escola de química, universidade federal do rio de janeiro, av. horácio macedo, 2.030, centro de tecnologia, bloco e, e-203, cidade universitária,cep 21941-909, rio de janeiro, brasil *instituto de química, universidade federal do rio de janeiro, av. horácio macedo, 2.030, centro de tecnologia, bloco a, a-302, cidade universitária, cep 21941-909, rio de janeiro, brasil **universidade federal fluminense, dep. engenharia química e de petróleo, r. passo da pátria, 156, bl e-226, são domingos , niterói, cep 24210-240, rio de janeiro, brasil corresponding author: e-mail: nbojorge@id.uff.br; tel.: +55-21-26295598 received: march 23, 2014; revised: june 7, 2014; published: december 6, 2014 abstract nowadays there is an increasing demand to develop new and robust biosensors in order to detect low concentrations of different chemicals, in practical and small devices, giving fast and confident responses. the electrode material was a polyaniline-graphite-epoxy composite (pani/gec). alcohol oxidase (aox) and horseradish peroxidase (hrp) enzymes were immobilized and the responses were tested by cyclic voltammetry. the conductivities for the composites of graphite/polyaniline were determined. the cyclic voltammograms allowed detecting ethanol in pure diluted samples in a range from 0.036 to 2.62 m. differential scanning calorimetry (dsc) and thermal gravimetry analysis (tga) were used to verify the thermal characteristics of the composites (0, 10, 20, 30 and 100 % of graphite). the imax value was determined for the dual enzyme biosensor (0.0724 a), and the k app , m as 1.41 m (with r 2 =0.9912). keywords cyclic voltammetry; ethanol; immobilized enzymes; pani/gce introduction ethanol is the most frequently analyzed aliphatic alcohol and several methods have been developed for its quantitative determination [1-3]. measurement of alcohol levels in liquors and alcoholic drinks is a common necessity as is clinical analysis of patient tissue samples. the method http://www.jese-online.org/ mailto:nbojorge@id.uff.br j. electrochem. sci. eng. 4(4) (2014) 165-175 hrp-aox-polyaniline-graphite composite biosensor 166 approved by the association of official analytical chemists [4] for quantitative volumetric determination of alcohol in beer, wine and distilled spirits is pycnometry, which is the most common method for determining solution density. this method is considered as reference and has the advantages of accuracy and no need for comparison against a standard solution. the principal disadvantage is that the methodology is laborious, requiring a significant amount of time for its performance. another disadvantage is that it requires pre-distillation, generally regarded as the first step in which error is introduced during the process of quantitative detection and analysis [5]. other more accurate analytical methods include spectrophotometry and chromatographic techniques: gas chromatography (gc) or high performance liquid chromatography (hplc). however, these methodologies are often less favorable due to high equipment prices and the need for well-trained operators. there is currently a movement toward replacing these methods with low-cost, fast and reliable electrodes working in conjunction with immobilized enzymes [6]. there is a growing need for the development of disposable devices for clinical and/or environmental monitoring. this need has stimulated the development of new technologies and methodologies that can efficiently monitor an increasing number of analytes on site in the environmental field or support clinical diagnoses as quickly and as cheaply as possible; offering even the possibility of on-site field monitoring. besides selectivity, an analytical device must also be sensitive. in this respect, biosensors have shown great potential in recent years and thus appear to be useful components of effective analytical tools [7-8]. biosensors that link enzyme catalyzed chemical reactions with amperometric detectors are having a great impact on fields such as environmental monitoring [9-10], analysis of the quality of food and beverages [11-12], biomedical monitoring process [13-14] and biomedicine [15]. these analytical tools, prepared by immobilization of enzymes on an electrode surface, are simple, sensitive and offer a fast response. the main problem that appears in the operation of these devices is the transfer of electrons from the active site of the enzyme to the electrode. immobilization of the biological material on the electrode surface constitutes a crucial step in development of the biosensor, since the enzyme’s structure must be maintained in order to enable its action on the sample of interest [16-18]. horseradish peroxidase (hrp) is widely used in enzyme-linked biosensors. however, there are at least two main drawbacks shown by this enzyme: (1) it exhibits a very broad specificity to reduce substrates [19-20], which results in low selectivity of the biosensor; (2) although it displays good stability at room temperature, it is unstable at high temperatures [21-22]. the co-immobilization of alcohol oxidase with horseradish peroxidase is expected to increase the selectivity and amplify the sensitivity of the biosensor for the quantitative determination of ethanol [23-24]. immobilization of dual enzymes provides an excellent basis for increasing the selectivity, sensitivity and the thermal stability of the biosensor, depending on the strategy adopted for immobilizing the enzymes [15]. the immobilized enzymes may be reused several times or employed in an economical continuous flow path. dual enzyme-linked sensors are amenable to automation for analytical measurements, scale up of enzymatic biotransformation reactors, or to recover a product with greater purity [19, 25]. xie, et al. [26] reported recent advances in enzyme immobilization technologies that enhance enzyme properties such as activity, stability, specificity and reduced inhibition effects. the authors suggest that in the future multi-enzyme sensors based on co-immobilization would be the solution to many of the applications for the biotechnology industry and analytical devices. a. c. o. santana et al. j. electrochem. sci. eng. 4(4) (2014) 165-175 doi: 10.5599/jese.2014.0057 167 the objective of this study was to characterize a composite-based on pani / epoxy / graphite and evaluate its performance as a substrate for horseradish peroxidase (hrp) and alcohol oxidase (aox) enzymes immobilized to an electrode creating a biosensor for ethanol detection. the development of such a method for the immobilization of multiple enzymes is highly attractive, especially for economic reasons because as enzymatic activity decays the support can be regenerated and reloaded with fresh enzyme. in fact, the cost of support is often a primary factor in the overall cost of the immobilized catalyst. in order to build the biosensor a composite prepared with graphite and an electron conductor polymer as polyaniline was studied and characterized in terms of its electrochemical conductance capacity and thermal stability [18,27]. we find critical compositions of the material that works with improved sensitivity over a relatively broad range of ethanol concentrations. experimental materials horseradish peroxidase (hrp; ec 1.11.1.7) was purchased from toyobo, brazil and alcohol oxidase (aox, ec 1.1.1.1, specific activity of 200 units/mg of protein), graphite powder and polyaniline (emeraldine salt) were purchased from sigma-aldrich. for the incorporation of enzymatic solutions, a 2.5 % (v/v) of glutaraldehyde (sigma-aldrich) and 1 mg/ml of protein albumin were used. the ethanol standard solutions were prepared with 0.1 m mono potassium phosphate buffer (ph 7.0). all reagents were of analytical-reagent grade. all solutions were prepared with distilled water. apparatus amperometric measurements were carried out using an autolab pgstat12 (ecochemie) connected to a personal computer via a serial rs232 port for data acquisition. the obtained amperometric alcohol dual enzyme sensors were evaluated by means of cyclic voltammetry in a three-electrode configuration with ag/agcl/kcl (3m) reference electrode and pt-wire counter electrode. when not in use, the electrode was stored dry at 4 °c in a refrigerator. the thermal properties (thermogravimetric analysis (tga) and differential scanning calorimetry (dsc)) for the composites of graphite : pani prepared with 0, 10, 20, 30 and 100 % of graphite were performed by ta instruments sdt q600. analyses were conducted in a 30 ml/min flow rate of air atmosphere, with a ramp of 5 °c/min from 30 to 800 °c. the electrical conductivities of the composites pellets were evaluated using the techniques of two electrodes, between which a pellet of known composition of the composite was fixed with the aid of a sleeve of teflon. the tests were done using a bench meter icel manaus md 6700 coupled to a computer. the disks pellets prepared with 0% and 100% of pure graphite and composites with 1, 3, 5, 10, 20, 30, 50, 70, 100% of graphite mixed with pani were measured. pure samples of pani and graphite were also determined. preparation and evaluation of the aox–hrp-based biosensors hrp (3.60 g) was dissolved in 30 ml of 50 mm phosphate buffer (pb, ph 7.0). after filtration and dialysis steps, a 0.133 mg/ml of hrp solution was mixed with aox (47 mg/ml) in buffer ph 7.0. the 10 % (w/v) of bovine serum albumin and 2.5% (v/v) of glutaraldehyde were also prepared in 50 mm pb (ph 7.0) solution. a 10 μl volume of bovine serum albumin and 10 μl of glutaraldehyde were deposited on the electrode surface sequentially. the excess of glutarj. electrochem. sci. eng. 4(4) (2014) 165-175 hrp-aox-polyaniline-graphite composite biosensor 168 aldehyde was rinsed off with water. teflon cylindrical electrodes (5.0 × 0.7 cm and 0.13 cm inside orifice) were used to construct the working dual enzyme biosensor with a 20 ml electrolytic cell and an ag/agcl reference electrode and a platinum counter-electrode. procedure for immobilization the methodology used was the ionic immobilization of aox and hrp enzymes on the electrode surface constructed using a graphite matrix with polyaniline and an epoxy resin. during the immobilization step, a solution containing 2.5 % (v/v) glutaraldehyde, 0.5 % (v/v) bsa and 97 % (v/v) enzyme solution containing 1100 μl hrp and 15 μl of aox was deposited on the electrode surface. the electrode was left at 4 °c for 24 hours [28]. measurement procedure cyclic voltammetry (cv) measurements on the electrode were performed in a 3-electrode system containing a ag/agcl/kcl 3m (microquímica®) reference electrode, coiled platinum wire (99.99 % pure) mounted at the end of a chemically-resistant epoxy rod as counter electrode in addition to the modified working electrode based on pani/gce. the potential was cycled between –400 and 400 mv vs. ag/agcl. determination of ethanol in samples ethanol (95 %) samples (0.15 ml) were diluted in a 10 ml flask with 0.1 m mono potassium phosphate buffer solution (ph 7.0). voltammetric determination was carried out by applying the standard addition method. diluted sample and standard ethanol solution (0.15 μl) were added to the voltammetric cell containing 10 ml of 0.1 m mono potassium phosphate buffer solution (ph 7.0). results and discussion study of differential scanning calorimetry and thermal gravimetry analysis the thermal stability of the graphite composite samples was analyzed by tg, derivatived thermogravimetric analysis (dtg) and dsc. results of tg and dtg analyses are presented in figure 1. the curves in figure 1 follow the mass as a function of temperature of composite samples containing 100 % graphite, 20 % graphite and 0% graphite (100 % polyaniline), respectively. the curve for the composite of graphite/pani (red dashed line) shows an intermediate stability between pure samples of graphite and pani polymer. the presence of the pani introduces four decomposition steps. in the first stage, beginning at 150 °c, there is a slow weight loss associated to the release of trapped water or organic solvents in the polymer structure. the second stage of weight loss is observed from 270 °c to 550 °c and is attributed to decomposition of the oligomers. the third decay, from 350 °c to 450 °c, was assigned to the thermal decomposition of the pani chains. the dtg curves fully support the above mentioned losses. the pure graphite sample decomposes above 600 °c, whereas the 20 % graphite composite presents four degradation steps (three attributed to the pure polyaniline and one to the pure graphite). similar results were found by kowner, et al. [29], bourdo, et al. [30] and mo, et al. [31]. a linear fitting between the data of polymer content estimated by tg and composition on a dry basis of raw materials in the composites was proposed, showing a good correlation coefficient (r2 = 0.9811). the difference between the values estimated from the correlation with those of the components in the composite has an average value of -0.1 % with standard deviation of 1.86 %. a. c. o. santana et al. j. electrochem. sci. eng. 4(4) (2014) 165-175 doi: 10.5599/jese.2014.0057 169 fig. 1. tg and dtg curves for the samples prepared with 100, 20 and 0 % of graphite. conductivity of graphite/pani composite song and choi [32] have reported that the most conductive form of pani is the fully protonated, half-oxidized emeraldine salt form. a decrease in conductivity was observed when the polymer was deprotonated or either fully oxidized or reduced. this work intends to develop a prototype composite-biosensor based on typical pani that maintains the conductivity. conductivity values were determined for different concentrations of graphite : polyaniline composites (1, 3, 5, 10, 20, 30, 50, 70, 100 % of graphite) and for the pure alcohol oxidase. as shown in figure 2 an increase in the conductivity of the composite samples was observed. fig. 2. electrical conductivities of pani/gec as a function of graphite concentration in the composites. also shown are samples with 100 % pani (green bar) and 100 % graphite (red bar). j. electrochem. sci. eng. 4(4) (2014) 165-175 hrp-aox-polyaniline-graphite composite biosensor 170 this is probably due to synergistic effects of mixing the conductive polymer pani and graphite powder. the mixture better supports electron transfer and, consequently, displays enhanced electrical conductivity. the conductivity values determined for the pure samples of graphite and pani were 1.82×10-3 s / cm and 4.64×10-4 s / cm, respectively. the maximum value was obtained with the 70 : 30 (graphite:pani) composite; higher than the value measured for the 100 % graphite sample. for the composites prepared with 1 to 10 % of graphite, there were not any significant variations in conductivity. a linear relationship between graphite content and conductivity was observed from 20 to 70 % with the conductivity values for 50 to 70 % of graphite surpassing those for the 100 % graphite sample. mo et al. [31] has detected an increase in the electrical conductivity as a function of graphite nanosheet content in a composite prepared with graphite nanosheets and pani. bourdo et al. [30] also found similar behavior for pure pani and graphite samples and for pani/graphite composites. in the present study, the 30 % pani composite compound was employed due to the improved performance of its electrical response. electrochemical behaviour of the biosensor the biosensor bi-enzymatic hrp/aox was characterized using cyclic voltammetry to demonstrate the electrochemical performance of the system. figure 3 shows the cyclic voltammograms obtained from 5 to 150 mv s-1 in a solution of 1mm k4fe(cn)6 mixture in 0.1 m kcl and phosphate buffer ph 7.0. the peaks currents of the cvs indicating quasi-reversible processes between fe(cn)64-/fe(cn)63couple and the electrodes at the faster scan rates each curve has the same form but it is apparent that the total current increases with increasing scan rate. this again can be rationalized by considering the size of the diffusion layer and the time taken to record the scan. clearly the voltammogram will be slower to record as the scan rate is decreased. hence the size of the diffusion layer above the electrode surface will be different depending upon the voltage scan rate used. in spite of that, working with lower scan rates a well-defined cathodic peak and a small anodic could be identified, and the scan rate of 10 mv s-1 applied to analyze the ethanol samples. so, the best quality voltammogram was obtained working with a scan rate of 10 mv s-1. therefore, that was the scan rate applied to analyze the ethanol samples fig. 3. cyclic voltammograms of the aox/hrp/graphite/pani in 0.1 m pbs, ph 7.0 at various scan rates (from inner to outer curves: 5, 10, 20, 50, 100, 150 mv s−1). a. c. o. santana et al. j. electrochem. sci. eng. 4(4) (2014) 165-175 doi: 10.5599/jese.2014.0057 171 figure 4 shows shows the current intensity for the calibration curve changed between 0.61 ma (0.316 m) to 0.25 ma (2.62 m) the concentration range of standard ethanol solutions used in the electrochemical measurements was 0.316 2.62 m (r2 = 0.991). this clearly demonstrates that the current density reduces linearly with increased ethanol concentration in the samples. this is attributed to an inhibition of the enzyme. the effect is especially evident at the higher ethanol concentrations, probably due to the reaction end-products (acetaldehyde) or external mass transfer limitations. a similar behavior was reported for aox and hrp that was covalently immobilized on controlled pore glass [23]. however, that study showed that all the supports exhibited less than 20 % of the specific activity of the free enzyme, as a consequence of conformational changes in the 3-d structure of the protein caused by the covalent binding of aox to the supports. in this work the enzymes were immobilized by adsorption, which is less aggressive than the covalent immobilization. however, the enzyme may be coupled to the support in a way that hinders the access of substrates to the active center, promoting the mass transfer limitations. the amperometric response exhibited by the different immobilized aox preparations was also very similar although the highest value was obtained when the support was activated using glutaraldehyde in phosphate buffer ph 7. sirkar, et al. [33] observed an increase in the current density (60 %) of the electrochemical biosensor response for a multilayer nanocomposite thin film using glutaraldehyde as a crosslinking agent in a trial for stabilizing the structure. the authors proposed that arginine and lysine residues of the enzyme react with amines present on the redox polymer and, as a consequence, the activity was maintained near 100 % for three weeks. fig. 4. ethanol biosensor calibration curve. scan rate 10 mv s-1. (n = four measurements). wu, et al. [34] reported an inverse calibration curve for oxygen consumption by a sensitive ethanol biosensor nanocomposite of carbon nanofiber with immobilized adh. they observed decreased oxygen consumption with the increase of ethanol concentration in the sample. chronoamperometric curves showed a decreasing response, upon addition of ethanol aliquots (0 112 μm) to static air-saturated ph 7.0 phosphate buffer saline. wen, et al. [35] reported an ethanol biosensor constructed with alcohol oxidase/chitosan immobilized eggshell membrane and a commercial oxygen sensor. those measurements were based on the depletion of dissolved j. electrochem. sci. eng. 4(4) (2014) 165-175 hrp-aox-polyaniline-graphite composite biosensor 172 oxygen upon exposure to ethanol solution (0.15 – 0.75 mm). al-mhanna and hueber [36] reported an economic system that worked with one enzyme in a differential ph measurement device for alcohol oxidase and -nicotinamide adenine dinucleotide (nadh+) reaction and obtained a logarithmic curve for ethanol concentrations against change in ph for standard samples. these authors described an inverse correlation between the signal response and the analyte concentration for the indirect detection measurements working with a wide range of ethanol standard concentration solutions (17.14 μm – 17.14 m). mackey, et al. [37] optimized the proportion of dual enzyme horseradish peroxidase:glucose oxidase biosensor working with ratios of 1 : 7 to 7 : 1, immobilized on a polyanilinepolyvinylsulphonate modified screen-printed carbon paste electrode and identified the proportion that produced the best response signal was 1 : 1. rondeau, et al. [38] identified the optimal proportion of two enzymes in the biosensor composite by monitoring electrical response signals to establish idealized conditions for glucose oxidase : horseradish peroxidase immobilized with a modified carbon paste for in order to increase the selectivity, sensitivity, accuracy and stability [38]. the intensity of the electrochemical signal response was analyzed by alpat and telefoncu [39] who measured the amount of alcohol dehydrogenase immobilized on the electrode surface (47.1 to 200 u cm-2) and found that the linear response was between 0.01 mm and 0.04 mm for 117.6 u cm-2. nicell and wright [40] reported the dependence of horseradish peroxidase activity over a wide range of hydrogen peroxide concentrations. they observed an increase in the inhibitory effect on the enzyme catalytic activity. the static procedures of the electrochemical measurements of this work for the ethanol concentration solutions (0.330 – 2.62 m) probably promotes an increase of the peroxide hydrogen concentration in the electrolytic cell, and hence the inhibition of the hrp. yotova and medhat [41] reported the inhibition effect in a multi-enzyme immobilized biosensor system constructed to analyze residue from pesticides with acetylcholinesterase and choline oxidase. the relative inhibition percentage of each measurement was calculated using the following equation: 0 0 , % 100 ipc ipc i = ipc  (1) where i is the relative inhibition; ipc0 is the initial inhibited cathode current intensity measured for the lower ethanol concentration and ipc the inhibited cathode current intensity determined for each sample. assuming a possible inhibition effect on the cyclic voltammetric response signal with the increase of the ethanol concentration in the sample, this treatment was adopted for this work. a linear correlation was observed, confirming the inhibitory effect of the ethanol on the enzyme. amine, et al. [42] published a review that discusses horseradish peroxidase among the enzymes that could be used for inhibition-based biosensors applied for food safety and environmental monitoring. kuusk and rinken [43] classified the carbaril inhibition of tyrosinase biosensor by excess substrate and considered the reasons behind their inability to determine low carbaryl concentrations by a classical steady state kinetic approach. the km and imax kinetics parameters were calculated from lineweaver–burk plots by using the relative inhibition values as described in equation 2: app m max ethanol max 1 1 1k = + ri ri c ri (2) a. c. o. santana et al. j. electrochem. sci. eng. 4(4) (2014) 165-175 doi: 10.5599/jese.2014.0057 173 where 1/cethanol is the concentration of the ethanol in the solution sample, ri and rimax represent the initial and the maximum relative inhibition current, respectively, and k app , m is the apparent michaelis constant. the lineweaver-burk plot for the dual enzyme aox-hrp biosensor showing 1/i versus 1/cethanol is illustrated in figure 5. the imax value determined considering the inhibition effect on the dual enzyme biosensor was 0.0724 μa, and the k app , m was 1.41 m (r 2=0.9912). fig. 5. line-weaver-burk plot for the bienzimatic aod-hrp biosensor for different ethanol concentration table 1 shows the analytical performance of the proposed ethanol biosensor towards ethanol detection compared with various electrochemical biosensors modified for dual enzymes that also reported k app , m and imax. despite the low affinity for substrate observed in this work, the sensitivity was higher when compared with those determined for both redox hydrogel dual enzyme films previously reported in the literature [44-45]. this suggests that the linear range and detection limit of the proposed ethanol biosensor mentioned above appear to be beneficial compared to other previously reported modified electrodes. table 1. comparison of analytical characteristics of ethanol dual enzyme biosensors. film/composite/enzymes i.d. / cm k app , m / mm imax / na sensitivity, na/m reference hrp+aox+pvi-os 0.305 4.71 813.95 0.17 [44] hrp/pvi10-os/peg-dge/aox/cp5 0.305 9.6 ± 0.3 572 ± 7 0.06 [45] pani-gec/hrp/bsa/aox 0.130 1.410 72.4 51.3. this work i.d. internal diameter; pvi poly(vinyl-imidazole; pvi10-os redox hydrogel synthesized; peg-dge poly(ethylene glycol) (400) diglycidyl ether; cp5 electrodeposition polymer; os complex: redox polymers synthesized (4,4'dimethylbipyridine); pani-gec: polyaniline in graphite epoxy composite; bsa bovine serum albumin. the applied potentials for all configurations are –50 mv vs. ag/agcl. j. electrochem. sci. eng. 4(4) (2014) 165-175 hrp-aox-polyaniline-graphite composite biosensor 174 conclusions the composite material prepared from differing proportions of graphite and pani displayed enhancement in the conductivity for compositions of less than 20 % graphite and a synergistic effect that increased its response for mixtures with more than 50 % of graphite. the thermal analysis techniques applied to characterize the prepared composites showed a good agreement with the original proposed formula composition. the electrochemical results confirm that it is possible to detect ethanol with this biosensor in the ethanol concentration range of 0.316 to 2.62 mol l-1 limited by a significant inhibition effect observed in the enzyme. acknowledgements: thanks to toyobo of brazil (enzyme horseradish peroxidase) and cnpq support from the announcement universal 2008/2010 and pibic. references [1] astm. d5501-12 standard test method, 100 barr harbor drive, west conshohocken, pa, usa, astm international (2012). 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[45] i. s. alpeeva, a. vilkanauskyte, b. ngounou, e. csöregi, i. y. sakharov, m. w. gonchar, microchim. acta 152 21-27 (2005). © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {potentiometric detection of low-levels of sulfamethazine in milk and pharmaceutical formulations using novel plastic membrane sensors} doi:10.5599/jese.575 17 j. electrochem. sci. eng. 9(1) (2019) 17-26; doi: http://dx.doi.org/10.5599/jese.575 open access: issn 1847-9286 www.jese-online.org original scientific paper potentiometric detection of low-levels of sulfamethazine in milk and pharmaceutical formulations using novel plastic membrane sensors saad s. m. hassan, ayman h. kamel, nada h. a. elbehery chemistry department, faculty of science, ain shams university, cairo, egypt corresponding authors e-mail: saadsmhassan@yahoo.com; ahkamel76@sci.asu.edu.eg received: june13, 2018; revised: july 29, 2018; accepted: july 30, 2018 abstract novel potentiometric sensors for selective screening of sulfamethazine (smz) in pharmaceutical preparations and milk samples were reported. the sensor membranes were made from pvc matrix doped with magnesium(ii)-, manganese(ii)and dichlorotin (iv)-phthalocyanines as ionophores and aliquat-336 and nitron/smz ion-pair complex as ion exchangers. these sensors revealed fast, stable and near-nernstian anionic response for the singly charged sulfamethazine anion over the concentration range 10-2 10-5 m. the sensors exhibited good selectivity towards smz over most known anions, excipients and diluents commonly added in drug preparations. validation of the proposed methods was demonstrated via evaluating the detection limit, linear response range, accuracy, precision (within-day repeatability) and between-day-variability. the sensors were easily interfaced with a double channel flow injection system and used for continuous monitoring of smz in drug formulations, spiked milk samples and biological tissues. the method offers the advantages of design simplicity, results accuracy, and automation feasibility. keywords sulfamethazine; potentiometric-ion sensors; automation; method validation introduction sulphonamides are classified and managed as antimicrobials for the treatment of food-producing animals such as cattle, sheeps, pigs and poultry [1,2]. side effects are correlated with high quantities of antibiotic residues in edible tissues such as resistance of microorganisms to antibiotic treatment, toxicological hazards, and allergenic effects [3]. for food safety, the maximum allowable quantities determined by the european community for sulfonamides in meat food products and milk is 100 µg/l [4]. sulphamethazine (smz) (also known as sulfadimidine, sdm) is an example of sulphonhttp://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:saadsmhassan@yahoo.com mailto:ahkamel76@sci.asu.edu.eg j. electrochem. sci. eng. 9(1) (2019) 17-26 potentiometric detection of sulfamethazine 18 amides, a broad-spectrum antibiotic used for the treatment of gastrointestinal and respiratory tract infections in livestocks. smz is categorized as a bacteriostatic drug which is very helpful for treatment of bacterial diseases in humans and other species caused by gram positive and gram negative bacteria [5]. it is widely used in veterinary practice for the treatment of coccidiosis in laying hens at the dose of 2 g/l for 6 consecutive days via the drinking water [6]. it is also used for therapeutic, prophylactic or as growth promoter and halt the growth of bacteria in animal production [3,7]. meat containing residual of smz can result in development drug resistance and hypersensitivity [8]. it has been reported that smz is the major issue in testing veterinary animal’s feeds. therefore, analytical methods are needed for detecting and quantification of smz residues in food animal products and pharmaceutical formulations. several methods have been reported for smz determination. these methods include colorimetry [9], immunoassay [1,10-13], gas chromatography [14-17], high performance liquid chromatography [18-22], thin layer chromatography [23], liquid chromatography [25-29], microbiological diffusion assay [29], microtitre plate assay [30], indirect atomic absorption spectrometry [31], ratiometric fluorescence with carbon and quantum dots [32], voltammetry [33], potentiometric sensors [34], piezoelectric sensors [35] and transmittance near infrared spectrometry [36]. most of these methods, however, have several drawbacks due to too long time consumption, labor-intensive effort and expensive cost. some of these methods need sophisticated instrumentation, suffer from a lack of selectivity, cover a narrow concentration range and need several manipulation steps for preparation and assessment procedures. on the other hand, potentiometric sensors are considered as viable and simple tools and have been used for the analysis of many types of pharmaceuticals [31-34]. these sensors have usually good performance characteristics and display useful analytical features [35-38]. however, little is known about their use for quantification of sulfamethazine. in the present work, novel potentiometric sensors are proposed for determination of smz. sensors are based on doping pvc membranes with ion exchangers and metal phthalocyanines. aliquat-336 and nitron ion association complexes with smz and charged mn(ii), mg(ii) and sn(iv) phthalocyanine ionophores are also used as sensing materials. these electroactive materials were dispersed in plasticized pvc membranes and used for static and hydrodynamic measurements of smz. the sensors were incorporated in a double channel flow injection system and used for continuous determination of smz in cow milk, chicken tissues and pharmaceutical formulations. experimental reagents and solutions analytical reagent grade chemicals were used in this work and de-ionized water (conductivity < 0.1 µs/cm) was employed for solutions preparation. high molecular weight polyvinyl chloride (pvc), o-nitrophenyloctylether (o-npoe), magnesium (ii)-, manganese (ii)and dichlorotin (iv)phthalocyanines, aliquat-336, tridodecyl-methylammonium chloride (tdmac) and tetrahydrofuran (thf) were obtained from fluka (ronkonoma, ny). nitron was purchased from riedel-de haën. pure grade of sodium sulfamethazine was obtained from sigma aldrich. 10-2 m nitron solution was prepared by dissolving the appropriate solid amount in 20 % acetic acid solution. 20 ml of 10-2 m nitron solution and 10 ml of 10-2 m smz were mixed together and stirred for 15 min. a brown precipitate is formed, filtered off, washed with de-ionized water, dried at room temperature and ground to a fine powder. the elemental analysis agreed with the composition [c20h17n4+][c13h13n3s] that confirms (1:1) stoichiometric ratio. stock solution of 10-1 m s. s. m. hassan et al. j. electrochem. sci. eng. 9(1) (2019) 17-26 doi:10.5599/jese.575 19 smz was prepared in de-ionized water. less concentrated smz standards were accurately prepared by dilution of the stock solution with a fresh 0.01 m na2so4 solution at ph 7. effect of interfering species was evaluated using 10-2 m solutions of sodium salts of phosphate, citrate, chloride, thiocyanate, salicylate, iodide, ascorbate, oxalate, tartrate and acetate. apparatus all potentiometric measurements were done with the electrochemical cell ag/agcl double junction reference electrode/sample test solution/smz selective membrane/10-3 m smz + 10-3 m of nacl/agcl/ag. an orion, 90-00-29, double junction electrode filled with 0.1 m lithium acetate in its outer compartment was used. potential differences between indicator and reference electrodes were measured by an orion digital ph/mv meter (type sa 720). the potentiometric signal output was transferred to a replacement point with six ways out; therefore, six sensors readings outplace in the same solution can be achieved. each way represented an electrical connector that provided suitable adaptation to each sensor. the ph was measured by a ross glass ph combination electrode (orion 81-02). spectrophotometric assays were carried out on a thermo scientific uv/vis evolution 300. sensor construction and electromotive force (emf) measurements three milligrams of metal-phthalocyanine ionophores were mixed with 126.4 mg of o-npoe plasticizer, 64.5 mg pvc and 1.2 mg tdmac and dissolved in 3 ml thf. the mixture was poured in a glass petri dish (3 cm diameter). a membrane consisting of nitron/smz ion-pair complex or aliquat336 was prepared by mixing 3 mg of the complex, 125.5 mg of o-npoe and 65.4 mg pvc. the mixture was dissolved in 3 ml thf and also poured in a glass petri dish. the cocktail solutions in petri dishes were left to evaporate overnight at room temperature to form thin plastic membranes. the membranes were removed and discs were cut out and glued into pvc body using thf. a mixture of 10-3 m of smz and 10-3 m of nacl was used as an internal reference solution and ~3 mm diameter ag/agcl coated wire was employed as an internal reference electrode. conditioning of the sensors was performed by soaking overnight in 10-4 m smz solution. the sensors were stocked in the same solution when they are not used. the smz sensors were calibrated by immersing them in conjunction with the reference electrode in a 25 ml beaker containing 10 ml of 10-2 m na2so4 solution of ph 7 as an ionic strength adjustor. portions (0.5-1.0 ml) of 10-4-10-1 m standard smz solutions were successively added and the potential response of stirred solutions was measured after stabilization to ±0.5 mv. a calibration graph was drawn by the emf readings put against the logarithm of smz concentrations. from the linear part of the obtained curve, the quantification of unknown concentrations of sulfamethazine drug can be obtained. flow injection setup and measurements a home-made tubular detector for smz was constructed as described previously [39]. the sensor was conditioned in 10-3 m aqueous smz solution for 24 h and was stored in the same solution when not used. a carrier solution consisting of 10-2 m na2so4 of ph 7 was propelled by means of a peristaltic pump through ptee tubing (1.13 mm). the sample loop (100 μl) of the injection valve was filled and the valve was rotated to allow the sample to be transferred by 10-2 m na2so4 stream (ph 7) to the flow with rate 3.5 ml/min. the potential outputs were recorded using data acquisition (eight-channel electrode-computer interface (nico-2000 ltd., london, uk) controlled by nico-2000 software). j. electrochem. sci. eng. 9(1) (2019) 17-26 potentiometric detection of sulfamethazine 20 analytical applications some commercially available sulphadimidine injection solutions containing 0.333 g smz/ml were analyzed. the vial contents were dispersed in water by sonication and diluted with 0.01m na2so4 to bring the concentration within the linear calibration range.1.0 ml aliquots of fresh cow milk samples were spiked with 9.0 ml aliquots of 0.5-10.0 µg smz/ml in 0.01m na2so4 of ph 7.0. the mixture was thoroughly homogenized in 15 ml screw capped falcon centrifuge tubes and sonicated for a period of 5 min to ensure convenient drug extraction. the potential readings were measured as previously described using smz sensors. the potential readings were recorded after equilibrium (10-20 s) and compared with the calibration plot. portions (1.0 g) of chicken muscles were homogenized and fortified with 5000 g/ml smz standard solution to give levels of 0.5, 0.75, 1.0 and 2.0 g/ml of smz. the spiked chicken samples were extracted with 3 ml of acetonitrile/water mixture (80:20, v/v) for 30 min at room temperature with continuous shaking. the sample mixture were centrifuged, filtered and dried under a steady flow of dry nitrogen gas. the residue was re-dissolved in 10 ml of 0.01 m na2so4 of ph 7.0. results and discussion sensors characteristics nitron, aliquat-336 and metal-phthalocyanines were utilized as sensing materials for sulfamethazine (fig. 1). the structure of these compounds form 1:1 complexes with smz as confirmed by elemental analysis and infrared spectrometry. figure1. structure of the proposed electroactive materials used for construction of sulfamethazine membrane sensors liquid-contact potentiometric sensors were constructed. the membrane sensors were prepared by incorporating 1.5 wt% of the sensing material in plasticized pvc matrix (33.6 wt% pvc and 64.4 wt% o-npoe solvent mediator). the sensors were tested and electrochemically evaluated at 25±1 °c according to the reported recommendations of iupac [40]. a cationic additive consisting of 0.5 wt% tdmac was added to membranes containing metal-phthalocyanines. the response features of these sensors are shown in table 1. it can be noticed that sensors with a membrane incorporating nitron/smz and aliquat-336 show calibration slopes of -41.70.6 and -63.90.8 mv/decade over the linear concentration range 7.7610-6-10-2 and 1.3810-5-10-2 m with lower detection limits of 0.86 and 1.36 µg/ml, respectively. sensors based on metal phthalocyanines (mgpc, mnpc and snpc), in the presence of tdmac as a cationic additive, exhibit calibration slopes of -65.3 0.4, -80.10.4 and s. s. m. hassan et al. j. electrochem. sci. eng. 9(1) (2019) 17-26 doi:10.5599/jese.575 21 -69.90.6 mv/decade over the linear concentration ranges 4.810-6-10-2, 2.610-6-10-2 and 7.9106-10-2 m with detection limits of 0.55,0.4 and 0.8 µg/ml, respectively. other performance characteristics of all investigated sensors are also presented in table 1. the time required for mgpc, mnpc and snpc based membrane sensors to reach values within ±1 mv of the final equilibrium potential after immersion in smz solutions, each having a 10-fold difference in concentration, varies from 10 s for >10-3 m to 30 s for <10-3 m smz. nitron/smz and aliquat-336 based sensors showed a response time of 20-30 s for 10-3-10-5 m. all the membrane sensors exhibit a day-to-day reproducibility of better than 0.6 mv for 10-2-10-5 m smz solutions. method validation validation of the proposed potentiometric methods for smz assessment was made by systematic measuring of the response range, lower detection limit (lod), accuracy (recovery), precision (r), within-day repeatability (cvw), between day-variability (cvb), linearity (correlation coefficient) and sensitivity (slope) over a period of 6 months. the results obtained on six batches (six determinations each) using the quality assurance standards [41] are depicted in table 1. these data support the application of the proposed new sensors for quality control assessment of drug formulations. table 1. performance characteristics of smz membrane sensors in 10-2 m na2so4 solution at ph 7.0 sensor parameter mgpc mnpc snpc aliquat-336 nitron/smz slope, mv/decade -65.3  0.4 -80.10.4 -69.90.6 -63.9  0.8 -4 1.7 0.6 correlation coefficient (r) -0.9996 -0.999 -0.999 -0.994 -0.999 linear range, m 4.78×10-6-10-2 2.63×10-6-10-2 7.94×10-6-10-2 1.38×10-5-10-2 7.76×10-6-10-2 detection limit, µg/ml 0.55 0.40 0.80 1.36 0.86 ph working range, 7 7 7 6.5–8 6.5–8 response time for 10-3 m, s ca.10 ca.10 ca.10 ca.10 ca.10 life span, week 8 8 8 8 8 standard deviation (σv), mv 1.1 0.7 0.8 1.3 1.1 accuracy, % 99.8 99.7 99.1 99.3 99.2 trueness, % 98.9 98.2 98.3 99.2 99.1 repeatability(cvw), % 0.3 0.7 0.5 0.7 0.4 between day-variability(cvb), % 0.8 0.7 1.2 1.3 0.4 accuracy and precision the agreement between the average concentration value obtained from 12 sets of potentiometric results for each sensor and the reference smz value obtained using the standard spectrophotometric method, was examined for the same smz solutions (2.0 g/ml of smz in 0.01m na2so4 at ph7.0). the standard deviation and coefficient of variation were compared. a comparison between the proposed potentiometric sensors and the standard spectrophotometric method was done. both methods were carried out using six portions of the same sample and each one in duplicate. the average mean smz value, standard deviation, and coefficient of variation are comparable (table 1). this reflects the response repeatability of the sensors and confirms the accuracy of the proposed method. linearity, limit of detection (lod) and limit of quantification (loq) linearity of the calibration graph, detection limit and quantification limit, were evaluated together with the linear regression analysis. the smz concentration was varied in the range from 10-6 m to 10-2 m. each concentration was measured in triplicate. from the calibration curves in figure 2, the linear ranges is 7.810-6-10-2 (2.1-2783 μg/ml), 1.410-5-10-2 (3.8-2783 μg/ml), j. electrochem. sci. eng. 9(1) (2019) 17-26 potentiometric detection of sulfamethazine 22 4.810-6-10-2 (1.3-2783μg/ml), 2.610-6-10-2 (0.7-2783 μg/ml) and 7.910-6-10-2 (2.2-2783 μg/ml) for nitron/smz, aliquat-336, mgpc, mnpc and snpc based membrane sensors, respectively. detection limits calculated according to iupac guidelines ranged between 0.4 and 1.4 µg/ml, respectively. fixed or proportional bias of the proposed sensors was checked by a simple linear regression for the measured concentrations. the slopes of the regression lines were near the same to those of the ideal value of unity (r2 = 0.999). the present potentiometric method shows no systematic difference between the assessed and expected concentrations within the test range. the statistical analysis for linearity measurements is tabulated in table 1. figure 2. potentiometric response of smz membrane based sensors using 0.01 m na2so4 at ph 7 specificity selectivity of ion-potentiometric sensors is quantitatively related to the equilibrium at the interface between sample and sensor membrane. the impact of various common anions on the response of smz sensors was investigated by measuring the selectivity coefficients (log kpot) of some species using the fixed solutions method [42]. the logarithmic values of log kpot were calculated by eq. (1) kpota,b = aa/(ab)za/zb (1) where ab is 1.010-3 m of the interfering species, za and zb are the ionic charges of main and interfering ions and aa is the intersection of the extrapolated linear portions of the plot emf versus the logarithm of smz concentration. in general, the values of log kpot showed the extent of preferential smz interaction over different ionic species. compounds that are commonly present in pharmaceuticals or biological samples were considered for this purpose. potentiometric selectivity coefficient (kpota,b)data are illustrated in table 2. as shown in table 2, sensors based on mgpc, mnpc and snpc ionophores have a relatively high selectivity toward smz compared to other hydrophobic anions such as clo4, salicylate, iand scn and other several common anions. the reason for the high selectivity of this electrode for the smz ion is thought to be due to a possible interaction of the anions with the central metal ion in the phthalocyanine ligand. s. s. m. hassan et al. j. electrochem. sci. eng. 9(1) (2019) 17-26 doi:10.5599/jese.575 23 table 2. potentiometric selectivity coefficients (kpotsmz,b) of smz membrane sensors in 10-2 m na2so4 at ph 7.0 potentiometric selectivity coefficient nitron/smz aliquat-336 snpc mnpc mgpc interfering ion -2. 8 -2.8 -2.6 -3.9 -3.8 po43 -1.3 -1.3 -1.4 -2.04 -2.03 cl -0.1 -0.1 -0.3 -1.29 -1.04 salicylate -1.6 +0.65 -1.1 -2.04 -1.28 scn -1.1 +0.67 -0.9 -2.32 -2.04 i -2.7 -2.9 -2.7 -3.87 -3.84 citrate -0.8 -1.4 -1.2 -1.45 -1.28 ch3coo -3.6 -2.5 -3.4 -4.47 -2.3 tartarate -1.6 -2.1 -1. 6 -2.56 -3.33 c2o42 -0.5 -1.04 -0.5 -1.28 -1.62 ascorbate it has already been reported that metal complexes of phthalocyanines are able tocoordinate with some analyte anions at the fifth and sixth axial positions of the carrier molecule, producing selective interaction and inducing the selectivity sequence for anions which deviates from the hofmeister series [43]. for sensors based on aliquat-336 and nitron/smz as ion exchangers, their selectivity behavior depends on the lipophilicity of the ion in the aqueous solution due to ion-exchange mechanism. so, these sensors exhibited severe interferences from highly lipophlic anions such as clo4-, salicylate and iions. from all said above, we can conclude that selectivity coefficient values obtained for mg(ii)-, mn(ii)-and sn(iv)phthalocyanines based membrane sensors apparently differ from the hofmeister selectivity pattern observed for nitron/smz and aliquat-336 based sensors. ruggedness (robustness) the sensitivity of the proposed method to variations of experimental conditions (temperature, ph, and sample size) was tested. the ruggedness test was done using "youden and steiner partial factorial design" where eight replicate analyses were conducted, and three factors are varied and analyzed [44]. the effect of ph variation on the sensors potentials was studied for the smz solution of 10-3 mol l-1. the ph was adjusted by small additions of the concentrated hcl or naoh solution and recorded by a combined glass-ph electrode. it was noticed that potential responses of the sensors are almost stable over the range of 6.5-8.0, with small potential variations within ± 2 mv. variation of the concentration of smz samples over the range 10-5-10-2 m did not affect the accuracy by more than 1%. change of the temperature of the test solution from 18-25oc slightly affected the results. the simplest form of the nernst equation is: e= eo+(0.065/n) log c. however, the 0.065/n part of the equation is a simplification of 2.303rt/nf. so, at 18 °c, 2.303rt/f = 0.060 volts and upon increasing the temperature to 25 °c, this value goes up to 0.065 volts. flow injection assembly fia is an extraordinary branch in analytical chemistry which is operating in different techniques of analysis. fia demonstrates many advantages like small volumes of samples that are quantified, fast operating, low cost, friendly to environment, applicability in the industrial field and easily automation. four different sensors were constructed as previously mentioned to detect smz concentration under hydrodynamic operation. a linear relationship between log [smz] concentrations and fia signals were obtained over the concentration range of 10-6-10-3 m using 0.01 m na2so4, ph 7 as shown in fig. 3. the optimum flow rate for measuring was chosen to be 3.5 ml/min. the sensors revealed a sub-nernstian response j. electrochem. sci. eng. 9(1) (2019) 17-26 potentiometric detection of sulfamethazine 24 with slopes of -53.7, -39, -43.2 and -37 mv/decade over a linear concentration range between 9.910-5, 10-5,1.110-5 and 10-4 m to 10-2 m, and detection limits of 17.5, 2.78, 2.78, 13.94 µg/ml for the aliquat-336, mgpc, mnpc and nitron/smz membrane-based sensors, respectively. general performance characteristics are shown in table 3. the lower sensitivity of fia measurements may be attributed to the small volume of the injectable sample, flow rate, and time taken for the sample to be reacted on the surface of the sensor. as shown in table 3, the sample frequency/hour is 60, 46, 48 and 50 sample/h for aliquat-336, mgpc, mnpc and nitron/smz membrane-based sensors, respectively. figure 3. fia signals for the evaluation of smz in 0.01 m carrier sodium sulphate solution ph 7.0, loop sample 100 μl, and flow rate 3.5 ml/min; (a) aliquat-336; (b) mgpc; (c) mnpc; and (d) nitron/smz membrane based sensors table 3. performance characteristics of smz membrane sensors plasticized with o-npoe under hydrodynamic mode (fia) of operation in 10-2m na2so4 buffer of ph 7.0 sensor parameter aliquat-336 mgpc mnpc nitron/smz slope, mv/decade -53.7 -39 -43.2 -37 correlation coefficient® -0.986 -0.988 -0.997 -0.979 linear range, m 9.910-5-10-2 1.0x10-5-10-2 1.1x10-5-10-2 1.0x10-4-10-2 detection limit, µg/ml 17.5 2.78 2.78 13.94 life span, week 8 8 8 8 optimum flow rate, ml/min 3.5 3.5 3.5 3.5 sample frequency, sample/h 60 46 48 50 s. s. m. hassan et al. j. electrochem. sci. eng. 9(1) (2019) 17-26 doi:10.5599/jese.575 25 smz assessment to test the applicability of the proposed method using the proposed sensors for determining smz, natural matrices such as drug formulations, milk or chicken muscle samples were taken. two commercial products containing smz, labeled 33.3 g/100 ml were collected from local markets for veterinary treatments. potentiometric determination of smz in triplicate under both static and hydrodynamic mode of operations showed results with an average recovery of 99.1 and 98.7 % and a mean standard deviation of ±1.2 and ±2.3 % for both static and hydrodynamic mode of operations (table 4). these data were compared with results obtained by uv-spectrophotometry [45]. an ftest revealed that there is no significant difference between the means and variances of two sets of results. quality control/quality assurance (qc/qa) of the method was tested by daily drug analysis over one month. r and x control charts [46] clearly indicated that all distribution measurements and range of assays data were under statistical control (lie between the warrant and control limits without any abnormalities). table 4. potentiomeric determination of smz in pharmaceutical preparations using smz membrane sensors commercial products* found, g/100ml potentiometry spectrometry [45] aliquat-336 mgpc/tdmac mnpc/tdmac snpc/tdmac nitron/smz batch fia batch fia batch fia batch fia batch fia sulphadimidine injection** 32.71.1 31.80.9 32.61.4 31.71.4 33.40.6 31.80.3 31.71.7 33.70.1 310.2 35.90.9 340.7 sulphadimidine injection*** 31.80.9 34.41.2 32.60.7 33.61.8 331.2 34.71.5 32.21.6 32.51.1 33.40.2 32.61.3 33.70.4 *labeled 33.3 g/100ml; **adwia, egypt; *** uccma, egypt the method was also tested for determining smz in milk and chicken muscles by spiking aliquots of different samples with the known standard of smz. the results showed an average recovery of 99.1 % with a relative standard deviation of ±0.8 %. results obtained for determination of smz in milk and chicken muscle samples using batch and fia are compared in table 5. table 5. assessment of smz in spiked samples with milk and chicken muscle using mnpc membrane-based sensor sample spiked, µg/ml *found, µg/ml batch fia cow milk 0.5 0.47±0.05 5 4.2±0.2 4.1±0.5 10 9.6±0.7 9.3±0.3 chicken muscles 0.5 0.42±0.04 0.75 0.68±0.07 1 0.93±0.06 2 1.8±0.1 *average of 5 measurements conclusions novel, simple and low cost potentiometric sensors were developed, characterized and used for static and continuous quantification of sulfamethazine drug. automatic determination of smz using a flow-through system coupled with a potentiometric detector proved to be an advantageous method over many other analytical methods. determinations of smz can be accomplished within a wide concentration range, regardless of the samples colors and turbidity. in addition, increased sensitivity provides improved precision, high sampling rates, low consumption of sample volume and better reproducibility. the 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s. kokot, chemometrics and intelligent laboratory systems 82 (2006) 241-247. 46. w. funk, v. dammann, g. donnevert, quality assurance in analytical chemistry, vch, new york, 1995. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) https://www.researchgate.net/scientific-contributions/2134621948_li_chen https://www.researchgate.net/scientific-contributions/2051587018_yeqing_xu https://www.researchgate.net/scientific-contributions/55734742_lin_sun https://www.researchgate.net/scientific-contributions/2125762696_jiahong_zheng https://www.researchgate.net/scientific-contributions/71363067_jiangdong_dai https://www.researchgate.net/scientific-contributions/36316482_chunxiang_li https://www.researchgate.net/scientific-contributions/51318295_yongsheng_yan https://www.researchgate.net/profile/jinbin_yuan?_sg=rngkhc9ibaml0ry-oyvabrvgqskt8j9eev2zcyokyzr00v58phngkqa5nx8gl9r53-b815k.33y1ytsq1mq2zakvd3yjbvza4c50-xdygy-5oywz-two77pb_1gb-jt-xq7ugz2gbunf3lacd6yjnzdorkbywa https://www.researchgate.net/scientific-contributions/39934960_shouzhuo_yao?_sg=rngkhc9ibaml0ry-oyvabrvgqskt8j9eev2zcyokyzr00v58phngkqa5nx8gl9r53-b815k.33y1ytsq1mq2zakvd3yjbvza4c50-xdygy-5oywz-two77pb_1gb-jt-xq7ugz2gbunf3lacd6yjnzdorkbywa http://creativecommons.org/licenses/by/4.0/) {facile one-pot synthesis of cuo nanospheres: sensitive electrochemical determination of hydrazine in water effluents:} http://dx.doi.org/10.5599/jese.1207 439 j. electrochem. sci. eng. 12(3) (2022) 439-449; http://dx.doi.org/10.5599/jese.1207 open access : : issn 1847-9286 www.jese-online.org original scientific paper facile one-pot synthesis of cuo nanospheres: sensitive electrochemical determination of hydrazine in water effluents n. m. abdul khader jailani1,, m. chinnasamy2 and n. s. k. gowthaman1, 1department of chemistry, hajee karutha rowther howdia college, uthamapalayam 625533, tamilnadu, india 2department of chemistry, theni college of arts & science, veerapandi 625534, tamilnadu, india corresponding authors:  jailani150677@gmail.com, tel.: +91-9442021072; gowthamkrishna178@gmail.com received: december 8, 2021; accepted: march 18, 2022; published: march 28, 2022 abstract hydrazine (hz) is massively used in several industrial applications. adsorption of hz through human skin creates carcinogenicity by disturbing the human organ system and thus, the quantification of hz levels in environmental water samples is highly needed. the present work describes the short-term development of copper oxide nanospheres (cuo ns) by onestep wet chemical approach and their implementation on glassy carbon electrode (gce) for the sensitive and selective quantification of the environmentally hazardous hz. the cuo ns formation was identified by x-ray diffraction (xrd), field-emission scanning electron microscopy (fe-sem), transmission electron microscopy (tem) and uv-visible spectroscopy. sem images exhibited the uniform cuo ns with an average size of 85 nm. the linker-free cuo ns modified gce offered high electrocatalytic activity against hz determination by showing the linear range determination in the range of 0.5 to 500 µm, with the detection limit of 63 nm (s/n=3), and sensitivity of 894.28 µa mm-1 cm-2. further, the developed hz sensor displayed excellent repeatability and reproducibility and was successfully exploited for the determination of hz in real environmental samples, implying that gce/cuo-ns is a confident and low-cost electrochemical platform for hz determination. keywords cuo nanospheres; electrochemical sensor; voltammetry; environmental hazard introduction metal oxides have received a huge consideration in sensing applications because of their large surface area derived high photo-electrocatalytic activity, cost-effectiveness and outstanding stability [1,2]. copper oxide nanomaterial (cuo), as a nonstoichiometric semiconductor possess a narrow bandgap of 1.2 to 1.6 ev. due to the miniaturization and tuning of its morphological and nanomaterial features, extensive research has been focused by the research community in several applications of cuo nanomaterials, including solar cells [3], electrochromic devices [4], electrocatalysis [5-7], photocatalysis [8], gas sensing application [9] and supercapacitors [10]. the userhttp://dx.doi.org/10.5599/jese.1207 http://dx.doi.org/10.5599/jese.1207 http://www.jese-online.org/ mailto:jailani150677@gmail.com mailto:gowthamkrishna178@gmail.com j. electrochem. sci. eng. 12(3) (2022) 439-449 one-pot synthesis of cuo nanospheres 440 friendly features of cuo nanomaterials such as low cost, high stability and durability, and ecofriendliness, motivated many researchers to develop cuo nanostructures by various synthetic routes, including hydrothermal synthesis, thermal oxidation and decomposition, vapor-phase synthesis, electron beam lithography and ultrasonic irradiation [11-13]. however, a prolonged synthetic procedure and the usage of surfactants and toxic chemicals confine the utility of cuo nanostructures in real-time applications. to overcome the aforementioned hurdles on cuo nanomaterials, this study reveals the one-step short-term surfactant-free synthetic approach for the preparation of homogenous cuo nanospheres (cuo ns) via wet chemical synthesis. the assynthesized cuo ns will be tested for their utility in real-time applications. hydrazine (n2h4, hz) is a significant chemical (bio) reagent and an effective reducing agent. it is utilized in many applications like fuel cells, chemical and pharmacy industries, pesticides, corrosion inhibitors, etc. hz is, however, a highly toxic, water-soluble, colorless and flammable compound that can stimulate various severe damages, including lungs, thyroid gland, liver, spleen, liver, temporary blindness, pulmonary edema, and dna and brain damages in the human body [14-21]. further, hz outflows from different industries to the environmental soil, water and air make hz highly hazardous [22]. therefore, the quantitative determination of hz is a highly important task. electroanalytical techniques showed many advantages, like easy handling, low cost, rapid response, label-free, less laborious, and high sensitivity and selectivity [14-21]. generally, the electrochemical determination of hz is based on its direct oxidation on the electrode surface, where the obtained oxidation current response is directly proportional to hz concentration. the catalytic performance of hz oxidation is highly dependent on the selection of electrode material. it has been reported that cuo facilitates the electron transfer at much lower over-potentials because of its excellent conductivity and large specific surface area [23-28]. few researchers have already developed cuo-based electrochemical hz sensor [29-36]. although the already reported sensors implied the importance of cuo in hz determination, the lack of beneficial analytical features, including sensitivity and detection limit, and expensive synthetic procedures, made them unsuitable for real-time applications. to achieve a simple, low-cost and reliable protocol for the development of hz electrochemical sensor, the present work describes the one-step short-term synthesis of cuo ns and its fabrication on glassy carbon electrode (gce) surface without the aid of a linker for the sensitive determination of an environmentally hazardous pollutant hz. the fabricated linker-free cuo ns/gce sensor is successfully utilized for hz determination in industrial effluent and environmental water samples. experimental preparation of cuo ns all chemicals used in this study were of analytical grade. the details of chemicals used in this study is given in supplementary material (sm). similar to the procedure employed for cuo nanoflakes preparation 6, cuo nanospheres synthesis follows dissolving 1.0 g of copper sulfate pentahydrate in 50 ml double distilled water (ddw), and subsequent addition of 10 ml of 25 % ammonium hydroxide and 0.1 g glucose with constant stirring. the above mixture was transferred into an rb flask and refluxed to 90-120 °c under vigorous stirring for 60 min. the added glucose could act as a reducing as well as shape directing agent in the formation of cuo ns. the reaction was cooled and washed thrice with ddw till the ph of the mixture reached to 6.5 to 7.0. then, the reaction mixture was ultra-centrifuged with 2000 rpm. finally, the product was vacuumized at 60 °c for 24 h to obtain a yellow solid product. the as-synthesized cuo ns was characterized by different n. m. abdul khader jailani et al. j. electrochem. sci. eng. 12(3) (2022) 439-449 http://dx.doi.org/10.5599/jese.1207 441 spectroscopic and microscopic studies and the detailed instrumentation is described in sm. figure 1 illustrates the cuo ns formation, while the corresponding chemical reactions are as follows: cuso4 (aq) + nh4oh (aq) → cu(oh)2 (s) + (nh4)2so4 (aq) (1) cu(oh)2 (s) → cuo (s) + h2o (2) figure 1. schematic representation of cuo ns preparation fabrication of cuo ns on gce prior to prepare the sensor electrode, gce (geometrical area = 0.07 cm2) was polished with 0.5  alumina and then ultrasonically cleaned in ethanol followed by ddw for 3 min and finally dried at room temperature. the cuo ns dispersion was prepared in ddw in the ratio of 1 mg ml-1 and the dispersion was ultrasonicated for 15 min. then, 10 µl of cuo ns dispersion was pipetted out, dropcasted on the surface of well-cleaned gce, and placed in an oven at 50˚c for 15 min. the resultant sensor electrode is represented as gce/cuo-ns and utilized for further electrochemical studies. all electrochemical studies were conducted at chi-660e workstation under an inert atmosphere. the detailed instrumentation is described in sm. preparation of environmental samples the industrial effluents and field water samples were collected from the nearby industries and fields and the collected samples were filtered to remove any solid particles present and diluted 10 times with phosphate buffer (pb) solution (0.1 m, ph 7). after the examination of environmental samples under optimized conditions using gce/cuo-ns sensor, known concentrations of hz were spiked into the samples, and the results obtained from the environmental samples were compared with the results of spiked samples. results and discussion characterization by xrd, ft-ir and uv-vis spectral studies crystallinity, purity and crystallite size of the as-synthesized cuo ns can be identified by xrd analysis. figure 2 displays the xrd pattern of cuo ns, which shows diffraction peaks at 22.31, 29.59, 36.66, 42.29, 61.67 and 73.50°, attributed to (100), (200), (211), (201), (123) and (402) monoclinic planes of cuo (jcpds no. 27-0150) [26]. all diffraction peaks of cuo belong to a single phase of the monoclinic crystal system. no other peaks observed for either cu(oh)2, or the precursor, indicate http://dx.doi.org/10.5599/jese.1207 j. electrochem. sci. eng. 12(3) (2022) 439-449 one-pot synthesis of cuo nanospheres 442 the formation of the pure monoclinic cuo ns. scherer’s formula is employed to find the crystallite size of the cuo ns by using the full-width half-maximum of (211) plane [37]: xs = kλ /  cos (3) where xs denotes the crystallite size, k represents scherer’s constant,  is wavelength,  is full-width half maximum of (211) plane and  denotes the diffraction angle. the average crystallite size of cuo ns is found to be 37.24 nm. figure 2. xrd patterns of cuo-ns ft-ir spectroscopy is useful to identify reaction progress by monitoring functional groups of the molecules involved and is utilized in this work to investigate the formation of cuo ns. the recorded ft-ir spectrum for cuo ns is given in figure s1. it shows four pronounced peaks around 700, 1378, 1620 and 3410 cm-1. the low intense peak at 700 cm-1, and the broad peak at 3410 cm-1 are respectively attributed to cuo metal-oxide vibration and hydroxyl groups, whereas the broad peak at 1620 cm-1 and a weak band at 1378 cm-1 are accredited to the o-h stretching and bending and ch vibration peaks, respectively. the less intense peak around 1100 cm-1 corresponds to the vibration of c-o coordinating with metal anions. further, the as-prepared cuo-ns was examined by uv-vis absorption spectroscopy to study its optical behavior. the cuo-ns displayed the absorption maximum at 285 nm (figure s-2a), which is correlated with the n-π* transition. the energy bandgap of the cuo ns was acquired from the uvvis studies by deriving the tauc plot (figure s2b) [38]. the optimal energy bandgap was estimated by eq. (4), ohν = a (hν-eg)1/2 (4) where, o stands for absorption coefficient, h denotes plank’s constant, ν represents photon frequency, a denotes proportionality constant and eg is the energy bandgap. the plot of (ohν)2 versus hν gives the bandgap energy of cuo-ns and it was found to be 2.15 ev (figure s-2b), indicating the direct bandgap semiconductor. morphological features of cuo-ns the surface features of the synthesized cuo-ns were characterized by microscopic tools such as transmission electron microscopy (tem) and field-emission scanning electron microscopy (fe-sem) n. m. abdul khader jailani et al. j. electrochem. sci. eng. 12(3) (2022) 439-449 http://dx.doi.org/10.5599/jese.1207 443 attached with an energy dispersive x-ray (edx) detector. granules of well dispersed homogeneous spherical cuo ns were clearly observed from the tem and fe-sem images (figure 3a and b) and the tem exhibited the average size of 35 nm for the cuo ns (figure 3a). the fe-sem image exhibited the homogeneous and highly dense growth of cuo ns with the typical thickness of 82.7 ± 0.3 nm (figure 3b). on the other hand, the edx analysis of cuo ns was carried out in the range of 0 – 10 kev energy region and the recorded edx point analysis profile is given in figure s3. the edx profile of cuo ns displayed the peaks for cu (0.90, 8.04 and 8.90 ev) and oxygen (0.51 ev) elements, confirming the presence of cu and o only in their stoichiometry. figure 3. (a) tem and (b) fe-sem images of cuo-ns characterization by electrochemical impedance spectroscopy (eis) changes in the electrode-electrolyte interface and kinetics of the electrode are mainly monitored by electrochemical impedance spectroscopy (eis). the kinetics of gce/cuo-ns electrode was analyzed by eis and the results are displayed in figure s4. the electron transfer probe, 5mm [fe(cn)63−/4−] in 0.1 m kcl, was used to analyze the reaction kinetics at gce/cuo-ns (figure s4). the randles equivalent circuit (rs[cpe-rct-w]) shown in the inset of figure s4, was fitted to the measured eis results. in the randles circuit, rs denotes solution resistance in a series connection with rct representing charge transfer resistance, and w standing for warburg diffusion impedance, which are both in parallel with a constant phase element (cpe) accounting for double-layer impedance. the charge transfer at the electrode-electrolyte interface is revealed by the diameter of the semicircle part of the nyquist plot. figure s4 displays the nyquist plots of bare gce and gce/cuons. the bare gce exhibited rct of 672 ω, whereas gce/cuo-ns displayed rct of 124 ω. about 5-fold lower resistivity delivered by gce/cuo-ns is mainly because of highly conductive cuo ns with homogenous morphology, which further facilitates electron transfer resulting in lower charge transfer resistance to the fabricated electrode. the above results suggested that the highly conductive cuo-ns on gce removes the resistance barrier at the electrode-electrolyte interface. further, the electrochemically active surface area (easa) of different cuo-ns fabricated electrode was calculated using the anson equation [37,39]. the easa of cuo-ns fabricated electrode was determined as 0.37 cm2. electrochemical response of cuo-ns fabricated sensor towards hz the electrochemical response of the cuo-ns fabricated sensor towards hz was analyzed by cyclic voltammetry (cv) in pb solution (0.1m, ph 7.0) in the potential window of -0.2 – 1.4 v at the scan rate of 50 mv/s and the results are displayed in figure 4a. in the absence of hz, neither bare gce 50 nm a b http://dx.doi.org/10.5599/jese.1207 j. electrochem. sci. eng. 12(3) (2022) 439-449 one-pot synthesis of cuo nanospheres 444 nor gce/cuo-ns (dotted line) showed any faradaic current in the given potential window, indicating the electrochemical inactivity of both electrodes. after adding of 1 mm hz into the pb solution, the bare gce (curve a) did not exhibit any significant response by showing any oxidation current for hz oxidation, indicating that the bare electrode is not a good catalyst for the oxidation of hz. when the cv of gce/cuo-ns was conducted in the presence of 1 mm hz (curve b), it exhibited a well-shaped oxidation with the peak potential of +0.96 v. it seems that the cuo-ns with higher conductivity and easa increased the catalytic activity by enhancing the hz oxidation current. although the hz oxidation mechanism depends highly on the nature of electrode and electrolyte, the following hz oxidation mechanism was proposed as reported early [40]: figure 4. cvs obtained for: (a) 1 mm hz at (a) bare gc and (b) gc/cuo-ns electrodes at 50 mv s-1 in 0.1 m pb solution (ph 7); (dotted line) in absence of hz at gc/cuo-ns electrode; (b) cvs of 1 mm hz at gc/cuo-ns electrode at different scan rates from 10–100 mv s-1 n2h4 + h2o ↔ n2h3 + h3o+ + e(slow) (5) n2h3 + 3h2o ↔ n2 + 3h3o+ + 3e(fast) (6) the rate-determining step is the slow step that proceeds by one-electron transfer (eq. 5), followed by three-electron transfer yielding n2 final product (eq. 6). therefore, the overall hz oxidation reaction leads to n2 and water formation as eco-friendly products, according to: n2h4 + 4h2o ↔ n2 + 4h3o+ + 4e(7) the electro-oxidation of hz at gce/cuo-ns is illustrated in figure 5. the interaction of cuo with hz can be proposed as follows: n2h4 + 2oh↔ [ho-n2h4-oh]2(8) [ho-n2h4-oh]2+ 4cuo ↔ 2cu2o + 2h2o + 2oh+ n2 (9) 2cu2o + 4oh↔ 4cuo + 2h2o + 4e(10) the fabricated cuo-ns provides favorable surface conditions through unique morphological features which facilitate electron transfer at the electrode-electrolyte interface region. the excellent electronic conductivity of cuo ns coupled with high surface area enables the reaction of hz analyte faster, along with the provision of continuous electron transport pathways. consequently, an intensive signal for the analyte is generated, making gce/cuo-ns be highly sensitive electrochemical hz sensor. n. m. abdul khader jailani et al. j. electrochem. sci. eng. 12(3) (2022) 439-449 http://dx.doi.org/10.5599/jese.1207 445 figure 5. electrochemical oxidation mechanism of hz at gce/cuo-ns figure 4b shows cvs of 1 mm of hz at gce/cuo-ns in pb solution (0.1 m, ph 7) at various scan rates of 10 to 100 mv s-1. while increasing the scan rate with the step of 10 mv s-1, the oxidation peak current of hz was also increased with a slight shift in the oxidation potential. the plot of oxidation current vs. square root of scan rate is linear with the regression coefficient r2 = 0.9993 (figure s-5). the obtained straight line and r2 value from the oxidation peak current vs. square root of scan rate plot suggested that the oxidation of hz is fast, i.e., a diffusion-controlled process. sensitive and selective determination of hz the sensitive determination of hz at cuo-ns fabricated electrode is conducted by differential pulse voltammetry (dpv) technique. figure 6a shows dpvs obtained for hz at gce/cuo-ns in pb solution (0.1 m, ph 7). the hz oxidation peak was observed at 0.7 v due to the initial addition of 10 µm hz. while increasing the concentration of hz from 0.5 to 500 µm, the oxidation peak current was increased linearly without shifting its oxidation potential. figure 6. (a) dpv responses of different concentrations of hz (0.5–500 µm) and (b) anti-interference activity of hz at gce/cuo-ns in 0.1 m pb (ph 7.0) solution the plot of oxidation peak current vs. hz concentration is linear with the regression equation defined as: ipa = 0.062c + 0.2207 and r2 = 0.9636 (figure s6). the obtained linear current response suggested that the cuo ns modified electrode exhibits excellent sensitivity against hz. the gce/cuo-ns sensor exhibited high sensitivity of 894.28 µa mm-1 cm-2 with the limit of detection http://dx.doi.org/10.5599/jese.1207 j. electrochem. sci. eng. 12(3) (2022) 439-449 one-pot synthesis of cuo nanospheres 446 (lod) of 63 nm (s/n = 3) in the 0.5–500 µm wide range of hz concentrations. the obtained sensitivity, lod value, and wide range of hz concentrations of linear response of cuo-ns fabricated sensor are generally superior for hz determination compared to some recently reported sensors (table 1) [29-36,41]. table 1. comparison of gce/cuo-ns properties for hz determination with various cuo-based sensors electrocatalyst medium linear range, μm detection limit, µm sensitivity, μa mm-1 cm-2 ref. cuo-nrsa/gceb/nafion naoh 0.1 – 1.0 0.01 3087 [29] cuo npsc/ild/cpee ph 9.0 0.05 – 150 0.03 1.474 [30] cuo/omcf–gce naoh 1-2.11×103 0.89 -[31] cuo/cntsg-rgog/gce ph 7.4 1.2 – 430 0.20 4.28 [32] cuo/si-p-nwsh/gce ph 7.4 1 – 5000 0.25 0.218 [33] rgo/cuo naoh 0.1 – 400 9.8 ´ 10-3 3.87 [34] cuo-nps/cpe naoh 100 – 800 -19.6 [35] cu/cu2o@carbon ph 7.4 0.25 – 800 0.022 2.37 [36] aunps-rgo@nfi ph 7.4 0.2 – 200 56 ´ 10-3 14.635 [41] cuo-ns/gce ph 7.0 0.5 – 500 63 ´ 10-3 894.28 this work ananorods; bglassy carbon electrode; cnanoparticles; dionic liquid; ecarbon paste electrode; eordered mesoporous carbon; fcarbon nanotubes; greduced graphene oxide; hnanowires; inickel foam; jnanospheres the specificity of gce/cuo-ns towards hz detection in the presence of possible interfering molecules and ions is examined by dpv analysis. the gce/cuo-ns showed well-defined oxidation peaks for each 10 µm hz addition in the dpv measurement. on the other hand, no significant increase of oxidation current was observed in the presence of 50-fold higher concentrated co-interfering agents. hydrogen peroxide (hp), 4-aminophenol (4-ap), 4-nitrophenol (4-np), resorcinol (rs), catechol (cc) and urea, metal ions of cadmium (cd) and lead (pb) and common ions of sulphate (so42-) were tested. the corresponding response current chart is displayed in figure 6b. the obtained results suggested that the fabricated gce/cuo-ns sensor possesses excellent specificity towards hz determination. the reproducibility and repeatability of the developed hz sensor was evaluated using three separately fabricated gce/cuo-ns by cvs with 50 µm hz in pb solution (0.1 m, ph 7) at a scan rate of 50 mv s-1. the cv responses of three different electrodes exhibited the oxidation of hz with 1.3 % relative standard deviation (rsd) oxidation current. also, three consecutive cv measurements were carried out using three different electrodes and the cvs exhibited concurrent oxidation current response for hz oxidation with an rsd of 1.5 %. further, the gce/cuo-ns was placed in pb solution (0.1 m, ph 7) after the cv measurements at room temperature. the cv response of the abovestored gce/cuo-ns was analyzed for the duration of two weeks. the hz oxidation current was decreased by about 1.4 ± 0.05 % in a week and 3.7 ± 0.05 % in two weeks compared to the initial current response. the obtained results are trustworthy for the excellent repeatability, reproducibility, and storage stability of the developed gce/cuo-ns-based hz sensor. real-time application of gce/cuo-ns the practical utility of the fabricated gce/cuo-ns based hz sensor was examined in real samples such as environmental field water and industrial effluents. these samples were obtained from field water and nearest industries. the procedure for the preparation and pre-treatment of the collected samples is given in section 2.3. dpv measurements were employed to analyze the collected environmental samples using gce/cuo-ns. the dpvs of the collected samples did not show any oxidation response in the potential window of +0.2 to 1.0 v, indicating that samples are free from n. m. abdul khader jailani et al. j. electrochem. sci. eng. 12(3) (2022) 439-449 http://dx.doi.org/10.5599/jese.1207 447 hz. at the same time, the hz spiked samples exhibited a well-defined oxidation peak for hz oxidation and the obtained oxidation currents are compared with the standard one. the gce/cuons offered 98-99 % recovery for hz determination in real sample analysis and the obtained analytical are tabulated (table 2). it is clear from the results listed in table 2 that gce/cuo-ns could deliver results that are sufficient for the environmental diagnosis of hz. consequently, being environmentally benign and economically viable, the proposed sensor could make a part of a robust platform for environmental pollutants determination. table 2. application of gce/cuo ns for hz determination in environmental samples sample hz added, µm hz found, µma recovery, %a field water 0.0 5.0 10.0 50.0 0.00 5.02 ± 0.09 9.48 ± 0.19 46.51 ± 1.15 100.40 ± 1.81 94.80 ± 1.93 93.02 ± 2.30 industrial effluent 0.0 2.5 10.0 30.0 100.0 0.00 2.47 ± 0.06 9.91 ± 0.15 28.41 ± 0.11 93.16 ± 0.37 98.80 ± 2.40 99.10 ± 1.53 94.70 ± 1.12 93.16 ± 0.37 arelative standard deviation in the end, the reliability of the present sensor was validated with the high-performance liquid chromatography (hplc) method. the results of hz determination in real samples obtained from the present method using gce/cuo-ns were found to be in close agreement with the results obtained via hplc. the obtained agreement suggests good practicability of the developed sensor for hz monitoring in real-time applications. conclusion in this study, a facile short-term one-step method was developed for cuo nanospheres (cuo ns) synthesis, which were then fabricated on 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commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1207 https://doi.org/10.1016/j.matlet.2011.02.013 https://doi.org/10.1039/c3nj40928g https://doi.org/10.1039/d0nj01715a https://doi.org/10.1016/j.microc.2020.105527 https://doi.org/10.1007/s00542-015-2726-x https://doi.org/10.1007/s13738-018-1416-x https://doi.org/10.1016/j.jcis.2019.04.063 https://doi.org/10.1007/s11581-019-03305-w https://doi.org/10.1039/c2nr32556j https://doi.org/10.1166/sam.2015.2025 https://doi.org/10.1016/j.electacta.2009.05.019 https://doi.org/10.1039/c5ay02122g https://doi.org/10.1039/c7ce01044c https://doi.org/10.1016/j.ultsonch.2019.104828 http://electrochemsci.org/papers/vol7/7076214.pdf https://doi.org/10.1039/c5ra06537b https://doi.org/10.1007/s10853-020-04684-6 https://creativecommons.org/licenses/by/4.0/) {simulation of corrosion protection methods in reinforced concrete by artificial neural networks and fuzzy logic:} http://dx.doi.org/10.5599/jese.1220 511 j. electrochem. sci. eng. 12(3) (2022) 511-527; http://dx.doi.org/10.5599/jese.1220 open access : : issn 1847-9286 www.jese-online.org original scientific paper simulation of corrosion protection methods in reinforced concrete by artificial neural networks and fuzzy logic alireza afshar1, ali shokrgozar2,, abdollah afshar3 and amirhossein afshar4 1department of civil and environmental engineering, george mason university, fairfax, va, usa 2department of civil and environmental engineering, idaho state university, 921 s 8th ave, mail stop 8060, pocatello, idaho, 83209 usa 3department of materials science and engineering, sharif university of technology, tehran, iran 4department of civil and environmental engineering, sharif university of technology, tehran, iran corresponding author:  shokali@isu.edu; tel:+1208252 8792 received: december 12, 2021; accepted: february 17, 2022; published: april 1, 2022 abstract in this study, the effect of protection methods regarding the corrosion decrement of steel in concrete was simulated by artificial neural networks (anns) and fuzzy logic (fl) approaches. hot dip galvanizing as a protective coating, ferrogard 901 corrosion inhibitor, a pozzolanic component, such as fly ash (fa) and micro-silica (ms), and eventually rebar aisi-304 were employed in concrete. reinforced concrete samples were held under impressed voltage of 30 v in 3.5 % nacl electrolyte for 350 hours toward a stainless-steel auxiliary electrode. corrosion currents have been modelled using feed forward back propagation anns and fl methods. the results demonstrate good consistency between corrosion data and simulated models. furthermore, the correlation coefficient criterion clearly indicates using pozzolanic materials, with a combination of ms and fa, can be introduced as one of the best corrosion protection methods, with a 35 % contribution factor in reinforced concrete. keywords simulation, corrosion, concrete, neural networks, fuzzy logic introduction ingress of water and aggressive fluids is the main reason responsible for major chemical and physical degradation of concrete pavements and structures, which decreases their durability and life span [1-11]. reinforced concrete has a great significance in the durability and stability of marine structures [12]. the premature deterioration of concrete buildings and infrastructure due to reinforcement corrosion is a severe challenge, both technically and economically [13,14]. reinforcement corrosion owing to passive layer disruption, γ-fe2o3h2o, accompanying chloride ion diffusion ones through concrete is started. different methods have been suggested to reduce the corrosion rate of steel reinforcements in concrete. the most important methods include pozzolanic additives, i.e., micro silica (ms) [15], fly ash http://dx.doi.org/10.5599/jese.1220 http://dx.doi.org/10.5599/jese.1220 http://www.jese-online.org/ mailto:shokali@isu.edu j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 512 (fa) [16,17] and granulated blast furnace slag [18], to prepare very dense and strong types of concrete, rebar or concrete coating [19-21], the alteration of the steel alloy composition [22], and corrosion inhibitor admixtures [23]. today, the combination of artificial neural networks (anns) and fuzzy logic (fl) methods as powerful tools to analyze and simulate phenomena are used to predict concrete features, such as compression strength. moreover, the ability to work with incomplete knowledge, prediction of reinforced concrete parameters, and having fault tolerance are some of the advantages of using these methods [24-33]. it is worth recalling that there are restricted studies on the simulation of the corrosion current of the rebar or the degradation of reinforced concrete by anns and fl. in this regard, topcu and coworkers have modeled both the compressive strength and the corrosion current of steel in concrete by anns regarding the addition of fly ash to the concrete mix design [34]. in another study, ukrainczyk and coworker have analyzed the relationships between numerous input parameters and observed damage owing to reinforcement corrosion with anns and the fuzzy prediction [35]. parthiban et al. have simulated the corrosion of steel in concrete through potential monitoring in accordance with astm c876 for a long time only by anns [36]. this study attempts to evaluate the influence of different protection methods on the enhancement of corrosion resistance of reinforced concrete by anns and fl approaches. these protection methods were applied by replacing stainless steel (aisi 304) with carbon steel rebar, coating carbon steel reinforcement by the hot dip galvanized film, the addition of pozzolanic materials, such as the ms and fa in the optimum content of 10 and 25 wt.% of cement, respectively, in concrete [37,38], and finally, the addition of the corrosion inhibitor admixture, ferrogard 901, to the concrete mixing design. in this research, the effects of the aforementioned parameters on the corrosion behavior of reinforcement and its durability in the simulated seawater solution (3.5 % nacl) [39] have been predicted by anns and fl and compared to the actual results of the accelerated corrosion test. aisi-304 rebar, hot dipping galvanized coating, accelerated corrosion test, and fl method are the parameters that have not been studied in the previous research. in all cases, the accelerated corrosion current at an impressed anodic potential of 32 v, applied between the stainless-steel counter electrode and the working electrode (rebar), was monitored at each 30 min interval by the designed data logger. eventually, the nonlinear response of the galvanic current, as well as the performance of each protective method to enhance the corrosion resistance of the reinforcement, was evaluated. the simulated results are in good consistency with the experimental ones. experimental materials in the experiments, tehran cement type ii represented in table 1, double washed sand in the range of 4.5 to 9 mm, river sand, ferrogard 901 as a corrosion inhibitor admixture (table 2), carbon steel, a-20 grade, and stainless steel, aisi-304 grade, as reinforcements (table 3) and 3.5 wt.% nacl solution for electrochemical tests was employed. besides, ms and fa, as shown in table 1, in the optimum content of 10 and 25 wt.% cement regarding the improvement of concrete properties were added to the mix design, respectively [40]. concrete mix design three types of concrete (table. 4) were prepared, which in the mixing patterns of a and b, controlling samples accompanied with concrete concerning corrosion inhibitor admixture with a w/c of 0.4 were presented. furthermore, in the third mixing design, the simultaneous effect of fa and ms at a percent of 25 and 10 wt.% cement was utilized, respectively. a. afshar et al. j. electrochem. sci. eng. 12(3) (2022) 511-527 http://dx.doi.org/10.5599/jese.1220 513 table 1. chemical composition of cement type ii, fa and ms ms fa cement type ii content, wt.% 3 4.62 62 cao 94 51.5 21 sio2 0.4 30.5 3.3 al2o3 0.7 6.7 1.5 fe2o3 0.1 3 5.5 mgo 0.15 0.4 1.7 so3 0.23 0.43 0.14 na2o 0.44 0.55 0.64 k2o 0.002 0.007 0.012 cl 2.87 2.17 7.13 loss of ignition 0.11 0.14 1.84 free lime 1.98 2.78 2.96 specific gravity table 2. inhibitor admixture specification ph working temperature. oc color density, kg/l chemical composition production name 10 1-35 green 1.06 2-dimethyl tetra amino ethanol sika ferrogard 901 table 3. chemical composition of reinforcements mo cr ni mn (max.) si (max.) p (max.) s (max.) c (max.) content, % 18-20 8-10.5 2 1 0.045 0.03 0.08 aisi 304 0.03 0.18-0.21 a-20 table 4. amounts of used materials in mixing designs concrete mixing design curing time, day amount of used materials, kg m-3 cement water ms fa sand crushed stone inhibitor sp1 a 28 400 160 736 1064 1 b 28 400 160 736 1064 12 c 28 400 216 0 100 736 1064 1.35 1superplasticiser (sp) was used 0.25 % by weight of binder (cement+fa+ms). sample preparation reinforcement preparation steel rebar with 10 mm of diameter was cut to 120 mm of length and so to intensify acid pickling and degreasing, reinforcements were ultrasonically cleaned in a 15 % hcl solution with 0.005 % urotropin (as a corrosion inhibitor) for 20 min and in acetone for 3 min, respectively. concrete molding concrete samples with 10×10×10 cm dimensions were prepared according to bs 1881 part 116 standards [41] to perform compression strength tests. samples of corrosion experiments with a cylindrical shape were prepared exactly such that the reinforcement with 10 mm diameter is centrally embedded (as shown in figure 1). to seal the reinforcement, teflon tape was utilized, and epoxy paint was used over it. the effective surface of the reinforcement in concrete was chosen to be 18.85 cm2 (s = 2πrh, where r = 5 mm and h = 60 mm). concrete samples after 24 hours were brought out of the 120 mm height molds and were placed in the curing room at a relative humidity of 90±5 for 28 days to complete the hydration reaction. the outer surface of the concrete was covered with a 2 mm height wax from both sides (top and bottom) to limit the diffusion of chloride ions only in the radial direction and also prevent corrosion in the common interfaces of concrete and reinforcement and atmosphere. http://dx.doi.org/10.5599/jese.1220 http://de.wikipedia.org/wiki/urotropin http://www.techstreet.com/products/1100509 j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 514 (1) epoxy coat used for reinforcement sealing (2) wax used in order to concrete sealing (3) effective surface (18.85 cm2) (4) concrete figure 1. schematic of concrete sample for the impressed current experiment reinforcement coating to create hot dip galvanized steel (hdgs), prepared rebar was placed in the ammonium chloride flux solution for 30 seconds and subsequently was immersed in pure zinc, with 0.3 % aluminum, a melt bath at 470 °c for 2 minutes. after quenching in water, the average thickness of the galvanizing from the 3 points was reported at approximately 100±5 µm. impressed voltage experiment an accelerated corrosion test under constant potential was accomplished through a dc power source, an experimental sample, and a plastic dish containing 3.5 wt.% nacl solution, two steel plates, and a designed data logger to collect the current data of reinforced concrete every five minutes [42-46]. indeed, a working electrode, reinforcement embedded in concrete as an anode, directly connected to the positive pole of the dc power source such that the auxiliary electrode, two stainless plates as a cathode, connected to the negative terminal, afterward a 30 v impressed voltage between the counter and the working electrode to fix stress is put on. the destruction happened when a longitudinal crack with a 0.5-1 mm width was observed in the concrete sample [47]. the impressed-voltage test setup was demonstrated in figure 2. figure 2. impressed voltage experiment setup for accelerating of steel corrosion in concrete a. afshar et al. j. electrochem. sci. eng. 12(3) (2022) 511-527 http://dx.doi.org/10.5599/jese.1220 515 fundamentals of intelligent systems anns structure artificial neural networks (anns) are mathematic algorithms based on the behavior of human brain abilities and belong to dynamic systems which, by processing on the experimental data, transfer hidden knowledge or rule behind the data to the network structure. then the neural network on the basis of acquired knowledge, can respond to a set of new data [47] and maintain a good connection between nonlinear data relations [34]. to accomplish a safe artificial neural network modeling, the following steps must be precisely defined [49]. these stages concern data gathering, defining input variables, analyzing, and preprocessing data, creating a network architecture, training the network, testing the trained network, and using the trained network for calculation and prediction. indeed, in designing the network architecture step, the number of hidden layers, the number of neurons in the hidden layers, and the transfer function type in the hidden layer must be created. no rule exists to determine the optimum amount of parameters for designing network, and they must be obtained via trial and error [50]. in the network training step, an algorithm undergoes the trend in which the matrix of connection weights and network bias vectors are adjusted. briefly, the training procedure is as follows: initial biases and connection weights are estimated, then with the output results of the network, initial biases and connection weights are modified so that it converges to real output results. this modification continues until the error reaches the desired value. the more the network is trained the more the error trend in each epoch tends to zero and when the error parameter reaches its minimum value, the training operation stops [37,49]. however, a low error is not always a good indicator of a better network. cross-validation is a highly recommended criterion to stop network training [51]. the training algorithm is shown in the flow chart of figure 3 [52]. among different ann architectures, the backpropagation learning algorithm is one of the simplest methods and has a more applicable learning algorithm, and is closer to human task behavior, such as predicting and categorizing. anns can be designed in different ways. one of the most popular ones is multilayer’s feed-forward neural networks [48,51]. this method is slow learning and needs more iterations before convergence. to obtain a performance index, a bellow performance index can be used. predicted and experimental values are compared with the root mean square error (rsme), the mean absolute percentage error (mape), and the correlation coefficient (r). rsme, mape and r are calculated by the equations (1) to (3): 2 i i 1 rmse t o p = − (1) i i i mape 100 t o o − = (2) 2 i i 2 i 1 t o r o  −  = −       (3) where ti is the experimental values, oi is the predicted values, and p is the number of data points [50]. in this work, the feed-forward backpropagation algorithm from the matlab neural network toolbox is used. http://dx.doi.org/10.5599/jese.1220 j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 516 figure 3. flowchart encompassing data pre-processing and anns training [52] fl background expert systems as technology started in the early 1950s and remained in research laboratories, and never broke through to the consumer market. fl theory which was introduced by lotfi a. zadeh extends conventional boolean logic (0 and 1) [53]. the fuzzy set theory provides a means to represent uncertainty, and everything is based on a degree. so, an object can be, to some extent, a member of a set. the membership function of a fuzzy set is allowed to have values between 0 and 1, which denotes the degree of membership of an element in a given set. the continuum of logical values between 0 (completely false) and 1 (completely true) are used. in contrast to the crisp theory with a sharp boundary, the fuzzy set theory has no sharp boundary. usually, fuzzy set and membership information is illustrated by equation (4): μa(x) = z (4) this says the membership (µ) of x in fuzzy set a is z [54]. the membership function can be an arbitrary curve, but there are several common membership functions for use in engineering applications, such as piecewise linear functions, the gaussian distribution function, the sigmoid curve, and quadratic and cubic polynomial curves, etc. fuzzy inference is the process of formulating the mapping from a given input to an output using fuzzy logic. there are two types of fuzzy inference systems used extensively: the mamdani-type and the sugeno-type. the sugeno or takagi-sugenokang (tsk) method of fuzzy inference was introduced in 1985 [53,54]. the output membership function of the sugeno-type fuzzy inference system is linear or constant. clustering involves the task a. afshar et al. j. electrochem. sci. eng. 12(3) (2022) 511-527 http://dx.doi.org/10.5599/jese.1220 517 of dividing data points into homogeneous classes or clusters so that items in the same class are as similar as possible and items in different classes are as dissimilar as possible. subtractive clustering is a fast, one-pass algorithm to estimate the number of clusters and cluster centers in the data set [53,55]. subtractive clustering is an effective approach to estimating the number of fuzzy clusters and cluster centers [56]. to predict the corrosion current by fuzzy logic experimental of corrosion current, the corrosion time and the concrete type are loaded in the matlab fuzzy logic tool, and the corrosion current model is constructed. detailed steps of the corrosion current model construction with fuzzy logic are described next. results and discussion compressive strength and corrosion analysis in concrete the results of the compressive strength experiment and the cracking initiation time of three mix proportions are depicted in table 5. according to the outcomes, the compressive strength of mix design c, the pozzolanic concrete, compared with the control concrete, mix proportion a, is decreased from 57.64 to 54.29 mpa, while the usage of corrosion inhibitor admixture, mix proportion c, suggests less decreasing of the compressive strength of concrete. it is worth mentioning that a similar decrement in concrete strength is also shown elsewhere [57,58]. indeed, completing the hydration reaction of pozzolanic materials is a time-consuming process and mainly using the pozzolanic component can delay the process [57]. however, seeing a porosity decrement, they are expected to increase the corrosion resistance of steel reinforcement in the concrete [59]. table 5. compressive strengths and damage occurrence times c b a mix proportion a-20 a-20 hdgs aisi-304 a-20 reinforcement type 183 154 38 56 26 cracking initiation time, h 54.29 56.12 57.64 concrete compressive strength, mpa in accordance with table 5, due to rebar corrosion in control, concrete destruction only can take place after 26 hours after applying a constant anodic voltage of 30 v. furthermore, corrosion initiation came about when the passive and protective surface layer on the reinforcement was broken, and the corrosion progression continued with high velocity through the environment to the reinforcement surface. basically, the formation of the corrosion products and subsequently the volume enhancement of corrosion layers approaching the reinforcement can be imposed a high degree of internal stress equal to 450 mpa to concrete and eventually bring about the failure of the concrete [44,60]. concrete failure is shown in figure 4. in the mixed designs of b and c, which were introduced as protection methods, the failure time of concrete was increased to 154 and 183 hours, respectively. it is important to mention that in this condition, a reinforcement type of a-20 grade was selected and only the effect of the concrete additive needs to be explained. fundamentally, the effect of the pozzolanic materials to improve the corrosion resistance of steel in concrete can be explained by the effective role of the pozzolanic component in closing or decreasing concrete porosity [61]. equivalently, ferrogard 901 corrosion inhibitor admixture, through its reaction with the free chloride ions and the decrement of their concentration over the rebar surface, reduces the corrosion product amount in reinforcement concrete [62]. http://dx.doi.org/10.5599/jese.1220 j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 518 figure 4. destruction of concrete samples after accelerated corrosion test the influence of changing the rebar type concerning hdgs and aisi-304 was evaluated at mix design a. outcomes directly extracted from table 3 obviously illustrate using hdgs and aisi-304 as an alternative to a-20 rebar can improve the destruction time to 38 and 41 hours, respectively. although stainless aisi-304 is more sustainable to pitting corrosion regarding chloride ions and easily broken the passivation film, some corrosion-resistant elements, such as ni, cr and mn, postpone corrosion advancement and, as a result, crack propagation can be delayed [63,64]. zinc coated steel rebar or hdgs shows approximately 38 hours of failure resistance to 30v impressed voltage in the nacl solution, which is clearly more than ordinary steel and less than stainless steel aisi-304. mainly zn element appears as a sacrificial anode and, with consumption during the electrochemical reaction, likely protects the steel from corrosive ions [65,66]. nn structure design and its parameters this study attempts to simulate the effect of time duration on steel corrosion in the concrete mix design, then evaluates the addition of pozzolanic materials type to concrete in the second model, and eventually compare the results with other proposed protection methods at the third network, i.e., as mentioned earlier changing the reinforcement type, coating, and influence of corrosion inhibitor. three models are discussed so that in the first network, the interval time imported as an input parameter and the galvanic corrosion current under a rapid chloride diffusion experiment acted as an output result in which one hidden layer with ten neurons and a log-sig transfer function has been used. figure 5 schematically represents the first neural network architecture. the second model is designed to simulate the pozzolanic additive in concrete. built anns structure in the first simulation was used to predict the missing corrosion current in the second simulation. this model is more general to predict corrosion current on the basis of more inputs, such as cement type, i.e., cement type ii, fa and ms, and corrosion time. thus, in this model, anns have 2 inputs and one output, and the built structure consists of one hidden layer with 10 neurons and a log-sig transfer function observed in figure 6. about 356 data points were obtained from experiments and 75 % of them were used for training and the remaining were used for testing and network validation. in the third prediction, the influence of changing reinforcement type, reinforcement coating, and the corrosion inhibitor admixture were verified, and in each case, the corrosion current was measured. in this model, anns have 4 inputs and one output and the built structure consists of one hidden layer with 20 neurons and the tan-sign transfer function, which can be observed in figure 7. a. afshar et al. j. electrochem. sci. eng. 12(3) (2022) 511-527 http://dx.doi.org/10.5599/jese.1220 519 figure 5. used system in the anns model for the first simulation figure 6. used system in the anns model for the second simulation figure 7. proposed architecture to simulate the third part http://dx.doi.org/10.5599/jese.1220 j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 520 about 2492 data points were obtained from experiments and 75 % of them were used for training and the remaining were used for testing and network validation. regarding the log-sig transfer function, the upper and lower limits of the network input and output must be between zero to one, so input and output data must be normalized. after network training, it must be tested. corrosion behavior analysis by anns all parameters in the three described simulations are summarized in table 6. indeed, results obtained by performance and error indicators were represented in table 6. table 6. performances of anns to predict the galvanic corrosion current mape mse rtotal rvalidation rtesting rtraining parameters 0.0520 4.9578 0.99648 0.99284 0.9952 0.99799 a m ix d e si g n (i) corrosion time s im u la ti o n 0.1101 4.5632 0.99632 0.99682 0.99699 0.99788 b 0.1600 4.0097 0.99654 0.99845 0.99861 0.99517 c 1.4789 7.9966 0.99595 0.99709 0.99641 0.99546 (ii) cement type 0.5834 5.4654 0.98933 0.99216 0.98476 0.99011 (iii) protection methods according to performance and error indicators, it can be stated that the ann model provides a precise answer for the corrosion current simulation. as an example, in case c in table 6, the simulation regression coefficient is r = 0.99654. since 0 ≤ r ≤ 1, the higher r represents a better match between the predicted corrosion current and the experimental corrosion current [67]. a typical result of the predicted corrosion current data and the experimental corrosion current data versus the corrosion time in mix design c according to the first simulation model are shown in figure 8. time, h figure 8. experimental and predicted corrosion current by ann versus time for first simulation (mix design c) also, the results of the predicted corrosion current data versus the experimental corrosion current data for simulations i and iii are typically shown in figure 9. the distribution between the predicted values and the experimental values has been modeled by the linear approach and its best linear equation was obtained. this equation is also shown in figure 9. it can be observed in figures 8 and 9 that trained anns structure in the nonlinear deviated regions predicts a successful and precise response. also, to demonstrate the precision of the proposed models, an error criterion as the difference between the predicted and experimental data versus time is suggested in figure 10, which illustrates the high precision of the modeled network. c o rr o si o n c u rr e n t, m a 0 a. afshar et al. j. electrochem. sci. eng. 12(3) (2022) 511-527 http://dx.doi.org/10.5599/jese.1220 521 a b experimental corrosion current, ma experimental corrosion current, ma figure 9. typical predicted data versus experimental corrosion currents by ann, also best linear fit, (a) first and (b) third simulation time, h figure 10. typical error of corrosion versus time of corrosion by anns for the second simulation fuzzy logic results similar to the earlier simulation, neural network data of each corrosion setup were combined, and a comprehensive data set was formed. data were grouped into two sets: training data (75 % of data) and testing data (25 % of data), which were randomly selected. training data were used to generate and train a sugeno-type fis and testing data were used to validate and verify the model. by extracting membership functions and fuzzy if-then rules, the takagi-sugeno fuzzy model was constructed. a comparison between predicted and experimental corrosion current results was made with the same performance and error indicators used in the ann simulation. the results of the trained network performance and error indicators are shown in table 7. table 7. performances of the fuzzy logic built r mape mse parameter 0.9992 0.9729 4.2166 training (i) corrosion time s im u la ti o n 0.9991 0.5394 5.2223 testing 0.9993 1.3245 3.9966 training (ii) cement type 0.9934 1.1974 4.5646 testing 0.9938 1.4799 1.0197 training (iii) protection methods 0.9937 1.2990 2.0200 testing results of the trained network performance and error indicators show the good accuracy of the fuzzy logic. also, to show the results and network performance predicted corrosion current versus the values of experimental corrosion current in training and testing is shown in figure 11. p re d ic te d c o rr o si o n c u rr e n t, m a e rr o r, m a r = 0.98933 y = x + 0.0036 r = 0.99654 y = 099x + 0.44 o data point --best lineart fit o data point --best lineart fit p re d ic te d c o rr o si o n c u rr e n t, m a 140 120 100 80 60 40 20 0 http://dx.doi.org/10.5599/jese.1220 j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 522 figure 11. typical predicted data versus experimental corrosion currents of fuzzy logic model: (a), (b) training and testing for the first model (c), (d) training and testing for the third model to show model precision, an error criterion, as the difference between predicted data and experimental data versus time, is shown in figure 12, which confirms the successfulness and high precision of the modeled network. time, h figure 12. error of corrosion versus time of fuzzy logic model the contribution factor (cf) measures the importance of the respective parameter in predicting the network’s output relative to other network input parameters. the higher the absolute sum of those weights is, the more the parameter contributes to classification. however, neural networks can also find patterns among several parameters, none of which is highly correlated with output, but which together form a pattern that uniquely determines the output [68]. the cf for individual input parameters in predicting the corrosion current was evaluated in figure 13. e rr o r, m a a. afshar et al. j. electrochem. sci. eng. 12(3) (2022) 511-527 http://dx.doi.org/10.5599/jese.1220 523 time concrete type figure 13. contribution factor for individual input parameters in predicting the damage of reinforcement concrete the cf predicts which model more effectively reduces corrosion damage. clearly, figure 13 demonstrates the effect of corrosion protective methods, such as the addition of fa and ms in the preferred percent to mix design, altering the reinforcement type, coating steel by the zinc element or the galvanizing method and corrosion inhibitor admixture to concrete. undoubtedly, all options besides interval time are so important in selecting the best state, but as described, the contribution of each approach to decrease corrosion here is important. predictions show using fa and ms as pozzolanic materials with a cf of approximately 35 % is the most important parameter to reduce steel corrosion in concrete. after that, controlling the elapsed time from construction is crucial. in other protective ways, utilizing the corrosion inhibitor, changing reinforcement with aisi-304, and coating the reinforcement with galvanizing are the next options using anns and fl in special concretes like reinforced self-compacting concrete (scc), including nano-clay particles and coated rebars (polyurethane and alkyd top coating) is an area that needs to be studied in the future. there are various publications in the supercapacitor in terms of energy density, different materials, microstructures, output current density, life cycle, etc. however, utilizing the anns and fl methods to predict the effects of the aforementioned parameters on the efficiency of supercapacitor and its practicality has not been studied yet, and it would be a great area to implement these methods [69-73]. conclusion steel corrosion in concrete is a complicated process that can be predicted through powerful artificial intelligence tools, such as anns and fl. in this study, the influence of different protection methods to enhance the corrosion resistance of reinforced concrete as addition of fa and ms in the preferred content to concrete, approximately 25 and 10 wt.% cement respectively, replacement stainless steel (aisi-304) with carbon steel, the coating of rebar by the hot dip galvanizing method, and the usage of ferrogard 901 to concrete mixing design, were analyzed and corrosion current results were simulated by anns and fl. the ann and fl are responsive even when the corrosion current shows deviational and nonlinear behavior. it can be concluded that anns and fl could be appropriate techniques to model the corrosion current of rebar in concrete. predictions from the three designed networks show the utilization of fa and ms as pozzolanic materials with a cf of approximately 35 % is the most important parameter to reduce steel corrosion in concrete. c o n tr ib u ti o n f a ct o r, % http://dx.doi.org/10.5599/jese.1220 j. electrochem. sci. eng. 12(3) (2022) 511-527 simulation of corrosion protection methods 524 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yu, electrochimica acta 364 (2020) 137231. https://doi.org/10.1016/j.electacta.2020.137231 [73] s. liu, s. sarwar, h. zhang, q. guo, j. luo, x. zhang, electrochimica acta 364 (2020) 137320. https://doi.org/10.1016/j.electacta.2020.137320 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1220 https://doi.org/10.1016/j.electacta.2020.137318 https://doi.org/10.1016/j.electacta.2020.137147 https://doi.org/10.1016/j.electacta.2020.137181 https://doi.org/10.1016/j.electacta.2020.137231 https://doi.org/10.1016/j.electacta.2020.137320 https://creativecommons.org/licenses/by/4.0/) electrodeposition of polyfunctional ni coatings from deep eutectic solvent based on choline chloride and lactic acid https://dx.doi.org/10.5599/jese.1451 1025 j. electrochem. sci. eng. 12(5) (2022) 1025-1039; https://dx.doi.org/10.5599/jese.1451 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrodeposition of polyfunctional ni coatings from deep eutectic solvent based on choline chloride and lactic acid dmytro uschapovskiy, viktoria vorobyova, georgii vasyliev and olga linucheva, national technical university of ukraine “igor sikorsky kyiv polytechnic institute”, 37, prospect peremohy, kyiv-56, 03056, ukraine corresponding author: g.vasyliev@kpi.ua; tel.: +38-096-924-9888; fax: +38-044-204-9773 received: june 8, 2022; accepted: september 9, 2022; published: september 13, 2022 abstract the process of electrodeposition of nickel coatings from electrolytes based on a deep eutectic solvent (des) mixture of choline chloride and lactic acid with a molar ratio of 1:3 was studied. the physicochemical properties and characteristics of des, namely, conductivity, ft-ir and nmr analysis were determined. ft-ir results confirmed that h-bonds occurring between two components in des were the main force leading to the eutectic formation. electrochemical techniques were used to characterize the deposition process and scanning electron microscopy was used to study the deposit morphology. based on polarization measurements, it has been found that at nicl2·6h2o content of 1.14 m and a temperature of 75 °c, the limiting current density of nickel electrodeposition was near 2 a dm-2. the polarization of the cathodic nickel deposition varied within -0.63 to 1.1 v at current density of 0.25 a dm-2 it has been shown that an increase of water content in the electrolyte does not significantly affect the current efficiency of the nickel electrodeposition process, which was in a range 85-93 %. however, the increase in water content contributes to the increase of heterogeneity and crystal grains size distribution of galvanic deposits. the established values of the wagner number indicate the predominance of the primary current density distribution in the process of electrodeposition of nickel coatings. galvanic coatings possess a highly developed nanostructured surface, exhibit increased capillary properties, and can be used as electrode materials for the process of electrolysis of water. keywords ni plating; current efficiency; polarization; crystal grains size; nanostructured surface introduction one of the modern directions of electroplating is the electrodeposition of coatings from nonaqueous electrolytes, in particular, based on deep eutectic solutions mixtures of organic compounds the so-called des [1-5]. the des formation process is coupled with the formation of https://dx.doi.org/10.5599/jese.1451 https://dx.doi.org/10.5599/jese.1451 http://www.jese-online.org/ mailto:g.vasyliev@kpi.ua j. electrochem. sci. eng. 12(5) (2022) 1025-1039 electrodeposition of polyfunctional ni coatings 1026 donor-acceptor bonds [6], and the melting point of the selected eutectic mixture is lower than the individual components. des-s are similar in their properties to ionic liquids, which makes promising their application in such areas of technical electrochemistry as electroplating [7-9] or chemical current sources [10-12]. electrolytes based on des are widely used for electrodeposition of metals, the main component-donor (hbd) of which is choline chloride. the following organic compounds can be acceptors (hba): organic acids, polyhydric alcohols, urea and its derivatives. a significant amount of printed works has been devoted to the electrodeposition of metal coatings from electrolytes based on des, such as ethaline (a mixture of choline chloride and ethylene glycol) [7,9,13-15] and reline (a mixture of choline chloride and urea) [16-18]. it is well known that galvanic nickel coatings, in terms of their physical, mechanical, and electrochemical properties, are multifunctional and are used both in the fields of special surface treatment of parts and of electrochemical energy conversion. in-depth investigation of nickel coatings' electrodeposition process from electrolytes based on des can expand their range of properties and applications. ethaline and reline are most often used as the basis of the electrolyte in the electrodeposition of nickel coatings, as evidenced by the data of most known works [7,9,13-18]. in particular, the disadvantages of the corresponding des include the following. although the operating temperature of the nickel electrodeposition process from ethaline-based solutions is 70 °c, the component (donor) of this solution is highly toxic ethylene glycol. the main disadvantage of reline-based electrolytes is the high operating temperature (90–100 °c), and hence the increased energy consumption of the process. thus, the urgent scientific task is to select a donor for the solvent of the nickel-plating electrolyte based on choline-containing des, which would meet the environmental safety conditions and cost-effectiveness of the nickel-plating process. a deep eutectic mixture based on choline chloride and lactic acid (lc) is of considerable interest [19-21]. these compounds were chosen because they can be obtained from natural sources, used as food additives, and generally recognized as safe (gras) [22,23]. the studies have shown that the molar ratio of choline chloride and lactic acid can vary from 1:1 to 1:3 without considerable reduction of hba:hbd interaction [24]. that is, the mass fraction of the cheaper and environmentally friendly component (donor) – lactic acid, is greater than for ethylene glycol in ethaline. electrodeposition of zinc from lc mixture formed by choline chloride - lactic acid (1:2) was studied [20]. the deposition process was conducted in a two-electrode cell. the low-carbon steel was used as the cathode and the zinc plate as the anode. the deposition of zn was carried out using a constant current mode. cyclic voltammetry results showed that the electrochemical stability window of lc is approximately 2.3 v. the onset of zn reduction (vs. ag/agcl) in lc occurs at e = −1.4 v and there is a clear current cross-over loop during the reduction process. the sem study showed that the morphology of the coatings evolves into domains of co-aligned platelets with a fast growth direction roughly perpendicular to the interface. this work is devoted to the investigation of the nickel coating electrodeposition process from the electrolyte based on a deep eutectic mixture of choline chloride and lactic acid with a molar ratio 1:3 and the investigation of the deposited ni coating properties as a promising electrode material for electrolysis of water. experimental choline chloride 99 % and lactic acid 90 % (acros organics), nickel chloride and potassium chloride (khimlaborreactiv) of reagent grade were used. the des was prepared by the heating method. briefly, choline chloride and lactic acid were mixed in sealed 100 ml glass flasks in molar d. uschapovskiy et al. j. electrochem. sci. eng. 12(5) (2022) 1025-1039 https://dx.doi.org/10.5599/jese.1451 1027 ratios. the mixtures were placed in a round-bottom flask and continuously stirred at 60 °c, 300 rpm in a magnetic stirrer until the mixture formed a clear solution. a des is often made with a mixture of salt and a hydrogen bond donor molecule which can form bonds with the halide in the salt, as shown in figure 1. figure 1. hydrogen bond donor (hbd) and acceptor (hba) studied in this work lead to the formation of (hydrogen bond between) choline chloride and lactic acid the synthesis of des was confirmed by the nuclear magnetic resonance (nmr) spectrometry method (bruker ascend 300). a nuclear magnetic resonance (nmr) spectrometer operating at 300 mhz was used to record the 1h nmr spectra of the des. all samples were dissolved in d2o. the chemical shift corresponding to the methyl group peak has been used as a reference. fourier transformed infrared (ft-ir) spectra of the dess were obtained using a bruker tensor 27 (bruker corp, massachusetts, usa). a total of 14 scans were measured at 4 cm-1 resolution with a scan rate of 22 scans min-1. measurements were done between 650 and 4000 cm-1 using air as a reference. the des for nickel plating electrolyte was prepared by fusing choline chloride with lactic acid in a molar ratio 1:3 at 70 °c. after homogenization of the system, nicl2·6h2o was dissolved in the obtained des to give a concentration of 1.14 m (h2o/ni molar ratio = 6). additionally, distilled water was added to the electrolyte in h2o/ni molar ratio = 12 and 18. to determine the width of the electrochemical window the cycling voltammetry technique was used. the electrochemical window determines the range of potentials where the electrolyte is neither oxidized nor reduced. traditional three-electrode electrochemical cell was used with a saturated silver chloride reference electrode (e = +0.2 v vs.nhe) (ssce). a saturated silver chloride reference electrode was connected to the cell via an intermediate beaker and a salt bridge filled with saturated kcl solution to prevent any mixing of solutions. the pair of platinum electrodes were polarized with direct current in the potential range -1.2 to +1.2 v vs.ssce. the scanning rate was 50 mv s-1. the scan was performed 3 times to assure data convergence. the cathodic potentiodynamic curves in the investigated ni-electrolyte were obtained on a working nickel electrode (99.98 %) of cylindrical shape, pressed into teflon. the surface of the working electrode was 0.5 cm2. before obtaining the curves, the working surface of the electrode was mechanically degreased with potassium carbonate and, after rinsing in distilled water, etched in 10 % hcl solution. nickel plate of electrodeposited nickel (99.98 %) was used as an auxiliary electrode. as a reference electrode, a silver wire (95.9 %) was used, which was placed directly in the cell. the potential of silver wire was measured vs. ssce electrode in the des solution and was found to be +0.1 v vs.ssce. polarization curves were obtained in a standard three-electrode cell. the potential values are given versus the potential of the silver wire. the scan rate was 10 mv s-1. the https://dx.doi.org/10.5599/jese.1451 j. electrochem. sci. eng. 12(5) (2022) 1025-1039 electrodeposition of polyfunctional ni coatings 1028 temperature was 75±3 °c. the polarization of the cathodic nickel deposition process (e) was determined at the current density of 0.25 a dm-2. the cathodic potentiodynamic curves of electrochemical evolution of hydrogen on electrodeposited nickel coatings were obtained in 0.1 m koh. a saturated silver chloride reference electrode was connected to the cell via two intermediate beakers and a luggin capillary. a platinum metal plate with an area of 0.53 cm2 was used as a counter electrode. the potential scan rate was 10 mv s-1. the temperature was 20 °c. the ametek versastat 3-200 digital potentiostat with versastudio 12 software was used to perform polarization measurements, as well as to measure the electrical conductivity of the investigated solutions. the resistance of the electrolyte was measured in a standard cell with two platinum electrodes, the area of each was 2 cm2, the interelectrode distance was 0.5 cm, the frequency of alternating current was 103 to 106 hz. the electrical conductivity of the electrolyte was calculated based on the values of the resistance of the electrolyte and the cell constant was determined using 0.01 m kcl. electrodeposition of nickel coatings was performed on rectangular steel samples of steel 08kp (european analogue fe37-3fn) with an area of 6 cm2. electrodeposition of nickel coatings was performed from an electrolyte prepared based on the investigated des. also, to compare the properties of the coatings obtained from the electrolyte based on des, matte and bright nickel coatings were tested, which were electrodeposited from the watts bath. for electrodeposition of bright coatings in the watts bath the saccharin at a concentration of 0.5 g dm-3 was added. operating current densities ranging from 0.5 to 2 a dm-2. electrodeposition was performed in a cylindrical glass cell with a cylindrical nickel anode made of nickel foil (99.98 %) 200 μm thick. the electrolyte temperature was 75 ± 3 °c. the current efficiency of the nickel electrodeposition process was determined based on the gravimetric method using analytical balance rawag as 220 r2. as a dc source, a rectifier b5-43 was used. the thickness of the electrodeposited coatings was 5 to20 μm. the investigations of capillary uplift of liquids on the surface of nickel coatings were performed by immersing samples with nickel coatings in a petri cup with ethyl alcohol (98 %) and distilled water. rectangular steel samples with electrodeposited nickel coatings were placed at an angle of 90° to the test liquid mirror. the depth of immersion of the sample edge was 3 mm. the height of the liquid uplift on the surface of the sample was measured and recorded 15 min after the start of exposure with an accuracy of 0.5 mm. sem-studies of the surface of electrodeposited nickel coatings were performed using tescan vega3 microscopes equipped with an edx analyzer bruker quantax eds and pem106-i equipped with an oxford hkl channel-5 edx analyzer. results and discussion characterization of deep eutectic solvents and des-based electrolyte from the ft-ir spectrum in figure 2, several bands characteristic groups of des scan are seen. the large band at 3432.9 cm-1 could be assigned to ο-h group. the bands appearing at 2920.7 and 2849.1 cm-1 correspond to c-h stretching vibration. the frequency at 3221 cm−1 is assigned to the o-h stretching frequency associated with oh-cl of choline chloride. the oh stretching region of lactic acid contains broad overlapping bands centered on 3420 cm−1 that is typical of carboxylic acid, forming strongly bonded dimer rings through intermolecular h-bonding between c-o and o-h groups. the outline of the bond may be due to the combined bonding of choline chloride and lactic d. uschapovskiy et al. j. electrochem. sci. eng. 12(5) (2022) 1025-1039 https://dx.doi.org/10.5599/jese.1451 1029 acid molecules overlapping in the o-h and n-h bond. two weak peaks were observed at 2987 cm-1 in des, indicating c-h stretching bonds, but shifted to a slightly lower wave number. figure 2. ft-ir spectrum of choline chloride – lactic acid the 1h nmr analysis has been used to study the ionization states of protons and determine the structure of the molecules (figure 3). the 1h nmr analysis on the synthesized choline chloride – lactic acid des was performed. the chemical shift for choline chloride appeared at δ = 4.95 ppm. the peak of lactic acid at 1.48 ppm has an integral value of 2.11, while at 3.44 ppm, labeled with number 9, the peak of choline chloride – lactic acid has an integral value of 2.00. thus, the molar ratio of the choline chloride – lactic acid is indeed 1:3. these findings explain that the oh groups of lactic acid may interact with choline chloride and then form intermolecular hydrogen bonds, which enhance the solvation capacity of des. d / ppm figure 3. 1h nmr spectrum of the des choline chloride – lactic acid figure 4 presents a typical cyclic voltammogram for des, which shows an electrochemical stability window (esw) on pt electrodes from about – 0.3 to + 1.0 v (electrode potential vs. ssce). pure solvents show the widest stability window, e.g., choline chloride-lactic acid esw exceeds 1.33 v. 0 20 40 60 80 100 120 5001000150020002500300035004000 t ra n s m it ta n c e , % wavenumber, сm-1 https://dx.doi.org/10.5599/jese.1451 j. electrochem. sci. eng. 12(5) (2022) 1025-1039 electrodeposition of polyfunctional ni coatings 1030 figure 4. cyclic voltammograms of choline chloride – lactic acid with various concentrations of water (0-25 %) investigation of nickel electrodeposition process according to [25], the addition of water to the nickel-plating electrolyte based on ethaline has a positive effect on the morphology of deposited metal. the effect of water content that corresponds to molar ratio h2o/ni [m/m] = 12 and 18 is especially noticeable. therefore, the effect of a similar amount of water was investigated in the electrolyte based on choline chloride and lactic acid. the polarization curves obtained in the investigated nickel-plating electrolyte are shown in figure 5. figure 5. the cathodic potentiodynamic curves (10 mv s-1) obtained in the nickel-plating electrolyte based on des. nickel content in the electrolyte is 1.14 m, molar ratio h2o/ni : 1: 6; 2: 12; 3: 18. dashed line shows current density (0.25 a dm-2) where the polarization value was determined as can be seen from figure 5, for curve 1 – the limiting current density of the cathode process of electroreduction of nickel in the water-free electrolyte is about 2 a/dm2. compact, gray, velvety nickel deposits were obtained in the current density range of 0.25 –2.0 a dm-2. the cathodic polarization e, determined for a cathodic current density of 0.25 a dm-2, is about –0.76 v in the electrolyte with molar ratio h2o/ni = 6. -4.0 -3.0 -2.0 -1.0 0.0 1.0 2.0 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 1.2 i / m a e / v vs. ssce 0 % 1 % 10 % 25 % -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 -1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.20.0 e / v vs. ssce j / a d m -2 2 1 3 d. uschapovskiy et al. j. electrochem. sci. eng. 12(5) (2022) 1025-1039 https://dx.doi.org/10.5599/jese.1451 1031 as the amount of water in the electrolyte increases, the molar ratio h2o/ni = 12, the e decreases to -0.63 v. the decrease of polarization of the cathodic process is consistent with an increase in the crystal grain size of the nickel deposits on average, from 0.5 to 1.5 μm (figures 6a, b). in general, the decrease of polarization of the cathodic process can be caused by the change in the structure of the des-based electrolyte because the addition of water changes the solvation number of nickel ions. as a result, the polarization of the nickel deposition process is lower than that of complex ions based on the studied des because ni ions are dissolved in water and not in des. a further increase in the amount of water to the molar ratio h2o/ni = 18 leads to the deeper hydrolysis of the system and the formation of sparingly soluble nickel compounds, which is reflected in the turbidity increase and partial sedimentation of agglomerated species. а b c figure 6. sem-images of the surface of galvanic nickel deposits with a thickness of 5 μm, obtained in the electrolyte based on des. nickel content in the electrolyte 1.14 m, molar ratio h2o/ni : 1: 6; 2: 12; 3: 18 the formation of sparingly soluble nickel compounds (ni salts) leads to partial blockage of the cathode and following an increase in the polarization of the cathodic process, approximately to -1.1 v (figure 5, curve 3). the increase in the limiting current density can be explained by partial sedimentation and an increase in the metal concentration near the cathode, which was located at a medium level relative to the height of the electrolyte column in the cell. partial blockage of the cathode leads to a decrease in the homogeneity of the crystal structure and the deterioration of the quality of the nickel deposits (figure 2, c). results of edx studies of the elemental composition of the sample surfaces with nickel coatings obtained in the studied electrolytes with different amounts of water are given in table 1. https://dx.doi.org/10.5599/jese.1451 j. electrochem. sci. eng. 12(5) (2022) 1025-1039 electrodeposition of polyfunctional ni coatings 1032 table 1. elemental composition of the surface of nickel coatings with a thickness of 5 μm depending on the amount of water in the electrolyte h2o/ni molar ratio 6 12 18 element element content, at.% ni 93.02 94.35 92.13 fe 5.19 3.28 2.46 o 1.79 2.37 5.41 as can be seen from table 1, the presence of iron (from steel base) in the coating indicates the porosity of the obtained coatings. as the amount of water in the solution increases, the oxygen content in the surface layers of the coating increases. this may be another confirmation of the fact that the increase in the amount of water in the system increases the amount of sparingly soluble oxygen-containing nickel compounds, which during electrodeposition, are incorporated in the coating. as shown by the results of determining the current efficiency (figure 7), the increase in water content in the test solution has little effect on the current efficiency of the nickel deposition process. the current efficiency varies within 80… 97 %, and the maximum values are observed at current densities of 0.5 to 1 adm–2. the main side cathodic process is the molecular hydrogen formation from protons, which is accompanied by intense gas evolution. the lack of a sharp decrease in the current efficiency of nickel deposition from the bath with h2o/ni molar ratio = 18 can also be explained by co-deposition with the metal of insoluble nickel compounds. figure 7. the effect of current density on efficiency for choline chloride lactic acid des used for ni electrodeposition, molar ratio h2o/ni 1: 6; 2: 12; 3: 18 an important parameter that determines the throwing power and, as a consequence, the uniformity of current density and coating thickness distribution on the cathode surface is the wagner number (w). this parameter can be determined by the equation (1) [26]: 1 d d d e j w h j h de   −   = =     (1) 70 80 90 100 0 0.5 1 1.5 2 2.5 с u rr e n t e ff ic ie n cy , % j / adm−2 1 2 3 d. uschapovskiy et al. j. electrochem. sci. eng. 12(5) (2022) 1025-1039 https://dx.doi.org/10.5599/jese.1451 1033 σ – specific electrical conductivity s cm-1; н – interelectrode distance, сm; de/dj – the slope of the polarization curve, ω сm2. accordingly, the greater the w, the more uniform and preferred is the secondary distribution of current density. in the case when w → 0, the uniformity of current and metal distribution decreases, approaches the primary and will be determined by the ratio of the geometric parameters of the galvanic cell. the determined values of electrical conductivity and the calculated de/dj and w values are given in table 2. wagner number was calculated since the interelectrode distance was constant and equal to 2 cm. the slopes were used for the cathodic branches of potentiodynamic curves (figure 5). also, according to the data [27], the corresponding electrochemical parameters are given for the watts electrolyte for comparison. table 2. electrochemical parameters of nickel-plating processes parameter electrolyte based on the investigated des with h2o/ni molar ratio* watts electrolyte**[27] 6 12 18 (de/dj) / ω cm2 6.68 2.96 0.96 2.10 σ / s сm-1 0.009 0.020 0.030 0.700 w 0.030 0.030 0.014 0.740  e/ v -0.76 -0.63 -1.1 -0.4 *temperature 75 °с; **temperature 50 °с. from table 2 it can be seen that with the increasing amount of water in the electrolyte, the polarization of the cathode decreases, and the electrical conductivity of the electrolyte increases. the values of the wagner number for the nickel electrolyte based on the investigated des are much smaller than for the conventional water-based watts electrolyte. the values of w in the investigated electrolyte based on des indicate that the electrodeposition will be dominated by the primary distribution of current density, which is determined by the ratio of the geometric parameters of the cell. regarding the possible field of application, such an electrolyte can be used in additive technologies of electrochemical 3d printing. in [28], it has been shown that in order to achieve the highest possible accuracy of printing (local electrodeposition of metal) and process control, the electrolyte must have a minimum throwing power, and the current distribution must correspond to the primary one. investigation of capillary properties of ni coatings deeper studies of the coating structure showed the presence of a cabbage-like structure with nanosized scaly crystalline groups, the thickness of which is less than 100 nm (figure 8). in general, the surface of the obtained coatings was gray, coarse-grained and velvety-like (figure 9a), and the coatings can be easily wetted with water. similar in appearance and structure tin coatings were already obtained in [29]. the appearance (velvet-like surface) and properties (good wettability of the surface with water) prompted further studies of the capillary properties of the obtained coatings (figures 9 and 10). as it was established during the measurement of the height of capillary uplift of ethyl alcohol (figure 9), on samples obtained from the investigated electrolyte, the capillary rise column reaches 6 mm at a coating thickness of 10 μm and 7-12 mm at a coating thickness of 20 μm. on the matte nickel, electrodeposited from the watts bath, the alcohol rises by 2 mm, and on the bright nickel, capillary uplift has not been observed. https://dx.doi.org/10.5599/jese.1451 j. electrochem. sci. eng. 12(5) (2022) 1025-1039 electrodeposition of polyfunctional ni coatings 1034 figure 8. the surface structure (in various magnifications) of the galvanic nickel coating with a thickness of 10 μm, obtained in the electrolyte based on des. nickel content in the electrolyte 1.14 m, in the form of nicl2·6h2o a b c d figure 9. samples with electrodeposited ni coatings placed in a container with ethyl alcohol: a, b – electrodeposited from the electrolyte based on des; c – bright nickel coating; d – matte nickel coating. coating thickness: a, c, d – 10 μm; b – 20 μm a b c figure 10. samples with electrodeposited ni coatings placed in a container with water: a – electrodeposited from the electrolyte based on des; b – bright nickel coating; c – matte nickel coating. the thickness of the coatings is 10 μm on the samples immersed in water (figure 10), the column of capillary rise of the liquid with a height of 5 mm is observed only on the coatings obtained from the investigated des-based electrolyte. the thickness of the coating has almost no effect on the height of capillary uplift. d. uschapovskiy et al. j. electrochem. sci. eng. 12(5) (2022) 1025-1039 https://dx.doi.org/10.5599/jese.1451 1035 based on the obtained results, it has been found that electrodeposited nickel coatings from the des-based electrolyte show the ability of capillary uplift of liquids such as water and ethyl alcohol, compared with conventional matte and bright coatings. this property makes the obtained coating a promising material for heat pipes and heat exchange surface modification [30,31]. the investigation of electrochemical hydrogen evolution reaction nickel electrodes are widely used, in particular, in the alkaline electrolysis of water. to reduce energy costs and increase process productivity, electrodes with a highly developed, microstructured, and bulky-porous surface are used [32-34]. the obtained nanostructured surface of nickel coatings that have been electrodeposited from an electrolyte based on des (figure 8) may indicate their prospects for use as cathode materials for the electrolysis of water. to investigate the electrochemical activity on the studied electrodes, a linear polarization scan with 10 mv/s scan rate curves was recorded in semilogarithmic coordinates (figure 11). figure 11. potentiodynamic polarization curves in 0.1 m koh and following electrodes: 1 pt; 2 electrodeposited ni from the electrolyte based on des; 3 electrodeposited matte ni from watts bath; 4 – electrodeposited bright ni from watts bath from figure 11, it can be seen that the lowest polarization of hydrogen evolution is observed on platinum, in the second place is nickel electrodeposited from des-based electrolyte, followed by galvanic matte and bright nickel. accordingly, at a cathodic current density of 1 ma cm-2 (log j = 0), the potentials of hydrogen evolution are: on platinum at -0.87 v vs. ssce, on ni electrodeposited from the electrolyte based on des at -0.94 v vs.ssce, on matte and bright nickel about 1 v vs.ssce. thus, the obtained data indicate the prospect of using galvanic nickel coatings, electrodeposited from an electrolyte based on a deep eutectic mixture of choline chloride and lactic acid, as cathode materials for electrochemical production of hydrogen. conclusions the deep eutectic mixture of choline chloride and lactic acid with a molar ratio of 1:3 was prepared by the heating method. the ft-ir analysis proved the formation of the combined bonding of choline chloride and lactic acid molecules and the 1h nmr analysis proved the molar ratio of the -3 -2 -1 0 1 2 -1.2-1.1-1-0.9-0.8-0.7 lo g ( j / m a cm − 2 ) e / v vs. nhe 1 2 3 4 https://dx.doi.org/10.5599/jese.1451 j. electrochem. sci. eng. 12(5) (2022) 1025-1039 electrodeposition of polyfunctional ni coatings 1036 choline chloride – lactic acid to be 1:3. the electrochemical stability window was found to be 1.33 v for pure des and reduces to 1.3 v when water was added in the amount of 25 vol.%. during the investigation of the process of electrodeposition of nickel coatings from the electrolyte based on a deep eutectic mixture of choline chloride and lactic acid, the following have been established. the surface of the electrodeposited coatings is light gray, like velvet. galvanic deposits of nickel are quite coarse-grained with a grain size of 0.5 to 2 μm. the surface of the crystal grains is highly developed and contains scaly crystals with a characteristic size of less than 100 nm. galvanic coatings, which were electrodeposited from the investigated electrolyte, show quite pronounced capillary properties in relation to water and ethyl alcohol. the highly developed surface structure of the obtained coatings makes them promising for use as an electrode material for hydrogen evolution reaction. the height of the capillary rise column on investigated coatings is 4 to 6 times higher than that on conventional matte and bright coatings obtained by watts bath. as the water concentration in the electrolyte increases (h2o/ni molar ratio increases from 6 to 8), the electrical conductivity of the system increases from 0.009 to 0.03 s/cm. however, the increase in the amount of water contributes to the deterioration and electrodeposition of more coarse-grained and less-ordered structures of nickel deposits. it has been established that in the range of current densities of 0.25 to 2 a dm-2 the current efficiency of nickel deposition process varies within 80 to 97 %. increasing the amount of water in the electrolyte does not significantly affect the current efficiency. based on polarization measurements and electrical conductivity measurements, the values of the wagner number are determined, which vary within 0.014 to 0.03. the obtained values indicate the predominance of the primary current density distribution during the electrodeposition of nickel from the electrolyte based on a deep eutectic mixture of choline chloride and lactic acid. this makes promising the use of the corresponding 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https://creativecommons.org/licenses/by/4.0/) {supercapacitor performance gains from structural modification of carbon electrodes using gamma radiations:} http://dx.doi.org/10.5599/jese.1224 485 j. electrochem. sci. eng. 12(3) (2022) 485-499; http://dx.doi.org/10.5599/jese.1224 open access : : issn 1847-9286 www.jese-online.org original scientific paper supercapacitor performance gains from structural modification of carbon electrodes using gamma radiations norliyana mustapar, mohd amir radhi othman, mohd sukor su'ait1, mohd suleman2, wai yin wong3 and kee shyuan loh3 department of applied physics, faculty of science and technology, universiti kebangsaan malaysia, 43600 ukm bangi, selangor, malaysia 1solar energy research institute (seri), universiti kebangsaan malaysia, 43600 ukm bangi, selangor, malaysia 2shibli national college, veer bahadur singh purvanchal university, uttar pradesh 222003, india 3fuel cell institute, universiti kebangsaan malaysia, 43600 ukm bangi, selangor, malaysia corresponding author: maro@ukm.edu.my; tel.: +603-8921-3474; fax: +603-8921-3777 received: december 31, 2021; accepted: february 16, 2022; published: march 10, 2022 abstract the performance of supercapacitors (sc) strongly depends on how their activated carbon (ac) electrodes were synthesized from precursor materials and pretreatments applied to them. this study investigates the effect of direct and filtered gamma radiations applied as pretreatments to the ac. the exposure doses used were from 0.1 kgy to 6 kgy. the high gamma-energy and high dose of the pretreatment broke the randomly orientated graphitic crystal lattices inside ac particles and disturbed the existing functional group populations. the filtered radiation pretreatment at 1 kgy, which contains a higher composition of secondary electrons than direct radiation pretreatment, yields ac with the best overall sc performance. the sc cell made from 1 kgy filtered radiation pretreatment ac showed higher specific capacitance 73.1 % (218.58 f g-1), specific energy 73.54 % (10.96 w h kg-1) and specific power of 8.36 % (155.67 w kg-1) compared to the sample without any radiation pretreatment. this study explicitly shows the benefit of secondary electrons in the radiation field, which produce decisively defect sites on the ac lattices for gains in sc performance. keywords supercapacitor cell; activated carbon; gamma pretreatment; secondary electrons radiation introduction an economic development based on non-renewable energy sources is unsustainable. unfortunately, such energy sources significantly influence the energy consumption in the world today as the demand for energy keeps increasing. therefore, renewable and green energy sources are attracting current energy research to reduce the dependence on unsustainable energy sources [1]. in the ecosystem of renewable and green energy sources, supercapacitor (sc), also known as ultracapacitor, http://dx.doi.org/10.5599/jese.1224 http://dx.doi.org/10.5599/jese.1224 http://www.jese-online.org/ mailto:maro@ukm.edu.my j. electrochem. sci. eng. 12(3) (2022) 485-499 supercapacitor performance 486 is a promising green energy storage device. sc is an ideal electrical energy storage device with moderate energy capacity and high-power delivery [2], long life cycle [3], low maintenance [4], desirable safety and fast dynamics of charge propagation compared to conventional energy storage devices. due to such benefits, scs are used primarily in hybrid electric vehicles, electric vehicles, fuel cell vehicles such as passenger cars, trains and trolleybuses [5]. the electrode material is one of the major components that determine the performance of sc. the commonly used electrode materials are based on carbonaceous materials such as carbon nanotubes [6], carbon nanofibers [7], carbon aerogel [8], graphene [9], and activated carbon (ac) [10]. the non-carbon-based materials (found in pseudocapacitors) include conducting polymers [11], and transition metals oxides such as nio [12], ruo2 [13], iro2 [14], and mno2 [15]. however, ac is the most widely studied electrode material due to its high porosity structure, high surface area, endurance, non-toxicity and potential to alleviate agro-industrial wastes [16]. the precursor materials of ac are naturally obtained from agricultural wastes, such as coconut shells [17], oil palm empty fruit bunches [18], cassava peel [19], banana fibers [20], jackfruit peels [21], sunflower seed shells [22], bamboos [23], rice straw [24], durian shells [25], and coffee beans [26] due to their low cost and abundant availability. the modification methods involved during the precursor preparation play an essential role in producing desired carbon electrode properties [27]. generally, the carbonization (in an inert gas at high temperature) and activation (physical or chemical) methods are used separately or in combinations to modify the ac electrode pore structure, surface area, and conductivity. however, the sc incorporating ac electrodes from biomass precursor has reached the limit of useable surface area for electrochemical performance gains. it has been reported that the performance of such scs does not linearly increase with the increase in surface area due to non-ideal pore size distribution [28]. thus, further viable treatment on the ac should focus on modifying carbon lattices and functional group populations. typically, functional groups present in ac from biomass precursors (coconut shell [29]) are associated with o-h bonds of hydroxyl groups, c-h bonds of methyl groups and c=c bonds. gamma radiation on carbonaceous materials has long been attributed to improving structural modifications in terms of the material disorder [30], increased pore width, and mesopore formation, contributing to a broader distribution of pore sizes [31]. hence, prior application of radiation treatment on the ac powder led to better sc performance [32]. however, most gamma energy does not deposit onto the ac powder during stochastic interactions (for uncharged particles such as photons) under direct gamma radiation. instead, the energy is carried away by scattering the knockedout secondary electrons. this effect intensifies for 1.25 mev-photon when the compton scattering interaction dominates, resulting in more energetic and long-range secondary electrons scattering. another issue is that ac materials have a low interaction cross-section with photons at the energy of 1.25 mev. the solution to these problems is to place a metallic layer as a filter (radiation attenuator) between the radiation source and ac powder, which acts as an additional interaction site for producing secondary electrons. the secondary electrons created from the metallic layer will reach the ac powder and interact deterministically (for charged particles such as energetic electrons) to create typical lattice defects, such as stone-wales defects and vacancy defects [33]. the defects created can influence the electrochemical property of carbonaceous materials [34]. this study used gamma treatment from a co-60 source with increasing doses between 0.1 and 6 kgy to radiate the ac commercial (acc) powder. the acc was exposed directly to gamma radiation (direct radiation), or the acc was placed inside a copper envelope before radiation (filtered radiation). this study investigates the microscopic impact of the radiation modalities on the acc n. mustapar et al. j. electrochem. sci. eng. 12(3) (2022) 485-499 http://dx.doi.org/10.5599/jese.1224 487 properties, such as pore networks, surface morphology, chemical bonds, and lattices. finally, the effect of the radiation modalities on electrochemical performance was investigated by using the acc as sc electrodes. we elucidated and differentiated the effect of the radiation modalities in producing radiation defects in the acc from the physical and electrochemical analyses. experimental powder irradiation setups a plastic bag filled with 0.5 g of acc powder (bravo green sdn bhd) was placed inside a revolving container (gammacell 220 excel) (figure 1a) in the direct radiation modality with a co-60 source (1.25 mev-photon). another plastic bag was enclosed in a copper foil envelope of 0.13 mm-thicklayer in the filtered radiation modality (figure 1a inset). the irradiation doses used were 0.1, 0.5, 1.0, 3.0, and 6.0 kgy for each radiation modality (the samples from direct radiation were denoted as accd-0.1, accd-0.5, accd-01, accd-03, and accd-06, respectively, and filtered radiation samples as accf-0.1, accf-0.5, accf-01, accf-03, and accf-06, respectively). figure 1b shows a schematic representation of gamma radiation on the samples. figure 1. (a) acc powder in a plastic bag ready for exposure to direct gamma radiation in the gammacell 220 excel. the plastic bag is enclosed with copper foil in the filtered radiation modality. (b) schematic representation of gamma radiation on the samples, where copper foil attenuates the primary radiation (curly red arrow) and produces secondary electrons, e(blue tracks). the copper foil is absent in the direct radiation, leaving mostly direct gamma interaction with acc powder preparation of supercapacitor cells the irradiated acc powders were further grounded and sieved (lab. tes sieve bs 140) until an average particle size less than 53 μm was obtained. the electrodes were prepared by mixing 90 wt.% acc powder, 5 wt.% carbon black (sigma-aldrich) and 5 wt.% pvdf-hfp (sigma-aldrich) to form a slurry at room temperature. the slurry was brush-painted on graphite tape (commercially availableziqrah scientific) with 1 cm2 of the cut area. the mass loading of active materials is about 4.2 mg on the electrode after drying. then, two symmetric electrodes were soaked in 6 m koh electrolyte before being made up into sc cell by sandwiching a polypropylene separator (25 μm thick, celgard 3501). http://dx.doi.org/10.5599/jese.1224 j. electrochem. sci. eng. 12(3) (2022) 485-499 supercapacitor performance 488 physical characterizations field emission scanning electron microscope (fesem, merlin, zeiss 2013) was used to observe acc, accd and accf powders morphologies. micrometric asap 2010 instrument using liquid nitrogen adsorbate at 77 k was used to determine the sample bet specific surface area (sbet) and pore distribution parameters such as micropore surface area (smicro), mesopore surface area (smeso), micropore volume (vmicro), mesopore volume (vmeso) and average pore diameter (dp). x-ray diffraction (xrd) patterns of acc, accd and accf samples were determined by x-ray diffractometer (bruker axs: model d8 advance) with cukα radiation (λ= 0.154 nm). the interplanar spacing (d002 and d100) for all accs powder was determined from equation (1) [35]: sin 2 d  = (1) where λ is the wavelength of the x-ray, and θ is the reflection at angles θ002 and θ100 for peaks (002) and (100), respectively. the layer stacking height (lc) from the (002) peak and longitudinal size of stack width (la) from the (100) peak were calculated from the debyescherrer equations (2) and (3) [35]: lc= 0.9 ( λ β002 cos θ002 ) (2) la= 1.84 ( λ β100 cos θ100 ) (3) where β002 and β100 is the full width at half maximum of the symmetrically shaped diffraction peaks of (002) and (100) planes, while θ002 and θ100 are (002) plane and (100) plane reflection angles, respectively. raman spectra were recorded from 650 to 4000 cm-1 using a 514 nm laser beam for selected samples (thermo scientific i model (dxr2xi)). the chemical structure of the selected samples was studied using fourier transform infrared spectroscopy (ftir, perkin elmer spectrum 400 ft-ir). electrochemical characterizations electrochemical performance of sc (two-electrode configuration) made from acc, accd and accf electrodes were investigated. in electrochemical impedance spectroscopy, eis (metrohm autolab b.v), measurements were carried out using 10 mv amplitude in a frequency range from 1 mhz to 0.01 hz. from eis data, the specific capacitance (csp) of the sc with two equal electrodes was calculated using equation (4): csp= 1 πf1z”1m (4) where f1 is the lowest frequency of measurement, z” is the imaginary impedance value () at f1, and m is the total mass of active material (g) of two electrodes. the sc charge storage capacity was studied using cyclic voltammetry, cv (metrohm autolab b.v) and csp for cv were calculated for two electrodes using equation (5) [36]: csp= 1 2mv(∆v) (∫ v1 v2 i(v)dv) (5) where ν (mv s-1) is the scan rate, ∆v (v) is the potential window, while ∫ i(v)dv is the total voltammetric charge bounded by the negative and positive sweeps (the area inside the cv curve). galvanostatic charge-discharge, gcd (5 v, 10 ma neware bts 3000 battery testing system) analysis was performed to produce csp, specific power (p) and specific energy (e) of the two electrodes sc based on equations (6) [37], and (7 and 8) [38]: n. mustapar et al. j. electrochem. sci. eng. 12(3) (2022) 485-499 http://dx.doi.org/10.5599/jese.1224 489 csp= 2i ( ∆v ∆t )m (6) p = vi m (7) e = vit m (8) where i is the discharge current, ∆v is the total potential difference during a complete discharge process, ∆t is the time for complete discharge, v is the cell operational voltage window, and t is discharging time (s). results and discussion physical characterizations selected acc samples were chosen based on the results of their sc electrochemical performance obtained earlier. in the fesem and n2 adsorption-desorption analysis, the selected samples include non-radiated and radiated samples of 0.5 and 1.0 kgy. in structural-sensitive characterizations such as xrd, raman spectroscopy and ftir analysis, the 6.0 kgy radiated samples were also included. fesem micrographs fesem characterization of surface morphologies of the selected samples shows that the acc naturally exhibited a porous texture with a network of open pores, as shown in figure 2. this richness of pore sizes in the samples is beneficial for sc electrodes as it provides good ionic transportation [39]. the electrode pores developed during the activation process result from the breakdown of the biomass precursor structure [40]. figure 2. fesem micrographs for: (a) acc-00 with complex open pores network; (b) accd-0.5 with surface roughness and some pores network extending deep into the particle; (c) accd-01 open pores network filled with smaller particles; (d) accf-0.5 same as (a); (e) accf-01 with granulation of smaller particles on the surface the morphological effect due to radiation exposures is not directly discernible through the fesem. however, the fesem does show that the distribution of granulations and the surface roughness of the samples are almost identical. the network of pores was confirmed through n2 adsorption-desorption analysis. http://dx.doi.org/10.5599/jese.1224 j. electrochem. sci. eng. 12(3) (2022) 485-499 supercapacitor performance 490 n2 adsorption-desorption the influence of gamma radiations on the porous texture of acc was captured in n2 adsorptiondesorption measurements (figure 3). the n2 adsorption-desorption isotherms of the selected samples show that the overall shapes are typical adsorption-desorption curves for ac materials [41]. the samples iupac curve classification is a combination of type i and type iv isotherms [42]. type i isotherm is characteristic of microporous samples [43], while type iv isotherm indicates the presence of mesopores [44]. the values of the pore structure parameters (table 1) from the adsorption isotherms showed a general trend which suggests that radiation exposures cause surface area to slightly decrease for radiated samples, compared to the non-irradiated sample [31]. however, the accf-01 sample has the most significant volume of smeso, which is essential for electrolyte ions transport [45] into micropore surfaces and improves power performance at high current densities. figure 3. nitrogen adsorption-desorption isotherms for selected acc, accd and accf samples table 1. pore parameters for selected acc, accd and accf samples radiation modality non-radiated direct radiation filtered radiation radiation dose, kgy acc-00 accd-0.5 accd-01 accf-0.5 accf-01 sbet / m2 g-1 1065.08 990.73 986.79 1010.07 1034.55 smeso / m2g-1 214.99 215.16 212.54 214.00 221.25 smicro / m2g-1 850.09 775.57 774.25 796.07 813.30 vmeso / cm3g-1 0.12 0.11 0.14 0.12 0.12 vmicro / cm3g-1 0.40 0.37 0.37 0.37 0.38 smeso / smicro 0.25 0.28 0.27 0.27 0.27 dp / nm 1.99 2.00 2.01 2.00 2.00 xrd analysis crystallographic structures from xrd for the selected acc powder samples are shown in figure 4. the existence of (002) and (100) broad peaks in figures 4(a) and 4(b) indicates that the samples are amorphous [46]. the diffraction pattern for the selected samples consists of two broad peaks, correlated with honeycomb-structure of carbon, located at around 24.6 and 43.7o, corresponding to the (002) and (100) peaks, respectively. the peak broadness represents a turbostratic or random layer lattice structure [47]. this turbostratic structure shows that the carbon layers are characterized by randomly shifted layers, unorganized stray carbons, local stacking faults, strain in the layers and varying interspacing values [48]. the structural features such as the interlayer spacings (d002 and d100), lc, la, the ratios of lc/la and lc/d002 of the samples calculated from equations (1)-(3) are shown in table n. mustapar et al. j. electrochem. sci. eng. 12(3) (2022) 485-499 http://dx.doi.org/10.5599/jese.1224 491 2. the radiated samples interlayer spacing (d002 and d100) values did not differ significantly from nonirradiated ones. there are significant increments of lc and decrements of la from accd and accf samples, especially the accd-01 and accf-01, compared to acc-00. the highest increment of lc at 1 kgy dose is associated with increased intermediate carbon plane distance held by van der waals force. this effect is attributed to the formation of defects and the appearance of intermediate dangling atoms between the carbon planes. the out-of-plane dangling atoms have a shorter distance to the next carbon plane. the decreasing of la at 1 kgy is due to the weakened of covalent (σ and π) bond represented by peak (100) of carbon plane [45]. hence, the ratio of lc/la and lc/d002 shows the largest value for samples accd-01 and accf-01. at a higher dose (6 kgy), more defects and dangling atoms formed, causing carbon planes to collapse and break apart at the defect points. hence, the new structures have fewer defects and the structural parameters resembling acc-00. the increment in lc, lc/la and lc/d002 values corresponds to the gain in specific capacitance of sc [49], already observed in the electrochemical characterizations section. this is due to the increase in lattice defects on accd and accf particles that indirectly affects pore distributions. figure 4. xrd pattern for: (a) direct radiation modality, accd, and (b) filtered radiation modality, accf, at different irradiation doses. the non-radiated acc-00 is shown on both sides for reference table 2. x-ray diffraction parameters for selected acc, accd and accf samples accs d002 / nm d100 / nm lc / nm la / nm lc / la lc / d002 acc-00 0.374 0.208 0.635 2.697 0.24 1.70 accd-0.5 0.370 0.208 0.713 2.481 0.29 1.92 accd-01 0.376 0.209 0.755 1.535 0.49 2.01 accd-06 0.372 0.209 0.536 2.361 0.23 1.44 accf-0.5 0.370 0.208 0.726 2.343 0.31 1.96 accf-01 0.378 0.209 0.866 1.651 0.52 2.29 accf-06 0.375 0.208 0.643 2.449 0.26 1.72 raman spectroscopy the results of raman spectroscopy analysis performed on the selected samples are shown in figure 5. raman spectra consist of three prominent characteristic peaks centered around (1340 to 1390 cm-1), (1590-1640 cm-1) and (2590-2620 cm-1), corresponding to d band, g band and 2d bands, respectively, which is typical for ac samples [50]. the d band represents defect/disorder carbon. the g band represents the in-plane vibration of the sp2. the 2d band represents graphitic carbon characteristics and its peak broadness is related to the number of carbon layers [51]. the degree of disorder of the sample is described by the intensity ratio of the d and g bands (id/ig). generally, id/ig values increased after exposure to radiation, where the sample exposed to http://dx.doi.org/10.5599/jese.1224 j. electrochem. sci. eng. 12(3) (2022) 485-499 supercapacitor performance 492 1 kgy recorded the highest increment (table 3). the reduction of id/ig values at overdose (6 kgy) suggests that the newly formed structures break at defect points with fewer defects than the 1 kgy sample. this trend is in good agreement with the xrd analysis (lc, lc/la and lc/d002) that showed maximums at 1 kgy dose and decrements for an overdose at 6 kgy. the intensity ratio between 2d and g peaks (i2d/ig) indicates the number of carbon layers present in carbon material [52]. the values of i2d/ig >2, 1< i2d/ig <2, i2d/ig <1 mean the presence of single-layer graphene, bilayer graphene, and three or more layers, respectively, in the turbostratic structure of the ac [53]. thus, table 3 shows the presence of bilayer graphene in all ac samples. figure 5. raman spectra for the selected samples: (a) direct radiation modality, accd, and (b) filtered radiation modality, accf, at different irradiation doses. the non-radiated acc-00 is shown on both sides for reference table 3. indicators for degree of disorder (id/ig) and layers (i2d/ig) accs id/ig band i2d/ig band acc-00 1.03 1.35 accd-0.5 1.08 1.28 accd-01 1.11 1.31 accd-06 1.09 1.16 accf-0.5 1.11 1.36 accf-01 1.12 1.40 accf-06 1.08 1.44 ftir analysis ftir analysis is based on the characteristic transmission of reference light from the molecular bonds of functional groups present in the selected samples. the effect of gamma radiation on molecular bonds for different doses and radiation modalities can be seen clearly after normalization at a specific point (here, at 1280 cm-1), as illustrated in figure 6. each ftir spectrum has a broad peak around 3321 cm-1, which is attributed to the o-h stretch of the hydroxyl functional groups [29]. the peak at 2067 cm-1 is associated with the c≡c alkyne group. this peak broadness indicates that c≡c alkyne vibration becomes weaker as it was disrupted after irradiation. three peaks observed at 1981, 1522 and 1812 cm-1 correspond to c-h aromatic compounds, c=c aromatic compounds and c=o carbonyl compounds, respectively. the peak at 1238 cm-1 is ascribed to c-o stretching, identified as lignin by shi et al. [54]. in the direct radiation modality, the radiation interacts stochastically with the accd samples due to the material low effective atomic number (z ≈ 6), which leads to low interaction cross-section at n. mustapar et al. j. electrochem. sci. eng. 12(3) (2022) 485-499 http://dx.doi.org/10.5599/jese.1224 493 1.25 mev energy photon (carbon mass attenuation coefficient, 5.6910-2 cm2 g-1) [55]. the stochastic effect is translated into interchanges of the order of ftir lines for each identified compound peak between accd samples and acc-00, figure 6(a). the interchanges of lines show a stochastic nature that breaks the identified bonds due to direct gamma interaction with the sample carbon materials (compared to non-radiated samples). however, the secondary electrons composition is more significant in the filtered radiation modality. it is due to more secondary electrons being created (from compton scattering) when the primary radiation gets attenuated in passing through the copper envelope. even though the copper envelope (z = 29) has a mass attenuation coefficient of 5.2610-2 cm2 g-1 at 1.25 mev-energy photon-like carbon, once the scattering of secondary electrons is created, they mostly move towards the accf samples. this is true given the proximity between the copper layer and accf and the high 1.25 mev-energy photons involved; the ejected secondary electrons move forwards with a small scattering angle with respect to the primary photon axis. the scattered secondary electrons interact deterministically with carbon materials (as charged particles always do) and have persistent bond breakings capability. this deterministic effect can be seen in figure 6(b), where the selected accf samples consistently have lesser transmittance than acc-00 for each identified compound peak. the accf-01 sample has more transparency than the acc-00 and accds (figure 6(a)) samples, suggesting that the distribution of functional groups in the accf-01 sample has been significantly modified. figure 6. ftir spectra for the selected samples: (a) direct radiation modality, accd, and (b) filtered radiation modality, accf, at different irradiation doses. the presence of acc-00 and accf-01 in both spectra is for comparison the reduction of c=c bonds in all radiation modalities is evident. it agrees with xrd and raman analyses that show the increase of defects in the carbon structure due to radiation modalities. however, as shown in figure 6(a), the effect from secondary electrons rich in filtered radiation modality (accf-01 sample) shows the most considerable reduction of c=c bonds. however, an unfamiliar peak in the acc samples appears at 2277 cm-1 associated with n=c=o isocyanates. the peak appearance is believed [56] to be due to the manufacturer nitric acid treatment given to the precursor of the acc. this peak usually appears in highly oxidized samples. electrochemical characterizations electrochemical impedance spectroscopy (eis) this type of electrochemical characterization investigates the favorable electrochemical properties of the accd and accf samples as sc electrodes compared to that of the acc sample. the nyquist plots of sc cells made from the accd, accf and acc samples shown in figure 7, consist of two regions with a semicircle at high frequencies and a straight-capacitive line at low frequencies. http://dx.doi.org/10.5599/jese.1224 j. electrochem. sci. eng. 12(3) (2022) 485-499 supercapacitor performance 494 this is a typical electric double-layer capacitance (edlc) behavior for sc with thin electrodes [57]. the semicircle represents charge-transfer resistance, rct, due to double-layer capacitance at the interface of electrolyte/electrode and the faradaic process [16]. the semicircle intercept with z’ at high frequencies (left side) is resistance rs, representing ionic resistance of electrolyte solution [58]. thus, the equivalent series resistance esr is given by esr = rs + rct. the eis resistance values of the samples are listed in table 4. figure 7. nyquist plots for sc cells made from: (a) direct radiation modality, accd samples, and (b) filtered radiation modality, accf samples, at different irradiation doses. the sc made from non-radiated acc-00 is shown on both sides for reference. inset of (a) shows the resistance parameters and lowest frequency point f1 on nyquist plot interestingly, the rct values for sc cells made from accd and accf electrodes are significantly lower compared to acc-00 sc cell and show a decreasing trend of rct and hence esr values, with increasing radiation doses until 1 kgy. this is beneficial to facilitate faster ion transport across the electrodeelectrolyte interface. it can be attributed to the difference in functional groups population that the accd and accf electrodes have compared to acc-00. at overdoses of 3 and 6 kgy, the rct of accd sc cells starts climbing up. however, for the accf sc cells, rct values increase for 3 kgy but drop back at 6 kgy overdoses. the breaking apart of carbon structure at overdoses increases the rct due to an increase of contact resistance among newly formed carbon structures except for accf-06. in the low-frequency region, the shorter straight-line in the nyquist plot means a lower z’’ value at f1, and therefore indicates an improved capacitive behavior (higher csp), as indicated by equation (4). the csp values (table 4) show an increasing trend with irradiation doses up to 1 kgy with maximum values of 121.9 f g-1 (accd-01 electrode) and 113.9 f g-1 (accf-01 electrode), before going down due to overdose at 3 and 6 kgy. table 4. electrical parameters of sc cells parameters non-radiated direct radiation filtered radiation acc-00 accd-0.1 accd-0.5 accd-01 accd-03 accd-06 accf-0.1 accf-0.5 accf-01 accf-03 accf-06 eis rs / ω 0.33 0.30 0.30 0.44 0.28 0.24 0.28 0.38 0.28 0.31 0.94 esr / ω 139.79 67.11 43.42 12.71 38.77 62.92 84.44 79.83 41.46 65.36 18.13 rct / ω 139.46 66.81 43.12 12.27 38.49 62.68 84.16 79.45 41.18 65.05 17.19 csp / f g-1 68.95 84.84 97.42 121.90 58.03 54.22 96.03 104.85 113.92 65.99 64.94 cv csp / f g-1 66.23 82.80 92.40 104.31 81.82 81.41 88.99 95.39 116.73 86.52 84.30 gcd csp / f g-1 58.80 85.95 96.04 107.18 61.13 53.23 91.49 98.87 218.58 73.28 71.99 p / w kg-1 142.65 142.97 144.87 153.5 145.33 136.4 146.12 147.57 155.67 143.22 136.71 e / wh kg-1 2.90 4.30 4.88 5.84 3.12 2.51 4.73 5.24 10.96 3.69 3.53 n. mustapar et al. j. electrochem. sci. eng. 12(3) (2022) 485-499 http://dx.doi.org/10.5599/jese.1224 495 cyclic voltammetry (cv) cv measurements were conducted within a potential window from 0 to 1 v and scan rates of 1 mv s-1. figure 8 shows that all sc cells have similar rectangular shapes, typical for an electrical double layer sc [59]. the capacitive performance (csp values were calculated using equation (5)) is dependent on irradiation doses and radiation modalities applied on the ac powder. the area of the cv-curve that determines the sc cells performance increases with increasing doses from 0 kgy (acc-00 electrode) until 1 kgy for the accd and accf electrodes. however, the sc cells performance decreased at overdoses of 3 and 6 kgy for both accd and accf electrodes. the accd-01 and accf01 sc cells exhibit the largest cv area than others, translating into the most significant csp values (table 4). at the low cv scan rate, the csp represents peak values because the conducting electrolyte ions have more time to penetrate deeper into available electrode pores and form electric double layers. the trend of increasing, decreasing and exhibition of maximum at 1 kgy of the csp values in cv analysis follows the eis analysis. figure 8. cyclic voltammograms at 1 mv s-1 for sc cells made from: (a) direct radiation modality, accd samples, and (b) filtered radiation modality, accf samples, at different irradiation doses. the sc made from non-radiated acc-00 are shown on both sides for reference galvanostatic charge-discharge (gcd) the current density is an essential factor for scs as it directly characterizes capacitive behavior. the gcd curves of the sc cells are shown in figures 9(a) and 9(b) at a current density of 0.7 a g-1. the curves show a triangular shape and exhibit small ir drops (the voltage drop due to inner resistance at the initial stage of the discharge process), which are typical for carbon-based materials. the charge-discharge capacity that determines the performance of the sc cells were calculated using equation (6), (7) and (8) for csp, p and e parameters, respectively. the upward trend (csp, p and e values) with irradiation doses is similar to previous eis and cv analysis; the values peaked at 1 kgy and decreased for 3 and 6 kgy (table 4). the relationship between e and p of the sc cells (figures 9(c) and 9(d)) shows the significant overall performance gain of sc cells made from accd01 and accf-01. http://dx.doi.org/10.5599/jese.1224 j. electrochem. sci. eng. 12(3) (2022) 485-499 supercapacitor performance 496 figure 9. gcd curves at 0.7 a g-1 for sc cells made from: (a) accd and (b) accf electrodes, where acc-00 are present on both sides for comparison; (c) and (d) their respective ragone plots conclusions based on the results obtained in this study, the electrochemical performance gain by accd and accf sc cells can be attributed to increasing lattice defects, mesopores improvement over micropores (smeso/smicro) and an increasing degree of disorder (id/ig). in visual identification through n2 adsorption-desorption isotherm curves and fesem pictures, it is hard to identify the radiation effect because it is the intrinsic nature of carbon structures. however, the radiation effects are elucidated in xrd (the changes in the lattice parameters), raman (defect parameter), ftir (reduction of individual functional group bonds) and in the electrochemical analysis. this study indicates that lattice defect modification in the ac is vital, as well as the ac electrode surface area in improving the sc performance. both accd and accf electrodes have almost equal sbet as the acc-00 electrode. however, the sc made from accd and accf electrodes have better overall performance than sc made from acc-00. the use of a filter to increase the composition of the secondary electron (as in accf) has shown significant improvement in the electrochemical performance (up to 51, 47 and 1.4 % increase in csp, e and p, respectively, in the gcd analysis) at 1 kgy of optimal exposure dose (accf-01 compared to the accd-01). the sc made from accf-01 has better overall performance than scs made from other electrodes such as accfs, accds and acc00. the defect created on the ac lattice is more assertive when deterministic radiation modality (filtered) is used than stochastic radiation modality (direct). it can be concluded that the importance of secondary electrons composition in the radiation field and the optimal dose of 1 kgy have an influential role in this study pretreatment of the acs. acknowledgment: we acknowledge the grant 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1224 https://doi.org/10.1142/s0217979208038119 https://doi.org/10.1149/1.2352197 https://doi.org/10.1088/1757-899x/73/1/012096 https://doi.org/10.1088/1757-899x/73/1/012096 https://doi.org/10.1051/matecconf/201815603018 https://doi.org/10.1016/s0378-7753(02)00108-8 https://doi.org/10.1016/s0378-7753(02)00108-8 https://doi.org/10.5714/cl.2008.9.2.127 https://doi.org/10.1039/c5ta04737d https://doi.org/10.1039/b613962k https://doi.org/10.1088/2043-6262/4/3/035012 https://doi.org/10.1016/j.egypro.2012.01.122 https://doi.org/10.1016/j.egypro.2012.01.122 https://nvlpubs.nist.gov/nistpubs/legacy/ir/nistir5632.pdf https://nvlpubs.nist.gov/nistpubs/legacy/ir/nistir5632.pdf https://doi.org/10.1016/j.jaap.2005.05.004 https://doi.org/10.1038/srep22062 https://doi.org/10.3390/nano8070533 https://doi.org/10.1016/j.jallcom.2018.04.123 https://creativecommons.org/licenses/by/4.0/) {modeling the effect of rib and channel dimensions on the performance of high temperature fuel cells-parallel configuration} http://dx.doi.org/10.5599/jese.907 59 j. electrochem. sci. eng. 11(1) (2021) 59-69; http://dx.doi.org/10.5599/jese.907 open access: issn 1847-9286 www.jese-online.org original scientific paper modeling the effect of rib and channel dimensions on the performance of high temperature fuel cells-parallel configuration vikalp jha and balaji krishnamurthy department of chemical engineering, bits pilani, hyderabad 500078, india corresponding author: balaji.krishb1@gmail.com received: september 19, 2020; revised: november 27, 2020; accepted: november 27, 2020 abstract this work investigates the effect of rib width, channel width and channel depth on the performance of a high temperature proton exchange membrane (ht-pem) fuel cell with parallel flow field configuration. simulation results indicate that the rib width has the maximum impact on the performance of the fuel cell. the lower the rib width, the better is performance of ht-pem fuel cell. changing the channel width seems to have a moderate effect, while changing the channel depth seems to have very limited impact on the fuel cell performance. the effect of various rib width and channel dimensions on the pressure drop across the channel is also studied. the concentration profile of the oxygen across the cathode gas channel is modeled as a function of the channel width and depth. modeling results are found to be in well agreement with experimental data. keywords fuel cell, modeling, rib width, channel width, pressure drop introduction improving the performance of a proton exchange membrane fuel cell (pemfc) through modification of the flow field design has already been studied in literature for low temperature fuel cell systems. yoon et al. [1] have studied the effect of rib width experimentally for 80 cm2 cell at different operating conditions. shimpalee and van zee [2] have studied the impact of channel path length on 200 cm2 cells. the authors postulate that the effect of varying the flow channel path length on the reaction area can affect the performance of the cell. shimpalee et al. [3] have also developed a model to study the impact of cross section dimension of channel and rib on the performance of low temperature pemfcs. hsieh and chu [4] have experimentally investigated the effect of channel and rib widths with an aspect ratio 0.5 to 2 for the serpentine flow field pemfc. manso et al. [5] analyzed the effect of the channel cross section aspect ratio for serpentine flow fields. the authors postulate that channel cross section aspect ratio has a direct effect on the performance of pemfcs. http://dx.doi.org/10.5599/jese.907 http://dx.doi.org/10.5599/jese.907 http://www.jese-online.org/ mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 11(1) (2021) 59-69 performance of high temperature fuel cells 60 wang et al. [6] have investigated the effect of channel to rib width ratio and geometric aspect ratio on the performance of pemfc. the authors have shown that for some conditions, the interdigitated design is superior to the parallel design. in addition, wang at al. ]7] have investigated the effect of sub rib convection on pemfc with serpentine flow channels. they found that at low operating voltages, the reduced channel aspect ratio improves the cell performance. chaudary et al. [8] have predicted that the effect of channel width is more pronounced than the land width. yu et al. [9] have mentioned that the land width has little influence on the performance of pemfc with interdigitated flow field. goebel [10] has recommended the minimum land fraction of 50 % for low contact resistance pemfcs. generally, there is a quite body of literature focusing on the effect of rib width and channel dimensions in low temperature fuel cells. on the other side, very little work has been done in the area of high temperature fuel cells, and just this will be the focus of the work presented in this paper. modeling procedure ht-pemfc schematic figure 1 shows the schematic of a parallel flow configuration in ht-pem fuel cell and cross section of flow channels. the fuel cell has a polybenzamidazole membrane. hydrogen and air are sent in the inlets of anode and cathode electrodes. the fuel cell is considered to operate at 130 oc. a b figure 1. (a) schematic of parallel flow configuration in ht-pem fuel cell, (b) cross section of flow channels. model assumptions: a) the model assumes steady state operating conditions. b) the model assumes isothermal conditions. the temperature is assumed as 130 c. c) the model assumes no phase change in a single-phase incompressible flow. d) all gases and gas mixture behaviors are considered ideal. e) crossover of reactants and water vapor through the membrane are neglected. f) the resistances due to gas diffusion layer (gdl) and catalyst layer are neglected. g) due to high temperature, water is considered as vapor. model equations: gas flow in flow channel path is governed by navier-stokes equations. mass and momentum conservation equations are given as follows:  = 0u (1) v. jha and b. krishnamurthy j. electrochem. sci. eng. 11(1) (2021) 59-69 http://dx.doi.org/10.5599/jese.907 61 ( )  = − − +u u pi (2) ( ) ( )   =  +  −    2 3 t u u u i (3) where  is density, u is velocity vector, p is pressure,  is viscous stress tensor, i is the identify tensor and  is dynamic viscosity. butler-volmer equation which describes kinetics of electrochemical reactions at anode and cathode electrode interfaces is given by    −     = + −          0r o exp exp ff i i rt rt (4) where i and io are current density and exchange current density, αr and α0 are anodic and cathodic transfer coefficients,  is overpotential, t denotes the operating temperature, while f and r is faraday and gas constant, respectively. for higher overpotential, tafel equation can be derived from butler-volmer equation in the following form:    =    0 ln i a i (5) where a denotes tafel slope. ohm’s law describes the charge transport in anode and cathode porous electrodes and electrolyte.  =l li q (6)  = − l l li (7)  =s si q (8)  = − s s si (9) where i, q, , and  denote current density, source mass term, conductivity, and potential for either electrolyte (subscript l) or solid electrode (subscript s). henry’s law describes hydrogen and oxygen concentrations at the interface of electrode and membrane: = 2 h h h h x p c k (10) = 2 o o o o x p c k (11) where xh and xo are hydrogen and oxygen mass fraction, while kh and ko are henry’s constant. brinkman equations (anode) brinkman equations are derived from navier-stokes equations for modelling of porous region maxwell-stefan equation describes transport of chemical species in ideal gas mixtures. ( ) ( )( ) ( )           −      =  − +  +  −  − + + +          1a m a a a a a f a a2 p p p p p 2 3 tu q u p i u u u i u u f (12)  =a mu q (13) brinkman equations (cathode) http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(1) (2021) 59-69 performance of high temperature fuel cells 62 ( ) ( )( ) ( )           −      =  − +  +  −  − + + +          1c m c c c c f c c2 p p p p p 2 3 t c u q u p i u u u i u u f (14)  =c mu q (15) where εp is gdl porosity, pc and pa are cathode pressure and anode pressure, uc and ua are cathode inlet velocity and anode inlet velocity,  denotes porous medium permeability,  denotes density and qm is mass source. boundary conditions applying the no slip boundary condition = =c a 0u u (16) cathode inlet – laminar inflow ( ) ( )         − +  +  −  =         entr c c c c entr p 2 3 t t p l p i u u u i p n (17) anode inlet – laminar inflow ( ) ( )         − +  +  −  =         entr a a a a entr p p 2 3 t tl p i u u u i p n (18) where lentr and pentr are entrance length and entrance pressure. the equations for species transport are given as follows: ( )  +  =i i ij u r (19)  = +i i in j u (20) where i j is flux density, u is velocity vector, 𝜌 is density and i  is mass fraction. numerical simulations three-dimensional computational domain of high temperature pem fuel cell (ht-pemfc) is shown in figure 2. all geometrical parameters and properties are listed in table 1. a b c figure 2. mesh distribution in computational domain of ht-pem fuel cell: (a) isometric view, (b) cross section, (c) top view. v. jha and b. krishnamurthy j. electrochem. sci. eng. 11(1) (2021) 59-69 http://dx.doi.org/10.5599/jese.907 63 in our optimization, various values (0.5, 1, 1.5 mm) of flow channel width, channel depth and rib width were considered for analysis. to study the effect of each of these three parameters individually, other two parameters were considered constant (1 mm). rib width is considered equally on both sides of the channel. in our analysis, three different channel lengths (19, 22, 25 mm) were considered to observe flow properties across the flow channel. structural hexahedral elements are used for meshing of computational flow domain. our computational domain consists of 24200 hexahedral mesh elements, 8486 boundary elements and 1024 edge elements. average skewness quality of our mesh elements is 1. mesh elements of our computational domain are adaptive with respect to various channel width, rib width and channel depth. a mathematical model is developed to analyze our computational domain in comsol 5.3a on viable concerns of ht-pem fuel cell, fluid dynamics, species transport and current distribution. model equations with initial and final boundary conditions are solved using appropriate solvers. results and discussion wang et al. [11] has postulated that at low current densities where water accumulates in gdl (gas diffusion layer), the under-rib convection plays a more important role in water removal than the pressure drop. water removal facilitates oxygen transportation and oxygen removal. at high current densities, when large amount of water accumulates in the channels, the pressure drop across the channels dominates water removal to facilitate oxygen transportation. this analogy can be extended to high temperature fuel cells for the removal of water vapor. liu et al. [12] postulated that water is mostly accumulated along the rib edges and at the corners by the channel wall and gdl. the authors postulated that a smaller channel width and smaller rib width enables better hydration of the membrane and easier discharge of water, leading thus to better fuel cell performance. a larger channel dimension gives enhanced surface area for reaction. however, a larger channel/rib dimension while enhancing the electrical and heat conduction in the fuel cell also reduces the ohmic drop across the cell. thus, the effects of rib and channel dimensions must be considered as a combination of these factors. figure 3 shows the simulated polarization curves describing the fuel cell performance as a function of different rib widths (0.5, 1, 1.5, 2 mm). for these simulations, the channel width and channel depth are kept constant at 1 mm. the graph shows that the fuel cell performance is the highest at the rib width of 0.5 mm, and the lowest at the rib width of 2.0 mm. the difference in the fuel cell performance for different rib widths is higher within the high current density zone, indicating mass transfer limitations and major role of water discharge. figure 3. variation of cell voltage with current density for various rib widths. http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(1) (2021) 59-69 performance of high temperature fuel cells 64 the effect of changing rib width is not seen to play significant role at lower current densities. also, it seems that the channel width plays a certain role, even at lower current densities figure 4 shows the effect of pressure drop for various channel lengths for different rib width values (0.5, 1, 1.5, 2 mm). the pressure drop is found to increase from 47 kpa when the channel length is 19 mm to 53 kpa at the channel length of 25 mm when the rib width is maintained at 0.5 mm. when the rib width is increased to 2 mm, the pressure drop is seen to change from 46.5 to 52 kpa for channel lengths changing from 19 to 25 mm. goebel [10] postulated that for greater pressure drop across the channel length, the greater is water transport, leading to better removal of water in a pemfc. this in turn leads to better oxygen consumption leading to better fuel cell performance. the pressure drop across the channel lengths when the rib width is changed from 0.5 to 2.0 mm, enables better water transport and hence better oxygen consumption, thus allowing better pemfc performance. smaller ribs also reduce the ohmic drop across the cell, enabling higher current densities. figure 4. pressure drop as a function of three different channel lengths (19 mm, 22 mm and 25 mm) for various rib widths. figure 5 shows the simulated polarization curves for varying channel widths. the figure shows that increasing values of the channel width plays a certain role in the performance of the fuel cell, particularly at higher current densities. it seems, however, that the channel width plays some role even at lower current densities. figure 5. polarization curves showing the variation of cell voltage with cell current density for various channel widths. v. jha and b. krishnamurthy j. electrochem. sci. eng. 11(1) (2021) 59-69 http://dx.doi.org/10.5599/jese.907 65 figure 6 shows the pressure drop across the length of the channel as a function of changing channel width. the figure shows that the pressure drop changes from 63 kpa at the channel length of 19 mm to 70 kpa at the channel length of 25 mm for the channel width of 0.5 mm. at the channel width of 1.5 mm, the pressure drop is found to change from 35 kpa at the channel length of 19 mm to 42 kpa at the channel length of 25 mm. thus, there is a significant effect of pressure drop on the water transport in the cathode gas channels, leading to significant fuel cell performance improvement particularly at high current densities. however, a wider gas channel also allows enhanced surface reaction on the cathode side enabling more oxygen consumption. however, the pressure drop seems to be a dominant effect and hence we see better performance when the channel width is 0.5 mm. figure 6. pressure drop at three different channel length values (19, 22 and 25 mm) for different channel widths. figure 7 shows the simulated polarization curves for varying rib/channel ratios. the graph shows that if the total channel width and rib width is fixed, with increasing values of the rib width, the fuel cell performance decreases. the highest performance is seen at the lowest value of the rib width. this indicates the effect of the electrical resistance of the rib width and the consequent effect on the ohmic drop in the pem fuel cell. figure 7. polarization curves for different channel width to rib ratios (channel width+rib width=2). figures 8a and 8b show the effect of change in oxygen concentration across the channel length when the channel width is changed from 0.5 to 1.5 mm. the channel length is this case is fixed at 25 mm. the oxygen consumption is seen to be higher when the channel width is 0.5 mm than 1.5 mm, clearly showing the effect of pressure drop across the cathode channel length. http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(1) (2021) 59-69 performance of high temperature fuel cells 66 a b figure 8. oxygen concentration profile: (a) along the cathode gas channel when the channel width is 0.5 mm, (b) in the cathode gas channel when the channel width is 1.5 mm. figure 9 shows the performance of the high temperature pemfc as a function of various channel depths. the figure shows that the channel depth plays very little role if any, in the performance of the pemfc. figures 10a and 10b show the oxygen concentration across the flow channel length as a function of two different channel depth values (0.5 and 1.5 mm). the channel length in this case is fixed at 25 mm. the figures show that the oxygen consumption is not changed much by changing the channel depth. figure 11 shows the effect of cathode channel length on the pressure drop in the cathode channel for a given current density (for three different channel widths). the graph indicates that the maximum pressure drop is obtained when the channel length is 25 mm. validation of simulation results with experimental data [13] is shown in figure 12. simulation results are well compared with experimental data. the parameters used for fitting experimental data are shown in table 1. v. jha and b. krishnamurthy j. electrochem. sci. eng. 11(1) (2021) 59-69 http://dx.doi.org/10.5599/jese.907 67 figure 9. variation of cell voltage with current density for various cell channel depths. a b figure 10. concentration profile of oxygen: (a) in the cathode gas channel for 0.5 mm channel depth, (b) in the cathode gas channel for 1.5 mm channel depth. http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(1) (2021) 59-69 performance of high temperature fuel cells 68 figure 11. effect of channel length on current density of the cell for three different channel widths. figure 12. validation of modeling predictions with experimental data. table 1. list of parameters and nomenclature notation values description xo 0.3 inlet oxygen mass fraction (cathode) xh 1.00 inlet hydrogen mass fraction (anode) wr, wch, hc / mm 0.5, 1, 1.5 rib width, channel width, channel depth vcell / v 0.4 cell voltage uc / m s-1 0.5 cathode inlet flow velocity ua / m s-1 0.2 anode inlet flow velocity t / oc 130 cell temperature l, s / s m-1 9.825, 222 membrane, gdl conductivity pa, pc / atm 1 reference pressure at anode and cathode µc / pa s 2.46×10-5 cathode viscosity µa / pa s 1.19×10-5 anode viscosity l / mm 19, 22, 25 cell length  / m2 10-12 gdl permeability f / c mol-1 96487 faraday’s constant r / j mol-1 k-1 8.314 universal gas constant hm, hcl / mm 0.1, 0.05 membrane, electrode thickness l 0.3 electrolyte phase volume fraction gdl 0.4 gdl porosity do2n2 / m 2 s-1 2.210-5(t/293.2)1.75 o2-n2 binary diffusion coefficient do2h2o / m 2 s-1 2.210-5(t/293.2)1.75 o2-h2o binary diffusion coefficient dn2h2o / m 2 s-1 2.210-5(t/293.2)1.75 n2-h2o binary diffusion coefficient dh2h2o / m 2 s-1 2.210-5(t/293.2)1.75 h2-h2o binary diffusion coefficient co2 ref / mol m-3 40.88 oxygen reference concentration ch2 ref / mol m-3 40.88 hydrogen reference concentration hgdl, hch / mm 0.38,1 gdl depth, channel depth v. jha and b. krishnamurthy j. electrochem. sci. eng. 11(1) (2021) 59-69 http://dx.doi.org/10.5599/jese.907 69 conclusions a three-dimensional numerical model is developed to study the effect of channel and rib dimensions on ht-pem fuel cell performance in a parallel flow configuration run at 130 ℃. simulation results indicate that changing the rib width (channel width and channel depth are kept constant) has the maximum effect on the fuel cell performance. changing the channel width (rib width and channel depth are kept constant) has a significant effect while changing the channel depth has negligible effect on the fuel cell performance. the effect of changing rib width, channel width and channel depth on the pressure drop across the channel is also studied. increasing pressure drop across the channel length facilitates water vapor transport and hence increases oxygen consumption. model results are found to compare well with experimental data. acknowledgement: the authors would like to acknowledge bits pilani, hyderabad and council for scientific and industrial research, csir grant no: (no:22/0784/19/emr ii) to support us in publishing this article. references [1] y. g. yoon, w. y. lee, g. g. park, t. h. park, t. h. yang, s. h. kim, electrochimica acta 50(2-3) (2004) 709-712. [2] s. shimpalee, j. w. van zee, international journal of hydrogen energy 32(7) (2007) 842-856. [3] s. shimpalee, s. greenway, j. w. van zee, journal of power sources 160(1) (2006) 398-406. [4] s. s. hsieh, k. m. chu, journal of power sources 173(1) (2007) 222-232. [5] a. p. manso, f. f. marzo, j. barrancco, x. garikano, m. garmendia mujika, international journal of hydrogen energy 37(20) (2012) 15256-15287. [6] x. d. wang, y. y. duan, w. m. yan, x. f. peng, electrochimica acta 53(16) (2008) 5335-5343. [7] x. d. wang, w. m. yan, y. y. duan, f. b. weng, g. b. jung, c. y. lee, energy conversion and management 51(5) (2010) 959-968. [8] m. z. chaudhary, o. genc, s. toros, international journal of hydrogen energy 43(23) (2018) 1079810809. [9] l. j. yu, g. p ren, m. j. qin, x. m. jiang, renewable energy 34(3) (2009) 530-543. [10] s. g. goebel, journal of power sources 196(18) (2011) 7550-7554. [11] c. wang, q. zhang, s. shen, x. yan, f. zhu, x. cheng, j. zhang, scientific reports 7 (2017) 43447. https://doi.org/10.1038/srep43447. [12] h. liu, p. li, k. wang, international journal of hydrogen energy 38(23) (2013) 9835-9846. [13] e. u. ubong, x. wang, z. shi, journal of the electrochemical society 156(10) (2009) b1276-b1282. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.907 https://doi.org/10.1038/srep43447 https://creativecommons.org/licenses/by/4.0/) {bsa-binding studies of 2and 4-ferrocenylbenzonitrile: voltammetric, spectroscopic and molecular docking investigations} http://dx.doi.org/10.5599/jese.861 335 j. electrochem. sci. eng. 10(4) (2020) 335-346; http://dx.doi.org/10.5599/jese.861 open access: issn 1847-9286 www.jese-online.org original scientific paper bsa-binding studies of 2and 4-ferrocenylbenzonitrile: voltammetric, spectroscopic and molecular docking investigations hacen benamara, touhami lanez and elhafnaoui lanez university of el oued, chemistry department, vtrs laboratory, b.p.789, 39000, el oued, algeria corresponding author: lanezt@gmail.com; tel.: +213-661655550 received: may 29, 2020; revised: june 11, 2020; accepted: june 15, 2020 abstract the binding affinity of 2-ferrocenylbenzonitrile (2fbn) and 4-ferrocenylbenzonitrile (4fbn) with bovine serum albumin (bsa) has been investigated by cyclic voltammetry, absorption spectroscopy and molecular modelling techniques. the results indicated that both of the two derivatives could bind to bsa and cause conformational changes with the order 2fbn > 4fbn. the voltammetric behavior of 2fbn and 4fbn before and after the addition of bsa suggests that the electrochemical reaction is kinetically controlled by the diffusion step and demonstrated that diffusion coefficients of 2fbn-bsa and 4-fbn-bsa complexes are lower than that of free compounds. molecular docking suggested that the binding mode of the two compounds to bsa is of hydrophobic and hydrogen bond interactions, moreover the ligand 2fbn additionally show a π-cation interaction. keywords cyclic voltammetry; ferrocene derivatives; binding constant; interactions; modelling; in silico; in vitro introduction nitriles and cyanides are compounds containing a -cn functional group in their molecular structure. in nitriles the -cn functional group is attached to an organic structure [1], but in cyanides, it is attached to an inorganic compound [2]. cyanides are toxic because they denote the highly toxic inorganic salts of hydrogen cyanide, while nitriles are not toxic because they do not release cyanide ions [3,4]. the non-toxic properties of nitriles encouraged researchers to study their pharmacochemistry as potential drugs. the prevalence of nitrile-containing drugs shows the biocompatibility of the nitrile functionality [5,6]. recently many pharmaceuticals drugs containing nitriles are either prescribed for many different types of diseases or are in clinical trial [6]. nitrile groups are usually known as hydrogen bond acceptors [7-9], many studies show the formation of hydrogen bonding between the nitrogen atom of the nitrile group and amino acids of http://dx.doi.org/10.5599/jese.861 http://dx.doi.org/10.5599/jese.861 http://www.jese-online.org/ mailto:lanezt@gmail.com j. electrochem. sci. eng. 10(4) (2020) 335-346 bsa-binding studies of 2and 4-ferrocenylbenzonitrile 336 protein backbone [10-13]. the bonds are often established between the nitrile and the expected hydrogen bond of the donor serine or arginine amino acids. serum albumin is the most important protein in blood plasma and plays a vital role in the transport and distribution of metals, fatty acids, hormones, and renders toxins harmless by transporting them to disposal sites. in addition, bsa binds to variety drugs at multiple sites in the body vascular system [14-16]. bsa interactions with small molecules have become increasingly important in pharmacochemistry and are commonly used as key steps in the construction of medicinal compounds [17-21]. the research in this field provides strong support for bsa-binding studies and a deeper understanding of the way medicaments target and bind receptors [22,23]. bsa interactions with small molecules also help in understanding the toxicity, pharmacokinetics, biochemistry, pharmacodynamics, and distribution of molecules in the organism. thus, the investigation of bsa interactions with small molecules has been an important analysis in medicinal chemistry and clinical medicine [24]. the pharmacochemistry of ferrocene derivatives has attracted the attention of many scientists, and their study has been encouraged by potential biological applications [25-27]. many ferrocene derivatives that have been studied in the last decade show important biological activities, such as cytotoxic [28-30], antimicrobial [31,32] and antitumor [33-38] activities. the incorporation of the ferrocenyl moiety into the structure of biologically active molecules may lead to the increase of their biological activities, based on the fact that nitriles-containing ferrocenes are expected to be more pharmacologically active than free nitriles. in this context the present study surveys the interaction of two nitrile-containing ferrocene with bsa by focusing on the roles of the cn functional groups. the study was carried out in 0.1 m 90 % dmf/buffer phosphate solution at ph 7.4 using voltammetric, spectroscopic and molecular docking techniques. experimental synthesis 2-ferrocenylbenzonitrile (2fbn) and 4-ferrocenylbenzonitrile (4fbn) addressed in this work are shown in figure 1a and 1b, and were synthesised by the coupling reaction between ferrocene and the diazonium salts of the corresponding cyanoaniline, as reported previously by our group [39]. the crystal structure of bsa was taken from protein databank (https://www.rcsb.org/, pdb id: 4f5s) (figure 1c). a b c figure 1. chemical structures of (a) 2-ferrocenylbenzonitrile, (b) 4-ferrocenylbenzonitrile and (c) structure of bsa (id: 4f5s) chemicals all reagents and solvents were of analytical grade and obtained from commercial sources and used without further purification. bsa was obtained from merck and used as received, while its https://www.rcsb.org/ h. benamara et al. j. electrochem. sci. eng. 10(4) (2020) 335-346 http://dx.doi.org/10.5599/jese.861 337 concentration was determined by the extinction coefficient of 44300 m-1 cm-1 at 280 nm [40]. all stock solutions were used within 5 days after preparation and stored at 4 °c until use. the phosphate buffer solution was prepared using sodium dihydrogen phosphate and disodium hydrogen phosphate (sigma aldrich) and double-distilled water. the physiological ph (ph 7.4) was maintained by this phosphate buffer. n,n-dimethylformamide (dmf) (hplc-grade; sigma-aldrich) was used as the solvent in voltammetric and spectroscopic assays. tetrabutylammonium tetra-fluoroborate (bu4nbf4) (electrochemical grade 99 %; sigma-aldrich) was used as the supporting electrolyte. nitrogen gas was provided from a cylinder (research grade (99.99 %); linde gaz algérie). materials and measurements voltammetric assays were performed using a pgz301 voltammeter running on voltamaster 4 v 7.08 software (radiometer analytical sas, france). the concentration of the supporting electrolyte was kept at 0.1 m all time. the air was removed from the solution by bubbling nitrogen gas through it. experiments were run in a three-electrode electrochemical cell containing a glassy carbon (gc) working electrode with a geometric area of 0.013 cm2, a platinum wire as counter (auxiliary) electrode and hg/hg2cl2 paste covered wire as reference electrode. absorption spectra measurements were conducted on a uv-vis spectrometer, (shimadzu 1800, japan), using the cell of length 1 cm. chemical structures of 2fbn and 4fbn were optimized by gaussian 09 program package [41], using density functional theory (dft) and the b3lyp level of theory [42,43] with 6-311++g(d,p) basis set. the molecular docking studies were done using autodock 4.2 docking software [44,45], all docking studies were executed on a pentium 2.20 ghz and ram 4.00 go microcomputer mb memory with windows 10 operating system. results and discussion cyclic voltammetric study bsa-2fbn and bsa-4fbn complexes in 0.1 m 90 % dmf/buffer phosphate solution at ph 7.4 were used. various concentrations of bsa were added into 12 ml solution of 100 µm of the ligand solutions, and the voltammograms were recorded in the potential range of 0.1 to 0.8 v vs. hg/hg2cl2 at 28 ± 1 °c. many studies on the interaction of bsa with small molecules in this potential range have been carried out using cyclic voltammetry techniques, and all these studies have shown that bsa does not show any adsorption on the bare electrode surface at this potential range [46-50]. adsorption of bsa can only appear at negative potential [51]. the cyclic voltammograms (figure 2) of 2fbn and 4fbn at different concentrations of bsa show respectively oxidation and reduction maximums in a reversible electrochemical process. addition of an increasing amount of bsa solution results in a decrease in peak current height with a positive shift in peak potential position. this decrease in anodic peak current height is exploited for the calculation of the binding parameters. the binding constant, kb, was calculated from the observed cyclic voltammetry data, using the following equation [52]: = − b bsa 0 1 log log +log i k c i i (1) http://dx.doi.org/10.5599/jese.861 j. electrochem. sci. eng. 10(4) (2020) 335-346 bsa-binding studies of 2and 4-ferrocenylbenzonitrile 338 where cbsa is bsa concentration, kb represents the binding constant, while i0 and i indicate the anodic peak current density of the free and bsa-bound compounds, respectively. 0.0 0.2 0.4 0.6 0.8 1.0 -6 -4 -2 0 2 4 6 8 10 12 c u rr e n t d e n s it y ,  a c m -2 potential, v 0.07 0.15 0.37 0.55 0.73 0.91 1.08 bsa-2fbn cbsa, m 0.0 0.2 0.4 0.6 0.8 1.0 -6 -4 -2 0 2 4 6 8 c u rr e n t d e n s it y ,  a c m -2 potential,v 0.07 0.15 0.22 0.37 0.55 0.91 1.08 bsa-4fbn cbsa, m figure 2. cyclic voltammograms of bsa–2fbn and bsa–4fbn complexes at different concentration of bsa. 2fbn and 4fbn concentrations were kept at 100 µm obviously, kb can be determined from the intercept of the plot of log (1/cbsa) vs. log (i/(i0-i)). these plots are for 2fbn and 4fbn represented in figure 3, from which the values of binding constants were determined as 7.05×105 m-1 for 2fbn and 3.44×105 m-1 for 4fbn. the binding free energy changes calculated using the equation g = -nrt lnkb are found equal to -33.94 and -32.14 kj mol-1, respectively. the order of magnitude and the sign of the obtained binding free energy indicate respectively the electrostatic mode and the spontaneity of interactions between the compounds and bsa. 0.0 0.2 0.4 0.6 0.8 1.0 1.2 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 lo g ( c b s a )1 /  m -1 log (i/(i 0 -i)) y = 1.1789x + 5.8484 r 2 = 0.998 a 0.2 0.4 0.6 0.8 1.0 1.2 1.4 5.8 6.0 6.2 6.4 6.6 6.8 7.0 7.2 lo g ( c b s a )1 /  m -1 log (i/(i0-i)) y = 1.40554x + 5.53709 r 2 = 0.998 b figure 3. plots of log (cbsa)-1 vs. log (i/(i0-i)) used to calculate bsa binding constants: (a) 2fbn, (b) 4fbn ratio of binding constants the ratio of the binding constants of the reaction of the reduced form fbn (fbn represents 2fbn or 4fbn) with bsa to that of the oxidized form [fbn]+, could be calculated from the voltammograms of figure 4, which represent the cyclic voltammograms of 100 µm solution of 2fbn and 4fbn in the absence and presence of 0.37 μm of bsa. the shift in the anodic and cathodic peak potential values caused by the addition of bsa can be used to calculate the ratio of binding constants [53]. h. benamara et al. j. electrochem. sci. eng. 10(4) (2020) 335-346 http://dx.doi.org/10.5599/jese.861 339 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -6 -4 -2 0 2 4 6 8 10 12 2fbn bsa-2fbn c u rr e n t d e n s it y ,  a c m -2 potential, v -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -6 -4 -2 0 2 4 6 8 4fbn bsa-4fbn c u rr e n t d e n s it y ,  a c m -2 potential (v) figure 4. cyclic voltammograms of the free compounds (100 µm) and their bsa complexes (0.37 μm), scan rate 100 mv s-1 in such case when both the anodic and cathodic peak potential values are shifted upon the addition of bsa, the following equilibriums can be applied [54]: figure 5. redox process of studied compounds with bsa, fbn represents 2fbn or 4fbn. the application of the nernst relation to the equilibriums of figure 5 produces the following equation: = = −− = 0 0 0 red b 0 0 f ox ( -bsa) ( )δ fbn fbn 0 06loge e k e e e . k (2) in equation (2), ef0 and eb0 are the formal potentials of the fbn+/fbn couple of free and bsabound compounds, respectively. the formal potential shift δe0 calculated on the basis of the voltammograms of figure 4, are summarized in table 1. the ratios of the binding constants were calculated from equation (2) by replacing δe0 taken from table 1. table 1. electrochemical data of free and bsa-bound 2fbn and 4fbn used to calculate the ratio of the binding constants. sample code epa / v epc / v e0 / v e0/ mv kred / kox 2fbn 0.502 0.369 0.4355 3.5 1.14 2fbn-bsa 0.527 0.351 0.439 4fbn 0.538 0.374 0.456 4.0 1.16 4fbn-bsa 0.572 0.348 0.46 the obtained ratios of the binding constants indicate that the reduced form of both 2fbn and 4fbn bind slightly stronger to bsa than their oxidized forms. diffusion coefficients the diffusion coefficients of the free and bsa-bound 2fbn and 4fbn compounds were obtained from their electrochemical behavior represented in figures 6 and 7. these cyclic voltammograms were obtained by varying the potential scan rates of 100 µm of the free compounds in the absence http://dx.doi.org/10.5599/jese.861 j. electrochem. sci. eng. 10(4) (2020) 335-346 bsa-binding studies of 2and 4-ferrocenylbenzonitrile 340 and presence of 0.91 µm of bsa. all the voltammograms present well-defined stable redox peaks attributed to the redox process of 2fbn and 4fbn. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -150 -100 -50 0 50 100 150 200 250 c u rr e n t d e n s it y , µ a c m -2 potential, v 100 200 300 400 500 v(mv.s -1 ) 2fbn 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -100 -50 0 50 100 150 c u rr e n t d e n s it y , µ a c m -2 potential, v 100 200 300 400 500 v(mv.s -1 ) 4fbn figure 6. cyclic voltammetric behavior of 100 µm of 2fbn and 4fbn in 0.1 m 90 % dmf/buffer phosphate solution at different scan rates. 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -150 -100 -50 0 50 100 150 200 c u rr e n t d e n s it y , µ a c m -2 potential, v 100 200 300 400 500 v(mv.s -1 ) bsa-2fbn 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 -100 -50 0 50 100 150 c u rr e n t d e n s it y , µ a c m -2 potential, v 100 200 300 400 500 v (mv.s -1 ) bsa-4fbn figure 7. cyclic voltammetric behavior of 0.91 µm of bsa-2fbn and bsa-4fbn in 0.1 m 90 % dmf/buffer phosphate solution at different scan rates diffusion coefficients of fbn-bsa in the solution with an excess of bsa were calculated from the voltammograms of figures 6 and 7 using the following randles–ševčik equation [55]: =  3 1 1 5 2 2 2 pa 2 69 10i . n scd v (3) in equation (3), ipa represents the anodic peak current (a), n is the number of electrons participated in the oxidation process, s is the surface of the working electrode (cm2), c is the concentration of the electroactive compounds (mol cm−3), d is the diffusion coefficient (cm2 s−1), and v is the scan rate (v s−1). the plots of ipa vs. v1/2 in figure 8 suggest that the oxidation reaction is diffusion controlled. the diffusion coefficients of the free and bsa-bound compounds were calculated from the slopes of linear regressions of the plots of ipa vs. v1/2. the lower diffusion coefficients of the bound compounds as compared to the free once, further confirm the interaction between the studied compounds and bsa (table 2). table 2. diffusion constant values of the free and bsa bound form of 2fbn and 4fbn. sample code equation r² d × 106/ cm2 s-1 2fbn y = 6.94x + 44.25 0.999 3.94 2fbn-bsa y = 5.78x + 42.97 0.999 2.73 4fbn y = 4.89x + 24.63 0.999 1.96 4fbn-bsa y = 4.22x + 26.40 0.999 1.46 h. benamara et al. j. electrochem. sci. eng. 10(4) (2020) 335-346 http://dx.doi.org/10.5599/jese.861 341 10 12 14 16 18 20 22 24 100 120 140 160 180 200 2fbn bsa-2fbn a n o d ic c u rr e n t d e s it y ,  a c m -2 v 1/2 / mv s -1 10 12 14 16 18 20 22 24 70 80 90 100 110 120 130 140 4fbn bsa-4fbn a n o d ic c u rr e n t d e n s it y ,   c m -2 v 1/2 / mv s -1 figure 8. ipa vs. v1/2 plots of 100 μm 2fbn and 4fbn in the absence and in presence of 0.91 μm bsa based on voltammograms in figures 6 and 7. absorption spectroscopic studies the interactions of 2fbn and 4fbn with bsa were also studied by absorption spectroscopic titration. the purpose of this study was to validate the results obtained from cyclic voltammetry assays. the experiments were carried out with 0.1 m 90 % dmf/buffer phosphate solution of ph 7.4. incremental portions of bsa solution from 0.34 to 0.70 μm for 2fbn and from 0.15 to 2.67 μm for 4fbn were added to the same solution containing 0.5 mm of 2fbn and 2 mm of 4fbn. the obtained mixture was scanned in the range of 300–600 nm. bsa does not show any absorption at this wavelength, while the strong peak which appeared at 434.5 nm (due to π→π* transition in the conjugated ring of ferrocene moiety) lowered in intensity upon continuous addition of bsa to 2fbn and 4fbn (figure 9). 300 350 400 450 500 550 600 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 cbsa, mbsa-2fbn a b s o rb a n c e , a .u wavelenght, nm 00 0.34 0.37 0.4 0.43 0.50 0.57 0.63 0.70 390 420 450 480 510 540 570 600 0.8 1.0 1.2 1.4 1.6 1.8 2.0 cbsa, m bsa-4fbn a b s o rb a n c e , a .u wavelenght, nm 00 0.15 0.29 0.44 0.73 1.00 1.28 1.42 2.06 2.67 figure 9. absorbance spectra of 2fbn-bsa and 4fbn-bsa complexes. 2fbn and 4fbn concentrations were kept respectively at 0.5 and 2 mm the binding constant kb was evaluated from the absorption data according to benesi-hildebrand equation [56]: 0 f f 0 b f b f b bsa 1      = + − − − a a a k c (4) in equation (4), a0 and a are absorbencies of the ligands and their complexes with bsa, respectively, while ɛf and ɛb are their extinction coefficients. the plot of a0/(a0-a) vs. 1/cbsa gave a http://dx.doi.org/10.5599/jese.861 j. electrochem. sci. eng. 10(4) (2020) 335-346 bsa-binding studies of 2and 4-ferrocenylbenzonitrile 342 slope of ɛf/(ɛb ɛf)kb, and intercept equal to ɛf/(ɛb ɛf), where kb is the ratio of the slope to the intercept (figure 10). the value of kb has been determined to be 7.18×105 for 2fbn-bsa and 2.79×105 m-1 for 4fbn-bsa. the corresponding free binding energies calculated using the equation g = -nrt lnkb were equal to -33.77 and -31.40 kj mol-1, respectively. these values are in good agreement with those obtained from cyclic voltammetry experiments. 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 a 0 /( a -a 0 ) cbsa -1 , m -1 y = -0.601x 0.4311 r 2 = 0.994 bsa-2fbn 0 1 2 3 4 5 6 7 -30 -25 -20 -15 -10 -5 0 bsa-4fbn a 0 /( a -a 0 ) cbsa -1 , m -1 y = -3.83771x -1.07128 r 2 = 0.994 figure 10. plots of a0/(a0-a) vs. cbsa-1 used to calculate the binding constants of bsa-2fbn and bsa-4fbn docking setup geometry optimization density functional theory (dft) was used for the optimisation without imposing any symmetry constraints and calculations were realized with the gaussian 09 package. the exchange functional of becke, and the correlation functional of lee, yang and parr (b3lyp) were employed with 6311++g(d,p) basis set. the optimized structures of the compounds are depicted in figure 11. 2fbn 4fbn figure 11. the optimized structures of 2fbn and 4fbn (ortep view 03, v1.08); color codes are carbon (grey), hydrogen (white), nitrogen (blue), iron (green). molecular docking studies in order to confirm and interpret the results of cyclic voltammetric and spectrophotometric measurements and recognize the way in which 2fbn and 4fbn bind to bsa, semi flexible docking was carried out using autodock 4.2 along with the autodock vina software. the crystal structure of bsa was taken from the protein databank (https://www.rcsb.org/, pdb id: 4f5s). the pdb file was imported into autodock tools, all hydrogen atoms and gassier charges were added. during all docking process, bsa kept rigid while all the bonds of the ligands were set free. the grid map with 0.375 å spacing and 126×70×100 points were generated. the docking experiment comprised of 100 docking https://www.rcsb.org/ h. benamara et al. j. electrochem. sci. eng. 10(4) (2020) 335-346 http://dx.doi.org/10.5599/jese.861 343 runs with 150 individuals and 2.500.000 energy evaluations. lamarckian genetic algorithm was used in the docking, and other parameters were set as default. the stable conformation which corresponds to the lowest binding energy was used for docking analysis. the visualization of the interaction was generated with plip web service (protein ligand interaction profiler). results from molecular docking suggest that hydrogen bonding, hydrophobic forces and π-cation interaction are involved in the binding process. figure 12 illustrates the interaction of 2fbn and 4fbn with the nearby residues in the active site of bsa. the interacting residues, distances, and type of interactions are summarised in tables 3 and 4. table 3. hydrophobic interactions between the ligands 2fbn and 4fbn with bsa. interaction type residue amino acid distance, ǻ bsa-bsa 80a leu 2.85 81a arg 3.45 88a ala 3.72 bsa-4fbn 115b leu 3.99 115b leu 3.57 122b leu 3.14 136b lys 3.94 137b tyr 3.15 140b glu 3.68 141b ile 3.58 160b tyr 3.96 table 4. hydrogen bonds between the ligands 2fbn and 4fbn and bsa. interaction type residue amino acid distance h-a, ǻ distance d-a, ǻ 2fbn-bsa 81a arg 3.00 3.45 82a glu 2.04 3.05 82a glu 3.22 3.71 4fbn-bsa 137b tyr 2.38 3.32 bsa-2fbn bsa-4fbn figure 12. best docking poses for bsa-2fbn and bsa-4fbn generated with plip web service illustrating the hydrophobic and h-bons interactions. elements colors: hydrogen, oxygen, nitrogen, and iron are represented in white, red, blue and brown, respectively color code: hydrogen bonds: bleu lines, hydrophobic interactions: gray lines, π-cation interactions: beige lines. http://dx.doi.org/10.5599/jese.861 j. electrochem. sci. eng. 10(4) (2020) 335-346 bsa-binding studies of 2and 4-ferrocenylbenzonitrile 344 the compound 2fbn formed three hydrogen bonds between amino acid residues arg-81 and glu-82 as donors and the polar groups of the ligand – nitrile group. 4fbn formed only one hydrogen bond between the residue tyr-137 as acceptor and the nitrogen of the 4fbn as a donor, table 4. the distances in table 4 are between hydrogen and the receptor atoms (h-a), and between donor and receptor atoms (d-a). furthermore, for the complex 2fbn-bsa, molecular docking results also suggested a π-cation interaction between the positively charged amino acid residue arg-81 and the benzene ring within a distance of 5.01 å. the ligand 4fbn interacted through hydrophobic interactions with the residues leu-115, leu122, lys-136, tyr-137, glu-140, ile-141, and tyr-160, the binding free energy of the docked structure of 2fbn and 4fbn ligands with bsa was found to be -32.89 and -31.26 kj mol-1, respectively. the binding constant kb calculated using the equation g = -nrt lnkb was found to be 5.77×105 and 2.99×105 mol-1, respectively. these results are supported by the absorption spectroscopy and cyclic voltammetry experiments. overall, molecular docking results are in good agreement with the data obtained from experimental assays. conclusions in the present work, we applied experimental and theoretical methods for the determination of binding proprieties of two nitrile-containing ferrocenes with bsa. binding free energies for the interaction of 2fbn and 4fbn with bsa, obtained from voltammetric experiments, were respectively -33.95 and -32.14 kj mol-1, and these values are in good agreement with those obtained from adsorption spectroscopic assays (-33.77 and -31.40 kj mol-1). the low diffusion coefficient values of the bsa-bound 2fbn and 4fbn as compared to the corresponding free 2fbn and 4fbn, further confirm the formation of the complexes 2fbn-bsa and 4fbn-bsa which diffuses more slowly compared to the free compounds due to their higher molecular weight. molecular docking study further confirms the obtained experimental results and allows the visualisation of interactions and determination of bonds length formed between the ligands and the amino acid residues of bsa. acknowledgements: the authors are grateful to the algerian ministry of higher education and research for financial support (project code: b00l01un390120150001). references [1] compendium of polymer terminology and nomenclature, iupac recommendations 2008, r. g. jones, j. kahovec, r. stepto, e. s. wilks, m. hess, t. kitayama, w. val matanomski (eds.). with advice from a. jenkins and p. kratochvil, 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[56] m. nie, y. wang, h. l. li: polish journal of chemistry 71(6) (1997) 816-822. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://creativecommons.org/licenses/by/4.0/) {improvement of the corrosion resistance of electrodeposited zn-fe by sol-gel conversion films:} http://dx.doi.org/10.5599/jese.1282 667 j. electrochem. sci. eng. 12(4) (2022) 667-683; http://dx.doi.org/10.5599/jese.1282 open access : : issn 1847-9286 www.jese-online.org original scientific paper improvement of the corrosion resistance of electrodeposited zn-fe by sol-gel conversion films céline arrighi1,2, yoann paint3, catherine savall2, juan creus2 and marjorie olivier1,3 1university of mons-umons, faculty of engineering, materials science department, place du parc 20, 7000 mons, belgium 2laboratoire des sciences de l’ingénieur pour l’environnement (lasie) umr cnrs 7356 la rochelle university, av. michel crépeau , 17042 la rochelle, france 3materia nova asbl, avenue copernic 1, 7000 mons, belgium corresponding author: marjorie.olivier@umons.ac.be; tel.: +32 65 37 44 31 received: february 4, 2022; accepted: march 25, 2022; published: april 27, 2022 abstract an aqueous hybrid inorganic/organic sol-gel solution composed of tetraethylorthosilicate (teos), methyltriethoxysilane (mtes) and (3-glycidyloxypropyl)trimethoxysilane (gptms) was applied on znfe (14 wt.% fe) electrodeposited on steel as a sacrificial layer. two precursor contents were studied: 10 (sg10) and 30 % (sg30). first, the morphology and thickness of the films were assessed by scanning electron microscopy (sem) observations. they revealed the presence of micro-cracks in the films without alkaline surface preparation due to the pyramidal shape of the znfe deposit. then, the corrosion resistance of the systems was determined by electrochemical impedance spectroscopy (eis) and neutral salt spray (nss) test. all results indicated an improvement in the corrosion resistance thanks to the presence of the sg films. however, the protection provided by the sg10 film did not permit to durably protect the znfe deposit. the combination of surface preparation and a sg30 film provided promising protection to the znfe deposit with an increase of the low-frequency modulus and a delay in corrosion product appearance during the nss test. keywords electrochemical impedance spectroscopy; zinc alloys; aqueous based solution introduction zn-ni (12-14 wt.% ni) electrodeposits are commonly used as sacrificial coatings to protect steel from corrosion [1,2]. however, researchers tend to focus on eco-friendlier alternatives because of the toxicity of nickel salts [3]. zn-fe electrodeposits could be potential candidates for the replacement of zn-ni ones [4–10]. nevertheless, it is essential to improve the corrosion resistance of zn-fe deposits in order to achieve the same performance as zn-ni coatings. sol-gel (sg) films, and particularly hybrid inorganic/organic ones, have been used for years to limit the corrosion of metallic systems. this solution was extensively studied for zn coatings, but less information can be found regarding zn alloys and particularly zn-fe ones. http://dx.doi.org/10.5599/jese.1282 http://dx.doi.org/10.5599/jese.1282 http://www.jese-online.org/ mailto:marjorie.olivier@umons.ac.be j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 668 su et al. [11] compared the effect of an inorganic (teos) and a hybrid inorganic/organic (teos + gptms) sg films deposited on galvanized steel. they studied the influence of sg films on the corrosion resistance and the paint adhesion of the systems. sem observations indicated that both sg films follow the morphology of the substrate (which was skin-passed). however, micro-cracks were observed for the inorganic sg film. the absence of cracks for the hybrid sg film was attributed to the presence of gptms. the neutral salt spray (nss) test revealed a delay in the appearance of corrosion products thanks to the presence of sg films with better performance for the hybrid sg film. corrosion products were first observed on the micro-cracks, explaining the better results obtained for the hybrid sg film. electrochemical tests were also performed, indicating a decrease in the corrosion current density thanks to sg films (from 6.00 µa cm-² for galvanized steel to 3.8 µa cm-² for inorganic film and 0.12 µa cm-² for the hybrid film). electrochemical impedance spectroscopy (eis) measurements were performed in 0.1 m nacl, and an electrical equivalent circuit (eec) was used to represent the behaviour of the system. this eec was composed of the electrolyte resistance; the sg film was represented by the pore resistance (rpore) and the coating cpe (qcoating) and the charge transfer at the interface was represented by the charge transfer resistance (rct) and the double layer cpe (qdl). all results indicated an improvement of the corrosion resistance by using the hybrid sg film with an increase in the charge transfer and the pore resistances. other authors obtained similar conclusions with inorganic or hybrid sg films, combined or not with corrosion inhibitors, on zinc [12–14], electrodeposited zinc [15], or galvanized steel [11,16–25]. regarding zinc alloys, dos santos [26] studied the influence of bis[3-(triethoxysilyl)propyl] tetrasulfide (btespts) or 1,2-bis(triethoxysilyl) ethane (btse) + btespts sg films combined with ce or la conversion layers, on the corrosion resistance of electrodeposited zn-fe (2 wt.%). eis measurements in 0.05 m nacl indicated an increase of the low-frequency modulus for the systems combining sg and corrosion inhibitors and systems with only conversion layers. sg can be prepared using alcohols since most precursors are non-miscible with water. however, it is possible to obtain aqueous sg. fedel et al. [16,17,20,27,28] published several studies with this kind of sg applied on galvanized steel in order to optimize the system (curing parameters, the addition of clay, etc). the matrix consisted of a mixture of teos, mtes and gptms. they used acidic catalysis with hydrochloric acid and 10 % in precursors. c. motte [29] studied the impact of an aqueous sg film composed of 10 % teos, mtes and gptms on the corrosion resistance of galvanized steel. she also incorporated clay (modified or not by cerium) in her work. a. nicolay [30] studied the influence of the precursors ratio (10, 20 and 30 %) and applied sg films on stainless steel. in this study, an aqueous sg was chosen to avoid using volatile solvents to obtain an ecofriendlier system. the aim of this study is to determine the corrosion resistance improvement brought by sg films deposited on znfe coated steel. experimental the substrate consisted of znfe (14 wt.% fe) coated st37 steel. a 15 µm znfe deposit was obtained by electrodeposition from an alkaline additive-free bath composed of 6.6 m of potassium hydroxide (alfa aesar), 0.3 m of zinc oxide (vwr chemicals) and 0.075 m of ferrous gluconate (alfa aesar) [4,31] maintained at 25 °c. the pulsed current was used with the following parameters: ton 4 ms, toff 16 ms and jp 125 ma cm-2 [31]. the roughness ra of the bare electrodeposited znfe coating was measured with a nanojura optical profiler. a ra value of 1.3 ± 0.2 µm was obtained. sol-gel solutions were obtained by mixing demineralized water and three precursors: teos (vwr chemicals), mtes (alfa aesar) and gptms (aldrich). two solutions containing 10 wt.% (sg10) or c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 669 30 % in precursors (sg30) were prepared. the composition of each matrix is given in table 1. pluronic123 (p123, aldrich) was added (0.03 wt.%) to improve the wettability of the solution on znfe deposits. the ph of the solution was adjusted at 3.5-4 with acetic acid (vwr chemicals). table 1. composition and dry extract of the two sg content, wt.% sg10 sg30 demineralized water 90 70 teos 3.3 10 mtes 3.3 10 gptms 3.3 10 dry extract 4.7 ± 0.1 14.0 ± 0.6 the viscosity of sg solutions was determined with anton paar mcr 302 equipment. shear stress (τ) in the function of the shear rate (�̇�) graphs were recorded at 25 °c with a shear rate varying from 0 to 1000 s-1 for a scan duration of 10 minutes. all sg solutions showed a newtonian behaviour, and their viscosity was indicated by the slope of the graph τ = f(�̇�). after 24 h of stirring, the viscosities of sg10 and sg30 solutions were respectively 1.39 mpa s and 3.00 ± 0.03 mpa s. surface preparation was used in some cases, consisting of an immersion of the samples for 30 s in a 10 g l-1 alkaline commercial gardoclean® solution maintained at 50 °c. samples were then rinsed with demineralized water and dried. sg films were deposited on znfe coated steel by dipcoating with a ksv nima 2 equipment. the withdrawal rate was 500 mm min-1. systems were then cured at 180 °c for 1 h in a thermoscientific oven. these curing parameters were optimized by a. nicolay in previous work [30]. the contact angle between a demineralized water droplet and the different systems (bare znfe and znfe + sg) was measured to determine the effect of the sol-gel film on the hydrophobicity of the sacrificial layer. the test was performed with a kruss dsa 10-mk2 equipment with a droplet volume of 1 µl. the corrosion resistance of the systems was assessed by electrochemical measurements (solartron analytical modulab) and by neutral salt spray (nss, q-fog ssp600) test. regarding electrochemical tests, a three-electrode system was used. the reference electrode was a saturated calomel electrode (sce), the counter-electrode was a platinum grid, and the working electrode was the substrate (1 cm² exposed). a flat cell containing 300 ml of 0.1 m nacl (vwr chemicals) was used. the ph of the electrolyte was adjusted at ph 7 with a naoh solution in order to perform experiments with the same initial ph. the open circuit potential (ocp) of samples was measured and eis was performed every 6 h until 3 days and then every 12 h until 7 days of immersion. eis was done with an rms amplitude voltage of 10 mv in the frequency range 105 – 10-2 hz. bare znfe and znfe + sg10 (with and without gardoclean®) were immersed for 3 days, while znfe + sg30 (with and without gardoclean®) were immersed for 7 days. experiments were reproduced to obtain two consistent results. bare znfe and znfe + sg were exposed to the nss test following iso 9227 standard. 3m tape was used to mask one side of the samples and their edges. a 7 cm² area was exposed. three samples of each condition were placed in a q-fog ssp600 climatic chamber maintained at 35 °c and exposed to a 50 g l-1 nacl solution. pictures of the samples were taken after 1, 2, 3, 7 and 14 days of exposure. the coverage of sg films on znfe deposits was assessed by sem observations on the surface and the cross-sections (obtained by cryofracture) of the samples. a hitachi su8020 equipment was used. http://dx.doi.org/10.5599/jese.1282 j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 670 sem observations were also coupled with edx measurements to analyse corrosion products formed on the surface of the samples, in terms of morphology and composition. this study focuses only on znfe deposits since zinc and/or galvanized steel have already been studied with this kind of sg film with promising results. results and discussion characterization of the coated systems table 1 presents the sem observations of the surface and the cross-section of znfe deposits with sg10 and sg30 films, with and without gardoclean® surface preparation. the surface of the znfe electrodeposit was covered with all sg films and znfe pyramids could still be observed. however, in the absence of gardoclean® surface preparation (figures 1(a) and (b)), some cracks were visible at the bottom of the pyramids, indicating that the sg films could not accommodate the morphology of the znfe deposit. when a gardoclean® surface preparation is performed before the application of sg films (figures 1(c) and (d)), an improvement was observed with the absence of cracks. sg30 film seemed thicker than sg10 one, especially in the valleys of the znfe deposit. this would be consistent with their difference in terms of viscosity. indeed, the thickness of the sg film can be calculated by the landau-levich relation (1). figure 1. sem micrographs of znfe deposits with sg10 (without (a) and with gardoclean® (c and e)) and sg30 (without (b) and with gardoclean® (d and f)) films. surface (a, b, c and d) and cross-section (e and f) observations c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 671 ( ) ( )    = 2/3 0 1/21/6 lv 0.94h g (1) where h is the thickness of the film, η is the solution viscosity, 0 is the withdrawal rate, lv is the surface tension and ρg is the gravity parameter. this relation indicates that for the same withdrawal rate, the higher the sol viscosity, the higher the film thickness. this was confirmed by the observation of the cross-section of the samples (figures 1(e) and (f)). thickness heterogeneities are observed, particularly between the top (about 1 µm or less) and the bottom of the pyramids (between 2 and 6 µm for sg30). however, the thickness of the film is higher for sg30 (reaching 6 µm). a decrease in the hydrophily of the znfe deposit was observed with the presence of both sg10 and sg30 films. indeed, the contact angle between the droplet and the znfe deposit could not be measured since the droplet immediately spread on the surface of the sample. thus, the contact angle was assimilated to 0o. however, in the presence of the sg films, this value was superior to 60o depending on the systems, as shown in table 2. fedel et al. [28] obtained similar values (between 61 and 67o) with an sg film containing 10 % in precursors. two hypotheses can explain the modification of the hydrophilic behaviour of the systems. first, sg films are composed of gptms and mtes. each of them presents an organic group. methyl group in mtes could decrease the hydrophilic aspect of the system. moreover, higher contact angle values were obtained for sg10 films, which could be due to the surface morphology. sem micrographs (figure 1) revealed that both sg10 and sg30 films follow the pyramidal shape of the znfe deposit. however, fewer pyramid peaks are visible in the case of sg30 film due to the levelling effect of the film. according to the literature, a hydrophobic behaviour can be obtained with a nanostructured morphology [32,33]. although znfe deposits are not nanostructured, the presence of more pyramid peaks for znfe + sg10 systems could decrease their hydrophilic behaviour. table 2. contact angles measured for the different coated systems, with and without gardoclean® contact angle, o without gardoclean® with gardoclean® sg10 72.2 ± 0.1 76.9 ± 0.3 sg30 62.6 ± 0.5 66.2 ± 0.4 electrochemical characterization the ocp was measured during the whole electrochemical experiments. after 3 days of immersion, the ocp value of bare znfe was about -0.92 v vs. sce. the ocp value was stable during the first 3 days of immersion for almost all systems. a shift of the ocp value was observed in the presence of sg films: +0.05 v for znfe + gardoclean® + sg10 and +0.13 and +0.09 v for znfe + sg30 without and with gardoclean®, respectively. these shifts indicate a good covering of the sg films. because of the similarities in behaviour between znfe + sg10 with and without gardoclean®, only znfe + gardoclean® + sg10 will be considered for the rest of the study. whereas both znfe + sg30 and znfe + gardoclean® + sg30 will be considered. corrosion resistance of bare znfe the bode diagrams obtained during eis measurements of znfe in 0.1 m nacl are presented in figure 2. an increase of the low-frequency modulus (figure 2(a)) until 48 h of immersion is observed (from about 1370 ω cm² after 6 h until about 4370 ω cm² after 48 h). then, the low-frequency modulus decreased, reaching about 2700 ω cm² after 72 h of immersion. these observations, associated with the shift of the time constant towards lower frequencies on the bode diagram http://dx.doi.org/10.5599/jese.1282 j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 672 (figure 2(b)), would indicate the formation of corrosion products, which was confirmed by the visual observations after the test (figure 2(c)). indeed, the surface of the samples is covered with white/bluish products, which could supposedly form a thin oxide layer. figure 2. evolution of bare znfe bode diagrams in the function of time in 0.1 m nacl: (a) modulus diagram and (b) phase diagram. (c) visual aspect of the samples after 3 days of immersion in 0.1 m nacl. the exposed surface is 1 cm². (d) electrical equivalent circuit used to represent the system this electrochemical behaviour can be represented by an electrical equivalent circuit (eec) (figure 2(d)) where rel is the electrolyte resistance and qznfe and rznfe are associated with the charge transfer at the interface with znfe deposit. data obtained from the eec (rel, rznfe, qznfe, n, which is the cpe parameter and χ²) are gathered in table 3. the use of a cpe [34–36] instead of a capacitance describes a non-ideal capacitive behaviour, considering the irregularities of the system (roughness, porosity, adsorption, etc.). the impedance of a cpe zcpe is given by relation (2):   =cpe 1 ( ) ( ) n z q j (2) where q and n are the cpe parameters. it is possible to calculate the capacitance of a film or a charge transfer with brug’s relation (3) [36]. ( ) ( ) − − = + 11/ 1 1 eff e t n n nc q r r (3) where ceff if the effective capacitance, q is the cpe value, re is the electrolyte resistance (called rel in this work), rt is the global resistance and n is the cpe exponent. in our case, rt>>re, consequently, the following relation can be used (4). ( ) = 11/ eff e n n nc q r (4) for bare znfe, the charge transfer resistance rt corresponds to rznfe (table 3). this value increases until 2 days of immersion (from 1 kω cm² to about 6 kω cm²), associated with an increase c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 673 in the capacitive aspect of the system (an increase of n). corrosion products could have a barrier effect, thus limiting dissolved oxygen from reaching the surface. after 3 days of immersion, a decrease of rznfe is observed until 3 kω cm², indicating a rupture of the protecting effect. this could be due to the formation of pulverulent products leading to a loss of adhesion. moreover, localized corrosion could take place in “less protected” areas. finally, chloride ions could contribute to the degradation of the protective film formed on znfe coating, although they are needed to form some corrosion products (particularly simonkolleite). table 3. data obtained from the eec model for the immersion of bare znfe in 0.1 m nacl rel / ω cm² rznfe / kω cm² qznfe / f cm-² sn-1 n χ² 10 3 6 h 108 1.22 809 0.68 2.83 12 h 113 1.53 908 0.68 2.11 1 d 117 2.04 862 0.76 4.54 2 d 121 5.96 838 0.85 3.60 3 d 109 3.26 928 0.82 3.77 influence of the presence of an sg film three different systems were investigated: znfe + gardoclean® + sg10 and znfe (+ gardoclean®) + sg30. the bode diagrams obtained during eis measurements of the systems in 0.1 m nacl, the visual aspects of the samples after the tests and the eec used to represent the systems are presented in figure 3. corrosion resistance of znfe + gardoclean® + sg10 the bode diagrams obtained during eis measurements of znfe + gardoclean® + sg10 in 0.1 m nacl are presented in figures 3(a) and (b). there is a decrease of the low-frequency modulus after 12 hours of immersion, indicating a loss of barrier properties. on the phase diagram (figure 3(b)), two time constants were observed until 12 hours of immersion: the high-frequency range associated with the sg film time constant and the middle frequency range associated with the sg/znfe interface. after one day of immersion, only one time constant is visible, which is larger than previously and probably corresponds to the gathering of the two time constants. after 2 days of immersion, this time constant is finer and is shifted towards lower frequencies, in the same range as the time constant observed for bare znfe after 6 hours of immersion (figure 2(b)). this shift was also observed for bare znfe and was attributed to the formation of corrosion products/oxide layer. at the end of the experiment, the presence of the time constant associated with the sg film can only be supposed. an eec with two cpe (figure 3(h)) was used to represent this system and values are gathered in table 4. this eec consists of the electrolyte resistance rel, the sg film characterized by rsg and qsg. r2 and q2 are associated with the oxide layer formed at the interface between the znfe deposit and the sg coating in certain circumstances. in this configuration, the charge transfer resistance rt would be characterized by r2. csg and c2 (tables 4, 5 and 6) correspond to the values obtained from eq. 4 and are effective capacitances. the different resistances rel, rsg and r2, the effective capacitances csg and c2, the cpe parameters n and χ² are gathered in tables 4, 5 and 6 depending on the system. the contribution of the sg film (about 2 kω cm² after 6 hours of immersion) is largely inferior to the one of the interfacial oxide layer (about 30 kω cm² after 6 h of immersion). these values are consistent with the literature [16,17]. rsg strongly decreases during the first hours of immersion, which is commonly observed in the literature [16,17,19,37,38]. moreover, the capacitance of this film increases with the immersion time, which could be explained by a water uptake or the break of si-o-si bindings. http://dx.doi.org/10.5599/jese.1282 j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 674 figure 3. evolution of different systems bode diagrams in function of time in 0.1 m nacl: (a and b) znfe + gardoclean® + sg10; (c and d) znfe + sg30 and (e and f) znfe + gardoclean® + sg30. (g) visual aspect of the different systems after 3 days (znfe + gardoclean® + sg10) or 7 days (znfe (+ gardoclean®) + sg30) of immersion in 0.1 m nacl. the exposed surface is 1 cm². (h) eec used to represent these systems c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 675 a decrease in the oxide layer resistance is also observed and could be attributed to an increase in the exposed surface due to the degradation of the sg film. after 2 days of immersion, csg and c2 capacitances are quite high and reach 100 and 25 µf cm-². consequently, a charge transfer phenomenon can be supposed leading to the formation of corrosion products. this hypothesis can be confirmed by the presence of corrosion products on the surface of the sample after 3 days of immersion (figure 3(g)). these images confirm the improvement in terms of corrosion resistance thanks to the sg film with the presence of intact areas. corrosion products do not form a homogeneous layer. indeed, some white/orange points are visible on the surface. the formation of these corrosion products could be explained by the micro-cracks observed in figure 1(a), allowing the electrolyte to reach the znfe deposit. table 4. data obtained from the eec model for the immersion of znfe + gardoclean® + sg10 in 0.1 m nacl rel / ω cm² rsg / kω cm² csg / µf cm-² n r2 / kω cm² c2 / µf cm-² n χ² 103 6 h 115 2.54 0.66 0.82 31.81 / 0.56 1.26 12 h 116 2.20 0.72 0.81 32.03 / 0.56 0.63 1 d 113 0.75 1.28 0.75 5.07 / 0.58 0.57 2 d 114 0.09 2.85 0.87 6.19 6.3 0.65 0.89 3 d 110 0.12 108 0.8 5.76 28.8 0.8 2.98 corrosion resistance of znfe + sg30 the bode diagrams obtained during eis measurements of znfe + sg30 in 0.1 m nacl until 4 days of immersion are presented in figures 3(c) and (d). without gardoclean® surface preparation, the time constant associated with the sg film strongly decreases during the first hours of immersion, which is consistent with the decrease of the low-frequency modulus. the time constant associated with the znfe/sg film interface becomes more intense and is shifted towards lower frequencies. these observations are consistent with the ones obtained for bare znfe and znfe + gardoclean® + sg10 and probably indicate the degradation of the sg film and the formation of corrosion products. this system can be represented by the same eec as the one used for the system znfe + gardoclean® + sg10. data obtained from the eec modelling are gathered in table 5. table 5. data obtained from the eec model for the immersion of znfe + sg30 in 0.1 m nacl rel / ω cm² rsg / kω cm² csg / µf cm-² n r2 / kω cm² c2 / µf cm-² n χ² 103 6 h 104 2.08 5.49.10-2 0.75 55.57 0.53 0.42 12 h 101 2.00 5.17.10-2 0.74 55.63 0.55 0.73 1 d 105 5.25 0.52 17.72 3.77 0.73 0.66 2 d 125 0.51 8.45.10-1 0.62 13.92 0.59 0.1 3 d 113 0.21 0.59 11.12 11.1 0.81 0.21 4 d 109 0.28 1.67 0.60 8.92 6.59 0.66 0.16 5 d 110 0.18 2.33 0.65 12.01 10.4 0.67 0.19 6 d 107 0.12 4.02 0.69 9.75 29.8 0.74 0.69 7 d 103 0.13 2.62 0.67 10.17 25.6 0.72 0.69 rsg values obtained after 6 h of immersion are in the same range as the one obtained previously for znfe + gardoclean® + sg10 (about 2 kω cm²). a sharp decrease in this value is observed until 2 days of immersion when it becomes inferior to 1 kω cm². the medium/low-frequency time constant was first associated with the interfacial oxide layer (with resistance like the one obtained for znfe + gardoclean® + sg10) and a few µf cm-² capacitances. however, the effective capacitance c2 increases from 3 days of immersion, reaching values superior to 10 µf cm-². consequently, this time constant http://dx.doi.org/10.5599/jese.1282 j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 676 was attributed to a charge transfer phenomenon, which could be confirmed by the presence of corrosion products on the surface of the samples (figure 3(g)). these white/orange corrosion products are spread on most of the surface. areas with bluish products are also observed and are quite similar to bare znfe samples after 3 days of immersion. finally, areas without corrosion products are detected. sem observations (figure 1(b)) highlighted the presence of cracks in the sg films. these cracks could allow the electrolyte to reach the znfe deposit, leading to the formation of corrosion products. corrosion resistance of znfe + gardoclean® + sg30 the bode diagrams of znfe + gardoclean® + sg30 are presented in figures 3(e) and (f). when a gardoclean® surface preparation is used before the application of the sg film, the time constant associated with the sg film varies essentially during the first day of immersion. this constant slightly decreases while the one associated with the znfe/sg film interface increases and is shifted towards lower frequencies. for the first time, the sg film time constant is still observed after 7 days of immersion, indicating the better performance of this system. finally, the low-frequency modulus is higher than the one obtained for the same system without gardoclean® and it remains stable during the whole test, indicating the better stability of this system. the eec (figure 3(h)) was used to represent the znfe + gardoclean® + sg30 during the 7 days of immersion and data obtained from the modelling are gathered in table 6. there is a decrease in rsg during the first days of immersion (from about 7 kω cm² after 6 h to about 3 kω cm² after 3 days). after 3 days of immersion, this value remains stable. the value obtained after 6 h of immersion is quite higher than the ones obtained for other systems, indicating a great improvement. the effecttive capacitance csg values are lower than the ones obtained for znfe + sg30 (table 5), indicating two things. first, the corrosion protection provided by this system is better than znfe + sg30. then, the sg film is thicker thanks to the gardoclean® treatment. indeed, the capacitance value is inversely proportional to the thickness of the film. the surface preparation could favour the formation of hydroxyl groups on the surface of the znfe coating, thus improving the adhesion of the sg film. moreover, an increase in the csg values is observed with the immersion time, which is consistent with results obtained for other systems. table 6. data obtained from the eec model for the immersion of znfe + gardoclean® + sg30 in 0.1 m nacl rel / ω cm² rsg / kω cm² csg / nf cm-² n r2 / kω cm² c2 / µf cm-² n χ² 103 6 h 111 6.92 15.3 0.83 241.12 / 0.55 1.25 12 h 112 6.11 1..8 0.82 323.69 / 0.58 1.27 1 d 104 3.74 17.9 0.81 365.55 0.61 0.63 1.6 2 d 100 2.99 18.7 0.79 538.64 1.11 0.66 2.69 3 d 98 2.68 20.6 0.79 946.97 1.11 0.66 2.76 4 d 100 3.10 21.8 0.78 1142.30 0.97 0.66 2.56 5 d 100 3.15 24.0 0.77 1486.60 0.91 0.66 2.41 6 d 96 2.49 28.7 0.76 976.87 1.13 0.67 2.19 7 d 95 2.40 33.5 0.75 832.02 1.25 0.68 2.25 regarding corrosion resistance, the contribution of the sg film is largely inferior to the one of the interfacial oxide layer. indeed, its resistance is superior to 240 kω cm² compared to about 7 kω cm² for the sg film. the effective capacitance c2 of this layer is about 1 µf cm-² during the whole test, indicating the absence of charge transfer. in this case, the oxide layer resistance r2 increases during the first days of immersion. this could be explained by an electrolyte infiltration reaching the oxide c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 677 layer and/or znfe deposit. the formation of oxides could reinforce the existing oxide layer and seal micro-defects in the sg film. this hypothesis could illustrate, on the one hand, the increase of the oxide layer resistance and, on the other hand, the stabilization of the sg film resistance. after 7 days of immersion, the surface of the samples is quite intact, showing that the combination of gardoclean® treatment and sg30 film considerably limits the degradation of znfe electrodeposit. sem observations after 7 days of immersion are presented in figure 4 for the znfe + gardoclean® + sg30 configuration. the sg film is observed on the whole exposed area. moreover, znfe pyramids (red circles in figure 4) are still visible under the sg film. a micro-crack (orange rectangle) is detected and could permit the infiltration of electrolytes and the formation of corrosion products. indeed, some corrosion products are present on the sem micrograph. however, they were not distinguished by the naked eye. figure 4. sem micrograph of a znfe + gardoclean® + sg30 after 7 days of immersion in 0.1 m nacl. the red circles indicate the presence of znfe pyramids. the orange rectangle highlights the presence of cracks in the sg film comparison of the different systems the evolution of the low-frequency modulus of bare znfe, znfe + gardoclean® + sg10 and znfe + sg30 (with and without gardoclean® surface preparation) is presented in figure 5. several areas were drawn, corresponding to different behaviours. the area i is associated with bare znfe. the low-frequency modulus increases until 2 days of immersion and then decreases until 3 days. the increase of the low-frequency modulus was attributed to the formation of corrosion products, while the decrease was explained by a loss of the protective aspect provided by these corrosion products. area ii corresponds to the systems znfe + gardoclean® + sg10 and znfe + sg30. during the first 12 hours of immersion, a great increase of the low-frequency modulus is observed, in comparison with bare znfe, indicating the barrier properties of the sg films. in this area, the low-frequency modulus rapidly decreases during the first 24 hours of immersion and then remains stable until 3 days of immersion. this evolution shows a loss of barrier properties associated with the sg film. although the low-frequency modulus is superior to the one of bare znfe, the presence of corrosion products on the surface of the samples highlights the lack of corrosion resistance. http://dx.doi.org/10.5599/jese.1282 j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 678 figure 5. evolution of the low-frequency modulus in the function of the immersion time in 0.1 m nacl for bare znfe, znfe + gardoclean® + sg10 and znfe + sg30 with and without gardoclean® finally, the evolution of znfe + gardoclean® + sg30 is shown in area iii. an increase of two decades in terms of low-frequency modulus is noted after 6 hours of immersion, in comparison with bare znfe. moreover, this value remains quite constant until 3 days of immersion, indicating the good corrosion protection provided by this system. salt spray test the evolution of bare znfe, znfe + gardoclean® + sg10 and znfe + sg30 with and without gardoclean® surface preparation after several days of neutral salt spray test is presented in figure 6. usually, two times are determined during zn coated steel exposed to salt spray test: the time before white rust appearance, the time before red rust appearance. however, in the case of znfe coatings, the appearance of red rust is not associated with the corrosion of the steel substrate. indeed, the degradation of the znfe deposit leads to the formation of iron and/or zinc products which are respectively red and white. visually, it is impossible to distinguish red corrosion products obtained from the corrosion of the znfe deposit from those obtained from the corrosion of the steel substrate. behaviour of bare znfe exposed to nss figure 6 shows that corrosion products, mostly white, are formed from the first day of exposure. these products are supposedly formed due to the dissolution of zinc from the znfe deposit. few orange areas are visible, but they become more intense after several days of exposure. after 7 days, iron corrosion products are majoritarian and form a voluminous layer. some of the corrosion products fell when samples were dried, indicating their lack of adhesion. sem observation of samples after different times of exposure permitted to identify corrosion products. figure 7 represents sem micrographs obtained for the different systems after different times of exposure. for bare znfe, after 3 and 7 days of exposure (figure 7(a) and (b) respectively), corrosion products with different morphologies were observed. hexagonal crystals could be associated with zinc corrosion products. among them, simonkolleite (zn5(oh)8cl2.h2o) presents hexagonal crystals [39,40]. c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 679 t0 1 day 2 days 3 days 7 days 14 days figure 6. evolution of the visual aspect of bare znfe, znfe + gardoclean® + sg10 and znfe + sg30 with and without gardoclean®, after different days of exposure in the nss test figure 7. sem micrographs of the different systems after exposure to nss test: (a and b) bare znfe after 3 and 7 days of exposure, respectively; (c and d) znfe + gardoclean® + sg10 after 14 days of exposure and (e and f) znfe + gardoclean® + sg30 after 7 days of exposure http://dx.doi.org/10.5599/jese.1282 j. electrochem. sci. eng. 12(4) (2022) 667-683 corrosion resistance of electrodeposited zn-fe 680 behaviour of znfe + gardoclean® + sg10 exposed to nss the surface of znfe + gardoclean® + sg10 is covered in corrosion products after only 1 day of exposure (figure 6). corrosion products are mostly white, but some orange areas are observed and become more present when exposure time increases. this result is consistent with the bode diagram evolution (figures 3(a) and (b)). indeed, it showed a large decrease in the sg film resistance (rsg) during the first 12 hours of immersion in 0.1 m nacl. moreover, sem micrographs (figure 1) revealed the presence of cracks in the sg deposit associated with small thicknesses (<1 µm) locally. however, the corrosion products layer seems less thick than on bare znfe. moreover, the system seemed quite stable until 3 days of exposure with few evolutions from day 1 to day 3, indicating that the sg film slows down the formation of corrosion products. after 7 days of exposure, the benefits of the sg films are perfectly visible since the corrosion products layer is far more important on bare znfe. figures 7(c) and (d) show the surface of the znfe + gardoclean® + sg10 system after 14 days of exposure, observed by sem. hexagonal crystals are observed with different thicknesses. figure 7(c) highlights the presence of pyramidal grains associated with the znfe coating. an edx mapping of this figure 7(c) is presented in figure 8, where area 1 corresponds to hexagonal crystals while area 2 is associated with pyramidal grains. the edx mapping confirmed that area 2 was still protected with the presence of si and the detection of fe. however, area 1 is rich in o and cl and fe and si are almost not detected. these results could indicate a degradation of the sg film and thus the corrosion of the znfe deposit, leading to the formation of corrosion products. finally, the formation of simonkolleite could be supposed due to the hexagonal shape of the crystals and the presence of cl. figure 8. edx mapping znfe + gardoclean® + sg10 after 14 days of exposure (figure 7 (c)) c. arrighi et al. j. electrochem. sci. eng. 12(4) (2022) 667-683 http://dx.doi.org/10.5599/jese.1282 681 behaviour of znfe + sg30 with and without gardoclean® figure 6 reveals an improvement in the corrosion resistance with the znfe + sg30 systems. indeed, few corrosion products are observed on the surface of the samples until 3 days of exposure. after 14 days of exposure, most of the surface is covered by orange corrosion products. however, the sg film considerably limits the formation of corrosion products since samples are less corroded after 14 days of exposure than bare znfe after 1 day of exposure. contrary to eis results, no improvement due to gardoclean® treatment was observed, which could be explained by the aggressiveness of the test. the sem observations of the surface of znfe + gardoclean® + sg30 after 7 days of immersion are presented in figures 7(e) and (f). figure 7(e) reveals the presence of hexagonal crystals that go through the sg film, which presents some cracks (indicated by red arrows). figure 7(f) shows an area entirely covered with corrosion products, mainly hexagonal crystals. similar products were observed for znfe + gardoclean® + sg10. conclusions an aqueous sg film composed of teos, mtes and gptms was deposited on znfe coated steel. due to the pyramidal shape of the znfe electrodeposit, cracks were observed in the sg films. however, the use of an alkaline surface preparation permitted to limit this phenomenon. thickness heterogeneities were also observed due to this pyramidal shape. electrochemical tests highlighted an improvement of the corrosion resistance of znfe deposit thanks to the sg films, with an increase of the lowfrequency modulus. but this effect lasted only for the system znfe + gardoclean® + sg30 showing a higher precursor content, which remained stable during 7 days of immersion in 0.1 m nacl. the same conclusions were obtained with an nss test, showing 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[40] s. cousy, n. gorodylova, l. svoboda, j. zelenka, chemical papers 71 (2017) 2325-2334. https://doi.org/10.1007/s11696-017-0226-4 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1282 https://doi.org/10.1016/j.electacta.2009.10.065 https://doi.org/10.1134/s2070205118030280 https://doi.org/10.1016/s0022-0728(84)80324-1 https://doi.org/10.1016/j.electacta.2007.05.022 https://doi.org/10.1007/s10971-018-4840-6 https://doi.org/10.1007/s11696-017-0226-4 https://creativecommons.org/licenses/by/4.0/) @article{arrighi2022, author = {arrighi, c{\'{e}}line and paint, yoann and savall, catherine and creus, juan and olivier, marjorie}, journal = {journal of electrochemical science and engineering}, title = {{improvement of the corrosion resistance of electrodeposited zn-fe by sol-gel conversion films:}}, year = {2022}, issn = {1847-9286}, month = {apr}, number = {4}, pages = {667--683}, volume = {12}, abstract = {an aqueous hybrid inorganic/organic sol-gel solution composed of tetraethylorthosilicate (teos), methyltriethoxysilane (mtes) and (3-glycidyloxypropyl)trimethoxysilane (gptms) was applied on znfe (14 wt.% fe) electrodeposited on steel as a sacrificial layer. two precursor contents were studied: 10 (sg10) and 30 % (sg30). first, the morphology and thickness of the films were assessed by scanning electron microscopy (sem) observations. they revealed the presence of micro-cracks in the films without alkaline surface preparation due to the pyramidal shape of the znfe deposit. then, the corrosion resistance of the systems was determined by electrochemical impedance spectroscopy (eis) and neutral salt spray (nss) test. all results indicated an improvement in the corrosion resistance thanks to the presence of the sg films. however, the protection provided by the sg10 film did not permit to durably protect the znfe deposit. the combination of surface preparation and a sg30 film provided promising protection to the znfe deposit with an increase of the low-frequency modulus and a delay in corrosion product appearance during the nss test.}, doi = {10.5599/jese.1282}, file = {:d\:/onedrive/mendeley desktop/arrighi et al. 2022 improvement of the corrosion resistance of electrodeposited zn-fe by sol-gel conversion films.pdf:pdf;:07_jese_1282.pdf:pdf}, keywords = {electrochemical impedance spectroscopy, aqueous based solution, zinc alloys}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1282}, } {tailoring surface properties of functionalized graphene papers aiming to enzyme immobilization:} http://dx.doi.org/10.5599/jese.1099 137 j. electrochem. sci. eng. 12(1) (2022) 137-151; http://dx.doi.org/10.5599/jese.1099 open access : : issn 1847-9286 www.jese-online.org original scientific paper tailoring surface properties of functionalized graphene papers aiming to enzyme immobilization jéssica luzardo1,2, danielle aguiar1, alexander silva1, sanair oliveira1, bráulio archanjo1, renata simão2, joyce araujo1, 1materials metrology division, national institute of metrology, quality and technology (inmetro), av. nossa senhora das graças, 50, cep 25250-020, duque de caxias, brazil 2nanotechnology engineering program pent, federal university of rio de janeiro, av. athos da silveira ramos, 149, cidade universitária, cep 21941-972, rio de janeiro, brazil corresponding author: jraraujo@inmetro.gov.br; tel.: +1-111-111-111; fax: +1-111-111-112 received: september 3, 2021; accepted: november 29, 2021; published: december 15, 2021 abstract the use of enzymes as catalysts requires recovery and reuse to make the process viable. enzymatic immobilization changes enzyme stability, activity, and specificity. it is very important to explore new substrates for immobilization with appropriate composition and structure to improve the efficiency of the immobilized enzymes. this work explores the use of two different graphene oxide papers, one produced by oxidation route (go) and the other by electrochemical synthesis (eg), aiming for β-galactosidase immobilization. the chemical and structural properties of these two papers were characterized by raman spectroscopy, x-ray photoelectron spectroscopy and x-ray diffraction. atomic force microscopy images showed that eg paper ensured more efficient immobilization of the enzymes on the surface of the paper. cyclic voltammetry was used to monitor the reaction of conversion of lactose to glucose in the free enzyme solution and graphene paper immobilized enzyme solutions. the cyclic voltammetry analysis showed that immobilized enzymes on go paper showed an improvement in the activity of β-galactose when compared to free enzyme solution, as well as enzyme immobilized on a glassy carbon electrode. keywords sensors; graphene oxide; β-galactosidase; glucose; lactose; cyclic voltammetry introduction enzymes are excellent biological catalysts, highly specific, and fundamental in biochemical reactions. they accelerate the speed of reactions without being consumed or modified [1]. industrially, enzymes have better characteristics than other chemical catalysts due to their higher specificity to the substrate, which promotes the production of only one biochemical reaction and, consequently, allows the synthesis of a specific product with no co-products formation. in addition, http://dx.doi.org/10.5599/jese.1099 http://dx.doi.org/10.5599/jese.1099 http://www.jese-online.org/ mailto:jraraujo@inmetro.gov.br j. electrochem. sci. eng. 12(1) (2022) 137-151 graphene papers for enzyme immobilization 138 they operate under mild reaction conditions, have a fast action, while the absence of toxicity decreases the environmental and toxicological problems [2]. the use of enzymes as biocatalysts in industries is currently a solution to many problems of modern organic chemistry, which attempts to carry out the most complex reactions under the rules of green chemistry [3]. this is the reason why enzymes are extensively used in the food industry [4-6], biofuels [7-9], textile industry [10-12], pharmaceutical [13-15] and many other industry applications. β-galactosidase, known as lactase, is an enzyme that has the function of hydrolyzing oligosaccharides, secondary metabolites and d-galactosyl residues of polymers [16]. lactase is used in industry to improve sweetness, solubility, flavor, and dairy product digestibility. the lactose enzymatic hydrolysis can eliminate serum disposal associated problems, crystallization in frozen concentrated foods and allow milk consumption by lactose-intolerant individuals [17]. despite unquestionable advantages, there are some practical problems with enzymes, as the high isolation cost, purification, and instability of enzyme structures once they are isolated from their natural environments, sensitivity to process conditions and the presence of substances that may act as inhibitors. enzymes also work dissolved in aqueous solutions in homogeneous catalysis systems and contaminate the product by hard recovering from the solution in the active state when aiming to posterior reuse [18]. these issues limit their applications in industrial processes and can be solved by enzymatic immobilization on substrates [19]. the use of substrates stabilizes the enzyme's structure, improving its activity. in comparison with free enzymes, immobilized enzymes are more resistant to environmental changes as temperature and ph. it is more important that these systems allow easy recuperation of both enzyme and product, multiple enzyme utilization, continuous operation of enzyme processes, rapid completion of reactions, and a broader range of bioreactor design [18]. β-galactosidase immobilization offers many advantages such as fast reaction, product-controlled formation, easy enzyme removal and adaptability to numerous engineering projects. the reactors containing immobilized β-galactosidases have been extensively studied due to their importance in the industrial production of free lactose milk and whey. lots of the enzyme reactors used in lactose hydrolysis include papers systems. in a paper bioreactor, the biocatalyst is confined in a well-defined space region by a selective paper or immobilized by adsorption or trapping inside the paper itself. the use of these systems is an effective technique. both lactose conversion and protein recovery can be performed in one step [17]. the development of nanostructured materials of different sizes and shapes aiming at enzyme immobilization has been investigated as an alternative to the solid bulk substrates [20]. graphene oxide can be considered the insulating and disordered analog of highly conductive crystalline graphene [21]. the nature of go is insulating, defective and has a heterogeneous chemical and electronic structure. its sp2 hybrid structure, as well as the presence of various types of functional groups containing oxygen at the basal plane and edges, allow go to interact with a wide range of organic and inorganic materials by non-covalent, covalent and/or ionic bonds interactions, which makes it a promising substrate for multivalent functionalization and efficient loading of small organic molecules and biomacromolecules [1,22]. go sheets are often an ideal substrate for the study of enzyme immobilization in nanostructured materials. go sheet contains oxygenated functional groups, making it an ideal substrate for enzyme immobilization without any superficial modifications or any coupling agents [23]. parvez et al. [24] reported a new way of graphene synthesis based on the electrochemical exfoliation of graphite. this method can produce high-quality thin graphene sheets in low reaction times and high yields [25]. in comparison to go, the graphene oxide produced by this method has a lower oxidation degree and higher defect density caused by the electrochemical process [26,27]. j. luzardo et al. j. electrochem. sci. eng. 12(1) (2022) 137-151 http://dx.doi.org/10.5599/jese.1099 139 in this work, we prepare two graphene-based papers with different hydrophilicity, go and eg, aiming to evaluate their efficiency as a support to provide β-galactosidase enzyme immobilization aiming to improve selectivity and activity of this biocatalyst in the reaction of conversion of lactose in glucose and galactose. surface properties of go and eg papers were studied by atomic force microscopy (afm), contact angle and x-ray photoelectron spectroscopy (xps) measurements. chemical and structural analysis of graphene papers were evaluated by raman spectroscopy and xray diffraction, respectively, while the morphology was evaluated in a helium ion microscope. the β-galactosidase mediated conversion of lactose in glucose, comparing different immobilization routes, as well as the enzyme in free solution, was evaluated by cyclic voltammetry analysis. experimental materials go synthesis was performed from expanded graphite purchased from nacional do grafite ltda (são paulo, brazil). sodium nitrate (nano3), potassium permanganate (kmno4), sulfuric acid (h2so4), hydrochloric acid (hcl) and hydrogen peroxide (h2o2) were purchased from sigma-aldrich (missouri, usa). the enzymes used in this work were the commercial samples: lacday, 10.000 fcc alu (tablet) and the deslac lactose drops 10.000 fcc alu. the working electrode was a glassy carbon electrode manufactured by metrohm, with a working surface diameter of 4 mm. eg synthesis was performed using a 0.25 mm thickness graphite sheet obtained from alfa aesar, milli-q water (millipore system), resistivity ≥ 18 mω cm2. the ammonium sulfate ((nh4)2so4) used in the carrier solution was obtained from sigma-aldrich (rio de janeiro, brazil). paper solutions were made from n, n'-dimethylformamide (99 %) obtained from sigma-aldrich. graphene oxide was produced using the modified hummers methodology [28]. concentrated h2so4 (92 ml) was added to a mixture of expanded graphite (2.0 g) and nano3 (1.0 g). after the mixture became homogeneous, kmno4 (3.0 g) was added slowly. the resulting mixture was then heated to 35 oc and stirred for 30 min. the water (300 ml) was added slowly to the mixture, promoting a highly exothermic reaction and self-heating to 98 oc. external heating was used to maintain the reaction temperature at 98 oc for 15 min and then, the mixture was cooled in an ice bath for 10 min. afterward, a solution of 10 ml of 30 % v/v h2o2 in 90 ml of water was added to stop the exothermic reaction. after the oxidation reaction ceased, a brown suspension was obtained and washed with 180 ml of water and 20 ml of 30 % (v/v) hcl solution to remove metallic ions. then, a filtration step was performed to separate the non-oxidized graphite. the filtered solution was left to settle for 24 h to remove the acid supernatant. the decanted part corresponding to the graphite oxide was thoroughly washed with water and centrifuged to remove the remaining acid solution. at each wash, the ph of the supernatant was monitored until a neutral value was reached. residual water was removed using a lyophilization process. the graphite oxide obtained in this step was exfoliated in an ultrasonic bath for 30 min. given the challenges regarding large-scale production of large-area defect-free graphene sheets, a new method of graphene synthesis based on the electrochemical exfoliation of graphite has been reported by parvez et al. [24]. based on this methodology, eg was synthesized by electrochemical exfoliation of graphite using a platinum wire as the counter electrode and a graphite foil as the working electrode. a solution of (nh4)2so4 0.1 mol l-1 was used as a carrier electrolyte. the potential of 10 v, applied between the graphite and platinum electrodes, during approximately 30 min, resulted in a complete exfoliation of graphite electrode in the electrolyte. afterward, eg sheets were separated by electrolyte filtration and dispersed in dimethylformamide (dmf) for 15 min. four http://dx.doi.org/10.5599/jese.1099 j. electrochem. sci. eng. 12(1) (2022) 137-151 graphene papers for enzyme immobilization 140 microliters of eg dispersion (10 mg ml-1) were dropped onto the surface of the glassy carbon electrode and then, the graphene-modified electrode was dried in an oven at 50 oc temperature. go and eg papers were produced using a vacuum filtration system where the dispersed graphene material was filtered. the paper thickness depends on the concentration and volume of the suspension. for eg papers, 30 ml of 4 mg ml-1 of eg solution in dmf were used. for the go papers preparation, we used 30 ml of a 1 mg ml-1 go solution in water. these solutions were put to filtration using an anodization paper (anopore disc) with 0.2 µm diameter. after complete filtration, the papers were dried in an oven at 60 oc until complete drying of adsorbed solvent. during the drying step, the graphene paper detached from the anodizing disk, being easily removed. the enzyme was immobilized by the adsorption method. for immobilization on the papers, pbs ph 4.5 solutions were prepared using β-galactosidase concentrations of 0.5, 2.0, 5.0 and 10.0 mg ml-1. the solid support (graphene paper) was placed in contact with the enzymatic solution for a period of 30 min under appropriate conditions that favor the enzymatic activity at ph 4.5 and 30 oc temperature. the remaining enzyme molecules (not adsorbed) was then removed from the surface by phosphate buffer washing. enzyme immobilization on the electrode surface was performed using 9.2 mg enzyme solution in pbs solution at ph 4.5. this solution was placed on the electrode surface and left in an oven at 60 oc until the solvent was completely dried. subsequently, the non-adsorbed enzyme molecules were then removed from the surface by phosphate buffer washing. characterization contact angle measurements were obtained using a ramé-hart, usa, 500 goniometer. distilled water (2 μl) was dripped onto eg and go papers. measurements were made at different paper positions to verify surface homogeneity. to allow the complete absorption of droplets on the paper surface, they were kept at rest for 1 min. the atomic force microscope (afm) (jpk nanowizard ii) was used to investigate the topography of papers before and after enzyme immobilization. sample images were obtained in intermittent contact mode using a bruker rtesp cantilever, with a resonant frequency of 273 khz and an elastic constant of 14 n m-1, determined by sader's method. raman spectroscopic analyzes were performed on a witec alpha 300 spectrometer with a 514.5 nm laser line using a 100 objective microscope. laser power was maintained below 0.1 mw (<2 w mm-2) to avoid local heat and damage to the samples. all spectra were acquired using 10 s integration time and ten accumulations of 100 to 3600 cm-1. the analyzed spectrum was obtained by the average of five measurements taken at random points to guarantee homogeneity in the results. x-ray photoelectron spectroscopy (xps) analyzes were performed in an ultra-high vacuum environment (p = 10-9 mbar) (scienta omicron, germany) using a non-monochromatic x-ray source, al anode (kα = 1486.7 ev), with 20 ma emission power and 15 kv voltage. the survey spectra were obtained with 160 ev analyzer step energy and 1 ev acquisition step. high-resolution spectra were collected in the c 1s region with 20 ev analyzer step energy and -0.05 ev acquisition step. x-ray diffraction (xrd) analyzes were performed on a d8-focus bruker diffractometer using nifiltered cu-kα radiation ( = 1.5406 å) using a scanning interval of 0.02o and 2 between 5 and 40o. a thin layer of the sample was prepared by ambient drying of the aqueous suspension on si plates. helium ion microscopy (him) images were performed by a zeiss orion nanofab, where a focused helium ions beam is used during this analysis, thus eliminating the need for samples coating on conductive metallic thin films. the images were performed at 30 kv and 0.4 pa. j. luzardo et al. j. electrochem. sci. eng. 12(1) (2022) 137-151 http://dx.doi.org/10.5599/jese.1099 141 the electrochemical measurements were performed to verify the catalytic activity as well as the selectivity of the immobilized enzyme on graphene, aiming to compare them with the activity and selectivity of free enzymes in solution. graphene, in this case, was not used in the form of papers as they were not designed to act as a support electrode. therefore, for the measurements mentioned above, the precursor eg samples were solubilized in dmf (4 mg ml-1) and deposited by casting over printed electrodes to determine the catalytic activity. cyclic voltammetry measurements were performed using an autolab potentiostat (pgstat 204 model) and nova 1.11 software was used for data acquisition. the tests were performed using the three-electrode cell, in pbs ph 7, in the potential range between -1.40 and 0.50 v, step potencial of 2.4 mv and step rate of 50 mv s-1. the working electrode used was a carbon screen-printed electrode (cspe), the counter electrode was a platinum wire, and the reference electrode was the ag/agcl electrode in kcl (3 mol l-1). all electroanalytical measurements were performed at room temperature. the presented voltammograms were built from the adjustment of the currents, obtained by subtracting its respective values at the potential of -0.1 v. the nomenclature of electrodes adopted throughout this work was: carbon screen-printed electrode (cspe), cspe with the enzyme immobilized on the surface (cspe-ie), electrochemically synthesized graphene modified carbon screen-printed electrode (egpe) and egpe with the immobilized enzyme on the surface (egpe-ie). electrochemical measurements were performed using a lactose solution and glucose (1 mmol l-1) solutions. all conditions tested concerning the condition of the enzyme in the reaction medium, being free solution or immobilized on the electrode surface, are described in table 1. cspe-lac and egpe-lac or cspe-glic and egpe-glic correspond to the electrodes in solutions of lactose or glucose respectively, without enzymes in the reaction medium, while cspe-fe and egpe-fe correspond to the electrodes with the free enzyme in solution. table 1. nomenclature of tested electrodes electrode solution enzyme nomenclature cspe lactose no cspe-lac cspe glucose no cspe-glic cspe lactose free cspe-fe cspe lactose immobilized cspe-ie egpe lactose no egpe-lac egpe glucose no egpe-glic egpe lactose free egpe-fe egpe lactose immobilized egpe-ie results and discussion morphology analysis (afm and him) immobilization was performed at different concentrations, while contact angle measurements and atomic force microscopy were made to verify the presence of enzymes on the paper surface. in this way, it was possible to obtain a qualitative analysis of the distribution of enzymes on the paper surfaces. atomic force microscopy (afm) is one of the micro and nanoscale surface imaging techniques. due to the direct probe-sample interaction, it is possible to determine certain mechanical, electrical, thermal and optical properties of the material [29]. the afm was performed to verify the surface characteristics of the produced papers. figures 1a and 1c show topography images obtained by afm of eg and go papers. in these images, one can see typical wrinkles of graphene and some higher http://dx.doi.org/10.5599/jese.1099 j. electrochem. sci. eng. 12(1) (2022) 137-151 graphene papers for enzyme immobilization 142 regions (120 nm for both go and eg), indicating that both papers have regions with different numbers of graphene layers stacked on top of each other. eg is known to have significant heterogeneity and may be more oxidized/exfoliated in some parts than others, and present some non-oxidized/non-exfoliated regions such as the original graphite [26,27]. the topography heterogeneity, as well as different types of defects (edge defects, vacancies, functional groups, heteroatoms) in graphene sheets, mainly generated by the electrochemical oxidation and exfoliation processes, create coordination sites with potential catalysis activity, as the catalytic performance depends on electronic mobility on the surface of materials [30,31]. figure 1. afm (left) and him (right) images of electrochemically produced graphene oxide paper (eg) (a) and (b); chemically produced graphene oxide paper (go) (c) and (d) him images of eg and go papers presented in figures 1b and 1d, show the typical structure of wrinkled graphene sheets before immobilization of the enzymes on the surface. in go paper (fig. 1d), it is possible to notice that the sheets are much more compacted, unlike the eg paper structure, where the chemical composition of the surface is much more porous (fig. 1b). through cross-section thickness evaluation of two different investigated graphene papers, we found 148 μm for eg paper and 23 μm for go paper, showing higher compaction in go paper. the thickness of graphene papers depends on different factors, i.e., chemical forces between carbon layers, as well as the presence or absence of intercalant ions. hence, the concentration of graphene in the precursor solution is not necessarily proportional to the graphene paper thickness. in the present case, as go has more intercalant oxygenated functional groups than eg, less material concentration in the solution is needed to form a graphene paper. besides that, the go paper morphology has a smaller sheet size than eg paper, which makes sense since the chemical oxidation process causes partial damage in the graphene sheets. this behavior was also observed in the afm images, which showed a more wrinkled graphene structure for eg than go paper. afm was used to evaluate the presence of enzymes on graphene paper surfaces after immobilization. through this technique, it was possible to understand how lactases are organized and distributed on the surface of each evaluated paper. figures 2a-d show the presence of j. luzardo et al. j. electrochem. sci. eng. 12(1) (2022) 137-151 http://dx.doi.org/10.5599/jese.1099 143 immobilized enzymes on the eg paper at different concentrations used: 0.5 mg ml-1 (fig. 2a), 2.0 mg ml-1 (fig. 2b), 5.0 mg ml-1 (fig. 2c) and 10.0 mg ml-1 (fig. 2d). it can be seen that there is a larger number of enzymes in these papers. as the concentration of enzymes in pbs buffer solution increases, they tend to agglomerate, which worsens their distribution on the paper surface. figure 2. afm (intermittent contact mode) images of graphene paper surfaces after enzyme immobilization, where eg with β-galactosidase enzymes are shown in: a) 0.5 mg ml-1, b) 2.5 mg ml-1, c) 5.0 mg ml-1 and d) 10.0 mg ml-1, and go with β-galactosidase enzymes are shown in: e) 0.5 mg ml-1, f) 2.5 mg ml-1, g) 5.0 mg ml-1 and h) 10.0 mg ml-1. afm images of go paper are shown in figures 2 e-h, for the enzyme concentration of 0.5 mg ml-1 (fig. 2e), 2.0 mg ml-1 (fig. 2f), 5.0 mg ml-1 (fig. 2g) and 10.0 mg ml-1 (fig. 2h). observing these images, it is possible to see the presence of enzymes on the surface but in smaller amounts compared to the eg paper. on eg papers, the enzymes are more dispersed, covering the paper surface (small white dots) better, while on go papers, clusters are formed (large white dots). since the lateral scale refers to heights, it is possible that in eg samples, these values do not vary so much, which is another evidence that there is a more homogeneous distribution of enzymes over the surface. in go samples, however, these heights are more inconsistent. this is because there are regions where enzymes are more agglomerated, and these clusters are formed in higher regions in relation to the support. consequently, the other regions tend to have smaller heights since most enzymes are not distributed on the surface but present in agglomerates. it is well known that enzymes in aqueous media like pbs buffer keep their hydrophobic sites closed [2,3,20]. however, in the presence of support with a hydrophobic surface, the enzyme opens this active site to adsorb on the support, a mechanism called interfacial activation of the enzyme [32]. eg paper has higher hydrophobicity than go paper, so it is expected that a larger number of enzymes would adsorb on eg paper surface, as could be seen from the afm images (fig. 2). to corroborate this result, water contact angle measurements were performed, and these results will be presented further. contact angle measurements the contact angle was used to verify the wettability of the paper surfaces. generally, the wettability of a solid surface is strongly influenced by its chemical composition and geometric structure (or surface roughness) and the angle that a drop of water makes with respect to a solid surface shows hydrophilicity or hydrophobicity of a material [33]. figure 3 shows the water contact angle (wca) measurements aiming to characterize the surface chemistry of the paper used as a support prior to enzyme immobilization. it was possible to observe that eg paper is less hydrophilic than go paper due to the higher value of eg paper contact angle http://dx.doi.org/10.5599/jese.1099 j. electrochem. sci. eng. 12(1) (2022) 137-151 graphene papers for enzyme immobilization 144 (55.6o). figures 3a and 3b compare eg paper contact angles with the value found for go paper (22.9o). the higher hydrophobicity of eg paper is due to the synthesis method. go paper that was synthesized via chemical oxidation route undergoes higher oxidation level than eg paper, which was synthesized via electrochemical oxidation route [34]. the wca measurements were also performed after the enzyme immobilization. in four tested concentration conditions, wca of eg papers are shown in figures 3c, 3d, 3e and 3f, while wca of go papers are shown in figures 3g, 3h, 3e and 3f. on these pictures, it was possible to notice significant changes in wca values after immobilization, especially in the go paper that before immobilization presented contact angle close to 20° (fig. 3b), and after immobilization reaches 74.1o (fig. 3i), in contrast with 55o of wca of eg paper before immobilization (fig. 3a) and 79.1° after immobilization (fig. 3 d). after the enzyme adsorption, the surfaces of these papers are modified, showing increased hydrophobicity. however, it is believed that with the increase of the enzyme concentration, a particle agglomeration occurs, leading to a poor distribution of the enzymes on the surface, which was already observed. fig. 3f shows the wca value for eg paper containing the enzyme in the concentration of 10 mg ml-1, where it is possible to notice that the particle distribution is so heterogeneous that in two measurements performed in different regions of the same surface, in one of them, the drop of water spread completely, and was not possible to measure the contact angle. in the other measurement, the drop presented a wca of 50.3o. the same effect of heterogeneity was observed for the go paper, where the highest enzyme concentration shows the lowest wca value, 21.7o (fig. 3j). this is supposed to be due to the formation of enzyme clusters because as concentration increases, the dispersion of enzymes becomes less homogeneous. figure 3. water contact angle measurements for: eg (a) and go (b) papers before enzyme immobilization; eg and go papers with enzymes on the surface in concentrations of: 0.5 mg ml-1 (c) and (g); 2.0 mg ml-1 (d) and (h), 5.0 mg ml-1 (e) and (i); 10.0 mg ml-1 (f) and (j) chemical and structural analysis (xps, raman and xrd) xps analysis provides valuable information on the oxidation level of eg and go papers. it can qualitatively estimate the percentage of carbon atoms in the basal plane and at the edges of the j. luzardo et al. j. electrochem. sci. eng. 12(1) (2022) 137-151 http://dx.doi.org/10.5599/jese.1099 145 graphene sheets. typically, epoxide, ether, phenol and hydroxyl groups are located at the basal plane, while carbonyl and carboxyl groups are located at the edges [35]. figures 4a and 4b show xps spectra of samples in the c1s region. 2 / o 2 / o figure 4. surface characterization of eg (left) and go (right) samples: xps spectra in c1s region (a) and (b); raman spectra (c) and (d); xrd patterns (e) and (f) the spectrum for c1s peak was decomposed into six components (gaussian/lorentzian function at 70:30 ratio) centered on the following energies: 284.6 ev (sp2, c=c bonding of graphite carbon chain), 285.5 ev (sp3, c-c bonding of graphite carbon chains) 286.7 ev (c-oh and coc, hydroxyl and epoxy groups, respectively), 287.4 ev (c=o, carbonyl), 288.8 ev (c-ooh, carboxyl) and 291.4 ev http://dx.doi.org/10.5599/jese.1099 j. electrochem. sci. eng. 12(1) (2022) 137-151 graphene papers for enzyme immobilization 146 (-* shake-up transition) that is a satellite peak component referring to the interaction of the valence shell electrons with the core electrons) [36]. xps survey spectra analysis shows that eg paper has a lower oxidation level than a typical graphene oxide material. eg showed the sp2/sp3 ratio of 1.5, while the sp2/sp3 ratio for go was 0.2. the satellite peak component, called “shakeup,” is a typical feature of delocalized electrons in aromatic systems [36]. this transition is used as an indication of graphitic structure, being present in sp2 carbon nanomaterials. the presence of ππ* shake-up satellite peak in c1s spectra of eg sample is due to the preservation of −sp2 delocalized electrons, which is absent in go as the defects generated by the oxidative process in the graphene sheets cause the loss of the delocalization of −sp2 electrons [37]. raman spectra of eg and go papers are shown in figures 4c and 4d. g band typically appears at 1580 cm-1, representing the plane elongation of carbon atoms in graphite structure [38-40]. the presence of disorder or defects in the graphene sheets, such as vacancies, oxygen-containing functional groups and/or adsorbed molecules, is revealed by the presence of d-band, centered close to 1350 cm-1 [41,42]. the bands centered close to 2700 and 2900 cm-1 are known as the secondorder contribution of d-band (named 2d) and the combination of g and d bands (named d + g), respectively [43]. the main parameters of d and g bands of eg and go papers were evaluated from raman spectra through peak fitting using lorentzian functions. table 2 presents the average values of the tuning parameters: frequency (f), fwhm (w) and relative intensity i(d)/i(g). the width of a raman band may be directly correlated with the disorder degree in a material [44]. the intensity ratio between the c-c stretching (g band) and the defect-induced (d band) modes can be used to measure crystallite sizes (la) only for samples with sizes larger than the phonon coherence length, which is found equal to 32 nm. in polycrystalline graphene systems, as the present case, the raman linewidth of the g band is ideal to characterize the crystallite sizes below the phonon coherence length, down to the average grain boundaries width, which is found to be 2.8 nm. eg sample presented lower g linewith than go, table 2, which indicates higher crystallite size in eg than go as well as lower defects density, following the model of la and ld described by cançado et al. [40]. table 2. mean values of d and g bands evaluated from the lorentzian peak fitting of raman spectra of eg and go samples sample fd / cm-1 wd / cm-1 fg / cm-1 wg / cm-1 id / ig eg 1344.1 96.4 1584.5 61.3 0.97 go 1347.6 126.9 1586.3 80.6 1.12 the xrd pattern of the eg powder sample (fig. 4e) shows the existence of the characteristic graphite bragg reflection plane (002), 2 = 26.5o for cukα radiation [46]. this value is equivalent to an interplanar distance around 0.34 nm. the wide peak centered at 26.5o may indicate some layers of graphene stacked in different orientations, as well as different interplanar spaces between them [46]. castro et al. [46] reported that the average lateral size of the initial graphene sheets is inversely correlated with the broadening of the xrd peak. this result reflects that smaller graphene sheets produce more unordered stacking sequences. in the case of the go paper (fig. 4f), a single peak in the region of 11o for cukα radiation can be seen, equivalent to an interplanar distance of 0.82 nm. this difference between interplanar distances between eg and go is due to the different oxygen intercalation between graphene sheets promoted by each synthesis process, which is much more intense in the chemical oxidative process than in electrochemical exfoliation. the permeation of oxygenated groups between the graphene sheets is responsible for this increase in the interplanar j. luzardo et al. j. electrochem. sci. eng. 12(1) (2022) 137-151 http://dx.doi.org/10.5599/jese.1099 147 distance. in the case of the eg paper, it is possible to notice that there are peaks also at smaller angles, corresponding to larger interplanar distances. electrochemical analysis previous results (contact angle and afm images) showed that eg paper exhibited a higher affinity to the enzyme when compared to go, resulting in more efficient enzyme immobilization. therefore, eg paper was chosen for catalytic tests aiming to the conversion of lactose into glucose. carbon electrodes are commonly used to perform electrochemical experiments. in addition, they have good electrical conductivity, thermal stability and robustness, and are highlighted as the most suitable materials for the design of electrodes with the modified surface [47]. electrode modification is as a way to add specific physical-chemical properties of the modifying agent to the original electrode, such as enhanced surface area, electrical conductivity, reactivity and selectivity [48]. to check if there was an enzyme action, it is necessary to know the peak position of lactose and some of the reaction products, which is glucose in this case. the voltammetry measurements were performed to detect the conversion of lactose to glucose, mediated by the β-galactosidase enzyme, responsible for the hydrolysis of lactose into glucose and galactose. two electrodes were tested, a carbon screen-printed electrode (cspe) and cspe modified with eg (egpe). oxidation peak positions of lactose and glucose analytes at these two electrodes are identified in table 3. table 3. peak positions of lactose and glucose analytes at cspe and egpe electrode peak potential, v lactose glucose cspe -0.489 -0.375 egpe -0.460 -0.337 knowing the peak positions of lactose and glucose makes it possible to track the catalytic reaction in different enzymatic conditions. the same initial test conditions were maintained for all voltammetry evaluations, with lactose and glucose concentrations of 1 mmol l-1. the lactose and glucose oxidation current peaks measured by cyclic voltammetry at cspe showed maximums at -0.489 and -0.375 v, respectively, while maximums at egpe were at -0.460 and -0.337 v, respectively (table 3). to understand the role of immobilization on the enzyme activity in this specific reaction of lactose conversion to glucose and galactose, the free enzyme activity was also monitored. corrected voltammograms (cf. experimental part) for cspe are shown in figure 5a, where it can be seen that when the enzyme is placed free in a solution containing lactose (green curve), there was a decrease in the lactose peak intensity compared to the response in solution without enzyme (red curve). this indicates a suppression in the lactose content in the solution, while a small shift of the oxidation peak (from -0.489 to -0.463 v) towards the glucose potential (-0.375 v) suggests that lactose hydrolysis is probably happening. this solution was stored, and a new measurement was performed after 48 hours. in the voltammetry experiment recorded in the solution containing the free enzyme after 48 h of reaction (orange curve), the peak appears centered at -0.395 v, referring to the glucose potential. the presence of this new peak and the absence of the lactose peak suggests that the enzyme present in the solution continued to perform its function of converting lactose into glucose. after 48 hours, all lactose has been converted and that is why the peak position was changed to glucose potential. the voltammetric responses evaluated with the egpe electrode under different conditions of enzyme immobilization are shown in figure 5b. the curve for the egpe-fe sample represents the condition when the enzyme is placed free in solution (green curve), and this voltammogram showed http://dx.doi.org/10.5599/jese.1099 j. electrochem. sci. eng. 12(1) (2022) 137-151 graphene papers for enzyme immobilization 148 the oxidation peak at -0.337 v referring to glucose oxidation, and no peak in the lactose region at 0.46 v (red curve). this shows that the conversion was efficient as there was no change in the peak height after 48 h of reaction (orange curve). when the enzyme is not immobilized but free in solution, its structure would not be stabilized to expose its active sites, which explains the low intensity observed. this is because even free in solution, the enzyme interacts with the electrode surfaces of cspe and egpe. the better interaction between the graphene at the egpe electrode surface with the enzyme's hydrophobic sites promotes the enzyme's stabilization since, in a biological environment, enzymes are typically linked to cellular structures aiming to their stabilization. figure 5. normalized voltammograms of cspe (a) and egpe (b) in: pure lactose solution (red), pure glucose solution (blue), lactose solution with free enzyme (green), lactose solution with free enzyme after 48 h of reaction (orange) and lactose with immobilized enzyme (purple) by comparing voltammograms of the enzyme immobilized on two electrodes, cspe and egpe, it was possible to observe that for cspe-ie (purple curve), no peak was detected, probably due to a poor chemical interaction between hydrophobic sites of the enzymes and cspe electrode surface. it is reasonable to assume that the enzyme's active site, responsible for the hydrolysis of the lactose to glucose, was inactivated by the linkage of the enzyme with cspe support, annulling the catalytic power of the enzyme. all modifications and measurements with the developed biosensors were done under strictly identical conditions. as expected, the reduction signal was improved when eg was present. using egpe-ie, it is possible to see that the conversion happens instantly with the appearance of the glucose peak at -0.273 v, corroborating the idea that eg improves the catalytic power of the β-galactosidase enzyme in the conversion reaction of lactose to glucose. as already discussed, enzymes interact with eg through their hydrophobic sites. when free in an aqueous solution, the enzyme tends to close its active sites due to the more hydrophilic environment. this process ends up interference with the conversion reaction of lactose into glucose. in the egpe-fe system, the enzymes that effectively participated in the conversion process were close to the surface of the modified electrode, which could interact with the surface and expose their active sites. thus, there is an increase in the glucose signal as soon as the electrode is placed in the solution since all electrode deposited enzymes have their active sites exposed and are prepared to start the conversion as soon as they come into contact with lactose. the glucose potential shift observed in the egpe-ie electrode voltammogram (fig. 5b, purple curve) is due to the higher energetic barrier in the reaction of conversion of lactose to glucose in the presence of the immobilized enzyme than in the case of free enzyme since the mobility of charge carriers is lower at the electrode surface (egpe-ie) than in electrolyte solution. the mobility of electrolyte ions is reduced in solid-state as the j. luzardo et al. j. electrochem. sci. eng. 12(1) (2022) 137-151 http://dx.doi.org/10.5599/jese.1099 149 graphene paper electrode surface, increasing the activation energy to convert lactose to glucose. consequently, the voltammetric peak is shifted from 0,337 v to 0,273 v, as observed in figure 5b. conclusions in this work, the synthesis of graphene papers was performed by two different routes: chemical synthesis and electrochemical exfoliation. the immobilization of the β-galactosidase enzyme was carried out on two synthesized papers having different surface chemical properties, eg (more hydrophobic) and go (more hydrophilic). the enzyme immobilization efficiency was dependent on the hydrophilicity of papers and the concentration of enzymes in the electrolyte solution. higher catalysis efficiency was observed when the eg paper was chosen as the support. compared to go paper, eg paper has more hydrophobic active sites, which are preferentially available to interact with the hydrophobic sites of enzymes. in addition, it was observed that high enzyme concentration in the solution is unfavorable since it promotes enzyme agglomeration and, consequently, a poor distribution over the substrate. cyclic voltammetry evaluation shows that graphene oxide produced by electrochemical exfoliation is the better substrate for immobilization. the electrochemical tests showed improved results for the egpe electrode in comparison with the cspe electrode. the immobilized enzyme on the egpe electrode showed instantaneous lactose to glucose conversion reaction, with the appearance of the glucose peak at -0.273 v, corroborating the idea that eg improves the catalytic power of the β-galactosidase enzyme. this result could be explained by considering that eg functional groups at the basal plane and edges of graphene defects help electron transfer to the redox sites of the enzymes, favoring the detection of products by the electrodes. in addition, the method used was adsorption to the substrate, which 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distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1099 https://doi.org/10.3389/fmats.2014.00039 https://doi.org/10.1021/la902496u https://doi.org/10.1038/351491a0 https://doi.org/10.1002/adma.200903696 https://doi.org/10.1002/adma.200903696 https://doi.org/10.14445/22315381/ijett-v49p220 https://doi.org/10.1038/nnano.2009.58 https://doi.org/10.1016/j.carbon.2015.11.059 https://doi.org/10.1016/j.tsf.2016.04.042 https://doi.org/10.1016/j.tsf.2016.04.042 https://doi.org/10.1063/1.4756995 https://doi.org/10.1016/j.ultsonch.2012.09.007 https://doi.org/10.1016/j.carbon.2015.08.020 https://doi.org/10.1021/nl201432g https://doi.org/10.1080/00018732.2011.582251 https://doi.org/10.1016/j.carbon.2018.11.070 https://doi.org/10.1016/j.carbon.2018.11.070 https://doi.org/10.1103/physrevb.82.125429 https://doi.org/10.1103/physrevb.61.14095 https://doi.org/10.1103/physrevb.61.14095 http://doi.org/10.20964/2018.01.02 http://doi.org/10.20964/2018.01.02 https://doi.org/10.1149/1.1838337 https://doi.org/10.1021/ac60362a043 https://doi.org/10.1021/ac60362a043 https://creativecommons.org/licenses/by/4.0/) development of an electrochemical sensor for the determina-tion of the total antioxidant capacity in berries based on boron doped diamond doi: 10.5599/jese.2012.0024 1 j. electrochem. sci. eng. 3(1) (2013) 1-9; doi: 10.5599/jese.2012.0024 open access : : issn 1847-9286 www.jese-online.org original scientific paper development of an electrochemical sensor for the determination of the total antioxidant capacity in berries based on boron doped diamond bruna pekec* , ***, birgit feketeföldi**, volker ribitsch**, astrid ortner*** and kurt kalcher* *institute of chemistry, department of analytical chemistry, karl-franzens university, graz, austria **joanneum research forschungsgesellschaft mbh materials, weiz, austria ***institute of pharmaceutical science, department of pharmaceutical chemistry, karl-franzens university, graz, austria corresponding author: e-mail: kurt.kalcher@uni-graz.at, stremayrgasse 16/iii, 8010; tel.: +43 (0)316 380 5310; 5313; fax: +43 (0)316 380-9845 received: august 28, 2012; revised: october 18, 2012; published: november 06, 2012 abstract many antioxidants can be electrochemically oxidized using graphite-based electrodes; nevertheless problems arise due to the strong adsorption of redox species at the sensing area. we have demonstrated that boron doped diamond (bdd) electrodes do not show this property, which can be exploited for the design of a new amperometric sensor for the quantification of antioxidants as “total antioxidant capacity” (aoc). as reference substances hydroquinone (hq) and 6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (trolox) were studied in more detail. the supporting electrolyte was a phosphate buffer solution (pbs, 0.1 mol/l, ph 7.0). the limits of detection (lod) were 1.5 mg/l and 2.5 mg/l for hq and trolox, respectively. the repeatability was 3 % rsd for concentration of 200 mg/l hq. the method could be applied for the determination of aoc in different berry samples, such as strawberry, blueberry, grape and bramble. a comparison with a standard photometric assay showed good correlation between both methods. the bdd sensor features good reproducibility without fatiguing over at least two months of operation. keywords boron doped diamond electrode; total antioxidant capacity; amperometric sensor; berry extracts. http://www.jese-online.org/ mailto:kurt.kalcher@uni-graz.at j. electrochem. sci. eng. 3(1) (2013) 1-9 sensor total antioxidant capacity 2 introduction the term “antioxidant” identifies a huge number of substances, in majority of cases phenolic or polyphenolic substances that inhibit the oxidation of molecules. they are strongly reducing agents that neutralize reactive oxygen species (ros). these ros are oxidizing radicals or molecules that are formed during the respiratory chain and cause damage on cell own material. antioxidants neutralize these species and therefore they are of enormous scientific interest. the free scavenger effect of antioxidants has been in use for many years to prolong the stability of lipid foods especially of edible oils. reports go back to 1947 where butylated hydroxyanisole was used as the first antioxidant for the prevention of the deterioration of fat-containing food [1-2]. nowadays antioxidants are also in the medicinal focus because of their tissue protecting effects by neutralization of ros. therapeutic progress has been made with many diseases that show a high degeneration rate, such are diabetes, coronary heart failure or alzheimer disease [3-5]. ever since in 2003 sinclair et al. found out that resveratrol, an antioxidant occurring in grape, extends the lifespan of yeast cells and causes a decrease of typical aging effects, antioxidants became also interesting for cosmetic branches and for anti-aging research [67]. antioxidants are mostly secondary plant metabolites. especially berries, such as grape, red or black currant, bramble or raspberries, are known to form high amounts of flavonoids. their synthesis is a response of the plant to stress attacks like strong light exposure or fungal or microbial infection. in the latter case, the plant uses them as phytoalexines for tissue protection. unfortunately, isolated antioxidants are highly sensitive to light, temperature and oxygen. although natural antioxidants are an interesting tool to prevent lipid products, e.g. oily food or cosmetics, to become rancid, their stabilization is still a problem to be solved. responding to a growing interest some analytical methods, mostly photometric assays, were developed. standardized techniques are the orac-assay (oxygen radical absorbance capacity) involving hydrogen transfer reactions, the folin-ciocalteu-assay and the teac-assay (trolox equivalent antioxidant capacity) that base on electron transfer reactions [8]. although these methods have been well approved they are rather complex, time-consuming and use high priced chemicals or test kits. most antioxidants show electrochemical activity and could be determined using voltammetric or amperometric methods. unfortunately, during the electrochemical process reduced and oxidized species adsorb very strongly on the surface of many electrode types, such as graphite or carbon paste electrodes (cpe). diamond electrodes offer an alternative approach. ever since in 1987 pleskov et al. [9] commenced studies on the electrochemistry of boron doped diamond (bdd) their development and fundamental research gained rising tendency. in contrast to graphite electrodes the c-sp 3 hybridization and tetrahedral bonding of boron doped diamond electrodes (bdde) leads to properties like chemical inertness, hardness, thermal conductivity and low fatigue [10]. their main field of application is the treatment of wastewater [11]. the potential window of bddes is much wider (-0.75 to +2.35 v) than of graphite electrodes (-0.5 to +1 v) because neither oxygen nor hydrogen evolution interfere in the analysis. because commercial bddes are usually synthesized by chemical vapor deposition (cvd) they contain non-diamond and metallic impurities [10]. also, surface oxygen can cause disturbances and it needs to be removed. for surface cleaning and pretreatment of bddes a few suggestions, like acid washing or anodic polarization, have been offered [12,13]. b. pekec et al. j. electrochem. sci. eng. 3(1) (2013) 1-9 doi: 10.5599/jese.2012.0024 3 in this work the pretreatment of boron doped diamond electrodes by polarization in na2so4 at +1 v, and the use of this electrode for the electrochemical determination of antioxidants in ethanolic berry extract, was investigated. experimental chemicals and materials the investigated target analytes were phenolic and polyphenolic ingredients of different berries (grape, bramble, blueberry, strawberry) that are known to show antioxidative potential [14]. therefore ferulic acid, vanillic acid, syringic acid, coumaric acid, sinapic acid, caffeic acid and phydroxybenzoic acid, all purchased from sigma-aldrich gmbh (vienna, austria), were selected. trolox [(±)-6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid] and hydroquinone (hq) were chosen as reference substances and were purchased, like all other standards, from sigmaaldrich gmbh (vienna, austria). to prepare the standard solutions the appropriate amount of substance was dissolved in deionized water. the aqueous solutions were prepared freshly before use, kept cool and protected from light. phosphate buffer (na2hpo4/nah2po4 0.1 mol/l, ph 7) was selected as electrolyte medium. apparatus all electrochemical experiments were performed using potentiostat (metrohm autolab ® pgstat302n) in a three electrode arrangement controlled by the corresponding software (nova 1.6). the bddes purchased from fraunhofer (michigan, usa), plated on titanium substrate with gold as a contact, a saturated calomel electrode (sce) and a platinum wire were used as working, reference and auxiliary electrode respectively. all potential values in this work are given against sce. cyclic voltammetry and hydrodynamic amperometry were chosen as techniques to obtain the electrochemical data. in the case of cyclic voltammetry the explored potential window was between -1 v and +1.2 v. the other operating parameters were: scan rate: 50.0 mv/s, step potential: 2.0 mv and time interval: 0.050 s. for hydrodynamic amperometry measurements the characteristic potential of the oxidation current peak was selected. measuring procedure the bdde was conditioned at +1 v for 1 h in 0.1 mol/l na2so4 solution prior to the start and also between the measurements to remove impurities and for the baseline correction. all experiments were carried out in 20 ml phosphate buffer (0.1 m, ph 7) after degassing with argon. aliquots of the standard solutions were added to the cell and cyclic voltammograms and hydrodynamic amperograms, respectively, were recorded by applying the aforementioned parameters. photometric analysis the photometric trolox equivalent antioxidant capacity assay (teac-assay) was selected as a reference method. according to the report of li et al. [15] the redox indicator 2,2´-azinobis-(3-ethylbenzthiazolin-6-sulfonic acid) (abts) forms a radical in contact with potassium persulfate. the abts radical cation is of green color which decreases in intensity upon addition of substances with antioxidative potential. the solution was diluted using phosphate buffer saline (ph = 7) until it showed an absorption of 0.700 ± 0.050 at 734 nm. to 3 ml of that solution 100 µl of the sample were added and the absorption decrease was measured. for the calibration trolox and hq j. electrochem. sci. eng. 3(1) (2013) 1-9 sensor total antioxidant capacity 4 solutions of 0.1, 0.2, 0.3 and 0.4 mol/l were used and the aoc of the berry samples was calculated as troloxor hydroquinone-equivalent (te, hqe). preparation of the samples the berry marcs (donated from grünewald fruchtsaft gmbh) were dried at 50 °c. for the recovery of the phenolic compounds the milled raw material was extracted with a mixture of absolute ethanol and water (75:25 v/v) at a 8:1 solvent-to-sample ratio (volume to mass of ovendried marc). the extraction was performed using sonication at a temperature of 50 °c for 15 min. after centrifugation (7000 rpm, 20 min) the extract was then filtered through a glass fiber filter (type ap2029325, millipore) and stored at 4 °c. results and discussion cyclic voltammetry cyclic voltammetry (cv) was basically used to determine the oxidation potential of the standard berry antioxidants with cpes and bddes (table 1). the quoted potentials correspond to the maximum of the oxidation current registered in the anodic scan. typical cvs of hydroquinone and trolox are shown in fig. 1. as can be seen the electrochemical behavior of both substances on both electrodes is rather similar with the exception of carbon paste electrode where the currents are significantly higher. this behavior is due to the adsorption of the analyte at cpe and therefore due to the strong  electron interaction of the aromatic ring with the graphite structure. both substances were used as reference compounds, by converting the response of other antioxidants to trolox(te) and hydroquinone-equivalents (hqe), respectively. figure 1. cyclic voltammograms of 30 mg/l (a) trolox and (b) hq measured in pbs (0.1 mol/l, ph 7) on• bdde and • cpe. the investigated antioxidants (table 1) produce typical peak currents in cv at bddes in the range of 4 to 8 ma mol -1 with the exception of hq which is about four times higher; probably because hq ideally interacts with the graphite surface finally yielding benzoquinone, whereas a b b. pekec et al. j. electrochem. sci. eng. 3(1) (2013) 1-9 doi: 10.5599/jese.2012.0024 5 other oxidants are reduced to the corresponding r-o radical with ensuing follow-up effects. again it is evident that carbon paste electrode favors adsorption of the oxidation product which exerts a positive effect for the first cycle in cyclic voltammetry but it is deteriorating the signals in further scans and in amperometry (see below) by blocking the active surface area. since the sensors based on boron-doped diamond should be used for alcoholic extracts of berries, the influence of ethanol on the signal was investigated. concentrations from 0.5 to 20 % ethanol were added to the supporting electrolyte; no influence on peak position or peak height was observed. table 1. oxidation potentials, epa, and peak currents, ipa, from cyclic voltammetry of typical antioxidants found in berries. concentration: 30, respectively 90 mg/l, supporting electrolyte: pbs (0.1 mol/l, ph 7); potential window: -1.5 to +1.5 v, scan rate: 50 mv/s. substance formula cpe bdde epa / v ipa / ma mol -1 epa / v ipa / ma mol -1 syringic acid 0.95 23.3 0.80 8.2 sinapic acid 0.70 15.2 0.65 4.3 caffeic acid 0.48 10.2 0.50 7.5 vanillic acid 0.88 78.6 0.88 4.9 ferulic acid 0.50 19.2 0.50 7.9 coumaric acid 0.90 37.5 0.75 5.1 p-hydroxybenzoic acid 0.90 148.5 1.0 6.9 trolox 0.50 14.2 0.50 6.2 ascorbic acid 0.61 56.4 0.54 36.4 hydroquinone 0.50 44.0 0.50 26.7 http://www.google.at/imgres?q=trolox&um=1&hl=de&sa=n&biw=1024&bih=743&tbm=isch&tbnid=kooh3age4ddodm:&imgrefurl=http://www.lookchem.com/trolox-c/&docid=fgka14gbpzmrdm&imgurl=http://www.lookchem.com/300w/casimage/2011-01-08-16/56305-04-5.gif&w=300&h=300&ei=ms29tqrihamybqii3jyd&zoom=1 http://www.labspaces.net/101359/new_year__new_vitamin_c_discovery__it__cures__mice_with_accelerated_aging_disease http://www.google.at/imgres?q=hydroquinon&um=1&hl=de&sa=x&biw=1024&bih=743&tbm=isch&tbnid=foisxc8dzfzt0m:&imgrefurl=http://dorisdalton.blogspot.com/2009/07/snow-white-and-her-seven-whitening.html&docid=ljd6zle81mowqm&imgurl=http://2.bp.blogspot.com/_kyhgz5nbxzk/sl9swhpmzii/aaaaaaaaak4/-48taoqdlg0/s320/hyroquinone.png&w=300&h=300&ei=qi69tuk9jmr1sgbrqmgaaw&zoom=1 j. electrochem. sci. eng. 3(1) (2013) 1-9 sensor total antioxidant capacity 6 hydrodynamic amperometry basic studies hydrodynamic amperometry was applied to the antioxidants listed in table 1 applying a working potential which corresponds to the maximum of the oxidation current in cv. after attaining a stable baseline the analytes were added to a stirred solution. figure 2 shows a comparison of the electrochemical behavior of hq between bdde (a) and cpe (b) electrodes under identical conditions. as can be seen a strong adsorption of the quinone on the cpe leads to a fast decrease of the signal due to blocking of the active electrode surface area with a very small current steps after ensuing additions. bdde does not show any obvious adsorption therefore, after addition of the electrochemically active analyte, the signal remains constant. repetitive additions lead to clear step-like increases of current. for this reason bdde seems to be an ideal electrode material for the determination of phenolic antioxidants due to the absent tendency for adsorption. a b figure 2. hydrodynamic amperograms of subsequent additions of 30 mg/l hq in phosphate buffer (0.1 m, ph 7) using (a) bdde and (b) cpe. working potential: +1v. validation of analytical parameters the analytical method based on hydrodynamic amperometry was evaluated with hydroquinone and trolox as standards. under optimized conditions the linearity range of the bdd-sensor was estimated to be between 10 and 400 mg/l for trolox and 10 to 300 mg/l for hq (fig. 3). the calibration curves can be described by the following regression equations: i hydroquinone / µa = 43.04 × γ / (mg/l) + 0.387 and itrolox / µa= 22.09 × γ / (mg/l) + 0.609, both with a mean correlation coefficient of r = 0.999. for the concentrations above 400 mg/l (trolox) and 300 mg/l (hq) the calibration graph levels off but still remains linear to concentrations above 1 g/l, in fact with a very small sensitivity. the limit of detection (lod) and the limit of quantification (loq estimated as the three-fold of lod) were determined as 1.5 mg/l and 4.5 mg/l for hq, and 2.5 mg/l and 7.5 mg/l for trolox according to the international conference on harmonisation, ich, quality guidelines for analytical evaluation [q2(r1)] guidelines for visual evaluation [16]. b. pekec et al. j. electrochem. sci. eng. 3(1) (2013) 1-9 doi: 10.5599/jese.2012.0024 7 the precision experiments (100, 200, 375, 625, 750 and 875 mg/l; n = 4) showed a relative standard deviation (rsd) of 3-8 % and a rsd of the slope of 3 %. the intercept of the calibration curves ranged from around 0.4 (hq) to 0.6 µa (trolox) with a rsd of about 5 % in different calibrations (n = 4). figure 3. calibration curve of the bdd-sensor 10-1200 mg/l concentration range of • hq and • trolox. application of the sensor in samples after it was demonstrated that the bddes showed only insignificant adsorption effects to the redox products of the investigated antioxidants they were applied to chosen samples. thus, ethanolic extracts were prepared using red grape, bramble, blueberry and strawberry. the extracts were injected directly to the analytical cell without further dissolution or preparation. 20 ml of phosphate bufferwas transferred into the electrochemical cell and after the baseline stabilization, 100 µl of the extract was injected. the extracts were analyzed using hydrodynamic amperometry at +1 v and the aoc was estimated as trolox equivalent (te) or hydroquinone equivalent (hqe) using the corresponding calibration curves. the capacities of the berry samples were determined in the range of 8 and 23 mm te/hqe (table 2) and are in a good agreement with the results of the photometric reference method. table 2. comparison of the electrochemical and photometric total antioxidant capacity measurements of berry extracts. trolox equivalent, mmol/l hydroquinone equivalent, mmol/l teac assay electrochemical bdd sensor teac assay electrochemical bdd sensor grape 23.4 ± 1.2 23.9 ± 1.2 22.6 ± 1.1 23.3 ± 1.2 bramble 12.4 ± 0.6 12.4 ± 0.6 12.1 ± 0.6 13.4 ± 0.7 blueberry 18.5 ± 0.9 18.1 ± 0.9 18.4 ± 0.9 17.9 ± 0.8 strawberry 8.9 ± 0.4 8.9 ± 0.4 8.8 ± 0.4 9.1 ± 0.5 figure 4 shows a comparison between electrochemical signals determined using the bdd sensor and the decrease of absorption (δa) measured using the teac-assay. it can be seen clearly that there is a directly proportional relation between the absorption signal decrease and j. electrochem. sci. eng. 3(1) (2013) 1-9 sensor total antioxidant capacity 8 amperometric currents. the method employing electrochemical sensors seems well applicable to detect the antioxidant capacity of ethanolic berry extracts. figure 4. comparison of electrochemical data recorded in current intensity and photometric results charted as decrease of absorption. our studies show that the evaluated trolox and hydroquinone equivalents from amperometric measurements with bddes are correlating well with the optical reference method (table 2). this was also verified with individual antioxidants (table 1) which produce the same antioxidant equivalent capacities with both procedures, amperometric and optical. maybe, in complex samples, substances which can be oxidized at an operational potential of 1.0 v could be present and do not contribute to the antioxidant capacity. in fact, we could not, so far, observe such a behavior with ethanolic extracts of berries. conclusions the investigations show that boron doped diamond used as working electrode in electrochemical analytics is an attractive tool because of its many advantages. due to its low feature for the adsorption of redox products it can be used for quantification of antioxidants even in complex material. the designed sensor is inexpensive because it can be used for repetitive measurements, easy to prepare, and with high reproducibility . acknowledgements: the financial support of the österreichische forschungsförderungsgesellschaft (ffg) for project “antiflavo” (no.827334) is highly acknowledged. references [1] e.r. sherwin, j. am. oil chem. soc. 55 (1978) 809-814 [2] e.n. frankel, food chem. 57 (1996) 51-55 [3] h. kaneto, y. miyagawa, t. matsuoka, y. fujitani, y. umayahara, t. hanafusa, y. matsuzawa, y. yamasaki, m. hori, diabetes 48 (1999) 2398-2406 [4] k.l. hamilton, j.l. staib, t. phillips, s.l. lennon, s.k. powers, free radical biol. med. 34 (2003) 800-809 [5] m.j. engelhart, m.i. geerings, a. ruitenberg, j.c. van swieten, a. hofman, j.c.m. wittermann, m.m.b. breteler, j. amer. med. assoc. 287 (2002) 3223-3229 [6] k.t. howitz, k.j. bittermann, h.y. cohen, d.w. lemming, s. lavu, j.g. wood, r.e. zipkin, p. chung, kisielewski, l.l. zhang, b. scherer, d.a. sinclaire, nature 425 (2003) 191-196 b. pekec et al. j. electrochem. sci. eng. 3(1) (2013) 1-9 doi: 10.5599/jese.2012.0024 9 [7] d. valenzano, e. terzibasi, t. genade, a. cattaneo, l. domenici, a. cellerino, curr. biol. 16 (2006) 296-300 [8] r.l. prior, x. wu, k. schaich, j. agr. food chem. 53 (2005) 4290-4302 [9] y.v. pleskov, a.y. sakharova, m.d. krotova, l.l. bouilov, b.v. spitsyn, j. electroanal. chem. 228 (1987) 19-27 [10] j.h.t. luong, k.b. male, j.d. glennon, analyst 134 (2009) 1965-1979 [11] m.a.q. alfaro, s. ferro, c.a. martinez-huitle, y.m. vong, j. brazil. chem. soc. 17 (2006) 227236 [12] m.c. granger, m. witek, j. xu, j. wang, m. hupert, a. hanks, m.d. koppang, j.e. butler, g. lucazeou, m. mermoux, j.w. strojek, g.m. swain, anal. chem. 72 (2000) 3793-3804 [13] h.b. suffredini, v.a. pedrosa, l. codognoto, s.a.s. machado, r.c. rocha-filho, l.a. avaca, electrochim. acta 49 (2004) 4021-4026 [14] m. monagas, b. bartolomé, c. gómez-cordovés, food sci. nutr. 45 (2005) 85-118 [15] h.b. li, k.w. cheng, c.c. wong, k.w. fan, f.chen, y. jiang, food chem. 102 (2007) 771-776 [16] validation of analytical procedures – methodology, federal register 62 (1997) 27463 © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ removal of arsenic and cod from industrial wastewaters by electrocoagulation doi: 10.5599/jese.2011.0002 55 j. electrochem. sci. eng. 1(1) (2011) 55-65; doi: 10.5599/jese.2011.0002 open access : : issn 1847-9286 www.jese-online.org original scientific paper removal of arsenic and cod from industrial wastewaters by electrocoagulation s. zodi, o. potier, c. michon, h. poirot, g. valentin, j. p. leclerc and f. lapicque laboratoire réactions et génie des procédés, cnrs – nancy université , ensicbp 20451, f-54001 nancy, france corresponding author: +33 0 383 175 266, francois.lapicque@ensic.inpl-nancy.fr received: march 21, 2011; revised: may 09, 2011; published: august 20, 2011 abstract the paper deals with the treatment of arsenic-containing industrial wastewaters by electrocoagulation. the waste issued from a paper mill industry downstream of the biological treatment by activated sludge was enriched with arsenic salts for the purpose of investigation of the treatment of mixed pollution. first, the treatment of single polluted waters, i.e. containing either the regular organic charge from the industrial waste or arsenic salts only, was studied. in the case of arsenic-containing waters, a broad selection of experimental data available in the literature was compiled and interpreted using an adsorption model developed previously. the same technique was used in the case of industrial waste. arsenic-enriched paper mill wastewaters with various amounts of as salts were then treated by electrocoagulation with fe electrodes. the set of data obtained were interpreted by a model developed on the basis of the separate models. the agreement between predicted and experimental variations of the as concentrations ranging from 0.3 µg/l to 730 µg/l showed that both the organic matter and as salt can be removed from the liquid independently from each other. keywords electrocoagulation; industrial wastewaters; arsenic; adsorption model introduction arsenic (as) is one of the most toxic elements that can be present in wastewaters and environment in general. therefore, the treatment of groundwater has been investigated for decades for the possible use of existing water stocks with appreciable as contents [1], in particular of those close to the mines, as reported by leist et al. [2]. http://www.jese-online.org/� mailto:francois.lapicque@ensic.inpl-nancy.fr� j. electrochem. sci. eng. 1(1) (2011) 53-63 removal of as and cod by electrocoagulation 56 treatment of as-containing solutions, being of natural origin or prepared by dissolving chemicals in pure water, has been tackled for years. it seems that as compounds particularly those in the pentavalent form exhibit interesting sorption capacities on fe(iii) hydroxides. adsorption also occurs on al(iii) hydroxides, although not as strongly as on fe(iii) species [1,3,4]. coagulation and electrocoagulation, therefore, seem interesting and suitable methods for the treatment of arsenic-containing species. the two treatment techniques do not differ very much. in both cases, the treatment consists of two steps: (i) supply of the liquid to be treated with fe(iii) species, either by fe electrode dissolution to fe(ii) and subsequent oxidation to fe(iii) upon air oxidation or by addition of fe salts, (ii) complexation-adsorption of as species on fe(iii) species. since the treatment is often carried out at ph above 3, trivalent fe species are in the form of solid hydroxide fe(oh)3. the largest part of research on this subject has been focused on potabilization of water [2,3,5,6]. although the content of as-salts in the analyzed waters was as large as 100 mg/l, the chemical oxygen demand (cod) content of the solutions to be treated was very low and the presence of other species could be disregarded. in this study, we investigated the electrocoagulation treatment of as-containing industrial wastes for abatement of the cod and as species by using dissolution of iron anodes. electrocoagulation technique relies on the destabilization of suspended matter by reduction of the absolute value of the zeta potential by the presence of electrogenerated trivalent metals. this technique has been used in and investigated for the treatment of various wastewaters [1,3,5,7,8]. the waste generated from paper mill industry has a slight arsenic content at ppb level. the addition of arsenic salts to the original waste makes it possible to investigate the treatment of mixed wastewaters. for this purpose, the published data on the treatment of single solutions of arsenic salts were compiled and analyzed using a simple model for electrocoagulation published previously [9] and relying upon two parameters. moreover, the treatment of the industrial wastewater with negligible concentrations of as allowed for the estimation of the two parameters related to the cod abatement. numerous electrocoagulation tests with the waste after the addition of various amount of as species were conducted. the interpretation of the data was made taking into account the presence of the two pollutants to examine whether the presence of as could affect cod removal in the electrocoagulation process, as it has been observed in the case of cr-containing industrial solutions [7]. experimental wastewater samples and chemicals the wastewater was collected from a local paper mill industry (clairefontaine, france) downstream of a primary settling stage and biological treatment by activated sludge. the waste already used in the previous investigation [10] had ph=7.7, cod=285 mg o2/l, turbidity = 35 ntu, and a very low as content near 3.8 µg/l. the cod level was mainly due to the presence of dissolved or suspended organic matters. because of the relatively high conductivity of the wastewater, 1.22 ms cm-1, no supporting electrolyte, such as sodium chloride, had to be added. the concentration of arsenic species was enhanced by adding small amounts of arsenic acid disodium salt, na2haso3 (purissimum, fluka), to the wastewater. arsenic concentration in the wastewater obtained was below 1 mg/l. the moderate cod of the industrial waste allows to have a reasonable organic pollution to as pollution ratio, so that interactions between as and the organic pollution could be investigated. electrochemical reactor treatment runs have been carried out batchwise by recirculation of 2.5 liters wastewater in the flow rig consisting of a reservoir tank, peristaltic pump, flow meter, and an electrochemical cell s. zodi et al. j. electrochem. sci. eng. 1(1) (2011) 53-63 doi: 10.5599/jese.2011.0002 57 [9,10]. the electrochemical cell had a rectangular cross-section and was provided with two flat facing fe electrodes, being 15x7 cm2. the electrode gap was maintained at 2 cm and the liquid was circulated at 300 cm3 min-1. runs were carried out for periods of time ranging from 45 to 90 minutes, depending on the current density applied, which ranged between 5 and 20 ma cm-2. the cell voltage was monitored along the run.. pollutants analysis and quantification liquid fractions of 10 cm3 were collected at regular intervals along the runs. the concentration values of the various species were corrected for the change in the liquid volume caused by the regular sampling. conductivity and ph were monitored using a multi-parameter instrument (consort c931). a small volume of each sample was acidified by nitric acid in order to determine the fe and as concentrations by icp-ms (series x7, thermo) after suitable dilution of the acidic solution. for arsenic species, a small liquid volume was submitted to microwave digestion using a start-d instrument prior to injection to the icp-ms instrument. the rest of each sample was allowed to settle for at least twelve hours before the analysis of the clear fraction was carried out. the turbidity was measured using an ir beam at 890 nm and detection of the light dispersion using a hanna ins. lp2000 spectrophotometer, and cod was determined by the standardized colorimetric method after high temperature oxidation with excess chromic acid and subsequent measurement of the optical density at 530 nm using a hach 2400 spectrophotometer. the estimated accuracy in the cod measurement was 25 ppm. electrocoagulation of the industrial waste the industrial waste with a low content of as species was submitted to electrocoagulation at two levels of the current density. as expected, the cod level was observed to decrease regularly with time in spite of the slight scattering of the data due to the uncertainty in the assay. moreover, electrocoagulation did not allow the entire removal of the organic charge since the final cod level was usually at approximately 50% of the initial value, even upon longer electrocoagulation runs. the concentration of dissolved iron increased regularly during the run. the overall current yield of the dissolution at time t was calculated as follows: fe fe fe [fe(iii)]t vn fφ m ait = (1) where v is the volume of the wastewater under treatment, nfe the number of electrons involved in the dissolution, taken as 2 because the anode dissolution leads to fe(ii) species, which rapidly oxidizes to fe(iii) in the presence of air oxygen. in equation (1), mfe is the molecular weight of iron, i the current density applied, and a the geometrical area (105 cm2). the current yield was observed to increase slightly over time, from 0.5 in the first minutes and reaching 0.8-0.85 after 30-40 minutes. this could be caused by partial inhibition of the iron electrode immersed in the liquid in the early stage of the run, as the electrode probably exhibited only moderate corrosive properties. as often reported, the reduction in cod level is mainly governed by the amount of coagulant generated at the anode surface (fig. 1). the experimental variation was modeled using a previous overall model [11] relying upon the instantaneous adsorption or complexation of the pollutant charge – expressed by the cod – with fe(iii) hydroxide. the presence of as species below 4 µg/l was neglected here. fe(iii) complexes organic matter, expressed by the cod level, s, as follows: fe(iii) + n1 s ↔ fes (2) j. electrochem. sci. eng. 1(1) (2011) 53-63 removal of as and cod by electrocoagulation 58 where coefficient n1 is expressed in mg/l o2 per mg/l fe species, since s is expressed by the cod level. for the sake of simplicity, the complex formed in the treatment is written fes, regardless of n1 value. in this section, fe(iii) and s represent free species. it can be observed at this level that numerous unknown elementary processes are involved in the overall treatment. moreover, it is not clear whether the abatement of the pollutant is governed only by absorption and what are the molecular structures of coagulants and pollutants. because of this, we defined a simplified apparent equilibrium constant k1 simply written as: [ ] [ ][ ]1 fes fe s k = (in l/mg) (3) where coefficient n1 is expressed in mg/l o2 per mg/l fe species, since s is expressed by the cod level. for the sake of simplicity, the complex formed in the treatment is written fes, regardless of n1 value. in this section, fe(iii) and s represent free species. it can be observed at this level that numerous unknown elementary processes are involved in the overall treatment. moreover, it is not clear whether the abatement of the pollutant is governed only by absorption and what are the molecular structures of coagulants and pollutants. because of this, we defined a simplified apparent equilibrium constant k1 simply written as: [ ] [ ] [ ] 0 1 fes s s n = + (4) [ ] [ ] [ ]fe fe fes t = + (5) where [s]0 expresses the initial cod level, and [fe]t represents the concentration of the generated fe(iii) covering both free and complexed fe(iii) forms. as a matter of fact, a fraction of the pollutant matter with the concentration [s]f cannot be treated, so only fraction ([s]-[s]f) has to be considered in the adsorption equilibrium constant. equations (2)-(5) led to the expression for the untreated s concentration: [ ] [ ]( ) [ ] [ ]( ) [ ]( )2 f0t f f0 0 1 1 1 1 1 f 4 s [s]fe fe1 1 s [s] s [s] [s] [s] 2 t n k n k k −    − − − + + − − + +          = + (6) fitting of the experimental variation of the cod led to the estimates for k1 and n1, even though a narrow-valley situation in the optimization procedure was encountered. as formerly observed in the treatment of other industrial wastes [12], moderate cod levels in relation to ratio (k1/n1) resulted in the higher accuracy obtained in determination of the ratio. nevertheless, the two parameters could be estimated at n1 = 2.1 mg o2 per mg fe, and k1 = 0.029 l/mg (fig. 1). electrocoagulation of pure solutions of arsenic salts before the compilation of published experimental data on as removal by electrocoagulation from waters possessing very low cod or turbidity levels, physicochemical properties of arsenic salts are briefly discussed. s. zodi et al. j. electrochem. sci. eng. 1(1) (2011) 53-63 doi: 10.5599/jese.2011.0002 59 figure 1. cod abatement from the industrial waste. the model considered is expressed by rel. (4) general features of as species in wastewaters arsenic can be encountered in the form of either organic compounds with methyl groups or inorganic species. the most stable forms in ores or waste are arsenious salts (the trivalent form) and arsenate salts (pentavalent form). as(iii) form is obtained from arsenious acid, haso2, or its hydrated form, h3aso3 [13]. the two acids exhibit weak acidity, with a pka near 9.2. as(v) is obtained from arsenic acid, h3aso4. the acidity constants of the triacid are near 2.25, 6.77, and 11.60 [8,14]. therefore, for electrocoagulation of as-containing wastewater with an initial ph near 7 and increasing up to 9 in the course of the treatment, as mainly has the following forms: h3aso3 with slight amounts of h2aso3 for as(iii) and h2aso4 and haso4 2ions for as(v). the latter two species are prone to adsorption on fe(iii) hydroxide [15,16]. pentavalent salts might be considered as oxidants, i.e. existing at appreciable extent under oxidizing conditions. pentavalent arsenic is in equilibrium with as(iii) according to: h3aso4 + 2 h + + 2 e ↔ haso2 + 2h2o e0 = 0.560 v (7) considering a solution with a ph 7, the equilibrium potential deduced from nernst law is 0.147 v/nhe. from a thermodynamic point of view, as(v) and as(iii) can be present in most aqueous media. it is generally admitted that as(v) is a predominant form in waters provided sufficient aeration. the iron anode dissolves to fe(ii). this reducing species can oxidize to fe(iii) according to: fe2+ ↔ fe3+ + e e0 = 0.771 v (8) comparison of the equilibrium potentials of the redox couples shows that as(v) cannot be reduced by fe(ii); the latter rapidly oxidizes to fe(iii) by air oxygen in aerated media. nevertheless, removal of as by adsorption or electrocoagulation is more efficient for as(v) than as(iii). in addition to its lower intrinsic toxicity, as(v) is thus a less hazardous form of arsenic to be used for treatment or consumption purposes. for this reason, in most investigations, pretreatment of wastewaters includes the preliminary step of as(iii) oxidation using hydrogen peroxide, fe(iii), or at the anode surface, as reported by ratna kumar et al. [4]. application of the electrocoagulation model to as removal the model was directly derived from that for cod abatement. fe(iii) complexes as species as follows: 100 150 200 250 300 0 200 400 600 800 1000 c o d / ( m g/ l) [fe] / (mg/l) 100 a/m2 150 a/m2 model j. electrochem. sci. eng. 1(1) (2011) 53-63 removal of as and cod by electrocoagulation 60 fe(iii) + n2 as(v) ↔ feas (9) where coefficient n2 is expressed in mg fe(iii) per mg as species. equilibrium constant k2 is simply written as: [ ] [ ][ ]2 feas fe as k = (in l/mg) (10) langmuir’s model has been used in several investigations for modeling the adsorption equilibrium of as species on fe(iii) hydroxide. mass balances on fe and as species were also written: [ ] 0 2 feas [as] [as] n = + (11) from the equations (10-12), the concentration of complex feas can be expressed as follows: [ ] [ ] [ ] [ ] 2 2 2 fe as feas 1 as t k k n = + (13) which appears perfectly similar to the conventional langmuir’s expression [ ] [ ] l as l as 1 as k q a = + (14) where qas is the amount of as adsorbed on fe hydroxides, the binding constant kl can be related to product k2[fe]t, and the sorbent capacity al to the ratio (k2 / n2). as with s, the concentration of free, untreated arsenic species is obtained by solving the system formed byequations (10-13): [ ] [ ] [ ] [ ] [ ] 2 0 0 0 2 2 2 2 2 fe fe 4 as1 1 as as [as] 2 t t n k n k k     − − + + − + +          = (15) as relevant studies indicate, as(v) species can be totally removed by electrocoagulation; therefore, its final concentration [as]f was neglected here. the model was applied to various data reported in the literature [1,4,15,17]. because the corresponding values for the concentration of generated iron species were rarely reported, it was assumed that iron anodes dissolved with a current efficiency equal to 0.8, as observed in previous investigations [9,10]. the postulated value is also in fair agreement with the value observed for the treatment of the industrial waste (see above section). the various data considered were approximately fitted to the model, yielding estimates for constants k2 and n2. fig. 2 shows the good correlation between theory and experimental data for as concentrations ranging from 100 µg/l to 30 mg/l within a factor 2, with k2 = 1.12 l/mg and n2 = 1.82 mg fe per mg as s. zodi et al. j. electrochem. sci. eng. 1(1) (2011) 53-63 doi: 10.5599/jese.2011.0002 61 figure 2. validation of the model for adsorption of as species onto fe hydroxide (rel. 15): data issued from refs. [1], [4], [15] and [17] are related to treatment of synthetic as solutions or groundwater. the dotted lines correspond to 50% and 200% of the diagonal expressing perfect agreement between the model and experiments. the existing scattering of the data is probably due to the lack of accuracy in the estimation of coagulant concentration. simulation tests actually showed that fe(iii) concentration had a noticeable impact on the predicted concentration of remaining as species. besides, the value for n2 could not be compared directly to the molar fe/as ratio values reported in the literature and discussed by hansen et al. [3], because the adsorption of as species, like other pollutants, obeys an adsorption isotherm law involving equilibrium between complexed and free species. therefore, the parameter n2 does not correspond to the fe/as ratio, which is deduced by global assessment of as removal depending on the amount of fe(iii) generated. removal of as-containing industrial waste experimental observations numerous batch runs have been carried out with as concentrations up to 700 µg/l. in most cases, iron dissolution in the industrial waste was shown to be unaffected by the presence of the arsenic salt (fig. 3). the concentration in fe species in the liquid increased regularly with the electrical charge passed, although at a slower dissolution rate in the first minutes of the run. this corresponds to the increase in current yield mentioned in a previous section with the raw industrial waste. in addition, the concentration of arsenic salt decreased with time. as for the cod and other features of the wastewater under treatment, the abatement of the toxic as-additives is governed by the amount of fe dissolved (fig. 4). nevertheless, removal of as(v) species is nearly complete with 150 mg/l fe dissolved, whereas the removal of the organic matter requires more significant dissolution (fig. 1). 0,01 0,1 1 10 100 0,01 0,1 1 10 100 [a s] th eo re t. / (m g/ l) [as]exp. / (mg/l) lakshmiparhiraj-25 martinez ratna kumar lakshmiparhiraj-10 parga model-50% model-200% j. electrochem. sci. eng. 1(1) (2011) 53-63 removal of as and cod by electrocoagulation 62 figure 3. anode iron dissolution in the treatment runs for various concentrations of as in the enriched paper mill wastewater. figure 4. typical variations of as species concentrations with the amount of dissolved iron in treatment runs of the as-enriched paper mill wastewater depending of operating conditions. modeling of as-removal from the industrial waste treatment of as-containing wastewaters was modeled assuming that the generated coagulant acts independently on the organic matter and as(v) species. s adsorbs on fe(iii) hydroxide according to parameters (k1, n1), whereas as(v) adsorbs on the solid coagulant with parameters (k2, n2). the model was, therefore, developed on the basis of equilibrium (2) and (8) with constants k1 and k2. mass balances were written for the pollutant charge expressed by s, as well as for as(v) and fe(iii). the two first balances are given in equations (4) and (11), respectively, whereas the conservation of fe species is written as: 0 50 100 150 200 250 300 0 500 1000 1500 2000 2500 3000 [f e] / ( m g/ l) charge passed, c iron dissolution36 µg/l-10 ma/cm2 290 µg/l-5 ma/cm2 290 µg/l-10 ma/cm2 450 µg/l-5 ma/cm2 450 µg/l-10 ma/cm2 0 100 200 300 400 500 0 50 100 150 200 250 300 [a s] / ( µ g/ l) [fe] / (mg/l) arsenic removal 36 µg/l-10 ma/cm2 290 µg/l-5 ma/cm2 290 µg/l-10 ma/cm2 450 µg/l-5 ma/cm2 450 µg/l-10 ma/cm2 s. zodi et al. j. electrochem. sci. eng. 1(1) (2011) 53-63 doi: 10.5599/jese.2011.0002 63 [fe]t = [fe] + [fes] + [feas] (16) taking into account the expressions of constants k1 and k2, the three mass balances lead to the equation: [ ] [ ] [ ]( )[ ] [ ] [ ] [ ] [ ] 1 20 f 0 t 1 2 1 2 s s fe as fe fe [fe] 1 fe 1 fe k k k k n n − = + + + + (17) from which the concentration of free fe(iii) hydroxide, [fe], can be obtained by numerical solution. the variation of concentration of arsenic species in time is deduced straightforwardly from [fe] using: [ ] [ ] [ ] 2 0 2 2 as as fe n n k = + (18) whereas the cod level is calculated according to: [ ] [ ] [ ] [ ]( ) [ ] 1 0 f f 1 1 s s s s fe n n k − = + + (19) the model required the values for parameters k1, k2, n1, and n2. the above model, assuming no interactions between the abatement of cod and as species, was applied to the set of data. fig. 5 establishes the acceptable validity of the model for as concentrations ranging from 0.3 µg/l to 700 µg/l. figure 5. validation of the model for as removal from as-enriched paper mill wastewater (rel. 12 and 13). most predicted values for the concentration lie between 50 and 200% of the experimental value. taking into account the uncertainty in the experiments, in particular in the chemical analysis of as in quite a large range of concentration and the existing uncertainty in the values for the parameters n2 and k2, the approach can be considered as successful. therefore, for the sample of 0,0001 0,001 0,01 0,1 1 0,0001 0,001 0,01 0,1 1 [a s] t h eo r. / ( m g/ l) [as] exp. / (mg/l) 14 µg/l 25 µg/l 36 µg/l 115 µg/l 290 mg/l 450 µg/l 700 µg/l diagonal 50% 200% j. electrochem. sci. eng. 1(1) (2011) 53-63 removal of as and cod by electrocoagulation 64 industrial waste considered, the highly toxic salt added seems to be removed from the wastewater independently of the organic matter contained in the waste. in addition, the simple equations developed here are sufficient to model the waste treatment. comparison of the experimental and predicted cod levels was not carried out since cod abatement was only slightly affected by the presence of as species, which was added in concentrations of nearly two orders below the cod level. conclusions arsenic species can be successfully removed from industrial waste by electrocoagulation using fe electrodes. as(v) ions adsorb on fe hydroxide in accordance with langmuir-like adsorption isotherm. the parameters of the adsorption equilibrium could be estimated from previously published data dealing with the treatment of waters with low cod levels. this equilibrium could be incorporated into a more general model for the abatement of both as and cod, assuming no interaction in the treatment of the two pollution sources. in the example of industrial waste issued by a paper mill industry, the model was shown to hold, allowing a fairly accurate prediction of the as species. however, the approach may not be successful for industrial wastewaters containing oxidizing or reductive agents, which could change the chemical state of the arsenic salt and then affect its adsorption onto the fe hydroxide particles. whatever the model, a thorough characterization of the waste to be treated is required prior to designing the electrocoagulation process. for such reactive wastes, further characterization of their chemical behavior seems to be absolutely necessary. acknowledgements: the authors are indebted to s. pontvianne for his thorough assistance in chemical analysis of the fractions collected. references [1] j.r. parga, d.l. cocke, j.l. valenzuela, j.a. gomes, m. kesmez, g. irwin, h. moreno, m. weir, j. hazard. mater. 124 (2005) 247-254. [2] m. leist, r.j. casey, d. caridi, j. hazard. mater. 76 (2000) 125-138. [3] h.k. hansen, p. nunez, d. raboy, i. schippacasse, r. grandon, electrochim. acta 52 (2007) 3464-3470. [4] p. ranta kumar, s. chaudhari, k.c. khilar, s.p. mahajan, chemosphere 55 (2004) 1245-1252. [5] j.a.g. gomes, p. daida, m. kesmez, m. weir, h. moreno, j.r. parga, g. irwin, h. mcwhinney, t. grady, e. peterson, d.l. cocke, j. hazard. mater. 139 (2007) 220-231. [6] g. dodbiba, t. nukaya, y. kamioka, y. tanimura, t. fujita, resour. conserv. recy. 53 (2009) 688-697. [7] p. canizares, f. martinez, c. jimenez, c. saez, m.a. rodrigo, j. hazard. mater. 151 (2008) 44-51. [8] b. merzouk, b. gourich, a. sekki, j.k. madani, m. chibane, j. hazard. mat. 164 (2009) 215222 [9] i. zongo, j.p. leclerc, h.a. maiga, j. wethe, f. lapicque, sep. purif. technol. 66 (2009) 159166. [10] s. zodi, j.n. louvet, c. michon, o. potier, m.n. pons, f. lapicque, j.p. leclerc, accepted in chem. eng. proc. proc. intens. [11] m. khemis, j.p. leclerc, g. tanguy, g. valentin, f. lapicque, chem. eng. sci. 61 (2006) 36023609. [12] i. zongo, a.h. maiga, j. wéthé, g. valentin, j-p. leclerc, g. paternotte, f. lapicque j. hazard. mater. 169 (2009) 70-76. [13] r.b. heslop and p.l. robinson, inorganic chemistry, elsevier, amsterdam (1967). s. zodi et al. j. electrochem. sci. eng. 1(1) (2011) 53-63 doi: 10.5599/jese.2011.0002 65 [14] r.c. weast, handbook of chemistry and physics, 76th edition crc press, boca raton (20052006). [15] j.f. martinez-villafane, c. montero-ocampo, a.m. garcia lara, j. hazard. mater. b172 (2009) 1617-1622. [16] a. maldonado-reyes, c. montero-ocampo, o. solorio-feria, j. environ. monit. 9 (2007) 1241-1247. [17] p. lakshmipathiraj, s. prabhakar, g.b. raju, sep. purif. technol. 73 (2010) 114-121 © 2011 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/� << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice relationship modelling for surface finish for laser-based additive manufacturing http://dx.doi.org/10.5599/jese.1286 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1286 open access : : issn 1847-9286 www.jese-online.org original scientific paper relationship modelling for surface finish for laser-based additive manufacturing samidha jawadeand ganesh kakandikar school of mechanical engineering school of mechanical engineering, dr. vishwanath karad mit world peace university, pune, maharashtra, india corresponding author: samidha.jawade@mitwpu.edu.in received: february 5, 2022; accepted: march 14, 2022; published: may 3, 2022 abstract nickel-based superalloys belong to a special class of high-performance alloys that feature large amounts of nickel. the uniqueness of superalloys lies in their design features, most notably mechanical strength, durability, etc. inconel 718 (in718) is a nickel-based superalloy that exhibits high tensile and impact-resistant properties, along with good high-temperature corrosion resistance. selective laser melting (slm) is additive manufacturing (am) technology aimed at melting and fusing metal powders using high power density lasers to produce precision functional parts. the accuracy and functional characteristics of manufactured parts are highly dependent on process parameters. the processing parameters that control the slm process comprise laser power (p), hatch spacing (hs), exposure time (t), and border power (bp). this work primarily focuses on finding the combined effect of these process parameters on the surface roughness (sr) and ultimate tensile strength (uts) of in718 manufactured by slm using the design of experiments (doe). keywords in718; selective laser melting; mechanical properties; surface roughness; inconel; superalloys introduction inconel 718 (in718) is an age-hardenable nickel-based superalloy [1]. the aerospace, energy, and automotive industries are accelerating specific applications of am technology in the manufacture of parts made of nickel-based superalloys [2,3]. the aerospace industry poses complicated technological needs, metal to work in extreme conditions, including cryogenic and high temperatures. because of these demands, nickel-based superalloys have been developed. applications for these materials are found in different areas like engine turbine blades, nuclear reactors, turbochargers, heat exchangers and petrochemical equipment etc. [4,5]. the in718 alloys show outstanding malleability and weldability, high tensile, fatigue, creep, and rupture strength has led to its widespread application [6,7]. previously, manufacturing industries depended upon subtractive manufacturing processes to make the products. nowadays additive manufacturing processes are replacing them [8-10]. additive manufacturing is the computer-controlled process of constructing a http://dx.doi.org/10.5599/jese.1286 http://dx.doi.org/10.5599/jese.1286 http://www.jese-online.org/ mailto:samidha.jawade@mitwpu.edu.in j. electrochem. sci. eng. 00(0) (2022) 000-000 modelling for surface finish 2 3d object by joining layers of material with the help of a laser on a build platform till the final product is finished [11,12]. today, in additive manufacturing, selective laser melting (slm) is one of the promising technology available for rapid prototyping and mass production. the use of metal alloys in the slm process to manufacture the parts used in different applications is limited [13-15]. for some reason, the industry is still hesitant to adopt the slm process used for the mass production of critical components. the most important concerns are a surface finish, microstructural inhomogeneity and microstructural defects observed in slm-generated parts that can affect mechanical properties badly [16,17]. the process parameters of selective laser melting play a huge role in determining the surface finish and the mechanical properties of the slm manufactured component [18]. although slm offers great flexibility in design, the cad geometry may get affected due to the changes in the process parameters [19,20]. process parameters also play an important role in determining the type of the grain's size and shape, melt pools and composition of different phases, which in turn dictates the mechanical properties such as surface roughness, and tensile strengths of the fabricated parts [21,22]. therefore, analysis of the process helps us understand the impact of these process parameters on microstructural and mechanical properties. this understanding of the process parameters shall provide feasibility to dynamically control the build process and thus, can be made more efficient, thereby reducing the microstructural defects [23]. this work mainly focused on the effect of process parameters to optimize the surface finish and mechanical properties of parts fabricated by slm. microstructural analyses are performed on the fabricated parts and they are followed by a mechanical investigation like surface roughness and strength. finally, the study is concluded by co-relating the effect of laser power, hatch spacing, exposure time, border power with the surface roughness (ra) and mechanical properties of in718. materials and methods materials and process parameters commercially available renishaw in718-0405 in powder form is selected for experimentation. based on the extensive literature review, the laser power, exposure time, border power and hatch distance were finally taken into consideration. melting of powder is crucial in slm and appropriate laser power ensures proper melting. exposure time ensures homogeneous melting at each layer. there is the possibility of uneven melting at borders. border power is an important parameter to ensure the same quality of melting at the surface as that of the core. hatch distance/ spacing is the separation between two consecutive laser beams. appropriate hatch distance ensures good quality of melting. as all the process parameters can be varied at three levels, and with the help of taguchi's design of experiment, the resulting orthogonal array l9 was selected for the further process. table 1 presents the selected parameters and their levels. table 2 presents the l9 orthogonal array. figure 1 shows the standard tensile test specimen drawing used for specimen manufacturing through the slm process. the specimen was manufactured using renshaw am 400 machines, shown in figure 2. figure 3 shows the component manufactured as per the l9 array for the experimentation. the optimization of process parameters in slm processes is important as the process parameters affect the microstructural property, compositional property, mechanical property, geometrical accuracy, and surface finish of the fabricated parts. over the years, very few researchers have tried to optimize these process parameters to reduce the variation and establish standard process parameters. as the slm process is affected by many parameters, a trial-and-error method would require a large number of tests, which is not suitable as it would be difficult to determine the s. jawade and g. kakandikar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1286 3 correlation between a specific parameter and its desired value [18]. it must also be noted that this technique is time-consuming and expensive. design of experiments (doe) techniques help as a practical alternative for the trial-and-error methods [19]. figure 1. tensile sample (astm e8) manufactured by slm figure 2. renishaw am 400 machine figure 3. tensile test specimens in l9 experimentation table 1. slm process parameters levels for manufacturing of tensile specimen parameters level 1 level 2 level 3 laser power, w 315 345 375 hatching distance, m 75 90 105 exposer time, µs 15 20 25 border power, w 275 325 375 table 2. l9 array of different slm process parameters sr. no. power, w hatch spacing, m exposer time, µs border power, w 1 315 75 15 275 2 315 90 20 325 3 315 105 25 375 4 345 75 20 375 5 345 90 25 275 6 345 105 15 325 7 375 75 25 325 8 375 90 15 375 9 375 105 20 275 http://dx.doi.org/10.5599/jese.1286 j. electrochem. sci. eng. 00(0) (2022) 000-000 modelling for surface finish 4 results and discussion performance measures surface roughness and tensile test were performed on selective laser melted inconel 718 (in718) specimens. the surface roughness is critical to its function and long-term performance. it is a key factor in mechanical properties and is driven by additive modality and process parameters. the surface finish influences the mechanical properties and aesthetics of components; hence surface roughness is part of the investigation. 2nd performance parameter was ultimate tensile strength. it is the maximum stress a material can withstand before breaking. table 3, figure 4 and figure 5 indicate the results of nine experiments as well as process variability for both performance measures. it is observed that surface roughness (ra values) varies between 8.46 µm to 9.51 µm. surface roughness master was used to measure the surface roughness. minimum surface roughness is observed in experiment no 6 specimens. ultimate tensile strength varies between 1045 to 1089 mpa. it is measured using universal testing machines. minimum uts was observed in the exp 7 specimen and maximum uts in experiment 9. error bar of surface roughness and ultimate tensile strength are shown in figures 6 and 7, respectively. figure 8 shows the specimen after the tensile test. table 3. design of experiment – performance measures sr. no. p / w hs / m t / µs bp / w sr / µm uts / mpa 1 315 75 15 275 8.85 1080 2 315 90 20 325 8.87 1078 3 315 105 25 375 8.65 1080 4 345 75 20 375 9.26 1081 5 345 90 25 275 9.17 1065 6 345 105 15 325 8.46 1059 7 375 75 25 325 9.11 1045 8 375 90 15 375 9.51 1082 9 375 105 20 275 9.37 1089 figure 4. surface roughness of in718 versus experiment no. figure 5. ultimate tensile strength of in718 versus experiment no. figure 6. error bar of surface roughness in718 s. jawade and g. kakandikar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1286 5 figure 7. error bar of ultimate tensile strength in718 figure 8. specimens after tensile test doe analysis results of the s/n ratio analysis, which indicate the influence of the process parameters i.e., laser power, exposure time, border power, and hatch spacing on the surface roughness and uts of the slm printed part, are presented in table 4. the process parameters with strong influence were identified using the difference between the maximum and minimum value, i.e., delta at the three levels in the experimentation. table 4. s/n ratio obtained after taguchi analysis sr. no. p / w hs / m t / µs bp / w s/n ratio of sr s/n ratio of uts 1 315 75 15 275 -18.938 60.670 2 315 90 20 325 -18.958 60.654 3 315 105 25 375 -18.740 60.670 4 345 75 20 375 -19.332 60.673 5 345 90 25 275 -19.247 60.553 6 345 105 15 325 -18.547 60.499 7 375 75 25 325 -19.190 60.382 8 375 90 15 375 -19.563 60.691 9 375 105 20 275 -19.434 60.739 tables 5 and 6 represent the crucial factors for surface roughness and uts. for surface roughness, laser power is the most influential parameter as compared to the other three. it is observed that for uts, border power is most influential, whereas exposure time has the second rank. the other two parameters are the least important. it was observed that laser power plays a big role in slm manufactured in718 parts to improve the surface roughness. the roughness analysis of slm fabricated parts established that the high values for ra were due to the presence of unmelted particles formed on the surface due to irregular melting and cooling processes. it was observed that maintaining the hatch spacing leads to a steady melt pool architecture, resulting in lower values for ra. balling effect also increases the surface roughness values and it can be reduced by the melting http://dx.doi.org/10.5599/jese.1286 j. electrochem. sci. eng. 00(0) (2022) 000-000 modelling for surface finish 6 and re-melting process which in turn enhances the values of ra by around 80 %. the balling effect is a problem that frequently occurs in the selective laser melting forming process and seriously affects the surface precision of the manufactured part. balling is a defect that can occur when the molten pool created during selective laser melting becomes discontinuous and breaks into separated islands. table 5. response table of signal to noise for surface roughness level p / w hs / m t / µs bp / w 1 -18.88 -19.15 -19.02 -19.21 2 -19.04 -19.26 -19.24 -18.90 3 -19.40 -18.91 -19.06 -19.21 delta 0.52 0.35 0.23 0.31 rank 1 2 4 3 table 6. response table of signal to noise for uts level p / w hs / m t / µs bp / w 1 60.66 60.58 60.62 60.65 2 60.58 60.63 60.69 60.51 3 60.60 60.64 60.54 60.68 delta 0.09 0.06 0.15 0.17 rank 3 4 2 1 microstructural analysis the xrd spectrum of the in 718 powder used as an interfacing layer is shown in figure 9; the spectrum is coincident with a solid solution of austenite (g) face-centered cubic (fcc) ni–cr matrix and shows the dominant presence of (111) plane. for the xrd analysis, ultima iv fully automated optical alignment machine was used. to examine the fracture surface fesem – fei nova nanosem 450 machine was used. fracture surfaces of the tensile specimen are shown in figures 10 and 11. experiment 9, tensile specimen showed ductile fracture characteristics, dimples on the fracture. in experiment 7, very small particle size was observed because of the non-equilibrium melting and solidification. figure 11 shows the fractography after the tensile test of experiment no 7-specimen investigation of the fractured samples revealed process-induced defects such as pores, unmelted and partially melted powder particles. figure 9. xrd analysis of in718 s. jawade and g. kakandikar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1286 7 figure 10. fractography of tensile specimen (in718) of experiment no.9 figure 11. fractography of tensile specimen (in718) of experiment no.7 figure 11 shows the fractography after the tensile test of experiment no 7-specimen investigation of the fractured samples revealed process-induced defects such as pores, unmelted and partially melted powder particles. it is likely that these defects serve as stress raisers and eventually lead to crack initiation and failure. the micro-pore act as a crack initiator. in tensile tests, due to stress concentration, microcracks usually start at these weakest locations and then propagate to accelerate fracture. simulation ‘simufact’ additive 2021 is a powerful software solution for the simulation of selective laser melting. 3d cad model of tensile specimen used for the simulation purpose, shown in figures 12, 13 and 14. after that, the model is imported into simufact additive (sa) software to generate mesh, support and define the material properties. simulation by changing process parameters was observed and experimental results were validated with the help of the simulation model. experimental results and simulation result shows a good agreement. yield stress, pa yield stress, pa figure 12. the contour map showing the yield stress simulation result at 315 w figure 13. the contour map showing the yield stress simulation result at 345 w yield stress, pa figure 14. the contour map showing the yield stress simulation result at 375 w conclusion the objective of this research is to correlate the effect of process parameters on the properties of in718 specimens using the taguchi method. surface roughness and ultimate tensile strength 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the processes of migration and diffusion in the systems with solid-state reagents} http://dx.doi.org/10.5599/jese.753 219 j. electrochem. sci. eng. 10(2) (2020) 219-227; http://dx.doi.org/10.5599/jese.753 open access: issn 1847-9286 www.jese-online.org original scientific paper on the processes of migration and diffusion in the systems with solid-state reagents viacheslav barsukov, volodymyr khomenko, oksana chernysh department for electrochemical power engineering & chemistry, kyiv national university of technologies and design, 2 nemyrovych-danchenko str., kyiv 01011, ukraine corresponding author: v-barsukov@i.ua; tel.: +380 -442-562-102; fax: +380-442-848-266 received: november 14, 2019; revised: january 10, 2020; accepted: january 14, 2020 abstract this paper deals with peculiarities of diffusion and migration in electrochemical systems with solid-state reagents (esssr). contradictions of the diffusion model are analyzed. it is the difference of applied potentials and the corresponding electric field strength in the bulk solid phase and at the interfaces which is the primary driving force of charge transfer in esssr. the time characteristic of diffusion processes is not comparable to the duration of electrode processes at charging/discharging of batteries and especially electrochemical capacitors. in many real systems involving esssr, the process of diffusion in solid phase is absent. examples of charge transfer processes in esssr (nickel hydroxide electrode, sparingly soluble quinoid compounds, li+ intercalation in graphite, etc.) are considered, and the processes are explained using the grothuss, tunnel and other migration mechanisms. it is shown in this paper that the linear relationship between peak currents in voltammetric curves and the square root of potential scan rate cannot be presented as an ultimate support of the diffusion model, but as а more universal property of esssr. in this aspect, the efficient diffusion coefficient, deff, could be at best discussed, not to distort the ideas of charge-transfer migration mechanisms in the esssr. keywords solid-state electrochemical systems; charge transfer models; mechanisms; macrokinetics. introduction solid-state insoluble and sparingly soluble reagents are common in electrochemical practice and are widely used as anode and cathode materials in many electrochemical power sources (eps), as insoluble anodes in electroplating, in corrosion protection systems, for electrochemical analyses, in electrochemical sensors and other fields of modern electrochemical science and engineering. however, the theory of charge transfer processes in solid-state systems is developed insufficiently and іs based mainly on the classical theoretical assumptions formulated essentially for solidhttp://dx.doi.org/10.5599/jese.753 http://dx.doi.org/10.5599/jese.753 http://www.jese-online.org/ mailto:v-barsukov@i.ua j. electrochem. sci. eng. 10(2) (2020) 219-227 systems with solid-state reagents 220 state reactions at the “electrolyte solution-solid phase” interface. in this context, the ideas about the diffusion of protons (or ions of lithium and sodium) in solid state, occurring for example in active materials of eps electrodes, can be frequently found in modern electrochemical literature [1-7]. moreover, it is usually believed a priori that it is just diffusion of cations in solid state that retards the discharge (charge) of electrodes. then, an attempt was made to calculate the coefficient of diffusion of protons (or ions of lithium and sodium) in solid state, d, based on experimental data, under the assumption that the following fick diffusion equation can be applied to experimental conditions [6]: j = d gradc (1) in eq. (1), j is the amount of material transported through unit area per second, while gradc is the gradient of ions concentration. it is supposed by fick diffusion equation (1) that the coefficient of diffusion d is a constant value for the selected material and fixed temperature. on the other hand, the current density is expressed as i =  grade (2) where  is conductivity of material, while grade is gradient of potentials. it is necessary to note that d values for some solid-state electrochemical systems that were already reported by different researchers and determined using different methods, commonly range over several orders of magnitude. for example, the attempts to determine d value of protons in the solid phase of nickel hydroxide electrode (nhе) using the above mentioned approach, led to d values ranging from 10-5 to 10-15… 10-16 сm2/s as it was shown by v. a. volynskij in [8,9] and especially in [8]. obviously, such evaluation with differences of 10-11 orders of magnitude indicates that the diffusion model applied to nhe is inappropriate. our earlier studies (for example [10]) showed that models should take into account the experimentally proved presence of interlayer water stabilized by the layered structure of nickel (ii, iii) hydroxides in the grain of nhe. then, the basically different mechanism of proton migration in the nhe grain, in particular, the grothuss mechanism of proton migration in the interlayer water becomes possible in accordance with following equations: niooh + h2o + e↔ ni(oh)2 + oh or niooh + h+ + e↔ ni(oh)2 (3) the migration model of nhe grain which was developed in [10,11], enables to explain the following main peculiarities of nhe behavior: reason for electrode passivation during the discharge; formally 1 eprocess at nhe; practically possible range of nhe cycling (up to 1.5 ̶ 2.0 e) depending on the electrode morphology and construction; influence of contaminants and activating additives (especially, co hydroxide) [12]. a similar “proton” mechanism is typical for sparingly soluble quinoid compounds, including anthraquinone-9,10 and its derivatives, p-chloranil, pentacenequinone-13,14, and some other representatives of this interesting class of materials. the reaction in such sparingly soluble electrochemically active compounds can be at the example of p-chloranil, described as follows: (4) a proper understanding of the charge transfer mechanism in solid quinones is hindered not only by the extremely low coefficient of proton diffusion, but also very low conductivity. it has already been reported [13,14] that quantum-mechanical tunneling is the most probable mechanism of v. barsukov et al. j. electrochem. sci. eng. 10(2) (2020) 219-227 http://dx.doi.org/10.5599/jese.753 221 hydrogen penetration through the potential barrier. the driving force of the process is the electric field strength in the solid phase, which distribution in the solid phase has a crucial effect on the macrokinetics of the process in the quinone grain. another example can be intercalation-deintercalation of li+ into graphite-carbon materials. despite obvious difficulties in modelling, the processes of intercalation-deintercalation of lithium cations into the anode or cathode material, are associated with different conditions of their movement in the solid phase and at solid-electrolyte interface (sei). therefore, it is difficult to imagine that such processes can be described by simple diffusion models. nevertheless, some authors used fick equation to calculate the diffusion coefficient. as it was shown by a. v. churikov in [15] and especially [16], the scatter of d values is again 10 orders, from 10-5 to 10-15 сm2/s for lithium ion intercalation-deintercalation reaction in different graphite-carbon active materials: 6c + li+ + e↔ lic6 (5) morphology of one or another carbon material can affect quantitative indicators of li+ ion transport to some extent. however, at the microscopic level, for all these materials, reaction (5) actually occurs in the similar graphene layer and therefore, a scatter of 10 orders of magnitude seems unlikely. there is no consistency in the reported descriptions of solid-state processes in a variety of layered cathode materials and for lithium batteries either. moreover, even if d value was measured in the same material by the same author using the same technique, its value changes by several orders of magnitude during the charge-discharge reaction. this indicates that d is not the constant coefficient, as the diffusion theory (1) suggests. in the attempt to justify the model of diffusion of protons (cations) in the solid phase of nhe, conducting polymers and some other systems with a solid-state reagent, some authors have cited the linear relationship existing between peak currents in voltammetric curves, im, and square root of potential scan rate, ν-1/2, as the way of proof. this type of potentiodynamic dependence gives evidence for the presence of diffusion limitation and mathematically, it is substantiated only for liquid-state reactions but not solid-state ones. here, we will prove theoretically and experimentally that the linear dependence of im vs. ν-1/2 and also linear dependence of potential difference formed between the anode and cathode peaks, em vs. ν-1/2, do not provide evidence for diffusion limitation in the reactions with a solid-state reagent, but rather indicate that there is а limited reserve capacity, qm of the active material in such systems. experimental to study a proton-conducting solid electrochemical system, p-chloranil was chosen. this sparingly soluble quinine shows linear current-voltage characteristic within almost the entire range of working potentials and an “ideal” reversibility [17]. p-chloranil was mixed with the battery graphite in a weight ratio of 1:1 and dispersed in a ball mill for 12 hours. disc electrodes with diameter of 16 mm and thickness of about 1 mm were pressed from the resulting mixture under the pressure of 105 n/m2. 1.5 m sulfuric acid served as an electrolyte and an insoluble antraquinone electrode was the reference electrode. the electrodes for lithium-ion batteries were produced by manual tape casting with different suspensions based on n-methylpyrrolidone (nmp) on aluminum foil. the mixture with nmp contained 85 wt.% active materials, 7 wt.% carbon percolator and 8 wt.% pvdf binder. as an active http://dx.doi.org/10.5599/jese.753 j. electrochem. sci. eng. 10(2) (2020) 219-227 systems with solid-state reagents 222 electrode material, li4ti5o12 (lithium titanate) from mti co. was chosen, while timical c65 conductive carbon black was used as a conductive additive. tape casting with 100 μm gap was carried out by doctor’s blade method. dry electrodes with a thickness in the range 20-80 μm were calendared by a hot rolling calendaring machine from mti. then the electrodes were cut into 16 mm disk samples. the samples were further dried at 120°c and transferred to the argon filled mbraun glove box for assembling laboratory cells. electrochemical tests were carried out using a coin-type cell (cr-2016) with lithium as reference and counter electrodes. the commercial separator (celgard 2325) was placed between electrodes to ensure electrical insulation. 1 m solution of lipf6 in a mixture of ethylene carbonate (ec) and dimethyl carbonate (dmc) (weight ratio ec:dmc of 1:1) served as the electrolyte. the electrochemical performance of materials was examined by cyclic voltammetry (cv) technique at different sweep rates (potentiodynamic curves). all electrochemical tests were performed using a multi-channel potentiostat/galvanostat vmp3 from princeton applied research (uk). results and discussion some theoretical approach the above mentioned approach towards postulating the diffusion model in a solid-state electrochemical system is not justified for the following few reasons: 1. difference of applied potentials and corresponding electric field strength in the bulk solid phase and at the interfaces are the primary driving force for charge transfer in any solid-state electrochemical system. so, charge transfer in the solid state in any case occurs mainly by migration but not diffusion. 2. typical diffusion processes in solid state is rather slow. for example, penetration of metallic ions into another metal on their direct “ideal” contact caused by a difference in concentrations, lasts for several months or even years (depending on nature of metals). therefore, the time characteristic of diffusion processes is not comparable to the duration of electrode processes at discharge (charge) of batteries (tens of minutes, hours) and especially electrochemical capacitors (fractions of second, seconds). 3. in many real systems involving solid-state reagents, the process of diffusion in solid phase is absent as such. in our opinion, the main reason for the observed variance of d values is that most of the solid-phase reactions involving charged particles like (3) and (4), proceed through migration mechanism, but not through diffusion mechanism. the driving force for these reactions is not a difference in concentrations, but a difference in potentials. therefore, applying diffusion approach and diffusion parameters to these reactions is not sufficiently justified. in many cases, diffusion processes which rates in solid phase are typically very low, are negligible compared with migration processes in real systems with a solid-phase component. in such systems, a considerable contribution to the charge-transfer is made by the grain microstructure of the active material, its defectiveness and porosity, presence of interlayer water (for aqueous systems) or interlayer electrolyte (for non-aqueous systems), respectively. suppose that in a certain potential range, e, the voltammetric characteristic of an electrode is linear and the electrode can be characterized by an effective conductance, g, according to δ i g e = (6) where the change in potential is counted off the equilibrium potential of the electrode, e0. v. barsukov et al. j. electrochem. sci. eng. 10(2) (2020) 219-227 http://dx.doi.org/10.5599/jese.753 223 the maximal charge-discharge capacity obtainable for a given electrode system during the linear sweep rate,  , can be calculated by the following equation: 2 m 0 1 d 2 τ q gvt t gντ= = (7) in eq. (7), t is time, and  is duration of peak recording in the voltammetric curve. on the other hand, the capacity can be calculated using the average current with taking into account the linearity section of the voltammetric curve as follows: m m 1 2 q i τ= (8) comparison of equations (7) and (8) and substitution of  in the expression obtained from eq. (7), gives the following linear relationship between the peak current and the square root of potential scan rate: m m2i gq ν= (9) the model considered suggests a similar linear dependence of the difference between the anode and cathode peaks, em, on the square root of potential scan rate m m 2 δ q e vτ ν g = = (10) these two equations were obtained without making any assumption about the mechanism of electrochemical process in the solid-state system (diffusion, migration, mixed or any other). consequently, the linearity of the relationship between the peak current and the square root of potential scan rate cannot be presented as a sufficient evidence of the diffusion model. indeed, the linearity of voltammetric characteristic of the electrode within a certain section is only a priory assumption of the model considered, which is fairly justified regardless of the mechanism of electrochemical process. if such a linear section is relatively small, the voltammetric characteristic can be always presented as a sequence of linear sections using the patch-linear approximation, and derived conclusions can be applied to every individual section. obviously, equations (9) and (10) will have a more complex nature. experimental cyclic voltammogramms of electrodes based on p-chloranil figure 1 shows potentiodynamic curves recorded at different scan rates for an electrode based on p-chloranil. it must be added here that bubbling of nitrogen helps to eliminate the possibly occurring side reaction of the corresponding hydroquinone oxidation by the oxygen dissolved in the acid: (11) as a result, the complete symmetry and closely spaced areas of anode and cathode peaks (charge and discharge capacities) which correspond to reaction (4) can be achieved (figure 1). experimental data of peak currents and potential differences evaluated from figure 1 and their statistical treatment for linear dependence on ν1/2 are shown in figures 2 and 3, where linear dependences essentially cover the whole range of working potentials and are in full agreement with linear models defined by eqs. (9) and (10). at the same time, as mentioned above, the hydrogen http://dx.doi.org/10.5599/jese.753 j. electrochem. sci. eng. 10(2) (2020) 219-227 systems with solid-state reagents 224 transfer in such systems is not inherently diffusion, but probably proceeds by tunneling and depends on the distribution of electric field strength in the grain of quinone. figure 1. cyclic voltammetric curves for electrode based on n-chloranil, recorded in 3 м h2so4 with continuously bubbling nitrogen at different potential scan rates: 1) 0.10, 2) 0.20, 3) 0.40, 4) 0.75 mv∙s-1 figure 2. anode (ima) and cathode (imc) peak currents (fig. 1) vs. ν1/2 figure 3. potential difference (e) between current peaks (fig. 1) vs. ν1/2. experimental cyclic voltammograms of electrodes based on lithium titanate figure 4 shows cyclic voltammograms for a cell with a cathode based on lithium titanate at different scan rates. the curves represent intercalation-deintercalation of lithium cations into lithium titanate. the cathode part of the curve refers to the lithium deintercalation, while the anode part describes intercalation of lithium cation into lithium titanate. with increasing scan rate, the height and area of the peak increases. this is explained by the fact that the peak area divided by the scan rate gives the capacity of active material, which must be a constant. i / a e / v imc i / a  e m / v v1/2 / v1/2 s-1/2 ima v1/2 / v1/2 s-1/2 v. barsukov et al. j. electrochem. sci. eng. 10(2) (2020) 219-227 http://dx.doi.org/10.5599/jese.753 225 e / v vs. li figure 4. cyclic voltammograms of lithium titanate electrode recorded at different scan rates: 1) 0.05, 2) 0.10, 3) 0.50, 4) 1.00 mv∙s-1 moreover, the anode peak shifts to the region of more negative potentials, whereas the respective cathode peak shifts toward more positive values. this is related to the fact that irreversibility of the process increases and manifests itself to a greater extent at high scan rates because lithium ions do not have enough time to deintercalate (intercalate) completely from the electrode. otherwise, irreversibility is usually related to the limitation caused by diffusion of lithium cations in the solid phase of materials. v1/2 / v1/2 s-1/2 v1/2 / v1/2 s-1/2 figure 5. anode (ima) and cathode (imc) peak currents (fig. 4) vs. ν1/2. figure 6. potential difference (em) between current peaks (fig. 4) vs. ν1/2 quantitative evaluation of the diffusion coefficient is virtually impossible due to small change in concentration. based on figure 4, peak currents and potential differences between peaks against sq. root of potential sweep rate are shown in figures 5 and 6. the shift of peaks within the potential scale with increasing scan rate is observed in figure 5 even at low potential sweep rates. this is why the assumption about the determining role of lithium ion diffusion in the solid state is not substantiated. as seen in figure 5, the peaks of the cathode and anode currents are linear functions of the square root of potential scan rate. supporters of the diffusion model [18] usually suggest that the charge transfer through the interface is a fairly quick process and postulate that the rate limitation is only due to the diffusion of lithium within the electrode material. then, by analyzing the slope of linear relation in figure 5, the diffusion coefficient, d, could be calculated. in this case, apart from imc i / a ima i / a  e m / v http://dx.doi.org/10.5599/jese.753 j. electrochem. sci. eng. 10(2) (2020) 219-227 systems with solid-state reagents 226 the above-mentioned contradictions, there arises a number of other problems. examples are different d values for the anode and cathode processes, or different d values obtained with changing thicknesses of the active material. the linear dependence of the cathode and anode peaks on the square root of potential scan rate (cf. fig. 5), can also be interpreted by the change in ohmic resistance of the solid phase and the effect of lithium migration under the electric field. recent successful usage of such a model for current distribution in a porous graphite electrode during the discharge process in lithium-ion battery or lithium-ion capacitor has been reported in [19]. conclusions a logical question put in this paper was: to which extent is it appropriate to use assumptions about diffusion and diffusion coefficient, d, for the processes being mainly of migration nature? formally, d can be connected to the mobility coefficient μ by the well-known einstein equation eff ktμ d e = (12) where kt is boltzmann factor, μ is mobility coefficient and e is electron charge. in this formalization, the efficient diffusion coefficient, deff, could be at best discussed, what would not distort the ideas of charge-transfer migration mechanisms in the systems containing a solid-phase reagent. in many real systems involving solid-state reagents, the process of diffusion in solid phase is absent as such. the existence of alternatives such as grothuss, tunnel, and other fast mechanisms of cation migration in various solid-state systems often makes unnecessary considering of the longduration of non-competitive and energy-consuming diffusion process of heavy, charged particles in the solid phase. in their attempt to justify the model of diffusion of protons (cations) in the solid phase, some authors have cited the linear relationship between peak currents, im, in cyclic voltammetric curves and the square root of potential scan rate, ν-1/2, as the proof. in this work, we have shown that the existing linear dependences of im and also of potential difference between the anode and cathode peaks, em, on ν-1/2 do not provide evidence for diffusion limitations in the reactions with a solidstate reagent, but rather that there is а limited stock of capacity, qm, of the active material in such systems. references [1] f. brieuc, g. dezanneau, m. hayoun, h. dammak, solid state ionics 309 (2017) 187-191. [2] b. shruthi, b.j. madhu, v. bheema raju, journal of energy chemistry 25 (2016) 41-48. [3] q. deyang, electrochimica acta 49 (2004) 657-665. [4] p. m. panchmatia, a. r. armstrong, p. g. brucec , m. s. islam physical chemistry chemical physics 16 (2014) 21114-21118. [5] w. pfleging, p. gotcu, applied science 9 (2019) 3588. [6] j. h. summerfield, c. n. curtis, international journal of electrochemistry (2015), id 496905, 10.1155/2015/496905. [7] c.h. hamann, a. hamnett, w. vielstich, in electrochemistry, wiley-vch, weinheim, new york, chichester, brisbane, singapore, toronto, 1998, p. 423. [8] v. a. volynskij, ju. n. chernyh, elektrohimija [russian journal of electrochemistry] 13 (1977) 11–16 (in russian). [9] v.a. volynskii, phd thesis, state university, saratov, 1977, p. 127 (in russian). v. barsukov et al. j. electrochem. sci. eng. 10(2) (2020) 219-227 http://dx.doi.org/10.5599/jese.753 227 [10] v. z. barsukov, l. n. sagoyan, b. e. rogoza, g. o. yarkovoy, l. i. zaslavskaya, in batteries for portable application and electric vehicles, c.f. holmes, a.r. landgrebe (eds.), pennington, nj, 1997, p. 823. [11] v. z. barsukov, b. e. rogoza, l. n. sagoyan, 34th ise meeting, extended abstracts, ise, erlangen, germany, 1983, p. 112. [12] v. z. barsukov, n. r. meshcherjakova, l. n. sagoyan, 36th ise meeting, extended abstracts, ise, salamanka, spain, 1985, p. 130. [13] o. s. ksenzhek, v. z. barsukov, v. v. matveyev, 29th ise meeting, extended abstracts, ise, budapest, hungary, 1978, 2, p. 709. [14] v. z. barsukov, v. v. matveyev, 33rd ise meeting, extended abstracts, ise, lyon, france, 1982, 2, p. 854. [15] a. churikov, a. ivanishchev, a. ushakov, journal of solid state electrochemistry 18 (2013) 14251441. [16] a. v. churikov, lecture at the memorial session devoted the memory of v.s. bagotskii, institute of physical chemistry and electrochemistry, moscow, russia, 22nd january 2013, 1-34 (in russian). [17] h. alt, h. binder, a. kouhling, g. sandstade, electrochimica acta 17 (1972) 873-887. [18] j. coelho, a. pokle, s.-h. park, n. mcevoy, n. c. berner, g. s. duesberg, v. nicolosi, scientific reports 7 (2017) 7614. [19] v. barsukov, f. langouche, v. khomenko, i. makyeyeva, o. chernysh, f. gauthy, journal of solid state electrochemistry 19 (2015) 2723-2732. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.753 http://creativecommons.org/licenses/by/4.0/) {anticorrosive polystyrene coatings modified with tannic acid on zinc and steel substrates:} http://dx.doi.org/10.5599/jese.1293 721 j. electrochem. sci. eng. 12(4) (2022) 721-730 http://dx.doi.org/10.5599/jese.1293 open access : : issn 1847-9286 www.jese-online.org original scientific paper anticorrosive polystyrene coatings modified with tannic acid on zinc and steel substrates julia both1, gabriella stefánia szabó2,, gabriel katona2 and liana-maria mureșan1 1babeş-bolyai university, faculty of chemistry and chemical engineering, department of chemical engineering, ro-400028, cluj-napoca, romania 2babeş-bolyai university, faculty of chemistry and chemical engineering, department of chemistry and chemical engineering, hungarian line of study, ro-400028, cluj-napoca, romania corresponding author: gabriella.szabo@ubbcluj.ro received: february 26, 2022; accepted: may 20, 2022; published: june 7, 2022 abstract polystyrene (ps) polymer layers were prepared by the sol-gel method and studied as anticorrosive barrier layers on carbon steel and zinc substrates. to increase the corrosion resistance of the coatings, two different approaches were considered: (i) the use of mesoporous silica-nanocontainers impregnated with a corrosion inhibitor (tannic acid) introduced into the polystyrene matrix and (ii) direct impregnation of polystyrene coatings with the same corrosion inhibitor. the impregnated nanocontainers were characterized by transmission electron microscopy. the thickness and the adhesion of the coatings were measured, and their corrosion behavior was investigated by electrochemical impedance spectroscopy. results showed that the used inhibitor slightly decreased adhesion, but significantly increased the corrosion resistance of the coatings. the direct introduction of tannic acid into the polymer matrix offers higher corrosion resistance than in the case of polystyrene coatings doped with impregnated silica nanocontainers. keywords polystyrene coatings; corrosion protection; tannic acid; mesoporous silica nanocontainers introduction corrosion is an inevitable phenomenon in the gradual deterioration of metals because of aggressive environments and researchers are constantly preoccupied with preventing or minimizing it [1]. methods of anticorrosion protection include the production of metal alloys (active protection) and the application of corrosion barrier layers on metals surface (passive protection). aiming to avoid health issues and environmental problems, the production of chromate-free coatings [2] and harmless, effective organic and inorganic barrier coatings [3,4] on metals have taken the place of previously performant technologies having carcinogen nature and general toxicity [5]. styrene, alongside a plethora of polymerizing agents forms polystyrene (ps), a vinyl polymer composed of a long hydrocarbon chain with phenyl groups attached to every other carbon atom. the most common use of polystyrene is in the production of plastics [6], and since it exhibits http://dx.doi.org/10.5599/jese.1293 http://dx.doi.org/10.5599/jese.1293 http://www.jese-online.org/ mailto:gabriella.szabo@ubbcluj.ro j. electrochem. sci. eng. 12(4) (2022) 721-730 modified polystyrene anti-corrosive coatings 722 interesting thermic characteristics, it is the precursor of styrofoam, a material widely used in various industries. in the corrosion protection domain, ps has mainly been used in the form of copolymers with different acids [7] or the form of a copolymer with cornstarch [8] for the corrosion protection of steel substrates. ps microcapsules containing silanol and ce (iii) inhibitors in epoxy coatings have also been reported as corrosion barrier systems [9]. ps, as any other coating matrix offers, the possibility of improvement by the addition of corrosion inhibitors. reported examples of corrosion inhibitors for steel and zinc are rare earth salts (such as cecl3 or lacl3) [10], organic compounds, such as benzotriazole [11], and gallotannins [12] such as gallic acid and tannic acid, with rust converter characteristics. tannic acid, a safe, non-toxic and environmentally friendly polyphenolic compound, forms complexes with the surfaces of the metal substrates generating an additional corrosion preventive thin film on the metallic structures [13]. these complex-forming characteristics of gallotannins, especially of tannic acid, make them eligible as inhibitors for the preparation of corrosion-resistant coatings. nanocontainers are widely used as corrosion inhibitor carriers in various systems. these nanoparticles offer ingenious solutions for the preparation of self-healing coatings by their versatility and the capability of controlled release of incorporated inhibitors. previous studies discuss the application of urea-formaldehyde microcapsules filled with epoxy resin for self-healing coatings on steel substrates [14]. mesoporous silica nanoparticles loaded with molybdate [15] or 2-mercaptobenzothiazole [16] were also reported for the corrosion protection of aluminum and steel alloys. the silica nanocontainers have also been reported in combination with tannic acid, used as a sealing agent, for benzotriazole impregnated mesoporous particles [17]. as mentioned above, applications of silica nanoparticles are widely discussed as inhibitor carriers in various coating systems. following that thought pattern, we aimed to study whether the introduction of tannic acid in the nanocontainers versus the direct introduction into the coating would have a more beneficial effect on the ps coating, which by itself had a measly protective effect. consequently, the aim of this study was the preparation of protective ps polymer coatings on zinc and steel substrates in the absence and in the presence of tannic acid as a corrosion inhibitor. the novelty of the research consists in the comparison between the characteristics of ps coatings prepared on zinc and mild steel by dip-coating, using two different approaches: (i) direct addition of tannic acid into the sol before polymerization of styrene and (ii) addition of tannic acid impregnated silica nanocontainers into the sol. the coatings were characterized by electrochemical impedance spectroscopy (eis), the synthesized mesoporous nanocontainers were analyzed by transmission electron microscopy (tem), and the prepared coatings were also subjected to adhesion and coating thickness measurements. experimental materials and methods zinc and mild steel substrates were used as metal substrates for the produced coatings. the used metal wafers were pre-treated: firstly, both metal substrates were abraded with rougher emery papers in the range of 800-1500, then polished with fine emery papers (grain size in the range of 2000-5000) followed by a degreasing step, prior to the application of the coatings. polystyrene sol was prepared from the 1:2 ratio of syntevene-404 (gallstaff multiresine) and dibenzoyl peroxide 97 % (alfa-aesar). mesoporous silica nanocontainers were synthesized from the mixture of tetraethyl orthosilicate (sigma-aldrich), absolute ethanol 99.9 % (sigma-aldrich), cetyltrimethylammonium bromide  98 % (sigma-aldrich), and sodium hydroxide 97 % j. both et al. j. electrochem. sci. eng. 12(4) (2022) 721-730 http://dx.doi.org/10.5599/jese.1293 723 (acs reagent). tannic acid (sigma-aldrich), as a corrosion inhibitor, was introduced in 1 wt.%, separately into the coating matrix, respectively into the synthesized silica nanocontainers. protective coatings were prepared using a homemade dip-coater. electrochemical measurements were effectuated on a autolab 302, with the use of a three-electrode cell in 0.2 g/l na2so4 (sigma-aldrich) electrolyte solution. the three-electrode cell consisted of a working electrode (zinc and mild steel substrates), a counter electrode (platinum wire) and a reference electrode (ag/agcl/kclsat). tem measurements were performed on a h-9500,100-300 kv hitachi high-tech global machine. adhesion was measured with a tqc adhesion test kit while coating thickness measurements were effectuated with the use of an elmatronic f / nf -1250 μm measuring instrument. synthesis of silica nanocontainers 0.5 ml of 2 m naoh was added to a mixture of 0.5 g of catb and 70 ml of water. the mixture was then heated to 80 °c and 4 ml of teos was added with continuous stirring. after stirring at 80 °c for two hours, an opaque, milky solution was obtained. the precipitated solution was filtered, following which the filtrate was rinsed with distilled water (2×5 ml) and ethanol (2×5 ml). after drying in an oven for approx. 24 hours, the filtration was sintered at 600 °c for 5 hours. the process leads to obtaining mesoporous silica nanocontainers. parts of the nanocontainers were added to an aqueous tannic acid solution, for impregnation. the nanocontainers were kept for 1 hour in the 1 wt.% tannic acid solution before being filtered and washed with both water and ethanol to remove the tannic acid from the exterior of the mesoporous silica particles. transmission electron microscopy measurements transmission electron microscopy (tem) measurements were performed on the mesoporous silica nanocontainers with a h-9500,100-300 kv hitachi high-tech global measuring instrument. tem analysis served to determine the diameter of the silica nanocontainers in order to establish whether the tannic acid molecule fits on the inside of the mesoporous silica particles. preparation of polystyrene precursor four different styrene precursor solutions were prepared. the four different solutions were based on a 1:2 mixture of styrene and its polymerizing agent, dibenzoyl peroxide. one solution was left for a polystyrene layer reference (ps); in the second, nanocontainers were added in a concentration of 1 % (ps+nc); tannic acid was added in a concentration of 1 % to the third (ps+ta), and nanocontainers impregnated with 1 % tannic acid (ps+ta+nc) were added to the last solution. in every case, following the addition of filler substances, the ps sol was placed in an ultrasonic bath for approximately 15 minutes to ensure desolvation and avoid agglomeration of the aforementioned substances. all four precursor solutions, based on preliminary studies, were allowed to polymerize for 10 days until they reached the optimum viscosity for layer drawing. preparation of coatings on zinc and mild steel substrates for both metal substrates, the same pretreatment steps were followed: the first step of the pretreatment was the sanding, followed by the degreasing of the metal wafers. the zinc plates were polished using a polishing machine and an organic-based polishing paste, then degreased in hydrochloric acid, rinsed with distilled water, sonicated in alcohol, and finally dried. in the case of mild steel sheets, as they proved to be more delicate to treatment with hydrochloric acid, the http://dx.doi.org/10.5599/jese.1293 j. electrochem. sci. eng. 12(4) (2022) 721-730 modified polystyrene anti-corrosive coatings 724 surfaces were first sanded with coarser sandpaper, then thoroughly washed with a detergent containing caustic soda, in order to get the pre-existing paraffin coating, of the mild steel, off. mild steel sheets were then rinsed in alcohol and dried in a manner similar to the zinc substrates. the layers were applied to the metal surfaces by the dip-coating method. two samples were prepared from each of the four sols to monitor reproducibility. the layers were drawn at a rate of 12 cm min-1. after layer drawing, thermal treatment at 80 °c for one hour was applied until the polymerization process was complete. the end of the polymerization was indicated by the hardening of the layer. adhesion and layer thickness measurements the adhesion measurements were performed with a robust grating cutter with an aluminum head, with replaceable blades called a tqc adhesion test kit. the cutter had self-aligning blades that followed the line of the surface. a symmetrical square mesh was scratched into the metal substrate. subsequently, a special adhesive tape meeting international standards was placed on the scratched surface of the plate. the number of incised squares remaining intact on the surface versus the number of squares that ripped off with the tape came up with the tape, the percentage of adhesion was calculated using the lattice-notch formula according to certain standard tables. the layer thickness was determined using an instrument that is able to read coating thicknesses on both metallic and non-metallic surfaces by magnetic induction or eddy current method for layers of any origin, namely an elmatronic f / nf -1250 μm measuring instrument. placing the apparatus perpendicular to the layered metal surface, layer thickness values for each of our different types of layers were determined. wettability measurements wettability measurements were carried out by the sessile-drop method on all substrates and coatings mentioned. the measurements were made by adding a 20 µl electrolyte droplet of 0.2 g/l na2so4 electrolyte solution of ph 5, in a saturated na2so4 vapor atmosphere. the resulting images were processed, and contact angle determination was made in imagej software (developed by wayne rasband). electrochemical characterization of the coatings electrochemical measurements were performed on a parstat 227 potentiostat, using a threeelectrode cell consisting of a working electrode (zinc and mild steel wafer), counter electrode (pt wire) and a reference electrode (ag/agcl/kclsat) immersed in 0.2 g/l sodium sulphate (sigmaaldrich) (ph 5) electrolyte solution. the layers were firstly characterized by open-circuit potential measurements (ocp) in order to determine the resting potential values of both the coated zinc and mild steel samples. ocp measurements were left to stabilize for 60 minutes each and offered a benchmark for all further electrochemical characterization. in a further study, electrochemical impedance spectroscopy (eis) measurements were performed in the 10 mhz to 100 khz frequency range with the use of a sinusoidal current (10 mv). results and discussion transmission electron microscopy analysis previously prepared silica nanocontainers were subjected to tem analysis and the obtained images are shown in figure 1. the purpose was to get information about the shape and diameter of the particles and to determine if they belonged to the nano range at all. furthermore, we also j. both et al. j. electrochem. sci. eng. 12(4) (2022) 721-730 http://dx.doi.org/10.5599/jese.1293 725 wanted to know whether the inhibitor molecule, more specifically the tannic acid molecule fits in, given that it is a molecule of 1.85×1.65×1.01 nm [18]. based on the tem images, we can state without any doubt that the diameter of the nanocontainers is in size range of 20 to 50 nm. this would theoretically allow the tannic acid molecule to be hosted by the nanoparticles. figure 1. transmission electron microscopy images reflecting a cluster of synthesised mesoporous silica nanocontainers at scales of (a) 200 nm respectively (b) 50 nm adhesion and coating thickness evaluation prior to the application of the ps-based layers and the measurement of their adhesion to the substrate, the surface of the plates was subjected to thorough cleaning and polishing operations. all these procedures were required in order to presumably promote a high level of adhesion of the protective coating. the purpose of measuring the adhesion was to obtain information about the quality of a protective coating, more precisely, how and to what extent it has established a highquality, long-lasting adhesion to the sheet to be protected. if adhesion proves inadequate, electrolyte infiltrations may occur beneath the layer, which can cause local corrosion, which is worse than corrosion that occurs in the complete absence of a protective coating. adhesion was quantified by coating type since there were no observed significant differences between the adhesion of the coatings on the two different metals. to calculate the adherence of the prepared coating types, the lattice-notch equation (1) was applied. in equation 1, the parameter a stand for the total of squares meshed into the coating, while b, for the total number of squares ripped off by the special tape. adhesion can parallelly be assessed with the help of so-called astm standards [19], which also take into consideration the measure by which the coating gets damaged following the tearing of the square mesh. these standards come as follows: 5b (no detachment), 4b (detachment of flakes, a maximum of 5 % damage), 3b (coating flaking around the edges, less than 15 % damage), 2b (detachment of ribbons with the cuts and squares, damage less than 35 %), 1b (greater detachment of the coating along the cuts, part of the squares gone, damage lesser than 65 %) and 0b (flaking that cannot be classified). adhesion 100 a b a − = (1) it can be observed from table 1 and figure 2 that the presence of tannic acid, although slightly reduces adhesion for both substrate types. the obtained adhesion results can be explained by the fact that the formation of metal tannates on the metal surface by a reaction between ta and the metal is a faster process than the polymerization of polystyrene. if so, the polystyrene adheres to an intermediate porous layer formed by a complex of iron or zinc oxides and tannic acid [13], which leads to poorer adhesion. http://dx.doi.org/10.5599/jese.1293 j. electrochem. sci. eng. 12(4) (2022) 721-730 modified polystyrene anti-corrosive coatings 726 a b c d e f g h figure 2. adhesion tests performed on both mild steel and zinc substrates coated with the following layers: (a) zn ps, (b) zn ps+nc, (c) zn ps+ta, (d). zn ps+ta+nc, (e) mild steel ps, (f) mild steel ps+nc, (g) mild steel ps+ta, (h) mild stel ps+ta+nc the layers thicknesses were determined for all the investigated coatings using an instrument that is able to read layer thicknesses on surfaces by magnetic induction or the eddy current method. the results are presented in table 1 and show that the addition of nc or ta in the ps layer leads to an increase of the coating thickness, which is approximately the same in all cases, around 20 µm. table 1. results of adhesion and layer thickness evaluation for the ps, ps+nc, ps+ta and ps+ta+nc coatings on zinc and mild steel substrates sample layer thickness, µm adhesion, % adhesion class (astm) zn/ps 8.0 95 4b zn/ps+nc 20.0 95 4b zn/ps+ta 21.8 91 3b zn/ps+ta+nc 21.8 87 3b mild steel/ps 8.0 ∼85 2b mild steel/ps+nc 20.0 ∼ 65 1b mild steel/ps+ta 21.8 ∼85 2b mild steel/ps+ta+nc 21.8 ∼85 2b in the case of the mild steel wafers, the coatings tore to a much greater extent, which led to the conclusion of a poorer adhesion than in the case of the zinc substrates. an explanation of this phenomenon can be that the mild steel has proven over the course of the experiment to be much more reactive to any corrosive factor than zinc, which possibly includes the additives of the ps. wettability measurements wettability measurements were performed on all types of coatings on both zinc and mild steel substrates. figure 2 contains the results of wettability measurements and the initial contact angle values of each coating on both zinc and mild steel substrates. tannin acid is a plant-derived polyphenolic substance that is hydrophilic, which explains the lower contact angle noticed in its presence in the coatings. same observation for the silica nc, which also increase hydrophilicity of the coating and thus, wettability. nevertheless, the decrease is not a significant one. j. both et al. j. electrochem. sci. eng. 12(4) (2022) 721-730 http://dx.doi.org/10.5599/jese.1293 727 figure 3. wettability measurements (at 0 min) effectuated with 20 µl droplet of 0.2 g/l na2so4 electrolyte solution on the following coatings: a. zn ps, b. zn ps+nc, c. zn ps+ta, d. zn ps+ta+nc, e. fe ps, f. fe ps+nc, g. fe ps+ta, h. fe ps+ta+nc electrochemical characterization figure 4 shows the complex plane representation of the eis spectra of the various layers deposited on zinc plates. as expected, the impedance value of the undoped polystyrene layer (ps) on zinc is higher than the impedance of the bare zinc plate. the presence of the empty silica nanocontainers in the polymeric coating (ps + nc) ruins the impedance of the ps by lowering its impedance values beneath those of the bare zinc plate. the nanoparticles seem to act as defects of the coating, favoring corrosion. low compatibility between ps and silica can also explain the poor corrosion resistance of (ps+nc) coatings. figure 4. eis plots of the zn ref, ps, ps+nc, ps+ta, ps+ta+nc coatings on zn substrates in 0.2 g/l na2so4 solution (ph 5) in the next step, silica nanocontainers impregnated with tannic acid were introduced into the polystyrene layer resulting (ps + nc + ta) coatings, and on the other hand, ps coatings were directly impregnated with tannic acid (ps+ta). http://dx.doi.org/10.5599/jese.1293 j. electrochem. sci. eng. 12(4) (2022) 721-730 modified polystyrene anti-corrosive coatings 728 the eis plots corresponding to the different coatings put in evidence the beneficial effect of tannic acid. tannic acid is known as an effective anticorrosive agent [20] that reacts with corrosion products formed on the metal surface and stabilizes them. this characteristic can be observed in this case as well. a comparison between the impedance spectra of zn/(ps + nc +ta) and zn/(ps+ta) coatings leads to the conclusion that ta is more effective when it is directly incorporated in ps (a larger capacitive loop is observed in the latter case). this is probably due to the fact that tannic acid is likely to remain adsorbed in a higher concentration in ps than in the presence of nanocontainers, which were washed with water after impregnation and some tannic acid may have been removed during the process. a second possibility is the better compatibility between ps and ta than between ps and silica. compared to the ps and (ps + nc) layers, tannic acid strongly exerted an inhibiting effect both in the (ps + ta + nc) and the (ps + ta) coatings. it can also be observed that in the presence of ta in the coatings, the shape of the spectra changes (a second capacitive loop is outlined), suggesting the change in the mechanism of the corrosion process. further research and modelling of the spectra will elucidate this aspect. figure 5 presents the impedance spectra of coatings deposited on mild steel substrates coated with the same aforementioned sols. figure 5. eis plots of the fe ref, ps, ps+nc, ps+ta, ps+ta+nc coatings on mild steel substrates in 0.2 g/l na2so4 solution (ph 5) the same resistance-improving effect of tannic acid, either embedded in silica nanocontainers or directly in the polymeric matrix, was observed compared to the undoped ps coating. at the same time, it should be mentioned that compared to the modified ps layers on zinc, the (ps+ta) and (ps+nc+ta) layers drawn on mild steel show lower impedance values. this observation was corroborated with literature data mentioning that tannic acid is a more efficient inhibitor on zinc than on mild steel [20]. a possible explanation in our case could be the fact that zinc substrates were much easier to polish in the pretreatment step, resulting in a smoother surface for the polymeric coating to adhere to. in many cases, zinc has better compatibility with materials, while mild steel oxidizes extremely quickly and often forms complexes not compatible with passive corrosion protective methods. on the mild steel substrates, the end result is the same as in the case of zinc: ta improves the corrosion resistance of the coatings, and the polystyrene layer impregnated directly with tannic acid has a higher anticorrosion resistance than that containing the ta modified silica nanocontainers. 0 10000 20000 30000 40000 50000 0 10000 20000 30000 40000 50000 fe ref ps ps+ta+nc ps+ta ps+nc -z " / ω c m 2 z' / ω cm2 0 1000 2000 3000 0 1000 2000 3000 z' / ω cm2 -z " / ω c m 2 fe ref ps j. both et al. j. electrochem. sci. eng. 12(4) (2022) 721-730 http://dx.doi.org/10.5599/jese.1293 729 conclusions the effect of tannic acid embedded in polystyrene coatings by two different ways on the anticorrosion protection of zinc and steel was investigated by electrochemical, layer thickness and adhesion measurements. on the one hand, silica nanocontainers of 20-50 µm diameter were impregnated with an aqueous solution of tannic acid, then introduced into a polystyrene coating matrix, and on the other hand, tannic acid was directly introduced into the polystyrene coating in the sol-preparation step. the results corresponding to doped polystyrene were compared with those for simple polystyrene coating and polystyrene containing empty silica nanocontainers. tannic acid showed an inhibitory effect in both systems compared to polystyrene layers with empty nanocontainers for zinc and steel. unfortunately, adhesion is reduced by the presence of tannic acid. this may be due to the fact that the formation of the tannate complex on the surface determines polystyrene to bind to an intermediate, porous layer, resulting in poor adhesion of ps coatings. however, the increased corrosion resistance makes doped ps layers promising candidates for low-cost, corrosion-resistant protective coatings. best corrosion-resistant properties were evident when tannic acid was directly introduced into the polystyrene coating in the sol-preparation step. acknowledgements: financial support from the program entrepreneurship for innovation through doctoral and postdoctoral research pocu/380/6/13/123886 is appreciated. julia both thanks for the financial support from the romanian ministry of national education within her phd stage. references [1] m. g. fontana, n. d. greene, corrosion engineering, print book, tata mcgraw-hill, new delhi, india, 2005. isbn-10: 0070214638 [2] r. g. buchheit, h. guan, s. mahajanam, f. wong, progress in organic coatings 47 (2003) 174-182. https://doi.org/10.1016/j.porgcoat.2003.08.003 [3] j. l. fang, y. li, x. r. ye, z. w. wang, q. liu, corrosion 49 (1993) 266-271. https://doi.org/ 10.5006/1.3316048 [4] g. szabó, e. albert, j. both, l. kócs, g. sáfrán, a. szöke, z. hórvölgyi, l. m. mureşan, surfaces and interfaces 15 (2019) 216-223. https://doi.org/10.1016/j.surfin.2019.03.007 [5] e. wierzbicka, b. vaghefinazari, s. v lamaka, m. l. zheludkevich, m. mohedano, 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©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.phpro.2011.06.084 http://www.ijnnonline.net/article_31222_802cf3b5ac5eb0991fd7c3ea52fa4cbe.pdf http://www.ijnnonline.net/article_31222_802cf3b5ac5eb0991fd7c3ea52fa4cbe.pdf https://doi.org/10.1039/c2nr11536k https://doi.org/10.1016/j.electacta.2018.12.062 https://doi.org/10.1039/c6ra15773d https://doi.org/10.1039/c6ra15773d https://doi.org/10.1380/ejssnt.2020.62 https://doi.org/10.1002/maco.201307277 https://doi.org/10.1002/maco.201307277 https://creativecommons.org/licenses/by/4.0/) @article{both2022, author = {both, julia and szabo, gabriella stefania and katona, gabriel and mureșan, liana maria}, journal = {journal of electrochemical science and engineering}, title = {{anticorrosive polystyrene coatings modified with tannic acid on zinc and steel substrates:}}, year = {2022}, issn = {1847-9286}, month = {jun}, number = {4}, pages = {721--730}, volume = {12}, abstract = {polystyrene (ps) polymer layers were prepared by the sol-gel method and studied as anticorrosive barrier layers on carbon steel and zinc substrates. to increase the corrosion resistance of the coatings, two different approaches were considered: (i) the use of mesoporous silica-nano­containers impregnated with a corrosion inhibitor (tannic acid) introduced into the polystyrene matrix and (ii) direct impregnation of polystyrene coatings with the same corrosion inhibitor. the impregnated nanocontainers were characterized by transmission electron microscopy. the thickness and the adhesion of the coatings were measured, and their corrosion behavior was investigated by electrochemical impedance spectroscopy. results showed that the used inhibitor slightly decreased adhesion, but significantly increased the corrosion resistance of the coatings. the direct introduction of tannic acid into the polymer matrix offers higher corrosion resistance than in the case of polystyrene coatings doped with impregnated silica nanocontainers.}, doi = {10.5599/jese.1293}, file = {:d\:/onedrive/mendeley desktop/both et al. 2022 anticorrosive polystyrene coatings modified with tannic acid on zinc and steel substrates.pdf:pdf;:10_jese_1293.pdf:pdf}, keywords = {polystyrene coatings, corrosion protection, mesoporous silica nanocontainers, tannic acid}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1293}, } mechanical and microstructural properties of yttria-stabilized zirconia reinforced cr3c2-25nicr thermal spray coatings on steel alloy http://dx.doi.org/10.5599/jese.1278 819 j. electrochem. sci. eng. 12(5) (2022) 819-828; http://dx.doi.org/10.5599/jese.1278 open access : : issn 1847-9286 www.jese-online.org original scientific paper mechanical and microstructural properties of yttria-stabilized zirconia reinforced cr3c2-25nicr thermal spray coatings on steel alloy sukhjinder singh1,, khushdeep goyal1 and rakesh bhatia2 1department of mechanical engineering, punjabi university, patiala, india 2yadavindra department of engineering, punjabi university guru kashi campus, damdama sahib, india corresponding author: sukhjindermoga@gmail.com; tel.: +91-9876785500 received: february 3, 2022; accepted: march 21, 2022; published: march 29, 2022 abstract in this research work, nano yttria-stabilized zirconia (ysz) reinforced cr3c2-25nicr composite coatings were prepared and successfully deposited on asme-sa213-t-22 (t22) boiler tube steel substrates using high-velocity oxy-fuel (hvof) thermal spraying method. different nanocomposite coatings were developed by reinforcing cr3c2-25nicr with 5 and 10 wt.% ysz nanoparticles. the nanocomposite coatings were analysed by scanning electron microscope (sem)/energy-dispersive x-ray spectroscopy (eds) and x-ray diffraction (xrd) technique. the porosity of yszcr3c2-25nicr nanocomposite coatings was found to be decreasing with the increase in ysz content, and hardness has been found to be increasing with an increase in the percentage of ysz in the composite coatings. the coating of 10 wt.% ysz-cr3c2-25nicr showed the lowest porosity, lowest surface roughness, and highest microhardness among all types of coatings. this may be due to the flow of ysz nanoparticles into the pores and gaps that exist in the base coatings, thus providing a better shield to the substrate material. keywords boiler steel tube; composite nanoparticles; coatings; surface roughness; porosity; hardness introduction traditional steels used in thermal power plants are susceptible to corrosion [1,2]. in the recent past, researchers have applied several types of coatings to improve the erosion and corrosion resistance of these steels [3-5]. thermal spray coating techniques are a key tool for developing coatings that improve component performance and longevity [6,7]. in recent years, these coatings have become more important. coatings with high anti-corrosion qualities have been developed as a result of advancements in powder manufacture and innovations in thermal spraying techniques [8-10]. in terms of substrate material chemical composition, these procedures have no special material limitations [11]. on boiler tube steel components, flame spraying, plasma spraying, arc spraying, http://dx.doi.org/10.5599/jese.1278 http://dx.doi.org/10.5599/jese.1278 http://www.jese-online.org/ mailto:sukhjindermoga@gmail.com j. electrochem. sci. eng. 12(5) (2022) 819-828 ysz reinforced cr3c2-25nicr thermal spray coatings 820 and high-velocity oxygen fuel (hvof) methods can generate coatings of a few millimeters thickness with a high microhardness value [12-15]. because of its cost-effectiveness and versatility, the hvof method has been classified as an adaptable technique [16-18]. the qualities of the substrate material are unaffected by the hvof coating procedure [19]. in the recent past, various researchers have used thermal spraying techniques to develop various types of coatings on boiler steels to increase their properties. the coatings produced by the thermal spraying method are porous in nature and have many local micro-cracks or through pores [20-22]. corrosive fluids and chemicals attack the substrate steels through these pores and micro-cracks. therefore, there is still scope for improvement in the mechanical and microstructural properties of these coatings [23-25]. many researchers have compared conventional coatings to nanostructured coatings, and many improvements in mechanical and microstructural properties of as-sprayed materials were observed, including an increase in microhardness, a decrease in porosity, a decrease in surface roughness, and a decrease in erosion rate, among other things [26-29]. many authors have reported the development of cr3c2-25nicr coatings on steel alloys, but literature related to nano yttria-stabilized zirconia (y2o3/zro2) (ysz) reinforced composite coatings is not available. therefore, there is scope to develop new nano yttria-stabilized zirconia (ysz) mixed cr3c2-25nicr nanocomposite coatings and subsequently deposit and investigate the microstructure, porosity, and microhardness of these newly developed composite coatings on boiler tube steel. in this research work, hvof sprayed 5 and 10 weight percent ysz-cr3c2-25nicr nano-coatings were developed and deposited on t22 boiler tube steel. the microstructure, porosity and microhardness of these newly developed composite coatings have been investigated. hvof thermal spraying technique was used in this research work because the coatings produced with the hvof method have high adhesive strength with the base material and also individual splats have high cohesive strength [30-31]. ksiazek et al. [20] observed that this spraying process provides homogeneous coatings having a low value of porosity along with high hardness. experimental substrate material the measured and nominal compositions of t22 steel are shown in table 1. the samples with dimensions of 22155 mm were manufactured from the boiler tube. silicon carbide paper was used to polish the cut samples. before applying different coatings, the samples were shot blasted with alumina powder of grit 45. table1 chemical composition of t-22 steel content, wt.% c mn si s p cr mo v ni fe nominal 0.15 0.3-0.6 0.5-1 0.03 0.03 1.9-2.6 0.07-1.13 --bal. actual 0.148 0.524 0.762 0.039 0.0129 2.692 0.928 0.031 0.019 bal. coating powders commercially available blend cr3c2-25nicr powder was mixed with 5 and 10 wt.% ysz using low energy ball milling to prepare different coating powders and the composition of different powders is shown in table 2. to prepare cr3c2-25nicr mixed with 5 wt.% ysz (y2o3/zro2) mixture, 950 g of cr3c2-25nicr was mixed with 50 grams of ysz. other compositions were prepared in a similar manner. the mixed powders were rolled for four hours ceaselessly at a speed of 200 rpm [32,33]. s. singh et al. j. electrochem. sci. eng. 12(5) (2022) 819-828 http://dx.doi.org/10.5599/jese.1278 821 table 2. composition of feed stock powders feed stock powder base powder cr3c2-25nicr content, wt.% content of reinforced ysz nano powder, wt.% cr3c2-25nicr 100 0 5 wt.% ysz(cr3c2-25nicr ) 95 5 10 wt.% ysz(cr3c2-25nicr) 90 10 formulation of coating the conventional cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocoatings were deposited on t22 boiler steel substrates with the hvof process at metallizing equipment company limited, jodhpur, india. the spraying process was carried out using a commercial hipojet-2100 device. before deposition of the coatings, the samples were grit blasted with alumina powder. coatings with a thickness of around 250 µm were deposited. the process parameters of the hvof spraying method are shown in table 3. during the spraying procedure, these process parameters were kept constant. table 3. hvof spraying process parameters oxygen flow rate 280 l min-1 fuel (acetylene) flow rate 100 l min-1 air-flow rate 600 l min-1 spray distance 200 mm powder feed rate 25 g min-1 fuel pressure 1.6 kg cm-2 oxygen pressure 3.00 kg cm-2 air pressure 4.40 kg cm-2 characterization of nanocomposite coatings the uncoated samples were cut into sections and mounted in epoxy. before the metallurgical inspection, the mounted samples were polished. the coatings were examined using xrd, sem/eds, and cross-sectional elemental analysis. the microhardness of the cross-section of all coated samples was assessed using a mitsubishi microhardness tester. on the coating–substrate interface, the microhardness was measured at specified intervals along the cross-section. leica image analyser software was used to evaluate the porosity of conventional cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coatings. before evaluating porosity, the specimens were polished. the pore area size is calculated using a computer-based porosity analysis technique that converts grey-level areas (pore areas) into a background that is different from the rest of the microstructure. the porosity value is then calculated by counting the number of pixels of background colour. for each type of coated specimen, the average of five porosity measurements was calculated. results coating thickness measurement the thickness of conventional cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coatings was measured with the help of minitest-2000 thin film thickness gauge (make: elektrophysik koln company, germany, precision ±1 µm) during the spraying process and are shown in table 4. the thickness of the coatings was evaluated along the cross-section of the specimens and the thickness has been found in the desired range [34-36]. http://dx.doi.org/10.5599/jese.1278 j. electrochem. sci. eng. 12(5) (2022) 819-828 ysz reinforced cr3c2-25nicr thermal spray coatings 822 porosity analysis thermal spray coatings are porous, and porosity has a significant impact on coating qualities. less porous coatings provide superior corrosion protection, according to the literature. the apparent porosity measurements of cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coated t22 steel specimens are shown in table 4. for all samples, the porosity values of the hvof-sprayed cr3c2-25nicr coating were less than 2 %. the numbers in table 4 show that as the ysz concentration in the nanocomposite coating increased, the porosity value also decreased. it is obvious that 10 wt.% ysz-cr3c2-25nicr coating has the lowest porosity value. the surface roughness values for cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite were found to be 3.75, 3.14 and 2.43 µm, respectively. table 4. average coating thickness and porosity values for different coatings substrate coating type coating thickness, µm porosity, % average surface roughness, μm asme sa213-t22 cr3c2-25nicr 251 1.81 3.75 5 wt.% ysz-(cr3c2-25nicr) 258 1.57 3.14 10 wt.% ysz-(cr3c2-25nicr) 252 1.25 2.43 microhardness measurement the microhardness profiles across the cross-section of cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coated specimens are shown in figure 3. the microhardness values of t22 steel were in the range of 242-318 hv. the microhardness measurements for cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coated specimens were in the range of 918-978 1018-1093 and 1088-1168 hv, respectively. it is clearly seen in figure 1 that with the increase of ysz in the cr3c2-25nicr matrix, the micro-hardness value increased. the nano ysz particles were able to increase the microhardness of hvof sprayed cr3c2-25nicr coatings. the microhardness profiles clearly show that the hardness through the coating cross-section is nearly uniform for all coated specimens. figure 1. microhardness profiles of cr3c2-25nicr, 5 wt.% and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coatings across the cross-section 0 200 400 600 800 1000 1200 1400 -120 -80 -40 0 40 80 120 160 200 m ic ro h a rd n e ss , h v distance from interface, µm cr3c2-25nicr 5wt.% ysz(cr3c2-25nicr) 10wt.% ysz(cr3c2-25nicr) s. singh et al. j. electrochem. sci. eng. 12(5) (2022) 819-828 http://dx.doi.org/10.5599/jese.1278 823 x-ray diffraction analysis the x-ray diffraction analysis for the surfaces of cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coated specimens was done, and the xrd patterns are shown in figure 2(a-c). xrd profile of the hvof sprayed cr3c2-25nicr coated t22 boiler tube steel sample showed chromium as the main phase, along with traces of ni. the xrd profile of the cr3c2-25nicr coating reinforced with and 10 wt. % ysz nanoparticles revealed that chromium and carbon are present as a major phase, and nickel, yttrium, and zirconium as minor phases. the increase in the formation of non-crystalline amorphous phases occurs because of very fast cooling during the spraying process. the presence of different phases and their proportion mostly depend on the process conditions at the time of the depositing of the coating powder on the base material. 2 / o  fe  c  cr  yzs  ni figure 2. xrd profile of: (a) cr3c2-25nicr; (b) 5 wt.% ysz-(cr3c2-25nicr); (c) 10 wt.% ysz-(cr3c2-25nicr) fe-sem/energy dispersive x-ray spectroscopy fe-sem micrographs with energy-dispersive x-ray spectroscopy analysis for hvof sprayed cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coatings on t22 boiler steel are shown in figure 3. the microstructure of cr3c2-25nicr coating is dense, consisting of interlocked particles with regular shape, as shown in figure 3(a). in the microstructure of the coating, several oxide stringers can also be visible. the ysz nanoparticles have been uniformly diffused in the cr3c2-25nicr matrix, as shown in figures 3(b) and 3(c). the dense and uniform layer of the coating was obtained by the reinforcement of nanoparticles of ysz. the microstructures reveal that uniform coalescence of nano ysz has occurred with base cr3c2-25nicr matrix in composite coating. as demonstrated in figures 3(b) and 3(c), energy dispersive spectroscopy examination revealed the elemental composition of the various coatings that were found to be comparable to that of the feedstock powder. eds analysis of cr3c2-25nicr coating revealed the presence of fe and si in the composition, which may be due to the diffusion of fe and si from the substrate to the coating matrix due to porosity in conventional coating. the microstructure of the cross section of cr3c2-25nicr, and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coatings on t22 boiler steel is shown in figure 4. the cross-sectional images indicate splat-like morphology of coatings, which might be due to the re-solidification of molten droplets. http://dx.doi.org/10.5599/jese.1278 j. electrochem. sci. eng. 12(5) (2022) 819-828 ysz reinforced cr3c2-25nicr thermal spray coatings 824 figure 3. fe-sem with eds analysis of hvof sprayed coatings: (a) cr3c2-25nicr; (b) 5 wt.% ysz(cr3c2-25nicr); (c) 10 wt.% ysz(cr3c2-25nicr) figure 4. cross-sectional morphology of hvof sprayed coatings: (a) cr3c2-25nicr; (b) 5 wt.% ysz(cr3c2-25nicr); (c) 10 wt.% ysz(cr3c2-25nicr) discussion the coating thickness of all coatings was measured along the cross-section of the coated specimens. the coating thickness was in the range of 251-258 μm, which was found to be in the desired (a) element content, wt.% c 17.30 cr 68.21 ni 14.41 fe 0.04 si 0.02 element content, wt.% c 16.38 cr 66.27 ni 17.28 fe 0.03 si 0.02 2 1 (b) element content, wt.% c 14.93 o 4.50 cr 68.22 ni 17.28 y 1.02 zr 7.99 si 0.01 element content, wt.% c 15.50 o 5.13 cr 66.17 ni 3.49 y 1.17 zr 8.17 si 0.01 2 1 element content, wt.% c 12.49 o 16.45 cr 44.59 ni 6.78 y 1.87 zr 17.67 element content, wt.% c 13.65 o 15.24 cr 46.98 ni 7.12 y 1.58 zr 15.37 (c) 2 1 s. singh et al. j. electrochem. sci. eng. 12(5) (2022) 819-828 http://dx.doi.org/10.5599/jese.1278 825 range as reported in the previous work for hvof coatings [37]. the porosity of the conventional cr3c225nicr coating on t22 boiler tube steel was found to be 1.81%, which further decreases with the addition of nano ysz to the cr3c2-25nicr coating matrix. the porosity values of 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite were observed as 1.57 and 1.25 %, respectively. the porosity of nanocomposite coatings decreased as the nanoparticles of nano ysz filled the pores and interlocked the grains of the cr3c2-25nicr coating matrix. improvement in surface roughness value was observed by the addition of the nanoparticles of ysz in the cr3c2-25nicr coating powder. the surface roughness values for the cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coated samples were found to be 3.75 µm, 3.14 and 2.43 µm, respectively. better surface characteristics were observed for 10 wt.% percent ysz reinforced cr3c2-25nicr nanocomposite coating as compared to the conventional cr3c2-25nicr coating, as the surface roughness decreased because of the addition of ysz nanoparticles. the decrease in porosity with the addition of carbon nanotubes has also been reported in the literature by khesri et al. [38] and guo et al. [39]. goyal and goyal [40] also reported that carbon nanotubes interlocked the particles of cr3c2–20nicr and improved the mechanical and microstructural properties of conventional cr3c2–20nicr coating. in comparison to the base material, the microhardness values of all coated specimens were found to be extremely high, as shown in figure 1. microhardness values for typical cr3c2–25nicr ranged from 918 to 978 hv. with an increase in nano ysz weight percent in the cr3c2–25nicr matrix, microhardness values improved even further. the inclusion of nanoparticles in the coating matrix improved indentation resistance. the high heat conductivity of ysz may result in increased melting and, as a result, an increased microhardness of ysz reinforced coatings. the nano ysz particles were uniformly scattered in cr3c2–25nicr matrix, filling the pores in the coating matrix, which resulted in a decrease in the porosity of the matrix. according to tian et al. [41], the increase in hardness can be attributed to a decrease in porosity of the coating matrix and may also be due to dispersion hardening. cr and ni are the main phases and ni is the minor phase as was identified by x-ray diffraction analysis of cr3c2–25nicr coated t22 boiler tube steel specimen. the identification of small peaks of fe and si in xrd spectra might be due to the diffusion of these elements from substrate alloy to the coating matrix due to porosity in the matrix. the xrd spectra of 5 wt.% and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coated samples revealed major phases of cr and ni, along with ysz in the coating matrix. the increase in the formation of noncrystalline amorphous phases occurs because of very fast cooling during the spraying process. stewart et al. [42] reported that the presence of different phases and their proportion mostly depend on the process conditions at the time of the depositing of the coating powder on the base material. fe-sem with eds analysis of conventional cr3c2-25nicr, 5 and 10 wt.% ysz reinforced cr3c2-25nicr nanocomposite coatings on t22 boiler steel showed that coatings obtained are dense, uniform and a proper coalescence of ysz with cr3c2-25nicr have been taken place and ysz particles have been distributed uniformly within the coating matrix. energy dispersive spectroscopy examination of all coatings revealed that the elemental composition of the various coatings was found to be comparable to that of the feedstock powder. eds analysis of cr3c2-25nicr coating revealed the presence of fe and si in the composition, which may be due to the diffusion of fe and si from the substrate to the coating matrix due to porosity in conventional coating. diffusion of base elements through pores/voids in the coating matrix has also been reported by various authors [3437]. the cross-sectional morphology of all coatings indicated dense and uniform misconstrue. all the coatings had a uniform and intact misconstrue. nano ysz coatings indicated that ysz particles had uniformly mixed in the base matrix throughout the matrix. reinforcement of nano ysz particles http://dx.doi.org/10.5599/jese.1278 j. electrochem. sci. eng. 12(5) (2022) 819-828 ysz reinforced cr3c2-25nicr thermal spray coatings 826 in cr3c2-25nicr coating had filled the gaps, reducing porosity which prevented the diffusion of base elements to the coating matrix, thereby increasing the microhardness of the coatings. the present study revealed that adding ysz nanoparticles to conventional coatings improved bonding at the substrate-coating interface, filled voids/pores in the coating matrix, enhanced microhardness, and resulted in dense and uniform coatings on boiler steel samples. conclusions the following conclusions are made from this experimental work: • the thickness of hvof sprayed cr3c2-25nicr, 5 wt.% and 10 wt.% ysz reinforced cr3c225nicr nanocomposite coatings was found to be in the range of 250-260 μm. • with the increase in ysz concentration in nanocomposite coating, the porosity value decreases. the 10 wt.% ysz-(cr3c2-25nicr) coating was discovered to have the lowest porosity value of 1.25%. a decrease in porosity resulted in an improvement in surface roughness values. • the highest microhardness was found for 10 wt.% ysz reinforced nanocomposite coating and was in the range of 1088-1068 hv. this might be due to the filling of pores/voids in the coating matrix by nano ysz particles. • xrd spectra of all nanocomposite coatings indicated the formation of non-crystalline amorphous phases due to very fast cooling during the spraying process. • sem/eds analysis of cr3c2-25nicr coating indicated the presence of fe and si, which might be due to the diffusion of these elements through pores in the coating matrix. 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https://dx.doi.org/10.1007/s12613-019-1742-8 http://dx.doi.org/10.4236/jmmce.2012.116041 https://doi.org/10.1108/acmm-06-2018-1954 https://doi.org/10.1016/j.surfcoat.2009.11.032 https://doi.org/10.1007/s00170-014-5661-6 https://doi.org/10.1080/02670844.2019.1662645 https://doi.org/10.1080/02670844.2019.1662645 https://doi.org/10.3390/coat‌ings803‌0112 https://doi.org/10.3390/coat‌ings803‌0112 https://doi.org/10.1016/s0043-1648%20(99)00032-0 https://creativecommons.org/licenses/by/4.0/) {a short introduction to digital simulations in electrochemistry: simulating the cottrell experiment in ni labview} doi:10.5599/jese.507 171 j. electrochem. sci. eng. 8(2) (2018) 171-181; doi: http://dx.doi.org/10.5599/jese.507 open access : : issn 1847-9286 www.jese-online.org original scientific paper a short introduction to digital simulations in electrochemistry: simulating the cottrell experiment in ni labview soma vesztergom eötvös loránd university, department of physical chemistry, pázmány péter sétány 1/a, 1117 budapest, hungary e-mail: vesztergom@chem.elte.hu; tel.: +36-20-461-2429 received: february 9, 2018; revised: march 28, 2018; accepted: march 28, 2018 abstract a brief introduction to the use of digital simulations in electrochemistry is given by a detailed description of the simulation of cottrell’s experiment in the labview programming language. a step-by-step approach is followed and different simulation techniques (explicit and implicit euler, runge–kutta and crank–nicolson methods) are applied. the applied techniques are introduced and discussed on the basis of padé approximants. the paper might be found useful by undergraduate and graduate students familiarizing themselves with the digital simulation of electrochemical problems, as well as by university lecturers involved with the teaching of theoretical electrochemistry. keywords explicit and implicit euler method; runge–kutta methods; crank–nicolson method; padé approximants. introduction at the 6th regional symposium on electrochemistry for south-east europe (6th rse–see), i gave a keynote lecture [1] on the kinetics of hydrogen evolution in mildly acidic solutions. under these chemical conditions — at low to moderate ph values (1 < ph < 4) — the reduction of h+ ions and that of water molecules both give a significant contribution to the measured cathodic current. in my talk, i presented digital simulations (and a simplified analytical model) that can describe polarization curves measured on rotating disk electrodes immersed into mildly acidic solutions. results presented in my talk at the 6th rse–see were published [2] just a few weeks before the conference. instead of further discussing these results, i chose to focus more in this paper on the applied method of digital simulation (ds). ds presents a powerful tool that can be used for the qualitative understanding, as well as for the quantitative description of electrode processes. when carrying out ds, we create a numerical http://dx.doi.org/10.5599/jese.507 http://www.jese-online.org/ mailto:vesztergom@chem.elte.hu j. electrochem. sci. eng. 8(2) (2018) 171-181 digital simulations in electrochemistry 172 model of the electrochemical system within a computer and allow this model to evolve according to a set of simple algebraic equations. these equations are derived from the physical laws that govern the electrochemical system under study. in effect, we carry out a simulation of the experiment with the aim to extract numeric representations of current, concentration profiles, potential transients, and so on [3]. the results of digital simulations can then be compared to measured quantities, so that the physical parameters of the system can be fine-tuned in subsequent simulations, until the measured data are reproduced at a desired accuracy. ds thus allows the determination of kinetic parameters, such as charge transfer rates and diffusion coefficients, etc. ds can yield important quantitative information even for complex electrochemical systems where the often-complicated set of partial differential equations, describing the transformation and movement of material, can be solved in closed form with difficulty or not at all [3]. in this paper i give a very brief and practice-oriented introduction to digital simulations and their use in electrochemistry. instead of reviewing the topic of digital simulations in detail — several good textbooks are dedicated to the subject and are recommended to the reader [3,4] — i choose a rather straightforward approach: that is, giving a step-by-step guide to the simulation of a simple electrochemical problem, that is known as cottrell’s experiment [5]. the paper is intended to be a starting point for undergraduate or graduate students, interested in digital simulations. furthermore, the paper may also be found useful by university lecturers, as it contains a simpler than usual description and classification of different partial differential equation solving methods, based on padé’s approximants. experimental cottrell’s experiment, first described in 1903 by f.g. cottrell, is one of the oldest problems in electrochemistry that has a well-known analytical solution. the simulation of cottrell’s experiment is thus expedient, since the success of any simulation can directly be tested [6] by comparison to analytically obtained results. in order to understand cottrell’s experiment, consider [7] a large planar electrode, of surface a, initially at rest, in contact with a semi-infinite layer of unstirred solution that contains some excess electrolyte and a small amount of a redox-active species r with a bulk concentration of c. at the instant t = 0, the electrode potential is suddenly changed to a value at which the species r is oxidized to some product o in the one-electron reduction r → o + e(1) and the concentration of r, close to the electrode surface, is brought to essentially zero. in this experiment, the reaction rate (and thus, the current) is determined by the rate of transport at which the species r is resupplied at the electrode surface. that is, the partial differential equation governing the system is that describing fick’s law,   =   2 2 . c c d t x (2) equation (2) is a second-order partial differential equation where c denotes the concentration and d the diffusion coefficient of the species r, while x and t denote the space and time variables, respectively. to obtain an analytical solution of equation (2), we take two boundary conditions into consideration: i.) that the concentration of the species r is zero at the electrode surface at any t > 0 and ii.) that the concentration at infinite distance from the electrode surface never changes and remains equal to c s. vesztergom j. electrochem. sci. eng. 8(2) (2018) 171-181 doi:10.5599/jese.507 173 figure 1. the graphical user interface (top) of a simple labview program that simulates cottrell’s experiment. snippets (code details) are shown in the blue boxes; see the text for explanation j. electrochem. sci. eng. 8(2) (2018) 171-181 digital simulations in electrochemistry 174 taking the above boundary conditions into consideration leads to the solution: ( )    =     , erf , 4 x c x t c dt (3) where the function erf(z) denotes gauss’s error function, defined by the integral: ( ) ( )= − 2 0 2 erf exp d . π z z u u the current density j(t), passing the electrode surface at any time t > 0 is thus: ( ) ( )  →   = =   0 lim , , πx d j t f d c x t f c x t (4) where f = 96485.3 c mol–1 is faraday’s constant. equation (3) makes it straightforward to calculate the concentration of species r at any given time t > 0 at a distance x measured from the electrode surface; while from equation (4), the wellknown cottrell equation, the current density can be calculated for any t > 0. these analytical formulae will be used below for testing the results of digital simulations. simulation approaches labview, developed by national instruments, is an easy-to-use, interactive, graphical programming language that helps users write sophisticated programs and applications in a short amount of time without needing a computer science degree [8]. the popularity of labview continuously increases amongst students and scientific researchers, also in electrochemistry, mostly due to its versatility in measurement automation. however, labview also provides simple means for digital simulations and i therefore chose to present the basics of simulation algorithms in this programming language. figure 1, discussed extensively below, shows the user interface of a program written for the simulation of cottrell’s experiment, as well as some details of the code. the main routine is that shown by the simulate.vi snippet. in most digital simulation schemes, we consider the electrolyte solution in terms of discrete and small (δx wide) volume elements, as illustrated by figure 2. when we apply this approach to the simulation of cottrell’s experiment in ni labview, we create a 1-dimensional array of concentrations, containing n entries, which we denote by c. at the start of the simulation process, all entries of the array c will be set equal to c (the bulk concentration). the initialization of the c array in labview is shown by the create c array.vi snippet of figure 1. each i entry of the array c (the index i can take values between 0 and n – 1 in labview) corresponds to the concentration of the species r at a certain distance from the electrode surface, as shown by figure 2. for accurate simulations, the δx width of the control volumes must be small, thus to include a big enough space in the simulations, a sufficiently high number of the control volumes is required. (see table 1 for numeric parameter values, used for the simulations presented in this paper.) when carrying out digital simulations, we iteratively modify the entries of the c array, as shown by the simulate.vi snippet of figure 1. each iteration step represents a δt time-step in which we replace the first element of c with zero (thereby realizing the boundary condition of the experiment) and then multiply the array (vector) c with a so-called stepper matrix, denoted here by s. as the iteration proceeds, we continuously “update” the c array, which at the end of the iteration will turn to be a discrete representation of the concentration profile, corresponding to the time t. s. vesztergom j. electrochem. sci. eng. 8(2) (2018) 171-181 doi:10.5599/jese.507 175 table 1. simulation parameters. symbol meaning value δx width of the control volumes 0.001 cm n number of the control volumes 100 d diffusion coefficient 10–6 cm2 s–1  c bulk concentration 10–6 mol cm–3 δt time-step varied between 1 ms and 5 s figure 2. scheme for the digital simulation of cottrell’s experiment the multiplication of the c array with the stepper matrix s in each iteration step mimics the role of transport in the simulation process, thus the construction of the stepper matrix has a deep impact on both the accuracy and the speed of the simulation. to understand the role of the stepper matrix s in digital simulations, let us consider figure 2, and assume that in each control volume shown, the concentration values are different. let us then try to estimate the changes of concentration in the control volume marked by the index i during a small time-step δt. fick’s first law of diffusion allows us to calculate the jleft and jright fluxes of material from the left-hand (i – 1) and the right-hand (i + 1) neighbours, directed to the ith control volume as: left 1 left δ1 δ δ i i n c c j d a t x − − = = − (5a) and right 1 right δ1 , δ δ i i n c c j d a t x + − = = − (5b) where δnleft and δnright denote the amount of substance arriving to the ith cell from its left and right neighbours, respectively. in each time-step δt, the amount of substance that enters the ith cell is = + left right δ δ δ ,n n n and taking into consideration the volume of the cell (aδx), this results in a ( )left right 1 1 2 δ 2 δ δ δ i i i i j j ac c c c d t a x x − + + − + = = (6) concentration change in the ith control volume. we note that the fraction on the right-hand side of equation (6) approximates the second spatial derivative of the concentration profile, and that equation (6) is in fact a discretized version, with respect to both space and time, of equation (2), fick’s second law. j. electrochem. sci. eng. 8(2) (2018) 171-181 digital simulations in electrochemistry 176 equation (6) dictates that in a time-step of δt duration, the change of concentration in the ith control volume depends on the concentrations in the ith, (i – 1)th and (i + 1)th control volumes, as well as on the model parameters d, δx and δt. equation (6) may be re-written in the form of a matrix-vector equation as: 2 δ δ δ d t x = c l c (7) by defining the so-called laplacian matrix l, an n × n matrix as: ( ), 1 if 1 deg if , 0 otherwise i k i k l i i k =   = − =   (8) where ( )deg i is the number of neighbours of the ith control volume. for the simulation scheme shown in figure 2, the laplacian — a negative semi-definite matrix — takes the following form: 1 1 0 0 0 0 1 2 1 0 0 0 0 1 0 0 . 0 0 1 0 0 0 0 1 2 1 0 0 0 0 1 1 −    −     =      −   −  l (9) note that equation (7) contains a finite difference (with respect to time) on its left-hand side, approximating a time derivative. for infinitesimally small time-steps, equation (7) may be rewritten as: 2 d , d δ d t x = c l c (10) which is a differential equation for the vector c. assuming that the concentration vector ( )kc is known at any kth step of the simulation, the +( 1)kc vector in the next step, δt time later, could be calculated by solving equation (10) in the form: ( ) ( ) ( )1 wexp , k k+ =c l c (11) where lw denotes the so-called weighted laplacian, defined as: w 2 δ . δ d t x =l l (12) the labview code used for the calculation of the weighted laplacian is shown by the weighted laplace matrix.vi snippet of figure 1. as can be seen in equation (11), the “ideal” choice for the stepper matrix s we introduced before would be that ( )= wexp ;s l the multiplication of the concentration vector c in each simulation step with this matrix could model the time evolution of the system, leaving the resolution of spatial discretization the only factor that limits the accuracy of calculations. the problem is, however, that the exponential of the weighted laplacian is not known and can only be approximated. one feasible way of approximating the exponential of a matrix is to truncate its taylor series at a given order; another, more accurate way, is to use its padé approximant [9]. s. vesztergom j. electrochem. sci. eng. 8(2) (2018) 171-181 doi:10.5599/jese.507 177 it is this latter approach which i will follow here, as many digital simulation techniques — such as the explicit and implicit euler, the runge–kutta and the crank–nicolson methods [4] — may be interpreted as techniques relying on the use of different padé approximants. given a function f(x) and two integers m ≥ 0 and n ≥ 0, the padé approximant of the function f(x) of order [m/n] is the rational function: ( ) 0 0 r , m j j j n j j j a x x b x = = =   (13) which agrees with f(x) to the highest possible order, which amounts to: ( ) ( )0 r 0f = (14a) ( ) ( )' 0 r' 0f = (14b) ( ) ( )'' 0 r'' 0f = (14c) ( ) ( ) ( ) ( )0 r 0 m n m n f + + = (14d) equation (14) is a system of equations, by solving which the parameters aj and bj of equation (13) can be determined up to one degree of freedom; it is usually assumed that b0 = 1. in what follows, i will use pm,n(x) to note the padé approximant of the exponential function exp(x). exact expressions for pm,n(x) are tabulated in table 2 for different values of m and n. some padé approximants to the exponential function are plotted, together with exp(x), in figure 3 to show the “goodness” of these approximations. padé approximants, when applied to matrices instead of scalars, form the basis of many known simulation techniques, as will be discussed below. figure 3. some padé approximants pm,n(x) of the function exp(x) j. electrochem. sci. eng. 8(2) (2018) 171-181 digital simulations in electrochemistry 178 table 2. some padé approximants pm,n(x) of the function exp(x) m\ n 0 1 2 3 0 1 1 1x− + − + 21 2 1 1x x − + − + 3 21 1 6 2 1 1x x x 1 +1x + − + 1 2 1 2 1 1 x x + − + 1 3 21 2 6 3 1 1 x x x + − + − + 1 4 3 2 31 1 24 4 4 1 1 x x x x 2 + + 21 2 1x x + + − + 21 2 6 3 1 3 1 1 x x x + + − + 21 1 12 2 21 1 12 2 1 1 x x x x + + − + − + 21 2 20 5 3 23 31 60 20 5 1 1 x x x x x 3 + + + 3 21 1 6 2 1x x x + + + − + 3 2 31 1 24 4 4 1 4 1 1 x x x x + + + − + 3 23 31 60 20 5 21 2 20 5 1 1 x x x x x + + + − + − + 3 21 1 1 120 10 2 3 21 1 1 120 10 2 1 1 x x x x x x 4 + + + + 4 3 21 1 1 24 6 2 1x x x x + + + + − + 4 3 231 1 4 120 15 10 5 1 5 1 1 x x x x x + + + + − + 4 3 21 1 1 2 360 30 5 3 21 1 30 3 1 1 x x x x x x + + + + − + − + 4 3 21 2 1 4 840 105 7 7 3 2 31 1 210 14 7 1 1 x x x x x x x the p1,0 (explicit euler) method the explicit euler method is the most straightforward one applied for the solution of partial differential equations. it is based on the p1,0(x) padé approximation ( )w wexp , + =l i l s (15) where i is the n × n identity matrix (see the identity matrix.vi snippet of figure 1). when plugged into equation (11), it yields that ( ) ( ) ( )1 w . k k+ = +c ι l c (16) the construction of the stepper matrix s, used for simulations based on the explicit euler method, is shown in the create stepper matrix.vi snippet of figure 1. note that as ( ) ( )+ = − 1 δ , k k c c c equation (16) can also be directly obtained by rearranging equation (10). the explicit euler method can thus be interpreted as approximating the differential d / dtc in equation (10) with the finite difference δ / δ .tc consequently, the explicit euler method, a linear approximation, may only yield accurate results when a small enough time-step δt is used. in-fact, when  2δ δ 2 ,t x d the explicit euler method fails to converge and gives erroneous results, as will be discussed later. the p2,0, p3,0 and p4,0 (runge–kutta) methods the overall error of the explicit euler method can be decreased by taking higher order terms into consideration. this can be achieved by using the p2,0, p3,0 and p4,0 padé approximants, which are in-fact taylor series expansions of different orders to the exponential function. this forms the basis of the so-called 2nd, 3rd and 4th order runge–kutta methods [10]. while the explicit euler method could be interpreted as a technique that takes the slope (but not the curvature) of the concentration vs. time dependence into account, runge–kutta methods aim to correct this error. when applying runge–kutta methods, the stepper matrix s is constructed as ( )  + + =2w w w 1 exp 2 l i l l s for the 2nd order, (17) ( )  + + + =2 3w w w w 1 1 exp 2 6 l i l l l s for the 3rd order, (18) and ( )  + + + + =2 3 4w w w w w 1 1 1 exp 2 6 24 l i l l l l s for the 4th order method. (19) s. vesztergom j. electrochem. sci. eng. 8(2) (2018) 171-181 doi:10.5599/jese.507 179 the construction of the stepper matrix s, used for simulations based on the 4th order runge– kutta method, is shown by the respective create stepper matrix.vi (detail) snippets of figure 1. note that — similarly to the euler method discussed above — runge–kutta methods are still explicit; that is, in each iteration step, they calculate the state of a system at a later time from the state of the system at the current time. the p0,1 (implicit euler) method the implicit (also called backward [4]) euler method differs largely from explicit methods. the method is implicit, meaning that it finds a solution for the state of the system at a later time by solving an equation that involves both the current state of the system and the later (yet unknown) state. the stepper matrix used by the implicit euler method, as also shown by the respective create stepper matrix.vi (detail) snippet of figure 1, is ( ) 1w wexp ( ) −  − =l i l s (20) note that due to the implicit nature of the method, the creation of the stepper matrix involves matrix inversion. this clearly requires some extra computation, yet implicit methods can become very useful for the solution of stiff problems where explicit methods require the application of impractically small δt values. as implicit methods are numerically stable, they can be applied with larger time-steps, and as usually the matrix inversion in equation (20) has to be carried out only once (at the beginning of the iteration), implicit methods still require small computation times. the p1,1 (crank–nicolson) method the method named after crank and nicolson [11] was developed for the numerical solution of partial differential equations, but it lies on the basis of the trapezium method of solving ordinary differential equations. the method is semi-implicit and is unconditionally stable; although when applied for stiff systems with large time-steps it may still be prone to spurious (decaying) oscillations. the stepper matrix used by the crank–nicolson method, as also shown by the respective create stepper matrix.vi (detail) snippet of figure 1, is ( ) 11 1w w w2 2exp ( )( ) . −  + − =l i l i l s (21) comparison of the different simulation methods as expected, applying padé approximations of different order for the construction of the stepper matrix s yields solutions that also behave differently. the efficiency of the methods was tested by simulating concentration profiles corresponding to t = 5 s, using the parameter values of table 1 and δt values ranging between 1 ms and 5 s. each simulated concentration profile was compared to the theoretically expected one, equation (3), and the square-root of the mean squared deviation was plotted as a function of the used time-step in figure 4. figure 4 clearly shows a major difference with respect to the stability of fully explicit (explicit euler and runge–kutta) and implicit (implicit euler and crank–nicolson) methods; at about  2 δ δ 2 ,t x d all explicit methods begin to diverge (see also figure 5). compared to the explicit euler, the 2nd order runge–kutta method brings a significant improvement of the error; this improvement is less significant for the 3rd and 4th order runge–kutta methods. the stability of the implicit euler and the crank–nicolson methods is better (see also figure 5), although the error of these methods is also significant at larger time-steps. j. electrochem. sci. eng. 8(2) (2018) 171-181 digital simulations in electrochemistry 180 figure 4. the error of six different digital simulation methods, as a function of the applied timestep. other simulation parameters are tabulated in table 1 figure 5. concentration profiles simulated using six different methods for t = 7 s, using a relatively large time-step (δt = 700 ms). faded thick curves show theoretical profiles, as calculated from equation (3). for the values of other simulation parameters, see table 1 summary in this paper i attempted to describe numerical methods used for the digital simulation of a rather simple, however instructive electrochemical problem, cotrell’s experiment. the same techniques may also be used — by modifying the near-electrode boundary condition — for the simulation of more complicated electrochemical experiments, such as cyclic voltammetry. the described simulation strategies may also be extended to take into account homogeneous reactions [12], or effects related to ohmic drop [13,14]. the further discussion of more complicated systems is, however, beyond the scope of this paper: my only intention here was to s. vesztergom j. electrochem. sci. eng. 8(2) (2018) 171-181 doi:10.5599/jese.507 181 give a starting point for undergraduate or graduate students who decided to familiarize themselves with digital simulations. accordingly, i attempted to describe basic numerical procedures in a simpler than usual manner, following a classification scheme based on padé’s approximants. readers interested in the topic of digital simulation in more detail are referred to other sources [3,4]. acknowledgements: i would like to thank my dear friends and colleagues éva valkó and norbert barankai for their valuable suggestions with which they aided this work. financial support from the national research, development and innovation office of hungary (under grant pd124079) is gratefully acknowledged. references [1] s. vesztergom, v. grozovski, g. g. láng, p. broekmann, 6th regional symposium on electrochemistry for south-east europe, on the electrolysis of dilute solutions of strong acids, balatonkenese, hungary, 2017, p. 103 (kn103). [2] v. grozovski, s. vesztergom, g. g. láng, p. broekmann, journal of the electrochemical society 164 (2017) 3171–3178. [3] a. j. bard, l. r. faulkner, electrochemical methods: fundamentals and applications, john wiley & sons, new york, 2001, pp. 785–807. [4] d. britz, j. strutwolf, digital simulation in electrochemistry, 4th edition, springer, berlin, 2016. [5] f. g. cottrell, zeitschrift für physikalische chemie 42 (1903) 385–431. [6] e. kätelhön, r.g. compton, analyst 140 (2015) 2592–2598. [7] a. j. bard, gy. inzelt, f. scholz (eds), electrochemical dictionary, springer, berlin, 2008, p. 163. [8] g. w. johnson, r. jennings, labview graphical programming, 4th edition, mcgraw-hill, new york, 2006. [9] g. a. baker, p. graves-morris, padé approximants, cambridge university press, cambridge, 1996. [10] k. a. atkinson, an introduction to numerical analysis, john wiley & sons, new york, 1989. [11] j. crank, p. nicolson, proceedings of the cambridge philosophical society, 43 (1947) 50–67. [12] s. vesztergom, n. barankai, n. kovács, m. ujvári, th. wandlowski, g.g. láng, acta chimica slovenica 61 (2014) 223–232. [13] s. vesztergom, n. barankai, n. kovács, m. ujvári, h. siegenthaler, p. broekmann, g. g. láng, journal of solid state electrochemistry 20 (2016) 3165–3177. [14] s. vesztergom, n. barankai, n. kovács, m. ujvári, h. siegenthaler, p. broekmann, g. g. láng, electrochemistry communications 68 (2016) 54–58. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {in-depth component distribution in electrodeposited alloys and multilayers} doi:10.5599/jese.480 49 j. electrochem. sci. eng. 8(1) (2018) 49-71; doi: http://dx.doi.org/10.5599/jese.480 open access : : issn 1847-9286 www.jese-online.org original scientific paper in-depth component distribution in electrodeposited alloys and multilayers lászló péter1,, kálmán vad2, attila csik2, rocío muñíz3, lara lobo3, rosario pereiro3, sašo šturm4, kristina žužek rožman4, györgy molnár5, katalin németh1, katalin neuróhr1, krisztina boros1, lajos pogány1, imre bakonyi1 1wigner research centre for physics, hungarian academy of sciences, konkoly-thege út 29-33, 1121 budapest, hungary 2institute of nuclear research of the hungarian academy of sciences, bem tér 18/c, 4026 debrecen, hungary 3department of physical and analytical chemistry, university of oviedo, julián clavería 8, 33006 oviedo, spain 4jožef stefan institute, jamova 39, 1000 ljubljana, slovenia 5institute of technical physics and materials science, centre for energy research, hungarian academy of sciences, konkoly-thege út 29-33, 1121 budapest, hungary corresponding authors e-mail: peter.laszlo@wigner.mta.hu; tel.: +36-1-392-2222, ext 3614; fax: +36-1-392-2768 received: november 30, 2017; accepted: january 8, 2018 abstract it is shown in this overview that modern composition depth profiling methods like secondary neutral mass spectroscopy (snms) and glow-discharge – time-of-flight mass spectrometry (gd-tofms) can be used to gain highly specific composition depth profile information on electrodeposited alloys. in some cases, cross-sectional transmission electron microscopy was also used for gaining complementary information; nevertheless, the basic component distribution derived with each method exhibited the same basic features. when applying the reverse sputtering direction to snms analysis, the nearsubstrate composition evolution can be revealed with unprecedented precision. results are presented for several specific cases of electrodeposited alloys and mulitlayers. it is shown that upon d.c. plating from an unstirred solution, the preferentially deposited metal accumulates in the near-substrate zone, and the steady-state alloy composition sets in at about 150-200 nm deposit thickness only. if there is more than one preferentially deposited metal in the alloy, the accumulation zones of these metals occur in the order of the deposition preference. this accumulation zone can be eliminated by well-controlled hydrodynamic conditions (like the application of rotating disc electrodes) or by pulse http://dx.doi.org/10.5599/jese.480 http://www.jese-online.org/ mailto:peter.laszlo@wigner.mta.hu j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 50 plating where the systematic decrease in the duty cycle provides a gradual transition from a graded to a uniform composition depth profile. the application of composition depth profile measurements enabled detecting the coincidence in the occurrence of some components in the deposits down to the impurity level. this was exemplified by the gdtofms measurements of ni-cu/cu multilayers where all detected impurities accumulated in the cu layer. the wealth of information obtained by these methods provides a much more detailed picture than the results normally obtained with bulk analysis through conventional integral depth profiling and help in the elucidation of the side reactions taking place during the plating processes. keywords alloy formation; near-substrate composition modulation; hydrodynamic conditions; component distribution correlations introduction electrodeposited metal coatings have been widely used for the corrosion protection as well as for the improvement of the appearance of the coated objects for more than a century. for both above mentioned purposes, the mean composition of the coating, the even lateral component distribution within the coating and the quality of the final surface (e.g., roughness and passivity) are the crucial parameters. nowadays, electroplating is much more than a workhorse of the coating industry. electrodeposition has found its role in the preparation of various nanostructures [1-9], whose functionality strongly depends on, e.g., the component distribution of the electroplated material also at the nanometer scale. inhomogeneities can influence various physical and chemical parameters such as adhesion of the coating, strain (that impacts hardness and yield strength), electrical resistivity, saturation magnetization, magnetostriction and hence the coercive field, band gap (of semiconductors), catalytic properties etc. therefore, the compliance of the component distribution of a coating to a predefined pattern is a prerequisite for achieving the desired functionality. in parallel to the emergence of electrochemical nanotechnologies, local composition analysis methods working at the nanoscale as well as the composition depth profile methods have undergone a significant development. concerning techniques that can be used for planar surfaces, most of the non-destructive composition depth profiling methods (such as x-ray and neutron reflectometry, ellipsometry and angle-resolved x-ray photoelectron spectroscopy /arxps/) are sensitive to a narrow range beneath the sample surface only (typically at most a few tens of nanometers). rutheford backscattering spectroscopy is capable of detecting the in-depth component distribution at a larger depth, nevertheless the depth resolution is limited. the common disadvantages of these methods (except for arxps) are that they require a preliminary assumption on the component/phase distribution that makes the model boundary conditions, a large number of input parameters necessary for the evaluation of the measurements and the lack of direct chemical information. the family of the destructive composition depth profile analysis methods is also quite complex, as presented in figure 1. all of the methods shown in figure 1 yield direct composition information, a few of them (auger/xps techniques) being also sensitive to the oxidation state of the elements. although the destructive nature of these methods leads to at least a partial consumption of the samples, all can be used without a preliminary assumption on either the sample structure or composition. l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 51 historically, the spontaneous variation of the alloy composition during d.c. plating was detected as the change of the average composition as a function of the thickness [10-14]. although this procedure is well indicative of a composition change, it is an integral method, which also means that its sensitivity to either local effects or subtle changes is very little. in contrast to integral methods, methods based on a local analysis exhibit a much higher sensitivity. this is true regardless of whether the information obtained either from the deposit itself (like in transmission electron microscopy /tem/ energy dispersive x-ray spectroscopy /edxs/) or from the composition of the sputtered material (with secondary neutral mass spectrometry /snms/ and with glow discharge – time-of-flight mass spectrometry /gd-tofms/). this is particularly important or even indispensable when a multilayer is deposited because a bulk analysis cannot reveal practically any detail of the component distribution. figure 1. organization chart of destructive composition depth profile analysis methods. methods used in the studies to be described later are labeled with a red square. key to acronyms not resolved in the figure: edxs: energy-dispersive x-ray spectroscopy; tem: transmission electron microscopy; sem: scanning electron microscopy; xps: x-ray photoelectron spectroscopy the goal of this study was twofold: (i) to establish general trends that serve as guidelines concerning the alloy composition change in the growth direction during the plating process; and (ii) to analyze the component distribution in two-pulse-plated multilayers, including major components and impurities. it was our intention to use the most advanced composition depth profiling methods and also to confront their results with each other for obtaining a detailed picture on the phenomena studied and, also, for comparing the peculiarities of some specific methods. experimental materials and chemicals plating baths were made of analytical grade chemicals. purified water with 18 mcm resistivity (elga purelab r7) was used in each case for the preparation of the solutions. when the solution to j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 52 be used contained an oxidation-sensitive component (like fe2+), the solution was freshly prepared every day. a new portion of bath was used in every experiment. a sacrificial anode was typically a sheet or a wire spiral made of the most noble metallic component to be deposited in order to avoid a spontaneous cementation process. since the sample studies were obtained with a large variety of baths, the solution components will be given in the sections where a specific group of deposit is reported. substrate samples were deposited onto si wafers that were pre-coated by evaporation. the si(100) wafers with approximately 3 nm root-mean-square roughness were cleaned but the native oxide layer was not removed. the evaporated coating consisted of a 5-nm chromium adhesion layer and a 20-nm copper conducting layer. electrodeposition apparatus the majority of the electrodeposition experiments were carried out in a 50-ml volume plexigrass cell [15] which is shown in figure 2a. the cathode was fixed in an upward-facing position to the bottom of the cell with a recess. in this cell, the solution was stagnant, and the even current distribution was provided by the recess and the parallel anode/cathode arrangement. the surface area was nominally 8 mm x 20 mm that might be a bit lower due to the compression of the gasket between the body of the cell and the cathode. the actual surface area of the deposit was measured after the removal of the cathode. a few experiments were also performed with a home-built rotating disc electrode (figure 2b). in these experiments, a 1-inch diameter pre-coated silicon wafer was fixed to a rotating ptfe cylinder of the same diameter. the electrical connection to the metallic coating of the wafer was provided with two clamps which also served to hold the wafer at the bottom surface of the electrode holder. the stainless-steel clamps were electrically sealed on the sides, hence minimizing the current passing elsewhere than through the wafer coating. the rotation rate of this electrode was less than 350 rpm. the counter electrode was a metal spiral at the bottom of the cell whose material was chosen according to the same principle as for the other cell. figure 2. a structure of the cell for experiments with a stagnant solution (after ref. 15). b construction of the rotating disc electrode for experiments with controlled hydrodynamics. the majority of the d.c.-plated samples were prepared with galvanostatic control; therefore, a reference electrode was only seldom used for checking the cathode potential variation during the deposition. multilayered samples were deposited in a mixed g/p deposition mode [16] where the l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 53 less and more noble metal layers were produced during the constant-current (g) and the constantpotential (p) pulse, respectively. the more noble metal (e.g. cu) was deposited with the limitingcurrent conditions, while the ratio of currents during the g and p pulses was used for the estimation of the deposit composition prepared during the g pulse (e.g. nixcuy). as power source for the sample preparation, three potentiostat/galvanostat units were used, all of them capable of operating without a pause between the pulses even during the g/p pulse sequence (electroflex ef453, iviumstat, ivium compactstat). sample preparation for in-depth composition analysis for snms depth profile analysis, samples were peeled off from the substrate. in order to carry out this process and to obtain a self-supporting sample with a sufficient mechanical strength, the deposition of the sample of interest was followed by the electrodeposition of at least one additional layer. after the deposition of the layer of interest onto the si/cr/cu substrate, the solution was changed without disassembling the cell, and a ni plating bath was used to obtain a ni layer of at least 3 micrometer thickness. in some cases, especially when the deposit of interest was rich in ni, a zinc interlayer was also deposited before the sample was finally covered with the nickel supporting layer. this made it possible to find the boundary of the layer of interest easily. after the coating process, the back side of the si wafer was scratched and then broken by bending it, the sample being always at the concave side where it could not be torn apart. then, the sample was gently separated from the si wafer. the separation took place at the weakest interface, which was the si/cr boundary in the case of si/cr/cu substrates. further details of the process can be found elsewhere [17-20]. this procedure led to samples where the sputtering could be started from the substrate side of the sample (reverse sputtering direction), and the initial roughness at the beginning of the sputtering process was essentially the same as that of the si wafer prior to the coating process. for the gd-tofms study [21], the sample remained on the si substrate and the deposit could be analyzed as it was obtained after the electrodeposition procedure. in this case, the sputtering direction was the conventional one; i.e., it started at the surface where the deposit formation finished. as the sample is deposited onto a si substrate and it is used for sealing the discharge chamber, the reverse sputtering arrangement could not be applied with this method. tem specimens were initially prepared from a pair of delaminated electrodeposited films, which were glued together and inserted between si wafer dummies. discs with 3 mm diameter were cut from the central area of these bulk specimens, mechanically ground to a final thickness of 20 μm and ion-milled using 4-kev ar+ to achieve thin, electron-transmissive areas located in the film regions. then, the tem specimens were further ion-milled at a low energy of 500 ev and an incident angle of 6° for 30 minutes to remove all contamination. analysis instruments snms depth profile analysis of the samples was performed with an ina-x type instrument (specs gmbh, berlin, germany) in direct bombardment mode by using ar+ ions with a fairly low energy for sputtering (typically with e[ar+]= 350 ev). the erosion area was confined to a circle of 2 to 3 mm diameter by means of ta masks. the lateral homogeneity of the ion bombardment was checked by profilometric analysis of the craters sputtered. a pulsed radiofrequency gd-tofms instrument (horiba scientific, france) was also used for depth profiling of some samples. the ion source was a copper-based 4 mm diameter anode with a 20 mm long flow tube. the sample was placed horizontally and rf power was supplied through the back of the sample. a quadrupole filter was placed just after the extraction cone, allowing j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 54 attenuation of the signal of up to four ion types of different masses in order to reduce the overload of the detector. tofms extraction frequency was set to 30 khz, which made it possible to obtain mass spectra in every 33 μs. data acquisition periods of 0.58 s were averaged for obtaining one data point in the depth profile. after the initial argon flush period, at 160 pa pressure and with 30 w of power, 1 ms pulse width and 25 % duty cycle were selected for sample analysis. for both sputtering-based analysis methods, the transformation of the intensity vs. time function to the mole fraction vs. depth function was carried out with a standard multi-matrix calibration procedure. the sputtering rate of the samples was measured with the help of a profilometer. the tem investigation was performed by a jeol jem-arm200f instrument, using the cold field emission source, equipped with an edxs system (centurio 100 mm2, jeol). the probe size for scanning tem (stem) imaging was set to 0.1 nm, with a current of 20 pa and a convergence semiangle of 24 mrad. stem images were acquired in a so-called bright-field (bf) and a high-angle annular dark-field (haadf) mode, respectively. the edxs spectrum images were recorded with a lateral probe size of 0.2 nm, under continuous scanning mode with a pixel dwell time of 25 microseconds and by using probe currents of 250 pa. results and discussion composition depth profile of d.c.-plated ni-cd alloys obtained from a stagnant solution during the study of the formation of binary alloys, first the composition depth profile of electroplated ni-cd alloys is analyzed where cd is a minority component of the alloy. nickel alloys have the advantage in such investigations that ni exhibits a relatively small exchange current density, which makes its deposition quite hindered. this prevents nickel from developing dendrites in a wide range of current density. as opposed to metals having a high exchange current density (like cu, ag or bi), the increment of the surface roughness of ni with increasing deposit thickness is slow. the limited roughening, together with the favorable mechanical properties of nickel, makes it an ideal major component for depth profiling studies of alloy formation. figure 3 summarizes the composition depth profile information obtained for various ni-cd alloys. samples were obtained from a watts-type bath (0.85 mol dm3 niso4, 0.15 mol dm-3 nicl2, 0.4 mol dm-3 h3bo3; ph 2.5) doped with the cdso4 (3–30 mmol dm-3) [19]. the current density that was used for the deposition of these samples (-19.5 ma cm-2) was much above the limiting current density of the cd deposition from a ni-free solution for all cd2+ concentrations used. this means that the cd deposition can be regarded as a diffusion-limited deposition process where the ni deposition absorbs the current that cannot be covered by the transport of the cd2+ ions. figure 3(a) shows the near-substrate quantitative composition depth profile of the main components. cr and cu signals come from the substrate layers that were detached from the si wafer. the good resolution of these layers indicates that the sample detachment from the substrate did not cause any significant damage to the sample. the decay of the cu mole fraction to the half of its maximum is approximately at the depth that is the sum of the nominal thickness of the cr and cu layers (5 nm + 20 nm, respectively). it can be well seen that the mole fraction of cd is about four times larger in the near-substrate zone than in the bulk of the sample. the cd accumulation is due to its preferred deposition, while the cd concentration decay is explained with the depletion of the solution layer near the cathode surface with respect to the fastreacting cd2+ ions. the decay of the mole fraction of the component with high relative deposition preference is the consequence of the interplay of the large reaction rate and the depletion of the bath with respect to this component in the unstirred solution. l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 55 figure 3. (a) reverse snms composition depth profile of a si/cr/cu//ni-cd sample; c(cd2+) = 10 mmol dm-3. (b) comparison of the cd mole fractions from the reverse depth profile measurements of alloys obtained with different cd2+ concentrations. the depth scale is corrected for the substrate thickness. (c) raw snms measurement data indicating the reverse depth profile with the signal intensities of all elements detected. in this experiments, the cd2+ ion concentration was c(cd2+) = 30 mmol dm-3. figure 3(b) indicates that the trend of the accumulation of the metal with higher deposition preference near the substrate is the same for a wide range of the concentration of its precursor j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 56 cation. however, some quantitative differences of the depth profile functions can also be observed. the relative accumulation of cd near the substrate (calculated as the ratio of the peak mole fraction and the mean mole fraction in the bulk) depends on the concentration. the smaller the cd2+concentration, the larger its relative accumulation, varying from 14 to 2.4 as c(cd2+) increases from 3 to 30 mmol dm-3. at the same time, the decay depth of the concentration maximum increases with the cd concentration (from 35 nm to about 110 nm as c(cd2+) increases by an order of magnitude from 3 to 30 mmol dm-3). the display of the data on a logarithmic intensity scale makes it possible to present the composition depth profile of all important components in a single plot, as shown in figure 3(c). the fast decay of the signal of the substrate components, cr and cu, to their background level is an indication of (i) the low surface roughness of samples peeled off from the si substrate; (ii) the lack of the redeposition of these components; and (iii) the complete removal of the products from the chamber after the sputtering of the corresponding layers finished. it can also be seen that co appears with the occurrence of the ni-rich layer. this is because co is a very common impurity in ni compounds and is deposited preferentially besides ni. therefore, a small concentration of co in the precursor materials manifests itself as a co signal in the snms measurement. it is worthwhile noting that the co intensity increases to its steady-state value in the same depth range in the nearsubstrate zone where the ni signal grows to its plateau value. although it is very tempting to treat the formation of dilute alloys merely by the mass transport effect of the precursor ions, the data for the ni-cd system also show that the phenomenon is by far not that simple. for instance, it can be seen from the quantitative analysis that the cd mole fraction is not linearly proportional to the cd2+ concentration (see figure 3(b)), although the mass transportbased contemplation would rationalize this view. besides the mass transport, various other factors that are difficult to treat simultaneously should be taken into account; i.e., the change in the deposition efficiency due to the alloy formation because of the impact of the alloying element on the hydrogen evolution overvoltage, structural effects during alloy formation etc. [22-24]. an earlier study showed that co as a minor component beside ni accumulates in the near-substrate zone in the same manner as cd does [19]. however, due to the formation of various structurally incoherent phases (with the most likely composition of cdni and cd5ni [23,25-27]), the ni-cd system is not suitable for a depth profiling study in a wide concentration range. comparison of d.c. plating and pulse plating: ni-fe alloys obtained from a stagnant solution the ni-fe system enables studying the composition change during codeposition in a wide concentration range because the structural incoherency does not prevent the formation of a compact deposit. iron is codeposited with nickel with the so-called anomalous codeposition mode [28-33], which means a high deposition preference of fe2+ ions. therefore, the baths that can be used for the deposition of ni-fe alloys are usually fairly dilute for fe2+ as compared to ni2+. in the present study, the following solution concentrations were applied [20]: 0.55 mol dm-3 niso4, 0.005-0.1 mol dm-3 feso4, 0.3 mol dm-3 na2so4 (supporting electrolyte), 0.1 mol dm-3 h3bo3 (buffering agent), 0.2 g dm-3 saccharin (stress reliever) and 0.03 g dm3 sodium dodecylsulfate (wetting agent). typical reverse composition depth profile curves obtained for ni-fe alloys are shown in figure 4, for both galvanostatic and potentiostatic deposition modes. it is obvious from the measurements presented in figure 4 that the enrichment of the preferentially deposited metal (here, fe) in the near-substrate zone is not the consequence of the choice of the regulated electrical parameter, but l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 57 it is a general feature of the fe-ni codeposition. this statement is well in accord with the reason of the enrichment as described above; i.e., it is related to the solution depletion, whichever deposition mode leads to the consumption of the precursor ions. it is also to be noted that the change in the deposition potential during the galvanostatic deposition or the change in the current density during the potentiostatic deposition is insignificant and cannot be related to the composition change. 0 100 200 300 0.0 0.2 0.4 0.6 0.8 1.0 cu fe m o le f ra c ti o n sputtering depth, nm ni potentiostatic deposition: e = -1.15 v vs. sce (a) 0 100 200 300 0 20 40 60 80 100 galvanostatic deposition: j = -20 macm -2 (b) cu fe ni m o le f ra c ti o n sputtering depth, nm figure 4. reverse snms composition depth profile of two si/cr/cu//ni-fe samples; c(fe2+) = 45 mmol dm-3 for both specimens. (a) potentiostatic deposition, (b) galvanostatic deposition. in both graphs, the unlabeled black line refers to cr. when pulse plating is applied, the composition depth profile function is nearly flat [20]. the small near-substrate composition transient can be minimized by decreasing the duty cycle. this provides that all circumstances, especially the electrolyte concentrations in the close vicinity of the substrate, recover to the same value as in the bulk solution by the start of the next deposition pulse. therefore, the material deposited during each current pulse will be identical in composition. the lower the concentration of the fe2+ ions in the solution, the smaller the duty cycle has to be in order to achieve the flat composition depth profile [20]. this is well understood since the smaller the composition of the ions of the preferentially deposited metal, the more the deposition approaches the diffusionlimited character (where the surface concentration of the reacting ions becomes zero). for the same j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 58 reason, the higher the on-time current density, the smaller the duty cycle has to be in order to achieve a flat composition depth profile. although the above trends are well established, no quantitative description is available at the time being for anticipating the necessary duty cycle range for suppressing the initial composition change below a particular predefined limit. figure 5 presents the summary of the data obtained for a number of pulse-plated samples. figure 5. reverse snms composition depth profile of three pulse-plated si/cr/cu//ni-fe samples. deposition conditions: c(fe2+) = 45 mmol dm-3, ton=0.1 s, toff = 0.4 s; jon = -26.7, -20 and -13.3 ma cm -2 for the solid black, dashed red and dotted blue curves, respectively. data were corrected for the substrate thickness and fe mole fractions are only displayed for the sake of simplicity. inset: dependence of the ratio of steady-state and initial fe mole fractions on the duty cycle for jon = -13.3 ma cm -2 (other deposition conditions are the same as for the main graph). composition depth profile of d.c.-plated ternary ni-co-fe alloys obtained from a stagnant solution for binary alloys, it was clear that the metal with high deposition preference accumulates near the substrate. for a ternary alloy, an order of deposition preference can be deducted from the composition data of d.c. deposits. for iron-group metal alloys, this deposition preference is as follows: fe>co>ni [34-36]. it is an open question how the order of the deposition preferences affects the composition evolution in the near-substrate zone. in this study, the solution contained 0.2 mol dm-3 niso4, 0.075 mol dm-3 coso4, 0.025 mol dm-3 feso4, 0.4 mol dm-3 h3bo3, 0.03 g dm-3 sodium dodecylsulfate and 0.2 g dm-3 saccharin, while the ph was set to 2.8 with sulfuric acid. the order of the concentration of the metal sulfates is the opposite as the deposition preference. this provides that the mole fractions in the deposit have comparable orders of magnitude. when sodium citrate was also added to the bath, its concentration was 0.2 mol dm-3, and the ph of the bath was 5.5 (which required no adjustment). some composition depth profile data for the near-substrate zone of electrodeposited ni-co-fe system are also available elsewhere [18,37], but these data are in contradiction to the long-range composition vs. thickness functions published in some other studies (e.g, fig 5 of ref. [38]). figure 6 presents the comparison of the reverse snms depth profile of two samples, deposited from a bath either without (figure 6(a)) or with (figure 6(b)) sodium citrate as complexing agent. in both measurements, the layer structure of the components originating from the substrate can be well seen, so much that the cu mole fraction during the sputtering of the cu layer achieves 1. this l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 59 indicates that the preparation of the samples led to undamaged specimens with intact cr and cu layers remaining on the deposit upon the removal from the si wafer. otherwise, the signals of the substrate layers and the deposit would smear out, strongly diminishing the apparent sharpness of the interfaces. the successful sample preparation also means that all differences found in the spectra are an inherent feature of the deposits and cannot be attributed to artefacts. 0 100 200 300 0.0 0.2 0.4 0.6 0.8 1.0 m o le f ra c ti o n sputtering depth, nm (a) bath without sodium citrate cu co fe ni 0 100 200 300 0.0 0.2 0.4 0.6 0.8 1.0 cu co fe (b) bath with sodium citrate m o le f ra c ti o n sputtering depth, nm ni 0 500 1000 1500 0.0 0.2 0.4 0.6 0.8 1.0 m ol e fr ac tio n sputtering depth, nm (c) bath without sodium citrate, full thickness, by omitting the substrate layer co fe ni figure 6. reverse snms composition depth profile of two d.c.-plated si/cr/cu//ni-co-fe samples with a current density j = -12 ma cm-2. (a) no sodium citrate, ph 2.8; (b) c(na3cit) = 0.2 mol dm -3, ph 5.5. (c) same profile as for (a) at the full depth scale. the scheme of the composition change is the same in the spectra of both d.c.-plated ni-co-fe deposits. namely, the deposition starts with an iron-rich material with an fe mole fraction of about 0.6, and the fe mole fraction starts to decay immediately. the sharpness of the fe peak at the substrate/deposit boundary is determined by the experimental transient during the snms measurement. the signal corresponding to co also exhibits a maximum, but the rise of the co mole fraction is far less fast as that of the fe, and the co mole fraction maximum follows the fe maximum delayed by about 20-30 nm. then, the co mole fraction also decreases, and the steady-state composition is achieved after some 150 nm deposit thickness. the data presented above (like several others, see [18,19]) indicate that the components of the electrodeposited alloys accumulate sequentially in the order of their deposition preference. the deposition rate of fe is high at the beginning, which manifests itself by a large fe mole fraction in the near-substrate zone of the deposit. since the fe2+ concentration in the bath is small, the depletion of the solution in the cathode vicinity leads to a decrease in the deposition rate, and the fe mole fraction declines. as the deposition rate of fe decreases, co starts replacing it, but the combination of deposition preference and depletion results in a co mole fraction maximum, too. the achievement of the steady-state is related to a stabilization of the depletion (composition profile of the precursor ions) in the solution that determines then the transport rate of the ions toward the substrate. the quantitative comparison of the samples deposited from citrate-free and citrate-containing solutions supports the view that the spontaneous initial composition change is related to the transport processes in the solution. the citrate ions form a complex with the majority of the metal cations, and the reason of its application to ni-co-fe baths lies in the beneficial impact on the bath stability and the resulting phase structure and magnetic properties of the deposits [39,40]. the j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 60 sodium citrate concentration is 2/3 of the total metal ion concentration in the present study. even under these conditions, the diffusion of the complexed precursor ions can be assumed to be slower than that of the non-complexed ones. hence, the decay rate of the mole fractions of the preferentially deposited metals (dy/dx where x is the thickness and y is the mole fraction) is larger if the transport rate is smaller. this can be well seen in the comparison of the profiles of the two alloys in figure 6. it is to be noted that the ph also differs for the two baths used in the above comparison. it is also known [41-43] that the mole fraction of fe and co in ni-co-fe deposits decrases with the increase in ph if ph >2.8. however, we can exclude that the effect of the ph is dominant in the composition change observed since the initial fe mole fraction is nearly the same. if the ph had a major impact on the composition, the near-substrate mole fractions should also change significantly, which was not observed. as figure 6(c) indicates, the composition depth profile can be uneven also after the decay of the initial transient. it can be easily observed that the composition variations are not a consequence of the random experimental errors since they are well correlated with each other. the changes in the mole fraction of fe and co exhibit the same direction, while the mole fraction of ni always shows a countermotion. further interesting details can be revealed from the interrelation of the mole fractions of the components of the samples if they are displayed as a function of each other. the mole fraction of fe was selected as the independent variable and all other mole fractions were plotted as a function of y(fe). this can be seen in figure 7 for two samples deposited from the same bath with different current densities. if the sample is deposited with a large enough current density (-24 ma cm-2, see figure 7), the co mole fraction is linearly proportional to the iron mole fraction: y(co) = ky(fe), and k  1/3. the value of the proportionality factor k can be compared to the c(co2+)/c(fe2+) ratio, which was 3. if both components were deposited at the diffusion limited rate, the concentration ratio should be close to the k factor since the transport coefficient of the me2+ ions with nearly the same size and weight must be very close to each other. therefore, we can conclude that at least the co deposition does not take place at the diffusion-limited rate. there must be, however, a governing factor in the codeposition kinetics which ensures this proportionality. at -24 ma cm-2, the extrapolation to zero fe2+ concentration leads to the result that the co mole fractions should be zero. this is clearly impossible since, in the absence of fe2+, the anomalous codeposition should lead to a relatively corich deposit. these data indicate indirectly that the anomalous codeposition involves the inhibition of the deposition of some metals of lower deposition preference, as it was also indicated by the measurements of podlaha et al. [30,36]. as the current density decreases (see data corresponding to -10.6 ma cm-2 in figure 7), the mole fraction of both fe and co increases. at the same time, the relationship between the mole fractions of the components remains linear, but the functional form is different: y(co) = y(co)0 + k’y(fe); i.e., the extrapolation to zero fe2+ concentration does not result in zero co content, and k’<k. this indicates that at a relatively low current density, the anomalous codeposition preference between co and ni can also manifest itself, and the dependence of the co content on the fe content becomes weaker. since ni is the metal of the weakest deposition preference but with the highest precursor ion concentration, its deposition can be regarded as the reaction that takes the “leftover” current not used by either fe or co deposition. hence, the countermotion of the ni mole fraction with both the fe and co mole fractions is the consequence of the deposition preference and the concentration ratios. l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 61 0.0 0.2 0.4 0.6 0.0 0.2 0.4 0.6 0.8 1.0 co: -10.7 macm -2 -24 macm -2 n i o r c o m o le f ra c ti o n fe mole fraction ni: -24 macm -2 -10.7 macm -2 figure 7. interrelation of the mole fractions of two d.c.-plated si/cr/cu//ni-co-fe samples obtained with two different current densities as indicated next to the datasets. the solution composition was the same as for figure 5(a) and the depth profiles were taken from snms measurements. dashed lines are a guide for the eye only and show the extrapolation of the trend of the corresponding experimental data. the small arrows next to the datasets indicate where the initial composition change decayed (data with larger fe mole fractions in the same dataset belong to the initial transient). it is to be solved yet what can cause a simultaneous increase/decrease in the fe and co concentrations in one particular sample in a wide range of current density. one can conclude that this behaviour is the result of the random fluctuation in the mass transport in the solution. if the surface concentration of the two preferentially deposited components, fe and co, cannot change simultaneously, the strong correlation between their mole fractions could not occur. it also has to be noted that this fluctuation must extend over the entire substrate surface area, otherwise the sputtering method carried out on spots of 2-3 mm diameter would not be able to reveal it. this problem is also worthwhile to be studied with a direct imaging method that reveals also the lateral composition map of the samples. tem analysis of d.c.-plated ni-co-fe alloys obtained from a stagnant solution the difference in the snms composition profile of the d.c.-plated and pulse-plated alloys indicate that the initial composition change observed for the d.c.-plated sample cannot be an instrumental artefact. nevertheless, it seemed to be worthwhile cross-investigating the results with a direct imaging method. this is why tem was applied for ni-co-fe samples deposited with analogous conditions than those studied with snms reverse depth profiling. in former reports describing a tem study of ni-co-fe samples [39,41,44-46], the near-substrate composition gradient shownin the previous chapter was not mentioned. the composition change in the near-substrate zone was observed also with tem and the features of the initial transition zone proved to be identical to those detected by snms. as it is shown in figure 8, the tem edxs results show consecutive maxima in the signal intensities of the components in the order of their deposition preference. the initial transition range was about 200 nm thick, which is only a bit larger than the thickness of these zones as established from snms measurements. the tem investigation could also confirm that spontaneous long-range composition oscillation can occur during the deposition of ni-co-fe samples from stagnant solutions. the spontaneous composition oscillation beyond the near-substrate zone ranged to a wider interval of the mole j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 62 fraction of the components if the current density was larger. at -20 ma cm-2 current density, the spontaneous composition oscillation is obvious from the elemental maps (see figure 9(a)). distance from the substrate, m figure 8. tem edxs study of a d.c.-plated ni-co-fe sample. the dashed lines correspond to the maximum of the signal intensity of the corresponding component in the near-substrate zone. (a) (b) 0.0 0.1 0.2 0.0 0.2 0.4 0.6 0.8 1.0 c o o r n i m o le f ra ct io n fe mole fraction co ni figure 9. (a) tem edxs elemental maps obtained for a d.c.-plated ni-co-fe sample. the substrate is on the right dark edge of the images. (b) mutual dependence of the mole fraction of the components of the same sample as calculated from the line scan perpendicular to the substrate. the elemental maps exhibit a complementary nature: light parts of the image show that the ni intensity always corresponds to dark parts of both the fe and co maps and vice versa. this indicates that there is a positive correlation between the mole fractions of co and fe, while the ni mole fraction has a negative correlation with that of the other components. by determining the mole fractions from a line scan performed perpendicular to the substrate, the mole fractions can be plotted in the same manner as in figure 7 and the result can be seen in figure 9(b). the similarity of l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 63 the character of the two figures is striking, which eliminates all doubts concerning the occurrence of any possible artefact in the snms measurements revealing spontaneous random concentration oscillations. however, there is a peculiar difference between the snms and the tem composition depth profiles. in both measurements, the y(co) = ky(fe) relationship holds, but the k parameter is very different (1/3 as derived from the snms measurements while nearly 2 from the tem measurements). this is not yet understood and may rely in the difference in the quantification procedures of the two methods. it has to be admitted that the preparation of a sample for tem studies is by far more cumbersome than simply inserting the same sample to the analytical instrument that performs the sputtering and the subsequent analysis. nevertheless, the tem elemental maps have also some advantage as compared to sputteringbased concentration depth profile measurements. namely, if the undulation of the concentration fluctuation fronts becomes comparable to the thickness of the zones with identical concentration, the sputtering-based measurements will show the mean composition only. the reason of this averaging is that the sputtering front can sample various zones at the same time due to the undulation of the zones with varying composition. the tem elemental maps can, however, yield fairly complex data: on the one hand, the concentration values as a function of the location and, on the other hand, the apparent roughness of the composition undulation fronts. therefore, the methods used provide information content of rather complementary nature. composition depth profile of ni-co-fe alloys plated under controlled hydrodynamics for controlling the convection, a rotating disc electrode system was used for the deposition of the samples. the key features of the setup were described in the experimental and are shown in figure 2b. the solution in this study contained the following components: 0.2 mol dm-3 niso4, 0.075 mol dm-3 coso4, 0.025 mol dm-3 feso4, 0.4 mol dm-3 h3bo3, 0.28 mol dm-3 nh4cl and 0.2 g dm-3 saccharin, while the ph was set to 2.8 with sulfuric acid. the ratio of the metal ions was the same as for the ni-co-fe baths mentioned before. the current density was -20 ma cm-2 and the electrode rotation was 70-210 rpm. first, a sample was deposited onto a si/cr/cu wafer in order to check the lateral homogeneity of the deposit. the analysis was performed with edxs on 200 m x 200 m spots along the radius of the wafer. the result is shown in figure 10. the composition was fairly stable along the radius and the deviations of the mole fractions of the components from their average values were uncorrelated with each other. this supports the view that the deviations should be attributed to random errors and no systematic trend can be established. the composition analysis indicates that the electrode rotation had a significant impact on the composition. samples prepared from a stagnant solution of nearly the same composition contained 50-70 at.% ni, 15-10 at.% co and 45-25 at.% fe [18] (the molar percentages are listed above in such an order that their sequence for a given element indicates the composition change upon increasing current density). in contrast, the deposit obtained from a well-stirred solution was rich in cobalt. this corresponds very well to the deposition preference and to the suppression of the role of the transport of the precursor ions. several samples were prepared with various current densities and electrode rotation frequencies for reverse depth profile analysis. a typical result of the snms measurements is presented in figure 11. j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 64 0.0 2.5 5.0 7.5 10.0 12.5 0 20 40 60 d e p o s it c o m p o s it io n , a t. % distance from the centre, nm co fe ni figure 10. result of the edxs analysis of a ni-co-fe deposit obtained by galvanostatic deposition on a rotating disc electrode (210 rpm). 0 400 800 1200 1600 0 20 40 60 80 100 cu cr zn ni fe c o n c e n tr a ti o n , a t. % sputtering depth, nm co figure 11. reverse snms composition depth profile curves of a ni-co-fe deposit obtained by galvanostatic deposition on rotating disc electrode after removing the si wafer substrate. the graph shows the complete structure with the substrate layers (cr and cu), the ni-co-fe deposit, the zn interlayer and the ni covering layer (support). an essentially even composition profile was obtained for each sample, and the composition fluctuation often observed in samples obtained from a stagnant solution was never detected. the apparent composition fluctuation was well within the usual error of the snms measurement and, hence, it is unrelated to the features of the samples. the variation of the mean composition with current density was an order of magnitude smaller than in a stagnant solution. the variation of the electrode rotation rate in the 70-350 rpm range did not have a significant impact on the composition. the composition change in the near-substrate zone was also small as compared to d.c.-plated samples obtained from stagnant solutions. although the mole fraction of iron was always slightly larger in the near-substrate region than in the bulk, the change in the fe concentration in the nearsubstrate zone was at most 6 at.% (c.f. 20-35 at.% change in samples obtained from stagnant l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 65 solutions). this mole fraction decay was compensated almost equally by a mole fraction increment of co and ni. the deposit thickness range in which this composition change took place shrunk in the stirred solution as compared to the stagnant one. while a 150-200 nm thick transition zone was observed for stagnant solutions, the transition zone thickness was less than 40 nm for samples deposited onto rotating discs. although the lack of a visible composition fluctuation in a set of samples cannot be taken as an absolute evidence for the complete elimination of the composition fluctuations, the full analysis of the composition profiles strongly underpins that the controlled hydrodynamics of the solution stabilizes the deposit composition and prevents the formation of a thick initial transition zone with large composition changes. hence, the role of the mass transport in the occurrence of uneven composition of the alloy deposits is strongly evidenced with the latter experiments. composition depth profile of ni-cu/cu multilayers [ni-cu/cu]n multilayers were deposited with the so-called g/p method [16] from an acetate/citrate bath [21,47]. by using this protocol, the alloy layer was obtained with a galvanostatic pulse, whose amplitude was suitable for the regulation of the composition of the ni-cu layer. the cu partial current density (jcu) was measured and then assumed to be constant also during the high-current pulse (ni-cu layer). hence, the nominal layer composition was calculated as follows: ycu = jcu/jtotal and yni = 1 ycu. the nickel mole fraction in the layers was set to 0.9, 0.75 and 0.4, and the corresponding layers will be denoted henceforth as high (h), medium (m) or low (l) ni-content layers, respectively. the cu layers were deposited with a potentiostatic pulse controlling the layer thickness with the real-time current integration during deposition. the cu deposition potential was chosen so that ni could neither deposit nor dissolve. further details of the sample preparation as well as the evaluation method of the gd-tofms spectra were published elsewhere [21]. since the low-pressure chamber of the gd device and the ambient-pressure environment is separated by the sample itself during the measurement, the foil preparation for the reverse sputtering method could not be used here, and hence, the analysis was performed with the conventional sputtering direction for deposits still on their si/cr/cu substrate (i.e., from the solution side toward the substrate). figure 12 shows a representative qualitative depth profile measurement for a multilayer with the l-m-h order of the ni layers (as counted in the order of their deposition). the profile well reveals the layered structure of the sample, and the relative peak intensities of the ni signals also indicate the sequence of the ni layers with various composition. the layer structure as estimated from the signal of different isotopes of the same element is identical. the curves shown in figure 12 all exhibit a long decay period after the sputtering of the corresponding layer finished. this is an inherent feature of the gd-tofms method. the long decay of the signals of each isotope explains why the ni signal cannot decrease to zero when the cu layer is sputtered and why the multilayer components are observed when the sputtering front reached the substrate. besides the features of the sputtering method itself, the long decay time of the signals can also be explained with the sputtering direction. since the conventional sputtering direction could only be used here, the sputtering started at a surface which exhibits a significant roughness. the resulting sputtering intensities are obtained as a convolution of the sputtering front to the layered structure, which leads to a widening of the wave corresponding to a particular isotope. therefore, the present measurements are not suitable for the direct comparison of the accuracy of the sputtering-based analysis methods themselves (snms and gd-tofms), but are highly influenced by the sputtering direction, too. j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 66 sputtering time, s figure 12. gd-tofms composition depth profile curve of a si/cr/cu//[ni-cu/cu]3 multilayer. thickness of the ni-cu and intermediate cu layers: 80 nm, thickness of the top cu layer: 40 nm. the order of the ni-cu layers follows the sequence of their deposition: l-m-h (from right to left in the graph). numbers next to the chemical symbols refer to the atomic weight of the isotope measured. an important advantage of the mass spectrometric analysis of the samples is that the major components and the impurities can be detected at the same time. figure 13 shows the spectra of the same multilayer as in figure 12 by indicating the signal intensities of different components. figure 13 clearly shows that metallic impurities that are less noble than cu are codeposited with ni. the signal intensity of co and fe shows the same variation at large scale as that of ni. both co and fe show an anomalous codeposition beside ni, but they cannot be deposited at the potential where cu is deposited. therefore, these elements accompany nickel. it is worthwhile drawing attention to some subtle details of figure 13. if the variation of the co and fe signals is scrutinized within one particular ni-rich layer, it can be seen that both of them decrease as the deposition proceeds. since both fe and co are preferentially deposited with ni and their ions are present in a small concentration, the bath is depleted with respect to their ions as the deposition of a particular ni-rich layer proceeds. therefore, the codeposition rate of fe and co is expected to decrease as the ni-rich layer grows. here, we can see the same phenomenon as the one discussed for ni-cd and ni-fe alloys in the previous sections. the accumulation of the preferentially deposited metals in the early phase of the deposition of the ni layer is so obvious that it can be detected also in the conventional sputtering direction, even though the resolution of the measurement in this sputtering mode is smaller than for the reverse sputtering mode. if we compare the co and fe signal intensities in the subsequent ni layers, we can see a decrease. this is related to the current density applied during the deposition of each ni layer. since the co2+ and fe2+ ions are present in the bath in small concentrations only, their deposition beside ni becomes diffusion limited. the larger the total current density, the larger the ratio of ni deposition rate as compared to that of the impurities. hence, the fe and co concentration in ni is expected to decrease as the current density increases. this can be well seen in the data of figure 13. 0 20 40 60 80 100 120 140 160 0 50 100 150 sputtering time, s s ig n a l in te n s it y , 1 0 6 c p s cu63 cu65 ni60 ni62 si28 cr52 l m h s ig n a l in te n si ty , 1 0 6 c p s l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 67 figure 13. gd-tofms composition depth profile curve of a si/cr/cu//[ni-cu/cu]3 multilayer by showing major and minor metallic components. top: linear intensity scale, bottom: logarithmic intensity scale. the sample is identical to that depicted in figure 12. the gd-tofms method was particularly suitable to detect non-metallic minority components, too. figure 14 shows the component depth profile of a finer multilayer structure with 40 nm thick layers. as the data show, the signal intensity of the minor components (c, na and cl) follow the same oscillation pattern as the multilayer and their intensity maxima coincide with those of cu. the composition depth profile spectra shown in figure 14 are not sufficient alone to determine what causes the correlation between the intensity change of some components. the most likely reasons can be (i) the preferred codeposition of the impurities with cu rather than ni, (ii) the adsorption and co-adsorption of the components of the solution on cu followed by their embedding by the growing layer, and (iii) the formation of cavities in cu with a higher probability than in ni. the codeposition theory seems to be rather unlikely since na cannot be deposited at all at the potential where cu is deposited, and chloride ions do not react at the cathode either. the adsorption of the citrate ions on copper is possible. since the citric acid has several carboxilate groups and at the ph of the solution the protons can be dissociated, attachment of the na+ ions by coulombic interaction is well possible. if the detachment of the citrate ion (or its fragment) cannot take place before it is embedded by the growing cu layer, all elements whose atoms are either present in the citrate ion itself or may accompany it can appear in the depth profile spectra. however, the simultaneous 0 20 40 60 80 0 40 80 120 160 sputtering time, s s ig n a l in te n s it y , 1 0 6 c p s ni60 ni62 co59 -20 ma (h) -6 ma (m) -2.25 ma (l) substrate growth direction 1.0e+03 1.0e+04 1.0e+05 1.0e+06 1.0e+07 1.0e+08 0 40 80 120 160 sputtering time, s s ig n a l in te n s it y , c p s ni60 co59 ni62 fe56 10 8 10 7 10 6 10 5 10 4 10 3 j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 68 occurrence of cl still requires another explanation. the cavity formation in the cu layer can explain the occurrence of all elements of the solution. however, in this case, sulfur should also be present in the cavities due to the relatively high sulfate ion concentration of the bath. in contrast with the expectation, the sulfur signal was around the background level throughout the measurement and did not exhibit a simultaneous oscillation with the rest of the solution components. this hints at the possibility that the cavity formation theory is to be abandoned, and the preferential occurrence of the impurities in the cu layer is to be explained with some specific chemical interaction between the growing cu surface and the solution components. figure 14. gd-tofms composition depth profile curve of a si/cr/cu//[ni-cu/cu]6 multilayer. thickness of the ni-cu and intermediate cu layers: 40 nm, thickness of the top cu layer: 20 nm. the order of the ni-cu layers follows the sequence of their deposition: h-m-l-h-m-l (from right to left in the graph). dashed lines are drawn to the maxima of the cu intensity and serve as a guide for the eye only. the two arrows indicate the initial surface of the substrate (right) and the final surface of the deposit (left). the quantitative analysis of the spectra shown in figure 14 is shown in figure 15. the quantitative analysis clearly shows the difference between the ni and total iron-group metal content of the nirich layers as a function of the ni concentration. the lower the ni concentration of the ni-rich layer (i.e., the lower the current density), the larger the contribution of co and fe to the total iron-group metal content of the ni-rich layers. the carbon concentration of the deposit is rather significant, ranging to 2 at.% in the cu layers. it is important to note that the carbon content of a deposit practically always remains hidden regardless of the nature of the major metallic constituents if the analysis is made with edxs in an electron microscope (in both sem and tem) since the carbon signal is typically attributed to surface impurities only. however, in the present measurement, the low intensity of the carbon signal during the sputtering of the ni layer indicates that the vacuum system is clean. the surface impurity occurs only at the beginning of the sputtering process and it decays fast. the oscillation of the carbon signal after the sputtering of the ni-rich layer just beneath the top cu layer must be attributed to the feature of the bulk of the sample. the bottom graph in figure 15 shows the interrelation of the c and cu content of the multilayer sample. as the carbon signal decays shortly after the start of the sputtering due to the removal of the surface impurities, the function travels nearly the same trajectory forth and back as the sputtering front reaches a cu and a ni-rich layer. this indicates that the incorporation of the carboncontaining impurity takes place in parallel with the deposition of cu and it is likely caused by a specific chemical interaction. 1.e+02 1.e+03 1.e+04 1.e+05 1.e+06 1.e+07 1.e+08 1.e+09 0 40 80 120 160 sputtering time, s s ig n a l in te n s it y , c p s cu ni c na cl 10 9 10 8 10 7 10 6 10 5 10 4 10 3 10 2 l. péter et al. j. electrochem. sci. eng. 8(1) (2017) 49-87 doi:10.5599/jese.480 69 figure 15. gd-tofms composition depth profile curve of a si/cr/cu//[ni-cu/cu]6 multilayer (same sample as for figure 14). top: quantitative depth profile; bottom: interrelation of the c and cu content of the deposit. dashed lines serve a guide for the eye only. conclusions it was shown that modern composition depth profiling methods like snms and gd-tofms can be used to gain highly specific composition depth profile information on electrodeposited alloys. with the reverse sputtering direction, the near-substrate composition evolution can be revealed with unprecedented precision. it was shown that upon d.c. plating from unstirred solution, the preferentially deposited metal accumulates in the near-substrate zone, and the steady-state alloy composition sets in at about 150-200 nm deposit thickness only. if there is more than one preferentially deposited metal in the alloy, the accumulation zones of these metals occur in the order of the deposition preference. this accumulation zone can be eliminated by various means. well-controlled hydrodynamic conditions (like the application of rotating disc electrodes) lead to a significant reduction of both the component accumulation and the thickness of the transition period. pulse plating has a similar impact where the systematic decrease in the duty cycle provides a gradual transition from a graded to a uniform composition depth profile. these observations can be very useful where attachment of a deposit to the substrate is highly impacted by the composition 0 0.2 0.4 0.6 0.8 0 200 400 600 depth, nm m o le f ra ct io n c si cu ni ni+co+fe 0 0.02 0.04 0.06 0.08 0 0.2 0.4 0.6 cu mole fraction c m o le f ra c ti o n surface contamination (beginning of the sputtering) j. electrochem. sci. eng. 8(1) (2018) 49-87 component distribution in alloys and multilayers 70 of the adjacent deposit layer. also, when the uniformity of the composition plays a role in the deposit properties (like the even composition for the coercive force of a magnetic metal), the proper choice of the deposition conditions can also be crucial. the comparison of the sputtering-based and tem edxs composition depth profile data demonstrated the complementary nature of such studies. the very systematic composition variation as shown up in the snms measurements performed over several mm2 surface area provides a clear evidence for that the samples are laterally homogeneous. however, the undulation of the concentration fluctuation fronts distorts the sputtering-based analysis, while they are visible in the cross-sectional tem analysis. the application of composition depth profile measurements makes it possible to detect the coincidence in the occurrence of some components in the deposits down to the impurity level. this was exemplified by the gd-tofms measurements of ni-cu/cu multilayers where all detected impurities accumulated in the cu layer. the wealth of information obtained by these methods provides a much more 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.12850/issn2196-0267.jept3300 http://creativecommons.org/licenses/by/4.0/) {theoretical study on quantum transport of carbon nanotubes for detecting toxic molecules: the role of dopants:} http://dx.doi.org/10.5599/jese.1243 431 j. electrochem. sci. eng. 12(3) (2022) 431-438; http://dx.doi.org/10.5599/jese.1243 open access : : issn 1847-9286 www.jese-online.org original scientific paper theoretical study on quantum transport of carbon nanotubes for detecting toxic molecules: the role of dopants department of applied chemistry, kyushu university, moto-oka, nishi-ku, fukuoka 819-0395, japan corresponding author: fujimoto.yoshitaka.093@m.kyushu-u.ac.jp received: january 20, 2022; accepted: march 1, 2022; published: march 15, 2022 abstract we study the effects of dopant atoms on quantum transport and adsorption properties of environmentally harmful (co2), toxic (co) and common (o2 and n2) molecules of carbon nanotubes (cnts) based on the first-principles electronic structure and quantum transport study. it is found that co and o2 molecules can bind on the boron (b)-doped (14,0) cnts with relatively large adsorption energies and short binding distances in air. we also studied the quantum transport of b-doped (14,0) cnts for the adsorption of the molecules. the quantum transport properties of the cnts are found to be sizably varied by the introduction of a dopant atom and the adsorption of the molecules. the nature of the variation of the quantum conductance induced by the doping as well as the molecular adsorption is discussed, and the possibility to individually detect toxic co and common o2 molecules under low bias voltages is also discussed. keywords first-principles electronic transport theory; conductance; doping; molecular/gas sensors introduction carbon nanotubes (cnts) have attracted much attention as potential electronic device materials to be used in field-effect transistors, sensors, etc. since they show excellent transport properties such as high carrier mobility and large current density [1,2]. nanotube-based molecular and/or gas sensors have been demonstrated with a fast response time and high sensitivity at room temperatures [3-7]. in molecular sensors, target molecules can be detected by gauging the change of conductivity of the nanotubes induced by the adsorption of molecules [3,4,8-12]. first-principles density-functional calculations suggested that a pristine cnt as well as graphene does not chemically but rather physically bind with several molecules such as o2 and no2, and it is surmised that the reactivity of the pristine cnts to adsorbates is weak [13-15]. therefore, the electronic structures, as well as the transport properties of the pristine cnts, are not changed largely by the adsorption of molecules [16]. doping with heteroatoms often modifies their electronic yoshitaka fujimoto http://dx.doi.org/10.5599/jese.1243 http://dx.doi.org/10.5599/jese.1243 http://www.jese-online.org/ mailto:fujimoto.yoshitaka.093@m.kyushu-u.ac.jp j. electrochem. sci. eng. 12(3) (2022) 431-438 quantum transport of carbon nanotubes for 432 structures as well as improves the chemical reactivity to the adsorbates around the dopant atoms [17]. boron and nitrogen are good dopants for carbon-based materials since they are neighboring atoms to carbon [18,19]. actually, it is shown that the adsorption energies of h atoms and nh3 molecules on b-doped and n-doped cnts are larger than those on pristine ones [20,21]. moreover, it is reported that the adsorption energies of various molecules on the b-doped graphene and cnts are enhanced than those on the n-doped ones [10,17]. several theoretical studies for electronic transports of cnts and graphene decorated by doping with heteroatoms and the introduction of defects have been reported [8,22], and however, their theoretical calculations are performed under the zero defect-density limitation. the transport properties of doped cnts with a finite defect density have to be considered to understand the effects of the dopant atoms and the molecular adsorption on the transport properties under more realistic situations. in this paper, we report the doping effect with a heteroatom and the adsorption effects of environmentally harmful (co2), toxic (co) and common (o2 and n2) molecules in air on the energetics and the quantum transport of b-doped (14,0) cnt based on first-principles electronicstructure and quantum transport calculations. it is found that co and o2 molecules are adsorbed on the b-doped cnts in the air with relatively large adsorption energies. it is also found that the quantum transport properties of nanotubes change by the introduction of the dopant atom and the adsorption of the molecules. scanning tunneling microscopy (stm) images for the adsorption of the molecules on the b-doped (14,0) cnts are demonstrated, and it is found that the b-atom dopant and the co and o2 molecules can be distinguished from one another in the stm images. methodology first-principles electronic-structure calculations have been performed within the densityfunctional theory (dft) [23]. the exchange-correlation effects are treated by the local density approximation (lda) parameterized by perdew and zunger [24-26]. the norm-conserving troulliermartins pseudopotentials are used to describe the interactions between the ions and the valence electrons [27]. for the calculation of the adsorption energy of the o2 molecule on the b-doped cnts, we use the local-spin-density approximation. we here use the b-doped (14,0) cnt, which has two-unit cells for the calculations of energetics and electronic structures regarding the adsorption of various molecules. to avoid any interactions between neighboring tubes, the supercell lattice constant of as long as 24 å (1 å = 0.1 nm) along the direction perpendicular to a tube direction is used. the wave functions in the kohn-sham equations are expanded using the plane-wave basis set with the cutoff energy of 50 ry (1 ry = 8.72 × 10-18 j) [28]. one-dimensional brillouin-zone integration is performed with 12 k-point samplings. the atomic geometries are optimized until the maximum values of the hellmann-feynman forces acting on all atoms are less than 0.05 ev/å (1 ev = 1.60 × 10-19 j). the adsorption energy (ea) is calculated by eq. (1): ea = etot -ecnt emol (1) where etot and ecnt are the total energies of b-doped (14,0) cnts with and without the molecules, respectively, and emol is the total energy of an isolated molecule. to obtain quantum transport properties, the scattering wave functions of the b-doped (14,0) cnts with and without molecules sandwiched between two semi-infinite b-doped (14,0) cnts are calculated based on the overbridging boundary-matching (obm) method [29-32]. for the calculation of the scattering wave functions, the real-space finite-difference approach is employed [33,34], and the grid spacings are taken to be 0.24 å which corresponds to the cutoff energy of 46.8 ry. the y. fujimoto j. electrochem. sci. eng. 12(3) (2022) 431-438 http://dx.doi.org/10.5599/jese.1243 433 conductance g(e) is calculated from the transmission coefficients ti(e) through the landauerbűttiker formula (2) [35]: 2 ii 2 ( ) ( ) e g e t e h =  (2) where i is the number of electron channels, and e and h denote the electron charge and the planck constant, respectively. the stm images are obtained based on the tersoff-hamann (th) approach [36], and the th method is known to be valid for many systems despite its simplicity [37]. in this approach, the tunneling current i(r) is approximated to be proportional to the local density of states (ldos) of the surface at the tip position integrated over a range of energy restricted by the applied bias voltage v, i.e. f f ( ) ( , )d e ev e i r r   +  (3) where ef denotes the fermi energy. the images obtained with negative and positive voltages reflect the occupied and unoccupied electronic states, respectively [38,39]. results and discussion atomic structures and energetics we here study the atomic structures of b-doped zigzag (14,0) cnts without the adsorbed molecules. unlike graphene, the zigzag carbon nanotubes have two kinds of c-c bonds around each atom. the two kinds of the b-c bond lengths of b-doped (14,0) cnt are found to be 1.46 and 1.49 å, and are considerably longer than two kinds of the c-c bond lengths (1.40 and 1.41 å) in a pristine (14,0) cnt. in addition, the two kinds of b-c bond lengths are longer and shorter, compared with that of b-doped graphene (1.47 å) [40,41]. table 1. adsorption energy ea and binding distance d from b atom for each molecule adsorbed on b-doped (14,0) cnts co co2 o2 n2 ea / ev -0.30 -0.01 -0.20 -0.21 d / å 1.54 2.62 1.66 3.37 we also study the adsorption properties of molecules, including environmentally toxic molecules on the b-doped (14,0) cnts. table 1 shows the adsorption energy and the binding distance between the molecule and the b atom in the b-doped (14,0) cnts, and the optimized atomic configurations of co and o2 molecules adsorbed on b-doped (14,0) cnts are shown in fig. 1. it is found that the co and o2 molecules are adsorbed with relatively large adsorption energies and short binding distances. it is also found that the b atoms in the b-doped cnts protrude from the tube surface when co and o2 molecules are adsorbed on the b-doped cnts [figs. 1(a) and 1(b)]. on the other hand, the co2 and n2 molecules weakly bind with relatively small adsorption energy as well as long binding distances. the adsorption energy of the co molecule on the b-doped (14,0) cnt is much smaller and larger than those on the b-doped (8,0) cnt and the b-doped graphene, respectively. thus, the adsorption energy would diminish as the tube diameter increases, and it is expected that the curvature of the nanotubes could improve the chemical reactivity to the adsorbates [8,10,21]. http://dx.doi.org/10.5599/jese.1243 j. electrochem. sci. eng. 12(3) (2022) 431-438 quantum transport of carbon nanotubes for 434 figure 1. optimized structures of (a) co and (b) o2 molecules on b-doped (14,0) cnts scanning tunneling microscopy scanning tunneling microscopy (stm) is one of the most effective methods to observe the local electronic structures of the surfaces at atomic levels [40,41]. figure 2 shows the stm images of bdoped (14,0) cnts with and without co and o2 molecules. one finds that there is a triangle-shaped hillock around the b atom in the b-doped cnt [fig. 2(a)]. it is also observed theoretically and experimentally in the b-doped graphene [17,41,42]. when the co molecule is adsorbed on the b-doped cnt, one can find that the gourd-shaped corrugation above the co molecule appears [fig. 2(b)]. on the other hand, there are two ring-like corrugations above the o2 molecule in the stm image when o2 molecule is adsorbed [fig. 2(c)]. in addition, it is interesting that the stm images at the bias voltages of +0.5 and -0.5 v are different from each other. thus, the b atom and the co molecule as well as the o2 molecule can be clearly identified in the stm images of b-doped (14,0) cnts. (a) boron (b) co (c) o2 v = +0.5 v v = 0.5 v figure 2. scanning tunneling microscopy of b-doped (14,0) cnts: (a) without any adsorbates; (b) with co molecule; (c) with o2 molecule. stm images are generated at bias voltages of +0.5 v (upper panel) and -0.5 v (lower panel) y. fujimoto j. electrochem. sci. eng. 12(3) (2022) 431-438 http://dx.doi.org/10.5599/jese.1243 435 quantum transport properties we examine how the quantum transport of (14,0) cnts is affected by the introduction of the b atom and the adsorption of molecules. figure 3 exhibits the quantum conductance g(e) of the (14,0) cnts as a function of energy e. the (14,0) cnt shows a semiconducting property with a band gap of 0.46 ev. the pristine cnt possesses quantized conductance and step-like structures, as shown in the black lines of figure 3. one finds that the pristine (14,0) cnt has the conductance of 2g0 near the valence-band and the conduction-band edges, where g0=2e2/h is the unit of the quantum conductance. interestingly, the conductance spectrum of the (14,0) cnt shows the symmetric behavior, whereas that of the (8,0) cnt possesses an asymmetric one: in the valence-band edge, both of the (14,0) and (8,0) cnts have 2 g0, while the (14,0) cnt has 2 g0 and the (8,0) cnt has 1 g0 in the conduction-band edge [8]. introduction of the b atom into the semiconducting cnts induces the acceptor states near the valence-band edge, and therefore the fermi level of the b-doped (14,0) cnt relatively moves toward the valence bands. as a result, the b-doped (14,0) cnt shows a p-type semiconducting property [see fig. 3]. when a co molecule is adsorbed on the b-dopant atom in the b-doped (14,0) cnt, one can find that the conductance spectrum largely changes: the adsorption of the co molecule overall reduces the conductance of the b-doped cnts, while the conductance of the b-doped (14,0) cnt is quantized. in the case of the adsorption of the o2 molecule, the conductance spectrum shows similar behaviors to that for the adsorption of the co molecule, but the slopes of both conductance spectra from e = 0 to -0.5 ev are different from each other. figures 4(a) and 4(b) show the scattering wave functions at the fermi energy for the adsorption of the co molecule and the o2 molecule, respectively. the b-doped (14,0) cnt has two conduction channels at the fermi energy, as already discussed [see fig. 3]. when the co molecule is adsorbed, electrons in one of two conduction channels are almost scattered and therefore, its scattering wave function is localized around the co molecule, and the other is composed of the delocalized π-orbital state [fig. 4(a)]. for the adsorption of the o2 molecule, one of two conduction channels is almost localized around the o2 molecule and the other is the extending π-orbital state as well [fig. 4(b)]. figure 3. quantum conductance of pristine (14,0) cnt and b-doped (14,0) cnt without any adsorbates, and with co and o2 molecules http://dx.doi.org/10.5599/jese.1243 j. electrochem. sci. eng. 12(3) (2022) 431-438 quantum transport of carbon nanotubes for 436 figure 4. iso-surfaces of squared scattering wave functions of b-doped (14,0) cnt with (a) co molecule and (b) o2 molecule at the fermi energy we now discuss the sensitivity of cnt-based molecular sensors and define the changes δg(e) / % = ((ga-gb )/gb) ×100, where ga and gb are the conductance of the b-doped (14,0) cnt with and without molecules, respectively. table 2 shows the conductance change δg(0) at the fermi energy (e = 0 ev). the changes δg(e = 0) for the adsorption of the co molecule and the o2 molecule are -48.6 and -38.8 %, respectively. it is therefore found that the co and o2 molecules adsorbed on the b-doped cnt are distinguishable without any applied bias voltages. table 2. changes in the conductance at the fermi energy [δg(0)] for the adsorption of co and o2 molecules on the b-doped (14,0) cnt. the changes in the conductance are calculated with respect to the b-doped (14,0) cnt co o2 δg(0) / % -48.6 -38.8 conclusions the effects of doping and adsorption of molecules on atomic structures, energetics, and quantum transport of b-doped (14,0) cnts have been studied based on our first-principles electronic structure and quantum transport studies. the co and o2 molecules are adsorbed on the b-doped cnts with relatively large adsorption energies, while co2 and n2 molecules are weakly adsorbed with long binding distances. the quantum conductance of (14,0) cnts regarding the adsorption of the molecules is examined. the introduction of the dopant atom and the adsorption of the molecules are found to sizably vary the quantum conductance of the cnts. the nature of the variation of the quantum conductance induced by the doping and the adsorption of the molecules is clarified in terms of the spatial distributions of the scattering wave functions and the possibility to selectively detect toxic co molecule and common o2 molecule in the air are discussed. the bdoped cnt could 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[1] have written a comprehensive review of the use of bi-layer electrolytes in sofcs. the authors have reviewed the performance of several bi-layer electrolyte combinations. cho at al. [2] have developed a high power density bi-layer electrolyte using gadolinium doped ceo2 (gdc) and yttria stabilized zro2 (ysz) layers. the authors postulate that ysz thin film serves as a blocking layer for preventing electrical current leakage in gdc layer and provides chemical and mechanical stability to the cell. heidari et al. [3] have evaluated the electrochemical performance of a solid oxide fuel cell using ysz and sdc (samaria doped ceria) bi-layer electrolyte. the authors postulate that ysz thickness of 3.5 µm ensures good chemical and mechanical stability of sofc. ahn et al. [4] have studied the performance of sofc using er2o3 stabilized bi2o3 (esb) and gdc bi-layer electrolyte. the authors postulate that bi-layer electrolyte can increase the open circuit potential of the cell. further, yahiro et al. [5] have found that a thin layer of ysz sandwiched between the anode and ydc (yttria doped ceria) can prevent reduction of ydc and increase the open circuit potential of sofc. several authors [6-8] have studied the impact of thin ysz layers effect on the blocking of the electronic current and protection of the inner layer of electrolyte. few mathematical models have already been developed to understand the effect of bi-layer electrolyte on the performance of sofc. virkar [9] has studied the oxygen partial pressure at the bilayer electrolyte interface using the equivalent circuit approach. chen [10] developed a bi-layer electrolyte model and studied sofc performance under different relative thicknesses. marques and navarro [11,12] have studied the performance of sofcs using bi-layer electrolyte by an electrochemical permeability model. shen et al. [13] have developed an analytical model for sofc, based on the nernst planck equation. the authors have studied the variation of the interfacial oxygen pressure in the bi-layer electrolyte as a function of ysz thickness and its effect on the performance of sofc. shen et al. [14] have also analysed the performance of sofc theoretically. the authors have developed an expression for the critical thickness ratio of the bi-layer electrolyte which is essential to maintain the chemical stability of electrolyte. chan et al. [15] have developed a model to study the performance of bi-layer electrolytes. the authors postulate that if a thin layer of ysz is deposited onto the substrate electrolyte in the thickness ratio of 1 to 10000, it could increase the interfacial oxygen partial pressure significantly in the bi-layer electrolyte and reduce the penetration of electronic current. studying the factors which influence the leakage current is very important in understanding the efficiency of sofc. several factors influence the leakage current in sofc such as thickness of ysz layer, temperature, voltage, ionic conductivity ratios of the bi-layer electrolyte, electronic conductivity ratios of the bi-layer electrolyte, etc. since the leakage current significantly affects the power density of sofc, studying the effect of these parameters on the leakage current and power density in addition to the interfacial oxygen pressure in the bi-layer electrolyte, is very important in understanding the overall performance of sofc. some of the earlier papers have studied the effect of ysz thickness on the power density and interfacial oxygen pressure in a bi-layer sofc. however, a study of some of the parameters which affect the leakage current in sofc with bi-layer electrolyte (especially effects of applied voltage, temperature, ionic conductivity ratio and electronic conductivity ratio of the bi-layer electrolyte) has not been studied by any previous model. in this paper, we have tried to address just these effects. model development and assumptions figure 1 shows the schematic of sofc with a bi-layer electrolyte. the total thickness of the bilayer electrolyte is given by t (t1+t2). the model considers gadolinia doped ceria (gdc) as the substrate electrolyte coated with a thin layer of yttria stabilized zirconia (ysz) on the anode side. b. krishnamurthy and h. ramasubramanian j. electrochem. sci. eng. 10(3) (2020) 293-304 http://dx.doi.org/10.5599/jese.770 295 the migration of the oxygen ions and diffusion of electron holes and electrons lead to the creation of leakage current (electronic current). figure 1. schematic of the bi-layer electrolyte sofc model assumptions a. the electronic current is assumed to be due to the transport of oxygen vacancies and electrons through the electrolytes. b. ionic and electronic current densities in the two electrolytes are assumed continuous across the electrolytes. c. the contact resistance between the electrolytes is assumed small and there is no reaction between electrolytes. d. the cell is assumed to operate isothermally. the concentration and activation overpotentials are neglected in the model. model equations the flux of the species in the bi-layer electrolyte is represented by the nernst planck equation =  −  i i i i i i -j zufc d c (4) where ji is the mole flux of the charge (o2and e-). the first term on the right-hand side is the migration term. here  refers to the electrostatic potential (the gradient of the electrostatic potential is used in the equation) and ci refers to the concentration of species i. the current arising from the motion of the charged particles is defined as: =  i ii f z j (5) for electroneutral electrolyte, the following equation is valid: = i i 0zc (6) the electronic conductivity in this model is assumed to obey 1/n power law of oxygen partial pressure where n = -4 while the ionic conductivity is constant for the electrolyte. thus [13,14], e =e0po2 -1/4 (7) http://dx.doi.org/10.5599/jese.770 j. electrochem. sci. eng. 10(3) (2020) 293-304 leakage current in a solid oxide fuel cell 296 where e0 is a coefficient which depends only on temperature. the ionic conductivities of two electrolytes are denoted as ο2-ι and ο2-ιi, respectively. similarly, electronic conductivities are denoted as eι and eιi, respectively. using the species concentration equations developed by shen et al. [13,14], the following expressions for ionic and electronic current densities can be found by applying transport equations:  − = 22e o o q t (v h v) i t (8)      − − −    − −   =           − +     −                2 2 2 221 1 4 4 1 o o th 2 e ii o o 1 ii e 1 2 eq e i 2 i 1 ( ) ( ) 1 ( 1) 1 o r p p v v r i t r r (9) where t is the total thicknesses of the electrolyte (both electrolyte 1 and electrolyte 2). the equivalent ionic conductivity is defined as [13-15]    =222eq 1 iio 1 2o o / + / t t t (10) where t1 and t2 are the thickness of electrolyte 1 and electrolyte 2, respectively. thus, the relationship between total current and operating voltage for sofcs with a bi-layer electrolyte can be written as: itotal = to2+ te (11) substituting eqs. (5) and (6) into eq. (8), gives      −− = + − +       2 2 222-1/4 -1/4 1 o 1 2 i ii i i 2 o total o 1 i 2o o e ( ( ) 1/ ( ) ) 1 ( 1) (1-1/ ) q e e t r p r pv h v i r rt (12) where   =        22eq i 1 o 1 bo -exp t t v h v q r t k t (13)   − − = 22 eq ii 2 o 2 b exp t o t v h v q r t k t (14) and = 2 2 ob ih ii t ln 4 o pk t v q p (15) expression (12) translates current density as a function of the operating voltage. when the partial pressures of oxygen at the electrodes are known, the total current density can be calculated directly using eq. (8). also, the output power density can be determined as: p = itotalv (16)] the efficiency of sofc can be calculated as  = 2total th o iv v i (17) the distribution of the oxygen partial pressure across the bi-layer electrolyte is given as [13-15] b. krishnamurthy and h. ramasubramanian j. electrochem. sci. eng. 10(3) (2020) 293-304 http://dx.doi.org/10.5599/jese.770 297   − − = 221 2 1 4 1 4 1 1 e o o 1 i ) ( ) -( -1) / ( )( ( )/ / / o o e i p x c p c i i (0 ≤ x ≤ t1) (18) − − − =  2 2 22 i i 2 i 1/4 e2 o 1 4 2 i 2 o 11 ( ) /( ) o / o e ic p x c p c i (t1 ≤ x ≤ t) (19) where  −    =      22 o 1 i b exp o i q c x k t (20)   = −     22o 2 ii o exp ( ) b i q c t x k t (21) the equations for leakage current density (eq. (9)) and interfacial oxygen partial pressures (eqs. (20) and (21)) are solved using matlab. results and discussion table 1 shows the list of parameters used for all simulations presented in figures 2-12. table 1. parameters used for calculations. t / k p1 / atm p2 / atm gdc ysz o2/ s cm-1 𝜎𝑒 0 / s cm-1 𝜎𝑂2− / s cm-1 𝜎𝑒 0 / s cm-1 1073 10-14 0.21 0.075 7.16  10-6 0.022 7.16  10-10 1023 10-14 0.1 0.0741 3.83  10-6 0.0148 3.83  10-10 973 10-14 0.1 0.0471 5.01  10-7 0.006 5.01  10-11 figure 2 shows simulations of the variation of leakage current density with increasing thickness of ysz layer (up to 0.15 µm) at three temperatures (1073, 1023, and 973 k). the figure shows that the addition of ysz bi-layer decreases the leakage current at all three temperatures. it is also seen that for the given set of parameters, the leakage current decreases with increase in the thickness of ysz layer until 0.05 µm, and then levels off. increasing thickness of ysz layer increases the overall cell resistance and leads to voltage losses in the cell. therefore, an optimum thickness of ysz thickness is required for enhanced power density. in their work, chan et al. [15] postulated that ysz layer having thicknesses in the range from 0.1 to 1 µm can effectively protect the substrate electrolyte and enhance the power density of the cell. figure 2. variation of leakage current density in sofc with thickness of ysz layer at different temperatures simulation results presented in figure 2 indicate that ysz layer thickness of 0.05 µm reduces the leakage current significantly and enhances the power density. the leakage current is also seen to http://dx.doi.org/10.5599/jese.770 j. electrochem. sci. eng. 10(3) (2020) 293-304 leakage current in a solid oxide fuel cell 298 increase with increase in temperature. the leakage current or the electronic current primarily happens because of diffusion of electrons and electron-holes across the electrolyte layer. the increase of temperature increases diffusion of these electrons and electron holes across the electrolyte layer thus increasing the leakage current. figure 3 shows the variation of leakage current density with the thickness of ysz layer for varying values of cell voltage (0.6, 0.7 and 0.8 v). it is seen that the leakage current density is higher at increasing cell voltages. the increasing cell potential across sofc creates an additional driving force for the transport of electrons and electron holes across the bi-layer electrolyte leading to an increase in the leakage current. figure 3. variation of leakage current density in sofc with thickness of ysz layer for different voltages applied across the cell figure 4 shows the variation of the cell power density with the thickness of ysz layer for three different temperatures. the figure shows that the overall cell power density increases with increase of temperature. at the beginning, the cell power density is seen to increase with addition of ysz layer, but when the thickness of the ysz layer becomes higher than 0.05 µm, the power density is seen to decrease slowly. it seems that increase of the ohmic resistance introduced by addition of ysz layer and the associated voltage drop cause reduction of power density. thus, it is very important to optimize the thickness of ysz layer to obtain the maximum power density in sofc. figure 4. variation of power density of sofc with thickness of ysz layer at different temperatures figure 4 is supported by figure 5 which shows the variation of power density with thickness of the ysz layer (variation of voltage). the figure clearly shows the peak power density occurring at a ysz thickness ranging from 0.02 µm to 0.05 µm. b. krishnamurthy and h. ramasubramanian j. electrochem. sci. eng. 10(3) (2020) 293-304 http://dx.doi.org/10.5599/jese.770 299 figure 5. variation of power density of sofc with thickness of ysz layer for different voltages applied across the cell figure 6 shows the variation of leakage current density with ysz thickness for different electronic conductivity ratios of gdc:ysz layer. the electronic conductivity ratio of two layers is varied as 1: 0.1, 1: 0.01 and 1:0.001. the total thickness of the bi-layer electrolyte is kept at 10 µm and the thickness of ysz layer is varied from 0 to 0.05 µm. the leakage current is found to vary significantly with decreasing values of electronic conductivity of ysz layer. it is seen that the lowest leakage current is obtained when the ratio is 1:0.001, indicating that the electronic conductivity of the additional layer should be much lower than the primary layer. typically the electronic conductivity of ysz layer is seen to be lower than gdc layer by value of 0.01 to 0.001. it is seen that when the electronic conductivity of ysz layer is significantly lower compared to gdc layer, the leakage current is found almost negligible even at every low thickness of ysz layer. figure 7 shows the corresponding variation of power density with electronic conductivity of the gdc: ysz layers varying in the same ratio. from figure 5, we see the highest power densities corresponding to the lowest leakage current density graphs. for 1:0.001 ratio of electronic conductivities of gdc: ysz, the peak power density is seen for ysz thickness of 0.02 µm. thus, it is important to use an additional layer with lower value of electronic conductivity since it minimizes the thickness of electrolyte required to minimize leakage current. this consequently reduces the ohmic drop and the voltage drop across the cell improving the power density. figure 6. variation of leakage current density of sofc with thickness of ysz layer for different electronic conductivity ratios of gdc:ysz layer it is seen in figure 8 that varying ionic conductivities of bi-layer electrolyte affects the leakage current but not to the extent of the electronic conductivity (cf. fig. 6). it is seen that when ysz has http://dx.doi.org/10.5599/jese.770 j. electrochem. sci. eng. 10(3) (2020) 293-304 leakage current in a solid oxide fuel cell 300 the lowest ionic conductivity compared to gdc, the leakage current is the lowest showing the effect of ionic conductivity on the leakage current. figure 7. variation of power density of sofc with thickness of ysz for different electronic conductivity ratios of gdc: ysz layer figure 8. variation of the leakage current density of sofc with thickness of ysz for different ionic conductivity of ysz layer figure 9 shows the corresponding power densities in a bi-layer sofc for variation of ionic conductivities. it is seen that changing of ionic conductivities has a substantial effect on the power densities. while the leakage current does not change substantially, the change in the power density of the bi-layer sofc seems to be substantial with change of the ionic conductivity figure 9. variation of power density of sofc with thickness of ysz for different ionic conductivities of ysz layer b. krishnamurthy and h. ramasubramanian j. electrochem. sci. eng. 10(3) (2020) 293-304 http://dx.doi.org/10.5599/jese.770 301 owing to the increase in the ionic conductivity of ysz layer, the electrical resistance of the ysz layer decreases, which leads to a substantial increase in the power density. it is known that in bilayer electrolyte in sofc, the inner electrolyte near the cathode can be unstable. to protect the unstable electrolyte near the cathode, the oxygen partial pressure at the interface of the bi-layer electrolyte must always be higher than its decomposition oxygen partial pressure [13-15]. thus predicting the interfacial oxygen pressure in a bi-layer electrolyte sofc becomes critical. figure 10 shows the interfacial oxygen partial pressure vs. distance along the electrolyte for different thicknesses of ysz layer. while the thickness of gdc layer is kept at 2.5 µm, the thickness of ysz layer is varied from 0.02 to 0.08 µm. the interfacial oxygen partial pressure is seen to rise steeply at the interface of gdc/ysz. it is also seen that the partial pressure in ysz layer increases with increasing thickness of ysz layer. figure 10. variation of interfacial oxygen pressure in sofc as a function of thickness of ysz layer thus, while we see that the power density is the highest when ysz layer is around 0.02 µm, the corresponding oxygen partial pressure is low. therefore, the thickness of ysz layer has to be optimized to get the maximum power density and also to enhance the interfacial oxygen partial pressure. simulation results indicate that the ideal thickness of ysz layer would be between 0.02 to 0.1 µm. chan [15] has predicted that for a substrate electrolyte thickness varying from 100 to 1 mm, a layer of ysz with thickness of 0.1 to 1 µm can effectively protect the substrate electrolyte and maintain a high current density in the cell. it is found that for the given set of parameters, thicknesses of 0.02 to 0.1 µm of ysz layer give the best power output and interfacial oxygen pressure. figure 11 shows the interfacial oxygen pressure as a function of distance along the electrolyte for different thickness ratios of gdc:ysz layer. the graph clearly shows that with decreasing thickness of ysz layer, the interfacial oxygen pressure decreases which corresponds to the graph in fig. 10. figure 12 shows the graphs of interfacial oxygen pressure as a function of distance along the electrolyte at different temperatures. it is seen that the interfacial oxygen partial pressure increases with increase in temperature. however, it is not possible to conclude that the stability of the unstable electrolyte increases with increase in temperature, since the stability of the electrolyte depends on other operating conditions as well. figure 13 shows the comparison of modelling predictions with experimental data (polarization curve taken from ref. 17. since the conductivity values of gdc and sdc are very close [13], we have modelled the polarization curve of a bi-layer electrolyte sofc using ysz (318 µm) and sdc (737 µm) at 650 oc. the list of parameters based on experimental data 13,17 are used and shown in table 2. modelling results are found to compare very well with experimental data. http://dx.doi.org/10.5599/jese.770 j. electrochem. sci. eng. 10(3) (2020) 293-304 leakage current in a solid oxide fuel cell 302 figure 11. variation of the interfacial oxygen pressure with distance along the electrolyte in sofc for different thickness ratio of gdc: ysz layer figure 12. variation of the interfacial oxygen pressure in sofc with distance along the electrolyte at different temperatures figure 13. comparison of polarization curves obtained by modelling predictions and experiment 17 table 2. list of parameters used for calculation of polarization curve in figure 13. parameter value sdc ionic conductivity, s/cm 0.019 sdc co-eff of electronic conductivity, s/cm 8.2110-8 ysz ionic conductivity, s/cm 0.003 ysz co-eff of electronic conductivity, s/cm 8.2110-10 ysz thickness, µm 318 sdc thickness, µm 737 total thickness, µm 1055 temperature, k 923 b. krishnamurthy and h. ramasubramanian j. electrochem. sci. eng. 10(3) (2020) 293-304 http://dx.doi.org/10.5599/jese.770 303 conclusion the effect of design and operational parameters on the leakage current of a bilayer sofc (specifically gdc : ysz bilayer) is modelled. simulation results indicate that for the given set of operating parameters, the peak power density is seen for ysz film thickness of 0.02 to 0.08 µm. the leakage current is seen to decrease with increasing thickness of ysz layer at various temperatures and cell voltages. the power density is seen to reach a maximum value with increasing thickness of ysz layer at various temperatures and cell voltages. however, increasing thickness of ysz layer is seen to reduce the power density, showing the effect of ohmic resistance. the ionic conductivity of ysz layer is found to play a minimal role, while the electronic conductivity of ysz layer is found to play a significant role in determining the leakage current. when the ratio of ysz:gdc electronic conductivity is low, the leakage current is found to be correspondingly low. the interfacial oxygen pressure which plays a major role in determining the stability of the inner electrolyte is found to increase with increasing thickness of ysz layer and is also found to increase with temperature. thus, it can be seen that the thickness of ysz layer has to be optimized to enhance the power density in a bi-layer sofc. nomenclature itotal total current density, a/cm2 ii current density of charge i, a/cm2 kb boltzmann constant, 1.380651023 j/k t total thickness of bi-layer electrolyte, m pi partial pressure of species i, atm p power density, w/cm2 q elementary charge, 1.602181019 c ti thickness of electrolyte i, m t absolute temperature, k v cell voltage, volt x position variable, m vth nernst voltage, volt zi number of proton charges carried by an ion ui mobility of ions, m2/ vs di diffusion coefficient of species i, m2/s subscripts o2 oxygen o2oxygen ion eelectron int measured interfacial parameters superscripts 1 anode parameters 2 cathode parameters i, ii electrolyte 1 or electrolyte 2 greek letter  efficiency of sofc  conductivity, s/m acknowledgements: the authors wish to acknowledge bits pilani, hyderabad for the support given for this article. http://dx.doi.org/10.5599/jese.770 j. electrochem. sci. eng. 10(3) (2020) 293-304 leakage current in a solid oxide fuel cell 304 references [1] b. shri prakash, r. pavitra, s. senthil kumar, s. t. aruna, journal of power sources 381 (2018) 136155. 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[17] s. shen, l. guo, h. liu, international journal of hydrogen energy 41 (2016) 3646-3654. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) voltammetric determination of hydroxylamine in water and waste water samples using a nio nanoparticle/new catechol derivative modified carbon paste electrode doi: 10.5599/jese.2013.0049 135 j. electrochem. sci. eng. 4(4) (2014) 135-144; doi: 10.5599/jese.2014.0049 open access: issn 1847-9286 www.jese-online.org original scientific paper voltammetric determination of hydroxylamine in water and waste water samples using a nio nanoparticle/new catechol derivative modified carbon paste electrode mahbobeh moazampour, fahimeh tahernejad-javazmi, maryam salimi-amiri*, hassan karimi-maleh and mehdi hatami** department of chemistry, graduate university of advanced technology, kerman, iran *department of physics, sari branch, islamic azad university, sari, iran **polymer research laboratory, university of bonab, bonab, iran corresponding author: e-mail: h.karimi.maleh@gmail.com tel.: +989112540112 received: february 21, 2014; revised: march 22, 2014; published: december 6, 2014 abstract a (9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboximido)-4-ethylbenzene-1,2-diol (ded) modified nio/nps carbon paste electrode “(ded/nio nanoparticle (nio/nps)/cpe) was constructed for determination of hydroxylamine (hx). the cyclic voltammogram showed that the electrocatalytic oxidation of hx at the surface of ded/nio/nps/cpe occurs at a potential of about 800 mv less positive than with an unmodified electrode. square-wave voltammetry results presented that the electrocatalytic oxidation peak currents of hx in ph 8.0 had two linear dynamic ranges in the range of 0.1 to 2.0 and 2.0 to 400.0 µm hx, with a detection limit of 0.07 µm. the kinetic parameters such as electron transfer coefficient (0.47) and rate constant (2.454 × 10 3 m -1 s -1 ) were determined for the chemical reaction between hx and ded. finally, this method was evaluated for the determination of hx in water and waste water samples. keywords hydroxylamine; nio nanoparticle; water and waste water analysis; sensor; voltammetry introduction hydroxylamine (hx) is known as a type of reducing agent and is widely used in industrial and pharmaceutical applications. it has been identified as a key intermediate in nitrogen cycles and nitrous oxide production [1]. the quantitative determination of hx is very important in both studies of biological processes and for industrial purposes. it has been confirmed that hx is produced during the reduction of nitrates by escherichia coli and torula yeast [2]. http://www.jese-online.org/ mailto:h.karimi.maleh@gmail.com j. electrochem. sci. eng. 4(4) (2014) 1350-144 voltammetric determination of hydroxylamine 136 electrochemical analysis is gaining significance within industrial process control, environmental monitoring and various pharmaceutical and biotechnology applications [3-7]. the use of unmodified electrodes for electrochemical detection has a number of limitations, such as low selectivity and sensitivity, poor reproducibility, slow electron transfer reaction, low stability over a wide range of solution compositions and the high overpotential at which the electron transfer process occurs [8-10]. chemical modification of inert substrate electrodes with redox active thin films offers significant advantages in the design and development of electrochemical sensors. in operation, the redox active sites shuttle electrons between the analyte and the electrodes with a significant reduction in activation overpotential [11]. a further advantage of chemically modified electrodes is that they are less prone to surface fouling and oxide formation, compared to inert substrate electrodes [12-14]. a wide variety of compounds have been used as electron transfer mediators for the modification of electrode surfaces in various procedures [15-17]. nanotechnology has become one of the most interesting disciplines in science and technology today. the intense interest in nanotechnology is being driven by various interesting fields and is creating a new industrial revolution [18]. nano-materials such as nanoparticles, carbon nanotubes or nanocomposite connected with biomolecules are being used for several bioanalytical applications [19-21]. electroanalysis is taking advantage of all the possibilities offered by nanomaterials that are easy to detect using conventional electrochemical methods. nanocomposite of a variety of shapes, sizes and compositions continues to change the field of bioanalytical measurement. in the present work, we describe the preparation and suitability of a ded modified nio/nps carbon paste electrode as a new electrode for electrocatalysis and determination of hx in an aqueous buffer solution. to demonstrate the catalytic ability of the modified electrode toward the electrooxidation of hx in real samples, we examined the utility of this method for the voltammetric determination of hx in water and waste water samples. experiment chemicals all chemicals used were of analytical reagent grade purchased from merck (darmstadt, germany), unless otherwise stated. doubly distilled water was used throughout. 1.0×10 –2 mol l –1 hx solution was prepared daily by dissolving 0.064 g hx in water and the solution was diluted to 100 ml with water in a 100 ml volumetric flask. the solution was kept in a refrigerator at 4 o c and in the dark. further dilution was made with water. phosphate buffer solutions (sodium dihydrogen phosphate and disodium monohydrogen phosphate, plus sodium hydroxide, 0.1 mol l –1 ) (pbs) with different ph values were used. high viscosity paraffin (d = 0.88 kg l –1 ) from merck was used as the pasting liquid for the preparation of the carbon paste electrode. spectrally pure graphite powder (particle size <50 µm) from merck was used as the substrate for the preparation of the carbon paste electrode as a working electrode. apparatus cyclic voltammetry (cv), chronoamperometry and square wave voltammetry (swv) were performed using an analytical system, autolab, with pgstat 302n (eco chemie, the netherlands). the system was run on a pc using gpes software. a conventional three-electrode cell assembly consisting of a platinum wire as an auxiliary electrode and an ag/agcl (kclsat) electrode as a s. kaushal at al. j. electrochem. sci. eng. 4(4) (2014) 1350-144 doi: 10.5599/jese.2014.0049 137 reference electrode was used. the working electrode was either an unmodified carbon paste electrode (cpe) or a ded/nio/nps/cpe. x-ray powder diffraction studies were carried out using a stoe diffractometer with cu–k radiation (l = 1.54 å). preparation of the modified electrode to prepare the modified electrode, 150.0 mg of nio/nps and 70.0 mg of ded was hand mixed with 780.0 mg of graphite powder using a mortar and pestle. using a syringe, 15 drops of paraffin were added to the mixture and mixed well for 55 min until a uniformly wetted paste was obtained. the paste was then packed into a glass tube. by pushing a copper wire down the glass tube into the back of the mixture, electrical contact was created. when necessary, a new surface was obtained by pushing an excess of the paste out of the tube and polishing it on weighing paper. the unmodified carbon paste electrode (cpe) was prepared in the same way without nio/nps and ded to the mixture, to be used for comparison purposes. preparation of real samples water samples were stored in a refrigerator immediately after collection. ten millilitres of the sample was centrifuged for 15 min at 1500 rpm. the supernatant was filtered using a 0.45 µm filter and then diluted three times with the pbs ph 8.0. the solution was transferred into the voltammetric cell to be analysed without any further pre-treatment. the standard addition method was used for the determination of hx in real samples. results and discussion nio/nps characterisation nio/nps were analysed by xrd analyses. the xrd pattern of nio/nps nanopowders in the 2 range of 10-80° is shown in fig. 1. figure 1. xrd patterns of as-synthesised nio/nps nanoparticles. j. electrochem. sci. eng. 4(4) (2014) 1350-144 voltammetric determination of hydroxylamine 138 figure 1 clearly proves the presence of nio/nps. an average diameter of as-synthesised nio/nps was calculated from the broadness peak (2 = 44°) by using the scherrer equation d = kλ/ cos , and measured about 25.0 nm. electrochemical investigation figure 2 depicts the cyclic voltammetry responses from the electrochemical oxidation of 400 µm hx at ded/nio/nps/cpe (curve c), ded/cpe (curve b), nio/nps/cpe (curve d) and unmodified cpe (curve e). as shown, the anodic peak potential for hx oxidation at ded/nio/nps/cpe (curve c) and at ded/cpe (curve b) was about 200 mv, while at nio/nps/cpe (curve d); the peak potential was about 1000 mv. at the unmodified cpe, the peak potential of hx was about 1050 mv (curve e). from these results, it was concluded that the best electrocatalytic effect for hx oxidation was observed at ded/nio/nps/cpe (curve c). figure 2. cyclic voltammograms of (a) the buffer solution at ded/nio/nps/cpe; (b) 400 µm hx at ded/cpe; (c) 400 µm hx at ded/nio/nps/cpe; (d) 400. µm hx at nio/nps/cpe; (e) 400 µm hx at cpe. conditions: 0.1 mol l -1 pbs (ph 8.0), scan rate of 20 mv s -1 . for example, the results show that the peak potential of hx oxidation at ded/nio/nps/cpe (curve c) shifted by about 800 and 850 mv toward less positive values when compared with nio/nps/cpe (curve d) and unmodified cpe (curve e), respectively. additionally, ded/nio/nps/cpe showed higher anodic peak current for the oxidation of hx compared to ded/cpe, indicating that the combination of nio/nps and the mediator significantly improved the performance of the electrode toward hx oxidation. in fact, ded/nio/nps/cpe in the absence of hx exhibited a wellbehaved redox reaction (figure 2a) in the buffer solution (ph 8.0). however, there was a drastic increase in the anodic peak current in the presence of 400 µm hx (curve c), which can be related to the electrocatalytic role of ded/nio/nps/cpe towards oxidation of hx. we observed a linear variation of the peak current with the square root of scan rate (ν 1/2 ) at scan rates ranging from 2-12 mv s –1 at ph 8.0 (figure 3). this result clearly indicates a diffusioncontrolled electrooxidation process [22]. s. kaushal at al. j. electrochem. sci. eng. 4(4) (2014) 1350-144 doi: 10.5599/jese.2014.0049 139 figure 3 plot of ipa versus ν 1/2 for the oxidation of 100.0 µm hx at various scan rates of (a) 2.0; (b) 3.0; (c) 5.0; (d) 8.0; (e) 12.0 mv s −1 in 0.1 mol l −1 phosphate buffer solution (ph 8.0) at nio/nps/ded/cpe. inset: cyclic voltammograms of 100.0 μm hx at various scans. to obtain information about the rate-determining step, a tafel plot was drawn, derived from points in the tafel region of the linear sweep voltammogram (figure 4). the slope of the tafel plot was equal to n(1−α)f/2.3rt, which resulted in 0.1115 v decade -1 [23]. therefore, we obtained the value of α being equal to 0.47. for further investigations, the value of α was calculated for the oxidation of hx at ph 8.0 for both the modified and unmodified paste electrodes using one other method (see equation 1): αnα = 0.048/(ep–ep/2) (1) where ep/2 is the potential corresponding to ip/2. the values for αnα were found to be 0.47 and 0.12 at the surface of ded/nio/nps/cpe and cpe, respectively. this result was also confirmed by the larger ipa values recorded during linear seep voltammetry at ded/nio/nps/cpe. chronoamperometric measurements of hx at ded/nio/nps/cpe were carried out for various concentrations of hx in buffered aqueous solutions (ph 8.0) by setting the working electrode potentials at 0.0 mv and 400 mv vs. ag/agcl/kclsat (figure 5a). for an electroactive material (hx, in this case) with a diffusion coefficient of d, the current observed for the electrochemical reaction at the mass transport limited condition was described using the cottrell equation. experimental plots of i vs. t -1/2 were employed, with the best fits for 300 µm of hx (figure 5b). the slope of the resulting straight line was then plotted against hx concentration (figure 5b). from the resulting slope and cottrell equation, the mean value of the d was found to be 2.1×10 −6 cm 2 s -1 [24]. j. electrochem. sci. eng. 4(4) (2014) 1350-144 voltammetric determination of hydroxylamine 140 figure 4. tafel plot for ded/nio/nps/cpe in 0.1 mol l −1 pbs (ph 8.0) with a scan rate of 8 in the presence of 100.0 µm hx. figure 5. a – chronoamperograms obtained at ded/nio/nps/cpe (a) in the absence and in the presence of (b) 300 μm hx at ph 8.0; b – cottrell’s plot for the data from the chronoamperograms; c – dependence of ic/il on t 1/2 derived from the chronoamperogram data; d – the charge-time curves (a′) for curve (a) and (b′) for curve (b). the rate constant for the chemical reaction between hx and redox sites in ded/nio/nps/cpe, kh, can be evaluated by chronoamperometry according to the method set out by galus [25]: ic/il = π 1/2 γ 1/2 = π 1/2 (kcbt) 1/2 (2) where ic is the catalytic current of hx at ded/nio/nps/cpe, il the limited current in the absence of hx and t is the time elapsed (s). the above equation can be used to calculate the rate s. kaushal at al. j. electrochem. sci. eng. 4(4) (2014) 1350-144 doi: 10.5599/jese.2014.0049 141 constant of the catalytic process kh. based on the slope of the ic/il versus t 1/2 plots (figure 5c), kh can be obtained for a given hx concentration. from the values of the slopes, an average value of kh was found to be kh = 2.454×10 3 m –1 s –1 . the value of kh also explains the sharp feature of the catalytic peak observed for catalytic oxidation of hx at the surface of ded/nio/nps/cpe. double potential step chronocoulometry, as well as other electrochemical methods, was in addition employed for the investigation of the electrode processes at ded/nio/nps/cpe. forward and backward potential step chronocoulometry on the modified electrode in a blank buffer solution showed very symmetrical chronocoulograms. these had about an equal charge consumed for both oxidation and reduction of the dedred/dedox redox system in ded/nio/nps/cpe. however, in the presence of hx, the charge value associated with forward chronocoulometry was significantly greater than that observed for the backward chronocoulometry (figure 5d). this behaviour is typical of that expected for electrocatalysis at a chemically modified electrode [26]. stability and reproducibility the repeatability and stability of modified electrode was investigated using cv measurements of 400.0 µm hx in a buffer solution. the relative standard deviation (rsd) for five successive assays was 1.4 %. when seven different ded/nio/nps/cpes were used, the rsd for ten measurements was 2.1 %. when the electrode was stored in the laboratory, the modified electrode retained 95 % of its initial response after a week and 92 % after 30 days (see figure 6). these results indicate that ded/nio/nps/cpe has good stability and reproducibility, and could be used for hx measurements. determination of hx individually square wave voltammetry (swv) was used to determine the concentration of hx. since square wave voltammetry has a much higher current sensitivity and better resolution than cyclic voltammetry, the swv was used for the determination of hx (figure 7 inset). the plot of peak current vs. the hx concentration consisted of two linear segments with slopes of 5.9035 and 0.1498 µa/µm at the concentration ranges of 0.1-2.0 µm and 2.0-400.0 µm, respectively (fig. 7). figure 6. cyclic voltammograms of 300 μm hx at a surface of ded/nio/nps/cpe in a 0.1 mol l −1 phosphate buffer solution (ph 8.0) at different times. j. electrochem. sci. eng. 4(4) (2014) 1350-144 voltammetric determination of hydroxylamine 142 the decreasing of sensitivity (slope) of the second linear segment was likely due to kinetic limitation. the detection limit was determined as 0.07 µm for hx based on ylod = yb+3σ. interference study and real sample studies analytical selectivity was one of the important parameters that affected the accuracy of the analysis. in order to evaluate the selectivity of the proposed method for the determination of hx, the influence of various substances as potentially interfering compounds, which can be present in the water and waste water samples with the determination of hx, were studied under optimum conditions with 1.0 µm hx at ph 8.0. the tolerance limit was taken as the maximum concentration of the foreign substances, which caused an approximate 3% relative error (in potential or current) in the determination. the result of interfering studied for some of the various substances in oxidation current and oxidation peak potential of hx showed that 1000-fold of ni 2+ , cn – , ca 2+ , mg 2+ , mn +2 , k + , na + , cl and scn – , 800-fold of glucose, sucrose, lactose and fructose did not affect the selectivity. figure 7. the plots of the electrocatalytic peak current as a function of hx concentration. inset shows the swvs of ded/nio/nps/cpe in a 0.1 mol l −1 phosphate buffer solution (ph 8.0) containing different concentrations of hx. from bottom-up corresponds to 0.0, 0.1, 0.5, 1.0, 2.0, 50.0, 100.0, 150.0, 200.0, 250.0, 300.0, 350.0 and 400.0 μm of hx. in order to demonstrate the applicability of the new sensor in determining hx in real samples, we used the new sensor in determining hx in tap water, river water, wastewater and well water. the determinations of hx in samples were carried out using the standard addition method (table 1). the accuracy of the method was examined by comparing the results obtained from this method with published methods for the determination of hx [2]. the results from the statistical calculation indicated good agreement between them for the mean values (t-test) and the precision (f-test) in the determination of hx in real samples for the three analyses. it was clear that a modified electrode was capable of voltammetric determination of hx, with high selectivity and good reproducibility. s. kaushal at al. j. electrochem. sci. eng. 4(4) (2014) 1350-144 doi: 10.5599/jese.2014.0049 143 table 1: determination of hx in water samples (n=3). sample added, µm founded, µm published method, µm fex ftab tex ttab tap water 1.00 1.05±0.10 0.98±0.11 6.3 19.0 1.2 3.8 well water 5.0 4.85±0.16 5.22±0.32 7.8 19.0 1.5 3.8 river water a 10.0 10.45±0.65 10.55±0.76 9.6 19.0 2.1 3.8 waste water 30.0 29.73±0.75 30.75±1.2 13.5 19.0 3.2 3.8 a tejan river, sari, iran fex calculated f value; ftab reported f value from f-test table with 95 % confidence level and 2/2 degree of freedom; tex calculated t; ttab reported t value from student t-test table with 98 % confidence level. conclusion a carbon paste electrode modified with nio/nps and ded was used for electrocatalytic determination of hx. the results showed that the oxidation of hx was catalysed at ph 8.0, whereas the peak potential of hydrazine was shifted by 800 mv to a less positive potential at the surface of ded/nio/nps/cpe. in addition, it was shown that hx can be determined using the square wave voltammetry technique. the detection limit (3σ) was 0.07 according to the swv method. the kinetic parameters, such as electron transfer coefficient, α (0.47) and a rate constant for the chemical reaction between hx and redox sites in ded/nio/nps/cpe, kh (2.454×10 3 m –1 s –1 ) were also determined using electrochemical approaches. finally, the electrocatalytic oxidation of hx at the surface of this modified electrode can be employed as a new method for the voltammetric determination of hx in real samples such as tap water, river water, 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[26] m. keyvanfard, s. sami, h. karimi-maleh, k. alizad, journal of the brazilian chemical society 24 (2013) 32-39. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {a simple and fast method to detect freebase cocaine in artificial saliva by square wave voltammetry (swv) using carbon paste electrode} http://dx.doi.org/10.5599/jese.849 361 j. electrochem. sci. eng. 10(4) (2020) 361-371; http://dx.doi.org/10.5599/jese.849 open access: issn 1847-9286 www.jese-online.org original scientific paper a simple and fast method to detect freebase cocaine in artificial saliva by square wave voltammetry (swv) using carbon paste electrode bruno ferreira, leandro oka duarte, érica naomi oiye, maria fernanda muzetti ribeiro, juliana midori toia katayama, pedro henrique barros oliva and marcelo firmino de oliveira geeqfor – grupo de eletroquímica, eletroanalítica e química forense, departamento de química faculdade de filosofia, ciências e letras de ribeirão preto, universidade de são paulo, 14040-901 ribeirão preto, são paulo, brasil corresponding author: marcelex@usp.br; tel.: +55 16 3315 9150 received: may 8, 2020; revised: july 2, 2020; accepted: july 5, 2020 abstract the consequences of consuming and commercializing illicit drugs including cocaine, constitute a serious problem for authorities and the whole society. cocaine is usually identified in the laboratory conditions by chromatographic or spectroscopic methods. electrochemical techniques have also gained prominence because they are fast and easy to use, have many applications, and provide reproducible and reliable results. therefore, in the present study, a voltammetric method was developed to detect freebase cocaine using carbon paste electrode and methanol as the main cocaine solvent. the developed method was applied to detect cocaine in the artificial saliva by the square wave voltammetry (swv). the current values increased linearly with the concentration of cocaine, which afforded construction of the analytical curve. the limit of detection (lod) and the limit of quantifycation (loq) were determined as 0.90 µg/ml and 2.41 µg/ml, respectively. for comparison purposes, hplcdad chromatographic method was also applied to detect cocaine. the corresponding analytical curve gave lod = 0.043 µg/ml and loq = 0.130 µg/ml. although showing better analytical results, hplc-dad method could not detect cocaine in saliva samples without previous treatment, what makes the electrochemical method much more attractive for this type of detection. keywords forensic chemistry; cocaine; saliva; square wave voltammetry http://dx.doi.org/10.5599/jese.849 http://dx.doi.org/10.5599/jese.849 http://www.jese-online.org/ mailto:marcelex@usp.br j. electrochem. sci. eng. 10(4) (2020) 361-371 detecting freebase cocaine in artificial saliva 362 introduction cocaine is a natural substance that is commonly found in erythroxylum coca leaves. this plant is widely cultivated, sold, and distributed in south american countries 1. cocaine stimulates the central nervous system and can cause cardiovascular damage, euphoria, and aggressiveness, which has turned it into a worldwide social problem 1,2. cocaine can be found in two main forms (figure 1), cocaine hydrochloride (hcl-salt form) and alkaline cocaine (freebase cocaine) 3. figure 1. molecular structure difference between freebase cocaine and cocaine hydrochloride. the routes of administration of cocaine rely on its chemical form. freebase cocaine for instance, is usually consumed through smoking, while its salt form is consumed by intranasal or injecting administration 1. in the human body, the cocaine metabolism can produce some biotransformation products such as benzoylecgonine (bec), ecgonine (ecg), and ecgonine methyl ester (eme) 4. the cocaine has half-life of approximately one hour 1-3. when consumed, this drug can concentrate in the tissues, increasing its detection window. currently, there are various literature studies involving cocaine detection in biological samples such as urine, plasma, and saliva 3. in the saliva, cocaine may be available up to 48 h after administration, and its concentration in this fluid is higher than the concentration of its main metabolite, benzoylecgonine 5,6. the use of saliva to detect narcotics is advantageous over other biological fluids, because saliva is easy to be sampling, has lower concentration of proteins and endogenous compounds than urine or blood, and its adulteration and contamination are less likely to occur since collection is usually done under supervision 7. therefore, government agencies use this matrix in the case of traffic accidents, for toxicological monitoring of employees and even for high-performance sports (e.g. anti-doping tests) 6. on the other hand, the use of saliva has limitations like small volume for detection, high possibility of contamination by exogenous substances (e.g. nicotine and food residues) and high viscosity 6,7. chromatographic techniques, especially gas or liquid chromatography coupled with mass spectrometry (ms), are commonly used to detect cocaine in saliva and other fluids because of their high sensitivity and selectivity 8. indeed, these techniques are often employed because of the biological matrix complexity and lower concentrations of the analytes. preparing the sample for injection into the chromatographic system is another important point. since these processes are generally long and costly, the search for new reliable methods that can detect cocaine and other narcotics in biological samples becomes an urgent matter 5,8. b. ferreira et al. j. electrochem. sci. eng. 10(4) (2020) 361-371 http://dx.doi.org/10.5599/jese.849 363 beside the chromatographic techniques, cocaine and its metabolites have already been accomplished using voltammetric techniques 4,9,10. the direct electrochemical determination of cocaine is widely performed in seized samples using a wide range of electrodes and voltammetric techniques (e.g. square wave voltammetry and cyclic voltammetry). for detection of its metabolites, few works have already appeared in the literature, where the main ones used mercury drop electrode as the working electrode 4,9,10. on the other hand, there is an apparent lack of research involving determination of cocaine in biological samples. in this specific area of toxicological analyses, the voltammetric techniques have high potential, since they have good reproducibility, high sensitivity and precision, and no need for biological sample pre-treatment procedures 11-14. it has already been shown that when carbon paste electrode was used as the working electrode for cocaine detection, the anodic scan refers to the oxidation of tertiary amine in cocaine molecule 11,15. an intermediate, imine, originates from this oxidation on the electrode surface when two electrons and one proton are lost 11,15. freebase cocaine is oxidized at potentials close to 1.1 v vs. ag/agcl 15. in the forensic field, cocaine detection is usually accomplished in countless seized samples, but few studies have been dedicated to the direct procedure of cocaine detection in biological matrices, particularly saliva 11. this work aims to detect freebase cocaine in artificial saliva by a direct method that uses a swv technique and unmodified carbon paste electrode. this procedure is advantageous because it allows simple and rapid cocaine detection based on the cocaine amine group oxidation 11. this work also compares the electrochemical results obtained by sw voltammetry with the results obtained by the high-performance liquid chromatography (hplc-dad) as one of the most frequently applied techniques for this purpose. experimental materials and reagents methanol and acetonitrile, both hplc grade, were purchased from honeywell. the other reagents were analytical grade. nitric acid, sodium phosphate dibasic (cinética química), potassium carbonate (sigma aldrich) and the salts of sodium (chloride, nitrite and hydroxide), obtained from merck, were used to prepare artificial saliva. for preparing supporting electrolyte (ph 9.5), the salts of perchlorate (synth) and potassium phosphate dibasic (sigma aldrich) were used. mixture of graphite powder (fisher scientific) and paraffin (isogama) were used to prepare the carbon paste electrode. for analyses of possible interference, stock solutions of caffeine at 485 µg/ml and lidocaine at 586 µg/ml were prepared by dissolving the respective standard powder in methanol. cocaine powder (99.8 % w/w) was acquired from the laboratory of toxicological analysis – institute of criminalistics – ribeirão preto, state of são paulo, brazil and purified following a procedure described in the literature 12. after this step, the stock solution of cocaine at 760 µg/ml was prepared by dissolving cocaine purified powder in methanol. apparatus electrochemical measurements were performed by the square wave voltammetry (swv) technique. µ-autolab iii potentiostat coupled to a microcomputer with nova software v. 1.11.2 (metrohm autolab) was employed. gas chromatograph coupled to a mass spectrometer (gc/ms) was conducted on a shimadzu model qp2010se (nakagyo-ku, kyoto, jpn) apparatus coupled to a single quadrupole mass http://dx.doi.org/10.5599/jese.849 j. electrochem. sci. eng. 10(4) (2020) 361-371 detecting freebase cocaine in artificial saliva 364 spectrometer with electron impact ionizer. the analysis was carried out in an rtx-5ms column (30 m  0.25 mm i.d., 0.25 µm film thickness) from restek corporation (bellefonte, pa, usa). an automated liquid chromatography coupled with diode array detector (lc/dad) system by thermo scientific dionex model ultimate 3000 hplc sq (waltham, ma, usa) equipped with an lpg-3400sd quaternary pump, a wps-3000rs autosampler, a tcc-3000sd column compartment, a dad-3000rs uv diode detector, and a c8 column (250  4.6 mm, 5 µm particle size) (nanoscience technologies) were used. manufacture of carbon paste electrode carbon paste electrode (cpe) was prepared using 3 g of graphite powder with 3 g of paraffine (50:50 w/w). graphite and paraffine were homogenized at high temperature (about 100 oc) until the paste was formed. afterwards, carbon paste was packed into a plastic cylindrical tube of a syringe form (figure 2). at the end of each measurement, the electrode surface was cleaned by a polishing paper. figure 2. carbon paste electrode prepared and employed in this research preparation and fortification of artificial saliva artificial saliva was prepared as follows: sodium hydrogen carbonate (4.20 g), sodium chloride (0.50 g), potassium carbonate (0.20 g), and sodium nitrite (0.03 g) were weight and put into a volumetric flask of 1000 ml with purified water and ph was adjusted to 5 with nitric acid 16. next, the artificial saliva was fortified with the standard of cocaine (760 µg/ml) previously purified as described above. the final concentration of cocaine in the electrochemical cell was 10 µg/ml. gc/ms analysis gc/ms was used to evaluate the presence of other adulterant molecules commonly found in the seized cocaine. this step is considered important because the seized cocaine was purified and the presence of adulterants or interferents in the sample could interfere with the electrochemical signal of cocaine. a solution of 20 µg/ml cocaine was prepared by diluting the stock solution with methanol, and 1 µl of this solution was injected in the splitless mode. the analysis conditions were 150 oc for 1 min, heating at 20 oc min-1 up to 280 oc, isotherm for 4 min, and heating at 10 oc min-1 up to 300 oc for 2 min. the injector and interface temperatures were 250 and 300 oc, respectively. the solvent delay was 2.5 min, and the ionization source was electron ionization (ie) operated at 70 ev. the spectra were obtained in the full scan mode. squared wave voltammetry analysis square wave voltammetry (swv) measurements were accomplished with a conventional threeelectrode cell of 5 ml capacity. carbon paste electrode (50:50 w/w) served as the working electrode, b. ferreira et al. j. electrochem. sci. eng. 10(4) (2020) 361-371 http://dx.doi.org/10.5599/jese.849 365 ag/agcl electrode with aqueous saturated kcl as the reference electrode, and a platinum spiral served as the auxiliary electrode. swv method was optimized in the supporting electrolyte solution of 0.1 mol/l nh4clo4 and 0.1 mol/l k2hpo4 by varying ph adjusted by sodium hydroxide solution to 8.0, 9.5 and 10.5, pre-concentration time (0, 30, 60, 90 and 120 s), pre-concentration potential (0.8, 0.9, 1.0, 1.1, and 1.2 v), frequency (25, 50, 75, 100, 150 and 150 hz), and amplitude (0.01 to 0.50 v). after the optimization step, the analytical curve was constructed by employing five concentration levels of cocaine: 2.54, 5.08, 7.62, 10.11, and 15.16 µg/ml. the voltammetric response using swv of freebase cocaine in artificial saliva was performed with the fortification of 10 µg/ml cocaine in the saliva solution and using optimized values of experimental parameters determined for freebase cocaine. hplc-dad analysis hplc-dad method was developed after the following parameters were evaluated: mobile phase composition (water/acetonitrile or methanol), ph (3.5 and 5.0), flow rate (0.6 and 1.0 ml min-1), detection wavelength (195, 235, and 275 nm), and temperature (24, 35, and 45 °c). the ph was adjusted with 0.1 % (v/v) phosphoric acid and triethylamine. the injection volume was fixed at 25 µl. the analytical curve was plotted by using six concentration levels of cocaine: 1.0, 2.0, 5.0, 10.0, 20.0, and 50 µg /ml. limits of detection (lod) and quantification (loq) the analytical parameters for both applied techniques were calculated from the corresponding analytical curves generated by the optimized methods. the analytical parameters are: method sensitivity, linearity range, and limits of detection (lod) and quantification (loq). lod and loq are defined as: lod = 3sd/m and loq = 10sd/m, where m is the sensitivity (slope) of the analytical curve and sd is the standard deviation of the response. results and discussion cocaine purity evaluation by gc/ms analysis from the gas chromatogram correlating the chromatographic peak with the mass spectrum (gc/ms) of freebase cocaine in methanol shown in figure 3, it can be concluded that there are no other organic compounds present in the sample. this would be important, because presence of other organic compounds may interfere with the cocaine electrochemical signal, i.e. these molecules can have anodic oxidation peak at the same potential as cocaine oxidation. cocaine detection method optimization by swv analysis after verifying the absence of any organic compounds in the previously purified cocaine using gc-ms, an initial screening analysis of freebase cocaine (10 µg/ml) was determined by swv. the obtained voltammetric signal was then compared to the voltammetric signals of the supporting electrolyte (0.1 mol/l nh4clo4 and 0.1 mol/l k2hpo4, ph 9.5), and pure methanol (figure 4). in these measurements, swv experimental parameters (amplitude signal, frequency, pre-concentration time, and pre-concentration potential) were not optimized. the cocaine voltammogram in figure 4 shows that the electrochemical response emerged at 1.08 v. this potential value is different from that of cocaine hydrochloride, because this form of salt has been protonated with addition of hcl and the potential for its oxidation must be higher compared to the other molecular form 17. in other words, because the cocaine in the salt form has already been protonated, more energy is needed for its oxidation compared to the freebase cocaine form. http://dx.doi.org/10.5599/jese.849 j. electrochem. sci. eng. 10(4) (2020) 361-371 detecting freebase cocaine in artificial saliva 366 figure 3. gas chromatogram of powder freebase cocaine in methanol and its respective mass spectrum showing cocaine as the only detectable molecule. figure 4. screening of freebase cocaine (10 µg/ml) detection on carbon paste electrode using swv in 0.1 mol/l nh4clo4 and 0.1 mol/l k2hpo4 as supporting electrolyte (red). sw voltammograms of 40 µl pure methanol (black) and pure supporting electrolyte (blue). experimental swv parameters: f =100 hz, a = 0.010 v, pre-concentration time of 60 s at 1.0 v. after the initial screening analysis of freebase cocaine, the supporting electrolyte solution ph (phse) and swv experimental parameters were optimized by varying ph, amplitude signal, frequency, pre-concentration time, and pre-concentration potential. initially, ph variation of the supporting electrolyte showed the highest oxidation peak at ph 9.5. therefore, this basic ph value for the supporting electrolyte was chosen for further experiments, and also because in acid ph, cocaine molecule could be protonated and more energy would be needed for oxidation of that form of cocaine. after ph optimization, the experimental swv parameters cited above where varied and optimized on the basis of the lowest potential and the highest current criteria. table 1 lists the optimized values of swv experimental parameters that were applied for construction of the analytical (calibration) curve. b. ferreira et al. j. electrochem. sci. eng. 10(4) (2020) 361-371 http://dx.doi.org/10.5599/jese.849 367 table 1. optimized values of swv experimental parameters for construction of freebase cocaine analytical curve experimental swv parameter values phse 9.5 amplitude, v 0.1 frequency, hz 75 pre-concentration time, s 30 pre-concentration potential, v 1.0 swv analysis sw voltammograms and the analytical curve for cocaine, obtained by swv technique with the optimized experimental parameters (table 1) are shown in figure 5. figure 5a shows the measured swvs and reveals that the current increased with increasing freebase cocaine concentration. figure 5b illustrates the analytical curve with the corresponding deviations set for the measured current at each concentration. lod and loq values were determined as 0.80 and 2.41 µg ml-1, respectively, while the correlation coefficient was equal to 0.9992. the analytical parameters determined from the analytical curve in figure 5b are summarized in table 2. figure 5. (a) sw voltammograms of freebase cocaine (concentrations: 2.54, 5.08, 7.62, 10.11, and 15.16 µg/ml) in the supporting electrolyte (0.1 mol/l nh4clo4 and 0.1 mol/l k2hpo4, ph 9.5) and (b) relation between peak current and concentration of freebase cocaine. swv experimental parameters are given in table 1 table 2. values of analytical parameters obtained from the analytical curve constructed for freebase cocaine using swv technique parameter values lod, µg/ml 0.90 loq, µg/ml 2.41 linear range, µg/ml 2.54 – 15.2 linearitya 0.9992 sensitivity, a ml µg-1 3.15×10-7 a pearson correlation coefficient study of interfering substances the study of interfering substances was conducted with caffeine (caf, 10 µg/ml) and lidocaine (lid, 10 µg/ml) present in the supporting electrolyte, using the sw voltammetry method developed in this work. the sw voltammogram in figure 6a refers to caffeine (black line) and shows that no oxidation peak emerged at 1.08 v. when freebase cocaine (coc, red line) was added to the solution already containing caffeine, the oxidation peak at about 1.10 v appeared, suggesting that caffeine http://dx.doi.org/10.5599/jese.849 j. electrochem. sci. eng. 10(4) (2020) 361-371 detecting freebase cocaine in artificial saliva 368 does not interfere with the voltammetric response of cocaine. this is supported by literature data showing also that in swv measurement of cocaine on unmodified carbon paste electrode, caffeine is not an interfering compound 19-21. sw voltammogram presented in figure 6b (black line) refers to lidocaine (10 µg/ml). two oxidation peaks are displayed at 1.20 v and around 0.9 v, respectively. when freebase cocaine (10 µg/ml) was added to the solution already containing lidocaine, sw voltammogram (red line) showed that the oxidation peak at 0.9 v was fully suppressed, leaving the peak at 1.14 v to be the single oxidation peak. figure 6. sw voltammograms of two main cocaine adulterants: (a) black voltammogram represents 10 µg/ml caffeine, and red voltammogram represents 10 µg/ml caffeine with addition of 10 µg/ml freebase cocaine (1:1). (b) black voltammogram represents 10 µg/ml lidocaine, and red voltammogram represents 10 µg/ml lidocaine with addition of 10 µg/ml freebase cocaine (1:1). supporting electrolyte: 0.1 mol/l nh4clo4 with 0.1 mol/l k2hpo4, ph 9.5. swv experimental parameters are given in table 1 hplc-dad method and analytical curve construction to compare the results of cocaine determination obtained by sw voltammetric method, the chromatography method (hplc-dad) was applied additionally. the optimized experimental parameters of hplc-dad method for the freebase cocaine detection are summarized in table 3. table 3. optimized experimental parameters used to construct hplc-dad analytical curve parameter results column c8 mobile phase water: acetonitrile (75:25) additions 0.1% phosphoric acid and triethylamine (ph 3.5) flow, ml min-1 1.0 column temperature, °c 25 wavelength, nm 195 injection volume, µl 25 the constructed analytical curve is shown in figure 7, while the corresponding analytical parameters for freebase cocaine detection are summarized in table 4. b. ferreira et al. j. electrochem. sci. eng. 10(4) (2020) 361-371 http://dx.doi.org/10.5599/jese.849 369 figure 7. hplc-dad analytical curve for freebase cocaine (concentrations: 1.0, 2.0, 5.0, 10.0, 20.0, and 50.0 µg/ml. optimized experimental parameters are given in table 3 table 4. values of analytical parameters obtained from the analytical curve constructed for freebase cocaine using hplc-dad technique parameter values lod, µg/ml 0.043 loq, µg/ml 0.130 linear range, µg/ml 1.0 50 linearitya, r 0.9996 sensitivity, mau ml µg-1 0.97 a pearson correlation coefficient identification of freebase cocaine fortified in artificial saliva using swv once the sw voltammetric method for cocaine detection was developed, the analysis of artificial saliva fortified with freebase cocaine is performed. freebase cocaine was detected in the artificial saliva by using swv technique with optimized voltammetric parameters given in table 1. figure 8 illustrates sw voltammograms for the freebase cocaine present in the artificial saliva and the analytical curve for freebase cocaine (figure 5b). figure 8a represents sw voltammogram of the supporting electrolyte (0.1 mol/l nh4clo4 with 0.1 mol/l k2hpo4, ph 9.5) containing artificial saliva only. figure 8b reveals the detection of 10 µg/ml freebase cocaine fortified artificial saliva in the supporting electrolyte. it is seen that an anodic peak emerged at the same potential (about 1.10 v) observed for a purified freebase cocaine standard (figure 4). figure 8c shows the analytical curve from which the concentration of freebase cocaine can be estimated from the current obtained for the saliva spiked with 10 µg/ml freebase cocaine. the peak current value measured in the solution containing artificial saliva and fortified with freebase cocaine (1.77 µa) was lower than the current value achieved with the freebase cocaine standard. this may happen due to the high concentration of various ions of different sizes in the artificial saliva solution, which may compete with cocaine for interaction with the active sites present on the carbon paste electrode surface 22. also, the optimization of the method performed for cocaine fortified in artificial saliva would possibly result by increase of the anode current. despite the decreased current, however, the voltammetric response is relatively quick because there was no need for some type of pre-treatment to make the analyte available for swv analysis. contrary to that, when detection of cocaine fortified in artificial saliva was carried out using the chromatographic method, the salt contained in the artificial saliva should be eliminated due to experimental limitation of this technique, what resulted in more costs and longer analysis time. http://dx.doi.org/10.5599/jese.849 j. electrochem. sci. eng. 10(4) (2020) 361-371 detecting freebase cocaine in artificial saliva 370 figure 8. sw voltammograms of (a) pure artificial saliva and (b) artificial saliva containing 10 µg/ml of freebase cocaine. (c) anodic peak current of 10 µg/ml freebase cocaine (1.77 µa) is denoted as blue point in the analytical curve of freebase cocaine. supporting electrolyte: 0.1 mol/l nh4clo4 and 0.1 mol/l k2hpo4, ph 9.5. swv experimental parameters are given in table 1 conclusions the fast and efficient procedure is developed for the squared wave voltammetric (swv) detection of freebase cocaine in artificial saliva, using the manufactured carbon paste electrode. the carbon paste electrode manufacture is easy and does not require expensive materials, what reduces the overall analysis cost. the electrochemical response of cocaine in the artificial saliva is in agreement with the literature, i.e., the oxidation potential of the tertiary aliphatic amine in the cocaine molecule is found at about 1.08 v vs. ag/agcl electrode. the selectivity for lidocaine and caffeine that are often found in cocaine samples, has been evaluated and confirmed. this opens up the possibility for future analysis of cocaine in real samples of saliva and other biological matrices. even though the values of lod and loq obtained by swv are worse than those obtained with hplc-dad technique, here established method shows certain advantages, such as simplicity, speed, less use of solvent, low-cost materials, and no need for sample preparation (liquid-liquid (lle) and solid phase extraction (spe)). all these make the electrochemical technique accessible to many laboratories where its application could reduce waste generation and costs. b. ferreira et al. j. electrochem. sci. eng. 10(4) (2020) 361-371 http://dx.doi.org/10.5599/jese.849 371 acknowledgements: the authors acknowledge the financial support of conselho nacional de desenvolvimento científico e tecnológico (cnpq), fundação de amparo à pesquisa do estado de são paulo (fapesp process 2016/23825-3), and coordenação de aperfeiçoamento de pessoal de nível superior (process capes pro forense 25/2014). finally, we would like to thank dr. cynthia maria de campos prado manso for revising and editing the text. references [1] r. a. goldstein, c. deslauriers, a. burda, h. johnson-arbor, seminars in diagnostic pathology 26(1) (2009) 10–17. 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[22] r. l. mccreery, chemical reviews 108 (2008) 2646–2687. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.849 https://creativecommons.org/licenses/by/4.0/) {the template-assisted electrodeposition of platinum nanowires for catalytic applicati} doi:10.5599/jese.495 205 j. electrochem. sci. eng. 8(3) (2018) 205-217; doi: http://dx.doi.org/10.5599/jese.495 open access : : issn 1847-9286 www.jese-online.org original scientific paper the template-assisted electrodeposition of platinum nanowires for catalytic applications soha mohajeri, abolghassem dolati, sahar hashemi daryan department of materials science and engineering, sharif university of technology, tehran, iran corresponding authors e-mail: smohajeri@alum.sharif.edu, dolati@sharif.edu received: january 6, 2018; revised: february 13, 2018; accepted: february 14, 2018 abstract template-assisted electrodeposition technique was applied to synthesize platinum nanowires (pt nws) on polycarbonate templates (pct) with pore diameters of 15, 50, and 100 nm for catalytic applications. influences of sulfuric acid added to the electrolyte, different potential scanning rates and different pore diameters of templates on the electrodeposition process of pt nws were investigated by electrochemical techniques, including voltammetry and chronoamperometry methods. it was confirmed that at lower scan rates and in acidic solutions, electrodeposition of platinum on templates with larger pores is controlled by diffusion. the potential range for deposition of pt nws was determined and the potentiostatic technique was utilized by applying various potentials of different durations to fabricate the nws. the morphological characteristics of pt nws were examined using the scanning electron microscopy (sem). it was shown that the growth of pt nws on pct 50 nm followed a pine-tree pattern, while the pt nws grew spherically on pct 100 nm. the uniform and compact shape of pt nws was verified by the transmission electron microscopy (tem). the catalytic activities of the prepared pt nws with the same exchanged charge density for hydrogen adsorption/desorption and methanol oxidation reactions were determined by the cyclic voltammetry (cv) testing, and the superior electrocatalytic performance was detected for pt nws prepared on pct 50 nm. this enhanced catalytic activity was attributed to the higher surface-to-volume ratio, larger electrochemical active surface area and higher density of exposed active sites accessible on the pine-tree morphology of these pt nws compared to the spherical structure of pt nws fabricated on pct 100 nm. this makes pt nws prepared on pct 50 nm to be a promising catalyst for direct methanol fuel cells (dmfcs). keywords platinum nanowires; polycarbonate templates; electrodeposition; hydrogen adsorption/desorption; methanol oxidation reaction http://dx.doi.org/10.5599/jese.495 http://www.jese-online.org/ mailto:smohajeri@alum.sharif.edu mailto:dolati@sharif.edu j. electrochem. sci. eng. 8(3) (2018) 205-217 platinum nanowires for catalytic applications 206 introduction in recent years, noble-metal nanostructures have attracted extensive research attention due to their promising potentials for numerous electrocatalytic applications including a variety of fuel cell systems, electrochemical biosensors and batteries [1]. in particular, the synthesis of platinumbased nanostructures such as nanotubes, nanoparticles and nanowires with specific structural features has been an area of considerable interest [2]. this happened owing to the enhanced catalytic capabilities of platinum for applications in direct methanol fuel cells (dmfcs) which are considered as highly promising power sources for electric vehicles and portable electronic devices [3]. limited supply and high cost of this precious metal, however, have impeded its commercial utilization. moreover, the formation of intermediate species in the process of methanol oxidation tends to poison the pt surface and diminish the oxidation rate, reducing thus the catalytic performance of this metal [4]. in order to improve the catalytic activity and the poisoning tolerance of pt catalysts, various researchers have focused their investigations on the replacement of zero-dimensional (0d) pt nanoparticles with one-dimensional (1d) pt nanowires (pt nws) [5,6]. several structural features offered by pt nws such as anisotropic morphology and high surface-tovolume ratio can enhance their mass-transport and electron-transfer kinetics, leading to the superior catalytic activity of pt for hydrogen evolution and methanol oxidation reactions [7]. although various techniques such as electron-beam deposition [8], solvothermal synthesis [9], physical vapor deposition [10], electron beam lithography [11] and plasma sputtering [12] have been applied to fabricate pt nws, electrodeposition [13] has been proven as the most commonly used deposition method due to its low cost and high productivity [14]. in particular, the templatebased electrodeposition of pt nws has been established as an effective method due to its simplicity and high-yielding process. this method provides a well-organized approach for the fabrication of isolated nanowire arrays with uniform diameters determined by the pore diameter of the template [15]. it is generally accepted that the diameter of nanowires plays a fundamental role in their functionality. as the diameter of nanowires decreases, their surface-to-volume ratio increases and consequently, the variation of the surface energy significantly influences the surface state and the related physical and chemical properties [16]. although many studies have investigated the template-based electrodeposition of pt nws, very few researchers have examined distinct morphological and structural properties of pt nws fabricated on templates with different pore diameters. also, a limited research has so far been reported on the electrocatalytic activity of pt nws for hydrogen adsorption/desorption and methanol oxidation reactions. the current study aims to examine the electrodeposition of pt nws on polycarbonate templates (pct) with different pore diameters and determine the dependence of their electrochemical behavior and morphological properties upon the pore size of templates. the work has been focused on the hydrogen adsorption/desorption on pt nws and the influence of their diameter on the methanol oxidation reaction performances. experimental the electrochemical deposition of pt nws was carried out in a typical three-electrode cell and the potential was recorded with reference to a saturated calomel electrode (sce). a pure platinum plate with the surface area of 2×1 cm2 was used as the counter electrode and polycarbonate templates (pct) with the surface area of 0.2×1 cm2, pore diameter of 15, 50, and 100 nm, pore length of 6 µ, and pore density of 108 pores/cm2 were employed as the working electrode. in order to impart conductivity to the working electrode, the solution sides of the templates were coated s. mohajeri et al. j. electrochem. sci. eng. 8(3) (2018) 205-217 doi:10.5599/jese.495 207 with 50 nm-thick au layer via sputtering. two electroplating baths were prepared and all the reagents were of analytical grade from merck. in the first bath, 0.05 m h2ptcl6 was added dropwise to deionized water-hydrochloric acid mixture (ph 1.6) under continuous stirring. in order to study the influence of sulfuric acid adding on the electrodeposition of pt nws, the second electroplating bath was prepared. 0.1 m h2so4 was added to 0.05 m h2ptcl6 solution with vigorous mixing during addition, and the final solution was added drop-wise to a mixture of deionized water and hydrochloric acid with already adjusted ph of 1.6. both electroplating baths were purged with n2 at room temperature for 2 h. to optimize the deposition potentials suited for the deposition of pt nws, linear sweep voltammograms and chronoamperometry curves were recorded by an eg&g princeton applied research (par) potentiostat and influences of 0.1m h2so4 added to the electrolyte, different potential scanning rates, and different pore diameters of templates on the electrodeposition process of pt nws were investigated. afterwards, the potential range for deposition of pt nws was determined and subsequently, the potentiostatic method was utilized by applying various potentials of different durations to synthesize the nanowires. the surface and cross-sectional morphologies of the obtained pt nws were examined by a scanning electron microscope (sem, tescan model), after dissolving the polycarbonate membranes by dichloromethane. a transmission electron microscopy (tem, em-10c model) analysis was used to evaluate the pt nws nanostructure. the electrocatalytic activity of pt nws obtained with the same exchanged charge density was assessed in a cell containing the pc templates filled with the pt nws (working electrode, 0.2×1 cm2) and a platinum sheet (counter-electrode, 2×1 cm2). the supplied bias and current were controlled by an autolab pgstat 302 electrochemical system. the cyclic voltammetry (cv) tests for hydrogen adsorption/desorption reactions were conducted in an argon-purged 0.5 m h2so4 solution at room temperature under continuous stirring and the obtained cv curves were used to estimate the electrochemically active surface area (ecsa) of pt nws catalysts. the methanol oxidation reaction activity of pt nws was studied by cv tests in a 0.5 m h2so4 electrolyte containing 2 m ch3oh, and the optimum diameter of nws for obtaining the maximal catalytic efficiency was determined. results and discussion electrochemical studies of the pt nws deposition the influence of sulfuric acid added to the plating bath on the electrodeposition of pt nws was studied. figure 1 demonstrates the cathodic voltammetry curves of pct 50 nm working electrode, measured in a pure bath and h2so4-containing solution in the potential range of 0 to −2000 mv vs. sce at the scan rate of 5 mv/s. it can be observed that in the presence of the sulfuric acid, higher current densities were measured and the curve shifts to more positive potentials. the higher value of slope indicates that the electrodeposition of pt ions occurs at a higher rate in a h2so4containing bath compared to the pure deposition bath, depicting that the sulfuric acid plays a catalytic role in the deposition of pt nws. in order to investigate the effect of various potential scan rates on the electrodeposition of pt nws, the linear voltammetry curves recorded in a h2so4-containing solution in the range of 0 to -1000 mv vs. sce at the scan rates of 20, 50 and 100 mv/s are illustrated in figure 2. two irreversible peaks can be detected in the cathodic branches of the curves obtained at the scan rates of 20 and 50 mv/s. the peaks are ascribed to the successive reduction of pt4+ to pt2+ ions and of pt2+ ions to pt, respectively, according to the following reactions: j. electrochem. sci. eng. 8(3) (2018) 205-217 platinum nanowires for catalytic applications 208 figure 1. effect of 0.1 m h2so4 added to the plating bath on electrodeposition of pt nws on pct 50 nm at the scan rate of 5 mv s-1 [ptcl6]2+2e→ [ptcl4]2+ 2cl(1) [ptcl4]2+2e→ pt + 4cl(2) a typical feature of the diffusion-controlled electrodeposition is that an increase of the scan rate shifts the reduction peaks to more negative potentials and causes higher current densities along with a simultaneous increase in the deposition rates. this behavior can be attributed to the fact that at higher scan rates, the cations have a lower opportunity for deposition on the surface and therefore need a higher energy and overpotential for deposition. figure 2, depicts such a feature, suggesting that the electrodeposition of pt is under the mass transfer control. with the increase of the scan rate from 20 to 50 mv/s, the mass transfer is enhanced, leading to the intensified adsorption of platinum ions on the working electrode. consequently, the potential difference between the cathodic peaks increases. finally, at the scanning rate of 100 mv/s, a single wide peak is observed due to the lack of the deposition duration that leads to the decrease of the deposited platinum species on the cathode. figure 2. linear sweep voltammograms of pt nws deposited on pct 50 nm at different scan rates. the influence of the template pore diameter (15, 50, and 100 nm) on the linear voltammetric curves in the range of 0 to -1000 mv vs. sce at a scan rate of 50 mv/s is depicted in figure 3. no diffusion peaks were observed for deposition on pct 15 nm and therefore, it can be concluded that the charge transfer governs the system kinetics. this can be attributed to the higher surfaces. mohajeri et al. j. electrochem. sci. eng. 8(3) (2018) 205-217 doi:10.5599/jese.495 209 to-volume ratio of the small pores with the diameter of 15 nm, which provides facile pathways for mass transfer and more effective exposure of active surface sites for the progressive ions flux to the cathode surface. thus, the diffusion of the solution species is fast and the deposition current is restricted by the electron transfer at the working electrode. this is a characteristic of deposition reactions in which the charge transfer is rate determining [17,18]. however, for pct 50 and 100 nm, diffusion controls the curves. the increase of pore diameters leads to the increase of the deposition current density and shifts the curves to more negative potentials. this can be due to the larger size of the pores and transference of more platinum ions into the pores, which requires higher overpotentials for reduction. figure 3. linear sweep voltammograms for electrodeposition of pt nws on templates with different pore diameters at scan rate of 50 mv s-1 figure 4 illustrates the potentiostatic i-t transients of pt nws deposited on pct 50 and 100 nm at −400 mv vs. sce. based on the characteristic shape of the chronoamperometry curves, a nucleation and growth mechanism can describe the electrodeposition process having the rate of growth limited by diffusion [19]. at the initial times, the current density increases due to the nucleation process and 3d growth of the nuclei. subsequently, diffusion controls the current drops and the deposition. finally, the current density remains relatively constant and filling up the pores occurs. these curves also reveal that the deposition trend does not change as the pores diameters increase from 50 to 100 nm. figure 4. chronoamperometry curves for deposition of pt nws on pct 50 and 100 nm at −400 mv vs. sce. j. electrochem. sci. eng. 8(3) (2018) 205-217 platinum nanowires for catalytic applications 210 morphological study of pt nanowires from the linear sweep voltammetry curves shown in figure 3, the reduction potential region for the deposition of pt nws on templates with pore diameters of 50 and 100 nm can be determined. due to the locations of peaks in the diffusion-controlled electrodeposition of nanowires, the values of −250 and−400 mv were determined as the starting potentials for the deposition of pt nws on pct 50 and 100 nm, respectively. the potentiostatic method was utilized to synthesize the nanowires by applying various potentials in the range of −250 to −400 mv for different durations. morphologies of pt nws deposited on the pct 50 nm from a h2so4-containing solution at different potentials in the range of −250 to −400 mv for 500 s were analyzed by sem. in order to study the surface morphology of prepared pt nws, the deposited samples were cut, and the surface areas of the templates were locally dissolved in ch2cl2. figures 5 (a)−(c) illustrate the cross-sectional sem images of pt nanowires grown on pct 50 nm at −250, −350 and −400 mv, respectively. figures 5 (d)−(f) show the sem images at a higher magnification, and it can be observed that the average lengths of the nanowires vary between 1 to 4.5 µ. according to figures 5 (a) and (d), it can be observed that at −250 mv, the growth of nanowires is uniform but due to the lack of deposition time, they do not meet the surface of the template. in figures 5 (b) and (e), it seems that at increased applied potential to −350 mv, the holes are not completely filled due to the short deposition time, but the growth of nanowires occurs simultaneously and uniformly. figures 5 (c) and (f) demonstrate that at more negative potential of −400 mv, due to the hydrogen evolution reaction, the efficiency of the nanowires deposition declines, leading to a less uniform growth of nanowires. (a) (b) (c) (d) (e) (f) figure 5. sem images of pt nws electrodeposited for 500 s on pct 50 nm at the potential of: (a), (d) −250, (b), (e) −350, and (c), (f) −400 mv. s. mohajeri et al. j. electrochem. sci. eng. 8(3) (2018) 205-217 doi:10.5599/jese.495 211 in order to examine the consequences of increasing the deposition duration on the surface and cross-sectional morphology of pt nws deposited on pct 50 and 100 nm, the potentials of −350 and −400 mv were applied for 3000 s. according to figures 6 (a) and (b), pt nws deposited on pct 50 nm at −350 mv, represent the tree branches and resemble pine needles. (a) (b) (c) (d) (e) (f) figure 6. surface and cross-sectional morphology of pt nws electrodeposited for 3000 s on pct 50 nm at the potential of (a), (b) −350, (c), (d) −400 mv, and on pct 100 nm at the potential of (e), (f) −400 mv. j. electrochem. sci. eng. 8(3) (2018) 205-217 platinum nanowires for catalytic applications 212 this pine-tree morphology can be attributed to the tendency of the multifaceted platinum nuclei to branch, spread, and dramatically overgrow on the surface. it is noteworthy to mention that unlike the nanowires of other fcc metals such as gold, copper, nickel, and cobalt, the pt nws do not follow a cauliflower growth pattern and do not tend to form caps on the surface of the templates. alternatively, they continue their overgrowth rather than overlapping, and manifest pine-tree structures that are single crystalline, and their facets grow close to the {001} plane [20]. as depicted in figures 6 (c) and (d), at the potential of −400 mv, the pt nws on the pct 50 nm do not grow uniformly along the pores, reaching the surface of the template and continuing their growth on the surface. as mentioned before, the evolution of hydrogen at more negative potentials accounts for the non-uniform growth of platinum nanowires. figures 6 (e) and (f) demonstrate the spherical growth of pt nws on the pct 100 nm. it was confirmed that the nucleation process of pt nws on both pct 50 and 100 nm follow the instantaneous mechanism in which the rate of new nuclei formation at the initial stage of deposition is high enough to immediately activate all the nucleation sites. however, the number of nuclei remains constant during the growth process and the 3d growth of the nuclei is limited by diffusion [21]. with regards to figures 6 (d) and (f), at the same deposition potential of −400 mv, the ratio of surface-to-volume of pt nws prepared on pct 50 nm is larger than those deposited on pct 100 nm, and the driving force inside the smaller pores is higher, leading to the increase of nucleation density inside the pores with the size of 50 nm. on the other hand, the mass transport inside smaller pores with the size of 50 nm is more limited by diffusion in comparison to the pores with the size of 100 nm. thus, the diffusion coefficient is smaller in pct 50 nm and the 3d growth of the nuclei follows a pine-tree pattern. as the 3d growth inside the pores of pct 100 nm is less restricted by diffusion and due to the lower concentration gradient inside larger pores, the nuclei are promoted to grow spherically on the pct 100 nm. the influence of the pore size of templates on the length of nanowires deposited under the potential of −400 mv during 2000 s is represented in figure 7. due to the dissolution of polycarbonate membranes by dichloromethane, the nanowires are randomly distributed on the surface. (a) (b) figure 7. surface morphology of pt nws electrodeposited at the potential of −400 mv for 2000 s on pct: (a) 50 and (b) 100 nm. s. mohajeri et al. j. electrochem. sci. eng. 8(3) (2018) 205-217 doi:10.5599/jese.495 213 the relatively short duration of applying the deposition potential resulted in the incomplete filling of the pores, and the average length of nanowires on pct 50 (fig. 7(a)) and 100 nm (fig. 7(b)) are 2.5 and 3.3 µ, respectively. since the system is controlled by diffusion at −400 mv, the efficient surface area and the diffusion coefficient are higher in bigger pores [13], leading to the formation of longer nanowires. tem characterization of pt nanowires tem images of pt nws deposited on the pct 50 nm at −400 mv for 1000 s are depicted in figure 8. according to figure 8 (a), under the magnification of 1500, the nanowires are uniform, and their diameter is 100 nm. due to the short deposition duration, the lengths of these nanowires are in the range of 1.5−2.5 µ, and they do not reach the thickness of template (6 µ). figure 8 (b) depicts a uniform and compact nanowire having a diameter between 100 to 120 nm at different points. since the end part of this nanowire is smaller and the deposition time is shorter than the required time for filling the pores, it can be concluded that this section belongs to the growing part of the nanowire. the dispersed grains inside the nanowire pertain to the growing nuclei and confirm that the nucleation and growth mechanism of nanowires are instantaneous and three dimensional, respectively. (a) (b) figure 8. tem images of pt nws electrodeposited for 1000 s at the potential of −400 mv on pct 50 nm under the magnification of: (a) 1500 and (b) 8800 electrocatalysis in acidic solutions in order to investigate the catalytic activity of pt nws 50 and 100 nm, the deposits obtained with the same exchanged charge density were compared. based on figure 4, the integration of chronoamperometric reduction current recorded over 3000 s for the pt nws 50 nm yielded the same amount of the exchanged charge as for the pt nws 100 nm recorded over 860 s. therefore, pt nws prepared at the potential of −400 mv for 3000 s on pct 50 nm and for 850 s on pct 100 nm were selected for studying the influence of nanowires diameter on their electrocatalytic activity. adsorption/desorption of hydrogen the cyclic voltammograms shown in figure 9 compare the hydrogen adsorption/desorption performance of pt nws deposited at the potential of −400 mv for 3000 s on pct 50 nm and for 850 s on pct 100 nm, in a 0.5 m h2so4 solution at a scan rate of 50 mv/s. both catalysts display j. electrochem. sci. eng. 8(3) (2018) 205-217 platinum nanowires for catalytic applications 214 peaks associated with the strong and weak adsorption/desorption of hydrogen molecules in anodic and cathodic sweeps in the sulfuric acid solution [22]. the existence of multiple peaks in both the cathodic and anodic scans, instead of one broad peak, reveals that various crystallographic planes of platinum participated in the reactions [23]. the upward and downward peaks are ascribed to hydrogen desorption (pd1 and pd2) and (pa1 and pa2) adsorption, respectively. since the hydrogen desorption peaks for pt nws 50 nm are approximately 10 to 30 mv more negative than that of pt nws 100 nm, it can be concluded that the hydrogen binding energy is lower on the nanowires prepared on pct 50 nm. the inferior hydrogen binding energy expedites the hydrogen desorption from the surface of nanowires and consequently, enhances the catalytic performance of nanowires. moreover, the potentials of hydrogen adsorption peaks on pt nws 50 nm are more negative, and their current values are higher compared with those of pt nws 100 nm. this implies that a great quantity of hydroxide species is produced on the electrode. furthermore, the integration area of hydrogen desorption peaks for pt nws 50 nm surpasses the pt nws 100 nm, indicating the higher amount of active surface area and thus, the more enhanced catalytic performance of the nanowires prepared on pct 50 nm. the electrochemical active surface area (ecsa) of pt nws was estimated from the area of desorption of atomic hydrogen on the cv curves in figure 9, between 0 and -300 mv, via the following equation [24]: hesca q mc = (3) in eq. (3), qh is the total charge (mc cm−2), m is actual pt loading (mg cm−2) on the substrate, and c signifies the charge required to oxidize a monolayer of hydrogen on a pt surface (0.21 mc cm−2). the ecsa values of pt nws 50 and 100 nm are calculated to be 51.32 and 44.56 m2g−1, respectively, indicating that pt nws 50 nm are capable of exposing sufficient active sites to guest molecules, leading to their superior electrocatalytic activity. this behavior can be due to the larger surface-to-volume ratio of pt nws 50 nm, as well as their unique pine-tree morphology (fig. 6) which improves the surface roughness and increases the surface tendency for reaction with hydrogen. figure 9. cyclic voltammograms of pt nws 50 and 100 nm in 0.5 m h2so4 solution at scan rate of 50 mv s -1 methanol oxidation the cv curves presented in figure 10 display the methanol oxidation performance of pt nws deposited at the potential of −400 mv for 3000 s on pct 50 nm and for 850 s on pct 100 nm, in a s. mohajeri et al. j. electrochem. sci. eng. 8(3) (2018) 205-217 doi:10.5599/jese.495 215 0.5 m h2so4 + 2 m ch3oh solution. it can be observed that in the anodic scans, the methanol oxidation reactions begin at 230 and 473 mv for pt nws 50 and 100 nm, respectively. the onset oxidation potential for pt nws 50 nm is less positive compared to that of pt nws 100 nm, indicating that the morphology of pt nws 50 nm reduces the required initial potential and promotes the methanol oxidation. with the increase of the potential in the anodic sweep, the methanol molecules are dissociatively chemisorbed on pt nanowires. as the potential is swept toward more anodic values, the anodic p1 peaks emerge at 920 and 1032 mv for pt nws 50 and 100 nm, respectively. these peaks are attributed to the production of intermediates, poisonous pt=c=o species, and carbon dioxide on the surface of the electrode. the lower value of the potential at p1 peak for pt nws 50 nm in comparison to pt nws 100 nm is indicative of the reduced required potential for methanol oxidation on smaller nanowires. moreover, the current value at p1 peak for pt nws 50 nm is higher than that of pt nws 100 nm, which can be justified by the unique pine-tree morphology of nanowires and their larger ratio of surface-to-volume, leading to the higher accessibility of these nanowires for trapping the guest molecules. thus, the adsorption of intermediate species on the surface of the pt nws 50 nm is enhanced and the methanol oxidation is intensified. figure 10. cyclic voltammetry curves for methanol oxidation on pt nws 50 and 100 nm in a solution containing 0.5 m h2so4 and 2 m ch3oh at scan rate of 50 mv s -1 in the backward scan, the cathodic p2 peaks can be observed at 756 and 867 mv for pt nws 50 and 100 nm, respectively, due to the removal of poisonous pt=c=o species. it is obvious that the p2 peak appears at a more positive potential and with a higher current value for pt nws 100 nm compared to pt nws 50 nm. this is due to the higher amount of pt=c=o species produced on pt nws 100 nm during the forward scan. therefore, their oxidation is more difficult and requires higher potential and current values. although pt nws act as effective catalysts for methanol oxidation, they tend to be poisoned by carbon monoxide [25]. the higher ratio of currents at p1 peak to p2 peak represents the improved tolerance of nanowires toward the poisoning by carbon monoxide [26]. regarding figure 10, this ratio is estimated to be 1.02 and 0.78 for pt nws 50 and 100 nm, respectively, demonstrating that the pt nws 50 nm possess an enhanced poisoning tolerance. this feature, in addition to the reduced onset and p1 peak potentials leads to the enhanced electrocatalytic activity of these nanowires. the superior electrocatalytic performance of the nanowires prepared on pct 50 nm is attributed to the higher value of surface-to-volume ratio of these nanowires, providing more uncovered active sites for trapping methanol molecules j. electrochem. sci. eng. 8(3) (2018) 205-217 platinum nanowires for catalytic applications 216 for their subsequent oxidation. furthermore, the pine-tree morphology of pt nws 50 nm provides enhanced electrocatalytic efficiency in comparison to the spherical morphology of pt nws 100 nm. conclusion the platinum nanowires with different diameters were fabricated by the template-assisted electrodeposition technique. the deposition process was characterized by the voltammetry method and it was confirmed that addition of sulfuric acid to the plating bath accelerates deposition of pt nws. the influence of different potential scan rates on the electrodeposition of pt nws was investigated. it was observed that at low scan rates, two cathodic peaks appeared in the cyclic voltammetric curves, implying that the diffusion-controlled reduction of platinum ions occurred through two successive steps. however, at high scan rates, the presence of one cathodic peak on the voltammograms proved the incomplete and direct reduction of ions. the linear voltammetric curves obtained for different templates revealed that while the deposition was under charge transfer control on pct 15 nm, the diffusion-controlled deposition occurred on pct 50 and 100 nm. morphological studies of nws synthesized by the potentiostatic deposition technique revealed that at short deposition durations and under less negative deposition potentials, the growth of nanowires occurred simultaneously and uniformly. less uniform growth of nanowires took place under more negative potentials due to the hydrogen evolution reaction. moreover, at longer deposition durations, the platinum nanowires on pct 50 and 100 nm possessed pine-tree and spherical structures, respectively. based on the cv curves obtained in acidic solutions for deposits with an identical exchanged charge, the nanowires grown on pct 50 nm exhibited enhanced catalytic performance toward hydrogen adsorption/desorption and methanol oxidation reactions compared to nanowires 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[26] c. xu, l. wang, r. wang, k. wang, y. zhang, f. tian, y. ding, advanced materials 21 (2009) 2165-2169. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {joint event: 7th regional symposium on electrochemistry south east europe (rse–see 7) and 8th kurt schwabe symposium editorial} 55 j. electrochem. sci. eng. 10(2) (2020) 55-64 open access: issn 1847-9286 www.jese-online.org editorial joint event: 7th regional symposium on electrochemistry south east europe (rse–see 7) and 8th kurt schwabe symposium višnja horvat-radoševića and zoran mandićb aruđer bošković institute, bijenička 54, zagreb, croatia bfaculty of chemical engineering and technology, university of zagreb, marulićev trg 19, zagreb, croatia this special issue of jese contains a collection of articles that were originally presented at 7th regional symposium on electrochemistry-south east europe (rse–see 7) that was held jointly with 8th kurt schwabe symposium, on 27-30 may 2019, in split, croatia. this gathering was intended to facilitate exchange of ideas and collaborations among experts, engineers and beginners in the field of electrochemistry. the joint 4-day symposium was organized by the association of south-east european electrochemists (aseee) http://www.aseee.eu and gathered 176 participants from 23 countries, giving in total 5 plenary talks, 10 keynote talks, 65 oral presentations and 64 poster presentations. also, the satellite student symposium with 14 contributions was associated to the main symposium event. the book of abstracts can be downloaded from here: http://www.aseee.eu/userfiles/rse7/book-of-abstracts.pdf. the conference was dedicated to the memory of late professor zdravko stoynov (1936–2017), a world distinguished bulgarian electrochemist and one of the founders of the regional cooperation of electrochemists, established in 2008 through rse–see chain of conferences. the conference was aimed to highlight advances and new findings in electrochemistry area and its impact on technology development. in the year of nobel prize–electrochemistry winners who developed li-ion batteries and created a rechargeable world, it is of special importance to emphasize that the present symposium topics were: batteries and electrochemical energy storage, corrosion science and protection, electrochemical engineering, electroanalytical chemistry and sensors, organic electrochemistry and bio-electrochemistry, physical electrochemistry, materials and electrochemical processing and general topics. this broadness of topics is reflected in this issue with papers not only on the basics of energy storage and energy storage devices such as batteries and fuel cells, but also on electrochemical sensing technologies, electrolytic production of metals, electrochemical synthesis for various purposes and corrosion inhibition. http://www.jese-online.org/ http://www.aseee.eu/ http://www.aseee.eu/userfiles/rse-7/book-of-abstracts.pdf http://www.aseee.eu/userfiles/rse-7/book-of-abstracts.pdf j. electrochem. sci. eng. 10(2) (2020) 55-64 editorial 56 extended versions of 4 plenary, 4 keynote and 6 contributed presentations can be found in this special jese issue, together with the list of authors and titles of abstracts presented at the conference. each published paper successfully passed the reviewing process that involved the participation of prominent international scientists. as the editors of this special issue of jese, we would like to express sincere appreciation to all those who had contributed in producing it. special thanks are going to the authors of contributions for their state-of-the-art articles and reviewers for their timely reviewing and valuable comments. we hope that readers will find this special issue interesting and valuable. we are also convinced that it will be beneficial for a wide audience in academy and industry. list of authors and titles of abstracts submitted to rse-see 7 and 8th kurt schwabe symposium (sorted alphabetically by the first author) plenary lectures (5) • miran gaberšček, sara drvarič talian, jože moškon, jan bitenc, alen vižintin, robert dominko. reaction and transport mechanisms in lithium-sulfur batteries. • francois lapicque. electrolytic iron production from alkaline suspension of solid oxides: compared cases of hematite, iron ores and iron-containing processing waste. • arjan mol (j.m.c. mol). in-situ electrochemical, microscopic and spectroscopic studies of local corrosion and buried metal-polymer interfaces. • pavel shuk, hans-ulrich guth, chad mcguire. gas sensing technologies in combustion: a comprehensive review. • daria vladikova, blagoy burdin. advancement in the electrochemical testing and characterization of power sources. keynote lectures (10) • jiří barek. novel electrode materials and arrangements for sensing biologically active organic compounds. • jenia georgieva, nina dimitrova, sotiros sotiropoulos, tzvetanka boiadjieva-scherzer. a new environment-sparing method for preparation of nanomaterials applicable in fuel cells and electrolyzers. • anton kokalj, ingrid milošev. molecular modeling of corrosion inhibitors: a search for physically sounds descriptors. • jiří ludvík, alan liška. stereoelectrochemistry of reducible calix[4]arenes. • nicolas murer, jean-paul diard, bogdan petrescu. the effects of time-variance on impedance measurements: examples on a corroding electrode and a battery cell. • géza nagy, livia nagy, andras kiss, daniel filotás, abdelilah asserghine, ricardo m. souto. results achieved fitting toand using secm in corrosion experiments. • nebojša d. nikolić. formation of disperse deposits by electrolysis: morphology, structure and mechanism. • helena otmačić ćurković, ekatarina kristan mioč, zana hajdari gretić. comparison of the protective properties of fatty acid films prepared by different application methods. • tamás pajkossy, soma vesztergom. a fifty-years old problem re-visited: the voltammograms of diffusion controlled reactions. • igor a. pašti. computationally driven development of novel electrocatalysts. v. horvat-radošević and z. mandić j. electrochem. sci. eng. 10(2) (2020) 55-64 57 oral presentations (65) • llona acznik, katarzyna lota. the influence of conductivity and ph on the performance of electric double layer capacitors working in neutral electrolytes. • veronica anăstasoaie, laura enache, mihaela cristea, raluca isopescu, gabriela stanciu, eleonora-mihaela ungureanu. electrochemical properties of substituted thioxothiazolidinazulene. • liana anicai, stefania costovici, aurora petica, sabrina rosoiu, aida g. pantazi, teodor visan. electrodeposition of sn binary and ternary alloys involving deep eutectic solvents. • josefina ballarre, tuba ademir, liliana liverani, aldo r. boccaccini. sol-gel spray and electrophoretic deposition techniques to enhance bioactive and antibacterial response of titanium implants. • viaceslav barsukov, v. khomenko, oksana butenko. on the validity of using the diffusion coefficient in characterization of charge transfer reactions in systems with a solid-phase reagent. • galin borisov, denis paskalev, katerina maksimova-dimitrova, elitsa petkucheva, evelina slavcheva. highly efficient carbon-free membrane electrode assemblies for electrolysis cells with anion exchange membrane (aemec). • blagoy burdin, dimitar bojchev, daria vladikova. introduction of differential resistance analysis in battery testing. • barbara burnat, andrzej leniart, sławomira skrzypek. functionalization of titanium dioxide coatings for biomedical applications – corrosion impact study. • manfred decker, arnold wezel, robert hasler, petra teichmann, michael schneider, winfried vonau. investigations about the simultaneous potentiometric determination of ph-value and potassium amount in freshly sampled soil on-site on the field. • nils donker, anastasiya ruchets, daniela schönauer-kamin, jens zosel, ulrich guth, ralf moos. pulse polarization measurements on the system pt|ysz by varying the polarization voltage. • matija gatalo, leonard moriau, francisco ruiz-zepeda, primož jovanovič, urša petek, nejc hodnik, marjan bele, miran gaberšček. co-assisted ex-situ chemical activation of gram-scale high-performance pt-cu/c orr electrocatalyst. • bernhard gollas, david fuchs, bernhard c. bayer. electrochemistry of macroscopic cvdgraphene on non-conducting substrates in deep eutectic solvents. • rui gusmão, zdeněk sofer, martin pumera. the advent of 2d layered materials for electrochemical energy applications. • thomas herl, daniel böhm, frank-michael matysik. electrooxidation of nucleobases on disposable electrodes investigated by electrochemistry-capillary electrophoresis-mass spectrometry. • alexander herms, jingying yao, jens zosel, wolfram oelßner, michael mertig. coulometric titration of oxygen in the picomol range. • nejc hodnik, matija gatalo, francisco ruiz-zepeda, andraž pavlišič, primož jovanovič, marjan bele, goran dražić, miran gaberšček. tracking and modeling of ptcu nanoalloy dealloying at the atomic-scale: spot the difference “game” at the atomic level. • magdaléna hromadová, viliam kolivoška, jakub šebera, táňa sebechlebská, jindřich gasior, štěpánka nováková lachmanová, gábor mészáros, marcin lindner, marcel mayor, michal valášek. effect of tripodal anchoring groups (meta versus para position) on the charge transport in single oligophenylene molecular wires. • jelena isailović, vasko jovanovski, primož jovanovič. development of hydrogen peroxide gas sensor for security applications. j. electrochem. sci. eng. 10(2) (2020) 55-64 editorial 58 • kaewta jetsrisuparb, sawitta triamthaisong, nontipa supanchaiyamat, andrew hunt, wirat jarernboon, chaiyaput kruehong, pornnapa kasemsiri, prinya chindaprasirt. porous composite electrode from pyrolyzed rice husk for rechargeable zinc battery. • primož jovanovič, kevin stojanovski, marjan bele, luka suhadolnik, goran dražić, gorazd koderman podboršek, miran gaberšček, nejc hodnik. novel methodology for investigating electrochemical gas evolution reactions: a case study of oxygen evolution reaction using floating electrode for effective oxygen bubble removal. • vasko jovanovski, nana ivana hrastnik, urša klun. insights into copper film electrodes for voltammetric determination of trace toxic elements. • jesper t.n. knijnenburg, napapon massa-angkul, kaewta jetsrisuparb. one-pot synthesis of cnt/ni electrocatalysts using electrophoretic deposition. • katharina kocher, viktor hacker. design of polymer coated catalysts for enhanced oxygen reduction reaction toward durable polymer electrolyte fuel cells. • veniamin kondratiev, svetlana eliseeva, ekaterina shkreba, mikhail kamenskii, elena tolstopjatova. improved electrochemical performance of lithium titanate anodes with conductive polymer binder. • dževad kozlica, ingrid milošev. the influence of surface preparation on efficiency of mercapto and phosphonic inhibitors adsorbed on aluminium and copper metal. • gyözö g. láng, mária ujvári. on the electrochemical reduction of molecular oxygen in coexisting equilibrium phases of solutions of limited miscibility. • katarzyna lota, ilona acznik, agnieszka sierczynska, lukasz kolanowski, grzegorz lota. the electrochemical properties of nitrogen-doped carbon materials. • peter machata, alexey popov, tyler t. clikeman, eric v. bukovsky steven h. strauss, olga v. boltalina. spectroelectrochemical studies of perfluoroalkylated derivatives of pahs. • andrei ionut mardare, cezarina cela mardare, c.hristoph cobet, achim walter hassel. combinatorial bandgap tuning of niobium-based anodic oxides for visible light application. • marija marguš, pierre-jean superville, gabriel billon. understanding the gold microwire electrode behavior for the detection of trace metals. • kurt mayer, viktor hacker. monitoring and evaluation of polymer electrolyte fuel cells under harmful operating conditions. • ko-ichiro miyamoto, tatsuo yoshinobu. in situ observation of crevice corrosion by a modified chemical imaging sensor system. • denise molinnus, petra siegert, holger s. willenberg, arshak poghossian, michael keusgen, michael j. schöning. adrenaline biosensing – from macroelectrode to chip-based biosensor to support tumor diagnosis • francisco montes, lucía yohai, marcela vazquez, maría b. valcarce. testing inorganic ions as “green” corrosion inhibitors for construction steel. • anne müller, susan sachse, manfred decker, frank-michael matysik, winfried vonau. comparison of different screen printed sensors based on carbon paste with incorporated prussian blue nanoparticles as electrode material for hydrogen peroxide detection. • georg pchelarov, dzhamal uzun, konstantin petrov. electrocatalysts incorporating higher fullerenes and dwcnts. • gonzalo perelstein, antonio da cunha, marcela vazquez, matías valdés. cu2znsns4 prepared sulfurizing a stack of electrodeposited metallic layers. • elitsa petkucheva, galin borisov, elefteria lefterova, evelina slavcheva. comparative study of nanostructured electrocatalytic materials on 3d nickel foam for efficient water splitting. • birgit pichler, bernhard gollas, viktor hacker. status, challenges and possible solutions for rechargeable zinc-air flow batteries. v. horvat-radošević and z. mandić j. electrochem. sci. eng. 10(2) (2020) 55-64 59 • johanna pilas, shahriar dantism, desiree röhlen, thorsten selmer, torsten wagner, michael keusgen, patrick wagner, michael j. schöning. electrochemical biosensors for on-line monitoring of biogas processes. • ivan pivac, frano barbir, jagoda radošević. rejuvenation phenomena of proton exchange membrane fuel cells. • lidija d. rafailović, christoph gammer, tomislav trišović, bernhard lutzer, christian rentenberger, aleksandar z. jovanović, igor a. pašti, hans-peter karnthaler. electrochemical approaches in synthesis of high surface area materials. • gergana raikova, blagoy burdin, emilia mladenova, milena krapchanska, daria vladikova, zdravko stoynov†. reversibility in a new design of solid oxide fuel cell. • timo raith, christian iffelsberger, stefan wert, frank-michael matysik. hydrodynamic scanning electrochemical microscopy. • rashko rashkov, vasil bachvarov, marina arnaudova, galin borisov. novel catalysts for water splitting. • saeedeh ravandi, tobias zenger, martin eibelhuber, achim walter hassel, andrei ionut mardare. surface pinning of voids in electromigration testing by anodization of combinatorial al-cu alloys. • dominik recktenwald, cezarina cela mardare, andrei ionut mardare, achim walter hassel. corrosion screening of dysprosium-magnesium-zinc alloys for bioresorptive implants. • peter rodič, dolores zimerl, ingrid milošev. hybrid sol-gel coatings modified with fluorinated (meth)acrylates for corrosion protection of aa2024-t3 in 0.5 m nacl. • anastasiya ruchets, nils donker, daniela schönauer-kamin, ralf moos, jens zosel, ulrich guth, michael mertig. selective multi-gas measurements with solid electrolyte cells operated by cyclovoltammetry. • denis sačer, suzana sopčić, davor antonić, davor čapeta, marijana kraljić roković. supercapacitors based on reduced graphene oxide hydrogels. • miglena slavova, elena mihaylova, emilia mladenova, blagoy burdin, borislav abrashev, daria vladikova. zeolite based gas-diffusion electrodes for secondary metal-air batteries. • romana sokolová, veronika majarová, michaela obluková, jan vacek, vladimir křen. on the reaction of polyphenols with copper dication. • ivan spajić, ingrid milošev. al2o3 and hfo2 films prepared by atomic layer deposition for protection of commercially pure titanium for biomedical applications. • sandra steiner, david moser, g. kothleitner, bernhard gollas. morphology control of aluminium anodes in secondary aluminium batteries. • anamarija stoilova, matko erceg, frano barbir. impact of elevated temperature on structural changes of perfluorosulfonated ionomer for polymer electrolyte membrane fuel cells. • krisztina j. szekeres, éva fekete, gyözö g. láng. investigation of electrochemical behaviour of bisphenol a on gold in contact with 0.1 m aqueous naclo4 solution. • árpád ferenc szőke, gabriella stefánia szabó, zoltán hórvölgyi, emőke albert, liana maria muresan. improving the anticorrosive properties of chitosan coatings by impregnation with inhibitors. • ludmila šimková, jiří ludvík. electrochemical investigation of fluorinated derivatives of 1,3diphenylisobenzofuran, chromophores for singlet fission. • jesus tartaj. fuel cells, a clean and efficient way of producing electricity: brief description of different types of fuel cells and their application. • urša tiringer, janez kovač, shoshan abrahami, joost van dam, ingrid milošev, arjan mol, tatsuo yoshinobu. in situ observation of crevice corrosion by a modified chemical imaging sensor system. the effect of pre-treatment on the surface chemistry and interfacial properties of sol-gel coating applied on aluminium and aluminium alloys. j. electrochem. sci. eng. 10(2) (2020) 55-64 editorial 60 • mária ujvári, soma vesztergom, szabolcs gyulai-gaál, gyözö g. láng. investigation of the vanadium redox system by dual voltammetry. • dzhamal uzun, elena razkazova-velkova, venko beschkov, konstantin petrov. an electrochemical process for the simultaneous purification of h2s and so2. • soma vesztergom, maría de jesús gálvez-vázquez, vitali grozovski, noémi kovács, peter broekmann. a model for hydrogen evolution on rdes: are the limiting current plateau lengths indicative of electrocatalytic activity? • daria vishenkova, elena korotkova, jiří barek. electrochemical behaviour of toluylene blue. • winfried vonau. kurt schwabe – name given of an international symposium poster presentations (65) • borislav abrashev, george pchelarov, konstantin petrov. gas diffusion electrodes (gdes) for mhair battery systems. • veronica anăstasoaie, laura enache, cristian omocea, liviu bîrzan, raluca isopescu, eleonoramihaela ungureanu. electrochemical behaviour of new (azulen-1-yldiazenyl)-heteroaromatic compounds containing 1,2,5-thiadiazol-3-yl moieties. • jasmina anojčić, valéria guzsvány, zoltán kónya, momir mikov. voltammetric characterization and trace level determination of dopamine by carbon paste electrode surface modified with oxidized multiwalled carbon nanotubes. • milka avramov ivić, jelena lović, nemanja trišović, jelena lađarević, aleksandar mladenović, milka jadranin, slobodan d. petrović, dušan mijin. the swv determination of esomeprazole at au and its degradation at irox followed by hplc and lc-ms. • josefina ballarre, aldo r. boccaccini, silvia m. cere. coatings of chitosan/gelatin/si-ge nanoparticles by electrophoretic deposition on surgical grade stainless steel: electrochemical and antibacterial behaviour. • krum banov, iliyan popov, todor petkov, branimir banov. comparative study of synthesis way influence on the electrochemical behaviour of nmc 811. • viacheslav barsukov, volodymyr khomenko, oksana butenko, ilona senyk. an interaction of nanostructured carbon based composites with electromagnetic waves in the wide frequency range. • teri bigham, charnete casimero, james dooley, nigel ternan, william snelling, james davis. electrochemical-biosensor for the detection of coliform presence in drinking water. • tanya boyadzhieva, violeta koleva, radostina stoyanova. effect of mg-doping on the electrochemical properties of namn1 mgxpo4 phospho-olivine electrodes. • tanya boyadzhieva, violeta koleva, radostina stoyanova. mixed fe/mn based phospho-olivines as electrodes in hybrid metal-ion batteries. • george-octavian buica, matei d. raicopol, nicoleta a. chira. modified electrodes with thiosemicarbazone receptors for metal ions complexation. • ivana cvijović-alagić, zorica cvijović, jelena bajat, marko rakin. corrosive wear degradation of ti-based implant alloy. • ana cvitešić kušan, irena ciglenečki. electrochemical behaviour of methyl 3-mercaptopropionate (3-mpa) at the hg electrode. • cornelia diac, cătălin ceauș, elena iulia cucolea, valeriu filip, șerban stamatin. ph-anion interplay in the electrochemical dissolution of platinum. • elena diacu, eleonora-mihaela ungureanu, alexandru anton ivanov, sabina-vasilica pistol, veronica anăstasoaie, cristian omocea, liviu bîrzan. study of electroactive properties of a new tetrazole azulene derivative for modified electrodes. v. horvat-radošević and z. mandić j. electrochem. sci. eng. 10(2) (2020) 55-64 61 • nina dimitrova, jenia georgieva, sotiros sotiropoulos, tzvetanka boiadjieva-scherzer. preparation of pt-iro2/tio2 bi-functional catalysts. • sanja eraković, miroslav pavlović, srećko stopić, miodrag mitrić, miroslava varničić, jasmina stevanović, vladimir panić, bernd friedrich. electrochemical performances of rare earth cobased mixed oxides and their application as supercapacitors and fuel cells. • matjaž finšgar, david majer, uroš maver, tina maver. sequential analysis of pb(ii) traces using unmodified and modified spe electrodes. • regina fuchs-godec. the synergistic inhibition of vitamin e and k3 in hydrophobic coatings on copper. • frank gerlach, kristina ahlborn, winfried vonau. new electrochemical and physical measurements on sensitive glasses in thin-film technology. • christian girginov, stephan kozhukharov. substitution of chromium conversion coatings (ccc) by cerium oxide layers (ceopl) deposition for protection of aluminium aircraft alloys. • petru ilea, marius ioan purcar, sorin-aurel dorneanu, alexandru horațiu marincaș. enhancement of the mass transport by numerical simulation in an electrochemical reactor with concentric cylindrical electrodes. • alexandru anton ivanov, elena diacu, eleonora-mihaela ungureanu, sabina-vasilica pistol, adinuta paun, magdalena-rodica bujduveanu. analytical applications of new chemically modified electrodes based on crown eter for heavy metal analysis. • melanie jablonski, lukas muschallik, johannes bongaerts, torsten wagner, thorsten selmer, petra siegert, michael keusgen, michael j. schöning. development of a field-effect biosensor to control the acetoin concentration during fermentation processes. • žana kajinić, jelena vešligaj turkalj, željka romić, anamarija stanković, maja dutour sikirić, martina medvidović-kosanović. electrochemical, zeta-potential and particle size characterization of groundwater samples. • saikrishnan kandaswamy, tanja vidaković-koch. frequency response analysis of oxygen reduction reaction in concentrated alkaline solutions. • barbara kapun, ingrid milošev. sol-gel coating based on zirconium(iv) isopropoxide and epoxysilane for corrosion protection of aluminium alloy in deep-sea environment. • boriana karamanova, svetlana veleva, antonia stoyanova, radostina stoyanova. electrochemical performance of symmetric supercapacitors based on carbon biomaterials in different electrolytes. • jozefina katić, ankica šarić, ines despotović, marin petković, katarina varzić, mirela leskovac, željka petrović. surface modification of ti dental implant by bioactive coatings: experimental and theoretical studies. • jozefina katić, zoran grubač, mirjana metikoš-huković. design of semiconductor (photo)catalysts and investigation of their electronic structure. • andrea kellenberger, nicolae vaszilcsin, delia duca, mircea laurentiu dan, narcis duteanu, svenja stiber, pia assmann, aldo s. gago, k. andreas friedrich. corrosion resistance of niobium coatings on copper bipolar plates for proton exchange membrane electrolyzers. • jong seok kim, dae won kim, tae hyun kim. lithium diffusion properties of fe3o4 nanoparticles as different morphologies for anode materials of lithium ion batteries. • myong hwan kim, youngmo goo, ji-young park, seung-eul yoo. a study on property and performance according to the pore structure of metallic porous separator for hydrogen fuel cell vehicles. • stephan kozhukharov, christian girginov, aleksandar tsanev, georgy avdeev, boriana tzaneva. evaluation of the electrochemical performance of ag incorporated aao layers after extended exposure to a model corrosive medium. j. electrochem. sci. eng. 10(2) (2020) 55-64 editorial 62 • mila n. krstajić pajić, sanja i. stevanović, vuk v. radmilović, piotr zabinski, nevenka r. elezović, velimir r. radmilović, snežana lj. gojković, vladislava m. jovanović. catalysis at nano level: promoting pt nanoparticle activity by au decoration. • yuliya kudryashova, dmitrii gryzlov, anna kuzmina, tatiana kulova, alexander skundin. synthesis and study of phosphorus sulfide-based electrodes for lithium-ion and sodium-ion batteries. • anna kuz’mina, il’ya gavrilin, yuliya kudryashova, tatiana kulova, alexander skundin. study of the lithium insertion into germanium nanofilaments. • andrzej leniart, mariola brycht, barbara burnat, sławomira skrzypek. application of a glassy carbon in the voltammetric determination of pesticides. • olga lipskikh, elena korotkova, alena nikolaeva. voltammetric determination of drug based on indomethacin menthyl ester. • jelena lović, vladimir jović. ni-sn coating as electrocatalyst for ethanol oxidation in alkaline solution. • gabrijela ljubek, nikola vugrinec, marijana kraljić roković. pedot-based composites prepared by one-step electrochemical method for supercapacitors applications. • vesna maksimović, milovan stoiljković, nebojša nikolić. electrodeposition of nico alloy powders with coral presentation-like structure. • borislav m. malinović, duško zorić, tijana djuričić. efficiency testing of commercial corrosion inhibitor for stainless steel en 1.4301 and copper en 13601 in simulated cooling water. • delyana marinova, rosica kukeva, ekaterina zhecheva, radostina stoyanova. modelling of sodium manganese-based sulfates as a new class of polyanion electrode materials. • delyana marinova, rosica kukeva, ekaterina zhecheva, radostina stoyanova. nickel-manganese sulfates as electrode materials for hybrid lithium–sodium ion cell. • bernhard marius, željko penga, viktor hacker. mitigating mass transport limitations of pefcs during dynamic operation. • dajana mikić, mia gotovuša, andreja kišić, helena otmačić ćurković. formation and stability of protective films of phosphonic acid on various metal substrates. • tomáš mikysek, milan sýs, miroslav novák. electrochemical study on biomimetic activity of selected copper complexes in comparison with enzymes. • vesna mišković-stanković, katarina nešović, ana janković, tamara radetić, aleksandra perićgrujić, maja vukasinović-sekulić, vesna kojić, kyong yop rhee. poly(vinyl alcohol)/chitosan hydrogels with electrochemically synthesized silver nanoparticles for wound dressing applications. • liana maria muresan, sorin dorneanu, eniko kovacs, simona varvara, petru ilea. voltammetric study of base metals recovery from brominated solutions used as lixiviants for waste printed circuit boards. • jan oberländer, julio arreola, michael keusgen. combined calorimetric gas sensor/spore-based biosensor for monitoring of gaseous sterilization processes. • michaela obluková, romana sokolová, ilaria degano, radomir čabala. electrochemical properties of 1-pentyl-3-(1-naphtoyl)indole and 1-pentyl-3-(2-methoxyphenylacetyl)indole. • tamara-rita ovari, gabriella szabo, gabriel katona, liana maria muresan. silica/graphene oxide composite coatings for corrosion protection of zinc. • marijana pantović pavlović, sanja eraković, miroslav pavlović, ljiljana veselinović, jasmina stevanović, vladimir panić, nenad ignjatović. surface modification of titanium implants by adherent hydroxyapatite/titanium oxide composite coatings using novel in-situ synthesis. • valentina popova, elena korotkova, jiří barek. electrochemical determination of nitric oxide in human macrophage. v. horvat-radošević and z. mandić j. electrochem. sci. eng. 10(2) (2020) 55-64 63 • gabrijela radić, ivan šajnović, marijana kraljić roković, željka petrović. reduced graphene oxide/α-fe2o3 fibres as active material for supercapacitor application. • matea raić, lara mikac, zoran mandić, vedran petrić, mile ivanda. silicon anode for lithium-ion batteries. • egon rešetar, damir iveković. magnetic biomass-derived pyrolytic carbon: an advanced electrode material for applications in bioelectrocatalysis and electrochemical sensing. • sanja stevanović, vladislava m jovanović. microwave-assisted polyol synthesis of pt based catalysts: enhanced methanol activity and co tolerance. • sanja šekuljica, valéria guzsvány, kurt kalcher. adsorptive stripping voltammetric determination of fungicide carbendazim by tetrabutylammonium chloride modified carbon paste electrode. • ivana škugor rončević, nives vladislavić, marijo buzuk, maša buljac, anđela lukin. corrosion protection of tin by some carboxylic acids in sodium chloride solution. • graziella liana turdean, eniko kovacs, sorin-aurel dorneanu. optimisation of experimental parameters for the electrochemical monitoring of the metals concentration during the waste printed circuit boards recycling process. • simona varvara, sorin aurel dorneanu, alexandru okos, roxana bostan, maria popa, liana maria muresan, petru ilea. an eis study of metals dissolution mechanism in bromide-based electrolytes used as lixiviants for waste printed circuit boards. • svetlana veleva, ekaterina zhecheva, paulina półrolniczak, antonia stoyanova, radostina stoyanova. electrochemical performance of nanotube sodium titanate in hybrid li-mg ion cells. • stefan wert, timo raith, frank-michael matysik. a simple approach for the preparation of ring ultramicroelectrodes in the context of combined scanning electrochemical microscopy and scanning ion conductance microscopy. • dolores zimerl, stojan stavber, ingrid milošev. from organic synthesis to testing of compounds with different anchor groups as corrosion inhibitors for aluminium. satellite student 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access: issn 1847-9286 www.jese-online.org original scientific paper formation and reduction of anodic film on polycrystalline bi electrode in pure methanol solutions anastos g. anastopoulos and athanasios a. papaderakis laboratory of physical chemistry, department of chemistry aristotle university of thessaloniki, 54124 thessaloniki, greece corresponding author: anasto@chem.auth.gr received: april 2, 2019; revised: may 4, 2019; accepted: may 20, 2019 abstract the processes of film formation and reduction of bismuth in pure methanol are phenomenologically studied by means of cyclic voltammetry, ac voltammetry and electrochemical impedance spectroscopy methods. film formation takes place under low electrode potentials within the potential range from -0.1 to about 0.2 v vs. ag|agcl resulting in the development of bi(ch3o)ads layer. the scan rate effect on the anodic current profile is interpreted in terms of a gradual variation of uncompensated resistance, accompanying the processes of film formation and reduction. phase sensitive ac voltammetry measurements suggest leaky insulating character of a thin anodic film in agreement with the results of electrochemical impedance experiments. keywords anodic film formation; bismuth; methanol, methoxylation introduction electrochemical studies have confirmed that in pure methanol solutions methoxy anions, resulting from the solvent self-dissociation, interact with various electrode materials viz. cu, ni, zn, fe ti, si and the valve metal al. this interaction leads to the formation of anodic surface films of the general type m-(ch3o)n, where m is the electrode material and n depends on the dissolution reaction of the electrode material and the specific reaction of film formation [1-3]. it is surprising that up to now, there is nothing reported in the literature about the electrochemical study of another valve metal bi in pure methanol solutions. on the contrary, long-lasting systematic electrochemical research has been devoted to the system bi/h2o. in aqueous solutions, bi2o3 is anodically formed on bi surface by diffusion and field assisted migration of bi3+ ions through the film under the influence of the so-called high field mechanism [4], resulting in both thickening and spreading of oxide islets. the cathodic reduction of bi2o3 [5] involves development of a metallic bi http://dx.doi.org/10.5599/jese.689 http://dx.doi.org/10.5599/jese.689 http://www.jese-online.org/ mailto:anasto@chem.auth.gr j. electrochem. sci. eng. 9(4) (2019) 223-230 anodic film on polycrystalline bi electrode 224 film at the oxide/electrolyte interface, which is extended towards the interior of bi2o3 layer. two possible reduction mechanisms are usually suggested, the first involving electron transfer in the film and reduction at the film/solution interface and the second one, involving ion transfer in the film and reduction at the metal/film interface [6]. bi2o3 oxide can be completely reduced by electrochemical means. bi2o3 anodic oxide films present electrical rectification by acting as insulators under anodic potentials and conductors under cathodic ones, respectively [7,8]. another important feature of bi2o3 layers is their photoconductivity in the visible region. bi2o3 layers present photocurrents of nand p-type, thus being characterized as amphoteric semiconductors. the query as to whether bi/ch3oh system possesses similar or different properties than bi/h2o has motivated us to get involved in the present work. experimental electrodes and chemicals a polycrystalline bi rod (alfa aesar, 99.99 %) embedded in a glass cylinder, sealed by suitable adhesive and covered with thermoplastic tube to leave a free disk shaped surface of 8 mm diameter at its end, was used as a working electrode. pt foil auxiliary electrode and ag|agcl reference electrode saturated with aqueous kcl, both placed in a single compartment of double walled electrochemical cell kept at 298 k, were used in all measurements. 0.1 m liclo4 in methanol is used as the base solution in all measurements. the working electrode was mechanically polished by emery paper of decreasing grain size. after that it was set for several minutes in an ultrasonic bath containing distilled water. then it was washed with the working solution and transferred rapidly to the electrochemical cell. the chemical reagents used without further purification are anhydrous liclo4 (aldrich 99.99 %) and methanol (lab-scan, purity >99 %, maximum water content 0.05 %). methods and instrumentation cyclic voltammetry and electrochemical impedance spectroscopy (eis) measurements were carried out by autolab pgstat302n electrochemical system connected to a pc running the autolab software. bi electrode activation was proved necessary to improve the reproducibility of all electrochemical measurements. therefore, bi electrode was activated by cyclic voltammetry, scanning the potential from -1.6 to 0.2 v and back to -1.6 v vs. ag|agcl with a rate of 0.025 v s-1. eis measurements were carried out within the frequency range of 10 khz to 0.1 hz with 4 mv ac signal amplitude at selected dc potentials. the reproducibility of all electrochemical impedance measurements was checked by the comparison of three successive measurements at the same potential. the linearity-stability conditions for eis measurements were checked by the kramers-kronig test incorporated in the autolab software. by this procedure, the sum of squares of differences (chi-square) between measured and calculated complex impedance values, i.e. real and imaginary impedance values were found to be of the order of 10-2 in all systems studied within the frequency range from 10 khz to 0.5 hz. at frequencies below 0.5 hz some eis measurements were omitted because the corresponding chi-square values were found not better than 10-1. phase selective ac voltammetric measurements were carried out by an experimental setup [9] comprising a lock-in amplifier (model sr830 from stanford research) and a potentiostat (model potentioscan wenking pos73 from bank elektronik), interfaced to a pc running labview 6.1. negative dc potential scan was applied in steps of 25 mv from 0.20 to -1.60 v vs. ag|agcl. each measurement was taken after 10 s of stopover at a. g. anastopoulos and a. a. papaderakis j. electrochem. sci. eng. 9(4) (2019) 223-230 http://dx.doi.org/10.5599/jese.689 225 each potential. the amplitude and the frequency of the ac signal were set to 4 mv and 80 hz respectively. the standard deviation between at least three successive measurements, expressed on a percent basis, was found equal to about 5 %. results and discussion cyclic voltammetry the cyclic voltammetry response of a polycrystalline bi electrode in pure methanol solution of 0.1m liclo4, obtained with an electrode potential scan rate ranging from 0.025 to 0.4 v/s is shown in figure 1. figure 1. cyclic voltammetry curves of bi in methanol solution of 0.1 m liclo4 at various scan rates: (1) 0.025, (2) 0.05, (3) 0.10, (4) 0.20, (5) 0.40 v/s. insets: anodic and cathodic peak currents vs. corresponding peak current potentials. during the anodic scan, the characteristic features of voltammetry curves are the following: at potentials positive to -0.40 v vs. ag|agcl, the anodic current increases leading to a single anodic current peak, which depending on the scan rate is located within the potential range (-0.25±0.07) v vs. ag|agcl. this peak marks the formation of initial nuclei of the anodic film. the absence of multiple anodic peaks indicates that successive layers are not developed. at potentials positive to the anodic peaks, the current vs. potential dependence presents a narrow and ill-defined plateau which tends to a new increase of anodic current at electrode potentials, ε ≥ 0.20 v vs. ag|agcl. this current increase is presumed to be related to the methanol oxidation on bi, the exact mechanism of which is neither known nor the subject of the present work. however, it can be reasonably assumed that e ≈ 0.20 v vs. ag|agcl is the positive limit of anodic film formation in the neutral methanol solution of 0.1 m liclo4. during the cathodic scan, the characteristic features of voltammetric curves are the following: the single cathodic current peak observed at -1.5 -1.0 -0.5 0.0 0.5 -6.0 -4.0 -2.0 0.0 2.0 4.0 6.0 -0.85 -0.80 -0.75 -0.70 -0.65 -4 -3 -2 -1 j / m a /c m -2 e / v vs. ag|agcl cathodic peaks -0.35 -0.30 -0.25 -0.20 -0.15 0 1 2 3 4 5 j / m a /c m -2 e / v vs. ag|agcl anodic peaks 2 1 3 4 5 j / m a c m -2 e / v vs. ag|agcl 5 4 3 2 1 http://dx.doi.org/10.5599/jese.689 j. electrochem. sci. eng. 9(4) (2019) 223-230 anodic film on polycrystalline bi electrode 226 (-0.75±0.07) v vs. ag|agcl is presumed to be related to the reduction process of the anodically developed film. at potentials negative to -1.40 v vs. ag|agcl, however, the hydrogen evolution manifests itself by increase of the cathodic current. the observed single anodic and cathodic peaks suggest that formation and reduction processes of the anodic film in the case of the system bi/ch3oh represent one step reactions. at potentials from about -0.1 v to 0.2 v vs. ag|agcl, where ill-defined anodic plateau of the curves of figure 1 is observed, anodic films of bi in methanol are developed. although the decomposition of methanol is a rather complicated process resulting in various ionic species of the general type chxo [1], the process of its self-dissociation can be simply described as: 2ch3oh ↔ ch3o+ ch3oh2+ (1) combination with the simple picture of voltammetric curves of figure 1, suggests that the process of anodic film formation of bismuth in pure methanol is presumably its surface methoxylation. at low positive and negative electrode potentials ch3oions are electrosorbed on the bi surface, which undergoes a single step anodic dissolution, as deduced by the single anodic peak. formation of bi+ is, similarly to that suggested for al [1], carried out according to the reaction: bi → bi+ + e (2) presumably, a thin film of the form bi(ch3o)ads is developed on the bare metal bi. single anodic peaks also suggest that bi+ is the final dissolution product and that further dissolution of bismuth is not detected in pure methanol. therefore, the anodic film formation of bi in pure methanol can be described by means of the following one-electron reaction [1]: bi+ + ch3o→ bi(ch3o)ads + e (3) correspondingly, during the cathodic scan, the reduction of bi(ch3o)ads film is reasonably expected to take place by means of the inverse reaction (3). according to the literature [10-13], a mechanistic interpretation of the nucleation and thickening of anodic films can be formulated by means of the relation between the anodic current profile and the electrode potential scan rate. in figure 1 we see that with increasing the scan rate, peak currents increase, and anodic peaks shift to more positive potentials, while cathodic peaks shift to more negative ones. peak potential and peak current values are linearly dependent on the square root of the scan rate as indirectly indicated by linear j peak vs. e peak plots shown in the insets of figure 1. such linear dependencies involving peak currents and peak potentials were already reported [14] for bi2o3 anodic film developed on bi surface in aqueous solutions. the formation and reduction mechanisms of bi2o3 include the occurrence of a solid state diffusion process [13,14] operating only in the case of thick films developed under high positive electrode potentials. this would not be the case for the thin anodic layer bi(ch3o)ads formed in pure methanol. in this case the linear dependence of e peak and j peak values on the square root of scan rate may be the result of a gradual variation of uncompensated resistance, accompanying the film formation and reduction according to the so called passivation model of müller, where the process of film formation is under the control of ohmic resistance [10]. phase selective ac voltammetry information about the structural characteristics of an electrochemical interface can be also acquired from differential capacitance measurements. however, over the whole potential range studied from 0.2 to -1.6 v vs. ag|agcl, the interface between the polycrystalline bi electrode and methanol solution may be not identified with the a. g. anastopoulos and a. a. papaderakis j. electrochem. sci. eng. 9(4) (2019) 223-230 http://dx.doi.org/10.5599/jese.689 227 model of ideal capacitor, because at the extremes of this range bi/methanol interface is not ideally polarizable. it must also be indicatively recalled that capacitance studies of single crystal bi electrodes in liclo4 containing methanol solution performed by estonian electrochemists [15], were carried out at electrode potentials ranging from -0.4 to -1.4 v vs. sce leaving outside the potentials where film formation and hydrogen evolution occur. in this respect, it is useful to look at the structural characteristics of bi/ch3oh interface by means of the phase selective ac voltammetry technique in terms of the potential dependence of the out of phase ac current, j90, decoupled from the in phase j0 component. j90 vs. e curves are typically free from faradaic contributions and describe the potential dependence of the resident interfacial charge, providing thus a picture of the structural condition of the interface. in figure 2, j90 vs. e dependence clearly shows potential regions where the anodic film is present and those where it is absent. figure 2. electrode potential, e, dependence of out of phase current, j90, at polycrystalline bi electrode in 0.1 m liclo4 methanolic solution. at potentials positive to -0.50 v vs. ag|agcl where the anodic film is present, j90 values show a weak potential dependence. according to the literature [16], potential independent capacitance may be expected for insulating films having thicknesses of about (1.5±0.5)x10-7cm. thus, the weak potential dependence of j90 current values shown in figure 2 may be considered as an indication for the presence of a leaky insulating film prior than passive or semiconductive layer of oxide nature. the minimum of j90 vs. e curve in figure 2 falls into the same potential range as cathodic peaks of cyclic voltammetric curves shown in figure 1. therefore, it may be assumed that the potential dependence of j90 from about -0.50 v to -0.85 v vs. ag|agcl represents a transition from the bi(ch3o)ads covered electrode to the bare metal bi. electrochemical impedance spectroscopy the elucidation of processes involved in the formation and reduction of bi(ch3o)ads surface film can be further assisted by electrochemical impedance measurements, carried out at selected electrode potentials. as shown in figure 3, all nyquist plots obtained at the vicinity of the anodic and cathodic peaks i.e. at -0.4 v and -0.68 v and also at -1.4 v vs. ag|agcl where hydrogen evolution is presumed, exhibit the form of flattened semicircles. -1.5 -1.0 -0.5 0.0 23 30 38 45 52 j 9 0 / μ a c m -2 e / v vs. ag|agcl http://dx.doi.org/10.5599/jese.689 j. electrochem. sci. eng. 9(4) (2019) 223-230 anodic film on polycrystalline bi electrode 228 -z’’ vs. z’ plots of figure 3 can be approximated by the randles circuit of the type rs(rctqdl) according to the circuit description code of boukamp [17]. this circuit incorporates solution and charge transfer resistances (rs and rct) and the parameter qdl of the constant phase element which resembles the contribution of double layer capacitance to the overall impedance. figure 3. nyquist plots of bi in methanol solution of 0.1 m liclo4, at various electrode potentials in v vs. ag|agcl:  -0.4,  -0.68,  -1.4. inset: bode phase diagrams at the same potentials. lines represent simulated responses fitted to experimental points. the corresponding bode phase diagrams shown in the inset of figure 3 clearly show a single rc time constant, with phase angles hardly approaching -60o. this is characteristic of a significant deviation from pure capacitive behavior, thus justifying the use of constant phase element instead of conventional capacitor. the results of circuit fitting to -z’’ vs. z’ data of figure 3 are provided in table 1. table 1. results of cnls fitting of circuit rs(rct qdl) to -z’’ vs z’ data e / v vs. ag|agcl rs / ω cm2 rct / kω cm2 qdl / ω-1 cm-2 sn n χ2 -0.40 55.5 8.8 0.54 10-4 0.75 0.085 -0.68 52.9 6.0 0.35 10-4 0.78 0.072 -1.40 54.7 2.1 0.63 10-4 0.83 0.067 the calculated values of rs and qdl are weakly dependent on electrode potential. the decrease of rct with increasing negative potential from -0.40 to -0.68 v vs. ag|agcl denotes the acceleration of the inversed reaction (3). this is not the case of the even lower value of rct at -1.4 v vs. ag|agcl because at this potential the more probable reaction is hydrogen evolution. the values of cpe exponent, n, point up to the significant deviation from the model of ideal capacitance. the validity of r(rq) circuit suggests the occurrence of reactions with a single electron transfer step as reaction (3) and its reverse. within the potential range -0.1 v to 0.2 v vs. ag|agcl, where the anodic film bi(ch3o)ads is present, nyquist plots obtained at various electrode potentials do not show noticeable features. however, at these potentials bode phase diagrams shown by the inset of figure 4 reveal two partially overlapping time constants. τhis is the most clearly seen in the bode plot measured at e = 0.15 v vs. ag|agcl, showing a deep at the frequency of 6.87 hz. 0 2 4 6 0 2 4 6 0 2 4 0 15 30 45 60 -φ / d e g log(f/hz ) -z ' '/ k ω c m 2 z '/ kω cm 2 a. g. anastopoulos and a. a. papaderakis j. electrochem. sci. eng. 9(4) (2019) 223-230 http://dx.doi.org/10.5599/jese.689 229 log (f / hz) figure 4. bode phase diagram of bi in methanol solution of 0.1 m liclo4, at e = 0.15 v vs. ag|agcl. insets: nyquist and bode plots at potentials ○ 0.05, ■ 0.10 and  0.15 v. lines are fitting results of r(q[r(rq)]) equivalent circuit to experimental points. among several equivalent circuits already used for the eis study of anodic oxides, the circuit rs(qdl[rct(rad qad)]) shown in figure 5, which has been suggested for the description of charge transfer process in the presence of adsorbed species [18], was applied. the results obtained after fitting the circuit in figure 5 to experimental impedance plots in figure 4, are presented in table 2. table 2. results of cnls fitting of circuit rs(qdl[rct(rad qad)]) to -z’’ vs. z’ data e / v vs. ag|agcl rs / ω cm2 rct / kω cm2 qdl / ω-1 cm-2 sn rad / kω cm2 qad / ω-1 cm-2 sn ndl nad χ2 -0.05 52.3 5.9 0.93 10-4 28.6 0.86 10-4 0.78 0.62 0.081 -0.10 52.5 7.4 0.67 10-4 23.8 0.61 10-4 0.890 0.75 0.088 -0.15 53.2 5.6 0.65 10-4 18.9 0.49 10-4 0.79 0.74 0.090 figure 5. electrical equivalent circuit used to fit impedance spectrum at potentials: 0.05, 0.10 and 0.15 v vs. ag|agcl. rs = solution resistance, rct = charge transfer resistance, rad= anodic film resistance. qad and qdl are parameters of constant phase elements corresponding to the anodic film bi(ch3ο)ads and bi/solution interface. in view of the bode plots of figure 4 and the circuit of figure 5, one of the two time constants may be related to the presence of bi(ch3ο)ads film by means of anodic film resistance and the other to the electron transfer associated with reactions (2) and (3) by means of charge transfer resistance. the results of table 2 reveal a high value of film resistance rad in agreement with the predicted leaky insulating character of the bi(ch3ο)ads layer. as expected, rct values are lower and of similar 0 2 4 6 0 15 30 45 60 0 2 4 15 30 45 60 -φ /d e g log(f/hz ) 0 2 4 6 8 10 0 2 4 6 8 10 -z " /k ω c m 2 z' /kω cm 2 -φ /d e g log(f/hz ) 6.87hz log (f / hz) http://dx.doi.org/10.5599/jese.689 j. electrochem. sci. eng. 9(4) (2019) 223-230 anodic film on polycrystalline bi electrode 230 magnitude with those of table 1. the values of cpe exponents, ndl and nad again point up to the significant deviation of both capacitor responses from that of ideal capacitance. conclusions the process of anodic film formation of bismuth in pure methanol is identified with its surface methoxylation. a single step dissolution of bi surface to bi+ is suggested. voltammetric results suggest that anodic film formation takes place within a narrow zone of low electrode potentials ranging from -0.1 v to 0.2 v vs. ag|agcl, by adsorption of –ch3ogroups, resulting in bi(ch3ο)ads film. weak potential dependence of the out of phase ac current within the above potential range suggests that this film is of leaky insulating character, which is also confirmed by the relatively high value of film resistance resulting from circuit fitting to eis measurements. references [1] j. banas, b. stypula, k. banas, j. światowska-mrowiecka, m. starowicz, u. lelek-borkowska, journal of solid state electrochemistry 13 (2009) 1669-1679. 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[13] z. grubač, m. metikoš-huković, electrochimica acta 43 (1998) 3175-3181. [14] d. e. williams, g. a. wright, electrochimica acta 21 (1976) 1009-1019. [15] k. lust, m. väärtnõu, e. lust, electrochimica acta 45 (2000) 3543-3554. [16] u. stimming, j. w. schultze, berichte der bunsengesellschaft für physikalische chemie 80 (1976) 12971302. [17] b. a. boukamp, solid state ionics 18-19 (1986) 136-140. [18] a. lasia, electrochemical impedance spectroscopy and its applications, springer, new york, usa, 2014. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {dft calculations and electrochemical studies on azulene ligands for heavy metal ions detection using chemically modified electrodes} doi:10.5599/jese.475 73 j. electrochem. sci. eng. 8(1) (2018) 73-85; doi: http://dx.doi.org/10.5599/jese.475 open access : : issn 1847-9286 www.jese-online.org original scientific paper dft calculations and electrochemical studies on azulene ligands for heavy metal ions detection using chemically modified electrodes amalia stefaniu1, maria-daniela pop2, georgiana-luiza arnold2, liviu birzan 3, lucia pintilie1, elena diacu4, eleonora-mihaela ungureanu2, 1national institute of chemical pharmaceutical research and development bucharest, 112 vitan av., 031299, bucharest, romania 2department of inorganic chemistry, physical chemistry and electrochemistry, university "politehnica" of bucharest, gh. polizu 1-7, 011061, bucharest, romania 3romanian academy, institute of organic chemistry “c. d. nenitzescu”, spl. independentei, 202b, 060023-bucharest, 35 p.o. box 108, romania 4department of analytical chemistry and environmental engineering, university "politehnica" of bucharest, gh. polizu 1-7, 011061, bucharest, romania corresponding author e-mail: eleonoramihaelaungureanu@gmail.com received: november 18, 2017; revised: january 13, 2017; accepted: january 14, 2017 abstract a computational study on three related derivatives of 5-[(azulen-1-yl)methylene]-2thioxoimidazolidin-4-one was conducted using density functional theory by calculating a series of molecular descriptors and properties of their optimized geometries (electrostatic and local ionization potentials, molecular frontier orbitals, etc.). thermodynamic properties (zero-point energy, enthalpy, constant volume heat capacity, entropy and gibbs energy) for these derivatives have been calculated and related to ligands electrochemical behavior. reduction and oxidation potentials have been correlated to their calculated energy levels for lumo and homo orbitals. chemically modified electrodes based on these derivatives have been tested in view of heavy metal ions recognition, and their detection limits have been correlated to the calculated values of electron affinity. keywords 5-[(azulen-1-yl)methylene]-2-thioxoimidazolidin-4-one derivatives; quantum mechanical calculations; electrochemical behavior; modified electrodes; heavy metal ions introduction azulenes have a push-pull structure, with a five-member cyclic moiety (electron-rich) connected to a seven-member cyclic moiety (electron-poor). azulene derivatives undergo irreversible http://dx.doi.org/10.5599/jese.475 http://www.jese-online.org/ mailto:eleonoramihaelaungureanu@gmail.com j. electrochem. sci. eng. 8(1) (2018) 73-85 azulene ligands for heavy metal ions detection 74 electrooxidations and irreversible or quasi-reversible reductions when polarized at anodic or cathodic potentials, respectively [1]. due to their low oxidation potential, they can lead to modified electrodes which could be used to build new electrochemical sensors. the sensors based on azulene derivatives are the basis of most recent methods for determination of heavy metals from water samples [2]. this method leads to comparable results to the best-known methods known to date, such as those based on iron oxide/graphene composite [3], bismuth nanoparticle-porous carbon paste [4], etc. the azulenes investigated in this study are derivatives of 5-[(azulen-1-yl)methylene]-2-thioxoimidazolidin-4-one (l1), shown in fig. 1, which have in their structure complexing imidazolidine group, specific for the recognition of heavy metal ions [5]. one of these ligands, l2, has been investigated in molecular recognition for building complexing chemically modified electrodes for heavy metals, and lead to promising results for the heterogeneous recognition of pb(ii) and cu(ii) ions [6]. similar azulenes with the thiourea moieties as complexing monomers have led to modified electrodes with good complexing abilities for heavy metals, with very good responses for cd(ii) and pb(ii) ions [7]. the present work reports on the investigation of l1 l3 compounds ability to coordinate heavy metals. chemically modified electrodes have been prepared from these ligands under similar conditions and used for heterogeneous recognition. their detection limits for some heavy metal ions (hg, pb, cd, cu) have been evaluated in order to be correlated to their molecular structural descriptors. density functional theory (dft) investigations on these azulenes derivatives have been performed to reveal some global reactivity parameters derived from frontier molecular orbitals energy diagram, to be linked with their ability to complex heavy metals, for electrochemistry applications. predicted structural characteristics are optimized, compared and correlated in terms of nmr chemical shifts, aiming to determine the structure thermodynamically favored by synthesis [5]. then, their computed parameters are obtained and analyzed in connection with some complexing and redox properties determined by electrochemistry. our interest concerns the use of l1 – l3 as ligands in heterogeneous recognition of heavy metals (fig. 1). the main parts of their structure are illustrated with different colors in fig. 1. they are the seven-member azulene ring (i), the five-member azulene ring (ii) and the imidazolidine ring (iii). l1 l2 l3 figure 1. 2d structure of investigated compounds experimental acetonitrile (ch3cn) and tetrabutylammonium perchlorate (tbap) from fluka were used as received for the solvent and supporting electrolyte. the azulene derivatives were synthesized according to the previous described method [5]. the electrochemical characterizations were carried out using a pgstat 12 autolab potentiostat connected to a three-electrode cell. the working electrode was a glassy carbon disk (3 mm diameter). the active surface was polished before each determination with diamond paste (1 μm) and cleaned with acetonitrile. ag/agno3 (10 mm) in 0.1 m tbap, ch3cn was used as reference a. stefaniu et alj. electrochem. sci. eng. 8(1) (2018) 73-85 doi:10.5599/jese.475 75 electrode and a platinum wire was used as auxiliary electrode. the electrochemical experiments were performed by cyclic voltammetry (cv), differential pulse voltammetry (dpv) and rotating disk electrode (rde). cv curves were recorded from 0.1 v/s to 1.0 v/s (for different scan rate). dpv curves were recorded at 0.01 v/s with a pulse height of 0.025 v and a step time of 0.2 s. rde curves were recorded at 0.01 v/s, for various rotation rates of the disk (between 100 and 2000 rpm). the potential was referred to the potential of the ferrocene/ferrocenium redox couple (fc/fc+) which in our experimental conditions was +0.07 v. the preparation of the modified electrodes has been done by controlled potential electrolysis in millimolar solutions of each ligand, in acetonitrile solutions containing tbap (0.1 m) as supporting electrolyte. all electrochemical experiments were performed at 25 °c under argon atmosphere. the experiments for heavy metals ions detection were carried out in a three-electrode cell (transfer cell). the working electrode was a glassy carbon disk (diameter of 3 mm) electrode modified with each ligand, the reference electrode was ag/agcl, 3 m kci, and the auxiliary electrode was a platinum wire. 0.1 m buffer acetate (ph 5.5) was used as supporting electrolyte. the recognition experiments were performed at 25 °c and under argon atmosphere. the buffer acetate solution was prepared from 0.2 m acetic acid and 0.2 m sodium acetate solutions. metal cation solutions were purchased from sigma aldrich and fluka: mercury (ii) acetate sigma aldrich, cadmium nitrate tetrahydrate sigma aldrich, copper (ii) acetate monohydrate fluka, lead (ii) nitrate sigma aldrich. heavy metals solutions with concentrations of 10−4 and 10−6 mol/l, were prepared by successive dilutions from 10-2 m stock solutions of mercury (hg), cadmium (cd), copper (cu) and lead (pb). the procedure to modify the glassy carbon electrodes consisted in electropolymerization of each ligand in millimolar solutions in 0.1 m tbap, ch3cn, followed by cleaning in acetonitrile, equilibration of the modified electrode in 0.1 m acetate buffer at ph 5.5, between -0.9 v and + 0.6 v vs. ag/agcl, over oxidation in 0.1 m acetate buffer at ph 5.5 between -0.2 v to + 1.2 v. the accumulation of heavy metal ions has been performed in the assay solutions containing heavy metal ions in different concentrations (starting from 10-10 to 10-4) for 20 minutes under magnetic stirring. this step has been followed by the stripping of the accumulated metal ions, after a reduction at -1.2 v for 120 s, in cv or dpv anodic scans with a scan rate of 0.01 v/s. the stripping currents for cd, pb, cu, and hg currents have been evaluated for each concentration. computational procedure details the calculations were carried out using spartan 14 software wavefunction, inc. irvine ca usa [8] on intel(r) core i5 at 3. 2 ghz cpu pc. first, the 3d cpk models of the studied compounds were generated. then, a systematic conformational search and analysis was achieved to establish the more stable conformers of each of the three derivatives of 5-[(azulen-1-yl) methylene]-2-thioxoimidazolidin-4-one, presenting the energy minima. the most stable conformer (lowest energy) was established using mmff (molecular mechanics force field for organic molecules developed by merck pharmaceuticals) model by refining the geometry for each studied compound. for these refined structures, a series of calculations of molecular properties and topological descriptors has been carried out using [9], software algorithm hybrid b3lyp model (the becke’s three parameter hybrid exchange functional with the lee-yang-parr correlation functional) [9-11] and polarization basis set 6-31g* [8,12] in vacuum, for equilibrium geometry at ground state. j. electrochem. sci. eng. 8(1) (2018) 73-85 azulene ligands for heavy metal ions detection 76 results and discussion quantum mechanical calculations the atomic numbering scheme given by the spartan 14 software for structures under study is shown at the beginning of the table 1. the parameters (bond lengths and dihedral angles) of the compounds l1-l3 have been obtained for the optimized geometries. they are listed in tables 1-2. the molecular structures comprise two coplanar cycles (i and ii) and the cycle iii, which can be rotated around c14-c13 bond, showing a small deviation from co-planarity given by the small values of the dihedrals (h7, c14, c13, c12) and (c3, c14, c13, n2). l3 conformer configuration presents also the torsion angles of 119.04° (c18, c15, c7, c4) and 61.03° (c16, c15, c7, c9). table 1. structure labels and predicted bond lengths (å) for l1 l3 compounds. l1 l2 l3 cycle of 7 atoms (i) c5-c1 1.391 1.406 1.408 c5-c8 1.399 1.410 1.409 c8-c9 1.397 1.394 1.395 c9-c7 1.398 1.404 1.398 c7-c4 1.397 1.393 1.400 c4-c0 1.392 1.411 1.391 c0-c1 1.487 1.495 1.496 substituents (methyl and isopropyl) of cycle i c5-c10: 1.520 c5-c11: 1.519 c9-c15: 1.518 c7-c15: 1.531 c4-c11: 1.518 c15-c18: 1.540 c15-c16: 1.541 cycle of 5 atoms (ii) c1-c3 1.429 1.441 1.431 c3-c6 1.424 1.416 1.426 c6-c2 1.391 1.389 1.385 c2-c0 1.411 1.409 1.424 substituents of cycle ii c10 1.502 chain between cycle ii and cycle iii c3-c14 1.434 1.444 1.441 c14-c13 1.357 1.357 1.358 cycle iii c13-n2 1.399 1.400 1.400 n2-c17 1.371 1.369 1.369 c17-n1 1.381 1.382 1.382 n1-c12 1.401 1.401 1.401 c12-c13 1.484 1.483 1.483 functional groups (c=o and c=s) of cycle iii c12-o1 1.217 1.217 1.218 c17-s1 1.657 1.658 1.659 a. stefaniu et alj. electrochem. sci. eng. 8(1) (2018) 73-85 doi:10.5599/jese.475 77 table 2. predicted dihedral angles values for l1 l3 compounds. l1 l2 l3 h13, c5, c1, c3): 0.28° (h7, c14, c3, c1): 13.76° (h7, c14, c13, c12): 2.79° (o1, c12, c13, c14): 0.69° (o1, c12, n1, h4): -0.50° (h4, n1, c17, s1): -0.96° (c6, c3, c14, c13): 15.25° (h14, c6, c3, c14): 2.46° (h5, n2, c13, c14): 9.80° (c3, c14, c13, n2): 2.95° (s1, c17, n1, h4): -0.96° (h5, n2, c17, s1): -8.32° (c10, c5, c1, c3): 2.20° (h7, c14, c3, c1): 26.71° (h7, c14, c13, c12): 1.82° (o1, c12, c13, c14): 1.75° (o1, c12, n1, h4): -0.61° (o1, c12, n1, h4): -0.66° (c6, c3, c14, c13): 28.93° (h14, c6, c3, c14): 0.86° (h5, n2, c13, c14): 7.69° (c3, c14, c13, n2): 5.77° (s1, c17, n1, h4): -0.66° (h5, n2, c17, s1): -7.77° (c11, c4, c0, c2): -0.82° (c11, c4, c7, h15): 0.93° (c15, c9, c8, h16): -1.75° (c15, c9, c7, h15): 0.74° (c10, c5, c8, h16): -1.53° (c10, c5, c1, c3): 2.20° c11, c5, c1, c3): 0.85° (h7, c14, c3, c1): 24.08° (h7, c14, c13, c12): 2.29° (o1, c12, c13, c14): 1.51° (o1, c12, n1, h4): -0.65° (h4, n1, c17, s1): -0.76° (c6, c3, c14, c13): 25.79° (h14, c6, c3, c14): 1.49° (h5, n2, c13, c14): 8.79° (c3, c14, c13, n2): 5.61° (s1, c17, n1, h4): -0.76° (h5, n2, c17, s1): -8.38° (c10, c2, c6, h14): -3.39° (c10, c2, c0, c4): -0.13° (c11, c5, c1, c3): 0.85° (c11, c5, c8, h16): 0.69° (c15, c7, c9, h17): -0.21° (c15, c7, c4, h12): 0.96° in table 3 are given the results of quantum chemical calculations for a) valuable predicted molecular properties and b) descriptors for l1 – l3 compounds, which provide information to conduct quantitative structure-property relationships (qspr) for z isomers. from table 3 it can be observed that area and volume for investigated compounds increase in the order: l1< l2 < l3, as expected, correlated with the molecular weight and presence of methyl and isopropyl substituents. the ovality index shows the same trend, suggesting the deviation of molecules from the spherical shape, considering the minimum surface for the spherical shape. this parameter is related to molecular surface area and van der waals volume and increases with the linearity of the structures in the same order as above-mentioned descriptors (area, volume). the polarizability increases in the order: l1 < l2 < l3, showing increased induction (polarization) interactions, resulting from an ion or a dipole inducing a temporary dipole in an adjacent molecule. table 3. predicted molecular properties and descriptors for l1-l3 compounds (z isomers). compound l1 l2 l3 a) molecular properties formula c14h10n2os c17h16n2os c19h20n2os energy, a.u. -1122.4149 1240.3606 1318.9892 energy (aq), a.u. 1122.4367 -1240.3835 1319.0106 solvatation energy, kj/mol 57.22 60.24 56.12 dipole moment, debye 5.86 8.24 8.26 b) qspr descriptors from cpk model area, å2 262.59 315.36 354.76 volume, å3 247.94 301.04 338.02 polar surface area, å2 36.350 37.106 37.037 ovality 1.38 1.45 1.51 log p 0.34 0.87 1.62 hbd count 2 2 2 hba count 4 4 4 polarizability, 10-30 m3 60.80 65.11 68.15 the same tendency for the dipole moment was observed, as expected. hydrogen bond donor (hbd) and acceptor (hba) counts have the same values for all structures under study, due to the j. electrochem. sci. eng. 8(1) (2018) 73-85 azulene ligands for heavy metal ions detection 78 presence of the same thioxoimidazolidine cycle iii, containing the same donors (nitrogen) and acceptors (sulphur and oxygen). the octanol-water partition coefficient (log p) and polar surface area (psa) are also evaluated and listed in table 3. log p values are positive, showing their good affinity for hydrophobic phases, and increase in the order: l1 < l2 < l3. l1 had also the smallest psa (polar surface area) value reinforcing the trend of log p; psa values for l2 and l3 are quite similar. homo – lumo analysis the molecular frontier orbitals (the highest occupied molecular orbital – homo and the lowest unoccupied molecular orbital – lumo) energies of the studied compounds (in z form) have been calculated (table 4) and their density distributions are given in the bottom (homo) and in the top (lumo) of fig. 2, respectively. analysis of fig. 2 shows the distribution of homo orbitals for all studied compounds is delocalized as follows: over atoms c7 and c8 from cycle i, over bonds c1-c3 and c0-c2 from cycle ii, over c13-c14 bond (between cycle ii and cycle iii) and over heteroatoms n2, o1 and s1 form cycle iii. for all compounds, lumo orbitals (fig. 2) are entirely delocalized over cycle i and cycle ii, while for l2 structure, they are distributed supplementary over functional groups c=o (o1), c=s (s1) and c12-c13 bond from cycle iii. the other occupied orbitals energy levels are listed in table 5, and they can be related with uv vis allowed transitions. l1 l2 l3 figure 2. homo (down) and lumo (up) molecular frontier orbitals and their energy gap for l1 l3. homo and lumo energies from table 4 have been examined in order to find the tendency of each ligand to donate electrons (to empty molecular orbitals with low energy of convenient molecules), or to accept electrons, respectively. the resulting band gap e (ehomo elumo) can be used to provide useful information on the chemical reactivity and kinetic stability of each ligand [13,14]. the band gap between the homo and lumo orbitals energy has been found to increase in the order: l3 < l2 < l1 (table 4). consequently, according the obtained values for e, l1 presents the lowest reactivity, followed by l2 and l3 (the most reactive). starting from homo and lumo energy diagram (fig. 2), other related descriptors have been calculated: ionization potential (i), electron affinity (a), electronegativity (χ), chemical hardness (η), global softness (σ) [15,16], chemical potential (µ), global electrophilicity index (ω), and also ionization potential (defined as i = ehomo) and electron affinity (defined as a = elumo), by applying koopmans’ theorem [17,18]. derived calculated quantum chemical parameters for the most stable conformer (z) of each studied compound are listed in table 4. e=2.91 ev e=2.74 ev e=2.87 ev a. stefaniu et alj. electrochem. sci. eng. 8(1) (2018) 73-85 doi:10.5599/jese.475 79 the parameters from table 4 are important global chemical reactivity descriptors of the studied molecules in terms of their reactivity and site selectivity. l1 has the highest ionization potential and electron affinity, which reflects his superior capability to interact with heavy metal cations and to accept one electron from a donor. in terms of chemical hardness, large e signifies hard molecules and small e refers to soft molecules. in this context, l1 presents the hardest structure. the values obtained for the electrophilicity index (ω) are not in the same order. it indicates an unexpected global electrophilic nature of these molecules, as measure of energy lowering due to maximal electron flow between donor and acceptor [19]. table 4. calculated quantum chemical parameters of the studied compounds table 5. predicted molecular orbital values (ev) for l1 l3 parameter\ligand l1 l2 l3 orbital ligand l1 l2 l3 ehomo / ev -5.41 5.19 5.08 homo -5.4 -5.2 -5.1 elumo / ev -2.50 2.32 -2.34 homo{-1} -6.1 -5.9 -5.9 e (ehomo-elumo) / ev 2.91 2.87 2.74 homo{-2} -6.5 -6.3 -6.3 i = ehomo / ev 5.41 5.19 5.08 homo{-3} -6.9 -6.6 -6.7 a = elumo / ev 2.50 2.32 2.34 homo{-4} -7.4 -7.2 -7.2 χ = ((i + a)/2) / ev 3.95 3.76 3.71 homo{-5} -7.6 -7.5 -7.4 η = ((i a)/2) / ev 1.45 1.44 1.37 homo{-6} -8.7 -8.0 -8.2 σ = l / η 0.68 0.70 0.73 homo{-7} -9.2 -8.9 -8.9  = (ehomo+elumo)/2 -3.95 -3.75 -3.71 homo{-8} -9.5 -9.1 -9.2 ω = 2 / 2 η 5.37 4.91 5.02 homo{-9} -9.7 -9.4 -9.2 lumo -2.5 -5.2 -2.3 lumo{+1} -2.3 -5.9 -2.2 molecular electrostatic potential and local ionization potential molecular electrostatic potential (mep) is a valuable descriptor for overall molecular charge distribution and for the way of electrophilic addition assessment [20,21]. graphical quantities resulted from quantum chemical calculations for mep and local ionization potential (lip) are given in fig. 3. these diagrams provide a visual representation of the chemically active site and comparative reactivity of atoms. figure 3. predicted molecular electrostatic potential (mep) and local ionization potential (lip) maps for l1-l3 mep for l1. mep for l2. mep for l3. lip for l1. lip for l2. lip for l3. j. electrochem. sci. eng. 8(1) (2018) 73-85 azulene ligands for heavy metal ions detection 80 in mep graphical representation from fig. 3, the red surface ( system) indicates a negative potential, suggesting that this region is attracted to a positive charge; the blue surface ( system) indicates a positive potential, thus, this region is attracted to negative charges. potential increases in the order: red < orange < yellow < green < blue. for all the studied compounds, the negative area is mainly localized over the oxygen (red region) and less on sulphur atoms (orange region). the maximum negative values (over sulphur oxygen) have been read and they vary as follow: -203 kj/mol to -154 kj/mol (for l1); -168 kj/mol to -151 kj/mol (for l2); -171 kj/mol to -153 kj/mol (for l3). the maximum positive regions (blue) are localized on the –nh groups from cycle iii, assigned to atoms h4 and h5, respectively, and vary between 154 and 204 kj/mol for all studied structures. for our further applications (complexation of heavy metals) the most interesting are orange and red regions (negative potential), which are most likely to be involved in the complexation event. regarding lip maps from fig. 3, the red regions suggest areas where electrophilic attack could happen, thus, atoms from which electron removal (ionization) is easy. by convention, in blue color are illustrated the regions where ionization is relatively difficult to occur. orange regions (the more susceptible to ionization) present close values between 6.9 and 7.02 ev for all studied compounds, so the results of the complexation event for similarly modified electrodes should be quite similar. thermodynamic properties thermodynamic properties such as zero-point vibrational energy, enthalpy, constant volume heat capacity, entropy and gibbs energy have been calculated with dft method for equilibrium geometry at ground state for z and e conformers, over a temperature range between 273.15 and 373.15 k. except for the zero-point energy (zpe), all depend on temperature. their calculated values are listed in tables 6 for 298.15 k. the values of enthalpy, constant volume heat capacity (cv), entropy (s) and free gibbs energy (g), increase with temperature, in agreement with the enhancement of the molecular vibrational intensities as the temperature rises [22]. obtained thermodynamic functions are given in table 7 and they are useful to estimate directions of further chemical reactions, by computing those to other thermodynamic functions and relationships. table 6. predicted thermodynamic properties at 298.15 k for z and e isomers of l1 –l3 parameter\ligand l1 l2 l3 zpe / kj mol-1 z e 554.47 554.41 775.17 774.98 924.20 925.53 h / a.u. z e -1122.1904 -1122.1918 -1240.0481 -1240.0423 -1318.6177 -1318.6127 cv / j mol -1 z e 182.76 182.11 228.52 228.27 256.73 256.17 s / j mol-1 z e 457.89 459.87 525.36 525.55 563.88 563.54 g / a.u. z e -1122.2424 -1122.2441 -1240.1078 -1240.1025 -1318.6818 -1318.6767 the dependence between calculated thermodynamic properties (enthalpy, constant volume heat capacity, entropy and gibbs energy) and temperature were fitted by quadratic equations and the correlation fitting factors (r2) vary between 0.9951 and 1. the obtained values for r2 show very good correlation of thermodynamic parameters and their fitting equations (table 7). the observed predicted and experimental values for chemical shifts from 13c and 1h nmr given in the supporting information, show good correlation for z conformer of all compounds. it leads to the assumption that z conformers are formed by synthesis, as assumed from the experimental procedure [5]. a. stefaniu et alj. electrochem. sci. eng. 8(1) (2018) 73-85 doi:10.5599/jese.475 81 table 7. fitted thermodynamic parameters for l1 –l3 function\ ligand l1 l2 l3 h / a.u. 10-7·t2+7·10-5·t-1122.2 (r2 = 0.9994) 9·10-8·t2+10-4·t-1240.1 (r2 = 0.9956) 2·10-7·t2+2·10-5·t-1318.6 (r2 = 1.0000) cv / j mol -1 -6·10-4·t2+0.5889·t+168.45 (r2 = 1) -5·104·t2+0.6806·t+211.89 (r2 = 1) 9·10-6·t2+0.7177·t+238.51 (r2 = 0.9955) s / j mol-1 67·10-4·t2-3.6019·t+937.43 (r2 = 0.9890) -104·t2+0.9517·t+274.33 (r2 = 1) -104·t2+0.9081·t+303.81 (r2 = 1) g / a.u. 3·10-8·t2-10-4·t-1122.2 (r2 = 0.9978) -9·10-8·t2-2·10-4·t-1240.1 (r2 = 0.9988) -2·10-7·t2-2·10-4·t-1318.7 (r2 = 0.9951) *all thermodynamic calculations were performed in gas phase. electrochemical characterization of compounds the electrochemical characterization of l1 l3 compounds has been performed and the redox potentials were evaluated from the electrochemical curves of l1 l3 azulene derivatives recorded on glassy carbon electrode by: cyclic voltammetry (cv), differential pulse voltammetry (dpv), and rotating disk electrode voltammetry (rde) in 0.1 m tbap, ch3cn. selected anodic and cathodic cv, dpv, and rde curves recorded in millimolar solutions (0 – 2 mm) of l1 l3 in 0.1m tbap in ch3cn have been shown in figs. 4-6. 0 200 0 40 -3 -2 -1 0 1 2 0 200 i /  a rde dpv 0.75 mm 0 mm cv e / v vs. fc/fc + c1 a1 1000 rpm l1 figure 4. cv, dpv, rde (1000 rpm) curves for l1 (0.75 mm) in 0.1 m tbap, ch3cn. 0 100 0 20 -3 -2 -1 0 1 2 -80 0 1 mm 0 mm c1 a1 rde dpv cv i /  a e / v vs. fc/fc + 1000 rpm l2 figure 5. cv, dpv, rde (1000 rpm) curves for l2 (1 mm) in 0.1 m tbap, ch3cn. j. electrochem. sci. eng. 8(1) (2018) 73-85 azulene ligands for heavy metal ions detection 82 0 100 0 20 -3 -2 -1 0 1 2 3 -200 0 1 mm 0 mm rde dpv cv a1 c1 e / v vs. fc/fc + i /  a 1000 rpm l3 figure 6. cv, dpv, rde (1000 rpm) curves for l3 (1 mm) in 0.1 m tbap, ch3cn. when comparing the curves obtained for these compounds, the following conclusions connected to the electrochemical behavior of l1, l2 and l3 can be drawn: • their oxidation potentials for the first oxidation peak (in v) are, respectively, 0.347, 0.297, and 0.266, being ranged in the order: l3 < l2 < l1, in correlation with homo energies (table 4). this fact is in concordance with the presence of the electron releasing alkyl groups on azulene moiety in case of l3 and l2. the presence of larger and bulky substituents conducts to an easier oxidation of the ligand. • their reduction potentials for the first reduction peak (in v) are, respectively, -1.598, -1.760, and -1.772, being ranged in the order: l3 < l2 < l1, in correlation with lumo energies (table 4). the small differences between the calculated lumo values and the experimental values of the reduction potentials could be explained by a strong interaction of the ligand with the solvent, which is neglected in quantum calculus. these values also are also connected to the presence of alkyl groups on the azulene structure. • all the electrochemical processes in figs. 4-6 are irreversible. while the vinylazulene moiety is more prone to oxidation, the vinylhydantoin part is more likely to be reduced. • all compounds seem to accumulate on the electrode surface (leading to polymer films) at more positive potentials then the first anodic peak (denoted a1 on each dpv curve), in agreement with the sharp decrease of the oxidation currents in rde and dpv curves after reaching this potential. electrochemical recognition using modified electrodes the compounds l1 l3 were used to prepare modified electrodes by electrochemical polymerization in similar conditions, as mentioned in the experimental part. the recognition experiments performed with l1, l2, and l3 modified electrodes lead to evaluate the detection limits for pb(ii), cd(ii), cu(ii) and hg(ii) ions. in fig. 7a are given the stripping curves obtained using poly l1 modified electrodes for the recognition of different concentrations of pb(ii), cd(ii), cu(ii) and hg(ii) ions, while in fig. 7b are given the dependences of the stripping peak currents on concentration for the metal ions, as resulted from fig. 7a, for the most efficient recognition events that has been found with this modified electrode for cd(ii) and pb(ii). a. stefaniu et alj. electrochem. sci. eng. 8(1) (2018) 73-85 doi:10.5599/jese.475 83 a b figure 7. stripping curves (a) using l1 modified electrodes for different concentrations of cd(ii), pb(ii), cu(ii) and hg(ii) ions and the corresponding calibration curves (b) for cd(ii) and pb(ii) ions. similar curves have been obtained for the modified electrodes prepared with l2 and l3 ligands, in similar conditions. the detection limits have been estimated for the electrodes modified with l1, l2 and l3 (table 8). these values show that the best ligand is l1, as by using this ligand, the lowest limits for all tested heavy metals have been obtained. table 8. estimated detection limits of investigated heavy metal ions for l1 –l3. heavy metal ion / ligand l1 l2 l3 pb(ii) 10-8 10-6 10-6 cd(ii) 10-8 10-6 10-6 cu(ii) 10-5 10-4 10-4 hg(ii) 10-5 10-4 10-4 a correlation between ligand molecular descriptors and the obtained detection limits is evident. the best behavior of l1 can be explained by the fact that l1 presents the smallest values for homo and lumo energies, and consequently, the highest ionization potential (5.41 ev) and the highest capability to accept electrons (electron affinity, a = 2.50 ev) among the three studied structures (table 4). also, judging by the molecular electrostatic potential, l1 structure shows the most negative potential over the oxygen atom, with the most negative value of -203 kj/mol, so it is the most capable to attract positive heavy metal ions charges, as previously discussed for fig. 3. the close values of detection limits for l2 and l3 are explained by the similar effect of substituents in l2 and l3, seen in their close values of electron affinity (table 4). consequently, their complexing capacity is similar, fact which is in a good agreement with the detection limits obtained for heavy metal ions using these ligands. conclusions chemical reactivity descriptors were predicted from the dft calculations. from molecular frontier orbitals energy level diagrams and gaps, it can be concluded that among all studied structures, l1 is the most stable compound. the oxidation potentials were found to be ranged in accordance with calculated homo energies trend, while the variation of reduction potentials versus lumo energies were perturbed by the occurrence of steric effects. thermodynamic analysis reveals that all thermodynamic calculated parameters, except the zeropoint energy (zpe), are increasing with temperature. the chosen fitting equations for thermo 0 5 10 0 5 cd pb i /  a c / mol l -1 cd pb j. electrochem. sci. eng. 8(1) (2018) 73-85 azulene ligands for heavy metal ions detection 84 dynamic properties are in a good agreement with calculated data. the nmr experimental and theoretical results supporting each other, being consistently supplemented by computed molecular descriptors and thermodynamic properties. the obtained characteristics are in a good agreement with the detection limits obtained for heavy metals using modified electrodes obtained from these ligands. the corresponding detection limits evaluated for pb, cd, cu and hg are of about 10-8 for pb and cd with l1 and of about 10-5 for cu and hg. these limits are larger for l2 and l3, which are very close due to the similarity of their structures. they are of about 10-6 m for pb and cd, and 10-4 m for cu and hg. this study contributes to the prediction of specific structures able to coordinate heavy metal ions in view of recognition. acknowledgements: the authors from “politehnica” university of bucharest are grateful for the financial support from: executive unit for financing education higher, research development and innovation (uefiscdi) project id pn-ii-ru-te-2014-4-0594 contract no. 10/2015, pn-ii-pt-pcca2013-4-2151 contract no. 236/2014 and romania–china bilateral project 68bm/2016 (ch 41.16.04); the authors from national institute for chemical pharmaceutical research and development – bucharest, romania, gratefully acknowledge financial support from project poc id p_40_406, smis 105542. references [1] e. m. ungureanu, a. c. razus, l. birzan, m.s. cretu, g.o. buica, electrochimica acta 53 (2008) 70897099. 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[22] k. chandrasekaran, r. t. kuma, spectrochimica acta, part a: molecular and biomolecular spectroscopy 150 (2015) 974-991. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {parametric modeling of microbial fuel cells} http://dx.doi.org/10.5599/jese.671 311 j. electrochem. sci. eng. 9(4) (2019) 311-323; http://dx.doi.org/10.5599/jese671 open access: issn 1847-9286 www.jese-online.org original scientific paper parametric modeling of microbial fuel cells amandeep singh and balaji krishnamurthy department of chemical engineering, bits pilani, hyderabad 500078, india corresponding author: balaji.krishb1@gmail.com received: february 27, 2019; revised: march 27, 2019; accepted: april 2, 2019 abstract microbial fuel cells use bacteria to generate electrical energy and are used for lower power density applications. this paper studies the effect of operational parameters on the performance of a microbial fuel cell. the effect of length of the anode compartment, inlet acetate concentration, acetate flow rate, temperature, thickness of the membrane and bio-film conductivity on the performance of the fuel cell is modeled. the thickness of the membrane is found to play a very limiting role in affecting the performance of the fuel cell. however, the length of the anode compartment, acetate flow rate and bio-film conductivity are found to play a significant role in the performance of the fuel cell. model results are compared with experimental data and found to compare well. keywords microbial fuel cell; hydrogen; bio-film; acetate; carbon dioxide; model introduction microbial fuel cells (mfcs) are multiphase systems involving biological and electrochemical processes to generate energy. these fuel cells convert chemical energy to electrical energy by the action of microorganisms. most mfc’s are constructed using a bio-anode, bio-cathode and a membrane separating two compartments. mfcs can be divided into two categories; mediated and unmediated. a mediated mfc uses a chemical (mediator) that transfers electrons from the bacteria in the cell to the anode. unmediated mfc uses electrochemically active bacteria to transfer electrons directly to the anode. mfcs have started to find their usage in wastewater treatment. modeling the effect of operational and design parameters on the performance of mfcs is critical for enhancing the performance of these fuel cells. the mfc performance expressed in terms of the polarization curve is affected by several factors, including rates of fuel oxidation, resistance of the circuit, size of the anode compartment, transport of protons through the membrane to the cathode, membrane thickness (resistance), oxygen supply, and the reduction reactions at the cathode. mathematical models help in studying the effect of http://dx.doi.org/10.5599/jese.671 http://dx.doi.org/10.5599/jese671 http://www.jese-online.org/ mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 9(4) (2019) 311-323 modeling of a microbial fuel cells 312 these operational parameters on the performance of mfc. quite a few models exist in the literature which studied the effect of operational parameters on the performance of mfc. the models which were developed to study mfc behavior can be primarily divided to anode based models and full cell models (which are focusing on both the anode and cathode). anode based models zhang and halme [1] have developed one-dimensional anode model based on double chamber configuration. picioreanu and scott [2] have developed a two-dimensional model to understand the anode microbial activity. the authors studied the relation between the growing bio-film thickness and the current output. marcus et al. [3] have proposed one-dimensional dynamic conduction based model to study the performance of mfc. the model postulates that conductivity of the bio-film is the major factor in a mfc performance. in both above references, the authors postulated that anode is the limiting factor in a mfc performance. merkey and choop [4] created a two-dimensional model to study the relationship between power output and the geometry of the cell. the model simulates the effect of bristle anodes and the way they should be placed in a grid and act on the energy output. sedaqatvand and mardanpoura [5] extended marcus’s model with a genetic algorithm which sought to minimize the difference between simulated and experimental data. jayasinghe and madhavan [6] simulated an mfc anode using a respiring bio-film. while the active bio-film invests the energy arising from the oxidation of the substrate, the respiring zone is in charge of energy conservation at the anode surface. pinto et al. [7] have modeled the anode of a mfc using different degradation mechanisms for decomposition of acetate substrate. models focused both on anode and cathode oliviera et al. [8] have developed 1-d mathematical model to study the temperature variation in a mfc. the authors also studied the effect of bio-film thickness on the mfc performance. cheng et al. [9] have developed a mathematical model to simulate the power loss and the potential drop caused by ohmic resistance of the carbon mesh. zeng et al. [10] have developed a mathematical model for a two-chamber mfc which studies the dynamic behavior of mfc. the authors suggested that the cathodic reaction is the most significant limiting factor in mfcs. cai and liu [11] have studied effects of bio-film porosity and external resistance on the performance of a mfc. the authors postulated that at the startup phase, bio-film porosity plays a crucial role in electricity generation. however, at the steady phase, bio-film porosity plays no role in electricity generation. mardanpour et al. [12] have studied the performance of a mfc as a function of bacterial transport parameters, bacteria activity, substrate variation, etc. the authors postulated that as the hydraulic diameter of the microchannel decreases, the power generation of the microfluidic mfc increases. ou and mench [13] have studied the effect of species diffusion and electrical migration on the performance of a mfc. the authors postulated that the electric field migration has a minimal impact on the performance of mfc. sobieszuk et al. [14] have studied influence of operational parameters on the mfc performance. the authors have shown that the hydraulic retention time is the most important parameter affecting the effectiveness of mfcs. yao and li [15] have developed twodimensional model to study the mfc performance. the authors postulated that the electrical conductivity of the bio-film plays an important role in the mfc performance. xia at al. [16] wrote a comprehensive review on mfcs. the authors qualified various models describing mfcs as the mechanism based models, bulk liquid models, electrochemical models, bio-film models, electrical models and application based models. gadkari and sadhukhan [17] presented a review on the mathematical models describing bio-electrochemical systems. along with reviewing the a singh and b. krishnamurthy j. electrochem. sci. eng. 9(4) (2019) 311-323 http://dx.doi.org/10.5599/jese.671 313 conventional models for a study of bio-electrochemical systems, the authors have also reviewed new models such as microbial electrolysis cell, microbial electro-synthesis and microbial desalination cell. capadaglio et al. [18] developed a mathematical model to study the application of mfc in wastewater treatment. the authors have reviewed the reactor configuration, electrochemical and microbiological characterization and operational schemes in the simulation of the mfc usage in wastewater treatment. while the above mentioned models already reported in the currently available and mfc related literature, give an overall perspective about the modeling work, we are primarily focused on developing of an anode model to study the effect of design and operational parameters on the performance of a mfc. the focus of this paper is to study the effect of operational parameters on the performance of the mfc seen through a polarization curve (cell voltage vs. current density) properties. the decrease in the fuel cell voltage is due to over-potential losses due to activation, ohmic losses and concentration polarization. concentration polarization losses occur at high current densities due to mass transport limitations. ohmic losses are primarily seen in the middle of the current density range. these are losses due to the resistance of the membrane, bio-film, etc. activation losses are usually seen at the beginning of the polarization curve where over-potential losses take place to enable the reaction to happen. therefore, the objective of this paper is to study the effect of operational parameters of mfc on polarization curves and thereby on the fuel cell performance. model development mfc can be considered as a reactor which transfers the chemical energy stored in an organic substrate into electricity due to the activity of enzymes/organisms. a schematic representation of the mfc is shown in figure 1 (marks are explained within nomenclature section). the fuel cell consists of an anode chamber with an electron current collector, a polymer electrolyte membrane and a cathode chamber with an electron current collector. the anolyte fuel, acetate reacts at the anode side with the bacteria to produce carbon dioxide and hydrogen ions. the carbon dioxide moves to the anode exit, while hydrogen ions diffuse across the membrane to the cathode. on the cathode side, the oxygen ions react with hydrogen to form water which is removed from the cathode. figure 1. schematic of a microbial fuel cell (mfc) http://dx.doi.org/10.5599/jese.671 j. electrochem. sci. eng. 9(4) (2019) 311-323 modeling of a microbial fuel cells 314 the transfer of electrons to the anode occurs by two mechanisms; a mediated electron transfermet and direct electron transfer-det. the anode and cathode reactions containing acetate as the substrate [1] are given by: (ch2o)2 + 2h2o → 2co2 + 8h+ + 8e (1) o2 + 4h+ + 4e-→ 2h2o (2) the overall reaction is given by: (ch2o)2 + o2 → 2co2 + 2h2o (3) assumptions 1. kinetics of the reactions on the anode is represented by tafel and monod equations. the monod equation is typically given by: max s s k s  = + (4) where µ is the specific growth rate of the microorganisms, µmax is the maximum specific growth rate of the microorganisms, s is the concentration of the limiting substrate for growth and ks is the half velocity constant. the monod equation describes substrate oxidation and microorganism growth. the units of µ, s and ks are time-1, (mol/volume) and (mass/volume), respectively. 2. the oxygen reduction reaction at the cathode is governed by tafel equation given by: eq 0 ln (1 ) rt i e e nf i − = − (5) where i and io are the current and exchange current densities for the oxygen reduction reaction. 3. the thickness of the bio-film is considered to be constant during simulations. this means that the overall rate of microbial growth through substrate utilization and the overall rate of bio mass loss are in equilibrium. 4. the transport of reactant species through the film is considered as one dimensional. 5. the effect of convection in the transport of reactant species in the bio-film and to the electrodes is neglected. the transport of reactant species in the bio-film and to electrodes is assumed to be diffusion, what means that fick’s diffusion laws can be used to evaluate the transport of these species. 6. only hydrogen ions are considered to be transported across the membrane. carbon dioxide, acetate, oxygen and water are assumed not transported across the membrane. 7. anode and cathode compartments are modeled as a continuous stirred tank reactor−cstr. this means that mass transport equations are primarily developed from cstr equations. the mass balance at the anode takes into account rates of reaction in the anode compartment, bio-film and at the electrode. 8. the carbon dioxide is considered to be dissolved in solution. hence the effect of co2 in the gas phase is considered negligible. model equations given that the anode compartment is modeled as cstr, the transient mass balance equation is given by: in out rxn d d m m m m t = − + (6) a singh and b. krishnamurthy j. electrochem. sci. eng. 9(4) (2019) 311-323 http://dx.doi.org/10.5599/jese.671 315 in eq. (4), mrxn represents the mass of reacting species, while min and mout represent the mass of species entering and leaving the system. the mass balance of acetate is given by [10]: 0 aca a a aac d ( ) d c q c c r t v = − − (7) the reaction rate of acetate oxidation (degradation) in the anode compartment defined by eq. (1) is given by: ac a a a a xac a exp a f c r k c rt k c      =    +   (8) in eq. (6), k represents the rate constant, caac represents the concentration of the acetate in the anode compartment and cx represents the concentration of the bio-mass in the anode compartment. a represents the anodic over-potential and ka represents the half velocity rate constant for acetate. the transient equations for the acetate mass balance in the anode compartment is obtained by inserting eq. (6) into eq. (5): ac 0 ac a a a a a xac ac a ( ) expa a dc f cq c c k c dt v rt k c      = − −    +   (9) the first term on the right hand side represents the mass flux of acetate in the anode compartment and the second term represents the reaction rate of acetate in the anode compartment. the reaction of acetate to form co2 is a bio-electrochemical reaction; the electrochemical part of the reaction is dealt with tafel equation and the bio-chemical reaction is dealt with monod equation. the acetate flux in the bio-film layer can be written as: b ab a a d d c n d x = − (10) eq. (8) assumes that the transport of acetate ions in the bio-film is a result of diffusive flux. the acetate ions are not a charged species (effects of potential are neglected) and the convective flux is neglected. the transient differential mass balance equation for any species is given by: a a d d c n r t x  = − −  (11) where the right side part represents the flux term and the reaction term. following eq. (9), the transient equation for acetate mass balance in the bio-film is given by: 2 ac bab ab a a a a x2 ac a a d d exp d d c c f c d k c t x rt k c      = −    +   (12) transport equations for the cathode the oxygen mass balance in the cathode compartment is given by [1]: ( )2 20 2 22 2cf o occ o occ 2o d d d d c cq c c d t a x = − + (13) the equation for the cell potential is given by [1]: vcell = ecell -ηa -ηc icellrcell (14) http://dx.doi.org/10.5599/jese.671 j. electrochem. sci. eng. 9(4) (2019) 311-323 modeling of a microbial fuel cells 316 in eq. (12), ηa and ηc are anode and cathode over-potentials and the last term refers to the ohmic loss due to the membrane resistance. the membrane resistance rcell [8] is given by:  = m cell t r (15) in eq. (13), tm is the membrane thickness and σ is the ionic conductivity of the membrane. the concentration of the acetate at the anode electrode/membrane interphase and the concentration of the oxygen at the cathode electrode/membrane interphase are assumed to be zero. numerical solution for analysis of different parameters, it is necessary to choose a method that is efficient and provides high convergence. the partial derivatives terms were transformed into algebraic terms using central difference and transient pde were converted into transient ode. they were then solved using matlab function ode15s. simple 1-d mesh was used and it was made enough fine that convergence is achieved. optimal node number was calculated on the basis of order of length involved by trial and error. results and discussion anode compartment length figure 2 shows a time variation of the cell current density for different anode compartment lengths varying from 0.1 to 0.025 m. it is seen that the cell current density increases with increasing length of the anode compartment (if the area is kept constant, this indicates an increasing volume of the anode compartment). for a given constant feed, figure 2 indicates that the length of the anode compartment plays a key role in the performance of the mfc. figure 2. time variation of current density for different lengths of anode compartment a higher length of the anode compartment indicates a higher reactor volume which drives the reaction rate. this decreases the activation over-potential, what is seen through the highest current density observed when the anode compartment thickness is 0.1 m. however, a large reactor volume also ensures that the concentration polarization is lower (at high current densities). this implies that when the anode compartment length is higher, presence of the excess reactant ensures that current a singh and b. krishnamurthy j. electrochem. sci. eng. 9(4) (2019) 311-323 http://dx.doi.org/10.5599/jese.671 317 can be drawn at extended time values. lorant et al. [19] have studied the effect of the cathode to anode surface ratio on the performance of mfc. the authors predicted that with increasing cathode/anode surface area, the power density of a mfc increases, indicating that the oxygen reduction reaction is the rate limiting step in a mfc. very little work has been done on understanding the effect of the anode surface area on the overall performance of a mfc. however, our results indicate that the anode surface area plays a significant role in the mfc performance. table 1 indicates the list of parameters used for simulation. table 1. table of model parameters taken from [8,10] for simulating graphs in figures 2-9 qaf 2.25×10-5 m3 h-1 vac, vcc 5.5×10-5 m3 ca0 1.56 mol m-3 k 0.207 mol m-3 h-1 αa 0.051 t0,tc 303 k taf, tcf 303 k ka 0.592 mol m-3 cx 0.4 mol m-3 as 5×10-4 m2 co20 (0.21×p)/(r×t) mol m-3 qc 1.11×10-3 m3 h-1 αc 0.44 ecell 0.77 v σ 3.6 s m-1 tm 1.778×10-4 m membrane film thickness figures 3 and 4 show polarization curves of mfc for different membrane film thicknesses. it can be seen from figure 3 that the membrane film thickness makes a very small impact on the performance of the fuel cell. figure 3. effect of membrane thickness on mfc performance figure 4 shows the effect of the membrane film thickness in the high current density area. it is seen that the membrane film thickness (ohmic resistance) plays a marginal role in the high current density region. the relationship between membrane resistance and membrane thickness is given by http://dx.doi.org/10.5599/jese.671 j. electrochem. sci. eng. 9(4) (2019) 311-323 modeling of a microbial fuel cells 318 eq. (13), which shows that the ohmic resistance due to the membrane is defined as the ratio of the thickness of the membrane to the ionic conductivity of the membrane. figures 3 and 4 indicate that if the ionic conductivity of the membrane is kept constant, the performance of the microbial fuel cell has little dependence on the thickness of the membrane. consequently, keeping the thickness of the membrane constant, varying the ionic conductivity of the membrane has little effect on the performance of the fuel cell. this indicates that the transport of hydrogen ions through the membrane is more dependent on other factors like concentration flux of hydrogen ions, potential gradient across the cell and the diffusion coefficient of hydrogen ions. figure 4. effect of membrane thickness on mfc performance bio-film conductivity figure 5 shows the effect of bio-film conductivity on the performance of mfc. the figure indicates that the performance of a microbial fuel cell is affected very strongly by the bio-film conductivity. whereas no effect of bio-film conductivity is observed on the activation part of the polarization curve, there seems to be a pronounced effect of bio-film conductivity in the high current density region of the polarization curve. malvankar et al. [20] have suggested that extra cellular electron transfer via conductive bio-film is the most efficient mechanism for high current density mfcs. increased bio-film conductivity lowers the resistance for electron transfer between the bio-film and the anode. it was also suggested that electron donor mass transfer resistance becomes significant at higher bio-film conductivity. the same authors [20] have further stated that the observed metallic like conductivity is associated with a network of microbial nanowires coursing through the bio-film. some experimental studies [21-23] have indicated that the bio-film conductivity does not play a major role in affecting the current density in a microbial fuel cell while other studies [20,24] suggested otherwise. an earlier modeling study [13] has indicated that the bio-film conductivity plays a major role in affecting the fuel cell current density. according to yao et al. [25], the conductivity of the bio-film directly affects a distribution of the solid potential. the authors postulated that the slope of the solid potential of the bio-film decreases with increase in conductivity which causes electrons to move more easily to the electrode surface. on the other hand, the liquid potential is a function of the bio-electrochemical reaction rate and increases with bio-film conductivity. thus, increasing the bio-film conductivity increases the overall rate of the bioelectrochemical reaction, leading to increase in performance of mfc, particularly in high current density regions. a singh and b. krishnamurthy j. electrochem. sci. eng. 9(4) (2019) 311-323 http://dx.doi.org/10.5599/jese.671 319 figure 5. effect of bio-film conductivity on mfc performance effect of temperature figure 6 shows that the performance of mfc is affected strongly by temperature. increasing of temperature not only drives up anodic and cathodic reaction rates, but also increases diffusion of hydrogen ions to the cathode, increasing thus the cathodic reaction rates and leading to significant improvement of mfc performance. figure 6. effect of temperature on mfc performance it seems that temperature shows somewhat higher impact in the lower current density region where the activation over-potential plays a major role, than in the higher current density region of the polarization curve. this has already been expected, since temperature plays a major role in affecting the rate constant and hence the rate of the reaction. behera et al. [26] postulated that the internal resistance of mfc is the highest when the temperature is at 20 °c, than progressively decreases with increasing temperature up to 40 °c, and does not change much after 40 °c. however, our simulation results indicate that the performance of mfc increases with temperature beyond 40 °c. since the effect of temperature affects other factors, including reaction rates at the anode and http://dx.doi.org/10.5599/jese.671 j. electrochem. sci. eng. 9(4) (2019) 311-323 modeling of a microbial fuel cells 320 cathode, it is very difficult to quantify the effect of temperature on the internal resistance of the cell using our simulation model. effect of anode surface area figure 7 shows the variation of current density for different inlet acetate concentrations, while figure 8 shows the variation of current density at constant acetate concentration, but different anode surface areas. combining these two graphs helps us to understand few results. figure 7. effect of anolyte concentration on mfc performance figure 8. effect of anode surface area on mfc performance the inlet acetate concentration plays a significant role in the low current density region. this indicates that the concentration of the acetate plays significant role in lowering the activation overpotential at the anode side (since we are looking at acetate concentration only, the cathodic reaction is not taken into account). figure 8 shows the variation of current density as a function of anode surface area for the given acetate concentration. the resulting graphs indicate that the area of the anode surface plays a major role in driving the current density in the high current density a singh and b. krishnamurthy j. electrochem. sci. eng. 9(4) (2019) 311-323 http://dx.doi.org/10.5599/jese.671 321 region (ohmic and concentration polarization regions). this indicates that a higher anode surface area really enhances the current in the mass transfer limited regions, possibly due to higher reaction rates. depending on the application of the microbial fuel cell, the fuel cell needs to be designed accordingly. very little data is available on the effect of acetate concentration on the performance of mfc. lorant et al. [19] proposed that efficiency of mfc can be fairly high at lower concentrations of acetate (what means that the power will not deteriorate rapidly at lower concentrations). our simulation results indicate a similar effect. comparison of modeling results with experimental data figure 9 shows the comparison of modeling results with experimental data [8,10]. the parameters used for data fitting are summarized in table 1. the following parameters were considered in simulations: temperature, inlet acetate concentration, anode flow rate, cathode flow rate, volume of the anode compartment and membrane thickness. the simulation results seem to capture experimental trends very well. however, a certain discrepancy between the simulation results and the experimental data can be observed. this could be due to certain assumptions made in the model. the assumption of constant bio-film thickness is one factor which could affect simulation results, as well as neglecting the effect of convection on the performance of mfc. although the effect of these assumptions could not be quantified, they could definitely affect the performance of a mfc. figure 9. comparison of simulation results with experimental data conclusion a mathematical model is developed to study effects of operational parameters on the performance of a microbial fuel cell. the effect of thickness of the anode compartment, membrane resistance, bio-film conductivity and inlet concentration of the acetate on the performance of the microbial fuel cell is studied. the membrane thickness and resistance are found to have minimal effect of the mfc performance. however, bio-film conductivity is found to have a significant impact on the polarization curve, particularly in the high current density region. increasing the area of the anode compartment seems to have a significant impact on the polarization curve in the high current region (mass transport limited region). model results are compared with experimental data and found to compare well. http://dx.doi.org/10.5599/jese.671 j. electrochem. sci. eng. 9(4) (2019) 311-323 modeling of a microbial fuel cells 322 nomenclature ac anode compartment ae anode electrode ap anode acrylic plate b bio-film cc cathode compartment ce cathode electrode cp cathode acrylic plate vac volume of the anode compartment, m3 na flux of acetate in the bio-film layer, mol m-2 s-1 dab diffusivity of the acetate in the bio film layer, m2 acc area of the cathode compartment, m2 s-1 ca concentration of acetate in the anode compartment, mol m-3 ca0 concentration of acetate at the entrance to the anode compartment, mol m-3 caac concentration of acetate at the exit of the anode compartment, mol m-3 cab concentration of acetate in the bio-film, mol m-3 caab concentration of acetate at the bio-film/ae interface, mol m-3 co2 concentration of oxygen in the cathode compartment, mol m -3 co2 0 concentration of oxygen at the cp/cc interface, mol m-3 co2 cc concentration of oxygen at the cc/ce interface, mol m-3 cx concentration of bio-mass in the anode compartment, mol m-3 f faraday constant, 94 485, c mol-1 k rate constant of the reaction, mol m-3 h-1. ka half velocity rate constant for acetate, mol m-3 r gas constant, 8.314 j mol-1 k-1 rcell cell resistance due to membrane,  t temperature, kelvin v volume, m3 vcell cell voltage, v t0 wall temperature on anode side, k tc wall temperature on cathode, k q flow rate, m3 h-1 tm membrane thickness, m taf temperature of the anode stream tcf temperature of the cathode stream as surface area of membrane, m2 e applied potential, v eeq standard potential of the redox reaction (v) αa anodic transfer coefficient αc cathodic transfer coefficient σ ionic conductivity of membrane, s m-1 ηa anode overpotential, v ηc cathode overpotential, v acknowledgements: we wish to acknowledge bits pilani, hyderabad for their help in publishing this article. a singh and b. krishnamurthy j. electrochem. sci. eng. 9(4) (2019) 311-323 http://dx.doi.org/10.5599/jese.671 323 references [1] x. c. zhang, a. halme, biotechnology letters 17 (1995) 809-814. 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[26] m. behrera, p.s.jana, m. m. ghangrekar, bioresource technology 101 (2010) 1183-1189. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.671 http://creativecommons.org/licenses/by/4.0/) the effect of process parameters on microstructure and corrosion behavior of aisi 4140 steel modified by pulse plasma treatment https://dx.doi.org/10.5599/jese.1259 923 j. electrochem. sci. eng. 12(5) (2022) 923-935; https://dx.doi.org/10.5599/jese.1259 open access : : issn 1847-9286 www.jese-online.org original scientific paper the effect of process parameters on microstructure and corrosion behavior of aisi 4140 steel modified by pulse plasma treatment yıldız yaralı özbek and ayşe şükran demirkiran sakarya university, engineering faculty, department of metallurgy and materials engineering, esentepe campus, 54187, sakarya, turkey and sakarya university, research, development and application center (sargem), sakarya, turkey corresponding author: yyarali@sakarya.edu.tr; tel +902642955619 received: january 27, 2022; accepted: july 19, 2022; published: september 14, 2022 abstract the pulse plasma system is a surface modification method applied to steel samples. in the present study, the effect of the nozzle distance and pulse number parameters were investigated on the modification process. aisi 4140 steel was preferred as the substrate for modification. in this system, the battery capacity and voltage were constant, and, were selected as 800 mf and 3000 v, respectively. the aisi 4140 steel samples were modified by applying 5, 10, and 15 pulses with 40, 50 and 60 mm nozzle distance. the molybdenum consumable electrode was used during the process. the modified surfaces were examined by optical and scanning electron microscope (sem) and analyzed by x-ray diffractometer (xrd). vickers microhardness on the cross-section surface of the samples was measured under a load of 50 g for 15 s. finally, the specimens were exposed to corrosion in 0.5 m nacl solution. corrosion tests were realized using the potentiodynamic polarization method. a modified layer on the steel was determined to consist of two layers, the compound layer and the diffusion layer. it was observed that the structure and thickness of the modified layer affect by pulse number and nozzle distance. new phases such as fe2n, fen, mon, and γ-fe in the modified layer have occurred. the hardness value of the treated sample has risen about 4-5 times than the untreated, depending on applied process parameters. in general, the pulse plasma treatment has improved the corrosion resistance of treated samples. it was observed that while intergranular corrosion has formed on the unmodified surface, pitting corrosion has appeared on the unmodified surfaces. keywords surface, hardness, coating, molybdenum introduction technological advances compel the use of materials in extremely aggressive conditions such as corrosive atmospheres and/or abrasive wear situations [1,2]. in recent years, a new process for surface modification of materials has emerged based on pulsed power beams [3,4]. https://dx.doi.org/10.5599/jese.1259 https://dx.doi.org/10.5599/jese.1259 http://www.jese-online.org/ mailto:yyarali@sakarya.edu.tr j. electrochem. sci. eng. 12(5) (2022) 923-935 microstructure and corrosion behavior of aisi 4140 steel 924 the pulse plasma process is especially a method used to improve the surface properties of work pieces of tool steels [2-5]. the pulse plasma process is described by the formation of high-gradient temperature fields concentrated in heat. compared with conventional surface treatment techniques, the pulse process has advantages in many ways [5,6]. for example, the surfaces of complex-shaped parts can be easily modified without residual tension stress. also, the pulse treatment is occurred in a very short time and easily. high energy density transferred to the surface by plasma pulse leads to ultra-high-speed heating followed by very fast cooling due to heat evacuation inward the bulk of substrate [7]. the result is a modification of the near-surface layer, i.e., changing its structure and properties [7-9]. the coating is formed by the vapors of inner chamber material as well as by the vapor and microdroplets of cathode material which are the result of high-current discharge inside the chamber. the cathode material dramatically influences the chemical composition and the structure of the coating. the consumable electrode (molybdenum or tungsten) is used to cathode plasma accelerators [6-19]. this situation facilitates the cathode surface melting, which leads to an increase in mass transfer by a micro-droplet fraction. when started by pulse treatment, a very rapid melt and solidification occur on the surface layer of samples. it is found that the pulse plasma process may lead to metastable structure in coating resulting from fast crystallization of plasma transferred material (i.e., “solute trapping” effect [20,21]). also, the high rates of ions can accelerate the formation of microcrystalline surface layers. during process time, the compound zone (white layer) and the diffusion zone on the modified layer occur. high surface residual stresses (compression stress) are generated in the compound zones, resulting from chemical composition gradients, stress fields around precipitates, volume changes, and thermal effects [5,6,8]. the source of alloying elements is a consumable electrode such as mo and w on treatment. these ionized products of electrode and nitrogen gases are used to modify the surface. these ionized products are dissolved or precipitated as iron-nitrides, alloys on the surface. the modified layers exhibit high hardness due to dispersed alloy nitrides and new phases in the matrix. with this development, the mechanical behaviors of the sample can be changed and improved [6,8,11]. depending on pulse plasma parameters, different plasma chemical reactions may occur, leading to a specific phase state of the coating. the process parameters affect the steel surfaces' final structure and mechanical properties. among these parameters, controlling gas diffusion through the nozzle distance, the number of pulses, and the battery capacity are the most critical parameters. the proper combination of these parameters would provide the best surface properties and decrease the processing time as an essential economic factor. the main aim of this work is to examine the microstructure, hardness, and corrosion properties of aisi 4140 steel surface modified by pulse-plasma treatment realized by applying different parameters. experimental preparation of the samples and modification process in the present study, aisi 4140 steel which is very cheap and is widely used in the industry, was selected as the substrate for surface modification. the chemical composition of aisi 4140 steel is presented in table 1. firstly, the specimens were cut out as square bars and subsequently machined and metallographically prepared to obtain the desired shape and surface roughness. the sample surfaces were sanded from 120 to 1200 mesh with sic sandpaper and polished with 0.05 µm alumina solution. the pulse plasma process was applied on clean surfaces prepared. the molybdenum (mo) consumable electrode was used in this treatment. y. yaralı özbek and a. ş. demirkiran j. electrochem. sci. eng. 12(5) (2022) 923-935 https://dx.doi.org/10.5599/jese.1259 925 in the pulse plasma system, seen in figure 1, the 3000 v high voltage and 800 mf capacity of the capacitor bank were selected. the power density of the heat flow is 107 w cm-2. this situation leads to evaporation of the dielectric walls of the discharge chamber and melting and evaporating of the electrodes (mainly the molybdenum). thus, plasma jet formation occurs. the molten cathode material is carried away by the gas stream and, possibly, partially enters the inner surface of the discharge chamber, which leads to its intense heating at the points of contact with the liquid metal. the high-speed shooting shows that when a gas outflows from the chamber within a few milliseconds, ejection of liquid metal drops occurs. following the surface layer's quick heating (105 to 107 k s-1), intensive cooling is ensured by rapidly removing heat from the mass. the parameters used in the pulse plasma process are presented in table 2. table 1. chemical composition of the aisi 4140 steel used as the substrate for surface modification element c si mn p s cr mo fe content, wt.% 0.39 0.30 0.80 0.035 0.015 0.90 0.13 balance figure 1. schematic presentation of the pulsed-plasma modification system table 2. the parameters of pulse plasma treatment and the sample codes parameters sample code 1 2 3 4 5 6 7 8 9 nozzle distance, mm 40 40 40 50 50 50 60 60 60 number of pulses 5 10 15 5 10 15 5 10 15 characterization of the modified surface after the pulse plasma treatments, all specimens were sanded with different mesh number sandpapers, polished with 3 µm diamond paste, and etched using the 2 % nital solution. the cross-section microstructures were examined using an optical microscope. besides, the morphology of pulse plasma-treated samples was observed by sem (jeol-jsm 6060 lv) and analyzed with a rigaku d/max/2200/pc brand device operating with cu-kα (λ = 0.154056 nm) radiation. vickers microhardness tests (shimadzu brand) on the polished cross-section surface of the samples were performed under a load of 50 g (hv0.050) for 15 sec. measurements were made as many times as possible from different parts of the specimen, and the arithmetic mean of the obtained values was taken. corrosion tests of the modified surface the untreated and the samples modified with different process parameters were cleaned in ethanol as ultrasonic, washed with deionized water, and finally dried with compressed air. 0.5 m https://dx.doi.org/10.5599/jese.1259 j. electrochem. sci. eng. 12(5) (2022) 923-935 microstructure and corrosion behavior of aisi 4140 steel 926 nacl (merck company) solution was prepared with distilled water. the specimens were isolated so that only the sample surface area of 0.8023 cm2 was provided to contact with the solution. corrosion tests were realized using the potentiodynamic polarization method. all electrochemical measurements were performed using gamry instruments pcl4/750 potentiostat/galvanostat/zra electrochemical analyzer system. an ag/agcl/kcl electrode was used as a reference electrode, and a graphite electrode was used as a counter electrode in corrosion experiments using a traditional threeelectrode system. the schematic representation of the corrosion cell used is presented in figure 2. all potentiodynamic polarization measurements were carried out at room temperature, a scanning rate of 5 mv s-1, and a scan range from -1 to +1 v. data were analyzed using the gamry echem analyst software package. the corrosion current (icorr), corrosion potential (ecorr), and corrosion rate (rcorr) values were determined with tafel extrapolation, which is a mathematical technique. equation (1) is used to determine the corrosion rate: corr corr i k ew r da = (1) where, rcorr / mm year-1 is the corrosion rate, icorr / µa is the corrosion current, k is a constant (0.00327 mm g year-1 µa-1 cm-1), ew / g is the equivalent weight, a / cm2 is the sample area and d / g cm-3 is the density [10]. all the tests were repeated three times to ensure the accuracy and stability of the experimental data. the mean value and standard deviation of the results were calculated. finally, the corrosion region of the samples was examined by sem (jeol-jsm 6060 lv). figure 2. the schematic representation of the corrosion cell used results and discussion microstructural characterization of the modified surfaces sem micrographs taken from the cross-section of the steel modified by applying 5, 10, and 15 pulses with 60 mm nozzle distance are presented in figure 3a-c. microstructural analyses show that morphological changes on the surface resulting from high power energy and thermal effect occur [8], and pulse plasma application resulted in micrographs having different contrast layers. it is seen that the modified layer consists of two layers, the compound layer and the diffusion layer, the pulse number affected the modification layer, and the thickness of the layer modified increased with the increase in pulse number. for example, the modified layer thickness of the sample subjected to 5 pulses with 60 mm nozzle distance is 34.26 μm, while the modified layer thickness of the sample subjected to 15 pulses with 60 mm nozzle distance was measured as 48.06 μm (figures 4 a and 4b). other researchers have reported similar results [1,2,9,11]. y. yaralı özbek and a. ş. demirkiran j. electrochem. sci. eng. 12(5) (2022) 923-935 https://dx.doi.org/10.5599/jese.1259 927 a b c figure 3. sem micrographs taken from the cross-section of the steel modified by applying (a) 5, (b) 10, and (c) 15 pulses with 60 mm nozzle distance sem micrographs taken from the cross-section of the steel modified by applying 10 pulses with 40, 50, and 60 mm nozzle distances are presented in figure 5a-c. similarly, like pulse number, the nozzle distance also affects the modification layer, and the thickness of the layer modified increases with the increase in nozzle distance. at the same time, the rapid solidification induced by pulse plasma leads to the formation of metastable structures, superfine grains, and fine dendritic structures in the modified layer [4,8,11]. it is known that instability of the surface structure increases with increasing diffusion of atoms, rapid heating and cooling results in the generation of high dislocation densities, fine grain, and defects on the surfaces of the work pieces [7,9,11-13]. the ɛ-nitrides develop on top of the γ1-nitrides and build a layer with thinner grains. figure 6 presents an sem micrograph showing the dendritic structure taken from the cross-section of the steel modified by applying 10 pulses with a 50 mm nozzle distance. some dendrites contain martenzite, which indicates a high cooling rate during solidification. cheng et al. [13] have stated that the interdendrites were characterized by long primary and secondary arms, and the secondary arm spacing was less than 0.5 µm. zhang, guana, and uglov have also reported similar results for the modification layer [13-17]. on the other hand, the fact that no cracks were seen in the micrographs indicates that the severe cooling of the pulse plasma modified specimens did not cause noticeable micro defects. diffusion layer compound layer diffusion layer compound layer diffusion layer compound layer https://dx.doi.org/10.5599/jese.1259 j. electrochem. sci. eng. 12(5) (2022) 923-935 microstructure and corrosion behavior of aisi 4140 steel 928 a b figure 4. optical micrographs taken from the cross-section of the steel modified by applying a 5, and b 15 pulses with 60 mm nozzle distance a b c figure 5. sem micrographs taken from the cross-section of the steel modified by applying 10 pulses with a 40, b 50 and c 60 mm nozzle distances 20 μm 34.26 μm compound layer diffusion layer 10 μm compound layer diffusion layer 48.06 μm diffusion layer compound layer diffusion layer compound layer diffusion layer compound layer y. yaralı özbek and a. ş. demirkiran j. electrochem. sci. eng. 12(5) (2022) 923-935 https://dx.doi.org/10.5599/jese.1259 929 figure 6. sem micrograph showing the dendritic structure taken from the cross-section of the steel modified by applying 10 pulses with a 50 mm nozzle distance phase analyses of the samples xrd analyses were performed to identify the phase content of the unmodified and the modified surfaces. the results obtained are presented in figure 7a-c. a b c figure 7. the xrd patterns of: a the unmodified surface, b the surfaces modified by applying 10 pulses with 50 mm nozzle distance, c the surfaces modified by applying 10 pulses with 50 mm nozzle distances as seen in figure 7a, the unmodified sample consists of only the α-fe phase having different planes. the xrd patterns of the surfaces modified by applying 5, 10, and 15 pulses with 50 mm nozzle distance given in figure 7b show that pulse plasma treatment changes the diffraction patterns of the samples, and the new phases form. the modified surfaces contain the nitride precipitates such as fe2n, fen, https://dx.doi.org/10.5599/jese.1259 j. electrochem. sci. eng. 12(5) (2022) 923-935 microstructure and corrosion behavior of aisi 4140 steel 930 and mon, besides α-fe and γ-fe. it is seen that while the intensity of α-fe phase decreases, the intensity of γ-fe phase and new phases occurred, increase. it is known that molybdenum, one of the new phases formed in the surface structure, is entered into the surface from the consumable electrode material [19]. it is known that the growth of the molybdenum nitride phase and austenite in the analyzed layer results from a deeper penetration of molybdenum and nitrogen into the bulk material and the increased carbon concentration, respectively [16]. similar results are also seen when nozzle distance increases (figure 7c). however, while the intensity of α-fe and γ-fe phases decreases, the intensity of the nitride-based phases increases as the nozzle distance increases. the increase in the amount of nitride-based phases depending on the increase in nozzle distance, may be due to nozzle focusing or recrystallization problems that may occur at smaller distances [5]. at the same time, the diffraction peaks are seen to widen. the broadening of the diffraction peaks with increased nozzle distance is an indicator of crystalline imperfections like grain boundary increased by small grain size and microstrain [15,18]. hardness of the samples it was determined that while the hardness value of the unmodified sample is 200 hv, the hardness values of the modified samples change between 530 and 1050 hv depending on applied process parameters. the surface hardness enhancement is mainly due to the increase of the nanosize grains, new phases, and individual columnar structure, which has dense structure [16-18]. the first reason for the increase in hardness values of the samples is grain size reduction due to the increased cooling rate within the cycles of pulses [17,18]. there are other reasons that cause grain size reduction, except for the cooling rate. the pulse plasma causes an increase in the energy accumulation and promotes grain growth. the surface layer stores too much energy due to the increased pulses. the evaporation becomes so intense that the outermost layer removes. in this case, new grains would be nucleated and grow up. simultaneously, the great thermal stress accumulated with the repeated irradiation of the surface would also be played a significant role in inducing grain refinement. guo et al. [20], who observed the formation of a modified layer, reported that rapid heating and solidification promoted heavy plastic deformation, which caused the formation of the dislocation cells by pulse plasma treatment. they stated that the formation of the nano-austenite structure might be attributed to the dissolution of the initial carbide particles and the subsequent stabilization of the γ-phase due to the increased carbon concentration. these effects are also among the factors that increase the hardness. the hardness value variations of the modified surfaces by applying 5, 10, and 15 pulses with 50 mm nozzle distance were presented in figure 8a. it is seen that the hardness values of the modified surfaces increased with increasing pulse numbers. hao reported that the increase of the pulse number or the energy density absorbed by the surface leads to an increase in the microhardness values [5,7]. they stated that the number of pulses was increased during the crystallization of the surface layer, which was beneficial in increasing the nucleation rate and advanced non-spontaneous nucleation. the greater the pulse numbers, the finer the grains become [16], and the hardness increases. the hardness value changes of the modified surfaces by applying 15 pulses with 40, 50, and 60 mm nozzle distances were presented in figure 8b. it is seen that the hardness values of the modified surfaces increase as the nozzle distance increases. this result could be explained by partial annealing taking place at the structure because of heating of the surface by pulses ignited close to the surface. y. yaralı özbek and a. ş. demirkiran j. electrochem. sci. eng. 12(5) (2022) 923-935 https://dx.doi.org/10.5599/jese.1259 931 also, process parameters affect the microstructure, and some new phases and structural transformations occur in the modified surface. the amount of these strong new phases precipitated in the matrix phase and structural transformations lead to an increase in the hardness value of the modified samples. a b figure 8. the hardness value variations of the modified surfaces as depending on a pulse number, b nozzle distance corrosion behavior of the samples the potentiodynamic polarization curves of the unmodified surface and the surfaces modified with different pulse numbers are given in figure 9a. in scan range from -1 to +1 v, while passivation does not occur at the surfaces modified by applying 15 pulses with 40 mm nozzle distance and substrate, in the modification that is not very significant is seen passivation at the surfaces modified by applying 5 pulses. the occurred passivation is not stable. figure 9b shows tafel curves which contain cathodic and anodic polarization curves of the unmodified and the surfaces modified by applying 15 pulses with different nozzle distances. although the scan range is extensive range from -1 to +1 v, no passivation at no samples is observed. the corrosion current was found by the tafel extrapolation method applied to tafel region of anodic and cathodic polarization curves in the e versus log i plots. the found values were used to calculate the corrosion rate (equation 1). the ecorr, icorr, and rcorr values, which specify the samples' corrosion behavior, are listed in table 3. as expected (equation 1), the corrosion rate increases as the corrosion current increases. it is seen that the modified surfaces' corrosion rate is lower than aisi 4140 substrate, and a significant improvement in the corrosion resistance takes place. pulse plasma parameters affect to corrosion behavior of the surfaces. the reason for the improvement in the corrosion resistance of the modified surfaces is microstructural changes, not passivation (figure 9). the pulse plasma leads to homogenization of the surface layer melted and the formation of supersaturated solid solution, which enhances the corrosion resistance of the modified sample [15,18-21]. the modified surfaces contain the nitride precipitates such as fe2n, fen, and mon. as the pulse number increases, the amount of nitride-based phases formed increases. it is evident that these phases having homogeneous distribution will provide resistance to corrosion. as a matter of fact, as shown in table 3 and figure 10a, the corrosion current reduces with increasing pulse numbers, and the corrosion rate depending on the corrosion current, decreases. as seen in figure 10b, the corrosion current and corrosion rate depending on the corrosion current, increase with increasing nozzle distance. we think that the reason for the corrosion rate that increases with the nozzle distance is the increase in the amount of nitride-based phases that will create a cathodic effect. while nitride-based phases provide resistance to corrosion up to a certain amount, they https://dx.doi.org/10.5599/jese.1259 j. electrochem. sci. eng. 12(5) (2022) 923-935 microstructure and corrosion behavior of aisi 4140 steel 932 accelerate corrosion above a certain amount. the reason for this is explained in the section on microstructural investigations of the corroded surfaces below [20,21]. a b figure 9. the potentiodynamic polarization curves of a the unmodified surface and the surfaces modified by different pulse numbers, b the unmodified surface and the surfaces modified by different nozzle distances table 3. potentiodynamic polarization test results of the corroded surfaces nozzle distance, mm pulse number ecorr / mv ± sd icorr / µa ± sd rcorr / mm year -1 ± sd 40 5 -981±6 11.13±0.831 0.164±0.016 40 15 -661±4 3.116±0.664 0.046±0.015 50 15 -926±5 6.615±0.830 0.097±0.012 60 15 -831±8 9.696±1.031 0.143±0.015 aisi 4140 -940±5 20.57±0.220 0.303±0.003 a b figure 10. change in corrosion rate and corrosion current depending on a pulse number, b nozzle distance the sem micrographs of some corroded surfaces are presented in figure 11. micro-cracks starting and extending along grain boundaries at the unmodified surface (figure 11a) observed. intergranular corrosion formed by the dissolution of grain boundaries has occurred on the unmodified surface. it is known that intergranular corrosion, also named intergranular attack, occurs when the grain boundaries of the material are more sensitive than inside of grain in a corrosive medium. the pittings formed on the modified surfaces draw attention in figure 11b-c, and y. yaralı özbek and a. ş. demirkiran j. electrochem. sci. eng. 12(5) (2022) 923-935 https://dx.doi.org/10.5599/jese.1259 933 it is seen that pitting corrosion has occurred. there are various causes of pitting corrosion formation. one of the formation causes of pitting corrosion is also that in multi-phase materials, some phases act as cathodes and some phases as anodes [17-21]. this situation creates a galvanic effect. when the cathode is large and the anode is small, the anode phase dissolves, occur cavities or holes. in the modified structure, nitride-based phases will act as the cathode, and fe will serve as the anode. large current density will occur on fe, and fe will corrode rapidly. in addition, cracks and rarely porous structures were also observed on the modified surfaces from place to place. a b c d figure 11. sem micrographs of the corroded surfaces: a substrate, b the surface modified by applying 15 pulses with 40 mm nozzle distance, c the surface modified by applying 10 pulses with 60 mm nozzle distance, drarely porous structures observed from place to place on the modified surfaces conclusions in this study, the microstructure, hardness, and corrosion properties of aisi 4140 steel modified by pulse-plasma treatment performed by applying different parameters were investigated. the modification process was realized in short times with a relatively simple operation. due to high power energy and thermal effects, morphological changes on the steel surface have occurred. a modified layer on the steel has been observed that consists of two layers, the compound layer and the diffusion layer. the structure and thickness of the modified layer affect by pulse number and nozzle distance. the fine grains, dendritic structures, and new phases such as fe2n, fen, mon, and γ-fe in the modified layer occur. the amount of these new phases increases with the rising of pulse numbers and nozzle distance. in this same way, hardness values of treated samples also change by https://dx.doi.org/10.5599/jese.1259 j. electrochem. sci. eng. 12(5) (2022) 923-935 microstructure and corrosion behavior of aisi 4140 steel 934 pulse number and nozzle distance and increase with the rising of pulse numbers and nozzle distance. while the hardness value of the unmodified sample is 200 hv, the hardness values of the modified samples change between 530 and 1050 hv depending on applied process parameters. the hardness value of the treated sample has risen about 4-5 times that of the untreated. in general, the pulse plasma treatment has improved the corrosion resistance of treated samples. the reason for this improvement has been 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the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://dx.doi.org/10.5599/jese.1259 https://doi.org/10.1080/10402000802369721 https://www.sciencedirect.com/journal/applied-surface-science/vol/315/suppl/c https://doi.org/10.1016/j.apsusc.2014.07.093 https://doi.org/10.1016/s0169-4332(01)00403-2 https://d.docs.live.net/595bad50a54dda5b/03_jese/02_prhvaceni/1259/54 https://doi.org/10.1016/j.net.2021.12.030 https://doi.org/10.1016/j.surfcoat.2020.126689 https://doi.org/10.1016/j.surfcoat.2021.127129 https://creativecommons.org/licenses/by/4.0/) optimization of parameters for dye removal by electro oxidation using taguchi design doi: 10.5599/jese.2014.0056 227 j. electrochem. sci. eng. 4(4) (2014) 227-234; doi: 10.5599/jese.2014.0056 open access: issn 1847-9286 www.jese-online.org original scientific paper optimization of parameters for dye removal by electro-oxidation using taguchi design mani nandhini, balasubramanian suchithra, ramanujam saravanathamizhan and dhakshinamoorthy gnana prakash department of chemical engineering, ssn college of engineering, kalavakkam, chennai 603110, india corresponding author: e-mail: thamizhan79@rediffmail.com; tel. +91 44 27469700; fax +91 44 27469772 received: april 18, 2014; revised: may 9, 2014; published: december 6, 2014 abstract the aim of the present investigation is to treat the dye house effluent using electro-oxidation and to analyse the result using taguchi method. l16 orthogonal array was applied as an experimental design to analyse the results and to determine optimum conditions for acid fast red dye removal from aqueous solution. various operating parameters were selected to study the electro-oxidation for the colour removal of the effluent. the operating parameter such as dye concentration, reaction time, solution ph and current density were studied and the significance of the variables was analysed using taguchi method. taguchi method is suitable for the experimental design and for the optimization of process variables for the dye removal. keywords electro-oxidation; taguchi design; colour removal; acid fast red; optimization introduction effluents discharged from textile industries have high intensity of colour, which leads to pollution. highly coloured wastewater can be treated by different methods such as biological treatment, chemical coagulation, activated carbon adsorption, ultrafiltration, ozonation, wet oxidation, photocatalysis, electrochemical methods etc. [1-5]. among these methods the electrochemical treatment has been receiving greater attention in recent years due to its unique features such as versatility, energy efficiency, automation and cost effectiveness [6]. the electrochemical technique offers high removal efficiencies and the main reagent is the electron called ‘clean reagent’ which degrades all the organics present in the effluent without generating any secondary pollutant or by-product/sludge. http://www.jese-online.org/ mailto:thamizhan79@rediffmail.com j. electrochem. sci. eng. 4(4) (2014) 227-234 dye removal using taguchi design 228 industrial wastewaters have been treated by electro-oxidation techniques and the operating parameters have been optimized by different techniques. the widely used technique is response surface methodology for the experimental design and process optimization. taguchi method is another method in the experimental design, proposed by genichi taguchi, contains system design, parameter design, and tolerance design. taguchi method was effectively used to improve the product or process effectiveness by using a loss function to attain the product quality in terms of the parameter design [7]. taguchi is the preferable technique among statistical experimental design methods since it uses a special design of an orthogonal array to study the effective parameters with a minimum number of experiments. this method helps researchers to determine the possible combinations of factors and identify the best combination. however, in industrial settings, it is extremely costly to run a number of experiments to test all combinations. the parameter design using taguchi method minimizes the time and experimental runs. in the design, ‘signal’ and ‘noise’ (s/n) represent the desirable and undesirable values for the output characteristics, respectively and the ratio is a measure of the quality characteristic deviating from the desired value. further an analysis of variance (anova) is used to determine the significant parameters. recently taguchi’s designs have been applied to various chemical and environmental engineering studies for the experimental design and for the optimization of the process variables. asghari et al. [8] used taguchi method to determine the optimum conditions for methylene blue dye removal from aqueous solutions using electrocoagulation. the authors found that the amount of electrolyte was the most significant parameter for the colour removal. srivastava et al. [9] used taguchi method to determine the optimum conditions for the orange-g dye removal from aqueous solution by electrocoagulation using iron plate electrodes. author also applied this methodology to optimize the process variables for the multi-component adsorption of metal ions onto bagasse fly ash and rice husk ash [10]. kaminari et al. [11] used taguchi method to determine the optimum condition of process variables and find the influencing parameters for the recovery of heavy metals from acidified aqueous solutions using electrochemical reactor. kim et al. [12] optimized the experimental process variable using taguchi technique for the nano-sized silver particles production by chemical reduction method. in another study mohammadi et al. [13] used taguchi method to determine the optimal experimental conditions for the separation of copper ions from a solution. moghaddam et al. [14] used taguchi method to design the experimental runs and optimize the parameters for the ammonium carbonate leaching of nonsulphide zinc ores. maria et al. [15] designed the experimental runs and optimized the process parameters using taguchi method for the adsorption of acid orange 7 dyes on guava seed. in that way taguchi method was used to study the electro oxidation of dye removal. the objective of the present study is to treat the acid fast red dye using electro-oxidation. the effect of experimental parameters such as initial dye concentration, reaction time, solution ph and current density on colour removal was investigated using an l16 orthogonal array. the taguchi experimental design has been used to determine the optimum conditions for the maximum colour removal of the dye from aqueous solutions. materials and methods all the chemicals used in this study were ar grade (merck). acid fast red was prepared by dissolving definite quantity of dye in the distilled water. to increase the conductivity of the solution of 1 mg l -1 nacl was used as supporting electrolyte for all experimental runs. the initial m. nandhini et al. j. electrochem. sci. eng. 4(4) (2014) 227-234 doi: 10.5599/jese.2014.0056 229 solution ph was adjusted by 0.1 m hcl or 0.1 m sodium hydroxide solution. experiments were repeated twice to minimize the experimental error. experimental setup the experimental setup of the batch reactor is shown in the figure 1. the volume of the reactor was 250 ml and electrodes used were fixed inside the reactor with 1 cm space between them. stainless steel sheet cathode and mesh type ruthenium oxide coated titanium anode were used. the void fraction of the mesh type anode accounts 20 % by area, which resulted in an effective anode area of 28 cm 2 (7×5 cm). the electrodes were connected to a 5 a, 10 v dc regulated power supply, through an ammeter and a voltmeter. the solution was constantly stirred at 200 rpm using a magnetic stirrer in order to maintain a uniform concentration. dc power supply was given to the electrodes according to the required current density and the experiments were carried out under constant current conditions. the samples were analysed for the colour removal using uv-vis spectrophotometer (jasco, v-570). the percentage colour removal was calculated by: colour removal, % = i t i 100 abs abs abs   (1) where absi and abst are absorbance of initial and at time t at the corresponding wavelength max. figure 1. schematic diagram of the experimental setup 1. magnetic stirrer 2. anode 3. cathode 4. dc power supply taguchi design the following procedure was adopted for the parameter design. 1. planning of experiment i. determine the experimental responses of the process. ii. determine the levels of each variable. iii. select a suitable orthogonal array table. the selection based on the number of variables and number of levels. iv. transform the data from the experiments into a proper s/n ratio. 2. implementing the experiment, based on design table. j. electrochem. sci. eng. 4(4) (2014) 227-234 dye removal using taguchi design 230 3. analyzing and examining the result i. anova analysis to determine the significant parameters in the process. ii. draw the main effect plot, s/n ratio plot, mean plot to analysis the optimal level of the control variables. the factors and levels chosen for the present experiment are shown in the table 1. l16 orthogonal array design was selected for the four variables with four different levels for the each factor. table 2 shows the taguchi design for the electro oxidation of acid fast red dye. each row represents one experimental run. based on taguchi design the experiments were carried out and the percentage colour removal was observed as response. the proposed design was an orthogonal array, for which each pair of the columns had all the possible combinations of levels. the s/n ratio characteristics can be divided into three categories when the characteristic is continuous: (i) nominal is the best characteristic 2 y 10 log s y n s  (2) table 1. variables and their values corresponding to their levels investigated in the experiments variables level 1 2 3 4 a dye concentration, mg l -1 25 50 75 100 b time, min 15 30 45 60 c ph 2 5 7 10 d current density, ma cm -2 5 7.5 10 12.5 table 2. experimental variables, their levels and results of conducted experiments corresponding to l16 experimental plan s. no levels run 1 run2 a b c d colour removal, % 1 1 1 1 1 78.50 78.40 2 1 2 2 2 81.82 81.20 3 1 3 3 3 86.42 86.40 4 1 4 4 4 89.75 90.00 5 2 1 2 3 70.08 69.98 6 2 2 1 4 84.41 84.15 7 2 3 4 1 77.20 78.01 8 2 4 3 2 91.53 91.50 9 3 1 3 4 71.55 71.54 10 3 2 4 3 68.21 68.12 11 3 3 1 2 86.78 86.78 12 3 4 2 1 83.44 83.58 13 4 1 4 2 52.05 52.19 14 4 2 3 1 56.70 57.25 15 4 3 2 4 91.66 91.89 16 4 4 1 3 96.32 96.80 m. nandhini et al. j. electrochem. sci. eng. 4(4) (2014) 227-234 doi: 10.5599/jese.2014.0056 231 (ii) smaller the better characteristics 2110 log s y n n    (3) (iii) larger the better characteristics 2 1 1 10 log s n n y    (4) where, y is the average of observed data, 2 ys the variance of y, n is the number of observations, and y the observed data. for each type of the characteristics, with the above s/n ratio transformation, the higher the s/n ratio the better is the result. the experimental data were analysed using minitab 14 (pa, usa) [16]. result and discussion main effect plot main effect plot for the percentage colour removal using electro oxidation of acid fast red dye is shown in the figure 2. the plot is used to visualize the relationship between the variables and output response. the effect of initial dye concentration on colour removal is shown by the factor ‘a’. the percentage colour removal of the dye increases with decrease in dye concentration. this is due to the fact that at higher initial dye concentrations, the intermediate products formed due to the degradation of dyes increase the resistance of current flow by blocking the electrode active sites, and thus, decrease colour removal. the effect of electrolysis time on colour removal is shown in the figure by a factor ‘b’. the percentage colour removal depends on the electrolysis time. when time increases the generation of ocl radical increases due to electro-oxidation which results increase the percentage colour removal. the effect of ph on the mean colour removal is represented by the factor ‘c’. as it is observed from the figure, colour removal increases with the decrease of ph. this is due the fact that the hydroxyl radical generation is high at acidic ph which results to increase the rate of colour removal. the effect of current density on the removal of dye is shown by the factor ’d’. the increase of current density increases the ocl generation hence the percentage colour removal increases. high concentrations of chloride ions and salts in water can improve the performance and effectiveness of the electro-oxidation process. various levels (1, 2, 3, 4) of the operating parameter (a, b, c, d) and their mean colour removal is shown in the table 3. it is observed form the table, level ‘1’ shows highest colour removal of 84.06 %, 84.37 % for the initial dye concentration and ph, respectively. level ‘4’ shows highest colour removal of 90.37 %, 86.52 % for electrolysis time and current, respectively. table 3. mean colour removal for electro oxidation of acid fast red level mean colour removal, % a b c d 1 84.06 68.04 86.52 74.14 2 80.86 72.74 81.71 77.98 3 77.50 85.64 76.62 80.29 4 74.36 90.37 71.94 84.37 j. electrochem. sci. eng. 4(4) (2014) 227-234 dye removal using taguchi design 232 figure 2. main effect plot for the percentage colour removal of acid fast red a: dye concentration parameter; b: time parameter; c: ph parameter; d: current density parameter signal to noise (s/n) ratio taguchi method was used to identify the optimal conditions and most influencing parameters on colour removal. in the taguchi method, the terms ‘signal’ to ‘noise’ ratio represent the desirable and undesirable values for the output response, respectively. the s/n ratios are different according to the type of output response. in the present case, larger s/n ratio is better for high colour removal. figure 3 shows the s/n ratio of dye removal using electro-oxidation. figure 3. s/n ratio plot for the percentage colour removal of acid fast red . a: dye concentration parameter; b: time parameter; c: ph parameter; d: current density parameter m e a n c o lo u r r e m iv a l, % m. nandhini et al. j. electrochem. sci. eng. 4(4) (2014) 227-234 doi: 10.5599/jese.2014.0056 233 it can be noticed that at higher s/n ratio better level for colour removal was achieved. it is observed that factor ‘a’, initial dye concentration, and factor ‘c’, ph, are required in a lower level and electrolysis time ‘b’ and applied current density ‘d’ are required at higher level for the maximum colour removal. optimization of process parameters based on s/n ratio the process parameters were optimized based on s/n ratio. lager the s/n ratio higher the percentage colour removal and vice versa. the values of the s/n ratios for the operating parameters are shown in the table 4. it is observed from the table that the s/n ratio of level ‘1’ 38.48, 38.72 shows the higher value for the factor ‘a’ and ‘c’, respectively. it shows that level ‘1’ gives the higher colour removal. the larger s/n ratios of factor ‘b’ and ‘d’ for the level ‘4’ are 39.11, 38.48, respectively. it shows that level ‘4’ gives the higher colour removal. table 4. s/n ratio for electro oxidation of acid fast red level s/n ratio of electro oxidation a b c d 1 38.48 36.56 38.72 37.31 2 38.11 37.13 38.20 37.64 3 37.74 38.64 37.54 38.00 4 37.10 39.11 36.97 38.48 analysis of variance (anova) analysis of variance (anova) was performed to see whether the process parameters were statistically significant or not. the f-test is a tool to check which process parameters have a significant effect on the colour removal. the p value less than 0.05 shows that the parameter is significant. anova table for the colour removal of the dye is shown in the table 5. it is observed form the table the most influential factor was electrolysis time because the p value is 0.049 with the corresponding sum of the square is higher compared to other variable in the table. then the less significant variables are ph, current density and initial dye concentration. table 5. anova table for the electro oxidation of acid fast red source degree of freedom sum of the squares mean of squares f p a 3 210.9 70.29 1.50 0.374 b 3 1330.5 443.48 9.46 0.049 c 3 476.8 158.94 3.39 0.171 d 3 220.2 73.40 1.56 0.361 residual error 3 140.7 46.90 total 15 2379 conclusion taguchi experimental design was used to determine the optimum operating conditions of the dye removal from aqueous solutions using electro oxidation. the significant variables were identified for the colour removal process. optimum levels for operating parameters can be simultaneously identified with the taguchi method. the advantage of the taguchi method is the reduction in time and minimization of the number of experimental runs. for the present study j. electrochem. sci. eng. 4(4) (2014) 227-234 dye removal using taguchi design 234 level ‘1’ is best for initial dye concentration (25 g l -1 ) and initial ph (2), level ’4’ is best for electrolysis time (60 min) and current density (12.5 ma cm -2 ) for the highest colour removal. it can be concluded that taguchi method is suitable for the experimental design and to optimize the process variable for the colour removal of the dye effluent. acknowledgement: the authors are grateful to the ssn trust for the financial support of this work. references [1] m. c. gutierrez, m. crespi, journal of the society of dyers and colorists 115 (1999) 342–345. 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[16] k. ravikumar, s. ramalingam, s. krishnan, k. balu, dyes and pigments 70 (2006) 18-26. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {application of fe3o4@sio2/go nanocomposite for sensitive and selective electrochemical sensing of tryptophan} doi:10.5599/jese.576 45 j. electrochem. sci. eng. 9(1) (2019) 45-53; doi: http://dx.doi.org/10.5599/jese.576 open access: issn 1847-9286 www.jese-online.org original scientific paper application of fe3o4@sio2/go nanocomposite for sensitive and selective electrochemical sensing of tryptophan hadi beitollahi1,, mohadeseh safaei2, masoud reza shishehbore2, somayeh tajik3 1environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran 2department of chemistry, faculty of sciences, islamic azad university, yazd branch, yazd, iran 3nanobioelectrochemistry center, bam university of medical sciences, bam, iran corresponding author: e-mail h.beitollahi@yahoo.com; tel. +98 3426226613 received: june 6, 2018; revised: july 30, 2018; accepted: july 30, 2018 abstract a simple strategy for determination of tryptophan (trp) based on fe3o4@sio2/go nanocomposite modified graphite screen printed electrode (fe3o4@sio2/go/spe) is reported. cyclic voltammetry (cv) and differential pulse voltammetry (dpv) were used to characterize the performance of the sensor. the fe3o4@sio2/go/spe displayed excellent electrochemical catalytic activities. the oxidation overpotentials of tryptophan decreased significantly and its oxidation peak current increased dramatically at fe3o4@sio2/go/spe. under the optimized experimental conditions tryptophan showed linear response over the range of 1.0-400.0 μm. the lower detection limit was found to be 0.2 μm for tryptophan. the practical application of the modified electrode was demonstrated by measuring the concentration of tryptophan in real samples. keywords tryptophan; fe3o4@sio2/go nanocomposite; graphite screen printed electrodes; voltammetry introduction tryptophan (trp) is a vital amino acid for humans and herbivores as the precursor of hormones, neurotransmitters and other relevant biomolecules. tryptophan is also a precursor of the neurotransmitter serotonin [1-4]. it has been implicated as a possible cause of schizophrenia in people who cannot metabolize it properly. when improperly metabolized, it creates a waste product in the brain that is toxic, causing hallucination, delusions depression, alzheimer’s and parkinson’s diseases. therefore, determination of tryptophan is very important in blood and urine sample in these disease. this compound is sometimes added to dietary, food products, pharmaceutical formulas due http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:h.beitollahi@yahoo.com j. electrochem. sci. eng. 9(1) (2019) 45-53 electrochemical sensing of tryptophan 46 to the scarcely presence in vegetables [5-8]. hence, a simple, fast, selective, sensitive and accurate method for the determination of this analyte is very important. methods for the determination of tryptophan are mainly based on hplc [9] and spectrophotometric procedures [10]. most of these methods involve laborious and slow procedures with the modification of tryptophan by numerous reagents. the electroanalytical methods, with respect to their sensitivity, accuracy, less expensive and simplicity have been more intentioned in recent years for analytes determination [11-14]. screen printed electrodes (spes) allowing the mass production of reproducible, economical and disposable devices. other substantial features of these spes which make them suitable for on-site analysis are related to their capability to be connected to portable instrumentation and its miniaturized size [15-18]. in order to improve the efficiency of usual electrodes for sensor applications, a conventional modification method is applied. modified electrodes not only render better electrocatalytic activity, higher sensitivity and selectivity, but also lower detection limit in comparison with traditional electrodes [19-26]. nano materials are recognized as engineered particles with considerable surface to volume ratio and dimension less than 100 nm [27-30]. application of nanoparticles in the construction of electrochemical sensors has increased recently [31-35]. graphene is a two dimensional (2-d) sheet of carbon atoms in a hexagonal configuration with atoms bonded by sp2 bonds. these bonds and this electron configuration provide this arterial with extraordinary properties, such as large surface area, theoretically 2630 m2 /g for a single layer, and double that of single walled carbon nanotubes (swcnts). it also shows excellent thermal and electrical conductivity. due to its unique electronic properties, large surface area, rich edge defects, a tunable band gap, room temperature hall effect, strong mechanical strength, high elasticity and thermal conductivity; it exhibits remarkable conductivity, and sensing capability [36-38]. magnetic iron oxide nanoparticles such as maghemite (γ-fe2o3) or magnetite (fe3o4), are used in decoration of graphene to obtain improved magnetic, optical, and electrochemical properties in graphene. this combination which leads to improve the properties of graphene, make it a great option for the modification of electrodes [39]. by showing advantageous properties of superparamagnetism, low toxicity and reusability, fe3o4 nanoparticles (fe3o4nps) have played a major role among the most widely used magnetic sorbents. application of bare fe3o4 nps, however, is limited owing to its oxidation characteristics and lack of target selectivity. thus, the bare fe3o4nps is required to be functionalized in order to protect them from oxidation and raise selectivity and adsorption efficiency of a target analyte. in addition, the silica (sio2) has been widely used as catalysis, electronic device and amorphous materials. it should be noted that sio2 has attracted much attention as sensing material in the design of biosensors [40-42]. the most promising and favourable coating material proved to be silica, as it not only protects magnetic nanoparticles against oxidation and agglomeration at wide ph range, but also improves their chemical stability. moreover, the surface of silica is often finished with a silanol group, which can react with various chemicals and silane coupling agents to conjugate with a variety of biomolecules and specific ligands. thus, sio2 layers have good compatibility and hydrophilicity and indispensable properties for the use of these materials in biomedical applications [43]. the present work aims to employ the electrochemical method for determination of tryptophan at the newly synthesized fe3o4@sio2/go nanocomposite modified screen printed electrode. the modified electrode was used successfully as an electrochemical sensor for determination of tryptophan in the real sample as a sensitive, selective, simple and rapid method. h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 45-53 doi:10.5599/jese.576 47 experimental apparatus and chemicals the electrochemical measurements were performed with an autolab potentiostat/galvanostat (pgstat 302n, eco chemie, the netherlands). the experimental conditions were controlled with the general purpose electrochemical system (gpes) software. the screen-printed electrode (dropsens, drp-110, spain) consists of three main parts which are a graphite counter electrode, a silver pseudo-reference electrode and a graphite working electrode (diameter: 1 mm). a metrohm 710 ph meter was used for ph measurements. tryptophan and all other reagents were of the analytical grade, and they were obtained from merck (darmstadt, germany). the buffer solutions were prepared from orthophosphoric acid and its salts over the ph range of 2.0-9.0. synthesis of fe3o4@sio2/go nanocomposite graphene oxide nano sheets were synthesized from natural graphite flakes based on the modified hummers and offeman’s method. the reduced graphene oxide (0.096 g) was dispersed in 40 ml of water, and the solution was kept in ultrasonic bath for 1 h. the mixture was further stirred vigorously for 30 min at 60 °c. next, 177 mg of fecl3·6h2o was added under the stirring. after the mixture was stirred vigorously for 30 min under the n2 atmosphere, 95 mg of feso4·7h2o was added and stirred under the n2 atmosphere for 30 min. finally, 30 ml of the 6 % nh4oh aqueous solution was added into the mixture, drop by drop, at 60 °c for 1 h and then reacted for another 2 h. the n2 atmosphere was used during the reaction to prevent critical oxidation. the reaction mixture was then centrifuged, washed with double distilled water, and then dried. the obtained black precipitate was fe3o4/go nanoparticles and was ready for use. core–shell fe3o4@sio2/go nanocomposites were prepared by growing silica layers onto the surface of fe3o4/go, as described by lu et al. [43]. twenty-five millilitres of ethanol, 1 ml of water, 1 ml of ammonium hydroxide, and 150 μl of tetraethyl orthosilicate (teos) were added in a 250 ml three-neck flask in a 40 °c water bath. fe3o4/go was added to the abovementioned solution under mechanical stirring. aliquots of the mixture were taken out after 12 h by centrifugation, and then washed with water and vacuum-dried at 60 °c overnight. the morphology of the product was examined by sem. fig. 1 depicts the sem pictures of fe3o4@sio2/go nanocomposites. preparation of the electrode the bare screen printed electrode was coated with fe3o4@sio2/go, as shown in the following. a stock solution of fe3o4@sio2/go in 1 ml of the aqueous solution was prepared by dispersing 1 mg of fe3o4@sio2/go with ultrasonication for 1 h, while 2 µl of aliquots of the fe3o4@sio2/go/h2o suspension solution was cast on the carbon working electrodes, followed by waiting until the solvent was evaporated at room temperature. fig. 1. sem image of fe3o4@sio2/go nanocomosite j. electrochem. sci. eng. 9(1) (2019) 45-53 electrochemical sensing of tryptophan 48 preparation of real samples urine samples were stored in a refrigerator immediately after collection. ten millilitres of the samples were centrifuged for 15 min at 2,000 rpm. the supernatant was filtered out by using a 0.45 µm filter. next, different volumes of the solution was transferred into a 25 ml volumetric flask and diluted to the mark with phosphate buffer saline (pbs) (ph 7.0). the diluted urine samples were spiked with different amounts of tryptophan. the tryptophan contents was analysed by the proposed method by using the standard addition method. the serum sample was centrifuged, and after filtering, diluted with pbs (ph 7.0) without any further treatment. the diluted serum sample was spiked with different amounts of tryptophan. the tryptophan contents was analysed by the proposed method by using the standard addition method. discussion electrochemical behaviour of tryptophan at the surface of various electrodes the electrochemical behaviour of tryptophan depends on the ph value of the aqueous solution. therefore, the ph optimization of the solution seems to be necessary in order to obtain the best results for the electrooxidation of tryptophan. thus, the electrochemical behaviour of tryptophan was studied in 0.1 m pbs at different ph values (2.0-9.0) at the surface of fe3o4@sio2/go/spe by voltammetry. the peak current of tryptophan is increased with the increase of ph and reached the highest value when the ph of pbs is 7.0. thus, the subsequent determination experiment was performed in 0.1 m ph 7.0 pbs (fig. 2). fig. 2. plot of ip vs. various ph (2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0 and 9.0) fig. 3 depicts the cv responses for electrooxidation of 100.0 μm tryptophan at the and unmodified spe (curve a), go/spe (curve b), sio2/go/spe (curve c) and fe3o4@sio2/go/spe (curve d) the peak potential occurs at 725 mv due to the oxidation of tryptophan, which is about 145 mv more negative than the unmodified spe. also, fe3o4@sio2/go/spe shows much higher anodic peak current for the oxidation of tryptophan compared to the other electrodes, indicating that the modification of the unmodified spe with fe3o4@sio2/go nanocomposite has significantly improved the performance of the electrode towards tryptophan oxidation. i p / m a h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 45-53 doi:10.5599/jese.576 49 e / mv vs. ag/agcl/kcl fig. 3. cvs of a) fe3o4@sio2/go/spe and b) unmodified spe in the presence of 100.0 µm tryptophan at ph 7.0. in all cases, the scan rate was 50 mv s-1 e / mv vs. ag/agcl/kcl fig. 4. cvs of fe3o4@sio2/go/spe in 0.1 m pbs (ph 7.0) containing 200.0 µm of tryptophan at various scan rates; numbers 1–6 correspond to 10, 30, 80, 100, 300 and 500 mv s-1, respectively. inset: variation of anodic peak current vs. square root of scan rate effect of scan rate the effect of potential scan rates on the oxidation current of tryptophan has been studied (fig. 4). the results showed that increasing the potential scan rate induced an increase in the peak current. in addition, the oxidation processes are diffusion controlled, as deduced from the linear dependence of the anodic peak current (ip) on the square root of the potential scan rate (ν1/2) for tryptophan. tafel plot was drawn from the data of the rising part of the current–voltage curves recorded at a scan rate of 10 mv s−1 for tryptophan. this part of voltammogram, known as the tafel region is affected by electron transfer kinetics between substrate (tryptophan) and fe3o4@sio2/go/spe. tafel slope of 0.1528 v/decade was obtained, which agree well with the involvement of one electron at the rate determining step of the electrode process [44], assuming charge transfer coefficients α = 0.61 for tryptophan. chronoamperometric measurements chronoamperometric measurements of tryptophan at fe3o4@sio2/go/spe was carried out by setting the working electrode potential at 0.75 v vs. ag/agcl/kcl (3.0 m) for various concentrations of tryptophan (fig. 5) in pbs (ph 7.0). for electroactive materials (tryptophan in this case) with a diffusion coefficient of d, the current observed for the electrochemical reaction at the mass transport limited condition is described by the cottrell equation [44]: i = nfad1/2cbπ-1/2t-1/2 i /  a i /  a i /  a v1/2 / (mv s-1)1/2 j. electrochem. sci. eng. 9(1) (2019) 45-53 electrochemical sensing of tryptophan 50 where d and cb are the diffusion coefficient (cm2 s-1) and the bulk concentration (mol cm−3), respectively. experimental plot of i vs. t−1/2 was employed with the best fits for different concentrations of tryptophan (fig. 5a). the slopes of the resultant straight lines were then plotted against tryptophan concentrations (fig. 5b). from the resultant slope and the cottrell equation, the mean value of d was found to be 2.5×10-5 cm2/s for tryptophan. t / s fig. 5. chronoamperograms obtained at fe3o4@sio2/go/spe in 0. 1 m pbs (ph 7.0) for different concentrations of tryptophan. the numbers 1–5 correspond to 0.1, 0.5, 1.0, 1.5 and 2.0 mm of tryptophan. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1–5. (b) plot of the slope of the straight lines against tryptophan concentrations. e / mv vs. ag/agcl/kcl fig. 6. dpvs of fe3o4@sio2/go/spe in 0.1 m pbs (ph 7.0) containing different concentrations of tryptophan. numbers 1–11 correspond to 1.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0, 100.0, 200.0, 300.0 and 400.0 μm of tryptophan. the inset shows the plot of the peak current as a function of the tryptophan concentration in the range of 1.0–400.0 μm. calibration plots and limits of detection the electrooxidation peak currents of tryptophan at the surface of fe3o4@sio2/go/spe can be used to determine tryptophan in the solution. since differential pulse voltammetry (dpv) has the advantage of having an increase in sensitivity and better characteristics for analytical applications, dpv experiments were performed by using fe3o4@sio2/go/spe in 0.1 m pbs containing various concentrations of tryptophan (fig. 6). the results show that the electrocatalytic peak currents of tryptophan oxidation at the surface of fe3o4@sio2/go/spe were linearly dependent on tryptophan concentrations over the range of 1.0-400.0 µm (with a correlation coefficient of 0.9981) while the detection limit (3σ) was obtained as 0.2 µm. these values are comparable with values reported by other research groups for determination of tryptophan (table 1). i /  a i /  a i /  a i /  a ctryptophan / m ctryptophan / m t-1/2 / s-1/2 s lo p e ,  a s -1 h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 45-53 doi:10.5599/jese.576 51 table 1. comparison of the efficiency of some modified electrodes used in the electro-oxidation of tryptophan electrode modifier lod, µm ldr, µm ref. glassy carbon silver nanoparticles-decorated reduced graphene oxide 7.5 10.0-800.0 [45] glassy carbon cu-nanoparticles incorporated overoxidized-poly (3-amino-5-mercapto-1, 2, 4-triazole) 0.16 4.0-144.0 [46] carbon paste carbon nanoparticles and reduced graphene oxide 65.0 80.0-1000.0 [47] glassy carbon poly (9-aminoacridine) functionalized multi-walled carbon nanotubes 0.8 1.0-500.0 [48] screen printed fe3o4@sio2/gr nanocomposite 0.2 1.0-400.0 this work real sample analysis the new fe3o4@sio2/go nanocomposite modified screen printed electrode was also applied for the determination of tryptophan in human blood serum and urine samples by using the standard addition method. differential pulse voltammetry (dpv) experiments were done for different real samples. the results for the determination of the tryptophan in real samples are given in table 2. satisfactory recoveries of the experimental results were found for tryptophan. the reproducibility of the method was demonstrated by the mean relative standard deviation (rsd). table 2. determination of tryptophan in human blood serum and urine samples. sample c / μm (n=5). recovery, % rsd, % spiked found human blood serum 0 7.5 7.6 101.3 3.2 12.5 12.2 97.6 2.8 17.5 17.9 102.3 1.8 27.5 27.3 99.3 2.4 urine 0 10.0 9.7 97.0 1.9 20.0 20.5 102.5 2.8 30.0 30.4 101.3 2.2 40.0 40.3 100.7 3.3 the repeatability and stability of fe3o4@sio2/go/spe the long-term stability of the fe3o4@sio2/go/spe was tested over a 3-week period. when cvs were recorded after the modified electrode was stored in atmosphere at room temperature, the peak potential for tryptophan oxidation was unchanged and the current signals showed less than 2.6 % decrease relative to the initial response. the antifouling properties of the modified electrode toward tryptophan oxidation and its oxidation products were investigated by recording the cyclic voltammograms of the modified electrode before and after use in the presence of tryptophan. cyclic voltammograms were recorded in the presence of tryptophan after having cycled the potential 20 times at a scan rate of 50 mv s−1. the peak potentials were unchanged and the currents decreased by less than 2.4 %. therefore, at the surface of fe3o4@sio2/go/spe, not only the sensitivity increase, but the fouling effect of the analyte and its oxidation product also decreases. j. electrochem. sci. eng. 9(1) (2019) 45-53 electrochemical sensing of tryptophan 52 conclusion the present study demonstrates the construction of fe3o4@sio2/go/spe and its application for the determination of tryptophan. the tryptophan oxidation was catalyzed at ph 7.0 and its peak potential were shifted to a less positive potential at the tryptophan surface. this modified electrode offers a considerable improvement in voltammetric sensitivity toward tryptophan, compared to the bare electrode. differential pulse voltammetry (dpv) exhibits a linear dynamic range from 1.0-400.0 μm and a detection limit of 0.2 μm for tryptophan. the modified electrode showed high stability, good reproducibility and fast response for the detection of tryptophan concentrations. moreover, the proposed method was applied to determination of tryptophan in real sample with satisfactory results. references [1] j. tashkhourian, m. daneshi, s.f. nami-ana, analytica chimica acta 902 (2016) 89-96. 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[48] s. güney, g. yıldız, electrochimica acta 57 (2011) 290-296. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {electrochemical determination of the levels of cadmium, copper and lead in polluted soil and plant samples from mining areas in zamfara state, nigeria} doi:10.5599/jese.460 167 j. electrochem. sci. eng. 7(4) (2017) 167-179; doi: http://dx.doi.org/10.5599/jese.460 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical determination of the levels of cadmium, copper and lead in polluted soil and plant samples from mining areas in zamfara state, nigeria modupe ogunlesi, wesley okiei, aderinsola adio-adepoju, michael oluboyo chemistry department, university of lagos, akoka, lagos, nigeria corresponding authors e-mail: mayogunlesi@yahoo.com; tel.: +2348033353238 received: october 10, 2017; revised: october 18, 2017; accepted: october 31, 2017 abstract the concentrations of lead, copper and cadmium in soil and plant samples collected from abare and dareta villages in anka local government area of zamfara state, nigeria have been electrochemically determined. the study was carried out because of the high mortality of women and children under five, reported for these areas in june 2010. the cause was ascribed to the lead poisoning which has been related to the mining and processing of gold-containing ores. linear sweep anodic stripping voltammetry technique was used with the glassy carbon working, ag/agcl reference and platinum auxiliary electrodes. voltammetric peaks for lead, copper and cadmium that were observed at 495 mv, -19.4 mv and -675 mv, respectively, have formed a basis for construction of the corresponding calibration plots. the concentrations (in mg/kg) of lead, copper and cadmium in the soil samples were found in the ranges of 18.99−26087.70, 2.96−584.60 and 0.00−1354.25, respectively. the concentration values for lead were far above already established usepa (2002) and who (1996) maximum permissible limits for residential areas. the concentrations of lead, copper and cadmium in the food samples ranged between 5.70−79.91, 11.17−41.21 and 0.00−5.74 mg/kg. several of these values are found well above the fao/who limits of 0.1, 2 and 0.1 mg/kg, respectively. the results indicate that in addition to the lead poisoning, copper and cadmium poisoning may also be responsible for sudden and high mortality in this population. keywords heavy metals; mining activities; soil; plants; anodic stripping voltammetry introduction in june 2010, the zamfara state ministry of health of nigeria was alerted by médecins sans frontières (msf) for an increasing number of several deaths and illness among women and children http://dx.doi.org/10.5599/jese.460 http://www.jese-online.org/ mailto:mayogunlesi@yahoo.com j. electrochem. sci. eng. 7(4) (2017) 167-179 determination of cd, cu and pb in zamfara state 168 under five years in some villages in bukkuyum and anka local government area. they contacted the united states centers for disease control (us cdc) who deployed a response team to assist in investigating the outbreak. at the same time, the blacksmith institute, usa, sent a team from terra graphics environmental engineering inc. to conduct an environmental assessment of the area. these teams worked with the national and state authorities, msf, and the country office of the world health organization (who). the investigations confirmed severe lead poisoning in more than 100 children in the villages of dareta and yargalma, having a mean blood lead concentration of 119 μg/dl. moreover, soil in and some residential areas in abare exhibited lead concentrations exceeding 100,000 mg/kg. a random sample of 56 children under 5 years from the villages of abare and dareta revealed that over 90 % of them had blood lead concentration  45 μg/dl and over 70 % had concentrations  0 μg/dl [1]. a high incidence of convulsions and deaths of young children had been recorded in these villages and lead poisoning was implicated. it is generally thought that lead was introduced in the environment by the illegal mining of gold ores. in the previous study [2], it was reported that our team visited abare and dareta villages and observed various processing activities of the ores which could promote metal contamination of the soil, water and vegetables and may constitute health hazards. these include handling the ores with bare hands, eating with unwashed hands, crushing the ores in residential areas, washing the ores in water bodies, exposure of wells to these ores and sometimes drinking polluted water. the concentrations of lead and copper in water from wells, boreholes, ponds and stream from these villages and environs were already reported [2]. our observations on the research visit to abare and dareta villages attributed pollution of soils in the residential areas and farms to the practices of bringing the ores to the residential areas where they are crushed, and irrigating of farmlands by the water already used in processing the ores. high concentrations of heavy metals in soil would increase potential uptake of these metals by plants and their entering into the food chain [3]. contamination of food chain becomes increasingly important in a view of its role in human health and nutrition [3-7]. the main threats to human health from heavy metals are associated with exposure to lead, copper, cadmium, mercury and arsenic. these metals may enter the human body through inhalation of dust, consumption of contaminated drinking water and ingestion of food plants grown in contaminated soils [8,9]. the uptake of metals by plants can be affected by several factors which include metal concentrations in soils, soil ph, soil texture, cation exchange capacity, organic matter content of soils as well as the age of the plants [10-12]. mining activities have been reported to have adverse effects on water resources and farmlands [13] and this could lead to accumulation of heavy metals in plants grown in such contaminated soils. the aim of this study is to ascertain the level of heavy metal contamination of the soils and plants such as guinea corn and maize seeds in abare, dareta and environs, using linear sweep anodic stripping voltammetry technique which offers high sensitivity and rapid response time. experimental collection of soil samples soil samples were collected from 30 sites in abare, dareta and environs in july 2010. sampling was done in a ‘w’ fashion at sites 10 m apart. soil samples were taken at 5 cm depth. the sampling sites designated da 01−10 and ab 05−10 were residential areas in dareta and abare, where grinding of the ore was carried out. the mining sites in dareta were designated da 11−15. the areas around a pond and at the bank of a stream in abare where the ores were washed are designated ab 01−05 m. ogunlesi et al. j. electrochem. sci. eng. 7(4) (2017) 167-179 doi:10.5599/jese.460 169 and ab 11−15, respectively. the coordinates of the sampling stations were recorded using a garmin 38 global positioning system (gps) device. about 500 g of each soil sample was collected in polythene bags and labeled. each sample was then air-dried, sieved (size 2.3 mm) and stored in an air tight glass jar. collection of plant samples samples of various plants and food items were collected from abare and dareta in july 2010. the items which included guinea corn and millet leaves were collected randomly from 50 plants in dareta. the guinea corn leaves and the millet leaves were each mixed thoroughly before analysis. elephant grass was obtained from the area of cattle grazing in abare. guinea corn and millet seeds were purchased from the homes in abare and dareta, while the maize and beans were purchased from the homes in abare. preparation of soil samples 1.0 g of each soil sample was added to 20 cm3 of a mixture of concentrated hno3 and h2o2 (30 %) (sigma-aldrich) in the ratio 3:1. the mixture was heated gradually to 120 °c and kept at this temperature for 2 hours till a clear solution was obtained. the solution was allowed to cool and filtered into a 50 cm3 standard flask and supplemented with deionized water before transferring to a pre-cleaned plastic bottle. preparation of plant samples the samples were air-dried in a dust-free environment for three weeks and pulverized using a domestic blender. the digestion of samples was carried out by sequential treatment with 65 % hno3 and hclo4 (sigma-aldrich) according to the method earlier reported [14]. 1 g of the fine powder of the sample was digested with 20 cm3 of 65 % hno3 at 80 °c for one hour. on cooling, 2 cm3 of 70 % hclo4 was added and digested again at 250 °c, until a clear solution was obtained. the mixture was allowed to cool and 20 cm3 of deionized water was added. it was filtered into a 100 cm3 standard flask, supplemented with deionized water and stored in a pre-cleaned plastic container for the heavy metals analysis which was carried out in triplicate within 24 h. voltammetric measurements a basi-epsilon potentiostat/galvanostat was used in the study. the concentrations of heavy metals were determined by linear sweep anodic stripping voltammetry technique [2,15]. the working electrode (3 mm diameter) was glassy carbon, while platinum electrode (1.6 mm diameter) served as the auxiliary electrode and ag/agcl as the reference electrode. the working electrode was polished with alumina powder to obtain a mirror-like image, washed with de-ionized water and placed in a pyranha solution for 2 min, washed with de-ionized water and dried. calibration curve standard solution of each metal was prepared by dissolving the weighed amount of the salt (0.1598 g of pb(no3)2), (0.3929 g of cuso45h2o) and 0.2745 g of cd(no3)24h20), (all from sigmaaldrich) in 100 cm3 of deionized water to give 1000 ppm of lead, copper and cadmium, respectively. from these solutions, serial dilutions were made with 0.1 m acetate buffer, ph 4.50 containing 80 ppm hg(no3)2 and 0.2 m kno3 to give the working concentrations of 250, 500, 1000, 2000 and 2500 ppb of each metal. these diluted solutions were used to obtain the calibration curves. for this purpose, 10 cm3 of each of the standard solution of the metal was transferred into the electrochemical cell and purged with nitrogen for 10 min. the pre-concentration of the metal was j. electrochem. sci. eng. 7(4) (2017) 167-179 determination of cd, cu and pb in zamfara state 170 carried out at -900 mv for 120 s with stirring and after a quiet time of 30 s, the stripping process was carried out by scanning the potential from -900 mv to 200 mv at the scan rate of 20 mv/s. peak currents for lead, copper and cadmium were observed at -495 mv, -19.4 mv and -675 mv, respectively. these peak current values were used to construct the calibration plots for determination of the concentrations of pb, cu and cd. analysis of samples 5 cm3 of each digested soil or plant sample was transferred into the electrochemical cell and 5 cm3 of 0.2 m acetate buffer, ph 4.50, containing 160 ppm hg(no3)2 and 0.2 m kno3 was added and mixed thoroughly. the solution was purged with nitrogen for 10 min and the potential scanned as described for the standard solutions. the values of the peak current obtained at -495 mv, 19.4 mv and -675 mv were used to determine the concentrations of pb, cu and cd in the samples. results similarly to already described determinations of calibration plots for pb and cu 2, figure 1 shows the overlay of voltammograms of standard solutions of cd, while figure 2 shows the calibration plot for cd in the concentration range of 250−2000 ppb. fig. 1. overlay of the voltammograms obtained for standard solutions of cd(no3)24h20 in 0.1m acetate buffer, ph 4.5 containing 80 ppm hg(no3)2 and 0.2 m kno3. cd concentration: 250 ppb (1), 500 ppb (2), 1000 ppb (3), 2000 ppb (4). fig. 2. calibration plot of peak currents at -675 mv against cd concentration in 0.1 m acetate buffer ph 4.50, containing 80 ppm hg(no3)2 and 0.2 m kno3. the voltammograms obtained for the determination of metals in soil samples ab 04 and ab 13 are shown in figures 3 and 4, respectively. figures 5 and 6 show voltammograms for the determination of pb, cu and cd in millet leaves and guinea corn from dareta. the results showing the concentrations of cu, cd and pb in soils from the various sampling sites are presented in table 1, while the corresponding results for the plants are listed in table 2. 0 5 10 15 20 25 30 35 40 45 0 1000 2000 3000 c u rr e n t a t -6 7 5 m v , μ a concentration of cadmium, ppb m. ogunlesi et al. j. electrochem. sci. eng. 7(4) (2017) 167-179 doi:10.5599/jese.460 171 fig. 3. voltammograms of the soil sample from abare ab 04 in 0.1 m acetate buffer, ph 4.5 containing 80 ppm hg(no3)2 and 0.2 m kno3. peaks a, b and c correspond to cd, pb and cu respectively. fig. 4. voltammogram of the soil sample from abare, ab 13 in 0.1 m acetate buffer, ph 4.5 containing 80 ppm hg(no3)2 and 0.2 m kno3. peaks a and b correspond to pb and cu, respectively. fig 5. voltammogram of millet leaves from dareta in 0.1m acetate buffer, ph 4.5, containing 80 ppm hg(no3)2 and 0.2 m kno3. peaks 1, 2, and 3 correspond to cd pb and cu j. electrochem. sci. eng. 7(4) (2017) 167-179 determination of cd, cu and pb in zamfara state 172 fig. 6. voltammogram of guinea corn seed from dareta in 0.1 m acetate buffer ph 4.5, containing 80 ppm hg(no3)2 and 0.2 m kno3. peaks 1 and 2 correspond to pb (-495 mv) and cu (-19.4 mv) table 1. concentrations of pb, cu and cd in soil samples from dareta and abare villages in zamfara state soil sample coordinate content of pb, mg/kg content of cu mg/kg content of cd, mg/kg da 01 (grinding site) n 120 01’50.2’’ e 0050 57’30.5’’ 1175.00 ± 8.40 36.604 ± 6.00 nd da 02(grinding site) n 120 01’42.6’’ e 0050 57’18.7’’ 40.04 ± 1.10 2.96 ± 0.28 nd da 03(grinding site) n 120 01’41.6’’ e 0050 57’18.7’’ 36.16 ± 0.18 10.86 ± 1.80 nd da 04(grinding site) n 120 01’29.6’’ e 0050 57’44.0’’ 18.99 ± 0.70 32.84 ± 0.50 nd da 05(grinding site) n 120 01’39.5’’ e 0050 57’32.1’’ 21.06 ± 0.24 6.56 ± 0.11 nd da 06(grinding site) n 120 01’51.8’’ e 0050 57’21.3’’ 26087.70 ±16.20 78.51 ± 1.66 nd da 07(grinding site) n 120 01’51.7’’ e 0050 57’21.2’’ 438.56 ± 2.20 9.73 ±1.27 nd da 08(grinding site) n 120 01’52.0’’ e 0050 57’20.9’’ 438.56 ± 1.30 9.73 ± 0.54 nd da 09(grinding site) n 120 01’51.7’’ e 0050 57’21.2’’ 701.30 ±0.91 33.26 ± 1.20 nd da 10(grinding site) n 120 01’51.8’’ e 0050 57’21.0’’ 339.46 ± 1.10 38.69 ± 1.10 nd da 11(mining site) n 120 01’15.8’’ e 0050 57’34.0’’ 1128.51 ± 26 123.08 ± 2.91 804.50 ± 5.60 da 12(mining site) n 120 01’15.3’’ e 0050 57’34.3’ 630.70 ± 1.22 54.30 ± 1.80 nd da 13(mining site) n 120 01’15.0’’ e 0050 57’34.0’’ 167.54 ± 1.00 54.30 ± 1.40 nd da 14(mining site) n 120 01’14.7’’ e 0050 57’34.0’’ 3760.96 ± 4.1 213.80 ± 2.80 1354.25 ± 7.50 da 15(mining site) n 120 01’14.9’’ e 0050 57’34.2’’ 481.14 ± 6.50 48.64 ± 1.80 nd ab 01(washing site) n 120 04’41.2’’ e 0050 57’29.1’’ 900.40 ± 1.30 92.53 ± 3.50 nd ab 02(washing site) n 120 04’42.1’’ e 0050 57’30.6’’ 4051.76 ± 28 458.37 ± 1.50 nd ab 03(washing site) n 120 04’41.5’’ e 0050 57’29.0’’ 606.14 ± 2.80 33.71 ± 2.50 nd ab 04(washing site) n 120 04’41.2’’ e 0050 57’29.7’’ 3720.62 ± 3.00 584.60 ± 2.85 1216.25 ± 9.50 ab 05(washing site) n 120 04’40.6’’ e 0050 57’29.2’’ 390.79 ± 5.10 50.91 ± 3.50 nd ab 06(grinding site) n 120 04’41.3’’ e 0050 57’26.2’’ 38.16 ± 0.87 15.16 ± 1.25 nd ab 07(grinding site) n 120 04’40.7’’ e 0050 57’26.2’’ 489.48 ± 1.80 17.19 ± 0.68 nd m. ogunlesi et al. j. electrochem. sci. eng. 7(4) (2017) 167-179 doi:10.5599/jese.460 173 soil sample coordinate content of pb, mg/kg content of cu mg/kg content of cd, mg/kg ab 08(grinding site) n 120 04’41.3’’ e 0050 57’26.4’’ 111.899 ± 2.10 24.35 ± 1.35 nd ab 09(grinding site) n 120 04’40.7’’ e 005057’26.8’’ 1615.79 ± 3.20 49.10 ± 3.50 nd ab 10(grinding site) n 120 04’41.3’’ e 0050 57’27.0’’ 207.89 ± 1.85 10.86 ± 1.67 nd ab 11(washing site) n 120 04’22.7’’ e 0050 57’28.3’’ 287.54 ± 0.87 92.69 ± 2.50 8.03 ± 1.35 ab 12(washing site) n 120 04’21.7’’ e 0050 57’27.6’’ 1323.68 ± 4.51 204.52 ± 3.45 nd ab 13(washing site) n 120 04’23.1’’ e 0050 57’28.8’’ 205.56 ± 1.25 40.04 ± 2.31 nd ab 14(processing site) n 120 04’22.9’’ e 0050 57’29.3’’ 390.79 ± 3.10 69.91 ± 1.40 nd ab 15(washing site) n 120 04’23.1’’ e 0050 57’29.5’’ 51.586 ± 1.20 23.55 ± 1.48 nd nd: not detected. table 2. concentrations of metals in samples of plants and food from abare and dareta, zamfara state. plant/food sample location content of pb, mg/kg content of cu mg/kg content of cd, mg/kg elephant grass abare 24.63 ±0.05 27.37 ± 0.08 nd guinea corn seeds abare 15.60 ± 0.15 11.17 ± 0.07 nd maize abare 24.56 ± 0.04 12.72 ± 0.06 5.74 ± 0.13 beans abare 10.64 ± 0.05 30.41 ± 0.10 nd millet seeds abare 5.70 ± 0.06 14.84 ± 0.06 nd guinea corn seeds dareta 79.91 ±0.07 41.21 ± 0.05 nd guinea corn leaves dareta 36.86 ± 0.10 19.05 ± 0.03 nd millet seeds dareta 22.29 ± 0.09 11.53 ± 0.10 nd millet leaves dareta 22.76 ± 0.09 40.58 ± 0.20 5.34 ± 0.08 fao/who (2011) -0.1 2 0.1 nd: not detected discussion soil analysis the concentration levels of pb, cu and cd in soil samples listed in table 1 can be considered based on the processes carried out at each specific location. da 01−10 are residences where grinding of ores is carried out by women and children. da 11−15 denote the mining sites located 3.5 km from dareta. sites ab 01−05 are located around a pond in abare. sites ab 06−10 are grinding areas located in residences in abare, while ab 11−15 are sites located by the stream in abare where the processors wash the powdered ores to obtain gold. data in table 1 show that in the residential areas in dareta (da 01−da 10), concentrations of pb were detected in the range 18.99-26087.70 mg/kg, cu was detected in the range 2.96−78.51 mg/kg, while cd was not detected at all. at the mining sites in dareta (da 11−da 15), the levels of pb and cu were in the range 167.54−3760.96 mg/kg and 48.64−123.08 mg/kg, respectively. cadmium was found at two mining locations in dareta (da 11 and da 14) in concentrations of 804.50 mg/kg and 1354.25 mg/kg, respectively. at the pond environ sites in abare (ab 01−ab 05), the levels for pb and cu were in the range 390.79−4051.76 mg/kg and 33.71−584.60 mg/kg, respectively, while cd was found at one site (ab 04) in the concentration of 1216.25 mg/kg. the levels of pb and cu at grinding j. electrochem. sci. eng. 7(4) (2017) 167-179 determination of cd, cu and pb in zamfara state 174 sites of residential areas in abare (ab 06−ab 10) were detected in the range of 38.16−1615.79 mg/kg and 10.86−49.10 mg/kg, respectively. cadmium was not detected in these samples. at the ore washing sites in abare (ab 11−ab 15) at the bank of a stream, the levels of pb and cu were detected in the range 51.59−1323.68 mg/kg and 23.55−204.52 mg/kg, respectively. cadmium in the concentration of 8.03 mg/kg was found at only one site in abare (ab 11). the permissible level of pb in soil of residential areas in the us is 400 mg/kg [16,17]. this value is exceeded in a home in dareta (da 01) and grinding sites in dareta (da 06−09), and is probably due to the volume of ore being processed over time. the concentration values for pb, cu and cd are comparable for the residential grinding sites da 01−05 and da 07−10 in dareta and ab 06−10 (36.16−1615.79 mg/kg) in abare. da 06 appears to be an isolated case. it is pertinent to note that even though there is no mine in abare, the inhabitants of the two villages appear to be processing about similar quantities of ores. with the exception of site da 13, the levels of pb at the mining sites in dareta (da 11−15) exceeded the usepa limit. it should be noted that the site da 14 is located at the entrance of the main mining site. the mining activities have severely polluted this site and are responsible for the very high level of pb observed in da 14 (3760.96 mg/kg). in the pond environment in abare (ab 01−05), the pb levels in the soil were very high, with 4051.76 mg/kg and 3720.62 mg/kg detected at sites ab 02 and ab 04, respectively. this may be due to the gathering of processors at these sites. the levels of pb were moderately high at ab 01 and ab 03 showing values of 900.40 mg/kg and 606.14 mg/kg, respectively. at powdered ore washing sites in abare (ab 11−15), high levels of pb were found at ab 12 (1323.68 mg/kg) and ab 15 (51.586 mg/kg). it is significant that these values were recorded in abare where there are no mines. however, the stream and pond in abare are both used by the two communities for the washing of the ores. the high concentration value of pb found at ab 15 maybe due to several processors who converge at this site. the permissible limit for cu in the soil of residential areas is 190 mg/kg [18]. data in table 1 show that cu levels in the soil samples from dareta were under the permissible limit except for da 14 located at the mining site, where 213.8 mg/kg was detected. in abare, this permissible limit was highly exceeded at pond sites (ab 02 and ab 04) with the values of 458.37 and 584.60 mg/kg, respectively and marginally exceeded in the residential processing site (ab 12) where 204.52 mg/kg of cu was detected. the permissible limit of cd in the soil is 12 mg/kg [18]. cadmium was not detected in soil samples taken from the residences in dareta (da 01−10), but two sites in the mining areas (da 11 and da 14) showed cd levels of 804 and 1354 mg/kg, respectively. these concentrations of cd were 67 and 113 times higher than the permissible limit. cadmium was also detected at two locations in abare (ab 04 and ab 11). high level of 1216.25 mg/kg found in ab 04 is 101 times higher than the permissible limit, while the level of cd in ab 11 is under the limit. it is pertinent to note here that all sites where cd was detected (da 11, da 14 and ab 04), exhibited high levels of pb too. in summarizing the results given in table 1, it can be said that more than 50 % of the soil samples collected from dareta and abare were contaminated by pb, 20 % by cu and 12 % by cd. this is in agreement with the joint united nations environment programmeoffice for the coordination of human affairs (unep/ocha) environmental unit (jeu) report [19] on lead pollution and poisoning crisis in zamfara state. this report showed that the top soil collected from various locations in dareta and abare were heavily polluted by pb. at a processing site near a mosque in abare, the pb level in the soil was reported as 1210 mg/kg, while for the top soil from a near site close to a private well, the pb level of 27000 mg/kg was reported. the unep/ocha study has also reported pb levels m. ogunlesi et al. j. electrochem. sci. eng. 7(4) (2017) 167-179 doi:10.5599/jese.460 175 of 23600 mg/kg in top soil from a grinding site in abare and 37500 mg/kg in top soil from a processing site near a well in dareta [19]. however, the unep/ocha study did not report any pollution of the soil by cu and/or cd. a study on the assessment of lead, mercury and arsenic levels in soils of dareta, bagega, sunke, abare and yargalma villages in north−western nigeria was reported in 2014 [20]. lead concentrations in soil samples were found to be in the range of 6.91−4157 mg/kg. the average concentration of pb in farmlands in abare was reported to be 515 mg/kg [21]. the results of all these studies, as well as those presented in this report show that top soil in several parts of abare and dareta residences are heavily polluted by pb as a result of mining activities. in a household survey carried out by blacksmith institute, 2011 in two villages, dareta and yargalma in anka area, it was reported that 66 % of the population in the villages undertook at least one mining activity within their residences [22]. these mining activities include crushing of the ore, grinding, washing, drying, extracting gold with mercury and/or melting gold. the results of the study show the impact of anthropogenic activities on abundance of potentially harmful elements in the soils of the area. the mining activities exposed the women and children to high levels of pb in the soil and this may be responsible for the mean blood lead concentration of 119 μg/dl found in children from these villages. blood levels as low as 10 μg/dl are associated with impaired neurological development in young children [23]. in the present paper, the levels of cu were generally found under the limit at most of the sites. however, prolonged exposure to these metals could lead to bioaccumulation. some of the most commonly reported adverse health effects of cu are gastrointestinal distress, nausea, vomiting, abdominal pain irritation of the respiratory tract, coughing, sneezing and pulmonary fibrosis [24]. the levels of cd found at two sites in dareta (da 11 and da 14) and one site in abare (ab 04) are excessively high and pose a health risk to the processors in these sites. the kidney is the main organ affected by the chronic cd exposure and toxicity. cadmium accumulates in the kidney which may result in renal failure [25]. in a study carried out in 2014, on discharges of potentially harmful elements (phes) in soils from various sites in anka local government area where artisanal mining for gold was carried out, concentrations of pb equal to 2637, 1960, 3920, 290 and 3326 mg/kg were reported for mine site, ore processing site and village square, respectively [26]. the corresponding values for cu at the same places were reported as 159, 159, 117, 29 and 223 mg/kg, while the corresponding values for cd were reported as 4.5, 8.7, 3.6, 0 and 6.9 mg/kg. in our study, the corresponding values for pb shown in table 1 for the mining sites (da 11da 15) and residential processing sites (da 01da 10 and ab 06ab 10) ranged between 167−3761 and 19−1615 mg/kg, respectively. here, the highest value of 26088 mg/kg was observed at only one location (da 06). the corresponding values for cu ranged between 49−214 mg/kg at the mining sites and between 3−49 mg/kg for the residential sites, respectively. it should be noted that 79 mg/kg of cu was detected at da 06. the corresponding values for cd ranged between 0−1354 mg/kg and are detected at the mining sites only. it must be noted here that the values of pb in the mine and processing sites in two studies are comparable, except for the high value of 26,088 detected at da 06. also, the values for cu in the mine sites in the two studies are not far apart, but the values for the processing site in our study are significantly lower than those reported in 2014 [26]. the values for cd for some mine sites in our study are significantly higher, while those for cu are lower than the values reported in 2014 [26]. however, no gps values were provided in the study carried out in 2014 [26]. j. electrochem. sci. eng. 7(4) (2017) 167-179 determination of cd, cu and pb in zamfara state 176 plants/food analysis concentrations of metals in the samples of plants and food presented in table ii show that concentrations of pb in the cereals; guinea corn, maize and millet seeds from abare were in the range of 5.70−24.56 mg/kg. the value for beans was 10.64 mg/kg, while the value for the elephant grass was 24.63 mg/kg. in dareta, the values of pb in guinea corn and millet seeds were 79.91 and 22.29 mg/kg, while the corresponding values found in guinea corn and millet leaves were 36.86 and 22.76 mg/kg, respectively. the concentrations of cu in the cereals in abare were in the range of 11.17−14.84 mg/kg, while the corresponding value for beans was 30.41 mg/kg and the value for elephant grass was 27.37 mg/kg. in dareta, the values of copper in guinea corn and millet seeds were 41.21 mg/kg and 11.53 mg/kg, respectively, while the levels of cu in guinea corn leaves and millet leaves were 19.05 and 40.58 mg/kg respectively. cadmium was not detected in guinea corn, millet, beans and elephant grass in abare, but the maize from abare was found to contain 5.74 mg/kg of cd. in dareta, cd was not detected in the cereals, but the level found in millet leaves was 5.34 mg/kg. guinea corn, maize and millet are the main food in this sub region, and therefore the concentrations of heavy metals in the food samples grown in the contaminated soils are pertinent to health. it is shown in table ii that the levels of pb in guinea corn and millet seeds in abare were 15.60 and 7.50 mg/kg, respectively, while the corresponding values for these samples in dareta were 79.91 mg/kg and 22.29 mg/kg, respectively. the values for dareta samples were understandably higher than the values for abare samples because the tonnage of gold ore processed in dareta is much higher than that of abare. the permissible limit of pb in cereals is 0.1 mg/kg [27]. this value is highly exceeded by factors in the range 156−799 for guinea corn seeds and 57−223 for millet seeds. the level of pb in the maize sample from abare exceeds the permissible level by a factor of 245. the corresponding factor in beans is 106. thus, all the cereals in abare and dareta are not good for human consumption because of the high level of pb. elephant grass, guinea corn leaves and millet leaves that are consumed by cattle also showed very high levels of pb. thus, pb can enter the food chain and will be transferred to humans who consume meat from such cattle. the permissible limit for cu in cereals is 2 mg/kg [27]. the value for millet seeds was 14.84 mg/kg and 11.17 mg/kg in guinea corn seeds in abare while in dareta, the value for millet seeds was 11.53 mg/kg and that for guinea corn seeds was 41.21 mg/kg. the values for millet seeds in abare and dareta were comparable but the value for guinea corn seeds in dareta of 41.21 mg/kg, is more than double the values for guinea corn and millet seeds in abare and the millet seeds in dareta. the value of 30.41 mg/kg for beans obtained in dareta was also much higher than those for the other food items except for guinea corn seeds. the values for the elephant grass in abare and the guinea corn and millet leaves from dareta also exceeded the permissible level by factors ranging from 10−20, if the permissible value of 2 mg/kg for cereal is adopted. cadmium was detected in the maize from abare and millet leaves in dareta, and both values exceeded the permissible limit of 0.1 mg/kg for cd by a factor of about 50. according to data in table 2, it seems that all the plant samples were polluted by pb and cu, while maize in abare and millet leaves in dareta were found to contain cd as a third contaminant. the samples that were collected from the field, namely elephant grass, guinea corn leaves and millet leaves had pb values in the range 22.76−36.86 mg/kg, while the food items purchased from homes with the exception of guinea corn from dareta had pb values in the range 5.70−24.56 mg/kg. the plants obtained from the fields would be contaminated by dusts from ore-processing. it should also be borne in mind that items kept in the residences could also be contaminated by dusts from orem. ogunlesi et al. j. electrochem. sci. eng. 7(4) (2017) 167-179 doi:10.5599/jese.460 177 processing activities in the residential areas. this is probably responsible for the value of 79.91 mg/kg for pb and 41.21 mg/kg for cu in guinea corn seeds obtained from dareta. the possible sources of heavy metal contamination of the food samples are: direct contact with the ores, uptake from polluted soils, uptake from metal-polluted air, deposition on the surface of plants and exposed food items and contact with polluted water used for irrigation. all these are potential sources of contamination in abare and dareta. however, irrigation was not observed in dareta. crops and vegetables grown in soils contaminated with heavy metals have been reported to accumulate higher levels of heavy metals than those grown in uncontaminated soils [28,29]. the consumption of the contaminated cereals, millet, guinea corn and maize seeds as well as beans by the inhabitants of dareta and abare will pose a risk to human health. the elephant grass obtained from dareta with mean pb concentration of 24.63 mg/kg is also unsafe for the livestock. in a study carried out on the human health risk characterization of lead pollution in plants harvested from contaminated farmlands of abare village, pb concentration of sorghum exceeded the fao/who limit by a factor of 3500 [21]. the time of sampling, however, was not stated. one of the most widely studied mechanisms of action of toxic metals is oxidative damage due to direct generation of free radical species and depletion of antioxidant reserves [30]. mercury, cadmium and lead can effectively inhibit glutathione peroxidase thereby reducing the effectiveness of this antioxidant defense system for detoxification [31]. some heavy metals act as molecular mimics of nutritionally essential trace elements and compete with metallic cofactors for entry into cells and incorporation into enzymes [32]. cadmium can compete with and displace zinc from proteins while lead and thallium are chemically similar to calcium and potassium [32-34]. lead toxicity in human adults can lead to poor muscle coordination, nerve damage to the sense organs and nerves controlling the body, increased blood pressure, hearing and vision impairment and decreased sperm count. in children it can lead to damage to the brain, nervous system, liver, kidney and death [35]. children less than seven years old are largely at risk [16]. the health implication of the results presented in this study is the possible occurrence of pb, cu and cd poisoning in all the people exposed to the ores. ores are processed in residential areas by women and older children. children including infants sometimes put unwashed hands in their mouths thus ingesting the ore dust. they also inhale the dust during processing of the ore. they have less tolerance to heavy metal toxicity and hence high mortality of exposed children is inevitable. the women are equally exposed and mortality though not as high as in children, would be expected. all the inhabitants ingest the contaminated foods and thus would be victims of heavy metal toxicity, namely lead and cadmium. lead toxicity is implicated in abdominal pain, confusion, anemia, seizures, coma and deaths [36] while cadmium toxicity is implicated in lung cancer, kidney and bone damage [37]. all reports on high mortality in zamfara state have attributed the deaths to the pb poisoning [38]. the electrochemical method used in this study has made it possible to establish the presence of other heavy metal pollutants, namely copper and cadmium which may also be implicated in the high mortality. conclusions linear sweep anodic stripping voltammetry has been found very useful in the identification and determination of heavy metal pollutants, copper, cadmium and lead in the soil and plants/food samples. presence of very high concentrations of these metals in the samples from several parts of abare and dareta regions reported in this study is related to the mining of ores at these locations j. electrochem. sci. eng. 7(4) (2017) 167-179 determination of cd, cu and pb in zamfara state 178 and may be implicated in the high mortality in zamfara state. it is obvious that to address this health risk, safe mining practices should be enforced and soil remediation of all these areas should be carried out. educating the people, especially children, on toxicity of heavy metals and monitoring of these toxic metals in their blood should also be carried out periodically. references [1] medecinssansfrontieres. msf briefing paper, (2012) 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[37] atsdr. agency for toxic substances and disease registry. toxicological profile for cadmium, us department of health and human services, atlanta, ga. 2012, pp 1-487. [38] who. world health organization. nigeria: mass lead poisoning from mining activities, zamfara state, 2010. http://www.who.int/csr/don/2010_07_07/en/ (accessed on 8th october, 2017). ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.atsdr.cdc.gov/toxprofiles/tp13-p.pdf http://www.who.int/csr/don/2010_07_07/en/ http://creativecommons.org/licenses/by/4.0/) on the stability of perchlorate ions against reductive attacks in electrochemical systems and in the environment doi: 10.5599/jese.2011.0003 1 j. electrochem. sci. eng. 1(1) (2011) 1-26; doi: 10.5599/jese.2011.0003 open access : : issn 1847-9286 www.jese-online.org review on the stability of perchlorate ions against reductive attacks in electrochemical systems and in the environment mária ujvári and győző g. láng eötvös loránd university, institute of chemistry, department of physical chemistry, laboratory of electrochemistry and electroanalytical chemistry, po box 32, budapest 112, h-1518, hungary marcsi@chem.elte.hu received: june 16, 2011; published: august 20, 2011 abstract the problems related to the electrochemical/electrocatalytic stability of perchlorate ions are reviewed in the light of recent experimental results. the electrocatalytic, catalytic, and electrochemical reduction processes are presented and the links between them are outlined. some possible mechanisms of the complicated reduction processes are discussed. various methods for the detection of reduction process are presented, e.g. voltammetry, impedance spectroscopy, and radiotracer methods. environmental aspects and some methods for perchlorate removal and wastewater treatment are briefly summarized. keywords perchlorate ions; environmental contamination; waste water treatment; electrocatalytic reduction; reduction mechanism; noble metals; corrosion 1. introduction based on the thermodynamic data presented in table 1, 4clo − should be instable against reductive attacks in acidic medium in a wide potential range. in contrast, in the electrochemical literature 4clo − are often considered to be very stable anions. therefore, perchlorate containing solutions are widely used as supporting electrolytes in electrochemical studies with various electrodes. among these investigations reports concerning dissolution, deposition, and corrosion of metals can also be found. in such studies, it is tacitly assumed that the reduction of the 4clo − is not taking place under the actual experimental conditions. however, evidence for the occurrence of the reduction process has been reported for rh [1-5], pt [6-9], wc [10], al [11], ti [12], ir [13], ru [14], re [15], tc [16], j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 2 and sn [17] electrodes. a survey of the literature can be found in recent reviews [18-21]. these observations prompt us to keep in mind the possibility of the occurrence of the reduction process at any electrode used in an aqueous acid medium containing perchlorate ions. table 1. standard potential values of reaction steps potentially involved in the reduction of 4clo * reaction e o / v 2clo +e = 2clo 0.93 + 3clo +2h +e = 2 2clo +h o 1.15 + 4clo +2h +2e = 3 2clo +h o 1.19 + 2clo +h +e = 2hclo 1.27 +hclo+h +2e = 2cl +h o 1.50 + 2hclo +2h +2e = 2hclo+h o 1.64 * selected constants. charlot, g.; collumeau, a.; marchon, m.j.c. oxidation-reduction potentials of inorganic substances in aqueous solution, butterworths, london, 1971. besides the significance of perchlorate reduction for the fundamental electrochemistry today, a very practical reason, the so-called perchlorate contamination challenge, came into foreground directing the attention towards the reductive elimination of perchlorate ions. perchlorates are used as an oxidizer component and primary ingredient in solid propellants for rockets, missiles, and fireworks. therefore, dissolved ammonium, potassium, magnesium, or sodium salts are present as contaminants in groundwater and surface waters originating from improper disposal of the solid propellants and from the wastewaters of the manufacturing plants. as the sorption or natural chemical reduction of perchlorate in the environment is not significant, perchlorates are exceedingly mobile in aqueous systems and can persist for many decades under typical ground and surface water conditions. according to different reports [22-27], a large number of water sources in the united states have been contaminated with perchlorate. however, the perchlorate problem is not located to the us only. this type of contamination constitutes a major problem in china [28-30] and many other countries in the world [31-33]. for example, in israel, an ammonium-perchlorate manufacturing plant that had been disposing untreated wastewater in four unlined ponds for 25 years caused extensive perchlorate contamination in the underlying aquifer. the perchlorate migration in the deep vadose zone has been found very low under natural recharge conditions in the semiarid region [34]. one of the main health hazards is connected with the very fact that perchlorate interferes with iodide uptake in the thyroid gland [35,36]. in large doses, it has been linked to anemia and fetal brain damage [30,35]. several technologies have been studied for the treatment of perchlorate contaminated water, such as chemical precipitation and reduction, bioreduction, anion exchanges, electroreduction, membrane filtration, electrodialysis, and photocatalytic reduction. even though some of these have been adopted for drinking water treatment, the optimization of cost, selectivity, and effective treatment is still a serious problem [37-39]. the electrochemical treatment method is one of the methods [20] considered applicable for solving the perchlorate contamination [24-26]. the elaboration of a treatment technology with dissolved complexes is significantly hindered by its high cost and significant difficulties with the regeneration or treatment of their end products. some recent results in waste water treatment are presented below. m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 3 the electrochemical and electrophotocatalytic reduction of the perchlorate ion to the chloride ion were assessed at laboratory scale [40]. the study was carried out in two-chambered batch reactor systems in which the cathodic and anodic compartments were separated by an ion exchange membrane. electrodes consisted of titanium coated with a thin film of small tio2 particles. test water systems were buffered to an acid ph and background electrolyte was added. for photolytic systems, concentrations ranged from 5 × 10-2 mol dm-3 to 5 × 10-7 mol dm-3. reduction initially proceeded rapidly but slowed with time. the percentage of perchlorate electrochemically reduced after 2 h was found to range from less than 1% at the highest concentration to 30-35 % at lower concentrations. the extent of photocatalytic reduction at high perchlorate concentrations was approximately five times greater than the extent of electrochemical reduction at the same concentrations. an additional fourfold improvement in reduction percentage was observed when the electrode was doped with vanadium. a mathematical model suggested that the limiting factor in perchlorate reduction was the competition among anions for active sites on the electrode surface. an electrically controlled anion-exchange process has been proposed to remove perchlorate ions from the industrial wastewaters [41]. a nanostructured conducting copolymer poly(anilineco-o-aminophenol) (panoa) was electrochemically synthesized on a glassy carbon (gc) electrode and its redox and anion exchange properties for perchlorate removal were investigated by cyclic voltammetry, ftir, and x-ray photoelectron spectroscopy (xps). ftir spectra demonstrated that 4clo − were immobilized in the copolymer film after its oxidation in a solution containing nacl and naclo4. the result of xps spectra showed that the copolymer films oxidized in a solution containing 0.10 mol dm-3 nacl and 0.010 mol dm-3 naclo4 at ph 5.7 and 9.0 had 4clo − / cl− molar ratio of 0.53 and 0.31, respectively, which were significantly higher than the 0.10 ratio in the tested aqueous solution. new, potentially green, and efficient synthetic routes for the remediation and/or re-use of perchlorate-based energetic materials have been suggested in two other studies [42,43]. four simple organic imidazoliumand phosphonium-based perchlorate salts/ionic liquids have been synthesized by simple, inexpensive, and nonhazardous methods, using ammonium perchlorate as the perchlorate source. by appropriate choice of the cation, perchlorate can be incorporated into an ionic liquid, which serves as its own electrolyte for the electrochemical reduction of the perchlorate anion. the electrochemical degradation of the hazardous perchlorate ion and its conversion to harmless chloride during electrolysis was studied using ir and cl-35 nmr spectroscopies. bioelectrochemical systems (bess) including microbial fuel cells (mfcs) and microbial electrolysis cells (mecs) integrate three important wastewater treatment options, namely, biological treatment, electrolytic dissolution, and electrochemical oxidation/reduction and are regarded as new sustainable and effective strategies for wastewater processing. an up-to-date review is provided on recent research and development in bess-based recalcitrant wastes treatment [44]. recently, the reduction of perchlorate in the cathode compartment of a bes has been described [45]. the authors were able to isolate one of the organisms (dechlorospirillum strain vdy) responsible for the perchlorate reduction. at a potential of -500 mv vs. ag/agcl reference electrode, the organism was capable of reducing perchlorate without the addition of a redox mediator. the results of these studies demonstrate that biological perchlorate remediation can be facilitated through the use of an electrode as the primary electron donor, and that continuous treatment in such a system approaches current industry standards. an mfc with a j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 4 denitrifying biocathode for perchlorate reduction has been investigated to identify putative biocathode-utilizing perchlorate-reducing bacteria (pcrb), which can utilize a cathode as an electron donor, and this process can be harnessed to treat perchlorate while producing usable electrical power [46]. perchlorate reduction has been observed with certain transition metal complexes and metal chelates, but the reactions are generally sluggish. a swift-oxo transfer reaction of perchlorate by rhenium oxazalin and thiazoline complexes has been reported quite recently [47,48]. the characteristic features of the new family of these molecular oxotransferases are their rapid kinetics and their ability in catalysing the very difficult reduction of perchlorate by atom transfer. new (re-pd) bimetallic heterogeneous catalysts [49,50] for the reduction of perchlorate in water with dihydrogen have been prepared in two ways: (1) by impregnating 5 wt % pd on activated carbon powder with one of the complexes trans-[reo2(py-x)(4)](+) (py-x = 4-substituted pyridine; x = h, me, ome, nme2) or (2) by adsorbing perrhenate onto the pd/c powder in the presence of the pyridine ligand under a hydrogen atmosphere. both sets of catalysts are highly active at ph 2.7-3.0 (hcl), with observed rates increasing with varying x in the order h < me < ome < nme2, which supports a rate-determining oxygen atom transfer reaction involving the re(v) centers [51]. granular activated carbon (gac) coated with iron compounds (ics) [52] or cetyltrimethyl ammonium bromide (ctab) [53] were synthesized to remove perchlorate from water via adsorption. laboratory-scale batch experiments were performed to study the factors affecting the perchlorate adsorption. in case of ics/gac, 97 % of the perchlorate ions were removed within 10 h at 90 °c. the experimental results also showed that feohso4 and fe2o3 nanoparticles have the function of perchlorate adsorption and play important roles in 4clo − removal. in case of ctab/gac, the optimal adsorption occurred at ph 2-3, and the mechanisms were associated with surface complexation, electrostatic interaction, and ion exchange. studies in heterogeneous reaction of dissolved perchlorate ions with metals open a new possible pathway for new approaches in the technology. in recent years, the possible application of iron has been investigated [54], including the rate and extent of perchlorate reduction on several types of iron metal in batch and column reactors. mass balances performed on the batch experiments indicate that perchlorate is initially sorbed to the iron surface, which is followed by a reduction to chloride. perchlorate removal was proportional to the iron dosage in the batch reactors, with up to 66 % removal in the period of 336 h in the highest dosage system (1.25 g ml-1). the most significant perchlorate removal occurred in solutions with slightly acidic or near-neutral initial ph values. elevated soluble chloride concentrations significantly inhibited perchlorate reduction, and lower removal rates were observed for iron samples with higher amounts of background chloride contamination. the reactive iron phase is neither pure zero-valent iron nor the mixed oxide alone. a mixed valence iron hydr(oxide) coating or a sorbed fe2+ surface complex represent the most likely sites for the reaction. the use of zero-valent iron for treating wastewaters containing rdx (cyclotrimethylene trinitramine) and perchlorate from an army ammunition plant (aap) in the usa at elevated temperatures and moderately elevated temperature with chemical addition was evaluated in batch and column experiments [55]. the interaction of a metal with dissolved perchlorate ions should be considered as a corrosion process, i.e. the overall transformation is composed of at least two or three reactions: + 2+ 2me+2h me +h→ (1.1) m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 5 + 2+ 4 24me+clo +8h cl +4me +4h o→ (1.2) from the electrochemical point of view, this formulation involves three charge transfer processes: 2+ -me me +2e→ (1.3) + 2 1 h +e h 2 → (1.4) + 4 2clo +8h +8e cl +4h o→ (1.5) this last step should be a very complex one composed of several elementary steps. the equations given above involve the coupling of the anodic dissolution of the metal with two cathodic processes: discharge of protons and reduction of 4clo − . in principle, the reduction of 4clo − could occur through a “catalytic mechanism” via the interaction with hydrogen atoms adsorbed on the metal surface. it follows from these considerations that the problem of perchlorate reduction constitutes an intersection of several branches of sciences: electrochemistry (anodic dissolution of metals, discharge of protons); corrosion science (coupling of cathodic and anodic processes); catalysis (electrocatalysis); and heterogeneous chemical processes. in this paper, we present the historical background of the relevant topics in the light of recent developments and some new results obtained from the studies of direct interaction of perchlorate ions with various metals. 2. heterogeneous catalytic/electrocatalytic aspects of perchlorate reduction in 1971, butula and butula [56] found that hclo4 could be reduced if pd/baso4 and rh/c catalyst was used in acetic acid at 60 °c. in the same decade, the hydrogenation of perchlorate ions was studied at different temperatures and different perchlorate and hydrogen ion concentrations [20]. it was demonstrated that 4clo − dissolved in acidic aqueous solutions could be reduced to cl− by molecular hydrogen in the presence of powdered platinum black catalyst [9]. considering the link between the liquid (aqueous) phase heterogeneous catalytic hydrogenation and electrocatalytic reduction, it was easy to draw the conclusion that electrocatalytic/electrochemical reduction of 4clo − could be performed at electrodes prepared from materials identical to the material of catalyst powders used in the catalytic hydrogenation [57]. electrodes with “catalytic” properties, such as pt, rh, pd, ir, and ru, have been in the focus of interest. there was, and perhaps still is, a general belief in the literature that perfect voltamograms reflecting h adsorption on these electrodes can be obtained in hclo4 solution. although in addition to pt and rh electrodes 4clo − reduction on ir [13] and ru [14] conveyed the warning to be cautious with noble metal electrodes when using hclo4 supporting electrolyte, most electrochemists in their various studies [58-60] carried out in the presence of hclo4 supporting electrolyte ignored the possible complications that should be ascribed to the occurrence of a reduction process involving 4clo − . in 1990, it became evident that the contradictory results obtained during previous decades could be explained if it was assumed that the reduction of the 4clo − was characteristic not only for the rhodized surfaces, but also for smooth polycrystalline surfaces and well-defined crystal faces j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 6 [3-5]. by this approach, almost all difficulties encountered in the interpretation of various phenomena could be eliminated during the last few years. thus far, the majority of the published experimental results were obtained by using combined voltammetric, amperometric, eqcm, impinging jet, and radiotracer methods [1,3,4,13,19,20,6165]. figure 1. voltamograms obtained for rhodized electrodes in different electrolytes; acid concentration 1 mol dm-3, sweep rate = 5 mv s-1. adapted from [61]. the distortion of voltammetric curves in the presence of 4clo − is the firm evidence of the occurrence of the reduction process. figure 1 shows the voltammetric curves obtained in 1 mol dm-3 h2so4, hf and hclo4 supporting electrolytes using rhodized electrodes [61]. in the case of h2so4 and hf, despite some differences between them, the voltammetric curves behave “normally” as expected. in contrast, the voltammetric curve obtained in the presence of perchloric acid is distorted. the negative peak in the course of the positive sweep clearly indicates the occurrence of a cathodic process, namely, the reduction of perchlorate ions. similar voltammetric curves were obtained with singleand polycrystalline smooth rh [3,4] and platinized pt [8] surfaces. electrochemical behavior of ir(100) in perchloric acid solutions has been characterized elsewhere [66]. the asymmetry of the cyclic voltammograms recorded in the double-layer region could be attributed to a slow reduction process, e.g. the reduction of perchlorate ions. the characteristic features common to these systems are as follows: (a) nowadays, there is a general agreement that the rate of reduction of 4clo − at noble metal and catalytic electrodes should be very low at both ends of the potential scale; consequently the i vs. e curve should go through a minimum. (b) the reduction process occurs almost exclusively during the positive sweep of the cyclic voltammogram. (c) generally, no reaction can be observed on the negative sweeps, but the shape, height, and position of the corresponding hydrogen peak differ significantly from those expected. m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 7 (d) the distortion of the anodic peak gradually disappears during subsequent cycles, but the anodic and cathodic peaks remain asymmetric. (e) the only reduction product found in the solution was cl− . (f) the very rapid decrease in the electrocatalytic activity with respect to 4clo − reduction can be explained by self-inhibition, namely, by the inhibiting action of cl− formed during reduction and adsorbed on the electrode surface. the higher the cl− concentration, the more pronounced is its inhibitive effect. (g) the desorption of cl− from the metal surface is a very slow process. for instance, no measurable desorption of cl− were detected until several minutes after switching the electrolyte solution from 1 mol dm-3 hf + 10-4 mol dm-3 hcl to 1 mol dm-3 hf [62,67]. according to the results presented in the literature, the rate of electrocatalytic reduction of perchlorates on rh is relatively high. the reduction process leading to the formation of cl− should be a very complicated reaction as eight electrons are involved in the overall process. it is difficult to believe that it can be performed without the formation of any stable intermediates or products of side-reactions. analogous to the reduction of perchlorate in homogeneous solution, the reduction of perchlorate at an electrode (or any surface) must involve oxygen atom transfer. therefore, the controversy regarding the mechanism of the reduction of 4clo − on rhodium is not surprising. according to the "classical" view, the rate-determining step in the electrocatalytic process can be described as a reaction of adsorbed 4clo − species with adsorbed h atoms [1,13,19,61,62,67]. in contrast, the reduction process can be considered as a slow decomposition of 4clo − species on the surface followed by fast reduction steps [63]. the latter approach is based on the view that the reduction rate attains a measurable level at potentials where the surface concentration of h is very low; consequently, it is unlikely that it can play any role. thus, it should be assumed that the rate-determining step is the decomposition of adsorbed 4clo − with the participation of a free adsorption site in its neighborhood. in accordance with the above considerations, three fundamental types of possible mechanisms are distinguished for the electrocatalytic reduction of 4clo − [13,20,21,61-63,65,67], namely the mechanism (a) involving protons and/or adsorbed hydrogen, the serial mechanism (b) and the mechanism (c) involving free-metal sites. mechanism (a) can follow two alternative paths: 4clo (sol) ⇌ 4clo (ads) ( )+ -h sol +e ⇌ h(ads) (a1) ( ) ( )-4clo ads +h ads → intermediates ( ) ( )-cl ads cl sol→ → or 4clo (sol) ⇌ 4clo (ads) ( )+ -4clo ads +h +e → intermediates ( )-cl ads→ (a2) -cl (ads)⇌ -cl (sol) mechanism (b): 4clo ⇌ ( )-4clo ads j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 8 ( ) ( ) ( )-4 3clo ads clo ads +o ads→ (slow) ( ) + 2o ads +2h +2e h o→ (fast) ( ) ( ) ( )-3 2clo ads clo ads +o ads→ (slow) ( ) + 2o ads +2h +2e h o→ (fast) ( ) ( ) ( )-2clo ads clo ads +o ads→ (slow) ( ) + 2o ads +2h +2e h o → (fast) ( ) ( ) ( )-clo ads cl ads +o ads→ (slow) ( ) + 2o ads +2h +2e h o→ (fast) where “(ads)” denotes that the species is adsorbed, while “(sol)” refers to components in the solution phase. according to this scheme, the whole process starting from perchlorate ion occurs on the surface and the role of desorption of intermediates can be neglected, i.e. their desorption rate is very low compared with the rate of the chemical transformation. mechanism (c): according to this mechanism, it should be assumed that the rate-determining step is the decomposition of adsorbed perchlorate ion if a free "active" site ( )⊗ is in its neighborhood. this assumption is the same as that made in [64] in connection with the reduction of n2o. the decomposition reaction can be written as: 4+clo⊗ → intermediates -cl→ on the other hand, it can be assumed that the surface concentration of active sites is determined by the following reactions: 2+h o⊗ ⇌ + --oh+h +e⊗ or -+oh⊗ ⇌ --oh+e⊗ in this case, the potential dependence of the surface concentration of free active sites plays an important role. in the reaction schemes proposed in the literature, the adsorption of perchlorate ion is an elementary step in the overall reduction. the decomposition of the adsorbed perchlorate ion is assumed to be slow; however, there are only speculations about the rates of the subsequent steps. for instance, in the mechanism (b) all decomposition steps are assumed to be slow, but no rigorous investigation of this problem has yet been performed. the electrochemical reduction of perchlorate ions on rhodium and ruthenium was investigated in a series of experiments (voltammetry, chronoamperometry, impedance spectroscopy) performed at different electrode potentials (e) and different temperatures (t). figure 2 shows cyclic voltammograms obtained for a rh rotating disc electrode in contact with a 3 mol dm3 hclo4 solution [67]. m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 9 figure 2. cyclic voltammograms obtained for a rh rotating disc electrode in contact with a 3 mol dm-3 hclo4 solution (geometric surface area of the electrode: a = 0.196 cm 2,rotation rate: ωr= 950 rpm, temperature: t = 25.0 °c). adapted from [67]. (ssce: sodium saturated calomel electrode) there is a distinct difference between the curves recorded at stagnant electrodes and the curves obtained with the rde. in case of stationary electrodes, the distortion of the anodic peak gradually disappears during subsequent potential cycles, i.e. the negative current during the positive sweep is continuously decreasing over the consecutive scans. in contrast, in case of rotating disc electrodes, the shape of the voltammogram does not change after the second or third cycle, and a negative current can be always observed during the positive sweep. it means that the desorption rate of -cl generated during the reduction process is significantly influenced by the hydrodynamic conditions, probably through desorption/diffusion coupling. this conception is supported by the results of chronoamperometric measurements carried out at different temperatures. the curves obtained at t = 45 °c are presented in figure 3 [67]. figure 3. chronoamperometric measurements. the current (i) as a function of time (t) at constant electrode potentials and at different electrode rotation rates. electrode potentials: a: 0.01 v vs. ssce; b: 0.06 v vs. ssce; c: 0.11 v vs. ssce. rotation rates (ωr): a: 0 rpm; b: 500 rpm; c: 1000 rpm; d: 2000 rpm; e: 3000 rpm; f: 4000 rpm. (geometric surface area of the electrode: a = 0.196 cm2, temperature: t = 45.0 °c) adapted from [67]. j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 10 during these experiments the current was recorded as a function of time at constant electrode potentials and at different electrode rotation rates. well-defined stationary currents were observed in all cases. the values of the stationary currents depend on the rotating rate of the rde, the electrode potential, and the temperature. it should be pointed out that the above measurements were carried out at potentials where the surface concentration of adsorbed h is very low, if existent. consequently, it is unlikely that it can play any role. thus, it should be assumed that the rate-determining step is the decomposition of adsorbed 4clo − with the participation of a free adsorption site. figure 4 shows impedance spectra of ru (deposited on gold) in 1 mol dm3 hclo4 solution. 0 500 1000 1500 2000 2500 3000 3500 4000 0 500 1000 1500 2000 2500 3000 3500 4000 2.6641 hz 0.02407 hz 0.28489 hz 0 rpm 500 rpm 1000 rpm 1500 rpm 2000 rpm -z '' / ω z ' / ω figure 4. impedance spectra (complex plane plot) recorded on ru in 1 mol dm3 hclo4 solution. the impedance of the electrode were measured at different rotating rates of the rde at the electrode potential of e = -0.15 v vs. ssce. (geometric surface area of the electrode: a = 0.196 cm2, temperature: t = 25 °c). (recent, unpublished results) the impedance of the electrode was measured at different rotating rates of the rde at the electrode potential of e=-0.15 v vs. ssce (t = 25 °c). the impedance measurements were started as soon as stationary conditions were reached (e.g. about 20 min after changing the rotation rate of the rde). the first observation is that the plots are very similar to what is expected qualitatively for the case of a single charge transfer step coupled with diffusion [68]. for all the spectra obtained in this study, only one time constant can be identified in the high frequency range, which suggests that the measured impedance response is dominated by a single (rate determining) charge transfer step. in the case of the reduction of perchlorate ions, the assumption that the -4 3clo clo→ transformation is the rate determining step seems to be a relevant and acceptable interpretation of the phenomena observed. experiments with -3clo strongly support this hypothesis. figure 5 shows cyclic voltammograms obtained for rh in contact with a solution containing 0.49 mol dm-3 naclo4 + 0.01 mol dm -3 hclo4, and for the same electrode in contact with a solution m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 11 containing 0.49 mol dm-3 naclo3 + 0.01 mol dm -3 hclo4. it can be clearly seen from the figure that in this potential range, the negative current is at least 2 orders of magnitude higher in the presence of -3clo than the current measured in the other solution. for comparison, the voltammetric curve obtained in 1 mol dm-3 hclo4 solution is also given in the figure. figure 5. cyclic voltammograms obtained for rh (1): in contact with a solution containing 0.49 mol dm-3 naclo4 + 0.01 mol dm -3 hclo4, and (2): for the same electrode in contact with a solution containing 0.49 mol dm-3 naclo3 + 0.01 mol dm -3 hclo4. (3): voltammetric curve obtained in 1 mol dm-3 hclo4 solution. (geometric surface area of the electrode: a = 0.196 cm2, temperature: t = 25.0 °c). adapted from [67]. electrocatalytic activity in perchlorate reduction was found in the case of some non-noble metals, such as tc [16], re [15], sn [17] and ti [12,69]. considering the very fact that tc is an artificial element, emitting β-radiation, the coupled electrochemical and radiochemical study of electrodeposition of tc species from hclo4 supporting electrolyte gave an interesting insight in the reduction of 4clo − . it was found that under potentiostatic conditions during the deposition of tc species, the increase in the radiation intensity, i.e. the increase of the amount of deposited material, is accompanied by a continuous increase of a cathodic current as shown in figures 6 and 7 (the amount of the electrodeposited tc species was determined directly by the measurement of the intensity of the radiation coming from the electrodeposited layer). so far, only the occurrence of 4clo − reduction has been suggested as acceptable explanation [16]. the voltammetric study of the system furnished unambiguous evidence proving the validity of this assumption (figure 8). it is well known [70] that the electrochemical behavior and properties of technetium and rhenium and those of their various ions are very similar. considering the similarity of tc and re, it was no wonder that the behavior found in the case of tc can also be observed for re and vice versa. in contrast to the behavior of platinized platinum electrodes, the catalytic activity of “rhenized” electrodes can be observed under potentiostatic or galvanostatic conditions for relatively long periods of time, although no real steady state can be attained [15], as is shown in figure 9 in the case of a potentiostatic experiment. j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 12 figure 6. γ vs. time (1) and (absolute value) current vs. time (2) curves obtained in the course of a simultaneous radiometric and electrochemical measurement of the deposition of tc species ( c 4tco =8×10-4 mol dm-3) from a 1 mol dm-3 hclo4 supporting electrolyte at e = 50 mv, (geometric surface area, 13 cm2). adapted from [20]. figure 7. the j current density vs. γ relationship obtained from data presented in figure 6. adapted from [20]. m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 13 figure 8. cyclic voltammetric curves at a tc-covered surface (γ = 2 × 10-8 mol cm-2) in 1 mol dm-3 h2so4 (1) and in 3 mol dm -3 hclo4 (2). sweep rate, 2.5 mv s -1 (geometric surface area, 13 cm2). adapted from [20]. figure 9. current vs. time curve obtained in 1 mol dm-3 hclo4 solution, at 90 mv, at a rhenized electrode, (geometric surface area, 2 cm2). adapted from [15]. the continuous decrease in the reduction current can be explained by two important effects: i) the self-inhibition of the process by the adsorption of -cl formed in the reduction (the current efficiency with respect to the formation of -cl is above 90 %). the effect of -cl was demonstrated in separate experiments by adding hcl in low concentration to the solution phase in the course of the reduction [15], and ii) deactivation of the electrode surface owing to the chemical transformation of the surface layer participating in the reaction. j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 14 the electrochemical behavior of tin in deaerated sodium perchlorate was studied using potentiodynamic and potentiostatic techniques [17]. the behavior of tin in sodium perchlorate was unexpectedly complicated by the reduction of the perchlorate anion. it was shown that the reduction process takes place within a potential region comprising the negative side of the double layer region and the positive side of the hydrogen region (-0.7 ≤ e ≤ -1.3 v). it was stated that the presence of oxide on the electrode surface favors the reduction reaction, which may occur in two steps: the formation of basic tin(ii) chloride followed by its reduction, producing chloride. similar mechanistic conclusions were drawn in the case of titanium electrode. it was reported that perchlorate ion is electrochemically reduced to chloride ion at an active titanium electrode in aqueous 1.0 m hclo4 [12]. it is assumed that the reduction occurs by direct reaction at the surface rather than a pathway involving catalysis by soluble titanium corrosion products. the reaction occurs by oxygen atom transfer to the titanium surface, and the implications of this mechanism for the surface composition of active titanium electrodes are discussed. chlorate ion is also reduced at titanium, and the rate coefficient for chlorate reduction is at least 105 times greater than that for the perchlorate reduction. in experiments with ti in contact with perchlorate and nitrate solutions, ti(iii) or ti(ii) were generated by applying anodic current [69]. these multivalent ti species are strong reducing agents and reduce perchlorate and nitrate. kinetic experiments were carried out in synthetic perchlorate, nitrate solutions, and tap water media at different perchlorate and nitrate concentrations. 3. reaction mechanisms and electrode impedance in another study [67], the experimental results obtained for the electrochemical reduction of perchlorate ions on rhodium have been explained by the following reaction mechanism. after a potential step from a potential at which the adsorption of 4clo − does not occur, the reduction process starts with the adsorption of perchlorate ions at free active sites on the metal surface occupied, e.g. by oh− . ( ) ( )-4clo sol +oh ads ⇄ ( ) ( )-4clo ads +oh sol the decomposition reaction can be written as: ( )-4clo ads → intermediates -cl (ads)→ ⇄ -cl (sol) after the production of a certain amount of -cl , practically all active sites are occupied by -cl or 4clo − , and a well defined stationary state with constant current is established. for the stationary state, the following reaction scheme can be suggested: ( ) ( )-4clo 0 +cl ads ⇄ ( ) ( )-4clo ads +cl 0 (3.s1) ( ) ( )+ -4 3 2clo ads +2h +2e clo ads +h o→ (slow) (3.s2) ( )-3clo ads → intermediates -cl (ads)→ (very fast) (3.s3) -cl (0) ⇄ -cl (sol) , (3.s4) or in a generalized form: ( ) ( )-a 0 +b ads ⇄ ( ) ( )-a ads +b 0 (3.1) m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 15 ( ) ( )+ 2a ads + h + e b ads + h o 2 n n n → (3.2) -b (0) ⇄ -b (sol) (3.3) where n is the number of electrons transferred from the metal phase to a− in forming one b− species, “(ads)” denotes here that the species is adsorbed on an active site, while “(0)” refers to components in the solution phase in the immediate vicinity of the electrode surface [71]. the rate of the first reaction (ν1) depends on the electrode potential (e), the concentration of a − and b− near to the electrode surface (ca(0) and cb(0)), the surface concentration of adsorbed a − and b− ions (γa and γb), respectively, [ ]1 1 a b a b, (0), (0), ,v v e c c γ γ= , (3.4) while the rate of the second reaction (ν2) depends on the electrode potential e, γa and the concentration of h+ (ch(0)) adjacent to the electrode surface: ( )2 2 a h, , 0v v e γ c =   . (3.5) according to the above considerations a b tγ γ γ+ = (3.6) where γt is the total surface concentration of the active sites. it can be assumed, that γt is constant at a given electrode potential. under steady-state conditions 1 2v v= . the transport equations for the (linear) diffusion of the species b− can be written in the form: 2 b b b 2 tx c c d t x    ∂ ∂ =   ∂ ∂   (3.7) where x is the distance from the electrode surface. if the system is perturbed with a sinusoidal signal of low amplitude, the variables can be written in terms of a stationary and a fluctuating component: ( ) ( ) ( ) ( )exp iξ x,t ξ x ξ x ω t= + δ , (3.8) where ( )ξ x represents the steady-state quantity, ( )ξ xδ is the amplitude of the fluctuation, ω is the angular frequency of the perturbing signal, t is the time, and ”i” is the imaginary unit. on making an appropriate taylor series expansion of the reaction rates with the plausible assumptions γt = constant, ( ) 1 a 0 0 v c  ∂ ≈  ∂  and ( ) 2 h 0 0 v c  ∂ ≈  ∂  , we obtain : ( ) ( ) ( ) b b bb 1 1 1 1 1 b b , b b ,, 1 0 1 b 2 b exp(i ) 0 exp(i ) exp(i ) 0 exp(i ) 0 exp(i ) exp(i ) c γ e ce γ v v v v v e ωt c ωt γ ωt e c γ v k e ωt k c ωt k γ ωt    ∂ ∂ ∂ = + δ + δ + δ =      ∂ ∂ ∂     = + δ + δ + δ (3.9) j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 16 b 2 2 2 2 b 2 3 4 b b exp(i ) exp(i ) exp(i ) exp(i ) γ e v v v v e ωt γ ωt v k e ωt k γ ωt e γ  ∂ ∂ = + δ − δ = + δ −  ∂ ∂    (3.10) each of the “rate constants” (k0 k4) appearing in equations (3.9) and (3.10) represents a partial derivative. the “faradaic” current density related to the charge-transfer reaction is given by if = nfν2. thus the linearized form of if becomes f 3 4 b( )i nf k e k γδ = δ − δ (3.11) since b 2 1 γ v v t ∂ = − ∂ , thus ( ) ( )b 3 0 2 4 b 1 biω γ k k e k k γ k cδ = − δ − + δ − δ . (3.12) the linearized form of eq.(7) is : ( )2 b b 2 i ( ) b c x ω c x d x ∂ δ δ = ∂ . (3.13) the above set of linearized equations (3.9) to (3.12) and the differential equation, (3.13) can be solved analytically [68,72-74] by using the following boundary conditions: at x = 0 b b 1 0 1 b 2 b (0) (0)b c j d v k e k c k γ x ∂δ δ = − = δ = δ + δ + δ ∂ (3.14) at x = δ ( )b 0c x δδ = = (3.15) where δ represents the diffusion layer thickness, and jb is the flux of species b −. the resulting expression for the “faradaic” admittance is quite complicated: ( ) ( ) ( ) 1 0 0 4 1 f 1 2 3 4 1 2 1 2 2 4 2 4 1 1 (1 ) i i k k k k k q y ω y ω y ω nfk nfk k k k k ω k k ω k k q k q k − − = + = − + + + + + + + − − (3.16) with ( ) ( ) exp 2 1 exp 2 1 ds sδ q sδ  + = − and b iω s d = . the first term of the right hand side of equation (3.16) can be rewritten as y1(ω) = 1/z1(ω), where the impedance function z1(ω) can be expressed by ( )1 3 4 4 1 4 3 2 3 2 3 b 1 1 1 1 tanh( ) i z ω nfk k k k k sδ ω nfk nfk k nfk k d s = + + − . (3.17) the second term can be written as y2(ω) = 1/z2(ω), where ( ) ( ) ( )2 4 1 12 0 4 0 b 0 4 tanh1 tanh sδk k k k s z ω sδ nf k k nfk d s nfk k + = − − . (3.18) m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 17 thus, the faradaic impedance of the electrode (zf) is given as ( ) ( ) ( )f 1 2 1 1 1 z ω z ω z ω = + . (3.19) in a general case, the impedance function completed by elements representing the ohmic resistance (ru) of the electrolyte solution and the double layer capacitance (cdl) can be written as: ( ) ( ) ( ) 1 t u dl 1 2 1 1 iz ω r ωc z ω z ω −   = + + +     . (3.20) the impedance function of the double layer in such type of electrochemical systems can be well approximated by zdl(ω) = (iωadl) -β, where β ≤ 1 (but it is close to 1) [75]. thus, ( ) ( ) ( ) ( ) 1 t u dl 1 2 1 1 i β z ω r ωa z ω z ω −   = + + +     , (3.21) in principle, it is possible to estimate the parameters of equation (3.21) from the measured impedance data by complex nonlinear fitting. however, the complete expression of the impedance (equation (3.20)) contains 10 free parameters, and the fitting in the case of such a high number of parameters is always a very difficult task. in addition, due to the mathematical structure of equation (3.21), the parameters are expected to be strongly correlated [76]. in order to overcome the difficulties concerning cnls fitting, simplifications in the mathematical expressions should be introduced. it may be assumed that k0 is very small, therefore z2(ω) can be neglected or approximated by an ohmic resistance: z2(ω)≈rk. on the other hand, by substituting ct 3 1 r nfk = , 4a 3 k c nfk = , 4a 2 3 k r nfk k = , 1 41 2 3 b k k p nfk k d = − , and 0 bp δ d= into equation (17), we have ( )1 ct ct a a a w0 a 1 1 1 1 1 1 1 1 1 i itanh( i ) i z ω r r ωc ωc r zp ω r p ω = + = + + + + + . (3.22) the equivalent circuit analog [77] corresponding to the analytical expression given by equation (3.21) is presented in figure 10. j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 18 figure 10. the equivalent circuit analog. detailed numeric results of the cnls fittings are presented in the study by ujvári et al [78]. surprisingly, despite of the great number of adjustable parameters, many of them could be determined with a good statistics (for instance ru, p0, ra, ca, pl, adl, β, ), and reasonable estimated mean values have been obtained for the others. figure 11. (a) impedance spectra (complex plane plots) recorded on rh in 1 mol dm3 hclo4 solution. the impedance of the electrode were measured at different rotating rates of the rde at the electrode potential of e = 0.01 v vs. ssce. (t = 25.0 °c). rotation rates (ωr): 1(): 500 rpm; 2 (): 1000 rpm; 3 (♦): 2000 rpm; 4 (): 3000 rpm. (b) – (c) bode diagrams (log|z| versus m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 19 log f and φ versus log f plots) at 4-4 different rotation rates of the rde. continuous lines: simulated curves calculated by using the optimized parameters. adapted from [67]. figures 11b and 11c show the bode diagrams (log|z| versus log f and φ versus log f plots) at 4-4 different rotation rates of the rde, respectively (e=0.01 v vs. ssce, t = 25 °c). the argand diagrams (complex impedance plane plots) are presented in figure 11a. in figures 11a-11c, beside the measured values (discrete points), the simulated curves calculated by using the optimized parameters are also displayed (continuous lines). the “visual” inspection of the transformed curves (bode plots) supplies more valuable information concerning the goodness of a fit than that of the complex plane plot, since complex plane plots usually show a rather “good” fit, while the transformed curves reveal the problems. the physical reality of the parameters is perhaps the most important criterion in checking the correctness of a model. the variation of the appropriate fitting parameters with rotating rates of the rde could be very informative. particularly the changes observed in ca and p0 are of special interest. ca increases monotonically with the rotation rate, as expected for decreasing concentration of chloride ions in the immediate vicinity of the electrode surface. the p0 vs. ωr -1/2 plot is linear. 4. reduction of perchlorate ions in the course of anodic dissolution and corrosion of metals most studies concerning the basics of the anodic dissolution of metals focused on relatively simple systems [79]. “simple systems” ideally consist of single-crystalline or polycrystalline, highly pure metals in acid aqueous solutions free of dissolved oxygen, surface-active substances, and complexing agents. the dissolution behavior in alkaline or neutral media, even in deaerated or aerated aqueous solutions free of surface-active and complexing substances, is difficult to study because of the easy formation and slow dissolution of protective layers. also, the influence of surface-active substances on the dissolution kinetics and mechanisms of metals is better known in acid than in neutral or alkaline environments. it is an almost general conclusion drawn from anodic dissolution studies that the nature of anions present in the electrolyte solutions affects the dissolution process to a more or less pronounced extent. in most mechanistic considerations, this effect is explained by the role of the specific adsorption of anions. accepting this assumption, and taking into consideration that the adsorbability of the anions on metals is very different depending on the nature of the anion, it would be important to study the dissolution process in the presence of anions with very low adsorbability in order to minimize the role of anion adsorption in the dissolution process, consequently, to obtain experimental data without the interference of anion specific adsorption. according to some very recent views, 4clo − are assumed to fulfill this requirement [80,81]. on the basis of the assumption of stability of 4clo − sophisticated electrochemical methods, such as eqcm and electrochemical impedance spectroscopy (eis), were used for the study of anodic dissolution and corrosion of various metals in perchloric acid solutions, for instance, in the case of nickel [82] and iron [80,81]. detailed mechanistic conclusions are drawn from such studies without taking into account the possible reduction of 4clo − under the conditions of the experiments. in a series of communications [83-85], the authors pointed out that the reduction of 4clo − takes place during the corrosion of cu, al, zn, ni, and fe in deoxygenated hclo4 solutions and emphasized that, for the interpretation of the results of corrosion or anodic dissolution studies in the hclo4/metal systems, this fact cannot be left out of consideration. presence of chlorate and j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 20 perchlorate ions leads to passivity breakdown and pitting corrosion of iron in sulfuric acid solutions [86]. the origin of the pitting has been attributed to cl− produced via the reduction of 3clo − and 4clo − by fe2+. similar results were obtained for al in [87]. the xps experiments detected perchlorate and chloride ions on the surface. both of these ions may play role in the passivity breakdown. oscillatory electrodissolution of al in perchloric acid solutions has been described [88]. according to the authors, current oscillations occurred over a large range of hclo4 concentration and over a wide scale of applied potential. during the anodic dissolution of the al electrode in hclo4 solutions, perchlorate ions were reduced to chloride ions, which induced pitting corrosion. the electrodeposition of cu on ru(0001) in perchlorate media has also been investigated [89]. xps was used to determine surface composition. the spectra show that 4clo − dissociates into adsorbed cl− and xclo − species. the composition of the passive layers formed on zn electrode in naturally aerated and deaerated 0.1 m kclo4 solution were studied using chronopotentiometry, electrochemical impedance, and x-ray photoelectron spectroscopic measurements (xps). cl− from the perchlorate reduction reaction were detected in the solutions during electrode polarization [90,91]. 5. interaction of 4clo with iron group metals in aqueous media the dissolution kinetics and mechanism of iron-group metals has been investigated in a great number of studies [79-82,92]. the kinetics of the anodic dissolution of iron, cobalt, and nickel was found to be quite similar and a common mechanistic picture can be used for the interpretation of phenomena occurring with these metals. in the 1960s, göhr and kruger investigated the behavior of cobalt in perchloric acid solution using measurements carried out under controlled potential and current [93,94]. the results were interpreted on the basis of thermodynamic data. it was concluded that at open circuit and at small polarizations cobalt corrodes exclusively with hydrogen evolution. the possibility of the reduction of 4clo − under these conditions was not considered as a physical reality. similar conclusions were drawn in case of other iron-group metals as presented in the survey article by lorenz and heusler devoted to the problems of the dissolution of iron-group metals [79]. on the other hand, it is an almost generally accepted view that the nature of the anions present in the solution phase in contact with the dissolving metal plays an important role in the kinetics and mechanism of the dissolution process. it is assumed that the main factor determining the effect of anions in these reactions is the adsorbability of the anion, i.e. the direct interaction of anions with the metal surface. generally, it is assumed that this interaction does not lead to the chemical transformation of the anion. it is also assumed that the adsorbability of 4clo − on iron-group metals is very low; thus, the adsorption of other anions can be studied in the presence of perchlorate supporting electrolyte without significant role of adsorption competition. however, the validity of this assumption could be questioned in the light of some recent studies [20] and revisiting some results reported in the literature more than fifty years ago [95,96]. as mentioned above, the problem of perchlorate contamination of ground waters is nowadays an important environmental issue [24,37-39], and several methods were suggested and elaborated for reductive elimination of 4clo − . one of these methods, reported quite recently, is the reduction with metallic iron. the results obtained from these practical studies can be m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 21 considered convincing evidence of the reduction interaction of fe with 4clo − . results with co prove that the rate and extent of the reduction process could also be very characteristic [97]. under suitable chosen experimental conditions, even this process could be the predominant reaction in the dissolution of metallic cobalt in the presence of 4clo − . in contrast to fe and co, a relatively slow reduction was found in the presence of ni [83]. nevertheless, it can be stated unambiguously that 4clo − dissolved in acidic aqueous solutions can be reduced with ni [83], fe [98], and co [97] metals. according to [99], significant reduction of perchlorate to chloride ions occurred on the nickel cathode in a cell with a nickel working electrode and a platinum counter electrode in concentrated solutions of hclo4. it has been shown that the mechanism of this process involves platinum deposited in small amounts on the cathode as a result of oxidation of the platinum anode in the perchloric acid solution. the reduction of perchlorate was accompanied by oxidation of the nickel cathode, which can be attributed to chlorate ions formed in the initial step of perchlorate reduction. to estimate the relative role of 4clo − reduction in the overall dissolution process of ni, fe, and co, we summarized some data obtained in previous studies of metal-perchlorate (perchloric acid) interaction (table 2). it may be seen that the rate of the formation of cl− from 4clo − is relatively low in the case of ni at ambient temperature, while for fe and co, depending on the experimental conditions, the reduction of 4clo − could play a substantial role in the overall process. considering this situation, it could be expected that the results of the usual corrosion measurements carried out in the presence of acidic perchlorate solution should reflect this role in the case of co and fe. table 2. interaction of ni, fe, and co with 4clo − at t = 25°c electrolyte a chemical amount corresponding to the mass of dissolved metal, mol b chemical amount of clformed during the dissolution of metal, mol b / a conversion* / % hclo4/m naclo4/m ni 3 3.57×10-2 7.5×10-5 2.1×10-3 0.84 1 1.26×10-2 2.6×10-5 2.0×10-3 0.80 fe 1 2 1.80×10-3 8.6×10-5 4.77×10-2 19.6 0.1 2.9 5.73×10-4 4.56×10-5 7.96×10-2 31.8 0.1 0.9 4.48×10-4 2.92×10-5 6.52×10-2 26.1 0.5 0 4.48×10-4 2.96×10-5 6.61×10-2 26.4 1 0 1.27×10-3 5.75×10-5 4.53×10-2 18.1 co 1 0 9.48×10-4 4.95×10-5 5.2×10-2 20.1 4 0 14.45×10-4 15.74×10-5 1.09×10-1 43.5 0.1 1.9 11.73×10-4 9.54×10-5 8.1×10-2 32.5 * the percentage of dissolved metal involved in 4clo − reduction (see text). the data presented in the table 2 were obtained using metal powders. thus, they should be considered as average values as during the dissolution process the parameters of the systems are changing continuously. some of these factors are as follows: i) the h+ concentration is changing according to the following equation j. electrochem. sci. eng. 1(1) (2011) 1-26 perchlorate ions in environment 22 ( )+ + 2+ 2+0 0h h cl co cl h co8 2 4 2c c c c c c c+= − − − = − (5.1) ii) the surface area of the metal powder is changing in the course of the dissolution process [100]. the change in the h+ concentration could be very important at low initial hclo4 concentrations. for instance, taking into consideration the data reported in table 2 for the dissolution of co at 0.1 mol dm-3 hclo4 concentration at 25 °c using 25 ml solution, the final h + concentration is ( )+ 3 3h 0.10 0.094 6 10 mol dmc − −= − = × . (5.2) this means that at the end of the dissolution experiment, the hydrogen ion concentration is more than one order of magnitude lower than that in the initial state. only in the case of high h+ concentration (4 mol dm-3) can be expected a practically constant acidity during the whole dissolution experiment. the problems caused by the h+ consumption are well reflected in the temperature dependence of the conversion of 4clo − to cl− in the presence of co powder. at low hclo4 concentrations (0.1 and 1 mol dm -3), the scattering of the data is remarkable, while in the case of a solution of 4 mol dm-3 hclo4, a monotonous increase can be observed. the change in the ph influences not only the rate of the perchlorate reduction – metal dissolution, but also the adsorption of the produced cl− . the adsorption phenomena were studied on powdered metals [100]. the principle of the method used was the measurement of radiation intensity originating from labeled adsorbed species on a powdered metal layer sprinkled on a thin plastic foil that serves simultaneously as the window for radiation measurement. the measurements were carried out at ambient temperature in an ar atmosphere. radiotracer experiments at different h+-ion concentrations can be seen in figure 12. the adsorption of cl− decreases with the ph. figure 12. the ph dependence of the adsorption of chloride ions (on relative scale) on co. it should be noted, however, that attempting the study of the adsorption of labeled cl− on powdered co in 0.5 m naclo4 solution at low ph values, for instance, ph ≈ 2 (by addition of hclo4) the count-rate vs. time curves (curves 1, 2 and 3 in figure 13) going through maximum were obtained. m. ujvári & g. g. láng j. electrochem. sci. eng. 1(1) (2011) 1-26 doi: 10.5599/jese.2011.0003 23 figure 13. count-rate vs. time curves obtained following the addition of 0.25 ml 1 mol dm-3 hclo4 to a solution of 0.5 mol dm -3 naclo4 + 1×10-4 mol dm-3 labeled -cl in contact with 0.5 g co powder. (1) first run, (2) second run, (3) third run, (4) addition of 1×10-4 mol nacl, (5) addition of 5×10-4 mol nacl. adapted from [98]. the decrease in the count-rate means that no equilibrium or steady state is attained with respect to the surface concentration of the isotope emitting the radiation. this phenomenon can be explained by the production of inactive cl− (i.e. by the decrease of specific activity of cl− species). the displacement of the active species by non-active ones occurs immediately at the surface and this could be the very reason for the rapid decrease of the count-rate in the case of curve (1). in the course of the corrosion, the ph also shifts from the initial ph ≈ 2 value to higher ones. in order to demonstrate that the decrease in the count-rate is mainly determined by the production of inactive cl− , hclo4 was added again to the solution phase shifting the ph to the original value. the results of this procedure presented by curves 2 and 3 in figure 13 show that the maximum in the count-rate is significantly lower than that for curve 1 and that the maxima are less pronounced in the two former cases. this observation is in agreement with the assumption of the production of cl− . it is evident that with decreasing specific activity the “isotopic dilution” caused by the production of further cl− becomes less and less pronounced and this is reflected by the position and shape of the maxima on curves 1, 2, and 3. curves 4 and 5 in figure 13 were obtained following the addition of inactive nacl to the solution phase. in this case, the “isotopic dilution” was carried by direct addition of inactive −cl . the effect observed is in accordance with the expectation. in the course of the corrosion of co powder, cl− are formed from the 4clo − . thus, the ph dependence presented in figure 12 was determined starting from ph = 6 and measuring the radiation intensity only a short period following a shift of ph to a preselected value. for more details about anion adsorption see ref. 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[101] g.g. láng, m. ujvári in perchlorates: production, uses and health effects, (l.e. matthews, ed.) nova publishers, in press, isbn: 978-1-61122-857-1 © 2011 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) a new amperometric glucose biosensor based on screen printed carbon electrodes with rhenium(iv) oxide as a mediator doi: 10.5599/jese.2012.0023 199 j. electrochem. sci. eng. 2 (2012) 199-210; doi: 10.5599/jese.2012.0023 open access : : issn 1847-9286 www.jese-online.org original scientific paper a new amperometric glucose biosensor based on screen printed carbon electrodes with rhenium(iv) oxide as a mediator albana veseli, ahmet hajrizi*, tahir arbneshi, kurt kalcher* department of chemistry, faculty of natural science, university of prishtina, nëna terezë, 10000, prishtina kosovo *institute of chemistry, analytical chemistry, karl-franzens university of graz, universitätsplatz 1, 8010 graz, austria corresponding author: e-mail: kurt.kalcher@uni-graz.at received: july 09, 2012; revised: september 14, 2012; published: november 10, 2012 abstract rhenium(iv)-oxide, reo2, was used as a mediator for carbon paste (cpe) and screen printed carbon (spce) electrodes for the catalytic amperometric determination of hydrogen peroxide, whose overpotential for the reduction could be lowered to -0.1 v vs. ag/agcl in flow injection analysis (fia) using phosphate buffer (0.1 m, ph=7.5) as a carrier. for hydrogen peroxide a detection limit (3σ) of 0.8 mg l-1 could be obtained. reo2-modified spces were used to design biosensors with a template enzyme, i.e. glucose oxidase, entrapped in a nafion membrane. the resulting glucose sensor showed a linear dynamic range up to 200 mg l-1 glucose with a detection limit (3σ) of 0.6 mg l-1. the repeatability was 2.1 % rsd (n = 5 measurements), the reproducibility 5.4 % (n = 5 sensors). the sensor could be applied for the determination of glucose in blood serum in good agreement with a reference method. keywords biosensor; glucose; hydrogen peroxide; rhenium(iv)-oxide; blood serum introduction nowadays, sensors and biosensors research, an ever expanding field of analytical chemistry, has been attracting scientists from many related disciplines such as biology, material sciences etc. [1]. chemical sensors and biosensors are offering alternative solutions, capable of satisfying the increasing demand for precise and fast analytical information through devices that require relatively simple instrumentation [2-4]. j. electrochem. sci. eng. 2(4) (2012) 199-210 amperometric glucose biosensor 200 electrochemical biosensors often rely on the use of oxidases or dehydrogenases as biological recognition element [5-8]. when using oxidases, hydrogen peroxide is often formed as an electrochemically active intermediate, whose detection is the crucial point of the design of corresponding biosensors. h2o2 can be oxidized or reduced at rather high positive or negative potentials with inherent risk to co-oxidize or co-reduce components of complex matrices, such as blood, urine or other biological fluids or extracts. in order to overcome the problem, mediators are used which react chemically with hydrogen peroxide and reduce high over potentials [6-9]. in many cases transition metal compounds are used which may change easily their oxidation states such as hexacyanoferrates [8-17], platinum metals or their oxides [18], and manganese dioxide [6-9,19-21]. rhenium shows quite a few oxidation states which could be involved in a reaction cycle where hydrogen peroxide is involved [22]. the main goal of this work was to investigate the electroanalytical behavior of reo2 in terms of acting as a catalyst for the determination of hydrogen peroxide as a basis for oxidase based biosensors. in particular, glucose oxidase was used as a template enzyme. experimental apparatus batch cyclic voltammetry and chronoamperometric measurements were performed using a potentiostat (palmsens, electrochemical sensor interface) connected to a laptop computer. carbon paste electrodes cpes (modified and unmodified) were used as working electrodes. as a reference electrode an ag/agcl electrode was used (3m kcl). all potentials referred to in this paper are against this reference electrode. a platinum wire served as a counter electrode. the flow injection system consisted of a high performance liquid chromatographic pump (510 waters, milford ma, usa) in connection with a system controller (waters 600e), a sample injection valve (5020 rheodyne, cotati, ca, usa), and a thin layer electrochemical detector (lc 4c, bas, west lafayette, indiana, usa) with a flow through cell (spacer thickness 0.19 mm; cc-5, bas) in combination with an electrochemical workstation (bas 100b). spce was used as the working electrode and an ag/agcl-electrode (3 m kcl) as the reference. the steel back plate of the thin layer cell served as the auxiliary electrode. reagents and solutions phosphate buffer (0.1 mol l-1) was prepared by mixing aqueous solutions (0.1 mol·l-1) of sodium dihydrogen phosphate and disodium hydrogen phosphate to produce solutions of the required ph (7.5). a stock solution of hydrogen peroxide (1000 mg l-1) was prepared freshly every day and solutions of lower concentrations were prepared immediately before use. also a stock solution of glucose (1000 mg l-1) was prepared with the corresponding working buffer solution, kept at room temperature overnight to facilitate α-ß-mutarotation, and stored at 4 oc when not in use. lower concentrations were prepared immediately before use. all chemicals were of analytical purity grade (p.a., fluka). a. vesel et al. j. electrochem. sci. eng. 2(4) (2012) 199-210 doi: 10.5599/jese.2012.0023 201 preparation of working electrodes carbon paste electrodes (unmodified); 1 g graphite powder and 360 µl paraffin oil were mixed in an agate mortar by gently stirring with a pestle until uniformity and compactness. for modified cpe 50 mg of reo2 were added per gram of graphite powder. screen printed carbon electrodes (modified): 0.05 g of rhenium(iv)-oxide was added to 1 g carbon ink (electrodag 421 ss, acheson); the oxide-ink mixture was stirred for 10-30 minutes with a glass-rod or stainless steel spatula and finally sonicated for 30 minutes in an ultrasonic bath (transsonic 700/h, elma®). the resulting mixture was immediately used for electrode fabrication using the semi-automatic screen printer (sp-200, mpm, ma-usa) and aluminum oxide support. the printed electrodes were dried at room temperature overnight before using for measurements. to prepare enzyme casting solutions, required volumes of ethanol (80 µl), nafion (5 % w/w in lower alcohols, 40 µl), water (80 µl) and god solution (5 % w/w, 40 µl) were mixed in the order listed in a plastic vial (1.5 ml micro centrifuge tubes, 616.201, greiner labor technic). five μl (unless otherwise specified) of the resulting mixture were directly applied onto the active area of the spce surface (≈0.40 cm2 area) and air dried. the electrode was inserted into the thin layer cell; electric contact was made with a crocodile clamp. procedures cyclic voltammograms were recorded from an initial potential of 0.80 v to a vertex potential of -1.00 v. the scan rate was 20 mv s-1; usually three cycles were recorded. hydrodynamic amperograms were recorded at potentials from -500 mv to 100 mv in increments of 100 mv. flow injections analyses were done at a potential of -100 mv if not stated otherwise. the flow rate of the pump was 0.40 ml min-1 and the injection volume of hydrogen peroxide and glucose solutions was 200 µl. analyses of samples blood samples were taken manually with a syringe and put in the plastic tubes containing edta (1.9 mg per ml of blood) as an anticoagulant. 1 ml of each sample was transferred into a 10 ml sterile pp-tube, diluted 10 times and centrifuged for 20 minutes at 4500 rpm. from the centrifuged supernatant serum the analyte solutions were prepared by diluting 40 and 100 times with phosphate buffer (0.1 m, ph 7.5). measurements were done in fia mode using the standard addition method by sequentially adding three times an amount of glucose standard (100 mg l-1) in phosphate buffer solution (0.1 m, ph 7.5), which corresponds roughly to half of the original glucose amount in the sample. after each addition the sample was re-measured. a commercial glucometer ascensia brio was used for glucose reference measurements using whole blood. results and discussion the studies were done with a few working electrodes: unmodified and reo2 modified carbon paste electrodes (cpe) and screen printed electrodes (spce) modified with reo2 as a mediator and glucose oxidase as a biocomponent (entrapped in a nafion film). j. electrochem. sci. eng. 2(4) (2012) 199-210 amperometric glucose biosensor 202 studies with hydrogen peroxide cyclic voltammetry h2o2 is electrochemically active with carbon paste electrodes, but shows high overpotentials for its reduction and also its oxidation as can be seen in fig. 1. at positive potentials h2o2 does not show any particular electrochemical activity at unmodified carbon paste electrodes. figure 1. cyclic voltammograms of h2o2 at an unmodified cpe; phosphate buffer solution 0.1 m, ph 7.5;fill curve blank, broken curve with 200 mg l -1 h2o2; conditions: einitial = 0.80v, efinal =-1.00v, scan rate 20 mv s -1. at negative potentials, similar behavior can be noticed. direct reduction of hydrogen peroxide occurs at rather negative potentials only. according to our knowledge, rhenium (reo2) has not yet been used as a modifier for carbon pastes or carbon inks. figure 2.cyclic voltammogram of a reo2-modified cpe; einitial = 0.80 v, efinal = -1.0 v; scan rate 20 mv s -1; support electrolyte 0.1 m phosphate buffer, ph 7.5. a. vesel et al. j. electrochem. sci. eng. 2(4) (2012) 199-210 doi: 10.5599/jese.2012.0023 203 figure 2 shows the cyclic voltammogram of a rhenium(iv)-oxide modified carbon paste electrode. no distinct peaks can be discerned in the investigated potential range. reduction occurs at positive potentials already and dominates at more negative values. re-oxidation on the other hand begins at slightly negative potentials already, but in fact does not get dominant even up to 0.7 v. the reduction current can be assigned to the reduction of re(iv) to re(iii) (probably present as re2o3 in slightly alkaline medium or even as repo4). re-oxidation in phosphate buffer solution seems electrochemically rather irreversible because it is much smaller than reduction showing a very broad signal extending from -0.3 v onward. in the presence of hydrogen peroxide a small additional reduction current occurs in cyclic voltammetry (fig.3). figure 3 cyclic voltammograms of a cpe modified with reo2 in the absence and in the presence of hydrogen peroxide; h2o2 200 mg l -1; conditions: e initial =0.80v, e final = -1.00v, scan rate 20 mv/s, support electrolyte 0.1 m phosphate buffer, ph 7.5. although the effect of the modifier is small in cv (but more pronounced in amperommetry, see below) it may be noted that in fact a catalytic effect on the reduction of h2o2 exists. basically two mechanisms can be assumed: rhenium(iv)-oxide (in the oxidation state +4) is reduced to trivalent rhenium (re2o3) which is oxidized to reo2 by hydrogen peroxide again (mechanism a, fig.1). thus, virtually more modifier (reo2) is present at the surface creating a higher reduction current. reo2 re o2 3 h o2 2 h o2 ereduction current a reo3 4 or reoreo2 h o2 2 h o2 ereduction current b fig.4. models for the catalytic action of reo2 on the reduction of h2o2; catalytic redox cycle of rhenium(iv)-oxide via trivalent rhenium and the action of h2o2 (a); catalytic redox cycle of rhenium(iv)-oxide via hexaor heptavalent rhenium and the action of h2o2 (b). j. electrochem. sci. eng. 2(4) (2012) 199-210 amperometric glucose biosensor 204 on the other hand it is basically also possible that reo2 is oxidized to reo3 or even perrhenate, reo4 -, a reaction which is known to proceed rather slowly in aqueous solution. in this case hydrogen peroxide would react with the original modifier, reo2, first, and then the reduction of the re(vi) or re(vii)-species would occur (mechanism b, fig.4). according to the fact that at negative potentials, where catalytic action can be noticed, reduction of reo2 is observable at the modified electrode alone, mechanism a seems more probable. hydrodynamic amperommetry figure 5 summarizes the hydrodynamic voltamperogram of the catalytic current caused by reo2 as a modifier compared with an unmodified cpe. the measerments were performed with stirred solutions as batch experiments, and the current change after addition of hydrogen peroxide was recorded. high signals can be obtained at potentials beyond -400 mv, but at such values the risk of co-reduction of other compounds in complex matrices is also high. nevertheless, at a potential of -100 mv there are significant current responses observable already, which can be exploited for quantitative analytical purposes. in fact, even higher operation potentials up to +100 mv could be used, but the current is significantly lower compared to the signal at -100 mv. with unmodified electrodes reduction currents of h2o2 can be observed under the given experimental conditions only at -400 mv and below (fig.5 curve b). figure 5. reduction current of h2o2 in hydrodynamic voltamperometry at a cpe modified with reo2 (a) compared to an unmodified cpe (b); supporting electrolyte sodium phosphate buffer (0.1 m, ph 7.5); hydrogen peroxide concentration 50 mg l-1. flow injection analysis (fia) fia was used to study the effect of reo2 on the reduction of h2o2 in detail and to optimize the experimental parameters. as screen printed carbon electrodes show higher robustness towards mechanical and chemical stress, but are otherwise comparable to carbon paste sensors due to their heterogeneous character concerning the electrode material, this type of electrode was used for further studies. figure 6 shows a typical amperogram with injection of hydrogen peroxide using a reo2-spce as a detector. a. vesel et al. j. electrochem. sci. eng. 2(4) (2012) 199-210 doi: 10.5599/jese.2012.0023 205 figure 6. amperogram obtained by fia with a spce modified with reo2; injection volume of h2o2 solution 200 µl; operating potential -100 mv; flow rate 0.4 ml min -1; carrier phosphate buffer (0.1 m, ph 7.5). the rhenium(iv)-oxide modified electrode responds very well and reproducible to hydrogen peroxide concentrations even in the low mg l-1-range. for optimization of the operating potential a hydrodynamic voltammperogram was recorded under fia conditions (fig.7). figure 7. dependence of the peak height of h2o2 on the working potential in fia with spces modified with reo2 as detector, flow rate 0.4 ml min -1, carrier 0.1 m phosphate buffer, ph 7.5, h2o2-solution 100 mg·l -1; injection volume 200 µl. reduction currents can be observed up to even 200 mv. when decreasing the potential to more negative values the signal (peak height of the reduction current) increases. apart from the reasons discussed already (increased risk of interference with increasing negative potentials) there is another disadvantage connected with highly negative operating potentials. the background j. electrochem. sci. eng. 2(4) (2012) 199-210 amperometric glucose biosensor 206 current strongly increases below -200 mv (not show) and even tends to vary during operation that the repeatability is significantly deteriorated. the signal-to-background ratio gets unfavorably low because the latter exceeds even the height of the signal. for these reasons -100 mv was chosen as the most favorable operating potential. figure 8. calibration curve for h2o2 within concentration 1-1000 mg·l -1at modified spce at 100 mv. supporting electrolyte phosphate buffer 0.1 m, ph = 7.5. a corresponding calibration curve for hydrogen peroxide is displayed in fig. 8. the linear dynamic range of the sensor (concentration range with linear relation between signal and concentration) is restricted to the lower concentrations of the analyte. linearity between concentration of hydrogen peroxide and signal exists from 0.6 to 20 mg l-1, with a correlation coefficient over 0.99. the detection limit (3σ), estimated from the standard deviation of fia-peaks at 5 mg l-1 h2o2, is 0.2 mg l-1. the repeatability of measurements for 100 mg l-1 is 3 % rsd (n = 5 measurements), and the reproducibility for 100 mg l-1 at different working electrodes (n = 5 electrodes) is 5.7 %. studies with glucose in order to check if the reo2 modified electrode may serve as a basis for the design of first generationbiosensors with oxidases producing h2o2, glucose oxidase was used as a template enzyme. it was immobilized on the surface of the reo2-modified spces using a nafion membrane [23]. the sensor responds well to injections of glucose under fia conditions. a typical calibration curve of the biosensor is shown is shown in figure 9. the range with a linear relation between current and concentration of glucose was found to extend up to 100 mg l-1 (fig. 9 insert) with a correlation factor r2=0.9999. the linear regression function is given in eqn. (1). i / na= 0.4867 c / mg l-1 + 2.4184 (1) a. vesel et al. j. electrochem. sci. eng. 2(4) (2012) 199-210 doi: 10.5599/jese.2012.0023 207 figure 9. calibration of glucose with a glucose oxidase/reo2-modified spce; working potential: -100 mv; flow rate: 0.4 ml/min; 100 µl injection volume. the detection limit (3σ) estimated from the standard deviation of fia-peaks for 20 mg l-1 glucose concentration is 0.6 mg l-1. the repeatability of measurements for 200 mg l-1 glucose was 2.1 % rsd (n = 5 measurements), and the reproducibility was 5.4 % rsd (n = 5 electrodes). table 1. comparison of limit of detection of different used modifiers modifiers detection limit of h2o2 detection limit of glucose references mg l-1 mg l-1 iro2 0.24 0.9 [26] fe3o4 0.2 0.5 [23] mno2 0.045 0.087 [24] sno2 15 6.8 [27] pto2 0.03 2.0 [26] cuo 0.03 [23, 25] reo2 0.2 0.6 this work pdo 0.8 0.83 [26] table 1 compares the lod values for sensors and biosensors modified with different metal oxides for the detection of h2o2 and glucose. it may be stated that reo2 is capable of detecting very low concentrations of hydrogen peroxide and glucose; it is comparable to fe3o4 [23] and super cedes platinum metal oxide [26]. interferences four common interferences, vitamin c, paracetamol, gentisic acid, and uric acid were tested if they interfere with the detection of glucose using the reo2-based biosensor. all investigated interferents give significant responses in the investigated concentration ratio. nevertheless, their concentration in blood may be expected to be much smaller so that the extent of influence on the signal of the analyte, glucose, is expected to be negligible (table 2). j. electrochem. sci. eng. 2(4) (2012) 199-210 amperometric glucose biosensor 208 table 2. relative signals of possible interferences in the current response of the biosensor in the absence of glucose; 100 % corresponds to the current of 200 mg l-1 glucose. flow rate: 0.40 ml min-1, operating potential -100 mv vs. ag/agcl; carrier: phosphate buffer (ph=7.5; 0.1 m), injection volume 100 µl interferent 50 mg l-1 100 mg l-1 500 mg l-1 % % % ascorbic acid 46.1 95.6 304 uric acid 38.0 45.4 166.0 paracetamol 21.8 21.5 34.7 gentisic acid 19.2 26.7 61.6 analysis of blood plasma samples the glucose biosensor described in this work was used to determine the glucose level in human serum. two samples (st1, sa1) were investigated. analyses were done with the standard addition method basically for two reasons: first to exclude effects of the matrix and interferences, and second to investigate if the linear relation between signal and different concentrations of glucose in the plasma is maintained. glucose concentrations obtained with the biosensor (blood plasma) were multiplied with a factor of 1.15 in order to convert them to whole blood concentrations [28, 29]. the results between the method employing the new biosensor and the reference are in very good agreement (table 3). sample sa1 is blood from a healthy person in the morning, sample st1 from the same person after eating a piece of sweet cake. table 3 comparison of glucose concentrations of two human serum samples measured using the new biosensor and the pocket glucometer ascensia brio. sample reo2 –based biosensor glucometer ascensia brio sa1 993 ± 26 mg l-1 980 ± 10 mg l-1 ml st1 1396 ± 32 mg l-1 1360 ± 20 mg l-1 ml conclusion the work presented here has clearly demonstrated that heterogeneous carbon sensors (carbon paste, screen printed carbon electrodes) with rhenium(iv)-oxide as a mediator exhibit good performance for the determination of hydrogen peroxide because the modifier lowers the overpotential of the analyte. a biosensor based on thick film technology with glucose oxidase as a template enzyme was developed. when using the screen-printed biosensor with flow injection analysis using phosphate buffer (0.1 m, ph 7.5) as a carrier with a flow rate of 0.4 ml min-1, a detection limit of 0.2 mg l-1 glucose could be achieved with a linearity range up to 20 mg l-1. thus, glucose can be determined in blood with this method. the influence of possible interferences (ascorbic acid, uric acid, paracetamol, and gentisic acid) on the determination of glucose was estimated. the extent of all investigated interferences is not fatal for the determination of glucose in human blood plasma. a. vesel et al. j. electrochem. sci. eng. 2(4) (2012) 199-210 doi: 10.5599/jese.2012.0023 209 acknowledgements: a.v. wishes to acknowledge support for this work by the öad-büro für austauschprogramme, österreichische rektorkonferenz and world university service-graz, austria, for a financial support. the authors are thankful to ceepus project cii-cz-0212-02-0809-m-28727 for mobility grants. references [1] x. zhang, h. ju, j. wang, electrochemical sensors, biosensors and their biomedical applications, academic press, new york, usa, 2008. 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[29] k. l. tieszen, 1st german diabetes diagnostics conference, diabetes stoffwechsel and herz, kohn, germany, may 5-7, 2006. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 25%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.6 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [4000 4000] /pagesize [612.000 792.000] >> setpagedevice {comparison of h2o2 screen-printed sensors with different prussian blue nanoparticles as electrode material} http://dx.doi.org/10.5599/jese.719 199 j. electrochem. sci. eng. 10(2) (2020) 199-207; http://dx.doi.org/10.5599/jese.719 open access: issn 1847-9286 www.jese-online.org original scientific paper comparison of h2o2 screen-printed sensors with different prussian blue nanoparticles as electrode material anne müller1, , susan sachse2, manfred decker1, frank-michael matysik3, winfried vonau1 1kurt-schwabe-institut für messund sensortechnik e.v. meinsberg, 04736 waldheim, germany 2hochschule mittweida university of applied sciences, 09648 mittweida, germany 3university of regensburg, 93053 regensburg, germany corresponding author: anne.mueller@ksi-meinsberg.de; tel.: +49 34327 608 104; fax: +49 34327 608 131 received: september 10, 2019; revised: november 7, 2019; accepted: november 7, 2019 abstract in order to determine hydrogen peroxide condensing from gaseous and liquid phases screen-printed electrodes with controlled and adjustable thickness, shape and size of the working electrode as well as electrode paste composition were investigated. for this purpose prussian blue (pb) nanoparticles with a different particle size distribution of 2030 nm (synthesized) and 60-100 nm (commercially available) were mixed with carbon paste and screen-printed on al2o3 templates to establish h2o2-sensitive electrode. these two types of screen-printed sensors were compared to the commercial one during measurements in h2o2/water solutions at concentrations between 10-5 and 10-2 m h2o2. the linear signal in the investigated concentration range was found only for the sensor with the commercially available pb particles. thus, this sensor prepared with pb particles of the size 60-100 nm showed the most reproducible and time-stable response versus the analyte in comparison to the others. this result offers the possibility to create sensors with adjustable design adapted to the concrete functionality. thin films of collecting electrolytes based on agarose gels were printed on the sensor structures. they showed a distinct response on the application of h2o2-containing aerosols and gaseous phase. keywords hydrogen peroxide; screen printing; carbon paste; agarose; hydrogel introduction the sensitive and selective determination of hydrogen peroxide (h2o2) is of great importance for biological, pharmaceutical, ecological and many other fields of application. especially in the decontamination of surfaces h2o2 plays an important role as a sterilant. depending on the requirements, a highly concentrated h2o2 solution is either passed through a vaporizer and then http://dx.doi.org/10.5599/jese.719 http://dx.doi.org/10.5599/jese.719 http://www.jese-online.org/ file://///srvdc2/homelw/mueller/dekosens/vorträge,%20schreiben%20usw/publikation/03/anne.mueller@ksi-meinsberg.de j. electrochem. sci. eng. 10(2) (2020) 199-207 h2o2 screen-printed sensors 200 introduced into the compartment to be sterilized with the aid of a carrier gas [1] or nebulized in the room supported by compressed air [2]. consequently, h2o2 is either only present in the gas phase or additionally solved in aerosol particles. the aim of research is therefore directed to the development of a cost-effective sensor that can reliably detect h2o2-amounts in the gas phase as well as in aerosols. for this purpose, a common method such as the determination of h2o2 in solution using electrochemical sensors with the redox mediator prussian blue (pb) could be modified with regard to the measurement of the analyte in the gas space. pb is known for its excellent catalytic activity and selectivity for the reduction of hydrogen peroxide [3]. pb is usually applied to carbon electrodes by electrochemical deposition [4,5], dip coating [6], inkjet printing [7,8], or immobilization on functionalized surfaces [9,10]. an alternative method suitable for mass production and thus costeffective is the screen printing of the pb working electrode similar to the procedure described by benedet et al. [11]. by this alternative it is possible to perform specific modifications to the composition of the ingredients of the paste for the working electrode and to tailor the sensor surface and design according to the respective measurement requirements. for the realization, pb nanoparticles were synthesized, freeze-dried and finally mixed with a carbon paste to screen print the working electrode. based on these electrode structures, preliminary experiments were performed with the pb electrode covered with agarose gel as a supporting electrolyte, and the resulting probe was used for the determination of h2o2-amounts in the gas phase. experimental materials the following chemicals were used: iron(iii) hexacyano-ferrate(ii) (alfa aesar, kandel, germany); hydrochloric acid (1 mol/l), hydrogen peroxide (30 vol.%) and potassium chloride from carl roth gmbh + co. kg, germany; carbon paste (bq242 conductor paste, dupont electronic materials ltd., uk); potassium hexacyanoferrate(ii) (fluka chemie ag, switzerland); iron(iii) chloride (riedel-de haёn laborchemikalien gmbh + co. kg, germany); agarose (biozym le, biozym scientific gmbh, germany) and hydrogen peroxide (interox sg-50, 49.5 % (w/w), solvay chemicals international sa, belgium). methods pb nanoparticles were synthesized according to chen et al. [9] by mixing 20 ml of a 5 mm potassium hexacyano-ferrate(ii) solution with 10 ml 0.1 m potassium chloride and 0.01 m hydrochloric acid. then 20 ml of 5 mm iron(iii) chloride were added and the mixed solution was stirred overnight. in order to dry the pb nanoparticle solution and to obtain a powder, the solution was first frozen for 20 min at a temperature of -80 °c and then dried for 22 h at -50 °c at a pressure of 1 mbar in a freeze dryer (alpha 1-2 ldplus, martin christ gefriertrocknungsanlagen gmbh, germany). both, the synthesized and the commercially available pb powder (iron(iii) hexacyano-ferrate(ii) ) were mixed with 10 % (w/w) to a carbon paste according to benedet et al. [11]. these mixtures were used to screen print the working electrode with a diameter of 5 mm onto a platinum lead previously applied to an alumina substrate. for the preparation of the screen-printed sensors an electrode arrangement comparable to the electrode arrangement of commercial sensors of the company dropsens (ds710, spain) was chosen. this enabled a direct comparison of the measurement results of the sensors with each other and provides information about the suitability of the different pb a. müller et al. j. electrochem. sci. eng. 10(2) (2020) 199-207 http://dx.doi.org/10.5599/jese.719 201 nanoparticles for electrode fabrication. this is the basis for changing the sensor structure with regard to the intended application. the electrochemical behavior of the electrodes in solution was studied with a potentiostat/galvanostat (autolab pgstat12, metrohm ag, the netherlands). the electrochemical cell was equipped with a three-electrode system, consisting of a pb-carbon working electrode, a screen-printed pt electrode as counter electrode, and an ag/agcl screen-printed electrode as reference electrode (see figure 1) figure 1. design of a screen-printed sensor with three-electrode system before starting an amperometric measurement the electrodes were electrochemically activated. for this purpose, a potential of 0 v vs. ag/agcl (0.1 m kcl) was applied for 5 min in phosphate buffer solution in absence of hydrogen peroxide. the phosphate buffer had a concentration of 0.1 m with 0.1 m kcl and a ph value of 6.36. this buffer was chosen because it has a sufficiently high concentration of kcl to ensure good conductivity and the ph value is low enough to avoid a signal drop [6,12]. during the amperometric measurements successive volumes of a hydrogen peroxide stock solution (0.1 m) were added into a phosphate buffer solution at a constant potential of 0 v vs. ag/agcl (0.1 m kcl). for measurements with decreasing hydrogen peroxide concentration, the sensors were removed, rinsed quickly with deionized water and then transferred to separately prepared solutions. in order to prepare the sensors for measurement of h2o2 in the gas phase and aerosols, the sensors were coated with agarose gel based on kcl solution as electrolyte. for this purpose, 0.2 g agarose was dissolved in 10 ml of a 0.2 molar kcl solution at a temperature of 90 °c and heated to 100 °c for 5 min similar to benedet et al. [11]. the gel was then applied by squeegees to the surface of the sensors fixed in a 40 °c-tempered manufactured holder, shown in figure 2. the reproducible placement of the gel layer directly on the sensor structure was achieved by a hole in the screen. the resulting gel layer had a thickness of 0.3 mm. for determinations of h2o2 in the gas phase and of the aerosols, a test chamber was used in which a 49.5 % h2o2 solution was nebulized using compressed air at a flow rate of 3 ml/min (see figure 3). the measurement was performed with a potentiostat (interface 1000, gamry instruments, usa) amperometrically at the potential of -0.5 v vs. ag/agcl reference electrode located on the structure. this reductive condition is considerably more negative than the reduction peak observed in the range between 0.3 v and 0 v. consequently, all presented pb is converted to its reduced and catalytically active form, the prussian white (pw) (equation 1). fe4iii[feii(cn)6]3 + 4e+ 4k+ → k4fe4ii[feii(cn)6]3 (1) http://dx.doi.org/10.5599/jese.719 j. electrochem. sci. eng. 10(2) (2020) 199-207 h2o2 screen-printed sensors 202 figure 2. manufactured sensor holder for gel coating the chamber was also equipped with a sensor for the detection of gaseous h2o2 from the company dräger (dräger-sensor h2o2 hc, drägerwerk ag & co. kgaa, germany). this sensor measures only gaseous h2o2 and served as a reference for monitoring of the beginning and the end of each dosing phase. during a measurement, a permanent airflow was passed through the test chamber to create a constant atmosphere of gaseous h2o2 and aerosols. figure 3. test chamber for the measurement of gaseous h2o2 and aerosols results and discussion the synthesis of pb yielded sphere shaped nanoparticles. according to the sem images, the pb nanoparticles had a particle size of 20-30 nm, shown in figure 4a. therefore, they were smaller than the commercially available pb particles with a size of 60-100 nm (figure 4b). both, the synthesized nanoparticles and the commercially available pb powder could be mixed without agglomeration into the carbon screen printing paste. the surfaces of the printed electrodes were comparable to those of a commercial screen-printed prussian blue/carbon electrode of the company dropsens, see figure 4c-e. the pb nanoparticles mostly covered the surfaces of the electrodes, so that the large slate-like particles of carbon paste were only visible in a few places. the images of the breaking edges of the working electrodes in figures 4f-h show that the particles are evenly distributed in the carbon paste over the entire layer thickness. the sensors only differed in the thickness of the printed carbon pb layer, approximately 25 µm for the sensors with synthesized and commercially available pb and 20 µm for the commercial sensor. a. müller et al. j. electrochem. sci. eng. 10(2) (2020) 199-207 http://dx.doi.org/10.5599/jese.719 203 figure 4. sem images of (a) synthesized pb-powder. (b) commercially available pb-powder. (c) electrode surface screen-printed with synthesized pb-powder. (d) electrode surface screenprinted with commercially available pb-powder. (e) electrode surface from a commercial sensor. (f) cross section of an electrode printed with synthesized pb. (g) cross section of an electrode printed with commercially available pb. (h) cross section of a commercial sensor both, the synthesized nanoparticles and the commercially available pb powder could be mixed without agglomeration into the carbon screen printing paste. the surfaces of the printed electrodes were comparable to those of a commercial screen-printed prussian blue/carbon electrode of the company dropsens, see figure 4c-e. the pb nanoparticles mostly covered the surfaces of the electrodes, so that the large slate-like particles of carbon paste were only visible in a few places. the images of the breaking edges of the working electrodes in figures 4f-h show that the particles are evenly distributed in the carbon paste over the entire layer thickness. the sensors only differed in the thickness of the printed carbon pb layer, approximately 25 µm for the sensors with synthesized and commercially available pb and 20 µm for the commercial sensor. in figure 5 the current densities of the sensors during the amperometric determination of hydrogen peroxide in different concentrations are monitored as a function of time. it must be noted that all measurements were carried out in unstirred media. therefore, all processes were diffusion controlled and the response times were extended. although this was rather unusual for amperometric measurements, the investigations were performed in this way to compare them with additional planned measurements, in which h2o2 was detected after diffusion from the gas phase into a thin electrolyte layer. all three sensor types show a clear response to the different h2o2 concentrations. the sensor printed with the commercially available pb powder and the commercial sensors had almost identical measurement curves and showed comparable current densities. http://dx.doi.org/10.5599/jese.719 j. electrochem. sci. eng. 10(2) (2020) 199-207 h2o2 screen-printed sensors 204 figure 5. amperometric measurements of sensors with synthesized pb and commercially available pb as well as a commercial sensor in 0.1 m phosphate buffer with 0.1 m kcl (ph 6.36) and different h2o2 concentrations at a polarization voltage of 0 v vs. ag/agcl, 0.1 m kcl both, for measurements with increasing and decreasing concentrations, a linear correlation between h2o2 concentration and the sensor signal at concentrations between 10-4 m and 10-2 m could be determined for all sensors with the exception of sensors with synthesized pb during measurement with decreasing concentration. for the sensor with commercially available pb, the linear correlation could even be extended to the concentration of 10-5 m. this can be seen in the calibration plots in figure 6. the mean values were calculated for data obtained after 10 min of measuring time per hydrogen peroxide solution. figure 6. mean current densities with the associated standard deviations for different concentrations of h2o2 (■ increasing concentration, ▲ decreasing concentration) for n = 3 the sensor, which was manufactured with synthesized pb powder, initially showed a comparable sensitivity to the other two sensor types. however, from a concentration of 10-2 m h2o2, the sensor increasingly lost its sensitivity and the remaining sensitivity dropped clearly. this is an indication that changes have occurred on the surface of the working electrode during the measurement. this could be due to impurities caused by kcl, a by-product of pb synthesis. edx studies of the synthesized and commercially available pb nanoparticles by an accelerating voltage of 20 kev a. müller et al. j. electrochem. sci. eng. 10(2) (2020) 199-207 http://dx.doi.org/10.5599/jese.719 205 showed an increased presence of kcl in the synthesized pb powder. kcl could be identified as plateshaped structures in the sem images in figure 4a, which was confirmed by edx studies (figure 7). cn (yellow-green) has been chosen for the detection of pb, because the iron ions were apparent through all other layers and therefore could not be assigned correctly. in the figure the potassium ions are marked blue and the chloride ions red. from this, it could be concluded that the lilac colored areas represent potassium chloride. figure 7. sem image (left) with edx analysis (right) of synthesized pb powder this kcl becomes detached from the surface of the electrode during the measurement and may carry along pb nanoparticles or reduce the stability of the pb paste composite. this could explain the reduction in selectivity as the measurement progresses. further investigations will show whether purification of the synthesized pb powder will reduce this process and whether these nanoparticles are suitable for the manufacture of screen-printed electrodes. the edx studies of the commercially available pb powder indicated reasons for this assumption. table 1 shows the standardized weight percentages of fe as a representative of pb and of k and cl within the different powders. table 1. chemical composition (edx) of samples in standardized wt.%. element content of synthesized powder, wt.%. content in commercially available powder, wt.%. k 29.91 0.1 cl 22.29 0.06 fe 16.90 35.32 consequently, using the commercially available pb powder, screen-printed sensors could be created that show a larger linear range than the commercial sensors while measurement in solution. the next step for sensor development was the integration of a suitable electrolyte which, in addition to the typical electrolyte properties, also showed chemical resistance against higher concentrations of h2o2, good permeability for h2o2, low evaporation rate and could be reproducibly applied to the electrode surface in thin layers. investigations showed that a polymer system known from sensor technology, agarose, fulfills these properties. therefore, an agarose hydrogel based on kcl solution could be suitable for the use as electrolyte for detection of h2o2 in the gas phase and aerosols [11]. the amperometric measurements of sensors coated with a 0.3 mm thick agarose layer in the test chamber when exposed to gaseous h2o2 and aerosols are shown in figure 8. the current density is monitored in dependency from the introduced gaseous h2o2-phase and aerosol particles (49.5 % http://dx.doi.org/10.5599/jese.719 j. electrochem. sci. eng. 10(2) (2020) 199-207 h2o2 screen-printed sensors 206 h2o2). the introduction of the gaseous h2o2 was controlled by a dräger-sensor (dotted line) and served as a monitoring tool for the start and end of the dosing phase. figure 8. amperometric measurements of gaseous h2o2 and aerosols with three screen-printed pb-sensors coated with agarose gel in the same way the dräger-sensor showed that a constant concentration of gaseous h2o2 of about 240 vol.-ppm was maintained over the dosing phase. in contrast, the screen-printed sensors covered with agarose showed a rapid signal increase after start of the introduction of h2o2. this sensor response pointed to a fast reaction from pw with h2o2 on the electrode surface (equation 2). k4fe4ii[feii(cn)6]3 + 2h2o2 → fe4iii[feii(cn)6]3 + 4k+ + 4oh(2) after the first minute the sensor signal insignificantly decreased, forming a plateau. this plateau indicated that the detection process was limited either by the kinetic of electron transfer for the electrochemical regeneration of pw or structural transformations of pb paste layer, or changes within the electrolyte during the lasting measurement process. the experiment showed that not all incoming h2o2 reacts with pw at the electrode. a considerable amount was decomposed at the sensor surface, creating o2 accumulating in the agarose gel. after oxygen saturation of the gel, bubbles are formed between the electrode surface and the gel layer. these cavities caused a detachment of the gel membrane resulting in a signal interruption. since the sensors coated with agarose showed promising results except for the formation of bubbles, it would be useful in further investigations to reduce the formation of the undesirable cavities by constructive changes or to simplify the gas escape from the electrolyte. conclusions with both, the synthesized and the commercially available pb powder, screen-printed electrodes could be successfully realized which respond sensitively to varying h2o2 concentrations comparable to a purchasable product. with the commercially available pb powder it was even possible to produce electrodes which show a linear response at h2o2 concentrations between 10-5 and 10-2 m that is a larger linear measuring range than of the commercial sensor. in the case of sensors with synthesized pb nanoparticles, a change at the working electrode occurred during the investigation, a. müller et al. j. electrochem. sci. eng. 10(2) (2020) 199-207 http://dx.doi.org/10.5599/jese.719 207 whereby the sensitivity decreased with continuing measurement. a reproducible determination of h2o2 over a longer period of time is currently not possible with this pb paste. further investigations will clarify what causes the changes on the surface of the working electrode in the sensors with synthesized pb powder. therefore, the synthesized pb powder will be purified from kcl. based on the research of the sensors with commercial prussian blue crystals covered with a kcl including agarose gel as the supporting electrolyte, it was shown that the determination of high concentrated gaseous h2o2 amounts and aerosol particles was possible with this sensor setup. further improvements have to be directed on the phase boundary electrode surface / gel electrolyte to avoid the formation of o2-bubbles by catalytic decomposition of hydrogen peroxide and to establish a rapid exchange of the solved analyte with pw particles. furthermore, changes in the electrode structure with the tested pb nanoparticles, as well as variations of the gel composition will be executed. this would allow an adaption of the sensor layout to the requirements for an aspired measuring of condensed h2o2 within a thin electrolyte film to monitor hydrogen peroxide in the gaseous phase. acknowledgements: the presented work is based on a project supported by the european regional development fund (efre) (fkz: 100270258). diese maßnahme wird mitfinanziert durch steuermittel auf der grundlage des vom sächsischen landtag beschlossenen haushaltes. references [1] s. govindaraj, m. s. muthuraman, international journal of chemtech research 8 (2015) 897-911. 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[12] l. wu, d. w. shoesmith, electrochimica acta 137 (2014) 83-90. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.719 http://creativecommons.org/licenses/by/4.0/) corrosion and wear protection of aisi 4140 carbon steel using a laser-modified high-velocity oxygen fuel thermal sprayed coatings http://dx.doi.org/10.5599/jese.1228 877 j. electrochem. sci. eng. 12(5) (2022) 877-888; http://dx.doi.org/10.5599/jese.1228 open access : : issn 1847-9286 www.jese-online.org original scientific paper microstructural and mechanical properties of cnt-reinforced zro2-y2o3 coated boiler tube steel t-91 sandeep kumar1,, rakesh bhatia2, hazoor singh2 and roshan lal virdi3 1department of mechanical engineering, guru kashi university, bathinda, punjab, india 2yadavindra department of engineering, punjabi university guru kashi campus, damdama sahib, india 3department of mechanical engineering, punjabi university, patiala, india full affiliation, address corresponding author: sandeep_ycoe@yahoo.com; tel.:+91-9041021227 received: january 1, 2022; accepted: may 24, 2022; published: july 26, 2022 abstract the main purpose of this study was to fabricate carbon nanotubes (cnts) reinforced zirconnium yttrium coatings on boiler tube steel and to investigate the microstructural and mechanical properties of these coatings. plasma sprayed conventional zro2-y2o3, zro2-y2o3 and 1 wt.% cnt and zro2-y2o3 and 4 wt.% cnt were prepared and deposited successfully on boiler tube steel material t-91 (astm sa-213) by plasma thermal spray technology. microhardness, porosity, xrd (x-ray diffraction), sem/edax (scanning electron microscopy/energy dispersive x-ray spectroscopy), x-ray mapping and crosssectional analyses were used to analyse the specimens. the hardness of cnt reinforced zro2-y2o3 increased with the increase in the percentage of cnt, whereas the porosity of the composite coatings decreased with the increase in the cnt percentage. the observed increase in hardness may be attributed to the content of cnt in the composite coating. the present research gives important information related to the fabrication and physical characteristics of cnt-reinforced zro2-y2o3 coatings deposited on t-91 boiler tube steel. keywords carbon nanotubes, plasma spray, porosity, hardness introduction in recent years, the importance of stainless steel alloys has attracted considerable attention because of their usage in a wide range of industrial applications to withstand high temperatures and a critical environment. stainless steel is used to perform in harsh environments, and this seems to be possible due to its excellent mechanical and thermal properties [1]. serious and never-ending hot corrosion problem occurs in boiler tubes, i.c. engines, power generation equipment, aircraft and gas turbines [2]. in these installations, corrosive salts are formed by low-quality fuels used, which contain na and v as impurities that can form na2so4 and v2o5 corrosive elements [3]. to http://dx.doi.org/10.5599/jese.1228 http://dx.doi.org/10.5599/jese.1228 http://www.jese-online.org/ mailto:sandeep_ycoe@yahoo.com j. electrochem. sci. eng. 12(5) (2022) 877-888 cnt-reinforced zro2-y2o3 coated steel t-91 878 enhance the corrosion resistance of the conventional steels used in thermal power plants, the researchers have applied many types of coatings on these steels. thermal spraying methods are one of the important methods of applying coatings on steel to increase its life span [4]. along with alloys, some reinforced coatings are employed to increase the resistance of the component against high-temperature oxidation [5]. in the recent past, there have been advancements in powder production technologies along with advancements in spraying methods, which have resulted in developing coatings with high corrosion resistance properties [6]. the spraying techniques do not limit the material selection in respect of coatings and substrate composition [7]. these techniques can apply coatings of a few millimeter thicknesses on the steel surface with high hardness. the flame spray, arc spray, plasma and hvof techniques are used to apply coatings on the boiler components [8,9]. defect-free plasma coatings are not possible because thermal spray coatings consist of pores and voids. these pores and voids originate at the splat boundaries during the spraying process. therefore, through these pores and voids, the corrosive elements penetrate towards the substrate material in the corrosive environment [10,11]. the carbon nanotubes (cnts) were developed and invented by iijima [12]. the carbon nanotubes have high strength and excellent thermal properties [13–16]. carbon nanotubes have more than five times elastic constant and more than 100 times tensile strength than conventional steels [17,18]. due to these properties, cnts are used as prospective reinforcement for fabricating composite materials [19]. a few researchers have used cnt-metal composite coatings to enhance the corrosion resistance of niand zn-based composite coatings [20]. cnt reinforced coatings act as an inert physical barrier to initiating and developing corrosion mechanisms. it is revealed in the literature that very few research has been conducted on the corrosion behavior of cnt reinforced coatings. some researchers have developed different cnts-al2o3 composite coatings to study their tribological behavior [21-25], but no studies on cnt zirconium yttrium (zro2-y2o3) composite coatings for high-temperature corrosion are available. in the present research work, plasma sprayed conventional zro2-y2o3, zro2-y2o3 and 1 wt.% cnt and zro2-y2o3 and 4 wt.% cnt coatings were deposited successfully, as coatings, on t-91 boiler tube steel material to protect in the high-temperature applications. experimental substrate material the fresh t-91 boiler tubes were procured from talwandi sabo thermal power plant situated at banawali, bathinda, punjab, india. the chemical composition of the base material analyzed using spectroscopy is shown in table 1. the specimens of 22×15×3 mm were cut from boiler tubes. the standard metallurgical procedures were followed to prepare the samples. due care was taken to avoid structural and metallurgical changes in the samples. the specimens were then polished using silicon carbide paper. before deposition of different coatings, the specimens were shot blasted using grit 45 alumina powder. table 1. chemical composition of t-91 composition content, wt.% c si mn p s cr mo ni v fe nominal 0.07~0.14 0.20~0.50 0.30~0.60 ≤0.02 ≤0.02 8-9.5 0.85-1.05 0.4 ≤0.4 bal. actual 0.12 0.42 0.42 0.016 0.004 8.354 0.93 0.2 0.05 bal. s. kumar et al. j. electrochem. sci. eng. 12(5) (2022) 877-888 http://dx.doi.org/10.5599/jese.1228 879 coating powders compositions of coating powders were prepared by blending conventional zirconium yttrium, zro2-y2o3 and 1 wt.% cnt, and zro2-y2o3 and 4 wt.% cnt in a laboratory ball mill, using a low-energy ball milling method. mixing of coating powders was done in a ball mill for 4 hours at 200 rpm [26]. powder particles were asymmetric in shape and size 45 ± 10 µm. the sem images of coating powders after mixing in a ball mill are shown in figures 1 (a)-(c). figure 1. sem micrographs of powders of (a) conventional zro2-y2o3; (b) zro2-y2o3 and 1 wt.% cnt and (c) zro2-y2o3 and 4 wt.% cnt formulation of coating the different cnt mixed zro2-y2o3 coatings were deposited on t-91 boiler steel substrates with a plasma spray process at metallizing equipment company pvt. limited, jodhpur, india. the process parameters selected for the plasma spray technique are shown in table 2. the process parameters were constant during the deposition of coatings. coatings of around 250-270 µm thickness were deposited by the plasma spray process. the macrographs of as-coated samples are shown in figure 2. figure 2. macrographs of (a) conventional zro2-y2o3; (b) zro2-y2o3 and 1 wt.% cnt; and (c) zro2-y2o3 and 4 wt.% cnt-coated steel specimen table 2. plasma spraying process parameters voltage, v current, a primary gas (argon) flow, l min-1 powder feed rate, g min-1 spray distance, cm 70 575 39 30 10.16 coating examinations the coated specimens were sectioned by a slow-speed diamond cutter at a controlled rate and then hot mounted in epoxy for cross-sectional examination. the mounted specimens were then polished by a standard polishing procedure for xrd, sem/edax and cross-sectional elemental analysis. the microhardness of coated samples was measured from the coating substrate interface [27,28]. the thickness of zro2-y2o3 cnt coated steel specimens was measured with the help of minitest 2000 thin film thickness gauge (make: elektro physik koln company, germany, precision ±1 µm). the image analysis method was adopted to measure the porosity of all the coated http://dx.doi.org/10.5599/jese.1228 j. electrochem. sci. eng. 12(5) (2022) 877-888 cnt-reinforced zro2-y2o3 coated steel t-91 880 specimens. a pmp3 inverted metallurgical microscope installed with leica image analyser software was used to measure the porosity of air plasma sprayed cnt-reinforced zirconium yttrium coatings on t-91 steel specimens. the porosity was evaluated of as-coated samples after polishing by a computer-based analyzing system. results coating thickness measurement the thickness of the coating was examined along the cross-section of the specimens. the thickness was found to be in the range of 225-250 µm and is reported in table 4. the coating thickness was found to be in parity with research work reported in the literature [16,29,30]. porosity analysis the thermal spray coatings are generally porous in nature due to splats and voids during deposition. the porosity strongly influences the properties of coatings [26,31]. it is concluded from the literature study that the lower the value of porosity better the corrosion resistance [32,33]. table 3 shows the porosity values of conventional zro2-y2o3 and zro2-y2o3-cnt coated t-91 steel specimens. the tabulated values clearly show that with the increase in cnt’s percentage in zro2y2o3, the porosity of the coatings decreases. the porosity was evaluated within the range of 3.45 to 4.25 % for the coatings under study. the specimen with zro2-y2o3 and 4 wt.% cnt showed the lowest value of porosity among various coatings under investigation. table 3. average coating thickness and porosity of different coatings substrate coating type coating thickness, µm porosity, % astm sa-213 t-91 zro2-y2o3 252 4.25 zro2-y2o3 and 1 wt,% cnt 254 3.62 zro2-y2o3 and 4 wt,% cnt 247 3.45 microhardness measurement figure 3 shows the microhardness profiles across the cross-section of conventional zro2-y2o3 and cnt coated samples. the microhardness measurement for conventional zro2-y2o3, 99 wt.% zro2y2o3 and 1 wt.% cnt, and 96 wt.% zro2-y2o3 and 4 wt.% cnt coated specimens were in the range of 789-796, 908-910 and 1047-1051 hv, respectively. figure 3. microhardness of cntzro2-y2o3 reinforced coatings across the cross-section 0 200 400 600 800 1000 1200 -80 -40 0 40 80 120 160 200 m ic ro h a rd n e ss , v ic k e rs distance from interface, µm zro₂-y₂o₃ zro₂-y₂o₃ and 2 wt.% cnt zro₂-y₂o₃ and 4 wt.% cnt s. kumar et al. j. electrochem. sci. eng. 12(5) (2022) 877-888 http://dx.doi.org/10.5599/jese.1228 881 it is observed clearly from figure 3 that with the increase in cnt content in the zro2-y2o3 matrix, the microhardness value of the coating increases. the graph shows the hardness profile along the cross-section of the substrate coating interface and hardness was found uniform across the surface of specimens. x-ray diffraction analysis the x-ray diffractions analysis for the surface of cntzro2-y2o3 coated specimens was done at iit roorkee. the diffraction patterns are shown in figures 4(a)-(c). the xrd curves of plasma-sprayed zro2-y2o3 coated t-91 boiler tube steel sample showed zro2-y2o3 as the main phase present during analysis. the xrd peaks of zro2-y2o3 and 1 wt.% cnt and zro2-y2o3 and 4 wt.% cnt composite coating also depicted zro2-y2o3 as the main phase with a small proportion of carbon, as shown in figure 4(b). c peaks indicate the presence of cnt content in the coating matrix peaks increased with an increase in cnt percentage, as shown in figure 4(c). the c peaks in the diffraction pattern indicated that the cnts were present in the pure carbon form in the coating matrix and had not reacted with zro2-y2o3 to form any complex compounds. figure 4. xrd profile of plasma sprayed: (a) conventional zro2-y2o3; (b) zro2-y2o3 and 1 wt.% cnt; (c) zro2-y2o3 and 4 wt.% cnt coated specimen sem micrographs with edax analysis of plasma-sprayed zro2-y2o3, zro2-y2o3 and 1 wt.% cnt and zro2-y2o3 and 4 wt.% cnt coatings on t-91 boiler steel are shown in figures 5(a)-(c). the coatings consist of interlocked particles having regular morphology with dense structure, as shown in sem images. the sem micrographs shown in figures 5(b) and 5(c) indicate that the reinforcement of cnt in the coating powders forms a uniform coalescence of zro2-y2o3 and cnt. edax analysis confirms the uniform dispersion of cnt in zirconia coating mixtures. plasma sprayed cnt reinforced coatings over t-91 steel formulate lamellar and dense coating microstructures. edax results show the rich presence of zr with o and some minor percentile of carbon in cnt-reinforced coatings. the http://dx.doi.org/10.5599/jese.1228 j. electrochem. sci. eng. 12(5) (2022) 877-888 cnt-reinforced zro2-y2o3 coated steel t-91 882 propositions of zr, o and c in ceramic coatings are nearly approaching the composition of the blended powders used for coating deposition. figure 5. sem/edax of: (a) conventional zro2-y2o3; (b) zro2-y2o3 and 1 wt.% cnt; (c) zro2-y2o3 and 4 wt.% cnt reinforced composite coatings cross-sectional morphology the plasma spray conventional and cnt-reinforced samples were examined for cross-sectional morphology with the variation of elemental composition at different points, as shown in figures 6(a)-(c). due to the solidification and de-solidification of the molten droplet on the surface of the substrate, steel splat-type morphology can be visualized. the elemental analysis indicates the presence of fe content in the substrate of coated specimens. with the reinforcement of cnt in the composite coatings, the traces of carbon content appear and the coatings show a rich percentage of zirconium, yttrium and oxygen. the content of zirconium, yttrium and oxygen were found uniform throughout the surface area, showing that all the composite coatings are uniform in elemental composition. s. kumar et al. j. electrochem. sci. eng. 12(5) (2022) 877-888 http://dx.doi.org/10.5599/jese.1228 883 figure 6. cross-sectional morphology and the elemental composition of: (a) conventional zro2-y2o3; (b) zro2-y2o3 and 1 wt.% cnt; (c) zro2-y2o3 and 4 wt.% cnt coatings elemental x-ray mapping the zro2-y2o3 and cnt-based zro2-y2o3 composite coatings have been analyzed using the elemental x-ray mapping technique as shown in figure 7, figure 8 and figure 9. in the elemental analysis of the substrate part of all specimens show the rich presence of iron while the reinforcedcnt based coating matrix of specimens shows the presence of zirconium in rich dominance in all xray mapping along with the minor presence of carbon as shown in figure 8 and figure 9. c o n te n t, w t. % c o n te n t, w t. % c o n te n t, w t. % http://dx.doi.org/10.5599/jese.1228 j. electrochem. sci. eng. 12(5) (2022) 877-888 cnt-reinforced zro2-y2o3 coated steel t-91 884 figure 7. x-ray mapping along the cross-sectional of conventional zro2-y2o3 coating figure 8. x-ray mapping along the cross-sectional of zro2-y2o3 and 1 wt.% cnt coating s. kumar et al. j. electrochem. sci. eng. 12(5) (2022) 877-888 http://dx.doi.org/10.5599/jese.1228 885 figure 9. x-ray mapping along the cross-sectional of zro2-y2o3 and 4 wt.% cnt coating discussion the coating thickness of all samples was measured along the cross-section of coated specimens. the range of coating thickness was between 250270 µm and was uniform throughout the surface. the coating thickness was in the desired range as reported in the literature for plasma spray coatings. in conventional zro2-y2o3 coating deposited over t-91 boiler tube steel, the porosity was found to be 4.25 percent which decreased with the addition of cnt in the zro2-y2o3 composite matrix. the cnts were able to lower the porosity of conventional zro2-y2o3 coating. the plasma-sprayed cnt formed a locked structure with zro2-y2o3 grains. the cnts were able to fill the pores, which helped to decrease the porosity of the zro2-y2o3 composite coating. there is a notable decrease in the value of porosity with an increase in the content of cnt content [16,34,35]. the microhardness values of conventional zro2-y2o3 coated t-91 steel specimen was in the range of 789-796 hv, as presented in figure 3. with the addition of cnt in the zro2-y2o3 composite matrix, the microhardness increased tremendously and was between 908-910 hv for zro2-y2o3 and 1 wt.% cnt coated t-91 specimen and 1047-1051 hv for zro2-y2o3 and 4 wt.% cnt coated t-91 specimen, respectively. the increase in microhardness results in enhancement in melting point and resistance to indentation [27,36,37]. the increase in microhardness results from cnt presence in the coating powder, which undergoes dispersion hardening. the improvement in microhardness of composite coating acknowledges the main role of cnt reinforcement [38]. zro2-y2o3 was the main phase identified by x-ray diffraction analysis of the zro2-y2o3 coated t-91 boiler steel specimen. the presence of carbon as small peaks in the cnt-based zro2-y2o3 coated specimens proves the presence of pure carbon in the form of cnt. this reveals that cnts were inert during the spraying and deposition process and have not chemically reacted with the zirconium and yttrium even at high temperatures of plasma spray during the coating process. the presence of carbon content in the x-ray diffraction graphs increases with the increase in the weight percent of cnt in the zro2-y2o3 composite matrix. with the plasma spray process, the composite coatings on substrate t-91 boiler steel material solidify faster. this results in the formation of metastable phases, as presented in x-ray diffraction graphs. similar findings were reported by researchers in their research work [28,39,40]. the results observed by http://dx.doi.org/10.5599/jese.1228 j. electrochem. sci. eng. 12(5) (2022) 877-888 cnt-reinforced zro2-y2o3 coated steel t-91 886 sem/edax analysis of conventional zro2-y2o3, zro2-y2o3 and 1 wt.% cnt and zro2-y2o3 and 4 wt.% cnt coated on t-91 boiler steel revealed that the coating is uniform and dense throughout the crosssection. cnt was completely coalescent with the zro2-y2o3 matrix in the composite coating. edax analysis at selected points reveals the presence of zirconium, yttrium and oxygen, along with carbon in cnt reinforced coating matrix. as the weight percentage of cnt in zro2-y2o3 coatings increases, the carbon content traces also increase. cnts form bridge-like stricture between coating matrix [28,39,41]. the results for cross-sectional morphology for cnt-based zro2-y2o3 composite coated t-91 specimens show a completely uniform and dense microstructure of coatings. there seems to be no gap between the interface of substrate and coating layer, which forms strong adhesion due to the good thermal properties of cnt [15]. the results shown by x-ray mapping help to identify the distribution of elements in coating throughout the cross-section. the mapping shows that fe was present in a rich percentage in the substrate zone. further coating zone of conventional zro2-y2o3 indicates the presence zr, y and o, whereas cnt-based zro2-y2o3 composite coatings show the presence of zr, y, o and c, which indicates the presence of cnt in the coating matrix. conclusions the following conclusions are made from this experimental work: • in the present research work, conventional zro2-y2o3 and cnt reinforced coatings having zro2-y2o3 and 1 wt.% cnt and zro2-y2o3 and 4 wt.% cnt coating were successfully deposited on t-91 boiler tube steel. • the coating formed had dense and uniform microstructure with coating thickness ranging from 250-270 µm. • the porosity of the coatings decreased with the increase in cnt weight percentage in the zirconium yttrium matrix, whereas the microhardness of the coatings increased with the addition of cnts. • the microhardness of 4 wt.% of cnt reinforcement coated specimen was recorded between 1047-1051 hv. due to the strong bond between zro2-y2o3 and cnt, the mechanical properties were improved. • scanning electron microscopy morphology shows a uniform 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(https://creativecommons.org/licenses/by/4.0/) https://www.sphinxsai.com/2014/ch_vol6_no11/1/(4579-4584)n14.pdf https://www.sphinxsai.com/2014/ch_vol6_no11/1/(4579-4584)n14.pdf https://doi.org/10.6180/jase.2017.20.4.05 https://doi.org/10.4236/jmmce.2012.116041 https://doi.org/10.1007/s11666-008-9258-1 https://doi.org/10.1111/ijac.13327 https://doi.org/10.1007/s00170-014-5661-6 https://doi.org/10.1016/s0921-5093(03)00256-9 https://doi.org/10.1007/s11665-014-1306-z https://doi.org/10.1021/la203056f https://doi.org/10.1016/j.surfcoat.2011.07.025 https://doi.org/10.1016/j.jmatprotec.2004.05.023 https://doi.org/10.1016/j.msea.2006.08.057 https://creativecommons.org/licenses/by/4.0/) {influence of the exchange current density and overpotential for hydrogen evolution reaction on the shape of electrolytically produced disperse forms} http://dx.doi.org/10.5599/jese.707 111 j. electrochem. sci. eng. 10(2) (2020) 111-126; http://dx.doi.org/10.5599/jese.707 open access: issn 1847-9286 www.jese-online.org review influence of the exchange current density and overpotential for hydrogen evolution reaction on the shape of electrolytically produced disperse forms nebojša d. nikolić ictm-department of electrochemistry, university of belgrade, njegoševa 12, p.o.b. 473, belgrade, serbia nnikolic@ihtm.bg.ac.rs; tel.: +381 11 337 03 90; fax: +381 11 337 03 89 received: july 4, 2019; revised: august 14, 2019; accepted: august 14, 2019 abstract in this study, comprehensive survey of formation of disperse forms by the electrolysis from aqueous electrolytes and molten salt electrolysis has been presented. the shape of electrolitically formed disperse forms primarily depends on the nature of metals, determined by the exchange current density (j0) and overpotential for hydrogen evolution reaction as a parallel reaction to metal electrolysis. the decrease of the j0 value leads to a change of shape of dendrites from the needle-like and the 2d fern-like dendrites (metals characterized by high j0 values) to the 3d pine-like dendrites (metals characterized by medium j0 values). the appearing of a strong hydrogen evolution leads to formation of cauliflower-like and spongy-like forms (metals characterized by medium and low j0 values). the other disperse forms, such as regular and irregular crystals, granules, cobweb-like, filaments, mossy and boulders, usually feature metals characterized by the high j0 values. the globules and the carrot-like forms are a characteristic of metals with the medium j0 values. the very long needles were a product of molten salt electrolysis of magnesium nitrate hexahydrate. depending on the shape of the disperse forms, i.e. whether they are formed without and with vigorous hydrogen evolution, formation of all disperse forms can be explained by either application of the general theory of disperse deposits formation or the concept of "effective overpotential". with the decrease of j0 value, the preferred orientation of the disperse forms changed from the strong (111) in the needle-like and the fern-like dendrites to randomly oriented crystallites in the 3d pine-like dendrites and the cauliflower-like and the spongy-like forms. keywords electrolysis; metal; morphology; powder particles; sem. http://dx.doi.org/10.5599/jese.707 http://dx.doi.org/10.5599/jese.707 http://www.jese-online.org/ mailto:name@address.domain j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 112 basic facts the disperse (powder, irregular) forms of metal deposits can be obtained by electrolysis from aqueous electrolytes and molten salt electrolysis [1,2]. the typical disperse forms obtained by electrolysis are: dendrites of different shape, crystals of regular and irregular shape, cobweb, needles, wires, carrot, globules, granules, cauliflower, the honeycomb, spongy, mossy, filaments, etc. the shape of disperse forms depends on the regimes and parameters of electrolysis, and the nature of metals. the both constant (potentiostatic and galvanostatic) and periodically changing (pulsating overpotential (po), pulsating current (pc) and reversing current (rc)) regimes of electrolysis are used for production of disperse forms. the main parameters affecting the shape of disperse forms are: the type and composition of electrolytes, temperature of electrolysis, the type of cathode, stirring of electrolyte, the addition of specific substances known as additives, etc. according to the exchange current density, melting point and overpotential for hydrogen evolution reaction, metals are classified into three classes [3]: a) class i, so-called normal metals like silver, cadmium, lead, tin and zinc. this group of metals is characterized by the high values of both the exchange current density (j0 > 1 a dm-2; j0 is the exchange current density) and overpotential for hydrogen evolution reaction, and low melting point, b) class ii, so-called intermediate metals like copper, gold and silver (ammonium electrolyte). this group of metals is characterized by moderate melting points, the medium exchange current density values (10-2 < j0 < 1 a dm-2), and the lower values of overpotential for hydrogen evolution than the normal metals, and c) class iii, so-called inert metals like nickel, cobalt, iron and platinum. this group of metals is characterized by the low values of the both exchange current density (10-2 > j0 > 10-12 a dm-2) and overpotential for hydrogen evolution reaction, and high melting points. a schematic illustration of position of the typical metals from each of these groups on the scale of the exchange current density is shown in fig. 1. the values of their exchange current densities are summarized in table 1. figure 1. a schematic position of metals on a scale of the exchange current density values (ag* the ammonium electrolyte) this mini author`s review gives a comprehensive survey of morphological characteristics of disperse forms of lead, silver and zinc (the normal metals), copper and silver (the intermediate metals) and nickel (the inert metal). nebojša d. nikolić j. electrochem. sci. eng. 10(2) (2020) 111-126 http://dx.doi.org/10.5599/jese.707 113 table 1. the values of the exchange current density for some technologically important metals class of metals the kind of metals the exchange current density, j0 / a dm-2 reference: pb j0 →  [4] normal metals ag 100 − 700 [5] zn 1.84 − 8.8; 0.8 − 37 [6,7] intermediate ag (ammonium electrolyte) 0.025 [8] metals cu 0.011 – 0.032 [1,9] inert metals ni 1.6  10-7 [1,9] class i, so-called normal metals the common characteristic of this group of metals is formation of disperse (powder, irregular) forms starting from small overpotentials, and the absence of formation of compact deposits without use of additives [1,2]. there is no unique and precise way for determination of the exchange current density values of this group of metals, and auxiliary ways are proposed for their estimation [4,6,7]. lead the processes of lead electrodeposition belong to the very fast electrochemical processes, and the estimated values of the exchange current density for pb tend to infinity [4]. pb electrodeposition occurs in the conditions of the mixed ohmic-diffusion control [2,10]. the ohmic control is defined by a straight-line dependence of current on overpotential. the ratio of the ohmic control to the overall control of the electrodeposition increases with increasing concentration of pb(ii) ions [10], and decreasing concentration of the supporting electrolyte (nano3) [11] (fig. 2). the inflection point at the polarization curve denotes the end of the plateau of the limiting diffusion current, and the fast growth of the current density with the increase of overpotential after the inflection point is observed (fig. 2). figure 2. polarization curves for pb electrodeposition from 0.10 m pb(no3)2 in 0.50 and 2.0 m nano3. figure 3 shows a typical disperse forms obtained under the different electrodeposition conditions. the regular hexagonal crystals are a characteristic of the ohmic control (fig. 3a).* the mixture of needle-like dendrites and crystals of irregular shape is obtained by electrodeposition at the * the exact experimental conditions for formation of this form, as well as all others forms shown in this study are given in the corresponding references indicated in figure captions. http://dx.doi.org/10.5599/jese.707 j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 114 overpotential belonging to the plateau of the limiting diffusion current density (fig. 3b). finally, the two-dimensional (2d) fern-like dendrites are formed by electrodeposition outside the plateau of the limiting diffusion current density in the zone of the fast increase in the current density with increasing overpotential after the inflection point (fig. 3c). depending on a concentration either pb(ii) ions or nano3, the very long needle-like dendrites can be formed (fig. 3d). the shape of regular hexagonal crystals does not depend on the type of electrolyte [12,13]. on the other hand, the shape of dendrites strongly depends on the type of electrolyte, and for example, dendrites formed from the complex electrolytes like acetate [12] and hydroxide [13] (fig. 3e) were more branchy structure than those obtained from the basic (nitrate) electrolyte (fig. 3c) [14]. following wranglen`s definition of a dendrite [15], dendrites obtained from the complex electrolytes usually belong to secondary (s) and tertiary (t) types, while those obtained from the nitrate electrolyte belong to primary (p) type [14]. according to wranglen [15], a dendrite is a skeleton of a monocrystal and consists of a stalk and branches, thereby resembling a tree. the dendrite consisted only of the stalk and primary branches is referred as primary (p) dendrite. if the primary branches in turn develop secondary branches, the dendrite is called secondary (s). the two-dimensional (2d) dendrite refers to dendrites with branches that lie in the same plane as that of the primary stalk [16]. the branches developed from the secondary branches are known as tertiary (t) ones, etc. aside from dendrites of different shape and regular and irregular crystals, the granules (fig. 3f and g) and the cobweb-like particles (fig. 3h) were also formed by pb electrodeposition. these particles are usually formed by pb electrodeposition from electrolytes with the low concentrations of pb(ii) ions [17,18]. irrespective of the type of electrolyte, the x-ray diffraction (xrd) analysis showed the predominant presence of pb crystallites oriented in the (111) plane in all types of pb particles [13,19,20]. this predominant presence of the crystallites oriented in the (111) plane can be attributed to the lower surface energy of this plane in relation to the other planes like (110) and (100) [21,22], where the values of the surface energy follow trend: γ111<γ100<γ110 , and  is the surface energy. pb crystallites were only oriented in the (111) plane in the regular hexagonal particles formed in the ohmic control, indicating that this particle type represents monocrystal of the (111) preferred orientation [19]. on the other hand, aside from pb crystallites oriented in the (111) plane, the presence of pb crystallites oriented in the other planes was observed in all types of dendritic particles, that can be considered as follows: due to different surface energy of crystal planes, electrodeposition rate on them are different, and for the fcc crystal lattice, follows a trend: (110) > (110) > (111) [23]. the (111) plane belongs to a slow-growing plane, and in the growth process, this plane survives [19]. pb crystallites oriented in this plane originate from growth centers present in the interior of crystals (“growth centre“). the other planes, such as (110) and (100), belong to the fast-growing planes, and in the growth process, these planes disappear. pb crystallites oriented in these planes originate from the growth centers present on the tips, corners and edges (“tip”, “edge” and “corner” types). in this way, the presence of pb crystallites oriented in these planes is obvious. silver the polarization curve for silver electrodeposition from the nitrate electrolyte (0.10 m agno3 in 2.0 m nano3) shows a relatively short plateau of the limiting diffusion current density in the range of overpotentials between 70 and 110 mv (fig. 4) [24,25]. nebojša d. nikolić j. electrochem. sci. eng. 10(2) (2020) 111-126 http://dx.doi.org/10.5599/jese.707 115 (a) (b) (c) (d) (e) (f) (g) (h) figure 3. the typical disperse forms obtained by pb electrodeposition from 0.10 m pb(no3)2 in 0.50 m nano3: (a) the regular hexagonal crystal ( = 20 mv) [11], (b) the irregular crystals ( = 50 mv) [11], (c) the fern-like dendrite ( = 80 mv) [11], (d) the needle-like dendrite (0.10 m pb(no3)2 in 4.0 m nano3;  = 50 mv) [11], (e) the very branchy 2d dendrite (0.10 m pb(no3)2 in 2.0 m naoh;  = 80 mv) [13], (f) and (g) granules (0.020 m pb(no3)2 in 2.0 m nano3;  = 5 mv) [17], h) cobweb-like particles (0.010 m pb(no3)2 in 2.0 m nano3;  = 62.5 mv) [18] morphological analysis of deposits of ag obtained at different overpotentials showed that granules were formed at the low overpotential (fig. 5a), the mixture of the needle-like dendrites and granules was formed at the overpotential inside the plateau of the limiting diffusion current density (fig. 5b), while the 2d fern-like dendrites were formed at the overpotential outside the plateau of the limiting diffusion current density in the zone of the fast increase in the current density with increasing overpotential after the inflection point (fig. 5c). http://dx.doi.org/10.5599/jese.707 j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 116 figure 4. the polarization curve for silver electrodeposition from 0.10 m agno3 in 2.0 m nano3. (a) (b) (c) figure 5. the disperse forms of ag obtained by electrodeposition from 0.10 m agno3 in 2.0 m nano3: (a) granules ( = 15 mv), (b) the mixture of the needle-like dendrites and granules ( = 90 mv), and (c) the 2d fern-like dendrite ( = 150 mv) [24]. the needle-like dendrites showed the strong (111) preferred orientation [24]. the strong (111) preferred orientation was also characteristic of the 2d fern-like dendrites, but with the larger ratio of ag crystallites oriented in (220), (200) and (311) planes than in the needle-like dendrites. zinc unlike lead and silver, the typical polarization curve for zinc electrodeposition from an alkaline electrolyte showed the well defined plateau of the limiting diffusion current density, in the range of overpotentials between 110 and 180 mv [6] (fig. 6). the filaments (fig. 7a), grouped into mossy or spongy-like particles (fig. 7b), are formed at the low overpotential. the large grains, referred as boulders, are formed at the overpotential close to the plateau of the limiting diffusion current density (fig. 7c). the 2d fern-like dendrites and irregular crystals are formed by electrodeposition at the overpotential inside the plateau of the limiting diffusion current density (fig. 7d). finally, the very branchy 2d fern-like dendrites are formed at the overpotential outside the plateau of the limiting diffusion current density in the zone of the fast increase in the current density with increasing overpotential after the inflection point at the polarization curve (fig. 7e and f). it is necessary to note formation of some positions at the electrode surface that are result of appearing of hydrogen evolution reaction as a parallel reaction to zn electrolysis at the high overpotentials (fig. 7e). nebojša d. nikolić j. electrochem. sci. eng. 10(2) (2020) 111-126 http://dx.doi.org/10.5599/jese.707 117 figure 6. the polarization curve for zn electrodeposition from 0.40 m zno in 6.0 m koh. (a) (b) (c) (d) (e) (f) figure 7. the disperse forms of zn obtained by electrodeposition from 0.40 m zno in 6.0 m koh: (a) filaments ( = 25 mv), (b) mossy ( = 25 mv), (c) large grains or boulders ( = 65 mv), (d) the mixture of the 2d dendrites and irregular crystals ( = 150 mv), (e) and (f) the 2d fern-like dendrites ( = 225 mv) [6]. the intermediate metals the disperse forms of metals from this group are formed in the activation-diffusion and diffusion controls of electrodeposition, as well as in the hydrogen co-deposition range (for copper) [2]. http://dx.doi.org/10.5599/jese.707 j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 118 silver (ammonium electrolyte) the polarization curve for silver electrodeposition from the ammonium electrolyte shows the well defined plateau of the limiting diffusion current density, in the range of overpotentials between 250 and 700 mv (fig. 8) [26]. figure 8. the polarization curve for ag electrodeposition from 0.10 m agno3 in 0.50 m (nh4)2so4 with the addition of nh3 in excess to dissolve silver sulfate precipitate (ammonium electrolyte). the three-dimensional (3d) pine-like dendrites constructed from the corncob-like forms as the basic element were formed by electrodeposition at the overpotential belonging to the plateau of the limiting diffusion current density ( = 650 mv; fig. 9a). the corncob-like forms consisted of small cauliflower-like agglomerates of approximately spherical grains [24]. the similar shape of the pinelike dendrites, but more branchy structure than those obtained by electrodeposition inside the plateau, was formed at the overpotential outside the plateau of the limiting diffusion current density ( = 1000 mv; fig. 9b). (a) (b) figure 9. the 3d pine-like dendrites formed by electrodeposition from the ammonium electrolyte at overpotentials of: (a) 650 mv, and (b) 1000 mv [26]. the ratios of ag crystallites oriented in (200), (220) and (311) planes increased with increasing overpotential of the electrodeposition, leading to almost randomly oriented ag crystallites in the pine-like dendrites formed at the overpotential outside the plateau of the limiting diffusion current density [24]. nebojša d. nikolić j. electrochem. sci. eng. 10(2) (2020) 111-126 http://dx.doi.org/10.5599/jese.707 119 copper the typical polarization curve for cu electrodeposition shows a wide and the well defined the plateau of the limiting diffusion current density (fig. 10). at the first sight, it is clear that the shape of this polarization curve was very similar to the one obtained for ag from the ammonium electrolyte. the plateau of the limiting diffusion current density corresponds to the range of overpotentials between 300 and 750 mv for solution containing 0.10 m cuso4 in 0.50 m h2so4, and no any difference in the length of the plateau of the limiting diffusion current density is observed with various concentrations of h2so4 as the supporting electrolyte [27]. figure 10. the polarization curve for cu electrodeposition from 0.10 m cuso4 in 0.50 m h2so4. the carrot-like forms, often with the sharp tips, (fig. 11a) and globules (fig. 11b) are formed in the mixed activation-diffusion control of the electrodeposition [2,28]. the cauliflower-like forms are formed at the beginning of the plateau of the limiting diffusion current density before the initiation of the dendritic growth (fig. 11c). the very branchy 3d pine-like dendrites constructed from the corncob-like forms as the basic element were formed at the overpotential belonging to the plateau of the limiting diffusion current density after an initiation of dendritic growth (fig. 11d). the corncob-like forms were constructed from small cauliflower-like agglomerates of approximately spherical grains. aside from the 3d pinelike dendrites, cauliflower-like agglomerates of cu grains, and holes which origin is of detached hydrogen bubbles were also formed at an overpotential of 625 mv which belonged to the plateau of the limiting diffusion current density (fig. 11e). the completely different situation was observed at the overpotential outside the plateau of the limiting diffusion current density. dendritic growth was completely inhibited, and the honeycomblike structure was formed [2, 26] (fig. 11f). the main characteristics of this structure type are holes formed by detached hydrogen bubbles and small agglomerates of approximately spherical grains formed around them. the absence of dendritic growth at an overpotential of 1000 mv and the cauliflower-like character of the particles obtained after a removing the deposit from the honeycomb-like structure is clearly visible from fig. 11g. the honeycomb-like structures are formed in the conditions of vigorous hydrogen evolution reaction, where vigorous hydrogen evolution changes the hydrodynamic conditions in the nearelectrode layer leading to the decrease in the thickness of the diffusion layer of the macroelectrode, the increase in the limiting diffusion current density, and to the decrease of the degree of the http://dx.doi.org/10.5599/jese.707 j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 120 diffusion control of the electrodeposition [29]. formation of the cauliflower-like forms (fig. 11g) instead of dendrites (fig. 11d) really indicates the lower degree of the diffusion control at an overpotential of 1000 mv than at 625 mv. (a) (b) (c) (d) (e) (f) (g) figure 11. the disperse forms of cu obtained by electrodeposition from 0.10 m cuso4 in 0.50 m h2so4: (a) the carrot-like forms ( = 210 mv) [28], (b) globules ( = 210 mv) [28], (c) the cauliflower-like forms ( = 550 mv) [31], (d) the 3d pine-like dendrites ( = 625 mv) [31], (e) the mixture of the 3d pine-like dendrites, holes and cauliflower-like agglomerates of cu grains ( = 625 mv) [31], (f) the honeycomb-like structure ( = 1000 mv) [31], g) the cauliflower-like particles obtained in the conditions of vigorous hydrogen evolution ( = 1000 mv) [31]. the amounts of evolved hydrogen during cu electrolysis at high overpotentials or current densities are quantified by determination of the average current efficiency of hydrogen evolution (i,av(h2)), and there is a critical value leading to the change of the hydrodynamic conditions in the nebojša d. nikolić j. electrochem. sci. eng. 10(2) (2020) 111-126 http://dx.doi.org/10.5599/jese.707 121 near-electrode layer. this value is estimated to be about 10.0 % [30]. the values obtained at overpotentials of 625 and 1000 mv were 2.0 and 46.8 %, respectively [31], indicating that the value obtained at the overpotential inside the plateau of the limiting diffusion current density was below, while the value obtained at the overpotential outside the plateau of the limiting diffusion current density was above the critical value for the change of hydrodynamic conditions in the near-electrode layer. in spite of very different macrostructure of the formed disperse forms (dendrites or cauliflowerlike forms), cu crystallites were random oriented in the both types of particles [31]. this random orientation can be attributed to the similar micro structure, i.e. to existence of spherical morphology in them. namely, the both types of disperse forms consisted of small agglomerates of approximately spherical grains at the micro level. the inert metals for this group of metals, there is a parallelism between metal electrodeposition process and hydrogen evolution reaction in the whole range of potentials and current densities. for that reason, it is no possible to record the polarization curve in a classical way, but it is possible with an application of ir drop compensation technique [2,32,33]. nickel the spongy-like particles are formed by ni electrodeposition in the powder production range (fig. 12). the main characteristics of this particle type are holes formed by detached hydrogen bubbles surrounded by cauliflower-like agglomerates of approximately spherical ni grains. it is clear that this particle type has the honeycomb-like structure. figure 12. the spongy-like particles of ni obtained by electrolysis from 0.10 m nicl2 + 1.0 m nh4cl + 0.70 m nh4oh under the condition of limiting diffusion current density [35]. there is no any difference among the spongy-like particles of ni formed from various electrolytes [2,33–36], and the spongy-like particles of the other metals from this group, like co [2,33,37], and fe [2,38]. it is understandable because this particle type is only determined by vigorous hydrogen evolution as a parallel reaction to metal electrolysis in the powder production range. crystallites of ni were random oriented in the spongy-like particles [36]. molten salt electrolysis the xrd analysis of products of molten salt electrolysis of magnesium nitrate hexahydrate showed that the mixture of mgo/mg(oh)2 was formed [39,40]. the network of the intertwined needles was formed in the upd (underpotential) region (fig. 13a). the holes formed by detached http://dx.doi.org/10.5599/jese.707 j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 122 hydrogen bubbles and very long needles often grouped into the flower-like aggregates were formed by molten salt electrolysis in the opd (overpotential) range (fig. 13b). the hole size decreased, while their number increased with increasing the overpotential of electrodeposition (13c and d). (a) (b) (c) (d) figure 13. morphologies of mgo/mg(oh)2 deposits: (a) the upd region; the network of the intertwined needle, (b) the opd region; very long needles grouped into the flower-like aggregates, (c) the dish-like hole;  = 200 mv, and (d) holes constructing the honeycomb-like structure;  = 1000 mv [39]. general discussion of the presented results the polarization curves for pb, ag, zn and cu normalized to the limiting diffusion current density values are shown in fig. 14. since the plateaus of the limiting diffusion current density at the polarization curves for ag from the nitrate electrolyte (ag(nit)) and pb are with a slope to the overpotential axis, the values of the current density corresponding to the inflection points are taken as the limiting diffusion current densities. the shape of polarization curves exclusively depends on the type of electrolyte, while only changes in the length of the plateau of the limiting diffusion current density were observed with various concentrations of both the depositing ions and the supporting electrolyte [6,10,11,27,30]. as a result of the decrease of the exchange current density (or the rate of electrodeposition), moving of the plateaus of the limiting diffusion current density towards the higher overpotentials of electrodeposition is observed. simultaneously, the change of shape of a dendrite from the needle-like and the 2d fern-like dendrites to the 3d pine-like dendrites was observed with the decreasing exchange current density values. the process of complex formation with depositing metal ions lowers the exchange current density value of metal, where the degree of decrease of the exchange current density depends on the strength of formed complex [12,13]. for example, in the case of ag, ag makes enough strong nebojša d. nikolić j. electrochem. sci. eng. 10(2) (2020) 111-126 http://dx.doi.org/10.5599/jese.707 123 complex with ammonium ions (this electrolyte is denoted with ag(am) in fig. 14) that a transfer of ag from the group of the normal to the group of intermediate metals is enabled. figure 14. the polarization curves for pb, ag, zn and cu normalized to the limiting diffusion current density values all shown shapes of dendrites follow the electrochemical definition of dendrites. from the electrochemical point of view, a dendrite is defined as an electrode surface protrusion which tip grows under activation control, while electrodeposition to the macroelectrode is predominantly under diffusion control [1,41–45]. this definition follows from an application of the general theory of disperse deposit formation, based on the concept of local diffusion fields formed around the tips and the top edges of surface protrusions formed in the initial stage of electrodeposition and buried deep in the diffusion layer of the macroelectrode. two types of local diffusion fields are responsible for formation and growth of disperse forms of various shape: (a) spherical – formed around the tips, and (b) cylindrical – formed around the top edges of growing protrusions. according to this theory, the radius of the spherical diffusion layer formed around the tip of the protrusion is equal to radius of the protrusion. during electrodeposition inside the plateau of the limiting diffusion current density, the outer limit of the diffusion layer of the macroelectrode was not disrupted. after the inflection point, the electrodeposition system remains diffusion controlled and the rapid increase in the current density with increasing the overpotential is a result of the fast growth of dendrites and a strong increase of the electrode surface area. then, using the electrochemical definition of a dendrite, it follows that this sudden and rapid increase of the current density with the increasing overpotential can be mainly ascribed to the activation controlled electrodeposition at the tips of the formed dendrites. it is very clear that the tips of both primary and secondary branches contribute to the overall control of electrodeposition process causing the disruption of the outer limit of the diffusion layer of the macroelectrode. it is no valid the general theory of disperse deposits formation for formation of the honeycomblike structures. this structure type is formed in the conditions of vigorous hydrogen evolution, and the concept of "effective overpotential" is proposed to explain formation of this structure type [2,29]. according to this concept, when hydrogen evolution is vigorous enough, then electrodeposition process occurs at some overpotential which is effectively lower than the specified one. from morphological point of view, it means that morphologies of metal deposits become similar to those obtained at some lower overpotentials where there is no hydrogen evolution, or it is too small to affect the hydrodynamic conditions in the near-electrode layer. formation of cauliflower-like http://dx.doi.org/10.5599/jese.707 j. electrochem. sci. eng. 10(2) (2020) 111-126 shape of electrolytically produced disperse forms 124 agglomerates of cu grains at the overpotential outside the plateau of the limiting diffusion current density (fig. 11g) instead of the dendrites formed inside the plateau of the limiting diffusion current density (fig. 11d) really indicated the lower degree of the diffusion control at the overpotential outside than inside the plateau. in this case, the fast increase in the current density with increasing the overpotential after the end of the plateau of the limiting diffusion current density is a result of vigorous hydrogen evolution. in spite of vigorous hydrogen evolution accompanied by formation of numerous holes from the detached hydrogen bubbles (the both dish-like holes and those constructing the honeycomb-like structures) [39], formation of very long needles of mgo/mg(oh)2 clearly indicates that this molten salt electrolysis process belongs to the very fast electrochemical process. conclusions the following conclusions concerning formation of disperse forms by electrolysis can be derived: a) the shape of disperse forms depends on parameters of electrolysis like kind of electrolyte, b) the shape of disperse forms depends on affiliation to the determined group of metals: i. normal metals: the needle-like and the 2d fern-like dendrites, crystals of regular and irregular shape, granules, cobweb, mossy, filaments, boulders, ii. intermediate metals: carrot-like and cauliflower-like forms, globules, the 3d pine-like dendrites, and iii. inert metals: the spongy-like particles, c) the decrease of the exchange current density leads to the change in the shape of dendrites from the needle-like and the 2d fern-like dendrites to the 3d pine-like dendrites, and d) vigorous hydrogen evolution changes a mechanism of formation of disperse forms from application of the general theory of disperse deposit formation to application of the concept of “effective overpotential”, and e) the decrease of the exchange current density leads 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[45] n. d. nikolić, k. i. popov, e. r. ivanović, g. branković, s. i. stevanović, p. m. živković, journal of electroanalytical chemistry 739 (2015) 137–148. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {electroorganic synthesis of disulfonamide substituted p-benzo-quinone by hydroquinone electrochemical oxidation} doi:10.5599/jese.555 1 j. electrochem. sci. eng. 9(1) (2019) 1-7; doi: http://dx.doi.org/10.5599/jese.555 open access: issn 1847-9286 www.jese-online.org original scientific paper electroorganic synthesis of disulfonamide substituted p-benzoquinone by hydroquinone electrochemical oxidation seba nassif1,, deeb bakeer1, rushdi madwar1, waleed khadam2, abeer nakhla2 1department of chemistry, faculty of science, al-baath university, homs, syria 2faculty of pharmacy, al-baath university, homs, syria. corresponding author e-mail: s_nassif81@hotmail.com received: june 4, 2018; revised: july 30, 2018; accepted: august 1, 2018 abstract this study illustrates electrochemical behavior of hydroquinone and 4-amino-6-chlorobenzene-1,3-disulfonamide in the phosphate buffer solution evaluated by cyclic voltammetry. it was found that the peak of the hydroquinone oxidation potential in the presence of 4-amino-6-chlorobenzene-1,3-disulfonamide is shifted to more positive values compared to hydroquinone alone. based on these results, the electrochemical synthesis of new disulfonamide substituted p-benzoquinone is proposed and carried out via electrochemical oxidation of hydroquinone in the presence of 4-amino-6-chlorobenzene-1,3-disulfonamide in the electrolytic cell. it has been concluded that hydroquinone is converted into disulfonamide substituted p-benzoquinone via an ece mechanism. the successful electrochemical synthesis was conducted in the water/ethanol mixture under green conditions without any toxic reagents or solvents and with high atom economy. keywords cyclic voltammetry; electrochemical synthesis; hydroquinone; disulfonamide introduction electrochemistry provides a versatile way for electrosynthesis of biologically active intermediates and kinetic studies of different reagents that are of pharmaceutical importance [1]. since electrochemical methods are simple and rapid, they are normally used to study electroactive compounds in pharmaceutical forms and physiological fluids. quinones are classified in a large group of natural pigments that show excellent photochemical properties [2] and act as intermediates in a biosynthesis of important antibiotics [3]. quinones exhibit biological activities such as antidiabetic [4] and are frequently used as charge transfer complexes [5]. also, the change from hydroquinone to quinine plays an important role in redox processes occurring in living organisms. quinones also act as electron–proton carriers for carrying oxygen in biochemical http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:s_nassif81@hotmail.com j. electrochem. sci. eng. 9(1) (2019) 1-7 synthesis of disulfonamide substituted p-benzoquinone 2 reactions [6]. there are many works addressed to electro-oxidation of hydroquinone and its derivatives followed by the addition reaction with different nucleophiles such as 1-methylindole– triphenylphosphine-1,3-dimethylbarbituric acid [7-9]. contrary to conventional organic synthetic methods that take place in organic solvents such as benzene, only few papers on the electrolytic synthesis of the sulfonamide derivatives have already been published.. this is somewhat strange because conventional organic synthetic reactions suffer from the necessity of heating, long reaction time and low atom economy [10,11]. sulfonamides are synthetic drugs that have various therapeutic potential uses such as antimetalloprotease [12], antibacterial, anti-diabetic [13], anti-carbonic anhydrase, diuretic [14], antithyroid, and antiviral activities [15]. based on all these information, we believed that the synthesis of an organic compound with a structure of both sulfonamide and quinone groups would be useful from the perspective of pharmaceutical properties. this idea encouraged us to investigate the electrochemical oxidation of hydroquinone in the presence of disulfonamide as a nucleophile. the reaction is carried out in a single step with high atom economy under ambient conditions using a carbon anode. experimental apparatus and reagents cyclic voltammetry experiments were performed using an ampere-metric station model (amel433analyser, milano, italy). the electrolytic cell with the glassy carbon working electrode was used. ag/agcl electrode as the reference, and a platinum wire as an auxiliary electrode were used. nmr 1h and 13c spectra were taken at 400 mhz bruker. infrared spectrum was taken on ft-ir-4100 from jasco. all chemicals including 4-amino-6-chlorobenzene-1,3-disulfonamide and hydroquinone were purchased from merck laboratories and were analytical grade materials. phosphate salt and solvents were all of pro-analysis quality and used without further purification. the water utilized in all studies was double-distilled and deionized. procedure all voltammetric experiments were performed in water (phosphate buffer, c = 0.2 m)/ethanol (50/50 v/v) solution. electroorganic synthesis of new substituted disulfonamide-para-benzoquinone was carried out in a single step, by using the electrolytic cell. in a typical procedure, 70 ml of phosphate buffer solution (0.2 m, ph 8.0) was added to the water/ethanol mixture (35/65 v/v). this mixture contains 0.05 m of hydroquinone and 0.05 m of 4-amino-6-chlorobenzene-1,3-disulfonamide. the total solution was electrolyzed in a divided cell equipped with a zinc cathode, two carbon rods-anode and dc power supply set at 0.08 a. current density was 6.41310-3 a/cm2. the reaction was electrolyzed with constant stirring using a magnetic stirrer for two hours. the progress of the reaction was monitored by tlc plate. then, evaporation of the solution was conducted and a brown colored product was obtained. this product was washed with ethanol and isolated at yield of 98 %. the product was characterized by spectroscopy (ir, 1h nmr). results and discussion electrochemical behavior of hydroquinone the cyclic voltammogram of glassy carbon electrode in a solution of hydroquinone (0.005 m) in a water (phosphate buffer, c = 0.2 m, ph 8.0)/ethanol mixture is shown in figure 1. anodic peak (a) s. nassif et al. j. electrochem. sci. eng. 9(1) (2019) 1-7 doi:10.5599/jese.555 3 at 95 mv and a corresponding cathodic peak (b) at -57 mv can be clearly observed. these peaks correspond to the conversion of hydroquinone to p-benzoquinone and vice versa, proceeding by a quasi-reversible two electrons process. e / v vs. ag/agcl figure 1. cyclic voltammogram hydroquinone (0.005m) on glassy carbon electrode. scan rate: 100 mv s-1 solvent water (phosphate buffer, c = 0.2m, ph 8.0)/ethanol. if, however, 1.0 mm solution of disulfonamide was put in the solution instead hydroquinone, not any peak was observed in the cyclic voltammogram recorded under same applied conditions. the effect of ph on the voltammetric behavior of hydroquinone was monitored and it was seen that the peak (a) shifts to the negative potential by increasing ph value. figure 2 represents the potential-ph diagram. e / v vs. ag/agcl figure 2. cyclic voltammogram of hydroquinone in phosphate buffer solution with various ph values on glassy carbon electrode. ph are 4.0,5.0,6.0,7.0 and 8.0, scan rate 100 mv s-1. the anodic peak potential (epa) is given by [16]:   pa pa(ph0) 2 303 ph 2 mrt e e . f i /  a i /  a j. electrochem. sci. eng. 9(1) (2019) 1-7 synthesis of disulfonamide substituted p-benzoquinone 4 where m is the number of protons involved in the reaction, epa(ph 0) is the anodic peak potential at (ph 0.0), while r, t, and f have their usual meanings. it is seen in figure 3 that epa is shifted to less positive potentials with the slope of 62.2 mv/ph. this slope is in agreement with the theoretical slope (2.303 mrt/2f) of 59 mv/ph for m = 2. based on this slope, it can be concluded that the electrode reaction is a two electrontwo proton process. the ratio of the number of protons to electrons (m/n) indicates the slope of the potential-ph diagram. upon increasing ph, the hydroxyl group dissociates to its anionic form. in other words, the proton of hydroxyl group does not participate in the electrochemical oxidation process. hence, the potential-ph diagram is independent of the hydroxyl proton. figure 3. potential – ph diagram of hydroquinone figure 4 shows cyclic voltammograms of hydroquinone with and without the presence of 4amino-6-chlorobenzene-1,3-disulfonamide. e / v vs. ag/agcl figure 4. cyclic voltammograms of: (1) hydroquinone, (2) hydroquinone in the presence of 4-amino-6-chlorobenzene-1,3-disulfonamide on glassy carbon electrode, scan rate: 100 mv s-1 solvent: water (phosphate buffer,c=0.2m,ph=8.0)/ethanol in the presence of sulfonamide, the current peak heights of hydroquinone are decreased and peak potentials are slightly shifted toward higher potentials, i.e. cathodic peak toward more y = -0.0622x + 0.7622 r² = 0.9803 0.2 0.3 0.4 0.5 0.6 3 4 5 6 7 8 9 e / v v s .a g /a g c l ph i /  a s. nassif et al. j. electrochem. sci. eng. 9(1) (2019) 1-7 doi:10.5599/jese.555 5 negative values while anodic peak shifted toward more positive values. obviously, the presence of sulfonamide affects the reaction of hydroquinone at gc electrode, probably due to their chemical or physical association in the solution decreasing diffusion coefficient. electroorganic synthesis in order to investigate the possibility of synthetic utilization of the reaction, a preparative electrochemical oxidation of hydroquinone in the presence of 4-amino-6-chlorobenzen-1,3-disulfonamide was carried out in a divided cell containing 0.025 m of hydroquinone, 0.05 m of 4-amino6-chlorobenzen-1,3-disulfonamide, and phosphate buffer solution (0.2 m, ph 8.0). the cell was equipped with a zinc cathode and two carbon rods anodes. after 576 c (as) of electricity passed, the reaction was stopped and the resulting product isolated as described in experimental section. the spectroscopic data of final product (ir, 1h nmr, 13c nmr) allowed us to propose the following pathway of the electrochemical oxidation of hydroquinone in the presence of 4-amino-6-chlorobenzen-1,3-disulfonamide: 1. firstly, hydroquinone oxidizes at the carbon anode by an electrochemical reaction into p-benzoquinone. then, this product reacts with disulfonamide to produce a substituted hydroquinone. the reaction follows the michael’s addition mechanism. 2. secondly, the substituted hydroquinone oxidizes into substituted p-benzoquinone in an electrooxidation reaction. this oxidation takes place at lower potentials than hydroquinone oxidation, what is due to the presence of the electron-donating amine functional group. hence, the overall reaction mechanism of the electrochemical synthesis followed the ece mechanism and can be described as follows: j. electrochem. sci. eng. 9(1) (2019) 1-7 synthesis of disulfonamide substituted p-benzoquinone 6 spectral data: 1h nmr (400 mhz, dmso-d6) δ ppm: 8.11(s,h, nh)7.3 (d, j=66hz, 2h, quinone)6.9 (s, h,aromatic) 6.6 (s, h, aromatic)3.3(s,2h, nh2)3.7(s,2h, nh2). 13c nmr(dmso-d6) δppm: 55.9, 104.3, 120.2, 129.6, 133.6,147.87, 158.13, 180.75,182 (c=o). ir (kbr) v/cm-1: 3407-3361 (nh2), 3279 (nh), 1630 1552 (c=o), 1170 (s=o). the atom economy was calculated for the synthesis substituted disulfonamide p-benzoquinone according to eissen and coworkers (eq. 1) [17]: atommass(indesiredproduct) atomeconomy,%= ×100 atommass(inreactant) (1) the calculated atom economy for the synthesis of substituted disulfonamide p-benzoquinone is 99.09 %. the high atom economy indicates that all atoms, except four hydrogen atoms from the starting materials are incorporated into the product. the substituted disulfonamide p-benzoquinone compound was tested to evaluate its antibacterial activity by agar-well diffusion method [18]. escherichia coli (e. coli gram-negative) was the bacteria used in this experiment. the created well was filled with 3 mg of substituted disulfonamide p-benzoquinone. the inhibition zone surrounding the wells (in millimeters) was measured to evaluate antibacterial activity. the results showed that e. coli has a little sensitivity to substituted disulfonamide p-benzoquinone. an outer membrane might be established and a set of multidrug resistance pumps in gram-negative bacteria, such as e.coli, are quite effective barriers for antimicrobial compounds [19]. conclusions in summary, the pharmacological properties of disulfonamide and quinone encouraged us to synthesize a new compound containing disulfonamide and p-benzoquinone by electrochemical method. the results of cyclic voltammetric studies illustrated the ece reaction mechanism. this mechanism started in electrooxidation reaction through hydroquinone oxidation to generate pbenzoquinone. then this middle product was attacked by disulfonamide to produce the final product. the synthesis of substituted disulfonamide p-benzoquinone was conducted in high atom economy and in accordance with the principles of green chemistry. these principles were the reaction in water/ethanol mixture instead of toxic solvents, running the reaction under room temperature, and the utilization of electrode as an electron source instead of the usage of any toxic reagents. s. nassif et al. j. electrochem. sci. eng. 9(1) (2019) 1-7 doi:10.5599/jese.555 7 references [1] h. lund and o. hammerich, organic electrochemistry 4th, crc press, new york, (2001), p.1-95. [2] s. patai, z. rappoport, the chemistry of the quinonoid compounds. john wiley and sons, london, (1974) 737-791. [3] s. patai, z. rappoport, the chemistry of the quinonoid compounds volume 2, john wiley and sons., london, (1988), p.1293-1349. [4] b. zhang, g. salituro, d. li. z. szalkowski, y. zhang, i. royo, d. vilella, m. t. diez, f.pelaez, c. ruby, r.l. kendall, x. mao, p. griffin, j. calaycay, j. r. zierath, j. v. heck, r. g. smith, d. e. moller, science journals 284 (1999) 974–977. [5] t. murata, y. morita, k. fukui, k. sato, d. shiomi, t. takui, m. maesato, h. yamochi, g. saito, k .a. nakasuji, chemie international edition 43 (2004) 6343–6346. [6] f. a. khan, s. choudhury, tetrahedron letters 51 (2010) 2541–2544. [7] d. nematollahi, v. hedayatfar, journal of chemical sciences 123 (2011) 709–717. [8] d. nematollahi, r. esmaili, journal of the iranian chemical society 7 (2010) 260-268. [9] d. nematollahi, h. goodarzi, journal of organic chemistry 67 (2002) 5036-5039. [10] f. ramirez, s. dershowitz, journal of the american chemical society 78 (1956) 5614. [11] a. solhy, w. amer, m. karkouri, r. tahir, a. e. bouari, a. fihri, m. bousmina, m. zahouily, journal of molecular catalysis 336 (2011) 8–15. [12] c. t. supuran, a. casini, a. scozzafava, medicinal research reviews 23 (2003) 535-558. [13] m. loubatieres-mariani, journal of social and biological structures 201 (2007) 121-125. [14] t. h. maren, annual review of pharmacology and toxicology 16 (1976) 309-327. [15] f. comby, j. f. lagorce, t. moulard, j. buxeraud, c. raby, veterinary research 24 (1993) 316-326. [16] d. nematollahi, r. esmaili, electrochimica acta 56 (2011),3899 [17] m. eissen, r. mazur, h. g. quebbemann, k. h. pennemann, helvetica chimica acta 87 (2004) 524– 535. [18] b. bonev, j. hooper, j. parisot, j. antimicrob. chemotherapy 61 (2008) 1295-1301. [19] m. m. cowan, clinical microbiology reviews 12 (1999) 564-582. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) ir-ni oxide as a promising material for nerve and brain stimulating electrodes doi: 10.5599/jese.2014.0059 85 j. electrochem. sci. eng. 4(3) (2014) 85-96; doi: 10.5599/jese.2014.0059 open access: issn 1847-9286 www.jese-online.org original scientific paper ir-ni oxide as a promising material for nerve and brain stimulating electrodes joan stilling, nehar ullah and sasha omanovic department of chemical engineering, mcgill university, 3610 university street, montreal, quebec, canada, h3a 0c5 corresponding author: e-mail: joan.stilling@mail.mcgill.ca; tel.: +1-587-894-0660; fax: +1-514-398-6678 received: august 11, 2014; published: september 22, 2014 abstract tremendous potential for successful medical device development lies in both electrical stimulation therapies and neuronal prosthetic devices, which can be utilized in an extensive number of neurological disorders. these technologies rely on the successful electrical stimulation of biological tissue (i.e. neurons) through the use of electrodes. however, this technology faces the principal problem of poor stimulus selectivity due to the currently available electrode’s large size relative to its targeted population of neurons. irreversible damage to both the stimulated tissue and electrode are limiting factors in miniaturization of this technology, as charge density increases with decreasing electrode size. in an attempt to find an equilibrium between these two opposing constraints (electrode size and charge density), the objective of this work was to develop a novel iridium-nickel oxide (ir0.2-ni0.8-oxide) coating that could intrinsically offer high charge storage capacity. thermal decomposition was used to fabricate titanium oxide, iridium oxide, nickel oxide, and bimetallic iridium-nickel oxide coatings on titanium electrode substrates. the ir0.2-ni0.8-oxide coating yielded the highest intrinsic (material property) and extrinsic (material property + surface area) charge storage capacity (csc) among the investigated materials, exceeding the performance of the current state-ofthe-art neural stimulating electrode, ir-oxide. this indicates that the ir0.2-ni0.8-oxide material is a promising alternative to currently used ir-oxide, pt, au and carbon-based stimulating electrodes. keywords neural stimulation electrodes; metal oxides; iridium; nickel; charge storage capacity http://www.jese-online.org/ mailto:joan.stilling@mail.mcgill.ca j. electrochem. sci. eng. 4(3) (2014) 85-96 ir-ni oxide as a material for stimulating electrodes 86 introduction with a recently emerging focus on the development of neural prostheses (technology which is able to interact with the body’s nervous system) to the utilization of electrical stimulation as a therapy for various neurological disorders, many efforts have been directed towards overcoming the extensive challenges related to electrical stimulation electrode design. controlled and selective stimulation of the human’s central and/or peripheral nervous system is the key to success for all neural stimulation treatment techniques, including deep brain stimulation (dbs), functional electrical stimulation (fes), bladder, intraspinal and epidural stimulation. dbs is used as a treatment for parkinson’s disease, epilepsy, essential tremor, dystonia, and medication resistant psychiatric diseases such as obsessive compulsive disorder (ocd) and depression, while fes, bladder, and spinal stimulation are utilized in stroke, multiple sclerosis, and spinal cord injury rehabilitation therapy[1-6]. the immensely widespread applicability of electrical stimulation in treating numerous pathologies demonstrates its incredible utility in successful medical device development. in all such applications, a neural electrode works as a bridge transferring information between the external electronic control device and the human biological system (neurons) [7]. neural prostheses based on stimulation and recording of neurons (i.e. the cells that transmit electrical information in the body) involve the use of electrodes, which are chronically interfaced to the nervous system. recording electrodes typically pick up information from sensory systems whereas stimulating electrodes, which are the focus of this article, often communicate with motor systems. metallic biomaterials have been the main material of choice for neural electrodes. for stimulation; titanium (ti), titanium nitride (tin), stainless steel (ss), platinum (pt), pt alloys, iridium (ir), ruthenium (ru), and rhodium (rh), which support charge injection by capacitive and faradic mechanisms are available, while materials used for recording electrodes include stainless steel, ptir alloys, ir oxide and ti-nitride [7, 8]. implantation of neural stimulators in each clinical application requires a safe protocol of operation involving their size, charge injection, and service time [9-11]. however, there are many challenges related to the development of such devices. one of the key problems with current electrodes is their lack of stimulus selectivity, which occurs as a result of large electrode size relative to the targeted neuronal population. for this reason, desirable electrodes are characterized by having a small enough geometric size for targeted and selective neuronal activation, while still possessing the ability to inject a sufficient charge density that does not induce harmful effects on neural function, cell structure, or the electrode itself. to address the issues mentioned above related to the neural stimulation electrodes, the biocompatibility properties of the material in response to stimulation can be investigated. metallic biomaterials offer many advantages over the other materials, such as high mechanical strength, wear resistance, inertness, and potential to produce an oxide layer on the surface of the material. however, metallic biomaterials are vulnerable to irreversible faradaic reactions such as water electrolysis, corrosion and the release of toxic ions through gas evolution or metal dissolution [10,12,13]. the performance of the electrode depletes with time as a result of the constantly changing and aggressive environment prone to corrosion in which the electrode is implanted. all of these susceptibilities may induce an immune or necrotic response in adjacent tissue, which can lead to fouling of the electrode surface and loss of functionality [12-14]. hence, a material with a relatively high corrosion resistance is a desirable feature for a stimulating electrode. it is also essential that the material has a high charge storage capacity; one that effectively stimulates an action potential in a nerve [15]. various noble metals such as pt and pt/ir alloys have a long history j. stilling et al. j. electrochem. sci. eng. 4(3) (2014) 85-96 doi: 10.5599/jese.2014.0059 87 of use as neural electrodes. however, the maximum charge injection (qinj) limit of pt (100-300 µc cm -2 geometrical area) is insufficient in most cases for nerve stimulation [7,16,17]. titanium is also used extensively in the biomedical field and neuroscience research due to its high corrosion resistance, biocompatibility, low cost and non-toxicity [18-20]. however, its low charge injection capacity limits its use as a neural stimulation electrode material. thus, faradaic electrode coatings based on films of ir oxide have been developed in response to the need for microelectrodes with higher charge injection capacities. activated iridium oxide film (airof) microelectrodes, prepared electrochemically, are widely used for neural applications as they offer a significant improvement in the charge injection limit (2-3 mc cm -2 geometrical area) with a reversible faradic reaction (ir 3+  ir 4+ + e ) when compared to other materials. however, activated iridium oxide delaminates under high current pulsing and deposits particles into the surrounding tissues. additionally, the brittle mechanical properties of iridium oxide make it very difficult to fabricate microelectrodes [7,10,21-24]. new methods have been adopted to address this problem by combining other precursors like ruthenium, titanium, tantalum and tin with iridium to form mixed metal oxide films. however, the long-term stability and higher charge storage/injection of these coatings still needs to be optimized for better performance [8,21,25-28]. some lower cost transition metal oxides such as nickel, manganese, cobalt and their composites have also been investigated as an alternative electrode material for neural stimulating electrode applications. nickel for example, has comparable electrochemical behavior to iridium, is low in cost, and readily available. yet, its nonbiocompatible structure, low corrosion resistance and low specific capacitance, limit the feasibility of using pure ni as a neural electrode coating material [12,29,30]. despite the drawbacks of pure nickel, it may be combined with other precursors (metals), such as iridium, to utilize its beneficial properties. the potential of bimetallic coatings such as this may help to solve some of the problems encountered with current electrodes. as outlined above, there is an evident need for the investigation into new materials for neural stimulating electrodes. the aim of this study was to alter electrode surface characteristics through the development of a novel nickel-iridium bimetallic coating on a titanium substrate. in an effort to reduce the aforementioned problems, electrodes were thermally prepared, electrochemically tested, and results were compared with the current industry standard (state-of-the-art), iridium oxide coating (control). experimental selection of electrode coating materials various metal oxide coatings were deposited on a titanium substrate and then characterized using a number of different electrochemical and surface characterization techniques. a titanium plate (99.2 %, metal basis) was used as the substrate. titanium was chosen due to its excellent biocompatibility characteristics, chemical stability, mechanical strength, and biocompatibility [31]. during experimentation, pure iridium oxide coatings were produced as a control sample; this is currently one of the most common materials applied to neural electrodes due to its high charge storage capacity when compared to other pure metals. however, this stimulating electrode suffers from a number of drawbacks, including loss of the charge-injection capability with time, delamination of the oxide coating, fouling, and high cost [7,32]. to address these issues and develop a better stimulating neural electrode, alternative metals were investigated. nickel was chosen on an empirical basis due to its stability and low cost [12]. in j. electrochem. sci. eng. 4(3) (2014) 85-96 ir-ni oxide as a material for stimulating electrodes 88 addition, other laboratories have previously demonstrated that this metal shows adequate electrochemical properties in terms of its redox and capacitive behavior, which are similar to that of iridium. both a one hundred percent nickel oxide coating and an uncoated titanium electrode substrate were created as additional controls for experimentation. in an attempt to obtain a coating that performs superior to both the pure nickel and iridium oxide in terms of coating stability and charge injection capability, an ir0.2-ni0.8-oxide coating was produced. this bimetallic coating combination of iridium with nickel is a novel idea for use in the application of neural electrodes. electrode preparation metal oxide coatings were formed through a thermal decomposition method on a flat titanium substrate (purity 99.2 % metals basis, alfa aesar 10398). to coat the titanium plates, a 0.15 m precursor solution was prepared by dissolving the corresponding metal precursor salts into isopropanol (purity 99.9%, fisher scientific a416-1). niso4× 6h2o (sigma aldrich 227676) was the precursor used to prepare the nickel oxide coating and ircl3 × 3h2o (acros organics 195500050) was used for the iridium oxide coating. it was assumed that the molar ratio of the metals in the precursor solution gave the same molar ratio in the electrode coating. titanium substrates used in all experiments were 1 cm-diameter discs with a thickness of 0.2 cm. in order to create a surface to which the precursor solution could adhere, the substrate was polished and etched. the titanium substrate plate was first wet-polished using 600-grit sic sandpaper [33]. then, the polished plate was rinsed thoroughly with abundant deionized water and sonicated for 30 minutes in a water bath to remove polishing residue. next, the polished plate was etched in a boiling solution of hydrochloric acid (33 wt. %, fisher scientific) and deionized water (1:1 by volume) for 30 minutes [34,35]. after etching, the plate was again thoroughly rinsed with deionized water and then dried in argon. the metal precursor coating solution was applied uniformly on the freshly prepared titanium substrate with a paint brush. after applying the first coating, the sample was placed in an oven at 383 k for 5 minutes (in order to vaporize the solvent), followed by annealing of the sample at 737 k in a furnace for 15 minutes. the sample was then removed from the furnace and allowed to cool for 5 minutes before another coating was applied. the same procedure was repeated six times in order to form six coatings on the titanium substrate. finally, the sample was annealed in the furnace at 737 k for a period of one hour to oxidize the coating [26,28,36-38]. electrochemical measurements a standard three-electrode electrochemical cell was employed in all electrochemical experiments. both a graphite counter electrode and saturated calomel reference electrode (sce) were utilized. all potentials in this paper are expressed with respect to sce. a titanium plate coated by ir-, nior ir0.2-ni0.8-oxide was applied as the working electrode (only one side of the titanium plate was coated with the metal–oxide coating). to utilize the coated side of the working electrode samples during experimentation, a custom-made electrode holder was used to expose a 0.785 cm 2 geometrical area of the working electrode to the supporting electrolyte. electrochemical experiments were employed in an aqueous 0.16 m nacl phosphate buffered solution at ph 7.4. the buffer solution, which is commonly used to simulate human-body fluids, was prepared by mixing appropriate amounts of sodium chloride (purity ≥99.5 %, fluka chemika 71381), sodium dihydrogen phosphate anhydrous (purity 99 %, fluka chemika 71496) and sodium phosphate dibasic (purity 99.5 %, fisher scientific s374). a 5 m naoh solution (fisher scientific ss256) was j. stilling et al. j. electrochem. sci. eng. 4(3) (2014) 85-96 doi: 10.5599/jese.2014.0059 89 used to adjust the ph of the buffer solution. in order to maintain an oxygen-free electrolyte, argon (99.998 % pure) was purged, both through the electrolyte 30 minutes prior to electrochemical measurements and during the electrochemical experiments. electrochemical measurements were performed using an autolab potentiostat/galvanostat/fra pgstat 30 controlled by fra2 and gpes v. 4.9 software. the general electrochemical behaviour and charge storage capacity of the uncoated ti (control) electrode and the ir-, niand ir0.2-ni0.8-oxide electrodes were determined using cyclic voltammetry (cv). to ensure complete characterization of the surface processes occurring at the electrodesolution interface and in the oxide phase, electrochemical impedance spectroscopy (eis) measurements were made in 0.16 m nacl phosphate buffer solution over seven frequency decades, from 100 khz to 50 mhz. the alternating current (ac) voltage amplitude of ±10 mv was employed. the corresponding spectra were modeled employing the two equivalent electronic circuits (eec) presented in figure 1. figure 1. (a) equivalent circuit model #1 figure 1. (b) equivalent circuit model #2 the values of solution resistance (rs), polarization resistance or total resistance to charge transfer (r = r1 + r2 for circuit in fig. 1(b)), and capacitance were determined by fitting the experimental data to either of the two models. a constant phase element (cpe), q, was utilized instead of pure capacitance; this was due to the distribution of the relaxation times as a result of heterogeneities present at the micro level, such as surface roughness [39]. in fig. 1(a), q represents the double-layer capacitance, while r represents the polarization resistance. in fig. 2(b), q1 and q2 are the double-layer and pseudo-capacitance, respectively (the latter related to the redox transitions in the oxide phase). r1 and r2 are the electron-transfer and mass-transport resistance, respectively (the latter related to transport of protons in the oxide phase to balance the metal oxide charge change). the ir0.2-ni0.8-oxide coating was best modeled using the equivalent circuit element shown in fig. 1(a), while the remaining coatings were modeled using the equivalent circuit model shown in fig. 1(b). surface morphology/topography surface morphology/topography of the various metal coatings was investigated using a phillips xl-30 field emission scanning electron microscope (fe-sem). results and discussion surface characterization by scanning electron microscopy (sem) in figure 2, scanning electron micrographs of the three different coatings applied to the titanium substrate are shown. in figure 2(a, b), a highly roughened ir0.2-ni0.8-oxide coating can be observed. this is shown by its highly tortuous, inhomogeneous surface architecture. the iridium oxide coating shown in figure 2(c, d) does not demonstrate as rough a surface as in figure 2(a, b). the surface has deep cracks uniformly dispersed throughout the coating, as indicated by the thin black regions present throughout the image. this surface topography/morphology is less favorable rs q r r2 rs q1 r1 q2 j. electrochem. sci. eng. 4(3) (2014) 85-96 ir-ni oxide as a material for stimulating electrodes 90 than that in figure 2(a, b), as less true surface area is available per geometric surface area to deliver charge. as a result, wider potential windows may be required to deliver a charge sufficient for neuronal stimulation. the nickel oxide coating shown in figure 2(e, f) demonstrates slight cracks throughout surface. however, the troughs do not appear as deep and wide as those present in the iridium oxide coating shown in figure 2(c, d). further, the nickel oxide coating’s small roughness value (lower true surface area) can be observed when compared to the ir0.2-ni0.8 oxide coating shown in figure 2(a, b). surface roughness has a strong effect on an electrode’s charge storage capacity, which will be demonstrated and discussed later in the article. figure 2. sem micrographs of the metal oxide coatings formed on a ti substrate after cv experiments in 0.16 m saline phosphate buffer solution at ph = 7.4, (a, b) ir0.2-ni0.8-oxide (c, d) iridium oxide (e, f) nickel oxide. electrochemical characterization by cyclic voltammetry cyclic voltammetry (cv) was used to assess the electrochemical redox behavior of the electrode coatings within the region between hydrogen and oxygen evolution; -1.0 v to 1.0 v. these measurements allowed for the characterization of the stimulation electrodes by their charge storage capacity (csc). csc is defined as the total amount of charge per unit geometric surface area (gsa), that may be stored reversibly in the electrode [13] and is sufficient enough to stimulate an action potential in a nerve without causing any damage to human tissues [7]. hence, in order to investigate whether the developed coatings are capable of delivering sufficient amount (a) (b) (c) (d) (e) (f) j. stilling et al. j. electrochem. sci. eng. 4(3) (2014) 85-96 doi: 10.5599/jese.2014.0059 91 of charge during stimulation, csc of various coatings were determined using cv data presented in figure 3. the presented behavior is typical of that displayed by purely capacitive and pseudocapacitive materials [7,17,40-43]. a few conclusions can be made by analyzing figure 3. first, there is a large difference in current response among the investigated materials; at a fixed potential, the recorded current increases in the order of ti< ni = ir < ir0.2-ni0.8-oxide, indicating that the charge storage capacity behaves in the same manner. second, when analyzing the ir0.2-ni0.8-oxide coating response, one can see that there are two redox peaks; an anodic peak at ca. 0.2 v and a cathodic peak at ca. -0.1 v. these peaks correspond to reversible redox transitions in the oxide phase, namely the ir(iii)/ir(iv) and ni(ii)/ni(iv) transitions [41-44]. these results demonstrate that the various electrode coatings behave quite differently when a charge is applied to the electrode. figure 3. cyclic voltammograms of ti-, ni-, irand ir0.2-ni0.8-oxide coatings recorded in 0.16 m saline phosphate buffer ph = 7.4 at a scan rate, sr = 100 mv s -1 . to evaluate the amount of csc that can be delivered during the anodic and cathodic polarization of the electrode, the corresponding anodic and cathodic areas under the curves were integrated and then normalized with respect to geometric surface area (gsa)in order to determine the extrinsic charge storage capacity (cscextrinsic). figure 4, presents cscextrinsic of the various metal oxide coatings investigated. the ir0.2-ni0.8-oxide coating has the highest cscextrinsic of all the materials tested, with a value of 30.3±4.8 mc cm -2 . ir-oxide, which is the current state-of-the-art neural stimulation material, demonstrated a significantly lower cscextrinsic than the bimetallic composite, yielding a value of 9.8±1.0 mc cm -2 . pure ni-oxide also failed to display high a csc and restrained to a value of 2.8±1.1 mc cm -2 , while the titanium (substrate) fell short of all other metal oxide coatings, exhibiting a csc of only 0.23 ±0.07 mc cm -2 . the fact that the titanium oxide sample has a very low csc value further indicates that adding a coating effectively increases the extrinsic csc of the electrode. additionally, it can be concluded that the data obtained from the samples with coatings are a result of the properties of the coating itself and not the titanium substrate. the highest extrinsic csc of ir0.2-ni0.8-oxide coating is most likely due to its large surface roughness value (determined in current studies, but not shown here). the fairly low percent relative standard deviations for all of the samples in figure 4 indicate that the coatings were able to store charge in a reproducible manner. to the best of authors’ knowledge, a higher cscextrinsic ir0.2-ni0.8-oxide ir-oxide ni-oxide ti-oxide j. electrochem. sci. eng. 4(3) (2014) 85-96 ir-ni oxide as a material for stimulating electrodes 92 value has not been reported in the literature for ir-ni-oxide coatings under the experimental condition applied. figure 4. total extrinsic charge storage capacity (cscextrinsic) of various metal oxide coatings determined from cvs in figure 3, for at least three sample (coating) replicates. the error bars represent the corresponding standard deviation. in order to investigate the charge storage capacity of the material alone (without considering the effect of surface area), the intrinsic charge storage capacity was further examined. for this purpose, a true electrode surface area was first determined form electrochemical impedance spectroscopy results. characterization by electrochemical impedance spectroscopy (eis) figure 5 represents an example of the eis spectra obtained for ir0.2-ni0.8-oxide coating at the 0.5 v. similar eis responses were obtained for all other oxide coatings, however they are not included in this analysis. to quantify the eis result, the experimental spectrum presented in figure 5 was modeled using non-linear least-squares fit analysis (nlls) software [45] and the electrical equivalent circuit (eec) presented in figure 1(a). the modeled data are represented by the dashed line in figure 5. it is evident that there is a substantial agreement between the experimental data (circles) and modeled data (dashed line), confirming the validity of the proposed eec in describing the impedance behavior of the investigated system under the given experimental conditions. the same eis analysis of the other developed metal oxide coatings was carried out, using the eec in figure 1(b). in order to determine the true electrode surface area of the metal oxide coatings, eec parameters were analyzed for the contribution of double-layer capacitance. low impedance is required for the stimulation of neural networks. since capacitance is inversely proportional to impedance, a large capacitance is desired [9]. to investigate the capacitive behavior of the developed coatings, a true value of the double-layer capacitance (cdl) was calculated using the equation proposed by brug et al. [46]: 1 1 n 1 1/n sdl [ ( ) ]c q r r      (1) j. stilling et al. j. electrochem. sci. eng. 4(3) (2014) 85-96 doi: 10.5599/jese.2014.0059 93 figure 5. nyquist plot of the ir0.2-ni0.8-oxide coating recorded in 0.16 m saline phosphate buffer at electrode potential of -0.5 v. the dashed line represents the simulated spectra obtained using the equivalent electrical circuit model in figure 1(a). cdl / mf cm -2 values for all coatings were calculated at -0.5 v and the resulting values are presented in figure 6. analysis of the figure demonstrates that the ir0.2-ni08-oxide coating has a capacitance value of 2.09±0.25 mf cm -2 . in contrast, iridium oxide (the state-of-the-art material for neural stimulation electrodes) has the value of 1.18±0.18 mf cm -2 , almost half of the value obtained for the ir0.2-ni08-oxide coating. furthermore, the pure ni-oxide coating yielded a value of 0.88±0.16 mf cm -2 , while the naked ti substrate exhibited the lowest value of all substrates tested, at a value of 0.005±0.002 mf cm -2 . this reflects the superiority of the ir0.2-ni0.8-oxide coating in terms of double layer capacitance and thus, high charge storage capacity. figure 6. dependence of the double-layer capacitance on the applied dc potential of various metal oxide coatings obtained by fitting eis spectra. 0 2 4 6 8 10 12 14 0 5 10 15 20 25 30 -z '' / k ω c m 2 z' / kω cm2 j. electrochem. sci. eng. 4(3) (2014) 85-96 ir-ni oxide as a material for stimulating electrodes 94 the next step in our analysis was to determine the intrinsic charge storage capacity (cscintrinsic) of the coatings. corresponding cscintrinsic values were calculated by normalizing the extrinsic charge storage capacity (cscextrinsic) values plotted in figure 4 with respect to true electrochemical active surface (teas) area of the electrode coating. this was done by employing the following equation [47,48]: extrinsic theoritical intrinsic dl csc csc c c  (2) where ctheoritical /25 f cm -2 represents the theoretical value of the double layer capacitance [13]. calculated values of cscintrinsic for all coatings are plotted in figure 7. figure 7. total intrinsic charge storage capacity (cscintrinsic) of various metal oxide coatings obtained from cvs recorded in 0.16 m saline phosphate buffer ph = 7.4, at a scan rate sr = 100 mv s -1 . it is evident from figure 7 that the ir0.2-ni0.8 oxide bimetallic coating has the highest intrinsic charge storage capacity as compared to other oxide coatings. lower cscintrinsic values indicate that large electrode sizes would be required in neural applications to deliver the required amount of charge. the high intrinsic csc ability of the ir-ni composite proves that it has potential to be a useful material in the effort of neural electrode miniaturization, while still providing the charge necessary for successful stimulation. conclusions in this study, the electrochemical properties of ir-, niand ir0.2-ni0.8-oxide coatings formed on a ti substrate were investigated in 0.16 m nacl phosphate buffered solution, at ph of 7.4, for the purpose of possible application of the materials in the technology of stimulating neural electrodes. sem results showed contrast between the stable, rough surface of the ir0.2-ni0.8 oxide coating and the niand ir-oxide coatings, which displayed the undesirable characteristic of slight cracking. in addition, it was also determined that the ir0.2-ni0.8-oxide coating had the highest charge storage capacity (csc) both intrinsically and extrinsically. the ir0.2-ni0.8-oxide coating’s extrinsic csc of 30 mc cm -2 proved to be higher than the csc offered by the current state-of-the-art neural j. stilling et al. j. electrochem. sci. eng. 4(3) (2014) 85-96 doi: 10.5599/jese.2014.0059 95 stimulating electrode, ir-oxide. eis results also demonstrated that the ir0.2-ni0.8-oxide coating was optimal, achieving the largest capacitance (lowest impedance) value. this novel bimetallic ir-ni-oxide coating serves as an extremely 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electrochemistry 36 (8) (2006) 83-888 [48] s. trasatti, o. a. petrii, journal of electroanalytical chemistry 327 (1-2) (1992) 353-376 © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {poly(vinyl alcohol)/chitosan hydrogels with electrochemically synthesized silver nanoparticles for wound dressing applications} http://dx.doi.org/10.5599/jese.732 185 j. electrochem. sci. eng. 10(2) (2020) 185-198; http://dx.doi.org/10.5599/jese.732 open access: issn 1847-9286 www.jese-online.org original scientific paper poly(vinyl alcohol)/chitosan hydrogels with electrochemically synthesized silver nanoparticles for wound dressing applications katarina nešović1, ana janković1, tamara radetić1, aleksandra perić-grujić1, maja vukašinović-sekulić1, vesna kojić2, kyong yop rhee3, vesna mišković-stanković1,3, 1university of belgrade, faculty of technology and metallurgy, belgrade, serbia 2university of novi sad, faculty of medicine, oncology institute of vojvodina, sremska kamenica, serbia 3kyung hee university, department of mechanical engineering, yongin, south korea corresponding author: vesna@tmf.bg.ac.rs; tel: + 381 11 3303 687; fax: + 381 11 3370 387 received: september 28, 2019; revised: october 25, 2019; accepted: october 28, 2019 abstract polymer-based hydrogel materials are excellent candidates for new-generation wound dressings with improved properties, such as high sorption ability, good mechanical properties and low adhesiveness. cross-linked hydrogel matrices also serve as excellent carriers for controlled release of antibacterial agents, such as silver nanoparticles (agnps), which are preferred over conventional antibiotics due to low propensity to induce bacterial resistance. in this work, we aim to produce novel silver/poly(vinyl alcohol)/chitosan (ag/pva/chi) hydrogels for wound dressing applications. the electrochemical agnps synthesis provided facile and green method for the reduction of ag+ ions inside the hydrogel matrices, without the need to use toxic chemical reducing agents. the formation of agnps was confirmed using uv-visible spectroscopy, scanning and transmission electron microscopy. release kinetics was investigated in modified phosphate buffer solution at 37 °c to mimic physiological conditions. release profiles indicated “burst release” behavior, which is beneficial for wound dressing applications. the antibacterial activity was evaluated against staphylococcus aureus and escherichia coli strains using disc-diffusion test, and non-toxicity of hydrogels was proved by dye-exclusion test. the obtained results confirmed strong potential of ag/pva/chi hydrogels for biomedical applications. keywords electrochemical synthesis; hydrogels; release kinetics; antibacterial activity; wound dressings http://dx.doi.org/10.5599/jese.732 http://dx.doi.org/10.5599/jese.732 http://www.jese-online.org/ mailto:vesna@tmf.bg.ac.rs j. electrochem. sci. eng. 10(2) (2020) 185-198 hydrogel wound dressings with agnps 186 introduction wound dressing materials are a major focus of biomedical and materials science research. the interest in improved material formulations is enhanced by growing problems related to treating deep and necrotic wounds and ulcers which are subject to major in-hospital infections, often caused by antibiotic and multidrug-resistant strains, sometimes with fatal consequences. many drawbacks of traditional dressing materials (mostly textile-based) [1], such as susceptibility to bacterial infection, poor regulation of moisture, sticking to the wound tissue and frequent need for replacement, have led to a reorientation of research efforts towards new-generation polymer-based materials which should alleviate some or all of these shortcomings. hydrophilic polymer gels are an excellent substitute for cotton-based dressings, as they provide a wealth of options to tailor such properties as adhesiveness, swelling and sorption, gas permeability, and moisture regulation [2]. biocompatibility is always the primary characteristic that must be evaluated before these materials could be considered for such applications. thus, several polymers (both natural-origin and synthetic) have emerged as acceptable candidates that satisfy this most important property. among those are biocompatible and often biodegradable polymers such as alginate, chitosan (chi), poly(vinyl alcohol) (pva), poly(vinyl pyrrolidone) (pvp), cellulose, etc. [3–11]. of course, apart from biocompatibility, hydrogel-based wound dressings must satisfy several other stringent requirements, e.g. sterility, non-toxicity/non-inflammatory characteristics, permeability, low adhesiveness, high sorption, moisture and temperature regulation, mechanical integrity [12]. particularly important aspect is the method of obtaining hydrogels, as the most widely-used chemical gelation/cross-linking techniques have proved dangerous due to the use of toxic chemical gelation agents, such as glutaraldehyde, therefore, there has been substantial research on finding other, green and non-toxic cross-linkers [13]. however, an even better option would be applying physical gelation methods (which fully eliminate the need for chemical agents), such as the freezing-thawing technique, whenever applicable. freezing-thawing is a green and simple method for obtaining hydrogels with high gelation degrees and has been widely applied for preparing poly(vinyl alcohol)-based hydrogels [14-16], although it was proved ineffective for other polymer types, such as pvp [14]. another important aspect of wound healing problems is related to widespread antibiotics use, which has led to an alarmingly high rate of emergence of resistant bacterial strains [17]. this problem has been aggravated by systemic non-specific drug administration that requires long treatment and high doses. hence, the research focus was shifted towards developing topical drug carriers, incorporating the antibacterial agents directly into dressing materials, thus enabling dosage decrease and allowing the drug to reach the infection site much quicker. additionally, research is increasingly shifting away from applications of antibiotics and towards finding alternative antibacterials with less inherent risks. thus metallic nanoparticles have emerged as strong candidates for wound-treatment applications [18]. silver nanoparticles (agnps) are, by far, the most widely-used due to strong antimicrobial activity coupled with unusual and highly-tailorable properties of silver on the nano-scale [19–21]. on a note related to efforts to eliminate the toxic chemical agents for gel formation, here exists a similar concern over chemical reduction methods for obtaining agnps. multiple studies were published on green reducing agents for agnp synthesis, but one of the more promising routes is recruiting the power of electrochemistry to obtain agnps, either directly in the hydrogel matrix or in the colloid form [15,22–25]. the electrochemical methods of agnp synthesis involve the reduction of silver ions using electrical current only or chemically-pure hydrogen gas which evolves due to side-reaction of water electrolysis in aqueous solutions; the final k. nešović et al. j. electrochem. sci. eng. 10(2) (2020) 185-198 http://dx.doi.org/10.5599/jese.732 187 biomaterial product is free from any side-products and thus completely non-toxic and safe for intended applications. in this work, we present a green method for obtaining poly(vinyl alcohol) and chitosan-based hydrogel materials, which are envisioned to be used as wound dressing material, by applying physical freezing-thawing technique. silver nanoparticles were synthesized via electrochemical reduction of silver ions inside the hydrogel matrix, thus obtaining a chemically-pure and non-toxic product. the successful incorporation of agnps was evaluated by uv-visible spectroscopy and electron microscopy; the prepared hydrogel discs were further characterized in terms of silver release behavior, as well as biological properties, i.e. antibacterial activity and cytotoxicity. experimental for the synthesis of silver/poly(vinyl alcohol)/chitosan hydrogels, the following chemical reagents were used: fully-hydrolyzed poly(vinyl alcohol) and partially deacetylated chitosan powders (sigma aldrich), glacial acetic acid (beta hem), silver nitrate (mp hemija), potassium nitrate (centrohem), deionized (di) water (millipore). the hydrogels were prepared following the method described in [15,25,26]. first, pva and chi powders were dissolved in di water and 2.0 vol% acetic acid solutjion, respectively. the dissolution of pva was carried out at an elevated temperature of 90 °c. after the dissolution of polymers and cooling of the pva solution, they were mixed gradually under continuous stirring to obtain uniform pva/chi dispersion. the amounts of chitosan were varied, yielding two different colloid dispersions with 0.1 wt% chi (pva/0.1chi) and 0.5 wt% chi (pva/0.5chi). the gelation was then carried out by casting the colloid dispersions in petri dishes and exposing them to five freezing (-18 °c for 16 h) and thawing (+4 °c for 8 h) cycles. thus were obtained pva/0.1chi and pva/0.5chi hydrogels. before the agnps synthesis, the above-described hydrogels were swollen in agno3 solutions supplemented with 0.1 m kno3 that served to improve electrical conductivity. the swelling was carried out for 48 h, after which the hydrogels were placed in an electrochemical cell depicted in figure 1, between two parallel pt plates that played the roles of working and counter electrodes, respectively. the electrodes were connected to a dc power source that supplied a 90 v constant voltage needed for agnps electrochemical synthesis. the total synthesis time was 4 min, during which the polarity of the electrodes was periodically switched (every 1 min), in order to dissolve the bulk ag layer that was deposited on the cathode surface as a side-reaction [25]. two different concentrations of agno3 swelling solution were used (0.25 mm and 3.9 mm), and the obtained agnpincorporated hydrogels were named accordingly, i.e. 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi, or 3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi. the hydrogels with higher amount of silver (3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi) were used primarily for physico-chemical characterization of the hydrogels, as the higher agnps concentration allows better evaluation of their properties, whereas the lower-agnp-content samples were chosen for biological evaluations, as it was confirmed in previous studies that this agnps concentration provided optimal balance between antibacterial properties and non-toxicity [23]. characterization the incorporation of agnps inside 3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi hydrogels was confirmed by uv-visible spectroscopy (uv-vis); the absorbance of hydrogel discs, dissolved in di water (1:7 w/v ratio), was measured in the wavelength range from 250 to 700 nm, using llg-unispec 2 (llg labware) instrument. scanning electron microscopy (sem) characterization was conducted in http://dx.doi.org/10.5599/jese.732 j. electrochem. sci. eng. 10(2) (2020) 185-198 hydrogel wound dressings with agnps 188 field emission leo supra 55 (carl zeiss) microscope at 10 kv acceleration voltage. high-resolution transmission electron microscopy (hrtem) was carried out in talos f200x g2 (fei) microscope at 200 kv. for hrtem, the samples were dissolved in di water (1:20 w/v ratio), dropped on 400-mesh copper tem grids and left to dry for 24 h. silver release the release of silver from the 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogel matrices was carried out according to our previously published procedure [15,26]. the hydrogel discs were immersed in 10 ml phosphate buffer (pb) solution (ph 7.4) and thermostated at 37 °c. the experiment lasted 28 days in total, during which the pb solution was removed from the vials and replaced with freshly prepared one. the removed solutions were kept in air-tight test-tubes at 4 °c before measurements. the concentrations of silver released into the pb solution were measured using pyu unicam atomic absorption spectrophotometer (philips). after 28 days, the hydrogels were also dissolved in 1:1 (v/v) hno3, and the measured concentrations represented the amount of silver that was retained inside the hydrogel after the 28-day release period. antibacterial properties the antibacterial activity of pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels was evaluated using disc-diffusion method, as described previously [15], with test strains staphylococcus aureus tl and escherichia coli atcc25922. hydrogels were sterilized under a uvlamp for 30 min. a 0.7 wt.% soft-top agar was inoculated with overnight bacteria cultures (number of bacteria ≈106 cfu ml−1) and poured into petri dishes with solidified nutrient agar base. later, the hydrogel discs were placed on the agar surface, and incubation was carried out for after 24 h at 37 °c, after which the widths of inhibition zones were measured. cytotoxicity the cytotoxicity of pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi, and 0.25ag/pva/0.5chi hydrogels was evaluated using dye-exclusion test (det), based on trypan-blue dye exclusion [27]. two fibroblast cell lines were used – mrc-5 (human-origin) and l929 (mouse-origin). viable cells (≈105 ml-1) were seeded in 12-well plates, along with hydrogel disc samples. the controls were cells without the samples. the cells with hydrogels were incubated for 48 h at 37 oc, in 5 % co2 atmosphere. after the incubation period, the cells were separated from hydrogel samples by trypsinization, and cells were counted using a trypan blue exclusion method. the obtained cell viability results were expressed as a percent of control. all experiments were carried out in triplicate, and the results were represented as the mean  standard deviation. statistical significance of det assay results was estimated using anova followed by a multiple-comparison (post-hoc) test with significance level decided at p < 0.05. results and discussion the pva/0.1chi and pva/0.5chi hydrogels were obtained by a physical cross-linking method, specifically through several cycles of freezing and thawing, during which the formation of the crosslinked polymer matrix occurred. this method is particularly advantageous for biological applications because it allows circumventing the issue of toxicity, which is always a risk in the case of chemically cross-linked hydrogels. pva polymer has been long known for its property to form elastic and wellstructured hydrogel networks through freezing-thawing technique [15,23,28–30], that can be tailored by modifying the duration, temperature, and the number of cycles to control the hydrogel k. nešović et al. j. electrochem. sci. eng. 10(2) (2020) 185-198 http://dx.doi.org/10.5599/jese.732 189 properties [28]. another important feature of physically cross-linked hydrogels is their thermoreversibility [31], which allows returning to sol state or dissolving them at elevated temperatures, thus significantly facilitating the handling and manipulation of the hydrogel. the gel formation is thought to occur via phase separation, dividing the solution into two regions, one with a high amount of polymer and a small amount of bound water, and the other vice versa [30–32]. thus, cross-link sites are formed in polymer-rich phase through hydrogen bonding or micro-crystallization, resulting in a polymer network with a high gelation degree, depending of course on the polymer concentration in the starting solution, as well as on the freezing and thawing experimental conditions [29,32]. our pva/0.1chi and pva/0.5chi hydrogels indeed had rather high gelation degrees of 94.0 % and 95.6 %, respectively, which were determined gravimetrically [25]. the synthesis of agnps in thus obtained physically cross-linked hydrogels was carried out by electrochemical method, well-described in our previous research [15,25–27,33]. hydrogel disc samples were swollen in agno3 solutions, as described in the experimental section above. swollen hydrogel discs were then placed in a glass cell between two platinum electrodes that were connected to a dc power supply, as depicted in figure 1. the cathodic reactions during the synthesis can be described by equations (1) and (2), where the first reaction is the reduction of silver ions, and the second is hydrogen evolution due to the use of the aqueous solution and high applied voltage of 90 v [25]. this intense h2 generation is what facilitates the formation of agnps inside the hydrogel, as the hydrogen bubbles easily permeate the hydrogel disc all across its volume, and thus the nucleation sites are formed through reaction (3) [34], with subsequent growth of nanoparticles taking place at these sites. ag+ + e→ ag (1) 2h3o+ + 2e– → h2 + 2h2o (2) h2 + 2ag+ → 2h+ + 2ag (3) reaction (1) mostly results in bulk silver layer deposition on the cathode surface (and perhaps the formation of a small amount of agnps on the hydrogel surface which is in immediate contact with the electrode). therefore the polarity of the electrodes was switched periodically in order to dissolve this ag layer from the cathode surface, as explained in experimental. figure 1. scheme of electrochemical synthesis of agnps in pva/0.1chi and pva/0.5chi hydrogels. http://dx.doi.org/10.5599/jese.732 j. electrochem. sci. eng. 10(2) (2020) 185-198 hydrogel wound dressings with agnps 190 thus obtained 0.25ag/pva/0.1chi, 0.25ag/pva/0.5chi, 3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi hydrogels were further characterized in order to confirm the formation of agnps, to evaluate the release of silver from their matrices and to confirm their antibacterial activity and non-toxicity. characterization of hydrogels uv-visible spectroscopy was employed to confirm the presence of agnps inside the nanocomposite hydrogels polymer matrices. silver nanoparticles exhibit strong absorption in the lower visible light region, which is absent from the spectra of the metal in bulk and is a consequence of the nature of nano-sized particles. the absorption is caused by matching resonance of the incident light frequency with collective oscillations of free electrons in the valence and conduction bands of silver atoms [35–37]. this phenomenon is known as localized surface plasmon resonance (lspr), and the wavelength (max), absorbance (amax), and full-width at half maximum (fwhm) of the resulting absorption band are dependent on the type, size, shape and degree of aggregation of the nanoparticles, as well as on the properties of the surrounding medium [35,37,38]. in the case of silver nanoparticles, the lspr band is most commonly found at wavelengths around ≈400 nm, with variations due to above-mentioned factors. the lspr peak in the uv-vis spectrum can, therefore, be used to evaluate the presence and the properties of agnps in the stabilizing matrix. figure 2 shows the typical uv-vis spectra of 3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi hydrogels, with prominent lspr peaks positioned at 396 and 407 nm, respectively, which confirmed the successful synthesis of agnps in the hydrogels’ matrices. the position and appearance of lspr peaks agreed with the expected typical spectra of spherical agnps [35]. the obtained lspr peak parameters, amax and max, were 0.21 a.u. and 396 nm for 3.9ag/pva/0.1chi and 0.29 a.u. and 407 nm for 3.9ag/pva/0.5chi. further, the uv-vis spectra can be used to estimate the average diameters of agnps, as the properties of lspr peaks also depend on the nanoparticles’ sizes, as already mentioned above. the agnps diameters (dagnp) could be estimated based on fwhm of the lspr peak [25,39]. the fwhm values were obtained by fitting the uv-vis spectra with gaussian-type curves after background subtraction (figure 2), and the obtained values were 68.2 nm for 3.9ag/pva/0.1chi and 79.8 nm for 3.9ag/pva/0.5chi. thus, agnp diameters for 3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi hydrogels were estimated to be 10.1 nm for 3.9ag/pva/0.1chi and 11.8 nm for 3.9ag/pva/0.5chi. these results indicated that the agnps diameters were in the range of ≈10 nm, which is beneficial for envisioned antibacterial applications. it was also observed that the maximum absorbance was higher in the case of 3.9ag/pva/0.5chi, indicating higher concentration of agnps incorporated inside the hydrogel with higher chitosan content. figure 2. uv-visible spectra of (a) 3.9ag/pva/0.1chi and (b) 3.9ag/pva/0.5chi hydrogels. k. nešović et al. j. electrochem. sci. eng. 10(2) (2020) 185-198 http://dx.doi.org/10.5599/jese.732 191 further confirmation of agnps incorporation and stabilization inside hydrogel matrices was obtained by electron microscopy. fesem micrographs of 3.9ag/pva/0.1chi and 3.9ag/pva/0.5chi hydrogels, presented in figure 3a and 3b, respectively, showed smooth hydrogel surface at high magnification, with agnps uniformly distributed in the polymer matrix. hrtem images (figure 3c and 3d) confirmed the crystalline structure of agnps which were mostly sized between 5 and 10 nm. however, hrtem also indicated the presence of larger nanoparticles, as well as some aggregation (not shown). this was also confirmed by dynamic light scattering analysis (dls) that pointed to monomodal size distribution along the hydrogel volume [25]. dls also provided statistically averaged values of agnps hydrodynamic diameters of 7.33 ± 0.088 nm for (3.9ag/pva/0.1chi) and 6.11±0.10 nm (3.9ag/pva/0.5chi), along with polydispersity indices of 0.211 and 0.269, respectively [25]. figure 3. (a, b) fesem and (c, d) hrtem micrographs of (a, c) 3.9ag/pva/0.1chi and (b, d) 3.9ag/pva/0.5chi hydrogels. silver release the release of the active antibacterial component from the hydrogel matrix, and the ability to control the release dynamics are important properties that are always evaluated when designing a new wound dressing material. the ability of the hydrogel to provide sufficient and sustained inflow of antibacterial agent to the wound site is also one of the factors that dictate durability of the dressing, i.e. determine the period after which the dressing should be replaced. usually, the release of drug from hydrogel matrix follows a two-stage profile, with the very fast initial release, known as “burst” period, and a latter slower release period when the remnants of the drug are released from the matrix. the agnps release from 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogel matrices followed this expected trend, as shown in their release profiles in figures 4a and 5a, respectively. the initial concentrations of silver in hydrogels were 81.5 mg dm-3 for 0.25ag/pva/0.1chi, and 87.2 mg dm-3 for 0.25ag/pva/0.5chi, and after this 28-day monitored period, the remaining concentrations were 21.1 and 21.0 mg dm-3, respectively (figures 4a and 5a). it can be observed that, even though the hydrogel with 0.5 wt% chi initially contained slightly higher amount of agnps http://dx.doi.org/10.5599/jese.732 j. electrochem. sci. eng. 10(2) (2020) 185-198 hydrogel wound dressings with agnps 192 (as also confirmed by uv-vis), the release profiles were similar for both hydrogels, and the remaining concentration was basically the same. the release profiles can be fitted with different models in order to elucidate kinetic and diffusion mechanisms, e.g. with such widely-used models as korsmeyer-peppas and makoid-banakar [25]. here, we will discuss several basic kinetic models, i.e. zero, first and second-order, in order to determine the dominant kinetic mechanism of silver release and to compare the obtained kinetic parameters. the zero-order kinetics represents a process where release rate is constant and independent of both time and concentration of the drug being released, and can be described by equation (4), where cag,0 is the initial concentration of silver in the hydrogel, cag is the concentration of silver remaining in the hydrogel after release time t and k0 is the rate constant of zero-order release. cag = cag,0 – k0∙t (4) on the other hand, the rate of first-order release process depends linearly on the concentration of agnps being released, while cag in the hydrogel decreases exponentially with time. such process can be represented by equation (5), where k1 is the rate constant of first-order release: cag = cag,0 ∙exp(– k1∙t) (5) the equation (5) can be rearranged by taking the logarithm of both sides of the equation, yielding the linear dependence of ln(cag)–t: ln cag = ln cag,0 – k1∙t (6) finally, in second-order processes, the release rate follows square-law dependence on the agnps concentration, whereas the concentration itself is inversely proportional to the release time. the second-order release kinetics can be described using the following equation: = + 2 ag ag,0 1 1 k t c c (7) where k2 is the second-order rate constant. the obtained release profiles of 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels and corresponding fits are presented in figures 4 and 5. using zero-order and first-order kinetic models, it was impossible to fit the entire release period, however, the profiles were divided into two, previously described release periods – initial burst release and subsequent slower process. these two periods could be fitted well with both zero and first-order kinetics, as shown in figures 4b and 4c (0.25ag/pva/0.1chi), as well as 5b and 5c (0.25ag/pva/0.5chi), respectively. as anticipated, the obtained rate constants in the initial period (k0 values of 1.510-3 and 1.610-3 mg dm-3 s-1 for 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi, respecttively, and k1 values of 2.010-5 s-1 for both hydrogels) were larger than those for release in later times (k0 = 0.310-3 mg dm-3 s-1 and k1 = 9.010-6 s-1 for both hydrogels), confirming the “burst effect” during short-time release. on the other hand, the second-order model perfectly fitted the obtained release profiles during the entire monitored period for both 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi (figures 4d and 5d, respectively), with r2 values of 0.991 and 0.995, indicating that the release of silver nanoparticles from these hydrogels follows second-order kinetics. the obtained value for secondorder rate constant was 3.010-7 dm3 mg-1s-1 for both 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels, pointing to relatively slow and gradual silver release over the 28days. the obtained silver release results confirmed a strong potential of 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels for wound dressing applications. the initial “burst release” period lasted even up to 5 days (figures 4a and 5a), enabling the prolonged use of hydrogel dressing, limiting the need for the replacement that could aggravate the newly formed tissue and cause damage to the wound. k. nešović et al. j. electrochem. sci. eng. 10(2) (2020) 185-198 http://dx.doi.org/10.5599/jese.732 193 figure 4. silver release of 0.25ag/pva/0.1chi hydrogel: (a) silver release profile, (b) zero-order fit, (c) first-order fit and (d) second-order fit. figure 5. silver release of 0.25ag/pva/0.5chi hydrogel: (a) silver release profile, (b) zero-order fit, (c) first-order fit and (d) second-order fit. the obtained silver release results confirmed a strong potential of 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels for wound dressing applications. the initial “burst release” period http://dx.doi.org/10.5599/jese.732 j. electrochem. sci. eng. 10(2) (2020) 185-198 hydrogel wound dressings with agnps 194 lasted even up to 5 days (figures 4a and 5a), enabling the prolonged use of hydrogel dressing, limiting the need for the replacement that could aggravate the newly formed tissue and cause damage to the wound. further, the governing mechanism of release was found to be second-order kinetics, where the time of release is inversely proportional to the concentration of agnps in the dressing, meaning that the release rate slows down in later periods, thus preventing potential “accumulated-effect” or repeated-dose toxicity. antibacterial activity the antibacterial efficiency of pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels was investigated using disc-diffusion test against two most common causes of wound infections – s. aureus and e. coli. the samples were incubated on top of bacteriainoculated agar plates, and inhibition zones were measured from photographs using autocad® software. the obtained widths of inhibition zones (minus the sample diameters, as the samples differed slightly in size) are summarized in table 1. all hydrogels exhibited antibacterial activity against s. aureus. the antibacterial activity of hydrogels without agnps occurred due to the presence of chitosan, which is known as a good antibacterial agent [40]. on the other hand, the presence of agnps improved hydrogels’ antibacterial efficiency, and the widths of inhibition zones increased to 0.9 mm for 0.25agpva/0.1chi and 1.3 mm for 0.25agpva/0.5chi (table 1). the 0.25agpva/0.5chi exhibited slightly more pronounced antibacterial action, evidencing the synergistic effect of chitosan and silver nanoparticles. however, in the case of e. coli, the antibacterial effect was not as pronounced. pva/0.1chi hydrogel did not exhibit any antibacterial activity, supposedly because the amount of chi was not enough to cause growth inhibition of e. coli strain, which is generally more resistant than our s. aureus strain. the inhibition zone of pva/0.5chi hydrogel was 0.7 mm wide, due to the higher amount of chitosan in the hydrogel. the widest inhibition zone was around 0.25ag/pva/0.1chi (1.2 mm) indicating that the incorporation of agnps improved antibacterial activity of the hydrogel, whereas it decreased for 0.25ag/pva/0.5chi (0.5 mm). the discrepancies in the width of inhibition zones are linked to the hydrogel samples swelling during the 24 h experimental period that comes as an artifact of soft-top agar moisture content. hydrogel samples swelling shielded the propagation and it was difficult to differentiate between the zones of bacterial sensitivity. table 1. widths of inhibition zones around the samples for pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels and for both tested bacterial strains (s. aureus and e. coli). sample inhibition zone width, mm s. aureus e. coli pva/0.1chi 0.6 / pva/0.5chi 0.7 0.7 0.25agpva/0.1chi 0.9 1.2 0.25agpva/0.5chi 1.3 0.5 overall, the obtained hydrogels exhibited antibacterial activity, which was also confirmed by the test in suspension from our previous work [25], where it was shown that 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels caused complete reduction of e. coli and s. aureus colonies after 1 h. these results indicated that the new wound dressing materials presented here could be considered as good candidates for antibacterial applications. superior antibacterial action of agnps compared to the bulk metal stem from advanced physiochemical and biological properties. the exact mechanism of action, although not conclusively k. nešović et al. j. electrochem. sci. eng. 10(2) (2020) 185-198 http://dx.doi.org/10.5599/jese.732 195 known, is thought to lean on two intertwined phenomena, first being the effect of direct contact between agnps and bacterial cell, while the second comes as the result of ion release-mediated antibacterial activity [41]. agnps can penetrate the bacterial membrane, causing physical damage, subsequent cellular contents leakage and bacterial death [42,43]. another significant contribution to enhanced agnps antibacterial activity is their size, as smaller nanoparticles provide for larger contact area for interaction with the bacterial cells [41]. inside the bacterial cell, agnps interact with cellular structures and proteins, lipids, enzymes, ribosomes and dna, inevitably leading to bacterial dysfunction and finally apoptosis [43–45]. agnps could also cause increased production of high amounts of reactive oxygen and free radical species and reduce the ability of bacteria to produce antioxidant enzymes [45]. the high-surface area agnps can also sustainably release of ag+ ions, which could affect atp production and inhibit normal cellular processes through binding to some membrane proteins or by chelating with nucleic acids [45]. thus, agnps possess exceptionally strong antibacterial activity achieved through the dual mechanism of direct interactions with bacterial cells, as well as the release of reactive silver ions. cytotoxicity the cytotoxicity of pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels was evaluated using dye-exclusion test, which is based on a principle that only non-viable or dead cells take up the trypan-blue dye, whereas the viable cells (with membrane integrity intact) do not [46]. the cell viability was expressed as a percentage of control, and the results are presented in figure 6. figure 6. det test of cytotoxicity towards mrc-5 and l929 cell lines for pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels. *significant differences (p < 0.05) were observed between the indicated samples, within the respective cell line. from the obtained data, it can be observed that both cell lines exhibited high survival rates in the presence of all investigated samples, confirming their non-toxicity towards both mice and human cells. the viabilities of l929 cell line were 96.21, 92.12, 88.0 and 88.15 %, and mrc-5 exhibited viabilities of 91.95, 85.31, 83.67 and 81.63 %, for pva/0.1chi, pva/0.5chi, 0.25ag/pva/0.1chi and http://dx.doi.org/10.5599/jese.732 j. electrochem. sci. eng. 10(2) (2020) 185-198 hydrogel wound dressings with agnps 196 0.25ag/pva/0.5chi, respectively. according to the statistical analysis, 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels exhibited significant decrease in viability (p < 0.05), compared to pva/0.1chi (figure 6), however, the cell survival rates were still higher than 80 % (almost 90 % for l929), so all hydrogels could be classified as non-cytotoxic or mildly cytotoxic, according to a cytotoxicity scale [47]. similar results were obtained using mtt assay [25], confirming strong biocompatibility of the synthesized hydrogels, and, coupled with their potent antibacterial activity, excellent potential for wound dressing applications. conclusions in this work, we have synthesized poly(vinyl alcohol) and chitosan polymer blend hydrogels aimed for wound dressing materials. the amount of chitosan in the hydrogel was varied (0.1 wt% and 0.5 wt%) in order to evaluate its effect on the properties of the obtained materials. thus, pva/0.1chi and pva/0.5chi hydrogels were obtained by green freezing-thawing method during 5 consecutive cycles, which yielded high gelation degrees of 94.0 % and 95.6 %, respectively. incorporation of silver nanoparticles was achieved in situ, inside the matrices of the obtained hydrogels, previously swollen in ag+ aqueous solution. the ag+ ions were reduced to agnps using the electrochemical method at a constant voltage of 90 v. the successful impregnation of agnps was confirmed by uv-visible spectroscopy and scanning electron microscopy, whereas transmission electron microscopy indicated the formation of agnps with diameters around ≈10 nm. the release of silver from 0.25ag/pva/0.1chi and 0.25ag/pva/0.5chi hydrogels was investigated over 28 days and pointed to two-stage release behavior, with initial “burst release” followed by a slower period after the amount of silver decreased in the hydrogel. the mechanistic study, carried 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(https://doi.org/10.1067/mpr.2000.107227). ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) https://doi.org/10.1016/0168-3659(91)90007-z https://doi.org/10.1016/0032-3861(87)90173-x https://doi.org/10.1016/0032-3861(87)90173-x https://doi.org/10.1016/j.compositesb.2017.12.017 https://doi.org/10.1016/j.compositesb.2017.12.017 https://doi.org/10.1021/jp074257w https://doi.org/10.1016/j.physe.2004.10.014 https://doi.org/10.1166/nnl.2012.1278 https://doi.org/10.1021/ja020393x https://doi.org/10.1016/j.physe.2004.07.010 https://doi.org/10.1016/j.eurpolymj.2015.06.008 https://doi.org/10.1016/j.eurpolymj.2015.06.008 https://doi.org/10.1016/s0168-1605(01)00717-6) https://doi.org/10.1016/j.apt.2017.11.028 https://doi.org/10.1007/s00284-017-1235-9 https://doi.org/10.1007/s00284-017-1235-9 https://doi.org/10.1016/b978-0-08-102579-6.00018-6 https://doi.org/10.1016/b978-0-08-102579-6.00018-6 https://doi.org/10.2147/ijn.s165125 https://doi.org/10.1016/b978-0-12-427150-0.50101-7 https://doi.org/10.1067/mpr.2000.107227 http://creativecommons.org/licenses/by/4.0/) {zeolite based gas-diffusion electrodes for secondary metal air batteries} http://dx.doi.org/10.5599/jese.763 229 j. electrochem. sci. eng. 10(2) (2020) 229-234; http://dx.doi.org/10.5599/jese.763 open access: issn 1847-9286 www.jese-online.org original scientific paper zeolite based carbon-free gas diffusion electrodes for secondary metal-air batteries miglena slavova, elena mihaylova-dimitrova, emiliya mladenova, borislav abrashev, blagoy burdin, daria vladikova acad. evgeni budevski institute of electrochemistry and energy systems, bulgarian academy of sciences, acad. g. bonchev str., bl. 10, sofia 1113, bulgaria corresponding author: mslavova@iees.bas.bg received: december 4, 2019; revised: february 25, 2020; accepted: february 26, 2020 abstract in recent years, secondary metal air batteries have received considerable attention as promising technology for energy storage in combination with renewable energy sources. the oxidation of carbon in conventional gas-diffusion electrodes reduces the life of the secondary metal-air batteries. replacement of the carbon-based material with zeolite is a possible solution for overcoming this problem which is the aim of this work. zeolite is a natural or synthetic porous material which provides the necessary gas permeability. the required hydrophobicity of the electrodes is ensured by mixing the zeolite with an appropriate amount of polytetrafluoroethylene following a specially developed procedure. the experiments are performed in a home designed test cell which ensures measurements in both half-cell and full cell configuration. in this study the testing is carried out in 3-electrode homemade half-cell configuration with hydrogen reference electrode. the cell was subjected to cycling at charge/discharge current ±2 ma cm-2 respectively. the obtained results show that the replacement of carbon with zeolite in the gas diffusion layer is a promising direction for optimization of the gas diffusion electrode. keywords polarization; renewable energy sources; polytetrafluoroethylene; charge/discharge cycling. introduction in order to turn renewable energy sources (res) into the main energy source it is necessary to store the produced energy, which requires "innovative solutions, next-generation technologies, including potentially revolutionary technologies" [1]. batteries have an important role in integrating and optimizing the application of res. one of the directions for new technological solutions is the development of rechargeable metal/air systems. research in this area paves the way for further industrial production. http://dx.doi.org/10.5599/jese.763 http://dx.doi.org/10.5599/jese.763 http://www.jese-online.org/ mailto:mslavova@iees.bas.bg j. electrochem. sci. eng. 10(2) (2020) 229-234 carbon-free gas diffusion electrodes 230 metal-air system is a hybrid electrochemical structure which combines a metal anode and gasdiffusion cathode, similar to that used in fuel cells. its main advantage is the use of oxygen from the atmospheric air [2]. the development of rechargeable metal-air batteries is a hot topic, due to the advantages of the system to ensure high energy density and capacity at low price [3-5]. however, there are still no effectively operating secondary metal-air batteries which makes this topic attractive for further research, focused on improvements of the recharging, enhancement of durability and reduction of cost [6]. the advantages of the system are: cheapness, non-toxic materials, operation in humid environments and water-based electrolytes, low self-discharge, recyclability, simple construction, long life and “flat“ discharge curves [7,8]. the components of the metal-air cell are: metal electrode, alkaline electrolyte, separator, and gas-diffusion electrode (gde) (figure 1). the gde consists of gas-diffusion layer (gdl) which ensures the needed oxygen and catalytic layer (cl) where the oxygen reduction/evolution takes place [6,9]. the classical gdl is made of a carbon-based material and some hydrophobic binding material, most commonly polytetrafluoroethylene (ptfe). an optimization in respect of gas permeability and hydrophobicity is an important step in its preparation. the electrochemical reaction occurs at the triple-phase boundary electrode/electrolyte/gas phase. the active material in gde is oxygen from the ambient air and thus the air electrode does not "exhaust", i.e. it should work until it is mechanically damaged which may be caused by the oxidation of carbon [6]. during discharge, the oxygen reduction reaction (orr) occurs, while the metal oxidizes and releases electrons which pass through the external circuit. in charge regime the opposite reactions with release of oxygen take place. for rechargeable gas diffusion electrode, a bifunctional catalyst should be used in cl layer. it has to combine good catalytic activity, electronic conductivity, good adhesion towards gdl and mechanical stability. figure 1. schematic illustration of rechargeable metal-air system the main problem with gde is high electrochemical carbon corrosion during oxygen evolution reaction (oer), which reduces its stability [10-15]. one of the possible solutions is the replacement of the carbon black in gdl with material having similar porous structure as zeolite. there is no information for such replacement in the literature, although zeolites are used in batteries as molecular sieves that can protect metal electrodes from rupture. the zeolite is alumosilicate with crystalline structure, which forms evenly distributed voids and channels 20 to 50 %. the aim of this work is to present innovative zeolite based carbon-free gas diffusion electrodes for rechargeable metal-air batteries. the selected approach replaces the carbon based gdl with the zeolite based one. m. slavova et al. j. electrochem. sci. eng. 10(2) (2020) 229-234 http://dx.doi.org/10.5599/jese.763 231 experimental the zeolite based gde consists of two layers porous zeolite based gdl and cl. the replacement of carbon black with zeolite of type clinoptilolite on gas diffusion electrode is described elsewhere [16]. for the preparation of zeolite based gdl, zeolite powder with grain size between 180 μm and 250 μm was mixed with 20-60 % ptfe and hot pressed (0.3 t/cm2) at 250 °c for 3 min. the bifunctional cl is a mixture of 20 wt.% co3o4, 70 wt.% ag and 10 wt.% ptfe, and its thickness is about 25 % of the total thickness of gde. the morphological studies were performed using electron microscope jeol 6390. the electrode working area was 10 cm2. for fast evaluation of hydrophobicity, an empirical “water drop” test was introduced according to which the water drop should stay on the outer gdl surface more than two hours for further testing of the sample (figure 2a). the second leaking test was performed in the home-made hydrophobicity test cell with electrolyte (6 m koh) for 100 hours (fig. 2b). figure 2. hydrophobicity testing: a) water drop test and b) real conditions test for quick evaluation of the sample gas permeability a simple testing system was used which gives more realistic picture reflecting the behavior of the percolating network. the methodology is based on measurements of the pressure p (mm h2o) dependence on the gas flow, qflow [ml/min], penetrating through porous media [17]. a new characteristic parameter, named permeability resistance rp (inversely proportional to the permeability) is introduced and the empirical criterion for the investigated system postulated as: the gas permeability should not less than 2.2 (mol / min) / cm2 air. the zeolite based gdes were electrochemically tested at room temperature in a homemade halfcell configuration with the working electrode area of 1 cm2 vs. hydrogen reference electrode (hre), 6 m koh аs the electrolyte, and mesh of stainless steel as the counter electrode located behind hre. volt-ampere curves (vac) were recorded using a solartron schumberger 1820 potentiostat and a tacussel (bi-pad) with a specialized electrochemical program, that allows for potential evaluation within a pre-defined range with a given speed and current flow registered. charge/discharge tests were performed with an eight channel galvanostate 54 (pmc). a cyclic condition was employed in the potential range -1.0 v to 2.0 v. the charge time was 45 min, which ensures charging limits from -1000 mv up to 1800 mv, selected in accordance with the further testing of new gde in full cell configuration with zn-electrode. the discharge time was 30 min and the charge/discharge current density was ±2 ma cm-2. conditions of the cycling: maximum half-cell voltage limit was 2000 mv; minimum half-cell voltage limit was -1000 mv. vac results were compared with those for a carbonbased gas diffusion etalon electrode (ee) [16] with the same cl. http://dx.doi.org/10.5599/jese.763 j. electrochem. sci. eng. 10(2) (2020) 229-234 carbon-free gas diffusion electrodes 232 results and discussion after the replacement of carbon black with zeolite in the classical gdl-recipe, ptfe-fibers net cover the zeolite grains, which is a prerequisite for sufficient hydrophobicity (figure 3). figure 3. sem image of zeolite-based gdl with 11 wt.% ptfe however, samples with the content of ptfe powder above 20 wt. % showed low air permeability and excellent hydrophobicity. the samples with 10 20 wt. % ptfe have good gas permeability and mechanical stability, but still insufficient hydrophobicity. for evaluation of the optimal gas permeability, a comparison of the zeolite based and that of the standard carbon based gdl permeability was used [17]. in order to fulfill both requirements, the teflonization procedure was modified and the second generation gdl was developed covering gdl with diluted ptfe emulsion. this impregnation procedure was performed in several cycles which ensured necessary hydrophobicity and gas permeability. the hydrophobicity increased as a result of the stepwise zeolite pores coated with ptfe. this technology was used for preparation of the second generation gde (sample ze 1) with bifunctional cl. although ze 1 has similar volt-ampere characteristics as ee until 7 ma/cm2 (figure 4), its durability is low – it operates only several cycles before wetting. for further improvement a third optimization of the teflonization procedure was performed – ptfe powder was replaced with ptfe emulsion. following this procedure, a third generation gde was introduced (sample ze 2). 0 2 4 6 8 10 12 14 400 600 800 1000 1200 1400 1600 ee ze 1 current density i / macm -2 u v s .r h e / m v oer orr j / ma cm-2 figure 4. volt-ampere characteristics during discharge (orr) and charge (oer) measured on gde with impregnated zeolite-based gdl (ze 1) compared with the etalon carbon based gde (ee) u / m v v s. r h e m. slavova et al. j. electrochem. sci. eng. 10(2) (2020) 229-234 http://dx.doi.org/10.5599/jese.763 233 as presented in figure 5, the current density of about 2-5 ma/cm2 is a promising initial result, which corresponds to the requirements for the final usage of zn/air batteries in solar farms where they shall operate in combination with other batteries/supercapacitors [6]. 0 2 4 6 400 800 1200 1600 ee ze 2 cycle 1 ze 2 cycle 78 orr current density i / macm -2 u v s .r h e / m v oer j / ma cm-2 figure 5. volt-ampere characteristics on the second generation zeolite-based gde (ze 2) during discharge (orr) and charge (oer) compared with the etalon carbon based gde (ee) for durability testing, the charge/discharge cycling was performed and showed a promising result for more than 150 cycles (figure 6). 0 30 60 90 120 150 180 -1000 0 1000 2000 3000 0 5 10 15 20 25 -1000 0 1000 2000 c e ll v o lt a g e / m v time / h first 20 charge/discharge cycles c e ll v o lt a g e / m v time / h ~150 cycles 2 ma cm 2 time, h figure 6. charge/discharge cycling of the third generation zeolite-based gde (ze 2) in half-cell configuration conclusions the first obtained results on the development of carbon free gde carried out by replacing carbon with zeolite are promising in respect to both – reached current densities and number of cycles. in addition, zeolites are easily available and cheap materials what makes them attractive for replacement of carbon-black in gdl for secondary metal-air batteries. u / m v v s. r h e c e ll v o lt a g e , m v http://dx.doi.org/10.5599/jese.763 j. electrochem. sci. eng. 10(2) (2020) 229-234 carbon-free gas diffusion electrodes 234 the next stage of the optimization is focused on further increase of the current density and number of charge/discharge cycles, i.e. durability and scaling up. acknowledgements: this work was supported by the bulgarian ministry 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[17] e. mladenova, d. vladikova, z. stoynov, a. chesnaud, a. thorel, m. krapchanska, bulgarian chemical communications 45(3) (2013) 366–370. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://ec.europa.eu/energy/sites/ener/files/documents/1_en_act_part1_v6_0.pdf https://ec.europa.eu/transparency/regdoc/rep/1/2016/en/1-2016-501-en-f1-1.pdf http://sintef.no/projectweb/zas/ https://doi.org/10.1007/s40243-014-0028-3 http://creativecommons.org/licenses/by/4.0/) {voltammetric and amperometric sensors for determination of epinephrine: a short review (2013-2017)} doi:10.5599/jese.569 27 j. electrochem. sci. eng. 9(1) (2019) 27-43; doi: http://dx.doi.org/10.5599/jese.569 open access: issn 1847-9286 www.jese-online.org short review voltammetric and amperometric sensors for determination of epinephrine: a short review (2013-2017) hadi beitollahi1,, mohadeseh safaei1, somayeh tajik2 1environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran 2nanobioelectrochemistry research center, bam university of medical sciences, bam, iran corresponding authors e-mail:  h.beitollahi@yahoo.com; tel.: +98 3426226613; fax: +98 3426226617 received: july 16, 2018; revised: september 13, 2018; accepted: septrember 19, 2018 abstract the present review focuses on voltammetric and amperometric methods applied for determination of epinephrine (ep) in last five years (2013-2017). occurrence, role and biological importance of ep, as well as non-electrochemical methods for its assessment, are firstly reviewed. the electrochemical behavior of ep is then illustrated, followed by a description of the voltammetric and amperometric methods for ep content estimation in various media. different methods for development of electrochemical sensors are reviewed, starting from unmodified electrodes to different composites incorporating carbon nanotubes, ionic liquids or various mediators. from this perspective, the interaction between functional groups of the sensor material and the analyte molecule is discussed, as it is essential for analytical characteristics obtained. the analytical performances of the voltammetric or amperometric chemical and biochemical sensors (linear range of analytical response, sensitivity, precision, stability, response time, etc.) are highlighted. numerous applications of ep electrochemical sensors in fields like pharmaceutical or clinical analysis where ep represents a key analyte, are also presented. keywords epinephrine; electrooxidation; voltammetry: amperometry contents 1. introduction --------------------------------------------------------------------------------------------------------------------28 2. epinephrine determination by non-electrochemical techniques ------------------------------------------------28 3. voltammetric and amperometric sensors ------------------------------------------------------------------------------29 3. 1. voltammetry/amperometry at bare/unmodified electrodes -------------------------------------------------------------------29 3. 2. voltammetry/amperometry at modified electrodes ------------------------------------------------------------------------------29 3. 2. 1. chemically modified electrodes ---------------------------------------------------------------------------------------------------29 http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:h.beitollahi@yahoo.com j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 28 3. 2. 2. modified electrodes with polymer ------------------------------------------------------------------------------------------------31 3. 2. 3. modified electrodes with carbon nanotubes ----------------------------------------------------------------------------------33 3. 2. 4. modified electrode with nanoparticles and nanocomposites -------------------------------------------------------------35 4. interferences from compounds present in biological media and pharmaceuticals ------------------------37 5. analytical performances of electrochemical epinephrine sensors ----------------------------------------------39 6. some applications of electrochemical epinephrine sensors in pharmaceutical and biological fluid analysis ---------------------------------------------------------------------------------------------------------------------------39 7. conclusions ---------------------------------------------------------------------------------------------------------------------40 abbreviation ------------------------------------------------------------------------------------------------------------------------41 references ---------------------------------------------------------------------------------------------------------------------------41 1. introduction epinephrine (ep), also called adrenaline, is an important catecholamine neurotransmitter in the mammalian central nervous system [1]. many life phenomena are related to the concentration of ep in blood. it also served as a chemical mediator for conveying the nerve pulse to efferent organs. medically, ep has been used as a common emergency healthcare medicine [2,3]. ep is used to stimulate heartbeat and to treat emphysema, bronchitis, bronchial asthma and other allergic conditions, as well as the eye disease, glaucoma. therefore, performing the research of ep has an important significance to medicine and life science [4]. ep is synthesized naturally in the body from l-tyrosine by the action of different enzymes. almost 50 % of the secreted hormone appears in urine as free and conjugated, 3 % as vanilmandelic acid (vam), the most abundant metabolite in urine [5]. only small amounts of free ep are excreted. meanwhile, ep is an electroactive compound and can be determined by electrochemical methods [6-11]. however, actual electrochemical detection of ep has two challenges. one is its low concentration level, while another challenge often encountered is the strong interference arising from electroactive compounds like norepinephrine (ne), dopamine (da), ascorbic acid (aa) and uric acid (ua) [6]. to resolve these problems, one of the most common routes is using a modified electrode to improve the measuring sensitivity of ep and minimize the interference of aa and ua to ep determination [7-12]. although many modified electrodes have been demonstrated to be effective for detecting ep, there is still a need to develop a new method with high efficiency and convenience for the detection of ep [13,14]. injectable ep solutions used by emergency medical personnel and hospitals are principally degraded via oxidation. this degradation can be accelerated by heavy metals, ultraviolet light, exposure to oxygen, and increased ph. typical preventive measures for hindering oxidative degradation use light-resistant containers, buffered solutions, and/or antioxidants [15-20]. due to the crucial role of ep in biochemistry and industrial applications, the determination of ep still presents research interest. quick monitoring of ep levels during production and quality control stages is important [21-24]. in this review, we investigate the latest progress in modification of electrodes and its improvement in detection of ep. 2. epinephrine determination by non-electrochemical techniques several methods have been reported for the determination of ep including high performance liquid chromatography (hplc) [25,26], hplc-mass spectrometry [27], fluorimetry [28], hplc optical fiber biosensor [29], capillary electrophoresis [30,31], flow injection [32,33], hplc with fluorimetric detection [34], chemiluminescence [35,36] and spectrophotometry [37,38]. h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 27-43 doi:10.5599/jese.569 29 3. voltammetric and amperometric sensors voltammetry is a potentiodynamic technique, based on measuring the current arising from oxidation or reduction reactions at the working electrode surface, when a controlled potential variation is imposed [39,40]. amperometry is based on the application of a constant potential to a working electrode, and the subsequent measurement of the current generated by the oxidation/reduction of an electroactive analyte [41-43]. 3. 1. voltammetry/amperometry at bare/unmodified electrodes bare electrodes without functionalization represent an interesting alternative, in particular when high sensitivity is not required. this approach has been realized by use of a simpler system, resulting in reduced costs for both production and use, and long-term stability. an electrochemical biosensor for the sensitive detection of ep was introduced by li et al. [44]. their results showed that the magnitude of the oxidation peak current of ep is related to many factors, including the ph value of the supporting electrolyte in the working electrode electrolytic cell, the acidity of the supporting electrolyte in the auxiliary electrode electrolytic cell, the distribution coefficients for different ep species, the properties of electrode surface charge and the molecular configuration of electroactive component. in performing experiments, ph of pbs buffer solution was kept at 7.0 in the working electrode electrolytic cell and hcl solution maintained at 1.0 mol l-1 in the auxiliary electrode electrolytic cell. the standard solutions of different amounts of ep were added to the working electrode electrolytic cell and the oxidation peak current of ep was recorded by cyclic voltammetry (cv). the range of 2.0×10-7-1.0×10-4 mol l-1, with a detection limit of 6.2×10-8 mol l-1 was obtained. satisfactory results have been achieved for the determination of ep in injection. the recovery of the standard addition was in the range of 95.0 -102.0 %. jemelkova et al. [45] reported the voltammetric behavior of ep investigated by differential pulse voltammetry (dpv) at a carbon paste electrodes (cpe) made with different carbon powders cr-2, glassy carbon (gc) microparticles, and single-wall carbon nanotubes (swnt). in briton-robinson (br) buffer solution (ph 6), the linear dependence was found for the determination of ep by the given method in the concentration ranges of 1×10-6-1×10-4 (cr-2), 1×10-6-1×10-4 (gc microparticles) and 4×10-6-1×10-4 (swnt) mol l-1. limits of detection were 8× 10-7, 8×10-7 and 2×10-6 mol l-1, respectively. the best results were obtained by employing the cpe containing carbon paste with 50 % (w/w) of swnt, which showed a linear dynamic range of 4×10-7-1×10-4 mol l-1 and a limit of detection 2×10-7 mol l-1. 3. 2. voltammetry/amperometry at modified electrodes the need for over-potential diminution and fouling minimization has required the electrode modification with a view to increase sensitivity and obtain more prominent peak separation. these properties are required mainly in complex media such as biological samples particularly prone to interferences, where ep coexists with other electroactive species. 3. 2. 1. chemically modified electrodes numerous electrochemical methods have been developed to determine ep on the basis of its electroactive nature. most of these methods, however, have two major problems in ep determination which reduce accuracy and sensitivity of the results. the first is that in a natural environment, ep often coexists with a high concentration of electroactive biomolecules like ua, da, ne, and aa that interfere with each other. the second problem of reported methods is that the product of ep oxidation (epinephrine chrome) can easily transform into polymers, which block its j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 30 further oxidation on the electrode surface. hence, despite of considerable investigations, the preparation of a sensitive sensor with satisfactory selectivity and low detection limit with high sensitivity is still of great interest. development and application of l-glutamic acid functionalized graphene nanocomposite modified gce for the determination of ep were reported by kang et al. [46] linear relationship between ep concentration and current response measured by dpv method was obtained in the range of 1×10-7 to 1×10-3 mol l-1 with a limit of detection of 3×10-8 mol l-1. the modified electrode was employed to determine ep in urine with satisfactory results. zhang and wang [47] have described β-mercaptoethanol self-assembled monolayer modified electrode, fabricated on a bare gold (me/au sams). the films accelerated the electron transfer as mediators, and showed an excellent electrocatalytic activity for the oxidation of ep. the electrochemical behavior of ep at me/au sams has been studied by cv and the electrocatalytic mechanism is explored. at potential of -0.044v (vs. sce) in the aqueous buffer (ph 4.0), the first oxidation wave was observed for ep at the modified electrode (electrochemical oxidation of leucoepinephrine to epinechrome). in contrast, the first oxidation wave was not observed for ne or da under same conditions. fabrication of modified gce for determination of ep in aqueous solutions was reported by ahmadian yazdely et al. [48]. their dpv results exhibited the linear dynamic range from 5.0×10-8 to 1.1×10−5 mol l-1 and detection limit of 2.3×10−8 mol l-1 for ep. in addition, the analytical performance of the modified electrode for quantification of ep in real samples was evaluated. sharath shankar and kumara swamy [49] have successfully investigated tetradecyltrimethyl ammonium bromide (ttab) surfactant immobilized at cpe which has been proposed for simultaneous investigation and determination of ep and serotonin (5-ht) in presence of aa. voltammetric techniques in the phosphate buffer solution (pbs) (ph 7.4) were applied. the anodic peak of ep was observed at 198 mv (vs. ag/agcl/kcl) at the scan rate 50 mv s-1. the interference studies showed that the modified electrode exhibits excellent selectivity for the determination of ep in the presence of large excess of aa and 5-ht. differences of the oxidation peak potentials for ep-aa and ep-5-ht were about 215 and 165 mv, respectively. detection limit of the modified electrode obtained by dpv technique was found to be 0.12 µmol l-1. the developed method was applied to the determination of ep in synthetic samples with satisfactory results. jahanbakhshi [50] reported a synthesis of mesoporous carbon foam (mcf) with particular properties due to simplistic and template-free procedure. the synthesized mcf was characterized by transmission electron microscopy, field emission scanning electron microscopy, x-ray diffraction and bet surface area techniques. porous mcf, with pore diameters of 5 to 10 nm resulted in extensive specific surface area that modifies the electrode surface. the obtained mcf was dispersed in the salep solution to prepare a stable suspension (s-mcf). the resultant composite was casted on the surface of gce to assemble the s-mcf modified gce electrode (s-mcf/gce). cv method was used to study electrochemical behavior and determination of ep was conducted by applying dpv method in the presence of ua. in the optimized conditions, the presented sensor was found able to detect the concentration range of 0.1-12 μmol l-1 with a limit of detection of 40 nmol l-1. the presented methodology possesses a reliable reproducibility, repeatability and stability in biological samples. sensitive and selective determination method for ep was developed by chandrashekar et al. [51] by immobilization of tx-100 surfactant on the bare cpe. the catalytic activity of the modified electrode for the oxidation of ep was determined using cvs recorded at different scan rates. the h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 27-43 doi:10.5599/jese.569 31 effect of the solution ph on the voltammetric response of ep was examined using the phosphate buffer solution. the tx-100/cpe demonstrated a good performance for the determination of ep in the concentration range from 10 to 50 μmol l-1, with a detection limit of 1×10−6 mol l-1. the application was conducted for the determination of ep in a human serum sample and the sensor was proven to be rapid, having excellent selectivity and repeatability. in the research of dehghan tezerjani et al. [52], an electrochemical sensor was constructed for determination of ep. the sensor was based on the cpe modified with graphene oxide (go) and 2-(5-ethyl-2,4dihydroxyphenyl)-5,7-dimethyl-4 h-pyrido (2,3-d) (1,3) thiazine-4-one (eddpt) as modifiers. the modified electrode was applied as an electrochemical sensor for oxidation of ep. under the optimum conditions, the overpotential value for ep oxidation decreased for about 279 mv at the modified cpe more than at non-modified cpe. also, the designed electrochemical sensor was applied to determine ep in the drug sample and for simultaneous determination of ep, act and da in human serum solutions. 3. 2. 2. modified electrodes with polymer in recent years, electrochemically modified electrodes with conductive or redox polymers have been widely used owing to their excellent and unique physical and chemical properties. this kind of modification is established as the best approach for selective determination of some biomolecules because the surface characteristic on the electrode can be modulated by introducing various chemicals with reactive groups. the polymer-modified electrodes showed broad potential windows and can still catalyze electrochemical reactions which have high overpotential and poor selectivity. electropolymerization of fuchsine acid (fa) was studied by taei et al. [53] on the surface of gce in different electrolyte media. then, a novel au-nanoparticle poly(fa) film modified gce (poly(fa)/aunp/gce) was constructed for the simultaneous determination of aa, ep and ua. in addition, for the poly(fa)/aunp/gce, oxidation peak potentials of aa-ep and ep-ua were found separated for 150 mv and 180 mv, respectively. at the same time, for the bare gce, not any separation was noticed. dpv results exhibited the linear dynamic range of 0.5-792.7 μmol l−1 for ep with the detection limit of 0.01 μmol l-1. the diffusion coefficient for the oxidation reaction of ep on aunp/poly (fa) film coated gc electrode was calculated as 2.6 (±0.10) × 10−5 cm2 s−1. li and wang [54] have investigated an electrochemical sensor based on the poly(guanine) (pga) modified gce that was fabricated by electropolymerization of guanine on the bare gce surface. this modified electrode exhibited good electrocatalytic property towards the oxidation of ep and ua in 0.1 mol l-1 pbs (ph 4.0), seen as enhanced peak currents and well defined peak separations. under optimum reaction conditions, oxidation peak currents of ep and ua were proportional to their concentrations in the range of 1.0×10-5 to 1.0×10-3 mol l-1 and detection limit of 1.8×10-6 mol l-1 was determined for both compounds. finally, this method was efficiently used for the determination of ep in ep injections. kocak and dursun [55] used a modified electrode that was fabricated by overoxidation of polymer film after electropolymerization of p-aminophenol on a bare gce. higher catalytic activity was observed for electrocatalytic oxidation of aa, ep, and ua in pbs (ph 7.4) at the overoxidized poly(p-aminophenol) film modified gce (ox-pap/gce), due to enhanced peak current and well defined peak separations compared to both bare gce and poly (p-aminophenol) film modified gce (pap/gce). devadas et al. [56] presented for the first time, a simultaneous voltammetric determination of ep and p-acetoaminophenol (ap) on the poly(curcumin) (1,7 bis((4-hydroxy-3-methoxyphenyl)-1,6 j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 32 -heptadiene-3,5 dione) modified gce. curcumin (cm) was polymerized onto the gce surface by simple electropolymerization process. low peak to peak (δep) separation of 60 mv was observed, indicating fast electron transfer between poly(cm) and the electrode surface. moreover, the poly(cm) modified gce exhibited enhanced electrocatalytic activity for ep in the linear range of 4.97-230.76 µmol l-1 and very low detection limit (lod) of 0.05 µmol l-1. taei and jamshidi [57] introduced a polymerized film of adizol black b (abb) on the surface of gce for the simultaneous determination of aa, ep and ua. this new modified electrode presented an excellent electrocatalytic activity towards the oxidation of aa, ep and ua by dpv method. the separation of the oxidation peak potentials for aa-ep and ep-ua were at about 180 and 130 mv, respectively. the diffusion coefficient for the oxidation reaction of ep at the poly(abb) film coated gce was calculated as 1.54 (±0.10)×10−4 cm2 s-1. ma et al. [58] demonstrated an electrochemical sensor based on the silver doped poly-l-cysteine film (ag-plc) that has been fabricated for simultaneous determination of da, ep and ua in the presence of aa. although voltammetric signals of da and ep were resolved at the bare gc electrode, the signals of da and ua were not resolved in a mixture. however, (ag-plc) modified electrode does not only separate voltammetric signals of da, ep and ua with potential difference of 390 and 135 mv between da-ep in the cathodic peak potential and ua-(da+ep) in the anodic peak potential respectively, but also shows higher electrocatalytic activity towards da, ua and ep in the presence of high concentration of aa. for ep, the linear range was determined from 5.00×10-6 to 1.10×10-4 mol l-1. the practical application for this modified electrode was demonstrated by determining the concentration of da, ua and ep in human urine samples. li and sun [59] introduced a novel paladium doped poly(l-arginine) modified electrode (pd-pla/gce), fabricated by electrochemical immobilization of the paladium doped poly (l-arginine) on a gce. this modified electrode was used for determination of ep by the cv method. the method was successfully applied to the determination of ep in injection with satisfactory results. a simple and sensitive poly(l-aspartic acid)/electrochemically reduced graphene oxide modified gce, poly(l-asp)/ergo/gce, has been constructed by electrochemical reduction of go that was drop coated on the gce within 2 mmol l-1 l-aspartic acid in pbs (ph 6). as suggested by mekassa et al 60, this procedure gives rise to in situ polymerization of l-aspartic acid on the ergo. significant enhancement of the peak current response of ep was observed, accompanied with a negative shift in the peak potential value at the composite modified electrode, compared to the bare electrode. real sample analysis was carried out in the pharmaceutical formulation of ep hydrochloride injection, which revealed good recovery results of 94–109 %. according to vieira da silva [61], the polymerization of ferulic acid (fa), forming poly(fa) on mwcnts modified gce was performed and the modified platform applied for simultaneous determination of nadh, ep and da. cv and ca methods were employed to investigate the electrocatalytic oxidation of nadh, ep and da on the modified electrode in aqueous solutions. the obtained analytical curve for ep showed linear range between 73-1406 μmol l−1. the detection limit was 22.2 μmol l−1 for ep. poly(ionic liquids), (pils), have been applied as the linkers between au nanoparticles and polypyrrole nanotubes (ppynts) for the synthesis of au/pils/ppynts hybrids. as was reported by mao et al. [62], due to the presence of pils, high density of well dispersed aunps was deposited on the surface of pils/ppynts by anion exchange with au precursor and in situ reduction of metal ions. the catalytic oxidation peak current obtained by dpv method increased linearly with increasing ep concentration in the range of 35-960 µmol l-1 with a detection limit of 298.9 nmol l-1, according to h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 27-43 doi:10.5599/jese.569 33 the criterion of a signal-to-noise ratio (s/n=3). these results suggested that this modified electrode shows excellent electrocatalytic activity towards this significant hormone in human life. 3. 2. 3. modified electrodes with carbon nanotubes carbon nanotubes (cnts) have attracted more attention in physical, chemical and material science fields due to their unique electrical conductivity, chemical stability and high mechanical strength and modulus. the subtle electronic properties of carbon nanotubes suggest that they are able to promote electron transfer when used as the electrode material in electrochemical reactions. these properties provided a new manner of electrode surface modification for designing new electrochemical sensors and novel electrocatalytic materials. in the research performed by apetrei [63], a biosensor comprising tyrosinase immobilized on a swcnts modified gce was developed for determination of ep. tyrosinase maintained high bioactivity on this nanomaterial by catalyzing the oxidation of ep to ep quinone, which was electrochemically reduced (-0.07 v vs. ag/agcl) on the biosensor surface. under optimum conditions, the biosensor showed a linear response in the range of 10-110 μmol l-1 and a limit of detection was calculated as 2.54 μmol l-1 with a correlation coefficient of 0.977 for ep. the repeatability, expressed as the relative standard deviation for five consecutive determinations of 10-5 mol l-1 ep solution, was 3.4 %. valentini et al. [64] used oxidized single wall carbon nanohorns (o-swcnhs) for the first time, in order to assemble chemically modified screen printed electrode (spe) that is selective towards the electrochemical detection of ep in the presence of serotonine-5-ht (s-5ht), da, ne, aa, act and ua. ep neurotransmitter was detected by using dpv in a wide linear range of concentrations (2-2500 μmol l-1) with high sensitivity, very good reproducibility (rsd ranging from 2 to 10 % for different spes), short response time for each measurement (only 2 s) and low detection limit (lod = 0.1 μmol l-1). a simple electrochemical sensor for ep has been developed by ghica and brett [65]. they modified a carbon film electrode (cfe) with mwcnts in a chitosan matrix. under optimum conditions (ph 7.0), the mwcnt/cfe electrode showed significant electrocatalytic oxidation of ep with a decrease of overpotential value for about 200 mv and 11-fold increase of the peak current value, compared to the unmodified cfe. the sensor exhibited excellent stability over a period of 6 months and was successfully applied to the analysis of injectable adrenaline solutions. the electrochemical behavior of a multi walled carbon nanotube paste electrode modified with 2-((7-(2,5-dihydrobenzylideneamino) heptylimino methyl) benzene-1,4-diol (dbhb) was studied by mazloum ardakani et al. [66]. cv method was used to study the electrocatalytic mechanism of ep electrooxidation at the modified electrode. catalytic rate constant and diffusion coefficient were obtained for oxidation of ep. by using dpv method, a highly selective and simultaneous determination of ep, acetaminophen and folic acid has been obtained at the modified electrode used as an electrochemical sensor. wu et al. [67] reported a sensor for ep that is based on ito electrode modified with mwcnts being pre-coated with a polymerized ionic liquid (pilmwnts). the chitosan film was electrodeposited on the ito electrode in the presence of ep and the pilmwnts. this film acts as an excellent recognition matrix due to excellent film forming ability and many functional groups that favor hydrogen bond formation with the target ep. the electrochemical response to ep was linear in 0.2 μmol l-1 to 0.67 mmol l-1 concentration range, and detection limit was as low as 60 nmol l-1 (at s/n =3). j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 34 wang et al. [68] demonstrated a modified gce that was covered with a layer of mwcnt coated with hexadecyltrimethyl ammonium bromide (ctab). the modified electrode showed excellent electrochemical catalytic properties for the redox reaction of ep and aa. in the presence of ctab, the peak separation between ep and aa can be broadened to 256 mv by the ctab. graphite paste electrode (gpe) modified with 1-butyl-3-methylimidazolium hexafluoro phosphate (bmimpf6) and mwcnts was prepared for simultaneous voltammetric determination of ep and xanthine (xn) by rajabi et al. [69]. the prepared electrode (bmimpf6-mwcnt/gpe) showed excellent catalytic activity in the electrochemical oxidation of ep and xn, leading to remarkable enhancement of the corresponding peak currents and lowering the peak potentials. the peak current values of linear sweep voltammograms increased linearly with ep concentrations in the range of 0.30-60 μmol l-1 in 0.1 mol l-1 pbs (ph 7.0). applicability of this modified electrode as the voltammetric biosensor was demonstrated by simultaneous determination of ep and xn in human urine, human blood serum and ampoule. in the study of babaei et al. [70], electrooxidation of ep, act and mefenamic acid (mef) has been investigated by application of nickel hydroxide nanoparticles/mwcnt modified gce (mwcnt-nhnps/gce) using cv and dpv methods. in another study, pradhan et al. [71] employed a composite electrode for the amperometric detection of ep. composite electrode was developed by electropolymerizing bromothymol blue (btb) on the cpe bulk modified with mwcnts. electropolymerization of btb on the surface of cpe involved much less energy compared to a cpe surface. the modification enhanced the current sensitivity of ep by 5.5 times as compared to the bare cpe. the sensor showed the optimum current response at physiological ph and the response was linear for the concentration of ep in the ranges 0.8-9.0 µmol l-1 and 10.0-100 µmol l-1, respectively. the detection limit was 8×10-7 mol l-1. the amperometric response of ep remained unaltered even in the presence of 50-fold excess of ua, aa and 100-fold excess of l-tryptophan, l-tyrosine, l-cysteine and nicotinamide adenine dinucleotide. this sensor showed stability, reproducibility, antifouling effects and was successfully applied for the determination of ep in blood serum and adrenaline injection. thomas et al. [72] developed an amperometric sensor for the determination of ep which was fabricated by modifying the cpe with pristine multi walled carbon nanotubes (pmwcnts). bulk modification, followed by a drop casting of sodium dodecyl sulfate (sds) onto the surface for its optimal potential application was performed. analytical applications of the modified electrode were demonstrated by determining ep in spiked blood serum and adrenaline tartrate injection. filho et al. [73] developed an electrochemical method for the single and simultaneous determination of da and ep in human body fluids, using a gce modified with nickel oxide nanoparticles and carbon nanotubes within a dihexadecyl phosphate film. swv and dpv methods were applied. by using dpv with the proposed electrode, a separation of ca. 360 mv between the peak reduction potentials of da and ep was present in binary mixtures. the detection limit of ep was determined as 8.2×10-8 mol l-1. koteshwara reddy et al. [74] checked out an efficient electrochemical sensor for selective detection of ep. it was fabricated with the aid of a functionalized mwcnt-chitosan biopolymer nanocomposite (chit-f cnt) electrode. mwcnts were successfully functionalized with the aid of nitric acid and confirmed by the raman spectral data. functionalized carbon nanotubes (f cnt) were dispersed in chitosan solution and the resulting bio nanocomposite was used for the fabrication of sensor surface by drop and cast method. electrochemical characteristics of the fabricated sensor were understood using cv and dpv analysis for the detection of ep in pbs (ph 7.4). h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 27-43 doi:10.5599/jese.569 35 3. 2. 4. modified electrode with nanoparticles and nanocomposites nanotechnology and nanoscience represent new and enabling platforms that promise to provide a broad range of novel uses and improved technologies for environmental, biological and other scientific applications. one of the reasons staying behind the intense interest is that nanotechnology permits the controlled synthesis of materials, where at least one dimension of a structure is less than 100 nm. nanostructured materials have also been incorporated into electrochemical sensors for biological and pharmaceutical analyses. while they offer unique advantages, including enhanced electron transfer, large edge plane/basal plane ratios and rapid kinetics of the electrode processes. in a study of sadeghi et al. [75], cpe was modified with zinc oxide (zno) nanoparticles and 1,3-dipropylimidazolium bromide was used as a binder. it was found that the oxidation of ep at the surface of modified electrode occurs at about 80 mv less positive potential than at unmodified cpe. dpv peak current values showed a linear relationship with concentration of ep in the range of 0.09800 μmol l-1, with a detection limit of 0.06 μmol l-1. the proposed sensor was successfully applied for the determination of ep in real samples. as suggested by babaei et al. [76], simultaneous determination of ep and act can be performed using a gce modified with a mwcnts, nickel hydroxide nanoparticles (nhnps) and mg-al layered double hydroxide (ldh) composite (mwcnts-nhnps-ldh/gce). based on dpv method, the oxidation of ep exhibited a dynamic range between 0.04-60 µmol l-1 and detection limit (3) of 11 nmol l-1. this method was used for the determination of ep in real samples, using the standard addition method. gold nanoparticles/polyaniline nanocomposite thin film was deposited on to the surface of gce by langmuir-blodgett (lb) technology to fabricate a new voltammetric sensor (gnps/pan-lbgce) for ep and ua detection, as was reported by zou et al. [77]. electrochemical behavior of ep and ua at the modified electrode was investigated in pbs (ph 6.6). silai et al. [78] have reported a modified electrode that was prepared by immobilizing ptnanoparticles into a chitosan film. the investigation of the influence of experimental conditions (scan rate, frequency, ph) on the electrochemical behavior of ep was realized by the cv method. novel mcm/zro2 nanoparticles modified cpe was fabricated and used by mazloum-ardakani et al. [79], in order to study the electrooxidation of ep and act and their mixtures. the modified electrode showed electrocatalytic activity toward ep and act oxidation with a decrease of the overpotential value by 173 mv for ep at the surface of the mz-cpe and an increase in peak current at ph 7.0. jin and zhang [80] used the nanogold modified gce obtained by electrodeposition, which can catalytically oxidize and accumulate ep. in this research, effects of changes of ph and concentration of pbs on the electrochemical behavior of ep were studied. this modified electrode could be applied for determination of ep in the presence of aa. dpv data showed that under optimal conditions, the obtained anodic peak currents were linearly dependent on the ep concentration in the range of 1.0×10-4-1.0×10-6 mol l-1. razavian et al. [81] employed electrochemical sensor that was developed and tested for detection of l-tyrosine in the presence of ep. the electrode was prepared by surface modification of a gce with nafion and cerium dioxide nanoparticles. the modified electrode exhibited a significant electrochemical oxidation effect of ep in a 0.2 mol l-1 britton-robinson (br) buffer solution (ph 2). the electro-oxidation peak current increased linearly with the ep concentration in the molar concentration range of 5 to 220 μmol l-1. by employing dpv method for simultaneous j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 36 measurements, two reproducible peaks for l-tyrosine and ep in the same solution with a peak separation of about 443 mv were detected. nitrogen doped three dimensional porous graphene (ng) modified electrode was fabricated by yang et al. [82]. the obtained data showed that electrooxidation of ep at the modified electrode is greatly facilitated, which was ascribed to the excellent properties of ng. the modified electrode was used for simultaneous determination of ep and metanephrine (mep). dpv peak currents of ep increased linearly with their concentration within the range of 1.0 μmol l-1 to 1.0 mmol l-1, with a sensitivity of 0.021 μa / (μmol l-1) for ep. the detection limit for ep was ascertained to be 0.67 μmol l 1. additionally, the detection of ep and mep was found possible in the presence of aa and ua. the modified electrode was applied to the detection of ep and mep in human plasma samples with recoveries from 98.9 % to 100.9 %, and ep hydrochloride injections with recoveries from 100.3 % to 104.6 %. chen and ma [83] used a graphene modified gce obtained via drop casting method and applied it to the simultaneous detection of ep, ua, and aa by cv method in a pbs solution (ph 3.0). the oxidation potentials of ep, ua, and aa at the graphene modified gce were 0.484, 0.650, and 0.184 v (vs. ag/agcl), respectively. peak separations between ep and ua, ep and aa, and ua and aa were about 166, 300, and 466 mv, respectively. a hybrid membrane, consisting of aminated graphene and ag nanoparticles, (agnps), was prepared on the surface of gce by the cv method, where aminated graphene (gr-nh2) acted as a matrix for immobilizing agnps. the morphology and electrochemical properties of this hybrid membrane were characterized together with the voltammetric behavior of ep in a study of huanhuanin et al. [84]. the membrane exhibited excellent eletrocatalytic activity for the redox reaction of ep and resolved the electrochemical response of ep and ua into two oxidation peaks. according to mak et al. [85], organic electrochemical transistors (oects) were found to be excellent transducers for various types of biosensors. it was highly sensitive ep sensor based on oects prepared on glass substrates by solution process. the device performance was optimized by immobilizing nafion and carbon based nanomaterials on the gate electrodes of the oects. the detection limit of the sensors was as low as 0.1 nmol l-1, which could cover the concentration level of ep in medical detections. in a study performed by beitollahi et al. [86], cpe modified with vinyl ferrocene (vf) and cnts was used for the sensitive and selective voltammetric determination of ep, which could be related to the strong electrocatalytic effect of the vf and cnt towards this compound. the mediated oxidation of ep at the modified electrode was investigated by cv. swv method of ep at the modified electrode exhibited linear dynamic ranges with a detection limit of 3.0×10-8 mol l-1. swv was also used for simultaneous determination of ep and tryptophan at the modified electrode. quantification of ep and tryptophan in some real samples was performed by the standard addition method. zhang et al. [87] described a facile preparation of polydopamine (pda)-nanogold composite modified gce used for the sensitive determination of ep, da, aa and ua simultaneously. under mild spontaneous reaction condition, da as a reducing agent and monomer and haucl4 as an oxidant trigger for da polymerization were mixed together with the source of gold nanoparticles to yield a composite of da polymer and gold nanoparticles. these composite particles were then anchored on gce by electropolymerization of the remaining da monomer. the resultant electrode exhibited excellent electrocatalytic redox activities toward ep, da, aa and ua. furthermore, although the oxidation peaks of ep and da at the modified electrode appeared at the same potential of 230 mv h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 27-43 doi:10.5599/jese.569 37 (vs. ag/agcl), three well defined oxidation peaks were generally obtained for aa, ep, da and ua (50, 230, 380 mv vs. ag/agcl). in a study of redin el al. [88], a green approach for the preparation of carbon black (cb) and electrochemically reduced graphene oxide composite (ergo) was described. electrochemical sensors were based on screen printed carbon electrodes (spces), fabricated on poly (ethylene terephthalate) (pet). the spce/cb-ergo sensor was tested with da, ep and paracetamol (pcm), exhibiting an enhanced electrocatalytic performance compared to the bare spce. in another study, gupta et al. [89] have synthesized nio/cnts nanocomposite and applied it for fabrication of nio/cnts nanocomposite modified cpe (cpe/nio/cnts) as swv sensor for the determination of ep. the electrooxidation signal of ep showed an irreversible response at 0.3 v (vs. ag/agcl). the oxidation current of ep was doubled compared to a cpe. at the best electrochemical conditions, the voltammetric oxidation signal of ep showed linear dynamic range (0.08-900.0 μmol l-1), with detection limit of 0.01 μmol l-1. electrochemical sensor developed by anithaa et al. [90] for the simultaneous determination of ep and xanthine is based on the gamma irradiated sds-wo3 nps. the fabricated sensor exhibited wide linear range (0.009-1000 μmol l-1) with low detection limit (1.8 nmol l-1) for ep. tsele et al. [91] studied electrochemical properties of functionalized mwcnt/polyaniline (pani) doped with metal oxide (tio2, ruo2) nanoparticles. successful syntheses of mwcnt, tio2, ruo2, pani, mwcnt-pani-tio2 and mwcnt-pani-ruo2 nanomaterials were confirmed using suitable characterization techniques. au-mwcnt-pani-tio2 and au-mwcnt-pani-ruo2 modified electrodes showed the best electron transport properties towards the oxidation of ep, compared with other electrodes investigated. the tafel values obtained in the presence of ep as 0.448 and 0.442 v/decade for au-mwcnt-pani-tio2 and au-mwcnt-pani-ruo2 electrodes respectively, suggested adsorption due to analyte oxidation intermediates products. the linear calibration plot for ep was obtained in the concentration range of 4.9 to 76.9 μmol l-1, while a limit of detection for au-mwcnt-pani-tio2 electrode was 0.16 μ mol l-1. 4. interferences from compounds present in biological media and pharmaceuticals interference studies were carried out with several chemical substances prior to the application of the proposed method for the assay of ep in urine samples and the injection solution. the potential interfering substances were chosen from the group of substances commonly found with ep in pharmaceuticals and biological fluids. in biological environments, aa is commonly present with ep and may be oxidized at similar potential as ep. in the research performed by kang et al. [46], cvs of ep and aa were respectively recorded at the l-glutamic acid-graphene/gce. the results showed that the oxidation peak of ep is not affected by presence of aa. this means that the modified electrode is able to distinguish ep from aa. the influence of various foreign species on the determination of 50.0 μmol l-1 ep, 100.0 μmol l-1 aa and 50.0 μmol l-1 ua was investigated by taei et al. [53]. the tolerance limit was taken as the maximum concentration of the foreign substance(s) which caused an approximately ±5 % relative error in the determination. it was also found that mg+2, ca+2, so4-2, br-, k+, no3, clo4glycine, glucose, sucrose, lactose, fructose, valine, aspartic acid, urea, and saturation of starch solution did not interfere with the determination of these compounds. however, greater amounts of cysteine (40-fold), oxalate ion (100-fold), and citric acid (30-fold) did cause interference in the simultaneous determination of ep, aa and ua by poly(fuchsine acid) modified gce. j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 38 as was reported by ahmadian yazdely et al. [48], the maximal tolerable concentration of foreign substances was, by using the thiourea modified gce, determined as 5.0×10−5 mol l-1 for glucose, uric acid, ascorbic acid, citric acid, dopamine, and na+, k+, cu2+, mg2+, no3and so42ions. in their study, li and wang [54] have illustrated that k+, na+, ca2+, mg2+, sucrose and glucose do not interfere significantly, while l-glutamic acid, cu2+ and fe2+ ions showed a certain effect on the examinations of ep and ua. sadeghi et al. [75] studied the influence of various substances as potential interfering compounds on the determination of ep by the swv method under optimum conditions. a study was performed by a novel biosensor based on zno nanoparticle/1,3-dipropylimidazolium bromide ionic liquid modified cpe. the tolerance limit was defined as the maximum concentration of the interfering substance like glucose, fructose, lactose, sucrose tryptophan, histidine, glycine, valine, methionine, lucine, alanine, phenylalanine, ca2+, li+, clo4 -, so42-, scn-, na+, mg2+, k+, aa, urea, cysteine and ua that caused an error less than 5 % for the determination of ep. the results showed that the peak current of ep was not affected by all conventional cations, anions and organic substances. as stated by babaei et al. [70], interferences of aa, l-glutamic, l-alanin, aspartic acid and aspirin in determination of ep were significant only at relatively high concentrations, confirming that the proposed nickel hydroxide nanoparticles/mwcnts modified gce (mwcnt-nhnps/gce) was likely to be free from interferences from common components of biological samples. wang et al. [68] have illustrated the influence of some metal ions and anions that usually exist in biological fluid on the determination of 5.0×10-5mol/l ep. if the ±5 % error was allowed, 5.0×10-3 mol/l of k+, na+, fe2+, mg2+, cl-, so42did not show obvious interference on a modified gce, fabricated by covering with a layer of mwcnts coated with hexadecyl trimethyl ammonium bromide (ctab). in another work, apetrei et al. [63] investigated the influence of various interfering agents on determination of ep. the interfering substances na+, s2o52-, cl-, urea, tartaric acid, hydrochloric acid, glucose and glycine did not show any influence on the biosensor response when detecting ep. an absence of significant modification of the peak current recorded in the presence of interfering species was demonstrated. therefore, tyrosinase immobilized on a single-walled carbon nanotube modified gce (tyrosinase/swcnt-gce) can be considered to be a good biosensor for recognition of ep. mazloum ardakani et al. [66] studied the electrochemical behavior of a mwcnt paste electrode modified with 2-((7-(2,5-dihydrobenzylideneamino) heptylimino) methyl) benzene-1, 4-diol (dbhb). influence of various foreign species like aa, da, ua, levodopa, n-acetyl and captopril at concentrations 5 times higher than ep did not show any interference in the determination of ep. the influence of various foreign species on the determination of 50 μm ep was investigated by mekassa et al. [60] under optimum experimental conditions. potentially interfering substances were chosen from the group of substances commonly found with ep in pharmaceutical formulations and biological fluids. the tolerance limit was defined as the maximum concentration of the foreign substance(s) that caused an approximately ±5 % relative error in the determination of ep. according to the obtained results, aa, citric acid, d-glucose, lactose, glycine, mg2+, ca2+, na+, and k+ did not show any interference effect in the determination of ep. study of vieira da silva [61] showed that influence of interference on the electrode response can be useful to set up the sample preparation with the goal to minimize their effects. interference from electroactive compounds typically present in a physiological sample (e.g., serotonin (ser), aa and h. beitollahi et al. j. electrochem. sci. eng. 9(1) (2019) 27-43 doi:10.5599/jese.569 39 ua) commonly hinders the accurate determination of ep. the selectivity of the sensor was examined in the presence of ser, aa and ua. 5. analytical performances of electrochemical epinephrine sensors the analytical performances of electrochemical methods depend on the sensor’s construction and some of the most illustrative examples are extensively reviewed in table 1. table 1. some analytical performances attained in electrochemical determination of ep. type of detection transducer linear response detection limit relative stand. dev. ref. voltammetry (cv, dpv) l-glutamic acid functionalized graphene nanocomposite, modified glassy carbon electrode 10-710-3 mol l-1 3×10-8 mol l-1 ≤ 3.4 % [46] voltammetry (dpv) β-mercaptoethanol self-assembled monolayer modified gold electrode 10-7-10-4 mol l-1 3.3×10-8 mol l-1 [47] voltammetry (cv, dpv) glassy carbon electrode modified with thiourea 5.0×10-8-1.1×10−5 mol l-1 2.3×10−8 mol l-1 1.2 % [48] voltammetry (cv, dpv) tetradecyltrimethyl ammonium bromide (ttab) surfactant immobilized carbon paste electrode 0.15 30 µmol l-1 0.12 µmol l-1 2.4 % [49] voltammetry (cv, dpv) mesoporous carbon foam modified glassy carbon electrode 0.1-12.0 µmol l-1 40 nmol l-1 [50] voltammetry (cv, dpv) au-nanoparticle poly-fuchsine acid film modified glassy carbon electrode (poly(fa)/aunp/gce) 0.5–792.7 μmol l−1 0.01 μmol l−1 0.37 % [53] voltammetry (cv, swv) poly (guanine) modified glassy carbon electrode (pga/gce) 10−5 10−3 mol l-1 1.8 × 10−6 mol l-1 [54] voltammetry (cv) single-walled carbon nanotube-modified glassy carbon electrode 10-110 μmol l-1 2.54 μmol l-1 3.4 % [63] voltammetry (swv) zno nanoparticle/1,3-dipropylimidazolium bromide ionic liquid-modified carbon paste electrode 0.09-800 μmol l−1 0.06 μmol l−1 3.2 % [75] voltammetry (cv, dpv) glassy carbon electrode coated with a novel mg–al layered double hydroxide– nickel hydroxide nanoparticles–multiwalled carbon nanotubes composite 0.04-60 µmol l-1 11.0 nmol l-1 ≤3.1 % [76] voltammetry (cv) gold nanoparticles/polyaniline langmuir–modified glassy carbonelectrode 4×10-7 10-5 mol l-1 8×10-8 mol l-1 1.97 % [77] voltammetry (dpas) mcm/zro2 nanoparticles modified carbon paste electrode 10-6-2.5×10-3 mol l-1 5.0×10-7 mol l-1 [79] voltammetry (cv) a graphene-modified glassy carbon electrode 0.20-100 µmol l-1 0.001 µmol l-1 [83] voltammetry (cv, dpv) aminatedgraphene and ag nanoparticles modified gce 0.916-184 μmol/l 2.0 nmol l-1 [84] voltammetry (cv, swv) vinylferrocene and carbon nanotubes (cnts)-modified carbon paste electrode 0.1-1000.0 µmol l-1 3.0 × 10-8 mol l-1 ≤ 2.5 % [86] 6. some applications of electrochemical epinephrine sensors in pharmaceutical and biological fluid analysis electrochemical ep sensors have widespread application in pharmaceutical and biomedical analysis, as shown in table 2. j. electrochem. sci. eng. 9(1) (2019) 27-43 determination of epinephrine 40 table 2. numerical data on ep content determined in various analysed systems. method electrode type analysed medium recovery, % reference cv, dpv l-glutamic acid functionalized graphenenanocomposite, modified glassy carbon electrode ep injection 100.4 [46] dpas β-mercaptoethanol self-assembled monolayer modified gold electrode ep injection 100.9 [47] cv glassy carbon electrode modified with thiourea ep injection 101.2 [48] dpv tetradecyltrimethyl ammonium bromide (ttab) surfactant immobilized carbon paste electrode ep injection 96.3 [49] dpv glassy carbon, electrode coated with a novel mg–al layered, double hydroxide–nickel hydroxide nanoparticles, multi-walled carbon nanotubes composite blood urine 97.6 98.2 [76] dpv gold nanoparticles/polyanilinelangmuir– modified glassy carbonelectrode serum 98.36 [77] dpv mcm/zro2 nanoparticles modified, carbon paste electrode ep injection 101.7 [79] dpv graphene-modified glassy carbon electrode urine 99.7 [83] dpv gce, modified by aminatedgraphene and ag nanoparticles serum 100.24 [84] dpv vinylferrocene and carbon nanotubes (cnts)modified carbon paste electrode ep injection ampoule 101.0 102.0 [86] 7. conclusions in past five years, utilization of electroanalytical methods for pharmaceutical analysis has significantly increased, especially for ep assessments. however, there is a limited number of publications concerning a combination of pre-concentration and electrochemical detection of ep. electrochemical techniques are often preferred to laborious instrumental methods for ep determination, which is due to the simplicity of procedure and instrumentation, minimum requirements with respect to sample pretreatment, as well as fast response, sensitivity and low cost. also, accurate results can be obtained in real time and complex media. different modalities of sensor development already described in the literature are presented, starting from bare to chemically modified sensors. recent advances imply the use of carbon nanotubes and various composites, for which large surface area and electrocatalytic activity greatly enhance the analytical signal, diminishes the peak potential corresponding to ep oxidation and solves peak overlapping problems in complex samples. provided that adequate pretreatment and cleaning steps are included, several examples of viable ep determination in various media performed by bare electrodes, even in the presence of interfering compounds are also presented. method performances and application areas depend on the chosen electrochemical technique. it can be generally concluded that different ways of construction and expected performances of sensor electrodes are adequate and tuned to the nature of the analysed compound and respective matrix. the nature of the electrode material and surface groups formed, as well as their interaction with analyte molecules, greatly influence the electrooxidation rate, as well as ph value of the analysed matrix, electrolyte type, and the peak potential and height. the mechanism and rate of electrooxidation are strongly dependent on the following factors: electrode nature and modifiers, electrode pre-treatment, surface 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[91] t. p tsele, a. s. adekunle, e. f. omolola, e. e. eno, electrochimica acta 243 (2017) 331-348. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {electrochemical behavior of sildenafil citrate at gold and cystein modified gold electrode in acid solution} doi:10.5599/jese.481 163 j. electrochem. sci. eng. 8(2) (2018) 163-170; doi: http://dx.doi.org/10.5599/jese.481 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behaviour of sildenafil citrate at gold and cystein modified gold electrode in acid solution jelena lović, nemanja trišović1, jelena antanasijević1, milka avramov ivić ictm, department of electrochemistry, university of belgrade, njegoševa 12, 11000, belgrade, serbia 1faculty of technology and metallurgy, university of belgrade, karnegijeva 4, belgrade, serbia corresponding author e-mail: jlovic@tmf.bg.ac.rs; tel.: +381-11-337-0389; fax: +381-11-337-0389; received: november 30, 2017; revised: february 14, 2018; accepted: march 21, 2018 abstract electrochemical behavior of sildenafil citrate (sc) at gold and cystein (cys) modified gold electrode (au/cys) in 0.1 m h2so4 was investigated by cyclic voltammetry (cv) and by square wave voltammetry (swv). effect of scan rate on the cvs of sc standard was performed in order to examine the mode of transport and irreversibility of process. sc as standard is determined by swv in acid solution at au electrode in a range: 10-3, 10-2, 0.1, 0.5, 1 μm and on au/cys in a range: 10-3, 10-2, 0.05, 0.1 μm. the presence of cys causes two-time larger peak currents and shifting of the incipient potential of the sc oxidation to 0.1 v in negative direction. in investigated range of concentration of sc standard in acid solution on modified and unmodified au electrodes swv method have excellent linear regression coefficient with value 0.997 promoting swv for reliable determination of sc. keywords sildenafil citrate standard; cystein modified electrode; sensitivity; quantitative analysis. introduction sildenafil citrate (sc) is chemically designated as 1-[[3-(6,7dihydro-1-methyl-7-oxo-3-propyl1h-pyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxyphenyl]sulfonyl]-4-methylopiperazine citrate. sc is the active ingredient of one of the widely used pharmaceutical formulations of viagra. studied initially for its use in treatment of hypertension and angina, viagra was very efficient for the treatment of erectile dysfunction. nonmedical use of sc has increased over the years [1,2]. sc treats and recovery of extreme tiredness after a long flight [3]. the most common adverse effects of sc use included nasal congestion, impaired vision (blurriness, loss of peripheral vision, photophobia) and headache. serious adverse effects include severe low blood pressure, heart attack, stroke, increased intraocular pressure, sudden hearing loss [4]. http://dx.doi.org/10.5599/jese.481 http://www.jese-online.org/ mailto:jlovic@tmf.bg.ac.rs j. electrochem. sci. eng. 8(2) (2018) 163-170 sildenafil citrate in acid solution 164 many various methods for determination of sildenafil citrate in pharmaceutical preparations and biological samples have been described. the most frequently used method for determination of sc is high performance liquid chromatography [5-7]. other reported methods for confirmation of sc in pharmaceutical preparations or biological fluids are optical methods such as uv-vis spectrophotometric methods [8], nmr spectroscopy [9] or electrospray tandem mass spectrometry (esi-ms-ms) spectrophotometry [10] and electroanalytical methods [11-15]. among electrochemical methods different electrodes and voltammetric techniques were used. the sc determination at ph 2.0 was performed by swv and adsorptive stripping techniques at hanging mercury drop electrode [16]. at glassy carbon electrodes (gc), cv and swv were applied in determination of sc in the mixture with paracetamol and carvedilol [17] and in biological and pharmaceutical formulations [14]. the cyclic and linear sweep voltammetric methods as well as differential pulse voltammetry (dpv) and swv at gc were applied for oxidative determination of sc in solutions containing 30 % (v/v) acetonitrile at different phs [18]. by swv at pb film modified gc a new method for determination of sc is developed [19]. different electrodes were used for electrochemical examination of sc. for example, electroanalytical methods were applied at boron-doped diamond and at diamond paste electrodes [12,20]. delolo et al. used gc modified with a chitosan-supported ruthenium film in order to prepare sensor for sc [13]. the aim of this work is to examine the electrochemical behavior of sc at au and au/cys in 0.1 m h2so4 by cv and swv. scan rate studies were carried out to assess whether the process on au and au/cys was under diffusion or adsorption-controlled. linear dependency of peak currents from concentrations is established using swv in order to investigate the sensitivity of the electrodes. also, the linear dependency was constructed as a calibration curve and tested for quantitative determination of sc. experimental sildenafil citrate, provided by hemofarm a.d. stada, was used as sc standard and as content of sildena® tablets (pharmaceutical formulation) with excipiences: lactose monohydrate, cellulose, microcrystalline, hydroxypropylcellulose, croscarmellose sodium, sodium stearyl fumarate, silica colloidal anhydrous, opadry ii blue, opadry fx silver. l-cysteine and h2so4 were purchased from sigma aldrich. for cv and swv measurements, pgz 402 volta lab (radiometer analytical, lyon, france) was used and the three-electrode electrochemical cell. polycrystalline gold (surface area 0.5 cm2) which served as the working electrode, was polished with diamond paste, cleaned with a mixture of 18 mω cm deionized water and sulfuric acid and further cleaned with 18 mω cm deionized water in an ultrasonic bath. a gold wire was used as the counter electrode and a saturated calomel electrode (sce) as the reference electrode. all potentials are given vs. sce. the electrolytes were deoxygenated by purging with nitrogen. electrode modification and stock solutions were prepared according to [21]. all the solutions used were prepared with high purity water (millipore, 18 mω cm resistivity). results and discussion the electrochemical behavior of sc on au and au/cys electrode was studied by cyclic voltammetry (cv) and its determination in pharmaceutical formulations was carried out by square wave voltammetry (swv). the cv of sc standard on gold electrode in 0.1 m h2so4 alongside the j. lović et al. j. electrochem. sci. eng. 8(2) (2018) 163-170 doi:10.5599/jese.481 165 voltammetric response of au electrode in blank solution (dot line) is presented in fig. 1. in the presence of sc, the cv was changed so that current increase occurs in the region of oxide formation. it can be said that highly developed gold oxide on surface is necessary as a catalyst for the beginning of the sc electrooxidation reaction. figure 1. cv of au electrode using 0.1 m h2so4 (dot line) and after the addition of 1 µm sc standard (full line), scan rate: 100 mv s-1. inset: cv of 1 µm sc standard on au (dash line) and au/cys (full line) electrode, scan rate: 20 mv s-1. in order to improve the activity, the electrode was modified with cysteine as was previously described [21]. in the inset of fig. 1 it is show that in the presence of cys the electrode becomes more active for the electrochemical oxidation of sc. the improved activity of the modified electrode in 0.1 m h2so4 could be due to the increase of the electrode surface area according to the observed differences in morphology of the gold surface modified with cys as was confirmed by optical microscope [21]. the role of cys can also be explained by the mechanism proposed by ozkan et al. for the electrochemical oxidation of sc on a gc electrode in an aqueous solution containing 30 % (v/v) acetonitrile [18]. the authors indicated that acid–base equilibrium precedes the electrooxidation of the piperazine moiety at ph 5.5. cys is chemisorbed on an au surface through the thiol group, while its amino and carboxylic groups remain available to interact with sc. as suggested by wang and du [22], it seems that cys act as a mediator of the electrode reaction and as an electron-transferring accelerator. possible mechanism of interaction between sc and au/cys is presented in detailed in [21], explaining the observed reactivity for sc electrooxidation. sildenafil citrate was used as sc standard and as content of sildena® tablets (pharmaceutical formulation) containing excipients. figure 2 show cvs of sc standard and sc in pharmaceutical formulation on modified and unmodified au electrode. overlap of cvs confirm that excipients are electrochemically inactive and did not affect the sc electrooxidation. furthermore, effect of the scan rate on the cvs of sc standard was studied on au/cys electrode as is presented in fig. 3. scan rate studies were carried out to assess whether the process on modified gold electrode was under diffusion or adsorption-controlled. j. electrochem. sci. eng. 8(2) (2018) 163-170 sildenafil citrate in acid solution 166 figure 2. cv of sc standard (full line) and sc tablet (dot line) on au electrode (a), cv of sc standard (full line) and sc tablet (dot line) on au/cys electrode (b); 1 µm sc + 0.1 m h2so4 scan rate: 100 mv s -1. figure 3. cvs of sc standard on au/cys electrode for different scan rates 20-140 mv s-1. figure 4 demonstrates the dependency of peak current and peak potential from scan rates recorded in the range 20-140 mv s-1 (data are taken from fig. 3). the peak current densities are linearly proportional to the scan rates suggesting adsorption-controlled electrode process (fig. 4a). furthermore, plot of logarithm of anodic peak current vs. logarithm of scan rate gave a straight line with a slope of 0.89 close to the theoretical value of 1.0, which is expected for an ideal reaction for the adsorption-controlled electrode process [23]. the peak potential was also dependent on scan rate. the plot of ep versus log v was linear suggesting that it is irreversible electrode process (fig. 4b). nevertheless, close inspection of the dependency of peak potential from scan rates in backward direction indicates partially deviation from linearity for slower scan rates suggesting the interference of some reaction intermediates on the electrode process. the same experiments were performed on au electrode exhibiting the same conclusion about the mode of j. lović et al. j. electrochem. sci. eng. 8(2) (2018) 163-170 doi:10.5599/jese.481 167 transport and irreversibility of process. effect of scan rate on sc at gc [14,18] and at the hanging mercury dropping electrode[24] demonstrate also the adsorption-controlled process. figure 4. electrochemical behavior of sc standard on au/cys electrode for scan rates 20-140 mv s-1, presented as dependence of peak current vs. v (a) and peak potential vs. log of scan rates (b). apart from apparent increase in the voltammetric response caused using higher scan rates, the significant improvement in sensitivity can be achieved by employing swv method. in order to establish the optimal parameters in swv pulse size, scan rate and accumulation time were varied. the peak current appeared with maximum values for an accumulation potential of -400 mv while the accumulation time of 60 s enables electrode surface saturation. with increasing scan rate, the peak current increases also but the peak becomes less sharp and poorly defined. higher peak current was observed by increasing the pulse size, but the background current also increases. the peak current increases linearly with step size up to 5 mv. thus, the best defined voltammetric signal for the determination of sc was recorded under the conditions: step size 5 mv, pulse size 25 mv, scan rate 10 mv s-1, accumulation time 60 s at 400 mv. sc standard is determined by swv in 0.1 m h2so4 at au in a range: 10−3, 10−2, 0.1, 0.5, 1 μm as is shown in fig. 5. it can be notice that the selected concentrations of sc standard start to oxidize with the beginning of oxide formation on gold electrode. sw voltammograms recorded at au electrode for increasing amount of sc standard concentrations showed a linear increase of anodic peak currents with the concentration (inset of fig. 5) with an excellent correlation coefficient (r = 0.997). obviously, the linear dependency can be used as a calibration curve for quantitative determination, providing a convenient way for quantitative analysis. j. electrochem. sci. eng. 8(2) (2018) 163-170 sildenafil citrate in acid solution 168 figur e 5. sw voltammograms of sc standard (10-3, 10-2, 5×10-2, 10-1, 5×10-1, 1.0 µm) on au electrode using 0.1 m h2so4. step size 5 mv, pulse size 25 mv and scan rate 10 mv s -1, accumulation time 60 s at -400 mv. inset: the linear dependency of anodic peak currents vs. concentration of sc standard. sc standard is determined by swv at au/cys electrode in a range: 10−3, 10−2, 0.05, 0.1 μm as is shown in fig. 6. the currents increase with increasing the concentration of sc standard, shifting the peak current towards less positive value. a linear increase of anodic peak currents with the concentration with an excellent correlation coefficient (r = 0.997) is observed and presented in inset of fig. 6. comparing the equations for linear dependency of peak currents from concentrations it can be notice that higher slope of calibration curve is obtained on au/cys electrode indicating the higher sensitivity of sw voltammetry for concentration detection. nevertheless, linear dependency can be obtained in extensive concentration range on au electrode regarding au/cys. figure 6. sw voltammograms of sc standard (10-3, 10-2, 5×10-2, 10-1, 5×10-1, 1.0 µm) on au electrode using 0.1 m h2so4. step size 5 mv, pulse size 25 mv and scan rate 10 mv s -1, accumulation time 60 s at -400 mv. inset: the linear dependency of anodic peak currents vs. concentration of sc standard. j. lović et al. j. electrochem. sci. eng. 8(2) (2018) 163-170 doi:10.5599/jese.481 169 better insight in the obtained activity of au and au/cys electrode in sc standard electrooxidation can be observed by overlapping of sw voltammograms for a chosen concentration, as is presented in fig. 7. comparing the results of sw voltammograms two times larger peak currents and 100 mv more negative incipient potential obtained on au/cys in regard to au electrode can be noticed. it could be explained by possible oxidation mechanism and by different electrode surfaces morphology [21]. concerning sw voltammogram of sc tablet on au/cys electrode, the same shape and peak position was obtained. examination of the electrochemical behavior of various concentrations of sc tablet by swv reveal excellent fitting in the calibration curve, as is presented in [error! bookmark not defined.]. besides, unknown concentrations of sc tablet are confirmed by hplc-uv method. it was highlighted that in contrast to hplc-uv, electrochemical method is more sensitive and can be used for the measurements of very low concentrations of analyte. figure 7. sw voltammograms of 0.1 µm sc standard on au and au/cys electrode and of 0.751 µm sc tablet using 0.1 m h2so4. step size 5 mv, pulse size 25 mv and scan rate 10 mv s -1, accumulation time 60 s at -400 mv. table 1 presents comparative characteristics of the quantitative determination of sc obtained with different electrochemical methods. the calibration curves provided reliable linear responses on a suitable range although with au/cys electrode linear concentration range was extent to a very low concentration (up to nanomoles per liter magnitude order). table 1. comparison of electrochemical methods for sc determination method electrode linear range, m lod, m reference adcsv bismuth film/gc 10-7-10-6 1.8x10-8 [25] potentiometry ppy/cit/graphite 3.4x10-5-1.7x10-3 3x10-5 [26] swv screen-printed gc 10-6-1.4x10-5 5.5x10-8 [14] swv au/cys 10-9-10-7 1.04x10-8 present work conclusions investigation of electrochemical behavior of sc at au electrode in acid solution reveal that highly developed gold oxide on surface is necessary as a catalyst for the beginning of the sc electrooxidation reaction. effect of the scan rate on the cvs of sc standard was studied on au/cys and au electrode suggesting adsorption-controlled electrode process since the peak current j. electrochem. sci. eng. 8(2) (2018) 163-170 sildenafil citrate in acid solution 170 densities are linearly proportional to the scan rates. also, the peak potential was linear dependent on logarithm of scan rate suggesting that it is irreversible electrode process. swv measurements indicate wider 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[26] l. m. ochiai, e. h. bindewald, p. mengarda, l. h. marcolino-junior, m. f. bergamini, journal of applied polymer science 133 (2016) 43762-43767. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.pfizer.com/ https://www.infona.pl/resource/bwmeta1.element.baztech-journal-0009-2223-chemia_analityczna/tab/jcontent https://www.infona.pl/resource/bwmeta1.element.baztech-journal-0009-2223-chemia_analityczna/tab/jcontent http://creativecommons.org/licenses/by/4.0/) {experimental and theoretical study on the corrosion inhibitor potential of quinazoline derivative for mild steel in hydrochloric acid solution} http://dx.doi.org/10.5599/jese.887 11 j. electrochem. sci. eng. 11(1) (2021) 11-26; http://dx.doi.org/10.5599/jese.887 open access: issn 1847-9286 www.jese-online.org original scientific paper experimental and theoretical study on the corrosion inhibitor potential of quinazoline derivative for mild steel in hydrochloric acid solution fidelis ebunta abeng1,, valentine c. anadebe 2, patience yake nkom1, kelechi j. uwakwe3 and enyinda goodluck kamalu4 1material and electrochemistry research group, department of chemistry, cross river university of technology. p. m. b. 1123, calabar-nigeria 2chemical engineering department, federal university ndufu alike, ebonyi state, nigeria 3university of chinese academy of sciences, dalian 116023, china 4chemistry department, federal university of agriculture, makurdi, benue state, nigeria corresponding author: fidelisabeng@yahoo.com, tel.: +234 08035664813 received: july 16, 2020; revised: september 28, 2020; accepted: october 3, 2020 abstract interaction of metal surfaces with organic molecules has significant role in corrosion inhibition of metals and alloys. more clarification, from both experimental and computational view is needed in describing the application of inhibitors for protection of metal surfaces. in this study, the surface adsorption and corrosion inhibition behavior of metolazone, a quinazoline derivative, on mild steel in 0.02, 0.04, 0.06, and 0.08 m hcl solutions were investigated. weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy techniques were used. the optimum inhibition efficiencies of 75, 82 and 83 % were found by these three techniques at the optimum inhibitor concentration of 500 mg/l and 303 k. scanning electron microscopy (sem) was used to confirm adsorption of quinazoline derivative on the surface of the mild steel. computational simulations were additionally used to give insights into the interaction between quinazoline inhibitor and mild steel surface. thermodynamic parameters of mild steel corrosion showed that quinazoline derivative functions as the effective anti-corrosive agent that slows down corrosion process. potentiodynamic polarization results revealed a mixed-type inhibitor, while the result of the adsorption study suggests that adsorption of the inhibitor on the mild steel surface obeys the physical adsorption mechanism and follows langmuir adsorption isotherm model. keywords molecular modeling; acid corrosion; adsorption, mild steel surface characterization; thermodynamics http://dx.doi.org/10.5599/jese.887 http://dx.doi.org/10.5599/jese.887 http://www.jese-online.org/ mailto:fidelisabeng@yahoo.com j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 12 introduction the extensive use of mild steel in the construction of petroleum refineries tanks, flow lines and transmission pipelines is as a result of ease of its fabrication and low cost [1]. at the other side, acid media are widely applied in the metallurgical industry for pickling and descaling of metals, chemical or electrochemical processes in oil refinery, and deactivation of equipment in atomic power establishment [2]. for solving the resulting corrosion problems for mild steel in acid environment in oil and gas industries, use of corrosion inhibitors is the best practical and cost-effective way. when added to an acidic environment in small amounts, inhibitors can appreciably reduce corrosion rates [1-3]. it has already been found that organic compounds which contain either lone pair-donating heteroatoms like n, o, s and p, or structural moieties with -electrons that interact with metal, demonstrate good inhibitive properties [2,4]. use of these compounds, however, has recently been restricted due to their high cost and toxicity to the environment [5]. therefore, the researchers practicing in the field of metallurgical and material industry, usually release fresh or new corrosion inhibitors, particularly those which are cheap and show highly mitigated competence and less amount of toxicity [6,7]. the present investigation aims to study the corrosion protection potential of metolazone drug, i.e. quinazoline derivative (chloro-2-methyl-4-oxo-3-o-tolyl-1,2,3,4-tetrahydroquinazoline-6-sulfonamide), for mild steel in hydrochloric acid. in general, metolazone is a pharmaceutical compound applied as diuretic, for preventing high blood pressure, and for other purposes. this compound, however, contains heterocyclic molecules, 𝜋-electrons, high molecular weight and functional groups which are all indicators of good corrosion inhibitors. many quinazoline derivatives were already investigated as corrosion inhibitors in different aggressive environments. thus, some chlorophenyl and nitrophenyl quinazoline derivatives were used as corrosion inhibitors for carbon steel in 2 m hcl solution by fouda et al. [8]. al-tamimy and al-majidi [9] have recognized corrosion inhibition property of 2-pentadecylquinazolinylthiourea as corrosion inhibitor for carbon steel in acidic medium. hashim et al. [10] synthesized three new quinazoline derivatives and studied their corrosion inhibition behavior on mild steel in h2so4 medium. recently, rbaa et al. [11] synthesized hydroxyl based quinazoline derivatives and proved excellent inhibition of corrosion at mild steel in hcl. chaouiki et al. [12] explored 3-cyclopropyl-3,4-dihydroquinoline-2(1h)-one as a new corrosion inhibitor for mild steel in hydrochloric acid solutions. corrosion inhibition behavior of 2-methyl quinazoline derivative has also been investigated by kadhim et al. [13]. ozturk [14] synthesized and studied corrosion inhibition behavior of two quinazoline derivatives di-cationic surfactants in acidic solutions. pradeep et al. [15] synthesized three new quinazoline derivatives and studied them as corrosion inhibitors for mild steel surface protection in hcl solution, using experimental and dft methods. fouda et al. [16] also documented quinazoline derivatives as green corrosion inhibitors for carbon steel in hydrochloric acid, employing weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy and scanning electron microscopy. the results of their investigation were similar to those obtained in the present study. in the view of above findings, the present study aims to investigate metolazone drug, that is a compound derived from quinazoline substituent, as the corrosion inhibitor for mild steel in hydrochloric acid solution by means of weight loss and electrochemical techniques. in addition, some density functional theoretical (dft) calculations are performed to locate the actual sites where inhibitor adsorption occurs on the mild steel surface. f. e. abeng et al. j. electrochem. sci. eng. 11(1) (2021) 11-26 http://dx.doi.org/10.5599/jese.887 13 experimental materials chemicals and reagents used for this study were supplied by vickam chemical co ltd, calabarnigeria, and were of analytical grade. the commercially available mild steel sheets (q235 steel) with purity of 98 % iron and thickness of 0.008 cm were used for the study. the sheets were mechanically press cut into various coupons of 540.008 cm for corrosion measurement. emery paper of different grades was used to smooth the surface of each coupon that was subsequently washed in ethanol and acetone for purification and allowed to be desiccated in air. preparation of hcl solution/inhibitor stock solution the concentration of corrosive medium used includes 0.02, 0.04, 0.06 and 0.08 m hydrochloric acid solution. these concentrations of hydrochloric acid were prepared by diluting the appropriate volumes of analytical grade concentrated hydrochloric acid (37 wt.%) with doubly distilled water using the standard solution formula. the inhibitor used in this study was metolazone drug of quinazoline derivative obtained from the local pharmacy (g & i pharmaceutical company, ikot ansa calabarnigeria) and was used without further purification. it is chemically designated as (chloro-2 -methyl-4-oxo-3-o-tolyl-1,2,3,4-tetrahydroquinazoline-6-sulfonamide). the molecular formula of the studied quinazoline derivative is c16h16cl n3o3s with molecular weight of 365.84 g mol-1. appropriate concentrations from 50 mg/l to 500 mg/l of the drug were diluted into already prepared hydrochloric acid solutions [17-19]. methods the methods employed in this study include the weight loss, electrochemical techniques, computational simulations and scanning electron microscope, sem. weight loss experiments the mild steel coupons of known weights were suspended in 200 cm3 solution of hcl in 250 cm3 beakers using a pyrex glass hook. the experiments were carried out in various concentrations of hcl (0.02 m, 0.04 m, 0.06 m and 0.08 m), in the absence and presence of studied inhibitor, at different temperatures (303, 313 and 333k). the test solutions were maintained in a thermostatic water bath for 12 hours, after which each coupon was washed, desiccated and reweighed. the weight loss (w) results were calculated from differences in two weights, that is the initial (w1) and final (w2) weight according to the eq. (1). thereafter the corrosion rate (cr) and inhibition efficiency (ie) were evaluated using eqs. (2) and (3) [20-22]. δw = w1w2 (1) cr = (δw / at) 1000 (2) blank inh blank cr -cr ie = 100 cr (3) electrochemical measurements all electrochemical measurements were performed with the aid of a potentiostat/galvanostat. platinum wire, saturated calomel electrode (sce) and mild steel coupon of exposed surface area of 1 cm2, were used as counter, reference and working electrode, respectively. the electrochemical experiments were performed in 0.02 m hcl solution, after 30 minutes immersion in the test solution at room temperature. polarization measurements were carried out from +250 to -250 mv vs. http://dx.doi.org/10.5599/jese.887 j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 14 corrosion potential (ecorr) at the scan rate of 0.5 mv s-1. eis measurement was performed using potentiostat/galvanostat in the frequency range of 100 khz to 0.05 hz with the signal amplitude of 5 mv [23-25]. scanning electron microscopy (sem) the morphological analysis of the corroded coupons was carried out at chemical engineering department ahmadu bello university, zaria, using the scanning electron microscopy sem (model mve016477830, manufactured by phenom-world eindhoven, the netherlands). sem images were taken after one day immersion of mild steel coupons in 0.02 m hcl solution in the absence and presence of the inhibitor at room temperature. computational chemistry study density functional theory (dft) methods of 4.0 material studio software were used in performing the quantum chemical modeling of the present study. this is because dft method is less time consuming and less expensive from computational point of view. the calculations were done using the becke’s three parameter hybrid method (b3) applying the correlation functional of lee yang and parr (lyp) (dft/b3lyp). results and discussion weight loss corrosion rates of mild steel in different concentrations of hydrochloric acid, based on weight loss measurements are presented in table 1. table 1. corrosion rates and inhibition efficiencies for mild steel in various concentrations of hcl with and without inhibitor, obtained from weight loss measurements. cinh/ mg l-1 chcl / m 0.02 0.04 0.06 0.08 0.02 0.04 0.06 0.08 corrosion rate, ×10-4 mg cm-2 h-1 inhibition efficiency, % 303 k blank 9.3 13.4 15.8 18.7 50 7.0 10.6 12.4 16.2 24.8 20.9 16.4 13.2 100 6.0 9.3 10.3 13.7 35.2 30.4 30.1 26.6 200 5.1 8.1 9.7 12.9 44.7 39.4 33.7 31.2 300 4.2 6.3 7.4 10.4 54.6 52.7 48.0 44.4 500 2.3 4.7 5.5 8.5 75.2 64.9 59.9 54.8 313 k blank 13.2 20.3 25.8 28.6 50 10.1 16.1 20.9 24.2 23.3 20.6 19.0 15.4 100 8.5 14.0 18.3 22.3 35.5 31.0 29.1 22.0 200 6.1 10.3 13.2 17.4 53.7 49.2 48.8 39.2 300 3.9 8.0 10.9 14.1 70.4 60.4 57.8 50.7 500 1.9 5.2 7.0 11.4 85.8 74.5 72.8 60.1 333k blank 17.0 23.2 27.0 32.1 50 13.2 18.1 22.6 27.4 22.4 22.0 16.3 14.3 100 10.1 15.0 18.4 23.1 40.6 35.3 31.9 27.7 200 9.2 14.7 18.0 21.9 45.7 36.6 33.3 31.5 300 6.1 12.4 15.2 19.6 64.0 46.6 43.7 38.6 500 4.4 8.3 11.8 15.3 73.9 64.1 56.3 52.0 f. e. abeng et al. j. electrochem. sci. eng. 11(1) (2021) 11-26 http://dx.doi.org/10.5599/jese.887 15 it is clearly seen that corrosion rate of mild steel increases with increase in the concentration of hydrochloric acid solution and decreases as the concentration of inhibitor increases. these results indicate an inhibitive potential of quinazoline derivative on mild steel corrosion. the inhibition efficiencies at various concentrations of hcl with concentrations of quinazoline derivative as inhibitor, were measured at different temperatures (303, 313 and 323k) and listed in table 1. results from table 1 show that the corrosion inhibition efficiency of quinazoline derivative on mild steel in hydrochloric acid increases with increased concentration of the studied inhibitor. also, inhibition efficiency decreases with a decrease in the concentration of the acid. this is because at higher concentrations of the corrodent, the inhibitor is attacked by the acid leading to reduction of the inhibitor effectiveness [26-28]. effect of temperaturethermodynamic/adsorption study the assessment of thermodynamic parameters is of great importance in the analysis of inhibitors adsorption on the metal surface. the thermodynamic parameters for the studied system were estimated using the following equations: () cr a e / rt ke= (4) a aδ δ cr e e s h r rtrt nh            = (5) in eq. (4), ea is activation energy of corrosion reaction, k is arrhenius constant, r is gas constant and t is absolute temperature. in eq. (5), n, h, δsa and δha are avogadro number, planck constant, activation entropy change, and activation enthalpy change, respectively. the plots of log cr and log (cr/t) as functions of the inverse of temperature are illustrated in figure 1, showing straight lines with linear regression coefficients close to unity for all inhibitor concentrations studied. the values of thermodynamic parameters for mild steel corrosion (ea, ha and sa) are calculated from the slopes and intercepts of linear relationships in figure 1 and are presented in table 2. a b figure 1. (a) arrhenius plots and (b) transition state plots for mild steel in 0.02 m hcl without and with different concentrations of quinazoline derivative inhibitor. data in table 2 revealed that the apparent ea value for corrosion in the blank solution is 60.2 kj mol−1, while in the presence of the quinazoline derivative, ea values range from 62 to 75 kj mol−1. it is evident that ea is higher in the solution with inhibitor than in the blank one. according to fouda et al. [8], higher ea value in presence of inhibitor indicates the physical adsorption mechanism of inhibitor on the mild steel surface. the increase in ea value can also be 1 1.5 2 2.5 3 3.5 3.05 3.1 3.15 3.2 3.25 3.3 3.35 lo g ( c r / m g c m -2 h -1 ) (103 / t) / k-1 blank 50 mg/l 100 mg/l 200 mg/l 300 mg/l 500 mg/l -1.8 -1.6 -1.4 -1.2 -1 -0.8 2.9 3 3.1 3.2 3.3 3.4 lo g ( c r t -1 /m g c m -2 h -1 ) (103 / t) / k-1 blank 50 mg/l 100 mg/l 200 mg/l 300 mg/l 500 mg/l http://dx.doi.org/10.5599/jese.887 j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 16 attributed to a noticeable decrease in the adsorption of the inhibiting molecules on the metallic surface with an increase in temperature [8]. this suggests that the energy barrier of the corrosion reaction rises in the presence of the quinazoline derivative [28]. the positive sign of enthalpy (ha) values is principally associated with endothermic nature of corrosion of mild steel, indicating slow dissolution of mild steel in the presence of the quinazoline derivative [8,28]. increase of ha values with increase of inhibitor concentration suggests slower dissolution of mild steel, or higher protective ability of inhibitor. as reported in table 2, the increase of negative values of δsa reflects an association rather than a dissociation step. this implies that a decrease in disorder occurs when reactants are transferred to the activated complex [28]. table 2. thermodynamic parameters for mild steel corrosion in 0.02 m hcl at different concentrations of quinazoline derivative inhibitor. cinh / mgl-1 ea / kj mol-1 r2 δha / kj mol-1 r2 δsa / jmol-1k-1 blank 60.2 0.966 49 0.699 -0.769 50 59 0.802 47 0.722 -0.642 100 56 0.651 45 0.542 -0.490 200 62 0.941 50 0.913 -0.652 300 75 0.979 64 0.971 -1.076 500 70 0.970 59 0.958 -0.786 potentiodynamic polarization figure 2 presents anodic and cathodic polarization curves of mild steel corrosion in 0.02 m hcl in the absence and presence of quinazoline derivative inhibitor. the polarization plots indicate that anodic and cathodic reactions in the blank acid solution and in solutions containing inhibitor follow tafel’s law. the inhibited solution, however, produced a pronounced effect by shifting ecorr into anodic direction, and decreasing anodic and cathodic current density values. this indicates that quinazoline derivative is a mixed-type inhibitor with predominant anodic effect. the corresponding polarization parameters namely, corrosion current density (jcorr) corrosion potential (ecorr), anodic and cathodic tafel slopes (a and c) were determined from figure 2 and listed in table 3. the increase in the inhibitor concentration decreases jcorr and ecorr values due to adsorption of inhibitor molecules on the mild steel surface, which slows down corrosion reaction. figure 2. potentiodynamic polarization plots of mild steel in 0.02 m hcl in absence and presence of various concentrations of quinazoline derivative inhibitor. the tafel constants (a and c) decreased with the increasing quinazoline derivative concentration, showing that quinazoline derivative controls both the anodic and cathodic reaction, but it is clearly seen -0.90 -0.80 -0.70 -0.60 -0.50 -0.40 -0.30 -0.20 -0.10 0.00 1.0e-08 1.0e-06 1.0e-04 1.0e-02 1.0e+00 e co rr / v v s. s c e jcorr / ma cm -2 blank 50 mg/l 100 mg/l 200 mg/l 300 mg/l 500 mg/l f. e. abeng et al. j. electrochem. sci. eng. 11(1) (2021) 11-26 http://dx.doi.org/10.5599/jese.887 17 in fig. 2 that the tafel plots are been sweep into the direction of the positive sites (anodic sites). the inhibition efficiency was calculated from polarization data by the equation [19,29-31]: corr(blank) corr(inh) corr(blank) 10ie 0 j j j  − =      (6) where jcorr(blank) and jcorr(inh) are the corrosion current density in the absence and presence of the inhibitor, respectively. table 3. polarization and tafel parameters for mild steel in 0.02 m hcl in the absence and presence of various concentrations of quinazoline derivative inhibitor. cinh / mg l-1 -ecorr/ mv jcorr / μa cm-2 c / mv dec-1 a/ mv dec-1 ie/ % blank 466 580 113 97 50 424 453 105 85 21.9 100 419 280 100 78 51.7 200 410 253 89 80 56.4 300 409 222 84 82 61.7 500 403 100 79 73 82.8 electrochemical impedance spectroscopy impedance spectra exemplified by nyquist plots obtained for mild steel in 0.02 m hcl solution in the absence and presence of various concentrations of quinazoline derivative are illustrated in figure 3a. depressed semicircles are generally seen in 0.02 m hcl solution over the frequency range studied, becoming smaller with increase of the inhibitor concentration. the electrochemical equivalent circuit (eec) employed to evaluate impedance parameter values is shown in figure 3b. this eec is composed from few elements due to the solution resistance (rs), charge transfer resistance of the interfacial corrosion reaction (rct) and the constant phase element (cpe) due to double-layer impedance. impedance of cpe (zcpe) is defined as [19,20,32-34]: zcpe = qdl-1(j𝜔)-n (7) a b figure 3. (a) nyquist impedance spectra of mild steel corrosion in 0.02 m hcl in absence and presence of quinazoline derivative; (b) eec used for eis data fittings. zcpe is described by two frequency independent parameters qdl and n, j2 = -1 while 𝜔 is angular frequency of measurement (𝜔 2= πf, where f represents frequency in hz). qdl is related to doublelayer capacitance (cdl), and n is the shifting parameter from ideal capacitive response due to the nature of the solid (mild steel) surface. the electrode impedance parameter values (rct, qdl, n), obtained by fitting eec (figure 3b) to measured impedance data (figure 3a) are listed in table 4. 0 20 40 60 80 100 120 0 50 100 150 -z im a g in a ry / ῳ cm 2 zreal / ῳ cm 2 blank 50 mg/l 100 mg/l 200 mg/l 300 mg/l 500 mg/l http://dx.doi.org/10.5599/jese.887 j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 18 data in table 4 show that rct values, determined by the diameters of semicircles in figure 3a, increase with increase in the concentration of inhibitor, with a corresponding decrease of qdl (cdl) values. these observations are attributed to adsorption of quinazoline derivative molecules directly onto the metal surface, preventing thus metal dissolution [32,33]. table 4. electrochemical impedance parameter values of mild steel in 0.02 m hcl in absence and presence of different concentrations of quinazoline derivative inhibitor. cinh / mg l-1 rct /  cm2 qdl / 10-3 -1 sn cm-2 n ie / % blank 20.10 219 0.89 - 50 25.56 170 0.87 21.4 100 30.02 150 0.85 33.0 200 34.52 130 0.86 41.7 300 68.20 66 0.86 70.5 500 120.40 40 0.87 83.3 the values of n are in the range of 0.85 to 0.89, signifying non ideal capacitive character which can be attributed to heterogeneity of the metal surface. if n values are reduced gradually with increase in the inhibitor concentration, this would be due to adsorption of the studied molecule on the steel surface [6,35]. in eis measurements, the inhibition efficiency of quinazoline derivative is usually calculated using the following equation: ct(blank) ct(inh) ie 1 100 r r = − (8) where rct(blank) and rct(inh) are charge transfer resistances for blank and inhibitor solution, respectively. adsorption considerations the mode of adsorption of quinazoline derivative inhibitor molecules on the mild steel surface was examined by different adsorption isotherm models. the model that best fitted adsorption behavior of the studied inhibitor was langmuir adsorption isotherm, evaluated using the following equation: c/ = 1/kads + c (9) in eq. (9), c is inhibitor concentration in the bulk electrolyte,  is degree of surface coverage equal to ie/100, while kads is the equilibrium constant of adsorption-desorption process obtained from the reciprocal intercepts of straight lines in figure 4 [36]. figure 4. langmuir adsorption isotherm for quinazoline derivative inhibitor on mild steel surface in different concentrations of hcl solution. 0 100 200 300 400 500 600 700 800 900 1000 0 200 400 600 (c / ) / m g l -1 c / mg l-1 0.02 m 0.04 m 0.06 m 0.08 m f. e. abeng et al. j. electrochem. sci. eng. 11(1) (2021) 11-26 http://dx.doi.org/10.5599/jese.887 19 the change of gibbs free energy of adsorption (gads) of the studied inhibitor molecules on mild steel surface was evaluated using the equation [6,22]: δgads═ -2.303 rt log (55.5 kads) (10) the values of gads calculated by eq. (10) are listed in table 5 for each concentration of the acid used, at different temperature values (303, 313, 323 k). negative signs of gads are obtained, and the values of -δgads are less than -20 kj mol-1, which are within the range of values that supports the mechanism of physical adsorption [37-39]. table 5. δgads values for adsorption of the studied inhibitor on mild steel in different concentrations of hcl solution and temperature range of 303 -333k. t / k chcl / m 0.02 0.04 0.06 0.08 δgads / kj mol-1 303 -14.1 -15.2 -16.8 -18.6 313 -16.8 -17.2 -18.0 -19.7 333 -16.2 -15.0 -17.5 -18.5 computational chemistry study the molecular properties of the studied inhibitor, i.e. quinazoline derivative compound were examined by quantum chemical calculations in order to provide a theoretical background for the experimental results [40,41]. figure 5 shows the optimized molecular structure, the highest occupied molecular orbital (homo) and the lowest unoccupied molecular orbital (lumo), of the studied molecule. the mechanism of adsorption on the metal surface involves donation and acceptance of electrons between the molecule and metal surface. homo orbital is associated with donation of electrons, while lumo orbital is associated with acceptance of electrons [15,26,42]. a b c d figure 5. (a) optimized structure, (b) homo, (c) lumo, and (d) molecular dynamic simulation of the inhibitor molecule on fe surface. atom legend: white = h; gray = c; red = o; blue = n. the results of important quantum chemical parameters of the studied molecule are presented in table 6. these parameters are of primarily importance because they relate molecular species http://dx.doi.org/10.5599/jese.887 j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 20 toward adsorption [4,6,35,36]. the energy barrier that must be overcome before an electron moves from homo to lumo level is described by the energy gap (δe = elumo-ehomo). usually, lower energy gap (δe) indicates good adsorption ability and hence good inhibition efficiency, because lower activation energy will be required to move an electron from the lowest occupied orbital [43,44]. calculations of the quantum chemical parameters like ionization potential (ip) and electron affinity (ea) were done by following equations: ip = -ehomo (11) ea = -elumo (12) electronegativity () and the absolute hardness (η) values according to the theory of pearson are given by the equations: ie+ea 2  = (13) ie-ea 2  = (14) table 6. calculated values of quantum chemical parameters of the studied inhibitor. ehomo / ev elumo / ev δe / ev ip / ev ea / ev  / ev  / ev σ / ev-1 δn ω / ev  / d -5.377 -2.605 2.772 5.377 2.605 3.991 1.386 0.721 1.085 5.746 10.823 the inverse of global hardness gives global softness (σ): homo lumo 1 2 e e   = = − − (15) the fraction of electrons transfer (δn) from the inhibitor molecule to the metal surface is estimated using the equation: ( ) m inh m inh δ 2 n     − = + (16) where m and inh represent electro-negativity of metal and inhibitor, while ηm and ηinh are the absolute hardness of metal (fe atoms) and inhibitor, respectively. the theoretical value of m for bulk metal is given as 7 ev, while ηm = 0 ev. note also, that the electrophilicity index (𝜔) has recently been defined as [15,43,45]: 2 2    = (17) when δn> 0, electron transfer takes place from inhibitor molecule to the metal surface. also, the electron donating ability of an inhibitor molecule is increased when δn < 3.6 [42-44,46]. dipole moment (μ) is mostly used for the prediction of corrosion inhibition efficiency, because it measures the polarity of a bond and is related to the electron distribution in a molecule. the higher is the dipole moment of an inhibitor, the stronger is the dipole-dipole interaction of the inhibitor with metal surface. this also indicates that the dipole moment reflects in higher inhibition efficiency [46]. this discussion is in line with the present study, because all calculated quantum chemical parameter values of this study given in table 6, are within the acceptable range of values already reported for some excellent corrosion inhibitors [6,43-45]. quantum chemical parameters presented in table 6 are global reactivity descriptors that describe the system as a whole. for analyzing local selectivity of inhibitor molecule, other descriptors should be used. in principle, fukui function shows the regions that gain or lose electron upon nucleophilic or electrophilic attacks. fukui function itself is based on the fact that if n-electron f. e. abeng et al. j. electrochem. sci. eng. 11(1) (2021) 11-26 http://dx.doi.org/10.5599/jese.887 21 system gains electron, it becomes n+1 electron system, while n-1 electron system means electron loss. based on this concept, three types of fukui function are known, i.e. electrophilic fukui function (f -) when it loses electron, nucleophilic fukui function (f +) when it gains electron and fukui function for radical attack (f o) [6,46,47]. calculated values of condensed electrophilic, nucleophilic fukui functions and bond lengths calculated by dft method are listed in table 7. a b figure 6. fukui function of quinazoline derivative: (a) f-, (b) f +. the highest negative values of (f -) show the preferred site for electrophilic attack and this occurs in the nitro group of studied heteroatom compound shown in figure 6, while the preferred site for nucleophilic attack is in alkenes carbon atom c7 of the studied molecule shown in figure 6. during adsorption or bond formation, the electrons are expected to transfer from homo to lumo [46,47. generally, shorter bond length is less reactive than longer bond length and also, multiple bond length is more reactive than the single bond length. based on this principle the bond lengths of the studied quinazoline derivative are seen as their degree of reactivity and are bolded in table 7 [43,44]. table 7. dft calculated values of bond lengths, and fukui (f -) and fukui (f +) indices for the studied heterocyclic inhibitor compound. atom bond length, å f f + c1 c1-c2 1.398 -0.001 -0.001 c2 c2=c3 1.396 0.004 0.005 c3 c3-c4 1.398 0.007 0.009 c4 c4=c5 1.404 0.004 0.005 c5 c5-c6 1.506 -0.002 -0.003 c6 c5-c7 1.420 -0.001 -0.002 c7 c7-n8 1.441 -0.014 -0.023 n8 n8-c9 1.475 0.045 0.055 c9 c9-c10 1.520 -0.02 -0.021 c10 n8-c11 1.375 -0.007 -0.01 c11 c11-c12 1.469 0.007 0.043 c12 c12-c13 1.392 0.023 0.019 c13 c13-n14 1.429 0.011 0.025 n14 c12=c15 1.408 0.042 0.049 c15 c1=c7 1.401 0.005 0.019 c16 c15-c16 1.401 0.001 0.002 c17 c16=c17 1.409 0.007 0.077 c18 c17-c18 1.397 0.023 0.046 s19 c18=c13 1.399 0.065 -0.007 n20 c16-s19 1.814 0.063 0.018 o21 s19-n20 1.758 0.181 0.053 o22 s19=o21 1.546 0.17 0.051 o23 s19=o22 1.546 0.027 0.077 c24 c11=o23 1.220 0.04 0.122 http://dx.doi.org/10.5599/jese.887 j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 22 mechanism of corrosion inhibition the actual corrosion inhibition mechanism in acidic (hcl) medium by an inhibitor molecule occurs through the following modes: (a) electrostatic attraction between the charged molecules and charged metal, (b) interaction of uncharged electron pairs of the molecule with metal, (c) interaction of  electrons with metal, (d) combination of (a) and (c) [48]. the inhibitor molecule may block anodic, cathodic, or both sites through the process of adsorption, preventing anodic metal dissolution and corresponding cathodic reaction (such as hydrogen evolution reaction produced when a metal is immersed in hcl solution). a small degree of hydration present in chloride ions might draw excess negative charges around the metal interface and promote more positively charged molecules of the inhibitor. the synergy between adsorbed chloride ions and quinazoline is lead to the adsorption process; quinazoline can be adsorbed on the metal surface, requiring the displacement of water molecules from the mild steel surface and the exchange of electrons between the atoms of nitrogen or oxygen and iron. the adsorption of the inhibitor molecule on the mild steel surface is due to the donor-acceptor interactions between π-electrons of the aromatic ring and the vacant d orbital of iron, figure 7 shows a schematic diagram of the proposed inhibition adsorption mechanism of quinazoline on mild steel in the solution of hcl. generally, corrosion of mild steel in the blank hcl solution is initially slowed down due to hcl ionization and formation of chloride ions which adsorb on the surface. the corrosion mechanism that has already been suggested for anodic iron dissolution in hcl solution can be described as 49,50: figure 7. schematic representation of adsorption modes of quinazoline derivative molecule at mild steel surface. fe + cl→ (fecl-)ads (18) (fecl-) ads → (fecl)ads + e(19) (fecl)ads + e→ fecl+ + e(20) fecl+ + e→ fe2+ + cl(21) anodic dissolution in hcl solution is accompanied by cathodic hydrogen evolution reaction according to the following reaction pathway [49,50]: fe + h+ → (feh+)ads (22) (feh+)ads + e→ (feh)ads (23) (feh)ads + h+ + e→ fe + h2 (24) f. e. abeng et al. j. electrochem. sci. eng. 11(1) (2021) 11-26 http://dx.doi.org/10.5599/jese.887 23 in acidic solution, inhibitor molecules can be protonated easily. physical adsorption may take place due to electrostatic interaction between protonated inhibitor molecule and negatively charged chloride species already present at the metal surface, while chemical adsorption is related to donor-acceptor interactions between inhibitor and low energy d-orbital of metal (fe). comparison of inhibition efficiency obtained by different methods inhibition efficiencies (ie) obtained from weight loss (wl), potentiodynamic polarization (pdp) and electrochemical impedance spectroscopy (eis) measurements are compared in figure 8, showing good correlation of the obtained results. figure 8. comparison of inhibition efficiencies (ie) obtained by different methods for mild steel in 0.02 m hcl at different concentrations of quinazoline derivative. the results obtained by all three techniques showed the same trends in the variation of corrosion rate and inhibition efficiency with increase of inhibitor concentration. certain differences observed in the results obtained from weight loss and electrochemical techniques may be attributed to the specificity of each particular technique. thus, weight loss measurements give average corrosion rates, while electrochemical measurements give instantaneous corrosion rates [51]. in table 8, corrosion inhibition performance of some other drugs already applied for mild steel corrosion protection [2631,52], are compared. it can be observed from data in table 8 that quinazoline, which is a derivative of metolazone drug, provides a reasonable corrosion inhibition efficiency behavior, compared to other drugs tested for corrosion inhibition of mild steel. table 8. comparative chart listing performances of some other drugs acting as corrosion inhibitors of mild steel. drug name electrolyte mode of adsorption optimum ie at inhibitor concentration ref. norfloxacin 0.1 m h2so4 langmuir adsorption isotherm 95.8 % at 0.5 g/l [26] metronidazole 0.5 m hcl mixed-type inhibitor / langmuir adsorption isotherm 80.0 % at 400 ppm [27] cephalexin 0.1 m hcl langmuir adsorption isotherm 80 % at 13.0×10-3 m [28] chloroquine diphosphate 0.1 m hcl langmuir adsorption isotherm 80 % at 4.65×10-4 m [29] tramadol 0.5 m hcl mixed-type inhibitor / langmuir adsorption isotherm 82.6 % at 2.16×10-3 m [30] dapsone 1 m hcl langmuir adsorption isotherm 93.3 % at 400 ppm [31] quinazoline derivative (metolazone) 0.02 m hcl mixed-type inhibitor/ langmuir adsorption isotherm 83.3% at 500 mg/l present study 0 10 20 30 40 50 60 70 80 90 50 100 200 300 500 ie , % concentration, mg l-1 wl pdp eis http://dx.doi.org/10.5599/jese.887 j. electrochem. sci. eng. 11(1) (2021) 11-26 corrosion inhibitor potential of quinazoline derivative 24 scanning electron microscopy (sem) the level of mild steel degradation was scrutinized using scanning electron microscopy (sem) test. as shown in figure 9, mild steel in the blank hcl solution exhibits visible and dark pits due the corrosive nature of acid (figure 9a), while the surface nature of the steel in the inhibited media shows a smooth and bright surface compared to the blank sample (figure 9b). this phenomenon of improved smoothness is attributed to adsorption of the active molecules of quinazoline derivative on the metal surface [50-53]. a b figure 9. sem images of mild steel surface after immersion period of 24 hours in: (a) uninhibited hcl solution and (b) inhibited hcl solution. conclusions a new insight into corrosion inhibition of mild steel in hcl solution by a novel drug molecule metolazone, a quinazoline derivative (chloro-2-methyl-4-oxo-3-o-tolyl-1,2,3,4-tetrahydroquinazoline-6-sulfonamide), was provided using weight loss, electrochemical and theoretical studies. the results obtained using different experimental techniques were found to be in good mutual agreement, and comparable with literature data on different drug molecules already tested for mitigation of mild steel corrosion. it was found that the studied quinazoline derivative behaves as a mixed type inhibitor, while its inhibition efficiency increases with increased concentration of the inhibitor. corrosion inhibition is realized by adsorption of quinazoline derivative on mild steel surface obeying langmuir adsorption isotherm. the negative sign of the free energy change of adsorption indicated that adsorption of the studied compound on mild steel surface is a spontaneous process proceeding by the physical adsorption mechanism. this was supported by the values of other thermodynamic corrosion parameters (activation energy, activation enthalpy and activation entropy) that suggest slowing down of corrosion by spontaneous adsorption of inhibitor. the computational quantum chemistry studies confirmed that the studied molecule is adsorbed on the metal surface through nitro functional groups. also, a pronounced effect of inhibitor adsorption on the mild steel surface in hcl solution was confirmed by sem images, showing surface smoothness being much 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[53] f. e. abeng, v. c. anadebe, v. d. idim, m. m. edim, south african journal of chemistry 73 (2020) 125-130. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.cdc.2019.100209 https://doi.org/10.1016/j.cdc.2019.100209 https://doi.org/10.1016/j.surfin.2020.100478 https://creativecommons.org/licenses/by/4.0/) {polymethionine modified carbon nanotube sensor for sensitive and selective determination of l-tryptophan} http://dx.doi.org/10.5599/jese.774 305 j. electrochem. sci. eng. 10(4) (2020) 305-315; http://dx.doi.org/10.5599/jese.774 open access: issn 1847-9286 www.jese-online.org original scientific paper polymethionine modified carbon nanotube sensor for sensitive and selective determination of l-tryptophan nambudumada s. prinith and jamballi g. manjunatha department of chemistry, fmkmc college, madikeri, mangalore university constituent college, karnataka, india corresponding author: manju1853@gmail.com;tel.: +91-08272228334 received: april 1, 2020; revised: may 11, 2020; accepted: may 11, 2020 abstract the electrochemically initiated catalytic oxidation of amino acid l-tryptophan (l-tpn) in phosphate buffer solution has been scrutinized using highly conductive polymethionine modified carbon nanotube paste sensor (pmetcntps) through cyclic voltammetry (cv) technique. compared to the bare carbon nanotube paste sensor (bcntps), pmetcntps exhibited a quantifiable current signal by cv method. pmetcntps was found sensitive to ltpn concentrations within the linear segment of detection range 1.5 8.0×10-5 m. by employing the calibration plot, the detection limit was determined as 6.99×10-7 m. in addition, pmetcntps was successfully exploited and validated in determining l-tpn in the pharmaceutical supplement. keywords electrochemical sensor; tryptophan; electropolymerization; cyclic voltammetry. introduction amino acids play a substantial role in biotic life and are necessary for human beings as a source of energy and for building up the proteins. tryptophan (l-tpn) is an essential proteinogenic amino acid responsible for serving as a precursor for the neurotransmitter drug serotonin, melatonin hormone and nicotinic acid [1,2]. therefore, the intake of l-tpn is important for humans because it has low storage in tissues and total concentration of l-tpn in the body is the least compared to all other amino acids. the small quantity of l-tpn which is necessary for human nutrition can be supplied from foods such as bananas, oats, tuna fish, milk, chocolates, dried prunes, chicken, peanuts, bread, cheese, turkey, supplements, and pharmaceutical formulations [3]. l-tpn has a major impact on the brain, since if not properly metabolized, a waste toxic product would be formed, leading to delusions and hallucinations [4]. it has already been pointed out that depletion of l-tpn levels is accompanied by weight gain, affecting the growth of kids, anxiety, chronic http://dx.doi.org/10.5599/jese.774 http://dx.doi.org/10.5599/jese.774 http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 10(4) (2020) 305-315 modified carbon nanotube sensor 306 insomnia, depression and pellagra [5–8]. at the other side, high intake of l-tpn leads to eosinophilia-myalgia syndrome [6]. therefore, the availability of rapid, facile, and sensitive methods to efficiently determine the biomarker is necessary and of great significance in diagnostic aids. a number of methods was already established to determine l-tpn, such as spectroscopy detection [9], fluorometric technique [10], high-performance liquid chromatography [11], capillary electrophoresis [12], chemiluminescence [13] and electroanalytical techniques [14–16]. the first five techniques provide accurate results but contain limitations due to poor sensitivity, nonspecificity, expensive equipment, grosseness, overlong time process, and waste of high amounts of solvents for the cleaning process. all these drawbacks impede applicability of these methods significantly. electroanalytical methods, however, are considered more appropriate for the determination of l-tpn, due to their low-priced equipment, high sensitivity, instantaneous response and easy and comfortable handling. among convenient electrode materials, carbon nanotubes (cnts) have already demonstrated a significant role in biomedical, biosensing and electrochemical sensing areas. cnts have peculiar tubular nanostructure, extraordinary chemical and electronic features, high conductivity, wide specific surface area, and possibility to alter sidewalls and ends by attaching functional groups. all these features contribute to high sensitivity and good compatibility of cnts used as sensor electrodes. the sidewalls and ends of cnts modified with conducting polymers have a pivotal role in recent years. the synergetic effect of two materials can enhance sensitivity, selectivity, transducing feature, biocompatibility and anti-fouling property [17–19]. in the current work, we have exploited the best features of both materials by fabricating the polymer film on a carbon nanotube sensor surface for improving the catalytic effect for the determination of l-tpn. electro-polymerization of the monomer l-methionine (l-met), an amino acid holding sulphur atoms in its structure, is utilized for the preparation of polymethionine carbon nanotube paste sensor (pmetcntps). the proposed sensor is applied for the electrochemical characterization of l-tpn and for its selective sensing in the presence of dopamine (da). the applicability of pmetcntps is also validated in determining l-tpn in a pharmaceutical capsule. experimental instrumentation cyclic voltammetry studies were carried out with the ch-instrument model chi-6038e (usa), associated with a standard three-electrode system and desktop for data storage. the unmodified and polymer-modified sensors were placed as the working electrode(s), while the saturated calomel electrode (sce), and pt wire served as reference and auxiliary electrode, respectively. in the present work, the current measurements were considered along with the background current. all the experimental activities were performed at room temperature. chemicals and chemical reagents l-tryptophan (l-tpn), l-methionine (l-met) and orthophosphoric acid were procured from molychem, mumbai, india. dopamine hydrochloride (da) was bought from sigma aldrich, and silicon oil from nice company, cochin, kerala. multiwalled carbon nanotubes (cnts) (length of 1030 µm and od 30-50 nm) were purchased from sisco research laboratory pvt. ltd., maharashtra, india. all chemicals were of analytical grade and utilized without any purification. 25×10-4 m l-tpn, 25×10-4 m da, and 25×10-3 m methionine stock solutions were prepared in deionized water. 0.1 m phosphate buffer solutions (pbs) of different ph values were prepared by mixing adequate volumes s. prinith and g. manjunatha j. electrochem. sci. eng. 10(4) (2020) 305-315 http://dx.doi.org/10.5599/jese.774 307 of 0.1 m disodium phosphate and 0.1 m monosodium phosphate and adjusted to acidic ph by 0.1 m orthophosphoric solution. development of bcntps bcntp sensor was prepared by employing cnt powder and silicon oil with the proportion of 60:40 (w/w) and blended in an agate mortar to achieve a perfect homogenized paste. a portion of this homogenized paste was filled into the cavity (3 mm in diameter) at the tip of teflon tube, and smoothed out the surface by rubbing it on the tissue paper. results and discussion electro-deposition of polymethionine on bcntps surface application of amino acid for electro-polymerization has a significant role in the modification of electrode surface [20]. methionine is α-amino acid containing sulphur atom which extends the electrode feature for sensing of electroactive species [21–25]. the electropolymerization of polymethionine on bcntps surface can be achieved easily by cv technique. the electrochemical deposition of polymethionine was developed in 0.1 m pbs of ph 7.0, containing 1 mm methionine. continuous potential was sweeped by 15 cycles within the potential domain -1.2 v to 1.6 v at the scan rate of 0.1 v s-1 (figure 1). after finishing of electro-polymerization, the fabricated sensor was washed gently with double distilled water to take off the remnants of unreacted methionine. thus prepared sensor electrode is depicted as pmetcntps. figure 1. cyclic voltammograms of electro-polymerization of 1 mm l-met in 0.1 m pbs of ph 7.0 at bcntps for 15 cycles at the sweep rate of 0.1 v s-1. fe-sem analysis of bcntps and pmetcntps understanding the morphology of sensing electrodes is vital as it has an impact on the kinetics of the sensor reaction, electroactive surface area, etc. surface morphology was scanned using fe-sem and presented in figure 2. as depicted in figure 2a, the surface of bcntps appears to have bundles of irregular tubular layouts. for pmetcntps, however, the surface morphology was changed significantly due to the physisorption of polymethionine on carbon nanotube paste sensor surface. it is noticeable from fe-sem image in figure 2b that pmetcntps surface is uniformly coated with polymethionine with the external average diameter of the nanotubes in the range of 51-56 nm. this http://dx.doi.org/10.5599/jese.774 j. electrochem. sci. eng. 10(4) (2020) 305-315 modified carbon nanotube sensor 308 suggests that surface roughness was increased drastically after coating with polymethionine that is uniformly covering the surface of bcntps and leading to electrostatic interaction between pmetcntps and l-tpn molecules. the surface formed is highly porous possessing wide surface area and enhancing the conductivity of pmetcntps. figure 2. fe-sem images for (a) bcntps and (b) pmetcntps assessment of surface area for bcntps and pmetcntps it is necessary to evaluate the active surface area of the electrode that is the referent electrode parameter responsible for the reaction rate. for this purpose, cv method was applied using the charged anionic ferrocyanide, available as the potassium salt k4fe(cn)6, and its redox reaction as a probe. generally, thin film of polymer has the ability to attract and bind the multiple charged complex molecules. it is observed in figure 3 that compared to bcntps, pmetcntps produced more prominent and quantifiable oxidation and reduction peaks of fe(cn)63-/4redox couple.the electrochemical active surface area was evaluated by employing randles – sevcik equation [26]: ip = 2.65×105 n3/2 a d½ ν½ c (1) figure 3. cyclic voltammograms (0.1 v s-1) of 1.0 mm k4fe(cn)6 in 0.1 m kcl at bcntps and pmetcntps. electrochemical activity of pmetcntps for determination of l-tpn in order to compare electrochemical properties of pmetcntps and bcntps for determining 0.1 mm l-tpn in 0.1 m pbs of ph 4.0, cv technique was employed, and the results are displayed in figure 4. a poor broad oxidation peak at 0.952 v bearing low current is spotted for l-tpn at bcntps s. prinith and g. manjunatha j. electrochem. sci. eng. 10(4) (2020) 305-315 http://dx.doi.org/10.5599/jese.774 309 which can be associated with slow electron transfer. at pmetcntps, however, the response for l-tpn is enhanced with peak potential shifted negatively to 0.916 v and with 6-fold increase of the current response. this proves that the polymer fabricated electrode, possessing a great surface area, exhibits the effective electrocatalytic phenomenon to the oxidation of l-tpn. it is worth noting that in the reverse sweep the reduction peak is absent, signifying the electrochemical behavior of ltpn as completely irreversible. further, to detect electrochemical sensitivity of pmetcntps to l-tpn, cvs were recorded in the presence and absence of l-tpn, and the results are depicted in figure 4. obviously, pmetcntps is very specific in detecting l-tpn, showing prominent anodic peak of high current. note that in absence of l-tpn, there is no anodic peak observed at all. figure 4. cvs (0.1 v s-1) in presence of 0.1 mm l-tpn in 0.1 m pbs of ph 4 at bcntps and pmetcntps, and for absence of l-tpn at pmetcntps. optimization of experimental parameters impact of ph of the supporting electrolyte generally, the ability of the electrode surface to protonate or deprotonate depends on the variation of ph value, what could affect the capability of an electrode in regulating the electrochemical reaction. therefore, the optimization of ph was examined by cv experiments at pmetcntps in 0.1 m pbs for different ph ranging from 3.0 to 7.0. the results presented in figure 5a and particularly figure 5b illustrate clearly that the maximum current is attained at ph 4.0 and declined gradually with further ph increase. thus, ph 4.0 of 0.1 m pbs was chosen and set for subsequent voltammetry experiments. as perceptible in figure 5a, the anodic peak is shifted towards negative potential by changing ph from 3.0 to 7.0, conveying the involvement of protons in the oxidation of l-tpn. the equation estimated from the plot of anodic peak potential (epa) versus ph shown in figure 5c is: epa= 1.1026 – 0.0454 ph (r = 0.976) (2) the slope estimated from eq. (2) is near to nernst value of 58.5 mv/ph, demonstrating that the electrochemical reaction of l-tpn carries two electrons and two protons process. http://dx.doi.org/10.5599/jese.774 j. electrochem. sci. eng. 10(4) (2020) 305-315 modified carbon nanotube sensor 310 figure 5. (a) cvs (0.1 vs-1) of 10 -4 l-tpn in 0.1 m pbs of different ph 3.0 -7.0 at the surface of pmetcntps; (b) triangular pyramid bar chart depicting ipa for different ph; (c) graphical representation between epa and ph impact of sweep rate the mechanism of electrochemical reaction involved could be attained from the affiliation of peak current and sweeping rate. the impact of different sweep rates was studied for l-tpn, employing cv experiments. it can be noticed in figure 6a that the current peak increased with increasing sweep rate. figure 6. (a) cvs at pmetcntps for 0.1 mm l-tpn in 0.1 m pbs of ph 4.0 at different potential scan rates (0.1, 0.125, 0.15, 0.175, 0.2, 0.225 v s-1); (b) plot of ipa vs. scan rate; (c) plot of ipa vs. square root of scan rate; (d) plot of log ipa vs. log  s. prinith and g. manjunatha j. electrochem. sci. eng. 10(4) (2020) 305-315 http://dx.doi.org/10.5599/jese.774 311 the linear dependence between peak current and sweep rate for l-tpn shown in figure 6b was evaluated as ipa= -3.48 + 520.4  [r = 0.988]. similar is valid for the peak current dependence on the square root of sweep rate for l-tpn presented in figure 6c, and evaluated to be ipa = -85.54 + 418.64  ½ [r = 0.994]. to differentiate between possible adsorption or diffusion controlled process, the plot of log (ipa) and log  was evaluated in figure 6d and the corresponding equation is calculated to be: log ipa= -3.23269 + 1.08 log  (r = 0.992) (3) the slope of the equation (3) is 1.08 which is almost equal to the theoretical value of 1, being characteristic for the adsorption-controlled process [27–29]. this confirms that the oxidation of ltpn is adsorption-controlled process. the number of electrons involved in irreversible oxidation of l-tpn was calculated using laviron expression [30,31]: ep = e 0 + (rt/αnf) ln (αnf/rtks) + (rt/αnf) ln  (4) herein, ep, e 0, r, t, f, α, n, and ks are peak potential, formal potential, gas constant, temperature, faraday constant, electron transfer coefficient, number of electrons involved, and heterogenous rate constant. the linear equation estimated from the plot of anodic peak potential (epa) versus ln (not shown in the figure) is defined as: epa = 1.0347 + 0.05525 ln  (r= 0.992) (5) using the slope value of 0.05525 and α assumed to be 0.5, the number of electrons was calculated to be 2.14. this implies that the electrocatalytic oxidation of l-tpn at pmetcntps exhibits a twoelectron transfer phenomenon. e0 was estimated as the intercept by plotting the graph epa vs. for  = 0. ks estimated from the intercept of eq. (5) is 2.18 s-1. the surface concentration (г) of pmetcntps was evaluated to be 3.764×10-9 mol/cm2 using the following equation [32]: г = q / nfa (6) in eq. (6), q is charge evaluated by integration of peak area in cv data as 1.778×10-5 a h, n is number of electrons, f is faraday constant (96 485.33 c mol-1) and a is surface area of pmetcntps (0.03 cm2). impact of number of electro-polymerization cycles of polymethionine on pmetcntps in order to figure out the impact of the number of potential cycles applied during electropolymerization of polymethionine on the anodic peak current of l-tpn, current responses at pmetcntps formed by different numbers of cycles of electro-polymerization were recorded. cvs presented in figure 7a show that as the number of polymerization cycles increased from 5 to 15. figure 7. (a) cvs (0.1 v s-1) for 0.1 mm l-tpn in 0.1 m pbs of ph 4.0 at pmetcntps formed by different number of electro-polymerization cycles on cntps; (b)typical conical chart representation of ipa vs. number of polymerization cycles http://dx.doi.org/10.5599/jese.774 j. electrochem. sci. eng. 10(4) (2020) 305-315 modified carbon nanotube sensor 312 the current peak was firstly enhanced dramatically, while for more than 15 cycles, the current peak was declined (figure 7b). this might be related to the increase of the polymer film thickness, resulting in the hindrance of charge transfer at the surface of the electrode. concentration variation of l-tpn at pmetcntps for the quantitative determination of l-tpn at pmetcntps, cyclic voltammetry was also employed. the voltammograms unveiled the linear dependence between peak current increase and increase of l-tpn concentration. as observed in figure 8, two linear segments were obtained in the range of 2 – 10 m and 15 80 m, respectively. here, the second linearity was chosen for the further analysis as it has the wide linear segment range described by the following regression equation: ipa = 36.37258 + 0.60017 ctpn (r = 0.997) (7) figure 8. calibration plot at pmetcntps showing anodic peak current vs. concentration of l-tpn detection limit (lod) and quantification limit (loq) were estimated to be 0.699 m and 2.33 m. lod and loq values were estimated applying the following equations[19,29]: lod = 3sd / m; loq = 10 sd / m (8) in eq. (8), sd is standard deviation evaluated from the blank and m is the slope of the calibration plot. lod values and linear ranges of concentration of l-tpn already obtained for various electrodes are compared in table 1, suggesting high efficiency of the present pmetcntp sensor. table 1. comparison of efficacy of pmetcntps with other reported sensors for determination of l-tpn modified sensor methods linear range, m detection limit, m ref. 1 cpe cv dpv 29.1 – 82.6 2.06 30 4.66 1.66 [33] 2 bdd dpv 20 -1000 10 [34] 3 gnp/cile swv 5 900 4 [35] 4 cile cv 8-1000 4.8 [36] 5 pmetcntps cv 15 80 0.69 this work cpe: carbon paste electrode; bdd: boron-doped diamond electrode; gnp/cile: gold nanoparticle – modified carbon ionic liquid electrode; dpv: differential pulse voltammetry; swv: square wave voltammetry. s. prinith and g. manjunatha j. electrochem. sci. eng. 10(4) (2020) 305-315 http://dx.doi.org/10.5599/jese.774 313 repeatability, reproducibility, and stability of pmetcntps the stability was inspected for repetitive 25 cycles for 0.1 mm l-tpn in 0.1 m pbs of ph 4.0 at the fabricated pmetcntps sensor. the resulting percentage of degradation is calculated by applying the following expression [33]: degradation, % = (ip(n) / ip(1)) 100 (9) in eq. (9), ip(n) is anodic peak current measured in nth measurement, while the first measured anodic peak current is labeled as ip(1). the calculated percentage of degradation is 1.05, suggesting high stability of the fabricated sensor electrode. the reproducibility and repeatability of pmetcntps were estimated as 4.3 and 1.6 %, respectively. simultaneous determination of l-tpn in presence of another electroactive species dopamine (da) is a neurotransmitter drug and one of the happy hormones. l-tpn is the precursor for another happy hormone and the neurotransmitter drug, serotonin. therefore, da and l-tpn coincide in the extra-cellular fluid of the central nervous system and serum. here, the fabricated working sensor pmetcntps was inspected for the simultaneous determination of these two electroactive species. it is observed and portrayed in figure 9 that by showing low and broad peak, bcntps clearly failed in simultaneous determining of two electroactive species. on the contrary, bcntps modified by polymethionine presents two well-separated and enhanced current signals for both da and l-tpn. this proves that pmetcntps is highly selective sensor. figure 9. cvs (0.1 v s-1) for the mixture of 0.1 mm l-tpn and 0.1 mm da in 0.1 m pbs of ph 4.0 at bcntps and pmetcntps. analysis of l-tpn capsule the practical applicability of pmetcntps was evaluated for the commercial medicinal capsule of l-tpn from advance nutratech. the amount of l-tpn in the capsule was labeled as 500 mg. the solution of specified concentration was prepared by a powdered capsule and examined by cv method. a known volume of l-tpn capsule solution was diluted with 0.1 m pbs of ph 4.0. the standard addition method was utilized for examining the pharmaceutical sample. the results are summarized in table 2, showing also high percentage recovery within the range of 97.5 to 98.6 %. http://dx.doi.org/10.5599/jese.774 j. electrochem. sci. eng. 10(4) (2020) 305-315 modified carbon nanotube sensor 314 table 2. application of pmetcntps for detection of l-tpn in l-tpn capsule. sample sample no. added l-tpn, m found l-tpn, m recovery, % l -tryptophan capsule 1 5.2 5.096 98 2 10.4 10.25 98.6 3 15.6 15.21 97.5 conclusions a new sensor electrode is prepared by electro-polymerization of l-methionine on carbon multiwalled nanotube paste surface (pmetcntps) and successfully applied for the determination of l-tryptophan (l-tpn). compared to the basic electrode, cyclic voltammetry results of the fabricated pmetcntps revealed increased current and negative shift of anodic peak, demonstrating good electrocatalytic activity toward electro-oxidation of l-tpn. the corresponding electrochemical reaction was found irreversible, carrying two-electron and two-proton transfer and controlled by the adsorption process. cyclic voltammograms measured under optimal experimental conditions, showed good linear dependence between oxidation peak current and concentration of l-tpn within wide linear range of 15 m – 80 m and possessing the detection limit of 0.69 m. the developed pmetcntps showed high sensitivity, high selectivity, and high specificity when l-tpn was measured in presence of dopamine (da). the projected sensor electrode disclosed high percentage recovery which may be applied in drug formulations and pharmacokinetics study. acknowledgements: we convey our gratitude to the vgst, bangalore under research project no.ksteps/vgst-kfist(li)2016-2017/grd-559/2017-18/126/333, 21/11/2017 for their financial support. references [1] m. kooshki, h. abdollahi, s. bozorgzadeh, b. haghighi, electrochimica acta 56 (2011) 8618–8624. 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[36] a. safavi, s. momeni, electroanalysis 22 (2010) 2848–2855. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.774 http://doi.org/10.3390/chemosensors/7020024 http://creativecommons.org/licenses/by/4.0/) novel electroanalysis of hydroxyurea at glassy carbon and gold electrode surfaces doi: 10.5599/jese.2014.0064 111 j. electrochem. sci. eng. 4(3) (2014) 111-121; doi: 10.5599/jese.2014.0064 open access: issn 1847-9286 www.jese-online.org original scientific paper novel electroanalysis of hydroxyurea at glassy carbon and gold electrode surfaces keerti m. naik and sharanappa t. nandibewoor p. g. department of studies in chemistry, karnatak university, dharwad 580 003, india corresponding author: e-mail: stnandibewoor@yahoo.com, tel.: +91 836 2770524; fax: +91 836 2747884 received: july 27, 2014; revised: september 15, 2014; published: september 22. 2014 abstract a simple and a novel electroanalysis of hydroxyurea (hu) drug at glassy carbon and gold electrode was investigated for the first time using cyclic, linear sweep and differential pulse voltammetric techniques. the oxidation of hu was irreversible and exhibited a diffusion controlled process on both electrodes. the oxidation mechanism was proposed. the dependence of the current on ph, the concentration, nature of buffer, and scan rate was investigated to optimize the experimental conditions for the determination of hu. it was found that the optimum buffer ph was 7.0, a physiological ph. in the range of 0.01 to 1.0 mm, the current measured by differential pulse voltammetry showed a linear relationship with hu concentration with limit of detection of 0.46 µm for glassy carbon electrode and 0.92 µm for gold electrode. in addition, reproducibility, precision and accuracy of the method were checked as well. the developed method was successfully applied to hu determination in pharmaceutical formulation and human biological fluids. the method finds its applications in quality control laboratories and pharmacokinetics. keywords hydroxyurea; voltammetry; glassy carbon electrode; gold electrode; electroanalysis; oxidation introduction synthesis, chemical analysis and testing of the drugs are important events in pharmaceutical laboratories. the many of the essential drugs has been reported by previous workers, which requires the development and validation of analytical methods for their analysis. the requirements of the quality of the drugs have increased tremendously due to the regulations led by regulatory departments like fda and ich. conventional methods have some shortcomings like time consumption and the use of costly and hazardous chemicals. this in turn motivated the analysts to develop and establish newer and faster methods of analysis of the quality of the drug http://www.jese-online.org/ mailto:stnandibewoor@yahoo.com j. electrochem. sci. eng. 4(3) (2014) 111-121 electroanalysis of hydroxyurea at gc and au electrode 112 substance and drug products. hydroxyurea (hu), the simplest, 1-carbon organic antitumor agent, is a member of the substituted urea group and is chemically known as hydroxycarbamide [1]. in 1981 it was reported to have antineoplastic activity against sarcoma [2]. presently, the primary role of hydroxyurea (scheme 1) in chemotherapy is the management of granulocytic leukemia and thrombocytosis. it has been used in combination with radiotherapy for carcinomas of the head and neck [3]. hu is used in the treatment of cancer [4], sickle cell anemia [5] and infection with the human immunodeficiency virus (hiv) [6]. hu is a potent, nonalkylating myelosuppressive agent that inhibits dna synthesis [7]. scheme 1. structural formula of hydroxyurea. only a few analytical procedures have been reported for the determination of hu. nuclear magnetic resonance spectroscopy [8], liquid chromatographic (lc) procedures have been recommended by the u.s. pharmacopeia [9] and others [10-12] for determination of hydroxyurea in pharmaceutical formulations and biological fluids. capillary gas chromatography (gc) with thermionic (n-p) specific detection has also been reported [13]. main problems encountered in using such methods are either the need for derivatization or the need for time-consuming extraction procedures. electrochemical methods may offer certain advantages, such as easier sample preparation, being less time-consuming and offering detection limits and dynamic range comparable to other analytical methods [14,15]. these methods have proven useful for the development of very sensitive and selective methods for the determination of organic molecules including drugs. redox properties of drugs can give insights into their metabolic fate or their in vivo redox processes or pharmaceutical activity [16]. electrochemical methods, especially differential pulse voltammetry (dpv) make it possible to decrease the analysis time as compared to the time exhaustive chromatographic methods [17]. the advantages of dpv over other electroanalytical techniques are greater speed of analysis, lower consumption of electroactive species in relation to the other electroanalytical techniques, and fewer problems with blocking of the electrode surface. to the best of our knowledge, there is no report on the electroanalytical method for the determination of hu using glassy carbon (gce) and gold (ge) electrodes until now. the aim of this study is to establish the suitable experimental conditions, to investigate the voltammetric behavior and oxidation mechanism of hu at gce and ge by cyclic, linear sweep and differential pulse voltammetric methods for the direct determination of hu in real samples like pharmaceuticals and human biological fluids. experimental reagents and chemicals hydroxyurea (hu) was obtained from sigma aldrich and used without further purification. a stock solution of hu (1.0 mm) was prepared in water and stored in a refrigerator at 4 o c. standard working solutions were prepared by diluting the stock solution with the selected supporting k. m. naik et al. j. electrochem. sci. eng. 4(3) (2014) 111-121 : 10.5599/jese.2014.0064 113 electrolyte. the phosphate buffers from ph 3.0 – 10.4 were prepared according to the method of christian and purdy [18]. the hu containing pharmaceutical product, hydrox-l, was purchased from a local pharmacy. other reagents used were of analytical grade. all solutions were prepared with millipore water. instrumentation electrochemical measurements were carried on a chi 630d electrochemical analyzer (ch instruments inc., usa). the voltammetric measurements were obtained in a 10 ml single compartment three-electrode glass cell with ag/agcl as a reference electrode, a platinum wire as counter electrode and a 2-mm diameter glassy carbon electrode and gold electrode as working electrodes. all the potentials are given against the ag/agcl (3 m kcl). ph measurements were performed with elico li120 ph meter (elico ltd., india). all experiments were carried at an ambient temperature of 25 ± 0.1 o c. analytical procedure polishing of the glassy carbon electrode (gce) and gold electrode (ge) was done on microcloths (buehler) glued to flat mirrors. al2o3 (0.3 μm) was used for polishing before each experiment. before transferring the electrode to the solution, it was rinsed thoroughly with doubly distilled water. after this mechanical treatment, the gce and ge were placed in 0.2 m phosphate buffer solution, and various voltammograms were recorded until a steady-state baseline voltammogram was obtained. the parameters for differential pulse voltammetry (dpv) were initial potential: 0.0 v; final potential: 1.2; increase potential: 0.004 v; amplitude: 0.05 v; frequency: 15 hz; quiet time: 2 s; sensitivity: 1 × 10 -5 a/v. area of the electrodes the area of the electrode was obtained by the cyclic voltammetry method using 1.0 mm k3fe(cn)6 as a probe at different scan rates. for a reversible process, the following randles-sevcik formula can be used [19]. ipa = 0.4463(f 3 / rt) 1/2 n 3/2 ad0 1/2 c0 υ 1/2 (1) where ipa refers to the anodic peak current, n is the number of electrons transferred, a is the surface area of the electrode, d0 is diffusion coefficient, υ is the scan rate, and c0 is the concentration of k3fe(cn)6. for 1.0 mm k3fe(cn)6 in 0.1 m kcl electrolyte, t = 298k, r = 8.314 j k -1 mol -1 , f = 96480 c mol -1 , n = 1, d0 = 7.6×10 -6 cm 2 s -1 , then from the slope of the plot of ipa vs. υ 1/2 relation, the electroactive area was calculated. in our experiment the slope was 2.46×10 -6 μa (v s -1 ) -1/ 2 and 2×10 -5 μa (v s -1 ) -1/ 2 and the area of electrodes were calculated to be 0.033 cm 2 and 0.0269 cm 2 for gce and ge. sample preparation two pieces of hu containing tablets were weighed and ground to a homogeneous fine powder in a mortar. a portion equivalent to a stock solution of a concentration of about 1.0 mm was accurately weighed and dissolved in water. the contents were sonicated for 20 min to affect complete dissolution. the excipient was separated by filtration and the residue was washed three times with water. the filtrate was diluted to 1.0 mm. appropriate solutions were prepared by taking suitable aliquots from this stock solution and diluting them with the phosphate buffer solutions. each solution was transferred to the voltammetric cell. the differential pulse j. electrochem. sci. eng. 4(3) (2014) 111-121 electroanalysis of hydroxyurea at gc and au electrode 114 voltammograms were subsequently recorded following the optimized conditions. the content of the drug in tablet was determined referring to the calibration graph or regression analysis. to study the accuracy of the proposed method and to check the interferences from excipients used in the dosage form, recovery experiments were carried out. the concentration of hu was calculated using standard addition method. plasma sample preparation human blood samples were collected in dry and evacuated tubes (which contained saline and sodium citrate solution) from a healthy volunteer. the samples were handled at room temperature and were centrifuged for 10 min at 1500 rpm for the separation of plasma within 1 h of collection. the samples were then transferred to polypropylene tubes and stored at 20 °c until analysis. the plasma samples, 0.2 ml, were deproteinized with 2 ml of methanol. after vortexing for 15 min, the mixture was then centrifuged for 15 min at 6000 rpm, and supernatants were collected. the supernatants were spiked with known amounts of hu. appropriate volumes of this solution were added to phosphate buffer as supporting electrolyte and the voltammograms were then recorded. results and discussion cyclic voltammetric behavior of hydroxyurea the electrochemical behavior of hu at gce and ge were studied using cyclic voltammetry (cv) at physiological ph = 7.0. the cyclic voltammograms obtained for 1.0 mm hu solution at a scan rate of 50 mv s -1 exhibit well-defined irreversible anodic peaks at 0.59, 0.78 and 0.91 v at glassy carbon electrode and 0.32, 0.84 and 1.13 v at gold electrode. the cathodic peak was appeared at 0.59 v corresponding to reduction of gold oxides [20]. the results are shown in figure 1. however, no peak was observed in the reverse scan, suggesting that the oxidation process is an irreversible one. there are two possibilities, either the charge transfer kinetics are slow at surfaces of gce and ge or products of the electron transfer were unstable and the reaction is accompanied by the fast chemical follow-up reaction resulting in electrochemically inactive products. figure 1. cyclic voltammograms obtained for 1.0 mm hu on glassy carbon electrode (gce) and gold electrode (ge): (a) hu on gce, (b) blank run of gce, (c) hu on ge and (d) blank run of ge in ph 7.0, 0.2 m buffer at ν = 50 mv s -1 . a c d b i / µ a e / v vs. ag/agcl k. m. naik et al. j. electrochem. sci. eng. 4(3) (2014) 111-121 : 10.5599/jese.2014.0064 115 influence of ph the electrode reaction might be affected by ph of the medium. the electrooxidation of 1.0 mm hu was studied over the ph range of 3.0 10.4 in phosphate buffer solution by cyclic voltammetry. a well-defined sharp oxidation peaks were appeared only in the ph range 7.0 10.4 (figures 2a and 2b). below the ph 7.0, the oxidation peak was not observed; hence the ph study was restricted only in the range from 7.0 to 10.4. however, with the increase in the ph of the solution, the oxidation peak current decreased continuously from 7.0 10.4. a stability study showed that the hu was unstable in aqueous solutions even at 4 °c [13] since it degrades with the production of hydroxylamine. the peak potential (ep) shifted towards less positive potentials with an increase in ph, suggesting the involvement of protons in the chemical process. from the plot of ipa vs ph of gce and ge it is clear that the peak height decreased with ph. since the best sensitivity was achieved at ph 7.0, this ph was selected for further experiments. a b figure 2. influence of ph on the shape of the peak in phosphate buffer solution at (a) 7.0, (b) 8.0, (c) 9.2 and (d) ph 10.4. for 1.0 mm hu on a gce, b ge. influence of scan rate useful information involving electrochemical mechanism can be acquired from the relationship between peak current and scan rate. therefore, the voltammetric behavior of hu at different scan rates from 10 to 50 mv s -1 was also studied using linear sweep voltammetry (figures 3a and 3b). scan rate studies were carried out to assess whether the processes on gce and ge were under diffusion or adsorption-controlled. the plot of square root of scan rate with the peak current showed a linear relationship in the range of 10 to 50 mv s -1 which is of diffusion controlled process [21]. a plot of logarithm of anodic peak current vs. logarithm of scan rate gave a straight line with a slope of 0.424 and 0.516 (figure 3c), which are close to the theoretical value of 0.5 for a purely diffusion-controlled process [22] which in turn confirms that the processes are diffusion controlled. the ep of the oxidation peak was also dependent on scan rate. the peak potential shifted to more positive values on increasing the scan rate, which confirms the irreversibility of the oxidation process, and a linear relationship between peak potential and logarithm of scan rate (figure 3d). for an irreversible electrode process, according to laviron [23] ep is defined by the following equation: ' p 2.303 2.303 log log o o rt rtk rte e nf nf nf        (2) a d a d i / µ a e / v vs. ag/agcl a d a d i / µ a e / v vs. ag/agcl j. electrochem. sci. eng. 4(3) (2014) 111-121 electroanalysis of hydroxyurea at gc and au electrode 116 a b c d figure 3. linear sweep voltammograms of 1.0 mm hu on a – gce and b – ge, with different scan rates, a e were 10, 20, 30, 40 and 50 mv s -1 , respectively; c dependence of the logarithm of peak current ip /10 -5 a on log of scan rate (υ / v s -1 ); 1.0 mm hu on (a) gce (log (ip / µa) = 0.424 log (υ / v s -1 ) + 1.110; r = 0.9810) and (b) ge (log (ip / µa) = 0.516 log (υ / v s -1 ) + 1.914; r = 0.9890); d relationship between peak potential ep / v and logarithm of scan rate log (υ / v s -1 ); 1.0 mm hu on (a) gce (ep / v = 0.059 log (υ / v s -1 ) + 0.998; r = 0.9860) and (b) ge (ep / v = 0.057 log (υ / v s -1 ) + 0.474; r = 0.9720). where α is the transfer coefficient, k o is the standard heterogeneous rate constant of the reaction, n is the number of electrons transferred, υ is the scan rate, and e o is the formal redox potential. other symbols have their usual meanings. thus, the value of αn can be easily calculated from the slope of ep vs. log υ. in this system, the slope is 0.059 and 0.057 for gce and ge, taking t = 298 k and substituting the values of r and f, αn was calculated. according to bard and faulkner [24] α can be given as p p/2 47.7 / mv e e    (3) where ep/2 is the potential where the current is at half the peak value. so, from this we obtained the value of α. further, the number of electrons (n) transferred in the electrooxidation of hu was also calculated using linear sweep voltammetry. the value of k o can be determined from the intercept of the above plot if the value of e o’ is known. the value of e o’ in equation (2) can be obtained a e i / µ a e / v vs. ag/agcl a e i / µ a e / v vs. ag/agcl log (υ / v s -1 ) lo g ( i p / 1 0 -5 a ) b a 0.0 0.5 1.0 1.5 2.0 2.5 -3 -2 -1 0 log (υ / v s -1 ) e p / v a b 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -3 -2 -1 0 k. m. naik et al. j. electrochem. sci. eng. 4(3) (2014) 111-121 : 10.5599/jese.2014.0064 117 from the intercept of ep versus υ curve by extrapolating to the vertical axis at υ = 0 [24]. all the values of αn, α, n, e o’ and k o obtained from linear sweep voltammetry are tabulated in table 1. table 1. the calculated values of αn, α, n, e o’ and k o for the electro-oxidation of hu by linear sweep voltammetry (lsv) at gce and ge parameters linear sweep voltammetry glassy carbon electrode gold electrode αn 1.0024 1.0375 α 0.5690 0.5860 n 1.76 1.78 e o’ / v 0.8740 0.3520 k o / min -1 4.910 × 10 3 5.530 × 10 3 calibration curve and detection limit to develop a rapid and sensitive voltammetric method for the determination of hu, differential pulse voltammetric method was adopted as the peaks obtained are better defined at lower concentration of hu than those obtained by cyclic voltammetry. according to the obtained results, it was possible to apply this technique to the quantitative determination of hu. the phosphate buffer solution of ph = 7.0 was selected as the supporting electrolyte for the quantification of hu as it gave a maximum peak current at ph = 7.0 for both gce and ge. differential pulse voltammograms obtained with increasing amounts of hu showed that the peak current increased with increasing concentration, as shown in figures 4a and 4b. the concentration of hu was varied from 0.01 to 1.0 mm. figure 4c shows that the graph of anodic peak current vs. concentration of hu shows two linear relationships in the range 0.01 to 0.08 and 0.2 to 1.0 mm. above 1.0 mm, deviation from linearity was obtained which might be due to the adsorption of hu or its oxidation products on the electrode surface. the decrease of sensitivity (slope) in the second linear range is likely to the kinetic limitations [25]. related statistical data of the calibration curves were obtained from the six different determinations. the detection limits (lod) and quantification (loq) in the lower range regions were given in table 2. table 2. the values of lod and loq for hu at gce and ge by using differential pulse voltammetric method glassy carbon electrode (gce) gold electrode (ge) linearity range, mm 0.01 to 1.0 0.01 to 1.0 number of data points 06 06 limit of detection (lod), µm 0.46 0.92 limit of quantification (loq), µm 1.54 3.08 repeatability – rsd, % 1.14 2.02 reproducibility – rsd, % 1.46 2.48 during the actual analysis, the analytical response was checked through the peak potential and its height. no change in peak potential was observed within an hour, while its height changed about ±1 % for five different quantitative determinations. this proposed method was better as compared with other reported methods [11,26]. j. electrochem. sci. eng. 4(3) (2014) 111-121 electroanalysis of hydroxyurea at gc and au electrode 118 to ascertain the repeatability of the analysis, six measurements of 1.0 mm hu solution were carried out using gce and ge at intervals of 30 min. the rsd value of peak current was found to be 1.14 % and 2.02 % respectively, which indicated that the methods had good repeatability. as to the reproducibility between days, it was similar to that of within day repeatability if the temperature was kept almost unchanged. a b c figure 4. differential pulse voltammograms of a – gce and b ge in hu solution at different concentrations: (a) 0.01, (b) 0.02, (c) 0.04, (d) 0.06, (e) 0.08, (f) 0.2 (g) 0.3, (h) 0.4, (i) 0.6, (j) 0.8 and (k) 1.0 mm c plot of peak current i / μa against the concentration of hu [hu] / mm (a) gce (ip / μa = 40.30 c / mm + 3.0870; r = 0.9880; ip / μa = 7.625 c / mm + 6.7590; r = 0.9910) and (b) ge (ip / μa = 28.87 c / mm + 1.5370; r = 0.9970; ip / μa = 5.473 c / mm + 3.1490; r = 0.9870). effect of excipients for the possible analytical application of the proposed method, the effect of some common excipients used in pharmaceutical preparations was examined. the tolerance limit was defined as the maximum concentration of the interfering substance that caused an error less than ±5 % for determination of hu. under the optimum experimental conditions, the effects of potential excipients on the voltammetric response of 1.0 mm hu as a standard were evaluated. the experimental results showed that hundred-fold excess of citric acid, dextrose, glucose, gum acacia, lactose, starch, and sucrose did not interfere with the voltammetric signal of hu. thus, the procedures were able to assay hu in the presence of excipients, and hence it can be considered specific. a k i / µ a e / v vs. ag/agcl a k i / µ a e / v vs. ag/agcl 0 2 4 6 8 10 12 14 16 18 0.0 0.5 1.0 1.5 [hu] / mm i / µ a a b k. m. naik et al. j. electrochem. sci. eng. 4(3) (2014) 111-121 : 10.5599/jese.2014.0064 119 determination of hu in pharmaceutical preparations and recovery test the proposed method was validated for the determination of hu in pharmaceutical preparations in tablets as a real sample by applying dpv using the standard addition method. the procedure for the tablet analysis was followed as described in sample preparation section. the results are in good agreement with the content marked in the label (table 3). recovery studies were carried out after the addition of known amounts of the drug to various pre-analyzed formulations of hu. the recovery in the sample was found to be 98.83 % with rsd of 1.37 %. table 3. determination of hu in pharmaceutical formulation samples using differential pulse voltammetric method labeled claim 500.0, mg added 20.0 mg gce ge gce ge amount found, mg a 499.2 492.5 19.5 19.2 recovery, % 98.83 98.50 97.78 96.11 rsd, % 1.37 1.87 3.51 3.54 bias, % -1.16 -1.50 -2.22 -3.88 a average of six determinations. detection of hu in spiked human plasma samples the applicability of the dpv to the determination of hu in spiked human plasma sample was investigated. the recoveries from human plasma were measured by spiking drug free plasma with known amounts of hu. the plasma samples were prepared as described in plasma sample preparation section. a quantitative analysis can be carried out by adding the standard solution of hu into the detect system of plasma sample. the calibration graph was used for the determination of spiked hu in plasma samples. the detection results obtained for four plasma samples are listed in table 4. table 4. determination of hu in spiked human plasma samples using differential pulse voltammetric method human plasma sample amount of spiked hu, 10-4 m amount of detected hu*, 10-4 m recovery, % rsd, % bias, % gce ge gce ge gce ge gce ge 1 0.3 0.3002 0.2998 100.1 99.95 2.92 3.12 0.067 -0.067 2 0.7 0.6863 0.6797 98.05 97.10 2.67 2.25 -1.96 -2.90 3 3.0 3.0407 3.0074 101.4 100.2 2.37 0.48 1.36 0.25 4 7.0 6.9620 6.9820 99.45 99.74 1.79 0.96 -0.54 -0.26 * average of six determinations detection of hu in urine samples the developed differential pulse voltammetric method was also applied for the determination of hu in spiked urine samples. the recoveries from urine were measured by spiking drug-free urine with known amounts of hu. the urine samples were diluted 100 times with the phosphate buffer solution before analysis without further pretreatments. a quantitative determination can be carried out by adding the standard solution of hu into the detect system of urine sample. the calibration graph was used for the determination of spiked hu in urine samples. the detection results of four urine samples obtained are listed in table 5. thus, satisfactory recoveries of the j. electrochem. sci. eng. 4(3) (2014) 111-121 electroanalysis of hydroxyurea at gc and au electrode 120 analyte from the real samples and a good agreement between the concentration ranges studied and the real ranges encountered in the urine samples when treated with the drug make the developed method applicable in clinical analysis. table 5. determination of hu in urine samples using differential pulse voltammetric method urine sample amount of spiked hu, 10 -4 m amount of detected hu*, 10 -4 m recovery, % rsd, % bias, % gce ge gce ge gce ge gce ge 1 0.3 0.3025 0.3092 100.8 103.1 2.32 4.12 0.83 3.07 2 0.7 0.6995 0.7101 99.94 101.4 0.34 1.41 -0.07 1.44 3 3.0 2.9424 2.9424 98.08 98.08 1.73 1.73 -1.92 -1.92 4 7.0 7.0799 6.9966 101.1 99.95 1.35 0.82 1.14 -0.05 * average of six determinations. conclusions the voltammetric oxidation of hu at glassy carbon and gold electrodes under physiological condition, i.e., ph 7.0 in phosphate buffer solutions has been investigated. hu undergoes two electron-two proton change and follows the diffusion-controlled process at both the electrodes. a suitable oxidation mechanism was proposed. the proposed method can be used successfully to assay the drug in pharmaceutical dosage form as well as in spiked real samples. high percentage recovery and study of excipients showed that the method is free from the interferences of the commonly used excipients in the formulations of drugs. the proposed method is suitable for quality control laboratories as well as pharmacokinetic studies where economy and time are essential. acknowledgment: keerti m. naik thanks ugc, new delhi for the award of research fellowship in science for meritorious students (rfsms). nomenclature hu = hydroxyurea gce = glassy carbon electrode ge = gold electrode 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[26] j. havard, j. grygiel, d. sampson, j. chromatogr. b. 584 (1992) 270-274. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {screening technique on the selection of potent microorganisms for operation in microbial fuel cell for generation of power} http://dx.doi.org/10.5599/jese.924 129 j. electrochem. sci. eng. 21(2) (2021) 129-142; http://dx.doi.org/10.5599/jese.924 open access: issn 1847-9286 www.jese-online.org original scientific paper screening technique on the selection of potent microorganisms for operation in microbial fuel cell for generation of power payel choudhury1,, biswanath bhunia2, tarun kanti bandyopadhyaya3 1department of electrical engineering, national institute of technology agartala, agartala-799046, india, email: payell.moon12@gmail.com 2department of bioengineering, national institute of technology agartala, agartala-799046, india, e-mail: bbhunia.bio@nita.ac.in 3department of chemical engineering, national institute of technology agartala, agartala-799046, india, e-mail: tarunkantibanerjee0@gmail.com corresponding author: payell.moon12@gmail.com received: november 6, 2020; revised: february 23, 2021; accepted: february 24, 2021 abstract this paper focuses on determination of the influence of electrochemically active microorganisms on the transmission of electrons from the respiratory enzymes to the electrode and assembling of exoelectrogens to the simulated wastewater medium. in this study, the total of eight microorganisms were experimentally tested to exhibit growth and high ironreducing ability in the absence of mediators. a major connection was observed between the growth and iron-reduction ability of the microorganism. the growth and iron-reduction ability were monitored experimentally over time. based on output data, the screening was done among eight different microorganisms, where escherichia coli -k-12 was chosen as the most potent microorganism for its wide application in a microbial fuel cell (mfc). in the present study, various biochemical process factors were optimized statistically using taguchi methodology for the rapid development of growth and iron-reducing assay conditions. the design of various experimental trials was carried out using five process factors at three levels with orthogonal arrays (oa) layout of l18. five process factors, including quantity of lactose, volume of trace element solution, inoculum percentage, ph, and temperature, were taken into consideration as imperative process factors and optimized for evaluation of growth of bacteria and iron reduction ability. the larger-is-best signal to noise (s/n) ratio, together with analysis of variance anova, were used during optimization. anticipated results demonstrated that the enhanced bacterial growth of 124.50 % and iron reduction ability of 112.6 % can be achieved with 8 g/l of lactose, 2 ml of trace element solution, 4 % (v/v) of inoculum, ph 7, and temperature of 35 oc. furthermore, the growth and iron reduction time profiles of escherichia coli-k12 were performed to determine its feasibility in mfc. http://dx.doi.org/10.5599/jese.924 http://dx.doi.org/10.5599/jese.924 http://www.jese-online.org/ mailto:payell.moon12@gmail.com mailto:bbhunia.bio@nita.ac.in mailto:tarunkantibanerjee0@gmail.com mailto:payell.moon12@gmail.com j. electrochem. sci. eng. 21(2) (2021) 129-142 microorganisms for operation in microbial fuel cell 130 open circuit voltage of 0.555 v was obtained over batch study on a single chamber microbial fuel cell (scmfc). keywords bioelectricity; exoelectrogenic bacteria; bacterial growth; iron-reducing ability; taguchi optimization; batch operation introduction it is a conventional statement that renewable energy sources are urgently necessary. the present necessity for fossil fuels is unsustainable due to their toxic waste and restricted supply [1]. one of the main strategies of using renewable energy sources and applying power-saving programs does not decrease focus on cutting the energy demand but increases the demand for stand-alone systems as alternates. therefore, use of renewable energy in the world increases in order to have a more supportable energy mix, which reduces greenhouse gas emissions, and also allows lower dependency on fossil fuels [2]. although much research is conducted in favor of renewable energy sources as alternative solutions, no one can completely replace fossil fuels. this proves that different alternative renewable sources are greatly needed to meet the energy demand. due to the recent invention that microorganisms which are exoelectrogenic in nature can be used to generate electricity from wastewater, and this organic matter has gained much attention in the present days. recently, the increased interest in microbial fuel cell (mfc) technology was highlighted as one of the most imperative renewable sources [3]. mfc not only generates a power, but at the same time stimulates bioremediation of the polluted sediments [4]. therefore, mfc can be used for detoxification to treat industrial wastewater containing heavy metals [5]. the innovation that microorganisms can be used to produce electrical current has led to increasing attention and the number of publications and research in the field of mfc [6,7]. this is mostly due to high importance of understanding the performance of mfc for continuous energy production and especially the role of various process factors on its performance [8,9]. in this study, the experiment was carried out with dairy wastewater taken as the substrate in mfc. since wastewater contains various organic substances, it was considered as an organic source. initially, the media was designed in the presence of simulated wastewater for the appropriate screening of the best microorganisms related to their growth and iron-reducing ability without any mediator. up to now, several bacteria like geobacter spp, rhodoferax sp., klebsiella sp., rhodopseudomonas sp., and dessulfobulbus sp., were found capable of transferring electrons without any mediator [10-12]. a total of eight microorganisms (bacillus subtilis, lactobacillus acidophilus, pediococcus acidophilus, staphylococcus aureus, zymomonas mobilis, klebsiella oxytoca, enterobacter aerogenes and escherichia coli -k-12) were chosen among potent catalysts for the generation of power in mfc. firstly, from the screening of their growth and iron-reducing ability, the best microorganisms among eight were selected for further statistical analysis. here, escherichia coli -k-12 proved to be the most potent microorganism with high growth and iron-reducing ability, although there was no normal protocol to check the growth and iron-reducing ability of bacteria which can be used commercially [13]. different investigations in laboratories have standardized regular procedures for the valuation of iron reduction. since the proper iron-reducing ability testing is a long time and costly procedure, here we used the recently developed speedy protocol [14] for checking the iron-reducing ability of escherichia coli -k-12. the growth of bacteria is linked with iron reduction ability and thus, the process factors such as lactose, trace element, inoculum percentage, ph, and temperature, would participate in the measurement of iron-reducing ability of microorganisms [15]. p. choudhury et al. j. electrochem. sci. eng. 21(2) (2021) 129-142 http://dx.doi.org/10.5599/jese.924 131 there is an enormous number of statistical methods carried out using the change of one variable per time (covt), but these methods are generally exhausting, time consuming and high-priced. szöllosi et al. [16] have reported a novel protocol for screening of biocatalyst, where covt method was used to control all parameters used in mfc. covt needs the maximum experimental number and so, an interaction effect among the process factors cannot be obtained. at the same time, it highlights the average performance of any process [17]. here, taguchi methodology (tm) is applied for optimization of biological and chemical reaction factors, specifically for escherichia coli -k-12. tm deploys a particular set of independent parameters, which are controllable and non-controllable over an accurate area of importance. hwang et al. [18] have already applied tm for engineering optimization. in the current work, various biological and chemical process factors were optimized using tm for the quick rise of growth and iron reduction profile of escherichia coli -k-12 [13]. escherichia coli -k-12 might be a potential candidate for a single chamber microbial fuel cell (smfc) for renewable energy applications [19]. the process efficiency of mfc can be affected by numerous factors such as type of microorganisms, mfc design, membrane type (pem), electrodes used, and several other factors. among all these, microbial activity is the essential factor, so the proper selection of the potential species is particularly important [20]. numerous experiments proved different methods to screen the exoelectrogenic bacteria [21], micro-fabricated mfc arrays [22], or using tungsten-oxide nanocluster as the probe [23]. these methods, however, are time-consuming and lengthy (5–6 days) to offer assessable information about the transfer of electrons by the microorganisms [24]. another limiting factor of these methods is the requirement for expensive equipment and materials. therefore, there is an increasing demand for novel and rapid screening methods in both research and development of mfcs. the main aim of this study was to develop a simple, affordable, and high sample throughput method for the screening of microorganism strains for mfc application. experimental culture management bacillus subtilis, lactobacillus acidophilus, pediococcus acidophilus, staphylococcus aureus, zymomonas mobilis, klebsiellaoxytoca, enterobacter aerogenes and escherichia coli -k-12 (mtcc1302) are eight microorganisms that were purchased from imtech chandigarh, india and examined in the present research. the growth of microorganisms was done on luria broth. the broth was set aside at ph 7.4 before sterilization. the incubation temperature was maintained at 35 oc. the microorganisms were repeatedly sub-cultured on the luria agar plate in seven days intervals. growth media the media for seed culture of all eight microorganisms was prepared [25]. briefly, the media was ready with 10 g/l of lactose, 0.2 g/l of nh4cl, 0.15 g/l of cacl2×2h2o, 0.33 g/l of kcl, 0.3 g/l of nacl, 3.15 g/l of mgcl2, 1.26 g/l of k2hpo4 and 0.42 g/l of kh2po4. in this work, the media of 50 ml was kept in 250 ml of erlenmeyer flask with a screw cap. the trace element solution was prepared with 20 g/l of znso4×7h2o, 10 g/l of h3bo3, 5 g/l of mncl2×7h2o, 5 g/l of feso4×7h2o, 1.5 g/l of cocl2×5h2o, 1.5 g/l of cuso4×5h2o and 1 g/l of na2moo4×4h2o. 1 ml of trace element solution was mixed with 50 ml of media, and ph was adjusted to 7.4 in the media. the conical flasks were plugged strongly with a screw cap to maintain oxygen absence in the media, placed inside incubator for 24 h and maintained at temperature of 35oc for all eight microorganisms. http://dx.doi.org/10.5599/jese.924 j. electrochem. sci. eng. 21(2) (2021) 129-142 microorganisms for operation in microbial fuel cell 132 microbial iron (iii) reduction study the iron reduction study for eight microorganisms was performed using the oversaturated solution of iron(iii)-citrate (5 g/l), supplemented with different broths, and used as the electron acceptor substance [26]. in this case, no methylene blue was added to the media as an electron shuttle (mediator). the anaerobic state was maintained in the media to provide the electronacceptor role of fe(iii) ions by using a screw cap. to seal the cap, parafilm was used on the cap and incubated at 35oc for all eight microorganisms. each day up to seven days, the samples were taken, and ph of samples was changed to ph 2 by addition of sulfuric acid. then, a coloring agent of ammonium-thiocyanate (nh4scn) (50 g/l) was added to the solution [27]. the final part of the sample was 200 times diluted. after thorough mixing, absorbance (a)of the solution was measured using spectrophotometer, covering the absorbance maximum of iron(iii)–thiocyanate complex within 300–600 nm range. statistical analysis for growth and iron-reducing ability of escherichia coli -k-12 the study was carried out using taguchi methodology (tm) and escherichia coli -k-12 was selected among eight microorganisms [28]. further analysis was based on the growth and iron reduction ability which were determined experimentally [29]. the assessment was done to understand the connection between the growth and iron reduction ability of escherichia coli -k-12 through active analysis. in this analysis, orthogonal arrays (oa) exhibit the changed experimental situation with the smallest amount of error. also, tm provides an advanced competence and parameters reproducibility for a range of experiment trials, and at the same time it reduces noise throughout optimization or analysis [30]. tm follows 4 individual steps, which are step by step described in figure 1. figure 1. four steps of experimental optimization included in taguchi methodology. the first step is the planning of experiments, the second step is experimenting, the third step is analysis of results, and the fourth step is validation of methodology. to bring about the wide-ranging of optimization, each step is interrelated with each other. p. choudhury et al. j. electrochem. sci. eng. 21(2) (2021) 129-142 http://dx.doi.org/10.5599/jese.924 133 investigational plan (step i) in this case, different process factors were selected, depending on the growth and iron reduction ability of escherichia coli -k-12. five different biological and chemical process factors are lactose, trace element, inoculum percentage, ph, and temperature, recognized firstly from seed culture [31]. the planning matrix was executed with suitable oas for the nominated factors with their consequent three levels [25]. here, three levels (low, mid and high) of five parameters were designed in the experiments for escherichia coli -k-12. data are summarized in table 1. table 1. trial range of five process factors (a-e) considered using taguchi methodology. factor code name range of variables low (1) mid (2) high (3) a lactose concentration, g/l 6 8 10 b volume of trace element solution, ml 1 2 3 c content of inoculum, % v/v 4 7 10 d ph 6 7 8 e temperature, oc 30 35 40 experiments (step ii) a variety of trials with process factors of different range arrangement are given in table 2 for escherichia coli -k-12, for which intensity of bacterial growth and iron reduction potential were calculated individually [32]. in every trial, the culture media of 50 ml was used. in table 2, the planning of the matrix was done with orthogonal array of five factors and three levels (35) which gives layout of l18. table 2. l18 oa (3 5) of design trials for five process factors (a-e) for growth and iron-reducing ability for escherichia coli -k-12. number of trials a b c d e growth of bacteria, cfu/ml × 1011 a460 nm (iron reducing ability) 1 1 1 1 1 1 0.81 0.211 2 1 2 2 2 2 1.52 0.415 3 1 3 3 3 3 0.74 0.191 4 2 1 1 2 2 1.98 0.505 5 2 2 2 3 3 1.22 0.306 6 2 3 3 1 1 0.89 0.221 7 3 1 2 1 3 0.77 0.212 8 3 2 3 2 1 1.11 0.299 9 3 3 1 3 2 1.16 0.312 10 1 1 3 3 2 1.09 0.289 11 1 2 1 1 3 1.01 0.269 12 1 3 2 2 1 1.00 0.256 13 2 1 2 3 1 0.94 0.234 14 2 2 3 1 2 1.44 0.378 15 2 3 1 2 3 1.49 0.381 16 3 1 3 2 3 1.10 0.282 17 3 2 1 3 1 0.86 0.221 18 3 3 2 1 2 0.89 0.242 the cell growth and iron reduction ability were determined separately after seven days of each trial for the microorganism, as per the protocol reported in the previous study [16,33]. all trials were performed 3 times, and mean values are shown in table 2. analysis of experimental data (step iii) to treat the obtained results, qualitek-4 software (nutek inc., mi, usa) was used. in step iii, the evaluation was done on bacterial growth and iron-reducing ability individually for the microorganism to examine the effects of individual process factors, and their interactive influence. moreover, http://dx.doi.org/10.5599/jese.924 j. electrochem. sci. eng. 21(2) (2021) 129-142 microorganisms for operation in microbial fuel cell 134 the performance of the total process, as well as the analysis of optimum conditions were done. the performance was analysed based on the larger-is-better s/n ratio for each trial. here, standard deviation (sd) is noise, and the anticipated value is signal [34]. hence, the ratio between anticipated and sd values defines signal-to-noise (s/n) ratio. thus, defined s/n ratio is used to calculate the quality characteristics of the output, represented by a deviancy from the anticipated value. loss function [l(y)] was used to calculate the quality characteristics of the output [35], which is computed by l(y)=k(y−m)2, where k = proportionality constant, m = target value and y = experimental data collected from each trial. the measurement of the loss function is performed using l(y) = k(1/y2) for quality appearances of output from each trial. here, the larger-is-better concept of s/n ratio is applied and hence, the projected loss function was set by ( ) 2 1 e l y ke y          = (1) where e (1/y2) was computed from n number of trials and written as 2 2 1 1 1 1n iy n y e =       =  (2) here, the mean square deviation (msd) represents the mathematical appearance for larger-isbetter s/n ratio, which was computed through the deviance from the target value: 2 1 1 1 10 log (msd)= -10 log n i s n n y =   = −       (3) validation of taguchi methodology (step iv) to authenticate the methodology assumed by predicted optimized conditions, the experiments were additionally performed individually for each microorganism. through tm, the results were then compared with predicted outputs known independently for each microorganism. wastewater the dairy wastewater used in our experiments was collected from gomati cooperative milk producers union limited, agartala. after the collection of dairy wastewater from gomati, storage of wastewater was done under the refrigerator for further use in the experiment. to achieve various chemical oxygen demand (cod) for the real dairy wastewater (rdw), it was further diluted with distilled water. the cod range in these experiments was fixed to 8000 mg/l after the statistical analysis. the standard methods were performed to achieve the correct cod measurement [36]. mfc setup and operating procedures batch study was conducted on 300 ml mfc with a working volume of 200 ml (figure 2). the compartment was acrylic having an anode, a membrane, and the cathode [37,38]. the anode was carbon cloth with a carbon-coat of 0.5 mg/cm2 and surface area of 50 cm2. nafion 117 (sainenergy fuel cell, chennai, india) was used as a membrane, and the electrode spacing was kept about 157 m, i.e. thickness of the membrane. the cathode (pt/c) was reinforced on carbon cloth with the loading of 0.5 mg/cm2 (sainenergy fuel cell, chennai, india). the cell was arranged in the order anode-membrane-cathode. the stainless-steel wire was attached to connect the electrodes, while digital multimeter is used to record the cell voltage (v) of scmfc. p. choudhury et al. j. electrochem. sci. eng. 21(2) (2021) 129-142 http://dx.doi.org/10.5599/jese.924 135 figure 2. batch operation on a single chamber microbial fuel cell (scmfc) results and discussion experimental results for growth and iron-reduction ability the experiments for growth and iron-reducing ability were done for all eight microorganisms. to understand the strength of all eight microorganisms, dilution plating was done, and strong ironreduction ability was checked in absence of a mediator for screening purposes [39]. the exception was found in the case of escherichia coli -k-12 [40]. as shown in table 3, escherichia coli -k-12 was found to have the highest growth as well as iron-reducing ability in the absence of mediator. thus, it proves to be an exoelectrogenic bacteria which indicates the production and secretion of exoelectrogens in the medium [41]. the most potent microorganisms were thus selected, and further statistical analysis was done based on their growth and iron-reducing ability tested experimentally. table 3. growth and iron(iii)-reduction ability of different microorganisms. s. n. microorganism bacterial growth iron(iii)-reduction without mediator 1. bacillus subtilis ++ + 2. lactobacillus acidophilus +++ -- 3. pediococcus acidophilus + -- 4. staphylococcus aureus ++ -- 5. zymomonas mobilis ++ -- 6. klebsiella oxytoca + -- 7. enterobacter aerogenes + -- 8. escherichia coli -k-12(mtcc-1302) +++ +++ bacterial growth: (++) minimum, (+++) maximum; iron(iii) reduction: (–) the change in absorbance was not detectable, (+) change less than 0.1, (++) change between 0.1 and 0.2, (+++) change more than 0.2. optimization outcome the optimization was done with the most potent microorganism among the selected eight. the estimation of growth and iron-reduction ability was done by the planning of individual process factors at their consigned levels [41]. data for escherichia coli -k-12 are shown in table 4. table 4. main effects of particular parameters for growth and iron-reducing ability of escherichia coli -k-12 serial no. factor code effect of individual factors in growth by s/n ratio effect of individual factors in iron reduction ability by s/n ratio level-1 level-2 level-3 l2-l1 level-1 level-2 level-3 l2-l1 1. a -0.019 2.101 -0.285 2.119 -11.626 -9.829 -11.785 1.796 2. b 0.455 1.346 -0.005 0.891 -11.253 -10.262 -11.726 0.99 3. c 1.248 0.234 0.313 1.014 -10.444 -11.395 -11.401 -0.951 4. d -0.504 2.438 -0.139 2.942 -12.081 -9.243 -11.916 2.837 5. e -0.654 2.271 0.18 2.924 -12.481 -9.239 -11.521 3.241 http://dx.doi.org/10.5599/jese.924 j. electrochem. sci. eng. 21(2) (2021) 129-142 microorganisms for operation in microbial fuel cell 136 influence of each factor the influence of individual process parameters at their consigned level used for evaluation of growth and iron-reduction ability, has been reported in table 2. results show that bacterial growth and corresponding iron reduction ability depend on all assigned process factors chosen in the present study. from table 4, it is observed that ph, temperature, lactose, and inoculum percentage are incredibly significant at level 2 among all designated biochemical process factors, and the trace element solution is maximum at level 1. it is also clear from level 3 that growth, as well as iron reduction ability is declined with an additional enrichment of all selected parameters except inoculum percentage and does not extensively affect the enrichment of trace element. therefore, the relative effect of individual factors was measured from (l2-l1). the better average magnitude in a variation of their effects designates a stronger influence in output. moreover, degradation of complex substance to simple products happens through the complex biochemical process, where terminal acceptor of an electron is ferric ion (fe3+) [17]. since ph of media controls the growth of bacteria, the iron reduction ability is indirectly controlled by the level of ph too. the second important parameter following ph, is temperature. generally, at a higher temperature, enzyme present inside the microbial system becomes deactivated, which indirectly inhibits the growth of bacteria. since growth and iron reduction capacity are interlinked to each other, temperature indirectly controls both [13]. in the present study, lactose is the sole carbon source, and the third important factor which supplies energy for the growth of bacteria. the destruction of growth, however, is found at higher and lower concentrations of lactose, what may be due to catabolic repression of monosaccharides [10]. the fourth important parameter is the inoculum percentage. the optimum inoculum percentage is required to attain maximum biomass after completing of a biochemical process. as the bacterial growth depends on the substrate accessibility in media, the suppressive growth was observed when a higher percentage of inoculum was added on it, what may be due to competitive inhibition [33]. thus, the enhancement of tracer amount in media does not significantly affect the growth and iron-reducing ability of the bacteria, and so, their requirement in media is low in comparison with other media components [16]. interaction between two factors severity index (si) was calculated using the tm individually for escherichia coli -k-12 [43]. the assessment of the interactive effect between two factors was carried out at different levels [44]. the results for escherichia coli -k-12 after the analysis are reported in table 5. interaction pairs are exposed in downward order of their si (0-100 %). si indicates the maximum angle along with all probable combinations of the line segments for interaction involving pairs of 3 level factors. thus, the distribution of the interacting process factors for which they are responsible is indicated in the columns shown in table 5. the 90o angle involving the lines for the parameters defines 100 % si interaction, while parallel lines stuck between them signifies 0 % si interaction. based on the first two levels, the best possible values are represented by the level of factors. it is evident from table 5 that the combination of trace element solution and inoculum percentage shows the highest interaction for both, growth and iron reduction ability. this combination is followed by the combination of lactose and inoculum percentage, trace element solution and ph, lactose and trace element solution, inoculum percentage and temperature, lactose and temperature, inoculum percentage and ph, ph and temperature, trace element solution and temperature and finally, lactose and ph. table 4 indicates that ph is designated as the highest impact parameter, p. choudhury et al. j. electrochem. sci. eng. 21(2) (2021) 129-142 http://dx.doi.org/10.5599/jese.924 137 followed by temperature (high impact parameter), lactose (moderate impact parameter), inoculum percentage (less impact parameter) and trace element solution (least impact factor) for iron reduction ability. it is evident that trace element solution and inoculum percentage are least and less impact parameters (table 4) but illustrates the highest interaction si in combination with them. the results of variance analysis demonstrate that yield of cell number and iron-reducing ability is dependent on the interaction of process factors for escherichia coli -k12 and quite independent of individual effects [45]. table 5. interactions estimated by severity index (si) growth s. no factors columns rc si (100 %) levels 1 volume of trace element solution × inoculum content 3 × 4 7 75.05 2, 2 2 lactose concentration × inoculum content 2 × 4 6 53.57 2, 1 3 volume of trace element solution × ph 3 × 5 6 44.06 1, 2 4 lactose concentration × volume of trace element solution 4 × 6 2 34.76 1, 2 5 inoculum content × temperature 2 × 3 1 33.14 2, 1 6 lactose concentration × temperature 2 × 6 4 19.07 2, 2 7 inoculum content × ph 4 × 5 1 16.63 1, 2 8 ph × temperature 5 × 6 3 14.75 2, 2 9 volume of trace element solution × temperature 3 × 6 5 10.02 2, 2 10 lactose concentration × ph 2 × 5 7 7.31 2, 2 iron reduction ability s. no factors columns rc si (100 %) levels 1 volume of trace element solution × inoculum content 3 × 4 7 69.13 2, 2 2 lactose concentration × inoculum content 2 × 4 6 61.10 2, 1 3 volume of trace element solution × ph 3 × 5 6 41.39 1, 2 4 lactose concentration × volume of trace element solution 2 × 3 1 35.47 2, 1 5 inoculum content × temperature 4 × 6 2 28.79 1, 2 6 lactose concentration × temperature 2 × 6 4 19.37 2, 2 7 inoculum content × ph 4 × 5 1 18.62 1, 2 8 ph × temperature 5 × 6 3 11.07 2, 2 9 volume of trace element solution × temperature 3 × 6 5 9.46 2, 2 10 lactose concentration × ph 2 × 5 7 7.83 2, 2 analysis of variance (anova) to improve the relative effect and comparable interactions of the process factors within the variation of results, trial data were examined by the analysis of variance, anova [35]. after anova analysis reported for escherichia coli -k12, the percentage of contribution of each process factor is presented in table 6. table 6. analysis of variance (anova). factor dof growth iron reduction ability sum of squares variance contribution, % sum of squares variance contribution, % lactose concentration, g/l 2 20.514 10.257 22.383 14.169 7.084 15.140 volume of trace element solution, ml 2 5.659 2.829 5.814 6.697 3.348 6.846 inoculum content, % v/v 2 3.819 1.909 3.762 3.645 1.822 3.458 ph 2 30.861 15.430 33.923 30.456 15.228 33.216 temperature, oc 2 27.246 13.623 29.891 33.278 16.637 36.348 other/error 7 1.559 0.222 4.227 1.850 0.264 4.992 total 17 89.661 100.000 90.099 100.000 http://dx.doi.org/10.5599/jese.924 j. electrochem. sci. eng. 21(2) (2021) 129-142 microorganisms for operation in microbial fuel cell 138 conversely, when these factors act jointly, they influence the maximum output. the on top of results are due to the outcome of several parameters collectively. optimal state maximum growth and iron-reducing ability are attained by the optimum values of selected parameters, which are for escherichia coli -k12 [31] reported in table 7. based on the excellence [46], quality selected for the analysis of the optimum situation was resolute. table 7 includes the ordinary presentation in calculating only considerable factors [47]. results illustrate that ph has maximum impact on the growth, while temperature achieves the maximum impact on the iron reduction of escherichia coli -k-12. table 7. optimum conditions and performance of growth and iron reduction ability for escherichia coli -k12. factor code factor value level contribution for growth from s/n ratio contribution for iron reduction ability from s/n ratio 1 lactose concentration, g/l 8 2 1.501 1.251 2 volume of trace element solution, ml 2 2 0.747 0.818 3 inoculum content, v/v % 4 1 0.649 0.636 4 ph 7 2 1.839 1.837 5 temperature, oc 35 2 1.671 1.841 total contribution from all factors 6.407 6.383 current grand average of performance 0.599 -11.080 expected result at optimum condition 7.006 -4.697 validation of experiments the frequency distribution plots for bacterial growth, as well as iron reduction ability for the current and improved conditions are shown in figures 3a and 3b, respectively. the yield of bacterial growth is increased from 0.98×1011 cfu/ml (s/n ratio is 0.599) to 2.2×1011 cfu/ml (s/n ratio is 7.006) after optimization of process factors. in general, 124.50 % increase of yield of bacterial growth is shown in figure 3a. tm also calculates that enhancement of iron reduction capacity may be carried out from 0.300 to 0.638 of absorbance as s/n ratio increases from -11.080 to -4.697. it is shown in figure 3b that 112.6 % of iron reduction ability is enhanced under the optimized condition. a b measured results [expt file qualitek 4] measured results [expt file qualitek 4] figure 3. performance distribution of improved and current condition of escherichia coli -k-12 for (a) bacterial growth and (b) iron-reducing ability f re q u e n cy d is tr ib u ti o n f re q u e n cy d is tr ib u ti o n p. choudhury et al. j. electrochem. sci. eng. 21(2) (2021) 129-142 http://dx.doi.org/10.5599/jese.924 139 to validate the experimental methodology, the experiments were carried out under predicted optimized conditions. it is found that the yield of bacterial growth is increased to 2.2×1011 (124.50 %), while iron reduction capacity is increased to -4.697 of absorbance (112.6 %) under predicted optimized condition. the variation of predicted data about bacterial growth and iron reduction ability from experimental data was found within the range of validation [49]. in figures 4a and 4b, time profiles of growth and iron-reducing ability of escherichia coli-k-12 are illustrated. figure 4a shows that in the growth of escherichia coli -k-12 there are three distinct phases [42]. at this point, three unconnected phases of iron-reducing consider absorbance changes at 460 nm (a460nm) for which, the raised time profile is found (figure 4b). therefore, figure 4b illustrates that up to 24 h of incubation, the iron-reducing ability of bacterial is almost constant with the samples collected. however, an important change of absorbance is found from 24 h (1 day) to 120 h (5 days) within the collected samples [50]. a b day day figure 4. a growth and b iron reduction time profiles of escherichia coli-k-12 under optimized condition the lower rate of shifting the absorbance is noticed from 120 h (5 days) to 168 h (7 days) of incubation for sample withdrawal. the present finding may be due to exhaustion of the main source of energy in the media i.e. lactose. in mfc, bacteria receive energy from carbon sources i.e., lactose or any organic substance in anaerobic condition, and transmission of electrons takes place from the anode to cathode which acts as an electron acceptor in presence of oxygen. microorganism usually develops -320 mv in the form of nadh in the anode, and +840 mv is gained by the cathode suitable for the shuttling of an electron. batch study for voltage generation the collected dairy wastewater was tested to find the chemical oxygen demand (cod), using the existing protocol [36] and found equal to 8010 mg/l. the value of cod obtained for simulated dairy wastewater (sdw) having 0.8 % (w/v) of lactose is equivalent to the real dairy wastewater (rdw). a little dilution of cod is done with distilled water with rdw and kept at 0.8 % (w/v). therefore, for real rdw, the value of cod was kept at 0.8 % (w/v) for batch experiments (up to 360 h) for 15 days. the highest ocv after 96 h of incubation was measured as 555 mv (figure 5). after a certain interval of time, the ocv is gradually decreased and the value of ocv was found to be lower after 180 h of mfc operation, i.e. equal to 500 mv [51]. c e ll n u m b e r, c f u /m l a b so rb a n ce a t 4 6 0 n m http://dx.doi.org/10.5599/jese.924 j. electrochem. sci. eng. 21(2) (2021) 129-142 microorganisms for operation in microbial fuel cell 140 time, h figure 5. maximum open-circuit voltage obtained during the batch process in scmfc the accessibility of substrate, ph of media, and incubation temperature are responsible for higher multiplication rate of bacteria [52]. the accessibility of substrate in mfc encourages bacterial growth and thus transfer of electrons in the system. since escherichia coli -k-12 is an exoelectrogenic bacteria, no mediator was used additionally [53]. as microorganisms need energy source, they use the substrate directly or indirectly. during batch operation, however, a huge amount of substrate hampers ocv generation. therefore, for further improvement of ocv, a systematic feeding strategy should be implemented to reach a constant and stable voltage in a microbial fuel cell. conclusion the current study investigates optimization of process factors, to get the progress of quick growth and iron reduction assay for escherichia coli -k-12 using taguchi methodology. the optimal value for each of five process factors has been projected using taguchi methodology by the experimentation with 18 trials. s/n ratio of larger-is-better concept was used to evaluate the main and interaction effects of process factors. it is also proven that iron reduction ability is associated with growth in the case of escherichia coli -k-12. lastly, it also proves to have a noteworthy growth rate in anaerobic conditions along with the presence of a strong oxidizing agent. thus, escherichia coli -k-12 was signified to be the most potential biocatalyst to produce energy in a single chambermfc. the highest ocv of 555 mv was measured with batch operation in scmfc for 15 days at ph 7, 35 oc of temperature, 8 g/l of lactose, 2 ml of trace element solution, and 4 % (v/v) of inoculum percentage. the yield of bacterial growth is increased to 2.2×1011 cfu/ml (s/n ratio is 7.006) 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[53] k. xiang, y. qiao, c. b. ching, c. m. li, electrochemistry communications 11(8) (2009) 1593-1595 https://doi.org/10.1016/j.elecom.2009.06.004. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1007/s12257-013-0429-7 https://doi.org/10.1007/bf03326265 https://doi.org/10.1039/b719955d https://doi.org/10.1016/j.mimet.2014.08.023 https://doi.org/10.1002/jctb.2446 https://doi.org/10.1016/j.bios.2012.05.046 https://doi.org/10.1021/es0499344 https://doi.org/10.1016/j.sna.2012.10.025 https://doi.org/10.1007/s11783-009-0028-1 https://doi.org/10.1681/asn.2014080842 https://doi.org/10.1039/c5an00492f https://doi.org/10.1016/j.ijhydene.2008.06.061 https://doi.org/10.1115/1.2919325 https://doi.org/10.1016/j.indcrop.2009.12.001 https://doi.org/10.1080/07408179208964244 https://doi.org/10.1016/j.elecom.2006.09.025 http://nopr.niscair.res.in/handle/123456789/14633 https://doi.org/10.1002/elsc.201000020 https://doi.org/10.1504/ijedpo.2019.101718 https://doi.org/10.1201/9780429448799 https://doi.org/10.1016/j.elecom.2007.12.009 https://doi.org/10.1016/j.elecom.2009.06.004 https://creativecommons.org/licenses/by/4.0/) surface protection of ss-316l with boron nitride based thin films using radio frequency magnetron sputtering technique http://dx.doi.org/10.5599/jese.1247 851 j. electrochem. sci. eng. 12(5) (2022) 851-863; http://dx.doi.org/10.5599/jese.1247 open access : : issn 1847-9286 www.jese-online.org original scientific paper surface protection of ss-316l with boron nitride based thin films using radio frequency magnetron sputtering technique mukhtiar singh, hitesh vasudev and maninder singh school of mechanical engineering, lovely professional university jalandhar delhi, grand trunk rd, phagwara, punjab 144001, india corresponding author: mukhtiar.16431@lpu.co.in received: january 22, 2022; accepted: march 21, 2022; published: july 4, 2022 abstract in the present work, the radio frequency (rf) magnetron sputtering process was used to develop boron nitride thin films on 316l stainless steel. the target material used in the experiment was a hexagonal boron nitride (c-bn) target. the deposition was performed in three different ar and n2 system mixing regimes. the composition and morphology of the coating developed at various n2 and ar plasma ratios were investigated using scanning electron microscopy (sem) and x-ray diffraction (xrd) techniques. the electrochemical corrosion test was used to investigate the boron nitride coating's corrosion behaviour. the goal was to study the changes in the ratio of n2 and ar during the process and to understand the structure of cubic boron nitride (c-bn) coatings. increased microstructure uniformity and further c-bn step creation with different quantities (q) qar/qn2 = 2 imply a fundamental strategy for creating improved cubic boron nitride films. keywords wear resistance; substrate; morphology; microstructure; corrosion resistance introduction the coating implies a substance added to other substances that affects the surface characteristics, such as colour, light, chemical attack, or wear resistance, without changing the bulk characteristics. thin films are often hetero-artificial materials formed by one of many deposition methods on a substrate, as reported by bello et al. [1]. the term coating often refers to paints like varnishes or enamels, but also the films used typical engineering applications like machine tools and automotive parts as protective coatings [2]. metal coatings are generally used to protect the surface of the material from corrosion [3-4]. the coatings adapted to protect the metallic substrates from corrosion are extremely important for the long-term efficiency and reliability of the coated parts and their product value. the cubic boron nitride(c-bn) is very well known for being one of the best materials after diamond [5]. due to its superior characteristics, such as optical transparency, hardness and thermal conductivity, it is an excellent material for the hard coating on various kinds http://dx.doi.org/10.5599/jese.1247 http://dx.doi.org/10.5599/jese.1247 http://www.jese-online.org/ mailto:mukhtiar.16431@lpu.co.in j. electrochem. sci. eng. 12(5) (2022) 851-863 surface protection of ss-316l with boron nitride 852 of tools [6]. it has two benefits over diamond: i) there is no diffusion of nitrogen and boron atoms into ferrous; ii) c-bn is chemically inert in oxygen atmospheres and more stable at elevated temperature against oxidation as compared to the diamond. thus, c-bn appears more appropriate for wearing a protective coating on steel substrates. boron nitride has been studied extensively because of its many superior characteristics such as high thermal conductivity and electrical resistivity and chemical inertness [7]. in general investigation, many parameters such as temperature, deposition and total flow rate have been examined to report their influence on the composition and crystalline structure of the material. in addition to the traditional chemical vapor deposition (cvd) process studies,plasma-enhanced chemical vapor deposition (pecvd) and low-pressure chemical vapor deposition (lpcvd) are also discussed in the literature. due to simple parameter optimization, cvd is one of the best techniques to develop a pure form of boron nitride for the desired applications. the response of ammonia gas and diborane mixtures in a hot-wall reactor on silicon substrates for chemical vapor deposition was studied [8]. the study was conducted with respect to the mechanism of deposition of boron nitride by cvd in a reactor at an elevated temperature, and subsequently, the effects of gas flow ratio and temperature (600 to 800 oc) on the formation were investigated. reaction mechanisms have shown variation at different temperatures [9]. at lower temperatures, an intermediate compound such as ammonia borane is formed, and at higher temperatures, borazine is formed. however, the proposed mechanisms could not find kinetic data and expression rates. the boron nitride based thin films from borazine with the help of cvd in a hot-wall chamber was developed at a temperature range between 850 and 900 oc with a pressure of 1 kpa and n2 as a carrier gas. the layers were evenly deposited on the substrate at 900 oc and partly hexagonal boron nitride (h-bn) was ordered. the coatings also possessed some n-h bonds at 800 oc. contrary to this, 900 oc is too high for borazine to decompose fully. as the temperature increases above 1400 °oc, boron nitride crystallisation increases. the impact of feed speeds, temperature and pressure of ammonia gases and triethylboron on the rate of deposition and bn thin film characterization on the crystal of silicon substrate by using the cvd technique was examined [10]. the temperature range was 850-1100 oc and the ambient pressure was kept at 133.32 pa carrier gases, hydrogen and argon gases were included. the triethylboron (teb) partial pressure affects the deposition rate at atmospheric pressure to the power of 0.7 w. owing to the rise in gas diffusivity, the deposition rate increases as the overall pressure decreases while the ammonia and teb partial pressure remains constant. first, the rate of deposition rises to 1050 oc and then reduces with high temperatures. also, xps spectra indicated that the coating contained carbide. xrd analysis revealed that the coating was turbostratic. traditional thermochemical treatments on stainless steel are associated with a loss of corrosion resistance as nitrogen, boron and carbon react with chromium to form nitrides/borides/carbides. this has resulted in the removal of chromium from the solid solution. the c-bn can be synthesised on various metals such as aluminium, gold or silver, as well as compound semiconductors such as silicon carbide or titanium nitride. despite the fact that films with a high c-bn content (i.e., >85 %) had been synthesised in certain circumstances and therefore, heteroepitaxial growth has not been achieved. the c-bn content of the films reduced as the metal substrate hardness decreased. the ductile metals are intended to absorb the stresses created in the developing film better, delaying or even inhibiting c-bn nucleation. the films produced with pure ar ion bombardment have low peak frequencies. the fact that the high cubic content bn films created by ar as a reactive gas frequently delaminated in the air has been reported [11]. in the cubic boron nitride thin film development; so far there have been numerous significant concerns [12]. the delamination of c-bn films from the m. singh et al. j. electrochem. sci. eng. 12(5) (2022) 851-863 http://dx.doi.org/10.5599/jese.1247 853 substratum is among the most important challenges. the c-bn films have been shown to have highpressure between 1 and 25 gpa. the surface oxidation due to humidity after air exposure is responsible for the delamination of non-stoichiometric c-bn films. the present study involves the development of thin films to counter the discussed issues by varying the process parameters. in the current work, boron nitride was deposited on the ss-316l using the rf sputtering technique. the process parameters were varied to deposit the thin coatings under different conditions called regimes and then subjected to various characterizations. the deposition was performed in three different ar and n2 system mixing regimes. the composition and microstructure morphology of the coating developed at various n2 and ar plasma ratios were investigated using scanning electron microscopy (sem) and x-ray diffraction (xrd) techniques. the electrochemical corrosion test was used to investigate the boron nitride coating's corrosion behaviour. experimental bn target details and rf magnetron sputtering in this research work, rf magnetron sputtering was used to deposit cubic (c-bn) films on ss-316l grade substrates. all the specimens were ultrasonically pre-treated for 3 minutes in a solutionofpetroleum ether before being rinsed with deionized (di) h2o. figure 1. depicts a diagram of the experimental system. a balanced plane magnetron and a broad electron beam source were mounted on the vacuum chamber, which has a diameter of 330 mm. as sputtering targets, a disc of h-bn (99.9 % purity) of 80 mm in diameter and 10 mm thick was employed. figure 1. the illustration of the rf sputtering experimental setup the bn target was placed on top of a water-cooled magnetron gun that was coupled via a network matching to radio frequency (13.56 mhz) generator. the hexagonal-boron nitride target was sputtered with a discharging power of 150 w in rf mode. the discharge current was controlled between 2 and 20 a, while the voltage was altered from 300 to 25 v. during the coating deposition, the beam of electrons was 100 ev energy. coatings were applied to the polished surfaces of ss316l substrates measuring 15×10×3 mm. before being placed in the chamber, specimens were cleaned ultrasonically in acetone solution. the operating chamber was lowered to 0.14 mpa. during ion etching, the sample pulse bias was 500 v (50 khz, 12.5 s), and the ion current density was 2 ma m-2. during the deposition of the coatings, the bias voltage was 200 v. an electron source was used to feed gas mixtures of ar/n2. http://dx.doi.org/10.5599/jese.1247 j. electrochem. sci. eng. 12(5) (2022) 851-863 surface protection of ss-316l with boron nitride 854 coatings were applied at the flow rates, qar/qn2? = 5, qar/qn2 = 2 and qar/qn2 = 1/5, with total gas pressures of 0.6, 0.7 and 0.9 mpa, respectively. following sputtering parameters were varied for three different deposition regimes, as shown in table 1. table 1. sputtering parameters used for different coating regimes rf frequency, mhz 13.56 target material h-bn,  = 80mm ar flow rate, sccm* 5 regime 1:n2 flow rate, sccm* 1 regime 2:n2 flow rate, sccm* 2.5 regime 2:n2 flow rate, sccm* 5 magnetron discharge power, w 150 deposition process time, h 5 *sccmstandard cubic centimeter per minute characterization of materials the characterization of substrates stainless steel (ss-316l) was carried out in order to determine its microstructure, grain size and element composition. figure 2a shows the typical austenitic microstructure of the ss-316l observed through fe-sem. the bse image of the ss-316l is shown in figure 2b. the elemental composition of (ss-316l) was checked by an optical spectrometer (make: metal vision, model: 1008i). the compositions of ss-316l are given in table 2. figure 2. microstructures of bulk stainless steel: (a) sem micrograph and (b) bse image table 2. substrate chemical composition of ss-316l element cr mo fe si c mn ni content, wt.% 17.3 2.66 bal 0.73 0.022 1.77 13 rf sputtering coated specimens at various sputtering regimes were mounted across the thickness using a low-speed diamond saw (ms-10, ducom, bangalore-india) and were cold-mounted in epoxy. the cold-mounted specimens were polished with different grades of emery papers. the metallurgical investigation of the coated specimens was performed by using an optical microscope, field emission scanning electron microscopy and x-ray diffraction technique. m. singh et al. j. electrochem. sci. eng. 12(5) (2022) 851-863 http://dx.doi.org/10.5599/jese.1247 855 x-ray diffraction (xrd) analysis the detection of various phases present in the coating at various sputtering parameter conditions was revealed by utilizing an xrd machine. the xrd patterns obtained through xrd machine were used to analyse the phases formed on the thin film surface and the feedstock powder. x-ray diffraction (xrd) was done with a rate of scanning of 1o min-1 and a 2 range of 20-80o on (bruker, axs diffractometer). the xrd was carried out with a cu-kα radiation source produced at 40 ma and 40 kv. the xrd spectra of thin bn films and the phases corresponding to the deposited coatings are shown in figure 3. the deposited coating of c-bn was identified as a significant phase by xrd. however, the surface contains the soft phase h-bn, as well as other phases such as turbostatic-boron(t-bn) nitride and amorphous-boron nitride. compared to the bn1 coating and bn3 coating, the bn2 coatings had higher c-bn peaks. the cubic phase of boron nitride has diffraction peaks of 44.356 and 52.407o in regime 1, whereas bn1 coatings were made up of softer boron nitride phases such orthorhombic-boron nitride, rhombohedral-boron nitride, and hexagonal-boron nitride with a small quantity of cubic-boron nitride phase [13]. the details regarding the formation of phases corresponding to the diffraction angles are shown in table 3. figure 3. x-ray diffraction pattern of (a) bn1 coatings corresponding to regime 1, (b) bn2 coatings corresponding to regime 2, and (c) bn3 coatings corresponding to regime 3 the orthorhombic-boron nitride and hexagonal boron nitride phases were identified in abundance in the bn3 coatings corresponding to regime 3. the bn2 coatings corresponding to regime 2 had more cubic boron nitride than the bn1 coating and bn3 coatings corresponding to regime 1 and regime 3, as demonstrated by xrd differentiation peaks analysis. the reason for this is that s bn2 coating is bombarded with high ar ions, resulting in extra ar ions present in the bn2 coatings formed in ar/n2 gas mixture. due to the presence of these argon ions present in the bn thin film coating, the compression stress arises during the hexagonal http://dx.doi.org/10.5599/jese.1247 j. electrochem. sci. eng. 12(5) (2022) 851-863 surface protection of ss-316l with boron nitride 856 boron nitride phase development on the substrate, facilitating c-bn nucleation and development [14]. scratch tracks demonstrated that bn2 thin film coatings have high hardness when compared to coatings placed in other coating parameter regimes. table 3. phases obtained during various regimes regime / coatings 2 / o phase regime 1 / bn1 20.45 orthorhombic-bn 25.436 orthorhombic-bn 27.343 orthorhombic-bn 26.76 h-bn 42.163 h-bn 59.718 rhombohedral-bn regime 2 / bn2 26.627 h-bn 29.578 orthorhombic-bn 43.193 c-bn 50.395 c-bn regime 3 / bn3 29.56 orthorhombic-bn 32.436 orthorhombic-bn 36.343 orthorhombic-bn 48.75 h-bn 59.89 h-bn 62.162 rhombohedral-bn ftir analysis ftir test was employed to detect if the phase-in deposited boron nitride films were hexagonal or cubic. figure 4 illustrates the infrared spectrum of bn coating on ss-316l specimens developed at various n2 and ar mixture ratios in transmission mode with a resolution of 4 cm-1. for the background spectra, uncoated ss-316l samples were employed. at the qar= 5 sccm and qn2 = 5 sccm flow rate, only hexagonal boron nitride absorption occurs at 1262 cm-1, but no cubic boron nitride absorption occurs at 1050-1100 cm-1. figure 4. ftir spectra of boron nitride thin coating of: (a) regime 1, (b) regime 2 and (c) regime 3 m. singh et al. j. electrochem. sci. eng. 12(5) (2022) 851-863 http://dx.doi.org/10.5599/jese.1247 857 at a flow rate of qar = 5 sccm and qn2 = 1 sccm, the c-bn content rises to 50 %. however, at a rate of flow of qar = 5 sccm and qn2 = 2.5 sccm, the c-bn content drops to 20 %. for cubic phase boron nitride deposition, the balance of boron and nitride atoms in the coating is critical. the mismatch of boron and nitride atoms in the thin coating prevents the formation of c-bn [15]. the boron and nitride atoms sputtered from the deposited coating at different rates when bombarded with argon ions. because the sputtering yield of nitride atoms is higher than that of boron atoms, the addition of optimal nitrogen gas compensates for the loss of nitride atoms in order to maintain the boron and nitride atoms balance in the film. according to the results of the experiments, the qar = 2 sccm and qn = 5 sccm flow rate gas composition is better for the c-bn synthesis. in bn2 samples, an absorbance peak for the cubic phase was observed at 1060 cm-1, whereas two absorbance peaks for the hexagonal phases were observed at roughly 780 and 1380 cm-1. the ftir results corresponding to regime 1, regime 2 and regime 3 are presented in figure 4. microstructural study of thin films specimen and elemental analysis figures 5-7 show the results of an eds and sem analysis of boron nitride thin coatings at three distinct regimes. as observed in figure 4, the thin boron nitride coatings at regime 1 have a significant quantity of boron and nitrogen on the boron nitride thin film coating. as a result, as shown in figure 5(a), the eds spectrum of the coating produced in regime 1 validates the elements contained in the coatings figure 5b. the eds and sem of boron nitride coatings produced in regime 3 are shown in figure 7. according to figures 5(a) of the eds, the amount of boron and nitrogen drops in this regime, as evidenced by the rectangle in fig. 5(b). figure 5. (a) sem illustration of boron nitride thin film at regime-1 (b) eds micrograph of bn1 thin film figure 6. (a) sem illustration of bn2 films (b) eds micrograph of bn2 films element content, wt.% b 30.2 fe 29.3 c 28.6 cr 7.3 ni 3.9 n 0.8 1 element content, wt.% b 14.4 fe 43.0 c 24.5 cr 10.4 ni 7.6 n 0.1 1 http://dx.doi.org/10.5599/jese.1247 j. electrochem. sci. eng. 12(5) (2022) 851-863 surface protection of ss-316l with boron nitride 858 the eds and sem images of boron nitride thin films on the bn2 coatings are shown in figure 6. corresponding to the sem micrograph presented in figure 6(a), the contents of nitrogen and boron have increased (boron to 32.80 wt.% and carbon to38.80 wt.%) eds presented in figure 6(b). the cross-section of the coating on the regime 1 sample is shown in figures 8(a-b). the morphology of the coating is irregular forms with a dendritic pattern. on the regime 2 samples, the morphology of the coating was observed to be considerably smoother, figures 8(c-d). in comparison to other regimes, the coating grain size on the regime 3 samples is bigger in figures 8(e-f). the bombardment ions energy is dissipated into phonons in what has been termed a thermal spike. the bn transforms from hexagonal phase to cubic phase at the action of the thermal spike, which results in very high temperature and pressure locally in a very brief period. figure 7. (a) sem illustration of bn3 films, and (b) eds micrograph of bn3 films figure 8. the cross-sectional sem micrographs at various magnifications of (a and b) bn-1 coating corresponding to regime 1, (c and d) bn-2 coating corresponding to regime 2; (e and f) bn3 coating corresponding to regime 3. at the low plasma energy and temperature, the bombardment ion energy is not high enough to make the hexagonal phase transformation to the cubic phase, as shown in regime 1. similarly, in element content, wt.% b 31.8 fe 12.3 c 39.3 cr 5.1 ni 2.6 n 0.1 m. singh et al. j. electrochem. sci. eng. 12(5) (2022) 851-863 http://dx.doi.org/10.5599/jese.1247 859 regime 3 the high plasma energy and temperature high ion energy makes deposited atoms re-sputter from the substrate, which is not favourable to the c-bn formation and leads to the decrease of c-bn content and poor surface morphology. at moderate plasma energy and temperature, there is sufficient bombardment ion energy, and surface morphology is relatively smoother in bn2 coatings. corrosion rate of the base metal and thin films corrosion is a destructive and unanticipated attack on the metal that usually starts on the surfaces. corrosion is a major source of electrochemical reactions on metal surfaces. as a result, electrochemical tests are ideal for corrosive examination [16,17]. the difference in potential between the working electrode and reference electrode in a solution of electrolyte (3.5 wt.% nacl) with no corresponding potential or current connection to the cell is known as open circuit potential (ocp), also known as potential of corrosion (ecorr). the ocp is estimated when the electrolyte reaches its stable state and the flow of current between the cathode and anode is null. at this moment, the half-rate reactions of reduction and oxidation are equal. in this approach, calculating the open-circuit potential is a step in evaluating the corrosion vulnerability of a material or the protective features of a coating [18]. in electrochemical investigations, a specimen of the coated substrate with a surface area of a few square cm was employed to assess the corrosion incidence of the metal in a corrosion test device. the coating specimens are dipped in a metal solution in the system under investigation. two additional electrodes are submerged in the fluid. a potentiostat controls all of the electrodes. the potentiostat allows samples to vary their potential in a phased manner and measure the current flow as a function of their potential. it provides icorr with immediate assurance at ecorr. in this study, electrochemical experiments on the base substrate were carried out. the alloy weights and material density equivalent are used as inputs in the polarisation tests (table 4). all potentials were measured against a reference electrode, which was a conventional calomel electrode (sce), and a reference electrode. coatings and base metals were used for the 0.38 cm2 corrosive region that served as the working electrode in this study. before conducting electrochemical testing, exposed portions were mechanically cleaned using emery paper with grits ranging from 100 to 8000, then acetone cleaning and hot air drying. the potentiodynamic polarisation curve was created at room temperature in a 3.5 wt.% nacl solution to evaluate the electrochemical response of the base metal and coatings. during the electrochemical tests, the potential was changed between -1 and +1 volt versus ecorr. a potentiostat was utilised to keep the electrode potential at 1 mv as a predefined value over a wide range of applied current, with a scanning rate of 1 mv s-1. the solution was used at room-temperature in each experiment. table 4. equivalent weight and density of the base material and coating density, g cm-3 equivalent weight, g mol-1 ss-316 7.99 27.56 bn 3.45 24.82 the electrochemical cell is coupled to a potentiostat (series g-1000; gamry instruments). the input data is displayed on the computer screen for setup, and an experiment to balance out the open circuit potential is started after waiting 1800 seconds. the test was completed in 1800 s for tafel and 5-10 minutes for polarisation. the output results (icorr, ecorr, and corrosion rate) as well as the output curve, were displayed on the screen. to get tafel constants, the tafel test was plotted by considering the three average values for each coating state. http://dx.doi.org/10.5599/jese.1247 j. electrochem. sci. eng. 12(5) (2022) 851-863 surface protection of ss-316l with boron nitride 860 potentiodynamic polarization test the polarisation curve given in figure 9 was used to study bn coatings and base metal in 3.5 wt.% nacl electrolyte solution. the electrochemical corrosion test was carried out on a 0.38-squaremeter surface area. the specimens were dipped in the electrolyte for the 1800 s before being tested for corrosion to determine the ocp value in a steady state. tafel extrapolation yielded a boron nitride based coating corrosion rate of 0.040 ma cm-2. table 5 lists the kinetic parameters of corrosion: corrosion potential (ecorr), corrosion rate (cr) and corrosion current (icorr). table 5. corrosion kinetic parameters specimens ecorr/ mv icorr / μa cr / ma cm-2 316l (stainless steel) -718 2.307 0.409 bn-1 -405 1.599 0.040 bn-2 -308 1.399 0.038 bn-3 -564 4.858 0.839 i / a figure 9. tafel plots of different boron nitride based coatings according to the data above, the bn3 sample has the highest corrosion rate of 4.625 mm year-1 in comparison to the bn-1 coatings and coatings, respectively. the corrosion rate for bn-2 is the lowest, 1.114 mm year-1 microstructural study of the corroded surfaces) the microstructural features of the tested base metal coated specimens and at different coatings conditions were determined by employing sem. the secondary electron images of the base metal corroded surface of the coated specimens are shown in figure 10. the formation of pits is significant on the surface. the medium reacts with the surface to cause the penetration of the medium in the subsurface of the materials and results in the formation of pits [19-21]. the corroded surfaces of the three regimes have been represented in figure 11. from figure 11 it is clear that the base metal was preferentially attacked during the corrosion testing, and the severe pits of size ranges 0.5 to 1 μm were observed in the microstructure of the corroded base metal surfaces. the formation of pits on the surface of different thin films can be observed in figure 11. regime 1 and regime2 showed less initiation of pits on the surface as compared to the coating developed in regime 3, figure 11(c). the e / v m. singh et al. j. electrochem. sci. eng. 12(5) (2022) 851-863 http://dx.doi.org/10.5599/jese.1247 861 deeper pits can be observed in the coating developed in regime 3. the high c-bn content in the coating has resulted in the lower corrosion rate in regime 2. figure 10. the sem micrograph of base metal after corrosion testing in 3.5 wt.% nacl solution figure 11.the sem micrographs of (a) bn2, (b) bn1, (c) bn3 coatings after subjected to 3.5 wt.% nacl solution conclusion a detailed description of different characterization techniques/tools (metallurgical, mechanical) used for finding the characteristics of developed bn thin film specimens has been presented in this study. the conclusions obtained from the current work are presented below: • the bn thin films were successfully deposited on ss-316 using rf sputtering technique by three different parameters (regimes). http://dx.doi.org/10.5599/jese.1247 j. electrochem. sci. eng. 12(5) (2022) 851-863 surface protection of ss-316l with boron nitride 862 • bn2 thin film has a higher cubic phase of boron nitride than the bn1 and bn3 samples attributed by xrd analysis. the deposited coating's c-bn is identified as a significant phase amongst the deposited thin films. • the scratch tracks have revealed that bn2 coatings have a high hardness and adhesion strength. • 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https://doi.org/10.1088/2051-672x/ac1044 https://doi.org/10.1007/s11668-020-01057-8 m. singh et al. j. electrochem. sci. eng. 12(5) (2022) 851-863 http://dx.doi.org/10.5599/jese.1247 863 [19] d. zhang, s. j. harris, d. g. mccartney, materials science and engineering a 344(1-2) (2003) 45-56. https://doi.org/10.1016/s0921-5093(02)00420-3 [20] d. chidambaram, c. r. clayton, m. r. dorfman, surface and coatings technology 192(2-3) (2005) 278-283. https://doi.org/10.1016/j.surfcoat.2004.08.072 [21] j. porcayo-calderon, o. sotelo-mazon, v. m. salinas-bravo, c. d. arrieta-gonzalez, j. j. ramos-hernandez, c. cuevas-arteaga, international journal of electrochemical science 7(2) (2012) 1134-1148. ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1247 https://doi.org/10.1016/s0921-5093(02)00420-3 https://doi.org/10.1016/j.surfcoat.2004.08.072 https://creativecommons.org/licenses/by/4.0/) analytical method for heavy metal determination in algae and turtle eggs from guanahacabibes protected sea park doi: 10.5599/jese.2014.0051 145 j. electrochem. sci. eng. 4(4) (2014) 145-154; doi: 10.5599/jese.2014.0051 open access: issn 1847-9286 www.jese-online.org original scientific paper analytical method for heavy metal determination in algae and turtle eggs from guanahacabibes protected sea park abel i. balbín tamayo, ana m. esteva guas, juan j. piña leyte-vidal and marcelo maccini* department of analytical chemistry, faculty of chemistry, university of havana, havana, 10400, cuba *department of food science, university of teramo, 64023, teramo, italy corresponding author: e-mail: ibrahim@fq.uh.cu; tel.: +53 873 82 22 received: february 4, 2014; revised: april 4, 2014; published: december 6, 2014 abstract a standard digestion method coupled to electrochemical detection for the monitoring of heavy metals in biological samples has been used for the simultaneous analysis of the target analytes. square wave anodic stripping voltammetry (swasv) coupled to disposable screen-printed electrodes (spes) was employed as a fast and sensitive electroanalytical method for the detection of heavy metals. the aim of our study was to determine cd, pb and cu by swasv in brown algae (sargasum natan) and green turtle eggs (chelonia mydas) using screen-printed electrodes. the method proved useful for the simultaneous analysis of these metals by comparison between two different procedures for preparing the samples. two different approaches in digestion protocols were assessed. the study was focused on guanahacabibes brown algae and green turtle eggs because the metal concentrations recorded in this area may be used for intraspecific comparison within the guanahacabibes protected sea park area, a body of water for which information is still very scarce. the best results were obtained by digesting biological samples with the epa 3050b method. this treatment allowed the fast and quantitative extraction from brown algae and green turtle eggs of the target analytes, with high sensitivity and avoiding organic residues, eventually affecting electrochemical measurements. keywords cadmium; copper; lead; sargasum natan; chelonia mydas eggs; square-wave anodic stripping introduction marine contaminations by anthropogenic chemicals pose one of the worst problems to coastal and estuarine ecosystems around the word. certain heavy metals have gained great significance in chemical and toxicological studies of the environment. among those heavy metals are cd and pb, http://www.jese-online.org/ mailto:ibrahim@fq.uh.cu j. electrochem. sci. eng. 4(4) (2014) 145-154 heavy metal determine in algae and turtles eggs 146 which are generally toxic even at very low levels, and potentially toxic metals, e.g. cu which also has indispensable essential properties with different threshold levels in different types of plants and organisms, including man. therefore, the evaluation of heavy metal concentrations in marine organisms constitutes an important area of research [1-4]. the use of marine organisms (algae, turtles eggs, fish, etc.) as bioindicators to trace metal pollution is very common these days [5-7]. macroalgae are able to accumulate trace metals, reaching concentrations that are thousands of times higher than the corresponding concentrations in sea water. algae accumulate only free metal ions, the concentrations of which depend on the nature of suspended particulate matter [8], which, in turn, is formed by both organic and inorganic complexes. moreover, algae satisfy all of the basic requirements of bioindicators: they are sedentary, their dimensions are suitable, they are easy to identify and collect, they are widely distributed, and they accumulate metals to a satisfactory degree [9]. on the other hand, many investigations have reported the accumulation of heavy metals in marine sea turtle having a long lifespan and occupying high trophic levels in the marine food chains, and showed the utility of this specie as a biological indicator of heavy metal pollution. the intentional killing of any living sea turtle is prohibited, except for research purposes, for which only very limited samples are available. hence, it is possible to estimate the concentration of heavy metals in the tissue of nesting female sea turtles by using their eggs [10-15]. for that reason, the eggs are a useful non-lethal indicator for monitoring heavy metals in the body of sea turtles. a wet-digestion procedure can be applied to all types of biological materials. in this procedure, small amounts of nitric and perchloric acids are added to the sample material. the overall reliability of the digestion method will follow the adequate mineralisation of samples, i.e. the levels of the heavy metals. if any metals were linked in their insoluble form, they are not of relevance for pollution control [16]. the digestion method involves the liberation of the analyte (metal) of interest from an interfering matrix using a reagent (mineral/oxidising acids or fusion flux) and/or heat. the utilisation of reagents (acids) and external heat sources can then cause problems. in elemental analysis, these problems are particularly focused on the risk of contamination and loss of analytes [17-19]. considering the low content of heavy metals in environmental samples, sensitive analytical methods are required. the heavy metal determination in organic samples can usually be carried out by atomic spectrometry: inductively-coupled plasma optical emission (icp-oes) or electrothermal and flame atomic absorption spectrometry (etaas and faas), although the detection limits are not sufficient when the concentrations are too low. however, many preconcentration techniques have been employed for analysing complex matrices and samples with low levels of metals. hence, square-wave anodic stripping voltammetry (swasv) includes a preconcentration step in situ; for this reason, this is an electroanalytical technique used for the analysis of traces metals in solution [20-27]. such a combination of an effective accumulation step with an advanced measurement procedure results in a very low detection limit, and makes stripping analysis one of the most important techniques in trace analysis. the coupling of disposable screen-printed electrodes with stripping techniques is a revolution in comparison with conventional stripping analysis: the design and operation are greatly simplified, in accordance with the requirements of a decentralised assay. the greater proportion of articles a. i. balbín tamayo at al. j. electrochem. sci. eng. 4(4) (2014) 145-154 doi: 10.5599/jese.2014.0051 147 have utilised the technique of stripping voltammetry, gaining detection limits in the low ng/ml (ppb) region [28]. screen-printed electrodes are planar devices realised by printing layers of different electroconductive and insulating inks with controlled thickness and shapes on a plastic substrate. in this work, the carbon surface of the screen-printed working electrode was employed as a substrate for a thin mercury film (tmf) [29]. the aim of our study was to apply a digestion method (epa 3050b) to determine cd, pb and cu by square-wave anodic stripping voltammetry in brown algae (sargasum natan) and green turtle (chelonia mydas) eggs, using a screen-printed electrode, and it demonstrated the usefulness of this method for the simultaneous analysis of cd, pb and cu by comparison between two different procedures for preparing the samples. experimental collecting and treatment of samples the study area was located in antonio beach guanahacabibes protected sea park. this site is situated in an area characterised by low anthropogenic activity [30]. the sample of brown algae (sargasum natan) was handpicked in the subtidal zone at a depth of about 2-3 m. care was taken to choose the sample to ensure that all were at a similar stage of development. the samples were washed in seawater at the sampling site and transferred to the laboratory in pre-cleaned polyethylene bags under refrigerated conditions. upon arrival at the laboratory, they were thoroughly cleaned and any sediment was carefully removed with nylon brushes under tap water for a few seconds. algal material was rapidly rinsed in deionised water (milli-q, millipore corp) to minimise any possible metal loss during the procedure and was then pulverised. finally, the samples were frozen and stored (4 °c) until analysis. the green turtle (chelonia mydas) egg samples were collected in the nesting area of this species. all samples were stored at 4 °c until chemical analysis, and then the eggshell, the albumen and the yolk were subsequently separated. the separation was carried out quickly to prevent thawing. samples were digested by two separate digestion procedures in order to select the simplest, which in turn would provide suitable analytical results: a) general acid digestion: a 1 g dried sample was placed in a teflon beaker; the acid digestion reagent (concentrated hno3) was added and the mixture was allowed to stand overnight. the sample was heated until the production of red no2 fumes had ceased. this mixture was digested via the addition of hclo4 and was heated until it had evaporated to a small volume. the samples were brought to an appropriate volume with a dilute acid solution (0.01 mol l -1 hcl). b) the method epa 3050b [17,18,31] was used to produce a transparent solution. this is a very strong acid digestion that will dissolve almost all elements that could become “environmentally available”. for the digestion of samples, a representative 1 g (dry weight) sample was digested with the repeated addition of nitric acid (hno3) and hydrogen peroxide (h2o2). the resultant solutions were diluted to a known volume with 0.01 mol l -1 hcl. for each analytical batch of samples processed, blanks were carried throughout the entire sample preparation and analytical process. these blanks will be useful for determining whether samples are contaminated, and are necessary to provide a realistic estimate of interferences that could be encountered in the analysis of test samples. j. electrochem. sci. eng. 4(4) (2014) 145-154 heavy metal determine in algae and turtles eggs 148 heavy metal determination all experiments were carried out using a palmsens portable electrochemical analyser (palmsens bv, houten, the netherlands). the conditions for square wave voltammetry striping onto a screenprinted electrode of carbon modified by plated hg films were: cd(ii), pb(ii) and cu(ii) analysis: conditioning potential (econd) 0.3 v for 60 s, deposition potential (edep) − 1.0 v for 300 s, equilibration time (teq) 30 s, sw amplitude (eamp) 28 mv, step potential (estep) 3 mv, frequency (f) 15 hz. electrodes were serigraphically screen-printed with a shape similar to that reported by palchetti [29]. they consisted of a round-shaped working electrode (diameter 3 mm), a graphite counter electrode and a silver pseudo-reference electrode. in addition, the silver electrical contacts were covered by a graphite layer in order to prevent oxidation phenomena during storage. graphite-based hg-modified screen-printed electrodes were used as the working electrode. these are based on the use of a special coating cellulose-derivative film deposited onto the graphite working electrodes containing a hg(ii) salt, as reported by meucci [32]. hg(ii) is reduced from the salt to the metallic form and the modified sensor can be then used for heavy metal accumulation and stripping. the use of this strategy allows the use of large amounts of hg solutions to be avoided, whilst retaining the high sensitivity which characterises mercury-coated electrodes [32]. each sensor was pre-treated in 0.1 mol l -1 hcl before being used for the first time, by applying ten cycles of square wave voltammetry (swv) using the following conditions: potential initial 1 v, potential final 0 v, scan rate 50 mv s -1 , sw amplitude (eamp) 28 mv, step potential (estep) 3 mv, frequency (f) 15 hz. this step is necessary to obtain a stable baseline. then, 0.1 mol l -1 hcl was used as the supporting electrolyte. all measurements were performed without removing oxygen from the solution. the measurements were performed by immersing the sensor in 5.0 ml of solution, with magnetic stirring during the conditioning and accumulation steps, whereas the square wave scan was performed without stirring. suprapure grade hydrochloric acid was purchased from merck. the water used for the preparation of solutions was from a milli-q system (millipore). the working standard solution of cd, pb and cu was prepared by diluting standard 1 g l -1 metal solutions with 0.01 mol l -1 hcl. statistical analysis for the statistical treatment, the experimental results followed the recommendations proposed by miller [33]. determinations of means, standard deviations, coefficients of variation and percentage recovered were performed using statistical software. lod: the limit of detection, expressed as the concentration cl, or the quantity ql, is derived from the smallest measure xl, that can be detected with reasonable certainty for a given analytical procedure. the value of xl is given by equation (1): xl = xbl + ksbl 1 loq: the lowest concentration of an analyte that can be determined with acceptable precision (repeatability) and accuracy under the stated conditions of the test. the ability to quantify is generally expressed in terms of the signal or analyte (true) value that will produce estimates with a specified relative standard deviation (rsd), which is commonly 10%. a. i. balbín tamayo at al. j. electrochem. sci. eng. 4(4) (2014) 145-154 doi: 10.5599/jese.2014.0051 149 results and discussion when measurements are made at low analyte levels, e.g. in trace analysis, it is important to determine the lowest concentration of the analyte or property value that can be confidently detected by the method, and the lowest concentration of analyte that can be determined with an acceptable level of repeatability, precision and trueness. the importance in determining this, and the problems associated with it, arises from the fact that the probability of detection does not suddenly change from zero to unity as some threshold is crossed. the detection and quantification limits for the general acid digestion and epa 3050b by square wave voltammetry striping methods are shown in table 1. with these procedures for preparing the samples, tiny, clear, well-separated signals corresponding to the different metals were recorded by swasv; no matrix effect and reproducible peaks and linear standard addition plots were observed in digested reagent blanks. the mean calculated detection limits method (based on three times the standard deviation of the blank signal) and quantification limits (based on ten times the standard deviation of the blank signal) for cd, pb and cu showed a marked improvement over those reported by wang, locatelli and palchetti [25,34,35] . taking into account the low detection limits, quantification limits and coefficient of variation (cv) in table 1, the general acid digestion and epa 3050b using square wave voltammetry anodic striping methods give good estimations for the metals analysed. table 1. detection limits, quantification limits and coefficient of variation for cd, pb and cu for general acid digestion and method epa 3050b by square wave voltammetry anodic striping. metal general acid digestion method epa 3050b lod, 10 -4 µg/gdry loq, 10 -4 µg/gdry cv, % lod, 10 -4 µg/gdry) loq, 10 -4 µg/gdry cv, % cd 12.5 15 1.6 13 19 1.5 pb 310 350 3.2 150 200 6.6 cu 210 240 4.7 118 130 3.4 determination of heavy metals in brown algae (sargasum natan) and green turtle (chelonia mydas) eggs all metal contents reported in this work refer to the initial dry mass. mean metal concentrations are reported as values with standard deviations. cd, pb and cu concentrations in brown algae and green turtle eggs are shown in table 2 and 3 for the general acid digestion and epa 3050b methods, respectively. the standard deviations of pooled samples refer to the variability within different replicates. for these procedures for preparing the samples, swasv recorded tiny, clear, well-separated signals corresponding to the different metals (figs. 1-4); no matrix effect, reproducible peaks and linear standard addition plots were observed in the digested biological matrix. different concentrations of cd, pb and cu were used to perform linear regression analysis for the utilised screen-printed electrodes. the linear regression analysis, generated by plotting the height of the peaks obtained for each concentration, gave the following equations:  general acid digestion of brown algae for cd: ip = 0.24 + 10.7 ccd, for pb: ip = 6 + 383 cpb for cu: ip = 10.8 + 356 ccu  general acid digestion of green turtle eggs j. electrochem. sci. eng. 4(4) (2014) 145-154 heavy metal determine in algae and turtles eggs 150 for cd: ip = 0.198 + 9 ccd, for pb: ip = 3.22 + 42 cpb for cu: ip = 3.5 + 44 ccu  epa 3050b of brown algae for cd: ip = 0.43 + 16 ccd, for pb: ip = 3.38 + 227 cpb for cu: ip = 3.95 + 59 ccu  epa 3050b of green turtle eggs for cd: ip = 0.27 + 6.6 ccd, for pb: ip = 7.64 + 84 cpb for cu: ip = 7.9 + 88 ccu e / v vs. ag/agcl figure 1. signals corresponding to standard addition the different metals concentration to brown algae samples digest by method general acid digestion. +0.02 ppm and +0.04 ppm of multistandard of cd(ii), pb(ii), cu (ii) e / v vs. ag/agcl figure 2. signals corresponding to standard addition the different metals concentration to green turtles eggs samples digest by method general acid digestion +0.01 pm and +0.02 ppm of multistandard of cd(ii), pb(ii), cu (ii). i p /  a i p /  a a. i. balbín tamayo at al. j. electrochem. sci. eng. 4(4) (2014) 145-154 doi: 10.5599/jese.2014.0051 151 e / v vs. ag/agcl figure 3. signals corresponding to standard addition the different metals concentration to brown algae samples digest by method epa 3050b. +0.01 ppm and +0.02 ppm of multistandard of cd(ii), pb(ii), cu (ii) e / v vs. ag/agcl figure 4. signals corresponding to standard addition the different metals concentration to green turtles eggs samples digest by method epa 3050b +0.02 ppm and +0.04 ppm of multistandard of cd(ii), pb(ii), cu (ii) in all cases, linearity ranging from 10 ppm to 30 ppm was obtained with a correlation ≥ 0.98 for all of the metals analysed. the values determined in the brown alga and turtle eggs generally had low values of standard deviation and a coefficient of variation of 5 % for both moist digestion techniques used to digest the brown seaweed and green turtle eggs; this is indicative of analytical quality results. i p /  a i p /  a j. electrochem. sci. eng. 4(4) (2014) 145-154 heavy metal determine in algae and turtles eggs 152 the samples were analysed by the method under validation both in its original state and after the addition (spiking) of a known mass of the analyte to the test sample. in the absence of reference materials, bias was investigated by spiking and recovery [22,35]. spiking/recovery studies are very strongly subjective; the recoveries that are significantly different from unity indicate that bias is affecting the method. better spiking/recovery data were obtained by the epa 3050b method, even though the poor recovery by general acid digestion was certainly an indication of a lack of trueness. the variation between the spiking/recovery data using different digestion methodologies may not only be due to volatility during digestion but could also be linked to the way in which these elements are attached to the biological matrix. on the other hand, the bioaccumulation of these metals in brown algae is influenced by numerous factors: ph, ligand concentration and type, and various sediment components [36-38]. trace metal concentrations reported in this study were of the same order of magnitude as those measured by other authors in uncontaminated sites [36,39]. these analysed metals in brown algae showed low anthropogenic activity. table 2. concentration of heavy metals in brown algae (sargasum natan) and in green turtle eggs (chelonia mydas) by general acid digestion metal brown algae (sargasum natan) green turtle eggs (chelonia mydas) means ± sd, µg/gdry cv, % recuperation surrogate recovery, %* means ± sd, µg/gdry cv, % recuperation surrogate recovery, %* cd 1.16 ± 0.06 5.0 89 2.2 ± 0.1 4.5 89 pb 0.79 ± 0.01 5.6 92 7.6 ± 0.2 2.6 90 cu 1.52 ± 0.01 1.0 78 7.7 ± 0.2 2.6 100 * recoveries for standard solutions are calculated by dividing the observed value by the expected value. the result is multiplied by 100 to give a percent recovery table 3. concentration of heavy metals in brown algae (sargasum natan) and in green turtle eggs (chelonia mydas) by epa 3050b metal brown algae (sargasum natan) green turtle eggs (chelonia mydas) means ± sd, µg/gdry cv, % recuperation surrogate recovery, %* means ± sd, µg/gdry cv, % recuperation surrogate recovery, %* cd 0.528 ± 0.004 2.9 94.3 4.6±0.2 4.3 100 pb 1.49 ± 0.01 4.3 100.0 8.9±0.3 3.1 101 cu 6.7 ± 0.1 1.5 100.0 8.7±0.2 1.8 101 * recoveries for standard solutions are calculated by dividing the observed value by the expected value. the result is multiplied by 100 to give a percent recovery green turtles are herbivorous and feed on macroalgae, occupying a trophic level lower than carnivorous turtles. in this specie, cu plays a crucial role in oxygen transport, energy production and enzyme activity, and it can be freely transferred from the mother to the egg [10]; however, although cd and pb do not perform any known role in biological systems, limited amounts were transferred, which suggests the relation of metal concentrations in egg. trace metal concentrations in turtle eggs reported in this study were the largest compared to those measured by other authors in sea turtles [10,12,15]. we hypothesise that the concentrations of cd, pb and a. i. balbín tamayo at al. j. electrochem. sci. eng. 4(4) (2014) 145-154 doi: 10.5599/jese.2014.0051 153 cu in turtle eggs could depend mainly on their feeding habits, as already suggested by other authors [10,40]. in addition to diet composition, age and gender could be important factors affecting metal excretion in egg. conclusions digestion techniques studied for the treatment of samples suggest that the epa3050b technique should be used for the simultaneous analysis of cd, pb and cu in in brown algae and sea turtle eggs by anodic stripping voltammetry with square wave, as it showed the highest percentage recovery values with low coefficients of variation. the concentrations in brown algae (sargasum natan) confirm that pollution in antonio beach 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[40] j. s. gray, mar. pollut. bull. 45 (2002) 46-52. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.dna.gov.ar/ciencia/santar04/cd/pdf/203bg.pdf http://creativecommons.org/licenses/by/4.0/ {investigating products of titanium dissolution in the presence of fluoride ions with dual dynamic voltammetry} doi:10.5599/jese.502 141 j. electrochem. sci. eng. 8(2) (2018) 141-149; doi: http://dx.doi.org/10.5599/jese.502 open access : : issn 1847-9286 www.jese-online.org original scientific paper investigating products of titanium dissolution in the presence of fluoride ions with dual dynamic voltammetry noémi kovács, laura sziráki, soma vesztergom, gyözö g. láng department of physical chemistry, laboratory of electrochemistry and electroanalytical chemistry, eötvös loránd university, pázmány péter sétány 1/a, 1117 budapest, hungary corresponding author’s e-mail: kovacsnoemi@chem.elte.hu; tel.: +36-1-372-2500/1527 received: january 15, 2018; accepted: february 21, 2018 abstract products of titanium dissolution in the active state are investigated in fluoride containing 1 m h2so4 solutions. the novel method of dual dynamic voltammetry, applied to ti disk/pt ring rotating ring–disk electrodes, is utilized for the simultaneous detection of different dissolution products. potential regions where certain products (primarily, tiiii and h2) are formed are identified by a 3d electrochemical map constructed based on the ring–disk electrode measurements. besides dissolution in the form of tiiii species and hydrogen evolution, the formation and prompt oxidation of tiii can also be presumed under the applied conditions. keywords rotating ring–disk electrodes; electrochemical map; cyclic voltammetry; dental implant materials; corrosion introduction due to its extensive application, the electrochemical stability of titanium has been the subject of investigations for more than fifty years [1–14]. the pure metal itself, and also some of its alloys are widely applied as materials of dental implants and braces. thus, being aware of the possible dissolution processes of ti, which can take place in acidic media and in the presence of fluoride ions, is paramount. assessing the possible corrosion products of ti alloys that contain materials with toxic forms is of special importance. accordingly, the literature of the topic is abundant and continuously expanding [1–22]. in this paper we demonstrate the applicability of a novel experimental method, dual dynamic voltammetry [23–25], for the detection of different dissolution products of titanium. the described technique is applicable to the vast majority of generator-collector systems, in what follows, however, we will concentrate on rotating ring–disk electrodes (rrdes). http://dx.doi.org/10.5599/jese.502 http://www.jese-online.org/ mailto:kovacsnoemi@chem.elte.hu j. electrochem. sci. eng. 8(2) (2018) 141-149 ti dissolution in the presence of fluoride ions 142 in conventional rrde experiments the potential (or the current) of at least one of the two working electrodes is held constant. it was recently shown, however, that independent and dynamic potential programs can also be applied simultaneously to the disk and the ring electrodes [23–26]. by means of dual dynamic potential control, the sensitivity of the rrde system can significantly be increased, and this control technique also enables the application of several new methods such as the so-called fast ring scanning technique [24]. in this paper, we apply this latter technique for investigating the dissolution of titanium in the presence of fluoride ions. since dual dynamic potential control is not feasible by the application of commercially available bipotentiostats, our measurements were carried out by using our self-built measurement station [27]. as it is going to be shown, fast ring scanning experiments on an rrde can be applied very effectively for studying the dissolution of titanium in acidic media, and in particular, for the detection of different dissolution products. in order to demonstrate the advantages of this technique, we chose a relatively simple target system: a ti disk/pt ring rrde immersed into 1 m h2so4 solutions, containing f– in different concentrations. titanium dissolution under similar conditions has been investigated before. titanium exhibits excellent corrosion resistance in sulfuric acid solutions, due to the easy formation of a highly protective oxide film (mainly tio2). this oxide coating possesses a rather high chemical stability [2,3], preventing the corrosion of the metal. however, researchers seem to agree that fluoride ions, when present in the electrolyte solution, can dissolve the protective oxide layer, exposing fresh titanium surface, and thus destroying the corrosion resistance [2,4,5]. titanium dissolution in sulfuric acid solutions can also be observed in the absence of f– [6–10], although at a considerably lower rate compared to f– containing media. dissolution in the oxidation state of tiiv is likely to occur at positive potentials, in the passive potential region [7], while the formation of tiiii and tiii species is supposed to occur at negative potentials, in the active potential region [6]. tiii can, however, easily be oxidized in aqueous media, resulting the formation of elemental hydrogen [6,11]. at negative potentials, evolution of hydrogen occurs in two parallel paths: one path involves the usual solvated proton and the other involves the adsorbed surface species (tioh)+ads, followed by a fast recombination reaction [11]. the aim of this work is to present how dual dynamic voltammetry, when used with rrdes, can allow the simultaneous detection of the different dissolution products of titanium, formed in a broad potential range. experimental measurements were carried out with rotating ring–disk electrodes in standard four electrode cells. in all cases, the cell contained a separate reference compartment being connected to the cell by a luggin capillary positioned close to the rrde surface. a nacl saturated calomel electrode (ssce) was applied as reference and two glassy carbon rods surrounding the rrde tip were connected and used as a counter electrode. during the presented measurements the electrode tip was rotated at 500 min–1. the 1 m h2so4 solution was made by dilution of concentrated sulfuric acid (merck) with milli-q water. the fluoride ion containing solutions were prepared by addition of a 1 m h2so4 + + 1 m naf solution, made from solid naf (vwr chemicals). solutions were purged by and maintained under argon. measurements were carried out in a water-jacketed glass cell at 37 °c. n. kovács et al. j. electrochem. sci. eng. 8(2) (2018) 141-149 doi:10.5599/jese.502 143 all the glass parts used in the experiments were previously immersed in piranha solution for several hours, rinsed with de-ionized water, and cleaned by steam. rrde measurements were controlled by our self-built electrochemical workstation [27]. this system relies on the state-of-the-art data acquisition devices of national instruments (pci-4461 and pci-6014 boards) that can be used in combination with analogue bi-potentiostats. for the measurements presented here, the measuring system was equipped with a pine model afcbp1 bi-potentiostat. the measuring system was controlled by a software written in the national instruments labview development environment. during the experiments we used the following three configurations. configuration a for the experiments concerning titanium dissolution in 1 m h2so4 solution and in 1 m h2so4 + + 20 mm naf solution we applied a pine afe6r1ptpk changedisk rotating ring–disk electrode tip containing a fixed platinum ring (ar = 0.1100 cm2) with a pine afed050p040 titanium disk (99.999 %, ad = 0.1963 cm2). the gap size between the ring and the disk was 0.75 mm. collection efficiency calculated from the geometric parameters is 25.6 % [28]. prior to measurements, the disk and the ring were separately polished by sic paper and diamond suspension (finest grain size: 1 μm). after polishing, the electrode surfaces were rinsed with pure ethanol and milli-q water. before recording the presented experiments, the titanium disk was left for 1 hour in 1 m h2so4 solution at ocp. meanwhile, high speed cyclic voltammograms were recorded at the ring in order to clean its surface and to gain a stable baseline cv shape for the collection experiments [24]. after the addition of the fluoride ion containing solution the disk was etched for 15 min at ocp, then we started to polarize it slowly towards positive potentials. the presented data are results of the first cathodic sweep. configuration b experiments investigating hydrogen evolution and oxidation on platinum were carried out with the pine afe6r1ptpk changedisk rotating ring–disk electrode tip containing a fixed platinum ring mentioned above. instead of the titanium disk insert, in this case we used a pine afed050p040 platinum (99.99 %) disk. the geometric parameters and the applied polishing procedure were the same as in case of configuration a. before recording the presented data, high speed cyclic voltammograms were simultaneously recorded at the disk and ring electrodes for 1 hour. configuration c for the experiments demonstrating the effect of fluoride ion concentration in the electrolyte we used a fixed “home-made” titanium (99.99 %) disk/platinum ring rotating ring–disk electrode (ad = 0.1963 cm2, ar = 0.2134 cm2) with collection efficiency of 37.5 %, calculated from the geometric parameters [28]. the gap size between the ring and the disk was 0.60 mm. prior to measurements, the surface of the electrode tip was renewed by lathe machining followed by rinsing with ethanol and milli-q water. the presented measurement series were carried out with step by step additions of fluoride ion containing solution. after every addition step, the titanium disk was etched for 15 min at ocp before starting the individual measurements. j. electrochem. sci. eng. 8(2) (2018) 141-149 ti dissolution in the presence of fluoride ions 144 figure 1. (a) control signal of the disk and the ring electrodes. while the disk electrode was polarized at a relatively slow sweep rate (1 mv/s), high speed cyclic voltammograms were recorded at the ring electrode (sweep rate: 100 mv/s). (b) measured current at the disk and the ring electrodes as a function of the electrode potential. the emphasized cyclic voltammograms on the ring (1–4) were recorded while the disk potential was at the marked values. (c) 3d representation of the measured data created by bilinear interpolation (grid resolution: 50 x 50 mv). the current measured on the ring electrode during the anodic ring scans is plotted as a function of the disk and ring potentials, edisk and ering. results and discussion for the investigation of titanium dissolution in the active potential region in the presence of fluoride ions, a special example of dual dynamic voltammetry, the fast ring scanning technique [24] was applied. the essence of this technique is that the disk electrode is polarized at a sufficiently low and the ring electrode at a sufficiently high scan rate. potential and current data measured this way can be used for the creation of a 3d map that may reveal the electroactive products or intermediates formed in the electrode processes taking place on the disk electrode. the results of the measurements with a ti disk / pt ring rrde in 1 m h2so4 + 20 mm naf and the applied potential program can be seen in figure 1. the disk electrode was polarized at 1 mv/s sweep rate from 100 mv vs. ssce, towards negative potentials. at the same time, cyclic voltammograms were recorded at 100 mv/s on the ring between potential limits of –200 mv and 1200 mv vs. ssce. these experiments were carried out using cell configuration a (described in the experimental section). due to the hydrodynamic flow created by rotation, the products formed on the disk reach the surface of the ring where the electroactive species can be detected. n. kovács et al. j. electrochem. sci. eng. 8(2) (2018) 141-149 doi:10.5599/jese.502 145 when using fast ring scanning, the current yielded by the detected species is superimposed on a baseline, the cyclic voltammogram of the ring. figure 2. difference (δiring) of the ring current and the reference anodic scan (eref, disk = 0 v vs. ssce) as a function of edisk and ering from two different perspectives: (a) and (b). in figure 1(b) it is apparent that the polarization of the disk towards negative potentials results in no significant change of the ring cvs; at least, until no current on the disk is measured (see point 1 on the disk voltammogram in figure 1(b) and the corresponding ring cv). as the disk current starts to increase (2), the positive potential parts of the ring cvs start to lift and, later on (3), the negative potential parts also start slowly to increase. after the disk current reaches its maximum, the positive potential parts of the ring cvs tend to fall back, while the negative potential parts keep increasing. in order to show all these changes at the same time, a 3d map can be constructed (figure 1(c)) by means of bilinear interpolation, using the measured disk potential, ring potential and ring current data. for details of the interpolation algorithm, see [29]. the ring scan marked as 1 in figure 1(c), recorded while the disk was at about 0 v vs. ssce, was chosen as reference and subtracted from the 3d map in order to obtain the “surface of changes”: the changes measured in the ring currents resulted by the polarization of the disk (δiring) as a function of the disk and ring potentials. figure 2 shows the electrochemical map constructed in this way from two different view-points. j. electrochem. sci. eng. 8(2) (2018) 141-149 ti dissolution in the presence of fluoride ions 146 for the identification of the peaks on figure 2, similar measurements were carried out in 1 m h2so4 solution with a ti/pt, and also with a pt/pt rrde using cell configuration a and b, respectively. figure 3 shows the results of these experiments. during the slow cathodic polarization of the platinum disk electrode in 1 m h2so4, the evolution of hydrogen starts to occur at suitably negative disk potentials. the hydrogen originating from the disk can be oxidized on the ring electrode, resulting in the changes of the measured pt cvs, as shown in figure 3(a). we note that similar changes in the ring cvs can be measured if we supply hydrogen from an outside source (gas bottle) to the electrolyte. at most potentials, where the platinum surface is active, the collected hydrogen causes an increase of the measured ring current. this increase seems to be independent of the electrode potential of the ring over a broad potential range. when the ring potential is high enough, however, so that the platinum surface is oxidized, the oxidation rate of hydrogen is much slower [30–32]. thus, the positive potential parts of the ring cvs hardly lift during the experiment. the situation is rather different in the case when a ti disk is applied in combination with a pt ring (1m h2so4). in case of a slow, negative-going polarization, the titanium disk begins to dissolve even in a fluoride-free electrolyte, as a result of a passive-active transition. this is shown by the moderate hump in the polarization curve of ti, shown in figure 3(b) at point 2. as a result, a moderate increase of current can be observed at positive ring potentials: an effect that was not present in the case of pt, and which is due to the collection of tiiii species [6]. at more negative potentials (see point 3), hydrogen evolution and slow titanium dissolution take place simultaneously [6,9] and current signals related to hydrogen detection dominate the ring cvs. figure 3. (a) hydrogen evolution resulted by a slow, negative-going scan on the platinum disk and oxidation of the produced hydrogen on the platinum ring of a pt/pt rrde in 1 m h2so4. (b) slow, negativegoing scan on the titanium disk and oxidation of the formed products and intermediates on the platinum ring of a ti/pt rrde in 1 m h2so4. in both cases the disk electrode was polarized towards negative potentials at 0.25 mv/s sweep rate while on the ring cyclic voltammograms were recorded between –200 mv and 1200 mv vs. ssce at 100 mv/s scan rate. based on the experiments presented in figure 3, we can come to the conclusion that the lift of the positive potential parts of the platinum cvs in figure 1(b) and 1(c) is caused mainly by the collection of tiiii species formed on the disk, while the lift of the negative potential parts of the ring cvs corresponds to the oxidation of hydrogen. n. kovács et al. j. electrochem. sci. eng. 8(2) (2018) 141-149 doi:10.5599/jese.502 147 figure 4. illustration of the effect of varying the fluoride ion concentration on the detectable ring current changes during fast ring scanning measurements. measurements were taken in 1 m h2so4 solutions containing naf at different concentrations as indicated. measurements were taken using configuration c, as described in the experimental section. a reference disk potential of 0 v vs. ssce was used for creating the presented surfaces. here we note that hydrogen detection on the ring electrode may either be a result of direct hydrogen formation or of tiii production on the disk. as tiii is unstable in aqueous environments, it is expected to react quickly with water and/or protons to form hydrogen and tiiii [11]. accordingly, at negative disk potentials, tiiii formed as a result of tiii oxidation may contribute to the tiiii collection signal. it is interesting to note that the 3d surfaces of figure 2 apparently exhibit three, relatively distinct features. while one of these clearly corresponds to the collection of h2 and another to that of tiiii species, the third peak-like feature —based on the results shown in figure 3— can be attributed to the combined collection of hydrogen and tiiii species. in our interpretation, figure 2 describes the following scheme: while at not very negative potentials, the main dissolution product is tiiii, at more negative potentials, the disk generates h2, tiiii and, probably, also tiii. as tiii is very unstable in aqueous solutions, it gets oxidized resulting the formation of tiiii and additional hydrogen. note that the ring signal corresponding to tiiii does not fully decay, not even at extreme cathodic disk polarization (figure 2(b)). thus it can be assumed that even at these very negative potentials some tiiii is formed on the disk or that —more likely— the disk generates tiii, the subsequent oxidation of which results in the collection of tiiii on the ring. in a narrow potential region on the ring, the collection of tiiii and hydrogen co-occurs, resulting in the peak-like feature marked as “tiiii + h2” in figure 2(b). the qualitative interpretation of the measurements, outlined above, remains the same for fluoride concentrations between 5 and 20 mm, although the intensity of the ring current signals (the amount of dissolved species) grows in strict correlation with the fluoride concentration. this is shown by figure 4 and can also be expected based on literature [5,14]. summary by using the method of dual dynamic voltammetry, applied to a ti disk/pt ring rrde, products formed during the polarization of titanium in 1 m h2so4 + 20 mm naf solution in a broad potential range were simultaneously detected. in the applied fast ring scanning technique, high speed cyclic voltammograms were recorded on the ring electrode while the disk electrode was polarized slowly from the edge of the passive potential region to the active potential region. as a j. electrochem. sci. eng. 8(2) (2018) 141-149 ti dissolution in the presence of fluoride ions 148 result, a 3d electrochemical map of the dissolution products was constructed, giving a comprehensive illustration of potential regions where the different products are formed and where they can be converted electrochemically. besides dissolution in the form of tiiii species and hydrogen evolution, the formation and prompt oxidation of tiii can be presumed under the applied conditions. acknowledgements: this work was supported by the únkp-16-3 and únkp-17-3 new national excellence program of the ministry of human capacities (elte/8495/60(2016) and elte/12422/82(2017)). financial support from the national research, development and innovation office & hungarian scientific research fund (nkfi otka k109036) is gratefully acknowledged. s. vesztergom gratefully acknowledges support from grant nkfi pd-124079 from the national research, development and innovation office, hungary. references [1] m. stern, h. wissenberg, journal of the electrochemical society 106 (1959) 755-759. [2] j. j. kelly, electrochimica acta 24 (1979) 1273-1282. [3] d. j. blackwood, l. m. peter, d. e. williams, electrochimica acta 33 (1988) 1143-1149. [4] m. j. mandry, g. rosenblatt, journal of the electrochemical society 119 (1972) 29-33. [5] z. b. wang, h. x. hu, c. b. liu, y. g. zheng, electrochimica acta 135 (2014) 526-535. [6] r. d. armstrong, j. a. harrison, h. r. thirsk, r. whitfield, journal of the electrochemical society 117 (1970) 1003-1006. [7] e. j. kelly, journal of the electrochemical society: electrochemical science and technology 123 (1976) 162-170. [8] j. a. harrison, d. e. williams, electrochimica acta 27 (1982) 891-895. [9] w. b. utomo, s. w. donne, electrochimica acta 51 (2006) 3338-3345. [10] d. devilliers, m. t. dinh, e. mahé, d. krulic, n. larabi, n. fatouros, journal of new materials for electrochemical systems 9 (2006) 221-232. [11] e. j. kelly, h. r. bronstein, journal of the electrochemical society: electrochemical science and technology 131 (1984) 2232-2238. 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[27] s. vesztergom, n. kovács, m. ujvári, g. g. láng, tm technisches messen 84 (2017) 683-696. n. kovács et al. j. electrochem. sci. eng. 8(2) (2018) 141-149 doi:10.5599/jese.502 149 [28] s. vesztergom, in reference module in chemistry, molecular sciences and chemical engineering, elsevier, amsterdam, netherlands, 2018, chapter id. 13563-2. [29] s. vesztergom, m. ujvári, g.g. láng, in voltammetry: theory, types and applications, y. saito, t. kikuchi, eds., nova science publishers, new york, usa, 2013, p. 249. [30] s. schuldiner, journal of the electrochemical society 115 (1968) 362-365. [31] j. bao, d. d. macdonald, journal of electroanalytical chemistry 600 (2007) 205-216. [32] a. e. von mengershausen, n. v. almeida, m. n. barzola, j. o. zerbino, s. m. esquenoni, m. g. sustersic, green and sustainable chemistry 4 (2014) 55-59. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {electrochemical behaviour of poly(3,4-ethylenedioxytiophene) modified glassy carbon electrodes after overoxidation − the influence of the substrate on the charge transfer resistance} doi:10.5599/jese.508 151 j. electrochem. sci. eng. 8(2) (2018) 151-162; doi: http://dx.doi.org/10.5599/jese.508 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behaviour of poly(3,4-ethylenedioxytiophene) modified glassy carbon electrodes after overoxidation − the influence of the substrate on the charge transfer resistance dora zalka, soma vesztergom, mária ujvári, gyözö g. láng institute of chemistry, laboratory of electrochemistry and electroanalytical chemistry, eötvös loránd university, pázmány p. s. 1/a, h-1117 budapest, hungary corresponding author e-mail: zalkadora@caesar.elte.hu; tel.: +36 1 372 2500 / 1527 received: february 12, 2018; revised: march 22, 2018; accepted: march 22, 2018 abstract time dependence of the electrochemical impedance of an overoxidized glassy carbon|poly(3,4-ethylenedioxytiophene) (pedot)|0.1 mol·dm-3 sulfuric acid (aq.) electrode has been investigated. to follow the changes occurring at the film/substrate interface after the overoxidation procedure, successive impedance measurements were carried out. although the system is intrinsically nonstationary, the charge transfer resistance (rct) corresponding to different time instants could be determined by using the socalled 4-dimensional analysis method. the same post-experimental mathematiccal/analytical procedure could be used also for the estimation of the charge transfer resistance corresponding to the time instant just after overoxidation of the pedot film. the increase of the charge transfer resistance of the overoxidized system with respect to that of the pristine electrode suggests that during overoxidation the electrochemical activity of the film decreases and the charge transfer process at the metal/film interface becomes more hindered. after the overoxidation procedure, when the electrode potential was held in the “stability region” (at e = 0.4 v vs. ssce in the present case) the rct decreased continuously with experiment time to a value somewhat higher than that of the pristine electrode. by comparing the properties of the gc|pedot|0.1 m h2so4 and the au|pedot|0.1 m h2so4 electrodes a possible mechanistic explanation for the observed behavior has been proposed. this is based on the assumption that in the case of the gc|pedot|0.1 m h2so4 electrode two processes may occur simultaneously during the impedance measurements: (a) reduction of the oxidized surface of the gc substrate, including the reduction of the oxygen-containing surface functionalities and (b) readsorption of the polymer chains (polymer chain ends) on the surface. keywords non-stationary system; 4-dimensional analysis method; substrate-polymer interaction; oxidation of glassy carbon. * this paper is dedicated to the memory of our friend and colleague professor zdravko stoynov in recognition of his great contribution to electrochemistry. http://dx.doi.org/10.5599/jese.508 http://www.jese-online.org/ mailto:zalkadora@caesar.elte.hu j. electrochem. sci. eng. 8(2) (2018) 151-162 pedot modified glassy carbon electrodes 152 introduction intrinsically conducting organic polymers such as polythiophenes, polyanilines and polypyrroles have been studied intensively during the last decades. poly(3,4-ethylenedioxythiophene) [1], often abbreviated as pedot, and its derivatives are relatively stable compared to other conducting polymers. the conjugated polymer backbone, consisting of alternating c-c double bonds, provides for -orbital overlap along the molecule. pedot can be doped with many anions, including macromolecular polyanions such as poly(styrene sulfonate) (pss). previous studies have shown that pedot is highly insoluble in most solvents, electroactive in aqueous solutions [2-4], and exhibits a relatively high conductivity. the behavior of pedot in electrochemical systems is a hot topic nowadays due to its important applications in a variety of fields. many studies have been performed recently to investigate the electrochemistry of pedot in more detail by using voltammetric techniques (in most cases cyclic voltammetry). it has been found in refs. [5-9] that at sufficiently positive electrode potentials, degradation of the polymer occurs. the oxidative degradation of polypyrroles, polythiophenes is known as „overoxidation”. that is, when the positive potential limit of the cyclic voltammogram (cv) is extended to the region in which the “overoxidation” of the polymer film takes place, an oxidation peak (without a corresponding reduction peak) appears in the 1st cv (with a maximum in the vicinity of 1.4 v vs. sce in 0.1 m h2so4 solution). the oxidative degradation of pedot has been studied most frequently using cyclic voltammetry, impedance spectroscopy (eis) or electron spin resonance (esr) spectroscopy [5,10-13]. on the other hand, in refs. [6-8] it has been shown that pedot films in modified electrodes undergo structural changes during the degradation process. the most probable stages involved in the overoxidation/degradation process are: 1) overoxidation results in stress generation in the pedot film [14]. 2) formation of cracks due to internal stress. 3) the products of the degradation of the polymer leave the polymer layer. 4) after the formation of the line cracks, the film stress is partially released. 5) the partial delamination of the polymer layer leads to the exposure of the underlying metal substrate to the electrolyte solution. nevertheless, the polymer film still present on the substrate after overoxidation remains electroactive; and the areas with the polymer form a barrier between the metal substrate and the surrounding electrolyte solution. apart from the morphological changes, overoxidation can also affect the charge structure of the polymer film. poly(3,4-ethylenedioxythiophene) is a redox conductive polymer that incorporates counterions from the electrolyte solution to maintain electroneutrality; thus, its charging processes involve a detectable counter-ion flux leaving the film [15]. the above conclusions seem to be confirmed by the experimental data, but they give no information about the processes occurring after overoxidation of the polymer film. for instance, it is known that the impedance spectra of overoxidized poly(3,4-ethylenedioxytiophene) (pedot) films on gold recorded in aqueous sulphuric acid solutions differ from those measured for freshly prepared films [12]. the most interesting feature is the appearance of an arc (or a “depressed semicircle”) at high frequencies in the complex plane impedance plot. the time evolution of the impedance spectra is another remarkable feature of the electrodes with overoxidized pedot films [16,17]. according to this observation, the charge transfer resistance (rct) at the (electronically conductive) substrate/polymer film interface decreases continuously over several hours when the potential is held in the “stability region” after overoxidation of the film. this means that the impedance spectra recorded using the consecutive frequency sweep d. zalka et al. j. electrochem. sci. eng. 8(2) (2018) 151-162 doi:10.5599/jese.508 153 mode (typical of eis) are corrupted by typical errors caused by the system evolution during the experiment. according to the usual interpretation of the concept of impedance, impedance is not defined as time-dependent and, therefore, there should not exist an impedance out of stationary conditions. this means that if the requirement of stationarity in impedance spectroscopy is in conflict with the essential properties of the object, the measured data points are not “impedances”, the obtained sets of experimental data are not “impedance spectra” and they cannot be used in any analysis based on impedance models. there are some methods proposed in the literature to deal with a non-stationary behavior. stoynov proposed a method of determining “instantaneous” (“calculated”, “corrected” or “synthetic”) impedance diagrams for non-stationary systems based on a four-dimensional approach [18-22] (see supplementary material). recently, the method have been successfully applied for the determination of the charge transfer resistance and some other characteristic impedance parameters of the gold|poly(3,4-ethylenedioxytiophene) (pedot)| sulfuric acid (aq) electrode corresponding to different time instants, including the time instant just after the overoxidation of the polymer film, demonstrating that the 4-dimensional analysis method can not only be used for the correction of the existing (experimentally measured) impedance data, but it opens up the possibility of the estimation of the impedance spectra outside the time interval of the impedance measurements. according to the results presented in [16,17], the high and medium frequency regions of the impedance spectra are stronger affected by the time evolution than the values measured at low frequencies. in ref. [17] the following explanation was given for the decrease of the charge transfer resistance with time after overoxidation: during overoxidation at sufficiently positive potentials a significant fraction of the polymer chains become detached from the substrate surface (i.e. “desorption”, “deactivation” or “delamination” occurs). in contrary this, at a less positive potential the readsorption of the polymer chains (polymer chain ends) becomes possible, that is the number of the polymer chains directly contacted with (adsorbed on) the substrate surface is increasing with the passing time. this means that during overoxidation the “effective” coverage of the substrate by the polymer decreases, and at less positive potentials the coverage may start to increase again. the direction of the change in rct is in complete agreement with this hypothesis. on the other hand, there are also objections that can be raised against the above arguments. for instance, an opponent could argue that the gold oxide layer formation on the gold substrate surface plays the most important role in the observed relaxation process by preventing adsorption (or readsorption) of the polymer chains on the surface. however, this would mean that the behavior of the pedot/gold system is special, and in case of other substrates (e.g. glassy carbon) the post-overoxidation properties of the pedot-modified electrodes are different. in the present study the electrochemical properties of poly(3,4-ethylenedioxythiophene) films deposited on glassy carbon substrates have been investigated in aqueous sulfuric acid solutions with the aim to reveal the similarities and differences between the behavior of pedot-modified glassy carbon (gc) and gold electrodes. experimental all electrochemical measurements were performed with a computer driven electrochemical workstation (zahner im6 controlled by a thales software package) in a three-electrode arrangement. j. electrochem. sci. eng. 8(2) (2018) 151-162 pedot modified glassy carbon electrodes 154 electrodeposition of pedot: poly(3,4-ethylenedioxytiophene) films were prepared by galvanostatic deposition on both sides of thin gold and glassy carbon plates from 0.01 mol·dm-3 ethylenedioxytiophene (edot) solution containing 0.1 mol·dm-3 na2so4 supporting electrolyte. analytical grade 3,4-ethylenedioxythiophene (aldrich), p.a. na2so4 (fluka), and ultra-pure water (specific resistance 18.3 mω cm) were used for solution preparation. all solutions were purged with oxygen-free argon (linde 5.0) before use and an inert gas blanket was maintained throughout the experiments. the deposition was performed with galvanostatic method. the gold or glassy carbon plate in contact with the solution served as the working electrode (we). a pt wire immersed in the same solution served as the counter electrode (ce), and a kcl-saturated calomel electrode (sce) as the reference electrode (re). a constant current density of j = 0.2 ma·cm-2 (i = 0.2 ma) was applied for 1000 s (the geometric surface areas of the working electrodes were a = 1.0 cm2). the film thickness was estimated from the polymerization charge by using the charge/film volume ratio determined earlier by direct thickness measurements [2,23,24]. the average thickness of the pedot film was about 0.8 μm, the structure of the pedot film was globular, cauliflower-like[8,9], i.e. the thickness of the film was non-uniform. cyclic voltammetry and impedance measurements: solutions used for cyclic voltammetric and impedance measurements were prepared with ultra-pure water and p.a. h2so4 (merck). the solutions were purged with oxygen-free argon (linde 5.0) before use and an inert gas blanket was maintained throughout the experiments. in the conventional three-electrode cell configuration the pedot-modified gold or glassy carbon substrate in contact with the solution was used as the working electrode (we), a bended gold plate immersed in the same solution as the counter electrode (ce), and a nacl-saturated calomel electrode (ssce) as the reference electrode (re). the counter electrode was arranged cylindrically around the working electrode to maintain a uniform electric field. the overoxidation of the pedot film was carried out in 0.1 mol·dm-3 h2so4 solution a day after the deposition. results and discussion cyclic voltammetry according to fig.1 cyclic voltammograms in the potential range -0.1 – 0.6 v vs ssce were recorded before the overoxidation. both in the cases of au |pedot and gc |pedot the cyclic voltammograms change significantly if the positive limit of the electrode potential is extended to 1.5 v vs. sce (“strong” overoxidation, curves 2–4 in fig. 1a and 1b). it is well known [5-9,12,25] that between –0.3 and 0.8 v vs. ssce the oxidation-reduction process of the pedot films is reversible. however, at more positive potentials irreversible degradation of the polymer layer occurs. as it can be seen in fig. 1a and in fig. 1b (curve 1), the cyclic voltammograms of pedotmodified electrodes show almost pure capacitive behavior in the potential range between -0.1 v and +0.6 v, i.e. if the positive potential limit is kept below 0.8 v vs. ssce. if the polarization potential exceeds this critical value an oxidation peak without corresponding reduction peak appears (see curves 2-4 in fig. 1a and 1b). as can be seen from from fig. 2 the charge consumed during the oxidation process is different for the two types of electrodes. according to the results the overoxidation is more intensive in the case of au|pedot electrodes than in the case of the gc|pedot system. the different shapes of the 2nd and 3rd cv-s may indicate that the anodic overoxidation results in a more passive surface in the case of the gc substrate. d. zalka et al. j. electrochem. sci. eng. 8(2) (2018) 151-162 doi:10.5599/jese.508 155 fig. 1. curve 1: cyclic voltammetric curves recorded before overoxidation a) au|pedot|0.1 m h2so4 b) gc|pedot|0.1 m h2so4 electrode in the potential range from -0.1 v to 0.6 v vs. ssce. curves 2-4: successive cyclic voltammograms („overoxidation cycles”) recorded on a) au|pedot|0.1 m h2so4 b) gc|pedot|0.1 m h2so4 electrode in the potential range from -0.1 v to 1.5 v vs. ssce. the sweep rate was ν = 50 mv/s in all case fig. 2 change of the (a) anodic and (b) cathodic charges with the number of overoxidation cycles. the decrease in the cathodic charges of the (over)oxidized polymer films are considerably smaller. fig. 3 shows the schematic representation of the suggested process in connection with the au|pedot system. in the beginning, i.e. in the pristine films, the sulfur atoms of the pedot chains are connected to the gold surface. as already mentioned in the introduction, during overoxidation at sufficiently positive potentials a significant fraction of the polymer chains become detached from the substrate surface. in parallel with this process the oxidation of the electrode leads to the formation of a gold oxide layer. this atomic oxide layer can block the active surface temporarily and ad interim hinders the adhesion interactions with the polymer chains. this effect may be operative in the present case too. on the other hand, after the overoxidation treatment, at a less positive potentials the readsorption of the polymer chains (polymer chain ends) becomes possible. this means that during overoxidation the “effective” coverage of the substrate by the polymer decreases, and at less positive potentials the coverage may start to increase again. the direction of j. electrochem. sci. eng. 8(2) (2018) 151-162 pedot modified glassy carbon electrodes 156 the change in rct is in agreement with this hypothesis. in the case of glassy carbon substrate, the overoxidation of pedot can take place in the same way in the polymer chains, however, no reduction peak can be observed in the cyclic voltammograms of the gc|pedot|0.1 m h2so4 electrode (see fig. 1b). this difference can be explained on the basis of the electrochemical properties of the two substrates. it is well known that the electrochemical behavior of gold in aqueous media has been thoroughly studied, and the voltammetric behavior of gold in acidic media can be explained by monolayer oxide formation/removal and adsorption phenomena. the structure and electrochemical characterization of vitreous or glassy carbon has been the subject of research since it was first produced in the early 1960s. some early structural models assumed that both complex sp2and sp3bonded carbon atoms were present, but this idea is now discarded and many of the properties suggest a 100 % sp2 bonding [27]. the jenkins–kawamura [28] model suggests that glassy carbon consists a long, narrow and imperfect aromatic ribbons. more recent studies have revealed that glassy carbon be made up of graphitic sp2 ribbons (i.e. the structure of glassy carbon consists of long, randomly oriented microfibrils, however, it has also been suggested that glassy carbon comprises a fullerene-type structure [29]) and hybridized carbon atoms with oxygen-containing groups (including oxygencontaining surface functionalities as hydroxyl, phenol, lactone, carbonyl, o-quinone and pquinone, etc.) along the edges of these ribbons where the graphite layer terminates [30,31]. pedot that contains ethylenedioxy-bridge and sulfur atom at the end of the polymer chain can easily grow on and strongly adhere to the gc surface. cyclic voltammograms recorded in 0.1 m sulfuric acid solution reveal that irreversible oxidation of the (bare) glassy carbon surface takes place at electrode potentials more positive than about 0.6 v vs. ssce (fig 4a). the negative limit of this potential range roughly coincides with the pedot overoxidation onset potential region therefore the surface oxidation may occur in parallel with the (gradual) desorption of the polymer chains. on the other hand, the smooth surface of the pristine polished gc becomes roughened with electrochemical oxidation [32,33], the surface becomes more and more porous and the effective surface area increases. the electrolyte solution can penetrate into the pores, which leads to the increase of the capacitance. the contribution of pseudo-capacity to double-layer charging/discharging currents originates mostly from fast faradaic reactions of redox active surface carbon oxygen containing species (e.g. hydroquinione/quinone couple), formed at the exposed edge plane sites of carbon [34]. as it can be seen in fig 4b there is no significant difference between cyclic voltammograms obtained before fig.3 the suggested mechanism of the desorption and re-adsorption polymer chains during the overoxidation process and after overoxidation (when the potential is held in the “stability region”). d. zalka et al. j. electrochem. sci. eng. 8(2) (2018) 151-162 doi:10.5599/jese.508 157 and after the oxidative treatment. (it should also be noted here that the same statement is true for the charge transfer resistances determined by impedance spectroscopy before and after the oxidation.) impedance measurements after overoxidation, subsequent impedance measurements were performed on the gc|pedot|0.1 m h2so4 electrode at the electrode potential e = 0.4 v vs. ssce over a frequency range from 100 mhz to 100 khz as described in the experimental section (starting at τ2 in fig. 5a). the fig. 5b and fig. 5c data points were measured using ac signal amplitude of 5 mv at 60 discrete frequencies in the frequency region investigated during each scan. the time moment corresponding to τ2 was taken as the starting time of the impedance measurements (“timestamp”: t = 0 s), and 63 successive impedance spectra were recorded. it is (somewhat arbitrarily) assumed that the overoxidaton process stopped completely at τ1, i.e. at t = -50.0 s and δτ = 50.0 s (see fig. 5a). the recording time of a "single" spectrum was about 346 s. in fig. 5b and in fig. 4b the results are presented in the complex plane (where re(z) is the real part and im(z) is the imaginary part of the complex impedance, respectively). for the sake of comparison sets of impedance data measured for an au|pedot|0.1 m h2so4 electrode after overoxidation under similar conditions are presented in fig. 5c. as we can see from figs. 5b and 5c, the pristine pedot films show very small charge transfer resistances and close to ideal capacitive characteristics. on the other hand, the diameters of the capacitive arcs at high frequencies (and hence the charge transfer resistances) increase significantly during overoxidation both in the case of gc|pedot|0.1 m h2so4 and au|pedot|0.1 m h2so4 electrodes. the analysis of the impedances obtained for the gc|pedot|0.1 m h2so4 electrode were carried out in the same way described in refs [16,17,21, see also supplementary material]. the calculation of the “instantaneous” impedance diagrams was carried out using the real and imaginary parts of the impedances measured at identical frequencies („isofrequential components”). the charge transfer resistances at t = 0 s has been obtained by extrapolation of the iso-frequency dependencies to this time instant. fig. 4. a) cyclic voltammograms recorded on bare glassy carbon in 0.1 m h2so4 vs ssce. curve 1 was recorded before, and curve 5 after the three subsequent oxidation cycles (curves 2-4). the sweep rate was 50 mv / s. b) complex plane impedance diagram of the bare gc in 0.1 m h2so4 recorded at 0.4 v vs. ssce before () and immediately after (⚫) three oxidation cycles. the impedance spectra were recorded at 60 discrete frequencies between 0.1 hz and 100 khz (amplitude: 5 mv rms). j. electrochem. sci. eng. 8(2) (2018) 151-162 pedot modified glassy carbon electrodes 158 fig. 5 a) the potential program applied to the gc|pedot|0.1 m h2so4 electrode. the impedance measurements started at τ2. τ1 is the estimated end of the overoxidaton process. δτ = 50 s. b) successive (measured) impedance diagrams recorded on the gc|pedot|0.1 m h2so4 and c) au|pedot|0.1 m h2so4 on the electrode at e= 0.4 v vs. ssce. the solid lines are to guide the eye only: not curve fits. the spectra signed by e in figs. 5 b and 5 c are calculated spectra corresponding to the presumed ends of the overoxidation processes, b) t = -50 s, c) t = -53 s, respectively. (the spectra e were calculated using the method of stoynov [16,17].) the values of the charge transfer resistances at t = 0 s and at the other time instants were estimated by fitting the high-frequency part of the “instantaneous” spectra with the impedance function (eq. 1) corresponding to the equivalent-circuit analog shown in the inset of fig. 6. u 1 ct 1 ( ) (i ) z r b r    − = + + (1) in eq. 1 rct is the charge transfer resistance and ru is the uncompensated ohmic resistance (which is mainly the resistance of the electrolyte solution between the working and the reference electrode). this simple equivalent circuit is based on the model proposed e.g. in [13,35], wherein the high frequency (“double layer”) capacitance (cd) at the substrate/polymer interface has been replaced by a constant-phase element (cpe), which more accurately imitates the nonideal (distributed) behavior of the double layer (zcpe = 1/b(i)-, where i is the imaginary unit, b and α are the cpe parameters, and ω is the angular frequency, respectively). the estimated values of the parameters (obtained by complex non-linear least squares (cnls) fitting program based on the gauss – newton – levenberg marquardt method with inverse magnitude weighting) are shown in table 1. the spectra calculated (simulated) by using the “bestfit” parameters are shown fig. 6. d. zalka et al. j. electrochem. sci. eng. 8(2) (2018) 151-162 doi:10.5599/jese.508 159 fig. 6. corrected complex plane impedance diagrams (“instantaneous impedances”) of the gc|pedot|0.1 m h2so4 system calculated for t = -50.0 s (e), t = 0 s (s1), t = 346.6 s (s2), t = 692.5 s (s3), t = 1039.4 s (s4), and t = 1386.7 s (s5), respectively. the solid curves were calculated (simulated) by using the “best-fit” parameters given in table 1. table 1. results of cnls fitting of impedance data of gc|pedot system: the estimated values of the parameters. ru is the uncompensated ohmic resistance (resistance of the solution), rct is the charge transfer resistance, b and α are the parameters of the cpe, δru, δrct, δb and δ are the corresponding 95 % confidence intervals. data set t / s ru / ω δru / ω rct / ω δrct / ω b / ω-1s δb / ω-1s  δ e -50.0 7.8 ±1.5 1391.4 ±1.3 2.91·10-5 ±0.12·10-6 0.878 ±0.045 s1 0 7.22 ± 0.42 1222.4 ±6.7 2.55·10-5 ±0.41·10-6 0.863 ±0.019 s2 346.6 6.15 ± 0.57 728.2 ±6.7 2.11·10-5 ±0.64·10-6 0.855 ±0.037 s3 692.5 5.96 ± 0.58 507.2 ±5.3 1.82·10-5 ±0.75·10-6 0.849 ±0.047 s4 1039.4 5.99 ± 0.51 384.9 ±4.3 1.93·10-5 ±0.76·10-6 0.845 ±0.051 s5 1386.7 5.69 ±0.54 258.6 ±3.6 1.76·10-5 ±0.95·10-6 0.831 ±0.070 s6 1765.0 5.83 ±0.42 219.9 ±2.9 1.76·10-5 ±0.87·10-6 0.829 ±0.066 s7 2113.5 5.93 ±0.33 190.7 ±2.4 1.76·10-5 ±0.74·10-6 0.828 ±0.059 s8 2464.7 5.39 ±0.56 172.6 ±3.1 1.57·10-5 ±0.16·10-6 0.807 ±0.097 s9 2823.9 5.56 ±0.42 153.7 ±2.5 1.59·10-5 ±0.99·10-6 0.809 ±0.084 s10 3173.1 5.88 ±0.30 138.2 ±1.9 1.62·10-5 ±0.82·10-6 0.810 ±0.071 s11 3522.3 5.73 ±0.20 125.6 ±1.5 1.63·10-5 ±0.61·10-6 0.811 ±0.054 s12 3872.1 5.88 ±0.14 114.9 ±1.1 1.65·10-5 ±0.47·10-6 0.812 ±0.043 some of the charge transfer resistance values (the rct vs. t plot) determined for the gc|pedot|0.1 m h2so4 by using the “instantaneous” impedances are shown in fig. 7a. it can be seen from table 1 (and from fig. 7a), that the highest value of rct is about (1391.4±1.3) ω. this value corresponds to the time instant just after overoxidation of the film. similarly to the au|pedot|0.1 m h2so4 electrode (see fig. 7b), starting from this value, rct decreases continuously with experiment time to a value somewhat higher than the charge transfer resistance of the j. electrochem. sci. eng. 8(2) (2018) 151-162 pedot modified glassy carbon electrodes 160 pristine electrode. it can clearly be concluded that the charge transfer resistances measured for the gc|pedot|0.1 m h2so4 electrode after overoxidation are significantly higher than those measured for the au|pedot|0.1 m h2so4 electrode, however, the relative rate of the relaxation process is apparently higher (the “half-life” of rct for gc|pedot is about t1/2,gc ≈ 400 s, while for au|pedot t1/2,au ≈ 860 s). fig. 7 time evolution of the charge transfer resistance during the experiment in the case of gc|pedot|0.1 m h2so4 electrode (a) and au|pedot|0.1 m h2so4 electrode (b, see insert). rct values were determined using the calculated impedance spectra. in both cases: the rct value designated by e corresponds to the time instant of the presumed end of the overoxidation processes: t = -50.0 s (gc|pedot) and t = -53.0 s (au|pedot). as it can be seen from the results, the direction, dynamics and nature of the temporal change in rct are consistent with the hypothesis outlined in [17] and in the introduction to this study. we can assume that during overoxidation a significant fraction of the polymer chains become detached from the substrate surface. the partial delamination of the polymer layer leads to the exposure of the underlying substrate material to the electrolyte solution. in areas where the substrate is in direct contact with the electrolyte solution, practically no charge transfer across the interface occurs (i.e. the local charge transfer resistance is close to infinite). nevertheless, the polymer film still present on the substrate after overoxidation remains electroactive. at the electrode potential of e = 0.4 v vs. ssce the readsorption of the polymer chains (polymer chain ends) becomes possible, i.e. the number of the polymer chains directly contacted with (adsorbed on) the substrate surface is increasing with the passing time. in other words, during overoxidation the “effective” coverage of the substrate by the polymer decreases, and at less positive potentials the coverage starts to increase again, leading to the decrease of the charge transfer resistance. the reasoning above leads us to the conclusion that in the case of the gc|pedot|0.1 m h2so4 electrode the number of irreversibly oxidized (desorbed or deactivated) polymer chains (or polymer chain ends) per unit area is greater than in the case of the au|pedot|0.1 m h2so4 electrode. however, this fact does not explain the apparent difference in the relative rates of decay of rct in the two systems. a plausible mechanistic explanation of this behavior is based on the differences in the chemical states of the substrate surfaces. in the case of au|pedot|0.1 m h2so4 after the last overoxidation cv (at e = 0.4 v vs. ssce, the electrode potential was kept at this value during the impedance specd. zalka et al. j. electrochem. sci. eng. 8(2) (2018) 151-162 doi:10.5599/jese.508 161 troscopy measurements, as well) the whole of the oxide is completely reduced, as is evident from the appearance of the reduction peak in fig. 1a (and is well known from the electrochemistry of gold [36,37]). on the other hand, it is quite clear from fig. 4 that immediately after an oxidation cycle up to a positive potential limit of e = 1.4 v vs. ssce, the electrochemical reduction of the surface of glassy carbon at 0.4 v vs. ssce is incomplete. this means, that in the case of the gc|pedot|0.1 m h2so4 electrode two processes may occur simultaneously during the impedance measurements: (a) reduction of the oxidized surface of the gc substrate, including the reduction of oxygen-containing surface functionalities and (b) readsorption of the polymer chains (polymer chain ends) on the surface. the extent of the first process will clearly influence the rate at which the other can proceed, manifesting in the increase in the rate of decay of the charge transfer resistance. nevertheless, it should be noted here, that other processes may also occur during the overoxidation process, i.e. chemical changes in the polymer chains that affect the extent of charging and the conjugation of the pedot chains and slow conformational relaxation phenomena of the polymer structure that may not be necessarily influenced by the substrate. conclusions the 4-dimensional analysis method, originally proposed by stoynov, was successfully applied for the determination of the charge transfer resistance of a glassy carbon|pedot|0.1 mol·dm3 sulfuric acid (aq.) electrode corresponding to different time instants after overoxidation. during the impedance measurements carried out almost immediately after the overoxidation procedure the electrode potential was held in the “stability region” of the system (at e = 0.4 v vs. ssce in the present case). the post-experimental mathematical/analytical procedure could be used also for the estimation of the charge transfer resistance corresponding to the time instant just after overoxidation of the pedot film. the decreasing capacitance and the increasing resistance suggest that during overoxidation the electrochemical activity of the film decreases and the charge transfer process at the metal/film interface becomes more hindered than in the case of pristine films. during the impedance measurements the charge transfer resistance decreased continuously with experiment time, the “starting” (calculated) rct value was approximately 1391 ω·cm2. after 50 seconds this value fell to about 1222 ω·cm2, i.e. it decreased by ~12 % in the first minute after the overoxidation procedure. by comparing the properties of the gc|pedot|0.1 m h2so4 and the au|pedot|0.1 m h2so4 electrodes a possible mechanistic explanation for the observed behavior has been proposed. this is based on the assumption that in the case of the gc|pedot|0.1 m h2so4 electrode two processes may occur simultaneously during the impedance measurements: (a) reduction of the oxidized surface of the gc substrate (including the reduction of oxygen-containing surface functionalities), and (b) readsorption of the polymer chains (polymer chain ends) on the surface. acknowledgements: support from the hungarian scientific research fund, the national research, development and innovation office nkfi (grants no. k 109036, vekop-2.3.2-16. references [1] c. kvarnström, poly(thiophene), in: 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[37] l. d. burke, p. f. nugent, gold bull. 31 (1998) 39–50. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) electroanalytical methods in characterization of sulfur species in aqueous environment doi: 10.5599/jese.2014.0053 155 j. electrochem. sci. eng. 4(4) (2014) 155-163; doi: 10.5599/jese.2014.0053 open access: issn 1847-9286 www.jese-online.org original scientific paper electroanalytical methods in characterization of sulfur species in aqueous environment irena ciglenečki, marija marguš, elvira bura-nakić and ivana milanović division for marine and environmental research, ruđer bošković institute, bijenička 54, 10 000 zagreb, croatia corresponding author: e-mail: irena@irb.hr; tel: 0038514561105; fax: 0038514680242 received: april 10, 2014; revised: june 13, 2014; published: december 6, 2014 abstract electroanalytical (voltammetric, polarographic, chronoamperometric) methods on an hg electrode were applied for studying of different sulfur compounds in model and natural water systems (anoxic lakes, waste water, rain precipitation, sea-aerosols). in all investigated samples typical hgs reduction voltammetric peak, characteristic for many different reduced sulfur species (rss: sulfide, elemental sulfur, polysulfide, labile metal sulfide and organosulfur species) was recorded at about 0.6 v vs. ag/agcl reference electrode. in addition, in anoxic waters which are enriched with sulfide and iron species, voltammetric peaks characteristic for the presence of free fe(ii) and fes nanoparticles (nps) were recorded at -1.4 v and around -0.45 v, respectively. depending on the used electroanalytical method and experimental conditions (varying deposition potential, varying time of oxidative and/or reductive accumulation, sample pretreatment i.e. acidification followed by purging) it is possible to distinguish between different sulfur species. this work clearly shows a large potential of the electrochemistry as a powerful analytical technique for screening water quality regarding presence of different reduced sulfur species and their speciation between dissolved and colloidal/nanoparticle phases. keywords voltammetry; chronoamperometry; speciation; reduced sulfur species; metal sulfide nanoparticles; hg electrode; anoxic water samples electrochemical measurements are along with icp-ms, the most used but challenging approaches in essential elements analysis and speciation in complex natural samples. there is a wide range of electroanalytical techniques for qualitative and quantitative determination of essential and potentially toxic elements in natural waters [1,2]. some examples include: potentiometry, polarography, voltammetry, chronopotentiometry, chronoamperometry, etc. these electrochemical methods, especially voltammetry, have appropriate features to be used as monitoring methods (early warning tools) for assessment of water quality in aqueous systems in general and will be key http://www.jese-online.org/ mailto:irena@irb.hr j. electrochem. sci. eng. 4(4) (2014) 155-163 electroanalysis in sulfur speciation 156 methods for trace pollutant analyses (sulfur species [3-17], organic compounds [18-20], trace metals [2-4, 8, 21-26], engineered and natural nanoparticles [27-34]). working electrodes, so called voltammetric sensors, have many embodiments that make them specific for detection of above listed natural and anthropogenically introduced compounds in natural environment, enabling their quantitative determination. electrochemical techniques offer increasing degree of accuracy, decreasing detection limits, simplicity, prompt response, ect. it involves dramatically lower costs than other techniques to reach same sensibility and with automated, portable instrumentation is suitable for fieldwork. in addition, many substances that are analyzed by other techniques use electrochemical detectors. voltammetry is the only technique allowing speciation and determination of the truly dissolved metal species without many sample handling [2,21-26]. speciation of a metal affects its biogeochemical cycling processes and its biological impacts. thus, electrochemical measurements in natural waters are essential in order to obtain more complete speciation information and to fully understand the geochemical cycling and bioavailability (toxicity) of trace metals. eu water quality guidelines are searching for new innovative methods for water quality monitoring, and electrochemistry in comparison with inductively coupled plasma mass spectrometry (icp-ms) and/or inductively coupled plasma/optical emission spectrometry (icp-oes) and diffusive gradients in thin-films (dgt) approach was found as preferable choice. besides, new investigations showed that voltammetry has a potential to be used in determination of metal nps, metal sulfide (ms) nps and aquatic colloids in natural waters [27-32]. growing evidence implies that ms nps of natural and anthropogenic origin exist in aquatic environments. these nps could play important role as mediators of the trace metal nutrition and toxicity. using different electrochemical methods it is possible to measure a variety of soluble and particulate sulfur compounds [3-17,32]. in this work voltammetric, polarographic and chronoamperometric measurements on a hg electrode were used for characterization and speciation of dissolved and particulate sulfur species, including thiols, hs , s 0 , ms nps (fes, pbs), sx 2 in different contrasting aqueous natural samples such as oxic/anoxic systems, rain precipitation and aerosols. experimental materials all chemicals used were reagent grade and were not further purified. stock solutions of sulfide, polysulfide, suspensions containing nps of fes and pbs were prepared as previously described [6, 7, 10-14, 32]. all measurements were performed in nacl (chemica, croatia) electrolyte solutions with ionic strengths ranging from 0.11 to 0.55 m nacl. in some experiments the nacl electrolyte was buffered with 0.03 m nahco3 (chemica, croatia).. instrumentation electrochemical measurements were performed with a bas-100-a chemical analyser, µ-autolab electrochemical instruments (eco chemie) and pgstat 128 n (metrohm, switzerland) connected to pencil like hmde and 663 va stand metrohm electrode (metrohm, switzerland) as a working electrodes, respectively. the reference electrode was an ag/agcl (3 m kcl) electrode connected to the solution via an electrolyte bridge, and a platinum electrode served as an auxiliary electrode. reduced sulfur species (rss) were determined by linear sweep and cyclic voltammetry (lsv, cv) [6,7,13] and by polarographic measurements [3] in fresh nonfiltered samples. in the case of cv and lsv the accumulation (ta = 0-120 s) of rss on the hg electrode surface with stirring was i. ciglenečki at al. j. electrochem. sci. eng. 4(4) (2014) 155-163 doi: 10.5599/jese.2014.0053 157 performed at the deposition potential of e =-0.20 v (vs. ag/agcl). after accumulation the potential was shifted in the negative direction (to e= -1.70 v vs. ag/ agcl) with a scan rate of 100 mv/s and hgs reduction peak at around -0.6 v, characteristic of many rss were recorded [6,7,13]. in the same cycle reduction peaks characteristics for the presence of metal sulfide layers and nps from the bulk of the solution were recorded at potential more negative than -0.6 v [10,11,17]. next, the solution was acidified with 30 μl of concentrated hcl (chemica, croatia) to ph ~2 and purged for 5 min. after restoring the original ph with naoh (chemica, croatia) the accumulation and scan steps were repeated. the result of the first measurement, prior to acidification, is assigned as total reduced sulfur species, rsst = h2s/hs + s 0 and the result of the second measurement is assigned to elemental sulfur, s 0 as model representative for non-volatile reduced sulfur species, rssnv [6,7,13]. for detection of s 0 and s 2 presence in polysulfide (alfa aesar, usa) containing solutions sampled dc polarography (sdc) or voltammetry at the hg electrode was performed with step potential of 0.0051 v, starting from -0.4 v (vs. ag/agcl) and shifting to more negative values. in chronoamperometry the detection potential at which current was measured as a function of time (i-t curves) was selected depending on the potential at which reduction of the nps from the bulk of the solution is proceeding [11,17,32]. in the case of pbs the used potential was -1.5 v. the scan lasted for 30 s and the sampling time was 0.1 s. the suspension of pbs nps was prepared by mixing the equimolar concentrations of pb 2+ and hs directly in the electrochemical cell [11]. during the ageing process the suspension was not stirred and recorded changes in the nps sizes were only due to aggregation caused by brownian motion. results and discusion typical voltammetric signal which can be found in an anoxic sulfide rich environment is presented in fig. 1. the obtained peak, usually in our papers designated as c2, represents the wellknown dissolution/reduction of hgs layer on the hg electrode surface [5-7,9-17]: e / v (vs. ag/agcl) -1,0-0,8-0,6-0,4-0,2 i / n a -120 -100 -80 -60 c2 1 2 e / v (vs. ag/agcl) -1,0-0,8-0,6-0,4-0,2 i / m a -30 -20 -10 0 c2 1 2 a b figure 1. lsv obtained from rogoznica lake water in the oxic (a) and anoxic bottom water layer (b), before 1) and 2) after acidification and purging with n2; (e = -0.2 v, ta = 120 s). the c2 peak increases with either sulfide or with s 0 addition and corresponds to 6.5 nm rssnv in a) and to mm rssv in b. the shift of this peak to a more negative potential after the acid-purge-base treatment is due to a final ph which is higher than the original ph. carbonate buffering in the sample is destroyed by acidification, so it is difficult to return the sample exactly to the original ph. j. electrochem. sci. eng. 4(4) (2014) 155-163 electroanalysis in sulfur speciation 158 hgs + h + + 2e → hs + hg 0 (1) this peak usually is taken as a measure for “free” and labile sulfur species content (h2s/hs /s 2, s 0 , sns 2, thiols, labile metal sulfide complexes and nanoparticles). in oxic water layers, in addition to c2, the peak at more positive potentials than -0.5 v can be frequently revealed. this peak usually corresponds to the presence of different organosulfur species (dms, 3-mercaptopropionat, thiocompounds) which at used experimental conditions oxidize the hg electrode but do not deposit hgs layer on its surface, therefore their peak appears more positively than c2 [9]. in cases when sample solution contains sulfide and metal ions (m 2+ ) which are present in an excess, depending on the electrochemical conditions (deposition potential, accumulation time) [15,17], the peak marked as c3 (figure 2) appears. this peak corresponds to formation of metal sulfide (ms) deposit (layers), in the given case pbs, due to electrochemical exchange reaction between hg 2+ from a hgs layer and the free m 2+ (pb 2+ ) ion from the solution [15,17]: hgslayer + m 2+ + 2e  mslayer + hg 0 (2) c3 might be easily misrepresented for the dissolved organosulfur species represented by c2 peak in figure 1. the ms layer stays on the hg surface without desorption up to potentials that are more negative than c2 peak (usually up to -1.6 v and more negative potentials) [14,17,32]. reduction of the ms (pbs) layers formed at c3 peak potential usually occurs at potentials of the peak c5 (figure 2, reaction 3). the potentials of both electrode reactions, and the formation and reduction of the ms layer, are shown to be directly controlled by the ms solubility products [15,17]. pbslayer +2e + h + → pb 0 + hs (3) figure 2. cv of solution containing 4x10 -5 m pb 2+ and 3x10 -5 m hs in 0.55 m nacl/0.03 m nahco3 electrolyte (e= -0.2 v; ta=60 s; v=100 mv/s). c4/a4 peaks in the figure 2, correspond to reduction/oxidation of the free metal on the hg, i.e. to the reduction of pb 2+ to pb 0 , while c6 in accordance with our previous work was ascribed to reduction of pbs nanoparticles (nps) from the bulk of the studied solutions [10,11,17]. these particles usually do not form ms layers. in the case of studied cds, pbs, ag2s, cu2s, hgs e / v (vs. ag/agcl) -1,6-1,4-1,2-1,0-0,8-0,6-0,4-0,2 i / n a -50 0 50 100 150 4.0x10 -5 m pb 2+ and 3.0x10 -5 m hs c2 c4 a4 c3 c5 c6 i. ciglenečki at al. j. electrochem. sci. eng. 4(4) (2014) 155-163 doi: 10.5599/jese.2014.0053 159 suspensions, depending on the solution conditions (concentration and ratio between metal and sulfide species, ionic strength, ph) larger nps will form and result in the appearance of the peaks similar to recorded c6 reduction peak. direct reduction of the formed nps, which is placed more negative than reduction process of the relevant ms layers [17] and/or reduction processes which occur on the nps surface upon collision with the hg electrode, and the potential where this process is occurring on the hg surface is successfully used as a background for further nps characterization by chronoamperometric measurements [32], as shown here later. in samples of anoxic seawater lake rogoznica lake (croatia), shown in figure 1, the peak at -0.5 v corresponds to the presence of organosulfur species (rssnv) which do not deposit hgs, and peak at -0.6 v corresponds to presence of rsstotal (all rss that deposit hgs). the major difference between oxic and anoxic rogoznica lake water layers is in the existence of volatile sulfide species (rssv) which are present in mm concentration in anoxic part mainly in the form of sulfide (hs ) and rssnv which presence is determined to be around 10 nm in oxic and 1-10 µm in anoxic water layers. the rssv can be removed by acidification and purging while nonvolatile species during acidification and purging procedure will remain in the sample and contribute to the c2 peak (figure 1b) [9,12,13,16]. with use of polarographic measurements on the hg electrode it is possible to distinguish further between detected rss on the polysulfide (sx 2), elemental s (s 0 ) and/or hs species without any pretreatment of the samples [3] (figure 3). figure 3. sdc polarographic curves recorded in: a) 0.55 m nacl solution containing 7 x 10 -6 m s4 2 and b) anoxic sample od rogoznica lake taken at 11 m depth. the voltammograms were recorded between –0.4 and -0.8 v vs. ag/agcl with potential steps of 1 mv. the voltammograms measured in the tetrasulfide containing solution (figure 3a) were characterized by the anodic and cathodic currents in the potential range from –0.5 v to –0.8 v, respecttively. the cathodic current is assigned to a reduction process given by the equation (4) [3,33,34], and it is a measure for the presence of s 0 in the polysulfide molecule:  2 2s 2 1 e h o hs ohn n n n n          (4) anodic current recorded at potentials more positive than –0.6 v is assigned to the well-known oxidation of the hg by hs according to equation (5) [3-17,33], and it can be taken as a measure for the s 2 presence in the molecule of polysulfide: hs hg hgs 2e h      (5) e / v (vs. ag/agcl) -0,8-0,7-0,6-0,5 i / n a -20 -15 -10 -5 0 5 3x s0 1x s27 x 10 -6 m s4 2electrolyte a e / v (vs. ag/agcl) -0,9-0,8-0,7-0,6-0,5-0,4-0,3 i / µ a -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 sulfide elemental sulfur electrolyte rogoznica lake sample,11m depth b j. electrochem. sci. eng. 4(4) (2014) 155-163 electroanalysis in sulfur speciation 160 ratio between cathodic and anodic currents in the studied case of tetrasulfide solution was roughly 3:1 indicating 3 s 0 and 1 s 2 in the molecule of s4 2. in rogoznica lake sample this ratio was much lower (1:6.5), pointing to a high excess of the free sulfide in the sample. common ratio between sulfide and elemental sulfur in the anoxic rogoznica lake samples is 10-15 to 1 in favour of sulfide [6,13]. similar voltammetric curves to rogoznica lake samples with revealed c2 rssnv peak can be found in rain precipitation and aerosols. usually in these samples rssnv are detected in much lower concentration range from 1-10 nm, while industrial waste samples could contain total rss from 10 µm up to mm concentration. in sulfide and iron rich natural samples such is anoxic water column of freshwater pavin lake in france, voltammetric curves similar to curves obtained in ms suspensions could be recorded (figure 4) [12,16]. besides c2 peak in such samples other relevant peaks can be seen: 1) c3/a3 peak couple can be attributed to transformation reaction of fes to the hgs [14,16,35]: feslayer + hg 0 → hgslayer + fe 2+ + 2e (6) and can be taken as an indication and rough measure for nanoparticulate fes [15,16]. 2) c4 is the well-known reduction peak of fe(ii) aqua ion, which is irreversibly reduced on the hg near -1.4 v [4,8,14,16,25]: fe 2+ + 2e → fe 0 (7) 3) the c1/a11 peaks arises from the reduction of fe 2+ or its labile complexes on the hg electrode surface modified by a fes layer [14,16,35 and references therein]. fe 0 deposited at the c1 would be oxidized in reverse scan at the a11 peak. figure 4. typical voltammetric curves of anoxic sulfide and iron rich samples taken from crater pavin lake, france at 71 m depth (e = -0.2 v, ta = 0 s for a, and e = -0.75 v, ta = 30 s for b. a3 peak is a rough measure for fes nanoparticles presence. in order to better characterize and possibly quantify concentration of the formed ms nps and to estimate its size ranges in water samples, additional chronoamperometric measurements were employed for characterization of fes nps recently. it was shown that recorded chronoamperemetric signals are carrying fes nps size, possibly charge and concentration information [32]. similar approach was adopted here for pbs nps characterization (figure 5). the pbs is chosen as one of the models because its redox chemistry is relative simple and not governed by multiple relevant redox states in comparison with cu and fe and in several anoxic e / v (vs. ag/ agcl) -1.8-1.6-1.4-1.2-1.0-0.8-0.6-0.4-0.2 i / µ a -6 -5 -4 -3 -2 -1 0 1 c3 c1 c4 a11 a3 a pavin lake sample, 71 m depth e / v (vs. ag/agcl) -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 i / µ a -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 a3 c2 b pavin lake sample 71 mdepth i. ciglenečki at al. j. electrochem. sci. eng. 4(4) (2014) 155-163 doi: 10.5599/jese.2014.0053 161 samples occasionally voltammetric peak at -1.2 v similar as shown in figure 2, was detected. in accordance with our previous studies [11,17] this peak is attributed to the reduction of the pbs nps according to reaction (8): pbs + 2e  pb(hg) + hs (7) consequently, the chronoamperometric measurements were started in the area of the c5 peak potential (figure 2) and the highest frequency of spike like signals were detected at -1.5 v. the recorded chronoamperometric curves at -1.5 v were characterized by the sharp reduction current transients with duration lasting from 100 ms and higher and peak heights in the range of 10 -10 to 10 -8 a. it is assumed that each spike represents a reduction of the pbs nps at the hg surface during collision according to reaction 8. figure 5. chronoamperograms for 5x10 -5 m pb 2+ and hs in 0.11 m nacl/0.03 m nahco3 recorded at -1.5 v (vs. ag/agcl) 5 min (black curve), 10 min (yellow curve) and 30 min (green curve) after mixing of pb 2+ and hs . recorded spikes appear to be sensitive on concentration of the pbs nps in the solution, ph changes, ionic composition and ageing time, similarly as obtained with fes nps. please be aware that in the case of fes nps, recorded spike like signals were caused by catalytic processes that occurred on the fes nps surface during collision with the hg electrode [32]. in figure 5, it is evident decrease in the signals frequency and increase of signals charge with ageing time of the pbs suspension. in the given time, size of the formed pbs nps, monitored by dynamic light scattering measurements were changed from 140 to 480 nm according to number size distribution. all above mentioned parameters (concentration, ph, suspension composition, and ageing time) highly influence physico-chemical properties of the formed nps, indicating a great potential of the chronoamperometric measurements for the characterization, quantification and sizing of all chalcogenides and other nps which behave similarly at the hg surface. t / s 0 10 20 30 40 50 60 -40 -30 -20 -10 5 min after mixing pb 2+ and hs 10 min after mixing pb 2+ and hs 30 min after mixing pb 2+ and hs i / n a j. electrochem. sci. eng. 4(4) (2014) 155-163 electroanalysis in sulfur speciation 162 conclusion in this work it is clearly shown how electrochemistry by choosing appropriate methodology and experimental conditions can be successfully used for characterization, speciation and deterimantion of different dissolved and colloidal sulfur species in natural waters including rain precipitation and aerosols. further, it appears that today, in the time of growing nanotechnology and production of different nps and nanomaterials, electroanalytical methods due to its relative simplicity and prompt response, low cost and relatively high sensitivity and selectivity, might be a good alternative analytical tools for characterization and possibly quantification of different nps in natural waters. this work is still challenging for the future. acknowledgements: this work is supported by the ministry of science and technology of the republic of croatia projects: ‘nature of organic matter, interaction with traces and surfaces in environment’ (number 098-0982934-2717) and ‘nanoparticles in aqueous environment: electrochemical, nanogravimetric,stm and afm studies’, a unity through knowledge fund, ukf project. references [1] k. rajeshwar, j. g. ibanez, environmental electrochemistry, academic press, san diego, ca, 1997. 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[35] k. winkler, t. krogulec, j. electroanal. chem. 386 (1995) 127-134. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {a novel dopamine electrochemical sensor based on la3+/zno nanoflower modified graphite screen printed electrode} http://dx.doi.org/10.5599/jese.674 187 j. electrochem. sci. eng. 9(3) (2019) 187-195; http://dx.doi.org/10.5599/jese.674 open access: issn 1847-9286 www.jese-online.org original scientific paper a novel dopamine electrochemical sensor based on la3+/zno nanoflower modified graphite screen printed electrode somayeh tajik1,2,, hadi beitollahi3, mohammad reza aflatoonian2,4 1bam university of medical sciences, bam, iran 2research center for tropical and infectious diseases, kerman university of medical sciences, kerman, iran 3environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran 4leishmaniasis research center, kerman university of medical sciences, kerman, iran corresponding author: mmtajik_s1365@yahoo.com received: december 4, 2018; revised: february 23, 2019; accepted: february 26, 2019 abstract flower-like la3+/zno nanocomposite was facile synthesized. a simple and ultrasensitive sensor based on graphite screen printed electrode (spe) modified by la3+/zno nanoflower was developed for the electrochemical determination of dopamine. the electrochemical behavior of dopamine was studied in 0.1 m phosphate buffer solution (pbs) using cyclic voltammetry (cv), chronoamperometry (ca) and differential pulse voltammetry (dpv). compared with the unmodified graphite screen printed electrode, the modified electrode facilitates the electron transfer of dopamine, since it notably increases the oxidation peak current of dopamine. also, according to cv results the maximum oxidation of dopamine on la3+/zno/spe occurs at 150 mv which is about 140 mv more negative compared with unmodified spe. under optimized conditions, the modified electrode exhibited a linear response over the concentration range from 0.15 to 300.0 μm, with a detection limit of 0.08 μm (s/n = 3). the proposed sensor exhibited a high sensitivity, good stability and was successfully applied for dopamine determination in dopamine ampoule, with high recovery. keywords dopamine; sensor; graphite screen printed electrode; la3+/zno nanoflower introduction dopamine (3,4-dihydroxyphenyl ethylamine), as an excitatory neurotransmitter in mammalian central nervous systems [1], plays an important role in several physiological activity such as mood, attitude and movement [2]. abnormal levels of dopamine may lead to neurological diseases, such http://dx.doi.org/10.5599/jese.674 http://dx.doi.org/10.5599/jese.674 http://www.jese-online.org/ mailto:mmtajik_s1365@yahoo.com j. electrochem. sci. eng. 9(3) (2019) 187-195 dopamine electrochemical sensor 188 as parkinson's disease and schizophrenia [3-5]. hence, sensitive and selective detection of dopamine is of great importance for the understanding and diagnostics of neurological diseases. significant effort has been applied to develop procedures for detection of dopamine, making use of chemiluminescence [6], fluorimetry [7], electrophoresis [8], colorimetry [9], high performance liquid chromatography [10], and electrochemical techniques [11]. as compared to other analytical methods, the electrochemical techniques employing a three electrode system have drawn a considerable attention because of the electroactive nature of dopamine, high selectivity, high sensitivity, low cost, reproducibility, simplicity and short operational duration [12-16]. screen-printed electrodes (spes), which are fabricated by printing several types of inks on a specific substrate have been considered superior. such spes have been recently employed as diagnostic tools for food poisoning, diseases, and environmental pollutants [17-21]. nowadays, the nanomaterials have been attracted much attention to modify the surface of electrodes for electrochemical detection of target analytes. thus, modified electrode surfaces offer high effective surface-to-volume ratio, possess a unique ability to enhance the electron transfer between redox centres in the analyte and electrode surfaces, provide a decrease of the overpotential of many analytes with respect to unmodified electrodes and show high catalytic efficiency [22-39]. zno is considered as one of the most important and functional metal oxide semiconductor material, due to its various fascinating properties and high technological applications. because of several interesting properties such as wide band gap (3.37 ev), large exciton binding energy (60 mev) at room-temperature (larger than several other important semiconductors such as gan and znse), thermal and mechanical stabilities, piezoelectric and pyroelectric properties, electrochemical and optoelectronic properties, biocompatibility, etc. [40-44]. due to various interesting properties, zno is widely used for variety of applications. therefore, the excellent properties and wide applications variety of zno nanomaterials such as nanorods, nanowires, nanobelts, nanoflowers, nanodisks, nanosheets, nanospirals, nanotubes and so on were prepared via several techniques and reported in the literature [40,45]. according to the previous points, it is important to create suitable conditions for analysis of dopamine in biological fluids. in this study, la3+/zno nanoflowers were used to improve the sensitivity of sensors for voltammetric determination of dopamine. the proposed sensor showed good electrocatalytic and accumulative effect on dopamine. in addition, the analytical performance of the suggested sensor for dopamine determination in real samples was evaluated. experimental chemicals and apparatus an autolab potentiostat/galvanostat (pgstat 302n, eco chemie, the netherlands) was employed to perform the electrochemical experiments and the system was controlled using a general purpose electrochemical system software. the screen-printed electrode (dropsens, drp-110, spain) consists of three conventional electrodes: graphite counter electrode, ag/agcl/kcl reference electrode and unmodified graphite working electrode. ph was measured by a metrohm 710 ph meter. dopamine and all other reagents were of analytical grade, and purchased from merck (darmstadt, germany). for the preparation of buffers, orthophosphoric acid and its salts were used to provide the ph range of 2.0–9.0. somayeh tajik et al. j. electrochem. sci. eng. 9(3) (2019) 187-195 http://dx.doi.org/10.5599/jese.674 189 preparation of la3+-doped zno nanoflowers all the chemicals used for the preparation of nano-powders, namely zinc acetate (zn(ch3coo)2×2h2o), lanthanum nitrate (la(no3)3×6h2o), thiourea ((nh2)2cs) and ammonia (25 % nh3), were of analytical grade. all the precursors were dissolved in deionized water. during the preparation of nano-powders, ammonia was used as a complexing agent. zno nanostructures were prepared by dissolving 0.46 mol of zinc acetate in 80 ml of deionized water, 0.0046 mol of lanthanium nitrate in 80 ml of deionized water, 0.18 mol of thiourea in 80 ml of deionised water and lastly by adding 19.76 ml of ammonia in 80 ml of deionised water. the amount of solutions of zinc acetate, thiourea and ammonia was held constant at a ratio of 1:1:1. then the zinc acetate solution was added in a beaker in the reaction bath, followed by adding thiourea and lanthanium nitrate solution in the same reaction bath and the mixture was stirred for a few seconds. lastly ammonia solution was added slowly into the mixture, while continuing stirring for 5 min. the temperature of the bath was then allowed to increase up to 80 °c. after that, the precipitates were formed, left overnight and filtered thereafter. the precipitates were then washed with ethanol. the obtained powders were dried at ambient conditions for several days. preparation of the electrode the bare graphite screen printed electrode was coated with la3+/zno nanoflower composite according to the following simple procedure: 1 mg la3+/zno nanoflower was dispersed in 1 ml aqueous solution within 45 min ultrasonication. then, 5 µl of the prepared suspension was dropped on the surface of carbon working electrode and left to dry at room temperature. preparation of real samples one milliliter of a dopamine ampoule (caspian tamin company, iran, containing 200 mg in 5 ml of dopamine hydrochloride) was diluted to 10 ml with 0.1 m pbs (ph 7.0) and then, different volumes of the diluted solution were transferred into each of a series of 25 ml volumetric flasks and diluted to the mark with pbs. the dopamine content was analyzed by the proposed method using the standard addition method. result and discussion electrochemical profile of dopamine on la3+/zno/spe since the electrochemical behaviour of dopamine is ph-dependent, it is necessary to obtain the optimized ph value in order to achieve the accurate results. experiments performed by use of the modified electrode at various ph values ranging from 2.0–9.0, revealed that the best results for electrooxidation of dopamine are obtained at ph 7.0. the cyclic voltammograms measured in 0.1 m pbs (ph 7.0) containing 60.0 μm dopamine, using la3+/zno/spe and bare spe are shown in figure 1. according to cv results, the maximum oxidation of dopamine on la3+/zno/spe occurs at 150 mv which is about 140 mv more negative compared with unmodified spe. effect of scan rate on the results according to figure 2, increase of the potential scan rate leads to enhanced oxidation peak current values (ip). in addition, there is a linear relationship between ip and the square root of the potential scan rate (ν1/2) which demonstrates that the oxidation and reduction of dopamine is diffusion controlled. http://dx.doi.org/10.5599/jese.674 j. electrochem. sci. eng. 9(3) (2019) 187-195 dopamine electrochemical sensor 190 fig. 1. cyclic voltammograms of (a) la3+/zno/spe and (b) bare spe in 0.1 m pbs (ph 7.0) in the presence of 60.0 μm dopamine at the scan rate 50.0 mvs-1. fig. 2. cyclic voltammograms of la3+/zno/spe in 0.1 m pbs (ph 7.0) containing 60.0 μm dopamine at various scan rates; numbers 1-9 correspond to 10, 30, 50, 70, 100, 200, 400, 600 and 800 mv s-1. inset: variation of anodic and cathodic peak currents vs. ν1/2. chronoamperometric (ca) analysis the analysis of ca for dopamine samples was performed by use of la3+/zno/spe vs. ag/agcl/kcl (3.0 m) at 0.2 v. ca results for different concentrations of dopamine in pbs (ph 7.0) are demonstrated in figure 3. the cottrell equation for ca analysis of electroactive moieties under mass transfer limited conditions is as follow [46]: i =nfad1/2cbπ-1/2t-1/2 somayeh tajik et al. j. electrochem. sci. eng. 9(3) (2019) 187-195 http://dx.doi.org/10.5599/jese.674 191 where d represents the diffusion coefficient, cm2 s-1, and cb is the bulk concentration of analyte, mol cm−3. experimental results of i vs. t−1/2 are plotted in figure 3a, with the best fits for different concentrations of dopamine. the resulted slopes corresponding to straight lines in fig. 3a were then plotted against the concentration of dopamine (figure 3b). the mean value of d was determined to be 1.15×10−6 cm2 s-1 according to the resulting slope and cottrell equation. fig. 3. chronoamperograms obtained at la3+/zno/spe in 0.1 m pbs (ph 7.0) for different concentrations of dopamine. the numbers 1–4 correspond to 0.1, 0.3, 0.6, and 1.0 mm of dopamine. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1–4. (b) plot of slopes of straight lines against dopamine concentration. calibration curves based on the resulting peak currents of dopamine by use of la3+/zno/spe, the quantitative analysis of dopamine was done in 0.1 m pbs (ph 7.0) (figure 4). the modified electrode (la3+/zno/spe) as a working electrode in the range of dopamine concentration in 0.1 m pbs was used in differential pulse voltammetry (dpv) due to the advantages of dpv including the improved sensitivity and better performance in analytical applications. according to the results, a linear relationship exists between the peak currents and concentrations of dopamine within the concentration range of 0.15 to 300.0 μm with the correlation coefficient of 0.9992. the detection limit was obtained as 0.08 μm. table 1. shows a comparison of the analytical figures of merit of the proposed method with some other modified electrodes for the determination of dopamine. http://dx.doi.org/10.5599/jese.674 j. electrochem. sci. eng. 9(3) (2019) 187-195 dopamine electrochemical sensor 192 fig. 4. dpvs of la3+/zno/spe in 0.1 m (ph 7.0) containing different concentrations of dopamine. numbers 1– 8 correspond to 0.15, 2.5, 10.0, 30.0, 60.0, 100.0, 200.0 and 300.0 µm of dopamine. inset: plot of peak current as a function of dopamine concentration in the range of 0.15-300.0 µm. table 1. comparison of the efficiency of some modified electrodes used in the electro-oxidation of dopamine electrode modifier method lod, m ldr, m ref. glassy carbon carbon nanohorns/poly(glycine) voltammetry 8-3.0×10 4-2.8×10 –6-1.0×10 47 carbon fiber reduced graphene oxide voltammetry 7−10×7.7 −42.24×10–−61.4×10 48 glassy carbon l-tyrosine (l-tyr) covalently functionalized graphene oxide (go) composite voltammetry 7-102.8× 4-10×5.0 –6-10×1.0 49 glassy carbon graphene quantum dots voltammetry 7−10×1.15 4−10×1.5 – 6−10×1.0 50 graphite screen printed /zno nanoflower3+la voltammetry 8−10×8.0 −43.0 × 10 – −71.5×10 this work interference study the influence of various substances as compounds potentially interfering with the determination of dopamine was studied in the presence of 75.0 µm dopamine. the tolerance limit was defined as the maximum concentration of the interfering substance that caused an error less than ±5 % in the determination of dopamine. according to the results, uric acid, l-serine, l-phenylalanine, ethanol, benzoic acid, isoproterenol, methanol, serotonin, l-asparagine, acetaminophen, nadh, saccharose, urea, glucose, l-lysine, l-glycine, carbidopa, caffeine, levodopa, fructose, l-threonine, lactose, l-histidine, l-proline, s2−, mg2+, so42−, nh4+, f− and al3+ did not show interference in the determination of dopamine. somayeh tajik et al. j. electrochem. sci. eng. 9(3) (2019) 187-195 http://dx.doi.org/10.5599/jese.674 193 analysis of real samples the applicability of this modified electrode in the determination of real samples was assessed through the determination of dopamine in dopamine ampoule using the described method. in order to perform this analysis, standard addition method was employed, and the results are listed in table 2. accordingly, the results of dopamine recovery are satisfactory, and the reproducibility of the results is proved by the mean relative standard deviation (r.s.d.). table 2. application of la3+/zno/spe for determination of dopamine in dopamine ampoule (n=5) c / µm (spiked) c / µm (found) recovery, % rsd, % 0 5.0 3.2 2.5 7.4 98.6 1.8 7.5 12.7 101.6 2.5 12.5 17.9 102.3 2.8 17.5 22.3 99.1 3.1 the repeatability and stability of la3+/zno/spe the long-term stability of the la3+/zno/spe was evaluated over 3-week period. after the modified electrode was stored for 3 weeks in atmosphere at room temperature, the experiments were performed again. according to cyclic voltammograms, the peak potential for dopamine oxidation remained unchanged and a decrement of less than 2.5 % compared with initial response was observed. the antifouling properties of the modified electrode towards dopamine 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[50] y. li, y. jiang, t. mo, h. zhou, y. li, s. li, journal of electroanalytical chemistry 767 (2016) 84-90. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.674 http://creativecommons.org/licenses/by/4.0/) {new electrochemical and physical measurements on sensitive glasses in thin-film technology} http://dx.doi.org/10.5599/jese.720 177 j. electrochem. sci. eng. 10(2) (2020) 177-184; http://dx.doi.org/10.5599/jese.720 open access: issn 1847-9286 www.jese-online.org original scientific paper new electrochemical and physical measurements on sensitive glasses in thin-film technology frank gerlach, kristina ahlborn, winfried vonau kurt-schwabe-institut für messund sensortechnik e.v. meinsberg, 04736 waldheim, germany corresponding author: frank.gerlach@ksi-meinsberg.de; tel.: +49 34327 608 116; fax: +49 34327 608 131 received: september 10, 2019; accepted: october 7, 2019 abstract all-solid-state sensors have several advantages compared to conventional ones. these include, e.g., miniaturization, planar design, location independence and an uncomplicated realization of sensors for high pressure and high temperature. therefore, a number of physical sensors, such as temperature sensors, pressure sensors, rotation angle sensors and force sensors are available in thin-film techniques. the presented paper shows the advantages to combination of thin and thick film techniques to manufacture electrochemical sensors, especially using pulsed laser ablation to create amorphous layers of glass. furthermore, it investigates the range of possibilities for characterization of bulk and surface properties with electrochemical methods and specialized techniques with thermally stimulated currents (tsc). keywords amorphous glass, pulsed laser ablation, electrochemical impedance spectroscopy, phsen-sor, redox sensor, thermally stimulated currents. introduction beside the advantages of all-solid-state sensors, it seems that special technological features lead to limited sensor capabilities because miniaturization and layer technologies reduce long-term stability and electrochemical performance compared to optical and conventional measurement technologies [1,2]. there are several demands of potential users for affordable, pressure-stable, low-service and low-maintenance, planar sensors. generally, with screen printing techniques and vacuum technologies a wide range of electrochemical, glass-based sensors can be realized [3-10]. by using amorphous ion-sensitive glasses and their stable material properties, a good sensor behavior can be expected. the necessary reference systems, produced in this technology, can be executed as a combination of different ion-selective glasses [11]. this simplified arrangement is suitable for special applications in http://dx.doi.org/10.5599/jese.720 http://dx.doi.org/10.5599/jese.720 http://www.jese-online.org/ mailto:frank.gerlach@ksi-meinsberg.de j. electrochem. sci. eng. 10(2) (2020) 177-184 sensitive glasses in thin-film technology 178 the process measurement technology and it provides stable measuring systems without disturbing outflow rates. such glass-based all-solid-state multi-parameter sensor systems are used in expensive syntheses and in microreaction technology for the production of special pharmaceuticals. although basing on the glass material, glass breakage and contamination are excluded and therefore such systems can be used in food industry without any problems. state of the art are enamel systems measuring potential difference of two different sensitive electrodes [12]. an adequate measuring system offers by [13] as a double-membrane glass electrode. this was specially designed for applications in the chlor-alkali industry. the reference system to be used here is a pna electrode which utilizes the stable sodium concentration in saline solution. an all-solid-state electrode likewise produced in thin-film technology is known as an ion-selective field effect transistor and is manufactured by means of classical semiconductor technology for example described by [14]. this manufacturing technology is very complex and focused on high volumes of sensors. to complete the measuring chain, a classical reference electrode is used here. in order to replace this in future measurements with an all-solid-state-system, it would be possible to carry out a differential measurement with a glass electrode reduced in terms of its parameterspecific sensitivity. this can minimize the systemic temperature dependencies and long-term drift by the symmetrical structure. the focus of research article is on the one hand the development of sensitive, amorphous glass layers [15] and on the other hand, the development of a defined and reproducible transition of the ion-conducting glass layers to the electron-conducting noble metal conductive paths [16]. since the chemical and electrochemical evaluation of the thin sensitive layers is very difficult, an attempt is made to extensively characterize the layer system using other integrative examination methods and thermal stimulations. experimental development of sensitive glasses ion-sensitive glasses are melted from powdery raw materials in high-temperature furnaces at 1200 1500 °c. in order to receive geometrically clearly defined shapes, this still hot melt is poured into graphite molds, as shown in figure 1. as a result, moldings (normally cylinders) are obtained. these are separated by means of a precision cutting saw with diamond grinding wheel (accutom50, struers, willich (d)) into individual slices with a thickness of 5 mm. applied to a target holder, they can be used in different vacuum coating systems. figure 1. (a) glass melt in graphite molds. (b) sliced casting body. (c) molded body (d) built in target at a target carousel f. gerlach et al. j. electrochem. sci. eng. 10(2) (2020) 177-184 http://dx.doi.org/10.5599/jese.720 179 fabrication of screen-printed chip structures extensive screening studies require a large number of prototypes. the basic chip structures can be cost-effectively provided by screen printing technique in research projects. to minimize surface roughness, alumina substrates are coated with a glaze paste. these chips are equipped with a conductive path made of precious metal (gold or platinum), which are obtained from an organometallic paste, with a thickness of 500 nm. finally, with the exception of the sensory surface and the contacts, the whole assembly is hermetically sealed with glass. as part of the investigations, two different layouts were developed and used. electrode structures, according to figure 2, with a diameter of 7.5 mm were used for the standard morphological and electrochemical investigations. figure 2. (a) overall arrangement of structure 1. (b) layout of the masked pick up electrode. (c) layer structure for the thermal simulations, the structures were equipped with a platinum heater, which was designed for temperatures up to 350 °c. the temperature control takes place by the evaluation of the resistance of a printed platinum conducting path. furthermore, the discharge electrodes were executed as interdigital structures, according to figure 3. thus, impedance spectra and polarization experiments can be realized at different constant temperatures. figure 3. (a) overall arrangement of structure 2. (b) layout of the masked interdigital electrodes. (c) layer structure http://dx.doi.org/10.5599/jese.720 j. electrochem. sci. eng. 10(2) (2020) 177-184 sensitive glasses in thin-film technology 180 preparation of sensitive glass layers by means of pulsed laser ablation the thin functional sensor layers were prepared by screen printing technique, sputtering methods and pulsed laser deposition (pld) [17]. for this purpose, the combined coating system „creamet 500 pld s2" with target carousel, substrate changer and mask changer was used. in this system, a sputtering system is also implemented for the deposition of intermediate layers (ti/tixoy) on the conductive path. pre-cleaning by means of plasma is also possible in the sputtering chamber. with integrated substrate handler and mask change system 2 sputter targets, 6 pld targets and altogether 5 changeable masks can be used and combined for the process. in order to ensure reproducible conditions during the processes, the laser entrance window is periodically cleaned by means of an ion source and the laser power is determined before and after a deposition process. the deposition of the amorphous sensitive glass layers takes place with pulsed laser ablation (pld) using an excimer laser compex pro 110 (coherent inc., santa clara (usa)). it operates at a wavelength of 248 nm (krf) and with a maximum pulse energy of 400 mj. the pulse length is at a maximum pulse rate of 100 hz, 20 ns. thus, a fluence of 5.3 j/cm2 at 320 mj is possible. figure 4. optical beam path in the pld chamber (a) excimer laser compex pro 110. (b) substrate holder with mask changer. (c) target carousel. (d) ion source results and discussion in addition to the analytical methods (especially sem, xrd, xps and µ-xrf) demonstrated elsewhere [18], the characterization of the properties of glass membranes was supplemented by electrochemical measurements. electrochemical impedance spectroscopy when transferring classical electrochemical sensor principles into all-solid-state embodiments, special attention is directed to the internal interface transitions. on one hand, a liquid inner filling with a buffer effect must be replaced by a solid direct contact and, on the other hand, a reproducible potential at the interface between an ion-conducting layer and an electron-conducting lead should f. gerlach et al. j. electrochem. sci. eng. 10(2) (2020) 177-184 http://dx.doi.org/10.5599/jese.720 181 be ensured. the use of intermediate layers has a decisive influence on the potential formation and the drift behavior. in the present work, the conductive path made of gold was electrochemically oxidized and a semi-conductive oxide layer of tixoy was implemented. these two processes were executed with the aim of achieving a better electrical transition between ion-conducting glass and electron-conducting precious metal. the investigations were carried out by means of a geometrically stable and reproducible three-electrode arrangement with a glass membrane electrode as measuring electrode. the reference electrode was a kcl-saturated silver/silver chloride electrode, and the counter electrode was a platinum sheet. an appropriate tool for the characterization of glass based thin film systems is the recording of electrochemical impedance spectra. the impedance spectra (figure 5) show an increased resistance in the layer structure au/auxoy/ph-glass. then again, inserting of tixoy semiconductive interlayers leads to a significant reduction in the total resistance of the measurement chain towards noninterlayer arrangements. figure 5. electrochemical impedance spectra of gold-based electrodes covered with ph-glass (with and without interlayers) – measured with three-electrode system consisting of working electrode, platinum electrode as counter electrode and ag/agcl/kclsat.-reference electrode the resulting spectra demonstrate that the total conductivity of all-solid-state configuration can be adjusted by chemical composition of the interlayers. therefore, new sensitive glasses with higher resistance can be used for innovative sensor applications. in this context, it was interesting to find out, that glasses tending to segregation can effectively transferred with vacuum coating processes so, the fabrication of completely glass-based lab-on-chip systems is conceivable. running-in behavior and potential stability electrode properties like running-in time, response time and sensor drift are particularly important for future users in terms of sensor acceptance. again, not only the sensitive membranes but also the internal intermediate layers have direct influence on these properties. in figure 6a it is shown, that the potential drift and the running-in behavior of the system au/tixoy/ph-glass is smaller than the system au/ph-glass. according to figure 6b, the electrode http://dx.doi.org/10.5599/jese.720 j. electrochem. sci. eng. 10(2) (2020) 177-184 sensitive glasses in thin-film technology 182 signals of system au/tixoy/ph-glass show smaller deviations in buffer solution of ph 6.86, measured vs. kcl-saturated silver/silver chloride reference electrode. figure 6. influence of intermediate layers. (a) running–in behaviors. (b) stability of the open circuit voltage during several consecutive measurements in buffer solution ph 6.87 sensor response and sensor slope similarly, the sensitive membrane and conduction transfer layers also influence the other sensor properties. by deliberately influencing material compositions and sensor design, these sensor properties can be adjusted in a wide range. the direct changes in sensor response behavior and sensor slope are shown in figure 7. although the response times are similar, with the same ph-sensitive membrane, only by using a semiconductive interlayer, the sensor slopes have been reduced by about 50 %. this makes it possible to develop inert glass-based reference electrodes with an extremely stable potential and low temperature drift. figure 7. (a) sensor response of au-electrodes covered with ph glass membranes without and with tixoy-interlayer. (b) sensor slope caused by change of sensor behaviors during repeated measurements in different ph-solutions thermally stimulated currents (tsc) in addition to usual analytical and electrochemical investigations, thermally stimulated characterizations of layer structures were carried out with the help of thermally stimulated currents (tsc) method. hereby, it is explicitly possible to deduce the integral total mobility of charge carriers, f. gerlach et al. j. electrochem. sci. eng. 10(2) (2020) 177-184 http://dx.doi.org/10.5599/jese.720 183 in particular of alkali and alkaline earth ions. this is important for the development of highly conductive ion-sensitive glasses. figure 8a shows the cyclic voltammograms recorded on a li-containing glass as a function of temperature deliver information about the ion mobility in sensitive glasses. the cyclic voltammograms at 250 °c of glasses with charge carriers having different ionic radii are presented in figure 8b. glasses with small ions (e.g. li-red curve) show higher currents than glasses with larger ones (e.g. la, nd-blue curve). with this method, the total conductivities of glasses with different composition can be determined at variable temperatures. figure 8. cyclic voltammograms recorded on sensitive glass layers. (a) variation of the temperature with integrated heater using li-glass (glass 137). (b) influence of glass conductivity by ion size and increased temperature (li-containing glass/red, la, nd-containing glass/blue). conclusions there is a growing interest in all-solid-state sensors for various applications in process measurement technology, microreactor technology and environmental technology. that`s why fundamental investigations into the development of sensitive membranes and on the mechanism of the behavior of the intermediate layers are necessary. it has been shown that the pld deposition process makes it possible to create homogeneous, amorphous glass structures. their electrochemical sensor behavior can be described with the nernst-equation. with the use of semiconducting intermediate layers, such as e.g. tixoy, it is possible to significantly reduce the total resistance of the device. this makes it possible to use the highimpedance, sensitive glasses for planar arrangements, such as lab-on-chip systems. it was demonstrated that glasses tending to segregation and therefore being unprocessable for the glassblower can be amorphously deposited in good quality by using vacuum technology (pld). various integral electrochemical methods (electrochemical impedance spectroscopy, thermally stimulated currents) were applied extensively to characterize even very thin layers. the targeted selection of glass compositions for sensitive membranes and the use of suitable intermediate layers allow the manipulation of sensor properties in such a way that it will also be possible to generate insensitive reference systems in the future. thus, all-solid-state measuring chains working in the differential method according to lübber's [11] are conceivable as planar arrangement, in a miniaturized design without damaging discharge of liquids. http://dx.doi.org/10.5599/jese.720 j. electrochem. sci. eng. 10(2) (2020) 177-184 sensitive glasses in thin-film technology 184 acknowledgements: das kurt-schwabe-institut für messund sensortechnik e.v. meinsberg wird mitfinanziert durch steuermittel auf der grundlage des vom sächsischen landtag beschlossenen haushaltes. references [1] k. schwabe, ph measurement, theodor steinkopff, dresden, d, 1976, pp. 120-186. [2] g. eisenman, glass electrodes for hydrogen and other cations, marcel dekker, ny, 1967. [3] r. e. belford, r. g. kelly, a. e. owen, thick film devices in chemical sensors, e. edmonds, ed., blackie and sons, london, 1988, pp. 236-255. [4] v. g. vlasov, fresenius z. anal. chem. 335 (1989) 92-99. [5] t. bachmann, j. spindler, f. gerlach, w. vonau, j. univ. of appl. sci. mittweida 6 (2006) 3-7. [6] t. bachmann, j. spindler, f. gerlach, w. vonau, patent de 10 2005 059 680 a1 (2005). [7] m. frumar, b. frumarova, p. nemec, t. wagner, j. jedelsky, m. hrdlicka, journal of non-crystalline solids 352 (2006) 544-561. [8] j.p. klook, l. moreno, a. bratov, s. huachupoma, j. xu, t. wagner, t. yoshinobu, y. ermolenko, y.g. vlasov, m.j. schöning, sensors and actuators b, 118 (2006) pp. 149-155. [9] o.g. pompilian, g. dascalu, j. mihaila, s. gurlui, m. olivier, p. nemec, v. nazabal, n. cimpoesu, c. focsa, applied physics a, 117 (2014) 197-205. [10] w. vonau, m. decker, j. zosel, k. ahlborn, f. gerlach, s. weißmantel, sensors & transducers, 193(10) (2015) pp. 93-99. [11] d.w. lübbers, the science of nature 49(21) (1962) 493. [12] pfaudler emaillierte ph-sonden, https://www.pfaudler-messtechnik.de/de/products/liquidanalysis/ph-measurement/ (2019). [13] ph-sensor inpro4850i, https://www.mt.com/de/de/home/library/technical-reports/processanalytics/ph_inpro4850i (2019). [14] analoger glasfreier ph-sensor tophit cps471, https://portal.endress.com/wa001/dla/5000315/8755/000/05/ti283cen_us_0510.pdf (2019). [15] l. florian, b. savu, f. sima, i.n. mihailescu, d. tanasković, d. janacković, digest journal of nanomaterials and biostructures 2(3) (2007) pp. 285-291. [16] w. vonau, f. gerlach, u. enseleit, j. spindler, t. bachmann, journal of solid state electrochemistry 13 (2009) 91-98. [17] f. gerlach, k. ahlborn, s. sachse, w. vonau, sensor & transducers 184 (2015) 39-44. [18] k. ahlborn, f. gerlach, w. vonau, international journal on advances in systems and measurements 10(1&2) (2017) 56-63. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) https://www.pfaudler-messtechnik.de/de/products/liquid-analysis/ph-measurement/ https://www.pfaudler-messtechnik.de/de/products/liquid-analysis/ph-measurement/ https://www.mt.com/de/de/home/library/technical-reports/process-analytics/ph_inpro4850i https://www.mt.com/de/de/home/library/technical-reports/process-analytics/ph_inpro4850i https://portal.endress.com/wa001/dla/5000315/8755/000/05/ti283cen_us_0510.pdf http://creativecommons.org/licenses/by/4.0/) {sol-gel synthesis and electrochemical performance of nico2o4 nanoparticles for supercapacitor applications} http://dx.doi.org/10.5599/jese.690 243 j. electrochem. sci. eng. 9(4) (2019) 243-253; http://dx.doi.org/10.5599/jese.690 open access: issn 1847-9286 www.jese-online.org original scientific paper sol-gel synthesis and electrochemical performance of nico2o4 nanoparticles for supercapacitor applications yong zhang1,2,, yi ru1, hai-li gao1,, shi-wen wang1, ji yan1, ke-zheng gao1, xiao-dong jia1, he-wei luo1, hua fang1, ai-qin zhang1 and li-zhen wang1 1department of material and chemical engineering, zhengzhou university of light industry, zhengzhou 450002, p.r. china 2henan provincial key laboratory of surface & interface science, zhengzhou university of light industry, zhengzhou 450002, p.r. china corresponding authors: zy@zzuli.edu.cn; gaohaili@zzuli.edu.cn received: april 4, 2019; revised: june 7, 2019; accepted: june 8, 2019 abstract in this work, nico2o4 nanoparticles with enhanced supercapacitive performance have been successfully synthesized via a facile sol-gel method and subsequent calcination in air. the morphology and composition of as-prepared samples were characterized using scanning electron microscopy (sem), transmission electron microscope (tem), x-ray diffraction (xrd), and raman spectroscopy (raman). the electrochemical performances of nico2o4 nanoparticles as supercapacitor electrode materials were evaluated by cyclic voltammetry (cv), galvanostatic charge/discharge (gcd) tests in 3 mol l-1 koh aqueous solution. the results show that as-prepared nico2o4 nanoparticles have diameters of about 20-30 nm with uniform distribution. there are some interspaces between nanoparticles observed, which could increase the effective contact area with the electrolyte and provide fast path for the insertion and extraction of electrolyte ions. the electrochemical tests show that the prepared nico2o4 nanoparticles for supercapacitors exhibit excellent electrochemical performance with high specific capacitance and good cycle stability. the specific capacitance of nico2o4 electrode has been found as high as 1080, 800, 651, and 574 f g-1 at current densities of 1, 4, 7, and 10 a g-1, respectively. notably, the capacitance retention rate (compared with 1 a g-1) is up to 74.1 %, 60.3 %, and 53.1 % at current densities of 4, 7, and 10 a g-1, respectively. after 100 cycles, higher capacitance retention rate is also achieved. therefore, the results indicate that nico2o4 material is the potential electrode material for supercapacitors. keywords supercapacitors, sol-gel method, nico2o4, nanoparticles, electrochemical performances http://dx.doi.org/10.5599/jese.690 http://dx.doi.org/10.5599/jese.690 http://www.jese-online.org/ mailto:zy@zzuli.edu.cn mailto:gaohaili@zzuli.edu.cn j. electrochem. sci. eng. 9(4) (2019) 243-253 nico2o4 nanoparticles for supercapacitors 244 introduction due to the rapid growth of global economy, depletion of fossil fuels and increasing environmental pollution, the search for “green” and renewable energy resources is one of the most urgent challenges facing us today. as one of the most promising energy storage devices, supercapacitors, also known as electrochemical capacitors or capacitors, are widely used in emergency power systems, hybrid electric vehicles, consumer electronics, industrial power systems, smart grids, aerospace, etc. this is due to their superior performances, such as high power density, nearly infinite cycle life, wide temperature range and high coulomb efficiency [1]. according to different storage mechanisms, supercapacitors are divided into two types [2]: electric double-layer capacitors (edlcs) based on charge storage mechanisms at the electrode/electrolyte interface, and pseudocapacitors based on additional reversible redox reaction(s) of electrode materials. the electroactive materials of edlcs are usually carbon-based materials [3] (such as activated carbon (ac), carbon nanotubes, carbon aerogels (ca), graphene, etc.), while the electrode materials of pseudocapacitors are mainly transition metal oxides and conductive polymers (such as ruo2 [4], nio [5], co3o4 [6], mno2 [7], ceox [8], nico2o4 [9], pr6o11@ni-co [10], nimoo4 [11], polyaniline [12], polypyrrole [13], etc.). pseudocapacitors can provide much higher specific capacitance than edlcs by using reversible faraday reactions on the electrode surface. supercapacitor devices are composed of four parts: electrode materials (positive and negative), electrolyte, separator and shell, among which electrode materials play the key role in their performance. unfortunately, the defects of electrode materials, such as low specific capacitance of carbon-based materials, poor electrochemical stability of conductive polymers, and poor electronic conductivity of transition metal oxides, seriously impede practical applications of supercapacitors [14]. among pseudocapacitive materials, ruo2, having specific capacity as high as 1580 f g-1 [15], is the most prominent electrode material for application in supercapacitors. however, the high cost and environmental toxicity hinder its extensive commercial application. therefore, great efforts have been devoted to search cheap alternative materials with good capacitive characteristics similar to ruo2, and especially those electrode materials with multiple oxidation states and high electronic conductivity. in recent years, binary metal oxide/hydroxides have shown much better electronic conductivity and higher electrochemical activity than single component oxides/hydroxides [16], making them one of the most promising electrode materials for supercapacitors. as one of the binary metal oxides, nico2o4 is generally considered as a mixed valence oxide with pure spinel structure, in which nickel occupies octahedral sites and cobalt distributes in octahedral and tetrahedral sites [17]. the solid phase redox couples of ni2+/ni3+ and co2+/co3+ in this structure, present better electrocatalytic activity than single nio and co3o4, the conductivity that is at least twice higher than for nio and co3o4, and capacitive performance comparable with the noble metal oxide ruo2. therefore, nico2o4 has attracted considerable attention in energy conversion/storage systems due to its excellent electrochemical properties, rich redox reactions involving different ions, complex chemical components and synergistic effects. hence, nico2o4 with ultra-high specific capacitance could be an alternative pseudocapacitive material to ruo2. currently, the spinel nico2o4 has been widely used in supercapacitors and lithium ion batteries, as electrocatalysts and magnetic materials, in photodetectors, ferrofluid technology, etc.[14,18]. based on the pseudocapacitor mechanism, a golden way to improve the redox kinetics is to create nanostructured electrode materials with large surface area for redox reaction and short transport paths for ions and electrons. therefore, the development of polymetallic oxide supercapacitor materials with special micro-nano structure and morphology is of great practical y. zhanga et al. j. electrochem. sci. eng. 9(4) (2019) 243-253 http://dx.doi.org/10.5599/jese.690 245 significance and challenge. thus far, various nico2o4 materials with different morphologies and nanostructures, such as nanowires [19], nanorods [20], urchin-like hollow microspheres [21], nanotubes [22], nanoneedles [23], nanosheets [24], etc., have been synthesized by various methods, including the sol-gel method [25], coprecipitation method [26], electrodeposition method [27], microwave method [28], and hydrothermal method [29]. in recent years, the ultracapacitors of nico2o4 have been studied [17,30]. however, due to the high calcination temperature, high crystallinity and low electrochemical activity, the specific capacitance of nico2o4 is low. therefore, the development of pure nico2o4 by sol-gel technology is considered as a simple, cheap and low energy consumption method for the preparation of mixed metal oxides, which can produce homogeneous multi-component metal oxide materials with high purity, small grain size, large specific surface area, and good electrical conductivity. in this work, we demonstrate use of a simple sol-gel method followed by calcination at moderate temperature of 350 °c, to prepare nico2o4 nanoparticles with unique characteristics. some techniques such as scanning electron microscopy (sem), x-ray diffraction (xrd), raman spectroscopy (raman), cyclic voltammetry (cv), and galvanostatic charge/discharge (gcd) methods were used to characterize the samples. the results show that the as-synthesized nico2o4 nanoparticles exhibit high specific capacitance, superior high-rate capability and long-life cycling stability. this is due to its small particle size, low crystallinity and high active materials utilization. moreover, nico2o4 material benefits from multiple oxidation states/structures which contribute by both nickel and cobalt ions. these results indicate that the as-prepared nico2o4 nanoparticles have the potential application in development of high performance supercapacitors. experimental synthesis of nico2o4 nanoparticles all reagents were of analytical level and used without further purification. figure 1 presents the schematic diagram of the fabrication process of nico2o4 nanoparticles. figure 1. schematic diagram of nico2o4 nanoparticles preparation firstly, 0.77 mmol of ni(no3)2·6h2o and 1.54 mmol of cocl2·6h2o were dissolved in 2.5 ml ethanol by ultrasonic treatment for 5 min, and then 0.025 g hexadecyl trimethyl ammonium bromide (ctab) and 2.0 g propylene oxide were added to the dispersion and followed by stirring for 8 h at 25 °c. after reaction, the gel product was collected and cleaned by deionised water and http://dx.doi.org/10.5599/jese.690 j. electrochem. sci. eng. 9(4) (2019) 243-253 nico2o4 nanoparticles for supercapacitors 246 ethanol and dried at 60 c for 12 h. finally, the light green powder of the precursor was calcined in air atmosphere at 350 °c for 3 h in a muffle furnace to obtain the final nico2o4 nanoparticles. structure characterization the morphologies and structures of the as-prepared nico2o4 nanoparticles were characterized by field emission scanning electron microscopy (sem, jeol jem-7001f, japan), transmission electron microscope (tem, jeol, jem-700, japan) and x-ray diffraction (xrd, d8 advance, bruker corporation). the composition and microstructure of the samples was performed on a raman spectrometer (raman, horiba, labram hr800). electrode preparation and electrochemical measurements the circular nickel foam with a diameter of 14 mm used as the working electrode current collector was firstly ultrasonicated for 10 min in 1 mol l-1 hcl solution, and then rinsed twice with acetone, ethanol and deionized water, respectively. then, the cleaned nickel foam was put into a vacuum drying oven and dried at 60 °c for 12 h. the working electrode was prepared as follows. firstly, the active material sample, carbon black and polyvinylidene fluoride (pvdf) were mixed with a mass ratio of 75:15:10 in an appropriate amount of n-methyl-2-pyrrolidone (nmp). the obtained mixture was ground to form the slurry, which was evenly coated on the surface of the treated nickel foam and dried at 60 °c for 8 h. the three-electrode system was used to determine the electrochemical performance of the working electrode. a platinum slice (2  2 cm) and hg/hgo electrode were used as the counter and reference electrode, respectively. all measurements were performed in 3 mol l-1 koh aqueous solution. the sample electrode was tested by using chi 660e electrochemical workstation (shanghai chenhuainstrument co., ltd.) in a three-port h-type cell for cyclic voltammetry (cv) and constant current charge-discharge (gcd) test. the scanning rates of cv tests were 2, 5, 10, 15 and 20 mv s-1 performed in the voltage scope of 0-0.5 v. gcd experiments were carried out in the potential range of 0-0.5 v with current densities of 1, 4, 7 and 10 a g-1, respectively. the specific capacitance (cm, f g-1) values were calculated from charge-discharge curves according to the following equation (1):  = =  m c i t c m m u (1) in eq. (1), i(a), m (g), △t (s) and △u (v) represent discharge current, mass of active electrode material, total discharge time and potential window, respectively. results and discussion structural characterization the surface morphologies of nico2o4 samples were investigated by sem with different magnifications. as shown in figure 2, the sample is composed of many regular and disordered nanoparticles with a diameter of about 20-30 nm. obviously, there are some loosely packed porous structures between the nanoparticles, with the size ranging from tens to hundreds of nanometers. this porous structure and interaction space between nico2o4 nanoparticles is beneficial for improving the specific surface area by enhancing the transport facility of ions, shortening the pathway of electron migration, maintaining the chemical stability during redox reactions, and improving the electrochemical performance. therefore, as obtained nico2o4 nanomaterial is a promising candidate electrode material for supercapacitors. y. zhanga et al. j. electrochem. sci. eng. 9(4) (2019) 243-253 http://dx.doi.org/10.5599/jese.690 247 a b c d figure 2. sem images of as-obtained nico2o4 nanoparticles at different magnifications: a ×30000; b ×50000; c ×60000; d ×80000. the detailed morphology, size and microstructure of the obtained nico2o4 nanoparticles were further examined by tem. as shown in figure 3(a) and (b), numerous nanoparticles are loosely packed together. in addition, figure 3(c) and (d) show tem magnification images of an individual diamond-like nico2o4 nanoparticles with a size of about 20-30 nm, in which the diamond-like structure contains a large number of nanopores. this unique porous structure will increase the contact area between the electrode and the electrolyte, which in turn will improve the electron and ion transport, improving thus electrochemical performance of the electrode [31]. the purity and crystal structure of the nickel foam substrate and as-prepared nico2o4 sample were determined by x-ray diffraction, and the corresponding xrd patterns are shown in figure 4. it is found that all peaks located at 2  = 44.51, 51.85 and 76.37 o can be perfectly indexed to (111), (200) and (220) planes of the nickel foam substrate (jcpds 04-0850), respectively. the major diffraction peaks at 2 = 31.15, 36.70, 44.62, 59.09 and 64.98o of nico2o4 (jcpds no. 20-0781) correspond to the (220), (311), (400), (511) and (440) crystal planes, respectively [32]. it can be observed that all peaks in the pattern corroborate well with the standard pattern of face-centered cubic spinel nico2o4 with a space group of fd3m. no excrescent peaks are detectable, indicating that the pure nico2o4 phase was successfully obtained. moreover, broad and weak diffraction peaks indicate that nico2o4 exhibited inferior crystallinity and nanocrystallinity, which are favorable for nico2o4 nanoparticles to exhibit better capacitive performance [25]. to further evaluate the phase formation and structural features of the prepared nico2o4 nanoparticles, raman spectroscopy was performed with a typical spectral range of 150-750 cm-1, and the typical raman spectrum of the sample is shown in figure 5. http://dx.doi.org/10.5599/jese.690 j. electrochem. sci. eng. 9(4) (2019) 243-253 nico2o4 nanoparticles for supercapacitors 248 a b c d figure 3. tem images of as-obtained nico2o4 nanoparticles at different magnifications 20 30 40 50 60 70 80 in te n si ty , a .u . 2 / (2 2 0 ) (3 1 1 ) (4 0 0 ) (5 1 1 ) (4 4 0 ) (1 1 1 ) (2 0 0 ) (2 2 0 ) nickel foam nico2o4 figure 4. xrd patterns of the nickel foam substrate and as-prepared nico2o4 nanoparticles y. zhanga et al. j. electrochem. sci. eng. 9(4) (2019) 243-253 http://dx.doi.org/10.5599/jese.690 249 200 300 400 500 600 700 10 15 20 25 30 35 40 45 50 in te n si ty , a .u . raman shift, cm -1 f 2g e g a 1g figure 5. raman spectrum of the as-prepared nico2o4 nanoparticles the stretching vibration peaks observed at ~182.2 , ~469.1, and ~661.1  cm-1 correspond to f2g, eg and a1g modes of nico2o4 respectively, confirming its single-phase formation of spinel structure. the observed mode of the phonon is attributed to the vibration of co-o and ni-o, respectively [33]. moreover, nico2o4 samples show only co-o and ni-o vibrations, indicating that the precursors of nickel and cobalt were completely decomposed at 350 °c, what is consistent with the literature [34] and implies that pure nico2o4 was formed after calcination. these results are well consistent with the xrd results, which further confirm the formation of nico2o4 nanoparticles. electrochemical characterization the electrochemical capacitive performance of the as-prepared nico2o4 sample was evaluated by cv measurements using a three-electrode system. figure 6 shows cvs recorded for nico2o4 nanoparticles in 3 mol l-1 koh aqueous electrolyte. the potential was scanned between 0 and 0.5 v at scanning rates of 2, 5, 10, 15 and 20 mvs-1, respectively. a pair of redox peaks can be detected in each voltammogram, indicating that the electrode capacitance is mainly based on the redox mechanism related to reversible redox reactions of ni2+/ni3+ and co3+/co4+, associated with anions oh[35]. as can be seen from cv curves in figure 6, the anodic peak potential at around ∼0.48 v and cathodic peak potential at ∼0.36 v are noticed at the scan rate of 2 mvs-1, which is close to the literature value. moreover, when the scanning rate increased from 5 to 20 mvs-1, the position of the anodic peak shifts slightly from 0.48 to 0.50 v, indicating that the electrode material resistance is relatively low and electrochemical reversibility is good [18]. to further estimate the supercapacitive performance of the as-prepared nico2o4 nanoparticles, gcd measurements were carried out within the potential range from 0 to 0.5 v at current densities of 1, 4, 7 and 10 a g-1, respectively. as shown in figure 7, the charge-discharge curves are non-linear with an obvious plateau at around 0.45 v and 0.39 v. this is well consistent with cv results and further verifies the pseudocapacitive behavior and good reversibility of the redox process at nico2o4 electrode [18]. http://dx.doi.org/10.5599/jese.690 j. electrochem. sci. eng. 9(4) (2019) 243-253 nico2o4 nanoparticles for supercapacitors 250 0.0 0.1 0.2 0.3 0.4 0.5 -10 -5 0 5 10 15 20 25 30 c u rr e n t d e n si ty , a g -1 potential, v vs. hg/hgo 2 mv/s 5 mv/s 10 mv/s 15 mv/s 20 mv/s figure 6. cv curves of as-prepared nico2o4 electrode in a three-electrode system scanned between 0 and 0.5 v at various scan rates 0 200 400 600 800 1000 1200 0.0 0.1 0.2 0.3 0.4 0.5 p o te n ti a l, v v s. h g /h g o time, s 1 a/g 4 a/g 7 a/g 10 a/g figure 7. gcd curves of as-prepared nico2o4 electrode at various current densities between 0 and 0.5 v based on the discharge curves, the specific capacitances of the as-prepared nico2o4 electrode were calculated according to eq. (1), and the results are illustrated in figure 8. specific capacitances of as-prepared nico2o4 are as high as 1080, 800, 651 and 574 f g-1 at discharge current densities of 1, 4, 7 and 10 a g-1, respectively. compared with the current density of 1 a g-1, the retained capacitances were still 74.1, 60.3 and 53.1 %, even the current density was increased to 4, 7 and 10 a g-1, what suggests good retention rate performance. the excellent electrochemical properties of nico2o4 nanoparticles can be attributed to the small particle size, which will increase the contact area of electrolyte/electrode, thus providing more active sites for rapid redox reactions which undoubtedly contributes to the high capacitance [35]. besides, the nanoporous structure between the particles also allows the electrolyte ions to be transported quickly into the bulk materials, what further enhances the electrode rate capability. y. zhanga et al. j. electrochem. sci. eng. 9(4) (2019) 243-253 http://dx.doi.org/10.5599/jese.690 251 0 2 4 6 8 10 500 600 700 800 900 1000 1100 d is c h a rg e s p e c if ic c a p a c it a n c e , f g -1 current density, a g -1 figure 8. specific capacitance of as-prepared nico2o4 electrode as a function of discharge current density the cycling stability of the as-prepared nico2o4 electrode in 3 mol l-1 koh aqueous electrolyte was examined by applying 100 cycles within the potential range of 0-0.5 v at current densities of 4, 7 and 10 a g-1, and the results are shown in figure 9. during the cycling, the specific capacitance of the as-prepared nico2o4 electrode remained almost unchanged at all current densities. the initial discharge specific capacitance of nico2o4 electrode is 800, 651 and 574 f g-1 at the current density of 4,7 and 10 a g-1, respectively. after 100 cycles, the specific capacitance decreases to 758, 597 and 521 f g-1, showing that the corresponding specific capacitance retention rates were 94.8, 91.7 and 90.8 % and indicating that nico2o4 electrode has good electrochemical stability. better cyclic stability can be attributed to the unique pore structure, which can be used as the “release zone” of ions and internal nanogrids that buffers possible volume changes due to ohions insertion/extraction process during cycling, improving thus its structural stability. 0 20 40 60 80 100 0 200 400 600 800 4 a/g 7 a/g 10 a/g d is c h a rg e s p e c if ic c a p a c it a n c e , f g -1 cycle number figure 9. cyclic stability of as-prepared nico2o4 electrode at various discharge current densities http://dx.doi.org/10.5599/jese.690 j. electrochem. sci. eng. 9(4) (2019) 243-253 nico2o4 nanoparticles for supercapacitors 252 conclusion by using ni(no3)2·6h2o and cocl2·6h2o as raw materials, a promising electrode material, nico2o4 nanoparticles, were synthesized successfully via the sol-gel process followed by calcining at 350 °c. in the unique structure of synthesized nanoparticles, there are obvious porous structures formed among large number of nanoparticles, which provide an effective way for rapid reversible reaction, improving thus electrochemical properties of nico2o4 material. electrochemical characterization showed high specific capacitance, good rate capability and good cycling stability of the prepared nico2o4 nanomaterial. high specific capacitance of 1080 f g-1 was achieved at the current density of 1 a g-1, while specific capacitances of 800, 651 and 574 f g-1were obtained at the discharge current densities of 4, 7 and 10 a g-1, respectively. after 100 cycles, the specific capacitance decreases to 758, 597 and 521 f g-1, suggesting the corresponding specific capacitance retention rates of 94.8, 91.7 and 90.8 %. the excellent electrochemical capacitive performance, low cost, simple and easy fabrication of the as-prepared nico2o4 nanoparticles render this material as the promising electrochemical capacitor electrode material. acknowledgments: this work is supported by the program for science & technology innovation team in universities of henan province (grant no. 16irtsthn016), the national natural science foundation of china (grant no. 21503193), and the national natural science foundation of china (grant no. u1404201). references [1] y. zhang, h. feng, x. wu, l. wang, a. zhang, t. xia, h. dong, x. li and l. zhang, international journal of hydrogen energy 34 (2009) 4889-4899. 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[35] y. zhu, x. ji, z. wu, w. song, h. hou, z. wu, x. he, q. chen and c. e. banks, journal of power sources 267 (2014) 888-900. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.690 http://creativecommons.org/licenses/by/4.0/) {chemisorption as the essential step in electrochemical energy conversion review} http://dx.doi.org/10.5599/jese.742 141 j. electrochem. sci. eng. 10(2) (2020) 141-159; http://dx.doi.org/10.5599/jese.742 open access: issn 1847-9286 www.jese-online.org review chemisorption as the essential step in electrochemical energy conversion ana s. dobrota, igor a. pašti university of belgrade – faculty of physical chemistry, studentski trg 12-16, 11158 belgrade, serbia corresponding author: igor@ffh.bg.ac.rs; tel.: +381-11-3336-625; fax: +381-11-2187-133 received: october 31, 2019; accepted: january 6, 2020 abstract growing world population and energy demands have placed energy conversion and storage into the very centre of modern research. electrochemical energy conversion systems including batteries, fuel cells, and supercapacitors, are widely considered as the next generation power sources. even though they rely on different mechanisms of energy conversion and storage, fundamentally these are all electrochemical cells, operating through processes taking place at the solid/liquid interfaces, i.e. electrodes. considering the interfacial nature of electrodes, it is clear that adsorption phenomena cannot be neglected when considering electrochemical systems. more than that, they are of crucial importance for electrochemical processes and represent an essential step in electrochemical energy conversion. in this contribution we give an overview of the phenomena underlying the operation of sustainable metal-ion batteries, fuel cells and supercapacitors, ranging from electrocatalytic reactions and pseudo-faradaic processes to purely adsorptive processes, emphasizing the types, roles and significance of chemisorption. we review experimental and theoretical methods which can provide information about chemisorption in the mentioned systems, stressing the importance of combining both approaches. keywords adsorption; reactivity trends; electrocatalysis; electrochemical power sources. introduction the surface of the material is its outlook to the external environment. therefore, many problems associated with modern materials can be solved by understanding the physical and chemical interactions that occur at materials interfaces. the nature and the state of the surface influence many properties of the material, including corrosion rate, catalytic activity, adhesive properties, wettability, contact potential, and others. these characteristics can be tuned by surface modification, for example surface functionalization or doping. let us look at a freshly prepared solid http://dx.doi.org/10.5599/jese.742 http://dx.doi.org/10.5599/jese.742 http://www.jese-online.org/ mailto:igor@ffh.bg.ac.rs j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 142 surface. if this surface is surrounded by a gaseous or liquid phase, it is exposed to the molecules of that phase. as a molecule approaches to a surface, its electron density redistributes in such a way that maximizes bonding to that surface. under normal conditions, this will result in rather quick coverage of the surface with these molecules, i.e. adsorption will occur. adsorption is an increase of the concentration of a substance at the interface of two phases. in electrochemistry, this is usually related to the solid|liquid interface. adsorption energy is a key quantity describing the strength of the interaction of molecules with the surface. while on single crystal surfaces, several high symmetry sites are available for adsorption, the preferential adsorption site is the one with the most exothermic adsorption. the way in which electron density redistributes in this process is determined by electronic structures of the surface and the adsorbate. depending on the type of forces responsible for adsorption, we can divide it into chemical and physical adsorption. chemical adsorption (chemisorption) is, according to iupac, adsorption which results from chemical bond formation (strong interaction) between the adsorbent and the adsorbate in a monolayer on the surface [1]. the forces responsible for chemisorption are the same kind as those that lead to the formation of chemical compounds. on the other hand, physical adsorption (physisorption, van der waals adsorption) is adsorption in which the forces involved are weak van der waals forces, which include dipole–dipole, dipole-induced dipole and london (instantaneous induced dipole-induced dipole) forces. they do not involve a significant change in the electronic orbital patterns of the species involved [2,3]. the problem of distinguishing between chemisorption and physisorption is the same as between chemical and physical interactions in general. the dividing line between the two is not completely sharp, and intermediate cases exist, e.g. adsorption involving strong hydrogen bonds, or weak charge transfer. however, chemisorption can be identified using some of the following criteria: (i) it is chemically specific (variations between substrates of different chemical composition, and between different surface planes of the same crystal); (ii) it leads to changes in the electronic structure of the adsorbate and adsorbent; (iii) the chemisorption elementary step often requires an activation energy; (iv) it is often dissociative (adsorbate dissociates into two or more fragments, both or all of which are bound to the surface of the adsorbent), and may not be reversible; (v) just one monolayer of adsorbate is usually formed. in general, chemisorption properties are determined by the compatibility of functionalities of adsorbents and adsorbates. now, let us look at an electrochemical system. each such system consists out of at least two pieces of a solid electronic conductor in electrical contact with each other through the electrolyte, from one side, and through an outer electronic conductor, from the other side of the cell. in such a way a closed electrical circuit is formed. the interface between the solid electronic conductor and the electrolyte (usually liquid ionic conductor) is the electrode of an electrochemical cell. considering the interfacial nature of the electrode, it is clear that adsorption phenomena are always taking place at the electrode and are of crucial importance for electrochemical processes and represent their essential step. the ever-growing energy demands have placed energy conversion and storage into the very centre of modern research. electrochemical energy conversion systems including batteries, fuel cells, and supercapacitors, are widely considered as the next generation power sources. even though they rely on different mechanisms of energy conversion and storage, fundamentally these all are electrochemical systems, operating through processes taking place at the electrodes. in batteries and fuel cells electrical energy is generated through faradaic processes, i.e. through redox reactions (electron transfer) at the electrodes. while the inside of a battery contains all that is needed for its operation, fuel cells obtain the electroactive species (species which undergo redox a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 143 reactions) from outside of the cell (in the case of hydrogen fuel cells oxygen from air and hydrogen from external storage unit), whereas the solid part of the electrode is just charge-transfer media (not changed in the overall reaction). on the other hand, electrochemical capacitors (supercapacitors, ultracapacitors) can be classified into two categories based on their energy storage mechanism: (i) electrical double layer (edl) capacitors, in which the capacitance is the result of pure electrostatic accumulation and disorganization of charges at the electrode, strongly dependent on the surface area of the electrode materials accessible to the electrolyte ions; (ii) pseudo-capacitors, in which fast and reversible faradic processes take place [4]. these two mechanisms can work simultaneously depending on the nature of electrode materials. electrochemical reactions take place at the electrode, i.e. at the solid/liquid interface. therefore, it is clear that charge transfer processes in some types of batteries and, generally, fuels cells represent cases of heterogeneous catalysis, in which the process rate is affected by the presence of a solid surface which mediates chemical transformations. during every catalytic cycle many bonds are cleaved and formed, but in the end the catalyst remains unchanged. a special case of heterogeneous catalysis in which the rate of the process is additionally influenced by the electrode potential and the electrode material properties is electrocatalysis. in this sense, virtually every electrochemical reaction is fundamentally electrocatalytic [5], excluding outer sphere reactions. for a faradaic electrochemical process to take place, at least three following steps have to occur: (i) transfer of electroactive species (reactant) to the electrode, (ii) charge transfer at the electrode, resulting in chemical transformation of the reactant to the product and (iii) transfer of the product away from the electrode. however, in majority of electrochemical reactions, interactions of reactants with the electrode or formation of adsorbed intermediates is the prerequisite for charge transfer. for example, this is the case for one of the most important electrochemical reactions, hydrogen evolution reaction (her), where adsorbed atomic hydrogen (hads) is formed in the process, in overall described as: 2h+ + 2e− → h2. an essential step in every heterogeneous catalytic reaction, and therefore in electrochemical energy conversion processes, is the chemical bond formation between the catalyst surface and the reactant/intermediate, i.e. chemisorption. in general, chemisorption properties are determined by the combination of electronic structures of the substrate and the adsorbate. as an adsorbate molecule approaches a surface, redistribution of electron density might result in weakening of some of the bonds inside the adsorbate molecule. in that way the solid catalyst activates the adsorbate, making it prone to chemical changes. the interaction of the reactants/intermediates with the catalyst has to be optimal in strength (sabatier principle, figure 1): if too weak, the reactants will not bind to the catalyst and the reaction will not be able to take place [6]. on the other hand, if the interaction is too strong, the catalyst active sites will be blocked by reactant, intermediate or product molecules, leaving no active sites available for new reactant molecules which would continue the reaction. in this contribution, we address chemisorption as the initial and fundamental step in electrochemical energy conversion and storage systems. we explore which adsorbates and adsorbents are of electrochemical interest and review experimental and theoretical methods used for exploring such systems, demonstrating how electronic structure calculations can provide fundamental, atomic-level understanding of the interaction of different types of adsorbates with various substrates of great importance in electrochemical energy conversion. we emphasize the importance of combining experimental and theoretical approaches in order to complete the adsorption puzzle. http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 144 figure 1. schematic view of the sabatier principle: finding the optimal bond strength. adsorbates of electrochemical interest carbon monoxide oxidation (co + ½ o2 → co2) is one of the simplest catalytic reactions, used as a prototype for understanding the oxidation of small organic molecules (e.g. methanol, ethanol, formic acid) used as the fuel in some fuel cells. it is also of great interest for the control of exhaust gases, air purification and sensors. three major types of co oxidation catalysts are used: platinum group metallic catalysts (whose surface might be oxidized), bulk oxide catalysts (e.g. ceo2), and oxide-supported metal clusters. therefore, their interaction with co molecule is of great practical interest. hydrogen fuel cells are a perspective replacement for fossil fuels which would diminish atmospheric pollution, giving water as the only by-product. in hydrogen fuel cells the oxygen reduction reaction (orr) plays an essential role in determining the cell performance. both in acidic and alkaline water environments orr can proceed through two paths: (i) the direct (4e−) pathway, and (ii) the indirect (2e−) pathway, as given in table 1. due to higher faradaic efficiency, the direct 4e− pathway is preferable. however, the mechanism of this pathway consists of a number of steps in which molecular oxygen has to dissociate at the surface and recombine with hydrogen ions to form water. therefore, dissociative chemisorption of o2 can be considered as a crucial step in orr. looking at the anode side, different fuels can be oxidized, starting with h2 or small organic molecules. in all the cases, anode reactions involve formation of adsorbed intermediates. another electrochemical reaction of great interest for hydrogen fuel cells is hydrogen evolution reaction (her). hydrogen is considered to be the future energy carrier. inside a fuel cell hydrogen undergoes oxidation, giving water as the final product, but there is the issue of providing and storing the hydrogen fuel in the first place. in acidic environment (table 1) the first her step is proton reduction, yielding adsorbed hydrogen (volmer reaction: h+ + e− → hads). two of so adsorbed atoms can recombine on the surface, forming h2 (tafel reaction: 2hads → h2), or one hads can undergo electrochemical desorption (heyrovsky reaction: hads + h+ + e− → h2). since hads formation is the first step in this mechanism, adsorption energy of atomic hydrogen is a valuable parameter for predicting material’s activity for her and h2 storage capability. table 1. oxygen reduction and hydrogen evolution pathways in acidic and alkaline solutions. reaction/pathway acidic environment alkaline environment direct orr (4e−) o2 + 4h+ + 4e− → 2h2o o2 + 2h2o + 4e− → 4oh− indirect orr (2e−) o2 + 2h+ + 2e− → h2o2 h2o2 + 2h+ + 2e− → 2h2o o2 + h2o + 2e− → ho2− + oh− ho2− + h2o + 2e− → 3oh− her 2h+ + 2e− → h2 2h2o + 2e− → h2 + 2oh− a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 145 in the case of metal-ion batteries, the interaction of the chosen metal with the electrode material is of key interest, as they operate through insertion/removal of the given metal ions at the cathode/anode. therefore, a key prerequisite for attaining high energy density (energy per unit volume) of a metal-ion battery is a cathode which accommodates and releases the metal ions quickly and in great capacity. battery operation (charge/discharge cycles) assumes a chemical change with known gibbs free energy change (δg), resulting in formation of well-defined phases. the open circuit potential of the cell can be related to δg of the reacting system. most widely used today are lithiumion batteries. however, their sustainability is in question due to the limited lithium availability and consequent expected price increase. therefore, intensive research efforts are made in the field of rechargeable metal-ion batteries involving more abundant light weight metals, such as sodium, magnesium, calcium, aluminium and zinc [7–10]. due to their multivalent nature, which means that multiple electrons are transported during the electrochemical charge/discharge reactions, mg, ca, al and zn metal anodes can be used to obtain higher energy density, improved safety and lower initial and cycle-life costs than state-of-art lithium batteries [10]. that is why understanding the nature of the interaction of the mentioned metals with potential electrode materials is crucial for further development of rechargeable metal-ion batteries. another important group of batteries are metal-air batteries, like lithium-air batteries, where metal lithium oxidizes on the anode, and oxygen undergoes reduction on the cathode, in the overall reaction: 2li + o2 → li2o2. in this case, cleaving of the o–o bond is not necessary, resulting in improved kinetics and re-chargeability, and not requirements for pt catalyst. however, metal-air batteries still face many practical problems restricting their massive production and use [7]. another important question regarding adsorption is whether universal relationships between the binding energies of different adsorbates exist. revealing such relationships would provide a simple tool for the assessment of surface reactivity towards various adsorbates using measurements with a single one. for some types of adsorbates and surfaces such interdependence has been demonstrated, as will be discussed later on. understanding adsorption: experimental and theoretical tools united each catalytic process involves multiple steps of bond breakage and formation. capturing each of these steps is very difficult even by the finest of experimental techniques. that is where the electronic structure calculations become involved, demonstrating their true power and complementarity with experimental methods [11]. the computational and experimental approaches in catalysis should not be considered separately, because only by combining them one can obtain full understanding of the given catalytic process. according to the classification given in ref. [12], experimental methods for investigating adsorption phenomena can be divided into three groups: (i) methods based on the measurement of changes in the electrical, magnetic and work function properties of the adsorbent during adsorption, (ii) methods based on the interaction between the adsorbate and radiation, electrons or ions, and (iii) spectroscopic methods. the first group includes measurements of the electrical conductivity of the adsorbent, hall effect, magnetic properties, and the work function [12]. for exploring surface bonds, ultraviolet photoelectron spectroscopy (ups) is often used [13]. ups operates on the same principles as x-ray photoelectron spectroscopy (xps), the only difference being that ultraviolet (instead of x-ray) ionising radiation is used to induce the photoelectric effect. the spectra are obtained by irradiation of a solid surface with x-ray/uv light and measuring number and kinetic energy of electrons emitted from the material. as the energy of the ultraviolet photons http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 146 is lower, most core level photoemissions are not accessible, and only the valence band region is probed. due to the short inelastic mean free path of free electrons within a solid (determined by the properties of the solid through which they are travelling, and electrons kinetic energy), ups is inherently surface sensitive, with an approximate information depth of 2-3 nm. therefore, a signifycant fraction of the spectral contribution can be associated with the adsorbate. while ups does not allow an atom-based view of bonding effects [14], x-ray emission spectroscopy (xes) and x-ray absorption spectroscopy (xas) provide just that, through measurements of occupied/unoccupied densities of states [13–15]. xas is a powerful tool, both element and site specific, for measuring the unoccupied density of electronic states of a material using high intensity x-ray radiation of tuneable energy. the fraction of x-ray photons absorbed by the samples electrons moving to the conduction band is measured, as their subsequent relaxation results in the emission of a photons having energy equal to the energy difference between the core level and excited states. xes is a complementary technique that probes the occupied density of electronic states of a material. another experimental technique for determination of surface structures, which can be used both for qualitative and quantitative analysis, is low energy electron diffraction (leed). in leed, a beam of electrons of a well-defined low energy (typically 20 200 ev) is directed normally onto the sample. the diffraction pattern is formed by the elastically-scattered electrons, while secondary electrons are removed by energy-filtering grids. the analysis of the diffraction spot positions yields information on the size, symmetry and rotational alignment of the adsorbate unit cell with respect to the substrate unit cell. single crystal adsorption calorimetry (scac) is a powerful method for measuring adsorption energies [16]. microscopic methods such as scanning tunnelling microscopy (stm) and atomic force microscopy (afm) can also provide valuable insights into the surface structures. stm is based on quantum tunnelling which occurs when the conducting tip scans the sample. while stm requires a conducting sample, afm is appropriate for the insulating ones as well. in contrast to experimental techniques, electronic structure methods provide atomic level resolution. however, the cost of these calculations requires introduction of significant approximations, and the results obtained cannot be considered as “absolute truth”. theoretical and experimental results should be taken together in order to have a picture as complete as possible. two theoretical electronic structure methods have made the largest impact in the field of catalysis: (i) high-level ab initio molecular orbital theory and (ii) density functional theory (dft). dft is based on two theorems given by hohenberg and kohn for the case of arbitrarily many electrons in a box subjected to external potential υ(r) [17,18]: (i) the system hamiltonian and its ground state are uniquely determined by the electron density (n(r)), and (ii) an universal functional (f[n]) exists, which determines the total energy of the system as: e[n] = ∫υ(r)n(r)dr + f[n]. the total energy has minimal value for the ground state n(r). this way, the scalar electron density was used [19] instead of the many-body wavefunction. kohn and sham have divided this functional into the kinetic energy of non-interacting electrons, the energy of electron-electron interactions, and the exchange-correlation energy (exc). in this way, the uncertainty is reduced to exc. kohn-sham self-consistent equations are solved iteratively, starting from a guessed n(r), resulting in the total ground state energy of the system. the adsorption energy (eads) can then be easily calculated as the difference in total energies between the final state (adsorbate adsorbed on the substrate) and the initial state (isolated adsorbate and substrate). although exc is expected to be a small part of f[n], it still makes dft somewhat approximate, and not fully first-principles. to account for the unknown exc, various approximations are used, including the local density approximation (lda), generalized gradient approximation (gga), hybrid functionals, and others. a proper choice of the electron exchange correlation functional is of great importance for accurate description of the chemia. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 147 sorption energetics [20]. even though that due to exchange-correlation treatment dft is more approximate than molecular orbital theory, it is more appealing for catalysis studies, as it offers the possibility of investigating more complex systems, closer to the real ones. today, dft is widely used as a numerical simulation tool for investigating systems consisting of up to 102 103 atoms (depending on the available computational resources), for example for large molecules, solid bulks and interfaces. moreover, its accuracy is high enough to explain and predict reactivity trends [11]. however, gga results in the neglect of dispersion interactions. this approach leads to an incorrect description of physisorption, which can be of interest when looking at o2 and alkali metals adsorption on some materials, species of great importance for fuel cell and metal-ion batteries applications. in fact, in physisorbed systems lda can give ‘better’ results than gga [21], but dispersion can be included in gga calculations through several corrections. theoretical modelling can be used for rational catalyst design and capturing general trends and principles underlying various catalytic processes at the electronic/atomic level [11]. however, the electronic structure calculations are the most demanding computational approaches used in (electro)catalysis research. a detailed analysis of reaction mechanisms for a particular catalytic reaction on many possible substrates can be extremely timeconsuming and impractical. additionally, for explicit treatment of an electrochemical interface, the existence of the solvent and the electrode potential would have to be taken into account, but there is no proper scheme to theoretically account for the electrode potential at the level which is affordable enough to allow routine use of such approach [22]. luckily, it seems that in most cases, the treatment of electrochemical reactions from the aspect of the electrocatalysis does not require an inclusion of the electrode potential. the presence of a solvent can also be disregarded, or included implicitly through corrective factors of the adsorption energies of the reaction reactants/intermediates [23]. this way, electrocatalytic reactions can be theoretically treated as reactions at the solid/gas interface. however, there is an enormous number of possibilities when it comes to systems which could be treated theoretically. therefore, it is practical to use some empirical and semi-empirical filters, e.g. element abundance, to reduce the number of systems requiring explicit dft treatment. an example of such pre-screening can be found in ref. [24]. microscopic properties of (electro)catalytic materials which can be associated with their performance according to kinetics of (electro)catalytic processes are called catalytic activity descriptors. they can be obtained by experimental and theoretical techniques and used for tuning the catalytic activity of a given material and establishing design concepts in search for new catalysts [25]. since in heterogeneous catalysis the interaction between the catalyst and reactants/ intermediates plays the paramount role for the catalytic performance, these descriptors are often corresponding adsorption energies, as will be shown later on. chemisorption on transition metal surfaces chemisorption trends on transition metal surfaces have been highly elaborated due to their significance in heterogeneous catalysis. for example, transition metal-based electrodes are typically used as catalysts for her in alkaline medium, while pt-group metals are the best catalysts for hydrogen and oxygen electrode reactions. hammer and nørskov have formulated a concept, known as the d-band model, which describes adsorption trends on transition metal surfaces using the position of the metals d-band [26]. this model provides a link between the electronic structure and the reactivity of transition metal surfaces, as well as their alloys and overlayers. the key parameter determining the adsorption strength, d-band centre, can be obtained both experimentally (from spectroscopical methods) and http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 148 by theoretical calculations [27–29]. according to d-band model, interaction of an adsorbate with the metal surface can be thought as a two-step process. first, the electronic state of the adsorbate widens due to its interaction with the metal wide, half-filled s-states (which are the same for all transition metals). next, the so-widened adsorbate state interacts with metal narrow d-state, giving rise to bonding and anti-bonding states. their relative occupancies, determined by the positions of the adsorbate state and the metal d-states, dictate the strength of the overall interaction. in order for all of this to occur, the adsorbate atom over which the bonding is to take place should have a radical state, i.e. an unpaired electron [30]. if not, rehybridization is needed to form such a radical state enabling the interaction with the surface [30]. of course, there are other scenarios as well. if instead of unpaired electrons the adsorbate possesses an unsaturated π-electron system (e.g. n2, o2, co, or unsaturated hydrocarbon molecules), binding to a metal surface is possible via virtual radical state created by partial mixing or excitation of the bonding and antibonding π and π* orbitals [31]. the mechanism of the virtual radical state formation depends on the π → π* excitation energy and determines the final adsorption geometry. complete cleavage of one π bond inside the adsorbate would leave both of the involved atoms “radicalized”, prone to bonding to the metal surface, resulting in horizontal adsorption. however, such bond cleavage would mean fully populating π* orbital, which is not favourable if π → π* excitation energy is relatively big (compared to the energy that would be obtained upon bonding). therefore, π-unsaturated molecules with smaller excitation energies, such as unsaturated hydrocarbons, will adsorb in parallel to the surface. this kind of bonding is explained in dcd (dewar, chatt and duncanson) model as donation of charge from the molecule highest occupied π-orbital into the metal, and a subsequent back-donation from filled metal states into the lowest unoccupied π*-orbital [32,33]. on the other hand, molecules with higher excitation energies (e.g. n2, co) will not undergo full bond breakage, π* will not be fully occupied, one atom will be more activated, and the molecule will adsorb vertically via that atom to the surface. for molecules that cannot engage in bonding neither through broken bonds nor virtually via π-electron system, rehybridization requires too much energy (compared to the energy obtained upon bonding) resulting in weak overall interaction with the surface. that is the case with molecules with lone pair electrons in rigid σ orbitals, e.g. h2o. the situation is similar for the case of saturated hydrocarbons, in which only small polarization rearrangements of the molecular orbital structure are energetically possible, resulting in weak physisorption [30]. regarding co oxidation on densely packed transition metal surfaces, three adsorption mechanisms have been proposed: (i) langmuir–hinshelwood [34], (ii) eley–rideal [11] and (iii) marsvan krevelen mechanism [35,36]. langmuir–hinshelwood model is most widely accepted and assumes adsorption of both co and o2 on the catalyst surface, but the adsorption of o2 is dissociative, yielding oxygen adatoms: o2 → 2oads. thus, adsorbed co and oads form co2 which desorbs from the surface (figure 2a). figure 2. different mechanisms of co oxidation on platinum group metal surface: (a) langmuir–hinshelwood mechanism of co and o2 co-adsorption, (b) eley–rideal mechanism of just o2 adsorption, and (c) mars-van krevelen mechanism for oxidized metal surfaces. reprinted from ref. [11], ©2014, akadémiai kiadó, budapest, hungary a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 149 within eley–rideal mechanism, co does not adsorb, but rather reacts from the gas phase with oads on the catalyst surface (figure 2b). if the metal surface is oxidized, the lattice o atoms of the first layer can be used for co oxidation (figure 2c). this results in vacancy formation and subsequent surface healing by o2 from the gas phase (mars-van krevelen mechanism). hydrogen adsorbed on transition metal surfaces is one of the most important intermediates in a large number of electrocatalytic processes. hydrogen atoms form moderately strong chemical bonds on transition metal surfaces (figure 3), thus being very reactive intermediates, but also very sensitive to catalytic poisons [5]. generally, adsorption energy decreases with an increase in the coverage, due to the repulsive adsorbate–adsorbate interaction and gradual stabilization of metal centres. the experimental and theoretical results for h adsorption on transition metal surfaces are in satisfactory agreement (figure 3). the differences between them lie within the energy window of only ~0.2 ev, which coincides with the accuracy of state of the art computational techniques used in material science studies [37]. figure 3. comparison of h adsorption energies on transition metal (111) surfaces obtained by both theoretical and experimental methods, in case of 0.25 ml surface coverage (t – top site, f – three-fold hollow fcc adsorption site). adapted from ref. [37], ©2011, pleiades publishing, ltd. in the case of her on metal surfaces in acidic solutions, trasatti has provided a link between the exchange current density and the metal-hydrogen (m−h) bond strength, in the form of the volcano curve (figure 4). since the highest exchange current density means the highest catalyst activity, the shape of the curve indicates that pt is the best single-metal catalyst for her under these conditions [38]. the search for new efficient electrocatalysts is motivated by the high price of pt. however, it should be noted that around the potential of hydrogen evolution some metals are covered by oxide (w, mo, etc.), some are covered by underpotential deposited h (strongly bound hydrogen; metals behaving this way are pt, pd, ir, etc.), while some are reduced, i.e., in metallic state [39]. this concept of her volcano received significant attention and it is widely accepted. recently, it was also demonstrated for her catalyst in alkaline media [40]. however, some new re-interpretations can be found, which also take into account the nature of her intermediate on different metal surfaces [41]. even so, formation of hads is the key step in this process. another important finding which can be mentioned is related to the scaling of adsorption energies of different molecules and molecular fragments on transition metal surfaces. for the cases of chx, (x ∈ {0, 1, 2, 3}), nhx, (x ∈ {0, 1, 2}), ohx, (x ∈ {0, 1}), and shx, (x ∈ {0, 1}) adsorbed on close-packed and stepped transition-metal surfaces, scaling relations between the adsorption energy of any of the http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 150 molecules considered with the adsorption energy of the corresponding central (c, n, o, or s) atom have been found (example shown in figure 5). figure 4. trasatti’s volcano plot for her in: (left) acidic solutions (j00 denotes the exchange current density, and em-h the energy of hydride formation) and (right) alkaline solutions (∆eh stands for hydrogen binding energy on monometallic surfaces, and i0 for the exchange current densities). reprinted from ref. [41], ©2014 quaino et al; licensee beilstein-institut; and ref. [42] with permission from the royal society of chemistry the scaling constant was found to depend only on x [43]. in the light of the adsorption scenarios presented in figure 4, it is clear that all considered molecular fragments are radical species, so similar adsorption mechanisms are operative for all of them. the power of such relations lies in the fact that with only a few explicitly calculated adsorption energies, one can estimate reaction mechanism of various (electro)catalytic reactions which involve these species. figure 5. correlation between the binding energies of ch3 (δech3) and c (δec) for adsorption in the most stable sites (triangles) and in the case where both ch3 and c have been fixed to the on-top site (squares). reprinted from ref. [43], ©2007 american physical society. as many of electrocatalytic reactions involve the use of expensive pt-group metals, one of the strategies to reduce the price is the use of core-shell or thin-layer catalysts [44]. it was recently shown that many properties of such thin films, including chemisorption properties, converge quickly with the thickness of the film and depend only on the lattice mismatch between the overlayer and the support. this means that from the difference between the lattice constants one can estimate chemisorption energies if data are known for pure overlayer [45] (figure 6). a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 151 figure 6. comparison of the calculated and predicted properties for pd3au trilayer on different substrates: co adsorption energies (left) and surface segregation energies in the presence of co (right). surface composition is defined by the symbol, while color defines the substrate (coefficients of determination r2(eads,co) = 0.99; r2(eseg,co = 0.94)). reproduced from ref. [45] with permission from the pccp owner societies. if such database is combined with scaling relations, it can be concluded that we already know a lot about the systems that have not been formed yet, even though these systems were not treated explicitly by theoretical calculations. so, the next step is the integration of many existing models and databases in a functional system which can be used to screen electrocatalysts for various reactions. chemisorption on metal oxides metal oxides are the most abundant materials in the earth's crust. they demonstrate a great variability in the crystal and electronic structure, as well as magnetic properties. surface properties of metal oxides can vary from typical insulator and semiconductor behaviour to metal-like behaviour. such diversity qualifies them for many technological applications, including those in catalysis [46,47], energy conversion [48], chemical sensors and environmental monitoring [49], corrosion [50], and ceramics [51]. a great variety in oxides properties is the reason why making general remarks about chemisorption trends on metal oxide surfaces is rather difficult, and why the situation is not as clear as in the case of (transition) metal surfaces. as an example, we take magnesium oxide (mgo), although it is an insulator and does not have many electrochemical applications. however, it is one of the most intensively studied oxide materials: its physical properties such as the simple rock salt structure, strong ionic bonding and high chemical stability, make it a perfect candidate for surface and adsorption studies. mgo(001) surface is especially widely used, as it is non-polar, easy to prepare and does not undergo large structural relaxation [52]. mgo is quite inert, but its surface can be made more reactive by introducing different types of defect into its structure [53–58], or growing it on a metal substrate [59–62]. let us consider dissociative adsorption of homonuclear diatomic molecule x2 on mgo(001), where x is non-metal from p-block of the periodic table of elements. in order to make a homolytic x−x bond break, adsorption should be parallel to the surface plane, so that both x atoms would interact with the surface. the energy spent for x−x bond cleavage is compensated by formation of new surface bonds. depending on the type of interaction, we can divide such molecules into three groups [53]: (i) weakly interacting with mgo (h2, n2, o2), (ii) strongly adsorbing on mgo(001) (b2, c2) and (iii) spontaneously dissociating on mgo(001) (f2). weak http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 152 adsorption is a consequence of very strong intramolecular bonds in these molecules, due to either fully filled bonding states (h2, n2), or partially filled antibonding states (o2) destabilizing the molecule on the surface. b2 and c2 possess partially filled bonding orbitals located in the same energy window as the mgo valence band, and these states participate in bonding with mgo(001) (figure 7). in case of f2 the empty σ* orbital located just below mgo valence band fills upon the interaction with mgo, due to charge transfer to electronegative f [53]. hence, it comes to a proper matching between adsorbate and substrate. figure 7. projected density of states (pdos) and integrated local density of states (ildos) for isolated (top) and adsorbed (bottom) b2 (left), c2 (middle) and f2 (right). density of states of clean mgo (scaled) is included. vertical dashed line denotes the highest occupied states of investigated molecules (top) or the highest occupied state of x2,ads+ mgo(001) complex. presented structures give ildos in the energy windows designated on pdos. reprinted from ref. [53], © 2014 elsevier b.v. the mechanism of co oxidation on metal oxide surfaces is generally of mars-van krevelen type, as the lattice o atom is available (figure 2c). dft calculations have confirmed this for the case of co3o4(110) surface, where cobalt reduction (co3+ + e− → co2+) acts as an electron sink, removing electrons upon co2 formation, and allowing re-oxidation of the surface [63]. similar was found for a rather good co oxidation catalyst, ceo2. as the mechanism assumes vacancy formation in the place of o, the vacancy formation energy can be used as a catalytic descriptor for co oxidation reaction [64]. this energy can be tuned by dopants and evaluated by electronic structure theory calculations. that way, one can optimize the catalyst to maximize its activity. again, sabatier-like principle is in action: o-vacancy formation energy should be low enough to enable the release of lattice o, but high enough to make the vacancy annihilation by o2 possible [65]. a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 153 oxide materials have found applications in her catalysis as well. at the moment, the focus is on interfacial effects where the presence of oxide/hydroxide interface with metal can boost hydrogen evolution through cooperative action in h2o activation [66,67]. chemisorption at the interface is the key step here, and the activity trends can be related to the chemisorption of hydrogen and formation of hads at the metallic phase. it also seems that in some cases, oxides could be put on the classical her volcano curve (figure 8) where hads bond strength with oxide surface can be taken as the descriptor of catalytic activity [68]. figure 8. her volcano curve in alkaline media with pdo located on it (the cases of 1 ml by hads on pdo(001) and pdo(100)). reprinted from ref. [68], © 2017 elsevier ltd. chemisorption on carbonaceous materials properly functionalized carbon-based materials can be used as either catalysts, or catalyst supports. for example, li can be inserted reversibly within most carbonaceous materials, but the insertion mechanism depends on the material type li intercalates in layered carbons such as graphite, and it adsorbs on the surfaces of single carbon layers. lithium also appears to reversibly bind near hydrogen atoms in carbonaceous materials containing substantial hydrogen [69]. for catalyst support applications, high conductivity and large specific surface area are of crucial importance. however, modifications might be necessary to achieve good tethering of catalyst nanoparticles to carbon surfaces. the most basic model for contemplating all carbon-based materials is graphene, a monolayer of carbon atoms arranged in a honeycomb lattice. since its experimental discovery in 2004, it has become one of the hottest topics in materials science. the 'graphene fever' has particularly influenced the world of electrochemical energy storage devices. better understanding of graphene reactivity would be useful for such applications, where graphene and other carbon materials are http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 154 key elements [70]. as mentioned before, in order to understand adsorption, it is important to consider electronic structure of both adsorbate and adsorbent. carbon atoms in graphene are sp2 hybridized, forming delocalized π electronic cloud on both sides of the basal plane. as a result, graphene is chemically inert. three high symmetry sites are available for atomic adsorption on pristine graphene: top (single-coordinated, above one c atom), bridge (two-coordinated, above the middle of a c−c bond) and hollow (six-coordinated, above the centre of c6 hexagon). the nature of graphene interaction with single atoms can be very divergent, depending on the element in question, and detailed data covering most of the periodic table of elements can be found in ref. [70]. for example, sodium preferentially adsorbs on the hollow site, transferring its charge to c atoms consisting c6 hollow, in an ionic interaction. as there is no accompanying π electronic system disruption nor graphene plane corrugation, such charge transfer results in an upshift of the fermi level [70]. the situation is similar for other alkali metals, with ns1 charge transfer. for alkaline earth metals (ns2 electronic configuration) and zn (filled d shell) the interaction is even weaker. on the other hand, atomic hydrogen adsorbs on c-top site, forming a covalent bond with c, causing significant sp2 → sp3 re-hybridization of the orbitals of c atoms surrounding the adsorption site, leading to basal plane deformation [71–73]. functionalization of the graphene basal plane with o-containing groups has been known to enhance h adsorption [73], making it comparable in strength to h adsorption on the coinage metal surfaces [37]. the reason for this enhancement is simple: o-groups introduce local disruptions of π electronic cloud of graphene, sp2 → sp3 rehybridization of c atoms to which they are bound, and layer corrugation. therefore, the first c neighbours of the c−o moieties will be prone to h bonding. such strong interaction indicates a possible application of o-functionalized graphene for hydrogen storage. oxygen functionalities can also improve graphene interaction with alkali metals, but in this case, the group type and concentration are of paramount importance [74,75], as the metal could interact strongly with the group, resulting in phase separation, lethal for electrochemical applications. scaling between adsorption energies of various simple adsorbates on oxidized graphene surfaces has been demonstrated [73], indicating similar nature of bonding for all of them. excellent correlations were observed (figure 9) with the coefficients of determination higher than 0.99, suggesting that stronger h adsorption at a given site is, stronger adsorption of other adsorbates at that site will be, as well. figure 9. correlation between h binding energy and the binding energies of cl, oh and pt adsorbates on oxidized graphene surfaces, with corresponding regression lines and coefficients of determination. reproduced from ref. [73] with permission from the pccp owner societies a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 155 scattering of data points is related to the effect of local geometries and the interaction of the adsorbate with o functional groups instead of the graphene basal plane, as well as different types of adsorption sites preferred by different adsorbates. when it comes to molecular adsorbates, o2 interacts rather weakly with pristine graphene. however, doping graphene with different heteroatoms like p or s significantly strengthens o2 bonding. these two dopants induce layer corrugation and transfer some of their charge to the surrounding c atoms, becoming in that way slightly positive and attractive for o2. this is, however, not the case with substitutionally introduced nitrogen. in the case of p-doped graphene, o–o bond is elongated by approx. 28 % upon adsorption (compared to the bond in the gas phase), while for sdoped graphene o2 can be considered completely dissociated (figure 10) [76]. this is precisely the type of molecule activation needed for catalytic purposes. also, it is a clear example of how alterations of the materials electronic structure affect its applicability for a chosen purpose. figure 10. relaxed structures of o2 adsorbed on pand s-doped graphene surfaces. bond lengths are given in angstroms. adapted from ref. [76] with permission from the pccp owner societies. chemisorption on supported metal clusters supported metal nanostructures are the most widely used type of heterogeneous catalyst in industrial processes. for example, pt nanoparticles supported by carbon are the catalyst of choice for fuel cell applications. dispersing the active catalyst component over a suitably chosen support enhances its active surface area and the number of active sites, resulting in increased catalytic activity. the size of metal particles is a key factor in determining the performance of such catalysts the specific activity per metal atom usually increases with decreasing size of the metal particles. lowering the coordination of a metal atom means leaving it less and less saturated, and therefore more (re)active. however, the surface free energy of metals increases significantly with decreasing particle size, causing aggregation of clusters. sometimes, the support can also contribute to the catalytic activity (the so-called active support). in such cases the interface between the support and the catalyst particle is also of great importance. although the macroscopic noble metals are inert and not susceptible to oxidation, their nano-sized forms are often very reactive and excellent catalysts. for example, pt-cluster size strongly influences co adsorption energy (figure 11). it is similar with au: when moving from the densely packed (111) surface to 12-atoms cluster, au evolves from inactive to the most active material towards co oxidation. a detailed dft study of co oxidation on au surfaces suggested that the oxidation of co on au nanoparticles dispersed over an inactive support occurs on au-steps in two consecutive stages: (i) co + o2 → co2 + o, and (ii) co + o → co2 [77]. in the case of active support, co oxidation follows the same steps occurring at au-support interface. from the mechanism, it is clear that the supported au cluster must be large enough to enable the co-adsorption of co and o2. in the case of mgo-support, dft calculations suggest that this is possible starting already with au-trimers [46]. http://dx.doi.org/10.5599/jese.742 j. electrochem. sci. eng. 10(2) (2020) 141-159 chemisorption in electrochemical energy conversion 156 figure 11. influence of pt-cluster size on dft-calculated co adsorption energy, with corresponding adsorption structures given as insets. adapted from ref. [78], with permission from pccp owner societies it is clear that very small metal clusters are in fact molecular systems and until their size is large enough (typically a few nm) it is not possible to talk about band structure, so the models used for transition metals do not necessarily work in these cases. also, there is a large number of low coordinated atoms which are often much more reactive than regular terrace sites. in these cases, xes and xas have been shown as extremely useful techniques which provide information about local bonding and valuable input for theoretical calculations. unfortunately, computational resources which are typically available do not allow exact treatments of nanoparticles (as they count thousands of atoms) and in order to model such systems one must simplify the model to obtain data regarding the adsorption of reactants and intermediates of investigated (electro)catalytic reaction. chemisorption on supported single atoms talking about supported metal clusters, we have mentioned that the surface free energy of metals increases significantly with decreasing particle size, promoting aggregation of metal atoms into small clusters. however, if the support material interacts with metal atoms strongly enough it will prevent the aggregation. the down limit is the single-atom catalyst (sac), with isolated metal atoms dispersed on an appropriate support (figure 12). sacs minimize the amount of the metal necessary for good catalytic activity, which is particularly important for supported noble metal catalysts, due to the abundance and price of metals. sacs offer great potential for achieving high catalytic activity, where isolated metal atoms anchored to support can act as active centres. figure 12. example structures of different types of sacs: single metal atoms (yellow) supported by (a) metal oxide, (b) metal surfaces, and (c) graphene. reprinted (adapted) with permission from ref. [79]. copyright (2013) american chemical society single supported metal atoms possess unique electronic properties different from those of metal nanocatalysts. the coordination of the single atom to the surface atoms of the support is critical, a. s. dobrota and i. a. pašti j. electrochem. sci. eng. 10(2) (2020) 141-159 http://dx.doi.org/10.5599/jese.742 157 and it cannot be discussed as an isolated single metal atom without the support. the bonding of metal atom to the support leads to charge transfer between metal atoms and the support due to different chemical potentials [79]. as a result, the anchored metal atom usually carries some charge, as was shown by various spectral measurements and computational studies. obviously, chemisorption is of crucial importance for sacs. first, strong-enough anchoring (to prevent clustering) of the metal to the support is needed, and next, optimal strength chemisorption of reactants/intermediates of the chosen catalytic reaction is necessary to provide good catalytic activity of sac. for now, there is no universal catalytic activity descriptor for sacs. conclusions chemisorption at electrified interface is the crucial step in virtually every electrochemical energy conversion process. hence, in order to progress further in materials development, we must understand how reactants and intermediates interact with electrodes. so far, a lot has been done for the case of metal surface, but much less is known regarding general trends in adsorption on metal oxides and carbon materials. to make the situation worse, realistic systems like supported metal clusters are extremely complicated to be treated as whole using modern computational techniques, while single atom catalysts cannot be considered separately from the support. for this reason, theory and experiment must be closely synchronized to provide atomic level understanding of chemisorption in energy conversion processes. acknowledgements: this work has been performed under the nato sps multi-year project g5729 “optimizing fuel cell catalyst stability upon integration with reforming oficer”. authors acknowledge the support provided by the serbian ministry of education, science and technological development (grant no. iii45014). references [1] j. g. calvert, pure and applied chemistry 62 (1990) 2167–2219. 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[79] x.-f. yang, a. wang, b. qiao, j. li, j. liu, t. zhang, accounts of chemical research 46 (2013) 1740– 1748. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.742 http://creativecommons.org/licenses/by/4.0/) {influence of aging on the heat and gas emissions from commercial lithium ion cells in case of thermal failure} doi:10.5599/jese.476 101 j. electrochem. sci. eng. 8(1) (2018) 101-110; doi: http://dx.doi.org/10.5599/jese.476 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of aging on the heat and gas emissions from commercial lithium ion cells in case of thermal failure michael lammer, alexander königseder, peter gluschitz, viktor hacker institute of chemical engineering and environmental technology, graz university of technology, inffeldgasse 25c, 8010 graz, austria corresponding authors e-mail: michael.lammer@tugraz.at; tel.: +43 316 873 8795; fax: +43 316 873 8782 received: november 22, 2017; revised: december 27, 2017; accepted: january 8, 2018 abstract a method for thermal ramp experiments on cylindrical 18650 li-ion cells has been established. the method was applied on pristine cells as well as on devices aged by cyclisation or by storage at elevated temperature respectively. the tested cells comprise three types of lini0.8co0.15al0.05o2 cells for either high power or high energy applications. the heat flux to and from the cell was investigated. degradation and exothermic breakdown released large amounts of heat and gas. the total gas and heat emission from cycled cells was significantly larger than emission from cells aged by storage. after aging, the low energy cell icr18650he4 did not transgress into thermal runaway. gas composition changed mainly in the early stage of the experiment. the composition of the initial gas release changed from predominantly co2 towards hydrocarbons. a comparable mixture of h2, co and co2 were emitted in all tests during thermal runaway. keywords battery characterisation; thermal runaway; gas analysis; heat quantification introduction recent events like burning phone batteries, notebook computers and battery fires on planes and electric car incidents have shown the intrinsic hazard potential connected to lithium ion battery systems [1–5]. the high energy density of these devices also implies the risk of catastrophic failure in case of malfunction. safety and risk related issues thus are not only of concern for end users, but also for producers of cells and battery packs and for transportation policy makers [6]. in li-ion cells the two highly reactive electrodes are separated by a thin polymer separator to prevent direct contact. ionic conductivity is achieved by an organic aprotic solvent containing the conducting salt. electrolyte flammability further increases the volatility of this particular electrochemical system. accumulation of heat within the cell, i.e. either excessive external heat influx or heat generation http://dx.doi.org/10.5599/jese.476 http://www.jese-online.org/ mailto:michael.lammer@tugraz.at j. electrochem. sci. eng. 8(1) (2018) 101-110 gas emissions from commercial lithium ion cells 102 surpassing the ability of dissipation, poses a serious threat to the integrity of the device [7,8]. thermally induced degradation processes comprise the evolution of gas from evaporation and degradation of the electrolyte, melting of the separator leading to internal short circuits, changes of the electrodes’ structure releasing oxygen and lithium respectively and other fast selfaccelerating exothermic reactions leading to thermal runaway [9–14]. lithium ion cells not only offer a higher energy density but also more resistance towards aging than other types of electrochemical secondary cells like pb-acid batteries and nimh systems [15–17]. aging affects the overall capacity by loss of active material and decay in conductivity and reactivity. after reaching their end of usability, li-ion cells are still reactive and potentially dangerous devices though. due to their importance and intrinsic risks, lithium ion cells and battery systems are subjected to rigorous testing. most of these tests provoke thermal issues by short circuiting, heating, decay of components or combinations of these damaging effects. widely applied mechanical testing methods involve dropping cells from defined altitudes (drop tests), crushing or piercing cells with conductive (nail tests) and non-conductive (wedge/crush tests) tools [6,18–20]. electrical testing is performed by overcharging, deep discharging and externally short circuiting the cells [6,20–22]. most thermal stress tests are conducted by heating the cells continuously (thermal ramp tests), step-by-step (heat-wait-seek tests) or subjecting the devices to thermal shock [6,9,10,23,24]. investigation of thermal characteristics, especially heat flux, is typically performed using calorimetric methods like cone calorimetry or accelerated rate calorimetry (arc) [8,24–28]. most commonly, tests are conducted on a single cell level [29–31], but fire tests on battery packs and full size electric vehicles are also reported in the literature [32]. in these tests, cone calorimetry is usually employed to gain insight into the burning and fire evolution under air atmosphere [33]. thermal degradation of battery components and single cells is performed using adiabatic calorimetry. the complex nature and high reactivity of lithium ion cells, especially during the fast gas release by rapid degradation reactions, raises the need for specialised equipment and specific operating procedures [33–35]. investigations into the behaviour of aged cells under thermal stress in comparison to pristine ones are part of this work. our group compared the heat and gas emissions of commercial cells of the cylindrical 18650 format in previous studies [9,10,36]. in this work, the tested cells comprise pristine devices as well as cells artificially fast-aged by cycling and cells aged by storing them at 60 °c. 80 % of remaining capacity was set as termination criterion for aging. this limit corresponds to the drop-out criterion for automotive application of li-ion cells. the fully charged cells (state of charge, soc = 100 %) are heated continuously by an external heat source in order to trigger thermally induced failure. the quantification of heat consumed and released from the device under test is performed at points of rapid temperature change. a time resolved acquisition of the venting behaviour enhanced the understanding of gas evolution. due to the quasi-adiabatic conditions at the points of interest, reproducible data has been collected. characteristic events like initial sudden gas release, the onset temperature of exothermic behaviour, the transgression into thermal runaway and the rapid cell deflagration have been identified. these events are subsequently used for accurate gas sampling to gain additional insight into the degradation of lithium ion cells at elevated temperature. experimental a custom made test rig for thermal ramp tests on cylindrical lithium ion cells of the 18650 format was used for all of the experiments. detailed information on the test rig has been published in rsc m. lammer et al. j. electrochem. sci. eng. 8(1) (2018) 101-110 doi:10.5599/jese.476 103 advances in 2017 [9].the test rig consists of a tube furnace equipped with a stainless steel tubular reactor for heating the cell. inert gas (n2) is used for purging the system and as a carrier gas stream for transporting the gaseous emissions towards the gas sampling device. characterisation of the emission profile and gas sampling are performed by separate experiments on the same cell type. no carrier gas is used for determining the emission profile. the test rig is flushed with inert gas and left at ambient pressure. by switching to a set of communicating vessels, gas emissions directly displace fluid from the tubes. the mass of the displaced water is directly quantified by weighing and directly corresponds to the volume of released gas. a time-resolved characterisation of the gas emission is achieved by this setup. gas sampling is achieved by a syringe pump withdrawing a sample from the off-gas stream and transferring it into a sample vial via a multi-port valve. after the sampling operation, the valve is automatically switched and a new vial becomes available for sampling. the previously filled vial is taken to the ex-situ gas analysis by micro-gc (agilent micro-gc 3000, usa). this method is used to collect gas samples at defined points of interest during the experiment. the first, sudden gas emission (first venting), the consecutive exothermic phase and the terminal venting after thermal runaway are considered points of interest in this work. figure 1 shows the generic profile of a thermal ramp test. figure 1. generic overview containing the characteristic events on the temperature curve and the corresponding gas emission profile the cells tested within this work were ncr18650bf, inr18650-35e and icr18650he4. all these cells consist of lini0.8co0.15al0.05o2 cathodes and graphite anodes. although ncr18650bf and inr18650-35e are similar in regards of energy density, the cells are working quite differently, as the three types of 18650 cells show different application profiles (table 1). cells were used in pristine or artificially aged condition. aging was carried out either by cyclisation or storing cells at 60 °c until a remaining capacity of 80 % was reached. the cells were cycled four times for determination of the capacity during storage. all cells were prepared by charging to 100 % of state of charge according to the manufacturer’s data sheet applying a constant current/constant voltage charging routine. a basytec battery test system (basytec gmbh, germany) was used for conditioning the cells. after charging, the plastic cover was removed, and the cell’s mass was recorded. three type-k thermocouples were fixed to the can by a sheet of glass fibre cloth. this sheet also acted as insulation material preventing short circuiting of the cell within the stainless steel sample holder. the sample holder both allowed reproducible positioning of the cell under test within the tubular reactor and leaving enough space for unhindered gas and particle emission. j. electrochem. sci. eng. 8(1) (2018) 101-110 gas emissions from commercial lithium ion cells 104 table 1. cells tested within the study; the application profile was established on base of the maximum discharge current and energy content calculated from manufacturer datasheets. cell mass, g capacity, ah energy, j application profile ncr18650bf 44.87 3.35 43404 long runtime at low power consumption inr18650-35e 47.70 3.35 45386 high power and high energy applications icr18650he4 45.50 2.50 32416 short runtime at high power consumption flushing the reactor and piping with inert gas (n2; 1 l min-1, 20 min) purged the system of air. the furnace was preheated to 80 °c. then a heating ramp of 70 w (corresponding to approx. 0.5 °c min-1) was applied until the terminal venting of the thermal runaway and cell deflagration occurred. continuous heating led to electrolyte degradation and evaporation, initiating pressure build-up within the can. to prevent uncontrolled cell rupture caused by the pressurisation of the cell by gaseous degradation products, cells of the cylindrical 18650 format are equipped with a safety rupture disk. breaking of this disk relieved critical pressure. this event of first gas release is denominated first venting within this work. further application of external heat influx induced a degradative condition where the cell itself became a heat source – the exothermic phase was reached. from the exothermic phase a rapid transgression into the thermal runaway was observable. as there are no specifications on thermal runaway of electrochemical devices found in literature, a self-heating rate of ≥2 °c min-1 had been chosen in this work to define the beginning of thermal runaway. this very fast, self-accelerating exothermic breakdown of cell components was characterised by a final release of large amounts of gas and heat, consuming the reactive components of the cell. after this deflagration event, the cell under test cooled down to reactor temperature, as indicated in figure 1. basic information on a cell type was gained by a test at ambient pressure under nitrogen atmosphere with no set gas flow. any gas release increased the pressure within the system, displacing water from the communicating tubes. this type of experiment yielded the gas emission profile as well as the thermal features of the cell, i.e. the critical temperatures of the first venting, onset of exothermic behaviour, transgression into thermal runaway and the maximum temperature during cell deflagration. previous studies of our group have shown that the characteristic events are reproducible at the temperatures found for each type of cell [9]. thus, the follow-up test was conducted using a nitrogen carrier gas stream of 70 cm³ min-1 to transfer the gaseous degradation products from the site of emission to the sampling device. an automated syringe pump was actuated at the characteristic temperatures to withdraw a gas sample at this point. the sample was fed into sealed and argon purged vials by means of a motorised multiport valve. this method combined the timed accuracy with the reproducibility of automatized gas sampling from the vent gas stream. once the individual vials were filled with vent gas, they were collected, and the ex-situ analysis was carried out on a micro-gc system set up for the quantification of h2, co, co2, ch4, c2h2, c2h4 and c2h6. designed as a two-column system, argon and helium were used as carrier gases within the gas chromatograph. this setup made argon a viable filling gas for the “empty” sample vials, as it did not interfere with the analysis. results and discussion thermal characterisation and characteristic events the characteristic events were determined by interpretation of the cell’s (self-)heating rate. the first venting was related to the appearance of a negative heating rate, as the joule-thomson effect m. lammer et al. j. electrochem. sci. eng. 8(1) (2018) 101-110 doi:10.5599/jese.476 105 cooled the cell. the point of inflexion in the rate vs temperature plot indicated the transgression into exothermic behaviour. a (self-)heating rate of ≥2 °c min-1 was defined to be the initiation of thermal runaway. an exemplary rate vs temperature plot is shown in figure 2. figure 2. rate plot of inr18650-35e (pristine), indicating the characteristic events and the corresponding phases of exothermic behaviour and thermal runaway (tr). calculation of the heat consumption and emission during the first venting incident and the thermal runaway was calculated from the temperature of the cell in relation to the thermal ramp at the respective points due to the quasi-adiabatic nature of these events. an overview of the cell temperature at the characteristic events is given in table 2. table 3 offers detailed information on the heat consumption and release as well as the gas emission at the respective events. figure 3 shows the comparison of the cells under test at different states of aging. aging effects are clearly visible in the trends of decreasing heat transfer during venting and thermal runaway respectively. the volumetric gas release does not show any clear trend. heat calculations were performed in reproducible manner in three distinct experiments for each cell. table 2. temperature of the tested cells at the characteristic events; status indicates whether a cell is pristine (a) or aged by either cyclisation (b) or storage at 60 °c (c); tvent relates to the cell temperature upon the first venting, tonset is the temperature during the exothermic onset, ttr is the temperature, at which thermal runaway was imminent and tmax is the maximum temperature recorded during thermal runaway. cell status tvent / °c tonset / °c ttr / °c tmax / °c ncr18650bf a 134 124 172 771 inr18650-35e a 135 115 171 695 icr18650he4 a 118 113 204 690 ncr18650bf b 139 125 177 681 inr18650-35e b 136 135 175 702 icr18650he4 b 116 182 203 373 ncr18650bf c 138 119 174 618 inr18650-35e c 136 135 172 687 icr18650he4 c 121 112 n.a. n.a. gas release during the first venting is roughly dependent on the capacity of the cell, though the venting characteristic of ncr18650bf (new) does not fit into the scheme. the heat consumption of the first venting (qvent) is highly consistent. though ncr18650bf and inr18650-35e have comparable energy content, inr18650-35e consumed considerably less energy for venting. the significantly lower energy content – indicating the lowest reactivity – of icr18650he4 was directly j. electrochem. sci. eng. 8(1) (2018) 101-110 gas emissions from commercial lithium ion cells 106 reflected by the lowest gas emission and heat consumption. the progress of the thermal runaway was consistent in regards of gas and heat release. inr18650-35e also showed lower emissions than the comparable ncr18650bf. table 3. heat and gas characteristics of the tested cells; status indicates whether a cell is pristine (a) or aged by either cyclisation (b) or storage at 60 °c (c); vvent and vtr give the amount of gas at the first venting and the thermal runaway respectively; qvent and qtr show the relevant heat transfer; qvent rel and qtr rel indicate the amounts of heat for the venting and thermal runaway relative to pristine cells; vtotal gives the total gaseous emission for the duration of the test cell status vvent / cm³ qvent / j qvent rel / % vtr / cm³ qtr / j qtr rel / % vtotal / cm³ ncr18650bf a 61 -212 5261 25541 5637 inr18650-35e a 167 -136 5491 23682 5680 icr18650he4 a 83 -103 2899 21126 3381 ncr18650bf b 212 -148 70 4967 22192 87 5246 inr18650-35e b 151 -134 99 5493 17165 72 5795 icr18650he4 b 31 -79 77 172 7876 37 378 ncr18650bf c 212 -135 64 5242 20624 81 5610 inr18650-35e c 67 -78 58 5816 14482 61 5879 icr18650he4 c 68 -60 58 n.a. n.a. n.a. 69 figure 3. volume of released gas and heat transfer for pristine devices (a), cells aged by cyclisation (b) and cells aged by storage at 60 °c (c). the left diagram compares the first venting incident; the right diagram compares the thermal runaway. note that heat transfer during venting is heat consumption; heat transfer during thermal runaway is heat release. this indicates an overall reduced reactivity of this type of cell. the low energy icr18650he4 degraded most severely during aging. it showed significantly lower gas and heat emissions and did not transgress into thermal runaway after aging by storage at 60 °c. though the tested cells have the same electrode chemistry, the respective behaviour under thermal strain is different. non-only does the electrochemically active material influence this behaviour, but also other components like the separator material and thickness, the susceptibility of the rupture disk, etc. separators with higher thermal stability prevent internal short circuits until higher temperatures are reached, making the cell itself more resistant towards thermally induced failure. venting at lower internal pressure eases the mechanical strain on the cell components. these factors have an influence on battery safety in addition to reactions within cells of similar chemical composition. in regard of reactivity loss, aging by storage at 60 °c appeared to have a more degrading effect than cyclisation. this is indicated by the lower heat consumption and emission of cells aged this way, as shown in table 3. total emission of gas over the duration of the experiment consisted of gas release from the m. lammer et al. j. electrochem. sci. eng. 8(1) (2018) 101-110 doi:10.5599/jese.476 107 first venting after the safety rupture disk had opened, the slight gas release during the exothermic phase and the violent venting marking the terminal phase of thermal runaway. the total emission of gas is thus larger than the sum of vvent and vtr. gas analysis at characteristic events gas samples were collected at the characteristic events of first venting, during the exothermic phase and after the thermal runaway. figure 4 depicts the vent gas composition for the characterristic stages of sampling (vertical separation) and different conditions (horizontal separation). figure 4. vent gas compositions at first venting, exothermic phase and thermal runaway, given for pristine specimens, cells aged by cyclisation and cells aged by storage at 60 °c. note that icr18650he4 did not go into thermal runaway after aging by storage. main component of the vent gas after the first release of gas was co2; icr18650he4 also released other gases like h2, co and ch4 in larger concentrations than 10 %. aging by storage drastically changed the vent gas composition at this stage, as most of the detected gas was c2h2 for all the cells. the exothermic phase was characterised by large quantities of co2. most aged cells also emitted 10 % or more of h2 and co. the cycled ncr18650bf did not exhibit any co but c2hydrocarbons like ethane and ethene. gas emissions during the final stage of thermal runaway were roughly similar for all the tested cells. icr18650he4 showed deviations here, as the cell released larger quantities of co2 after aging by cyclisation and did not go into thermal runaway after aging by storage at 60 °c. this means that no gas sampling was carried out. gas evolution is determined by numerous factors. decay and regeneration of the sei layer at various states of health, reaction temperature as well as pressure within the cells influence the gas evolution and composition. degradation of components like the organic electrolyte and consecutive gas formation within the cell during aging is not observable by the test setup, until the rupture disk gives in. a gas mixture formed during aging as well as degradation by elevated temperature is collected by the system. co2 is mainly deriving from decarboxylation of the organic carbonate electrolyte; co is attributed to oxidation of carbonaceous species by oxygen released from the oxide electrode. the h2 emission appears to be a product of the reduction of h2o deriving from the j. electrochem. sci. eng. 8(1) (2018) 101-110 gas emissions from commercial lithium ion cells 108 combustion of carbohydrates. detailed mechanisms on the gas formation within the complex system of a li-ion cell under thermal stress were not in the direct focus of this work and will be the aim of future investigations. safety considerations in regard to safety, cells of large heat consumption during venting are considered as safer due to the possibility of cool-down below a critical level. low emissions of heat during the thermal runaway also mark safer cells, as less heat is then spread through a battery pack or system. the evolution of gas also has to be considered, as large quantities may both exert mechanical strain on components and pose a danger due to toxic effects, fire and explosion. large quantities of gas containing high concentrations of flammable gases, e.g. h2, co and hydrocarbons, which are in part highly toxic (e.g. co), were detected in the vent gas of all cells. thermal runaway is thus considered a highly dangerous condition also in regards of gas emission. even cells which have reached the drop-out criterion of 80 % remaining capacity are still potentially hazardous. safety is not determined by a universal feature – critical conditions transform devices regarded as safe into critically unsafe pieces of technology. conclusions lithium ion cells are highly susceptible to thermal stress. inducing thermal stress by means of a tubular furnace allowed studying the associated effects in a detailed and reproducible way. operation of the test rig under ambient pressure prevented pressure induced side reactions and created a safe and easy way to determine a gas emission profile. an estimation of the characteristic events was achieved based on these data. these events comprise the first venting, i.e. the release of gas from the previously sealed cell, the exothermic phase when the cell itself becomes a heat source and the thermal runaway. thermal runaway of electrochemical devices is not defined in literature, thus a self-heating rate of ≥2 °c min-1 is used in this study. three types of 18650 format cells were conditioned for operation – ncr18650bf, inr18650-35e and icr18650he4. tests were conducted on pristine cells, devices aged by cyclisation and devices aged by storing them at 60 °c. all tests were carried out on fully charged cells. the characteristic events were verified by investigating the (self-)heating rate of the cell. a sudden drop to negative rates was directly related to joule-thomson cooling initiated by gas expansion form the pressurised can. the inflexion in the heating rate signified the transgression into self-heating and thus the onset of the exothermic phase. thermal runaway started at a self-heating rate of 2 °c min-1 and progressed towards total breakdown of the cell and its components, releasing large quantities of gas and heat. the heat consumed for venting and emitted in the thermal runaway had been calculated from the respective temperatures relative to the thermal ramp. aging by storage at 60 °c showed the largest influence on the reactivity in regards of the heat and gas emissions. cells aged this way showed the lowest emission of heat in comparison to new cells – 81 % for ncr18650bf, 61 % for inr18650-35e and no thermal runaway in case of icr18650he4. gas release was only affected in the low energy cell icr18650he4. this device degraded the most and after aging by cyclisation it released 6 % of the volume compared to the pristine specimen. as for icr18650he4 aged by storage no thermal runaway was observable, no gas emission took place. we suppose that cells of similar chemistry and energy density react differently because of nonelectrochemically active components like the separator setup and also because of the early venting behaviour. thermally resistant separators shift the initiation of catastrophic internal short circuit to m. lammer et al. j. electrochem. sci. eng. 8(1) (2018) 101-110 doi:10.5599/jese.476 109 higher temperature; venting releases mechanical strain on the cell components and also cools the cell. investigation on these components is going to be conducted in the future. gas samples were taken at the characteristic events and analysed ex-situ. analysis showed fairly similar behaviour for all of the cells within the same condition. aging by storage at 60 °c led to the emission of a large proportion of c2h2 during the first venting. c2h2 is a gas observed only in traces in all other tests and situations. the evolution of c2-hydrocarbons appears to be tightly connected to the degradation of electrolyte. the quantity of hydrocarbon emission is larger in cells aged by storage. as these cells exhibit comparable temperature at the first venting incident, it is assumed that the formation is favoured by degradation before the event, i.e. during the aging process. the main component of the vent gas during thermal runaway was co, making the vent gas a flammable and toxic mixture. it poses a high risk of fire and explosion in regard to the high temperature associated with the event of thermal runaway. battery safety cannot be attributed to single features. in respect to cool-down capability during venting, heat emission during thermal runaway and gas evolution during the thermal breakdown, cells can be defined as safe. as not all types of cell show all safety criteria, e.g. a large cool-down as well as low heat emission and the release of low quantities of mainly co2, cells are not universally safe. two methods for aging li-ion cells to reach the automotive drop-out criterion of 80 % remaining capacity were applied. aging did decrease the heat release from the cells, but still they posed a threat in case of thermally induced failure. the low capacity icr18650he4 cells exhibited the least reactivity after aging by storage, making them somewhat safer under these conditions. this of course comes at the price of low energy density in the first place. acknowledgements: this work and the project isalib is funded by the austrian ministry of transport, innovation and technology (bmvit) and the austrian research promotion agency (ffg) through the program “mobilität der zukunft” (2014-2017). support by our research and industry partners kompetenzzentrum das virtuelle fahrzeug and samsung sdi is gratefully acknowledged. references [1] k. bullis, mit technol. rev. 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[36] a. w. golubkov, d. fuchs, j. wagner, h. wiltsche, c. stangl, g. fauler, g. voitic, a. thaler, v. hacker, rsc adv. 4 (2014) 3633–3642. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {borax buffer solution for ph measurement: homogeneity and stability studies and its application in the proficiency testing program} doi:10.5599/jese.467 261 j. electrochem. sci. eng. 8(3) (2018) 261-270; doi: http://dx.doi.org/10.5599/jese.467 open access : : issn 1847-9286 www.jese-online.org technical report borax buffer solution for ph measurement: homogeneity and stability studies and its application in the proficiency testing program nurhani aryana, ayu hindayani, oman zuas, fransiska s. h. krismastuti metrology in chemistry research group, research centre for metrology-indonesian institute of sciences (rcm-lipi), building no. 456, kawasan puspiptek, serpong 15314, tangerang selatanbanten, indonesia corresponding author e-mail: oman.zuas@lipi.go.id; tel.: +6221-7560533; fax: +6221-7560064 received: october 19, 2017; revised: december 8, 2017; accepted: december 9, 2017 abstract this paper describes the preparation of borax buffer solution and its application as proficiency testing (pt) test samples in a pt scheme. the test samples were made gravimetrically from 0.01 m sodium tetraborate decahydrate and ph of the test samples were measured by using a differential potentiometric cell. homogeneity and stability of test samples were evaluated in accordance to iso13528:2015. the results obtained revealed that test samples met the pt requirement criteria in terms of stability and homogeneity. the prepared pt test samples were used in the 2016 pt scheme. from the results received from 50 participants, it was found that about 44 % of participants achieved satisfactory results, 38 % achieved unsatisfactory results, while the remaining participants (about 18 %) showed a questionable result. keywords sodium tetraborate decahydrate; ph measurements; proficiency testing scheme; iso 17043; iso 17025 introduction metrology in chemistry laboratory of indonesian institute of sciences (mic lab-lipi) is an accredited proficiency testing (pt) provider in indonesia having responsibility for all tasks in the pt scheme development and operation within the institute. based on iso 17043:2010, the pt scheme is to design and operate for one or more rounds in a specific area of testing, measurement, calibration or inspection [1]. the pt is widely known as a tool to evaluate the performance of participant’s laboratories for accreditation process purposes [2]. for the participant, in particular, there are some benefits in joining the pt, including to confirm the competence of the participant, http://www.jese-online.org/ mailto:oman.zuas@lipi.go.id j. electrochem. sci. eng. 8(3) (2018) 261-270 borax buffer solution for ph measurement 262 to identify the possible problem related to testing or measurement, to compare methods and procedures used, to improve performance and to boost confidence to conduct a specific measurement, to educate staff, and to assure the quality of the test result as required by iso 17025:2008 [3-5]. the pt program at the mic lab-lipi has been conducted since 2013. this is in the mic lab-lipi’s annual agenda to disseminate the pt test samples together with the reference standard solution, to different testing laboratories in indonesia. the pt program covers various commodities and parameters including metals in drinking water, wastewater, fish flour and rice flour, pesticide in tea, food additive in soy sauce, and buffer solution. all these pt samples were prepared and measured by mic lab-lipi. ph measurement is also one of the parameter in the pt program of mic lab-lipi because the ph measurement is very common and widely used in quantitative measurements in the field of chemical analysis [6]. in addition, the data obtained from ph measurement is widely used as the basis for making environmental-related decisions and for the evaluation of product quality in industries [7]. in 2013, the range of ph measurement was between acid and neutral (ph 4.00-7.00). for the acid ph, 70 % of participants achieved satisfactory result, while 80 % participants got satisfactory result for measuring the neutral ph. in 2014-2015, the range of the ph measurement was between ph 1.00-5.00 (acid ph). in 2014, 54 % of participants got satisfactory result and it was increased up to 91 % in 2015 [8]. in 2016, mic lab-lipi provided a pt item in the range of ph alkaline to assess the performance of participant laboratories. the pt samples were borax buffer solution and it was distributed to registered participant laboratories. however, before distribution, two specified tests have to be conducted including homogeneity and stability testing in order to meet the criteria of the pt samples with the purpose to reduce the bias of the measurement result [1,9]. in this paper, preparation of the borax buffer solution for ph measurement including its homogeneity and stability evaluation, as well as its application as samples in the pt scheme are presented and the results are discussed. experimental materials all chemicals were of the analytical grade and used as received without any further purification. platinum tetrachloride (ptcl4) anhydrous (57.5 % pt), lead (ii) acetate (99.5 % purity), nitric acid (hno3, 65 % purity) and hydrochloric acid (hcl, 37 % purity) were purchased from merck, germany. hydrogen (h2) gas (99.9 % purity) was purchased from sii gas indonesia. sodium tetraborate decahydrate (99.6 % purity) was purchased from wako, japan, while srm® 187e was purchased from nist, usa. demineralized water (0.05 μs cm-1) was produced from a thermo scientific barnstead smart2 pure water purification system and used in all experiment runs. equipment equipment used were the analytical balance with accuracy 1 mg (pr5003 dualrange, mettler toledo switzerland) and flowmeter (adm1000, agilent). a set of differential potentiometric cell systems include a digital multimeter 0.01 mv (34461a, agilent technologies sdn bhd, malaysia), digital thermometer (mkt50, anton paar gmbh, germany), water bath 0.1°c (thomas tn22d, japan), chiller (thomas trl-117nf, japan), baucke cell, chamber, and pt electrode. a magnetic n. aryana et al. j. electrochem. sci. eng. 8(3) (2018) 261-270 doi:10.5599/jese.467 263 stirrer (cimarec 2, barnstead thermolyne corp. usa), direct current (dc) power supply 0.01 a (ad8723d, china), stopwatch, hdpe bottles 25 l and 250 ml, clean glassware and clean spatula. preparation of pt samples the pt samples were prepared gravimetrically from sodium tetraborate decahydrate 0.01 m. this gravimetric preparation could eliminate the need to weigh exactly the predetermined mass of solid samples. 3.83 g of sodium tetraborate decahydrate was transferred into a clean and dry beaker glass (1 l) then diluted using fresh demineralized water until the total weight obtained was 1002.85 g. the solution containing sodium tetraborate decahydrate was shaken thoroughly using magnetic stirrer until the solid was totally dissolved. the procedure was repeated as much as 19 times. after that, the solution was transferred to hdpe bottle 25 l. after homogenization process in one night, the solution was transferred into smaller and clean hdpe bottle 250 ml. the volume of sample has been estimated enough for the participant laboratories to conduct duplicate measurement and their quality control. the label of the bottle was code by lbb16-xxx (lbb is code for borax buffer solution, 16 is code for the year 2016 and xxx was a number of the bottle). each bottle containing borax buffer solution was packaged and sealed using a plastic seal. preparation of standard solution the standard solution was made from srm® 187e. the similar procedure was applied for preparing the standard solution as described above. shortly, 3.83 g of srm® 187e was transferred into a clean and dry beaker glass (1 l), diluted using fresh demineralized water and stirred thoroughly. the prepared standard solution was placed into a beaker glass and was tightly covered to avoid against co2 contamination. preparation of pt electrode a pt electrode was used for the ph measurement by the secondary method using differential potentiometric cell. in order to achieve sufficiently small bias of potentials value and to avoid possible interference during the ph measurement, the pt electrode was firstly cleaned by removing impurities on the electrode surface using hot aqua regia solution [10,11]. after that, the pt electrode was rinsed with demineralized water. the cleaned pt electrode was then platinized by an electrolytic method using dc power supply and solution containing a mixture of platinum tetrachloride (ptcl4), lead (ii) acetate and hydrochloric acid (hcl). the platinization process was conducted under the current of 0.25 a for 2.5 minutes. under these conditions, the pt electrode having a potential bias below 3 μv could be achieved [10]. ph measurement the ph values of all pt samples were measured using differential potentiometric cell, called baucke cell. the baucke cell was firstly introduced in 1994 [6]. a schematic diagram of the differential potentiometric cell system used in this measurement is shown in figure 1. the shape of the cell is like u-shaped cell that is separated by a sintered glass disk in the middle, so it also called a two-half cell. in this two-half cell system, one cell contains the standard solution (srm® 187e) and platinized pt electrode. another cell contains the pt sample of borax buffer solution and platinized pt electrode. the potential difference between the standard solution and pt sample solution were assigned as e. after that, the hydrogen (h2) gas was fed into each cell under the flow rate of 500 ml/h [10]. it should be noticed that h2 gas was pre-humidified before entering the baucke cell by passing h2 into two chambers containing borax buffer solution [6]. both, cell and the cell chamber j. electrochem. sci. eng. 8(3) (2018) 261-270 borax buffer solution for ph measurement 264 were placed into the water by keeping its temperature at 25 °c using chilled circulation system. the platinized pt electrodes were then connected to a digital multimeter. under stable condition (1 hour is required to stabilize the potential), e was observed and recorded every 10 minutes. the ph value of pt sample (borax buffer solution sample) was calculated using eq. (1). phs = php – (e f / rt ln10) (1) where phs is a ph value of pt sample, php is ph value of standard solution, e is potential difference between the standard solution and pt sample (v), f is faraday constant (96485.332 c mol-1), r is the universal gas constant (8.3144621 j k-1 mol-1), and t is temperature of measurement (k). figure 1. a schematic diagram of differential potentiometric cell system: a h2 gas, b flowmeter, c1 and c2 chambers containing borax buffer solution, d baucke cell and the platinized pt electrodes, e digital multimeter, f digital thermometer, g chilled circulation system homogeneity testing all the pt samples were evaluated for their homogeneity before distributing to the participants. the homogeneity test was conducted in accordance to annex b of iso 13528:2015 and the results were statistically evaluated by f-test using one-way analysis of variance (anova). ten bottles of the prepared pt sample were selected by stratified random sampling and were measured in duplicate using differential potentiometric cell under repeatable condition. based on the annex b of iso 13528:2015, the calculation was conducted as follows: define the sample averages (�̅�𝑡 ) + = t,1 t,2 t 2 x x x (2) where xtis represents the proficiency test item (t = 1, 2..., g), and the between test portion ranges (wt) wt=ǀxt,1 xt,2ǀ (3) calculate the general average (�̿�) 1 1 g t t x x g = =  (4) estimate the standard deviation of sample averages (sx) n. aryana et al. j. electrochem. sci. eng. 8(3) (2018) 261-270 doi:10.5599/jese.467 265 ( ) 2 1 1 g t x t x x s g= − = −  (5) and the within sample standard deviation (sx): 2 t w 1 2 g t w s g= =  (6) finally estimate the between sample standard deviation (ss) as: 2 2 w s x 2 s s s= − (7) the estimated value of s 2 s becomes negative when ss is relatively smaller than sw. this can be expected when pt samples are highly homogeneous [12]. stability testing two different types of stability testing were performed i.e., one is stability monitoring to simulate the transport condition where the samples were stored at 40 °c for 4 weeks and another is stability monitoring during the pt scheme. the stability testing was performed according to the annex b of iso 13528:2015. two bottles of the pt sample were selected for stability monitoring at 40 °c and then the pt samples were measured consecutively after 2, 3, and 4 weeks. another three bottles of pt sample were selected randomly for stability monitoring during the pt scheme in conjunction with the due date of participants reports submission. the ph measurement for the stability monitoring was conducted by duplicate by using differential potentiometric cell. the pt samples are considered to be adequately stable if: 1 2 pt 0.3y y −  (8) where 1y is the average ph value of the homogeneity check, 2y is the ph value from stability check and pt the standard deviation of pt. in this pt scheme, the value of pt was set 0.03 according to iupac recommendation as a maximum measurement uncertainty for the glass electrode method with two-point calibrations (bracketing procedure)[13]. based on mic lab-lipi’s experience in organizing previous 2 rounds of ph measurement scheme by applying such 𝜎𝑝𝑡 value, it was found that the value 0.03 can fit for this purpose and can be used as the performance evaluation criteria for ph measurement. application the prepared borax buffer solution was used as the pt samples. these pt samples were found homogenous and stable at high temperature for 1 month and they were distributed on the pt scheme during a scheduled technical meeting. for some laboratories who can not join the technical meeting, the samples were dispatched by a courier. each participant received one bottle containing about 250 ml of tetraborate buffer solution (lbb16). each bottle of lbb16 has a specific number, example no. lbb16-001. each laboratory received one bottle with a different number. results and discussion the borax buffer solution is a very common and widely used as a ph standard solution for the calibration of the glass electrode. the borax buffer solution has long time storage characteristic until 2 years under closed bottle [14]. in addition, the borax buffer solution could be used directly and j. electrochem. sci. eng. 8(3) (2018) 261-270 borax buffer solution for ph measurement 266 stored at room temperature for long period without decomposition [15]. before used as the pt sample, the borax buffer solution was evaluated for its homogeneity and stability in order to assure that all pt participants can receive the same pt sample with sufficiently homogeneous and stable properties; thus all pt results can be compared [1, 12]. homogeneity testing homogeneity is defined as a material condition having uniform in term of its structure or composition-related specified properties [16]. the homogeneity assessment of pt samples was conducted before distribution to the pt participants [1,17], with the primary aim to verify that all units of the pt samples can be considered identical when a batch of certification is envisaged [18]. homogeneity of borax buffer solution was checked by measuring the ph values of 10 bottles taken by random sampling and using the differential potentiometric cell. the results of duplicate analysis are listed in table 1. it can be seen from data in table 1, that the general average of ph value for 10 pt samples is 9.186. this value was put as the assigned value for the pt scheme 2016 and denoted as 1y in the stability monitoring using eq. (8). table 1. ph values of 10 selected bottles with borax buffer solution no. of bottle ph measurement 1 measurement 2 1 9.188 9.185 13 9.192 9.185 25 9.187 9.185 37 9.187 9.187 49 9.187 9.184 61 9.189 9.185 73 9.185 9.184 85 9.187 9.184 97 9.185 9.186 109 9.184 9.187 general average of ph value ( x ) 9.186 after getting the ph values from 10 measurements, the homogeneity testing was statistically evaluated by the f-test using one-way analysis of variance (anova) and the results are tabulated in table 2. as can be seen from table 2, the value of f is smaller than fcritical (f < fcrit), meaning that the homogeneity of pt samples is found insignificant. insignificance in homogeneity indicates that no significant difference exists between pt samples (different bottles) and within the pt sample (the same bottle). therefore, the pt samples were found homogenous. table 2. anova results of homogeneity testing of pt samples source of variation ssa dfb msc fd p-valuee fcrit between groups 2.745e-05 9 3.05e-06 0.616162 0.760518 3.020383 within groups 4.95e-05 10 4.95e-06 total 7.695e-05 19 athe sums of squares, bthe associated degrees of freedom, cthe mean squares, df test indicated that the result of the homogeneity is insignificant if f < fcrit, with the critical value of f for α =5 %, and egives the level for which the observed f = fcrit [19]. n. aryana et al. j. electrochem. sci. eng. 8(3) (2018) 261-270 doi:10.5599/jese.467 267 in addition to anova evaluation, the homogeneity of pt samples was evaluated by using another statistical analysis (eq. 7) according to the annex b of iso 13528:2015 and the results are shown in table 3. it can be seen in table 3, that the estimated value of 𝑠𝑠 2 is found negative, showing that ss value is relatively smaller than sw. this finding indicates that pt samples were highly homogeneous. this result agrees with the anova evaluation. table 3. result of homogeneity testing of pt samples according to annex b of iso 13528:2015 no. of bottle ph tx wt 2 t w ×10-6 x ( tx x− ) 2×10-7 sx ×10 -3 sw×10 -3 2 s s × 10-6 meas. 1 meas. 2 1 9.188 9.185 9.187 0.003 9.0 9.186 1.22 1.13 2.31 -1.39 13 9.192 9.185 9.189 0.007 49.0 55.2 25 9.187 9.185 9.186 0.002 4.0 0.225 37 9.187 9.187 9.187 0.000 0.0 7.22 49 9.187 9.184 9.186 0.003 9.0 4.23 61 9.189 9.185 9.187 0.004 1.6 7.22 73 9.185 9.184 9.185 0.001 1.0 27.2 85 9.187 9.184 9.186 0.003 9.0 4.23 97 9.185 9.186 9.186 0.001 1.0 4.23 109 9.184 9.187 9.186 0.003 9.0 4.23 stability testing stability testing is another criterion for obtaining good and suitable pt samples. the stability defines the ability of a reference material, when kept under a certain conditions, to preserve a pointed property value within specified limits and period of time [16]. there are two types of stability tests for pt samples. firstly, stability testing at elevated temperature was performed to elucidate whether any degradation is occurred during transport. this testing is usually conducted within a short time period, typically not longer than 4 weeks. in this case, the stability was monitored for sample transportation purposes and it was checked for samples stored for 4 weeks at 40 °c. from data presented in table 4, it can be seen that pt samples of borax buffer solution have fulfilled the criteria of stability testing according to iso 13528:2015 and eq. (8). it means that the pt item of borax buffer solution is stable at 40 °c for 4 weeks. table 4. stability monitoring data for pt sample at 40°c week ph 1 2y y− , where 1 9.186y = criteria iso 13528:2015 1 2 pt 0.3y y −  where pt = 0.03, so: 1 2 0.9y y−  measurement 1 measurement 2 average ( 2y ) 2 9.186 9.185 9.186 0 ok 3 9.183 9.186 9.185 0.001 ok 4 9.181 9.181 9.181 0.005 ok secondly, the stability testing of pt samples was conducted during the pt scheme. it is a time period in which the pt participants should perform their analyses in order to assure that the ph value of pt sample will not change during the measurement at the participant’s laboratory. the criteria for the stability monitoring were already discussed above together with eq. (8), while the results are tabulated in table 5. from table 5, it can be observed that pt samples met the stability j. electrochem. sci. eng. 8(3) (2018) 261-270 borax buffer solution for ph measurement 268 testing criteria according to iso 13528:2015, indicating that pt samples were stable during the pt scheme. table 5. data for the stability monitoring of the 2016 pt sample no. of bottle ph 1 2y y− , where 1 9.186y = criteria iso 13528:2015 1 2 pt 0.3y y −  where pt = 0.03, so: 1 2 0.9y y−  measurement 1 measurement 2 average ( 2y ) 38 9.181 9.179 9.180 0.006 ok 64 9.182 9.179 9.181 0.005 ok 115 9.179 9.179 9.179 0.007 ok application the pt program for the ph measurement was successfully conducted. after the pt samples met the criteria for homogeneity and stability monitoring for transportation, the pt samples were distributed to each participant by dispatching using a courier in the same day. the participants were asked for checking the physical conditions of the pt sample. the mic lab-lipi as the pt provider assured that the samples were received in good conditions. participants were requested to conduct the ph measurement for the given pt sample by using their routine method. one month is given time for the participants to conduct their measurements and to report their results. there were 50 laboratories who participated in the pt scheme. all pt results were statistically assessed through the statistical evaluation. the performance score was typically derived by comparing the difference between the values of participants and assigned value together with standard deviation [12]. the assigned value of pt sample was determined by the differential potentiometric cell at 25 °c, using a secondary method of ph measurement. the ph measurement was traceable to srm® 187e and the assigned value of this scheme was obtained from the average of ph value that was 9.186. all submitted data from the participants were then analysed by using the software analyticalquality-assurance for interlaboratory studies, prolab plus. in order to avoid wrong data input, all data were imported automatically by using the ringdat.exe application as a part of the reporting procedure used in prolab software. the performance of the participant laboratory was assessed by comparing the submitted results against assigned value using the z-score method. the z-score (zi) of each participant result was calculated using eq. (9). zi = (xi-xpt) / pt (9) in eq. (9), xi is the submitted result by individual participant, xpt is the assigned value of the test material, pt is the standard deviation for proficiency assessment (the value of this scheme was 0.03). the interpretation of z-score is as follows: a) a result that gives |z-score|≤ 2.00 is considered to be satisfactory, given by symbol “ok”. b) a result that gives 2.00<|z-score|< 3.00 is considered to be questionable, given by symbol “$”. c) a result that gives |z-score|≥ 3.00 is considered to be unsatisfactory, given by symbol “$$”. the z-score results for 50 participants presented in figure 2 can be summarized as follows: 22 laboratories (44 %) achieved the satisfactory result, 19 laboratories (38 %) reported the unsatisfactory result, while 9 laboratories (18 %) predicated the questionable result. taking this z-score result into consideration, one can notice that ph measurements conducted in some participant laboratories are still needed to be improved. in addition, investigation regarding the possible mistake sources are essential to be performed. n. aryana et al. j. electrochem. sci. eng. 8(3) (2018) 261-270 doi:10.5599/jese.467 269 figure 2. plot of z-score of participants results of ph measurement conclusions the prepared pt samples (borax buffer solution) have fulfilled the criteria for homogeneity and stability according to iso 13528:2015. the homogenous and stable pt samples were distributed to the participant laboratories for the pt 2016 program. the results of evaluation regarding to each participant’s laboratory performance showed that 44 % of participant laboratories achieve the satisfactory result, 38 % of participant laboratories achieve the unsatisfactory result and 18 % of participant laboratories achieve the questionable result. these results imply that ph measurements conducted in these laboratories are still needed to be improved. it is also required to investigate the possible source of mistakes. acknowledgements: the authors would like to thank mrs. nuryatini, mrs. eli susilawati and mr. sujarwo for helping in the laboratory experiment, data collection and discussion. references [1] sni iso/iec 17043: conformity asessment-general requirements for proficiency testing (2010). [2] y. mitani, j. v. lara-manzano, a. rodriguez-lopez, accreditation and quality assurance 13 (2008) 421426. [3] ilac secretariat, benefits for laboratories participating in proficiency testing programs, ilac, silverwater, australia, 2008. [4] i. mann, b. brookman, selection, use and interpretation of proficiency testing (pt) schemes, second edition, eurachem, 2011. [5] sni iso/iec 17025: general requirements for the competence of testing and calibration laboratories (2008). [6] f. b. gonzaga, j. c. dias, d. jehnert, b. werner, k. schräpler, l. vyskočil, electroanalysis 25 (2013) 19551959. [7] critical factors affecting ph measurement, http://www.jumo.ch/media/pdfs/press/whitepaper/jumo_critical_factors_ph_measurement.pdf (accessed on 31 august 2017) [8] mic laboratory, report of citric buffer solution (lbs15), lipi, serpong, indonesia, 2015. [9] m. f. palupi, r. a. sari, u. partriana, d. widyarimbi, e. rusmiati, et al., national technical meeting for agricultural functional, proceedings, bogor, indonesia, 2006, p 37-42. [10] f. g. k. baucke, journal of electroanalytical chemistry 368 (1994) 67-75. -4 -3 -2 -1 0 1 2 3 4 lc 0 0 9 2 lc 0 0 8 1 lc 0 0 0 5 lc 0 0 0 8 lc 0 0 3 2 lc 0 0 2 3 lc 0 0 2 7 lc 0 0 4 0 lc 0 0 7 8 lc 0 0 5 1 lc 0 0 9 0 lc 0 0 2 0 lc 0 0 6 6 lc 0 0 2 6 lc 0 0 5 0 lc 0 0 4 6 lc 0 0 5 2 lc 0 0 1 1 lc 0 0 6 1 lc 0 0 6 8 lc 0 0 6 5 lc 0 1 0 7 lc 0 0 2 8 lc 0 0 3 1 lc 0 0 3 7 lc 0 0 3 9 lc 0 0 8 0 lc 0 1 1 1 lc 0 0 1 4 lc 0 0 8 5 lc 0 0 7 2 lc 0 0 8 4 lc 0 0 1 2 lc 0 0 8 7 lc 0 0 9 8 lc 0 1 0 3 lc 0 1 0 9 lc 0 0 4 9 lc 0 1 1 2 lc 0 0 0 1 lc 0 0 1 3 lc 0 0 6 9 lc 0 0 4 8 lc 0 0 7 4 lc 0 0 7 9 lc 0 0 8 8 lc 0 0 7 3 lc 0 0 9 9 lc 0 0 9 7 lc 0 1 0 4 ring test: als16, tbs16, lbb16, pkm16 method: iso 13528 sample: tetraborate buffer (lbb16) measurand: ph no. of laboratories: 50 assigned value: 9.186 (reference value) mean: 9.134 target s.d.: 0.03 (reference value) rel. target s.d.: 0.33% (reference value) range of tolerance: 9.126 – 9.246 (|z-score| ≤ 2.00) prolab plus laboratory j. electrochem. sci. eng. 8(3) (2018) 261-270 borax buffer solution for ph measurement 270 [11] y. d. mustopo, effect of time on thickness and adhesivity layer in electroplating process of chrome decorative without basic layer with copper basic layer and copper-nickel, department of mechanical engineering, technical faculty of uns, solo, indonesia, 2011. [12] iso 13528: statistical methods for use in proficiency testing by interlaboratory comparison (2015). [13] r. p. buck, s. rondinini, a. k. covington, f. g. k baucke, c. m. a. brett, m. s. camoes, m. j. t. milton, t. mussini, r. naumann, k. w. pratt, p. spitzer, g. s. wolsin, pure and applied chemistry 74 (2002) 21692200. [14] p. spitzer, accreditation and quality assurance 6 (2001) 55-60. [15] federman neto, a., a. d. l. borges, m. a. s. lavrador, journal of basic and applied pharmaceutical sciences 27 (2006) 63-72. [16] j. c. gust, 4th ilac proficiency testing consultative group, a discussion of stability and homogeneity issues in proficiency testing for calibration laboratories, vienna, autria, 2006. [17] r. iavetz, statistical manual-chemical proficiency testing, australian government department of industry, innovation and science, national measurement institute, australia, 2016. [18] j. pauwels, a. lamberty,h. schimmel, accreditation and quality assurance 3 (1998) 51-55. [19] j. c. ulrich, j. e. d. s. sarkis, 16th international congress of metrology, estimation of uncertainty to the homogeneity study in the certification of a candidate certified reference material in fish matrix, paris, france, 2013. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {impact of the final thermal sealing of combined zinc/cerium oxide protective coating primers formed on low carbon steel:} http://dx.doi.org/10.5599/jese.1297 685 j. electrochem. sci. eng. 12(4) (2022) 685-701; http://dx.doi.org/10.5599/jese.1297 open access : : issn 1847-9286 www.jese-online.org original scientific paper impact of the final thermal sealing of combined zinc/cerium oxide protective coating primers formed on low carbon steel stephan kozhukharov1,, christian girginov2, nelly boshkova3 and alexandar tzanev4 1lamar laboratory, university of chemical technology and metallurgy, sofia, bulgaria 2department of physical chemistry, university of chemical technology and metallurgy, sofia, bulgaria 3institute of physical chemistry – bulgarian academy of science, sofia, bulgaria 4institute of general and inorganic chemistry – bulgarian academy of science, sofia, bulgaria corresponding author: s.kozhukharov@uctm.edu; tel.: +359 899 837 282; received: february 13, 2022; accepted: may 25, 2022; published: june 13, 2022 abstract the final sealing possesses a proven beneficial effect on the protective properties of anodic oxide films on aluminum. in this sense, the present research is devoted to the evaluation of the impact of this procedure on the barrier ability of combined zn/ce oxide layers deposited on low carbon steel samples. for this purpose, four samples were submitted to galvanic zinc deposition, followed by spontaneous formation of cerium oxide primer layer (ceopl). afterwards, two of the samples underwent thermal sealing in boiling water in order to enhance their barrier ability. its evaluation was performed by two electrochemical methods: electrochemical impedance spectroscopy (eis) and potentiodynamic scanning (pds) after 24 hours of exposure to a diluted model corrosive medium (mcm). other instrumental methods were used in order to describe the effect of this final procedure on the color characteristics and hydrophobicity of the films. the results were collected from multiple tests, followed by statistical data treatment. in addition, the surfaces of the obtained films were submitted to direct observation by scanning electron microscopy (sem), coupled with energy dispersion x-ray (edx). their composition was determined by means of x-ray photoelectron spectroscopy (xps). the acquired data have revealed a detrimental effect of the final sealing in boiling water. it was expressed by the loss of the barrier properties of the zn/ceopl films, combined with additional decolorization and hydrophilization. finally, the mechanism of this detrimental effect was determined by further sem, edx and xps analyses. keywords zn-galvanization; cerium oxide primer layers (ceopl); thermal sealing; barrier properties; surface analysis introduction unlike high-performance steels, the low-carbon ones are exceptionally susceptible to uniform corrosion even under moderate conditions. furthermore, it is well known that the corrosion products of iron (i.e., red rust) do not form dense layers, enabling continuous access of corrosive http://dx.doi.org/10.5599/jese.1297 http://dx.doi.org/10.5599/jese.1297 http://www.jese-online.org/ mailto:s.kozhukharov@uctm.edu j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 686 species to the metallic surface beneath them. hence, low-carbon steel corrodes until the complete destruction of the respective iron-based works/products. for this reason, the corrosion protection of these materials is indispensable. on the one hand, galvanic zinc plating is a classical approach for processing low-carbon steel. it provides reliable corrosion protection of the steel substrates [1,2] and a suitable base for the deposition of other types of protective layers [3-5]. besides, the optimization of the electrodeposition of zn-based layers still remains a topic of intensive research [6,7]. in this sense, the main approaches for this optimization are through enhancement of the electrolyte composition [8-10] or by applying different electrodeposition regimes [11-13]. on the other hand, the successful formation of cerium conversion coatings on steel substrates was reported recently through both spontaneous [14-16] and electrochemical [17] deposition methods. furthermore, this type of coatings is also proposed for further enhancement of zinc-phosphate finishing of steel [18]. another recent trend is the formation of combined ce/ni [19] or ce/v [20] conversion coatings on steel and magnesium alloys, revealing the versatility of this class of coatings and the possibilities to be combined with other elements and to be deposited on other metals and alloys. following these trends, the present brief research aims to assess and compare the basic physical characteristics of the obtained zn/ceopl double-layer coatings with and without additional thermal sealing in boiling water. besides, the color-related features and wettability data were acquired after multiple measurements in order to be submitted to statistical data treatment. in addition, the mechanism of the impact of the final thermal treatment procedure was analyzed by detailed topological and compositional analyses. experimental electrochemical film deposition the electrodeposition of the zinc coatings was performed on low-carbon steel plates (dimensions 30x30x1 mm) in a glass electrolytic cell with a volume of 300 ml. the applied current density was 2 a dm-2, the ph was adjusted to 5 and the temperature was held in the range 20-25 °c. metallurgical zn was used for the anode. the electrolyte for the zn film deposition was composed of two inorganic substances: znso4.7h2o (175 g dm-3) and (nh4)2so4 (25 g dm-3) with two organic additives: a mixture of polyethylene glycol and benzoic acid (40 g dm-3) and benzalacetone, dissolved in ethanol (10 g dm-3). this electrolyte was prepared following the optimal results reported in [21]. the further cerium oxide primer layers were formed by electroless deposition, again following the optimal compositions reported in previous works [22,23]. a mixture of (nh4)2ce(no3)5 and cecl3 (both with a concentration of 0.025 mol dm-3) with the addition of 10 ml dm-3 30 % h2o2 as deposition activator was used for the spontaneous formation of the cerium conversion coatings. the duration of this process was 15 min, at 50 ± 1 °c. all the described procedures were performed on four samples, two of which were subjected to additional treatment in boiling distilled water for another 15 min. this approach has enabled to obtain two types of zn/ceopl coating primers: (i) two reference samples, without additional treatments, denoted as ref1 and ref2 and (ii) two thermally sealed specimens, marked as ts1 and ts2, respectively. electrochemical assessments the electrochemical measurements were performed after 24-hour exposure to a 0.01 m nacl solution, at room temperature, using a standard three-electrode flat corrosion test cell, according s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 687 to iso 16773-2:2016, with a volume of 100 ml. this rather low concentration of nacl in the model corrosive medium allows for distinguishing the differences among the acquired data from the respective samples. these assessments were performed by means of electrochemical impedance spectroscopy (eis) and potentiodynamic scanning (pds). the eis spectra were acquired in 50 data points, distributed in the frequency range from 10 khz to 0.01 hz by applying an excitation signal of 10 mv, according to the open circuit potential (ocp). the respective ocp value was determined immediately prior to the acquisition of each spectrum. the pds curves were acquired in the potential range from -50 to +500 mv with respect to the reference electrode, with a potential sweep of 10 mv s-1. this relatively high sweep rate allows for acquiring a rather wide range of pds curves (total width of 550 mv) for quite a short time (55 s) of polarization. these electrochemical measurements were performed with an autolab pg-stat30, equipped with a fra2 frequency response analyzer (metrohm, netherlands). all measurements were performed against a ag/agcl/3m kcl reference electrodepositioned 10 mm above the horizontal corrosion test areas (1 cm2). the electrochemical cells were completed with a cylindrical pt-mesh, serving as a counter-electrode mounted around the reference electrode. color characteristics the color characteristics of all samples were measured by operating a lovibond rt100 tintometer. three independent measurements were performed on different points on the surface of each sample. the objective was to determine the data repeatability acquired from one and the same sample surface in order to detect any color variation. contact angle measurements the respective contact angles were measured using a “theta lite” high-precision optical device, a product of biolin scientific (uk), coupled with specialized “one attention” software (finland). the constant volume of the drops was ensured using a “gastight 1001” precise screw syringe, a product of hamilton co. (usa). the measurements were performed on three different points on the sample surfaces, similar to those in the previous section. statistical data treatment procedures the electrochemical measurements were performed on two samples from each type. the eis spectra were submitted to kramers/kroenig test (known as k/k test procedure), in order to determine the correspondence between the actual acquired eis spectra from the samples and these of the used model equivalent circuit (mec). the test procedure is based on the presentation of each data point (n) of the corresponding eis spectrum as a response of the respective rc circuit. the equivalent circuit for the execution of the k/k test is illustrated in figure 1. figure 1. equivalent circuit for k/k test procedure, where in the present case n = 50 [24], r denotes the resistance of the electrolyte the acquired values from the k/k test procedure are listed in table 1, together with the determined mec component values. http://dx.doi.org/10.5599/jese.1297 j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 688 the pds curves were submitted to tafel slope analyses. next, the differences in the data acquired from every two samples were submitted to a simplified method, used in a previous work [25], based on eq. (1): + − = 2 1 2 1 2 2 mv mv mv mv av (1) where av is the average value; mv2 is the higher measured value, whereas mv1 is the lower measured value, respectively. the data for characterization of the color characteristics and wettability of the investigated surfaces were acquired from three measurements for each sample. this approach allowed to obtain a total of 6 values for each parameter. the color characteristics were quantified according to cie (l*a*b*)-system, developed by international commission on illumination (abbreviated cie) in 1976 [26]. there, the parameter l* (luminosity) varies from 0 to 100 and accounts for the color brightness. the other vector parameters, a* and b*, vary from -128 to 127, showing the transition from yellow to blue and red to green, respectively. the acquired values of these color parameters were submitted to statistical data treatment, performed in 7 consecutive steps, described in the algorithm used in [27] and illustrated in figure 2. figure 2. algorithm for statistical data treatment s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 689 this algorithm was applied for six measurements (n = 6) for each group (i.e., three measurements on two samples) and the value of the student criterion was selected to be t = 5.959 [24], at probability p = 0.1 %. the obtained values for the contact angles were also submitted to this statistical analysis in order to determine the impact of the thermal treatment on the surface properties of the zn/ceopl films, as well as their uniformity (from the respective standard deviations). topological and compositional analysis of the surface this analysis was performed by means of three instrumental methods: (i) scanning electron microscopy (tescan, sem/fib lyra i xmu), (ii) energy dispersive x-ray map analysis (quantax 200 xray spectrometer, brueker) and (iii) x-ray photoelectron spectroscopy (vg escalab ii). the latter uses alkα radiation with an energy of 1486.6 ev, with a chamber pressure of 1.33x10-7 pa. the c1s line of adventitious carbon at 284.6 ev was used as an internal standard to calibrate the binding energies. the photoelectron spectra were corrected by subtracting a shirley-type background and were quantified using the peak area and scofield’s photo-ionization cross-section. the accuracy of the binding energy (be) measurements was ± 0.2 ev. results and discussion electrochemical assessment the eis spectra, shown in figure 3, undoubtedly reveal the detrimental effect of the treatment of the already deposited zn/ceopl films in boiling water. indeed, the bode plots show that the |z| / (f) dependence acquired for the reference samples lies above the thermally treated ones. the same trend is observable for the -phase shift / (f) diagrams. it does not reach -90o, revealing that the electric double layers formed between the sample surfaces and the liquid mcm are strongly irregular and do not form perfect capacitance. the phase shift of the reference samples is almost twice as higher (reaching about 60°) as this of the thermally treated (at around 30°). consequently, the final treatment by boiling water leads to deterioration of the obtained zn/ceopl films. the deterioration of the zn/ceopl layer is even more obvious when comparing the respective nyquist plots. there, the semi-circles of the reference samples are quite larger than those of the thermally treated ones. indeed, the value of the real part z’ of ref1 and ref2 reaches 45 and 35 kω cm2, whereas the semi-circles of ts1 and ts2 do not reach even 10 kω cm2. figure 3. eis spectra in nyquist (a) and bode (b) plots of the investigated samples (symbols denote experimental data, while lines the respective fitted spectra) -z '' /  c m 2 http://dx.doi.org/10.5599/jese.1297 j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 690 the spectra, shown in figure 3, were submitted to further analysis by means of fitting to a suitable model equivalent circuit (mec). it was composed of three elements, which correspond to the resistance of the model corrosive medium (rmcm), the constant phase element of the interface between the sample surface and the medium (cpeedl) and the charge transfer resistance (rct). this mec consists of a simple randles cell, which perfectly reveals the actual behavior of conducting layers exposed to an electrolyte [28]. the acquired numerical data are summarized in table 1. table 1. numerical data acquired by fitting of the obtained eis spectra to the mec, illustrated in the inset of figure 3 sample rmcm / ω cm2 cpeedl / sn ω-1 cm-2 10-6 n rct / kω cm2 ref1 636.00 ± 10.78 28.13 ± 0.78 0.76 ± 0.01 50.10 ± 1.51 ref2 670.00 ± 14.70 29.17 ± 1.07 0.75 ± 0.01 39.60 ± 1.41 ts1 687.00 ± 6.44 159.30 ± 4.05 0.78 ± 0.01 6.30 ± 0.13 ts2 687.00 ± 6.44 214.40 ± 4.37 0.78 ± 0.01 3.41 ± 0.05 the values of rmcm are noticeably similar for all samples. their somewhat high values are a consequence of the relatively low nacl concentration of the mcm. the constant phase element of the electric double layer (cpeedl) is by an entire order of magnitude higher for the samples submitted to final thermal sealing in boiling water. this fact reveals deterioration of the capacitive reactance caused by the thermal treatment. consequently, the boiling water causes partial leaching of the cerium oxides/hydroxides formed on the surface of the zn/ceopl coating. this suggestion is confirmed by the rct values, which for the thermally treated specimens are also lower by an entire order of magnitude than those of the reference samples. hence, the average rct value for the reference samples is about 44.85 kω cm2, whereas, for the thermally sealed ones, it reaches 4.86 kω cm2. the lower rct values, in this case, are an indication of the loss of insulating properties of the ce-oxides and hydroxides, which compose the ceopl layer, probably due to its partial leaching. each eis spectrum acquisition was followed by performing a potentiodynamic scan (pds), the obtained curves are presented in figure 4. figure 4. pds curves acquired from the investigated specimens after 24 hours of exposure to 0.01 m nacl model corrosive medium figure 4 shows that the pds curves of the reference samples lie below those of the samples, treated in boiling water. besides, an overlapping of the anodic branches is observed for the reference samples, unlike the thermally sealed ones. this fact reveals that probably the treatment for s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 691 15 minutes in boiling water results in the partial removal of ce-compounds from the zn/ceopl films. these inferences are confirmed by the results of the tafel slope analysis, summarized in table 2. table 2. numerical results acquired from the tafel slope analysis of the pds curves recorded after 24 hours of exposure to the model corrosive medium parameter sample ref1 ref2 ts1 ts2 polarization resistance, kω cm2 49.18 39.09 6.31 3.67 average value 44.16 ± 5.05 4.99 ± 1.32 corrosion potential, mv -878 -895 -903 -932 average value -886.50 ± 8.50 -917.50 ± 14.50 indeed, the data in table 2 show that the analysis of respective pds curves confirms the results of the eis spectra. the values of the polarization resistance of the investigated samples are almost equal to the charge transfer resistance (rp ≈ rct). the average values of the corrosion potential, calculated from eq. 1, are close to -900 mv. the simultaneous appearance of distinguishable rp values for the respective sample groups and the relatively similar corrosion potentials was the reason for determining the effect of the final thermal sealing on other physical properties of the obtained zn/ceopl films. color characteristics and wettability the color parameters presented in table 3 reveal that the zn/ceopl coatings do not suffer any drastic changes in brightness (l*) and in the red/green vector (a*). only weak brightening (from l*av = 75.95 ± 0.01 of the references to l*av = 78.56 ± 0.01 for the thermally treated samples) was observed as a result of the probable partial detachment of the yellow cerium oxide/hydroxide deposits provoked by the formation of water vapor bubbles in the boiling water. as a result, uncovered white areas of the electrodeposited zn layer appear, resulting in the observed brightening. table 3. color characteristics of the investigated surfaces sample measurement number l* a* b* ref1 1 76.72 -0.33 54.86 2 76.86 -1.61 54.10 3 75.32 -0.29 53.65 ref2 1 74.20 -2.39 55.77 2 76.26 -2.54 52.09 3 76.33 -1.57 52.78 average value 75.95 ± 0.01 -1.455 ± 0.006 53.875 ± 0.006 ts1 1 75.23 -6.03 -2.48 2 80.73 -7.39 -2.00 3 75.88 -6.45 -1.54 ts2 1 79.05 -7.25 -5.08 2 83.62 -6.93 -0.21 3 76.86 -5.67 -0.28 average value 78.56 ± 0.01 -6.62 ± 0.01 -1.92 ± 0.09 the values of a* tend to zero for both types of samples. this means that there are no traces of red-rust, despite the definitely acidic properties of the ce-containing deposition solutions evinced in previous works [29] and confirmed by other authors [30]. the insignificant transition of the red/green color vector (from a*avg = -1.455 ± 0.006 for the references to a*avg = -6.62 ± 0.01 for the http://dx.doi.org/10.5599/jese.1297 j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 692 thermally treated ones) is most probably due to the appearance of spots of feoh2 caused by the boiling water, as this compound is green in color. however, a sharp decay of the values of the yellow/blue vector (from b*avg = 53.875 ± 0.006 to about b*avg = -1.92 ± 0.09) shows a transition from yellow to gray. indeed, the gold-like look described by other authors, as well [31,32], has disappeared due to the thermal treatment. similar observations have been reported with respect to the post-treatment of a cerium conversion coating on aa2024-t3 by boiling in dilute nh4h2po4 solution [33]. since the zn/ceopl films are intended for initial layers of multilayered coating systems, such as the proposed in [34,35], their surface properties are of key importance for the adhesion of the subsequent intermediate and finishing layers. in this sense, systematic measurements of the contact angle were performed in order to define whether the elaborated zn/ceopl films are more suitable for hydrophilic or for hydrophobic coating layers. this additional analysis is rather important in order to avoid potential blistering or delamination across the zn/ceopl coating primer and the further organic, inorganic, or hybrid coating layers. figure 5 represents the results of the systematic drop tests performed on the investigated samples. the images clearly show that the reference samples (ref1 and ref2) possess relatively hydrophobic properties, whereas the thermally treated ones (ts1 and ts2) are hydrophilic. figure 5. profiles of the interfaces between the surfaces of the investigated samples and distilled water drops after 10 s of contact the obtained results, shown in figure 5, were further submitted to statistical analysis, following the seven-step procedure illustrated in figure 2. the results show that the reference samples possess almost hydrophobic properties, with the average wetting angle avg tending to 90°, whereas the value of avg of the thermally sealed ones is below 40o. all results, including the raw data (the measured values) and these from the statistical treatment, are presented in table 4. the surprising detrimental effect of the final thermal treatment has imposed the need to analyze the cause of its occurrence. in this sense, it was of interest to determine the deterioration mechanism by performing additional topological and compositional analyses, which are interpreted in the next paragraph. s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 693 table 4. data on the wetting angle of the investigated samples sample measurement number left angle, o right angle, o ref1 1 75.07 62.95 2 88.94 91.09 3 83.13 88.84 ref2 1 81.52 86.09 2 92.83 90.34 3 73.05 78.55 average value θavg = (82.70 ± 0.01) o ts1 1 38.52 46.34 2 51.79 63.26 3 37.73 35.04 ts2 1 28.07 30.28 2 36.05 35.64 3 25.78 28.28 average value θavg = (38.07 ± 0.01) o surface topological and compositional analysis this analysis was performed by scanning electron microscopy (sem) combined with energy dispersion x-ray (edx) spectroscopy. both the sem images and the edx maps reveal that the uniform zn/ceopl films undergo cracking due to the submission to the final sealing procedure, as shown in figure 6. figure 6. lowand high-resolution sem images and edx maps of zn/ceopl coating primer before (a) and after (b) the additional sealing there are several possible reasons that could cause the cracking of the zn/ceopl films during the final sealing in boiling water. for instance, the difference in the thermal expansion coefficients of the steel substrate, the primary zn layer and the ce-oxide/hydroxide coating result in tension stress across the interfaces between the layers. these stresses appear at the beginning of the thermal sealing, as well as after this process when the sample undergoes re-cooling. during these stages, the temperature of the sample changes in the range between 20 and 100 °c. in addition, the cracks that have already appeared expand due to water uptake and also due to the formation of water vapor inside them. hence, the already penetrated water forms vapor bubbles, exfoliating parts of the zn/ceopl layer. http://dx.doi.org/10.5599/jese.1297 j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 694 the additional cross-sectional sem images in figure 7 reveal that the film deterioration proceeds mainly at the interface between the cerium oxide/hydroxide layer and the galvanic zn sublayer. figure 7. cross-sectional sem images of zn/ceopl layers before (a) and after (b) the thermal sealing as it can be seen from the figure, the electroplated zn layer is much thicker (being about 12 μm) than the ceopl upper layer (approximately 1 μm). consequently, the difference between the thicknesses of these layers is an additional reason for the preferential deterioration exactly at the interface between these layers. besides, as a rule of thumb, metals like the fe substrate and the zn galvanic layer possess rather higher thermal expansion coefficients compared to the ce-oxide/hydroxide upper layer, since its composition resembles this of ceo2 based glasses and ceramics. further, comparing positions (a) and (b) of figure 7, it becomes clear that in the former case, the coating is composed of two distinguishable layers, whereas in the latter case, the thermal sealing causes obvious deterioration. another probable reason is the change in the chemical composition of the layer due to the hydration of both ce-oxides and metallic zn underneath. this results in swelling of the film and further increase of mechanical stresses in its structure. both phenomena, film cracking and hydration, result in changes in the physical properties described in the previous sections. indeed, the cracks facilitate water uptake of the thermally treated (ts1 and ts2) layers, making them more hydrophilic compared to the reference samples (ref1 and ref2). of course, this effect is further enhanced by the hydrated surface obtained during the final sealing. besides, the loss of yellow tonality results from the partial leaching of the ce-oxide/hydroxides from the zn/ceopl films because of water penetration and bubble formation during the film treatment in boiling water. the potential impact of these probable film deterioration factors was further analyzed by x-ray photoelectron spectroscopy. specimens of both types were submitted to xps analysis in order to elucidate the deterioration mechanism caused by the final thermal treatment procedure. the elemental compositions of the reference and thermally treated samples in atomic percentages are presented in table 5. table 5. element composition of zn/ceopl coating primers before and after the final thermal sealing established by xps content, at.% content, %* sample c o zn ce (total) ce3+ ce4+ reference 45.1 37.4 3.5 14.0 11.5 88.5 thermally sealed 30.9 48.0 17.9 3.2 23.8 76.2 *both contents are calculated as a part of the total ce-content s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 695 the data in table 5 undoubtedly reveal that the thermal sealing causes depletion of ce with simultaneous zn enrichment. indeed, the concentration of cerium decreases by more than four times, while the concentration of zn increases by almost 6 times. both phenomena reveal the separation of ceopl proposed in the previous paragraph, which reveals areas of the surface of the galvanic zn layer. based on the data in this table, it could be inferred that 12.3 at.% of ce(iv) oxides and hydroxides undergo a reduction with respect to ce(iii) compounds. this result is acquired by calculation of the integrated peak area at 916.7 еv, following the equation of pardo et al. [36]. this peak for both samples is illustrated in figure 8a (third peak marked as ce4+ at 916.7 ev). ce3d core photoelectron spectra in figure 8a are characterized by a complex structure due to the hybridization of the cerium ions with the ligands of oxygen orbitals and partial occupation of the 4f valence orbital. as a result, a spin-orbital splitting appears in the doublet peaks, whereupon each doublet has an additional structure, owing to the effect of the final state [37]. the decrease in the intensities of the ce peaks confirms the decrease in the total ce content. binding energy, ev binding energy, ev figure 8. ce3d characteristic peaks for ceria (a) and deconvolution of the characteristic peaks of o1s (b) figure 8b shows the deconvolution performed on the o1s spectra of both types of samples. the o1s core electron spectrum of the reference sample is divided into four peaks. the first peak, situated at 529.2 еv is characteristic of the presence of oxygen ions bonded with се [38]. the second one, at 530.3 ev, could be ascribed to the occurrence of oxygen atoms coordinated with zn atoms in the zno lattice [39]. the third (at 531.2 ev) and fourth (at 532.8 ev) peaks in figure 8b are related to the presence of hydroxide ions and both physically adsorbed and crystal hydrate water [40], respectively. in the case of the thermally sealed coating primer, the occurrence of chemically bonded oxygen (се-o and zn-o) appears. the peaks for zno (at 530.5 ev) are much more intensive with respect to those of the reference sample. at the same time, the ce-o peak (528.9 еv) decreases after the thermal sealing. hence, obviously, parts of the ceopl upper layer undergo detachment, uncovering areas http://dx.doi.org/10.5599/jese.1297 j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 696 covered by the inner galvanic zn layer. the quantity of the онˉ-groups at 531.8 ev also decreases, as a result of the thermal treatment. surprisingly, the peak characteristic for adsorbed water disappears. this is possible due to the high temperature of the sample during its withdrawal from the boiling water and its cooling to room temperature is accompanied by intensive water evaporation. the zn2p core photoelectron spectra are shown in figure 9a. the zn2p peak is positioned at 1021.4 ev, and its spin-orbital splitting is 23.1 еv. in the case of the thermally treated samples, a weak shift of the zn2p peaks appears towards higher binding energies. here, the value of the calculated modified auger parameter is 2010.0 ev for the reference sample, compared to 2010.3 ev, calculated for the thermally treated specimen. following al-gaashani et al. [41], it could be inferred that these characteristic peaks reveal the conversion of the zn-galvanic layer to zno during the formation of zn/ceopl. bibding energy, ev binding energy, ev kinetic energy, ev figure 9. zn2p core electron spectra of the reference and the thermally sealed samples (a), deconvolution of the characteristic peaks for zn2p (b) and auger znlmm spectra (c) the peaks of zn2p3/2 spectra in figure 9b are relatively wide and asymmetric, showing the coexistence of various zn-compounds. for this reason, the zn2p3/2 spectrum presented in fig 9b was deconvoluted, which led to the detection of three individual peaks at 1020.1 1020.5 ev, 1021.3 1021.6 ev and 1022.5 1022.7 ev, respectively. the low binding energy of the first peak shows weak bonds between zn ions and other positively charged particles, such as се4+/се3+ or carbon [42], whereas the other two peaks are typical for zno and zn(oh)2. the percentage ratios of these peaks to the total zn-content for the reference sample are as follows: 15 % for the peak at 1020.1 еv, 59 % for the one at 1021.3 еv and 26 % for the third at 1022.5 еv. for the thermally treated sample, these percentages are 21, 61 and 18 %, respectively. consequently, some quantities of metallic zn in the zn/ceopl layer undergo oxidation during the thermal treatment. this conclusion is further confirmed by the pattern of the auger peaks (figure 9b). the comparison of the c1s spectra for the reference and the thermally sealed samples in figure 10 reveals the appearance of an additional peak at 283.5 ev. this peak suggests the appearance of carbide compounds [43,44] due to the additional thermal treatment. the other three peaks, s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 697 positioned at 284.5 ev, 285.3 285.8 ev and 285.5 ev are characteristic of the occurrence of a variety of (с–с), (c–oh) or (c–o–c) bonded carbons and even (c=o) bonds [45]. binding energy, ev figure 10. c1s spectra of the investigated samples the occurrence of a considerable amount of carbon originates from the used organic additives described in the experimental part. finally, no fe2p spectrum was detected even at passing energy of 50 (i.e., with a more sensitive analysis). this fact indicates that the inner zn layer continues to efficiently protect the low carbon steel, even after the partial destruction of the upper ceopl layer during the thermal treatment. after collecting all assessment data, a conceptual model was created, illustrated in figure 11. figure 11. schematic illustration of the deterioration processes caused by the thermal treatment of the zn/ceopl coating primer: 1) low carbon steel substrate; 2) inner galvanic zn layer; 3) ceopl outer layer; 4) boiling water; 5) ambient air and 6) corrosion products of the zn-galvanic layer http://dx.doi.org/10.5599/jese.1297 j. electrochem. sci. eng. 12(4) (2022) 685-701 zn/ce oxide protective coating primers 698 the immersing of the zn/ceopl coated samples into boiling water causes tensile stresses (indicated with the letter f in figure 11) due to the dilatation of the metallic substrate and the galvanic layer. these stresses appear as a consequence of the difference in the thermal expansion coefficients of the respective layers (figure 11a). probably, the reduction of ce(iv) oxides and hydroxides to the respective ce(iii) compounds also contributes to the ceopl cracking, causing defects in its structure. afterwards, the entrapment of boiling water through the resulting ruptures in the ceopl leads to partial dissolution of the organic compounds (see the experimental section) and localized corrosion on the surface of the zn layer. besides, water vapor bubbles appear on the boundaries between the galvanic zn layer and the subsequent ceopl. the growing vapor bubbles and the expansion of the zno and zn(oh)2 corrosion products result in further enhancement of the mechanical stress inside the ceopl cracks. as a result, these ruptures convert to centers of ceopl delamination (figure 11b). similar undercoating wedge effects, which favor corrosion, are observed and explained in detail by other authors [46-48], as well. besides, re-deposition of zn-corrosion products occurs on the intact areas surrounding the delaminated areas. this re-deposition is favored by the outward hydrodynamic fluxes promoted by the boiling water bubbles. coincidently, direct detachment of ceopl fragments is also possible among the adjacent cracks, again enhanced by the bubbling of the boiling water (figure 11c) inside these cracks. finally, the areas of the zn layer, uncovered by the ceopl detachment, become centers of zn-corrosion (figure 11d). here, it should be appointed that the zn oxidation (corrosion) could be further enhanced by the reduction of ce(iv) compounds to ce(iii) ones. it should also be noted that the present conceptual model excludes any direct ceopl dissolution effects due to the extremely low solubility of the ce(iii)/ce(iv) oxides and hydroxides, established by aramaky [49] and confirmed by the model, proposed by scholes et al. [50]. after the withdrawal of the thermally treated samples from the boiling water, the high temperature results in intensive evaporation of water from their surface. this accelerated drying probably has an additional contribution to the resulting zn/ceopl deterioration. finally, the proposed conceptual model completely explains the changes in the contents of all the elements shown in table 5. conclusions systematic comparative research of the impact of the final thermal sealing is performed on zn/ceopl coating primers deposited on low carbon steel substrates. these combined coating primers were formed by deposition of galvanic zn film, followed by the formation of cerium oxide primer layer (ceopl). the results from the electrochemical tests performed by means of eis and pds in a dilute model corrosive medium have shown obvious deterioration of the barrier ability of the thermally sealed layers. the detrimental effect of this final procedure was registered by the sharp change of the eis spectra and confirmed by their further analysis using a suitable model equivalent circuit. furthermore, the potentiodynamic curves have revealed a sharp drop in the polarization resistance by an entire order of magnitude due to the thermal sealing. the comparative statistical analysis of the data acquired for the physical properties (color characteristics and wettability) of the investigated samples has shown an obvious loss of the goldlike look and hydrophobicity of the zn/ceopl films after the thermal sealing. s. kozhukharov et al. j. electrochem. sci. eng. 12(4) (2022) 685-701 http://dx.doi.org/10.5599/jese.1297 699 the topological and compositional analyses (via sem and edx) have undoubtedly revealed the appearance of cracks and ruptures in the zn/ceopl films, combined with partial exfoliation of the formed ce-oxides and hydroxides. the research activities were completed by detailed xps analysis in order to describe the alteration of the contents of all elements composing the combined zn/ceopl coating primer. the results have shown depletion of the ce content, accompanied by partial reduction of ce(iv) to ce(iii) compounds. an increase in zn content was registered due to uncovering of areas of the galvanic zn layer caused by ceopl cracking, delamination and detachment. these phenomena are followed by partial superficial zn corrosion. no traces of fe were found, evincing that the zn-ce-o deterioration proceeds on the interface between the zn galvanic layer and the finishing ceopl. the combination of all used analytical methods has enabled to determine the mechanism of the detrimental effect of the final sealing on the barrier properties and the physical characteristics of the obtained zn-ce-o coating primers. it was established that severe cracking appears due to mechanical tensions in the film structure and is combined with an alteration of its chemical composition. summarizing all the obtained results, a brief conceptual model was created and even 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{kozhukharov, stephan v. and girginov, christian and boshkova, nelly and tzanev, alexandar}, journal = {journal of electrochemical science and engineering}, title = {{impact of the final thermal sealing of combined zinc/cerium oxide protective coating primers formed on low carbon steel:}}, year = {2022}, issn = {1847-9286}, month = {jun}, number = {4}, pages = {685--701}, volume = {12}, abstract = {the final sealing possesses a proven beneficial effect on the protective properties of anodic oxide films on aluminum. in this sense, the present research is devoted to the evaluation of the impact of this procedure on the barrier ability of combined zn/ce oxide layers deposited on low carbon steel samples. for this purpose, four samples were submitted to galvanic zinc deposition, followed by spontaneous formation of cerium oxide primer layer (ceopl). afterwards, two of the samples underwent thermal sealing in boiling water in order to enhance their barrier ability. its evaluation was performed by two electrochemical methods: electrochemical impedance spectroscopy (eis) and potentiodynamic scanning (pds) after 24 hours of exposure to a diluted model corrosive medium (mcm). other instrumental methods were used in order to describe the effect of this final procedure on the color characteristics and hydrophobicity of the films. the results were collected from multiple tests, followed by statistical data treatment. in addition, the surfaces of the obtained films were submitted to direct observation by scanning electron microscopy (sem), coupled with energy dispersion x-ray (edx). their composition was determined by means of x-ray photoelectron spectroscopy (xps). the acquired data have revealed a detrimental effect of the final sealing in boiling water. it was expressed by the loss of the barrier properties of the zn/ceopl films, combined with additional decolorization and hydrophilization. finally, the mechanism of this detrimental effect was determined by further sem, edx and xps analyses.}, doi = {10.5599/jese.1297}, file = {:d\:/onedrive/mendeley desktop/kozhukharov et al. 2022 impact of the final thermal sealing of combined zinccerium oxide protective coating primers formed on low ca.pdf:pdf;:08_jese_1297.pdf:pdf}, keywords = {zn, barrier properties, cerium oxide primer layers (ceopl), galvanization, surface analysis, thermal sealing}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1297}, } {editorial} doi:10.5599/jese.513 1 j. electrochem. sci. eng. 8(1) (2018) 1-2; doi: http://dx.doi.org/10.5599/jese.513 open access : : issn 1847-9286 www.jese-online.org editorial this special issue of the journal of electrochemical science and engineering (jese) is dedicated to selected papers contributed to the 6th regional symposium on electrochemistry – south east europe (rse–see), held in balatonkenese, hungary, from 1115 june 2017. this issue constitutes part 1 of the proceedings of the symposium (6 articles). part 2 will appear in the next issue of jese. following five successful symposia in croatia (rovinj, red island, 2008), serbia (belgrade, 2010), romania (bucharest, 2012), slovenia (ljubljana, 2013) and bulgaria (pravetz, 2015), the 6th regional symposium on electrochemistry – south east europe (rse–see) was hosted in hungary, supported by the electrochemical societies of the region. the scientific theme of the meeting “renaissance of electrochemistry in the 21st century and its effect on the development of south east europe” underscored the key role of electrochemistry as a fundamental discipline bringing together societal needs from the one side and technology from the other side. over the past 10 years this series of symposia has attracted an international group of scientists and engineers with broad experience in electrochemical measurement and modeling. building on already established traditions, rse-see6 crossed regional borders, creating a common platform for scientists and experts from regional and international research and industrial institutions who brought their extensive knowledge, novel ideas, and boundless enthusiasm to the conference venue. more than 110 participants from 24 countries presented papers (66 talks including 4 plenary and 7 keynote lectures, 40 posters) and discussed recent progress during seven sessions (electrochemical energy storage, batteries, fuel cells and supercapacitors; analytical electrochemistry, sensors and biosensors, electrochemistry at interfaces, thermodynamic, kinetic and methodological aspects; electrocatalysis, new, non-noble metal catalysts, electrochemistry of new functional materials, conducting polymers and composites, general session). we would like to acknowledge the financial and other support of the hungarian academy of sciences, the international society of electrochemistry, the furukawa electric group, jászplasztik, ametek, zahner, metrohm, palmsens, ivium and paks nuclear power plant. we wish to express our gratitude to the members of the scientific and organizing committees for their continued dedication and support. we would like to sincerely thank the authors for their contribution, the reviewers for their commitment and professional work and the jeseeditorial office for their invaluable assistance in the proceedings publication. the next (7th) regional symposium on electrochemistry – south east europe will be held in croatia in 2019. gyözö g. láng, guest editor http://dx.doi.org/10.5599/jese.513 http://www.jese-online.org/ j. electrochem. sci. eng. 0(0) (2018) 00-00 editorial 2 members of the rse-see6 local organizing committee: györgy inzelt (honorary chairman), tamás pajkossy and gyözö g. láng (co-chairs), soma vesztergom (secretary). (homepage: http://rse-see2017.hu/ . photos: dóra zalka and krisztina j. szekeres) ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://rse-see2017.hu/ http://creativecommons.org/licenses/by/4.0/) {glutathione detection at carbon paste electrode modified with ethyl 2-(4-ferrocenyl-[1,2,3]triazol-1-yl)acetate, znfe2o4 nanoparticles and ionic liquid:} http://dx.doi.org/10.5599/jese.1230 209 j. electrochem. sci. eng. 12(1) (2022) 209-217; http://dx.doi.org/10.5599/jese.1230 open access : : issn 1847-9286 www.jese-online.org original scientific paper glutathione detection at carbon paste electrode modified with ethyl 2-(4-ferrocenyl-[1,2,3]triazol-1-yl)acetate, znfe2o4 nanoparticles and ionic liquid hadi beitollahi1,2,, somayeh tajik3, mohammad reza aflatoonian4 and asghar makarem 1school of medicine, bam university of medical sciences, bam, iran 2environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran 3research center of tropical and infectious diseases, kerman university of medical sciences, kerman, iran 4leishmaniasis research center, kerman university of medical sciences, kerman, iran 5department of rehabmanagement, university of social welfare and rehabilitation sciences, tehran, iran corresponding author:  h.beitollahi@yahoo.com; tel.: +983426226613 received: january 2, 2022; accepted: february 13, 2022; published: february 21, 2022 abstract the purpose of the present study was to introduce a newly designed approach for determination of glutathione using modified carbon paste electrode with znfe2o4 nanoparticles, ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) and ethyl-2-(4-ferrocenyl -[1,2,3]triazol-1-yl)acetate (efta/zfo/il/cpe). according to the results from the electrochemical experiments, oxidation current of glutathione on the modified electrode surface was incremented and its oxidation potential was decreased compared to bare cpe. a linear response was observed for the electrode at different glutathione concentrations (0.2 to 300.0 μm). keywords chemically modified electrode, cyclic voltammetry, differential pulse voltammetry, real sample analysis introduction glutathione (gsh,γ-glutamyl-l-cysteinyl-glycine) is an important tripeptide thiol within a eukaryotic and mammalian cell and contributes to several biological functionalities (e.g., expressing gene, synthesizing dna and protein, producing cytokine, and toxin metabolism). studies found that glutathione exists in the tissues of mammals and plants at concentrations of about 1 to 10 mm. unusual levels of glutathione are one of the indexes of different illnesses, including alzheimer's http://dx.doi.org/10.5599/jese.1230 http://dx.doi.org/10.5599/jese.1230 http://www.jese-online.org/ mailto:h.beitollahi@yahoo.com j. electrochem. sci. eng. 12(1) (2022) 209-217 glutathione detection at carbon paste electrode 210 disease, diabetes, parkinson diseases (pd), aids, arthritis, atherosclerosis, and several kinds of cancers. therefore, it is of high importance to develop a fast and efficient analytical method for glutathione determination for early medical diagnoses and the prevention of initial outset of side effects [1-3]. there are numerous potential analytical methods for glutathione determination such as spectrophotometry, titrimetry, mass spectrometry, high-performance liquid chromatography, and spectrofluorimetry [4-8]. however, a majority of the mentioned techniques generally have a lot of shortcomings, including complex and costly devices, laborious processes, and the requirements of the trained staff. therefore, developing of a simple, fast, inexpensive, and accurate technique should be of key importance in this case [9-23]. electrochemical methods are methods of choice due to their inherent properties, such as cost-effectiveness, easy operation, rapid response, sensitivity and selectivity [24-39]. due to the inherent sluggishness of heterogenous charge transfer of glutathione at carbon and metallic electrodes, its electrochemical oxidation/reduction is usually performed at modified electrodes coated with the substances which serve as electron transfer mediators, increasing the charge transfer rate. modifiers are usually prepared in the form of nanomaterials [40-50] which, apart from charge transfer mediation properties, take advantage of their very high surface area, increasing the selectivity and the sensitivity of the method [51-54]. various nanoparticles are currently being applied in sensors’ construction as modifiers. among them, zinc ferrite nanoparticles caught significant attention in nano-medicine because of low zn2+ toxicity. this is especially desirable for biocompatible mri contrast agents in medicine due to the high toxicity of existing contrast agents. on the other hand, permissible dosages for znfe2o4 nanoparticles are 18 and 15 mg/day for fe and zn, respectively, making them a suitable option for mri contrast agents. this is considerably higher than other biocompatible materials [55]. efficient mediator needs to have a low relative molar mass while being reversible, fast reacting, regenerated at low potentials, ph independent, stable in both oxidized and reduced forms, unreactive with oxygen and nontoxic. among the most successful mediators are those based on ferrocene and its derivatives that meet the above criteria [56]. ionic liquids (ils) have been generating increasing interest over the last decade. ionic liquids have a great potential for possible electrochemical applications due to the high thermal stability, no volatility, high polarity, large viscosity, high intrinsic conductivity, and wide electrochemical window [56]. in this study we focused on the electrochemical oxidation of glutathione using ethyl-2-(4-ferrocenyl-[1,2,3]triazol-1-yl)acetate (efta) as a mediator in a composite electrode consisting of efta/znfe2o4 nps/ionic liquid modified carbon paste electrodes surfaces (efta/zfo/il/cpe). experimental chemicals and instruments a potentiostat/galvanostat autolab pgstat 302n instrument equipped with a general-purpose electrochemical system (gpes) software has been utilized to measure electrochemical parameters. an ag/agcl/kcl (3.0 m) electrode, a platinum wire, and efta/zfo/il/cpe were used as the reference, auxiliary and working electrodes, respectively. a digital ph-meter (metrohm 710) was employed for measuring the ph values. h. beitollahi et al. j. electrochem. sci. eng. 12(1) (2022) 209-217 http://dx.doi.org/10.5599/jese.1230 211 the chemicals and solvents were used without further purifications from aldrich. the buffer solutions of different ph values (between 2.0 and 9.0) were prepared from orthophosphoric acid and its salts. preparation of modified electrode efta (0.01 g) was mixed manually with graphite powder (0.95 g) and znfe2o4 nanoparticles (0.04 g). then certain levels of ionic liquid and liquid paraffin were added and blended for 20 minutes. the uniform wet paste was appended into the bottom of a tube made up of glass of 15 cm length and 3.4 mm in diameter. electrical contacts were created by positioning copper wires inside the carbon paste. additionally, extra paste was inserted into the tube and polished by a weighing paper to establish a fresh surface. results and discussion electrocatalytic oxidation of glutathione at efta/zfo/il/cpe figure 1 shows cyclic voltammograms of the efta/zfo/il/cpe with (trace a) and without (trace b) added glutathione in the solution. for comparison, the response of unmodified cpe in the presence of glutathione is also shown (trace c). the redox reaction shows one reversible redox peak pair at the potentials of 330 mv. according to the literature the peaks correspond to one-electron reduction/oxidation of ferrocene moiety. by adding the glutathione into the solution, anodic current is significantly increased while cathodic current disappeared. such behavior is a characteristic of a mediated electron transfer confirming the hypothesis of a mediated oxidation reaction of glutathione by efta at modified cpe. the reaction mechanism of glutathione at efta mediated electrode is described by scheme i. ph dependence of the glutathione oxidation at modified electrode shows mediated current maximum in neutral solution so the medium of ph 7.0 was used for further study. figure 1. cvs of (a) efta/zfo/il/cpe in 0.1 m pbs (ph 7.0) consisting of 100.0 µm glutathione, (b) efta/zfo/il/cpe in 0.1 m pbs (ph 7.0), and (c) un-modified cpe in 0.1 m pbs (ph 7.0) with 100.0 µm glutathione http://dx.doi.org/10.5599/jese.1230 j. electrochem. sci. eng. 12(1) (2022) 209-217 glutathione detection at carbon paste electrode 212 linear sweep voltammetry (lsv) method was performed to assess the scan rate effect on electrochemical oxidation of glutathione in 0.1 m phosphate buffer solution (pbs) at a ph of 7.0. as seen in figure 2 an increase of the scan rate from 5 mv/s to 100 mv/s caused an increase of the oxidation current and a slight positive shift of oxidation peak potential. the plot of the oxidation peak current (ip) vs. the square root of scan rate (ν1/2) is linear (figure 2 inset) indicating a diffusioncontrolled oxidation process [57]. in order to determine the diffusion coefficient of glutathione, potential steps from 0 to 380 mv were taken at several concentration of glutathione and current versus time were recorded (figure 3). from the linear plots of i vs. t-1/2 (figure 3 inset), at various concentrations indicated in the figure, the mean diffusion coefficient of 3.0 ×10-6 cm2/s was calculated. scheme 1. electrocatalytic oxidation mechanism of glutathione at modified electrode figure 2. lsvs of efta/zfo/il/cpe in 0.1 m pbs (ph 7.0) with 100.0 μm glutathione at different rates of scans. values 1 to 6 respectively is corresponding to 5, 10, 30, 50, 75 and 100 mv s-1. inset: changes in the anodic peak currents against ν1/2 h. beitollahi et al. j. electrochem. sci. eng. 12(1) (2022) 209-217 http://dx.doi.org/10.5599/jese.1230 213 figure 3. chronoamperograms gained at efta/zfo/il/cpe in 0.1 m pbs (ph 7.0) at various levels of glutathione concentration. 1 to 5 is corresponding to 0.1, 0.55, 0.75, 0.9, and 1.2 mm of glutathione. insets: up plots of i versus t-1/2 achieved by chronoamperegrams 1–5; down plot of the straight line slop vs. glutathione concentrations electroanalysis of glutathione the peak current of glutathione using efta/zfo/il/cpe was utilized for quantitative analysis of glutathione. since dpv has benefits in terms of the greater sensitivity and better informative application features, the adjusted electrode has been employed as the working electrodes in analysing dpv. regarding the dpv of glutathione using the efta/zfo/il/cpe, linear response was observed in a range from 2.0×10-7 3.0×10-4 m with the correlation coefficient of 0.9991 (figure 4). in addition, the related limit of detection of 0.07 µm has been obtained. table 1. presents a comparison of the analytical figures of merit of the proposed work with other modified electrodes for the detection of glutathione. figure 4. dpvs of efta/zfo/il/cpe in 0.1 m (ph 7.0) with various levels of glutathione concentration. 1 to 8 is corresponding to 0.2, 5.0, 15.0, 40.0, 70.0, 100.0, 200.0 and 300.0 µm of glutathione. inset: dpvs of efta/zfo/il/cpe in 0.1 m (ph 7.0) with distinct levels of glutathione concentration http://dx.doi.org/10.5599/jese.1230 j. electrochem. sci. eng. 12(1) (2022) 209-217 glutathione detection at carbon paste electrode 214 table 1. comparison of the efficiency of some modified carbon paste electrodes used in the electro-oxidation of glutathione, used method voltammetry modifier lod, µm ldr, µm ref. 2cbfaz 0.02 0.05-200.0 [17] ferrocene 2.1 2.2-35.00 [58] 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https://doi.org/10.1016/j.cca.2006.03.006 https://creativecommons.org/licenses/by/4.0/) {fabrication of superhydrophobic surfaces by laser surface texturing and autoxidation:} http://dx.doi.org/10.5599/jese.1260 639 j. electrochem. sci. eng. 12(4) (2022) 639-649; http://dx.doi.org/10.5599/jese.1260 1 2 open access : : issn 1847-9286 3 www.jese-online.org 4 original scientific paper 5 fabrication of superhydrophobic surfaces by laser surface 6 texturing and autoxidation 7 vijay kumar, rajeev verma and harish kumar bairwa 8 industrial and production engineering department, dr b r ambedkar national institute of 9 technology jalandhar, punjab, india-144011 10 corresponding author: vknitj94@gmail.com 11 received: january 7, 2022; accepted: march 11, 2022; published: may 4, 2022 12 13 abstract 14 the creation of superhydrophobic surfaces (shs) has received exceptional thought from the 15 entire research community due to its notable application in varied fields such as anti-icing, 16 self-cleaning, drag reduction, anti-bacterial, and oil-water separation. the superhy-17 drophobic (sh) conditions for a surface can be attained through the consolidation of a low 18 surface energy surface with appropriate micro/nano-surface roughness through texturing. 19 motivated by the sh nature of lotus leaf and petal effect, microstructures have been 20 prepared in this work on a metal surface by a fiber laser marking machine at 35 w. the 21 textured surfaces with a different pitch to diameter (p/d) ratio (2.0-0.70) have been turned 22 into hydrophobic and finally sh, after storing in an ambient environment for a few days due 23 to oxide layer deposition on the textured surface. in this study, the maximum contact angle 24 achieved by textured geometry after 30 days of auto-oxidation was 158.6 o. further, test 25 results showed that the fabricated surfaces have a high potential to maintain their sh 26 nature even after the harsh condition of applications. 27 keywords 28 antibacterial; oxide layer deposition; texturing; micro/nano-structure; self-cleaning 29 30 introduction 31 recently, the transformation in the wettability condition of metallic surfaces from hydrophilic to 32 superhydrophobic (sh) has received considerable attention from researchers due to its numerous 33 applications such as an anti-bacterial layer, anti-frosting, self-cleaning, and drag-reducing over-34 lay [1]. the micro/nanostructures of insect wings and plants leaf have been saved as biomedical 35 bodies for researchers to create such surfaces on metallic/alloy surfaces primarily to reduce 36 maintenance costs. they show unique wetting behaviours such as sh and oil-water separation 37 properties. the wettability behaviour of shs is likely to be influenced by the micro/nano dual 38 structure as well as the surface chemistry of the substrate surface [2]. 39 http://dx.doi.org/10.5599/jese.1260 http://dx.doi.org/10.5599/jese.1260 http://www.jese-online.org/ mailto:vknitj94@gmail.com j. electrochem. sci. eng. 12(4) (2022) 639-649 fabrication of superhydrophobic surfaces 640 the surface wettability can be categorized into three categories: hydrophilic, hydrophobic, and 40 sh properties, which depend on the water contact angle (wca) of the surface [3]. that is, the surface 41 with wca less than 90o is a hydrophilic surface, wca lies between 90 to 150o for a hydrophobic 42 surface, whereas wca greater than 150o is referred to as shs. the above states of the surface are 43 highly influenced by the surface interface energy of water droplets and solid surfaces. the 44 wettability of surfaces has been studied by three states: the wenzel state, the cassie-baxter (cb) 45 state, and the metastable state [4,5]. the wenzel model considers water droplets seeping between 46 surface asperities, while the cb model discusses water droplets settling onto irregularities and 47 trapped air in the rough surface irregularities [6]. the schematic diagram of the above three 48 categories is shown in figure 1. among the above three infiltration states, the cb state is more 49 appropriate for the investigation of the shs [7,8] since it is valid for rough and chemically 50 homogeneous surfaces and to air entrapment inside the irregularities of rough surfaces. 51 52 figure 1. infiltration states in wenzel, cb and metastable states 53 shs has the potential application to reduce corrosion behavior and is advantageous in self-54 cleaning and offers antiseptic environments [9]. there are two essential aspects to enhancing the 55 sh behaviour of a surface that is low surface energy treatment and the creation of micro/nano-56 hierarchical structure. to improve the intrinsic sh behaviour of the surface, laser surface texturing 57 has been a relatively new technique for creating a dual structure on metallic surfaces, which is 58 responsible for the making surface non-sticky. so far, to produce such micro/nano-dual scale 59 structures on a surface, a great effort has been made lately by researchers and this has been a 60 motivation to discover a newer technique, which is easy and eco-friendly [10]. previously, several 61 techniques have been proposed and investigated to achieve high wca and low ca hysteresis, such 62 as chemical etching [11], plasma etching [12], shot blasting [13], machining [14], lithographic 63 patterning [15], sol-gel technique [16], electrodeposition [17], and plasma fluorination technique 64 [18]. however, the previously reported techniques have some disadvantages that limit their 65 application, for example, the intricate processing steps and low durability. to create shs, some 66 chemical modifications such as coating of fluorosilane and fluoropolymer are used as the top layer, 67 which significantly improves the hydrophobic behavior of the surface [19]. however, these 68 fluorinated coatings are harmful to the environment and also may be easily peeled off from the 69 substrate interface due to poor mechanical interlocking and thermal stability. to overcome the 70 above shortcomings, laser surface texturing has emerged as one of the best techniques to produce 71 a hierarchical structure with well-defined surface roughness as a requisite favorable condition for 72 attaining sh behavior. laser texturing produces required surface roughness and spontaneous 73 hydrophobization and carbon absorption onto the textured surfaces with a multimodal roughness 74 that looks passive, environmentally friendly, and naturally reproducible [20]. recently, ma et al. 75 fabricated shs by creating a circular texturing pattern of different p/d ratios, which were highly 76 durable [21]. moreover, creating an shs by optimizing p/d ratios for different materials became an 77 interesting area for researchers due to its high durability and eco-friendly nature. 78 wenzel state cb state metastable state v. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 639-649 http://dx.doi.org/10.5599/jese.1260 641 very little literature has been reported on the fabrication of shs by direct laser treatment on 79 steel surfaces. the fabrication of superhydrophobic or hydrophobic surfaces by direct laser texturing 80 (dlt) eliminates the use of any hazardous chemical treatment. the creation of shs by dlt and 81 deposition of oxide layer due to heat-treatment during texturing (autoxidation) is highly responsible 82 for better durability and maintaining their behavior against harsh environmental conditions. 83 in this work, the fiber laser marking machine of 50w was employed to create a micro/nano-84 hierarchical structure on aisi 420 steel substrate. to attain a stable sh condition, the textured 85 surfaces of different p/d ratios such as 2.0, 1.0, 0.95, 0.90, 0.80, and 0.70 were left in an open 86 environment for a few days. the surface characterization through scanning electron microscopy 87 analysis, tape peel test, and sandpaper abrasion test were employed to evaluate the soundness of 88 the developed shs. 89 experimental 90 surface preparation 91 the aisi 420 steel plate was procured from the local market of jalandhar and its chemical 92 composition was evaluated by spectroscopic analysis at the central institute of hand tools, 93 jalandhar, punjab, india (table 1). to perform micro-texturing on the aisi 420 steel sheet was 94 sectioned to the size of 40405 mm. subsequently, the sectioned surface was polished with 95 abrasive sandpaper of p200, p400, p800, p1200, and p1500 followed by washing with ethanol and 96 deionized water. before laser texturing, all samples were dried in an electric oven at 80 oc for 30 97 minutes to remove moisture from the surfaces. 98 table 1. spectroscopic analysis of aisi 420 steel 99 element c cr mn si p s fe content, wt.% 0.13 12.60 0.85 0.73 0.03 0.05 balance laser surface texturing 100 the fiber laser marking machine (figure 2) with a 50 w laser source was applied to micro-texture 101 the circular pattern of diameter 100 μm and pitch varying from 200 to 70 μm as tabulated in table 2. 102 103 figure 2. laser texturing machine and its texturing parameters (ezcad software) 104 http://dx.doi.org/10.5599/jese.1260 j. electrochem. sci. eng. 12(4) (2022) 639-649 fabrication of superhydrophobic surfaces 642 the scanning speed used was 50 mm/s, a frequency of 50 khz, and a maximum power of 35 w 105 with a laser wavelength of 1064 nm. the same parameters were repeated three times for every 106 sample to achieve the surface roughness (ra) between 8 and 14 μm, as seen in table 2. from table 2, 107 it has been observed that ra values of the textured geometry increase with a decrease in p/d ratio, 108 which could be ascribed to the decrease in the flat area of the textured geometry. with the increase 109 of the p/d ratio, the untextured surface area (outside the cylinder) increases, and the mean surface 110 roughness decreases. 111 table 2. parameters of texturing pattern and its surface roughness values, d = 100 m 112 sample no. p / μm p /d ra / μm 1 200 2 9.10 2 100 1.00 9.15 3 95 0.95 9.79 4 90 0.90 10.23 5 80 0.80 11.02 6 70 0.70 11.29 surface characterization 113 the surface morphology of the textured surface was characterized using scanning electron 114 microscopy (sem). the wettability and durability of the texture geometry were investigated by 115 assessing the mechanical resistance by a small-scale laboratory testing, such as tape peeling, 116 abrasive paper abrasion, and contact angle hysteresis of water droplets deposited on the textured 117 surface. the droplet contact angle is defined as the angle at which the water-air interface intersects 118 the surface, which characterizes the wettability tendency of liquid droplets. the roll-off angle 119 represents the slope of a surface from that a drop starts rolling and eventually falls. wettability 120 conditions of superhydrophobicity are usually accepted when the contact angle is greater than 150o 121 and also the roll angle is lower than 10o [22]. the wca value has been measured for each experiment 122 and co-related with the sem topography and the degree of hydrophobicity. we used a technique 123 like a sessile contact angle approach and for this purpose, an image was captured of liquid droplets 124 on the textured surface using a high-resolution (micro-lens) camera. then the image processing was 125 carried out to assess the contact perspective as described above. depending on the degree of rolling, 126 we incrementally tilted the samples to determine the angle when the drop started rolling off from 127 the surface. for this purpose, a drop of water of 10 μl volume was released from a micro-syringe 128 from a height of 10 mm, and the drop image was captured and analyzed using image analysis 129 software under ambient conditions (at 5-days interval). 130 mechanical testing 131 tape peel test 132 generally, a tape peeling test is carried out to investigate the behavior of superhydrophobic 133 surfaces after being glued to other surfaces, which indicates the loss of a degree of wettability of 134 the sh surface. for the tape-peeling test, the highly adhesive and pressure-sensitive tape is used for 135 uniformity of adhesion. in this work, cellofix test tapes were used for uniformly adhering to the 136 textured surface with the application of an external load of 80 g rolling onto the entire surface for 137 proper adherence with the sh surface. the glued tape was later lifted at an angle of 180o to the 138 surface [8,10,23]. the wettability of the surfaces was measured after every 10 repetitions of the 139 test. this test was measured until the sh surface showed very low wettability loss. 140 v. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 639-649 http://dx.doi.org/10.5599/jese.1260 643 sandpaper abrasion test 141 a sandpaper abrasion test was carried out to evaluate the durability of the sh surfaces against 142 the damage from abrasive particles, which shows the wettability changes after a number of abrasion 143 cycles. in this test, the sandpaper of grit size p800 was put on the sh surface to a length of 40 mm 144 facing abrasive particles towards the sh surface. applied 100 g external load on top of the sandpaper 145 and sandpaper was pulled horizontally with a constant velocity of approximately 10 mm/s. this test 146 was repeated for 50 cycles and the wca of abraded surfaces was measured at intervals of 10 147 abrasion cycles [24-26]. 148 results and discussion 149 in the present experiment, we have successfully produced metal textured surfaces with uniform 150 sh properties using laser texturing (figure 3). 151 152 figure 3. water droplets exhibiting uniform hydrophobic and sh properties of textured surfaces 153 after 19 days of exposure to the open environment 154 influence of laser texturing on surface morphology 155 it has been well established that the morphology of the surface and the roughness have a strong 156 influence on surface wettability. in general, it has been reported in the literature that two states, 157 namely wenzel and cassie-baxter (cb), are used to explain the liquid-solid interaction: (1) the liquid 158 fills the valleys of a rough solid surface known as the wenzel state, and (2) air entering the valleys 159 of the rough solid surface called cb state. when processing line by line (two laser beams separated 160 by distance) during laser texturing, the laser beams move and remove material (ablation) from the 161 surface by melting and evaporating the metal, creating micro-patterned structures. to produce 162 metallic surfaces with sh properties, the surface must become one of three structures: 163 nanostructures, microstructures, or dual structures [27]. in this work, the circular-shaped texturing 164 with different p/d ratios (2.0-0.7) was fabricated on steel samples by a fibre laser texturing machine. 165 during pilot experimentation, sh behaviour on the samples was performed to optimize the p/d 166 ratio. from the study, it was found that textured geometry with a p/d ratio of 0.7 resulted in the 167 maximum wca, demonstrating the most encouraging results, whereas textured geometry with a 168 p/d ratio 2 and more did not show many promising wca results due to more flat surfaces present 169 on top of the sample. based on this preliminary study, our further investigation was mainly focused 170 on textured geometry with a p/d ratio of 0.7, and a sample of texturing with a p/d ratio of 2 was 171 discarded from the further investigation. 172 a distribution of multi-model and regular pattern geometry has been produced after laser 173 texturing, as shown in the sem image of the textured substrates at different magnification levels. 174 the laser-processed substrate surface produced a hierarchical structure, a nanostructure in micro-175 protrusion pores and depressions, due to the evaporation and inherent laser bulging effect. 176 http://dx.doi.org/10.5599/jese.1260 j. electrochem. sci. eng. 12(4) (2022) 639-649 fabrication of superhydrophobic surfaces 644 figure 4 shows the sem image of the texture pattern structure with a p/d ratio of 0.7 at 177 magnification levels of 200 and 500, respectively. the sem images show the formulation of a coral-178 like surface structured with mushroom-like nano-protrusions on micro-cavities arranged in random 179 alignments. these uniform/deterministic micro-cavities-oriented structures consisted of tiny cavities 180 with dimensions in the micron to nano range that created an ultrafine porous structure [28]. further 181 studies show that these structures were reproducible and well controllable by adjusting the laser 182 parameters. after the laser ablation process, the wca on all textured geometries was measured and 183 left at ambient conditions to further study the wettability behaviour with time. further investigation 184 of wca ensured that the surface wettability of laser-ablated surfaces increases with time. 185 186 figure 4. sem images of a textured surface with a p/d ratio of 0.7 at different magnification levels 187 effect of auto-oxidation on wettability 188 the topography of the textured surface depends on the laser power and the intensity of the beam. 189 it is well known that spatial depth and heat zone increase with increasing laser intensity. as the laser 190 intensity doubles, the heat-affected region lengthens three times. the result is that the texturing 191 pattern's size increased and a distorted type of heat-affected zone was created [2,29]. one advantage 192 of pattern distortion is that it creates a nano protrusion and a high heat-affected zone is highly 193 responsible for the oxide formation and carbon abstractions from the environment and surface 194 dissolution. after laser texturing on the specimens, wca was measured on the textured patterns and 195 all specimens were subjected to exposure to an open environment for further wettability studies time 196 the contact angle was measured at time intervals of five days, as shown in table 3. 197 table 3. wettability measurement on the laser texture aisi 420 steel 198 time interval. day p / d 0.7 0.8 0.9 0.95 1.0 wca, o 1 97.90 85.36 77.23 70.9 70.2 5 125..42 101.21 92.32 88.12 86.3 10 131.48 115.23 105.41 100.32 98.25 15 145.49 129.3 121.23 114 101 19 158.59 135.23 123 114.45 101 199 after 15 days, it was observed as 145.45o and after 19 days, the fabricated surface showed sh 200 behaviour with a contact angle of 158.6o in textured geometry with a p/d ratio of 0.7, whereas no 201 significant change in the contact angle was observed after 19 days, as shown in figure 6. the 202 v. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 639-649 http://dx.doi.org/10.5599/jese.1260 645 variation in wettability over time is due to a change in surface chemistry and a key factor could be 203 ascribed to the accumulation of carbon to the textured substrate with active magnetite, as shown 204 in table 4 [30]. this accumulation of carbon particles from the air on the textured geometry and air 205 entrapment inside the texturing pattern creates low surface energy on the surfaces. as a result, 206 when a higher energy liquid is dropped on this surface, it is repelled from the surface and tries to 207 attain a spherical shape due to the dominant molecular cohesive forces among the water droplets. 208 further, it was observed, that after a small surface inclination, the water droplets roll off easily from 209 the surfaces. the eds analysis of textured geometry with a p/d ratio of 0.7 (figure 5) after 19 days 210 shows an enormous chemical composition alteration of the surface, as shown in table 4. the higher 211 chemical composition alteration especially carbon and oxygen, may be caused due to surface 212 burring due to ablation during laser texturing and oxidation due to heat treatment during texturing 213 followed by auto-oxidation. from table 4, trends of increasing percentage composition of lower 214 melting points elements have been observed after laser texturing. this may be due to the lower 215 melting points element diffusing out to the surface by heat treatment during texturing due to high 216 laser fluence. the high increment of carbon composition and introduction of oxide formation on the 217 textured geometry could be accounted for by the creation of low surface energy. 218 219 figure 5. energy-dispersive x-ray spectroscopy (eds) on the textured surface table 4. chemical composition of texture surface before and after texturing before texturing after texturing element content, wt. % element content, wt. % c 0.13 c 6.35 p 0.035 p 0.33 s 0.026 s 0.44 si 0.59 si 4.58 cr 13.68 cr 9.67 fe 84.56 fe 59.61 o 17.47 ca 1.55 total 100.00 100.00 micrographs of the shs were examined by sem and eds analysis at 10 kv at the nittr in 220 chandigarh, india, to examine the surface morphology and chemical composition of the surfaces. 221 the ability of water repellence increases with decreasing p/d value of the laser-ablated surface due 222 to increased surface roughness and the same behaviour for the other values of the p/d ratio was 223 observed. there was no significant change in the contact angles of the irradiated surface with time 224 after a stable equilibrium was reached. the wca of water droplets was calculated on laser irradiated 225 surfaces (p/d = 0.7, 0.8, 0.90, 0.95, and 1.0) ablated with the same laser fluence and left at ambient 226 conditions for 19 days. it could be acknowledged from the results obtained that the pulse 227 superposition implemented in the ablation process could play an essential role in the surface 228 topography produced by the laser texturing process. as the p/d ratio of the circular texture pattern 229 decreased from 1.0 to 0.7, the surface roughness (9.15 μm to 11.29 μm) also increased as shown in 230 table 2. the increase of ra value with dual structure is highly responsible for the improvement in 231 hydrophilic to sh behaviour. further, with the increase of the p/d ratio from 0.7 to 2.0, the surface 232 microstructure becomes very flat and there are not many nano-scale multi-modal structures present 233 on the surface to offer hydrophobicity. the decrease in cas and the high adhesive force of these 234 surfaces can be explained according to the wenzel model. as illustrated in figure 6, the lower wca 235 http://dx.doi.org/10.5599/jese.1260 j. electrochem. sci. eng. 12(4) (2022) 639-649 fabrication of superhydrophobic surfaces 646 on textured geometry except/compared to p/d ratio 0.7 was due to the water droplet could get into 236 the groove of the rough solid surface partially or completely. 237 238 figure 6. wca of textured aisi 420 steel specimens with different p/d ratios during 20 days interval 239 mechanical test 240 to determine the functional working efficiency of the fabricated shs, mechanical reliability is also 241 a critical parameter. in general, the shs artificially produced by chemical methods are relative of 242 low durability and can lose their ability to repel water through scratching or abrasion, causing 243 excessive damage to the surface chemistry and structure [24]. small scale laboratory tests prevalent 244 to ascertain the durability of shs were conducted, namely sandpaper abrasion tests, tape peel tests, 245 and water repellency over time. in this study, sh surfaces show marginal loss of wettability after 50 246 test repetitions. it was observed that samples with a p/d ratio of 0.7 had exhibited some decreasing 247 trend of wca during the first 40 test cycles of the tape peel test and a similar diminishing trend was 248 observed in the sandpaper abrasion test. a very small wettability change of the surface had been 249 observed in the first 10 cycles of the abrasive paper scratch test. whiles, a sharp drop in wca 250 (figure 7) from 157.40 to 144.30 was observed between 10 to 50 cycles as the structure of the 251 textured substrate was damaged. the decreasing trends of wca were observed after increasing the 252 number of testing cycles because when the number of cycles increases, the nano-protrusion over 253 micro-structures gets damaged after every repetition. however, the surface remains hydrophobic 254 after 50 cycles of test and it marginally lost its ability to repel water. 255 the promising results of the study show that these low water adhesion shs can be used in many 256 engineering fields, such as the ‘mechanical hand’ to transfer small water droplets without any loss 257 or contamination for micro sample analysis. these shs can also be used to store any type of liquid 258 solution in small volumes where no loss is required. moreover, such surfaces can also be used to 259 transfer liquid droplets from low adhesion surface to high adhesion surface without any loss. 260 70 80 90 100 110 120 130 140 150 160 1 5 10 15 20 c o n ta ct a n g le , time interval, day 0.7 0.8 0.9 0.95 1.0 ra = 11.29 ra = 11.02 ra = 10.23 ra = 9.79 ra = 9.15 p/d: v. kumar et al. j. electrochem. sci. eng. 12(4) (2022) 639-649 http://dx.doi.org/10.5599/jese.1260 647 261 figure 7. wca measurement on the surface after mechanical testing (p/d = 0.70) 262 conclusion 263 it has been shown that a compact and relatively inexpensive fiber laser texturing machine can be 264 used to produce shs on aisi 420 steel substrates that exhibit hydrophobic properties immediately 265 after direct laser texturing under atmospheric conditions. the wettability of the ablated surface 266 enhances with time and the surfaces become sh after 19 days of autoxidation. to evaluate the 267 mechanical stability of the developed shs, two different test methods were performed, the first 268 being the sandpaper abrasion test and the second being the tape-peel adhesion test. following are 269 the main finding of the study: 270 • sem and eds analysis showed that laser texturing was mainly responsible for the changes in 271 surface morphology and scatting surface chemistry. the results showed that both the surface 272 chemistry and the 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liu, h. gao, s. li, z. han, l. ren, chemical engineering journal 337 (2018) 697-708. 345 https://doi.org/10.1016/j.cej.2017.12.139 346 347 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1260 https://doi.org/10.3390/app10082678 https://doi.org/10.1088/2051-672x/ac4321 https://doi.org/10.1016/j.cej.2017.12.139 https://creativecommons.org/licenses/by/4.0/) @article{kumar2022a, author = {kumar, vijay and verma, rajeev and bairwa, harish kumar}, journal = {journal of electrochemical science and engineering}, title = {{fabrication of superhydrophobic surfaces by laser surface texturing and autoxidation:}}, year = {2022}, issn = {1847-9286}, month = {may}, number = {4}, pages = {639--649}, volume = {12}, abstract = {the creation of superhydrophobic surfaces (shs) has received exceptional thought from the entire research community due to its notable application in varied fields such as anti-icing, self-cleaning, drag reduction, anti-bacterial, and oil-water separation. the super­hy­drophobic (sh) conditions for a surface can be attained through the consolidation of a low surface energy surface with appropriate micro/nano-surface roughness through texturing. motivated by the sh nature of lotus leaf and petal effect, microstructures have been prepared in this work on a metal surface by a fiber laser marking machine at 35 w. the textured surfaces with a different pitch to diameter (p/d) ratio (2.0-0.70) have been turned into hydrophobic and finally sh, after storing in an ambient environment for a few days due to oxide layer deposition on the textured surface. in this study, the maximum contact angle achieved by textured geometry after 30 days of auto-oxidation was 158.6 o. further, test results showed that the fabricated surfaces have a high potential to maintain their sh nature even after the harsh condition of applications.}, doi = {10.5599/jese.1260}, file = {:d\:/onedrive/mendeley desktop/kumar, verma, bairwa 2022 fabrication of superhydrophobic surfaces by laser surface texturing and autoxidation.pdf:pdf;:05_jese_1260.pdf:pdf}, keywords = {anti, bacterial, cleaning, micro/nano, oxide layer deposition, self, structure, texturing}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1260}, } effective numbers of electrons as a criterion of carbon suitability as a hemosorbent doi: 10.5599/jese.2011.0005 27 j. electrochem. sci. eng. 1(1) (2011) 27-37; doi: 10.5599/jese.2011.0005 open access : : issn 1847-9286 www.jese-online.org original scientific paper effective numbers of electrons as a criterion of carbon suitability as a hemosorbent mogely khubutiya, boris grafov*, mikhail goldin**, alexei davydov*, vladimir kolesnikov*** and mark goldin n.v. sklifosovsky research institute of emergency medicine, sukharevskaya pl. 3, moscow, 129010, russia *a. n. frumkin institute of physical chemistry and electrochemistry, russian academy of sciences, leninskii pr. 31, moscow, 119991, russia **liberty university, 1971 university blvd., lynchburg, va 24502-2213, usa ***d.i. mendeleev university of chemical technology of russia, miusskaya pl. 9, moscow, 125047, russia corresponding author: e-mail: mgoldin@liberty.edu; tel.: +1-434-582-2210; fax: +1-434-582-2488 received: may 20, 2011; revised: august 01, 2011; published: august 20, 2011 abstract the number of medical applications of electrochemistry has grown in recent years due to the increased applications of electrochemical concepts to various systems of the organism. this includes electrochemically controlled hemosorption detoxification, where the removal of toxicants is controlled by changing the potential of the hemosorbent. it is important to avoid faradaic processes in the course of hemosorption, which can lead to the addition of electrochemically modified toxicants to blood. the probability of their occurrence should depend on the open-circuit potential of the activated carbon. in order to elucidate the identification of faradaic reactions, a model system was investigated. the adsorption of copper ions, tert-butanol and acetone on samples of electrochemically modified ag-3 activated carbon hemosorbent with various open-circuit potentials was used as a model of the detoxification processes. the effective number of electrons transferred in the elementary act of adsorption was shown to be a non-zero quantity for all three cases, which corroborates an electrochemical mechanism of hemosorption. for the cupric ions, ranges of open-circuit potentials were identified corresponding to different mechanisms of adsorption and faradaic processes. keywords activated carbon; thermally expanded graphite; hemosorption; open-circuit potential; effective number of electrons j. electrochem. sci. eng. 1(1) (2011) 27-37 criterion of carbon suitability 28 introduction a continual increase of the number of applications of electrochemistry in modern medical technologies is primarily due to the broader use made of theoretical electrochemical concepts in the functioning of various systems of the human organism. of note here are the works of sawyer et al. [1-2] on electrochemically induced intravascular thrombosis (blood clotting) by introducing anodically polarized metal conductors into damaged blood vessels in order to attract negatively charged blood components to the electrode and stop the hemorrhage. these investigations were continued by guglielmi, et al. and applied for the treatment of aneurisms [3-6]. at the present time, one of the leading applications of electrochemical concepts in medicine is the ideas of nordenström and his model of homeostasis as a biologically closed electric circuit [7], in which blood flowing through the blood vessels is treated as a conductor of electrical signals. vascular walls and other tissues possess electrochemical potentials whose values depend on the state of the tissue (i.e., normal, inflamed, damaged, etc.). potentials measured with platinum electrodes implanted into the tissue can serve as a source of information about the presence or absence of certain pathological states in the vicinity of the implanted electrode. based on the above model, nordenström proposed and realized an electrochemical method of treatment for malignant carcinomas by passing dc current through the affected areas of tissue in order to impose certain potentials upon it [8,9]. another novel medical treatment also has electrochemical basis, viz., detoxification by means of electrochemically controlled hemosorption [10,11]. in this electrochemical model, the hemosorbent was considered to be a porous electrode immersed in aqueous 0.15 m sodium chloride solution. based on such an approach, an important conclusion was reached as to the mechanism of interaction between the negatively charged formed elements of blood and the activated carbon hemosorbent. the sign and magnitude of the charge of the hemosorbent surface is determined by its potential. the latter, in turn, depends on the nature of the activated carbon material, since the composition of the electrolyte (blood) remains virtually unchanged. the electrochemical model of hemosorption has made it possible to find a new approach to identifying new promising materials suitable as potential hemosorbents. this is important, since the requirements towards “good” hemosorbents are quite rigorous and somewhat selfcontradictory: on the one hand, such a material must be indifferent towards blood, while on the other it should maintain high adsorption activity towards the toxicants being removed from blood. by treating the hemosorbent/blood interface as an electrochemical system, authors [10,11] were able to assert the possibility of controlling the removal of toxicants from blood via polarization of the hemosorbent (which is usually a porous carbon material). an important aspect of this, however, is the mechanism of interaction between the carbon material and the electrolyte (blood), since adsorption on activated carbon may include the occurrence of faradaic processes. such processes may occur in cases where the potential of the hemosorbent in blood is comparable to the reduction or oxidation potential of the toxicant. it is well known that faradaic processes (especially electrooxidation) can lead to highly undesirable effects in blood. for example, anodic oxidation of proteins leads to their denaturation [12]. at the same time, frumkin showed [13] that the potential of an electrode depends on the adsorption of organic and inorganic substances on it. unfortunately, electrochemical data are rarely given or utilized in carbon adsorption research. for instance, authors of [14] considered the properties of the carbon/electrolyte interface when certain cations are adsorbed; however, while the authors [14] recounted frumkin’s conception of mogely khubutiya et al. j. electrochem. sci. eng. 1(1) (2011) 27-37 doi: 10.5599/jese.2011.0005 29 activated carbon as an oxygen electrode, they considered chromene-like groups that generate resonance-stabilized carbonium ions to be the initiators of adsorption activity of carbons [15]. thus, consideration of carbon potential may help elucidate the mechanism of adsorption on carbons and reveal the presence or absence of faradaic processes during it. for this purpose the mathematical expressions given in [16] can be used, which are based on frumkin’s conception of activated carbon as a perfectly polarizable electrode, i.e. an electrode whose state is fully dependent on the amount of electrical charge consumed [13]. the total charge q of a perfectly polarizable electrode is a function of both electrode potential e and the amount г of adsorbed electrochemically indifferent surface-active substance. since the measurements are being made in an open circuit, an assumption can be made that the total charge of the carbon q is constant q = q(e,г) = const (1) and therefore its total differential is equal to zero: γ d d dγ 0 γ e q q q e e ∂ ∂   = + =   ∂ ∂    (2) in line with [16,17], (∂q/∂e)г = с∞, i.e. the capacitance of the electrode at a sufficiently high frequency; and –(∂q/∂г)e = nf. therefore: с∞ de – nf dг = 0 which can be rearranged as с∞ de = nf dг isolating n, one would obtain: d dγ c e n f ∞= (3) finally, if the assumption is made that the ratio de/dг ≈ ∆e/∆г, i.e., replacing the first derivative of potential with respect to adsorption by the finite ratio of changes in e and г, the following expression is obtained, from which the effective number of electrons can be estimated: n = (с∝∙∆e)/(f∙∆г), (4) where n – effective number of electrons needed for the elementary act of adsorption and/or faradaic process, с∞ (f) – differential capacitance of the adsorbent, ∆e / v – ocp shift corresponding to a relatively short period of time t, f (= 96500 c mol-1) – faraday’s constant, ∆г / mol – change in the adsorption (gibbs’ surface excess) of the adsorbate during time t. equation (3) can be used for computations of the effective number of electrons. thus, the empirical data needed to calculate the value of n are the open-circuit potentials of activated carbon and corresponding values of adsorption. the value of differential capacitance с∞ = 100 f was used for 1 g of activated carbon according to the data in [18]. an approximation was made that the capacitance с∞ of the adsorbent is weakly dependent on the electrode potential and gibbs’ adsorption of the adsorbate. with the above considerations, the effective number of electrons transferred in the elementary act of adsorption process or faradaic process was chosen as the criterion of suitability of carbon materials for use as hemosorbents: viz., the virtual absence of faradaic processes points to the suitability of such hemosorbents. the adsorption of certain undissociated organic toxicants and j. electrochem. sci. eng. 1(1) (2011) 27-37 criterion of carbon suitability 30 cupric ions was chosen for a model-based investigation of the applicability of the concept of effective charge transfer for hemosorption. experimental an ipc-pro l potentiostat (volta co. ltd, russia) was used for measurements of polarization curves; it was also utilized for ocp measurements. a saturated silver/silver chloride electrode was used as the reference electrode in all measurements. adsorption of acetone and 2-methyl-2propanol (tert-butanol, tb) was investigated using gas chromatography. an sri 310c chromatograph (sri instruments, usa) with a 1 m hayesep-d packed column and a catalytic combustion detector, as well as a shimadzu gc17 chromatograph with a carbowax 20m phase, were used. 0.150 m nacl or 0.200 m na2so4 solutions were used as a supporting electrolyte in adsorption and potential measurements of the organic compounds and in measurements of the initial carbon potentials. adsorption of organic compounds and cupric ions was investigated on ag-3 activated carbon; the ocp value of the initial sample measured in 0.200 m na2so4 was 50 mv (vs. ag/agcl). modification of the ag-3 carbon was carried out according to [10,11] in the cell shown in fig. 1. the granules of initial activated carbon were placed in the anode (1) and cathode (2) chambers separated by perforated discs made from a dielectric material (8), with the granules subsequently compressed by the current-carrying discs (6) by turning the bolts (7). a peristaltic pump was used to pass an aqueous sodium chloride solution through the assembled cell at a flow rate of 50 ml/min. carbon was modified galvanostatically with a current of 0.75 a for 10 to 90 min, with subsequent washing of the cathodically and anodically modified carbons to a ph = 7. as a result of the modification, carbons with ocp values in the range between +475 mv and –775 mv (vs. ag/agcl) were obtained and used for further experiments. figure 1. flow-through cell for the electrochemical modification of activated carbon: 1 – anodic chamber, 2 – cathodic chamber, 3 – electrolyte inlet, 4 – electrolyte outlet, 5 – alternative inlet for solution or reference electrode, 6 – stainless steel perforated current-carrying disc, 7 – stainless steel bolt, 8 – perforated polymer disc, 9 – housing, 10 – covers, 11 – screw, 12 – rubber gasket. mogely khubutiya et al. j. electrochem. sci. eng. 1(1) (2011) 27-37 doi: 10.5599/jese.2011.0005 31 the amount of copper adsorbed on activated carbon was determined spectrometrically by measuring the concentration change of cu2+ ions in aqueous 0.100 m cuso4 solution on scanning uv/visible spectrophotometers genesys 10uv (thermo scientific, usa) at λmax = 808 nm in the form of copper sulfate and beckman-coulter du 800 at λmax = 607 nm in the form of an amino complex. carbon ocp was recorded simultaneously with solution sampling. contact time of the activated carbon with the solution was 30 min. figure 2. non-flow-through cell for measurement of granulated carbon ocp: 1 – clamp cover, 2 – housing,3 – cylindrical chamber, 4,5 – teflon bolts and nuts, 6,7 – current-carrying stainless steel meshes. carbon ocp was measured in a non-flow-through apparatus described in [19], shown in fig. 2. this apparatus was immersed in solution with constant stirring at 100 rpm using a magnetic stirrer. carbon granules were placed in the cylindrical chamber (3) of the apparatus and compressed against the stainless steel meshes (6,7) by the cover (1). the current-carrying meshes and the reference electrode were connected to the potentiostat, and the time dependence of ocp values was recorded. since the surface area of the carbon is 1000 m2/g, while the surface area of the steel mesh is on the order of 10–4 – 10–5 m2, the potential measured is that of the carbon and not of the current-carrying steel. cyclic voltammetry was used to obtain a potentiodynamic charging curve for computation of differential capacitance of ag-3 activated carbon. a single granule of ag-3 carbon was placed in a three-electrode cell. thermally expanded graphite was used as the current collector and the auxiliary electrode. the cyclic voltammogram was collected at a scan rate of 0.5 mv/s in the range between –800 mv and +600 mv. results and discussion as noted above, carbon modification in the present work was performed according to the method previously developed in [10,11]. it was shown in [11] that anodic and cathodic treatment of carbon led to significant changes in the composition of its surface functional groups (table 1). as seen from the data shown in table 1, cathodic treatment of activated carbon drastically reduced the amount of acidic functional groups on carbon surface, while anodic treatment increases their amount. this trend is observed regardless of the acidity of the electrolyte used for modification. thus, cathodic treatment results in the reduction of oxygenated surface functional groups, while anodic treatment leads to the oxidation of carbon atoms. ocp data for unmodified and electrochemically modified ag-3 carbon samples during contact with solutions of acetone (in 0.150 m nacl) and tb (in 0.20 м na2so4) were obtained as a time dependence, with corresponding concentration changes of the organic compounds in solution. a typical graph of ocp and adsorption, calculated from chromatographic data, is shown in fig. 3 (for tb on unmodified ag-3); similar dependences were obtained for all other samples. zero j. electrochem. sci. eng. 1(1) (2011) 27-37 criterion of carbon suitability 32 adsorption corresponds to the initial potential of ag-3 in the experiment in the investigated solution. table 1. influence of electrochemical modification of activated carbon on its surface functional groups [11]. type of treatment amount of functional group, mmol g-1 carboxyl lactone phenol initial untreated carbon 0.47 0.10 0.18 nacl solution (ph = 11.5) cathodic 0 0 0 anodic 0.41 0.30 0.32 nacl solution (ph = 1.4) cathodic 0.15 0.04 0.20 anodic 0.51 0.16 0.20 figure 3. dependence of the ocp of ag-3 carbon on tb adsorption from 0.20 м na2so4. initial tb concentration: 77 g/l. differential capacitance the differential capacitance of the ag-3 activated carbon was computed based on the potentiodynamic data. an 8.0 mg carbon sample was used, and potentials were scanned in the range between –700 mv and +500 mv at a scan rate of 0.5 mv/s. the resulting dependence of potential on differential capacitance is shown in fig. 4. a minimum at cmin = 78.7 f per gram and a maximum at cmax = 121 f per gram were observed. the average value of differential capacitance from the calculated results was cavg = 95.4 f per gram of carbon, which closely matches the literature data [18]. thus, the value of capacitance с∞ = 100 f per gram of carbon was used in all further computations. r² = 0.9958 140 160 180 200 220 240 260 280 300 -1,7 -1,6 -1,5 -1,4 -1,3 -1,2 -1,1 -1 -0,9 -0,8 e/ m v ( vs a g/ a gc l) log(γ/mol) mogely khubutiya et al. j. electrochem. sci. eng. 1(1) (2011) 27-37 doi: 10.5599/jese.2011.0005 33 figure 4. differential capacitance per gram of ag-3 activated carbon as a function of potential. adsorption of organic molecules based on the final ocp and adsorption values for unmodified and electrochemically modified ag-3 carbon samples, effective numbers of electrons were calculated for acetone and tb adsorption. the results are summarized in tables 2-3. it should be noted that these calculations reflect the integral character of effective charge transfer in the range from einitial to efinal. table 2. adsorption of tert-butanol on ag-3 carbon. potentials reported vs. ag/agcl. mcarbon/g einitial/mv efinal/mv ∆e/mv ∆г/mmol n 2.0157 -670 -590 +80 12 0.014 1.8550 -267 -8 +259 12 0.043 2.0461 0 +165 +165 11 0.046 1.9979 +35 +280 +245 9.7 0.046 1.9763 +85 +300 +215 9.9 0.035 2.1517 +280 +450 +170 13 0.034 1.6763 +380 +575 +195 11 0.018 1.6747 +425 +540 +115 10. 0.014 table 3. adsorption of acetone on ag-3 carbon. carbon mass m = 10.00 g. (potentials vs. ag/agcl). einitial/mv efinal/mv ∆e/mv ∆г/mmol n +440 +580 +140 72 0.020 +310 +460 +150 88 0.018 +190 +290 +100 95 0.011 +102 +190 +88 110 0.0084 -96 +45 +141 110 0.013 -144 -10 +134 98 0.014 0 20 40 60 80 100 120 140 -800 -600 -400 -200 0 200 400 600 c /f g -1 e/mv (vs. ag/agcl) experimental data moving average trend j. electrochem. sci. eng. 1(1) (2011) 27-37 criterion of carbon suitability 34 calculated values for the effective number of electrons in the course of adsorption of organic molecules on ag-3 carbon were quite small (0.0084 to 0.046). this may indicate that faradaic processes do not contribute significantly to the adsorption process of these substances. importantly, adsorption of all of these compounds on ag-3 carbon is accompanied by a positive shift in carbon ocp, which points to an electrochemical mechanism of adsorption (since changes in an electrochemical parameter are observed). adsorption of cupric ions similar adsorption experiments were performed on ag-3 carbon samples in aqueous cuso4. the most important results of these experiments are summarized in table 4. a typical time dependence of the ocp and concentration (for unmodified ag-3) is shown in fig. 5. fig. 6 shows the dependence of the “local” effective number of electrons (calculated between data points) on potential in the course of the experiment described in fig. 6. discrete values of (e, n) correspond to the elapsed time, as shown next to each point in the figure. it can be seen from fig. 6 that the process of adsorption reaches equilibrium after about 30 min, as the “local” effective number of electrons transferred reaches zero (within the precision of the experimental data). table 4. integral values of the effective numbers of electrons n in the course of cu2+ ion adsorption on ag-3 carbon with various initial potentials. carbon masses (mcarbon) as shown below. (potentials vs. ag/agcl) mcarbon/g einit/mv efinal/mv ∆e/mv г/mmol n 5.022 –775 40 815 2.1 2.07 4.990 –700 40 740 1.8 2.13 5.007 –580 40 620 1.5 2.15 5.001 –470 47 517 1.4 1.91 figure 5. time dependence of the ocp of unmodified ag-3 carbon and the adsorption of cupric ions from solution. 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 0,50 0 20 40 60 80 100 120 140 160 180 0 5 10 15 20 25 30 35 γ /m m o l e/ m v ( vs .a g/ a gc l) t/min carbon ocp cu adsorption mogely khubutiya et al. j. electrochem. sci. eng. 1(1) (2011) 27-37 doi: 10.5599/jese.2011.0005 35 figure 6. dependence of the effective number of electrons on the ocp of unmodified ag-3 carbon during cu2+ adsorption. figure 7. dependence of the integral effective number of electrons on the initial open-circuit potential of carbon for cu2+ cation adsorption on ag-3 carbon. for initial carbon potentials in the range between –775 and –470 mv (vs. ag/agcl), the data obtained (table 4 and fig. 7) shows effective numbers of electrons n ≈ 2. this corresponds to the two-electron faradaic open-circuit process of copper electroreduction on the carbon: r’—c + cu2+ + h2o → r’—c=o + cu 0 + 2h+ (5) this is also corroborated by the appearance of a characteristic reddish-colored copper deposit on the carbon granules after cathodically modified ag-3 is immersed in aqueous cuso4. thus, it is possible to detect the presence of faradaic processes in the course of adsorption by measuring ocp and calculating the effective number of electrons. (it should also be noted that, while noble t = 2 min t = 5 min t = 10 min t = 20 min t = 30 min -0,05 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 0,40 0,45 100 110 120 130 140 150 160 170 n e/mv (vs. ag/agcl) 1,00 1,20 1,40 1,60 1,80 2,00 2,20 2,40 2,60 2,80 3,00 -800 -750 -700 -650 -600 -550 -500 -450 n einit/mv (vs. ag/agcl) j. electrochem. sci. eng. 1(1) (2011) 27-37 criterion of carbon suitability 36 metals are known to deposit onto carbons [20], the literature does not relate this to the ocp of carbon.) probability of one-electron copper reduction importantly, one should consider the probability of the one-electron reduction process in the course of adsorption (as a competing process, with possible further transformation of cu+): cu2+ + е– → cu+ (6) cu+ + е– → cu0 (7) the determination of cu+ in solution is rather difficult, since it quickly disproportionates in solution: 2cu+ → cu2+ + cu0 (8) in electrolytic refining of copper [21], bulk [cu+] concentration for aqueous cuso4 in equilibrium with metallic copper was ca. three orders of magnitude below [cu2+], i.e. virtually negligible. other investigations [22-26], however, suggest a stepwise mechanism for the deposition of cupric ions/ionization of metallic copper, with excess cu+ in the interphase region as an intermediate between cu0 and cu2+. while a polarographic method of cu+ determination in solution has been proposed [27], it involves the addition of organic ligands that might interact with activated carbon, making this method impractical for the present work. in addition, if cu+ only exists in excess near the interface, as suggested by [22-26], it would be difficult to detect it at an additional electrode removed from the vicinity of the carbon surface. thus, it is not possible to elucidate the involvement of cu+ in the investigated processes. however, regardless of mechanistic details, faradaic reduction of cu2+ to metallic cu0 undoubtedly occurs in the above experiments; this is also corroborated by the calculated values of n. conclusions thus, experimental data on the effective number of electrons unequivocally confirm an electrochemical mechanism of hemosorption. all effective numbers of electrons were calculated assuming that the electric double layer capacitance remained constant despite significant potential changes of the carbon. for the model processes of acetone and tb adsorption it was shown that the adsorption of an organic toxicant on ag-3 activated carbon took place virtually without any electron transfer, i.e. faradaic processes did not occur, while the carbon potential did change in the course of adsorption. the interaction of copper cations with the surface of activated carbon depended on carbon potential: for potentials more negative than –470 mv (vs. ag/agcl), the two-electron faradaic process of copper deposition cu2+ +2e– → cu0 occurred, as reflected by calculated values of the effective number of electrons. therefore, it is possible to determine the mechanism of adsorbate interaction with an activated carbon hemosorbent by performing open-circuit potential and adsorption measurements. the effective number of electrons can be calculated and utilized as a criterion of hemosorbent suitability, since it allows the detection of faradaic processes between the hemosorbent and adsorbate. it is important that the phenomenon of partial transfer of charge allows working hemosorbens to be reclassified according to their electric effectiveness. acknowledgements: this study is supported by funds from the center for research and scholarship fund of liberty university. grant # 021_030110. we wish to thank the anonymous referee for a thorough reading of and valuable suggestions for the present paper. mogely khubutiya et al. j. electrochem. sci. eng. 1(1) (2011) 27-37 doi: 10.5599/jese.2011.0005 37 references [1] p.n. sawyer, j.w. pate, am. j. physiol. 175 (1953) 113–117 [2] p.n. sawyer, j.w. pate, c.s. weldon, am. j. physiol. 175 (1953) 108–112 [3] g. guglielmi, f. viñuela, i. sepetka, v. macellari, j. neurosurg. 75 (1991) 1–7 [4] g. guglielmi, f. viñuela, j. dion, g. duckwiler, j. neurosurg. 75 (1991) 8–14 [5] g. guglielmi, f. viñuela, in neurosurgery clinics of north america: endovascular approach to central nervous system disease, l.n. hopkins, ed., saunders, philadelphia, pa, 1994, vol. 5, p. 427–435 [6] g. guglielmi, in: neuroimaging clinics of north america: interventional neuroradiology, f. viñuela, j. dion, g. duckwiler, eds., saunders, philadelphia, pa, 1992, vol.2, p. 269–278. 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a.a. murtazina, a.s. kolosov, elektrokhimiya 5 (1969) 106-108 [in russian] [27] j.a. altermatt, s.e. manahan, anal. chem. 40 (1968) 655-657 © 2011 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) {a comparative study of chemical and physical properties of copper and copper alloys affected by acidic, alkaline and saline environments} http://dx.doi.org/10.5599/jese.877 373 j. electrochem. sci. eng. 10(4) (2020) 373-384; http://dx.doi.org/10.5599/jese.877 open access: issn 1847-9286 www.jese-online.org original scientific paper a comparative study of chemical and physical properties of copper and copper alloys affected by acidic, alkaline and saline environments samiul kaiser1 and mohammad salim kaiser2, 1department of civil engineering, bangladesh university of engineering and technology, dhaka1000, bangladesh 2directorate of advisory, extension and research services, bangladesh university of engineering and technology, dhaka-1000, bangladesh corresponding author: mskaiser@iat.buet.ac.bd; tel.: +88-02-9663129; fax: +88-02-9665622 received: june 11, 2020; revised: june 29, 2020; accepted: july 1, 2020 abstract chemical and physical behavior including corrosion performance, thermal conductivity and visual color change of the copper-based alloys brass and bronze have been studied prior and after corrosion in acidic, alkaline and saline media. the concentrations of 0.5 m h2so4, 0.5 m naoh and 0.5 m nacl were used in which copper and copper-alloy samples were immersed and left to corrode at room temperature for 28 days. the experiments were performed prior and after corrosion, using conventional gravimetric measurements accompanied with measurements of thermal conductivity, microstructure and optical properties. the color change of different samples was also studied through tristimulus color parameter (l*, a* and b*) values. it is concluded that the corrosion rate of copper and copper alloys is greater in acidic than in salt and alkaline media. this is due to the extent of disruption of the passive film formed on the surfaces. in the cases of alkaline and salt media, the passive films on the surface remain stable to a large extent. small increase of thermal conductivity takes place due to formation of a very thin film of oxide and hydroxide bonded to the surface. the environment also affects the color of copper and copper alloys by chemical changes like oxidation and formation of different intermetallics on the surfaces. a microstructural study of experimental materials confirms that corrosion after 28 days results in formation of pores on the surfaces in acidic environment, and passive film that grows thicker on the surfaces in alkaline and saline environments. aluminum oxide that is more stable than zinc oxide causes better anti-corrosion performance and minimal color variation of bronze compared to brass, especially in acidic environment. keywords copper-alloys; corrosion; oxidation; thermal conductivity; color; microstructure http://dx.doi.org/10.5599/jese.877 http://dx.doi.org/10.5599/jese.877 http://www.jese-online.org/ mailto:mskaiser@iat.buet.ac.bd j. electrochem. sci. eng. 10(4) (2020) 373-384 properties of copper and copper alloys 374 introduction pure metals like aluminum, copper, iron, etc. are found quite soft in texture. the strength of these materials is gained by adding other elements to the pure metals. in this way alloys are formed [1-3]. several properties of metals like strength, hardness, corrosion resistance, electrical and thermal conductivity, resistance, change in color, etc. can be increased or reduced through alloying [4-6]. in the case of copper, zinc, tin, aluminum, nickel, manganese, and silicon are usually used in an approximate order of increasing effectiveness to form copper alloys. brasses and bronzes are considered to be the most renowned families of copper-based alloys which are used frequently in daily works. brass consists of copper and zinc, while bronze is an alloy of copper combined mostly with tin, but also with other metals like aluminum, phosphorus, manganese, and silicon. brass and bronze are used in various works, based on their properties. these alloys are used extensively in production of heavy load bearing machine, high-speed operation of shaft sliding sleeves and bush bearings, automobiles, ships, metallurgy, machinery and other industrial fields. at the same time, the application of those alloys is also found in ornamental and building construction industries. varying amounts and proportions of alloying elements are found in those alloys. thus, a wide range of properties and variation in color are seen due to the variation of mixtures [7,8]. brass and bronze are used frequently in different environments and for various purposes as they show outstanding corrosion behavior. corrosion defines the degradation of metallic materials which is due to various electrochemical and chemical reactions. corrosion may result into component failure in different industrial environments and it is affected by various environment and metal related factors [9,10]. it is well established in the literature that addition of a substance to improve one property may result by unintended effects on other properties. although a lot of information can be found about the effects of various alloying elements on the mechanical properties of cu-based alloys, there is no systematic information on simultaneous changes of other properties [11-13]. therefore, the present study is focused on evaluating the environmental impact on the physico-mechanical properties of cu and cu alloys, brass and bronze. this investigation aims to study the effects of ten weight percent (10 wt%) of alloying elements al and zn, before and after corrosion in 0.5 m acidic, alkaline and saline solutions. experimental the present study involves copper, brass and bronze samples which were prepared through casting of commercially pure copper, zinc and aluminum. a clay-graphite crucible in a natural gas fired pit furnace was used for melting these experimental alloys. full chemical compositions of the experimental samples of copper, brass (10.3 wt% zn) and bronze (9.6 wt% al), showing contents of al, zn, pb, sn, fe, ni, mn, si, cr, sb and cu elements, are given elsewhere [10,14]. the oxide layers from the surface of the cast samples were removed and pieces of 15 x 20 x 150 mm were cut using a shaper machine. cold rolling of the cast alloys was carried out with a laboratory scale rolling mill of 10hp capacity at 50 % of reduction. in every pass, about 1 mm of deformation was given and as a result thickness of the samples was reduced to 7.5 from 15 mm. from the cold rolled samples 5 x 20 x 20 mm coupons were obtained and sanded mechanically with emery papers of rough and finish grade of 1500 grits. the ageing treatment was conducted in an electrical resistance furnace at different temperatures for one hour. microhardness measurements were carried out on the differently aged samples using a micro vickers hardness tester where a load of 1 s. kaiser and m. s. kaiser. j. electrochem. sci. eng. 10(4) (2020) 373-384 http://dx.doi.org/10.5599/jese.877 375 kg and dwell time of 10 seconds were used. a total of seven readings from different locations were taken for every aged sample and average value was calculated. corrosion behavior was tested for 50 % cold rolled cast samples of 3 x 3042 mm size that were artificially aged for 60 min at 300 °c. prior to use, the samples were sanded with emery paper and cleaned with ethanol. afterwards, the samples were dried in forced air, weighted (initial weight, wint) and exposed to different solutions for periods up to 28 days. stagnant solutions of 0.5 m h2so4, 0.5 m naoh, and 0.5 m nacl were used as aggressive media. after the designated exposure in the solution medium, the samples were removed from the test solution, thoroughly washed with a distilled water, dried well in forced air and weighted again (final weight, wfin). the weight loss, δw, was calculated in accordance with [15]: − = int finδ (w w ) w a (1) where δw = weight loss (mg/cm2), wint = initial weight prior to the immersion (mg), wfin = final weight after exposure (mg) and a = area (cm2). the corrosion rate, kcorr (mm/year), was evaluated on the basis of = corr δk w k td (2) where k = unit conversion constant (87.6 for mm/year unit), t = time of exposure (h) and d = density of metal (g/cm3). an electric conductivity meter, type 979 was used for measuring the electrical conductivity of the alloys after different immersion conditions. thermal conductivity was calculated through the wiedemann–franz law from measured electrical conductivity data [16]. the images of the corroded samples were taken with a digital single-lens reflex (dslr) camera to examine the visual changes. the sample images were analyzed through matlab software for determining the tristimulus color parameter (l*, a* and b*) values. the changes of these values with respect to the immersion time in different environments were shown graphically. the washed and dried samples were observed by optika microscope and some selected photomicrographs were taken. results and discussion gravimetric analysis of corrosion effect the change of corrosion rate with immersion time for copper, brass and bronze samples, previously aged at 300 °c for one hour, were calculated using eq. (1) and eq. (2). the values of corrosion rate in 0.5 m solutions forming acidic, alkaline and saline environments are plotted in figure 1(a), 1(b) and fig. 1(c), respectively. the corrosion rate kcorr for all experimental material samples is the highest in 0.5 m h2so4. the results observed in h2so4 media are ascribed to the high aggressive attack of sulphide ions, leading to the disruption of passive films formed on the metal surfaces. however, the corrosion rate is slightly lower than that observed at the earlier stage of immersion, suggesting that the passive films formed on the surface of the experimental samples are stable [17,18]. in the case of alkaline solution, the higher corrosion rate at the initial stage could be due to the segregation of copper oxide formed in the matrix. the lower corrosion rate is ascribed to the protective nature of copper hydroxide film formed on the metal surface [19]. after initial time period, corrosion rates start to decrease with time and attain a roughly constant value for all samples. http://dx.doi.org/10.5599/jese.877 j. electrochem. sci. eng. 10(4) (2020) 373-384 properties of copper and copper alloys 376 a b c figure 1. variation of corrosion rate of copper, brass and bronze with immersion time in (a) acidic, (b) alkaline and (c) saline environment for 28 days in the case of salt solution, lower corrosion rates are observed for all three materials. in the early stages of corrosion in 0.5 m nacl solution, an oxide film grows on the surfaces. the result in the salt solution seems neutral, i.e., neither weight gain nor weight loss was observed. the lower corrosion rate is accredited to the formation of passive films on the surface of the samples, which lead to the temporary seizure of the corrosion attack, while the higher corrosion rate is due to the gradual breakdown of the passive films by the corrosion attack within several days [20,21]. pure copper in acidic solution form cu2o and cuso4, while brass and bronze form additional zno and znso4, and al2o3 and al2(so4)3, respectively. dissolution of cuso4 and znso4 in acidic solution is higher than al2(so4)3, what caused lower corrosion rate of bronze metal. in addition, al2o3 film inhibits the dissolution of the bronze metal. in alkaline and saline media, pure copper forms cuo, while brass and bronze form additional zno and al2o3 layers on the surfaces. these layers are very much protective, providing enhanced corrosion resistance [22,23]. thermal conductivity time changes of thermal conductivity of copper, brass and bronze samples previously immersed in acidic, alkaline and saline media are plotted in figure 2(a), 2(b) and fig. 2(c), respectively. in each case, the thermal conductivity of the sample increased initially due to the formation of oxide layer on the surface which caused higher conductivity. after prolonged exposure, the thermal conductivity of the 0 5 10 15 20 25 30 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 c o rr o s io n r a te k c o rr , m m /y e a r immersion time, days copper brass bronze 0 5 10 15 20 25 30 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 c o rr o s io n r a te k c o rr , m m /y e a r immersion time, days copper brass bronze 0 5 10 15 20 25 30 -0.05 0.00 0.05 0.10 0.15 0.20 0.25 c o rr o s io n r a te , k c o rr , m m /y e a r immersion time, days copper brass bronze k co rr / m m y e a r1 k co rr / m m y e a r1 k co rr / m m y e a r1 https://www.sciencedirect.com/topics/engineering/corrosion-resistance s. kaiser and m. s. kaiser. j. electrochem. sci. eng. 10(4) (2020) 373-384 http://dx.doi.org/10.5599/jese.877 377 experimental materials in acid solution slightly decreased due to the diffusion of aggressive ions leading to porosity of the surface. in naoh solution, thermal conductivity decreased because of the formation of black corrosion product layer that becomes thicker with increase of immersion time. a trend of initial conductivity increase, followed by a decrease is observed also in the saline solution within 28 days. copper forms a very thin film of copper oxide and hydroxide bonded to its surface, increasing the thermal conductivity. the corrosion attack is restored after some days due to the gradual disruption of the passive films which caused conductivity decrease [21,24]. brass and bronze form additional zinc oxide and aluminum oxide, respectively that together with copper oxide caused small effects on thermal conductivity. a b c figure 2. variation of thermal conductivity of copper, brass and bronze with immersion time in (a) acidic, (b) alkaline and (c) saline environment for 28 days optical observations dslr camera optical images of the experimental samples of copper, brass and bronze aged at 300 °c for one hour before and after corrosion in acidic, alkaline and salt media, were taken at different immersion times and recorded in figure 3. the images show that polished copper is of a reddish-brown color, the color of brass moves from red to gold, while the color of aluminium bronze is dull gold depending on the levels of zinc and aluminium added to alloy. the surface colors of samples obviously change with increase of immersion time, what points toward the formation of films on the surfaces in different environments. 0 5 10 15 20 25 30 50 100 150 300 350 400 t h e rm a l c o n d u c ti v it y , w /m .k immersion time, days copper brass bronze 0 5 10 15 20 25 30 50 100 150 300 350 400 t h e rm a l c o n d u c ti v it y , w /m .k immersion time, days copper brass bronze 0 5 10 15 20 25 30 50 100 150 300 350 400 t h e rm a l c o n d u c ti v it y , w /m .k immersion time, days copper brass bronze http://dx.doi.org/10.5599/jese.877 j. electrochem. sci. eng. 10(4) (2020) 373-384 properties of copper and copper alloys 378 figure 3. dslr camera images following color changes of polished copper, brass and bronze samples prior and after corrosion in acidic, alkaline and saline environment for 1 and 28 days figure 3 shows that the change of surface color before and after corrosion is more noticeable in the alkaline solution than other environments. in the acidic solution, a mixed film composed of copper oxide and copper sulphate is formed. however, in the presence of the aggressive sulphate ions, the passive film may breakdown continuously and so, the change in color is minimal. after prolonged immersion time, the passive films might be stable, causing higher color variation. after corrosion for either 1 or 28 days in naoh solution, the surface colors of all samples have changed s. kaiser and m. s. kaiser. j. electrochem. sci. eng. 10(4) (2020) 373-384 http://dx.doi.org/10.5599/jese.877 379 drastically because of thick black corrosion product layers formed on all surfaces. in the saline environment, copper forms a thin layer of copper oxide and hydroxide which is bonded to its surface. at higher immersion time, the corrosion attack is restored after some days, causing the gradual disruption of the passive film [25,26]. for this reason, the color change is minimal in the saline environment. brass and bronze form zinc oxide and aluminum oxide, respectively, with copper oxide formed in all environments. aluminum oxide is more protective and so, small variations of bronze color are seen, especially in acidic solution. in alkaline and saline environments, these additional oxides accelerate the color variation. the color change of all three alloys in alkaline solution after 1 day is very strong, but after 28 days the variation is almost indifferentiable due to formation of thick layer of corrosion products at the surfaces. the color change of all samples is examined by tristimulus color parameter, l*, a* and b* values. l* represents the lightness, where darkest black is at l* = 0, and brightest white at l = 100, while a* represents the green–red component of the color, with green and red in the negative and positive direction, respectively. analogously, b* represents the blue–yellow component of the color, with blue and yellow in the positive and negative direction, respectively. the graphical representations of change of these values with immersion time are shown in figures 4-6. the variations of l* value with the immersion time in acidic, alkaline and saline solution are shown in figure 4(a), 4(b) and fig. 4(c), respectively. a b c figure 4. change of tristimulus color parameter l* (0 = black; 100 = white) of copper, brass and bronze samples with immersion time in (a) acidic, (b) alkaline and (c) saline environment for 28 days 0 5 10 15 20 25 30 0 10 20 30 40 50 60 l * fr o m b la c k ( 0 ) to w h it e ( 1 0 0 ), a u immersion time, days copper brass bronze 0 5 10 15 20 25 30 0 10 20 30 40 50 60 l * fr o m b la c k ( 0 ) to w h it e ( 1 0 0 ), a u immersion time, days copper brass bronze 0 5 10 15 20 25 30 0 10 20 30 40 50 60 l * fr o m b la c k ( 0 ) to w h it e ( 1 0 0 ), a u immersion time, days copper brass bronze l* / a . u . l* / a . u . l* / a . u . http://dx.doi.org/10.5599/jese.877 j. electrochem. sci. eng. 10(4) (2020) 373-384 properties of copper and copper alloys 380 from the graphs shown in figure 4, it is clear that l* value is higher in acidic solution and lower in alkaline solution, while l* value in saline solution is higher than alkaline solution but lower than acidic solution. in acidic solution, the thicker blue layer of hydrated copper sulphate salt (cuso4×h2o) film is formed by a chemical reaction at the surface, what caused decreased l* value. its intensity is lower because this kind of the oxide film is dissolved more quickly in the acidic solution [27,28]. in the case of alkaline solution, copper forms cu2o and light blue precipitate of cu(oh)2 which are deposited on the surface of the oxide as reported previously [19,29]. these layers drastically change l* value of the experimental materials. similarly, in the saline solution, one has to consider formation of brownish corrosion film consisted of cuo, cu2o and greenish particles, dicopper chloride trihydroxide cu2(oh)3cl [20,30]. additional zno and al2o3 are formed with cuo for brass and bronze respectively as discussed earlier. these oxides protect from corrosion, and protect the color of the experimental materials surfaces. a b c figure 5. change of tristimulus color parameter a* (from green (-) to red (+)) of copper, brass and bronze with immersion time in (a) acidic, (b) alkaline and (c) saline environment for 28 days the a* and b* values vs. immersion time graphs for different environments are presented in figure 5 and figure 6, respectively. the graphical scenario shows greater values of a* and b* for acidic and saline solutions, but lower values in the alkaline solution. quick dissolvement of oxide film in the solution results by less variation of the sample from the original in case of acidic solution. thus, for all three materials, red and yellow components are more dominant and so a* and b* values are higher for acidic solution. similarly, the values are higher in the saline condition because of less 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 35 a * fr o m g re e n ( -) t o r e d ( + ), a u immersion time, days copper brass bronze 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 35 a * fr o m g re e n ( -) t o r e d ( + ), a u immersion time, days copper brass bronze 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 30 35 a * fr o m g re e n ( -) t o r e d ( + ), a u immersion time, days copper brass bronze a * / a . u . a * / a . u . a * / a . u . s. kaiser and m. s. kaiser. j. electrochem. sci. eng. 10(4) (2020) 373-384 http://dx.doi.org/10.5599/jese.877 381 deviation from original colors. but in alkaline condition, deposition of copper oxides provides darker layers on the surface where the green and blue components are dominant. this is the reason of lower values of a* and b* in alkaline condition for all three materials. in the case of acidic conditions, a* values of brass and bronze are lower than copper because of the presence of zno for brass and al2o3 for bronze, which both increase green component of the color. for the same reason, b* value is higher for these two alloys rather than pure copper as yellow component is increased. in alkaline environment all three materials have almost same type of a* and b* values as all the oxide layers sustain over the materials. in salt solution all three materials retain almost their original colors. a b c figure 6. change of tristimulus color parameter b* (from blue (-) to yellow (+)) of copper, brass and bronze with immersion time in (a) acidic, (b) alkaline and (c) saline environment for 28 days optical microscopy the worn surfaces of aged copper, brass and bronze samples before and after immersion in different corrosive environments are presented in figure 7. before corrosion test, all polished materials showed some scratches on the surfaces which may be formed by the sandpaper during surface preparation. it is known that addition of alloying elements changes the grain structure of all alloys. this change in microstructure can be characterrized by the increased dark tone or lighter tone in the micrograph of the experimental materials. immersed in 0.5 m of acidic, alkaline and saline solution for 1 day, some defects like pits are generally formed in the conversion layers of the experimental materials, but the quantity of the pits per area is relatively low. 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 b * fr o m b lu e ( -) t o y e ll o w ( + ), a u immersion time, days copper brass bronze 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 b * fr o m b lu e ( -) t o y e a ll o w ( + ), a u immersion time, days copper brass bronze 0 5 10 15 20 25 30 -5 0 5 10 15 20 25 b * fr o m b lu e ( -) t o y e ll o w ( + ), a u immersion time, days copper brass bronze b * / a . u . b * / a . u . b * / a . u . http://dx.doi.org/10.5599/jese.877 j. electrochem. sci. eng. 10(4) (2020) 373-384 properties of copper and copper alloys 382 figure 7. optical micrographs of the polished copper, brass and bronze prior and after corrosion in acidic, alkaline and saline environment for 1 and 28 days. the images of the alloys studied after 28 days of exposure in 0.5 m h2so4 solution clearly show presence of pores due to the uniform degradation of the alloys [31]. the aggressive attack of the sulphide ions disrupts the passive film formed on the surface of the alloys. the images referring to 100 m s. kaiser and m. s. kaiser. j. electrochem. sci. eng. 10(4) (2020) 373-384 http://dx.doi.org/10.5599/jese.877 383 naoh solution shows a black corrosion product layers on the alloys surfaces which grow thicker with the corrosion proceeding. it is worth noting that the corrosion layer formed in the alkaline solution seems more compact and homogenous [32]. also, this layer is not porous. the images obtained in nacl solution show presence of passive films on the alloys surface. only some spots are observed due to the gradual disruption of the passive film [33]. some variation is observed in cases of brass and bronze. in acidic solution, bronze forms al2o3 and al2(so4)3 which dissolution rate is lower than cu2o, and cuso4 formed by pure copper, and zno and znso4 formed by brass. as a consequence, the minimum pores are formed on the bronze surfaces. brass and bronze form additional zno and al2o3 layer, respectively, with cuo on the surfaces in alkaline and saline media. due to this protective layer, brass and bronze surfaces show the extra mark of corroded products [23]. conclusions copper and copper-based alloys brass and bronze samples showed higher corrosion rate in acid solution compared to alkaline and saline environments, what is due to gradual disruption of passive films and segregation of the copper oxide film in the matrix. it seems that al2o3 layer is especially protective in acidic environment, causing the lower corrosion rate of bronze. the thermal conductivity of all three experimental materials increased slightly after corrosion in acid, basic and saline environments due to the formation of oxide layers which decreased surface roughness. the tristimulus color parameter l*, a* and b* values increase in acidic solution due to formation of blue layer of hydrated copper sulfate (cuso4×h2o) salt film that is dissolved quickly from the surface. the additional production of zno in brass and al2o3 in bronze is also responsible for deviations of l*, a* and b* values from pure copper. in alkaline solution, light blue precipitate of copper hydroxide cu(oh)2 is deposited on the surfaces, what decreases the tristimulus color parameter. similarly, in the saline solution there is formation of greenish particles, di-copper chloride trihydroxide cu2(oh)3cl, but the gradual breakdown of the passive films brought about by the corrosion attack within several days causes a lower variation in color than in alkaline media. evidence of formation of crystallographic pitting is found in nacl solution for exposure times up to 28 days, while an intensive pit formation is observed in the acidic medium. aluminum forms a 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[33] h. nady, m. m. el-rabiei, g. m. abd el-hafez, journal of bioand tribo-corrosion 3 (2017) art. 6. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license ( https://creativecommons.org/licenses/by/4.0/) https://www.sciencedirect.com/science/journal/03054403 callto:45(1)%202011%2052-61 https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahukewjjweje56tyahwgno8khrp3cgiqfggxmae&url=http%3a%2f%2fstudylib.net%2fdoc%2f6655580%2fjese_0043---journal-of-electrochemical-science-and&usg=aovvaw3iua13s8eqbz8kw5qg5ie5 https://www.sciencedirect.com/science/journal/0010938x https://www.researchgate.net/scientific-contributions/2074987726_mohsin_e_al-dokheily?_sg%5b0%5d=jnjmmuygoy3vejb_ubjj4axnacezc-uwh1agymee15ypch5c1ciasojyoazmizc5rb_-3dg.ie2amqtuupq_rnlnjmlre6vm7xjebs1gdwceonv--_u-1welouoejwopmvdntico9mxmaplpyid0ehin88btyw&_sg%5b1%5d=-b50zegyq1wjluav5axxhs2e-ctqljrx1-gcg89agvsl0bnvsnu2jqzzgmuk3kb7iqwu1hk.45i5k3eggp6jq_u0llnwoxg2bgcl1fpje0cd8phjob3can7ghbaxbgng3fjofdlm6_6w1b2hv0w_wgtcxzpyfw https://www.researchgate.net/profile/husam_kredy?_sg%5b0%5d=jnjmmuygoy3vejb_ubjj4axnacezc-uwh1agymee15ypch5c1ciasojyoazmizc5rb_-3dg.ie2amqtuupq_rnlnjmlre6vm7xjebs1gdwceonv--_u-1welouoejwopmvdntico9mxmaplpyid0ehin88btyw&_sg%5b1%5d=-b50zegyq1wjluav5axxhs2e-ctqljrx1-gcg89agvsl0bnvsnu2jqzzgmuk3kb7iqwu1hk.45i5k3eggp6jq_u0llnwoxg2bgcl1fpje0cd8phjob3can7ghbaxbgng3fjofdlm6_6w1b2hv0w_wgtcxzpyfw https://www.researchgate.net/scientific-contributions/2074949539_rasha_n_al-jabery?_sg%5b0%5d=jnjmmuygoy3vejb_ubjj4axnacezc-uwh1agymee15ypch5c1ciasojyoazmizc5rb_-3dg.ie2amqtuupq_rnlnjmlre6vm7xjebs1gdwceonv--_u-1welouoejwopmvdntico9mxmaplpyid0ehin88btyw&_sg%5b1%5d=-b50zegyq1wjluav5axxhs2e-ctqljrx1-gcg89agvsl0bnvsnu2jqzzgmuk3kb7iqwu1hk.45i5k3eggp6jq_u0llnwoxg2bgcl1fpje0cd8phjob3can7ghbaxbgng3fjofdlm6_6w1b2hv0w_wgtcxzpyfw https://www.sciencedirect.com/science/article/pii/s0010938x69801010#! https://www.sciencedirect.com/science/article/pii/s0010938x69801010#! https://www.sciencedirect.com/science/journal/0010938x https://www.sciencedirect.com/science/journal/0010938x/9/9 {v2o5 as magnesium cathode material with extended cyclic stability} http://dx.doi.org/10.5599/jese.769 257 j. electrochem. sci. eng. 10(3) (2020) 257-262; http://dx.doi.org/10.5599/jese.769 open access: issn 1847-9286 www.jese-online.org original scientific paper v2o5 as magnesium cathode material with extended cyclic stability charalampos drosos1,, benjamin moss2, andreas kafizas2,3, and dimitra vernardou4, 1delta nano – engineering solutions ltd., cyprus 2department of chemistry, molecular science research hub, imperial college london, white city campus, london, w12 0bz, u.k. 3grantham institute for climate change, imperial college london, south kensington, london, sw7 2az, u.k. 4department of electrical & computer engineering, school of engineering, hellenic mediterranean university, 710 04 heraklion, crete, greece corresponding authors: info@delta-nano.com; a.kafizas@imperial.ac.uk; dvernardou@hmu.gr received: december 24, 2019; revised: february 19, 2020; accepted: february 22, 2020 abstract in this work, the electrochemical performance of aerosol-assisted chemical vapour deposited vanadium pentoxide cathodes at 600 °c, is presented. the as-grown oxides indicate specific discharge capacity of 300 ma h g-1 with capacity retention of 92 % after 10000 scans, coulombic efficiency of 100 %, noble structural stability and high reversibility. the present study shows the possibility to grow large-area magnesium cathode material with extended cycle stability via utilization of an aqueous electrolyte under a corrosive environment. this enhanced performance may be a combination of electrode morphology and adherence, when compared to previous work employing electrode growth temperature at 500 °c. keywords magnesium ion batteries; chemical vapor deposition; electrode morphology; coating adherence; corrosive environment. introduction aqueous metal-ion batteries (aibs) are auspicious alternatives for large-scale energy storage since the water-based battery system is low cost, environmentally friendly, safe and does not require rigorous manufacturing conditions (elimination of glove box in the production line) [1]. additionally, aibs have no special operational and maintenance requirements and do not require regular charge because of low self-discharge [2]. http://dx.doi.org/10.5599/jese.769 http://dx.doi.org/10.5599/jese.769 http://www.jese-online.org/ mailto:info@delta-nano.com mailto:a.kafizas@imperial.ac.uk mailto:dvernardou@hmu.gr j. electrochem. sci. eng. 10(3) (2020) 257-262 v2o5 as magnesium cathode material 258 in this perspective, multivalent-ion technologies (mg2+, ca2+, al3+) have been proposed as replacements for li-ion technology because they can store more charge per ion at the cathode, leading to larger energy density than the univalent ion batteries [3]. among them, magnesium-ion batteries have attracted attention due to the high abundance of magnesium (1.94 %) [4], high volume specific capacity (3833 mah cm-3) [5] and low reduction voltage (-2.37 v vs. she) [6]. in our previous work [7], the viability of a technology that is proven to be industrially competitive – that of cold-wall aerosol assisted chemical vapor deposition was established for the growth of vanadium pentoxide (v2o5) with excellent performance as cathodes for aqueous magnesium-ion batteries (specific discharge capacity 427 ma h g-1 with capacity retention of 82 % after 2000 scans). the cathodes presented an increase of current intensity as the scan number proceeded to 500, remaining however unchanged after 1000 scans. inspired by the respectable results of mg2+ ion battery chemistry, herein we would like to report that this path can be further exploited and enhanced for high-performance cathode materials. in this work, we demonstrate that a mixed phase of α-v2o5 and β-v2o5 deposited at 600 °c (i.e. higher substrate temperature than before [7]) is capable of reversibly inserting magnesium ions showing a specific discharge capacity of 300 mah g1 with capacity retention of 92 % after 10000 scans, which may be a combination of electrode morphology and adherence. this particular cathode offers a safe and economical energy storage solution to large-scale applications with extended cyclic stability. experimental synthesis of v2o5 cathodes thin layers of v2o5 were deposited at 600 °c, following the same procedure of aerosol-assisted chemical vapor deposition (aacvd) as previously published [7], having similar characteristics in color and stability. physical characterization x-ray diffraction (xrd) was implemented to study the structure of v2o5 coatings on a modified bruker-axs d8 diffractometer. field-emission scanning electron microscopy (fe-sem) and x-ray photoelectron spectroscopy (xps) were utilized to evaluate the surface morphology and chemistry of the material in its initial state, after mg2+intercalation and 10000 continuous mg2+ intercalation/deintercalation scans. the processing parameters and the instruments employed were the same as those reported in [7]. electrochemical performance the aacvd coating, pt foil (with surface area 1 cm2) and ag/agcl/kcl (0.1 m) were acted as working, counter and reference electrodes [7,8], respectively. an aqueous solution of 0.075 m mgcl2 was conducted as the electrolyte in all measurements for the scan rate of 10 mv s-1 and potential ranging -1.5 v to +1.0 v. cyclic voltammograms, potentiometric measurements and electrochemical impedance spectroscopy (eis) studies were performed in an electrochemical workstation (pgstat302n) [7].in particular, eis curves were performed for alternating current (ac) amplitude of 5.0 mv and set potential of -1.0 v over the frequency range of 100 khz 10 mhz. results and discussion the crystal structure of the coating presents three peaks at 12.6, 21 and 30.7o with respective miller indices (200), (001) and (301) (figure 1a). the first one is consistent with the formation of β-v2o5 [9], c. drosos et al. j. electrochem. sci. eng. 10(3) (2020) 257-262 http://dx.doi.org/10.5599/jese.769 259 while the other two with α-v2o5[10]. regarding the rest of the peaks (■) are due to the fto glass substrate [11]. to further investigate the surface chemistry, xps indicates in figure 1b binding energies at 517.3 (v 2p3/2), 524.9 (v 2p1/2) and 530.2 ev (o 1s), which correspond to v5+ and o2of v2o5 [12,13]. additionally, there is a peak at 532.6 ev associated with the presence of chemisorbed oh groups on the surface of the coating [12]. increasing the substrate temperature from 500 °c [7] to 600 °c, leads to the improvement in the crystallinity (i.e. (001) peak intensity increases) as expected for a conventional activated process. higher temperatures enhance surface mobility, which in turn improves coating crystallization. the co-existence of αand β-v2o5 is supported by sem analysis indicating the grain structure recorded at magnification of ×50k (inset of fig. 1a). it is clearly observed that the coating exhibits a compact structure with well distributed grains as compared with pure αv2o5 [7]. the grains have irregular shape presenting some degree of agglomeration. figure 1.xrd of v2o5 coating grown on fto glass substrate by aacvd at 600 °c (squares represent reflections from fto glass substrate) (a). xps depth profile and fe-sem image at ×50000 of v2o5 coating as inset (b). the electrochemical intercalation of mg2+ into v2o5 coatings was studied in a three-electrode electrochemical cell using an aqueous solution of 0.075 m mgcl2 as electrolyte. to examine if mg2+ ions intercalate into the lattice of the host material, the state of the vanadium was examined by xps. figure 2a inset shows mg 1s spectrum, which presents one peak at 1304 ev [14]. figure 2. xps depth profiles (a) and fe-sem image (b) at ×50000 after mg2+ intercalation into the host of v2o5 coating. http://dx.doi.org/10.5599/jese.769 j. electrochem. sci. eng. 10(3) (2020) 257-262 v2o5 as magnesium cathode material 260 the clear magnesium signal indicates that metallic mg is indeed intercalated from the electrolyte. the core level xps spectrum of o 1s indicates two peaks, which are located at 530.1 and 532.5 ev due to o2in v2o5 and oh groups on the surface of v2o5 [12], respectively. after mg2+ intercalation in the host of v2o5, the xps spectrum resembles that of v2o5 in figure 1a and only minor morphological changes observed (figure 2b). the stability of v2o5 was evaluated via cyclic voltammograms for 10000 continuous mg2+ intercalation/deintercalation scans (figure 3a) in the aqueous mgcl2 0.075 m electrolyte sweeping the potential from -1.5 v to +1.0 v (vs. ag/agcl/kcl(0.1 m)). the voltammograms show one oxidation peak at -0.61 v with the reverse reaction occurring, as presented by the corresponding reduction peak at -1.10 v. these peaks are observed after the first scan due to the activation of intercalation/deintercalation processes into v2o5 host cathode material. the current density was found to decrease from the first to the 2000th scan and remained constant afterwards, presenting an extended cyclic stability compared to the previously published work [7]. the specific capacity of v2o5 cathode was evaluated under the constant specific current of 15 a g1 and applied potentials ranging from -1.5 v to +0.7 v for the first and the 10000th scan (figure 3b). a peak at approximately -1.12 v is indicated, employing the one-step deintercalation (discharge) process for the specific current applied. additionally, a faint plateau is shown at approximately +0.5 v suggesting the one-step intercalation (charge) process. the specific discharge capacity was estimated to be 300 ma h g-1 with capacity retention of 92 % after 10000 scans and coulombic efficiency of 100 %. the capacity performance of this electrode is enhanced as compared to aacvd v2o5 at 500 °c [7], fevo4 in 1 m mg so4 (300 ma h g-1, efficiency: 100 % after 50 cycles [15]) and magnesium manganese oxide sieves in 0.5 m mgcl2 (300 ma h g-1, efficiency: 100 % after 300 cycles [16]). finally, the shape of the curves remained unchanged for the first to the 10000th proposing good electrode stability. rate capability of the electrodes was studied at specific currents of 3.9 a g-1 up to 15 a g-1, and then returned to 15 a g-1 (figure 3 c). the electrode shows the highest specific discharge capacity at 15 a g-1 (300 ma h g-1), which could still deliver the same value when the specific current returned to 15 a g-1 exhibiting capacity retention of 100 %. this performance confirms the upright structural stability and high reversibility of the cathode. the specific discharge capacity shows a rising trend with increasing specific currents, which is similar with the aacvd v2o5 at 500 °c [7]. impedance responses of aacvd v2o5 cathode were evaluated by nyquist plots for 1, 4000 and 8000 scans (figure 3d). the experimental data were fitted by the electrical equivalent circuit (eec) shown in figure 3d inset. eec consists of two parallel r-c circuits in series, for which a discussion still exists on the attribution of different elements to different processes in intercalation battery electrodes. here, r1 is ascribed to the electrolyte and fto resistances, r2 to the contact resistance between fto and v2o5, which could possibly be influenced with artefacts due to presence of ce and re in three-electrode cell, and r3 to the charge transfer reaction resistance at v2o5/redox electrolyte interface [17,18].the first semi-circle (high-frequency) slightly changes upon different scan numbers, while the second semi-circle (low-frequency)is larger from the first to the 8000th scan showing however the percentage increase is smaller after 4000 scans (26 %). the impedance shape of the curves is not the same with those reported in [7] due to the differentiation of materials structure and morphology at higher growth temperature. ragone plot is used to relate the power density with the energy density using the total weight of the active material (0.0028 g), the cell voltage and the capacity based on charge/discharge curves at various current densities. the vanadium pentoxide cathode achieved a high value of 448 wh kg-1 at 24200 w kg-1. assuming that the cathode takes up 50 wt.% of a cell, the magnesium-ion cell with c. drosos et al. j. electrochem. sci. eng. 10(3) (2020) 257-262 http://dx.doi.org/10.5599/jese.769 261 v2o5 cathode can deliver a 224 w h kg-1, which is comparable with li-ion batteries (<300 w h kg-1cell) and li-s batteries (<500 w h kg-1cell) [19]. figure 3. cyclic voltammetry curves of aacvd v2o5 for 1, 1000, 2000, 4000, 8000 and 10000 scans in 0.075 m mgcl2 at scan rate of 10 mv s -1(a). specific capacity for v2o5 cathode for a potential ranging -1.5 v to +0.7 v under constant specific current of 15 a g-1 for 1 and 10000 scans (b).rate capability of the electrode at specific current values of 3.9 a g-1 up to 15 a g-1 and then back to 15 a g-1(c). nyquist plot of measured (symbols) and fitted (solid lines) of v2o5 electrode after 1, 4000 and 8000 scans. electrical equivalent circuit is also indicated as inset (d). ragone plot based on the mass of the active material, cell voltage and capacity values obtained in figure 3b (e). xps depth profiles and fe-sem image at ×50000 of v2o5 as inset, after 10000 continuous mg 2+ intercalation/deintercalation scans (f) finally, the surface morphology and chemistry of v2o5 cathodes were studied after 10000 continuous intercalation/deintercalation scans. the grains appear elongated due to changes associated with mg2+ intercalation/deintercalation processes in v2o5 (figure 3f, inset). the presence of v2o5 is http://dx.doi.org/10.5599/jese.769 j. electrochem. sci. eng. 10(3) (2020) 257-262 v2o5 as magnesium cathode material 262 still supported after 10000 scans (figure 3f) with lower intensity as compared with figure 1b), whereas the peak due to chemisorbed oh groups at 532.6 ev disappears upon cycling due to the weak interaction of water with the metal oxide. conclusions this paper demonstrates the utilization of cold-wall aacvd to grow v2o5 electrodes as cathodes for magnesium ion batteries. furthermore, the growth temperature increase to 600 from 500 °c (reported previously) is found decisive forv2o5 cathode with extended cyclic life. this enhancement may be a combination of the co-existence αand β-v2o5 along with the adherence of the coating. in particular, the electrode indicated a specific discharge capacity of 300 ma h g-1 with capacity retention of 92 %, coulombic efficiency of 100 %, good structural stability and comparable specific energy values with li-ion and li-s batteries under a corrosive environment. this was obtained for the first time to the best of our knowledge. references [1] h. ao, y. zhao, j. zhou, w. cai, x. zhang, y. zhu, y. qian, journal of materials chemistry a 7 (2019) 18708-18734. 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[18] j. park, j. woo choi, w. kim, r. lee, h. chul woo, j. shin, h. kim, y. jun son, j. young jo, h. lee, s. kwon, c. -l. lee, g. young jung, rsc advances 9 (2019) 14868-14875. [19] p. g. bruce, s. a. freunberger, l. j. hardwick, j.-m. tarascon, nature materials 11 (2012) 19-29. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {atomic layer deposited v2o5 coatings: a promising cathode for li-ion batteries} http://dx.doi.org/10.5599/jese.708 21 j. electrochem. sci. eng. 10(1) (2020) 21-28; http://dx.doi.org/10.5599/jese.708 open access: issn 1847-9286 www.jese-online.org original scientific paper atomic layer deposited v2o5 coatings: a promising cathode for li-ion batteries martyn pemble1,2, ian povey1, dimitra vernardou3, 1tyndall national ιnstitute, university college cork, lee maltings, prospect row, cork, ireland 2school of chemistry, university college cork, cork, ireland 3department of electrical & computer engineering, school of engineering, hellenic mediterranean university, 710 04 heraklion, crete, greece corresponding author: dvernardou@staff.hmu.gr; tel.: +30 2810 379753 received: july 5, 2019; revised: september 14, 2019; accepted: september 25, 2019 abstract a modified, thermal atomic layer deposition process was employed for the pulsed chemical vapour deposition growth of vanadium pentoxide films using tetrakis (dimethylamino) vanadium and water as a co-reagent. depositions were carried out at 350 °c for 400 pulsed cvd cycles, and samples were subsequently annealed for 1hour at 400 °c in air to form materials with enhanced cycling stability during the continuous lithium-ion intercalation/deintercalation processes. the diffusion coefficients were estimated to be 2.0410-10 and 4.1010-10 cm2 s-1 for the cathodic and anodic processes, respectively. these values are comparable or lower than those reported in the literature, indicating the capability of li+ of getting access into the vanadium pentoxide framework at a fast rate. overall, it presents a specific discharge capacity of 280 ma h g-1, capacity retention of 75 % after 10000 scans, a coulombic efficiency of 100 % for the first scan, dropping to 85 % for the 10000th scan, and specific energy of 523 w h g-1. keywords pulsed-cvd; vanadium pentoxide; li+ intercalation/deintercalation; cyclic voltammetry; cycling stability; electron transport properties introduction the global energy crisis and growing ecological concerns have led to intensive development associated with energy use and environmental preservation [1]. for this reason, the need for clean and efficient energy storage has become paramount. new energy industries including electric vehicles (evs) and hybrid electric vehicles (hevs) need large scale energy storage systems (esss) http://dx.doi.org/10.5599/jese.708 http://dx.doi.org/10.5599/jese.708 http://www.jese-online.org/ mailto:dvernardou@staff.hmu.gr j. electrochem. sci. eng. 10(1) (2020) 21-28 atomic layer deposited v2o5 22 with advanced safety, high reliability, high energy density, high power density, low cost and minimal environmental impact [2]. among the various esss, rechargeable lithium-ion batteries (libs) are the most promising candidates owing to their high energy density, long cycle lives and environmental friendliness [3]. nevertheless, one of the key obstacles encountered in the commercialization of libs is the development of electrode materials. in particular, the determining factor for the overall energy density of libs is the capacity of the cathode, which is lower than that of the anode [4]. hence, it is important to find cathodes with high reversibility, high rate capability and extended cycle stability. vanadium pentoxide (v2o5) is a well-known cathode material for libs due to its natural abundance, low cost, relatively easy synthesis and processing, and most importantly its high theoretical capacity of ≈ 440 ma h g-1 [5,6]. however, v2o5 in the bulk form suffers from poor rate capability and cycle stability due to its intrinsic structural instability and low li+ diffusivity (≈ 10-12 cm2 s-1) [7,8]. to address these issues, v2o5 has been synthesized into various nanostructures in order to improve the cycling performance and the rate capability [9-11]. v2o5 nanostructures offer many advantages [12] such as the short diffusion length facilitating the transport kinetics of electrons and li+. the small particle size and the large electrode/electrolyte contact area enhance the utilization of active materials accommodating more effectively the strain of li intercalation/deintercalation. this paper deals with the atomic layer deposition (ald) for the growth of v2o5 as the cathode material. this method has several advantages [13] compared to other conventional thin-film deposition techniques, such as sol-gel, physical vapor deposition and chemical vapor deposition including the conformality, digital thickness control down to the sub-å level, low substrate temperature, uniform and pinhole-free films and low impurity content. the first application of ald for libs was initially reported in 2000 [14]. since then, there are several research papers dealing with ald v2o5 as the cathode material. crystalline v2o5 was readily obtained by ald without any thermal post-treatment from vanadium (v) oxytri-isopropoxide [vo(oc3h7)3, vtop] and ozone (o3) in a temperature window of 170-185 °c [15,16]. these films presented an initial specific discharge capacity of 147 ma h g-1and capacity retention of 100 % after approximately 100 cycles. additionally, α-v2o5 has been synthesized on carbon nanotubes (cnts) by ald presenting an initial specific discharge capacity of 450 ma h g-1, which drops to approximately 200 ma h g-1 after 100 scans [17]. one can then observe that most studies have been focused on organic electrolytes, and rarely on aqueous electrolytes. in this paper, we present the electrochemical performance of ald v2o5 in an aqueous solution of 0.001 mol cm-3 licl showing excellent rate capability and cycling stability (capacity retention of 75 % after 10000 scans). experimental growth procedure thin films were deposited using a cambridge-nanotech fiji ald system on ito coated glass substrates with a sheet resistance of 30-60 ω/cm2 (sigma-aldrich). tetrakis (dimethylamino) vanadium (iv) (v(nme2)4) (strem us, min. 95 % tdmav) and water were used as the vanadium source and coreagent in a thermal ald process. depositions were carried out at 350 °c for 400 pulsed-cvd cycles using a nanolaminate method. pulse durations for tdmav and water were 300 and 100 ms respectively and all pulses were separated by 10 second purges. samples were subsequently annealed for 1h at 400 °c in air. further details of the film deposition method have previously been reported [18]. m. pemble et al. j. electrochem. sci. eng. 10(1) (2020) 21-28 http://dx.doi.org/10.5599/jese.708 23 characterization techniques x-ray diffraction (xrd) was performed using a rigaku (rint 2000) diffractometer with cu kα for 2 = 10.00-50.00o, a range of step sizes and time step 30 s/o for a glancing angle 0.5o. raman measurements were performed with a nicolet almega xr micro-raman system operating at wavenumber range 100-1100 cm-1 using a 473 nm laser. the morphology of the films was studied by scanning electron microscopy (sem) with a fei quanta feg 650 sem and the surface elemental estimation was evaluated using energy dispersive x-ray spectroscopy (eds) analysis in a jeol jsm-7000 microscope. electrochemical evaluation of the samples was performed in a three-electrode cell as reported previously [19-22] using 0.001 mol cm-3 licl aqueous solution as an electrolyte for 10000 continuous li+ intercalation/deintercalation scans at 10 mv s-1. the counter electrode was pt, the working electrode was ald v2o5 and all potential values were taken with respect to a standard ag/agcl reference electrode. the geometrical area of the working electrode (i.e. ald samples) was 1 cm2. additionally, cyclic voltammograms were obtained at scan rates of 1.5, 5, 10, 20 and 50 mv s-1 for the diffusion coefficient estimation. chronopotentiometric measurements were performed at a constant specific current of 450 ma g-1 and at potential ranging from -0.8 v to +1.0 v. finally, electrochemical impedance spectroscopy (eis) studies were obtained by pgstat302n, and the same instrument was used in all electrochemical measurements. nyquist plots were acquired for the alternating current (ac) amplitude of 5.0 ma and set potential +0.3 v over the frequency range of 100 khz to 10 mhz. results and discussion morphology as shown in figures 1 and 2, there is little to distinguish among the un-annealed and annealed samples, with the samples showing signs of some degree of texturing. eds analysis confirms the presence of vanadium oxide (figure 1 (c)). the thickness of v2o5 was estimated to be 22 ± 3nm (fig. 2). energy, kev figure 1. sem micrographs of un-annealed (a) and annealed at 400 °c for 1 h (b), v2o5 filmsfor a magnification 50,000 and a bar equal with 2 μm. eds spectrum for annealed v2o5 film (c) http://dx.doi.org/10.5599/jese.708 j. electrochem. sci. eng. 10(1) (2020) 21-28 atomic layer deposited v2o5 24 figure 2. cross-sectional tem micrographs of v2o5 film annealed at 400 °c for 1 hour (1 v2o5; 2 ito surface layer of substrate) structure xrd patterns are presented in figure 3(a). films prepared by thermal ald were amorphous ‘asgrown’ and a post annealing step was therefore required to produce crystalline films with enhanced stability. as shown in figure 3, in the case of annealed v2o5 sample, three peaks assigned to orthorhombic α-v2o5 [7] were observed according with joint committee on powder diffraction standard no. 41-1426. the intensity of the peaks is low due to the small thickness of v2o5 layer. raman spectrum of the annealed v2o5 sample is in excellent agreement with that reported previously [23] (figure 3(b)) indicating all characteristic vibrations with no shift. figure 3. xrd pattern (a) and raman spectrum (b) of annealed v2o5 samples at 400 °c for 1 h cyclic voltammetry cyclic voltammograms in the form of current density-potential curves are presented in figure 4 in order to illustrate the stability and reversibility of the samples after sweeping the potential from -1 to +1 v and back for 10000 continuous li+ intercalation/deintercalation cycles. from figure 4, it may be seen that the voltammogram consists of three cathodic peaks at +0.28 v, +0.04 v and -0.91 v, which were accompanied by a three-step color change of v2o5, yellow→lightblue→green, suggesting that v5+ is being reduced to v4+ and v3+. the reverse reactions also take place, as indicated by three corresponding anodic peaks at -0.47, +0.37 and +0.54 v. the current density was found to decrease from 0.5 ma cm-2 (first scan) to 0.25 ma cm-2 (2500th scan) and remained constant afterwards, indicating the prolonged stability of the electrode as supported by the voltammograms showing no evidence of degradation after cycling for 10000 times. the sample presents enhanced electrochemical activity as compared with the literature [24,25]. it was not possible to study the electrochemical performance of the ‘as-grown’ v2o5 for more than five scan numbers because it fell apart in the electrolyte due to the poor stability. m. pemble et al. j. electrochem. sci. eng. 10(1) (2020) 21-28 http://dx.doi.org/10.5599/jese.708 25 figure 4. cyclic voltammograms displayed as current density vs. potential curves of v2o5 film, scan rate of 10 mv s-1 and varying number of continuous li+ intercalation/deintercalation scans up to 10000. the electrolyte utilized in all cases was 0.001 mol cm-3 licl aqueous solution cyclic voltammograms recorded at different scan rates are presented in figure 5(a). from figure 5(a), it may be seen that the peak current increases with scan rate, while the shape of the curves is essentially retained suggesting the operation of fast li+ intercalation/deintercalation reactions [26]. furthermore, a linear relationship is obtained from the plot of peak current density as a function of the square root of scan rate suggesting diffusion-controlled processes (figure 5(b)). the li+ diffusion coefficient can be estimated according to equations (1) and (2) which, however, should be taken with certain reserve [20]: ip =d1/2 2.72105 n3/2acν1/2 (1) 5 1 2 3 22 72 10 / /. a d n ac =  (2) in eqs. (1)and (2), ip is the peak (anodic and cathodic) current, a; d is the diffusion coefficient, cm2 s-1; n is the number of electrons; a is the crosssectionalarea, cm2; c is the concentration of li+, mol cm-3; v is the scan rate, v s-1 and a is the slope obtained from figure 5(b). in this study a and n have unity values, while c was declared to be 0.001 in equation (2). the diffusion coefficient was found to be 2.0410-10 and 4.1010-10 cm2 s-1 for the cathodic and anodic processes, respectively. by comparison with the values taken from the literature in table 1, it may be concluded that the v2o5 films produced in this work show comparable or lower diffusion coefficients as compared with other materials. figure 5. cyclic voltammograms displayed as current density vs. potential curves of the annealed v2o5 film recorded at 1.5, 5, 10, 20 and 50 mv s-1 (a). anodic (squares) and cathodic (circles) peak current density values as a function of the square root of scan rate (the fitting is also indicated) (b) http://dx.doi.org/10.5599/jese.708 j. electrochem. sci. eng. 10(1) (2020) 21-28 atomic layer deposited v2o5 26 table 1: diffusion coefficient values of other materials in aqueous electrolytestaken from the literature. the chronopotentiometric curves of v2o5 films under the constant specific current of 450 ma g-1 and potential ranging from -0.8 v to +1.0 v vs. ag/agcl are presented in figure 6(a). it is indicated that the discharging process shows three plateau regions at approximately -0.64 v, +0.17 and +0.37 v indicating the three-step process as also observed in cyclic voltammetry analysis. the specific discharge capacity reported for v2o5 is found to be 280 ma h g-1 with capacity retention of 75 % after 10000 scans, which is comparable with others reported in the literature, i.e. 140 [16], 350 [17] and 368 ma h g-1 [30] with a capacity retention close to 100 % for shorter scan numbers as compared with our work. nevertheless, figure 6(b) presents the trend of specific discharge capacity as a function ofthe scan number. the continuous li+ intercalation/deintercalation processes cause dissolution of v2o5 film resulting in the fading of discharge capacity from the first to the 2500th scan, which however remains stable enough for 10000 scans. additionally, figure 6(b) indicates the coulombic efficiency as estimated from the specific discharge capacity divided by the specific charge capacity. it is found to be 100 % for the first scan,dropping to 85 % for the 10000th scan. figure 6. chronopotentiometric curves for v2o5 film under specific current of 450 ma g -1 and potential ranging from -0.8 v to +1.0 v (a). specific discharge capacity and coulombic efficiency as a function of the scan numbers (b) the specific discharge capacity was evaluated at different specific currents ranging from 450 ma g-1 to 1500 ma g-1 (figure 7(a)). it is shown that the specific discharge capacity drops to 225 ma h g-1 at the highest specific current (1500 ma h g-1), presenting retention of 80 %. furthermore, the cathode can still deliver a specific discharge capacity of 278 ma h g-1, when the specific current returned to 450 ma g-1, showing good structural stability. nyquist plots consist of semicircles in high-frequency regions and sloped lines in low-frequency regions (figure 7 (b)). these correspond to the charge transfer resistance at the ald v2o5 cathode/electrolyte interface and warburg impedance associated with li-ion diffusion in v2o5. the material diffusion coefficient, cm2s-1 reference 3d-printed graphene 1.9110-7 [19] mno2 0.882 10-11 [20] nanorutile tio2 7.010-8 [27] nano-cao-sno2 1.0 x 10-14 [28] cs4pbbr6 7.3410-8 [29] v2o5 4.1010-10 this work m. pemble et al. j. electrochem. sci. eng. 10(1) (2020) 21-28 http://dx.doi.org/10.5599/jese.708 27 plots were fitted based on a circuit mentioned elsewhere [29]. the charge transfer resistance from the 1st (5 ω) to the 10000th scan (12 ω) endow the sustainable performance of the ald v2o5. figure 7. rate capability of v2o5 film at specific current ranging from 450 ma g -1 to 1500 ma g-1 and then back to 450 ma g-1 (a). electrochemical impedance spectra of the first scan (black) and the 10000th scan (red) (b). conclusions a thermal ald process for growth of v2o5 films was developed using tetrakis (dimethylamino) vanadium and water as a co-reagent. the process was subsequently modified in order to anneal v2o5 films with the aim of increasing stability of films on the substrate. dense, uniform, crack-free crystalline films were produced. the diffusion coefficient was 2.0410-10 and 4.1010-10 cm2 s-1 for the cathodic and anodic processes, respectively, allowing li+ access into v2o5 at a fast rate. overall, the ald v2o5 is a promising cathode for libs because it combines useful characteristics such as good stability, high specific discharge capacity of 280 ma h g-1 with capacity retention of 75 % and coulombic efficiency of 85 % after 10000 scans, high specific energy of 523 w h g-1 and ease of electron transport. the dissolution of v2o5 is a problem in the initial scans, which is restrained for longer periods benefiting this material as an electrode in libs. acknowledgements: the support of science foundation ireland principal investigator grant, depo-man (15/ia/3015) is gratefully acknowledged. references [1] g. cai, x. wang, m. cui, p. darmawan, j. wang, a. l. -s. eh, p. s. lee, nano energy 12 (2015) 258-267. 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[30] x. chen, e. pomerantseva, k. gregorczyk, r. ghodssi, g. rubloff, rsc advances 3 (2013) 4294-4302. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {the effect of different factors on electrochemical obtaining of alloys re-te-cu} http://dx.doi.org/10.5599/jese.895 107 j. electrochem. sci. eng. 11(2) (2021) 107-114; http://dx.doi.org/10.5599/jese.895 open access: issn 1847-9286 www.jese-online.org original scientific paper effects of different factors on electrochemical obtaining of re-te-сu alloys elza a. salakhova, dilgam b. tagiyev, parvana e. kalantarova, ramila e. huseynova and i̇rana i̇. cabbarova institute of catalysis and inorganic chemistry of azerbaijan national academy of sciences, az1143, h. javid ave. 113, baku, azerbaijan corresponding author: elza_salahova@mail.ru received: august 2, 2020; revised: february 7, 2021; accepted: february 15, 2021 abstract based on the study of volt-ampere dependencies during electroreduction of rhenium (vii), tellurium (iv) and copper (ii) ions from hydrochloric acid electrolyte on pt electrode, the conditions of deposition of nano-coatings of ternary re-te-cu alloy were determined. the influence of various factors such as amounts of individual components and total concentrations of components in the electrolyte solution, temperature, concentration of hydrochloric acid, etc., on the composition and quality of coatings was studied. it was established that with the increase of rhenium concentration in the electrolyte, as well as increase of temperature, the rhenium content in deposits increased. the morphology of films deposited on platinum substrate was studied using the scanning electron microscopy. based on the experimental data, the following electrolyte composition (mol/l) is recommended for obtaining the semiconductor re-te-cu alloy containing 40-50 wt.% re: (6.9·10-4-6.9·10-3) kreo4 + (9·10-4-1.8·10-2) teo2 + (6·10-4-1.2·10-2) cucl2·2h2o +2 hcl, t = 75 c. keywords rhenium alloys; ternary alloys; thin coatings; electrochemical deposition; electrolyte composition introduction one of the most important tasks of metal science is to: a) study the structure and properties of refractory metals and alloys, b) develop technological schemes for obtaining pure refractory metals and metal products, and c) study their operational characteristics in various devices and tools. the general aim is to scale up the application of refractory metals and alloys, since materials (particularly refractory metals) often play a decisive role in the development of entire industries of new technology. refractory metals have unique properties that make them indispensable in many http://dx.doi.org/10.5599/jese.895 http://dx.doi.org/10.5599/jese.895 http://www.jese-online.org/ mailto:elza_salahova@mail.ru j. electrochem. sci. eng. 11(2) (2021) 107-114 electrochemical obtaining of re-te-сu alloys 108 aspects of new technology. they have high melting, boiling and recrystallization temperature as well as low vapor pressure, which all allow use of these metals for long-term operation at high temperatures in deep vacuum. rhenium (re) is a heat-resistant metal that has widely been used as high-tech material with exceptional properties. this metal has some specific properties and is used in various fields of semiconductor industry for space technology, electronics, etc. rhenium is an important component of heat-protection shields for reducing the temperature of objects returned from the space. also, common filaments produced from tungsten coated with rhenium increase the service life of lamps, while very thin rhenium coatings are found highly effective for the corrosion protection of metals. rhenium chalcogenides are semiconductor materials, which can be used as photosensitive elements in the visible spectrum [1-5]. rhenium alloys with sulfur are in the form of thin coatings used as photosensitive materials in the semiconductor engineering. also, rhenium-sulfur alloys are frequently used as efficient catalysts in dehydrogenation of alcohols. various methods have already been used to produce thin films of semiconducting rhenium alloys. analysis of all known methods of obtaining thin layers of rhenium chalcogenides showed clearly that the electrochemical method is the most promising and economical [6-12]. therefore, the search and development of new electrochemistry related techniques to produce thin semiconductor films of rhenium chalcogenides on various substrates including metal substrates, is considered not only practically, but also theoretically relevant [13-18]. the present work is devoted to the electrochemical deposition of ternary semiconductor re-cute coatings from hydrochloric acid electrolyte, and the study of the effects of various factors on the composition and quality of deposited ternary alloys. experimental polarization curves were measured in a glass cell equipped with 50 ml of glass jacket. polarization measurements were carried out with platinum electrode having the surface of 0.15 cm2, welded to a glass tube. silver/silver-chloride electrode (ag/agcl) was used for polarization measurements as the reference electrode. platinum plate with the surface area of 2.0 cm2, having thus several times higher area then the working electrode, was used as the counter electrode. with the aim of studying the kinetics of co-electrodeposition of rhenium with some chalcogens, the method of cyclic voltammetry was applied using iviumstat. for the composition analysis, co-electrodeposition was performed on pt substrate with the surface area of 2.0 cm2 under galvanostatic conditions. for the structure, morphology and elemental analyses, co-electrodeposition under potentiostatic conditions was performed. deposition time ranged from 30 to 60 min. to check the scientific reproducibility, each experiment was performed twice. the working temperature for electrodeposition was 75 °c, except for experiments when the effect of temperature was followed. temperature was regulated with the accuracy of ± 0.1 °c using u-10 thermostat. morphologies of deposited films and electrode surfaces were studied using jeol jsm7600f scanning electron microscope (sem) at various magnifications, while the elemental analysis was carried out using oxford x-max 50 detector (eds). the sample was scanned in secondary electron mode at an acceleration voltage of 15 kev. x-ray diffraction analysis of films was performed using dron-5 at cu k α-radiation. the amount of copper was determined separately by an atomic-absorption spectrophotometer aas-1n carl zeiss jena. the amounts of rhenium and tellurium were determined separately by thiourea complex, using colorimetric method on specord 50 plus. current output was detected with copper coulombmeter by weighing method and was calculated with regard to the composition of deposit. ph of solutions was determined at ph/mv/temp az 86551. the composition of e. a. salakhova et al. j. electrochem. sci. eng. 11(2) (2021) 107-114 http://dx.doi.org/10.5599/jese.895 109 re–te–cu alloy was analyzed as follows: firstly, the electrochemically obtained cathode deposit was heated and dissolved in 10 ml of nitric acid, and after vaporizing several times in water steam, 5 n nh3po4 is added. then, rhenium was separated from tellurium and copper by extracting with isoamyl alcohol. when both te and re were separated, they are determined at specord 50 plus using spectrophotometric method. determination of copper in re-te-cu system was conducted at aa 280 fs fast sequential atomic absorption spectrometer varian. results and discussion to study the co-deposition of rhenium, tellurium and copper, the deposition processes of these metals from a chosen electrolyte must be studied separately. for this purpose, electrolytic depositions of rhenium, tellurium and copper from 2 m hcl solution were studied by cyclic voltammetry (cv) technique. in chloride electrolyte, rhenium is present in the form of perrhenate, reo4(vii), ions and reduction of rhenium is proceding in several stages [7-10]. it is seen in figure 1(a) that at potentials preceeding hydrogen evolution reaction, the reduction of reo4ions in the strong acidic electrolyte goes in stages. reduction in a order re(vii)→re(vi)→re(iv)→re(0) leads to the formation of intermediate oxide layers, what is confirmed by x-ray analysis and red and blue colors in deposits. by studying the effects of the concentration of rhenium, temperature of electrolyte, and concentration of hcl solution, the optimum conditions were selected for obtaining high-quality rhenium deposits from chloride electrolyte. electrolytic deposition process of tellurium from 2 m hcl solution was also studied by cyclic voltammetry, and typical cv curve is shown in figure 1(b). optimum conditions for deposition of tellurium from hcl solution were determined by studying effects of concentration of tellurium, and temperature on polarization curves. reduction of tellurium ions is proceeding according to the following way: the first stage of reduction process is determined by formation of elementary tellurium te(0) from hteo2+(iv) ions present in highly acid electrolyte. in the second stage, divalent tellurium (te2)-2(-ii) is formed, while in the third stage of electrode process, the formation of hydrogen was observed. in the reverse scan, a peak at about 0.3 v denotes anodic dissolution of the formed tellurim film. cyclic polarization curve presented in figure 1(c) shows the reduction process of copper ions at the cathode. optimum conditions for deposition of copper were determined by studying the effects of copper ion concentration in the electrolyte, temperature, and acidity of electrolyte. stepwise reduction of cu(ii)→cu(0) is observed before hydrogen evolution reaction, what is probably due to the slowness of cu(ii)→cu(i) step caused by stable cu(ii)clx complexes formed in chloride solutions. one stripping wave is observed in the anodic curve of copper at -0.05 v due to oxidation of copper. to study co-deposition of re-te-cu components, an electrolyte containing all three components was used, and polarization curves of co-deposition are plotted in figure 1(d). when observing polarizarion curves of re-cu-te in figure 1(d), one can clearly see three anodic stripping waves. wave i in anodic curve can be related to oxidation of cu, wave ii to oxidation of te, and wave iii to anodic dissolution of new triple re-te-cu alloy film. in order to obtain thin semiconductor coatings of rhenium-copper-tellurium alloy from chloride electrolyte and to find optimal conditions for co-deposition, effects of various factors were studied: concentrations of components, temperature, current density of deposition, and concentration of hcl solution. as was expected, the composition and quality of re-cu-te alloy films are highly affected by rhenium content in the electrolyte (table 1). http://dx.doi.org/10.5599/jese.895 j. electrochem. sci. eng. 11(2) (2021) 107-114 electrochemical obtaining of re-te-сu alloys 110 a b potential, v vs. ag/agcl potential, v vs. ag/agcl c d potential, v vs. ag/agcl potential, v vs. ag/agcl figure 1. cyclic polarizing curve (0.005 v s-1) of rhenium (a), tellurium (b), copper (c), rhenium-tellurium-copper (d) on pt electrode in the electrolyte containing (mol/l): (a) 6.9·10-3 kreo4 + 2 hcl; (b) 1.8·10-2 teo2 + 2 hcl; (c) 1.2·10-2 cucl2·2h2o + 2 hcl; (d) 6.9·10-3 kreo4 +1.8·10 -2 teo2 +1.2·10 -2 cucl2·2h2o + 2 hcl; t=75 °c table 1. composition of re-cu-te alloy films deposited on pt electrode from 2m hcl containing different concentrations of rhenium and constant concentrations of copper and tellurium. t = 75 oс, ik = 20 ma cm -2 electrolyte concentration, mmol l-1 content, wt.% appearance of coating kreo4 cucl2 teo2 re te cu 0.2 1.5 1.23 40 33 27 dark-gray, matte 0.6 1.5 1.23 42 32 26 dark-gray, matte 1.0 1.5 1.23 44 31 25 black, shiny 1.4 1.5 1.23 46 30 24 black, shiny 1.5 1.5 1.23 49 24 26 black, shiny it was also established that at higher content of rhenium in the electrolyte, the rhenium content in the alloy increased with an increase of the electrolyte temperature (table 2). high quality deposit coatings in the form of thin films are obtained at 75-80 °c. at 25-45 oc, however, re-cu-te alloy with an excess of amorphous tellurium was obtained on the cathode, while at the temperature above 80 oс, the quality of deposits deteriorates. therefore, all experiments were performed at 75-80 oс. the effect of deposition current density on the composition and quality of re-cu-te alloy deposits was studied at 75 °с on pt cathode (table 3). as the current density of deposition increased, the rhenium content in the alloy increased from 40 to 50 wt.%. this can be explained by the fact that as the current density increases, the reduction of rhenium accelerates compared with the reduction of tellurium, and therefore the rhenium content in the alloy increases. c u rr e n t, m a c u rr e n t, m a c u rr e n t, m a c u rr e n t, m a e. a. salakhova et al. j. electrochem. sci. eng. 11(2) (2021) 107-114 http://dx.doi.org/10.5599/jese.895 111 table 2. composition of re-cu-te alloy films deposited on pt electrode (ik = 20 mа/cm 2) from 2m hcl containing constant concentrations of rhenium, copper, and tellurium at different temperatures electrolyte concentration, mmol l-1 temperature, °с content, wt.% appearance of coating kreo4 cucl2 teo2 re te cu 1.4 1.5 1.23 25 42 30 28 dark-gray, matte 1.4 1.5 1.23 45 44 32 24 dark-gray, matte 1.4 1.5 1.23 65 45 31 24 black, shiny 1.4 1.5 1.23 75 46 30 24 black, shiny 1.4 1.5 1.23 90 48 26 26 black, shiny table 3. composition of re-cu-te alloy films deposited on pt electrode from 2m hcl containing constant concentrations of rhenium, copper, and tellurium at different deposition current densities. t = 75 °с; ckreo4 = 1.4 mmol l -1; ccucl2 = 1.5 mmol l -1; cteo2 = 1.23 mmol l -1 ik / mа cm-2 content, wt.% appearance of coating re te cu 5 42 30 28 dark-gray, matte 10 44 32 24 dark-gray, matte 15 45 31 24 black, shiny 20 46 30 24 black, shiny 25 47 27 26 black, shiny the effect of hcl solution concentration on the composition and quality of re-cu-te alloys was also studied (table 4). since re-cu-te alloy film is deposited from an acidic electrolyte, the effect of acid concentration on the composition and quality of alloys has its importance. the composition of re-cu-te cathode deposit is also highly dependent on the concentration of the acid in electrolyte. with the increase of hcl concentration in electrolyte, rhenium content in the alloy increases, but tellurium content decreases. table 4. composition of re-cu-te alloy films deposited on pt electrode (iк = 20 mа/cm 2) from different concentrations of hcl containing constant concentrations of rhenium, copper, and tellurium t = 75 °с; ckreo4 = 1.4 mmol l -1; ccucl2 = 1.5 mmol l -1; cteo2 = 1.23 mmol l -1 chcl / mol l-1 content, wt.% appearance of coating re te cu 1.0 42 30 28 dark-gray, matte 1.5 44 30 26 dark-gray, matte 2.0 46 30 24 black, shiny 2.5 47 28 25 black, shiny 3.0 48 26 26 black, shiny morphologies of re-cu-te films deposited on platinum substrates were studied by a scanning electron microscope. it was established generally that depending on the material of substrate, thin coatings with different compositions and sizes were obtained on the cathode when prepared at the same experimental conditions. however, coatings with grain sizes of 80-150 nm were obtained on pt substrate electrode. sem image and elemental composition of re-cu-te film deposited on pt substrate are shown in figures 2 and 3. upon thorough investigation, a structure similar to nanostructure and formation of nanosized islets on a platinum sample is observed. islets do not have a certain shape; they have forms of an egg, ellipse, and various other shapes. x-ray pattern of re-cu-te thin film deposited on pt electrode and annealed at 500 °c for 2 hours is presented in figure 4. http://dx.doi.org/10.5599/jese.895 j. electrochem. sci. eng. 11(2) (2021) 107-114 electrochemical obtaining of re-te-сu alloys 112 figure 2. sem image re-te-cu thin film deposited on pt substrate energy, kev figure 3. elemental analysis of re-te-cu thin film deposited on pt electrode 2 /o figure 4. x-ray pattern of deposited re-cu-te thin film annealed at 500 °c for 2 hours it was found that depending on the electrolyte composition and current density during electrolysis, the substances comprising one and two phases are obtained. at optimum deposition conditions, the in te n si ty , a . u . e. a. salakhova et al. j. electrochem. sci. eng. 11(2) (2021) 107-114 http://dx.doi.org/10.5599/jese.895 113 obtained re-te-cu thin films on a platinum electrode contains oxide compounds. to purify the composition of films from various oxides, the substances obtained electrochemically in re-te-cu system were annealed at 500 °c for 2 hours and an x-ray pattern was recorded. intensity of the peaks becomes much more visible after annealing. this may be due to annealing of oxides in films. conclusions based on the study of volt-ampere dependencies during co-electroreduction of rhenium (vii), tellurium (iv) and copper (ii) ions from chloride electrolytes on pt electrode, the conditions of deposition of nano-coatings of ternary re-te-cu alloy system were determined. the influence of various factors on the composition and quality of coatings was studied. these factors were concentrations of individual components and total concentration of components in the electrolyte, temperature of electrolyte, current density of deposition and concentration of hcl solution. it was established that with the increase of rhenium content in the electrolyte and temperature, the rhenium content in deposits increases. based on the experimental data, the following electrolyte composition was recommended for obtaining the semiconductor re-te-cu alloy on pt electrode in the electrolyte solution containing 40-50 wt.% of re: (6.9·10-4-6.9·10-3) m kreo4 + (9·10-4-1.8·10-2) m teo2 + (6·10-4-1.2·10-2) m cucl2·2h2o, + 2m hcl. t=75 °c. the morphology of films deposited on pt substrate electrode is followed by the scanning electron microscope, showing formation of nanosized islets with grain sizes of 80-150 nm. references [1] e. f. speranskaya, electrochemistry of rhenium, gylym, alma-ata, 1990. p. 253. 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[17] s. de, w. d. sides, t. brusuelas, q. huang, journal of electroanalytical chemistry 860 (2020) 113889 https://doi.org/10.1016/j.jelechem.2020.113889. [18] b. p. hahn, k. j. stevenson, electrochimica acta 55(22) (2010) 6917-6925 https://doi.org/10.1016/j.electacta.2010.05.001. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://www.journalcra.com/ https://doi.org/10.1016/j.jelechem.2020.113889 https://doi.org/10.1016/j.electacta.2010.05.001 https://doi.org/10.1016/j.electacta.2010.05.001 https://creativecommons.org/licenses/by/4.0/) {comparative analysis of adsorption and corrosion inhibitive properties of ethanol extract of dialium guineense leaves for mild steel in 0.5 m hcl} doi:10.5599/jese.486 219 j. electrochem. sci. eng. 8(3) (2018) 219-226; doi: http://dx.doi.org/10.5599/jese.486 open access : : issn 1847-9286 www.jese-online.org original scientific paper comparative analysis of adsorption and corrosion inhibitive properties of ethanol extract of dialium guineense leaves for mild steel in 0.5 m hcl shola elijah adeniji, bamigbola abiola akindehinde department of chemistry, ahmadu bello university zaria, nigeria corresponding author e-mail: shola4343@gmail.com; received: december 4, 2017; revised: march 16, 2018; accepted: march 19, 2018 abstract adsorption and corrosion inhibitive properties of ethanol extract of dialium guineense leaves for mild steel in 0.5 m hcl was studied using the gravimetric method. the results showed that the ethanol extract of dialium guineense leaves is a good corrosion inhibitor for mild steel in 0.5 m hcl. the inhibition efficiency was found to increase with increase in the concentration of ethanol extract of dialium guineense leaves up to the maximum of 92 %, but at the same time it decreased as the temperature was increased. corrosion inhibition by the extract of dialium guineense leaves is carried out by adsorption mechanism with the kinetics of corrosion following the pseudo first order reaction with high correlation. thermodynamic consideration revealed that adsorption of the ethanol extract of dialium guineense leaves on mild steel surface is an exothermic and spontaneous process that fitted the langmuir adsorption isotherm. the values of activation energy and gibb’s free energy were found within the range of limits expected for the mechanism of physical adsorption. keywords corrosion rate; dialium guineense; inhibitor efficiency; langmuir isotherm; mild steel introduction corrosion is a deterioration of a metal due to its interaction with the environment such as water, acid, alkaline, air etc. due to corrosion, many useful properties of a metal such as malleability, ductility and electrical conductivity are lost. metals are particularly prone to corrosion in hydrochloric and sulphuric acids that are in turn widely used for acid pickling, industrial acid cleaning, acid descaling and oil-well cleaning [1]. in such conditions, use of inhibitors became one of the most practical methods for protection against corrosion. synthetic organic compounds are widely used as corrosion inhibitors for the prevention of corrosion of many metals and alloys in http://dx.doi.org/10.5599/jese.486 http://www.jese-online.org/ mailto:shola4343@gmail.com j. electrochem. sci. eng. 8(3) (2018) 219-226 corrosion inhibitive properties of dialium guineense 220 various aggressive environments. some of the synthetic organic compounds that show good anticorrosive activity are, however, highly toxic and can cause hazards to both human and the environment during their application [2]. the environmental hazards caused by the use of synthetic inhibitors have always been a global issue. in order to avoid these hazards, the researchers have focused their attention on developing cheap, non-toxic, biodegradable and environment friendly natural products of plant origin as corrosion inhibitors [3]. plant extracts, including that of dialium guineense leaves, have already been reported to be environmental friendly, readily available, renewable and acceptable source of a wide range corrosion inhibitors by some researchers [4–18]. the antioxidant and antimicrobial activities of dialium guineense leaves extract has also been reported [19]. the present study aims to investigate inhibitive properties of dialium guineense leaves extract for corrosion of mild steel at 303 k and 333 k, using the gravimetric technique and experiments performed with and without the extract present in 0.5 m hcl. experimental techniques preparation of plant extract (inhibitor) fresh leaves of dialium guineense were obtained from ahmadu bello university botanical garden. the leaves were firstly rinsed with distilled water to remove foreign particles and then sun dried, ground and soaked in ethanol for 2 days. after 2 days, the sample was cooled and filtered. the filtrate was subjected to evaporation at 79 °c in order to leave the sample free from ethanol. different concentrations of the extract were prepared by dissolving 0.1, 0.2, 0.3, 0.4, and 0.5 g of the extract in 1dm3 of 0.5 m hcl, respectively. preparation of metal specimen material used for the study was a mild steel sheet of the following composition (wt): mn (0.6), p (0.36), c (0.15) and si (0.03) and fe (98.86). the sheet was cut into coupons, each of dimensions 2×4×0.1 cm. each coupon was polished with emery papers of different grit sizes in order to obtain a smooth surface, degreased with ethanol, cleaned with acetone and allowed to dry in air before its preserving in a desiccator. gravimetric measurements the mild steel coupon was accurately weighed and fully immersed in 100 ml of corrosive solution (without or with the inhibitor present in 0.5 m hcl) in an open beaker. the beaker was covered with the aluminum foil and maintained at 303 k for different time intervals. after every 24 hours, the corrosive product was removed from the coupon which was further rinsed in distilled water, degreased with ethanol, cleaned with acetone and allowed to dry in air before the re-weight. the experiment was repeated at 323 k and the weight loss was determined in triplicate. determination of weight loss the weight loss (w) was calculated using the following formula: w = wi wf (1) where wi is the initial weight in g of the coupon before immersion, while wf is the final weight in g of coupon after immersion. determination of corrosion rate the corrosion rate (cr) was calculated using the following formula: s. e. adeniji and b. a. akindehinde j. electrochem. sci. eng. 8(3) (2018) 219-226 doi:10.5599/jese.486 221 1 87.6 cr, mm year w dat − = (2) where w is the weight loss in g, d is density in g cm-3, a is the area of specimen in cm2 and t is time of immersion in hours. determination of inhibition efficiency the inhibition efficiency (ie) was calculated using the following formula: 1 2 1 cr cr ie, % 100 cr − = (3) where ie is inhibition efficiency, cr1 is corrosion rate in the absence of inhibitor, while cr2 is corrosion rate in the presence of inhibitor. determination of surface coverage the surface coverage ( ) was calculated using the following formula: 0 1 0 w w w  − = (4) where w0 is the weight loss in absence of inhibitor, while w1 is the weight loss in presence of inhibitor. results and discussion mild steel corrosion behavior was investigated in the absence and presence of dialium guineense extract in 0.5 m hcl, using the gravimetric method. the variation of weight loss with immersion time for the corrosion of mild steel in 0.5 m hcl containing different concentrations of ethanol extract of dialium guineense leaves at 303 k is shown in figure 1. figure 1: variation of weight loss with immersion time for corrosion of mild steel in 0.5 m hcl containing different concentrations of ethanol extract of dialium guineense leaves at 303 k. it is seen in figure 1 that the weight loss of mild steel increases with increase in immersion time but decreases as the concentration of the ethanol extract of dialium guineense leaves increases. this implies that presence of the extract retards corrosion of the mild steel in 0.5 m hcl. as shown in figure 2, the corrosion rate of mild steel in 0.5 m hcl calculated by eq. (2), decreases with increase in the concentration of the ethanol extract of dialium guineense leaves, suggesting again that the j. electrochem. sci. eng. 8(3) (2018) 219-226 corrosion inhibitive properties of dialium guineense 222 extract inhibits corrosion of the mild steel in 0.5 m hcl. also, corrosion rates at all concentrations of inhibitor increase with the temperature increase, what is due to the increase in the average kinetic energy of the reacting molecule and similar to the already reported results [20,21]. variations of the inhibition efficiency calculated by eq. (3) for different concentrations of the ethanol extract of dialium guineense leaves at 303 k and 323 k are presented in figure 3. significant differences between inhibition efficiency of the ethanol extract of dialium guineense leaves determined at 303 k and 323 k for each concentration of the inhibitor, suggest that the mechanism of adsorption of the inhibitor on the mild steel is physical adsorption [22]. for a physical adsorption mechanism, inhibition efficiency of an inhibitor decreases with temperature while for a chemical adsorption mechanism, inhibition efficiency increases with rise of the temperature. figure 2. variation of corrosion rate with concentration of ethanol extract of dialium guineense leaves at 303 k and 323 k. figure 3: variation of inhibition efficiency of ethanol extract of dialium guineense leaves with concentration at 303 k and 323 k. kinetic consideration the kinetic study on the corrosion inhibition of the corrosion of mild steel in solution of 0.5 hcl by ethanol extract of dialium guineense leaves suggests a run of the pseudo first order reaction according to the following rate constant equation: f 1 i 1 ln w k t w   =     (5) in eq. (5), k1 is the rate constant for the first order reaction, wf is the final weight of coupon after immersion, wi is the initial weight of coupon before immersion, while t is immersion time. the consistency of k1 values listed in table 1 suggests that corrosion of mild steel in either blank solution of 0.5 hcl or with different concentrations of the ethanol extract of dialium guineense leaves is the first order reaction. table1. rate constant of corrosion for different concentrations of dialium guineense leaves extract at different time intervals time, h k1 / h-1 concentrations of dialium guineense leaves extract, mg l-1 0 100 200 300 400 500 24 0.00574 0.00215 0.00179 0.00150 0.000906 0.000418 48 0.00577 0.00225 0.00181 0.00151 0.000908 0.000425 72 0.00581 0.00241 0.00181 0.00151 0.000915 0.000435 96 0.00583 0.00245 0.00185 0.00153 0.000921 0.000441 120 0.00585 0.00256 0.00185 0.00154 0.000927 0.000452 s. e. adeniji and b. a. akindehinde j. electrochem. sci. eng. 8(3) (2018) 219-226 doi:10.5599/jese.486 223 alternatively, k1 can be determined graphically using eq. (5) written in the following form: 1 f i log log 2.303 k t w w= − (6) plot of log wf versus t yields a straight-line graph which is for different concentrations of the extract of dialium guineense leaves presented in figure 4. the half-life (t1/2) was determined using the following equation: 1/2 1 0.693 t k = (7) calculated values of k1 and t1/2 are presented in table 2. from the result obtained, it can be seen that ethanol extract of dialium guineense leaves is a good corrosion inhibitor. figure 4. variation of log wf vs. immersion time for the corrosion of mild steel in 0.1 m hcl containing different concentrations of ethanol extract of dialium guineense leaves. table 2. kinetic parameters of mild steel corrosion inhibited by ethanol extract of dialium guineense leaves. concentration, mg l-1 slope k1 / h-1 t1/2 / h ea / kj mol-1 blank -0.0026 0.00599 115.69 10.66 100 -0.0011 0.00253 273.4 14.56 200 -0.0008 0.00184 376.63 15.56 300 -0.0005 0.00152 455.92 21.19 400 -0.0004 0.000921 752.44 38.91 500 -0.0002 0.000461 1503.3 63.43 effect of temperature the effect of temperature on the corrosion of mild steel in 0.5 m hcl in absence and presence of inhibitor was investigated using the logarithmic form of the arrhenius equation: a2 1 1 2 1 1 log 2.303 ecr cr r t t     = −        (8) in eq. (8), cr1 and cr2 are corrosion rates of mild steel at absolute temperatures t1 (303 k) and t2 (323 k), respectively. ea is the activation energy for the reaction and r is the molar gas constant 8.314 j k-1 mol-1). as can be seen in table 2, the values of activation energy calculated by eq. (8) range from 14.66 to 63.43 kj mol-1. these values are found higher than the value of 10.66 kj mol-1 obtained for the blank solution, which indicates an inhibition of corrosion of the mild steel in 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 j. electrochem. sci. eng. 8(3) (2018) 219-226 corrosion inhibitive properties of dialium guineense 224 0.5 m hcl, caused by presence of the extract. the activation energies are also found lower than the threshold values of 80 kj mol-1 required for the mechanism of chemical adsorption. this implies that adsorption of ethanol extract of dialium guineense leaves on the mild steel surface obeys the mechanism of physical adsorption [22]. thermodynamic consideration the heat of adsorption (qads) for the ethanol extract of dialium guineense leaves on mild steel was calculated using equation (9): 2 1 1 2 ads 2 1 2 1 2.303 log log 1 1 t t q r t t          = −     − − −     (9) where 1 and 2 are the degree of surface coverage of the inhibitor at the absolute temperature t1 (303 k) and t2 (323 k), respectively and r is the molar gas constant. the values of qads calculated using eq. (9) are reported in table 3. these values are negative and range from −9.80 to −52.97 kj mol-1, indicating that adsorption is an exothermic process. the values of free energy of adsorption (∆gads) for the ethanol extract of dialium guineense leaves are calculated using the following equation [23]: ∆gads = −2.303 r t log (55.5kads) (10) where r is the molar gas constant, t is absolute temperature, 55.5 is the molar concentration of water in mol l-1 and kads is the equilibrium constant of adsorption defined as [24]: ads (1 ) k c   = − (11) values of ∆gads calculated using eq. (10-11) are reported in table 3. the values of ∆gads range from −6.76 to −17.05 kj mol-1 and tend to be more negative with rise in the concentration of the inhibitor. this indicates that adsorption of the ethanol extract of dialium guineense leaves on the mild steel surface is spontaneous and the strength of adsorption increases with increase in concentration of the inhibitor. it is also a significant to note that values of ∆gads that are less negative than -40 kj mol-1 suggest the mechanism of physical adsorption. table 3. thermodynamic parameters for adsorption of ethanol extract of dialium guineense leaves on mild steel. concentration, g l-1 qads / kj mol-1 ∆gads / kj mol-1 303 k 323 k 0.1 -9.80 -18.43 -6.76 0.2 -8.25 -16.95 -8.83 0.3 -13.85 -16.65 -10.58 0.4 -33.85 -17.45 -12.83 0.5 -52.97 -18.80 -17.05 adsorption isotherm adsorption isotherms are very important in characterizing inhibition of the corrosion reaction. the most frequent isotherms used are langmuir, freundlich, temkin, flory-huggin, frumkin, parsons, bockris-swinkel and dublin-radushkevich. in this study, the langmuir isotherm was found suitable for the experimental findings and is used to describe the adsorption characteristics of the inhibitor. s. e. adeniji and b. a. akindehinde j. electrochem. sci. eng. 8(3) (2018) 219-226 doi:10.5599/jese.486 225 langmuir isotherm langmuir adsorption isotherm is expressed by the following equation: log (c/ ) = log c + log (1/kads) (12) where kads is the equilibrium constant of adsorption, c is bulk concentration of adsorbing (inhibitor) species and  is the degree of surface coverage. as shown in figure 5, a plot of log c/ versus log c yields a straight-line graph. -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 log (c / g l-1) figure 5. langmuir isotherm for the adsorption of ethanol extract of dialium guineense leaves on mild steel surface the linear plots presented in figure 5 having slopes and correlation coefficients (r2) listed in table 4, indicate that adsorption of the inhibitor is highly consistent with the langmuir adsorption isotherm. the values of kads at two temperatures were determined from the intercept lines on the log (c/ ) axis and listed in table 4, together with ∆gads values for the inhibitor adsorption on the surface of mild steel, calculated using eq. (10). table 4: langmuir adsorption parameters and free energy of adsorption of ethanol extract dialium guineense leaves on mild steel surface t / k slope r2 log kads kads ∆gads / kj mol1 303 0.8432 0.9977 -0.0071 0.9838 -10.08 323 0.8204 0.9968 -0.0611 0.8688 -10.41 conclusion ethanol extract of dialium guineense leaves effectively inhibited corrosion of mild steel in 0.5 m hcl. inhibition efficiency of the extract increased with increase of concentration of dialium guineense leaves but decrease with rise in the temperature. this suggests that corrosion inhibition can be attributed to the physisorption mechanism. the mechanism of physisorption was further confirmed by examining the values of apparent activation energies and heat of adsorption. the adsorption of the ethanol extract of dialium guineense leaves inhibitor on mild steel surface is an exothermic and spontaneous process that fits the langmuir adsorption isotherm. references [1] j. a. selvi, s. rajendran, v. g. sri, a. j. amalraj, b. narayanasamy, portugaliae electrochimica acta 27 (2009) 1-11. 303 k 323 k 0.0 -0.2 -0.4 -0.6 -0.8 j. electrochem. sci. eng. 8(3) (2018) 219-226 corrosion inhibitive properties of dialium guineense 226 [2] a. chinweuba, journal of natural science research 4 (2014) 3180-3186. 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[21] n. ibisi, v. uwakwue, global journal of environmental science and technology 2 (2015) 355-359. [22] a. fouda, h. tawfik, a. badr, advances in materials and corrosion 1 (2012) 1-7. [23] a. hamdy, n. s. el-gendy, egyptian journal of petroleum 22 (2013) 17-25 [24] e. a. noor, journal of applied electrochemistry 39 (2009) 1465-1475. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) wastewater treatment by multi-stage batch adsorption and electrochemical regeneration doi: 10.5599/jese.2012.0019 223 j. electrochem. sci. eng. 2 (2012) 223-236; doi: 10.5599/jese.2012.0019 open access : : issn 1847-9286 www.jese-online.org original scientific paper wastewater treatment by multi-stage batch adsorption and electrochemical regeneration fadhil m. mohammed‡, edward p. l. roberts, andrew k. campen* and nigel w. brown* school of chemical engineering and analytical science, university of manchester. the mill, oxford road, manchester m13 9pl, uk; ‡ current address: ministry of science and technology, baghdad, iraq *arvia technology ltd, daresbury innovation centre, keckwick lane, daresbury, cheshire wa4 4fs, uk. corresponding author: e-mail: edward.roberts@manchester.ac.uk; tel.: +44-161-306-8849; fax: +44-161-306-9321 received: may 23, 2012; revised: august 18, 2012; published: november 10, 2012 abstract the removal and destruction of a tri-phenyl methane dye, acid violet 17 (av17), from aqueous solution by adsorption and electrochemical regeneration was studied using a graphite intercalation compound (gic) adsorbent. it was demonstrated that the adsorbent could be regenerated by anodic oxidation of the adsorbed dye in a simple electrochemical cell. the gic adsorbent recovered its initial adsorption capacity after 40 to 60 min of treatment at a current density of 10 ma cm−2, corresponding to a charge of 12 to 18 c g−1 of adsorbent. the charge passed is consistent with that expected for mineralisation of the dye, suggesting that the dye was removed and destroyed with high charge efficiency. the energy cost of the regeneration was found to be around 120 j per g of adsorbent regenerated or 115 j per mg of the av17 dye removed and destroyed. a model describing the process of wastewater treatment by multiple cycles of adsorption and electrochemical regeneration, based on adsorption isotherm data, has been developed and validated. it was found that relatively modest improvements in the adsorption capacity of the adsorbent material could significantly improve the process performance. keywords adsorption; electrochemical regeneration; graphite intercalation compound; tri-phenyl methane dye; acid violet. introduction dyestuff removal has been one of the most persistent problems in wastewater treatment in the last thirty years. recent methods which have been investigated for dye removal include adsorpj. electrochem. sci. eng. 2(4) (2012) 223-236 wastewater treatment adsorption and regeneration 224 tion, ion exchange, chemical oxidation, precipitation and biological treatment [1,2]. there are ever more dyes available commercially, and due to their complex structure and synthetic origin, most are difficult to decolorize [3]. therefore, it is necessary to remove them from liquid wastes at least to a limit accepted by national and international regulatory agencies before discharge. adsorption is a widely used technique for the removal of dyes from wastewater and can be effective for overall treatment, particularly if the sorbent is cheap, does not require a pre-treatment step before application and is easy to regenerate [4]. adsorption on activated carbon is a technology which has been widely studied to remove dyes from wastewater but the high capital and regeneration cost of activated carbon has led to the search for low cost materials. in the last twenty years, many investigators have studied the feasibility of inexpensive, commercially available materials, that are easy to regenerate and re-utilized as many times as possible [5]. in recent years many inexpensive, widely available materials have been investigated as adsorbents to remove dyes from contaminated water. wood, fly ash, coal, zeolite, silica, and agricultural wastes have all been tried with varying degrees of success [6-14]. the adsorption of dyes from aqueous solutions onto these inexpensive adsorbent materials focused on the determination of the capacity, kinetics, equilibrium isotherms and the effect of different parameters such as thermodynamics, ph, bonding mechanisms, and desorption [14-16]. however, only limited application of such data has been directed to the modelling of the batch adsorption / regeneration process for wastewater treatment. electrochemical regeneration of activated carbon adsorbents was demonstrated by narbaitz and cen in 1994 [17] and there have been several recent studies of this process [18-20]. however, the process is not economic as regeneration is slow and the energy cost is high [21], due to the poor conductivity of the activated carbon bed. recent work has shown the potential of graphite intercalation compound (gic) adsorbent materials that can be electrochemically regenerated very rapidly and cheaply [21-24]. conventional adsorption processes are designed based on the single use of a batch of adsorbent. the rapid electrochemical regeneration process discussed above opens up the possibility of using a batch of adsorbent for multiple cycles of adsorption with regeneration carried out on-site. in this way the adsorbent can be reused many times. in addition, a treatment process can be envisaged where the water is treated by the same batch of adsorbent through several cycles of adsorption and regeneration. this approach has been used to treat radioactive organic waste in the nuclear industry [25]. in this study we aim to carry out the first detailed study of this multistage adsorption and electrochemical regeneration process. experimental materials the adsorbate used in this study was acid violet 17, a powdered, anionic tri-phenyl methane dye, which has three substituent groups and is a mono sodium salt. it was supplied by kemtex educational supplies ltd under the trade name kenanthrol violet 2b at a dye content of about 22 %. analysis is by uv/vis spectroscopy at a λmax of 542 nm, as described in our earlier paper [26]. the adsorbent used was a graphite intercalated compound (gic), nyextm 1000, supplied by arvia technology ltd., which contains around 94 wt. % carbon and has a particle size range of 100 – 700 µm [26]. f. m. mohammed et al. j. electrochem. sci. eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 225 adsorption/electrochemical regeneration methodology a laboratory scale sequential batch rig (see fig. 1) was used for both adsorption and electrochemical regeneration, which were carried out at the ambient laboratory temperature of 20°c. in each experiment 100 g of gic adsorbent was mixed with 1 l of water in the adsorption zone by sparging air into the bottom of the rig. after adsorption, the adsorbent was allowed to settle into the anodic compartment of the electrochemical regeneration zone. the adsorbent bed was in contact with a graphite anode and was separated from the perforated stainless steel cathode (316l with open area 33%, 3 mm holes) by a microporous polyethylene separator (daramic 350, grace gmbh). the separator acts as a barrier to minimize the transport of electrolyte through the membrane and to ensure that the gic bed does not contact the cathode, as this would short circuit the electrochemical cell. the cathode compartment was filled with 0.3 wt% nacl solution to provide good conductivity, and the ph of this solution was adjusted to below 2 using hydrochloric acid to ensure that the membrane was stable. the anode, cathode and membrane of the electrochemical cell had dimensions of 10 cm by 7 cm, and the gap between the anode current feeder and the membrane was 2.2 cm. the 100 g of adsorbent used formed a bed of depth 5 cm in the anode compartment. a dc power supply was used to apply a current of 0.5 a to the cell, corresponding to a current density (from studies by brown, 2005) of 10 ma cm−2 (based on the membrane area). ideally electrochemical regeneration should occur throughout the bed depth in the anode compartment. although it is difficult to measure the current distribution in the packed bed electrode, previous studies with gic adsorbents have examined the effect of bed thickness [22] or separated and tested layers of adsorbent in the regeneration bed [24]. these studies have indicated that complete regeneration can be achieved with bed depths of up to around 20 mm. prior to each experiment 100 g of fresh adsorbent was mixed with 1 litre of clean water for 30 min before being allowed to settle into the electrochemical regeneration cell. the water was drained off and a current of 0.5 a was applied for 30 min in order to oxidise any organic impurities present on the surface of the adsorbent. a volume of 1 l of solution containing 120 mg l−1 of av17 was then added to the cell. this concentration was selected in order to saturate the adsorbent and to give a significant equilibrium concentration. this was necessary to ensure that the equilibrium concentration achieved with the regenerated adsorbent can be accurately measured. the adsorbent and av17 solution were mixed by sparging air into the cell for 120 min. this adsorption time was found to be sufficient to ensure that equilibrium was reached at these conditions. once the adsorption stage was complete, the air was switched off and the adsorbent particles settled into the anodic compartment of the electrochemical cell. the treated liquid was drained off and a sample was taken and analysed by uv/visible spectroscopy in order to determine the loading of av17 dye on the adsorbent. the bed was regenerated for a period of between 10 and 120 min. after regeneration the supernatant was drained from above the bed of gic adsorbent and the adsorption stage was repeated using a fresh av17 solution (120 mg l−1) which was added to the cell. the adsorption stage was repeated by sparging with air for a period of 120 min. a sample of the solution obtained after adsorption was analysed by uv/vis spectroscopy, as described above. the performance of the regeneration was characterised using the ‘regeneration efficiency’, obtained by comparing the equilibrium adsorbent loading achieved before and after regeneration: / %  100 (1) j. electrochem. sci. eng. 2(4) (2012) 223-236 wastewater treatment adsorption and regeneration 226 where qi and qr are the equilibrium loading (qe) of av17 on the adsorbent (mg g −1) obtained before and after regeneration, respectively, calculated using equation (2). (2) where c0 and ce are the initial and equilibrium concentration of av17 in solution respectively (mg l−1), v is the volume of solution (l), and w is the mass of adsorbent used (g). figure 1. laboratory scale sequential batch rig for electrochemical regeneration of the gic adsorbent: schematic diagram of the side (a) and front (b) elevation of the rig, and (c) schematic diagram showing a cross section of the electrochemical regeneration zone. (c) electrochemical regeneration zone catholyte adsorbent bed anode membrane cathode hydrogen air in regeneration zone adsorption zone (b) front view 10 cm 37 cm 30 cm 12.3 cm 2.2 cm 11.5 cm air in (a) side view f. m. mohammed et al. j. electrochem. sci. eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 227 a similar technique was used for multi-stage batch adsorption and electrochemical regeneration, but in this case the solution was not drained off after the adsorption cycle. in this case, the initial concentration of av 17 was much higher, 668 mg l−1 as this solution was treated by a series of adsorption/regeneration cycles. for these experiments, the amount of gic adsorbent was increased to 125 g, which formed a bed of 7 cm depth in the anode compartment. in addition, the adsorption time was reduced to 60 min and the regeneration time was 30 min. results and discussion adsorption isotherm the design of a batch adsorber system requires knowledge of the equilibrium isotherm to understand the adsorption process behaviour and provide fundamental physiochemical data for evaluating the maximum capacity of the adsorbent [27]. batch sorption studies have been performed previously to investigate the kinetic and equilibrium isotherm of the adsorption of the acid violet 17 [26]. the experimental data were found to fit the langmuir isotherm model, equation (3). l e e e1 bk c q bc = + (3) where kl and b are the langmuir constants related to the capacity of the adsorbent (mg g −1) and the intensity of adsorption, (l mg−1), respectively. there is little data available in the literature on the adsorption of av17 by activated carbon, but there is some data on other dyes such as methylene blue [28]. the comparison of langmuir isotherm constants for adsorption of organic dye calculated in this work with those determined by [28] for activated carbon are shown in table 1. the low surface area of the adsorbent leads to a relatively low adsorption capacity, as indicated by the relatively low value of kl. however, if the adsorptive capacity (kl) is normalised with the specific surface area, it is found that the gic adsorbent is able adsorb a higher mass of dye per unit area than the activated carbons studied (albeit for a different adsorbate). this suggests that either the gic has more adsorption sites per unit area or (more likely) that some of the activated carbon surface area in the micropores and is not accessible to the dye molecules. table 1. comparison of langmuir constants and surface area for adsorption of methylene blue (mb), and av17 onto activated carbons [28] and the gic adsorbent nyextm 1000 [29], respectively, at room temperature (23°c) and normal ph. adsorbent adsorbate langmuir constant surface area (bet), m2 g−1 adsorptive capacity, mg m−2 kl / mg g −1 b / l mg−1 gic (nyextm 1000) av17 0.987 0.31 1.0 0.987 activated carbon (pac1) mb 307 0.12 863.50 0.36 activated carbon (pac2) mb 345 0.15 857.14 0.4 activated carbon (f400) mb 455 0.2 1216.4 0.37 the essential characteristic of the langmuir isotherm shape on whether adsorption is ‘favourable’ or ‘unfavourable’ can be classified in terms of dimensionless separation factor or equilibrium parameter, rl , equation (4) [30]. j. electrochem. sci. eng. 2(4) (2012) 223-236 wastewater treatment adsorption and regeneration 228 l mx 1 1 r bc = + (4) where cmx is the maximum solution concentration studied. the separation factor rl indicates the isotherm shape according to table 2. table 2. effect of separation factor on isotherm shape [30]. value of rl type of isotherm rl>1 unfavourable rl=1 linear 0<rl<1 favourable rl=0 irreversible the separation factor for the gic adsorbent (b = 0.31 l mg−1) has been calculated at range of maximum concentrations of av17 as shown in figure 2. it can be observed from this figure that the value of rl lies between 0 and 1 at all initial dye concentrations, confirming the favorable uptake of the av17 by the gic adsorbent. figure 2. separation factor for acid violet 17 onto the gic adsorbent nyextm 1000 at 20°c. using the langmuir model to determine the solid phase loading qmx in equilibrium with cmx: l mx mx mx1 bk c q bc = + (5) thus: e mx mx mx e 1 1 eq c bc q c bc + = + (6) defining a dimensionless solid phase concentration as q = (qe/qmx), and a dimensionless liquid phase concentration as c = (ce/cmx), we obtain: f. m. mohammed et al. j. electrochem. sci. eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 229 ( )l l 1 c q r r c = + − (7) measured values of q for a range of values of c are plotted in figure 3, where the maximum equilibrium concentration was cmx = 46 mg l −1. the experimental data was fitted to equation (7) by finding the value of rl that gave a minimum in the sum of the absolute error (sae) between the (q, c) data and equation (7), indicating a value of rl = 0.065. similar results have been reported for separation factors for the adsorption of the dye av17 onto activated carbon prepared from sunflower seed hull [16] and orange peel [14]. figure 3. isotherm shape for adsorption of av17 onto the gic adsorbent nyextm 1000 at room temperature (23°c) and the curve obtained using equation (7) with rl = 0.065. adsorption and electrochemical regeneration the liquid phase concentration of av17 dye after adsorption was found to be in the range 13.2 to 15.4 mg l−1, corresponding to a loading of av17 on the gic adsorbent of 1.07 to 1.05 mg g−1. this was slightly higher than the value of kl (0.987 mg g −1) from the langmuir adsorption isotherm [29], possibly due to natural variability of the adsorbent, attrition or surface modification of the adsorbent during the washing procedure. it was found that the full adsorption capacity could be recovered after 40 min of electrochemical regeneration, corresponding to a charge passed of 12 c g−1 (figure 4). further increases in the regeneration time led to an increase in the adsorption capacity to a value around 10 % higher than the original capacity. there are a number of possible reasons for this increase in adsorption capacity after regeneration. previous studies [24] have suggested that the electrochemical regeneration can lead to a slight roughening of the adsorbent surface, and thus an increase in the specific surface area available for adsorption. another possibility is the presence of oxidising species formed in the adsorbent bed during regeneration which may react with the organic dye leading to an apparent increase in adsorption capacity. for regeneration times less than 40 min, the full capacity of the adsorbent was not recovered. this suggests that some av17 or its breakdown products remained adsorbed on the surface of the gic adsorbent. when the full adsorption capacity is recovered, the av17 dye is either fully mineraj. electrochem. sci. eng. 2(4) (2012) 223-236 wastewater treatment adsorption and regeneration 230 lised, or breakdown products are formed which do not adsorb on the adsorbent and are thus released into the water. chemical oxygen demand (cod) analysis of treated water (unpublished data from arvia technology ltd) have not found organic breakdown products in solution, suggesting that breakdown products remain adsorbed until full mineralisation is achieved. it is possible to estimate the charge required to fully mineralise the dye; however this will depend on the products formed. the maximum charge required can be estimated by assuming complete anodic oxidation of the av17 to carbon dioxide, sulphate and nitrate: c41h44n3nao6s2 + 93h2o → 41co2 + 230h + + na + + 2so4 2− + 3no3 − + 224e− (8) alternatively a lower estimate of the charge required for mineralisation can be obtained by assuming that the products were carbon monoxide, sulphide and nitrogen: c41h44n3nao6s2 + 35h2o → 41co + 114h + + na + + 2s 2− + 1.5n2 + 111e − (9) the theoretical charge, q, for mineralisation of the av17 dye can be estimated from: e w q nf q m = (10) where n is the number of electrons required per molecule of dye oxidised, f is faraday’s constant (96487 c mol−1), and mw is the molecular weight of the dye (761.9 g mol−1). the value of qe was 1.05 mg g−1, and using n = 111 and 224 [based on equations (4) and (5) respectively] the theoretical charge required for mineralisation of the av17 would be between 15 and 30 c g−1. comparison of these values with the data in fig. 4 suggests that the charge efficiency for the anodic oxidation is high, but further work is needed to determine whether any breakdown products are present in the treated water. figure 4. regeneration efficiency as a function of charge passed during electrochemical regeneration of the gic adsorbent nyextm 1000 loaded with 1.06 mg g−1 of the organic dye av17, using a current density of 10 ma cm−2 and a bed depth 2.2 cm. f. m. mohammed et al. j. electrochem. sci. eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 231 the cell voltage was not continuously monitored, but was observed to be relatively stable at around 6 v throughout regeneration. the cell voltage is dominated by ohmic losses in the membrane and in the water in the anode compartment. slight variations in the cell voltage were observed, presumably associated with changes in the conductivity of the water in the anode compartment (for example due the decrease in ph caused by the electrochemical processes) or the formation of gas bubbles leading to a decrease in effective conductivity. it was thus not possible to use the cell voltage to determine the completion of the regeneration. based on a cell voltage of 6 v, the energy cost of the regeneration was 120 j per g of adsorbent regenerated or 115 j per mg of av17 removed and destroyed. multi-stage batch design model batch adsorption and regeneration process are usually carried on small volumes of wastewater. however, the efficiency of dye removal can be improved by carrying out the treatment using multi-stage adsorption/regeneration system. an adsorption isotherm can be used to predict the design of a single stage batch adsorption system [31-36]. a wastewater treatment process consisting of a series of adsorption and regeneration stages can be considered to be a multi-stage equilibrium operation. the adsorption isotherm can thus be used to design a multi-stage adsorption / regeneration process as shown in figure 5. figure 5. schematic diagram of two stages in a multi-stage batch adsorption and regeneration system, considered as a multi-stage equilibrium process. the design objective is to reduce the dye concentration in the feed (of volume v) from c0 to cn (mg l−1), reusing the adsorbent (of mass w) in each adsorption stage after regeneration. the dye mass balance for batch adsorber system (n) in figure 5 can be written as: v ce,n−1 + w qr = v ce,n + w qe,n (11) for the electrochemical regeneration process used in this study, we can assume that the regeneration efficiency was 100% (based on the data shown in figure 4), so qr = q0 = 0 , and equation (11) thus becomes: v (ce,n−1 − ce,n) = w qe,n (12) the experimental adsorption data were found to fit the langmuir isotherm model (equation 3) for adsorption of av17 on the gic adsorbent. thus the langmuir equation can be used to substitute for qe,n, giving: qe,n w ce,n-1 v qe,n-1 w batch adsorption n-1 ce,n v water ce,n-2 v qr w electrochemical regeneration sorbent qr nyextm1000 w batch adsorption n j. electrochem. sci. eng. 2(4) (2012) 223-236 wastewater treatment adsorption and regeneration 232 , 1 , , , ( ) 1 e n e n l e n e n c cw v bk c bc − −=     +  (13) equation (13) can be rearranged to a quadratic form and solved to give: 2 l , 1 l e ,n 1 e ,n 1 , 1 1 4 2 e n e n w w bk bc bk bc bc v v c b − − −    − − + − + +       =  (14) for the first adsorption cycle (n = 1), ce,n−1 in equation (14) is replaced by the initial concentration c0. equation (14) was used to determine the concentration after each stage of treatment for a given feed concentration c0 and mass of adsorbent (w) using the langmuir isotherm constants for the gic / av17 system: b = 0.31 l mg−1 and kl = 0.987 mg g −1. for a given feed concentration, number of adsorption stages and required percentage dye removal, the solution was iterated numerically using a forward derivative newton’s method with a convergence limit of 10−4 in order to calculate the amount of adsorbent, w (g), required. the amount of adsorbent w was estimated for a five stage adsorption regeneration system to achieve av17 dye removals ranging from 10 to 99 %, for a range of initial concentrations (55 680 mg l−1), and the results obtained are plotted in figure 6. the results indicate that the amount of adsorbent required increases linearly with the percentage dye removal, up to removals of around 90 to 95 %. this can be explained by observation that the adsorbent approaches its maximum loading in each of the adsorption cycles, so the amount of dye removed per gram of adsorbent is approximately constant. however, to achieve very high removals the dye concentration in the final adsorption stage will be low, so that the adsorbent used in this stage will not be fully saturated. the amount of adsorbent required thus begins to increase more rapidly as the final concentration decreases towards zero. figure 6. the adsorbent dose required as a function of the dye removal for a range of initial dye concentrations using a five stage batch adsorption and regeneration system. f. m. mohammed et al. j. electrochem. sci. eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 233 for validation of the model, an experimental study of a five stage adsorption – regeneration process was carried out for the adsorption of av17 (with an initial concentration of c = 668 mg l−1) onto the gic adsorbent (with an adsorbent dose of 125 g l−1). the values of the percentage removal achieved (rn) after each stage obtained from the model and the experiment (equations 14 and 15) were compared, as shown in figure 7. the percentage removal of av17 was observed to increase from 19 to 93 % in stages 1 to 5, and there was good agreement between the experimental data and the model. from the experimental data, the adsorbent loading after each adsorption cycle was found to be approximately 1±0.05 mg g−1, consistent with the capacity of adsorbent obtained from the isotherm study [24]. this confirms that for the conditions used in this experiment the adsorbent was approaching saturation with av17 after each adsorption cycle. 0 n n 0 0% 10/ c c r c − = (15) the adsorbent material investigated in this study has a very low adsorbent capacity, and there is significant potential to develop new adsorbent materials suitable for electrochemical regeneration but with increased adsorptive capacity compared to the gic adsorbent nyextm 1000. recent studies have shown that higher specific surface area adsorbent materials similar to the gic used in this study can offer improved adsorption capacity [37,38]. in order to evaluate the benefits of increased adsorptive capacity, the effect of the value of the langmuir kl on the multi-stage adsorption-regeneration process was investigated. in this case, the number of adsorption/regeneration stages required to achieve 99.9 % av17 removal (with an initial concentration of 1 g l−1) was calculated for an adsorbent dose of 125 g l−1 and for a range of values of kl (see figure 8). as kl was increased, it was found that stage number of stages of adsorption-regeneration required decreased such that a doubling of the adsorbent capacity leads to a reduction in the number of stages required from 10 to 5. a doubling of the adsorbent capacity has been demonstrated [37,38] and an order of magnitude increase in kl can readily be envisaged, which would reduce the number of stages required to only 2. this would have a significant impact on the economics of the process. figure 7. performance of batch adsorption and electrochemical regeneration for removal of av17 using the gic adsorbent nyextm 1000 with five stages adsorption and regeneration. the initial av17 concentration was 680 mg l−1 and the adsorbent dose was 125 g l−1. j. electrochem. sci. eng. 2(4) (2012) 223-236 wastewater treatment adsorption and regeneration 234 figure 8. effect of adsorptive capacity (kl) on the number of stages required for 99.9% removal of av17 (with an initial concentration of 1 g l−1) using multi-stage adsorption-regeneration with an adsorbent dose of 125 g l−1. 125 g l−1. conclusions it has recently been shown that low capacity unexpanded gic adsorbents can be regenerated rapidly and cheaply, so that they can be regenerated and reused on-site [23,25,26]. this study describes the first detailed study of the adsorption/regeneration characteristics of a graphite intercalation compound. the laboratory scale batch regeneration tests demonstrated that the gic adsorbent loaded with the organic dye acid violet 17 could be fully regenerated with a charge of around 15 c g−1. this charge is of the correct order of magnitude to suggest that the dye was mineralised during regeneration. the energy cost of the regeneration was found to be around 120 j per g of adsorbent regenerated or 115 j per mg of av17 removed and destroyed. the effective treatment of wastewater containing a dissolved organic contaminant by multistage batch adsorption with electrochemical regeneration has been demonstrated for the first time. multistage adsorption / regeneration was shown to be effective for the removal of high concentrations of av17 dye using the gic adsorbent, exploiting its rapid electrochemical regeneration. removal of 93 % of the dye from a solution containing 668 mg l−1 of av17 was achieved after five cycles of treatment with 125 g l−1 of the gic adsorbent. a design model for the multi-stage adsorption and electrochemical regeneration process based on the langmuir equilibrium isotherm has been developed and validated for removal of a dye. this model was used to predict the dose of adsorbent required to achieve a range of dye removals for a given number of adsorption / regeneration cycles. the model was found to be in good agreement with the experimental results obtained for five stages of adsorption / regeneration. it was found that the predicted number of stages of batch adsorption / regeneration required to achieve 99.9 % av17 removal was halved when the adsorptive capacity of the adsorbent was doubled. this finding confirms that development of adsorbents which can be regenerated electrochemically with increased capacity compared to the gic used in this study could significantly improve the economics of the process. f. m. mohammed et al. j. electrochem. sci. eng. 2(4) (2012) 223-236 doi: 10.5599/jese.2012.0019 235 acknowledgements: the authors would like to thank the uk engineering and physical sciences research council, arvia technology ltd and the iraqi ministry of higher education and scientific research for their financial support for this research. references [1] p.a. pekakis, 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [4000 4000] /pagesize [612.000 792.000] >> setpagedevice hardness and electrochemical behavior of ceramic coatings on inconel doi: 10.5599/jese.2011.0007 19 j. electrochem. sci. eng. 2(1) (2012) 19-31; doi: 10.5599/jese.2011.0007 open access : : issn 1847-9286 www.jese-online.org original scientific paper hardness and electrochemical behavior of ceramic coatings on inconel c. sujaya, h. d. shashikala, g. umesh, a. c. hegde* department of physics, national institute of technology karnataka, surathkal, srinivasnagar 575025, karnataka, india * department of chemistry, national institute of technology karnataka, surathkal, srinivasnagar 575025, karnataka, india corresponding author: e-mail: sujayachendel@gmail.com; tel.: +91 824 2474000 ext 3278; fax: +91 824 2474033 received: april 18, 2011; revised: october 13, 2011; published: march 22, 2012 abstract thin films of ceramic materials like alumina and silicon carbide are deposited on inconel substrate by pulsed laser deposition technique using q-switched nd: yag laser. deposited films are characterized using uv-visible spectrophotometry and x-ray diffraction. composite microhardness of ceramic coated inconel system is measured using knoop indenter and its film hardness is separated using a mathematical model based on area-law of mixture. it is then compared with values obtained using nanoindentation method. film hardness of the ceramic coating is found to be high compared to the substrates. corrosion behavior of substrates after ceramic coating is studied in 3.5% nacl solution by potentiodynamic polarization and electrochemical impedance spectroscopy measurements. the nyquist and the bode plots obtained from the eis data are fitted by appropriate equivalent circuits. the pore resistance, the charge transfer resistance, the coating capacitance and the double layer capacitance of the coatings are obtained from the equivalent circuit. experimental results show an increase in corrosion resistance of inconel after ceramic coating. alumina coated inconel showed higher corrosion resistance than silicon carbide coated inconel. after the corrosion testing, the surface topography of the uncoated and the coated systems are examined by scanning electron microscopy. keywords microhardness; alumina; silicon carbide; pulsed laser deposition; potentiodynamic polarization; electrochemical impedance spectroscopy. j. electrochem. sci. eng. 2(1) (2012) 19-31 ceramic coatings on inconel 20 introduction nickel-base superalloy inconel (nicrfe alloy) is commonly used in aircraft gas engines, rocket engines, nuclear-power plants, petrochemical equipment and offshore industries [1,2]. in these applications, components must have reliable chemical and mechanical stability at high operating temperatures. pitting corrosion of inconel has also been reported [3] when used in components operating in corrosive medium. to overcome these problems, the surface of the alloy is usually coated with ceramic materials. coatings of ceramic materials are especially suitable for the protection against wear and corrosion [4,5,6]. alumina (al2o3) and silicon carbide (sic) are the ceramic materials widely used for surface coatings. these coatings can be produced by many chemical and physical vapor deposition techniques. one of them, pulsed laser deposition (pld), is considered as flexible, simple and easily controllable method for producing thin films [7]. indentation microhardness is a reliable test method for the evaluation of the mechanical property of the coatings. the knoop indenter is used for microhardness measurements. the knoop indentation depth is shallower [approximately 1/30 the long diagonal] than the vickers indentation depth [approximately 1/7 the average diagonal] when the same load is applied on the material. hence, the knoop indentation is better suited for testing of thin coatings. hardness of thin films has combined effect of the substrate and the film that leads to the so-called “composite hardness”. the composite hardness includes the component of the substrate hardness depending on the relative depth of penetration of the indenter and mechanical properties of both, the film and the substrate. film hardness is separated from the composite hardness based on the area law of mixtures. film hardness can also be determined by nanoindentation technique in which loaddisplacement curve is obtained by depth sensing indentation technique. in the present study, efforts were made to get adhesive films of al2o3 and sic using pld technique on the fine polished inconel substrate at 4500c. usually thicker ceramic films are deposited at temperatures higher than 6000c. in the present investigation adhesive films of lesser thickness (about 0.5 μm) were deposited at lower temperature (450 °c) and appreciable increase in microhardness and corrosion resistance after coating was observed. processing parameters, like laser fluence, substrate target distance, and substrate temperature during the deposition are standardized using multiple trials. composite microhardness of al2o3 and sic coatings are measured using knoop indenter. its film hardness is separated using a model based on johnson and hogmark [8], after including the effect of indentation size effect (ise) [9], and compared with the values obtained using nanoindentation technique. a corrosion behavior of the coated substrate is studied in 3.5% nacl solution by potentiodynamic polarization and electrochemical impedance spectroscopy (eis) measurements. experimental the substrate used is inconel 601. the substrates are sequentially polished with sic waterproof abrasive paper from 320 grit to 1500 grit size and with al2o3 suspension of 1, 0.5, 0.3 μm size to an average roughness (ra) of 40 nm. the polished surfaces are cleaned with acetone in ultrasonic bath for 20 minutes before the deposition of coating on the substrate. a q-switched nd: yag laser (laser spectra) of wavelength 1064 nm and frequency 10 hz was used for deposition. laser beam of energy of 135 mj focused to 0.1 cm diameter using a spherical lens of focal length 50 cm was impinged onto the rotating al2o3 target. inconel substrates of size 2×2 cm2 were used for coating. sintered al2o3 and sic pellets of thickness 0.4 cm and diameter 4 cm were used as targets. the rotation speed of 40 rotations per minute was used c. sujaya et al. j. electrochem. sci. eng. 2(1) (2012) 19-31 doi: 10.5599/jese.2011.0007 21 to avoid the crater formation. the incident angle between laser beam and target surface was kept at 45°. to obtain a uniform film thickness, the substrate was placed at 8 cm distance from the target surface. the ablation was carried out in vacuum under the pressure of 10-3 pa, and at a substrate temperature of 450 °c. figure 1 shows the schematic representation of the pld system used in the present study. figure 1. schematic representation of the pld system. measurement techniques the average surface roughness was measured using 3-d optical surface profilometer (vecco). stylus profilometer (taylor hubson) was used to measure the film thickness. optical absorption measurements were performed on al2o3 and sic coated glass substrate at room temperature using uv-visible spectrophotometer (ocean optics). xrd analysis (jeol) was carried out by utilizing cukα radiation on ceramic coated substrates. microhardness tester (clemex) with knoop indenter was used to measure the composite hardness. loads of 25, 50, 100, 200, 300 and 500 g were employed at a dwell time of 10 seconds. at each load 10-12 indentations were made on the sample surface. the average microhardness at each load was calculated. in the knoop hardness measurement, the knoop hardness is obtained from the following relation 2 14229 p hk l = (1) where hk is knoop hardness in gpa, 14229 is a constant dependent on the indenter geometry. p is the applied load in n, l is the length of long diagonal of the resultant rhombic impression in μm. the microhardness measurement of the coating is the composite hardness of the film-substrate composite system. to separate the composite hardness into its constituents, a model based on area i.e., area “law-of mixture” approach is used. it was originally proposed by jonsson and hogmark [8] for vickers indentation and later it is adapted to the geometrical configuration of the knoop indenter [10]. the composite hardness, hc of the film-substrate system is expressed as sf c f s aa h h h a a = + (2) j. electrochem. sci. eng. 2(1) (2012) 19-31 ceramic coatings on inconel 22 where a is the contact area; h is the hardness; the subscripts c, f and s are related to the composite, film and the substrate, respectively and a = af + as. by means of simple geometrical relations depending on the knoop indentation, equation for af / a can be written as 2 2 f 2 2a ct c t a l l = − (3) where l is the length of the large diagonal, t is the thickness of the film, c = 2.908 for brittle film on the soft surface or c = 5.538 for soft film on the soft surface [11]. usually, measured hardness varies with load due to ise which has not been taken into account in jonsson and hogmark model. jonsson and hogmark model is improved by incorporating the ise taking into account a linear relation between hardness and the reciprocal indentation depth [12]. to obtain the true hardness of the film, the hardness variation with the applied load is written as f f fo b h h l = + (4) s sos b h = h + l (5) where hfo and hso are the absolute hardness of the film and the substrate respectively, bf and bs are constants and l is the indentation diagonal length. substituting equation (3), (4) and (5) in (2) and neglecting second order 1/l terms, the composite hardness becomes c c so b h h l = + (6) where bc = bs + 2ct(hfo hso). to evaluate hso and bs values, hardness measurements on substrate was performed. the experimental plot hs versus 1/l is approximated well by a linear regression. from the intercept and slopes of these plots, hso and bs of inconel are obtained as 3.6 gpa and 37.98 gpa m-1, respectively. the experimental data on the composite hardness of the filmsubstrate system, hc is plotted against 1/l and the intrinsic hardness of the film (hfo) was calculated from the slope of the regression line using equation (6). film hardness obtained from the above model was compared with the film hardness obtained using nano-indentation method. generally, it is not possible to measure the properties of a thin film independent of the substrate, using conventional testing equipment, unless coating is very thick. to determine the film hardness, the indentation depth should be 10% of the film thickness. in such cases nanoindenter was used. common feature of this method is that it continuously monitors the load and the displacement as the indentation is produced. the feature of a continuous depth and load recording allows thin film properties to be obtained directly from the data without the need to measure indentation diagonals. the data were analyzed with the oliver and pharr method [13]. hysitron tribo indenter was used for the nanoindentation. the corrosion behavior of the coated and uncoated substrates were studied using tafel polarization studies by immersing samples in 3.5% nacl solution in open air and at room temperature, using potentiostat/galvanostat from gamry instruments (pci 4g750-47065). all electrochemical measurements were performed using conventional three electrode cell, using a platinum plate as an auxiliary electrode, saturated calomel electrode (sce) as a reference electrode. the exposed area working electrode to the corrosive medium was 1cm2. the sample was cleaned in distilled water before loading it to the sample holder. the sample was placed in such a way that luggin c. sujaya et al. j. electrochem. sci. eng. 2(1) (2012) 19-31 doi: 10.5599/jese.2011.0007 23 capillary of the reference electrode was close to the working electrode and this arrangement was used for all the tests. the open circuit potential (eocp) or the steady state potential is maintained to study the potentiodynamic polarization. after achieving the stable eocp, the upper and the lower potential limits of the linear potential sweep were set at ± 250 mv vs eocp. the sweep rate was 1 mv s-1. the tafel plot was drawn using the obtained electrochemical data. the corrosion potential (ecorr) and the corrosion current density (icorr) are deduced from the tafel plot (logi vs. e plot). the corrosion current density is obtained using the stern–geary equation [14]: corr p b i r = (7) where b=ba bc /2.303(ba+ bc) and where b is called stern–geary constant, ba and bc are tafel slopes for the anodic and cathodic reactions respectively and rp is the polarization resistance expressed in ω cm2. the corrosion rate (cr) in micrometer per year (y) was calculated from the faraday’s law [15] using the following equation corr3.27i ewcr d = (8) where ew is the equivalent weight of the testing material in grams and d is the density of the testing sample in g cm-3. by substituting the value of icorr, cr was calculated. impedance measurements were conducted using the frequency response analyzer (fra). the spectrum was recorded in the frequency range 10 mhz 300 khz. the applied alternating potential had the amplitude of 10 mv. after each experiment, the impedance data were displayed as nyquist and bode plots. a circuit description code (cdc) is assigned for the acquired data and the acquired data are curve fitted and analyzed using zsimpwin software. results and discussion optical characterization of the coatings adhesive coatings of al2o3 and sic were obtained on inconel substrate by using the standardized parameters described in the previous section. the optical band gap energy and urbach energy calculations for sic and al2o3 coated on the glass substrate were carried out using optical absorption data. the tauc’s plot is used to obtain the optical band gap energy (eg) [16]: gconst( ) ph h eα ν ν= − (9) where eg is the optical band gap energy and hν is the photon energy of the incident radiation, the exponent p in the equation is 2 for the indirect optical transition and p=1/2 for the direct optical transition. the energy band gap eg of sic and al2o3 film is obtained by extrapolating linear region of the curve obtained by plotting (αhν)1/2 as a function of hν in the case of sic (indirect band gap) and (αhν)2 versus hν in the case of al2o3 (direct band gap) as shown in figures 2 and 3. the eg values are found to be 3.32 ev for al2o3 and 2.07 ev for sic film. these values match with the reported values of 3.2 ev for al2o3 [17] and 2-2.9 ev for sic [18]. generally, amorphous films have low band gap energies compared to crystalline films. j. electrochem. sci. eng. 2(1) (2012) 19-31 ceramic coatings on inconel 24 figure 2. absorption coefficient versus energy for the al2o3 film. figure 3. absorption coefficient versus energy for the sic film. the obtained coated films are amorphous in nature. this was confirmed by optical absorption data. at longer wavelength in the spectrum, there is an absorption tail described by urbach exponential law [19], which describes the amorphous nature of the film. the urbach energy is related to the absorption coefficient as u oe h eh ν α ν α= (10) where αo is a constant. the urbach energy eu is the measure of a disorder in the system. figures 4 and 5 correspond to the plot of lnα as a function of hν. the value of urbach energy was calculated by fitting the linear region of figures 4 and 5 applying the method of least squares to the straight line equation (lnα = hν /eu + constant) and was found to be 1.49 ev for al2o3 and 1.39 ev for sic and film. urbach energy eu is the measure of structural disorder in the system. the value of urbach energy obtained for sic and al2o3 confirms that the obtained films are amorphous in nature, which is confirmed also by the absence of sharp xrd peaks. c. sujaya et al. j. electrochem. sci. eng. 2(1) (2012) 19-31 doi: 10.5599/jese.2011.0007 25 figure 4. lnα as a function of energy (hν) for al2o3 film. figure 5. lnα as a function of energy (hν) for sic film. hardness measurements figures 6a and 6b show the dependence of the measured composite film-substrate hardness on the inverse of indentation diagonal for al2o3 and sic coatings on inconel. a least-square fit to the plots of the equation 6 results in the slope bc= [bs + 2ct(hfo-hso)]. the value of c is chosen depending on the film whether it is brittle or soft, since the nature of the films depends on their bulk material. its value is 2.908 for brittle film and 5.538 for soft film [11,20]. to verify whether the film is brittle or soft we observed the nature of indentation on the surface of the coating. if the film is brittle it accommodates the indenter by crack formation. if it is a soft film, it plastically deforms to match the shape of the indenter. in the present study, there was no cracking of the film observed in al2o3 and sic deposited substrates even at high load (10 n) which is shown in figure 7. a soft nature of the film is due to the amorphous structure of the coated film [21,22]. since the deposition was carried out at low temperature (450 °c), deposited ceramic films were not transformed into crystalline form as confirmed by optical absorption and xrd data. from the above reasoning, assuming the film to be soft, the c value is selected as c = 5.538. using the value of bs and hso of substrate, the intrinsic hardness of the films was calculated using j. electrochem. sci. eng. 2(1) (2012) 19-31 ceramic coatings on inconel 26 the equation 6. the slope of the straight line obtained by plotting hc versus 1/l (figure 6) was used to calculate the intrinsic film hardness hfo. the intrinsic hardness of the coated films is listed in table 1. the film hardness gives of higher value than that of the substrate. nano-indentation measurements were performed on the surfaces of both, uncoated inconel and with al2o3 and sic, coatings in order to determine the hardness of coating. figure 8 compares the indentation load-depth curve for inconel substrate and al2o3 film and sic film of thickness 0.5 μm on inconel. the indentation is carried out at the peak load of 1000 µn. the difference in hardness of the two samples is clearly evident from the difference in the depth obtained at the maximum load. the uncoated sample exhibits a larger indentation depth at the maximum load. in order to find the intrinsic hardness of the coated film an indentation depth should be 0.1 of the film thickness. the load was selected in such a way that its indentation depth was less than 0.05 µm on the coated substrate. table 2 compares the hardness value obtained using mathematical model, and the nano-indentation method and it shows that the two values are in a good agreement. figure 6. knoop hardness variation with 1/l on a) al2o3 and b) sic coated film on inconel. figure 7. knoop indentation on a) al2o3 b) sic coated film on inconel substrate at 10n load. table 1. hardness data of coatings on inconel coating material t / μm bs / gpa m -1 bc / gpa m -1 2c hso / gpa hfo / gpa al2o3 0.5 37.98 56.90 11.076 3.61 7.0 sic 0.5 37.98 64.74 11.076 3.61 8.6 c. sujaya et al. j. electrochem. sci. eng. 2(1) (2012) 19-31 doi: 10.5599/jese.2011.0007 27 figure 8. load variations with nanoindentation depth a) substrate inconel b) al2o3 and c) sic coating on inconel. table 2. comparison of hardness values obtained using a model and nanoindentation method. material surface roughness, nm thickness of the film, µm hardness, gpa from model nanoindentation al2o3 on inconel 42.46 0.5 7.0 7.74 sic on inconel 40.46 0.5 8.6 8.65 potentiodynamic polarization measurements the tafel plots obtained for inconel, inconel with al2o3 and inconel with sic coatings of thickness (t) 0.5 μm in 3.5 % nacl solution at 30 °c are shown in figure 9. the corrosion potential (ecorr), the corrosion current density (icorr) and the corrosion rate are deduced from the tafel (logi vs. e) plots. the values are listed in table 3. for the inconel substrate the icorr is approximately 1.007 na cm-2 and it decreases to 0.235 na cm-2 for alumina coatings and to 0.850 na cm-2 for sic coatings. corrosion rate is directly proportional to icorr.. it is established that cr reduced drastically after ceramic coating. alumina coated substrates exhibit excellent corrosion resistance. corrosion rate after al2o3 coating reduced to 1/4 of that of inconel, whereas sic coated samples reduced to 1/2 of inconel. the improvement in the corrosion resistance of the substrate after ceramic coating is due to the fact that ceramic coating passivates the surface of the substrate and prevents the corrosion attack by electrolyte. generally, in physical vapor deposited coatings, electrolytes enter into the substrate through the defects like pores and micro cracks and cause the corrosion of the substrate [23]. the protective coatings must fulfill the two main requirements to achieve the remarkable effect on corrosion resistance: a strong adhesion to the substrate and a low density of pores and cracks. the absence of pores and micro cracks completes surface coverage and good adhesion of the ceramic film coating to the substrate obtained by pld technique enhances the corrosion resistance of the substrate. enhanced corrosion rate of coatings on inconel in the present study indicates that the coated films are having negligible concentration of pores and cracks. j. electrochem. sci. eng. 2(1) (2012) 19-31 ceramic coatings on inconel 28 figure 9. potentiodynamic polarization curves of a) inconel, b) al2o3 coated inconel and c) sic coated inconel. table 3. potentiodynamic polarization measurements. inconel sic al2o3 ba / mv dec -1 110.9 125.9 62.59 bc / mv dec -1 44.9 78 165.5 icorr / na cm -2 3.69 1.007 0.235 ecorr / mv -328.0 -372.0 -321.0 cr /µm year-1 44.00 29.95 12.78 rp / mω cm -2 3.76 20.8 83.9 electrochemical impedance spectroscopy studies figures 10, 11 and 12 show the impedance response of inconel, inconel with al2o3 and inconel with sic coatings as nyquist and bode plots. the eis data obtained from these plots are listed in table 4. the nyquist plots show unfinished semicircles what is attributed to the charge transfer controlled reactions [24]. bode plots in the form of phase angle vs. log ω showed that the phase angles remain almost constant between higher and lower frequencies. z’ ’ × 1 07 / ω c m 2 z × 107 / ω cm2 z’ ’ × 1 07 / ω c m 2 z × 106 / ω cm2 figure 10. nyquist plots of: a) inconel, b) sic and c) al2o3 coatings. c. sujaya et al. j. electrochem. sci. eng. 2(1) (2012) 19-31 doi: 10.5599/jese.2011.0007 29 figure 11. bode plots (phase angle vs. logω) of a) inconel, b) sic and c) al2o3 coatings. figure 12. bode plots (log|z| vs. logω) of a) inconel, b) sic and c) al2o3 coatings. the simplified equivalent circuits are shown in figure 13. the values of solution resistance (rs), charge-transfer resistance (rct), pore resistance (rp), total capacitance (q) of the constant phase element (cpe) are listed in table 4. the rct increases in the following order: inconel substrate < < sic coated inconel < al2o3 coated inconel, which shows that the alumina coated inconel has the highest corrosion resistance. the values of rct are strongly dependent on the passive film characteristics and are an indication for corrosion of materials. this supports the results obtained using potentiodynamic polarization measurements. log|z| vs. logω plots of fig. 12 show that the absolute impedance increased in the same order as mentioned above. the equivalent circuit for inconel is shown in fig. 13(a). it consists of a double layer capacitance, which is parallel to the charge transfer resistance, both of which are in series with the solution resistance between the working electrode (we) and the tip of the luggin capillary. the double layer capacitance provides information about the polarity and the amount of charge at the substrate/electrolyte interface. for better quality of fitted results, pure capacitance is replaced with constant phase element (cpe) that accounts for deviation from ideal dielectric behavior. its admittance, y=z-1, is defined as: y(ω) = q(jω)n (11) where q is an adjustable parameter used in the non-linear least squares fitting and n is also an adjustable parameter that always lies between 0.5 and 1. the value of n is obtained from the slope of log|z| versus log ω plot (figure 12). the phase angle ( ) can vary between -90° (for a perfect capacitor (n = 1)) and 0 (for a perfect resistor (n = 0)). the cdc for the equivalent circuit proposed for inconel is r(q)r. when the sample is immersed in the electrolyte, the defects in the coating provide the direct diffusion path for the corrosive media. in this process the galvanic corrosion cells are formed and the localized corrosion dominates the corrosion process. in such cases, electrochemical interface can be divided into two sub-interfaces: electrolyte/coating and electrolyte/substrate. the proposed equivalent circuit for such a system is shown in figure 13(b). the parameters in the equivalent circuit rp and qc are related to the properties of the coating and the electrolyte/coating interface reactions. rct and qdl are related to the charge-transfer reaction at the electrolyte/substrate interface. the cdc of the proposed equivalent circuit for the coated sample is r(q[r(qr)]). from the eis data given in table 4, it is evident that qc decreased from sic coating to alumina coating, indicating that sic coatings contain relatively more pores and less dense microstructure as compared to alumina coatings. j. electrochem. sci. eng. 2(1) (2012) 19-31 ceramic coatings on inconel 30 figure 13. a) equivalent circuit to fit the electrochemical impedance data of the inconel substrate and b) equivalent circuit to fit the electrochemical impedance data of the coated substrate. scanning electron microscopy (sem) studies after the corrosion testing, the surface morphology of the uncoated and the coated system are studied by sem. the corrosion pits were observed on the uncoated inconel after the corrosion test (figure 14a). corrosion pits are not found in al2o3 and sic coated samples, (figures 14b and 14c)). pits are formed as a result of corrosion attack on the uncoated surface. the improvement in the corrosion resistance is due to adherent thin al2o3 film coating deposited on the surface of the substrate which will not allow the direct contact of the electrolyte with the substrate. edax analysis indicates the presence of sodium (na) and chlorine (cl) in the regions where the pits are formed. similar conclusion can be drawn in corrosion behavior of sic coated inconel. table 4. eis data obtained by equivalent circuit simulation of sic and al2o3 coatings. sample rs ω cm-2 qdl ω-1sn ndl rct mωcm-2 qcoat ω-1sn nc rpore mωcm-2 inconel 15.46 2.07 0.8 0.89 inconel/sic 14.66 0.212 0.89 21.07 0.73 0.87 7.66 inconel/al2o3 15.82 0.16 0.99 108 0.085 0.74 18.6 figure 14. surface morphology of the a) inconel substrate b) alumina c) sic coated substrate surface. c. sujaya et al. j. electrochem. sci. eng. 2(1) (2012) 19-31 doi: 10.5599/jese.2011.0007 31 conclusions adhesive thin films of al2o3 and sic have been deposited on inconel substrate by pld technique using nd: yag laser of wavelength 1064 nm. knoop microhardness of the film-substrate system gives the composite microhardness. film hardness can be separated from the composite hardness using a model based on area law of mixtures. it is nearly equal to the film hardness measured using nanoindentation technique. the potentiodynamic polarization and the eis measurements showed that al2o3 and sic coatings exhibit better corrosion resistance as compared to that of substrate. acknowledgements: the authors would like to thank indian space research organization (isro) for financial assistance in carrying out this research and to vikram sarabhai space centre (vssc), trivandrum for supplying substrate materials. references [1] s.h. jeong, c.w. cho, z. lee, tribol. int. 38 (2005) 283–288 [2] c.j. wang, s.m. chen, surf. coat. tech. 201 (2006) 3862–3866 [3] y.i. kim, h.s. chung, w.w. kim, j.s. kim, w.j. lee, surf. coat. tech. 80 (1996) 113-116 [4] c. cibert, h. hidalgo, c. champeaux, p. tristant, c. tixier, j. desmaison, a. catherinot, thin solid films 516 (2008) 1290-1296 [5] m. zaytouni, j.p. rivière, wear 197 (1996) 56-62 [6] y. li, j. yao, y. liu, surf. coat. tech. 172 (2003) 57-64 [7] t.j. zhu, l. lu, m. o. lai, appl. phys. a 81(4) (2005) 701-714 [8] b. jönsson, s. hogmark, thin solid films 114 (1984) 257-269 [9] g. guillemot, a. iost, d. chicot, thin solid films 518 (2010) 2097-2101 [10] a. iost, scripta 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [425.197 850.394] >> setpagedevice {root cause analysis of the corrosion-related coiled tubing failure:} http://dx.doi.org/10.5599/jese.1237 501 j. electrochem. sci. eng. 12(3) (2022) 501-510; http://dx.doi.org/10.5599/jese.1280 open access : : issn 1847-9286 www.jese-online.org original scientific paper root cause analysis of the corrosion-related coiled tubing failure sanja martinez1, fuad khoshnaw2, vuokko heino3, sara fahmi4, talal a. aljohani5, and sally elkatatny6 1research laboratory for corrosion engineering and surface protection recorr, department of electrochemistry, faculty of chemical engineering and technology, university of zagreb, croatia 2school of engineering and sustainable development, faculty of computing, engineering and media, de montfort university, leicester, united kingdom 3vtt technical research centre of finland ltd, p.o. box 1000, 02044, vtt, finland 4materials management consultant, infosys ltd., 6002 rogerdale road, houston, usa 5materials science research institute, king abdulaziz city for science and technology (kacst), riyadh, 12354, saudi arabia 6mechanical engineering department, faculty of engineering, suez canal university, ismailia, 41522, egypt corresponding author: taljohani@kacst.edu.sa received: february 3, 2022; accepted: march 1, 2022; published: march 13, 2022 abstract coiled tubing (ct) is widely used in the oil and gas industry. however, corrosion-related failures are frequently reported. research into the causes of failures leads to improvement in the design of components and processes. in this study, a new ct sample and a ct sample with perforated wall that had failed after a few acidizing operations were selected for analysis. scanning electron microscope (sem) images at the fracture site showed that ct damage was caused by the low cycle fatigue. in addition, light and scanning electron microscopy (sem) showed that a corrosion pit acted as the initiator of the crack. elemental analysis using energy dispersive x-ray spectroscopy (eds) indicated the presence of an iron oxide layer and a layer associated with the sb containing inhibitor. the corrosion damage investigation showed that the internal ct wall pits likely formed during storage due to the acidizing operations in the areas where the remaining liquid was still at the tube bottom. keywords coil tubing; corrosion; failure analysis; microbiological induced corrosion introduction coiled tubing technology (ct) is widely used in well operations [1,2]. ct application can be divided into two general categories, fluid pumping and mechanical applications. ct is frequently affected by failures due to various causes [3,4]. therefore, knowledge of different nomenclatures is required to http://dx.doi.org/10.5599/jese.1237 http://dx.doi.org/10.5599/jese.1280 http://www.jese-online.org/ mailto:taljohani@kacst.edu.sa j. electrochem. sci. eng. 12(3) (2022) 501-510 root cause analysis of the corrosion 502 diagnose the causes of failure. the causes considered to be the main reasons for the failure of ct can be classified as follows [4-6]: • mechanical damage: premature failure of the ct string caused by mechanical means such as fatigue, buckling, yielding, etc. • corrosion: premature failure of the ct string caused by corrosive environments during well servicing and workovers. • manufacturing defect: defects are generated during the original manufacturing process of the coiled tubing, such as material defect, temper embrittlement or cold weld. • human error: faults or mistakes during coiled tubing string operations. • microbiological induced corrosion (mic): involves microbes or bacteria within contact with the coiled tubing. • external abrasion: a localized reduction in wall thickness, generally caused by friction between the coiled tubing and the wall of the well. • hydrogen cracking due to h2s: cracks after the material is exposed to h2s in the well. ct corrosion occurs in contact with the atmosphere, pumped and production fluids. corrosion can significantly shorten tubing life and must be prevented. in particular, localized corrosion can be a trigger for fatigue cracking growth during cycling. in addition, corrosion can affect the strength and pressure resistance of the pipes. statistical analysis of the ct failure causes was conducted [7] showing that mechanical damage, corrosion, manufacturing flaws, and human error were the main diagnoses for ct failures from 1994 to 2005. these four major causes also represented 80 to 90 % of the failures from 2006 to 2017. van arnam et al. [8] reported that corrosion can occur on ct strings from the day of production if they were not properly protected with a corrosion inhibitor. when the temperature of ct reaches the dew point the moisture condenses on the ct surface immediately causing rusting. the rust layer consists of hydrated iron oxides, fe(oh)2 and fe(oh)3, or their dry counterparts, hematite (fe2o3) and magnetite (fe3o4). in addition, the presence of chlorides, sulfates or carbon dioxide increases the corrosion rate. to prevent corrosion, several corrective actions are taken, including revising n2 purging practices, trail runs with new inhibitors, and seeking feedback from operations to optimize inhibition plans while not increasing operational burden [7,9,10]. nevertheless, the combination of mechanical and corrosive actions during operations eventually leads to the development of fatigue cracks. it has been found that the growth of fatigue cracks is greater at low-stress cycles, which allows more time for the corrosion process [11]. unpredictable ct service life is a serious economical and safety issue for the industry. it results from the inadequate in-service inhibition and lacking tubing maintenance programs that do not include storage corrosion prevention. maximizing ct service life requires effective corrosion control on both sides of the ct wall, during storage and in service. experimental materials and methods the current study examined the new and the failed ct samples. the failed ct sample, taken from the wellbore after only few well acidizing jobs with 15 % hcl, contained a perforated wall that was later cut open to expose the fracture area that was also studied. the specified composition of the ct-80 material of the tubes is illustrated in table 1. s. martinez et al. j. electrochem. sci. eng. 12(3) (2022) 501-510 http://dx.doi.org/10.5599/jese.1237 503 table 1. elemental composition of the coiled tubing material element c mn p s si cu ni cr mo content, wt.% 0.06 0.88 0.012 0 0.15 0.2 0.14 0.47 0.14 bruker vertex 70 ftir was used to measure the ftir spectra of the corrosion products. a scanning electron microscope with energy dispersive spectrometry sem fei feg250quanta/ oxford eds pentafet was used to study the surface morphology and elemental composition of the fracture. a duramin 2 struers microhardness tester was used to test vickers microhardness on the base metal and near the fracture surface. results and discussion visual examination of a new ct sample revealed no corrosion damage to the tubing, as shown in figure 1. a uniform black coating, visually consistent with the corrosion protection applied by the ct manufacturer, was visible on both sides of the ct wall. in addition, surface rust is visible on the exterior wall of ct (figure 1a) where the black coating is likely mechanically damaged. no rust was observed on the interior wall of ct (figure 1b). the black coating was not damaged because the interior wall is protected from mechanical impact. a b figure 1. new coiled tubing: a outer surface and b inner surface the used ct sample from the well had a metallic appearance on the outside with no visible corrosion products. on the inside, the sample was covered by a layer of brown corrosion products (figure 2). a black-green corrosion product streak was observed longitudinally across approximately 20 % of the inner diameter. figure 3 shows the corrosion pits and figure 4a shows the cracks, both of which occur only within a black-green area. one of the cracks observed on the inside protruded through the wall of ct and was visible on the outside surface, as shown in figure 4b. for close observation and examination, the sample was cut open to expose the fracture surface (figure 5a). the edge of the corrosion damage in figure 5b is the source of the striation marks and therefore likely served as the fracture origin. in the central part of the wall, a large area of striation marks perpendicular to the fracture direction was observed. the striations represent the propagation front of the fatigue crack and indicate that the fracture was caused by material fatigue. the y-shaped crack ends shown in figure 4b, also support the fatigue hypothesis. in addition, ridges have formed along the inner edge of the fracture parallel to the direction of fracture propagation, which can be http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(3) (2022) 501-510 root cause analysis of the corrosion 504 attributed to micro-cracking outside the central fracture plane. terraces (stepped structures) are formed along the edges of the fracture, which are due to the initial development of the fracture in different planes. figure 2. black-green corrosion product streak on a coiled tubing sample from the well a b figure 3. corrosion pits on the inner wall of the coiled tubing a b figure 4. cracks on a the inner surface and b the outer surface the uniformity of striations indicates the homogeneity of the metallurgical structure of the material. the spacing between the striations usually gives information about the growth rate of the cracks (µm/cycle). cracks perforated crack black-green corrosion product s. martinez et al. j. electrochem. sci. eng. 12(3) (2022) 501-510 http://dx.doi.org/10.5599/jese.1237 505 a b figure 5. light microscope photographs of a fracture surface and b fracture initiation site the number of striations is greater than the number of tube straightening and bending cycles, which may indicate a corrosion-related fatigue fracture. as the tip region of the fracture is repeatedly covered with a layer of corrosion products, the growth of the striation mark slows down. it can be argued that the numerous striation marks are due to the acceleration and deceleration of the growth of a fracture during each cycle. becker and lampman [12] reported that under many loading conditions, the pre-existing macroscopic crack-like defect blunts with increasing load and a new crack form by ductile tearing, creating a striation zone. consequently, continued increase in loading causes the crack propagation mechanism to change to cleavage or quasi-cleavage. the internal surface shown in figure 3a was examined under a metallographic microscope (figure 6) and the corrosion pits contained a black corrosion product. brown-orange corrosion product was seen above the black one on the surface of the shallower defects. the appearance of the corrosion damage suggests atmospheric corrosion of the steel with unobstructed oxygen flow outside the pit and a de-aerated space at the bottom of the pit. the corrosion, more pronounced on the inner diameter, is in good agreement with still and rolovic [13]. a b figure 6. the corroded inner surface at different zones the polished and etched used ct had a fine-grained microstructure (astm sizes 13 to 14) with evenly distributed areas of ferrite and pearlite and some bainite, as shown in figure 7a. short banded structures of pearlite are observed in some places in the centre of the wall section (figure 7b). no cracks are visible on the metallographic samples. http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(3) (2022) 501-510 root cause analysis of the corrosion 506 a b figure 7. fine-grained structure of quasi-polygonal ferrite with uniformly distributed areas of pearlite and bainite the layers on the inner surfaces of the new and used ct samples were rubbed with kbr, which was later pressed into pellets for ftir analysis. the ftir spectrum of the new ct sample, figure 8a, shows no clear bands of iron oxides, confirming the absence of corrosion on the inner diameter of the new ct sample. the observed bands indicate the presence of an organic corrosion inhibitor used by the manufacturer of the ct. the ftir spectrum of the used sample ct is shown in figure 8b and indicates the presence of antimony oxide. antimony salts were used as a component of the acidizing corrosion inhibitor and the inner surface of ct is partially covered with deposited metallic antimony, which oxidizes to antimony oxide when exposed to air. a t ra n sm it ta n ce , % wavenumber, cm-1 b t ra n sm it ta n ce , % wavenumber, cm-1 figure 8. ftir spectra for a new tubing and b used tubing from the well figure 9a shows a cracked but layer of antimony at the inner surface of the used ct (measurement point #1), located away from the analysed crack. the inner surface near the crack in s. martinez et al. j. electrochem. sci. eng. 12(3) (2022) 501-510 http://dx.doi.org/10.5599/jese.1237 507 figure 9b shows that no continuous antimony layer is present (measurement points #3 to #5), and in particular, no antimony is found within the pit (measurement point #2) a b figure 9. fine-grained structure of quasi-polygonal and needle ferrite with uniformly distributed areas of perlite and bainite the eds was performed at different locations on the fracture surface (figure 10). representative eds spectra are shown in figure 11, and the results are summarized in table 2. the eds elemental analysis of the fracture indicates elements that can originate from the metal substrate (fe, cr, si, c) or the environment (o, c, s, si, cl, sb). carbon can originate from the organic component of the corrosion inhibitor. small amounts of sulfur were detected during the test. the sulfur could be from h2s from the environment and/or microbiological corrosion induced by sulfate-reducing bacteria (srb). however, the amount of sulfur is minimal and the source of the sulfur could not be clearly determined from this analysis. the presence of sulfides was not detected by the lead acetate paper test. figure 10. eds spectra locations at the fracture surface of the perforated sample http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(3) (2022) 501-510 root cause analysis of the corrosion 508 figure 12 shows the locations where the hardness measurements were made, and the results are summarized in table 3. the hardness values were relatively consistent and agreed well with the declared value of 91.87 hrb (190 hv) of the ct-80 tubing parent material. given the relatively small grain size (astm 13 to 14) and the fact that hardness decreases with decreasing grain size, this is the expected value of the measured hardness. h. ghiasi [14] reported that the microstructure affects the improvement of properties and fatigue life. by decreasing the grain size, the mechanical properties and fatigue life increased. a b figure 11. representative eds spectra of the fracture surface in figure 10 at different locations: a -location 1 and b location 5 s. martinez et al. j. electrochem. sci. eng. 12(3) (2022) 501-510 http://dx.doi.org/10.5599/jese.1237 509 table 2. eds elemental analysis of the fracture surface at locations 1 through 6 in figure 10 content, wt.% element fracture surface location #1 #2 #3 #4 #5 #6 c 9.38 7.34 6.30 5.86 8.09 4.41 o 15.04 8.35 38.67 22.38 23.78 7.81 si 0.32 0.62 0.42 0.22 cr 0.58 0.67 0.52 fe 74.04 83.03 54.26 11.61 50.14 3.24 s 0.10 0.18 cl 0.09 sb 0.44 0.34 60.14 17.06 84.53 figure 12. vickers microhardness examination sites table 3. vickers hardness values near the fracture at different locations location hv location hv 1 204 4 201 2 203 5 199 3 206 �̅� ± 𝜎 202.6  2.7 conclusions • it can be concluded that pitting corrosion can occur on the inner surface of the tubing for two reasons: − during well acidizing, due to incomplete corrosion inhibition in 15% hcl, − during storage, between acidizing operations when the remaining liquid settles at the tubing bottom and causes pitting corrosion to occur. • given the appearance and location of the corrosion damage, the most likely cause of the fatigue cracks are corrosion pits that appear in the area of accumulation of residual electrolyte in the tubing during storage in atmospheric conditions. • the pits at the inner wall serve as initiators of fatigue cracks and fractures that progress in the cycle of straightening and bending of the tubing. • to prevent storage corrosion, it is necessary to properly maintain the tubing between acidizing operations by the following procedures: − nitrogen purging, preferably using a polyurethane ball to expel electrolytes from the tubing and subsequent capping the tubing ends, − application of vapor phase corrosion inhibitors. fracture surface 1 5 http://dx.doi.org/10.5599/jese.1237 j. electrochem. sci. eng. 12(3) (2022) 501-510 root cause analysis of the corrosion 510 references [1] j. abdo, a. al-shabibi, h. al-sharji, tribology international b 82 (2015) 493-503. https://doi.org/10.1016/j.triboint.2014.01.028 [2] j. li, j. misselbrook, m. sach, journal of canadian petroleum technology 49 (08) (2010) 6982. https://doi.org/10.2118/113267-pa [3] l. garner, l. vacik, s. livescu, d. blanco, spe/icota coiled tubing and well intervention conference and exhibition, houston, texas, usa, 2016 (spe-179101-ms). https://doi.org/ 10.2118/179101-ms [4] r. hampson, e. jantz, t. seidler, spe/icota coiled tubing and 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h. luft, e. kee, s. m. tipton, spe/icota coiled tubing and well intervention conference and exhibition, the woodland, texas, usa, 2007 (spe-107113ms). https://doi.org/10.2118/107113-ms [11] d. bronde, r. edwards, a. hayman, d.hill, s. mehta, t. semerad, oilfield review 6(2) (1994) 4-18. https://www.slb.com/-/media/files/oilfield-review/p04-18 [12] w. t. becker, s. lampman, fracture appearance and mechanisms of deformation and fracture, in: volume 11, failure analysis and prevention, w. t. becker, r. j. shipley eds., asm international, 2002, 559-586. https://doi.org/10.31399/asm.hb.v11.a0003537 [13] j. w. still, r. rolovic, a study of atmospheric corrosion of coiled tubing and its inhibition, corrosion 2003, san diego, california, usa, 2003 (nace-03593). isbn: 03593 2003 cp [14] h. ghiasi, scientia iranica 25 (4) (2018) 2155-2161. https://dx.doi.org/10.24200/sci.20 18.20677 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.triboint.2014.01.028 https://doi.org/10.2118/113267-pa https://doi.org/10.2118/179101-ms https://doi.org/10.2118/179101-ms https://doi.org/10.2118/179078-ms https://doi.org/10.2118/179078-ms https://doi.org/10.2118/194306-ms https://doi.org/10.2118/173658-ms https://doi.org/10.2118/189914-ms https://doi.org/10.2118/189914-ms https://doi.org/10.2118/60744-ms https://doi.org/10.1051/e3sconf/202123001018 https://doi.org/10.2118/107113-ms https://www.slb.com/-/media/files/oilfield-review/p04-18 https://doi.org/10.31399/‌asm.hb.‌v11.‌a0003537 https://dx.doi.org/10.24200/sci.2018.20677 https://dx.doi.org/10.24200/sci.2018.20677 https://creativecommons.org/licenses/by/4.0/) preparation of mesoporous carbon/polypyrrole composite materials and their supercapacitive properties doi: 10.5599/jese.2011.0001 67 j. electrochem. sci. eng. 1(1) (2011) 67-73; doi: 10.5599/jese.2011.0001 open access : : issn 1847-9286 www.jese-online.org original scientific paper preparation of mesoporous carbon/polypyrrole composite materials and their supercapacitive properties wu-jun zou, shan-shan mo, shuang-li zhou, tian-xiang zhou, nan-nan xia and ding-sheng yuan department of chemistry, jinan university, guangzhou 510632, china corresponding author: e-mail: tydsh@jnu.edu.cn; tel.: +86-20-85220597; fax: +86-20-85221697 received: march 18, 2011; revised: may 19, 2011; published: august 20, 2011 abstract we synthesized mesoporous carbons/polypyrrole composites, using a chemical oxidative polymerization and calcium carbonate as a sacrificial template. n2 adsorptiondesorption method, fourier infrared spectroscopy, and transmission electron microscopy were used to characterize the structure and morphology of the composites. the measurement results indicated that as-synthesized carbon with the disordered mesoporous structure and a pore size of approximately 5 nm was uniformly coated by polypyrrole. the electrochemical behavior of the resulting composite was examined by cyclic voltammetry and cycle life measurements, and the obtained results showed that the specific capacitance of the resulting composite electrode was as high as 313 f g−1, nearly twice the capacitance of pure mesoporous carbon electrode (163 f g–1). this reveals that the electrochemical performance of these materials is governed by a combination of the electric double layer capacitance of mesoporous carbon and pseudocapacitance of polypyrrole. keywords mesoporous carbon/polypyrrole composites; chemical oxidation; calcium carbonate; sacrificial template; specific capacitance; pseudocapacitance introduction mesoporous carbon (mc) is one of the most important carbon materials with high surface area, uniform pore size, and good electric conductivity. during the past ten years, many efforts have been made to develop simple and efficient methods for the preparation of mc materials and to improve their properties for various applications. recently, one-step soft template methods for mc preparation have gradually replaced time-consuming and tedious two-step hard mesoporous http://www.jese-online.org/� j. electrochem. sci. eng. 1(1) (2011) 65-71 mc/ppy composites capacitance 68 silica templates, such as sba-15, mcm-41, and mcm-48 [1,2]. the mcs and their composites are broadly applied to electrochemical capacitors [1,3], catalyst supports [4] and adsorbents [5,6], and magnetic separation [7]. compared with mesopourous carbon materials, conducting polymers (cps) possess pseudocapacitance, which is almost 10-100 times higher than the capacitance of electrochemical double-layer capacitors (edlcs) [8]. polypyrrole (ppy), one of cps, has drawn a lot of attention for supercapacitor applications [9,10], mainly due to its oxidation-reduction properties, high conductivity in doped state, high specific capacitance, good environmental stability, and especially facile synthesis [11]. however, the shortcomings of cps, such as low surface area and constrained power-output properties [12], markedly restrict their application to electrochemical capacitors. hence, the most effective way is to use mc materials to improve the property of the cps electrode. a new class of composite materials has originated from the combination of mcs and cps [13]. choi et al. introduced a conducting polymer layer into the pore surface of mesoporous carbon via vapor infiltration of a monomer and, by subsequent chemical oxidative polymerization, obtained mesoporous carbon-polypyrrole composites [14]. the maximum specific capacitance of these composites is 274.5 f g−1. pacheco-catalán et al. synthesized mesoporous carbon/conducting polymer composites by adsorption of different monomers (aniline, pyrrole, thiophene, and 3-methyltiophene) in the gas phase onto the carbon surface, followed by oxidative chemical polymerization [15]. the electrochemical performance of carbon/polypyrrole composites electrode showed that it had a low specific capacitance (maximum value: 83.8 f g−1) and stable cycle life in the potential range of 0 to 1v. however, all those materials showed low specific capacitance. recently, we prepared mesoporous carbon by onestep method [16]. the obtained results revealed that mcs display good capacitive behavior with high reversibility and reproducibility due to their unique large mesopore size, which is favorable for fast ionic transport. however, the specific capacitance of mcs reaches only 163 f g–1, which limits their application to electric vehicles and high power electronic devices. in this study, we used ppy growth on the surface of mc via simple chemical oxidative polymerization with calcium carbonate as the sacrificial template to improve the capacitance of carbon materials. the resulting composite material combined double layer capacitance of mc with pseudocapacitance of ppy. experimental chemicals pyrrole monomer (98%, aldrich chemical co.) was distilled under reduced pressure, transferred into a refrigerator, and stored under nitrogen until further use. the diameter of nanoscale calcium carbonate was 80 nm and the bet surface area was 25 m3 g-1. sodium 4-methyl benzene sulfonate (tsona) was purchased from sinopharm chemical reagent co. ltd, shanghai, china. the other starting materials in this work were of analytical grade. synthesis of mesoporous carbons/polypyrrole composites mcs were synthesized from f127/silica/butanol according to the procedure described previously [16]. typically, 2 cm3 sulfuric acid (98 wt%) and 9.3 cm3 butanol (buoh) were directly added into a clear solution containing 2.5 g f127 and 120 g deionized water at 318 k. after stirring for 1.5 h, 5 cm3 tetraethyl orthosilicate (teos) was added, vigorously stirred at 318 k for 24 h, and aged at 373 k for 24 h. the f127/silica/butanol composites were collected by filtration and dried at room temperature for 12 h and at 433 k for 6 h. finally, the obtained composites were transferred into a tube furnace and carbonized under pure n2 atmosphere at 1123 k for 2 h, followed by the treatment with diluted hf solution. the chemical oxidative polymerization of pyrrole was performed using a modified procedure reported previously [17]. first, 0.3 g tsona, 0.5 g caco3, 25 g cacl2, 20 cm 3 c2h5oh, and 0.5 g silica/carbon were mixed into 40 cm 3 deionized water with magnetic stirring under n2 atmosphere for 30 min; the nanoscale caco3 served as a wu-jun zou et al. j. electrochem. sci. eng. 1(1) (2011) 65-71 doi: 10.5599/jese.2011.0001 69 core, and tsona was used as anionic surfactant. then 0.3 cm3 pyrrole was added by a syringe. after 10 min, 40 cm3 of 0.02 mol dm-3 fecl3 solution was slowly added as oxidant into the reaction vessel. the polymerization was carried out for 12 h in an ice-bath with magnetic stirring and maintained under n2 atmosphere. the mesoporous carbon/polypyrrole composite was filtered and rinsed several times with distilled water and ethanol to remove retained pyrrole monomer and oxidant. the as-synthesized powder was transferred into hcl solution for caco3 removal for 24 h. the mixture was filtered and rinsed several times again with distilled water. then the silica/carbon/ppy composite was treated by diluted hf solution to remove the silica and dried in vacuum at 333 k for 12 h. the product was denoted as mp, containing an approximate mc:ppy weight ratio of 6:4. under the influence of the lewis acid fecl3 and inhibition by abundant cacl2, caco3 was dissolved, slowly releasing co2, which contributed to the holes formation in the composite during the overflow process. actually, caco3 as a core was the sacrificial template. characterization the morphologies of mc and as-synthesized composite were examined using a high-resolution transmission electron microscopy (tem, jeol jem-2100, 200 kv). nitrogen sorption isotherms of samples were measured by a micromertics tristar 3000 analyzer at 77 k. the ft-ir measurements on samples were performed using nicolet 6700 ft-ir spectrometer. the electrochemical behavior of both mc and mp was investigated by cyclic voltammetry (cv) conducted on a chi 660 b electrochemical workstation (chenhua instruments co., shanghai, china). the measurements were carried out in a standard three-electrode cell system. a pt-foil modified by either mc or mp was used as the working electrode, saturated calomel electrode was used as the reference electrode, and a pt-foil as the auxiliary electrode. the working electrodes were prepared by mixing active materials (8 mg), acetylene black as a conductive reagent, and 5%-nafion as a binder (80:10:10 wt %) and dispersed in absolute ethanol in ultrasonic bath. the dispersion was coated onto pt foil drop by drop and then dried at 353 k. the cv experiments were carried out in aqueous solution of 1 mol dm-3 h2so4. results and discussion fig. 1a and 1b show the n2 adsorption-desorption isotherms and bjh pore size distribution of mc and mp. the two samples are found to yield a type iv isotherm with a type h2 hysteresis loop near relative pressure of 0.50 in the desorption branch, which is associated with sharp capillary condensation taking place in mesopores [18]. 0.0 0.2 0.4 0.6 0.8 1.0 100 200 300 400 500 600 700 800 900 relative pressure a a ds or pt io n vo lu m e, c m 3 g -1 adsorption desorption 0.0 0.2 0.4 0.6 0.8 1.0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 0.05 0.10 0.15 0.20 0.25 0.30 dv / d lo gd pore size / nm b ad so rp tio n vo lu m e, c m 3 g -1 relative pressure adsorbtion desportion figure 1. nitrogen adsorption-desorption isotherms and bjh pore size distribution (inset) of a) mc and b) mp. j. electrochem. sci. eng. 1(1) (2011) 65-71 mc/ppy composites capacitance 70 the results show that the mesoporous structure of mc was maintained after loading of ppy. however, bet surface area of mp was considerably lower. the specific surface areas of mesoporous carbon and mp were 1041 and 207 m2 g–1, respectively (table 1). the decrease of specific surface area is mainly attributed to the mesopores of the support being partially covered or filled by ppy. therefore, pore volume and average pore size also decrease from 1.18 cm3 g–1 and 5.2 nm to 0.22 cm3 g–1 and 4.9 nm, respectively. this indicates that ppy was distributed evenly over the surface of mc. in order to gain further insights into the structure of mesporous carbons, we investigated their appearance on the synthesized materials by tem (fig. 2). the disordered mesoporous size of mc with around 5 nm, which is clearly visible in fig. 2a, is in agreement with the data of the adsorption-desorption measurements (fig. 1). fig. 2b clearly shows the mesoporous structure of mc covered over with transparent films. in comparison with fig. 2a, we may draw a conclusion that the transparent film is ppy and that it has been successfully loaded on the mc. figure 2. tem images of a) mc and b) mp. the functional groups of mc, ppy, and mp are characterized by ftir spectroscopy (fig. 3). the characteristic peak at 3423 cm–1 is observed in the ir spectrum of mc and ascribed to –oh. the small peaks at 2920 and 2850 cm–1 originate from the stretching vibrations of c–h bond. the peaks at 1731, 1632, and 1402 cm–1 could be assigned to the stretching vibration in carboxyl groups and c=o, c=c, and c–o bonds. the bands at 1537 and 1452 cm–1 in ppy and mp spectra are due to the typical pyrrole ring vibration of pure ppy and bands of =c–h in plane vibration at 1298, 1089, and 1032 cm–1 [19]. the peaks at 3434 and 1631 cm–1 correspond to n–h and c=c stretching vibrations, respectively. the spectrum of mp is very similar to that of ppy, verifying that ppy has been successfully applied onto mc. however, the peak at 1632 cm–1 for c=o disappeared due to the combination of mc and ppy. similarly, the peak for n-h stretching exhibits a red-shift phenomenon due to the interaction with the reactive hydroxyl functional groups [20]. fig. 4 shows cvs of mc and mp in 1 mol dm-3 h2so4 at different scan rates. cvs of mc electrode contain redox peaks and deviate from the rectangular shape (fig. 4a). with increasing scan rates, the redox current evidently increases, indicating that it has good rate capability. ftir spectra confirm the existence of the abundant functional groups, for example c=o and o-h bonds, on the surface of mc. these groups are supposed to remarkably improve the hydrophilicity and wettability of the surface of mc and are beneficial for the aqueous electrochemical capacitors. the redox peaks might originate from the transformation of the o–h bond of mc into the c=o and vice versa during charge and discharge processes. in addition, the c=o bond of mc interacts with the h+ to become –oh in the reduced state, which would be re-oxidized into c=o during the discharge. wu-jun zou et al. j. electrochem. sci. eng. 1(1) (2011) 65-71 doi: 10.5599/jese.2011.0001 71 4000 3500 3000 2500 2000 1500 1000 60 65 70 75 80 85 90 95 1036 1134 1300 1452 1543 1634 28522923 1032 1089 1298 1452 1537 1631 2852 2923 3430 3434 14021632 1731 2850 2920 3423 mp ppy mc tr an sm itt an ce , % wavenumber, cm-1 figure 3. ft-ir spectra of mc, ppy and mp. however, the cvs of mp present a steep increase in the current range from 0.0 to 0.1 v, which is an important behavior in supercapacitors [14], as shown in fig. 4b. cv curves of the composites do not exhibit redox peaks as pronouncedly as in the case of mc. this is because the π-bonded surface of the mc may interact strongly with the conjugated structure of ppy, especially through the pyrrole ring [21], or n-h of ppy can interact with the reactive hydroxyl functional groups of mc [20]. 0.0 0.2 0.4 0.6 0.8 -10 -5 0 5 10a cu rr en t d en si ty , a g -1 potential, v vs sce 2 mv s-1 5 mv s-1 10 mv s-1 20 mv s-1 50 mv s-1 0.0 0.2 0.4 0.6 0.8 -16 -8 0 8 16b cu rr en t d en sit y, a g -1 potential, v vs sce 2 mv s-1 5 mv s-1 10 mv s-1 20 mv s-1 50 mv s-1 0 200 400 600 800 1000 80 100 120 140 160 180 200 220 240c c / f g1 cycle number mc mp figure 4. cvs of a) mc and b) mp in 1 mol dm-3 h2so4 electrolyte at different scan rates; c) cycle life of mc and mp in 1 mol dm-3 h2so4 electrolyte at the scan rate of 20 mv s –1. j. electrochem. sci. eng. 1(1) (2011) 65-71 mc/ppy composites capacitance 72 the gravimetric specific capacitance (c) of electrode is calculated according to the eq. (1) from charge-discharge data of cvs: vw ti wv q c ∆ d∫== (1) where i, w and v are the voltammetric current, the mass of active materials, and the total potential of electrochemical window, respectively. the calculated specific capacitances are listed in table 1. the maximum specific capacitance of mp reaches as high as 313 f g–1 at the scan rate of 2 mv s–1, which is nearly twice the specific capacitance of mc (163 f g–1). although the surface area of mc dramatically decreases due to the coverage of ppy, leading to the edlc loss, the pseudocapacitance from the faradic reaction on the ppy contributes much more to the total capacitance, leading to a higher specific capacitance of mp. due to the different synthesis methods and electrolytes and measured potential windows, it is difficult to compare our results with those reported in the literatures. therefore, for comparison purpose, we synthesized the ppy using the same method and measured the electrochemical properties in 1 mol dm-3 h2so4. the specific capacitance of pure ppy was 328.4 f g-1 at a scan rate of 2mv s-1. if the contribution of pure mc and pure ppy were calculated, the total was only 229 f g-1. however, in this study, we obtained a higher result that reached 313 f g−1, revealing the intensely synergetic effect between mc and ppy. fig. 4c shows a cycle life of mc and mp in 1 mol dm-3 h2so4 electrolyte at scan rate of 20 mv s–1. the specific capacitance of mp drops from 238 f g–1 down to 181 f g–1 after 1000 cycles, corresponding to a 23.9% loss. table 1. pore structure parameters of the mc and mp and the specific capacitances of mc and mp electrodes calculated from cvs in 1 mol dm-3 h2so4 electrolyte sample surface area m2 g-1 pore volume cm3 g-1 pore size nm c / f g-1 2 mv s-1 5 mv s-1 10 mv s-1 20 mv s-1 50 mv s-1 mc 1041 1.18 5.2 163 150 139 125 103 mp 207 0.22 4.9 313 281 261 238 203 conclusions mc/ppy composites were successfully synthesized using a chemical method of oxidative polymerization. the resulting modified electrode showed some properties similar to the combination of the double layer capacitance of mc and pseudocapacitance of ppy. the results obtained by cyclic voltammetry demonstrated that the specific capacitance of the mp electrode was as high as 313 f g−1 compared to 163 f g−1 of mc electrode. in conclusion, it seems that the mp is a promising electrode material in supercapacitor field. acknowledgements: the study was financially supported by the national natural science foundation of china (20876067 and 21031001) and the fundamental research funds for the central universities (21609203). references [1] j. lee, s. yoon, t. hyeon, s.m. oh and k.b. kim, chem. commun. 21 (1999) 2177-2178 [2] k.t. lee, x.l. ji, m. rault and l.f. nazar, angew. chem. int. ed. 121 (2009) 5661-5665 [3] j.x. chen, n.n. xia, t.x. zhou, s.x. tan, f.p. jiang and d.s. yuan, int. j. electrochem. sci. 4 (2009) 1063-1073 [4] s.h. joo, s.j. choi, i. oh, j. kwak, z. liu, o. terasaki and r. ryoo, 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice a comparison of pitting susceptibility of q235 and hrb335 carbon steels used for reinforced concrete doi: 10.5599/jese.2012.0025 11 j. electrochem. sci. eng. 3(1) (2013) 11-17; doi: 10.5599/jese.2012.0025 open access : : issn 1847-9286 www.jese-online.org original scientific paper a comparison of pitting susceptibility of q235 and hrb335 carbon steels used for reinforced concrete guoliang zhan, jiancheng luo*, shaofei zhao*, weishan li* new century concrete of guangdong foundation co., ltd, guangzhou 510660, china *school of chemistry and environment, south china normal university, guangzhou 510006, china corresponding author: e-mail: liwsh@scnu.edu.cn; tel.: +86-020-39310256; fax: +86-020-39310256 received: july 16, 2012; revised: october 16, 2012; published: november 13, 2012 abstract the phase structure and the pitting susceptibility of two carbon steels, q235 and hrb335, used for reinforced concrete, are investigated by phase observation, polarization curve measurements, electrochemical impedance spectroscopy, and mott-schottky analysis. it is found that q235 is ferrite and hrb335 is pearlite. q235 is more susceptible to chloride ions leading to pitting than hrb335. the polarization curves show that the breakdown potential of the passive film in saturated ca(oh)2 solution containing 0.4 m nacl is 0 v for q235 and 0.34 v for hrb335. the mott-schottky analyses show that passive films formed on q235 and hrb335 in saturated ca(oh)2 solution containing chloride ions behave like an n-type semiconductor. the passive film formed on q235 has a higher donor density, which explains why q235 is more susceptible to pitting than hrb335. keywords pitting susceptibility; carbon steel; phase structure; polarization curve; electrochemical impedance spectroscopy; mott-schottky. introduction reinforced concrete is widely used as building material because the corrosion resistance of the embedded carbon steel for the reinforcement plays a significant role in the life of reinforced concrete. in a high quality concrete, the embedded carbon steel prevents corrosion by forming a passive film on a steel surface, which slows down the access of oxygen, moisture, and various aggressive species to the interface between steel and concrete [1]. among all the aggressive species, chloride ions exhibit the strongest attack on passive film. chloride ions may be introduced to the concrete from raw material, such as water and sand, or they can penetrate from the outside in highway viaducts where de-icing salts are used as well as in marine structures [2]. in practice, http://www.jese-online.org/ mailto:liwsh@scnu.edu.cn j. electrochem. sci. eng. 3(1) (2013) 11-17 pitting susceptibility of q235 and hrb335 12 the corrosive attack due to chloride penetration usually leads to pitting corrosion, which causes catastrophe because its initiation and propagation is difficult to predict. the pitting susceptibility of carbon steel is related to the microstructure and composition of the steel and inhibitors used for the formation of passive films [3-12]. various carbon steels usually are combined for use because of the need for strength and tenacity. for example, two kinds of carbon steels, q235 and hrb335, are usually used together in reinforced concrete structures to improve the strength of concrete. it is necessary to understand the pitting susceptibility of various kinds of carbon steel for their successful application in reinforced concrete. the aim of this work is to understand the difference in pitting susceptibility between q235 and hrb335. experimental electrodes and solutions the composition of q235 and hrb335 is given in table 1. the steel specimens (φ 0.8 cm × 0.5 cm) were embedded in epoxy resin, with a test area of 0.5 cm 2 . prior to each measurement, the working surface of specimens was polished with different sic2 abrasive papers and al2o3 powder of 0.05 μm, then degreased with ethanol and rinsed with de-ionized water successively. the saturated ca(oh)2 solution was prepared as simulated concrete pore solution (sps) with de-ionized water. nacl was added to form the solution containing chloride ions. all the chemicals used were of analytical grade. table 1. composition of q235 and hrb335, wt. % samples c mn p s si cr ni al as q235 0.15 0.326 0.039 0.03 0.115 0.024 0.032 0.127 0.028 hrb335 0.26 0.698 0.046 0.05 0.324 0.033 0.039 0.016 0.026 phase-structure observation optical microscope (eclipse 50ipol, nikon corporation) was used to observe the microstructure of q235 and hrb335. before the observation, the specimens were etched with 0.2 % nital for 10s, degreased with ethanol and rinsed with de-ionized water. electrochemical measurement the electrochemical measurements were carried out with pgstat-30 (autolab, eco chemise b. v. company). a classical three electrodes electrochemical cell was used. a platinum sheet with a geometric area of 1 cm 2 was used as the counter electrode and a saturated calomel electrode (sce) was used as the reference electrode. all the potentials in this paper are versus the sce. the electrolyte was deareated with nitrogen for 30 min before each measurement, and all the electrochemical measurements were carried out at ambient temperature without stirring. the potential scanning rate used in the polarization curve measurements was 0.5 mv s -1 . the impedance measurements were carried out in frequency range from 100 khz to 0.01 hz. in the mott-schottky measurement, the frequency used was 1 khz and the potential step was 5 mv. prior to each measurement, the working electrode was firstly kept at -1.15 v for 30 min, then at 0 v for 60 min to form a passive film, and finally it was stabilized at open circuit potential (ocp) for 30 min. g. zhan et al. j. electrochem. sci. eng. 3(1) (2013) 11-17 doi: 10.5599/jese.2012.0025 13 results and discussion phase structure fig. 1 presents the phase structure of q235 (a) and hrb335 (b). both, q235 and hrb335 have two phases, pearlite and ferrite [11,13]. however, the proportion of ferrite and pearlite is different in hrb335 and q235. ferrite phase prevails in q235 while in hrb335 prevails mainly pearlite. the composition of two carbon steel is similar except for the contents of mn and si (table 1), which might account for the difference in microstructure between two carbon steels. from the different microstructure of the two carbon steels, it can be expected that they exhibit different pitting susceptibility [3,9]. figure 1. phase structure of q235 (a) and hrb335 (b) polarization curve in order to understand the formation process of passive film and to determine the passive potential range, a potential scan ranging from -0.6 to 0.65 v is performed. fig. 2 presents the formation of passive film on q235 and hrb335 obtained in the simulated concrete pore solution (sps). it can be seen from fig. 2 that both electrodes, q235 and hrb335, exhibit a similar formation process of passive film. a cathodic process takes place when the potential is scanned from point a (-0.6v) to b (-0.41 v) for q235, and from point a (-0.6 v) to b’ (-0.45 v) for hrb335. this cathodic process should be ascribed to the hydrogen evolution reaction, because the experiments were carried out under nitrogen atmosphere and there were no other reducible species in the solution. when potential is scanned from point b (-0.41 v) and b’ (-0.45 v) toward positive potential, the current in both cases (q235 and hrb335 electrodes) increases till the potential approaches point c (-0.18 v). from point c (-0.18 v) to point d (0.65 v) the electrodes reach passive region. both electrodes, q235 and hrb335, have similar passive current density of about 4 μa cm -2 . the sharp increase of the current density at point d (0.65 v) is ascribed to the o2 evolution, which depends on ph of the solution [13]. the current plateau of the anodic polarization curve indicates the formation of passive film within the potential domain from -0.18 to 0.65 v for both carbon steel electrodes. fig. 3 shows the polarization curves of q235 and hrb335 in the sps containing 0.4 m chloride ions. before the measurement, the two carbon steel electrodes were passivated in sps, in absence of chloride ions, at the potential of 0 v for 60 min. in sps containing 0.4 m chloride ions, the breakdown of passive films on both electrodes, q235 and hrb335, occurs at the potential, which is indicated by a drastic increase of current. however, the breakdown potential values of passive a b j. electrochem. sci. eng. 3(1) (2013) 11-17 pitting susceptibility of q235 and hrb335 14 films were different, i.e. about 0 v for q235 and 0.34 v for hrb335. therefore, the passive film formed on q235 is more susceptible to chloride ions than the one formed on hrb335 electrode. -8.5 -8.0 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 b a b' c d e / v vs . s c e log i / a cm -2 q235 hrb335 figure 2. polarization curves of q235 and hrb335 in sps in absence of chloride ions (scan rate: 0.5 mv s -1 ). -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 -0.4 -0.2 0.0 0.2 0.4 0.6 log i / a cm -2 e / v vs . s c e q235 hrb335 figure 3. polarization curves of q235 and hrb335 in sps containing 0.4 m chloride ions (scan rate: 0.5 mv s -1 ). electrochemical impedance fig. 4 presents the nyquist plots of passivated q235 and hrb335 in the sps containing 0 and 0.4 m chloride ions at open circuit potential. it can be seen from fig. 4 that two electrodes have similar behavior which does not involve any diffusion process, although there is a significant difference in polarization impedance for the electrodes in the solution with and without chloride ions. a passivated electrode can be modeled by the equivalent circuit of fig. 5. in fig. 5, rs represents the solution resistance; rf and q are the resistance and the space charge layer capacitance of the passive film. the element q is usually represented by the constant phase element (cpe) in which n is in the range between 0.5 and 1 due to the surface heterogeneity and surface roughness of the passivated electrodes [14]. the impedance of a cpe is given by   1 n0 cpe yz jw      (1) g. zhan et al. j. electrochem. sci. eng. 3(1) (2013) 11-17 doi: 10.5599/jese.2012.0025 15 0 50 100 150 200 250 300 0 50 100 150 200 250 12.56 mhz 39.80 mhz hrb335 q235 -z '' /  c m 2 z' / k cm 2 0 5 10 15 20 25 0 2 4 6 8 10 12 z' / k cm 2 10.00 mhz 62.70 mhz hrb335 q235 -z '' /  c m 2 figure 4. nyquist plots of passivated q235 and hrb335 electrodes in sps: (a) without chloride ions; (b) containing 0.4 m chloride ions the capacitance element q (cpe) is pure capacitance when n = 1 and pure resistance when n = 0. the results obtained from fitting with fig. 5 are shown by solid lines in fig. 4 and by the data in table 2. it can be seen from table 2 that, in the solution without chloride ions, the film resistance (rf) of q235 is smaller than the one of hrb335, indicating that the passivated q235 tends to react more easily than the passivated hrb335. when chloride ions are added in the solution, the resistance of the film decreases for both carbon steel electrodes. figure 5. equivalent circuit for a passivated electrode. mott-schottky analysis passive films of most metals behave as semiconductor [15-18]. in the high frequency domain, the mott-shottly approach is a good tool for characterizing the semiconducting properties of a passive film [19,20]. when the frequency used for the impedance measurement is high enough, potential dependence of the capacitance of space-charge layer (csc) is expressed by mott-schottky relationship [5]: for n-type semiconductor            fb2 sc r 0 d 1 2 kt e c e n e (2) for p-type semiconductor             fb2 sc r 0 a 1 2 kt e c e n e (3) where e is electron charge (1.6×10 -19 c), εr is dielectric constant, taken as 15.6 [17]. ε0 is the vacuum permittivity (8.85×10 -14 f cm -1 ), nd is donor density, na is acceptor density, e is the applied potential, φfb is flat-band potential, k is boltzmann constant (1.38 × 10 -23 j k -1 ) and t is absolute temperature. nd and na can be determined from the slope of the linear relationship between csc -2 and e, while φfb is obtained from the extrapolation to csc -2 = 0. rs q rf rs q rf j. electrochem. sci. eng. 3(1) (2013) 11-17 pitting susceptibility of q235 and hrb335 16 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 100 150 200 250 300 350 400 e / v vs. sce c -2 / f -2 m 4 q235 hrb335 figure 6. mott-schottky plots of passivated q235 and hrb335 in the sps containing 0.4 m chloride ions. fig. 6 shows mott-schottky plots of passivated q235 and hrb335 electrodes in sps containing 0.4 m chloride ions. it can be seen from fig. 6 that, at the potentials lower than 0.35v, csc -2 increases with increasing the potential. there is a positive linear relationship between csc -2 and e in the potential range from 0 to 0.15 v for both electrodes, indicating that both passive films are ntype semiconductors [4]. the donor densities values calculated from the slope of the linear relationship between csc -2 and e in fig. 6 are shown in table 2. it is found that the donor densities values are very high, i.e. in the order of 10 26 m -3 . this order is characteristic of heavily doped and disordered passive films, which was also found by cheng and luo (10 26 m -3 ≤ nd ≤ 10 27 m -3 ) [21]. a donor density of a passive film determines its pitting susceptibility. the larger the donor density is, the more susceptible to pitting of the passive film is. the donor density of passive film formed on q235 is 4.42 × 10 26 m -3 , which is higher the one of hrb335 (2.29 × 10 26 m -3 ), confirming that q235 is more susceptible to pitting than hrb335. table 2. fitting results of the experimental data in fig. 4 and fig. 6 samples cnacl / m rf / kω cm 2 y 0 /µs cm -2 s n n nd / 10 26 m -3 q235 0 285.3 10.95 0.9607 q235 0.4 6.0 18.42 0.9236 4.42 hrb335 0 402 10.81 0.9610 hrb335 0.4 22.5 12.55 0.9512 2.29 conclusions based on the results from phase observation, potentiodynamic polarization, electrochemical impedance measurements and mott-schottky analysis of passive films of two carbon steel electrodes, it can be concluded that q235 is more susceptible to pitting than hrb335. q235 has a phase structure of ferrite, which tends to form a less stable passive film and thus is more susceptible to chloride ions than hrb335, whose phase structure is pearlite. acknowledgements: this work was supported by the natural science foundation of guangdong province (grant no. 10351063101000001) and the joint project of guangdong province and g. zhan et al. j. electrochem. sci. eng. 3(1) (2013) 11-17 doi: 10.5599/jese.2012.0025 17 ministry of education for the cooperation among industries, universities and institutes (grant no. 2011b090400633). references [1] a.a. almusallam, constr. build. mater. 15 (2001) 361-368 [2] m. ormellese, m. berra, f. bolzoni, t. pastore, cement concrete res. 36 (2006) 536-547 [3] w.s. li, j.l. luo, j. mater. sci. lett. 21 (2002) 1195-1198 [4] w.s. li, j.l. luo, corros. sci. 44 (2002) 1695-1712 [5] w.s. li, n. cui, j.l. luo, electrochim. acta 49 (2004) 1663-1672 [6] w.s. li, s.q. cai, j.l. luo, j. electrochem. soc. 151 (2004) b220-b226 [7] j. lu, w.s. li, j.l. luo, corros. eng. sci. techn. 43 (2008) 208-212 [8] w.s. li, j.l. luo, int. j. electrochem. sci. 2 (2007) 627-665 [9] a. pardo, m.c. merino, a.e. coy, r. arrabal, f. viejo, e. matykina, corros. sci. 50 (2008) 823834 [10] e.e abd el aal, s. abd el wanees, a. diab, s.m. abd el haleem, corros. sci. 51 (2009) 16111618 [11] f. zhang, j.s. pan, c.j. lin, corros. sci. 51 (2009) 2130-2138 [12] l. hamadou, a. kadri, d. boughrara, n. benbrahim, j.p. petit, appl. surf. sci. 252 (2006) 4209-4217 [13] d. trejo, p.j. monteiro, cement concrete res. 35 (2005) 562-571 [14] m. cai, s.m. park, j. electrochem. soc. 143 (1996) 3895-3902 [15] a.m.p. simoes, m.g.s. ferrira, b. rondot, m. cunhabelo, j. electrochem. soc. 137 (1990) 8287 [16] g. cooper, j.a. turner, a.j. nozik, j. electrochem. soc. 129 (1982) 1973-1977 [17] k. azumi, t. ohtsuka, n. sato, j. electrochem. soc. 134 (1987) 1352-1357 [18] u. stimming, electrochim. acta 31 (1986) 415-429 [19] l. hamadou, a. kadri, n. benbrahim, corros. sci. 52 (2010) 859-864 [20] f. di quarto, m. santamaria, corros. eng. sci. techn. 39 (2004) 71-81 [21] y.f. cheng, j.l. luo, electrochim. acta 44 (1999) 2947-2957 © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ electrochemical extraction of oxygen using pem electrolysis technology doi: 10.5599/jese.2012.0016 211 j. electrochem. sci. eng. 2 (2012) 211-221; doi: 10.5599/jese.2012.0016 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical extraction of oxygen using pem electrolysis technology boulbaba eladeb, caroline bonnet, eric favre and françois lapicque laboratoire réactions et génie des procédés, cnrsuniversité de lorraine, ensic, 1 rue grandville, f-54001 nancy, france corresponding author: e-mail: francois.lapicque@ensic.inpl-nancy.fr; tel.: 33 (0) 383 17 52 66 received: march 13, 2012; revised: july 12, 2012; published: november 10, 2012 abstract electrochemical extraction of oxygen from air can be carried out by chemical reduction of oxygen at the cathode and simultaneous oxygen evolution by water anode oxidation. the present investigation deals with the use of an electrolysis cell of pem technology for this purpose. a dedicated 25 cm2 cell provided with a commercial water electrolysis mea and titanium grooved plates has been designed for continuous operation at pressures close to the ambient level. the mea consisted of a nafion 117 membrane sandwiched between a pt/c cathode and a non-supported pt-ir anode. oxygen partial consumption in long-term runs was evaluated by analysis of the outlet air by gas chromatography, depending on the cell voltage or the current density and the excess in air oxygen fed to the cathode. runs over more 50 hours indicated the relative stability of the components used for current densities ranging from 0.1 to 0.2 a cm-2 with high efficiency of oxygen reduction. higher current density could be envisaged with more efficient mea’s, exhibiting lower overpotentials for oxygen evolution to avoid too significant degradation of the anode material and the membrane. interpretation of the data has been carried out by calculation of the cathode current efficiency. keywords oxygen extraction; pem electrolysis; electrode stability; pt-ir anode; current efficiency 1. introduction extraction of oxygen from air can be used for various applications, as follows. for instance oxygen impoverished air reduces the oxidation content from the atmosphere over food for the sake of higher preservation in its packing. lower oxygen contents in air also reduce the corrosion rate at non-noble metals pieces. another potential application is the preparation of the gas phase to be injected to biological reactors – upon addition of carbon dioxide – for the possible growth of j. electrochem. sci. eng. 2(4) (2012) 211-221 extraction of oxygen using pem technology 212 microorganisms such as bacteria, fungi or microalgae or undifferentiated cells of higher-grade vegetal organisms in dedicated bioreactors. various techniques for oxygen removal from a gas phase to be used for biological applications or food processing can be listed as follows. the easiest technique, which can be qualified as indirect, simply consists of adding of pure nitrogen into the air stream: the cost of pressured nitrogen renders the technique somewhat expensive for practical applications. conventional, non electrochemical techniques for oxygen removal rely upon the following various principles: (i) oxygen combustion by a gas microburner, however other gases of significant safety or environment impact such as carbon monoxide or nitrogen oxides can be formed; (ii) selective reaction of oxygen upon addition of reducing agents e.g. sulfites that have been used for decades for food preservation: however the sulfate produced by oxygen oxidation of sulfite have to be removed from the medium; (iii) selective adsorption on a suitable compound which has nevertheless to be regenerated for its possible reuse; (iv) membrane separation of oxygen from carbon dioxide and nitrogen at temperatures close to ambient: however, because of the intermediate behaviour of oxygen in the three-species, this technique cannot be used for biological applications. besides, various electrochemical techniques can be used for oxygen extraction from an air flow, as reviewed by winnick [1]. in a pioneering paper, langer and haldermann [2] showed that oxygen could be extracted from air and recovered in the form of pure oxygen in a two-compartment electrolytic cell, in the presence of acidic or alkaline solutions and separated by a conventional cationic membrane: cathode o2 + 4h + + 4e → 2 h2o (1) anode 2 h2o → o2 + 4h + + 4e (2) the equilibrium voltage of such a cell is nil, assuming similar conditions of h+ activity and oxygen pressure in the two compartments; the cell voltage is the sum of the two overpotentials – in absolute valueand the ohmic drop of the cell. the two reactions involved are nevertheless slow, even at platinum surfaces, and the cell voltage attains easily 1 v even at low current densities. since then, the technique was improved by wynween and montgomery [3]. in the eighties, fujita et al. [4] improved the efficiency of the technique by using a nafion membrane, an air cathode whereas the anode was platinum particles deposited on the membrane surface: this technology derived from pem fuel cell technology allowed current density up to 200 ma cm-2 for cell voltage at 1.4 v for continuous operation: the corresponding energy consumption for the oxygen extraction was near 4.7 kwh kg-1 o2. it can be observed that the process consumes less energy than the conventional electrolysis, with specific energy consumption near 6.7 kwh kg-1 o2 for a cell voltage of 2 v, even though the current densities of the two processes differ quite a lot. more recently, general electrics [5] developed an oxygen supply system for high-altitude aircraft relying upon similar technology: oxygen at high pressures could be produced with cell voltage claimed to be near 1 v at 125 ma cm-2. the electrochemical separation of oxygen can be carried out upon specific diffusion in mixed, non stoichiometric oxides exhibiting oxygen vacancies in the lattice, as described in several patents [6-8]. the technology used derived from solid oxide cells: oxygen is reduced to o2ions at the cathode and migrates to the anode for back oxidation to oxygen; temperatures higher than 500 °c are required for sufficient diffusivity of o2ions in the non-stoichiometric oxide mixture. however the current density is usually below 100 ma cm-2. b. eladeb et al. j. electrochem. sci. eng. 2(4) (2012) 210-221 doi: 10.5599/jese.2012.0016 213 oxygen extraction by its reduction to hydrogen peroxide on a graphite-based cathode through exchange of two electrons only was imagined by tseung and jasem [9] cathode o2 + h2o + 2e → ho2 + oh(3) as expected form the electron number, the energy consumption could be reduced to 2.7 kwh kg-1 o2 in a concentrated koh solution. peroxide decomposes chemically to oxygen and peroxide on the surface of nico2o4 mesh, whereas oxygen is formed at the nico2o4 anode after anode 2 oh→ ½ o2 + h2o + 2e (4) brillas et al. [10] developed the above technique by using carbon-polytetrafluorethylene air-fed cathode. peroxide anions ho2 are formed at the carbon-based cathode, whereas oxygen evolution occurs at the anode by oxidation of both ho2 and ohanions. pure oxygen is produced with a voltage lower than in conventional water electrolyzers, because of the lower standard potential of oh-/ h2o2, which further reduces the energy consumption. however, the current density is limited below 0.2 a cm-2 by the finite concentration of peroxide in the solution. recently, we demonstrated the use of a pemfc for oxygen extraction, the anode reaction being hydrogen oxidation [11]. the technique was shown to be of relatively high efficiency, with current density up to 0.6 a cm-2, i.e. 0.015 mol o2 s -1 per m2 membrane. long-term tests confirmed the validity of the method, which however requires the presence of hydrogen to feed the fuel cell. to avoid this drawback, it was preferred to replace hydrogen oxidation at the anode by oxygen evolution. the present investigation deals with the use of an electrolysis cell of pem technology involving reactions (1) and (2) at the electrodes. a dedicated 25 cm2 cell provided with a commercial water electrolysis mea has been designed for operation at pressures close to the ambient level. oxygen partial consumption in long term tests was evaluated by analysis of the outlet air, depending on the cell voltage or the current density and the excess in air oxygen fed to the cathode. interpretation of the data has been conducted by calculating the cathode current efficiency. 2. experimental section 2.1. electrochemical cell and measurement devices the 25 cm2 cell has been designed and built up from regular pem technology. the cell consisted of two ti bipolar plates being 10 mm thick: the flow pattern of serpentine type with five 1×1 mm2 parallel channels with 1mm broad edge was designed and machined by the mechanical workshop of the lab. the grooved part of the plate was covered by a 0.5 µm gold layer by ion sputtering. gas diffusion layer at the cathode was of carbon material (sgl 30 bc, ubzm, germany) including a macroporous carbon fiber paper and a carbon black microporous layer (mpl), whereas a one mm thick high porosity ti fleece was used at the anode. the mea (fumatech) designed for pem water electrolysis, consisted in a 117 nafion membrane, a pt/c (0.4 mg cm-2) cathode and a nonsupported pt/ir (0.45 mg cm-2) anode (figure 1). all experiments were conducted at temperature levels ranging from 50 to 80°c. liquid water was driven to the anode compartment by a peristaltic pump at 5 cm3 min-1 and preheated before entering the cell. purified compressed dry air at flow rate ranging from 20 to 200 stp cm3 min-1 was humidified in lab-made trickled bed at rh=62 %. the cell was operated either at fixed voltage or at controlled current density using a pgstat 30 autolab potensiostat connected to a 20 amp. autolab booster. j. electrochem. sci. eng. 2(4) (2012) 211-221 extraction of oxygen using pem technology 214 figure 1. schematic view of the 25 cm2 cell for oxygen extraction prior to its use for oxygen extraction, the mea was run up by carrying out water electrolysis at 80°c: the anode was fed with hot water and nitrogen at 40 stp cm3 min-1 was fed to the cathode. as recommended by the supplier the mea was conditioned by 24 hour long operation at 2.0 v for maturation of the anode ir/pt catalyst. 2.2. current versus voltage measurements current density vs. voltage curve was established either in potensiostatic or in galvanostatic modes, so that the cell voltage was below 1.4 v for long term runs. because the reactions of interest are oxygen reduction and evolution, the two electrode potentials are distributed around the equilibrium potential of h2o/o2 couple. moreover the two reactions are relatively slow processes and exhibit absolute overpotentials in the same order of magnitude, it can therefore be estimated that the maximal cell voltage corresponds to anode potentials in the order of 1.8 v vs. nhe i.e. in a domain where corrosion phenomena at the anode can become significant and affect the stability of the anode catalyst. most measurements were carried out with air but oxygen was also used for comparison. 2.3. oxygen extraction runs and gas analysis oxygen extraction was achieved through numerous runs conducted at fixed current density for which the cell voltage was below 1.4 v as justified above. taking into account faraday’s law and the oxygen content in air ( 2 in oy =0.21), the inlet oxygen flow rate was large enough for all runs. the cell voltage at fixed current and air flow rate was monitored for lapses of time ranging from 4 to 24 hours: in most cases, the voltage attained a steady value within ten to thirty minutes. the gas leaving the cathode was stored in a 400 cm2 polymeric sampling bag. this bag has been thoroughly emptied first, the outlet was then sampled for a couple of minutes and the bag was purged again. the second, longer sampling procedure for analysis was then achieved. after sufficient filling, the sampling gas was shut. the decrease in temperature to the ambient level was b. eladeb et al. j. electrochem. sci. eng. 2(4) (2012) 210-221 doi: 10.5599/jese.2012.0016 215 the cause of water condensation and the bag slightly shrunk. the ambient temperature, tamb, recorded for all runs ranged from 19 to 27°c. analysis of the gas containing in the bag, i.e. nitrogen, oxygen and slight amount of water vapor was carried out off line by gas chromatography using a molecular sieve microcolumn with a tcd detector in the µgc equipment (varian 490 gc). three replicate analyses were conducted for each sample. the tcd detector was calibrated by analysis of six synthetic air-nitrogen mixtures of oxygen concentration perfectly known and ranging from 5 to 21 vol. %. the vapor amount in the gas mixture at tamb was estimated using antoine’s law: 2h o amb 1737.93 log 5.20389 39.485 p t = − − (5) where the partial pressure of water is expressed in atm and tamb is in k. the composition of the dry outlet could then be deduced from ohy 2 . 3. experimental results and interpretation 3.1. electrochemical behavior of the cell steady values of the current or the voltage – depending on the electrical mode were usually obtained within 15 minutes or so and for voltammetric measurements the voltage or current level was changed every 30-60 minutes. the mea response could slightly depend on the long-term run carried out immediately before the measurement. temperature was shown to exert a moderate effect (data not shown) and the results presented here were obtained at 60°c. at a given voltage gas flow rate exerted also a moderate influence on the cell current at a given cell voltage, as exemplified by figure 2. over a given flow rate depending on the current density, the effect is however of reduced importance. the observed phenomenon probably expresses the partial control by oxygen diffusion from the air fed to the cathode. figure 2. effect of the air flow rate on the current density recorded at fixed cell voltage. as expected, current density is an increasing function of the cell voltage (figure 3). feeding the cell with pure oxygen largely increases the performance of the cell, with for instance a current density twofold larger with pure oxygen than upon air feed at 1v. in comparison to what occurs with air pure oxygen accelerates charge transfer rates in both oxygen reduction and evolution, j. electrochem. sci. eng. 2(4) (2012) 211-221 extraction of oxygen using pem technology 216 while suppressing the gas-side mass transfer occurring in the presence of air nitrogen. current density up to 0.34 a cm-2 could be measured at 1.2 v with pure oxygen. the voltammetric data were compared to those obtained by langer [2] or by fujita [4] in figure 3. in spite of the different i-v profiles reported, the available current densities are in the same order of magnitude over 1 v, also with better performance with pure oxygen. the differences between the three sources of data can be explained by the different features of the electrochemical cells: in addition to the likely different ohmic resistances, the electrodes differed a lot, in particular the anode, consisting of deposited pt in the quoted papers and a pt-ir mixture in the present investigation. figure 3. current density versus cell voltage at steady state. present work with air at 200 stp cm3 min-1 and oxygen at 120 stp cm3 min-1; comparison with literature data. besides, the cell behaviour in long term runs was examined during oxygen extraction runs. as shown by figure 4 for fixed values of the cell voltage, the current monitored remained at a stable level for periods up to 50 hours. figure 4. current density in the oxygen extraction cell operated at fixed voltage. b. eladeb et al. j. electrochem. sci. eng. 2(4) (2012) 210-221 doi: 10.5599/jese.2012.0016 217 3.2. model of the cell oxygen extraction a simple zero-d model has been written for the cathode compartment of the cell. the inlet variables are the molar flow rate of dry air inairf and the cell current i, whereas outlet variables are the molar flow rate of oxygen outo2f and the temperature of the sampling gas, tamb: water was assumed to be at vapor-liquid equilibrium at tamb. faraday’s law is written for oxygen consumption at the cathode according to reaction (1): 2 2 out in o o 4 i f f φ = − ℑ (6) where φ is the current efficiency of oxygen reduction and ℑ is the faraday’s constant. the inlet oxygen flow rate is related to the inlet air flow as follows: 2 2 2 in in in in in o air n air of f f f y= − = (7) where fraction ino2y refers to the oxygen content in air. the oxygen molar fraction in the sampling bag is defined as: 2 2 out oout o out t f y f = (8) where subscript t refers to the total number of moles in the gas, covering nitrogen, oxygen and water vapor. finally the total molar flow rate of gas is related to the flow of dry gas and the vapor molar fraction in the bag: 2 2 in out air out air t h o amb h o amb 4 1 1 i f f f y t y t φ − ℑ= = − − (9) for the sake of simplicity, the ambient temperature is no more mentioned in the following. the fraction of vapor in the sampled gas was calculated by antoine’s law. finally, the stoichiometric factor λ of fed air oxygen related to the flow of oxygen consumed by reaction (1) with a current efficiency at unity is defined by the relation: 2 2 in in in o o air 4 i f y f λ= = ℑ (9) from relations (6) to (9) the current efficiency of oxygen reduction φ is expressed as follows: 2 2 2 2 2 out o h o in o out o h o 1 1 y y y y y λ − − φ = − − (10) for current efficiency equal to unity, the molar fraction of the oxygen in the sampling bag can be directly expressed as a function of operating parameter λ as: ( ) 2 2 2 out o h o in o 1 1 1 y y y λ λ − = − − (11) j. electrochem. sci. eng. 2(4) (2012) 211-221 extraction of oxygen using pem technology 218 3.3. experimental data and comparison with model predictions the oxygen molar fractions determined by gas chromatography are compared to the values predicted by eq. (11) in figure 5, considering the actual tamb value. good agreement between theory and practice is generally observed, even though the predicted line is slightly below the experimental data, indicating that the current efficiency could be somewhat lower than unity. for very large λ values, corresponding to large excess in oxygen, air is only slightly depleted in oxygen and the outlet gas fraction of oxygen is little different from the inlet value. conversely, with low excess of oxygen, high abatement of oxygen from the fed air can be obtained, as expected. figure 5. oxygen molar fraction at the outlet of the cell versus the stoichiometric factor of air oxygen λ finally, because the outlet molar fractions of oxygen are in the same order of magnitude as the inlet fraction, in particular for large excess in fed air, the uncertainty in the determination of the current efficiency had to be estimated. the related calculations reported in the appendix show that high values of the current and moderate values for stoichiometric factor are favorable for higher accuracy in the determination of φ: although the inlet flow rate of air could be moderate, the outlet molar fraction of oxygen is noticeably different from the inlet fraction, which prevails on the overall estimate for the uncertainty. on the contrary, for large λ values, the outlet molar fraction of oxygen is little below the inlet fraction and, from the expressions of terms a and b, the determination of the current efficiency is little accurate, with uncertainty exceeding 30 %. the variations of the current efficiency with the stoichiometric factor of air oxygen and current density are shown in figure 6 and 7 respectively, in the form of the maximum and the minimum estimates for each experiment. in spite of a significant dispersion, the data clearly show that the current efficiency is somewhat lower than unity. the separate analysis of the effect of operating conditions on the current efficiency is rendered uneasy because of the large dispersion of the data in the representations (φ vs. λ) or (φ vs. i) in figure 6 and 7. this apparent dispersion is mainly caused by the uncertainty in the determination of the current efficiency but also by the slight interaction between the two parameters. nevertheless the stoichiometric factor of air oxygen exerts a negative influence on the current efficiency (figure 6) whatever the current density applied. the positive effect of current density on φ appears clearly with very little effect from λ (figure 7): for current density over 100 ma cm-2, the current efficiency is near or larger than 0.9, which is promising for the investigated process. b. eladeb et al. j. electrochem. sci. eng. 2(4) (2012) 210-221 doi: 10.5599/jese.2012.0016 219 figure 6. current efficiency for oxygen reduction in the cell versus stoichiometric factor λ: maximum and minimum estimates figure 7. current efficiency for oxygen reduction in the cell versus the current density: maximum and minimum estimates 4. conclusions and significance the validity of the oxygen extraction using pem electrolysis technology has been validated in long term runs for current density up to 200 ma cm-2, with a cell voltage near 1.3 v. assuming that the reductive extraction is carried out with a current efficiency of 90 %, the corresponding energy consumption can be estimated to be near 4.4 kwh kg-1 o2. although the state of health of the pem j. electrochem. sci. eng. 2(4) (2012) 211-221 extraction of oxygen using pem technology 220 electrodes could not be evaluated by measurements of the electrode active surface [12], as done for pemfc, no real decay in activity could be observed after more than 400 hour tests with the same mea. considering the current progresses in water electrolysis technology, the current efficiency can be expected to attain 100 %, which would result in energy consumption near 4 kwh kg-1 o2. besides, the current improvement in the electrodes of pem electrolysers is to allow substantial reduction in cell voltage. however, to our point of view, the main advantage offered by pem electrodes is that the cathode reaction involving air oxygen should be carried out with current density to 1 a cm-2, provided that the cell voltage does not exceed 1.4 v to limit degradation issues at an acceptable level: progress in electrolysis mea has to be achieved for this purpose. in contrast, the reduction from a liquid phase as done through the peroxide route cannot be conducted for current densities over 100 or 200 ma cm-2. although the pem technique is not to allow lower energy consumption, higher production rates can be expected. long-term tests will be carried out at larger current densities, i.e. higher cell voltages, together with investigation of ageing phenomena, to evaluate the highest current density offered by the technique. further work has to be done by improving developing electrode materials exhibiting faster kinetics for both oxygen reduction and evolution. acknowledgements: the authors are indebted from region lorraine and institut carnot iceel which funded the investigation cost and b. eladeb’s phd grant. appendix: uncertainty in determination of the current efficiency relation (10) shows that efficiency φ is a function of parameters or variables λ, outo2y and oh2y . the differential of the current efficiency was expressed as a function of partial derivatives with respect to the three above variables: 2 2 2 2 out o h oout o h o d d d dy y y y λ λ    ∂φ ∂φ ∂φ φ = + +        ∂ ∂ ∂      (a1) derivation of the current efficiency with respect to λ, outo2y and oh2y followed by algebraic rearrangement led to: 2 2 2 2 2 2 2 out o h oout o h oout in o o h o d dd d 1 1 1 1y y y y y b ay y b a λ λ   φ   = + − + −      φ       (a2) with 2 2 2 out o h o in o 1 y a y y = − − (a3) and 2 2 out o h o1b y y= − − (a4) factor λ is linked to the molar flow rate of fed oxygen, which is proportional to the volume flow rate: the relative uncertainty of l can therefore be estimated by that for the inlet air flow rate. the flow meters employed for the measurements had a 1 % relative uncertainty in a large range of flow rates, i.e. for flow rates larger than 2 % of the full scale flow rate maxairq . below this threshold value, larger uncertainty can be expected. the following expressions were then considered: b. eladeb et al. j. electrochem. sci. eng. 2(4) (2012) 210-221 doi: 10.5599/jese.2012.0016 221 in max air air in in air air 0.02 1% q q q q λ λ δδ = = for in maxair air0.02q q< (a5) in air in air 1% q q λ λ δδ = = for in maxair air0.02q q> in practice, because of the flow rate scale considered, the relative uncertainty varied from 3 to 10 %. moreover, in spite of the replicate analysis in the gas chromatograph and the thorough calibration procedure, it can be estimated that the relative error in the analytical determination is near 1 %: 2 2 out o out o 1% y y δ = (a6) finally, the uncertainty in oh2y is due to uncertainty in the ambient temperature. considering that this temperature was estimated within 1 °c and using antoine’s law (5), the relative uncertainty in the molar fraction of vapour could be estimated in the temperature range considered: 2 2 h o h o 4.5% y y δ = (a7) applications of relations (a2)-(a7) yielded estimates for the relative uncertainty on the current efficiency, in the range 2-30 % for most cases. references [1] j. winnick, “electrochemical separation of gases”, in “advances in electrochemical science and engineering”, edited by h. gerischer and c.w. tobias, vol. 1, vch, new-york (1990), 210-248. [2] s.h. langer and r.g. haldemann, j. phys. chem. 68 (1964) 962-963. [3] r.a. wynveen and k.m. montgomery, j. electrochem. soc. 114 (1967) 589-592. [4] y. fujita, h. nakamura, t. muto, j. appl. electrochem. 16 (1986) 935-940. [5] j.w. harrison, asme 75-enas-51 (1975). [6] w.n. lawless, us patent n° 4,462,891 (july 31, 1984); 4,547,277 (oct. 15, 1985). [7] l.g. marianowski and r.j. remick, us patent n° 4,589,296 (1989). [8] m. riecke, us patent n° 6,632,400 b2 (2003). [9] a.c. tseung, s.m. jasem, j. appl. electrochem. 11 (1981) 209-215. [10] e. brillas, a. maestro, m. moratalla, j. appl. electrochem. 27 (1997) 83-92. [11] s. altmeyer, e. favre, c. bonnet, p. carré, f. lapicque « etude de procédés pour application au flux gazeux d’un photobioréacteur de culture d’algues », report 08 gps 155, nancy (jan. 2009) [in french]. [12] a. pozio, m. de franceco, a. cemmi,f. cardellini and l. giorgi, j. power sources 105 (2002) 13-19. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 25%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.6 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [4000 4000] /pagesize [612.000 792.000] >> setpagedevice {studies on electrodeposited zn-fe alloy coating on mild steel and its characterization} doi:10.5599/jese.565 9 j. electrochem. sci. eng. 9(1) (2019) 9-16; doi: http://dx.doi.org/10.5599/jese.565 open access: issn 1847-9286 www.jese-online.org original scientific paper studies on electrodeposited zn-fe alloy coating on mild steel and its characterization ramesh bhat, ampar chitharanjan hegde1 department of chemistry, nmam institute of technology, nitte-574110, india 1electrochemistry research laboratory, department of chemistry, nitk, surathkal, srinivasnagar 575 025, india corresponding author: e-mail: rameshbhat@nitte.edu.in; tel.: +91-8861037310; fax: 08254-281265 received: june 27, 2018; revised: july 19, 2018; accepted: august 19, 2018 abstract chloride bath containing zncl2 ∙7h2o, fecl2 ∙h2o and a combination of sulphamic acid and citric acid (sa+ca) were optimized for electrodeposition of bright zn-fe alloy coating on the mild steel. bath constituents and operating parameters were optimized by the hull cell method for highest performance of the coating against corrosion. the effect of current density and temperature on deposit characteristics such as corrosion resistance, hardness, thickness, cathode current efficiency and glossiness, were studied. potentiodynamic polarization and electrochemical impedance spectroscopic (eis) methods were used to assess corrosion behaviour. surface morphology of coatings was examined using scanning electron microscopy (sem). the zn-fe alloy with intense peaks corresponding to zn (100) and zn (101) phases, evidenced by x-ray diffraction (xrd) study, showed the highest corrosion resistance. a new and economical chloride bath for electrodeposition of bright znfe alloy coating on mild steel was proposed and discussed. keywords zn-fe alloy; chloride bath; sulphamic/citric acid; corrosion; xrd study introduction electrodeposition of metals and alloys has become extensively used in many industries, showing distinctive advantages compared to most of other finishing technologies [1]. implementation of zn and zn alloy coatings appears to be one of most original and simplest methods for attainment of high protection against corrosion [2]. the most commonly used metals for zn alloying are of the fe group [3-5]. zn-fe alloy coatings are characterized by excellent corrosion resistance and good weldability, formability and paint ability. they have numerous important applications such as in chemical and galvanic processes within automobile and aerospace industries [6]. behaviour of zn, fe http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:rameshbhat@nitte.edu.in j. electrochem. sci. eng. 9(1) (2019) 9-16 electrodeposited zn-fe alloy coating 10 and zn-fe alloy deposited onto copper from acid chloride solutions containing edta and boric acid was investigated by cyclic voltammetry and steady state polarization techniques [7]. electrodeposition of zn-fe alloys from a chloride-based electrolyte has already been studied using electrochemical polarization techniques and other instrumental methods [8]. it was found that the coating of zn-fe alloy is formed following the mechanism of nucleation and subsequent growth. the zn2+ ions in the electrolyte were found to inhibit deposition of fe, while fe2+ ions promote deposition of zn. it was also found that co-deposition of zn and fe behaves anomalously [9]. sulphate containing bath was developed for the preparation of zn-fe alloy coatings by yang et al. [10]. the effects of citric acid and sodium citrate used respectively as a buffer and complexing agent were studied by diaz et al. [11]. it was found that the kinetics of deposition is governed by the interfacial ph. hegde et al. [12] showed that transition of metal alloys shows anomalous co-deposition, occurring as a result of changes in the applied current density during electrodeposition of zinc–nickel, zinc-iron and zinc-nickel alloys in the acidic medium. eliaz et al. [13] concluded that deposition of iron group metals shows good corrosion resistance compared to the single metal deposition. electrodeposition of zn-fe alloys is generally classified as anomalous co-deposition process due to the preferential deposition of zn as less noble metal [2]. several hypotheses have been proposed to explain this anomalous co-deposition [2,14]. many extensive research works have been reported on deposition mechanism of zn-fe group metal alloys, concentrating on anomalous co-deposition and dependency of deposit characteristics on the bath constitution and operating parameters. not any work, however, has been reported on optimization of zn-fe alloy bath containing sulphamic acid (sa) in conjunction with citric acid (ca) for obtaining bright zn-fe alloy coatings. in the present study, the role of sa and ca used in a combination, on electrodeposition of zn–fe alloy and its corrosion protection performance are investigated. the main focus of this work is related to optimization of the bath composition, operating parameters and characterization of the coatings. techniques such as scanning electron microscopy (sem) and x-ray diffraction (xrd) were used to characterize the coatings. experimental plating solutions were prepared from laboratory grade chemicals and distilled water. standard hull cell of 267 ml capacity was used to optimize bath constituents. all depositions were carried out at 303 k and ph 3.0 (except during its deviation). polished mild steel plates were used as cathode (surface area 7. 5 cm2) and a pure zinc plate with the same exposed surface area was used as anode. a pvc cell of 250 cm3 in capacity was used with the cathode-anode space of about 5 cm. all depositions were carried out galvanostatically under constant temperature and ph for duration of 10 minutes using power source (n6705c, keysight technologies). a combination of sulphamic acid and citric acid, (sa+ca), was used as an additive for improving the brightness and homogeneity of the coating. the constituents and deposition parameters were optimized on the basis of coating appearance and corrosion resistance. in order to study the electrochemical properties of coatings, potentiodynamic polarization and electrochemical impedance spectroscopy (eis) measurements were performed. all electrochemical tests were carried out using a potentiostat/galvanostat (ch instruments) and a three-electrode cell. the working electrode was the coated metal specimen. the counter electrode was a platinum electrode with the same surface area as the working electrode. all electrochemical potentials in this work are referred to the ag/agcl electrode. the 5 % nacl solution was used as a corrosion medium throughout the study. potentiodynamic polarization study was carried out in a potential ramp of r. bhat and a. c. hegde j. electrochem. sci. eng. 9(1) (2019) 9-16 doi:10.5599/jese.565 11 0.250 v to +0.250 v around the open circuit potential (ocp), at the scan rate of 1 mv s-1. corrosion rates were determined by the tafel’s extrapolation method. impedance measurements were carried out over the frequency range of 100 khz to 20 mhz, using sine waves of 10 mv amplitude. the microstructure of deposits was examined using sem (jsm-6380 la from jeol, japan). x-ray diffraction (xrd) patterns were collected by jdx-8p jeol, japan, with cukα radiation (k = 1.5418 å) as the x-ray source. while the thickness of the coatings was calculated from faradays law, it was verified using digital thickness tester (coat measure m&c, iso-17025). the fe content in the coating was estimated by spectrophotometer method [14]. the hardness of the deposits (~19 µm thickness) was measured by vickers method, using micro hardness tester (clemex). the cathode current efficiency (cce) of the deposition was determined by knowing the mass and composition of the deposit [2]. the brightness of deposit was measured using gloss meter (nova-elite, 600). results and discussion hull cell study the bath composition and operating parameters of zn-fe bath have been optimized by usual hull cell method [2] at 1.0 a cell current and temperature 30 °c. varieties of deposits having greyish white/bright/porous bright appearance were obtained over the wide range of current density of 1.0-5.0 a dm-2. effect of each bath constituent on hull cell panels were examined in terms of their appearance and surface morphology. the composition and operating parameters of the optimal bath are collected in table 1. table 1. composition and operating parameters of optimized bath for electroplating zn-fe alloy on mild steel bath composition amount, g l-1 operating parameters zinc chloride ferric chloride sodium acetate sulphamic acid citric acid 70 20 60 1.0 4.0 ph 3.0 temperature: 303 k anode: pure zinc current density: 3.0 a dm-2 effect of current density the current density over wide range (1.0-5.0 a dm-2) was found to have a predominant influence on many characteristics of plated deposits. appearance and corrosion performance of deposit formed at each current density are described with data listed in table 2. at low current density, the bath produced greyish white coating with ~2.64 wt.% fe and a porous bright deposit with ~9.81 wt.% fe at high current density. a sound bright deposit with ~5.87 wt.% fe was found at 3.0 a dm-2. increase in fe content with current density is attributed to the rapid depletion of more readily depositable zn2+ ions at the cathode coating [2]. as shown in table 2, the hardness of zn-fe alloy deposit increased together with fe content in the deposit, hardness may ascribed to higher current density of iron compared to zinc (dfe= 7.90 g cm-3 and dzn= 7.14 g cm-3). at high current density, the coating was very thick with increased hardness. thick and porous bright deposit formed at very high current density is due to the metal hydroxide formation caused by rapid evolution of hydrogen during plating. the reflectance of zn-fe alloy coatings at different current densities were also tested and it was found that the deposit formed at very low current density showed lower glossiness, whereas at the optimal current density the glossiness was found to be maximum. at higher current density, glossiness decreased due to increased porosity and thickness. the thickness of deposits was found to increase j. electrochem. sci. eng. 9(1) (2019) 9-16 electrodeposited zn-fe alloy coating 12 substantially with current density, thickness may be due to the adsorbed metal hydroxide at the cathode, caused by the steady increase of ph due to cathodic evolution of hydrogen gas. under all conditions of deposition, cathodic current efficiency (cce) of the bath is found high. i.e. >85 %. a slight decrease in cce with increasing current density as observed in table 2, may be due to the excessive hydrogen evolution during plating. table 2. effect of current density on ph, fe content, cce, thickness, hardness, glossiness and appearance of zn-fe electrodeposited from optimized bath at 303 k j / a dm-2 ph fe content, wt.% cce, % thickness, µm vickers hardness (hv200) glossiness appearance of deposit 1.0 3.0 2.64 87.6 7.2 135 109.0 greyish white 2.0 3.0 4.71 90.5 10.6 158 117.0 bright 3.0 3.0 5.87 93.6 15.8 181 156.1 bright 4.0 3.0 8.19 87.0 17.9 195 138.6 bright 5.0 3.0 9.81 85.8 21.5 199 146.8 porous bright effect of temperature temperature is also found to play outstanding role on the composition and appearance of the deposit, as is the case of zn-fe alloys. as shown in table 3, the deposit was found greyish white (due to high fe content) at low temperature, and silver bright (due to low fe content) at high temperature, respectively. this may be ascribed by the fact that at elevated temperature, more readily depositable metal (zinc) are favoured to be replenished at the cathode film. table 3. effect of temperature on fe content in the deposit plated at 3.0 a dm-2 from bath having optimal composition t / k fe content in deposit, wt.% appearance of deposit 283 6.02 greyish 293 6.08 bright 303 5.87 bright 313 3.78 bright 323 2.14 porous bright corrosion study potentiodynamic polarization study electroplated specimens were subjected to corrosion study and corrosion data of zn-fe coatings corresponding to different current densities are given in table 4. the increase of corrosion rate (cr) with current density is due to the changed phase structure caused by high fe content in the deposit. high cr observed at very low current density is due to increased fe content. this may be due to the tendency of bath to transit from anomalous to normal co-deposition [15]. table 4. corrosion parameters of zn-fe alloy coatings plated at different current density in potential ramp of -0.250 v to +0.250 v from ocp at scan rate of 1 mv s-1 in 5 % nacl j / a dm-2 icorr / µa cm-2 ecorr / v vs. ag/agcl c / mv dec-1 a / mv dec-1 corrosion rate x10-2, mm year-1 1.0 12.36 -1.259 41.0 20.4 13.2 2.0 6.083 -1.240 54.6 37.8 9.42 3.0 1.666 -1.233 22.7 16.6 3.36 4.0 4.072 -1.239 29.3 20.1 6.38 5.0 9.011 -1.259 145.4 50.3 12.13 r. bhat and a. c. hegde j. electrochem. sci. eng. 9(1) (2019) 9-16 doi:10.5599/jese.565 13 large variation in the tafel’s slope of cathodic polarization shown in figure 1, indicates that the corrosion rate is controlled more by the cathodic reaction. it was observed that the deposit formed at 3.0 a dm-2 with ~5.87 wt.% fe showed the lowest corrosion rate (3.3610-2 mm year-1). hence it is inferred that at 3.0 a dm-2, the added (sa+ca) combination has reduced the availability of free fe2+ ions in the solution by proper complexation, improving thus homogeneity of coatings and reducing the corrosion rate. figure 1. tafel plots for zn-fe alloy coatings formed at different current densities from the optimal bath at scan rate of 1 mv s-1 electrochemical impedance study eis is useful technique for ranking coatings, assessing interfacial reactions, quantifying coating breakdown, and predicting the lifetime of coating/metal systems. advantages of this ac technique over dc techniques include the absence of any significant perturbation of the system, applicability to the assessment of low conductivity media and existence of frequency components that may provide mechanistic information. the eis responses of zn-fe alloy coatings, deposited from the optimal bath at different current densities, are shown in figure 2. figure 2. electrochemical impedance spectra (100 khz–20 mhz) of zn-fe coatings electrodeposited at different current densities. j. electrochem. sci. eng. 9(1) (2019) 9-16 electrodeposited zn-fe alloy coating 14 usual semicircle response due to corrosion can be noticed for each curve. the same origins of all curves are due to the solution resistance that is nearly identical in all cases, as the same bath chemistry and cell configuration were used. since diameters of semicircles are related to the corrosion resistance, it may be noted that the highest corrosion resistance is exhibited by the coating deposited at the optimal current density, i.e. at 3.0 a dm-2. cyclic polarization study the cyclic polarization behavior shown in figure 3 confirmed the formation of both air formed oxide layer and corrosion product layer. in the potential range of -0.3 v to -0.7 v, the current density of backward scanning was higher than that of the forward scanning, indicating the breakdown of the air formed oxide layer, while in the potential range from -0.7 v to -1.0 v it was lower than that of forward scanning, is due to formation of corrosion product initiated by pitting. figure 3. cyclic polarization curve of zn-fe alloy electrodeposited at 3.0 a dm-2 at scan rate of 1 mv s-1 in 5 % nacl x-ray diffraction analysis (xrd) the phases of the electrodeposited zn-fe alloy are very complicated, depending on their chemical compositions [9]. xrd patterns of zn-fe alloys presented in figure 4 shows the formation of coatings having different phase structures, depending on the current density at which they were deposited. the xrd peaks reveal that zn-fe alloys having range of compositions is formed and many phases coexist. it may be noticed that only relative intensities of few phases of zinc like (100), (101), (200) and (110) change among coatings deposited at different current densities, and hence different compositions were generated [9]. it was found that zn-fe alloy deposited at 3.0 a dm-2 (with the lowest cr) exhibits two intense peaks corresponding to zn with (100) and (101) phases. sem analysis sem analysis showed that current density plays a significant role on the surface morphology and homogeneity of the deposit. variation in the surface morphology with current density is shown in figure 5. the coating was found to be very thin at 2.0 a dm-2 (figure. 5a) and bright at optimal current density 3.0 a dm-2 (figure 5b). at high current density, however, the deposit was found to be porous as shown in figure 5c. r. bhat and a. c. hegde j. electrochem. sci. eng. 9(1) (2019) 9-16 doi:10.5599/jese.565 15 figure 4. x-ray diffraction profiles of zn-fe coating electrodeposited on mild steel from optimized bath at different current densities figure 5. scanning electron microscopy (sem) images of zn-fe alloys deposited at (a) 2.0; (b) 3.0 and (c) 4.0 a dm-2 conclusions • an electrolytic bath has been proposed for deposition of bright and corrosion resistive zn-fe alloy coatings on mild steel using sa and ca as additives. at all conditions, the electroplating j. electrochem. sci. eng. 9(1) (2019) 9-16 electrodeposited zn-fe alloy coating 16 process followed the anomalous co-deposition with preferential deposition of zinc. the effect of temperature on the deposition process showed that the co-deposition of metals is diffusion controlled. the optimum condition for electroplating of zn-fe alloy is found at the cathode current density 3.0 a dm-2, bath temperature 30 °c and ph 3.0. • at the optimum bath composition and processing parameters, the zn-fe alloy coating showed maximal corrosion resistance and minimal corrosion rate (3.36 ×10-2 mm year-1). • the current density was found to play an important role on the production and properties of the deposit. no transition current density at which the co-deposition behaviour changed from the anomalous to normal type was observed over the wide range of current densities (1.0-5.0 a dm-2). • increase of fe (less readily depositable metal) content in the deposit with temperature confirmed the diffusion controlled deposition process. • the cyclic polarization study showed that improved corrosion resistance is due to barrier effect of oxide layer and corrosion products. • xrd study revealed that zn-fe alloy coating with the highest corrosion resistance corresponds to zn with (100) and (101) phases. references [1] j. b. bajat, s. stankovic, b. m. jokic, journal of solid state electrochemistry 13 (2009) 755 [2] a. brenner, electrodeposition of alloys: principles and practice, academic press, new 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {remediation of clay soil contaminated with lead nitrate using washing-enhanced electrokinetic technique} doi:10.5599/jese.571 63 j. electrochem. sci. eng. 9(1) (2019) 63-73; doi: http://dx.doi.org/10.5599/jese.571 open access: issn 1847-9286 www.jese-online.org original scientific paper remediation of clay soil contaminated with lead nitrate using washing-enhanced electrokinetic technique mahdi karkush, shahad ali college of engineering, university of baghdad, iraq corresponding author: e-mail mahdi_karkush@coeng.uobaghdad.edu.iq; tel.: +9647801058893 received: july 23, 2018; revised: october 8, 2018; accepted: november 13, 2018 abstract the remediation of clay soil contaminated with lead nitrate has been investigated in details by using the electrokinetic technique enhanced by using purging solutions, mid compartment, and washing technique. the intact soil samples are obtained from al-ahdab oil field located in the southeast of iraq. the soil samples are contaminated synthetically with two different percentages of lead nitrate (6.67 and 20 g/kg) and kept for 30 days. the mid compartment is used to reduce the existing paths of contaminants from the soil. purging solutions in the anode, mid, and cathode compartments are used to control the ph value, while the activated carbon is used to prevent the reverse electroosmotic flow from cathode to anode. the main results of electrokinetic experiments, such as variations of electrical current and ph with time, and the accumulated volume of electroosmotic flow are presented and discussed. it was shown that increasing of the concentration of lead causes increase of the electrical current generated during the remediation process. as a consequence, the intensity of chemical reactions occurring in the anode, mid, and cathode compartments are also increased. the removal efficiency of lead from soil samples ranged between 12.4 and 21 %. the washing process is found beneficial in reducing the period of remediation but does not affect the removal efficiency. keywords heavy metals; lead; contaminated soil; soil remediation; washing; electrokinetic introduction the contamination of soil is a significant problem that was increased in recent years because of development of industrial, agricultural, and military activities. heavy metals (hms) are considered as the major contaminants which have diverse effects on geotechnical properties of soil. the effects of contaminants depend on several factors such as their mobility and chemical activity in the soil, especially when the content of contaminants is above certain levels [1,2]. hms usually adhere to the http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:mahdi_karkush@coeng.uobaghdad.edu.iq j. electrochem. sci. eng. 9(1) (2019) 63-73 remediation of clay soil 64 soil particles and thus become immobile. this process is called sorption and describes the distribution of hms between the solid and pores solution. contamination of soil with hms causes reduction of shear strength parameters of soil. also, contamination of soil with hms leads to increased compressibility and maximum dry density, but decreased optimum moisture content and permeability of soil [1]. costs and removal efficiency are the major factors in selection of the remediation technique for removing contaminants from the soil. also, sites contaminated with more than one type of contaminants require more than one type of remediation technique that should be applied simultaneously or in a sequence. generally, the process of removing hms from the soil is considered difficult, because the metals are not biodegradable or soluble in water, except mercury and selenium which could degrade and volatilize by microorganisms. the electrokinetic method (ek) has already been found suitable for the remediation of low permeability contaminated soils through applying dc current between two electrodes installed in the medium. there are generally three phenomena occurring during the ek process: electroosmosis, electromigration and electrophoresis [3]. the removal efficiency of hms from a soil depends on several factors such as the type and composition of hm in the soil matrix and the type of soil [4,5]. during the ek process, the electrolysis occurs at the electrodes, generating hydrogen ions at the anode and hydroxide ions at the cathode [6,7]. the hydrogen ions move from the anode toward cathode while hydroxide ions move from the cathode toward anode. the advance of the acid fronts is faster than advance of the basic front migration, because the mobility of h+ is greater than ohfor about 1.76 times [8]. only few studies, however, investigated effects of hms on geotechnical properties of the soil. li et al. [9] studied the removal efficiency of cadmium (ii), lead (ii), and chromium (iii) from a sandy soil. they used a conductive solution to separate the soil and cathode compartments, in order to enhance the removal efficiency which can be higher than 90 %. reddy and chinthamreddy [4] studied the effects of different purging solutions on removal efficiency of hms from the soil contaminated with different percentages of cr, ni, and cd. the removal efficiency of contaminants was found very low when the tap water was used as a purging solution, but increased when 1 m of acetic acid and 0.1 m of ethylene diamine tetraacetic acid were used as the purging solution in the cathode compartment. the maximum removal efficiency was obtained when water was used as a purging solution at the beginning of the test in anode and cathode compartments, being followed by using acetic acid as the purging solution in the cathode and naoh solution in the anode compartments. jensen et al. [10] demonstrated that soils with high content of carbonate compounds produced negative effects on the removal efficiency of lead from the soil. karkush and altaher [7] studied the remediation of the clayey soil contaminated with several concentrations of total petroleum hydrocarbons by using washing-enhanced electrokinetic technique. the purging solution used to enhance ek consisted of 30 % of ethanol and 70 % of deionized water. the results of tests demonstrated the removal efficiency of 15 % after 10 days of remediation. in the present study, the washing-enhanced ek technique was used in the remediation of a finegrained soil contaminated with two different percentages of lead nitrate. the ek technique was enhanced by using purging solutions and the mid compartment to increase the removal efficiency of lead from soil. also, the activated carbon was used to prevent the reverse electroosmotic flow. experimental soil sampling and materials the intact soil samples were obtained from an open pit excavated to a depth of 3 m below the existing ground level (egl) in the site of al-ahdab oil field located in the southeast of iraq. depending m. karkush and s. ali j. electrochem. sci. eng. 9(1) (2019) 63-73 doi:10.5599/jese.571 65 on the unified soil classification system (uscs), the intact soil was classified as silty clay of high plasticity (ch). the fine-grained soil is highly affected by contaminants, due to its large specific surface area, dynamic crystalline structure and charged particles [11,12]. two quantities of pb(no3)2 (100 and 300 g) were added to soil samples in order to study the impact of lead nitrate on the chemical and physical properties of soil and measure the removal efficiency of the washingelectrokinetic remediation technique. the ek process was enhanced with purging solutions, activated carbon, and mid compartment. lead nitrate is soluble in water, having solubility of 565 g/l. nitric acid (hno3) and sodium hydroxide (naoh) were used as purging solutions to control the ph value in the cathode and anode compartments, respectively. after several electrokinetic tests, activated carbon was used for preventing reverse electroosmotic flow occurring from anode to cathode. the physical and chemical properties of contaminant and purging solutions used in the compartments of electrokinetic cell are given in table 1. table 1. properties of contaminant and purging solutions. chemical compound molar weight, g/mol density, g/cm3 concentration ph electrical conductivity μs/cm pb(no3)2 331.2 4.53 6.67, 20 g/kg naoh 39.99 2.13 0.1mol/l 12.3 11150 hno3 63.01 1.51 0.001mol/l 2.5 664 preparation of contaminated soil samples two disturbed soil samples were placed in plastic containers and soaked with contamination solutions, covered with tightened covers and left for one month to ensure chemical reactions between the contaminant and solid skeletons of the soil. the soil sample of 15 kg weight was put in each container and covered with the contamination solution consisted from lead nitrate and distilled water. two quantities (100 and 300 g) of lead nitrate were mixed with 10 liters of distilled water. the distilled water was necessary for dissolution of pb(no3)2 and the final contaminant solution had to have sufficient volume to cover the soil sample and provide an adequate column (about 3 cm) above the soil surface. in such a way, contaminant could penetrate deeper in the soil [1,13]. the soil samples used in this study were designated with the symbols given in table 2. table 2. designation of soil samples. symbol definition concentration of lead nitrate in soil, g/kg m0 intact soil sample 0 m1 soil sample contaminated with 100 g of pb(no3)2 6.67 m2 soil sample contaminated with 300 g of pb(no3)2 20 chemical and physical properties of soil samples the chemical and physical properties of intact and contaminated soil samples were measured according to astm standards [14]. the chemical properties of soil samples were important for analysis of chemical reactions between the contaminant and mineral composition of soil. the measured chemical properties were the contents of trioxide sulfate (so3), chloride ions (cl), silicon dioxide (sio2), calcium oxide (cao), organic matter (omc), gypsum, total soluble salts (tss), and ph value. as shown in table 3, the results of chemical tests showed significant changes in contents of these components, resulting from the impacts of lead nitrate. https://en.wikipedia.org/wiki/silicon_dioxide https://en.wikipedia.org/wiki/silicon_dioxide https://en.wikipedia.org/wiki/calcium_oxide j. electrochem. sci. eng. 9(1) (2019) 63-73 remediation of clay soil 66 table 3.chemical properties of tested soil samples. soil sample content, % ph so3 cl sio2 cao omc gypsum tss m0 0.036 0.5442 32.37 18.31 0.620 0.04 3.60 7.6 m1 0.221 0.2650 32.63 16.61 0.513 1.081 3.45 8.4 m2 0.346 0.3290 33.54 18.24 0.682 3.65 3.00 8.2 the physical properties of soil samples listed in table 4 involve the particle-size distribution, atterberg’s limits (ll and pl), specific gravity (gs), maximum dry density (ρdmax), optimum moisture content (ωopt), and permeability of soil (k). also, the results demonstrated that presence of lead in soil samples tended to increase the percentage of particles that had size higher than 0.005 mm [1]. caco3 and omc were the main compounds in the cementation between the particles of soil and are responsible for both, the sticking and stability of soil particles [15]. the lead has a greater adsorption ability on the surfaces of clay particles than other hms, and formation of cation bridges among the particles of clayey soil resulted in increased stability of aggregate matrix. table 4. physical properties of tested soil samples. soil sample content, % ll, % pl, % gs ρdmax / g cm-3 ωopt / % k×10-8 / cm sec-1 sand silt clay m0 0.020 25.98 74 55 27 2.74 1.678 21.6 3.22 m1 0.102 29.898 70 45 26 2.78 1.720 18.5 2.60 m2 0.273 43.373 56 43 21 2.83 1.760 17.0 1.86 washing-enhanced electrokinetic remediation technique electrokinetic remediation technology has great capability for treatment of low permeability soils contaminated with hms and/or organic compounds [16]. several studies showed that using water as an electrolyte is not effective to give high removal percentage, especially when the soil is contaminated with more than one type of contaminant. the ek technique, however, can be improved by different ways, such as increasing of treatment duration, increasing electric potential gradient, and using solvents or surfactants. it has also been found that the use of purging solutions is the most promising strategy for obtaining high removal efficiency [17]. the selection of enhancing agent depends on the type and concentration of contaminant and the type of soil. soil washing is one of technologies that has been used in remediation of contaminated soils, and is based on isolating and extracting the most contaminated fraction of the soil. in the soil washing process, several extracting fluids containing a chemical reagent (acid/base, surfactant, chelating agent, salt, or redox agent) is used to transfer metals from the soil into an aqueous solution. soil washing can be used alone or linked with other treatment techniques. in this study, the soil washing was linked with the ek technique to enhance the removal efficiency of contaminant from the soil. the components and dimension of the ek test set up is shown in figure 1. in the present study, the ek technique was enhanced by four ways. the first was installing of the mid-compartment to reduce the travel path required for exit of the contaminant particles from the soil. the second way is use of the activated carbon (ac) to keep the flow from anode to cathode and prevent reverse electroosmotic flow. the ac compartment is 5 cm in length, 10 cm in width and 10 cm in height. the third way was use of purging solution (0.001 m hno3 in anode compartment put at initial state and followed by m. karkush and s. ali j. electrochem. sci. eng. 9(1) (2019) 63-73 doi:10.5599/jese.571 67 addition of 0.1 m naoh and 0.001 m hno3 in mid and cathode compartments, respectively). the fourth way of enhancement was utilization of the washing process. figure 1. schematic diagram of the electrokinetictest setup. the major components of the system were power supply, pump station, and electrokinetic rectangular cell of dimensions 50 cm in length, 10 cm in width and 10 cm in height. the ek cell consisted of five compartments, for the soil sample, for the activated carbon, the mid compartment between soil and ac and two others for electrodes (anode and cathode). each electrode compartment comprised a valve to manage the flow into the cell, a graphite electrode, filter paper and porous stone. the filter paper and porous stone were used to prevent soil particles from entrance into anode or cathode reservoirs. gas vent tubes were provided in electrode compartments to release gases resulting from the electrolysis reactions and collect any liquid that was removed with the gases. a ph meter was used to measure changes in ph of the solution in the cathode, mid, and anode compartments and soil after remediation. the effluent from cathode compartment was collected to determine the accumulated volume of electroosmotic flow (eof). the size, shape, and arrangement of electrodes and the distance between them affect also the removal efficiency. only few researches have been conducted to select the optimum values for these parameters. the dimensions of electrodes used in the present study were 2 cm in thickness × 10 cm in width and 10.5 cm in height. also, the voltage gradient through the soil sample will affect the movement of ions between opposite charged electrodes and the transport of charge by electroosmosis. a number of studies reported the voltage gradient in the range of 1–3 vdc/cm [18,19]. the choice of the most suitable voltage gradient relies on soil properties and contamination type, where soils of high electrical conductivity need more charge than soils of low electrical conductivity. increase of the voltage gradient will increase transport rates, i.e. ionic migration and electroosmosis rates. at the other side, increase of the voltage gradient results in increase of the electric current, the cost of the process, and the produced heat. therefore, the voltage gradient used in this study was set to 1.3 vdc/cm. j. electrochem. sci. eng. 9(1) (2019) 63-73 remediation of clay soil 68 testing procedure the testing procedure can be described by the following steps: 1) preparing the ek cell (electrodes, purging solutions,a device used to control the hydraulic gradient). 2) preparing the contaminated soil sample in the ek cell according to the field unit weight and natural moisture content. in front of the electrodes, the filter paper and porous stone were placed. 3) the electrodes compartments were filled with purging solutions. the anode reservoir was filled with 0.001 m hno3 at the beginning of test, and then 0.1 m naoh was added to maintain the ph higher than 2. the cathode and mid reservoirs were filled with 0.001 m hno3 from the beginning of the experiment. 4) the graphite electrodes (anode and cathode) were linked to the power supply with a voltage gradient equal to 1.3 vdc/cm. 5) the electric current, the volume of electroosmosis flow (eof) and ph in the anode, mid, and cathode compartments were measured every hour during the testing period. 6) the test was stopped when the electrical current became constant or no change in eof was observed. 7) at the end of each test, the soil specimen and ac are extruded from the cell by hands. the soil specimen was sectioned into five parts and each part was weighed and subsequently preserved in a glass container. for each soil section, ph was measured according to astm (d4972) and the residual concentration of lead was measured using aas. results and discussion the results of ek experiments measured with time involve: (1) the variation of electric current; (2) the rate of eof; (3) ph values determined in compartments and soil sample, and (4) the lead content in the remediated soil samples. electrical current the electrical current in the soil sample m2 was equal to zero at the first 30 hours of the ek test, then started to increase steadily, reached its maximum value and then decreased until a constant value was reached. for the soil sample m1, however, the electric current rose rapidly in the first hour of testing, reached its maximum and then continued steadily to the end of test. increasing the concentration of ionic species in each experiment caused rising of the electrical current due to generation of h+ and ohions at anode and cathode respectively. increasing the mobility of soil solids due to the presence of ions causes rising of the electrical current produced during treatment process. the migration of these ions towards the electrodes during the treatment reduces the electric current [9,20,21]. generally, the values of electrical current reached a peak value because dissolved salts that are connected with dry soil particles increased the quantity of ions in the pore solution [13,20]. the time required to get a stable current or reach the maximum value was variable and depended on migration rates of ions from solution to the soil column [23]. the profiles of electrical current vs. time are shown in figure 2. m. karkush and s. ali j. electrochem. sci. eng. 9(1) (2019) 63-73 doi:10.5599/jese.571 69 figure 2. variation of electrical current with time. ph value the values of ph measured in anode, mid, and cathode compartments during the period of testing and their variations with time are shown in figure 3. figure 3. variation of ph value of electrolyte solutions at three different compartments with time. 0 100 200 300 400 0 50 100 150 200 250 c u rr e n t, m a elapsed time, h m1 m2 0 3 6 9 12 15 0 50 100 150 200 250 p h elapsed time, h anode compartment m1 0 3 6 9 12 15 0 50 100 150 200 250 p h elapsed time, h anode compartment m2 0 3 6 9 12 15 0 50 100 150 200 250 p h elapsed time, h mid compartment m1 m2 0 3 6 9 12 15 0 50 100 150 200 250 p h elapsed time, h cathode compartment m1 0 3 6 9 12 15 0 50 100 150 200 250 p h elapsed time, h) cathode compartment m2 j. electrochem. sci. eng. 9(1) (2019) 63-73 remediation of clay soil 70 application of voltage gradient to the soil specimen that produced low dc current leads to the production of hydrogen and hydroxyl ions at electrodes. consequently, an acidic medium at the anode and alkaline medium at the cathode were generated [24,25]. when ph reached the value about 2, naoh was added to the solution in the anode compartment, so the value of ph increased. the values of ph in the mid compartment were inversely proportional to the concentration of contaminant in the soil. ph value in the mid compartment depended on the quantity of accumulated salts which was higher in the soil sample m2 than m1. increasing the sediment of salts in the compartment led to the lowering of the ph value. in general, the ph value at anode during the test should have decreased, but in the present study, the value of ph sometimes increased and oscillated between 6-12, what may be attributed to the applied hydraulic gradient (washing process). in the cathode compartment, the value of ph was equal to zero at the beginning of testing what was due to the effect of ac which prevented the rapid change of ph value. after that, ph value began to increase and reached its maximum value due to the generation of hydroxide ions (oh-). the increase of electroosmotic flow towards the cathode led to lowering of ph value near the cathode because of the opposite migration of ions [26,27]. the lowering of ph value at the anode and rising at the cathode were accompanied by propagation of an acid front into the soil pores from the anode and a base front from the cathode. this process can significantly affect the soil zeta potential drop, as well as the solubility, ionic state and charge, and level of adsorption of the contaminant [28]. the variation of ph value of soil along the distance from anode to cathode is shown in figure 4. figure 4. profile of ph value of tested soil samples. electroosmosis flow (eof) accumulative volumes of effluent from soil samples m1 and m2 are shown in figure 5. according to the electroosmotic phenomenon, the water carries lead nitrate from anode to cathode compartment. the high concentration of lead nitrate may lead to the reverse flow from cathode to anode compartment. also, the electroosmotic flow can be reversed, what occurs when the charge of soil surface is converted from negative to positive. the surface charge depends on ph and ionic strength and so, the high concentration of lead nitrate leads to increase of ionic strength that influenced the surface charge and zeta potential. lowering the ph value of the soil causes alteration of the net surface charge of soil from negative to positive, what is due to the sorption of hydrogen ions on the surface of soil particles. in the soil with a positive surface charge, the direction of electroosmotic flow changed from the cathode to the anode, and so it is important to keep the negative charge of the soil surface to avoid the reversed eof. also, the ph value of the soil should 7 7.5 8 8.5 9 0 0.2 0.4 0.6 0.8 1 s o il p h normalized distance from anode m1 m2 m. karkush and s. ali j. electrochem. sci. eng. 9(1) (2019) 63-73 doi:10.5599/jese.571 71 be maintained low enough to keep all contaminants in the dissolved phase [28,29]. the results of tests demonstrated that increase of the concentration of lead nitrate in the soil, increased the rate of eof. this leads to increase of the ionic migration due to rise of the produced electrical current. also, it is important to notice that eof started 80 hours after the beginning of the experiment, what is due to low solubility of lead nitrate in water. figure 5. accumulative volume of eof in cathode compartment versus time removal efficiency the residual concentrations of lead in the soil were measured using atomic absorption spectroscopy (aas) device. the initial concentration of lead in the soil is calculated by eq. (1): pb atomicmass of lead×usedquantity / %= 100 atomicmass of leadnitrate c (1) the initial concentration of lead in the soil samples m1 and m2 were found equal to 4.17 and 12.512 g/kg, respectively. the removal efficiency is obtained by eq. (2).  i f i er c / c c (2) in eq. (2), er is the the removal efficiency of lead from soil, while ci and cf are initial and final concentration of lead in the soil (g/kg). the removal efficiency of lead from soil samples m1 and m2 was determined as 12.4 and 21 %, respectively. figure 6 shows the residual concentrations of lead in the soil after ek remediation. figure 6. variation of the residual concentration of lead in soil sample with distance 0 100 200 300 400 0 50 100 150 200 250 a cc u m u la te d e o f v o lu m e , m l elapsed time, h m1 m2 0 2 4 6 8 10 12 0 0.2 0.4 0.6 0.8 1 r e si d u a l c o n ce n tr a ti o n , g / k g normolized distance from anode m1 m2 j. electrochem. sci. eng. 9(1) (2019) 63-73 remediation of clay soil 72 conclusions the lead nitrate as a soil contaminant has diverse impacts on the chemical and physical characteristics of tested soil samples. the impacts of contamination depend on the concentration and type of contaminant, as well as the type of soil, where geotechnical properties of soil are highly influenced with increasing the concentration of lead nitrate in the soil. from the results of this study, the following conclusions can be drawn out: 1) the electrokinetic method is very effective technique to remediate low permeability soils. 2) the ph value of soil increased in the soil specimen when moving towards the cathode compartment. 3) the volume of eof increased with increasing the concentration of lead in the soil. 4) the removal efficiency of lead from soil samples m1 and m2 is 12.4 and 21 %, respectively. the low removal efficiency can be attributed to the high molar weight of lead nitrate which prevents moving and transportation of contaminant particles in the ek process. the extraction efficiency can be improved by increased the value of voltage gradient or by using another effective purging solution to remove lead from the contaminated soil. 5) using of activated carbon is very effective to prevent reverse eof. the ability of activated carbon to adsorb a contaminant increases with increased concentration of contaminant in the soil. 6) using the mid compartment to enhance the ek technique is very useful in shortening the exiting path of contaminant particles from the soil sample. for the concentration of lead in soil increased over 6.67 g/kg, the voltage gradient must be kept less than 1.3 vdc / cm to avoid increasing temperature of ek cell what may cause a damage. also, when the temperature exceeds 55 °c in the cathode compartment, production of gases is increased, what can be poisonous and have detrimental effects to the human health. references [1] m. o. karkush, a. t. zaboon, h. m.hussien, coupled phenomena in environmental geotechnics, taylor & francis group, london (2013) 599-607. 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[29] d. h. kim, b. g. ryu, s. w. park, c. i. seo, k. baek, journal of hazardous materials 165(1-3) (2009) 501-505. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {study of electrochemical deposition of ni-mo thin films from alkaline electrolytes} http://dx.doi.org/10.5599/jese.912 39 j. electrochem. sci. eng. 11(1) (2021) 39-49; http://dx.doi.org/10.5599/jese.912 open access: issn 1847-9286 www.jese-online.org original scientific paper study of electrochemical deposition of ni-mo thin films from alkaline electrolytes ulviyya magsud gurbanova1,, dunya mahammad babanly1,2, ruhangiz gurmuz huseynova1 and dilgam babir tagiyev1 1institute of catalysis and inorganic chemistry named after acad. m. nagiyev, azerbaijan national academy of sciences, az 1143, baku, h. javid 113, azerbaijan 2french azerbaijani university (ufaz), az1010, baku, nizami 189, azerbaijan corresponding author: uqurbanova92@gmail.com, phone: +994702756030 received: october 7, 2020; revised: december 4, 2020; accepted: december 5, 2020 abstract the process of co-deposition of ni with mo from alkaline electrolytes was studied by taking linear and cyclic polarization curves of pt electrode at various concentrations of initial components and potential scan rates. solutions of na2moo4∙2h2o and niso4∙7h2o were used as sources of ions of the main components in nh4oh electrolyte. it was found that codeposition of nickel with molybdenum goes through the oxide formation stage, and a solid solution of these two metals is deposited on the cathode surface. the film obtained at the constant current on ni electrode under optimal conditions was found amorphous, but additional thermal treatment at 500 °c for one hour made it polycrystalline. this was confirmed by peaks in x-ray diffraction pattern, corresponding to nimoo4, ni, and moo3. the proposed electrolyte and electrolysis conditions allow to obtain thin films with molybdenum content ranging from 17.1 to 50.9 at.%. the co-deposition of ni with mo is limited by diffusion of these ions to the cathode surface. the knowledge of the mechanism of co-deposition of ni and mo will make possible the selection of optimal conditions for deposition of alloys of the required composition with satisfactory electrocatalytic properties. keywords ni-mo co-deposition, electrodeposition, potentiodynamic polarization, ammonium hydroxide, platinum substrate, nickel substrate introduction nickel-molybdenum alloys have multifunctional properties, and their catalytic activity in the hydrogen evolution process is one of the most studied topics in the literature [1-3]. ni-mo alloys can be obtained by several methods among which metallurgical ones are not convenient due to easy http://dx.doi.org/10.5599/jese.912 http://dx.doi.org/10.5599/jese.912 http://www.jese-online.org/ mailto:uqurbanova92@gmail.com j. electrochem. sci. eng. 11(1) (2021) 39-49 electrochemical deposition of ni-mo thin films 40 oxidation and high melting point of molybdenum. powder metallurgy and mechanical alloying [4], spark plasma sintering [5], and laser sintering [6] are actual methods, widely used to obtain ni-mo alloys. all these methods, however, are expensive in comparison with the electrochemical synthesis of ni-mo alloys. obtaining of molybdenum alone by electrodeposition from aqueous solutions is very difficult, what can be facilitated by its co-deposition with metals of the iron group (fe, ni, co), i.e. in the presence of one of these components. when studying the co-deposition of two metals, it would be necessary to study their behavior separately in the same electrolyte, and record deposition polarization curves of each metal under the same conditions. the electrodeposition of molybdenum from tartrate electrolytes was studied in [7], and from alkaline electrolytes in [8]. deposition of nickel from an alkaline citrate electrolyte was studied in [9], and from an alkaline electrolyte in [10]. although nickel possesses electrocatalytic properties and due to these properties is often used in industry, the overvoltage of hydrogen evolution (her) on the nickel electrode is quite high. this causes an increase in the energy consumption during its use. in order to reduce overvoltage, nickel is often doped with other elements such as mo [11], co [12], p [13], cu [14], and some others. besides, the results of water electrolysis on titanium and molybdenum electrodes from neutral (nacl) and alkaline (naoh) electrolytes showed that addition of mo (vi) ions into the electrolyte has activated her [15]. at high polarization, mo-containing films were formed at the cathode by electroreduction of mo (vi) ions. presence of mo-containing film reduces the overvoltage for her and increases the electrode activity, what is exclusively due to the catalytical activity of the deposited film [15]. a number of scientific works has already been devoted to the co-electrodeposition of ni-mo alloys. in their study of the mechanism of this process, landolt and co-workers assumed that codeposition of these two metals proceeds by the induced co-deposition mechanism [11,16]. several works were devoted to the study of morphology and phase composition of ni-mo films [17-20]. x-ray diffraction analysis revealed that ni-mo alloys electrodeposited from citrate electrolyte (ph 8.5-9.5) contain ni-mo solid solution. clear diffraction peaks were observed for films having 12-30 wt.% of mo, being more intensive at decreased (down to 12 wt.%) content of molybdenum in deposits [17]. it was also found that ni-mo deposit is a solid solution with grain sizes ranging from 4 to 17 nm (average size of 5 nm), i.e. the electrodeposited ni-mo alloy is almost amorphous. a similar conclusion was given after xrd analysis of ni-mo alloys electrodeposited from pyrophosphateammonium chloride electrolyte (ph 8.5) [18]. the results of the energy-dispersive x-ray spectroscopy were firstly demonstrated in [19,20], according to which, electrodeposited ni-mo alloys with high mo content contain up to 50 at.% of oxygen. a significant atomic percentage of oxygen (30 50 at.%) was also found during the co-deposition of nickel and molybdenum in all electrodeposited samples [21,22]. during calculation of the composition of ni-mo alloy, authors usually considered only the percentage of main components of ni and mo, neglecting the oxygen content in deposits. it was concluded that alloys containing a large amount of mo are mixtures of ni and some polyvalent oxides of mo (iv) or mo (vi). all heretofore presented works devoted to the electrochemical synthesis of ni-mo alloys, however, are unsuccessful about obtaining thin films with molybdenum content higher than 30 at.% and also, corrosion resistances of the obtained alloys are not high enough. the present contribution is devoted to the study of the co-deposition process of nickel and molybdenum from alkaline electrolytes, carried out by taking cyclic and linear polarization curves. prepared electrolytes and conditions of electrolysis allowed deposition of films with a molybdenum content up to 50.9 at.%. u. m. gurbanova et al. j. electrochem. sci. eng. 11(1) (2021) 39-49 http://dx.doi.org/10.5599/jese.912 41 experimental in the study of co-deposition of nickel and molybdenum, niso4∙7h2o of the indian central drug house (p) ltd. company was used as a source of nickel ions, and na2moo4∙2h2o of the indian firm qualikems fine chem pvt. ltd. was applied as a source of molybdenum ions. both salts were dissolved in nh4oh of the indian firm qualikems fine chem pvt. ltd., boric acid (cdh ltd) was added to the electrolyte as a buffering agent, and nicl2∙6h2o (cdh ltd) was introduced into the electrolyte to reduce the formation of metal oxides. ph of all electrolyte solutions was 11.2. except pure ammonia electrolyte solution containing 0.1m h3bo3 + 7m nh4oh, the following alkaline electrolyte solutions for individual deposition of ni, mo and co-deposition of ni-mo were prepared: 1) 0.107 m niso4⋅7h2o + 0.13 m nicl2∙6h2o + 0.1 m h3bo3 + 7 m nh4oh. 2) 0.124 m na2moo4⋅2h2o + 0.1 m h3bo3 + 7 m nh4oh. 3) 0.107 m niso4⋅7h2o + 0.124 m na2moo4⋅2h2o + 0.13 m nicl2∙6h2o + 0.1 m h3bo3 + 7 m nh4oh. for some experiments, the concentrations of na2moo4⋅2h2o in the alkaline electrolyte solution 3) were changed between 0.057 and 0.138 m. cyclic and linear polarization curves were recorded on iviumstat electrochemical interface potentiostat, using a classic three-electrode cell with a capacity of 100 ml, and equipped with a water jacket. platinum wires with an area of 0.2 cm2 and 0.4 cm2, were used as working electrodes, and platinum plate with an area of 4 cm2 as the auxiliary electrode. silver/silver chloride (ag/agcl) electrode was applied as the reference electrode and all potentials were given relative to this electrode. to study the properties of deposited films by scanning electron microscopy (sem) and energy – dispersive x –ray analysis (eds) methods on a d2 phaser from bruker (cu кα; ni filter) were used. for these experiments, deposition of ni-mo film from the corresponding electrolyte solution was carried out on ni cathode by the galvanostatic method at 25 °c. the current density of 25 ma/cm2 for 15 hours was applied to the cell, where platinum plate served as the anode. the deposited nimo film was annealed in air at 500 °c for 1 hour. results and discussion open circuit potential value of the platinum electrode in ammonia electrolyte containing complex nickel compounds was, depending on the concentration of niso4∙7h2o in the electrolyte, recorded between 0.2 v and 0.4 v vs. ag/agcl. the first traces of nickel on platinum electrode appear at the potential of -0.08 v. at the other side, the open circuit potential value of molybdenum in ammonia solution fluctuates, depending on the concentration of na2moo4∙2h2o, between 0.50 and 0.56 v. the deposition of molybdenum oxide occurs at small polarization of 0.04 v, while its first traces appear on the surface of the electrode at the potential of -0.06 v. the peak of the polarization curve during the deposition of mo corresponds to the potential of -0.25 v. it can be stated that in the ammonia electrolyte, deposition potentials of nickel and molybdenum are quite close each to other. polarization curves of the deposition of nickel, molybdenum, and co-deposition of nickel with molybdenum on pt electrode from corresponding alkaline electrolytes are presented in figure 1(a). for the comparison purpose, the polarization curve of pt electrode in pure ammonia electrolyte is presented in figure 1(b). it can be seen from figure 1(a) that the co-deposition of nickel with molybdenum occurs at a potential remarkably close to both, nickel and molybdenum deposition potential values. also, the cathodic polarization curve of co-deposition of ni-mo alloy (curve 3) is placed between polarization curve of nickel (curve 1) and molybdenum (curve 2) deposition. as already known, such an arranhttp://dx.doi.org/10.5599/jese.912 j. electrochem. sci. eng. 11(1) (2021) 39-49 electrochemical deposition of ni-mo thin films 42 gement of polarization curves indicates formation of a solid solution between alloy components [23]. at the potentials -0.75 and -0.79 v on curves 3 and 2, respectively, hydrogen evolution occurs in two stages. the presence of both metals in the composition of the deposited film was confirmed by chemical analysis and data obtained by scanning electron microscope (sem) discussed below. figure 1. linear polarization curves (40 mv/s) of pt electrode for: a) ni2+ reduction from electrolyte solution 1) (curve 1), mo6+ reduction from electrolyte solution 2) (curve 2), co-deposition of ni2+ with mo6+ from electrolyte solution 3) (curve 3), and b) pure ammonia electrolyte. cyclic voltammetry (cv) curve of co-deposition of nickel and molybdenum from the alkaline electrolyte solution 3) is, together with cv response of pt electrode in pure ammonia electrolyte, presented in figure 2. it is obvious that cv of co-deposition has two main sections. the first section corresponds to the co-deposition of nickel and molybdenum, which occurs at the potential of -0.28 v. when the potential reaches -0.8 to -0.9 v, hydrogen evolution occurs in two stages, which is indicated by formation of peaks on the cathodic polarization curve at -0.81 and -0.87 v, respectively. on the anodic sweeping curve, two small plateaus appear on the anodic curve at potentials -0.11 and 0.22 v, respectively. the first plateau corresponds most likely to the anodic dissolution of the deposited film, while the second plateau can be associated with the dissolution of nickel, present as a separate phase in ni-mo thin film. presence of ni was confirmed by x-ray phase analysis. figure 2. cyclic voltammetry curve (100 mv/s) of pt electrode for co-deposition of ni and mo from electrolyte solution 3) (curve 1) and pure ammonia electrolyte (curve 2). in order to accurately determine processes occurring on the cathodic surface during co-deposition of nickel and molybdenum, the potential of inversion from cathodic to anodic direction was set at either -0.28 v or -0.8 v. firstly, the cathodic polarization curve was scanned down to the potential value of -0.28 v, then the process was stopped and after 5 minutes the anodic curve was taken. figure 3(a) shows that at the potential of 0.26 v, a plateau appears on the anodic curve, which corresponds u. m. gurbanova et al. j. electrochem. sci. eng. 11(1) (2021) 39-49 http://dx.doi.org/10.5599/jese.912 43 to the dissolution of thin ni-mo film. if, however, the cathodic scan was stopped at -0.87 v, cv presented in figure 3(b) shows that no peaks are displayed on the anodic curve before oxygen evolution emerged at 0.7 v. this suggests that at -0.81 and -0.87 v mainly hydrogen is released. figure 3. anodic polarization curves (100 mv/s) of pt electrode in electrolyte solution 3), after being kept for 5 minutes at negative potential limit of: (a) -0.28 v, (b) -0.8 v. as already known, at metals (me) in alkaline solutions, hydrogen evolution occurs in two steps according to the following reactions [24,25]: me(h2o) + e→ me(h) + oh(1) me(h) + me(h) → 2me +h2 (2) the first step (1) describes formation of adsorbed hydrogen, me(h), at a metal surface. the second step (2) of the hydrogen evolution reaction can alternatively proceed according to another mechanism: me(h) + h2o + e→ me + h2 + oh (3) in figure 4, cathodic potentiodynamic polarization curves of co-deposition of nickel and molybdenum are given for various concentrations of molybdenum ions and concentrations of remaining ions kept constant. an increase in the concentration of na2moo4∙2h2o leads to a rise in the peak height corresponding to the alloy formation. deposition of elements to the alloy film occurs at the potential of  -0.32 v. figure 4. cathodic polarization curves (100 mv/s) of pt electrode for co-deposition of ni and mo at various concentrations of na2moo4∙2h2o in electrolyte solution 3): 0.057 m (curve 1), 0.069 m (curve 2), 0.092 m (curve 3), 0.138 m (curve 4). the mechanism of the release of molybdenum is reported in [26], according to which, a thin film composed of a mixture of trivalent molybdenum and bivalent nickel hydroxides is formed on the http://dx.doi.org/10.5599/jese.912 j. electrochem. sci. eng. 11(1) (2021) 39-49 electrochemical deposition of ni-mo thin films 44 cathode. presence of metal-hydroxide depositor provides the necessary permeability of the film, so that molybdenum ions can penetrate to the cathode and can be deposited on it. at the same time, the film protects released molybdenum atoms from reverse oxidation with electrolyte. other researchers believe that co-deposition of mo with other metals occurs in two stages. during the first stage (slow), molybdenum oxide (moo2) transforms into a mixed oxide (moo2ni4) by the following reaction: 4моо2 + 4ni2+ + 8e= 4moo2ni4 (4) during this slow reaction, the mixed oxide forms a compound on the surface that inhibits hydrogen evolution [27]. then, the mixed oxide is electrochemically reduced with the participation of nickel ions to form ni3mo alloy: моо42+ 3ni2+ + 8е+ (4+n)h = ni3mo +4oh+ nhаds (5) reaction (4), proceeding slowly in combination with a relatively fast reaction (5) can be the main reaction in the co-deposition of nickel with molybdenum to form ni3mo alloy. ernest and holt 28 proposed a two-stage mechanism of the co-deposition of mo with other metals (m), the mechanism of which can be depicted as follows: moo42+ 4h2o + ne= mo(oh)(6-n) + (2+n) oh(6) mo(oh)(6-n) + (6-n) h + m = m⋅ mo + (6-n) h2o (7) it is known that in strongly alkaline solutions, molybdenum is in the form of monomolybdate ions [moo4]2and at high concentrations of molybdenum, only a small part of molybdate ions reaching the cathode surface is reduced to the metallic mo. the rest is deposited in the form of multivalent oxides, most of which is moo2 [29]. in the co-deposition of ni with mo, molybdate ions are electrochemically reduced to moo2 according to the following reaction: moo42+ 2h2o + 2e= moo2 + 4oh(8) due to the fact that moo2 has catalytic properties in the reaction of hydrogen evolution, the chemical reduction of molybdenum by atomic hydrogen occurs at the cathode, followed by adsorption of mo on nickel electrode with the formation of induced ni-mo alloy [30]. figure 5 shows the results of x-ray phase analysis of thin ni-mo films deposited on ni substrate by the galvanostatic method at ik = 25 ma/cm2 for 15 hours. whereas fresh deposit didn’t show resolvable diffraction pattern, after annealing in air at 500 °c for 1 hour, the x-ray diffraction pattern shows clear peaks corresponding to ni, moo3, and nimoo4. the appearance of peaks corresponding to nimoo4 phase further confirms that nimoo4 phase is formed due to presence of already formed moo2 phase. apparently, the co-deposition of nickel with molybdenum occurs through the stage of formation of oxides of these metals, as a result of which a solid solution is formed. when the films are annealed at 500 °c for 1 hour, a solid-phase reaction occurs between nickel oxides and molybdenum oxides, resulting in the formation of nimoo4 phase, seen in the x-ray diffractogram shown in figure 5. the moo3 phase, however, is most likely formed as a result of induced deposition from molybdate ions according to the following reaction: moo42+ ni2+ + h2o + 2e→ moo3 + ni + 2oh (9) it should be noted that formation of nimoo4 phase which contributes to high catalytic activity of ni-mo alloys, was already obtained as a result of double annealing for the first time for 5 hours at 300 and again at 600 °c for another 5 hours [22]. in our study, this compound appears in the diffraction pattern upon annealing for 1 hour at 500 °c. induced co-deposition is accompanied by the evolution of hydrogen, but part of the current consumed in this process is so small that can be neglected. u. m. gurbanova et al. j. electrochem. sci. eng. 11(1) (2021) 39-49 http://dx.doi.org/10.5599/jese.912 45 figure 5. x-ray diffraction pattern of ni-mo deposit annealed at 500 °c for 1 hour. co-deposition of ni with mo was performed on ni electrode from electrolyte solution 3) at ik = 25 ma/cm 2 for 15 hours. figure 6 shows cyclic voltammograms of co-deposition of ni with mo as a function of the potential scan rate. these curves can be divided into two groups. the first group, represents the curves recorded at potential scan rates from 10 to 20 mv/s (curves 5 and 6), while the second group consists of curves recorded at scan rates of 40 mv/s and higher (curves 1-4). figure 6. cyclic voltammograms of pt electrode for co-deposition of ni with mo from alkaline electrolyte solution 3) at various potential scan rates (mv/s): 100 (curve 1), 80 (curve 2), 60 (curve 3), 40 (curve 4), 20 (curve 5), 10 (curve 6). ni −nimoo4 sio2 moo3 http://dx.doi.org/10.5599/jese.912 j. electrochem. sci. eng. 11(1) (2021) 39-49 electrochemical deposition of ni-mo thin films 46 on the curves belonging to the first group, only one peak followed by a plateau appears on the anode component at the potential of -0.13 v, corresponding to the dissolution of a thin ni-mo film. at potential scan rates above 40 mv/s, another peak followed by new plateau appears at the potential of 0.23 v. it is quite probable that at 0.23 v nickel dissolves and enters to the alloy as a separate phase, which is confirmed by x-ray phase analysis shown in figure 5. morphologies and chemical compositions of fresh and annealed ni-mo film deposited on ni electrode are presented in figure 7. figure 7. morphology and composition of thin ni-mo film deposited on ni electrode under conditions described in fig. 5: (a) not subjected to annealing, (b) annealed at 500 °c for 1 hour. according to figure 7(a), the freshly deposited, i.e. not annealed film consists of 58.39 at.% nickel, 24.45 at.% molybdenum, and 17.16 at.% oxygen. surface image of the sample shows that ni-mo deposit is amorphous and composed of small particles. this was a reason why the xrd pattern of this deposit did not yield any consistent results. comparison of figure 7(a) and (b) shows that for the films subjected to annealing, the particle size was increased as a result of sintering at high temperatures. also, some nickel peaks disappeared or decreased, the molybdenum content also decreased, and the oxygen content increased. the composition of the films changes insignificantly after annealing and consists of 52.31 at.% nickel, 21.08 at.% molybdenum, and 26.61 at.% oxygen. in all cases, atmospheric oxygen also participates in the reaction, which leads to an increase in the amount of oxygen after annealing and decrease in the content of molybdenum and nickel in the deposits, what agrees well with literature data [22,31]. figure 8 shows linear polarization curves of pt electrode for co-deposition of ni with mo as a function of the potential scan rate (). based on the data in figure 8, the dependence of limiting u. m. gurbanova et al. j. electrochem. sci. eng. 11(1) (2021) 39-49 http://dx.doi.org/10.5599/jese.912 47 current (il) on  0.5 was plotted and shown in figure 9. the rectilinear dependence occurs when the deposition rate is controlled by diffusion of ions to the cathode surface [32]. these data are in good agreement with data obtained in [21,22,33,34]. figure 8. linear cathodic polarization curves of pt electrode for co-deposition of ni with mo from electrolyte solution 3) at various scan rates (mv/s): 10 (curve 1), 20 (curve 2), 40 (curve 3), 60 (curve 4), 80 (curve 5), 100 (curve 6). figure 9. dependence of il on  1/2 for data taken from fig. 8. conclusions it was found that during co-deposition of nickel with molybdenum, a solid solution of these two metals is formed, and the deposited alloys are amorphous. co-deposition passes through the stage of formation of oxides of these two metals. annealing at 500 °c for one hour makes sample polycrystalline and nimoo4 compound appeared in the x-ray diffraction pattern. the process of co-deposition is controlled by diffusion of ions to the cathodic surface. the knowledge of the mechanism of codeposition of nickel with molybdenum will make it possible to select optimal conditions for the deposition of alloys of strictly required composition with satisfactory electrocatalytic properties. the catalytic activity of amorphous ni-mo film is higher than film subjected to annealing, what is due to the large real surface of amorphous films. http://dx.doi.org/10.5599/jese.912 j. electrochem. sci. eng. 11(1) (2021) 39-49 electrochemical deposition of ni-mo thin films 48 references [1] b. e. conway, l. bai, m. a. sattar, international journal of hydrogen 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[34] v. d. jović, b. m. jović, u. lačnjevac, g. branković, s. bernik, a. rečnik, electrochimica acta 55(13) (2010) 4188-4193 https://dx.doi.org/10.1016/j.electacta.2010.02.065. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.912 https://doi.org/10.1149/1.2086788 https://doi.org/10.1149/1.2428912 https://doi.org/10.1149/1.2428912 https://doi.org/10.1007/bf01007931 https://dx.doi.org/10.1149/1.2428691 https://dx.doi.org/10.1149/1.2428691 https://dx.doi.org/10.1016/s1388-2481%20(99)%2000137-x https://dx.doi.org/10.1016/s1388-2481%20(99)%2000137-x https://dx.doi.org/10.1016/j.tsf.2011.10.024 https://dx.doi.org/10.1016/j.tsf.2011.10.024 https://dx.doi.org/10.1016/j.electacta.2008.11.068 https://dx.doi.org/10.1016/j.electacta.2010.02.065 https://creativecommons.org/licenses/by/4.0/) {influence of intensity of ultrasound on morphology and hardness of copper coatings obtained by electrodeposition:} http://dx.doi.org/10.5599/jese.1290 603 j. electrochem. sci. eng. 12(4) (2022) 603-615; http://dx.doi.org/10.5599/jese.1290 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of intensity of ultrasound on morphology and hardness of copper coatings obtained by electrodeposition ivana o. mladenović1, jelena s. lamovec2, dana g. vasiljević radović1, vesna j. radojević3 and nebojša d. nikolić1, 1university of belgrade, institute of chemistry, technology and metallurgy, njegoševa 12, 11 000 belgrade,serbia 2university of criminal investigation and police studies, cara dušana 196, zemun, 11 000 belgrade, serbia 3university of belgrade, faculty of technology and metallurgy, karnegijeva 4, 11 000 belgrade, serbia corresponding author: nnikolic@ihtm.bg.ac.rs; tel.: +381-11-337-03-90; fax: +381-11-337-03-89 received: february 8, 2022; accepted: february 25, 2022; published: february 27, 2022 abstract the influence of various intensities of ultrasound applied for the electrolyte stirring on morphological and mechanical characteristics of electrolytically produced copper coatings has been investigated. the copper coatings produced by the galvanostatic regime of the electrodeposition from the basic sulphate electrolyte and the electrolyte with added levelling/brightening additives at the low temperature were characterized by scanning electron microscopy (sem) and atomic force microscopy (afm) techniques (surface morphology and topography, respectively) and by vickers microindentation (hardness). the roughness of coatings increased with the increasing intensity of ultrasound, indicating that morphology of the coatings worsened with the enhanced application of ultrasonic waves. this is attributed to the strong effect of ultrasound on hydrodynamic conditions in the near-electrode layer, which is manifested by the increase of share of the activation control in the mixed activation-diffusion control of electrodeposition with increasing the intensity of ultrasound. the concept of "effective overpotential" originally proposed to explain a change of surface morphology in the conditions of vigorous hydrogen evolution is also applicable for a change of morphology of cu coatings under the imposed effect of ultrasonic waves. hardness analysis of the coatings showed that an intensity of applied ultrasound did not have any significant effect on the hardness, especially for the cu coatings produced from the basic sulphate electrolyte. keywords copper; surface characterization; scanning electron microscopy; atomic force microscopy; topography; composite hardness model http://dx.doi.org/10.5599/jese.1290 http://dx.doi.org/10.5599/jese.1290 http://www.jese-online.org/ mailto:nnikolic@ihtm.bg.ac.rs j. electrochem. sci. eng. 12(4) (2022) 603-615 morphology and hardness of copper coatings 604 introduction copper attracts huge attention from both scientific and technological communities owing to its unique characteristics, such as excellent thermal and electrical conductivities, malleability, catalytic activity, superior mechanical, anticorrosion and antimicrobial features [1-3]. these characteristics of copper enable its wide application in various industries, such as aerospace, automotive, microelectronics, telecommunications, medicine, energy sciences and technologies, etc. for all these purposes, the main methods of cu synthesis in different forms are electrodeposition, electroless plating, chemical vapor deposition (cvd), physical vapor deposition (pvd), thermal spray and sputtering techniques [2]. the electrodeposition technique is a very suitable way to obtain copper of desired characteristics suitable for application in above mentioned purposes [4]. copper is possible to obtain in various morphological forms starting from compact to powdered forms by the easy selection of parameters and regimes of the electrodeposition [4-6]. the technologically important cu electrodeposition processes include electroplating with a high aspect ratio [7,8], formation of functional thin films and coatings, and creation of nanoparticles and powders [4,9-15]. although acid sulphate electrolytes based on copper(ii) sulphate and sulphuric acid are the most often used electrolytes for cu electrodeposition, some other types, such as cyanide and fluoborate, also found application in the electrodeposition processes [16]. the concentrations of main components of electrolyte but also the addition of specific substances in electrolytes like additives for levelling and brightness, also play an important role in the final quality of electrolytically produced coatings. the other electrodeposition parameters determining the quality of deposits are the temperature of the electrolyte, electrodeposition time (or thickness of deposit), type of used cathode, mixing of electrolyte during electrodeposition process, rate of circulation of electrolyte, etc. [4,10,17-20]. all these parameters affect the quality of coatings produced by both constant (potentiostatic and galvanostatic (dc)) and periodically changing regimes (pulsating overpotential (po), pulsating current (pc) and reversing current (rc)) of the electrodeposition [4]. although electrodeposition processes are often performed in silent electrolytes, stirring of electrolyte is desirable to obtain smooth and uniform coatings, especially when electrodeposition process performs by the constant regimes. the most often used ways for electrolyte stirring are magnetic stirring [10] and application of ultrasound [21-23]. it was shown that the application of various ways of electrolyte stirring affects mass transfer, nucleation, and growth, and finally, morphology and structure of electrolytically synthesized coatings. in this study, we examine the effect of various intensities of applied ultrasound (or power of ultrasound) on the morphology of cu coatings produced in the galvanostatic (dc) regime. the two types of electrolytes were analysed: the basic sulphate electrolyte and an electrolyte with the addition of levelling and brightening additives. mechanical characteristics of the produced cu coatings are also considered. characterization of the cu coatings was performed by sem and afm (surface characterization) techniques and by vickers microindentation (mechanical characterization). experimental copper was electrodeposited galvanostatically at a current density of 50 ma cm-2 in an open cell of cylindrical shape. the cu coatings thicknesses from 16.6 µm were electrodeposited at the low temperature (t = 7.0  1.0 °c) from the following electrolytes: a. 240 g/l cuso4·5 h2o + 60 g/l h2so4; (electrolyte i) and b. 240 g/l cuso4∙5h2o + 60 g/l h2so4 + 0.124 g/l nacl + 1 g/l peg 6000 (polyethylene-glycol) + 0.0015 g/l mpsa (3-mercapto-1-propanesulfonic acid); (electrolyte ii). i. o. mladenović et al. j. electrochem. sci. eng. 12(4) (2022) 603-615 http://dx.doi.org/10.5599/jese.1290 605 for the preparation of the electrolytes, doubly distilled water and chemicals of p.a. quality (sigma aldrich company) were used. electrodeposition was performed using a brass of (2.0  1.0) cm2 surface area as a cathode (260 1/2 hard, astm b36, k&s engineering, 250 µm thick; composition: 66 % cu, 34 % zn), while an anode was a copper foil of cylindrical shape positioned close to the wall of the cell. the electrolytes were stirred by an application of ultrasound using an ultrasonic cleaner (model hd 2200 bandelin / berlin, germany) of 20 khz ± 500 hz frequency (fs) equipped with a standard sonotrode (cylindrical probe of 13 mm tip diameter), with power (i) of 200 w in the continuous mode. ultrasonic treatments were performed using amplitude (pi) of 10, 30 and 50 % maximal value. since the power of the ultrasonic device was 200 w, the values of pi of 10, 30 and 50 % corresponded to a power of 20, 60 and 100 w, respectively. the volume of the electrolyte was 100 ml with an immersion depth of the horn of 3 cm. for sonication of the electrolyte in the range of volumes between 20 and 900 ml, the maximum amplitude is 165 µm, representing 100 %. working with such intensive electrolyte stirring requires temperature regulation and a system with a more adequate cooling medium. in order to mitigate temperature oscillations, electrodeposition was performed at the low temperature using ice for cooling of the electrolyte and regulation of the working temperature. the experimental setup for cu electrodeposition in the presence of ultrasound is shown in figure 1. figure 1. the experimental setup for electrodeposition in the presence of ultrasound the obtained cu coatings were characterized by scanning electron microscopy (sem) and atomic force microscopy (afm) techniques. for morphological analysis, sem microscope model jeol jsm-6610lv (jeol ltd., tokyo, japan) equipped with energy-dispersive x-ray spectroscopy (eds) oxford instruments inca was used. the topographic analysis of coatings was investigated using an atomic force microscope in the non-contact mode (afm-auto probe cp research; tm microscopes-veeco instruments, santa barbara, ca, usa). the scan area was (20  20) m2. histogram analysis of the coatings was done by an application of accompanying afm software. hardness analysis of the coatings was done by vickers microindentation with applied loads between 0.049 and 2.452 n, and a dwell time of 25 s. vickers microhardness tester leitz kleinert prufer durimet i (leitz, oberkochen, germany) was used for the determination of hardness. the measured (or composite) hardness of the coating is defined by eq. (1) [9]: 2c 8544.1 d p h = (1) http://dx.doi.org/10.5599/jese.1290 j. electrochem. sci. eng. 12(4) (2022) 603-615 morphology and hardness of copper coatings 606 in eq. (1), hc (in pa) is a measured or composite hardness, p (in n) is an applied load, and d (in m) is a size of diagonal made in a coating during indentation. the hardness of the cathode (brass) was determined by the psr (proportional specimen resistance) model [24], and a value of 1.41 gpa [25] was obtained. the hc values usually include contributions of hardness both cathode (substrate) and coating, causing a necessity of application of composite hardness models for a determination of absolute (or real) hardness of coating 9,10,25. results and discussion morphological and hardness analyses of the cu coatings obtained from the basic sulphate electrolyte (electrolyte i) figure 2 shows morphologies and eds spectrums of cu coatings obtained at a current density of 50 ma cm-2 from the basic sulphate electrolyte with applied intensities of ultrasound corresponding to pi of 10 % (i.e. 10 % of maximum ultrasound amplitude) (figure 2a and b) and 50 % (figure 2c and d). the coatings are fine-grained and similar to each other at the macro level, with visible boundaries among grains. according to expectations, eds analyses of the obtained coatings identified only the presence of copper. a) b) energy, kev c) d) energy, kev figure 2. morphology and eds spectrum of copper coatings obtained at a current density of 50 ma cm-2 from electrolyte i with an intensity of ultrasound corresponding to: (a) and (b) pi = 10 %, (c) and (d) pi = 50 % the closer analysis of structural characteristics, but with included cu coating obtained with an intensity of ultrasound corresponding to pi of 30 %, was done applying the afm technique. figure 3 shows the 3d (three-dimensional) images and the corresponding histograms of topography obtained with an intensity of ultrasound of 10 % (figure 3a and b), 30 % (figure 3c and d) and 50 % (figure 3e and f). the values of the arithmetic average of the absolute (ra) roughness for the given cu coatings are given in table 1. the increase of roughness of the coatings with increasing the intensity of applied ultrasound is noticed by analysis of data given in table 1. i. o. mladenović et al. j. electrochem. sci. eng. 12(4) (2022) 603-615 http://dx.doi.org/10.5599/jese.1290 607 a) b) c) d) e) f) figure 3. the 3d (three dimensional) images (left) and histograms of topography (right) for the cu coatings obtained at 50 ma cm-2 from the basic sulphate electrolyte with an intensity of ultrasound corresponding to: (a) and (b) pi = 10 %, (c) and (d) pi = 30 % and (e) and (f) pi = 50 %. table 1. the values of the arithmetic average of the absolute (ra) roughness and meyer`s index (m) for the cu coatings electrodeposited from the basic sulphate electrolyte with various intensities of applied ultrasound (pi) pi / % 10 30 50 ra / nm 138.1 181.2 212.4 m 0.4503 0.4594 0.4750 the dependencies of measured composite hardness (hc) on the rid (relative indentation depth) for the given coatings are shown in figure 4a. the relative indentation depth (rid) is defined by the http://dx.doi.org/10.5599/jese.1290 j. electrochem. sci. eng. 12(4) (2022) 603-615 morphology and hardness of copper coatings 608 ratio between the depth of an indentation (h) and thickness of a coating (), i.e. as rid = h/ [9]. a depth of indentation, h is related with a diagonal size, d as h = d/7. with an application of loads above some critical value, a substrate (cathode) commences to contribute to a composite hardness, and for that reason, it is necessary to apply a composite hardness model for a determination of the absolute (or real) hardness of the coating. for electrolytically produced cu coating/brass systems, the chicot-lesage (c-l) composite hardness model [26-28] showed very valuable for a determination of the absolute hardness of cu coatings [9,10,25,29]. a detailed description of the cl model is given in ref. [9]. a) b) c) figure 4. the dependencies of: (a) the composite (hc) and (b) the calculated coating (hcoat) hardness on the rid for the cu coatings electrodeposited from the basic sulphate electrolyte with various intensities of ultrasound, and (c) the dependencies of (/d)m on the rid for the same cu coatings the calculated coating hardness (hcoat) values by the application of the c-l composite hardness model on the rid are given in figure 4b. it can be clearly seen from figure 4a and b that the dependencies of both hc and hcoat on the rid have “s” shapes. both coating hardness values grow with increasing the rid value reaching maximum at about 0.40, and then, the sharp fall occurred approaching the values of both hc and hcoat to a value of hardness of brass substrate (hs) of 1.41 gpa [25]. the beginning of a contribution of the substrate to composite hardness is estimated by a determination of the dependencies of the (/d)m − the rid, where m is a composite meyer`s index calculated by a linear regression performed on all experimental points for the given coating− cathode (substrate) systems [9,24]. the calculated m values are added to table 1, while the dependencies of the (/d)m − the rid are given in figure 4c. taking into consideration that the c-l model is valid up to i. o. mladenović et al. j. electrochem. sci. eng. 12(4) (2022) 603-615 http://dx.doi.org/10.5599/jese.1290 609 (/d)m = 1 [26-29], the critical or limiting rid value ((rid)c) of 0.14 was obtained (see figure 4c). for rid values above 0.14 (rid  0.14), a substrate (brass) significantly commences to contribute to composite (measured) hardness, and then, the calculated values obtained by application of the c-l model correspond to the absolute (or real) hardness of the cu coatings. for rid values below 0.14 (rid  0.14), an absolute hardness of the cu coatings corresponds to the measured (composite) hardness. morphological and hardness analyses of the cu coatings obtained from the electrolyte with levelling/brightening additives (electrolyte ii) the morphology and eds spectrums of the cu coatings obtained at a current density of 50 ma cm-2 from the electrolyte containing levelling and brightening additives with pi of 10 and 50 % are shown in figure 5. at first sight, certain differences among them are visible. the coating obtained with pi of 10 % is very smooth without visible boundary among grains (figure 5a), while that obtained with an intensity of 50 % can be characterized as fine-grained since certain boundaries among grains can be noticeable (figure 5c). in both coatings, eds spectrums identified only the presence of copper (figure 5b and d). a) b) energy, kev c) d) energy, kev figure 5. morphology and eds spectrum of copper coatings obtained at a current density of 50 ma cm-2 from electrolyte ii with an intensity of ultrasound corresponding to: (a) and (b) pi = 10 %, (c) and (d) pi = 50 % the afm analysis, including the 3d afm images and histograms of the topography, for the cu coatings obtained with an intensity of ultrasound of 10, 30 and 50 % is shown in figure 6. the values of ra roughness for these coatings are given in table 2. similar to the cu coatings obtained from the basic sulphate electrolyte, the roughness of the cu coatings obtained from the electrolyte with additives increased with increasing the intensity of ultrasound. http://dx.doi.org/10.5599/jese.1290 j. electrochem. sci. eng. 12(4) (2022) 603-615 morphology and hardness of copper coatings 610 a) b) c) d) e) f) figure 6. the 3d (three dimensional) images (left) and histograms of topography (right) for cu coatings obtained at 50 ma cm-2 from the electrolyte containing levelling/brightening additives with an intensity of ultrasound corresponding to: (a) and (b) pi = 10 %, (c) and (d) pi = 30 % and (e) and (f) pi = 50 % table 2. the values of the arithmetic average of the absolute (ra) roughness and meyer`s index (m) for the cu coatings electrodeposited from the electrolyte with added additives for levelling/brightening in the presence of various intensities of ultrasound. pi / % 10 30 50 ra / nm 26.87 53.07 90.5 m 0.4407 0.4565 0.4725 the dependencies of the measured (composite) hardness (hc) and the calculated coating hardness (hcoat) by the c-l model on the rid for the cu coatings obtained from the electrolytei. o. mladenović et al. j. electrochem. sci. eng. 12(4) (2022) 603-615 http://dx.doi.org/10.5599/jese.1290 611 containing additives are shown in figures 7a and b, respectively. the “s” shape of curves is not mentioned in this case. the probable reason for it lies in the fact that a value of a brass hardness of 1.41 gpa was not exceeded for these coatings. anyway, it is necessary to note that hc and hcoat values for the given coatings were smaller than those obtained from the additive-free electrolyte. a) b) c) figure 7. the dependencies of: (a) the composite (hc) and (b) the calculated coating (hcoat) hardness on the rid for the cu coatings electrodeposited from the electrolyte containing the levelling/brightening additives with various intensities of ultrasound, and (c) the dependencies of (/d)m on the rid for the same cu coatings the dependencies of the (/d)m the rid for the given coatings are shown in figure 7c, while the calculated values of m index used in a determination of these dependencies are added to table 2. using the same procedure as applied for the cu coatings obtained from the additive-free electrolyte, the limiting or critical rid value ((rid)c) of 0.14, separating the zone in which the composite hardness can be equalled with the coating hardness and the zone requiring an application of the c−l model for a determination of the absolute hardness of the cu coatings, was also obtained. discussion of the presented results formation of morphologies of cu coatings obtained from electrolyte i and ii with various intensities of ultrasound can be explained as follows: a current density of 50 ma cm-2 belongs to the mixed activation-diffusion control of the electrodeposition 9. in this control of the electrodeposition, deposition overpotential,  is given by eq. (2) [4,30]: )/(1 1 ln 3.2 ln 3.2 l c 0 c jj b j jb η − += (2) http://dx.doi.org/10.5599/jese.1290 j. electrochem. sci. eng. 12(4) (2022) 603-615 morphology and hardness of copper coatings 612 where bc is the cathodic tafel slope, j0 is the exchange current density and jl is the limiting diffusion current density. the deposition overpotential consists of the activation (act) and the diffusion (diff) parts. the activation part (act) required for the charge transfer is presented by eq. (3) 4,30: 0 c act ln 3.2 j jb η = (3) while the diffusion part (diff) is given by eq. (4) 4,30: )/(1 1 ln 3.2 l c diff jj b η − = (4) and it is due to mass transfer limitations (diffusion limitation primarily). the imposed ultrasound only affects the diffusion part of deposition overpotential. the imposed ultrasound causes a stirring of electrolyte in the near-electrode layer leading to a decrease of diffusion layer thickness and up to an increase of the limiting diffusion current density. this process leads to a decrease of diffusion part in overall deposition overpotential, i.e. up to a decrease of the degree of diffusion control in the mixed activation-diffusion control. this decrease enhances with an increase of intensity of applied ultrasound. the concept of „effective overpotential“ can be proposed to explain the change of surface morphology under an imposed ultrasound. this concept was originally proposed to explain the morphology of cu deposits formed under vigorous hydrogen evolution 31. in conditions of vigorous hydrogen evolution, the electrodeposition process occurs at overpotential, which is effectively lower than the specified value, and this overpotential is denoted as „effective“ value. according to this concept, vigorous hydrogen evolution causes a stirring of the electrolyte in the near-electrode layer, causing the decrease of thickness of diffusion layer, the increase of the limiting diffusion current density, and as a result of this process, the decrease of the degree of diffusion control of electrodeposition. from the morphological point of view, it means that morphologies of metal deposits become similar to those obtained at the lower overpotentials at which there is no hydrogen evolution, or it is very small. this concept applies to all cases where there is a change of hydrodynamic conditions in the nearelectrode layer, like the case of imposed magnetic fields 30. then, the effect of various intensities of ultrasound on the morphology of cu coatings (figures 2,3,5 and 6) can be considered as follows: with an increase of intensity of applied ultrasound, it intensifies an electrolyte stirring in the nearelectrode layer, enhancing the decrease of the diffusion layer thickness and the increase of the limiting diffusion current density. hence, the larger intensity of ultrasound, the smaller degree of diffusion control is. in eq. (2), as already mentioned, the activation part does not depend on whether ultrasound is imposed to the electrodeposition process or not. with the decreasing diffusion part in eq. (2) caused by the application of ultrasound, it increases the contribution of activation control in the mixed activation-diffusion control. this can be easily confirmed by analysis morphologies of cu coatings obtained from the basic sulphate electrolyte (figure 2a and c). the larger contribution of the activation control in the mixed activation-diffusion control in the coating obtained with the larger intensity of imposed ultrasound (figure 2c) can be proved by appearing of the larger and welldefined grains in this coating (see parts in circles in this figure). namely, it is well known 4 that large and regular grains are characteristic of the activation control of electrodeposition. a similar effect on the morphology of copper coatings is observed by the regime of pulsating current (pc) regime 9. analysing the copper coatings obtained at various average current densities, i. o. mladenović et al. j. electrochem. sci. eng. 12(4) (2022) 603-615 http://dx.doi.org/10.5599/jese.1290 613 which belonged to the mixed activation-diffusion control, formation of coatings with larger and well defined crystal grains was observed at the average current density which corresponded to the very beginning of the activation-diffusion control. the formation of coatings with such characteristics is explained by the increase of ratio of activation control in the mixed activation-diffusion control of electrodeposition with decreasing the average current density. although certain differences in morphology of cu coatings obtained under various intensities of ultrasound are obtained, a noticeable difference in their hardness is only observed between the coatings obtained from the basic sulphate electrolyte and those obtained from the electrolyte with additives. the absence of any noticeable difference with various intensities of ultrasound can be attributed to small differences in the surface morphology reached by application of ultrasound, as well as to a sensitivity of the vickers microindentation technique used for a determination of the coating hardness. namely, for the applied loads in the (0.049 2.452 n) range, the size of the diagonal of indent was in the (9.78 56.02 m) range. analysing morphologies of cu coatings shown in figures 2, 3, 5 and 6, it is clear that these surface morphologies were not significantly sensitive for this method of a determination of coating hardness. anyway, the critical or limiting rid value ((rid)c) of 0.14, separating the zone in which the composite hardness can be equalled with the coating hardness (negligible effect of the cathode hardness on the composite hardness) and the zone of a necessary application of the c−l model for a determination of the absolute hardness of the cu coatings (the strong effect of the cathode hardness on the composite hardness) was confirmed for all analysed cu coatings. the fact that this value was obtained for the cu coatings produced by various regimes of electrodeposition, such as the pulsating current (pc) [9,32] and the constant galvanostatic (dc) [10] regimes, as well as by varying all relevant parameters of electrodeposition in the dc regime, such as the type of cathode [10], the thickness of coating [25], the type of electrolyte [10] and kind of electrolyte stirring [10] clearly indicates that (rid)c value of 0.14 has universal character for electrolytically produced copper coatings. conclusions the cu coatings produced by the dc regime from electrolytes without and with additives for levelling and brightening were characterized by sem and afm techniques (surface characterization) and by vickers microindentation (mechanical characterization). the various intensities of ultrasound were used for the electrolyte stirring. on the basis of the obtained results, it follows: o the increasing intensity of ultrasound worsens the quality of the coatings. it is manifested by the increase of roughness of coatings, and it is valid for both types of cu coatings, i.e. for those obtained from the electrolytes both without and with added additives. o the change in the quality of coatings is explained by a strong effect of ultrasonic waves on hydrodynamic conditions in the near-electrode layer. increasing the intensity of ultrasound, it increases a share of the activation control in the mixed activation-diffusion control, which is accompanied by the formation of larger and well-defined grains. o mathematical set equations are used to explain a change of surface morphology under the influence of ultrasonic waves. it is shown that the concept "effective overpotential" originally proposed for copper electrodeposition under vigorous hydrogen evolution can be also applied for cu electrodeposition under imposed ultrasonic stirring of the electrolyte. o the hardness of cu coatings obtained from the electrolyte without additives was larger than those obtained from the electrolyte with additives. the use of various intensities of ultrasound did not significantly affect hardness of the coatings, especially for those obtained from the basic sulphate electrolyte. http://dx.doi.org/10.5599/jese.1290 j. electrochem. sci. eng. 12(4) (2022) 603-615 morphology and hardness of copper coatings 614 acknowledgment: this work was funded by ministry of education, science and technological development of republic of serbia (grants no. 451-03-68/2022-14/200026 and 451-03-68/202214/200135). references [1] copper plating service, https://www.sharrettsplating.com/coatings/copper (accessed on 27 september 2021). 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nikolić, journal of the serbian chemical society 72 (2007) 787-797 https://doi.org/ 10.2298/jsc0709787n [31] n. d. nikolić, k. i. popov, lj. j. pavlović, m. g. pavlović, journal of electroanalytical chemistry 588 (2006) 88-98. https://doi.org/10.1016/j.jelechem.2005.12.006 [32] i. o. mladenović, j. s. lamovec, d. g. vasiljević-radović, v. j. radojević, n. d. nikolić., journal of the serbian chemical society 87 (2022) 899-910 https://doi.org/10.2298/jsc2110 14105m , ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1290 https://doi.org/‌10.1016/j.ultsonch.2020.105193 https://doi.org/‌10.1016/j.ultsonch.2020.105193 https://doi.org/10.1016/0921-5093(91)90753-a https://doi.org/10.1016/0921-5093(91)90753-a https://doi.org/10.3390/met11010111 https://doi.org/10.1016/j.tsf.2005.09.194 https://doi.org/10.1016/j.surfcoat.2005.01.056 https://doi.org/10.1016/j.surfcoat.2005.01.056 https://doi.org/10.1016/0040-6090(94)06239-h https://doi.org/10.1016/0040-6090(94)06239-h http://dx.doi.org/10.20964/2020.12.01 http://dx.doi.org/10.20964/2020.12.01 https://doi.org/10.2298/jsc0709787n https://doi.org/10.2298/jsc0709787n https://doi.org/10.1016/j.jelechem.2005.12.006 https://doi.org/10.2298/jsc211014105m https://doi.org/10.2298/jsc211014105m https://creativecommons.org/licenses/by/4.0/) @article{mladenovic2022, author = {mladenovi{\'{c}}, ivana o and lamovec, jelena s and radovi{\'{c}}, dana g vasiljevi{\'{c}} and radojevi{\'{c}}, vesna j and nikoli{\'{c}}, neboj{\v{s}}a d}, journal = {journal of electrochemical science and engineering}, title = {{influence of intensity of ultrasound on morphology and hardness of copper coatings obtained by electrodeposition:}}, year = {2022}, issn = {1847-9286}, month = {feb}, number = {4}, pages = {603--615}, volume = {12}, abstract = {the influence of various intensities of ultrasound applied for the electrolyte stirring on morphological and mechanical characteristics of electrolytically produced copper coatings has been investigated. the copper coatings produced by the galvanostatic regime of the electrodeposition from the basic sulphate electrolyte and the electrolyte with added levelling/brightening additives at the low temperature were characterized by scanning electron microscopy (sem) and atomic force microscopy (afm) techniques (surface morphology and topography, respectively) and by vickers microindentation (hardness). the roughness of coatings increased with the increasing intensity of ultrasound, indicating that morphology of the coatings worsened with the enhanced application of ultrasonic waves. this is attributed to the strong effect of ultrasound on hydrodynamic conditions in the near-electrode layer, which is manifested by the increase of share of the activation control in the mixed activation-diffusion control of electrodeposition with increasing the intensity of ultrasound. the concept of "effective overpotential" originally proposed to explain a change of surface morphology in the conditions of vigorous hydrogen evolution is also applicable for a change of morphology of cu coatings under the imposed effect of ultrasonic waves. hardness analysis of the coatings showed that an intensity of applied ultrasound did not have any significant effect on the hardness, especially for the cu coatings produced from the basic sulphate electrolyte.}, doi = {10.5599/jese.1290}, file = {:d\:/onedrive/mendeley desktop/mladenovi{'{c}} et al. 2022 influence of intensity of ultrasound on morphology and hardness of copper coatings obtained by electrodeposit.pdf:pdf;:03_jese_1290.pdf:pdf}, keywords = {copper, atomic force microscopy, composite hardness model, scanning electron microscopy, surface characterization, topography}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1290}, } {electrodeposition of the sb2se3 thin films on various substrates from the tartaric electrolyte} http://dx.doi.org/10.5599/jese.676 1 j. electrochem. sci. eng. 10(1) (2020) 1-9; http://dx.doi.org/10.5599/jese.676 open access: issn 1847-9286 www.jese-online.org original scientific paper electrodeposition of sb2se3 thin films on various substrates from the tartaric electrolyte vusala asim majidzade1,, akif shikhan aliyev1, parvin heydar guliyev2, dunya mahammad babanly1,3 1institute of catalysis and inorganic chemistry named after acad. m. nagıyev azerbaijan national academy of sciences (anas), baku, az1143, azerbaijan 2nakchivan state university, nakchivan ar, az 7012, azerbaijan 3french azerbaijani university (ufaz), baku, az1010, azerbaijan corresponding author: vuska_80@mail.ru, phone: +99450 640 02 25 received: march 4, 2019; revised: july 1, 2019; accepted: july 2, 2019 abstract the present contribution is devoted to the electrochemical deposition of sb2se3 thin films from tartrate electrolyte. the study was conducted by potentiodynamic, potentiostatic and galvanostatic methods carried out under different conditions at pt, cu and ni electrodes. the kinetics and mechanism of the electroreduction of antimony and selenite ions in the tartaric acid were studied separately for the electrochemical deposition. comparison of the obtained polarization curves showed that co-deposition occurs between electroreduction potentials of antimony and selenium, indicating depolarization electrode effect for antimony ions. the influence of electrolyte composition, ph, current density, temperature, etc. has been studied. on the basis of cyclic polarization, x-ray phase and sem-edx analyses, it is found that sb-se thin films are deposited on pt and ni electrodes, but not on cu electrode. black, uniform, crystalline and shiny films of the stoichiometric composition of sb2se3 compound are deposited on pt and ni electrodes within the 338-348 k temperature interval, ph 1.85, current density of 2.5-3.0 a/dm2, and annealing temperature of 703 k. experiments were carried out using the optimal electrolyte composition containing 0.05 m sbocl + 0.05 m h2seo3 + 0.007 m c4h6o6. keywords antimony triselenide; semiconductor; electrochemical deposition; polarization; tartaric acid introduction metal chalcogenides represent an important class of functional semiconductor materials possessing highly usable properties [1-3]. these materials are widely used as catalysts for photovoltaic cells, photoresistors, and laser materials because of their photosensitive and http://dx.doi.org/10.5599/jese.676 http://dx.doi.org/10.5599/jese.676 http://www.jese-online.org/ mailto:vuska_80@mail.ru j. electrochem. sci. eng. 10(1) (2020) 1-9 sb2se3 thin films 2 thermoelectric properties [4-10]. the economic drive to make efficient, low-cost solar cells requires sustained research on new absorber materials for thin-film photovoltaics [11-13]. antimony selenide (sb2se3) is considered one of the most promising semiconductor materials of the type a2b3 with a relevant bandgap of ~1.78 ev for solar cell applications [14]. this gives a solid argument for the production of this type of semiconductor materials which complements the field of application in electrochemistry. relatively lower prices of sb and se, as well as a low toxicity of sb2se3 merit its exploration for photovoltaic application. there are different methods for obtaining thin films of metal-chalcogenides, including chemical vapor deposition, pyrolytic decomposition, thermal evaporation, hydrothermal synthesis, inert gas condensation and electrochemical method [15-25]. a highly crystalline sb2se3 sample was obtained by thermal evaporation and chemical precipitation method in a nitrogen atmosphere [9,26]. in a different work, sb2se3 was prepared by adding ethylene glycol to antimony oxide, and sodium selenosulfate. after 10-12 h at 423 k temperature, nanorods of sb2se3 with 10-15 mm length and 200-450 nm width were obtained [27]. all above-mentioned methods, however, are time, finance and effort consuming. the electrochemical method is more accurate, time-saving, and efficient method for the synthesis of metal chalcogenides including sb2se3 [7,8,28,29]. usually, this method is used to get thin layers of the required composition from electrolytes of different compositions in a short time and at relatively low temperatures. one of the advantages of this method is the possibility to alter the composition of the obtained layers by changing the composition of electrolyte and conditions of the electrolysis. the nucleation and the growth mechanisms of antimony selenide (sb2se3) on indium-doped tin oxides (ito) and fluorine tin oxide (fto) substrates were achieved using the electrochemical method [7,30]. to obtain high-quality sb2se3 thin films with an electrochemical method, more accurate experimental results in this field of research are still required. this work is a part of our systematic investigation in the area of electrodeposition of thin films of semiconductor chalcogenides [31-37]. therefore, the present contribution is devoted to exploring the optimal conditions for obtaining high-quality films of antimony selenide via studying the effect of the substrate material on the electrodeposition process. experimental part the composition of an electrolyte solution used in our experiments was 0.01 – 0.09 m sbocl (supplied by 2a pharmachem, usa), 0.01 – 0.09 m h2seo3 (supplied by reachim ltd, russia) and 0.007 m c4h6o6 (tartaric acid) (supplied by indiamart, india). the cyclic polarization curves were performed via "iviumstat electrochemical interface" potentiostat. during experiments, a three-electrode electrochemical cell with 100 ml volume, pt wire of 210-6 m2 surface and cu and ni plates of 110-4 m2 surface area were used as working electrodes. the silver-silver chloride (ag/agcl/kcl) was used as a reference electrode and a pt sheet as a counter electrode. the temperature was regulated by the utu – 4 universal thermostat. the phase composition of the prepared thin films was analyzed by "d2 phazer" x-ray phase analyzer (cukα; ni filter) of the "bruker" company. morphology, topography and chemical elemental composition of the deposited samples were measured by "carel zeiss sigma" scanning electron microscope (sem) provided with edx unit. at the beginning of the experiments, pt, ni, and cu electrodes were purified in the concentrated nitric acid and then washed with bi-distilled water. ni-electrodes were additionally subjected to electrochemical polishing in a mixed solution of sulfuric, phosphoric and citric acids at certain conditions (t = 293–303 k, j = 0.5 a/dm2, τ = 180 s) and washed with distilled water. vusala asim majidzade et al. j. electrochem. sci. eng. 10(1) (2020) 1-9 http://dx.doi.org/10.5599/jese.676 3 results and discussion as is already known, the composition and quality of the obtained thin layers considerably depend on the composition of electrolyte and concentration of primary components during electrodeposition. in the present study, sb-se thin films were electrochemically deposited from tartrate electrolyte by galvanostatic and potentiostatic methods. we carried out the deposition process by changing certain conditions, such as current density, concentration of components, temperature, etc. different substrate electrodes, i.e. pt, cu, and ni have been used to obtain the stoichiometric compound sb2se3. as a result, the composition of the electrolyte and the condition of electrolysis were optimized to the necessary extent. initially, pt electrode was used to study the kinetics and mechanism of the electrochemical codeposition process. experiments were carried within 0.01 – 0.1 m concentration range of sbocl and h2seo3 acid in the presence of the tartaric acid, wherein the ph value varied in the interval of 1.65-2.1. the potentiodynamic method was used to record the cyclic polarization curves on the pt electrode. after that, thin sb-se films were deposited on ni and cu electrodes under optimal experimental conditions. as can be seen from figure 1, in comparison with other processes, the electroreduction of selenite ions occurs in stages. according to the reference [38], the standard electrode potential of selenite ions is more positive than antimony ions. figure 1. cyclic polarization curves (i vs. e) of electroreduction of selenite and antimony ions in tartaric acid and co-deposition of sb and se on different electrodes. composition of the electrolyte (m): 1) 0.007 c4h6o6, on pt electrode; 2) 0.05 h2seo3 + 0.007 c4h6o6 on pt electrode; 3) 0.05 sbocl + 0.007 c4h6o6, on pt electrode; 4) 0.05 sbocl + 0.05 h2seo3 + 0.007 c4h6o6, on pt electrode; 5) 0.05 sbocl + 0.05 h2seo3 + 0.007 c4h6o6, on ni electrode; 6) 0.05 sbocl + 0.05 h2seo3 + 0.007 c4h6o6 on cu electrode. t = 298 k, ν = 0.02 v s -1. http://dx.doi.org/10.5599/jese.676 j. electrochem. sci. eng. 10(1) (2020) 1-9 sb2se3 thin films 4 this affects co-electrodeposition and formation of the compound sb2se3 on pt and ni electrodes, which occurs between the potentials of the electroreduction of the antimony ions (-0.46 v) [32] and selenite ions (0.4 v) [33]. co-deposition is indicated at -0.44 v, suggesting depolarization, i.e. decrease of the cathodic potential of electrode for antimony deposition. starting from -0.44 v, the surfaces of pt and ni electrodes become completely covered with the black sb-se layer. as can be seen from the curve 6 in figure 1, formation of sb-se compound on cu electrode does not occur. further studies of the process of electrolysis on ni-electrodes were carried out using both galvanostatic and potentiostatic methods. the films obtained by the galvanostatic method at current density of 2-6 a/dm2 from an electrolyte of 0.05 мsbocl + 0.05м h2seo3 + 0.007 мc4h6o6 were thermally treated at tthermal = 673-773 k temperature range, in an argon atmosphere. the processes of electrodeposition and heat treatment lasted for 60 min each. the purpose of heat treatment was to obtain crystalline films of sb2se3. before and after annealing, the obtained thin films were investigated using x-ray phase, sem and elemental analyses. the results of sem-edx analysis presented in figure 2 show that thin films obtained at a current density of 2-3 a/dm2 are close to the stoichiometric composition. figure 2. sem-edx results of sb-se layers electrodeposited on ni-electrode: a) galvanostatic method, electrolyte composition 0.09 м sbocl + 0.01 м h2seo3 + 0.007 m c4h6o6, current density = 2.5 а/dm2, t = 298 k, ph 2.1, tthermal = 703 k; b) galvanostatic method, electrolyte composition 0.05 m sbocl + 0.05 m h2seo3 + 0.007 m c4h6o6, current density = 2.5 а/dm 2, t = 298 k, ph 1.85, not thermally treated; c) potentiostatic method, electrolyte composition 0.05 m sbocl + 0.05 m h2seo3 + 0.007 m c4h6o6, e = -0.55 v, t = 338 k, ph = 1.85, tthermal = 703 k. vusala asim majidzade et al. j. electrochem. sci. eng. 10(1) (2020) 1-9 http://dx.doi.org/10.5599/jese.676 5 the content of antimony and selenium in the film is 61.91 at. % and 38.09 at. %, respectively. it was shown previously that the antimony content in the deposited film decreases with an increase of current density to 6 a/dm2 [34]. the co-deposition of sb-se layers on ni electrode by the potentiostatic method was carried out at 298-358 k temperature range. obtained results show that the stoichiometric sb2se3 samples are formed starting from 338 k. this is evidenced by the x-ray phase results of samples (figure 3). figure 3. x-ray phase analysis of sb-se films formed by potentiostatic deposition on ni-electrode. the composition of electrolyte: 0.05 м sbocl + 0.05 m h2seo3 + 0.007 m c4h6o6, e = -0.55 v, time of electrolysis = 60 min, t = 338 k, ph = 1.85, tthermal= 703 k. it was established that co-deposition of sb-se thin films at 298 k occurs at –0.6 v, while at 358 k, the potential of co-deposition is shifted to –0.36 v, suggesting that an increase in temperature has a positive effect on co-precipitation. during these experiments, the color of the samples varied from golden red to black. it was also observed that the composition of layers obtained within the temperature range 338 – 348 k is closer to the stoichiometric one (figure 2c). adhesion of films obtained at t > 348 k, however, deteriorates and therefore falls from the surface of the electrode. sb-se thin films were also deposited on cu electrode. this process was carried out under the galvanostatic regime at 2–2.5 a/dm2 current density in different concentrations of the electrolyte. the obtained samples were further annealed at 723 k in an argon environment. as shown in figures 4 and 5, the results of x-ray phase analysis on cu-electrode differ from the results obtained on ni electrodes. the duration of electrolysis and heat treatment on cu-electrodes lasted for 60 min each. as can be seen from figures 4 and 5, the electrodeposition of antimony does not occur on the surface of cu electrode. in all studied concentration intervals of the initial components, the chemical compound cu2se is formed. in our opinion, this is because selenium is deposited at more positive potential as compared to sb, and the electrode surface is completely covered with selenium. therefore, the formation of cu2se compound occurs after heat treatment and the interaction of antimony with selenium does not occur. http://dx.doi.org/10.5599/jese.676 j. electrochem. sci. eng. 10(1) (2020) 1-9 sb2se3 thin films 6 figure 4. a) sem, b) edx and c) x-ray phase analysis of electrodeposited layers on cu electrode. electrolyte composition: 0.025 м sbocl + 0.075 м h2seo3 + 0.007 м c4h6o6, time of electrolysis = 60 min, current density = 2.5 a/dm2, t = 298 k, ph 1.73, tthermal = 703 k the main results of present experiments are summarized in table 1, showing that the chemical composition of sb-se films is closer to the stoichiometric one when they are deposited on the surface of pt and ni electrodes. the compound sb2se3 does not form on the cu electrode. thus, the obtained results have proved that the electrode support material has a considerable influence on the co-deposition process, and that pt and ni electrodes are more effective for co-deposition of sbse thin films than cu electrode. table 1. chemical compositions of sb2se3 thin films formed by galvanostatic deposition* from tartaric electrolyte containing different concentrations of sb and se on different substrate electrodes. no. composition of electrolyte, м ph electrodes pt ni cu content, atomic % sbocl h2seo3 sb se sb se sb se 1 0.01 0.09 1.65 28.19 71.81 21.76 78.24 0 70.73 2 0.025 0.075 1.73 50.29 49.71 43.05 56.95 0 58.19 3 0.05 0.05 1.85 61.38 38.62 61.91 38.09 0 47.47 4 0.075 0.025 1.98 82.30 17.70 75.99 24.01 0 35.31 5 0.09 0.01 2.1 95.34 4.66 99.47 0.53 0 22.00 *current density = 2.5 а/dm2, time of deposition = 60 min, t = 338 k, tthermal=703 k. vusala asim majidzade et al. j. electrochem. sci. eng. 10(1) (2020) 1-9 http://dx.doi.org/10.5599/jese.676 7 figure 5. a) sem, b) edx and c) x-ray phase analysis of electrodeposited layers on cu electrode. the electrolyte composition: 0.05 m sbocl + 0.05 m h2seo3 + 0.007 m c4h6o6, time of electrolysis = 60 min, current density = 2.5 a/ dm2, ph 1.85, tthermal= 703 k. conclusions the electrochemical deposition of sb2se3 thin films has been performed by potentiostatic and galvanostatic methods, under various experimental conditions on pt, cu, ni electrodes. the study was conducted by changing the concentration of electrolyte, ph, current density, temperature, etc. the obtained results of cv, x-ray phase and sem-edx have shown that thin films of sb2se3 compounds are deposited on pt and ni electrodes, but not on cu electrode. it was found that black, uniform, crystalline and shiny films of the stoichiometric composition of sb2se3 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[38] handbook of electrochemistry, a. m. sukhotin (ed.), leningrad: khymia, 1981. p. 488 (in russian). ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.676 http://creativecommons.org/licenses/by/4.0/) {advances in understanding li battery mechanisms using impedance spectroscopy review} http://dx.doi.org/10.5599/jese.734 79 j. electrochem. sci. eng. 10(2) (2020) 79-93; http://dx.doi.org/10.5599/jese.734 open access: issn 1847-9286 www.jese-online.org review advances in understanding li battery mechanisms using impedance spectroscopy jože moškon1, sara drvarič talian1, robert dominko1, miran gaberšček1,2, 1department of materials chemistry, national institute of chemistry, hajdrihova 19, 1000 ljubljana, slovenia 2faculty of chemistry and chemical technology university of ljubljana, večna pot 113, 1000 ljubljana, slovenia corresponding author: miran.gaberscek@ki.si; tel.: +386-1-4760-320 received: october 7, 2019; revised: january 8, 2020; accepted: january 8, 2020 abstract the use of impedance spectroscopy in the field of modern batteries is demonstrated on three systems: lithium ion batteries represented by lithium iron phosphate (lfp) and lithium cobalt oxide (lco) electrodes, a porous carbon cathode in contact with polysulfides and a metallic lithium anode exhibiting dendritic growth. in all cases, systematic experiments are shown where the type and composition of electrochemical cell is varied in order to identify the main processes contributing to the impedance response. the experiments are upgraded with appropriate models, using mainly the transmission line approach. the approach allows establishment of clear correlations between the composition and morphology of electrodes on one hand and the measured impedance features on the other. as the transmission line models are based on the use of physically well-defined elements, the approach allows a quantitative description of the main processes (diffusion, reaction, migration across films and contacts) taking place in modern battery electrodes. keywords lithium ion batteries; lithium sulfur batteries; transmission line model; porous electrode model; dendrites diffusion; electrochemical reaction. introduction lithium batteries have been successfully marketed since 1991 when sony introduced the first rechargeable li battery based on lithium cobalt oxide as cathode and graphite as anode material. later on, many other materials have been proposed both as a cathode or anode in order to further improve the battery performance. despite the commercial success and continuous improvement of li batteries, the mechanisms occurring in these devices have remained only partially explained. whereas the development of new methodologies (microscopy, various spectroscopies etc.) has http://dx.doi.org/10.5599/jese.734 http://dx.doi.org/10.5599/jese.734 http://www.jese-online.org/ mailto:miran.gaberscek@ki.si j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 80 provided significant insight into local phenomena in certain electrode materials, evaluation of the overall electrochemical performance of given electrode still mainly relies on the use of conventional electrochemical methods and approaches. among those, impedance spectroscopy is known to be the most accurate and also most powerful as regards the discrimination of various electrochemical steps occurring within the electrochemical cell. this short review paper demonstrates how impedance spectroscopy can help to improve the understanding of main mechanisms taking place in typical lithium battery cells. for the sake of some generality, we present typical results obtained on three significantly different electrode types: (i) insertion cathodes, (ii) sulfur cathodes having different morphologies and (iii) a metallic lithium anode. most of the results presented here are related to our own past experiments and modeling, occasionally other results are also referred to and commented on. results and discussion insertion electrodes insertion electrodes are still the most popular type of li battery electrodes [1,2]. they predominantly consist of active particles of different shapes with the sizes between ca. 10 nm to several micrometers. the active particles are bound together by a polymeric binder so that a porous matrix is formed. during the cell manufacture, the pores are filled with electrolyte that contains ions of active species. some carbon black or a similar electronic conductor is also added in order to ensure sufficient electronic conductivity of the whole electrode matrix. the electrode composite is pressed on a current collector which mainly consists of copper (for anodes) or aluminium (for cathodes). the main transport-reaction steps taking place in insertion electrodes are schematically shown in figure 1a (for clarity the binder and conductive additive are not shown). briefly, during discharge the electrons move from the metallic collector (foil) across the conductive paths (mostly across conductive additives) to each individual active particle (step a in figure 1a). ions are provided from the electrolytes side. both electrons and ions are simultaneously inserted into the active particles (step b in figure 1a). finally, ions and electrons move simultaneously into the inner of active particles (step c). however, this movement of ions and electrons inside active particles might proceed in very different ways, depending on the system. for example, in the conventional lico2 (lco) cathode, this movement can be described as rather simple chemical diffusion process (at least in certain potential range of operation). at the other extreme, in lifepo4 (lfp) electrode, this movement involves complicated phase transformations such as spinodal decomposition. furthermore, these reactions may depend on particle size, reaction rate and other parameters. due to these differences we will in continuation include typical results on both extreme insertion materials, lco and lfp. first we focus on a typical impedance spectrum of a lfp electrode (figure 1b). we see two main features in this figure: a red high-frequency (hf) arc and a blue low-frequency (lf) line. additionally, we may see that hf arc does not approach the origin of graph (the 0,0 point) but forms a small intersection at extrapolated highest frequencies. this part (small intersection) is usually ascribed to the resistance of electrolyte. as this resistance is comparatively small, it usually does not represent a big contribution to the total impedance of cell. the situation may change under high rates where the species from electrolyte may be consumed or formed at very high rates. this case, however, is not considered here. thus it remains to find the origin of the other two features (hf arc, lf line). one way to find the meaning of these features and correlate them to the general scheme in figure 1a is to look into previous results published by other authors and assume the present case is similar jože moškon et al. j. electrochem. sci. eng. 10(2) (2020) 79-93 http://dx.doi.org/10.5599/jese.734 81 to previous ones. another approach, which is more consuming but also much more reliable, is to make dedicated experiments that will reveal the actual correlation between the scheme in figure 1a and the measurement in figure 1b. figure 1. a) three main steps of transport/reaction mechanism in insertion electrodes; b) typical impedance response of lithium iron phosphate (lfp) electrode; c) effect of external loading (mechanical pressure) on the hf impedance arc of lfp electrode in pouch cell (soft cell enabling application of external mechanical load); d) effect of external loading on cell capacity. the figure presents selected unused results from the study published in ref. [3]. an example of such dedicated experiment is shown in figure 1c. the pouch cell containing two lfp cathodes was pressed externally and the impedance response measured before and after exerting the pressure (for details see ref. [3]). as regard the impedance measurements, they were carried out with different equipments, depending on the type of cell and frequency range investigated: in the frequency range of 1 mhz to 20 hz, we used hewlet packard 4284a instrument, while in the frequency range of 100 khz to 0.001 hz we used solartron si 1260 coupled with par eg&g 283 potentiostat/galvanostat. the galvanostatic characterization was performed using vpm3 potentistat/galvanostat at room temperature. we may see that the use of mechanical pressure on soft pouch cell decreases considerably the size of hf arc. making many sets of such experiments and varying the pressure in a controlled and repeatable way [3], we assumed that hf arc might correspond to some contacting, as a contact resistance is known to be dependent on the pressure between contacting phases. however, looking at figure 1a, one sees two types of contacts in a typical battery cell: (i) the one between the current collector (here aluminium foil) and the electrode composite, and (ii) the contacts between the individual active particles constituting the electrode composite. thus a further dedicated experiment was required, as exemplified in figure 2. from figure 2 we can see that if we apply a conductive paste between the current collector and the composite electrode, the hf arc more or less disappears – even if no pressure is exerted on the charge, a s http://dx.doi.org/10.5599/jese.734 j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 82 pouch cell [3]. this experiment indicates quite convincingly that the hf arc must be due the poor contact between the current collector and the electrode composite. after application of ag the contact becomes more or less invisible due to improved electronic conductivity across that contact. this assumption was additionally confirmed by several other sets of experiments as discussed in our previous paper [3]. figure 2. improving (decreasing) the contact resistance between aluminium substrate and electrode composite by applying conductive ag paste to that interface (adapted and redrawn based on the results of ref. [3]). based on the results shown and discussed above, one might wonder if the finding about the origin of hf arc also holds for other cathode systems, for example the more conventional lithium cobalt oxide, lco. thus we performed a similar experiment using the variable external pressure also on the lco system (figure 3a) (for details see ref. [4]). however, already a quick look at the typical spectrum, we notice that lco electrode gives an impedance response that contains two arcs, denoted as a and b. at least two obvious questions arise: which of the arcs is due to the contact between the collector and electrode composite (if any) and what is the meaning of the other arc? the experiment with the variable pressure reveals that only the size of arc a significantly changes with pressure whereas the size of arc b is more or less pressure-independent. this strongly indicates that the arc a must be due to the contact resistance – similarly as hf arc in the case of lfp. indeed this assumption was confirmed by a more detailed analysis, as shown in our previous work [4]. still, the question remains what is the meaning of arc b? based on the peak frequency and the associated capacitance we assumed that one explanation could be that the arc b represents the resistance of insertion reaction (step b in figure 1a) coupled with the double layer capacitance of the interfacial region between the electrolyte and active particles. however, this assumption needed to be confirmed/refuted by some dedicated experiments. jože moškon et al. j. electrochem. sci. eng. 10(2) (2020) 79-93 http://dx.doi.org/10.5599/jese.734 83 figure 3. a,b) typical effect of external loading (mechanical pressure) on the impedance response of a lithium cobalt oxide (lco) electrode. note the absence of effect on arc »b«. c) effect of changing the electrolyte concentration on arc b of lco electrode. the electrolyte consisted of a lipf6 salt in ec:dec (ethylene carbonate/diethyl carbonate) (1:1 by volume). an example of such experiment is shown in figure 3c. we prepared several cells that contained as similar as possible lco electrodes but in each cell the concentration of electrolyte was different. we found that any decrease in the electrolyte concentration led to a significant (and systematic) increase of arc b. simultaneously, the electrolyte concentration had a significant impact on the peak frequency of arc b and thus on the associated capacitance. a more detailed analysis showed that all the trends were consistent with the main assumption, namely that arc b was due to insertion reaction coupled with double layer capacitance [4]. based on some additional dedicated experiments and analyses shown in previous works [5-7], we could construct a transmission line (figure 4) that could describe quite accurately all of the measured impedance spectra of insertion electrodes. the physical meaning of individual transmission line elements is described in figure 4. based on this description we can say that in both materials, lfp and lco, the hf arc is due to the coupling between elements r0 and c0, i.e. the resistance and capacitance due to the contact between current collector and electrode composite. furthermore, one can conclude that arc b in the case of lco is due to the sum of discretized elements r2,i and c1,i, i.e. the resistance due to insertion reaction coupled with the capacitance due to the double layer between the electrolyte and the active particles. as mentioned before, resistance due electrolyte, r4,i, appears as the high-frequency intersection. what has not been resolved, however, is the meaning of the low-frequency line observed especially clearly in the case of lfp (figure 1b, blue line). if we look closer to that part of impedance response, we in fact see a sort of hump at the beginning of line. also, we can see that the line is quite inclined, i.e. it is not vertical. if it was vertical and without a hump we could assume that it must be due to some big capacity in system, such as the chemical capacity of insertion material itself. if the inclination was close to 45 degrees, we could ascribe it more or less directly to a warburg impedance, possibly due to the chemical diffusion in the solid active particles, as indeed indicated in the proposed transmission line, i.e. the element zw,i in figure 4. http://dx.doi.org/10.5599/jese.734 j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 84 figure 4. top: a scheme of a typical porous insertion electrode indicating resistive and capacitive contributions to impedance response: c0 – capacitance due to contact between current collector and electrode composite; r0 – resistance due to contact between current collector and electrode composite; r1,i – electronic resistance of carbon coating; c1,i – capacitance due to electrolyte/active particle interface; r2,i – resistance due to insertion reaction; r3,i –resistance due to contact between active particles; r4,i –resistance due electrolyte; zw,i – warburg impedance due to diffusion of active species inside active particles; rel – electrolyte resistance. bottom: a transmission line model corresponding to the proposed scheme. again, to get further insight into the meaning of this lf line we performed additional systematic experiments. for example, figure 5a shows that the inclination of the lf line changes with state of charge of the cathode whereby it approaches the expected 45-degree case as the soc increases towards the medium values (half of total charge). an analysis based on the proposed transmission line showed that such a behavior could mean that during charging the averaged diffusion coefficient inside active particles is changed [4]. figure 5. measured (a) and simulated (b) impedance spectra of lfp electrode at two different states of charge (corresponding to two different potentials). note the decreasing angle as the soc increases. the effect is reproducible and reversible which indicates a unique correlation between the angle and soc. the figure presents selected unused results from the study published in ref. [4] indeed such variation of diffusion coefficient was earlier shown by other authors for different insertion systems [5-7] and is generally attributed to the formation of non-ideal mixtures (occupied jože moškon et al. j. electrochem. sci. eng. 10(2) (2020) 79-93 http://dx.doi.org/10.5599/jese.734 85 vs non-occupied sites with li during insertion or de-insertion of lithium). however, a more mechanistic explanation that would correlate this variation with soc is still missing so further model experiments are needed to fully understand this phenomenon. as a whole, the information collected so far enables one to make several conclusions as regards the meaning of measured impedance features in insertion electrodes. figure 6 summarizes such conclusions by connecting the regions in an insertion electrode where a given process occurs with the corresponding impedance features. the latter are, in turn, also uniquely connected to the elements of transmission line model. such a scheme might represent a good starting point for studying any insertion material. however, similarly as shown above, at least some model experiments (systematic/dedicated experiments) need to be performed to confirm the validity of the scheme and, possibly, identify specific feature(s) for given system. figure 6. left: schematic and sem image of a typical insertion electrode; right: transmission line model and general simulated impedance response of insertion electrode. the correlations between the image, transmission line and impedance response are clearly indicated. litium sulfur system: sulfur electrode (porous carbon in contact with polysulfides) li-s battery is a highly promising candidate to replace the current lithium ion systems [8-10]. however, there are still several open questions regarding the very basic operation of this complex system. impedance spectroscopy could help answer some of them. as mentioned before, significant progress can only be expected if many sets of dedicated experiments are carried out and the results are combined with the findings obtained using other methods. here we show how it is possible to get additional insight into selected cathode mechanisms by using different electrode morphologies such as a flat electrode or a porous electrode with well-defined morphology. the cathode of li-s system is usually composed of sulfur that is embedded into the pores of a highly porous conductive substrate such as a porous carbon. during cell operation the initial solid sulfur gradually electrochemically reduces into soluble polysulfides with progressively shorter http://dx.doi.org/10.5599/jese.734 j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 86 chains. thus, most of the capacity obtained represents the electrochemical reduction of polysulfide species. as many of these species are soluble, they can move away from the carbon surface. if so, they can later be reduced only if they diffuse back to the surface. this complicates the chargedischarge mechanism which is also reflected in the corresponding impedance spectra. figure 7 shows a typical development of impedance spectra during discharge of a conventional li-s cell. the development during charging (not shown) is similar, only going to backward direction, as expected for a reversible reaction. in any case, what we can see in figure 7b is that most of the measured impedance responses are dominated by the low frequency response consisting of tail (a curved line) with progressively higher impedances. an analysis of this response appears to be rather difficult and ambiguous. the development during charging (not shown) is similar, only going to backward direction, as expected for a reversible reaction. in any case, what we can see in figure 7b is that most of the measured impedance responses are dominated by the low frequency response consisting of tail (a curved line) with progressively higher impedances. an analysis of this response appears to be rather difficult and ambiguous. figure 7. a) typical charge-discharge curve of li-s cell. the spikes represent the points where the cell operation was stopped and impedance spectra measured (after stabilization). the corresponding spectra obtained during the discharge are shown in panel b). the ensaco 350g carbon (imerys) and a corresponding sulfur composite (ensaco 350g/s) were prepared in a c:s=1:2 mass ratio. the composite was mixed with a printex (degussa) conductive additive and a pvdf binder (8:1:1) in (nmp n-methyl-2-pyrrodione). sulfur loading was approx. 1 mg/cm2. the electrolyte was 1 m litfsi (lithium bis(trifluoromethanesulfonyl)imide) in tegdme (triethylene glycol methyl ether):dol(dioxolane) 1:1 (v:v). the separator was celgard 2400. the figure presents selected unused results from the study published in ref. [11] we have thus proposed to simplify the conventional li-s cell in three ways [11,12]: (i) we replaced the porous cathode with a flat glassy carbon, (ii) we removed the lithium anode and used another glassy carbon to form a symmetrical carbon-carbon cell, (iii) instead of using sulfur we used different polysulfides to study the various stages of charge-discharge process. typical results and a basic analysis are shown in figure 8. a comparison between the impedance spectrum shown in figure 8c and the spectra displayed in figure 7b shows several pronounced differences. firstly, the impedance spectrum in figure 8c consists of a much bigger high-frequency arc (several thousands ) as opposed to the much smaller (50-100 ) arc in conventional cell. jože moškon et al. j. electrochem. sci. eng. 10(2) (2020) 79-93 http://dx.doi.org/10.5599/jese.734 87 figure 8. a) schematic presentation of the flat symmetrical cell used to study the basic reaction-diffusion properties of various polysulfides; b) a typical reaction-diffusion scheme that could be studied in proposed cell (e.g. reduction of s42to s22-); c) measured impedance spectrum (red open circles) of reaction under (b) and the corresponding fit (blue circles) using the randles circuit shown in panel (d). impedance spectra of the symmetrical glassy carbon cells were measured at ocv with the same voltage amplitude (10 mv, rms). the low-frequency limit was varied and extended down to 0.1 mhz. adapted partial results from ref. [11] secondly, at low frequencies the spectrum in figure 8c consists of rather small arc that extends to poorly defined shape at lowest frequencies. analysis [11] showed that the latter feature was an artefact – most likely due to tendency of polysulfides to disproportionate. by contrast, the other features (big and small arc) could very well be fitted with the conventional randles circuit as demonstrated by the good match between blue and red circles. making many such experiments involving different polysulfides, we could determine quite accurately the main reaction (exchange current density, i0) and transport (chemical diffusion coefficient, dchem) parameters, as shown in detail in our previous paper [11,12]. once the basic intrinsic reaction and transport parameters of different polysulfides were known, we could move back to more complex electrode configuration. specifically, we retained the symmetrical cell configuration but replaced the flat electrodes with porous ones. we used porosity of different degrees in order to decouple the effect of porosity itself from the other electrode parameters. in this brief overview we only show selected results on unconventional porous electrodes consisting of a carbon felt with intermediate level of specific surface area (15 m2 g-1) (figure 9). room temperature impedance spectra of the cells were measured on vmp3 potentiostat/galvanostat (bio-logic) at ocv in the range from 1 mhz to 0.1 mhz (or 20 µhz) with 10 (5) points per decade and an amplitude of 10 mv (rms). the general impedance response of porous electrodes involving surface reaction and diffusion in pores (corresponding well to the situation in the present carbon-polysulfide system) has been derived analytically [13,14]. two typical impedance spectra predicted using the analytical expression are displayed in figure 9b. http://dx.doi.org/10.5599/jese.734 j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 88 figure 9. a) schematic of a carbon-carbon cell where porous carbon felt (15 m2 g-1) was used as a model porous electrode; b) theoretical impedance response of a porous battery electrode (with surface reaction) as predicted by the analytical model in ref. [14]; c) measured impedance response of symmetrical carboncarbon cell with polysulfides as active substance – low frequencies; d) measured impedance response of symmetrical carbon-carbon cell with polysulfides as active substance – high frequencies (adapted from ref. [15]); attempts to fit the spectra using ordinary voigt circuit (colored parts) were not entirely successful it can be seen that impedance spectra consist of a high frequency arc with more or less expressed 45-degree feature at the highest frequencies. at low frequencies there is an additional arc but typically with a smaller size. also, we can see that the size of both arcs depends on the electrode thickness: the size of hf arc decreases with thickness, while the size lf arc increases. comparing the theoretical predictions to the actual measurements we can see that hf arc follows quite well the predicted trends. however, there is a large mismatch between the prediction and the measured behavior of lf arc: firstly, it is much bigger (about 2 orders of magnitude) than the predicted arc and secondly, its size does not increase with electrode thickness. further analysis in which we systematically changed the concentration of catholyte solution, the type and thickness of separator etc. [15] revealed that in fact, the theoretically predicted low-frequency arc was masked by the response of catholyte-soaked separator. in other words, the diffusional processes occurring in the separator prevailed over those occurring in the porous cathode itself. as we could not find any model for this particular situation, we built-up our own model (figure 10) based on the well-known transmission line approach developed for porous electrochemical systems [16]. the validity of model was first checked for the case where the response of separator was neglected (set to zero). we found a complete correspondence between the model shown in figure 10 and the recent analytical solution [14]. then the response of separator was plugged in, and the predictions of the whole model compared to the actual measurements. typical simulations are displayed in figure 11. jože moškon et al. j. electrochem. sci. eng. 10(2) (2020) 79-93 http://dx.doi.org/10.5599/jese.734 89 figure 10. general transmission line scheme that takes into account diffusion in separator (right part, as denoted), diffusion in porous cathode (elements z1, z2, z3) and reaction in porous cathode (elements zl). electronic conductivity of carbon matrix (elements ze) is set to zero for simplicity (however, in principle could be of any value). more detailed meaning of all the elements can be found in our previous article [15]. figure 11. low (top) and high (bottom) frequency response of porous cathode in contact with separator as predicted by the model in figure 10. we can see that now the low-frequency response is very similar to the measurements (cf. figure 9 c,d) while, as expected, the good prediction of hf arc from the analytical solution is also retained. in fact, a more detailed analysis shows that the chemical capacity of lf arc (related to its peak frequency) is a sum of the values for the porous cathode and porous separator. this theoretical prediction was fully confirmed in all measurements on different cells carried out in this study. http://dx.doi.org/10.5599/jese.734 j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 90 metallic lithium anode unlike the sulfur cathode of the li-s system, the metallic lithium anode has been subject of intense investigation for more than 40 years [17,18]. in organic electrolytes, lithium gets spontaneously passivated by a surface layer that conducts li ions but is more or less non-conductive for electrons and hence its widely accepted name »solid electrolyte interface« (sei) [17-20]. however, the nature of sei as well as the surface of li anode in general is significantly modified during repetitive cycling [21,22]. various proposed mechanisms exist that describe li morphology obtained after certain number of cycling. here we assume and treat only the most general features that are common to many investigations: (i) continuous dissolution and re-deposition of lithium leads to a porous morphology of superficial lithium (called dendrites) as the dissolution-deposition process does not proceed uniformly across the surface; (ii) the porous li structures (dendrites) are passivated on their surface with sei-like film; (iii) there are at least two types of dendrites, the socalled »live dendrites« and »dead dendrites«; the former still contain a significant amount of metallic lithium, whereas the latter contain a small amount of electrically disconnected lithium or even no lithium but only sei-like products. due to the complex surface morphology of cycled lithium anode, it is not surprising that impedance spectra are also quite complex such as demonstrated in figure 12. in that experiment, a symmetrical li-li cell was cycled as indicated on the left part of the figure. the cells for this experiment were made according to the pouch technology using two lithium electrodes and celgard 2320 separator. figure 12. development of impedance spectra with the number of cycles (shown on top left) on symmetrical li-li cell. for clarity only selected cycle numbers are displayed. see development of unusual shapes at low frequencies representing the development of dendrites. the figure presents selected unused and/or adapted results from the study published in ref. [23] the electrolyte used was 20 µl of 1 m litfsi in tegdme:dol 1:1 (v:v). the electrochemical experiments on the cells were done with a combination of impedance spectroscopy measurements and stripping and deposition cycles on li||li symmetrical cells. the cells were connected to a vmp3 biologic potentiostat/galvanostat within 10 minutes of cell assembly. they were left to stabilize for 24 hours, during which impedance spectra were measured. after this initial period, li||li cell jože moškon et al. j. electrochem. sci. eng. 10(2) (2020) 79-93 http://dx.doi.org/10.5599/jese.734 91 underwent stripping and deposition at a constant current rate for 2 hours. the current for the discharge/charge cycles was either 0.2 ma (“low current cells”) or 1.0 ma (“high current cells”). after stripping and deposition of lithium, the cells were left to relax for 15 minutes, after which impedance spectra were measured. all of impedance spectroscopy measurements were done in the range of 1 mhz to 1 mhz with an amplitude voltage of 10 mv (rms). as seen from figure 12, during cycling the impedance response of the cell changes significantly. hf arc is gradually decreasing whereas lf arc transforms into an extended shape typical for cases where several processes closely overlap. in order to check the validity of such measurements (that potentially include some uncompensated dynamics and additionally is carried out to very low frequencies), we carried out a couple of standard testings such as (i) measurements at different values of excitation signal (testing the linearity of response), (ii) repetition of measurements at given steady state conditions (checking the time invariance of the system) and (iii) kramers-kronig analysis (for details see ref. [23]). based on the assumptions i-iii stated above we constructed a transmission line that captures the main known morphological features of li-li cell (figure 13). figure 13. top: transmission line model proposed for lithium anode containing dendrites. live and dead dendrites are considered separately as they have different basic properties (see main text). bottom: simulations using the proposed transmission line; different level of coupling between live dendrites, dead dendrites and separator are displayed; most measurements show a close coupling corresponding either to the dashed or solid graphs (compare with the shapes seen in figure 12). briefly, the main feature that generates the first and the second dotted arc are the porous live dendrites. it is quite remarkable that in fact a porous live lithium filled with electrolyte is conceptually almost identical to a porous carbon electrode containing polysulfides. this is due to the fact that in both cases the reaction proceeds on a conductive porous surface whereas at low http://dx.doi.org/10.5599/jese.734 j. electrochem. sci. eng. 10(2) (2020) 79-93 advances in understanding li battery mechanisms 92 frequencies, diffusion of active species inside the pores (the second arc) is the predominating process in both cases. however, the main difference is that the cycled lithium electrode also contains some dead dendrites where no surface reaction can take place so only diffusion across the pores is seen (see second compartment of scheme in figure 13). this part contributes to another, third arc which however is quite strongly coupled to the second one (because in both cases the main diffusional processes are similar). finally, at lowest frequencies (the third compartment in figure 13) the arc due to diffusion in separator occurs. as the thickness and porosity of the separator are similar as in live or dead lithium, this arc is also strongly coupled to both dendritic features. as a whole, this strong coupling gives an extended shape as indeed observed in the actual measurements (compare the simulations in figure 13 with some of the measured shapes in figure 12, especially the ones after 20th, 30th and 40th cycle where dendrites have already fully evolved). conclusions in this brief review we showed how impedance spectroscopy can be used for systematic investigation of selected mechanisms occurring in modern batteries. in the first part we focused on explanation of the main impedance features observed in well-known li ion insertion systems such as lithium iron phosphate (lfp) and lithium cobalt oxide (lco). then we demonstrated the use of impedance spectroscopy on a post lithium ion system, specifically lithium-sulfur battery. we showed that the complex response of the sulfur cathode can be better understood if the usual porous electrode morphology is replaced with a flat electrode and a symmetrical cell instead of asymmetrical is used. once the main transport-reaction mechanism has been identified, the flat electrode can be replaced by a porous in order to take into account the effect of porosity. there, one of the main issues is how to connect (couple) the response of a porous electrode with that of porous separator. we have managed to carry out this coupling by proposing an appropriate transmission line model. the latter can describe all the measured features of a porous carbon electrode in contact with polysulfides. finally, we extended the same basic concept of transmission line for porous electrodes to a cycled lithium electrode. namely, it appears that the dendrites formed during charge-discharge represent a conceptually similar framework for transmission line modeling as the porous carbon electrode with surface reactions (such as in the case of polysulfides). the proposed transmission line is able, for the first time, to quantitatively – and with physically sound elements reproduce the response of dendritic lithium. acknowledgements: this work was financially support by slovene research agency and through helis project (european union's horizon 2020 research and innovation program under grant agreement no. 666221). references [1] m. winter, r. j. brodd, chemical reviews 104 (2004) 4245–4270. 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[23] s. drvarič talian, j. bobnar, a. r. sinigoj, i. humar, m. gaberšček, journal of physical chemistry c, 123 (2019) 27997-28007. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.734 http://creativecommons.org/licenses/by/4.0/) {zdravko stoynov – the scientist who created new scientific horizons} http://dx.doi.org/10.5599/jese.775 65 j. electrochem. sci. eng. 10(2) (2020) 65-78; http://dx.doi.org/10.5599/jese.775 open access: issn 1847-9286 www.jese-online.org review zdravko stoynov – the scientist who created new scientific horizons daria vladikova institute of electrochemistry and energy systems, bulgarian academy of sciences, 10 acad. g. bonchev st., 1113 sofia, bulgaria corresponding author: d.vladikova@bas.bg received: january 7, 2020; accepted: january 14, 2020 abstract this paper is in the memory of the big bulgarian scientist professor zdravko stoynov. an original and non-standard personality with the spirit of a gentleman, he turned all that he touched into a masterpiece. his innovative thinking which made him one of the world’s leaders in electrochemical impedance spectroscopy, chiseled away at the scientific taboos and created new scientific horizons. zdravko stoynov left us an abundant heritage – his spirit, his interminable capacity to wonder, his developments and inventions – all that he has taught us and brought up with. it has no dimensions but rather carries a unique value. keywords electrochemical impedance spectroscopy; 4-d analysis; rotating fourier transforms; differential impedance analysis; differential resistance analysis. about zdravko stoynov zdravko stoynov left our world abruptly – engulfed in the ebullience of his latest series of advanced inventions. however, he left us an abundant heritage – his spirit, his interminable wonder and scientific curiosity which gave birth to remarkable developments and inventions which we must preserve, cherish, and augment. an original and non-standard personality with the spirit of a gentleman, he turned all that he touched into a masterpiece. his innovative thinking often made him feel like a scientific loner. some of his developments were highly evaluated only 15-20 years after their invention, while others are still waiting to be accepted and implemented. at the age of 24, zdravko stoynov builds the first in bulgaria analog computer to calculate differential equations to the 12th order. for over half a century right until his passing, he continues to create and develop algorithms & solutions, which he integrates into his nonstandard equipment, to unveil new key phenomena in electrochemical power sources – the primary zone of his professional interest. http://dx.doi.org/10.5599/jese.775 http://dx.doi.org/10.5599/jese.775 http://www.jese-online.org/ mailto:d.vladikova@bas.bg j. electrochem. sci. eng. 10(2) (2020) 65-78 zdravko stoynov 66 zdravko stoynov’s scientific achievements are not only numerous and significant, but also cross between different disciplines to redefine problems outside normal boundaries and reach solutions based on a new understanding of complex processes a novel approach in modern science which usually involves different teams of scientists from diverse backgrounds and fields. zdravko stoynov’s research combined electrochemistry with engineering, mathematics, computer sciences… he suited the equipment he created to his experimental goals – from the development of unique hardware and software to original methods for data analysis. for deeper insight into the underpotential prenucleation of ag on dislocation free surface of single crystal face which could be observed in potential range 2-6 mv, he constructed an ultra-sensitive potentiostat with a voltage range of 10 mv and precise control, stability and sensitivity of the potential, thus ensuring amplitude of the ac signal of 0.05 mv. his precise equipment and methodology were able to register impedance of a single dislocation. zdravko stoynov’s extensive inter-disciplinary knowledge and skills are reflected in his academic degrees: ph.d. in engineering (automation of the experiment) from the bulgarian academy of sciences (bas), doctor of technical sciences (technical cybernetics) from swiss federal institute of technology zurich, and doctor of chemical sciences from bas. 2010: zdravko stoynov in his cabinet as director of the institute of electrochemistry and energy systems, bas, sofia, bulgaria there is an unavoidable question put by many of zdravko stoynov’s colleagues: what is the source of such limitless scientific curiosity and inexhaustible constructive energy to define new topics and undertake seemingly insoluble problems, transforming them into fertile and attractive research fields? the answer can be found in the words of the renowned physical chemist ivan stransky: “a scientist must possess the capability to wonder. he has to see voids where others see nothing. ” these words are perfectly suitable for zdravko stoynov who managed to preserve and develop his childish curiosity and disregard for limits and taboos which, combined with exceptional intelligence and multidisciplinary knowledge accumulated during the years, made him an extraordinary scientist. the scientist zdravko stoynov graduated from the technical university of sofia in 1960 by constructing the first analog computer in bulgaria as his master’s thesis. in the next 2 years, he established the “computing and modeling laboratory” at the technical university and constructed a larger analog computer to calculate non-linear differential equations up to the 120th order [1]. at the same time, daria vladikova j. electrochem. sci. eng. 10(2) (2020) 65-78 http://dx.doi.org/10.5599/jese.775 67 he developed non-standard equipment for researchers at the bas. as a result, in 1963 he was introduced to evgeni budevski, who in that time feverishly worked with his team on growing a dislocation-free single crystal, an extremely sensitive object. if they succeeded, the first “perfect crystal” in the world would be produced, experimentally confirming the stransky-kaischew theory of two-dimensional nucleation. thus, zdravko stoynov became a part of a talented world-renowned research team on the verge of significant scientific discovery. in a matter of months, he considerably increased sensitivity, precision, and stability of the measuring equipment and although coming from a completely different scientific field, he started to participate actively in the scientific work. in a contribution about academician evgeni budevski’s life and scientific achievements [2], zdravko stoynov describes expressively the atmosphere, emotions, and dedication of the entire team during those exiting times. in 1965, thirty-three years after the publication of the stransky-kaischew theory, its experimental proof was finally obtained. first in the world, budevski’s team successfully grew the dislocation-free single crystal. this accomplishment has sparked extensive electrochemical studies in a new, experimentally unexplored system with completely different behaviour from systems with dislocations in the crystal lattice. zdravko stoynov is in the heart of this research, developing models for surface diffusion and monoatomic layer growth [3,4]. with such a strong scientific start, what to do next? the exceptional engineering and mathematical background give zdravko stoynov the unique independence to develop targeted task specific equipment. from then, he has the freedom to choose what scientific peak to overcome next, and he selected a difficult and little exploited, but yet exciting field, full of potential and enormous opportunities – electrochemical impedance spectroscopy (eis). in parallel, he continues to work in a pure engineering field automation of the scientific experiment. in 1967, the central laboratory of electrochemical power sources (cleps) was established, with director prof. evgeni budevski. the young experienced engineer zdravko stoynov, a head of the newly founded department of electrochemical methods, builds the needed equipment. thus, cleps laboratories started operating with automated powerful testing stations which could not be purchased on the market and were also implemented in the bulgarian lead acid battery plants. currently the equipment of department of electrochemical methods covers the range from 0.1 to 1000 a. zdravko stoynov’s computerized testing laboratory in cleps (1975) http://dx.doi.org/10.5599/jese.775 j. electrochem. sci. eng. 10(2) (2020) 65-78 zdravko stoynov 68 electrochemical impedance spectroscopy gradually, the development of non-standard original equipment for specialized electrochemical studies turned from a target into a tool, while the objectives shifted towards the invention of sophisticated experimental methods. zdravko stoynov started to develop innovative approaches and techniques for electrochemical studies and characterization of different systems. despite his interest and work in various scientific fields, his greatest and undying love remained to be electrochemical impedance. very soon he becomes one of the world leaders in eis which today is evaluated as a powerful method for thorough investigations of a wide scope of systems and processes. what has attracted zdravko stoynov to eis method? ... the unique information opportunities and numerous challenges that he tried to overcome during the course of over half a century to his last breath. nowadays, measuring electrochemical impedance with frequency response analyzers is the mainstream method used in thousands of laboratories. however, only the few know that zdravko stoynov implemented this equipment into electrochemical measurements after a visit to the solartron company in 1968. the same year, the first two frequency response analyzers were bought and introduced by prof. israel epelboin in his laboratory in paris (cnrs-upmc, université pierre et marie curie) and zdravko stoynov in cleps (bas). the next challenge was “impedance of batteries” – an experiment which correlates with both the applied and research activities of cleps, and which was considered impossible at that time. however, zdravko stoynov starts to systematically chisel away at the scientific taboos…. till his last day…. with never ending enthusiasm and dedication. for a start, he introduces measurements out of equilibrium. today this is customary, but 40 years ago it was unacceptable. he builds specialized equipment and with his colleagues at cnrs in paris measure, for the first time in the world, impedance of batteries– galvanostatically and quasi-stationary [5]. the year is 1977. today, 40 years later, every company for electrochemical equipment produces systems for impedance testing of batteries. obviously, science does not allow “taboos” and despite big and powerful expert teams that work today, it also needs pioneers. first publication on impedance of batteries, ref. [5] in 1977; 27 years later at the poemes workshop in bulgaria, the co-authors zdravko stoynov and hissasi takenouti continue to enthusiastically discuss impedance (2004). the next challenge comes from the non-steady state of batteries during impedance measurements. the solution is the development of the non-stationary (4-d) impedance analysis [6-13] which is quickly evaluated and adopted for testing of lithium-ion batteries by mitsubishi [14,15], and later for studies of other time evolving processes [16,17]. recently, 4-d impedance analysis was included in the software portfolio of bio-logic [18]. daria vladikova j. electrochem. sci. eng. 10(2) (2020) 65-78 http://dx.doi.org/10.5599/jese.775 69 zdravko stoynov and massimo schiavetti in front of stoynov’s prototype equipment for „in operando“ impedance testing of large batteries for accumulation of energy from solar cells (enelpisa, 2014). other innovative approaches the list of zdravko stoynov’s innovative approaches for electrochemical data analysis with enhanced information capabilities is long. differential coulometry spectroscopy (dcs) [19] is introduced in the analytical package of the software version v11.01 of bio-logic science instruments [20]. differential impedance analysis (dia) in which structural identification which does not require preliminary hypothesis, is applied for the first time in impedance data analysis [21-27]. rotating fourier transforms (rft) is initially developed during his doctorship in zurich [28,29] he concentrates on it again in his last and very creative year introducing new opportunities for impedance analysis at low and ultra-low frequencies where “many important and still hidden interesting phenomena can be measured precisely“ [30]. his goal is the development of a new, “4th generation” marketable impedance analyser, applying rft and mrft (multiple rft). he was expecting this to happen in the near future and was working hard to accelerate its coming. zdravko stoynov’s latest invention is differential resistance analysis based on original data processing from current-voltage curves. this method surprises and fascinates with simplicity and originality that provide over 10 times higher sensitivity when assessing degradation rate. this approach could eliminate some difficult and risky accelerated stress tests on batteries and fuel cells [31-33]. bio-logic science instruments (grenoble 2017): zdravko stoynov explaining visually his rotating fourier transforms http://dx.doi.org/10.5599/jese.775 j. electrochem. sci. eng. 10(2) (2020) 65-78 zdravko stoynov 70 international activities although engulfed in science, zdravko stoynov is no less valued as a “social scientist” with an acute sense of establishment, with excellent teaching skills, and with a neverending enthusiasm to educate and pass on his knowledge and know-how. he is one of the international impedance society founders a similarly minded enthusiastic group of colleagues fascinated by the potential of electrochemical impedance. for over 30 years, the society has been gathering within the framework of two international conference chains: • the international electrochemical impedance spectroscopy symposia (eis), dating from 1989, • the international electrochemical impedance analysis symposia (eia), dating from 1988. 10th international symposium on electrochemical impedance spectroscopy (eis), a toxa, galicia, spain (2016). lecture on rotating fourier transforms (zdravko stoynov’s last participation in eis) on a regional level, zdravko stoynov was one of the driving forces behind the establishment of the regional symposium of electrochemistry south-east europe (rse-see) in 2007, and acted as its co-organizer in bulgaria, 2015. the 7th symposium held in may 2019 in croatia was dedicated to his memory. starting in 1996, zdravko stoynov also establishes the national electrochemical event with international participation “sofia electrochemical days”. till his last day, zdravko stoynov continued to invigorate these scientific forums with his innovative and avantgarde ideas. recognition it is a quite rewarding experience to be able to reap what you have sown. zdravko stoynov could reap the respect and appreciation of his friends and colleagues from iees, bas, and all over the world, who joined together physically and virtually to celebrate his 80th birthday, turning it into a joyous and heartwarming event. the bulgarian academy of sciences honored his exceptional service with the special medal of the president of bas, crowning and completing his bas honors collection. the international electrochemical community and the bulgarian colleagues expressed their respect and gratitude to zdravko stoynov, who was named by his associate digby mcdonald from berkeley-california as “a giant in the electrochemical impedance spectroscopy”. the special issue of bulgarian chemical communications has been dedicated to his 80th anniversary and includes 30 publications by scientists from 13 countries. philosophers say that a man dies only when the memory of him is gone. the memory of zdravko stoynov will not fade. it will not be gone. it remains strong within the large international family of his students, colleagues, and friends from all over the world who honor, respect, and appreciate him. they named the electrochemical impedance analysis symposium and the internet center for daria vladikova j. electrochem. sci. eng. 10(2) (2020) 65-78 http://dx.doi.org/10.5599/jese.775 71 impedance spectroscopy after him. the “zdravko stoynov” prize in his honor will be awarded at each meeting of the eia to foster and promote creative out-of-the-box thinking in young researchers. the memory of zdravko stoynov is persistent, as are his deeds. all that he has sown and nurtured will continue to bear fruit into the future. “zdravko stoynov” young scientist prize awarding of “zdravko stoynov” prize to its first recipient at the 11th international symposium of electrochemial impedance anlysis (camogli, italy, nov. 2017) zdravko stoynov’s methods non-stationary (4-d) impedance analysis based on the classical method of the transfer function, electrochemical impedance spectroscopy gives a local, linearized and full (in a frequency aspect) description of the investigated electrochemical system, which is assumed to be in steady-state. however, batteries behave as big statistical systems with distributed parameters on a macro and micro scale. during cycling, processes of mass and energy transfer take place. they change the object’s structure and parameters. thus, batteries show nonsteady state behavior, which is the basic restriction for correct impedance studies. for further development of batteries, a liberation from the restriction of steady-state condition is generally needed. this motivated zdravko stoynov to develop the non-stationary (4-d) impedance analysis an approach that eliminates the delay errors during frequency sweep by introducing the time for each measurement as the fourth parameter in the experimental data set [6-9] (figure 1). figure 1. simulated complex-plane impedance diagrams of time evolving simple faradaic reaction deformed by measurement delay errors: a) iso-frequency dependence for the lowest frequency; b) fifth iso-frequency dependence (starting from the lowest frequency). reproduced from ref. 13 with permission from d. vladikova this approach improved significantly the impedance studies of batteries and other non-steady state systems [10-13]. zdravko stoynov approximates every iso-frequency dependence with a formal model and applies two orthogonal iso-frequency cubic splines for the real and the imaginary http://dx.doi.org/10.5599/jese.775 j. electrochem. sci. eng. 10(2) (2020) 65-78 zdravko stoynov 72 components, respectively. the interpolation (or extrapolation) for a given time, tj, and a set of frequencies ωi, gives the corresponding estimates of the real and imaginary parts. thus, the data of each time evolving diagram are reconstructed, and the corresponding impedance diagrams already represent the impedances measured virtually at the same time for all frequencies. 4-d impedance analysis was introduced in the battery testing software and successfully applied in li-ion battery research [14,15,18], and also for studies of some other time-evolving systems [16,17]. rotating fourier transform (rft) non-steady state condition of electrochemical systems during impedance measurements penetrates deeper, affecting the fourier transform (ft), which is the mathematical kernel of every impedance analyzer of our days. the ft is the best estimator of sinusoidal signals in steady-state conditions. however, out of those conditions, the ft produces specific errors dependent on the frequency. after an in-depth analysis of those errors, zdravko stoynov succeeds to develop theoretically an advanced generalized form of this transform – the rotating fourier transform (rft). this is the topic of his doctoral dissertation in the federal institute of technology – zurich [28,29]. the implementation of the rft in impedance spectroscopy provides for precise measurements of time-evolving systems out of steady state conditions. it opens a new horizon for studies of batteries and fuel cells, corrosion and numerous other systems, far beyond the usually applied frequency range where “4-d impedance method seems to be the only method to easily remove the effect of time-variance” [18]. during the last year of his life, zdravko stoynov refocuses on further theoretical and experimental development of his advanced mathematical rft tool, introduced 30 years ago. he describes it as a “powerful engine for non-stationary impedance spectroscopy which opens up the exploration of the low and infra-low frequencies where many important and interesting phenomena, still hidden, can be measured precisely”. expecting the development of new “4th generation” marketable impedance analysers through the application rft and mrft (multiple rft) in the near future, he labours over both the mathematical tool and his experimental verification (figure 2) in order to accelerate the coming of this “near future” and to be able to see it. an unedited version of his last unpublished work on this topic, written months before his passing, is presented in the proceedings of the 10th international symposium of electrochemical impedance analysis (section “letters to the editor”) [30]. his colleagues from iees are very much hopeful and will support the continuation of his efforts and the realization of his idea. figure 2. complex-plane impedance diagrams of li-ion battery 2000 mah during charge with 100 ma: (a) produced by conventional impedance analyser (solartron 1260) using fourier transform. frequency range 1 khz – 1 mhz, 5 points/decade, ac signal 11 ma; (b) produced by rotating fourier transform of the data from fig. 2a. frequency range 10 mhz to 1 mhz, 5 points/decade [30]. differential coulometry spectroscopy (dcs) battery testing is an important application-oriented tool for evaluation of their operational capability and performance as sources of energy and power. the typical testing results give general daria vladikova j. electrochem. sci. eng. 10(2) (2020) 65-78 http://dx.doi.org/10.5599/jese.775 73 information that represents the overall behaviour and thus does not support the analytical understanding of the processes taking place. to fill this gap, zdravko stoynov develops differential coulometry spectroscopy (dcs) [19], which extracts valuable information about the batteries internal kinetics, electrochemical design, and life degradation. the dcs technique requires measurements of the voltage changes with time followed by precise analysis with the technique of spectral transform (st) of aperiodic functions [24,26]. in order to separate the thermodynamic kinetics from the masking dynamic effect of the intrinsically connected transport limitations, very slow-rate galvanostatic testing (charge/discharge) is required. in principle, the galvanostatic mode of testing is a sweep coulometry and provides for the evaluation of the quantity of electricity necessary for the propagation of the investigated process or its steps. the voltage-time (u/t) dependence (figure 3a) of an electrochemical cell measured during charge (or discharge) can be easily converted into a voltage – quantity of electricity dependence. the final relation can be presented as capacity/voltage (c/u) applying the st tool (figure 3b). the coulometric peak analysis provides for capacity evaluation of the different charge or discharge steps. the mapping of the peaks position and coulometric capacity and the analysis of their time dependence can be used for evaluation of battery state of health (soh). the dcs can also provide valuable information about the degradation processes. it is useful tool for electrochemical studies of rechargeable batteries based on intercalation materials. recently, dcs was included in the analysis package of biologic science instruments’ ec-lab software version v11.01 [20]. a b figure 3. four volts li-ion sample with nominal capacity of 2200 mah: a) slow sweep voltage–time charge curve (h = 100 hours); b) capacity spectrum of the slow sweep voltage–time charge and discharge curves (h = 100 hours). reproduced from ref. 19 with permission from d. vladikova differential impedance analysis (dia) the introduction of differential impedance analysis (dia) method for electrochemical systems is one of the most advanced inventions of zdravko stoynov, in which for the first time the structural identification approach is applied in eis. as a transfer function, electrochemical impedance provides a local, linear, and complete description of the system under investigation. however, this information has to be extracted from the experimental data, since impedance does not directly measure the processes under study, i.e. the physical reality, but gives information about the response of the system on an external perturbation. the most commonly used approach for analysis of experimental data is the development of one or several working models followed by their validation. the model structure is chosen a priori, and thus the identification procedure is only parametric. the main efforts towards the improvement of the impedance data analysis are focused on the advancement of the identification procedure [33]. http://dx.doi.org/10.5599/jese.775 j. electrochem. sci. eng. 10(2) (2020) 65-78 zdravko stoynov 74 in technical cybernetics, the structural identification approach is applied for data analysis. it does not require an initial working hypothesis and provides both structural and parametric identification. zdravko stoynov develops the technique of differential impedance analysis (dia), introducing the structural identification approach in the frequency domain [21-27]. this is one of his biggest breakthroughs and example of his out-of-the-box thinking. dia procedure starts with the initial set of experimental data (angular frequency, real and imaginary components of the impedance), and thus can be applied to previously measured data. the kernel is the local scanning analysis, performed with the so-called local operating model (lom) with a simple structure consisting of a resistance r1 in series with a parallel connection of capacitance c and a second resistance r2. the effective time-constant t = rc is also introduced as a lom parameter. in contrast to the classical parametric identification performed over the entire frequency range which estimates the parameters’ vector as a preliminary chosen model structure, the identification of the lom parameters’ vector by local analysis is carried out within a narrow frequency range, regarded as an operating window. when the window width is a single frequency point, the solution is purely deterministic. the following steps describe the procedure of the structural and parametric identification: • scanning with the lom throughout the whole frequency range with a scanning window of a single frequency point; • parametric identification of the lom at every working frequency. since the number of the independent data available inside the observation window is smaller than the number of unknown parameters, the initial set of impedance data is extended with two additional terms – the derivatives of the real and imaginary components of the impedance with respect to the frequency. thus, the set of equations determining the lom parameters estimation becomes solvable; • frequency analysis of the lom parameters’ estimates (figure 4). figure 4. dia of a three-step reaction: (a) complex-plane impedance diagram; (b) temporal plots (c) spectral plots; d) time-constant spectra of motor oil mobil delvac mx 15w40: new (▀); after 10000 km. (▀). reproduced from ref. [26] with permission from d. vladikova the results are presented in so called temporal plots, which give the frequency dependence of the lom parameters estimates (figure 4b). if the lom parameters’ estimates are frequency independent in a given frequency region, the sub-model corresponding to this frequency segment daria vladikova j. electrochem. sci. eng. 10(2) (2020) 65-78 http://dx.doi.org/10.5599/jese.775 75 follows the lom structure, i.e. it is described with a parallel combination of capacitance and resistance, which determine a time-constant. thus, the number of the plateaus gives the number of the time-constants in the model (figure 4b). the results may be presented in the more illustrative spectral form by the spectral transform procedure which converts the plateaus into spectral peaks by accumulating similar values of the respective parameters (figure 4c) [24,26,27]. in principle the spectral transform is not simply a presentation of the same data in different coordinates. by its nature it is a type of regression analysis, filtering the statistical noises coming from the object itself and from the performed measurements, as well as the non-statistical noises. its high power of filtration is eliminating the possible wild points and the resulting “black” noise, since the presence of an outlier introduces a fuzzy low intensity line, located away from the spectral kernel of the assembly, representing the investigated phenomenon. the amplitude of the spectral line is proportional to the frequency range with similar values of the corresponding parameters estimates. the presence of frequency dependence in the temporal plots marks either the regions of mixing between two phenomena, or frequency-dependent behaviour. instead of spectral peaks, the spectra form fuzzy spectral “tails”. those segments are additionally examined by secondary dia [25-27]. the procedure, known as differential temporal analysis, includes the differentiation of the lom parameters’ estimates with respect to the log frequency. it recognizes frequency-dependent elements (cpe, warburg, bounded warburg etc.), as well as their presence in more complicated models. with dia, zdravko stoynov opened new horizons for the impedance spectroscopy, increasing its objectivity, reliability and information capabilities (figure 4d). the method is highly appreciated by the experts in the field of impedance spectroscopy and presented in recent monographs and surveys [35-38]. as an advanced development, dia gradually enters in the impedance analysis and is already successfully applied for studies of electrochemical energy sources and other systems [39-48]. differential resistance analysis zdravko stoynov’s final invention, developed in the last months of his life, is related to the most severe hurdles facing the deployment of batteries and fuel cells – lifetime and durability, and more precisely – their reliable evaluation.” the state-of-the-art degradation already requires long-term testing, which can take several years – a huge restriction to market deployment. for the moment, the problem-solving approach consists in the development of accelerated stress test (ast) procedures and protocols, which should shorten the testing time in conditions activating the same aging mechanisms as in non-accelerated testing. this is a critical moment, since the measured degradation should be transferred to the real-life behaviour of the tested system. thus, the selection of the acceleration parameters and the level of acceleration are of critical importance and may differ significantly between systems and applications. zdravko stoynov offers a new and original approach that decreases the testing time by increasing the sensitivity and accuracy of the standard monitoring and diagnostic tools, providing reliable quantitative information. for this purpose, he introduces new performance indicator with increased sensitivity in respect to degradation evaluation. it is based on analysis of the standard currentvoltage characteristics, which describe the interrelation between the power components – voltage and current. one effective direction which zdravko stoynov develops is the analysis of the differential resistance an approach which is not still effectively exploited. the form of the volt-ampere characteristics depends on the operating parameters and the degradation processes. under constant operating conditions, its changes are due to degradation. the next step is the introduction of an algorithm for their characteristic description. for this http://dx.doi.org/10.5599/jese.775 j. electrochem. sci. eng. 10(2) (2020) 65-78 zdravko stoynov 76 purpose, zdravko stoynov developed differential analysis of current-voltage curves (diva). the method operates with the differential resistance rd = du/di, i.e. with the derivative of the potential u with respect to the current i, which changes its value according to the working point, following the shape of the volt-ampere curve [31-33]. the analysis of the differential resistance (dra) as a function of the applied current rd = f (i) (figure 5a) works with derivatives, which increases the sensitivity in respect to the small deviations which should be recorded. the new functional dependence defines a new characteristic point for the cell performance the minimum of the differential resistance rd,min. in general, it reflects the state of health (soh) at constant operating conditions, since it is determined by the intrinsic properties of the system and not by the external conditions (for instance current load). it is supposed that for deeper insight, impedance measurements should be performed also in a working point corresponding to rd,min. figure 5. differential resistance analysis schematic representation: a) u-i curve (-) and r vs. i dependence (-); b) spectral transform the impedance of the investigated electrochemical system depends on the selected working point, i.e. measurements carried out in different working points are not comparable. usually when the degradation rate is under studies, they are performed at constant current corresponding to the one selected for the testing. however, the extraction of rd,min defines a working point which is constant in respect to the internal state of the system, i.e. to its state of health, characterized with the smallest internal differential resistance for the conditions of the i-u curve. in addition, due to the analysis of derivatives, the changes in rd,min will be observable much earlier than changes in other parameters, which are used for evaluation of soh. the introduction of a spectral transform procedure additionally increases the noise immunity and sensitivity of the method (figure 5b) [32,33]. the intensity of the individual spectral peak (a ω-1 cm-4) is proportional to the length of the current range with similar values of the differential resistance. the operation with derivatives increases the data noise. in order to provide high quality data, z. stoynov developed recommendations for current-voltage measurements when applying diva [49]. the differential resistance analysis, which uses the new characteristic indicator rd,min, is several times more sensitive than the classical approach for measurement of the degradation rate based on the voltage change over time at constant current [30-32]. figure 6 demonstrates the high selectivity and sensitivity of the method that makes visible changes in the system which are unnoticeable in the current-voltage characteristics and ensures their quantitative evaluation. the combination of differential resistance analysis with impedance measurements in selected characteristic working points can provide information about the origin of the dominating degradation processes. daria vladikova j. electrochem. sci. eng. 10(2) (2020) 65-78 http://dx.doi.org/10.5599/jese.775 77 d figure 6. differential resistance analysis of a solid oxide fuel cell: (a) i-u curve measured in the beginning of the testing and after 600 hours; b) spectral transform of the differential resistance rd. instead of conclusion the memory of zdravko stoynov is persistent, as are his methods. all that he has sown and nurtured will continue to bear fruit into the future. references [1] p. lazarova, annual of the national polytechnic museum 10 (1980) 149 (in bulgarian). [2] z. stoynov, w. obretenov, bulgarian chemical communications 49 c (2017) 42-50. [3] e. budevski, w. bostanov, t. vitanov, z. stoynov, a. kotzewa, r. kaischew, electrochimica acta 11 (1966) 1697. [4] e. budevski, w. bostanov, z. stoynov, a. kotzewa, r. kaischew, physica status solidi 13 (1966) 577-588. [5] m. keddam, z. stoynov, h. takenouti, journal of applied electrochemistry 7 (1977) 539-544. [6] z. stoynov, b. savova-stoynov, journal of electroanalytical chemistry 183 (1985) 133-144. [7] b. savova-stoynov, z. stoynov, electrochimica acta 37 (1992) 2353-2355. [8] z. stoynov, electrochimica аcta 38 (1993) 1919. [9] z. stoynov, b. savova, journal of electroanalytical chemistry 112 (1980) 157-161. [10] z. stoynov, in nato science series 3. high technology, c. julien, z. stoynov (eds.), kluwer academic publishers, 85, 2000 p.359. [11] z. stoynov, b. savova-stoynov, t. kossev, journal of power sources 30 (1990) 275-285. 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[42] d. vladikova, z. stoynov, a. barbucci, m. viviani, p. carpanese, g. raikova, in new developments in advanced functional ceramics, l. mitoseriu (ed.), kerala, india, 2007, ch. 16, p. 457. [43] a. kirchev, journal of power sources, 170 (2007) 495-512. [44] a. kirchev, journal of power sources, 179 (2008) 808-818. [45] a. kirchev, journal of power sources, 191 (2009) 82-90. [46] d. vladikova, z. stoynov, g. raikova, a. thorel, a. chesnaud, j. abreu, m. viviani, a. barbucci, s. presto [47] p. carpanese, electrochimica acta 56 (2011) 7955-7962. [48] d. vladikova, z. stoynov, a. chesnaud, a. thorel, m. viviani, a. barbucci, g. raikova, p. carpanese, m. krapchanska, e. mladenova, international journal of hydrogen energy, 39 (2014) 21561-21568. [49] z. stoynov, d. vladikova, b. burdin, in handbook of test procedures and protocols, chapter 3: endurance, procedures and protocols, http://www.durablepower.eu/index.php/handbook. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.1557/adv.2017.592 http://dx.doi.org/10.1557/adv.2017.592 http://www.durablepower.eu/index.php/handbook http://creativecommons.org/licenses/by/4.0/) {staircase voltammetry of dissolved redox couple in a thin layer twin electrode cell} http://dx.doi.org/10.5599/jese.713 11 j. electrochem. sci. eng. 10(1) (2020) 11-19; http://dx.doi.org/10.5599/jese.713 open access: issn 1847-9286 www.jese-online.org original scientific paper staircase voltammetry of dissolved redox couple in a thin layer twin electrode cell milivoj lovrić divkovićeva 13, zagreb 10090, croatia corresponding author: mlovric@irb.hr received: august 10, 2018; accepted: september 18, 2019 abstract a model of reversible and quasi-reversible electrode reaction of dissolved redox couple is developed for the staircase voltammetry and the twin electrode thin layer cell. it is assumed that the neutral molecule is oxidized to a monovalent cation. the calculations were performed for the absence of supporting electrolyte and for its various concentrations. the influence of migration of cations and of irω drop on the peak currents and peak potentials was investigated. also, the kinetically controlled electrode reactions were simulated. these reactions can be distinguished from the reactions influenced by the resistance in the solution. keywords steady state voltammetry; two electrodes cell; migration; theory introduction there is a considerable interest in the investigation of the influence of supporting electrolyte on the mass transfer [1-4] and the charge transfer [5] on microelectrodes [6-8] and rotating disk electrodes [9,10]. these measurements can be also performed in the thin layer cells [11-15], particularly in those that operate without a reference electrode [16-18]. these twin electrode cells are useful for the analysis of solutions of redox couples [19-21]. in this paper, a diffusion and migration in the micrometer-type cell are simulated for the staircase voltammetry that appears in the modern digital instrumentation [22-27]. the model an electrode reaction of a neutral compound and its cation in the twin electrode thin layer cell is investigated. red ↔ ox+ + e (1) http://dx.doi.org/10.5599/jese.713 http://dx.doi.org/10.5599/jese.713 http://www.jese-online.org/ mailto:mlovric@irb.hr j. electrochem. sci. eng. 10(1) (2019) 11-19 staircase voltammetry 12 initially, a solution contains equal concentrations of the compound and the salt of its cation. a certain inert electrolyte mx may be also present in the solution. the solvent is polar and supports the dissociation of ionic compounds oxa and mx. the model applies, for instance, to the mixture of ferrocene and ferrocenium tetrafluoroborate [28]. as the difference between the potentials of two electrodes increases, the first electrode acts as an anode and the second one as a cathode. at the anode the compound is oxidized and at the cathode the cation is reduced. the current depends on the concentrations of ionic species ox+, a-, m+ and x-. for the sake of simplicity, it is assumed that all ionic mobilities are equal: uion = fd / rt (2) in eq. (2), d is a common diffusion coefficient. the electric potential gradient in the film depends on the ohmic resistance:  / x = ir / l (3) where i is a current and l is the film thickness. the resistance is estimated using the relationship between the equivalent conductivity at infinite dilution and the concentration of ions: −=  1 1 -1 ω 0 dr s x (4) ( ) + −= + + ++ -2 * * * *ox m x /af d c c c c rt (5) where s is the electrode surface area and *yc is initial concentration of ions. also, an electroneutrality in the solution is assumed. if the electrode reaction (1) is fast and reversible, the diffusion and migration of electroactive species are defined by the following differential equations and the initial and boundary conditions: cred/ t = d2cred/ x2 (6) cox/ t = d2cox / x2 – uion( / x)(cox / x) (7) t = 0, 0  x  l: =* *ox redc c , e = 0 (8) t > 0, x=0: = −   * * 0 ox, =0 red, =0 1exp ( ) /x xc c f e e rt (9) e1 – e0 = e / 2 – ir / 2 (10) d(cred / x)x=0 = l / fs (11) d(cox / x)x=0 = l / fs + uion( / x)cox, x=0 (12) t > 0, x = l: cox, x=l = cred, x=l exp[f (e2 e0) / rt] (13) e2 e0 = -e / 2 + ir / 2 (14) d(cred / x)x=1 = l / fs (15) d(cox / x)x=l = l / fs + uion( / x)cox, x=l (16) e is a potential difference between two electrodes. equations (6) and (7) are solved by the finite difference method [29]. by the combination of conditions (9) – (12) a non-linear equation for current is obtained. it appears in the form: (a + bx) exp(dx) = p – qx – rx2 (17) all parameters are positive and a < p. so, the solution of this equation can be found numerically, as the cross section of the exponential curve and the parabola. the dimensionless current  is defined as a ratio of the current and the limiting current that appears under steady state conditions: m. lovrić j. electrochem. sci. eng. 10(1) (2019) 11-19 http://dx.doi.org/10.5599/jese.713 13 * ss red2 /i fsdc l= (18) in staircase voltammetry the scan rate is defined as a ratio of the potential increment and the duration of step:  = de /  (19) it was assumed that de = 1 mv if v  1 v/s and that  = 1 ms if v > 1 v/s. in the calculation the dimensionless diffusion coefficient dt / x2= 0.4 and the time increment t = 10-4 s were used. also, the distance between electrodes was divided in 100 space increments. results and discussion the relationship between dimensionless currents and the potential difference in the twin electrode thin layer cell in the absence of supporting electrolyte is shown in figure 1. all curves tend to the limit that indicates the establishment of steady state in the cell. at higher scan rates voltammograms exhibit maxima, while at lower scan rates they resemble polarographic wave. e / v fig. 1 dimensionless staircase voltammograms of electrode reaction (1). [mx]* = 0 and v / vs-1 = 2 (1), 1 (2), 0.5 (3), 0.2 (4) and 0.01 (5). these characteristics are caused by the development of concentration gradients that are shown in figure 2. at the potential difference of maximum, the diffusion layers are extended to the middle of cell, but they do not overlap. one can notice that the gradients of cation are lower than the gradients of compound, which is caused by the migration of cations. under steady state the gradient of compound is constant, and its concentration is defined by the equation: =*red red/ 2 /c c x l . the concentration of cation is given by the equation: = − −*ox ox/ 2 exp( / ) / ( 1)c c x l e (20) it is the solution of the differential equation: = − +*ox ox oxd / d 2 / /c x c l c l (21) and the boundary condition: = * ox ox 0 d l c x c l (22) 0.1 0.2 0.3 0.4 0.5 0.5 1 1.5 2  e / v 1 2 3 4 5  http://dx.doi.org/10.5599/jese.713 j. electrochem. sci. eng. 10(1) (2019) 11-19 staircase voltammetry 14 a x / l b x / l fig. 2 dimensionless concentrations of the reduced and oxidized components of electrode reaction (1). [mx]* = 0, v = 1 v/s and δe / v = 0.130 (a) and 1 (b). figure 3 shows logarithmic analyses of responses to low scan rates. if the step duration 𝜏 is so long that the steady state is established before the current is measured, the response is defined by the following equation:  = {exp(fe / 2rt) 1] / [1 + exp(fe / 2rt] (23) this equation can be transformed into the logarithmic form: e1 – e0 = 0.059 log [1 + ) / (1 )] (24) the straight line (1) in figure 3 satisfies eq. (24), but the curves (2) and (3) have lower slopes at the smallest potential difference. the curve (3) corresponds to the curve (4) in figure 1. one can notice that the latter current potential curve is steeper than the curve (5) in this figure. this is the reason of the lower slope in figure 3. 0.2 0.4 0.6 0.8 1 0.5 1 1.5 2 x / l c / c * cox cred 0.2 0.4 0.6 0.8 1 0.5 1 1.5 2 x / l c / c * cox cred c / c* c / c* m. lovrić j. electrochem. sci. eng. 10(1) (2019) 11-19 http://dx.doi.org/10.5599/jese.713 15 log [(1 + ) / (1 )] fig. 3. logarithmic analyses of the responses of electrode reaction (1). [mx]* = 0 and v / vs-1 = 0.01 (1), 0.1 (2) and 0.2 (3) under transient conditions the responses are characterized by maxima. figure 4a shows the relationship between the logarithms of peak current and scan rate. black circles can be connected by the straight line (1): log max = 0.425 log  + 0.238 (25) this result is in agreement with the observation that the maximum develops before the diffusion layers are overlapped. the potentials of maxima increase with the increasing scan rate, as can be seen in fig. 4b. the asymptote (1) is defined by the equation: emax / v = 0.144 log  + 0.086 (26) as the electrode reaction (1) is reversible, this relationship can be ascribed to the influence of ir drop. figure 5 shows voltammograms in the presence of various concentrations of supporting electrolyte. they differ in the properties of maxima. the peak current is the highest if [mx]* / *redc = 10 and it stagnates if this ratio is higher than 1000. the potential difference of maximum is 0.068 v if [mx]*/ *redc > 100. these changes can be explained by the diminished ir drop. it seems that the contribution of migration is significant if the concentration of supporting electrolyte is low, but it vanishes in highly concentrated electrolytes. these effects were investigated further by the variation of scan rate in fully supported systems. the results are presented in figure 4, as black squares. firstly, the peak potentials are independent of scan rate. their average value is 0.074 v, which is shown by the straight line (2) in figure 4b. furthermore, the peak currents can be approximated by the linear relationship (see the line (2) in fig. 4a): log max = 0.454 log  + 0.255 (27) this slope is closer to 0.5 than the slope 0.425 that appears in the absence of supporting electrolyte. the difference between these two slopes is ascribed to the ir drop. however, in the thin layer cell one cannot expect that the current depends on the square root of scan rate, as in the case of semi-infinite diffusion. these calculations show that the electrode reaction (1) is controlled only by the diffusion if the concentration of supporting electrolyte is thousand times higher than the concentration of the electroactive redox couple. this is in agreement with literature data [1,6,10]. 1 2 3 4 0.05 0.1 0.15 0.2 (e 1 e 0 ) / v log((1+ ) / (1)) 1 2 3 (e 1 e 0 ) / v http://dx.doi.org/10.5599/jese.713 j. electrochem. sci. eng. 10(1) (2019) 11-19 staircase voltammetry 16 a log ( / v s-1) b log ( / v s-1) fig. 4. dependence of the logarithm of the maximum current (a) and the potential difference of the maximum (b) on the logarithm of scan rate. [mx]*/ *redc = 0 (1) and 10 3 (2). the straight lines are linear approximations in the last part of this paper, the influence of the kinetics of electrode reaction is investigated. it is assumed that the supporting electrolyte is present in high concentration, so that the migration of electroactive cations can be neglected. under these conditions eqs. (9) and (13) are replaced by the well-known butler-volmer equations. some results are shown in figure 6. voltammograms depend on the normalized kinetic parameter =*s s( / ) / 100k k l , where ks is the standard rate constant of electrode reaction, and on the transfer coefficient α. if l = 50 m, the constant *sk = 10 -3 s-1 corresponds to ks = 5×10-4 cm s-1. the main difference between figure 6 and figure 1 is the inflexion point at 0.2 v in figure 6. all curves in figure 1 are convex between zero and the maximum, while in figure 6 the curves start as concave. -0.25 0 0.25 0.5 0.75 1 0.2 0.4 0.6 0.8 log  max log(v / vs-1) 1 2 -0.25 0 0.25 0.5 0.75 1 0.1 0.15 0.2  e max / v log(v / vs-1) 1 2 lo g  m a x  e m a x / v m. lovrić j. electrochem. sci. eng. 10(1) (2019) 11-19 http://dx.doi.org/10.5599/jese.713 17 e / v fig. 5. influence of the concentration of supporting electrolyte on the voltammograms of electrode reaction (1). v = 1 v/s and [mx]*/ *redc = 0 (1), 1 (----), 2 (-.-.-), 10 (-..-..-) and 5000 (5) e / v fig. 6. dimensionless staircase voltammograms of quasireversible electrode reaction (1) in a great excess of supporting electrolyte. *sk = 10 -3 s-1, α = 0.5 and  / vs-1 = 0.01 (1), 0.5 (2), 1 (3) and 2 (4) figure 7 shows that this change of curvature has the consequence on logarithmic analyses. they all tend to the straight line with the slope of 0.118 v for α = 0.5. however, below 0.1 v all curves have slopes higher than the limiting one. the effect of too high scan rate is added to the kinetic effect. the peak currents and peak potentials of voltammograms that are shown in figure 6 satisfy the following relationships: max = 0.47 log  + 0.10 (28) emax = 0.118 log  + 0.383 (29) the latter one can be used for the estimation of the transfer coefficient together with the logarithmic analysis. 0.1 0.2 0.3 0.4 0.5 0.5 1 1.5 2  e / v 1 5 0.2 0.4 0.6 0.8 1 0.5 1 1.5 2 1  e / v 2 3 4   http://dx.doi.org/10.5599/jese.713 j. electrochem. sci. eng. 10(1) (2019) 11-19 staircase voltammetry 18 log [1 + ) / (1 )] fig. 7. logarithmic analyses of responses of quasi-reversible electrode reaction (1). *sk = 10 -3 s1, α = 0.5 and  / vs-1 = 0.01 (1), 0.2 (2) and 0.4 (3). conclusions this simplified model shows that the staircase voltammetry can be applied to the electroactive redox couple in the twin electrode thin layer cell in the absence of supporting electrolyte. under this condition the response is influenced by an ir drop. at high scan rates the voltammograms of reversible electrode reaction exhibit maxima. the potentials at which these maxima appear depend linearly on the logarithm of scan rate. in the presence of supporting electrolyte in very high concentration, the potentials of maxima are independent of scan rates. this is an evidence of reversible charge transfer. presented calculation show that under steady state conditions the concentration profile of ionic product is not linear because of migration. all responses tend to the limiting current that is caused by these concentration profiles. if the electrode reaction is not reversible, but the ir drop can be neglected, the logarithmic analysis of the response recorded under steady state conditions is a curve that tends to the straight line with the slope equal to rt / αf. in the beginning of this curve, the slope is higher. the logarithmic analysis of the response of reversible electrode reaction is not a curve, but the straight line with the slope rt / f. references [1] n. p. c. stevens, a. m. bond, journal of electroanalytical chemistry 538 – 539 (2002) 25-33. 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[29] d. britz, j. strutwolf, digital simulation in electrochemistry, springer, berlin, germany, 2016. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.713 http://creativecommons.org/licenses/by/4.0/) {internal resistance and temperature change during over-discharge of lead-acid battery} doi:10.5599/jese.469 129 j. electrochem. sci. eng. 8(2) (2018) 129-139; doi: http://dx.doi.org/10.5599/jese.469 open access : : issn 1847-9286 www.jese-online.org original scientific paper internal resistance and temperature change during overdischarge of lead-acid battery balázs broda, györgy inzelt institute of chemistry, laboratory of electrochemistry and electroanalytical chemistry, eötvös loránd university, pázmány p. s. 1/a, h-1117 budapest, hungary corresponding authorse-mail: balazsbroda@gmail.com received: october 26, 2017; revised: january 29, 2018; accepted: january 29, 2018 abstract in recent decades, more and more electronic systems (start-stop, drive-by-wire, brakeby-wire) have been developed in the automotive industry therefore reliable power sources are necessary. it is essential to understand thoroughly the detailed behaviour of the battery to increase its efficiency, stability and monitorability which is the most popular field nowadays. over-discharge plays an important role in aging because it increases the probability of initiation of grid corrosion, sulfation and loss of active mass. in this work, the effects of over-discharge of lead-acid battery have been investigated via internal resistance increase and temperature change separately for both the negative and the positive electrode. most of the measurements were carried out in a prepared test cell (which contained a negative and a positive plate, an ag│ag2so4 reference electrode, a shunt for measuring current accurately) connected to a dummy battery and an electronic load. keywords energy storage; deep discharge; aging; battery sensor introduction despite of the fact that the lead-acid battery is a 150-year-old system, its over-discharge characteristic is a less investigated area. during this process, different reactions can occur which lead to the aging of the battery (for example sulfation, loss of active mass or loss of water) [1-4]. in the 21st century, lead-acid batteries are still used in most of cars for starting the gasoline or diesel engine and for the operation of the electronic system (drive-by-wire, brake-by wire, start-stop systems) too. in this case, reliable and well-monitorable power sources are necessary. for this purpose, better understanding of over-discharge is still necessary. at the end of the 20th century, garche et al. [5] estimated the basic reactions during overdischarge. the cell voltage, the potential of the positive and the negative electrode, the intensity http://dx.doi.org/10.5599/jese.469 http://www.jese-online.org/ mailto:balazsbroda@gmail.com j. electrochem. sci. eng. 8(2) (2018) 129-139 over-discharge of lead-acid battery 130 of gas evolution and the density of sulfuric acid at the top of the cell were measured. it was stated that the basic reaction during discharge proceeded to a lesser extent and was replaced by other reactions (for example increased gas evolution). in case of positive electrode, the capacity loss is mainly caused by mechanical stress, because of different molar volumes of the reactants (vlead-dioxide:vlead-sulfate:vlead=1.4:2.7:1.0). in case of negative electrode, the main reason of the aging is the irreversible oxidation of the expanders. this capacity loss was investigated by blank et al. [6] setting depth of discharge (dod) from 100 % to 130 %. it was stated that significant part of the initial capacity was lost because of overdischarge just after a few cycles. the amount of decreasing was closely related to dod level, at higher dod, the loss of capacity was also larger. it is also stated by the authors that the area of deep discharge has so far been mostly neglected in published research apart from fundamental material investigations [7]. in this paper, we present the effects of over-discharge on the internal resistance of the negative and the positive electrode and the whole cell and on the temperature of the electrolyte at different places of the cell (in the vicinity of the negative and the positive electrode and in the bulk solution). experimental two types of batteries were investigated. we used a furukawa hidash battery (type: flooded, voltage: 12 v, capacity: 48 ah) for the internal resistance measurements and a test cell which contains a negative and a positive plate of a dry unibat battery (type: flooded, voltage: 12 v, capacity: 9 ah) and 650 ml 37 m/m% sulfuric acid for temperature experiment. internal resistance measurement the potential of the positive and the negative electrode against ag│ag2so4 reference electrode and another cell voltage were measured by using labjack data acquisition device. we apply 4.8 a current [which is the c/10 current for a battery which capacity (c) is 48 ah] until it reduced to the 10 % of its initial value. during this procedure, we measured the impedance of the battery every 10 minutes at 1 khz excitation frequency which is good approximation to the battery's internal resistance (figure 1). the 1 khz-resistance r1khz is defined as the real part of the battery impedance at 1 khz frequency. this is a practical definition, since it is quickly measurable with an active excitation device. the standard handheld battery testers usually measure this value. [8] figure 1. measured impedance spectrum of the examined cell during discharge (nyquist plot) b. broda and g. inzelt j. electrochem. sci. eng. 8(2) (2018) 129-139 doi:10.5599/jese.459 131 temperature experiment an ag│ag2so4│saturated aqueous k2so4 solution (0.69 v vs. she) reference electrode and j-type thermocouples were used. the schematic representation of the experimental system is shown in figure 2. four thermocouples were used to measure the temperature at different places in the cell, their positions are presented in figure 3. we used a heat chamber to hold the external temperature constant (25 °c). figure 2. experimental system figure 3. temperature measurement the procedure is summarized in table 1. table 1. procedure of the measurements operation (details) circumstances (t = 25 °c) 1 preparation cell preparation – 2 immersion ucell > 2.1 v 3 pretreatment charge ilim = 0.16 a; ulim = 2.4 v; t = 2 h 4 discharge i = 0.16 a; t = 1 h 5 charge ilim = 0.16 a; ulim = 2.4 v; t = 2 h 6 discharge i = 0.16 a; t = 1 h 7 charge ilim = 0.16 a; ulim = 2.4 v; t = 2 h 8 discharge i = 0.16 a; t = 1 h 9 achieving full charge charge ilim = 0.16 a; ulim = 2.4 v; i < 0.016 a 10 delay – 11 capacity measurement discharge i = 0.16 a; ucell < 1.75 v 12 „over-discharge” setting dod (100·x%) discharge i = 0.16 a; t = tcapacity measurement · (x-1) 13 delay t = 1 h 14 „recharge” charge ilim = 0.16 a; ulim = 2.4 v; t = 16 h 15 delay – j. electrochem. sci. eng. 8(2) (2018) 129-139 over-discharge of lead-acid battery 132 in step 12, x can be 1.0, 1.1 and 1.2, which means that the dod level is 100 %, 110 % and 120 %. the duration of step 12 is the product of the duration of step 11 (tcapacity measurement) and x-1. results and discussion internal resistance measurement negative electrode in figure 4 (regarding to the end of step 11, step 12 and 13 and the beginning of step 14 of table 1), it can be seen that during the over-discharge (between ca. 3-8 h) the negative electrode’s internal resistance changed to nearly 100 times of its initial 2-3 mω value when a large change in the potential of the negative electrode occurred. when the potential has become practically constant (there is only a small decrease because of the uncertainty of potential measurement), the electrode's internal resistance has begun to decrease at a similar rate to the previous one and reached about 2 times of its initial value. the internal resistance of the negative electrode hasn’t changed until the end of discharge, but when the current has become zero (between ca. 8-9 h) a significant increase could be observed. this process lasted until the charging has been started. during recharging (between ca. 9-16 h), the internal resistance highly decreased and returned to its initial value after several minutes. figure 4. change of the potential of the negative electrode (multiplied by minus 1) and internal resistance during a discharge-charge cycle during discharge (between ca. 0-3 h), porous metallic lead is converted to lead sulfate at the negative electrode (reaction 1). pb + hso4− → pbso4 + h+ + 2 e− (reaction 1) the specific volume of pbso4 is approximately 3 times higher than the specific volume of pb and its conductivity is lower by orders of magnitude [9]. thus, the porosity decreases during this process as well as the pores become clogged (so the diffusion coefficient of the different species in the electrolyte decreases). at the same time, sulfuric acid reacts with lead, that’s why its concentration becomes smaller and the amount of acid required for the reaction isn’t available on the surface of the active material. as the result of these two processes, the potential of the negative electrode and the internal resistance highly increase. b. broda and g. inzelt j. electrochem. sci. eng. 8(2) (2018) 129-139 doi:10.5599/jese.459 133 the nernst equation provides an approximate calculation of the potential of the negative electrode. although the system is not in equilibrium during discharge, this relationship can be used to interpret the direction of the processes. + 4 4 4 4 pbso h pbso /pb pbso /pb pb hso ln 2 o a art e e f a a − = + (1) where epbso4/pb is the potential of the negative electrode, e o pbso4/pb is the standard electrode potential of the pbso4/pb system (-0.356 v vs. she), r is the gas constant, t is the temperature of the system, f is the faraday constant and apbso4, apb, ah+, ahso4are the activities of the given species. based on equation 1, if the lead ion or the sulfate ion activity changes on the electrochemically active surface of the electrode, the potential of the negative electrode will change too. the concentration (also the activity) of the sulfuric acid decreases continuously during discharge, due to this, the potential also gradually increases. the reason of the high change at the end of discharge is the decrease of both species’ activity. at more positive potential, other reactions can take place. the most significant reactions are the following: pbso4 + 2 h2o → pbo2 + hso4− + 3 h+ + 2 e− (reaction 2) 2 h2o → 4 h+ + o2 + 4 e− (reaction 3) in reaction 2 lead sulfate with high resistance and specific volume is converted to lead dioxide. due to this reaction, the electrode's internal resistance is decreased and the average size of the pores is increased. in reaction 3 (which is significant when the polarity reversal of the negative electrode takes place), oxygen gas evolves, which phenomena was observed by garche et al. [5]. on the one hand, these gas bubbles mix the electrolyte, so the inhomogeneities can be eliminated. on the other hand, a non-conductive layer of gas is formed on the active surface resulting in increased internal resistance. if all electrochemically active lead reacted due to the reaction 1, the potential changes (even the potential of the positive electrode can be reached) and the reactions 2 and 3 take place. based on this, the over-discharge of the negative electrode is very similar to the charge/over-charge of the positive electrode. during this process, the internal resistance of the electrode doesn’t change significantly until the end of over-discharge. after over-discharge, the system is left alone for 2 hours and synproportionation can occur (reaction 4), in which the remaining lead content of the active mass reacts with lead-dioxide generated during over-discharge (reaction 2). pb + pbo2 + 2 hso4− + 2 h+ → 2 pbso4 + 2 h2o (reaction 4) in this case, the concentration of sulfuric acid decreases, lead and lead dioxide with lower resistance and specific volume converted to lead sulfate with high resistance and specific volume. based on these phenomena, pores can become clogged and the internal resistance can significantly increase. during the recharge, both lead sulfate and lead dioxide can be converted into lead with better conductivity and lower specific volume. although the transformation of the whole amount of lead sulfate requires a long time, the internal resistance significantly decreases during ca. 10 minutes. this phenomenon can be explained with the opening of pores and higher sulfuric acid concentration inside them which resulted in low resistivity connectors to the grid. several hours later, the internal resistance reaches to its initial value. j. electrochem. sci. eng. 8(2) (2018) 129-139 over-discharge of lead-acid battery 134 positive electrode the observed phenomena during discharge of the positive electrode (figure 5) were similar to the previously mentioned ones. in figure 5, the end of step 11, step 12 and 13 and the beginning of step 14 of table 1 are shown. the internal resistance increased continuously as the cell lost its capacity (“normal” discharge between ca. 0-3 h) and when high decrease of the potential occurred, it increased by approximately two orders of magnitude compared to the initial value. during over-discharge (between ca. 3-8 h), it approached its initial value, and after that, when discharge was over (between ca. 8-9 h), we experienced some increase again. after starting the recharge (between ca. 9-16 h), it reached its initial value in about half an hour. figure 5. change of positive electrode’s potential and internal resistance during a dischargecharge cycle in this case, the potential of the positive electrode can also be estimated from the nernst equation. + 2 4 2 4 2 4 4 2 3 pbo h hso pbo /pbso pbo /pbso 2 pbso h o ln 2 o a a art e e f a a − = + (2) where epbo2/pbso4 is the potential of the positive electrode, e o pbo2/pbso4 is the standard electrode potential of the pbo2/pbso4 system (+1.685 v vs. she), r is the gas constant, t is the temperature of the system, f is the faraday constant and apbo2, apbso4, ah2o, ah+, ahso4are the activities of the given species. this case is a little bit more complex than the case of negative electrode. although, the reason of the potential’s monotone decrease is the sulfuric acid consumption, but the fall of potential can be caused by several processes: the decrease of activity of lead-dioxide (in extreme case when the battery is “fully” discharged, almost the whole surface of the active material is covered by leadsulfate, so the activity of the lead-dioxide must be less than one) or sulfuric acid or the increase of activity of water next to the electrochemically active surface (the activity of the water in such a concentrated electrolyte is less than one [10]). it can be stated that the potential doesn’t change dramatically when the over-discharge of the positive electrode completed. there may be two b. broda and g. inzelt j. electrochem. sci. eng. 8(2) (2018) 129-139 doi:10.5599/jese.459 135 different explanations: reaction 4 isn’t significant during over-discharge at all or it becomes significantly important before the process is over. it can be distinguished betweem the two possibilities by analyzing the results of the resistance measurement. looking at the change in internal resistance of the positive electrode, the phenomenon is similar to the case of the negative electrode. the initial increase is caused by clogged pores and high resistivity lead sulfate. the interpretation of sudden decrease in the potential is practically the same as mentioned before, but there is an important difference in the further part of overdischarge, the internal resistance begins to increase before the process is over. based on this, it is concluded that the reaction 4 becomes significant before over-discharge is over. cell in figure 6, the voltage of a whole cell is shown (regarding to the end of step 11, step 12 and 13 and the beginning of step 14 of table 1). besides the previously mentioned phenomena, interesting behavior could be observed after the cell reversal (between ca. 5-8 h) and after the over-discharge was finished (between ca. 8-10 h). the internal resistance of the cell didn’t decrease during the over-discharge period as much as shown in the previous sections. this phenomenon can be justified by the worse structural properties of the active material. if the pores inside the electrodes and the electrochemically active surface of them are smaller, the effects of the same processes can be much larger as it can be seen until the beginning of recharge (between ca. 5-10 h). after the end of discharge, the internal resistance increased just as in the case of the negative and positive electrode, but the rate was much higher than it can be expected, which also refers to the poor structural properties of the electrode. but after this region, the resistance decreased (between ca. 8-10 h). if we consider that the impedance measurement has an impact on the system, it can be explained by the discharge current which was applied in every 10 minutes for a short time resulting in a decrease of internal resistance (the pulsating curve also suggests this phenomenon). figure 6. change of cell’s voltage and internal resistance during a discharge-charge cycle based on this, it can be said that during the measurements when current flowed through the system, this phenomenon hadn’t caused an error, because the current was applied for a short time. it was only significant when neither discharge nor charge was executed. in the experiments j. electrochem. sci. eng. 8(2) (2018) 129-139 over-discharge of lead-acid battery 136 shown in figure 4 and figure 5, this phenomenon was negligible because the system was not in a labile state contrast to the measurement shown in figure 6. temperature measurement over-discharge in figure 7, it is shown that during the negative electrode polarity reversal, at first the value of that thermocouple changes (tnegative) which is located in the direct vicinity of the negative electrode and the other ones follow later. the temperature near the positive electrode (tpositive) and in the bulk electrolyte (telectrolyte) starts to increase 15 minutes after the negative electrode polarity reversal. according to our expectations, tnegative changed the biggest (0.4 °c), tpositive and telectrolyte increased by 0.3 °c. the phenomenon regarding to the positive electrode polarity reversal was essentially the same. figure 7. temperature change due to over-discharge (potential of negative and positive electrode, temperature in the vicinity of the negative and the positive electrode, in the electrolyte and in the heat chamber) these changes may not seem very significant at first sight, but if they are converted for the original cell, the effects are much higher. in table 2, the most important differences between the test cell and the original cell are summarized. table 2. parameters of the cells. test cell original cell amount of electrolyte, ml 650 75 number of negative electrodes 1 5 number of positive electrodes 1 4 measured capacity, ah ca. 3 ca. 8 based on the measured values by the thermocouple placed in the electrolyte, it can be assumed that the temperature of the whole cell has increased by more than 0.3 °c. during overdischarge if the same processes take place, the original cell generates about 3 times as much heat b. broda and g. inzelt j. electrochem. sci. eng. 8(2) (2018) 129-139 doi:10.5599/jese.459 137 as the test cell based on the capacity differences. the cell voltages are practically the same in these cases, but approximately 3 times higher charge goes through the original cell than the test cell. the generated heat can be calculated as the product of voltage and charge based on equation 5. we need to estimate the heat capacity of the electrodes. for this calculation, we used the following molar heat capacity values: 2 j/(mol·°c) for lead, 103 j/(mol·°c) for lead sulfate and 61 j/(mol·°c) for lead dioxide. the weight of the negative electrode is approximately 40 g of which the lead grid is 12 g and the active mass is the remaining 30 g (we suppose that it contains mostly lead at the beginning of the measurement). the positive electrode’s weight is approximately 50 g, of which the lead grid is 12 g and the active mass is 38 g (we suppose that it is mostly lead dioxide at the beginning). by the time the cell was discharged, approximately 3 ah of charge was passed through it. according to the faraday law, it is possible to calculate how much active mass was converted during discharge procedure (table 3, table 4). table 3. transformation of active material and sulfuric acid in the test cell during discharge negative electrode positive electrode electrolyte compound lead lead sulfate lead dioxide lead sulfate sulfuric acid water mbeg / g 28 0 38 0 308 524 mtrans / g -12 +17 -13 +17 -11 +2 mend /g 16 17 25 17 297 526 we used the following data for the calculation: ρ (37 m/m% sulfuric acid) = 1.28 g/cm3; m(pb) = 207.2 g/mol; m(pbso4) = 303.3 g/mol; m(pbo2) = 239.2 g/mol; m(h2so4) = 98.1 g/mol; m(h2o) = 18.0 g/mol; f = 96500 c/mol. table 4. transformation of active material and sulfuric acid in the original cell during discharge. negative electrode positive electrode electrolyte compound lead lead sulfate lead dioxide compound lead lead sulfate mbeg / g 140 0 152 0 36 61 mtrans / g -31 +45 -36 +45 -29 +5 mend /g 109 45 116 45 6 66 based on the measured capacity of the original cell, it is stated that approximately 8 ah charges could pass through the cell until reaching the fully discharged state. at that time, the mass fraction of the sulfuric acid in the test cell was 36 m/m% [heat capacity=2.9 j/(g·°c)] and it was 9 m/m% in the original cell [heat capacity=3.9 j/g·°c). [11] based on table 3, table 4 and the above-mentioned data, the heat capacity of the cells can be calculated (active mass+grid+electrolyte): ctest cell=2410 j/°c, coriginal cell=374 j/°c. we can make a calculation to estimate the generated heat (qtest cell) in the test cell: qtest cell = ctest cell ttest cell = 2410 j / oc × 0.3oc = 723 j  qoriginal cell/3 (3) where δttest cell is the temperature change during the negative electrode polarity reversal, qorginal cell is the generated heat in the original cell due to the previously mentioned effect. j. electrochem. sci. eng. 8(2) (2018) 129-139 over-discharge of lead-acid battery 138 based on this, we can roughly calculate how many degrees of celsius the original cell will warm with the same current density and process: o original cell original cell o 732 j 3 5.8 c j 364 c t q   =  = (4) the temperature in the original cell would increase by about 6 °c due to the electrode polarity reversal. however, it is important to note that this will be the average value in the whole cell, so inside the electrode, the temperature can rise significantly higher which has a very significant role in aging processes. [12] it can be assumed that not the different reactions cause the temperature rise, because the system cools down later and it approaches its initial state in the vicinity of the electrodes’ surface, although the reactions don’t change significantly at that time. however, the high resistance increase can cause this phenomenon. the initial resistance of the cell was 0.02 ω, the current (i) during the whole procedure was 0.16 a, the duration (t) of the temperature rise (also the duration of the high resistance increasing) was approximately 1 hour. the generated heat (q) can be estimated from equation 5. q = p t = ucell i t = r i2 t (5) where p is the electrical power, ucell is the cell voltage and r is the resultant resistance. we can calculate the resultant resistance from equation 6. 2 2 723 j 8 (0.16 a) 3600 q r i t s = =    (6) although the calculation contains many approximations and estimates, it shows that the resultant resistance, as in the previous section, increased by more than two orders of magnitude during over-discharge process (when the potential of the electrode and the temperature greatly changed at the same time). “recharge” the recharging of an over-discharged cell is shown in figure 8. figure 8. temperature change due to “recharge” (potential of negative and positive electrode, temperature in the vicinity of the negative and the positive electrode, in the electrolyte and in the heat chamber) b. broda and g. inzelt j. electrochem. sci. eng. 8(2) (2018) 129-139 doi:10.5599/jese.459 139 during the recharging, temperature rise was also experienced which was similar to the previously described one (during over-discharge) and the heat capacity of the cell was practically the same. based on these results, we can state that the internal resistance of the examined cell increased by several orders of magnitude compared to its initial value during the recharging. conclusions by investigating a lead-acid test cell, it was proved that the internal resistance highly increased during the cell reversal. this process and the increasing gas evolution caused significant temperature to rise inside the electrode undergoing the reversal and consequently in the whole cell. this is a very reproducible phenomenon which has a serious influence on aging specifically on the loss of capacity because there is high temperature increase and gas flow at the same time. acknowledgements: financial and spiritual support from the bolyai collage is gratefully acknowledged. references [1] p. ruetschi, journal of power sources 127 (2004) 33-44. [2] t. b. reddy, linden’s handbook of batteries, elsevir, new york, usa, 2002 [3] j. garche, e. karden, p. t. moseley, d. a. j. rand, lead-acid batteries for future automobiles, elsevir, amsterdam, netherland, 2017 [4] l. g. mallison, ageing studies and lifetime extension of materials, springer, oxford, united kingdom, 2001 [5] j. garche, a. jossen, h. doring, journal of power sources 67 (1997) 201-212. [6] t. blank, j. badeda, j. kowal, d. u. sauer, ieee conference, deep discharge behavior of lead-acid batteries and modeling of stationary battery energy storage systems, scottsdale, usa, 2012, p. 1-4. [7] d. pavlov, lead-acid batteries: science and technology, elsevier, oxford, united kingdom, 2011, p. 178. [8] g. bárány, lead-acid battery state detection for automotive electrical energy management, phd dissertation, budapest university of technology and economics, 2015, p. 70-81. [9] h. bode, lead-acid-batteries, john wiley & sons, new york, united states, 1977 [10] k. r. bullock, journal of power sources 35 (1991) 197-223. [11] j. e. kunzler, w. f. giauque, journal of the american chemical society 74 (1952) 3472-3476. [12] a. jossen, h. karl, g. lehner, f. hummel, 9. symposium photovoltaische solarenergie, betriebserfahrungen mit unterschiedliechen bleibatterien, bad staffelstein, germany, 1994, p. 271. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {gas sensing technologies in combustion: a comprehensive review} http://dx.doi.org/10.5599/jese.743 103 j. electrochem. sci. eng. 10(2) (2020) 103-110; http://dx.doi.org/10.5599/jese.743 open access: issn 1847-9286 www.jese-online.org review gas sensing technologies in combustion: a comprehensive review ulrich guth1,, pavel shuk2, chad mcguire2 1technische universität dresden, d-01062 dresden, germany 2rosemount measurement and analytical, emerson electric corp, shakopee, mn 55379, usa corresponding author: ulrich.guth@chemie.tu-dresden.de; tel.: +49 351 46337597; fax: +49 351 46337164 received: october 31, 2020; revised january 9, 2020; accepted: january 20, 2020 abstract irrespective of the change in kind of energy to renewables there are many processes in which combustion of conventional fuels and biofuel are necessary. for cement and glass production, paper fabrication and air traffic it is absolutely essential to control the combustion processes by intelligent sensors in order to maximize the efficiency and to minimize the emission of harmful substances such as nox and co. mainly oxygen sensors based on solid electrolytes and calorimetric sensors using the heat formation by catalytic combustion of co and hydrocarbons with two resistance temperature detectors (rtd) are utilized. recently, tunable diode laser spectroscopy (tdls) became attractive in chemical plants. several analytical companies are offering in-situ or extractive laser analyzers for combustion gases. keywords combustion control; in situ sensors for oxygen and hydrocarbons; solid electrolyte sensors; calorimetric sensors; tunable diode laser spectroscopy. combustion process the combustion of fuel is a complex process which has to be carried out in a small operation window concerning the air/fuel ratio. by reasons of economy as well as restriction of law the main components as oxygen and unburnt species (co and hydrocarbons) have to be measured in situ to gain real-time information. nox as the main pollutant must be measured at the end of exhaust to ensure the regulations by law. an overview on the concentrations of combustion flue gases and the small operation window is given in figures 1 and 2. due to the high viscosity of flue gases the concentration of oxygen and co is local-dependent (stratification) so that several sensors have to be installed to obtain an average value of oxygen and combustibles (figure 3). http://dx.doi.org/10.5599/jese.743 http://dx.doi.org/10.5599/jese.743 http://www.jese-online.org/ mailto:ulrich.guth@chemie.tu-dresden.de j. electrochem. sci. eng. 10(2) (2020) 103-110 gas sensing technologies in combustion 104 figure 1. combustion flue gas diagram and a need to measure o2, co, ch4 [1] figure 2. active combustion control (after [2]) there are several technologies for combustion process gases monitoring (figure 4). only few of them, such as electrochemical high temperature sensors, calorimetric sensors and laser techniques, are suitable to measure in situ across stack application. the last one is expensive as compared with solid electrolyte sensors and it needs some effort for maintenance. figure 3. combustion process regulation: flue gas stratification (o2 content, %) [3] figure 4. sensor principles for combustion control solid electrolyte sensors electrochemical oxygen sensors based on zirconia solid electrolytes, which are widely used today in different industrial applications and for controlling fuel/air ratio in internal combustion engines (lambda sensor), were first introduced 60 years ago by peters and möbius (germany) [4] and weissbart and ruka (westinghouse, usa) [5]. an overview on the basic principles of solid electrolyte sensors is given in figure 5 [6]. common to all these sensors is the high operation temperature which is necessary to ensure high electrochemical kinetics for the oxygen exchange within the electrodes and sufficient conductivity of the solid electrolyte. on the other hand, it allows a high operation temperature to be measured in hot gases such as in or near the combustion chamber. most industrial zirconia oxygen sensors are based on electrochemical cells with stabilized zirconia solid electrolytes operating in potentiometric mode [7-12]. this electrochemical cell is made of zirconia solid electrolyte (mostly yttrium stabilized zirconia, ysz) and two platinum electrodes printed and sintered on the opposite sides of a dense zirconia ceramics and exposed to the process and reference gases, respectively. gas-tight sintered ceramics is formed as tubes, discs, planar substrates or thick films. zirconia sensors are supplied as steel or ceramic probes up to 4 m in length. 0.0 5.0 10 15 20 25 30 -5 0 5 10 g a s c o n c e n tr a ti o n ( % ) oxygen concentration (%) n 2 co 2 co o 2 % deficiency % excess h 2 o current combustion control with o 2 : 2...6% o 2 in 0.05...0.5% so 2 10...18% co 2 10…15% h 2 o 2…20g/m 3 dust co mbustion control with o 2 & co <2% o 2 , <200ppm co ch 4 might be present if fuel wasn't ignited ulrich guth et al. j. electrochem. sci. eng. 10(2) (2020) 103-110 http://dx.doi.org/10.5599/jese.743 105 the service life depends on application temperature and loads of pollutants and can be up to 510 years. several producers from us (emerson, ametec), japan (yokogawa) and germany (enotec, zirox, stg) are on the market. ysz based oxygen sensors can be applied not only for optimization of combustion, but also to measure in processes in which reducing or oxidizing gases are necessary in order to ensure a certain oxidation state. that is important for the production of ceramics, porcelain, bricks. figure 5. basic working principles of solid electrolyte sensors for oxygen containing gases. the equations refer to the measured signals [6] oxidizing and reducing states differ in nernst’s voltage (figure 6). at the lambda-leap (λ = 1) the equilibrium potential changes abruptly. in the 1960s and 1970s the limits of measurement in terms of temperature and oxygen partial pressure have been determined [7]. sometimes the load of the flue gas with sulfur cause problems regarding the poisoning of electrode material [13]. hence, nowadays there are only few research activities in this field [14]. for automotive applications, lambda probes (oxygen sensors) are based on the potentiometric or combined potentiometric and amperometric dual sensing technologies [8,9]. such sensors are produced in millions of pieces by thick film techniques in a co-firing process. recently, mixed potential sensors for co and hydrocarbons were developed [15,16] and are commercially available in germany by lamtec and enotec. the components in flue gas like co, hydrocarbons (hxcy), nitric oxides (nox) and residual oxygen (o2) to be measured do not equilibrate at temperatures < 700 °c in the vicinity of the hot electrode. thus, gas components which are not thermodynamically stable are electrochemically active. in hydrocarbons and oxygen containing atmosphere at least two electrode reactions can take place: oxygen electrochemical reduction and electrochemical oxidation of hydrogen or other combustibles (figure 7). the measured open circuit voltage, also called mixed potential, does not obey the nernst equation but depends logarithmically on the concentration of co, methane or hydrocarbons. low catalytic activity materials such as gold or gold platinum alloys are usually employed for these sensors combined with catalytically active pt-electrode. in many cases composite materials consisting of gold and oxides show a high sensitivity to co as can be seen in figure 8. furthermore, complex perovskite-type oxides are very promising sensitive materials (figure 9). http://dx.doi.org/10.5599/jese.743 j. electrochem. sci. eng. 10(2) (2020) 103-110 gas sensing technologies in combustion 106 figure 6. measurement in reducing and oxidizing gases figure 7. comparison of mechanism between an equilibrium electrode (left) and a mixed potential electrode (right) it was established that beside the morphology, the electronic and ionic defects in such substituted oxides play an important role for sensitivity and selectivity optimization and trimming [17-19]. the main drawback of all mixed potential sensors is their quite high cross-sensitivity to sulfur dioxide as well as long term stability in severe combustion flue gas environment [1]. often carbon monoxide equivalent expression “coe” is applied to these sensors meaning co concentration response used for the calibration. figure 8. composite materials sensitivity to co [15] figure 9. sensitivity of different electrode materials to combustibles [17] calorimetric catalytic sensors for combustibles (co, ch4) in sensors using calorimetric effects by catalytic turnover of combustibles such as co or methane, two resistance temperature detectors (rtd) are compared. on of this is covered with catalytically active layer, e.g., platinum. the catalyst film is merged with rtd using highly thermally conductive packaging oxide materials to ensure more efficiently heat transfer of the combustion reaction on catalyst film to rtd. the surface of the other one is catalytically inactive. the reaction heat of the active resistor leads to an increase of temperature linearly proportional to the concentration of combustibles [20,21]. 0 0.2 0.4 0.6 0.8 1 1.2 l a 0 .9 8 s r 0 .0 2 c r 0 .9 g a 0 .1 o 3 - s e n si ti v it y ( m v /p p m ) l a 0 .9 9 5 s r 0 .0 0 5 c r 0 .8 g a 0 .2 o 3 - l a 0 .9 5 s r 0 .0 2 c r 0 .7 g a 0 .3 o 3 - l a 0 .9 5 s r 0 .0 2 c r 0 .8 g a 0 .2 o 3 - l a 0 .9 5 s r 0 .0 2 c r 0 .9 g a 0 .1 o 3 - l a c r 0 .8 g a 0 .2 o 3 - l a c r 0 .9 g a 0 .1 o 3 - a u 0 .8 (g a 2 o 3 ) 0 .2 a u 0 .8 (i n 2 o 3 ) 0 .2 a u 0 .8 (n b 2 o 5 ) 0 .2 electrode materials co sensitivity 600oc, 3% o 2 + 10ppm co + n 2 0 0.5 1 1.5 2 2.5 3 a b c d e f g h i electrode material d u /d m v /p p m  co propane propene toluene a = la0.995sr0.005cr0.8ga0.2o3- b = la0.98sr0.02cr0.9ga0.1o3- c = la0.95sr0.02cr0.7ga0.3o3- d = la0.95sr0.02cr0.8ga0.2o3- e = la0.95sr0.02cr0.9ga0.1o3- f = lacr 0.8ga0.2o3- g = lacr0.9ga0.1o3- h = au/20%in2o3 i = au/20%nb2o5 8.5 ulrich guth et al. j. electrochem. sci. eng. 10(2) (2020) 103-110 http://dx.doi.org/10.5599/jese.743 107 oxidation reaction occurs between combustible species and oxygen with defined heat released is depending highly on the combustible concentration. calorimetric catalytic co-sensor can measure co-concentration down to ppm level with detection limit 5…10 ppm co. this technology is permitting fast and precise (figure 10) <±25 ppm co or 5 % of the reading measurements in flue gas up to 1750 °c temperature. 0 200 400 600 800 1000 0 10 20 30 40 rosemount calorimetric co sensor stability: 0 ppm ± 5 ppm 500 ppm ± 4 ppm 1000 ppm ± 6 ppm ~35 s for 90% response [c o ] (p p m ) t (min) 200 400 600 800 1000 ppm co 125 500 00 0 5 10 15 20 25 30 0.0 0.5 1.0 1.5 2.0 2.5 up down e ( m v ) [co] (%) co sensor linearity ( >0.999) with ~0.0012 mv/ppm sensitivity 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 200 400 600 800 1000 [co] (ppm) figure 10. calorimetric catalytic co-sensor response at low co concentration [1] figure 11. calorimetric co-sensor linearity [1] calorimetric co-sensor has good >99.9 % linearity (figure 11) and reproducibility (figure 12) in a wide co concentration range up to 2.5 % co but to improve accuracy a calibration would be always recommended using upper limit of co concentration measurement range. calorimetric catalytic cosensor is highly flow-sensitive and gas flow stabilization and a flow sensor should be implemented to improve co-sensor reliability in extractive combustion analyzer [1]. 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 40 80 120 160 [c o ] (% ) t (min) 0.5% 1.0% 2.0% 2.5% co 0 2.5% co figure 12. calorimetric catalytic co-sensor response at high co concentration conventional catalysts for calorimetric methane sensors and combustion reaction are based on noble metals, i.e., palladium (pd) or platinum (pt) deposited on oxides such as cerium oxide (ceo2) or metals. a calorimetric catalytic ch4 sensor has a good linearity (figure 13) and 30 s response for 90 % signal (figure 14). flue gas flow control extractive environment would contribute to ch4 http://dx.doi.org/10.5599/jese.743 j. electrochem. sci. eng. 10(2) (2020) 103-110 gas sensing technologies in combustion 108 detection reliability with better accuracy and reproducibility in the very challenging temperature, pressure, flow and chemicals flue gas combustion environment [20]. only two available major methane gas sensing technologies – catalytic calorimetric and tdls/qcls – have so far found practical application in instrumentation on market because they produce quite reliable and accurate (about 2...5 % error) values in the extractive or across the stack mode. 0 1 2 3 4 5 6 0 1 2 3 4 5 6 calorimetric ch 4 -sensor linearity: 0.9995 [c h 4 ] (% ), m e a su r e d [ch 4 ] (%), applied figure 13. calorimetric catalytic ch4-sensor linearity [20] figure 14. calorimetric catalytic ch4-sensor response [20] tunable diode laser spectroscopy (tdls) for co, ch4 and oxygen measurements tunable diode laser (tdl) and quantum cascade laser (qcl) spectroscopies are innovative optical measurement techniques utilizing lasers with different wavelength (figure 15), e.g., tunable diode and quantum cascade [22-27]. tdl and qcl spectroscopies are highly distinguished from the conventional process photometry with the laser’s ability to be scanned across the narrow combustion species absorption peaks many times per second by trimming the current through the laser. with a typical scan in the range of 0.2 to 0.3 nm, the laser would provide much higher selectivity in applications. the ir absorption spectrum of oxygen (o2), carbon monoxide (co) and methane (ch4) is like a fingerprint, providing gas identification at different detection wavelengths (see figure 16 for o2). figure 15. ir spectrum for different gases [27] figure 16. oxygen absorption a-band [28] the detection wavelengths and limits of some combustion gases are summarized in table 1. a series of process adaption application tools were developed for in-situ, extractive or in crosspipe installations. the probe design would not require a special optical path alignment but would deliver single point measurement. in the probe design the laser source and detector are located in the probe housing with the laser beam being reflected at the probe end, back to the detector by 0 20 40 60 80 100 0 10 20 30 40 50 60 70 s ig n a l (% ) t (s) 5% ch 4 0% calorimetric sensor response with t 90 ~30s 0.5 1 1.5 2 2.5 0 2 4 6 8 10 ir wavelength (m) o 2 0.76m h 2 o 1.39m co 1.57m no 1.80m co 2 1.96m co 2.33m ch 4 1.65m ulrich guth et al. j. electrochem. sci. eng. 10(2) (2020) 103-110 http://dx.doi.org/10.5599/jese.743 109 quartz prism, with max operation temperature of <250 °c or by gold mirror with max operation temperature of <430 °c [20]. table 1. combustion gases absorption lines and detection limits [20] target gas wavelength (µ) detection limit (ppm) methane (ch4) 1.65 (tdl) 7.9 (qcl) 0.15 0.003 carbon monoxide (co) 1.57 2.33 4.80 30 0.5 0.01 oxygen 0.76 8.956 carbon dioxide (co2) 1.96 3 nitric oxide (no) 1.8 2.65 60 1 nitrogen dioxide (no2) 0.68 0.3 water (h2o) 1.39 0.06 with a unique folded-path design, the probes can be installed in almost all pipes and stacks with no alignment needed. the tdl or qcl analyzer can serve as a probe using deflection mirror or quartz prism (figure 17) for single point near real-time measurement or would provide extractive measurement across the duct or pipe (figure 18). temperature and pressure variation in the process has to be compensated and might cause an additional error in the measurements. ir light reflection at high temperatures, combined with wide background radiation from the fire box and process windows fouling, might bring additional challenges in many applications. there are quite accurate tdl analyzers with ±1-2 % error and ppm or even sub-ppm detection limits in some applications. figure 17. in-situ probe, 250-430°c limit (au-mirror or quartz prism) figure 18. across the stack installation [29] however, laser analyzers are much more expensive with additional expensive and very timeconsuming installation compared to the zirconia o2-analyzers. tdl o2, co and ch4 analyzers are mostly covering special chemicals production and very high >1000°c temperature applications. conclusions after more than 50 years, zirconia potentiometric oxygen gas sensing technology is still dominating combustion market with the high reliability, accuracy and low price. state of the art is the simultaneous measurement of oxygen and coe by means of one and the same probe. calorimetric catalytic and mixes potential sensors are implemented for coe measurement. tdls/qcls are also applied for co and for oxygen measurements. they are about 5 times more expensive compared to the zirconia o2-analyzers and additional require very expensive installation. tdls o2 analyzers mainly cover the production of specialty chemicals and very high >1000 °c http://dx.doi.org/10.5599/jese.743 http://www.yokogawa.com/an/faq/tdls/spec_1.htm j. electrochem. sci. eng. 10(2) (2020) 103-110 gas sensing technologies in combustion 110 temperature applications. tdls/qcls and calorimetric catalytic sensors are used in combustion process for methane measurement. only two methane gas sensing technologies – catalytic calorimetric and 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http://www.yokogawa.com/an/faq/tdls/spec_1.htm http://creativecommons.org/licenses/by/4.0/) {synthesis and characterization of a novel non-enzymatic glucose biosensor based on polyaniline/zinc oxide/multi-walled carbon nanotube ternary nanocomposite} http://dx.doi.org/10.5599/jese.666 207 j. electrochem. sci. eng. 9(2) (2019) 207-222; http://dx.doi.org/10.5599/jese.666 open access: issn 1847-9286 www.jese-online.org original scientific paper synthesis and characterization of a novel non-enzymatic glucose biosensor based on polyaniline/zinc oxide/multi-walled carbon nanotube ternary nanocomposite soha mohajeri, abolghassem dolati, khashayar yazdanbakhsh department of materials science and engineering, sharif university of technology, azadi ave., p.o. box 11155-9466, tehran, iran corresponding authors: smohajeri@alum.sharif.edu; dolati@sharif.edu received: february 17, 2019; revised: march 28, 2019; accepted: march 28, 2019 abstract novel polyaniline/zinc oxide/multi-walled carbon nanotube (pani/zno/mwcnt) ternary nanocomposite was fabricated as a non-enzymatic glucose biosensor. thermal chemical vapor deposition (cvd) process was employed to synthesize vertically aligned mwcnts on stainless steel substrates coated by co catalyst nanoparticles. in order to fabricate sensitive and reliable mwcnt-based biosensors, nanotubes density and alignment were adjusted by varying the cvd reaction time and cobalt sulfate concentration. the fabricated nanotubes were modified by zno particles through the potentiostatic electrodeposition technique. optimal electrodeposition potential, electrodeposition time, and electrolyte concentration values were determined. the optimized zno/mwcnt nanocomposite was reinforced by polyaniline (pani) nanofibers through the potential cycling technique, and the morphology, elemental composition, and phase structure of the fabricated nanocomposites were characterized by scanning electron microscopy (sem), energy dispersive x-ray spectroscopy (edx), and x-ray diffraction (xrd), respectively. the sensing mechanism of the pani/zno/mwcnt electrode for the electrochemical detection of glucose was investigated, and the limit of detection and sensitivity values of the designed sensor were determined. the fast response time of the ternary nanocomposite-based sensor as well as its satisfactory stability and reproducibility, makes it a promising candidate for non-enzymatic detection of glucose in biomedical, environmental, and industrial applications. keywords multi-walled carbon nanotubes; zinc oxide nanoparticles; polyaniline nanofibers; electrodeposition; non-enzymatic glucose sensor http://dx.doi.org/10.5599/jese.666 http://dx.doi.org/10.5599/jese.666 http://www.jese-online.org/ mailto:smohajeri@alum.sharif.edu mailto:dolati@sharif.edu j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 208 introduction electrocatalytic oxidation of glucose is of crucial importance not only to clinical diagnostic applications, but also to the ecological fields, wastewater treatment, and food industries [1-4]. the most commonly used amperometric glucose detectors are enzymatic biosensors in which glucose oxidase enzyme catalyzes the oxidation of glucose to gluconolactone [5]. however, due to the intrinsic nature of enzymes, such enzyme-based sensors are susceptible to variations in ph, temperature, humidity, and toxic chemicals, and simultaneously suffer from instability, high cost, and complex immobilization procedures [6]. thus, considerable attempts were made to develop economical, highly sensitive, fast and reliable non-enzymatic glucose sensors [7]. the enzyme-free detection of glucose on noble metals, thin film alloys, and metallic nanoparticles was extensively explored; however, these sensors undergo surface poisoning by adsorbed intermediates, leading to their poor sensitivity, selectivity, and stability [8-10]. nevertheless, non-enzymatic electrochemical glucose sensors based on carbon nanomaterials, particularly carbon nanotubes and graphene, have attracted huge attention due to their unique properties including high surface area, chemical and electrochemical stability, exceptional electrical conductivity, superior biocompatibility, and ease of modification [11,12]. as compared to traditional electrodes, multi-walled carbon nanotubes (mwcnt) based electrochemical sensors present lower charge-transfer resistance, faster response time, and higher sensitivity for the direct oxidation of glucose owing to their one-dimensional hollow tubular nanochemistry [13]. high-quality and well-aligned mwcnt arrays can be fabricated by thermal chemical vapor deposition (cvd) process which is uniquely superior to the other synthesis methods of carbon nanotubes such as arc discharge and laser-ablation. this method involves the catalytic decomposition of a gaseous carbon precursor at an elevated temperature on the surface of a substrate coated with transition catalysts such as iron, cobalt, or nickel [14]. to gain greater control over the performance of the mwcnt-based electrochemical sensors, low-density vertically aligned mwcnt forests can be synthesized by adjusting the cvd reaction time, temperature, pressure, gas flow rate, nature of the metal catalyst, concentration of the catalyst precursor, and so forth [15]. recently, various modified carbon nanotube electrodes have been prepared and utilized for biosensing applications [16,17], among which mwcnt-based sensors modified with zinc oxide as an electroactive metal oxide have been the subject of much interest [18]. zno nanostructures can be efficiently applied for the modification of mwcnt-based glucose biosensors as they possess extraordinary properties including biocompatibility, nontoxicity, high electrochemical activity, intrinsic hydrophilicity, chemical and photochemical stability [19]. moreover, mwcnt can be modified by conducting polymers to take advantage of their synergic effect, and significantly enhance electrochemical activity and sensitivity of the modified sensing platform [20]. polyaniline (pani) is exceptionally unique among the class of conducting polymers, and it has been extensively applied in electrochemical sensors by virtue of its excellent conductivity, environmental stability, high polymerization yield, and low cost [21]. although both zno/mwcnt and pani/mwcnt nanocomposites have been studied for enzymatic glucose detection, none of these binary hybrid materials have been examined as non-enzymatic glucose biosensors. moreover, no report is available on the development of pani/zno/mwcnt ternary nanocomposite as an electrochemical biosensor with enhanced sensitivity towards the enzyme-free glucose detection. in the current study, we combined the advantageous features and synergetic effects of mwcnt, zno, and pani with the aim of the direct electrochemical oxidation of glucose in alkaline medium. firstly, lowdensity vertically aligned mwcnt-based electrodes were fabricated by thermal cvd process and subsequently, the synthesized nanotubes were modified by zno nanoparticles and pani nanofibers soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 209 via potentiostatic electrodeposition and in-situ electropolymerization techniques, respectively. the objective of this paper was to optimize the process parameters in order to synthesize pani/zno/mwcnt ternary nanocomposite with an optimal morphological structure and enhanced electrocatalytic performance towards glucose sensing. furthermore, this study aimed to explore the sensitivity, detection limit, response time, stability, and reproducibility of the developed biosensor for non-enzymatic glucose detection in alkaline electrolytes. experimental stainless steel type 304 substrates were polished to a 1200 grit finish with sic abrasive paper, degreased with alcohol, and ultrasonically rinsed in distilled water. in order to activate the surface of the stainless steel and remove the passive protective chromium oxide layer, the substrates were treated in 9 m sulfuric acid solution (h2so4 98 %, merck) for 10 min. prior to the synthesis of carbon nanotubes, thin cobalt layers were deposited onto the substrates from aqueous solutions containing various concentrations of coso4∙7h2o (0.01, 0.05, 0.1, 0.15, and 0.25 m), 1 m na2so4, 0.5 m h3bo3, and double distilled water. all reagents were analytical grade purchased from merck. a typical threeelectrode cell consisting of a 0.5 × 0.5 cm2 stainless steel plate as the working electrode, a 2 × 1 cm2 platinum plate as the counter electrode, and a saturated calomel electrode (sce) as reference was employed. electrodeposition was conducted at 25 °c using an eg&g par potentiostat model 273a at the potential of −1.1 v for 2 s. vertically aligned multi-walled carbon nanotubes (mwcnt) were synthesized by thermal chemical vapor deposition (cvd) of ethylenediamine as a precursor. for this purpose, cobalt-coated substrates were placed in the middle of a 50 mm diameter quartz tube reactor of 1100 mm length which was preheated to 750 °c. simultaneously, to prevent the oxidation of the cobalt film, the reactor was purged by n2 gas. thereafter, ethylenediamine was introduced into the quartz tube at a flow rate of 400 ml min−1 while n2 was being bubbled through the ethylenediamine container. ultimately, after two different reaction times of 20 and 30 min, the reactor was cooled down to room temperature under an inert nitrogen atmosphere, and mwcnts were fabricated. in order to investigate the influence of various cobalt sulfate concentrations and cvd reaction times on the morphology and density of the developed nanotubes, a tescan vega ii scanning electron microscope (sem) was employed and the optimal operating parameters were determined. the electrodeposition of zno on low-density vertically aligned mwcnt was conducted using an autolab pgstat302n system in a conventional three-electrode configuration, where a mwcnt-coated electrode, a platinum wire, and saturated calomel electrode (sce) served as working, counter, and reference electrodes, respectively. zno/mwcnt composites were synthesized in electrolytes containing different concentrations of zn(no3)2 (0.025, 0.05 m) and kcl (0.05, 0.1 m) by applying various deposition potentials (-0.15, -1, -1.5, -1.8 v) for different durations (10 and 20 min), and the optimum process parameters were ascertained through the morphological analyses. the optimized zno/mwcnt nanocomposite was reinforced with polyaniline (pani) nanofibers by immersing the electrode in a solution containing 0.025 m aniline (c6h5nh2 99 %, sigma aldrich) and 0.25 m sulfuric acid (h2so4 98 %, merck), and sweeping the potential for 6 cycles between of -0.2 to +0.8 v vs. sce at a rate of 50 mv s−1. the obtained pani/zno/mwcnt electrode was subsequently dried at ambient temperature and its morphology was examined by sem. the elemental composition and phase structure of the optimized zno/mwcnt and pani/zno/mwcnt nanocomposites were analyzed by energy dispersive x-ray spectroscopy (edx, shimadzu 720/800hs/900hs) and x-ray diffraction (xrd, philips pw-3040), respectively. the sensing properties of the fabricated electrodes were assessed towards the detection of glucose in alkaline media. electrochemical experiments were carried out via http://dx.doi.org/10.5599/jese.666 j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 210 an eg&g par potentiostat model 273a in a three-electrode cell including the unmodified mwcnt, zno/mwcnt, or pani/zno/mwcnt electrode as the working electrode, a platinum wire as the counter electrode, and sce as reference. the electrolyte contained different concentrations of sodium hydroxide (naoh 98 %, merck) and glucose (c6h12o6 99.5 % sigma-aldrich). cyclic voltammetry (cv) and linear sweep voltammetry (lsv) tests were carried out in the range of 0 to 1.5 v for various naoh concentrations (10-300 mm), scan rates (10-500 mv s−1) and glucose concentrations (0.1-6 mm). the oxidation mechanism of glucose at the fabricated sensors was identified, and the sensitivity and detection limit of the pani/zno/mwcnt electrochemical biosensor were measured. the response time of the ternary nanocomposite-based biosensor was estimated through amperometric measurements, and its stability and reproducibility were evaluated. results and discussion influence of different operating parameters on the growth of mwcnt cobalt nanoparticles deposited on the stainless steel substrates are efficient catalysts for the growth of mwcnt in good alignment by the chemical vapor deposition of ethylenediamine. while the thickness of the cobalt film determines the diameter and density of the vertically aligned mwcnt, the cvd reaction time controls the length of nanotubes [22]. figure 1 displays the sem images of mwcnt synthesized at 750 °c for 30 min on a stainless steel substrate at two different magnifications. prior to the synthesis of mwcnt, a thin cobalt layer was electrodeposited as an active catalyst on the substrate at the potential of -1.1 v for 2 s from a bath containing 0.25 m coso4 ∙7h2o. it can be observed that high concentration of cobalt sulfate in the deposition bath leads to the formation of a relatively thick co film, where the growth of high-density mwcnt is favored. moreover, since the cvd reaction time is prolonged to 30 min, a relatively long forest of disordered mwcnt is produced, and the nanotubes are deformed and tangled together due to the local bending of their tips. a b figure 1. (a) low magnification and (b) high magnification sem images of mwcnt grown on cobalt-coated stainless steel substrate at 750°c for 30 min. cobalt catalyst nanoparticles were electrodeposited at the potential of -1.1 v vs. sce for 2 s from a bath containing 0.25 m coso4∙7h2o. in order to fabricate sensitive and reliable mwcnt-based biosensors, it was crucial to carefully tune the density and alignment of mwcnts. thus, the cobalt sulfate concentration and cvd time were reduced while the deposition potential and deposition time of the cobalt film were kept constant at -1.1 v and 2 s, respectively. the sem images of the fabricated nanotubes are shown in figure 2. according to figure 2a, as the concentration of cobalt sulfide is deceased to 0.01 m while soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 211 the cvd reaction time is kept at 30 min, the thickness of the deposited catalyst layer reduces, creating islands of randomly oriented mwcnt. figure 2b demonstrates that increasing the concentration of cobalt sulfide from 0.01 to 0.05 m slightly enhances the distribution of nanotubes, but the alignment of mwcnt is still not satisfactory. with a further increase of the concentration to 0.15 m, although the alignment of mwcnt is substantially improved due to the formation of a highdensity mwcnt forest, there is not an appropriate spacing between neighbouring tubes (figure 2c). this structure is not desirable for using mwcnt as electrochemical sensors and biosensors because the densely packed nanotubes will block the transfer of the electrons and increase the response time of the sensing platform. as depicted in figure 2d, a concentration value between 0.05 and 0.15 m, i.e. 0.1 m, can decrease the density of active sites for the growth of low-density mwcnt and contribute to the synthesis of an aligned mwcnt forest. besides, since the cvd reaction time is decreased from 30 to 20 min, tubes of shorter height are fabricated and the distortion and bending of the tips are prevented. in this case, the average diameter and length of mwcnts are estimated to be 100 nm and 2 μm, respectively. thus, coso4∙7h2o concentration of 0.1 m and the cvd reaction time of 20 min are selected as the optimal operating parameters for the fabrication of perfectly aligned arrays of low-density mwcnt for biosensing applications. a b c d figure 2. sem images of mwcnt grown on cobalt-coated stainless steel substrate at 750 °c for various cvd times and coso4∙7h2o concentrations: (a) 30 min, 0.01 m, (b) 30 min, 0.05 m, (c) 30 min, 0.15 m, and (d) 20 min, 0.1 m. modification of mwcnt with zinc oxide low-density vertically aligned mwcnt were modified by zno structures via electrodeposition technique. the potentiostatic method was utilized to synthesize zno/mwcnt composites by applying http://dx.doi.org/10.5599/jese.666 j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 212 various potentials in the range of -0.15 to -1.8 v for different durations in solutions containing varying concentrations of zn(no3)2 and kcl. afterwards, the influence of operating parameters on the morphological and structural properties of the fabricated zno-modified mwcnt was investigated. the electrochemical deposition of zinc oxide from zinc nitrate aqueous solutions at different applied potentials initiates with the reduction of nitrate ions to nitrite and hydroxide ions. the generated hydroxide ions react with zinc ions, leading to the precipitation of zinc hydroxide on the surface of mwcnt, which is spontaneously dehydrated into zinc oxide. these processes can be represented by the following reactions [23]; no3− + h2o + 2e− → no2− + 2oh− (1) zn2+ + 2oh− → + zn(oh)2 (2) zn(oh)2 → zno + h2o (3) the electrodeposition potential plays a critical role in determining the morphological structure of zno/mwcnt composite. to optimize the potentials suited for the electrodeposition of zno onto carbon nanotubes, the potentials of -0.15, -1, -1.5, and -1.8 v were applied for 20 min in a solution containing 0.05 m zn(no3)2 and 0.1 m kcl. the morphologies of the synthesized zno/mwcnt composites were analyzed by sem, as shown in figure 3. based on figures 3 (a) and (b), when the electrodeposition potential of -0.15 v was applied, the stainless steel substrate and the sidewalls of mwcnts have a relatively high interfacial tension, leading to an increase of the nucleation energy barrier [24]. therefore, only a few fine zno crystallites can nucleate on the surface of mwcnts and undergo three-dimensional growth, yielding to formation of micro-sized bundles of zno particles that are non-uniformly distributed on the surface. the obtained zno structure does not provide the composite with a high surface area-to-volume ratio which is essential for biosensing applications, and thus is not considered as a favorable sensing platform. figure 3(c) displays that at the deposition potential of -1 v, the mwcnts are uniformly covered with two-dimensional zno hexagonal plates that are perpendicular to the sidewalls of the nanotubes. the high-magnification image in figure 3(d) shows that the adjacent hexagonal plates of zno intercross each other, and create a thin and porous layer with a flake-like structure which is not appropriate for non-enzymatic biosensing applications. as illustrated in figures 3(e) and (f), at the potential of -1.5 v, zno is deposited on individual mwcnt as interconnected nanoparticles rather than a thin layer, with a good uniformity and moderate density. the formation of this nanostructure is attributed to the relatively high electrodeposition potential which reduces the interfacial tension of the mwcnt and increases the number of active nucleation sites [25]. hence, higher number of zno crystallites can adsorb and nucleate on the surface of mwcnts, producing more refined structure of zno nanoparticles with the average diameter of 115 nm. according to figures 3(g) and (h), a homogeneous array of zno nanoparticles is deposited on mwcnt at the potential of -1.8 v, while the size of zno nanoparticles, estimated as 110 nm, does not significantly change as compared to the size of nanoparticles deposited at -1.5 v. this can be ascribed to the evolution of hydrogen at more negative potentials, which adsorbs on the surface of mwcnt or growing zno crystals, thereby suppressing the further adsorption of zn2+ and oh− ions and hindering the growth of zno nanoparticles. a b soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 213 c d e f g h figure 3. sem micrographs of zno/mwcnt nanocomposites deposited from solutions containing 0.05 m zn(no3)2 and 0.1 m kcl during 20 min at the deposition potential (v vs. sce) of: (a), (b) -0.15; (c), (d) -1; (e), (f) -1.5; and (g), (h) -1.8 v vs. sce. http://dx.doi.org/10.5599/jese.666 j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 214 moreover, it can be observed that the density of deposited particles increased at -1.8 v due to the higher deposition rate at more negative potentials. accordingly, the attachment of too many zno nanoparticles onto mwcnt may reduce the electronic transfer properties of nanotubes and impair their capability as biosensors [26]. thus, the potential of -1.5 v is selected as the optimal deposition potential. in order to efficiently tailor the density of zno nanoparticles at the electrodeposition potential of -1.5 v, the concentrations of zn(no3)2 and kcl in the deposition bath were reduced to 0.025 m and 0.05 m, respectively, and the deposition time was kept 20 min. figures 4 (a) and (b) depict the influence of the electrolyte concentration on the surface morphology of zno/mwcnt nanocomposites. from these figures it is obvious that lowering the concentrations of zinc nitrate and potassium chloride increases the spacing between zno nanoparticles and reduces the mean diameter of nanoparticles to 90 nm, contributing to the non-enzymatic biosensing properties of the prepared electrode. nevertheless, nonspherical zno particles with sharp facets are not uniformly distributed over the surface of mwcnts, making it partially uncovered with zinc oxide. thus, lowering the electrolyte concentration is not an effective approach for decreasing the density of the deposited nanoparticles. to resolve this issue, the deposition time was reduced to 10 min, while the concentration of zn(no3)2 and kcl was kept at 0.05 m and 0.1 m, respectively. based on figures 4(c) and (d), the density of zno nanoparticles decreased after shorter electrodeposition time, and the deposited particles are more consistent in size and shape. in this case, most of the particles have a quasi-spherical shape, with a mean diameter of 60 nm, are dispersed evenly on the surface of mwcnt. a b c d figure 4. sem images of zno/mwcnt nanocomposites deposited at −1.5 v vs. sce for: (a), (b) 20 min in 0.025 m zn(no3)2 + 0.05 m kcl; (c), (d) 10 min in 0.05 m zn(no3)2 + 0.1 m kcl. soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 215 from the above analysis, it is concluded that the surface morphology of zno/mwcnt nanocomposites can be optimized if the electrodeposition potential of -1.5 v is applied for 10 min in a solution containing 0.05 m zn(no3)2 and 0.1 m kcl. reinforcement of zno/mwcnt nanocomposites with polyaniline a polyaniline (pani) film was deposited on the optimized zno/mwcnt-based electrode by sweeping the potential for 6 cycles in the range of -0.2 to +0.8 v with a scan rate of 50 mv s−1, in a solution containing 0.025 m aniline and 0.25 m sulfuric acid. the fabricated pani-reinforced electrode was subsequently dried at room temperature and its surface morphology was examined. figures 5(a) and (b) demonstrate that electro-polymerization of aniline occurred successfully and a compact layer of the pani nanofibers perfectly covered zno-modified carbon nanotubes, creating an entangled network which can further enhance the electrical conductivity of mwcnts [27]. moreover, based on data already reported in the literature [28], it can be concluded that the fabricated pani/zno/mwcnt nanocomposite has a core-shell structure. the effective interaction between π-bonds in the aromatic ring of the polyaniline and the graphitic structure of carbon nanotubes and zinc oxide nanoparticles causes pani chains to be adsorbed on the surface of the zno/mwcnt nanocomposite, thus forming a tubular pani shell that surrounds zno-modified mwcnt. a b figure 5. (a) low magnification and (b) high magnification sem images of pani/zno/mwcnt nanocomposite elemental and structural analyses the elemental analyses of the optimized zno/mwcnt and pani/zno/mwcnt nanocomposites were carried out using edx measurements, as shown in figure 6. a b figure 6. edx spectra of (a) zno/mwcnt and (b) pani/zno/mwcnt nanocomposites http://dx.doi.org/10.5599/jese.666 j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 216 figure 6(a) represents the strong peaks of zinc and oxygen elements, confirming the presence of zinc oxide nanoparticles within zno/mwcnt nanocomposite. figure 6(b) exhibits the nitrogen and carbon peaks for pani/zno/mwcnt nanocomposite, and the detected atomic ratios of these two elements correspond to polyaniline according to c24h20n4 formula. however, the edx method does not enable the detection of hydrogen atoms on polyaniline aromatic rings. the expected peaks for zinc and oxygen can be observed as well, and it can be concluded that the pani/zno/mwcnt nanocomposite sample contains zinc oxide and polyaniline compounds. the xrd patterns of the optimized zno/mwcnt and pani/zno/mwcnt nanocomposites are illustrated in figure 7. for zno/mwcnt nanocomposite, diffraction peaks at 2 = 43.1, 44.6, 50.7, and 74.5° correspond to the reflections of (100), (101), (102), and (112) planes of carbon (jcpds no. 41-1487), respectively, indicating the crystallinity of the multi-walled mwcnts. the graphitic peak at 2  = 26.4° can be attributed to the (002) reflection of mwcnt while it is relatively broad as compared to other carbon peaks. the lower intensity of this peak is indicative for a less number of adjacent graphite layers present in mwcnts, and increased degree of nanotubes alignment [29,30]. moreover, the peaks at 2θ values of 30.75, 36.20, 56.45, 62.72, and 66.25° are assigned to the (100), (101), (110), (103), and (200) crystal planes of zno nanoparticles, respectively. these diffraction peaks can be indexed to the standard hexagonal wurtzite-structured zno nanoparticles (jcpds no. 36-1451). for the pani/zno/mwcnt nanocomposite, a sharp diffraction peak emerges at 2θ value of 26.28° due to an overlap between (002) peak of mwcnt and (200) peak of pani, resulting in an intense peak of the composite. this result further confirms that a homogeneous coating of pani nanofibers is well dispersed on zn-modified mwcnts, as has already been demonstrated by the sem images in figure 5. this xrd pattern also exhibits a characteristic peak corresponding to the (311) plane of cobalt catalyst nanoparticles (jcpds 43-1003) that are encapsulated into mwcnt during their synthesis. 2 / o figure 7. xrd patterns of zno/mwcnt and pani/zno/mwcnt nanocomposites. non-enzymatic electrochemical detection of glucose the electrocatalytic activities of the unmodified mwcnt, zno/mwcnt, and pani/zno/mwcnt electrodes towards the oxidation of glucose were evaluated using cv measurements in the potential soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 217 range of 0 to 1.5 v. figure 8 (a) displays the obtained cvs at the examined surfaces in a solution containing 100 mm naoh and 10 mm glucose at a scan rate of 100 mv s−1. a large oxidation wave with a peak potential at about 0.85 v is sensed by the unmodified mwcnts due to the electrocatalytic oxidation of glucose by carbon nanotubes with minor contributions from co catalyst nanoparticles. it is worth mentioning that during the synthesis of mwcnt arrays on co catalyst-coated stainless steel substrate, cobalt is likely to be trapped in graphite-like planes within the carbon nanotube or along the tube axis, thereby promoting the electrochemical oxidation of glucose on mwcnts [31]. a b figure 8. cyclic voltammetric responses at scan rate of 100 mv s−1 of: (a) mwcnt, zno/mwcnt and pani/zno/mwcnt electrodes in 100 mm naoh + 10 mm glucose solution and (b) pani/zno/mwcnt electrode in a solution containing various concentrations of naoh and 1 mm glucose (inset shows the amplified cv curve obtained in a solution containing 10 mm naoh). for zno/mwcnt electrode, the anodic current starts to increase at about 0.42 v with the appearance of an obvious oxidation peak at around 1.1 v. this electrode shows a higher current response and a faster electron transfer kinetics as compared to the unmodified mwcnt electrode due to the electrocatalytic active sites provided by zno nanostructures. zno nanoparticles play a crucial role in the oxidation of glucose; however, they do not directly react with glucose in the nonenzymatic sensors but they accelerate the electron transfer from glucose to mwcnt. thus, the http://dx.doi.org/10.5599/jese.666 j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 218 electron-transfer ability and the electro-catalytic performance of mwcnt towards the oxidation of glucose can be significantly enhanced by incorporating zno nanoparticles. the pani/zno/mwcnt electrode produces an anodic current at 0 v followed by a negligible peak at 0.25 v which is attributed to the formation of the emeraldine salt [32]. with the increase of the potential in the anodic sweep, an oxidation wave appears due to the oxidation of glucose with a peak potential at about 1.08 v. the higher oxidation currents from the pani/zno/mwcnt electrode in response to the glucose oxidation and the significant increase in the peak current as compared to the zno/mwcnt and unmodified mwcnt electrodes are ascribed to the synergistic effect of pani, zno, and mwcnts. moreover, the conducting behavior of pani nanofibers with large surface area and fast charge transport ability can efficiently enhance the sensitivity of the fabricated glucose biosensor [33]. the absence of cathodic peaks in the cvs indicates that the oxidation of glucose on all the three electrodes is completely irreversible. the glucose sensing behavior of the pani/zno/mwcnt electrode was further studied in naoh solutions of different concentrations in the presence of 1 mm glucose, as shown in figure 8 (b). this figure demonstrates that in alkaline media, the rates of electron transfer reactions of glucose occurring at the electrode increase with naoh concentration (i.e. ph value) in the range of 10 to 300 mm. however, high naoh concentrations generate many intermediate reactants that might interact with the electrode materials and hinder the glucose reaction [34], so that oxidation peaks cannot be clearly observed in the cvs. thus, the enhanced current responses and electron transfer rates recorded for higher concentrations of naoh impair the detection capability of the sensor and are not beneficial for the sensing performance of the electrode. based on the inset in figure 8(b), the oxidation peak is more pronounced at low naoh concentration of 10 mm, and the sensitivity of the electrode is much higher than that obtained in solutions of higher concentrations. hence, 10 mm naoh solution is selected as the optimized supporting electrolyte for the electrocatalytic oxidation of glucose. further assessment of the glucose oxidation process was conducted by exploring the influence of different scan rates (10, 50, 100, 300, and 500 mv s−1) on the linear sweep voltammetric response of the pani/zno/mwcnt electrode in a solution containing 10 mm naoh and 1 mm glucose. figure 9(a) exhibits that upon increasing the scan rates, the oxidation peak current densities increase, while the glucose oxidation peaks shift to more positive potentials. the inset in this figure illustrates a linear relationship between the anodic peak current density (ip) and the square root of the scan rate (vl/2). the regression line equation is: ip / ma cm−2 = 0.5756 v1/2 / (mv s−1)1/2 + 2.343; r2 = 0.9968. these results reveal that the oxidation process is controlled by diffusion of glucose molecules to the electrode surface, rather than adsorption. the oxidation peaks in figure 9 (a) can be ascribed to the following reaction [35]: glucose + o2 → gluconolactone + h2o2 (4) as the potential is swept to more oxidizing values, gluconolactone is spontaneously converted to gluconic acid [36] according to: gluconolactone → gluconate− + h+ (5) figure 9(b) illustrates lsvs of pani/zno/mwcnt electrode recorded at the scan rate of 100 mv s−1 in 10 mm naoh solution containing different concentrations of glucose from 0.1 to 6 mm. it can be observed that with an increase in the glucose concentration, the oxidation peak current density at about 1.08 v shows an increasing trend. as depicted in the inset in figure 9 (b), the variation of the peak current density vs. glucose concentration exhibits two separate linear regions in the concentration ranges of 0.1-1 mm and 1-6 mm, respectively. the corresponding regression equations soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 219 are ip / ma cm−2 = 7.8307 c / mm + 2.1088 (r2 = 0.995) and ip / ma cm−2 = 1.6731 c / mm + + 8.454 (r2 = 0.998), respectively. from the slopes of the corresponding calibration plots, the sensitivity of the sensor is calculated to be 7.8307 ma mm−1 cm−2 and 1.6731 ma mm−1 cm−2 for the glucose concentration between 0.1 to 1 mm and 1 to 6 mm, respectively. based on the signal to noise ratio of 3, the detection limit of 0.1 μm was obtained. the fabricated pani/zno/mwcnt sensor electrode shows better performance and higher sensitivity for the non-enzymatic determination of glucose in comparison to other previously reported non-enzymatic glucose biosensors that focused on carbon nanotubes, zno nanoparticles, and pani nanofibers-modified electrodes [37-39]. such enhanced sensitivity can be attributed to the unique properties of carbon nanotubes, zinc oxide nanoparticles, and polyaniline nanofibers and their synergistic effect which results in a significant improvement of the electrochemical activity of the modified electrode for the detection of glucose. a b figure 9. linear sweep voltammograms of pani/zno/mwcnt electrode in 10 mm naoh solution containing (a) 1 mm glucose solution at different scan rates from 10 to 500 mv s−1 (inset shows the oxidation peak current density vs. the square root of the scan rate), (b) different concentrations of glucose from 0.1 to 6 mm at the scan rate of 100 mv s−1 (inset shows oxidation peak current density vs. glucose concentration) figure 10 presents the steady-state current-time response of the pani/zno/mwcnt electrode in 10 mm naoh with the addition of 0.1 mm glucose at the oxidation potential of 1.08 v. the response time (i.e. the time required to reach 95 % of the maximum steady-state current) is < 5 s, http://dx.doi.org/10.5599/jese.666 j. electrochem. sci. eng. 9(2) (2019) 207-222 novel non-enzymatic glucose biosensor 220 indicating rapid current response of the electrode to the injection of glucose due to the facile electron transfer process through the pani/zno/mwcnt nanocomposite. figure 10. amperometric response of pani/zno/mwcnt electrode at 1.08 v vs. sce in 10 mm naoh with addition of 0.1 mm glucose the stability of the pani/zno/mwcnt electrode was explored by measuring its current response within two weeks. the sensor was stored in air at ambient conditions and its current response to 1 mm glucose was tested every day. after 15 days of storage, the current response of the electrode only decreased 6.3 % compared to the initial response, revealing the long-term stability of the developed biosensor. the reproducibility measurement was carried out by comparing the current responses of six modified electrodes fabricated under the same conditions towards the detection of 1 mm glucose dissolved in 10 mm naoh. the relative standard deviation (rsd) of responses was estimated to be 3.2 %, indicating the satisfactory reproducibility and reliability of the synthesized sensor. conclusions in this work, we successfully developed a non-enzymatic electrochemical glucose sensor based on pani/zno/mwcnt ternary nanocomposite. in order to decrease the density of mwcnt and synthesize aligned nanotubes by thermal cvd process on cobalt-coated substrates, coso4×7h2o concentration of 0.1 m and cvd reaction time of 20 min were selected to yield the formation of perfectly aligned arrays of low-density mwcnt that were efficient for biosensing applications. the optimized zno/mwcnt nanocomposite, in which 60 nm quasi-spherical zno nanoparticles were uniformly dispersed on the surface of nanotubes, was fabricated at the electrodeposition potential and time of −1.5 v and 10 min, respectively, in a solution containing 0.05 m zn(no3)2 and 0.1 m kcl. the electrochemical sensing properties of zno/mwcnt nanocomposite was improved by a thin layer of pani through the potential cycling technique, and sem, edx, and xrd confirmed the effective structural modification of the surface by pani nanofibers. cv measurements conducted in solutions containing glucose and sodium hydroxide revealed that the electrocatalytic performance of mwcnt towards the oxidation of glucose can be significantly enhanced by incorporating zno nanoparticles and pani nanofibers. the oxidation of glucose on the designed pani/zno/mwcnt sensor involved a diffusion-controlled process, with sensitivities of 7.8307 ma mm−1 cm−2 and soha mohajeri et al. j. electrochem. sci. eng. 9(2) (2019) 207-222 http://dx.doi.org/10.5599/jese.666 221 1.6731 ma mm−1 cm−2 for the glucose concentrations between 0.1 to 1 mm and 1 to 6 mm, respectively, and 0.1 mm limit of detection. the fabricated ternary nanocomposite-based sensor exhibited fast response time < 5 s, as well as high stability of 93.7 % and reproducibility of 3.2 %, thereby validating the unique capabilities of the designed pani/zno/mwcnt electrode as an electrochemical sensing platform for non-enzymatic glucose detection. acknowledgements: this research was financially sponsored by the department of 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[40] k. ghanbari, z. babaei, analytical biochemistry 498 (2016) 37–46. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) effect of corrosion on flexural bond strength doi: 10.5599/jese.2014.0052 123 j. electrochem. sci. eng. 4(3) (2014) 123-134; doi: 10.5599/jese.2014.0052 open access: issn 1847-9286 www.jese-online.org original scientific paper effect of corrosion on flexural bond strength akshatha shetty, katta venkataramana and k. s. babu narayan department of civil engineering, nitk surathkal-575025, india corresponding author: e-mail: akshathashetty16@gmail.com received: october 17, 2014; revised: april 3, 2014; published: september 22, 2014 abstract corrosion is one of the main causes affecting durability of structures. corrosion effects on structures cannot be ignored and replaced. to understand the performance of structures there is a need to study the rate at which different corrosion levels occur. hence the present investigation has been taken up to study the behaviour of nbs (national bureau of standard) beam specimens made up of ordinary portland cement (opc) and portland pozzolona cement (ppc) concrete matrix were subjected to accelerated corrosion for different corrosion levels of 2.5 % to 10 % at 2.5 % interval. results are compared with those for control beam specimen. it is observed that bond stress value decreases with the increase in corrosion levels. also corrosion leads to the decline of load carrying capacity. keywords opc concrete; ppc concrete; corrosion level; nbs beam; strain; corrosion current density introduction reinforcement corrosion has been identified as being the predominant deterioration mechanism for reinforced concrete structures, which seriously affects the serviceability and safety of structures. chloride ions from the external environment diffuse through concrete to the steel surface, leading to the depassivation of the protective layer and ultimately to the initiation of reinforcement corrosion. corrosion consumes iron of the reinforcing bar progressively thereby reducing the cross sectional area. corrosion increases its volume 2 to 6 times than that of the original steel; it causes volume expansion developing tensile stresses in concrete [1]. the corrosion of rebar in concrete is generally considered as an electrochemical process [2]. with attention of researchers focusing towards the prediction of the residual life of reinforced concrete structures affected by reinforcement corrosion, the use of electrochemical techniques for the determination of relevant parameters in this regard becomes a major area of study. http://www.jese-online.org/ mailto:akshathashetty16@gmail.com j. electrochem. sci. eng. 4(3) (2014) 123-134 concrete beam corrosion 124 therefore the electrochemical techniques are widely used for the study of rebar corrosion in laboratories together with their application to real life structures [3]. the bond between steel and concrete is the mechanism which allows for effective force transfer. the load applied to the concrete member is transferred to the reinforcing steel through bond [4]. bond of deformed bars is developed mainly by the bearing pressure of the bar ribs against the concrete [5]. one of the most important prerequisites of reinforced concrete construction is adequate bond between the reinforcement and the concrete. hence present study aims to investigate the effect of corrosion on performance of reinforced concrete beams, in particular on the bond strength and load carrying characteristics and deflection characteristics and maximum crack width. following are the main highlights of the study: 1. accelerated corrosion using impressed current technique was adopted for achieving the desired corrosion levels. 2. nbs rc beams have been cast and tested for bond strength by two-point loading flexural strength. 3. opc and ppc concrete were used for preparing the beam specimens. experimental details of experiments, materials used, and method of testing are explained below. materials materials used for the experimental investigation are tested as per code provisions: cement ordinary portland cement and portland pozzolona cement were used. cements used for the study are tested as per is: 8112-1989 [6] for opc and is 1489 (part-i):1991 [7] for ppc recommendations. setting time, specific gravity and compressive strength of results are tabulated in table 1 and table 2 respectively. table 1. test results on characteristics of opc sl no test parameters results as per is 8112:1989 (specifications of 43 grade opc) 1 initial setting and final setting time 75 min and 260 min not less than 30 min. and not more than 600 min 2 specific gravity 3.1 3 compressive strength: 3 days 24.09 n/mm 2 not less than 23 n/mm 2 7 days 34.48 n/mm 2 not less than 33 n/mm 2 28 days 46.85 n/mm 2 not less than 43 n/mm 2 fine aggregate physical tests on fine aggregates were conducted as per is 383-1970 [8] and specific gravity, water absorption and moisture content and grading results are presented in table 3. coarse aggregate size of aggregate used was 20 mm downsize and 12.5 mm down size angular type coarse aggregate. physical tests on aggregates were conducted as per is 383-1970 [8]. specific gravity, water absorption and moisture content results are tabulated in table 4. a. shetty at al. j. electrochem. sci. eng. 4(3) (2014) 123-134 doi: 10.5599/jese.2014.0052 125 table 2. test results on characteristics of ppc sl no test parameters results as per is 1489 (part-i): 1991 (specifications of portland pozzolana cement) 1 initial setting and final setting time 76 min and 270 min not less than 30 min. and not more than 600 min 2 specific gravity 2.91 3 compressive strength: 3 days 16.64 n/mm 2 not less than 16 n/mm 2 7 days 26.54 n/mm 2 not less than 22 n/mm 2 28 days 39.84 n/mm 2 not less than 33 n/mm 2 table 3.test result of fine aggregate used for concrete mix sl no test parameters results 1 specific gravity 2.6 2 water absorption 2.0 % 3 moisture content 5.0 % 4 grading zone i table 4. test result of 20mm down size used for concrete mix sl no test parameters results 1 specific gravity 2.8 2 water absorption 0.5 % 3 moisture content nil 4 shape angular reinforcing steel tensile strength of reinforcing steel bar was tested using universal testing machine (utm). stress-strain curves for 25 mm thermo mechanically treated (tmt) fe-415 reinforcing steel bar was obtained by plotting tension test data. typical stress-strain curve for 25 mm bar is shown in figure 1. the yield strength and ultimate strength are 485 n/mm 2 and 589 n/mm 2 respectively. mix design details according to the codal recommendation of is 456-2000 [9] minimum concrete grade to be adopted in coastal environment is m30 and maximum water cement ratio of 0.45. target strength of 30 n/mm 2 was considered for the study. to control the proportion of water in freshly mixed concrete is to specify an upper limit for “slump”. hence a slump range of 50-60 mm was selected for the present study. mix design calculations were made as per is 10262-2009 [10]. test results of materials were used in the calculation of determination of mix proportions. after several trials the mix proportion of 1:1.77:2.87 was achieved with an addition of 2 ml/kg of commercially available chemical admixture for both opc and ppc concrete. preparation of test specimens for the present study national bureau of standard (nbs) beam specimens of size 2.44 × 0.457 × 0.203 m (figure 2) was used [11]. before placing of concrete in beam molds, one j. electrochem. sci. eng. 4(3) (2014) 123-134 concrete beam corrosion 126 blue color multi-strand copper wire of 4 cm 2 cross sectional area was connected at one end of rebar, and soldered to reinforcement bar and also covered with m-seal to prevent corrosion at that area. this wire is protruded to the surface level to induce electric current. similar steps were followed at the other end and one black color, multi-strand copper wire of 2 cm 2 cross sectional area was connected to reinforcement bar, which helps in the monitoring process of corrosion rate. figure 1. stress-strain curve for 25 mm diameter tmt fe-415 reinforcing steel bar figure 2. reinforcement details of nbs beam specimen [11] a. shetty at al. j. electrochem. sci. eng. 4(3) (2014) 123-134 doi: 10.5599/jese.2014.0052 127 accelerated corrosion technique under natural conditions the process of reinforcement corrosion is very slow; so laboratory studies need an acceleration of corrosion process to achieve a short test period. this can be accomplished by applying a constant potential or an electric current of constant magnitude to the embedded steel [12]. hence accelerated corrosion technique was used in the present study to achieve different degree of corrosion levels. after curing of beam specimens for 28 days, specimens were lifted and shifted to the corrosion tank to induce desired corrosion levels. electrochemical corrosion technique was used to accelerate the corrosion of steel bars embedded in beam specimens. specimens were partially immersed in a 5% nacl solution for duration of 8 days; direction of current was arranged such that, rebars embedded inside the concrete specimens served as anode. steel plate which was placed along the length of beam functions as cathode. current required achieving different corrosion levels can be obtained using faraday’s law (eq. 3) [13]. based on the calculation amount of 2.5 to 10 a current at the variation of 2.5 a was applied to obtain the required corrosion level i.e. 2.5 to 10 % at the variation of 2.5 % respectively. for each trial, three specimens were considered. a schematic representation of corrosion test set-up used for accelerated corrosion process is shown in figure 3. figure 3. schematic representation of accelerated corrosion of beam specimen calculation of amount of current required to obtain different corrosion levels from faraday’s law, i f corr ( )w w f i dlwt   (1) i f i ( ) 100 w w w    (2) eq (2) in eq. (1) i app corr 100 w f i i dlwt     (3) j. electrochem. sci. eng. 4(3) (2014) 123-134 concrete beam corrosion 128 where icorr = corrosion current density; iapp = applied current;  = degree of corrosion wi = initial weight of steel (20,000 g); f = 96 487 as; d = bar diameter; l = length of bar; w =equivalent weight of steel (27.925 g); t = time in seconds. corrosion rate measurements after completion of accelerated corrosion, corrosion rate was measured with applied corrosion monitoring (acm) instrument (figure 4) based on linear polarization resistance (lpr) method to assure the achieved desired degree of corrosion levels in beam specimens after inducing corrosion for a particular duration. figure 4. corrosion monitoring set up corrosion current density was calculated by using the stern-geary formula [14]. corr p b i r (4) where, icorr = corrosion current density, μa/cm 2 ; rp= polarization resistance, kω cm 2 ; b = 26 mv (for steel in active condition this value is normally used) [15]. test setup used for flexural bond study test set up used for the present study is shown in figure 5a. beam specimens were tested under two point loading condition. the load was applied at 15 kn increments. proving ring of 50 tonne capacity was used to note the applied load. strain value recordings have been done by using demec gauges at every load interval. positions of demec targets have been shown in figure 5b. maximum crack width was measured using crack microscope (figure 6). deflection recordings were done by using dial gauges. dial gauges were indicated in figure 7. a. shetty at al. j. electrochem. sci. eng. 4(3) (2014) 123-134 doi: 10.5599/jese.2014.0052 129 figure 5(a). test set up of nbs beam specimen figure 5(b). demec buttons were at 100 mm c/c figure 6. concrete crack microscope figure 7. positions of dial gauges determination of bond stress average bond stress values were obtained from eq. (6). 1 s bd d4 f l    (6) where, diameter of bar 1 1 100 p    (7) bd = average bond stress, n/mm 2  = initial diameter 25 mm 1 = reduced diameter values are presented in table 5. j. electrochem. sci. eng. 4(3) (2014) 123-134 concrete beam corrosion 130 p = weight loss in percentage ld = embedment length of the bar (747 mm) from the test setup fs = steel stress values have been obtained for initial and final strain values (slip region) of different corrosion levels from stress corresponding to strains at that load level. table 5. reduced bar diameter for different levels of corrosion corrosion level, % bar diameter, mm 0 25.00 2.5 24.69 5 24.37 7.5 24.04 10 23.72 results and discussion compressive strength of control cube after 28 days of curing was 35 n/mm 2 and 33 n/mm 2 for opc and ppc concrete respectively. measurement of corrosion current density corrosion rate is measured in terms of corrosion current density, icorr, and is a quantitative index, which represents an overall estimate of the corrosion attack on reinforcement. corrosion current density values were calculated for each specimen from acm instrument for different grids and average value was considered for different corrosion levels to calculate the weight loss, %. obtained corrosion levels or weight loss for applied current are shown in figure 8. it is seen that corrosion levels increases linearly with the increase in the applied current. from figure 8 corrosion levels for the applied current, a, can be calculated as: y= 0.96x 0.22 r² = 0.995 (ppc concrete) (8a) y = 0.98x 0.54 r 2 = 0.998 (opc concrete) (8b) where; x = applied current, a and y = obtained corrosion level, %. experimental investigation on ultimate load carrying capacity and bond stress of rc member in the present study, 3 control specimens and 12 corroded specimens (3 sets each for 2.5 %, to 10 % at 2.5 % variation of corrosion) were considered for opc and ppc concrete beam specimens. ultimate load carrying capacity, deflection and crack propagation of nbs beams effect of corrosion on ultimate load carrying capacity is shown in figure 9. as the degree of corrosion level increases load carrying capacity decreases (figure 9). it is also observed that for every percentage increase in corrosion level there is about 1.6 % decrease in load carrying capacity. from figure 10 it is noticed that as the corrosion level increases deflection value increases. this is due to as the corrosion level increases cross sectional area reduces and leads to reduction in stiffness of reinforcement. from figure 11 it is observed that at low corrosion levels (0 to 5%) crack width is less but as the corrosion level increases (7.5 % to 10 %) crack width increases rapidly. from figure 12 it is seen that as the load level increases strain value increases linearly in the initial stage. a. shetty at al. j. electrochem. sci. eng. 4(3) (2014) 123-134 doi: 10.5599/jese.2014.0052 131 figure 8. variation of corrosion levels with applied current figure 9. effect of corrosion on ultimate load carrying capacity figure 10. effect of corrosion levels on central deflection of nbs beam figure 11. effect of corrosion levels on maximum crack width of nbs beam j. electrochem. sci. eng. 4(3) (2014) 123-134 concrete beam corrosion 132 then at higher corrosion levels, rate of increase of strain is higher for the same increment of load level compared to lower levels of corrosion. control beam specimen performs better at increased corrosion levels. it is also observed that there is a sudden increase in strain values observed for the applied load interval, in all beam specimens (figures 12 to 15). bond characteristic is dependent on characteristics of steel at interface and not in the body of reinforcement. from the stress strain curves it is clear that the sudden increase in strain levels without increase in stress is indicative of slip (sudden increases in strain value is much lesser than the yield strength of 25 mm diameter) characteristics owing to drop in bond. higher the accelerated corrosion at surface lower is the stress level at which such slip occurs. figure 12. effect of different levels of corrosion on stress strain behavior of opc concrete beam specimens at left side loading point (l); where, trltension side reinforcement level figure 13. effect of different levels of corrosion on stress strain behavior of opc concrete beam specimens at right side loading point (r); where, trl-tension side reinforcement level figure 14. effect of different levels of corrosion on stress strain behavior of ppc concrete beam specimens at right side loading point (r); where, trl-tension side reinforcement level figure 15. effect of different levels of corrosion on stress strain behavior of ppc concrete beam specimens at left side loading point (l); where, trl-tension side reinforcement level bar stress and bond stress performance of nbs beam from table 6 and table 7 it is observed that as the corrosion level increases bond stress value decreases. percentage reduction in bond stress for different levels of corrosion i.e. 2.5 %, 5 %, 7.5 % and 10 % with respect to control specimen were 6.59 %, 13.17 %, 21.56 % and 29.34 % respectively for opc concrete beam specimens and for ppc concrete beam specimens 4.32 %, 10.65 %, 18.93 % and 26.04 % respectively. a. shetty at al. j. electrochem. sci. eng. 4(3) (2014) 123-134 doi: 10.5599/jese.2014.0052 133 from figure 16 and figure 17 it is exhibited that bond stress approximately drops for about 2.6 % and 2.1 % for (initial strain value) opc and ppc concrete beam specimen and also 2 % and 2.1 % (final strain value) for opc and ppc concrete beam specimens respectively for every percentage increase in corrosion level. table 6. bar force, reduced diameter and bond stress performance for different degree of corrosion in nbs beam bar force and bond stress values at slip region in strain values initial strain values final strain values corrosion level, % micro strain stress in bar, n/mm 2 reduced diameter, mm bond stress, n/mm 2 micro strain stress in bar, n/mm 2 reduced diameter, mm bond stress, n/mm 2 0 700 199.21 25.00 1.67 1330 367.96 25.00 3.08 2.5 660 188.85 24.69 1.56 1275 353.58 24.69 2.92 5 600 177.59 24.37 1.45 1210 337.08 24.37 2.75 7.5 560 163.25 24.04 1.31 1110 310.00 24.04 2.49 10 505 148.13 23.72 1.18 1050 293.93 23.72 2.33 table 7. bar force, reduced diameter and bond stress performance for different levels of corrosion in ppc concrete beam specimens bar force and bond stress values at slip region (ppc concrete beam specimens) initial strain values final strain values corrosion level, % micro strain stress in bar, n/mm 2 reduced diameter, mm bond stress, n/mm 2 micro strain stress in bar, n/mm 2 reduced diameter, mm bond stress, n/mm 2 0 710 201.75 25.00 1.69 1400 387.00 25.00 3.24 2.5 690 196.46 24.69 1.62 1345 371.88 24.69 3.07 5 645 185.04 24.37 1.51 1280 354.85 24.37 2.89 7.5 585 170.13 24.04 1.37 1165 325.13 24.04 2.62 10 530 154.94 23.72 1.23 1085 303.13 23.72 2.41 figure 16. effect of corrosion levels on bond stress of slip region at final strain values figure 17. effect of corrosion levels on bond stress of slip region at final strain values bond stress values for different degree of corrosion can be calculated from following equations obtained from figure 16 and figure 17, where x = corrosion levels, % and y = bond stress, n/mm 2 . at initial slip point j. electrochem. sci. eng. 4(3) (2014) 123-134 concrete beam corrosion 134 (opc concrete) y = -0.049x + 1.678 r 2 = 0.996 (9a) (ppc concrete) y = -0.046x + 1.718 r 2 = 0.982 (9b) at end slip point (opc concrete) y = -0.076x + 3.099 r 2 = 0.992 (10a) (ppc concrete) y = -0.084x + 3.269 r 2 = 0.990 (10b) conclusions based on the detailed experimental investigations, following conclusions are drawn: reinforcement corrosion leads to the decline of load carrying capacity of nbs rc beam specimens. for every percentage increase in corrosion level, there is about 1.6 % decrease in load carrying capacity. for increasing corrosion level, strain values increase in the initial stages. then at higher corrosion levels rate of increase of strain is higher for the same increment of load level, compared to the lower levels of corrosion. crack width is less at the lower level of corrosion (0 to 5 %), after that it increases rapidly (7.5 to 10 %). reinforcement corrosion causes degradation of bond behavior. the strain value becomes large due to corrosion and the larger the corrosion lesser the bond stress value. proposed regression equation is very much useful for quick assessment to predict the bond strength values for different corrosion levels in structure. though the state of the art states about blended cements the research results done by opc concrete is very much useful for the already existing structures subjected to corrosion. references [1] s. bhaskar, b. h. bharatkumar, g. ravindra, m. neelamegam, journal of structural engineering (csir-serc), 37 (2010) 37-42. [2] t. maheswaran, j. g. sajavan, journal of material concrete research, 56 (2004) 359-366. [3] c. andrade, c. alonso, journal of construction building materials, 10 (1996) 315-328. [4] r. park, t. paulay, reinforced concrete structures, john wiley & sons, inc. new york (1975). [5] l. a. lutz, p. gergely, aci materials journal, 11 (1967) 711-721. [6] is 8112: 43 grade ordinary portland cement – specification bureau of indian standards (1989). [7] is 4031: methods of physical tests for hydraulic cement, bureau of indian standards (1988). [8] is 383: indian standard specification for coarse and fine aggregates from natural sources for concrete (1970). [9] is 456: indian standards code of practice for plain and reinforced concrete (2000). [10] is 10262: recommended guidelines for concrete mix design, bureau of indian standards (2009). [11] r. j. paul, master thesis, department of civil engineering and applied mechanics, mc gill university canada (1978) p. 66. [12] s. care, a. raharinaivo, cement and concrete research, 37 (2007) 1598–1612. [13] s. ahamad, the arabian journal of science and engineering, 34 (2009) 95-104. [14] b. pradhan, b. bhattacharjee, construction and building materials, 23 (2009) 23462356. [15] m. g. fontana, corrosion engineering, tata mcgraw-hill education (2005). © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {a novel vitamin b9 sensor based on modified screen-printed electrode} http://dx.doi.org/10.5599/jese.878 1 j. electrochem. sci. eng. 11(1) (2021) 1-9; http://dx.doi.org/10.5599/jese.878 open access: issn 1847-9286 www.jese-online.org original scientific paper a novel vitamin b9 sensor based on modified screen-printed electrode maryam zare and hamid sarhadi department of food science, bam branch, islamic azad university, bam, iran corresponding author: sarhadihamid2020@gmail.com; tel.: +98 3444342489; fax: +98 3444342489 received: june 14, 2020; revised: july 14, 2020; accepted: september 16, 2020 abstract in the field of determination of vitamin b9 (folic acid, fa), we have described the development of a sensitive electrochemical sensor through promoting the screen-printed electrode (spe) and taking the advantage of zinc ferrite magnetic nanoparticles (znfe2o4mnps). cyclic voltammetry (cv) experiments demonstrated the powerful activity of znfe2o4mnps/spe for electrooxidation of fa by showing the prominent oxidation peak at 600 mv vs. ag/agcl. by differential pulse voltammetry (dpv) measurements, a linear relation between current response and concentration of vitamin b9 was determined in the range of 1.0–100.0 µm, and detection limit is found to be 0.3 µm (s/n=3). except high sensitivity, the developed sensor demonstrated high stability, reproducibility and repeatability, and was also successfully applied to specify fa in real samples of vitamin b9 tablets and human urine. keywords voltammetric sensor; folic acid; modified electrode; znfe2o4 magnetic nanoparticles introduction vitamin b9 is folic acid (fa), n-[4-[[(2-amino-4-oxo-1,4-dihydropteridin-6-yl)methyl]amino] benzoyl]-l-glutamic acid, which can be dissolved in water and commonly found in fresh fruits, vegetables, liver, and yolk [1,2]. fa is necessary in many biological functions and is a significant compound in the process of repair of dna, synthesis and methylation of dna, and synthesis of red blood cells in human’s body [3-5]. the deficiency of fa in human’s body can cause many disorders such as higher risk of neural tube defects, hypomethylation, colorectal cancer and, it can induce proto-oncogene expression causing cancer [6-8]. hence, the specification of fa is considered necessary in pharmaceutical, food, and clinical samples. most of literature investigations on determination and quantification of fa are using fluorimetry [9], capillary electrophoresis [10], spectrophotometry [11], flow injection analysis [12], http://dx.doi.org/10.5599/jese.878 http://dx.doi.org/10.5599/jese.878 http://www.jese-online.org/ mailto:sarhadihamid2020@gmail.com j. electrochem. sci. eng. 11(1) (2021) 1-9 vitamin b9 sensor based on screen-printed electrode 2 high performance liquid chromatography [13] and chemiluminescence analysis [14]. all these techniques are commonly expensive, show low sensitivity, take long time for each testing, and require sample pretreatment by organic solvent extraction. therefore, using new alternative techniques is supposed to be very useful. in recent years, electrochemical methods and particularly voltammetry, have attracted significant attention due to their benefits like sensitivity, simplicity, low cost, accuracy and selective specification of electroactive species [15-28]. in addition, the usage of electrochemical approach towards detection of fa is generally promising because fa is the electrochemically active compound [29,30]. among many potentially useful electrodes, screen-printed electrodes (spes) provide many benefits for electrochemical sensors, such as low cost, portability, ease of chemical modification, rapid response, and flexible design [31-36]. for determining the trace level of fa, however, the signal to noise ratio of the bare, i.e. unmodified spe is not high enough. therefore, the scientists tried to modify bare spe surfaces by different sorts of modifiers. among modifiers, nano-materials can be a good choice due to their high surface-to-volume ratio which will significantly increase the voltammetric response. also, presence of nanomaterials leads to faster electron transfer between the electrode and analyte [37-48]. in order to construct electrochemical sensors and biosensors, many nanomaterials like metal oxides, metals, semiconductor nps, and carbon-based materials have already been employed [49-51]. among them, znfe2o4 magnetic nps showed promising electronic and magnetic characteristics and thermal stability, which are commonly appropriate to a broad range of usage like drug delivery technology, magnetic resonance imaging (mri), and gas sensing [52-55]. the main goal of the present work is to take advantages of both spe and znfe2o4 magnetic nps and combine them in a sensor for specifying the amounts of vitamin b9 (fa) in various real samples. experimental chemicals and apparatus autolab potentiostat/galvanostat (pgstat 302n, eco chemie, the netherlands) was utilized for electrochemical measurements. general purpose electrochemical system (gpes) software was used for controlling experimental conditions. spe (dropsens; drp-110: spain) possessed three electrodes, i.e. an unmodified graphite working electrode, graphite counter electrode and ag/agcl reference electrode. metrohm 710 ph meter was utilized in order to determine ph. fa and all the remaining reagents were of analytical grade and provided by merck (darmstadt, germany). orthophosphoric acid and the related salts were utilized to prepare the buffer solutions of ph in the range of 2.0–9.0. znfe2o4 nanoparticles were synthesized according to the literature [56]. preparation of real samples drug-free human urine specimens were stored in a refrigerator immediately after collection. firstly, 10 ml of each sample was used and centrifuged at 2000 rpm for 15 minutes and the supernatant passed through 0.45 µm filter. next, various quantities of the treated urine samples were taken and transferred to a flask and diluted by utilizing a phosphate buffer solution (pbs) with ph of 7.0. for these samples, various quantities of fa were spiked. the standard addition approach was utilized to determine fa concentrations. m. zare and h. sarhadi j. electrochem. sci. eng. 11(1) (2021) 1-9 http://dx.doi.org/10.5599/jese.878 3 five fa tablets labelled as 100 mg per tablet, tehran chemie pharmaceutical co., iran) were firstly grounded to a powder. afterwards, the tablet solution was prepared via dissolving 500 mg of the powder in 25 ml water through ultra-sonication. various volumes of the diluted solution have been transported to 25 ml volumetric flasks and diluted to the marks with pbs, ph 7.0. fa contents were analysed by the recommended technique via the standard addition method. results and discussion electrochemical profile of fa at znfe2o4 mnps/spe the mechanism of fa oxidation is schematically drawn in figure 1 where electrochemical oxidation of fa has been proposed as 2e-/2h+ reaction, what suggests ph dependent electrochemical response 57. therefore, finding of the optimum ph value would be required for achieving precise outputs. the modified electrodes were tested at ph values ranging between 2.0 and 9.0. the most acceptable output for fa electrooxidation was observed at ph 7.0. figure 1. electrochemical mechanism for oxidation of folic acid [57]. figure 2 represents cyclic voltammograms (cv) of bare spe (curve a) and znfe2o4 mnps modified spe (curve b) in 0.1 m pbs (ph 7.0) containing 60.0 μm of fa. based on cv outcomes, it may be stated that much higher oxidation peak current of fa was recorded for znfe2o4 mnps/spe, proving thus higher sensitivity of modified electrode than bare spe. also, peak potential for modified spe occurred at 590 mv, what is about 70 mv more negative in comparison to the unmodified spe. this suggests that modifying spe with znfe2o4 mnps improved the catalytic activity of the electrode towards fa. http://dx.doi.org/10.5599/jese.878 j. electrochem. sci. eng. 11(1) (2021) 1-9 vitamin b9 sensor based on screen-printed electrode 4 figure 2. cvs (50 mv s-1) of a) spe and b) znfe2o4 mnps/spe in presence of 60.0 µm of fa in 0.1 m pbs (ph 7.0). effect of scan rate figure 3 presents the effect of potential scan rate changes on the oxidation currents of vitamin b9 (fa), recorded by linear sweep voltammetry (lsv). the results show increase of anodic currents with potential scan rate, and only minor shifts of peak potential values. it can be observed in the inset of figure 3 that the peak current (ip) and the square root of the potential scan rate (ν1/2) are linearly associated, what demonstrates that the oxidation process of fa is fast and almost diffusioncontrolled process. figure 3. lsvs of znfe2o4 mnps/spe in 0.1 m pbs (ph 7.0) containing 40.0 μm fa at different scan rates (1-10 correspond to ν = 5, 10, 20, 40, 60, 80, 100, 200, 300 and 400 mv s-1). inset: variation of anodic peak current vs. ν1/2. m. zare and h. sarhadi j. electrochem. sci. eng. 11(1) (2021) 1-9 http://dx.doi.org/10.5599/jese.878 5 chronoamperometric analysis for chronoamperometric analysis, the working electrode potential was set at 0.64 v for each sample solution containing different concentration of vitamin b9 in 0.1 m pbs (ph 7.0), and the outputs are shown in figure 4. generally, the current for an electroactive material such as vitamin b9 at the mass transport limited condition, could be illustrated by cottrell's equation: i =nfad1/2cbπ-1/2t-1/2 (1) in eq. (1), n, cb, d and a refer to the number of electrons in redox reaction, bulk concentration of analyte (mol cm−3), diffusion coefficient (cm2s−1) and surface area of the electrode (cm2), respectively. the optimized fitting results of i vs. t−1/2 linear plots based on the experiments are, for different vitamin b9 samples, presented in figure 4a. the slopes of the obtained straight lines vs. concentration of vitamin b9 are shown in figure 4b. figure 4. chronoamperograms of znfe2o4 mnps/spe in 0.1 m pbs (ph 7.0) for various concentrations of vitamin b9 (1 to 4 correspond to 0.1, 0.5, 1.0, and 1.5 mm of fa). inset (a): current vs. t-1/2 plots achieved from chronoamperograms 1 to 4. inset (b): slopes of straight lines in fig. 4b vs. vitamin b9 concentrations. considering cottrell equation (1), the resulting slopes in figure 4, n=2 (figure 1), and geometric surface area of we of 0.0341 cm2, d value for fa in pbs solution was calculated equal to 6.3×10-5 cm2 s−1. calibration plot and limit of detection vitamin b9 (fa) was quantitatively analyzed using differential pulse voltammetry (dpv), because dpv technique offers several benefits, like better sensitivity for the analytical utilization. dpvs of the modified electrode (znfe2o4 mnps/spe) for different concentrations of vitamin b9 (1.0 to 100.0 μm) in 0.1 m pbs (ph 7.0) are shown in figure 5. it is obvious from the inset of figure 5 that the oxidation peak current and concentration of vitamin b9 are linearly correlated in the concentration range between 1.0 to 100.0 μm, with the correlation coefficient 0.9994. limit of detection (lod) is found equal to 0.3 μm. http://dx.doi.org/10.5599/jese.878 j. electrochem. sci. eng. 11(1) (2021) 1-9 vitamin b9 sensor based on screen-printed electrode 6 figure 5. dpvs of znfe2o4 mnps /spe in 0.1 m pbs (ph 7.0) containing diverse concentrations of vitamin b9 (1 to 8 correspond to 1.0, 5.0, 10.0, 20.0, 40.0, 60.0, 80.0 and 100.0 µm) inset: peak current plot vs. vitamin b9 concentrations from 1.0-100.0 µm. stability and repeatability of znfe2o4 mnps/spe stability of znfe2o4mnps/spe was tested by maintaining the proposed sensor electrode for 15 days in pbs solution of ph 7.0. prior and after immersion, cyclic voltammograms were recorded in the solution containing 30.0 µm of fa to obtain information on the stability of the prepared sensor electrode. a slight decrease in voltammetric response equal to 2.1 % was observed comparing with the initial response. this confirms that znfe2o4mnps/spe is highly stable sensor electrode. the anti-fouling characteristic for the oxidation of fa were tested by cyclic voltammetry of the modified spe before and after its use in the presence of fa. cyclic voltammograms were recorded after 15 potential cycles performed at 50 mv s−1 in the presence of fa. it was found that the current values were reduced less than 2.3 %, while the peak potential is not changed. these experimental results approved high electrode repeatability. analysis of real samples for assessing the usefulness of here proposed znfe2o4 mnps/spe sensor for fa determination, the analyses in some real samples were performed. quantifications of fa in vitamin b9 tablets and human urine samples were performed using the standard addition method, and the obtained analytical results are presented in table 1. it must be added here that the recovery of fa was found satisfactory, as well as reproducibility which was demonstrated on the basis of mean relative standard deviation (rsd) (1.7-3.5). m. zare and h. sarhadi j. electrochem. sci. eng. 11(1) (2021) 1-9 http://dx.doi.org/10.5599/jese.878 7 table 1. analytical results of znfe2o4 mnps/spe for determination of fa in vitamin b9 tablet and urine samples (n=5). sample concentrations, µm recovery, % rsd, % spiked found vitamin b9 tablet 0 6.0 2.9 2.5 8.6 101.2 3.4 5.0 10.8 98.2 1.8 7.5 14.0 103.7 2.2 10.0 15.9 99.4 3.0 urine 0 5.0 4.9 98.0 1.7 10.0 10.3 103.0 2.2 15.0 15.2 101.3 3.5 20.0 19.8 99.0 2.6 conclusions a disposable electrochemical sensor for detection of vitamin b9 (folic acid, fa) was developed using a screen-printed electrode (spe) modified by znfe2o4 magnetic nanoparticles (nps). based on the advantages of spe technology in combination with excellent electrocatalytic properties of magnetic znfe2o4nps, a highly suitable sensor for fa was developed. modified spe showed significantly improved oxidation peak current in comparison with unmodified spe. the electrochemical efficiency of the modified sensor was assessed using dpv, showing a wide linear range (1.0-100.0 µm) and low detection limit of 0.3 µm fa. testing performed with the developed znfe2o4 mnps/spe showed appropriate sensitivity, reproducibility, stability and repeatability of sensor electrode for exact determination of fa. determination of vitamin b9 in some real samples (vitamin b9 pharmaceutical tablets and human urine) performed using the developed sensor and standard addition method, showed very acceptable results. 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dopamine and uric acid hadi beitollahi1,2,, somayeh tajik3, mohammad reza aflatoonian4 and asghar makarem 1school of medicine, bam university of medical sciences, bam, iran 2environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran 3research center of tropical and infectious diseases, kerman university of medical sciences, kerman, iran 4leishmaniasis research center, kerman university of medical sciences, kerman, iran 5department of rehabmanagement, university of social welfare and rehabilitation sciences, tehran, iran corresponding author:  h.beitollahi@yahoo.com; tel.: +983426226613 received: january 2, 2022; accepted: february 13, 2022; published: february 21, 2022 abstract this research applied a nanostructured electrochemical sensor with a screen-printed electrode (spe) for examining the dopamine (da) electrocatalytic oxidation when uric acid (ua) was present. cu(salophen) nanostructured modified spe (cu(salophen)/spe) was employed to investigate the electrochemical behavior of da. at optimal ph (ph 7.0), oxidation of da at the modified electrode takes place at a potential around 100 mv less positive than at the unmodified spe. chronoamperometry was used to determine the diffusion coefficient of da (d = 1.96×10-5 cm2 s-1). differential pulse voltammetry (dpv) showed linear response in the range between 0.2-450.0 μm for da. the limit of detection (lod) of da was computed to be 0.05 μm. moreover, cu(salophen)/spe was employed for determining da in the presence of ua using dpv. the dpv results showed that at the modified electrode, two well-separated oxidation peaks of da and ua could be obtained at potentials of 180 and 325 mv, respectively. this separation forms the basis for the codetection of these two materials on the surface of cu(salophen)/spe. this sensor was then employed to determine da and ua in real specimens. keywords dopamine; uric acid; screen printed electrode; voltammetry, chemically modified electrodes, real sample analysis http://dx.doi.org/10.5599/jese.1231 http://dx.doi.org/10.5599/jese.1231 http://www.jese-online.org/ mailto:h.beitollahi@yahoo.com j. electrochem. sci. eng. 12(1) (2022) 199-208 cu(salophen) modified screen-printed electrode 200 introduction considerable benefits for designing and developing electrochemical sensors are provided by chemically modifying inert substrate electrodes via mediators [1-8]. redox-active sites facilitate the electron transfer rate decreasing the activation overpotential [9-21]. another benefit of the chemically modified electrodes is that they have less susceptibility to surface fouling and oxide formation in comparison with inert substrate electrodes [22-33]. recently, authors have increasingly focused on nanostructured materials, such as nanomaterials, nanoparticles, nanowires, and nanotubes, because of their specific physicochemical features, which may offer a prominent and functional ground for electroanalysis, especially when designing modified electrodes for electrochemical sensing [34-42]. dopamine (da) is a catecholamine neurotransmitter in the brain of mammals' central nervous system. da is a chemical secreted through neural cells for sending signals to other nerve cells. hence, it functions as a chemical messenger, which contributes significantly to the performance of hormonal, renal, and cardiovascular systems. there is a relationship between unusual levels of da and neurological abnormalities, including parkinson’s disease and schizophrenia [43]. a key catabolic product of purine nucleoside, guanosine, and adenosine is uric acid (ua, 2,6,8-trihydroxypurin). direct conversion of purines obtained from the dietary nucleic acids catabolism to ua is also completed. a number of illnesses are caused by abnormal amounts of ua in body fluids. hence, analytical determination of ua in biological samples is of key importance in preventing in treating these illnesses. among vast analytical procedures for the determination of ua, electrochemical techniques stand out due to their simplicity and versatility [44]. it is of high importance to simultaneously detect da and uric acid, because above compounds are essential biomedical compositions with a prominent contribution to the human metabolism. as we know, there are only a few studies conducted on the simultaneous detection of da and ua via modified spes. transition metal (e.g. copper, cobalt and iron) schiff base complexes are well recognized for their excellent electrocatalytic properties toward the detection of many important analytes. the electrochemical application of schiff base complexes of various metals is reviewed for a wide range of applications such as electrocatalyst for novel sensors development and ion-carriers in ion-selective electrodes. the selectivity pattern in the electrochemical responses of such chemically modified electrodes can be influenced by the structural characterization of the modifier species [45]. copper comples nanostructures are known to show attractive properties in electrode modification by improving the analytical sensitivity and selectivity during last few years [46]. the aim of this research was to investigate the suitability of a cu(salophen) / spe as a novel electrode in da and ua oxidation. afterward, the analytical response of the modified electrode was evaluated to quantify da in the presence of ua. eventually, the novel electrochemical sensor was utilized for detecting da and ua in real specimens. experimental chemicals and apparatus a potentiostat/galvanostat autolab pgstat 302n instrument equipped with a general-purpose electrochemical system (gpes) software has been utilized to measure electrochemical parameters. the screen-printed electrode was purchased from dropsens (drp-110, spain). a digital ph-meter (metrohm 710) was employed for measuring the ph values. da, ua and each remaining reagent were h. beitollahi et al. j. electrochem. sci. eng. 12(1) (2022) 199-208 http://dx.doi.org/10.5599/jese.1231 201 of analytical grade. furthermore, they were purchased from merck. finally, the phosphate buffer solution (pbs) were prepared from the orthophosphoric acid and the respective salts in a ph 2.0 9.0. synthesis of cu(salophen) the synthesis procedure of the salophen ligand is the same as described in the literature [47]. cu(salophen) has been procured via an easy low-temperature synthesis path at atmospheric pressures by reacting salophen ligands and copper chloride under reflux. in general, mixing and sonication of 1 mmol cucl2 6h2o, 1mmol salophen ligand, and 20 ml methanol were done (two hours, 60 °c). in addition, purification of the resulting green solid has been done by a 2-phase procedure through double solvent extraction with methanol and water. eventually, drying the solid was done in a vacuum desiccator at a temperature of 80 °c for two hours before another analysis or application. electrode preparation cu(salophen) was utilized for coating a bare spe. a stock solution of complex in 1 ml aqueous solution was prepared by adding of 1 mg complex. then 2 µl of it was casted onto the carbon working electrode. afterward, the electrode was put aside till the solvent evaporation was accomplished at room temperature. preparation of real samples da ampules (labeled 200 mg per 5 ml, caspian tamin co.; iran) were used. the ampule solution was diluted to 10 ml of pbs. different volumes of the diluted solution have been poured into a 25 ml volumetric flask. the analysis of da was performed by the standard addition method. urine was used for the analytical determination of da in biological samples. urine samples were stored in a refrigerator upon the collection. then, centrifugation of 10 ml of the specimens was done for 15 min at 2,000 rpm. afterward, a 0.45 µm filter was used for purifying the supernatant. for the next step, a various contents of the solution were transferred into a 25 ml cell. then, its dilution was completed to the mark with pbs (ph of 7.0). the diluted urine specimens were spiked with various levels of da and ua. finally, the recommended process has been used to analyze the amounts of da and ua employing the standard addition technique. electrochemical measurements all the experiments were performed at room temperature. the oxidation signals of dopamine were obtained using dpv, with a scan rate of 50 mvs−1 between -0.05 and +0.4 v in 0.1 m pbs. the data were baseline corrected with gpes software. repetitive measurements were carried out by repeating the above assay formats. results and discussion cu(salophen) structure and morphology figure 1 shows ft-ir spectrum of the synthesized cu(salophen). on spectrum the bands of o-h, c=n, c=c, c-o, and c-h vibrations emerge at 3358-3086, 1668-1609, 1503, 1293-1053, and 846, 762, 629 cm-1, respectively. moreover, cu(salophen) spectra exhibit new absorption bands in the region of 495 and 440 cm-1 that can be attributed to cu-cl and cu-n stretching modes [47]. scanning electron microscopy (sem) has been used to examine the product morphology. figure 2 depicts sem images of cu(salophen). according to the image, cu(salophen) shows plate-like nonagglomerated morphology with plane sizes less than 45 nm. http://dx.doi.org/10.5599/jese.1231 j. electrochem. sci. eng. 12(1) (2022) 199-208 cu(salophen) modified screen-printed electrode 202 wavenumber, cm-1 figure 1. ft-ir spectra of cu(salophen) figure 2. sem micrographs of the cu(salophen) for additional confirmation of the compositions of the resulting products, edx analysis was used (figure 3a). energy, kev figure 3. a edx spectra of cu(salophen); b, c and d map images of nanocrystal complex http://dx.doi.org/10.5599/jese.1231 203 h. beitollahi et al. j. electrochem. sci. eng. 12(1) (2022) 199-208 figure 3a shows the predominance of cl and cu atoms in the analysed sample. the map images of the complex are shown in fig. 3b. electrochemical behaviour of da at the surface of different electrodes figure 4 gives cyclic voltammetry (cv) response for electrochemical oxidation of 100.0 µm da at the unmodified spe (curve a) and cu(salophen)/spe (curve b). the effect of the surface is clearly visible on the obtained cyclic voltammograms. it is obvious that modified electrode facilitates the electron transfer and that the obtained cyclic voltammograms approach reversible behaviour. the potential of the anodic peak is approximately 300 mv at the bare spe surface (curve b) and 200 mv on the surface of cu(salophen)/spe (curve a). figure 4. cvs of cu(salophen)/ spe (a) unmodified spe (b) in 0.1 m pbs (ph 7.0) containing 100.0 µm da. in all cases the scan rate were 50 mv s-1 it is widely known that the electrochemical oxidation of da is sensitive to the ph of the aqueous solution. thus, optimizing the solution ph is crucial for the electroanalysis of da. hence, cv was used to explore the dopamine electrochemical behaviour in 0.1 m pbs at various ph-values (2.0 < ph <9.0) at cu(salophen)/spe surface. the results showed that the neutral conditions favour the da electrochemical oxidation at the cu(salophen)/spe surface compared to the basic or acidic media. therefore, ph 7.0 has been selected as an optimized ph for electrochemical da oxidation at cu(salophen)/spe surface. impact of scan rate we examined the impact of the potential scan rate on the cyclic voltammograms of da (fig. 5). figure 5. cvs of cu(salophen)/spe in 0.1 m pbs (ph 7.0) containing 100.0 μm da at various scan rates (10, 50, 100, 200, 300 and 400 mv s-1). inset: variation of anodic and cathodic peak current vs. ν1/2 y = 1.1422x + 2.8147 r2 = 0.9994 y = -0.928x + 3.5357 r2 = 0.9855 http://dx.doi.org/10.5599/jese.1231 j. electrochem. sci. eng. 12(1) (2022) 199-208 cu(salophen) modified screen-printed electrode 204 with the increase of scan rate, the height of the current peaks also increases. as demonstrated in the inset of figure 5 the anodic and cathodic peak currents linearly depend on the square root of the potential scan rate (ν1/2), indicating diffusion control of the reaction [48]. chronoamperometric analysis the scan rate dependence allowed us to calculate the diffusion coefficient by a chronoamperometric technique. to determine it, a potential step from 0 v to the potential of 0.25 v was applied (figure 6). from the slope of the straight lines of i versus t-1/2 and using cotrell equation the diffusion coefficient value for da was estimated at 1.96×10-5 cm2 s-1 [48]. figure 6. chronoamperograms obtained at cu(salophen)/spe in 0.1 m pbs (ph 7.0) for different concentrations of da (0.1, 0.8, 1.8 and 3.0 mm). insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-4. (b) plot of the slope of the straight lines against da concentration calibration curve it should be noted that the peak current of da using cu(salophen)/spe has been utilized for quantitative analysis of da. since dpv has benefits in terms of the greater sensitivity and better application features, the modified electrode was employed as working electrode in analysing of da. regarding the dpv of da using the cu(salophen)/spe, linear response was obtained in a concentration range from 0.2 to 450.0 µm and 0.9995 correlation coefficient. in addition, the related lod has been 0.05 µm. simultaneous analysis of da and ua the analysis was performed to assess the applicability of cu(salophen)/spe for simultaneous detection of da and ua in 0.1 m pbs (ph 7.0). the dpv oxidation peak currents were enhanced linearly with coelevation of da and ua concentrations, almost with no change in their oxidation peak potentials (figure 7). moreover, two distinct separated oxidation signals were found for da (180 mv) and ua (325 mv) based on the difference in their dpvs, which were enough for their simultaneous detection on the modified electrode surface. y = 15.147x + 17.528 r2 = 0.9993 s lo p e ,  a s -1 /2 h. beitollahi et al. j. electrochem. sci. eng. 12(1) (2022) 199-208 http://dx.doi.org/10.5599/jese.1231 205 fig. 7. dpvs of cu(salophen)/spe in 0.1 m pbs (ph 7.0) containing different concentrations of da and ua from inner to outer: 5.0 + 5.0, 40.0 + 50.0, 90.0 + 100.0, 250.0 + 300.0 and 450.0 + 600.0 m respectively. insets (a) plot of ip vs. da concentration and (b) plot of ip vs. ua concentrations. analyzing real sample for estimating analytical usability of the recommended technique, it has been utilized for detecting da and ua in the real samples. table 1 reports the outputs. findings showed good recovery and mean relative standard deviation (rsd) of the experimental outputs for da and ua. table 1. determination of da and ua in da ampoule and urine samples (n=5). sample c / m recovery, % rsd, % spiked found da ua da ua da ua da ua da ampoule 0.0 0.0 7.5 2.4 5.5 10.0 12.8 10.2 98.5 102.0 1.8 2.4 10.5 15.0 18.2 15.1 101.1 100.6 2.6 3.1 15.5 20.0 22.8 19.7 99.1 98.5 3.1 2.9 20.5 25.0 27.6 25.1 98.6 100.4 3.2 1.9 urine 0.0 0.0 10.0 15.0 10.1 14.9 101.0 99.3 1.8 2.9 20.0 25.0 19.7 25.7 98.5 102.8 3.1 3.1 30.0 35.0 29.8 34.8 99.3 99.4 3.3 1.7 40.0 45.0 40.2 45.2 100.5 100.4 2.6 2.6 conclusion a spe modified with cu(salophen) has been built in this research to investigate electrochemical behavior of da. electrochemical behaviors of da at cu(salophen)/spe surface revealed that catalyzing da electrooxidation has been done at ph 7.0. the modified electrode substantially removes the coincided voltammetric peaks of da and ua so that it shows higher selectiveness in dpv measurements of da and ua into their mixed solution. ultimately, the modified electrode has been investigated to detect da and ua in real specimens. the constructed sensor had some advantages such as 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https://doi.org/10.1016/j.snb.2008.03.034 https://doi.org/10.1016/j.matpr.2020.07.020 https://doi.org/10.1021/ic8017985 https://creativecommons.org/licenses/by/4.0/) {the effects of time-variance on impedance measurements: examples of a corroding electrode and a battery cell.} http://dx.doi.org/10.5599/jese.725 127 j. electrochem. sci. eng. 10(2) (2020) 127-140; http://dx.doi.org/10.5599/jese.725 open access: issn 1847-9286 www.jese-online.org original scientific paper the effects of time-variance on impedance measurements: examples of a corroding electrode and a battery cell nicolas murer, jean-paul diard and bogdan petrescu bio-logic sas, 4 rue de vaucanson, 38170 seyssinet-pariset, france corresponding author: nicolas.murer@bio-logic.net, tel.: +334 76 98 68 31 received: september 17, 2019; accepted: october 10, 2019 abstract when performing electrochemical impedance spectroscopy (eis) measurements on a system, we must make sure it fulfills certain conditions. one of them is that it should be stationary that is to say, steady-state and time-invariant. commonly studied systems are time-variant, for example a corroding electrode or a battery under operation. a corroding electrode sees its polarization resistance decrease with time. a passivating electrode sees its polarization resistance increase with time. these phenomena cause a deformation of the nyquist impedance at low frequencies. this result was first simulated and validated by experimental measurements on a corroding steel sample undergoing uniform corrosion. the effect of performing impedance measurements on a discharging battery was also shown. several methods are available to check and correct time-variance. the nonstationary distortion (nsd) indicator is used to separate valid and invalid data samples and the so called “4d impedance” method can easily produce instantaneous impedance data. keywords eis; non-stationarity; corrosion; in operando; battery cells; 4d impedance; nsd. introduction for a valid impedance measurement, the system under investigation should be linear, stable, causal and stationary [1,2]. these properties, which imply stationarity can be tested and checked using kramers-kronig [1] and hilbert transforms [3]. in this paper, the term “stationarity” comprises steady-state and time-invariance. steady-state is the state of a system after its transient state. for example, an r/c circuit with a definite time-constant is submitted to a potential or current step. its response will vary over time after reaching a steady-state. another example is any system submitted to a series of ac potential or current modulations of different frequencies, which is basically what an electrochemical http://dx.doi.org/10.5599/jese.725 http://dx.doi.org/10.5599/jese.725 http://www.jese-online.org/ mailto:nicolas.murer@bio-logic.net j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 128 impedance spectroscopy (eis) measurement is. when the frequency is being switched from one value to another, the system response is transient. time-variance is the property of a system whose parameters defining its transfer function change with time. for example, a corroding electrode whose polarization resistance changes over time, either because of corrosion or of passivation, is a time-variant system. a discharging battery also sees its various resistances (charge transfer, diffusion) evolve during discharge. sometimes steadystate and time-invariance are difficult to distinguish. the most classical use of eis in corrosion is for the determination of the polarization resistance rp using the wagner-traud relationship [4-8]. a corroding system is specifically a non-stationary system after the first instant of immersion. the change in parameters can greatly affect the impedance data especially at lower frequencies. in the field of batteries, eis measurements during continuous charge or discharge, or in operando, are performed to investigate how a battery alters while it is being operated [9-14]. the problem in this case is to know which impedance data can be considered valid, or more importantly, which frequency data should be considered erroneous because of the time-variance of the system. this problem of non-stationarity has been known for quite some time now [11] and various methods have been proposed to check and correct the effect of non-stationarity on impedance measurements. in this paper, first we tried to simulate the effect of a polarization resistance that changes with time on the impedance data of a simple corroding system whose dc characteristic follows the wagner-traud relationship. the deformation of the impedance diagrams at low frequencies were confirmed by experimental data obtained in acidic media on a stainless steel sample. secondly, impedance data were measured on a discharging battery, to show the effect of time-variance. finally, several methods are presented demonstrating how to check and correct the effect of timevariance: i) using galvano-controlled eis instead of potentio-controlled eis, ii) performing two successive eis measurements, iii) using a non-stationarity distortion indicator to separate valid and invalid impedance data and iv) using the 4d impedance representation to easily plot instantaneous impedance diagrams. simulation conditions let us consider a simple corroding system for which the steady-state behavior is described by the wagner-traud relationship, derived from the butler-erdey-grúz-volmer relationship, better known as the butler-volmer relationship: i = icorr (exp(b1(e – ecorr) – exp(-b2(e ecorr))) (1) where i is the current in a, icorr the corrosion current in a, e the potential in v, ecorr the corrosion potential in v, b1 and b2 the tafel slopes in v-1. assuming this relationship holds when the system is submitted to a dynamic perturbation and that this perturbation π = e – ecorr is small enough, the faradaic impedance of the considered system is the charge transfer resistance rct expressed as: ct f corr 1 1 2 2 1 exp( π)+ exp( π)) r z i (b b b b = = − (2) more details on this calculation can be found in [15,17]. adding the electrolyte resistance rω in series and the double layer capacitance cdl in parallel, the simplest way to model a corroding n. murer et al. j. electrochem. sci. eng. 10(2) (2020) 127-140 http://dx.doi.org/10.5599/jese.725 129 electrode is to use the equivalent electrical circuit shown in fig. 1 with its corresponding impedance nyquist diagram (fig. 1). figure 1. left equivalent circuit corresponding to an electrode in solution whose dc behavior follows the wagner-traud expression; right corresponding nyquist representation of its impedance. in the above example rct = rp. the analytical expression of the impedance z of a tafelian system in solution is known and can be simulated using mathematica [17]. p ω dl p ( )= 1 r z j r j c r   + + (3) where j = -1, ω the modulation pulse in rad s-1, rω the electrolyte resistance in ω and cdl the double layer capacitance at the electrode interface in f. two cases were simulated, first a polarization resistance that is decreasing with time and second a polarization resistance that is increasing with time following these relations: rp(t) = rp(0) kt (4) rp(t) = rp(0) – kt2 (5) where rp(0) is the value of the polarization resistance at the beginning of the simulation (t = 0) in ω, and k a constant factor in ω s-1/² for eq. (4) and in ω s-² for eq. (5). experimental conditions eis measurement on a corroding sample potentio-controlled eis measurements were performed on a large mild steel sample (undisclosed composition) using a coating cell, a carbon rod electrode, an ag/agcl reference electrode and 0.1 m h2so4 solution. six eis successive measurements were performed using a bio-logic sp-200, ec-lab® software. a 20 mv voltage amplitude modulation was applied around ocv with frequencies ranging from 200 khz to 10 mhz with 6 points per frequency decade. for each frequency, the result was an average of two measurements. eis measurements on discharging battery cell a 26650 lfp battery cell produced by a123 was used. a discharge of -100 ma during 130 s was performed followed by a galvano-controlled eis (geis) measurement with a dc current of -100 ma and an ac current modulation of -200 ma at frequencies ranging from 1 khz to 10 mhz with 6 points per frequency decade. using a loop technique, the experiment, composed of a discharge followed by a geis, was performed nine times. http://dx.doi.org/10.5599/jese.725 j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 130 results simulation the following values were used: rω = 50 ω, rp(0) = 500 ω, k = 1.5 ω s-1/2 (decreasing rp(t)) or 0.5 10-5 ω s-2 (increasing rp(t)), cdl = 2 x 10-2 f, fmax = 10 hz and fmin = 10−3 hz. all frequencies are logarithmically distributed over 8 points per decade. impedance data are calculated over frequency values fi, which are used to calculate t: n n n i i=1 i 1 1 i t( f ) t( f ) f = = =  (6) with t the time in s and f the frequency in hz. please note that this formula assumes that for each frequency, only one period is used for the simulation, which is not the case in a real measurement, where at high frequencies, generally several periods of the signal are used to measure the impedance. the obtained results are shown in figure 2. e) figure 2. nyquist representation of impedance data obtained using eq. (3) and parameters written above with dots: rp(t); thin blue line: rp(0) ; thick orange line: rp(t = tmax) (a) for a decreasing rp(t) scanning from fmin to fmax (b) for a decreasing rp(t) scanning from fmax to fmin (c) for an increasing rp(t) scanning from fmin to fmax (d) for an increasing rp(t) scanning from fmax to fmin (e) 3d representation of the results shown in (d). the size of the dots increases with time z’ /  z’ /  z’ /  z’ /  z’ /  -z ” /  -z ” /  -z ” /  -z ” /  -z’’ /  n. murer et al. j. electrochem. sci. eng. 10(2) (2020) 127-140 http://dx.doi.org/10.5599/jese.725 131 it can be seen in figure 2 that as time goes by, the data points move slowly from the thin blue curve to the thick orange curve, which is to be expected, as the thin blue curve is the instantaneous impedance diagram at t = 0 and the thick orange curve is the instantaneous impedance diagram at t = tmax. with a decreasing rp(t) and scanning from high to low frequencies the nyquist graph is “folding over” smaller re z values at lower frequencies (fig. 2a). a similar shape can be seen with an increasing rp(t) when scanning from low to high frequencies (fig. 2c). with a decreasing rp(t) and scanning from low to high frequencies, the nyquist graph forms a tail over larger re z values, at lower frequencies ie longer times (fig. 2b). the same shape can be seen with an increasing rp(t) and scanning from high to low frequencies (fig. 2d). figure 2e is a pseudo 3d representation of figure 2d. corrosion experiment the results are shown in figure 3. please note that the frequency sweep was performed from high to low values, corresponding in the simulation data to figures 2b and 2d. it can be seen in figure 3a that right after immersion (graph 1 in fig. 3a), the instantaneous polarization resistance seems to increase and then to rapidly decrease, hence the occurrence of a loop at lower frequencies (fig. 3b). the next five impedance diagrams show the “fold-over” of the values at lower frequencies, which, according to figure 2a is characteristic of a decrease of the instantaneous polarization resistance. it should be noted that unlike the simulated data, the measured data show an inductive behavior at lower frequencies, with values that have a positive imaginary part. we can assume, considering the ph of the electrolyte, that a volmer-heyrovský mechanism is taking place: it is a common two-step mechanism for hydrogen evolution, with an adsorption step and dihydrogen release step. it was calculated that the impedance of such a reaction shows an inductive loop at lower frequencies [18]. however, the general trends predicted by the simulation (fig. 2) seem to be validated by experimental data (fig. 3a). a) b) z’ /  z’ /  figure 3. (a) nyquist representation of the impedance data (200 khz – 10 mhz frequency range) obtained on a mild steel sample (undisclosed composition) just after immersion in 0.1 m h2so4. the six measurements were performed one after another. (b) close-up on the low frequency part of the first obtained graph battery experiment figure 4 shows the successive impedance diagrams obtained on the batteries under discharge. before each impedance measurement, the battery is being discharged during 130 s at -100 ma. the 0.50 0.00 -0.50 -1.00 -1.50 -2.00 -z ” /  -z ” /  http://dx.doi.org/10.5599/jese.725 j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 132 impedance measurement is performed with galvanic control hence there is no discontinuity between the discharge and the impedance measurement. figures 4a to 4e show the first five successive measurements. figure 4f is the last measured impedance diagram, i.e. the tenth impedance diagram. figure 4. nyquist representation of the impedance data obtained on a lfp 26650 battery from a123 using the conditions shown in fig. 4 (1 khz – 10 mhz frequency range) (a)-(e) cycles 1-5 (f) cycle 10. each impedance measurement was preceded by a discharge at -100 ma during 130 s. the dc potential evolution during the discharge and the geis experiment are shown in fig. 5 a) b) 0.00 0.02 0.04 0.00 0.02 0.04 z’ /  z’ /  c) d) 0.00 0.02 0.04 0.00 0.02 0.04 z’ /  z’ /  e) f) 0.00 0.02 0.04 0.00 0.02 0.04 z’ /  z’ /  re ( z ) / oh m 0,040,020 im ( z ) /o h m 0,04 0,03 0,02 0,01 0 re ( z ) / oh m 0,040,020 im ( z ) /o h m 0,04 0,03 0,02 0,01 0 re ( z ) / oh m 0,040,020 im ( z ) /o h m 0,04 0,03 0,02 0,01 0 re ( z ) / oh m 0,040,020 im ( z ) /o h m 0,04 0,03 0,02 0,01 0 re ( z ) / oh m 0,040,020 im ( z ) /o h m 0,04 0,03 0,02 0,01 0 re ( z ) / oh m 0,040,020 im ( z ) /o h m 0,04 0,03 0,02 0,01 0 0.04 0.03 0.02 0.02 0.00 -z ”/  0.04 0.03 0.02 0.02 0.00 2 -z ”/  0.04 0.03 0.02 0.02 0.00 -z ”/  0.04 0.03 0.02 0.02 0.00 -z ”/  0.04 0.03 0.02 0.02 0.00 -z ”/  0.04 0.03 0.02 0.02 0.00 -z ”/  0.04 0.03 0.02 0.02 0.00 0.04 0.03 0.02 0.02 0.00 2 n. murer et al. j. electrochem. sci. eng. 10(2) (2020) 127-140 http://dx.doi.org/10.5599/jese.725 133 the first three diagrams are quite different and then the data seem to stabilize. figure 5 shows the potential evolution during the discharge and the impedance measurement for cycles 1 to 5 (fig. 5a to 5e) and cycle 10 (fig. 5f). we can see that the potential mostly decreases during the first three cycles and then starts to stabilize. figure 5. dc potential evolution obtained on a lfp 26650 battery from a123 during eis measurements preceded by a discharge at -100 ma during 130 s (a)-(e) cycles 1-5 (f) cycle 9. the dc potential evolution during the discharge is shown by thick markers and during the impedance measurement by a thin line a) b) time, s time, s c) d) time, s time, s e) f) time, s time, s t i m e / s 5 0000 < e w e > /v 3,55 3,5 3,45 3,4 3,35 gcpl.mpr : ew e vs. time, loop 0 geis.mpr : <ew e> vs. time, cycle 1 # t i m e / s 5 0000 < e w e > /v 3,55 3,5 3,45 3,4 3,35 gcpl.mpr : ew e vs. time, loop 1 geis.mpr : <ew e> vs. time, cycle 2 # t i m e / s 5 0000 < e w e > /v 3,55 3,5 3,45 3,4 3,35 gcpl.mpr : ew e vs. time, loop 2 geis.mpr : <ew e> vs. time, cycle 3 # t i m e / s 5 0000 < e w e > /v 3,55 3,5 3,45 3,4 3,35 gcpl.mpr : ew e vs. time, loop 3 geis.mpr : <ew e> vs. time, cycle 4 # 3.55 3.50 3.45 3.40 3.35 3.55 3.50 3.45 3.40 3.35 3.55 3.50 3.45 3.40 3.35 3.55 3.50 3.45 3.40 3.35 3.55 3.50 3.45 3.40 3.35 3.55 3.50 3.45 3.40 3.35 e w e / v e w e / v e w e / v e w e / v e w e / v e w e / v  gcpl mpr: ewe vs. time, loop 0  gcpl mpr: ewe vs. time, loop 1 − geis mpr: ewe vs. time, cycle 1# − geis mpr: ewe vs. time, cycle 2#  gcpl mpr: ewe vs. time, loop 2  gcpl mpr: ewe vs. time, loop 3 − geis mpr: ewe vs. time, cycle 3# − geis mpr: ewe vs. time, cycle 4#  gcpl mpr: ewe vs. time, loop 4  gcpl mpr: ewe vs. time, loop 8 − geis mpr: ewe vs. time, cycle 5# − geis mpr: ewe vs. time, cycle 9# http://dx.doi.org/10.5599/jese.725 j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 134 the comparison of figures 4 and 5 shows that the unusual impedance values at low frequencies are due to the time-variance of the system, which is directly related to the dc current applied to the battery. it is quite difficult to interpret these data, and the peculiar shape of the impedance data at lower frequencies is not related to a specific electrochemical phenomenon but to the change of the systems parameters and a transient behavior. which data can be reliably interpreted and considered as valid? how can i make sure that my interpretation of the data is not erroneous, for example that the time-variance effect is not interpreted as an inductive loop? in the next part of this paper, we will present some tools that will help address such problems. discussion: how to check and correct time-variance? perform successive eis measurements an obvious and easy means of checking time-variance is to perform successive eis measurements, scanning frequencies either in the same direction each time or reversing the order, that is to say from high to low and back to high frequencies. figure 3a is a good example of such a measurement. perform eis with galvano control impedance spectroscopy can be performed by controlling the potential, which means that the input signal is a potential modulation around a dc potential and the response of the system is a current modulation around a dc current. in this case, the transfer function is not the impedance but the admittance of the system. however, the input modulation can also be an ac current modulation around a dc current, in which case the response is an ac potential modulation around a dc potential. in this case, the transfer function is the impedance of the system. the impedance diagrams shown in fig. 3 were obtained using the potential-controlled eis (peis). figure 6 shows the evolution of the dc current during this experiment, for which the potential modulation is applied around the open circuit potential (ocp) measured just before the beginning of the measurement. time, s figure 6. evolution of dc current and dc potential during the six potentio-controlled eis experiments shown in fig. 3. it can be seen in figure 6, that the potential is maintained at a constant level over the whole experiment duration but that the dc current is rapidly moving away from zero, meaning that as the time goes by, the sample is anodically polarized due to the evolution of its free corrosion potential t i m e / s 5 0000 < e w e > /v -0,8 -0,85 -0,9 -0,95 < i> /m a 1,2 1 0,8 0,6 0,4 0,2 0 -0.80 -0.85 -0.90 -0.95 1.2 1.0 0.8 0.6 0.4 0.2 0.0 e w e / v i / m a n. murer et al. j. electrochem. sci. eng. 10(2) (2020) 127-140 http://dx.doi.org/10.5599/jese.725 135 (or ocp). this phenomenon is illustrated in figure 7a, which shows that if the steady-state i vs. e characteristic of a corroding sample is moving towards more cathodic potentials, the initial ocp becomes an anodic potential. potentio-controlled (pc) impedance measurements are not performed around the initial ocp but around a specific operating point on the anodic part of the steady-state curve. on the contrary, as illustrated in figure 7b, if the impedance measurement is performed using current control (or galvano-control gc) and around zero current (which is equivalent to ocp), even though the steady-state curve is changing and moving towards cathodic potentials, the modulation is still performed around the same point on the steady-state curve. figure 7. description of the effect of (a) potential control (pc) and (b) galvano control (gc) when the steady-state curve of the system is moving towards more cathodic potentials. only galvano control at zero current allows to follow the change of the corrosion potential. the effect of using a controlled current instead of a controlled potential can be seen in figure 8. figure 8a shows the impedance diagram measured while the corrosion potential and the corrosion current change as shown in figure 7a: the blue i vs. e characteristic is at t = 0 and the orange i vs. e characteristic is at t = tmax. figure 8b shows the same impedance diagram but measured using a controlled current. in both cases, the effect of the system’s time-variance can be seen at lower frequencies, but this is reduced in the case of a controlled current and is only due to the corrosion current change and not to the corrosion potential change. the fact that the sample is not anodically polarized reduces the extent of the time-variance of the system. 0 r rp,tmax rp,0 0 r rp,tmax rp,0 z’ z’ figure 8. nyquist representation of simulated impedance data obtained with (a) potential control (pc) and (b) galvano control (gc) when the steady-state curve of the system is moving towards more cathodic potentials. in galvano control the time-variance is only due to the change of corrosion current but not to the change of corrosion potential. the parameter values are: rω = 20 ω; cdl = 3 × 10-3 f. rp(t) is given by the eq. (2) with icorr(t) and ecorr(t) changing linearly with time. et=tmax et=0 et=tmax et=0 e e -z ” -z ” http://dx.doi.org/10.5599/jese.725 j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 136 non-stationary distortion (nsd) indicator in this paper, as explained in the introduction, what we mean by non-stationarity is two-fold: i) the fact that the system is in a transient regime that it has not yet reached its steadystate. its transfer function stays the same throughout the experiment but its steady-state response is not reached instantaneously, it is lagging. ii) the fact that the system sees its transfer function or the values of the parameters constituting its transfer function change over time. this is what we called throughout the paper time-variance. both phenomena have a specific effect on the response signal, which can be seen on the impedance diagram, but also on its fourier transform (ft), which gives a frequency representation of the signal. the ft of the response signal of a stationary system will show a line at the same frequency as the input signal (fig. 9). the response of a non-stationary system, whether it is in a transient state (fig. 10a) or time-variant (fig. 10b), will show lines not only at the same frequency as the input signal but also at adjacent frequencies. time frequency figure 9. time and frequency representation of the response of a stationary system to a single sinus wave with a frequency f (taken from [19]) a) time frequency b) time frequency figure 10. time and frequency representation of the response of (a) a non-steady state system and (b) a time-variant system to a single sinus wave with a frequency f (taken from [19]). the amplitude of the adjacent lines around the signal response at the fundamental frequency is proportional to the extent of non-stationarity of the system. n. murer et al. j. electrochem. sci. eng. 10(2) (2020) 127-140 http://dx.doi.org/10.5599/jese.725 137 we can introduce an indicator that can be used to quantify the non-stationarity of the signal, whether it is due to a transient state or to a time-variance. we call this indicator nsd which is calculated as follows: 2 2 -δ δ 1 nsd( )= f f f f f f y y y + + (7) where yf is the amplitude at the stimulation or fundamental frequency, y²f-δf and y²f+δf are the amplitudes at the adjacent frequencies to the fundamental and δf is the spectral frequency resolution. figure 11a shows the nsd for the impedance measurements shown in figure 4 and again here in figure 11b. log (f / hz) z’ /  figure 11. (a) nsd of the current response as a function of the frequency and (b) the corresponding impedance measurements. the 1 hz boundary is also shown on both graphs, which could be used as a threshold value to separate stationary and quasi-non-stationary impedance data. the nsd is dependent on the frequency. if the measurement is fast enough, it is not affected by the non-stationarity, especially by the time-variance of the system. a nsd value as low as 2.5 % can have a significant effect on the shape of the impedance diagrams as it can be seen in figure 11. the nsd is used to determine a lower frequency limit for usable data, ie data that can be considered quasi-instantaneous, quasi-time-independent or quasi-steady-state. in figure 11 this limit is fixed at 1 hz and we consider that below this frequency the impedance measurements are affected by nonstationary phenomena. figure 12 shows the nsd on the potential response for the impedance data measured on a discharging battery shown in figure 4. it is clear that the conditions are highly non-stationary with a nsd reaching a maximum of 120 % at around 100 mhz for the first measurement (fig. 12a) where the dc potential changes the most (fig. 5a). the maximum nsd is reduced drastically for the second measurement reaching 50 % at 10 mhz (fig. 12b). the maximum nsd does not go over 4 % at all frequencies from the fifth measurement (fig. 12e). at this point, the potential does not change along the measurement (fig. 5e), we have reached the nominal potential of the battery. at the ninth measurement, the nsd is very close to 0 % for all frequencies (fig. 12f). in the same way as for the impedance measurement on a corroding system, we can determine a lower frequency limit below which we consider that the measurements are not valid and cannot be interpreted without accounting for a time dependence. in the case of the measurement on the battery cell, this lower limit would be 10 hz. it is indicated as a straight line in figure 12. we have seen in this part that the nsd indicator could be used to indicate the non-stationarity of the system, which can lead to strong deformation of the impedance data and wrong n s d i , % -z ” /  2.5 2.0 1.5 1.0 0.5 0.0 http://dx.doi.org/10.5599/jese.725 j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 138 interpretation. in the next part we will explain the “4d impedance” method to correct the effect of time-variance on impedance data and lead to data that can be considered as valid or quasistationary and rightly interpreted. (a) (b) log (f / hz) log (f / hz) (c) (d) log (f / hz) log (f / hz) (e) (f) log (f / hz) log (f / hz) figure 12. nsd of the potential response from impedance measurements shown in fig. 7. (a)-(e) cycles 1-5 (f) cycle 9. even if the dc potential does not seem to change the effect of time-variance is still detectable thanks to nsd. it seems only data from 10 khz to 10 hz can used. “4d impedance” this method was first introduced by stoynov in his seminal paper [9]. firstly, the acquired impedance data need to be represented as a function of time, as it was first shown in fig. 2e. figure 13a shows the results from fig. 3a represented as a function of time. secondly, all data points obtained at the same frequency but at different times are interpolated to produce what we could name a temporal impedance envelope (fig. 13b). for each frequency, z’ and -z’’ are expressed as a function of time. l o g ( f r e q / hz ) 20-2 n s d e w e /% 120 100 80 60 40 20 0 geis.m pr nsd ew e vs. log (f req), cycle 1 l o g ( f r e q / hz ) 20-2 n s d e w e /% 120 100 80 60 40 20 0 geis.m pr nsd ew e vs. log (f req), cycle 2 l o g ( f r e q / hz ) 20-2 n s d e w e /% 120 100 80 60 40 20 0 geis.m pr nsd ew e vs. log (f req), cycle 3 l o g ( f r e q / hz ) 20-2 n s d e w e /% 120 100 80 60 40 20 0 geis.m pr nsd ew e vs. log (f req), cycle 4 l o g ( f r e q / hz ) 20-2 n s d e w e /% 120 100 80 60 40 20 0 geis.m pr nsd ew e vs. log (f req), cycle 5 l o g ( f r e q / hz ) 20-2 n s d e w e /% 120 100 80 60 40 20 0 geis.m pr nsd ew e vs. log (f req), cycle 9 n s d e w e , % n s d e w e , % n s d e w e , % n s d e w e , % n s d e w e , % n s d e w e , % n. murer et al. j. electrochem. sci. eng. 10(2) (2020) 127-140 http://dx.doi.org/10.5599/jese.725 139 thirdly, the instantaneous impedance is calculated using the interpolations obtained before. the instantaneous impedance diagram is considered as a cross-section of the temporal impedance envelope. in figure 13c, three cross-sections are shown which are considered to be the instantaneous impedance diagrams corresponding to graphs 2, 4 and 6 in fig. 3a, or more importantly, as impedance data of the system in the state it is at the start of frequency scans 2, 4 and 6. these data, corrected for time-variance, can now be considered valid or quasi-stationary and could be interpreted using equivalent circuit modelling for example. this method has been recently successfully applied on various materials such as poly(oethoxyaniline) [20] and poly(3,4-ethylenedioxythiophene) [21] modified electrodes. (a) (b) (c) z’ /  figure 13. (a) impedance data from fig. 6a plotted as a function of time. (b) same gra phs as before but with interpolating iso-frequency curves in dots. (c) calculated instantaneous impedance diagrams at times corresponding to the first points of graphs 2, 4 and 6 in fig. 6a. conclusions in this paper, we have tried to show the effect of system time-variance on impedance measurements and to present some tricks and methods to both detect and correct these effects. using a calculation software (mathematica [19]) we first simulated the effect of a time-changing polarization resistance on the low-frequency impedance of a simple corroding system. experimental data obtained on a mild steel sample in sulfuric acid showed similar trends than the simulated ones. we also showed some data obtained on a discharging battery. impedance data can be considered stationary when the nominal voltage of the battery is reached. several methods available to check that impedance measurements are performed on a stationary system or not were presented: i) performing successive measurements; ii) using the nsd indicator. this indicator is also used to determine a lower frequency limit below which the data cannot be considered as valid. it is also recommended in corrosion to use galvano-controlled eis so that the change of corrosion potential is accounted for. z’ /  z’ /  -z’’ /  -z’’ /  -z ” /  t / min t / min http://dx.doi.org/10.5599/jese.725 j. electrochem. sci. eng. 10(2) (2020) 127-140 time-variance in impedance measurements 140 finally, the “4d impedance” method was described in detail, which seems to be the only method to easily remove the effect of time-variance. references [1] m. urquidi-macdonald, d. macdonald, electrochimica acta 35 (1990) 1559-1566. [2] k. darowicki, journal of electroanalytical chemistry 486 (2000) 101-105. [3] c. a. schiller, f. richter, e. gülzow, n. wagner, physical chemistry chemical physics, 3 (2001) 374378. [4] c. wagner and w. traud, zeitschrift für elektrochemie und angewandte physikalische physik 44 (1938) 391-402. [5] m. stern and a. l. geary, journal of the electrochemical society 104 (1957) 56-63. [6] i. epelboin, m. keddam, h. takenouti journal of applied electrochemistry 2 (1972) 71-79. [7] f. mansfeld in advances in corrosion science and technology m. g. fontana et al. eds., plenum press, new york, u.s.a., 1976, p. 163. [8] c. gabrielli, m. keddam, h. takenouti in treatise on materials science and technology; corrosion: aqueous processes and passive films, j. c. scully, ed., academic press inc., london, u.-k., 1983, p. 395 [9] z. b. stoynov, b. s. savova-stoynov, t. kossev, journal of power sources 30 (1990) 275-285. [10] f. berthier, j.-p. diard, a. jussiaume, j.-j. rameau, corrosion science, 30 (1990) 239-247. [11] m. keddam, z. b. stoynov, h. takenouti, journal of applied electrochemistry 7 (1977) 539-544. [12] m. itagaki, k. honda, y. hoshi and i. shitanda, journal of electroanalytical chemistry 737 (2015) 7884. [13] m. itagaki, n. kobari, s. yotsuda, k. watanabe, s. kinoshita, m. ue, journal of power sources 135 (2004) 255–261. [14] j.-p. diard, b. le gorrec, c. montella, journal of power sources, 70 (1998) 78-84. [15] c. montella, j.-p. diard, b. le gorrec, exercices de cinétique électrochimique ii. méthode d’impédance, hermann eds., 2005, p. 148, p. 217. [16] m. e. orazem, b. tribollet, electrochemical impedance spectroscopy, wiley & sons eds., 2008, p. 165. [17] wolfram research, inc., mathematica, version 12.0, champaign, il (2019). [18] j.-p. diard, p. landaud, b. le gorrec, c. montella, journal of electroanalytical chemistry 255 (1988) 1-20. [19] bio-logic website, support section https://www.bio-logic.net/wp-content/uploads/wp2systems.pdf (05/09/2019). [20] v. horvat-radošević, k. kvastek, k. magdić košiček, bulgarian chemical communications 49 (2017) 119-127. [21] d. zalka, s. vesztergom, m. ujvári, g. g. láng, journal of eletrochemical science and engineering 8 (2018) 151-162. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) https://www.bio-logic.net/wp-content/uploads/wp2-systems.pdf https://www.bio-logic.net/wp-content/uploads/wp2-systems.pdf http://creativecommons.org/licenses/by/4.0/) the effect of cetyltrimethylammonium bromide on size and morphology of zno and cuo doi: 10.5599/jese.2014.0063 97 j. electrochem. sci. eng. 4(3) (2014) 97-110; doi: 10.5599/jese.2014.0063 open access: issn 1847-9286 www.jese-online.org original scientific paper the effect of cetyltrimethylammonium bromide on size and morphology of zno and cuo anantha n. subba rao, venkatesha t. venkatarangaiah department of p.g. studies and research in chemistry, school of chemical sciences, kuvempu university, shankaraghatta-577451, karnataka, india corresponding author: e-mail: drtvvenkatesha@yahoo.co.uk, tel.: +91-9448855079; fax: +91-08282-256255 received: april 1, 2014; revised: september 15, 2014; published: september 22, 2014 abstract the nanoparticles (np) zno and cuo were synthesized by electrochemical-thermal method. the influence of cetyltrimethylammonium bromide (ctab) on size and morphology of np was evaluated. they were characterized by powder x-ray diffraction spectroscopy (xrd), scanning electron microscopy (sem), uv-visible absorption spectroscopy. the average crystallite size and the average grain size of np decreased with ctab concentration. the ctab significantly affected the morphology of cuo and zno np. the regular spindle shape of cuo transformed into irregular spherical shape and the homogeneity in the morphology of spherical zno np was lost with increase in ctab concentration. the effect of morphology and size of zno on its photocatalytic activity was evaluated by subjecting methylene blue (mb) dye to photocatalytic degradation under the irradiation of uv light. the color removal of mb dye during electrolysis was monitored by uv-visible spectroscopy. the highest photocatalytic activity was noticed for zno 10 mm ctab. keywords electrolysis; photocatalyst; morphology; methylene blue; degradation; surfactant; nanoparticles; color removal; absorbance; band gap introduction the tio2, sno2, fe2o3, pdo, wo3, ga2o3, in2o3, zro2, cuo and zno are amongst the extensively studied transition metal-oxide semiconducting nanomaterials and have potential applications in optics, optoelectronics, microelectronics, biosensors, gas sensors, chemical sensors, switching, magnetic, luminescent, electrical and acoustic devices, solar cells, catalysis and powder metalhttp://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 98 lurgy [1-4,5]. the cuo and zno nanomaterials are particularly of special interest as they are cheap, easy to prepare by simple, low cost methods at normal temperatures [6]. they exhibit diverse nanostructure configurations and show superior performance in many applications. cuo is a p-type semiconductor with band gap of 1.8 to 2.5 ev [2,7]. it is an excellent solar absorber with low emittance, which makes it a suitable candidate for solar cells as thermal collector [8]. the suspension of cuo nps in the heating or cooling fluids substantially increases the thermal conductivity [9]. cuo is highly responsive in non-enzymatic glucose sensing and h2s gas sensing [1,10-12]. cuo on ceria shows high catalytic activity for the oxidation of no, co and dehydrogenolysis of ch4 [13]. zno on the other hand, is extensively studied and widely recognized for its photocatalytic activity. zno, with a high surface reactivity owing to large number of active sites, has emerged to be an efficient photocatalyst [14,6]. the nanocrystalline zno is non-toxic and has the ability to kill or restrain bacteria [4]. it has a wide band gap of 3.2 ev and large exciton binding energy of 60 mev, which makes it a preferable material in low-voltage and short wavelength optoelectronic devices such as light emitting diodes and diode lasers [5]. the applicability of zno is widening with the extending knowledge of its physico-chemical properties. zno nanomaterials are also used in solar cells, luminescent, electrical and acoustic devices, chemical sensors, catalysis, electronics, gas sensor devices, optoelectronics, transducers [3,4]. the synthesis of zno and cuo thus acquires special interest. the common chemical synthetic methods adopted in the preparation of these nanomaterials are precipitation, sol-gel, hydrothermal, solvothermal, solution combustion, thermal decomposition, simple hydrolysis, wetchemical, microemulsion, non-microemulsion, microwave assisted solvothermal, ultrasonic radiation precipitation, mechanical milling, electrochemical, chemical vapor deposition (cvd), laser vaporization, exfoliation, laser ablation methods and so on [2]. however, these methods involve organic solvents, rigorous reaction conditions, demand sophisticated instruments and few of these techniques require longer time duration affording less yield than expected [4]. the organic solvents can be replaced completely with aqueous medium using electrochemical method. electrochemical method is fast, cheap, and easy to control and provides appreciable yields even at normal temperature and mild conditions. it was reetz and co-workers who proposed the sacrificial anode electrochemical route for the synthesis of nano metal clusters and colloids and showed that it is possible to control the particle size by adjusting the applied current density [15-17]. gao-qing et al. demonstrated that the size and shape of the synthesized cuo nanocrystals could be controlled by changing the solvent system [12]. mahamuni and co-workers prepared zno quantum dots using the sacrificial anode electrochemical method [18]. the zno [19], cu2o [20], cuo [21], au/agi [22] have been successfully synthesized by electrochemical method. we have recently synthesized hexagonal shaped cuo np in bulk [2], cubic shaped zn2so4 [23] and large size zno np [3,4] by hybrid sacrificial anode electrochemical-thermal route. there are many factors to be monitored in the electrochemical synthesis to control the size and shape of nps. the current density, distance between the electrodes, solvent system, temperature, supporting electrolyte and stabilizing agent are the most critical parameters which dictate the size and morphology of resulting nps. the role of stabilizing agent is to bind to the surface of np, decrease the surface energy, control growth and shape evolution and prevent the agglomeration of nps [24]. cetyl trimethylammonium bromide (ctab) a cationic surfactant is the most commonly used stabilizing agent in controlling the shape and size of nps [24-29]. oscar et al. [24] reported that in the synthesis of gold nanorods, ctab dictates the crystal growth along the faces by binding a. n. subba rao et al. j. electrochem. sci. eng. 4(3) (2014) 97-110 doi: 10.5599/jese.2014.0063 99 to the surfaces along the sides of nanorod. reetz and co-workers have demonstrated the role of surfactant (ctab) in the formation of nanoclusters and nanocolloids [15-17]. in this paper, we have systematically studied the effect of ctab concentration in the electrolyte bath, on the morphology and size of zno and cuo metal oxide nps prepared by hybrid sacrificial anode electrochemical-thermal route. furthermore, the photocatalytic degradation of methylene blue dye on zno np was studied in order to evaluate the effect of size and shape of the nps on its photocatalytic activity. experimental procedure materials and synthesis of zno and cuo np zinc and copper metal plates (99.99 % pure) were purchased from sisco research laboratories, mumbai. ar grade nahco3, nano3, ctab and methylene blue (mb) dye purchased from s. d fine chemicals ltd. india, were used without further purification. the electrolyte solution was prepared in millipore water (specific resistance > 18 mω at 25 °c, millipore elix 3 water purification system, france). the electrolysis was carried out under constant current supply from potentiostat / galvanostat source (model ps-618 chemilink systems, navi mumbai, india). the experimental procedure for the electrochemical-thermal synthesis of zno np was followed as mentioned in our previous work [4]. the two zn metal plates with dimension 4 x 4 x 0.1 cm 3 , were dipped in 1 m hcl for 30 seconds and washed thoroughly with millipore water. these plates were then immersed in a rectangular electrolytic glass cell containing 250 cm 3 of 60 mm nahco3 with or without ctab. three different concentrations of ctab (5 mm, 10 mm and 15 mm) were used. the np prepared in presence of ‘x’ mm concentration of ctab is hereafter represented as mo/x, where m is zn or cu. the distance between the two electrodes was maintained to be 5 cm. the electrolysis was carried out for 90 minutes under constant current density of 4.9 ma cm -2 with constant stirring at the rate of 500 rpm. the electrolytic bath temperature was maintained around 26 ± 1 °c. the ph of the electrolytic bath was noted before and after electrolysis. the resulting colloidal solution was filtered and the solid residue thus obtained was calcined at 300 °c for 1 hour. the same procedure was followed in the preparation of cuo np, except that the electrodes were pretreated cu plates and the electrolyte was 60 mm nano3. as a pretreatment, the cu metal plates were dipped in 5 % hno3 for 1 minute. characterization of np the crystallite size of the nps was evaluated by powder x-ray diffraction (xrd) pattern using x’pert pro diffractometer (phillips, cu-kα radiation, λcu= 1.5148 å) working at 30 ma and 40 kv recorded in the 2 range between 10° and 90° (scan rate 1° min -1 ). the scherrer equation was made use in estimating the average crystallite size. the scanning electron microscope (philips xl 30) fitted with an energy dispersive x-ray (edax) analyzer recorded the morphology and composition of the synthesized np. the band gap of the synthesized nps was evaluated by the uvvisible absorption spectrum of solid zno and cuo nps using uv-visible spectrophotometer (hr 4000 uv-vis spectrophotometer, uv-vis-nir light source, dt-mini-2-gs, jaz detector). photocatalytic degradation of mb dye to evaluate the photocatalytic activity of zno np, 50 mg l -1 mb dye solution was prepared using millipore water. 30 mg zno nanoparticle was added to 30 ml mb solution and stirred for 2 hours in dark to establish equilibrium between adsorption/desorption of mb on nps. then, this j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 100 solution was exposed to continuous flash of uv light for duration of 10 minutes. a high pressure mercury vapor lamp (hpml) of 125 w (phillips, india) was jacketed in a quartz tube provided with inlet and outlet for water circulation to avoid rise in temperature of the solution. the uv-lamp radiated predominantly at 365 nm (3.4 ev) and photon flux of 5.8×10 -6 mol of photons sec -1 . 0.5 ml of the irradiated sample was removed, diluted with 1 ml millipore water and subjected to centrifugation for 5 minutes. the supernatant liquid was taken for uv-visible spectroscopic analysis. after 2 minutes, the uv light irradiation was again continued for 10 minutes and then sample was removed. the total time of uv light irradiation on the sample was 60 minutes. results xrd analysis the xrd patterns obtained for zno nps prepared in presence of different concentrations of ctab are given in fig. 1. 2, degree figure 1. the xrd pattern of a) zno/0; b) zno/5; c) zno/10; d) zno/15 the positions and intensities of the diffraction peaks of zno nps are in good agreement with the literature values of hexagonal phase zno (jcpds card no. 36-1451) with primitive lattice structure. the crystallite size of the nps was calculated from full wave half maximum intensity (fwhm) of each peak using scherrer’s formula and the average crystallite size was determined. cos k d     in the above formula, d is the diameter of the crystallite size, k the shape factor (the typical value is 0.9), λ the wavelength of the incident beam,  the broadening of the diffraction line measured in radians at fwhm and  is the bragg’s angle. the crystallite size of zno nps calculated using the xrd data is given in table 1. a. n. subba rao et al. j. electrochem. sci. eng. 4(3) (2014) 97-110 doi: 10.5599/jese.2014.0063 101 table 1. the average crystallite size of zno nps prepared in presence of ctab zinc oxide np crystallite size, nm family of crystallographic planes {hkl} and corresponding 2 values {100} {002} {101} {102} {110} {103} {200} {112} {201} average 31.8° 34.5° 36.3° 47.6° 56.7° 62.9° 66.4° 68.1° 69.2° zno/0 44 98 45 28 106 55 23 40 47 48 zno/5 44 44 41 23 66 27 35 31 24 34 zno/10 25 31 23 16 39 18 24 21 24 23 zno/15 16 20 27 13 11 20 23 20 24 18 the crystallite size of np markedly decreased with increase in concentration of ctab in the electrolyte bath. the width of the peaks appreciably increased suggesting the reduction in the crystallinity of zno with increase in ctab concentration. 2, degree figure 2. the xrd pattern of a) cuo/0; b) cuo/5; c) cuo/10; d) cuo/15 the xrd patterns of synthesized cuo (fig. 2) are in close match with the xrd pattern of jcpds card no. 48-1548 with monoclinic (tenorite) phase crystallite structure. the crystallite size of cuo nps calculated using the xrd data is given in table 2. the crystallite size of cuo nps also decreased with increase in ctab concentration. j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 102 table 2. the average crystallite size of cuo np prepared in presence of ctab. copper oxide np crystallite size, nm family of crystallographic planes {hkl} and corresponding 2 values {110} {111} {111} {202} {020} {202} {113} {311} {220} {311} {222} average 32.1° 35.1° 38.5° 47.8° 53.7° 57.9° 61.1° 65.3° 68.1° 71.9° 74.9° cuo/0 22 49 44 14 15 24 24 25 11 22 18 31 cuo/5 36 26 36 16 14 20 20 20 30 26 15 26 cuo/10 09 22 22 13 10 24 15 18 18 13 13 16 cuo/15 18 33 11 17 16 14 33 17 09 12 22 19 sem/edax analysis the changes in the morphology of zno (fig. 3) and cuo (fig. 4) nps with ctab concentration are evident from the sem images. from fig. 3 and fig. 4, it can be noticed that the change in the morphology of both cuo and zno crystals is not obvious within 5 mm ctab, but the tendency from regular shapes to irregular ones increases with the increase of ctab concentration. the spherical shape of zno crystals was though retained, the irregularity in the shape increased with increase in ctab concentration. the shape of cuo crystals changed from nanospindles (fig. 4a and fig. 4b) to irregular nanospheres (fig. 4c and fig. 4d). similar changes in the morphology of cuo crystals were observed by gao-qing yuan et al. [12] on varying the solvent system. the average grain size of zno and cuo nps as depicted from sem images is given in table 3. figure 3. sem images of zno np; a) zno/0; b) zno/5; c) zno/10; d) zno/15 a. n. subba rao et al. j. electrochem. sci. eng. 4(3) (2014) 97-110 doi: 10.5599/jese.2014.0063 103 figure 4. sem images of cuo np; a) cuo/0; b) cuo/5; c) cuo/10; d) cuo/15 table 3. the average grain size of zno and cuo nps prepared in presence of ctab metal oxide np average grain size, nm 0 mm ctab a 5 mm ctab 10 mm ctab 15 mm ctab zno 32-42 31-40 30-69 25-29 cuo 60-65 85-95 18-40 27-64 a the ctab concentration in electrolyte bath figure 5. edax analysis of zno np; a) zno/0; b) zno/5; c) zno/10; d) zno/15 the shape and size of cuo/0 and cuo/5 nanospindles was homogeneous. the length of the nanospindles ranged between 430 nm to 580 nm and the rigidity enhanced from cuo/0 to cuo/5. j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 104 the cuo/10 and cuo/15 nps showed irregular morphologies (nanospindle, nanosphere, nanoplate). in case of zno nps, the homogeneity in the spherical shape was lost as the ctab concentration was increased in the electrolyte bath. there was only slight reduction in the grain size from zno/0 to zno/15. the stoichiometry of metal ion and oxygen in the synthesized zno and cuo nanomaterials was determined by edax analysis (fig. 5 and fig. 6). no peaks corresponding to impurities were found suggesting that the synthesized zno and cuo nanomaterials are pure. figure 6. edax analysis of cuo np; a) cuo/0; b) cuo/5; c) cuo/10; d) cuo/15 uv-visible spectroscopy to better understand the optoelectronic properties of np, the band gap energy was calculated from their absorption curves using tauc plots. the uv-visible absorption spectra of zno and cuo nps are given in fig. 7a and fig. 7b respectively. firstly, the absorption coefficient (α) was calculated using eq. (1):  / nm      / nm figure 7. uv-visible absorption spectra of a) zno nps and b) cuo nps synthesized in presence of different concentrations of ctab 2.303 a d   (1) where a is absorbance, d is the thickness of sample. to determine the band gap of np, a plot of (αhν) n vs. eg was made (tauc plot). the value of ‘n’ is dependent on the mode of electronic a. n. subba rao et al. j. electrochem. sci. eng. 4(3) (2014) 97-110 doi: 10.5599/jese.2014.0063 105 transition; n = 2 for direct band transitions and n = 1/2 for indirect transitions. for both zno and cuo, it is known that the electronic transition is direct. therefore, eq. (2) is valid for both zno and cuo np: (αhν) 2 = b (hν eg) (2) where, b is a constant, hν is the photonic energy. the band gap (eg) was obtained by extrapolating the linear part of the graphics to the axis of the abscissa [30]. h / ev h / ev figure 8. tauc plots for the evaluation of band gap of samples a) zno/0; b) zno/5; c) zno/10; d) zno/15 h / ev h / ev figure 9. tauc plots for the evaluation of band gap of samples a) cuo/0; b) cuo/5; c) cuo/10; d) cuo/15 j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 106 table 4. eg values for different zno and cuo nps obtained from tauc plot c (ctab) / mm eg / ev zno cuo 0 3.18 2.34 5 3.19 2.32 10 3.17 2.42 15 3.18 2.40 the tauc plots for different zno and cuo nps has been presented in fig. 8 and fig. 9 respectively and corresponding band gap values have been summarized in table 4. the band gap of cuo nps increased with increase in the concentration of ctab in the electrolytic bath during synthesis and it was found to be in between 2.32 – 2.42 ev, which is higher than that reported by wang et al. [31]. however, such notable variation in the band gap of zno nps was not observed. the average band gap of zno np was 3.18 ev; where the literature reports a 3.2 ev band gap [3]. photocatalytic degradation of mb dye the photocatalytic degradation of methylene blue dye was monitored by uv-visible spectroscopy. the variation in the absorption intensity of the methylene blue dye on uv irradiation in presence of 30 mg of zno nps is presented in fig 10. the degradation of methylene blue was expressed in terms of % color removal by recording the changes in absorption intensity at the characteristic peak position (661 nm) in the visible region.   / nm  / nm   / nm  / nm figure 10. the variation in the uv-visible absorption intensity of methylene blue dye with time on electrolysis. uv irradiation in presence of 30 mg zno nps; a) zno/0; b) zno/5; c) zno/10; d) zno/15 a. n. subba rao et al. j. electrochem. sci. eng. 4(3) (2014) 97-110 doi: 10.5599/jese.2014.0063 107 the eq. (3) was used to calculate the color removal: o t o [ ] color removal, % 100 a a a    (3) where, ao and at are the absorbance at 661 nm at 0 minutes and t minutes respectively. the color removal of methylene blue achieved in presence of zno photocatalyst was in the following order; zno/10 > zno/15 > zno/0 = zno/5. the color removal achieved after 60 minutes of uv irradiation was evaluated and given in table 5. table 5: photocatalytic degradation of methylene blue dye zno np color removal, % a zno/0 13 zno/5 13 zno/10 33 zno/15 22 a color removal achieved after 60 minutes irradiation the cuo was also tested for its photocatalytic activity. the cuo np under the irradiation of uv light (predominantly at 365 nm) was ineffective as a photocatalyst to degrade the mb dye. the color removal of mb achieved in presence of cuo np and uv irradiation was insignificant. discussion it is crucial to understand the mechanism of formation of zno and cuo in order to control their size and shape. the electrolytes used in the electrolyte bath should enhance the conductivity and assist the formation of metal hydroxides. the nahco3 and nano3 have been used in the synthesis of zno and cuo np respectively [2-4,12]. these electrolytes facilitate the formation of metal hydroxides along with enhancing the conductivity of the electrolyte bath. the following reactions take place during the electrolysis which leads to the formation of zno np in the electrolyte bath containing nahco3 electrolyte [eqs. (4) to (7)]; zn  zn 2+ + 2e (on anode), e° = 0.76 v (4) nahco3 + h2o  naoh + co2 (in electrolyte solution) (5) zn 2+ + 2 oh  zn(oh)2 (in electrolyte solution) (6) zn(oh)2  zno + h2o (7) in presence of nano3, the cuo is generated according to the following reactions [12] [eqs. (8) to (11)]; cu  cu 2+ + 2e (on anode), e° = 0.34 v (8) no3 + h2o + 2e  no2 + 2 oh (on cathode), e° = 0.1 v (9) cu 2+ + 2 oh  cu(oh)2 (in electrolyte solution) (10) cu(oh)2  cuo + h2o (11) the mechanism of np crystal growth is usually explained as oriented aggregation in the electrolyte solution. the metal ions generated at the anode react with the oh ions in the electrolyte bath to form corresponding nano-sized amorphous metal hydroxide precipitate j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 108 (cu(oh)2 or zn(oh)2). a part of the precipitate undergoes dehydration to give metal oxide in the electrolyte bath (cuo or zno) [2-4]. however, complete conversion of all metal hydroxide into metal oxide is ensured by calcinating the sample at elevated temperatures (in this case, 300 °c). the metal oxide thus generated acts as seed for further directional aggregation of metal oxides to form precisely oriented nanostructures [12]. the advantage in electrochemical synthesis is that the precursor metal ion is slowly released from the anode metal. the current density determines the number of ions released per unit time and their availability in the electrolyte bath for further reaction. however, in chemical methods (like solvothermal and precipitation) the entire precursor metal ion is made available for reaction. the nucleation and crystal growth are the two processes which decide the particle size. if the nucleation is fast, the crystal growth is hindered and the particle size eventually remains small. these two processes occur at controlled rate in electrochemical method. the crystal growth and shape evolution is controlled by the stabilizing agent when it is present in the electrolyte bath. in this case, ctab binds to the surface of the nuclei and prevents agglomeration or dictates the orientation of incoming species [12]. in the present study, the concentration of ctab was more than 6 times its critical micelle concentration (cmc) i.e., 0.8 mm [24]. the ctab micelles existing at this concentration of ctab tend to act as template for the shape evolution of nps. the effect of ctab concentration on the morphology and size of both zno and cuo nps is clearly evident from the sem images and the crystallite size calculated using xrd data. as mentioned earlier, from the sem images (fig. 3 and fig. 4), the change in morphology of both cuo and zno crystals is not obvious within 5 mm ctab. but, the tendency from regular shapes to irregular ones increased beyond the ctab concentration ≥ 10 mm. the morphology of cuo nps was much affected by the ctab concentration as compared to zno nps. the spindle shape of cuo nps increased in length and gained rigidity in presence of 5 mm ctab as noticed in the sem images. however, when the ctab concentration was raised to 10 mm and 15 mm, the spindle shape of cuo nps transformed into irregular spherical shape with reduced grain size. on the other hand, the zno nps retained their spherical shape irrespective of the ctab concentration. however, the homogeneity in the shape of zno nps was disrupted. the grain size of both zno and cuo nps decreased with increase in ctab concentration. from these observations, it can be deduced that the minimum concentration of ctab required to influence the shape and size of zno and cuo np crystal is ≥ 10 mm i.e., more than 10 times its cmc value. the effect of ctab concentration on the band gap energy and absorption edge in the uv region for zno nps was insignificant. however, there was a hike of 0.1 ev in the band gap of cuo nps on increasing the concentration of ctab from 0 mm to 15 mm. the decrease in size of cuo nps led to the increment in band gap. the absorption edge in the uv region for cuo nps altered with the grain size which is intern dependent on the ctab concentration. the influence of changes in the morphology and size of zno nps reflected on its photocatalytic activity. the electrons from the valence band (vb) of zno nps leap to the conduction band (cb) on uv light irradiation. vacancies created in the valence band (holes) are represented as hvb + and the excited electrons are given the symbol ecb . the photogenerated positive vacancies in the vb have high oxidizing potential. these holes can react with the adsorbed water to produce hydroxyl radical and the photogenerated electrons lead to the formation of other oxidizing agents such as superoxide anions (·o2 ), hydrogenperoxide (h2o2), hydroxyl radicals (·oh), hydrogen dioxide anion (ho2 ) and hydroperoxy radicals (·ho2) as given in eqs. (12) to (18) [7]; mo + hν  mo (hvb + + ecb ) (12) a. n. subba rao et al. j. electrochem. sci. eng. 4(3) (2014) 97-110 doi: 10.5599/jese.2014.0063 109 mo (hvb + ) + h2o  mo + ·oh + h + (13) mo (ecb ) + o2  mo + ·o2 (14) mo (ecb ) + ·o2 + 2h +  mo + h2o2 (15) mo (ecb ) + h2o2  mo + ·oh + oh (16) o2 + h2o2  ·oh + oh + o2 (17) o2 + h +  ·oh2 (18) these oxidants are powerful enough to completely incinerate the organics into co2 and h2o. the photocatalytic reaction efficiency increases with increase in surface area available for the adsorption of target molecule. as the size of zno nps decreased with increase in ctab concentration, the surface area available for the adsorption of dye molecules also increased. the number of mb molecules adsorbed on the zno/15 and zno/10 nps was higher than that on zno/0 and zno/5 nps. the % color removal of mb dye achieved after 60 minutes uv light irradiation was thus highest with zno/10 and zno/15 nps. conclusion the zno and cuo nps of varying size and morphology were successfully synthesized by sacrificial anode electrochemical-thermal method. the crystallite size of both zno and cuo reduced with increase in concentration of ctab in the electrolyte bath during electrolysis. the crystallite size of zno nps was between 48-18 nm and for cuo it varied between 31-19 nm. the zno/0 and zno/5 nps exhibited homogeneous size and morphology. the homogeneity of zno nps was lost with increase in ctab concentration in the electrolyte bath during electrolysis and led to the formation of irregular sized nanostructures of reduced size in presence of 10 mm and 15 mm ctab. similar effects were observed for the cuo nps. the cuo nps of uniform size and morphology were obtained in the absence of ctab and in presence of 5 mm ctab. the rigidity of cuo nps enhanced in presence of 5 mm ctab. however, on increasing the ctab concentration to 10 mm and 15 mm the spindle shape of cuo nps disrupted to form irregular nanostructures. the ctab adsorbs on the surface of the metal oxide nuclei and restrains the growth of nps by reducing the surface energy and preventing further agglomeration. ctab tends to gain spherical structure in aqueous solution at these concentrations and act like template for the evolution of spherical nps of irregular morphology. it can be deduced from the results that the minimum concentration of ctab influencing the evolution of shape and size of the np is ≥ 10 mm (greater than 10 times the cmc value of ctab). the effect of size and shape of zno nps on the absorption edge and band gap was insignificant. the band gap of zno nps lied between 3.17 – 3.19 ev. the band gap of cuo nps showed considerable changes with size. a hike of 0.1 ev was observed due to the reduction in size of cuo nps with increase in ctab concentration. the photocatalytic activity of zno nps was tested for the degradation of mb dye. the zno/10 nps showed comparatively higher photocatalytic activity and a color removal of 33 % was achieved using this nps with the uv light irradiation for 60 minutes. acknowledgement: this work was supported by university grants commission, new delhi, govt. of india under the grant ref: f. no. 41-231/2012 (sr) dated 16/07/2012]; and department of chemistry, kuvempu university. authors thank mr. manu kumar k n, mr. kiran kumar k s and mr. nagaraja h n for their valuable assistance. j. electrochem. sci. eng. 4(3) (2014) 97-110 effect of ctab on cuo and zno nanoparticles 110 references [1] j. song, l. xu, c. zhou, r. xing, q. dai, d. liu, h. song, appl. mater. interfaces. 5 (2013) 12928−12934. 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[31] h. wang, j. z. xu, j. j. zhu, h. y. che, j. cryst. growth. 244 (2002) 88–94. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {copper-nickel oxide nanofilm modified electrode for non-enzymatic determination of glucose} http://dx.doi.org/10.5599/jese.699 245 j. electrochem. sci. eng. 10(3) (2020) 245-255; http://dx.doi.org/10.5599/jese.699 open access: issn 1847-9286 www.jese-online.org original scientific paper copper-nickel oxide nanofilm modified electrode for non-enzymatic determination of glucose mahsa hasanzadeh1,, zahra hasanzadeh1, sakineh alizadeh2, mehran sayadi3,, mojtaba nasiri nezhad4, reza e. sabzi1 and sajjad ahmadi1 1department of analytical chemistry, faculty of chemistry, university of urmia, urmia, iran 2faculty of chemistry, bu-ali sina university, hamadan, iran 3department of food safety and hygiene, school of health, fasa university of medical science, fasa, iran 4department of chemical engineering, urmia university of technology, urmia, iran corresponding authors:  mahsa.hasanzadeh95@gmail.com; tmf_fmt3000@yahoo.com received: may 22, 2019; revised: december 20, 2019; accepted: december 22, 2019 abstract cuxo-nio nanocomposite film for the non-enzymatic determination of glucose was prepared by the novel modifying method. at first, anodized cu electrode was kept in a mixture solution of cuso4, niso4 and h2so4 for 15 minutes. then, a cathodization process with a step potential of -6 v in a mixture solution of cuso4 and niso4 was initiated, generating formation of porous cu-ni film on the bare cu electrode by electrodeposition assisted by the release of hydrogen bubbles acting as soft templates. optimized conditions were determined by the experimental design software for electrodeposition process. afterward, cu-ni modified electrode was scanned by cyclic voltammetry (cv) method in naoh solution to convert cu and ni nanoparticles to the nano-scaled cuxo-nio film. the electrocatalytic behavior of the novel cuxo-nio film toward glucose oxidation was studied by cv and chronoamperometry (cha) techniques. the calibration curve of glucose was found linear in a wide range of 0.04–5.76 mm, with a low limit of detection (lod) of 7.3 µm (s/n = 3) and high sensitivity (1.38 ma mm-1 cm-2). the sensor showed high selectivity against some usual interfering species and high stability (loss of only 6.3 % of its performance over one month). the prepared cuxo-nio nanofilm based sensor was successfully applied for monitoring glucose in human blood serum and urine samples. keywords cuxo; nio; nanofilm; cu electrode; modifying method; non-enzymatic glucose sensor. introduction in electro-biochemical analysis, it is generally believed that sensing interfaces play key roles in the enzyme-less detection of biomolecules like glucose, ascorbic acid (aa), dopamine (da) and uric acid http://dx.doi.org/10.5599/jese.699 http://dx.doi.org/10.5599/jese.699 http://www.jese-online.org/ mailto:mahsa.hasanzadeh95@gmail.com mailto:tmf_fmt3000@yahoo.com j. electrochem. sci. eng. 10(3) (2020) 245-255 non-enzymatic determination of glucose 246 (ua) [1]. glucose detection is essential in clinical diagnosis, biotechnology and food industry [2]. over past decades, a number of studies was performed on alleviating drawbacks of enzymatic glucose sensors [3]. the majority of known amperometric biosensors for glucose monitoring use glucose oxidase (god) or glucose dehydrogenase enzymes [4,5]. the most common and serious problem is insufficient stability resulted from the nature of the enzymes which could hardly be restrained. although god is quite stable compared to other enzymes, glucose sensors based on god are always exposed to a possibility of thermal and chemical deformation during fabrication, storage or use. furthermore, god quickly loses its activity below ph 2 and above ph 8, and also, temperatures above 40 °c can cause fatal damages [3]. ionic detergents also deactivate god. at the other side, glucose dehydrogenase enzyme showed the problem of effectiveness from interfering species like aa, da, etc. [6,7], unless a selective membrane for reducing effect of interfering species was used [8]. to overcome the above obstacles, non-enzymatic glucose sensors are developed coming closer to practical applications [6]. non-enzymatic glucose sensors have received significant interest due to their advantage of thermal and chemical stability [7]. in order to prepare a non-enzymatic glucose sensor, nanostructured particles have attracted extensive scientific and industrial interests due to their unique electronic, optical and catalytic properties [9]. moreover, when utilizing nanomaterials, it becomes possible to minimize the influence of interfering substances such as aa and ua [10]. various nanostructured materials have been proposed as new opportunities to develop novel non-enzymatic glucose sensors [6] such as carbon nanotubes [11], metals [12,13], metal oxides such as nio [14-17] or cu2o [18], metal composites such as cu-ni [19], etc. different methods were already reported for preparations of different forms of cu-ni film for determining various materials, such as preparation of cu-ni alloys used in electrochemical oxygen analysis [20] or preparation of cu and ni nanoparticles deposited on ito electrode for non-enzymatic electrochemical carbohydrates sensor applications [21]. in addition, cu@ni/mwcnt nanocomposite was prepared for simultaneous electrochemical determination of guanine and adenine [22], cu@ni core–shell nanoparticles/reduced graphene oxide nanocomposite for non-enzymatic glucose sensor [23], and non-enzymatic multispecies sensor based on cu-ni nanoparticle dispersion on doped graphene [19]. the goal of this work is preparation of the novel cu-ni nanofilm on an inexpensive electrode like cu, which should serve as fast, low-cost and easy method for the non-enzymatic glucose determination. a novel procedure of cuxo-nio nanocomposite fabrication is proposed, involving hydrogen bubbles evolution serving as soft templates for electrodeposition of porous cu-ni nanofilm. finding of optimized conditions for electrodeposition is based on a famous statistical method called the response surface methodology (rsm). rsm method is widely applicable in electrochemistry and other research fields for gaining some important information for optimizing experimental conditions by considering the interactions between model factors instead individual variables. deposition potential, time of deposition and volume combination of ni-cu solutions are chosen as model factors in electrodeposition. the prepared novel nanocomposite cuxo-nio film is studied by cv and cha techniques and tested for amperometric determination of glucose in practical samples of human blood serum and urine. experimental reagents and chemicals cuso4×5h2o, niso4×6h2o, naoh, aa, ua, da and h2so4 were all purchased from merck co. blood serum samples were obtained from a local hospital. for glucose determination, 0.1 m naoh m. hasanzadeh et al. j. electrochem. sci. eng. 10(3) (2020) 245-255 http://dx.doi.org/10.5599/jese.699 247 solution was used as the electrolyte. double distilled (dd) water was applied in preparing all solutions. apparatus and electrochemical measurements electrochemical experiments were performed in a three-electrode cell, µautolab type iii (netherlands). agilent technologies dc power supply n5752a (usa) was used for applying -6 v potential. cu rod electrode (3 mm in diameter), ag/agcl (kcl saturated) electrode and a piece of pt wire were used as working, reference and counter electrode, respectively. ultrasonicator (pars nahad co., iran) was applied to clean the electrode surface from impurities. field emission scanning electron microscopy (fesem) images were obtained by a mira3 tescan sem system [razi metallurgical research center (rmrc), iran]. x-ray diffraction (xrd) measurements were done at urmia university. optimized conditions were determined by the experimental design. all calculations and programming for the experimental design were performed in matlab (hyper-cube inc. version10) software. the essential regression and experimental design for chemists and engineers (eregress) excel were added in software. all experiments were conducted in stationary solutions except tests relevant to a hydrodynamic cha method, which were conducted using a magnetic stirrer with a constant rotation speed of 150 rpm. all parameters affecting the experiments including concentrations of solutions, volume ratios of solutions, electrolyte concentrations, applied step potential and its duration time, were optimized. all experiments were performed at room temperature (25 °c) and repeated at least three times. modified electrode fabrication bare cu electrode was firstly polished with 400 and 2000 grit emery papers respectively, and washed to remove impurities. then, the electrode was immersed into dd water and ultrasonicated for few minutes. afterwards, the surface of the electrode was washed with dd water again to clean any surface impurity. after polishing and cleaning of the surface, the electrode was anodized electrochemically by cha method with an applied potential of +1.5 v for 40 s in 0.1 m naoh. then, the electrode was washed carefully with dd water and dried in air. before electrodeposition of cuni film, the anodized cu electrode was kept for 15 minutes in a solution mixture of 0.3 m cuso4, 0.3 m niso4 and 0.5 m h2so4 (5:5:2 v/v), which was referred to as precursor solution, for effective contacting with electrode surface. then, a step potential of -6 v was applied for 70 s in a solution mixture of 0.3 m cuso4 and 0.3 m niso4 (1:1 v/v). during the cathodic electrodeposition process, the porous cu-ni film was generated on the bare cu electrode surface with the concurrent release of hydrogen bubbles acting as soft templates. in the next step, cu-ni film modified electrode was scanned by cv method in the potential range of -0.2 to 0.8 v in 0.1 m naoh to convert cu and ni nanoparticles to the nano-scaled cuxo-nio film. at the end of the synthesis procedure, cuxo-nio modified cu electrode was carefully rinsed with dd water, dried and kept in the air for further experiments. eregress software was utilized to determine the optimum deposition conditions, including applied potential and its duration, as well as relative volume percent of 0.3 m cuso4 and 0.3 m niso4 as model factors. the central composite design was used to prepare a model for obtaining optimized conditions. results and discussion characterization of nanostructured modified cu electrode a novelty of this work is related to a newly proposed method for preparing cu-ni nanocomposite film on the electrode surface, including a novel mixing method for modifier solutions. the method is http://dx.doi.org/10.5599/jese.699 j. electrochem. sci. eng. 10(3) (2020) 245-255 non-enzymatic determination of glucose 248 very fast, straightforward and cheap compared to other methods which have been previously used for fabricating composite materials, like fabrication of cu-ni by mechanically alloying [24] or some other techniques [25,26]. it must be stressed that here proposed method could also be applied in producing other electroactive composites for detecting other materials. the surface examination of cuxo-nio/cu electrode under fesem proved formation of nano-sized and non-uniform porous structures (figure 1). figure 1. sem images of cuxo-nio film at different magnifications the use of porous films with non-uniform pores resulted in better efficiency compared to traditional porous films [27]. presence of nanoparticles deposited on cu electrode is clearly demonstrated in figure 1. the obvious porosity of the electrode surface is due to the hydrogen bubble evolution. by applying a high negative step potential for a limited time, a deposition of nano-scaled particles is generally expected because particles could not expand their size and participate in the nucleation process. in the present case, nanoparticles had just 70 s to deposit on the electrode surface at -6 v and therefore, they deposit along each other in a sloppy manner. the phase characteristics of the modified electrode are identified from its xrd pattern. as depicted in figure 2 the diffraction peaks of the modified film are well indexed as monoclinic cuo (jcpds no. 45-0937), cu2o (jcpds no. 34-1354), nio (jcpds no. 47-1049), cu (jcpds no. 04-0836) and ni (jcpds no. 04-0850), respectively. figure 2. xrd pattern of cuxo-nio film m. hasanzadeh et al. j. electrochem. sci. eng. 10(3) (2020) 245-255 http://dx.doi.org/10.5599/jese.699 249 optimization of electrodeposition process by experimental design method central composite experimental design (ccd) was applied for simultaneous optimization of experimental parameters. ccd method is useful to determine the number of experiments that must be performed for the optimization of variables (factors) and responses. ccd uses the response surface methodology (rsm) to optimize experimental conditions. rsm is a widely used mathematical and statistical method for modeling and analyzing a process in which the response of interest is influenced by various variables and the goal of this method is to optimize the response. a key aim of experiment was to determine how significant each factor is. it is discussed how to design an experiment that allows sufficient degrees of freedom to determine the significance of a given factor. in the following section, the procedure of proving significance of each factor is explained. there are many situations, in which this information would be useful. after checking different parameters in ccd, one model is defined for obtaining the best conditions of synthesis, which get the response as a function of effective parameters. multiple linear regression (mlr) method is used for modelling and the coefficients are calculated. the effect of three independent electrodeposition variables (e, t and v) on the system response is approximated by the second degree polynomial (mlr) equation: response = b0 + b1e + b2t + b3v + b4e2 + b5et + b6t2 + b7v2 (1) according to eq. (1), the response for optimization of three factors, i.e. applied potential (e), applied potential duration (t) and relative volume percent (v), involves b0 as an intercept or average, b1e, b2t and b3v as linear terms depending on each of three factors, b5et interaction term between factor e and t, and also b4e2, b6t2, and b7v2 quadratic terms depending on each of three factors. greater coefficients in eq. (1) show a more effective parameter and positive signs of coefficients describe that with their increasing, the response is increased as well. some of the obtained characteristics of the designed model calculated by experimental design software (eregress) are: b0 = -0.179, b1 = -0.06307, b2 = 0.00406, b3 = 0.00642, b4 = -0.00659, b5 = -0.000217, b6 = -3.92673×10-5, b7 = -6.68436×10-5. since coefficients b2 and b3 have the highest values, t and v are defined as the most effective factors in the response. in this work, eregress software was used for interpreting the effect of both first and second order of parameters. the minimum, intermediate, and maximum values of each variable are labeled as −1, 0, and +1, respectively. if p-value (probability value) for each variable was greater than 0.05, it would not have significant effect in the model and takes zero value. if p-value was greater than 0.05 but the higher order of this factor had p-value less than 0.05, the variable was not removed. table 1 shows r2 (coefficient of determination), that was improved by eliminating some unimportant variables (p>0.05). r2 is a statistical measure that shows the proportion of the variance for a dependent variable that is affected by all independent variables in a regression model. in other words, r2 assumes that every independent variable explains the variation in the dependent variable. table 1. coefficients of determination of prepared statistical model r2 adjusted r2 predicted r2 0.946 0.908 0.747 the adjusted r2, however, increases only if the new term improves the model more than would be expected by chance. therefore, in statistical modeling, adjusted r2 was also determined. the predictive power of the model has been proved by the predicted r2 value. this parameter was obtained from the sum of squares of predicted errors. comparing of r2, adjusted r2 and predicted r2 can be a convenient technique to understand the conformity of a model. in a good model these http://dx.doi.org/10.5599/jese.699 j. electrochem. sci. eng. 10(3) (2020) 245-255 non-enzymatic determination of glucose 250 three parameters should not be too different. as shown in table 1, r2, adjusted r2 and predicted r2 have acceptable values for the model conformity. for obtaining the optimum conditions of studied parameters, the surface response plots (plots show response as surfaces) were constructed. the optimum values can be derived using these plots. in these plots, all the minimum or maximum values of response for relative volume (v / %), e and t were investigated. also, the relationship between the dependent and independent variables can be understood by these plots. since the model has more than two factors, one factor was always kept constant. the example of surface response plot is shown in figure 3. this figure demonstrates the interaction between relative volume (v) and e when t was kept constant. response numbers show how much the factors influence another variable. maximum response for each factor was selected according to surface response. as a general result, the optimized values for the applied potential, deposition time and relative volume percent were determined to be -6 v, 70 s and 50 % (1:1 v/v), respectively where the current or response was maximum in the corresponding surface response plots. figure 3. surface response representation related to rsm for optimizing two variables (e and relative volume, when t was kept constant). cyclic voltammograms and electro-oxidation behavior of glucose at cuxo-nio modified cu electrode cyclic voltammograms in figure 4 show that cu (a), ni (b) and cu-ni (c) particles can deposit separately or along each other at -6 v. however cyclic voltammogram for cuxo-nio shows higher current of the overall redox process. maybe the synergistic effect of ni and cu particles along each other is the reason for this phenomenon. it must be stressed here that according to our previous experiments performed under optimum conditions, presence of 0.3 m niso4 in the deposition solution of 0.3 m cuso4 resulted in an increased current of the redox peaks in cyclic voltammograms and chronoamperograms. cyclic voltammograms in figure 4 are in accordance with other studies [1,28]. it is seen in figure 4a, that the modified cuxo electrode provides broad anodic and cathodic peaks attributed to successive cu+/cu2+ and cu2+/cu3+ redox pairs in the alkaline solution within the specified potential region [28]. figure 4b shows ni2+/ni3+ redox pair for nio film, what is in well agreement with ref. [14]. therefore, cu+/cu2+, cu2+/cu3+ and ni2+/ni3+ redox pairs would play the main role in redox process of cuxo-nio film modified electrode. figure 5 compares the electro-oxidation behaior of cuxo-nio film electrode in alkaline medium with and without presence of glucose in the potential range of -0.2 to 0.8 v. m. hasanzadeh et al. j. electrochem. sci. eng. 10(3) (2020) 245-255 http://dx.doi.org/10.5599/jese.699 251 figure 4. cyclic voltammograms of (a) cuxo (b) nio and (c) cuxo-nio film electrode in 0.1 m naoh (ν = 0.05 v/s) figure 5. comparison of the electro-oxidation behavior of cuxo-nio film electrode in 0.1 m naoh in(a) absence and (b) presence of 2.87 mm glucose (ν = 0.05 v/s) oxidation of glucose started at about 0.45 v and caused an increase in current as observed. by adding only 2.87 mm glucose, a current difference of about 0.43 ma has been observed, what is observable on the current scale of ma and in accordance with chronoamperograms demonstrated below. the electrons resulting from the oxidation of glucose transfer directly to the electrode surface and result in the increase of anodic current at approximately 0.45 to 0.8 v during scanning. during the potential scan, the modified electrode surface can be oxidized, while the deprotonation of glucose molecules in an alkaline medium trigger isomerization to an enediol form, which then oxidizes to gluconolactone and then further hydrolyzes to gluconic acid [28]. amperometric detection of glucose at cuxo-nio modified cu electrode figure 6 illustrates chronoamperograms of 0.05 m glucose at different electrode modifying stages (before modifying, after anodizing and after electrodeposition), showing the effect of each modifying step. figure 6. cha detection of 0.05 m glucose at (a) bare cu electrode, (b) anodized cu electrode, (c) anodized and cathodized cu electrode http://dx.doi.org/10.5599/jese.699 j. electrochem. sci. eng. 10(3) (2020) 245-255 non-enzymatic determination of glucose 252 according to figure 6, the best chronoamperogram with the highest current response is obtained after fabricating cuxo-nio modified cu electrode. finally, the fabricated non-enzymatic glucose sensor is applied for real samples such as human blood serum and urine at optimized condition using the cha technique. the data of glucose evaluation in real samples obtained using the proposed sensor and data obtained from the local hospital were in full accordance. the optimized step potential for chronoamperometric measurements of glucose was determined as 0.6 v. the chronoamperogram related to the addition of glucose into stirred 0.1 m naoh solution is shown in figure 7a. it is obvious that increase of the concentration of glucose in the electrolyte resulted in increase of the amperometric current value. figure 7. (a) cha response of cuxo-nio modified cu electrode upon optimized conditions after successive additions of glucose into 0.1 m naoh, (b) relationship between current and glucose concentration relevant to figure 7a, and (c) the corresponding calibration curve for glucose. the proposed sensor, however, lost its performance after the exposure to high concentrations of glucose, what is probably due to destruction of its active sites. thus, the sensor shows decreased current values at high concentrations of glucose. figure 7b demonstrates the relationship between currents and glucose concentrations, while figure 7c shows the corresponding calibration plot for glucose concentration in a linear range from 0.04 to 5.76 mm. low limit of detection (lod), equal to 0.0073 ± 4.16×10-4 mm and limit of quantification (loq) of 0.0243 ± 1.38×10-3 mm (signal/noise (s/n) = 3, n= 3 and confidence limit= 95 %) were determined. the calculated sensitivity of the proposed sensor was 1.38 ma mm-1 cm-2 which is signifycantly higher compared to data reported in previous studies for coooh nanosheets (0.34 ma mm-1 cm-2) [29], or nionps/go (1.09 ma mm-1 cm-2) [30]. in this work, the obtained lod of 7.3 µm is much better than m. hasanzadeh et al. j. electrochem. sci. eng. 10(3) (2020) 245-255 http://dx.doi.org/10.5599/jese.699 253 obtained for cuxo-cu (49 µm) and gold nanotube array electrodes (10 µm) [7,31]. the calibration plot linearity range for our sensor is between 0.04 and 5.76 mm which is an order of magnitude higher than for cuo nanoleaves/mwcnts (0–0.9 mm) [32]. results obtained after comparison of the performance of fabricated glucose sensor with other enzyme less glucose sensors are shown in table 2. it is seen in table 2 that here proposed sensor is superior among most of other compared sensors, what is mainly due to its high sensitivity, low lod and wide linear range. table 2. analytical data for glucose evaluation of the proposed cuxo-nio/cu nanosensor and other modified electrodes working electrode linear range of concentration, mm sensitivity, ma mm-1 cm-2 lod, μm ref.a cuxo-nio/cu 0.04–5.76 1.38 7.3 this work cuo nanoleaves/mwcntsb 0–0.9 0.6643 5.7 [32] cuxo/polypyrrole 0–0.8 0.232 6.2 [33] nio-cu 0.01–2.14 4.02 1.7 [14] cu cubes/mwcnts 0.5–7.5 0.922 2.0 [34] cuxo 0–6 1.62 49 [7] platinum oxide layers 1–10 0.0056 800 [35] sio2/c/cuo 0.02–20 0.472 0.06 [36] atcsncc spheres 1–8.1 1.968 0.19 [37] pt@cnosd 2–28 0.0216 90 [38] au/niau 0.005−31 0.483 1.0 [39] au@cu2o 0.05−2.0 0.715 18 [40] cu@ni 0.001–4.1 0.78 0.5 [23] cuni-ngre/gce up to 20 7.143 10 [19] polyaniline/zinc oxide/ mwcnts 0.1–1, 1–6 7.83, 1.67 0.0001 [41] areference, bmulti-walled carbon nanotubes, cag@tio2 core-shell nano composite, dcarbon nano-onions, ecuni-nitrogen doped graphene. selectivity and poison resistance of cuxo-nio nanosensor selectivity against interfering species such as aa, ua and da is crucial for a sensor. amperometric currents recorded upon addition of glucose and these interfering species are depicted in figure 8, showing the favorable selectivity of the proposed nanosensor for glucose monitoring. for most of the non-enzymatic glucose sensors based on precious metals and alloys, however, the activity can be easily lost due to the poisoning by chloride ions [9]. the amperometric current for cuxo-nio modified electrode toward glucose detection, however, is found almost constant in 0.1 m naoh in the presence and absence of 0.15 m nacl. therefore, the proposed sensor has demonstrated good poison-resistance ability towards chloride ions. figure 8. cha response of cuxo-nio modified electrode upon addition of glucose, ua, aa and da of equal concentrations (0.05 m) and equal volumes of 1 ml into 0.1 m naoh upon optimized condition. http://dx.doi.org/10.5599/jese.699 j. electrochem. sci. eng. 10(3) (2020) 245-255 non-enzymatic determination of glucose 254 stability and reproducibility of cuxo-nio nanosensor stability and reproducibility should be checked for any proposed sensor. in the present paper, stability of cuxo-nio modified cu electrode was monitored over the one month period. the sensor was kept in the refrigerator (4 °c) and evaluated each day for ten consecutive days and every five days over the next twenty days. after monitoring for one month, the sensor lost only 6.3 % of its efficiency in glucose detection. moreover, we studied the reproducibility of the developed nanosensor by recording the amperometric current responses towards 0.6 mm glucose. data of 7 parallel tests for each analyte showed a relative standard deviation rsd, % of 5.1 ± 4.02 × 10-3. thus, the proposed non-enzymatic sensor showed favorable stability and reproducibility for glucose determination. conclusion a new, simple, and low-cost electrochemical method is proposed for preparation of cuxo-nio nanofilm on a bare cu electrode for enzyme-free glucose sensing. the high electro-catalytic behavior could be attributed to the large electrochemical surface of the modified electrode, resulted from electrodeposition of nanostructured particles. moreover, the fabricated sensor showed high sensitivity, a wide linear range, high stability (only 6.3 % decrease in performance in one-month time period), favorable precision and accuracy. the results from the determination of glucose concentration in blood serum and urine samples complied with those obtained from the local hospital. thus, this easily fabricated cuxo-nio nanosensor, could be applied as a practical sensor for routine analysis of glucose in clinical samples. acknowledgements: the authors appreciate fasa university of medical sciences for financial supports of this work. references [1] x. niu, y. li, j. tang, biosensors and bioelectronics 51 (2014) 22-28. 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[41] s. mohajeri, a. dolati, k. yazdanbakhsh, journal of electrochemical science and engineering 9(2) (2019) 207-222. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.699 http://creativecommons.org/licenses/by/4.0/) preparation and characterization of ruo2/polyaniline/polymer binder composite electrodes for supercapacitor applications doi: 10.5599/jese.2012.0010 41 j. electrochem. sci. eng. 2(1) (2012) 41-52; doi: 10.5599/jese.2012.0010 open access : : issn 1847-9286 www.jese-online.org original scientific paper preparation and characterization of ruo2/polyaniline/polymer binder composite electrodes for supercapacitor applications suzana sopčić, marijana kraljić roković and zoran mandić university of zagreb, faculty of chemical engineering and technology, marulićev trg 19, hr-10000 zagreb, croatia corresponding author: e-mail: zmandic@fkit.hr; tel.: +385-1-4597164 received: january 4, 2012; revised: february 23, 2012; published: march 22, 2012 abstract the composite electrodes consisting of amorphous and hydrous ruo2, polyaniline and polymeric binder, nafion® or poly(vinilydene fluoride) were prepared. the electrochemical and pseudocapacitive properties of the prepared electrodes were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. the results show that the responses of composite electrodes are very sensitive to the presence of individual components and their respective ratio in the mixture. the difference in the electrochemical behavior was explained by the different physico-chemical properties of the polymeric binders. keywords ruo2; polyaniline; supercapacitors, composite electrodes, nafion ®. introduction transition metal oxides such as ruthenium and manganese oxides are increasingly investigated as active electrode materials for electrochemical supercapacitors. due to variety of different solid forms and a range of oxidation states they usually show almost flat current profile during linear polarization experiments. among transition metal oxides showing this type of behavior, ruthenium(iv) oxide, ruo2, stands out since it exhibits very reversible and reproducible solid state reactions involving redox transitions between ru(iii) and ru(iv) redox states. the redox response is stable enough allowing the oxide to undergo several thousands of charging/discharging cycles without significant deterioration of its charge storage ability and kinetics [1]. however, the gravimetric capacitance of ruo2 is very sensitive to the degree of its hydration and crystallinity and j. electrochem. sci. eng. 2(1) (2012) 41-52 ruo2/pani composite for supercapacitors 42 in order to be used in widespread applications of ru-based supercapacitors, its utilization should be maximized. it has recently been demonstrated that ru(iii)-ru(iv) solid state redox transition of ruo2 consists at least of two redox reactions taking place at different rates during its charging and discharging [2]. fast reaction includes the simultaneous proton/electron exchange according to the generally accepted mechanism: ruox(oh)y + ze + zh+ ⇔ ruox-z(oh)y+z (1) whereas additional reaction was proposed to proceed with the incorporation of oxygen into the hydrous ruo2 matrix during oxidation (equation 2). the latter reaction was recently defined as dissociative adsorption of water [3]. 2ruo2 + 2e + 2h+ ⇔ ru2o3 + h2o. (2) equations (1) and (2) stipulate that both electronic conductivity and proton availability are necessary for good performance of ruthenium oxide in supercapacitor applications. since the hydration of ruthenium oxide promotes the proton conductivity, while at the same time it hinders the electron transport especially among the ruthenium oxide particles, it is obvious that the compromise between these two effects has to be made. crystalline forms of ruo2 give very low gravimetric capacitances due to insufficient hydration and limited rates of proton transport into the deeper layers of oxide, whereas on the other hand, the capacitances up to 720 f/g were achieved with hydrous and amorphous ruo2 prepared by the sol-gel synthesis method [4]. it is now well established and confirmed by several authors that, in order to achieve the maximal utilization of ruthenium oxide and extract as much charge as possible, two parameters, electronic and protonic conductivity, should be optimized. this is usually done by annealing the amorphous ruo2·xh2o at appropriate temperatures [5-13]. several strategies were attempted to improve further the ruo2 utilization and to increase its gravimetric capacitance. one strategy includes the different ruo2 preparation methods [4, 8-11,14-23], whereas several authors recognized the importance of both intraand inter-particle electronic conductivities and prepared the composite materials of ruo2 with various forms of carbon [6,18,19,27-28]. it was observed that nano-structured ruo2 particles were decisive factor in achieving the high gravimetric capacitance [29,30]. carefully prepared electrodes with low ruo2 loadings and good electric contact between ruo2 and underlying conductive support gravimetric capacitances of ruo2 as high as 1500 f/g and even higher can be achieved [6,7,13]. another class of materials showing the promising pseudocapacitive properties is conducting polymers. in contrast to redox mechanism of the transition metal oxides, electrochemical reactions of conducting polymers are usually accompanied by the exchange of anions with surrounding medium. among these materials, polyaniline (pani) is the most frequently investigated due to simple synthesis, rich chemistry and electrochemistry along with the stable and reproducible behavior. the gravimetric capacitance of pani can reach up to 400 f/g and higher comprising a combination of pseudocapacitance and double layer capacitance [31]. although the potential range of pseudocapacitive behavior of pani is limited between the two redox transitions, leucoemeraldine/emeraldine and emeraldine/pernigraniline, it could nevertheless be expected that the combination of pani with ruo2 might lead to the possible synergic effects resulting in the composite material with improved characteristics for supercapacitor applications. s. sopčić et al. j. electrochem. sci. eng. 2(1) (2012) 41-52 doi: 10.5599/jese.2012.0010 43 several routes for the preparation of ruo2/pani composite materials were attempted. song et al [32] deposited electrochemically the hydrous ruo2 from rucl3 solutions on the casted pani/nafion® support. deposition of pani by electropolymerization in the pores of irox [33] was also carried out. the preparation of ruo2–poly(3,4-ethylenedioxythiophene) nanocomposite for supercapacitor purpose was reported [34]. recently, the procedure based on the chemical reaction between ruo2 attached at the electrode surface and aniline from the solution has been developed [35]. this system showed good long-term stability and preserved electrochemical activity at higher ph values. in this paper, we describe the preparation of the ruo2/pani composite by slightly different approach. electrochemical deposition of pani was carried out from aniline solution onto the glassy carbon (gc) electrode covered by casted hydrous ruo2 particles. pani was deposited in the pores of and between ruo2 particles, what should lead to an increase in overall capacitance of the ruo2 electrode by the contribution of the pani pseudocapacitance. equally important should be the expected effective increase in the inter-particle electronic conductivity which was shown in some cases to be the limiting factor in achieving high capacitance of high-power supercapacitors. to be as close as possible to practical applications, ruo2 electrodes were made by mixing the oxide with the polymeric binder of variable amount. two frequently encountered polymer binders, nafion® and poly(vinylidene fluoride) (pvdf) were used. due to their different physico-chemical properties, it is expected that the binders should affect the overall performance of the composite gc/ruo2/binder/pani electrodes differently. thus, the aim of this work was to prepare the composite electrodes with variable amounts of ruo2, pani and binders and to investigate their electrochemical and supercapacitor properties. the optimization of the composition of the composite electrode to achieve maximum capacitance and power was also attempted. the intention was to reveal the role of the individual components in the overall behavior of the composite electrode. experimental chemicals and materials the following analytical grade chemicals were purchased: ruo2 in hydrated and amorphous form, nafion® 117 (5 % solution in isopropyl alcohol) and poly(vinylidene fluoride) (pvdf) from aldrich chemie, n-methyl pyrrolidone (nmp) from merck, isopropyl alcohol from t.t.t. (croatia), aniline and h2so4 from fluka. all solutions were prepared from bi-distilled water. x-ray diffraction and thermogravimetric analysis of the ruo2 sample were published elsewhere [13]. preparation of composite electrodes particles of the hydrous ruthenium oxide, ruo2·xh2o, were dispersed in corresponding binder solution using an ultrasonic bath. two binders were used, nafion® in isopropyl alcohol and pvdf in nmp. homogenized suspensions were casted on the glassy carbon electrode (gc), which was previously polished by al2o3 slurry, washed by bi-distilled water and degreased in ethanol. the electrodes were dried 24 h at room temperature in open air in the case of nafion® and in vacuum in the case of pvdf. various quantities of polyaniline (pani) were electrodeposited onto the composite electrodes by cyclic voltammetry from the solution of 0.1 m aniline in 0.5 m h2so4, in the potential range from -100 to 900 mv vs. ag/agcl reference electrode with the scan rate of ν = 50 mv/s. the exact compositions of electrodes and their assignations are given in tables 1 and 2. the same electrode compositions were achieved only in two cases (electrodes xx22 and xx23) j. electrochem. sci. eng. 2(1) (2012) 41-52 ruo2/pani composite for supercapacitors 44 since it was not possible to obtain compact composite layer with amount of pvdf higher than 0.05 mg and with amount of nafion® smaller than 0.025 mg. table 1. composition of composite electrodes with nafion® as a binder. electrode m(ruo2) / mg m(nafion ®) / mg nafion®/ruo2 ratio rn14 0.04 0.10 2.50 rn15 0.04 0.40 10.0 rn22 0.10 0.025 0.25 rn23 0.10 0.05 0.50 rn24 0.10 0.10 1.00 rn25 0.10 0.40 4.00 rn35 0.46 0.40 0.86 table 2. composition of composite electrodes with pvdf as a binder. electrode m(ruo2) / mg m(pvdf) / mg pvdf/ruo2 ratio rp11 0.04 0.01 0.25 rp21 0.10 0.01 0.10 rp22 0.10 0.025 0.25 rp23 0.10 0.05 0.50 rp31 0.46 0.01 0.02 electrochemical characterization of composite electrodes two electrochemical methods, cyclic voltammetry (cv) and electrochemical impedance spectroscopy (eis), were used to investigate the electrochemical and capacitive properties of prepared composite electrodes. the measurements were done in three-electrode system where the working electrode was as prepared composite on gc substrate, the counter electrode was a pt-foil and the reference electrode was ag/agcl (3m kcl). cv experiments were carried out in an one-compartment cell by the potentiostat/galvanostat par, model 263a whereas eis measurements were carried out with lock-in amplifier eg&g par, model 5210, connected to the potentiostat. electrochemical properties of the prepared composite electrodes were investigated in 0.5 m h2so4 in the potential range from -100 to 1000 mv with the scan rate of ν = 50 mv/s. eis measurements were carried out at dc potential of edc=0.45 v vs. ag/ agcl, with the amplitude of 5 mv in the frequency range from 100 khz to 10 mhz. gravimetric capacitances were determined on the basis of ruo2 content. results and discussion the results obtained indicate that the responses of the composite electrodes are very sensitive not only to their constituents but also to the quantities of the individual components as well as to their ratios. figures 1a and 1b show cyclic voltammograms of ruo2/nafion ® composite electrodes of different compositions, rn15 and rn23, respectively, during several cycles of pani electrodeposition. a brief inspection of the figures 1a and 1b reveals the importance of nafion® on the characs. sopčić et al. j. electrochem. sci. eng. 2(1) (2012) 41-52 doi: 10.5599/jese.2012.0010 45 teristics of composite electrodes. the specific currents and consequently the charge extracted from rn23 electrode are at least an order of magnitude higher than currents obtained for rn15 electrode. in addition, rn15 composite electrode shows close to rectangular shape prior to the pani deposition (fig. 1a, black line), which is characteristic for capacitive behavior. as the amount of ruo2 is increased in rn23 with respect to rn15 electrode, larger deviations from rectangular shape were observed (figure 1b). these deviations might be the consequence of the poor interparticle contacts among oxide particles of the rn23 electrode since it contains lower nafion® content comparing to the rn15 electrode (table 1) leading to the increased overall ohmic resistance. pani deposition is also influenced by the composition of the composite electrode. the pani growth seems to be much faster in the case of rn15 compared to rn23 composite electrode with well resolved current peaks corresponding to the well known leucoemeraldine/emeraldine and emeraldine/pernigraniline redox transitions superimposed onto the ruo2 capacitive current. in the case of rn23 electrode, the pani deposition causes only negligible change in cyclic voltammograms (figure 1b). -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -4 -2 0 2 4 i / a g1 e / v 0 1 2 3 4 cycle no.: a) -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -30 -20 -10 0 10 20 30 b) i / a g -1 e / v 0 5 10 15 cycle no.: figure 1. cyclic voltammograms of a) rn15 and b) rn23 composite electrode during several voltammetric cycles of the pani electrodeposition. scan rate = 50 mv/s. cyclic voltammograms of the composite electrodes with pvdf (figure 2, black lines) as a binder show very broad anodic peak at about 0.5 v, which is commonly observed in the cyclic voltammograms of hydrous ruo2·xh2o [6-8,25]. it is usually explained by the reversible processes which accompany the sorption of protons and water molecules [38,39]. in contrast to the composite electrodes prepared with nafion® as a binder, pvdf influences the specific currents in the opposite way. with the decrease in ruo2 content relative to the pvdf, an increase of specific currents was achieved. cyclic voltammograms recorded during the deposition of pani through several voltammetric cycles are also shown in figures 2a and 2b. in the first few cycles of pani deposition the specific current was somewhat decreased, while at later stages the continuous specific current increase was observed. the effect of pani deposition is even more pronounced for rp21 electrode since high currents characteristic for ruo2 disappear at the potentials higher than 850 mv, at which the conductive emeraldine form of pani transforms into the non-conductive pernigraniline state. it seems that in this case non-conductive pernigraniline form increases inter-particle resistance and resistance between particles and the current collector resulting in the sudden current decrease in the pernigraniline potential region. it is interesting to note that such behavior was not obtained in the case when nafion® was used as a binder (figure 1). further synthesis of pani leads for all j. electrochem. sci. eng. 2(1) (2012) 41-52 ruo2/pani composite for supercapacitors 46 compositions of pvdf electrodes to the overall current increase due to the contribution of pani redox reactions. obviously, the polymerization process as well as the resulting properties of the electrodes with pvdf binder is different from those obtained when nafion® is used. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -45 -30 -15 0 15 30 45 a) 0 1 8 13 i / a g -1 e / v cycle no.: -0.2 0.0 0.2 0.4 0.6 0.8 1.0 -45 -30 -15 0 15 30 45 b) i / a g -1 e / v 0 5 10 15 cycle no.: figure 2. cyclic voltammograms of (a) rp21 and (b) rp23 composite electrodes during several cycles of pani deposition. scan rate = 50 mv/s. although the shape of cyclic voltammograms and current heights can reveal important information about the system studied, in the investigations aimed to the development of electrodes for supercapacitor applications more important quantity is the gravimetric capacitance, cs. the gravimetric capacitance for the investigated composite electrodes could be determined from the cyclic voltammograms according to the equation 1. the results obtained for the nafion® and pvdf composite electrodes are shown in figures 3a and 3b, respectively, as plots of gravimetric capacitance versus voltammetric cycles of pani deposition. )2( s em q c δ = (1) q is the sum of anodic and catodic charges obtained by the integration of cyclic voltammograms in the potential interval δe, and m are the mass of casted ruo2. the data shown on figure 3 could be analyzed from two different standpoints. the data values belonging to y-axis correspond to the gravimetric capacitances of electrodes composed of various ruo2/nafion® (figure 3a) and ruo2/pvdf (figure 3b) ratios. the curves representing the dependence of gravimetric capacitance on the number of voltammetric cycles reveal the influence of pani on the overall capacitance of the particular composite electrode. the analysis of the obtained results indicates the significant influence of the binder, its chemical structure or/and its physico-chemical properties on the overall behavior of ruo2 composite electrode. while nafion® exerts a negative influence, i.e. gravimetric capacitance decreases with the increase of nafion® content relative to ruo2, the gravimetric capacitance of ruo2/pvdf composite electrodes shows completely opposite behavior (figure 3c). with the increase of pvdf content relative to ruo2 the overall capacitance increases. the highest ruo2 gravimetric capacitance of 526 f g -1 was obtained for the rp11 composite electrode. the formation of pani on the surface and inside the pores of ruo2/nafion® composite electrodes contributes to the gravimetric capacitances although for some composite electrodes (rn22, rn23, rn24, rn35) slight decrease of the overall charge was observed after a few voltammetric cycles. the trend depends on nafion®/ruo2 ratio and decrease in charge is obtained in the cases when this ratio was less than 1. s. sopčić et al. j. electrochem. sci. eng. 2(1) (2012) 41-52 doi: 10.5599/jese.2012.0010 47 in contrast to the ruo2/nafion® composite electrodes, deposition of pani on ruo2/pvdf electrode leads to the capacitance decrease in the first few voltammetric cycles but later on the capacitances steadily increase indicating the contribution of pani redox reactions to the total charge of the composite electrodes. 0 2 4 6 8 10 12 0 25 50 200 400 600 0 1 2 3 4 5 200 220 240 260 a) c s / f g -1 rn15 rn25 rn35 rn24 rn14 rn22 rn23 n 0 2 4 6 8 10 12 14 16 18 20 0 100 200 300 400 500 600 700 b) c s / f g -1 n rp11 rp21 rp31 rp22 rp23 0.0 0.1 0.2 0.3 0.4 0 100 200 300 400 500 600 c) c s / f g -1 m (binder) / mg pvdf nafion® figure 3. dependence of the specific capacitance, cs, on the number of voltammetric cycles of pani deposition, n, for a) ruo2/nafion ® composite electrodes, b) ruo2/pvdf composite electrodes, c) the comparison of cs dependence for the two composite electrodes (n = 0), m(ruo2) = 0.1 mg in order to get a deeper insight into the origins of such a peculiar behavior of the ruo2 electrode and the role individual components play in the electrochemical properties of the composite electrodes, further investigations by the electrochemical impedance spectroscopy (eis) were carried out. in figures 4a and 4b bode plots for the various compositions of ruo2/nafion ® and ruo2/pvdf electrodes are shown, respectively. depending on the frequency range three regions of impedance response could be distinguished, resistive behavior at high frequencies, capacitive-like response at low frequencies and, in some cases, diffusion-like behavior in the intermediate frequency range. these impedance responses were usually observed for the porous electrodes involving ruo2 pseudocapacitive behavior [9,38,39]. diffusion characteristics are frequently explained by the rate-limiting transport of ions, mainly protons, into the deeper layers of porous and hydrous ruo2. figure 4a displays impedance spectra of three composite electrodes prepared with nafion® as a binder with the same amount of nafion® and various amounts of ruo2. depending on the composite, various processes dominate the impedance spectra. all three composites show capacitive response at low frequencies. however, for the electrodes with relatively low ruo2 content, the onset of the capacitive responses falls in the range of 1-10 hz. for rn35 electrode, the electrode with very high ruo2 and low nafion ® content, in the same frequency range a process j. electrochem. sci. eng. 2(1) (2012) 41-52 ruo2/pani composite for supercapacitors 48 controlled by diffusion appears and the onset of the capacitive response is shifted toward lower frequencies. on the other hand, the electrodes with very low ruo2 content (rn15) exhibit in the high frequency range a behavior which could be modeled with parallel resistive/capacitive combination. this is probably due to increased inter-particle resistance caused by their separation by nafion® chains or poor contact between oxide particles and gc support. 0.01 0.1 1 10 100 1000 10000 100000 0 1 2 3 4 5 6 a) rn15; 0.04 mg ruo2 rn25; 0.1 mg ruo2 rn35; 0.46 mg ruo2 f / hz lo g (iz i / w c m 2 ) 0 30 60 90 -φ / de g 0.01 0.1 1 10 100 1000 10000 100000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 lo g (iz i / w c m 2 ) rp11; 0.04 mg ruo2 rp21; 0.1 mg ruo2 rp31; 0.46 mg ruo2 f / hz 0 30 60 90b) φ / de g figure 4. bode plots of a) ruo2/nafion ® and b) ruo2/pvdf composite electrodes as impedance spectra were concerned, composite electrodes with pvdf as a binder (figure 4b) behaved differently from the ruo2/nafion ® composites. none of the ruo2/pvdf composites showed resistive/parallel combination in high-frequency region of impedance spectra similar to rn15 electrode. however, diffusional process in the intermediate frequency range was observed for almost all composites. pani deposition causes the eis spectra of ruo2/nafion ® and ruo2/pvdf composite electrodes to change. the changes in the impedance spectra during each successive voltammetric cycle are shown in figures 5a and 5b, respectively. the changes depicted in the figure 5 are mostly located in the medium, diffusion region of the frequency range. however, surprising observation was that in the case of ruo2/nafion ® composite, each successive cycle of pani deposition promotes diffusion process (figure 5a), while on the other hand, diffusion process usually observed for ruo2/pvdf electrode is hindered and starts to disappear with pani deposition (figure 5b). 0.01 0.1 1 10 100 1000 10000 100000 0 1 2 3 4 lo g (iz i / w c m 2 ) a) 0 1 2 3 4 f / hz 0 30 60 90 -φ / de g 0.01 0.1 1 10 100 1000 10000 100000 0.0 0.5 1.0 1.5 2.0 2.5 3.0 lo g (iz i / w c m 2 ) b) 0 1 2 8 18 f / hz 0 30 60 90 -φ / de g figure 5. bode plots of the composite electrodes, a) rn15 and b) rp21 at different voltammetric cycles of pani deposition specific capacitance values obtained from eis (table 3) are in a very good agreement and indicate the same trend as cs values obtained from cyclic voltammetry. from the table 3 it is also evident that cs values obtained from eis are higher compared to cyclic voltammetry values. these s. sopčić et al. j. electrochem. sci. eng. 2(1) (2012) 41-52 doi: 10.5599/jese.2012.0010 49 values are obtained using complex non-linear least square (cnls) fitting of data in zsimpwin program. the electrical equivalent circuit used in modeling procedure consisted of the parallel combination of warburg diffusion impedance, zw, and constant phase element, q, all in series with the solution resistance, rs. some cs are estimated from c=1/ωz’’ at 0.01 hz, and for the two results (rn35 and rp11) it was not possible to obtain good fit or to estimate cs. this could be explained by the fact that diffusion behavior which dominates in wide frequency range influences capacitive response in the low frequency range. table 3. cs values obtained from cv and eis of the ruo2/binder electrodes for various compositions electrode cs / f g -1 (cv) cs / f g -1 (eis) rn14 224 275 rn15 10.4 12.4 rn22 482 687 rn23 391 446 rn24 423 519 rn25 199 219 rp21* 381 671 rp22 409 526 rp23 437 770 rp31* 155 330 *cs (eis) values are estimated from c = 1/ωz’’ at 0.01 hz. the reason for such an unusual behavior of the ruo2/binder/pani electrodes is obviously due to the specific inter-molecular interactions among various components. although at the first glance two binders have similar chemical structure, there is, in fact, a tremendous difference in physico-chemical properties between them. nafion® consists of two distinct domains, hydrophilic one around sulphonic groups, which exist at the end of the side chains of the hydrophobic polymer backbone. this is in contrast to the pvdf composed of completely hydrophobic chains. the sulphonic groups in nafion® are able to interact with charged species and hydrophilic environment. when the ruo2/nafion ® composite is formed, it is reasonable to assume that the sulphonic groups are oriented toward the hydrous ruo2 surface effectively enveloping and blocking the solid particles with nafion® chains, as depicted in figure 6. the blocking effect of nafion® is demonstrated by the decrease of gravimetric capacitance (figures 3a and 3c) as well as by the disappearance of processes related to the diffusion of ions into the deeper layers and pores of ruo2 (figure 4a). the deposition of pani on the ruo2/nafion ® composite electrode removes nafion® from the surface of ruo2 due to energetically more favored ionic attraction between positive pani backbone and sulphonic groups. in fact, sulphonic groups might play a role of counter-ions during pani synthesis as well. the result is the liberation of the ruo2 surface by pani deposition enabling its unhindered oxidation and reduction including the transport of species into deeper layers what is manifested as a diffusion appearing in the middle frequency range during pani deposition (figure 5a). pvdf binder, due to its hydrophobicity, forms separate isolated domains within its composite with ruo2 and does not hinder the reactions of ruo2 active sites by blocking the oxide surface (figure 6). pvdf has no affinity toward pani either and the deposition of pani leads to the coverage of ruo2 surface interfering with the ruo2 redox reactions, especially those requiring j. electrochem. sci. eng. 2(1) (2012) 41-52 ruo2/pani composite for supercapacitors 50 transport of ionic species. as a result, the disappearance of diffusion characteristic was observed (figure 5b) and total charge extracted after the initial decrease continuously increases due to the contribution of pani redox reactions. a) b) figure 6. schematic view of pani growth within the composite electrodes of: a) ruo2/nafion ® and b) ruo2/pvdf. the data presented in this paper reveal the importance of understanding of all properties of the components involved and interactions they undergo with each other in construction of the ruo2 electrode with desired properties for supercapacitor applications. in order to prepare efficient capacitive electrode, one has to keep in mind its particular application. if high power supercapacitors are aimed, the electrodes exhibiting no diffusion processes, such as rn15 and rn25, are favorable. on the other hand, if high energy devices are needed, ruo2/pdvf/pani composite electrodes are preferable. finally, in high energy devices for the applications with intermittent high current demand, the electrodes with the low nafion® contents would be the composition of choice. conclusions for the construction of active electrodes for supercapacitor applications consisting of hydrous ruthenium oxide, with or without polyaniline, one has to pay particular attention to the exact composition and amount of additives used in the manufacturing process. different physicochemical properties of polymeric binders influence the overall behavior of composite electrodes differently. nafion consisting of perfluorinated chains having hydrophilic sulphonic groups attached on side-chains tends to block the surface of ruo2, preventing deeper layers charging/discharging reactions. this decreases the overall capacitance of the ruo2 but on the other hand enables its applications in the high-power devices. s. sopčić et al. j. electrochem. sci. eng. 2(1) (2012) 41-52 doi: 10.5599/jese.2012.0010 51 poly(vinlydene fluoride) forms separate domains within the composite electrode leaving the surface of ruo2 free for the ionic transport which accompanies charging/discharging reactions. as a result, higher amount of ruo2 material is utilized and consequently higher energy electrodes can be constructed, but with limited ability for high power applications. acknowledgements: this work is supported by the national research project no 125-1252973-2576 from the ministry of science, education and sports of the republic of croatia. the authors wish to thank višnja horvat radošević and krešimir kvastek for fruitful discussion related to this work. references [1] k.h. chang, c.c. hu, appl. phys. lett. 88 (2006) 193102 [2] s. sopčić, m. kraljić roković, z. mandić, a. róka, g. inzelt, electrochim. acta 56 (2011) 35433548 [3] p. kurzweil, j. power sources 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice {comparative study of al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 electrode prepared by hydrolysis coating technology} http://dx.doi.org/10.5599/jese.624 85 j. electrochem. sci. eng. 9(2) (2019) 85-97; http://dx.doi.org/10.5599/jese.624 open access: issn 1847-9286 www. jese-online. org original scientific paper comparative study of al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 electrode prepared by hydrolysis coating technology daxian zuo1, cuiping wang1, guanglei tian2, kangying shu2, xingjun liu1,3, 1college of materials and fujian provincial key laboratory of materials genome, xiamen university, xiamen, 361005, p. r. china 2college of material science and engineering, china jiliang university, hangzhou, 310018, p. r. china 3department of materials science and engineering, harbin institute of technology, shenzhen, guangdong, 518055, p. r. china corresponding author: lxj@xmu.edu.cn; tel.: +86-592-2187888 received: october 8, 2018; revised: january 18, 2019; accepted: january 19, 2019 abstract in this work, al, si and ti oxides are used to modify the surface of lini0.6co0.2mn0.2o2 (ncm622) electrode through the hydrolysis coating technology. sem and tem results revealed that three prepared oxide layers have different uniformity and morphology. also, charge-discharge results showed different initial discharge capacity and cycle ability of three different oxide coatings. it is shown that when the temperature is increased from 25 to 50 °c, the capacity retention of al2o3-coated ncm622 is reduced by only 4 %, what demonstrated the best ability of this oxide to restrain cycle deterioration. additionally, when the charge cutoff voltage is increased to 4.6 v, al2o3-coated ncm622 showed 74 % of capacity retention. as the number of charge-discharge cycles increases, the dissolution of some transition metal ions may be restrained by al2o3 layer. generally, the enhanced electrochemical performance of al2o3-coated ncm622 could be ascribed to the suppression of mutual reaction between electrode and electrolyte and improvement of structural stability of the material by al2o3 coating. keywords lithium ion battery; nickel-rich layered cathode; oxide coating layer; electrochemical performance; polarization resistance http://dx.doi.org/10.5599/jese.624 http://dx.doi.org/10.5599/jese.624 http://www.jese-online.org/ mailto:lxj@xmu.edu.cn j. electrochem. sci. eng. 9(2) (2019) 85-97 al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 86 introduction in recent years, lithium ion batteries with high energy density have attracted much attention in the field of mobile devices such as notebook computers, cellular phones, and digital cameras [1-4]. especially cathode materials play a key role in obtaining high energy density. among them, the nickel-rich layered cathode materials, linixcoym1-x-yo2 (x>0.5) with α-nafeo2 structure [5-8], are considered as promising candidates to satisfy high capacity and rate capability criteria. for the nickel-rich layered cathode materials, however, although the discharge capacity would increase with amount of ni2+, formation of the hybrid between ni2+ and li+ would result in a cycle decay [9,10]. whereas at present improvement of the charge voltage may be a key research direction for cathode materials, it can generally be expected that significant increase of interfacial resistance between nickel-rich layered cathode materials and electrolyte [11] would also lead to rapid capacity decay. additionally, the rate capability and cycle stability at elevated temperatures were found poor for this material [12]. all these drawbacks limit further application of nickel-rich layered cathode materials. to resolve these problems, a lot of efforts have been made to ameliorate the surface structure of nickel-rich layered cathode materials by various approaches [13-15]. thus, the surface coating has generally been recommended as a simple and effective technique to improve the rate capability and cycleability at elevated temperatures, inhibit cation dissolution into electrolyte and suppress the growth of polarization impedance [16,17]. besides, the presence of the coating layer could also restrain the interfacial reaction between electrode and electrolyte, and protect the electrode material from the hf attack during the charge-discharge process [18,19]. therefore, better performance is expected to be achieved by surface coating, even in conditions of higher upper limit voltage and increased temperature. ncm622 is a representative kind of nickel-rich layered cathode materials with a high initial discharge capacity, approximately 200 mah g-1 [20]. unfortunately, just like other nickel-rich layered cathode materials, ncm622 showed many intrinsic drawbacks of linio2 [21], such as thermal instability and poor cycle stability. so far, the commercialized application of this cathode material has been inhibited by its severe capacity fading during high temperature and voltage cycling. to solve these problems, the surface coating has been proved to be an effective technology. metal oxides such as al2o3 [22], zro2 [23], v2o5 [24], and zno [25] have been reported as very effective coating materials. it has already been shown [26-28] that for different oxide coating materials, the uniformity, thickness and structure characteristic of each coating layer are different, what would lead to different electrochemical performances. therefore, it becomes necessary to make a comparative study of the relationship existing between each oxide coating layer and electrochemical behavior of the coated ncm622 electrode. unfortunately, there are only few literature data on this aspect. in this study, a commercial sample of ncm622 is coated with al2o3, sio2 and tio2, by using the hydrolysis coating technology. corresponding structures and electrochemical properties of ncm622 coated by these three oxides are examined, and the correlation between oxide coating layers and electrochemical performance of the coated ncm622 is discussed in detail. experimental sample preparation commercial ncm622 (ningbo jinhe new materials co. , ltd, china) was used as the pristine material. aluminum isopropoxide (aip), tetrabutyl titanate (tbt) and tetraethyl orthosilicate (teos) daxian zuo et al. j. electrochem. sci. eng. 9(2) (2019) 85-97 http://dx.doi.org/10.5599/jese.624 87 were used as precursor reagents for the corresponding oxide coatings. throughout this paper, pristine ncm622, al2o3-coated ncm622, sio2-coated ncm622 and tio2-coated ncm622 are marked as p-ncm622, a-ncm622, s-ncm622 and t-ncm622, respectively. since it was shown previously [17] that the cathode material with oxides and active material in the weight ratio of 1:99 has better electrochemical performance, in this work, the oxide-coated ncm622 materials with oxide and ncm622 in the same weight ratio (1:99) were synthesized by the hydrolysis coating technology. figure 1 describes schematically the coating procedure. figure 1. schematic illustration of oxide coating layer formation on ncm622. the stoichiometric amount of the precursor reagent was firstly dissolved in ethanol, and polyvinyl pyrrolidone (pvp, 0.8 g) was added into the solution. then the ncm622 powder (39.6 g) was dispersed in the solution, and stirred at room temperature for 1 h. after that, 50 ml of distilled water/ethanol (volume ratio=1:9) was added to the mixed solution slowly during 1 h with vigorous stirring, and the solution ph value was adjusted to 10.0 by 0.1 m nh3×h2o solution. the mixed solution was then continuously stirred at 60 °c until the solvent was evaporated. at last, the product was washed, dried and then treated at 600 °c for 4 h, giving the final sample. for comparison purpose, the pristine ncm622 sample was subjected to the same procedure, but without “active component” as aluminum, silica or titanium compounds. structure and morphology characterizations powder x-ray diffraction (xrd) data were collected using x-ray diffractometer (thermo arl, x’ tra) equipped with a ni-filtered cu-kα radiation at 40 kv and 20 ma for structure analysis. also, the diffraction patterns were recorded between scattering angles of 10 and 90° in steps of 0.02°. the surface morphology of all samples was observed using a scanning electron microscope (sem, hitachi su8010) and transmission electron microscope (tem, jeol jem 2100). additionally, the electron diffraction spectroscopy (eds) mapping was examined to confirm the surface element distribution, together with sem in a large field of view. electrochemical characterizations electrochemical performances of all samples were examined using cr2430-type coin cells. a metallic lithium foil was used as the negative electrode. the positive electrodes were prepared by mixing 90 wt% of active material (commercial ncm622 particles or as-prepared oxides-coated ncm622), 5 wt% of acetylene black conductive additive, 5 wt% of polyvinylidene fluoride (pvdf) http://dx.doi.org/10.5599/jese.624 j. electrochem. sci. eng. 9(2) (2019) 85-97 al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 88 binding agent and proper amount of n-methyl-2-pyrrolidone (nmp) solvent to form the slurry. the as-prepared black slurry was pasted on the current collector (aluminum foil) and followed by drying at 90 °c in a vacuum oven overnight. electrodes were cut into small wafers with diameter of 16 mm. the active material surface loadings on al foils were about 7-8 mg cm-2. the electrolyte solution consisted of 1 mol dm-3 lipf6 dissolved in ethylene carbonate (ec)/dimethyl carbonate (dmc) (1:1, v/v). a polypropylene microporous film (cellgard 2300) was used as a separator. the cells (cr 2430) were assembled in an argon-filled glove box with water/oxygen content less than 0.1 ppm (super 1220/750, mikrouna). galvanostatic electrochemical test data were collected on a multichannel battery cycler (ct2001a, wuhan land electronic co. , ltd) at different temperatures. the impedance spectroscopy (eis) and cyclic voltammogram (cv) data were collected at an electrochemical workstation (chi 660e, ch instrumenets). cv curves were carried out at the scan rate of 0.1 mv s-1 between 3.0 and 4.6 v (vs. li/li+). eis were measured with a voltage of 5 mv amplitude in the frequency range of 100 khz to 0.01 hz, and carried out in a self-made threeelectrode electrochemical cell with lithium metal as counter and reference electrodes. results and discussion structure and morphology characterizations the xrd patterns of ncm622 coated with al, si and ti oxides are compared with those of pristine ncm622, respectively. as shown in figure 2, xrd patterns of all samples are in good accordance with a hexagonal α-nafeo2 layered structure belonging to the r-3m space group, and no obvious impurities and secondary phase are observed. figure 2. xrd patterns of (a) p-ncm622, (b) a-ncm622, (c) s-ncm622 and (d) t-ncm622. besides, the well-defined doublets of 006/102 and 108/110 for all samples indicate that these materials have a well-developed layered structure [29-31]. for three oxide-coated ncm622 samples, however, there is no diffraction peaks of al2o3, sio2 and tio2 observed in each xrd pattern, which may be attributed to low amount of oxide existing on the surface of ncm622.moreover, crystal lattice parameters listed in table 1 were calculated from xrd patterns mentioned above. as daxian zuo et al. j. electrochem. sci. eng. 9(2) (2019) 85-97 http://dx.doi.org/10.5599/jese.624 89 is seen in table 1, xrd patterns of three oxide-coated particles show no significant changes in a and c lattice parameters and their c/a ratios, suggesting that the crystal structure of ncm622 is not affected by each oxide coating. table 1. lattice parameters, c/a and i(003)/i(104) values of samples. sample lattice parameters, å c/a i(003)/i(104) a c p-ncm622 2.8711 14.2248 4.9545 1.31 a-ncm622 s-ncm622 t-ncm622 2.8738 2.8725 2.8725 14.2404 14.2324 14.2197 4.9553 4.9547 4.9503 1.37 1.35 1.32 morphology of all samples is characterized by sem, and the images are shown in figure 3. it can be seen that particles of all samples present irregular spherical shapes, and particle sizes remained unchanged after surface modifications with different oxides. figure 3. sem, tem and corresponding eds-mapping results: (a) p-ncm622; (b,e) a-ncm622; (c,f) s-ncm622; (d,g) t-ncm622 samples. moreover, as shown in figure 3a, the surface of pristine ncm622 is relatively smooth and seems rather "clean". for three oxide-coated ncm622 samples, however, there is an obvious difference in the surface morphology. as shown in figure 3b, many small particles are attached on the surface of ncm622, forming a small number of al agglomerates. similarly, the surface of tio2-coated ncm622 is coated by some small particles and becomes rough (fig. 3d). as seen from figure 3c, the surface of sio2-coated ncm622 reveals no distinguishable difference compared with the rather smooth pristine ncm622.additionally, eds mapping has been carried out to identify the elements on the http://dx.doi.org/10.5599/jese.624 j. electrochem. sci. eng. 9(2) (2019) 85-97 al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 90 surface of oxide-coated ncm622 particles. as shown in the middle part of figure 3, homogeneous distributions of al, si and ti are observed for the oxide-coated ncm622.all coatings covered the whole surface homogeneously, proving that all oxides coating layers are successfully formed on the surface of ncm622 material. figures 3e-g present tem images of three oxide-coated samples, where it can be seen that in each case, a complete thin oxide layer with a 5-10 nm thickness is formed on the surface of sample. for tio2 coating, however, a thin floc-like layer which makes the morphology different than that of sio2 and al2o3 coatings can be noticed in figure 3g. the surface differences among three distinct oxide coatings may give rise to different electrochemical performances of the coated ncm622. electrochemical characteristics to investigate the influence of oxide coating layer on the electrochemical performance of ncm622 cathode material, three oxide-coated samples and pristine counterpart cells with li anodes were tested by galvanostatic charging/discharging at 0.1 and 1 c (1 c = 160 ma g-1) in the voltage range of 3.0-4.4 v, at different temperatures (25 and 50 °c). figure 4a illustrates the initial charge/discharge curves measured at 0.1 c, where the discharge capacity of the pristine sample is 191 mah g-1. the initial discharge curve of al2o3-coated ncm622 is basically identical with that of the pristine sample, and the discharge capacity is also about 191 mah g-1.for sio2 and tio2 coatings, however, initial discharge capacities are slightly decreased to about 180 and 177 mah g-1, respectively. as shown in figure 4b, the initial coulombic efficiency does not change much after oxide coatings. figure 4. (a) initial charge-discharge curves of all samples in the voltage range of 3.0-4.4 v at 0.1 c. (b) the corresponding initial coulombic efficiency. figures 5a and 5c depict initial charge/discharge curves measured at different temperatures (25 and 50 °c) at 1 c, showing almost the same phenomena as in figure 4. differences between initial discharge capacities may be related to the surface morphology of ncm622 and the properties of each oxide itself what could result in different resistances to li ion migration or electron transfer in three oxide coating layers and lead to diverse electrochemical performance. compared with other oxide coatings, it seems that al2o3 coating layer formed on the surface of ncm622 is the most favorable for li ion transport and electron transfer. figures 5b and 5d reveal changes in cycle performance for three oxide-coated samples, performed between 3.0 and 4.4 v at 1 c and high and low temperatures, respectively. under the test condition of 25 °c, the discharge capacity of pristine ncm622 is weakly decreased during daxian zuo et al. j. electrochem. sci. eng. 9(2) (2019) 85-97 http://dx.doi.org/10.5599/jese.624 91 cycling, and the initial discharge capacity and capacity retention ratio of pristine sample are 177 mah g-1 and 94 %, respectively (shown also in table 2). when the test temperature increased to 50 °c, however, the capacity retention ratio dramatically decreased to 70 %, i. e. the capacity retention ratio is reduced by 24 %. figure 5. initial charge-discharge curves of all samples in the voltage range of 3.0-4.4 v at 1 c at different temperatures (a): 25 °c, (c): 50 °c. cycle performance curves of all samples at (b): 25 °c, (d): 50 °c. the appearance of deterioration phenomenon may be attributed to high temperature aggravation of the side reactions between the cathode surface and the electrolyte and the structural damage of ncm622 during charge-discharge cycling, which will hinder li ion migration and thus reduce the cycle stability. in comparison with the pristine one, the oxide-coated samples display capacity retention rates of 96, 95 and 97 % for a-ncm622, s-ncm622 and t-ncm622 after 50 cycles at 25 °c (fig. 5b), which shows similar curve trends with the pristine sample. that is to say, at low temperature, the oxide coating effect is not conspicuous after repeated charge-discharge cycling. however, at high temperature, the cycle performance of oxide coated-samples is largely enhanced related to the pristine electrode (fig. 5d). moreover, data listed in table 2 indicate that when the temperature increased from 25 to 50 °c, the reduction of capacity retention ratios for oxide coated samples is less than of the pristine one. this suggests that oxide coating could restrain the cycle deterioration at high temperature. especially, for a-ncm622 sample, the reduction of capacity retention ratio is only 4 %, which is less than for other two samples. it seems that under the condition of high temperature cycling, the al2o3 coating layer could better inhibit side reactions between electrode and electrolyte and thus show its superiority among all oxide-coated samples treated here. figure 6 exhibits the rate capabilities of all samples at various discharging rates performed between 3.0 and 4.4 v. http://dx.doi.org/10.5599/jese.624 j. electrochem. sci. eng. 9(2) (2019) 85-97 al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 92 table 2. electrochemical performance of lithium-ion cells with p-ncm622, a-ncm622, s-ncm622 and t-ncm622 at different temperatures after 50 cycles between 3.0 and 4.4 v at 1 c. sample capacity, mah g-1 pncm622 ancm622 sncm622 tncm622 25 °c initial discharge capacity 177 178 174 168 capacity retention, % 94 96 95 97 50 °c initial discharge capacity 194 195 190 183 capacity retention, % 70 92 87 81 25→50 °c reduction of retention, % 24 4 8 16 figure 6. discharge rate performance of all samples. as seen from figure 6, as the current density increased (from 0.1 c to 5 c), the discharge capacity of pristine sample decreased rapidly, and its capacity retention is only 57.31 %. the distinct decrease in discharge capacity of pristine ncm622 sample may be due to the slow kinetics at high rates, and destruction of the surface resulting from the side reactions between the active material and electrolyte. in contrast, after oxide coatings, the capacity retention increased to about 80 %, which shows better rate capability. compared with s-ncm622 and t-ncm622 samples, a-ncm622 electrode delivered the highest discharge capacity of 191 mah g-1 at 0.1 c, and still has capacity retention of 84.97 % when the cell was discharged at 5 c. in order to better elucidate the possible role of oxide coating, the cycle stability of ncm622 material at high cutoff voltage is measured at the rate of 1 c. as shown in figure 7a, when the charge cutoff voltage increased to 4.6 v, the capacity retention rate of the pristine sample dropped dramatically, retaining only 41 % after 50 cycles. this is attributed to the cation dissolution into electrolyte and interfacial reaction, what is schematically sketched in figure 7b. daxian zuo et al. j. electrochem. sci. eng. 9(2) (2019) 85-97 http://dx.doi.org/10.5599/jese.624 93 figure 7. (a) cycle performance curves of all samples between 3.0 and 4.6 v at 1 c. (b) schematic diagram of interfacial reactions between electrode and electrolyte. high cutoff voltage will aggravate decomposition of organic solvents and further promote mutual reaction between electrode and electrolyte. also, as the charge-discharge cycle increases, more transition metal ions may dissolute into electrolyte and the attack action of hf generated in electrolyte during cycling could be more prominent. for three different oxide coating samples, however, the downward trend is also different. compared with s-ncm622 and t-ncm622 samples, a-ncm622 sample showed excellent capacity retention, which still retained 81 % in the voltage range from 3.0 to 4.6 v. besides, under different charge cutoff voltages, the specific capacity of ancm622 sample is also higher than of other samples. as the charge cutoff voltage increased from 4.4 v to 4.6 v, this gap becomes obviously wide. cycling performances of a-ncm622 sample described in the present report and few earlier reports on oxide-coated lini0.6co0.2mn0.2o2 electrodes are compared in table 3. some differences observed among different literature data can be connected with different oxide coating layers that are strongly related to unique chemical and physical properties of each oxide itself [38]. therefore, after coating with different oxides, the cycle life of the cathode material can be extended diversely. table 3. cycling performance of different oxides coated lini0.6co0.2mn0.2o2 electrodes cathodes and reference coating material cycling rate, c / cutoff voltage, v initial discharge capacity, mah g-1 capacity retention ratio, % surface loading, mg cm-2 ncm622 (this article) al2o3 1.0 /3.0-4.4 178 96 (50 cycles) 7-8 ncm622 [32] al2o3 1.0 /3.0-4.3 164 95 (30 cycles) 2-3 ncm622 [33] sio2 0.5 / 3.0-4.3 165 95 (50 cycles) 2-3 ncm622 [34] tio2 1.0 / 2.5-4.3 163 85 (100 cycles) 2-3 ncm622 [35] tio2 1.0 / 3.0-4.5 177 88 (50 cycles) 2-3 ncm622 [36] co3o4 1.0 / 2.8-4.6 189 60 (100 cycles) 2-3 ncm622 [37] zro2 0.1 / 2.8-4.3 174 83 (100 cycles) 4 to further explore possible reasons of different electrochemical performances observed for three different oxide-ncm622 samples, eis tests were performed after initial cycle performed between 3.0 and 4.4 v at 1 c. as shown in figure 8, all impedance curves presented as nyquist plots (z’’ vs. z’) can be distinguished in two sections. depressed semicircle responses are observed in higher frequency region, while sloping line responses are observed in lower frequency region, respectively. diameter of a semicircle can be regarded as the charge transfer resistance value (rct), which is an important parameter determining the difficulty of charge transfer [39-41]. and the cpe represents the http://dx.doi.org/10.5599/jese.624 j. electrochem. sci. eng. 9(2) (2019) 85-97 al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 94 corresponding constant phase element. the impedance parameters are calculated by the simplified electrical equivalent circuit (eec) shown in figure 8a. figure 8. (a) eec performed to fit (b) nyquist plots of p-ncm622, a-ncm622, s-ncm622 and tncm622 after initial cycle between 3.0 and 4.4 v at 1 c. in table 4, the calculated values of resistive impedance parameters rct and rs are listed, where rs is resistance at the highest frequency, determined mostly by the electrolyte solution resistance. apparently, the charge transfer resistance value of pristine sample is 52.8 ω, while after coating with oxides, charge transfer resistance values become higher, attaining values of, 83.9, 151.8 and 278.6 ω for a-ncm622, s-ncm622 and t-ncm622, respectively. table 4. resistive impedance parameter values obtained by fitting of eec in fig. 8a to impedance spectra in fig. 8b sample p-ncm622 a-ncm622 s-ncm622 t-ncm622 rs / ω 3.3 4.7 12.6 3.9 rct / ω) 52.8 83.9 151.8 278.6 this phenomenon could be attributed to the inactive oxide layers, which may be subjected to diffusion limitation and thus to an increase of impedance. additionally, the value of rct for a-ncm622 samples is the smallest among all coated samples, which may be more beneficial to li+ insertion/extraction compared with other two oxide-coated samples. to better understand the effect of three different oxide coating layers on electrochemical properties in the charge-discharge processes, the cyclic voltammetry curves of all samples were additionally measured. as shown in figure 9, a pair of sharp redox peaks is observed within 3.6-4.0 v, which is related to the oxidation and reduction reactions between ni2+/ni3+ and ni4+ [42]. besides, there is only one pair of redox peaks appeared in the first five cycles, which suggest that the phase transition from hexagonal to monoclinic is not taking place at 3.0-4.6 v, and the oxide coating layer has no negative effect on the electrochemical reaction. according to the relevant literature [43,44], the magnitude of the potential gap between the anodic and cathodic peak (δv) can be used to qualitatively describe the reversibility/polarization of the electrochemical process. the potential gap values (0.069, 0.128 and 0.198 v) of redox peaks for daxian zuo et al. j. electrochem. sci. eng. 9(2) (2019) 85-97 http://dx.doi.org/10.5599/jese.624 95 three coated ncm622 samples are considerably smaller than those of the pristine sample (0.280 v), indicating that the interfacial polarization of cathode is inhibited by oxide coating. al2o3-coated ncm622 electrode exhibits the lowest potential gap, thereby obtaining the lowest polarization. figure 9. cv curves at scan rate of 0.1 mv s-1of (a) p-ncm622, (b) a-ncm622, (c) s-ncm622 and (d) t-ncm622. this suggests that reversibility of the al2o3-coated electrode is the best among all sample electrodes. therefore, cv results clearly demonstrated that lower polarization and better reversibility of li ions insertion/de-insertion for al2o3 coating sample can be achieved, which is well in line with its superior electrochemical properties. conclusions in this paper, oxide coating layers of al, si and ti were successfully prepared on the surface of lini0.6co0.2mn0.2o2 (ncm622) electrode by the hydrolysis coating technology. the relationship between the kind of oxide coating layer and electrochemical behavior of coated ncm622 electrode was investigated. electrochemical measurements demonstrated that compared with sio2 and tio2coated samples, ncm622 electrode coated with al2o3 layer exhibited higher discharge capacity and better cycle stability. it was also found that cycle stability of this electrode can be significantly improved at increased test temperature and cutoff voltage. the improved electrochemical performance of al2o3 coated electrode is mainly attributed to the suppression of mutual reaction between electrode and electrolyte. thus, as the charge-discharge cycle increases, the dissolution of some transition metal ions may be restrained by the al2o3 coating layer. additionally, it was shown that al2o3 coating layer could better decrease the polarization gap accompanying the chargedischarge processes and achieve better reversibility of li ions intercalation/deintercalation. the http://dx.doi.org/10.5599/jese.624 j. electrochem. sci. eng. 9(2) (2019) 85-97 al2o3, sio2 and tio2-coated lini0.6co0.2mn0.2o2 96 obtained results suggest that al2o3 modification of the surface by this convenient method may play a promoting role for application of ncm622 cathode material in lithium ion batteries. acknowledgements: this research was supported by national natural science foundation of china (no. 51571168), the national key r&d program of china (no. 2016yfb0701401), and the ministry of science and technology of china (no. 2014dfa53040). references [1] d. larcher, j. m. tarascon, nature chemistry 7 (2015) 19-29. 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[44] d. wang, x. li, z. wang, h. guo, z. huang, l. kong, j. ru, journal of alloys and compounds 647 (2015) 612-619. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.624 http://creativecommons.org/licenses/by/4.0/) {redox mediation at poly(o-aminophenol) coated electrodes: mechanistic diagnosis from steady state polarization curves} doi:10.5599/jese.582 271 j. electrochem. sci. eng. 8(4) (2018) 271-280; doi: http://dx.doi.org/10.5599/jese.582 open access : : issn 1847-9286 www.jese-online.org original scientific paper redox mediation at poly(o-aminophenol) coated electrodes: mechanistic diagnosis from steady state polarization curves gabriel ybarra1,, carlos moina1, maría inés florit2, dionisio posadas2 1instituto nacional de tecnología industrial, av. gral. paz 5445, cc 157, wab1650b san martín, argentina 2instituto de investigaciones fisicoquímicas teóricas y aplicadas (inifta), facultad de ciencias exactas, universidad nacional de la plata, suc. 4, cc 16, (1900) la plata, argentina corresponding author e-mail: gabriel@inti.gov.ar; tel./fax: +51-11-4724-6333 received: march 7, 2018; accepted: march 12, 2018 abstract in this work, the mediated reduction of fe(cn)63and fe3+ in poly(o-aminophenol) coated electrodes is analyzed by means of a diagnosis diagram based on the features of the steady state current-potential curves. this analysis allows to identify the current determining process and to reproduce the experimental characteristics of the polarization curve from the relevant kinetic and thermodynamic parameters with a minimum amount of experimental measurements. keywords current rectification; diagnostic diagrams; electroactive polymers; redox polymers introduction redox mediation is a process of indirect electron transfer between a redox species in solution and an electrode with the necessary participation of a third actor, a redox mediator [1-5]. redox mediators can be surface immobilized sites which can be oxidized or reduced electrochemically by controlling the potential of the supporting electrode. the redox sites of the mediator may then oxidize or reduce other redox species in the solution. direct oxidation or reduction at the electrode surface can be inhibited either because of intrinsically slow heterogeneous electron transfer kinetics or because close approach of the soluble redox species to the electrode is prevented. redox polymers, such as poly(o-aminophenol) (pap) [2] and os(ii) bipyridyl polyvynilpyridine [6], can act as this kind of surface-immobilized redox mediators. redox mediation has attracted the attention of many researchers [4-15], mainly because the possibility of tuning the electrochemical reactivity of the coated electrode towards a specific end, http://www.jese-online.org/ mailto:name@address.domain http://dx.doi.org/10.5599/jese.582 j. electrochem. sci. eng. 8(4) (2018) 271-280 poly(o-aminophenol) coated electrodes 272 both by choosing appropriate redox mediators and by controlling the film characteristics. it is also possible to design mediators with certain specific properties, such as ion selectivity, conductivity, formal potential, etc. several models have been developed to explain the obtained i-e curves and the mechanism of mediation. most of them employ the limiting current obtained in steady state conditions as the only criteria to determine the current determining process. in a previous work [1], we have presented a model, based on the previous work of laviron [16] and murray [17], which allows to interpret most of the experimental facts, as well as to elaborate diagnostic criteria from which the kinetic parameters can be estimated. in this work, we show how those diagnostic criteria can be applied to the case of the reduction of fe(cn)63and fe3+ mediated by a redox polymer. the approach presented here provides a considerable knowledge of the system’s electrochemical behavior with a minimum amount of measurements. experimental analytical grade chemicals were used as received. the solutions were prepared with water purified from a millipore milli-q system. the working electrode was an au disk with a diameter of 5.0 mm. a par model 273a potentiostat was used for the voltammetric measurements. the counter electrode was a pt foil of about 5.0 cm2. a 1.0 m nacl normal calomel electrode was used as a reference electrode and the values of the electrode potential were converted to the saturated calomel electrode (sce) scale. the polymeric films were electropolymerized as indicated elsewhere [2] and all measurements were carried out using 0.1 m hclo4 as supporting electrolyte. films of different thicknesses were obtained by increasing the electropolymerization time. the charge under the cathodic peak was employed as a measure of the polymer thickness according to a relation obtained by ellipsometry [18]. theoretical framework we have presented elsewhere [1] a model based on previous treatments [16,17] to analyze current-potential curves of redox mediation at redox polymers coated electrodes. this model considers that any observed current under steady state conditions follows the process schematically shown in figure 1. r oe r o k a electrode film solution         k d,o k d,r k c d e /l x = lx = 0 figure 1. schematic representation of the mediation process in redox polymer coated electrodes. oxidized (o) or reduced (r) redox species in solution must reach the polymer-solution interface where an electron transfer reaction takes place with the redox sites in the polymer (ω and ρ stand for the oxidized and reduced species in the polymer). in the solution, species o and r are transported to and from the polymer-solution interface. in steady state conditions, this is usually carried out by controlling the fluxes of o and r with a rotating disk electrode (rde). after the electron exchange reaction at the polymer-solution interface takes place, charge is transported in g. ybarra et al. j. electrochem. sci. eng. 8(4) (2018) 271-280 doi:10.5599/jese.582 273 the polymer film by electron hopping, which can be described as a diffusion process. finally, there is another electron transfer reaction at the polymer-electrode interface, which is assumed to be fast enough as not to be the current determining process. this model is limited to cases were redox species in solution do not penetrate significantly inside the polymer film and can be ascribed to s mechanisms in the albery’s treatment [18]. as has been show before by laviron [16], there are no thermodynamic restrictions for redox mediation by redox polymers. nevertheless, coating the electrode with a redox polymer introduces additional processes for the reduction and oxidation of redox agents in solution and, consequently, the i-e responses of coated electrodes may differ considerably from those obtained in naked electrodes. let us begin considering the electron exchange reaction taking place at the polymer-solution interface:  + o ⇌  + r (1) whose equilibrium constant can be expressed as a function of the formal electrode potentials of both redox couples, e°’o/r and e°’/, as: o' o' o/r ω/ρr o ω exp ρ / e ec k c rt nf          (2) unless the value of e°’o/r is close to that of e°’/, the equilibrium constant will have either a considerably high or a considerable low value and therefore eq. 1 will be displaced towards the left side (reactants) or the right side (products). we will focus here on mediation processes with high equilibrium constants, as they apply to the cases which will be presented in the results and discussion section. a high equilibrium constant (k1) implies that the reduction of o is greatly favored from a thermodynamic point of view. however, the kinetics of the redox mediation process involves several steps and the actual i-e response is determined by all the processes shown in figure 1. each of these steps is characterized by parameters which can be expressed as characteristic current values: s ood,od, cnfaki  (3) s rd,rd,r cnfaki  (4) lcnfadi /tee  (5) s otckc ccnfaki  (6) s rtaka ccnfaki  (7) id,r and id,o are related to the transport rate of redox species o and r in solution, ika and ikc to the charge transfer reaction rate at the polymer-solution interface and ie to the electronic transport rate in the polymer. cos and crs are the concentration of o and r at the bulk of the solution and ct = c + c, the total concentration of redox sites in the mediator (a complete list of symbols is given at the end of the article). with these five characteristic current parameters, a general relationship for i-e curve under steady state can be worked out: j. electrochem. sci. eng. 8(4) (2018) 271-280 poly(o-aminophenol) coated electrodes 274  ka kc e d,r e d,o 1 1 1 i i i i i i i i i i i                                 (8) where  = c0/ct is the fraction of oxidized mediator at the metal-polymer interface. we may consider that any of the three processes schematically shown in figure 1 can be the one determining the limiting current. we will denominate case r to the conditions in which the electron transfer at the polymer-solution interface is the current determining process (c.d.p.), case e to the one which is controlled by slow charge transport in the film, and case d to the one which is determined by slow mass transport in the solution. in all cases, the resulting i-e curve obtained from eq. 8 has the usual sigmoidal shape. however, its location in the potential scale may vary between eo’o/r and eo’/, and the limiting current may vary from null to a certain value which is related to the most sluggish process(es) from those shown in figure 1. it is interesting to note that the identification of the c.d.p. can be achieved with a minimum of experimental measurements (as few as two) by observing the influence of the film thickness on the value of the half-wave potential e½ and the limiting current. figure 2 shows the diagnostic flow chart which has been worked out to identify the c.d.p. figure 2. diagnostic flow chart based on the consideration of half-wave potential and the film thickness for the mediation of the reduction of an oxidized agent under the conditions stated in the upper right square. it is also possible to construct a kinetic diagram, such as the one shown in figure 3, similar to others previously reported [16]. this kind of diagrams is useful to visualize how experimental variables, such as the concentration of the external redox couple, film thickness and rotation frequency, affect the overall process of mediation. these plots are constructed using characteristic parameters of the system. we have chosen to plot log (ikc/ie) as a function of log (ikc/id,o) as shown g. ybarra et al. j. electrochem. sci. eng. 8(4) (2018) 271-280 doi:10.5599/jese.582 275 in figure 3. for instance, the effect of increasing the film thickness is to slow down the charge transport in the film and correspond to a vertical displacement in the kinetic diagram. therefore, r→e or d→e transitions could be observed by increasing the film thickness. likewise, increasing the rotation frequency of the rotating disk electrode is associated to a horizontal displacement, and d→r or d→e transitions are to be expected. the use of the diagrams such as those presented in figure 2 and 3 provide a straightforward insight into the mediation processes of different systems as is shown in the next section. figure 3. kinetic scheme showing the three different regions of control of the limiting current. results and discussion we will begin considering the pap|fe(cn)63-/4system. figure 4a shows the steady state currentpotential curves obtained for the redox couple fe(cn)63-/4in 0.1 m hclo4 for a naked and a papcoated electrode with different thickness at a constant rotation frequency. both cathodic and anodic currents can be observed in the naked electrode, and the limiting current value corresponds to that obtained for the diffusion of species in solution. on the contrary, only the cathodic wave, which corresponds to the reduction of fe(cn)63-, can be observed in pap coated electrodes and it is displaced towards more negative potentials when compared to the naked electrode. besides, it can be seen that the limiting current depends on the film thickness. we will interpret the obtained results within the frame of the diagnostic flow chart based on the half-wave potential value. the formal potential of the quasi-reversible couple in the redox polymer pap has been estimated to be +0.050 v vs. sce [2]. it can be seen in figure 4a that pap-coated electrodes present only the cathodic wave and that the half-wave potential is displaced towards more negative values as compared with the curve obtained in naked electrodes. in fact, the e½ obtained with the coated electrode coincides with the value of the formal potential of pap. it can also be seen that the limiting current depends on the film thickness; the thicker the film, the lower the limiting current. based on these features of i-e curve and following the diagnosis diagram (figure 2), we reach the conclusion that the electron transport within the film is the current determining step (case e). j. electrochem. sci. eng. 8(4) (2018) 271-280 poly(o-aminophenol) coated electrodes 276 figure 4. (a) experimental steady state i-e curves obtained in a 5 mm fe(cn)63+ 5 mm fe(cn)64+ + 0.1 m hclo4 solution for a au pristine electrode (□) and coated with pap with a thickness of 88 nm (○) and 125 nm (), at a rotation frequency of 1200 rpm. full lines show simulated i-e curves. (b) levich plot for a pristine (––) and pap coated au electrode with different film thickness: 163 nm (□), 88 nm (), 51 nm (○), obtained in a 5 mm fe(cn)63+ 5 mm fe(cn)64+ 0.1 m hclo4 solution. a levich plot obtained for films with three different thicknesses is shown in figure 4b. as can be seen, the limiting current increases linearly at low rotation frequency and reaches a constant value for a given thickness. this behavior can be explained by considering that the c.d.p. is the diffusion of fe(cn)63in the solution at low rotation frequency and then the c.d.p. is the electronic transport in the polymer film at high rotation frequency. the transition between both regimes is abrupt. as long as fe(cn)63flux does not exceed the electron transport capacity in the film, the limiting current will equal the limiting current observed in naked electrodes; once it is exceeded, the limiting current equals the electron transport current, ie from eq. 5. it is possible to simulate i-e curves by employing eq. 8 and the values of characteristic currents (ie, id,o and id,r) as experimental parameters. being the kinetics of electron transfer much faster than diffusion process, kinetic parameters can be neglected from eq. 8 (ik>>ie, ik>>id); electron transport characteristic current values can be obtained from figure 4b (ie = 0.341 ma and ie = 0.524 ma for the thickest and the thinnest films, respectively); finally, diffusion characteristic current can be obtained from measurements carried out on naked electrodes (id,o = 0.698 ma and id,r = 0.625 ma). these parameters, together with the value of the formal potentials for pap and fe(cn)63-/4couples, are enough to simulate the i-e curve. the simulated curves are shown in figure 4a. let us now consider the pap|fe3+/2+ system, with 5 mm fe3+/2+ as the external redox couple in a 0.1 m hclo4 solution. the redox couple has a formal potential of e°’(fe3+/2+) = +0.510 v vs. sce. the i-e curves are shown in figure 5a, both for naked and pap-coated electrodes. the i-e response for pap-coated electrodes shows no anodic currents and the cathodic limiting current is independent of the film thickness. figure 5a shows the steady state i-e curves for the reduction of fe3+ in 0.1 m hclo4 obtained for an au electrode and a pap coated electrode at the same rotation frequency. at first sight, the features of these curves are quite similar to those found for the reduction of fe(cn)63-, with a displacement of the cathodic wave towards more negative potential. we will employ the diagnosis criteria based on the half-wave potential to sort out the mechanism. g. ybarra et al. j. electrochem. sci. eng. 8(4) (2018) 271-280 doi:10.5599/jese.582 277 figure 5.(a) experimental steady state i-e curves obtained in a 10 mm fe(iii) + 0.1 m hclo4 solution for a au pristine electrode (○) and coated with pap of 116 nm (up triangle) y 249 nm (down triangle), at a rotation frequency of 300 rpm. full lines show simulated i-e curves. (b) koutecky-levich plot for pap coated electrodes in a 10 mm fe(iii) + 0.1 m hclo4 solution. film thickness: 43 nm (□), 116 nm (), 249 nm () and 117 nm (○) firstly, it can be noted that the half-wave potential has the same value of the pap redox couple. secondly, the limiting current is insensitive to the film thickness. keeping this in mind, we may now follow the diagnosis diagram and reach to the conclusion that i-e curve is a typical case r where the c.d.p. is the electron transfer reaction between the polymer and the redox species. the koutecky-levich plots, ilim,cat-1 vs. -1/2 (figure 5b), provide additional information for systems which can be ascribed to case r. it can be shown from eq. 8 that, under circumstances that apply in this case, the origin ordinate equals ikc-1. as was done with the pap|fe(cn)63-/4system, the i-e curve can be simulated employing as experimental parameters the value of the characteristic current for the electron transfer kinetics (ikc-1) and the diffusion one (id,r), and the formal potential for the pap couple and the fe2+/3+ redox couple (see figure 5a). finally, it worth noting that it is possible to locate both studied systems pap|fe3+/2+ and pap| fe(cn)63-/4in a kinetic diagram, employing estimated values for the kinetic constants and diffusion coefficients [2]. the axis in the kinetic diagram corresponds to non-dimensional parameters formed by constants (electron transfers constant, diffusion coefficient) and variables (frequency of rotation of the rde, film thickness). while the involved constants depend exclusively on the system under study, variables can be modified within a certain range. thus, there is a window accessible for any given system. in figure 6 we show the estimated windows for both systems. in the case of the pap|fe3+/2+ system, electron transfer at the polymer-solution interface is so sluggish that this process will always have a determining influence on the observed current (case r). on the contrary, for the pap|fe(cn)63-/4-, the electron transfer is so rapid that other processes will limit the current: either the transport of redox agents in solution (case d) or the electron transport in the polymer film (case e). as a matter of fact, it is possible to move from one regime to the other by controlling the experimental variables. we may change from case d to case e either by increasing the rotation frequency (i.e. moving horizontally in the kinetic diagram) or by increasing the film thickness (moving vertically in the kinetic diagram). in both cases, the transition is abrupt, as shown in figure 4b. j. electrochem. sci. eng. 8(4) (2018) 271-280 poly(o-aminophenol) coated electrodes 278 figure 6. localization in the kinetic diagram for the reduction of fe(iii) species in the two pap-fe(ii/iii) systems presented in this work. conclusions in this work we have analyzed the i-e response of a redox polymer (pap) modified electrode in steady state towards the mediated reduction of fe3+ and fe(cn)63-. the available set of diagnostic criteria is widened with the addition of the position of the half-wave potential. we have shown that the analysis of the complete curve allows identification of current determining process, to obtain the kinetic and transport parameters and to simulate the experimentally obtained current-potential curves with a set of characteristic current values which can be easily assessed. acknowledgements: this work was financially supported by the consejo nacional de investigaciones científicas y técnicas, the agencia nacional de promoción científico tecnológica, the universidad nacional de la plata, the comisión de investigaciones científicas de la provincia de buenos aires and the instituto nacional de tecnología industrial. m. i. f. and d.p. are members of the cic and conicet. list of symbols  fraction of oxidized redox sites  reduced redox site in the polymer  oxidized redox site in the polymer  electrode rotation frequency a electrode area c concentration d diffusion coefficient e electrode potential e°' formal potential e½ half-wave potential f faraday constant ie characteristic electronic current id characteristic diffusion current g. ybarra et al. j. electrochem. sci. eng. 8(4) (2018) 271-280 doi:10.5599/jese.582 279 ik characteristic kinetic current ilim limiting current k electron transfer rate constant k equilibrium constant l film thickness n number of exchanged electrons o oxidized species in solution r reduced species in solution r gas constant t absolute temperature x distance from the electrode-polymer interface subscripts and superscripts 0 metal-polymer interface a anodic c cathodic eq thermodynamical equilibrium condition d diffusion e electronic l polymer-solution interface o soluble oxidized species r soluble reduced species s solution t total redox sites current determining processes case e control by slow electron transport in the polymer case d control by slow soluble species diffusion in the solution case r control by slow electron transfer reaction between soluble species and redox species in the polymer references [1] g. o. ybarra, c. moina, m. i. florit, d. posadas, journal of electroanalytical chemistry 609 (2007) 129139. 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[19] w. j. albery, a.r. hillman, journal of electroanalytical chemistry 170 (1984) 27. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) silver ion recognition using potentiometric sensor based on recently synthesized isoquinoline-1,3-dione derivatives doi: 10.5599/jese.2012.0017 143 j. electrochem. sci. eng. 2 (2012) 143-154; doi: 10.5599/jese.2012.0017 open access : : issn 1847-9286 www.jese-online.org original scientific paper silver ion recognition using potentiometric sensor based on recently synthesized isoquinoline-1,3-dione derivatives ajar kamal, rajiv kumar puri, raghu raj, vipan kumar and rakesh kumar mahajan department of chemistry, ugc-centre for advanced studies, guru nanak dev university, amritsar143005, india corresponding author: e-mail: rakesh_chem@yahoo.com; tel.: +91-183-2502223; fax +91-183-2258820 received: march 27, 2012; revised: may 22, 2012; published: august 30, 2012 abstract the four derivatives of isoquinoline-1,3-dione based on β-lactum (i-iv), have been explored as neutral ionophores for preparing poly(vinylchloride) based polymeric membrane electrodes (pme) selective to silver(i) ions. the addition of sodium tetraphenylborate (natpb) and dioctylsebacate (dos) as a plasticizer was found to improve the performance of ion selective electrodes. the best performance was obtained with pme-1 based on ionophore i having composition: ionophore (9.2 mg), pvc (100.1 mg), dos (201.1 mg) and natpb (1.5 mg) in 5 ml tetrahydrofuran. the electrode response was linear with nernstian slope of 58.44 mv/decade in the concentration range of 1.0 x 10-1 m to 5.0 x 10-6 m and detection limit of 5.83 x 10-6 m. it performs satisfactorily over wide ph range of 1.0-5.5. the proposed sensor can be used over a period of more than three months without any significant drift in potential and shows good selectivity to silver(i) ion over a number of cations especially with no interference of mercury(ii) ions. sharp end point was obtained when the sensor was used as an indicator electrode for the potentiometric titration of silver(i) ions with chloride ions and therefore this electrode (pme-1) could be used for quantitative determination of silver(i) ion in synthetic water, silver foil and dental amalgam samples. keywords isoquinoline-1, 3-diones, silver(i) ion, potentiometric sensors, selectivity, indicator electrode. j. electrochem. sci. eng. 2(3) (2012) 143-154 silver ion recognition isoquinoline-1,3-dione 144 1. introduction silver has wide commercial importance in photography, medicine, optics, electronics, and production of currency, jewelry and silverware as well as high capacity silver-zinc and silvercadmium batteries [1,2]. because of its antibacterial properties, silver compounds have been used to disinfect drinking water and water for recreational purposes, in dental, pharmaceutical, antibacterial, anti-hiv preparations and implanted prostheses [3]. soluble silver salts are lethal and slowly absorbed by body tissues, with consequent bluish or blackish skin pigmentation (argyria), and causes severe corneal injury if liquid comes in contact with the eyes. it may produce other toxic effects including kidney, eye, lung, liver and brain damage as well as change in blood cells. metallic silver, however, appears to pose a minimal risk to health [4]. it has been found that an excess of silver is toxic to fish and micro-organisms at a concentration as low as 0.17 µg l-1 [3]. it is therefore crucial to closely monitor the activity of silver in the environment. there exist several techniques for silver metal ion determination, such as inductively coupled plasma atomic emission spectrometry [5,6], inductively coupled plasma mass spectrometry [7,8] and flame absorption spectrometry [9,10], but these techniques are normally time consuming, involving sample manipulation, and they are relatively expensive. ion-selective electrodes (ises) have been established for many years as powerful analytical tools. carrier-based ises have found widespread use for the direct determination of various ionic species in complex samples [11-15]. the ise is an ion analysis technique that provides unique characteristics such as selectivity, good precision, simplicity and low cost [16-19]. ises have been successfully applied for trace analysis of ions of environmental and physiological importance [20-22]. the determination of silver(i) ions in presence of mercury(ii) ions is often a challenging job. silver selective potentiometric sensors based on ion carriers like podands, crown ethers and calixarenes derivatives have been reported [2,23-32]. in addition, bis-pyridinetetramide macrocyclic [33], bis-dialkyldithiophosphate derivative [34] and o,o,o-trialkylphosphorothioates [35] have been reported as effective ionophores to construct silver(i) ion selective electrodes. these ionophores are generally hampered with mercury(ii) ions interference. in this work, we report the use of isoquinoline-1,3dione derivatives based on β-lactum as a highly selective neutral ion carrier for detection of silver(i) ions using membrane ion selective electrodes. the need for highly sensitive and selective determination of silver(i) ions arises from silver’s economic value and the toxicity toward human beings. in present work, an attempt has been made to improve the sensitivity and selectivity of silver(i) ions ise towards various secondary ions particularly mercury(ii) ions. the electrochemical selectivity for various metal ions, the effect of membrane components, internal solution and ph of sample solution on the electrode response was investigated. further, the electrode was also used as indicator electrode in titration experiments. 2. experimental 2.1. materials all reagents used were of analytical grade, and doubly distilled deionised water was used to prepare solutions. all isoquinoline-1,3-dione derivatives i.e. (4rs,4asr)-4-[(sr)phenyl(p-tolylamino)methyl)-4a,7,8,8a-tetrahydro-4h-isoquinoline-1,3-dione [i], (4rs,4asr)-4-[(sr)p-tolyl(p-tolylamino)methyl)-4a,7,8,8a-tetrahydro-4h-isoquinoline-1,3-dione [ii], (4rs,4asr)-4-[(sr)4-chloro-phenyl(ptolylamino)methyl]-4a,7,8,8a-tetrahydro-4h-isoquinoline-1,3-dione [iii], (4rs,4asr)-4-[(sr)4-chlorophenylamino(p-tolyl)methyl]-4a,7,8,8a-tetrahydro-4h-isoquinoline 1,3-dione [iv] were synthesied as a. kamal et al. j. electrochem. sci. eng. 2(3) (2012) 143-154 doi: 10.5599/jese.2012.0017 145 reported in literature [36] and they are shown in fig. 1. the plasticizers bis-2-ethylhexylsebacate (dos), bis-2-ethylhexylphthalate (dop), bis-2-ethylhexyladipate (doa), dibutylphthalate (dbp) and tributylphosphate (tbp) and high molecular weight poly(vinylchloride)(pvc) were used as received from fluka. anion excluder sodium tetraphenylborate (natpb) was obtained from aldrich while silver nitrate and other metal nitrates received from merck were used without any further purification. 2.2. preparation of pvc membrane the general procedure for preparation of pvc membrane was to mix thoroughly pvc, plasticizer, additive and varying amount of isoquinoline-1,3-dione based ionophore in about 5 ml of tetrahydrofuran. mixture was shaken vigorously and clear solution was poured into petri-dish (50 mm diameter). solvent was allowed to evaporate at room temperature. the resulting membrane of 0.4 mm thickness was cut to size, attached to pvc tube with help of pvc glue and conditioned with metal ion solution (10-2 m) for about 24 hours till it gave reproducible and stable potential. h n h n o o cl ch3 h n h n o o h3c (i) (ii) h n h n o o h3c ch3 h n h n o o h3c cl (iii) (iv) figure 1. structures of isoquinoline-1,3-dione receptors i-iv 2.3. emf measurements silver/silver chloride electrodes with 3 m kcl as salt bridge were used as internal and external reference electrodes. the electrochemical cell assembly used for the study was: ag-agcl 3 m kcl 1.0 x 10-2 m agno3 pvc membrane test solution 3 m kcl ag-agcl all the measurements of electrode potential were made with equiptronics model eq-602 potentiometer. the ph measurements were made using elico li model-120 ph meter. j. electrochem. sci. eng. 2(3) (2012) 143-154 silver ion recognition isoquinoline-1,3-dione 146 3. results and discussion 3.1. potentiometric response for multiple-ions using polymeric membrane electrodes in preliminary experiments, different pvc-based ion selective membrane sensors, using isoquinoline-1,3-dione based receptor i as ion carrier, were prepared by mixing 100.1 mg pvc, 201.1 mg dos and 9.2 mg receptor i. membranes so prepared were conditioned with 10-2 m solution of different ions for 24 hours. their potential responses were noted against 10-1 m to 10-8 m solutions of respective analyte ions to determine their potentiometric response for different metal ions (figure 2). silver(i) ions were found to give better response in terms of slope, working concentration range and lower detection limit as compared to other metal ions. results obtained indicate that electrode based on receptor i has preferential affinity for silver(i) ions and it can be employed as an ionophore for the preparation of silver(i) ion selective electrodes. the preferential affinity of receptor i for silver(i) ions can be due to the presence of nitrogen and oxygen as donor atoms. log ([mn+] / m) figure 2. potentiometric response of polymeric membrane electrodes based on isoquinoline-1 3-dione receptor i for various metal ions 3.2 potential response for silver(i) ions using polymeric membrane electrodes based on the results obtained from the preliminary investigation with receptor i, it was decided to employ isoquinoline-1,3-dione based receptors i-iv as ion carrier for the preparation of silver(i) ion selective electrodes. response curves of polymeric membrane electrodes pmes 1-4 incorporating receptors i-iv, respectively, as ion carriers are shown in fig. 3. the composition and the response characteristics obtained for the polymeric membrane electrodes pmes 1-4 are shown in table 1. -300 -150 0 150 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 hg2+ co2+ pb2+ cd2+ cu2+ mg2+ fe3+ li+ na+ k+ zn2+ nh 4 + ag+ ca2+ e / m v a. kamal et al. j. electrochem. sci. eng. 2(3) (2012) 143-154 doi: 10.5599/jese.2012.0017 147 table 1. composition and response characteristics of silver(i) ion selective electrodes based on isoquinoline-1,3-dione receptors i-iv as ionophores the silver(i) ion ises pme-2, pme-3 and pme-4 exhibited nernstian slopes of 52.72, 49.62 and 67.47 mv dec-1 , respectively, in the concentration range of 1.0×10-1 m to 1.0×10-5 m silver(i) ions and produced lower detection limits of the order of 9.54×10-6 m, 1.15×10-5 m and 1.07×10-5 m, respectively. electrode pme-1 based on receptor i shows nernstian slope of 58.44 mv dec-1 over concentration range of 1.0×10-1 m to 5.0×10-6 m silver(i) ions with 5.83×10-6 m as lower detection limit. log ([ag+] / m) figure 3. potentiometric responses of silver(i) selective polymeric membrane electrodes based on isoquinoline-1,3-dione receptors i-iv as ionophores -300 -250 -200 -150 -100 -50 0 50 100 -8 -6 -4 -2 0 pme-1 pme-2 pme-3 pme-4 e / m v s. no. pvc, mg plasticizer, mg natpb, mg ionophore, mg internal solution, m linear range, m detection limit , m slope, mv dec-1 pme-1 100.1 201.1 dos 1.5 9.2 i 1.0x10-2 1.0x10-1-5.0x10-6 5.83x10-6 58.44 pme-2 100.6 199.8 dos 1.5 9.0 ii 1.0x10-2 1.0×10-1-1.0×10-5 9.54×10-6 52.72 pme-3 100.0 201.7 dos 1.6 9.2 iii 1.0x10-2 1.0×10-1-1.0×10-5 1.15×10-5 49.62 pme-4 100.3 199.6 dos 1.6 8.9 iv 1.0x10-2 1.0×10-1-1.0×10-5 1.07×10-5 67.47 pme-5 100.2 201.0 dos 1.5 9.2 i 1.0x10-3 1.0×10-1-1.0×10-5 1.01x10-5 53.20 pme-6 100.0 201.2 dos 1.5 9.2 i 1.0x10-1 1.0×10-1-5.0×10-5 8.12x10-5 44.96 pme-7 100.0 201.0 dos 1.5 7.0 i 1.0x10-2 1.0×10-1-1.0×10-5 3.16×10-5 44.50 pme-8 100.5 200.2 dos 1.6 5.2 i 1.0x10-2 1.0×10-1-1.0×10-5 1.07×10-5 49.09 pme-9 101.0 201.5 dos 1.5 3.2 i 1.0x10-2 1.0×10-1-5.0×10-6 6.45×10-6 27.52 pme-10 100.4 200.7dos 3.0 9.0 i 1.0x10-2 1.0×10-1-1.0×10-5 1.05×10-5 50.31 pme-11 100.8 200.3 dos 9.1 i 1.0x10-2 1.0×10-1-5.0×10-6 4.36×10-6 38.09 pme-12 100.0 200.0 dbp 1.5 9.2 i 1.0x10-2 1.0×10-1-1.0×10-5 1.90×10-5 29.25 pme-13 100.0 200.3 doa 1.5 9.2 i 1.0x10-2 1.0×10-1-5.0×10-6 8.91×10-5 71.86 pme-14 99.9 200.1 tbp 1.6 9.0 i 1.0x10-2 1.0×10-1-1.0×10-5 5.01×10-6 77.56 pme-15 100.6 201.4 dop 1.6 9.1 i 1.0x10-2 1.0×10-1-1.0×10-5 1.77×10-5 17.39 j. electrochem. sci. eng. 2(3) (2012) 143-154 silver ion recognition isoquinoline-1,3-dione 148 3.3. effect of internal solution the effect of the concentration of internal solution on the potential response of the pme-1, pme-5 and pme-6 for silver(i) ion based on receptor i was studied. the internal solution concentration of silver(i) ion was varied from 1.0×10-1 m to 1.0×10-3 m and the potential response of pme-1 was observed as shown in table 1. the concentration of internal solution has been found to affect the slope and the working range of the electrode. further, it was found that the best results, in terms of nernstian slope and the working concentration range , were obtained with the internal solution of concentration 1.0×10-2 m. thus, 1.0×10-2 m solution was found to be quite appropriate for the smooth functioning of the proposed polymeric electrode pme-1. 3.4. effect of ionophore content the influence of the ionophore content on the response behavior of silver(i) ion selective electrode incorporating receptor i as an ion carrier was studied by preparing membrane electrodes pme-1, pme-7, pme-8 and pme-9, using varying amount of receptor i (table 1). polymeric membrane electrode pme-1 containing 9.2 mg of receptor i exhibited nernstian slope of 58.44 mv dec-1 over the wide concentration range of 1.0×10-1 m to 5.0×10-6 m with lower detection limit of 5.83 x 10-6 m, whereas, polymeric membrane electrodes pme-7, pme-8 and pme-9, containing 7.0 mg, 5.2 mg and 3.2 mg of receptor i, exhibited sub-nernstian slopes over comparatively narrow concentration range with lower detection limit of 3.16×10-5 m, 1.07×10-5 m and 6.45×10-6 m, respectively (table 1). the best result was obtained in case of pme-1 containing 9.2 mg content of receptor i. 3.5. effect of additive content the presence of lipophilic negatively charged additive improve the potentiometric behavior of cation-selective electrodes by reducing ohmic resistance, increasing the membrane sensitivity in case of ionophore whose extraction capability is poor [37] and by catalyzing the exchange kinetics [38]. thus the presence of lipophilic negatively charged additive improve the response behavior and selectivity of ion selective electrodes [37,39,40]. in our study sodium tetra phenyl borate (natpb) was incorporated as an additional membrane component (table 1). polymeric membrane electrodes pme-1 and pme-10 were prepared by using 1.5 mg and 3.0 mg of natpb and pme-11 was prepared without any additive (natpb). response characteristics for the polymeric membrane electrodes pme-1, pme-10 and pme-11 incorporating varying amount of natpb are given in table 1. it is clear from table 1 that 1.5 mg natpb is the optimal content required for proper functioning of receptor i based silver(i) ion selective electrode. it may be due to the fact that 1.5 mg of natpb is the appropriate amount for the charge compensation of counter ion in the membrane and thus facilitates the process of the ion charge transduction. on the other hand, pme-11 which contains no lipophilic additive shows sub-nernstian slope. that may be due to the presence of anionic impurities within the polymer matrix [41]. 3.6. effect of nature of plasticizer the dielectric constant of the membrane phase, the mobility of ionophore molecules and the state of ligands are dependent on the nature of the plasticizer incorporated in the membrane [14,42]. the selectivity and the sensitivity of the selective electrode strongly depend on the membrane composition and the nature of plasticizer used [11,43]. the effect of plasticizer on the potential response of silver(i) ion ise incorporating isoquinoline-1,3-dione receptor i as an ion carrier was studied by preparing different membrane sensors containing plasticizer such as dos, a. kamal et al. j. electrochem. sci. eng. 2(3) (2012) 143-154 doi: 10.5599/jese.2012.0017 149 dbp, doa, tbp, dop and the results are summarized in table 1. the electrodes pme-13 and pme-14, prepared by incorporating doa and tbp, exhibited super-nernstian slopes (71.86, 77.56 mv dec-1), while dbp and dop incorporating electrodes pme-12 and pme-15 exhibited sub-nernstian response (29.25, 17.39 mv dec-1). the response of membrane sensor pme-1 incorporating dos as a plasticizer was found to be better in terms of nernstian slope of 58.44 mv dec-1 and wide linear concentration range of 1.0×10-1 m to 5.0×10-6 m. 3.7. effect of ph the potential response of an ion selective electrode is affected by ph of the analyte ion solution. the ph dependence of the polymeric membrane electrodes pmes 1-4 containing receptors i-iv as ionophores were examined at 1.0×10-2 m concentration of silver(i) ions as shown in fig. 4. the ph values were adjusted using conc. nitric acid and hexamine and the potential was measured after each addition. the electrode potential for all four electrodes remained constant over a ph range 1.0 to 5.5, which was the working ph range of the proposed silver(i) ion electrode. at lower ph<1 deviation in potential may be due to the hydrogen error of the indicator electrode, and the sharp change at higher ph values may be due to the formation of some hydroxyl complexes of the charge transport process by the membrane, thereby causing interference. figure 4. effect of ph on potential response of silver(i) ion selective polymeric membrane electrodes pme 1-4 based on isoquinoline-1,3-dione receptors i-iv as ionophores 3.8. response time and life time of proposed electrode the response time of an ion selective electrode is the average time required for attaining the equilibrium value of the potential. in this study the practical response time was recorded from lower to higher successive concentrations of silver(i) ions solutions (10-5 m to 10-1 m). a potential versus time trace for electrode pme-1 is shown in fig. 5. it is evident that the electrode reached the equilibrium response in a very short time of less than 12 seconds and no change was observed up to 5 minutes. the life time of the present electrode was more than three months and electrode -300 -250 -200 -150 -100 -50 0 50 0 1 2 3 4 5 6 7 8 pme-1 pme-2 pme-3 pme-4 e / m v ph j. electrochem. sci. eng. 2(3) (2012) 143-154 silver ion recognition isoquinoline-1,3-dione 150 potentials were reproducible. after more than three months the selectivity and sensitivity of polymeric membrane started degrading. it may be due to the leaching out of ionophores from the membrane. the response time of the proposed sensor pme-1 is compared with the best silver(i) ion-selective electrodes reported earlier in literature (table 2). it is clear from table 2 that proposed electrode is better as compared to other reported electrodes in regard to nernstian slope, lower detection limit, ph range and working concentration range. table 2. a comparative study of the proposed silver(i) ion selective electrodes with previously reported ion selective electrodes for silver(i) ion ionophore linear range, m detection limit, m ph range slope, mv dec-1 response time, s ref. no. schiff-base (n2e,n2’e)-n2,n2’bis(thiophen-2-ylmethylene)-1,1’-binaphthyl-2,2’-diamine. 1.0×10-71.0×10-2 -a 2.0-6.0 57.40 9 [44] calix[4]arene. 1.0×10-51.0×10-2 a >2 56.00 a [45] schiff-base–p-tert butylcalix[4]arene. 1.0×10-51.0×10-1 6.3×10-6 1.0-6.0 58.90 30 [32] calix[4]arene. 1.0×10-1 1.0×10-4 1.0×10-4 a 50.01 3 [46] schiff-basen,n,-bis(pyridine2-ylmethylene)benzene-1,2diamine. 1.0×10-71.0×10-1 a 3.0-8.0 58.60 10 [47] isoquinoline-1, 3-diones 5.0×10-61.0 x 10-1 5.83×10-6 1.0-5.5 58.44 <12 this work a not available in the literature figure 5. response time of silver(i) ion selective electrode pme-1, based on isoquinoline-1,3-dione receptor i for step changes in concentration of silver(i) ion. (a) 1.0×10-5 m (b) 1.0×10-4 m (c) 1.0×10-3 m (d) 1.0×10-2 m (e) 1.0×10-1m. 3.9. electrode selectivity the most important characteristic of any ion-selective sensor is its response to the primary ions (a) in the presence of the secondary ions (b) in the solution, which is expressed in terms of the -250 -200 -150 -100 -50 0 50 100 -100 0 100 200 300 400 500 600 e / m v t / s [e] [d] [c] [b] [a] a. kamal et al. j. electrochem. sci. eng. 2(3) (2012) 143-154 doi: 10.5599/jese.2012.0017 151 potentiometric selectivity coefficients ( pota,bk ). in the present work the selectivity coefficients ( pota,blogk ) were determined using fixed interference method (fim) based on the semi empirical nikolsky-eisenman equation [48]. in fim, the selectivity coefficient was evaluated from potential measurements in solutions containing a fixed concentration of interfering cation (1.0×10-2 m) and varying amount of silver(i) ions. the potential values obtained are plotted versus the activity of the primary silver(i) ions. the potentiometric selectivity coefficients for proposed silver(i) ion selective electrodes pmes 1-4 were evaluated by fim as shown in figure 6. comparing selectivity data of polymeric membranes pmes 1-4, it has been found that the pot ag ,b logk + values for most of the secondary ions are better in case corresponding to pme-1. as seen from fig. 6 the silver(i) ion electrode pme-1 is significantly more selective to silver(i) ions over all the interfering ions. although, the selectivity coefficient value for mercury(ii) ion is higher than the values for other secondary metal ions ( 2 pot ag ,hg logk + + = -2.81) still, it has been found that a mercury(ii) ion does not interfere in the normal functioning of the proposed silver(i) ion selective electrode pme-1. moreover, it is found that most of the studied cations such as k+, zn2+, fe3+, cu2+, co2+, pb2+, nh4 +, al3+, mg2+, na+, li+ and cd2+ would not cause any interference in determination of silver ions, even if present in large amount. figure 6. selectivity of silver(i) ion sensors based on isoquinoline-1,3-dione receptors i-iv -5 -4 -3 -2 -1 0 1 k+ zn2+ fe3+ cu2+ co2+ pb2+ nh 4 + al3+ mg2+ ag+ hg2+ cd2+ li+ na+ pme 1 pme 2 pme 3 pme 4 j. electrochem. sci. eng. 2(3) (2012) 143-154 silver ion recognition isoquinoline-1,3-dione 152 4. analytical application the optimized proposed polymeric membrane sensor pme-1 was found to work well under laboratory conditions. practical utility of the proposed sensors was tested by using them as indicator electrode for potentiometric titration of silver solution (1.0×10-2 m) with sodium chloride solution (1.0×10-2 m) as shown in fig.7. the plot is of sigmoidal shape and the inflexion point of the plot corresponds to 1:1 stoichiometry of the agcl precipitation. therefore, the end point and the amount of silver(i) ions in the solution can be accurately determined by extrapolation of the three linear portions of the titration plot. also, pme-1 was employed for the quantitative determination of silver(i) ion content in synthetic water (1 and 2), silver foil (3) and dental amalgam (4) samples. in case of silver foil, the sample was prepared by desolving it in diluted nitric acid (3 ml of double distilled water and 5 ml of conc. hno3), and then diluted to 100 ml by adding double distilled water. silver(i) ion content in these samples was also determined volumetrically. the results summarized in table 3 make it clear that both of these methods of analysis give compatible results providing the practical significance of proposed electrode. figure 7. potentiometric titration of 1.0 x 10-2 m silver(i) solution with 1.0 x 10-2 m nacl using pme-1, based on isoquinoline-1,3-dione receptor i table 3. estimation of silver(i) ion content in synthetic water, silver foil and dental amalgam samples sample silver content, m volumetric method estimated using pme-i compatibility, % synthetic water (1) 3.72×10-3 m 3.86×10-3 m 103.76 synthetic water (2) 6.69×10-4 m 6.58×10-4 m 98.36 silver foil (3) 9.13×10-3 m 9.23×10-3 m 98.91 dental amalgam(4) 1.67×10-2 m 1.61×10-2 m 96.41 -30 0 -25 0 -20 0 -15 0 -10 0 -5 0 0 5 0 0 10 20 30 40 50 60 e / m v v o lu m e of n a c l so lu tion a d d ed , m l a. kamal et al. j. electrochem. sci. eng. 2(3) (2012) 143-154 doi: 10.5599/jese.2012.0017 153 5. conclusion isoquinoline-1,3-dione based receptor i has been found to be novel carrier of silver(i) ions in the pvc based polymeric membrane pme-1. the main advantage of the electrode is the simplicity of construction. it has good response properties and performance, with nernstian response (58.44 mv/decade) in the concentration range of the 1.0×10-1 m to 5.0×10-6 m and detection limit of 5.83×10-6 m. the sensors have working ph range of 1.0 to 5.5 with fast response time of less than 12 seconds. the selectivity coefficient indicated that pme-1 sensor is significantly selective to silver(i) ion over many different secondary ions. even mercury(ii) ions were not found 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[48] d. g. hall, j. phys. chem. 100 (1996) 7230-7236. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 /emitdscwarnings false /endpage 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice electrochemical mediated oxidation of phenol using ti/iro2 and ti/pt-sno2-sb2o5 electrodes doi: 10.5599/jese.2014.0069 259 j. electrochem. sci. eng. 4(4) (2014) 259-270; doi: 10.5599/jese.2014.0069 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical mediated oxidation of phenol using ti/iro2 and ti/pt-sno2-sb2o5 electrodes jéssica pires de paiva barreto, elisama vieira dos santos, mariana medeiros oliveira, djalma ribeiro da silva, joão fernandes de souza* and carlos a. martínez-huitle federal university of rio grande do norte, ccet institute of chemistry, campus universitario, lagoa nova cep 59.072-970, rn, brazil *federal university of rio grande do norte, ccet – department of chemical engineering, campus universitario, lagoa nova cep 59.072-970, rn, brazil corresponding author: e-mail: carlosmh@quimica.ufrn.br; tel.: +55-84-9181-7147; received: october 13, 2014; published: december 6, 2014 abstract the indirect electrochemical oxidation of phenol was studied at ti/iro2 and ti/pt-sno2-sb2o5 electrodes by bulk electrolysis experiments under galvanostatic control. the obtained results clearly shown that the electrode material was an important parameter for the optimization of such processes determining their mechanism and oxidation products. different current efficiencies were obtained at ti/iro2 and ti/pt-sno2-sb2o5, depending on the applied current density in the range from 10, 20 and 30 ma cm −2 . the effect of the amount of dissolved nacl was studied also. it was observed that the electrochemical processes (direct/indirect) favor specific oxidation pathways depending on electrocatalytic material. phenol degradation generates several intermediates eventually leading to complete mineralization, as indicated by the results obtained with the high-performance liquid chromatography (hplc) technique. keywords phenol; anode material; chlorine active species; indirect electrochemical oxidation. introduction phenol is an aromatic compound and it is a hygroscopic crystalline solid at ambient temperature and pressure. when pure, solid phenol is white but is mostly colored due to the presence of impurities. phenol is very soluble in ethyl alcohol, in ether and in several polar solvents, as well as in hydrocarbons such as benzene. in water it has a limited solubility and behaves as a weak acid [1]. more phenol than is usually found in the environment has been found in surface waters and http://www.jese-online.org/ mailto:carlosmh@quimica.ufrn.br j. electrochem. sci. eng. 4(4) (2014) 259-270 electrochemical oxidation of phenol 260 260 surrounding air that were contaminated when phenol was released from industries and commercial products containing phenol. it has been found in materials released from landfills and hazardous waste sites, and it has been found in the groundwater near these sites [2,3]. the interest in developing new and more efficient methods for destruction of hazardous waste such as phenol [4] and the conversion of mixed waste to low-level-toxicity waste has significantly increased. these wastewaters are difficult to be effectively treated by conventional biological methods, for this reason, advanced oxidation processes (aops) have been developed to treat these bio-refractory organic wastewaters [1-11]. in this frame, the electrochemical oxidation of the model substrates has been investigated using several anodic materials, generally metal oxides like iro2, pbo2, sno2 and sno2–sb2o5 [5,6]. dimensionally stable anodes (dsas) belong to a particular category of electrodes, constituted of a ti-support coated by noble metal oxides, which confer the enhanced catalytic activity towards chlorine evolution and oxygen evolution reaction (o.e.r). to date, dsas have been largely employed in the chloro-alkali industry, due to their excellent catalytic property and service life [12,13]; however, significant performances have been obtained when these have been used for electrochemical treatment of industrial effluents [14]. in the case of active chlorine, the interest in this oxidant is based on the ubiquitous presence of chloride ions in a certain number of effluents and natural waters, making possible the involvement of active chlorine during electrochemical treatment using principally dsa electrodes; and the chemistry and electrochemistry of higher oxidation states for chlorine close to neutral ph [15,16]. the electrochemical treatment of phenol has been already studied by other authors [2-4,9-11, 17,18]. using nacl solutions to degradate phenol by direct anodic reaction and/or through the mediation of active chlorine [19,20]; the possible role of the clion during the process was not taken into account by the authors [19,20]. therefore, the electrochemical degradation of phenol was studied at ti/iro2 and ti/pt-sno2-sb2o5 electrodes by varying operating conditions such as current density and cl concentration. the intermediate species formed at each one of the anodes were compared. experimental chemicals the compounds used such as phenol, catechol, hydroquinone, oxalic acidic were of analytical grade. the chromatographic elution solvents were of hplc grade (merck). the stock solutions (1000 mg l -1 ) of phenol and its degradation products were prepared from certified reference standards (purity > 98 %), with the dissolution in methanol hplc grade. all electrolyte solutions were prepares using purified milli-q water system with a conductivity of 0.1 µs cm -1 . electrochemical measurements electrochemical analyses were performed with an autolab model pgstat320n (metrohm). quasi-steady polarization curves were carried out at a scan rate of 2.5 mv s −1 and with a 0.45 mv step potential, in solutions of nacl at different concentrations. experiments were carried out in a conventional three-electrode system, and measurements were performed in a range from 0.0 v to 3.5 v. ti/iro2 and ti-pt-sno2-sb2o5, with an exposed geometric area of ca. 1.0 cm 2 , were used as the working electrode, while a platinum wire and an ag/agcl (kcl 3 mol l −1 ) electrode were employed as the auxiliary and reference electrodes, respectively. j. p. de paiva barreto et al. j. electrochem. sci. eng. 4(4) (2014) 259-270 doi: 10.5599/jese.2014.0069 261 electrolytic systems bulk oxidations were performed in an electrolytic flow cell with a single-compartment with parallel plate electrodes [21]. circular electrodes (ti/iro2 and ti-pt-sno 2 -sb2o5 electrode) were used as anodes exposing to the effluent a nominal surface area of 63.5 cm 2 . in all cases, a ti disc was used as the cathode. the inter-electrode gap was 10 mm. for the electrochemical flow cell, inlet and outlet were provided for effluent circulation through the reactor; the solution of phenol was stored in a thermoregulated glass tank (1 l) and circulated through the cell using a peristaltic pump, at a flow rate of 151 dm 3 h −1 , which allowed a mass transfer coefficient (determined using the ferri/ferro-cyanide redox couple) of 2.0×10 −5 m s −1 [22]. the oxidation experiments of phenol were performed under galvanostatic conditions (using a power supply minipa-3305m) at 25 °c for studying the role of applied current density (j = 10, 20 and 30 ma cm -2 ) adding 20 and 30 mm of cl for studying the effect of cl-mediated approach. analytical methods the oxidation intermediates, produced during the electrolysis experiments at both anodes, were analyzed by hplc. chromatographic separations were performed on an analytical column supelcosil-c18 (5 µm, 25 × 46 mm) at room temperature and with an uv detector at λ = 225 nm. generated carboxylic acids were detected and quantified using an ultimate tmaqc18 5 m (25 × 46 mm) column at room temperature and photodiode array detector set at λ = 210 nm. for these analyses, a 70:30 (v/v) methanol/water mixture at 0.5 ml min -1 for the elution was used. the flow rate of the mobile phase was 1.5 ml min -1 . spectrophotometric measurements (uv–vis) were also performed using a shimadzu model uv-160 spectrophotometer. experimentally, degradation of phenol was monitored from the abatement of their chemical oxygen demand (cod). values were obtained, using a hanna hi 83099 spectrophotometer after digestion of samples in a hanna thermo-reactor, in order to estimate the total current efficiency (tce), using the following relationship [23]:  0 fcod cod tce, % 100 8 fv i t        (1) where cod0 and codf are chemical oxygen demands at times t=0 (initial) and f (final time) in g o2 dm −3 , respectively; i the current (a), f the faraday constant (96,487 c mol −1 ), v the electrolyte volume (dm 3 ), 8 is the oxygen equivalent mass (g eq. −1 ) and δt is the total time of electrolysis, allowing for a global determination of the overall efficiency of the process. additionally, the limiting current can be estimated from the value of cod using the equation 2 for electrochemical oxidation of phenol [24,25]. lim m( ) 4 cod( )i t fak t (2) where ilim(t) is the limiting current (a) at a given time t, 4 the number of exchanged electrons, a the electrode area (m 2 ), f the faraday’s constant, km the average mass transport coefficient in the electrochemical reactor (m s −1 ) and cod(t) the cod, mol o 2 m −3 at a given time t. the energy consumption (ec) per volume of phenol oxidized was estimated and expressed in kwh dm -3 . the average cell voltage, during the electrolysis, is taken for calculating the energy ec, as follows: cenergy consumption 3600 e it v   (3) j. electrochem. sci. eng. 4(4) (2014) 259-270 electrochemical oxidation of phenol 262 262 where t is the time of electrolysis (s); δec / v and i / a are the average cell voltage and the electrolysis current, respectively; and v is the sample volume (dm 3 ). results and discussion polarization curves in the presence of halide based on the considerations about the possible effect of halide on the oxygen evolution reaction (o.e.r.) polarization curves were recorded in the absence and in the presence of different concentrations of cl − . the results obtained in the presence of chloride ions (10 to 80 mg l -1 ), at both anode materials, are shown in fig. 1. in the case of the ti/iro2 anode, in absence of chloride in solution, the oxygen evolution reaction is attained around 1.7 v. after that, the whole polarization curve is modestly shifted to less positive potentials (up to 1.5 v), when the concentration of nacl is increased. this behavior is due to the increase of the importance of the cl2/h2o system [23,26], favoring the production of active chlorine species than the oxygen evolution reaction. under these conditions, a fast incineration of a number of organic substrates can be favored during mediated electrochemical process due to the production of oxychlororadicals, often assumed as intermediates in the chlorine evolution reaction. figure 1. current-potential curves in the presence of different amounts of nacl on ti/iro2 and ti-pt-sno2-sb2o5 anodes at scan rate of 2.5 mv s −1 . black curve: water with lower conductivity. similar experiments were carried out using ti-pt-sno2-sb2o5 anode in the presence of cl − , as shown in fig. 1, employing the same range of cl − concentrations. in that case, at very small nacl concentration (10 mg l −1 ), a relevant shift to less positive potentials was observed of i/e curves. above 20 mg l −1 , the anode potential becomes increasingly buffered by the halide electroactivity. this behavior can be attributed to an interaction between anode surface and cl − to form active chlorine species (desirable and undesirable, such as cl  , cl2, clo2 − and clo3 − , clo4 − , respectively) with this active material [16]. according to the electrode nominal composition, it suggests a mixbehavior as active or non-active anode due to the presence of sno2 in its surface. in fact, the oxygen evolution reaction is achieved at more positive potentials than that observed at ti/iro2 anode. however, the performances of ti-pt-sno2-sb2o5 anode are not comparable with the performance of an ideal non-active anode like diamond electrode [26]. it indicates that, the concentration of halide in solution increases the importance of cl2/oxy-chloro radicals system depending on the electrocatalytic material and this behavior plays an important role in relation with the oxygen evolution reaction, influencing on the efficiency of electrochemical approach adopted [27-29]. j. p. de paiva barreto et al. j. electrochem. sci. eng. 4(4) (2014) 259-270 doi: 10.5599/jese.2014.0069 263 bulk electrolysis the experiments were performed at 25 °c varying cl concentration ions (20 and 30 mm) in solution and applied different current density (10, 20 and 30 ma cm -2 ) in order to evaluate the elimination of organic load. fig. 2 shows the performances of each one of the electrocatalytic material used as a function of applied current density and electrolyte concentration during the cod removal. it was observed that efficiency of cod removal was dependent on the applied current density and cl concentration. in fact, when 10, 20 and 30 ma cm -2 were applied by using ti/iro2, the initial cod (338 mg l -1 ) was reduced 262 mg l -1 ; 182 mg l -1 and 140 mg l -1 with 20 mm of cl in solution, while at 30 mm of cl , cod decays to 193 mg l -1 , 123 mg l -1 and 121 mg l -1 for 10, 20 and 30 ma cm -2 , respectively, after 120 min of electrolysis. for ti-pt-sno2-sb2o5 anode, under similar conditions, cod concentration was reduced from 338 mg l -1 to 223, 45 and 96 mg l -1 at 20 mm of cl and 158; 18 and 98 mg l -1 when 30 mm of cl was added by applying 10, 20 and 30 ma cm -2 , respectively. based on the cod removal reported in fig. 2, the increase in the applied current density contributes to the degradation of phenol and the intermediates generated during the electrolysis, thanks to the action of active chlorine species produced on the electrodes surface. however, the results reveal that at ti-pt-sno2-sb2o5 there is greater reduction in cod than that achieved at ti/iro2 (fig. 2), principally at 20 ma cm -2 . conversely, when 30 ma cm -2 was applied, the elimination of cod decreased due to the promotion of cl2 rather than the production of active chlorine species. figure 2. effect of nacl for cod removal at different concentrations during mediated electrochemical oxidation of phenol by using (a) ti/iro2 and (b) ti-pt-sno2-sb2o5 anodes. operational conditions: [phenol]0= 100 mg l −1 , initial cod = 338 mg l -1 , t = 25°c. this behavior suggests that phenol oxidation depends on the nature of the anode material due to the efficient production of active chlorine species on anode surface [23, 26-29]. ti/iro2 and ti-pt-sno2-sb2o5 materials are classified as active anodes [5,6] because these electrocatalytical materials are characterized by strong electrode-hydroxyl radical interaction, resulting in a low chemical reactivity for organics oxidation. this problem can be avoided when cl-mediated approach is used [14]. generally, under favorable ph conditions and nacl in solution, electrochemical oxidation via •oh radicals is not the only oxidation mechanism that occurs on the dsa anodes [14]. in this case, chlorohydroxyl radicals are also generated on anode surface and consequently oxidizing organic matter (equations 4 and 5) [30,31]: j. electrochem. sci. eng. 4(4) (2014) 259-270 electrochemical oxidation of phenol 264 264 h2o + m + cl → m[•cloh] + h + + 2е(4) r + m[•cloh] → m + ro + h + + cl (5) reactions between water and radicals near to anode surface can yield molecular oxygen, free chlorine, and hydrogen peroxide (6, 7 and 8) [32]: h2o + m[•oh] → m + o2 +3h + + 3е (6) h2o + m[•cloh] + cl → m + o2 +cl2 + 3 h + + 4е (7) h2o + m[•oh] → m + h2o2 + h + + е (8) furthermore, hypochlorite can be formed as follows (9 and 10) [31]: cl2(diss) + h2o → hocl + h + + cl (acidic medium) (9) cl2(diss) + 2oh → clo + cl + h2o (alkaline medium) (10) therefore, indirect oxidation results in reduction of organic pollutants such as phenol thanks to the participation of active chlorine species electrochemically formed [15,23,31]. oxidants are quite stable and migrate in the solution bulk, and then, these indirectly oxidize the effluent, favored by hydrodynamic configuration of electrochemical cell. the efficiency of indirect oxidation depends on the diffusion rate of oxidants in the solution, concentration of oxidants, and ph of solution [15]. for the electrochemical flow cell used in this study, the mass transfer coefficient was 2.0×10 −5 m s −1 , and the limiting current (for both anodes) results in an average value of 0.88 a, according to eq. 2. this current is lower than all the currents applied in this work (1.27–1.90 a), suggesting that the oxidation under these experimental conditions could occur under mass transport control since 120 min of electrochemical treatment. these assumptions are in agreement with the studies published by cañizares and co-workers [33]. uv spectroscopic characteristics of the electrochemical oxidation of phenol the oxidation of phenol at ti/iro2 and ti-pt-sno2-sb2o5 electrodes was also monitored by spectrophotometric measurements, which allow a straightforward way to follow the elimination of phenol. thus, uv spectra for phenol degradation at ti/iro2 and ti-pt-sno2-sb2o5 electrodes, at 20 mm and 30 mm of cl in solution at 25°c by applying 20 ma cm −2 , are shown in figure 3. an inspection of these uv spectra allowed confirming that phenol was more rapidly removed at ti/iro2 (fig. 3a and 3b, at 20 mm and 30 mm of cl in solution by applying 20 ma cm −2 ) than at tipt-sno2-sb2o5 electrodes under the same experimental conditions (fig. 3c and 3d). moreover, the bands in fig 3a and 3b are completely different after 5 minutes of electrolysis due to formation of reaction intermediates, confirming the fast degradation of phenol. these major changes in uvvis bands are not observed in figs. 3c and 3d. this behavior depends on the effective electrochemical production of active chlorine species at both anodes. perhaps, at iro2, metal cations in the oxide lattice may reach higher oxidation states under anodic polarization stabilizing •oh radicals and cl ions on its surface [6,32,34], which favors the o2 and cl2 evolution at the expense of the electrochemical incineration reaction. this feature avoids the formation of enough active chlorine species that promote an efficient degradation of phenol, generating different aromatic intermediaates. however, it is not possible to reliably ensure that phenol is degraded because the products and/or intermediates formed during the process may have a higher molar absorptivity () and absorb in the same wavelength region of phenol. j. p. de paiva barreto et al. j. electrochem. sci. eng. 4(4) (2014) 259-270 doi: 10.5599/jese.2014.0069 265 figure 3. spectrophotometric measurements, as a function of time, during cl-mediated electrochemical treatment of phenol (100 mg l -1 ) by applying 20 ma cm -2 at 25°c. ti/iro2: (a) 20 mm of cl and (b) 30 mm of cl . ti-pt-sno2-sb2o5: (c) 20 mm of cl and (d) 30 mm of cl . conversely, at ti-pt-sno2-sb2o5 electrode (fig. 3c and 3d), the elimination of phenol is attained by the reactive chlorine species such as chlorine and hypochlorous acid or hypochlorite ion (cl2, hclo and ocl − ) that react rapidly with organics mainly by the reactions in solution [23,35,36]. in acidic conditions, free chlorine is the dominant oxidizing agent, while in slightly alkaline conditions hypochlorite, chloride ions and hydroxyl radicals are all generated in relevant concentrations [26,31]. in fact, ph typically varies between 5.5 and 6.2 throughout the course of the reaction for phenol. then, this ph behavior suggests the participation of active chlorine oxidants, confirming the increase on cod removal rate when ti-pt-sno2-sb2o5 electrode was used. distribution of by-products of phenol oxidation as stated above, phenol could be transformed into different intermediates or carbon dioxide during the process and such differences would not be apparent from the timeor charge-course of this parameter. for this reason, identification of some intermediates was performed by hplc. the change in concentration of phenol (initial concentration of 100 mg l -1 ) and intermediates in the course of indirect electrochemical treatment, by applying 20 ma cm -2 at 25°c, is shown in fig. 4ad. as can be observed, the degradation of phenol (main by-product formed) is influenced by the amount of cl as well as the electrocatalytic material used. in fact, 84 % and 88 % of phenol was removed by using ti/iro2 anode after 120 min of electrolysis when 20 and 30 mm of cl in solution were used, respectively. conversely, the decrease of phenol on ti-pt-sno2-sb2o5 was about 93 % and 83 % by applying 20 ma cm -2 for 20 and 30 mm of cl , respectively. as shown in fig. 4, the intermediates concentration produced on ti/iro2 and ti-pt-sno2-sb2o5 electrodes are different. j. electrochem. sci. eng. 4(4) (2014) 259-270 electrochemical oxidation of phenol 266 266 hydroquinone, which is aromatic intermediate, was principally found in high concentration at the experiments performed with ti/iro2 anode. hplc results indicated the transformation of the phenol by electrolysis to hydroquinone, which was decomposed to other forms. this behavior was observed at both anodes where similar by-products were formed. however, at ti-pt-sno2-sb2o5, all the intermediates are quasi-completely mineralized to co2 and h2o due to the attack of active chlorine species. figure 4. intermediates formed during cl-mediated oxidation of phenol (100 mg l -1 ) by applying 20 ma cm -2 using ti/iro2 ((a) 20 mm of cl and (b) 30 mm of cl ) and ti-pt-sno2-sb2o5 ((c) 20 mm of cl and (d) 30 mm of cl ) anodes. hplc retention times: benzoquinone: 1.9 min, catechol: 3.05 min; phenol: 3.5 min; hydroquinone: 2.76 min and oxalic acid: 2.3 min. moreover, it is found that the amount of oxalic acid produced on ti-pt-sno2-sb2o5 is larger than that at ti/iro2, suggesting a better grade of mineralization. it is important to indicate that, unidentified intermediates were produced; however, these by-products were predominantly generated at ti/iro2 as showed by their chromatographic area (fig. 5). for ti/iro2, aromatic compounds concentration is not reduced with the action of active chlorine, finding difficult to break the aromatic ring and favoring the conversion of phenol to other aromatic compounds. based on the results obtained, the effect of nacl on ti-pt-sno2-sb2o5 is much more evident and efficient to produce strong oxidants that promote the mineralization of phenol and by-products on the bulk of solution. j. p. de paiva barreto et al. j. electrochem. sci. eng. 4(4) (2014) 259-270 doi: 10.5599/jese.2014.0069 267 figure 5. chromatographic areas of unidentified intermediates produced during indirect electrochemical oxidation of phenol at ti/iro2 (20 mm of cl and 30 mm of cl ) and ti-pt-sno2-sb2o5 (20 mm of cl and 30 mm of cl ) anodes. kinetic and activation energy analysis to study the kinetics of the overall reaction involved in the disappearance of phenol and intermediates by indirect electrochemical oxidation, the decay of phenol concentration under different nacl concentration of electrolyte was considered. results given in fig. 6a and 6b were further analyzed using kinetic equations related to different reaction orders. good linear plots were fitted to a pseudo-first-order reaction (ln(c0/ct) vs. time) for ti/iro2 and ti-pt-sno2-sb2o5 electrodes. for ti/iro2, apparent constant rates (kapp) of 0.020 min -1 (r 2 = 0.97) for 20 mm of cl and 0.015 min -1 (r 2 = 0.95) for 30 mm of cl were estimated; while for ti-pt-sno2-sb2o5, 0.027 min -1 (r 2 = 0.97) for 20 mm of cl and 0.017 min -1 (r 2 = 0.98) for 30 mm of cl were achieved. these figures confirm that the indirect electrochemical oxidation is faster at ti-pt-sno2-sb2o5 anode, respect to the behavior attained at ti/iro2, where the process is disfavored when an increase in cl ion is performed. it can be due to the preferential production of cl2 gas, as discussed on polarization curves results [26]. in this form, lower concentrations of active chlorine are produced, favoring the electrochemical conversion of phenol to aromatic compounds. considering the kinetic model pseudo-second order ((1/c0)-(1/ct) vs. time) and concentration cl ions in the medium, these show suitable linearity when compared to pseudo-first order model. figure 6. kinetic analysis for the pseudo-first-order in(c0/ct) and pseudo-second-order (1/c0)-(1/ct)) reaction for phenol decay during its indirect electrochemical oxidation by active chlorine species. j. electrochem. sci. eng. 4(4) (2014) 259-270 electrochemical oxidation of phenol 268 268 the coefficients of pseudo-second order were about 7.0110 -4 min -1 (r 2 = 0.99) and 2.510 -4 min -1 (r 2 = 0.99) for concentration 20 and 30 mm of cl , respectively, using ti-pt-sno2-sb2o5. conversely, values of 4.510 -4 min -1 (r 2 = 0.97) and 2.410 -3 min -1 (r 2 = 0.98) at concentration 20 and 30 mm of cl where estimated at ti/iro2 electrode. then, the kinetic data obtained by pseudo-second order model indicate that the indirect oxidation is controlled by the rate at which organic molecules are carried from the bulk to the electrode surface, but when the concentration of final intermediates increase, their rate is limited by diffusion control. however, the principal step for the electrochemical approach is the production of active chlorine species, as indicated by pseudo-second order model. finally, it is very important to estimate the treatment applicability, and thus, table 1 reports the current efficiency and energy consumption (kwh dm −3 ) at 10, 20 and 30 ma cm −2 , after 120 min of electrochemical treatment. as can be observed, ti-pt-sno2-sb2o5 consumed relatively less electrical energy than ti/iro2 anode, but different current efficiencies were achieved after 120 min of electrolysis at both anode materials. table 1. energy requirements and total current efficiency for elimination of phenol at different applied current densities by cl-mediated electrochemical oxidation. current density, ma cm -2 current total efficiency, % energy consumption, kwh dm -3 ti/iro2 ti-pt-sno2-sbo5 ti/iro2 ti-pt-sno2-sb2o5 10 59 72 36 28 20 39 56 42 40 30 26 49 69 54 conclusions the cl-mediated electrochemical oxidation of phenol was investigated under galvanostatic conditions at ti/iro2 and ti-pt-sno2-sb2o5 electrodes, as a function of applied current density and amount of nacl dissolved. a partial elimination of the organic pollutant was achieved at ti/iro2, while a quasi-complete electrochemical elimination takes place at ti-pt-sno2-sb2o5 anode. the influence of the anode material on the elimination of phenol seems to be very important due to an efficient production of active chlorine species during electrolysis. in fact, ti-pt-sno2-sb2o5 showed good electrocatalytic activity to promote the electrochemical generation of active chlorine, as indicated by potentiodynamic measurements, and as a consequence of this characteristic, significant removal efficiency of phenol and its by-products was attained. the effect of chloride on the electrooxidation of organics with ti/iro2 and ti-pt-sno2-sb2o5 depends mainly on the reaction between electrogenerated •oh and cl − ions or the conversion of chloride ion to chlorine which is further hydrolyzed to other active species [26,37]. at the same time, the production of cl to active chlorine is directly related to different experimental conditions, but it is principally dependent on the concentration of free •oh radicals, the increase of the importance of the cl2/h2o system and the interaction of cl and •oh radicals with the anode surface. because, increasing the concentration of cl ions in solution, elimination of cl from solution by cl2 is favored [23,37,38]. acknowledgements: e. v. dos s. gratefully acknowledges the programa de recursos humanos – petrobras (pbfrh-22) for her phd fellowship and support provided by the núcleo de processamento primário e reuso de água produzida e resíduos (nupprar-ufrn) for analyzing the j. p. de paiva barreto et al. j. electrochem. sci. eng. 4(4) (2014) 259-270 doi: 10.5599/jese.2014.0069 269 samples electrochemically treated. the authors thank the financial support provided by cnpq and petrobras. they also thank to the dott. christian urgeghe (industrie de nora s.p.a. milan, italy) for providing ti/iro2 and ti-pt-sno2-sb2o5 electrodes. references [1] j. e. amore, e. hautala, journal of applied toxicology 3 (1983) 272–290. 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[38] p. cañizares, c. sáez, a. sánchez-carretero, m.a. rodrigo, journal of applied electrochemistry 39 (2009) 2143. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {one-step synthesis of mesoporous carbon/ iron sulfide nanocomposite for supercapacitors} doi:10.5599/jese.572 55 j. electrochem. sci. eng. 9(1) (2019) 55-62; doi: http://dx.doi.org/10.5599/jese.572 open access: issn 1847-9286 www.jese-online.org original scientific paper in-situ synthesis of mesoporous carbon/iron sulfide nanocomposite for supercapacitors xiaolong yu, bogang li college of materials science and engineering, sichuan university, chengdu 610065, china corresponding author: e-mail libogang@scu.edu.cn received: august 7, 2018; revised: october 20, 2018; accepted: october 21, 2018 abstract mesoporous c@fexsy composite as a negative electrode for supercapacitors was synthesized via a one-step hydrothermal treatment followed by an electrodeposition process and its electrochemical properties were studied. compared with bare carbon sphere, the electrochemical performance of c@fexsy composite was significantly improved, with a high specific capacitance (267.45 f/g), good rate performance (201.08 f/g at 2.5 a/g), and superior cycling stability (almost no capacitance degradation after 1000 cycles). the results show that the obtained c@fexsy composite is a promising negative electrode material for supercapacitors. keywords carbonaceous materials; porous structure; hydrothermal treatment; electrochemical properties; energy storage introduction supercapacitors have attracted tremendous attention in recent years owing to their high power density, short charging time, and good stability [1–3]. carbonaceous materials have been widely adopted as negative electrode materials for supercapacitors because of their low cost, good electrical conductivity, and long cycle lifetime. however, carbonaceous materials exhibit limited double-layer capacitance, which limits further development of high energy density supercapacitors [4,5]. one of the effective approaches to enhance the energy density of supercapacitors is to improve the specific capacitance of carbonaceous materials [6–8]. carbon spheres (cs) with their large surface area, high porosity, and fine pore size is a promising carbonaceous material for supercapacitors. nevertheless, the low capacitance of cs limits its practical applications [9,10]. to improve the capacitance, researchers have recently focused on developing new negative electrode materials by activation pretreatment, doping modification, and loading of metal-base materials [5,11–13]. among the metal-base materials, fe-based materials are http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:libogang@scu.edu.cn j. electrochem. sci. eng. 9(1) (2019) 55-62 mesoporous carbon/iron sulfide nanocomposite 56 promising candidates because of their suitable working window in negative potential, abundance, low cost, large theoretical capacitance, and nontoxicity [14–17]. fe-based materials are combined with carbonaceous materials to explore an effective method to prepare composites; this not only could improve the rate performance of fe-based materials, but also enhance the specific capacitance of carbonaceous materials. in this work, cs was grown in situ on nickel foam (nf) using a one-step hydrothermal process, and iron sulfide was selected as fe-based material. a new approach was found to synthesize c@fexsy composite by using the cyclic voltammetry (cv) electrochemical deposition method to introduce iron sulfide onto cs. the electrical performance of c@fexsy was investigated, and the results indicated that the c@fexsy composite exhibited a significantly improved electrochemical performance than bare cs. the findings of this study would open up new possibilities for the design of carbon-based composites for high-performance supercapacitors. experimental materials commercial nf was used as the current collector. anhydrous dextrose, fecl3·6h2o, thiourea, and the other reagents in this experiment were used without further purification. deionized water was used to prepare solutions. synthesis of carbon spheres the cs was prepared by hydrothermal treatment. nf (2×4 cm2) was treated with hydrochloric acid (3 mol/l), acetone, and absolute ethanol in an ultrasound bath for 10 min, respectively, and then washed with deionized (di) water. anhydrous dextrose (10.8 g) was dispersed in 60 ml of di water and stirred at 25 c until a clear solution was obtained. the solution was transferred into a 100 ml teflon-lined autoclave. a piece of nf was placed vertically in the teflon-lined stainless-steel autoclave, soaked in the solution, at 180 c for 4 h. subsequently, cs precursors were formed on the nf. then, the product was washed with di water followed by vacuum drying at 60 c for 12 h and heat treatment at 800 c for 1 h in a n2 atmosphere to remove oxygen-containing groups on the surface of the cs. synthesis of c@fexsy composite c@fexsy was obtained through an electrochemical deposition method. the experiment was carried out using an electrochemical workstation with a three-electrode cell. cs grown on nf was used as the working electrode, and a platinum plate and a saturated calomel electrode (sce) were used as the counter electrode and the reference electrode, respectively. fecl3·6h2o (1, 1.5, 2, 2.5, and 3 mmol) and thiourea (5 mmol) dissolved in 100 ml di water was used as the deposition bath. the deposition process was carried out via cv at 5 mv/s for 6 cycles in the voltage range of -1.2 to 0.2 v. the as-prepared c@fexsy composites with 1, 1.5, 2, 2.5, and 3 mmol of fecl3·6h2o are henceforth referred to as c@fexsy-1, c@fexsy-1.5, c@fexsy-2, c@fexsy-2.5, and c@fexsy-3, respectively. the as-prepared c@fexsy-n (n = 1, 1.5, 2, 2.5, and 3) was then carefully rinsed with di water and dried in a vacuum oven at 60 c for 12 h. characterization and electrochemical measurements the morphology and the microstructure were observed by scanning electron microscopy (sem, hitachi, s-4800). the crystal structure was characterized using an x-ray diffraction (xrd, rigaku) system with cu k irradiation. the chemical composition of the sample was investigated by x-ray x. yu and b. li j. electrochem. sci. eng. 9(1) (2019) 55-62 doi:10.5599/jese.572 57 photoelectron spectroscopy (xps, escalab 250xi, thermo scientific). the structure of c@fexsy-2 was further explored by raman spectroscopy (renishaw invia rm200). the brunauer-emmett-teller (bet; gemini vii) specific surface areas of the samples were determined from n2 adsorption data in the relative pressure ranging from 0.1 to 1.0. electrochemical characterization was carried out using a chi 660d electrochemical workstation (chenhua, shanghai) at 25 c. the electrochemical characterization includes cv, galvanostatic charge-discharge (gcd), and electrochemical impedance spectroscopy (eis), using a three-electrode cell with 3.0 m koh as the electrolyte, materials grown on nf (1×1 cm2) as the working electrode, hg/hgo as the reference electrode, and pt foil as the counter electrode. cv was performed in the voltage window from -1 v to 0 v at scan rates of 5, 10, 20, 30, and 50 mv/s. gcd was carried out between -0.95-0 v at current densities of 0.25, 0.5, 1.0, 1.5, 2.0, and 2.5 a/g. eis was evaluated in the frequency range of 105 to 0.01 hz. the dc potential and the ac amplitude for eis measurements are -0.01 v and 5 mv, respectively. results and discussion morphology and structural analysis the sem images of cs and c@fexsy-2 are presented in figure 1. highly dense css are distributed on the nf substrate (figure 1a). these css exhibit smooth surfaces and they are connected with each other. this provides high volumetric specific surface area and good mass transport property, which are useful for supercapacitor application. the size distribution of the carbon particles is within the range of 1-2 μm. figure 1b shows that the c@fexsy-2 composite maintains the spherical structure, and many fexsy nanoparticles are dispersed on the surface of the carbon particles. this provides a large surface area for easy diffusion of the electrolyte toward the electrode surface and contributes to the enhancement of specific capacitance. figure 1. (a) sem image of the carbon spheres/nf. (b) sem image of c@fexsy-2/nf. the xrd patterns of cs and c@fexsy-2 (figure 2a) showed similar characteristics. a broad diffraction peak is observed at approximately 2 = 23 besides peaks of ni, corresponding to (002) plane of graphitic structure. the decreased peaks for c@fexsy-2 indicate the amorphous nature of fexsy. figure 2b shows the raman spectrum of the cs and c@fexsy-2 samples. both of them exhibit two characteristic peaks at 1345 and 1597 cm-1, corresponding to d peak from amorphous structure of carbon and g peak from graphitic structure of carbon, respectively[18,19]. the id/ig ratio of c@fexsy-2 is 0.97, a little higher than 0.90 of cs, which indicates the c@fexsy-2 has a small number of defects. the xps data of c@fexsy-2 are shown in figure 2c-d. the peaks at approximately 723 ev j. electrochem. sci. eng. 9(1) (2019) 55-62 mesoporous carbon/iron sulfide nanocomposite 58 (fe 2p1/2), 713 ev (fe 2p3/2) and 164 ev in the fe 2p spectra and the s 2p spectra are assigned to amorphous iron sulfide [20,21]. these results are in good agreement with the sem observations and further confirm the presence of c@fexsy-2 on the composite electrode. figure 2. (a) xrd patterns of cs/nf and c@fexsy-2/nf electrode. (b) raman spectra of cs and c@fexsy-2. (c)-(d) fe 2p and s 2p xps spectra of c@fexsy-2, respectively. the nitrogen adsorption-desorption isotherms and brunauer-joyner-halenda (bjh) pore diameter distribution of c@fexsy-2 are shown in figure 3. the sample exhibits a typical type iv isotherm and shows a hysteresis loop at medium relative pressure (figure 3a), which reveals the existence of relatively mesoporous pores. the hysteresis loop exhibits h2 type characteristics, indicating that the sample has a wide pore size distribution. the bet surface area of c@fexsy-2 is 117.74 m2/g. the bjh pore diameter distribution of c@fexsy-2 (figure 3b) shows that the pore diameter distribution is within the range of 5-10 nm, which further demonstrates that the sample has a mesoporous structure. from the above results, it is evident that the c@fexsy-2 electrode material has a high specific surface area, and the mesoporous structure could facilitate infiltration of the electrolyte and shorten electron and ion transport distances during energy storage. this would increase the surface utilization of the active material and improve the capacitance of the electrode material. x. yu and b. li j. electrochem. sci. eng. 9(1) (2019) 55-62 doi:10.5599/jese.572 59 figure 3. (a) bet of c@fexsy-2. (b) pore diameter distribution of c@fexsy-2. electrochemical performance analysis the electrochemical performance of the composites was estimated by cv and gcd measurements. figure 4a shows the gcd curves of cs and c@fexsy-n (n = 1, 1.5, 2, 2.5, and 3) at a current density of 0.25 a/g. the charge/discharge curves are nearly linear in shape, indicating ideal electrical double-layer capacitive behavior. the symmetric triangle characteristics reveal the excellent electrochemical reversibility of the samples with high coulombic efficiency. the c@fexsy-2 sample exhibited the longest charge-discharge time, and hence, the largest specific capacitance. the specific capacitance derived from galvanostatic tests can be determined using the following equation [22]:    i t c m v (1) where c is the specific capacitance, i is the discharge current (a), ∆t is the discharge time, m is the mass of active materials loaded on the working electrode, and ∆v is the potential window. figure 4. (a) gcd curves of cs and c@fexsy-n (n = 1, 1.5, 2, 2.5, and 3) at a current density of 0.25 a/g. (b) specific capacitances of cs and c@fexsy-n based on the gcd curves. (c) cv curves of cs and c@fexsy-2 at a scan rate of 20 mv/s. j. electrochem. sci. eng. 9(1) (2019) 55-62 mesoporous carbon/iron sulfide nanocomposite 60 as plotted in figure 4b, all the c@fexsy samples exhibit a higher specific capacitance than the cs electrode (158.01 f/g); c@fexsy-2 has the highest specific capacitance of 267.45 f/g at 0.25 a/g. it is considered that the loading mass of fexsy affected the capacitance, and c@fexsy-2 has the optimum loading mass. the reason might be that fexsy deposited on the surface of cs provides a porous structure, which further increases the specific surface area of the electrode material and thus enhances the capacitance. however, owing to the poor conductivity of fe-base materials, when the loading mass of fexsy further increases, the capacitance of the electrode material decreases. figure 4c shows the cv curves of cs and c@fexsy-2 at a scan rate of 20 mv/s. as expected, the c@fexsy-2 composite shows a much higher capacitance than cs. the electrochemical performance of c@fexsy-2 for supercapacitor applications was investigated in koh solution with a three-electrode system. as shown in figure 5a, cv test was carried out at various scan rates of 5-50 mv/s. the quasi-rectangular shape of the cv curves with an obvious hump shape indicates the effect of electrical double-layer capacitance. moreover, the rectangular shape of the curve even at a high scan rate of 50 mv/s suggests the outstanding capacitive behavior and high ionic conductivity of c@fexsy-2. the inconspicuous redox peaks may be induced by fexsy. these results demonstrate that c@fexsy-2 provides numerous accessible pores and paths for ion transfer. figure 5b-c show the discharge curves of c@fexsy-2 and cs at different current densities. all the curves exhibited excellent linear response, implying the ideal electric double-layer capacitive behavior of the samples. it is evident from figure 5d that c@fexsy-2 has a much higher capacitance than cs at all current densities. remarkably, the c@fexsy-2 electrode exhibited capacitance retention of 75 % when the current density was increased from 0.25 to 2.5 a/g, indicating its outstanding rate capability. the cycling stability of c@fexsy-2 was evaluated for 1000 cycles at a high charging-discharging current density of 2.5 a/g. as shown in figure 5e, the c@fexsy-2 electrode does not show capacitance degradation, and the charge/discharge curves in the last ten cycles are almost identical to those in the first ten cycles. after 1000 cycles, the capacitance retention of the c@fexsy-2 electrode is even 103 % of the initial capacitance, indicating its superior cyclic performance. the reason for the slight increase in the capacitance may be the gradual entry of the electrolyte into the inner micropores of the c@fexsy-2 electrode after several cycles, which increases the contact area of the electrode and electrolyte, thus enhancing the capacitance. eis was used to gain a deep understanding of the rate capability of the c@fexsy-2 electrode. figure 5f shows the nyquist plots of c@fexsy-2 and cs. it can be seen that the nyquist plot is mainly composed of two parts: a semicircle in the high frequency range and a vertical line in the low frequency region. the inset in figure 5f shows the magnified portion of the nyquist plot near the origin, providing more details on the electrode impedance behavior. the x-intercept of the high frequency semicircle provides the value of equivalent series resistance (rs) of the electrochemical system [23,24]. it is observed that the rs values of c@fexsy-2 and cs are 0.60 and 0.71 ω, respectively. the diameter of the high frequency semicircle corresponds to the charge transfer resistance (rct). the rct of c@fexsy-2 and cs was calculated to be 0.12 and 0.50 ω, respectively. as for c@fexsy-2, the low resistances are associated with fexsy nanoparticles on the carbon substrate, which provide a larger electrolyte/electrode interface, facilitating the electrode material wetting and the ion transfer. the nyquist plot of c@fexsy-2 exhibits an obvious warburg 45° line region, corresponding to the semi-infinite ion diffusion resistance, which reflects a lower ion diffusion rate within the porous c@fexsy-2 electrode [25,26]. in the low-frequency region, the straight line represents the good capacitive behavior of the electrode and the more parallel to the virtual axis, x. yu and b. li j. electrochem. sci. eng. 9(1) (2019) 55-62 doi:10.5599/jese.572 61 the closer to the double-layer capacitance [27]. the nyquist plot in the low frequency range of c@fexsy-2 shows a less vertical line, which is an indication that c@fexsy-2 has diffusion-controlled faradaic processes due to the higher contribution of warburg impedance [28-30]. figure 5. (a) cv curves of c@fexsy-2 at different scan rates. (b)-(c) gcd curves of c@fexsy-2 and cs at different current densities, respectively. (d) specific capacitance of c@fexsy-2 and cs at different discharge current densities. (e) cycling performance of the c@fexsy-2 electrode at a current density of 2.5 a/g. (f) electrochemical impedance spectra of c@fexsy-2 and cs. conclusions in this study, a mesoporous negative electrode c@fexsy composite was successfully fabricated on nf using a hydrothermal method followed by an electrodeposition treatment. the electrochemical performance of the c@fexsy-2 electrode was significantly higher than that of bare j. electrochem. sci. eng. 9(1) (2019) 55-62 mesoporous carbon/iron sulfide nanocomposite 62 cs. the hybrid electrode demonstrated a high specific capacitance (up to 267.45 f/g), good rate performance (201.08 f/g at 2.5 a/g), and superior cycling stability (almost no capacitance degradation after 1000 cycles). the excellent electrochemical behavior could be attributed to the unique hybrid electrode design and the enhancement in the surface area. the present study provides an adaptable method for novel design of negative electrode materials for supercapacitors, extending the potential applications of carbon-based devices. acknowledgements: the authors greatly appreciate the support of xinyue huang, xingye tong, and xiaoming tu. references [1] x. yuan, b. tang, y. sui, s. huang, j. qi, y. pu, f. wei, y. he, q. meng, p. cao, journal of materials science: materials in electronics 3 (2018) 1-13. 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[30] x. du, c. wang, m. chen, y. jiao, the journal of physical chemistry c 113(6) (2009) 2643-2646. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {enhancement of the cerium oxide primer layers deposited on aa2024-t3 aircraft alloy by preliminary anodization} doi:10.5599/jese.478 113 j. electrochem. sci. eng. 8(2) (2018) 113-127; doi: http://dx.doi.org/10.5599/jese.478 open access : : issn 1847-9286 www.jese-online.org original scientific paper enhancement of the cerium oxide primer layers deposited on aa2024-t3 aircraft alloy by preliminary anodization stephan kozhukharov1,, christian girginov2 1lamar – laboratory, university of chemical technology and metallurgy – sofia, 8 "kliment okhridsky" blvd. 1756, sofia, bulgaria 2department of chemical sciences, university of chemical technology and metallurgy – sofia, 8 "kliment okhridsky" blvd. 1756, sofia, bulgaria corresponding authors e-mail: stephko1980@abv.bg; tel.: +359 899 83 72 82 received: november 28, 2017; revised: february 2, 2018; accepted: february 8, 2018 abstract the possibility for combination between anodized aluminum oxide (aao) and cerium oxide primer layer (ceopl) for elaboration of efficient protective coatings for aa2024-t3 aircraft alloy is proposed in the present research. the combined aao/ceopl coating characterizations include electrochemical impedance spectroscopy (eis) combined with linear voltammetry (lva), for extended times (until 2520 hours) to a model corrosive medium (3.5 % nacl). topographical and cross-sectional (sem and edx) observations were performed in order to determine the aao/ceopl film thickness and composition. the aao/ceopl layer durability tests were confirmed by standard neutral salt spray (nss). the data analysis from all the used measurement methods has undoubtedly shown that the presence of aao film significantly improves the cerium oxide primer layer (ceopl) protective properties and performance. keywords aa2024-t3 alloy; anodization; cerium oxide primer layers; barrier ability; durability introduction although the anodization process is well investigated for pure aluminum [1], its highly doped alloys reveal significant deviations from the expected behavior, due to their compositional and structural heterogeneity [2-9]. in the latter case, the intermetallic inclusions entirely predetermine the mechanism and kinetics of both the primer protective layer deposition and the corrosion processes. thus, the method applied for preliminary surface treatment completely predetermines the chemical composition of the al-oxide surface layer in sense of correlation between anhydrous aluminum oxide al2o3 and crystalline boehmite (alooh), the hydroxide and adsorbed water contents, etc. [10,11]. besides, the surface oxide layer composition, thickness and properties are http://dx.doi.org/10.5599/jese.478 http://www.jese-online.org/ mailto:stephko1980@abv.bg j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 114 of crucial importance for the primer coating layer adhesion [12,13]. thus, in a previous work it was demonstrated that the anodization in sulfuric acid enables the growth of rather thick anodized alumina oxide (aao) layers [14], efficiently compensating the aa2024-t3 heterogeneity. on the other hand, the cerium conversion coatings (cecc), appear to be an efficient environmentally compliant chromium substitute [15,16]. however, it is widely accepted to treat the metallic substrate only by chemical or mechanical methods [14,16], prior to cecc deposition. consequently, the preliminary anodization prior to cerium oxide primer layer (ceopl) deposition should have a remarkable beneficial effect on the performance of the obtained aao/ceopl conjunction. the obtained double layer coating systems should possess significantly extended durability and enhanced barrier properties, providing reliable and durable corrosion protective ability. the expected synergism between the aao and ceopl is not limited only to the additive effect between the aao and ceopl barrier properties but it also includes ceopl adhesion enhancement, due to the aao porosity, providing enlarged contact surface area. furthermore, the porous thick aao provide uniform ce-oxide distribution and thicker ceopl layers. in this case, according to various authors the ceopl layers should possess significant adhesion, predetermined by the al–o–ce covalent bonds composing the interfaces of the entire al/aao/ceopl conjunction [17-21]. finally, it is worth mentioning that the anodization procedure is easily controllable and provides aao parameters variation via relatively simple regime parameter control. as a conclusion of all the above-mentioned facts, the combination between aao and ceopl should provide rather extended corrosion protective capabilities, being really successful strategy for efficient corrosion protection of various aluminum and other metallic substrates. the aim of the present study is to determine the impact of the preliminary anodization process duration on the subsequent cerium oxide layer (ceopl) properties and behavior in a model corrosive medium. experimental four sets of three aa2024-t3 plates were submitted to a sequence of procedures for repeatable aao/ceopl bi-layer primer coating deposition. initially, each sample was etched in 50g/l naoh alkaline medium at 60 °c, and desmutted in diluted hno3 (1:1 v/v). each procedure was performed for 2 min, with subsequent vigorous washing with tap water for at least 2 min. afterwards, the preliminary treated specimens were additively cleaned by distilled water and mounted in a two-electrode cell with similar design as described in a previous work [2]. this cell ensured uniform anodization of circular area of a 4 cm2. the cathode was platinum mesh and the electrolyte was composed of 15 %wt. h2so4 solution. the anodization procedures were performed in galvanostatic regime (15 ma cm-2), in triplicate for 12, 24, or 48 min at room temperature and continuous stirring at 120 rpm. this procedure was followed by spontaneous ceopl deposition for 4 min at 60 °c, in cerium containing aqueous mixture. it was preliminary prepared by mixing of 16.8 g. of 98 % anhydrous diammonium pentanitrocerate ((nh4)2ce(no3)5, fluka chemica -switzerland), 20.0 g. nacl (sigmaaldrich) and 5.0 ml. of 30 % h2o2 (sigma-aldrich) in a volumetric flask (1000 ml.). this stock solution was used for all the depositions done. the specimen assignations are shown in table 1. electrochemical studies were performed periodically (once a week), during continuous exposure to the 3.5 % nacl model corrosive medium. the measurements were performed using electrochemical impedance spectroscopy (eis), from 10 khz to 10 mhz, distributed in 7 frequency steps per decade. s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 115 table 1. assignations of the investigated specimens ceopl coated sets of samples samples no references anodized for 12 min anodized for 24 min. anodized for 48 min 1 2 3 1s0 2s0 3s0 1s12 2s12 3s12 1s24 2s24 3s24 1s48 2s48 3s48 the ac signal amplitude was 15 to 45 mv, according to open circuit potential (ocp). its amplitude was determined as the lowest possible value, enabling the acquisition of readable impedance spectrum. it is worth noting that when the amplitude was too high, inductance appeared, indicating electrode polarization. the eis spectra, as well as all the rest electrochemical measurements were recorded versus (ag/agcl 3 m kcl) reference electrode. the eis spectra acquisitions were followed by linear voltammetry. the voltammetric measurements were performed in subsequence of cathodic (from 30 to –600 mv) and anodic (from –30 to 600 mv) ranges, both at 10 mv s-1 scan rate. these measurements were performed in similar cells, but without stirring. besides, the exposed area was lower (2 cm2), in order to avoid undesirable edge effect phenomena. the resulting film morphology and thickness was determined by scanning electron microscopy, (sem) performed by tescan, sem/fib lyra i xmu working at 30 kv. the sem observations were combined by elemental analysis, with energy dispersion spectroscopy (edx) using energy dispersion spectrometer quantax 200 of bruker. in order to determine the coating performance in severe service conditions, neutral salt spray (nss) test procedure was performed, according to iso 9227 standard. this procedure was performed on nine square samples of the same alloy, but with larger dimensions (60×60 mm). prior to the test procedures, aao/ceopl double layered coatings were deposited following the procedures described above. the surface areas of the double coatings were with 40 mm of diameter, (providing 12.50 cm2). the test procedure was performed for one complete week cycle (168 h) in vsn 1000 humidity chamber (heraeus-vötsch gmbh, germany) by spraying of 5 % nacl aqueous solution with ph range between 6.5 and 7.2. the samples were positioned at 20 ± 5 о of slope. the in-chamber humidity was high enough to enable water to condense of at least 1 tо 2 ml h-1. the corrosion impact rate was observed visually and estimated as a number of localized corrosion damaged domains from the entire exposed surface, according to the bulgarian national standard, bds 15258-81, method с. results and discussion performance in a model corrosive medium the performance of the obtained aao/ceopl double layers was determined by electrochemical measurements executed once of week during extended exposition of the investigated samples to 3.5 % nacl model corrosive medium. this approach enabled determination of the corrosion protective properties of the obtained oxide bilayers, conditionally divided into: barrier ability and durability [22]. barrier ability this property is actually the capability of any coating to obstruct the corrosive species access to the metallic surface. it can be evaluated by the electrical resistance increase, determined by electrochemical measurements (eis and lva), performed at the initial exposition period. although the initial measurements were performed at the 24th hour of exposure to the nacl solution, it was preferred to represent the results acquired after 168 hours of exposition. after one week of exposure, the reference samples, (prepared by ceopl deposition without preliminary anodization) were obviously affected by pitting corrosion unlike the double layered specimens. the average eis spectra, acquired after 168 hours of exposure are shown in fig. 1. j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 116 figure 1. eis spectra acquired after 168 hours of exposure for the investigated specimens for ceopl deposited coatings after 0, 12, 24 and 48 minutes of preliminary anodization. as can be seen from fig 1, the average spectra obtained for the respective sample sets are rather distinguishable among themselves. the already corroded references (0 minutes of anodization) show simple spectra. their nyquist plots consist of slightly depressed semi-circles and warburg diffusion tails (i.e. sloped straight lines). besides, the phase shift/frequency lines in the bode plots showed only one, very narrow minimum at about 10 hz. the maxima in the spectra of the combined coating layers were wide (due to the overlapping of at least two peaks). obviously, the appearance of additional peak is a contribution of the aao formed during the anodization. this supplemental layer contributes for the increase of the total impedance logarithm (log|z|) values, registered at 10 mhz, as well. these values increased from 104 ω cm2 for the references to almost 106 ω cm2 for the combined coatings. the impedance spectra acquired after 168 hours of exposure were fitted to two different equivalent circuits (fig. 2a, and b). figure 2. equivalent circuits used for eis-data fitting rel – 3.5 % nacl electrolyte resistance; qceopl/aao/edl – total cpe of the ceopl, aao, and the metal/electrolyte electric double layer; rceopl/aao/ct total resistance of the ceopl, aao, and the charge transfer across the metal/electrolyte interface; wdiff – warburg element for description of the diffusion of the corrosive species across the ceopl defects; qceopl/aao – cpe of the aao/ceopl interface; raao/ceopl resistance of the aao/ceopl interface; qedl – cpe of the electyric double layer the metal/electrolyte interface; rct charge transfer across the metal/electrolyte interface thus, the spectra of the corroded reference samples have simple shapes, corresponding to only one rq unit (parallel resistance and constant phase element circuit), which includes a warburg diffusion element, related to the diffusion limitations against the access of corrosive species towards the metallic surface, across the surface layer defects. all the rest specimens (with additional aao underlayer) revealed two consecutive rq units. s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 117 each one of the acquired impedance spectra, obtained after 168 hours of exposure was submitted to numerical fitting analysis by the equivalent circuits (fig. 2). the obtained data for the impedance spectra elements are shown in tables 2 and 3. table 2. fitting results of the eis spectra recorded after 168 hours for the referent ceopl coated specimens element unit sample 1s0 sample 2s0 sample 3s0 rel qcoat+oxy+ct n rcoat+oxy+edl wdiff ω cm2 sn ω-1 cm-2 ω cm2 s0.5 ω-1 cm-2 19.44 ± 0.59 0.76 ± 0.03×10-4 0.83 ± 0.95 17.98 ± 1.40×103 3.68 ± 0.68×10-4 19.18 ± 0.49 1.19 ± 0.04×10-4 0.82 ± 0.91 12.56 ± 0.87×103 6.50 ± 1.30×10-4 20.02 ± 0.59 1.12 ± 0.05×10-4 0.81 ± 0.01 7.68 ± 0.46×103 12.70 ± 0.29×10-4 the numerical data for the referent samples (table 2) show relatively low deviations. these specimens, with clear corrosion features (i.e. multitudes of pits) have shown total admittance between 0.29×10-4 and 1.12×10-4 sn ω-1 cm-2 and ohmic resistance between 7.68 and 19.98 kω cm2. these values are in the typical range for corroded bare aa2024-t3 alloy, established in previous studies [23-25]. besides, the data fitting for these specimens required addition of warburg element. this fact is an evidence of free diffusion of corrosive species across both the native al-oxide and ceopl layers. both these findings indicated that the directly deposited ceopl layer does not possess any protective activity after 168 hours of exposure to 3.5 % nacl model corrosive medium. that was the reason for termination of the exposure tests after this period. on the other hand, the data results for all the ceopl coated specimens after anodization were more appropriate for fitting to the other equivalent circuit (fig. 2b), composed by two-time constants. it should be noted that these time constants are not related to the interfaces between the aao and ceopl layers, but rather these correspond to the interfaces between the aao/ceopl conjunction, the aa2024-t3 metallic base and the aqueous nacl electrolyte. probably, this conjunction is composed by covalent ce-o-al bonds, suggesting that there is not clear interface between the aao and ceopl layers. besides, both aao and ceopl layers are composed by metallic oxides with apparent dielectric properties. due to these reasons, the equivalent circuit used for the reference specimens (fig. 2a) was inappropriate for eis data result fitting of the spectra acquired for the preliminary anodized aa2024-t3 samples. the second equivalent circuit (fig. 2b) has shown to be more suitable and the fitting results are shown in table 3. the data acquired by the eis spectra fitting to the equivalent circuit (fig 2b) possess different deviations rates among the results for each sample. thus, the ceopl specimens coated, after only 12 min of anodization possess remarkable deviations, in the range of entire orders of magnitude. this fact indicates that this anodization time is obviously insufficient for creation of thick enough aao films with uniform coverage. besides, probably the randomly distributed intermetallic inclusions of the alloy substrate interrupt this film. the comparison between the charge transfer resistance (rct) data in table 3 with the combined resistance (rcoat+oxy+edl), in table 2 show obvious barrier ability elevation after the preliminary anodization. consequently, the observed barrier ability improvement after anodization is an indication for the beneficial role of the preliminary anodization procedure on ceopl performance. following the concept for island growth of ce oxide layers [26], it should be assumed that the ceopl formation begins from exactly these intermetallics. besides, the most active ceopl growth initiation centers should be the copper rich s-phase and θ-phase inclusions [3], according to the following reactions [3, 26]: j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 118 table 3. fitting results of the eis spectra acquired after 168 hours of exposure for the anodized, ceopl coated specimens 12 minutes of anodization element unit sample 1s12 sample 2s12 sample 3s12 rel qcoat+oxy n rcoat+oxy qedl n rct ω cm2 sn ω-1 cm-2 ω.cm2 sn ω-1 cm-2 ω cm2 15.36 ± 0.21 4.35 ± 0.06×10-5 0.78 ± 0.002 23.71 ± 0.96×103 30.92 ± 5.18×10-5 0.75 ± 0.08 2.42 ± 0.24×105 22.78 ± 2.54 0.79 ± 0.10×10-5 0.77 ± 0.02 6.72 ± 1.15×103 2.89 ± 0.55×10-5 0.67 ± 0.04 0.91 ± 0.08×105 33.06 ± 8.95 0.15 ± 0.05×10-5 0.68 ± 0.003 7.68 ± 0.34×103 0.05 ± 0.08×10-5 0.95 ± 0.01 78.40 ± 5.42×105 24 minutes of anodization element unit sample 1s24 sample 2s24 sample 3s24 rel qcoat+oxy n rcoat+oxy qedl n rct ω cm2 sn ω-1 cm-2 ω cm2 sn ω-1 cm-2 ω cm2 16.46 ± 1.69 2.52 ± 0.48×10-6 0.71 ± 0.01 2.39 ± 0.44×103 1.31 ± 0.42×10-6 0.86 ± 0.04 4.91 ± 0.19×105 45.86 ± 6.28 1.78 ± 0.32×10-6 0.76 ± 0.02 1.36 ± 0.20×103 1.15 ± 0.24×10-6 0.87 ± 0.03 7.58 ± 0.31×105 17.28 ± 1.73 0.90 ± 0.09×10-6 0.71 ± 0.02 4.62 ± 0.23×103 0.88 ± 0.08×10-6 0.94 ± 0.02 30.64 ± 1.32×105 48 minutes of anodization element unit sample 1s48 sample 2s48 sample 3s48 rel qcoat+oxy n rcoat+oxy qedl n rct ω.cm2 sn ω-1 cm-2 ω.cm2 sn ω-1 cm-2 ω cm2 27.44 ± 6.91 6.52 ± 0.02×10-6 0.58 ± 0.003 3.74 ± 0.18×103 0.90 ± 0.10×10-6 0.97 ± 0.02 2.70 ± 0.06×105 25.82 ± 12.12 2.43 ± 0.09×10-6 0.63 ± 0.004 2.60 ± 0.06×103 1.36 ± 0.07×10-6 0.96 ± 0.009 11.16 ± 0.31×105 71.60 ± 17.95 0.96 ± 0.07×10-6 0.67 ± 0.008 5.74 ± 0.14×103 1.93 ± 0.07×10-6 0.94 ± 0.009 111.20 ± 1.22×105 oxygen reduction: o2 + 2h2o + 4e→ 4oh(1) formation of intermediated complex species: 2ce3+aq. + h2o2 + 2oh→ ce(oh)22+aq. (2) reaction of the obtained complex ions with hydroxyl ions near to the metallic surface: ce(oh)22+aq. + 2oh→ ce(oh)4 (3) subsequent conversion of the cerium hydroxides to the respective oxides: ce(oh)4 → ceo2 + 2h2o (4) according to the authors, these reactions proceed accompanied by the following supplemental reaction of direct interaction of the ce3+ with the hydroxyl ions: ce3+ + 3oh→ ce(oh)3 (5) however, all this chain of consecutive reactions is strongly dependent on the available anodic area, composed by the al-matrix under the oxide layer defects, since it provides the electrons necessary for the oxygen reduction and the resulting in ohion generation. namely, the predominant hydroxyl ion generation (i.e. alkalization) near these intermetallics causes ce(oh)3 and/or ce(oh)4 precipitation. consequently, the oxygen reduction cathodic reaction rate on the intermetallics is strongly dependent on the anodic al-dissolution (reaction 6): al0 → al3+ + 3e(6) applying this model for the ceopl deposited aa2024-t3 samples after anodization, it can be anticipated that the aao film hinders the ceopl formation, because it covers the al-matrix anodic s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 119 area, necessary to supply electrons for the cathodic ohgeneration reactions, and the resulting ce-hydroxide precipitation. nevertheless, this concept is in contradiction with the data in table 3. all the acquired resistance data (i.e. rcoat+oxy and rct) reveal remarkably distinguishable dissipations among the specimens of each group (e.g. 12, 24 and 48 min of anodization). simultaneously, the result deviations possess very similar deviation rates and orders of magnitude among the respective sample groups. thus, no clear trend of these values was observed, although the obvious oxide layer thickness increases within the anodization procedure continuation, established in a previous work [27]. these facts provide undoubted evidence for the low oxide layer density due to its probably porous structure. in addition, the difference between the equivalent circuits suitable for the eis-data fitting of the references and the anodized specimens reveal completely different mechanism of ceopl deposition after aao formation. following the concepts of arenas and damborenea [28], it can be assumed that the aao/ceopl conjunction is alumina layer with partial substitution by ce-ions. obviously, the ceopl layers deposited on thicker aao layer, formed at extended anodization times possess better coverage. indeed, conde et al. [29] remark that the ceopl layer formation is based on chemisorption processes on the superficial hydrated oxide layer of the aluminum, as follows: -al-oh2+ ↔ -aloh + h+ (7) -al-oh + ce3+ ↔ [-al-oce(iii)]2+ + 2h+ (8) consequently, ceopl deposition mechanism alteration occurs on the more completely hydrated aao layers, occupying the entire metallic surface. thus, the ceopl deposition follows the mechanism, proposed by conde et al. [29], described by reactions (7) and (8). these trends of barrier ability enhancement by the preliminary anodization have been evinced by the concordance of the results acquired by both the eis data analysis and the polarization curves, described below. fig. 3 illustrates the general trends (of current densities decrease, combined by corrosion potential (ecorr) shift in positive direction) for the polarization curves recorded after 168 hours of exposure. potential, v potential, v figure 3. cathodic (a) and anodic (b) polarization cures recorded after 168 hours of exposure the polarization curves have been submitted to further tafel plot analysis for determination of ecorr and polarization resistance (rp). the numerical data of the obtained results for all the investigated specimens are represented in table 4. some trends can be seen from the inferred, regardless the considerable result dissipation. the largest ecorr difference between the cathodic and anodic curves belongs to the reference samples, showing that the relaxation time of about hours is not sufficient for ecorr value restoration, after the cathodic curve acquisition. unexpecc u rr e n t d e n si ty a c m -2 c u rr e n t d e n si ty a c m -2 j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 120 tedly, the ecorr values of the anodic curves are with about 100 mv more positive, although cathodic polarization of the specimens. thus, during the cathodic curve acquisition, the investigated specimens (being negatively charged) attract cations from the model corrosive medium, like na+ and h3o+, shifting ecorr in positive direction. this assumption is additionally confirmed by the occurrence of warburg diffusion elements, commented in the previous section. table 4. tafel plot analysis of the obtained results from the polarization cures, recorded after 168 hours of exposure cathodic anodic ecorr / v vs. ag/agcl rp / kω cm2 ecorr / v vs. ag/agcl rp / kω cm2 1s 2s 3s 1s 2s 3s 1s 2s 3s 1s 2s 3s ref. 12 min 24 min 48 min -0.658 -0.628 -0.633 -0.673 -0.685 -0.595 -0.607 -0.617 -0.718 -0.611 -0.773 -0.639 17.8 300.0 480.0 1980.0 14.0 240.0 280.0 3800.0 8.6 160.8 196.0 2200.0 -0.557 -0.589 -0.571 -0.676 -0.565 -0.631 -0.615 -0.653 -0.536 -0.608 -0.685 -0.614 18.4 248.6 600.80 2345.0 10.0 220.0 380.0 3600.0 8.4 170.0 214.0 2400.0 the cathodic curves acquired from the reference specimens approximated straight lines, possessing indistinguishable curvature. their slopes are probably result of electrolyte ohmic drop, and diffusion hindering in the electrolyte bulk. all other polarization curves obtained from the preliminary anodized samples had more similar ecorr values, with deviations of about 35 to 40 mv. besides, their cathodic curves stay at lower current densities compared to those of the reference samples. consequently, the aao/ceopl conjunctions possess much better insulating properties, compared to the reference ceopl monolayers. this inference is additionally confirmed by the relatively higher rp values of the combined layer coatings, although the considerable value deviations among the samples. in all cases, the ceopl coated specimens after 48 min of anodization possess at least an order of magnitude higher rp values, compared to those, anodized for only 12 min. the specimens treated for 24 min have intermediate position, being similar either to the former, or to the latter specimens. the higher barrier ability of the aao/ceopl layers suggests extended durability, and this fact was the reason to continue the experiments, related to the sample exposure to the model corrosive medium. durability – this property was evaluated by further extension of the sample exposure until corrosion pits appear, combined with regular eis and lva measurements, according to the regimes, described in the experimental section. since this approach has provided a huge number of data collections, only the final measurements will be commented in detail. the sample durability of the specimens increases progressively with the anodization process duration (and the aao thickness, respectively). thus, the reference ceopl monolayers have already failed until the 168th hour of exposure, whereas the aao/ceopl layers, obtained by 48 min of anodization have resisted whole 2,520 hours in the model corrosive medium (fig. 4) the eis spectra recorded for the final exposure times were appropriated for fitting to the equivalent circuit shown in fig. 2b. since the ceopl coated specimens, after 24 min of anodization fall between the anodized for 12 and 48 figure 4. durability diagram of the investigated specimens s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 121 min, their results were not submitted to fitting. furthermore, the significant difference of the exposure times registered for these samplers (e.g. the anodized for 24 min) should lead to remarkable deviation among the respective eis data fitting. the results, obtained from the final eis spectra fitting are represented in table 5. it is worth mentioning, that at the final measurements (fig. 4) all the preliminary anodized specimens possessed superior charge transfer resistance (rct) values (table 5), compared to those of the references after 168 hours of exposure (table 2). although the fact that the equivalent circuit was appropriate for data fitting in both cases of the specimens, preliminary anodized for 12 and 48 min, their spectra had rather distinguishable shapes (fig. 5). since the specimens anodized for 24 min have shown great deviations among themselves, their spectra are not shown. figure 5. averaged eis spectra bode plots of ceopl deposited coatings after 12 and 48 minutes of anodization, acquired at the end of the exposition to the model corrosive medium both the real (z’) and the imaginary (-z”) values of the smples anodized for 48 min are much superior to these of the samples with 12 min of anodization (fig. 5a). the real impedance component reaches 250 kω cm2 for the thicker aao layer, whereas in the other case, its value is only 25 kω cm2. similar difference is observable for the logarithm of the total impedance in the respective bode plot (fig. 5b). the φ(f)-dependence recorded after 2,520 hours of exposure have two clearly distinguishable maxima, related to the aao and ceopl layers, at the lower and the higher frequency ranges. these maxima are almost completely overlapped in the eis spectra acquired after (504/672 hours of exposure) of the samples anodized for 12 min. this fact indicates that the coatings deposited after 48 min of anodization possess residual protective capabilities, although the interruption of their integrity after 2520 hours of exposure to the model 3.5 % nacl corrosive medium. the eis results are further confirmed by the respective lva measurements, performed after the sample exposure, shown in fig. 6. potential, v potential, v figure 6. cathodic (a) and anodic (b) polarization curves acquired at the end of the exposure of ceopl coated samples after 12 and 48 minutes of anodization c u rr e n t d e n si ty a c m -2 c u rr e n t d e n si ty a c m -2 p h a se sh ift, o j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 122 the cathodic polarization curves acquired after the corrosion tests for the specimens anodized for 12 and 48 min overlap, where there is obvious difference between the respective anodic curves. this fact is indication for the almost equal cathodic activity in the former case. however, there is a sharp current rise, in the anodic curve of the sample anodized for 12 min, as a result of localized corrosion activity as is proposed elsewhere [30]. consequently, the corrosion processes proceed with almost the same rate, but with a different mechanism. table 5. fitting results for the anodized ceopl coated specimens at the final exposure measurements 12 minutes of anodization element unit sample 1s12 after 504 hours sample 2s12 after 504 hours sample 3s12 after 672 hours rel qcoat+oxy n rcoat+oxy qedl n rct ω cm2 sn ω-1 cm-2 ω cm2 sn ω-1 cm-2 ω.cm2 14.18 ± 0.20 0.68 ± 0.01×10-4 0.86 ± 0.03 22.08 ± 1.00×103 0.51 ± 22.96×10-3 0.95 ± 0.09 49.24 ± 15.20×103 18.92 ± 0.67 2.00 ± 0.06×10-5 0.82 ± 0.05 44.94 ± 2.36×103 3.42 ± 12.92×10-4 0.96 ± 0.01 10.20 ± 68.63×103 57.48 ± 16.28 1.44 ± 0.09×10-6 0.66 ± 0.06 11.62 ± 0.74×103 0.78 ± 0.08×10-6 0.92 ± 0.02 3.52 ± 0.21×103 48 minutes of anodization element unit sample 1s48 after 2520 hours sample 2s48 after 2520 hours sample 3s48 after 2520 hours rel qcoat+oxy n rcoat+oxy qedl n rct ωcm2 sn ω-1 cm-2 ω cm2 sn ω-1 cm-2 ω cm2 187.20 ± 10.95 0.681 ± 0.06×10-5 0.65 ± 0.01 15.06 ± 4.29×103 0.81 ± 0.43×10-6 1.00 ± 0.11 21.84 ± 11.40×105 85.60 ± 10.46 3.22 ± 0.09 ×10-6 0.57 ± 0.03 23.04 ± 0.96×103 0.94 ± 0.07×10-6 0.99 ± 0.02 12.30 ± 0.27×105 48.56 ± 3.88 4.10 ± 0.43×10-5 0.69 ± 0.01 19.36 ± 7.22×103 2.37 ± 1.08×10-6 1.00 ± 1.48 71.60 ± 6.29×104 the anodic areas of the specimens, anodized for 48 min suffer uniform corrosion and the current density is distributed equally on the entire aluminum matrix, probably due to the initial galvanic corrosion stage. on the other hand, strong localized corrosion occurs for the samples anodized for 12 min, after only 504/672 hours of exposure. this fact shows that the durability of the samples anodized for 48 min exceeds 2,520 hours, whereas the other samples (anodized for 12 min) have already failed. all the polarization curves, recorded at the sample exposure termination, were submitted to further analysis and the obtained results for 12 and 48 min of anodization are summarized in table 6. table 6. tafel plot analysis of the polarization curves, recorded at the end of the exposure period cathodic anodic ecorr / v vs. ag/agcl rp / kω cm2 ecorr / v vs. ag/agcl rp / kω cm2 12 min 1s12 2s12 3s12 1s12 2s12 3s12 1s12 2s12 3s12 1s12 2s12 3s12 504 h 6,672 h -0.673 -0.637 -0.641 32.0 62.0 17.8 -0.568 -0.655 -0.699 16.4 42.0 1610.0 48 min 1s48 2s48 3s48 1s48 2s48 3s48 1s48 2s48 3s48 1s48 2s48 3s48 2,520 h -0.623 -0.656 -0.627 142.0 460.0 38.0 -0.624 -0.627 -0.538 134.0 1380.0 960.0 in both cases of the ceopl coated samples anodized for 12 and 48 min, the rp values are by entire orders of magnitude higher than those of the reference samples, registered after 168 hours of exposure (tables 4 and 6). furthermore, the values of the samples anodized for 48 min are similar or slightly higher than those of the treated for only 12 min. the rp values of the anodized specimens do not vary remarkably for the entire exposition period. besides, this trend is in s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 123 concordance with the eis data fitting (tables 3 and 5). all these facts indicate that both sets of anodized specimens possess residual barrier ability even after extended exposure periods. this conclusion was additionally confirmed by the morphological observations, which show weaker corrosion impact of the preliminary anodized samples. scanning electron microscopy and edx-chemical analysis the ceopl coated aa2024-t3 specimens after 12, 24 and 48 min of anodization were submitted to systematic sem observations before (fig. 7) and after (fig. 8) the corrosion tests, described in the previous sections. the comparison among the positions in fig. 7 reveal clear differences among the ceopl layers, deposited after different anodization durations. although the ceopl layers were deposited, following the same procedure, these possess rather distinguishable morphologies, predetermined by the aao underlayer formation process duration. obviously (positions (a) and (b)), the aao formed for only 12 min is disrupted over the alloys’ intermetallic particles locations. that is the reason for the occurrence of brighter coarse particles in the locations of the coating disruptions. for comparison, such intermetallics were not observed in the other cases (24 and 48 min). another distinguishable feature observed in the case of 48 min of anodization (positions (c) and (f)) is the occurrence of net of cracks throughout the entire aao/ceopl layer surface. this coating cracking is an indirect indication for a probably greater ceopl layer thickness, achieved after 48 min of anodization. besides, the cracks are not deep enough to cross the aao underlayer, because these aao/ceopl conjunctions have shown remarkable durability, as was already commented. the concavities on the metallic surface observed for all the samples are result of the preliminary sample treatment procedures, as is discussed in previous works [31 33]. similar systematic sem observations have been done on the already corroded specimens. (i.e. ceopl coatings after 0, 12, 24, and 48 min of anodization and subsequent corrosion tests for 168, 504, 1008 and 2520 hours of exposure). the obtained sem images are shown in fig. 8. figure 7. low (a, b, c) and high (d, e, f) resolution sem images of the investigated specimens after 12 (a, d), 24 (b, e) and 48 (c, d) minutes of exposure the sem images in fig. 8 clearly demonstrate the lack of corrosion pits on the sample, anodized for 48 min, whereas in all the other cases occurrence of such pits was observed. consequently, the aao/ceopl combined layers coatings, formed at the largest anodization duration possess durability superior to 2,520 hours. in addition, these samples did not show any remarkable differences before and after the corrosion tests (fig. 7 and 8). this fact is additional evidence for the superior sample durability. j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 124 figure 8. low resolution sem images of ceopl coatings deposited after: (a) – 0, (b) – 12, (c) – 24 and (d) – 48 minutes of anodization the sem observations were followed by edx chemical analyses, in order to illustrate the coating chemical compositions, the elements distribution regularity, and the aao/ceopl coverage rate before the corrosion test procedures (fig. 9 and 10). figure 9. edx map analyses of ceopl coated aa2024-t3 samples, after (a) 12, (b) – 24 and (c) – 48 minutes of anodization the edx map analyses reveal uniform element distribution, indicating uniform coverage of the metallic surface by the aao and the further ceopl layers. the basic alloy (al, cu and mg), aao (al, s and o) and ceopl (ce and o) elements were selected for observation. the edx images clearly reveal mg re-deposition and sulfur entrapment in the aao/ ceopl layer composition. unlike the results from previous works [8] the ceopl layers deposited on the already anodized aa2024-t3 samples cover completely the metallic surface possessing dense structure and uniform element distribution (fig. 10). the sem cross-sectional observations (fig. 11) reveal gradual increase of the oxide layer thickness from 8 to 18 µm, within increasing of the anodization process duration from 12 to 48 min, respectively. s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 125 figure 10. element distribution of ceopl coated aa2024-t3 samples, after (a) – 12, (b) – 24 and (c) – 48 minutes of anodization the correlation between the anodization process duration and the resulting aao/ ceopl layer thickness reveals an aao thickness growth rate of about 600 nm min-1. an interesting trend was established when the coating thickness and durability (section 3.1) were compared. for instance, the mathematical ratio between the durability test data (504 h) and the film thicknesses (7.5 µm) of the aao/ceopl obtained after 12 min of anodization results in 70 h µm-1. for the thickest film (18.8 µm), obtained after 48 min of anodization with 2,520 h of durability this relation has 134 h µm-1. this difference leads to the assumption that the anodization procedure proceeds with simultaneous film densification. figure 11. cross-sectional sem images of aao/ceopl double layered coatings deposited after: (a) – 12, (b) – 24 and (c) – 48 minutes of anodization another interesting point is the element distribution inside the aao/ceopl film. the edx map analyses reveal relatively equal ceriumand sulfur distributions across the investigated films (fig. 12). j. electrochem. sci. eng. 8(2) (2018) 113-127 cerium oxide deposited on aa2024-t3 aircraft alloy 126 figure 12. cross-sectional edx maps of aao/ceopl double layered coatings deposited after: (a) – 12, (b) – 24 and (c) – 48 minutes of anodization nss-test procedures the excellent performance of the aao/ceopl combined coating primers obtained by 48 min (18.8 µm) of anodization was undoubtedly confirmed by the nss test procedures, because no corrosion pits were not registered after entire 168 hours of spraying with relatively concentrated nacl solution. conclusions the possibility for combination of anodized alumina oxide (aao) and cerium oxide primer layer (ceopl) for elaboration of efficient protective coatings for aa2024-t3 aircraft alloy was proposed. the correlation between the anodization process duration (the resulting film thickness, respectively) and protective capabilities was evaluated by systematical characterization procedures. these procedures included electrochemical measurements (eis, lva), combined with morphological observations (sem) and chemical element distribution (edx). the protective properties of all investigated coatings were evaluated by extended exposure (up to 2,520 hours) to a 3.5 % nacl model corrosive medium and regular electrochemical measurements. additional cross-sectional sem and edx observations were performed in order to determine the aao/ ceopl film thickness (related to the anodization duration). the obtained results regarding significant aao/ceopl layer durability were confirmed by standard neutral salt spray (nss) test. the combination among these characterization methods has enabled to create complementary image for the coating properties and their performance. in addition, the correlations between the applied deposition procedures and the resulting aao/ceopl coating layers were determined. the measurement results have shown that the aao/ceopl conjunctions possess remarkable corrosion protective capabilities, unlike the reference ceopl monolayers. the eis spectra of the aao/ceopl conjunctions registered after 168 hours of exposure reveal much higher resistance values, compared to the referent ceopl. furthermore, the eis spectra fitting required equivalent circuit for combined aao/ceopl layers, different from those for ceopl reference coatings. this fact has revealed completely different performance in the model corrosive medium for both kinds of coatings. these conclusions from the eis spectra analysis were confirmed by the respective polarization curves. the rp values of the aao/ceopl layers were by entire orders of magnitude higher than those of the ceopl references. the layers’ durability increases progressively with the anodization duration. the specimens anodized for 12 minutes have resisted only about 600 hours, whereas 48 min of anodization has led to more than 2,520 hours of exposure until pitting appearance. the further data analysis, based on the film thickness determination has led to the inference that the coating durability increases progressively with the thickness growth (the ratio between the coating durability (600 to 2,520 hours) versus the film thickness (from 7.5 to 18.5 µm). s. kozhukharov and c. girginov j. electrochem. sci. eng. 8(2) (2018) 113-127 doi:10.5599/jese.478 127 the cross-sectional edx map analyses have undoubtedly evinced the uniform distribution of the aao/ceopl combined layer components (o, al, s and ce). the nss test procedures have undoubtedly confirmed the excellent performance of the thickest aao/ceopl combined coatings (with 18.5 µm, obtained by 48 min of anodization). acknowledgements: the authors are grateful for the funding of this research to the bulgarian national scientific research fund, under contract nfni: дм-19/6 2017. references [1] w. j. stepniowski, m. moneta, k. karczewski, m. michalska-domanska, t. czujko, j.m.c. mol, j. g. buijnsters, j. electroanal. chem. 809 (2018) 59-66. 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[33] e. matter, s. kozhukharov, ann. proceeds. univ. rousse (bulgaria), 49 (2010) 14-19, acces via: http://conf.uni-ruse.bg/bg/docs/cp10/9.1/9.1-2.pdf ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://conf.uni-ruse.bg/bg/docs/cp10/9.1/9.1-2.pdf http://creativecommons.org/licenses/by/4.0/) ni-moo2 cathodes for hydrogen evolution in alkaline solutions. effect of the conditions of their electrodeposition doi: 10.5599/jese.2012.0027 29 j. electrochem. sci. eng. 3(1) (2013) 29-36; doi: 10.5599/jese.2012.0027 open access : : issn 1847-9286 www.jese-online.org original scientific paper ni-moo2 cathodes for hydrogen evolution in alkaline solutions. effect of the conditions of their electrodeposition gvozden tasić, borka jović*, uroš lačnjevac*, nedeljko krstajić** and vladimir jović* university of belgrade, vinča institute, department of physical chemistry, 11001 belgrade, p.o. box 522, serbia *institute for multidisciplinary research university of belgrade, 11030 belgrade, p.o. box 33, serbia **faculty of technology and metallurgy university of belgrade, 11000 belgrade, karnegijeva 4, serbia  corresponding author: e-mail: e-mail: vladajovic@imsi.rs; tel.: +381-11-3303-688; fax: +381-11-3055-289 received: august 29, 2012; revised: november 2, 2012; published: december 4, 2012 abstract the electrodeposition of ni-moo2 composite coatings is a specific process taking place under defined hydrodynamic conditions. in this work the influence of hydrodynamics, current density, and moo2 concentration on the electrodeposition of ni-moo2 coatings from a suspension of moo2 particles in a watt’s type bath were investigated by comparison of their polarization characteristics for the hydrogen evolution reaction (her) in 1 m naoh at room temperature. it was shown that electrolyte and air flow rates influence the process of coating electrodeposition at low concentrations of moo2. with increases in moo2 concentration the current density became the main parameter influencing the coatings’ properties. the best coating, with the lowest over-voltage for the her, was obtained from the suspension containing 3 g dm -3 of moo2 particles. the over-voltage of this coating was 57 mv lower for the her at j = –0.3 a cm -2 (the current density used in industrial applications) than that for the commercial de nora’s ni + ruo2 electrode. keywords ni-moo2; composite coatings; electrodeposition; her; morphology. http://www.jese-online.org/ mailto:vladajovic@imsi.rs j. electrochem. sci. eng. 3(1) (2013) 29-36 ni-moo2 cathodes for hydrogen evolution 30 introduction one of about 90 hydrogen production routes [1] is alkaline water electrolysis [2]. it is a wellestablished process that is technologically very simple and produces very clean gases, but it is also energy-inefficient, slow, and expensive. one of the ways to increase the energy efficiency of electrolysers is by developing advanced electrocatalytic materials in order to reduce the electrode over-voltage. in conventional water electrolysis, the cathode is usually made of stainless steel or nickel-based materials (raney ni), and operates in 6–9 m koh solution in a temperature range of 60–90 o c. the overall energy efficiency of electrolysis is related to the hydrogen evolution reaction (her). in chlor-alkaline electrolysis, her, as the cathodic reaction in 32 wt.% naoh at 90 o c, is also responsible for the energy efficiency of electrolysers [3]. hence, for industrial application, advanced electrocatalytic materials for her are of great importance to increase energy efficiency. since porous ni (raney ni) electrodes showed extensive deactivation during electrolysis, several approaches to design and produce new electrocatalytic cathode materials have been presented in the past [4–20]. one of these approaches is based on the brewer intermetallic bonding theory, which predicts a synergetic activation effect whenever metals of the left half of the d-series are alloyed with metals of the right half of the d-series [4–10]. accordingly, an increase in electrocatalytic activity has been widely observed for such materials: ti–ni [11], pt–mo [12], ni– mo [9], ni–co–lani5 [13], ni–ruo2 [14], and ni–transition metals (fe, mo, w) [15]). recently, particular attention has been given to the replacement of expensive ni + ruo2 [14] composite coating with coatings obtained by the electrodeposition of ni with molybdenum oxide species [16–20]. it was shown that ni–moo3 [16,20] composite coatings, electrodeposited from a suspension containing different concentrations of moo3 particles in a watt’s type bath, possess good catalytic activity, but their properties during the “service life” test were not good enough for application in industrial electrolysis [18]. on the other hand, ni–moo2 composite coatings electrodeposited from a suspension containing 3 g dm -3 moo2 particles in a solution of 0.2 m nicl2 + 2 m nh4cl were found to be a promising replacement [17,18] for commercial ni + ruo2 coating obtained by a similar electrodeposition procedure. taking into account that the commercial ni + ruo2 cathodes were electrodeposited by incorporation of ruo2 particles in an ni matrix, in this work an attempt was made to produce ni– moo2 composite coatings from a suspension of moo2 particles in a watt’s type bath under different conditions (different concentrations of moo2 particles, different current densities, and different hydrodynamic conditions) and to compare their polarization characteristics for her in 1 m naoh with that for the commercial ni + ruo2 coating. in this way most of the possibilities of obtaining ni–mo–oxide coatings by electrochemical deposition would be fulfilled. experimental electrodeposition of the ni–moo2 composite coatings the moo2 powder was synthesized by a rheological phase reaction route [18]. the average size of moo2 powder particles, determined with a brookhaven instruments light-scattering system equipped with a bi-200sm goniometer, a bi-9000at correlator, a temperature controller, and a coherent innova 70c argon-ion laser, amounted to about 200 nm. dynamic light scattering measurements were performed using 135 mw laser excitation at a wavelength of 514.5 nm and a detection angle of 90°. all samples were electrodeposited onto ni 40 mesh from the suspension of moo2 powder particles (1–3 g dm -3 ) in a watt’s type electrolyte containing 210 g dm -3 niso4 × 7h2o, 60 g g. tasić et al. j. electrochem. sci. eng. 3(1) (2013) 29-36 doi: 10.5599/jese.2012.0027 31 dm -3 nicl2 × 6h2o, 60 g dm -3 na2so4, and 30 g dm -3 h3bo3 at ph = 3.8 and t = 50 o c in the pilot plant cell (fig. 1a,b,c). the electrolyte was circulated with the pump, while the flow rate was measured with the flow meter (fig. 1a). additional mixing of the electrolyte was provided by the air flow (measured with the flow meter, fig. 1b) through two pipes with small openings facing the bottom of the cell in order to remove eventually precipitated molybdenum oxide particles from the bottom of the cell and to force particles to float and circulate with the electrolyte (fig. 1b,c). the temperature of the electrolyte was kept constant by the thermocouple, heater, and control unit (as shown in fig. 1a). the ni 40 mesh cathode (dimensions: 5 × 6 cm 2 ) was connected to a ni holder (frame, fig. 1d) and placed between two ni anode plates (18 × 22 cm, fig. 1b). a homemade power supply, with a ripple smaller than 1 %, was used to apply the required current/voltage [16,17]. all ni 40 mesh substrates were first cathodically degreased in a solution of 30 g dm -3 naoh, 30 g dm -3 na2co3, and 40 g dm -3 na3po4 × 12h2o for 5 min at j = –100 ma cm -2 and t = 60 o c. neutralization was performed in a solution of 20 g dm -3 h2so4 for 5 min. the ni 40 meshes were then shortly etched in a 1:3 mixture of h2o:hno3 and washed with distilled water before the electrodeposition of the ni–moo2 composite coatings. her investigations all experiments were carried out in 1 m naoh at room temperature in a three-compartment electrochemical cell with two pt mesh counter electrodes placed parallel (in separate compartments) to the working electrode and a saturated calomel electrode (sce) as the reference electrode, connected to the working electrode (in the central compartment) by means of a luggin capillary. the solution of 1 m naoh was made from extra pure uv water (smart2pureuv, tka) and p.a. chemicals. all experiments were performed with the potentiostat reference 600 and phe 200 or dc 105 software (gamry instruments). polarization curve measurements electrodes were first submitted to hydrogen evolution at a constant current density of j = –0.3 a cm -2 for 800 s (step 1), followed by her at a constant potential at which the cathodic current density for her was higher than –0.3 a cm -2 (step 2) for 800 s. after this pre-electrolysis, polarization curves were recorded by sweeping the potential at 1 mv s -1 from the potential applied in step 2 to the potential value of –1.10 v and recording the current density. the potential values were corrected for the ir drop by the current interrupt technique (dc 105). sem and eds analysis of the ni–moo2 composite coatings the appearance of the surface and the chemical composition of the coatings were investigated by sem (vega ts 5130 mm, tescan) equipped with an energy-dispersive x-ray spectroscope (eds; incapentafet-x3, oxford instruments). results and discussion electrodeposition of ni–moo2 samples samples 1–12 were deposited with a charge of 60 c cm -2 , while samples 13–21 were deposited with a charge of 120 c cm -2 . the conditions of electrodeposition are given in table 1, while j. electrochem. sci. eng. 3(1) (2013) 29-36 ni-moo2 cathodes for hydrogen evolution 32 corresponding values of potential taken from the polarization curves for her in 1 m naoh at j = –0.3 a cm -2 are presented in the last column of table 1. (a) (b) (c) (d) figure 1. (a) schematic presentation of the system for electrodeposition of coatings. (b) system for additional mixing of electrolyte with air bubbling through two pipes on the bottom of the tank. (c) position of anodes and cathode. (d) position of the mesh cathode on the ni frame. before the addition of moo2 powder particles to the electrolyte, electrodeposition of several pure ni samples in a watt’s type bath was performed at j = –50 ma cm -2 with different electrolyte and air flow rates. in all cases good quality deposits were obtained with a high current efficiency (> 85 %). when the first amount of moo2 powder particles of 1 g dm -3 was added to the electrolyte, electrolyte was cycled at the highest rate of 20 dm 3 min -1 (with the pump, fig. 1a) and mixed with the highest air flow of 20 dm 3 min -1 (using a compressor, fig. 1b) for several hours. electrodeposition of ni–moo2 composite coating started after 24 h of electrolyte aging (following experience from our previous work [17]). the electrolyte flow rates were set at two values, 10 and 20 dm 3 min -1 , while the air flow rates were 1, 5, 10, and 20 dm 3 min -1 . three values of the deposition current densities (jdep) were used: –25 ma cm -2 , –50 ma cm -2 , and –100 ma cm -2 . g. tasić et al. j. electrochem. sci. eng. 3(1) (2013) 29-36 doi: 10.5599/jese.2012.0027 33 polarization curves for the best samples four samples were deposited in the suspension containing 1 g dm -3 moo2. their polarization curves for her were similar, with the best one being that for sample 4. all samples possessed lower over-voltage for her than ni 40 mesh with about 50 m of pure ni deposited from the watt’s type bath (ni). the best sample (4) was deposited at the highest current density and flow rates of electrolyte and air (see table 1). nine samples were deposited in the suspension containing 2 g dm -3 moo2. the best polarization characteristics were recorded for sample 13 and the worst ones for sample 11, while all others were placed between these two. eight samples were deposited in the suspension containing 3 g dm -3 moo2. the best polarization characteristics were recorded for sample 19, while all others were placed close to the best one. the best polarization characteristics for samples deposited from all three suspensions are presented in fig. 2, together with polarization characteristics for the deposited ni coating (ni) and commercial de nora’s ni + ruo2 (dn) coating. table 1. conditions for electrodeposition of ni-moo2 coatings. sample moo2 g dm -3 –jdep ma cm -2 electrolyte flow rate, dm 3 min -1 air flow rate, dm 3 min -1 e(-0.3 a cm-2) v 1 1 25 10 1 -1.480 2 25 10 5 -1.446 3 50 10 1 -1.437 4 50 10 5 -1.437 5 2 25 10 1 -1.466 6 25 10 5 -1.469 7 50 10 5 -1.400 8 50 10 1 -1.430 9 50 10 10 -1.400 10 50 10 20 -1.434 11 50 20 10 -1.463 12 50 20 5 -1.420 13 50 20 5 -1.393 14 3 50 20 5 -1.343 15 50 20 10 -1.324 16 25 20 10 -1.358 17 100 20 10 -1.309 18 100 20 10 -1.267 19 50 20 10 -1.270 20 50 10 5 -1.284 21 25 10 5 -1.295 dn -1.327 j. electrochem. sci. eng. 3(1) (2013) 29-36 ni-moo2 cathodes for hydrogen evolution 34 figure 2. the best polarization curves recorded in 1 m naoh at room temperature for samples deposited in the suspensions containing 1 g dm -3 moo2 (sample 4), 2 g dm -3 moo2 (sample 13), and 3 g dm -3 moo2 (sample 19). polarization curves for deposited ni (ni) and commercial de nora’s ni + ruo2 electrode (dn). morphology of electrodeposited samples the typical morphology of the electrodeposited samples is presented in fig. 3a. two different types of deposits could be clearly seen: a bright, compact deposit, rich in ni, around the position where the ni wires crossed, and a dark deposit characterized by a large number of cracks, rich in mo, located on the rest of the surfaces of the ni wires. the ni-rich and mo-rich parts of the surfaces were analysed by eds, as shown in fig. 3b. the results of the eds analysis are presented in table 2. figure 3. (a) sem of the surface of a typical deposit; (b) eds analysis of ni-rich and mo-rich surfaces. g. tasić et al. j. electrochem. sci. eng. 3(1) (2013) 29-36 doi: 10.5599/jese.2012.0027 35 table 2. chemical compositions of sample surface presented in fig. 3b. spectrum no. o content, at. % ni content, at. % mo content, at. % average values o content, at. % ni content, at. % mo content, at. % 1 36.45 50.57 12.98 37 51 12 2 30.57 58.24 11.19 6 45.43 44.31 10.26 3 75.14 4.08 20.77 76 5 19 4 76.04 4.74 19.22 5 76.10 5.13 18.77 as can be seen, spectra 1, 2, and 6 correspond to the ni-rich surface, while spectra 3, 4, and 5 correspond to the mo-rich surface. similar morphologies and compositions were obtained for samples electrodeposited from ammoniacal nicl2 solution [17–19], except that a larger part of the surface of the ni–moo2 composite coatings obtained in our previous work [17–19] was richer in ni, while a smaller part contained a high percentage of mo. in chlor-alkali electrolysis the efficiency of the cathodes results from the combination of a certain activity and stability at the high current densities (3–6 ka m -2 ) used in technological applications. one of the main reasons for the loss of activity and stability of cathodes during longterm operation is the so-called polarity inversion of the electrodes which takes place during the replacement of old electrodes of an electrolyser with new ones in zero-gap cells. during this operation anodes and cathodes are short-circuited, causing a reverse current flow which may damage the cathodes and negatively affect their activity for her [3]. manufacturers can predict how often this operation should be performed during a certain period of time and, in accordance with that, design appropriate accelerated “service-life” tests for cathodes. the service-life test of cathode materials that are promising for use in industrial chlor-alkali electrolysis is the subject of only two papers in the literature [18,21]. the procedure for testing the service life is based on a sequence of galvanostatic polarizations in the her range and cyclic voltammetries (cvs) in a wide potential range, inducing hydrogen evolution at its negative limit and oxygen evolution at its positive limit (reproducing the conditions of polarity inversion [18,21]). as shown in our previous paper [18] the main reason for the loss of activity of ni–moo2 composite coatings is the dissolution of mo-rich parts of the coating during the anodic polarization. although the morphology and composition analysis of ni–moo2 composite coatings obtained in this work indicates the presence of large amount of deposit with mo-rich parts, their performance during the service-life test will be the subject of our further work. conclusions in this work the influence of hydrodynamics, current density, and moo2 concentration on the electrodeposition of ni-moo2 coatings from a suspension of moo2 particles in a watt’s type bath was investigated by comparison of their polarization characteristics for her in 1 m naoh at room temperature. it was shown that electrolyte and air flow rates influence the process of coatings electrodeposition at low concentrations of moo2, while at higher concentrations of moo2 the current density became the main parameter influencing the coatings’ properties. the best coating was obtained from the suspension containing 3 g dm -3 of moo2 particles. this coating had a 57 mv lower over-voltage for the her at j = –0.3 a cm -2 than a commercial de nora’s ni + ruo2 electrode. j. electrochem. sci. eng. 3(1) (2013) 29-36 ni-moo2 cathodes for hydrogen evolution 36 acknowledgements: the authors are indebted to the ministry of education and science of the republic of serbia (project no. 172054) for the financial support of this work as well as to the department for the research and development of de nora industries s.p.a. for providing them with the commercial de nora’s cathode for hydrogen evolution in the chlor-alkali plants. references [1] c.j. winter, j. nitsch, hydrogen as an energy carrier: technologies, systems, economy, springer–verlag, berlin, new york, 1988 [2] j.o. jensen, v. bandur, n.j. bjerrum, s.h. jensen, s. ebbesen, m. mogensen, n. tophoj, l. yde, pre–investigation of water electrolysis, energinet.dk, 2008 [3] c. iwakura, m. tanaka, s. nakamatsu, h. noue, m. matsuoka, n. furukawa, electrochim. acta 40 (1995) 977-982 [4] l. brewer, in alloying, j.l. walter, m.r. jackson, c.t. sims, ed(s)., asm international, new york, usa, 1988, p. 1 [5] l. brewer l, p.r. wengert, metall. trans. 4 (1973) 83–103 [6] l. brewer, science 161 (1968) 115–122 [7] k.a. gschneider, in solid state physics: advances in research and applications, f. seitz, d. tunold, ed(s)., academic press, new york, usa, 1964, p. 275 [8] m.m. jakšić, č.m. lačnjevac, b.n. grgur, n.v. krstajić, j. new mat. electrochem. systems 3 (2000) 131-144 [9] j.m. jakšić, n.v. krstajić, b.n. grgur, m.m. jakšić, int. j. hydrogen energ. 23 (1998) 667-681 [10] p. sabatier, la catalyse en chimie organique, librairie polytechnique, paris, france, 1913 [11] n.v. krstajić, b.n. grgur, n.s. mladenović, m.v. vojnović, m.m. jakšić, electrochim. acta 42 (1997) 323-330 [12] b.n. grgur, n.m. marković, p.n. ross, j. phys. chem. b 102 (1998) 2494-2501 [13] g. wu, n. li, c.s. dai, d.r. zhou, mater. chem. phys. 83 (2004) 307-314 [14] a.c. tavares, s. trasatti, electrochim. acta 45 (2000) 4195-4202 [15] e. navarro-flores, z. chong, s. omanovic, j. mol. catal. a 226 (2005) 179-197 [16] n.v. krstajić, lj. gajić-krstajić, u. lačnjevac, b.m. jović, s. mora, v.d. jović, int. j. hydrogen energ. 36 (2011) 6441-6449 [17] n.v. krstajić, lj. gajić-krstajić, u. lačnjevac, b.m. jović, s. mora, v.d. jović, int. j. hydrogen energ. 36 (2011) 6450-6461 [18] v.d. jović, u. lačnjevac, b.m. jović, n.v. krstajić, electrochim. acta 63 (2012) 124-130 [19] u.č. lačnjevac, b.m. jović, v.d. jović, n.v. krstajić, j. electroanal. chem. 677–680 (2012) 3140 [20] borka m. jović, uroš lačnjevac,vladimir d. jović, ljiljana gajic-krstajić, nedeljko v. krstajić, j. serb. chem. soc., 77 (2012) 211-224 [21] a.l. antozzi, c. bargioni, l. iacopetti, m. musiani, l. vazquez-gomez, electrochim. acta 53 (2008) 7410-7416 © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {microfluidic paper based membraneless biofuel cell to harvest energy from various beverages} http://dx.doi.org/10.5599/jese.687 49 j. electrochem. sci. eng. 10(1) (2020) 49-54; http://dx.doi.org/10.5599/jese.687 open access: issn 1847-9286 www.jese-online.org original scientific paper microfluidic paper based membraneless biofuel cell to harvest energy from various beverages prakash rewatkar, sanket goel mems and microfluidics lab, department of electrical and electronics engineering, birla institute of technology and science (bits) pilani, hyderabad campus, hyderabad, 500078, india corresponding author: msanketgoel@gmail.com; tel.: + 91-40-66303686 received: march 14, 2019; revised: july 8, 2019; accepted: july 9, 2019 abstract the present work establishes the cost-effective and miniature microfluidic self-pumping paper based enzymatic biofuel cell (p-ebfc). the developed y-shaped p-ebfc consists of buckeye composite multiwall carbon nanotube (mwcnt) buckypaper (bp) based bioanode and bio-cathode that were immobilized with electro-biocatalytic enzymes glucose oxidase (gox) and laccase, respectively. the electrocatalytic activity of enzymes on electrode surface is confirmed using cyclic voltammetry (cv) technique. such immobilized bio-anode and bio-cathode show exquisite electrocatalytic activity towards glucose and o2, respectively. most appealingly, p-ebfc can directly harvest energy from widely available beverages containing glucose such as mountain dew, pepsi, 7up and fresh watermelon juice. this could provide potential application of p-ebfc as a portable power device. keywords paper based enzymatic biofuel cell (p-ebfc); beverages; carbon nanotubes (cnt); buckypaper (bp); cyclic voltammetry introduction in recent decades, research towards enzymatic biofuel cell (ebfc) has drawn remarkable attention and is generally considered as an efficient energy conversion device. energy conversion has been carried out by utilizing naturally procurable enzymes as bio-electrocatalyst to catalyze the fuel oxidation [1-3]. in ebfc, fuel (glucose) is oxidized at the anode side and oxidant such as o2 is reduced at the cathode side, producing a potential difference between bio-electrodes. due to biocompatibility, normal operating conditions (neutral ph and room temperature) and high enzyme selectivity, ebfc devices have widen the range of their application toward implantable devices, security applications, and harvest energy by using supercapacitors [4,5]. previously reported ebfcs are usually fabricated from a polymer material used for cell and carbon-based material used for http://dx.doi.org/10.5599/jese.687 http://dx.doi.org/10.5599/jese.687 http://www.jese-online.org/ mailto:msanketgoel@gmail.com j. electrochem. sci. eng. 10(1) (2020) 49-54 biofuel cell to harvest energy from beverages 50 electrodes with the purpose of increasing the surface to volume ratio (svr) [6]. such devices are, however, bulky, complex and rigid in structure. to overcome these drawbacks, light-weight, low-cost, flexible and biocompatible paper based enzymatic biofuel cell (p-ebec) was recently developed, and is gaining more popularity in the scientific community. in addition, the self-pumping via capillary transport mechanism eliminates the need of external pressure pump and a system for moving fluid towards bio-electrodes. even though an enormous development has been carried out towards the study of p-ebfc, several challenges still need to be fixed before it can be used in practical applications. from this perspective, extensive research has been carried out to miniaturize ebfc without compromising on the performance and sustainability. in previously reported paper based platforms, carbon-based electrode materials, expensive equipment, and time-consuming and complex additional redox co-factor based electrochemistry for the immobilization of bio-electrodes were used for the development of miniaturized power devices [7-11]. in addition, there are reports about ebfc that can harvest energy from human physiological fluidics such as blood, saliva, serum, urine and some commercial beverages [12-17]. however, the utilization of such fuels is expensive and usually needs the complicated extraction methods. therefore, a requirement to achieve miniaturization, simplified structure, and time-efficient and cost-effective redox co-factor free electrochemistry to fabricate bio-electrodes and p-ebfc is still actual. in earlier studies, our group made some attempts to overcome these challenges by establishing redox co-factor free biochemistry for enzyme immobilization and fabricating bio-electrodes [18]. to continue our work, a buckypaper (bp) is used as bio-electrodes by covalent immobilization with glucose oxidase (gox) and laccase without any redox co-factor. these immobilized bp exhibited excellent performance towards electrochemical redox reactions and provided potential application as portable and low-cost power device [19]. in order to test the developed p-ebfc for its potential to deliver the power from commercially available beverages, various commercial beverages are specifically utilized as substitutes for glucose. the obtained results realized cost-effective, handheld, green, renewable and easily accessible features of developed p-ebfc. experimental materials and reagents multiwall carbon nanotube (mwcnt) based buckeye composite based buckypaper (bp) was purchased from (nanotechlabs, inc.), usa. whatman #1 filter paper was purchased from (whatman). glucose oxidase (gox) from aspergillus niger, laccase from trametes versicolor and other analytical grade chemicals like d-(+)-glucose, p-benzoquinone, 2, 2-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid) (abts), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (edc), n-hydroxy succinimide (nhs), monosodium phosphate (nah2po4), disodium phosphate (na2hpo4), hydrochloric acid (hcl), sodium hydroxide (naoh), isopropyl alcohol (ipa) and acetone were purchased from sigma-aldrich. commercial beverages (mountain dew, pepsi, 7up and fresh watermelon juice) were purchased from a local market. throughout experimentation, 18.2 mω cm ultrapure milliq water (millipore) was used. 0.1 m phosphate buffer solutions (pbs) (ph 7.0 and 5.0) were used for bio-anode and bio-cathode electrochemical analysis. the p-ebfc construction is described in two sections: firstly, the fuel cell design and fabrication with mini platform, and secondly, the fabrication and immobilization of bp based bioanode and biocathode. p. rewatkar and s. goel j. electrochem. sci. eng. 0(0) (2019) 000-000 http://dx.doi.org/10.5599/jese.687 51 microfluidics fuel cell design and fabrication figure 1 illustrates a digital photo of the paper based enzymatic biofuel cell (p-ebfc). the y-shaped microchannel design (whatman paper) was cut using a commercially available computer controlled graphtec cutter and plotter (graphtec ce-2000, japan). this 50 mm long and 5 mm wide y-shape design comprised of two inlets for forwarding fuel (glucose) and oxidant (o2) due to the capillary action mechanism and harvest energy up to long duration. a small jig to hold bfc was designed using 3d printer (flashforge usa). this mini-platform consisted of two reservoirs to store electrolytes (anolyte and catholyte) and to transport electrolyte towards electrodes. this platform was also harnessed to hold p-ebfc which was adhered to glass side using a double-sided tissue tape. as shown in figure 1, external connections were established using alligator clips put at the ends of bio-electrodes. figure 1. digital image of paper based enzymatic biofuel cell (p-ebfc) operating under different beverages fabrication and immobilization of bp based bioelectrodes the buckeye composite mwcnt based bp was used to fabricate bio-electrodes. the fabrication and enzymes immobilization process were carried out on the basis of the previously published work by our group [18,20]. in brief, for the modification of bio-anode, bp was firstly cut to 15 x 8 mm dimensions and cleaned with isopropyl alcohol (ipa) to remove the contamination and then preserved in hot air oven (90 °c) for 2 hours. for the activation of carboxylic group, the fabricated bp was submerged in edc (30 mm)/nhs (90 mm) in 10 ml of milliq water (37 °c for 2 hours). thereafter, gox enzyme solution was prepared by dissolving 5 mg/ml pbs (0.1 m, ph 7.0) solution. developed bp based anode (15 × 8 mm) was immobilized into gox enzymes solution and left at room temperature for two hours. for bio-cathode, a similar procedure was carried out till the stage of immersion of bp in edc (30 mm) / nhs (90 mm) solution for 2 hours for the activation of the carboxyl group. thereafter, the bio-cathode was fabricated by dipping bp in the laccase enzyme solution (5 mg/ml pbs (0.1 m ph 5.0)) and kept under proper ventilation at room temperature for 2 hours. as shown in figure 1, these modified bio-electrodes were assembled in parallel on both sides of the y-shaped microchannel, keeping 1 mm between them. electrochemical measurements the electrochemical analysis was carried out with a conventional 3-electrode cell and a computer controlled potentiostat/galvanostat sp-150 (bio-logic science instruments, france). ag/agcl http://dx.doi.org/10.5599/jese.687 j. electrochem. sci. eng. 10(1) (2020) 49-54 biofuel cell to harvest energy from beverages 52 (3 m nacl) and platinum were used as reference and counter electrodes, respectively. in electrochemical measurements, cyclic voltammetry (cv) and open circuit potential (ocp) techniques were explored for fabricated bio-anode and bio-cathode. subsequently, the polarization performance of the p-ebfc was carried out. all the experiments were conducted at 10 mv/s scan rate at room temperature. results and discussion in the electrochemical studies, the cyclic voltammograms (cv) of immobilized bio-electrodes, measured at the scan rate of 10 mv/s and room temperature were carried out. in brief, for the bioanode analysis, 40 mm glucose and 1 mm p-benzoquinone was added into 5 ml pbs (0.1 m ph 7.0). as shown in figure 2 (a), a very small catalytic oxidation current peak can be noticed in the absence of glucose (red line), while a prominent catalytic oxidation current peak is noticed in presence of glucose (blue line). this peak corresponds to the electrocatalytic oxidation reaction of gox with a redox mediator and reveals the direct electrocatalytic activity of gox enzymes on the surface of bp. from the cv in fig. 2a, the maximum current density is estimated as 9.32 ma/cm2 at 0.33 v. figure 2. cyclic voltammetry (scan rate = 10 mv/s) of enzymes immobilized bio-electrodes at room temperature: (a) gox modified bio-anode in presence and absence of 40 mm glucose in 5 ml pbs (0.1 m, ph 7.0), (b) laccase modified bio-cathode in presence (air purged) and absence of oxygen (nitrogen purged) in 5 ml pbs (0.1 m, ph 5.0). similar cv measurements of bp based bio-cathode were evaluated in 5 ml pbs (0.1 m, ph 5.0), containing laccase with 1 mm abts as a cathodic mediator, in presence and absence of oxygen. as shown in figure 2(b), the small electrocatalytic reduction current peak can be noticed in the presence of laccase saturated with nitrogen gas (red line). more prominent electrocatalytic reduction current peak can clearly be observed in the presence of laccase saturated with air (blue line), confirming the successful immobilization of laccase enzymes on bp surface. from the cv in figure 2b, the maximum current density is estimated to be 2.25 ma/cm2 at 0.43 v. 3d printed mini-platform was realized to integrate the modified bioelectrodes on y-shaped paper microchannel and assemble the complete p-ebfc. for analyzing power performance, the polarization study was carried out using potentiostat. to characterize and analyze the harvested power from p-ebfc, widely available soft-beverages (mountain dew, pepsi, and 7 up) and fruit juice (fresh watermelon) containing glucose were used as a fuel feedstock. according to the tables of ingredients of different beverages, glucose contained in soft-drinks extracted less power compared p. rewatkar and s. goel j. electrochem. sci. eng. 0(0) (2019) 000-000 http://dx.doi.org/10.5599/jese.687 53 to the fresh watermelon juice. this observation may be attributed to the higher glucose concentration and additional complex additive components [21]. therefore, the output power was enhanced by diluting these beverages at a certain ratio. the polarization performance of all beverages is shown in figure. 3, and data are summarized in table 1, proving that beverages with additional glucose have strong potential to harvest more energy. figure 3. polarization performances of different beverages: (a) fresh watermelon juice, (b) 7 up, (c) mountain dew, and (d) pepsi table 1. summary of the polarization performance of different beverages beverages open circuit potential, v current density, µa/cm2 power density, µw/cm2 fresh watermelon juice 0.6 47 14.5 7 up 0.31 90 13.5 mountain dew 0.39 60 12 pepsi 0.32 41 6.15 conclusions herein, a portable, low-cost, simple structured and miniature microfluidic y-shaped p-ebfc is demonstrated using buckypaper (bp) bio-electrodes. the capillary flow mechanism and selfpumping features of p-ebfc eliminated the need for a membrane between the anolyte and catholyte regions. in addition to the power outputs from glucose, p-ebfc are shown to have the potential to directly harvest energy from commercially available beverages containing glucose. this http://dx.doi.org/10.5599/jese.687 j. electrochem. sci. eng. 10(1) (2020) 49-54 biofuel cell to harvest energy from beverages 54 simple, miniaturize and cost-effective p-ebfc has utilization potential for portable bioenergy device, which is the main subject of ongoing work in our lab. acknowledgements: the work was financially supported to a research initiation grant (rig) to sanket goel. references [1] d. kashyap, p. s. venkateswaran, p. k. dwivedi, y. h. kim, g. m. kim, a. sharma, s. goel, international journal of nanoparticles 8 (2015) 61-81. 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[21] x. li, l. zhang, l. su, t. ohsaka, l. mao, fuel cells 9 (2009) 85-91. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) {electrodeposition of bi-se thin films involving ethylene glycol based electrolytes} http://dx.doi.org/10.5599/jese.914 51 j. electrochem. sci. eng. 11(1) (2021) 51-58; http://dx.doi.org/10.5599/jese.914 open access: issn 1847-9286 www.jese-online.org original scientific paper electrodeposition of bi-se thin films involving ethylene glycol based electrolytes sevinj piri javadova, vusala asim majidzade, akif shikhan aliyev, asmat nizami azizova and dilgam babir tagiyev institute of catalysis and inorganic chemistry named after acad. m. nagıyev azerbaijan national academy of sciences (anas), baku, az 1143, azerbaijan corresponding author: vuska_80@mail.ru, phone: +99450 640 02 25 received: october 10, 2020; revised: january 5, 2021; accepted: january 6, 2021 abstract the work is devoted to the electrochemical deposition of bi-se thin films from ethylene glycol-based electrolytes. the studies have been carried out by potentiodynamic and galvanostatic methods under various conditions, using pt and ni electrodes. by recording cyclic and linear polarization curves, the potential ranges of deposition of thin bi-se films on pt (-0.75 to -1.2 v) and ni (0.2 to -0.85 v) electrodes were determined. a comparison of the polarization curves of two electrodes showed that co-electrodeposition of bi and se occurs in approximately the same potential range. in order to find the optimal mode and composition of the electrolyte, the effect of various factors (concentration of initial components, temperature, etc.) on the process of co-electrodeposition of bi with se was studied. in addition, the samples of bi-se thin films obtained on ni electrodes using the galvanostatic method were studied by scanning electron microscope (sem) and x-ray phase analysis. the results of x-ray phase analysis confirmed the formation of thin bi2se3 films with and without additional heat treatment step. elemental analysis of obtained films carried out by eds shows that films contained 62.79 wt.% bi and 37.21 wt.% se. keywords bismuth (iii) selenide; thin films; co-deposition; ethylene glycol; polarization introduction the use of thin semiconductor films is widespread in the modern world [1-5]. they are applied in various fields of technology, microelectronics, and electronics for the production of photoresistors, laser materials, optical recording devices, etc., and also in solar cells for energy production and storage [6-7]. therefore, finding new promising and optimal methods and conditions for obtaining of semiconducting thin films is quite important task. http://dx.doi.org/10.5599/jese.914 http://dx.doi.org/10.5599/jese.914 http://www.jese-online.org/ mailto:vuska_80@mail.ru j. electrochem. sci. eng. 11(1) (2021) 51-58 electrodeposition of bi-se thin films 52 bismuth selenide (bi2se3) is a layered semiconductor with a straight band gap 0.3-0.4 ev in the monocrystalline form and 0.9-2.3 ev in a thin film [8-10]. bi2se3 is important member of a2vb3vi family of compounds and has high absorption coefficient in the visible and near-infrared regions. as an important n-type chalcogenide, bi2se3 has many important characteristics such as high electrical conductivity [11], conspicuous thermoelectric property [12], photosensitivity [13], electrochemical property [8], and photoconductivity [14]. besides, bi2se3 is a popular topological insulator [15-17] and has unique property of conducting surface states and isolated bulk states. up to now, to synthesize bi2se3, numerous methods, such as pulsed magnetron sputtering, molecular beam epitaxy, organometallic chemical vapor deposition, hydrothermal deposition, sequential ionic layer absorption and reaction, chemical deposition, electrochemical deposition, etc. have been used [18-25]. electrochemical obtaining of semiconductor chalcogenide films is enlarging day by day, mostly due to several advantages of the deposition method. these advantages include short deposition time periods and reasonable operation temperatures. moreover, through the proper selection of electrolyte composition, various types of the alloy may be obtained, allowing also the covering of large and complex substrates. also, the parameters that affect electrochemical deposition process can be easily controlled, carrying out the studied process in the strictly desired direction. application of electrochemical deposition does not require complex instruments, devices and procedures, and is therefore considered economically beneficial. the present research aims to obtain thin films of semiconductor material bi2se3 by the electrochemical deposition method. also, this work would be a certain continuation of our previous work about the electrochemical obtaining of chalcogenide compounds [2,4,26-31]. experimental the recording of potentiodynamic polarization curves was carried out using a potentiostat "iviumstat electrochemical interface", equipped with a computer programmed by the iviumsoftware. during potentiodynamic experiments, a three-electrode electrochemical cell with a volume of 100 ml was used. as a working electrode, pt wire with an area of 0.3 cm2 and a ni-electrode with an area of 2 cm2 were taken. a saturated silver/silver chloride (ag/agcl/kcl) electrode was used as a reference electrode, and a pt plate with an area of 4 cm2 was used as an auxiliary electrode. experiments were performed at 313-323 k. the study of the morphology and determination of the elemental composition (energy dispersive spectroscopy, eds) of the electrodeposited bi-se samples were carried out using the scanning electron microscope of carl zeiss sigma brand (sem). the phase composition of the obtained thin layers was determined using an x-ray phase d2 phazer analyzer from a bruker company, germany (cukα filter, ni). the electrolyte used in the experiments was composed from 0.1 м bi(no3)3·5h2o (supplied by jsc vekton, russia) and 0.03 м h2seo3 (supplied by llc reachim, russia), dissolved in ethylene glycol (supplied by pjsc sibur-neftechim, russia). since pt electrodes require periodic cleaning, before the experiments they were cleaned in concentrated nitric acid, and then rinsed with bi-distilled water. after each experiment, pt electrode was kept for 30 minutes in the boiling nitric acid that contains a small amount of ferric chloride. after that, it was thoroughly washed with tap water, and then with distilled water. at the end, pt electrode surface was rinsed with alcohol or acetone [32]. ni electrodes were firstly polished electrochemically in the concentrated hno3 acid, and then reduced in a solution consisting of h2so4, h3po4, and citric acids (t = 293-303k, i = 500 ma/cm2, τ = 180 s). at the end, they were thoroughly washed with distilled water [32]. s. p. javadova et al. j. electrochem. sci. eng. 11(1) (2021) 51-58 http://dx.doi.org/10.5599/jese.914 53 results and discussion electrodeposition of two components is more complex than deposition of individual components, because co-deposition requires stricter control of the electrolyte composition and deposition conditions. therefore, prior to co-deposition experiments, it would be necessary to study deposition of initial components separately. figure 1 shows cyclic polarization curves of pt electrode in the pure electrolyte (ethylene glycol), electroreduction of individual components (bi and se), and co-electrodeposition of bi with se. in comparison with an aqueous electrolyte, reduction processes in non-aqueous media occur at more negative potentials and with lower currents [33,34]. this is due to very low electrical conductivity of non-aqueous solutions, what prevents rapid arrival of ions to the electrode surface. in pure ethylene glycol solution (curve 1), pt electrode is in almost polarizable state, and ions are accumulated at the surface forming electrical double-layer. the reduction of selenite (seo32-) ions to se2ions (curve 2) occurs in one stage with the observation of a cathodic plateau at potentials of -0.3 to -0.5 v, which should be assigned to the reduction of selenite ions. electrochemical reduction of bismuth (bi3+) ions (curve 3) occurs within the potential interval of -0.05 to -0.75 v. at the potential of -0.05 v, bi3+ ions are reduced to bi0, and the first trace of bismuth appears on the pt surface and with increasing potential, in a more negative direction the deposition of bi is accelerated, and pt surface becomes covered with a black layer. figure 1. cathodic polarization curves (0.02 v s-1) of pt electrode in: ethylene glycol (c6h8o7) (curve 1), 0.03 m h2seo3 +c6h8o7 (curve 2), 0.07 m bi(no3)3×5h2o + c6h8o7 (curve 3) and 0.07 m bi(no3)3×5h2o + 0.03 m h2seo3 + c6h8o7 (curve 4); t = 298 k. co-electrodeposition of bismuth with selenium (curve 4) from ethylene glycol on pt electrode occurs within the potential range of -0.05 to -1.5 v in two stages. starting from -0.05 v and down to -0.6 v, the electroreduction of bi3+ ions occurs, while at about -0.6 v, co-deposition of bi and se http://dx.doi.org/10.5599/jese.914 j. electrochem. sci. eng. 11(1) (2021) 51-58 electrodeposition of bi-se thin films 54 occurs. with further potential decrease down to -1.2 v, electrochemical deposition is accelerated, the current consumed for the process is increased, and the electrode becomes completely covered with a black layer. in comparison with deposition potentials of individual bi (-0.05 v) and se (-0.3 v), their co-electrodeposition occurs at more anodic potential of -0.6 v. the process of co-electrodeposition of bismuth with selenium has also been studied on a porous nickel electrode. from the comparison of cathodic polarization curves of pt (figure 1, curve 4) and ni (figure 2) electrodes in ethylene glycol containing bi and se ions, it is clear that co-electrodeposition of bi and se on ni electrode occurs by forming the cathode plateau at potentials of -0.4 to -0.85 v. after determining the potential range of co-deposition, the effect of various factors on the process was studied by the potentiodynamic method to find the optimal electrolysis mode and electrolyte composition. as is already known, one of the main factors affecting the electrochemical process is temperature. the effect of temperature on the co-deposition process was studied in the range of 298-338 k. figure 2. cathodic polarization curve (0.02 v s-1) of co-deposition of bi with se on ni electrode. electrolyte composition (м): 0.07 bi(no3)3×5h2o + 0.03 h2seo3 + c6h8o7; т = 298к. figure 3. cathodic polarization curves (0.02 v s-1) of co-deposition of bi with se on pt electrode from ethylene glycol solution containing (0.07 m bi(no3)3×5h2o + 0.03m h2seo3) at t/k = 298 (1), 308 (2), 318 (3), 328 (4), 338 (5). cathodic polarization curves of the experiments shown in figure 3 demonstrate that with increasing of temperature, the beginning of co-electrodeposition of bismuth with selenium is shifted positively to 0.0 v, while the current spent on the process increased up to -20 ma. the influence of the concentration of initial components was studied separately. the effect of the concentration of bismuth ions was investigated in the range of 0.05-0.11 mol/l, keeping the concentration of selenium ions constant. the research results are presented in figure 4, showing that with an increase in the concentration of bi ions, the potential of electrodeposition moved towards a positive direction. also, with an increase in the number of bismuth ions in the electrolyte, co-deposition was accelerated, and the current consumed on the process increased. a similar situation is seen in figure 5 showing cathodic polarization curves of co-electrodeposition process of bi and se for different concentrations of selenite ions at the constant concentration of bismuth ions. the influence of the concentration of selenite ions was studied in the range 0.018-0.054 mol/l. as is seen from figure 5, with an increase in the concentration of selenite ions, the process of co-deposition also accelerates. in this case, however, acceleration affects detrimentally onto stoichiometric composition and quality of the obtained thin layers. hence, it has been determined by s. p. javadova et al. j. electrochem. sci. eng. 11(1) (2021) 51-58 http://dx.doi.org/10.5599/jese.914 55 experiments that thin films close to the stoichiometric composition are obtained at 0.07 mol/l concentration of bismuth ions and 0.03 mol/l concentration of selenite ions. figure 4. cathodic polarization curves (0.02 v s-1) of co-deposition of bi with se on pt electrode from ethylene glycol solution containing 0.03 m h2seo3 and different concentrations of bi(no3)3×5h2o (m): 0.05 (1), 0.07 (2), 0.09 (3), 0.11 (4); т = 298 к. figure 5. cathodic polarization curves (0.02 v s-1) of co-deposition of bi with se on pt electrode from ethylene glycol solution containing 0.07 m bi(no3)3×5h2o and different concentrations of h2seo3 (m): 0.018 (1), 0.03 (2), 0.042 (3), 0.054 (4); т = 298 к. after determining the potential range and studying the influence of some factors on the process of co-deposition of thin bi-se films, the samples were deposited on ni electrode in the galvanostatic mode at the current density of 9 10 ma/cm2. figure 6 shows x-ray diffraction pattern, morphology, and elemental composition of the obtained bi-se film. figure 6. (a) x-ray pattern, (b) morphology and (c) eds spectrum of bi-se film formed on ni electrode by co-deposition from ethylene glycol containing 0.07 m bi(no3)3×5h2o+0.03 m h2seo3; т = 298 к. http://dx.doi.org/10.5599/jese.914 j. electrochem. sci. eng. 11(1) (2021) 51-58 electrodeposition of bi-se thin films 56 using x-ray analysis, it has been established that bi2se3 thin films are obtained as a result of these experiments. the diffraction peaks observed in figure 6a at 2 angles close to 18, 29, 38 and 58, respectively are proper to rhombohedral bi2se3. clear peaks observed at 2 45 and 52 belong to ni substrate. the size of grains is changed within a fraction of 1 to 5 microns that is shown in figure 6b. the variation in the size of various grains is due to high porosity of ni substrate that allows them to grow freely during deposition. the elemental content presented in figure 6c indicates that the film mainly consists of bi and se, what demonstrates its chemical purity. eds data show that film contains 62.79 wt.% bi and 37.21 wt.% se. the presence of nickel atoms in the spectrum is due to ni substrate. as is commonly seen from figure 6, the results of freshly deposited film confirm the deposition of bi2se3 films. figure 7 presents the results for the deposited film thermally treated at 673 k in argon medium for 30 minutes. x-ray pattern (figure 7a), sem image (figure 7b) and eds results (figure 7c) show not much changes compared to figure 6. it seems, however, that after the thermal treatment, bi2se3 film becomes more crystalline. therefore, depending on the field of application, thin semiconductor films of bi2se3 can be obtained by either heat treatment or without it. figure 7. (a) x-ray pattern, (b) sem image and (c) eds spectrum of bi2se3 thin film presented in figure 6 after heat treatment at 673 k. conclusion electrochemical co-deposition of bismuth and selenium on pt and ni electrodes from ethylene glycol was investigated by the potentiodynamic method. co-deposition of bismuth with selenium from ethylene glycol on pt electrode occurs at the potential -0.6 v. on ni electrode, co-deposition is carried out with a cathode plateau formed in the potential range of -0.4 to -0.85 v. to find the s. p. javadova et al. j. electrochem. sci. eng. 11(1) (2021) 51-58 http://dx.doi.org/10.5599/jese.914 57 optimal mode and composition of the electrolyte, the influence of various factors (concentration of initial components, temperature, etc.) on the co-deposition process was studied. according to the experimental results, the electrolyte with the composition of 0.07 bi (no3)3×5h2o + + 0.03 h2seo3 + c6h8o7 and t = 298 k can be used to obtain thin films of bi2se3 with a composition close to the stoichiometric ratio. the deposits were also exposed to thermal treatment at 673 k in argon atmosphere. for both fresh and thermally treated samples, formation of thin bi2se3 films was confirmed. films that undergone thermal treatment, however, was found more crystalline. acknowledgement: this work was carried out with the financial support of the national academy of sciences of azerbaijan within the framework of research programs in priority areas in 2019-2020. references [1] r. henríquez, c. vasquez, n. briones, e. muñoz, p. leyton, e. a. dalchiele, international journal of electrochemical science 11 (2016) 4966-4978 https://dx.doi.org/10.20964/2016.06.17. 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[34] v. a. majidzade, p. h. quliyev, y. babayev, a. sh. aliyev, nakhchivan state university scientific works 80 (2016) 140-144. . ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.electacta.2009.06.089 https://doi.org/10.1016/j.electacta.2009.06.089 https://doi.org/10.1039/b306662m https://doi.org/10.1016/j.jssc.2004.06.042 https://doi.org/10.1039/c3ce42056f https://doi.org/10.1016/s0169-4332(01)00461-5 https://doi.org/10.1016/s0927-0248(97)00237-7 https://doi.org/10.32737/2221-8688-2018-3-331-336 https://doi.org/10.32737/2221-8688-2018-3-331-336 https://doi.org/10.1134/s0020168519100108 https://www.researchgate.net/deref/http%3a%2f%2fdx.doi.org%2f10.32737%2f0005-2531-2019-1-6-13?_sg%5b0%5d=fgx4qfdupfjykca_mo2dctprkv4_z_fyddhw3eyl--mcj4w6h7mw5nxe7mrk-ra56zgxhqsnnmiflozdp-axxrb9fw.2wceelnyg2qtivwbfbtg_liziotrx61cklelauxt6qv9y6l2mm64yvl9oiugdux48g15f4k5kwbtvxr4eabvbw https://www.researchgate.net/deref/http%3a%2f%2fdx.doi.org%2f10.32737%2f0005-2531-2018-3-6-10?_sg%5b0%5d=o8ksyzvf7dgnoqvtkra0mm0x-veiivja3p_tbxd9av6zpg8zlswpyb_hsxydqxkmanp5nx9uydh69faatexh3nrxna.xstlcuayazgl6g5tdjzzrt8pgm5qroqd6jrxnci3otqeeyigkmxyvatbvippe4vyrwdcf4lf14nl_wdr21xfla https://doi.org/10.5599/jese.490 https://creativecommons.org/licenses/by/4.0/) influence of ceramic separator’s characteristics on microbial fuel cell performance doi: 10.5599/jese.2014.0047 315 j. electrochem. sci. eng. 4(4) (2014) 315-326; doi: 10.5599/jese.2014.0047 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of ceramic separator’s characteristics on microbial fuel cell performance anil n. ghadge, mypati sreemannarayana, narcis duteanu* and makarand m. ghangrekar department of civil engineering, indian institute of technology, kharagpur -721302, india *university “politehnica” of timisoara, industrial chemistry and environmental engineering, 2 victoria sq., 300006 timisoara, romania corresponding author: ghangrekar@civil.iitkgp.ernet.in; tel.: +91-3222-283440 received: january 27, 2013; revised: february 15, 2014; published: december 6, 2014 abstract this study aimed at evaluating the influence of clay properties on the performance of microbial fuel cell made using ceramic separators. performance of two clayware microbial fuel cells (cmfcs) made from red soil (cmfc-1) typically rich in aluminum and silica and black soil (cmfc-2) with calcium, iron and magnesium predominant was evaluated. these mfcs were operated under batch mode using synthetic wastewater. maximum sustainable volumetric power density of 1.49 w m -3 and 1.12 w m -3 was generated in cmfc-1 and cmfc-2, respectively. during polarization, the maximum power densities normalized to anode surface area of 51.65 mw m -2 and 31.20 mw m -2 were obtained for cmfc-1 and cmfc-2, respectively. exchange current densities at cathodes of cmfc-1 and cmfc-2 are 3.38 and 2.05 times more than that of respective anodes, clearly indicating that the cathodes supported much faster reaction than the anode. results of laboratory analysis support the presence of more number of exchangeable cations in red soil, representing higher proton exchange capacity of cmfc-1 than cmfc-2. higher power generation was observed for cmfc-1 with separator made of red soil. hence, separators made of red soil were more suitable for fabrication of mfc to generate higher power. keywords cation exchange capacity; coulombic efficiency; charge transfer resistance; charge transfer coefficient; exchange current density; power density; wastewater treatment introduction recently considerable attention is being paid on the two major problems of the world, which are namely maintaining quality of water body and energy crisis. solution to these problems could be provided by microbial fuel cell (mfc) to treat organic matter present in wastewater and http://www.jese-online.org/ mailto:ghangrekar@civil.iitkgp.ernet.in j. electrochem. sci. eng. 4(4) (2014) 315-326 ceramic separator in microbial fuel cell 316 simultaneously produce bio-electricity [1-5]. although, considerable progress has been achieved in the performance of a mfc in the past ten years, one of the main challenge for commercializing scalable mfcs is the high cost and low mechanical strength of the separator materials used for fabrication of this device. tian et al. [6], demonstrated that placing of an anaerobic membrane filtration process sequentially with an mfc accomplished efficient nutrients removal with low propensity of membrane fouling. it was reported that the use of poly(tetrafluoro-ethylene) (ptfe) layered activated charcoal electrode and zirfon® as separator, improved mfcs performance and can be used to replace costly polymeric membrane and expensive catalyst in mfcs [7]. proton exchange membrane (pem) such as nafion [8], nano-composite membrane made of sulfonated polymer (ether ether ketone) and montmorillonite clay [9], nano-composite membranes of nafion and montmorillonite clay [10] were used in the mfcs to separate anodic chamber from cathodic chamber. however, these polymeric membranes or composite membranes are costly; hence, limit the practical application of the mfcs. ceramic membranes are found to be promising materials in mfcs because of their low production cost and better structural strength, thus, providing an alternative for the costly polymeric membranes [11-15]. application of such ceramic membranes in mfc has been practiced since last ten years and its utility has been demonstrated through different studies. this was the first attempt where park and zeikus [11] developed a porcelain septum separator for single chambered mfc using 100% kaolin and found comparable performance with that dual chambered mfc. a three layered cathode composed of a cellulose acetate film, a ceramic membrane, and a porous graphite plate to create a single chamber mfc that linked with solar cell to enhance power generation [12]. behera and ghangrekar [13] studied the effect of different thickness of such ceramic membrane on performance of the dual chambered mfc, and reported better power output for mfc having smallest thickness of the membrane. use of terracotta pot for making single chamber mfc, after coating outer surface with conductive graphite paint, demonstrated coulombic efficiency of 21 ± 5 % with power density of 33.13 mw m -2 [14]. more recently, winfield et al. [15] compared performance of mfcs made from terracotta and earthenware by considering wall thickness, porosity and cathode hydration. more porosity of earthenware proved to be the better material compared to terracotta. however, these studies do not include the effect of soil ph, conductivity and cation exchange capacity (cec) on the performance of mfcs. for effectual use of such ceramic membranes, made from clay minerals, they should have higher cation exchange capacity. the existence of the ph dependent charge portion of the cation exchange capacity of soils is widely accepted for many years [16]. electrical conductivity of the soil is the measure of salt concentration in the soil solution. bulk electrical conductivity of soil is generally assumed to be dominated by the electrical conductivity of the soil solution, with perhaps a small contribution from surface charges associated with soil solids [17]. in mfcs, the rate of proton consumption at the cathode is often higher than the transfer rate through the membrane [18,19]. hence, for enhancing power generation of this device the separator used should offer higher rate of proton/cation transfer. the transfer of protons from a protonated species to an uncharged molecule at the surface of the clay mineral is an important process [20]. the soil used for making ceramic separator in mfc participates in exchange of cations from anodic chamber to cathodic chamber. protons released during the oxidation of organic matter from the anodic chamber are being adsorbed on to the surface of the soil by replacing the loosely held cations. the layered silicate clay minerals like smectite clays, show attractive hydrophilic properties and good thermal stability at high temperature [21]. the layered a. n. ghadge at al. j. electrochem. sci. eng. 4(4) (2014) 315-326 doi: 10.5599/jese.2014.0047 317 silicates commonly used for proton exchange membrane fuel cell applications are montmorillonite made of silica tetrahedral and alumina octahedral sheets which has advantageous hygroscopic properties [22,23]. the cations ca 2+ , mg 2+ , k + and na + are called the base cations and h + and al 3+ are called acidic cations. the acidity of the soil is the amount of the total cation exchange capacity (cec) occupied by the acidic cations [24]. more than proton, this cation migration also affects the performance of cathode, hence overall performance of mfc. porosity of the soil represents the hydraulic conductivity which depends upon the pore throat radii of clay materials. typically clays have very low hydraulic conductivity due to their small throat radii. for mfc made with such clayware separator, different soil porosities play a vital role in the seepage of substrate from anodic to cathodic chamber [25]. apart from loss of fuel, this may lead to the availability of organic matter at higher concentration on the cathode, supporting heterotrophic bacterial growth on cathode and thereby reducing cathode potential. under such circumstances, the cathode often gives negative potentials (vs. ag/agcl), than the positive potential it is expected to give, while using oxygen as an electron acceptor [26]. hence, hydraulic conductivity and cation exchange capacity are the important properties of the materials to be selected as a separator in mfc. the objective of this study was to investigate the effect of different soil properties like ph, conductivity, porosity, cation exchange capacity on the performance of mfcs having ceramic separators made from two different soils. in addition, the electrode reaction kinetics was investigated for assessing performance of these mfcs. experimental construction of microbial fuel cell the study was carried out using dual-chambered clayware microbial fuel cells (cmfcs). the anodic chambers of these cmfcs were made up of baked clayware pot and the wall material of the pot (about 5 mm thick) itself acted as a separator allowing transfer of protons from anode to cathode. the pots were made from the red soil (typically rich in aluminum and silica) in cmfc-1 and black soil (rich in calcium, iron and magnesium predominant) in cmfc-2. the anodic chamber of the cmfc-1 and cmfc-2 had a liquid volume capacity of 550 ml and 700 ml, respectively. cathodic chambers in both the mfcs were made up of plastic container having 5 liter capacity. although there is difference in anodic chamber volume of both the mfcs, however, cathodic chamber volume of 5 litre, which was kept same in both the mfcs. it is important to note here that the rate of proton transfer largely depends on the separator area to anodic chamber volume ratio (sa/v). in the present study, this ratio was 83.3 and 86.5 m 2 /m 3 , respectively, for cmfc-1 (separator made of red soil) and cmfc-2 (separator made of black soil), which indicates that there was no significant difference in sa/v ratio. carbon felt (panex®35, zoltek corporation) with 230 cm 2 and 261 cm 2 projected surface areas were used as cathode in cmfc-1 and cmfc-2, respectively. anodes in cmfc-1 and cmfc-2 were made from stainless steel mesh having total surface area of 268 cm 2 and 304 cm 2 , respectively. an aquarium aerator was inserted at the bottom of cathodic chamber to supply air continuously with an aquarium air pump (sobo aquarium pump, china). the connections between two electrodes were made with concealed copper wire through external resistance of 100 ω. j. electrochem. sci. eng. 4(4) (2014) 315-326 ceramic separator in microbial fuel cell 318 inoculation and operation of cmfcs anaerobic mixed sludge collected from septic tank was used as an inoculum in the anodic chamber of the cmfcs. when mixed anaerobic sludge is used as source of inoculum, it contains both electrogenic as well as non-electrogenic (mostly methanogenic) bacteria. in the anodic chamber of mfc, it is necessary to dominate electrogenesis to obtain higher coulombic efficiency. methanogens in the mfcs compete for substrate and electrode space with electrogenic bacteria and reduce the power output. therefore, the inoculum sludge was given a heat pre-treatment (heated at 100 °c for 15 min) to suppress methanogens and required amount of sludge was added to the anodic chamber [27]. synthetic wastewater containing sodium acetate as a source of carbon with chemical oxygen demand (cod) of about 3000 mg l -1 was used in this study. the sodium acetate medium was prepared by adding 3843 mg l -1 ch3coona, 4500 mg l -1 nahco3, 954 mg l -1 nh4cl, 81 mg l -1 k2hpo4, 27 mg l -1 kh2po4, 750 mg l -1 cacl2.2h2o, 192 mg l -1 mgso4.7h2o and trace metals like fe, ni, mn, zn, co, cu, and mo as per the composition given by [27]. the feeding frequency of 5 days was adopted. these cmfcs were operated at temperatures varying from 33 to 37 °c under batch mode. the ph of tap water used as catholyte remained in the range of 8.2-8.5; whereas, anolyte ph was in the range of 7.1-7.4. analysis and calculations the ph and conductivity of anolyte and catholyte was measured using ph meter (cyber scan ph 620) and tds meter (cyber scan cd 650, eutech instruments, singapore), respectively. cod concentrations were measured according to apha standard methods [28], using closed reflux method. the performance of cmfcs was evaluated in terms of voltage (u) and current (i) measured using a digital multimeter with data acquisition unit (agilent technologies, malaysia) and converted to power according to p = ui, where p = power, w; i = current, a; and u = voltage, v. power density and power per unit volume were calculated by normalizing power to the anode surface area and net liquid volume of anodic chamber, respectively. the current density id was calculated using d ext d u i a  r (1) where, rext is the external resistance (ω) and ad (m 2 ) is the surface area of the anode. polarization studies were carried out by varying the external resistance from 10000 to 10 ω using the resistance box (gec 05 r decade resistance box) and cell voltages (u) were recorded. internal resistance of the cmfcs was measured from the slope of the line from plot of voltage versus current [29]. columbic efficiency (ce) was determined by integrating the current measured over time, t, and compared with the theoretical current on the basis of cod removal and calculated as [1]: 0 d = t m i t ce fbv cod  (2) where, v is the volume of the anodic chamber of mfc; m = 32, molecular weight of oxygen; f, faraday’s constant = 96485 c mol -1 ; b = 4, the number of electrons exchanged per mole of oxygen; δcod is the difference in the influent and effluent cod for time t. a. n. ghadge at al. j. electrochem. sci. eng. 4(4) (2014) 315-326 doi: 10.5599/jese.2014.0047 319 analysis of the soil properties the ph and the conductivity of the soil samples were measured according to the indian standard method of test for soils. indian standard is: 2720 (part 26) – 1987 was used for determination of ph value [30] and conductivity of the soil was measured according to is 14767: 2000 [31]. cation exchange capacity of the soil was measured according to indian standard, is: 2720 (part 24) – 1976 [32]. chemical constituents for the red and black soils were obtained through the x-ray fluorescence (xrf) analysis. porosity of the soil was indirectly measured from the percentage water absorbed by the clayware pot made from respective soils after immersing in water for 24 hours. reaction kinetics at electrodes tafel plot, as derived from equation (3) [33], was employed to measure the reaction kinetics for working electrode (anode and cathode) and ag/agcl was used as the reference electrode. the reference electrode was placed in the working chamber during the measurements. 0 ln f rt i i               (3) where i0 is exchange current density, i is the electrode current density (ma m -2 ),  is the electron transfer coefficient, r is the ideal gas constant (8.31 j mol -1 k -1 ), f is the faraday’s constant (96,485 c mol -1 ), t is the absolute temperature, k and η is the activation overpotential. purpose of using tafel plot is to calculate the i0 and  value. the i0 is a fundamental parameter in the rate of electro-oxidation or electro-reduction of a chemical species at an electrode at equilibrium. the charge transfer resistance (rct) was calculated from the following equation: ct 0 rt r nfi  (4) where, n is the number of electrons. results and discussion physico-chemical properties of the soil used in cmfcs the soils used for manufacturing the pots showed different ph. the electrical conductivity and cation exchange capacity of the red soil is higher than that of the black soil, indicating usefulness of the former in the clayware separator application (table 1). however, the porosity of the pot made from black soil was higher than the pot made from red soil. higher porosity may allow the anolyte to come to the cathode resulting in not only the physical substrate loss but also it will allow oxygen to penetrate in anodic chamber, reducing coulombic efficiency of the system due to direct oxidation of the substrate. table 1. physical, chemical and electrical properties of red and black soil used for making separators sl. no soil properties red soil (cmfc-1) black soil (cmfc-2) 1 ph 7.4 8.5 2 porosity, % 11.6 17.6 3 electrical conductivity, ms cm -1 2.403 0.045 4 cation exchange capacity (cec), mmol (kg soil) -1 125 20 j. electrochem. sci. eng. 4(4) (2014) 315-326 ceramic separator in microbial fuel cell 320 wastewater treatment after inoculating the anodic chamber of the cmfcs with heat pretreated anaerobic mixed consortia, synthetic feed was supplied and wastewater treatment performance of the cmfcs under different feed cycles was observed. average cod removal efficiency of 78.9 ± 3.9 % and 89.6 ± 3.2 % was observed in cmfc-1 and cmfc-2, respectively. cod removal efficiency in the cmfc-2 was higher than the cmfc-1. it was observed that porosity of clayware pot used in cmfc-2 was 52 % higher (table 1) than that of cmfc-1, because of which probably it has permitted more diffusion of oxygen from cathodic chamber to anodic chamber to support aerobic oxidation of fraction of substrate present in anodic chamber to establish higher cod removal efficiency. the oxygen diffusion coefficient in the range of 5.38 x 10 -6 to 6.67 x 10 -6 cm 2 s -1 is reported in early studies for this clayware separator by behera and ghangrekar [13]. in addition, due to more porosity of separator used in cmfc-2, exchange of water molecules due to osmosis across the membrane might have diluted the anolyte, resulting in higher cod removal rate. electricity generation performance of cmfcs was evaluated by measuring the open circuit voltage and operating voltage. the current and voltage gradually increased with time of operation. the maximum voltage across 100 ω resistance of 286 mv and 280 mv was observed in cmfc-1 and cmfc-2, respectively. cmfc-1 generated a maximum sustainable power density (normalized to the anode surface area) and volumetric power (normalized to the working volume of anodic chamber) of 30.5 mw m -2 and 1.49 w m -3 (fig. 1), respectively; whereas, cmfc-2 generated power density of 25.7 mw m -2 and volumetric power of 1.12 w m -3 . the power produced by cmfc-1, made from red soil, was 1.33 times higher than the cmfc-2 wherein the separator was made from black soil with lower cec and electrical conductivity. it is interesting to note here that in spite of having higher separator area and more liquid volume (anodic chamber) for cmfc-2, it generated less power compared to cmfc-1. figure 1. volumetric power density of cmfc-1 and cmfc-2 0 2 4 6 8 10 12 14 16 18 20 22 0.4 0.6 0.8 1.0 1.2 1.4 1.6 p o w e r d e n si ty , w m -3 time, days mfc-1 mfc-2 a. n. ghadge at al. j. electrochem. sci. eng. 4(4) (2014) 315-326 doi: 10.5599/jese.2014.0047 321 effect of chemical properties of soil used for making separator on electricity generation the cec of red soil is 6.25 times (table 1) higher than black soil, indicating more number of exchange sites are available for the transfer of cations in red soil. due to availability of more exchange sites in cmfc-1, better transfer of the protons occurred to improve the power generation in cmfc-1 compared to cmfc-2. in addition, the xrf data (table 2) confirms that the aluminum content of the red soil is more than the black soil which makes the red soil more acidic than black soil. the ph of the red soil (table 1) was lower than black soil confirming that the red soil is more acidic and has high capacity to hold the h + ions which improved the performance of cmfc-1 in terms of power generation. electrical conductivity is the measure of salt concentration in the soil solution. soils high in smectite often exhibit high electrical conductivity due to water associated with the clays. the soil with high montmorillonite mineral can act as better proton exchange material due to its hydrophilic nature and the high cation exchange capacity [34]. the conductivity of soil used as separator in cmfc-1 is almost 53.4 times (table 1) more compared to cmfc-2, authenticating utility of the red soil for making separator to harvest more power from the cmfcs. table 2. chemical compounds present in red and black soil sl. no compound content, % sl. no compound content, % red soil black soil red soil black soil 1 na2o 3.95 0.273 14 co 0.406 0.441 2 mgo 0.654 3.86 15 ni 0.004 0.006 3 al2o3 26.3 21.6 16 cu 0.274 0.282 4 sio2 57.5 53.4 17 zn 0.005 0.023 5 p2o5 1.13 0.204 18 ga 0.001 0.001 6 so3 0.258 0.162 19 rb 0.007 0.008 7 k2o 1.78 0.798 20 sr 0.004 0.017 8 cao 0.791 10.4 21 y 0.004 0.002 9 fe2o3 4.70 6.75 22 zr 0.013 0.010 10 cl 1.45 0.071 23 nb 0.001 0.0005 11 ti 0.658 1.45 24 ba 0.012 0.020 12 cr 0.01 0.01 25 ce 0.021 0.023 13 mn 0.067 0.093 26 pb 0.003 0.002 coulombic efficiency coulombic efficiency compares the recovery of the coulombs through the external circuit against theoretical coulombs that is present in the organic matter. cmfc-1 showed average ce of 7.69 ± 1.52 %, whereas in cmfc-2 it was 6.39 ± 1.40 %. higher ce of cmfc-1 than cmfc-2 might have been due to the difference in the cec of red and black soil and also due to more diffusion of oxygen in case of black soil due to high porosity. in mfcs higher ce is reported with pure inoculum culture and with synthetic wastewater [35]. j. electrochem. sci. eng. 4(4) (2014) 315-326 ceramic separator in microbial fuel cell 322 polarization and internal resistance polarization curve helps to understand the performance of mfc in terms of power generation and internal resistance. it represents the cell voltage and power density as a function of the current density. figure 2 shows power and polarization curves obtained using variable resistor box for cmfc-1 and cmfc-2. figure 2. polarization curve for cmfc-1 and cmfc-2 during polarization, the maximum power density observed for cmfc-1 was 51.65 mw m -2 (e = 0.204 v, rext = 30 ω) and that of cmfc-2 it was 31.20 mw m -2 (e = 0.217 v, rext = 50 ω). this indicates that the higher cec of red soil supported better proton transfer from the anode to the cathode in cmfc-1. conversely, lower power output observed in cmfc-2 could be attributed to the lesser cec of black soil used for making separator (table 1). it is well documented that the cation exchange capacity of soil plays vital role in the proton transfer mechanism in soil [36]. internal resistances of cmfc-1 and cmfc-2 measured from the slope of the plot of voltage vs. current were 36 ω and 56.5 ω, respectively. in the region of low current density (fig. 2) rapid voltage drops were observed in both the mfcs, and in the region of high current density, voltage decreased linearly at lower rate. lower proton transfer rate and low conductivity of black soil used for making separator of cmfc-2 increased the internal resistance. electrode potential electrode potentials represent the energy level of the electrons at anode and cathode. electrons move from area of higher potential energy to area of lower potential energy. as the anode has a higher potential energy so electrons move from anode to cathode through an external circuit. during polarization, cathode of cmfc-1 well supported for the reduction reaction up to 0.5 ma current at 1000 ω external resistance. however, the cathode potential of cmfc-2 dropped to zero (vs. ag/agcl) at 0.25 ma current at 2100 ω external resistance, showing inefficiency of cathode for reduction reaction at higher current (fig. 3). 0 50 100 150 200 250 300 350 400 0 100 200 300 400 500 600 700 cmfc-2 cell voltage cmfc-1 cell voltage cmfc-2 power density cmfc-1 power density current density, ma m -2 v o lt a g e , m v 0 5 10 15 20 25 30 35 40 45 50 55 p o w e r d e n si ty , m w m -2 a. n. ghadge at al. j. electrochem. sci. eng. 4(4) (2014) 315-326 doi: 10.5599/jese.2014.0047 323 figure 3. change of the cathode and anode potentials during polarization in cmfc-1 and cmfc-2 the open circuit potentials (ocp) for anode observed before polarization (vs. ag/agcl) for cmfc-1 and cmfc-2 were -610 mv and -580 mv, respectively. during polarization, increase in anode potentials was observed in both the mfcs due to transfer of electrons from anode to cathode, thus positive overpotential was observed. however, the anode potentials during polarization in both the mfcs were only slightly increased, indicating better stability of the anodes. electrode kinetics tafel plot analyses were carried out to determine the exchange current density (i0), charge transfer coefficient () and charge transfer resistance (rct). the values obtained from these analyses are summarized in table 3. based on the tafel-type linear equation obtained from the graphs (figs. 4a, 4b), the slope is f/rt and the y-axis intercept is the logarithm of the exchange current. for anodic reaction at 25°c, the slope of tafel plot is b = 0.059 / 1  (5) and at the same time the slope for cathodic reaction is b = 0.059 / (6) the value of i0 represents the rate of exchange current density at equilibrium state when the reaction overpotential is zero. higher the exchange current (i0) faster is the reaction rate, resulting in a lower activation energy barrier of forward reaction [37]. the electrode materials used in both the cmfcs were same, however different reaction kinetics at electrodes was observed. the reactions at cathodes were faster than anode. presence of very high actual surface area of the carbon felt material resulted in producing a low cathodic overpotential [38]. comparing the i0 values for the reduction reactions at cathode of cmfc-1 and cmfc-2, the cmfc-1 with separator made from red soil had better performance. it indicates that the reactions at cathode of cmfc-1 were faster; might be due to the higher transfer of h + ions and other cations in cmfc-1, enhancing the reaction rates at cathode. comparing the anodes of both the cmfcs, reactions at the anode of cmfc-2 were slightly faster than cmfc-1. apart from the differences in the cec, the reactions at cathode of cmfc-2 were slower than cmfc-1. this could be probably due to the higher porosity of 0 2 4 6 8 10 12 -600 -500 -400 -300 -200 -100 0 100 200 300 e le ct ro d e p o te n ti a l, m v current, ma cmfc-1 cathode potential cmfc-1 anode potential cmfc-2 cathode potential cmfc-2 anode potential j. electrochem. sci. eng. 4(4) (2014) 315-326 ceramic separator in microbial fuel cell 324 the clayware separator used in cmfc-2, due to which the substrate exchange occurred and oxygen supplied in the cathodic chamber was utilized by the substrate. the exchange current densities of cmfc-1 and cmfc-2 cathodes were 3.38 and 2.05 times more than that of the respective anodes, clearly indicating that the reaction at cathode was much faster than anode. a b figure 4. tafel plots for a cathode of cmfc-1 and cmfc-2, and b anode of cmfc-1 and cmfc-2 according to the butler–volmer model of electrode kinetics, the charge transfer coefficient () is used to describe the symmetry between the forward and reverse electron transfer steps and the magnitude of  ranges in value from 0 to 1. charge transfer coefficient signifies the fraction of the interfacial potential at an electrode-electrolyte interface that helps in lowering the free energy barrier for the electrochemical reaction. lower electron transfer coefficient indicates less activation energy required for the electron transfer, resulting lower activation loss [37]. charge transfer resistance (rct) represents the capability to resist the transfer of charge from electrodeelectrolyte interface. it is interesting to note that the rct and  (table 3) of cathodes for both the mfcs were much lesser than the anode of both the mfcs. -250 -200 -150 -100 -50 0 0.0 0.5 1.0 1.5 2.0 cathode cmfc-1 cathode cmfc-2 overpotential, mv ln ( i / m a m -2 ) 0 20 40 60 80 100 120 140 160 2 3 4 5 6 7 8 overpotential, mv anode cmfc-2 anode cmfc-1 ln ( i / m a m -2 ) a. n. ghadge at al. j. electrochem. sci. eng. 4(4) (2014) 315-326 doi: 10.5599/jese.2014.0047 325 table 3. tafel analysis of cmfc-1 and cmfc-2 parameter cmfc-1 cmfc-2 anode cathode anode cathode exchange current density (i0), ma m -2 0.60 2.03 0.74 1.52 charge transfer coefficient () 0.30 0.032 0.45 0.038 charge transfer resistance (rct), ω m 2 10.70 3.16 8.67 4.22 the rct and  values for cathode of cmfc-1 were lower than cmfc-2, which supports that the clayware membrane made from red soil supported better reaction at the cathode. this is because of high cation transported from cmfc-1 to the cathode side, increased the rate of electrochemical transformation with lower electrical energy loss, thus charge transfer coefficient gets reduced. lower rct for anode of cmfc-2 than anode of cmfc-1 indicated that the anode of cmfc-2 was performing slightly better, as also evident from the exchange current density. however, due to limitations of the cathodic reactions the overall performance of cmfc-2 was inferior as compared to cmfc-1. conclusions properties of the clayware separator such as cec, ph and electrical conductivity influenced the performance of mfcs. the power generation of mfc having separator made from red soil was better than the black soil, due to high cec, low ph and higher electrical conductivity of the red soil. results of tafel plots showed that lower exchange current density and higher charge transfer resistance of anodes compared to cathodes, contributed towards more activation loss in both the mfcs. in spite of similar electrode materials in both cmfcs, variation in electrode kinetics accentuate effect of properties of separator on the performance of cmfcs. detailed studies on the mineral composition of soils are required to enhance the cec for further improving power generation of mfc made with such low cost clayware separator. development of such efficient and cheaper separator material will help in drastically reducing fabrication cost of mfc for field implementation. acknowledgement: grants received from department of science and technology, government of india (file no. dst/tsg/nts/2010/61) to undertake this work is duly acknowledged. references [1] b. e. logan, b. hamelers, r. rozendal, u. schröder, j. keller, s. freguia, p. aelterman, w. verstraete, k. rabaey, environ. sci. technol. 40 (2006) 5181-5192. 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[38] q. deng, x. li, j. zuo, a. ling, b.e. logan, j. power sources 195 (2010) 1130-1135. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {insights into electrochemical dealloying of cu out of au-doped pt-alloy nanoparticles at the sub-nano-scale} doi:10.5599/jese.487 87 j. electrochem. sci. eng. 8(1) (2018) 87-100; doi: http://dx.doi.org/10.5599/jese.487 open access : : issn 1847-9286 www.jese-online.org original scientific paper insights into electrochemical dealloying of cu out of au-doped pt-alloy nanoparticles at the sub-nano-scale matija gatalo1,2, primož jovanovič1, francisco ruiz-zepeda1, andraž pavlišič3, ana robba1,2, marjan bele1, goran dražić1, miran gaberšček1,2, nejc hodnik3, 1department of materials chemistry, national institute of chemistry, hajdrihova 19, si-1000 ljubljana, slovenia 2faculty of chemistry and chemical technology, university of ljubljana, vecna pot 113, si-1000 ljubljana, slovenia 3department of catalysis and chemical reaction engineering, national institute of chemistry, hajdrihova 19, si-1000, ljubljana, slovenia corresponding authors e-mail:  nejc.hodnik@ki.si; tel.: +386 1 4760 212 received: december 6, 2017; revised: december 30, 2017; accepted: january 8, 2018 abstract pt alloy nanoparticles present the most probable candidate to be used as the cathode cathodic oxygen reduction reaction electrocatalyst for achieving commercialization targets of the low-temperature fuel cells. it is therefore very important to understand its activation and degradation processes. besides the ones known from the pure pt electrocatalysts, the dealloying phenomena possess a great threat since the leached lessnoble metal can interact with the polymer membrane or even poison the electrocatalyst. in this study, we present a solution, supported by in-depth advance electrochemical characterization, on how to suppress the removal of cu from the pt alloy nanoparticles. keywords electrocatalysis; efc-icp-ms; il-tem; pt alloy; orr introduction the motivation of scientist, in general, is to improve the life quality of people. still our development and ability to live without a shortage of food, clean water, energy, etc. are dependent on raw materials resources and economics. however, on the other hand, it is limited by the pollution and negative climate changes. since it has been already recognized that the pollution caused by humankind indeed affects the environment, it is now time to shift the balance towards the sustainable technologies and renewable energy. in the energy sector, this means replacement of fossil fuels by electricity coming from the sun and wind. since this energy sources are intermittent, http://dx.doi.org/10.5599/jese.487 http://www.jese-online.org/ mailto:nejc.hodnik@ki.si j. electrochem. sci. eng. 8(1) (2018) 87-100 dealloying of cu out of au-doped pt-alloy 88 the current grand challenge is to store this clean energy to match the consumption demand. this is achieved by energy conversion like batteries, flow batteries, pump hydro, compressed air, flywheels, etc. or conversion to hydrogen via electrolyzers. hydrogen can then be elegantly converted back to electricity via proton exchange membrane fuel cells (pem fc) with only byproducts being water and heat [1-3]. this is very useful in the transportation and stationary sector. in order to increase the pem fc efficiency, pt, as cathodic oxygen reduction reaction (orr), is alloyed with less noble 3d metals like co, ni and cu [1,4-12]. initial performances of pt-alloys are already surpassing the us department of energy targets [1]. however, their stability is still under debate and is the topic of many current and future studies [13-20]. three issues are foreseen for ptalloys: (i) loss of initially superior orr activity [13], (ii) formation of unstable porous nanoparticles [9,15] and (iii) poisoning of the pem fc by the leached less noble metal cations [12,21-23]. it is known that leached metals can cause the increase of the resistance of the polymer membrane and the lowering of the performance of the anode electrocatalysts due to the poisoning of the pt surface [12,23]. in this study, we have examined the effect of au doping on dealloying of cu out of highly active ptcu3 alloy nanoparticles supported on high surface area carbon (hereinafter referred to as audoped ptcu3/c). although this effect is already known in the literature [6,14,24-26], we provide additional insight into this very effective less noble metal removal suppression tactic. we utilize state-of-the-art advanced electrochemical techniques like thin film rotating disc electrode (tf-rde), electrochemical flow cell coupled to inductively coupled plasma to mass spectroscopy (efc-icp-ms) and identical location (scanning) transmission electron microscopy (il-(s)tem). experimental synthesis: the starting material ptcu3/c – intermetallic partially ordered (ordered shell, disordered core) ptcu3 nanoparticles that are tightly embedded (anchored) into modified carbon support were prepared via a modified sol-gel synthesis using a gelatin precursor [5,14,17,27,28]. briefly, the synthesis consists of two vital steps, the first being annealing of a non-noble metal precursor (e.g., a cu precursor) together with gelatine and carbon black to obtain embedded nanoparticles in a porous carbon matrix. in the second part, the cu from the composite is partly galvanically replaced by a pt precursor and annealed for the second time. the starting material was prepared in a 5 g batch. from the starting material, we have then prepared two completely comparable materials which differ only in the small amount of noble metal addition (au or pt). in both cases, 100 mg of the starting material was dispersed in 500 ml of 5 mm kcl and purged with ar for 1 hour while stirring in order to remove the oxygen from the suspension. afterwards, while continuously purging the starting material suspension with ar, 500 ml of 40 µm solution of au or pt metal salt solution (haucl4 or k2ptcl4. approximately 4 mg of au or pt) was added drop-wise in a timeframe of 2 hours in order to galvanically displace superficial cu atoms found at selected spots of the nanoparticles surface. both noble metal decorated materials were then filtrated, re-dispersed in 0.1 m koh, filtrated again and re-dispersed 3 additional times in hot mili-q water followed by filtration. after the last filtration, the materials were dried overnight at 50 °c. both noble metal decorated materials were then placed together in the same furnace and subjected to an additional thermal annealing step at 500 °c for 3 hours in a co-containing atmosphere (two separate batches of annealing). prior to thermal annealing in co, the samples were first purged with ar overnight to ensure an oxygen-free atmosphere. furthermore, the m. gatalo et al. j. electrochem. sci. eng. 8(1) (2018) 87-100 doi:10.5599/jese.487 89 samples were first heated to only 120 °c for 1 hour in order to remove any residual moisture from both samples before moving to 500 °c. two materials were obtained – (i) au-doped ptcu3/c and (ii) pt-doped ptcu3/c. xrd the powder x-ray diffraction (xrd) measurements of all samples were carried out on a siemens d5000 diffractometer with cu kα1 radiation (λ = 1.5406 å) in the 2θ range from 10° to 60° with the 0.04° step per 1 s. samples were prepared on zero-background si holder. electrochemical evaluation via thin film rotating disc electrode (tf-rde): electrochemical measurements were conducted in a two-compartment electrochemical cell in a 0.1 m hclo4 (merck, suprapur, 70 %) electrolyte with a conventional three-electrode system controlled by a potentiostat (compact stat, ivium technologies). ag/agcl was used as a reference and a pt wire as a counter electrode. the working electrode was a glassy carbon disk embedded in teflon (pine instruments) with a geometric surface area of 0.196 cm2. prior to any experiment, the two-compartment electrochemical cell was boiled in 18 mω cm-1 mili-q water for 1 hour, and the electrode was polished to mirror finish with al2o3 paste (particle size 0.05 µm, buehler) on a polishing cloth (buehler). after polishing, the electrodes were rinsed and sonicated in 18 mω cm-1 mili-q water for 5 minutes. 20 µl of 1 mg ml-1 water-based well-dispersed catalysts ink was pipetted on the glassy carbon electrode completely covering it and dried under ambient conditions. such preparation resulted in the loading of approximately 25 µgpt cm-2 and was found to be optimal for obtaining high specific activities at 0.9 vrhe. after drying, the electrode was mounted on the rotator (pine instruments). the electrode was placed in ar saturated electrolyte under potential control at 0.05 vrhe. all electrocatalysts were electrochemically activated for 200 cycles between 0.05 and 1.35 vrhe with a scan rate of 300 mv s-1. degradation measurements were performed after electrochemical activation, each time with a fresh thin film of the electrocatalyst material. the materials were degraded for 10 000 cycles between 0.4 and 1.x vrhe (x = 0, 2, 4) and for 50 000 cycles between 0.4 – 1.0 vrhe. after electrochemical activation and after degradation we exchanged the electrolyte and measured both co stripping and orr polarization curve. orr polarization curves were measured in an oxygen saturated electrolyte with rotation at 1600 rpm in the same potential window with a scan rate of 20 mv s-1. after subtraction of background current due to capacitive currents, kinetic parameters were calculated at 0.9 vrhe. ohmic resistance of the electrolyte was determined and compensated for as reported in [29] electrochemical surface area (esa) was determined by integrating the charge in co stripping experiments as described in [30]. all potentials are given against the reversible hydrogen electrode (rhe), which was measured before the start of the experiment and at the end. at the end of each experiment, the thin film on the rde was dispersed in 0.5 ml of isopropanol with the help of an ultrasound bath and later deposited on a tem grid for ex-situ tem analysis or wiped from the rde with a carbon conductive adhesive tapes for sem-edx analysis. scanning electron microscopy: energy dispersion x-ray spectrometry (sem-edx): ex-situ sem-edx analysis was carried out with supra 35 vp (carl zeiss) that is equipped with energy dispersion x-ray spectrometer inca 400 (oxford instruments). the operational voltage was set to 20 kv, the distance between the gun and the sample was set to 8.5 mm for optimal signal acquisition and areas for sem-edx analysis were selected under magnification of 20 000. for statistical relevance, 3 different areas were measured for each sample. j. electrochem. sci. eng. 8(1) (2018) 87-100 dealloying of cu out of au-doped pt-alloy 90 transmission electron microscopy (tem): transmission electron microscopy was carried out in a probe cs-corrected scanning transmission electron microscope jeol arm 200 cf equipped with an sdd jeol centurio energy-dispersive x-ray (edx) spectrometer. the operation voltage was set to 200 kv. high angle annular dark field (haadf) images were taken with 68 and 180 mrad for inner and outer semiangles. convergence angle was set to 25 mrads. identical location transmission electron microscopy (il-tem)): the electrocatalyst suspension (1 mg ml-1) was ultrasonicated for 15 minutes and diluted 10 times (100 ul of the suspension, 900 ul of mili-q water). after additional 5 minutes of ultrasonication (diluted suspension), 5 ul of the suspension was deposited on au-grid (plano au finder grid (nh7) coated with a lacey carbon film). once dried, the grid was inspected under tem. several spots were registered in the finder grid under tem and stem. locations of the spots were recorded at different magnifications to facilitate the afterward tracking. edx maps and line scans were performed spending the minimum time to avoid any possible beam damage. the grid was afterward removed from the microscope to perform the electrochemical tests as following. after the inspection, the grid was mounted on a glassy carbon disc, embedded in teflon (pine instruments) with a geometric surface area of 0.196 cm2. electrochemical activation was conducted in a two-compartment electrochemical cell in a 0.1 m hclo4 (merck, suprapur, 70 %) electrolyte with a conventional three-electrode system controlled by a potentiostat (compact stat, ivium technologies). the electrode was mounted on the rotator (pine instruments). the electrode was placed in ar saturated electrolyte under potential control at 0.05 v (vs. rhe). the electrocatalyst on the au-grid was electrochemically activated for 200 cycles between 0.05 and 1.35 vrhe with a scan rate of 300 mv s-1. all potentials are given against the reversible hydrogen electrode (rhe). after electrochemical activation, the au-grid was dipped into fresh mili-q water and left to dry at room temperature. once dried, the gird was once again inspected under tem and stem. il was identified by the low magnification images which hold marked locations of previously analyzed areas on the tem grid labeled with letters. the images and the edx spectra are then taken under the same conditions as previously. electrochemical flow cell coupled to icp-ms (efc icp-ms) a replica of basi electrochemical flow cell (cross-flow cell kit mw-5052) made of tecapeek material was coupled with an agilent 7500ce icp-ms instrument (agilent technologies,palo alto, usa) equipped with a micromist glass concentric nebulizer and a peltier-cooled scott-type doublepass quartz spray chamber. a forward radio frequency power of 1500 w was used with the following ar gas flows: carrier 0.85 l min-1, makeup 0.28 l min-1, plasma 1 l min-1, and cooling 15 l min-1. hclo4 (0.1 mol l-1, merck, suprapur, 70 %) carrier solution for the electrochemical experiments was pumped at 263 l min-1 by using a peristaltic pump into the basi ec flow cell and then directly into the nebulizer of the icp-ms. the carrier solution and electrochemical cell were electrically grounded with respect to the icp-ms to minimize the possibility of icp-ms response spikes from static charging at the peristaltic pump rollers. the icp-ms was tuned for high sensitivity to obtain the best possible signal-to-noise ratio for the measurements. the concentration of catalyst suspension was set to 1 mg ml-1. the suspension was drop-casted by micropipette over one of the gc electrodes and stabilized by a 5 µl of nafion® (5 wt.% water suspension) diluted by isopropanol (1/50). second gc electrode was used as a counter electrode. the orientation of the working electrode (we) and counter electrode (ce) was adjusted so that we was placed after ce in direction of the electrolyte m. gatalo et al. j. electrochem. sci. eng. 8(1) (2018) 87-100 doi:10.5599/jese.487 91 flow. an electrochemical protocol performed consisted of 200 cycles (300 mv s-1) between 0.05 and 1.35 vrhe. monte carlo simulations kinetic monte carlo (kmc) modeling was performed by simply applying arrhenius equation for the overall bond energy of the atom (first: activation energy is depended on bond type; second: the overall bond energy of the atom is rising with number of neighbors). in the iterative loop atom was randomly chosen to perform specific process (dissolution or diffusion) which corresponds to updated crystal mesh. the loop was running until the specific dealloying time was achieved (for more details look in ref. “atomically resolved dealloying of structurally ordered pt nanoalloy as oxygen reduction reaction electrocatalyst”). by varying initial chemical compositions, atoms distribution and shape/size of the nanoparticles, obtained by tem and edx observations, kmc simulations resulted in different morphological changes. results and discussion we prepared three ptcu3/c analogs: (i) the starting or as-prepared ptcu3/c, (ii) pt-doped ptcu3/c and (iii) au-doped ptcu3/c. for the preparation of the last two samples, the first one was used as a precursor. this means that all samples have the same particle size distribution. since pt and au are more noble than cu [31] the galvanic displacement can occur. therefore, some surface superficial cu is replaced by pt and in the last sample by au cations form the solution. this results in the removal of less than 1 % of cu. the additional annealing step is needed for the formation of the pt or ptau skin [14]. for this reason, we can also consider that the composition of the three samples is very similar. the idea behind doping of ptcu3/c nanoparticles skin was to study the effects caused by very small additions of au, and also pt as a reference point. first cyclic voltammograms (fig. 1a), which is a part of the electrochemical activation (ea) protocol of 200 cycles, 300 mv s-1, 0.05 – 1.35 vrhe, performed with tf-rde shows comparison of cu dealloying in the first cycle of the ea. by measuring the charge obtained in the first cycle of ea (fig. 1a), we notice that in the case of both au and pt decoration, the total amount of dealloyed cu is smaller compared to the as-prepared ptcu3/c. this is a direct proof of successful galvanic displacement of superficial cu. the success of galvanic displacement is further confirmed by the presence of an additional au peak at approximately 38.5 o 2(fig. 1c, dash blue), while we cannot observe any additional peaks in the case of pt decoration which points towards too small size of the pt clusters (fig. 1b, dash red). surprisingly, there is a visible difference in the intensity of both pm3m peaks (ptcu3 ordered phase) upon decorating with both pt and au. after annealing, au doping is shown to have an effect on the bulk structure of the ptcu3 nanoparticles by further decreases the intensity of the peaks corresponding to the pm3m phase, while also slightly broadening the peaks corresponding to the fm3m phase (fig 1c, blue). the phenomena could be directly correlated to the doping process itself, as it could point towards au acting as an impurity in the crystal lattice of the ptcu3 nanoparticles and consequently decreasing the average size of a domain. one can also notice that the peak for pure au disappeared, while there is an appearance of weak new peaks that most likely correspond to the small number of small ternary nanoparticles. the same phenomena cannot be noticed in the case of pt doping, where we don’t observe any major changes to the crystal lattice upon annealing (fig. 1c, red). although we are dealing with very similar size distribution, we observe a huge difference in the case of the electrochemically active surface area (esa) (fig. 1d). this is due to the decreased degree of porosity in the case of the au-doped ptcu3/c analog [25]. j. electrochem. sci. eng. 8(1) (2018) 87-100 dealloying of cu out of au-doped pt-alloy 92 figure 1. (a) first cyclic voltammogram of ea of (i) as-prepared ptcu3/c, (ii) decorated ptcu3/c and doped ptcu3/c analogs, (b) and (c) powder xrd comparison (colors and patterns match the legend in figure a) and (d) esa measurement (from co stripping) after ea of (i) au-doped ptcu3/c and (ii) pt-doped ptcu3/c. formation of porous nanoparticles in the case of pt-doped ptcu3/c analog results in the increase in the esa. this surface area is, however, not stable due to the coarsening of the porous structure even at mild degradation simulations till 1 vrhe [15]. this is nicely seen from the fig. 2 where esa of both analogs after degradation is limiting towards a similar value. fig. 3a shows the drop of the specific activity of both doped analogs. au-doped ptcu3/c analog exhibits about 25 % lower initial specific activity. we presume that this is due to the increased activity of the porous nanoparticles on account of the confinement effect [32]. however, as can be seen in the same figure, the specific activity is decreasing faster for the pt-doped sample. this is because of the increased loss of cu and the fact that porous particles are not stable [15]. the trend indicates that further degradation would result in the faster loss of specific activity, as well as esa of the pt-doped ptcu3/c analog. removal of the cu from the two doped ptcu3 electrocatalysts was measured by energy-dispersive x-ray spectroscopy detector in the sem (sem-edx). in fig. 4 it is clearly seen that au-doped ptcu3/c analog losses much less cu upon ea and also upon all follow-up degradation protocols. efc-icp-ms measurements (fig. 5a) directly prove the enhanced retention of cu from the au-doped ptcu3/c analog. leached cu cations present a serious threat to the real application of pem fc. it can increase the internal resistance of polymer membrane and can also poison the electrocatalyst active pt surface [1–4]. and on the other hand, retention of cu is essential to sustain enhanced orr activity via ligand and/or strain effects [13]. the negative effect of cu dealloying under long-term pem fc operating conditions (e.g., simulated via degradation protocol of 50 000 cycles, 0.4 – 1.0 vrhe, 1 v s1) can be visible in the change of the slope of orr polarization curve (fig. 5b) in the case of pt-doped ptcu3/c analog. m. gatalo et al. j. electrochem. sci. eng. 8(1) (2018) 87-100 doi:10.5599/jese.487 93 figure 2. the absolute decrease of esa of the two doped ptcu3 analogs: au-doped ptcu3/c and pt-doped ptcu3/c upon different degradation protocols. (a) comparison of esa for both analogs when varying the upper potential limit of the degradation protocol (0.05 – 1.x vrhe, x = 0, 2, 4), while keeping number of cycles constant and (b) comparison of both analogs when varying the number of cycles (10 000 and 50 000), while keeping the upper potential limit of the degradation protocol constant. both analogs were electrochemically activated prior to each degradation protocol. such a change in slope can be explained by the redeposition of dealloyed and subsequently underpotentially deposited cu (cuupd) [21]. furthermore, when comparing co stripping of pt-doped ptcu3/c, a slight but clear decrease in esa is observed (fig. 5c,d). that further proves the sintering of the pores even under mild degradation conditions. on the other hand, the slope of orr polarization curve in the case of au-doped ptcu3/c analog hasn’t changed (fig 5b). this further proves the benefit of higher cu retention. additionally, full esa has been retained (fig. 5d) in comparison to the co stripping experiment after ea (fig. 5c). figure 3. loss of specific activity of the two doped ptcu3 analogs au-doped ptcu3/c and pt-doped ptcu3/c upon different degradation protocols. (a) shows comparison of specific activity for both analogs when varying the upper potential limit of the degradation protocol (0.05 – 1.x vrhe, x = 0, 2, 4), while keeping number of cycles constant and (b) shows comparison of both analogs when varying the number of cycles (10 000 and 50 000), while keeping the upper potential limit of the degradation protocol constant. both analogs were electrochemically activated prior to each degradation protocol. j. electrochem. sci. eng. 8(1) (2018) 87-100 dealloying of cu out of au-doped pt-alloy 94 figure 4. sem-edx measurements of cu loss, % of the two doped ptcu3 analogs: au-doped ptcu3/c and ptdoped ptcu3/c upon different degradation protocols. (a) shows comparison of cu loss, % for both analogs when varying the upper potential limit of the degradation protocol (0.05 – 1.x vrhe, x = 0, 2, 4), while keeping number of cycles constant and (b) shows comparison of both analogs when varying the number of cycles (10 000 and 50 000), while keeping the upper potential limit of the degradation protocol constant. both analogs were electrochemically activated prior to each degradation protocol. figure 5. (a) shows efc-icp-ms measurement of cu dealloying from the two doped ptcu3/c analogs during 200 cycles of ea (300 mv/s, 0.05 – 1.35 vrhe). (b) shows polarization orr curves after both the activation and degradation experiment in o2 saturated electrolyte (0.1 m hclo4). co stripping of pt & au-doped ptcu3/c electrocatalysts after (c) 200 cycles of ea (0.05 – 1.35 vrhe, 300 mv s -1) and after (d) 50 000 cycles of degradation (0.4 – 1.0 vrhe, 1 v s -1). transmission electron microscopy investigation confirms the results above on the sub-nanoscale. pt-doped ptcu3/c analog exhibits extensive porosity formation and shrinkage in diameter as a result of cu dealloying. in fig. 6 il-stem haadf images of the same pt-doped nanoparticle before and after activation dealloying is shown. line scan edx profiles clearly confirm the formation of m. gatalo et al. j. electrochem. sci. eng. 8(1) (2018) 87-100 doi:10.5599/jese.487 95 pores. careful observation of the pt line profile (red curve in fig. 6) reveals the formation of the pt surface enrichment. kinetic monte-carlo simulations of dealloying process of the same size ptcu3 nanoparticle are in fascinating accordance with the il-stem images. as an input parameter, the ptcu3 nanoparticle already had two monolayers of pt on the surface. if either none, one or three monolayers were chosen, results did not resemble the ones observed in the il-stem images [25]. for this reason, we presume that as-synthesized pt-doped sample has, on average, already 2 monolayers of pt on the surface. this atomistic insight into ptcu3 dealloying was only possible due to the coupling of the advanced characterization techniques with computer simulations. figure 6. il-stem edx line scan (top), haadf image (middle) and kinetic monte carlo simulations (bottom) of the same pt-doped ptcu3 electrocatalyst before and after ea. an increase in the thickness of the pt skin is observed after ea, additionally, faceting and pore formation are observed. for the au-doped ptcu3/c analog no porosity is observed (fig. 7). this is in complete accordance with the previous results showing much lower cu removal. careful inspection of the il-stem haadf (yellow dot circle around the particle in fig. 7) also reveals small shrinkage and the formation of facets [13]. edx cu line profile indicates that negligible amount of cu was removed at the activation protocol, unlike in the case of the pt-doped sample. again, the formation of pt or probably ptau skin is evident. monte-carlo simulation indicates that 4 monolayers of noble atoms (pt and au) are present on the surface. as an approximation, here the surface diffusivity of au and pt were chosen j. electrochem. sci. eng. 8(1) (2018) 87-100 dealloying of cu out of au-doped pt-alloy 96 to be the same. however, it is known from the literature that au surface mobility is faster compared to pt and has, therefore, better ability to “heal” the surface [33]. therefore it is less probable that the porosity formation takes place even with the same thickness of pure pt layer [17]. the formation of the facets is also seen in the monte carlo simulation. figure 7. il-stem edx line scan (top), haadf image (middle) and kinetic monte carlo simulations (bottom) of the same au-doped ptcu3 electrocatalyst before and after electrochemical activation. an increase in the thickness of the pt skin is observed after ea, but only faceting and no pore formation is observed. the formation of the ptau skin is also observed in the il-stem and il-edx elemental map images of the same au-doped ptcu3 nanoparticles before and after electrochemical activation protocol in fig. 8. in the il-stem images, the formation of more defined edges and also facets is seen (i.e., faceting) [13,16]. this is in accordance with fig. 7. in il-stem haadf the intensity of the edges is brighter, which indicates the presence of the metal with higher z like pt and au. this is confirmed by the il-edx elemental map. careful observation also reveals the enrichment of the pt on the surface even before the dealloying. this is in accordance with the monte carlo simulation results in fig. 7. m. gatalo et al. j. electrochem. sci. eng. 8(1) (2018) 87-100 doi:10.5599/jese.487 97 figure 8. il-stem bright field (top), il-stem high angle annular dark field (middle) and edx elemental mapping (bottom) of the same au-doped ptcu3 electrocatalyst before and after ea. notice the etching of the surface in the particles and the increase of pt skin thickness after ea. in the fig. 9 the exact atomic composition of the representative activated au-doped ptcu3 nanoparticle is measured with longer edx elemental mapping and edx line scan exposure. this was, however, avoided in the il-edx experiments to prevent alteration or damage of the nanoparticles structure by the electron beam. this is also the reason why au signal could not be distinguished from the dominant pt signal. au is distributed over the entire crystal lattice of the nanoparticle. however, it can be presumed from the flat shape [34] of the au line scan (fig. 9c) that the concentration of au lowers when moving from the surface towards the core. if au was uniformly j. electrochem. sci. eng. 8(1) (2018) 87-100 dealloying of cu out of au-doped pt-alloy 98 distributed, au line scan would resemble the shape of the cu scan. we can also distinguish that the surface is formed from at least 5 monolayers of ptau skin (fig. 9b). figure 9. (a) edx elemental mapping, (b) haadf intensity line profile across particle and (c) line elemental scans of the au-doped ptcu3 dealloyed nanoparticle. conclusions two completely comparable materials which differ only in the small amount of noble metal addition (au or pt) have been prepared from the initial 5 g bath of highly active intermetallic ordered ptcu3/c electrocatalyst. in contrast to pt doping (specific activity ≈ 2 ma cm-2pt), au doping of the ptcu3/c electrocatalyst results in approximately 25 % lower initial specific activity, as well as lower esa due to inhibition of porosity formation during 200 cycles of ea (0.05 – 1.35 vrhe, 300 mv s-1). however, upon the degradation till 1.2 vrhe or higher, the difference between both pt and au-doped ptcu3/c analogs becomes negligible because au-doped ptcu3/c analog exhibits much better stability, or in other words much lower stability of pt-doped ptcu3/c analog. the increased stability of au-doped ptcu3/c analog is due to the prevention of the porosity formation and due to higher cu retention. the most impressive result of the current study is the much-improved retention of cu that presents potentially serious issues in the real pem fc applications. m. gatalo et al. j. electrochem. sci. eng. 8(1) (2018) 87-100 doi:10.5599/jese.487 99 acknowledgements: authors gratefully acknowledge the financial support of the slovenian research agency (arrs) through the research core funding programme p2-0152, p2-0393 and project z2-8161. references [1] m. k. debe, nature 486 (2012) 43-51. 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[33] l. dubau, j. durst, f. maillard, l. guétaz, m. chatenet, j. andré, e. rossinot, electrochimica acta 56 (2011) 10658-10667. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) electrochemical treatment of acid red 1 by electro-fenton and photoelectro-fenton processes doi: 10.5599/jese.2014.0058 235 j. electrochem. sci. eng. 4(4) (2014) 235-245 ; doi: 10.5599/jese.2014.0058 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical treatment of acid red 1 by electro-fenton and photoelectro-fenton processes camilo gonzález-vargas, ricardo salazar and ignasi sirés* ,  laboratorio de electroquímica medio ambiental, leqma, departamento de ciencias del ambiente, facultad de química y biología, universidad de santiago de chile, usach, casilla 40, correo 33, santiago, chile *laboratori d’electroquímica dels materials i del medi ambient, departament de química física, facultat de química, universitat de barcelona, martí i franquès 1-11, 08028 barcelona, spain corresponding author: e-mail: ricardo.salazar@usach.cl; tel.: +56-2-27181134 corresponding author: e-mail: i.sires@ub.edu; tel.: +34-93-4039243; fax: +34-93-4021231 received: july 23, 2014; published: december 6, 2014 abstract small volumes (100 ml) of acidic aqueous solutions with 30-200 mg l -1 toc of the toxic azo dye acid red 1 (ar1) have been comparatively treated by various electrochemical advanced oxidation processes (eaops). the electrolytic system consisted of a bdd anode able to produce  oh and an air-diffusion cathode that generated h2o2, which subsequently reacted with added fe 2+ to yield additional  oh from fenton’s reaction. under optimized conditions (i.e., 1.0 mm fe 2+ , 60 ma cm -2 , ph 3.0, 35 ºc), the analysis of the initial rates for decolourization and ar1 decay assuming a pseudo-first-order kinetics revealed a much higher rate constant for photoelectro-fenton (pef, ~ 2.7x10 -3 s -1 ) compared to electro-fenton (ef, ~ 0.6x10 -3 s -1 ). mineralization after 180 min was also greater in the former treatment (90 % vs 63 %). the use of uv radiation in pef contributed to fe(iii) photoreduction as well as to photodecarboxylation of refractory intermediates, yielding a mineralization current efficiency as high as 85% during the treatment of solutions of 200 mg l -1 toc. primary reaction intermediates included three aromatic derivatives with the initial naphthalenic structure and four molecules only featuring benzenic rings, which were totally mineralized in pef. keywords air-diffusion cathode; azophloxine; boron-doped diamond (bdd); e128; eaops; decolourization; food azo dye; mineralization; red 2g http://www.jese-online.org/ mailto:ricardo.salazar@usach.cl mailto:i.sires@ub.edu j. electrochem. sci. eng. 4(4) (2014) 235-245 acid red 1 by electro-fenton and photoelectro-fenton 236 introduction in recent years, great attention has been paid to the electrochemical advanced oxidation processes (eaops) based on fenton’s reaction chemistry for water decontamination [1]. among them, electro-fenton (ef) and photoelectro-fenton (pef) have been the most studied methods for the treatment of waters polluted by organic pollutants such as pharmaceuticals [2-4], pesticides [5-7] and synthetic dyes [8-11] due to their outstanding performance. in ef and pef, h2o2 is continuously electrogenerated in the reaction cell from the two-electron reduction of injected or flushed gaseous o2 as follows [1]: o2 + 2 h + + 2 e  → h2o2 (1) the use of carbonaceous cathodes, particularly in the form of gas-diffusion electrodes (gdes), promotes the production of a high yield of h2o2 at high rate. therefore, the current efficiency for reaction (1) on gdes turns out to be the highest among all tested materials. the activation of h2o2, which is a weak oxidant, is achieved in the presence of a small amount of metal catalyst, especially fe 2+ , at ph around 3. under such conditions,  oh is produced in the bulk from fenton’s reac tion (2) [1]: fe 2+ + h2o2 → fe 3+ +  oh + oh  (2) in pef, the contaminated acidic solution is irradiated with artificial uv light, thus causing: (i) the photoreduction of fe(oh) 2+ , which is the main fe 3+ species at ph 3, via photo-fenton’s reaction (3), and (ii) the photodecarboxylation of some fe(iii)-carboxylate complexes, which are quite refractory to oxidation by  oh formed from fenton’s reaction, via reaction (4) [1]. while reaction (3) contributes to form additional  oh as well as to maintain the fenton’s cycle by continuously regenerating fe 2+ , reaction (4) makes it possible to reach great mineralization degrees. fe(oh) 2+ + hν → fe 2+ +  oh (3) [fe(oocr) 2+ ] + hν → fe 2+ + co2 + r  (4) for some pollutants, the performance of ef and pef can be further enhanced by using a large o2-overpotential anode such as boron-doped diamond (bdd). this material favours the electrooxidation (eo) of the organic molecules by action of  oh adsorbed on the anode surface via reaction (5) [12]: bdd + h2o → bdd(  oh) + h + + e  (5) acid red 1 (ar1, also called amido naphtol red g, red 2g, food red 10 or azophloxine, see chemical structure in table 1) is a synthetic monoazo dye. azo dyes constitute the most comprehensive and varied family among all synthetic organic dyes available in the industry for dyeing all kinds of fabrics [13]. until 2007, ar1 was the preferred dye for baby food colouring due to the resulting intense red colour, but the scientific panel of the european food safety authority (efsa) on food additives, flavourings, processing aids and materials in contact with food was then asked to re-evaluate this dye due to food safety concerns related to its high toxicity and carcinogenic effects [14]. based on this report, the european union has agreed with its suspension as food colouring, as published in the official journal [15]. not surprisingly, it is also banned in other countries outside europe. at present, it is known that ar1 is extensively metabolized to aniline, it may interfere with blood haemoglobin and it is suspected of being a carcinogen [16]. it is therefore one of the dyes c. gonzález-vargas et al. j. electrochem. sci. eng. 4(4) (2014) 235-245 doi: 10.5599/jese.2014.0058 237 that the hyperactive children’s support group recommends to be eliminated from the diet of children. in contrast, ar1 has become one the most used dyes in chile and over the world for dyeing polyester-, nylon-, celluloseand acrylic-based fibers. consequently, if such industrial wastewaters are not conveniently treated before discharge, their impact on the surrounding ecosystems may be dramatic. some few studies have focused on the fate of ar1 upon application of non-electrochemical advanced oxidation processes (aops) [17-19]. regarding the electrochemical technology, only its electrochemical reduction on an activated carbon fiber cathode has been reported [20], but as far as we are concerned its removal by ef and pef with bdd or other anodes has not been reported yet. these latter processes are of great interest for the removal of food azo dyes, as some of us recently demonstrated for the treatment of tartrazine solutions [21]. in this work, the decolourization, ar1 decay and mineralization profiles resulting from the treatment of synthetic aqueous solutions of 100 ml of ar1 by several eaops have been investigated. bdd and gde, both of 2.5 cm 2 , have been used at constant current upon addition of 50 mm na2so4 as supporting electrolyte at ph 3. the optimization of iron catalyst concentration and applied current has preceded the treatment of solutions with up to 200 mg l -1 toc content by pef. in addition, chromatographic analyses allowed the identification of aliphatic and cyclic byproducts formed during the cleavage of the ar1 structure. experimental chemicals ar1 (disodium 8-acetamido-1-hydroxy-2-phenylazonaphthalene-3,6-disulfonate, c18h13n3na2o8s2, ci 18050, 60% purity) was purchased from sigma-aldrich. anhydrous sodium sulfate used as background electrolyte and iron(ii) sulfate heptahydrate used as catalyst in ef and pef were of analytical grade from merck. solutions were prepared with bidistilled water and their ph was adjusted to 3 before the electrolyses with analytical grade sodium hydroxide or sulfuric acid from merck. other chemicals were obtained from merck and sigma-aldrich. electrolytic system the electrolyses were performed in an open, undivided cell containing a 100 ml solution and featuring a double jacket for circulation of external thermostated water at 35 ºc (wisecircu® wcb-11 water bath). the solution was stirred with a magnetic bar at 800 rpm to ensure good mixing and transport of reactants. the cell contained a 2.5 cm 2 si/bdd thin-film electrode from adamant® (500 ppm b) as the anode and a 2.5 cm 2 carbon-ptfe air-diffusion cathode from electrocell®. the cathode was fed with compressed air flowing at 1 l min -1 for h2o2 generation. the trials were performed at constant current provided by an mcp m10-qd305 power supply. in pef, the solution was irradiated with a black ray b100ap lamp. equipment and analytical procedures the solution ph was measured on an extech 321990 ph-meter. samples withdrawn at regular time intervals from electrolyzed solutions were neutralized at ph 7-8 to stop the degradation process and filtered with 0.45 μm ptfe filters from whatman before analysis. the decolourization of ar1 solutions was monitored from the absorbance (a) decay at the maximum visible wavelength (max) of 520 nm, measured from the spectra recorded on a cary 1e uv/vis spectrophotometer (varian). the percentage of colour removal or decolourization efficiency was then determined as follows [21]: j. electrochem. sci. eng. 4(4) (2014) 235-245 acid red 1 by electro-fenton and photoelectro-fenton 238 colour removal, % = 0 t 0 100 a a a  (6) where a0 and at denote the absorbance at initial time and after an electrolysis time t, respectively. the mineralization of solutions was monitored from their total organic carbon (toc) abatement, determined on a vario toc select analyzer. from these data, the mineralization current efficiency (mce) at a given current (i / a) and electrolysis time (t / h) were estimated as follows [1]: mce, % = s exp 7 ( ) 100 4.32 10 nfv toc mit   (7) where f is the faraday constant (96487 c mol -1 ), vs is the solution volume (l), ∆(toc)exp is the experimental toc decay (mg l -1 ), 4.3210 7 is a conversion factor (3600 s h -1  12000 mg mol -1 ) and m is the number of carbon atoms of ar1 (18 atoms). the number of electrons (n) consumed per each dye molecule was taken as 98 considering that its mineralization leads to carbon dioxide and nitrate and sulfate ions as follows: c18h13n3na2o8s2 + 45 h2o  18 co2 + 3 no3  + 2 so4 2 + 2 na + + 103 h + + 98 e  (8) ar1 decay was followed by reversed-phase high performance liquid chromatography (hplc) with a waters 625 lc fitted with a hibar® rp-18e 5 µm, 150  4 mm, column at 25 ºc and coupled with a photodiode array detector set at  = 520 nm. the mobile phase was a 70:30 (v/v) acetonitrile/1.0 mm ammonium acetate (ph 4) mixture at 0.5 ml min -1 . generated carboxylic acids were detected by ion-exclusion hplc using the same lc fitted with a bio-rad aminex hpx 87h, 300 × 7.8 mm, column at 25 °c and setting the array detector at  = 210 nm. the isocratic elution at 0.6 ml min -1 with 4 mm h2so4 as the mobile phase yielded good peaks for maleic (tr = 8.8 min), oxamic (tr = 10.6 min), malic (tr = 11.3 min), formic (tr = 14.9 min) and acetic (tr = 16.1 min). the cyclic and/or aromatic intermediates were analyzed by gas chromatography coupled to mass spectrometry (gc-ms). several electrolyses were carried out under different experimental conditions for short and long times. the final solutions were collected together until reaching 500 ml, which were then extracted three times with 30 ml ch2cl2. the resulting organic solution (90 ml) was dried with anhydrous na2so4, then filtered and completely evaporated in a rotary to obtain a pale yellow solid that was further analyzed. results and discussion influence of the experimental parameters on the degradation of acid red 1 by ef process solutions containing 300 mg l -1 ar1 (i.e., 0.59 mm ar1 or 100 mg l -1 toc) were electrolyzed at 60 ma cm -2 in the presence of different amounts of fe 2+ as catalyst. as can be seen in fig. 1, the absence of fe 2+ (so-called eo process) caused the slowest decolourization and toc abatement, only reaching 70 % colour removal after 70 min and 25 % toc decay after 180 min. under the present eo conditions, given the weak oxidation power of h2o2, the organic matter can be mainly degraded by bdd(  oh) formed via reaction (5). this radical tends to be very active towards the initial pollutants and their by-products because it is weakly physisorbed on the anode surface and it is generated at a very positive potential. moreover, it is known that hydroxyl radicals can react at high rate with all double bonds in the aromatic rings and, especially, with the –n=n– bond. however, since bdd(  oh) is confined to the anode vicinity, the degradation process becomes c. gonzález-vargas et al. j. electrochem. sci. eng. 4(4) (2014) 235-245 doi: 10.5599/jese.2014.0058 239 severely limited by mass transport, thus being needed a much longer electrolysis time to effectively destroy the molecules in a batch system without recirculation like the one tested here. in contrast to the previous finding, within the same time period the presence of fe 2+ allowed the complete decolourization as well as a greater mineralization in all cases, which can be accounted for by the crucial contribution of  oh formed in the bulk from fenton’s reaction (2). as can be seen in fig. 1a, the increase in fe 2+ concentration from 0.1 to 0.5 and then to 1.0 mm clearly accelerated the colour removal, being necessary 70, 60 and 40 min, respectively, to get colourless solutions. figure 1. effect of fe 2+ concentration on (a) decolourization efficiency at 520 nm and (b) toc removal with electrolysis time for the electro-fenton treatment of solutions of 300 mg l -1 ar1 in 0.05 m na2so4 at ph 3.0, 35 °c and 60 ma cm -2 . the rising catalyst content had a positive effect on the mineralization profiles as well, since 47, 54 and 63 % toc removal was attained after 180 min using 0.1, 0.5 and 1.0 mm fe 2+ . in contrast, further increase to 1.5 mm was detrimental, eventually leading to slower colour removal and only 59 % toc abatement at 180 min. this phenomenon can be mainly explained by the larger extent of parasitic reactions causing the consumption of  oh, particularly by fe 2+ . it must be noted that the mineralization was always partial, with a tendency to reach a plateau owing to the plausible j. electrochem. sci. eng. 4(4) (2014) 235-245 acid red 1 by electro-fenton and photoelectro-fenton 240 accumulation of refractory intermediates (as confirmed later on) that could not be oxidized by  oh in the bulk and were very slowly destroyed by bdd(  oh). in conclusion, 1.0 mm was chosen as the optimum fe 2+ concentration for subsequent tests. the effect of current density within the range 8-80 ma cm -2 , carried out under conditions shown in fig. 1 with 1 mm fe 2+ as the optimized amount, is depicted in fig. 2. these trials aimed at exploring the possibility of enhancing the decolourization and mineralization kinetics, which is based on the fact that the applied current determines the yield of bdd(  oh) formed via reaction (5) as well as that of  oh via reaction (2) because it depends on the h2o2 formation rate and fe 2+ regeneration rate. figure 2. effect of current density on (a) decolourization efficiency at 520 nm and (b) toc removal with electrolysis time for the electro-fenton treatment of solutions of 300 mg l -1 ar1 in 0.05 m na2so4 with 1.0 mm fe 2+ at ph 3.0 and 35 ºc. a progressive increase in current density from 8 to 60 ma cm -2 caused the acceleration of both, decolourization and mineralization. this can be easily explained by the faster generation of bdd(  oh) on the anode and  oh in the bulk. note that even the lowest current densities were able to yield the complete decolourization at long electrolysis time. however, further increase to 80 ma cm -2 was detrimental since it caused a slower colour removal and led to a lower toc c. gonzález-vargas et al. j. electrochem. sci. eng. 4(4) (2014) 235-245 doi: 10.5599/jese.2014.0058 241 removal. this negative effect arises from the shift in cathode potential to unfavourable values that promoted the reduction of o2 to h2o over reaction (1) and hindered the conversion of fe 3+ to fe 2+ . as a matter of fact, h2o2 concentration analyzed during the electrolyses at 60 and 80 ma cm -2 reached 25 mm and 15 mm, respectively. thus, 60 ma cm -2 was chosen as the optimum value. effect of uva light as discussed in the introduction, for most of the contaminated solutions studied in the past it was possible to enhance the degradation process by irradiating them with uv light, which favoured the oxidation of pollutants and their by-products due to the action of reaction (3) and (4). in the present study, no direct photolysis of ar1 by uv light was observed, since its peak during the hplc analyses remained unchanged. this ensured the photostability of ar1 during the electrolyses run under pef conditions. for this purpose, the ar1 solutions were treated as done in the previous ef experiments but incorporating the uv lamp near the cell. as shown in fig. 3a, solutions of 300 mg l -1 ar1 treated by pef under the optimized conditions described before (i.e., 1.0 mm fe 2+ and 60 ma cm -2 ) were more quickly decolourized compared to ef trials, being required 50 min instead of 60 min to become colourless (see fig. 1a for comparison). the key contribution of photoreduction reaction (3) favoured the faster regeneration of fe 2+ , which then was able to accelerate the production of  oh from reaction (2). on the other hand, pef also yielded a much larger mineralization after 180 min, reaching 90 % owing to photodecarboxylation reaction (4). as reported elsewhere [1], some of the reaction intermediates can form stable complexes with fe(iii) that can be effectively degraded only upon action of uv photons since  oh and bdd(  oh) are much less effective. fig. 3a also depicts the decay of ar1 monitored by hplc during the same experiment. its profile is quite similar to colour removal profile, which means that no other coloured by-products were formed during the treatment. assuming a pseudo-first-order kinetics, an apparent rate constant (kapp) of 2.74×10 -3 s -1 for ar1 decay was determined. the decay of the dye was also similar to the colour removal trend for ef treatment (not shown), revealing a much smaller kapp = 0.59×10 -3 s -1 . therefore, the beneficial synergy between  oh, bdd(  oh) and uv light for the decontamination of ar1 solutions is demonstrated. due to production needs, actual wastewaters may present a significant variation in the dye content over time and thus, it is mandatory that the water treatment technology is flexible enough to be adapted to such changes. the effect of ar1 concentration on the mineralization profile vs. time is shown in fig. 3b. a similar toc removal was attained after 180 min for solutions containing 30 and 100 mg l -1 toc. in contrast, only 75 % mineralization was reached for solutions with 200 mg l -1 toc, which is simply due to the much larger number of organic molecules to be degraded in the latter case. but, an important feature to be highlighted is the progressively increasing slope of the curves (i.e., larger mineralization rate) upon increase of ar1 concentration, which can be related to the more efficient reaction between  oh/bdd(  oh) and the organic molecules. indeed, low ar1 concentrations cause the waste of radicals in self-destruction and other side reactions, whereas high organic contents lead to effective oxidation reactions. this is clearly demonstrated in fig. 3c, which compares the evolution of mce vs time for several eaops. for solutions with 100 mg l -1 toc, the efficiency increases in the sequence eo < ef < pef, with maximum values of 10, 25 and 55 %, respectively. as discussed before, this can be related to the more favorable synergy between different oxidants in the case of pef. in addition, a greater mce resulted from the treatment of larger ar1 concentrations in pef, reaching 85 % during the j. electrochem. sci. eng. 4(4) (2014) 235-245 acid red 1 by electro-fenton and photoelectro-fenton 242 treatment of 200 mg l -1 toc, thus confirming the lower extent of parasitic reactions that cause radical waste. note that mce tends to decrease at long electrolyses time, which can be explained by (i) the formation of more resistant intermediates and (ii) the mass transport limitations related to low organic loads. figure 3. (a) decolourization efficiency at 520 nm and percentage of ar1 removal with electrolysis time for the photoelectro-fenton treatment of solutions of 300 mg l -1 ar1 in 0.05 m na2so4 with 1.0 mm fe 2+ at ph 3.0, 35 °c and 60 ma cm -2 . (b) toc abatement vs. time for the same experiment compared to trials at different ar1 concentrations. (c) mce for trials shown in (b) compared to eo (0 mm fe 2+ ) and ef (1.0 mm fe 2+ ) treatments shown in fig. 1b. c. gonzález-vargas et al. j. electrochem. sci. eng. 4(4) (2014) 235-245 doi: 10.5599/jese.2014.0058 243 identification of reaction intermediates table 1 summarizes the seven aromatic intermediates identified during pef treatments. table 1. structures of acid red 1 and its degradation intermediates identified by gc-ms analysis. chemical name structure m/z acid red 1 509.42 naphthalene derivatives disodium 8-acetamido-1-hydroxy-2-(4hydroxyphenylazo)naphthalene-3,6-disulfonate 524.12 disodium 1,2-dihydroxy-3phenylazonaphthalene-4,7-disulfonate 468.37 (5-phenylazo-3,4,6trihydroxynaphthalene)sulfonic acid 362.33 benzene derivatives sodium (3,5-dihydroxy-2phenylazo)benzenesulfonate 316.26 sodium (1-hydroxy-2-[4-hydroxyphenylazo]-2oxo)ethanesulfonate 282.21 2-(3,5-dihydroxyphenylazo)-1-hydroxy-2-oxoethanesulfonic acid 276.22 hydroquinone 110.11 j. electrochem. sci. eng. 4(4) (2014) 235-245 acid red 1 by electro-fenton and photoelectro-fenton 244 as can be seen,  oh and bdd(  oh) led to the hydroxylation of the benzenic and naphthalenic rings of ar1 to yield three naphthalene derivatives. the subsequent action of radicals onto ar1 and/or onto those intermediates led to the formation of four benzene derivatives. the accumulation of these aromatic intermediates would be dangerous due to their inherent high toxicity and thus, pef treatments had to be prolonged until their complete disappearance. the progressive cleavage and oxidation of the aromatic intermediates gave rise to the formation of short-chain aliphatic carboxylic acids, as also described elsewhere for other pollutants [22]. five c1-c4 acids were identified by ion-exclusion hplc, namely maleic, oxamic, malic, formic and acetic. as explained from fig. 3, pef ensured the almost complete removal of all these acids since the toc in the final solutions was <10 % after 180 min. conclusions pef technology is confirmed as a very powerful alternative for giving response to environmental concerns related to water contamination by organic pollutants. this process allows a much faster destruction of ar1 as well as a more significant and efficient toc removal compared to eo and ef, thus becoming a promising technology for the treatment of industrial wastewaters containing this azo dye. the toxic intermediates formed during the first degradation stages are completely transformed into aliphatic molecules, which are slowly converted to co2 and h2o. the use of renewable energy such us sunlight in sunny countries like chile and spain, giving rise to the socalled solar photoelectro-fenton (spef) process, would be an interesting feature for real-scale application. acknowledgements: the authors thank conicyt (chile) for support under fondecyt grant 1130391 and dicyt-usach, as well as for the phd fellowship n° 21130071 awarded to c. gonzálezvargas. references [1] e. brillas, i. sirés, m. a. oturan, 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[21] a. thiam, m. zhou, e. brillas, i. sirés, applied catalysis b: environmental 150-151(2014) 116–125 [22] m. a. oturan, m. pimentel, n. oturan, i. sirés, electrochimica acta 54 (2008) 173–182 © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://www.sciencedirect.com/science/article/pii/s1762585804000189 http://www.sciencedirect.com/science/article/pii/s1762585804000189 http://creativecommons.org/licenses/by/3.0/ {single and multi-frequency impedance characterization of symmetric activated carbon single capacitor cells} doi:10.5599/jese.536 183 j. electrochem. sci. eng. 8(2) (2018) 183-195; doi: http://dx.doi.org/10.5599/jese.536 open access : : issn 1847-9286 www.jese-online.org original scientific paper single and multi-frequency impedance characterization of symmetric activated carbon single capacitor cells suzana sopčić1, davor antonić1, zoran mandić1, krešimir kvastek2, višnja horvat-radošević2, 1university of zagreb, faculty of engineering and technology, department of electrochemistry, marulićev trg 19, 10000 zagreb, croatia 2ruđer bošković institute, bijenička c. 54, 10000 zagreb, croatia corresponding author e-mail: vhorvat@irb.hr received: april 9, 2018; revised: may 4, 2018; accepted: may 10, 2018 abstract electrochemical impedance spectroscopy (eis) technique is used for characterization of single cell symmetric capacitors having different mass loadings of activated carbon (ac). relevant values of charge storage capacitance (ct) and internal resistance (esr) were evaluated by the single frequency and multi-frequency analyses of measured impedance spectra. curve fittings were based on the non-ideal r-c model that takes into account the parasitic inductance, contributions from electrode materials/contacts and the effects of ac porosity. higher ct and lower esr values were obtained not only for the cell with higher mass of ac, but also using the single vs. multi-frequency approach. lower ct and higher values of esr that are generally obtained using the multi-frequency method and curve fittings should be related to the not ideal capacitive response of porous ac material and too high frequency chosen in applying the single frequency analysis keywords double-layer capacitor; electrical response; porous electrodes; storage capacitance, internal resistance introduction electrochemical capacitors/supercapacitors are high power energy storage devices composed basically from two electrodes built-up from an active material put on current collectors and immersed in a liquid ionic conductor (electrolyte) 1−8. in these devices, energy is stored reversibly or quasy-reversibly by either ion adsorption at the electrode/electrolyte interfacial regions (electrochemical double layer capacitors − edlc), or ion adsorption combined with some fast surface redox (faradaic) reaction (supercapacitors − sc). various types of edlc/scs were already derived, including symmetric and asymmetric devices. symmetric edlc/scs utilize the same http://dx.doi.org/10.5599/jese.536 http://www.jese-online.org/ mailto:vhorvat@irb.hr j. electrochem. sci. eng. 8(2) (2018) 183-195 impedance characterization of single capacitor cells 184 electrode material (usually carbon) for both electrodes, while asymmetric types use different materials for electrodes, including a battery-like electrode in asymmetric hybrid devices 8−10. charging/discharging of an edlc/sc device containing purely capacitive electrodes is initiated by on/off switching of dc voltage (v) and related to separation of charges in double-layers formed at each electrode in contact with electrolyte. the scheme of the single cell of an edlc/sc device containing two separated and equal material electrodes (positive and negative) in an electrolyte is presented in figure 1. figure 1. schematic presentation of symmetric single cell capacitor the charge storage ability (or capacity) of a capacitive cell is determined by the positive and negative electrode capacitance values (c+ and c−), what makes the total capacitance (ct) of cell to be defined as: 1/ct = 1/c− + 1/c+ (1) the overall performance of an edlc/sc capacitive device is usually described in terms of the rated charge storage capacitance (ct), dc voltage (v) and electrical series resistance (esr) values. esr comprised the internal resistance of a device including resistances of current collectors, electrodes, electrolyte, separator and all contact regions formed in between. the values of these three characteristic parameters (ct, v and esr) are essential for estimation of important comparative topics for edlc/sc energy devices such as specific energy (esp =ctv2/2n) and the maximal deliverable specific power (psp(max)=v2/4nesr). here, n denotes the normalization parameter such as mass, volume, surface area, etc. use of high surface area porous carbon materials, conductive electrolytes with high breakdown potential and highly ion-permeable separators have strongly been recommended for attaining beneficially high ct and v and low esr values, i.e. high energy and high power edlc/sc devices 11−19. determinations of ct and esr values for devices containing new materials and also upon conventional testing of edlc/scs have usually been performed using different electrochemical techniques. the most commonly used are cyclic voltammetry (cv) and galvanostatic charging/discharging measurements 16−26. by using cv experiments, ct values are estimated from the constant (capacitive) currents measured within a voltage window (v) at low scan rates. by use of charging/discharging experiments performed at some constant current value within the voltage window (v), ct values are obtained from measured v vs. time linear plots. at the same time, the voltage drop appearing at the initiation of the constant current discharge is commonly used for determination of esr. s. sopčić et al. j. electrochem. sci. eng. 8(2) (2018) 183-195 doi:10.5599/jese.536 185 electrochemical impedance spectroscopy (eis) 25,27−29 is another electrochemical technique that can provide both, ct and esr values at various voltage (v) levels of a capacitive device that should be in causal, stable, linear and time invariant conditions. eis is usually performed by applying a small sinusoidal (current or potential) perturbing signal and measuring impedance (z) or admittance (y) vector as a function of the angular frequency (). impedance vector is defined as: z = 1/y = zei = z' + iz'' (2) in eq. (2),zand  are magnitude and phase angle of impedance vector, while z' and z'' are the real and imaginary parts in its complex number presentation. impedances of capacitive devices are commonly measured in the linear frequency (f = /2) range between 50 khz − 0.01 hz. impedance spectra of edlc/sc capacitive systems have generally been presented as nyquist (z’’ vs. z’) 11,12,17-19,21,23,24,26,28,30−42, bode (logzand  vs. log ) 19,24,26,30,31,33, 38,42 and complex capacitance (y/i = c) plots. in this last case, complex capacitance spectra can be presented as c’’ vs. c’ and/or c’’ and c’ vs. log  (log f) plots 12,18,31,33,34,37,40,42. in applying of either single frequency or multi-frequency eis method for characterization of capacitive devices, esr and ct values are usually obtained using the one time (lumped) constant r−c model 1,2,6,28,37 for a capacitive device, having the impedance/frequency response defined as: z() = esr + 1/ict (3) according to eq. (3), a capacitive device will behave as a pure resistor at high and pure capacitor at low frequencies, with some transition region in between. the product of esr and ct determines the time constant capacitor value, esr  ct = c. this time constant characterizes the rate of a capacitor charging up to about 63 % of its final capacity (or about 37% of its depletion on discharging) and is important merit in comparing edlc/sc devices. esr and ct values can be estimated directly from measured impedance values at high (esr = z'(→)) and low (ct = −1/z’’(→0)) frequencies, respectively. the other way of identifying esr and ct parameters is the curve fitting procedure. this procedure is usually made by assigning the impedance function defined by eq. (3) across the entire range of measured frequencies, where all departures can easily be detected through the least square approach. departure from eq. (3) has usually been noticed through appearance of inductive impedance (having positive imaginary part) at the highest frequencies 11,17,21,24,35,36,41 and another one impedance showing characteristic −45 phase angle in the transition region between pure resistive and pure capacitive impedance responses 2,18,24,31−36,38−41. whereas inductive impedance response has usually been observed for low impedance (highly capacitive) systems, the characteristic −45 phase angle impedance response is almost always noticed in impedance spectra of capacitive devices with highly porous electrodes. therefore, this impedance was usually ascribed to a distributed electrode structure, reactivity and/or porosity effects 2,35,40. all these observations changed the ideal r−c model described by eq. (3) to the one which takes into account the parasitic inductance and the effects of electrode porosity according to the following relation 11,18,24,35,36,41,43,44: z()= zl() + rhf + zs() (4) eq. (4) shows that the total impedance of a non-ideal capacitive device is composed from three impedances, i.e. inductive impedance (zl), high frequency resistance (rhf) and pore impedance (zs). frequency response of an inductive impedance is defined as zl() = il, where l is inductance that is measurable at high frequencies as l = z’’/ 21,24,43. frequency response of zs, however, is more complex as it should describe the frequency dependence of a capacitance generated at pore walls, j. electrochem. sci. eng. 8(2) (2018) 183-195 impedance characterization of single capacitor cells 186 as well as ionic resistance distributed within electrode pores. in order to account for −45 phase angle at high-to-medium frequencies that is followed by pure capacitive response at lower frequencies, various models of zs() have already been recommended in the literature. some of these models include either a pure capacitor in series with two or more parallel rc combinations 24,36,41,43,44, or the vertical ladder network 45, both mimicking different time constants in the pores. the most frequently, however, the concept based on the de levie’s theory of porous and rough electrodes 46 has been applied by which zs(i) is ascribed to the impedance/frequency response of a single rail rc transmission line element (tle) 2,29,35,36,39,43,44,47. whereas ct value is beneficially increased by using highly porous carbon as electrode material, appearance of an additional resistance distributed within the pores is generally considered detrimental. this is because any additional resistance would increase internal resistance, esr = rhf + rs() 31−33 and make the high-frequency resistance rhf = z'( →) in eq. (4) to be only a part of esr. for some capacitive cells, however, a semicircle impedance response has additionally been noticed at higher frequencies, i.e. between rhf and characteristic −45 and/or vertical shape of zs() [18,23,32,33,39,40,43]. this impedance loop has increased esr by another resistance component and shifted the capacitive response to smaller frequencies [32,33]. some between many explanations for such impedance response were ascribed to the charge transfer of possible pseudocapacitive reaction, formation of a passive layer at current collectors, formation of the interfacial region between current collector and electrode material or even pure dielectric polarization effects of the bulk solution [18,32,44]. in spite of diverse interpretation, a semicircle in impedance spectra of capacitive devices can simply be accounted for by a resistor-capacitor (rc) combination, defining its impedance/frequency response as zrc()=rrc/(1+irrccrc) 32,39,40,45. thereby, eq. (4) that has already been defined as the sum of three impedances becomes enlarged by inclusion of zrc() impedance according to: z()= zl() + rhf + zrc() + zs() (5) impedance spectra defined by eq. (5) is composed of four characteristic frequency regions and described by six impedance parameters (l, rhf, rrc, crc, rs and cs) that all can be determined by the curve fitting procedure. in these conditions, ct = cs and esr = rhf + rrc + rs(). in using the single frequency method, however, just contributions of different resistance components to esr are the reason why esr should inevitably be defined at lower frequencies 17,19 and why esr = z' at f = 1 khz has been recommended for determination of esr of technical capacitor devices using the single frequency eis method 22,27. hereinafter, impedance characterization of the single cell (−)ac//ac(+) capacitors, prepared within realization of the esu-cap project of the croatian national foundation will be elaborated. single and multi-frequency analyses will be performed and several approaches to analyze impedance data will be applied. the final results concerning total storage capacity (ct) and internal resistance values (esr) will be compared and discussed. experimental preparation of single cell ac//ac capacitors the electrodes for ac//ac single cell capacitors were prepared from a slurry of the activated carbon (ac), carbon black and polyvinylidene difluoride (pvdf). the pre-determined weights of the slurry were coated onto al foils pieces of 2 cm2, dried and hot-pressed, forming electrodes with 6.5 s. sopčić et al. j. electrochem. sci. eng. 8(2) (2018) 183-195 doi:10.5599/jese.536 187 and 20.5 mg of active material. the single cell capacitor assembly was performed within a high purity argon filled glove-box. glass-fiber was inserted as a separator between two electrodes and solution of 0.25 mol dm-3 tetraethylammonium tetrafluoroborate (teabf4) in acetonitrile (acn) served as the electrolyte. the capacitors containing electrodes with 3.2 and 10.2 mg cm-2 of the total active material were denoted as ac-1 and ac-2, respectively. all details of preparation were described previously 48. electrochemical impedance spectroscopy among various possibilities and equipment for impedance measurements 49,50, the present impedance measurements were performed using the biologic potentiostat sp-200 under ec-lab software. the ac//ac cells were measured in discharged states at the open circuit voltage (v = 0.00 v) after being charged/discharged in 5 cycles between 0.00 and 2.7 v, at 4 ma cm-2 for ac-1 and 10 ma cm-2 for ac-2 cell, respectively. ac signal of  14 mv amplitude was applied and impedances at 10 measured points per decade were measured in the range of 1106 to 0.01 hz. prior measurements the cells were left for about one hour to attain steady-state which was controlled by subsequent impedance measurements of each cell. impedance results were analyzed using the zview (scribner ass.). the complex non-linear least squares (cnls) program using a presumed model and modulus weighting mode was applied for impedance data fittings. ct and esr values obtained by the single frequency method served as initial values for the fitting procedure and all parameters were stated free in calculations. statistical criteria for defining reasonable fits were small ( 910−4) standard deviation of the overall fit, 2: ' ' 2 '' '' 2 (exp) ( ) (exp) ( )2 2 1 ( ) ( ) ( )n k k cal k k cal k k cal z z z z z=  − + −  =      (6) in eq. (6), z', z'', k and n are real and imaginary impedances, particular frequency of measurement and total number of measured frequencies (data points), respectively. z is impedance magnitude, while “exp” and “cal” denote measured and calculated quantities. more details of corrections and statistical criteria for reasonable fits of impedance spectra are described elsewhere 51. results and discussion single-frequency impedance analysis figure 2 shows nyquist (z’’ vs. z’) plots of two tested ac//ac single capacitor cells containing two near equal electrodes with different active material mass loadings (ac-1 and ac-2). both nyquist plots showed near vertical lines, indicating capacitive impedance responses for both ac//ac cells. higher impedance observed for the electrode with less mass of active material (ac-1) is in agreement with literature data on similar ac//ac cells 31. esr values that according to eq. (3) should be obtained as z'( →), are at f = 1105 hz estimated as 3.07  for the ac-1 and 2.11  the ac-2 cell. ct values calculated using ct = −1/z’’ at the lowest measured frequency of 0.01 hz were estimated to be 0.082 f and 0.297 f for the ac-1 and ac-2 cells, respectively. from these ct values, specific capacitance per unit mass of one electrode, calculated using csp (f g-1) = 4  ct/m, where m denotes the total mass of both electrodes 20, were 25 f/g for the ac-1 and 29 f/g for the ac-2 cell, respectively. in another way of impedance data presentation, the concept of complex capacitance is applied due to the following capacitive impedance definition [27,29]: z() = y()−1 =ic()−1 (7a) j. electrochem. sci. eng. 8(2) (2018) 183-195 impedance characterization of single capacitor cells 188 figure 2. nyquist (z’’ vs. z’) plots of ac-1 and ac-2 capacitor cells the complex capacitance (c =y/i) has also vector properties and in terms of real and imaginary components can be defined as: c() = c’() − ic’’() (7b) for the pure r−c model defined by eq. (3), c’() and c’’() are defined as: c’() = −ct /(1+2ct2esr) (8a) c’’() =  ct esr/(1+2ct2esr) (8b) in this context, c’() defined by eq. (8a) is a term related to the energy stored, while c’’() defined by eq. (8b) is a term related to the energy dissipation [18,23,31,34,38,42]. as shown in fig. 3a, c’’ vs. c’ formed semicircle shapes for both ac//ac cells, tending to zero at high and ct values at low frequencies. in accordance with eq. (8a), ct values can be scanned directly from fig. 3b as c’ at →0. such determined values were 0.082 f and 0.295 f for ac-1 and ac-2 cells, respectively. as expected, these results are equal to that calculated above using z’’ value at 0.01 hz. time constant (c) values of 0.38 s and 0.77 s for ac-1 and ac-2 cells respectively, were estimated using the relation c = (c)-1, where c is the cutoff frequency at top of semicircles in fig. 3a, or from maximums of c’’ vs. log  curve shown in fig. 3c 18,23,33. more commonly discussed and 2 times higher values of the relaxation time constants defined as 0 = (f0)-1 12,18,31,34,38,40,42 were estimated from the maximums of c’’ vs. log f curves shown in fig. 3c as 2.39 s and 4.83 s, respectively. both relaxation time values (c and 0) can also be obtained from the phase bode plots shown in figure 4, by determining frequency values f0 =0/2 at which z’ = z’’ and  = −45 [30,33,40] as 0.42 and 0.21 hz, respectively. using the relation esr=0/ct, esr values were calculated as 4.63  and 2.61 , respectively. somewhat higher esr values than were estimated as z’ values at 1105 hz, suggested some extent of departure between measured data and eq. (3). s. sopčić et al. j. electrochem. sci. eng. 8(2) (2018) 183-195 doi:10.5599/jese.536 189 figure 3. a) c’’ vs. c’, b) c’ vs. log  and c) c’’ vs. log  and log f dependences of ac-1 and ac-2 capacitor cells bode presentation of a pure r−c combination described by eq. (3) is expected to show a linear magnitude line with 0 phase angle at higher frequencies and a sloping linear line with −90 phase angle at lower frequencies. certain discrepancies from this expectance can be noticed in figure 4 as appearance of phase angles different than 0 (including positive values) at the highest frequencies of impedance spectra of both single capacitor cells. figure 4. bode (log z and  vs. log ) plots of ac-1 and ac-2 capacitor cells figure 5 shows more precisely that almost ideal resistive and/or capacitive responses predicted by eq. (3) and labeled by dashed lines are obtained only within limited ranges of frequencies 21,24,28,30,35. vertical lines in fig. 5a shows that frequency independent z’ values, suggesting j. electrochem. sci. eng. 8(2) (2018) 183-195 impedance characterization of single capacitor cells 190 almost pure resistive cell responses, are for both ac//ac cells obtained within limited regions of high to medium frequencies. figure 5. a) z’ and b) log z’’ vs. log  dependences of ac-1 and ac-2 capacitor cells the estimated esr values at the recommended 1 khz frequency 22,24,28,33 (denoted by the arrow in figure 5a) are higher than those obtained as esr = z’ at f = 1105 hz and are equal to 4.36  and 2.29  for the ac-1 and ac-2 cells, respectively. fig. 5b shows that the sloping linear lines, suggesting pure capacitive responses are observed at   100 ( 15 hz) for the ac-1 cell and at   10 ( 1.5 hz) for the ac-2 cell, respectively. pure capacitive responses squeezed to low frequency region have already been noticed in figure 4. all these results suggest that the r−c model described by eq. (3) is insufficient to present impedance of ac//ac capacitor cells but can be used as an approximation at rather low frequencies only. therefore, in using the single frequency method, ct should be estimated at the lowest measured frequency, while esr values should not be estimated at too high frequencies 24,31−33,35. multi-frequency impedance analysis as has already been stated in the experimental part, impedance spectra of both ac//ac cells were measured over eight decades of frequencies (106−0.01 hz). the results of cnls fittings of eq. (3) to measured impedance spectra of both ac//ac cells are listed in table 1. in spite of acceptable standard deviations obtained for ct and esr parameters values, huge 2 values have indicated the complete fail of the r−c model for both ac//ac capacitor cells. as is also shown in table 1, similar has happened after use of the r−cpe model, where pure capacitive impedance in eq. (3) is replaced by impedance of the constant phase element (cpe) 26,40,43. impedance of cpe, zcpe () defined by eq. (9) has usually been applied in order to account for some inclination from the ideal vertical capacitive line in nyquist plots and phase angle different than −90 in bode plots and ascribed to surface inhomogeneities of various kinds 26,40,43. zcpe () = 1/t(i) (9) in eq. (9), zcpe () is described by two frequency independent parameters (t and ), where t becomes pure capacitive parameter c for  = 1. for   1 and in the terms of eq. (3), the effective capacitance value, (ct)c, can be calculated using eq. (10) 52,53: (ct)c = t1/  esr (1)/ (10) s. sopčić et al. j. electrochem. sci. eng. 8(2) (2018) 183-195 doi:10.5599/jese.536 191 table 1. results of cnls fittings of c−r and r−cpe models to impedance spectra (106−0.01 hz) of ac//ac capacitor cells. ac//ac model esr /  ct / f t / fs-1  *(ct)c / f 2 ac-1 eq. (3) 4.060.08 0.0800.003 ----11 eqs. (3)+(9) 4.010.08 -0.0770.002 0.950.02 0.072 6.38 ac-2 eq. (3) 2.330.03 0.2860.008 ----8.23 eqs. (3)+(9) 2.280.03 --0.2610.008 0.920.01 0.249 3.57 *(ct )c calculated by eq. (10) fails of either the r−c model described by eq. (3) or r−cpe model described by eq. (3) combined with eq. (9) in the curve fittings of impedance data measured at frequencies from 106 to 0.01 hz, pointed to another impedance(s) influencing the measured impedance spectra of both ac//ac cells. these additional impedances can clearly be noticed in the enlarged view of high-to medium frequency parts of nyquist plots presented in figure 6. for both ac//ac cells shown in figure 6, inductive impedance responses at the highest frequencies (region i) are followed by prominent semicircle paths (region ii). some differences between two cells, however, can be noticed at lower frequencies. whereas for the ac-1 cell, the semicircular shape dominated down to 100 hz after which an almost vertical (capacitive) response is observed, for the ac-2 cell, a semicircular shape is observed down to 1000 hz and then followed by the characteristic −45 phase angle type of response (region iii) being prominent down to 1 hz. only below 1 hz, a capacitive line (region iv) becomes dominating for this ac//ac cell. slow transition, or prominent −45 phase angle response (region iii) for the ac-2 capacitor cell can be related to an inhibited transport of electrolyte within a porous structure of active material having higher thickness generated by higher mass loading. figure 6. high-to-medium frequency region of nyquist plots of ac-1 and ac-2 capacitor cells nyquist plots with four characteristic shapes (i−iv), i.e. (inductive, semicircular, −45 and capacitive), have already been observed for edlc/sc devices 35,43 and discussed here together with eq. (5). in applying eq. (5) as the impedance model equation in the fitting procedure, zs () should be primarily defined. to account for the characteristic impedance −45 shape at j. electrochem. sci. eng. 8(2) (2018) 183-195 impedance characterization of single capacitor cells 192 intermediate frequencies followed by an almost purely capacitive response at the lowest frequencies, the tle concept will be applied. in such a way, zs() in eq. (5) is described in terms of the tle impedance, z()tle, defined by the following relation 2,35,36,40,43,44,47: zs() = z tle ()= rs (is)−p coth (is)p (11) for highly electronically conducting porous layer, rs and cs = s/rs in eq. (11) denote the total electrolyte resistance within pores and double-layer capacitance induced at pore walls. in the ideal case, the exponent p = 0.5, but due to the same reasoning as for cpe impedance defined by eq. (9), the exponent p in eq. (11) can be lower than ideal 0.5 35,40, thus making cs to be similar in its explanation to the cpe parameter t 51. the characteristic time constant (s) that is determined by pore and electrolyte properties is directly responsible for the characteristic shape of impedance/frequency response. at two  limits, eq. (11) with p=0.5 becomes reduced to 2,35: zs(→) = (rs/cs)0.5 (i)−0.5 (12a) zs(→0) = rs/3 + 1/(i)cs (12b) now, it is easy to show that at frequencies higher than s = 1/s, eq. (12a) defining the phase angle of −45 is operative, while at frequencies lower than s = 1/s, eq. (12b) is operative describing the ideal r−c response having the phase angle of −90. eq. (5) combined with eq. (11) is generally described by six frequency independent impedance parameters (l, rhf, rrc, crc, rs and cs) that are for not ideal impedance responses increased for two more parameters ( and p). the values of these eight frequency independent impedance parameters obtained by cnls fittings of eq. (5) involving zs() defined by eq. (11) and zrc() corrected by eq. (9), to measured impedance spectra of two ac//ac cells are listed in table 2. the corresponding fitted curves are drawn by full lines in figures 4 and 6. table 2. results of cnls fittings of the eq. (5) combined with eq. (11) to measured impedance spectra of ac//ac capacitor cells ac//ac cell 107l/h rhf /  106trc / fs-1  rrc /  rs /  s / s p *ts / fs 2p-1 2 ac-1 2.600.01 1.240.03 4.60.4 0.82 3.100.04 0.710.04 0.0480.003 0.48 0.068 6.910-4 ac-2 2.770.01 1.00.1 3.10.4 0.85 1.20.1 1.570.02 0.4280.006 0.49 0.273 5.310-4 *ts calculated as s/rs well fitted results indicated by acceptable 2 values and low errors of all individual parameters have pointed that eq. (5) was a reasonable model function applied in the impedance analysis of two ac//ac capacitor cells. ct values calculated from ts and p values from table 2 using eq. (10) with  = 2p are listed in table 3, together with esr values calculated as esr = rhf + rrc + rs/3. the corresponding data obtained by the single-frequency method and impedance data plotted in figures 2, 3 and 5 are also listed in table 3. table 3. ct and esr values of ac-1 and ac-2 capacitor cells obtained by single and multi-frequency methods method ac-1 ac-2 frequency range ct / f esr /  ct / f esr /  single frequency (fig.3) -0.082 (0.01hz) 3.07 (105hz) 0.297 (0.01hz) 2.11 (105hz) single frequency (fig.6) -0.082 (0.01hz) 4.36 (103hz) 0.297 (0.01hz) 2.33 (103hz) single frequency (fig.4) -0.082 (0.01hz) 4.63(0.42hz) 0.297 (0.01hz) 2.61 (0.21hz) curve fitting by eqs. (5)+(11) 1106−0.01 hz 0.065 4.58 0.271 2.72 s. sopčić et al. j. electrochem. sci. eng. 8(2) (2018) 183-195 doi:10.5599/jese.536 193 comparison of ct values listed in table 3 shows that generally lower ct values were for both ac//ac capacitor cells obtained by the curve fitting procedure performed using eq. (5) combined with eq. (11), than by using the single frequency method and usual approximation ct =−1/z’’ at f = 0.01 hz. almost 10−20% lower ct values obtained for two cells using the multi-frequency method should be ascribed to influence of not ideal responses of capacitances induced at pore walls of ac electrode materials and seen through 2p  1 (cf. table 2). esr values determined by the single frequency method showed significant dependence on the chosen frequency of evaluation, suggesting significant contribution of distributed electrolyte solution resistance within porous ac electrode material to the evaluated data. the present results also show that for here characterized ac//ac cells, the recommended f = 1 khz is too high for proper esr evaluations. only when the frequency of esr evaluation was rather low, esr values become similar to that determined by the multi-frequency analysis and curve fitting procedure, where distributed resistance effects have already been taken into account by applying eq. (5) combined with eq. (11). conclusions the characteristic values of total charge capacitance (ct) and internal resistance (esr) of two single cell ac//ac capacitors with different quantities of active electrode materials were analyzed using the single and multi-frequency analyses of measured impedance spectra. single frequency ct and esr values were determined on the basis of ideal c−r model, using impedance magnitude values measured at low and high (or already recommended) frequencies, respectively. multi-frequency analysis involving curve-fitting procedure was performed over all measured frequencies and based on the non-ideal r−c model. non-ideal r−c model assumed various impedance contributions including inductive, resistive inherent to the electrode material and/or contacts and that of ac electrode porosity. although close ct and esr values were generally obtained, not any single frequency result was found to agree with the multi-frequency fitting results. beneficially higher ct and lower esr values were generally obtained using the single frequency method, what can be explained by an assumption of ideal capacitive response and too high frequency value of esr evaluation. at the other side, lower ct and higher esr values obtained using the multi-frequency analysis are based on anticipated contributions from porosity and pore surface inhomogeneity of ac electrode material, which all have been taken into account by the non-ideal r−c model used for curve fittings. acknowledgements: this work is supported by the croatian science foundation under the project esup-cap (ip-11-2013-8825). the authors wish to acknowledge prof. miran gaberšček and dr. jože moškon (national institute of chemistry, ljubljana, slovenia) for their delightful collaboration in this project. references [1] b. e. conway, electrochemical supercapacitors: scientific fundamentals and technological applications, kluwer academic publishers/plenum press, new york, usa, 1999. 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http://dx.doi.org/10.5599/jese.637 open access: issn 1847-9286 www.jese-online.org original scientific paper carvacrol electrochemical reaction characteristics on screen printed electrode modified with la2o3/co3o4 nanocomposite sayed zia mohammadi1,, hadi beitollahi2, tahereh rohani1, hossein allahabadi1 1department of chemistry, payame noor university, tehran, iran 2environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran corresponding author: szmohammadi@yahoo.com, tel.: +98 3433353591, fax: +98 3433353621 received: november 16, 2018; revised: january 15, 2019; accepted: january 17, 2019 abstract electrochemical characteristics of carvacrol were investigated on a screen-printed electrode (spe) modified with la2o3/co3o4 nanocomposite by using voltammetric techniques, which displayed a well-defined peak for sensitive carvacrol determination in phosphate buffer solution (pbs) at ph 7.0. la2o3/co3o4 nanoparticles demonstrated suitable catalytic activity for carvacrol determination by differential pulse voltammetry (dpv) technique. besides, determination of carvacrol in a real samples was recognized in the light of electrochemical findings and a validated voltammetric technique for quantitative analysis of carvacrol in a real formulation was proposed. the dpv peak currents were found to be linear in the concentration range of 10.0 to 800.0 μm. the limit of detection (lod) was found to be 1.0 μm. keywords carvacrol, electrochemical reaction, la2o3/co3o4 nanocomposite, differential pulse voltammetry introduction carvacrol (5-isopropyl-2-methylphenol) is the main thyme oil constituent, crucial oil of origanum vulgare, and plants including thymus vulgaris [1]. this compound is an ordinary additive applied in numerous foods (as flavorings), pharmaceuticals and perfumes due to its antitussive, anticarcinogenic, antioxidant, antibacterial, anticancer and antifungal features [2-5]. as the standardization of pharmaceutical compounds is based on their carvacrol content, the development of a novel validated technique for its quantification is a challenging requirement [6,7]. numerous analytical methods such as gas chromatography (gc), thin-layer chromatography (tlc) combined with densitometry, gas chromatography-mass spectrometry (gc-ms) and highperformance liquid chromatography (hplc) with fluorimetric detection have been employed for carvacrol measurement in many matrixes [8-11]. in some cases, lower content of carvacrol in http://dx.doi.org/10.5599/jese. http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:szmohammadi@yahoo.com j. electrochem. sci. eng. 9(2) (2019) 113-123 carvacrol electrochemical reaction 114 complicated matrices makes an emergency task to the preliminary application of preconcentration and separation procedures [12]. as compared with above mentioned methods, which typically require time-consuming and complex sample pretreatment procedure involving separation, extraction, and adsorption, electrochemical sensors have been documented as more authoritative tools [13-16]. the advantages of electrochemical sensors are the possibility of miniaturization and portability, selectivity, sensitivity, a wide linear range, minimal space and power requirement and cost-effective instrumentation. electrochemical sensors have a leading position among all currently accessible sensors that have already reached the commercial stage and found a vast range of important applications in the fields of clinical, industrial, environmental and agricultural analyses [17,18]. compared with bare electrodes, modified electrodes have some advantages. initially, the analyte overpotential can be decreased together with the possible interfering background current. secondly, the current signal response can be improved and therefore, a lower detection limit can be attained. thirdly, adsorption of analytes and products can be omitted [19-22]. consequently, the sensitivity and selectivity of the analysis by using modified electrodes can be significantly increased [23,24]. in addition, nanomaterial-modified electrodes have many benefits over traditional material–modified electrodes [25]. firstly, nanomaterials ensure the vast specific surface area for carvacrol determination on the electrodes. secondly, some semiconductor nanomaterials, especially metal oxides, may act as promoters of electrochemical communication, accelerating the electron transfer rate between active sites and electrodes. thirdly, nanomaterials can assist the electrode to preserve its characteristics for a long period of time [26]. so, a variety of nanomaterials have been synthesized and characterized for the electrochemical analysis [27-32]. nanostructured materials are one of the most promising supporting materials for surface modification of electrodes, due to their unique properties, such as good biocompatibility, high surface-to-volume ratio, enhanced magnetic and electrical properties. metal oxides of the nanometer size exploited in nanomaterial-modified electrodes for electroanalysis, have some distinct advantages such as low influence of the solution resistance, high mass transport rate, low detection limit and better signal-tonoise ratio compared to the conventional electrodes [33-38]. thus, la2o3/co3o4 nanocomposite has already been employed for carvacrol sensor transducer surface which facilitates the electron transfer, enhances the effective electroactive surface area and improves the carvacrol detection limit. nowadays, application of screen-printed electrodes (spes) for analyte analysis is in the main research focus in electrochemical sensors area, what is due to their reliability, mass production, reproducibility, and low cost [39]. as ductile devices, spes can be drawn in different shapes and made from various materials [40]. this study will focus on the design and fabrication of disposable spe modified with la2o3/co3o4 nanocomposite for electrochemical detection of carvacrol. special emphasis will be put on the sensor fabrication procedure, operating details and performance characteristics for carvacrol monitoring. electrochemical properties of fabricated sensor propose sensitive voltammetric determination of carvacrol with advantages of low detection limit, respectable stability and rapid response towards electro-oxidation of carvacrol compared to bare spe. to the best of our knowledge, this investigation is the first report describing the application of spe modified with nanoparticles for carvacrol determination in real-samples. experimental reagents carvacrol was purchased from sigma-aldrich (tulsa, ok, usa). sodium dihydrogen phosphate, sodium monohydrogen phosphate, orthophosphoric acid and sodium hydroxide were provided sayed zia mohammadi et al. j. electrochem. sci. eng. 9(2) (2019) 113-123 http://dx.doi.org/10.5599/jese.637 115 from merck (darmstadt, germany). doubly distilled, deionized water was employed in all experiments. all other chemicals exploited in this study were of analytical reagent grade and purchased from merck (darmstadt, germany). stock solutions of carvacrol were prepared by dissolving the appropriate compound amount in acetonitrile. working solutions were prepared by accurate dilution from stock solutions of these compounds using phosphate buffer solution (pbs). pbs was employed as the supporting electrolyte. apparatus the spe (dropsens, drp-110, spain) consists of three main parts which are silver pseudoreference electrode, graphite counter electrode, and graphite working electrode, respectively. voltammetric measurements (cyclic voltammetry and differential pulse voltammetry) were done using a potentiostat/galvanostat (pgstat 302n, autolab, eco-chemie, the netherlands) with the software gpes, version 4.9. metrohm 710 ph meter, equipped with the glass electrode, was used for ph measurements. the surface morphology of the composites was analyzed with kyky, em 3200 scanning electron microscopy (sem). xrd studied were conducted on a rigaku d/max 2500 v instrument with a graphite monochromator and a cu target. the ft-ir spectrophotometer used for the studies was a bruck equinox 55 and kbr pellet technique was used for loading the samples into the instrument. la2o3/co3o4 nanocomposite synthesis all chemicals used for the preparation of the nano-powders, namely cobalt acetate (co(ch3coo)2.2h2o), thiourea ((nh2)2cs) and ammonia (25 % nh3), were of analytical grade. all the precursors were dissolved in deionized water. during the preparation of the nano-powders, ammonia was used as a complexing agent. co3o4 nanostructures were prepared by dissolving 0.46 mol of cobalt acetate in 80 ml of deionized water, 0.18 mol of thiourea in 80 ml of deionised water and lastly by adding 19.76 ml of ammonia in 80 ml of deionised water. the amount of solutions of cobalt acetate, thiourea and ammonia was held constant at a ratio of 1:1:1. then the cobalt acetate solution was added in a beaker in the reaction bath, followed by adding thiourea solution in the same reaction bath and the mixture was stirred for few seconds. lastly, ammonia solution was added slowly into the mixture, while continuing stirring for 5 min. the temperature of the bath was then allowed to increase up to 80 °c. after the precipitates were formed, they were left overnight and filtered thereafter. the precipitates were then washed with ethanol. the obtained powders were dried at ambient conditions for several days. la2o3/co3o4 nanocomposites were prepared by la2o3 nanoparticles onto the surface of the co3o4 hexagonal nanosheets. a solution of 0.2 mol of la(no3)3,6h2o (50 ml) was mixed with 0.1 g co3o4 hexagonal nanosheets. then appropriate amount of naoh solution (1.5 m) was added dropwise into the solution until the ph value of the mixture reached to 10. the precipitate was brought to a reaction temperature of 80 °c and stirred for 1 h. after the reaction was completed and cooled to room temperature, the resulting products were centrifuged for 15 min at 3000 rpm. finally, the precipitate was washed several times with water and vacuum dried at 60 °c overnight. the obtained black precipitate was la2o3/co3o4 nanocomposite and was ready for use. real samples preparation urine samples were retained in a refrigerator immediately after collection. 10 ml of the samples were centrifuged for 15 min at 2000 rpm. the supernatant was filtered out by using a 0.45 µm filter. next, different volumes of the solution were transferred to a 25 ml volumetric flask and diluted to the mark http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 113-123 carvacrol electrochemical reaction 116 with pbs (ph 7.0). the diluted urine samples were spiked with different amounts of carvacrol. the carvacrol contents were analysed by the proposed method and standard addition procedure. spe electrode fabrication the bare screen-printed electrode was coated with la2o3/co3o4 nanocomposite, as exposed in the following. the stock solution of la2o3/co3o4 nanocomposite in 1 ml of the aqueous solution was prepared by dispersing 1 mg of la2o3/co3o4 nanocomposite with ultrasonication for 1 h, while 2 µl of aliquots of la2o3/co3o4/h2o suspension solution was cast on the carbon working electrodes, followed by waiting until the solvent was evaporated at 25 °c. results and discussion nanostructures characterization figure 1 displays the ft-ir spectrum of la2o3/co3o4 nanocomposite sample in the frequency range of 400–4000 cm-1. the ft-ir spectrum of la2o3/co3o4 nanocomposite shows strong vibrational bands in the lower frequency regions (at around 520 and 449 cm-1), ascribable to the vibration of metal–o. the absorption seen at ~3417 cm-1 is thought to be due to the symmetric vibration of the -oh groups of absorbed h2o molecules [41]. fig. 1. ft-ir image of la2o3/co3o4 nanocomposite from the x-ray diffraction (xrd) pattern of la2o3/co3o4 nanocomposite shown in figure 2, it can be seen that all major diffraction peaks match well with the standard peak of this sample. for la2o3/co3o4 nanocomposite, diffraction peaks at 31.7, 36.1, 44.5, 59.9 and 64.9° (jcpds 74-2120) can be indexed to (220), (311), (400), (511) and (400) planes of co3o4, while diffraction peaks at 15.1, 28.5, 29.1, 39.8, 49.1 and 55.2° (jcpds 41-4019) can be indexed to (100), (002), (101), (102), (211) and (201) plane of la2o3, respectively [42,43]. the morphology of the product was examined by sem. figure 3 shows that co3o4 nanosheets and la2o3 nanoparticles constitute la2o3/co3o4 nanocomposite. sayed zia mohammadi et al. j. electrochem. sci. eng. 9(2) (2019) 113-123 http://dx.doi.org/10.5599/jese.637 117 fig. 2. xrd pattern of la2o3 /co3o4 nanocomposite fig. 3. sem image of la2o3/co3o4 nanocomposite. electrochemical behaviour of fabricated sensor the ph value of the aqueous solution is effective on the electrochemical behaviour of carvacrol. therefore, ph of the solution was optimized to obtain highly accurate results for carvacrol electrooxidation. accordingly, the electrochemical behaviour of carvacrol at the surface of la2o3/co3o4/spe was studied by voltammetry in 0.1 m pbs at ph values between 2.0 and 9.0. it was revealed that neither acidic nor basic medium was appropriate for the electro-oxidation of carvacrol at the surface of la2o3/co3o4/spe and best results were obtained at neutral conditions. accordingly, the ph 7.0 was chosen as the optimum ph for electro-oxidation of carvacrol at the surface of la2o3/co3o4/spe. figure 4 demonstrates the cv responses for electro-oxidation of 400.0 m carvacrol at la2o3/co3o4/spe (curve a) and unmodified spe (curve b). the peak potential corresponding to the carvacrol oxidation occurs at 400 mv, which is about 180 mv more negative than for the unmodified spe. likewise, the anodic peak current for the oxidation of carvacrol at la2o3/co3o4/spe is much higher in comparison with the unmodified spe, indicating thus effectiveness of spe modification with la2o3/co3o4 nanocomposite in the carvacrol oxidation process. based on the obtained results, the oxidation of carvacrol at the surface of la2o3/co3o4/spe can be considered as a simple http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 113-123 carvacrol electrochemical reaction 118 electrochemical reaction. also, the effect of potential scan rates on the oxidation current of carvacrol was assessed and shown in figure 5. according to the results, increase of the potential scan rate causes an increment in the peak current. the linear relationship between anodic peak current (ip) and the square root of the potential scan rate (ν1/2) during the oxidation of carvacrol demonstrates that the oxidation process is under the control of diffusion (fig. 5). from rising sections of the current–voltage curve recorded at 10 mv s−1, the tafel curve of analyte is constructed and plotted n figure 6. the kinetics of electron transfer in the electrode reactions influences tafel regions of the current-potential curve. the estimated tafel slope is 0.1583 v implying that the rate of electrode process is determined by the electron transfer step (rds) with charge transfer coefficient (α) of 0.63 [44]. fig. 4. cvs of la2o3/co3o4/spe (a) and unmodified spe (b) in 0.1 m pbs (ph 7.0) containing 400.0 µm ca. scan rate = 50 mv s-1 fig. 5. cvs of la2o3/co3o4/spe in 0.1 m pbs (ph 7.0) containing 500.0 μm ca at various scan rates (10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mv s-1). inset: variation of anodic peak current vs. ν1/2. sayed zia mohammadi et al. j. electrochem. sci. eng. 9(2) (2019) 113-123 http://dx.doi.org/10.5599/jese.637 119 fig. 6. cv for la2o3/co3o4/spe in 0.1 m pbs (ph 7.0) with a scan rate of 10 mv s-1 in the presence of 500.0 μm ca. the inset shows the tafel plot derived from the cv. chronoamperometric investigation chronoamperometric measurements of carvacrol at la2o3/co3o4/spe were carried out by setting the working electrode potential at 0.5 v vs. ag/agcl/kcl (3.0 m) for various concentrations of carvacrol in pbs (ph 7.0) and the results are shown in figure 7. the determination of diffusion coefficient for electroactive moieties (carvacrol in this case) with a diffusion coefficient of d, is performed using the cottrell equation [44] which under the mass transport limited condition is defined as: i = nfad1/2cb π-1/2 t-1/2 fig. 7. chronoamperograms of la2o3/co3o4/spe in 0.1 m pbs (ph 7.0) for different concentrations of ca. the numbers 1-4 correspond to 0.1, 0.5, 1.0 and 2.0 mm of ca. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms. (b) plot of the slope of straight lines against ca concentration. http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 113-123 carvacrol electrochemical reaction 120 in eq. (2), n, a, d and cb are number of transferred electrons, electrode area (cm2), diffusion coefficient (cm2 s-1) and bulk concentration of electroactive species (mol cm−3), respectively. experimental plots of i vs. t−1/2 are for various concentrations of carvacrol presented in figure 7a, while slopes of the resultant straight lines are plotted versus carvacrol concentrations in figure 7b. from the resultant slope and the cottrell equation, the mean value of d for carvacrol was determined as 3.3×10-6 cm2 s-1. the working curve and detection limit the higher sensitivity of pulse techniques with respect to cv is due to the reduction of charging currents. the electro-oxidation peak current of acetyl carvacrol at the surface of la2o3/co3o4/spe can be used to determine carvacrol in the solution. due to the higher sensitivity and better performance of dpv in analytical applications, dpv experiments were carried out using la2o3/co3o4/spe in 0.1 m pbs (ph=7.0) containing various concentrations of carvacrol. the results shown in figure 8 reveal that at the surface of la2o3/co3o4/spe, there is a linear dependency between electrocatalytic peak currents of carvacrol oxidation and its concentration within the range of 10.0−800.0 m. the correlation coefficient is 0.9998, and the detection limit (3σ) was obtained as 1.0 µm. fig. 8. dpvs of la2o3/co3o4/spe in 0.1 m pbs (ph 7.0) containing different concentrations of ca (10.0, 50.0, 75.0, 100.0, 200.0, 400.0, 500.0, 600.0 and 800.0 μm). inset: plot of i vs. ca concentrations. scan rate = 50 mv s-1. interference studies the influence of various substances as compounds potentially interfering with the determination of carvacrol was studied under optimum conditions. the potentially interfering substances were chosen from the group of substances commonly found with carvacrol in pharmaceuticals and/or biological fluids. the tolerance limit was defined as the maximum concentration of the interfering substance that caused the error less than ±5 % in the determination of carvacrol. according to the results, l-lysine, glucose, nadh, acetaminophen, uric acid, l-asparagine, l-serine, l-threonine, l-proline, l-histidine, l-glycine, l-phenylalanine, lactose, saccarose, fructose, benzoic acid, methanol, ethanol, urea, caffeine, dopamine, epinephrine, norepinephrine, serotonin, ascorbic acid, isoproterenol, levodopa, carbidopa, mg2+, al3+, nh4+, fe+2, fe+3, f−, so42− and s2− did not show interference in the determination of carvacrol. sayed zia mohammadi et al. j. electrochem. sci. eng. 9(2) (2019) 113-123 http://dx.doi.org/10.5599/jese.637 121 the repeatability and stability of la2o3/co3o4/spe the generation of reproducible surface of la2o3/co3o4/spe was examined by cvs data obtained in 0.1 m pbs (ph 7.0) containing ca. the calculated rsd for various parameters was accepted as the criteria for satisfactory surface reproducibility (2-4 %). this degree of reproducibility is virtually the same as that expected for the renewal of ordinary screen-printed electrode surface. in addition, the long-term stability of la2o3/co3o4/spe was tested over 3-week period. when cvs were recorded after the modified electrode was stored in atmosphere at room temperature, the peak potential for carvacrol oxidation was unchanged and the current signals showed less than 2.7 % decrease of the initial response. the antifouling properties of modified electrode towards carvacrol oxidation and its oxidation products were investigated by recording the cvs of modified electrode before and after using in the presence of carvacrol. cvs were recorded in the presence of carvacrol for 20 cycles at a scan rate 50 mv s–1. the peak potentials were unchanged, and the currents decreased by less than 3.9 %. therefore, at the surface of la2o3/co3o4/spe, not only the sensitivity increase, but the fouling effect of the carvacrol and its oxidation product also decreases. however, we regenerated the surface of la2o3/co3o4/spe before each experiment. analysis of real samples the analytical practicality of the proposed method was evaluated through the quantification of carvacrol in urine samples by using the standard addition method. the outcomes for the carvacrol determination in real samples are presented in table 1. the experimental results showed acceptable recoveries for carvacrol. the reproducibility of the method was investigated by the mean relative standard deviation (rsd). table 1. determination of carvacrol in urine samples (n=5) using la2o3/co3o4/spe. spiked concentrations, µm found concentrations, µm recovery, % rsd, % 0 7.5 7.6 101.3 1.8 12.5 12.4 99.2 2.1 17.5 17.3 98.8 3.1 22.5 22.7 100.9 2.7 conclusions for the first time, la2o3/co3o4 nanocomposite modified-spe demonstrates an outstanding behavior toward carvacrol oxidation in an aqueous pbs (ph 7.0) solution. based on the electrochemical oxidation, the quantitative measurement of carvacrol in real samples was developed by a simple, rapid, selective and sensitive 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[44] a. j. bard, and l. r. faulkner, electrochemical methods fundamentals and applications, second ed., wiley, new york (2001). ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese. http://creativecommons.org/licenses/by/4.0/) electrokinetics and soil decontamination: concepts and overview doi: 10.5599/jese.2014.0054 297 j. electrochem. sci. eng. 4(4) (2014) 297-313; doi: 10.5599/jese.2014.0054 open access: issn 1847-9286 www.jese-online.org review electrokinetics and soil decontamination: concepts and overview mohammed a. karim department of civil and construction engineering, southern polytechnic state university (spsu), 1100 south marietta parkway, marietta, georgia 30060, usa e-mail: mkarim@spsu.edu & makarim@juno.com, phone: (678) 915-3026 (off.); (804) 986-3120 (cell) received: february 12, 2014; revised: may 17, 2014; published: december 6, 2014 abstract electrokinetic decontamination and extraction have been proven to be one of the most viable, cost effective and emerging techniques in removing contaminants, especially heavy metals from soils for about last five decades. basic concepts and an overview of the electrokinetic extraction processes and their potential applications in geotechnical and geoenvironmental engineering have been reviewed based on the literature and presented in this paper. primarily, theoretical and laboratory experimental studies related to electroreclamation of soils are summarised in brief with basic concepts of electrokinetic processes. the paper has been divided into different sections that include history of electrokinetics, background and concepts, modelling, parameter effects, instrumentation, contaminant extraction, field applications, and summary and recommendation. based on the review it is obvious that the field application of electrokinetic technology to remediate heavy metal contaminated soils /sediments is very limited and site specific. additional laboratory studies and more pilotand full-scale information from field applications are critical to the further understanding of the technology and to customize the process in different field conditions. keywords electrokinetic decontamination; heavy metals; site remediation; soil; edta; soil ph; electro-osmosis; electrophoresis; streaming potential; ion migration; sediment potential; zeta potential; electrolysis; electrokinetic modelling introduction contaminant such as heavy metals removal from solid porous medium such as soils and sediments has been a technological challenge for engineers and scientists for the past several decades. a variety of remedial options exist to cleanup a hazardous waste site; however, the http://www.jese-online.org/ mailto:mkarim@spsu.edu mailto:makarim@juno.com j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 298 technological challenge, efficiency, and costs of these options may vary widely. conventional ground burial and land disposal are often economical, but they do not provide a permanent solution, and in some cases they are not necessarily the most effective solutions. for removing contaminants such as organics and inorganics from solid porous media, the most common ex-situ methods employed include soil washing, and ligand extraction. ex-situ methods may not be technologically challenged that much; however, they suffer from several problems. apart from the generic problems of any ex-situ process, i.e., the need to excavate the media and place it in an external reactor, the above mentioned processes suffer from several disadvatages [1]. several in situ methods include vacuum extraction, thermal desorption, hydraulic fracturing, electrokinetic decontamination (including the "lasagna" process), biotreatment, immobilization by encapsulation, and placement of barrier systems are already in use to some extent for soil and sediment remediation and decontamination. most of these processes are employed for removal of organics present in soils or sediments. among these in-situ methods electrokinetic decontamination (ekd) processes are in use for the past five decades in different applications. the major advatages of the ekd processes include (a) they can be implemented in-situ with minimal disruption, (b) they are well suited for fine-grained, heterogenous media, where other processes can be ineffective, and (c) accelerated rates contaminant extraction and transport may be achieved. the basic concepts and an overview of the ekd processes and their real life applications, as of now, in geotechnical and geoenvironmental engineering have been reviewed and presented in this paper. primarily, theoretical and laboratory experimental studies related to ekd of soils and sediments are presented in brief with basic concepts of electrokinetic processes. history the movement of water through capilary and pores as a result of the application of electric potential is known as electrokinetic phenomena and this phenomena was first described by f. f. reuss in russia in 1808. this phenomenon was first treated analytically by helmholtz in 1879, which was later modified by pellat in 1903 and smoluchowski in 1921. this phenomenon is widely known as the helmholtz-smoluchowski model which relates electro-osmotic velocity of a fluid of certain viscosity and di-electric constant, through a charged porous medium under an electric gradient. the helmholtz-smoluchowski model is the most common theoretical description of electro-osmosis and is based on the assumption of fluid transport in the soil or sediment pores due to transport of the excess positive charge in the diffuse double layer towards the cathode [2]. it applies to systems with pores that are large relative to the size electric diffuse double layer and provides with reasonable predictions for electro-osmotic flow in most soils. the rate of electroosmotic flow is controlled by the coefficient of electro-osmotic permeability of porous media and the balance between the electrical force on the liquid and the friction between the liquid and the surface of the particles of the porous media. the first application of electrokinetics was made by casagrande in 1939 for consolidation and stabilization of soft fine-grained soils. numerous laboratory studies and a very few field applications have been conducted to investigate the electrokinetic processes to date. the areas in which electrokinetics have been applied successfully to some extent include increasing pile strength, stability of soil during excavation and embankments, increasing flow rate of petroleum production, removal of salts from agricultural soils, removal of metalic objects from the ocean bottom, injection of grouts, microorganisms and nutrients into the subsoil strata of low permeability, barriers and leak detection systems in clay liners, dewatering of clayey formations during excavation, control and decontamination of m. a. karim j. electrochem. sci. eng. 4(4) (2014) 297-313 doi: 10.5599/jese.2014.0054 299 hazardous wastes, removal of chemical species from saturated and unsaturated porous medium, removal of gasoline hydrocarbons and trichloroethylene from clay and removal or separation of inorganic and organic contaminants and radionuclides. background and concepts electrokinetic processes are a relatively new and promising technology being investigated for their potential applications in hazardous waste management specifically in case of high clay containing soils. united state environmental protection agency (usepa) has designated electrokinetic method as a viable in-situ process and interested parties are attempting to apply this method at contaminated sites which have inherently low permeability soils and otherwise difficult to decontaminate. electrokinetic flows occur when an electric gradient is applied on a soilfluid-contaminant system due to existence of the diffuse double layer at the soil particle surface – pore fluid interface. several electrokinetic phenomena arise in clay when there are couplings between hydraulic and direct current (dc) electrical driving forces and flows. those phenomena can broadly be classified into two pairs by the driving forces causing the relative movement between the liquid and the solid phases. the first pair consists of electro-osmosis and electrophoresis, where the liquid or the solid phase moves relative to the other under the influence of an imposed electrical potential. the second pair consists of streaming potential and migration or sedimentation potential, where the liquid or the solid phase moves relative to the other under the influence of hydraulic or gravity force and thus inducing an electrical potential. those four electrokinetic phenomena in clay are depicted in fig. 1 [3]. fig. 1. electrokinetic phenomena in clay j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 300 the detailed description of these flow processes and the associated complicated features generated by electrochemical reactions are given by several authors [4-23]. the use of electrokinetics in sealing leaks in geomembrane and compacted clay liners has been explained in detail by a few authors [24-28]. potential applications of electrokinetics in geotechnical and geoenvironmental engineering are described elaborately by multiple authors [21,22,27,29,30-34]. some of the applications, as appropriate, are reviewed and included in the subsequent sections. the extraction technique, variably called electrokinetic remediation, electroremediation, electroreclamation, electrorestoration, electrochemical soil processing or electrochemical decontamination, uses low level constant voltage dc power supply, potential gradients in the range of 20–200 v m -1 [35] or alternatively a constant current density in the range of 0.025–5 a m -2 [31] between the electrodes placed at the end of the contaminated soil sample. when an electric field is imposed to a wet soil mass, positive ions are moved toward the cathode (the negative electrode) and the negative ions toward the anode (positive electrode) as illustrated in fig. 2 [36]. because of the isomorphous substitution and the presence of broken bonds in the soil structures, excess mobile cations are required to balance the negative fixed charges on the soil particle surfaces. therefore, mobile cations exert more momentum to the pore fluid than do mobile anions. as a result there is a net movement of fluid relative to soil particles under the influence of imposed electric potential gradient which is called electro-osmosis (field-induced convection of water through a porous medium with a surface charge). unlike water flow under pressure, electro-osmosis depends on the electric current through the soil, the flow resistance of soil, and the frictional drag exerted by the migrating ions in the water molecule and this flow originates at the electric double layer of the soil pores. the electrokinetic flow rate qeo in a porous medium of length l, porosity n, area a and degree of saturation s, may be presented by the following equation [37]: d o s eo s d r q i nas l     (1) where d is the potential at the slipping plane, o is the permeability of free space, d is the dielectric constant of the pore fluid,  is the pore water viscosity, is is the current carried by surface conductance and rs is the surface resistance of the porous medium i.e. soil. fig. 2. concept of electrokinetic extraction of contaminants m. a. karim j. electrochem. sci. eng. 4(4) (2014) 297-313 doi: 10.5599/jese.2014.0054 301 when the electrokinetic technique is applied without conditioning of the process fluid at the electrodes, which is termed as unenhanced electrokinetic remediation, the applied electric current leads to electrolysis reactions at the elctrodes, generating an acidic medium at the anode and an alkaline medium at the cathode [38]. the electrolysis reactions of the primary electrodes are presented in the following equations: anode reaction: 2h2o 4e  o2 + 4h + , e o = -1.229 v (2) cathode reaction: 2h2o + 2e  h2 + 2oh , e o = -0.828 v (3) where e o is the standard reduction electrochemical potential, which is a measure of the tendency of the reactants in their standard states to proceed to products in their standard states. although some secondary reactions might occur at the cathode because of their lower electrochemical potential, the water reduction half reaction (h2o/h2) is dominant at early stages of the process. within the first few days of the process, electrolysis reaction drops the ph at the anode below 2 and increases the ph at the cathode above 10, depending the total current applied [9]. the following are the secondary reactions that may exist depending upon the concentration of available species: h + + e  (1/2) h2 (4) m n+ + ne  m (5) m(oh)n(s) + ne  m + noh (6) where m refers to metals. the acid medium (eq. 2) generated at the anode advances through the soil toward the cathode by ionic migration and electro-osmosis due to electrical gradient, pore fluid flow due to any externally applied or internally generated hydraulic gradient and diffusion due to the chemical gradients developed in the system. the base developed at the cathode initially advances toward the anode by diffusion and ionic migration. however, the counterflow due to electro-osmosis retards the back-diffusion and migration of the base front. the advance of this front is slower than the advance of the acid front because of the counteracting electro-osmotic flow and also because the ionic mobility of h + is about 1.76 times that of oh . as a result, the acid front dominates the chemistry across the specimen except for small section of the specimen close to the cathode, where base front prevails [21,35]. as the acid buffer capacity of soil or sediment is low, acid front moving through the soil lowers the system ph. since most heavy metals are soluble in an acidic environment, this lowering of ph promotes desorption of heavy metals from the soil and solubilization of metal ions. ions in dissolved phase can be removed effectively by the combined actions of electro-osmosis and ion migration. however, the presence of heavy molecular weight organic matter (humus substances) within the soil pores may reduce the mobility of the heavy metals due to the formation of organometallic compounds. under these circumstances, enhanced electrokinetic remediation could be necessary. numerous studies have been conducted to date using different chelating and complexation agents to enhance the remedial techniques [39-52]. the particular use of the enhancing and conditioning agents are reviewed and included in the appropriate sections. modeling electrokinetics electrokinetic modeling is based on the applicability of coupled flow phenomena for fluid, solute, current and temperature flow through porous media under the influence of hydraulic, j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 302 electrical, concentration, and thermal gradients, respectively. the governing equations for these analyses generally have been formulated on the basis of the postulates of irreversible thermodynamics and the applicability of the onsager reciprocal relations under the assumption of isothermal conditions [14,16], although equation formulation on the basis of continuity considerations has also been shown [53,54]. the state-of-the-art in modeling electrokinetic remediation is represented by the one-dimensional finite element model for coupled multicomponent, multispicies transport under electrical, chemical and hydraulic gradients described in a study conducted by alshawabkeh and acar [54]. this study compared the predictions of pb removal using the model with the results of pilot scale study involving electrokinetic extraction of pb from a spiked kaolinite sand mixture. multidimensional models for multi spices transport have been developed by several reserachers [55-57]. a study conducted by haran et al. [58] developed a mathematical model for decontamination of hexavalent chromium from low surface charged soils. they simulated the concentration profiles for the movement of ionic species under a potential field for different time period. the model predicted the sweep of the alkaline front across the cell due to the transport of oh ions. a comparison of chromate concentration profiles with experimental data for 28 days of electrolysis showed a good agreement. a numerical model of transport and electrochemical processes was extended for the first time to incorporate complexion and precipitation reactions in a study by jacobs et al. [59]. their model confirmed that the isoelectric focusing could be eliminated and high metal removal efficiencies could be achieved by washing the cathode. in order to describe the transport and reaction processes in a porous medium in electrical field, one-dimensional numerical models have been developed by several authors [60-62]. in several studies, choi and lui [63-66] developed a mathematical model for the elctrokinetic remediation of contaminated soils assuming the contaminants are mostly heavy metals, water is in excess, the dissociation-association of water into hydrogen and hydroxyl ions is rapid, and that electro-osmosis is significant when compared to electromigration (field-induced transport of ions in an electrolyte as defined earlier) as a transport mechanism. the analytical steady state solutions of electroplating and transport in binary electrolyte arising from electrochemistry were provided in several articles by several authors [67-70]. electrolysis and isoelectric focusing effects were also theoretically analyzed by various researchers [68-71]. modified finite difference model of electrokinetic transport in porous media was developed and numerical solutions were provided in studies [60,72]. an assessment of available multispecies transport model and an investigation of long-time behavior of multi-dimensional electrophoretic models were done in couple of studies [9,73]. the quantitative determination of potential distribution in stern-gouy double layer model was elaborated by shang et al. [74]. the analytical and numerical steady state solutions for electrochemical processes with multiple reacting species were provided in articles [75,76]. shackelford [77] summarized the modeling electrokinetic remediation. in his review he emphasized that the prediction of multi-component, multi-species transport with chemical reactions through soil medium represents one of the challenging modeling endeavors in environmental geotechnics. he compared his statement with studies conducted by acar and alshawabkeh [78] and mentioned that this study provided some insight of the advances along these lines. however, he stressed on the additional effort that is needed in evaluating the potential limitations in modeling these electrokinetic processes in terms of the assumptions inherent in the models and field-scale applications. m. a. karim j. electrochem. sci. eng. 4(4) (2014) 297-313 doi: 10.5599/jese.2014.0054 303 instrumentation electrokinetics has many applications in geo-environmental and geotechnical engineering. for the measurements of electrokinetic properties of soil and soil remediation processes, individual researchers have designed their own apparatuses of various shapes, sizes and materials for different purposes. some significant experimental apparatuses used for geotechnical and geoenvironmental engineering investigation have been reviewed in detail by yeung [13]. a number of important apparatuses that have been used for soil remediation by electrokinetics are mentioned here. the apparatuses currently available for the purpose of electrokinetic remediation include those developed at louisiana state university [31,79], lehigh university [52,80,81], university of texas at austin [11,82], the university of california at berkeley [3,83], massachusetts institute of technology [38, 47, 59], texas a & m university [6], the technical university of denmark [84,85], vanderbilt university, nashville, tennessee [86], royal institute of technology, stockholm, sweden [87-89], university of south carolina [58] and many others. a comprehensive review of the apparatus used in the ekd experiments has been presented by yeung et al. [6]. however, it is obvious from the literature that most of these apparatuses are used for the remediation of finegrained soils by electro-osmosis. none of them except the last three are used for the decontamination of course-grained soils such as sandy/salty soils, where the electro-osmosis is ineffective [90]. it is reported that the last two instruments have been successfully used to decontaminate sandy soils using electrolysis and electro-migration. parameter effects the important parameters of ekd processes are electric gradient, system ph, electro-osmotic flow, ion-migration, zeta potential, electro-osmotic permeability, and current density. all of these parameters play important role in the process efficiency, soil decontamination, and ultimately the cost. therefore, parameter optimization should be an important part the process performance. in general, the application of electric gradient induces electric current density and promotes the electrolysis reactions at anode and cathode. electric current results in generation of protons (h + ) at the anode (eq. 2) that migrate together with the metal cations to the negatively charged cathode (fig. 2) for removal and processing. a very low voltage can serve the purpose of electrolytic reactions and create low ph solution in the anode. so determination of optimum electric gradient or current density is important as higher electric gradient or current density may increase the cost of the process and create higher gases in anode and cathode which may require careful watch and become difficult to maintain experiments. electro-osmotic flow is the prevalent parameter for the low permeable soils having high surface charges whereas ion-migration may be the driving force for high permeable soils having low surface charges. system ph contributes to the dissolution of metal precipitates and depends on the type of contaminants and their salts present in the soils. most of the metal salts may be soluble in a ph range of 2 to 4. therefore, bringing the soil ph below 2 may not be necessary to optimize the removal efficiency. it is reported that the values of hydraulic conductivity of different soils can differ by orders of magnitude; however, those of coefficients of electro-osmotic conductivity are generally between 1 × 10 -5 and 10 × 10 -5 cm 2 v -1 s -1 and are relatively independent of soil type. thus, an electric gradient is much more effective driving force than a hydraulic gradient for moving fluid through fine-grained soils of low hydraulic conductivity [6,9,83]. korfiatis et al. [91] used an experimental approach to assess the relative magnitudes of hydraulic and electro-osmotic permeability under application of hydraulic or electric gradients or both and to study the extent of ph changes during j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 304 the electro-osmotic process. the practical and theoretical aspects of ion exchange resins and membranes have been investigated by hansen [85,92]. acar et al. [30] estimated the electroosmotic permeability in kaolinite to be in the range of 0.80 × 10 -5 to 3.0 × 10 -5 cm 2 v -1 s -1 which is within the range reported in the literature. the zeta potential of most soils, except for quartz, is negative, because soil surfaces carry a negative charge that causes the electro-osmotic generally from anode to cathode. the ph and ionic strength of the pore fluid may affect the value of zeta potential and zeta potential is reported to decrease linearly with logarithm of the ph of the porous medium [2]. high acidic solution causes the zeta potential to become less negative and even to attain positive values at low ph. as a result flow rates have been reported to decrease if the ph of the electrolyze is depressed below neutral and to increase at alkaline ph values [47,93]. the effect of zeta potential on electro-osmotic permeability has further been investigated by shang [94]. the steady state and limiting current conditions are investigated by dzenitis [95]. influence of current density and system ph on electro-remediation of kaolinite clay was investigated by rahman [45] and hamed and bhadra [93] and soil saturation effect on electrorestoration was investigated by puppala [46]. the effects of temperature on electrokinetic remediation on low permeability soils are explored by penn [96]. the effects of electrokinetics in complex natural sediments are explained by grundl and reese [97]. shang et al. [98] investigated the effects of polarization and conduction on clay-water-electrolyte systems. shri ranjan and karthigesu [99] devised a capillary flow meter for measuring the hydraulic conductivity of clay under the applications of low gradients. a theoretical and experimental basis on electrokinetic sedimentation is explained by shang [5]. reddy et al. [100] investigated the effects of soil composition on the electrokinetic extraction of chromium (vi). they used three kinds of soil minerals such as kaolin, glacial till, and na-montmorillonite in their study. their study found that the adsorption and removal of cr (vi) are greatly dependent on the compositions of the soil minerals. contaminant extraction there are some cases where unenhanced electrokinetic extraction is ineffective for soil remediation. in this situation chelating and conditioning agents are used to enhance the process which is termed as enhanced electrokinetic remediation. the most commonly used chelating and conditioning agents are ethylene diamine tetraacetate (edta), hcl, acetic acid, iodine-iodide etc. a few important studies using enhanced and unenhanced electrokinetic process have been reviewed and presented below. however, only the studies related to heavy metal removals were reviewed and reported here. heay metal removal with enhanced process in a study conducted by cameselle and reddy [101] found that electro-osmotic flow under applied electric potential depends on a number of soil, contaminant and applied electric potential conditions. electro-osmotic flow induced in the same direction of metal or complexed metal ions transport can enhance heavy metal removal. in case of hydrophobic organic contaminants, periodic voltage application combined with the use of a solubilizing solution is shown to create sustained electro-osmotic flow and enhanced contaminant removal. the suggested to validate the optimum conditions determined from laboratory investigations for generating significant electroosmotic flow through field pilot-scale demonstrations. joseph et al. [41] investigated the feasibility m. a. karim j. electrochem. sci. eng. 4(4) (2014) 297-313 doi: 10.5599/jese.2014.0054 305 of mobilizing precipitate heavy metals from soil by ionic migration using edta. they used edta solution to catholyte where it solubulizes the precipitated metals. the resulting complexes are then transported to the anode. the removal efficiencies were found to be very close to 100 % for zn and pb. a feasibility study of using surfactants and organic acids sequentially and vice versa during ekd was evaluated by reddy et al. [102] for removal of both heavy metals and pahs from clayey soils. they selected kaolinite as a model clayey soil and spiked it with phenanthrene and nickel at concentrations of 500 mg kg -1 dry each to simulate typical field mixed contamination. they performed bench-scale electrokinetic experiments with the sequential anode with 1 m citric acid followed by 5 % igepal ca-720, 1 m citric acid followed by 5 % tween 80, and 5 % igepal ca-720 followed by 1 m citric acid. the migration and removal efficiency of panathrene in the first two sets of tests were found to be very low. but overall the sequential use of 5 % igepal ca 720 followed by 1 m citric acid appeared to be an effective remedial strategy to remove coexisting heavy metals and pahs from clayey soil. the effect of edta in removing pb and zn from millpond sludge during ekd was investigated by karim and khan [39]. they conducted several experiments with distilled water and dilute edta solutions with strengths of 0.05 m and 0.125 m. the beneficial effects of using edta that were observed in this investigation are edta substantially increased the electro-osmotic flow in the millpond sludge indicating that it could significantly reduce the duration of ekd, a significantly higher percentage of pb and zn removal from the solid phase due to the complexation of edta with these heavy metals, and edta was able to prevent the precipitation of metals near the cathode electrode typically observed in ekd process. yeung et al. [103] studied the basic pb-edta complexion reactions and their influence on electrokinetic extraction process. their main focus was on edta enhanced electrokinetc extraction of lead from milwhite and georgia kaolinite and the acid/base buffer and sorption capacities of these soil minerals. their study revealed that more than 90 % of lead was migrated toward the cathode with a lower voltage applied across the sample within a shorter duration of treatment. allen and chen [48] investigated the extraction of lead from the contaminated new jersey and delaware soils with edta. the investigation found almost 100 % extraction of lead from new jersey soil at a 10 -3 m concentration of edta and at 10 -3 m or lower concentration of edta, the recovery of lead that had been added to the delaware soil was greater than that of new jersey soil that had been previously contaminated at level of ph 4.30.1. li et al. [88,89] suggested a new approach in electrokinetic decontamination in which a conductive solution was inserted between the cathode and the soil to be treated. by this approach, the ph in the soil can be kept low so that no metal precipitation would occur near the cathode. this would eliminate the isoelectric focusing effect. their study found the metal removal efficiencies of more than 96 % for both copper and zinc. a similar approach was suggested by shapiro et al. [104] in which acetic acid was used to rinse the catholyte to reduce the ph near cathode. cox et al. [105] studied the remediation of mercury from soils using iodine-iodide as a chelating agent and found it to be very effective. acar and alshawabkeh [78] investigated the feasibility and efficiency of transporting pb under an electric field with a constant current. the tests were conducted with a pb concentration of 856 mg kg -1 and 1,553 mg kg -1 respectively. the third test was conducted on a 1:1 mixture of kaolinite and sand with pb concentration of 5,322 mg kg -1 . their study found that 55 % of pb mobilized inside the soil precipitated within the last 2 cm close to the cathode, 15% were left in the soil before reaching this zone, 20 % precipitated on the fabric separating the soil from cathode, and 10 % were unaccounted. ellis et al. [106] studied the release of cadmium, chromium, copper, lead, and nickel from soil collected from a superfund site near seattle, washington. they conducted both batch j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 306 equilibrium and column studies using edta alone and edta followed by hydroxylamine hydrochloride, to reduce iron oxides in the soil. results of their batch and column tests showed that edta was able to remove more than 90 % pb and 60 % cd. huang et al. [107] found that the removal of zn (ii) from solids is independent of types of solids. the addition of edta resulted in a shift of maximum zn (ii) adsorption to the acidic ph range, and reduction of zeta potential and overall zn (ii) removal in presence of edta was significantly reduced at alkaline ph range and slightly enhanced in the acidic range. klewick and morgan [108] explored the rates of decomposition of complexes for manganese in the +iii oxidation state as a function of the complexing ligand, the total ligand: manganese concentration ratio and the ph. three ligands were chosen, edta was one of them. the rate of appearance of the mn (iii) complex decreased with increasing ph over the range of 6 to 8. mcardell et al. [109] studied cobalt-edta complexation generated on site at oak ridge, tn shallow landfills. their study confirmed the ability of edta to solubilize mineral surface-bond co (iii). davis and singh [110] studied the several chemical washing procedures for zn (ii) contaminated soil to determine the metal extraction efficiency from using specific extractants such as acid solution, edta, diethylenetriamine pentaacetic acid (dtpa), and chlorine. their study found 79 % removal of zn(ii) with 0.001 m edta, 85 % with 0.003 m edta for ph around 2; 79 % with 0.001 m dtpa, 90% with 0.003 m dtpa for a ph of 2, and 85 % with 0.003 m dtpa for a ph of 6. they also found that about 99 % of zn(ii) was in the form of zn-edta complex at ph level 6. amrate et al. [111] tested the removal of lead from an algerian contaminated soil (with pb concentration ≈4.43 mg/g of soil) sited near a battery plant using edta at various concentrations (0.05–0.20 m). they applied a constant voltage corresponding to nominal electric field strength of 1 v cm -1 for duration of 240 hours. results of contaminant distribution across the experimental cell have shown efficient transport of lead toward the anode despite the presence of calcite (25 %) and the high acid/base buffer capacity of the soil. they modified the cell by adding extra compartments and inserting cation exchange membranes (neosepta cmx) to avoid ligand loss, which would be anodically oxidized. they found simultaneous recovery of edta and lead from their chelated solutions. reddy et al. [112] conducted batch and electrokinetic experiments to investigate the removal of three different heavy metals, chromium (vi), nickel (ii), and cadmium (ii), from a clayey soil by using edta as a complexing agent. their batch experiments revealed that high removal of these heavy metals (62–100 %) was possible by using either a 0.1 m or 0.2 m edta concentration over a wide range of ph conditions (2–10). however, the results of the electrokinetic experiments using edta at the cathode showed low heavy metal removal efficiency. they used edta at the cathode along with the ph control at the anode with naoh which increased the ph throughout the soil and achieved high (95 %) cr (vi) removal, but the removal of ni (ii) and cd (ii) was limited due to the precipitation of these metals near the cathode. their finding was that the low mobility of edta and its migration direction, which opposed electro-osmotic flow, prevented edta complexation from occurring. they also found many complicating factors that affected edta-enhanced electrokinetic remediation and suggested further research to optimize this process to achieve high contaminant removal efficiency. heay metal removal with unenhanced process a comprehensive treatise on removal of pb (ii) from kaolin is reported by hamed [34] and hamed et al. [79]. the process removed about 75 % to 95 % of pb (ii) at concentrations up to 1500 g g -1 across the test specimen at a energy expenditure of 29–60 kwh m -3 of soil processed. li et al. [88] examined the efficiency of electro-migration process in removing pb (ii), cd (ii) and m. a. karim j. electrochem. sci. eng. 4(4) (2014) 297-313 doi: 10.5599/jese.2014.0054 307 cr (iii) from sandy soils. their study showed the removal efficiencies more than 90 % for all three metals. hamed and bhadra [93] studied the effect of current density and influent ph on electrokinetic processing. their study results revealed that flow rate increases as the current density increases and the electro-osmotic flow increases gradually between ph of 2 to 10 and sharply between ph of 10 to 12. acar and alshawabkeh [78] investigated the feasibility and efficiency of transporting lead under electric field conducting three pilot-scale tests with leadspiked kaolinite at an electrode spacing of 72 cm. in their tests program, a constant current of density 133 a cm -2 was applied. out of three tests, two of them were conducted with a lead concentration of 856 mg kg -1 and 1,533 mg kg -1 respectively. the third test was conducted on a 1:1 mixture of kaolinite and sand with lead concentration of 5,322 mg kg -1 . their study found that 55 % of lead removal across the soil precipitated within the last 2 cm close to the cathode, 15 % left in the soil before reaching this zone, 20 % precipitated on the fabric separating the soil from cathode and 10 % unaccounted. hansen et al. [84] investigated the removal of cu, cr, hg, pb and zn from sandy loam by electrodialysis. their study found that decontamination of soil was to an extent lower than the recommended critical values for metal concentration in soil. the elctrochemical analysis of ion-exchange membrane with respect to a possible use in electrodialytic decontamination of soil polluted with heavy metals was also studied by hansen et al. [85]. their study revealed that cation-exchange membranes show the transport number of average 0.97 in nacl and cacl2 solutions and anion-exchange membranes about 0.95 in nacl, cacl2 and zncl2 solutions. one-dimensional experimental studies were conducted by yeung et al. [113] and darilek et al. [27,28] to examine the feasibility of using electrophoresis to repair in-service leaking surface impoundment lined by geomembranes. their studies were concentrated on the effect of clay type, clay particle concentration in the suspension and the electric field strength on the cake formation mechanism. acar et al. [114] investigated the removal of cd (ii) from saturated kaolinite under the application of electric current and found to remove more than 95 % of cd (ii) within 10 days of experiment. the effect of various sites and operating conditions on the efficacy of metal removal by electromigration was investigated by hicks and tondorf [38] and pamukcu and wittle [81]. pamukcu et al. [52] investigated the feasibility of electro-osmosis to remove zinc from soil since it was listed among the 129 priority pollutants by epa and is known to possess moderate noncarcinogenic toxicity and is found frequently in the soil in contaminated sites. their finding was encouraging in zinc migration to the cathode chamber. reddy and chinthamreddy [115] studied the migration of hexavalent chromium, cr (vi), nickel, ni(ii), and cadmium, cd (ii), in clayey soils that contain different reducing agents under an induced electric potential. they conducted benchscale electrokinetic experiments using two different clays, kaolin and glacial till, both with and without a reducing agent. the reducing agent used was either humic acid, ferrous iron, or sulfide, in a concentration of 1,000 mg kg -1 . they spiked the soils with cr (vi), ni (ii), and cd (ii) in concentrations of 1000, 500 and 250 mg kg -1 , respectively, and tested under an induced electric potential of 1 v dc cm -1 for duration of over 200 hours. their study found that the reduction of chromium from cr (vi) to cr (iii) occurred prior to electrokinetic treatment and the extent of this cr (vi) reduction was found to be dependent on the type and amount of reducing agents present in the soil. the maximum reduction was found to be occurred in the presence of sulfides, while the minimum reduction was found to be occurred in the presence of humic acid. their study concluded that significant removal of the contaminant from the soils was not achieved and suggested additional research to determine strategies by which contaminant migration may be enhanced and ultimately lead to significant contaminant removal. ricart et al. [116] investigated j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 308 the feasibility of electrokinetic remediation for the restoration of polluted soil with organic and inorganic compounds had been development and evaluated using a model soil sample. they prepared model soil was prepared with kaolinite clay artificially polluted in the laboratory with chromium (cr) and an azo dye: reactive black 5 (rb5). they focused on the electromigration of cr in a spiked kaolinite sample in alkaline conditions. despite of the high ph registered in the kaolinite sample (around ph 9.5), they reported that cr migrated towards the cathode and it was accumulated in the cathode chamber forming a white precipitate. the removal was not complete, and only 23 % of the initial cr was retained into the kaolinite sample close to the cathode side. they also reported that the electrokinetic treatment of a kaolinite sample polluted with both cr and rb5 yielded very good results. the removal of cr was improved compared to the experiment where cr was the only pollutant, and rb5 reached a removal as high as 95 %. rb5 was removed by electromigration towards the anode, where the dye was degraded upon the surface of the electrode by electrochemical oxidation. chromium (cr) was transported towards the cathode by electromigration and electro-osmosis. the concluded that the interaction among rb5 and cr into the kaolinite sample prevented premature precipitation and allow cr to migrate and concentrate in the cathode chamber. the removal of pah and metal contaminants from a former manufactured gas plant polluted soil was studied by reddy et al. [117] and found that the removal is influenced by the type of flushing solution and application of voltage gradient. igepal surfactant was shown to remove pahs, while edta chelant was shown to remove heavy metals. sequential application of surfactant and chelant removed both pahs and heavy metals present in the soil and the efficacy of the process depends on the order of flushing. application of voltage gradient is found to retard the removal of pahs and enhance the removal of metals from the soil. their experiments conducted only for a short duration and suggested to run the experiments for longer duration to establish this as a potent technology for the remediation of soil contaminated by mixed wastes. the study suggested that soil composition can have a profound effect on the contaminant removal; therefore, site-specific soil investigations must be conducted to develop sequential process that will be effective to remove mixed contaminants from the soil. it is apparent to say that enhanced electrokinetic removal technology has been more effective in removing heavy metals from low permeability soils compared to unenhanced electrokinetic removal technology as the enhanchment agents eliminate the ph jump topwards the cathode region and be able to break the organometalic complexes in samples where organic matters are present. edta, a chelating agent that is readily available and environmentally benign and does not interact with soils, seems to be the best enhancing agent, especially to break the organometalic complexes. many of the chelating agents other than edta are ionic and can, in principle, be introduced into the soil by ionic migration. allen and chen [48] have shown that edta is an excellent solubilizing agent for many metals including pb and zn. it is of interest that edta has been used medically to promote removal of lead from the human body and also as an additive to render floor polishes with zinc binders amenable to detergent washing [41]. edta is a tetraprotic acite abbreviated as h4y where y denotes the ethylenediaminetetraacetate ion edta 4. it is slightly solution in water and the four stepwise dissociation constants of the parent acid to yield h3y , h2y 2, hy 3 and y 4 ions are 1.00 × 10 -2 , 2.16 × 10 -3 , 6.92 × 10 -7 and 5.50 × 10 -11 , respectively [48]. it implies that h2y 2 and hy 3 species are major edta anions adsorbed [107]. each edta 4 ion can attach to a metal ion at six different sites since each of four acetate groups and the two nitrogen atoms have free electron pairs available for coordinate bond formation as shown in fig. 3 [118]. m. a. karim j. electrochem. sci. eng. 4(4) (2014) 297-313 doi: 10.5599/jese.2014.0054 309 fig. 3. configuration of metal-edta complexes unless the ph is very high, the edta will not be completely deprotonated. in fact, this is the reason for the high solubility of metal-edta complexes. the complexation of metals by edta is dependent on ph. with a metal ion m, it can form a complex my, a protonated complex mhy, a hydro complex my(oh)n and a mixed complex of the form myx where x is a unidentate ligand. field applications in most practical applications of electrokinetics, the anodes are iron or aluminum rods and the cathodes are steel tubes. sometimes graphite electrodes are also used for both anodes and cathodes. lageman et al. [119] reported the results of field applications in the netherlands. these studies demonstrated about 60 % of zn removal at a concentration of 70 g g -1 from sandy clay soils; 80 % of as removal at a concentration of 90 g g -1 from heavy clayey soils and 75 % of pb removal at a concentration of 340 g g -1 from dredged sediment. the energy expenditure ranged from 60 to 220 kwh m -3 of soil processed. banerjee et al. [120] applied the electrokinetic extraction process in conjunction with the pump-and-treat method in a abandoned industrial hard-chrome plating facility superfund site in corvallis, oregon, usa. their study demonstrated that chromium removal slightly increased, but they didn’t provide any numerical value of removal efficiency. they primarily concluded that ion migration plays a significant role in the decontamination process. in another field study conducted at stadskanaal, the netherlands [121], it is reported that at an energy expenditure of 20 kwh m -3 of soil, pb concentration reduced to 120 mg kg -1 , cd 150 mg kg -1 , and zn 320 mg kg -1 ; at 65 kwh m -3 of soil, pb concentration reduced to 90 mg kg -1 , cd 50 mg kg -1 , and zn 120 mg kg -1 ; and at 180 kwh m -3 of soil, pb and zn concentrations reduced to less than 10 mg kg -1 and cd less than 2 mg kg -1 . in all cases the initial concentrations of pd, cd and zn were 210 mg kg -1 , 300 mg kg -1 , and 480 mg kg -1 , respectively. however, a number of problems not encountered in the laboratory studies arose in the field trails, e.g., presence of unexpected large objects (> 10 cm) buried in the soil. summary and recommendation an overview and concept of electrokinetic extraction processes and their potential applications in geotechnical and geoenvironmental engineering have been reviewed and presented. historically, the success of electrokinetics in soil restoration and decontamination in terms of inorganic contaminants (i.e. heavy metals) has demonstrated its ability to be one of the most cost effective and viable in-situ remediation processes compared to the conventional remediation technologies such as soil washing, ligand extraction, vacuum extraction, thermal desorption, hydraulic fracturing, biotreatment, immobilization by encapsulation, and placement of barrier ooch2c h h ch2coo n c c n ooch2c h h ch2coo m n+ j. electrochem. sci. eng. 4(4) (2014) 297-313 electrokinetics and soil decontamination 310 systems. based on the literature review and researches, it is obvious that the field application of electrokinetic technology to remediate heavy metal contaminated soils /sediments is very limited and site specific. additional laboratory studies and more pilotand full-scale information from field applications are critical to the further understanding of the technology and to customize the process in different field conditions. references [1] m. a. karim, l. i. khan, soil sediment contam. 20(7) (2011) 857-875. 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[121] r. lageman, environ. sci. technol. 27(13) (1993) 2648-2650. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ electrochemical degradation of reactive brilliant red k-2bp on ti/rutiirsnmn oxide anode in a batch cell doi: 10.5599/jese.2012.0022 171 j. electrochem. sci. eng. 2 (2012) 171-183; doi: 10.5599/jese.2012.0022 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical degradation of reactive brilliant red k-2bp on ti/rutiirsnmn oxide anode in a batch cell luo jiancheng*, yang jie*, li weishan*,**,***, huang qiming*,**,*** and xu hongkang**** *school of chemistry and environment, south china normal university, guangzhou 510006, china **key laboratory of electrochemical technology on energy storage and power generation of guangdong higher education institutes, south china normal university, guangzhou 510006, china ***engineering research center of materials and technology for electrochemical energy storage (moe), south china normal university, guangzhou 510006, china ****dongguan hongjie environmental technologies ltd, dongguan 523039, china corresponding author: e-mail: liwsh@scnu.edu.cn; tel.: +86-020-39310256; fax: +86-020-39310256 received: july 06, 2012; published: november 10, 2012 abstract electrochemical degradation of reactive brilliant red k-2bp on ti/rutiirsnmn oxide anode in chloride containing solution was investigated by voltammetry and electrolysis in a batch cell. it is found that the degradation mechanism of k-2bp on ti/rutiirsnmn oxide anode involves an indirect electrocatalytic oxidation, in which k-2bp is oxidized by the electrochemically generated active chlorine. this degradation reaction follows pseudo-first order reaction kinetics. ti/rutiirsnmn oxide exhibits excellent electrocatalytic activity toward the generation of active chlorine from chloride. hence, k-2bp can be electrochemically degraded effectively in chloride containing solution. the decolorization efficiency was found to increase with the decrease in ph and with the increase in current density, nacl concentration, temperature, and flow rate of the solution. keywords decolorization; electrooxidation; active chlorine introduction dye effluent in textile industry is an environmental concern due to its huge quantity, dark color, low biodegradability, and potential toxicity to aquatic life. various treatment methods have been proposed and/or tested to effectively degrade the effluent, including biological treatment [1-7], j. electrochem. sci. eng. 2(4) (2012) 171-183 degradation of reactive brilliant red k-2bp 172 photocatalysis [8-10], ozonation [11,12], wet oxidation [13,14], fenton reaction [15,16], electrochemical coagulation [17-19], and electrochemical oxidation [20-31]. compared with other methods for the treatment of the textile effluent, electrochemical technologies are easy to operate, highly efficient and environmentally friendly. electrooxidation has been widely used to treat various industrial wastewaters, including the wastewaters containing dye, urea, phenol, molasses, microcystins, tannery, olive oil, pesticide, etc. [20,26-32]. there are two mechanisms for the electrooxidation treatment of wastewater, direct and indirect oxidation. in the direct oxidation mechanism, pollutants are oxidized directly on the anode surface. in that case, there is no any other substance involved, except the electrons in the electrooxidation of pollutants. however, pollutants and their degraded products are usually adsorbed on the anode, leading to the low degradation efficiency and the complicated treatment technology for the anode cleaning. in the indirect mechanism, pollutants are oxidized in a bulk solution by a medium, such as active chlorine and hydroxyl radicals electrogenerated in-situ on the anode surface, and thus the adsorption of pollutants and their degradation products can be avoided. the medium is utilized immediately after its electrogeneration. therefore, indirect electrooxidation can lead to a high efficiency of degradation. dimensionally stable anode (dsa) was originally designed for chlor-alkali industry [33]. dsa has long lifetime and it is much cheaper than noble metal anodes such as platinum. it has been widely used in a wastewater treatment research and development. rutheniumand iridiumoxides are the main composition of dsas. they exhibit good electrocatalytic activity for active chlorine generation and oxygen evolution. based on these two oxides, many composite oxides have been developed for dsa use. for example, ruthenium-based oxides, especially ti/ti0.7ru0.3o2, have been reported to treat the refractory effluent [26,28,30,31]. reactive brilliant red k-2bp (k-2bp) is an azo-dye which is extensively used in the textile industry. its molecule structure is shown in fig. 1. k-2bp is resistant to either biological or photodegradation. the solution containing k-2bp is brilliant red and can be distinguished even in trace concentration by naked eye. figure 1. molecule structure of reactive brilliant red k-2bp in this work, a coating on titanium substrate of composite oxides based mainly on ru and ir, ti/rutiirsnmn oxide, was used as an anode to degrade k-2bp by the indirect electrooxidation with electrochemically generated active chlorine as the medium. the parameter effects of electrolytic conditions on the degradation efficiency were investigated and the degradation mechanism was discussed. experimental reagents and solutions deionized water was used for the preparation of all solutions. sodium hydroxide and sulfuric acid were used to adjust ph of the solution. sodium chloride was used for the generation of active so3na n n nao 3s so 3na oh hn n h n n n cl cl l. jiancheng et al. j. electrochem. sci. eng. 2(4) (2012) 171-183 doi: 10.5599/jese.2012.0022 173 chlorine. sodium sulfate and sodium nitrate were also used as electrolytes for comparison. all the reagents were analytical reagents and purchased from sinopharm chemical reagent co., ltd, china. the commercially used textile azo dye, mainly containing reactive brilliant red k-2bp, was purchased from the local market. all the chemicals were used as received without further purification. electrolysis cell the experiments were carried out in an un-divided filter-press cell (fig. 2) with constant current at fixed temperature of 30 °c except otherwise stated. the dye-containing solution was supplied from the feed tank (250 ml) to the electrolysis cell by a peristaltic pump. the treated water can be recycled back to the solution tank if necessary. constant current was provided by a d. c. power supply (363, eg&g). a magnetic stirrer was used to keep the solution mixed and a thermostatic water bath was employed to control the temperature. the solution volume was 200 ml with 100 mg/l k-2bp. the anode (ti/rutiirsnmn oxide) was supplied by guangzhou etsing plating research institute, which was prepared by coating the composites (the compounds containing ru, ti, ir, sn, and mn) on ti plate and treated under high temperature. the cathode was made from stainless steel (sus304). both electrodes, anode and cathode were of the same size of geometric area having 21 cm2 exposed to the solution. the gap between the two electrodes was 10 mm. before each set of experiments, the anode was immersed in absolute alcohol for 5 min and then rinsed with deionized water. the cathode was polished with sandpaper (2000), then immersed in 1.0 m hcl solution for 5 min, and finally rinsed with deionized water. 1. d. c. supply 2. magnetic stirrer 3. peristaltic pump 4. un-divided filter-press cell 5. feed tank 51.sampling 52. inlet 53. outlet figure 2. schematic diagram of experimental setup for the degradation of k-2bp analytical procedure at desired time interval, the dye solution was sampled and filtered through a 0.45 μm millipore membrane. several drops of saturated sodium sulfite solution were added into the sample to prevent further oxidation of the dyestuffs and their intermediates by residual chlorine. the samples were analyzed immediately after their collection using a shimadzu uv-1700 double-beam spectrophotometer. a v on o ff + tem stir tem ctrl stir ctrl 2 1 3 45 51 52 53 a v on o ff + tem stir tem ctrl stir ctrl 2 1 3 45 a v on o ff +a va v on o ffon o ff ++ tem stir tem ctrl stir ctrl tem stir tem ctrl stir ctrl temtem stirstir tem ctrltem ctrl stir ctrlstir ctrl 2 1 3 45 51 52 53 j. electrochem. sci. eng. 2(4) (2012) 171-183 degradation of reactive brilliant red k-2bp 174 the concentration of k-2bp in the samples was determined from the uv-vis absorbance recorded at 520 nm (max) and the degradation efficiency (a / %) was calculated according to [28]: 0 t 0 ( ) / % 100 a a a a − = where a0 and at are the absorbance of the solution before and after electrolysis, respectively. voltammetric measurements in order to understand the degradation mechanism of k-2bp on ti/rutiirsnmn oxide anode, linear and cyclic voltammograms were obtained by using a three-electrode cell (solartron 1470e, solartron co., ltd.). a pt plate was used as the counter electrode, a saturated hg/hg2cl2 electrode (sce) was used as the reference electrode, and ti/rutiirsnmn oxide disk with a diameter of 1 cm was used as the working electrode. the potentials reported are with respect to the sce. all the electrochemical measurements were carried out at room temperature. linear voltammetry was performed in 0.5 m h2so4 solutions with and without 1.0 m nacl from 0.5 v to 1.35 v at a can rate of 1 mv s-1. cyclic voltammetry was performed in 0.5 m h2so4 + 1.0 m nacl solutions with and without 200 mg l-1 k-2bp between 0.5 v and 1.15 v at 1 mv s-1. the cyclic voltammograms reported in this paper are the results of the 5th cycle. before the experiments, the working electrode was immersed in absolute alcohol for 5 min, rinsed by deionized water and kept in the experimental solutions for 30 min. results and discussion effect of salts in order to enhance the diffusion and adsorption of dyestuffs onto the textile, abundant inorganic salts, such as nacl, na2so4 and nano3, are added into the dyestuff slurries. this technology results in the real textile effluent containing various electrolytes. on the other hand, the existence of salts improves the conductivity of the effluent and reduces the electric energy needed for the electrochemical treatment of the effluent. therefore, in this paper the effect of various salts on degradation efficiency of k-2bp on ti/ruirsnmnti anode was investigated. as shown in fig. 3, the decolorization efficiency of k-2bp in both na2so4 and nano3 solutions increases slowly and linearly with treatment time. the decolorization efficiency of k-2bp is only 50.9 % after one hour electrolysis in both na2so4 and nano3 solutions. however, the decolorization efficiency of k-2bp in nacl solution increases drastically in the first 20 min, then increases slowly in the later stage and reaches almost 100 % after 40 min. this indicates that nacl favors the degradation of k-2bp. the mechanism can be explained as follows. in the nacl-free solution, water is decomposed on the surface of metal oxide anode, producing hydroxyl radicals, and the dye is degraded into co2 by hydroxyl radicals subsequently (eqs. 1 and 2): h2o – e →·oh + h+ (1) ·oh + dye – e→ h2o + h+ + co2↑ (2) the reaction of water decomposition on anode to form hydroxyl radicals is usually accompanied by the oxygen evolution reaction (eq. 3), which reduces formation efficiency of the hydroxyl radicals and thus leads to the low degradation efficiency of k-2bp in both na2so4 and nano3 solutions. 2h2o – 4e → 4h+ + o2↑ (3) l. jiancheng et al. j. electrochem. sci. eng. 2(4) (2012) 171-183 doi: 10.5599/jese.2012.0022 175 0 10 20 30 40 50 60 0 20 40 60 80 100 d ec o lo ri za ti o n , % time, min. nacl nano 3 na 2 so 4 figure 3. effect of supporting electrolyte (se) on the decolorization efficiency of k-2bp. (j = 30 ma cm-2, cse = 4.0 g l -1, flux = 50 ml min-1, initial ph = 7.0) the composite of ru, ir, sn, mn, and ti oxides exhibits high activity for the oxidation of chloride ions to form active chlorine. in the nacl-containing solution, chloride ions are oxidized on this kind of composite oxide anode, forming active chlorine [22,27,29] (eq. 4). 2cl– 2e→ cl2↑ (4) the active chlorine, electrogenerated in-situ on an anode surface, transforms into a strong oxidant, such as hclo, which oxidizes dyes into small molecules or even carbon dioxide (eq. 5). therefore, higher degradation efficiency of the dye on the anode in the nacl-containing solution can be expected than that in nacl-free solution. hclo + dye (and its oxidation intermediates) → h2o + cl+ co2↑ (5) fig. 4 presents the linear voltammograms of ti/ruirsnmnti oxide electrode in 0.5 m h2so4 solution with and without nacl. 0.6 0.8 1.0 1.2 1.4 0 5 10 15 20 b c u rr en t, m a potential, v vs. sce a: without nacl b: with nacl a figure 4. linear voltammograms of ti/ruirsnmnti oxide electrode in the solutions with and without nacl j. electrochem. sci. eng. 2(4) (2012) 171-183 degradation of reactive brilliant red k-2bp 176 in the nacl-free solution, the decomposition potential of water is 1.231 v. however, when chloride ions are added, the decomposition potential shifts negatively to 1.142 v and the current increases significantly. this indicates that chloride ions can be oxidized more easily on ti/ruirsnmnti oxide electrode than the water decomposition, thus the ti/ruirsnmnti oxide shows a good electrocatalytic activity toward the oxidation of chloride ions. fig. 5 presents the voltammograms of ti/ruirsnmnti oxide electrode in 0.5 m h2so4 + 1.0 m nacl solutions with and without 200 mg/l k-2bp. for the electrode in the solution without k-2bp, the current increases sharply at the potential higher than 1.1 v during the forward scanning. based on the observation of fig. 4, it follows that the oxidation current is corresponding to the formation of active chlorine from oxidation of chloride ions. active chlorine can be detected during the backward scanning, as shown by the current peak at about 1.09 v. for the electrode in the solution with k-2bp, the current for the oxidation of chloride ions is similar to that in the solution without k-2bp, but the peak current for the reduction of active chlorine is reduced. this suggests that k-2bp is not oxidized directly on the ti/ruirsnmnti anode, but indirectly by active chlorine. 0.4 0.6 0.8 1.0 1.2 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 b c u rr en t, m a potential, v vs. sce a: without k-2bp b: with k-2bp a figure 5. cyclic voltammograms of ti/ruirsnmnti oxide electrode in 0.5 m h2so4 + 1.0 m nacl solutions with and without 200 mg l -1 k-2bp effect of current density charge transfer step is a critical process in electrocatalysis, where the current density at the anode surface determines the rate of the electrode reaction. the effect of current density on the decolorization efficiency was investigated. fig. 6 presents the obtained results. it can be seen from fig. 6 that in the first 15 min, when the applied current density rises from 5 to 20 ma cm-2, the decolorization efficiency increases drastically from 50 to 85 %. there is an optimal decolorization efficiency at 20 ma cm-2. a further increase of current density makes no obvious improvement in the decolorization efficiency. this phenomenon can be explained as follows. when the applied current density is low, the production rate of active chlorine is less than its consumption rate for the oxidation of k-2bp, resulting in the low decolorization efficiency. when the applied current density is high, the production rate of active chlorine increases and the decolorization efficiency is improved. as the applied current density increases further, the concentration of chloride ions on anode surface is deficient and active chlorine is no more available for oxidizing the k-2bp. l. jiancheng et al. j. electrochem. sci. eng. 2(4) (2012) 171-183 doi: 10.5599/jese.2012.0022 177 0 10 20 30 40 50 60 0 20 40 60 80 100 d ec o lo ri za ti o n , % time, min. 5ma/cm2 10ma/cm2 20ma/cm2 30ma/cm2 40ma/cm2 figure 6. effect of current density on the decolorization efficiency of k-2bp (cnacl=4.0g/l, flux=50ml/min, initial ph=7.0) effect of nacl concentration fig. 7 presents the effect of nacl concentration on the decolorization of k-2bp. it can be seen from fig. 7 that in the broad range of nacl concentration, from 0.5 to 5.0 g l-1, k-2bp can be removed completely, even at low nacl concentration. for instance, when the dose of nacl is 0.5 g l-1, the decolorization efficiency reaches 90 % in the first 25 min, and in the later 15 min the decolorization efficiency reaches 100 %. by increasing the dose of nacl from 0.5 to 2.0 g l-1 the decolorization rate is improved. however, a further increase of the nacl dose does not improve the decolorization rate any further. 0 10 20 30 40 50 60 0 20 40 60 80 100 d ec o lo ri za ti o n , % time, min. 0.5g/l 1.0g/l 2.0g/l 3.0g/l 4.0g/l 5.0g/l figure 7. effect of nacl concentration on the decolorization efficiency of k-2bp(j=30 ma/cm2, flux=50 ml/min, initial ph=7.0) the electrooxidaton process of k-2bp on ti/ruirsnmnti anode is determined not only by the charge transfer step, but also by the mass transfer step. at low nacl concentration, the production rate of active chlorine is less than its consumption rate with k-2bp, leading to low decolorization j. electrochem. sci. eng. 2(4) (2012) 171-183 degradation of reactive brilliant red k-2bp 178 rate. the increase of nacl dose can reduce the effect of the mass transfer process and increases the formation rate of active chlorine. when the dose of nacl is 2.0 g l-1, the formation rate of active chlorine matches the reaction rate between active chlorine and k-2bp. further increasing of the dose of nacl does not increase the decolorization rate, because the formation of active chlorine is controlled by the charge transfer step. effect of initial ph fig. 8 presents the effect of the initial ph of the solution on the decolorization efficiency. as shown in fig. 8, the initial ph of the solution affects the decolorization efficiency of k-2bp significantly. in the ph range between 1.0 and 11.0, the decolorization efficiency of k-2bp increases with the decrease of the initial ph of the solution. 0 10 20 30 40 50 60 0 20 40 60 80 100 d ec o lo ri za ti o n , % time, min. ph1.0 ph3.0 ph5.0 ph7.0 ph9.0 ph11.0 figure 8. effect of initial ph on the decolorization efficiency of k-2bp(j=30 ma/cm2, cnacl=4.0 g/l, flux=50 ml/min) in aqueous solution, the active chlorine in-situ electrogenerated is in the form of hypochlorous acid and hypochlorite ion, which reach an equilibrium (eq. 6): hclo ⇄ clo+ h+ (6) according to the le chatelier’s principle, the increase of the hydrogen ion concentration shifts the equilibrium to the left in favor of forming hypochlorous acid. the standard formation potential of hypochlorous acid and hypochlorite ion is 1.49 v and 0.81 v, respectively, and the potential is a function of hydrogen ion concentration in solution (eqs. 7 and 8): φ(hclo/cl-) = φθ(hclo/cl-) + rt/2f ln([h+][hclo]/[cl-]) (7) φ(clo-/cl-) = φθ(clo-/cl-) + rt/2f ln([clo-]/[oh-]2[cl-]) (8) therefore, oxidative ability of hypochlorous acid is much stronger than of hypochlorite ion, and the smaller the value of ph is, the stronger is the oxidative ability of hypochlorous acid [30,34-37]. effect of flow rate the effect of solution flow rate on decolorization efficiency of k-2bp was considered in the range from 10 to 70 ml min-1. fig. 9 presents the obtained results. it can be seen from fig. 9 that decolorization efficiency increases as the flow rate increases from 10 to 50 ml min-1. further increase in the flow rate does not affect the decolorization efficiency significantly. apparently, the l. jiancheng et al. j. electrochem. sci. eng. 2(4) (2012) 171-183 doi: 10.5599/jese.2012.0022 179 formation process of active chlorine is also controlled by the diffusion of chloride ions. the diffusion layer thickness is proportional to the flow rate, i.e. the faster flow rate favors the diffusion of chloride ions to the electrode surface. however, at high flow rate, turbulent flow appears and the effect of flow rate cannot be observed. 0 10 20 30 40 50 60 0 20 40 60 80 100 d ec o lo ri za ti o n , % time, min. 10ml/min 30ml/min 50ml/min 70ml/min figure 9. effect of flow rate on the decolorization efficiency of k-2bp (j = 30 ma cm-2, cnac l= 4.0 g l -1, initial ph = 7.0） effect of temperature fig. 10 presents the effect of temperature on the decolorization efficiency of k-2bp. as shown in fig. 10 the increasing temperature leads to the enhancement of the decolorization efficiency. the time of complete decolorization is 35 min at 303 k, while it reduces to 20 min at 343 k. the enhancement of decolorization efficiency of k-2bp can be ascribed to the improvement of the charge transfer rate and the reaction rate of the active chlorine with k-2bp in the solution. 0 10 20 30 40 0 20 40 60 80 100 d ec o lo ri za ti o n , % time, min. 303k 313k 323k 333k 343k figure 10. effect of temperature on the decolorization efficiency of k-2bp ( j= 30 ma cm-2, cnac l= 4.0 g l -1, flux = 50 ml min-1, ph = 7.0) j. electrochem. sci. eng. 2(4) (2012) 171-183 degradation of reactive brilliant red k-2bp 180 reaction kinetics the variation of k-2bp concentration (absorbance) with time at different temperatures was modeled by the zero order, the first order and the second order reaction. the obtained results are shown in table 1. it can be seen from table 1 that the correlation coefficient (r2) of the first order kinetics model is about 0.999, far higher than those of zero and second order models. this indicates that the decolorization process of k-2bp follows the pseudo-first-order kinetics. therefore, the decolorization rate equation can be expressed by (eq. 9): t t obs 0 0 ln ln c a k t c a = = − (9) where t is the reaction time (min), c0 and ct are the concentrations (mg l -1) of k-2bp at the initial and at t, a0 and at are the absorbance of k-2bp at the initial and at t, and kobs is the pseudo-firstorder kinetic constant (min-1). table 1. results of fitting of the kinetics equation for zero-order, first-order and second-order of reactive brilliant red k-2bp at various temperatures temperature, k zero order second order first order r2 r2 r2 kobs/min -1 303 0.84361 0.91002 0.99933 0.15119 313 0.79883 0.87378 0.99946 0.16941 323 0.76557 0.79324 0.99987 0.19533 333 0.71530 0.71310 0.99903 0.23650 343 0.67154 0.71861 0.99574 0.28096 fig. 11 shows the linear relationship between the decolorization time t and ln (at/a0). fig. 12 presents the relationship between rate constant kobs and temperature. it can be seen that the rate constant increases with temperature. 0 5 10 15 20 -6 -4 -2 0 ln ( a /a 0 ) time, min. 303k 313k 323k 333k 343k linear fit of 303k linear fit of 313k linear fit of 323k linear fit of 333k linear fit of 343k figure 11. first-order kinetic fitting results of decolorization rate of k-2bp at various temperatures l. jiancheng et al. j. electrochem. sci. eng. 2(4) (2012) 171-183 doi: 10.5599/jese.2012.0022 181 300 310 320 330 340 350 0.15 0.20 0.25 0.30 k o b s / m in -1 temperature, k figure 12. relationship between rate constant kobs and temperature uv-visible spectroscopy in k-2bp there are aromatic rings, naphthalene rings, triazine rings and -n=ngroups. -n=nis the chromophore group and the bridge between the aromatic ring and naphthalene to form a big bond conjugate system. fig. 13 presents the variation of k-2bp uv-visible spectrum with time. k2bp is characterized by three absorbance peaks of 235 nm, 285 nm and 370 nm in ultraviolet region and only one peak of 520 nm in visible region. the absorbance in the visible region should be ascribed to the chromophore group of -n=n-, and the absorbance in ultraviolet region should be ascribed to the aromatic ring, triazine ring and naphthalene ring, respectively [38-40]. it can be seen from fig. 13 that the intensity of all the absorbance peaks decreases rapidly with time and these peaks disappear completely in 30 min. this indicates that in the nacl-containing solution, ti/ruirsnmnti anode can remove the color of k-2bp completely, and the triazine ring, aromatic ring and naphthalene ring can be cleaved into aliphatic acid with low molecular weight or even completely mineralized. 200 300 400 500 600 700 800 0 1 2 3 4 a b so rb a n ce , a . u . wavelength, nm 0min 5min 10min 15min 20min 25min 30min figure 13. uv-vis spectrum variation of k-2bp during electrolysis in nacl solution j. electrochem. sci. eng. 2(4) (2012) 171-183 degradation of reactive brilliant red k-2bp 182 conclusions ti/ruirsnmnti oxide anode can be used to degrade reactive brilliant red k-2bp effectively in the solution containing chloride ions. the degradation mechanism of k-2bp is an indirect electrocatalytic oxidation, in which k-2bp is oxidized by the electrogenerated active chlorine. this degradation reaction follows pseudo-fist-order-reaction kinetics and can be improved by increasing current density and nacl concentration, enhancing the temperature and flow rate, and lowering the ph of the solution. acknowledgements: this work was financially supported by natural science foundation of guangdong province (grant no. 10351063101000001) and combination project of enterprise, university and scientific research of guangdong province (grant no. 2010b090400019). references [1] h. salvado, m.p. gracia, j.m. amigó, water res. 29 (1995) 1041-1050. 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[40] x.j. lu, b. yang, j.h. chen, r.sun, j. hazard. mater. 161 (2009) 241-245. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true /dscreportinglevel 0 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice {determination of lead(ii) and cadmium(ii) in water lily stems using a bismuth film electrode} http://dx.doi.org/10.5599/jese.651 153 j. electrochem. sci. eng. 9(3) (2019) 153-164; http://dx.doi.org/10.5599/jese.651 open access: issn 1847-9286 www.jese-online.org original scientific paper determination of lead(ii) and cadmium(ii) in water lily stems using a bismuth film electrode monthira somkid, pipat chooto analytical chemistry division, department of chemistry, faculty of science, prince of songkla university, hat yai, songkhla, 90112, thailand corresponding author: pipat.c@psu.ac.th; tel.: 66 7428 8431; fax: +66 7455 8841 received: december 29, 2018; revised: february 28, 2019; accepted: march 4, 2019 abstract mesoporous silica nanoparticles and chitosan were combined with ex-situ bismuth film modified glassy carbon electrode (nanosio2–cts/bife) for simultaneous determination of pb(ii) and cd(ii) in hcl/kcl ph 2.2. differential pulse anodic stripping voltammetry (dpasv) measurements, based on formation of ion-associates between chlorocomplexes and protonated amino groups (–nh3+) of chitosan at the electrode surface, were explored. under optimum conditions, the linear correlation coefficient was 0.998 for both pb(ii) and cd(ii) after 240 s of deposition time. the limits of detection (lod) and quantification (loq) were 1.0 g/l and 3.3 g/l for pb(ii) and 0.5 g/l and 1.6 g/l for cd(ii), respectively. the method exhibited satisfactory reproducibility and the results comparable to icp-oes. also, dpasv at nanosio2– cts/bife was applied successfully in determining pb(ii) and cd(ii) in water lily stems in thale noi. keywords anodic stripping voltammetry; mesoporous silica nanoparticles; chitosan introduction thale noi is a freshwater lake at the uppermost part of songkhla lake in the south of thailand, between 7°45'-55' n and 100°05'-15 e. its area is approximately 27 square kilometers and the depth 1.5 meters on average [1]. people around thale noi are living on crafts, especially handicrafts made from krajood (lepironia articalata). to make the products colorful, the plant needs to be dyed with the substances containing toxic heavy metals including lead (pb), chromium (cr), cadmium (cd), copper (cu) and nickel (ni) [2], and the resulting wastewater contaminated with heavy metals is usually released to water resources of thale noi. phytoremediation is direct use of plants to clean up contaminated water, soils and sediment. this technology is cost effective and environmentally friendly [3]. in addition, the roots of aquatic plants http://dx.doi.org/10.5599/jese.651 http://dx.doi.org/10.5599/jese.651 http://www.jese-online.org/ file:///d:/dokumenti/dropbox/03_jese/0_web/articles/olf/doc/pipat.c@psu.ac.th j. electrochem. sci. eng. 9(3) (2019) 153-164 determination of pb and cd in water lily stems 154 such as water lily (macrophytes) have an ability to absorb and accumulate heavy metal from polluted water resources [4]. water lily (nymphaea lotus) is an herbaceous water plant with widespread distribution in thale noi. one of the edible parts of water lily is a stem which can be used in cooking a variety of foods. therefore, growing of water lily in the soil contaminated with heavy metals makes a possible risk to human health. previous researches concerning thale noi were addressed to determination of lead and chromium concentrations in water [5], as well as lead and arsenic in water, sediment and water plants, including esthwaite waterweed (hydrilla verticillata), coontail hornwort (ceratophyllum demersum), blue water lily (nymphaea stellata) and water lily (nymphaea lotus)[6]. according to the studies on heavy metal contents in water lily plants, high ability for uptake of heavy metals including lead (pb), cadmium (cd), iron (fe), arsenic (as), copper (cu), nickel (ni) and mercury (hg) was also found [7]. the techniques that have been used for heavy metal determination include atomic absorption spectrometry (aas), inductively coupled plasma-mass spectrometry (icp-ms), inductively coupled plasma-optical emission spectrometry (icp-oes) and electroanalytical techniques. although being commonly used, spectroscopic techniques have disadvantages of high cost and need for preconcentrating and extracting steps, which are tedious with the risk of loss and contamination [8]. electroanalytical techniques, in particular anodic stripping voltammetry (asv) at modified electrodes, could be considered as a preferable choice due to more satisfactory detection limit, higher sensitivity, lower cost, faster operation, higher degree of simplicity and greater capability in simultaneous analysis [9]. a great variety of electrode materials have already been investigated due to their crucial role in improving analytical performances of the method. compared with other materials, bismuth offers a number of superb characteristics covering sensitivity, low toxicity, simplicity in preparation, no oxygen interference and also, well-defined and well-separated analytical signals [10]. application of bi as a modifying agent started with detection of lead [11]. its versatile applications have thoroughly been reviewed [12], whereas its few recent uses are also referred here. particle sizes were found to play important role in thermodynamic properties of bi nano-oxide which can be applied in a variety of areas including analysis and energy [13]. despite specific treatment to form alloys, the nanoporous bismuth modified electrode was found to provide better life time and precision [14]. in forming thin film with tin, bismuth alloy electrodes with different compositions were studied in order to obtain greater accuracy in simultaneous trace metal analysis [15]. it was also shown that bi nanoparticles decorated in graphene nanotubes in the screen-printed electrode form can be used for hg detection [16]. more importantly, a mesoporous nano-silica modified bismuth film was found to increase the surface area and enhance the sensitivity [17]. an additional substance selected as an adhesion reagent for mesoporous nano-silica is chitosan. this natural product extracted from the shell of shrimps, crabs and insects has both hydrophilic and hydrophobic groups. the chitosan amino group has pka about 6.5 [18], suitable for its protonation under acidic or neutral conditions. these reasons prove chitosan as water soluble, bioadhesive and easily binding to negatively charged metal ion complexes. since chitosan has special characteristics of biocompatibility, biodegradability, non-toxicity and adsorption properties, several reports have already been published, involving chemical modification of electrodes by chitosan [19] and their application in determination of various metals [20-22]. monthira somkid and pipat chooto j. electrochem. sci. eng. 9(3) (2019) 153-164 http://dx.doi.org/10.5599/jese.651 155 the principle of the method involving anodic stripping voltammetry (asv) at a chitosan modified electrode is in accordance with that proposed by khaled et al. [23] and involves attraction of mcl42 to the electrode surface where metal ions are reduced by applying the negative potential. in the next step, the metal ions are re-oxidized, resulting in stripping current peaks as an analytical signal. selective current enhancement could be explained by the electrostatic attraction between protonated amino groups (–nh3+) of chitosan and negatively charged metal chlorocomplexes. to picture the whole process, this enhancement starts with the protonation of amino groups to increase density of active sites on the electrode surface. metal chlorocomplexes then attach to these positive sites to balance the charge. after that upon potential control, the metal ions are firstly reduced and finally stripped out of the surface in the form of chlorocomplexes once again. the present research is focused on the use of bismuth film modified with mesoporous silica nanoparticles and chitosan for simultaneous determination of pb(ii) and cd(ii) ions by asv technique. the modified electrode is applied to water lily stems to pave the way for monitoring the metals in local raw materials for food products. experimental reagents all the reagents were of analytical reagent grade. mesoporous silica nanoparticles were of 200 nm particle size and 4 nm pore size. low molecular weight chitosan was purchased from sigmaaldrich. standard solutions of pb(ii) and cd(ii) (1000 mg/l) were from fluka. bismuth salt (bi(no3)3×5h2o) was dissolved to get 0.01 mol/l stock solution. a supporting electrolyte hcl/kcl was obtained by combining 0.1 mol/l hcl with 0.1 mol/l kcl and ph was adjusted to 2.2 by ph meter. double deionised water (milli-q millipore 18.2 mω cm was used in preparing all aqueous solutions. before use, all glassware was soaked overnight in a 10 % v/v nitric acid and then rinsed with deionized water. instruments all voltammetric experiments were conducted on a powerlab 2/20 potentiostat (adinstrument, australia) with echem software version 2.1.16. a glassy carbon electrode (gce) (diameter of 3 mm, bas) was used as a working electrode, ag/agcl (3m kcl) as a reference electrode and pt wire as a counter electrode. all electrodes were put in 50 ml cell for electrochemical measurements and the solution was stirred with a magnetic stirrer without any deaeration. ph of solutions was measured by ph meter model 510 (eutech instruments, usa). icp-oes (perkin-elmer, usa) was used to validate sample analysis by the proposed method. ex-situ preparation of bismuth film modified glassy carbon electrode (bife) glassy carbon electrode (gce) was polished with 0.05 μm al2o3 on a polishing kit to completely shiny surface. then, the electrode was sonicated in distilled water to get rid of alumina and any other contaminants from the surface. the bismuth film was deposited on gce surface without removing oxygen by immersing in 1 mol/l hcl solution containing 2 mmol/l bi(iii) ions and electrolyzed at -0.80 v vs. ag/agcl for 150 s while stirring the solution. after the electrode was modified with bismuth, it was rinsed with distilled water and dried. http://dx.doi.org/10.5599/jese.651 j. electrochem. sci. eng. 9(3) (2019) 153-164 determination of pb and cd in water lily stems 156 preparation of nanosio2–chitosan/bismuth film modified glassy carbon electrode (nanosio2–cts/bife) to obtain nanosio2–chitosan as a modifying solution, 10 ml chitosan solution (0.2 % (w/v) in 2 mol/l acetic acid) was prepared and ph was adjusted to 6.0 with 1 mol/l naoh. then 2 mg nanosio2 was added with mechanical stirring at room temperature to obtain a highly viscous solution. after that, 2 l drop of nanosio2–chitosan was placed with a micropipette (brand, usa) onto the gce modified with bismuth film and allowed to dry in the air. voltammetric procedure standard solutions of pb(ii) and cd(ii) were added into a three-electrode electrochemical cell containing hcl/kcl (ph 2.2), and stirred under -1.1 v for 240 s. after that, the stirring was stopped and 10 s of quiet time was spent before differential pulse anodic stripping voltammetry (dpasv) was applied with a potential scan from -1.1 to -0.4 v, pulse amplitude 50 mv, pulse duration 20 ms and step potential 5 mv. in order to obtain the best pulsing wave forms, square wave (swasv) measurements were additionally performed. sampling, sample storage and pretreatment water lily samples shown in figure 1 were collected from five sites in thale noi denoted in figure 2. the stems of water lily samples were cut by using plastic-ceramic knife, kept in polyethylene resealable zipper plastic bags, labelled and later transported to the laboratory by keeping in the icebox. on arrival at the laboratory, the samples were cleaned with distilled water to eliminate any adhering soil or dust particles, stored in polyethylene resealable zipper plastic bags and then frozen at -5 °c. next, the samples were defrosted, rinsed three times with distilled water, cut by using plasticceramic knife and placed in different petri dishes before being dried in an oven at 105 °c until reaching constant weights. the dried samples were ground to powder with non-metal mortar and pestle and finally collected in polyethylene bags before analysis. figure 1. water lily in thale noi showing stems, flowers and leaves figure 2. location of sampling sites within thale noi sample digestion by water bath digestion method water lily stem samples were further digested using water bath digestion method [6]. 0.5 g dry sample was transferred to a test tube, followed by adding 2.0 ml of 65 % of nitric acid (hno3). the test tube was then placed in the water bath, heated at 95 °c about 1.5 hours and left to cool at room monthira somkid and pipat chooto j. electrochem. sci. eng. 9(3) (2019) 153-164 http://dx.doi.org/10.5599/jese.651 157 temperature. after that, 0.5 ml 30 % hydrogen peroxide (h2o2) was added, placed in the water bath, heated at 95 °c about 30 min, left at room temperature, filtered through whatman no. 1 and diluted with ultrapure water to 10 ml and finally analyzed by icp-oes. for dpasv, 1 ml of sample solution was added into the electrochemical cell and adjusted to 50 ml with hcl/kcl (ph 2.2). results and discussion optimization of voltammetric procedure table 1 shows the list of operating parameters for the comparison between the square wave (swasv) and differential pulse (dpasv) anodic stripping voltammetry (asv) modes in the analysis of 20 g/l pb(ii) and cd(ii) at the modified gce. table 1. comparison between operating parameters of square wave (sw) and differential pulse (dp) modes of asv technique parameter swasv dpasv accumulation step deposition potential, mv deposition time, s equilibration time, s measuring step potential window mv frequency, hz step potential, mv amplitude, mv -1000 180 10 -1000 to -300 25 hz 5 mv 25 mv -1000 180 10 -1000 to -300 5 50 as shown in figure 3 in terms of currents, dpasv exhibited higher sensitivity than swasv mode, and therefore dpasv was selected for further experimentation. figure 3. comparison of peak currents between swasv and dpasv modes for 20 g/l pb(ii) and cd(ii) in 0.1 mol hcl/kcl (ph 2.2). potential and time of pre-plated bismuth film modified electrode bi film formation on gce was optimized using 2 mmol/l bi(iii) solution in 1 mol/l hcl as a plating solution [24] and the effects of plating potential (from -0.6 to -1.0 v) on the electrochemical performance were investigated. at more negative deposition potentials, the peak currents of pb(ii) and cd(ii) increased as shown in figure 4a. at the potential more negative than http://dx.doi.org/10.5599/jese.651 j. electrochem. sci. eng. 9(3) (2019) 153-164 determination of pb and cd in water lily stems 158 -0.8 v, the electrode did not provide higher currents due to the overloaded bismuth. therefore, -0.8 v vs. ag/agcl was selected as the optimum plating potential. the plating time is crucial in controlling bismuth film thickness. its effect was studied in the range of 60–300 s. as shown in figure 4b, pb(ii) and cd(ii) peak currents increased with longer bismuth deposition time up to 150 s and after that, peak currents were lowered due to the film saturation for both metals. the plating time of 150 s was therefore chosen for further investigations. figure 4. influence of plating potential (180 s of deposition time) (a) and plating time (deposition potential -1.0 v) (b) of bismuth film formation upon dpasv signals of 20 g/l pb(ii) and cd(ii) in 0.1 m hcl/kcl (ph 2.2). other dpasv parameters as in table 1. effect of chitosan the effectiveness of modification by chitosan was investigated by varying its concentration in the modifying solution with a fixed content of mesoporous silica nanoparticles (nanosio2) of 2 mg. figure 5 shows that the peak currents of both metals were higher with increasing concentration of chitosan and reached the maximum at 0.20 % (w/v). figure 5. effect of concentration of chitosan upon dpasv signals of 20 g/l pb(ii) and cd(ii) in 0.1 m hcl/kcl (ph 2.2) and other parameters as in table 1. further increase of the concentration of chitosan suppressed both peak heights, probably due to lower conductivity of chitosan. therefore, 0.20 % (w/v) of chitosan in the modifying solution was chosen in all subsequent work. monthira somkid and pipat chooto j. electrochem. sci. eng. 9(3) (2019) 153-164 http://dx.doi.org/10.5599/jese.651 159 effect of hcl/kcl solution ph ph of hcl/kcl solution is an important parameter in supporting the deposition of pb(ii) and cd(ii) on nanosio2-cts/bife. as shown in figure 6, by varying ph from 1.7 to 2.4 for 20 g/l pb(ii) and cd(ii), both metal ions seem to remain in the form of mcl42in hcl/kcl ph 2.2 which is suitable for sorption on protonated amino groups (nh3+) in chitosan through anion exchange/ electrostatic attraction. thus, 0.1 m hcl/kcl ph 2.2 was selected for further experiments. figure 6. effect of ph upon dpasv signals of 20 g/l pb(ii) and cd(ii) in 0.1 m hcl/kcl with other conditions specified in table 1. optimization of voltammetric parameters due to the fact that asv is mostly based on the electrolytic deposition of analytes onto the electrode for preconcentration, the operational parameters such as optimum deposition potential and optimum deposition time that would result in the best current response need to be investigated. the deposition potential was varied between -0.8 to -1.3 v with 180 s deposition time with the results shown in figure 7a. at deposition potentials from -0.8 to -1.1 v, the currents increased for both analytes and then decreased due to hydrogen evolution at higher negative potentials. consequently, the deposition potential of -1.1 v was selected for the following experiments. figure 7. influence of deposition potential (a) and deposition time (b) on dpasv signals of 20 g/l pb(ii) and cd(ii) in 0.1 m hcl/kcl (ph 2.2) with other conditions specified in table 1. http://dx.doi.org/10.5599/jese.651 j. electrochem. sci. eng. 9(3) (2019) 153-164 determination of pb and cd in water lily stems 160 the effect of pb(ii) and cd(ii) deposition time was investigated within 60-480 s at deposition potential of -1.1 v as shown in figure 7b. at longer deposition time, higher sensitivity was obtained, as expected due to a greater amount of analyte deposited on nanosio2-cts/bife. at times longer than 240 s, however, the sensitivity did not increase much because of the saturated electrode surface. the deposition time of 240 s was therefore chosen as the optimum value. for further optimization, dpasv operating parameters including step potential and pulse amplitude, were varied to observe the stripping response of 20 g/l pb(ii) and cd(ii). the best results were obtained for the step potential of 5 mv and pulse amplitude of 50 mv, which were taken as optimum values. the optimum operation conditions all studied parameters affecting analytical performance and the optimum conditions for pb(ii) and cd(ii) determination by dpasv using nanosio2-cts/bife as the working electrode are collected in table 2. table 2. optimized operating conditions for dpasv at nanosio2-cts/bife parameter optimum value studied range plating potential for ex-situ bife formation, v plating time for ex-situ bife formation, s deposition potential for pb2+ and cd2+ detection, v deposition time for pb2+ and cd2+ detection, s ph of 0.1 mol/l hcl/kcl chitosan concentration, % (w/v) equilibration time, s step potential, mv pulse amplitude, mv -0.8 150 -1.1 240 2.2 0.20 10 5 50 -0.6 – -1.0 60–300 -0.8 – -1.3 60 – 420 1.7 – 2.4 0.10–0.30 0–40 1–20 50–150 interference studies the effect of other metal ions on the peak currents of pb(ii) and cd(ii) was evaluated under optimized conditions by detecting 20 g/l pb(ii) and cd(ii) in presence of interfering ions and the results are listed in table 3. table 3. interference from other metal ions (200 g/l) on stripping peak currents of 20 g/l pb(ii) and cd(ii) interference contribution, % (ip pb(ii) = 100 %) contribution, % (ip.cd(ii) = 100 % ) ca2+ mg2+ zn2+ ni2+ cu2+ hg2+ fe3+ 0 0 -7.08 -5.03 -31.00 -5.95 +2.84 0 0 -5.40 -7.27 -42.97 -6.02 +0.35 ip: peak current most metal ions under study provided less than 10 % current response change and they considered to be not interfering. presence of cu(ii), however, contributed more than 30 % change for both pb(ii) and cd(ii) signals. to improve the sensitivity by eliminating cu(ii) interference in asv, 0.01 mmol/l of ferrocyanide can be added as a masking agent acting via complex formation with cu(ii). monthira somkid and pipat chooto j. electrochem. sci. eng. 9(3) (2019) 153-164 http://dx.doi.org/10.5599/jese.651 161 analytical characteristics of the method calibration curve the calibration curve was obtained using dpasv at nanosio2-cts/bife performed under optimized experimental conditions in hcl/kcl, ph 2.2 solution, containing 2-40 µg/l pb(ii) and cd(ii) and shown in figure 8. as shown in figure 9, the linear correlation coefficient (r2) of 0.998 was achieved for both pb(ii) and cd(ii) after 240 s of deposition time. slight difference of the slopes for two ions revealed minor matrix effect and suggested the use of standard addition. figure 8. dpasvs obtained upon increasing of pb(ii) and cd(ii) in hcl/kcl (ph 2.2): 0.0, 2.0, 5.0, 10.0, 20.0, 30.0 and 40.0 g/l at optimized measurement parameters listed in table 2 figure 9. calibration graphs for pb(ii) and cd(ii) determinations limit of detection (lod) and limit of quantification (loq) for the blank solution spiked with 2.0 g/l of pb(ii) and cd(ii) standard solution, the limit of detection (lod) and limit of quantification (loq) of 0.3 g/l and 1.1 g/l for pb(ii) as well as 0.1 g/l and 0.4 g/l for cd(ii), respectively, were obtained. http://dx.doi.org/10.5599/jese.651 j. electrochem. sci. eng. 9(3) (2019) 153-164 determination of pb and cd in water lily stems 162 reproducibility the reproducibility of the developed nanosio2-cts/bife electrode was investigated by measuring ten replicates of 20 g/l pb(ii) and cd(ii) in 0.1 mol/l hcl/kcl (ph 2.2). the relative standard deviations (rsd) for pb(ii) and cd(ii) currents were found to be 2.26 and 1.71 %, respectively. recoveries as shown in table 4, the recovery range obtained by spiking both metal ions in the extracted solution were 93.3 –101.9 % for pb(ii) and 99.3 –103.7 % for cd(ii) table 4. recovery test for the studied method using water lily stem samples (n = 3) spiked with 2.0, 20.0 and 40.0 µg/l of pb(ii) and cd(ii) cpb(ii) / µg l-1 recovery, % ccd(ii) / µg l-1 recovery, % sample nd nd sample + 2.0 µg/l 1.9 93.3 2.1 103.7 sample + 20.0 µg/l 20.4 101.9 19.9 99.3 sample + 40.0 µg/l 40.1 100.3 40.1 100.2 table 5. comparison of the proposed method with recent literature data of pb(ii) and cd(ii) analysis electrode method deposition time, s linear range, µg l-1 lod, µg l-1 ref pb(ii) cd(ii) pb(ii) cd(ii) 1 bi/gce swasv 250 5-150 5-150 1.9 3.2 [25] 2 bi/omc-mw/gcpe swasv 150 1-70 1-70 0.08 0.07 [26] 3 biocl/mwcnt/gce swasv 120 5-50 5-50 0.57 1.2 [27] 4 l-cys/gr-cs/gce dpasv 120 1.04-62.1 0.56-67.2 0.12 0.45 [28] 5 mwcnt/poly(pcv)/gce dpasv 420 1.0-200.0 1.0-300.0 0.4 0.2 [29] 6 bi2o3/gce swasv 60 2-250 1-150 0.26 0.52 [30] 7 spce–bi2o3–pss–cnp dpasv 420 15-45 15-45 0.10 0.27 [31] 8 bi/mwcnt-ebp-na/gce swasv 120 1–50 1–50 0.08 0.06 [32] 9 bi/mgf-nafion/gce dpasv 360 0.5-110 2-70 0.1 0.5 [33] 10 pa-pani/gce dpasv 120 0.1-60 0.05-60 0.05 0.02 [34] 11 nanosio2-cts/bife dpasv 240 0-40 0-40 0.3 0.1 this work bi/omc-mw/gcpe: bismuth film/ordered mesoporous carbon molecular wire modified graphite carbon paste electrode biocl: bismuth-oxychloride l-cys: l-cysteine gr-cs: graphene-chitosan mwcnt: multiwall carbon nanotube poly(pvc): poly(pyrocatechol violet) bi2o3/gce: graphite-composite electrodes bulk modified with bi2o3 spce–bi2o3–pss–cnp: screen-printed carbon electrodes-bismuth oxide-polystyrene sulfonate carbon nanopowder mwcnt-ebp-na: multi-walled carbon nanotube-emeraldine base polyaniline-nafion mgf-nafion: mesoporous graphene framework-nafion pa-pani: phytic acid-polyaniline nanofibers cts: chitosan analytical characteristics of the proposed method in comparison with other recent studies are compared in table 5. the present method is proved to have comparable deposition time and linear ranges, but relatively better lod and loq for both metals. with simplicity, speed and costmonthira somkid and pipat chooto j. electrochem. sci. eng. 9(3) (2019) 153-164 http://dx.doi.org/10.5599/jese.651 163 effectiveness, the developed method is a good alternative for metal ion analysis, especially in further continuing research and development. analytical application the nanosio2-cts/bife was applied to determine the pb(ii) and cd(ii) in water lily stems samples by dpasv under optimum conditions. as shown in table 6, with standard addition, the method provided well analytical performance and gave results comparable to those measured by icp-oes, proving thus satisfactory accuracy and reliability. therefore, the nanosio2-cts/bife is found effective in determining pb(ii) and cd(ii) at trace levels in real samples. table 6. concentration of pb(ii) and cd(ii) in water lily stems (mg/kg) determined by dpasv and icp-oes methods (n = 3) sites sampling site longitude and latitude cpb(ii) / mg kg-1 ccd(ii) / mg kg-1 dpasv icp-oes dpasv icp-oes 1 khlong ban bon e 100 7 40 n 7 47 21 0.11 0.10 < loq < loq 2 khlong na lam e 100 9 33 n 7 48 07 < loq < loq < loq < loq 3 khlong nang riam e 100 11 6 n 7 47 05 < loq < loq < loq < loq 4 khlong yuan e 100 7 58 n 7 46 14 0.29 0.28 0.10 < loq 5 ao na klang e 100 7 40 n 7 46 43 0.16 0.16 0.07 < loq conclusions a simple and accurate electrochemical method to determine pb(ii) and cd(ii) was proposed using the nanosio2-cts/bife combined with dpasv technique, giving well-defined stripping peaks for both metals. the chitosan selected as strong adhesion reagent for silica nanoparticles coating is also cheaper than the normally used nafion. under optimized conditions, the linear correlation coefficient (r2) was 0.998 for both metals with 240 s deposition time. the limit of detection (lod) of 0.3 g/l was obtained for pb(ii) and 0.1 g/l for cd(ii). the method was successfully applied for determination of pb(ii) and cd(ii) in water lily stems with the results in good agreement with those obtained by icp-oes. acknowledgements: the authors would like to acknowledge the support from graduate school and faculty of science of prince of songkla university. we also wish to thank scientific equipment center for the icp-oes analysis and mr. chai sunwanchart, chief of the thale noi non-hunting area, national parks, wildlife and plant conservation department for facilitating sample collections. references [1] h. segers, p. pholpunthin, international journal of limnology 33 (1997) 13-21. 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[34] h. huang, w. zhu, x. gao, x. liu, h. ma, analytica chimica acta 947 (2016) 32-41. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://www.sciencedirect.com/science/journal/00134686 http://www.sciencedirect.com/science/journal/00134686/132/supp/c http://creativecommons.org/licenses/by/4.0/) experimental and theoretical study on corrosion inhibition of mild steel by meso-tetraphenyl-porphyrin derivatives in acid solution https://dx.doi.org/10.5599/jese.1400 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; https://dx.doi.org/10.5599/jese.1400 open access : : issn 1847-9286 www.jese-online.org original scientific paper experimental and theoretical study on corrosion inhibition of mild steel by meso-tetraphenyl-porphyrin derivatives in acid solution messaoud meraghni1,2, touhami lanez2,, elhafnaoui lanez2, lazhar bechki3 and ali kennoufa2 1university of el oued, process engineering department, faculty of technology, b.p.789, 39000, el oued, algeria 2vtrs laboratory, department of chemistry, faculty of sciences, university of el oued b.p.789, 39000, el oued, algeria 3university of ouargla, chemistry department, po box 511, 30000, ouargla, algeria corresponding author: touhami-lanez@univ-elourd.dz received: june 12, 2022; accepted: august 23, 2022; published: august xx, 2022 abstract the inhibition effect of meso-tetraphenyl-porphyrin (tpph2), meso-tetra4-methophenylporphyrin tpph2(p-me), and meso-tetra4-actophenyl-porphyrin (tacpph2) on the corrosion of xc52 mild steel in aerated 0.5 m aqueous sulfuric acid solution was studied by potentiodynamic polarization experiments and quantum chemical calculations. results from potentiodynamic polarization showed that inhibition efficiency of three compounds increased upon increasing of the inhibitor concentration and they are acting as mixed type inhibitors, having dominant anodic reactions. adsorption of all compounds follows the langmuir adsorption isotherm with moderate values of free energy of adsorption. quantum chemical calculation using dft/b3lyp method confirmed a strong bond between meso-tetraphenyl-porphyrins and mild steel surface. the inhibition mechanism was also determined by the potential of zero charge (pzc) measurement at the metal/solution interface. keywords low carbon steel; potentiodynamic polarization; quantum chemical method, potential of zero charge introduction mild steel is made from iron with low carbon content of approximately 0.05-0.30 % by weight, where other elements such as manganese and silicon may be also added. the presence of carbon content improves toughness and corrosion-resistance of mild steel compared to pure iron. mild steel is one of the most widely used materials in industry such as electronics, and the manufacture https://dx.doi.org/10.5599/jese.1400 https://dx.doi.org/10.5599/jese.1400 http://www.jese-online.org/ mailto:touhami-lanez@univ-elourd.dz j. electrochem. sci. eng. 00(0) (2022) 000-000 corrosion inhibition of mild steel in acid solution 2 of integrated circuits, as well as a construction material for pipeline transport production and processing related industries [1,2]. as many other metals, mild steel is usually exposed to corrosion in various industries. these exposures can make changes in the properties of the metals and thus to unexpected failures of materials in service. therefore, a metal surface should be protected from corrosion attack, and the most efficient method of metal protection against aqueous acidic corrosion is utilization of organic inhibitors molecules. despite the fact that mild steel has numerous applications, it shows weak resistance to corrosion attack in corrosive aqueous media [3-5]. consequently, the development of protective materials able to increase the corrosion resistance of mild steel is mandatory. in this case also, the use of organic compounds as corrosion inhibitors is one of the most widespread protective methods against aqueous corrosion. organic inhibitors are usually organic molecules containing heteroatoms such as nitrogen, oxygen and sulphur that result in the enhanced adsorption onto a metal surface [6,7]. also, the presence of aromatic cycles and aliphatic chains in the molecular structures of inhibitors improve their adsorption at a metal surface. in such a manner, a metal surface becomes isolated from the corrosive media and consequently, its corrosion resistance is improved [8,9]. porphyrins are a class of cyclic tetrapyrroles which possess a highly conjugated, heterocyclic macrocycle. their 18π electron structure gives rise to their remarkable stability, their structures are formed by four pyrrole subunits connected together via methine bridges. the presence of four nitrogen atoms in their skeletons qualifies them to be potential corrosion inhibitors. additionally, the fully aromatic character of meso-tetraphenyl-porphyrin derivatives facilitates the mobility of electrons in the rings, which makes possible their application in many fields such as electrochemistry and catalysis [9], photomedicine [10] and photosynthesis [11]. in the present work, the corrosion inhibition efficiency of 5,10,15,20-tetraphenylporphyrin (tpph2), 5,10,15,20-tetra-p-tolylporphyrin (tpph2(p-me)), and 1,1',1'',1'''-(porphyrin-5,10,15,20tetrayltetrakis(benzene-4,1-diyl))tetrakis(ethan-1-one) (tacpph2) on xc52 steel in sulfuric acid solution has been studied using potentiodynamic polarization measurements and quantum chemical calculations. experimental materials and sample preparation sulfuric acid (sigma aldrich) used as corrosive medium in this study was of analytical grade and used as sourced without further preparation. the chemical composition of the xc52 mild steel used in this study is: 0.065 wt.% c, 0.245 wt.% si, 1.685 wt.% mn, 0.002 wt.% p, 0.001 wt.% s, 0.042 wt.% cr, 0.005 wt.% cr, 0.026 wt.% ni, 0.042 wt.% al, 0.010 wt.% cu, 0.067 wt.% nb, 0.019 wt.% ti, 0.014 wt.% v and the remainder fe. it was obtained as platelets from anabib ltd (ghardaïa, algeria). electrochemical measurements the electrochemical measurements were conducted on a pgz301 potentiostat (radiometer analytical sas, france) connected to a standard three-electrode electrochemical cell assembly composed of a saturated calomel electrode (hg/hg2cl2/cl-) and a platinum wire as the reference and auxiliary electrode, respectively. mild steel (xc52) rod with 0.707 cm2 of exposed surface area served as the working electrode. all experiments were performed in atmospheric conditions m. meraghni et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1400 3 without stirring. prior to each assay, the working electrode was immersed in the test solution until a stable value for the open circuit potential was obtained (40 to 60 minutes). synthesis the synthesis of meso-tetraphenyl-porphyrin derivatives (tpph2, tpph2(p-me), and tacpph2) used in this work as potential inhibitors of corrosion of xc52 mild steel in aqueous sulfuric acid solution, was performed following our previously reported procedure [10]. molecular structures of three synthesized porphyrin derivatives are shown in figure 1. tpph2 tpph2(p-me) tacpph2 figure 1. chemical structures of meso-tetraphenyl-porphyrin derivatives a stock solution of each inhibitor (300 ppm) was prepared by weighing 300 mg of synthesized material dissolved in one liter of 0.5m sulfuric acid. other concentrations (10 – 100 ppm) were obtained from the stock solution following successive dilution. the xc52 mild steel electrode used for electrochemical assays was prepared, degreased, and cleaned as previously reported [11,12]. structure optimizations were run using density functional theory (dft) implemented in gaussian 09 package [13]. all calculations were carried out with the unrestricted becker’s three parameter hybrid exchange functional [14] combined with lee-yang-parr nonlocal correlation function, abbreviated as b3lyp [15–17] with basis set 6-311++g(d,p) [18–20]. https://dx.doi.org/10.5599/jese.1400 j. electrochem. sci. eng. 00(0) (2022) 000-000 corrosion inhibition of mild steel in acid solution 4 results and discussion potentiodynamic polarization study potentiodynamic polarization curves were used to study the corrosion inhibition of xc52 mild steel in aerated 0.5 m aqueous sulfuric acid solution, in the absence and presence of different concentrations of tpph2, tpph2(p-me), and tacpph2 at 25±1 °c. the obtained tafel curves are shown in figure 2. -650 -600 -550 -500 -450 -400 -350 -4 -3 -2 -1 0 1 2 lo g ( j / m a c m -2 ) e / mv blank 10 ppm 20 ppm 30 ppm 40 ppm 50 ppm 60 ppm 70 ppm 80 ppm 90 ppm 100 ppm tpph 2 -650 -600 -550 -500 -450 -400 -350 -4 -3 -2 -1 0 1 2 lo g ( j / m a c m -2 ) e / mv blank 10 ppm 20 ppm 30 ppm 40 ppm 50 ppm 60 ppm 70 ppm 80 ppm 90 ppm 100 ppm tpph 2 (p-me) -700 -600 -500 -400 -300 -5 -4 -3 -2 -1 0 1 2 lo g ( j / m a c m -2 ) e / mv blank 10 ppm 20 ppm 30 ppm 40 ppm 50 ppm 60 ppm 70 ppm 80 ppm 90 ppm 100 ppm tacpph 2 figure 2. potentiodynamic polarization curves of mild steel in 0.5 m h2so4 in the absence (blank) and presence of different (10-100 ppm) concentrations of tpph2, tpph2(p-me) and tacpph2 figure 2 shows that in the presence of inhibitors, all anodic tafel slopes were lower compared to the blank solution, thus showing the effect of the inhibitors on the metal dissolution reaction. in contrast, cathodic tafel slopes were less affected. this infers that compounds in the present study were inhibitors of mixed type, having dominant anodic reaction [21]. the corrosion current densities were determined from the intersection of anodic and cathodic tafel slopes, while inhibition efficiency (ie / %) was calculated from the corrosion current density of xc52 mild still electrodes in the absence and presence of the inhibitor using the equation (1) [22]:  − =     0 0 100 j j ie j (1) where j0 and j are the current density values in the absence and presence of inhibitor respectively. corrosion current density values and ie calculated by eq. (1), are for different concentrations (10-100 ppm) of three inhibitors presented in table 1. lo g ( j / m a c m -2 ) lo g ( j / m a c m -2 ) lo g ( j / m a c m -2 ) m. meraghni et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1400 5 table 1. inhibition efficiencies of different concentrations of tpph2, tpph2(p-me), and tacpph2 inhibitors toward xc52 mild steel corrosion in 0.5 m h2so4 c / ppm tpph2 tpph2(p-me) tacpph2 j / ma cmie / % j / ma cm-2 ie / % j / ma cm-2 ie / % -0.6727 -0.6727 -0.6727 - 10 0.4311 36 0.4039 40 0.4002 41 20 0.3988 41 0.3908 42 0.3142 53 30 0.3669 45 0.3621 46 0.2075 69 40 0.3308 51 0.3284 51 0.1808 73 50 0.3003 55 0.2915 57 0.178 74 60 0.2502 63 0.2414 64 0.1341 80 70 0.2365 65 0.2313 66 0.1279 81 80 0.2283 66 0.2194 67 0.1223 82 90 0.2194 67 0.2091 69 0.1097 84 100 0.2088 69 0.1988 70 0.1031 85 it is clearly seen from table 1 that the current density values decrease considerably with increasing concentration of the inhibitors due to the formation of a barrier film on the mild steel surface. moreover, for all three inhibitors, inhibition efficiency increases with concentration, while the maximum ie of 85 % is observed for 100 ppm of tacpph2, indicating significant protection of the mild steel from corrosion. anodic and cathodic tafel slopes as shown in table 2, in solutions without and with different concentrations of inhibitors, the values of anodic tafel slope (a) varied from 48 to 72 mv for tpph2, from 52 to 71 mv for tpph2(p-me), and from 48 to 80 mv for tacpph2. the values of cathodic slopes (c) varied from -121 to -160 mv for tpph2, from -135 to -193 mv for tpph2(p-me), and from -149 to -194 mv for tacpph2. these limits represent typical anodic and cathodic tafel slopes that were reported in the literature for mild steel in acidic solutions [23,24]. as it can be observed from table 1 and 2, the variations of anodic and cathodic tafel slopes caused by the addition of the inhibitors, affected both the corrosion potential and the anodic and cathodic current densities. as a increases, the potential on the anodic mild steel surface also increases; however, the potential distribution along the cathodic surface is not affected significantly. therefore, as a increases, the potential difference between the anodic and cathodic regions decreases, and this results in lower corrosion rates. table 2. anodic and cathodic tafel slopes for the mild steel immersed in 0.5 m h2so4 medium for 30 min c / ppm tpph2 tpph2(p-me) tacpph2 ecorr / mv a / mv dec-1 c / mv dec-1 ecorr / mv a / mv dec-1 c / mv dec-1 ecorr / mv a / mv dec-1 c / mv dec-1 00 -516 72 -154 -503 71 -183 -501 80 -188 10 -511 65 -150 -499 66 -193 -492 63 -183 20 -506 56 -140 -497 61 -173 -468 69 -191 30 -511 56 -140 -493 63 -187 -485 56 -160 40 -512 57 -140 -495 61 -192 -481 61 -194 50 -510 56 -138 -493 59 -185 -480 62 -185 60 -505 53 -137 -489 58 -176 -498 57 -165 70 -503 54 -141 -507 64 -156 -492 53 -149 80 -512 56 -135 -490 57 -182 -480 60 -185 90 -509 53 -137 -487 52 -154 -497 60 -170 100 -504 48 -121 -498 55 -135 -499 48 -175 https://dx.doi.org/10.5599/jese.1400 j. electrochem. sci. eng. 00(0) (2022) 000-000 corrosion inhibition of mild steel in acid solution 6 adsorption isotherm in order to find out the mode of adsorption of three porphyrin derivative inhibitors on the surface of xc52 mild steel and the adsorption isotherm that fits the experimental results, the θ/cinh values were plotted versus inhibitor concentration (cinh) for all investigated compounds (figure 3). the obtained straight lines follow the langmuir adsorption isotherm that is given by the equation (2) [25]:  = +inh inh ads 1c c k (2) where θ is surface coverage degree, cinh is concentration of tested inhibitor compounds, and kads is adsorption equilibrium constant. the surface coverage degree (θ) was calculated based on the assumption that the inhibition efficiency (ie) is due mainly to the blocking effect of the adsorbed inhibitor molecules on the metal surface. hence, θ is given by the equation (3) [26]: 0 1 100 ie j j  = = − (3) the values of adsorption equilibrium constant (kads) obtained from intercepts of linear lines at θ/cinh axes in figure 3, are listed in table 3. the results demonstrate that all values of the linear correlation coefficients (r2) and all slopes are almost equal to one, which confirms that adsorption of all three studied inhibitor molecules in 0.5 m aqueous sulfuric acid on the surface of the x52 mild steel obeys langmuir adsorption isotherm. 0 20 40 60 80 100 120 140 160 180 60 80 100 120 140 160 180 200 220 240 experimental linear fit c /   m c / m tpph 2 0 20 40 60 80 100 120 140 160 60 80 100 120 140 160 180 200 220 experimental linear fit c /   m c / m ptth 2 (p-me) 0 20 40 60 80 100 120 140 20 40 60 80 100 120 140 160 experimental linear fit c /   m c / m tacpph 2 figure 3. langmuir’s adsorption plots of tpph2, tpph2(p-me) and tacpph2 on mild steel in 0.5 m h2so4 solution c  -1 /  m c  -1 /  m c  -1 /  m m. meraghni et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1400 7 table 3. langmuir adsorption isotherm and thermodynamic parameters for adsorption of tpph2, tpph2(p-me) and tacpph2 in 0.5 m h2so4 on xc52 mild steel at 25 °c molecule equation r2 kads / m-1 δg°ads / kj mol-1 tpph2 y = 1.1239x + 51.40 0.999 1.94×104 -34.4 tpph2(p-me) y = 1.0967x + 46.27 0.999 2.16×104 -34.7 tacpph2 y = 1.0438x + 17.44 0.999 5.73×104 -37.1 the standard free energy of adsorption (δg°ads) is obtained using the equation (4) [27]: goads = -rt ln (55.5kads) (4) where r is the gas constant (8.32 j mol-1k-1) and t is absolute temperature (298 k). the values of δg°ads calculated using eq. (4) are listed in table 3. generally, for ∆g°ads values of around -20 kj mol-1 or less negative, adsorption is regarded as the physisorption, those around -40 kj mol-1 or higher, the adsorption is regarded as the chemisorption [28]. in the present study the values of ∆g°ads suggest that adsorption of all studied inhibitors at xc52 mild steel surface is physisorption. inhibition mechanism the mechanism of the corrosion inhibition is generally based on the physical adsorption of inhibitor molecules onto the metal surface. this type of adsorption arises from the electrostatic attractive forces between protonated form of inhibitor molecule and the electrically negative charged surface of the metal. the surface charge of the metal can be attributed to the electric field existing at the metal/solution interface. this surface charge at the open circuit potential can be calculated using the equation (5) [29]: er = ecorr – eq=0 (5) where er is referred to as antropov’s rational potential or potential on the correlative scale, eq=0 is the potential of zero charge, and ecorr is the corrosion potential. if er is negative, the electrode surface in this case has a negative net charge and the adsorption of the protonated molecule is favourized [26]. the recommended value of pzc in sulphuric acid for mild steel is equal to -0.129 v vs. sce [30,31]. nominating this value into eq. (5) and considering ecorr = -512 mv (table 2), er was calculated as -383 mv. the obtained negative value of er indicates that investigated compounds in 0.5 m sulphuric acid are protonated and subsequently act as cations and adsorb electrostatically on the negatively charged surface of the xc52 mild steel. note that for any other ecorr value taken from table 2, a negative value of er would also be obtained. to make evidence of the protonation of porphyrins in the acidic corrosive medium, some uv-vis spectra of tpph2, tpph2(p-me) and tacpph2 in dmso and 0.5 m h2so4 were taken. the obtained electronic absorption spectra (figure 4) consist of two distinct regions. the first appears at around 410-416 nm which involves the transition from the ground state to the second excited state, and this band is called the soret band. the second region consists of a weak transition to the first excited state in the range between 512 and 650 nm, and these bands are called the q bands. the soret band of tpph2, tpph2(p-me) and tacpph2 in dmso is centered at 410, 412, and 416 nm respectively, and the q-bands are all located between 512 and 650 nm [32]. the change in spectra upon addition of diluted acid is attributed to the attachment of protons to two imino nitrogen atoms of the free-base [33]. other evidence of the physical adsorption of inhibitor onto the xc52 mild steel surface is the increasing of the polarization resistance (rp) upon increasing the inhibitor concentration. the polarization resistance can be calculated using the stern–geary equation (6) [34]: https://dx.doi.org/10.5599/jese.1400 j. electrochem. sci. eng. 00(0) (2022) 000-000 corrosion inhibition of mild steel in acid solution 8 300 400 500 600 700 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 a b s o rb a n c e , a .u . wavelength, nm h 2 so 4 0.5 m dmso tpph2 300 350 400 450 500 550 600 650 700 750 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 tpph2(p-me) a b s o rb a n c e , a .u . wavelength, nm h 2 so 4 0.5 m dmso 300 350 400 450 500 550 600 650 700 750 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 tacpph2 a b s o rb a n c e , a .u . wavelength, nm h 2 so 4 0.5 m dmso figure 4. uv-visible spectra of tpph2, tpph2(p-me) and tacpph2 in dmso and 0.5 m h2so4 ( )      = =  + p corr2.3 a c a c e r i i (6) table 4 summarized the polarization resistance values obtained from tafel extrapolation method for selected inhibitor concentrations. the increasing values of the polarization resistance upon increasing the inhibitor concentration reflects adsorption of the inhibitor onto the metal surface which passivates efficiently active sites and inhibits corrosion [35,36]. table 4. polarization resistance and tafel slopes of tpph2, tpph2(p-me) and tacpph2 in 0.5 m h2so4 compound ecorr / mv a/ mv dec-1 c/ mv dec-1 rp / ω cm 2 30 ppm tpph2 -511.3 56 -139.7 47.64 tpph2(p-me) -493.3 62.9 -186.9 62.7 tacpph2 -484.6 55.9 -159.9 87.66 40 ppm tpph2 -512.4 56.8 -139.9 50.79 tpph2(p-me) -495.6 61.4 -191.8 66.08 tacpph2 -481.3 61.5 -194.3 117.28 50 ppm tpph2 -510.0 56.5 -138.4 54.25 tpph2(p-me) -493.4 59.2 -185.4 73.43 tacpph2 -479.6 62.1 -185.3 128.44 60 ppm tpph2 -505.5 53 -147.3 83.5 tpph2(p-me) -489.7 58.1 -176 90.0 tacpph2 -497.9 56.9 -165.1 162.5 m. meraghni et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1400 9 spectroscopic analysis uv–visible spectroscopy technique was used to determine the surface adsorption of the inhibitor molecules. the analysis was done before and after each corrosion assay. the uv–vis. spectra of inhibitors in both cases are shown in figure 5. the inhibitor solutions before the immersion of the metal show adsorption peaks at 452, 439, and 426 nm which correspond to the inhibitors tpph2, tpph2(p-me) and tacpph2, respectively. it is clear from figure 5 that this peak reallocated after the corrosion assessment. all spectra show a remarkable change in the adsorption band, which is associated with adsorption of inhibiting molecules on xc52 mild steel surface [37,38]. 350 400 450 500 550 600 650 700 750 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 a b s o rb a n c e , a .u . wavenumber, nm before immersion after immersion tpph2 300 400 500 600 700 800 0.0 0.1 0.2 0.3 0.4 0.5 0.6 a b s o rb a n c e , a .u . wavenumber, nm before immersion after immersion tpph2(p-me) 300 350 400 450 500 550 600 650 700 750 800 0.0 0.1 0.2 0.3 0.4 before immersion after immersion a b s o rb a n c e , a .u . wavelength, nm tacpph2 figure 5. uv–vis spectra of tpph2, tpph2(p-me) and tacpph2 before and after immersion of xc52 mild steel specimen in 0.5 m h2so4 for 36 h, at 298 k. molecular orbital analysis the highest occupied molecular orbital energy (ehomo) and the lowest unoccupied molecular orbital energy (elumo) are respectively connected with the electron donating and withdrawing capacities of a compound. the less negative ehomo and more negative elumo are related to low chemical stability and high chemical reactivity because of ease transition of electrons [39]. a smaller energy gap δe (elumo ehomo) is often interpreted by stronger chemisorption bond and consequently higher inhibition efficiency [40,41]. in order to obtain more information about the frontier molecular orbitals and consequent inhibitory action of the investigated compounds, theoretical study based on molecular orbital analysis was performed. the ehomo and elumo of the investigated compounds were obtained using density functional theory (dft) without imposing any symmetry constraints, and calculations were realized with the gaussian 09 package [10]. the exchange functional of becke and the correlation functional of lee, yang and parr (b3lyp) were employed with 6-311++g(d,p) basis sets. the obtained https://dx.doi.org/10.5599/jese.1400 j. electrochem. sci. eng. 00(0) (2022) 000-000 corrosion inhibition of mild steel in acid solution 10 contour diagrams of homo and lumo are shown in figure 6, and the values of the energy of frontier orbitals are reported in table 5. according to figure 6 and data table 5, the compound tacpph2 has the lowest energy gap value (2.6009 ev) which is the reason for its highest inhibition efficiency. on the other hand, compound tpph2(p-me) has slightly lower energy gap value (2.7056 ev) than tpph2 (2.7247 ev), which explains the slightly higher inhibition efficiency of tpph2(p-me) compared to tpph2. gauss-sum 2.2 program [42] was used to calculate group contributions to the molecular orbitals (homo and lumo) and prepare the density of states (dos) plot for the highest and lowest energy gap of tpph2, tpph2(p-me) and tacpph2 shown in figure 7. the dos spectra were generated by convoluting the molecular orbital information with gaussian curves of unit height. lumo  homo tpph2 tpph2(p-me) tacpph2 figure 6. contour diagrams of homo and lumo of tpph2, tpph2(p-me) and tacpph2 table 5. energy (ev) of homo, lumo, and energy gap of tpph2, tpph2(p-me) and tacpph2. parameters compounds tpph2 tpph2(p-me) tacpph2 ehomo / ev -5.2934 -5.1696 -5.5688 elumo / ev -2.5688 -2.4640 -2.9679 δe /ev 2.7247 2.7056 2.6009 molecular electrostatic potential map analysis molecular electrostatic potential (mep) map gives an idea about the chemical reactivity of the studied inhibitors. also, mep shows the preferred site for the electrophile attack which is colored in red around the amine functions in compounds tpph2 and tpph2(p-me). the nitrogen atom in these two compounds is the most preferred site for the nucleophilic attack (figure 8). the mep map analysis also shows four red surfaces on each acetyl group of the compound tacpph2, and this may be the reason of high corrosion efficiency of this compound. figure 8 also shows that the compound tacpph2 shows the maximal negative potential value (-0.0674 a.u.) and the highest positive potentiality value (+0.0674 a.u.). m. meraghni et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1400 11 figure 7. density of states diagrams and homo-lumo energy gap of tpph2, tpph2(p-me) and tacpph2 +0.0383 -0.0383 +0.048 -0.0489 +0.0674 -0.0674 tpph2 tpph2(p-me) tacpph2 figure 8. molecular electrostatic potential map of the studied tpph2, tpph2(p-me) and tacpph2 inhibitors conclusion in this work, three meso-tetraphenyl-porphyrin derivatives (tpph2, tpph2(p-me) and tacpph2) were tested as corrosion inhibitors of xc52 mild steel in 0.5 m sulphuric acid solution. the results of potentiodynamic polarization showed that the use of tacpph2 can decrease the corrosion of xc52 mild steel by up to 81%, whereas for the tpph2 and tpph2(p-me), the inhibition efficiency reaches 65%. three investigated meso-tetraphenyl-porphyrin derivatives showed dominant anodic reaction. the adsorption of meso-tetraphenyl-porphyrins investigated obeys langmuir adsorption isotherm. also, the order of magnitude of adsorption energy indicates that physical adsorption occurs. https://dx.doi.org/10.5599/jese.1400 j. electrochem. sci. eng. 00(0) (2022) 000-000 corrosion inhibition of mild steel in acid 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www.jese-online.org original scientific paper study of oxygen evolution reaction on thermally prepared xptoy-(100-x)iro2 electrodes ollo kambiré1,, lemeyonouin a. g. pohan2, konan h. kondro3 and lassiné ouattara3, 1ufr sciences et technologies, université de man, bp 20 man, côte d’ivoire 2ufr sciences biologiques, université peleforo gon coulibaly de korhogo, bp 1328 korhogo, côte d’ivoire 3laboratoire de constitution et réaction de la matière, ufr ssmt, université félix houphouëtboigny de cocody, abidjan, 22 bp 582 abidjan 22, côte d’ivoire corresponding authors: kambireollo@yahoo.fr; ouatlassine@yahoo.fr received: march 9, 2020; revised: june 21, 2020; accepted: july 10, 2020 abstract the mixed coupled xptoy-(100-x)iro2 electrodes (x = 0, 10, 20, 30, 40, 50 60, 70, 80, 90 and 100) were thermally prepared at 450 °c on titanium supports. the prepared electrodes were firstly physically characterized by scanning electron microscopy (sem), x-ray diffraction (xrd) and x-ray photoelectron spectroscopy (xps). afterwards, electrochemical characterizations were performed by voltammetric (cyclic and linear) methods in different electrolyte media (koh and hclo4). it is shown that the prepared electrodes are composed by both ptoy (platinum and platinum oxide) and iro2 (iridium dioxide). for xptoy-(100-x)iro2 electrodes having higher content of iro2, more surface cracks and pores are formed, defining a higher surface area with more active sites. higher surface area due to presence of both ptoy and iro2, is for xptoy-(100x)iro2 electrodes in 1 m koh solution confirmed by cyclic voltammetry at potentials of the oxide layer region. for all prepared electrodes, voltammetric charges were found higher than for ptoy, while the highest voltammetric charge is observed for the mixed 40ptoy-60iro2 (x = 40) electrode. the tafel slopes for oxygen evolution reaction (oer) in either basic (0.1 m koh) or acid (0.1 m hclo4) media were determined from measured linear voltammograms corrected for the ohmic drop. the values of tafel slopes for oer at ptoy, 90ptoy-10iro2 and iro2 in basic medium are 122, 55 and 40 mv dec-1, respectively. for other mixed electrodes, tafel slopes of 40 mv dec-1 were obtained. although proceeding by different oer mechanism, similar values of tafel slopes were obtained in acid medium, i.e., tafel slopes of 120 60 and 39 mv dec-1 were obtained for ptoy, 90ptoy-10iro2 and iro2, and 40 mv dec-1 for other mixed electrodes. the analysis of tafel slope values showed that oer is more rapid on coupled mixed electrodes than on pure ptoy. for mixed xptoy-(100-x)iro2 electrodes, oer is more rapid when the molar http://dx.doi.org/10.5599/jese.806 http://dx.doi.org/10.5599/jese.806 http://www.jese-online.org/ mailto:kambireollo@yahoo.fr mailto:ouatlassine@yahoo.fr j. electrochem. sci. eng. 10(4) (2020) 347-360 oxygen evolution reaction on pt-ir oxide electrodes 348 percent of ptoy meets the following condition: 0 ˂ x ≤ 80. this study also showed that the mixed coupled electrodes are more electrocatalytically active for oer than either ptoy or iro2 in these two media. keywords platinum oxide, iridium dioxide, mixed pt-ir oxide, oxygen evolution, tafel slope, electrocatalytic activity introduction currently, climate changes and energy shortage are some of the greatest challenges for humanity [1-3]. increasing carbon dioxide (co2) levels in the atmosphere cause severe issues such as global warming, energy security, and sustainability. these issues stimulated researchers to find new clean technologies that are high in efficiency and low in cost [4]. to fulfill this goal without destroying the environment, alternative methods of energy generation, conversion, and storage need to be employed [5-8]. as a sustainable and environmentally friendly alternative, water electrolysis combined with renewable electricity sources, such as wind and solar power, is expected to provide a practical solution. however, the major drawback of technologies such as solar or wind energy is that they cannot function continuously. thus, renewable energy from water splitting by the electrochemical route is considered as one of the important techniques to provide a solution for climate change and energy shortage. the overall water splitting reaction includes the hydrogen evolution reaction (her) and oxygen evolution reaction (oer). to realize the practical and sustainable application of electrocatalytic hydrogen/oxygen evolution reactions (her/oer), an efficient, durable, and economical electrocatalyst is demanded triggering the reaction with minimal overpotential and fast kinetics [9-11]. according to the literature, the high-performance electrocatalysts are mainly based on noble metals such as for example, platinum [12,13]. from the cost perspective, however, it is highly desirable to develop electrocatalysts based on metal oxides. the most usually targeted oer electrocatalyst oxides are iridium oxide (iro2) and ruthenium oxide (ruo2) [14-16]. ruthenium oxide exhibits several interesting properties, and has the advantages of high conductivity and good electrochemical reversibility [17-19]. however, the ruthenium oxide electrode is not applicable for long-term operations and dissolves during oer. iridium oxide has attracted considerable attention due to its unique advantages including excellent chemical stability and sensitivity, impressive electrocatalytic activity, and sufficient electric conductivity [20-22]. besides, the iridium oxide is known for its superb biocompatibility and longterm stability [23-27]. it was already found that when iro2 is mixed with pt oxide, pt-ir oxide shows enhanced electrocatalytic activity and rapid oxygen evolution reaction [28,29]. to further research about electrocatalytic properties of mixed ptoy-iro2, it appears necessary to optimize the ratio of ptoy and iro2 on this electrode. in the present work, the proportion of each element (pt and ir) of the mixed electrode has been varied and the influence of these variations on the kinetics of oer is studied. mixed xptoy-(100-x) iro2 electrodes (x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100) where x represents the molar percent of ptoy precursor, have been prepared thermally at 450 °c and the kinetics of oer is followed on these electrodes in 0.1 m koh and 0.1 m hclo4 media, respectively. experiments metallic supports (or substrates) of titanium plates of dimensions 16160.5 mm were used. these supports were pretreated by sandblasting and rinsing to remove all the impurities and to o. kambiré et al. j. electrochem. sci. eng. 10(4) (2020) 347-360 http://dx.doi.org/10.5599/jese.806 349 increase the roughness which improves adhesion of the deposit. the surfaces were degreased and cleaned with isopropanol, followed by rinsing with distilled water. precursors of h2ptcl66h2o (fluka, 99 %), and h2ircl6h2o (fluka, 99 %) were applied by brushes to the surfaces of the pretreated electrodes. these precursors were diluted in isopropanol (fluka, 99.5 %). two precursor solutions of h2ptcl6h2o and h2ircl6h2o were prepared separately at 23.30 g/l and 23.25 g/l, respectively. then, mixtures of these two solutions were prepared, respecting the proportions by moles number of ptoy and iro2 for the coupled mixed electrodes. the concentrations of precursors in the mixture which served for the preparation of the mixed xptoy-(100-x)iro2 electrodes are grouped in table 1. the solvent of the deposit was evaporated by baking in the oven at 80 °c for 10 min. then the deposit underwent thermolysis in the furnace for 15 min at 450 °c. several layers of deposit were thus produced before being calcined in the furnace at 450 °c for one hour. the mixed coupled xptoy-(100x)iro2 electrodes (x = 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100) were prepared by this method, where x is referred to the molar percent of ptoy precursor. for the mixed electrodes, a mixture of different percent molars of the corresponding precursors was used. the deposit loading was about 5 g m-2 on each titanium substrate. table 1. concentrations of h2ptcl6h2o and h2ircl6h2o in the mixed coupled precursors used for preparation of mixed electrodes electrodes concentration of h2ptcl6h2o, m concentration of h2ircl6h2o, m 10ptoy-90iro2 0.0045 0.0405 20ptoy-80iro2 0.0090 0.0360 30ptoy-70iro2 0.0135 0.0315 40ptoy-60iro2 0.0180 0.0270 50ptoy-50iro2 0.0225 0.0225 60ptoy-40iro2 0.0270 0.0180 70ptoy-30iro2 0.0315 0.0135 80ptoy-20iro2 0.0360 0.0090 90ptoy-10iro2 0.0405 0.0045 high-resolution images of the prepared electrode surfaces were realized by scanning electron microscopy (sem, zeiss, supra 40vp). the crystalline structure was examined by x-ray diffraction (xrd) using a siemens equipment with a cu cathode. the xps analysis was carried out with a kratos axis-ultra spectrometer using a monochromatic al kα x-ray source, operated at 15 kv, and a pass energy of 20 ev. the electrochemical measurements (cyclic and linear sweep voltammetry) were carried out using a conventional three-electrode electrochemical cell and autolab pgstat 20 (ecochemie). the prepared xptoy-(100-x)iro2 electrodes, having geometric surface area of 0.785 cm2, served as the working electrodes (we). the reference electrode (re) was a saturated calomel electrode (sce), mounted in a luggin capillary and placed close to we by a distance of 1 mm. the counter-electrode (ce) was the spiral platinum wire, having the area at least hundred times larger than we. all potential values in this manuscript are given against the reversible hydrogen electrode (rhe) using the following relation: erhe = esce + 0.059 ph + 0.244 v (1) all electrochemical experiments were made at the ambient temperature of 25 °c. http://dx.doi.org/10.5599/jese.806 j. electrochem. sci. eng. 10(4) (2020) 347-360 oxygen evolution reaction on pt-ir oxide electrodes 350 results and discussion physical characterization of prepared electrodes the prepared electrodes were analyzed by scanning electron microscopy. the surface micrographs of iro2, 40ptoy-60iro2, 70ptoy-30iro2, and ptoy electrodes are presented in figure 1. the micrograph of pure iro2 in figure 1a shows superimposed layers of iridium dioxide, cracks and some pores, what is in accordance with the literature [24]. the cracks observed on the surface of pure iro2 occurred during the cooling of the deposit due to the mechanical stresses produced by the difference in the coefficients of thermal expansion of the substrate and the deposit. in figure 1b, sem image of the electrode containing 40ptoy-60iro2 shows that the surface of this deposit appears more continuous and the cracks are narrower in comparison with pure iro2. also, some small pt crystals dispersed on the surface of the deposit can be observed in figure 1b. sem image of the deposit containing 70ptoy-30iro2 is presented in figure 1c, showing a porous and very rough surface with almost non-existent cracks. at this stage, the layer of the deposit resembles that of ptoy [25]. on ptoy electrode surface shown in figure 1d, there are no cracks but the grains are strongly bonded to each other, thus showing the porous and rough nature of the deposited ptoy. these observations indicate that when the molar percentage of ptoy in the deposit increases, the surface structure of the deposit tends to be more like that of ptoy. figure 1. scanning electron micrographs of iro2 (a), 40ptoy-60iro2 (b), 70ptoy-30iro2 (c) and ptoy (d) xps spectra of ptoy, 50ptoy-50iro2 and iro2 electrodes are presented in figure 2. on the xps spectrum of ptoy, one can notice the presence of three intense peaks at 70, 312 and 335 ev, which all characterize platinum presence on titanium. carbon is also present with a peak observed at 286 ev at all presented electrodes. presence of carbon could be due to the poisoning of the measuring device, isopropanol used in the precursor preparations, and/or contamination in the oven used to prepare the electrodes. two very close peaks are observed at 218 and 224 ev. these two peaks characterize the presence of platinum oxide. in the xps spectrum of the mixed electrode (c) (d) (a) (b) o. kambiré et al. j. electrochem. sci. eng. 10(4) (2020) 347-360 http://dx.doi.org/10.5599/jese.806 351 50ptoy-50iro2 in addition to platinum peaks observed at 70, 312 and 335 ev, there are some new peaks at 61, 296 and 494 ev. these three new peaks also exist in xps spectrum of pure iridium oxide, approving the presence of iridium oxide in the mixed electrode. figure 2. xps spectra of ptoy, 50ptoy-50iro2 and iro2. the xrd spectrum of ptoy presented in figure 3 provides more details on the bulk structure of the platinum coating. the xrd pattern of ptoy shows two peaks at 2 = 40 and 68°, which are assigned to the metallic pt (jcpds no 4-802). the peaks located at 35 and 63° correspond to pto2 (jcpds no 38-1355). the peaks located at 2 = 40 and 53o could be assigned to ti [29]. those xrd results indicate that the coating of ptoy is a mixture of metallic pt and pto2. in other words, the deposit is a mixture of pt with zero-valence and its oxidized form, what also confirms the xps results of ptoy. generally, both xps and xrd results reveal that platinum present at the titanium surface exists in different oxidized states. 2 / o figure 3. xrd spectrum of ptoy electrochemical characterization of prepared electrodes with ferri/ferrocyanide redox couple figure 4a shows the cyclic voltammograms of the mixed 60ptoy-40iro2 electrode in 1 m koh, containing different concentrations of potassium ferri-ferrocyanide couple (1 100 mm). these voltammograms showed oxidation peaks in the forward potential scans and reduction peaks in the http://dx.doi.org/10.5599/jese.806 j. electrochem. sci. eng. 10(4) (2020) 347-360 oxygen evolution reaction on pt-ir oxide electrodes 352 reverse potential scans. there is an increase in the current density of both peaks with increase of the concentration of potassium ferri-ferrocyanide couple. the current densities of the oxidation and reduction peaks were plotted as a function of the concentration of potassium ferri-ferrocyanide for 10ptoy-90iro2, 50ptoy-50iro2, and 90ptoy-10iro2 electrodes, and presented in figures 4b and 4c. the obtained curves are straight lines that pass through the origin of the coordinate system. note that similar results were obtained with other mixed electrodes studied here (curves not shown). the equations of the lines make it possible to determine the anode (da) and cathode (dc) diffusion coefficients, using the following equation [30,31]: jp = 2.69 105 n3/2ad1/2c1/2 in eq. (2), jp is peak current density, n is number of exchanged electrons, a is geometric surface area, c is concentration of reaction species, d is their diffusion coefficient, and v is scan rate. figure 4: cyclic voltammetry of mixed 60ptoy-40iro2 electrode in 1 m koh containing different concentrations of ferri-ferrocyanide redox couple at 20 mv s-1 (a), anodic peak current densities (b) and cathodic peak current densities (c) against concentrations of [fe(cn)6]3–/[fe(cn)6]4– at mixed 10ptoy-90iro2, 50ptoy-50iro2 and 90ptoy-10iro2 electrodes. the values of diffusion coefficients of ferri/ferrocyanide couple listed in table 2 were determined by eq. (2) with n = 1, and are of the same order of magnitude as those from the literature [33]. the certain difference observed between the values determined in this work and literature data could be linked to the use of the geometric surface and not the real surface of the electrode in the calculation. it is clear from table 2 that d values are somewhat lower for mixed xptoy-(100-x)iro2 electrodes than those obtained on pure iro2 and ptoy. the ratios of the absolute value of the anode and cathode peak current densities are also shown in table 2. a value close to 1 was obtained for each electrode. these results confirm an almost reversible behavior of the potassium ferri-ferrocyanide couple on all here studied electrodes. o. kambiré et al. j. electrochem. sci. eng. 10(4) (2020) 347-360 http://dx.doi.org/10.5599/jese.806 353 table 2: kinetic parameters of ferri/ferrocyanide redox reaction in 1 m koh at xptoy-(100-x)iro2 electrodes electrochemical characterization of prepared electrodes in koh medium figure 5a shows cyclic voltammograms of ptoy and iro2 in the basic medium (0.1 m koh) in the oxide stability potential zone (0.77 to 1.47 v vs. rhe). for pure ptoy, a presence of some reduction current can be noticed from 0.77 to 0.91 v vs. rhe. this can be ascribed to the reduction of oxides formed in the forward potential scan [33] and also during the electrode calcination. a rectangular shape from 0.91 to 1.47 v/rhe is characterized by an almost constant current density and found similar to that observed previously [29]. in comparison to iro2 electrode, ptoy electrode has much lower voltammetric charge. at the other side, iro2 electrode possesses very high voltammetric charge and its voltammetric response is generally characteristic for heat-prepared dsas (dimensionally stable anodes) [29,35,36]. much improved voltammetric charge of the pure iro2 electrode can be associated with the pseudocapacitive reactions at iro2 and also, due to the presence of cracks and pores that increased the number of electrode active sites at the surface [35]. figure 5b shows cyclic voltammograms of mixed xptoy-(100-x)iro2 electrodes in 0.1 m koh medium from 0.77 to 1.47 v/rhe. the voltammograms of mixed electrodes have shapes similar to ptoy, indicating that ptoy is present on all the prepared mixed electrodes. however, the voltammetric charges of these electrodes are higher than that of ptoy. this increase in the voltammetric charge is due to the presence of iro2 on the surface of mixed electrodes. at mole percentages of ptoy greater than 50 %, the voltammetric charge is lower than that of iro2 but still greater than that of ptoy. for mole percentages of ptoy less than 50 %, the voltammetric charge is higher than that of ptoy and iro2. the highest voltammetric charge is observed for ptoy with mol percentage of 40 %. the high voltammetric charge of mixed electrodes shows the increase in active sites due to the presence of both ptoy and iro2. figure 5. cyclic voltammograms (10 mv s-1) in 0.1 m koh of ptoy and iro2 electrodes (a) and xptoy-(100-x)iro2 electrodes (b). electrodes |jpa / jpc| da / 10-6cm2 s-1 dc / 10-6cm2 s-1 ptoy 1.03 9.43 9.77 90ptoy-10iro2 1.00 5.19 6.11 80ptoy-20iro2 1.01 5.24 5.88 70ptoy-30iro2 0.99 3.27 5.02 60ptoy-40iro2 1.02 3.62 5.04 50ptoy-50iro2 1.04 4.57 6.33 40ptoy-60iro2 1.00 2.70 4.60 30ptoy-70iro2 0.99 2.50 4.29 20ptoy-80iro2 1.01 4.47 5.20 10ptoy-90iro2 1.03 4.37 3.65 iro2 1.06 12.29 12.92 http://dx.doi.org/10.5599/jese.806 j. electrochem. sci. eng. 10(4) (2020) 347-360 oxygen evolution reaction on pt-ir oxide electrodes 354 the studies were also carried out at the lower scan rate because, at a low scan rate, the system is in a quasi-stationary state. in figure 6, the linear voltammograms obtained with different xptoy-(100-x)iro2 electrodes in the basic medium of 0.1 m koh are presented. these voltammograms show that the potential at j = 1 ma cm-2 related to oxygen evolution varies from one electrode to another. the oer potential was also determined at j = 10 ma cm-2 for each electrode. the potential values of all prepared xptoy-(100-x)iro2 electrodes at these two current density values are presented in table 3. figure 6. linear voltammograms (5 mv s-1) of prepared electrodes in 0.1 m koh medium table 3. potentials of oxygen evolution reaction on various xptoy-(100-x)iro2 electrodes in 0.1 m koh at j = 1 ma cm-2 and j = 10 ma cm-2, scan rate = 5 mv s-1. electrode e / v vs. rhe (j = 1 ma cm-2) e / v vs. rhe (j = 10 ma cm-2) ptoy 1.679 1.908 90ptoy-10iro2 1.521 1.710 80ptoy-20iro2 1.488 1.578 70ptoy-30iro2 1.473 1.571 60ptoy-40iro2 1.470 1.576 50ptoy-50iro2 1.468 1.568 40ptoy-60iro2 1.460 1.544 30ptoy-70iro2 1.453 1.531 20ptoy-80iro2 1.464 1.543 10ptoy-90iro2 1.467 1.557 iro2 1.489 1.576 data in table 3 show that oer potentials for xptoy-(100-x)iro2 electrodes determined at j = 1 ma cm2 and j = 10 ma cm-2 vary in a similar way. the highest potential values observed at any current density for ptoy electrode suggest that it is less electrocatalytically active for oer than other electrodes in the basic medium. also, it seems that 90ptoy-10iro2 electrode is more electrocatalytically active than ptoy, but less than iro2 and other mixed electrodes. on the other hand, the electrodes with ptoy percentage lower or equal to 80 % have a potential related to oer close but less than pure iro2. it is clear that the coupling of ptoy with iro2 has improved electrocatalytic activity of the electrode. the electrodes which have ptoy percentage lower or equal to 80 % are the most electrocatalytically active for the oxygen evolution in the basic medium. the tafel slope of oer on an electrode can be abnormally high, especially within a high potential domain. according to kapalka et al. [37], the increase of the tafel slope is due to the ohmic drop. o. kambiré et al. j. electrochem. sci. eng. 10(4) (2020) 347-360 http://dx.doi.org/10.5599/jese.806 355 the ohmic drop between the anode and the cathode is due to the geometry of the cell, solution conductivity, presence of bubbles in solution, and accumulation of gas bubbles produced on the surface of the electrode (blockage of the surface) [38,39]. here, the ohmic drop corrections of the measured polarization curves were carried out according to the method already described in the literature [28,29]. briefly speaking, the overpotential (η / v) observed in the experiment can be defined by  = a + b ln j + jr (3) where a / v is tafel constant, b / v dec-1 is tafel slope, j / a cm-2 is current density and r / ω cm-2 is the total area-specific, uncompensated resistance of the system, assumed to be constant. the derivative of eq. (3) concerning current density, results in eq. (4) from which b and r can be easily obtained by plotting dη / dj as a function of 1 / j. d/dj =(b / j) +r (4) the estimation of r allows correcting the experimental overpotential by subtracting the ohmic drop jr according to the following equation: corr =  jr (5) where ηcorr stands for the corrected overpotential. during the calculations, the derivatives dη / dj were replaced by their finite elements δη ⁄ δj estimated from each pair of consecutive experimental points. after correcting the ohmic drop for each electrode in the electrolyte used, the tafel slope lines were determined from the corrected e vs. log j dependences that are for some xptoy-(100-x)iro2 electrodes presented in figure 7a. figure 7. tafel lines (a) and tafel slopes (b) of different xptoy-(100-x)iro2 electrodes in 0.1 m koh. note that tafel slopes vary from electrode to electrode, suggesting that the kinetics of oer varies from one electrode to another thus, it can be seen from figure 7a that the tafel slope of pure iro2 electrode is 40 mv dec-1. according to figure 7b, tafel slope remains almost constant at 40 mv dec-1 for all mixed electrodes which have ptoy percentage lower or equal to 80 %. this result indicates that oer at iro2 and all these mixed electrodes proceeds practically with the same kinetics. at the other side, tafel slopes of 55 and 122 mv dec-1 were respectively obtained for 90ptoy-10iro2 and ptoy electrodes. these results suggest that the electrodes which have the voltammetric response close to ptoy have also high tafel slope similar to ptoy. this is the case for electrodes whose surfaces are rich in ptoy. also, when the surface is dominated by iro2, the tafel slopes tend towards that of pure iro2. generally, the values of the obtained tafel slopes suggest that the kinetics of oer on mixed electrodes is faster on the electrodes having the percentage of ptoy lower or equal to 80 % than on the electrodes with the percentage of ptoy higher than 80 %. http://dx.doi.org/10.5599/jese.806 j. electrochem. sci. eng. 10(4) (2020) 347-360 oxygen evolution reaction on pt-ir oxide electrodes 356 the oer mechanism in the basic media is taken from the literature on mixed ptoy and iro2 electrodes [32], and described by following four steps s + oh→ soh + e(6) soh + oh→ so+ h2o (7) so→ so + e (8) 2so → 2s + o2 (9) where s stands for electrode active sites, while oh, oand o represent adsorbed intermediates. the mechanism described by eqs. (6-9) predicts the tafel slope of 120 mv dec-1 when step described by eq. (6) is the rate determining step (rds) 0 mv dec-1 when step (7) is rds, 40 mv dec-1 when step (8) is rds, and 15 mv dec-1 when step described by eq. (9) is rds. tafel slopes presented in figure 7b suggest that mixed xptoy-(100-x)iro2 electrodes have probably the same reaction pathway for oer, but different rate determining steps. for ptoy anode, the first step described by eq. (6) can be considered as the rate determining step, because the observed tafel slope is close to 120 mv dec-1. on the pure iro2 and the anodes with ptoy percentage less or equal to 80 %, the observed tafel slope is 40 mv dec-1, indicating that the step described by eq. (8) could be considered as the rate determining step. for the electrode containing 90 % of ptoy, the second step described by eq. (7) will be considered as the rate determining step, because its tafel slope was found close to 60 mv dec-1. electrochemical characterization of prepared electrodes in perchloric acid medium the oxygen evolution has also been studied in an acid medium. the linear voltammograms of mixed xptoy-(100-x)iro2 electrodes in 0.1 m hclo4, recorded in a quasi-stationary manner are presented in figure 8. figure 8. linear voltammetry (5 mv s-1) of xptoy-(100-x)iro2 electrodes in 0.1 m hclo4 figure 8 shows that the onset potential related to the oxygen evolution (the potential of the sharp current increase) varies from one electrode to another but is the highest for ptoy electrode. the potential values at j = 1 ma cm-2 and j = 10 ma cm-2 related to oer were determined for all xptoy-(100-x)iro2 electrodes and the obtained values are represented in table 4. data in table 4 show that potential values of oer vary from one electrode to another similarly and regardless of the current density. ptoy electrode has the highest potential related to oxygen evolution, suggesting that ptoy is the least electrocatalytically active electrode for oer in an acid medium. except for the electrode which contains 90 % ptoy, the mixed xptoy-(100-x)iro2 electrodes have lower onset potentials related to oer than either pure ptoy or pure iro2. this pointed out that the mixed pt-ir oxide electrode in an acid medium is generally more electrocatalytically active for o. kambiré et al. j. electrochem. sci. eng. 10(4) (2020) 347-360 http://dx.doi.org/10.5599/jese.806 357 o2 evolution than its pure components. these results are fully consistent with those observed for oer in the basic medium. table 4. potentials of oxygen evolution reaction on various xptoy-(100-x)iro2 electrodes in 0.1 m hclo4 at j = 1 ma cm-2 and j = 10 ma cm-2 and scan rate = 5 mv s-1 electrode e / v vs. rhe (j = 1 ma cm-2) e / v vs. rhe (j = 10 ma cm-2) ptoy 1.571 1.714 90ptoy-10iro2 1.470 1.587 80ptoy-20iro2 1.434 1.479 70ptoy-30iro2 1.428 1.475 60ptoy-40iro2 1.430 1.475 50ptoy-50iro2 1.438 1.475 40ptoy-60iro2 1.431 1.458 30ptoy-70iro2 1.424 1.454 20ptoy-80iro2 1.429 1.461 10ptoy-90iro2 1.441 1.481 iro2 1.450 1.514 the tafel slopes for oer in 0.1 m hclo4 were determined for all studied xptoy-(100-x)iro2 electrodes. as shown in figure 9a, plotting of the corrected e vs. log j gave straight lines in the tafel region. the particular values of tafel slopes for all studied xptoy-(100-x)iro2 electrodes are recorded in figure 9b. figure 9. tafel lines (a) and tafel slopes (b) of different xptoy-(100-x)iro2 electrodes in 0.1 m hclo4 at 5 mv s-1 tafel slope of 120 mv dec-1 was obtained for pure ptoy, while 60 mv dec-1 was obtained for the mixed electrode with 90 % of ptoy. the other mixed electrodes and pure iro2 gave values of 40 mv dec-1. all these values obtained in the acid medium (0.1 m hclo4) are almost identical to those obtained in the basic medium (0.1 m koh). also, the tafel slopes show that the oxygen evolution reaction in the acidic and basic medium is faster on the electrodes which have a percentage in ptoy lower or equal to 80 % than on the electrodes which have a percentage in ptoy higher than 80 %. the obtained tafel slopes, together with the literature results [27,29,40] point towards the following mechanism of oer on the prepared electrodes in an acid medium: s + h2o → s-oh + h+ + e(10) s-oh → so + h+ + e(11) 2so → 2 s + o2 (12) where s stands for electrode active sites, while oh and o represent adsorbed intermediates. the mechanism described by eqs. (10-12) predicts the tafel slope of 120 mv dec-1 if the first step described by eq. (10) is the rate determining step (rds), 40 mv dec-1 if the second step described by eq. (11) is rds, and 30 mv dec-1 if the third step described by eq. (12) is rds. http://dx.doi.org/10.5599/jese.806 j. electrochem. sci. eng. 10(4) (2020) 347-360 oxygen evolution reaction on pt-ir oxide electrodes 358 for oer on ptoy in hclo4 medium, the rate determining step is the first step described by eq. (10), because the tafel slope is 120 mv dec-1. for the pure iro2 electrode and the mixed xptoy-(100-x)iro2 electrodes except that with 90 % ptoy, the rate determining step is the second step described by eq. (11), because their tafel slopes are very close to 40 mv dec-1. with the platinumiridium dioxide coupling with a proportion of 90 % ptoy, however, the tafel slope is 60 mv dec-1. this specific value is between that obtained for pure ptoy (120 mv dec-1) and iro2 (39 mv dec-1), but closer to iro2 than ptoy. this result is in agreement with the voltammogram for 90ptoy-10iro2 electrode that is closer to iro2 than ptoy. generally, tafel slopes of mixed xptoy-(100-x)iro2 electrodes are close to that of iro2 and it is just iro2 which determines the oxygen evolution rate. the rate determining step is similar to that obtained with iro2 electrode, but oer is slower on 90ptoy-10iro2 anode than iro2 anode. the possible reason could be the mixed kinetics due to the adsorption of unstable species such as s-oh* (equations 13a and 13b), or the synergistic effect of ptoy and iro2 [22]. the mechanism of oer involving eq. (13) instead eq. (10) accounts for possible surface chemical rearrangements. therefore, the elemental step described by eq. (13b) probably controls the overall rate of oer [41,42]. s + h2o → s-oh* + h+ + e(13a) s-oh* → s-oh (13b) in each specific solution, ptoy, iro2 and mixed coupled xptoy-(100-x)iro2 electrodes studied here, generally follow the same oxygen evolution reaction pathway described by equations (6-9) or (10-13), but with different rate determining steps. note that oer reaction pathways described by equations (6-9) and equations (10-12) are generally known as the oxide decomposition path and the electrochemical oxide path, respectively [43]. the electrodes with molar percentage of ptoy lower or equal to 80 % have the lowest tafel slope values. this shows that the kinetics of oer is faster for these electrodes than for the electrodes which have ptoy percentage greater than 80 %. this result is in agreement with the voltammetric curves where the onset potential related to oxygen evolution reaction on these electrodes is lower than that obtained with the electrodes with ptoy content greater than 80 % mol. it is clear from these results that the electrodes which have a voltammetric behavior close to that of iridium oxide have faster oer kinetics than the electrodes which have the ptoy content greater than 80 % mol. when the mixed oxide surface is dominated by certain metal oxide, the slope of tafel tends towards that oxide. these observations agree with the scanning electron micrographs because on the electrodes having content less or equal to 80 % there are prominent pores and cracks on these electrodes. this has increased the number of active sites, hence the rapid kinetics of oxygen production. conclusions the physical (sem, xps and xrd) and electrochemical (cyclic and linear voltametries) characterizations of mixed coupled deposits of xptoy-(100-x)iro2 with different molar percent of ptoy precursor (x = 0, 10, 20, 30, 40, 50 0, 70, 80, 90 and 100) produced thermally on titanium supports, have shown rather different properties. presence of pt in different oxidation states (pt and pto2) and presence of pure iro2 is indicated for mixed coupled xptoy-(100-x)iro2 deposits, while their surface morphology was changed from the highly cracked and porous surface characteristic for pure iro2 (x = 0) to more continuous and much less cracked ptoy (x = 100). all prepared mixed coupled xptoy-(100-x)iro2 electrodes showed the metallic character by supporting perfectly the reversible ferri/ferrocyanide reaction taken as the redox probe. o. kambiré et al. j. electrochem. sci. eng. 10(4) (2020) 347-360 http://dx.doi.org/10.5599/jese.806 359 voltammetric charges measured in the oxide region of potentials for all mixed coupled xptoy-(100-x)iro2 electrodes in 0.1 m koh were found higher than for pure ptoy. for mixed electrodes having the mol percentage of ptoy equal or less than 50 % (x  50), the voltammetric charge is found even higher than for pure iro2. the highest voltammetric charge was observed for the mixed of 40ptoy-60iro2 electrode, i.e. the electrode having ptoy mol percentage of 40 % (x = 40). this could be ascribed to the increase of electrochemical surface area of the electrode due to the presence of both ptoy and iro2. in either basic (0.1 m koh) or acid (0.1 m hclo4) media, all mixed xptoy-(100-x)iro2 electrodes were found more electrocatalytically active for oxygen evolution reaction (oer) than pure ptoy electrode. the mixed electrode having mol percentage of ptoy equal or lower than 80 % (x  80) showed similar, or for (x  40) even better electrocatalytic activity for oer than pure iro2. tafel slopes of mixed xptoy-(100-x)iro2 electrodes obtained from linear voltammograms corrected for ohmic drops, suggest faster oer on coupled mixed xptoy-(100-x)iro2 electrodes having (x  80) than pure ptoy. this suggests that just presence of iro2 determines high oxygen evolution rate at coupled mixed xptoy-(100-x)iro2 electrodes. references [1] d. gielen, f. boshell, d. saygin, nature materials 15 (2016) 117–120. 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[43] s. trasatti, in: encyclopedia of electrochemical power sources, 1st edition, j. garche, c. dyer, p. moseley, z. ogumi, d. rand, b. scrosati (eds.), elsevier science, amsterdam, vol. 2, 2009, pp. 49–55. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://creativecommons.org/licenses/by/4.0/) binder-less activated carbon electrode from gelam wood for use in supercapacitors doi: 10.5599/jese.2012.0028 37 j. electrochem. sci. eng. 3(2) (2013) 37-45; doi: 10.5599/jese.2012.0028 open access : : issn 1847-9286 www.jese-online.org original scientific paper binder-less activated carbon electrode from gelam wood for use in supercapacitors nirwan syarif* , **, ivandini a. tribidasari* and widayanti wibowo* *department of chemistry, faculty of mathematics and natural sciences, university of indonesia, indonesia **department of chemistry, faculty of mathematics and natural sciences, sriwijaya university, indonesia  corresponding author: e-mail: e-mail: nnsyarif@gmail.com; tel.: +62-711-580269; fax: +62-711-580069 received: august 8, 2012; revised: november 30, 2012; published: : april 19, 2013 abstract this work focused on the relation between the porous structure of activated carbon and its capacitive properties. three types of activated carbon monoliths were used as the electrodes in a half cell electrochemical system. one monolith was produced from activated carbon and considered to be a binder-less electrode. two others were produced from acid and high pressure steam oxidized activated carbon. the micrographs clearly indicate that three electrodes have different porous structures. both porosity and surface area of carbons increased due to the formation of grains during oxidation. this fact specified that an acid oxidized carbon monolith will have relatively higher capacitance compared to non-oxidized and steam oxidized monoliths. maximum capacitance values for acid, steam oxidized and non-oxidized electrodes were 27.68, 2.23 and 1.20 f g -1 , respectively. keywords pyrolysis; voltammogram; surface oxidation; capacitance; micropore. introduction supercapacitors have become the focus attention of many researchers for decades due to their potential application as energy storage devices in relation to their high energy density, great power density and long cycle life. much effort has been done to develop advanced carbons to meet the demand for high performance supercapacitors [1]. fossiland organic-based carbonaceous materials are amongst the most widely used starting or precursor materials for supercapacitor electrodes because they are relatively inexpensive and easy to fabricate. these electrode materials http://www.jese-online.org/ mailto:nnsyarif@gmail.com j. electrochem. sci. eng. 3(2) (2013) 37-45 binder-less carbon electrode in supercapacitors 38 also have large specific surface areas, good electrical conductivity and excellent chemical stability. the carbonaceous materials extracted from biomass such as fruit peels, beans, plant shells, bamboo, woods, natural fibers and bagasse have been studied for supercapacitor carbon electrodes. the fabrication of carbon electrodes for supercapacitors, normally requires an addition of a polymer matrix in order to bind together the carbon particles into monolithic (pellet) form. this addition blocks a part of the carbon pores [2] and additionally causes an increase in the electrical resistivity [3]. furthermore, the use of polymer binder makes the product more expensive and environmentally less friendly. therefore, the carbon electrode without the use of any binding agent is highly recommended in order to be potentially used in supercapacitor applications, because they might have lower resistance in the absence of a polymeric binder and they are also of lower cost [4]. in most cases, the physicochemical properties of the carbon electrode, such as its structure, electrical conductivity and porosity, was improved by an activation process. the physical activation is normally carried out by treatment of the carbonized sample using gas atmosphere (such as co2, n2 and water steam) under a moderately higher temperature of 800 – 1000 c to improve the internal porosity [5]. in this paper, we report the preparation and characterization of free binder activated carbon monolith electrodes prepared from gelam wood, which were activated by physical process under n2/co2 gas and followed by surface oxidation using nitric acid and high pressure steam. it should be emphasized that the use of this activated carbon for supercapacitor was not yet reported. experimental activated carbon and electrode preparation activated carbons were prepared from the gelam wood piles obtained from local timber industry. non-bark wood piles sizes 10 cm length and 5 cm diameter was used in the production of activated carbon rod. pyrolysis was conducted after preheating process and followed by second pyrolysis for activation process. wood was preheated in the oven at 230 – 260 °c for a half hour. wood were carbonized by heating from 40 to 700 °c at rate of 3 °c per minute under atmosphere of nitrogen flow and controlled for ½ 1 hour at peak temperature. nitrogen was flowed to the reactor at rate of 1 ml min -1 . the heating procedure was repeated but using co2 as the atmosphere to produce activated carbon. this activated carbon was labeled as ccp. ccp was cut into pellets of 5 mm in thickness by using a mini-circular electric knife. ccp was subjected for surface oxidation using concentrated nitric acid and high pressure steam, both labeled as ccphno3 and ccpwot respectively. activated carbons were oxidized in reflux system for 3 hours using concentrated nitric acid at 60 c. the high pressure steam oxidation was conducted in the hydrothermal reactor for 16 hours at 200 c. the products were washed with abundant de-mineralized water and dried. these three types of activated carbon pallets were used as electrodes. surface characterization the characterization of activated carbons was carried out through physical adsorption of nitrogen gas at 77 k, using a conventional volumetric system (micromeritics asap 2400 instrument). the surface area based on nitrogen isotherms was calculated using the bet equation, and using the “t” method. scanning electron microscopy (sem) and energy dispersive x-ray analysis (edx) were used to observe the structural morphology and composition of activated carbon, respectively. hitachi s-4000 field emission sem was used for this analysis, operated at 30 kev. uncoated samples were subjected for sem observation. n. syarif et al. j. electrochem. sci. eng. 3(2) (2013) 37-45 doi: 10.5599/jese.2012.0028 39 the boehm titration method was applied to determine the acid and basic sites in the activated carbons [6]. the total acid sites were neutralized with 0.1 m naoh solutions, and the basic sites were neutralized with a 0.1 m hcl solution. the reaction between reagents and acidic oxygenatedfunctional groups on the surface is based on the difference strength of the acid/base. the strength order of acidic and basic groups is as follow: carboxyl > lactone > phenol. the carboxylic and lactonic sites were titrated with a 0.1 mna2co3 solution, the carboxylic sites were titrated with a 0.1 m nahco3 solution and the phenolic sites were determined by the difference. for each determination, 50 ml of the solution was added to one gram of activated carbon in a glass container. the container was inserted into vortex shaker for over one day. a 10 ml sample was titrated with 0.1 m of hcl or naoh solutions. electrochemical measurement electrochemical tests of the electrodes were conducted using the cyclic voltammetry technique using a three electrode configuration on a potentiostat instrument (edaq pty ltd, model ea161). ag/agcl electrode was used as reference electrode and a platinum rod was used as the counter electrode. cyclic voltammetry tests were conducted at scan rates of 2, 5, 10, 20, 50, and 100 mv s 1 with potential windows ranging from -1 to 1 v in 1, 2, and 4 mol l -1 koh or h2so4 aqueous solution as electrolytes. considering that the anodic voltammetric charges and cathodic voltammetric charges are not the same in the shape of voltammogram curve, the average specific capacitance of the electrode was calculated using integral area approximation       avg idvq c w v s vw (1) where δq is the total amount of the charge accumulated over a potential window, δv, w is the mass of active material in one electrode, i is the current, and s is the potential scan rate. hence ∫i dv is integral area of voltammogram curve. results and discussion pore structures and chemical characteristics three carbon electrodes, namely ccp, ccphno3 and ccpwot, were prepared from gelam wood activated carbon. the pores and grains were formed during preparation affecting the properties of electrodes. pores act as the entrance of small molecules penetrated deeper into the micro pores [7]. penetrating water steam and gases made some cracks and led to changed porous structures [8]. the porous structure of carbon can be characterized by the adsorption isotherms and pore size distribution plot [9]. the adsorption isotherms plot for the three electrodes (figure 1a) are type i represented by a flat curve, indicating micropores. the broadening of the knee in the very low relative pressures range in ccphno3 indicates that the microporosity of the carbon became wider upon acid oxidation treatment (figure 1a (b)). acid oxidation burst opening pore in carbon surface and increasing pore volume two times larger. in the other hand, some micro pores vanished (figure 1a (c)) and pore size distribution became narrow (figure 1b (c)) upon hydrothermal treatment due to fracture of the bulky network and the pore walls [10]. this fracturing blocked some pores and affected the pore structures and surface functional groups of carbon. j. electrochem. sci. eng. 3(2) (2013) 37-45 binder-less carbon electrode in supercapacitors 40 figure 1. (a) nitrogen adsorption-desorption on the surface of electrodes (b) pore size distribution for (a) ccp, (b) ccphno3 and (c) ccpwot. table 1 shows the changes in surface area, pore volume, and surface functional groups as a result of oxidation treatments. ccphno3 had the highest bet surface (337.1 m 2 g -1 ), and a total pore volume measured at p/p0 = 0.95 of 0.185 cm 3 g -1 , most of the pores being micropores (81.50 %), with an average pore width of 1.1 nm. on the other hand, ccpwot exhibited the lowest bet surface (88.800 m 2 g -1 ), with a much lower total pores volume (0.014 cm 3 g -1 ) and a narrower pore width (1.408 nm). a pore size distribution in the range of micropore in which larger than the size of two solvated ions, was then identified as a way of improving the energy density and the power capability [11], whereas mesopores improved the ionic mass transport inside the bulk of carbon. therefore, it was assumed that a good balance between microand mesoporosity was needed to maximize capacitance [12]. some capacitance values from the literature are n. syarif et al. j. electrochem. sci. eng. 3(2) (2013) 37-45 doi: 10.5599/jese.2012.0028 41 provided to clarify the relationship between pore structure and capacitance. it can also be seen from the table 1 that the some capacitance values obtained in this study are smaller than those obtained from the literatures. table 1. surface and electrochemical properties of the ccp, ccphno3 and ccpwot electrodes. electrode sbet / m 2 g -1 vtotal / cm 3 g -1 micro pore content, % functional groups concentration, mmol g -1 csp,max f g -1 fenolic lactonic carboxylic base ccp 149.848 0.093 56.249 0.200 3.85 0.00 0.24 1.22 ccphno3 337.116 0.185 81.499 0.100 3.08 0.20 0.19 27.68 ccpwot 88.800 0.014 28.306 0.200 3.40 0.10 0.22 2.23 carbon from sucrose [13] 2749.0 2.01 233 carbon from sago waste [14] 1408.2 0.897 64.1 carbon from coal [14] 179.2 0.073 16.2 sbet – specific surface area; vtotal total pore volume, measured at p/p0=0.995. furthermore, the specific surface area of the ccphno3 electrode as listed in table 1 is 337.116 m 2 g -1 , which is higher than that of the ccp and ccpwot electrode, i.e. 149.85 and 88.80 m 2 g -1 , respectively. the boehm analysis reveals that the three electrodes have similar amounts of functional groups, except for the carboxylic and lactone groups. a computational study was undertaken on the effect of the two functional groups on the structure of graphite basal. the study found that the cooh functional groups that fill the vertical plane will bend graphite layer led to reduced effectiveness of electron transfer between layers. these functional groups improved capacitive processes of the carbon [15], and facilitated the penetration of the electrolyte ions into the interior pores. although the ccpwot electrode has many more functional groups, however, many more pores were vanished and the surface area was decreased, leading to a decrease in the capacitance of electrode. furthermore, changes in the carbon pore structure can be clearly seen as surface morphology by using electron microscope. the micrographs clearly indicate that the three electrodes have different surface morphologies. figure 2a and 2d show the analysis of the ccp microstructure by sem. they revealed continuous and smooth surface of the ccp with less grain boundaries wood (figure 2a and 2d) due to the limited process of opening pore in line with the nature of the wood. the other sem pictures show the opening pores in the surface of the carbon which occurred when ccp was treated with nitric acid (ccphno3), especially the oxidation of low molecular weight carbon. these sem images of ccphno3 show irregular sizes and flaky grains in which the structures tend to form voids between the grains (figure 2b and e). the number of grains and micropores of ccphno3 are higher than those of ccpwot and ccp, due to the reaction between low molecular weight carbon fragments with nitrogen. the oxidation reaction generated volatile molecules that leave the pores behind after volatilization. furthermore, sem images of ccpwot show more regular size and relatively uniform (figure 2c and f). this porous structure, i.e. pore volume, surface area and pore size distribution along with functional groups on the surface of carbon are regarded as the important factors which determined the properties of capacitive processes, i.e. ionic accessibility, electrical double layer and redox reactions [16-18]. j. electrochem. sci. eng. 3(2) (2013) 37-45 binder-less carbon electrode in supercapacitors 42 figure 2. sem images for ccp (a and d), ccphno3 (b and e), and ccpwot (c and f) electrochemical performance cyclic voltammetry tests were conducted at scan rates of 2, 5, 10, 20, 50, and 100 mv s -1 with potential windows ranging from -1 to 1 v versus ag/agcl in 1, 2, and 4 mol l -1 koh or h2so4 aqueous solution to quantify the electrochemical performance of ccp, ccphno3 and ccpwot electrodes. as shown in figure 3 and 4, no pronounced reversible reduction oxidation peak can be observed from voltammograms for the three electrodes. the shape of the voltammograms was different from one to another. figure 3. cv curve of carbon electrodes (a) ccp, (b) ccphno3 and (c) ccpwot in the h2so4 1m (a) and 4m (b). scan rate was 20 mv s -1 . inset: magnification of voltammogram (c). -1000-500 0 500 1000 -40 -20 0 20 40 -1000-500 0 500 1000 -1000 0 1000 -2 0 2 i / m a e / mv (a) (b) (c) a -1000 0 1000 -2 0 2 (c) (b) (a) b n. syarif et al. j. electrochem. sci. eng. 3(2) (2013) 37-45 doi: 10.5599/jese.2012.0028 43 the voltammogram of the ccpwot electrode was relativelylower compared to the other two curves. therefore the oxidation treatment under water steam reduced the capacitance. on the other hand, the highest current of ccphno3 electrode points to the largest capacitance. the changes in capacitance occurred on the electrode after the oxidation process was associated with changes in pore volume of carbon, as previously described. oxidation using strong acid clarified the existence of the faraday process (redox reaction) that occurred at the carbon as described by other authors [19]. the voltammograms shape of ccphno3 in both acid and base electrolytes are much the same, which means that the tendency to establish a redox reaction was similar. the voltammogram area in figure 3 shows the decrease in capacitance at high concentrations of h2so4. by contrast, the capacitance of the electrodes increased at a low concentration koh (figure 4). figure 4. voltammogram of carbon electrodes (a) ccp, (b) ccphno3 and (c) ccpwot in the koh 2m (a) and 4m (b). scan rate was 5mv s1 . inset: magnification of voltammogram (c). calculations of specific capacitances for all electrodes at various scan rates in the h2so4 solution are presented in figure 5. at the 5 mv s -1 scans rate in h2so4, the ccp electrode had a specific capacitance of 1.2 f g -1 , while at a scan rate of 100 mv s -1 this was 0.22 f g -1 . capacitance values of the ccpwot electrode (figure 5 b) at 5 – 10 mv s -1 were lower than that of ccp (figure 5 a), but were higher at 20 – 100 mv s -1 . the capacitance values of ccp and ccpwot electrodes were much smaller than those of the ccphno3 electrode (figure 5 c). maximum capacitance for nonoxidized, steam and acid oxidized electrodes were 1.22, 0.63 and 2.57 f g -1 , respectively. these values occurred at 5 mv s -1 . plots of the electrodes specific capacitances vs. scan rate in koh are presented in figure 6. capacitances of ccp in koh were much the same as those in h2so4 both at low and high scan rates. contrary to what occurred in the h2so4, the capacitance values of the ccpwot electrode (figure 6b) were higher than those of ccp (figure 6a), both at low and high scan rates. capacitance values of ccp and ccpwot electrodes were also much smaller than those of the ccphno3 electrode. plot of specific capacitance of ccphno3 in koh was discrete (figure 6c) due to instability of the system, especially at a scan rate higher than 10 mv s -1 . a stable voltammogram j. electrochem. sci. eng. 3(2) (2013) 37-45 binder-less carbon electrode in supercapacitors 44 was only obtained at a scan rates higher than 40 mv s -1 at 1m koh. capacitance value for acid and steam oxidized electrodes were 2.23 and 27.68 f g -1 , respectively. figure 5. plot of specific capacitance of carbon electrode (a) ccp, (b) cppwot and (c) ccphno3 versus scan rate in h2so4 (a) 4m, (b) 2m and (c) 1m. figure 6. plot of specific capacitance (csp)of carbon electrode (a) ccp, (b) cppwot and (c) ccphno3 versus scan rate in koh (a) 4m, (b) 2m and (c) 1m. the presence of the micropores in the activated carbon are unique for gelam wood. combining their uniqueness, simplicity of the preparation method and the abundance availability, makes these electrode materials relatively inexpensive and environmentally friendly. thus gelam wood activated carbon has the potential to be developed as electrodes in electrochemical capacitors. n. syarif et al. j. electrochem. sci. eng. 3(2) (2013) 37-45 doi: 10.5599/jese.2012.0028 45 conclusions the three types of electrodes, namely ccp, ccphno3 and ccpwot, prepared from gelam wood activated carbon, have different surface morphologies which basically contained many micropores. ccp has continuous smooth surface and contains very few grain boundaries. on the other hand ccpwot and ccphno3 have rough surfaces and contain many cracks. the different oxidation methods using nitric acid and water steam gave different effects on the surface area of the carbons. acid nitric oxidation led to a larger pore volume and surface area, while water steam oxidation led the opposite effect. in addition, ccphno3 had relatively large amounts of micropores. the presence of surface oxides was important factor in improving the capacitance of electrode and increased the effective utilization of the pore structure. ccphno3 had the highest capacitance value of 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croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {electrochemical oxidation and electroanalysis of paracetamol on a boron-doped diamond anode material in aqueous electrolytes} http://dx.doi.org/10.5599/jese.933 71 j. electrochem. sci. eng. 11(2) (2021) 71-86; http://dx.doi.org/10.5599/jese.932 open access: issn 1847-9286 www.jese-online.org original scientific paper electrochemical oxidation of paracetamol on boron-doped diamond electrode: analytical performance and paracetamol degradation kouakou e. kouadio1, ollo kambiré2, konan s. koffi 1, lassiné ouattara1, 1laboratoire de constitution et réaction de la matière, ufr ssmt, université félix houphouëtboigny de cocody, abidjan, 22 bp 582 abidjan 22, côte d’ivoire 2ufr sciences et technologies, université de man, bp 20 man, côte d’ivoire corresponding author: ouatlassine@yahoo.fr received: november 16, 2020; revised: february 6, 2021; accepted: february 7, 2021 abstract electrochemical oxidation of paracetamol on boron-doped diamond (bdd) anode has been studied by cyclic voltammetry and preparative electrolysis. quantification of paracetamol during electrolysis has been mainly realized by differential pulse voltammetry technique in the britton-robinson buffer solutions used as the supporting electrolyte. various parameters such as current intensity, nature of the supporting electrolyte, temperature, and initial concentration of paracetamol have been investigated. the electrochemical characterization by the outer sphere fe(iii)/fe(ii) redox couple has also been performed, showing the metallic character of bdd electrode. the obtained linear dependency of the oxidation peak current intensity and paracetamol concentration indicates that bdd electrode can be used as an electrochemical sensor for the detection and quantification of paracetamol. the investigation of paracetamol degradation during preparative electrolysis showed that: (i) the degradation rate of paracetamol increases with increase of current intensity applied; (ii) for the initial concentrations of 10, 6 and 1 mm of paracetamol, its oxidation rate reaches 60, 78 and 99 % respectively, after 1 h of electrolysis in 0.3 m h2so4 (ph 0.6) at applied current density of 70 ma cm-2; (iii) at temperatures of electrolyte solution of 28, 55 and 75 °c, paracetamol oxidation rate reached 85, 92 and 97 % respectively, after 2 h at applied current density of 70 ma cm-2. from the investigation of the effect of ph value of electrolyte solution, it appears that oxidation of paracetamol is more favorable in acidic solution at ph 3 than solutions of higher ph values. keywords boron-doped diamond; paracetamol; electrolysis; voltammetry; oxidation; degradation. http://dx.doi.org/10.5599/jese.933 http://dx.doi.org/10.5599/jese.932 http://www.jese-online.org/ mailto:ouatlassine@yahoo.fr j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 72 introduction paracetamol, known also as acetaminophen, is one of the most commonly used analgesics (pain relievers) and antipyretics (anti-fever) for humans today [1-3]. it is present in doliprane, efferalgan or di-antalvic, which are the most sold drugs in pharmacies. efficacy, good tolerance, pharmacokinetic profile, cost and acceptability make paracetamol to be the first-line drug for treatment of mild to moderate pains. paracetamol is generally available in 500 mg tablets, and also in the soluble form (sachets, effervescent tablets), or liquid preparations for young children [2,4]. the intensive use of paracetamol leads to its presence in the environment, and more specifically in the surrounding waters. this makes a real problem for humans and other living things, since paracetamol breaks down into very toxic metabolites [5-8]. a general awareness is therefore necessary in order to propose effective methods for detection, quantification and decontaminating wastewaters containing paracetamol. according to the literature [9-12], physical, biological and chemical methods are usually used in wastewaters treatment. thus, activated carbon is widely used to treat organic and inorganic pollutants [13,14] and also, to immobilize metal ions such as copper, zinc, cadmium or chromium [15]. the main limitation of such treatment lies in the fact that in any case, pollutants are not degraded, but concentrated on the activated carbon which must be subsequently treated in order to regenerate it. biological methods for wastewater treatment are based on the use of microorganisms under aerobic or anaerobic conditions [16]. faced with toxic compounds that are difficult to biodegrade, however, these microorganisms proved to be rather inefficient. due to shortcomings of these treatment methods, over the past decade much research was focused on the new class of oxidation techniques, known as the advanced oxidation processes (poa). poa is designed to remove organic and inorganic materials in water and wastewater by the oxidation process. this technology has already shown its effectiveness in the treatment of toxic and biologically refractory organic pollutants. along to all these treatments, the electrochemical treatment must also be considered. compared to other conventional processes such as fenton process for water and wastewater treatment and processes using ultraviolet radiation [17-19], the electrochemical treatment has three major advantages: high efficiency, rapid reaction, and easy implementation. in addition, electrochemical oxidation processes consume few chemicals and produce no waste. according to the literature, the electrochemical method has shown its effectiveness in the treatment of wastewater containing organic pollutants [20-22]. the effectiveness of the electrochemical treatment depends strongly on the properties of electrode used as the anode in the case of electrolysis [10,23,24]. in this work, the boron-doped diamond (bdd) electrode is used. bdd has remarkable optical, electronic, and mechanical qualities, thus showing a great aptitude for various applications in different fields. according to literature, bdd can allow complete mineralization of organic compounds [21,24]. electrochemical methods such as square wave voltammetry (swv), cyclic voltammetry (cv) and differential pulse voltammetry (dpv) have already been used for the determination of concentrations of organic compounds [1,25-29]. also, it was already shown that dpv provides good limit of detection for paracetamol using boron-doped diamond electrode [1,25]. in this study, the follow-up of paracetamol concentration during its oxidation will be performed by dpv technique, using bdd electrode as the sensor. https://en.wikipedia.org/wiki/wastewater https://en.wikipedia.org/wiki/oxidation k. e. kouadio et al. j. electrochem. sci. eng. 11(2) (2021) 71-86 http://dx.doi.org/10.5599/jese.907 73 experimental electrochemical measurements the voltammetric measurements were performed using an autolab pgstat 20 (ecochemie) connected to a potentiostat equipped with usb electrochemical interface. this system is connected to a three-electrode single compartment glass cell and a computer for data storage and processing. gpes 4 software was employed to get voltammograms. the glass electrochemical cell consisted of saturated calomel electrode (sce) and platinum wire as the reference and counter electrode, respectively. bdd electrode with a surface of 1 cm2 was used as the working electrode. all ph values were measured with a ph meter. all potential values are reported against standard hydrogen electrode (she), calculated according to the following relation: eshe = esce + 0.25 (1) electrodes boron doped diamond (bdd) electrodes were prepared by hot-filament chemical vapor deposition (hf-cvd) on low resistivity (1-3 mω cm) p-si wafers (siltronix, diameter 10 cm, thickness 0.5 mm). the process gas was a mixture of 1 % ch4 in h2 containing trimethylboron. film was grown at a rate of 0.24 μm h-1. the film thickness was about 1 μm. chemicals paracetamol was purchased from a pharmacy in abidjan. all chemicals used in the experiments were reagent grade or higher, and used as received without any further purification. sulfuric acid (96 %), iron (ii) sulfate (99 %) and sodium hydroxide (98 %) were obtained from panreac quimica sau. orthophosphoric acid (85 %) and boric acid (99. 5 %) were obtained from emsure brand. ammonium iron (iii) sulfate (99 %) was obtained from merck, while acetic acid (99.7 %) was obtained from emparta. the working solutions for voltammetric investigations were prepared by dilution of the stock solution. all solutions were protected from light and used within 24 h to avoid decomposition. distilled water was used to prepare supporting electrolytes. britton-robinson buffer solution preparation britton-robinson buffer solution, 0.04 m, was prepared in order to maintain ph value of the samples taken during the electrolysis within a very restricted range. for this purpose, acetic acid, orthophosphoric acid and boric acid were used in the respective amounts of 4.6 ml, 5.4 ml, and 4.9713 g per 2 l of solution 30. the preparation of this buffer solution was done as follows: a certain amount of water in a glass beaker was heated to a temperature of around 70 °c using a water bath. then, boric acid was added into heated water and left to cool. the cooled solution of boric acid was transferred into 2000 ml flask containing above-mentioned quantities of acetic acid and orthophosphoric acid, filled with distilled water to mark and homogenized. in 99 ml of this britton-robinson buffer solution, 1 ml of paracetamol samples taken from electrolysis at pre-established time intervals were added. after homogenization of solution, differential pulse voltammograms (dpvs) were recorded using electrochemically pretreated bdd electrode, in order to determine paracetamol concentrations. preparative electrolysis for preparative electrolysis, bdd was used as the anode and zirconium plate as the cathode. these two electrodes of 16 cm2 each were separated by 2 cm, what delimited the cell-volume to 32 cm3. the solution was recirculated using a mini peristaltic pump of the watson marlow 505u http://dx.doi.org/10.5599/jese.907 https://en.wikipedia.org/wiki/ammonium_iron(iii)_sulfate j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 74 type. the flow rate of the solution was 2.7 ml s-1. fabrimex 141 type voltage generator allowed to keep the current constant during the electrolysis. batch mode setup was used for electrolysis. setup for analytical measurements prior to the application of differential pulse voltammetry, bdd electrode was electrochemically pretreated. cyclic voltammetry was applied by seven cycles aiming to purify bdd in 0.5 m sulfuric acid with previously cathodically pretreated bdd electrode at −2 v during 90 s [31]. well cleaned electrode was immersed in the britton-robinson buffer solution without paracetamol and anodically pretreated at 2 v for 15 s. finally, the pretreated bdd electrode was used to analyze the content of paracetamol using dpv technique in different supporting electrolytes after homogenization via a magnetic bar coupled to a magnetic stirrer. dpv curves were obtained under the following conditions: ep = -2 v; tp = 90 s; modulation time ≥ 0.05 s; potential scan rate = 70 mv s-1. for the determination of limits of detection (lod) and quantification (loq) of paracetamol, the following equations were used: lod = 3 sd / b (2) loq = 10 sd / b (3) where sd is the standard deviation of blank current signals and b is the slope of the calibration curve [26]. results and discussion electrochemical characterization of bdd electrode in order to investigate the electrochemical behavior of the prepared bdd electrode, fe(iii)/fe(ii) redox couple was used, and the influence of its concentration (20-50 mm) on the electrode response was studied in 0.3 m sulfuric acid solution as the supporting electrolyte. the obtained results are presented in figure 1(a), showing oxidation peak in the forward potential scan and reduction peak in the backward potential scan. the anodic and cathodic peaks correspond to the oxidation and reduction of the redox couple considered. the peaks observed are typical for electrodes that are good electronic conductors [24,31,32]. figure 1. (a) cyclic voltammetry (50 mv/s) of bdd electrode at different concentrations of fe(iii)/fe(ii) redox couple in 0.3 m h2so4; (b) anodic and cathodic peak current intensity against fe(iii)/fe(ii) concentration; t = 25 °c figure 1(a) also shows that the current intensities of the anodic and cathodic peak increase in absolute values with increase of the concentration of redox species. anodic and cathodic peak current intensities plotted as a function of redox species concentration, led to straight lines passing k. e. kouadio et al. j. electrochem. sci. eng. 11(2) (2021) 71-86 http://dx.doi.org/10.5599/jese.907 75 through the origin of the axes (figure 1(b)). these results show that oxidation and reduction reactions of redox species are responsible for the appearance of these peaks. the results are similar to that obtained with good electronic conductor electrodes such metallic electrodes. thus, the metallic character of the used bdd electrode is generally approved [24,31]. the proportionality observed between the current intensity and concentration of the redox couple studied suggests that bdd electrode can principally be used as an electrochemical sensor for detection and quantification of paracetamol, and possibly other pharmaceutical organics in aqueous solutions [1,31]. cyclic voltammetry investigations were carried out at several potential scan rates for an equimolar concentration (50 mm) of the redox couple fe(iii)/fe(ii). figure 2(a) shows the obtained cyclic voltammograms. in this figure, the current intensities of the anode and cathode peak increase in absolute values with increase of potential scan rates. in figure 2(b), the anodic and cathodic peak current intensities have been represented as a function of the square root of the potential scan rate. it appears that the peak current intensities are proportional to the square root of the potential scan rates, since straight lines passing through the origin of the axes were obtained. this confirms the quasi-reversible nature of fe(iii)/fe(ii) redox reaction on bdd, what has already been observed previously in our laboratory [24,31]. the linearity between the peak current intensity and the square root of the potential scan rates indicates that the oxidation-reduction kinetics at the electrode is diffusion-controlled [24]. e / v vs. she v½ / mv½ s-½ figure 2. (a) cyclic voltammetry of fe(iii)/fe(ii) redox couple (50 mm) in 0.3 m h2so4 on bdd at different potential scan rates; (b) anodic and cathodic peak current intensities against square root of the scan rate; t = 25 °c cyclic voltammetry of paracetamol oxidation the electrochemical behavior of bdd in 0.3 m sulfuric acid solution in the presence of several paracetamol concentrations at the potential scan rate of 100 mv s-1 is presented in figure 3(a). this figure shows increase in the current values around 0.9 v, followed by an irreversible oxidation peak at 1.05 v. this peak characterizes the anodic oxidation of paracetamol. figure 3(a) shows that as the paracetamol concentration increases, the current intensity of the oxidation peak also increases. this indicates the direct oxidation of paracetamol on bdd electrode. the plot of the oxidation peak current intensity against the concentration of paracetamol shown in figure 3(b), led to the straight line passing through the origin of the axes. the slope of this straight line is about 0.4 ma mol-1, with determination coefficient of 0.99. hence, bdd electrode can be a good candidate for detection and quantification of paracetamol. http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 76 figure 3. (a) cyclic voltammograms (100 mv s-1) of bdd electrode at different concentrations of paracetamol in 0.3 m h2so4 solution; (b) anodic peak current intensity as a function of paracetamol concentration; t = 25 °c effect of the supporting medium in order to determine the ideal supporting electrolyte for the quantification of paracetamol during electrolysis, calibration curves of paracetamol in sulfuric acid, orthophosphoric acid and britton-robinson buffer solutions were carried out. figure 4(a) represents dpv curves for paracetamol concentrations varying from 2 to 91 µm in 0.3 m sulfuric acid solution. it can be noticed in figure 4(a) that the peak potential values of paracetamol oxidation are approximately equal, whereas the intensity of the oxidation peak current increases with the increase of paracetamol concentration. in figure 4(b), the peak current intensity is plotted against the concentration of paracetamol, showing two linear regions. one linear region is characteristic for low concentrations of paracetamol below 10 μm, and the other for concentrations of paracetamol higher than 10 μm. in the insert of figure 4(b), the straight line determined in the low concentration range of paracetamol is shown. its equation was evaluated as: i =2.4698 c + 1.3764 with determination coefficient of r2 = 0.972. the limits of detection and quantification were determined using equations (2) and (3), and the obtained values lod = 0.37 μm and loq= 1.23 μm. figure 4. (a) differential pulse voltammograms of bdd electrode for different concentrations of paracetamol in 0.3 m h2so4; (b) current intensity calibration curve as a function of paracetamol concentration; t = 25 °c orthophosphoric acid (0.3 m) was also chosen as the supporting electrolyte containing several concentrations of paracetamol. the obtained dpv results are shown in figure 5(a). here also, the k. e. kouadio et al. j. electrochem. sci. eng. 11(2) (2021) 71-86 http://dx.doi.org/10.5599/jese.907 77 current intensity of paracetamol oxidation peak increases with the increase of concentration of paracetamol. figure 5(b) presents the peak current intensity as a function of paracetamol concentration. the obtained results are quite similar to these obtained in sulfuric acid. in orthophosphoric acid supporting electrolyte, two linear regions, one for low concentrations and the other for high concentrations of paracetamol, were also obtained. in the low paracetamol concentration range (insert of figure 5(b)), the straight line is characterized by the following equation: i = 1.9232 c + 4.6364 with r2 = 0.997. in orthophosphoric acid supporting electrolyte containing paracetamol, lod and loq of paracetamol were estimated as 0.47 and 1.57 µm, respectively. figure 5. (a) differential pulse voltammograms of bdd electrode for different concentrations of paracetamol in 0.3 m h3po4; (b) current intensity calibration curve as a function of paracetamol concentration; t = 25 oc figure 6(a) highlights dpv curves obtained in 0.04 m britton-robinson buffer solution. figure 6. (a) differential pulse voltammograms of bdd electrode for different concentrations of paracetamol in 0.04 m britton-robinson buffer solution; (b) current intensity calibration curve as a function of paracetamol concentration; t = 25 oc here also, the oxidation peak potential values associated with each concentration of paracetamol are equal, while the peak current intensity increases linearly with paracetamol concentration (figure 6(b)). this straight line passes through the origin of the axes and its equation is evaluated as: i = 0.8615 c + 2.7177 with determination coefficient of 0.999. lod and loq values for paracetamol in britton-robinson buffer solution were calculated as 0.167 and 0.559 μm, respectively. http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 78 from the results presented in figures 4-6, it appears that the britton-robinson buffer solution leads to the best calibration curve, which explores the highest paracetamol concentration range (2.0-130.66 μm) and shows the lowest lod and loq values (0.167 and 0.559 μm). as have already been shown for electroanalytical determinations of some other drugs [33,34], britton-robinson buffer solution is confirmed also in our experimental conditions as the most suitable solution for monitoring concentrations of paracetamol during its electrolysis. up to now, paracetamol was determined by various electrochemical methods using various electrode materials. a comparison of our findings with some literature data [31,35-44] is presented in table 1. data in table 1 clearly show that the detection limit of 0.167 m determined in this work is in accordance or even better than other literature data, indicating that our determination method could be a reliable method for the determination not only of paracetamol, but also traces of some other organic pharmaceutical compounds present in wastewaters. in further experiments, this method will be used to follow the evolution of paracetamol concentration during its degradation. table 1. efficiency of some modified electrodes in determination of paracetamol by various voltametric methods electrode method linear working range of paracetamol concentration, μm detection limit of paracetamol concentration, μm reference pani/mwcnt/gce swv 1 100 0.25 [35] c60/gce dpv 50 1500 5 [36] nafion/tio2-graphene/gce dpv 1 100 0.21 [37] aunp-pga/swcnt dpv 8.3 145.6 1.18 [38] pay/nano-tio2/gce dpv 12 120 2.0 [39] mip dpv 1 4000 0.33 [40] nevirapine/cpe dpv 20 250 0.77 [41] gr–cs/gce dpv 1-100 0.3 [42] cs/cpe swv 0.8 to 200 0.51 [43] pmmcntpe cv 2.0-50 0.38 [44] bdd dpv 1.98 13.87 0.16 [31] bdd dpv 2.0-130.66 0.167 this work gce: glassy carbon electrode; cpe: carbon paste electrode; bdd: boron doped diamond electrode; dpv: differential pulse voltammetry; swv: square-wave voltammetry; cv: cyclic voltammetry; pga: poly (glutamic acid); pay: poly (acid yellow 9); swcnt: single-walled carbon nanotubes; pani-mwcnt: polyaniline-multi-walled carbon nanotubes composite; mip: molecular imprinted polymeric micelles; gr-cs: graphene-chitosan composite; cs: chitosan; pmmcntpe: poly(methyl) orange modified carbon nanotube paste electrode. electrolysis of paracetamol the effect of current density on paracetamol oxidation during electrolysis was also studied. for such a purpose and in order to determine the optimal current density, the electrolysis experiments were conducted in 0.3 m h2so4 solution under current densities of 20, 50, 70 and 100 ma cm-². dpvs of paracetamol samples in britton-robinson buffer solution, subjected previously (10 mm) to electrolysis in 0.3 m h2so4 at different current density values and taken after different electrolysis time, are shown in figure 7. in this figure, differential pulse voltammograms show a decrease of oxidation current peak intensities of paracetamol with the electrolysis time. the decrease of the peak current intensity is rapid and highly marked as the applied current density increases. the oxidation peak current of paracetamol is observed at around 0.9 v. at higher electrolysis time, however, another current peak appears at 0.55 v. this new peak firstly increases in intensity and then decreases until it disappears. k. e. kouadio et al. j. electrochem. sci. eng. 11(2) (2021) 71-86 http://dx.doi.org/10.5599/jese.907 79 figure 7. differential pulse voltammograms of bdd electrode in britton-robinson buffer solution containing paracetamol (10 mm) subjected to electrolysis in 0.3 m h2so4 at: (a) 20 ma cm -2; (b) 50 ma cm-2; (c) 70 ma cm-2; (d) 100 ma cm-2; t = 25 °c flow rate = 2.7 ml s-1 the effect of the current density on the change of paracetamol concentration during electrolysis was also determined. figure 8(a) shows the normalized concentration (c/c0) of paracetamol as a function of the electrolysis time for each of the current density explored. note that c/c0 denotes paracetamol concentration at any time to that at the initial time. exponential decrease of the normalized paracetamol concentration versus the electrolysis time is observed, regardless of the applied current density. the shapes of these curves are in accordance with these described in literature [24] insofar the applied current densities are higher than the initial limiting current density which is equal to 6 ma cm-2. in this case, the oxidation process paracetamol is under mass transfer control. according to our results, the oxidation of paracetamol becomes more rapid as the current density increases from 20 to 100 ma cm-2. the evolution of the current ratio i/im of oxidation peak currents of intermediates during the electrolysis was determined for all applied current densities. figure 8(b) presents the obtained results, showing that at 20 ma cm-2 the production of secondary species is very low compared to those obtained under 50, 70 and 100 ma cm-2. as shown in figure 8(b), higher is the current density, the greater is the production of these species and quicker is their complete mineralization. http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 80 figure 8. (a) influence of current density on paracetamol oxidation (10 mm) in 0.3 m sulfuric acid; (b) influence of current density on oxidation of reaction intermediates; t = 28 °c; flow rate = 2.7 ml s-1 figure 9 highlights the evolution of solution color during the electrolysis. starting from the colorless solution at the beginning of the experiment, the solution became colored during the experiment, and returned to the colorless solution at the end of the electrolysis. the total discoloration of the solution after 2.5 h of electrolysis may indicate the effectiveness of the treatment at 70 ma cm-2. as mentioned above, before electrolysis, paracetamol solution was colorless and after 0.5 h it becomes colored (yellow color) with an increasing intensity after 1 h of electrolysis. this is followed by a gradual discoloration of the solution, and after 2.5 h, the complete discoloration occurred. appearance and disappearance of the yellow color during electrolysis could be related to the production of intermediates which undergoes additional oxidation. 0 h 0.5 h 1.0 h 1.5 h 2.0 h figure 9. photos of the evolution of the solution color due to oxidation of intermediates during electrolysis of paracetamol in 0.3 m h2so4 at 70 ma cm -2; flow rate = 2.7 ml s-1 current efficiencies of paracetamol oxidation were evaluated as a function of the electrolysis time. the obtained results are presented in figure 10, showing decrease of oxidation current efficiency with time. the oxidation current efficiencies obtained after 2.5 h of electrolysis at 20, 50, 70 and 100 ma cm-2 were 52, 27.5, 22.85 and 19.11 %, respectively. these data show that as the current density increases, a significant part of the current is lost by the production of some side reactions. low current efficiencies obtained at high current density can be explained by a limitation by mass transport of paracetamol due to parallel formation and oxygen bubbles evolvement during electrolysis. the paracetamol oxidation is therefore more and more favorable as the current density decreases. the influence of paracetamol concentration on the extent of its oxidation during electrolysis was also studied. figure 11(a) illustrates the results obtained under the current density of 70 ma cm-2 at ph 0.6. it is obvious from this figure that the ratio of paracetamol concentration at any time to that at the initial time (c/c0t) decreases with time. after 1 h of electrolysis, the determined oxidation efficiencies for 10, 6 and 1 mm of paracetamol, were found to be 60, 78 and 99 %, respectively. i / i m k. e. kouadio et al. j. electrochem. sci. eng. 11(2) (2021) 71-86 http://dx.doi.org/10.5599/jese.907 81 figure 10. influence of current density on current efficiency for paracetamol oxidation (10 mm) in 0.3 m h2so4, ph = 0.6; t = 25 °c; flow rate = 2.7 ml s -1 the paracetamol conversion (degradation) rate (0) is followed at any time of the electrolysis by the following equation:  = −      o 1 100 o c c (4) the histogram presented in figure 11(b) shows that oxidation of paracetamol is more favored at low concentrations, suggesting thus more difficult oxidation at increased concentrations of paracetamol. increase in paracetamol concentration implies increase in the number of collisions between the molecules itself, decreasing at the same time their interaction with hydroxyl radicals (oh∙) that are mediators in the oxidation process. the produced quantity of oh∙ becomes insufficient for total mineralization of paracetamol molecules present on the electrode surface, because of very short lifetime and possible recombination of hydroxyl radicals [21]. figure 11. (a) influence of paracetamol concentration on oxidation by electrolysis in 0.3 m sulfuric acid, ph 0.6, (b) degradation rates for different paracetamol concentrations after 1 h of electrolysis; j = 70 ma cm-2; t = 28 °c; flow rate = 2.7 ml s-1 the influence of the concentration of paracetamol on the current efficiency of oxidation is also studied and the results are presented in figure 12. this figure shows that when the concentration of paracetamol decreases, the current efficiency of oxidation increases. quick reactions take probably place in the reaction medium between paracetamol molecules and oxidative species (oh∙ in this case), because of high availability of the organic compound. therefore, the competitiveness of unwanted reactions becomes weak, and several hydroxyl radicals can react with paracetamol molecules. 100 90 80 70 60 50 40 http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 82 figure 12. influence of paracetamol concentration on current efficiency of oxidation at various electrolysis time in 0.3 m sulfuric acid, ph 0.6, j = 70 ma cm-2; t = 28 oc; flow rate = 2.7 ml s-1 studies focusing on the influence of the initial ph value of the solution were already carried out by large number of environmental scientists in order to weigh its importance in the effectiveness of degradation of pollutants [45,46]. conflicting results were generally obtained, what is in large part due to the difference in structure of the studied compounds and nature of the electrode material used [47]. therefore, it becomes necessary to investigate, the effect of the solution initial ph on the electrochemical oxidation of paracetamol on bdd electrode. figure 13(a) shows the obtained results in the sulfuric acid medium, with initial ph values adjusted to 0.6, 3 and 7 with hcl or naoh solutions. figure 13(a) shows that paracetamol oxidation reaction is more rapid at ph 3 than at ph 0.6 and 7. mineralization rates of paracetamol determined after 1 h of electrolysis at ph 0.6, 3 and 7 were 79, 84 and 74 %, respectively. the high performance of bdd electrode observed in the acidic solutions could be mainly due to the oxidation of paracetamol by the electrogenerated hydroxyl radicals. in the acidic medium, hydrophobicity that results from bdd surface termination changes at high potentials and hampers adsorption of hydroxyl radicals on bdd surface. thus, hydroxyl radicals are left to react actively with the organic compounds. besides this indirect oxidation process of paracetamol with oh∙, a direct electron transfer reaction between paracetamol and the surface of bdd occurs [48]. moreover, it is shown in figure 13(b), that in the course of the electrolysis process of paracetamol, high amounts of intermediates are produced. figure 13. (a) influence of ph on paracetamol (10 mm) oxidation by electrolysis in 0.3 m sulfuric acid; (b) evolution of the ratio of oxidation peak intensities during electrolysis; j = 70 ma cm-2; t = 28 oc; flow rate = 2.7 ml s-1  ph 0.6  ph 3.0  ph 7.0  ph 0.6  ph 3.0  ph 7.0 i / i m k. e. kouadio et al. j. electrochem. sci. eng. 11(2) (2021) 71-86 http://dx.doi.org/10.5599/jese.907 83 because of the importance of temperature in the physico-chemical analysis and treatment of wastewaters [8,49-51], the effect of temperature on paracetamol oxidation process was also studied in this work. figure 14(a) presents the obtained results, showing that increase of temperature promotes paracetamol oxidation. histogram of paracetamol degradation rates determined after 2 h of electrolysis are presented in figure 14(b). it is obvious that at 28, 55 and 75 oc, 85, 92 and 97 % of paracetamol were degraded, respectively. the exponential decrease of paracetamol concentration during oxidation at all investigated temperatures is characteristic of a pseudo first order reaction. figure 15 was built on the basis of figure 14. from the obtained straight lines in figure 15(a) that match the pseudo first order reaction model, the kinetic constants of paracetamol oxidation reaction at different temperatures were determined. thus, at 28, 55 and 75 oc, the determined kinetic constants (k) were 0.9474, 1.1789 and 1.8469 h-1, respectively. figure 14. (a) influence of temperature on paracetamol concentration during electrolysis in 0.3 m h2so4 ph 0.6; (b) degradation rate for different electrolyte temperatures after 2 h of electrolysis at j = 70 ma cm-2; flow rate = 2.7 ml s-1 figure 15. (a) -ln (c/c0) as a function of time of paracetamol electrolysis in 0.3 m h2so4 at different temperatures; (b) influence of temperature on current efficiency of paracetamol oxidation at j = 70 ma cm-2; flow rate = 2.7 ml s-1 these values undoubtedly show that the increase in temperature accelerates and therefore promotes degradation kinetics of paracetamol. it seems that increase in disorder takes place in the reaction medium when the temperature increases, and such effect promotes a higher abatement rate of paracetamol. this situation leads to an increase in shocks between molecules and interactions between species of the medium, accelerating thus paracetamol oxidation by hydroxyl radicals. the influence of temperature of the supporting electrolyte on the current efficiency of paracetamol oxidation was also investigated. the obtained results are shown in figure 15(b), suggesting  t = 28 oc  t = 55 oc  t = 75 oc  t = 28 oc  t = 55 oc  t = 75 oc -l n ( c / c 0 ) 100 95 90 95 80 75 http://dx.doi.org/10.5599/jese.907 j. electrochem. sci. eng. 11(2) (2021) 71-86 oxidation of paracetamol on b-doped diamond electrode. 84 that the current efficiency increases with the supporting electrolyte temperature. this result shows that increasing the temperature of the support electrolyte helps to reduce the current loss during the electrolysis. conclusion the electrochemical characterization of bdd electrode showed its almost metallic character, indicating that it can be used as the electrochemical sensor for detection and quantification of paracetamol in different solutions. determination of paracetamol using differential pulse voltammetry was shown to be more reliable in britton-robinson buffer solution than in sulfuric and orthophosphoric acids. oxidation peak of paracetamol was obtained at around 0.9 v vs. she, while after the electrolysis in 0.3 m h2so4, another peak related to the oxidation of paracetamol appeared at around 0.55 v vs. she. varying the current density of electrolysis from 20 to 100 ma cm-2, the total and rapid mineralization of paracetamol was observed at the highest current density and the process was under the mass transport control. by varying the initial concentration of paracetamol during electrolysis, it was shown that paracetamol oxidation kinetic is rapid at low paracetamol concentrations. this study also showed that the oxidation of paracetamol is favored in an acidic medium, particularly at ph 3. thus, the oxidation efficiency reached 99 % for 1 mm of paracetamol in 0.3 m h2so4, ph 3, after 1 h of electrolysis under the current density of 70 ma cm-2. it was also found that increase of temperature promotes the degradation rate of paracetamol, reaching 97 % at 75 °c after 2 h of electrolysis at the current density of 70 ma cm-2. acknowledgement: we are thankful to the swiss national funds for its financial support. they funded the project (iz01z0_146919) which durability helped this work to be undertaken. references [1] m. k. konan, l. ouattara, revist-revue ivorienne des sciences et technologie 34 (2019) 44-66 https://revist.net/revist_34/revist_34_4.pdf. 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https://www.researchgate.net/scientific-contributions/15846920_m_clement https://www.researchgate.net/profile/olivier_thomas10 https://doi.org/10.1016/j.envint.2009.07.001 https://doi.org/10.1007/s13201-019-1043-4 https://doi.org/10.1007/s13201-019-1043-4 https://doi.org/10.15406/bij.2020.04.00159 https://doi.org/10.15406/bij.2020.04.00159 https://creativecommons.org/licenses/by/4.0/) doi: 10.5599/jese.2012.0008 67 j. electrochem. sci. eng. 2(2) (2012) 67-75; doi: 10.5599/jese.2012.0008 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical studies of nevirapine, an anti-hiv drug, and its assay in tablets and biological samples nagappa laxman teradal, shankar narayan prashanth, jaldappagari seetharamappa department of chemistry, karnatak university, dharwad-580 003, india corresponding author: e-mail: jseetharam@yahoo.com received: november 6, 2011; revised: february 17, 2012; published: june 18, 2012 abstract the electrochemical oxidation of nevirapine, an anti-hiv drug, at a glassy carbon electrode has been studied by voltammetric techniques. nevirapine showed one well defined irreversible oxidation peak with a potential of 0.749 v in phosphate buffer at ph 10. the effects of different electrolytes, ph and scan rate on the electrochemical behaviour of nevirapine were examined to determine the optimum reaction conditions. the oxidation peak current was found to vary linearly with the concentration of nevirapine in the range of 5.0 – 350 µm. the limit of detection and limit of quantification values were calculated and found to be 1.026 µm and 3.420 µm, respectively. the low relative standard deviation values of inter-day and intra-day assays highlighted the good reproducibility of the proposed method for assay of nevirapine. further, a sensitive and accurate differential pulse voltammetric method was developed for the determination of nevirapine concentrations in pharmaceutical formulations. keywords nevirapine, non-nucleoside inhibitor, voltammetric investigations, pharmaceutical formulations. introduction antiretroviral drugs are used for the treatment of infections by retroviruses, primarily human immunodeficiency viruses (hiv) that can lead to acquired immunodeficiency syndrome (aids). presently, four classes of antiretroviral drugs are available viz., (i) nucleoside/tide reverse transcriptase (nrti) inhibitors, (ii) non-nucleoside (nnrti) inhibitors, (iii) protease (pi) inhibitors and (iv) fusion inhibitors [1]. nevirapine (nvp), (11-cyclopropyl-4-methyl-5, 11-dihydro-6hdipyrido [3,2b:2′,3′-e][1,4]diazepin-6-one) (figure 1) is a nnrti with activity against human immunodeficiency j. electrochem. sci. eng. 2(2) (2012) 67-75 electrochemical studies of an anti-hiv drug 68 virus type 1 (hiv-1) [2]. nvp binds directly and reversibly to the catalytic site of the reverse transcriptase enzyme, and therefore interferes with viral rna to dna-directed polymerase activities [3]. it is recommended for treating hiv infections in combination with other reverse transcriptase inhibitors such as stavudine and lamivudine [4-7]. figure 1. structure of nevirapine electrochemical methods have proven to be sensitive and reliable for the determination of numerous electroactive compounds [8-10]. under some circumstances, electrochemical methods can offer optimal solutions for drug analysis. simplicity, low cost and relatively short analysis times make electrochemical techniques more useful for routine analytical applications. a literature survey reveals that no attempt has been made up to the present date to investigate the electrochemical behavior of nvp, and determine it in pharmaceutical formulations by voltammetric methods. a few chromatographic methods viz., hplc, lc-ms or lc-ms/ms have been reported for the determination of nvp [11-16]. these methods require long analysis times, elaborate extraction and purification steps, or on-line sample extraction, and are relatively costly. hence, the aim of the present work is to develop a simple, sensitive and accurate electrochemical method for the analysis of nvp, in bulk and dosage forms. the proposed method is more sensitive compared to reported methods [11,12]. experimental reagents and solution preparations a stock solution of nvp (2.5 mm) was prepared in methanol water (20:80, v/v) and stored in a refrigerator at 4 °c. working solutions of the drug were prepared daily by diluting the stock solution with the selected supporting electrolyte. in the present study, two different buffers viz., britton robinson buffer (ph 2 10) and phosphate buffer (ph 3 10.6) were used as the supporting electrolytes. all other chemicals used in this investigation were of analytical grade. apparatus and procedure electrochemical studies were carried out on a chi-1110a electrochemical analyser (ch instruments ltd. co., usa, version 4.01) electrode system consisting of a glassy carbon electrode (gce) (3 mm diameter) as the working electrode, a platinum wire as the counter electrode and an ag/agcl reference electrode. ph measurements were made on a eq-610 ph meter (equiptronics, india). the gce was polished using 0.3 micron al2o3 before each measurement. after polishing, the electrode was rinsed thoroughly with water and then used for analysis. n. l. teradal et al. j. electrochem. sci. eng. 2(2) (2012) 67-75 doi: 10.5599/jese.2012.008 69 the parameters for differential pulse voltammetry (dpv) were: pulse amplitude 50 mv; pulse width 30 ms; and scan rate 20 mv s-1 maintained. assay of tablets tablets of nvp (each containing 200 mg of nvp) were obtained from local commercial sources. ten tablets were finely powdered, and a portion of this powder equivalent to 2.5 mm of nvp was weighed and transferred into a 25 ml calibrated flask containing a methanol-water mixture (20:80, v/v). it was then sonicated for 15 min to effect complete dissolution, and diluted to volume with the same solvent. suitable amounts of this solution were taken and analyzed. the amount of nvp in the tablet was calculated using a calibration graph or regression equation. determination of nvp in human urine and plasma samples a spiked urine sample was obtained by treating a 1.8 ml aliquot of urine with 200 μl of nvp solution (2.5 mm). a suitable aliquot of the spiked urine was then diluted with phosphate buffer without any pre-treatment to prepare appropriate sample solutions, and their differential pulse voltammograms were recorded under optimized conditions. for the determination of nvp in plasma, spiked serum samples were prepared following the procedure reported earlier [17]. serum samples obtained from healthy individuals (after obtaining their written consent) were stored frozen until assay. for the assay of nvp in plasma, 1 ml of nvp solution (5 mm) was added to 1 ml of untreated plasma. the mixture was vortexed for 30 s. in order to precipitate the plasma proteins, the plasma samples were treated with 500 µl of 15% hclo4. afterward, the mixture was vortexed for 30 s and then centrifuged at 5000 rpm for 5 min. an appropriate volume of supernatant liquor was transferred to a voltammetric cell containing phosphate buffer at ph 10, and voltammograms were recorded. the voltammograms of blank samples (without nvp) did not show any signal that could interfere with this direct determination. the content of the drug in plasma was determined by referring to a calibration graph or regression equation. results and discussion voltammetric behavior of nvp at a gce the electrochemical behavior of nvp at a gce was investigated by cv. nvp showed one oxidation peak at 0.749 v in phosphate buffer at ph 10 with a scan rate of 100 mv s-1 (figure 2). no peak was observed in the reverse scan, suggesting that the oxidation of nvp on the gce was irreversible. multi-sweep cyclic voltammograms of nvp (data not shown) revealed a significant decrease in peak current, indicating fouling of the electrode surface due to adsorption of nvp or its oxidation product. effect of different electrolytes and ph on electrooxidation of nvp the influence of ph on the peak current (ip) and peak potential (ep) was investigated in phosphate (ph 3.0 – 10.6) and br (ph 2 – 12) buffers. in both buffers, the oxidation peak current of nvp decreased in the ph range of 3 – 7 (data not shown). above ph 7, the peak current increased up to ph 10.6 (figure 3). furthermore, a sharper, well defined peak was noted in phosphate buffer at ph 10; hence, we have used this buffer throughout the study. j. electrochem. sci. eng. 2(2) (2012) 67-75 electrochemical studies of an anti-hiv drug 70 figure 2. cyclic voltammogram of a) 50 µm nvp in phosphate buffer at ph 10 and b) blank (buffer solution). figure 3. effect of ph on peak potential (■) and peak current (▲) for 50 µm nvp figure 3 shows the effect of ph on peak potential (■) and peak current (▲) of 50 µm nvp in phosphate buffer. the peak current of nvp was observed to be maximal at ph values between 8 and 10. this might be attributed to the deprotonation of radical cations in the basic medium, leading to the formation of radical species. this in turn facilitated the oxidation of nvp. this was less pronounced at ph values 7 and 9; hence, the peak heights were found to be minimal. the potential, v vs. ag/agcl c u rr en t, µ a b a 1 2 3 4 5 6 7 2 4 6 8 10 12ph ep /v 0.2 0.4 0.6 0.8 1 1.2 1.4 i p / µ a e p / v i p / μa ph n. l. teradal et al. j. electrochem. sci. eng. 2(2) (2012) 67-75 doi: 10.5599/jese.2012.008 71 anodic peak potential of nvp was shifted towards less positive potential with increasing ph of the buffer. this revealed that the ph of the supporting electrolyte exerted a significant influence on electrooxidation of nvp on the gce, and involvement of a proton in the oxidation process. the plot of peak potential versus ph gave a slope of 63.8 mv ph-1, which is close to the expected value of 59 mv ph-1 for participation of equal numbers of protons and electrons in the nvp oxidation process [18–21]. the corresponding equation is shown below: ep = 1.403 – 0.0638 ph: r 2 = 0.991. further, the number of electrons transferred, n, was determined by the peak width, w1/2 at half height. this was found to be around 83 mv in all the electrolytes, which is close to the theoretical value of 90 mv expected for an electrochemical reaction involving transfer of a single electron [22]. hence, we propose the electroxidation of nvp involves the transfer of one electron. based on the above results, a probable mechanism for electrooxidation of nvp is proposed (scheme 1). it is proposed that the secondary ring nitrogen of nvp undergoes a single electron oxidation to yield a radical cation, which is further deprotonated to form a radical. then, the free radical readily combines with another radical to form a dimerized product. this scheme is also in agreement with an earlier report [23]. scheme 1. probable reaction mechanism for electrooxidation of nvp. influence of scan rate on electrooxidation of nvp we examined the influence of the scan rate on the electrochemical behavior of nvp, to understand the nature of the electrode process. for this, we recorded cyclic voltammograms of 50 µm nvp at a gce at different scan rates (figure 4). the oxidation peak current of nvp was noted to increase with increasing scan rate, with a positive shift in the peak potential. the plot of values of log ip versus log ν in the scan rate range of 10–350 mv s -1 yielded a straight line with a slope of 0.57. this value is close to the theoretical value of 0.5 expected for an ideal reaction condition for a diffusion-controlled electrode process [24]. h + j. electrochem. sci. eng. 2(2) (2012) 67-75 electrochemical studies of an anti-hiv drug 72 figure 4. cyclic voltammograms for the oxidation of nvp at different scan rates (1–12): 10, 20, 40, 60, 80, 100, 120, 140, 180, 220 and 260, 300, 350 mv s−1. analytical applications calibration curve considering the electrochemical oxidation of nvp at a gce, an analytical method was developed using dpv. differential pulse voltammograms of nvp at different concentrations are shown in figure 5. under the optimized conditions, a linear relationship between the peak current and drug concentration was observed in the range of 5.0 – 350 µm (table 1). above a concentration of 350 µm, linearity was lost, probably due to adsorption of nvp on the electrode surface. validation of the optimized procedure for quantitative assay of nvp was examined via evaluation of lod, loq, accuracy, precision and recovery values. the lod and loq values were calculated using the equation below [25]: lod= 3 s / m, loq= 10 s / m where s is the standard deviation of the peak currents (five runs), and m is the slope of the calibration curve. the lod and loq values were found to be 1.026 µm and 3.420 µm, respectively (table 1). low values of lod and loq confirmed the sensitivity of the proposed method. the interday reproducibility of the method was examined by recording the voltammograms of six replicates at 150, 200 and 250 µm. these yielded rsd values of 2.65, 2.50 and 2.24 %, respectively. further, the rsd values for intra-day assay reproducibility in 150, 200 and 250 µm solutions (n = 6) were found to be 2.33, 2.28 and 2.97 %. the corresponding results are shown in table 1. the low values of rsd confirm the good precision of the proposed dpv method for assaying nvp. the major advantages of the proposed method are its simplicity, ease of performance, and sufficient sensitivity for nvp. c u rr en t, µ a potential, v vs. ag/agcl n. l. teradal et al. j. electrochem. sci. eng. 2(2) (2012) 67-75 doi: 10.5599/jese.2012.008 73 figure 5. differential pulse voltammograms for increasing concentrations of nvp in phosphate buffer at ph 10 at a gce. scan rate: 10 mv s-1; pulse amplitude: 50 mv; and pulse width: 30 ms. nvp concentration was maintained at (1) 5.0, (2) 10, (3) 50, (4) 100, (5) 150, (6) 200, (7) 250, (8) 300, (9) 350 µm. table 1. characteristics of the calibration plot for nvp. dpv linearity range, µm 5.0 350 lod, µm 1.026 loq, µm 3.420 inter-day assay rsd* , % 2.65 intra-day assay rsd*, % 2.97 * for 50 µm nvp determination of nvp in urine and blood samples the proposed method was applied to the determination of nvp in spiked urine samples of healthy volunteers, but not to urine samples of patients treated with nvp. the recoveries from the urine samples were measured by spiking drug free urine with known amounts of nvp, and differential pulse voltammograms were then recorded. the amounts of nvp in the spiked urine samples were then evaluated from the calibration graph. the results of the analysis are listed in table 2. the average recovery values, higher than 98.94 %, and rsd values less than 2.57 %, indicate the high accuracy and precision of the proposed method. further, the proposed method was also applied to the assay of nvp in spiked human serum samples of healthy volunteers, but not to patient serum samples. for this experiment, drug free serum samples were spiked with 150, 200 or 250 µm of nvp; differential pulse voltammograms were then recorded. the amount of nvp in each serum sample was calculated from the calibration plot. the results of the analysis are summarized in table 2. the percent recovery of nvp was determined by c u rr en t, µ a potential, v vs. ag/agcl j. electrochem. sci. eng. 2(2) (2012) 67-75 electrochemical studies of an anti-hiv drug 74 comparing the peak currents of the drug in serum samples with those of pure drug using the calibration curve. table 2. results of analysis of nvp in spiked human urine and serum samples. urine samples nvp added, µm n amount found, µm average recovery, % rsd, % 150 5 148.90 99.26 2.57 200 5 197.89 98.94 2.45 250 5 249.78 99.91 2.25 serum samples 150 5 149.96 99.97 2.31 200 5 198.02 99.01 2.28 250 5 248.91 99.56 2.07 analysis of nvp in tablets the practical analytical application of the dpv method was further established by determining nvp concentrations in tablets. the corresponding results of the analysis are shown in table 3. the low rsd values again highlighted the reproducibility of the results. recovery studies were carried out using a standard addition method. known quantities of pure nvp were mixed with defined amounts of pre-analyzed formulations; then the mixtures were analyzed as before. the total amount of the drug was then determined, and the amount of drug added was calculated by the difference. the high percentage of recovery indicates that the commonly encountered excipients in the formulation did not interfere with the proposed method. table 3. determination of nvp in tablets. nevimunea neviretrob labeled amount, mg 200 200 amount found, mg 198.7 199.8 recovery, % 99.35 99.90 rsd,c% 1. 95 1. 80 pure nvp added to tablet solution, mg 25 25 found, mg 25.06 24.92 recovery, % 100.1 99.7 rsd, % 1.89 2.05 amarketed by cipla. ltd., india; bmarketed by alkem (cytomed) ltd., india; caverage of six 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[25] s. skrzypek, w. ciesielski, a. sokolowski, s. yilmaz, d.kazmierczak, talanta 66 (2005) 1146 1151. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 25%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.6 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions true 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [4000 4000] /pagesize [612.000 792.000] >> setpagedevice {electrochemical and quantum chemical parameters of (-)-(s)-9-flu¬oro-2,3-dihydro-3-methyl-10-(4-methyl-1-pipera-zinyl)-7-oxo-7h-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acidas anti-corrosive agent for api 5l x-52 steel} http://dx.doi.org/10.5599/jese.744 235 j. electrochem. sci. eng. 10(3) (2020) 235-244; http://dx.doi.org/10.5599/jese.744 open access: issn 1847-9286 www.jese-online.org original scientific paper electrochemical and quantum chemical parameters of (-)-(s)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7h-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acidas anti-corrosive agent for api 5l x-52 steel fidelis e. abeng1,2,, magdalene e. ikpi2, victor e. okpashi3, onumashi a. ushie4 and mbang e. obeten1 1material and electrochemistry research group, department of chemistry, cross river university of technology, calabar, p. m. b 1123 calabar, nigeria 2corrosion and electrochemistry research laboratory, department of pure and applied chemistry, university of calabar, p.m.b. 1115 calabar, nigeria 3department of biochemistry, cross river university of technology, p. m. b.1123 calabar, nigeria 4department of chemical science, federal university wukari, nigeria corresponding author: fidelisabeng@yahoo.com received: november 1, 2019; revised: january 5, 2020; accepted: january 6, 2020 abstract the inhibitive action of (-)-(s)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7 -oxo-7h-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid (levaquin) on api 5l x-52 steel in 2 m hcl solution was investigated using potentiodynamic polarization method and quantum chemical study. levaquin drug showed good inhibition efficiency of 88 and 95 % at 303 and 323 k, respectively. the results of experimental measurements revealed that levaquin drug works as a mixed type inhibitor. langmuir thermodynamic model was tested to describe the mode of inhibitor adsorption on the steel surface. the quantum chemical calculations confirmed the efficacy of levaquin drug as a corrosion inhibitor. keywords corrosion; density functional theory; potentiodynamic polarization; organic inhibitor; adsorption. introduction corrosion is a destructive attack on a material caused by its reaction with environment. the severe consequence of corrosion has become a problem to the society. corrosion causes waste of valuable resources, loss or contamination of products, and reduction in efficiency of the cost maintenance. corrosion destroys the safety of metal structures and reduces technological progress. http://dx.doi.org/10.5599/jese.744 http://dx.doi.org/10.5599/jese.744 http://www.jese-online.org/ mailto:fidelisabeng@yahoo.com j. electrochem. sci. eng. 10(3) (2020) 235-244 levaquin as anti-corrosive agent 236 therefore, some control measures are needed to be implemented in order to inhibit corrosion, thereby prolonging the life span of metal structures. use of organic corrosion inhibitors is the most efficient and economical approach among all anticorrosive methods. these inhibitors, however, cannot be used for large scale corrosion inhibition because of the growing ecological awareness about their high hazardous environmental implication [1]. thus, some nonor low-toxic alternatives must be developed to replace traditional hazardous inhibitors. in recent years, some pharmaceutical drugs were recommended for such a purpose such as azithromycin [2], amoxicillin [3], cefixime [4], ciprofloxacin [5], amlodipine[6], oxytetracycline [7], doxycycline [8], norfloxacin [9], enrofloxacin [10], amifloxacin [11], pefloxacin [12], penicillin v [13], ampicillin [14], cloxacillin [15] and flucloxacillin [16]. all these pharmaceutical compounds are rich in heteroatoms like nitrogen, sulphur and oxygen,  bonds and/or aromatic rings, which are major adsorption centres in good corrosion inhibitors. one of the most widely used metals is carbon steel, i.e. an alloy of iron and carbon which corrodes in the form of rusting. rusting is the process of oxidation in which iron combines with water and oxygen to form corrosion products (rust). the present study aims to investigate (-)-(s)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7h-pyrido[1,2,3-de]-1,4-benzoxazine-6-carboxylic acid (levaquin) as corrosion inhibitor for api 5l x-5 carbon steel in 2 m hcl solution, using potentiodynamic polarization method and density functional theory (dft) for the calculation of quantum chemical parameters. experimental acid-inhibitor solution preparation levaquin is an antibiotic drug used to treat several bacterial infections. it belongs to the class of fluoroquinolones and has molecular formula c18h20fn3o4 and molecular weight of 361.368 g/mol. the tablets of levaquin were supplied from peace land pharmaceutical shop in calabar-nigeria. 500 mg of the drug was ground and dissolved in one liter of prepared 2 m hcl solution. the resultant solution was filtered to obtain the stock solution of concentration 500 ppm. solutions of concentrations 50, 100, 200 and 300 ppm were prepared from this stock solution using serial dilution method [17,18]. sample preparation api 5l x-52 steel specimen with chemical composition shown in table 1 was obtained from a cylindrical pipeline. the steel specimen was mechanically press cut into coupons of dimensions 111 cm. the coupons were polished using different grades of emery paper to obtain mirror polished surfaces. copper wires were soldered to the coupons and mounted in epoxy resin to produce the working electrode. prior to running electrochemical experiments, the coupons were once again polished, rinsed in distilled water, degreased in acetone and air dried. table 1. chemical composition of api 5l x-52 steel element c mn p s si v cr ni mo al nb ti fe content, %wt 0.22 1.40 0.025 0.015 0.45 0.15 0.20 0.20 0.08 0.03 0.15 0.04 97.04 electrochemical experiments potentiodynamic polarization experiments were carried out using a conventional three-electrode electrochemical cell assembly. steel coupons with an exposed surface area of 1 cm2 was used as the working electrode, saturated calomel electrode (sce) as the reference electrode and platinum plate f. e. abeng et al. j. electrochem. sci. eng. 10(3) (2020) 235-244 http://dx.doi.org/10.5599/jese.744 237 as the counter electrode. the polarization measurements at 303 and 323 k, as well as estimations of corresponding polarization resistance values were performed using gamry electrochemical analyzer. potentiodynamic current-potential curves were recorded by changing the electrode potential automatically at the scan rate of 0.5 mv s-1, from 0.25 v below to 0.60 v above the corrosion potential (ecorr). before each run, the working electrode was immersed in the test solution for 30 minutes to attain steady state. the corrosion rate (cr) of the metal was examined through the corrosion current density (jcorr) using the equation correwcr= kj d (1a) where k is conversion factor (3272 mm a-1 cm-1 y-1), ew is equivalent weight of corroding metal in g equivalent, and d is density of the metal in g cm-3. the inhibition efficiency (ie) was evaluated from jcorr values using the following equation [19-21] corr(blank) corr(inh) corr(blank) ie 100 j j j − = (1b) where jcorr(blank) and jcorr(inh) are corrosion current densities measured in the blank solution and solution containing inhibitor, respectively. quantum chemical study the density functional theory (dft) was used for the quantum chemical calculations. materials studio 4.0 software with a basis set of 6.311 g and hybrid functional of b3lyp were used to treat the exchange-correlation interaction of electrons. results and discussion potentiodynamic polarization measurements polarization plots forapi 5l x-52 steel coupons measured in 2 m hcl solution in the absence and presence of different concentrations of levaquin (50, 100, 200, 300 and 500 ppm) and at two temperatures (303 and 323 k) are presented in figures 1 and 2. j / a cm-2 figure 1. potentiodynamic polarization curves for carbon steel in 2 m hcl in the absence and presence of denoted concentrations of levaquin at 303 k e / v v s. s c e http://dx.doi.org/10.5599/jese.744 j. electrochem. sci. eng. 10(3) (2020) 235-244 levaquin as anti-corrosive agent 238 in both cases, cathodic and anodic branches of tafel polarization curves in the presence of levaquin are shifted towards lower current density values, which may be a result of the adsorbed inhibitor molecules [10]. it can also be deduced from the plots in figures 1 and 2 that the studied molecules inhibit corrosion by controlling both cathodic and anodic reactions, i.e. as mixed type inhibitor [19]. in other words, addition of inhibitor reduces anodic dissolution of api 5l x-52 steel and also retards the cathodic reaction. electrochemical parameters deduced from the polarization curves measured at two temperatures are listed in table 2. j / a cm-2 figure 2. potentiodynamic polarization curves for carbon steel in 2 m hcl in the absence and presence of denoted concentrations of levaquin at 323 k table 2. electrochemical parameters obtained from tafel plots t / k c / ppm a / mvdec-1 c /mv dec-1 ecorr/ mv jcorr / μacm -2 cr, mm y-1 rp / ω cm2  ie, % 303 blank 102 454 -446 560 6.498 10633 -- 50 72 84 -412 98 1.137 22357 0.83 83 100 71 84 -422 92 1.067 23137 0.84 84 200 71 79 -417 80 0.928 24053 0.86 86 300 64 90 -408 71 0.824 29029 0.87 87 500 61 111 -402 65 0.754 41503 0.88 88 323 blank 116 217 -442 3560 41.307 5433 -- 50 76 391 -333 3370 39.102. 9469 0.05 5 100 61 249 -333 2040 23.670 5284 0.43 43 200 53 541 -326 1770 20.537 22229 0.50 50 300 74 100 -405 622 7.217 4752 0.83 83 500 56 87 -425 188 2.181 8524 0.95 95 it can be observed from data in table 2 that increase in the concentration of inhibitor results in decreased corrosion current density and corrosion rate, respectively. simultaneous increase of inhibition efficiency (ie) or degree of surface coverage ( ), as well as polarization resistance (rp) suggest that the studied compound blocked the available surface area of steel [19-23]. note that the values of  listed in table 2 were determined using equation (1b) divided by 100. effect of temperature temperature dependence gives some insight into the mechanism of inhibitor adsorption, because increase of inhibition efficiency of an inhibitor with increase in temperature predicts e / v v s. s c e f. e. abeng et al. j. electrochem. sci. eng. 10(3) (2020) 235-244 http://dx.doi.org/10.5599/jese.744 239 chemical adsorption mechanism. at the other side, physical adsorption mechanism is feasible for decreasing inhibition efficiency at increasing temperature [3]. hence lower inhibition efficiency at higher temperature (table 2) suggests that a physical adsorption mechanism is generally operative in inhibition of steel corrosion by levaquin, except at its highest concentration. activation energy has also been used to predict the mechanism of adsorption as it depends on temperature. generally, when the activation energy of an inhibitor is higher than that of the blank solution, physical adsorption mechanism is implied, whereas when the activation energy is lower than that of uninhibited solution, chemical adsorption is probable adsorption mechanism. therefore, increase of the inhibition efficiency with increase in temperature corresponds to decrease of corrosion activation energy in the presence of inhibitor, suggesting thus the chemisorption mechanism. in reverse, decrease of inhibition efficiency with increase of temperature corresponds to increase in corrosion activation energy in the presence of inhibitor, suggests physical adsorption mechanism [23]. the apparent activation energies (ea) for dissolution of api 5l x-52 steel in 2 m hcl in the absence and presence of the inhibitor were calculated from the condensed arrhenius equation [24]: a2 1 1 2 cr 1 1 log = cr 2.303 e r t t     −        (2) where r is gas constant, while cr1 and cr2 are corrosion rates at temperatures t1 and t2, respectively. the calculated ea values are listed in table 3, showing generally higher values in presence of inhibitor than in blank solution, except for the highest concentration of levaquin. in agreement with previous observations on ie temperature changes, it seems that physisorption mechanism is operating at lower concentration of levaquin. heat of adsorption (qads) was evaluated using the following equation: 2 1 1 2 ads 2 1 2 1 2.303 log log 1 1 t t q r t t            = −       − − −       (3) where 1 and 2 are degree sof surface coverage at temperatures t1 and t2, respectively. the estimated values of qads are also listed in table 3. the negative values of qads are consistent with an inhibitor that is physically adsorbed on the metal surface [23,25]. table 3. calculated values of activation energy (ea) and heat of adsorption (qads) c / ppm ea / kj mol-1 qads / kjmol-1 1 2 blank 73.33 --- 50 140.71 -57.71 0.83 0.05 100 122.57 -78.11 0.84 0.43 200 123.14 -72.53 0.86 0.50 300 86.42 -15.34 0.87 0.83 500 43.03 -35.07 0.88 0.95 adsorption isotherm a possible way of discussing the mechanism of corrosion inhibition is through consideration of adsorption of organic compounds. the adsorbed compounds block the surface of the metal from corrosive agents, reducing thus the corrosion process. adsorption provides information about interaction among adsorbed molecules, as well as their interaction with the metal surface. adsorption isotherm is very important in proposing the mechanism of heterogeneous organoelectrochemical reaction on solid surfaces [26]. different adsorption isotherm models were fitted http://dx.doi.org/10.5599/jese.744 j. electrochem. sci. eng. 10(3) (2020) 235-244 levaquin as anti-corrosive agent 240 to the experimental data, and it was found that langmuir adsorption isotherm fitted well to the experimental data (figure 3). langmuir adsorption isotherm is defined as 2 ads 1c c k = + (4) where c is bulk concentration of adsorbed (inhibitor) species, while kads is equilibrium adsorption constant (l mol-1), equal to the reciprocal of the intercept of straight line in figure 3. figure 3. langmuir adsorption isotherm plot for the adsorption of levaquin on api 5l x-52steel in 2 m hcl as shown in table 4, the values of slopes and correlation coefficient (r2) illustrate that at both temperatures, the experimental data agree very well with linear relationships. the thermodynamic adsorption parameter known as free energy of adsorption (δgads) was calculated at different temperatures using the equation δgads= -2.303rt log 55.5 kads (5) where 55.5 is concentration of water in mol l-1, r is gas constant and t is absolute temperature table 4. calculated values of free energy of adsorption t / k kads slope δgads / kj mol-1 r2 303 0.183 1.126 -18.41 0.995 323 0.0062 0.717 -66.87 1 the negative signs of δgads in table 4 indicate that adsorption of levaquin on the surface of api 5l x-52 steel is spontaneous. generally, the values of δgads higher than -40 kj mol-1 reflect chemisorption which involves charge sharing or transfer from inhibitor molecules to the metal surface to form a coordinate bond type, while values of δgads below -40 kj mol-1 suggest electrostatic interaction between metal surface and charged organic molecules in the bulk of the solution identified as physisorption [23]. the calculated δgads values presented in table 4 are between -18.41 and -66.87 kj mol-1, indicating that the adsorption mechanism of levaquin on api 5l x-52 steel in 2 m hcl solution is physical adsorption at 303 k and chemical adsorption at 323 k. quantum chemical calculations quantum chemical calculations were employed to gain insight into the inhibition mechanism of levaquin drug by examining the structure-reactivity correlation of the compound [1,19]. figure 4 shows a complete geometric optimization of the studied molecule, while figure 5 displays the frontier molecular orbitals namely, the highest occupied molecular orbital (homo) and the lowest y = 1.1266 x + 5.4727 r² = 1 y = 0.7174 x + 160.79 r² = 0.9953 0 100 200 300 400 500 600 0 200 400 600 (c / ) / p p m c / ppm 303 k 323 k f. e. abeng et al. j. electrochem. sci. eng. 10(3) (2020) 235-244 http://dx.doi.org/10.5599/jese.744 241 unoccupied molecular orbital (lumo), present in studied compound. the calculated quantum chemical parameters obtained according to [16-24] are listed in table 5. figure 4. optimized structure of levaquin molecule homo lumo figure 5. frontier molecular orbital (homo and lumo) structures of levaquin table 5. calculated quantum chemical parameters for levaquin ehomo /ev elumo /ev δe / ev ip, ev ea, ev  /ev  / ev  μ / d  δn /e -6.817 -3.315 3.502 6.817 3.315 3.066 1.751 0.571 12.28 7.327 0.501 frontier molecular orbital calculations the values of energy of the highest occupied molecular orbital (ehomo) and energy of the lowest unoccupied molecular orbital (elumo) were obtained from dft analysis. ehomo is often connected with the electron donating ability of a molecule, while elumo shows its electron accepting ability. according to the results presented in table 5, higher energy value of ehomo reveals the ability of the studied molecule to donate electron to an empty molecular orbital. thus, increase in ehomo value facilitates adsorption of inhibitor [26-27], while the energy of lumo shows the potential of the molecule to accept electrons. energy gap energy gap can also be used to predict inhibition efficiency of a compound or to develop a theoretical model for explaining the structure and confirmation barrier in many molecular systems. energy gap (δe) is calculated using the following equation [28-29]; δe = elumoehomo (6) from our study the value of δe in table 5 is within the range of values of good corrosion inhibitors [29]. http://dx.doi.org/10.5599/jese.744 j. electrochem. sci. eng. 10(3) (2020) 235-244 levaquin as anti-corrosive agent 242 ionization potential and electron affinity ionization potential (ip) is the maximum energy necessary to remove an electron from many electron atoms in a gas phase, whereas electron affinity (ea) is the energy released when an electron attaches to a gas phase atom. according to the following equations ip = -ehomo (7) ea = -elumo (8) ip is related to ehomo while ea is related to elumo. the calculated values of ip and ea are listed in table 5 [30-31]. electronegativity electronegativity reflects the power of an electron or group of atoms to attract electrons towards itself. the koopman’s theorem was used to estimate the electronegativity () of the studied compound using the following equation χ = ½(ehomo + elumo) (9) the higher the value of χ, the more effective is the inhibitor and vice versa. data in table 5 shows that χ value is within the range of effective inhibitors. these results agree with the experimental results of other studies [24,31]. global hardness and softness global hardness (ƞ) measures the resistance of an atom to charge transfer, whereas global softness (σ) describes the capacity of an atom or group of atoms to receive electrons [3,25]. the expressions for ƞ and σ are given as follows ƞ = ½(ehomo – elumo) (10)  = 1/ƞ (11) a hard molecule requires a large δe and a soft molecule requires a small δe. soft molecules could therefore easily offer electrons to an acceptor system that make them more reactive than hard molecules. furthermore, adsorption may occur at the point of a molecule where absolute softness is high [26,32]. table 5 shows that the values of global hardness and global softness are within the range of effective corrosion inhibitors. dipole moment the dipole moment (μ) is another index that is often used for the prediction of corrosion inhibition process. it is the measure of polarity in a bond, related to the distribution of electrons in a molecule. therefore, inhibitors with high dipole moment form strong dipole-dipole interactions with the metal, ensuring a strong adsorption on the surface of the metal and leading to higher inhibition efficiency. it was observed that the dipole moment value for levaquin shown in table 5 correlates with the experimental results reported elsewhere [32-35]. global electrophilicity index global electrophilicity index () is estimated by combining the electrophilicity and chemical hardness parameters according to the following relation; 2 2 χ ω η = (12) a high value of electrophilicity index describes a good electrophile while a small value describes a good nucleophile [25-32]. the results presented in table 5 show good correlations with the f. e. abeng et al. j. electrochem. sci. eng. 10(3) (2020) 235-244 http://dx.doi.org/10.5599/jese.744 243 experimental result of the study. the fraction of transferred electrons (δn) can be evaluated using the following equation: ( ) inh fe inh δ 2 w χ n η η − = − (13) where w is work function, χinh is electronegativity of inhibitor, while ƞfe and ƞinh are the global hardness of fe and inhibitor, respectively. the electronegativity χ was calculated using eq. (9), the value of ƞfe was taken as 0 ev mol-1, while ƞinh was calculated using eq. (10). to calculate n, the work function with the value of w = 4.82 for fe (110) surface was applied. for the value of δn > 0, the electron transfer takes place between an inhibitor molecule and the metal surface, while for δn< 3.6, the electron donating ability of an inhibitor molecule is increased. all the calculated values of this study are within the range of values already reported for some excellent corrosion inhibitors [27-28,36]. conclusions (-)-(s)-9-fluoro-2,3-dihydro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-7h-pyrido[1,2,3-de]-1,4benzoxazine-6-carboxylic acid was tested as corrosion inhibitor process on api 5l x-52 steel in 2 m hcl solution, using potentiodynamic polarization and quantum chemical calculations. the following conclusions can be derived from the present study. 1. inhibition efficiency of levaquin increases with increase in its concentration. 2. adsorption behaviour of levaquin follows langmuir adsorption isotherm and depending on temperature, reflects either physical or chemical adsorption mechanism. levaquin is good corrosion inhibitor which acts as a mixed type inhibitor. 3. results of calculations of quantum chemical parameters carried out in this study are in good agreement with experimental results. references [1] y. qiang, s. zhang, b. tan, s. chen, corrosion science 133 (2018) 6-16. 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[34] v. c. anadebe, o. d. onukwuli, m. omotioma, n. a. okafor, south african journal of chemistry 71 (2018) 51-61. [35] t. he, w. emori, r. h. zhang, p. c. okafor, m. yang c. r. cheng bioelectrochemistry, 130 (2019) 107332107342 [36] m. ramezanzadeh, g. bahlakeh, b. ramezanzadeh, journal of molecular liquids 292 (2019) 111387111396. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {electroactive properties of a new azulene thioxo-imidazolidin-4-one ligand for modified electrodes} http://dx.doi.org/10.5599/jese.727 209 j. electrochem. sci. eng. 10(2) (2020) 209-217; http://dx.doi.org/10.5599/jese.727 open access: issn 1847-9286 www.jese-online.org original scientific paper electroactive properties of a new azulene thioxo-imidazolidin-4-one ligand for modified electrodes elena diacu1, adinuta paun1, veronica anastasoaie1, georgiana-luiza arnold-tatu1, liviu birzan2, eleonora-mihaela ungureanu1, 1university "politehnica" of bucharest, faculty of applied chemistry and material science, 1-7 gheorghe polizu, 011061, bucharest, romania 2romanian academy, institute of organic chemistry “c. d. nenitzescu”, spl. independentei, 202b, 060023-bucharest, 35 p.o. box 108, romania corresponding author: eleonoramihaelaungureanu@gmai.com; tel.: +40 21 4023977 received: september 19, 2019; revised: october 7, 2019; accepted: october 17, 2019 abstract the electrochemical characterization of a new synthesized azulene compound (z)-5-((5-isopropyl-3,8-dimethylazulen-1-yl)methylene)-2-thioxoimidazolidin-4-one (l) using cyclic voltammetry, differential pulse voltammetry and rotating disk electrode is presented. chemically modified electrodes were obtained by successive scanning or by controlled potential electrolysis using different electrode potentials or charges. the new modified electrodes were tested in solutions containing different concentrations of the following heavy metals: cadmium, lead, mercury and copper. a good analytical response was obtained for pb2+. keywords (z)-5-((5-isopropyl-3,8-dimethylazulen-1-yl)methylene)-2-thioxo-imidazolidin-4-one, electrochemical characterization, chemically modified electrodes, heavy metals recognition introduction azulenes have a five member (electron rich) cyclic moiety connected to a seven member (electron poor) cyclic moiety. azulene derivatives present an irreversible electrooxidation, and an irreversible [1] or quasi reversible reduction [2]. there are many studies performed on the properties of azulene polymers formed by electrochemistry. they have characteristics very similar to those chemically synthesized. researches were carried out to prepare polyazulene films or metal complexes of azulene derivatives [3-5]. our research team is working on developing sensors based on azulene derivatives for heavy metals recognition [6-8]. classical techniques used for the determination of heavy metal ions are: spectrometry (graphite furnace atomic absorption spectroscopy, flame atomic absorption spectroscopy, inductively coupled plasma optical emission spectrometry), and chromatography (gas chromatography, high performance http://dx.doi.org/10.5599/jese.727 http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:eleonoramihaelaungureanu@gmai.com j. electrochem. sci. eng. 10(2) (2020) 209-217 azulene thioxo-imidazolidin-4-one ligand 210 liquid chromatography). these methods are precise, but they require complex equipment (expensive), and well-trained staff [9-13] to maintain and use them. stripping analysis using chemically modified electrodes (cmes) is an alternative to these classical methods, offering the following advantages: higher selectivity due to the specific interactions between the metal ions and complexing matrix, limited chemical interferences, and capability to sense metals as hg or ag [5,14]. the present work reports the study of electroactive properties of (z)-5-((5-isopropyl-3,8-dimethylazulen-1-yl)methylene)-2-thioxo-imidazolidin-4-one (l), in view of building new sensors for heavy metals recognition. the study of its electroactive properties aims to get the values of the most important parameters for the electrochemical preparation of chemically modified electrodes based on this ligand. the structure of this new azulene thioxo-imidazolidin-4-one derivative is given below (fig. 1). the study performed using cyclic voltammetry, differential pulse voltammetry and rotating disk electrode was a subject of a phd thesis [15] defended in our group. figure 1. structure of (z)-5-((5-isopropyl-3,8-dimethylazulen-1-yl)methylene)-2-thioxo-imidazolidin-4-one experimental this azulene derivative (l) was synthesized according to the previously published procedure [16]. acetonitrile (sigma aldrich, electronic grade 99.999 % trace metals) and tetra-n-butylammonium perchlorate (tbap) (fluka, puriss, electrochemical grade >99 %) were used as solvent and supporting electrolyte, respectively, without further purifications. sodium acetate (roth, 99.99 %) and acetic acid (fluka, >99.0 %, trace select) were used for preparing acetate buffer solution. pb(no3)2 (sigma-aldrich, +99.99 % trace metal basis), cd(no3)2 4h2o (sigma-aldrich, +99.0 %), cu(ch3coo)2h2o (fluka, +99.0 %) and hg(ch3coo)2 (sigmaaldrich, +99.99 % trace metal basis) were used as received. distilled water was obtained using a millipore direct – q 3uv water purification system (18.2 mω cm). the electrochemical experiments were performed using a pgstat 12 autolab potentiostat connected to a three-electrode cell. ag/10 mm agno3 in 0.1 m tbap, ch3cn, was used as reference electrode, and a platinum wire served as counter-electrode. the working electrode was a glassy carbon disk (3 mm diameter) polished with diamond paste (0.25 μm) and cleaned with the acetonitrile before each experiment. cyclic voltammetry (cv) was run usually at 0.1 v/s or at different scan rates (0.1 – 1.0 v/s) to see the influence of this parameter on cv curves. differential pulse voltammetry (dpv) was performed with a scan rate of 0.01 v/s with a pulse height of 0.025 v and a step time of 0.2 s. rde curves were usually recorded at 0.01 v/s, and at different rotation rates. all potentials were referred to the potential of ferrocene/ferrocenium redox couple (fc/fc+) which in our conditions was +0.07 v. electrochemical experiments of heavy metal ions detection have been performed in 0.1 m acetate buffer (ph 5.5) solution as the supporting electrolyte. heavy metal ion solutions of different concentration (10-4 – 10-8 m) were freshly prepared by successive dilutions from a stock solution (10-2 m) in water. all experiments were performed at room temperature (25 °c) under argon atmosphere. e. diacu et al. j. electrochem. sci. eng. 10(2) (2020) 209-217 http://dx.doi.org/10.5599/jese.727 211 results and discussion electrochemical characterization of l the electrochemical behavior has been studied by cv, dpv and rde in 0.1 m tbap, ch3cn using millimolar solutions of l. all the curves were recorded starting from the stationary potential. cv and dpv anodic and cathodic curves obtained on glassy carbon electrode for different concentrations of l in 0.1 m tbap, ch3cn are presented in figure 2. the current peaks are increasing with l concentration. two significant peaks (a1, a2) can be seen in the anodic domain and four peaks (c1 – c4) in the cathodic domain from the dpv curves. cv curves show the two main peaks (a1 and a2). the peak a4 can be attributed to some adsorption processes, as the current does not systematically increase with the ligand concentration. in the reduction domain, only three peaks are clearly defined (c1, c2, c3). figure 2. dpv (0.01 v/s) and cv (0.1 v/s) curves at different concentrations of l figure 3 presents rde anodic and cathodic curves at different rotation rates (500 – 1000 rpm) for l (1 mm). in comparison with dpv anodic and cathodic curves for l at 1 mm. rde curves present one main anodic wave, and two main reduction waves. figure 3. rde anodic and cathodic curves at different rotation rates (up) and dpv anodic and cathodic curves (down) for l (1 mm) http://dx.doi.org/10.5599/jese.727 j. electrochem. sci. eng. 10(2) (2020) 209-217 azulene thioxo-imidazolidin-4-one ligand 212 they are denoted according to the notation given for the peaks of dpv curves. in the oxidation domain the current drops suddenly after the potential of the peak a1 reaching a very low current value. this behavior is characteristic for the electrode coverage with an insulating film. one isosbestic point (ip) appears at about 1.2 v, in the anodic domain. rde currents increase with the rotation rate. figure 4 shows cv anodic and cathodic curves at different scan rates (0.1 – 1 v/s) in 1 mm solution of l for the first anodic and cathodic peaks. the inset shows the linear dependences of the total peak currents (for a1, a1’ and c1) vs. the square root of the scan rate. the currents linearly increase with the scan rate. even if traces of oxygen are present in solution, this increase is evident (table 1). the shape of a1 peak is characteristic for a quasi-reversible system. the diffusion coefficient of l (dl) was estimated from the slope of the a1 peak current using the randles-sevcik equation for one electron transfer. the obtained value is quite high (dl = 4.310-5 cm2/s), in comparison with that of a fast system such as ferrocene (dferrocene = 2.610-5 cm2/s) [17]. figure 4. cv anodic and cathodic curves at different scan rates for cl = 1mm; inset: linear dependences of the total peak currents vs. the square root of the scan rate table 1. equations of the linear dependences of the total anodic (ipa) and cathodic (ipc) peaks currents (in µa) vs. the square root of the scan rate (in v s-1) for l ipa1 vs. v 1/2 ipc1 vs. v 1/2 ipa1 = -7.57 + 123.91 v 1/2 r2 = 0.997 ipc1 = 0.64 – 69.05 v 1/2 r2 = 0.997 ipa1’= 9.22 – 24.54 v 1/2 r2 = 0.892 figure 5 shows cv anodic and cathodic curves at different scan domains in 1mm solution of l. table 2 presents the peak potentials from dpv and cv curves at 1 mm, and the peak processes assessment. in the anodic and cathodic domains, the processes are quasi-reversible or irreversible. table 2. peak potentials from dpv and cv curves in 1 mm solution of l and characteristics of their processes peak e / v vs. fc/fc+ process characteristics dpv cv a1 0.27 0.32 quasi-reversible a2 0.49 0.53 quasi-reversible a3 1.31 a4 1.98 irreversible c1 -1.70 -1.78 irreversible c2 -2.04 -2.30 irreversible c3 -2.58 -2.66 quasi-reversible c4 -2.89 quasi-reversible e. diacu et al. j. electrochem. sci. eng. 10(2) (2020) 209-217 http://dx.doi.org/10.5599/jese.727 213 figure 5. cv (0.1 v/s) anodic and cathodic curves at different scan domains for l=1 mm modified electrodes based on polyl to detect heavy metal ions, such as cadmium, lead, mercury and copper, polyl glassy carbon modified electrodes were prepared. polyl films were obtained either by successive scanning or by controlled potential electrolysis (cpe) in millimolar solutions of l in 0.1 m tbap, ch3cn. evidence on films formation on the electrode surface was obtained by transferring the modified electrodes in a solution of ferrocene (0.5 mm) in 0.1 m tbap, ch3cn. figure 6a shows the cv curves recorded during the preparation of the modified electrode by successive scans in the domain of the anodic processes (proved to lead to insulating films). the formation of a film on the electrode surface was confirmed by transferring the modified electrode in a solution of ferrocene. figure 6b shows the cv curves recorded after the transfer of the modified electrode obtained by 20 successive scans (in 1 mm solution of l in 0.1 m tbap, ch3cn). the signal for ferrocene couple on the modified electrode is slightly diminished in intensity in comparison with that recorded on the bare electrode, indicating the formation of a thin layer film. a b figure 6. cv curves (0.1 v/s) during the preparation of the modified electrode by 20 successive cycles in 1 mm solution of l in 0.1 m tbap, ch3cn (a); cv curves (0.1 v/s) recorded in 1mm ferrocene solution in 0.1 m tbap, ch3cn on the bare (dotted line) and modified (solid line) electrode obtained after 20 successive cycles (as shown in fig. 6 a) (b) http://dx.doi.org/10.5599/jese.727 j. electrochem. sci. eng. 10(2) (2020) 209-217 azulene thioxo-imidazolidin-4-one ligand 214 in figure 7, cv curves in 1 mm ferrocene solution are presented for the modified electrodes obtained by cpe at 0.82 v and 1 mc (solid line) in comparison with those generated by the bare electrode (dotted line). the cv curve for the modified electrode transferred to the ferrocene solution is slightly modified relative to that for the bare electrode, which indicates the deposition of a thin layer film on the electrode surface at the charge of 1 mc. figure 7. cv curves (0.1 v/s) recorded in 1mm ferrocene solution in 0.1 m tbap, ch3cn on the bare (dotted line) and modified (solid line) electrode prepared by cpe at 0.82 v and 1 mc heavy metals recognition by polyl the interaction between the polyl modified electrode and cations occurs through the complexing units of thiohydantoin which are connected to polyazulene structure in polyl. it depends both on ph and metal affinity. in basic media, it is expected that the thiohydantoin moiety ionizes forming an anion which interacts with the metal (limiting structure 1 in scheme 1 for the ligand). a thiophilic metal, such as ag+, hg2+, pb2+, etc. is expected to bind to a negative sulfur atom (limiting structure 3 in scheme 1), while an oxophilic one to a negative o or n atom (limiting structures 1 or 2 in scheme 1). in neutral media, as are the majority of the organic solvents, it is possible that strong interaction takes place between metal ions and negatively polarized electronegative atoms, as described. a number of 5-(pyridylmethylidene)-2-thiohydantoins (ligand similar to that described by us) complexes with heavy metals such as cu2+, ni2+, co2+, revealed a double coordination of the metal by sulfur and 3-nitrogen atoms [18]. such complexes were observed also at other similar compounds, especially to p-dimethylbenzylidenerhodanin, which is widely used for the absorption of toxic metals in solutions with very low concentrations or for their dosage, for example ag+ ion [19], au3+, hg2+ ions [20] or other thiophilic ions [21]. scheme 1 e. diacu et al. j. electrochem. sci. eng. 10(2) (2020) 209-217 http://dx.doi.org/10.5599/jese.727 215 polyl glassy carbon modified electrodes were prepared by controlled potential electrolysis at different potentials and charges. they were obtained from 1 mm solution of l in 0.1 m tbap, ch3cn using a charge of 2 mc. the modified electrodes obtained by cpe were further introduced into the transfer cell containing 0.1 m acetate buffer at ph 5.5. the electrode equilibration was done during 15 cv cycles between -0.9 to + 0.6 v. then the overoxidation during 5 cv cycles between -0.2 v to and +1.2 v was performed. after that, the modified electrodes were introduced in assay solutions containing a mixture of heavy metal ions at different concentrations (between 10-8 and 10-4 m), where they were maintained under magnetic stirring for 10 minutes. finally, the modified electrodes were transferred in a cell containing 0.1 m acetate buffer at ph 5.5. during a further dpv experiment, the potential of -1.2 v was applied for 120 s (when the ions accumulated in the complexing film were reduced). then the electrode was polarized in an anodic scan and the stripping currents for the heavy metals dissolution were recorded. figure 8a shows the stripping curves obtained for modified electrodes prepared at different potentials using a charge of 2 mc. a b figure 8. anodic stripping curves recorded on polyl modified electrodes obtained by cpe at different potentials vs. fc/fc+. the modified electrodes where kept for 10 minutes in 10-4 m solution of all ions (cd(ii), pb(ii), cu(ii), hg(ii)) (a); graph of the dpv peak currents for pb(ii) vs. cpe potential (v) at which the electrodes were modified for a thickness corresponding to 2mc (b) the heavy metals appear at the following potentials: ecd = -0.83 v; epb = -0.50 v; ecu = -0.05 v; ehg = +0.24 v. the recognition of all cations was possible in the order: lead > copper > cadmium > > mercury. for the films of 2 mc the best response in detection of lead was obtained for the films prepared at 0.81 v or at 1.9 v (fig. 8b). figure 9a shows the stripping curves obtained on the modified electrodes after their introduction into solutions containing mixtures of heavy metal cations at different concentrations (between 10-8 and 10-4 m). figure 9b provides linear dependences of dpv stripping currents on heavy metal ions concentrations in accumulation solutions. all the cations were recognized at concentrations higher than 10-5 m. the stripping currents increase with concentration values lower than 10-5 m. for higher concentrations, the stripping current values decrease or remain constant, indicating a saturation of the binding sites. at a lower concentration (10-7 m), only lead was detected. their analytical signal values vary in the following order: lead> copper> mercury> cadmium. when using this polyl modified electrode, lead shows the best response from all the investigated cations. figure 10a shows the stripping curves obtained by applying the standard addition method on glassy carbon electrodes modified by cpe at 0.81 v, 2 mc (in 1 mm l). the solutions with the http://dx.doi.org/10.5599/jese.727 j. electrochem. sci. eng. 10(2) (2020) 209-217 azulene thioxo-imidazolidin-4-one ligand 216 concentration of 10-6 m mixture of cd (ii), cu (ii), hg (ii), pb (ii) and various concentrations of pb(ii) (between 210-6 m pb (ii) and 6x10-6 m pb (ii)) were examined. a b figure 9. anodic stripping curves recorded in acetate buffer at ph 5.5 on polyl modified electrodes after their immersion in solutions containing different concentrations of cd(ii), pb(ii), cu(ii) and hg(ii) ions in water (accumulation time 10 min); the modified electrodes were obtained by cpe (0.81 v vs. fc/fc+, 2 mc) (a); dependences of the dpv stripping currents on heavy metal ions concentrations (b) a b figure 10. anodic stripping curves recorded in acetate buffer at ph 5.5 on polyl modified electrodes after their immersion in solutions containing different concentrations of pb(ii) (between 10-6 and 610-6 m) and a constant concentration of cd(ii), cu(ii) and hg(ii) (10-6 m) ions in water (accumulation time 10 min); the modified electrodes were obtained by cpe (0.81 v vs. fc/fc+, 2 mc) (a); dependences of the dpv pb peak stripping area vs concentration of pb in accumulation solution (b) figure 10 b shows the graph of the pb(ii) peak area at different concentrations of pb(ii) in the accumulation solution. a linear increase of the peak area for lead ions is noticed (correlation coefficient 0.973). conclusions the electrochemical characterization of (z)-5-((5-isopropyl-3,8-dimethylazulen-1-yl)methylene)2-thioxo-imidazolidin-4-one (l) was performed by cyclic voltammetry, differential pulse voltammetry and rotating disk electrode methods. the anodic and cathodic processes specific for l have been characterized and the conditions for obtaining electrodes modified with l were identified. the films formation on the surface of the electrode was confirmed by the transfer of modified electrodes in ferrocene solutions. chemically modified electrodes were prepared by electropolymerization of l at potentials of the second oxidation peak and tested for heavy metal e. diacu et al. j. electrochem. sci. eng. 10(2) (2020) 209-217 http://dx.doi.org/10.5599/jese.727 217 cations recognition. when using the polyl modified electrode, lead showed the best response of all the investigated cations, followed by copper, cadmium, and mercury. this is a 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[21] m. t. certi mazza, l. de cicco, g. de rosa, r. de rosa, r. caramazza, bollettino della societa italiano di biologia sperimentale 72 (1996) 79–86. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.727 https://www.ncbi.nlm.nih.gov/pubmed/?term=certi%20mazza%20mt%5bauthor%5d&cauthor=true&cauthor_uid=8771907 https://www.ncbi.nlm.nih.gov/pubmed/?term=de%20cicco%20l%5bauthor%5d&cauthor=true&cauthor_uid=8771907 https://www.ncbi.nlm.nih.gov/pubmed/?term=de%20rosa%20g%5bauthor%5d&cauthor=true&cauthor_uid=8771907 https://www.ncbi.nlm.nih.gov/pubmed/?term=de%20rosa%20r%5bauthor%5d&cauthor=true&cauthor_uid=8771907 https://www.ncbi.nlm.nih.gov/pubmed/?term=caramazza%20r%5bauthor%5d&cauthor=true&cauthor_uid=8771907 http://creativecommons.org/licenses/by/4.0/) {spray coating formation of stearic and octadecylphosphonic acid films for corrosion protection of cupronickel alloy was studied in this work as a more practical alternative to widely studied dip-coating method. protective properties of organic films forme} http://dx.doi.org/10.5599/jese.739 161 j. electrochem. sci. eng. 10(2) (2020) 161-175; http://dx.doi.org/10.5599/jese.739 open access: issn 1847-9286 www.jese-online.org original scientific paper protective films of stearic and octadecylphosphonic acid formed by spray coating ekatarina kristan mioč, helena otmačić ćurković laboratory for corrosion engineering and surface protection – recorr, department of electrochemistry, faculty of chemical engineering and technology, university of zagreb, marulićev trg 19, 10000, zagreb, croatia corresponding author: helena.otmacic@fkit.hr; tel.: +385-1-4597-117 received: october 22, 2019; revised: january 18, 2020; accepted: january 28, 2020 abstract spray coating formation of stearic and octadecylphosphonic acid films for corrosion protection of cupronickel alloy was studied in this work as a more practical alternative to widely studied dip-coating method. protective properties of organic films formed under various experimental conditions were examined by electrochemical studies in 3% nacl solution as a corrosive medium. polarization resistance measurements as well as electrochemical impedance spectroscopy were employed to follow in time the corrosion behaviour of cupronickel alloy modified by studied organic acids. it was found that among examined experimental parameters, time elapsed between two sprays and number of sprays have the strongest influence on the film stability and its protective properties. this study confirmed that it is possible to form by spray coating the films of stearic and octadecylphosphonic acid with protective properties that resemble to those of the films prepared by dip-coating method. differences in corrosion behaviour of samples protected with stearic and octadecylphosphonic acid were attributed to difference in the bond strength between substrate and each organic acid. studied samples were also examined by the scanning electron microscopy. fourier transformed infrared spectroscopy studies showed that crystalline structure dominates in studied films, while contact angle measurements confirmed that modified cupronickel alloy surface exhibits hydrophobic properties. keywords corrosion; electrochemical impedance spectroscopy; polarization measurements, ftir, cupronickel alloy. introduction modification of surface properties of metallic or semiconductor substrates by long-chain organic molecules has been thoroughly studied in recent years due to interest for its application in various http://dx.doi.org/10.5599/jese.739 http://dx.doi.org/10.5599/jese.739 http://www.jese-online.org/ mailto:helena.otmacic@fkit.hr j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 162 fields such as organic electronics, sensors or corrosion protection [1-7]. molecules with sufficiently long alkyl chain usually possess the self-assembling property, i.e. they spontaneously form ordered structures in a form of self-assembled monolayers or multilayers [8,9]. such thin films present a barrier between the substrate and environment. if the film has a low number of defects and is strongly attached to metallic substrate it can provide good corrosion protection to a metal in various corrosive environments. it has been shown that long-chain organic acids such as carboxylic, phosphonic or hydroxamic acids, can form such barrier layers on different metals [6,7,10-15]. in most of cases, self-assembled monolayers/multilayers were prepared by dip-coating method. although this method is characterized by good reproducibility, its drawback is that it is time consuming and requires high amounts of solution. spraying method is often used for organic coatings as it enables their quick application on metallic objects of different dimensions. despite advantages over dip-coating method, there are only few works where spraying method was used for formation of self-assembled monolayers [10,11,13,16,17]. studies on films formed by dipcoating method showed that many different parameters should be taken into account in obtaining a layer with small number of defects and high crystallinity [15]. among the most important parameters is the adsorption time, as a certain time is always needed for molecules to reach the metal surface, adsorb on it and organize in well-ordered structures [18]. studies performed on nickel oxide modification by alkylphosphonic acids self-assembled monolayers also showed that wellordered monolayers can be formed by both immersion and aerosol spraying methods, if adequate conditions for monolayer formation were selected [11]. in our previous studies it was shown that multilayer films of stearic acid (sa) [14,15] or octadecylphosphonic acid (odpa) [7] formed by immersion method provide high and durable corrosion protection to copper-nickel alloy (cuni) in chloride medium. the aim of this work is to determine under which conditions sa films that provide good corrosion protection to underlying cuni substrate, can be formed by spraying method. then, similar experimental conditions were applied for preparation of odpa films. experimental sample preparation investigations were performed on cupronickel alloy (70cu30ni) obtained from goodfellow inc., uk. the specimens were cut-out from a cupronickel rod with 1.3 cm diameter and 0.5 cm in thickness. in order to prepare working electrodes for electrochemical measurements on the backside of these plates, a copper wire was soldered and then, the electrodes were embedded into epoxy resin. the exposed surface area of the working electrodes was 1.33 cm2. prior to all investigations and surface modifications, the electrodes were abraded with emery paper grade 800, 1200, 2500, and polished with α-al2o3 particle size 0.1 μm, degreased with ethanol in ultrasonic bath and rinsed with re-distilled water. stearic acid and octadecylphosphonic acid (odpa, 97 %) were obtained from sigma-aldrich corp. usa, while sodium chloride (p.a.) and ethanol (96 % p.a.) were obtained from lachner d.o.o. croatia. solutions for electrochemical measurements were prepared with deionized water. ethanolic solutions of 0.01 mol dm-3 stearic and octadecylphosphonic acid were used for spray application. the first step was to optimize the method of preparing the films with stearic acid, and after specific preparation procedure was chosen, it was applied for preparation of odpa films. the film formation was conducted according to the experimental procedures presented in table 1. they consisted of three steps: substrate oxidation which was conducted in order to obtain reproducible oxide layer for 24 h at 80 °c, followed by acid adsorption from etoh solution of sa or odpa by spraying method, e. k. mioč and h. otmačić ćurković j. electrochem. sci. eng. 10(2) (2020) 161-175 http://dx.doi.org/10.5599/jese.739 163 and final drying step, 5 h at 50 oc (sa) or 80 oc (odpa) [7,14]. initially, all the samples were sprayed 5 times while the influence of temperature (a) and time (b) between two consecutive spraying on the protective properties of sa film were studied. in the next step, the influence of the number of sprays (c) on the protective properties of the formed film was examined. for comparison, a blank sample (cuni) was prepared, on which native oxide layer was formed during 24 hours at 80 °c. the oxidation and drying were conducted in an air convection oven with temperature control accuracy of 0.5 °c. table 1. investigated film preparation procedures by spraying method sample temperature, °c time, min number of sprays a) influence of substrate temperature t25 25 60 5 t40 40 60 5 b) influence of time between spraying t10 25 10 5 t30 25 30 5 t60 25 60 5 c) influence of the number of sprays 2x 25 30 2 5x 25 30 5 10x 25 30 10 electrochemical measurements electrochemical investigations were conducted in a three electrode cell (250 cm3) containing 3 % nacl solution. a platinum foil and saturated calomel electrode were used as the counter and reference electrode, respectively. studies were conducted at room temperature, 24±2 °c. the polarization measurements were performed in a wide (± 150 mv vs. open circuit potential) and narrow (±20 mv vs. open circuit potential) potential range, at a potential scan rate of 0.166 mv s-1. the electrochemical impedance spectroscopy (eis) was performed at open circuit potential (eocp) in the frequency range 100 khz to 10 mhz, 5 points per decade, and 10 mvrms ac amplitude. the obtained impedance spectra were interpreted on the basis of electrical equivalent circuits using zsimpwin software. in general, χ2 value was always below 5·10-4. the electrochemical measurements were performed using a bio-logic sp-300 potentiostat. all measurements were conducted in triplicate. surface studies contact angle (ca) measurements, fourier transform infrared spectroscopy (ftir) and scanning electron microscopy (sem) were performed with the aim of surface layer characterization. the contact angle measurements on bare cupronickel and sa and odpa treated cuni samples were conducted using a goniometer dataphysics contact angle system oca 20, with a drop of 2 μl water under the ambient atmospheric conditions. all measurements were conducted in at least ten points. sem morphology analysis was performed with vega 3 sem tescan at an acceleration voltage of 10 kv. ftir measurements were carried out by attenuated total reflectance fourier transform infrared spectroscopy (atr ftir), using a spectrum one ftir spectrometer from perkin elmer, with the scan range from 4000 – 650 cm-1, having a resolution of 0.5 cm-1, and the results shown in this paper were averages of 25 scans. results and discussion in order to determine the experimental conditions at which sa film with excellent barrier properties is formed, protective properties of differently formed samples were tested by polarization measurements in 3 % nacl solution. http://dx.doi.org/10.5599/jese.739 j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 164 polarization measurements of sa treated samples influence of temperature between spraying since our previous studies on samples prepared by immersion method showed that adsorption of sa and odpa is enhanced at elevated temperature [7,14,15], there was a need to investigate the influence of temperature on the adsorption step in the spraying method. according to the literature [19], lower activation energy is required for a molecular gas or liquid precursor dispersed in small droplets at room temperature to bind by the chemisorption on a heated surface of the substrate, than for the heated solution and cold substrate. accordingly, it is expected that heating of the substrate during spraying could improve adsorption of molecules on the substrate. moreover, by spraying droplets of a room temperature in comparison to that of elevated temperature, the solvent evaporation during spraying is reduced. the solvent evaporation changes the droplet size, which affects the concentration and spatial arrangement of a drop, resulting in uneven spraying of droplets and disordered layer on the substrate surface [20,21]. for above mentioned reasons, sa solution was not heated as in the case of immersion method [7,14]. instead of that, between two consecutive sprays, t40 sample was heated at 40 °c for 1 h, after which it was sprayed with a solution of room temperature. the sample thus prepared was compared to t25 sample that was left at room temperature. the protective properties of such prepared samples were tested by polarization in a wider range of potentials after 1 h of exposure to corrosive solution (figure 1a). polarization measurements were also conducted in a narrow potential range during 14 days of exposure of samples to corrosive medium. from such obtained polarization curves, polarization resistance values were calculated (figure 1b). (a) (b) figure 1. influence of temperature between spraying: (a) polarization curves recorded after one-hour immersion in 3 % nacl and (b) polarization resistance as a function of immersion time for blank and treated samples table 2 presents corrosion parameters, i.e. corrosion potential (ecorr), corrosion current density (jcorr), anodic and cathodic tafel slopes (ba and bc), obtained from the polarization curves (figure 1a) by tafel extrapolation method. from the values of corrosion current densities of blank (jcorr0) and treated (jcorrt) samples, corrosion inhibition efficiency (i) was calculated according to the following equation: 0 t corr corr i 0 corr j / % 100 j j = (1) e. k. mioč and h. otmačić ćurković j. electrochem. sci. eng. 10(2) (2020) 161-175 http://dx.doi.org/10.5599/jese.739 165 table 2. corrosion parameters obtained by tafel extrapolation method from polarization curves given in fig. 1a values in parentheses represent standard deviations sample ecorr / mvvs. sce jcorr / μa cm-2 ba / mv dec-1 -bc / mv dec-1 i / % cuni -177 (6) 1.12 (0.10) 78 (21) 95 (5) – t25 -232 (6) 0.03 (0) 43 (3) 109 (5) 97 (0) t40 -232 (9) 0.06 (0.01) 46 (3) 124 (22) 94 (1) from the polarization curves presented in figure 1a and corrosion parameters given in table 2, it is clear that corrosion current density of both sa treated samples is significantly lower compared to that of the blank sample. the protective films on t25 and t40 samples acted as barrier toward oxygen diffusion, thus slowing down cathodic corrosion reaction, while anodic corrosion reaction decreased to a lesser extent. this effect was observed in most of papers investigating use of selfassembled monolayers (sams) in corrosion protection [22,23]. for application of sams in corrosion protection, it is not sufficient to determine only the initial protection level but also to verify if the protection remains satisfactory with passing of time. for that reason, polarization dc measurements in narrow potential range were conducted over longer period. the polarization resistance (rp) values determined from polarization measurements are given in figure 1b. it can be seen that both t25 and t40 samples initially exhibited higher rp values compared to the untreated sample, but over time there was a decrease in these values. decrease in polarization resistance in time can be associated with dissolution of upper layers of the protective sa film, or formation of pores in the film. however, the minimum rp values observed after 14 days of immersion in 3% nacl solution are twice than of the untreated sample. the t25 sample showed the highest initial polarization resistance values with higher reproducibility than for t40 sample. such results can be related to slower solvent evaporation at lower temperatures which then enable longer time for organization of sa molecules on cuni substrate, i.e. molecular motion and stacking into a more compact film. study of spread coating of odpa on mica showed that for longer time of adsorption (before solvent evaporation), less porous and more ordered film was formed [24]. this was found when deposition was conducted by low concentration odpa solution containing dissolved single molecules, and also in the case of more concentrated solution containing odpa micelles. therefore, even though heating of the substrate during spraying can improve adsorption of molecules on a substrate [19], faster evaporation of the solvent can lead to poorer organization of molecules in the layer and consequently to formation of less protective odpa film. influence of time between spraying after examining the substrate temperature effect, the next parameter tested was the time elapsed between two consecutive sprays. for that purpose, samples were sprayed at room temperature five times, but between two consecutive sprays samples were left in air for 60 minutes (t60 sample), 30 minutes (t30 sample), or 10 minutes (t10 sample). in this way, it was attempted to determine the minimum time required for the preparation of stable and protective layers. their protective efficiency was examined by polarization measurements as presented in figure 2, while corrosion parameters obtained from polarization curves are presented in table 3. from figure 2a it can be seen that polarization curves obtained for all sa treated samples exhibit lower values of current density compared to the untreated sample, with a significant reduction of cathodic current density resulting in the displacement of the corrosion potential in the cathodic direction. http://dx.doi.org/10.5599/jese.739 j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 166 (a) (b) figure 2. influence of time between spraying – results of polarization measurements: (a) polarization curves recorded after one hour immersion in 3 % nacl, and (b) polarization resistance as a function of immersion time for blank and treated samples. table 3. corrosion parameters obtained by tafel extrapolation method from polarization curves given in fig. 2a values in parentheses represent standard deviations sample ecorr / mvvs. sce jcorr / μa cm-2 ba / mv dec-1 -bc / mv dec-1 i / % t10 -203 (5) 0.28 (0.02) 37 (9) 147 (23) 75 (2) t30 -234 (12) 0.02 (0.01) 39 (2) 89 (7) 98 (1) t60 -232 (6) 0.03 (0) 43 (3) 109 (5) 97 (0) the largest decrease can be observed for t30 sample which indicates formation of an effective barrier to oxygen diffusion towards the metal surface, probably due to the formation of a layer with less defects and pores. from corrosion parameters shown in table 3, it can also be noted that the lowest value of corrosion current density and hence the lowest corrosion rate and the highest corrosion inhibition efficiency was obtained for t30 sample. similar results were obtained by testing the durability of protection by polarization resistance measurements in time, as can be seen in figure 2b. from the obtained results it can be seen that 10 minutes was not sufficient time between two sprays, while time longer than 30 minutes was even not required, as it didn’t contribute to improvement of protective properties of sa film. influence of the number of sprays the next step in optimizing the spraying method was to examine how many sprays are needed to obtain good corrosion protection. as aforementioned, it is expected that thicker sa layers would provide better corrosion protection. however, if such layers are porous and with high number of defects, their protective effect is not very high. therefore, our aim was to prepare sufficiently thick protective layer, but with low degree of porosity in order to achieve good corrosion protection. therefore, the prepared samples were sprayed at room temperature two (2x sample), five (5x sample) and ten times (10x sample). between the individual spraying, samples were left for 30 minutes at room temperature. the results of the polarization measurements are shown in figure 3, and the parameters obtained by tafel extrapolation method from polarization curves are shown in table 4. e. k. mioč and h. otmačić ćurković j. electrochem. sci. eng. 10(2) (2020) 161-175 http://dx.doi.org/10.5599/jese.739 167 (a) (b) figure 3. influence of the number of spraying – results of polarization measurements: (a) polarization curves recorded after one hour of immersion in 3 % nacl, and (b) polarization resistances as a function of immersion time for blank and treated samples table 4. corrosion parameters obtained by tafel extrapolation method from polarization curves given in fig. 3 values in parentheses represent standard deviations sample ecorr / mvvs. sce jcorr / μa cm-2 ba / mv dec-1 -bc / mv dec-1 i / % 2x -232 (10) 0.23 (0.03) 48 (8) 124 (61) 80 (2) 5x -234 (12) 0.02 (0.01) 39 (2) 89 (7) 98 (1) 10x -225 (22) 0.01 (0.01) 42 (2) 184 (29) 98 (1) figure 3a shows that the cathodic current density was reduced for all treated samples, while the slightest reduction could be observed for 2x sample, which is in accordance with the fact that by increasing the number of sprays, a larger amount of sa is deposited on the surface thus resulting in a thicker protective layer. it is interesting to note that 10x sample showed the greatest anodic inhibition, probably because thicker multilayer structure caused more difficult penetration of water and chloride ions towards the substrate surface. from the values of corrosion parameters (table 4), decrease of corrosion current density with an increase of the number of sprays can be observed. from the dependence of the polarization resistance on corrosive media exposure time shown in figure 3b, it can be seen that although 10x sample initially shows the best results, on the third day of immersion, its rp value is already equated with 2x sample. the possible reason of such result is that at the beginning, a very thick layer on 10x sample was formed due to the large number of sprays, but as the molecules in the multilayer structure were not tightly bounded, the outer layers were removed with time. therefore, it can be concluded that the best results were achieved when the samples were sprayed 5x. electrochemical impedance spectroscopy measurements on sa treated samples in order to better understand time evolution of polarization resistance values, eis measurements were conducted. figure 4 presents eis spectra of studied samples for the first (figure 4a and 4b) and 14th day (figure 4c and 4d) of exposure to 3 % nacl solution. http://dx.doi.org/10.5599/jese.739 j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 168 (a) (b) (c) (d) figure 4. eis – nyquist and bode plots: (a) and (b) after one hour, and (c) and (d) 14th day of exposure to 3% nacl. symbols experimental data; solid lines modelled data according to electrical equivalent circuits given in fig. 5 (a) (b) figure 5. equivalent electrical circuit used for fitting the impedance data for fitting impedance data of the blank sample it was necessary to use the electrical equivalent circuit with two time constants (figure 5a). due to the frequency dispersion, the capacitor element in electrical equivalent circuit was replaced with the constant phase element (cpe). the impedance of cpe (zcpe) is defined as zcpe = 1/[q(jω)n] (2) where constant q is the frequency-independent parameter of cpe [25] which represents pure capacitance when n = 1. proposed 2rq circuit (figure 5a) used for fitting eis spectra consists of rf – qf and rct – qdl couples. rf – qf couple describes a time constant observed at high frequencies, where rf corresponds to the resistance of the oxide film and qf is related to the capacitance of the film associated with its thickness and dielectric property. rct – qdl couple presents the corrosion reaction at the metal substrate/solution interface, where rct is charge transfer resistance and qdl is related to e. k. mioč and h. otmačić ćurković j. electrochem. sci. eng. 10(2) (2020) 161-175 http://dx.doi.org/10.5599/jese.739 169 the double layer capacitance. the electrolyte resistance between the working and reference electrodes is represented by rel. for eis spectra of sa treated samples measured on the first day of exposure to 3 % nacl, the same model (figure 5a) was used. in this case, however, rf represented resistance of the pores in stearic acid film and qf was related to the capacitance of the surface film. after 14 days of exposure to corrosive medium (3 % nacl), some changes in eis spectra were observed (figure 4c and 4d). firstly, the difference between impedance modulus of blank and protected samples was not as high as at the beginning of the experiment. secondly, the third maximum in the phase angle plot appeared at high frequencies. in our previous work on stearic acid films [14], appearance of this time constant was attributed to transformation of the organic film into porous outer layer and more compact inner layer. for that reason, impedance spectra were modelled with the equivalent circuit presented in figure 5b, where rf,o – qf,o represents resistance of the pores and capacitance of the outer porous film, and rf,i – qf,i represents resistance and capacitance of the inner compact layer. eis parameters obtained by fitting selected models to experimental data are presented in tables 5 and 6. note that the values for outer film parameters are omitted in table 6, because rf,o values were low for all studied samples such that their contribution to overall corrosion resistance is practically negligible. table 5. impedance parameters for the first day of exposure to 3 % nacl values in parentheses represent standard deviations sample qf / µs sn cm-2 nf rf / kω cm2 qdl / µs sn cm-2 ndl rct / kω cm2 cuni 60.71 (11.22) 0.86 (0.01) 4.16 (1.63) 291.70 (8.06) 0.50 (0) 5.55 (0.50) t40 0.21 (0.16) 0.90 (0.07) 29.15 (17.11) 0.51 (0.33) 0.65 (0.04) 679.70 (498.34) t25/t60 0.15 (0.04) 0.94 (0.01) 21.72 (8.51) 0.89 (0.80) 0.64 (0.06) 657.73 (111.58) t30/5x 0.13 (0) 0.95 (0) 34.87 (22.78) 0.26 (0.13) 0.65 (0.05) 943.73 (234.92) t10 0.42 (0.18) 0.90 (0.01) 24.28 (23.48) 1.11 (0.59) 0.63 (0.07) 247.67 (184.95) 2x 0.49 (0.48) 0.89 (0.04) 72.82 (24.18) 1.12 (1.42) 0.69 (0.02) 386.61 (305.09) 10x 0.11 (0.02) 0.97 (0) 101.73 (13.97) 0.12 (0.03) 0.67 (0.08) 2471.33 (439.67) by examining data in table 5, it can be observed that the difference in impedance parameters of t40 and t25 is not significant. on the contrary, parameters of samples prepared with different times elapsed between two sprays are quite different. firstly, t10 samples showed higher qf values than t30 and t60 samples. taking in account that all samples were prepared by five times (5x) spraying, it is expected that the similar amount of sa was deposited on the metal surface and as qf is reversely proportional to film thickness, its values should be the same for these three samples. as this is not the case, the probable explanation is that sa layer on t10 sample was more porous, which enabled penetration of higher amounts of water into the film and resulted in increase of its dielectric constant. this is also in accordance with higher qdl and lower rct values obtained for t10 compared to t30 and t60 samples. when examining the number of sprays, it is evident that the highest resistance values (both rf and rct) and the lowest capacitance values (qf and qdl) were obtained for the sample with the highest number of sprays, 10x. from data presented in table 6, it can be observed that two weeks (14 days) exposure of samples to 3 % nacl solution resulted in decrease of almost all resistive elements values and increase of capacitive elements values when compared to data in table 5 for the first day of exposure. such changes can be attributed to ingress of water into the sa film and dissolution of outer film layers. http://dx.doi.org/10.5599/jese.739 j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 170 table 6. impedance parameters for the 14th day of exposure to 3 % nacl values in parentheses represent standard deviations sample qf,i / µs sncm-2 nf,i rf,i / kω cm2 qdl / µs sn cm-2 ndl rct / kω cm2 cuni 58.95 (0.78) 0.86 (0.01) 6.06 (5.14) 66.72 (30.09) 0.52 (0.03) 17.88 (11.20) t40 16.83 (5.46) 0.80 (0.04) 1.36 (0.34) 23.15 (10.80) 0.65 (0.09) 63.49 (6.48) t25/t60 13.21 (4.14) 0.80 (0.08) 6.11 (5.58) 9.57 (3.87) 0.66 (0.14) 84.09 (15.29) t30 /5x 13.01 (6.54) 0.85 (0.01) 23.58 (14.16) 24.33 (12.46) 0.52 (0.03) 94.86 (34.46) t10 17.46 (2.73) 0.76 (0.05) 50.00 (39.33) 358.40 (238.52) 0.83 (0.12) 36.09 (14.44) 2x 19.68 (2.49) 0.85 (0.01) 14.11 (7.05) 19.40 (1.72) 0.50 (0) 73.27 (15.43) 10x 14.76 (0.93) 0.84 (0.02) 18.50 (16.34) 27.08 (8.61) 0.50 (0) 61.46 (21.88) for most of the samples, however, both resistance values (rf,i and rct) were still higher than those obtained for the blank sample, thus indicating that sa films provided corrosion protection even after two weeks of exposure to corrosive medium. the most significant decrease of rf,i and rct values was observed for 10x sample that exhibited the highest resistive values for the first day of exposure 3 % nacl (table 5). the possible explanation for such behaviour would be that by increase of the film thickness, the number of defects in the film is also increased, especially in the outer layers, enabling thus their easier desorption from the surface. obtained results confirm that the most protective sa film was formed on the sample t30/5x. comparison of stearic and octadecylphosphonic acids films after examining the parameters of spray deposition of sa, odpa films were prepared in the same way (t30/5x procedure), except at the final drying step that was conducted at 80 oc as applied in immersion deposition of odpa [7]. such prepared samples were immersed in 3 % nacl solution and electrochemical measurements were conducted. the results of the polarization measurements are shown in figure 6, and corrosion parameters obtained by tafel extrapolation method from polarization curves are shown in table 7. (a) (b) figure 6. comparison of equally prepared sa and odpa films – results of polarization measurements: (a) polarization curves recorded after one hour immersion in 3 % nacl, and (b) polarization resistance as a function of immersion time for blank and treated samples. e. k. mioč and h. otmačić ćurković j. electrochem. sci. eng. 10(2) (2020) 161-175 http://dx.doi.org/10.5599/jese.739 171 from polarization curves given in figure 6a it can be observed that while the protective films of sa significantly decreased cathodic current densities and only slightly anodic current densities, odpa films reduced both anodic and cathodic current densities. apart from acting as a barrier film, it may be assumed that odpa is adsorbed on anodic sites of alloy thus preventing their dissolution. protective efficiency of odpa film was slightly lower than that of sa film. also, on the 1st day of exposure to 3% nacl, sa film showed higher polarization resistance values compared to odpa film. however, contrary to a continuous decrease in rp values of sa films in time, rp values of odpa films, increased on the third day of immersion, probably due to the reorganization of molecules in the film into the more stable configuration. later on, rp values of odpa samples decreased in time but remained always higher than those obtained for sa samples. studies were also conducted by eis in order to better understand corrosion behaviour of odpa samples (figure 7). table 7. corrosion parameters obtained for sample with odpa protective layer. values in parenthesis represent standard deviations sample ecorr / mv vs. sce jcorr / μa cm-2 ba / mv dec-1 -bc / mv dec-1) i / %) odpa -185 (28) 0.11 (0.07) 43 (4) 69 (3) 90 (6) (a) (b) figure 7. nyquist (a) and bode (b) impedance spectra for odpa treated samples for the 1st and 14th day of immersion in 3 % nacl solution. eis spectra given in figure 7 confirm findings from polarization measurements that odpa films initially provided lower corrosion protection than sa films (figure 4), but this protection was much more stable in time. the shapes of phase angle log frequency curves of bode plots in figure 7b reveal that it was necessary to use the electrical equivalent circuit with three time constants to adequately fit eis spectra measured for both the first and the last day of exposure to 3% nacl. for this purpose, the electrical equivalent circuit given in figure 5b was used and the obtained impedance parameter values are presented in table 8 (except for rf,o – qf,o elements). table 8. impedance parameters for odpa treated samples sample qf,i / µs sncm-2 nf,i rf,i / kω cm2 qdl / µs sn cm-2 ndl rct / kω cm2 odpa-1st day 2.11 (0.15) 0.66 (0.01) 47.97 (25.74) 1.75(0.98) 0.66 (0.08) 405.83 (78.16) odpa-14th day 3.58 (1.92) 0.71 (0.04) 1.66 (0.75) 4.81 (0.78) 0.69 (0.04) 212.37 (6.86) therefore, it appears that odpa films were initially characterized by the presence of porous outer layers which might explain the lower protection exhibited by odpa than sa films. however, by examining the impedance parameter values for the 14th day of exposure to 3 % nacl, it is clear that http://dx.doi.org/10.5599/jese.739 j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 172 rct value for odpa sample was twice higher than for sa sample, while qdl values of odpa sample were five times lower than for sa sample. taking into account that polarization measurements (figure 6a) showed that unlike sa, odpa exhibited strong anodic current inhibition, and also that phosphonic acids are stronger than carboxylic acids, it may be proposed that odpa molecules were more strongly bounded to cuni surface than sa molecules. thus, despite the formation of more porous surface layer, dissolution of strongly bounded odpa molecules was more difficult than dissolution of sa molecules. results obtained in this work are similar to results obtained in our previous study [7], where odpa films were prepared by dip-coating method, except for the fact that for dip-coating method, presence of porous outer layer was not clearly observed for the first day of exposure to 3 % nacl. however, the rp values observed in this work for odpa films formed by spraying are only slightly higher than those observed for odpa films formed by dip-coating [7]. when comparing the results obtained for sa films prepared by dip-coating [14,15] and spraying method, it appeared that films obtained by spraying initially exhibited higher corrosion protection than films obtained by dipcoating method. however, the resistance of sprayed samples decreased more significantly in time, eventually reaching the values obtained for dip-coated samples. therefore, for preparation of protective sa and odpa films, spraying method can be used instead of dip-coating method. the thicknesses of obtained coatings can be roughly estimated from the film capacitance (cf) values as cf value would be reversely proportional to the film thickness. according to brug et al. [31], the pure capacitance (c) value can be calculated using the relationship c = (qrel1-n)1/n. for sa film obtained by optimal preparation procedure (5x), the calculated cf was about 0.13 f cm-2. in our previous work [15], the thickness of sa film obtained by dip-coating was determined by ellipsometry. it was found that sa film exhibiting cf value of 0.88 f cm-2 has the thickness of about 18 nm. thus, one can estimate that sa film-5x has the thickness of approximate 120 nm. in addition, it has to be taken into account that cf value doesn’t depend solely on the film thickness but also on dielectric constant of the film, which significantly depends on the amount of the water in film (film porosity). anyway, it appears that films formed by spraying method were thicker than those formed by dip-coating method. this could explain better initial corrosion protection offered by spray deposited sa films. for odpa film, inner and outer films were observed with qf,o much lower than qf,i. however, the values of nf,i and nf,o were found lower than 0.75 which is considered as a threshold limit for the use of brug equation and cf calculation. surface studies ftir spectroscopy was used to confirm stearic and phosphonic acid film formation and to analyse the alkyl chain ordering in the films. characteristics of the methylene (ch2) stretching region of the ftir spectrum indicate whether or not an ordered film was present on the surface. a well-ordered film has been characterized as having alkyl chains in all trans configuration on the substrate with characteristic peaks at νch2asymm ≤ 2918 cm −1 and νch2symm ≤ 2850 cm −1. shifts of these peaks to higher wavenumbers indicate a disordered film with gauche interactions in the alkyl chain [26-28]. although, ftir has been mainly used to characterize monolayer films, it was also applied in characterization of self-assembled and langmuir-blodgett multilayer films [29,30]. ftir spectra presented in figure 8 exhibit adsorption peaks characteristic for well-ordered films for both studied samples. since the layers obtained in our study where not monolayers but multilayers, ftir spectra cannot be used for accurate determination of odpa bonding to cuni surface as vibrations of upper layers dominate in the obtained spectra. e. k. mioč and h. otmačić ćurković j. electrochem. sci. eng. 10(2) (2020) 161-175 http://dx.doi.org/10.5599/jese.739 173 figure 8. ftir spectra of sa and odpa layers deposited on cuni alloy by selected spraying method the water contact angle (ca) is commonly used to determine the wettability of modified surface but is also considered as a measure of the layer crystallinity. well-ordered and densely packed sams [11,26,30] have shown the water contact angle of around 108°. the water contact angles determined on sa and odpa treated samples (figure 9) showed such ca values indicating that the alkyl tail group was oriented towards environment. (62  3)o (107  2)o (108  1)o cuni sa odpa figure 9. comparison of water contact angles on non-treated cuni alloy and samples treated with sa and odpa by selected spraying method freshly prepared sa and odpa samples were examined by means of scanning electron microscopy, afterwards samples were exposed to 3 % nacl solution for 14 days and examined by sem again (figure 10). cuni sa odpa figure 10. sem images of non-treated cuni alloy and equally prepared sa and odpa films before and after exposure to 3 % nacl solution for 14 days b e fo re e x p o su re t o n a c l n a c l a ft e r e x p o su re t o n a c l fo r 1 4 d a y s d a y s cuni sa odpa http://dx.doi.org/10.5599/jese.739 j. electrochem. sci. eng. 10(2) (2020) 161-175 protective films of stearic and octadecylphosphonic acid 174 comparing the morphology of stearic and octadecylphosphonic acid films, it can be seen that after preparation of the sample, the stearic acid films appeared more uniform. however, after 14 days of exposure to corrosive media, sa films exhibited large number of cracks in the film, while odpa films visually didn’t change as much. conclusion studies conducted in this work confirmed that spraying method can be used to prepare protective films of sa and odpa on cuni alloy and that adequately prepared films exhibit similar properties to those observed for films formed by dip-coating method. several parameters of spraying deposition method were examined, among which time elapsed between two sprays and number of sprays have the strongest influence on the film stability and its protective properties. comparison of similarly prepared sa and odpa films showed that initially, sa films provide better corrosion protection than odpa films. prolonged exposure to corrosive medium, however, showed faster decline of sa film protective properties than that of odpa film, what is related to stronger attachment of phosphonic acids on cuni surface. it was also shown that spray formed protective films provide corrosion protection to the underlying cuni alloy even after 14 days of continuous exposure to corrosive medium. acknowledgements: the research leading to these results has received funding from croatian science foundation under grant agreement 9.01/253. references [1] c. queffelec, m. petit, p. janvier, d. a. knight, b. bujoli, chemical reviews 112 (2012) 3777 – 3807. 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[31] g. j. brug, a. l. g. van der eeden,m. sluyters-rehbach, j. h. sluyters, journal of electroanalytical chemistry 176 (1984) 275-295. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.739 http://creativecommons.org/licenses/by/4.0/) voltammetric folic acid sensor based on nickel ferrite nanoparticles modified-screen printed graphite electrode: http://dx.doi.org/10.5599/jese.1607 1215 j. electrochem. sci. eng. 12(6) (2022) 1215-1224; http://dx.doi.org/10.5599/jese.1607 open access : : issn 1847-9286 www.jese-online.org original scientific paper voltammetric folic acid sensor based on nickel ferrite nanoparticles modified-screen printed graphite electrode peyman mohammadzadeh jahani1 and mohammad reza aflatoonian2, 1school of medicine, bam university of medical sciences, bam, iran 2leishmaniasis research center, kerman university of medical sciences, kerman, iran corresponding author: m.aflatoonian97@gmail.com received: november 19, 2022; accepted: november 26, 2022; published: november 30, 2022 abstract in this study, an electrochemical sensor for the quantification of folic acid with voltammetric detection in physiological conditions was constructed. for this purpose, nickel ferrite (nife2o4) nanoparticles were used to modify the surface of a screen-printed graphite electrode (nife2o4/spge) and applied in the determination of folic acid. the modified electrode displays a strong electrochemical response to folic acid. folic acid was determined electrochemically using the differential pulse voltammetry (dpv) technique with a detection limit of 0.09±0.001 µm in 0.2–147.0 µm linear range in phosphate buffer solution (pbs) at ph 7.0 with this nife2o4/spge sensor, which has the best electron transfer rate. also, the sensitivity of the modified electrode was obtained as 0.1139 µa µm-1. the nife2o4/spge sensor was successfully applied for the determination of folic acid in real samples. keywords modified electrode, electrochemical sensor, electrocatalytic activity, magnetic nanoparticles introduction vitamins are a group of organic compounds essential in very small amounts for the body's normal functioning [1]. folic acid (n-[p-{[(2-amino-4-hydroxy-6-pteridinyl) methyl] amino}benzoyl]-l-glutamic acid), also called pteroylglutamic acid (pteglu), is a water-soluble vitamin of b complex family. it is most commonly referred to as vitamin b9 [2,3]. folic acid is an important substance for keeping the activity and health of critters and is essential for cell growth and division of the human body. it participates in lots of bodily reactions and mainly in the synthesis of nucleic acid and some important substances and promoting the synthesis of protein from amino acid [4-6]. research over the past decades has shown that deficiency in folate concentration leads to neural tube defects in newborns and an increased risk of megaloblastic anemia, cancer, coronary heart disease, alzheimer’s disease, neurological disorders, and cardiovascular disease in children and adults. furthermore, the requireement of folate increases during periods of rapid cell division and it is essential for pregnant women [7,8]. hence, analytical methods for the determination of this important bioelement are needed. http://dx.doi.org/10.5599/jese.1607 http://dx.doi.org/10.5599/jese.1607 http://www.jese-online.org/ mailto:m.aflatoonian97@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1215-1224 folic acid sensor based on nickel ferrite 1216 several methods have been proposed for the determination of folic acid in real samples, including spectrophotometry [9], high-performance liquid chromatography [10], capillary electrophoresis [11], fluorimetric [12], colorimetry [13], and flow injection chemiluminescence method [14]. however, in most of the cases reported above, prior steps are required before the actual determination of folic acid. also, these techniques consume a long time for analysis, are subject to interferences and require expensive reagents. these disadvantages do not make them applicable for rapid analytical determination. it is well known that electrochemical methods are simple and inexpensive, in which analytical techniques require a small amount of sample. electrochemical sensors have attracted wide attention due to their facile fabricating processes, quick surface renewal, high sensitivity, selectivity, low background current, fastness, a wide range of potential ranges, compatibility and reproducibility [15-29]. the concept of modified electrodes is an exciting development in the field of electrochemistry. the electrocatalysis of slow electron transfer reactions is perhaps the most important feature of chemically modified electrodes. such electrodes enhance the electron transfer rate by reducing the overpotential associated with a reaction. the importance of modified electrodes in electrochemical sensors is because of their high electron transfer rate, high sensitivity, selectivity and stability in analyzing the electrochemical behavior of the analyte. so it is very important to develop highly sensitive and precise analytical methods and material with good conductivity to modify electrode to detect the concentration of analyte effectively [30-41]. during the last years, the research outcomes related to nanomaterials have increased in different application fields due to the development in the preparation and application of these new materials [42-46]. nanomaterials offer unique and specific electroanalysis properties only found in nanoscale materials. these properties derive from the enhanced diffusion of the target analyte based on convergent rather than linear diffusion, with a high surface area, enhanced selectivity, catalytic activity, and a high signal-to-noise ratio [47-51]. magnetic nanoparticles provide significant levels of new functionality for electrochemistry due to their high surface area, effective mass transport, catalysis and control over the local microenvironment [52,53]. magnetic nanoparticles (nps) with the general formula mfe2o4 (m = fe, ni, co, cu, mn, etc.) are the most popular materials in analytical biochemistry, medicine, removal of heavy metals and biotechnology and have been increasingly applied to immobilize proteins, enzymes, and other bioactive agents due to their unique advantages. nickel-ferrite is one of the most malleable and important spinel compounds due to its typical ferromagnetic properties and high electrochemical stability. moreover, nife2o4 nps exhibit a high surface area and low mass transfer resistance. it is expected that nife2o4 could also be used as an electrocatalyst apart from its electronic and magnetic applications due to its conducting nature [54-56]. in this work, a screen-printed graphite electrode modified with the nife2o4 magnetic nanoparticles was used for sensitive voltammetric determination of folic acid and the modified electrode exhibited excellent electrocatalytic activity to folic acid. experimental apparatus and chemicals all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab. the measurement cell consisted of spge (dropsens; drp-110: spain) containing a graphite counter electrode, a graphite working electrode, and a silver pseudo-reference electrode. p. mohammadzadeh jahani and m. r. aflatoonian j. electrochem. sci. eng. 12(6) (2022) 1215-1224 http://dx.doi.org/10.5599/jese.1607 1217 solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). all chemicals used were of analytical grade and were used as received without any further purification and were obtained from merck. orthophosphoric acid was utilized to prepare the phosphate buffer solutions (pbs), and sodium hydroxide was used to adjust the desired ph values (ph range between 2.0 and 9.0). preparation of modified electrode nife2o4/spges were prepared by modifying the bare working electrode of an spge using the dropcasting method. briefly, 4 µl of the solution of nife2o4 nps (1 mg/ml) were dropped onto the working electrode surface and dried at room temperature. the obtained electrode was noted as nife2o4/spge. the surface area of nife2o4/spge and the bare spge were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using randles-ševčik formula for nife2o4/spge, the electrode surface was found to be 0.109 cm2 which was about 3.5 times greater than bare spge. results and discussion electrochemical behavior of folic acid at the surface of various electrodes the effect of the electrolyte ph on the oxidation of 100.0 μm folic acid was investigated at nife2o4/spge using dpv measurements in the pbs in the ph range from 2.0 to 9.0. according to the results, the oxidation peak current of folic acid depends on the ph value and increases with increasing ph until it reaches the maximum at ph 7.0 and then decreases at higher ph values. the optimized ph corresponding to the higher peak current was 7.0, indicating that protons are involved in the reaction of folic acid oxidation. scheme 1 demonstrates the electrooxidation process of folic acid. scheme 1. the electoroxidation reaction of folic acid figure 1 displays cyclic voltammetric responses from the electrochemical oxidation of 100.0 μm folic acid at the surface of nife2o4/spge (curve b) and bare spge (curve a). figure 1. cyclic voltammograms of a) bare spge, b) nife2o4/spge in the presence of 100.0 μm folic acid in 0.1 m phosphate buffer solution, ph 7.0 http://dx.doi.org/10.5599/jese.1607 j. electrochem. sci. eng. 12(6) (2022) 1215-1224 folic acid sensor based on nickel ferrite 1218 the results showed that the oxidation of folic acid is very weak on the surface of the bare spge, but the presence of nife2o4 nps in spge could enhance the peak current and decrease the oxidation potential (decreasing the overpotential). a substantial negative shift of the currents starting from oxidation potential for folic acid and a dramatic increase of the current indicates the catalytic ability of nife2o4/spge to folic acid oxidation. the results showed that the use of nife2o4 nanoparticle (curve b) definitely improved folic acid oxidation characteristics, partly due to the excellent characteristics of nife2o4 nps, such as good electrical conductivity and high chemical stability. effect of scan rate on the determination of folic acid at nife2o4/spge the influence of potential scan rate (ν) on ip of 70.0 μm folic acid at the nife2o4/spge was studied by linear sweep voltammetry (lsv) at various sweep rates (figure 2). as shown in figure 2, the peak currents of folic acid grow with the increasing scan rates and there are good linear relationships between the peak currents and ν1/2 (square root of scan rate) (figure 2 inset). the regression equation is ipa= 1.1019 ν1/2 +0.8618 (r2= 0.9986), indicating the oxidation process of 70.0 μm folic acid at the nife2o4/spge was diffusion-controlled. figure 2. linear sweep voltammograms of 70.0 μm folic acid at nife2o4/spge at different scan rates, 1-7 correspond to 5, 10, 50, 100, 200, 300 and 400 mv s−1 in 0.1 m phosphate buffer solution, ph 7.0. inset shows the plot of ipa versus ν 1/2 for the oxidation of folic acid at nife2o4/spge to obtain further information on the rate-determining step, the tafel plot for oxidation of 70.0 µm folic acid at the surface of nife2o4/spge using the data derived from the raising part of the current-voltage curve has been recorded in figure 3. figure 3. linear sweep voltammograms for 70.0 μm folic acid with 5 mv s-1 scan rate.inset: the tafel plot derived from the rising part of the corresponding voltammogram p. mohammadzadeh jahani and m. r. aflatoonian j. electrochem. sci. eng. 12(6) (2022) 1215-1224 http://dx.doi.org/10.5599/jese.1607 1219 using the slope of tafel at a scan rate of 5 mv/s, the value of the electron transfers coefficient (α) was determined as 0.68. chronoamperometric studies the electrochemical oxidation of folic acid by a nife2o4/spge was also studied by chronoamperometry. chronoamperometric measurements of different concentrations of folic acid at nife2o4/spge were done by setting the working electrode potential at 650 mv (figure 4). figure 4. chronoamperograms obtained at the nife2o4/spge in 0.1 m phosphate buffer solution, ph 7.0 for different concentrations of folic acid. numbers 1-9 correspond to 0.1, 0.25, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2 and 1.5 mm of folic acid. (a) plots of i vs. t-1/2 for electrooxidation of folic acid obtained from chronoamperometry. (b) plot of the slope of the straight lines against folic acid concentration in chronoamperometric studies, we have determined folic acid's diffusion coefficient, d. the experimental plots of i versus t−1/2 with the best fits for different concentrations of folic acid were employed (figure 4 a). the slopes of the resulting straight lines were then plotted versus the folic acid concentrations (figure 4 b), from whose slope and using the cottrell equation (1): i =nfad1/2cbπ -1/2t -1/2 (1) we calculated a diffusion coefficient of 6.610-5 cm2 s-1 for folic acid. calibration curve and limit of detection since dpv has a much higher current sensitivity than cyclic voltammetry, we used the dpv method for the determination of folic acid. figure 5 (inset) shows dpvs of different concentrations of folic acid and the obtained calibration curves (step potential = 0.01 v and pulse amplitude = 0.025 v). the results showed a linear segment for folic acid concentration from 0.2 to 147.0 μm folic acid (figure 5), with a regression equation of ip = 0.1139cfolic acid+ 1.1599 (r2= 0.9992, n = 8). the detection limit, lod, was obtained by using the equation (2): lod = 3sb /m (2) where sb is the standard deviation of the blank response (n = 15) and m is the slope of the calibration plot. the limit of detection was determined to be 0.09±0.001 μm for folic acid. http://dx.doi.org/10.5599/jese.1607 j. electrochem. sci. eng. 12(6) (2022) 1215-1224 folic acid sensor based on nickel ferrite 1220 in addition, table 1 shows that the nife2o4/spge can compete with other sensors for the determination of folic acid [4, 57-60]. figure 5. differential pulse voltammograms of the nife2o4/spge in 0.1 m phosphate buffer solution (ph 7.0) containing different concentrations of folic acid. numbers 1 to 8 correspond to 0.2, 5.0, 15.0,40.0, 60.0, 80.0, 100.0 and 147.0 µm of folic acid. (b) plot of the voltammetric peak current as a function of folic acid concentration table 1. linear range and lod obtained at the nife2o4/spge for the determination of folic acid compared with other sensors electrochemical sensor method linear range, μm lod, µm ref. multi-wall carbon nanotube/glassy carbon electrode square wave stripping voltammetry 0.3 80.0 0.134 4 gold nanoclusters/activated graphene/multi-wall carbon nanotube nanocomposite/glassy carbon electrode square wave voltammetry 10.0 170.0 0.09 56 poly(o-methoxyaniline)-gold (poma-au) nanocomposite/glassy carbon electrode differential pulse voltammetry 0.5 900.0 0.090 57 platinum nanoparticles/graphene nanoplatelets/multiwalled carbon nanotubes/β-cyclodextrin composite/carbon glass electrode cyclic voltammetry 20.0 500.0 0.48 58 fe3o4 nanoparticles@molecularly imprinted polymergraphene oxide/carbon paste electrode square-wave adsorptive voltammetry 2.5 48.0 0.65 59 nife2o4/spge differential pulse voltammetry 0.2 147.0 0.09 this work real sample analysis to investigate the applicability of the proposed sensor for the voltammetric determination of folic acid in real samples, we selected urine and folic acid tablet samples for the analysis of folic acid contents. the folic acid contents were measured after sample preparation using the standard addition method. the results are given in table 2. according to the table, the recovery values within 97.5 to https://www.sciencedirect.com/topics/chemistry/nanoparticle https://www.sciencedirect.com/topics/chemistry/molecularly-imprinted-polymer https://www.sciencedirect.com/topics/chemistry/graphene-oxide https://www.sciencedirect.com/topics/chemistry/voltammetry p. mohammadzadeh jahani and m. r. aflatoonian j. electrochem. sci. eng. 12(6) (2022) 1215-1224 http://dx.doi.org/10.5599/jese.1607 1221 103.3 % confirm the powerful ability of nife2o4/spge for the determination of folic acid in real samples. table 2. aapplication of nife2o4/spge for determination of folic acid in real samples (n=3) sample c / µm recovery, % rsd, % spiked found urine 0 4.0 3.9 97.5 2.2 8.0 8.1 101.2 3.0 folic acid tablet 0 4.0 3.4 2.0 6.2 103.3 1.9 3.0 6.9 98.6 2.7 conclusion in this work, a simple, rapid and sensitive electrochemical detection method has been developed for the determination of folic acid. nife2o4 nanoparticles modified spge as a voltammetric sensor to improve the detection sensitivity. the sensitivity (0.1139 µa µm-1), detection limit (0.09 ± 0.001 µm) and linear response range (0.2 to 147.0 µm) for the nife2o4/spge modified electrode make it an efficient way for determination of folic acid. real sample applications were carried out to prove the applicability and precision of the novelty-produced electrode. the amount of folic acid in real samples was obtained satisfactorily with high recovery values by the standard addition method. references [1] v. d. vaze, a. k. srivastava, electrochimica acta 53(4) (2007) 1713-1721. 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on nickel ferrite nanoparticles modified-screen printed graphite electrode:}}, year = {2022}, issn = {1847-9286}, month = {nov}, number = {6}, pages = {1215--1224}, volume = {12}, abstract = {in this study, an electrochemical sensor for the quantification of folic acid with voltam­metric detection in physiological conditions was constructed. for this purpose, nickel ferrite (nife2o4) nanoparticles were used to modify the surface of a screen-printed graphite electrode (nife2o4/spge) and applied in the determination of folic acid. the modified electrode displays a strong electrochemical response to folic acid. folic acid was determined electrochemically using the differential pulse voltammetry (dpv) technique with a detection limit of 0.09±0.001 µm in 0.2–147.0 µm linear range in phosphate buffer solution (pbs) at ph 7.0 with this nife2o4/spge sensor, which has the best electron transfer rate. also, the sensitivity of the modified electrode was obtained as 0.1139 µa µm-1. the nife2o4/spge sensor was successfully applied for the determination of folic acid in real samples.}, doi = {10.5599/jese.1607}, file = {:d\:/onedrive/mendeley desktop/jahani, aflatoonian 2022 voltammetric folic acid sensor based on nickel ferrite nanoparticles modified-screen printed graphite elect.pdf:pdf;:www/jese_v12_no6_1215-1224.pdf:pdf}, keywords = {modified electrode, electrocatalytic activity, electrochemical sensor, magnetic nanoparticles}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1607}, } {electrolytic iron production from alkaline suspensions of solid oxides: compared cases of hematite, iron ore and iron-rich bayer process residues} http://dx.doi.org/10.5599/jese.751 95 j. electrochem. sci. eng. 10(2) (2020) 95-102; http://dx.doi.org/10.5599/jese.751 open access: issn 1847-9286 www.jese-online.org original scientific paper electrolytic iron production from alkaline suspensions of solid oxides: compared cases of hematite, iron ore and iron-rich bayer process residues abdoulaye maihatchi1,2, marie-noëlle pons1, quentin ricoux2, frederic goettmann2, francois lapicque1, 1reaction and chemical engineering laboratory, cnrs-university of lorraine, 1 rue grandville, 54000 nancy, france 2extracthive, centre de recherche cea, bat. 51, 30591 marcoule, france corresponding author:  francois.lapicque@univ-lorraine.fr received: november 15, 2019; revised: january 7, 2020; accepted: january 7, 2020 abstract iron can be produced by the direct electrochemical reduction of hematite particles suspended in hot, concentrated naoh solutions. because various other iron sources can be considered, the present work was aimed at investigating the electrolytic treatment of the “red mud” generated by the bayer process for alumina preparation from bauxite. such sources contain very high amounts of impurities, in particular silicon and aluminium oxidebased minerals, in addition to other mineral phases. electrolytic reductive treatment of the industrial red mud sample was shown to be possible but with both lower current density and current efficiency than for pure hematite. after deposition tests at a fixed current density, further experiments in simulation tests have been carried out for better understanding. in particular, hematite particles were tested with and without impurities introduced in the solution. presence of little soluble impurities at the particle surface appear to hinder the reactivity of the suspended particles at the cathode surface, whereas sidehydrogen reaction still occurs. keywords iron production; electrodeposition; iron ores; red mud; surface reactivity. introduction iron production by reduction of hematite or other iron oxides by coal carbon at high temperature is per se a significant source of co2. although reported decades ago, the electrolytic alternative has appeared an attractive secondary way for iron production with far lower co2 emissions and possibly lower energy demand per ton of iron produced. most works conducted until recently concerned the http://dx.doi.org/10.5599/jese.751 http://dx.doi.org/10.5599/jese.751 http://www.jese-online.org/ mailto:francois.lapicque@univ-lorraine.fr j. electrochem. sci. eng. 10(2) (2020) 95-102 electrolytic iron production 96 direct reduction of hematite (-fe2o3) particles suspended in a hot concentrated alkaline solution [1-6]. it was shown that the reduction of trivalent iron proceeds through a particular pathway with formation of intermediate magnetite only upon direct contact of the hematite particle with the cathode: metal iron can then be produced from concentrated suspensions of hematite in 50 wt.% naoh at 110 °c at 1000 a/m2 with a current efficiency near 80 %. this particular mechanism involves the adsorption of hematite particles on the cathode surface, as shown by zeta potential measurements in such concentrated naoh media, and complementary electrochemistry tests [7,8]. direct reduction of dissolved fe(iii) at concentrations at a few 10-3 m also occurs but at far lower current density and with poor current efficiency [2], corresponding to production rates far lower than that through the above solid-solid electrochemical reaction. because hematite can be considered as an already refined, reactive iron-containing mineral, other iron oxides e.g. goethite [9] and magnetite [9,10] have been tested for electrolytic production of metal iron. as a matter of fact, the equilibrium – thermodynamic cell voltage for the reduction of the above metal iron oxides at 110 °c were calculated to be at 1.25 v ± 50 mv [11] depending on the iron oxide. nevertheless, tests conducted at similar electrolyte composition, temperature and cell voltage led to the following performance: hematite > goethite >> magnetite for both the cell current and the current efficiency. apart from the various existing iron oxides, the cases of rawer materials such as iron ores before removal of the gangue after their extraction has also been studied [11]. feeding the electrolytic cell with iron ore concentrate particles in the alkaline medium would avoid the stage for gangue dissolution, then reducing the cost of electrolytic iron production. with concern to ironcontaining waste, the waste of bayer process for purification of bauxite – often called “red mud” represents a huge potential source of iron since its contents in hematite vary from 35 to 60 %, depending on the process conditions and on the bauxite ores considered [12]. in addition to their very high production rate at approx. 70 million tons per year, the alkaline feature of the concentrated suspensions of iron mud, which should be disposed of, represents a potential hazard to environment, as observed by the ruptures of lagoon dams in hungary and in brazil in the last decade [13]. despite various works and processes developed for the valuable use of red mud [12], their year production rate largely exceeds that of their use in industry. we investigated here at the lab scale the potential of the electrochemical reduction of red mud suspension, making profit of their alkaline nature in the preparation of the strongly alkaline electrolytic medium. the manuscript presents the electrolytic treatment of this solid after thorough characterization of the mineral phases contained, with their potential and limits, in comparison to the “ideal” case of hematite. experimental section chemicals used sodium hydroxide solutions have been either taken from vwr prolabo chemicals 47–51 wt.% solutions of by dissolution of vwr sodium hydroxide pellets, in both cases without further purification. more dilute solutions have been prepared by dilution of mother naoh solutions. hydrochloric and nitric acids respectively at 37 and 65 wt.% were analytical grade solutions from vwr or emsure. sodium metasilicate as a solid powder was from alfa aesar and kaolinite (al2o3 2sio2, 2h2o) also solid was a technical vwr prolabo product. two raw materials that can be suitable for iron production by electrolysis in an alkaline medium have been used: abdoulaye maihatchi et al. j. electrochem. sci. eng. 10(2) (2020) 95-102 http://dx.doi.org/10.5599/jese.751 97 • commercial hematite (vwr), i.e. -iron oxide fe2o3; • red mud samples (alteo, france) recovered after production of alumina from bauxite by the bayer process by partial dissolution of silica and silicocaluminates in 15-25 wt.% naoh liquors at temperature and pressure up to 200 °c and 20 bars, respectively. lab-scale electrolytic cells and deposition tests because of the presence of suspensions of little soluble particles, electrodeposition experiments have been carried out with thorough stirring of the medium by a 3.5 cm magnet at 600 rpm in a cylindrical double walled reactor with oil recirculation via a thermostated bath (huber). the volume of the reactor was equal to 0.8 l. the 19.4 cm2 graphite cathodic rod was surrounded by a ti/pt grid (area near 150 cm2), allowing fairly uniform deposition rate on the cathode surface. the electrochemical reduction of the red mud sample was compared to the reference case of commercial hematite conducted in the same cell. temperature was fixed at 110 oc under steady nitrogen stream for all experiments. red mud (150 g) was suspended in 450 ml 12.5 m naoh solution: the appreciable alkali content of the red mud partly compensated the naoh concentration here somewhat lower than that in former works [2-4,9]. runs were carried out a fixed current using a bio-logic sp150 potentiostat coupled to a 10 a booster (bio-logic). after deposition, the cathode was carefully rinsed with deionised water, then dried with a paper cloth and weighted. eventually the deposit was detached from the graphite rod and submitted to microscopic observations and analysis (see following section). current efficiency was estimated from the weight of the solid recovered: as a matter of fact, the deposits were shown to consist of iron at more than 97 wt.%, which justified the estimation method. most tests were replicated: deviations below 5 % were usually reported for the current efficiency. characterization techniques of the solids particle size distributions of the solid phases investigated have been measured using a granulometer with laser diffraction (mastersizer 2000, malvern ltd): hematite particles consisted of fines with a diameter in the order of one micrometre or below, and larger particles with an average size near 15 µm (figure 1). the red mud investigated had a very broad size distribution, from less than 1 µm up to 300 µm or so for agglomerates of appreciable mechanical stability (figure 1). figure 1. particle size distribution of the two fe-rich solids investigated 0 2 4 6 8 10 12 0.1 1 10 100 1000 v o lu m e p e rc e n ta g e particle size, µm hematite red mud v o lu m e f ra ct io n , % http://dx.doi.org/10.5599/jese.751 j. electrochem. sci. eng. 10(2) (2020) 95-102 electrolytic iron production 98 chemical characterization of the two materials have been conducted either by x-ray fluorescence or by icp-oes (agilent technologies, s100) analysis of the liquid solutions produced after alkaline fusion with lithium salts (libo2 and libo3) at 1050 oc and dissolution into nitric acid. identification of the various phases present in the two solids was conducted by x-ray diffraction with use of topas® software for quantification. the metal deposits were observed by sem-edx (jsm6490, jeol ltd) and xrd (bruker d8 advance), respectively for chemical analysis of the surface and phase identification, in complement to quantitative xrd and icp analysis. the equivalent viscosity of suspension of red mud in alkaline media was measured by using a brookfield viscosimeter. chemical and phase composition of the solids tested the weight contents in the various metals or elements have been expressed as their oxide contents in table 1. titanium is very significant in the red mud and ti-oxide minerals contained in the starting bauxite are very little soluble in alkaline media. the appreciable amounts of sodium, calcium and potassium in the red mud, can originate from the bayer treatment. finally, vanadium, a transition metal with multiple valences was also found and assayed in the red mud fraction. table 1. weight contents of the various elements (in their oxide equivalents) in the solids. the sum of the contents can slightly differ from unity content, wt.% fe2o3 sio2 al2o3 cao tio2 na2o k2o mno2 v2o5 hematite red mud 98.67 52.7 0.08 4.19 0.21 13.9 0.22 4.1 0.04 7.40 2.10 14.0 0.36 0.28 iron is mainly in the form of hematite in the two fractions investigated. presence of goethite is visible in the xrd spectra of the red mud (figure 2). quartz (silica) is also present in the red mud. aluminium contained in the red mud was in the form of cancrinite na8(al6si6o24)(oh)2.04, (h2o)2.66 and liottite (na, ca, k)24(sial)36o72(so4clf)10 (figure 2). figure 2. xrd spectra of the two fe-rich solids investigated presumably, these two crystalline aluminates were formed from kaolinite, or mixed aluminates and silicates contained in the native bauxite through formation of an amorphous phase. this phase rapidly dissolves while forming crystalline phases, e.g. cancrinite whose solid entities gradually hematite 2 the ta, a rb it ra ry u n it s red mud 1 hematite 2 hydroxycancrinite (alsi)n(oh)x 3 goethite 4 silicon oxide 5 liotite na-ca-k (alsi)nom(so4-f-cl) 6 perovskite catio3 7 calcite caco3 2 the ta, a rb it ra ry u n it s hematite 2 the ta, a rb it ra ry u n it s red mud 1 hematite 2 hydroxycancrinite (alsi)n(oh)x 3 goethite 4 silicon oxide 5 liotite na-ca-k (alsi)nom(so4-f-cl) 6 perovskite catio3 7 calcite caco3 2 the ta, a rb it ra ry u n it s abdoulaye maihatchi et al. j. electrochem. sci. eng. 10(2) (2020) 95-102 http://dx.doi.org/10.5599/jese.751 99 mature in the alkaline solution during the bayer process [15,16]. calcium in the red mud is also visible as calcite and calcium titanium oxide (perovskite) catio3, as shown in figure 2. solubility of particular minerals in the electrolytic solutions at 110 oc and for various naoh concentrations have been predicted by simulation using phreeqc® software with minteq.v4 database [14]. for this purpose, the chemical composition of the commercial hematite of the red mud sample (table 1) was used. results and discussion deposition tests from red mud and hematite first deposition tests from red mud suspensions were done at 1000 a/m2 and led to faradaic efficiencies in the order of 20 %. tests with other red mud samples produced in canada and south korea led to comparable results, in spite of the partly different natures of impurities (data not shown). it was then preferred to test lower current density levels: as shown in figure 3, the faradaic efficiency of fe deposition from red mud suspension decreases with the current density, peaking at 71 % at 45 a/m2: although fair, this result corresponds to very low production rates from the metallurgical waste. in comparison, hematite particles suspended in a 12.5 m naoh solution can be reduced to iron with a current yield larger than 80 % for current density in the range 200 1000 a/m2. the poor performance of fe deposition exhibited by the red mud is presumably related to the high contents of the various impurities (table 1), most of them being crystallized minerals, e.g. cancrinite, calcite or calcium titanium oxide. figure 3. current efficiency of fe deposition with hematite and red mud depending on the applied current density to conclude, the lower, but unequal performance exhibited by the two solids led us to investigate its possible causes. presence of these impurities can affect the reduction efficiency through their inhibiting effect either at the solid surface (solid form effect), or after their dissolution into the electrolytic bath, that might lead to surface inhibition. successive series of experiments have therefore been carried out for better understanding. dissolution of impurities into the electrolytic bath preliminary calculations with phreeqc on the red mud sample together the results of xrd measurements led to consider the presence of hematite, goethite, cancrinite and perovskite for 0 20 40 60 80 100 10 100 1000 fe f a ra d a ic e ff ic ie n cy , % current density, a/m2 hematite red mud http://dx.doi.org/10.5599/jese.751 j. electrochem. sci. eng. 10(2) (2020) 95-102 electrolytic iron production 100 naoh concentration ranging from 1 to 12 m. the solubility of these various phases in the 12.5 m naoh solution is shown in figure 4, in comparison with that of kaolinite (al2si2o7,2h2o) hematite and goethite were found to have a solubility near 2.3 10-3 and 1.5 10-3 m respectively, in good agreement with the experimental determinations reported in [2,9]. solubilities of cancrinite, titania and calcium titanium oxide (perovskite) were predicted at 0.90x10-3, 1.15x10-3 and 0.46x10-3 m. thus, considering the high solid/liquid ratio used here, the three minerals can be considered as dissolving to insignificant extent. kaolinite is actually highly soluble, with a solubility near 1.9 m under these conditions. these predictions have been made assuming that the mineral phases are accessible to the alkaline solution, and not entrapped in complex agglomerates by refractory solid phases. traces of kaolinite remaining in the red mud after the bayer process – not quantified – might dissolve in the course of electrodeposition tests. however, the single silicate and aluminate ions produced are also to crystallize in the formation of little soluble cancrinite and sodalite as explained above [15,16]. figure 4. solubility of mineral phases present in the red mud samples placed in naoh 12.5 m and at 110 oc effect of introduced impurities in the electrolytic bath the principle of this series of experiments is to perform the electrochemical reduction of hematite after addition of salt impurities contained in the red mud and to compare the run performance with the data obtained with pure hematite or with the less refined solids. to emulate the solid, the effect of two impurities, namely silicoaluminates and vanadium, has been studied here. vanadium ions have actually been reported as hindering the deposition of particular metals e.g. gallium [17] even at concentrations far below 1 g/l, presumably because of the capacity of vanadium to generate galvanic cells with antagonistic reactions at the two electrodes resulting in very poor production rates. kaolinite has been employed as silicoaluminate source. in both cases, impurities have been introduced in a concentration range exceeding the actual species content in the red mud. the presence of kaolinite reduced significantly the faradaic efficiency conducted at 41 a/m2 (figure 5a). this effect might be caused by the progressive formation of cancrinite and sodalite on the surface of the particles [18] – this could not be directly evidenced then resulting in lower faradaic efficiency, found at 38 % upon addition of 30 wt.% kaolinite. another effect of kaolinite addition is the significantly increased viscosity of the suspension, as shown in figure 6: presence of 30 % kaolinite in the solid introduced made the suspension viscosity at 110°c increase from 2.8 mpa s to approx. 16 mpa s (figure 6) the effect of kaolinite addition has been also tested at higher current density (figure 5b). with 10 wt.% kaolinite – a content in the order of that of (si-al)-based impurities, the faradaic efficiency s o lu b il it y , m m o l / l s o lu b il it y , m o l / l 3.2 2.4 1.6 0.8 3.0 2.5 2.0 1.5 1.0 0.5 abdoulaye maihatchi et al. j. electrochem. sci. eng. 10(2) (2020) 95-102 http://dx.doi.org/10.5599/jese.751 101 was found at 62 and 70 % at 400 and 1000 a/m2 respectively: although lower than the reference value without silicoaluminate, the faradaic efficiency is nevertheless far larger than that with the red mud investigated. figure 5: effect of silicoaluminate added on the current yield for fe deposition: (a) low current density; (b) at higher current density, with 10 % silicoaluminate, with comparison of data with hematite and red mud figure 6: equivalent viscosity of suspensions of solids (1 g / 3 ml solution) in naoh 12.5 m at 110 °c. vanadium in the form of v2o5 was added at various contents to the electrolytic bath, with v element content ranging from 150 to 1500 mg/l. the lowest content did not really affect fe deposition conducted at 1000 a/m2, with faradaic efficiency near 85 %. increasing vanadium concentration to 500 and 1500 mg/l resulted in slightly lower faradaic efficiency near 76 %. although not favourable, the presence of vanadium species cannot actually explain the low deposition rates of iron with red mud. discussion and conclusion replacing hematite by less pure fe-containing minerals can be considered, principally in view to cheaper processes for fe electrodeposition from alkaline media, but the case study reported here clearly shows the poor potential of red muds for fe deposition. the red mud sample considered for such electrolytic treatment is first less electrochemically reactive, and with far lower current efficiency. the presence of impurities in the solids investigated seems to act mainly on the surface reactivity, as formerly reported in [18]. it could be supposed that the presence of the tiand si-oxides phases on the hematite particle surface would modify its zeta potential, then affecting the efficiency of particle adsorption on the cathode surface. 0 20 40 60 80 100 0 5 10 15 20 25 30 35 fe f a ra d a ic e ff ic ie n cy , % amount of silicoaluminate added to hematite suspension, wt% deposition at 41 a/m2(a) 0 20 40 60 80 100 0 5 10 15 20 25 30 35 fe f a ra d a ic e ff ic ie n cy , % amount of silicoaluminate added to hematite suspension, wt% deposition at 41 a/m2(a) v is co si ty , m p a s http://dx.doi.org/10.5599/jese.751 j. electrochem. sci. eng. 10(2) (2020) 95-102 electrolytic iron production 102 whereas iron ores instead of refined hematite had been shown to be of interest for fe deposition from alkaline media [11], beneficiation of red mud by an electrochemical route still appears difficult, principally because of the low reactivity of the solid particles and the huge cost for particle activation through dissolution of the refractory minerals present. acknowledgements: the phd (ab. m.) project conducted for the work presented here have been partly by french anrt agency, together with the two industrial partners involved. thanks are also due to n. maurice, p. louis and j.f. rémy (lrgp) for helping in the mineral characterization and analysis. references [1] d. oster, electrochemical reduction of suspended iron oxides suspended in water-sodium hydroxide mixture (between 25 and 140 oc). investigation of conditions for production of electrolytic iron, phd dissertation, univ. paris [in french] (1979). [2] a. allanore, h. lavelaine g. valentin, j. birat, f. lapicque, journal of the electrochemical society 154 (2007) e187-193. [3] a. allanore, experimental study of the production of electrolytic iorn in alkaline media: mechanisms of oxides reduction and design of a cell. phd dissertation, inpl nancy [in french] (2007). [4] a. allanore, h. lavelaine, g. valentin, j. birat, p. delcroix, f. lapicque, electrochimica acta 55 (2010) 4007-4013. [5] b. yuan, o. e. kingstein, g. m. haarberg, journal of the electrochemical society 156 (2009) d64-d69. [6] s. gu, x. zou, x. lu, applied mechanics in materials 595 (2014) 8-13. [7] m. siebentritt, p. volovich, k. ogle, g. lefevre, colloids and surfaces a: physicochemcial and engineering aspects 440 (2014) 197-201. [8] a. allanore, j. feng, h. lavelaine, k. ogle, journal of the electrochemical society 157 (2010) e24-e30. [9] v. feynerol, h. lavelaine, p. marlier, m. n. pons, f. lapicque, journal of applied electrochemistry 47 (2017) 1339-1350. [10] j. f. moteiro, y. a. ivanova, a. v. kovalesky, d. k. ivanou, j. frade, electrochimica acta 193 (2016) 284292. [11] v. feynerol, treatment of iron ores by alkaline leaching followed by their electrolysis in an alkaline medium. phd dissertation, university of lorraine [in french] (2018). [12] ab. maihatchi, m.n. pons, q. ricoux, f. goettmann, f. lapicque, submitted to journal of environmental management (2019). [13] w. m. mayes, i. t. burke, h. i. gomes, a. d. anton, m. molnar, v. feigl, e. ujaczki, journal of sustainable metallurgy 2 (2016) 332-343 [14] d. l. parkhurst, k. l. kipp, p. engesgaard, s. r. charlton, geochimica and cosmochimica acta 69 (2005) supplement s, a156-a156 [15] m. c. barnes, j. addai-mensah, a. r. gerson, colloids and surface a: physicochemical and engineering aspects 147 (1999) 283−295. [16] h. peng, d. seneviratne, j. vaughan, industrial and engineering chemistry research 57 (2018) 14081416. [17] l. liu, m.y. wang, z. wang, y. zhang, hydrometallurgy 146 (2014) 76-81. [18] b. rusch, k. hanna, b. humbert, colloids and surface a: physicochemical and engineering aspects 353 (2010) 172-180. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {enhanced voltammetric detection of paracetamol by using carbon nanotube modified electrode as an electrochemical sensor} http://dx.doi.org/10.5599/jese.717 29 j. electrochem. sci. eng. 10(1) (2020) 29-40; http://dx.doi.org/10.5599/jese.717 open access: issn 1847-9286 www.jese-online.org original scientific paper enhanced voltammetric detection of paracetamol by using carbon nanotube modified electrode as an electrochemical sensor madikeri m. charithra, jamballi g. manjunatha department of chemistry, fmkmc college, mangalore university constituent college, madikeri, karnataka, india corresponding author: manju1853@gmail.com; tel.: +91-08272228334 received: august 28, 2019; revised: november 25, 2019; accepted: november 25, 2019 abstract new aspects associated with electro-catalytic activity of poly(methyl orange) modified carbon nanotube paste electrode (pmmcntpe) towards the detection of paracetamol (pc) which is typically used worldwide as a pain reliever, were explored through implementation of cyclic voltammetry (cv) and differential pulse voltammetry (dpv) techniques. bare carbon nanotube paste electrode (bcntpe) was modified by methyl orange using the electropolymerizing method. the effect of ph and influence of potential scan rate were resolved by means of cv technique. it was found that under optimized experimental conditions, pmmcntpe imparts the analytical curve for pc in the concentration range of 2.0×10-6 – 5.0×10-5 m with detection limit of 3.8×10-8 m and limit of quantification of 1.2×10-8 m. the proposed sensor exhibited acceptable reproducibility, admirable stability, and adequate repeatability. the interference study of pc with dopamine (da) and folic acid (fa) showed good selectivity of the designed sensor. the feasibility of the constructed electrochemical sensor to detect pc was successfully tested in some pharmaceutical formulations. keywords voltaammetric studies; electropolymerization; methyl orange; paracetamol; detection limit introduction the quality of human health was vastly upgraded in last fifty years, what is predominantly due to well-developed medicines, education, and lifestyle. at the present time, an increase in daily use of pharmaceuticals to prolong life and avoid or cure various diseases, has additionally been recorded [1-2]. one of the most frequently used drugs is painkillers. among several painkillers, paracetamol (pc) has commonly been used for fever of viral and bacterial sources and pain relief [3]. pc (acetaminophen, n-acetyl-p-aminophenol) diminishes mild-moderate pain related with headache, migraine, toothache, backache, muscular aches, and postoperative pains [4-6]. it is the best http://dx.doi.org/10.5599/jese.717 http://dx.doi.org/10.5599/jese.717 http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 10(1) (2020) 29-40 voltammetric detection of paracetamol 30 substitute for patients who are sensitive to aspirin, which is also used as an analgesic [7-8]. normally, pc does not have any harmful side effects, as it is completely metabolized into inactive metabolites that can easily be excreted via urine [9], whereas the overdoses of pc can lead to hepatic-toxicity and nephron-toxicity [10-13]. due to its growing use, defecated pc together with pc disposal in manufacturing industrial effluents raise its presence in water source systems [14]. to control and evade these problems, the monitoring of pc becomes essential. drug analysis in biological systems provides valuable information in pharmacokinetic studies, and clinical diagnosis. the literature surveys revealed a number of already developed techniques for determining pc in different samples [15]. these techniques include titrimetric determination [16– 18], liquid chromatography-mass spectroscopy [19-20], high performance-liquid chromatography [21-22], liquid chromatography [23], capillary electrophoresis [24], chemiluminescence [25], uv–vis spectrometry [26-27], infrared spectroscopy [28], and flow [29-30] and batch [31] injection analyses. these methods offer high sensitivity, selectivity and accuracy, but difficulties of their use such as time-consuming measurement process, highly expert technicians required for the operation, and high cost instruments, limit their extensive applications. the electrochemical approach, however, provides simple, quick, and selective analyte detection with inexpensive and durable instruments. recently, electrochemical methods have attracted great attention because compared to traditional methods, they are simpler and more sensitive. also, electrochemical methods offer excellent stability and repeatability and require short analytical time. the construction of novel electrochemical sensors based on carbonaceous materials such as carbon nanotubes (cnts), graphene and graphite, has an outstanding importance in the development of electroanalytical techniques. this is due to their uncomplicated preparation, low cost, high conductivity, and biocompatibility. cnts are one of the carbonaceous materials that have already been well inspected for the design of electrochemical sensors with fabulous physical and chemical properties, including high conductivity and large surface area [32-33]. for further enhancing of electrode properties, however, electrode surfaces should be modified. to modify cnt paste electrode (cntpe), electro-polymerization is one of frequently used modification methods in which a certain thickness of polymer films on the electrode is obtained by regulating the number of cycles or potential values applied to an electrode. the modified cntpe is worthy for enhancing electrode conductivity and stimulating the transfer of electrons [34-35]. the present work targets the fabrication of the electrochemical sensor based on the bare cntpe (bcntpe) modified with methyl orange and the fabricated pmmcntpe sensor was implemented to the detection of pc by cv technique. the voltammetric response of pc at pmmcntpe showed a quasi-reversible redox behavior and remarkable improvement in current sensitivity. according to reports available in the literature, bcntpe modified with methyl orange as an electrochemical sensor was not yet utilized for the determination of pc using cv technique. the constructed sensor was successfully employed to the determination of pc, dopamine (da) and folic acid (fa) simultaneously. the electro-oxidation mechanism of pc is shown in scheme 1. scheme 1: electro-oxidation mechanism of pc m. m. charithra and j. g. manjunatha j. electrochem. sci. eng. 10(1) (2020) 29-40 http://dx.doi.org/10.5599/jese.717 31 experimental reagents and solutions pc was procured from tokyo chemical industry company ltd. (japan). cnts were purchased from the sisco research laboratories pvt. ltd. mumbai, maharasthra (od: 30-50 nm and length: 10-30 µm). silicone oil and methyl orange were obtained from nice chemicals, india. monosodium hydrogen phosphate (nah2po4) and disodium hydrogen phosphate (na2hpo4) were acquired from molychem, india. all analytical grade chemicals were used without any further treatment. stock solutions of pc (25×10-4 m) and methyl orange (25×10-4 m) were prepared in distilled water. pbs (0.2 m) solutions of proper ph for particular experiments were prepared by mixing appropriate amounts of 0.2 m nah2po4 and 0.2 m na2hpo4 and used as supporting electrolytes. all voltammetric experiments were carried out at the room temperature (25±1 °c). instrumentation the voltammetric measurements were performed by using an electrochemical working station chi-6038e (ch-instruments-usa), integrated to the personal computer. the electrochemical analyzer was fitted with an electrochemical cell consisting of three-electrode compartment that comprised bcntpe or pmmcntpe as a working electrode, a platinum wire as an auxiliary electrode, and the standard calomel electrode (sce) as a reference electrode. all the potential values were referred against sce. the eq-610 model digital ph meter was used to provide the solution with the relevant ph for the experiment. all peak current measurements were taken with the background current. the dst purse lab, mangalore university, provided the fesem characterization of the bare and modified electrodes. fabrication of pmmcntpe in the first stage of preparation of the modified electrode, bcntpe was constructed by hand mixing of cnts and silicone oil in the ratio 60:40 in an agate mortar and grinding with the help of pestle until a homogenous cnt paste was obtained. the resultant homogenous cnt paste was then firmly packed into the cavity of the teflon tube and smoothed on a tissue paper to obtain the uniform surface. a copper wire was introduced into the teflon tube to provide the electrical contact. afterwards, pmmcntpe was fabricated by electrodeposition of methyl orange on the surface of the prepared bcntpe. electrodeposition was carried out in 0.2 m pbs of ph 7.0 in the presence of 10-4 m methyl orange, for 15 continuous potential cycles between -0.6 to 1.4 v at a scan rate 0.1 v s-1. after electrochemical polymerization, the surface of the prepared pmmcntpe was thoroughly washed with the distilled water and used for the further electrochemical analysis. results and siscussion electrochemical polymerization of methyl orange on bcntpe methyl orange (mo) is a redox indicator undergoing polymerization. electrochemical polymerization of mo was carried out on the surface of bcntpe between -0.6 v and 1.4 v for 15 cycles at the scan rate 0.1 v s-1, and an adequate polymer growth was attained. figure 1 shows cyclic voltammograms (cvs) of the electrochemical polymerization of 10-4 m mo in 0.2 m pbs of ph 7.0 indicating satisfactory development of the polymer on the surface of bcntpe. two anodic peaks and one reduction peak in cvs are related to the redox behavior of mo. the electrochemical polymerization of mo has been already reported [36-38]. http://dx.doi.org/10.5599/jese.717 j. electrochem. sci. eng. 10(1) (2020) 29-40 voltammetric detection of paracetamol 32 figure 1. electrochemical polymerization of methyl orange at bcntpe surface morphology characterization of bcntpe and pmmcntpe the surface morphologies of bcntpe and pmmcntpe were probed with fesem. figures 2(a) and 2(b) present fesem images of bcntpe and pmmcntpe, respectively. the fesem image of bcntpe discloses a rough surface morphology with a tube-like cnt structure, while the smooth surface morphology in fesem image of pmmcntpe shows formation of uniform polymeric film of mo on the surface of bcntpe. therefore, by comparing fesem images of bcntpe and pmmcntpe, it can clearly be seen that electrodeposition of mo on the surface of bcnte was successfully achieved forming poly-mo film by electrochemical polymerization. figure 2. fesem magnified image of (a) bcntpe and (b) pmmcntpe electrochemical behavior of pc at bcntpe and pmmcntpe characterized by cv technique the electroactivity of pc compound on pmmcntpe has been analyzed by means of cv method. figure 3 presents cyclic voltammetric responses of 10-4 m pc at pmmcntpe (dashed line) and bcntpe (solid line) in 0.2 m pbs of ph 7.5, recorded at the scan rate 0.1 v s-1 in the potential window 0 v to 0.8 v. bcntpe offers less significant oxidation peak with a current of 24.2 µa without a reduction peak. pmmcntpe, however, shows boosted electrocatalytic behavior towards the electro-oxidation of pc with an oxidation peak at 0.357 v having current value of 72.9 µa and a welldefined reduction peak at 0.217 v with current of 41.2 µa, respectively. the redox peak currents of m. m. charithra and j. g. manjunatha j. electrochem. sci. eng. 10(1) (2020) 29-40 http://dx.doi.org/10.5599/jese.717 33 pc at pmmcntpe are higher than the redox peak currents of pc at bcntpe. also, ∆ep is 0.14 v, what suggests a quasi-reversible redox performance of the pc on the surface of pmmcntpe [39]. hence, pmmcntpe shows a good electrochemical sensing performance towards the redox behavior of pc. figure 3. cvs of pc (10-4m) at bcntpe (solid line), and pmmcntpe (dashed line) in 0.2 m pbs of ph 7.5, at the scan rate 0.1 v s-1 electrochemical response of pc on pmmcntpe the electrochemical response of pc at pmmcntpe was inspected with cv technique. figure 4 presents cvs at pmmcntpe without (curve a) and with (curve b) pc in 0.2 m pbs of ph 7.5. cvs were recorded at the scan rate 0.1 v s-1 and potential scanned between 0 v to 0.8 v. in the presence of pc, oxidation peak appeared at 0.357 v with a current of 72.9 µa while without pc in the solution, redox peaks were not observed. therefore, the above result suggests that the analyte pc could be detected by pmmcntpe by utilizing cv method. figure 4. cvs of pmmcntpe without pc (curve a) and with 10-4 m pc (curve b) in 0.2 m pbs of ph 7.5, at the scan rate 0.1 v s-1. http://dx.doi.org/10.5599/jese.717 j. electrochem. sci. eng. 10(1) (2020) 29-40 voltammetric detection of paracetamol 34 effect of ph variation the ph of the supporting electrolyte is one of the main experimental parameters affecting the voltammetric response of pc at pmmcntpe. therefore, an optimum ph is required to maximize the intensity of redox peak currents. figure 5(a) illustrates voltammetric responses of pc in 0.2 m pbs of ph varying from 6.5 to 8.0, at the scan rate 0.1 v s-1. as can be deduced from cvs in figure 5(a), the current signal of pc at pmmcntpe increases with elevating ph value from 6.5-7.5, and after that, the current signal decreases with further increase of ph value. this is clearly shown in figure 5(b) from the graphical plot of ipa. vs. ph. thus, ph 7.5 was selected as the optimum ph value for further experiments. in figure 5(c), the graph of epa vs. ph is plotted, showing a linear relationship between the oxidation peak potential and ph value in the range 6.5 to 8.0. the displayed linear regression equation is: epa / v ꞊ 0.0705 0.0453 ph (r2 ꞊ 0.9968) figure 5: (a) cvs of 10-4 m pc at pmmcntpe in 0.2 m pbs at ph varying from 6.5-8.0, at the scan rate 0.1 v s-1, (b) ipa, vs. ph dependence, (c) epa vs. ph dependence impact of scan rate figure 6(a) presents cvs of 10-4 m pc at pmmcntpe in 0.2 m pbs of ph 7.5 at various scan rates () ranging from 0.1-0.3 v s-1. the rise in the potential scan rate leads to successive increase in peak current value and at the same time, oxidation peak potential is shifted to more positive potentials. this can also be inferred from the graphical plot of ipa vs. square root of scan rate shown in figure 6(b), where the linear regression equation is represented as ipa / µa ꞊ 0.3227+ 0.0351 (v1/2) (r2 ꞊ 0.9995) linearity between ipa and square root of  indicates that the redox process of pc at the surface of pmmcntpe is diffusion controlled [40-41]. the graphical plot of log ipa vs. log v was constructed and shown in figure 6(c). good linearity was obtained and the linear fitted equation is expressed as log (ipa / µa) ꞊ 5.3634+ 0.6336 log v (r2 ꞊ 0.9959) m. m. charithra and j. g. manjunatha j. electrochem. sci. eng. 10(1) (2020) 29-40 http://dx.doi.org/10.5599/jese.717 35 the obtained slope from the graph is 0.63, which is very close to the speculative value 0.5 [42]. this suggests that the redox process of pc at pmmcntpe is a diffusion-controlled process. figure 6: (a) cvs of 10-4 m pc at pmmcntpe in 0.2 m pbs of ph 7.5, recorded at different scan rates ranging from 0.1-0.3 v s-1 (b) ipa vs. square root of scan rate (c) log ipa vs. log v dependence voltammetric determination of pc on pmmcntpe the main aim of this study is to develop the sensing electrode based on pmmcntpe to detect low pc concentrations. therefore, electro-oxidation of pc at pmmcntpe in 0.2 m pbs of ph 7.5 was carried out by varying its concentration in the range from 2×10-6 m – 1.1×10-4 m. figure 7 displays the graphical plot of ipa vs. concentration of pc, specifying that by increasing the concentration of pc peak current also increases, showing the linear relationship between ipa and concentration of pc. the linearity is obtained in the range of 2.0×10-6 m – 5.0×10-5 m, and the linear fitted equation is expressed as ipa / µa ꞊ 0.3035 + 4.0305 m (r2 ꞊ 0.9992) the limit of detection (lod) and limit of quantification (loq) were calculated by using the equation lod ꞊ 3×s/m and loq ꞊ 10×s/m, respectively, where s is the standard deviation of the blank and m is the slope of the calibration curve [43]. the calculated lod and loq are 3.85×10-8 m and 1.28×10-7 m, respectively. a comparison of linear ranges and lod values for pc determination with those obtained for some other electrodes [44-51] is listed in table 1 to assess the efficiency of the here fabricated sensor http://dx.doi.org/10.5599/jese.717 j. electrochem. sci. eng. 10(1) (2020) 29-40 voltammetric detection of paracetamol 36 figure 7. the graphical plot of ipa vs. concentration of pc table 1. comparison of linear ranges and detection limits for pc at different electrodes serial no. electrode linear working range of pc, µm detection limit of pc, µm references 1 aunp-pga/cnt 8.3-140 1.2 [44] 2 pay/nano-tio2/gce 12-120 2.0 [45] 3 pani-mwcnt 1.0-100 0.25 [46] 4 pedot-gce 1.0-100 0.40 [47] 5 chcfe-cpe 1.0-10 0.46 [48] 6 bdde 50-83 0.49 [49] 7 ru-cce 1.99-31 0.58 [50] 8 ni-al-hcf-gce 3.0-1500 0.80 [51] 9 pmmcntpe 2.0-50 0.38 present work aunp-pga/cnt: glutamic acid and gold nanoparticles on a single-walled carbon nanotube film; pay: poly(acid yellow 9); gce: glassy carbon electrode; pani-mwcnt: polyaniline–multi-walled carbon nanotubes; pedot: poly(3,4-ethylenedioxythiophene); chcfe-cpe: cobalt hexacyano ferrate carbon paste electrode; bdde: boron doped diamond electrode; ru-cce: arene-ruthenium (ii) complex carbon ceramic electrode; ni-al-hcf: hexacyanoferate(iii) intercalated ni al layered double hydroxide. repeatability, reproducibility and stability the repeatability of pmmcntpe was scrutinized by four repetitive measurements with the same electrode, resulting with the rsd of 2.2 %. the reproducibility of pmmcntpe was inspected by four repetitive measurements with different pmmcntpe electrodes, what resulted in the rsd of 3.5 %. both the repeatability and reproducibility exposed rsd less than 4 %, indicating thus good repeatability and reproducibility of the proposed sensor. the stability of the designed sensor was assessed by 30 successive potential cycles. the percentage degradation was calculated by using the following formula degradation, % ꞊ (ipn / ip1)100 where ipn and ip1 are the last and first anodic peak current, respectively [52]. the calculated percentage degradation is 94 %. this suggests that the modified electrode retained its activity even after 30 cycles. good stability, excellent reproducibility and acceptable repeatability confirm the excellent performance of the proposed sensor. m. m. charithra and j. g. manjunatha j. electrochem. sci. eng. 10(1) (2020) 29-40 http://dx.doi.org/10.5599/jese.717 37 voltammetric response of pc at pmmcntpe by dpv technique the voltammetric response of pc at pmmcntpe was also recorded via differential pulse voltammetry (dpv) technique. figure 8 depicts differential pulse voltammograms (dpvs) of the electro-oxidation of 10-4 m pc in 0.2 m pbs of ph 7.5 at the sweep rate 0.1 v s-1 on the surfaces of pmmcntpe and bcntpe. at the surface of the pmmcntpe, an enhanced current sensitivity was observed for the electro-oxidation of pc, while for bcntpe in identical conditions, inferior voltammetric response was obtained. the peak current of 71.9 µa at peak potential of 0.268 v was obtained for the electrochemical oxidation of pc at pmmcntpe. thus, good electro-catalytic activity towards the electro-oxidation of pc at pmmcntpe is confirmed via dpv approach too. figure 8. dpvs recorded for 10-4 m pc at bcntpe and pmmcntpe in 0.2 m pbs of ph 7.5, at the sweep rate 0.1 v s-1. interference study the selectivity of the designed pmmcntpe sensor was examined by carrying out the interference study through cv and dpv methods. folic acid (fa) and dopamine (da) are commonly coexisting with pc in biological samples. thus, the simultaneous detection of pc, fa, and da was performed. figure 9(a) shows cvs of pmmcntpe in 0.2 m pbs of ph 7.5 with simultaneous presence of pc (10-4 m), da (10-4 m) and fa (10-4 m). figure 9(b) shows cvs for the concentration of pc varied in the range of 10-4 -3.5 ×10-4 m and concentrations of da and fa kept constant (10-4 m). in all cases, the scan rate 0.1 v s-1 was performed within the potential window -0.2 v to 1.0 v. three wellseparated and sharp peaks for da, pc, and fa were gained in cvs of figures 9(a) and (b). furthermore, by increase of pc concentration from 10-4 to 3.5 ×10-4 m, peak current of pc also increases, what can clearly be observed in figure 9(b) and is also shown by plotting the graph of ipa vs. concentration of pc in figure 9(c). all these above-mentioned results suggest that good selectivity of the designed sensor has been achieved via cv technique. simultaneous detection of pc, da, and fa was also performed by means of dpv technique. dpv of simultaneous detection of pc (10-4 m), da (10-4 m) and fa (10-4 m) in 0.2 m pbs of ph 7.5 is displayed in figure 10(a), where distinct peaks for da, fa and particularly for pc can be observed. figure 10(b) depicts dpvs for the variation of pc concentration from 10-4 to 3.5 ×10-4 m without changing the concentration of da and fa equal to 10-4 m for each. the graphical plot of ipa vs. concentration of pc shown in figure 10(c) implies that for the increasing concentration of pc, peak http://dx.doi.org/10.5599/jese.717 j. electrochem. sci. eng. 10(1) (2020) 29-40 voltammetric detection of paracetamol 38 current also increases. this approves that the fabricated sensor detects pc in the presence of interference compounds da and fa. figure 9. cv at pmmcntpe in 0.2 m pbs of ph 7.5, recorded at the scan rate 0.1 vs-1 in presence of: (a) pc (10-4 m ), da (10-4 m ) and fa (10-4 m), (b) da (10-4 m), fa (10-4 m), and pc of varying concentration from 10-4-3.5 ×10-4 m, (c) ipa vs. concentration of pc. figure 10. dpvs of pmmcntpe in 0.2 m pbs of ph 7.5, recorded at the scan rate 0.1 v s-1 for simultaneous detection of pc, da and fa at: (a) pc (10-4 m), da (10-4 m) and fa (10-4 m) (b) da (10-4 m), fa (10-4 m) and pc varied from 10-4-3.5 ×10-4 m, (c) graphical plot of ipa vs. concentration of pc real sample analysis the workability of the constructed pmmcntpe sensor for the analysis of pc was assessed by detecting pc in some pharmaceutical formulations, such as paracetamol and dolo tablets that were procured from the local drugstore. the tablets were weighed and ground into a fine powder using m. m. charithra and j. g. manjunatha j. electrochem. sci. eng. 10(1) (2020) 29-40 http://dx.doi.org/10.5599/jese.717 39 the pestle in a mortar. solutions were prepared from powdered tablets and analyzed by cv technique. the cvs were documented in 0.2 m pbs of ph 7.5 at the sweep rate of 0.1 v s-1. a good recovery ranges from 95 to 103 % was accomplished by the developed method. conclusions in this study, pmmcntpe was prepared as electrochemical sensor which could serve for a sensitive and selective determination of pc. the fabricated pmmcntpe disclosed good electrocatalytic activity towards the oxidation of pc with cv and dpv techniques. the pmmcntpe sensor boosted the analytical sensitivity of pc by yielding low detection limit of 3.8×10-8 m. the sensor unveiled an excellent stability, repeatability, and reproducibility. in addition, the fabricated sensor was implemented to the simultaneous detection of pc, da, and fa, showing significant 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[52] j. g. manjunatha, sensing and bio-sensing research 16 (2017) 79-84. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {a sensitive electrochemical sensor based on polypyrrole/electrochemically reduced graphene oxide for the determination of imidacloprid} http://dx.doi.org/10.5599/jese.630 143 j. electrochem. sci. eng. 9(3) (2018) 143-152; http://dx.doi.org/10.5599/jese.630 open access: issn 1847-9286 www.jese-online.org original scientific paper a sensitive electrochemical sensor based on polypyrrole/electrochemically reduced graphene oxide for the determination of imidacloprid chenglong chen, zhen han, wu lei, yong ding, jingjing lv, mingzhu xia, fengyun wang, qingli hao school of chemical engineering, nanjing university of science and technology, nanjing, 210094, china corresponding authors: leiwuhao@njust.edu.cn; tel.: +86-25-84315943; fax: +86-25-84315190 received: october 30, 2018; revised: february 7, 2019; accepted: february 7, 2019 abstract the glassy carbon electrode (gce) was modified by electrochemically reduced graphene oxide (ergo) and polypyrrole (ppy) prepared by simple cyclic voltammetry (cv) electropolymerization. the ppy/ergo modified electrode (ppy/ergo/gce) was used as a platform of electrochemical sensor to detect imidacloprid (imi) insecticide. cv and differential pulse voltammetry (dpv) were chosen as the methods to investigate of the electrochemical behavior of imi on ppy/ergo/gce surface. scanning electron microscopy (sem) and raman spectra were utilized to describe the morphology and structure of the modified electrode. experimental parameters were optimized, such as the number of polymerization cycles, scan rate and the ph value of electrolyte. under the optimized conditions, when the concentration of imi was in the range of 1-10 μm and 10-60 μm, the increase of reduction peak current was linear with the concentration of imi, and the low detection limit was found to be 0.18 μm (s/n = 3). results showed that ppy/ergo/gce demonstrated satisfactory reproducibility and stability, and has great potential in actual sample testing. keywords electro-polymerization; modified electrode; imidacloprid insecticide; electrochemical behavior; actual samples introduction as a widespread used neonicotinoids insecticide, imidacloprid (1-[(6-chloro-3-pyridinyl) methyl]n-nitro-2-imidazolidinimine) has great effective on the pest control by directly acting on the central nervous system of insects [1,2]. even though imi is low toxic to mammals, it can move off-site with the underground water and persist as long as several years [3]. the potential contamination and the http://dx.doi.org/10.5599/jese.630 http://dx.doi.org/10.5599/jese.630 http://www.jese-online.org/ mailto:leiwuhao@njust.edu.cn j. electrochem. sci. eng. 9(3) (2018) 143-152 sensor for the determination of imidacloprid 144 high risk to bees and aquatic invertebrates [4] make the determination of imi an urgent task since 2013. among diverse analytical approaches [5-8], the electrochemical method is considered highly efficient, mostly due to its simplicity, sensitivity and feasibility. most of the reported imi electrochemical sensors have been focused on the fabrication of the modified electrocatalytic nanomaterials and comprehensive synthesis procedures, such as preparation of gold nanoparticles/paminothiophenol/β-cyclodextrin polymer/reduced graphene oxide or imprinted poly(ophenylenediamine) membranes at reduced graphene oxide [9,10]. the sensing platform based on the simple electro-polymerization, however, has rarely been mentioned. since graphene was firstly reported in 2006 [11], it has attracted great and increasing interest in various areas for its unique mechanical, electronic, thermal properties, large specific surface area and other remarkable characteristics [12]. graphene oxide (go) is one of the most attractive derivatives of graphene, which is highly water-soluble and contains large number of oxygencontaining groups (epoxy, carboxyl, hydroxyl, etc.) on its surface and edges [13,14]. graphene can be chemically reduced from go via thermal [15,16] or electrochemical [17] reduction, and the product is called reduced graphene oxide (rgo). rgo is also widely used to modify electrodes. it can be used as an electrode modification material alone, or as a component of a composite material in a sensor preparation, what is all due to its unique two-dimensional structure and rich oxygencontaining functional groups [18]. compared with the chemical method, the electrochemical reduction process forming electrochemically reduced graphene (ergo) is quick, simple and nontoxic [19]. likewise rgo, the surface charge of ergo is partially negative in neutral electrolyte because of the oxygen-containing functional groups [20]. however, the molecular imi is also of negative surface charge, what would bring electrostatic repulsion and hinder the adsorption and interaction between imi and ergo modified electrode. to overcome this drawback, polypyrrole (ppy) was introduced into the electrode modification due to its positively charged nitrogen atoms [21,22]. the positively charged nitrogen atoms would provide interaction sites to enhance the interaction of imi through electrostatic attraction. besides, ppy shows remarkable sensing properties for its unique π-conjunction structure, perfect conductivity and abundant active sites. moreover, the amine group (–nh–) on the pyrrole ring may lead to enhancement of the sensing performance [23,24]. however, as far as we know, there is no any report in the literature about electrochemical sensor for determination of imidacloprid based on ppy/ergo composites. in this study, a simple electropolymerization method has been used to prepare the glassy carbon electrode modified with ppy/ergo composite for the detection of imi. ergo doped into ppy would not only compensate low conductivity of ergo, but also remarkably promote the electron-transfer ability. cv and dpv techniques have been used to investigate electrochemical behavior of imi at ppy/ergo/gce. experimental reagents and apparatus imi was purchased from aladdin reagents (shanghai aladdin biochemical technology co., ltd.). all reagents were of analytical grade and used without further purification. milli-q water purifying system (18 mω cm) was used for the preparation of deionized water which was used in all experiments. citric acid/disodium hydrogen phosphate buffer solution (cps) and sodium dihydrogen phosphate/disodium hydrogen phosphate buffer solution (pbs) were employed as electrolyte in the electrochemical characterizations. all experiments were performed at room temperature. chenglong chen et al. j. electrochem. sci. eng. 9(3) (2018) 143-152 http://dx.doi.org/10.5599/jese.630 145 electrochemical characterizations were carried out with a three-electrode system by a chi660d electrochemical workstation (chenhua instrument. shanghai co., ltd., china). bare and modified gce were used as the working electrode, while a saturated calomel electrode (sce) and a platinum wire electrode were employed as reference and auxiliary electrodes, respectively. sem (japan electron optics laboratory co., ltd) characterization was carried out by jeol-6380lv, and raman spectrum were conducted on renishaw invia raman microprobe using a 514.5 nm argon ion laser. preparation of ppy/ergo/gce as reported in our previous works [25], go was prepared from graphite powder via a modified hummer’s method [26]. the gce was polished on the fur with an alumina powder/water and then ultrasonically cleaned in deionized water before modification. before the polymerization, the homogenous go-pyrrole dispersion was prepared with 1 ml go (5 mg ml-1), 20 μl pyrrole and 5 ml water by 30 min ultrasonication. go doped ppy was electro-polymerized onto the electrode surface through cv method in the go-pyrrole dispersion with potential scanning between -0.2 and 1.0 v at 100 mv s-1. then, go was electrochemically reduced in 0.1 m pbs (ph 7.0) by performing 5 cycles of cv in the potential range from 0 to -1.7 v at 100 mv s-1. for comparison, ppy modified gce (ppy/gce) was prepared by the same procedure in acetonitrile solution containing 0.1 m liclo4 and 0.1 m pyrrole solution without go. ergo modified gce (ergo/gce) was obtained from go/gce by the same procedure as described above. electrochemical measurements electrochemical behavior of imi at various electrodes was characterized by cv and dpv methods in cps (dpv conditions: potential increase, 0.004 v; amplitude, 0.05 v; pulse width, 0.05 s; pulse interval, 0.2 s). electrochemical impedance spectroscopy (eis) was used for the electron transfer property characterization of the modified electrodes in 0.1 m kcl solution with 5 mm [fe(cn)6]3-/4 (frequency range: 106 0.1 hz, amplitude: 5 mv). possibly dissolved oxygen was expelled by nitrogen flow before every experiment. results and discussion characterizations of ppy/ergo/gce the light yellow ppy/go film was slowly deposited on the gce through cv method as previously described. the electropolymerization cv curves at gce in the go-pyrrole dispersion are presented in figure 1. as shown in figure 1, during 10 cycles of electro-polymerization, the anodic current increased while the initial oxidation potential continuously shifted negatively, indicating the successful growth and thickening of the ppy/go film. the morphologies of the coated ppy and ppy/ergo film on gces are shown in figure 2 (a and b). it could be seen from the sem images that ppy film deposited on the surface of the bare gce has a relatively compact nanoparticle structure and some obvious ridges (fig. 2a), while ppy/ergo composite shows different morphology with ppy nanograins growing on the surface of ergo sheets (fig. 2b). ppy/ergo sheets are randomly arranged on the surface of gce to form a large amount of nanopores and micropores, which can definitely lead to a larger specific surface area (fig. 2b). favourable combination of ppy and ergo and strong affinity of ppy/ergo to the surface of gce are beneficial to the diffusion of imi from the solution to the electrode surface, and also facilitate fast exchange and transfer of electrons. http://dx.doi.org/10.5599/jese.630 j. electrochem. sci. eng. 9(3) (2018) 143-152 sensor for the determination of imidacloprid 146 figure 1. cv curves at gce in the go-pyrrole dispersion (20 μl pyrrole and 1 mg ml-1 go) at a scan rate of 100 mv s-1 in the potential range between -0.2 and 1.0 v figure 2. sem of ppy (a) and ppy/ergo (b); (c) raman spectra of ergo, ppy/ergo and ppy ; (d) nyquist plots in 0.1 m kcl containing 5.0 mm [fe(cn) 6] 3−/4− recorded at bare gce, ppy/ergo and ppy/go modified gces (initial potential: 0.18v) raman spectra of pure ppy, ergo and ppy/ergo are demonstrated in figure 2c. for pure ppy, two strong peaks at 1335 and 1567 cm-1 represent the ring stretching mode and the backbone stretching mode of c=c bonds in ppy, respectively. peaks at 971 and 1051 cm-1 correspond to the ring deformation and the c-h in-plane deformation of ppy, respectively. as for ergo, the characteristic d and g bands could be observed at 1347 and 1584 cm-1, respectively [27,28]. d band chenglong chen et al. j. electrochem. sci. eng. 9(3) (2018) 143-152 http://dx.doi.org/10.5599/jese.630 147 exhibits the sp3 defects in the sp2-hybridized carbon network, while g band indicates doubly degenerate optical phonon of e2g symmetry [29]. compared with the spectra of ppy and ergo, d and g bands of ppy/ergo shift to 1350 and 1590 cm-1 respectively, and the characteristic peaks of ppy appear at 983 and 1049 cm-1 respectively. these shifts indicate an interaction between ppy and ergo sheets. therefore, the raman spectrum of ppy/ergo composite reveals the presence of ergo-doped ppy structures. this is in good agreement with raman spectra results reported elsewhere [30]. electrochemical characteristics of ppy/ergo modified gce eis was utilized for the characterization of electron transfer capability. figure 2d shows the nyquist plots of the bare gce, ppy/go/gce and ppy/ergo/gce in 0.1 m kcl with 5 mm [fe(cn)6]3−/4−. large well-defined semicircle can be observed at higher frequencies in the nyquist plot of bare gce, while the plot of ppy/go/gce shows a smaller semicircle, revealing the smaller interface impedance value of ppy/go/gce than gce. this could be due to the fact that large surface area and high conductivity of ppy/go promoted electron transfer. for the electrode modified with ppy/ergo, a much smaller semicircle can be observed in figure 2d, suggesting further decrease of interface impedance after reduction of ppy/go. this indicates that ergo plays a significant role in improving the electron-transfer process, what results in a superior electrochemical performance of ppy/ergo/gce over ppy/go/gce and gce. electrochemical behavior of imi at ppy/ergo modified gce electrochemical behavior of imi at differently modified gce were investigated with cv and dpv. figure 3 shows cvs (a) and dpvs (b) at ppy/ergo/gce (a) and bare gce (b) in cps buffer solution containing 0.1 mm imi. for comparison, cvs and dpvs of ppy/ergo/gce in the blank cps are also shown in figures 3a(c) and b(c) (scan rate: 100 mv s-1; ph: 6.7). figure 3. cvs (a) and dpvs (b) of ppy/ergo/gce (a, c) and bare gce (b) in presence (a, b) and absence (c) of 0.1 mm imi in cps. scan rate: 100 mv s-1, ph: 6.7. as demonstrated in figure 3, under given experimental conditions, no reduction peaks are observed for ppy/ergo/gce in the blank cps (fig. 3c). in presence of imi, however, for both bare gce (fig. 3b) and ppy/ergo/gce (fig. 3a), there is only one single reduction peak in cv and dpv, respectively. this suggests that nitro groups of imi exhibit completely irreversible reduction process at the gce and its modified electrodes. http://dx.doi.org/10.5599/jese.630 j. electrochem. sci. eng. 9(3) (2018) 143-152 sensor for the determination of imidacloprid 148 irrespective of either cv or dpv applied, imi reduction peak current values at ppy/ergo/gce are prominently higher than those at the bare gce (figs. 3a and 3b). at the same time, compared with the bare gce, peak potential values are positively shifted at ppy/ergo/gce (from -1.18 to -1.10 v in cv; from -1.12 to -1.03 v in dpv). both suggest the satisfactory electrocatalytic activity of ppy/ergo towards the reduction of imi and high conductivity of the composite film. these electrosensing improvements can be attributed to a good combination of ppy with ergo, and large surface area of the film with a porous structure. the effect of polymerization cycles the effect of the number (2, 5, 10, 15, 20) of polymerization cycles on the determination of imi at ppy/ergo/gce was optimized by cvs and dpvs in cps ph 6.7 containing 0.1 mm imi. as shown in figure 4 (a and b), the highest peak current value was obtained with ten polymerization cycles both in cv and dpv. with the further increase of the cycle number, the peak current values decreased apparently. the growing ppy/ergo film would conversely hinder the electron transport as well as the analyte adsorption. therefore, the optimized number of polymerization cycles is 10. figure 4. cvs of 0.1 mm imi in ph=6.7 cps at ppy/ergo/gce with various polymerization cycles (2, 5, 10, 15, 20). scan rate: 100 mv s-1. the effect of scan rate cvs of 0.1 mm imi at ppy/ergo/gce obtained with the scan rate (ν) ranging from 10 to 200 mv s-1 in cps ph 6.7 are presented in figure 5a. as the scan rate increased, the reduction peak currents of imi increased, while the peak potentials shifted negatively. the linear relationships between scan rate and the peak potential/current values are further depicted in figure 5b-5c. figure 5b demonstrates the linear relationship between the cathode peak current values of imi and the square root of the scan rate (v1/2) within the range of 10 to 50 mv s-1. the obtained regression equation of ipc (µa) vs. v 1/2 (mv s-1)1/2 is: ipc = 1.234 v1/2+ 0.6415 (r = 0.9993) (1) figure 5b and eqn. (1) indicate that the reduction reaction of imi on ppy/ergo/gce is a typical diffusion-controlled process at low scan rates [31]. for high scan rates (50-200 mv s-1), the peak current values of imi are linear with the scan rate (fig. 5c). the generated regression equation between ipc / µa and v / mv s-1 is: ipc = 0.05043 v + 7.089 (r = 0.9989) (2) chenglong chen et al. j. electrochem. sci. eng. 9(3) (2018) 143-152 http://dx.doi.org/10.5599/jese.630 149 figure 5c and eqn. (2) suggest that at the high scan rate, the reduction reaction of imi is a typical adsorption-controlled process. figure 5. cvs acquired at a ppy/ergo/gce in ph 6.7 cps containing 0.1 mm imi at different scan rate (ν) of 10 -200 mv s-1 (a) and the influence of scan rate on peak current (b, c). the effect of ph on cv behavior of imi influence of ph on the cv behavior of 0.1 mm imi on ppy/ergo/gce was also studied and the results are presented in figure 6. as ph was increased from 2.7 to 6.7, the reduction peak current values increased accordingly (fig. 6a). when ph increased further up to 7.2, the reduction peak current value showed an obvious drop (fig. 6b). therefore, ph 6.7 was chosen as the optimized ph value in every electrochemical measurement. for the peak potential of the reduction peak, the change in potential was linear with the ph value (fig. 6c). the linear regression equation between epc(v) and ph was obtained as: epc = -0.03078 ph 0.9524 (r = 0.9946) (3) figure 5c and eqn. (3) suggest participation of protons during the electro-catalyzed reduction reaction of imi. the slope of the regression line is equal to -2.303mrt/αnf, in which, m and n are the number of protons and electrons, respectively. therefore, m/αn was calculated as 0.52 (in the reaction, α = 0.5 and n = 4 [31]). therefore, the number of the involved protons (m) is 1.2, which is approximately equal to 1. thus, the electro-catalyzed reduction of imi at ppy/ergo/gce is fourelectrons and one-proton process. http://dx.doi.org/10.5599/jese.630 j. electrochem. sci. eng. 9(3) (2018) 143-152 sensor for the determination of imidacloprid 150 figure 6. the influence of cps ph on the reduction peak current (a, b) and peak potential (a, c) of 0.1 mm imi at an optimized ppy/ergo/gce. dpv determinations of imi enhancement of the electrochemical signal observed at ppy/ergo/gce compared with the bare gce and ppy modified gce, makes ppy/ergo to be an attractive electrocatalytic candidate for the determination of imi. dpv technique, that is usually considered to have a higher sensitivity than cv, was utilized here for the quantitative determination of imi under optimized conditions. figure 7a shows well-defined dpv reduction peaks at ppy/ergo/ergo in cps, ph 6.7 containing different concentrations of imi. due to weak adsorption of imi on the surface of the modified electrode, the reduction peak current values increased when imi concentration gradually increased from 1 μm to 60 μm. moreover, the peak current values were found proportional to the concentration of imi. finally, two linear relationships between dpv peak current value and imi concentration were extracted, covering two linear ranges of 1-10 μm and 10-60 μm, respectively. the corresponding linear equations between ipc(µa) and c (µm) are: ipc = 0.1265 c + 0.3279 (r = 0.9996) (4) ipc = 0.03983 c + 1.192 (r = 0.9996) (5) the low detection limits are by eqns. (4) and (5) determined as 0.18 µm and 1.2 µm (s/n=3), respectively. as compared with other literature work presented in table 1, here proposed dpv sensing method with ppy/ergo/gce shows relative low detection limit, demonstrating thus clearly high potential of this electrode for sensitive determination of imi. it is apparent from figure 7 and equations （4）and（5）that there is a significant change in slopes of linear lines at a single specific concentration, and the slope at high concentrations is much smaller than at low concentrations. we speculate that this phenomenon is due to the poor conductivity of imi, and the accumulation of imi at high concentrations hinders the exchange of electrons between itself and the electrodes. in this way, the sensitivity of the sensor is reduced, so the slope at high concentration is lower than that at low concentration. table 1 comparison of electrochemical sensors for imi electrode method lod, µm references agnds/gns/gce dpv 0.81 [32] nagnf/ntio2nf/gce cv 0.63 [10] poly(o-phenylenediamine)-rgo/gce lsv 0.40 [33] poly(carbazole)/chemically reduced graphene cv 0.22 [25] oxide/gce dpv 0.44 [25] β-cyclodextrin polymer/gce cv 0.10 [34] go/gce cv 0.36 [35] ppy/ergo/gce dpv 0.18 this work chenglong chen et al. j. electrochem. sci. eng. 9(3) (2018) 143-152 http://dx.doi.org/10.5599/jese.630 151 figure 7. dpvs (a) of imi with different concentrations (1-60 µm) at ppy/ergo/gce in cps solution (ph 6.7). (b) relationship between the dpv peak current and imi concentration reproducibility, repeatability and interference for the sake of demonstrating the accuracy and practicability of the method proposed, the reproducibility of the as-fabricated ppy/ergo/gce was investigated by comparing the peak current of ten experiments performed successively in cps containing 0.1 mm imi. the relative standard deviation (rsd) was 2.2 %, which reflected that this method has the outstanding reproducibility. under optimal conditions, after storing the electrode at room temperature for seven days, we did not notice any obvious change in the peak current. the interference test was performed in the presence of the inorganic ions and some pesticides, for instance 0.1 m k+, na+, ce2+, zn2+, pb2+, cl−, no3−, so42−, carbendazim, trichlorfon and glyphosate. the reduction signal did not change considerably, indicating that all these inorganic ions and pesticides have no effect on the determination of imi and illustrating that the electrochemical sensor based on ppy/ergo/gce exhibits high selectivity. conclusion in this work, ppy/ergo modified gce was successfully used as a platform for electrochemical sensing of imidacloprid insecticides. the recorded cvs showed a remarkably enhanced electrochemical response of imi at ppy/ergo/gce. under the optimized conditions and using dpv technique, two linear ranges for imi determination were obtained between 1.0-10 µm and 10 60.0 µm respectively. the limit of detection was 0.18 µm. these results indicated that ppy/ergo is good functional material, suitable for sensing of imi with high sensitivity, selectivity, and stability. furthermore, here proposed sensing method is rapid, reliable and easy for manipulation. therefore, the proposed strategy based on a ppy/ergo modified electrode could be a promising approach for quantitative detection of imidacloprid insecticide. acknowledgements: the work was supported by the national natural science foundation 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zhang, j. zhou, j. xie, g. diao, electrochimica acta 108 (2013) 1-9 [35] w. lei, z. han, w. si, q. hao, y. zhang, m. xia, f. wang, chemelectrochem 1 (2014) 1063-1067 ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {mild steel corrosion inhibition by synthesized 7-(ethylthiobenzimidazolyl) theophylline} http://dx.doi.org/10.5599/jese.952 97 j. electrochem. sci. eng. 11(2) (2021) 97-106; http://dx.doi.org/10.5599/jese.952 open access: issn 1847-9286 www.jese-online.org original scientific paper mild steel corrosion inhibition by 7-(ethylthiobenzimidazolyl) theophylline n’guessan yao silvère diki1,, nagnonta hippolyte coulibaly2, kadjo françois kassi3 and albert trokourey1 1laboratoire de constitution et réaction de la matière, ufr ssmt, université félix houphouëtboigny, 22 bp 582 abidjan 22, côte d’ivoire 2ufr sciences et technologies, université de man, bp 20 man, côte d’ivoire 3laboratoire de thermodynamique et de physico-chimie du milieu, ufr sfa, una, côte d’ivoire corresponding author: dickiensil2@gmail.com received: january 11, 2021; revised: april 3, 2021; accepted: april 7, 2021 abstract the corrosion inhibition of mild steel by 7-(ethylthiobenzimidazolyl) theophylline (7-etbt) in 1 m hcl medium was investigated through weight loss and tafel polarization techniques within a temperature range of 298 to 318 k. the inhibition efficiency depends on the concentration of 7-etbt and reaction system temperature. the maximum inhibition efficiency values of 90.73 and 87.06 %, respectively, were estimated using both weight loss and tafel polarization techniques at 298 k. the results suggest spontaneous and predominant physical adsorption of 7-etbt on the metal surface which obeys langmuir isotherm model. furthermore, tafel polarization method revealed that 7-etbt is a mixed-type inhibitor. potentiodynamic polarization results are in accordance with weight loss data to a good extent. keywords organic inhibitor; inhibition efficiency; weight loss, tafel polarization; physical adsorption introduction corrosion of metallic materials in acidic solution is of huge concern and its inhibition has been deeply investigated [1,2]. hydrochloric acid is widely used in various technological processes in industry including pickling baths, extraction and processing of oil and gas and in other chemical and petrochemical industries [3,4]. corrosion of mild steel is important and expensive problem in the industries and represents a significant portion of losses as a result of lost production, inefficient operation, and high maintenance cost. it has been found that one of the best methods for protecting metals against corrosion involves the use of inhibitors which are mostly organic compounds. organic http://dx.doi.org/10.5599/jese.952 http://dx.doi.org/10.5599/jese.952 http://www.jese-online.org/ mailto:dickiensil2@gmail.com j. electrochem. sci. eng. 11(2) (2021) 97-106 mild steel corrosion inhibition 98 compounds contain electronegative functional groups and π-electrons in triple or conjugated double bonds that slow down the corrosion rate [5-7]. many organic compounds have been investigated as corrosion inhibitors for different types of metals including mild steel [8-10]. with increased awareness towards environmental pollution and control, the search for less toxic and environmentally friendly corrosion inhibitors is becoming increasingly important. thus, researchers have focused their works on several synthesized compounds, for which non-toxicity tests are carried out during their synthesis [11-14]. a detailed literature review showed no data available regarding the behavior of 7-etbt as corrosion inhibitor for mild steel. herein, for the first time, we report the use of 7-etbt as a corrosion inhibitor for mild steel in one molar hydrochloric acid medium. the aim of the present paper is to evaluate the behavior of the studied inhibitor against mild steel corrosion in 1m hcl, by analyzing both thermodynamic data and potentiodynamic polarization parameters. the chemical structure of 7-etbt is shown in scheme 1. scheme 1. chemical structure of 7-(ethylthiobenzimidazolyl) theophylline experimental mild steel specimen and reagents the test samples used in corrosion studies were cut from a piece of mild steel into coupons of dimensions 1×1×1 cm. mild steel was of the following chemical composition (wt.%) (c: 0.17, mn: 0.03, si: 0.14, s: 0.028, p: 0.033 and fe: balance). 7-etbt, a beige color chemical with molecular formula c16h16 n6o2s, was provided by the laboratory of organic chemistry and natural substances, felix houphouet-boigny university. its molecular structure was identified by 1h nmr and 13c nmr spectroscopy: -rmn 1h (400 mhz, dmso-d6, δ / ppm): 3.22 (s, 6h, -ch3-); 3.74 (dd, j=6.6, 5.1 hz, 2h, n-ch2-); 4.60 (dd, 2h, -ch2-s); 7.04 (dd, 2h, har); 7.32 (dd, 2h, har); 7.96 (s, 1h, n=ch-). -rmn 13c (dmso, δ / ppm): = 27.56 (ch3-); 29.22 (ch3-); 32.00 (n-ch2-); 46.49 (-ch2-s); 150 (car); 149.34 (car); 148.34 (car). the corrosive aqueous solution of 1 m hcl was prepared by dilution of analytical grade 37 % hcl purchased from sigma-aldrich chemicals with bi-distilled water. acetone of purity 99.5 % was also purchased from sigma-aldrich chemicals. 1 m hcl solutions without or with different quantities of 7-etbt, ranging from 0.02 to 2 mm were then prepared. weight loss measurements each test was conducted in 50 ml of aerated and unstirred 1 m hcl solution without or with desired concentrations of the tested inhibitor for 1 h immersion time at different temperatures. temperature was regulated by a water-controlled thermostat (memmert, precision ±0.5 oc). prior experiments, mild steel coupons were polished with different emery papers, washed thoroughly n’. y. s. diki et al. j. electrochem. sci. eng. 11(2) (2021) 97-106 http://dx.doi.org/10.5599/jese.952 99 with bi-distilled water, degreased and dried with acetone. the obtained samples were then kept in an oven (memmert) at 70 °c and weighed accurately. after 1 h of immersion in the corrosive media with or without 7-etbt, the specimens were carefully washed in bi-distilled water, dried and reweighed accurately. triplicate experiments were performed in each case and the mean value of the weight loss was reported. from the weight loss measurements, the corrosion rate (cr), degree of surface coverage ( ), and inhibition efficiency (ie) were calculated using following equations [15]: m cr st  = (1) 0 0 c cr cr r  = (2)       0 0 = 100 cr cr ie cr (3) where cr0 and cr (expressed in mg h-1 cm-2) are respectively corrosion rate without and with 7-etbt, m is weight loss, s is total surface of steel specimen and 𝑡 is immersion time. electrochemical measurements electrochemical experiments were carried out at the ambient temperature (298 k) in a conventional three-electrode glass cell of 100 ml cell capacity. mild steel specimen served as the working electrode (we) having the exposed surface of 1.000±0.001 cm2, with the rest being covered by a commercially available polymeric resin. a platinum electrode and a saturated calomel electrode (sce) coupled to a fine luggin capillary served as the counter (ce) and reference (re) electrode, respectively. all potential values given in this study are referred to sce reference. solutions were aerated, unstirred and prepared like these used for weight loss measurements. all measurements were performed with the autolab pgstat 20 potentiostat (ecochemie, utrecht netherlands) controlled by gpes 4.4 software. origin 6.0 software was used for plotting, graphing and fitting data. before each measurement, the mild steel working electrode was prepared as described for weight loss measurements and immersed in the test solution for 30 min to establish steady state at the open circuit potential (eocp). tafel polarization curves were carried out by linear potential sweeps at the scan rate of 1 mv/s, applied from the cathodic potential of –1000 mv to anodic potential of +1000 mv with respect to eocp. each experiment was repeated at least three times using always a fresh mild steel electrode. only well reproducible results were reported. the inhibition efficiency (ie) was calculated according to: 0 inh corr corr 0 corr 100 j j ie j       − = (4) where jcorr0 and jcorrinh are referred to the corrosion current density in solutions without and with the inhibitor, respectively. results and discussion weight loss measurement the weight loss method has found broad practical application. a major advantage of this method is its relative simplicity and availability. in addition, this method uses a direct parameter for the quantitative evaluation of corrosion, the loss in weight of the metal [16]. the data obtained from weight loss measurements for the corrosion rates of mild steel samples in 1m hcl solutions with and without 7-etbt at different temperatures are presented in figures 1 and 2. http://dx.doi.org/10.5599/jese.952 j. electrochem. sci. eng. 11(2) (2021) 97-106 mild steel corrosion inhibition 100 figure 1. evolution of corrosion rate of mild steel with temperature at different concentrations of 7-etbt as displayed in figure 1, the curves showed that corrosion rate of mild steel in the studied medium increases with increasing temperature. however, the increase in 7-etbt concentration (from 0.02 to 2 mm) is accompanied by decrease of corrosion rate over the temperature range (298-318 k). these results highlight the fact that adsorption of inhibitor on the metallic surface increases with increase in inhibitor concentration. the evolution of inhibition efficiency calculated using equation (3) versus temperature is illustrated in figure 2. figure 2. inhibition efficiency vs. temperature for different concentrations of 7-etbt as shown in figure 2, the inhibition efficiency decreases with the rise in temperature for the inhibitor concentration range studied. according to the literature [17], the decrease in inhibition efficiency with increase of temperature indicates that the process of adsorption of the inhibitor on the corroding metal surface is physisorption. this effect can be also attributed to the increase in the solubility of the protective films and of any reaction products precipitated on the surface of the metal [18], thereby increasing the metal surface exposure to corrosive attack when the temperature rises. all these observations indicate that the studied inhibitor acts as an effective inhibitor of mild steel corrosion over the concentration range studied. this behavior could be explained by the formation of a barrier which separates the metal from the acidic solution. adsorption considerations attempts were made to fit degree of surface coverage ( ) values to three isotherms including langmuir, el-awady and flory-huggins. the best fit according to the strong correlation (r2 > 0.995) and slopes of straight lines close to unity over the temperature range studied were obtained with langmuir adsorption model. in the rearranged form, langmuir adsorption isotherm is expressed as: ie / % n’. y. s. diki et al. j. electrochem. sci. eng. 11(2) (2021) 97-106 http://dx.doi.org/10.5599/jese.952 101 inh inh ads 1c c k = + (5) where kads is adsorption equilibrium constant. the curve of cinh / as a function of cinh (inhibitor concentration) is shown in figure 3. figure 3. langmuir adsorption isotherm for 7-etbt on mild steel in 1m hcl at different temperatures regression parameters related to langmuir adsorption isotherm are gathered in table 1. table 1. regression parameters of langmuir adsorption isotherm of 7-etbt on mild steel surface in 1 m hcl t / k r2 slope intercept 298 0.9960 1.1080 0.0351 303 0.9955 1.1396 0.0370 308 0.9958 1.1653 0.0376 313 0.9953 1.1911 0.0407 318 0.9951 1.2171 0.0424 in order to assess the strength of interactions between inhibitor molecules and the metal surface, the values of adsorption equilibrium constant kads were calculated using intercepts of straight lines on cinh /-axis. the calculated adsorption equilibrium constant was related to the standard free energy of adsorption δg0ads by the following equation [19]: g0ads = -rt ln (55.5 kads) (6) in equation (6) [20], 55.5 is concentration of water in mol l-1, t is absolute temperature while r is universal gas constant. the values of g0ads and other adsorption thermodynamic functions are summarized in table 2. table 2. adsorption thermodynamic functions of 7-etbt on mild steel surface in 1m hcl t / k kads / m -1 g0ads / kj mol-1 h0ads / kj mol-1 s0ads / kj mol-1 298 28490.03 –35.35 –7.52 93.40 303 27027.03 –35.81 308 26595.74 –36.36 313 24570.02 –36.74 318 23584.91 –37.22 in our work, the computed values of g0ads for 7-etbt were ranging from –35.35 to –37.22 kj mol-1 which indicated that adsorption of 7-etbt on the mild steel surface may involve physisorption as well as chemisorption [21,22]. decrease in values of kads with increasing temperature suggest that http://dx.doi.org/10.5599/jese.952 j. electrochem. sci. eng. 11(2) (2021) 97-106 mild steel corrosion inhibition 102 desorption process is enhanced with increase in temperature [22]. the large negative values of g0ads reveal that the adsorption process occurs spontaneously and the adsorbed layer on the metallic surface is highly stable [23]. the standard adsorption enthalpy change (h0ads) and the standard adsorption entropy change (s0ads) are correlated with the standard gibbs free energy (g0ads) according to the following equation: g0ads = δh0ads tδs0ads (7) figure 4 presents plots of g0ads versus temperature. the values of h0ads and s0ads obtained by linear regression are listed in table 2 h0ads value is negative, showing an exothermic process. according to the literature [24], an exothermic process means either physisorption or chemisorption. therefore this result confirms that the process of adsorption is both physisorption and chemisorption [25]. s0ads value is positive, meaning that disorder increases during the adsorption process. this situation can be explained by the desorption of water molecules replaced by the inhibitor. figure 4. δg0ads vs. temperature for adsorption of 7-etbt on mild steel in 1 m hcl effect of temperature and activation parameters of the corrosion process the activation energy (ea) can be calculated by using arrhenius equation: log ( ) log 2 303 aecr a . rt = − (8) where a is the arrhenius pre-exponential constant. plot of log cr versus 1 / t yields a straight line (figure 5), where slope and intercept define ea / 2.303 r and log a values, respectively. (1000/t) / k-1 figure 5. arrhenius plots for mild steel corrosion in 1 m hcl without and with of 7-etbt n’. y. s. diki et al. j. electrochem. sci. eng. 11(2) (2021) 97-106 http://dx.doi.org/10.5599/jese.952 103 the other activation parameters for the corrosion process were calculated from the arrhenius equation: a aδ δlog log 2 303 2 303 * *cr r s h t h . r . rt                = + −  (9) where sa* is the change in apparent activation entropy, h*ais the change in apparent activation enthalpy,  is avogadro number and ℎ is planck constant. figure 6 presents plots log (cr/t) versus 1/t for mild steel corrosion in blank solution and solutions with different concentrations of 7-etbt. the slope –ha* /2.303rt and the intercept (log (r / h) + sa* / 2.303 r) of each straight-line lead to the values of activation enthalpy change and activation entropy change (table 3). figure 6. transition state plots for mild steel corrosion in 1 m hcl without and with 7-etbt table 3. activation parameters for mild steel corrosion in 1 m hcl without and with 7-etbt solution concentration, mm ea / kj mol -1 δh*a / kj mol -1 δs*a / j mol -1 k-1 0.0 (blank) 92.23 92.57 69.66 0.02 104.27 101.58 97.31 0.10 104.90 102.21 100.08 1.00 105.68 102.99 101.19 2.00 116.52 113.81 131.62 from table 3, it seems that ea and ha* values are varied in the same manner, i.e. they increase with increase of inhibitor concentration, what is probably due to the thermodynamic relation between them (ha* = ea-rt). also, it can be seen that the values of ea are higher in inhibited than uninhibited (blank) solutions. on the other hand, higher values of ea in the presence of inhibitor compared to that in its absence, and decrease of the inhibition efficiency (ie) with increase in temperature can be interpreted as an indication of predominant physical adsorption process [26-28]. moreover, the positive signs of ha* reflect the endothermic effect of mild steel dissolution process. the value of sa* is also higher for inhibited than uninhibited solutions. this phenomenon shows that disorder increases on going from reactant to activated complex. this might be the result of the adsorption of organic inhibitor molecules from the acidic solution which could be regarded as a quasi-substitution process between the organic compound in the aqueous phase and water molecules at the metal surface [29]. tafel polarization potentiodynamic polarization curves were recorded to obtain information about the influence of 7-etbt on anodic and cathodic processes at mild steel in the test solution. polarization curves for lo g ( c r t -1 / m g h -1 c m -2 k -1 ) http://dx.doi.org/10.5599/jese.952 j. electrochem. sci. eng. 11(2) (2021) 97-106 mild steel corrosion inhibition 104 mild steel in 1 m hcl without and with 7-etbt at different concentrations are shown in figure 7. the anodic and cathodic current–potential curves were extrapolated up to their intersection points, where corrosion current density (jcorr) and corrosion potential (ecorr) are evaluated. the effect of increasing concentration of 7-etbt on the anodic and cathodic polarization curves of mild steel in 1 m hcl solution at 298 k is also presented in figure 7. increasing of 7-etbt concentration moved both cathodic and anodic branches of polarization curves toward lower current densities, what reveals that inhibitor molecules are adsorbed on the metal surface. figure 7. potentiodynamic polarization curves obtained for mild steel in 1 m hcl at 298 k, without and with 7-etbt electrochemical parameters associated with tafel polarization measurements such as anodic and cathodic tafel slopes (ba and bc), corrosion potential (ecorr) and corrosion current density (jcorr) were estimated from polarization curves and listed in table 4, together with ie values calculated using equation (4). as is presented in table 4, the slopes of cathodic and anodic tafel lines are almost constant and independent on the inhibitor concentration, meaning that the inhibiting action of the tested compound occurred by simple blocking of the available surface area. in other words, the inhibitor decreased the surface area available for anodic dissolution and hydrogen evolution reactions, without affecting their reaction mechanisms. table 4. potentiodynamic polarization data for corrosion of mild steel in 1 m hcl solutions without and with different concentrations of 7-etbt at 298 k cinh / mm ecorr / mv vs. sce jcorr / μa cm-2 ba / mv dec-1 -bc / mv dec-1 ie / % 0 (blank) –453.53 451.06 104.85 115.31 0 .02 –469.42 135.05 108.13 164.02 70.06 0 .1 –470.30 116.84 117.52 159.60 74.10 1 –472.61 94.81 121.93 168.38 78.98 2 –482.68 58.35 129.80 172.29 87.06 conclusions from the results of the present study, it can be concluded that 7-etbt is the efficient corrosion inhibitor of mild steel in 1 m hcl, reaching the inhibition efficiency of almost 90 % for 2 mm of 7-etbt at 298 k. the inhibition efficiency of 7-etbt is 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[29] l. afia, r. salghi, a. zarrouk, h. zarrok, o. benali, b. hammouti, s. s. al-deyab, a. chakir, l. bazzi, portugaliae electrochimica acta 30(4) (2012) 267-279 https://doi.org/10.4152/pea.201204267. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.jelechem.2007.10.014 https://doi.org/10.1149/1.1391837 https://doi.org/10.5599/jese.585 https://doi.org/10.1007/s10570-008-9226-4 https://doi.org/10.9790/5736-0910011725 https://doi.org/10..9790/5736-1104012436 https://doi.org/10.4152/pea.201204267 https://creativecommons.org/licenses/by/4.0/) theoretical and electrochemical analysis of l-serine modified graphite paste electrode for dopamine sensing applications in real samples: https://dx.doi.org/10.5599/jese.1390 1243 j. electrochem. sci. eng. 12(6) (2022) 1243-1250; https://dx.doi.org/10.5599/jese.1390 open access : : issn 1847-9286 www.jese-online.org original scientific paper theoretical and electrochemical analysis of l-serine modified graphite paste electrode for dopamine sensing applications in real samples revanappa santhosh kumar1, gururaj kudur jayaprakash2,3,, siddalinganahalli manjappa1, mohan kumar4 and avvaru praveen kumar5 1department of chemistry, university b.d.t. college of engineering, visvesvaraya technological university, davangere 577004, karnataka, india 2laboratory of quantum electrochemistry, school of advanced chemical sciences, shoolini university, bajhol, himachal pradesh, 173229, india 3department of chemistry, nitte meenakshi institute of technology, bangalore, karnataka, 560064, india 4department of chemistry, pes institute of technology and management, sagar road, guddada arakere, kotegangoor, 577204, shivamogga, india 5department of applied chemistry, school of applied natural science, adama science and technology university, p o box 1888, adama, ethiopia corresponding author:  rajguru97@gmail.com; tel.: +91-953-876-2343 received: may 27, 2022; accepted: june 8, 2022; published: july 4, 2022 abstract in this study, the carbon paste electrode (cpe) was modified by grinding l-serine in a pestle and mortar. l-serine (l-s) was shown to be an effective electrocatalyst at the modified cpe (mcpe) interface for detecting dopamine (da). l-smcpe showed excellent activity to detect da in commercial injection samples with a recovery range of 98.9 to 100.5 %. theoretical studies were used to understand the electrocatalysis of l-serine at the atomic level using frontier molecular orbitals (fmo) and analytical fukui assay. according to theoretical findings, the amine group of lserine works as an extra oxidation site (reason for enhanced reduction peak da) and the carboxylic acid group acts as an additional reduction site (reason for enhanced oxidation peak da) at the lsmcpe interface. keywords amino acid; redox reaction; quantum chemical modelling; voltammetry; sensor; dopamine introduction dopamine (da) is a monoamine neurotransmitter that plays a variety of physiological activities in humans and animals. it is a basic organic molecule from the catecholamine family. the detection of da in the human body is crucial. da is also involved in the control of heart rate and blood pressure [1,2]. da is involved in a person's movement, mood, and conduct [3]. an inadequate da level https://dx.doi.org/10.5599/jese.1390 https://dx.doi.org/10.5599/jese.1390 http://www.jese-online.org/ mailto:rajguru97@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1243-1250 l-serine modified graphite paste electrode 1244 in the human body can lead to serious health issues such as restless leg syndrome, huntington's disease, schizophrenia, senile dementia, and parkinson's disease [4]. da level in urine samples of humans is utilized as a biomarker to study renal and cardiovascular illnesses. therefore, proper maintenance of the da level in the human body is necessary. because of the relevance of da, the creation of novel dopamine detecting sensors is highly valued for therapeutic applications [5,6]. for da detection, a variety of methods have been developed such as capillary electrophoresis [7], fluorescence-based sensing [8,9], colorimetry [10], and fluorescence [11,12]. on the other hand, their protocols are complex, costly, time-consuming, and frequently need specialist equipment. electrochemical methods, however, provide several benefits, including quick and highly sensitive reactions, the convenience of usage, and low cost. because of the electroactive nature of da, its determination using electrochemical techniques is a key scientific issue. over the last few decades, the sensitivity of da determination has increased significantly. carbon paste electrodes (cpes) offer few beneficial properties, including repeatability, stability, and surface renewability, which make them one of the most appealing working electrodes [13,14]. due to their low cost relative to other materials, cpes are becoming more widely used in fields such as pharmaceutical, biological, and environmental investigations. physical or chemical treatments can improve electrochemical characteristics of cpe such as adsorption capacity, selectivity, and sensitivity [15]. to create a novel sensor with appropriate electrochemical characteristics, a cpe matrix must be modified. earlier, a lot of research has been published on carbon-based electrodes for sensing applications [16-24]. on the atomic scale, the quantum chemistry approach, such as density functional theory (dft) can be utilized to quantify the electron transfer (et) of electrode catalysts. as a result, electroanalytical data can be supported and explained using basic principles and dft. in catalytical research, a combination of pre-et (frontier molecular orbital theory (fmo)) and post-et (fukui functions) would be more useful [13,14,25,26]. in this study, we have modified the cpe by grinding l-serine in a pestle and mortar. the l-serine molecule redox reactive sites and mediating mechanism were predicted using conceptual dftbased quantum chemical modelling. the nucleophilic and electrophilic regions of l-serine are identified using the frontier molecular orbitals (fmo), highest occupied molecular orbital (homo), and lowest unoccupied molecular orbital (lumo) densities. the results of the donor-acceptor interactions were further evaluated utilizing analytical fukui functions to corroborate the findings. experimental chemicals and reagents the analytical grade chemicals were used without further purifications. the sodium dihydrogen orthophosphate monohydrate, silicone oil, dopamine, l-serine, disodium hydrogen orthophosphate, and k4fe(cn)6 were purchased from sigma-aldrich himedia. graphite powder was purchased from loba chemicals. preparation of bare carbon paste electrode (bcpe) the bare graphite paste was made by extensively hand mixing the carbon (graphite) powder and the binder (silicon oil), in an agate mortar with a pestle in a 75:25 (w/w) proportion for 25 minutes until the paste was uniform and homogeneous. then, the graphite paste was filled into a 3 mm hole teflon tube. the electrode surface was wiped smoothly on the butter paper for a clean, consistent, and even surface [13-15]. r. s. kumar et al. j. electrochem. sci. eng. 12(6) (2022) 1243-1250 http://dx.doi.org/10.5599/jese.1390 1245 preparation of l-serine modified carbon paste electrode (l-smcpe) l-smcpe was prepared by hand mixing/grinding 2 to 10 mg of l-serine with carbon paste in an agate mortar. then, the graphite paste containing l-serine was filled into a 3 mm hole teflon tube and smoothen on the butter paper. electrochemical cell the potentiostat model chi-660c was used in the tests (ch instrument-660 electrochemical workstation). three-electrode cell was applied, where working electrodes were bare cpe (bcpe) and l-serine modified cpe (l-smcpe), the reference electrode was saturated calomel electrode (sce), and the counter electrode was a platinum rod. computational methods to build model geometries, we used the sinapsis program [27] and density functional theory (dft) [auxiliary density perturbation theory] as implemented in the demon2k [28-30] software, together with pbe [31,32] correlation functions and the tzvp [33] basis set. the fmo [highest occupied molecular orbital (homo) and lowest unoccupied molecular orbital (lumo)] and analytical fukui functions were visually shown using sinapsis [27]. results and discussion cyclic voltammetric response of dopamine at l-serine modified carbon paste electrode figure 1 depicts cyclic voltammograms (cvs) for 10 μm da in 0.1m phosphate buffer solution (pbs) of ph 7.4 at bcpe (red line) and l-smcpe with different quantities of l-serine, at a scan rate of 100 mv s-1. at the bcpe, redox peak currents are low with higher separation of peak potentials (∆ep) when compared to l-serine mcpe. e / v vs. sce figure 1. cvs of 10 μm da in 0.1 m pbs, ph 7.4 at bcpe (red line) and mcpe with different quantities of l-serine (2-10 mg) in table 1, ∆ep and ipa values for da are compared for bcpe and mcpe with different quantities of l-serine. from figure 1 and table 1, it can be seen that mcpe with 6 mg of l-serine has displayed http://dx.doi.org/10.5599/jese.1390 j. electrochem. sci. eng. 12(6) (2022) 1243-1250 l-serine modified graphite paste electrode 1246 the highest redox current value and lower ∆ep. therefore we have considered 6 mg l-smcpe for further analysis. table 1. ∆ep and ipa values of bcpe and mcpe with different quantities of l-smcpe (data taken from fig. 1) el. no. electrode ∆ep / mv ipa / a 1 bcpe 98 7.157 2 2 mg l-smcpe 70 10.84 3 4 mg l-smcpe 67 15.24 4 6 mg l-smcpe 62 17.46 5 8 mg l-smcpe 61 16.83 6 10 mg l-smcpe 61 16.87 computational studies of l-serine when l-serine is bound by physisorption to the surface of graphite electrode, the electrocatalytic activities of l-smcpe towards da are boosted. for mathematical modelling purposes, a monomer of l-serine is considered. to determine which atoms of l-serine are involved in the redox electron transfer processes, the frontier molecular orbitals were estimated and utilized the fukui concept to predict the redox electron transfer sites. l-serine homo is present in the amine group (figure 2a), while the lumo is in the carboxylic acid group (figure 2b). therefore, according to fmo findings, the amine group in l-serine is engaged in oxidation, whereas the carboxylic acid group is engaged in reduction. the fukui function is frequently used in electrochemistry to understand redox reaction pathways [13,14,25,26]. simulations based on the fukui function can be utilized in chemical and electrochemical applications to locate electron transfer sites [13,14]. the fukui function [34] can be defined using equation (1):      → +  −      0 ( ) ( ) ( ) lim n n n n f n r r r (1) here ρ(r) is the electron density, n denotes the number of electrons in the system, and the + and signs denote electron addition and removal, respectively. a b homo [iso=0.05; grid=0.2] lumo [iso=0.05; grid=0.2] figure 2. average reactive orbital space of frontier molecular orbital of l-serine r. s. kumar et al. j. electrochem. sci. eng. 12(6) (2022) 1243-1250 http://dx.doi.org/10.5599/jese.1390 1247 the analytical fukui findings for l-serine are displayed in figure 3. figure 3a displays f-(r) and f+(r) (figure 3b) plots of the l-serine surface, respectively. the amine group of l-serine functions as an oxidation site and the carboxylic group of l-serine functions as a reduction site. a b f-(r) [iso=0.0; grid=0.2] f+(r) [iso=0.03; grid=0.2] figure 3. analytical fukui isosurface results [h = white, c = grey, n = blue, and o = red] fmo analysis provides information on reactive redox sites without accounting for electronic relaxation. analytical fukui's findings, however, provide details on the places that would suffer the most redox reaction changes while accounting for relaxation effects. hence, fmo and fukui researches should be correlated to more precisely forecast redox reactivity locations. in the current studies, both fmo and fukui, studies are in agreement with each other. as a result, we get more consistent results that amine groups are oxidation centers and carboxylic acid groups are reduction centers. effect of da concentration using the cv approach, the influence of da concentration was investigated at the surface l-smcpe electrochemical sensor in 0.1 m pbs, ph 7.4. the rise in anodic peak current with increasing da concentration (10 to 70 μm) is shown in figure 4. a linearity was observed between anodic peak current (ipa) and da concentration, with a corresponding linear regression equation: ipa = 0.448896cda + + 5.1907, with r2 =0.99532. e / v vs. sce figure 4. cvs of different da concentrations (10 -70 m) at l-smcpe in 0.1m pbs, ph 7.4 http://dx.doi.org/10.5599/jese.1390 j. electrochem. sci. eng. 12(6) (2022) 1243-1250 l-serine modified graphite paste electrode 1248 effect of scan rate the impact of changing the scan rate (150 to 350 mv s-1) on the oxidation peak current of da in 0.1 m pbs as the supporting electrolyte is displayed in figure 5. it can be seen that over the range of 150 to 350 mv s-1, the oxidation peak current rises linearly with the scan rate. e / v vs. sce figure 5. cvs of 10 μm da at l-smcpe in 0.1m pbs, ph 7.4 at different scan rates the graph of anodic peak current vs. scan rate shown in figure 6 reveals a linear relation with a correlation coefficient of 0.9974. this suggests that da oxidation at l-smcpe is adsorption controlled process. v / mv s-1 figure 6. anodic peak current vs. scan rate (data from fig. 5) determination of da in real samples the analysis of da in the commercial injection sample was carried out at l-smcpe in order to evaluate the reliability of the proposed approach by using the standard addition method. the results summarized in table 2 reveal that the found values are close to the labeled content with a recovery range of 98.9 to 100.5 %. therefore l-serine mediated et can detect da in injection samples effectively. r. s. kumar et al. j. electrochem. sci. eng. 12(6) (2022) 1243-1250 http://dx.doi.org/10.5599/jese.1390 1249 table 2. determination of da in injection samples cda / m recovery, % number spiked found 1 10 10.05 100.5 2 20 19.78 98.90 3 30 29.98 99.93 conclusion in the current work, the cpe was modified by grinding l-serine (eco-friendly modifier) in a pestle and mortar. l-serine showed excellent electrocatalytic at the l-smcpe interface for sensing da in real injection samples with a recovery range of 98.9 to 100.5 %. the catalytical activity of l-serine was theoretically examined using fmo and analytical fukui analysis. theoretical observation proved that the amine group of l-serine acts like an additional oxidation site (reason for enhanced 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https://doi.org/10.1007/s42452-020-2785-1 https://doi.org/10.1016/j.cplett.2021.139295 https://doi.org/10.1039/c8nj03679a http://sinapsis.sourceforge.net/ http://demon-software.com/public_html/index.html http://demon-software.com/public_html/index.html https://doi.org/10.1002/wcms.98 https://doi.org/10.1063/1.3036926 https://doi.org/10.1103/physrevlett.77.3865 https://doi.org/10.1103/physrevlett.80.891 https://doi.org/10.1139/v92-079 https://doi.org/10.1021/ja00326a036 https://creativecommons.org/licenses/by/4.0/) @article{kumar2022, author = {kumar, revanappa santhosh and jayaprakash, gururaj kudur and manjappa, siddalinganahalli and kumar, mohan and kumar, avvaru praveen}, journal = {journal of electrochemical science and engineering}, title = {{theoretical and electrochemical analysis of l-serine modified graphite paste electrode for dopamine sensing applications in real samples:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {6}, pages = {1243--1250}, volume = {12}, abstract = {in this study, the carbon paste electrode (cpe) was modified by grinding l-serine in a pestle and mortar. l-serine (l-s) was shown to be an effective electrocatalyst at the modified cpe (mcpe) interface for detecting dopamine (da). l-smcpe showed excellent activity to detect da in commercial injection samples with a recovery range of 98.9 to 100.5 %. theoretical studies were used to understand the electrocatalysis of l-serine at the atomic level using frontier molecular orbitals (fmo) and analytical fukui assay. according to theoretical findings, the amine group of l-serine works as an extra oxidation site (reason for enhanced reduction peak da) and the carboxylic acid group acts as an additional reduction site (reason for enhanced oxidation peak da) at the l-smcpe interface.}, doi = {10.5599/jese.1390}, file = {:d\:/onedrive/mendeley desktop/kumar et al. 2022 theoretical and electrochemical analysis of l-serine modified graphite paste electrode for dopamine sensing applic.pdf:pdf;:www/jese_v12_no6_1243-1250.pdf:pdf}, keywords = {amino acid, dopamine, quantum chemical modelling, redox reaction, sensor, voltammetry}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1390}, } electrochemical sensing of caffeic acid antioxidant in wine samples using carbon paste electrode amplified with cdo/swcnts http://dx.doi.org/10.5599/jese.1701 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1701 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical sensing of caffeic acid antioxidant in wine samples using carbon paste electrode amplified with cdo/swcnts zahra arab1, sara jafarian1, hassan karimi-maleh2,, leila roozbeh nasiraie1 and mohammad ahmadi3 1department of food science and technology, nour branch, islamic azad university, nour, iran 2department of chemical engineering, quchan university of technology, quchan 9477177870, iran 3department of food hygiene, ayatollah amoli branch, islamic azad university, amol 4615143358, iran corresponding authors: sara_jafary2002@yahoo.com; ,h.karimi.maleh@gmail.com tel.: 0098 9123110253 received: february 11, 2023; accepted: may 22, 2023; published: june xx, 2023 abstract an electrochemical sensor was introduced as an analytical tool for monitoring caffeic acid in food samples. this analytical tool was amplified by cadmium oxide decorated on single wall carbon nanotubes as a new catalyst and showed a powerful ability to sensing of caffeic acid in food products. the presence of cadmium oxide decorated on single wall carbon nanotubes catalyst improved the oxidation signal of caffeic acid about 2.4 times at optimum conditions. the ph investigation confirmed that the redox reaction of caffeic acid was ph dependent and showed maximum sensitivity at ph 7.0. the paste electrode amplified with cadmium oxide decorated on single wall carbon nanotubes was successfully monitored caffeic acid in the concentration range 0.02–200 µm with a detection limit of 9.0 nm, respectively. the standard addition strategy showed a recovery range of 97.96 – 102.59 % to the measurement of caffeic acid in fruit juice, white and red wine that was acceptable for the fabrication of a new analytical tool in food monitoring. keywords food sensor, nanocomposite, electroanalysis, modified electrodes, real sample analysis introduction antioxidants, as one of the most famous substances used in food, prevent the adverse effects of free radicals [1,2]. antioxidants can be found as natural or artificial and are important in food safety [3,4]. although the body directly confronts some free radicals, the wide range of radicals makes it important to get antioxidants through food [5]. caffeic acid is one of the famous natural antioxidants found in various beverages and wine samples [6,7]. caffeic acid can cause mild effects on insomnia http://dx.doi.org/10.5599/jese.1701 http://dx.doi.org/10.5599/jese.1701 http://www.jese-online.org/ mailto:sara_jafary2002@yahoo.com mailto:h.karimi.maleh@gmail.com j. electrochem. sci. eng. 00(0) (2023) 000-000 sensing of caffeic acid antioxidant in wine 2 and excessive consumption is not recommended [8]. therefore, monitoring of caffeic acid in food products was investigated by some analytical methods such as hplc [9], lc-ms/ms [6], tlc-densitometry [10] and electrochemical sensors. electrochemical sensors are a useful and powerful strategy for sensing food products and especially food antioxidants [11-14]. easy modification to create highly selective and sensitive tools in food products analysis is the main advantage of electrochemical methods compared to other analytical strategies in sensing electrochemically active materials [15-20]. different modifiers were suggested for the amplification of electrochemical sensors, such as 2d nanomaterials, nanocomposites [21], ionic liquids, composites, conductive polymers, biological compounds, etc. [22-24]. between them, nanomaterials showed more advantages due to easy diversity and high electrical conductivity [25,26]. many research efforts focused on the modification of electrochemical sensors to trace-level analysis in complex matrixes [27-30]. nanotechnology changed the approach to pure science and creating unique properties in new materials [31-37]. nanomaterials showed unbelievable properties and a wide range of applications in different branches of science [38-42]. the application of nanomaterials in the fabrication of modified sensors is increasing significantly [43]. this issue relates to high electrical conductivity and easy modification of nanomaterials with different functionalized groups. in this study, a nanostructure-based sensor (cadmium oxide decorated on single wall carbon nanotubes (cdo/swcnts) modified paste electrode (pe) (cdo/swcnts/pe) in this case) was suggested as a simple and economical type of sensor for easy monitoring of caffeic acid in food products and results showed the high ability of this sensor. experimental materials cadmium acetate, swcnts-cooh, and sodium hydroxide were purchased from sigma-aldrich company and used to synthesize cdo/swcnts nanocomposite with the recommended procedure by cheraghi and taher [44]. graphite powder (99.99 %) and paraffin oil were purchased from merck company and used to fabricate paste electrodes. phosphoric acid, sodium dihydrogen phosphate, disodium hydrogen phosphate anhydrous, and trisodium phosphate were purchased from across company and used to prepare phosphate buffer solution (pbs). instruments all electrochemical signals were recorded by an ivium-vertex machine connected to ag/ag/kclsat, pt wire, and cdo/swcnts/pe as reference, counter, and working electrodes, respectively. a ph meter model 780 ph meter, metrohm was used to prepare pbs solution. fabrication of cdo/swcnts/pe the working electrode (cdo/swcnts/pe in this case) was fabricated after optimization of cdo/swcnts compared to graphite powder in the presence of caffeic acid in ph 7.0 as the optimum condition. the maximum sensitivity was obtained in the presence of 9.0 wt.% cdo/swcnts nanocomposite at the surface of the paste electrode (figure 1). therefore, 910 mg + 90 mg cdo/swcnts was mixed in mortar and pestle and converted to paste using paraffin oil and hand mixing for 30 min. zahra arab et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1701 3 mass of nanocatalyst in electrode matrix, mg figure 1. the oxidation current of caffeic acid at surface of paste electrode modified with different values of cdo/swcnts in carbon paste matrix real sample preparation 10 ml of white, fruit juice and red wines were selected and centrifuged at 4000 rpm for 15 min and, after filtration, diluted into 10 ml pbs (ph 7.0) and transferred to an electrochemical cell for the analysis of caffeic acid using the standard addition method. results and discussion ph optimization caffeic acid is a phenolic antioxidant and according to reported papers and scheme 1 [45], its oxidation in signal changes with the changes in the ph of the solution, and optimization of this factor is so important in the first step. scheme 1. redox reaction of caffeic acid therefore, linear sweep voltammograms of caffeic acid were recorded in the ph range of 5.0 – 9.0 and the results are shown in figure 2 inset. results showed that maximum oxidation current to http://dx.doi.org/10.5599/jese.1701 j. electrochem. sci. eng. 00(0) (2023) 000-000 sensing of caffeic acid antioxidant in wine 4 caffeic acid was observed at ph = 7.0 (figure 2), and this condition was selected as the optimal condition for monitoring caffeic acid in food samples. catalytic activity of senor the oxidation signal of 500 µm caffeic acid at the surface of pe (figure 3, curve a) and cdo/swcnts/pe (figure 3, curve b) was recorded and results showed oxidation currents of 7.97 and 19.2 µa at oxidation potentials of 563 and 460 mv, respectively. figure 2. current-ph curve to electrooxidation of 500 µm caffeic acid. inset) linear sweep voltammograms of 500 µm caffeic acid in the ph range 5.0 to 9.0. figure 3. ls voltammograms 500 µm caffeic acid at surface of a) pe and b) cdo/swcnts/pe. inset) current density diagram relative to ls voltammogarms zahra arab et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1701 5 comparing these values clearly showed that the presence of cdo/swcnts at the surface of pe plays a catalytic role in the redox behavior of caffeic acid. on the other hand, the active surface area of pe and cdo/swcnts/pe were determined to be about 0.09 and 0.17 cm2, respectively. the active surface was determined by the recording of cyclic voltammograms of [fe(cn)6]3-/4in the presence of 0.1 m kcl and randles–sevcik equation.. this confirms cdo/swcnts increase the active surface area of the electrode. the current densities relative to two electrodes was shown in figure 3 inset. scan rate investigation the role of scan rate on the potential and current of caffeic acid signal was investigated and results showed in figure 4 inset. the results showed a positive shift in the oxidation potential of caffeic acid and also increase in the oxidation current at the surface of cdo/swcnts/pe at the scan rate range 5 – 80 mv s-1. the linear relationship between current of caffeic acid and 1/2 with an equation of i = 1.61971/2 +4.8775 (r2= 0.9944) confirms a diffusion-controlled process. also, a positive shift in oxidation potential indicates a kinetic limitations of the redox reaction of caffeic acid at the surface of cdo/swcnts/pe. analytical parameters the limit of detection and linear dynamic range are two main analytical factors in the design of new types of analytical methods. the linear dynamic range between 0.02 to 200 µm with equation i = 0.0481c + 0.7410 (r2 = 0.9911) was determined. a detection limit of 9.0 nm was determined to for this method of caffeic acid at the surface of cdo/swcnts/pe at optimum conditions using equation ylod = 3σ/m. figure 4. i 1/2 curve to electro-oxidation 450 µm caffeic acid at surface of cdo/swcnts/pe. inset) lsv 450 µm caffeic acid at scan rates a) 5; b) 20; c) 40; d) 60 and e) 80 mv s-1 selectivity of cdo/swcnts/pe to sensing of caffeic acid the selectivity of cdo/swcnts/pe as a new catalyst for sensing of 15.0 µm caffeic acid was investigated at ph 7.0 as the optimum condition. the results were reported with an acceptable error of 5 % in table 1. data were repeated five times and confirmed the high selectivity of cdo/swcnts/pe as a new sensor for sensing caffeic acid in an aqueous solution. http://dx.doi.org/10.5599/jese.1701 j. electrochem. sci. eng. 00(0) (2023) 000-000 sensing of caffeic acid antioxidant in wine 6 table 1. interference study to sensing 15.0 µm caffeic acid using cdo/swcnts/pe species tolerance limit (interference mass, g / caffeic acid mass, g) li+, k+, brand na+ 1000 sucrose, glucose, fructose and lactose 600 vitamin b9 and glycine 400 starch saturated determination of caffeic acid in real samples the fruit juice and wine samples were prepared according to the reported procedure in section 2.4 and transferred to an electrochemical cell for electrochemical monitoring by standard addition methods. the results were repeated three times and compared with one published sensor and data shown in table 2. f-test and t-test were used to investigate accuracy data and results clearly confirmed cdo/swcnts/pe's high ability to sense caffeic acid in real samples. table 2. determination of caffeic acid using cdo/swcnts/pe (n=3) sample content of caffeic acid, m recovery, % ftab fexp ttab texp added founded founde by hplc white wine --3.11±0.13 2.96±0.13 ---------- 10.00 13.45±0.34 13.67±0.55 102.59 19.0 6.7 3.8 1.5 red wine --2.85±0.22 2.94±0.13 ---------- 5.00 7.69±0.45 8.05±0.49 97.96 19.0 7.3 3.8 1.8 fruit juice --<lod <lod ---------- 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https://doi.org/10.1016/s0039-6028(01)01596-5 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.5796/denka.21.367 https://doi.org/‌10.2109/‌jcersj.96.74 https://doi.org/‌10.2109/‌jcersj.96.74 https://www.tun.com/blog/university-of-texas-po‌werful-and-longlasting-battery/ https://www.tun.com/blog/university-of-texas-po‌werful-and-longlasting-battery/ https://dx.doi.org/10.1103/physrev.71.717 https://dx.doi.org/10.1063/1.1702952 https://dx.doi.org/10.1063/1.1702952 https://dx.doi.org/10.1380/jsssj.21.791 https://doi.org/10.1016/s0039-6028(01)01596-5 https://creativecommons.org/licenses/by/4.0/) {affordable voltammetric sensor based on anodized disposable pencil graphite electrodes for sensitive determination of dopamine and uric acid in presence of high concentration of ascorbic acid} http://dx.doi.org/10.5599/jese.783 263 j. electrochem. sci. eng. 10(3) (2020) 263-279; http://dx.doi.org/10.5599/jese.783 open access: issn 1847-9286 www.jese-online.org original scientific paper affordable voltammetric sensor based on anodized disposable pencil graphite electrodes for sensitive determination of dopamine and uric acid in presence of high concentration of ascorbic acid preethi sankaranarayanan and sangaranarayanan m. venkateswaran department of chemistry, indian institute of technology madras, chennai, tamil nadu 600036, india corresponding author: preethi92.us@gmail.com; tel.: +919500955589 received: february 3, 2020; revised: february 28, 2020; accepted: march 2, 2020 abstract a simple, disposable and low cost voltammetric sensor based on the anodized pencil graphite electrode (apge) for the simultaneous determination of dopamine (da) and uric acid (ua) is demonstrated. the physico-chemical properties of the pencil graphite electrode (pge) before and after anodization were analyzed using ft-ir, ft-raman, sem and eis characterization techniques. in comparison to pge, apge exhibited excellent electrochemical activity towards the simultaneous detection of da and ua with peak-topeak separation of about 0.18 v even in the presence of high concentration (2 mm) of ascorbic acid (aa). the discrimination of apge towards aa was rationalized through the absence of favorable surface interactions between oxygen rich functional groups on the surface of apge and aa. using dpv without any pre-concentration step and under optimized conditions, apge displayed a linear range of 1 – 80 μm with an estimated limit of detection (lod, 3σ/m) of 0.008 μm and 0.014 μm for da and ua, respectively. moreover, a higher sensitivity in comparison to other previously reported pretreated pencil graphite electrodes was observed for da (34.32 μa/μm) and ua (12.33 μa/μm). the practical applicability of apge was demonstrated through the estimation of da in human blood serum and ua in urine samples. keywords anodization; pencil graphite electrode; dopamine; uric acid; ascorbic acid. introduction neurotransmitters are an important class of biomolecules that play a vital role in control and effective functioning of the central nervous, cardiovascular, renal and hormonal systems [1,2]. http://dx.doi.org/10.5599/jese.783 http://dx.doi.org/10.5599/jese.783 http://www.jese-online.org/ mailto:preethi92.us@gmail.com j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 264 dopamine (da) is one of the neurotransmitters in the family of catecholamines, which exists as organic cations in brain tissues and body fluids. abnormal or low level of da in the human body leads to many psycho-somatic disorders [3.4]. like da, uric acid (ua) is another class of biomolecules which forms in the body as one of the principal end-products of the purine metabolism. several diseases such as lysch – nyhan syndrome, hyperuricaemia and gout are the consequences of abnormal levels of ua [5-10]. therefore, accurate detection of da and ua in body fluids like blood serum and urine is of great clinical importance for an effective point of care analysis. in the recent past, due to their fast processing time and cost effectiveness, electroanalytical techniques have been favored over other traditional analytical techniques for the detection of da and ua [11-13]. however, electrochemical detection of these two compounds at conventional electrodes such as bare carbon electrode has the following difficulties [14-16]: 1. da and ua coexist with electrochemically active ascorbic acid (aa) in biological fluids, thereby, causing mutual interference during detection. 2. da and ua have similar oxidation potentials, thereby rendering their electrochemical detection relatively challenging task. 3. the concentration levels of ua and aa in body fluids are significantly higher in comparison with da. 4. during electrochemical detection of these compounds, their oxidation products adsorb or electropolymerize on the electrode surfaces, thereby reducing their reproducibility and reusability. in order to overcome the above-mentioned drawbacks of bare carbon electrodes, many researchers have employed novel materials as chemical modifiers [17], including polymeric materials [18,19], ionic liquids [20], carbon nanotubes (cnts) [20-23], nanoparticles (nps) [24] and nanocomposites [25,26]. from this perspective, the simplest way in which a bare carbon electrode can be chemically modified is electrochemical pretreatment by applying high potential using either chronoamperometry (ca) or cyclic voltammetry (cv) technique. during the anodization process (application of high positive potential) of bare carbon electrode, oxygen rich groups such as -oh, -cooh, etc. [5,27-29], are formed on the electrode surface. due to the polar nature of these oxygen rich groups, the anodized carbon electrodes exhibit affinity towards adsorption of polar molecules and also enhance the wettability of electrodes [5,27,30]. furthermore, at neutral or physiological ph, da and ua exist as positively charged while aa as negatively charged ions [31,32]. therefore, these molecules can be selectively determined at anodized carbon electrodes because of the inherent polar nature. among various carbon electrodes, pencil graphite electrodes (pges) have recently received widespread attention. due to their sp2 hybridized carbon, pges exhibit good conductivity, adsorption properties, low background current, high sensitivity, and ease of preparation and surface modification [24,33-37]. moreover, in comparison to glassy carbon electrodes (gces), pges offer easy renewability of the electrode surface, which plays an important role in subsequent analysis [24]. however, only few reports are available wherein pretreated pges have been utilized for the detection of da and ua [11,38,39]. for example, ozcan et al. have developed the electrochemically over-oxidized pge modified by poly(pyrrole-3-carboxylic acid) for the detection of da in blood serum samples, where chronoamperometry was employed for carrying out the anodization [38]. further, alipour et al. reported a pre-treated pge for the simultaneous determination of da and ua in biological samples wherein, cyclic voltammetry was employed to prepare anodized pges [11]. though, these studies have reported good figures of merit, the analyses were preceded by a pre-concentration step and exhibited short linear range of calibration plots. p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 265 hence, in the present study we have developed anodized disposable pencil graphite electrodes (apges) for the selective and sensitive determination of da and ua in the presence of high concentration of aa without any pre-concentration step. pges were anodized using a simple chronoamperometric method and formed apges were subsequently analyzed using various material characterization techniques. the discriminating nature of apge has been rationalized through the formation of oxygen containing groups on the electrode surface. experimental chemicals and reagents k2hpo4 (fisher scientific, 99.5 %), kh2po4 (srl, 99.5 %), hcl (merck, >35 %), naoh (aldrich, >97 %), k4[fe(cn)6].3h2o (merck, 99 %), k3[fe(cn)6].3h2o (merck, 99 %), kno3 (merck, >98 %), ascorbic acid (srl, 99.7 %), uric acid (srl, 99 %), d-glucose (srl, 99 %), dopamine hydrochloride (alfa aesar, 99 %), kcl (srl, >99.5 %), nacl (ar, acs, exiplus, 99.9 %) and na2so4 (ar, acs, exiplus, 99.9 %) were of analytical grade and used as received. the phosphate buffer solution (pbs, 0.1 m) was freshly prepared using 1 m of k2hpo4 and 1 m of kh2po4 with triple distilled water. prior to analysis, 10 ml of 0.1 m pbs was purged with argon for 15 minutes to dissolved oxygen. the ph of 0.1 m pbs was adjusted using either 5 m hcl or 5 m naoh. stock solutions of 0.02 m uric acid, 0.1 m ascorbic acid and 0.01 m dopamine were freshly prepared using 0.1 m pbs (ph 7.0). electrochemical measurements differential pulse voltammetry (dpv), chronoamperometry (ca), cyclic voltammetry (cv) and electrochemical impedance spectroscopy (eis) were carried out using conventional three-electrode assembly in the chi660a electrochemical workstation (ch instruments, usa). the pencil graphite leads (hb, 0.5 mm diameter, 60 mm height, camlin), pt gauze and saturated calomel electrode (sce) were used as working, counter and reference electrodes, respectively. anodization of pge prior to the anodization, pges were mechanically polished with a clean white paper (75 gsm grade) and washed thoroughly with water. an apparent surface area of 0.19 cm2 of pge was exposed and the remaining area was covered using a scotch tape except a small portion exposed for connecting the leads. pges were anodized by ca and the experimental parameters such as anodization potential, time and supporting electrolyte were optimized for maximum sensing performance. as the first step, three different supporting electrolytes, namely 0.1 m pbs (ph 7.0), 0.1 m hcl (ph 1.1) and 0.1 m naoh (ph 13.0) were used for anodization of pges at 2.0 v for 120 s. secondly, five different anodization potentials (1.6, 1.8, 2.0, 2.2 and 2.4 v) and time applied (60, 120, 180, 240 and 300 s) for anodization were examined in 0.1 m pbs (ph 7.0). pge anodized at 2 v for 120 s in 0.1 m pbs (ph 7.0), was chosen as the optimized condition (vide infra) for anodization and the electrode hereafter will be referred as anodized pge (apge). these apges were then thoroughly washed with water, dried and subsequently used as working electrodes for further studies. spectroscopic, morphological and electrochemical characterizations fourier transform infrared (ft-ir) and fourier transform raman (ft-raman) spectroscopies were used to characterize the electrodes. ft-ir spectra were recorded with the help of kbr pellets in jasco 4100 spectrometer. in order to make the pellets, 0.5 cm of pge (bare/anodized) was broken and grinded along with kbr. raman spectra were acquired using bruker rfs 27: standalone ftraman spectrometer. the surface morphology (sem) and electron dispersive analysis of x-rays http://dx.doi.org/10.5599/jese.783 j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 266 (edx) of both electrodes were carried out using fei quanta feg200 at an acceleration voltage of 20 kv in high vacuum mode. prior to the spectroscopic and morphological measurements, the electrodes were dried using a heat gun and stored in a vacuum desiccator for 48 hours to remove the moisture. the electrochemical characterizations of pge and apge were carried out in 0.1 m pbs (ph 7.0) solution containing 10 mm [fe(cn)6]4-/3(1:1) using cv and eis. for cv, the potential was scanned from -0.2 to 0.6 v at different scan rates (5 to 50 mv/s). eis measurements were carried out at 0.26 v in the frequency range of 104 to 10−1 hz with the amplitude of 5 mv. the electrical equivalent circuit fittings were carried out using the default fitting software provided with the instrument. simultaneous detection of da & ua in the presence of high concentration of aa the simultaneous detection of da and ua at apge was carried out in 0.1 m pbs (ph 7.0) using dpv in the presence of 2 mm of aa. dpv experiments for sensing were carried out by scanning the potential from −0.2 to 0.6 v, while the other parameters such as increment voltage, amplitude, pulse width, pulse period and quiet time were kept constant at 0.004 v, 0.05 v, 0.05 s, 0.5 s and 2 s, respectively. the corresponding calibration plots were constructed after triplicate measurements. real sample analysis for real sample analysis, the human blood serum and urine samples were obtained from the apollo clinical laboratory, iit-madras, chennai. blood serum and urine samples were respectively diluted to 500 and 1000 times in 0.1 m pbs (ph 7.0) prior to the measurement. results and discussion spectroscopic characterization it is a well-established fact that anodization of carbon surface leads to the creation of surface defects due to the formation of oxygen rich functional groups such as -cooh, -oh, etc. [27,40]. as a result, sp2 hybridized lattices of graphite get distorted because of sp3 hybridized carbons of these functional groups. the changes such as formation of new functional groups and distortion of graphite lattice sites are effectively captured in ft-ir and ft-raman spectroscopy, respectively. infrared spectroscopy is an excellent tool for finding out the formation of new functional groups. ft-ir spectra for pge before and after anodization process in 0.1 m pbs (ph 7.0) are given in figure 1a. figure 1. (a) ft-ir and (b) ft raman spectra for pge and apge p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 267 it can be inferred from figure 1a that oxygen rich functional groups have formed on pge as a result of anodization. the characteristic peaks at 3144 cm-1 (o–h stretching from cooh), 1662 cm-1 (c=c stretching), 1394 cm-1 (c=c bending) and 1088 cm-1 (c–o stretching from cooh) indicate the successful modification of pge surface with these functional groups [41,42]. furthermore, ir peak intensities of apge are significantly higher than that of pge. it can be inferred from figure 1a that oxygen rich functional groups have formed on pge as a result of anodization. the characteristic peaks at 3144 (o–h stretching from cooh), 1662 (c=c stretching), 1394 (c=c bending) & 1088 cm-1 (c–o stretching from cooh) indicate the successful modification of pge surface with these functional groups [41,42]. also, ir peak intensities of apge are significantly higher than that of pge. in order to investigate the variations in the graphitic structures, raman spectroscopy is the characterization tool of choice. once the graphitic lattice is altered after anodization, they exhibit characteristic d and g vibrational bands at wavenumbers 1330 cm-1 for sp3 hybridized (disordered, tetragonal) and 1570 cm-1 for sp2 hybridized (ordered, hexagonal) carbons present in them. furthermore, the ratio of intensities of d band to g band i.e., id/ig provides a measure of the disorderliness present in the graphite lattice [43]. moreover, greater id/ig value is an indication of more disorderliness. in general, it has been reported that the oxygen rich functional groups such as quinone, phenol and alcohols are responsible for d band of oxygen containing graphitic materials [41,44-47]. figure 1b depicts raman spectra for pge and apge which show d and g bands at 1351 and 1569 cm-1, respectively. moreover, id/ig was observed to be 0.589 and 0.594 for pge and apge, respectively. hence, it is evident from id/ig ratios for pge and apge that, apge exhibits more disorderliness due to anodization. morphological characterization the morphological variations of pges after anodization were analyzed using sem technique. sem images for pge and apge are shown in figures 2a & 2b, respectively. it can be inferred from figure 2 that the roughness of the graphite surface has significantly increased after anodization. furthermore, the formation of oxygen containing groups after anodization is further corroborated from the presence of oxygen elemental peak in edx spectrum for apge (figure 2d) which is absent for pge (figure 2c). electrochemical characterization electrochemical behaviors of pge and apge were evaluated using the standard [fe(cn)6]3-/4redox couple and the cyclic voltammograms of both these electrodes are shown in fig. 3a. cyclic voltammograms recorded at different scan rates for apge are shown in figure 3b. in comparison with the voltammetric response of pge, the peak current of apge was found to be higher thereby indicating an enhanced electron transfer after anodization. moreover, the peak currents of both pge and apge were found to increase with the scan rate. the electrochemical active surface areas (ecasa) for pge and apge were calculated from ip vs. ν1/2 plots (fig. 3c) using the randles-ševcik equation: 5 3 2 1 2 1 2 p s2 69 10 / / /i . n d ac =  (1) where ip is the peak current in a, n is the number of electrons in the redox process (here n=1), d is the diffusion coefficient of [fe(cn)6]3-/4(1.3210-6 cm2/s), cs denotes the bulk concentration of [fe(cn)6]3-/4(10 mm, 1:1) and ν is the scan rate. the parameter a in equation (1) denotes ecasa that was found to be 0.36 cm2 and 1.32 cm2, respectively, for pge and apge. furthermore, the roughness factors for pge and apge were determined using ecasa to geometrical surface area ratio which was estimated to be 1.78 and 6.48, respectively. this considerable increase in the roughness of apge is in concordance with the observations in sem micrographs (figure 2b). http://dx.doi.org/10.5599/jese.783 j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 268 figure 2. scanning electron micrographs for (a) pge and (b) apge. the corresponding edx spectra for (c) pge and (d) apge figure 3. cyclic voltammograms in 0.1 m pbs (ph 7.0) containing 10 mm (1:1) [fe(cn)6] 3-/4-for (a) apge and pge at 5 mv/s, and (b) apge at various scan rates ranging from 5 mv/s to 50 mv/s. (c) variation of ipa with ν 1/2 for apge in order to understand the variation in the electron transfer kinetics of pge before and after anodization, eis was employed. nyquist plots measured for pge and apge in 0.1 m pbs (ph 7.0) containing 10 mm (1:1) [fe(cn)6]3-/4at 0.26 v are shown in figure 4a. the distinct semicircular shape in the nyquist plot for apge at higher to medium frequencies corresponds to the electron transfer limited process while the linear part extending to lower frequencies indicates the process limited by diffusion. nyquist plot of pge, however, exhibits a prominent semicircular shape without a contribution of linear part due to diffusion impedance. the nyquist plots obtained were fitted using modified randles electrical equivalent circuits shown in figures 4b and 4c, respectively for pge and apge. the constant phase element (cpe) was chosen as double-layer capacitive element to account for various inhomogeneities arising from the nature of the electrode and its porosity [48]. warburg element (w) denotes impedance due to diffusion, while rs and rct denote resistances due to the electrolyte solution and charge transfer at the electrode-electrolyte interface, respectively. p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 269 frequency dependences of cpe impedance (zcpe) and warburg impedance (zw) are defined as: dl dl cpe 1 ( ) n cpe j z = (2) 0 5 (1 ) . wz j w  − = − (3) figure 4. (a) nyquist plots for pge and apge measured in 0.1 m pbs (ph 7.0) containing 10 mm [fe(cn)6] 4-/3 (1:1) at 0.26 v in the frequency range of 104 to 10−1 hz. electrical equivalent circuits for (b) pgeand (c) apge in equations (2) and (3), cpedl, ndl and w are frequency independent impedance parameters, while j is imaginary number and ω is radial frequency of measurements. impedance parameter values obtained by fitting electrical equivalent circuits to the nyquist plots are given in table 1. table 1. summary of impedance parameter values obtained by fitting equivalent circuits in figure 4b-c to nyquist plots in figure 4a electrode rs/ ω cpedl, µs s n ndl w / s s 1/2 rct / ω ko / 10 −6 cm s−1 pge 11.92 11.64 0.854 -1113 6.64 apge 16.91 226.9 0.713 0.007 100.8 19.99 from the charge transfer resistance (rct) values, the standard heterogeneous rate constants (ko) were estimated using equation (4) and are provided in table 1: o 2 2 ct s rt k n f ar c = (4) it can be inferred from table 1 that ko for apge is 3 times higher than for pge which in turn is responsible for its enhanced electrochemical activity (vide infra). electrochemical behavior of apge towards da and ua in order to compare the performance of pge and apge towards da and ua detection, voltammetric techniques were employed. the cyclic voltammetric responses of 10 µm da (figure 5a) and 20 µm ua (figure 5c) were recorded in 0.1 m pbs (ph 7.0) at the scan rate of 25 mv/s. it can be inferred from figures 5a & 5c that quasi reversible behavior was observed for both da & ua at apge. on the other hand, at pge, an irreversible cyclic voltammogram was observed for both species. furthermore, the anodic peak at apge for da and ua was found to be centered at 0.15 v and 0.33 v, respectively. also, the corresponding current responses at apge for da and ua were 12 and 10 times higher than at pge. http://dx.doi.org/10.5599/jese.783 j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 270 figure 5. cyclic voltammograms of (a) 10 µm of da and (c) 20 µm of ua in 0.1 m pbs (ph 7.0) at pge and apge. corresponding differential pulse voltammograms for (b) da and (d) ua since differential pulse voltammetry (dpv) is known to be more sensitive analytical technique than cv, dpv responses for da (fig. 5b) and ua (fig. 5d) at apge were compared with pge. at apge, the oxidation peak potential was observed at 0.11 v for da and 0.31 v for ua. however, extremely weak dpv signals for da and ua oxidation were observed at pge. unlike cvs shown in figs. 5a & 5c, the oxidation peak currents in dpv for da and ua at apge were found 92 and 112 times higher than at pge (figs. 5b & 5d). the enhanced current response for da and ua at apgeis attributed to its increased ecasaand the presence of non-covalent interactions (vide infra). thus, it is evident from the above discussion that apge can be effectively employed for the detection of da and ua. effect of ph the effect of ph of pbs on the detection of da and ua was investigated using dpv in the ph range of 5.0 – 9.0 and the corresponding voltammograms are shown in figures 6a & 6c. the anodic peak currents of both da and ua increase, reach the maximum at ph 7.0, and for higher ph decrease subsequently. the increment in the peak currents from ph 5.0 to 7.0 is attributed to the proton transfer during the oxidation process. on the other hand, the decrease in the peak currents from ph 7.0 to 9.0 is due to the facile oxidation of hydroxyl groups and the instability of the oxidized form of da and ua [49]. furthermore, the oxidation peak potentials (ep) of da and ua were found to be dependent on ph values as observed in figures 6b & 6d. it can also be deduced from these figures that with every p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 271 unit increase of ph, the ep shifts towards negative potential region. the linear regression equation for the dependence of ep with ph agrees with the nernst equation viz. ep = eo’-0.0591 ph (5) where e0’ is the formal potential. since the slope values observed here for da (-60.4 mv/ph) and ua (-58.6 mv/ph) are in close agreement with the theoretical slope value of equation (5), the same number of electrons and protons is involved in the oxidation process. the electrochemical oxidation of da and ua taking place at apge is illustrated in scheme 1. based on the highest current response, ph 7.0 has been chosen as the optimized ph for subsequent experiments. figure 6. differential pulse voltammograms of apge at different ph values in 0.1 m pbs containing (a) 10 µm of da and (c) 20 µm of ua. variations in ep with change in ph for (b) da and (d) ua scheme 1. schematic representation of electrochemical oxidation mechanism of da and ua at apge http://dx.doi.org/10.5599/jese.783 j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 272 optimization of experimental parameters prior to the simultaneous determination of da and ua at apge, various experimental parameters such as kind of supporting electrolyte, potential and time used for anodization were analyzed via dpv current responses of various apges in solutions containing 20 μm of either da or ua. before optimizing the anodization potential and time, the effects of supporting electrolytes were studied using 0.1 m pbs, 0.1 m naoh and 0.1 m hcl while the applied potential and time were kept constant at 2 v and 120 s, respectively. the corresponding dpv responses are shown in figure 7a for da and figure 7d for ua. since the maximum current response was observed in 0.1 m pbs for both da and ua, it was chosen as the supporting electrolyte in the subsequent studies. figure 7. differential pulse voltammograms for apge prepared in (a,d) different supporting electrolytes, and (b,e) in 0.1 m pbs (ph 7.0) at different applied anodization potentials and (c,f) different anodization times. solutions contained(a-c) 20 µm of da and (d-f) 20 µm of ua. for optimizing the anodization potential, the time of anodization was kept constant at 120 s while the applied potential was varied from 1.6 v−2.4 v in 0.1 m pbs. the maximum current response was observed for apge anodized at 2 v in the case of both da (figure 7b) and ua (figure 7e). furthermore, the anodization time was optimized by varying the time of anodization from 60 − 300 s, while keeping the anodization potential constant at 2 v. the maximum current response was observed for apge anodized at 2 v for 120 s for both da (figure 7c) and ua (figure 7f). dpv results depicted in figure 7 can be understood as follows: due to severe anodization in basic solution (0.1 m naoh), more oxygen containing functional groups may form, thereby resulting in sluggish electron transfer. on the other hand, the extent of anodization is comparatively lower in acidic condition (0.1 m hcl) which results in poor current response for the oxidation of da and ua. however, in neutral conditions optimal surface oxidation of graphite occurs which in turn is responsible for the maximum current response. at higher anodization potential or time, the graphite surface gets oxidized excessively. this leads to low current response for da and ua. on the contrary, due to insufficient formation of oxygen containing functional groups at lower anodization potential or time, the current observed in voltammograms for da and ua oxidation is poor. thus, from the maximum current responses observed in figures 7a-f, the optimized conditions for anodization potential, anodization time and supporting electrolyte were chosen as 2 v, 120 s and 0.1 m pbs (ph 7.0), respectively. p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 273 mechanistic investigation of da and ua oxidation at apge the effect of scan rate (ν) changes was investigated at apge in 0.1 m pbs (ph 7.0) containing either 100 μm of da (figure 8a) or ua (figure 9a) by varying scan rates from 5 to 500 mv/s. the logarithm of peak currents (log ipa) of da and ua increases linearly with logarithm of scan rate (log ν) (figures 8b & 9b) suggesting that the redox processes taking place at apge may be adsorption or diffusion controlled [50]. furthermore, an excellent linearity between log ipa and log ν with the slope value close to unity for da (figure 8b) and 0.5 for ua (figure 9b) was observed. these slope values indicate that adsorption and diffusion-controlled electron transfer process is predominant for da and ua, respectively. figure 8.(a)cyclic voltammograms of apge electrodes containing 100 µm da in 0.1 m pbs (ph 7.0) at scan rates ranging from 5 – 500 mv/s,(b) variation of log ipa with log ν, and variation of ep-e o’ with log ν figure 9. (a) cyclic voltammograms of apge electrodes containing 100 µm ua in 0.1 m pbs (ph 7.0) at scan rates ranging from 5 – 500 mv/s, (b) variation of log ipa with log ν, and variation of ep-e o’ with log ν according to laviron’s formalism [51,52], when δep>(0.20/n) v, a linear plot of ep(a,c)-eº’ vs. log ν would be observed. furthermore, from the intercepts of ep(a,c)-eo’ vs. log ν plots, the electron transfer coefficient (α) and the apparent surface electron transfer rate constant (ks) can be obtained using the following equations: a c 1     = − (6) a c s (1 ) = = nf nf k rt rt    − (7) in equations (6) and (7), νa and νc denote the intercepts for anodic and cathodic process, respecttively, n is the number of electrons involved in the redox process, while other symbols have their usual significance. here, eo’ corresponds to the formal potential or the mean value of the anodic and cathodic peak potentials obtained at 5 mv/s. the number of electrons (n) involved in the redox process was calculated to be 2.01 and 1.55 for da and ua respectively, using equation (8) at 5 mv/s. http://dx.doi.org/10.5599/jese.783 j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 274 p 0 059 δ . e n = (8) from figures 8c & 9c, an excellent linearity between ep(a,c)-eo’ and log ν can be evidenced for da and ua, respectively. the values calculated using equations (6-8) are summarized in table 2. it can be deduced from table 2 that ks for da is higher than the value observed for poly(l-aspartic acid) modified gce (1.13 s-1) [53]. on the other hand, ks for ua is greater than the value found for modified carbon nanotube paste electrode (1.17 s-1) [54]. table 2. summary of kinetic parameters for da and ua at apge estimated from laviron’s theory parameters da ua α 0.58 0.60 n 2.01 1.55 ks/cm s -1 4.87 3.69 selective determination of da and ua at apge in the presence of high concentration of ascorbic acid since ascorbic acid (aa)ispresent at high concentration levelsin human body fluids, it is imperative to carefully evaluate the ability of apge to selectively determine da and ua in the presence of aa. moreover, it is the prime objective of the current study. the comparison of differential pulse voltammograms of da, ua and aa measured separately and at mixtures are shown in figure 10. it is evident from figure 10 that apge doesn’t produce any signal for aa even if its concentration is high (2 mm). thus, it is apparent from these voltammograms that apge is able to detect da and ua selectively without any interference from aa. figure 10. comparison of various differential pulse voltammograms at apge in 0.1 m pbs (ph 7.0) and denoted concentrations of da, ua and aa the discriminating ability of apge towards da, ua and aa can be understood in the following way [11,38,43]. the anodization process creates negative surface charge on apge due to the formation of oxygen rich functional groups. it is well known that da and ua exist in their protonated forms at neutral or physiological ph, while aa exists in deprotonated form. p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 275 furthermore, during the positive potential scan the negatively charged oxygen rich groups on apge surface favor the formation of hydrogen bonds with both da and ua. since, the voltammetric current response for da oxidation is higher than that of ua (figure 10), there may be another non-covalent interaction like π-π interaction in addition to the existing electrostatic and hydrogen bonding interactions between apge and da. on the other hand, the imperceptible voltammetric response observed for aa in figure 10 is an indication of the absence of any such interactions between aa and apge surface. the electrode mechanism described above is concisely summarized in scheme 2. scheme 2. plausible mechanism for the selective detection of da and ua in the presence of aa at apge simultaneous detection of da and ua at apge the main objective of this study was to evaluate the ability of apge to simultaneously determine da and ua in the presence of aa. in this regard, three differential pulse voltammetric experiments were carried out as follows: (i) 10 µm of da and 2 mm of aa is kept constant while the concentration of ua is changed, (ii) 20 µm of ua and 2 mm aa is kept constant and the concentration of da is varied, (iii) both da and ua concentration is simultaneously altered in the presence of 2 mm aa. the corresponding differential pulse voltammograms are given in figures 11a-c. it can be inferred from figures 11a-c that with the increase in the concentration of da and ua, the peak currents increase linearly. furthermore, the linear calibration plots constructed from these dpv responses are given in figures 11d-f. on close inspection of these calibration plots, it can be deduced that, in all three cases two linear ranges were observed in the concentration regions of 1-10 µm and 20-50 µm, respectively. the linearity observed in figures 11d-f indicates the excellent ability of apge to simultaneously determine da and ua even in the presence of high concentration of aa without any cross interference. the results obtained in the present study are compared in table 3 with these obtained for other chemically modified carbon electrodes already reported in the literature for the simultaneous detection of da and ua. http://dx.doi.org/10.5599/jese.783 j. electrochem. sci. eng. 10(3) (2020) 263-279 sensitive determination of dopamine and uric acid 276 figure 11. dpvs obtained at apge in 0.1 m pbs (ph 7.0) with different concentrations of (a) da containing 20 µm of ua and 2 mm of aa, (b) ua containing 10 µm of da and 2 mm of aa, and (c) da and ua containing 2 mm of aa. concentrations varied in (a-c) were 1, 2.5, 5, 7.5, 10, 15, 20, 30, 50, 60 and 80 µm. corresponding calibration plots for (a), (b), and (c) are given in (d), (e) and (f). table 3. comparison of the analytical performance of apge for da and ua determination with some previously reported modified electrodes modified electrode pretreatment method/ conditions pretreatment solution pre-conditioning linear range, μm lod, μm sensitivity, μa/μm# ref. da ua da ua da ua ctabpani/ac/gce 0.1 m pbs (ph 7.0) no 0.3 – 20 1 – 20 0.06 0.20 1.21 0.47 [12] gce cv / 0 to 0.9 v at 30 mv/s 0.5 m naoh no 3.0–30 5.0–70 2.67 4.7 0.52 0.19 [56] gce cv / 0.0 to +2.0 v at 100 mv/s 0.1 m h 2 so 4 no 1.97–9.88 19.7–98.8 3.16 0.83 [57] poly(ebt)/ gpe cv / -0.4 to 1.4 v at 100 mv/s 0.05 m h2so4 no 100 – 300 200 – 2000 [58] ppge cv / 1.5 to 2.0 v at 100 mv/s 0.1 m pbs (ph 5.0) yes 0.15–15 0.3–150 0.033 0.12 0.18 0.02 [11] sgpge ca / 3 v for 150 s 2 m naoh yes 0.15 – 45 0.0082 20.81 [59] p(p3ca) / eopge ca / +1.8 v for 60 s 0.1 m pbs (ph 7.4) yes 0.01 – 7.5 0.0025 29.50 [38] etpge cv / -0.3 to 2.0 v at 50 mv/s 0.1 m h3po4 yes 0.01-5 0.0031 17.19 [39] apge ca / + 2.0 v for 120 s 0.1 m pbs (ph 7.0) no 1 – 80 1 – 80 0.008 0.014 34.32* 12.33* this work ctab-pani/ac/gce – cetyl trimethylammonium bromide – polyaniline / activated charcoal / glassy carbon electrode; gce glassy carbon electrode; poly(ebt)/gpe – poly(eriochrome black t) / graphite pencil electrode; ppge pretreated pencil graphite electrode; sgpge – surface-graphenization pencil graphite electrode; p(p3ca) / eopge – poly(pyrrole-3-carboxylic acid) / electrochemically over-oxidized pencil graphite electrode; etpge – electrochemically treated pencil graphite electrode; # slope of the calibration plot; * from the slope of the calibration plot in the low concentration range. it can be inferred from table 3 that apge exhibits higher sensitivity without any preconditioning of the electrode in comparison to other pretreated pges and gces. by comparing the results of other pretreated electrodes in table 3, it is obvious that the electrodes anodized using chronoamperometry displayed better sensitivity than electrodes modified with cv. this may be attributed to more effective oxidation of carbon surface at constant potential than during a potential sweep. lod values were calculated using 3σ/m. here, σ is the standard deviation of the mean value for ten dpvs of the blank solution determined according to iupac regulations [55] and m denotes the slope estimated from the regression equation of the corresponding calibration plot. p. sankaranarayanan and m. venkateswaran j. electrochem. sci. eng. 10(3) (2020) 263-279 http://dx.doi.org/10.5599/jese.783 277 repeatability, reproducibility and stability in order to access the applicability of any electrochemical sensor, repeatability, reproducibility and stability are important figures of merit apart from sensitivity and selectivity of modified electrodes. the repeatability of apge was determined from 10 consecutive dpv responses in 0.1 m pbs (ph 7.0) containing 50 μm da and ua. the relative standard deviation (rsd) of 4.78 % for da and 5.29 % for ua, is an indication of excellent repeatability of the modified electrode. for reproducibility study, differential pulse voltammograms of five identically prepared apges were obtained in 0.1 m pbs (ph 7.0) containing 50 μm da and ua. an excellent reproducibility in the electrode preparation was apparent from rsd of 6.62 and 8.37 % respectively for da and ua. the stability of apge was determined from dpv response for 50 μm da and ua in 0.1 m pbs (ph 7.0) after one month of storage at room temperature. the long-term stability of apge was evident from the very minor decrement in the oxidation current response for da (2.85 %) and ua (3.32 %). real sample analysis the standard spike and recovery analysis were employed for the determination of da in blood serum and ua in human urine samples using dpv. the recovery results are given in table 4 and it can be seen that apge was able to determine da and ua with a satisfactory recovery in the range of 98-100 %. thus, apge can be utilized for real-time detection of da and ua. table 4. results for spike and recovery analysis of da and ua in blood serum and human urine samples using apge sample analyte amount, µm recovery, % added found blood serum da 30 29.81 99.36 50 50.32 100.60 70 69.24 98.91 human urine ua 0 0.15 30 30.13 100.40 50 50.11 100.20 70 70.08 100.11 conclusions the suitability of a disposable electrochemical sensor for da and ua using electrochemically anodized pencil graphite electrode (apge) was successfully demonstrated. apge was found to simultaneously distinguish between da and ua even in the presence of 2mm of aa, thereby indicating its exceptional selectivity. moreover, the proposed apge sensor exhibited excellent analytical performance with satisfactory results. unlike previously reported results on pretreated pencil graphite electrodes, the proposed apge displayed higher sensitivity with low limits of detection without any preconditioning potential. from the good recovery results observed for da in human blood 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[58] u. chandra, b. e. k. swamy, o. gilbert, s. reddy, b. s. sherigara, american journal of analytical chemistry 2 (2011) 262–269. [59] x. fan, y. xu, t. sheng, d. zhao, h. yuan, f. liu, x. liu, x. zhu, l. zhang, j. lu, microchimica acta 186 (2019) 324–332. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.783 http://creativecommons.org/licenses/by/4.0/) electrochemical decolourization of reactive black 5 in an undivided cell using ti and graphite anodes: doi:10.5599/jese.164 145 j. electrochem. sci. eng. 5(2) (2015) 145-156, doi: 10.5599/jese.164 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical decolourization of reactive black 5 in an undivided cell using ti and graphite anodes: effect of polypyrrole coating on anodes snehal a. popli and upendra d. patel*, civil engineering department, c. s. patel institute of technology, charotar university of science & technology, gujarat, india *civil engineering department, faculty of technology & engineering, the m. s. university of baroda, gujarat, india corresponding author: patelupendra@gmail.com; tel: +91-9687961022 received: april 2, 2015; revised:august 5, 2015; published: august 26, 2015 abstract electrochemical decolourization of azo dye reactive black 5 (rb5) was studied using ti, graphite, and polypyrrole coated ti (ppy-sls-ti) and graphite (ppy-sls-g) anodes in the presence of sodium chloride as an electrolyte to investigate the effect of polypyrrol coating. the colour removal efficiencies were 52.6 %, 96.3 %, 51.6 %, and 41.0 % respectively for above anodes, at the end of 90 minutes of electrolysis at current density of 5 ma cm-2. the presence of polypyrrol coating resulted in lower decolourization than that in the absence of coating. the major characteristic absorption peaks of rb5 at 595 and 312 nm present initially, decreased consistently and no new peak emerged during electrolysis using uncoated ti and graphite anodes which indicated direct and/or indirect oxidation playing major role in decolourization. in case of ppy-sls-ti and ppy-sls-g anodes, a new peak at 254 nm emerged distinctly which was determined to be vinylsulfone (vs), an amine. generation and accumulation of vs suggested that oxidation of rb5 was suppressed while reduction at cathode played important role in rb5 decolourization using ppy coated anodes. the results obtained in the present study may lead to development of anodes for undivided cell, on which, the extent of oxidation may be controlled by a suitable coating such as ppy. keywords electrochemical oxidation; azo dye; vinyl sulfone; polypyrrol coating introduction dye wastewater is an environmental concern due to its huge quantity, variable nature, low biodegradability, chemical composition, and toxicity to aquatic life [1]. in the textile industry, 60 % http://www.jese-online.org/ mailto:patelupendra@gmail.com j. electrochem. sci. eng. 5(2) (2015) 145-156 electrochemical decolourization using ti & graphite anodes 146 of the dyes used are azo dyes, in which the azo (–n=n–) group is connected to an aromatic compounds forming a chromophoric group responsible to impart a characteristic colour to the dye. reactive dyes are widely used due to their relatively easy application in the dyeing process [2]. for the removal of dyes from wastewater, different techniques such as chemical precipitation, chemical oxidation, adsorption, and biological processes have been developed [3]. the electrochemical technologies (electrochemical oxidation (eo), electrochemical reduction (er), electrocoagulation (ec), indirect electrooxidation with strong oxidants, and emerging photoassisted electrochemical treatments for treating dye wastewaters have been proposed [4]. the application of these technologies is benefiting from advantages such as versatility, environmental compatibility, and potential cost effectiveness. for electrochemical treatment of industrial wastewaters, direct and indirect eo processes are widely used. in a direct eo process, pollutants are first absorbed on the anode surface and then destroyed by the electron transfer reaction. different anode materials, such as dimensionally stable anodes, noble metals, carbon-based anodes, metal oxides, and boron-doped diamond electrodes have been explored for degradation of dyes. in an indirect eo process, strong oxidants, such as ozone, chlorine, hypochlorite, and hydroxyl free radicals are generated electrochemically which, in turn, destroy the pollutants by the oxidation reactions. all of the oxidants are generated in situ and are utilized immediately [5-7]. graphite and metal oxide coated ti electrodes have been shown to be able to decolourize various dyes [8-11]. metal oxides can be coated directly on a substrate or through a conducting polymer already coated on the anode surface. conducting polymers are permeable to electroactive species and they are easy to coat on various substrates. conducting polymers with porous structures and high surface areas, such as polyaniline and polypyrole, are usually employed as matrix to incorporate noble metal catalysts for the electro-oxidation of small molecules such as hydrogen, methanol and formic acid [12,13]. polypyrrole (ppy) is a conducting polymer and can be formed chemically or electrochemically through oxidative polymerization of pyrrole, the final form of ppy is a long conjugated backbone as shown in figure 1. electrochemical synthesis is the most common method as it is simpler, quick and perfectly controllable [10]. figure 1. chemical structure of polypyrrole the electrical and physical properties of the polymeric films are considerably influenced by the conditions under which they are prepared, such as the electrochemical method of polymerization, concentration of monomer and doping agent and other synthesis conditions. in its neutral state the polymer is not conducting and only becomes conducting when it is oxidized and have a conductivity of 10-3 to 102 (ω cm)-1 [14-16]. use of electrodes coated with metal-nanoparticlesincorporated ppy as cathodes for electrochemical reduction (er) reactions have been reported. for example, sun et al. [17] demonstrated use of pd-ppy-ctab-ni foam cathode in a divided cell for er of 2,4-dichlorophenol (2 4-dcp). 2,4-dcp was dechlorinated to phenol and the cathode could be reused for 10 dechlorination cycles. similarly, er of nitrate and nitrite on ppy-cu cathode s. a. popli et al. j. electrochem. sci. eng. 5(2) (2015) 145-156 doi:10.5599/jese.164 147 in a divided cell is reported by nguyen et al. [18]. however, behaviour of ppy coated electrodes as anode has not been studied to the best of our knowledge. the main aim of the present study was to compare the performance of ti, graphite, and ppy coated ti (ppy-sls-ti) and graphite (ppy-sls-g) as anodes for electrochemical decolourization of rb5 in an undivided cell to investigate the influence of ppy film on the extent and mode of decolourization. materials and methods chemicals experimental chemicals including sulphuric acid (h2so4, 98 %), sodium lauryl sulphate (sls, nac12h25so4), sodium carbonate (na2co3), sodium sulphate (na2so4), sodium chloride (nacl), and oxalic acid (h2c2o4 x 2h2o) were of analytical standard and supplied by s d fine chem ltd., mumbai. pyrrole (c4h5n) was obtained from spectrochem pvt. ltd. ahmedabad. titanium material grade 2 (300×450×0.5 mm) was supplied by shree ambe engineering, vadodara. in all the experiments, the size of anodes (ti, graphite, ppy-sls-ti, or ppy-sls-g) was 4×3 cm. two stainless steel (ss) plates of the same size were used as cathodes. reactive black 5 (rb5) dye was supplied by a local dye industry. it was used without further purification. all solutions were prepared using distilled water. table 1. general characteristics of reactive black 5 (rb 5) [19, 20]. parameters value molecular formula c26h21n5na4o19s6 molecular weight 991.82 g/mol max 595 nm molecular structure some common information about rb5 is given in table 1. a calibration plot of absorbance at 595 nm versus various rb5 concentrations was constructed and found to be linear within the concentration range of interest with r2 value of 0.9985. preparation of ppy-coated ti and graphite electrodes before ppy coating, ti plate was pre-treated first. ti plate was placed in 0.3 mol l-1 na2co3 solution at 90 oc for 30 min to remove surface grease. then it was placed in 0.1 mol l-1 boiling oxalic acid for 30 min to remove surface oxides. finally, it was rinsed thoroughly with distilled water and soaked in ethanol for later use. during electropolymerization of ppy-sls composite film, the ti plate was used as anode, and the ss plate was used as cathode. ppy-sls film was formed on the surface of ti plate by electrodepositing in a 250 ml mixed solution containing 0.3 mol l-1 h2so4, 0.1 mol l-1 distilled pyrrole (py), and 1.0 g l-1 sls for 45 min with the electrodeposition current of 60 ma. the mixed solution was deoxygenated by ar for 5 min before the commencement of electrodeposition. ppy-sls/graphite plate was prepared similarly by taking graphite j. electrochem. sci. eng. 5(2) (2015) 145-156 electrochemical decolourization using ti & graphite anodes 148 plate which was polished by using sand paper and washed with distilled water prior to polymerization. figure 2 shows the ti plate before and after electro-polymerization of pyrrol. figure 2. ti (a) and ppy-sls-ti (b) plates experimental procedure experiments were carried out in a undivided electrolysis cell prepared using a 500 ml glass beaker with one central ti plate or ppy-sls/ti plate or graphite plate or ppy-sls/graphite plate as anode, and two ss plates as cathodes. 250 ml of electrolyte solution containing 50 mg l-1 of rb5 dye and 4.0 g l-1 of nacl was used in all the experiments. driving force for decolourization was provided by direct current power supply (aplab regulated d.c. power supply, l3202). the current density was set at 5 ma cm-2. unless and otherwise stated, samples were withdrawn at regular interval of 5, 10, 15, 20, 30, 45, 60, and 90 min and analysed for the colour removal. percentage of colour removal was calculated as: colour removal, % = ((c0-c)/c0) x 100 (1) where, c0 = absorbance at time t0, c= absorbance at time t. analytical methods uv/vis spectra of the collected samples were recorded in the 200 800 nm range using a cary 60 uv/vis spectrophotometer, purchased from agilent technologies. the scanning electron microscopy sem (philips esem edax xl30) was used to examine the surface morphology of the ppy coated ti plate. the plate was cut in size of 1×1 cm and air dried before the analysis. high performance liquid chromatography (hplc) analysis was performed using a perkin elmer series 200 chromatographic system equipped with diode array detector. the chromatographic separation was performed on a c-18 (250×4.6 mm) with 5 µm packing and was used as stationary phase.two solvent mixtures were used (a: 95 % h2o, 5 % acetonitrile, 25 mm ammonium acetate; b: 50 % h2o, 50 % acetonitrile, 25 mm ammonium acetate) with the following conditions: 020 min 100 % a; 20-25 min 50 % a and 50 % b; 25-30 min100 % a; at 1 ml min-1. the detection wavelength was set at 595 and 254 nm. chromatographs were obtained for reaction solutions after one hour of electrolysis using graphite ppy-sls-g anodes. results and discussions characterization of ppy coated ti plate figure 3 shows sem image of ppy-sls-ti plate. clusters of ppy with pores of different sizes may be noted. it may be observed that the surface is very rough, which may provide high surface area for reaction. s. a. popli et al. j. electrochem. sci. eng. 5(2) (2015) 145-156 doi:10.5599/jese.164 149 figure 3. sem image of ppy coating on ti plate electrochemical decolourization of rb5 using ti and ppy-sls-ti anodes time course profiles of rb5 decolourization during electrolysis using ti and ppy-sls-ti anodes are depicted in figure 4(a). it may be noted that the extent of decolourization at the end of 90 min electrolysis was almost same (51-52 %) for ppy-coated and uncoated ti anodes. this suggests that ppy coating did not have any appreciable effect on extent of decolourization. the corresponding first order kinetic plots shown in figure 4(b) show that initial rate of decolourization was faster in case of ti as compared to ppy-sls-ti. the initial rapid decolourization in case of ti may be attributed to direct and indirect oxidation followed by slower oxidation due to formation of protective layer of tio2 on anode surface. in case of ppy-sls-ti, it was observed that after 60 minutes of electrolysis, the ppy layer started getting peeled off from the plate and was found suspended in the solution. figure 4. time course profile (a) and first order kinetic profile (b) of rb5 decolourization using ti and ppy-sls-ti anodes (sample 250 ml, cell : undivided, rb5 concentration mg l-1, anode: ti plate or ppy-sls/ti plate, cathode : ss plate, electrolyte: nacl, electrode gap : 1 cm) electrochemical decolourization of rb5 using graphite and ppy-sls-g anodes electrolysis using graphite as an anode has been extensively studied. graphite is not only less expensive, but also lasts long as anode. time course profiles of rb5 decolourization during eo j. electrochem. sci. eng. 5(2) (2015) 145-156 electrochemical decolourization using ti & graphite anodes 150 using graphite and ppy-sls-graphite anodes are shown in figure 5(a). almost complete decolourization (96.3 %) of rb5 was achieved at the end of 90 min of reaction using graphite anode. this result is quite comparable to the results of similar studies reported in published literature. for example, the decolourization of acid brown and reactive blue dyes by anodic oxidation in a batch recycle electrochemical reactor using graphite plate as anode and ss plate as cathode was studied by el astoukhy et al. [12]. under the optimized condition and with 100 mg l-1 dye concentration, 100 % colour removal and 89 % cod removal was achieved [12]. the decolourization of acid orange 7 using graphite plate was reported to achieve more than 90 % efficiency after 80 minutes of electrolysis [9]. figure 5. time course profile (a) and first order kinetic profile (b) of rb5 decolourization using graphite and ppy-sls-g anodes (sample 250 ml, cell : undivided, rb5 concentration mg l-1, anode: graphite plate or ppy-sls/g plate, cathode : ss plate, electrolyte: nacl, electrode gap : 1 cm) on the other hand, rb5 decolourization in case of ppy-sls-graphite anode was limited to only 41 % at the end of 90 min. these results clearly indicate that ppy layer suppressed eo of rb5 on graphite anode. moreover, ppy layer started getting peeled off after 30 minutes of reaction and ppy flakes were seen suspended in the reactor. figure 5(b) shows that the rate of first order reaction was very rapid in case of graphite anode. a comparison of figures 4 and 5 shows that the presence of ppy either retarded the rate and/or decreased the extent of decolourization of rb5 irrespective of the support material, i.e. graphite or ti. also, graphite as anode is much more efficient than ti, probably due to formation of oxide layer on ti which prevents direct and indirect oxidation of rb5 on ti. table 2 compiles the results of rb5 decolourization using various anodes in the present study. table 2. comparison of performance of various anode materials for decolourization of rb5 (cathode – ss; current density 5 ma cm-2; electrolysis time – 90 min) anode plate colour removal, % reaction rate constant, min-1 ti plate 52.65 0.0237 ppy-sls/ti plate 51.65 0.0125 graphite plate 96.30 0.0514 ppy-sls/graphite plate 41.01 0.0101 s. a. popli et al. j. electrochem. sci. eng. 5(2) (2015) 145-156 doi:10.5599/jese.164 151 mechanism of electrochemical decolourization of rb5 at various anodes the transformation of a dye in an undivided cell may be due to its direct or indirect oxidation or reduction depending upon the working electrode. on the other hand, in a divided cell, oxidation occurs in anode compartment, simultaneously with reduction in cathode compartment.. it has been reported that all peaks of uv/vis spectrum of a dye consistently decrease with time without formation of any new peak in case of oxidation in a divided cell. del rio et al. [21] studied the electrochemical oxidation and reduction of reactive orange 4 (ro4) using ti/sno2-sb-pt and ss as anode and cathode, respectively, and reported that no new peak was generated during the oxidation of ro4 in the time-dependent uv/vis spectra. mendez-martinez et al. [22] studied the electrochemical reduction and oxidation pathways of rb5 with ni-pvc composite cathode and ni wire mesh anode in an undivided cell and found that rb5 spectra reduces with time and no new peaks were formed during the oxidation process. costa et al. [23] reported decrease in the absorbance values of peaks during the electrochemical oxidation of acid black -210 using boron doped diamond electrode as an anode. similarly, kariyajjanavar et al. [24] reported consistent decrease in all the peaks and the absence of formation of any new peaks in an undivided cell during the degradation of novacron deep red c-d (ndrcd) and novacron orange c-rn (nocrn) reactive azo dyes from aqueous solution using graphite electrode. on the other hand, reduction of an azo dye is normally associated with formation of a new peak in uv/vis spectrum as a result of generation of amine due to cleavage of azo link/s. del rio et al. [25] studied the electrochemical reduction of reactive orange 4 (ro4) and reported that the decolourization took place via cleavage of azo bonds and a new peak was formed at 246 nm which was identified as 2-amine-1,5-napthalenedisulfonic acid as one of the amines. vidhu and phillip [26] studied the catalytic reductive degradation of methyl orange by sodium borohydride and biosynthesized silver nanoparticles. the authors reported that during the decolourization, the main chromophoric peak of mo at 465 nm gradually disappears and a new peak emerges at 247 nm. in our case as seen from figure 6, uv/vis spectrum of rb5 at t= 0 is represented mainly by two peaks viz. 595 nm and 312 nm. it may be noted from table 1 that each molecule rb5 consists of h acid (1amino-8-hydroxynaphthalene-3,6disulfonic acid) coupled with two molecules of vs (4-aminophenyl sulfonyl-hydroxy ethyl sulfato ester sodium salt) through azo (-n=n) bonds [20]. the peak at 595 nm corresponds to chromophoric group –n=nand the peak at 312 nm corresponds to naphthalene (h-acid) rings. the dye decolourization, besides occurring at anode surface through direct oxidation, may also occur through indirect oxidation in the presence of nacl as an electrolyte. the anodic oxidation of chloride ions results in chlorine evolution as shown in equations (2-4). in water, chlorine rapidly disproportionate, yielding hypochloric acid and hypochlorite ion [25], which can rapidly oxidize organic compounds. anode: 2cl¯ → cl2 + 2e¯ (2) bulk solution: cl2 + h2o → hocl + h+ + cl¯ (3) hocl⇄h+ + ocl¯ (4) as seen from the figure 6, it was observed that with graphite as anode, the peaks at 595 and 312 nm decreased consistently with time. moreover, no new peak emerged except that uv absorption in 200-260 nm range slightly increased probably due to formation and accumulation of less conjugated, partially oxidized intermediates. this observation indicates that oxidation of rb5 was a predominant mechanism responsible for decolourization with graphite as an anode in an undivided cell. j. electrochem. sci. eng. 5(2) (2015) 145-156 electrochemical decolourization using ti & graphite anodes 152 figure 6. uv-vis spectra of rb5 at various time points during electrolysis using graphite anodes a careful observation of uv-vis spectra (figure 7(a)) of rb5 obtained in a similar manner using ppy-sls-g anode, shows that while the peak at 595 nm consistently decrease with time, a new peak emerges and increases with time at 254 nm indicating the formation of an amine. similar phenomenon was also observed when using ppy-sls-ti as anode. (a) (b) figure 7. (a) uv-vis spectra of rb5 decolourization using ppy-sls/g anode, (b) time course profile of rb5 decolourization and absorbance at 254 nm s. a. popli et al. j. electrochem. sci. eng. 5(2) (2015) 145-156 doi:10.5599/jese.164 153 figure 7(b) shows a time-course profile of rb5 decolourization and concomitant increase in the absorbance at 254 nm. the chemical reduction of dye takes place producing hydrazo compound in the partial reduction, and aromatic amines in total reduction. equation 5 shows generation of expected amines during the reduction of rb5. during the reduction it generates two moles of vs and one mole of h-acid. vs has been shown to accumulate when rb5 is subjected to chemical reduction process such as electrocoagulation with iron sacrificial anode [20], electroreduction [27] or anaerobic biodegradation [28-30]. thus, it is interesting to note that vs is generated during rb5 oxidation in an undivided electrolytic cell with sodium chloride as an electrolyte. these reaction conditions (nacl as an electrolyte and undivided cell) would otherwise promote oxidation of rb5 as shown in figure 6 and discussed in the published literature. this suggests that under identical reaction conditions, the presence of ppy coating suppressed oxidation and promoted reduction of rb5 on cathode resulting in generation and accumulation of an amine, vs. (5) additional evidences to support the above-mentioned hypothesis were obtained using hplc analysis. figure 8 shows hplc chromatographs of pure vs (figure 8(a), vs: 50 mg/l, retention time 3.57 min.), reaction solution after 60 min of electrolysis using graphite as anode (figure 8(b)), and reaction solution after 60 min of electrolysis using ppy-sls-g as anode (figure 8(c).it may be noted from figure 8b (which represents electrochemical oxidation in an undivided cell in the presence of nacl as an electrolyte) that the peak at 28.25 min which corresponds to rb5 was completely disappeared; and no new peak/s emerged. a small peak appears at 2.53 min in figures 8b and 8c which may be an inherent impurity in the rb5 sample. figure 8c shows that rb5 removal is partial as rb5 peak at 28.25 min exists. it is important to note that a new peak at 3.2 min appears which closely matches with the retention time of 3.57 min of pure vs. slight difference in the retention times may be due to the difference between ph of the pure vs solution and reaction solution or slight difference in the chemical structures of pure vs and vs generated during electrolysis. it is reported that in reductive systems such as bimetallic [31,32], and palladium-hydrogen (pd-h2) [33] systems, electrons specifically attack electron-withdrawing groups such as c-cl in chlorinated compounds or –n=ngroups in azo dyes, respectively, resulting in dechlorination and cleavage of azo bonds. thus, vs, an amine which is a product of reductive cleavage of –n=nas shown in equation 5, generated and accumulated when ppy coated anodes were used. j. electrochem. sci. eng. 5(2) (2015) 145-156 electrochemical decolourization using ti & graphite anodes 154 figure 8. hplc chromatograms of reaction solutions at various conditions: (a) pure vs, 50 mg l-1 (b) reaction solution after 60 min of electrolysis using graphite as anode (c) reaction solution after 60 min of electrolysis using ppy-sls-g as anode. conclusions  graphite plate as anode achieved the highest rb5 decolourization of 96.3 % in 90 minutes electrolysis time with a first order reaction rate constant of 0.2161 min-1. on the other hand, initially higher rate of decolourization slowed down greatly probably due to formation of oxide layer in case of ti anode. in te n si ty , a .u . s. a. popli et al. j. electrochem. sci. eng. 5(2) (2015) 145-156 doi:10.5599/jese.164 155  the presence of ppy coating suppressed the rate and extent of rb5 decolourization for both graphite and ti anodes. lower decolourization and hence lower oxidation in case of ppy coated plates may be attributed to 1) lesser affinity of rb5 molecules for oxidation on ppy, and/or 2) lesser affinity of chloride ions to get oxidized at ppy and produce oxidative species, or 3) consumption of oxidative chlorine species for oxidation of ppy rather than rb5.  major peaks at 312 and 595 nm in the uv-vis spectra present at the beginning of the reaction, decreased consistently with time and no new peaks were generated in case of graphite as an anode. based on similar observations in published literature it was concluded that direct/indirect oxidation of rb5 may be responsible for the decolourization in a divided cell with nacl as an electrolyte and bare graphite as anode.  rb5 decolourization using ppy coated graphite was much slower and incomplete as compared to uncoated graphite. also, a new peak at 254 nm emerged and increased in the uv-vis spectra during electrolysis. hplc analysis suggested that the compound having absorbance at 254 nm may be vs which is an amine generated due to reductive cleavage of azo bonds.  it may be concluded that ppy coating on graphite suppressed direct/indirect oxidation of rb5 on anode in an undivided cell in the presence of nacl as an electrolyte. the results reported here may be used to fabricate anodes for an undivided cell wherein the oxidation of target compound is to be suppressed and reduction is to be promoted. references: [1] y. zheng and a. wang, journal of material chemistry 22(2012) 16552-16559. 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[33] u. d. patel and s. suresh, (us patent), us 8,496,837 b2 (2013). © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) 0 http://creativecommons.org/licenses/by/4.0/ {role of the newly synthesized brightener in modification of surface properties of zn-ni alloy electrodeposited on steel substrate} http://dx.doi.org/10.5599/jese.606 175 j. electrochem. sci. eng. 9(3) (2019) 175-185; http://dx.doi.org/10.5599/jese.606 open access: issn 1847-9286 www.jese-online.org original scientific paper role of the newly synthesized brightener in modification of surface properties of zn-ni alloy electrodeposited on steel substrate jyoti s. kavirajwara, basavanna shivarudraiahb, , yanjerappa arthoba nayakac adepartment of chemistry, the oxford college of engineering, bangalore-560068, karnataka, india bdepartment of chemistry, btl institute of technology & management, bangalore-560099, karnataka, india cdepartment of chemistry, kuvempu university, shankaraghatta, shimoga-577451, karnataka, india corresponding author: drsbasavanna@gmail.com; tel.: +91 9901984626 received: august 17, 2018; revised: march 3, 2019; accepted: march 4, 2019 abstract in the present study, a new brightener was synthesized by condensation of salicylaldehyde and cysteine hydrochloride (sc). to examine the influence of sc on the nucleation mechanism of zn-ni alloy, electrodeposition, cyclic voltammetric and chronoamperometric studies were carried out. the model of schariffker and hills was used to analyze current transients which explained the electrocrystallization process of zn-ni alloy. it is revealed that zn-ni electrocrystallization process in presence of sc is regulated by instantaneous nucleation mechanism. the corrosion studies were done for the bright and dull zinc-nickel alloy coatings in 3.5 wt.% nacl solution, using potentiodynamic polarization and electrochemical impedance spectroscopic techniques. the phase structure, surface morphology and brightness of the deposit were characterized by means of x-ray diffraction (xrd) analysis, scanning electron microscopy (sem) and reflectance studies. these studies revealed the role of sc in producing a bright zn-ni alloy coating on mild steel substrate and also showed its improved corrosion resistant nature. keywords nucleation; electrocrystallization; electrodeposition; zinc-nickel, schariffker and hills model introduction zinc and its alloy coatings are finding numerous applications in different industries like automotive, electrical, aerospace etc., as sacrificial metallic coatings for the protection of steel components [1]. zinc-nickel alloys obtained by electrodeposition process with the amount of nickel varying from 8 to 14 % by weight, facilitate corrosion protection and have other physical properties http://dx.doi.org/10.5599/jese. http://dx.doi.org/10.5599/jese.606 http://www.jese-online.org/ mailto:drsbasavanna@gmail.com j. electrochem. sci. eng. 9(3) (2019) 175-185 surface properties of zn-ni alloy 176 five to six times superior to pure zinc coatings [2]. amongst all commonly electroplated alloys, zincnickel alloy is the most commonly exploited alloy in commercial applications. this is due to its ecofriendly and economic nature as compared to toxic and expensive cadmium coatings [1]. electrodeposition of zn-ni alloys is interesting, because these alloys exhibit significantly higher corrosion resistance and better surface morphology than pure zinc [3]. although nickel is more noble metal than zinc, the co-deposition of zinc and nickel in zn-ni alloy is anomalous [4-5] and a higher percentage of zinc is present in the final deposit. according to lin’s conclusion, anomalous electrodeposition of zinc-nickel alloy is caused by the slow kinetics and hydrogen evolution at the nickel deposit. zinc-nickel alloy provides a sacrificial protection to steel [6]. many studies have been carried out to understand the characteristics of zn-ni alloy deposit. it has already been found that characteristics of the deposited coating depend on the applied voltage, current density, bath composition, ph, additives, temperature etc. the phases and the microstructure of the surface of deposited zn-ni alloy are other important characteristics which control the corrosion resistance and other mechanical properties [7]. brighteners are often added into the plating bath solution of common metals such as zinc, nickel or cobalt to improve properties of the final deposit and to make the surface more durable, compact and uniform for better performance in terms of corrosion protection [3,8]. brighteners are usually added into the electrolytic bath at very low concentration, in parts per million, but their presence in the bath promotes formation of smooth and shiny coatings [9]. experimental bath solutions were freshly prepared using analytical grade chemicals in distilled water. the standard hull cell of 267 ml capacity was used to optimize the bath constituents and bath variables. the hull cell experiments were carried out without agitation [10]. ph value of the plating bath solution was measured using a digital ph-meter and adjusted with 10 % v/v h2so4 or sodium bicarbonate solution. zinc plate of 99.99 % purity was used as anode and activated each time by immersing in 10 % v/v hcl for 4-5 seconds, followed by washing with the running water. the mild steel plates of standard hull cell size were mechanically polished to obtain smooth surfaces and degreased by dipping in the boiling trichloroethene. the scales and dust on the steel plates were removed by dipping in 10 % v/v hcl solution and then subjected to electro-cleaning process. thereafter, steel plates were washed with distilled water, dried and then used for the experiments. after the experiment was finished, the plate was removed from the solution and subjected to bright dip in 1 % v/v hno3 solution for 3-5 seconds, followed by water washing and drying. the condensation product was synthesized from equimolar amounts of salicylaldehyde (1 mm) with chemical formula c7h5o (ar grade, s.d fine chemicals) and cysteine hydrochloride (1 mm) with chemical formula c3h10clno3s in ethanol medium (50 ml) under reflux condition for 3 hours at 343 k [11]. the completion of the reaction was confirmed by thin layer chromatography (tlc). the resulting product was diluted to 100 ml with distilled water and a known amount of this solution was added to the plating bath solution. the bath solution was stirred for 30 min before conducting the experiments, in order to properly mix the condensation product with the plating bath solution. the cyclic voltammetry and chronoamperometric studies were performed using chi660d electrochemical workstation with a three-electrode system. a graphite electrode of geometrical area 0.02 cm2, a platinum wire and a saturated calomel electrode (sce) were used as the working, counter and reference electrodes respectively. before conducting each experiment, the working electrode was polished to a mirror finish with 0.05 µm alumina. jyoti s. kavirajwar et al. j. electrochem. sci. eng. 9(3) (2019) 175-185 http://dx.doi.org/10.5599/jese.606 177 the corrosion behavior of zn-ni alloy coatings obtained from the plating bath in presence and absence of sc were studied in 3.5 wt.% nacl solution by galvanostatic polarization method and electrochemical impedance spectroscopic technique, using the electrochemical workstation (instrument model chi660d). a three-electrode assembly was used. the working electrode (we), i.e. zn-ni coated mild steel plate was masked with a lacquer to expose 1 cm2 area. a platinum foil was used as a counter electrode (ce) with saturated calomel as a reference electrode (re). polarization experiments were performed in the potential range between -0.9 v and -1.4 v at the scan rate of 0.01 v s-1. impedance spectra were measured with the perturbation signal amplitude of 0.005 v at frequencies between 100 khz and 1 hz. the composition and surface morphology of deposits were analyzed using sem images. x-ray diffraction studies were used to find the preferred orientation of zn-ni alloy crystallites and the phases present in the alloy deposit, both in presence and absence of sc. the x-ray diffractometer with a cukα radiation was used to obtain xrd spectra. jcpds powder diffraction file cards were used for phase identification. the preferred orientation of the deposits was determined by markesan’s method, using the following equation for calculating the texture coefficient (tc) =   o c o (hkl) (hkl) 100 (hkl) (hkl) i i t i i (1) where i(hkl) is the peak intensity of zn-ni electrodeposit and ∑i is the sum of the intensities of independent peaks. the index ‘o’ refers to the intensities for the standard zn-ni sample (jcpds 10-0209). the orientation with the maximum texture coefficient value is the preferred orientation of the zn-ni alloy crystallites in the deposit [12]. the percentage reflection of the deposit was determined using ocean optics usb 4000 spectrophotometer, referenced against a silver mirror. the reflectivity of silver mirror was set at 100 % and the measurements were carried out at different surface points of the deposited sample. results and discussion electrodeposition process the bath constituents and operating conditions were optimised by hull cell experiments to get a bright deposit over wide current density range [13]. the plating bath composed of zinc sulphate, nickel sulphate, sodium sulphate, cetyltrimethylammonium bromide (ctab) and boric acid. zinc sulphate and nickel sulphate were used as the main electrolytes, while sodium sulphate was used as an additional electrolyte which increased conductivity of the plating bath solution. ctab was used as a wetting agent, while boric acid was added to improve the morphology, brightness and adhesion of the deposited nickel. from the available literature it has also been found that boric acid can act as a very good catalyst by lowering the over-voltage, what prefers deposition of nickel instead hydrogen evolution. also, the literature revealed that boric acid plays an important role because it forms a complex, from which nickel can be, discharged more easily [14]. during the hull cell experiments, a particular concentration at which good bright deposit is obtained was fixed as the optimum concentration. the procedure was repeated for all bath constituents and operating conditions. the optimized bath composition is given in table 1. the basic bath gave coarse dull deposit within the current density range of 0.5 – 3 a dm-2.the optimized bath solution gave mirrorbright deposit within the current density range of 1 – 6 a dm-2. http://dx.doi.org/10.5599/jese.xxx j. electrochem. sci. eng. 9(3) (2019) 175-185 surface properties of zn-ni alloy 178 table. 1 optimized bath composition and operating conditions bath constituents concentration operating conditions znso4.7h2o 0.5 m anode: zinc plate (99.99 %) niso4.6h2o 0.1 m cathode: mild steel plate na2so4 0.29 m ph 3.5 h3bo3 0.26 m temperature= 298 -303 k ctab 0.01 m plating time: 10 min brightener (sc) 40 ml l-1 bright current density range: 1-6 a dm-2 voltammetric response cyclic voltammetry was used to define the major characteristics of zn-ni alloy deposition process with and without sc as an additive. the results are shown in figure 1. in absence of sc during cathodic scan, the cathodic peak potential is placed at -1.17 v (e1). this peak is associated with simultaneous reduction of both zn2+ and ni2+ ions to zinc and nickel atoms, respectively. on reversing the sweep direction, two current crossovers appear in the cathodic region. the more cathodic potential at which the crossover occurs is known as the nucleation overpotential (eη) [13]. the second crossover at zero current region (-1.02 v) is known as crossover potential (eco). two crossovers are characteristic of three dimensional (3d) and subsequent crystal growth process. in the absence of sc in the optimized bath solution, four oxidation peaks (a1, a2, a3 and a4) are seen in the voltammogram. figure1. typical cyclic voltammogram of zn-ni alloy coating deposited on graphite electrode from optimized zinc-nickel bath solution (table 1), in presence (solid line) and absence (dotted line) of sc, in the potential range of 0 v to -1.42 v at scan rate of 0.05 vs-1 the multiple peaks detected during the electrochemical oxidation of alloys are due to dissolution of metals in the alloy via different intermediate phases [13]. four anodic dissolution peaks for zn-ni alloy correspond to the dissolution of three phases in zinc-nickel alloy deposit. they are η-phase (zn), -phase (ni3zn22) and γ-phase (ni5zn21). the first anodic peak (a1) at the potential of -0.89 v corresponds to the oxidation of zinc from the η-phase, which is almost pure zinc phase. the second (a2) and the third (a3) anodic peaks at potentials -0.76 v and -0.54 v respectively, correspond to dissolution of zinc from δ-phase and γ-phase respectively. the fourth anodic peak (a4) at nobler potential (-0.27v) corresponds to the dissolution of nickel from ,  and  phases [3,9,15]. thus, the voltammetric response gives information regarding the characteristics of the components of zn-ni jyoti s. kavirajwar et al. j. electrochem. sci. eng. 9(3) (2019) 175-185 http://dx.doi.org/10.5599/jese.606 179 alloy and the structure of deposited phases. as also shown in figure 1, in presence of sc, the plating bath cathodic potential (e2) was shifted negatively to -1.41 v, which indicates that sc acts by adsorbing on the cathodic surface. adsorbed sc created a barrier near the electrode surface that inhibits the discharge of metal ions, leading to a decrease in the grain size of the deposit [13]. the voltammogram in figure 1 also shows two broad anodic peaks (a2 and a4) at -0.73v and -0.25 v. the first broad peak (a2) corresponds to oxidation of zinc from the η-phase (zincrich phase) and the second broad peak (a4) corresponds to the dissolution of nickel from η and γ phases. it can also be noticed that anodic peaks are slightly shifted to more positive potential in presence of sc. this indicated that oxidation of zn-ni alloy becomes more difficult due to the adsorption of sc and hence, the coating appears more smooth, compact and uniform. similar results were observed in the literature [13]. chronoamperometric analysis chronoamperometric analysis was performed to identify the nucleation mechanism at different potentials. the primary factor leading to the generation of fine-grained deposit is the formation of fresh nucleation sites and their growth rate. chronoamperometric technique has proved to be a powerful way for explaining the mechanism of new phase formation (electrocrystallization process) [16]. for most of metals, the charge transfer rate is high, and the continuous growth of formed nuclei is entirely mass transfer controlled. according to the theory, two cases of nucleation can be considered based on the rate of nucleation. in the first case, at increased nucleation rate all nuclei are instantly formed after the potential step and their number remains constant during the growth process. this is called instantaneous nucleation and is described by the following equation:  1     = − −    1/2 1/2 ! exp( ) ( ) zfd c j n kdt t (2) in eqn. (1), j! = current density related to the geometric area of the electrode surface, n is the total number of nuclei formed, zf is the molar charge of the deposited species, d is the diffusion coefficient, c is the bulk concentration (mol dm-3) and k is the numerical constant calculated from the following equation: 8    =     1/2 cm k (3) in eqn. (3), m and  are the molar mass and metal density, respectively. in the second type of nucleation, at decreased nucleation rate, the nuclei are continuously formed during the entire time, before overlapping of the diffusion hemispheres around the growing nuclei. this process is called progressive nucleation [16]. in this type of nucleation, the metal clusters of different sizes are formed, especially at the initial time (t). it is described by the following equation:         −  =              1/2 2 1/2 ' ! 1-exp ( ) 2 zfd c an k dt j t (4) in eqn. (4), no is the number density of substrate active sites, a is the steady state nucleation rate constant per site, while k' is the numerical constant and is given by: 8 1/2 4 '= 3 cm k         (5) http://dx.doi.org/10.5599/jese.xxx j. electrochem. sci. eng. 9(3) (2019) 175-185 surface properties of zn-ni alloy 180 figure 2 shows the current versus time (i-t) transients, recorded during metal reduction in the potential range from -1.12 to -1.18 v in absence of sc and -1.34 to -1.46 v in presence of sc in the plating bath. the transients exhibit the typical shape for a nucleation process with threedimensional growth of nuclei limited by diffusion of the electroactive species [16]. at short times an increase in the current density is observed, corresponding to beginning and growth of the nuclei. at later stages, diffusion zones of adjacent nuclei overlap, and current density reaches a maximum followed by a decaying portion, converging to a limiting current corresponding to linear diffusion of electroactive ions to a planar electrode (cottrell equation). figure 2. current transients for zn-ni deposition at different potentials from optimized plating bath solution (table 1) in (a) absence and (b) presence of sc. the model of 3d nucleation with hemispherical diffusion control of the growing 3d clusters, describes the kinetics of phase formation of growing clusters of zn-ni alloy crystallites in early stages, when diffusion of the depositing species from the bulk of the solution to the electrode/solution interface is the slow step of the process, considering the eventual overlap of diffusion zones and the development of nucleation exclusion zones around already established nuclei [17]. two limiting cases can be observed: instantaneous nucleation where all nuclei are formed immediately after the potential step and progressive nucleation where the number of nuclei increased during the whole deposition process. different theoretical models of nucleation were developed and tested in the last two decades in order to predict the behavior of potentiostatic current transients. a convenient method to identify the nucleation mechanism was proposed by schariffker and hills [16]. the method is based on rendering the curves dimensionless by referencing the current, i, at any instant of time to the maximum current, imax, and the time, t, to the time of maximum current, tmax. the following equations describe instantaneous eqn. (6) and progressive eqn. (7) nucleation:                      22 max max max 1.9542 = 1-exp -1.2564 / i t i t t t (6) 2 2 2 max max max 1.2254 = 1exp -2.3367 / i t i t t t                      (7) non-dimensional plots obtained using experimental and theoretical data for zn-ni alloy deposition in absence and presence of sc are shown in figure 3 at different potentials. jyoti s. kavirajwar et al. j. electrochem. sci. eng. 9(3) (2019) 175-185 http://dx.doi.org/10.5599/jese.606 181 t / tmax t / tmax figure 3. non-dimensional (i/imax) 2 vs. t/tmax plots for zn-ni deposition at different potentials from optimized plating bath solution (table 1) in (a) absence and (b) presence of sc. the following mechanism explains the anomalous behaviour of fe group metal ions on the electrode surface: 2h2o + 2e→ h2 + 2oh(8) m2+ +oh→ m(oh)+ (9) m(oh)+ → m(oh)+ads (10) m(oh)+ads + 2e→ m + oh(11) where m indicates nickel, iron and cobalt atoms. although many attempts have been made to explain the anomalous co-deposition of alloys, there is still no universally accepted theory [18]. the reduction rate of m mainly depends on the stability of m(oh)+ads or m(oh)+. the stability of zinc and nickel metal mono hydroxide ions or metal hydroxides can be sorted in the following order: ni(oh)+ > zn(oh) +. as elucidated above, the amount of nickel in the deposit is much lower than in the plating bath solution. also, the concentration of nickel ions near the cathode surface should be maintained at a constant value during the entire electroplating process [9]. the nucleation process of zn-ni deposit is always associated with release of hydrogen which is seen from equation (8). hence the experimental (i/imax)2 should be much larger than the theoretical model [3]. as seen in figure 3(a), in absence of sc, the rising part of the transients are located near the instantaneous nucleation curve at lower potentials. in presence of sc shown in figure 3(b), the normalized curves are located on the theoretical instantaneous nucleation curve under studied potentials. this result confirmed 3d instantaneous nucleation mechanism of zn-ni alloy deposition in presence of sc. another diagnostic criterion given by schariffker and hills nucleation model is based on the rising part of the current-time transient curve, which is in fact the analysis of initial stage of deposition [16]. it is possible to represent i versus t1/2 for instantaneous and i versus t3/2 for progressive nucleation. based on this model, i versus t1/2 for instantaneous nucleation are shown in figure 4. figure 4(a) shows linearly fitted plots of i vs. t1/2 in absence of sc, which indicate that at higher negative potentials the nucleation occurs by instantaneous nucleation mechanism. in figure 4(b), linearity is also obtained for all i vs. t1/2 plots, which indicates that even in presence of sc, the nucleation continues to occur by instantaneous mechanism at all studied potentials. e / v e / v (i / i m a x) 2 (i / i m a x) 2 http://dx.doi.org/10.5599/jese.xxx j. electrochem. sci. eng. 9(3) (2019) 175-185 surface properties of zn-ni alloy 182 figure 4. i vs. t1/2 plots for initial transient portions of zn-ni deposition at different potentials in (a) absence of sc and (b) presence of sc. data are taken from figures 2a and 2b. impedance and polarization studies corrosion resistance studies were carried out in 3.5 wt.% nacl electrolyte solution using electrochemical impedance spectroscopy (eis) and polarization (tafel extrapolation method) techniques. the results are presented in figure 5. a b 0 50 100 150 200 250 0 -20 -40 -60 -80 -100 -120 z' /  cm 2 a b z '' /  c m 2 (a) figure5.(a) nyquist impedance plots and (b) tafel polarization curves in the potential range -0.8v to -1.4v and scan rate 0.01vs-1 of zn-ni coatings on mild steel in 3.5 wt. % nacl solution. zn-ni electrodeposits are obtained in a absence and b presence of sc in the optimized plating bath solution (table 1). inset in fig. 5(a) shows electrical equivalent circuit used to simulate measured eis data. nyquist plots of zn-ni deposits obtained in absence and presence of sc in the optimized bath are compared in figure 5(a). the charge transfer resistance values of deposits prepared in absence and presence of sc were calculated with the help of zsimpwin 3.21 software as 164 and 313 ω cm2. this revealed that the bright coating increased the corrosion resistance of mild steel substrate, what is in good agreement with the polarization results shown in figure 5(b) obtained for dull and bright coatings on mild steel substrate. polarization data revealed decrease in icorr. value from 73.11 µa cm-2 (dull deposit) to 10.16 µa cm-2 (bright deposit). also, in presence of sc, ecorr.values are shifted to nobler direction, indicating reduction in corrosion rate and thereby increase in the corrosion resistance. these results suggest that the bright deposit can successfully act as a protective layer for mild steel substrate and can also improve the corrosion resistance of the decorative bright coating. w qdl rct rs jyoti s. kavirajwar et al. j. electrochem. sci. eng. 9(3) (2019) 175-185 http://dx.doi.org/10.5599/jese.606 183 surface morphology sem analysis and reflectance studies figure 6 shows the sem images of zn-ni alloy electrodeposits obtained in absence and presence of sc in the optimized bath solution. the deposit obtained in absence of sc shows coarse grained structure having randomly distributed crystallites, whereas the bright deposit shows fine-grained, uniform, smooth and compact deposit. this indicates that sc promotes grain refinement by increasing the number of nucleation sites and retarding the random growth of nuclei during deposition. a b figure 6. sem images of zn-ni coatings electrodeposited at 4 adm-2 from the optimized plating bath solution (table 1) in (a) absence and (b) presence of sc the ideal reflectance and the degrees of total reflection as a function of wavelength of visible light for zn-ni alloy electrodeposits formed in presence and absence of sc are shown in figure7. from the reflectance spectra, it is observed that bright deposits obtained in presence of sc in plating bath increases percentage of reflection in the visible region. this result confirmed that sc acts as a good brightener for zn-ni alloy coatings. figure 7. reflectance spectra of zn-ni coatings electrodeposited at mild steel electrode at 4 a dm-2 from the optimized plating bath solution (table 1) in (a) absence and (b) presence of sc x-ray diffraction analysis xrd patterns of dull and bright zn-ni alloy deposits are presented in figure 8, showing formation of lines corresponding to -phase zn (101) and additional lines that can be indexed considering a -ni5zn21 phase. all peaks correspond exclusively to zinc metal and zinc-nickel alloy, which means that there are no impurities in the deposit. it is also observed that there is an increase of the intensity -ni5zn21 phase line obtained in presence of sc in the plating bath solution. this increase in the intensity is due to the increase in percentage of nickel. http://dx.doi.org/10.5599/jese.xxx j. electrochem. sci. eng. 9(3) (2019) 175-185 surface properties of zn-ni alloy 184 figure 8. x-ray diffraction patterns of zn-ni electrodeposits observed in (a) absence and (b) presence of sc in the optimized plating bath solution (table 1) to know the preferred orientation of zn-ni alloy deposit, the texture coefficient was calculated for each peak in diffraction patterns. in dull deposit, the texture coefficient value was found to be maximum for (611) plane. hence (611) was the preferred orientation of the crystallites in dull deposit. in the case of bright deposit, the texture coefficient value was maximum for (630) plane. hence (630) was the preferred orientation of the crystallites in bright deposit. the adsorption of sc on different plane, changes the preferred orientation of the crystallites in the deposit from (611) to (630), which in turn alters the surface morphology of the deposit and thereby improves the corrosion resistant property. conclusions the newly synthesized brightener (sc) gives smooth, uniform, fine-grained and mirror-bright znni alloy coating electrodeposited from the optimized plating bath solution. the voltammetric response gives information regarding the components of the deposit and structure of deposited phases present in zn-ni alloy coating in presence and absence of sc. the chronoamperometric studies revealed that the mechanism of zn-ni alloy electrodeposition in presence of sc follows three dimensional (3d) instantaneous nucleation mechanism. corrosion studies confirmed that presence of sc in the plating bath improves corrosion resistance and hence protection ability of zn-ni alloy coating. sem and reflectance studies also confirmed that the presence of sc in zinc-nickel plating bath promotes the formation of smooth, shiny, compact and corrosion resistant coating. the performed studies suggest that the newly synthesized brightener (sc) can be conveniently used as a brightener in decorative applications and also for increasing the corrosion resistance of steel components in engineering and electroplating applications. acknowledgements: the authors are thankful to btlitm and kuvempu university, department of science and technology for providing the lab facilities to carry out this work. availability of data and materials – the data used and analyzed during the current study are available from the corresponding author on reasonable request. jyoti s. kavirajwar et al. j. electrochem. sci. eng. 9(3) (2019) 175-185 http://dx.doi.org/10.5599/jese.606 185 references [1] s. rashmi, l. elias, a. c. hegde, engineering science & technology 20 (2017) 1227-1232. https://doi.org/10.1016/j.jestch.2016.10.005 [2] s. b. ramesh, k.u. bhat, a.c. hegde, analytical& bioanalytical electrochemistry 3 (2011) 302-315. 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[17] a. e. alvarez, d. r. salinas, journal of electroanalytical chemistry 566 (2004) 393-400. https://doi.org/10.1016/j.jelechem.2003.11.051 [18] a. bai, c. c. hu, electrochimica acta 50 (2005) 1335–1345. https://doi.org/10.1016/j.electacta.2004.07.055 ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.xxx https://doi.org/10.1016/j.jestch.2016.10.005 https://doi.org/10.1007/s10800-009-9907-1 http://jes.ecsdl.org/content/162/9/d412.full https://file.scirp.org/html/9-7701149_38134.htm http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.619.2785&rep=rep1&type=pdf https://doi.org/10.1007/bf02711243 https://doi.org/10.1007/s10800-011-0263-6 http://nopr.niscair.res.in/handle/123456789/13703 https://doi.org/10.1021/ic00254a013 http://jes.ecsdl.org/content/136/8/2314.abstract https://www.ias.ac.in/article/fulltext/boms/037/05/1137-1146 http://dx.doi.org/10.4061/2011/742191 https://doi.org/10.1016/j.apsusc.2005.01.161 https://doi.org/10.1016/j.jelechem.2003.11.051 https://doi.org/10.1016/j.electacta.2004.07.055 http://creativecommons.org/licenses/by/4.0/) {electrochemical corrosion performance of si-doped al-based automotive alloy in 0.1 m nacl solution:} http://dx.doi.org/10.5599/jese.1373 565 j. electrochem. sci. eng. 12(3) (2022) 565-576; http://dx.doi.org/10.5599/jese.1373 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical corrosion performance of si-doped al-based automotive alloy in 0.1 m nacl solution maglub al nur1, akib abdullah khan1, somlata dev sharma1 and mohammad salim kaiser2, 1department of mechanical engineering, bangladesh university of engineering and technology, dhaka-1000, bangladesh 2directorate of advisory, extension and research services, bangladesh university of engineering and technology, dhaka-1000, bangladesh corresponding author: mskaiser@iat.buet.ac.bd; tel.: +88-02-9663129; fax: +88-02-9665622 received: may 6, 2022; accepted: june 6, 2022; published: june 13, 2022 abstract the aim of this study is to investigate the electrochemical corrosion behavior of al-si automotive alloys with different levels of si doping in 0.1 m nacl solution at room temperature. the study was performed by electrochemical method, using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. the condition of surfaces was characterized by both optical and scanning electron microscopy. both the eis and tafel analyses revealed that the corrosion resistance was improved with the addition of si up to the eutectic point, what is due to the formation of protective oxide films. the higher si added alloys showed lower values of current density, while the corrosion potential was shifted to a more positive direction. for higher si added alloys, a higher amount of mg2si was formed as precipitates, which tend to form oxides such as sio2 and mgo, further protecting the surfaces from corrosion. it can be observed from the micrographs that the scratches from polishing are removed after corrosion. additionally, the sem images reveal that corroded surfaces appear to have pits that are less noticeable in alloys with a greater si content, suggesting thus the formation of a protective layer of oxides. keywords al-si automotive alloy; protective oxide layer; corrosion potential; polarization resistance; microstructure introduction automotive industry focus on lightweight materials like aluminium-silicon alloys for manufacturing automotive components like engine blocks, cylinder heads, pistons, intake manifolds and other parts, replacing steel and cast iron [1-3]. these lightweight materials not only increase fuel efficiency but also enhance properties such as high elongation values, high strength-to-weight ratio, and excellent corrosion resistance [4,5]. recently, diesel and direct fuel gasoline engines were developed, which have a high specific power. because of working in diverse environments, these http://dx.doi.org/10.5599/jese.1373 http://dx.doi.org/10.5599/jese.1373 http://www.jese-online.org/ mailto:mskaiser@iat.buet.ac.bd j. electrochem. sci. eng. 12(3) (2022) 565-576 corrosion of si doped al-based automotive alloy 566 engines have a significant performance influence on piston materials [6]. corrosion is the process through which materials, especially metals, gradually deteriorate due to chemical or electrochemical reactions with their surroundings. corrosion of engine components results in loss of strength, hardness, toughness, and other desirable mechanical properties, which deteriorates engine performance. pure al is highly resistive to corrosion due to a passive film formed on its surface when exposed to the atmosphere. however, in the case of al alloys, corrosion resistance decreases as various alloying elements are added, such as cu, mg and fe. these alloying elements have their effects on the corrosion properties of al-si alloys. one important parameter affecting mechanical properties and corrosion resistance is the silicon content [7,8]. when silicon is added as the major alloying element, the alloy goes into the 4xxx series. the alloy can be hyper-eutectic, eutectic, or hypoeutectic, depending on the percentage of silicon in it. if there is 12.6 wt.% si in the al-si alloy, it is the eutectic alloy. the alloy is hypo and hyper eutectic if the percentage of si in the al-si alloy is lower and higher than 12.6 wt.%, respectively [9,10]. the mechanical properties of al-si alloys are enhanced in higher si added alloys, but after the eutectic composition, the strength properties start to deteriorate, as coarse polyhedral particles appear after eutectic composition, which is in fact, primary silicon [11]. if cu is present in the experimental alloys, it helps resist corrosion by forming a protective oxide layer, approximately 900 to 1000 nm thick [12]. the addition of si along with mg into the alloy creates the precipitates of mg2si, which facilitate the formation of oxides like sio2 and mgo as protective layers. these oxides further protect the surfaces from corrosion [13]. iron as an alloying element tends to form a iron-rich layer in fe2o3, which diminishes corrosion resistance [14]. the automotive alloy can get in contact with many solutions. the piston and cylinder of the engine continuously are in contact with engine fuels. often, some unexpected chemicals may mix with the fuel. sodium chloride can enter the fuels as sodium careers [15,16]. also, sodium chloride is present in engine cooling water, which has a deleterious effect on engine materials [17]. one of the key factors of nucleation of pitting is the adsorption of aggressive ions such as clinto the faults in the protective layer and their penetration and accumulation in these imperfections [18,19]. there are some studies about the role of si as an alloying element on the corrosion resistance, and the effect of nacl as a corrosive medium for al-si alloys. role of other alloying elements in automotive al-si alloys, such as cu, mg, fe, ni, etc., and investigations of their influence on mechanical and corrosion properties are ongoing. however, there is limited work on the effect of si on the corrosion behavior of automotive alloys, keeping the percentages of other alloying elements constant. the purpose of the present study is to investigate the corrosion behavior of automotive alloys with various levels of si, including an alloy with si as trace impurity, keeping the concentrations of other constituent elements constant, in nacl solution of 0.1 m strength. the electrochemical investigations have been carried out using the electrochemical impedance spectroscopy (eis) method, which provides values of solution resistance, polarization resistance, and double-layer capacitance. besides, the potentiodynamic polarization technique is also used, which provides the values of corrosion current, corrosion potential and corrosion rate. experimental to study the corrosion behavior of al-based automotive alloy, si of the different amounts was added to the alloy. for this purpose, commercially pure aluminium (al 99.750), copper (cu 99.997), magnesium (mg 99.80) and al-50 wt.% si master alloy ingots were used. the levels of cu and mg were kept constant to investigate the effect of si as an alloying element. fe, ni, sn, sb, etc, were m. al nur et al. j. electrochem. sci. eng. 12(3) (2022) 565-576 http://dx.doi.org/10.5599/jese.1373 567 also present in the alloys as trace impurities. the shimadzu pda 7000 optical emission spectrometer was used to determine the chemical composition of the test alloys. the main constituents of five alloys are given in table 1. table 1. chemical composition of experimental alloys (wt.%) alloy content, wt.% si cu mg fe ni al alloy 1 0.244 2.158 0.767 0.211 0.199 bal alloy 2 3.539 2.309 0.784 0.273 0.217 bal alloy 3 6.149 2.113 0.754 0.301 0.264 bal alloy 4 12.656 2.130 0.770 0.311 0.277 bal alloy 5 17.851 2.190 0.755 0.321 0.281 bal a natural gas-fired pit furnace and a graphite crucible were used to melt these alloys. to avoid oxidation during melting, a suitable flux cover was used. for the casting, a mild steel mold of 20  200 300 millimeters was preheated to 250 °c. a muffle furnace was used for homogenization, kept at 450 °c for 12 hours, and then air-cooled to relieve stress. the homogenized samples were solutionized at 535 °c for 2 hours. then the alloys were quenched in salt water to get a super saturated single-phase region. afterwards, the alloys were machined to remove the natural oxide layer. rectangular samples of the alloys (5  5  12 mm) were machined from the cast products. to achieve a smooth and refined surface, samples were polished using 320, 600, 800, 1200 and 2000 grit emery sheets. all the samples were subjected to age hardening at 200 °c for 4 hours to get the maximum strength [20,21]. a typical digital microscope was used for microstructural observations. the whole experiment was carried out at room temperature, at 25 ± 1 °c. the densities as well as equivalent weights of the alloys were calculated from the chemical composition. the electrochemical impedance spectroscopy of the experimental alloys was performed using a computerized ch instrument – electrochemical workstation. analytical reagent grade nacl was dissolved in deionized water to prepare 0.1 m solution of nacl. the tests were carried out in a standard three-electrode glass cell with a capacity of 100 ml. ag/agcl-kcl was used as a reference electrode, a platinum electrode was employed as a counter electrode and an experimental alloy sample was used as a working electrode. only 4  4 millimeters of the surface of each working electrode was exposed to nacl solution, while the remaining surface was coated with pvc heat shrink tube. a thin wire was connected to the working electrode to complete the circuit. the working electrode was immersed in 100 ml of nacl solution, and about 30 min of time interval was given to the system to attain a steady-state. the ocp values of samples were determined by measuring the voltage between the working and reference electrodes by the electrochemical workstation. before eis analysis, the sinusoidal voltage amplitude was set to 5 mv, and the frequency range from 100 khz to 0.2 hz was explored. nacl solution was refilled periodically during the experimental runs to ensure more precision. the eis experiments were performed at open circuit potential values, and impedance data were collected for the above-mentioned frequency range. afterwards, analogous electrical circuits were designed using the data analysis program ec-lab analyst. equivalent circuits were matched to experimental data, and the circuit with maximum accuracy was selected. thus, the values of the respective circuit components, i.e., solution resistance (rs), corrosion resistance (rp), and effective double-layer capacitance (cp(eff)), were calculated. moreover, the goodness of fits was determined from discrepancies between the experimental and fitted data. http://dx.doi.org/10.5599/jese.1373 j. electrochem. sci. eng. 12(3) (2022) 565-576 corrosion of si doped al-based automotive alloy 568 using the same experimental setup as for the eis, the potentiodynamic polarization experiments were performed similarly. the potential range against the reference electrode was adjusted at -1 to +1 v. the scan rate was set at 0.50 mv s-1. following the conclusion of the experiment, the relevant tafel polarization plots were plotted. for each of the experimental alloys, the corrosion potential (ecorr) in mv, the corrosion current (icorr) in a, and the corrosion rate in millimeters per year were determined using the tafel polarization plot. after the experiment, the sample surfaces were degraded and examined using a conventional optical microscope and a scanning electron microscope. the formula for determining the corrosion rate in mm year-1 is in accordance with astm standard g 102 defined as: corrcorrosion rate = i k ew a (1) where icorr = corrosion current in a, k = constant defining the units of corrosion rate in mm year-1 (here, k = 3272 mm (a cm year)-1 from astm standard g 102), ew = equivalent weight in gram/equivalent, ρ = density of alloy in g cm-3 and a = surface area in cm2. results and discussion impedance measurements results calculated by fitting the equivalent circuit to experimental data are listed in table 2. a few circuits were modeled to fit experimental eis data, and the best-fitted one is presented in figure 1. rp denotes the polarization or corrosion resistance equivalent to charge transfer resistance, rs denotes the ohmic solution resistance of the electrolyte and cp(eff) represents effective electrical double layer capacitance, which impedance is sometimes replaced by the impedance of the constant phase element. it can be noted that the values of rs are negligible compared to those of rp. polarization resistance is related to the working electrode, while solution resistance, here of 0.1 m nacl, is related mostly to the uncompensated resistance of the electrolyte solution between working and reference electrodes. results of previous studies indicated that polarization resistance is inversely related to the corrosion rate of a metal, which gives information about the rate of reactivity of the surface with the environment [22-25]. from the data given in table 2, it can be seen that the value of polarization resistance increases for higher silicon-added alloys up to the eutectic composition of 12.7si. in other words, the corrosion rate decreases up to the eutectic point, making 12.7si the most corrosion resistant alloy among the samples. according to previous studies, the ocp value increases or shifts toward a more positive value as corrosion resistance increases. hence, a more positive value of ocp means higher corrosion resistance [22]. from the experimental data, the values of the open circuit potential shift to a more positive direction for higher si-added alloys. the effective double-layer capacitance (cp(eff)) value decreases to 12.7si, then it increases to 17.9si. figure 1. equivalent electrical circuit used to fit eis data of automotive al-si alloys m. al nur et al. j. electrochem. sci. eng. 12(3) (2022) 565-576 http://dx.doi.org/10.5599/jese.1373 569 table 2. test results from electrochemical impedance spectroscopy (eis) and ocp values alloy ocp, v vs. sce rs / ω rp / ω cp(eff) / µf goodness of fit alloy 1 (0.2si) -0.66048 42 14040 12.91 0.04703 alloy 2 (3.5si) -0.65451 55.97 16947 12.75 0.07666 alloy 3 (6.1si) -0.64811 67.56 18121 12.62 0.06434 alloy 4 (12.7si) -0.63539 66.91 23708 11.62 0.0257 alloy 5 (17.9si) -0.62005 65.72 18379 21.51 0.06588 figure 2 shows nyquist diagrams of the experimental impedance measurements for all the automotive al-si alloys immersed in 0.1 m nacl solution. the real (zreal) and imaginary (-zimaginary) parts of the impedance are represented at the x and y axes, respectively. for each of the experimental alloys, the imaginary component of the impedance (-zimaginary) against the real part (zreal) is derived using the capacitive-resistive model. a single full semicircular appearance on the nyquist diagram indicates that the corrosion of al–si alloy is regulated by the charge transfer process [26]. small distortion in diagrams may be due to frequency dispersion [25]. figure 2 shows nyquist plots that are slightly depressed, indicating that they are not perfect semicircles as expected by eis theory. this discrepancy can be explained by the non-ideal behavior of the double layer as a capacitor [27,28]. figure 2. nyquist plots for experimental automotive alloys in 0.1 m nacl solution at room temperature the impedance data obtained are displayed as bode plots in figure 3. the bode magnitude plot displays the impedance modulus versus frequency (figure 3(a)), and the bode phase plot displays the phase angle versus frequency (figure 3(b)). a b figure 3. bode plots for experimental alloys: (a) bode magnitude plots (|z| vs. frequency)), (b) bode phase plot (phase angle vs. frequency) 0 5000 10000 15000 20000 0 5000 10000 15000 20000 -z im a g in a ry / ω zreal / ω 0.2si 3.5si 6.1si 12.7si 17.9si 0.1 1 10 100 1000 10000 100000 10 100 1000 10000 100000 |z | / ω f / hz 0.2si 3.5si 6.1si 12.7si 17.9si 0.1 1 10 100 1000 10000 100000 0 10 20 30 40 50 60 70 80 90 100 -φ / ° f / hz 0.2si 3.5si 6.1si 12.7si 17.9si http://dx.doi.org/10.5599/jese.1373 j. electrochem. sci. eng. 12(3) (2022) 565-576 corrosion of si doped al-based automotive alloy 570 potentiodynamic polarization analysis the tafel plots or potentiodynamic polarization curves of the aluminium-silicon alloys with varying concentrations of silicon are shown in figure 4. table 3 displays the parameters obtained from the potentiodynamic polarization analysis. icorr represents the corrosion current, which is the dissolution current at the corrosion potential. the corrosion potential, ecorr is a mixed potential in which the rate of anodic dissolution equals the rate of cathodic reactions, and there is no net current flowing in or out of the electrode. this potential is also known as open circuit potential or rest potential [29]. ecorr represents the ability of metal and nonmetal surfaces to lose electrons in the presence of an electrolyte, and the corrosion rate is shown in mm/y, which is the amount of corrosion loss in thickness per year. it can be interpreted that both the corrosion current and corrosion rate decrease as the concentration of si is increased up to 12.7 wt.%, and then increase again for the alloy with 17.9 wt.% si. according to the chemical composition, there is about 0.8 wt.% mg in each of the alloys and so, in higher si added alloys, the amount of mg2si formed is higher. this intermetallic phase is added to a protective layer as oxides, which disrupt the corrosion process [13]. the results from previous investigations prove the presence of al2o3 when electrochemical corrosion occurs at surfaces of al-si alloys in 0.1 m nacl [30]. again, si addition may allow the formation of fe2sial7 interdiffusion layer, limiting the growth of fe-al intermetallic phases [30]. however, such intermetallic layers only provide barrier protection, not cathodic protection [31]. to be added, the thickness of the diffusion layer has been considered an important factor for better corrosion performance, as the al-si coating is susceptible to microcrack due to different thermal expansion coefficients between coating and substrate. table 3. corrosion parameters resulted from potentiodynamic polarization analysis alloy icorr / µa ecorr / mv corrosion rate, mm year-1 alloy 1 (0.2si) 20.178 -597.538 0.8858 alloy 2 (3.5si) 18.156 -583.639 0.7954 alloy 3 (6.1si) 17.321 -583.112 0.7575 alloy 4 (12.7si) 13.491 -574.055 0.5865 alloy 5 (17.9si) 17.845 -577.849 0.771 figure 4. potentiodynamic polarization curves of automotive al-si alloys with varying levels of si tested in 0.1 m nacl 10-7 10-6 10-5 10-4 10-3 -0.70 -0.65 -0.60 -0.55 -0.50 e / v v s . s c e i / a 0.2si 3.5si 6.1si 12.7si 17.9si m. al nur et al. j. electrochem. sci. eng. 12(3) (2022) 565-576 http://dx.doi.org/10.5599/jese.1373 571 the microcracks do not propagate through thick diffusion layers [32,33]. the mg2si phase allows cathodic protection, enhancing the anti-corrosion performance of al-si alloys. from the values of corrosion potential (ecorr), it is found that the values shift towards a more positive direction for higher si added alloys, up to a eutectic point, which indicates better corrosion performance in higher si added alloys [34]. an exception is noted in the case of 17.9si added alloy. the probable reason is that beyond the eutectic composition, more primary si is created in the matrix, which has needles or plate-like al-si eutectic. these structures are dispersed in such a way that creates pores on the surfaces of the alloys. as a result, excess corrosion occurs at the sites where pores exist. again, because of mg2si particles, the alloy is more prone to localized corrosion [35]. the minor scatter in the potentiodynamic curves could be due to surface conditions such as surface porosity, the existence of un-melted powder, and the density of melt pool borders at the surface [36]. optical micrographic investigation figure 5 shows optical micrographs of the polished surfaces of 0.2si, 3.5si, 6.1si, 12.7si and 17.9si doped experimental alloys before and after corrosion in 0.1 m nacl solution. not enough information can be extracted from these types of unetched microstructures, but some apparent observations can be made. there are al-rich dendritic matrix, eutectic si, and some different intermetallic phases in these microstructures. in this type of image, precipitations of fe and cu with al appear in a darker tone, whereas si and mg with al appear in a lighter tone. as a result of the varying quantities of si, the alloys display different levels of tone, which is visible in figure 5. as the experimental alloys went through the electrochemical corrosion process, corrosion was observed on the surfaces at different intensities according to si concentration. optical micrographs of the surfaces of the alloys after corrosion presented in figure 5 show concentrated attacks on different locations of each experimental alloy surface, generated by corrosion products dissolving into the surrounding environment, i.e., 0.1 m nacl solution. comparing the micrographs of the surfaces of the alloys before and after corrosion, it is visible that the polished marks completely disappeared after corrosion. the corroded surfaces seem to have pit formations that are lower for higher si-added alloys. the factor controlling this characteristic might be the intermetallics containing si, one of which is mg2si. this intermetallic tends to form mgo and sio2 during corrosion, serving as protective films which are thicker for higher si-added alloys [13,30]. besides, al2o3 and al(oh)3 are also formed as a protective coating in every experimental automotive alloy [30,37,38]. the formation of pits may be attributed to the fact that the intermetallics fall off the surfaces due to the dissolution of the surrounding matrix. it is noted that the surface of the 17.9si added alloy exhibits the most obliteration among the surfaces of all experimental alloys, which is due to the increased amount of primary silicon in that alloy. increased surface pinholes can be discerned on the surfaces of the hypereutectic alloy, which may be attributed to intermetallic particles created by high impurity. these surface pinholes indicate an excessive pitting corrosion mechanism [39]. sem and edx observations figure 6 shows the surface morphology of corroded surfaces of various si-added experimental alloys after electrochemical corrosion in 0.1 m nacl solution using sem images. sem photographs prove the corrosion of intermetallic compounds on the surface of each of the experimental alloys. in higher si added alloys, the rough polish marks are more visible, except in the 17.9si alloy, as corrosion is more prominent in that alloy. moreover, numerous surface defects as pinholes are also identified in the sem image of 17.9si added sample, similar to the optical image. http://dx.doi.org/10.5599/jese.1373 j. electrochem. sci. eng. 12(3) (2022) 565-576 corrosion of si doped al-based automotive alloy 572 before corrosion after corrosion alloy 1 alloy 2 alloy 3 alloy 4 alloy 5 figure 5. microstructure of polished al-si alloys before and after corrosion in 0.1 m nacl solution 150m m. al nur et al. j. electrochem. sci. eng. 12(3) (2022) 565-576 http://dx.doi.org/10.5599/jese.1373 573 figure 6. sem images and edx spectra of al-si alloys after exposure to 0.1 m nacl solution: (a) 0.2si, (b) 3.5si, (c) 6.1si, (d) 12.7si, (e) 17.9si (a) (b) (c) (d) (e) http://dx.doi.org/10.5599/jese.1373 j. electrochem. sci. eng. 12(3) (2022) 565-576 corrosion of si doped al-based automotive alloy 574 selective dissolution of primary aluminium phases over a depth of a few micrometers creates the pinholes [39]. the images prove that there are localized corrosion and discrete pits on the surfaces of the samples. more pits can be identified in the lower si added alloys, indicating their less resistivity to corrosion. pit development is most likely the result of intermetallic compounds dissolving from the alloy surface into the surrounding environment. it is also possible that the pits are formed as a result of selective dissipation of the alloy second phase particles. because there was no dissolution of corrosion products into the surrounding matrix, the majority of corrosion products are visible on the surfaces in sem photographs. the corrosion products that occur on the surfaces are dispersed unevenly. a protective layer is also observable on the surface of every corroded alloy, which seems to be thicker in higher si-added alloys. overall, the sem images dictate that the corrosive property is improved as si is added into automotive alloys if other constituent alloying elements are kept fixed up to the eutectic point. from the quantitative edx analysis the alloys show following chemical compositions in wt.%: 0.2 si added alloy 1: 27.25 % o, 0.46 % mg, 0.74 % si 3.5 si added alloy 2: 14.75 % o, 0.74 % mg, 4.64 % si 6.1 si added alloy 3: 13.70 % o, 0.50 % mg, 9.81 % si 12.7 si added alloy 4: 11.37 % o, 0.55 % mg, 21.17 % si 17.9 si added alloy 5: 10.73 % o, 0.58 % mg, 21.53 % si one interesting observation is that the percentage of oxygen decreases at higher si-added alloys. as wt.% of si is increased, more elements from the surface are depleted as oxides, and the corrosion products remain on the surface, which is the reason for the higher percentage of oxygen in lower silicon-added alloys. edx data shows the oxygen percentage decreases even after the eutectic point, the trend not aligning with the previous results. the probable reason is that oxides are formed due to si, which follows the same trend in the hypereutectic alloy. but the corrosion is not protected the same way after the eutectic point for the surface defects created. another observation is that the wt.% of si and mg is more than the wt.% found from the chemical composition determined by the spectrometer in every alloy. although other elements are dissolved into the solution, increased si does not dissolve and forms mg2si, which is involved in protecting the materials from corrosion, as discussed earlier. conclusions based on the above discussions, the following conclusions are emphasized: alloys with higher si content have better corrosion resistance up to eutectic composition if other components are kept constant. it occurs due to the formation of protective layers of sio2 and mgo by mg2si, which is more pronounced in higher si-added alloys. beyond eutectic composition, the corrosion resistance tends to fall since the plate-like morphology of hyper-eutectic intermetallic results in surface pinholes. it is observed that the surfaces are demolished to some extent after corrosion. again, observing the optical micrographs and sem images, the surface deterioration seems lesser in higher si added alloys but exceptionally higher in 17.9si added alloy. oxide layers are formed after corrosion for all alloys, but the layers are thicker in higher si-added alloys, which is visible in both optical and sem images. the percentage of oxygen on the surface of an alloy is less for higher si added alloy, as less corrosion product is formed. m. al nur et al. j. electrochem. sci. eng. 12(3) (2022) 565-576 http://dx.doi.org/10.5599/jese.1373 575 differences in electrochemical behavior appeared due to differences in their chemical compositions and microstructures. the percentage 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electrochemical behavior of the stainless steel aisi 304 0.01 m chloride solution interface wejdene mastouri, luc pichon, serguei martemianov, thierry paillat, anthony thomas institut pprime, université de poitiers-cnrs-ensma, upr 3346, poitiers, france corresponding author: anthony.thomas@univ-poitiers.fr received: november 20, 2018; accepted: february 2, 2019 abstract stainless steels are broadly used thanks to their specific physical properties such as their high corrosion resistance in poorly aggressive solutions. however, only few studies have been reported in the literature concerning their electrochemical behavior in low concentration electrolytes medium. accordingly, the present work aims to study the immersion time influence on the solid-liquid interface properties of the austenitic stainless steel aisi 304l, immersed in a low-concentrated (0.01 m) sodium chloride (nacl) solution. the electrochemical behavior of the interface was evaluated by electrochemical impedance spectroscopy (eis) and open circuit potential (ocp) monitoring. the morphological features and the modification of the surface composition were evaluated by optic microscopy, scanning electron microscopy, energy dispersive x-ray spectrometry, atomic force microscopy, white light interferometry and x-ray photoelectron spectroscopy. it was determined by ocp measurement that the characteristic time of the interface stabilization is very long (several months). after an immersion of 2 months in nacl solution, a second time constant on impedance phase diagram appears. surface characterizations reveal a significant modification of the morphology and chemistry of the aisi 304l surface that can be linked to ocp/eis observations. it can be noticed that the repeatability deviation of the eis method was about 1 % while its reproducibility deviation was estimated to 35 %. keywords stainless steel aisi 304l; low concentration sodium chloride solution (0.01 m); electrochemical impedance spectroscopy; x-ray photoelectron spectroscopy; immersion time effect introduction austenitic stainless steels (ass) are widely used nowadays for a great number of applications thanks to their ability to resist in corrosive environments while keeping good mechanical properties. http://dx.doi.org/10.5599/jese. http://dx.doi.org/10.5599/jese. http://www.jese-online.org/ mailto:anthony.thomas@univ-poitiers.fr j. electrochem. sci. eng. 9(2) (2019) 99-111 electrochemical behavior of the stainless steel aisi 304 100 the corrosion resistance of ass originates from the protective chromium-rich passive film formed spontaneously on the surface when it is exposed to air. the passive film consists of an external part enriched in iron/chromium hydroxides and an internal part enriched in oxides [1,2]. its thickness is limited to few nanometers and its composition depends on the conditions of preparation and on the surrounding medium [3]. however, the passive film is heterogeneous and presents a highly disordered or even amorphous structure [4]. the presence of this passive oxide layer, usually with a semiconductor electric behavior, has to be obviously taken into account in the study of the solid/liquid interface. when an alloy surface is immersed in liquid, serious difficulties such as non-stability, roughness or non-uniformity of the surface may be encountered during its study. these issues consequently induce a low repeatability and reproducibility of the measurements. in this work, the studied interface is composed of a polycrystalline austenitic stainless steel aisi 304l (ass 304l) immersed in a low concentrated sodium chloride solution (0.01 m). the goal of this paper is to investigate the immersion time influence on the interface properties by combining electrochemical and surface characterization techniques. last decades, stainless steel/electrolytic solution interface has been extensively studied in corrosive conditions, with high concentrations of the electrolyte [5,6]. nonetheless, to our knowledge, only few studies concern the electrochemical behavior of stainless steels in contact with low concentration electrolytes and in an electrical potential range where the contribution of faradic reactions may be considered weak [7,8]. the morphology and the chemical composition of ass 304l samples surfaces were examined by optic microscopy (om), scanning electron microscopy (sem), energy dispersive x-ray spectrometry (eds), atomic force microscopy (afm), white light interferometry (wli) and x-ray photoelectron spectroscopy (xps). experimental material and solution ass 304l is an iron-based alloy with two main alloying elements: chromium and nickel. the studied ass 304l is mainly crystallized in the austenitic face centered cubic (fcc) γ phase, with grain sizes of few tens of micrometers. some different precipitates (carbides, mns, …) exist, as well as some residual ferritic α phase grains. the samples for electrochemical measurements consist of 5.0 × 4.0 mm (diameter × thickness) cylinders. the chemical composition of this steel provided by the supplier goodfellow® and confirmed by eds (except the carbon content) is shown in table 1. table 1. chemical composition of the ass 304l used in this study content, wt.% fe cr ni mn c mo bal. 18.14 8.08 1.77 0.024 0.3 to obtain reproducible electrochemical results, the ass 304l electrode was carefully prepared by a dedicated polishing process. samples were initially mechanically polished with increasing grade (500 to 2000) silicon carbide (sic) papers, then with 6 to 1/4 μm diamond suspension abrasives. to eliminate the polishing residues, specimens were degreased in a commercial rbs™ 25 detergent solution, washed with tap water, distilled water and then rinsed with ethanol in ultrasonic bath. they were finally dried under a low nitrogen flow at ambient temperature. all the electrochemical experiments were carried out in 0.01 m nacl solutions with ph equal to 6 ± 0.5 at t = 20 °c. the reason of using these molar concentrations is to avoid corrosion problems. wejdene mastouri et al. j. electrochem. sci. eng. 9(2) (2019) 99-111 http://dx.doi.org/10.5599/jese.639 101 the solutions were prepared with ultra-pure water, produced and purified with millipore-milli-q system, and were used immediately after their preparation to avert contamination. electrochemical experiments all electrochemical experiments were carried out in a conventional three-electrode cell. the cell is a cylindrical pyrex® glass of 100 ml capacity, double walled, allowing water circulation to keep the temperature of the cell constant at 17.0 ± 0.2 °c thanks to a thermostatic bath. before each measurement, the cell was cleaned with pure water and rinsed with the electrolytic nacl solution. a saturated ag/agcl and a platinum slab of 2 cm2 were used as the reference electrode and counter electrode, respectively. all potential measurements presented in the present study are thus referred to the ag/agcl potential. the working electrode was a polished ass 304l cylinder. it was fixed in a ptfe holder, keeping only the top surface of the cylinder (0.2 cm2 area) exposed to the electrolyte. the working electrode was embedded on a rotating disk electrode (rde) providing a controlled liquid flow to avoid diffusion-limited phenomena. before starting each electrochemical experiment, the electrolyte was previously deaerated by nitrogen bubbling for 20 min in order to reduce the dissolved oxygen content. the nitrogen circulation at the edge of the free surface of the solution was maintained during all measurement duration. a solartron® multistat 1480 potentiostat/galvanostat, coupled with a model 1260 frequency response analyzer was employed to perform the electrochemical measurements. the monitoring of ocp evolution over few hours was performed after various immersion times: less than 25 minutes, 3 days and 2 months. electrochemical impedance spectroscopy (eis) experiments were carried out by applying, at open circuit potential (0 v) vs. ag / agcl, a perturbative sinusoidal potential with amplitude of 10 mv. the impedance response was measured over frequencies from 104 hz to 0.5 hz. surface characterization a nikon® optical microscope and a jeol-ttls 7001f field emission gun scanning electron microscope (feg-sem) equipped with an energy dispersive spectroscopy (eds) microanalysis hardware (oxford instruments) were used in order to examine the morphology and the chemical composition of the surface before and after immersion in chloride containing solutions. the roughness was evaluated by atomic force microscopy (afm) on a nanoscope iii apparatus (digital instruments), operating in tapping mode on a 4040 µm² surfaces, and by white-light interferometry (wli) on a talysurf ccl microscope (taylor hubson ltd) on 350350 µm² surfaces. the surface roughness of each specimen has been evaluated as the root mean square (rms) value rq. surface analyzes by xps were realized with a riber division instruments spectrometer using a non-monochromated mg kα x-ray line (hν = 1250 ev) obtained with an accelerating voltage of 12 kv and a current intensity of 15 ma. the xps analysis was performed under pressure of about 10-11 torr. the incidence angle is 20° relative to the sample surface whereas the emitted photoelectrons are collected at  = 20° from the surface normal direction. although they may not be the most adapted to this study, these angles are fixed by the set-up geometry and cannot be modified. the deconvolution of the peaks was performed with unifit 2007® software. the shirley method was used for background subtraction. the energy positions were corrected in reference to the 1s carbon peak (284.6 ev binding energy) in order to eliminate any perturbation (charge effects, surface state, etc.). for each experimental peak, the fitting was realized with a small number of contributions (typically 3 or 4), using a gaussian–lorentzian product function, with relatively large fwhms (full width at the half maximum) up to few ev, and deviation from a central binding energy up to 0.8 ev. http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 99-111 electrochemical behavior of the stainless steel aisi 304 102 by considering that the cr and fe metallic contributions (fe0 and cr0) in the fe2p and cr2p peaks are exclusively coming from the bulk material under the oxide, the thickness of the passive film, ox / å, can be evaluated from the eq. (1) [9]. 1      +  = +    +     ox ox ox ox m m fe fe cr cr cr fe cr fe ox m m m m ox ox fe fe cr cr cr fe cr fe cos ln s n i s n i n n s n i s n i n n (1) 𝜆 / å is the average inelastic mean free path of photoelectron in the passive film (12.3 å).  is the collection angle (20° from the normal surface). sfe(0.188) and scr (0.155) are the sensitivity factors of fe and cr elements, respectively. ifeox and icrox, are respective area values of the oxide+hydroxide contributions in the fe and cr peaks. ifem and icrm are the area values of the metallic contribution (fe0 and cr0), respectively. nfeox and nfeox / mol m-3 are molar concentrations of fe and cr metal elements in passive film considered as a fe2o3-cr2o3 mixture (the fe/cr ratio in the oxidized film is given by the xps quantification). nfem and ncrm / mol.m-3 are molar concentrations of fe and cr metal elements in the unoxidized (bulk) alloy (ass 304l). no other special preparation of the surface (e.g. surface cleaning by ion beam sputtering) than the aforementioned post-polishing cleaning was carried out to obtain an analysis reflecting more closely the state of our surface before/after immersion. all physical and electrochemical measurements were performed before 48 hours interval after surface preparation. results and discussion surface analysis figures 1a and 1b show the surface of the ass 304l observed with optic microscopy (om) and scanning electron microscopy (sem) before immersion in the nacl solution and the electrochemical tests. the surface preparation allows to distinguish the presence of precipitates and  grains with different crystallographic orientations (figure 1a). depending on the samples, some traces of residual scratches may still be visible at different scales of observation. a large density of inclusions (highlighted by the red circle) could also be observed on the ass 304l surface (figure 1b). these inclusions have an average size ranging between 0.5 and 2 µm and are randomly distributed over the entire surface. the chemical analysis obtained by eds (figure 1c) revealed that the composition of these inclusions is richer in mn and s (or unlikely mo) compared to the spectrum obtained on a γ phase grain. a study on a 304l stainless steel immersed in alkaline solutions of naoh + koh in the presence of chlorides proposed by freire et al. [6] highlights the presence of similar inclusions on the surface. according to this study as well as others [10], these inclusions have been identified as sulfides mixtures of mns, crs and/or fes. the rms roughness value measured by wli on several samples varies between 15 and 35 nm from one sample to another. the average roughness was evaluated to be 25 ± 10 nm. the average roughness value obtained by afm is lower (13 ± 7 nm) mainly because the studied surface is smaller: 0.12 mm² in wli versus 1.6.10-3 mm² in afm. the chemical composition of the passive film formed in presence of ambient air was assessed by xps. the global surveys are presented in figure 2a. only the elements iron, chromium, oxygen and carbon can be clearly identified. the preponderance of the elements (c, o) is naturally relative to the oxide film and to the organic pollution of the surface. this is unavoidable with the preparation method used before xps analysis (samples are kept in air for several hours and no sputtering was performed to clean the surface). wejdene mastouri et al. j. electrochem. sci. eng. 9(2) (2019) 99-111 http://dx.doi.org/10.5599/jese.639 103 figure 1. (a) optical microscope image (nomarsky constrast), (b) scanning electron microscopy (secondary electron) image of ass 304l surface, (c) eds spectra registered on the austenitic grains (spectrum 1) and on the precipitate (spectrum 2) the other components of the alloy (ni, mn…) are not sufficiently visible outside the background noise because of a too low signal/background ratio. indeed, nickel should appear at a binding energy of about 800 ev. however, in our case study, the ni signal is quite low. its weak signal can be explained by a relatively low concentration in the bulk material (few at.%), a significant organic pollution at the surface which shielded the signal but also by its low participation in the oxide layer probed by the xps. moreover, according to veleva et al. [11], it is probably hidden by the auger features of the other dominant metal oxides. figure 2 (b-e) shows the high resolution spectra of the normalized peaks fe 2p3/2, cr 2p3/2, o 1s and c 1s. xps peak position and quantitative contents of the elements are listed in table 2. based on the deconvolution of the spectra, the passive film formed on the ass 304l surface in contact with air is composed of oxides (feo, fe2o3, and cr2o3) and hydroxides (feooh and cr(oh)3) of the two main metallic elements, fe and cr [12,13]. in the quantitative analysis (excluding carbon contaminants), a preponderance of chromium (3.14 at.%) on iron (1.82 at.%) was found in the layer probed by the xps. the ratio fe/cr (all contributions combined) is about 0.58, whereas the expected value is 3.9 for the bulk material. this reflects the important role of chromium in the development of the native and passive oxide films. http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 99-111 electrochemical behavior of the stainless steel aisi 304 104 figure 2. (a) low resolution survey xps normalized spectrum and high resolution, (b) fe 2p3/2, (c) cr 2p3/2, (d) o 1s, (e) c 1s xps normalized spectra of the ass 304l surface after mechanical polishing. experimental data are represented as round dots and lines refer to the different contributions and to the global optimized simulation in the hypothesis that fe0 and cr0 contributions are coming from the bulk material, and that oxides and hydroxides contributions are coming from the passive layer, the oxidized film thickness, calculated from the eq. (1), corresponds approximately to 3.8±0.8nm. the uncertainty given here comes from the different possible hypotheses on the nature of the oxides and hydroxides (especially on their density). wejdene mastouri et al. j. electrochem. sci. eng. 9(2) (2019) 99-111 http://dx.doi.org/10.5599/jese.639 105 table 2. parameters used for the deconvolution of xps spectra: the elements compositions, fe/cr ratio and thickness value of the passive film formed on ass 304l in the air and after different immersion times ass 304l, in contact with air ass 304l, after immersion in 0.01 m nacl solution (t = 0) ass 304l, after immersion in 0.01 m nacl solution (t =2 months) element detected species binding energy, ev element content by species, at.% element content, at.% binding energy, ev element content by species, at.% element content, at.% binding energy, ev element content by species, at.% element content, at.% fe 2p3/2 fe0 (metal) 706.1 0.50 1.82 705.6 0.19 1.30 705.7 0.11 0.55 feo 707.6 0.51 707.6 0.51 707.6 0.23 feooh/fe2o3 709.4 0.80 709.3 0.60 709.4 0.21 cr2p3/2 cr0 (metal) 573.6 0.47 3.14 573.9 0.46 1.89 573.0 0.28 2.42 cr2o3 575.6 1.43 575.1 0.67 575.2 1.06 cr(oh)3 577.0 1.24 576.4 0.77 576.5 1.09 o 1s feooh 529.1 18.64 46.28 529.0 13.13 48.42 529.0 11.81 29.10 fe2o3 530.5 15.30 530.6 17.81 530.4 8.77 cr2o3/cr(oh)3 531.7 7.95 531.5 10.19 531.6 5.47 h2o 532.7 4.39 532.5 7.28 532.7 3.05 c 1s c -c 284.6 36.14 48.76 284.6 30.46 48.39 284.6 45.55 67.94 c o 285.4 9.85 285.2 10.06 285.2 18.77 c = o 288.1 2.77 288.0 1.87 287.4 4.61 fe / cr 0.58 0.69 0.23 ox / nm 3.8  0.8 3.5  0.8 4.1  0.8 electrochemical impedance spectroscopy (eis) the impedance experimental diagrams obtained on the ass 304l alloy tested in the nacl solution are shown in figure 3. they highlight similar behavior as those reported in the literature for stainless steel electrodes immersed in a low concentration electrolytic solution [7,8]. in the quantitative analysis (excluding carbon contaminants), a preponderance of chromium (3.14 at.%) on iron (1.82 at.%) was found in the layer probed by the xps. the ratio fe/cr (all contributions combined) is about 0.58, whereas the expected value is 3.9 for the bulk material. this reflects the important role of chromium in the development of the native and passive oxide films. on the nyquist diagrams in figure 3 (a,d), a near-vertical line corresponding to a predominantly capacitive behavior is obtained. the line tilt reflects the presence of secondary faradic reactions arising from impurities in the electrode and the electrolyte. by increasing the frequency, the interface switches from a capacitive to a resistive behavior characterized by a phase (-) approaching zero and a constant impedance module (fig. 3 b,c,e,f). the phase angle evolution with frequency presented in figure 3 (c,f) shows that the phases are lower than 90°, for all impedance spectra. such behavior can be interpreted as a deviation from the ideally polarized blocking electrode behavior. repeatability and reproducibility tests reproducibility and repeatability constitute a major challenge during solid/liquid interfaces studies. to show the repeatability, at least three series of measurements were performed the same day, one after the other, on the same sample, with the same electrolytic solution. for the reproducibility, it consists to reproduce each electrochemical measurement several times on several different specimens changing each time the used solution. for this purpose, three samples were identically prepared, on different days and consequently under slightly different conditions (room temperature, products amount used for polishing, etc.). http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 99-111 electrochemical behavior of the stainless steel aisi 304 106 figure 3. (a-c) repeatability, (d-f) reproducibility of electrochemical impedance measurements obtained with ass 304l in nacl solution (0.01m) at e= 0 v vs. ag/agcl: (a,d) nyquist and (b,c,e,f) bode plots. according to figure 3 (a,b,c) the repeatability deviation is about 1 %, demonstrating very good repeatability of the results with unique sample. considering the reproducibility tests shown on figure 3 (d,e,f), the obtained deviation remains high in the order of 35 %. the high dispersion can be related to the nature of the material used which is a polycrystalline alloy of different elements, presenting heterogeneities at the surface and different grains with variable crystallographic orientations. the actual state of the surface, especially its morphology, cannot therefore be strictly the same for different samples. other factors could also generate a decrease in reproducibility such as the quality of the electrolyte. wejdene mastouri et al. j. electrochem. sci. eng. 9(2) (2019) 99-111 http://dx.doi.org/10.5599/jese.639 107 effect of immersion time immersion time is one of the most influential parameter on interface properties. results on stainless steel related to short-term tests have been widely discussed in literature [5-6,14-15], however little has been carried out for relatively long contact periods between the electrolyte and the metallic material [16,17]. open circuit potential evolution figure 4 illustrates the ocp evolution for the ass 304l in the nacl solution measured just after the immersion in liquid (t=0), after an immersion of 3 days (t = 3 days) or after an immersion time up to 2 months (t = 2 months). the alloy exhibits an increase of ocp with time. nonetheless, a nonstabilized profile and strong fluctuations are remaining even after 3 days of immersion. thus, the time required to reach the stationary state is relatively long (higher than several days). for the test realized after a 2 months’ immersion, the ocp values evolve smoothly between -0.1 and 0.075 v vs ag/agcl during the first 6 hours (21600 s) and stabilize around 0.075 v over the other 25 hours of measurements (115000 s). figure 4. (a) open circuit potential evolution of the ass 304l from 0 to 2 months of immersion in the nacl solution (0.01m), (b) zoom on the first 6000 s of measurements the ocp stabilization at a value of 0.075 v vs. ag/agcl was verified only after a long period of 2 months. although a follow-up of the ocp evolution at different times between 3 days and 2 months would be necessary to accurately define the ocp stabilization, this result suggests that the time constant of the interface stabilization is in the order of at least several days. according to mohammedi et al. [5], ocp values stabilize at higher immersion times due to the nonlinear rate of oxide film growth. impedance spectroscopy analysis after long-term solution immersion eis data were recorded after two different immersion times, t = 0 and t = 2 months and presented in figure 5. bode plots show that the first time constant is shifted to higher frequencies and a second time constant τ2 appears at low frequency (< 10 hz) when the immersion time is increased from 0 to 2 months. this behavior can be related to the passive film. indeed, a film evolution according to the immersion time can be noted. time constant τ2 can also be attributed to the charge transfer related to the electrical double layer (edl) developing at the passive film/nacl interface. surface analysis of passive films on stainless steel ass 304l after long-term solution immersion the sem images of the ass 304l surface morphology after the first immersion (less than 25 minutes, t=0) does not present any new features compared to the initial surface (figure 6a). http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 99-111 electrochemical behavior of the stainless steel aisi 304 108 a b c figure 5 (a) nyquist and (b-c) bode plots obtained for ass 30nacl solution (0.01m) after different immersion times a b figure 6. scanning electron microscopy (secondary electron) image of ass 304l surface after different immersion times (a) t = 0 s, (b) t = 2 months wejdene mastouri et al. j. electrochem. sci. eng. 9(2) (2019) 99-111 http://dx.doi.org/10.5599/jese.639 109 the small particles (few nanometers size) with high bright contrast are likely relative to deposition of some dust coming from the air or the liquid. some are also visible on the sample immersed for 2 months (figure 6b). however, an immersion for 2 months induced a clear change in the sample surface. the surface density of the dark contrast inclusions is higher on the observed zone: this may be attributed to the inhomogeneity of the initial material, as such an increase was not observed on the whole surface. however, it is not excluded that some new species (oxide precipitates, hole…) appeared during long immersion in liquid thanks to the pitting corrosion or oxidation. around these small inclusions with full dark contrast (attributed to the sulfides inclusions in the initial surface), new areas with intermediate dark contrast were formed over few square micrometers after the immersion of 2 months. indeed, the sulfides inclusions (mns) are demonstrated to play a major role in the corrosion behavior of stainless steels in (high concentration) nacl solution, as favored locations to initiate the pitting corrosion [6,18]. although the involved mechanism is still discussed, mns inclusions would enable a preferential adsorption of chloride ions compared to the surrounding passivated γ phase. depending on several factors (chloride ions concentration, shape and composition of inclusions…), this can lead to dissolution of the sulfides precipitates as well as the initiation of a preferential oxidation of the nearby material [19]. these changes naturally affect the electrochemical behavior of ass 304l in ocp and eis measurements. despite this visible change in surface morphology, the rms roughness value measured on ass 304l surface exposed to nacl solution for two months was not changed. it remains about 25 ± 5 nm. the xps spectra and the deduced chemical analysis are presented in figure 7 and table 2. the xps results revealed that the same elements were detected for the film formed in the air, after immersion at t = 0 or t = 2 months. the main evolution with the immersion time is the variation of the fe-total /cr-total ratio: from the initial value of 0.58, the immersion first slightly increases it up to 0.69 but the 2 months’ immersion induces reduction of the ratio to 0.23. the film is then significantly depleted in iron, probably by a phenomenon of iron hydroxides dissolution or a favored chromium oxide growth. secondly, the fe0 contribution around 706 ev in the fe 2p3/2 signal is modified by the immersion. compared to the hydroxide/oxide peaks, its relative proportion (~28 %) is reduced to 15-19 % after immersion (figure 7b and table 2). this evolution, in conjunction with the aforementioned fe/cr ratio decrease, confirms the dissolution of fe-hydroxide/oxide in the growing passive layer. comparatively, no significant modification with immersion time can be observed in the shape of the cr 2p3/2 peak (figure 7c), with a clear predominance of criii. the calculated thickness ox of the oxidized layer is not significantly evolving and remains roughly constant around 4 nm. finally, the altered thickness and chemistry of the passive oxide film, as well as the localized oxidation/corrosion pits observed by sem, consequently involve the modifications of the physical/chemical surface properties. for instance, the electrical conductivity and also the interaction with species in the liquid (adsorption and desorption) are parameters that clearly influence the electric double layer (edl) formation, its characteristics, and thus the electrical response in eis measurements. the appearance of a second time constant in the phase bode plot (figure 5b) is then related to these modifications although additional simulation with an equivalent electrical circuit is required to link the observed modifications with the eis results. http://dx.doi.org/10.5599/jese. j. electrochem. sci. eng. 9(2) (2019) 99-111 electrochemical behavior of the stainless steel aisi 304 110 figure 7 (a) low resolution and high resolution, (b) fe 2p3/2, (c) cr2p3/2, (d) o 1s, (e) c 1s xps normalized spectra of the surface films formed on ass 304l in the air and after different immersion times. round dots are experimental data and the colored continuous lines are the simulated spectra. the black lines correspond to the different simulated contributions of the xps spectra measured after a 2 months immersion (t=2 months) conclusions in the present work, the immersion time influence on the characteristics of the ass 304l alloy in the nacl solution (0.01m) was studied using surface analysis (om, sem, afm, wli, xps) and electrochemical characterization techniques (eis, ocp). the following conclusions can be drawn: • with all the precautions taken concerning the metal samples preparation by polishing and well controlled manipulations, the repeatability deviation is 1 % but that of reproducibility is 35% for the eis method. this poor reproducibility has been attributed to unstable surface states related wejdene mastouri et al. j. electrochem. sci. eng. 9(2) (2019) 99-111 http://dx.doi.org/10.5599/jese.639 111 to transient phenomena or solid surface inhomogeneities. the ocp evolution with the immersion time is confirming the transient regime over at least several days. • surface characterization and electrochemical results proved that the long-term immersion modifies the composition and the electrochemical response of the ass 304l surface. after two months solution immersion, the impedance diagrams show the appearance of a second time constant at low frequencies. it is attributed to the modifications of the oxide film properties and/or of the charge transfer in the edl. indeed, sem and xps analysis confirm a significant modification of the surface morphology (corrosion pits, localized oxidation) and an iron depletion (by dissolution) in the thicker passive oxide layer after long immersion times. acknowledgments: this work was funded by the french government program “investissements d’avenir” (labex interactifs, reference anr-11-labx-0017-01). references [1] m. ibrahim, s. s. abd el rehim, m. m. hamza, materials chemistry and physics 115 (2009) 80-85. 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[11] l. veleva, m. a. alpuche-aviles, m. k. graves-brook, d. o. wipf, journal of electroanalytical chemistry 537 (2002) 85-93. [12] s. tang, o. j. kwon, n. lu, h. s. choi, surface and coatings technology 195 (2005) 298-306. [13] k. brunelli, s. gottardello, e. napolitani, b. dal bianco, r. bertocello, m. magrini, m. dabalà, materials chemistry and physics 136 (2012) 1073-1080. [14] j. pellier, j. geringer, b. forest, wear 271 (2011) 1563-1571. [15] l. freire, m. j. carmezim, m. al ferreira, m. f. montemor, electrochimica acta 55 (2010) 6174-6181. [16] c. compere, n. le bozec, proceeding of the first stainless steel congress in thailand, thailand, 159 (1997) in efc working party report on 'microbially induced corrosion’. [17] i. costa, c. v. franco, c. t. kunioshi, j. l. rossi, corrosion 62 (2006) 357-365. [18] m. a. baker, j. e. castle, corrosion science 34 (1993) 667-682. [19] p. schmuki, h. hildebrand, a. friedrich, s. virtanen, corrosion science 47 (2005) 1239-1250. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese. http://creativecommons.org/licenses/by/4.0/) one-pot synthesis of crystalline structure: nickel-iron phosphide and selenide for hydrogen production in alkaline water splitting http://dx.doi.org/10.5599/jese.1696 563 j. electrochem. sci. eng. 13(3) (2023) 563-573; http://dx.doi.org/10.5599/jese.1696 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical obtaining of rhenium-molybdenum alloys elza salakhova, dilgam tagiyev, parvana kalantarova, kamala ibragimova, yilmaz alizade, ramila huseynova and i̇rana cabbarova institute of catalysis and inorganic chemistry, anas, h. javid ave. 113, baku az1143, azerbaijan corresponding authors: elza_salahova@mail.ru received: february 7, 2023; accepted: june 8, 2023; published: june 27, 2023 abstract based on the study of current-voltage dependencies during the co-deposition nt electrical reduction of rhenium and molybdenum from sulfuric acid on a pt electrode, conditions for deposition of alloy nanocoatings of the re-mo system were established. the influence of various factors, such as the content of components in the electrolyte, current density, temperature, acidity of solutions, etc., on the composition and quality of coatings was studied. it was established that with an increase in the content of rhenium in the electrolyte and an increase in current density, the content of rhenium in the alloy is increased. the composition and morphology of re-mo thin films electrodeposited on a platinum electrode were analyzed. the phase composition of the obtained films was determined by xrd using a diffractometer, and the study of the morphology of re-mo films on platinum and nickel substrates was performed using a scanning electron microscope. based on this study, the optimal conditions for the deposition of molybdenum with rhenium and the required composition of the electrolyte were selected. for obtaining rhenium-molybdenum alloys containing 50-80 wt.% re, the following electrolyte composition can be recommended: 0.0015m na2moo4 + 0.0035m kreo4 + 2m h2so4, ph 0.4; t = 75c, the electrode pt. keywords binary alloys; rhenium alloy, thin coatings, electrochemical deposition; electrolytic bath composition introduction the unique physical and chemical properties of rhenium make this metal promising for use in various high-tech industries, such as aviation, rocket engines, nuclear power engineering, electronics, biomedicine, and heterogeneous catalysis [1-10]. studies of the process of electrodeposition of rhenium and its alloys are primarily of practical but also theoretical interest [11-15]. the practical significance of research in this area is associated mainly with the growth in the production of rhenium and, consequently, with the need to address the issue of the optimal technology for the separation of rhenium from solutions in the form of pure http://dx.doi.org/10.5599/jese.1696 http://dx.doi.org/10.5599/jese.1696 http://www.jese-online.org/ mailto:elza_salahova@mail.ru j. electrochem. sci. eng. 13(3) (2023) 563-573 electrochemical obtaining of rhenium-molybdenum alloys 564 metal or alloys. in addition, it is required to find the possibilities of using rhenium and its alloys as galvanic coatings for various purposes [16-18]. from the theoretical point of view, the process of electrodeposition of rhenium and its alloys is also of interest, since rhenium, like many other refractory metals, is released from the anions (reo4-), and the mechanism for the separation of metals from anions is complex and insufficiently studied. therefore, the data obtained on the example of rhenium may also be useful for the analysis of similar processes. rhenium is a strong, ductile, refractory metal with a hexagonal close-packed crystal structure. it has the second-highest melting point among all metals (after w) of 3157 to 3181 °c. molybdenum, in turn, is a very universal refractory metal exhibiting high melting point and high heat resistance. in addition, it could be easily mechanically processed. molybdenum and rhenium in pure forms on the cathode can be obtained only with low current efficiency [19-27]. rhenium has a unique effect on lowering the transition temperature of tungsten and molybdenum. the mechanism of action of rhenium on these metals has not yet been fully elucidated. rhenium is characterized by very high solubility in transition metals. alloys of rhenium with molybdenum and tungsten are of great practical interest since rhenium has an exceptional influence on their deformability and mechanical properties. previously [17,18,28], we obtained coatings of rhenium alloy with chalcogens by the electrochemical method. as is known from the literature, rhenium can be deposited from both acidic and alkaline electrolytes [9-14]. the purpose of this work is the electrochemical obtaining of new nanomaterials based on re-mo alloys on various substrates. for this purpose, a study of cathodic processes during the reduction of molybdenum and rhenium in the sulfuric acid on pt and ni electrodes is carried out. in addition, the possibility of obtaining thin re-mo alloy coatings from sulfuric acid containing various concentrations of na2moo4 and kreo4 as metallic precursors at ph 0.4 and 75 °c is studied in detail. the optimization of electrodeposition operation parameters for the alloy, as well as characteristics of the obtained re-mo deposits, are also discussed. experimental preparation of solutions and electrolysis mode the following reagents were used in the given work: kreo4 (99 %), h2so4 (99 %), na2moo4 (98 %), hno3 (99 %), (nh2)2cs (99 %), sncl2·2h2o (98 %), h3po4 (99 %), cro3 (99 %), na2so3·7h2o (98 %), all by sigma-aldrich. the indicated reagents were purified before use. hydrochloric and sulfuric acids (chemically pure) were exposed to double distillation. to obtain the re–mo alloy, the electrolyte was prepared as follows: the given amounts of kreo4 and na2moo4 were loaded into the volumetric flask, followed by their dissolution in the concentrated h2so4 in the required amount, and diluted with distilled water to a certain volume. this solution served as the stock solution, and all used solutions were prepared by further dilution of a certain amount of this solution. kreo4 is very slightly soluble in acid, so solutions were prepared by heating up to 75 °c. in order to determine the chemical and phase composition and microstructure of the obtained alloys, platinum and nickel electrodes having a constant geometrical area of 4 cm2 were used, while galvanostatic and potentiostatic depositions were performed. polarization measurements were carried out using the platinum cathode, with a surface of 0.15 cm2, soldered into a glass tube. before immersion in the solution, the electrode was e. salakhova et al. j. electrochem. sci. eng. 13(3) (2023) 563-573 http://dx.doi.org/10.5599/jese.1696 565 mechanically polished, degreased, boiled in 30 % nitric acid, and washed with an appropriate electrolyte. before use, the nickel electrode was mechanically grinded, degreased with vienna lime (used for cleaning and polishing metal, having the composition: cao, mgo and fe2o3), electrochemically polished, and then washed twice with distilled water. during polarization measurements, a platinum working electrode was also used. an ag/agcl-kcl electrode was used for polarization measurements as the reference electrode. the auxiliary electrode was a platinum wire with a surface several times higher than the surface of the working electrode. in order to study the kinetics of electrochemical processes during the co-electrodeposition of rhenium and molybdenum, the cyclic voltammetry technique was used, involving an iviumstat potentiostat. the polarization curves were recorded using a glass cell equipped with a 50 ml glass jacket and with the platinum electrode used. to check the reproducibility of the results, each experiment was carried out in duplicate. the temperature in the electrolytic cell was maintained constant with an accuracy of 0.1 °с by a u-10 thermostat. current output was detected with a copper coulometer by weighing method and calculated regarding the composition of deposit. the ph of solutions was determined using ph/mv/tempaz86551 equipment. determination of thin alloy coatings composition the composition of cathodic re–mo alloy deposits was determined as follows. the cathodic deposit obtained by the electrochemical method was dissolved in 10 ml of concentrated nitric acid under heating. after repeated evaporation in a water bath, 1.67 m (5 n) h3po4 solution was added. rhenium was separated from molybdenum by extraction with isoamyl alcohol and then rhenium and molybdenum were determined by spectrophotometry taking into consideration the thiourea complex of rhenium and the thiocyanate complex of molybdenum using a specord-50 plus equipment. then, it was photometrically measured to the zero solution at 390 nm in a cuvette 11 cm. the content of rhenium was determined by a calibration curve built using standard solutions of rhenium. in a flask with a capacity of 50 ml, a certain amount of a re-containing solution was added, 10 ml of concentrated hydrochloric acid, 10 ml of a 5 % aqueous solution of thiourea, 2 ml of a 20 % solution of tin chloride in concentrated hydrochloric acid, bring to the mark with distilled water and mix. after an hour, solutions are photometered on specord 50 plus instrument using a blue light filter in cuvettes 1-10 cm relative to distilled water. the three-electrode cell contained the electrode under study, an auxiliary platinum electrode with an area of 4 cm2, and a silver chloride reference electrode (ag/agcl-kcl). to study the structure and composition of deposited films, deposition was carried out on pt and ni substrates with an area of 4.0 cm2. platinum electrode was used as an anode for potentiostatic and galvanostatic depositions. the working temperature for electrodeposition was 75 °c and the deposition time was 60 min. to study the morphology of films on platinum and nickel substrates, the electrode surface was examined on a jeol jsm7600f scanning electron microscope at various magnifications and, accordingly, was subjected to elemental analysis using an oxford x-max 50 detector. x-ray diffraction analysis of the obtained films was carried out on a dron-5 setup with cu k α-radiation. the films were obtained using galvanostatic mode in stationary conditions. for chemical analysis, the cathode deposit was dissolved on heating in concentrated hno3. the amount of rhenium and molybdenum was determined separately by the thiourea complex using the colorimetric method on specord 50 plus instrument. http://dx.doi.org/10.5599/jese.1696 j. electrochem. sci. eng. 13(3) (2023) 563-573 electrochemical obtaining of rhenium-molybdenum alloys 566 results and discussion in order to study the kinetics of the co-electrodeposition of re with mo, the obtained polarization curves of individual components were compared with those obtained during their co-deposition. for this purpose, the influence of various factors on the electrodeposition of rhenium and molybdenum in the test solution was studied. the potentiodynamic polarization curve, presenting the reduction of molybdate ions from the investigated solution, is shown in figure 1a. in the potential range of -0.1 to 1.0 v vs. ag/agcl, with the increasing negative value of electrode potential, there was an increase in the cathode current density of the reaction of incomplete reduction of molybdate ions. as is known from the literature, molybdenum is deposited from both acidic and alkaline electrolytes [19-27]. in this work, the choice of sulfuric acid was made based on the fact that it is possible to obtain high-quality molybdenum deposits from this electrolyte, while it is not always possible to obtain high-quality films from molybdate solution in a very acidic electrolyte using sulfuric acid. also, it was established by preliminary experiments that high-quality films can be obtained from sulfuric acid even at very low concentrations of molybdenum in the electrolyte. for this purpose, cathode processes were studied at different concentrations of molybdenum in sulfuric acid on a pt electrode. figure 1b shows the cyclic polarization curves presenting the reduction of molybdenum from molybdate solution in a very acidic electrolyte using sulfuric acid at various scan rates. a b potential, v vs. ag/agcl potential, v vs. ag/agcl figure 1. cyclic voltammograms of molybdenum deposition on pt electrode in 2 m h2so4, ph 0.4, 75°c at: (a) scan rate 5 mv s-1 and following concentrations of na2moo4 (1) 0.02 m; (2) 0.015 m; (3) 0.0015 m and (b) 0.0015 m na2moo4 at different cycles (1-3) usually, after immersing the pt-electrode in sulfuric acid for 30 minutes, the electrode potential takes a constant value equal to 0.500.05 v, which does not change with a change in the concentration of mo in the solution. the main role in establishing the stationary potential is played not by the equalization of the concentration ratios in the cathode layer but by the balance between the electrode surface and solution, due to the presence of an oxide film and sparingly soluble compounds on the cathode surface. as is known from the literature, rhenium electrolytes are acidic electrolytes. the electrochemical behavior of rhenium in sulfuric acid solutions was studied in [2,3]. during electrolysis from acidic rhenium-containing solutions, depending on the conditions of electrolysis and the composition of the electrolyte, coarse-grained or fine-grained deposits of rhenium are deposited on the cathode. figure 2 shows the polarization curves of rhenium deposition on the pt electrode from sulfuric acid. as is known from the literature [6-8], in the sulfuric acid, rhenium is in the form of re o4and its c u rr e n t, m a c u rr e n t, m a e. salakhova et al. j. electrochem. sci. eng. 13(3) (2023) 563-573 http://dx.doi.org/10.5599/jese.1696 567 reduction probably consists of several separate processes. the discharge of re o4ions is carried out in stages, and each of the stages of the formation of intermediate products is undoubtedly characterized by a certain electrode potential. this electrode potential becomes more negative in the process of reduction, which can also contribute to the convergence of deposition potentials of rhenium and molybdenum. thus, the reduction of rhenium in strongly acidic electrolytes proceeds in stages through the formation of an intermediate oxide film, as evidenced by the presence of red and blue precipitates in the resulting film. figure 2a shows the polarization curve of rhenium deposition from the sulfuric acid electrolyte, while figure 2b shows rhenium deposition from sulfuric acid at different concentrations of rhenium in the sulfuric acid. a b potential, v vs. ag/agcl potential, v vs. ag/agcl figure 2. cyclic voltammograms (5 mv s-1) of rhenium deposition on pt electrode in 2 m h2so4, ph 0.4, 75 °c and presence of: (a) 0.00084 m kreo4 and (b) various concentrations of kreo4 (1) 0.00069 m; (2) 0.0035 m; (3) 0.035 m the influence of various factors on the co-deposition of rhenium and molybdenum was also studied. current density, temperature, ph, rhenium and molybdenum concentration in solution, sulfuric acid concentration, and electrodes were varied in order to obtain better quality and nanolayers. it turned out that all these factors affect the composition and quality of the alloy coating in different ways. based on these studies, the optimal conditions for the deposition of solutions of molybdenum with rhenium and the required composition of the electrolyte were selected. figure 3 shows the polarization curves during the co-deposition of rhenium with molybdenum from a sulfuric acid solution. as shown in figure 3, the alloy formation curves differ from the polarization curves of individual components shown in figures 1 and 2. while the polarization curves of rhenium show one wave, these curves of the alloy show two sections of the limiting currents. it is possible that during co-electrodeposition, the reduction of rhenium and molybdenum is accelerated, as rhenium catalyzes the reduction of molybdenum. this indicates once again that the reduction potentials of rhenium and molybdenum are approaching each other [19,21]. with such an arrangement of polarization curves, the potentials for the realization of components converge, and depolarization of both components occurs when they are released into the alloy due to the heat of the formation of the compound or solid solution. indeed, the analysis of cathodic deposits at current densities of 10 to 35 ma cm-2 showed deposits close to the corresponding compositions of solid solutions or re-mo compounds. the proof that compounds or a solid solution based on the re-mo alloy are formed on the cathode follows directly from the cyclic polarization curves shown in figure 3. c u rr e n t, m a c u rr e n t, m a http://dx.doi.org/10.5599/jese.1696 j. electrochem. sci. eng. 13(3) (2023) 563-573 electrochemical obtaining of rhenium-molybdenum alloys 568 a b potential, v vs. ag/agcl potential, v vs. ag/agcl figure 3. cyclic voltammograms (5 mv s-1) of rhenium and molybdenum co-deposition on pt electrode in 2 m h2so4, ph 0.4, 75 °c, in presence of 0.0015m na2moo4 and: (a) 0.035m kreo4; (b) various concentrations of kreo4 (1) 0.035 m; (2) 0.0035 m; (3) 0.00069 m as can be seen from figure 3, on the current-voltage curves of the anodic part of rhenium and molybdenum, respectively, almost one oxidation wave is observed related to the oxidation of rhenium and molybdenum. the wave observed at the potential of 0.3 v is undoubtedly due to the anodic dissolution of rhenium, and the wave at the potential of 0.4 v is due to the anodic dissolution of molybdenum. thus, the obtained data prove again that in the co-deposition of rhenium with molybdenum, the main role is played by the chemical activity of rhenium, associated with its tendency to reduce. in this case, the deposition of both components, forming the alloy is accompanied by depolarization. depolarization during the release of rhenium into the alloy is associated with the formation of a chemical compound of molybdenum with rhenium. table 1 shows the data on the dependence of the alloy composition on the concentration of rhenium in the electrolyte. as can be seen from table 1, with an increase in the concentration of rhenium in the electrolyte, the content of rhenium in the alloy increases. the chemical composition of alloy was determined both chemically and as a result of edx. high-quality alloys are obtained at the temperature of 75 °c and 0.004 m kreo4 in the electrolyte. table 1. dependence of the composition of re–mo alloy on the concentration of rhenium in 2 m h2so4 containing constant concentration of molybdenum. alloy is formed on pt electrode at current density ik = 25 ma cm-2 and 75 °c electrolyte concenration, m content, wt.% deposit appearance kreo4 na2moo4 h2so4 re mo 0.003 0.0015 2 69 31 dark-grey, smooth 0.0035 0.0015 2 70 30 dark-grey, smooth 0.004 0.0015 2 72 28 dark-grey, smooth 0.0045 0.0015 2 76 24 dark-purple, smooth 0.005 0.0015 2 78 22 dark-grey, smooth the study of the re-mo electrochemical deposition process shows that the composition and quality of the resulting coatings mainly depend on the current density. it has been shown in table 2 that with an increase in current density from 5 to 30 ma cm-2, the amount of rhenium in the alloy increases from about 50 to about 80 wt.%. as expected, the composition and quality of re-mo alloys are strongly affected by temperature. it has been established that the rhenium content in the alloy increases with an increase in temperature. with an increase in temperature, the value of the limiting current increases, probably c u rr e n t, m a c u rr e n t, m a e. salakhova et al. j. electrochem. sci. eng. 13(3) (2023) 563-573 http://dx.doi.org/10.5599/jese.1696 569 due to diffusion changes in the near-cathode sheath. high-quality deposits in the form of thin films are obtained at the temperature of 75 to 80 °c, and at the temperature of 25 to 45 °c, loose deposits of re-mo alloy are obtained on the cathode. table 3 shows the data on the dependence of the composition of the alloy on the temperature of the electrolyte. table 2. dependence of the composition of re–mo alloy on the current density of pt electrode in 2 m h2so4 containing constant concentrations of molybdenum and rhenium, at 75 °c electrolyte concentration, m content, wt.% current density, mа сm-2 deposit appearance kreo4 na2moo4 h2so4 re mo 0.0035 0.0015 2 52 48 5 dark-grey, smooth 0.0035 0.0015 2 58 42 15 dark-grey, smooth 0.0035 0.0015 2 62 38 20 dark-grey, smooth 0.0035 0.0015 2 69 31 25 dark-grey, matt 0.0035 0.0015 2 76 24 30 dark-grey, smooth table 3. dependence of the composition of re–mo alloy on temperature of the electrolyte composed from 2 m h2so4 and constant concentrations of re and mo, during deposition on pt electrode at current density jk = 25 ma cm-2 electrolyte concentration, m content, wt.% temperature, °c deposit appearance kreo4 na2moo4 h2so4 re mo 0.0035 0.0015 2 50 50 25 dark-grey, smooth 0.0035 0.0015 2 56 44 45 dark-grey, matt 0.0035 0.0015 2 60 40 60 dark-grey, smooth 0.0035 0.0015 2 69 31 75 dark-grey, matt 0.0035 0.0015 2 75 25 85 dark-grey, smooth the paper also considers the influence of the duration of deposition at different current densities and electrolyte ph on the properties of the rhenium-molybdenum alloy. it has been established that changes in the ph of the solution significantly affect the kinetics of cathodic reduction of re-mo alloys, the quality of the deposit, and the effective current efficiency. in acidic environments (up to ph 3.0), the release of metal is accompanied by the intense release of hydrogen, which reduces the current efficiency and the content of molybdenum in the alloy and worsens the quality of the precipitate. increasing the ph of the solution above 6.0 makes it possible to increase the content of molybdenum in the alloy but reduces the current efficiency from 40 to 5 %. a study of the surface of coatings of re-mo alloys obtained by co-deposition from electrolytes with different ph values showed that uniform, shiny coatings are formed at ph from 0.2 to 3.0. it was found that at a low concentration of one of the components, the alloy is a solid solution based on the second component. therefore, in this case, when studying the kinetics and mechanism of molybdenum deposition, the main attention was paid to those factors that contribute not only to obtaining high-quality deposits but also significantly shift the potential of a more noble metal to the negative side (or the deposition of a more noble metal is accompanied by high polarization). the polarization curves of re-mo alloys using a ni working electrode are presented in figure 4. as can be seen from the figure, the cathodic reduction of the re-mo alloy is located in the region of more positive potentials than the potentials of deposition of individual elements. with the shift of the cathode curve to the region of more positive potentials, the value of the limiting current increases. a similar effect is explained by the release of energy during the formation of a chemical compound. http://dx.doi.org/10.5599/jese.1696 j. electrochem. sci. eng. 13(3) (2023) 563-573 electrochemical obtaining of rhenium-molybdenum alloys 570 potential, v vs. ag/agcl figure 4. cyclic voltammograms (5 mv s-1) of ni working electrode in 2 m h2so4, 0.0015 m na2moo4 and 0.0035 m kreo4, ph 0.4; 75 °c chemical and x-ray diffraction analysis established that at a potential of -0.1 v, an alloy is obtained on the cathode, the composition of which corresponds to the re-mo. morphologies of re-mo films deposited on platinum substrates were studied by a scanning electron microscope. it was established generally that depending on the material of the substrate, thin coatings with different compositions and sizes were obtained on the cathode when prepared at the same experimental conditions. however, coatings with grain sizes of 80-150 nm were obtained on pt substrate electrode. examples of the sem image and elemental composition of remo film deposited on pt substrate are shown in figure 5 and figure 6, respectively. upon thorough investigation, a structure similar to nanostructure and formation of nanosized islets on a platinum sample is observed. islets do not have a certain shape, but they have forms of an egg, ellipse, and various other shapes. x-ray pattern of re-mo thin film deposited on pt electrode and annealed at 500 °c for 2 hours is presented in figure 7. it was found that depending on the electrolyte composition and current density during electrolysis, the substances comprising one phase are obtained. at optimum deposition conditions, the obtained re-mo thin films on a platinum electrode contain oxide compounds. to purify the composition of films from various oxides, the substances obtained electrochemically in re-mo system were annealed at 500 °c for 2 hours and an x-ray pattern was recorded. intensity of the peaks becomes much more visible after annealing. this may be due to the annealing of oxides in films. it follows from the sem data that the film consists of 76.9 wt.%, re, and 23.1 wt.% mo. figure 6 shows the microstructure of re-mo thin coatings obtained by the electrochemical method. in addition to potassium perrhenate and sodium molybdate, ammonium sulfate, citric or boric acid were also introduced into the composition of electrolytes for the deposition of rhenium alloys with molybdenum, but these solutions did not affect the composition and quality of the alloy. the content of molybdenum salt in the electrolyte is usually 0.015 m na2moo4, and the perrhenate concentration is 0.0035 m kreo4. electrolytes operate in a wide ph range (from 3.0 to 0.1), and the cathodic current density usually does not exceed 40 ma cm-2. the electrolyte temperature in the range of 50-75 °с contributed to the production of shiny precipitates. thus, on the basis of experimental data, for the production of rhenium-molybdenum alloys containing 45-80 wt.% re, the following electrolyte composition (mol l-1) is recommended: 0.0015 m na2moo4 + 0.0035 m kreo4 + 2 m h2so4; ph 0.4; scan rate 5 mv s-1; t = 75 °c. c u rr e n t, m a e. salakhova et al. j. electrochem. sci. eng. 13(3) (2023) 563-573 http://dx.doi.org/10.5599/jese.1696 571 figure 5. sem image of re-mo thin film deposited on pt electrode (production of rhenium-molybdenum alloys containing 45-80 wt.% re from 0.0015 m na2moo4 + 0.0035 m kreo4 + 2m h2so4; ph 0.4; t = 75 °c) energy, kev figure 6. elemental analysis of re-mo thin film deposited on pt electrode (production of rheniummolybdenum alloys containing 45-80 wt.% re from 0.0015 m na2moo4 + 0.0035 m kreo4 + 2m h2so4; ph 0.4; t = 75 °c) 2 / ° figure 7. x-ray pattern of deposited re-mo thin film on pt electrode and annealed at 500 °c for 2 hours (production of rhenium-molybdenum alloys containing 45-80 wt.% re from 0.0015 m na2moo4 + 0.0035 m kreo4 + 2 m h2so4; ph 0.4; t = 75 °c) http://dx.doi.org/10.5599/jese.1696 j. electrochem. sci. eng. 13(3) (2023) 563-573 electrochemical obtaining of rhenium-molybdenum alloys 572 conclusions 1. based on the study of cyclic voltammograms recorded on pt electrode during co-electroreduction of rhenium and molybdenum ions from rhenium and molybdate solution in a very acidic electrolyte using sulfuric acid, the 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1696 http://dx.doi.org/10.1149/1.3489532 https://doi.org/10.1039/c39940002617 https://doi.org/10.1134/s1023193511080118 https://doi.org/10.4236/ampc.2012.24b064 https://aip.scitation.org/author/christopher+hudson%2c+david https://doi.org/10.1063/1.4883115 https://doi.org/10.1063/1.4883115 https://doi.org/10.55‌99/‌jese.219 https://doi.org/10.55‌99/‌jese.219 https://doi.org/10.1016/j.electacta.2010.05.001 https://doi.org/10.1016/j.electacta.2010.05.001 https://doi.org/10.1016/j.surfcoat.20‌17.05.024 https://doi.org/10.1016/j.surfcoat.20‌17.05.024 https://doi.org/10.1080/00084433.2018.1511252 https://doi.org/10.1080/00084433.2018.1511252 https://doi.org/10.14288/1.0216002 https://www.chemicalpapers.com/file_access.php?file=471a28.pdf https://www.chemicalpapers.com/file_access.php?file=471a28.pdf https://inis.iaea.org/search/search.aspx?orig_q=author:%22hubli,%20r.c.%22 https://doi.org/10.1016/j.surfcoat.2014.11.073 https://doi.org/10.1134/s0033173206040084 http://dx.doi.org/10.4236/msce.2015.310001 https://creativecommons.org/licenses/by/4.0/) doi: 10.5599/jese.2012.0013 77 j. electrochem. sci. eng. 2 (2012) 77-90; doi: 10.5599/jese.2012.0013 open access : : issn 1847-9286 www.jese-online.org original scientific paper corrosion inhibition of carbon steel by extract of buddleia perfoliata roy lopes-sesenes, jose gonzalo gonzalez-rodriguez, gloria francisca dominguez-patiño*, alberto martinez-villafañe** universidad autonoma del estado de morelos, ciicap, av. universidad 1001, 62209-cuernavaca, mor.,mexico *universidad autonoma del estado de morelos, facultad de ciencias biologicas, av. universidad 1001, 62209-cuernavaca, mor., mexico **centro de investigaciones en materiales avanzados, miguel cervantes 120, chihuahua, mexico corresponding author: e-mail: ggonzalez@uaem.mx tel/fax (777) 3297084 received: march 23, 2012; revised: may 2, 2012; published: june 18, 2012 abstract buddleia perfoliata leaves extract has been investigated as a carbon steel corrosion inhibitor in 0.5 m sulfuric acid by using polarization curves, electrochemical impedance spectroscopy and weight-loss tests at different concentrations (0, 100, 200, 300, 400 and 500 ppm) and temperatures, namely 25, 40 and 60 °c. results showthat inhibition efficiency increases as the inhibitor concentration increases, decreases with temperature, and reaches a maximum value after 12 h of exposure, decreasing with a further increase in the exposure time. it was found that the inhibitory effect is due to the presence of tannines on this extract. keywords corrosion inhibitor, buddleia perfoliata, eis, polarization curves. introduction due to currently imposed requeriments for eco-friendly corrosion inhibitors, there is a growing interest in the use of natural products such as leaves or seeds extracts. some papers have reported the use of natural products for mild steel corrosion inhibition in different environments [1-23]. this is due to the fact that sinthetic inhibitors are, among other factors, expensive and highly toxic. among the so-called “green inhibitors” there are organic compounds, such as ascorbic acid, succinic acid, tryptamine, caffeine, etc., that act by adsorption on metal surface. additionally, some other natural products such as black pepper, azadirachta indica, gossipium hirsutum, j. electrochem. sci. eng. 2(2) (2012) 77-90 corrosion inhibition by buddleia perfoliata 78 guanadine, occimum viridis, talferia occidentalis and hibiscus sabdariffa, were used. for instance, oguzie carried out the inhibitive action of leaf extracts of sansevieria trifasciata on aluminium corrosion in 2 m hcl and 2 m koh solutions by using the gasometric technique [20]. results indicated that the extract functioned as a good inhibitor in both environments and inhibition efficiency increased with the increase of the inhibitor concentration. a mechanism of physical adsorption is proposed for the inhibition behaviour. the adsorption characteristics of the inhibitor were approximated by freundlich isotherm. in another work by chauhan et al. [3] the inhibition effect of zenthoxylum alatum plant extract on the corrosion of mild steel in 5 % and 15 % aqueous hydrochloric acid solution has been investigated by weight-loss method and electrochemical impedance spectroscopy (eis). the corrosion inhibition efficiency increased by increasing the plant extract concentration till 2400 ppm. the adsorption of this plant extract on the mild steel surface obeys the langmuir adsorption isotherm. okafor et. al. [4] studied the inhibitive action of leaves, seeds and a combination of leaves and seeds extracts of phyllanthus amarus on mild steel corrosion in hcl and h2so4 solutions using weightloss and gasometric techniques. the results indicate that the extracts functioned as a good inhibitor in both environments, and inhibition efficiency increases with extracts concentration. the adsorption characteristics of the inhibitor were approximated by temkin isotherm. the corrosion efficiency of these extracts is normally ascribed to the presenceof complex organic species such as tannins, alkaloids and nitrogen bases, carbohydrates and proteins as well as their acid hydrolysis products. the genus buddleia, included in the family loganiaceae, and previously classified in a family of its own, the buddlejaceae, is now classified in the family scrophulariaceae. native to asis, africa, north and south america, buddleia is a genus containing 100 species, 50 being distributed in america, of which 16 grow in mexico [22]. buddleia species are widespread and share some remarkable similarities in their medicinal uses. this may well indicate the presence of the same or similar compounds with a particular pharmacological action. a patterns is emerging about the composition of these compounds; flavonoid and iridoid glycosids being the major seccondary metabolites that have been isolated to date [23]. buddleia perfoliata became officialy recognized in the 1930 mexican pharmacopoeia where it was shown to have antisudorific activity [24]. this plant also contains essential oil, tannic, gallic and oxalic acids [25]. in folk medicines, it is used in the treatment for tuberculosis as well as for catarrh, ptyalism and headaches [25]. in the present paper, we evaluated the inhibitory effect of buddleia perfoliata in the corrosion of 1018 carbon steel in 0.5m h2so4 by using both gravimetric and electrochemical techniques. experimental procedure corrosion tests were performed on coupons prepared from 1018 carbon steel rods containg 0.14 % c, 0.90 % mn, 0.30 % s, 0.030 % p and as balance fe, encapsulated in commercial epoxic resin with the exposed area of 1.0 cm2. the aggressive solution, 0.5 m h2so4 was prepared by dilution of analytical grade h2so4 with double distilled water. dried buddleia perfoliata leaves (38.7 g) were soaked in 300 ml of methanol during 24 h and refluxed during 5 h obtaining a solid, which was weighted and dissolved in methanol untill this was completely evaporated and used as a stock solution and then for preparation of the desired concentrations by dilution. methanol is commonly used in the obtaining green inhibitors [2-11] and since it completely evaporates, there is no risk of toxicity. weight-loss experiments were carried out with steel rods of 2.5 cm in length and 0.6 cm diameter abraded with fine 1200 grade emery paper, rinsed with acetone, and exposed to the aggressive solution during 72 h. after a total exposition time of 72 h, specimens r. lopes-sesenes et al. j. electrochem. sci. eng. 2(2) (2012) 77-90 doi: 10.5599/jese.2012.0013 79 were taken out, washed with distilled water, degreased with acetone, dried and weighed accurately. specimens were hanging by nylon fibers which were introduced through a hole of 0.5 mm in diameter, drilled in one end of the specimen. tests were performed by triplicate at room temperature (25 °c), 40 and 60 °c by using a hot plate. corrosion rates, in terms of weight loss measurements, δw, were calculated as follows: δw = (m1 – m2) / a (1) were m1 is the mass of the specimen before corrosion, m2 the mass of the specimen after corrosion, and a the exposed area of the specimen. for the weight loss tests, inhibitor efficiency, ie, was calculated as follows: ie = 100 (δw1 δw2)/δw1 (2) where δw1 is the weight loss without inhibitor, and δw2 the weight loss with inhibitor. specimens were removed, rinsed in water and in acetone, dried in warm air and stored in desiccators. specimens were weighed in an analytical balance with a precision of 0.1 mg. surface analysis of corroded specimens was carried out by a scanning electronic microscope (sem). electrochemical techniques employed included potentiodynamic polarization curves and electrochemical impedance spectroscopy (eis) measurements. in all the experiments, carbon steel electrode was allowed to reach stable open circuit potential value, ecorr. each polarization curve was recorded three times at constant sweep rate of 1 mv s-1 at the interval from -1000 to + 1500 mv in respect to the ecorr value. measurements were obtained by using a conventional three electrodes glass cell with two graphite electrodes symmetrically situated, and a saturated calomel electrode (sce) as a reference electrode with a lugging capillary bridge. corrosion current density values, icorr, were obtained by using tafel extrapolation. inhibitor efficiency, ie, was calculated as follows: ie = 100 (icorr1 icorr2)/icorr1 (3) where icorr1 is the corrosion current density value without inhibitor, and icorr2 the corrosion current density value with inhibitor. eis tests were carried out three times at ecorr by using a signal with amplitude of 10 mv in a frequency interval of 100 mhz 100 khz. an acm potentiostat, controlled by a desk top computer was used for the polarization curves, whereas for the eis measurements, a model pc4 300 gamry potentiostat was used. results and discussion the effect of buddleia perfoliata concentration in the polarization curves at 25 °c is shown in fig. 1, whereas electrochemical parameters for these curves are shown in table 1. it can be seen that steel exhibits an active-passive behavior with and without inhibitor; in the uninhibited solution, the steel shows an increase in the anodic current density, but around 400 mv a region where the current remains more or less stable, similar to a passive region, apperas although very unstable since some anodic transients can be seen due to the brakedown and repair of this incipient passive layer. this unstable region dissappears with the addition of 100, 200 or 300 ppm of buddleia perfoliata and the passive zone becomes stable, but with a further increase of the inhibitor concentration this unstable regions apperas once again. it is clear that the addition of buddleia perfoliata has caused a clear decrease in both the anodic and cathodic branch of the polarization curves, and this effect is more pronounced as the inhibitor concentration increases; from table 1 it can be seen that as soon as the extract is added to the electrolyte, the ecorr value j. electrochem. sci. eng. 2(2) (2012) 77-90 corrosion inhibition by buddleia perfoliata 80 becomes more negative and the icorr value decreases, reaching its lowest value when 500 ppm of inhibitor is added. 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 -1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600 800 1000 400 ppm 500 ppm 0 ppm 100 ppm 200 ppm 300 ppm e / m v v s. s c e j / ma cm -2 figure 1. effect of buddleia perfoliata concentration in the polarization curves for 1018 carbon steel corroded in 0.5 m h2so4 at 25 °c table 1.electrochemical parameters obtained from polarization curves at 25 °c. estimated error in potential is ± 6 mv, in current density is ± 5 x 10-3 ma cm-2 and in resistance is ± 1 x 10-3 ω cm2. cinh ppm ecorr mv vs. sce icorr ma cm-2 ipas ma cm-2 βa mv dec-1 βc mv dec-1 rp ω cm2 0 -473 0.09 50 125 -120 60 100 -503 0.07 0.06 118 -115 102 200 -497 0.07 0.05 108 -115 145 300 -491 0.06 0.04 65 -110 240 400 -460 0.05 25 60 -105 565 500 -478 0.05 27 50 -105 600 as expected, the polarization resistance value, rp, increases as the inhibitor concentration increases, reaching a maximum value when 500 ppm of inhibitor is added. in addition to this, passive current density decreases as the buddleia perfoliata concentration increases up to 300 ppm, but increasing again with a further increase of the inhibitor concentration. the cathodic slope was practically unaffected by the addition of the buddleia perfoliata, which indicates that hydrogen evolution reaction is diminished exclusively by the surface blocking effect of adsorbed inhibitor [15]. regarding the anodic region of the potentiodynamic polarization curves, there is clearly an active-passive behavior either in presence or in absence of the inhibitor. also, the currents remains almost the same in all cases in the active dissolution region of the metal, but it decreased in the passive region when the inhibitor is added. this behavior could be related to a change in the anodic reaction mechanism (iron dissolution) which is corroborated by a decrease in the anodic tafel slope with increasing concentration of buddleia perfoliata [15]. r. lopes-sesenes et al. j. electrochem. sci. eng. 2(2) (2012) 77-90 doi: 10.5599/jese.2012.0013 81 nyquist and bode diagrams for 1018 carbon steel exposed to 0.5 m h2so4 with different buddleia perfoliata dosis is shown in fig. 2. nyquist diagram, fig. 2a, shows a single deppressed, capacitive semicircle with its center in the real axis regardless of the inhibitor concentration, indicating that the corrosion process is under charge transfer control from the metal to the electrolyte through the electrochemical double layer. this type of plot is characteristic of solid electrodes and it is often ascribed to dispersion effects, which are attributed to roughness and inhomogeneities of the surface during corrosion [24,25]. 0 50 100 150 200 250 300 350 400 450 500 550 600 650 0 -50 -100 -150 -200 -250 -300 -350 -400 -450 -500 -550 -600 -650 z i m / ω c m 2 zre / ωcm 2 100 ppm 200 ppm 300 ppm 400 ppm 500 ppm0 ppm a) 0.1 1 10 100 1000 10000 0 -20 -40 -60 -80 f / hz 0 ppm 100 ppm 200 ppm 300 ppm 400 ppm 500 ppm ϕ / ° b) figure 2. effect of buddleia perfoliata concentration in the a) nyquist and b) bode diagrams for 1018 carbon steel corroded in 0.5 m h2so4 at 25 °c this behavior is not affected by the presence of the inhibitor, indicating the activationcontrolled nature of the reaction; the semicircle diameter increases with the inhibitor concentration, reaching a maximum value with 500 ppm of inhibitor. these results support those obtained from the polarization curves and confirm the inhibitor adsorption onto carbon steel surface. the intersection of of the semicircle with the real axis at high frequencies provides a value of the solution resistance of 4.3 ω cm2. the semircle diameter is related to the charge transfer resistance, rct, inversely proportional to the icorr value, thus, the lowest corrosion rate is attained with 500 ppm, as indicated by the polarization curves in fig. 1 and table 1. for the uninhibited solution, an rct value of 52 ω cm 2 was found. bode diagram, fig. 2b, shows a single peak around 200 hz, which shifts towards higher frequency values as the inhibitor concentration increases up to 500 ppm. equivalent electric circuit used to simulate the eis data for 1018 carbon steel exposed to 0.5 m h2so4 with different buddleia perfoliata dosis is shown in fig. 3. in fig. 3, rs represents the solution resistance, rct the charge transfer resistance or the resistance to the flow of electrons from the metal to the electrolyte, and cdl the capacitance of the electrochemical double layer, or the capacity to storage charge in this layer. however, one has to account for the inhomogeneity of the surface-electrolyte system. when a non-ideal frequency response is present, it is commonly accepted to employ distributed circuit elements in an equivalent circuit. the most widely used is a constant phase element (cpe) or time constant, which has a non-integer power dependence on the frequency. often a cpe is used in a model in place of a capacitor to compensate for nonhomogeneity in the system. since only a single peak exists in bode diagram, fig. 2b, only one time constant is needed to simulate the eis data. table 3 summarizes the calculated parameters to simulate the eis data using circuit shown in fig. 3. it can be seen that the rct value reaches its j. electrochem. sci. eng. 2(2) (2012) 77-90 corrosion inhibition by buddleia perfoliata 82 highest value between 400 and 500 ppm, whereas the electrochemical double layer capacitance, cdl, attains its lowest value at these inhibitor concentrations, which may be attributed to the adsorption of the components in the buddleia perfoliata extract onto the metal/electrolyte interface or to an increase of the double layer [12]. figure 3. electric circuit used to simulate the eis data table 3. calculated parameter values used to simulate the impedance data for eis data estimated error in resistance is ± 1 x 10-3 ω cm2 whereas for capacitance it is ± 1 x 10-8 f cm-2 cinh / ppm rs / ω cm 2 rct / ω cm 2 cdl/ f cm -2 0 4.9 52 5.3 x 10-5 100 6.9 102 7.8 x 10-6 200 5.2 145 5.3 x 10-6 300 4.9 240 4.1 x 10-6 400 4.4 565 1.7 x 10-6 500 4.2 600 4.7 x 10-7 alternatively, the double layer capacitance, cdl, was calculated from the equation bellow: cdl = 1/2πfmaxrct (4) where fmax is the frequency value at which the imaginary component of the impedance is maximal. for the uninhibited solution, a cdl value of 530 μf cm-2 was found. table 3 gives the results for the rs, rct and cdl values for the solution with and without inhibitor. it is important to note that theincrease of the inhibitor concentration brings an increase in the charge transfer resistance value and a decrease in the double layer capacitance. the decrease in the cdl value can be interpreted as due to the adsorption of the inhibitor onto the electrode surface [22]. the double layer formed at the metal-solution interface is considered as an electric capacitor, whose capacitance decreases due to the displacement of water molecules and other ions originally adsorbed on the electrode by the inhibitor molecules, forming a protective film. the thickness of the film formed increases with increasing concentration of the inhibitor, since more inhibitor adsorbs on the surface, resulting in lower cdl values. the stability of any film formed by the inhibitor was evaluated by plotting the nyquist diagram at different times during 24 h, as shown in fig. 4. this figure shows that the semicircle diameter remains more or less constant during 8 h, and after this time, the semicircle diameter decreases continuously as time elapses. the change in the rct and cdl values calculated from this figure are shown in fig. 5, where it is evident that the rct values remains more or less constant during the first 8 h, but after this, its value starts to decrease, indicating that the inhibitor remains more or less stable on the steel surface during this time. the cdl value increases as time elapses, indicating a decrease in the inhibitor film thickness. r. lopes-sesenes et al. j. electrochem. sci. eng. 2(2) (2012) 77-90 doi: 10.5599/jese.2012.0013 83 0 100 200 300 400 500 600 0 -100 -200 -300 -400 -500 -600 20 h 16 h 12 h 8h z i m / ω c m 2 zre / ω cm 2 0 h 4 h figure 4. change of the nyquist diagram with time for 1018 carbon steel corroded in 0.5 m h2so4 at 25 °c with the addition of 500 ppm of buddlia perfoliata 0 5 10 15 20 25 100 200 300 400 500 600 700 time, h r ct / ω c m 2 10 -6 10 -5 10 -4 c d l / f cm -2 figure 5. change of the charge transfer resistance, rct, and double layer capacitance values, cdl, with time for 1018 carbon steel corroded in 0.5 m h2so4 at 25 °c with the addition of 500 ppm of buddleia perfoliata the rct values were used to calculate the ie according to the equation: ie = 100 (rct2 rct 1) / rct 2 (5) where rct1 and rct2 are the charge transfer resistance values for the uninhibited and inhibited solution, respectively. the results, together with those obtained by polarization curves (tafel method), weight-loss and polarization resistance measurements are given in table 2. it can be clearly seen that all different techniques show that the corrosion inhibition efficiency increases with the increase of inhibitor concentration, reaching a maximum value at 500 ppm of inhibitor. the discrepancy in the ie values obtained from different techniques can be interpreted as the result of different measurements time. therefore, these results suggest, once again, the formation of an insoluble inhibitor film due to the adsorption of inhibitor onto carbon steel surface. j. electrochem. sci. eng. 2(2) (2012) 77-90 corrosion inhibition by buddleia perfoliata 84 table 2. efficiency of buddleia perfoliata as an inhibitor of 1018 carbon steel in 0.5 m h2so4 calculated with different techniques. estimated error in efficieny values is ± 3 %. cinh / ppm ietafel slope , % ieweight–loss , % ieeis , % iepol. resistance , % 100 22 10 47 41 200 22 15 65 58 300 33 18 75 69 400 44 22 84 80 500 44 27 88 84 in order to evaluate the adsorption process of buddleia perfoliata on the 1018 carbon steel surface, langmuir, temkin and frumkin adsorption isotherms were obtained according to the following equations: langmuir: θ/1-θ = kcinh (6) temkin: log (θ/cinh) = log k gθ (7) frumkin: log (θcinh)/(1-θ) = log k + gθ (8) where θ is the surface coverage, k is the adsorption-desorption equilibrium constant, cinh is the inhibitor concentration and g is the adsorbate interaction parameter. the three isotherms tested fitted well the data obtained, as can be seen in the fig. 6 indicating that buddleia perfoliata is adsorbed onto the carbon steel surface. however, the isotherm which gave the best r2 value, 0.996, was the frumkin one. from the frumkin isotherm, the adsorption-desorption equilibrium constant k was determined as 3.785 l mg-1 leading to an adsorption free-energy value of -37.4 kj mol-1. generally, values of the adsorption free-energy much less than -40 kj mol-1 have typically been correlated with the electrostatic interactions between organic molecules and charged metal surface (physisorption) whilst those values in the order of -40 kj mol-1 are associated with charge sharing, or charge transfer from the organic molecules to the metal surface (chemisorption) to form a co-ordinate type of bond [26].the negative value of the free-energy of adsorption value means that the adsorption process is spontaneous, while the value around -40 kj mol-1 indicates that buddleia perfoliata was chemisorbed on steel surface. the temkin isotherm, fig. 5 b, also shows a good correlation with the experimental data, and the negative value of the slope indicates the existance of a repulsive lateral interaction in the adsorption layer [26]. the effect of temperature on the corrosion of carbon steel in the uninhibited and inhibited 0.5 m h2so4 solutions was studied using both, potentiodynamic polarization curves and eis tests. fig. 7 shows the effect of temperature on the polarization curves for uninhibited and inhibited solution containing 500 ppm of inhibitor, respectively. it was found that the corrosion rate of steel in both, uninhibited and inhibited, solutions increases as the temperature increases. however, the extent of the rate increment in the inhibited solution is higher in the uninhibited than in the inhibited solution. this suggests that the corrosion inhibition might be caused by the inhibitor adsorption onto the steel surface from the acidic solution, and higher temperatures might cause a stronger adsorption of the inhibitor on the steel surface. r. lopes-sesenes et al. j. electrochem. sci. eng. 2(2) (2012) 77-90 doi: 10.5599/jese.2012.0013 85 0 100 200 300 400 500 0 2 4 6 8 10 θ/ (1 -θ ) c inh / ppm langmuir r 2 =0.930 a) 0.3 0.4 0.5 0.6 0.7 0.8 0.9 -2.8 -2.7 -2.6 -2.5 -2.4 -2.3 -2.2 -2.1 temkin lo g( θ/ c i n h ) θ r 2 =0.951 b) 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 2.5 3.0 3.5 4.0 frumkin lo g( θ c i n h )/ (1 -θ ) θ r 2 =0.996 c) figure 6. a) langmuir b) temkin and c) frumkin adsorption isotherm for 1018 carbon steel in 0.5 m h2so4 at 25 °c with the addition of buddleia perfoliata the apparent activation energy, ea, associated with 1018 carbon steel in uninhibited and inhibited acid solution was determined by using an arrhenius-type plot according to the following equation: log icorr = -ea / 2.303rt + log f (9) where icorr is the corrosion current density value, r is the molar gas constant, t is the absolute temperature and f is the frequency factor. arrhenius plots of log icorr against t -1 for 1018 carbon steel in 0.5 m h2so4 in absence and presence of buddleia perfoliata are shown in fig. 8. the apparent activation energy obtained for the corrosion process in the inhibitor-free, uninhibited acid solution was found to be 83.9, and 63.9 kj mol-1 in the presence of the inhibitor. notably, the energy barrier of the corrosion reaction decreased in the presence of the inhibitor, which can be due to the chemisorption of the inhibitor on the steel surface. j. electrochem. sci. eng. 2(2) (2012) 77-90 corrosion inhibition by buddleia perfoliata 86 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 -1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600 800 1000 e /m v v s sc e j / ma cm -2 25 0 c 40 0 c 60 0 c 0 ppm a) 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 -1400 -1200 -1000 -800 -600 -400 -200 0 200 400 600 800 1000 e / m v v s sc e j / ma cm -2 25 0 c 40 0 c 60 0 c 500 ppm b) figure 7. effect of temperature in the polarization curves for 1018 carbon steel corroded in 0.5 m h2so4 a) wihout and b) with 500 ppm of buddleia perfoliata 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 lo g (i co rr / m a c m -2 ) 1000 t -1 / k -1 0 ppm, slope= -7.78 500 ppm, slope= -3.34 figure 8. arrhernius plots for log (icorr) vs. 1000t -1 for 1018 carbon steel corroded in 0.5 m h2so4 wihout and with 500 ppm of buddleia perfoliata r. lopes-sesenes et al. j. electrochem. sci. eng. 2(2) (2012) 77-90 doi: 10.5599/jese.2012.0013 87 in a way similar to the polarization curves, the effect of temperature on the nyquist diagrams for uninhibited and inhibited solutions are shown in figs. 9 and 10, respectively. in both cases it can be seen that the semicircle diameters, and thus, the rct values, decreased as the temperature increased, which was more evident for the inhibited solution. an arrhernius plot of log 1/rct against t-1 for 1018 carbon steel in 0.5 m h2so4 in absence and presence of buddleia perfoliata is shown in fig. 10, where apparent activation energy obtained for the corrosion process in the free acid solution was found to be 4.31 and 3.5 kj mol-1 in presence of the inhibitor. 0 30 60 0 -30 -60 z i m / o h m c m 2 z re / ω cm 2 25 0 c 40 0 c 60 0 c 0 ppm 0 100 200 300 400 500 600 0 -100 -200 -300 -400 z i m / ω c m 2 z re / ω cm 2 25 0 c 40 0 c60 0 c 500 ppm b) figure 9. effect of temperature in the nyquist diagrams for 1018 carbon steel corroded in 0.5 m h2so4 a) wihout and b) with 500 ppm of buddleia perfoliata according to popova et al. [27] lower ea values in solutions in presence of buddleia perfoliata indicate a specific type of adsorption of the inhibitor, while szauer and brandt [28] associate this behavior with the chemisorption of the inhibitor to the metal surface. taking into consideration these references and the ea value calculated from the arrhenius plots, the action of buddleia perfoliata as a corrosion inhibitor for 1018 carbon steel in acid solution can be attributed to a strong type of chemisorption of the inhibitor onto metal surface. 3.0 3.1 3.2 3.3 -6 -5 -4 -3 lo g (1 /r ct / ω -1 cm -2 ) 1000 t -1 / k -1 0 ppm, slope = -7.11 500 ppm, slope = -3.5 figure 10. arrhernius plots for log (1/rct) vs. 1000 t -1 for 1018 carbon steel corroded in 0.5 m h2so4 without and with 500 ppm of buddleia perfoliata some micrographs of 1018 carbon steel specimens, after being exposed to corrosion in 0.5 m h2so4, with and without additions of buddleia perfoliata, are shown in fig. 11. for the uninhibited solution (fig. 11 a), only a surface showing uniform corrosion can be seen, but after addition of j. electrochem. sci. eng. 2(2) (2012) 77-90 corrosion inhibition by buddleia perfoliata 88 200 ppm of buddleia perfoliata (fig. 11 b), a porous, non-protective layer of corrosion products can be seen; however, after addition of 400 ppm of inhibitor (fig. 11 c), this layer is much more compact, but it still shows some cracks, indicating that it is not protective enough; finally, at the same inhibitor concentration of 400 ppm but at 60 °c (fig. 11 d), the layer of corrosion products becomes more porous than the one at 25 °c, and becomes less protective, as indicated by all data. a b c d figure 11. micrographs of 1018 carbon steel corroded in 0.5 m h2so4 with addition of a) 0, b) 200 c) 400 ppm at 25 °c and d) 400 ppm at 60 °c of buddleia perfoliata the use of buddleia perfoliata in traditional medicine has been atributed to the presence of some flavonoids as well as some essential oil, tannic, gallic and oxalic acid [23-24]. uv-visible spectra analysis were performed for the acidic solution containing the extract before and after the corrosion test. for the extract before the corrosion test, the uv-spectrum shows an absorption peak at 360 nm (fig. 12) corresponding to tannins; after the addition of the extract to the acidic solution, tannins are hydrolyzed producing galic and ellagic acids [25]. condensed tannins, called pro-antocianidines, are polymers of flavonoids. formation of a complex formed with fe2+ ions and ohgroups present in condensed tannins are responsible for the corrosion protection of the metal. as the temperature increases, the degree of polymerization increases and the formed species are more complex and more easily oxidized until the formed corrosion products are detached from the surface and corrosion protection decreases. r. lopes-sesenes et al. j. electrochem. sci. eng. 2(2) (2012) 77-90 doi: 10.5599/jese.2012.0013 89 200 400 600 800 1000 1200 0 1 2 3 4 5 6 wavelength, nm pure extract a b so rb an ce , a .u . acid+400 ppm inhibitor before test acid+400 ppm inhibitor after test figure 12.uv visible spectra of pure buddleia perfoliata extract, 0.5 m h2so4 + 400 ppm of inhibitor before and after the corrosion test. conclusions a study of buddleia perfoliata leaves extract as corrosion inhibitor for 1018 carbon steel in 0.5 m h2so4 has been investigated by using electrochemical techniques and weight-loss tests. results have shown that buddleia perfoliata leaves extract acts as a good inhibitor, and its efficiency increases with increasing the concentration up to 500 ppm but it decreases by increasing the temperature, and remains on the metal surface no more than 12 h. it was found that the inhibitory effect is due to the presence of tannines from this extract which form a protective layer by reacting with fe2+ ions, and which are chemisorbed onto the metal surface following frumkin type of adsorption isotherm and the corrosion reaction energy 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[25] a. avila, j. guillermo, a. romo de vivar, biochem. system. ecol. 30 (2002) 1003-1005 [26] s. martinez, i. stern, appl. surf. sci. 199 (2002) 83-90 [27] a. popova, e. sokolova, s. raicheva, m. christov, corros. sci. 45 (2003) 33-58 [28] t. szauer, a. brandt, electrochim. acta 26 (1981) 1253-1256 © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 25%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.6 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent 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<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> /enu (use these settings to create adobe pdf documents best suited for high-quality prepress printing. created pdf documents can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [4000 4000] /pagesize [612.000 792.000] >> setpagedevice mno2 nanorods modified screen-printed electrode for the electrochemical determination of sudan dye in food sample: http://dx.doi.org/10.5599/jese.1415 1121 j. electrochem. sci. eng. 12(6) (2022) 1121-1131; http://dx.doi.org/10.5599/jese.1415 open access : : issn 1847-9286 www.jese-online.org original scientific paper mno2 nanorods modified screen-printed electrode for the electrochemical determination of sudan dye in food sample peyman mohammadzadeh jahani1 and sayed ali ahmadi2, 1school of public health, bam university of medical sciences, bam, iran 2department of chemistry, kerman branch, islamic azad university, kerman, iran corresponding author: ahmadi.iauk59@gmail.com received: june 19, 2022; accepted: july 1, 2022; published: july 30, 2022 abstract a novel mno2 nanorods modified screen-printed electrode was fabricated and used as a voltammetric sensor for sudan determination. mno2 nanorods were characterized using field emission-scanning electron microscopy (fe-sem). electrochemical measurements were performed using cyclic voltammetry (cv), linear sweep voltammetry (lsv), differential pulse voltammetry (dpv), and chronoammperometry (ca). the mno2 nanorods on the electrode surface act as an excellent catalyst for the sudan oxidation reaction. our modified electrode presents good electrocatalytic activity toward sudan, a short response time of <10 s, a low detection limit of around 0.08 µm, and linear detection range from 0.25 to 300.0 µm. keywords voltammetric sensor; cyclic voltammetry; electroanalysis; sudan red introduction all food dyes can be divided into two basic, general categories: natural dyes and artificial dyes. the natural food dyes are derived from grapes, saffron, paprika, carrots, beets, and algae and are used to color a variety of foods. the artificial food dyes (afcs), mostly derived from petroleum, contain a single or more azo functional groups (–n=n–), most frequently connecting the two aromatic parts. people associate specific colors with specific flavors, therefore, the colors of food can affect their perception of taste, especially their perception of sweets and beverages. artificial dyes may improve on natural variations in color, may enhance colors that occur naturally, or may provide color to colorless and “fun” food, thereby making it appear more attractive and appetizing, e.g., adding a red, yellow or green color to gummy sweets, which would naturally be colorless [1,2]. sudan red i (1-phenylazo-2-naphthol), a synthetic azo dye, is considered to be a genotoxic carcinogen and classified as a category 3 carcinogen by the international agency for research on cancer (iarc) (1975). its presence is prohibited in foodstuffs for any purpose at any level worldwide. unfortunately, a variety of foodstuffs contaminated with sudan dyes and para red, particularly http://dx.doi.org/10.5599/jese.1415 http://dx.doi.org/10.5599/jese.1415 http://www.jese-online.org/ mailto:ahmadi.iauk59@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1121-1131 mno2 nanorods spe for determination of sudan dye 1122 sudan red i, have been detected throughout europe and asia [3-5]. high-performance liquid chromatography-mass spectrometry (hplc–ms) has been proven to be an excellent method for the direct determination of sudan reds (i, ii, iii, and iv). although sensitive and specific, this method can be time-consuming and expensive. therefore, it is necessary to develop a rapid and economical method for detecting sudan red i [6]. the electrochemical sensors are the most developed analytical tools for detecting the analyte. electrochemical detection has the advantages of simple operation, low cost, and easy to carry, but the specific recognition of the electrochemical method in detection is poor. the chemical modification of inert substrate electrodes offers significant advantages in the design and development of electrochemical sensors [7-26]. a further advantage of chemically modified electrodes is that they are less prone to surface fouling and oxide formation compared to inert substrate electrodes [27-39]. in this context, electrochemical sensors based on screen-printed electrodes (spes) have gained increasing interest as analytical tools for food analysis since spes provide great advantages that make these kinds of sensors have the important characteristics of ideal sensors: ease of use, low cost, and portability [40-42]. so, the screen-printed technology has significantly contributed to the transition from the traditional unwieldy electrochemical cells to miniaturized and portable electrodes that meet the needs for on-site analysis. although a screen-printed electrode (spe) is not as robust as a conventional electrode, such as glassy carbon or gold disk, and the surface of its working electrode is not as perfect as the one of a mirror-like polished solid electrode, the advantages of spes regarding cost and size led to their increasing use in the last years as transducers in sensing. the use of spe-based sensors in the control of food spoilage as complementary analytical tools to the conventional methods allows a rapid screening at any point of the food production chain, preventing the occurrence of foodborne illnesses and the reduction of food waste [43]. the development of nanoscience and nanotechnology has allowed trials to apply different nanomaterials to fabricate chemically modified electrodes [44-53]. in recent years, various nanomaterials have been used singly or in composite form to modify electrodes [54-60]. manganese dioxide (mno2), as a type of transition metal oxide, has received much more attention for its role as an electrode material for sensors, batteries and supercapacitors attributed to its good electrochemical performance in the neutral electrolyte, high theoretical capacity and nontoxicity [61]. in this research work, an electrochemical sensor modified with mno2 nanorods (mno2–nrs) was designed using a screen-printed electrode for the determination of sudan. experimental apparatus and chemicals all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab. the measurement cell consisted of spe (dropsens; drp-110: spain) containing a graphite counter electrode, a graphite working electrode, and a silver pseudo-reference electrode. solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). sudan and other chemicals used were analytical grade and were purchased from merck. orthophosphoric acid and its salts (nah2po4, na2hpo4 and na3po4) were utilized to prepare the phosphate buffer solutions (pbss), and sodium hydroxide was used for adjusting the desired ph values (ph range between 2.0 and 9.0). mno2 nanorods (mno2-nrs) were synthesized in our laboratory. so that first, 0.316 g of kmno4 was dissolved in deionized water (30 ml) under stirring. then, 1.4 ml of hcl (3.0 m) was added into the above solution under stirring for 30 min. p. mohammadzadeh jahani and s. a. ahmadi j. electrochem. sci. eng. 12(6) (2022) 1121-1131 http://dx.doi.org/10.5599/jese.1415 1123 the achieved solution was transferred into a teflon lined autoclave at 160 °c for 6 h. next, it was cooled down at room temperature to gather the products via centrifugation, followed by washing with ethanol and deionized water several times. the product was finally dried in an oven at 60 °c for 12 h. figure 1 shows the fe-sem image of mno2-nrs. figure 1. fe-sem image of mno2-nrs preparation of mno2-nrs/spe first, 1 mg of prepared mno2-nrs was added into an aqueous solution (1 ml), followed by sonication for 30 min to give a homogeneous solution. then, 4 μl of mno2–nrs were dispersed on the surface of spe dropwise. following the solvent's evaporation, the sensor's surface was washed several times with deionized water to clean free modifier molecules and subsequently air-dried. the obtained electrode was noted as mno2-nrs/spe. the schematic of spe surface modification for the oxidation of sudan is shown in scheme 1. the surface areas of the mno2-nrs/spe and the unmodified spe were obtained by cv using 1 mm k3fe(cn)6 at various scan rates. using the randles–ševčik equation for mno2-nrs/spe, the electrode surface was found to be 0.122 cm2 which was about 3.9 times greater than un-modified spe. scheme 1. schematic of spe surface modification for the oxidation of sudan http://dx.doi.org/10.5599/jese.1415 j. electrochem. sci. eng. 12(6) (2022) 1121-1131 mno2 nanorods spe for determination of sudan dye 1124 results and discussion electrochemical behavior of sudan at the surface of various electrodes the effect of the electrolyte ph on the oxidation of 70.0 μm sudan was investigated at mno2nrs/spe using differential pulse voltammetry (dpv) measurements in the pbs in the ph range from 2.0 to 9.0. according to the results, the oxidation peak current of sudan depends on the ph value and increases with increasing ph until it reaches the maximum at ph 7.0 and then decreases at higher ph values. the optimized ph corresponding to the higher peak current was 7.0, indicating that protons are involved in the reaction of sudan oxidation. the electrochemical behavior of sudan was also investigated by cv. the cyclic voltammetry obtained using the bare spe (trace b) and mno2-nr/spe (trace a) in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm sudan is shown in figure 2. on a bare spe, a signal with a low oxidation current of 3.2 μa was obtained with a peak potential of 680 mv. in contrast, mno2-nrs/spe exhibited an enhanced sharp anodic peak current (ipa =11.0 μa) at a much lower overpotential ep = 610 mv. these results confirmed that the mno2-nrs/spe improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of sudan. figure 2. cyclic voltammograms of (a) mno2–nrs/spe and (b) bare spe in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm sudan at the scan rate 50 mvs-1 effect of scan rate on the determination of sudan at mno2–nrs/spe the influence of the scan rate () on the peak currents (ipa) of sudan at mno2-nrs/spe was investigated by lsv (figure 3). figure 4 shows the voltammetric response of 100.0 μm sudan at mno2nrs/spe at different scan rates in the range of 5 to 400 mv/s. the oxidation peak current of sudan increases linearly with increasing scan rate. a linear regression equation was obtained from the plot ipa vs. 1/2 (square root of scan rate) as follows; ipa = 1.4523 1/2 + 0.8751 (r2 = 0.9990) for the oxidation process, which indicates that the reaction of sudan at mno2-nrs/spe is diffusion controlled. p. mohammadzadeh jahani and s. a. ahmadi j. electrochem. sci. eng. 12(6) (2022) 1121-1131 http://dx.doi.org/10.5599/jese.1415 1125 figure 3. linear sweep voltammograms of mno2-nrs/spe in 0.1 m pbs (ph 7.0) containing 100.0 μm sudan at various scan rates; 1-7 correspond to 5, 10, 50, 100, 200, 300 and 400 mv s-1 figure 4. plot of anodic peak current vs. ν1/2 at different scan rates in the range of 5 to 400 mv/s chronoamperometric analysis the analysis of chronoamperometry for sudan samples was performed using mno2-nrs/spe at 0.66 v. the chronoamperometric results of different concentrations of sudan in pbs (ph 7.0) are demonstrated in figure 5. the cottrell equation for the chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is as in equation (1): i = nfad1/2cbπ-1/2t-1/2 (1) http://dx.doi.org/10.5599/jese.1415 j. electrochem. sci. eng. 12(6) (2022) 1121-1131 mno2 nanorods spe for determination of sudan dye 1126 where d represents the diffusion coefficient (cm2 s-1), and cb is the applied bulk concentration (mol cm-3). experimental results of i vs. t-1/2 were plotted in figure 6a, with the best fits for different concentrations of sudan. the resulting slopes corresponding to straight lines in figure 6a were then plotted against the concentration of sudan (figure 6b). the mean value of d was determined to be 4.5×10-5 cm2/s according to the resulting slope and cottrell equation. t / s figure 5. chronoamperograms obtained at mno2-nrs/spe in 0.1 m pbs (ph 7.0) for different concentrations of sudan. the 1-4 correspond to 0.1, 0.5, 1.0 and 1.5 mm of sudan t-1/2 / s-1/2 csudan / mm figure 6. (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-4. (b) plot of the slope of the straight lines against sudan concentration (0.1-1.5 mm) y = 7.2555x + 1.142 r2 = 0.9974 s lo p e , m a s -1 /2 p. mohammadzadeh jahani and s. a. ahmadi j. electrochem. sci. eng. 12(6) (2022) 1121-1131 http://dx.doi.org/10.5599/jese.1415 1127 calibration curve because dpv commonly has a higher sensitivity than cyclic voltammetry, the dpv technique was applied for the quantitative detection of sudan. figure 7 shows the differential pulse voltammograms of sudan at various concentrations using mno2-nrs/spe (step potential of 0.01 v and pulse amplitude of 0.025 v). as seen, the oxidation peak currents of sudan enhance gradually by increasing its concentration. the oxidation peak currents (ipa) show a good linear relationship with the concentrations of sudan ranging from 0.25 to 300.0 μm. the linear equation is ipa = 0.0952csudan + 1.296 (r2 = 0.9995) (figure 8). also, the limit of detection, cm, of sudan was calculated using the equation (2): cm=3sb/m (2) where, m is the slope of the calibration plot (0.0952 μa/ μm) and sb is the standard deviation of the blank response obtained from 15 replicate measurements of the blank solution. the detection limit of 0.08 μm was obtained for the determination of sudan using this method. figure 7. dpvs of mno2-nrs/spe in 0.1 m (ph 7.0) containing different concentrations of sudan. numbers 1-8 correspond to 0.25, 2.5, 10.0, 30.0, 70.0, 100.0, 200.0 and 300.0 µm of sudan csudan / mm figure 8. plot of the electrocatalytic peak current as a function of csudan in the range of 0.25-300.0 µm http://dx.doi.org/10.5599/jese.1415 j. electrochem. sci. eng. 12(6) (2022) 1121-1131 mno2 nanorods spe for determination of sudan dye 1128 analysis of real samples the real samples for the analysis were prepared and quantified by the dpv method. the developed sensor was applied to detect sudan in tomato paste and ketchup sauce samples. the results are summarized in table 1. each measurement was repeated 3 times. the recovery and relative standard deviation (rsd) values confirmed that the mno2-nrs/spe sensor has a great potential for analytical application. table 1. the application of mno2–nr/spe for determination of sudan in real samples (n=3) sample concentration, µm recovery, % rsd, % spiked found tomato paste 0 4.0 3.9 97.5 3.2 6.0 6.3 105.0 1.8 ketchup sauce 0 5.0 5.1 102.0 2.2 7.0 6.9 98.6 2.9 conclusion modifications of the screen-printed electrode with mno2 nanorods for sensing sudan in food samples were investigated. mno2-nrs/spe electrodes were used as sudan sensors by using cv, lsv, ca and dpv techniques. the results showed that the electrodes gave linearity of 0.25 to 300.0 µm, and a detection limit of 0.08 µm. the diffusion coefficient for sudan using mno2-nrs/spe, 4.5×10-5 cm2/s was obtained. the analysis of real food samples spiked with sudan gave satisfactory results with recovery values between 97.5 and 105.0 %. references [1] n. vladislavić, m. buzuk, i. š. rončević, s. brinić, 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distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1415 https://doi.org/10.22034/chemm.2020.113388 https://doi.org/10.1016/j.fct.2022.112864 https://doi.org/10.5599/jese.774 https://doi.org/10.33945/sami/chemm.2020.4.5 https://doi.org/10.1039/c8nj05581e https://doi.org/10.1002/elan.200804403 http://www.echemcom.com/article_96649.html https://doi.org/10.22034/chemm.2020.109975 https://doi.org/10.1016/j.snb.2017.05.124 https://creativecommons.org/licenses/by/4.0/) @article{ahmadi2022, author = {ahmadi, sayed ali and jahani, peyman mohammadzadeh}, journal = {journal of electrochemical science and engineering}, title = {{mno2 nanorods modified screen-printed electrode for the electrochemical determination of sudan dye in food sample:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {6}, pages = {1121--1131}, volume = {12}, abstract = {a novel mno2 nanorods modified screen-printed electrode was fabricated and used as a voltammetric sensor for sudan determination. mno2 nanorods were characterized using field emission-scanning electron microscopy (fe-sem). electrochemical measurements were performed using cyclic voltammetry (cv), linear sweep voltammetry (lsv), differential pulse voltammetry (dpv), and chronoammperometry (ca). the mno2 nanorods on the electrode surface act as an excellent catalyst for the sudan oxidation reaction. our modified electrode presents good electrocatalytic activity toward sudan, a short response time of <10 s, a low detection limit of around 0.08 µm, and linear detection range from 0.25 to 300.0 µm.}, doi = {10.5599/jese.1415}, file = {:d\:/onedrive/mendeley desktop/ahmadi, jahani 2022 mno2 nanorods modified screen-printed electrode for the electrochemical determination of sudan dye in food sampl.pdf:pdf;:www/jese_v12_no6_1121-1131.pdf:pdf}, keywords = {sudan red, voltammetric sensor, cyclic voltammetry, electroanalysis}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1415}, } experimental and theoretical investigations of new schiff base compound adsorption on aluminium in 1 m hcl http://dx.doi.org/10.5599/jese.1405 975 j. electrochem. sci. eng. 12(5) (2022) 975-987; http://dx.doi.org/10.5599/jese.1405 open access : : issn 1847-9286 www.jese-online.org original scientific paper experimental and theoretical investigations of new schiff base compound adsorption on aluminium in 1 m hcl hojat jafari1,, elham ameri1,, fariba soltanolkottabi1, avni berisha2 and mahamadou seydou3 1department of chemical engineering, shahreza branch, islamic azad university, p.o. box 311-86145 shahreza, iran 2department of chemistry, faculty of natural and mathematics science, university of prishtina, 10000 prishtina, kosovo 3université de paris, cnrs, itodys umr 7086, 15 rue j.a. de baïf, f-75013 paris, france corresponding authors:  hojatjafari80@yahoo.com, ameri@iaush.ac.ir received: june 13, 2022; accepted: july 7, 2022; published: august 11, 2022 abstract the new schiff base, 2,2'-((1z,1'z)-(((propane-1,3-diylbis(oxy))bis(2,1-phenylene))bis(methanylylidene))bis(azanylylidene))diethanol, was investigated as a corrosion inhibitor of aluminium in 1 m hcl. polarization and electrochemical impedance measurements were used for this purpose. polarization curves showed that the compound is a mixed-type corrosion inhibitor. also, the results showed an increase in inhibition efficiency as the concentration of the compound increased. the maximum corrosion inhibition efficiency of approximately 81 % was reached at the concentration of 2 mg/l of the inhibitor. the results of the density functional theory method were consistent with the experimental results. the surface morphology of the samples was examined under atomic force microscopy. keywords acid corrosion; potentiodynamic polarization; atomic force microscopy; density functional theory. introduction corrosion of aluminium is a common process [1]. aluminium is an active metal, but its corrosion resistance is high in most situations due to the production of aluminium oxide layer on its surface [2,3]. this oxide forms a protective layer resistant to most acidic and neutral solutions but highly vulnerable in hcl conditions [4]. as a result, numerous treatments and investigations are required to mitigate aluminium corrosion to some level [5]. to address this issue, an efficient and practical approach to avoiding metal corrosion has usually been employed, i.e., the use of inhibitors [6,7]. corrosion inhibitors are chemical compounds that, when introduced to a corrosive medium at low quantities, decrease or prevent metal corrosion [8-10]. research on corrosion inhibition has been ongoing for years [10-14]. one of the most popular strategies for changing the active corrosion http://dx.doi.org/10.5599/jese.1405 http://dx.doi.org/10.5599/jese.1405 http://www.jese-online.org/ mailto:hojatjafari80@yahoo.com mailto:ameri@iaush.ac.ir j. electrochem. sci. eng. 12(5) (2022) 975-987 new schiff base compound adsorption on aluminium 976 process to inactivity is to add an organic inhibitor [15]. in industries, adding inhibitors is particularly prevalent since it can be done without pausing or affecting the process. researchers are currently looking for a safe inhibitor that has the best effect on corrosion prevention [5-7]. the success of such large-scale inhibitor applications is dependent on slowing down the corrosion process. at present, several organic chemicals are commonly utilized in industry to reduce corrosion and manage alkaline and acidic environmental conditions [16-21]. up to now, many organic compounds have been used to prevent aluminium corrosion in acidic solutions [1,3-5]. schiff bases are corrosion inhibitors due to the presence of the c = n group, an electron cloud on the aromatic ring, and electronegative n, o, and s atoms in the molecule [22,23]. in the present paper, corrosion inhibition of a new schiff base on aluminium was investigated in 1m hcl. using polarization method and atomic force microscopy, the corrosion protection effectiveness of a schiff base named 2,2'-((1z,1'z)-(((propane-1,3-diylbis(oxy))bis(2,1-phenylene))bis(methanylylidene)) bis(azanylylidene))diethanol (ppmd) for aluminium in 1m hcl was evaluated. in addition, dft calculations were employed in conjunction with other quantum chemical methods in order to find consistency with the ppmd inhibitory performance. experimental aluminium samples cut out to squared specimens with an area of 1 cm2 were cooled in cold epoxy resin before being polished with emery paper grades ranging from 220 to 2000 and washed with distilled water and acetone. then, the samples were dried with air and stored in a desiccator until used for polarization tests and surface analysis. chemical composition of al in wt.% was: al (99.38 %), fe (0.25 %), cu (0.04 %), ti (0.06 %), mn (0.06 %), si (0.14 %), mg (0.03 %) and zn (0.04 %). for corrosion tests, a solution of 1 m hcl was prepared by diluting analytical grade 37 % hcl acid with double distilled water. later, the inhibitor with concentrations of 0.5, 1.0, 1.5 and 2.0 mg/l was dispersed in the corrosion test solution for electrochemical studies. ligand 2,2'-((1z,1'z)-(((propane1,3-diylbis(oxy))bis(2,1-phenylene))bis(methanylylidene))bis(azanylylidene))diethanol was prepared according to literature method [24]. 2-[3-(2-formyl phenoxy)propoxy] benzaldehyde (0.284 g, 1 mmol) and ethanol amine (0.122 g, 2 mmol) were mixed and heated under reflux for 3 hours in ethanol (30 ml). the solution was refined and the filtrate was reduced to cca 10 cm3. yield: 85 %. anal. calc. for c21h26n2o4: c, 68.09; h, 7.07; n, 17.28. found: c, 67.74; h, 7.17; n, 17.39. ir (kbr, cm-1): 1638 (ʋ c=n schiff base), 1490 ( c=c), 3365 ( oh). 1h nmr (dmso, ppm, 300 mhz): δ 2.36 (m, 2h), 3.71 (t, 4h), 3.82 (t, 4h), 4.25 (t, 4h), 6.99 (m, 2h), 7.40 (m, 2h) 7.96 (m, 2h), 7.99 (m, 2 h), 875 (s, 2h). 13c nmr (dmso, ppm, 300 mhz): δ 62.48, 63.57, 65.03, 76.73, 77.05, 112.20, 121,09, 125.62,125,74, 12684, 127.40, 132.16 (aromatic rings), 158.88 (schiff base). the chemical structure of the prepared schiff base is shown in figure 1. figure 1. chemical structure of schiff base h. jafari et al. j. electrochem. sci. eng. 12(5) (2022) 975-987 http://dx.doi.org/10.5599/jese.1405 977 autolab instrument was used to perform polarization tests. the tests were performed with a standard cell having a platinum wire as an auxiliary electrode and a saturated calomel electrode (sce) as a reference electrode. the experiments were performed at room temperature. at the beginning of each experiment, an interval of about 30 minutes was applied to stabilize the potential of the electrode. the measurements were performed within a potential range of about -900 to -300 mv compared to the open circuit potential, with a scan rate of 1 mv/s. electrochemical impedance tests were performed after 30 minutes of immersion in the solutions. for this purpose, a three-electrode electrochemical cell, including counter electrode (graphite), reference electrode (sce) and the working electrode (al), is connected to a computer-controlled autolab potentiostat/galvanostat system (pgstat 302) and tested at the open circuit potential in the frequency range of 100 khz to 10 mhz, and ac peak amplitude of 10 mv. atomic forced microscopy was used to study the morphology of sample surfaces. in this method, al samples were immersed for 24 hours in 1 m hcl solution without inhibitor and also in a solution containing 2 mg/l of inhibitor at room temperature. the samples were then removed from the solution and washed with distilled water and acetone and scanned by atomic force microscopy (afm) nanosurf easyscan2. to optimize adsorption of the corrosion inhibitor on metal surface atoms and prove that they are effective, it is crucial to properly examine their electronic and molecular properties. it is crucial to focus on the frontier molecular orbital as a result (fmos). the inhibitor should affect the metal surface, in theory. in this respect, theoretical knowledge is essential to estimate the geometrical structure of adsorption and other corrosive species on the surface of metal atoms. the outcomes of this investigation, which combins molecular dynamics with monte carlo simulation, will be covered in depth in the following section. fmos were investigated using the materials studio program dmol3 module. the geometrical properties of all inhibitors were optimized using the m06-l [25-28] functional in combination with a double numerical basis set and polarization to comprehend better electronic interaction and correlation (dnp) [28]. the self-consistent field converged at an energy change of less than 10-7 au. the conductor-like screening model (cosmo) [29-31] was employed in this study to account for the solvent-water effect. furthermore, potential interactions between the inhibitor molecule and al surface atoms were examined using a monte carlo (mc) approach with the adsorption locator module in material studio software (materials studio package, biovia, dassault systèmes, san diego, usa, 2017). a cell with dimensions of 2.86342.86342.3370 nm and a vacuum slab 4.0 nm further than al (111) surface. the mc simulations were carried out by continuously loading the inhibitor molecules into a modelled cell with corrosive species such as 750 h2o, 10 h3o+, and 10 clusing the adsorption locator module in material studio. specifically, the top layer of the al (111) slab was chosen as the target atoms during the mc simulation because it was thought to have plausible sites for adsorption of the inhibitor molecules. after the simulation, the equilibrium adsorption configuration with the lowest energy was explored and assessed. molecular dynamic (md) simulations were used to simulate inhibitor adsorption on the metal surface. the density functional theory (dft), mc, and molecular dynamics simulations for this work were performed using material studio. the compass forcefield in material studio's forcite module was used to change the geometries of the inhibitor, water, and hydronium ion (h3o+) [32-37]. md simulations were provided here to investigate the interactions between a particular inhibitor, metal surface, and aqueous hydrochloric acid. following that, the solution slabs were optimized. http://dx.doi.org/10.5599/jese.1405 j. electrochem. sci. eng. 12(5) (2022) 975-987 new schiff base compound adsorption on aluminium 978 second, nine atom layers of thickness were used to construct an al slab with a (111) plane. this slab is referred to as al (111). this plane surface was used for the computations due to its dense structure and low energy surface. following its development, the simulation box was geometrically optimized in order to avoid unfavorable chemical combinations and produce an energetically minimal model for future simulations. during the optimization process, the 'smart' algorithm was used. optimization was successful since the temperature and energy fluctuation curves remained unchanged. our next step was to run md simulations in material studio's forcite module at 295 k using the compass forcefield on the optimized simulation box with all al (111) atoms frozen but the two top layers (with a time step of 1.0 fs). the simulation was run for 1.5 ns [34,35]. using the trajectory of the md simulation, the radial distribution function (rdf) was generated in order to predict how the inhibitor molecule adsorbs. results and discussion polarization diagram figure 2 shows the polarization diagram of aluminum in 1m hcl without the presence of inhibitor and with different concentrations (0.5 – 2 mg/l) of inhibitor. log (j / a cm-2) figure 2. tafel plots of al in 1.0m hcl without and with corrosion inhibitor: blank (1); 0.5 (2); 1.0 (3); 1.5 (4); 2.0 (5) mg/l electrochemical parameters, including corrosion potential (ecorr), corrosion current density (jcorr) and anodic and cathodic tafel slopes (a, c) are estimated from figure 2, and shown in table 2. the degree of surface coverage () and inhibitory efficiency (ie / %) are calculated by the following equations [12,13]: 0 corr corr 0 corr j j j  − = (1) e / v v s. s c e h. jafari et al. j. electrochem. sci. eng. 12(5) (2022) 975-987 http://dx.doi.org/10.5599/jese.1405 979 0 corr corr 0 corr 100 j j ie j − = (2) where j0corr and jcorr are corrosion current densities in solution without inhibitor and in the presence of inhibitor, respectively. as shown in table 1, the corrosion current density decreases with increasing inhibitor concentration. table 1. results derived from tafel plots in fig. 2 concentration, mg/l -ecorr / mv jcorr / ma cm -2 a (±1) / mv dec -1 -c (±1) / mv dec -1  (±10-2) ie / % 0.0 729 28.1 221 223 --------- 0.5 732 17.8 203 219 0.37 37 1.0 756 12.7 197 281 0.55 55 1.5 767 8.4 195 211 0.70 70 2.0 773 5.3 194 238 0.81 81 also, with increasing schiff base concentration, both tafel anode and cathode slopes didn’t show any specific trend. this confirms that the inhibitor prevents corrosion by covering the active points on the metal surface [22]. the value of corrosion potential in the presence of inhibitor is reduced compared to the solution without schiff base and with increasing concentration of the inhibitor. on the other hand, the corrosion potential is shifted to higher negative values when the inhibitor concentration rises (figure 2), indicating a certain inhibitor impact on the cathodic branch of the polarization curve. table 1 shows that the effectiveness of the inhibition is improved with increasing ligand concentration. due to its clear impact on both the cathodic reaction (hydrogen and oxygen reductions) and the anodic reaction (metal dissolution), the inhibitor was of the mixed type, although the cathodic reaction is more clearly affected. in other words, the anodic dissolution energy gain is less than the hydrogen reduction energy increase. in addition, the corrosion potential changes slightly with increasing concentration, indicating a mixed mechanism of corrosion inhibition [19-21]. eis test figure 3 shows the nyquist diagrams of the aluminium electrode in hcl solution without inhibitor and with different concentrations of inhibitor. z’ /  figure 3. nyquist plots of al in 1.0 m hcl in the absence and presence of the corrosion inhibitor: blank solution (1) and 0.5 (2), 1.0 (3); 1.5 (4); 2.0 (5) mg/l z  /  http://dx.doi.org/10.5599/jese.1405 j. electrochem. sci. eng. 12(5) (2022) 975-987 new schiff base compound adsorption on aluminium 980 in agreement with already reported impedance spectra of aluminium aa2219-t6 alloy in hcl solution with and without ferrocenyl schiff bases 5, impedance spectra in figure 3 show semicircle responses at higher to medium frequencies and inductive responses at lower frequencies. as shown in figure 3, the radius of the semicircle increases significantly with the increasing concentration of the inhibitor. it has already been established that the appearance of a semicircle in impedance spectra of aluminium corrosion in acid media is primarily connected with double layer capacitance (cdl) and charge transfer resistance (rct) at the aluminium/corrosive electrolyte interface 4,5. a significant increase of circle diameters in figure 3 suggests that with increasing inhibitor concentration, charge transfer resistance (rct) increases, meaning that the degree of corrosion of the aluminium in solution decreases. as the schiff base concentration increases, the double-layer capacitance (cdl) value decreases. this indicates that inhibitory molecules replace water molecules on the metal surface. the decrease in cdl values caused by adsorption of inhibitor indicates that the exposed surface area decreased. on the other hand, a decrease in cdl, can result from a decrease in local dielectric constant and/or increase in the thickness of the electrical double layer, which suggests that schiff base inhibitors act by adsorption at the metal-solution interface. surface morphology study the morphology of the surface of al samples in 1 m hcl solution was examined by afm in the presence of 2 mg/l schiff base and without schiff base at 25 °c and after 24 hours of immersion. the results are shown in figure 4. a b figure 4. afm of aluminium electrode after 24 h of immersion in 1 m hcl solution: (a) without inhibitor; (b) with 2mg/l of inhibitor in the absence of an inhibitor, a rough surface is observed due to rapid corrosion by 1 m hcl. corrosion, in this case, is relatively uniform and there is no sign of local corrosion. in the presence of the inhibitor, the aluminium surface roughness is reduced, which indicates the formation of a film on the metal surface. dft analysis a conformer search (boltzmann jump technique; the number of conformers: 8000; utilizing the compass ii forcefield) was undertaken in the early phase of the procedure in order to acquire the lowest feasible starting energy for the molecule while speeding up dft computations [29-42]. calculations for dft were started with the lowest-energy conformer shown by the image in figure 5. h. jafari et al. j. electrochem. sci. eng. 12(5) (2022) 975-987 http://dx.doi.org/10.5599/jese.1405 981 figure 5. energies of the conformer search and the resultant structure of the inhibitor the σ-profile (figure 6) reveals the charge density distribution on the molecule's surface and may be used to evaluate the solubility of the conjugate in various solvents (in our case, water) [29,43]. the sigma-profile charge density curve is generated using cosmo model simulations. partially charged atomic nuclei are utilized in cosmo to illustrate the electrostatic potential [44]. figure 6 indicates that the inhibitor is both acceptor and donor of h-bonds. since water molecules establish hbond acceptor/donor connections when an inhibitor is dissolved, its solubility is regulated by this capacity. screening charge density, e nm2 figure 6. σ-profile of the inhibitor o and n heteroatoms, which are near the inhibitor homo electron density, are also near the lumo electron density, which is represented in figure 7. homo electron density is diffused towards the molecule heteroatoms (o and n), meaning that these molecules can transmit electrons to the al surface, as can be noticed in figure 7. coating a protective organic layer on metal surfaces arises from this electron sharing, which shields the metal from corrosion [41]. electron acceptors termed lumos are connected to parts of the inhibitor that receive electrons from an electron surface rich in metals, such as that found in al [44]. r e la ti v e e n e rg y o f th e c o n fo rm e r, j m o l-1 s ig m a p ro fi le , n m 2 http://dx.doi.org/10.5599/jese.1405 j. electrochem. sci. eng. 12(5) (2022) 975-987 new schiff base compound adsorption on aluminium 982 figure 7. homo, lumo and esp surface of the inhibitor adsorption on the metal surface is expected to be considerably increased due to the interchange of lone pair electrons between heteroatoms (n and o) and the vacant al d-orbital, which results in a minor boost in surface absorption potential [43]. table 2 lists the most often reported descriptors (the formulae used to construct them may be found elsewhere [41]). a better understanding of the adsorptive behavior of inhibitors may be achieved by analyzing the dft simulations of inhibitor adsorptive reactivity [41]. due to their low electron affinity and high ionization potential, it is commonly thought that the adsorption of inhibitors onto the al(111) surface is supported by its capacity to exchange electrons with the al surface (table 2) [41]. chemical softness and hardness are predicted values representing the inhibitor adsorption affinity for the metal surface, as are high values of chemical softness. inhibitors reflect the capacity to accept electrons from the al surface [40]. table 2. calculated theoretical chemical parameters for the inhibitor descriptor inhibitor homo, ev -5.7460 lumo, ev -2.2190 ∆e(homo-lumo) / ev 3.527 ionization energy (i / ev) 5.7460 electron affinity (a / ev) 2.2190 electronegativity (χ / ev) 3.9825 global hardness (η /ev) 1.7635 chemical potential (π / j kg-1) -3.9825 global softness (σ /ev) 0.5671 global electrophilicity (ω /ev) 4.4968 electrodonating (ω-) power, ev 6.7085 electroaccepting (ω+) power, ev 2.7260 net electrophilicity (∆ω+-), ev 2.5769 fraction of transferred electrons (∆n /ev) 0.0787 energy from inhb to metal (∆n/ev) 0.0109 ∆e back-donation / ev -0.4409 e n e rg y , e v h. jafari et al. j. electrochem. sci. eng. 12(5) (2022) 975-987 http://dx.doi.org/10.5599/jese.1405 983 mulliken atomic charges (mac) are a reliable and persuasive indication of the inhibitory sites (atoms) involved in metal adsorption. there are numerous possibilities for a description of the reaction between atoms on the al surface with the inhibitor molecules [42]. figure 8 illustrates the inhibitor mac values. the inhibitor o and n atoms have strong negative charges, indicating that these centers have the maximum density of electrons and are consequently the most effective in attaching to metal surfaces. figure 8 displays the molecular electrostatic potential (mep) of the inhibitors in different concentrations (the region in red). figure 8. optimized structures of the inhibitor and their mulliken atomic charges (mac) mc and md simulations using the al(111) surface as a starting point, the adsorption energetics of the system may be calculated readily. it is feasible to compute the adsorption energy (eads) by using the following equation (3) [42]: eadsorption = eal(111)inhibitor – (eal(111) + einhibitor) (3) where eal(111)inhibitor is the total energy of the simulated system, eal(111) and einhibitor is the total energy of the al(111) surface and the corresponding free inhibitor molecules. after mc calculations were completed successfully, a comprehensive investigation of the inhibitor adsorption geometry was done to validate the findings. the mc simulation capacity to establish equilibrium may be validated using the steady-state energy levels. the system minimal energy state was obtained roughly halfway through the experiment. as demonstrated in figure 9, the real arrangement of the adsorbent inhibitors is represented on a simulated al (111) plane. figure 9. mc and md possess the lowest adsorption configurations for inhibitor adsorption in the simulated corrosion media on the al substrate http://dx.doi.org/10.5599/jese.1405 j. electrochem. sci. eng. 12(5) (2022) 975-987 new schiff base compound adsorption on aluminium 984 al (111) surface is being decorated by the inhibitor in virtually (in the case of md) a parallel direction to the inhibitor. we suggest that this adsorption pattern on the al (111) plane may be induced by an inhibitor molecule's backbone binding the plane's surface atoms (overseen by heteroatoms). the molecules' inclination to expose their heteroatoms and electron rings to the surface leads to adsorption, which provides the molecules with their adsorptive abilities [42]. inhibitor's adsorption reflects a relatively large eads (figure 10) on the metal surface. because of the extraordinarily high adsorption energies, the inhibitor has a strong adsorption interaction with the metal. using this contact, the metal surface is protected from corrosion by the development of a protective layer [41]. eads / j mol -1 figure 10. distribution of adsorption energies of the inhibitor obtained from mc calculations md is extensively considered a correct representation of the adsorption dynamics [32,33,35,39]. it is evident that after several hundreds of ps of nvt simulation, the inhibitors (figure 9) adopt a somewhat flat structure on one side of the molecule rings onto the metal surface and is strongly adsorbed onto the al surface. as demonstrated in figure 11, the adsorption energy is rather high, with a minor decline in energy as temperature increases, which is consistent with the experimental findings. simulation time, ps figure 11. interaction energy of the inhibitor at the al surface during the second half of md p e a k i n te n si ty , a .u . eads = 50.23 j mol-1 a d so rp ti o n e n e rg y, j m o l-1 h. jafari et al. j. electrochem. sci. eng. 12(5) (2022) 975-987 http://dx.doi.org/10.5599/jese.1405 985 corrosion inhibitor adsorption on metal surfaces may be explored utilizing the rdf analysis of the md trajectory obtained during corrosion simulations, which is a basic and uncomplicated way [42]. adsorption processes on metal surfaces may be determined from the rdf graph if peaks occur at a given distance from the metal surface [42]. heights are believed to reflect chemisorbable processes when detected in the region of 0.1–0.35 nm in length; nevertheless, for physical adsorption, rdf peaks are expected to be present at distances larger than 0.35 nm. al surface and inhibitor heteroatoms (n and o) showed rdf peak values at distances of less than 0.35 nm (figure 12) [42]. in this scenario, the inhibitor appears to be strongly interacting with the metal surface, as suggested by its comparatively high negative adsorption energy (as obtained for mc and md) value and rdf peaks. r / nm figure 12. rdf of heteroatoms (o and n) for the inhibitor on al surface accomplished from md trajectory analysis conclusion in this study, the effects of corrosion inhibition and the ability of adsorption of a new schiff base (ppmd) on the al surface were investigated and the following results were obtained: the organic compound ppmd has inhibition properties in 1 m hcl medium. afm analysis supports the creation of a preventive layer of inhibitor on al surface, which facilitates the lowering of corrosion rate. this inhibitor affects both anodic and cathodic branches and 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1405 https://doi.org/10.1080/00268976.2017.1333644 https://doi.org/10.1002/jcc.20782 https://doi.org/10.1002/wcms.1338 https://doi.org/10.1155/2019/5126071 https://doi.org/10.1002/app.49673 https://doi.org/10.1016/j.molliq.2021.117333 https://doi.org/10.17675/2305-6894-2021-10-1-23 https://doi.org/10.1016/j.molliq.2021.115413 https://doi.org/10.1002/qua.26600 https://doi.org/10.1016/j.molliq.2020.114958 https://doi.org/10.1016/j.molliq.2021.117886 https://doi.org/10.1007/s10800-021-01556-y https://doi.org/10.1016/j.molstruc.2022.132425 https://doi.org/10.1007/s40735-022-00679-9 https://doi.org/10.1002/maco.201106141 https://doi.org/10.1016/j.porgcoat.2016.08.008 https://doi.org/10.1155/2019/5126071 https://creativecommons.org/licenses/by/4.0/) effect of koh concentration on corrosion behavior and surface morphology of stainless steel 316l for hho generator application http://dx.doi.org/10.5599/jese.1615 451 j. electrochem. sci. eng. 13(3) (2023) 451-467; http://dx.doi.org/10.5599/jese.1615 open access : : issn 1847-9286 www.jese-online.org original scientific paper effect of koh concentration on corrosion behavior and surface morphology of stainless steel 316l for hho generator application basori basori1,2, wan m. f. w. mohamad2,3,, muhd r. mansor2,3, noreffendy tamaldin2,3, agung iswadi1, maman k. ajiriyanto4 and ferry budhi susetyo5 1department of mechanical engineering, faculty of engineering and science, universitas nasional, sawo manila, pasar minggu, jakarta 12520, indonesia 2fakulti kejuruteraan mekanikal, universiti teknikal malaysia melaka, hang tuah jaya, 76100 durian tunggal, melaka, malaysia 3centre for advanced research on energy, universiti teknikal malaysia melaka, hang tuah jaya, 76100 durian tunggal, melaka, malaysia 4research center for nuclear fuel cycle and radioactive waste technology – national research and innovation agency (brin), 15314, indonesia 5department of mechanical engineering, universitas negeri jakarta, jakarta 13220, indonesia corresponding author: farid@utem.edu.my received: november 23, 2022; accepted: march 6, 2023; published: march 19, 2023 abstract hydrogen production could be enhanced by increasing the potassium hydroxide (koh) concentration, but higher koh concentrations result in higher corrosion rates. therefore, a deep investigation of the electrochemical behavior of stainless steel (ss 316l) in the koh solution is needed. this study investigates the influence of koh concentrations on the electrochemical behavior, surface morphology, structure, and sample phases of ss 316l. the investigations were conducted by some electrochemical techniques, uv-vis, scanning electron microscopy-energy dispersive spectroscopy (sem-eds), and x-ray diffraction (xrd). the corrosion rate was found to increase, and solution resistance to decrease with increasing koh concentration. samples tested in 5, 30, and 50 g l-1 of koh showed corrosion rates of 0.457, 2.362, and 5.613 µm year-1, respectively. a wide passive region and the noblest pitting potential were noticed for the sample with 5 g l-1 of koh. moreover, mott-schottky plots and characteristic wavelengths of uv-vis suggest the formation of iron and chromium oxides by the passivation of samples. the sem analysis showed a dynamic change of surface morphology from the lowest to the highest concentration with the intergranular corrosion found at the grain boundaries area. in conclusion, concentrations < 50 g l-1 koh could be recommended since they would support the optimum remaining life of ss 316 l plates in hho generators. keywords corrosion rate; koh media; passive film; hydrogen generator http://dx.doi.org/10.5599/jese.1615 http://dx.doi.org/10.5599/jese.1615 http://www.jese-online.org/ mailto:farid@utem.edu.my j. electrochem. sci. eng. 13(3) (2023) 451-467 corrosion behavior of stainless steel 316l 452 introduction fossil fuels are about 82 % of the world's energy demand for a combustion engines. fossil fuels generate the main greenhouse gas problem contributing to global warming and climate change [1]. with the increase in energy demand, innovations must be developed and produced to supply clean and affordable energy. hydrogen is a promising renewable source of clean energy and environmentally friendly [2-4]. hydrogen indeed could play an essential role as one of the possible solutions towards climate change adaptation and decarbonization agenda. many studies have been done for years to develop an optimum system for hydrogen production. one study in turkey analyzed the cost of hydrogen production via an electrolysis system supplied by wind energy [5]. another study emphasized that hydrogen produced from renewable energy could help to overcome the intermittency of renewable sources, which could be a problem in isolated territories. thus, a life cycle assessment covering hydrogen production and consumption in an isolated area was performed [6]. another study also provided an analysis of the life cycle assessment for hydrogen production using the electrolysis system by considering the improvement of material demand, system efficiency, and cell lifetime [7]. this study indicated that the high material demand with high system efficiency (hdhe) reduces carbon emission by up to 15.5 % compared to the current state of the electrolysis system. hydrogen could be produced through a water electrolysis system. apart from the electrolysis system types, several previous studies were focused on increasing oxy-hydrogen (hho) gas production, rather than the performance of the material used for the generator system itself [8-12]. hho gas production can be optimized by determining electrolyte concentration, operating temperature, and large cell gap. choodum et al. [9] reported that using a closed-loop hho production system, the number of cells and applied current were the two main parameters affecting the hho gas production rate. based on the numerical simulation, it was discovered that by using 200 cells with 40 a of current at 80 °c, 64 l min-1 of hydrogen gas could be produced [9]. subramanian and thangavel [10] also reported that increased electrolyte concentration from 0.25 m to 1.0 m of sodium hydroxide (naoh) enhances conductivity and decreases electrical potential. similar to that report, the influence of variation in electrolyte concentration and applied current was also studied to find the optimum configuration of the water electrolysis system for hho gas production [12. it was found that a combination of 0.05 m naoh concentration with applied current of 15 a produced the highest volume of hho gas up to 0.925 l min-1 with 89.13 % of electric current efficiency [12]. besides naoh, potassium hydroxide (koh) is preferred for hydrogen production because it has higher conductivity than naoh due to the ionic conductivity of k+ being higher than na+ [13-15]. moreover, the overall production efficiency of hydrogen in 3 m koh and 3 m naoh at standard conditions resulted in an efficiency of 90.25 and 83.33 %, respectively [14]. hydrogen production through koh increased with an increase in the koh concentration. an increase from 1 to 2 mol of koh could increase gas production from 55 to 75 cm3 min-1 at 10 v and 2 a [16]. moreover, purwondho et al. [17] conducted hydrogen gas production by 5, 10, 15, 20, and 25 g l-1 of koh, resulting in approximately 14, 26, 27, 31, and 62 ml min-1, respectively. increasing the koh concentration could also increase the corrosion rate of electrodes [18]. as discussed by subramanian and ismail [19], the cost associated with the periodic replacement of electrodes due to corrosion is one of the crucial challenges to be mitigated in the water electrolysis system for the hho generator. various electrodes, such as stainless steel, ss 316l, ss 304, and copper (cu) alloys, are commonly used for hydrogen production. a comparison of the corrosion behavior of these materials in water electrolysis using koh solution was carried out, and ss 316 l exhibited the highest corrosion b. basori et al. j. electrochem. sci. eng. 13(3) (2023) 451-467 http://dx.doi.org/10.5599/jese.1615 453 resistance among three metals tested [20]. olivares-ramírez et al. [21] studied different stainless steel in koh and naoh solutions. they found that ss 316 l is the perfect electrode material in koh, due to producing a similar volume of hydrogen gas for less time. this phenomenon is due to the higher nickel content and stabilized oxide resistivity [22,23]. furthermore, ss 316 l has density of 7.99 g cm-3 and equivalent weight of 27.56 g mol-1 [24]. this study investigates the effect of koh solution on the electrochemical behavior of ss 316 l, surface morphology, phases, and structure. varying concentrations of koh solution are used to compare factors in identifying the most favorable koh concentration for ss 316 l. the use of koh solution with various concentrations was also recommended by purwondho et al. [17], who observed higher gas production by increasing koh concentration. investigating ss 316 l in koh solution with varying concentrations is expected to discover a new promising option for metal plates and electrolyte solution for utilization in the hho system generator. thus, the investigation of ss 316 l performance against corrosion carried out in this study would contribute to a better understanding and improve the feasibility of the electrolysis system for the hho generator. experimental materials the ss 316 l plate with a thickness of 0.3 mm was used as an experimental specimen. ss 316 l was chosen as a substrate due to its superior corrosion resistance compared to alloy 304/304 l. its excellent properties make it a suitable candidate for an electrode plate applied to hho generator systems. the chemical composition of the ss 316 l is shown in table 1. samples were subjected to the open circuit potential measurements, potentiodynamic polarization, electrochemical impedance spectroscopy, and mott-schottky test in koh solutions (sigma aldrich) of different concentrations. electrochemical measurements are referred to treated samples, and the untreated (blank) sample is used for the comparison. ss 316 l samples in concentrations of electrolyte of 5, 30, and 50 g l-1 koh, were denoted as k5, k30, and k50, respectively. the concentration of the electrolyte solution affects the production of hho gas. a few studies revealed that the production of hydrogen gas increased with the increasing alkaline concentration of koh solution [25-27], and consequently, the electrode plates used in high concentrations may be degraded due to corrosion. therefore, in this study, electrochemical corrosion tests were carried out at intervals of 5 to 50 g l-1 to determine the level of corrosion resistance of ss 316 l plates. table 1. chemical composition of ss 316 l element cr ni mn mo c fe content, wt.% 16.33 9.97 2.06 1.95 <0.1 balance electrochemical measurements the ss 316 l plates were cut and then mounted in a holder with an exposure area of 1 cm². the samples were polished with grit 1000 up to 2000 and cleaned with acetone before open circuit potential (ocp) measurement was conducted. to reach steady voltage, ocp measurement was performed during 7200 s in 5, 10, and 50 g l-1 of koh, respectively. a potentiodynamic polarization method was carried out to determine the corrosion resistance of the ss 316 l. the potentiodynamic polarization was conducted in koh solutions with varying concentrations (5, 30, and 50 g l-1). the potentiodynamic polarization test used the gamry reference 600 http://dx.doi.org/10.5599/jese.1615 j. electrochem. sci. eng. 13(3) (2023) 451-467 corrosion behavior of stainless steel 316l 454 potentiostat. the saturated calomel electrode (sce) was used for the reference electrode (re), and platinum wire was used for the counter electrode (ce). the potentiodynamic polarization measurements were performed from -0.250 to +1.500 v with a scan rate of 2 mv s-1. it was already reported that a slower scan rate (0.1 mv s-1) could lead to a significant change in the interfacial structure [28] and a more corroded surface [29]. on the other hand, the higher scan rate could cause a disturbance of the charging current and lead to misinterpretation of data [28,29]. therefore, 2 mv s-1 was expected to be the appropriate scan rate at the potential range used in this study (-0.250 to +1.500 v). the corrosion rate of samples was calculated by the following expression [30]: corrcmjcr n = (1) where cr / mm year-1 is corrosion rate, c = 3.27 is constant, m / g mol-1 is atomic weight, jcorr / a cm-² is corrosion current density, ρ / g cm-3 is the density of material sample and n is the number of electrons involved. the electrochemical impedance spectroscopy (eis) measurements were also carried out by applying gamry potentiostat reference 600 equipped for eis measurements. eis was measured at ocp, in the frequency range of 100 000 0.2 hz and alternating signal of 10 mv amplitude. the mottschottky plots were determined by eis equipment at the frequency of 1000 hz and potential changes from -2 to +2 v. all electrochemical measurements were performed at 30 °c. for uv-vis spectroscopy, shimadzu uvmini 1240 was used for measurement at wavelengths 207-250 nm. surface morphology observation surface morphology was observed using scanning electron microscopy with energy dispersive spectroscopy (sem-eds). thermo-scientific sem-eds of the phenom pharos type was used with a magnification of 2500. these observations were conducted after potentiodynamic polarization experiments were performed in 5, 30, and 50 g l-1 of koh, respectively. x-ray diffraction (xrd) analysis xrd (panalytical with co k radiation) was conducted with a step size of 0.02° from 30° until 70°. xrd was conducted for an untreated (blank) sample and treated samples after potentiodynamic polarization experiments were performed in 5 and 50 g l-1 of koh, respectively. a high score plus software was used to find sample patterns. results and discussion open circuit potential (ocp) the measurement of ocp as a function of time is shown in figure 1. ocp of k5 sample slowly increases, while ocps of k30 and k50 increase sharply to get a steady state. overall, all curves show the same tendency to increase ocp to a more positive value. it is well known that a rise of ocp to positive values indicates the formation of passive and protective film, while its moving to a negative direction would indicate broken film, unformed film, or dissolution of the film [31]. ocp of k50 reached a steady state at 4500 s, while other samples needed time over 7200 s. this means that for k50 sample, a protective passive film on the ss 316 has been formed in 4500 s [32]. this behavior indicates that decreasing koh concentration shifts ocp to a more positive value. wojciechowski et al. [33] found similar behavior when ocp of ss 316 l was performed in 1 and 6 m koh. it can also be noticed in figure 1 that the initial ocp value of the k30 and k50 samples is more negative than b. basori et al. j. electrochem. sci. eng. 13(3) (2023) 451-467 http://dx.doi.org/10.5599/jese.1615 455 k5, indicating that the k5 sample possessed a thicker native oxide film than others [34]. moreover, at all immersion times, k5 showed more noble ocp than others [33], which suggests that a more protective passive layer is formed on the surface ss 316 l showing less corrosion rate [31]. the passive film could slow down the dissolution of steel due to the stability of the passive film [35,36]. this result fully agrees with corrosion rates obtained by the potentiodynamic polarization tests described in the following section. figure 1. open circuit potential curves of ss 316 l in various koh solutions potentiodynamic polarization potentiodynamic polarization was performed to find out the electrochemical behavior of the samples. figure 2 represents the electrochemical polarization of ss 316 l in various concentrations of koh. three different potential regions are shown in the potentiodynamic polarization curves, which include: (i) active region, (ii) passive region, and (iii) transpassive region [37,38]. it seems that the k5 sample has a wider passive region than other samples. based on the ocp, k5 is nobler and could have a more protective passive layer. protective passive film is related to the surface complex oxide, which is confirmed by eds and mott-schottky results. oxide on the ss 316 l is associated with cr2o3, nio, and fe2o3 [37,39]. the k5 sample has less corrosion rate than other samples because it has a wider passive region. as we know, passivation is one of the most influential factors in the corrosion resistance of the sample [40]. based on figure 2, an increase in the electrolyte concentration leads to a decrease in the passive area region. krawczyk et al. found that passive behavior is associated with the sudden drop of current density with the potential, occurring as a function of the electrolyte concentration [41]. commonly before passivation, a metal dissolution, i.e., uniform corrosion process, occurs in the active loop on the ss 316 l. a primary surface film formed during the active loop consists of cr2o3 [41]. sample k30 and k50 have an active loop that is not prominent for the k5 sample. k5 sample produces a pitting potential (epit) at the highest value of around 0.8 v. epit is the lowest potential where the passive film is still stable prior to breakdown potential where aggressive species in the electrolyte solution could penetrate the films [34]. the breaking down of the film is indicated by the significant current increase. thus, the higher the epit value, the sample is more resistant to corrosion [41]. this behavior may be due to the surface with more oxygen based on the eds observation shown below. http://dx.doi.org/10.5599/jese.1615 j. electrochem. sci. eng. 13(3) (2023) 451-467 corrosion behavior of stainless steel 316l 456 figure 2. potentiodynamic polarization curves of ss316l samples the polarization curves in figure 2 show that the concentration of koh affects the corrosion rate. the higher the concentration of koh, the higher the corrosion current or corrosion rate. hamidah et al. [18] also reported that the higher the concentration of koh, the higher the corrosion rate of ss 316. the higher concentration of koh produces a polarization curve that shifts further to the right, indicating an increase in corrosion current, as shown in figure 2. this argument can be corroborated by the corrosion rate data in table 2, showing the highest corrosion rate of 5.613 µm year-1 for the k50 sample. in comparison, the k5 produced the lowest corrosion rate of 0.457 µm year-1. this finding is aligned with a previous study that found a higher corrosion rate of ss 316 l at higher concentrations of electrolyte solution [37]. table 2. parameters determined from potentiodynamic polarization curves and corrosion rates calculated by eq. (1) sample ecorr / v vs. sce icorr / na corrosion rate, µm year-1 k5 -0.380 19.65 0.457 k30 -0.375 103.20 2.362 k50 -0.370 245.50 5.613 the hho gas formation rate from 1 to 2 mol concentration increases linearly [16]. moreover, other research was conducted on hho gas production using 5, 10, 15, 20, and 25 g l-1 of koh, resulting in approximately 14, 26, 27, 31, and 62 ml min-1 [17]. an increase in the production of hho gas is needed, but it is necessary to pay attention to the impact on the damage or corrosion experienced by the electrode (ss316l plate). therefore, corrosion analysis is needed to determine the optimum concentration in the electrolysis of the hho gas production process. corrosion analysis using tafel extrapolation for a concentration of 5 50 g l-1 can explain that a concentration range of 5 50 g l-1 produces a fairly low corrosion rate < 7.620 µm year-1. performing a higher concentration than 50 g l-1 might have a worse impact, such as pitting corrosion, that could affect the whole hho system. electrochemical impedance spectroscopy (eis) figure 3 presents nyquist impedance plots of ss 316 l samples measured at ocp in various concentrations of koh. the imaginary impedance against the real component showed a single capacitive b. basori et al. j. electrochem. sci. eng. 13(3) (2023) 451-467 http://dx.doi.org/10.5599/jese.1615 457 arc for all specimens. the capacitive arc is highest for k5 and lowest for the k50 sample, but the differences are small. generally, a high capacitive arc corresponds to low metal dissolution since the diameter of the arc is related to the charge transfer resistance at ss 316 l-koh solution interface [42]. figure 3. nyquist plots of ss316l samples the experimental results of eis in the form of bode diagrams are presented in figure 4. the total impedance behavior presented as the log of impedance magnitude and phase angle against the log of applied frequency is shown in the bode plot. at high frequencies, the solution resistance (rs) could be assessed from log |z| horizontal line (zero phase angle). as expected, rs is dependent on the concentration of koh and is the highest for k5 (less conductive) and lowest for k50 (more conductive) sample. capacitive charging of the solid-liquid interface occurs at medium to lower frequencies and is seen as about -1 sloping line in the log |z| plot, coupled with a phase angle tending to -90°. native (oxide) passive film properties of the surface layer could be provided by this capacitive impedance. a prominent and near equal capacitive impedance response for all samples suggests low corrosive systems, i.e., systems with very high charge transfer (rct) values. in impedance plots, rct contributes to the overall impedance at low frequencies. in figure 4, due to very high rct values for all samples, their contribution is best seen from the decrease of phase angles within a decade of the lowest frequencies. the value of rct could be, together with other impedance parameters, determined by impedance curve fitting procedure and corresponding electric equivalent circuit (eec) [43]. resistance (rs and rct) values of ss 316 l are presented in table 3. the determination of rs and rct values was performed by fitting the eec drawn in figure 4 to experimentally measured impedance spectra. the solution resistance (rs) and charge transfer resistance (rct) values decrease with the increase in koh concentration. the decrease in the charge transfer resistance of the samples suggests a rise in the corrosion rate. this is supported by potentiodynamic polarization results, where the corrosion rate increases with increased koh concentration. table 3. eis resistance results sample rs /  rct / k k5 86.51 95 k30 16.36 73 k50 11.63 61 http://dx.doi.org/10.5599/jese.1615 j. electrochem. sci. eng. 13(3) (2023) 451-467 corrosion behavior of stainless steel 316l 458 figure 4. bode plots of ss 316 l samples mott-schottky analysis figure 5 presents the mott-schottky curves of ss 316 l in various concentrations of koh. measurements in both positive and negative potential directions were conducted. however, there is a difference in semiconductor properties between k5 and k30 or k50 samples. sample k5 has a turning potential of -0.75, -0.5 and 0.5 v vs. sce. both samples k30 and k50 have a turning potential of 0.5 v vs. sce if any. the negative slope in the second region is attributed to p-type, and the positive slope in the first and third regions is indicated as n-type [44]. in previous studies, curves were measured in the positive direction [45,46]. therefore, in the present work, the positive direction was used to explain the semiconducting properties of the ss 316 l in various concentrations of koh. figure 5. mott-schottky plots of ss 316 l samples the curve increases linearly when the potentials are between +0.50 and +1.75 v vs. sce. this behavior is due to the space layer of the oxide film of ss 316 l (passive film) formed with the applied potential according to the following n-type and p-type equations (2) and (3) [39,47]: fb2 2 0 d 1 2 kt e e ec en a   = − −    n-type (2) b. basori et al. j. electrochem. sci. eng. 13(3) (2023) 451-467 http://dx.doi.org/10.5599/jese.1615 459 fb2 2 0 a 1 2 kt e e ec en a   = − −    p-type (3) where 0 = 885.4 pf cm-1is permittivity of free space,  = 12 is relative dielectric constant, e = 1.610-19 c is charge of the electron, nd / cm-3 is donor density, na / cm-3 is acceptor density, a / cm2 is surface area of the sample, e / v is applied potential, efb / v is flat band potential, k = 1.3810-23 j k-1 is boltzmann constant,; and t / k is absolute temperature. potentials around 0 to +0.5 and +0.5 to +1.75 v vs. sce represent p-type and n-type, respectively. at the flat band potential (p-type), cr2o3 is forming. for lower potential (n-type), nio and cr2o3 are created, and then for higher potential (n-type), fe2o3 forms [39,48,49]. this means that between 0 and +1.75 v, cr2o3 and fe2o3 are probably formed, which can be supported by eds results presented below, which show the presence of chromium, iron, and oxygen on the surface of treated ss 316. uvvis measurements uv-vis of koh solutions has been conducted, and the results are presented in figure 6. uv-vis was recorded in various concentrations of koh after potentiodynamic polarization measurement to prove the possible presence of particles in the solution [50]. figure 6. uv-vis measurements of koh solutions samples of concentrations of 5, 30, and 50 g l-1 koh showed wavelength peaks at 210, 250, and 210 nm after being used for potentiodynamic polarization measurement. maximum absorbance and wavelength are different for various samples because the uv-vis plot shape is sensitive to particle size [51] and their concentration. el kemari et al. have found that increased nio concentration increases absorbance at similar wavelengths [52]. based on the uv-vis, some particles are shown in the solution after potentiodynamic polarization. it is known that cr2o3, nio, and iron oxide wavelength intensities are around 250, 330, and 200-220 nm, respectively [52–54]. therefore, those particles as oxides are obviously present in the koh solution after potentiodynamic polarization tests. surface morphology observation using sem-eds figure 7 shows the surface morphology of treated ss 316 l. sem-eds tests were carried out to examine the influence of increasing koh concentration on the surface morphology and composition http://dx.doi.org/10.5599/jese.1615 j. electrochem. sci. eng. 13(3) (2023) 451-467 corrosion behavior of stainless steel 316l 460 of the samples. it is found that the surface changed gradually, exhibiting white rust forms on sample surfaces. as defined by nasrazadani et al. [55], white rust (oxide) occurs in the grain boundaries area. du et al. [56] found that white rust accumulating at the grain boundaries may refer to intergranular corrosion. white rust is associated with oxide and in figure 7, it can be especially seen at grain boundaries of k5 and k50 samples. according to the mott-schottky measurement, different oxides could be specified by p-type on n-type properties. in the voltage range voltage between -2 and +2 v, sample k5 clearly showed n-type and p-type properties. in addition, the surface of the k30 sample was found to be more homogeneous than the k50 sample. a similar finding was reported by galindo-luna et al. [57], who found that naoh-h2o solution with a concentration of 50 wt.% caused detached flakes on the steel surface. in comparison, a more homogeneous and compact surface was found on the sample using a concentration of 40 wt.% [57]. figure 7. sem micrographs of ss 316l samples: (a) k5, (b) k30, and (c) k50 the eds analysis was carried out to identify the phases on the ss 316l sample surfaces at the grain boundaries area, as shown in figure 8. the results for all samples are in general agreement with the composition of ss 316l given in table 1, showing the presence of fe, cr, ni, and mo as main surface elements and oxygen as the additional element. the content of the oxygen element for the k5 sample was higher than other samples, which refers to a better passivation process on the k5 sample. for the k5 sample, some other additional elements (al, si, k, ca) appeared on the surface. interestingly, the potassium (k) element could not be found on the surfaces of the k30 and k50 samples. hamidah et al. [18,20] found that the percentage of k element deposited on the surface of the sample tested for corrosion in 0.27 m koh was about 2.15 wt.%, comparable with 1.9 wt.% found in this study. eds chemical composition testing carried out in this study was based on the spot mode by selecting only a few specific locations. this means that the chemical composition results might not fully represent the whole sample surface. also, the washing process performed after corrosion tests could cause the elimination of the k element for k30 and k50 samples, where k was not identified by eds. the identified element k at the k5 sample is considered an impurity based on the solution settled and trapped on the surface of the corroded sample or at the grain boundaries. the grain boundaries of the ss 316 l sample are the parts that have high energy compared to the grain or matrix areas. consequently, they are more easily corroded and allow the presence of elemental deposits from electrolytes in the grain boundary area. however, it is worth noting that based on the sem micrographs and eds analysis, the k30 sample exhibited better surface morphology than the k50 sample. b. basori et al. j. electrochem. sci. eng. 13(3) (2023) 451-467 http://dx.doi.org/10.5599/jese.1615 461 figure 8. eds analysis of ss316l samples: (a) k5, (b) k30, and (c) k50 structure analysis of corroded surfaces using xrd the electrochemically treated and untreated samples were analyzed by xrd, as shown in figure 9. the xrd curves showed peaks around 51° and 59.7° identified as austenite (γ-fe). the diffraction patterns for all samples are the same, suggesting the presence of only the γ-fe phase. the corrosion process in the koh solution did not produce a new phase or compound, although the sem-eds results showed the presence of oxygen elements on the surface. because the oxygen content was relatively low, the corrosion product in the form of a new compound or phase was not identified. treated samples, however, showed decreased peak heights compared to the untreated sample. chai et al. [58] found that surface modification causes more roughness that can lead to a decrease of peak heights. it seems that in the present experiments, treated k5 and k50 samples changed their http://dx.doi.org/10.5599/jese.1615 j. electrochem. sci. eng. 13(3) (2023) 451-467 corrosion behavior of stainless steel 316l 462 surface roughness by corrosion during potentiodynamic polarization tests, which resulted in a decrease of xrd peaks at around 51° and 58°. figure 9. xrd patterns of untreated and treated ss 316 l samples conclusions the influence of the concentration of koh on the morphology, structure, phase, and corrosion behavior of ss 316l was studied using surface and electrochemical methods. the conclusions of the research are presented as follows. 1. the corrosion rate of the ss 316l plate increased along with an increase of koh concentration from 5 to 50 g l-1. 2. a wider passive region and more noble pitting potential was found for the ss316l in the lowest concentration of koh solution (5 g l-1) compared to higher concentrations (30 and 50 g l-1). 3. the solution resistance (rs) and charge transfer resistance (rct) determined by eis were decreased with the growth of koh concentration. these results suggested a rise in solution conductivity and corrosion rate of ss 316l with an increase in koh concentration. 4. at potentials between 0 and 1.75 v vs. sce, mott-schottky plots suggested the formation of cr2o3 and fe2o3 passive films on sample surfaces. m-s response was found much more prominent for a lower concentration of koh (5 g l-1), suggesting the formation of a well-defined semiconductive passive film in this case. 5. as seen by uv-vis, some particles are present in the koh solution after the potentiodynamic polarization test. on the basis of characteristic wavelengths, these particles could be identified as fe, ni and cr oxides. 6. the intergranular corrosion phenomenon was prominent at the grain boundaries area, as seen by white rust in sem images. at these places, eds analysis suggested the presence of oxygen in addition to fe, ni and cr as main components. the content of oxygen was higher for the solution containing 5 g l-1 koh than for other solutions. 7. the corrosion process in the koh solution, however, did not produce a new phase or compound on the ss 316l surface. only the austenite phase was detected for all concentrations of koh solution, while decreased xrd peak heights of treated samples indicate an increase in surface roughness of ss 316l. 8. on the basis of obtained results, utilization of < 50 g l-1 of koh concentration could be recommended for the hho generator with ss 316l plates. low corrosion rates in these cases b. basori et al. j. electrochem. sci. eng. 13(3) (2023) 451-467 http://dx.doi.org/10.5599/jese.1615 463 would support the optimum useful life of the ss 316l plate electrodes in the hho generator system. acknowledgement: the authors would like to thank the faculty of mechanical engineering, universiti 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http://dx.doi.org/10.5599/jese.1638 421 j. electrochem. sci. eng. 13(2) (2023) 421-436; http://dx.doi.org/10.5599/jese.1638 open access : : issn 1847-9286 www.jese-online.org original scientific paper post-heat treatment of electrochemically carburized low-carbon steel chan on sow1, wong min jin karen1, saffuan awg bahrin2, bih lii chua1, gan jet hong melvin3, and nancy julius siambun3, 1faculty of engineering, universiti malaysia sabah, jalan ums, kota kinabalu 88400, sabah, malaysia 2mechanical engineering department, politeknik kota kinabalu, jalan politeknik, kota kinabalu industrial park, kota kinabalu 88460, sabah, malaysia 3centre of research in energy and advanced materials, faculty of engineering, universiti malaysia sabah, jalan ums, kota kinabalu 88400, sabah, malaysia corresponding authors:  melvin.gan@ums.edu.my; nancyjs@ums.edu.my received: december 14, 2022; accepted: february 14, 2023; published: february 19, 2023 abstract limited studies are available on post-heat treatment (tempering/annealing) of electrochemically carburized low-carbon steel, which can relieve internal stresses induced by the quenching process. in this study, the electrochemical carburization was carried out using the electrolyte mixture of sodium carbonate (na2co3) and sodium chloride (nacl) under a co2 gas environment and 800 °c. the samples were then quenched in either water or oil. the peak hardness of the water-quenched sample (wq) was higher than the oil-quenched sample (oq). comparatively, post-heat treated (tempered and annealed) samples showed lower peak hardness compared to quenched samples. an optical microscope was used to observe microstructural changes, while x-ray diffraction (xrd) was used to examine metal phases within all samples. the full width at half maximum (fwhm) of the martensite peak supported the stress relief in both tempered and annealed samples. scanning electron microscope (sem) with energy dispersive x-ray (edx) was applied to determine the elemental composition of as received and electrochemically carburized and quenched lowcarbon steel samples. the carbon content of the wq sample was relatively higher than the oq sample, whereas the tempered samples showed higher carbon content compared to the annealed samples, but both were lower than for quenched samples. electrochemical carburization increased the carbon content and improved the hardness, while the tempering or annealing process relieved internal stresses that resulted in the hardness reduction. keywords steel; electrochemical carburization; molten salt; quenching; tempering; annealing http://dx.doi.org/10.5599/jese.1638 http://dx.doi.org/10.5599/jese.1638 http://www.jese-online.org/ mailto:melvin.gan@ums.edu.my mailto:nancyjs@ums.edu.my j. electrochem. sci. eng. 13(2) (2023) 421-436 post-heat treatment of low-carbon steel 422 introduction the hardness of steel is defined as the ability to withstand localised deformation or resist the change in shape [1,2]. several studies claimed a linear correlation between yield strength and tensile strength with the hardness of steel [3]. furthermore, the behaviour of wear is affected by high hardness, seen as less abrasion on the parts subjected to high friction stress, resulting in longer service life of the steels [4]. low-carbon steels with less than 0.3 wt.% carbon content consisted mostly of ferrite. ferrite, which is the softest phase of steel, makes low-carbon steel have high machinability but low hardness [5]. the hardness of low carbon steels has usually been increased by the carburization process in molten salt, which increases the carbon content at the surface. in the early stage of molten salt carburization, the conventional carburization used a cyanide bath (ba(cn)2) as the electrolyte under heating for a period of time. however, the production of toxic discharge from this process led to the utilization of other types of electrolytes. thus, an alternative method, electrochemical carburization, known as molten salt electro-carburization, was introduced to replace conventional carburization [6]. the molten salt electrochemical carburization was implemented to increase the carbon content on the surface of low-carbon steel and improve its hardness, wear resistance, and contact fatigue [6]. furthermore, the continuous carbon dioxide (co2) supply into the system during the electrochemical carburization process successfully improved the hardness of the carburized mild steel [7]. the internal strain, induced due to the carbon atoms interstitially present in the crystal lattice of iron after the electrochemical carburization, caused the increment in hardness [8]. moreover, the improved hardness of electrochemically carburized mild steel showed the reduced severity of adhesive wear and the tendency of the worn surface to fracture, resulting in a longer lifespan of mild steel [9]. the rapid cooling process (known as the quenching process) also improves microhardness, related to the resistance to slip and dislocation, by altering the primary phases [10]. heat treatments involve heating the steel to a certain temperature and eventually quenching it to room temperature in order to produce the desired microstructure of steel [11]. various cooling rates of different quenching media (water, oil, etc.) will affect the hardness of steel, where a fast cooling rate provides better hardness performance [12]. the effect of quenching after the electrochemical carburization process improves hardness due to the formation of martensite [13]. it was already found that a high level of residual stresses was induced after the quenching process due to the severe thermal gradient at the surface and core of the workpiece [14]. in addition, the presence of residual macro-stresses after cutting led to the distortion of the steel [15]. a high level of residual stress distorts the manufacturing process, which results in cracking and general instability of the steel. it was shown that residual stress leads to the cracking of metal in the corrosive environment. moreover, a fast cooling process such as quenching will cause quench cracks [16]. hence, an idea was triggered to apply the post-heat treatment of low-carbon steel after electrochemical carburization and quenching to relieve the quenched steel's internal and residual stresses and improve its properties. it was shown that mechanical properties such as toughness, hardness and ductility could be improved by the post-heat-treatment by relieving internal stresses [10]. the tempering process diminishes the brittleness and improves the strength of the metal, while the annealing process refines the grain structure, which makes metals more ductile and softer. the postheat treatment of metal is almost similar to the post-weld heat treatment (pwht), which was used to reduce the residual stress and achieve better performance of the weld [17]. the pwht ensures dimensional stability and reduces residual stress along the heat-affected zone of weldment, which c. o. sow et al. j. electrochem. sci. eng. 13(2) (2023) 421-436 http://dx.doi.org/10.5599/jese.1638 423 improves welded joints' performance. this is similar to the residual stress relief performed through the post-heat treatment of quenched steel [18]. in this study, the low-carbon steel was electrochemically carburized using na2co3-nacl electrolyte in the carbon dioxide environment. the effect of quenching medium, water or oil, was investigated for electrochemically carburized samples. due to their preferable cooling rate, water and oil were selected to obtain the desired hardness of carburized samples. post-heat treatment, tempering or annealing, was conducted after the quenching process to relieve the stress in the samples. hardness, microstructure, structural determination, and elemental analysis of the electrochemically carburized low-carbon steel were performed to examine the effect of both quenching media and two different post-heat-treatment procedures (tempering and annealing). to the best of our knowledge, there is a limited number of studies on the post-heat-treatment of electrochemically carburized low-carbon steel, which triggered the idea of this study. measurements of the hardness of electrochemically carburized low carbon steel samples quenched in different quenching media and post-heat-treated (tempering/annealing) were carried out, together with microscopic images, metal phase analysis and elemental determination. the combination of heat treatment processes such as electrochemical carburization, quenching and post-heat treatment (tempering/annealing) are expected to provide an alternative method to improve the properties of low carbon steel aside from the conventional heat treatment. experimental electrochemical carburization (electro-carburization process) low carbon steel samples with dimensions of 2.5×2.5×2.5 cm (length×width×height) were utilized in this study. each sample surface was further ground to remove contaminants and rust. after grinding, the sample was connected to the cathode prior to the electrochemical carburization process. the low-carbon steel sample acted as a cathode, while the stainless-steel rod was used as an anode. the distance between electrodes was about 2.3 cm. the electrolyte used was a mixture of sodium carbonate (na2co3, bendosen, purity ≥ 99.0 %) and sodium chloride (nacl, bendosen, purity ≥ 99.0 %) with a molar ratio of 4:1 (na2co3: nacl). a total of 650 g salt was mixed and transferred into a 750 ml stainless-steel crucible. the schematic diagram of the setup for the electrochemical carburization process is shown in figure 1. a chamber was fabricated using stainless steel with a gas inlet and outlet to ensure the continuous flow of carbon dioxide gas (co2) in the chamber during the electrochemical carburization process. the top of the chamber was covered using alumina wool to minimize heat loss during the process. the crucible containing the electrolyte was placed in the stainless-steel crucible holder, then the holder was inserted in the middle of the chamber. a lid connected to the cathode, anode, and thermocouple was used to close and tightly seal the chamber. the electrochemical carburization was carried out under the constant temperature of 800 °c for 3 h in the continuous supply of co2 (200 ml min-1) with the constant voltage of 4.0 v supplied by the dc power supply (agilent e3633a). the reactions involved during the electro-carburization are as follows, equations (1)-(4) [7,8,19]. co32+ fe + 4e fe-c + 3o2 (1) o2+ co2(g)  co32(2) 2o2 o2(g) + 4e(3) co2(g) + fe  fe-c + o2(g) (4) http://dx.doi.org/10.5599/jese.1638 j. electrochem. sci. eng. 13(2) (2023) 421-436 post-heat treatment of low-carbon steel 424 figure 1. schematic diagram of the setup for electrochemical carburization process based on the reaction (1), as the voltage was applied through the electrodes, the carbonate ions (co32-) in the electrolyte decomposed into carbon atoms and oxide ions (o2-) at the cathode, where the sample is connected. the o2ions react with either co2 to form co32(reaction (2)) or discharge at the anode as o2 gas (reaction (3)). reaction (4) shows the overall reaction during the electrochemical carburization process with the product of fe-c, which is the carbon dissolved in the cathode sample [19]. after the electrochemical carburization, the samples were quenched in two types of quenching media: water and oil. the quenched samples were further tempered or annealed at 600 °c for 1 hour for the post-heat-treatment process. the tempered samples were cooled at ambient temperature, while the annealed samples were left to cool in the furnace. as received and electrochemically carburized samples treated in different conditions were abbreviated as: as-received (ar), water quenched only (wq), water quenched and tempered (wq-t), water quenched and annealed (wq-a), oil quenched only (oq), oil quenched and tempered (oq-t) and oil quenched and annealed (oq-a), respectively. characterization of hardness, structure and composition of samples the hardness from the outer surface towards the core of ar, wq, wq-t, wq-a, oq, oq-t and oq-a samples was measured using vickers hardness tester (mitutoyo avk-c21) with the applied load of 10 n and 10 s of indentation time [8,13,19]. the hardness from the outer surface towards the core (1000 µm) was taken with an interval distance of 100 µm as shown in figure 2. figure 2. schematic illustration of the hardness test for as-received and electrochemically carburized samples c. o. sow et al. j. electrochem. sci. eng. 13(2) (2023) 421-436 http://dx.doi.org/10.5599/jese.1638 425 microstructure analysis was conducted using the optical microscope (20× and 50× magnification, olympus bx60m). prior analysis, the cross-section area and slightly polished outer surface of the sample was etched with nital solution (4 wt.% nitric acid in ethanol) for 30 s. x-ray diffraction (xrd, rigaku smartlab,  = 0.15406 nm, cu kα radiation, 40 kv, 50 ma) was utilized to characterize the metal phase of samples in a continuous mode over a range of 2 = 25 to 70° with a scan speed of 4.00° min-1 and step width of 0.01°. elemental determination for the samples was examined using a scanning electron microscope (sem, hitachi s3400n) equipped with energy dispersive x-ray (edx, quantex esprit 1.9) under magnification of 3000, electron acceleration voltage of 15 kv and 10 mm working distance. results and discussion hardness and case hardening of as received and carburized samples the electrochemically carburized samples underwent rapid cooling in two quenching media: water and oil. after the quenching process, the post-heat-treatment of tempering or annealing was carried out to relieve the residual stress of the electrochemically carburized samples. figure 3 shows the average hardness measured from the outer surface (0 µm) to the depth of 1000 µm for the asreceived (ar) and electrochemically carburized samples after quenching and post-heat treatment in different conditions. figure 3. average hardness from the outer surface to the core of received and electrochemically carburized samples it is seen in figure 3 that the outer surface (zero distance) hardness for all carburized samples was lower than the peak hardness detected at 100 µm depth. hardness is gradually reduced from the peak and then levelled towards the 1000 m depth. the point where the hardness started to level was estimated as the case depth. the case depth of water and oil-quenched samples (wq and oq) was in the range of 600 to 700 µm. post-heat treated (tempered/annealed) samples exhibited similar case depth in the range of 450 to 550 µm. at 100 µm, the quenched samples exhibited relatively higher hardness than post-heat treated samples. the average hardness of ar sample was 177±9 hv. wq exhibited the highest peak hardness of 702±5 hv compared to other samples. comparatively, the peak hardness of wq is found to be lower than the values obtained for 3 hour electrochemical carburization process studied by liew et al. [19] (795 hv with 660 µm case depth at 4.5 v) and bahrin et al. [13] (1440 hv with 800 µm case depth at 4.0 v), with different experimental settings. the variation in peak hardness might be due to the distance between electrodes [13]. a smaller gap between the electrodes increases the transfer rate http://dx.doi.org/10.5599/jese.1638 j. electrochem. sci. eng. 13(2) (2023) 421-436 post-heat treatment of low-carbon steel 426 of ions, which leads to higher carbon diffusion at the cathode. moreover, the difference in the voltage applied during the electrochemical carburization process affected the hardness of the sample, too [8]. high voltage increased the charge and accelerated the reaction (1) in the molten salt bath, where the high activity of co32caused the presence of more carbon at the cathode. in the study of dewangan et al. [20], low-carbon steel was heated to 727 °c and then water quenched to room temperature. it was found that the brinell hardness number (bhn) of the water-quenched sample was 357 bhn, within the range of 370 to 380 hv. another heat treatment study on heating of aisi 1020 to 750 °c and water quenching resulted in the rise of the hardness of the sample from 191.7 hv (as received) to 254.3 hv [21]. a sample that underwent an electrochemical carburization process possessed higher surface hardness than the sample that was not electrochemically carburized, suggesting that electrochemical carburization produces high surface hardness. the higher hardness of electrochemically carburized samples has to be related to the carbon content formed at the cathode by equation (1). both ways of post-heat treatment of the wq sample caused the reduction of peak hardness. thus, the wq-t sample showed a peak hardness of 623±9 hv, while wq-a showed 444±1 hv. both peak hardness values are lower compared to the wq sample. the annealing process reduced the peak hardness more than the tempering process. the peak hardness of wq-t was higher than wq-a due to the cooling process during the tempering or annealing processes. in the tempering procedure, the sample was cooled at ambient temperature, whereas in annealing, the sample was left and cooled inside the furnace. thus, the tempering procedure involved a faster cooling process, which caused insufficient time for carbon to react with oxygen in the atmosphere, and thus carbon became trapped within the sample, forming martensite. the annealing process involves a slower cooling process inside the furnace, where carbon might have sufficient time to react with oxygen [22]. the faster cooling process increased the hardness of the sample by delaying the formation of ferrite and promoting the formation of pearlite and martensite [23]. typically, the range of high to low hardness is martensite > pearlite > ferrite [22]. the hardness of wq-t and wq-a samples was lower than wq due to the stress relief and the structure rearrangement that lowers distortion occurring after martensite formation. for noncarburized samples, almost similar patterns can be observed with the exception that the overall hardness is comparatively lower than that of electrochemically carburized samples. for instance, according to kanwal et al. [24], the initial hardness of low carbon steel in their study was 173.5 hv, while after heating at 960 °c and water quenching, the hardness was increased to 476.0 hv, and after the further tempering process at 550 °c, the hardness was reduced to 181.7 hv. the peak hardness of the oq sample was 571 ± 3 hv. the peak hardness of oil-quenched samples was lower than water-quenched samples due to the difference in the cooling rate of the quenching medium. similar results were reported by other studies, too [13]. at a slow cooling rate, the formation of pearlite is generated, which has been restricted for a high cooling rate. therefore, most of the martensite has been formed through the high cooling rate of water quenching [25]. martensite is formed with a highly distorted lattice structure, resulting in a high hardness of the sample [12]. thus, the oq sample, which experienced a slower cooling quenching process, consisted of pearlite and less amount of martensite, while the wq sample consisted of a higher amount of martensite. consequently, a lower peak hardness is shown for oq compared to the wq sample. similar to wq samples, oq samples also showed a reduction of peak hardness after tempering or annealing processes. the peak hardness of post-heat-treated samples decreased from 571 ± 3 hv to 495 ± 4 hv for oq-t and to 367 ± 6 hv for oq-a. the peak hardness of the wq-t sample was higher c. o. sow et al. j. electrochem. sci. eng. 13(2) (2023) 421-436 http://dx.doi.org/10.5599/jese.1638 427 than the wq-a sample due to the compressive stress. the compressive stress was due to the fast cooling in the tempering process, leading to higher peak hardness [22]. the increase of compressive stress increased the hardness of the sample, which will be further discussed below regarding fwhm results [23]. in general, the post-heat treatment process reduces the peak hardness of quenched samples, regardless of the quenching medium. even though tempering or annealing lowered the hardness of the quenched sample, the hardness of both tempered and annealed samples was generally higher than the ar sample and provided better ductility (better machinability) compared to quenched samples. when the hardness was measured towards the core, it can be observed that the hardness decreased and eventually came close to the hardness of ar. in addition, the hardness decreased with the relaxation through the annealing process due to the reduction of dislocation density and strain hardening [26]. similarly, the hardness also decreased due to the rearrangement of dislocations after stress-relieving tempering [27]. it is noticeable that all samples (except ar) showed lower surface hardness than the peak hardness at 100 m depth. further investigation via metallographic analysis and xrd was carried out. a discussion of the findings will be elaborated in the next section. structure and composition of as-received and electrochemically carburized samples figure 4 shows the optical microscopic images of the cross-section microstructure of ar and electrochemically carburized samples, indicating carbon penetration from the outer surface (right) towards the core (left). as visible in figure 4(a), the ar sample is dominated by ferrite (white phase), while the presence of pearlite (dark phase) is visible on the grain boundaries only. pearlite consists of lamellas of ferrite and cementite [28]. nevertheless, the lamellar structure is not clearly visible in the image due to the relatively close orientation between ferrite and cementite. another study also showed the presence of ferrite and pearlite in plain low-carbon steel [29]. thus, the ar sample showed high ductility and low hardness due to the dominance of ferrite compared to pearlite [28]. figure 4. optical microscope images on the cross-section microstructure of (a) as-received and (b g) electrochemically carburized samples http://dx.doi.org/10.5599/jese.1638 j. electrochem. sci. eng. 13(2) (2023) 421-436 post-heat treatment of low-carbon steel 428 the quenched electrochemically carburized samples (figure 4(b) and (e)) and post-heat-treated samples (figure 4(c-d) and (f-g)) showed a prominent dark microstructure compared to the ar sample, with the saturated dark region or dark layer near the surface. the dark microstructure consisted of phases containing high carbon content. as already reported by others, the dark microstructure layer consisted of martensite [13]. martensite was formed when the carbon atoms became trapped within the octahedral interstitial sites of martensitic crystal structure [23]. optical images at two different magnification levels (20 and 50) for cross-section microstructure of ar, wq and oq samples are presented in figure 5, showing the presence of martensite within the dark layer microstructure. figure 5. optical microscope images at different magnifications (20 and 50) of cross-section microstructure for ar, wq and oq samples according to figure 5, the microstructure of the ar sample consisted of a majority of ferrite (light phase) and pearlite (dark phase). for 20 magnification of wq and oq samples, there is a dark layer microstructure near the surface (right), not observed for the ar sample. the dark layer microstructure indicates the depth of the carbon diffusion layer from the surface to the core. from images with a 50 magnification level shown in figure 5, the martensite present in needle-like or plate-like structures can be seen in both wq and oq samples. this indicates that due to the formation of martensite, the hardness for both wq and oq would be higher than for the ar sample. the estimated dark layer distance from the surface to the core for wq and oq samples was between 230 to 245 m, while for quenched and post-heat-treated samples (wq-t, wq-a, oq-t and oq-a), between 310 to 340 m. in other words, tempering and annealing processes allow deeper distribution of carbon towards the core, seen as thicker dark layers for post-heat treated samples compared to quenched samples (figure 4). in the re-heating process, the concentration difference of carbon was created between the inner part and outer part of the sample. this led to carbon diffusion, seen as dark microstructure diffusing to either the core region or to the surface, where it would cause decarburization [30]. on the other hand, the hardness of martensite depends c. o. sow et al. j. electrochem. sci. eng. 13(2) (2023) 421-436 http://dx.doi.org/10.5599/jese.1638 429 on the carbon content, and consequently, dispersion of carbon from the dark microstructure layer to the core would cause lower hardness of the post-heat-treated samples. moreover, the peak hardness of all carburized samples in figure 3 was within the dark layer distance. this is due to the presence of the martensite phase in the dark region, which improved the hardness of the samples compared to the rich ferrite phase in ar. based on the observation of microstructures from the surface towards the core, the dark microstructure (carbon-rich phase) is still visible, especially on grain boundaries of quenched and post-heat-treated samples. moving further inwards to the core, the dark microstructure is gradually reduced, suggesting a gradual reduction of the concentration of carbon too. reduction of carbon concentration would result in a decrease in hardness, which is in full agreement with hardness profiles shown in figure 3. it is noticeable that the hardness of wq-t, wq-a, oq-t and oq-a samples have levelled to 177±19 hv (ar average hardness) after 550 m depth. it is interesting that even after 700 m depth, the hardness of wq and wq-t samples remained higher than 177±19 hv, suggesting that carbon was distributed deeper towards the core. the outermost dark microstructure layer near the outer surface showed a grey layer for postheat treated samples. this is due to the oxidation of steel at high temperatures during either the tempering or annealing process. the presence of oxygen during the cooling process of tempering and annealing caused the reaction between oxygen and highly oxidized element such as ferrite to form iron oxide [19]. this led to a lower surface hardness of post-heat-treated samples than the peak hardness (at 100 µm). in other words, for post-heat-treated samples (tempered or annealed), some extent of decarburization occurred. based on zorc et al. [30], the oxidation and decarburization of the steel take place simultaneously in the oxidizing atmosphere during the annealing process. at high temperatures, the difference in carbon concentration between the inner parts and the steel surface caused carbon diffusion from the inner part toward the surface. the carbon atoms then reacted with oxygen gas and formed carbon monoxide and carbon dioxide [30]. at temperatures higher than 570 °c, iron typically oxidizes to form iron oxide. decarburization happened when equilibrium carbon potential is achieved between the chemical potential of carbon in the atmosphere and carbon on the surface of the annealed sample [30]. campos et al. [31] found that decarburization of the steel occurred for quenched and tempered samples, for which the decarburized layer was observed and affected the microhardness of the sample. therefore, the decarburization of samples might decrease the carbon content and lower the content of martensite on the surface, thus causing a reduction of surface hardness. further investigation was carried out to analyse the phases present inside the dark region. the surface of the ar and electrochemically carburized samples were polished using 1 µm size diamond paste to remove approximately 30 µm of the sample surface. samples were etched using a nital solution to unveil the microstructure at 30 µm depth. the corresponding optical images are displayed in figure 6. the microstructure of the ar sample shown in figure 6(a) consisted mostly of ferrite, which contributes to the metal softness but exhibits good strength and moderate ductility. the microstructure of wq shown in figure 6(b) is dominated by the plate-like structure of martensite, which demonstrates higher hardness but a brittle feature. the hardness of the martensite depends on the carbon content of the steel, as higher carbon content contributes to high hardness. when the water-quenched sample was tempered, the martensite was transformed into tempered martensite, as shown by the image of the wq-t sample in figure 6(c). since the tempered martensite has a lower hardness than martensite [32], the hardness of the wq-t sample was found to be lower than the wq sample. http://dx.doi.org/10.5599/jese.1638 j. electrochem. sci. eng. 13(2) (2023) 421-436 post-heat treatment of low-carbon steel 430 figure 6. optical microscope images of slightly polished surface for (a) as received and (b g) electrochemically carburized samples when the water-quenched sample was annealed, soft ferrite was present among the martensite phase [10], as shown in the microstructure of the wq-a sample in figure 6(d). the presence of the soft ferrite decreased the hardness of wq-a compared to the wq sample. as shown in figure 6(e), the oq sample consisted of a dispersion of fine ferrite within the martensite microstructure. the presence of fine ferrite caused lower hardness of oq compared to the wq sample. tempering or annealing processes on oq samples resulted in the formation of pearlite grains and coarse ferrite. choi et al. [33] found that fine ferrite grains consisted of equiaxed and polygonal grain shapes that exhibited higher elastic modulus and hardness compared to coarse ferrite. this was due to the grain boundary that acted as a barrier of dislocation movement [33]. hence, the presence of coarse ferrite in oq-t and oq-a samples contributed to the lower hardness compared to oq samples. the overall result is that the quenching process contributed to the high hardness due to the transformation into harder martensite. formation of martensite, however, caused high distortion in the microstructure of the sample and therefore induced internal stresses, which led to low ductility and toughness of the sample. the tempering process was used to relieve the internal stress across the lath boundaries by allowing the local rearrangement of atoms [34]. aside from tempering, annealing was also used to increase the ductility of the sample, soften the sample, relieve internal stresses and refine the structure to become homogeneous [35]. further investigation using xrd (figure 7) was carried out on the surface of ar and electrochemically carburized samples. in accordance with the image of ar in figure 4(a), the xrd peaks of the ar sample showed the presence of only ferrite (jcpds 06-0696) [36]. the xrd peaks for ferrite and martensite are located at 2 = 44.7 and 44.6°, respectively, which are almost the same peak [36,37]. based on figure 6, the ar sample consisted mostly of ferrite, whereas wq and oq are dominated by martensite. the martensite peak (jcpds 44-1292) [37] at 2 = 44.6° (110) was visible for all electrochemically carburized samples (wq, wq-t, wq-a, oq, oq-t and oq-a), while c. o. sow et al. j. electrochem. sci. eng. 13(2) (2023) 421-436 http://dx.doi.org/10.5599/jese.1638 431 martensite peak at 2 = 64.9° (200) was detected only for wq and oq, which are only quenched, and not post-heat treated samples. it is noticeable that peaks of iron oxides, i.e., magnetite, fe3o4 (jcpds 65-3107) [38] and hematite, fe2o3 (jcpds 33-0664) [39] were detected in all samples that underwent post-heat treatment. the presence of iron oxides (magnetite and hematite) in post-heat-treated samples supported the occurrence of decarburization and oxidation during the cooling process, where the samples were cooled in an oxidizing atmosphere, as discussed in the previous section. 2 / ° figure 7. x-ray diffraction (xrd) analysis for the outer surface of as received (ar) and electrochemically carburized samples (ar, wq, wq-t, wq-a, oq, oq-t and oq-a) the full width at half maximum (fwhm) at 2 = 44.6° (martensite) for the electrochemically carburized samples is summarized in figure 8. according to fu et al. [15], the fwhm intensity of martensite is proportional to the hardness value. thus, a higher fwhm value of martensite indicates a higher hardness value. it is shown in figure 8 that the wq sample has the highest fwhm martensite intensity, followed by wq-t, oq, wq-a (and oq-t) and oq-a samples. the fwhm martensite intensity is comparable to the hardness value of electrochemically carburized samples, as shown in the hardness profiles in figure 3. it is interesting to mention that the stress relieved through the post-heat treatment was more prominent for wq samples, where significant changes can be observed in figure 8. for instance, the fwhm value for wq-a and oq-t samples was similar, but in figure 3, wq-a showed higher surface hardness than the oq-t sample. this is due to the significant stress relieved from wq to wq-a sample, while the stress relieved from oq to oq-t sample was not so distinct. nevertheless, the fwhm values functioning as hardness indicators support hardness profiles, as shown in figure 3. the hardness of electrochemically carburized quenched samples was reduced after the post-heat treatment. the fwhm value was found to be proportional to the residual stress [40], where high fwhm indicates the increment of the micro-strain along with the decrease of domain size of the metal, which could be relieved through the annealing process. in other words, the annealing process decreased the micro-strain while increasing the domain size (region inside the grains), reducing hardness [15]. http://dx.doi.org/10.5599/jese.1638 j. electrochem. sci. eng. 13(2) (2023) 421-436 post-heat treatment of low-carbon steel 432 figure 8. full-width half maximum (fwhm) for the electrochemically carburized samples at 2 = 44.6° (martensite) priyadarshini et al. [41] stated that the annealing process decreased the hardness of the sample due to the release of internal stresses. zhang et al. [42] also found that fwhm is related to the grain distortion, residual stress, and dislocation density of the surface materials since higher fwhm showed higher hardening of the measured surface. the strain and surface defects were decreased after the tempering process [42]. in this work, tempering and annealing processes successfully relieved the stresses on the sample surface since lower fwhm was recorded for tempered or annealed samples (after either water or oil quenching). sem images with edx mapping of oxygen and carbon were carried out on the outer surface of ar, wq, wq-t and wq-a samples, as shown in figure 9. figure 9. sem image with edx mapping of (a) ar, (b) wq, (c) wq-t and (d) wq-a samples c. o. sow et al. j. electrochem. sci. eng. 13(2) (2023) 421-436 http://dx.doi.org/10.5599/jese.1638 433 the oxygen mapping of post-heat-treated samples is in agreement with xrd results. the oxygen mapping (blue colour) of tempered (figure 9(c)) and annealed (figure 9(d)) samples covered more area (concentrated) compared to wq and ar samples. moreover, the dark area (without oxygen) was more obvious for wq and ar samples. it is also apparent that the wq-a sample had higher oxygen concentration compared to wq-t. the edx analysis showed the carbon content on the outer surface of ar (2.79 wt.%), wq (5.24 wt.%), wq-t (5.16 wt.%), wq-a (2.94 wt.%), oq (4.73 wt.%), oq-t (2.78 wt.%) and oq-a (2.63 wt.%), respectively. higher carbon content was in accordance with the higher surface hardness of samples. the edx mapping also showed that, comparatively, wq had higher carbon distribution (intense red colour). the red colour in wq is more prominent compared to others, which indicates a high content of carbon that can be matched to the surface hardness depicted in figure 3. conclusion different post-heat treatment procedures were applied to electrochemically carburized and quenched low-carbon steel samples in order to improve steel properties. electrochemical carburization was performed in na2co3-nacl molten salt at 800 °c in the presence of co2, and samples were quenched in either water or oil. post-heat treatment was performed by either tempering involving faster or annealing involving a slower cooling process. post-heat-treated samples generally showed lower peak hardness values than only quenched samples. among post-heat-treated samples, annealed samples showed lower peak hardness than tempered samples. in difference to as received sample that showed only ferrite and quenched samples that showed only martensite phase, xrd of post-heat-treated samples showed a mixture of martensite, ferrite, and iron oxide (hematite and magnetite). the corresponding full width of half maximum (fwhm) values for the martensite xrd peak showed the relieving of stress after either tempering or annealing, which pointed again to the lower hardness of post-heat-treated samples. optical images indicated a dark layer extending from the surface to the core of samples, that was 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https://www.ripublication.com/ijame-spl/ijamev4n7spl_07.pdf https://doi.org/10.1080/02670836.2017.1355584 https://creativecommons.org/licenses/by/4.0/) theoretical and electrochemical analysis of l-serine modified graphite paste electrode for dopamine sensing applications in real samples https://dx.doi.org/10.5599/jese.1391 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; https://dx.doi.org/10.5599/jese.1391 open access : : issn 1847-9286 www.jese-online.org original scientific paper tool wear rate during electrical discharge machining for aluminium metal matrix composite prepared by squeeze casting: a prospect as a biomaterial arvinder singh channi1, harminder singh bains2, jasmaninder singh grewal3, vettivel singaravel chidambaranthan4, raman kumar5, 1department of mechanical engineering, ikg punjab technical university, jalandhar, punjab, india 2department of mechanical engineering, university institute of engineering and technology, panjab university ssg regional centre hoshiarpur, punjab, india 3department of mechanical and production engineering, guru nanak dev engineering college, ludhiana, punjab, india 4department-mechanical engineering, ccet (degree wing), chandigarh, india 5department of mechanical and production engineering, guru nanak dev engineering college, ludhiana, punjab 141006, india corresponding author: sehgal91@yahoo.co.in received: may 29, 2022; accepted: june 11, 2022; published: august 29, 2022 abstract squeeze casting was used to prepare aluminium alloy-6061/titanium diboride (tib2) to create a composite with varied tib2 quantities. the composite's metallographic structure, tensile strength, and hardness have been explored. the tensile strength and hardness of produced metal matrix composites increased when a particle of tib2 was increased from 5 to 10 vol.%. this study uses electrical discharge machining (edm) to investigate the output response tool wear rate (twr). variables in edm operation were investigated, such as current, pulse on time, and voltage gap. the experiments were designed using the box–behnken strategy. statistical approaches were used to analyse the experiments. at ideal settings for tib2 concentrations of 5 and 10 vol.%, twr was 0.2146 and 0.1749 mm3 min-1 and surface roughness was 2.47 and 3.03 µm, respectively. tib2 is utilized in automobile disc brakes, an industry where components slide against each other. the aluminium alloy-6061/titanium diboride has many applications as a biomaterial and is a good prospect. keywords box behnken design; hardness; tensile strength; eds; response surface methodology; desirability analysis introduction metal matrix composites (mmcs) are increasingly used in structure, wear, thermal, transport, and electrical applications. composites have better strength-to-weight and strength-to-cost ratios than similar monolithic conventional alloys [1]. aluminium-based particulate matrix composite https://dx.doi.org/10.5599/jese.1391 https://dx.doi.org/10.5599/jese.1391 http://www.jese-online.org/ mailto:sehgal91@yahoo.co.in j. electrochem. sci. eng. 00(0) (2022) 000-000 twr during edm for al metal matrix composite 2 composites have been recognized as an important class of materials used in the aircraft, vehicle, chemical, and automotive sectors due to their enhanced strength, high elastic modulus, and continued increased abrasion resistance over traditional base alloys [2]. there are three production procedures for aluminium mmcs liquid state, semisolid, and powder metallurgy [3]. first, the ceramic particles are integrated into a molten metallic matrix in liquid state techniques, and the mmc is cast. stir casting is a liquid state composite material fabrication process that involves mechanically stirring a ceramic particle into a molten matrix metal. the liquid composite material is subsequently cast using typical casting processes [4]. aluminium 6061 is a low-density, more significant, and wear-resistant metal alloy. to counteract this fault, tib2 additives are formed to the alloy, enhancing its hardness, young's modulus, and abrasive wear resistance [5]. aluminium alloys are reinforced with ceramic materials such as sic, tic, tib2, zrb2, aln, si3n4, al2o3, and sio2. tib2, a reinforcing ceramic particle with a high young's modulus, low density, and better vickers hardness, is particularly appealing, thermal conductivity, high electrical conductivity, high elastic modulus, hardness, high melting point, superior wear resistance, and good thermal stability [6]. studies show that tib2 particles do not react with aluminium [7]. as a result, brittle reactions at the reinforcement–matrix contact are avoided [8]. the behaviour of aluminium reinforced with tic, tib2, b4c, and sic was compared utilizing the powder metallurgy approach [9]. tib2 outperformed the other reinforcements in terms of mechanical properties. tib2 is frequently used in advanced engineering ceramics due to its unique qualities [10]. the bioceramics are either strongly connected to the body’s possessed materials or are nighindestructible metal oxides [11]. dental and bone implants made of ceramics are increasingly widely used in healthcare. surgical cermets are frequently utilized. bioceramic coatings are routinely used on joint replacements to minimize wear and inflammation [12]. bioceramics are also used in pacemakers, kidney dialysis machines, and respirators, among other medical applications [13]. bioceramics are intended for endovascular circulation systems such as dialysis or designed bioreactors; otherwise, they are most commonly employed as implants. because of their physicochemical properties, ceramics have many uses as biomaterials [14]. alumina (al2o3) is often used in bioceramics because it lasts longer than the patients. middle ear ossicles, ocular prostheses, electrical insulating for implantable devices, catheter holes, and countless prototypes of implanted technologies such as cardiac pumps can all benefit from the material [15]. nevertheless, porous materials have lesser mechanical strength than bone, making porous implants fragile. in addition, the implant can induce mechanical stresses at the tissue interface because the elastic modulus of ceramic materials is often higher than that of the surrounding bone tissue [16]. biologically active ceramics have already seen particular usage responsible for biological reactivity, in contrast to their typical qualities. instances include calcium phosphates, oxides, and hydroxides [17]. bioceramics are suitable for medical use because they are anticorrosive, biocompatible, and attractive. bioinertness and noncytotoxicity are two properties of zirconia ceramic. carbon is another mechanical option that includes the after-bone and blood compatibility, no tissue reactivity, and no cell toxicity [18,19]. due to the apparent geometric structure sophistication, high cutting costs, and high tool wear, manufacturing materials with reinforcement using traditional machining processes is difficult. as an outcome, non-traditional cutting processes for shaping metal matrix composites for a complex die contour have been developed [20]. electric discharge machining (edm)has become one of the most prevalent non-conventional methodologies. electrically conductive materials are machined with edm employing finely regulated sparks between an electrode and the workpiece in a dielectric fluid a. s. channi et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1391 3 media [21]. using an artificial bee colony technique, researchers looked at numerous process parameter combinations to find the best material removal rate mrr and surface roughness (ra) for edm of en31 tool steel. they also looked into the effects of machining parameters on machining performances, finding that when t–on and current increased, so did mrr and ra [22]. the edm technique investigated the effects of voltage, current, t–on, and duty factor on mrr, tool wear rate (twr), and ra. they devised a mathematical model for determining the average ra, difficulty, and mrr. the calculated values are incredibly close to the experimental findings. as a result, the mathematical model can be utilized to analyse mrr and average ra responses for parameter investigation [23]. in wire edm of ss304, the output response of ra and kerf-width were investigated for optimization [24]. according to anova results, t–on and peak currents substantially impact the surface quality of 2312 hot wrought steel [25]. in edm process settings, the machining performance of aa7075 reinforced with sic mmc with a copper tool electrode was investigated. t–on is essential, contributing 57.9 % to performance measurements [26]. the central composite design optimized process parameters while considering mrr, twr, and ra during edm. mrr increases as current increases, and twr increases as peak current increases [27]. a genetic algorithm was utilized to explore the cutting properties of edm during the machining of al6351-b4c-sic mmc. the results reveal that current contributed 68.92 % in predicting ra [28]. the impact of edm process parameters on an mmc of aa 6061/sic with a tool made of copper concluded that if the current was higher and voltage was lower, mrr reached its maximum. conversely, when both current and voltage are low, ra was improved [29]. the aa-6061/10% al2o3 metal matrix composite machining properties were investigated using edm. they managed the process parameters to achieve the highest mrr and lowest twr possible. analysis of variance is used to determine the optimal value of input parameters [30]. the effect of mmc of al+tib2 fabricated by in-situ process and machined by edm indicates that mrr increases with the current, while it decreases with the increases in reinforcement [31]. in machining al/cu/tib2 in-situ mmcs, a multi-criteria optimization technique was used to optimize the edm parameter. mrr, twr, and ra are reaction parameters, while discharge current, pulse on time, and pulse off time are machining parameters. the study reveals that the optimized results agree with the confirmation run [32]. the effect of the edm on the machining of the al+tib2 using the copper electrode for mrr, twr, and ra indicates that mrr and twr increase by increasing the current and spark on time. it reduces with the increase of spark-off time, while ra depends on the applied current and increases with the increase in the applied current [33]. many studies were conducted on composite materials with edm and achieved significant results [18,19,34-36]. a literature survey revealed that machining research for al/tib2 composites had been conducted in some of the studies. on the other hand, the edm of al/tib2 squeeze cast composite on twr has received little attention. this research aims to govern the effects of edm process parameters such as current, pulse-on time, and voltage gap on twr. the box-behnken method is used for the design of experiments. response surface methodology (rsm) models were developed, supported with anova, and verified with residual plots. the hardness and tensile strength (ts) were reported for a fabricated composite of aa 6061 with 5 and 10 vol.% tib2. experimental composite preparation aluminium 6061 alloy is one of the most extensively used materials in the al6xxx series due to its high strength, outstanding workability, and excellent corrosion resistance [37]. on the other hand, https://dx.doi.org/10.5599/jese.1391 j. electrochem. sci. eng. 00(0) (2022) 000-000 twr during edm for al metal matrix composite 4 tib2 is a well-known ceramic material with exceptional properties [38-40]. as a result, the study's matrix is al alloy 6061 reinforced with tib2. hindalco's aa 6061 ingots were used as the matrix material, sliced into small pieces, and melted using tib2 of less than 10 microns. the sem of al 6061 is shown in figure 1(a), and eds is shown in figure 1(b). figure 1(c) shows the sem of tib2, and figure 1(d) depicts the eds of tib2. a b energy, kev c d energy, kev figure 1. (a) sem of al 6061, (b) eds of aa 6061, (c) sem of tib2, (d) eds of tib2 eds analysis energy dispersive x-ray synthesis, or eds analysis, is also known as energy dispersive x-ray microanalysis. it's an analytics method for identifying a sample's chemical composition or characterization. it is based on an interaction between an x-ray emitter and a specimen [41-43]. when applied to the ready samples with completely varied percentages of tib2, the eds assessment provides the following results, as shown in figure 2, and constituents of the eds of aa6061 + tib2. figure 2(a) aa6061+ 5 vol.% tib2 indicates the presence of aluminium, titanium, boron, and magnesium. figure 2(b) aa6061 + 10 vol.% tib2 leads the aluminum, titanium, boron, chromium, and copper. figures 2(c) and 2(d) show eds layered images showing ti and boron with uniform distribution. mechanical testing mechanical testing is a technique for determining a material's mechanical characteristics. it is used to assess a material regardless of its geometry and under specific topological situations [44-46]. hardness and tensile strength test the influence of tib2 particles on matrix hardness was investigated using a vickers hardness tester on polished samples of produced aa 6061/tib2 specimens. the test was carried out following the astm e384-10 standard. in addition, hardness tests were performed at three different sites to safeguard the hard reinforcement particles from the indenter resting effect. the hardness of aa 6061 + 5 vol.% tib2 and aa6061 + 10 vol.% tib2 are shown in table 1. the hardness of tib2 increases a. s. channi et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1391 5 as the percent volume of tib2 increases. the outcome of ts is depicted in table 2. it is clear that with the increase in the content of the reinforcement, there is an increase in ts. a energy, kev b energy, kev c 100 nm d 100 nm figure 2. eds of composite specimens (a) aa6061 + 5 vol.% tib2; (b) aa6061 + 10 vol.% tib2; layered eds image for (c) ti and (d) boron ti b https://dx.doi.org/10.5599/jese.1391 j. electrochem. sci. eng. 00(0) (2022) 000-000 twr during edm for al metal matrix composite 6 table 1. microhardness reading of mmc sample microhardness, hv trial 1 trial 2 trial 3 mean aa6061+ 5 vol.% tib2 100.8 102.3 101.7 101.6 aa6061+ 10 vol.% tib2 150.6 151.6 154.7 152.3 table 2. tensile strength of mmc sample tensile strength, mpa aa6061+ 5 vol.% tib2 142.4 aa6061+ 10 vol.% tib2 195.1 design of experiments the box-behnken design (bbd) is a second-order response surface methodology (rsm) model that is the focus of most rsm research [29]. the bbd only requires three levels for each element to fit a second-order regression model (quadratic). the bbd chose a middle ground between the components' initial low and high levels [47-49]. face points, rather than corner points, are used in the bbd, resulting in fewer experimental runs. the experiments were designed using the bbd approach with design expert software 7.0.0 version and three process parameters with 17 runs to examine the impact of edm parameters on mrr. • peak current (a), three levels 1, 2, 3 at 3, 4, and 5 a • pulse on time (b), three levels 1, 2, 3 at 10, 15, and 20 µs • voltage gap (c), three levels 1, 2, 3 at 15, 30, and 45 v a die sinking edm of electronica, pune, india, with model electrica rt 20, was used to conduct the testing. it has a work tank that measures 650×400×280 mm and a working table that moves 200 mm in the x-direction, 280 mm in the y-direction, and 300 mm in the z-direction. the mmc of aa 6061 + 5 vol.% tib2 and aa6061 + 10 vol.% reinforcement has a 50 mm outer diameter, and two cylindrical rods with diameters of 23 mm and cylindrical rod slices of 3 mm were produced from this casting using wire cut edm with brass wire, 0.25 mm diameter, wire tension of 7 to 8 n, and distilled water as a dielectric. as a result, the edm machining raw material is 23×3 mm in size. the copper electrode was 4 mm in diameter. the dielectric fluid was gandhar oil refinery limited’s divyol seo 25 oil, with flash and fire points of 112 and 124 oc, respectively. at 20 and 40 °c, the fluid has a viscosity of 3.48 and 2.35 mm2 s-1, respectively, and a boiling point of 240 °c. measurement of tool wear rate twr was calculated for each experimental trial using the weight loss method during edm machining, as shown in equation (1) [30]. a computerized mass balance with a 0.001 g precision is used to estimate the beginning and end weights of the specimens. each test sample's measurements were taken three times to ensure accuracy.  − = initial finaltwr w w (1) were w is weight,  is density,  is time results and discussion modelling and anova of tool wear rate the response surface methodology (rsm) was utilized to develop a model [50,51] for twr while edm machined aa6061 + tib2 mmc material. twr could be estimated using the mathematical a. s. channi et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1391 7 model of peak current a, pulse on time b, and voltage gap c. to construct quadratic twr models, the test plans were created. twr was parametrically characterized, and a link to machining parameters was attempted. the developed twr model for aa 6061 + 5 vol.% tib2 is shown by equation (2) and for twr aa6061 + 10 vol.% tib2 in equation (3). the model's relevance, the importance of distinct aspects, and the lack of fit were determined using statistical testing. twr (aa 6061+ 5 vol.% tib2) = 0.25 + 0.025a + 2.875×10-3b 5.000×10-4c 0.016a + 0.013ac 10-3b + 0.018a2 + 0.019b2 0.013c2 (2) twr (aa6061 + 10 vol.% tib2) = 0.23 + 0.069a + 8.500×10-3b + 1.750×10-3c + + 2.750×10-3ab 2.500 ×10-4ac 5.750×10-3bc + 0.024a2 2.975×10-3b2 + 2.500×10-5c2 (3) anova is a family of standardized models and accompanying estimates processes for analyzing differences between means [52,53]. it's used in various settings to see if there's a significant difference between means of distinct groups [54-56]. for example, table 3 shows anova for twr: rsm for twr of 5 vol.% tib2 (partial sum of squares-type iii). table 4 shows anova for twr: rsm model for twr of 10 vol.% tib2 (partial sum of squares-type iii). table 3. anova for twr: rsm for twr of 5 vol.% tib2 (partial sum of squares type iii) source sum of squares df mean square f value p-value prob>f model 0.010 9 1.134×10-3 35.56 < 0.0001 significant a 4.950×10-3 1 4.950×10-3 155.21 < 0.0001 significant b 6.613×10-5 1 6.613×10-5 2.07 0.1931 c 2.000×10-6 1 2.000×10-6 0.063 0.8095 ab 1.056×10-3 1 1.056×10-3 33.12 0.0007 significant ac 7.290×10-1 1 7.290 22.86 0.0020 significant bc 4.000×10-6 1 4.000×10-6 0.13 0.7337 residual 2.233×10-4 7 3.189×10-5 -- lack of fit 4.525×10-5 3 1.508×10-5 0.34 0.7996 non significant pure error 1.780×10-4 4 4.450×10-5 -- cor total 0.010 16 --- std. dev. 5.647×10-3 --r2 0.9786 mean 0.26 --adj r2 0.9511 cv, % 2.19 --pred r2 0.9039 press 1.002×10-3 -- the model f-values of 35.56 and 72.21, respectively, indicate that the model is significant for 5 and 10 vol.% tib2. due to noise, there is only a 0.01 % chance that a "model f-value" this large will occur. the edm parameters' significance is revealed by p values less than 0.05. twr of 5 vol.% tib2 a, ab, and ac are significant, but twrs of 10 vol.% tib2 a and b are significant. the lack of nonsignificant results in the developed modes indicates they are good. f-values of 0.34 and 0.52 for twr of 5 vol.% tib2 and 10 vol.% tib2 show no substantial lack of fit. the lack of fit at 5 vol.% tib2 may be attributed to noise in 79.96 % of cases and at 10 vol.% tib2 in 68.95 % of cases. excellent concordance is seen between the observed r-square values of 0.9511 for 5 vol.% tib2 and 0.9756 for 10 vol.% tib2 and the expected values of 0.9039 and 0.9400, respectively. https://dx.doi.org/10.5599/jese.1391 j. electrochem. sci. eng. 00(0) (2022) 000-000 twr during edm for al metal matrix composite 8 table 4. anova for twr: rsm model for twr of 10 vol.% tib2 (partial sum of squares type iii) source sum of squares df mean square f value p-value prob>f model 0.041 9 4.578×10-3 72.21 < 0.0001 significant a 0.038 1 0.038 600.76 < 0.0001 significant b 5.780×10-4 1 5.780×10-4 9.12 0.0194 significant c 2.450×10-5 1 2.450×10-5 0.39 0.5539 ab 3.025×10-5 1 3.025×10-5 0.48 0.5120 ac 2.500×10-7 1 2.500×10-7 3.943e003 0.9517 bc 1.323×10-4 1 1.323×10-4 2.09 0.1919 residual 4.438×10-4 7 6.340×10-5 -- lack of fit 1.250×10-4 3 4.167×10-5 0.52 0.6895 nonsignificant pure error 3.188×10-4 4 7.970×10-5 -- cor total 0.042 16 --- std. dev. 7.962×10-3 -r2 0.9893 mean 0.24 --adj r2 0.9756 cv, % 3.28 --pred r2 0.9400 press 2.498×10-3 -- assessment of actual and predicted twr the predicted versus actual responses were displayed to verify the appropriateness of the created model. the rsm model's effectiveness for the replies was assessed by inserting the data into the constructed model [57,58]. the experimental results are compared to the predicted data from the analysis, as shown in figure 3. a b figure 3. plots for twr (a) aa6061 + 5 vol.% tib2(mmc) and (b) aa 6061 + 10 vol.% tib2 (mmc) the proposed quadratic models for responses performed admirably, with a little lack of fit, and had no difficulty predicting response values, as indicated by the smallest range of projected data points in both situations. the anticipated value points were uniformly distributed, near the parity line, and fit in a straight line. the model's robustness gains confidence as a result of this. normal probability plots a normal probability map is a visual tool detecting significant deviations from normality. examples include outliers, skewness, kurtosis, the need for transformations, and mixtures. they are raw data, model fit residuals, and predicted variables [59-61]. if the data are roughly normally distributed, the sorted data are plotted versus values chosen to make the final image look near a straight line. variations from a straight line indicate discrepancies [51,62]. for example, the normal 1 actual twe, mm3 min-1 2 design-expert® software tool wear rate color points by value of tool wear rate: 0.321 0.215 actual tw r (m m 3/m in) p re d ic te d t w r ( m m 3 /m in ) 0.21 0.24 0.27 0.30 0.33 0.21 0.24 0.27 0.29 0.32 p re d ic te d t w e , m m 3 m in -1 a. s. channi et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1391 9 probability plot of residuals for twr for aa6061 + 5 vol.% tib2 mmc is shown in figure 4(a) and for twr for aa6061 + 10 vol.% tib2 mmc in figure 4(b). most plotted points form a reasonably linear pattern, with minimal deviations from the straight line indicating consistent data, indicating the model's competence. a b figure 4. normal probability plots for twr (a) aa6061 + 5 vol.% tib2 (mmc), (b) aa 6061 + 10 vol.% tib2 (mmc) optimization of edm for developed mmc by desirability analysis desirability analysis solutions for twr for 5 and 10 vol.% tib2 are shown in table 5, with the top five results to choose the best value. table 5. desirability solutions for the twr aa6061 + 5 vol.%tib2 number current, a time, µs voltage, v twr, mm3 min-1 desirability 1. 3.32 14.01 44.73 0.2146 1.000 selected 2. 3.30 14.09 44.68 0.2145 1.000 3. 3.02 13.32 43.35 0.2134 1.000 4. 3.11 12.99 44.35 0.2117 1.000 5. 3.04 14.27 44.11 0.2133 1.000 aa6061 + 10 vol.%tib2 1. 3.13 10.16 15.59 0.1749 1.000 selected 2. 3.00 10.92 18.67 0.1760 1.000 3. 3.08 10.06 21.98 0.1768 1.000 4. 3.05 11.26 17.90 0.1777 1.000 5. 3.02 10.49 21.91 0.1767 1.000 the optimal value for mmc of aa 6061 + 5 vol.% tib2 has a current of 3.32 a, a duration of 14.01 µs, and a voltage gap of 44.73 v, resulting in a twr of 0.2146 mm3min-1. an optimal value for the mmc of aa6061 + 10 vol.% tib2, with a current of 3.13 a and a time of 10.16 µs with a voltage gap of 15.59 v, yields a twr of 0.1749 mm3min-1. perturbation plot the perturbation plot can be used to compare all the components' effects at a single point in the solution space. only one parameter is changed across the response spectrum, while the others remain constant. for example, figure 5 shows the perturbation plot for twr of aa6061 + 5 vol.% https://dx.doi.org/10.5599/jese.1391 j. electrochem. sci. eng. 00(0) (2022) 000-000 twr during edm for al metal matrix composite 10 tib2 and aa6061 + 10 vol.% tib2, where a represents peak current, b represents pulse on time, and c indicates voltage gap. twr rises as current rises, and pulse on-time slightly impacts twr, whereas voltage has a much smaller impact. a b figure 5. perturbation plots for twr (a) aa6061 + 5 vol.% tib2 (mmc); (b) aa 6061 + 10 vol.% tib2 (mmc) contour plots a contour plot is a graphical technique for displaying a 3-dimensional surface in a 2-dimensional format by showing contours. a contour plot is a 2-d version of a 3-d surface plot [63-65]. for example, it is evident from figure 6 for 5 and 10 vol.% tib2 that as the current and time increase, there is an increase in the tool wear. furthermore, it is evident from figures 6(a) and 6(b) that with the increase in the current tool wear rate increases, the blue area indicates less twr, and the red indicates more twr. a b figure 6. contourplots for twr (a) aa6061 tib2 (5 vol.%) mmc, (b) aa 6061 tib2 (10 vol.%) mmc surface roughness at optimal variables and comparative analysis the tool wear and surface roughness for 5 vol.% tib2 is 0.2146 mm3min-1 and 2.47 µm, respecttively. tool wear and surface roughness for 10 vol.% tib2 is 0.1749 mm3min-1 and 3.03 µm. therefore, it indicates that as the reinforcement grows, the tool wear and surface roughness decreases, indicating that as the reinforcement increases, the mrr decreases, directly affecting the twr and ra; similar results were also reported [55,56]. the comparative results for twr, ra, hardness and tensile strength are shown in figure 7 and optimal edm variables. a. s. channi et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 https://dx.doi.org/10.5599/jese.1391 11 figure 7. comparative results for aa6061 + tib2 (5 vol.%) mmc, and aa 6061 + tib2 (10 vol.%) mmc conclusions squeeze casting made the al alloy 6061 mmcs with 5 and 10 vol.% tib2 reinforcements. squeeze cast specimens have less porosity than squeezing because they are created with a squeeze motion. chemical composition revealed by microstructure and eds analysis included ti, boron, and other elements. because of its exceptional characteristics, al mmc has a well-known use. the experiments were designed using the bbd approach to explore the impact of edm parameters, including peak current, pulse on time, and voltage gap on twr. various analyses of twr's optimization and modelling lead to meaningful results. • the results of squeeze casting reveal that when the tib2 increased from 5 to 10 vol.%, the hardness increased from 101.6 to 152.3 hv, while tensile strength increased from 142.4 to 195.1 mpa, respectively. • the edm parameters for mmc of aa 6061 + tib2 with 5 and 10 vol.% reinforcement casting reveal that as the current increases, so does the surface roughness. in addition, twr was observed to rise when pulse on time increased. • the rsm f-values of 35.56 for twr of 5 vol.% tib2 and 72.21 for twr of 10 vol.% tib2 indicate that the model is significant, and the predicted r2 of 0.9039for twr of 5 vol.% tib2, and 0.9400 for twr of 10 vol.% tib2 are in reasonable agreement with the corrected r2 of 0.9511 and 0.9756, respectively. • desirability analysis reveals that the optimal value for mmc of aa 6061 + 5 vol.% tib2 has a peak current of 3.32 a, pulse on time of 14.01 µs, and a voltage gap of 44.73 v, giving a twr of 0.2146 mm3min-1. an optimal value for the mmc of aa6061 + 10 vol.% tib2 with a peak current of 3.13 a and a pulse on time of 10.16 µs with a voltage gap of 15.59 v produces a twr of 0.1749 mm3min-1. acknowledgements: the authors express deep gratitude to i. k. gujral punjab technical 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©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.3390/ma15041287 https://doi.org/10.1177/0020294020947126 https://doi.org/10.1177/2633366x20963137 https://doi.org/10.1016/j.matlet.2021.130769 https://doi.org/10.1002/pc.25683 https://doi.org/10.3390/ma13122729 https://doi.org/10.1016/j.jmrt.2020.12.114 https://doi.org/10.1016/j.procir.2021.01.170 https://doi.org/10.1016/j.matpr.2021.04.624 https://doi.org/10.37628/ijma.v6i1.632 https://doi.org/10.1177/14644207211054143 https://doi.org/10.1080/2374068x.2020.1835013 https://doi.org/https:/doi.org/10.1002/pc.25683 https://creativecommons.org/licenses/by/4.0/) {an overview of recent developments of carbon-based sensors for the analysis of drug molecules:} http://dx.doi.org/10.5599/jese.999 161 j. electrochem. sci. eng. 11(3) (2021) 161-177; http://dx.doi.org/10.5599/jese.999 open access: issn 1847-9286 www.jese-online.org review an overview of recent developments of carbon-based sensors for the analysis of drug molecules pemmatte a. pushpanjali, jamballi g. manjunatha and nagarajappa hareesha department of chemistry, fmkmc college, madikeri, mangalore university constituent college, karnataka, india corresponding author: manju1853@gmail.com; tel.: +91-08272228334 received: april 21, 2021; revised: july 5, 2021; accepted: july 6, 2021; published: july 20, 2021 abstract this review summarizes some recent developments in the fabrication of modified sensors and biosensors using carbon-based materials. the great potential of carbon-based electrodes as sensing platforms is exciting due to their unique electrical and chemical properties, high accessibility and high biocompatibility. carbon-based materials are particularly interesting due to almost infinite possibility of their functionalization with a wide variety of organic molecules, biologically important compounds and pharmaceuticals. this review is specifically focused on recent developments in the utilization of various carbonbased electrodes as sensing devices for the electrochemical investigation of drug molecules. various voltammetric techniques considered in this effort include linear sweep voltammetry (lsv), cyclic voltammetry (cv), differential pulse voltammetry (dpv), square wave voltammetry (swv), and square wave adsorptive stripping voltammetry (swadsv). the carbonbased electrode materials considered in this review comprise carbon paste, carbon nanotubes, graphite, graphene, and glassy carbon. the analytes chosen are some routinely used drugs such as paracetamol (pc), diclofenac sodium (dcf), 5-fluorouracil (5-fu), cetirizine (ctz) and salbutamol (sal). all here reported sensing electrodes produced very good results in electrochemical investigations of these drug molecules. keywords electrochemical investigations; modified carbon electrodes; sensing device; voltammetry techniques introduction drugs are chemical substances of specific structures, which administered to a body, interact and bind with certain proteins and cells and affect physiological function by altering the biological structure. this is the common thought staying behind all medicine. there are no drugs, however, being so specific to interact with a certain type of cell or protein, what is the main reason of various http://dx.doi.org/10.5599/jese.999 http://dx.doi.org/10.5599/jese.999 http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 11(3) (2021) 161-177 carbon-based sensors for drug molecules 162 side effects [1]. since drugs are routinely used for treatments of various diseases, it is generally considered very essential to develop a convenient method for their determination. among various analytical techniques, electroanalytical techniques possess high flexibility, sensitivity, simplicity, rapidity, and are easy to handle. under accessible operating conditions, these techniques are found appropriate for the analysis of food, pharmaceutical and clinical samples [2‒5]. most of the analytical methods for investigations of drug molecules already reported in the literature, such as high-performance liquid chromatography (hplc) [6], capillary zone electrophoresis [7], fluorescence spectroscopy [8], and spectrophotometry [9], are expensive compared to electroanalytical methods. all these methods, however, are inappropriate for routine analyses because they need expensive instrumentation, tedious sample preparation, complex pretreatment, solvent extraction, elongated analysis time, derivatization, and expert analysts handling with a complex instrumentation [10,11]. the electroanalytical performance is the most appropriate analytical platform for sensitive and selective investigation of commercial and real drug samples. electroanalytical methods use easy instrumentation and show fast responses. they are also highly sensitive, steady, simple, accurate, specific, economical and need small amounts of sample [12‒14]. in the context of electroanalytical methods aimed to the analysis of drug molecules, the most important topic is preparation of modified sensing electrodes via numerous modulating mediators and methods. such sensor electrodes show the boosted sensitivity and respectable steadfastness for the analysis of drug molecules using some specific electrochemical methods [15‒18]. the modification of electrode surfaces has usually been done by some important methods such as electro-deposition, drop-casting, and electrochemical polymerization. these methods need surface modifying agents such as metal-based nanomaterials, carbon-based nanostructures, organic molecule functionalized nanostructures, surfactants (anionic, cationic and nonionic), dyes and organic molecules (like amino acids, vitamins, neurotransmitters, etc.) [19‒21]. the surface modification of bare electrode materials enhances the electrochemical sensing activity due to application of elevated catalytic action, and generates more active sites, higher electron affinity, lower ionization potential, and higher electronic and ionic conductivity [23‒25]. electroanalytical approach with carbon-based modified electrodes is extensively used in the exploration of new drug compounds. in that, voltammetric and amperometric techniques are usually used for recording of current-voltage responses of drug molecules at fabricated electrode surfaces [26‒28]. depending on the mode of potential change, voltammetric techniques include various types such as linear sweep voltammetry (lsv), cyclic voltammetry (cv), normal pulse voltammetry (npv), differential pulse voltammetry (dpv), differential pulse adsorptive stripping voltammetry (dpadsv), square wave voltammetry, (swv), and square wave adsorptive stripping voltammetry (swadsv). carbon-based electrodes are currently in widespread use in electroanalytical chemistry for many different applications. this is due to their large potential window, low cost, rich surface chemistry, low background current, chemical inertness, and congruity [29‒35]. carbon is a distinctive and necessary element in our world. remarkably, for the human body, it is the second most common element after oxygen [36]. carbon science is very popular today in the field of nanoscience, materials science, engineering and technology. carbon nanostructures are considered as different low-dimension allotropes of carbon including graphite, activated carbon, carbon nanotubes (cnts), and graphene [37,38]. most of recently reported data on the application of these carbon materials for the electrochemical sensing of some commonly applied drug molecules (paracetamol, diclofenac, 5-fluoro uracil, cetirizine, and salbutamol), are discussed in this review. p. a. pushpanjali et al. j. electrochem. sci. eng. 11(3) (2021) 161-177 http://dx.doi.org/10.5599/jese.999 163 carbon-based sensors for electrochemical study of drug molecules electrochemical analysis of paracetamol paracetamol (pc) is one of the most frequently using painkillers to relieve pain, and it shows both analgesic and antipyretic activities [39]. electrochemical sensing of pc is known to be very interesting due to its routine usage for various treatments. usually, pc has no adverse side effects as it is entirely metabolized into inactive metabolites, which can be quickly excreted by urine [40]. however, the overdose of this drug may lead to inflammation of the pancreas, depletion of glutathione, fulminating hepatotoxicity, kidney problems and nephrotoxicity [41]. the possible redox reaction of pc at an electrode surface is shown in scheme 1. scheme 1. possible redox reaction of pc at electrode surface in regard to electrochemical investigation of pc, charithra et al. [42] investigated the electrochemical activity of pc at carbon nanotube paste electrode (cntpe) modified with poly (methyl orange) using cv and dpv techniques, in phosphate buffer solution (pbs) of ph 7.5. cv response produced well resolved redox peaks at 0.35 and 0.21 v vs. sce, due to the diffusion (dfn) controlled redox reaction of pc at the electrode surface. linearity between anodic peak current and concentration of pc was procured in the concentration range of 2.0×10-6 to 5.0×10-5 m with limit of detection (lod) of 3.8×10-8 m. the offered sensor showed excellent stability (sbt) of 94.0 %, suitable reproducibility (rpd) (relative standard deviation (rsd) = 3.5 %) and repeatability (rpt) (rsd = 2.2 %). the developed electrochemical sensor showed brilliant sensing performance for the determination of pc, even at simultaneous presence of dopamine (da) and folic acid (fa). the developed electrode was successfully applied to detect pc in some pharmaceutical formulations. in the research performed by zhang et al. [43], a new electrochemical sensor was fabricated for the detection of pc, based on glassy carbon electrode modified with poly (caffeic acid) and zn/nizif-8-800. cv and dpv techniques were applied in pbs of ph 7.5, and the fabricated electrode was additionally characterized by electrochemical impedance spectroscopy (eis), x-ray diffraction (xrd) and scanning electron microscopy (sem). the electrochemical reaction taking place at the electrode surface was found adsorption (ads) controlled. the electrode exhibited good reproducible characteristic with rsd of 2.5 %. dpv responses were recorded by increasing the concentration of pc from 0.08 μm to 1000 μm and lod was evaluated as 0.029 μm. the developed novel electrochemical sensor was successfully applied for the determination of pc in medicinal dosage forms, and human urine samples with satisfactory recoveries. http://dx.doi.org/10.5599/jese.999 j. electrochem. sci. eng. 11(3) (2021) 161-177 carbon-based sensors for drug molecules 164 the study was conducted by gholivand and his co-researcher [44] for the determination of pc in pbs of ph 7.0 using glassy carbon electrode modified by polyluminol/functionalized multi-walled carbon nanotubes (mwcnts) and applying cv and swadsv techniques. the redox reaction of pc at the electrode surface was found ads controlled. the prepared electrode surfaces were additionally characterized by sem and eis techniques. under optimized conditions, two linear concentration ranges (0.04-32.2 and 32.2-172.2 μm) were obtained, and lod was determined from lower concentration range as 0.025 μm. the prepared electrode exhibited high sbt of 95.0 %, and high rpt (rsd = 2.1 %). the applicability of projected sensors was checked by determinations of pc in serum and urine samples. as reported by charithra et al. [45], pc was electrochemically studied using cntpe modified with poly (alizarin carmine) sodium lauryl sulfate, having electroactive surface area (esa) of 0.04 cm2. cv technique was applied in pbs of ph 7.0. the electrode reaction was dfn controlled, and exhibited enhanced sbt (94.0 %), rpt (rsd = 3.5 %) and rpd (rsd = 4.1 %). the proposed electrode was successfully utilized to detect pc, da and estriol simultaneously. a fine linear fit was obtained at pc concentrations from 4.0-100 μm, and lod of 0.06 μm was accomplished. the developed sensor was effectively applied for tablet sample analysis. the analytical characteristics of some other carbon-based sensors utilized in the study of pc and reported recently in the literature, are summarized in table 1. table 1. analytical performances of different carbon-based electrodes for pc determination electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % sa ref. ch-cpe swv 0.097 6.6–500 0.66 dfn [46] tc8a/aunps/mwcnt dpv cv 1–150 1‒100 0.2 0.5 ads 85.6; -; 4.4 da [47] nf/go-coopd/gce amp 0.15 0.04-800 0.012 dfn 97.3; 1.6; 2.81 [48] mwcnts/graphite/gce swv 0.47–13.2 0.157 dfn -; 2.05; 1.35 [49] aunps@tc8a/gn/gce dpv 0.5-150 0.1 85; -; 4.4 da [50] gpucspe cv 1.00–100 0.818 [51] nano-tio2/[bmim]bf4 ionic liquid /gce swv 0.05-50 0.01 -; 4.8; 5.02 [52] graphene-chitosan nanocomposite/gce dpv 0.124 1.00–100 0.3 ads 96.4; 3.0; 2.5 [53] p(4aba)/ecrgo/gce dpv 0.27 0.1-65 0.01 ads 94.8; 4.0; 3.5 da [54] pedt/goc/gce cv 10-60 0.57 [55] graphene-modified carbonpaste electrode swv 2.5-143 0.6 ads [56] poly(taurine)/tio2-graphene nanocomposite/gce dpv 0.1-90 0.5 dfn 95.75; 3.29; .81 caffeine [57] poly(caffeic acid)/gce swv 0.2-10 0.026 ads -; 3.7; aa, da, ua, p-amino phenol [58] fe3o4@sio2nanoparticles– coated pdadmac cnt/gce dpv 10-110 0.039 [59] tio2–graphene/poly (methyl red) composite film/gce dpv 0.25-50 0.025 ads 96.48; 1.92; 2.82 [60] mwcnt-alumina coated silica nanocomposite/gce swv 0.05-2.0 0.05 dfn [61] nafion/tio2–graphene/gce dpv 1-100 0.21 ads 93.0; -; 3.6 aa, da [62] graphene/gce swv 0.1-20 0.032 ads 94.5; -; 4.6 aa, da [63] pencil graphite electrode atsdpv 0.05-2.5 0.025 -, -; 3.1 [64] erglnionp/gce dpv 0.04-100 0.02 dfn 91.4; -; 2.0 [65] swcnt–gnh /gce dpv 0.099 0.05-64.5 0.038 dfn 95.0; 3.7; 2.5 da [66] swcnt/cce dpv 0.04-8.5 0.025 dfn 98.0; -; 4.0 [67] mcngpsce dpv 0.02-0.1 0.034 [68] p. a. pushpanjali et al. j. electrochem. sci. eng. 11(3) (2021) 161-177 http://dx.doi.org/10.5599/jese.999 165 electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % sa ref. rgcabe npv 6.6-66.0 0.132 98.0; 2.6; [69] n-(3,4-dihydroxyphenethyl)-3,5-dinitrobenzamide mwcnt/cpe dpv 0.084 15-270 10.0 97.0; 2.2; n-acetyl cystine [70] gce dpv 4-100 0.36 dfn -; 1.39; 1.94 [71] zro2/cpe dpv 1-2500 0.912 dfn fa, noreinephrine [72] poly(patton and reeder’s reagent) /mcpe dpv 0.7-100 0.53 [73] ch-cpe: coffee husks modified carbon paste electrode; tc8a/aunps/mwcnt:thiolated calix [8] arene/gold-nanoparticles/multi-walled carbon nanotubes; nf/go-coopd/gce: nafion/graphene oxide carboxylated pd/glassy carbon electrode; gpucspe: graphite-polyurethane composite/screen printed electrode; p(4aba)/ecrgo: poly(4-aminobenzoic acid)/electrochemically reduced graphene oxide; pedt/goc: poly(3,4ethylenedioxythiophene) /graphene oxide composites; pdadmac: poly(diallyldimethylammonium chloride); aunps@tc8a/gn/gce: gold nanoparticles (aunps) modified by thiolated calix[8]arene graphene nanosheets; atsdpv: adsorptive transfer stripping dpv; erglnionp: electrochemically reduced graphene loaded nickel oxide nanoparticles; swcnt-gnh: single walled carbon nanotubes-graphene nanosheet hybrid; cce: carbon ceramic electrode; mcngpsce: micro crystalline natural graphite polystyrene composite electrode; rgcabe: renewable glassy carbon annular band electrode; amp: amperometry; rds: rate determining step; sa: simultaneous analysis; aa: ascorbic acid; ua: uric acid electrochemical determination of diclofenac diclofenac (dcf), commonly designated as voltaren, is a non-steroidal anti-inflammatory drug taken to reduce inflammation and pain [74]. it is commonly given in clinical medication for the treatment of tuberculosis, urinary tract infection and menstrual pains [75]. on the other side, dcf might cause life-threatening problems, like cardiac arrest and stroke, particularly when the patient is using it for a long time. it may also trigger harmful effects on humans, such as aplastic chlorosis, gastrointestinal problems and changes in kidney and liver functions [76]. therefore, it is essential to determine trace amounts of dcf in biotic and medication samples, for development and treatment purposes. scheme 2 represents the redox reaction of dcf at an electrode surface. scheme 2. proposed redox mechanism of dcf at electrode surface in this regard chethana et al. [77] studied the electrochemical action of dcf using tyrosinemodified carbon paste electrode (esa = 0.014 cm2) by cv and dpv techniques in pbs of ph 7.0. at the prepared electrode surface, the electrochemical reaction of dcf is proceeding through dfn limited step. the resulting peak current values obtained from dpv responses were increased linearly http://dx.doi.org/10.5599/jese.999 j. electrochem. sci. eng. 11(3) (2021) 161-177 carbon-based sensors for drug molecules 166 within dcf concentration range from 10 to 140 μm, with lod of 3.28 μm. the applicability of the designed sensor was checked by determinations of dcf in human urine and pharmaceutical samples with 99.5 % recovery. altai et al. [78] developed a novel electrochemical sensor for the analysis of dcf based on f-mwcnts and gold–platinum bimetallic nanoparticles modified gold electrode. electrochemical properties of the fabricated sensor were evaluated by cv and dpv techniques in pbs having ph of 7.0, and the redox reaction at the electrode surface was found ads controlled. the surface features of the prepared electrodes were additionally examined by sem and energy dispersive x-ray diffraction (edx) methods. the calibration curve of dcf was linearly fitted over the concentration range of 0.5 μm to 1000 μm, giving lod of 0.3 μm. the proposed electrode showed good rpt (rsd = 1.7 %) and rpd (rsd = 2.6 %) towards dcf determination. the prepared sensor was employed to determine dcf content in real samples. pushpanjali et al. [79] described poly(l-methionine) modified carbon nanotube-based platform (esa = 0.043 cm2) for the enhanced sensitive determination of dcf. cv and dpv techniques were used, and the electrode reaction is found ads controlled. the developed electrodes were also characterized by field emission-sem (fe-sem) and (eis) techniques. under the optimal investigation conditions, dcf offered the linear range from 2 μm and 50 μm and lod was assessed as 0.10 μm. the fabricated electrode showed great sensitivity, high sbt (87.0 %), and good rpd (rsd = 3.64 %) and rpt (rsd = 2.97 %). the compliance of the projected sensor was confirmed by the valuation of dfc in tablet samples. analytical performances of some other carbon-based electrodes recently reported in the literature for determination of dcf are summarized in table 2. table 2. analytical performances of different carbon-based electrodes for dcf determination electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % sa ref. dynw/cpe swv 0.01-1.0 0.002 [80] swcnt modified eppge swv 0.02-1.5 0.02 ads 96.18; -; 1.26 [81] eppg swv 0.01-1.0 0.006 ads [82] mwcnt-il/cce dpv 0.36 0.05-50.0 0.018 ads 95.0; 3.0; indomethacin [83] mwcnt-il/cce dpv 0.05-20.0 0.027 dfn 95.0; -; 4.0 [84] mwcnts/cu(oh)2 nanoparticles/il dpv 0.012 0.18-119 0.04 dfn -; 0.99; 2.13 naproxen [85] il/cntpe dpv 0.50-300.0 0.2 dfn 98.0; 1.5; 2.5 [86] dna-go/gce dpv 1.0-130.0 dfn [87] il/mwcntpe swv 0.23 0.3‒750 0.09 dfn 98.0; -; 2.4 [88] cntpe npv 2‒100 0.8 -; -; 1.5 [89] vinylferrocene/mwcntpe swv 5‒600 2 -; 1.9; morphine [90] mwcnts-surfactant composite gce lsv 0.072 0.17-2.5 0.08 dfn 94.4; 2.10; 2.96 [91] cu-doped zeolite-expanded graphite-epoxy electrode dpv 0.3-20 0.05 ads & dfn [92] ion selective electrode ptm 10-10000 4.0 [93] bismuth film electrode amp 6-50 4.3 -; 3.27; [94] pedot/tio2 [bmim]c1/cpe dpv 10-100 0.011 dfn ua, aa [74] core shell nanostructured modified carbon electrode swv 0.083 0.01-300 0.034 ads 97.5; 4.0; 2.4 [75] dynw: dysprosium nanowire modified; eppge: edge plane pyrolytic graphite electrode; il: ionic liquid; dna-go: dnagraphene oxide; pedot: poly(3,4-ethylene-dioxythiophene); bmim: 1-butyl-3-methylimidazoliumchloride; ptm: potentiometry. electrochemical scrutiny of 5-fluorouracil uracil (ur) is a naturally occurring pyrimidine base found in rna [95]. nucleic acid biosynthesis in tumors is preferably carried out using ur. a huge number of ur derivatives are already reported p. a. pushpanjali et al. j. electrochem. sci. eng. 11(3) (2021) 161-177 http://dx.doi.org/10.5599/jese.999 167 as anti-cancer agents, among which 5-fluorouracil (5-fu) is of great importance [96]. 5-fu is amongst the most impactful drugs for the treatment of cancer, and also has antibacterial activity in a biological state [97]. however, an overdose of 5-fu has an adverse effect, including the metabolic build-up, which causes neurotoxicity and has a great impact on morbidity and mortality rates in humans. therefore, it is necessary to determine 5-fu in pharmacological and human fluids [98]. the plausible redox mechanism of 5-fu at an electrode surface is illustrated in scheme 3. scheme 3. probable redox mechanism of 5-fu at electrode surface bukkitgar and his co-researcher [99] studied the electrochemical behavior of 5-fu at cpe modified with glucose in pbs of ph 7.0, using cv and dpv techniques. the redox reaction of 5-fu was found dfn controlled. the experimental results showed the applicability of electrode for electrochemical investigation of 5-fu with lod of 0.0051 μm. the developed electrode exhibited good sensitivity and selectivity towards 5-fu detection. the practical application of the prepared novel electrode was demonstrated for the analysis of the pharmaceutical and urine samples. in this context, jyothi et al. [100] investigated 5-fu at mwcnts – paraffin oil paste electrode (esa = 0.135 cm2) in pbs of ph 7.0, using cv and dpv techniques under optimum conditions. the redox reaction of 5-fu at the electrode surface was observed to be dfn controlled. the peak current values were found linear in the concentration range from 0.1 μm to 5.0 μm with lod of 0.039 μm. the electrode showed the outstanding selectivity and sensitivity. the proposed technique was pronounced appropriate for quality control laboratories, real sample clinical study, and pharmacokinetics. in addition, xin et al. [101] described glassy carbon electrode modified with bromothymol blue and mwcnts (esa = 0.172 cm2), applied in a study of the electrochemical behavior of 5-fu performed by cv method. this reaction was found controlled by ads kinetic. the calibration graph was plotted over the logarithmic concentration range of 0.8 μm to 5000 μm with lod of 0.26 μm. the developed method afforded the benefits of time-savings, high rpt (rsd = 3.7 %), high rpd (rsd = 5.4 %) and simplicity to determine 5-fu in injection samples. rahimi-nasrabadi et al. [102] constructed cpe modified with nanoparticles of praseodymium erbium tungstate (esa = 0.093 cm2) for the sensitive voltammetric detection of 5-fu, using cv and swv techniques. this reaction was found dfn controlled. the linear 5-fu concentration range of 0.01–50 μm was used to evaluate lod of 0.98 nm by swv technique. the developed novel electrode gained an excellent sensitivity, along with improved rpd (rsd = 3.6 %), rpt (rsd = 1.02 %) and short response time. http://dx.doi.org/10.5599/jese.999 j. electrochem. sci. eng. 11(3) (2021) 161-177 carbon-based sensors for drug molecules 168 analytical properties of some other carbon-based sensors already reported in literature for the electrochemical analysis of 5-fu, are collected in table 3. table 3. analytical performances of different carbon-based electrodes for 5-fu determination electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % ref. methylene blue/cpe dpv 0.155 0.1-40 0.00204 dfn -; -;1.4 [103] il/cpe dpv 50-800 0.013 dfn 95.9; 2.3; 3.1 [104] rgo/chitosan/gce swv 0.192 0.01-0.11 0.00493 dfn -; -; 0.54 [105] cpe dpv 0.1-40 0.01225 dfn 97.5; -; 3.5 [96] znfe2o4/ilcpe swv 0.1-1400 0.07 dfn -; 3.9; 3.6 [97] mwcnts/btb/gce cv 0.172 0.8-5000 0.26 [101] gce/ctab dpv 0.02-0.6 0.02013 dfn [106] porphyron/au nanoparticles/cpe dpv 0.075 29.9-234 0.66 dfn -; 2.09; 2.20 [107] gce/p(bcp)/dna dpv 7.6-383.8 2.38 dfn -; 3.9; 3.6 [108] gqd/bpbr/cpe swv 0.26 0.001-400 0.0005 dfn 92.3; -; [119] cunps/mwcnt/il/ chitosan/gce dpv 1-110 0.15 dfn 93.0; 2.5; [110] pnipam/pedot/gce dpv 0.03-182 0.015 (40 °c) 0.37 (25 °c) dfn 98.6; -; 2.21 [111] cpe: carbon paste electrode; rgo: reduced graphene oxide; btb: bromothymolblue; ctab: cetyltrimethyl ammonium bromide; p(bcp): poly(bromocresol purple); gqd/bpbr: graphene quantum dots/1-butylpyridinium bromide; pnipam: poly(n-isopropylacrylamide) 2.4. electroanalysis of cetirizine cetirizine (ctz) is one of the antihistamine drugs that many people have used in medicine and pharmacy [112]. ctz is chemically recognized as 2-[2-[4-[4-chlorophenyl]-phenylmethyl] piperazin1-yl] ethoxy] acetic acid [113]. it is usually prescribed for allergic symptoms such as runny nose, itching, hives, eye irritation, body aches, etc. [114]. however, it is understood that its overdose causes mild drowsiness, headache, dry mouth, and fatigue [115]. therefore, it is essential to determine ctz in biological samples. the reaction mechanism of ctz at an electrode surface is presented in scheme 4. scheme 4. proposed reaction mechanism of ctz at electrode surface as reported by girish et al. [116], ctz was determined via cv method at poly (dl-valine) modified mwcnt paste sensor (esa = 0.13 cm2) in pbs of ph 7.0, and the corresponding reaction at the electrode surface was found dfn controlled. the fabricated sensor showed the linearity range at ctz p. a. pushpanjali et al. j. electrochem. sci. eng. 11(3) (2021) 161-177 http://dx.doi.org/10.5599/jese.999 169 concentrations from 2.0 to 80.0 μm, with lod equal to 0.11 μm. the fabricated sensor was fruitfully used to simultaneously detect ctz, riboflavin and pc. the sensor surface was additionally examined by fe-sem micrographs. the modified sensor exhibited very good rpd (rsd = 1.24 %), sbt (93.15 %) and was used for the assessment of ctz in pharmaceutical pills with satisfactory results. roopa et al. [117] developed mwcnts modified gce (esa = 0.144 cm2) for the determination of the electrocatalytic activity of ctz using cv technique, and the respective electrode reaction is found ads controlled. lod was evaluated by plotting the graph of peak currents versus different concentrations of ctz in the range of 0.5 to 10.0 μm and calculated as 0.07 μm. the offered technique has significant benefits over all other current methods regarding sensitivity, rpd (rsd = 2.48 %), precision, time-saving and minimal susceptibility. the introduced sensitive and convenient electrochemical method was successfully extended to ctz detection in pharmaceutical and urine sample trials. the study carried out by gholivand et al. [118] for the detection of ctz at gce modified with a nanocomposite of chitosan, mwcnts and ionic liquid (esa = 0.086 cm2) using dpadsv technique, showed ctz redox reaction at the electrode surface controlled by ads step. eis, cv and sem techniques were additionally used to characterize the proposed electrodes. the linear fit for ctz determination was obtained over the concentration range of 40 nm–480 μm, with lod of 0.008 μm. the fabricated electrode showed outstanding sbt (96.32 %), rpt (rsd = 1.77 %), rpd (rsd = 2.64 %) and was used for the assessment of ctz in serum and tablet as a real sample study. yaragatti et al. [119] have fabricated graphene oxide modified carbon paste sensor (esa = 0.083 cm2) to analyze the behavior of ctz electrochemically in pbs of ph 6.0, using cv technique. the linear response was observed in the calibration curve for concentrations of ctz in the range of 0.01-0.4 μm with lod of 0.0019 μm. the electrode reaction was found controlled by ads step. modification of cpe with graphene oxide particles exhibited remarkable reproducibility and accuracy. some already reported analytical abilities for a number of other carbon-based electrochemical sensors applied for the electrochemical investigation of ctz are collected in table 4. table 4. analytical performances of different carbon-based electrodes for ctz determination electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % sa ref. mwcnt/ptnps/cpe dpadsv 0.081 0.19-193 0.058 ads pc, phenylephrine [113] nanoclay/cpe swv 0.098 0.1-0.3 0.00848 ads [114] gce cv dpv 20-100 4.3 4.5 dfn [120] cb/gce swadsv 0.49-10.8 0.40 ads -; 4.68; hydroxyzine [121] pretreated graphite pencil electrode swv 0.5-10 0.16 dfn 91.7; -; [122] β-cyclodextrin/gce swv 0.02-6.0 0.00037 [123] ru-tio2/mwcnts swv 0.042 0.03-1.0 0.0031 dfn 97.0; 2.8; 2.6 [124] bentonite clay/cpe dpv 0.083 0.08-1.0 0.059 dfn 97.8; 2.82; 2.5 [125] carbon paste membrane sensor ptm 0.1-60 7.0 -; 0.9; [126] plmcntpe cv 0.039 5-50 0.17 dfn 89.0; 3.29; 3.94 pc [127] plmcntpe: poly(leucine) modified carbon nanotube paste electrode electrochemical study of salbutamol salbutamol (sal) is ([1-(4-hydroxy-3-hydroxymethylphenyl)-2-(t-butylamino) ethanol]) and is also referred as albuterol. sal is selective agonist of 2-adrenergic receptor widely used to treat airway obstruction in patients with bronchial asthma, emphysema, chronic obstructive pulmonary disease, lung disorders and also blood potassium level balance [128‒131]. sal is commonly used in humans as a tocolytic agent and also in veterinary medicine [132]. high doses of this medication can have a lipolytic effect. its presence is most common in liver and meat, and it may be harmful to humans who http://dx.doi.org/10.5599/jese.999 j. electrochem. sci. eng. 11(3) (2021) 161-177 carbon-based sensors for drug molecules 170 are consuming those as a food supplement [133]. however, owing to its misuse as a stimulant and anabolic agent, a high dose of sal is banned in sports. therefore, sal containing inhaler is permitted to use for athletes with asthma or asthma-induced exercise [134]. hence, the determination of sal is of high importance. the oxidation mechanism of sal at an electrode surface is shown in scheme 5. scheme 5. oxidation mechanism of sal at electrode surface wei and his lab mates [135] determined sal in pbs of ph 7.0 using mwcnt film coated gce and swv technique. the electrode reaction with respect to sal was found ads controlled. the response of sal was found linear in the concentration range from 0.8 μm to 10.0 μm with lod of 0.2 μm. the method of detecting sal in pharmaceutical formulations was successfully carried out at the fabricated electrode. the developed electrode showed elevated sbt (96.4 %) and rpt (rsd = 4.2 %). a rapid and effective electrochemical method for the determination of sal was introduced by abdol et al. [136], using iron titanate nanopowder-modified cpe in phosphate nitrate solution having ph 7.5, and dpadsv technique. the electrode exhibited a linear response for sal in the concentration range of 0.2–25 nm with lod of 90 pm. the proposed electrode was found stable (90.8%) and reproducible (rsd 4.32 %). it was used satisfactorily for the detection of sal in human serum samples. meareg et al. [137] determined the electrochemical behavior of sal at poly(4-amino-3-hydroxynaphthalene) sulfonic acid modified gce in pbs of ph 7.0 and cv technique. the electrode reaction of sal was observed to be ads controlled. using dpv technique, the authors obtained the linear dependence of peak current on sal concentration in the range of 0.2–8 μm, with lod of 0.068 μm. the proposed method was successfully applied for the determination of sal in medicinal dosages. alcian blue modified cpe for the voltammetric examination of sal performed by cv technique was reported by chitravathi et al. [138]. the authors gained the linearity in the concentration range of 0.5-18.0 μm, with lod of 0.06 μm. from the scan rate study, the electrode reaction was evaluated to be dfn controlled. the proposed method was successfully extended for sal determination in tablet samples with acceptable recovery. also, the developed electrode was found to be highly stable (97.55 %). analytical properties of some other carbon-based electrochemical sensors, recently reported in the literature for the electrochemical investigation of sal, are collected in table 5. table 5. analytical performances of different carbon-based electrodes for sal determination electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % sa ref. nano-au/mwcntsnf/gce lsv 0.09–7.0 0.05 ads 94.5; 4.6; 4.3 [139] c60/gce swv 0.42–8.35 0.17 dfn [140] p. a. pushpanjali et al. j. electrochem. sci. eng. 11(3) (2021) 161-177 http://dx.doi.org/10.5599/jese.999 171 electrode method esa, cm2 linear range, μm lod, μm rds sbt; rpt (rsd); rpd (rsd), % sa ref. poly taurine/zro2/gce lsv 5–220 0.02 ads -; 4.9; 3.12 ractopamine [141] smwcnt-nf/gce amp 0.10-0.30 0.10 ads 94.7; -; ractopamine [142] gce dpv 3.02-123.0 0.51 [128] graphene-au/gce dpv 0.05-10.0 90.9; 3.1; epinephrine [129] graphene-nf/gce dpv 0.4-30 0.11 dfn 96.99; -; 4.09 clenbuterol [143] p(aminosulfonic acid)/gce cv 2-100 0.65 96.8; 1.85; [144] swcnt/eppge swv 0.21-10.57 0.018 dfn -; .94; 2.40 [145] mno2/rgo@ni foam dpv 0.042-1.463 0.023 96.6; 3.5; ractopamine [146] nscuhcfe-cnt/gce cv 5-25 70.0; -; [147] pt-graphene-pt/gce dpv 0.682 0.03-180 0.009 dfn 96.0; -; 3.06 [148] graphene-pedot/pss cv 5-550 1.25 95.5; -; 7.5 [149] mip/cpe dpv 0.001-0.055 0.0006 [150] agpd/anti-sal/gce lsv 0.00004-0.41 0.000006 -; 4.7; 4.7 clenbuterol; ractopamine [151] smwcnt: mixture of swcnt and mwcnt; nscuhcfe: nanostructured copper hexacyanidoferrate; pss: poly(styrene-sulfonate). conclusions this article describes some recent developments of carbon-based electrodes for their possible utilization as sensors for drug molecules. as a high-performance material, carbon and its derivatives occupy a special place in pharmaceuticals, because of their extremely beneficial properties in many ways. the purpose of this review is to demonstrate the application of carbon-based electrodes for the analysis of some drug molecules, what can be also used in proof-of-principle research studies of pharmaceutical compounds in their dosage forms and biological samples. several forms of carbonbased electrodes have emerged over the last few years for measurements of diverse targets in biologic fluids and different dosage forms. the synergetic impact of carbon-based materials with some combination of various modifiers such as metal nanoparticles, conductive polymers, or surfactants, enhances the competence of the electrode in the electrochemical determination of bioactive and electroactive molecules. the utilization of carbon-based materials in the development 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https://doi.org/10.1016/j.bios.2013.04.041 https://creativecommons.org/licenses/by/4.0/) external control of anodic dissolution mechanisms of 100cr6 in nitrate/chloride mixed electrolytes doi: 10.5599/jese.2011.0004 39 j. electrochem. sci. eng. 1(1) (2011) 39-54; doi: 10.5599/jese.2011.0004 open access : : issn 1847-9286 www.jese-online.org original scientific paper external control of anodic dissolution mechanisms of 100cr6 in nitrate/chloride mixed electrolytes andreas lesch, gunther wittstock, chris burger*, benjamin walther* and jürgen hackenberg* department of pure and applied chemistry, cis center of interface science, faculty of mathematics and natural sciences, carl von ossietzky university of oldenburg, d–26111 oldenburg, germany *robert bosch gmbh, stuttgart, germany corresponding author: gunther.wittstock@uni-oldenburg.de; tel.: +49 441 7983971; fax: +49 441 7983979 received: june 17, 2011; published: august 20, 2011 abstract the anodic dissolution of 100cr6 steel in neutral electrolytes containing sodium chloride and sodium nitrate was investigated potentiodynamically and galvanodynamically with a rotating disc electrode at room temperature. the total concentration of the mixed electrolyte was 3 mol l-1 with variation of chloride/nitrate mole ratios. the potentiodynamic linear sweep voltammograms (lsvs) in mixed electrolytes are similar to the lsvs in pure chloride electrolyte at lower current densities and switch to behaviour observed in pure nitrate electrolytes at higher current densities. provided that both anions are present, it seems that the dissolution reactions at the steel anode are determined by the interface layer only. the effect of these layers on surface quality and current efficiency was also investigated in a flow channel applying galvanostatic pulses. an evidence for different dissolution mechanisms can be seen with an important influence of duty cycle and flow conditions. this allows external control of the desired dissolution mechanism in mixed electrolytes. keywords electrochemical machining; mixed electrolyte; anodic dissolution; rotating disc electrode; flow channel; galvanostatic pulses 1. introduction electrochemical machining (ecm) is often applied to deburring and shaping components made of e.g. iron and steels in aqueous inorganic salt electrolytes[1,2]. shape and surface of work pieces are modified by high rate anodic dissolution at high current densities (10-150 a cm-2) with a special j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 40 designed cathode as tool. the resulting gap width is usually below one millimetre. high electrolyte flow velocities (5-50 m s-1) are required to remove reaction products and joule heat. the precision of pulsed ecm is higher compared to direct current ecm because reaction products can be removed during pulse off time [3]. sodium chloride and sodium nitrate are often used as electrolytes in industrial ecm because of low costs and labour safety aspects. anodic dissolution of iron and many steels in pure sodium chloride and in pure sodium nitrate electrolytes has been investigated in great detail [4-9]. the dissolution mechanisms in both electrolytes differ strongly and result in very different features of the machined work piece. in chloride media an active anodic dissolution takes place whereas nitrate anions cause a passivation of the substrate surface and transpassive dissolution can be observed. methods of characterizing the dissolution mechanism are anodic polarization curves, analysis of the resulting surface quality and weight-loss measurements in order to determine the current efficiency. the anodic dissolution of pure iron in chloride electrolyte is divalent over a wide range of current density and the current efficiency is closely to 100 % [5]. hydrogen is developed in the counter reaction at the cathode [10]. in nitrate media the evolution of oxygen takes place as a second import anodic reaction and the anodic dissolution of iron leads simultaneously to fe2+ ions and fe3+ ions [8,11,12]. at low current densities the iron dissolution efficiency is close to zero because the anodic current originates from oxygen evolution. with increasing current density the current efficiency of the anodic iron dissolution increases up to 90 % but oxygen evolution is still present [11]. the reduction of nitrate anions takes place at the cathode [1]. the anodic dissolution of iron and many steels is mass transport-controlled and reaction products enrich at the anode interface [5,6]. electrochemically inert metal carbides cause an apparent higher current efficiency for carbide-containing steels because the measured weight-loss includes the mass of carbide particles that were detached but not dissolved during the ecm treatment [13,14]. qualitative anodic dissolution models explain the observed dissolution behaviour and the resulting surfaces. a two-layer model explains the dissolution in nitrate electrolyte [8,15,16]. because of the current density-dependent divalent/trivalent dissolution of iron, different iron oxides (fe2o3 and fe3o4) are formed and cover the steel surface as a thin film. an adherent liquid polishing film consisting of supersaturated iron nitrate hydrates is built at the oxide/electrolyte interface and causes a decreasing oxygen evolution rate [11,16]. in chloride media a weakly attached, easily removable black, carbide-rich solid surface film and a thin polishing film consisting of fecl2, fe(oh)2 and feo (fexoyclz) are formed at the active-steelanode/electrolyte interface [14]. in nacl electrolyte the dissolution rate is higher than in nano3 electrolyte due to active dissolution and high current efficiency while in nano3 electrolyte a much better dimensional control can be reached. the anodic dissolution of mild steel in chloride/nitrate mixed electrolytes has been described by hoare, laboda and mao [17,18]. from their steady-state anodic polarization curves, film stripping and current efficiency measurements, they concluded that chloride anions cause only a localized attack of passive films formed in the presence of nitrate anions on the steel surface. depending on the chloride/nitrate ratio, the chloride anions lower the oxidation power of the nitrate anions, prevent the formation of a compact, protective film and only a non-protective salt film is present and the resulting surfaces are severe cratered. at lower potentials an active dissolution caused by the chloride anions takes place while passivation and transpassive dissolution dominates the behaviour at more positive potentials. with increasing chloride/nitrate ratio, the work piece surface is passivated at higher potentials [18]. chloride/nitrate mixed electrolyte have been used andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 41 in pulse and pulse reverse through-mask ecm of channels for flow field in stainless steel bipolar plates for pem fuel cells [19], where the process improved the surface roughness of the channels at the expense of the process time and the rib/channel width uniformity compared to the process in pure chloride electrolyte. according to the reported results from mao et al [18] we describe in this paper more detailed insights and process conditions under which the dissolution mechanism of soft annealed steel in chloride/nitrate mixed electrolytes can be externally controlled, i.e. switched between two mechanisms. the influence of varying flow conditions at a rotating disc electrode and in a flow channel as well as the influence of applied potentiodynamic, galvanodynamic and galvanostatic pulse series show clearly the possibility to switch in one and the same electrolyte exclusively by external control (current or voltage, hydrodynamics) between both dissolution mechanisms allowing fast machining rates using the chloride mechanism and high precision by the nitrate mechanism in order to obtain crater-free surfaces. experimental specimens spheroidization-annealed 100cr6 cylinders (annealed to globular cementite, 5 mm diameter) were used as specimens from which the cross sectional surfaces were investigated. the 100cr6 steel is mainly composed of approximately 96 wt. % iron, 1.5 wt. % chromium and 1 wt. % carbon. carbon is contained as c in the steel matrix (20 % of total c content), as globular fe3c (40 % of total c content) with diameters of approximately 1-3 μm and as 100 nm-slices fe2.5c (40 % of total c content). the metal carbides were considered to be electrochemically inert under the applied conditions. directly before and after the experiments the specimens were washed with distilled water and isopropanol. electrolytes aqueous solutions of sodium chloride and sodium nitrate served as electrolytes. the total salt concentration ctot was set to 3 mol l -1 with variation of chloride/nitrate mole ratio. the chloride/nitrate concentration ratio is given as the relative chloride concentration θ (eq. 1). c θ c − = cl tot (1) the electrolyte temperature was (23±1) °c. with increasing θ the measured conductivity increases from 156 ms cm-1 (θ = 0) up to 194 ms cm-1 (θ = 1). rotating disc electrode (rde) the measurements of potentiodynamic and galvanodynamic scans or cyclic voltammograms (cvs) were performed with a rotating disc electrode and potentiostat voltalab 80 pgz402 & voltamaster4 software (radiometer analytical sas). the front surfaces of the 100cr6 specimens were ground mechanically with sic papers and the cylinder barrels were masked with an insulating galvanic polyester tape before each measurement. these samples were used as working electrodes (we) in a three electrode cell completed by a platinized titanium net as counter electrode (ce) and an ag/agcl/sat. kcl reference electrode (re). we and ce had an electrode distance of approximately 1 cm. the scan rate for potentiodynamic experiments was set to 50 mv s-1. in this paper all potentials e are referred to the standard hydrogen electrode (she). under the j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 42 typical rotation rate of 3000 rpm laminar flow conditions are expected. incipient turbulences must, however, be expected because of grinding marks at the work piece surface, roughness due to anodic dissolution and recess of the surface below the galvanic polyester tape used to mask the side walls of the sample cylinders. flow channel a flow channel in form of a parallel plate reactor was used. the flow velocity was set to 7 m s-1 assuming turbulent flow conditions [20]. mechanical grinding was done for preparation of the specimens. a steel cathode with 5 mm diameter was positioned directly opposite to the 100cr6 specimen with a gap of 0.75 mm before start of anodic dissolution. the electrodes were fixed and, therefore, the gap width increased during experiments. with this two-electrode configuration galvanostatic pulses up to 40 a cm-2 were applied with a custom made galvanostat (plating electronic gmbh, germany). no current was applied during pulse off time. the pulse times were in the range of milliseconds. the duty cycle γ is on on off t γ t t = + (2) ton is the pulse on time and toff is the pulse off time. an averaged current id can be calculated with γ and pulse current ip (eq. 3 [21]). on d p p on off t i i i γ t t = ⋅ = ⋅ + (3) multiplying id with measuring time ttot gives flowing charge q (eq. 4): q = id·ttot (4) the theoretical weight loss δmtheo was calculated by the faraday law (eq. 5): d tot theo i t m q m m z f z f δ ⋅ ⋅ ⋅ = = ⋅ ⋅ (5) where m is the molar mass of iron and f is the faraday constant. the dissolution valence z was taken as 2 in all calculations knowing that the valence is between 2 and 3 in nano3 electrolyte depending on current density [11]. current efficiency η is then given by equation 6: tot theo 100 δm η %. δm = ⋅ (6) where δmtot is the total experimental mass loss of the specimens which was obtained as mass difference before and after the dissolution. duty cylces γ of 0.625, 0.5 and 0.167 were applied with charges q between 50 and 300 c. surface characterisation observation with optical microscopes was used for rough classification of the dissolution mechanisms. the topography of the surface was measured with confocal microscope nanofocus μsurf (nanofocus ag), for which regions in the middle of the surface were chosen at which minor streaming artefacts occur. the 3d average roughness sa was then calculated with software nanoexplorer xt. the topography was filtered in waviness and roughness by a robust gauss-filter andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 43 with a cut-off wavelength of 0.25 mm. sa is the arithmetic mean of the absolute distances of the surface points from the mean plane and is given by equation 7: ( )a 1 n l j i j i s z x ,y nl =  (7) n and l are the indexes of measured points in xand y-direction. this method facilitates a relative comparison of the resulting surface roughnesses. additionally, the resulting surfaces were examined with scanning electron microscope (sem) evo 50 vpx (carl zeiss ag). an everhartthornley-detector and an acceleration voltage of 20 kv were used for 1000× enlargement with a work distance of 10 mm. results and discussion sharp transition from active to transpassive dissolution at resting electrode linear sweep voltammograms (lsv) were recorded between -0.5 v and 7.2 v. figure 1 shows the lsvs in pure nacl electrolyte (θ = 1), in pure nano3 electrolyte (θ = 0) and in mixed electrolytes with θ = 0.67, 0.5 and 0.33 at a resting electrode. figure 1. lsvs in 3 m nacl, 3 m nano3 and in three chloride/nitrate mixed electrolytes; static we; scan rate 50 mv s-1; relative chloride concentrations θ: 1 (1), 0.67 (2), 0.5 (3), 0.33 (4), 0 (5). the curves of the pure electrolytes show the well known behaviour of active (nacl) and transpassive dissolution (nano3) [1]. in nacl electrolyte a limiting current is reached because the concentration of reaction products increases to saturation. thus a limiting current plateau is built until the overlimiting current is reached and the current increases again. in nano3 electrolyte fluctuating current densities are caused by the evolution of oxygen at the we which sets in at 1.8 v. the curves that were measured in the three mixed electrolytes overlap with each other. they agree qualitatively with earlier reports about anodic dissolution in mixed electrolytes [18]. surprisingly, there is no strong influence of the relative chloride ion concentration on the anodic dissolution at the resting we in figure 1. at low potentials active dissolution takes place, indicated j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 44 by an increasing current density from 0 v on. the surface is suddenly passivated at approximately 1.6 v and the current density decreases to nearly 0 a cm-2, oxygen evolution starts at 1.8 v and transpassive dissolution sets in at higher current densities. with respect to the models of metal/electrolyte interface [11,14,16] the following mechanistic model can be proposed. at lower current densities which occur at potentials below 1.6 v at the resting electrode, the dissolution mechanism must be dominated by the chloride anions and the lsv is very similar to the curve in pure chloride solution. in this paper this section of lsvs will be called “chloride curve”. due to active dissolution, the chloride polishing film is formed. the nitrate anions are larger than chloride anions. most likely their incorporation in the polishing film changes the structure of the polishing film so that no diffusion controlled current regime (plateau in the pure nacl electrolyte) is observed in mixed electrolytes. the higher the current density, the higher is the active dissolution rate of the steel. chloride anions have to diffuse from the bulk of the electrolyte through the diffusion layer to the steel surface. when the current density becomes higher than approximately 1.4 a cm-2 not enough chloride anions reach the steel surface. therefore, the nitrate polishing film composed of iron nitrate hydrates is formed. this film could withdraw the water from the chloride polishing layer. hence, iron chloride could be precipitated and could be displaced by the nitrate polishing film and the anodic dissolution is dominated by the nitrate anions at potentials higher than 1.8 v. hence, the transpassive dissolution sets in at higher current densities at e > 1.8 v and this section of lsvs is called “nitrate curve” below. the transition from active to transpassive dissolution can also be observed visually (see figure sm-11 in supporting information). during active dissolution the assumed reaction products creep down as a green viscose film. after passivation, the evolution of oxygen breaks off the visible flakes of the black carbide-containing layer from the anode surface. it is suggested that this surface layer is equal to the one in pure nacl electrolyte. haisch et al. [7] investigated the surface layer obtained in pure nacl electrolyte by energy dispersive x-ray microanalysis. they reported that the particles of this layer mainly consist of fe, cr and c and suggested that they originate from the carbides in the steel matrix [7]. transpassive dissolution can be observed by the formation of oxygen bubbles and yellow-brown colouration of the solution. influence of anion transport on the transition from active to transpassive dissolution at rde for further experiments the we was operated as rde in order to improve the removal of oxidation products and simulate the electrolyte flow under technical ecm conditions. this leads to an increasing of limiting current in nacl electrolyte [22] and to a less fluctuating current density in nano3 electrolyte in corresponding lsvs at 3000 rpm (figure 2). in pure 3 m nacl electrolyte no limiting current plateau is observed within the resulting current density range (curve 1). the electrolyte changes its colour from colourless to green and green precipitation occurs. in pure nano3 electrolyte the current density rises linearly at 1.8 v (curve 14). because transpassive dissolution sets in at even higher current densities, the colourless electrolyte changes to yellow-brown and yellow-brown precipitations are observed. with decreasing θ, the activation potential for the chloride dominating activation potential is shifted to more anodic potentials. the nitrate anions disturb the activation [23]. with respect to the proposed mechanistic model, increasing θ leads to higher current densities at which the change of dominating mechanism occurs due to the transport of chloride anions from the bulk of the electrolyte through the diffusion layer to the steel surface. with increasing θ formation of the chloride polishing film can be maintained to higher current densities until replacement by andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 45 the nitrate polishing film and commencement of transpassive dissolution. in this paper the term passivation is not used for this phenomenon because the current density does not fall to nearly zero for θ ≥ 0.07. transpassive dissolution sets in directly. the lsvs in mixed electrolytes with θ > 0.83 indicate no sharp decrease in current density. both mechanisms compete and the relatively small concentration of nitrate anions cannot create an abrupt change of the dominating mechanism. curve 13 shows that for θ = 0.03 the flux of chloride ions is too small to induce and maintain active anodic dissolution in comparison with θ = 0.05 (curve 12). the insert in figure 2 shows separate forward and backward lsv in mixed electrolyte with θ = 0.5 arranged like a cv. the backward lsv corresponds completely to the backward lsvs which are obtained in pure nano3 electrolyte due to stable transpassive dissolution. figure 2. lsvs; rotation rate of we 3000 rpm; scan rate 50 mv s-1; relative chloride concentrations θ: 1 (1); 0.9 (2); 0.87 (3); 0.83 (4); 0.77 (5); 0.67 (6); 0.5 (7); 0.33 (8); 0.13 (9); 0.1 (10); 0.07 (11); 0.05 (12); 0.03 (13); 0 (14). insert: forward and backward lsv in mixed electrolyte with θ = 0.5. the transport of chloride anions and, therefore, the transition from chloride curve to nitrate curve can also be controlled by rotation rate. increasing the rotation rate for a given θ = 0.5 increases the current density at which the dissolution mechanism changes (figure 3) due to a higher flux of chloride ions to the surface. this is in agreement with the proposed mechanistic model. the mass flux of chloride anions jcl to the disc surface can be estimated by using equation 8 [24]: j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 46 figure 3. lsvs in chloride/nitrate mixed electrolyte; θ = 0.5; scan rate 50 mv s-1; rotation rates: 0 rpm (1); 100 rpm (2); 500 rpm (3); 1000 rpm (4); 2000 rpm (5); 3000 rpm (6), 4000 rpm (7). 2 3 1 6 1 2 cl cl cl 0 62 / / /j . d ν ω c− − − −= ⋅ ⋅ ⋅ ⋅ (8) where dcl= 2.032·10 -5 cm2 s-1 [25] is the diffusion coefficient of clanions at infinite dilution in water. the cinematic viscosity ν = 1·10-2 cm2 s-1 [24] is assumed to be the one of water. ω is the angular velocity of the rde and cclthe bulk concentration of chloride anions. with equation 8 only the diffusion is considered whereas migration is neglected. in the mixed electrolyte with θ = 0.5 the concentration of chloride is 1.5 mol l-1. with rotation rate 3000 rpm and ω = 2·π·3000 rpm = 314.2 s-1, jclis 2.64·10 -5 mol cm-2 s-1 calculated from equation 8. this calculation does not consider the larger existing nitrate anions in the solution. the nitrate anions could act as obstacles for the chloride anions flux but migration could lead to an increase of jcl-. therefore, the calculated mass flux of chloride anions is only an approximation. the dissolution rate of iron jfe2+ at the surface can be calculated by using equation 9, which is derived from faraday law (eq. 5): 2 cl ,lim fe j j f z − + = ⋅ (9) where jcl-,lim = 3.6 a cm -2 is the current density at which the active dissolution mechanism cannot be maintained any longer. the calculated value jfe2+ = 1.87·10 -5 mol cm-2 s-1 is in agreement with the proposed model considering that a fe2+ flux of 1.87·10-5 mol cm-2 s-1 would require a clflux of 2·1.87·10-5 mol cm-2 s-1 to form stoichiometrically fecl2. the active dissolution in mixed electrolytes is mass transport-controlled with respect to chloride ions. from the lsv experiments it is concluded that under rde conditions a sharp distinction is possible between active (chloride anion-dominated mechanism) and transpassive (nitrate anion-dominated mechanism) anodic dissolution mechanisms. andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 47 switching between both mechanisms by potential control at rde sequential lsvs which means in this paper that several lsvs are applied directly one after the other show that switching between the two dominating dissolution mechanisms is easily possible by potential control. the start potential can be chosen in the area of the chloride or the nitrate curve. figure 4. in sequence (1) to (5) directly successive measured lsvs with potential limits 4.2 v to 5.2 v (1); -0.5 v to 1.9 v (2); 4.2 v to 5.2 v (3); 0.7 v to 1.7 v (4); 4.2 v to 5.2 v (5); complete lsv (6); θ = 0.5; rotation rate 3000 rpm; scan rate 50 mv s-1. insert: sequence (1) enlarged. curve 1 of figure 4 starts in the potential region of the nitrate curve. after a short initial rise of the current density j (figure 4 insert), j decreases and the curve follows the nitrate curve. the initial rise of j after switching from active to transpassive dissolution is reproducibly observed (curves 3, 6 in figure 4). the reason of this effect is still unknown. maybe chloride anions and nitrate anions which are initially present at the anode cause a situation where both mechanisms take place simultaneously. after the chloride anions are consumed, the dissolution follows the nitrate curve. curve 2, initiated at 0 v, was applied directly after curve 1 and follows exactly the chloride curve. the surface is activated. stepping the potential to 4.2 v, i.e. to a section of the lsv dominated by the transpassive dissolution, causes a change of dominating mechanism indicated by the large decrease of current density within a very short transition time (curve 3). the resulting current density after that switch is so high that the consumption of chloride ions cannot be maintained by mass transport and active dissolution ceases. curve 4 starts within the section of the chloride curve (at 0.7 v). an instantaneous slope can be seen before the current density increases linearly with increasing potential. this slope fits to the chloride curve in the complete lsv but the current density is approximately 1 a cm-2 lower. it seems that the chloride curve is shifted to more anodic potentials for presently unknown reasons. one explanation could be that an acidification caused by oxygen evolution at anode/electrolyte interface influences the processes and shifts the chloride curve. a passivation is observed at approximately 1.5 v in this curve. so the next linear sweep in section of nitrate curve is following the expected behaviour of transpassive dissolution (curve 5). j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 48 switching between both mechanisms by current density control at rde programmed current chronopotentiometry (pcc) [26], i.e. the linear variation of the current in a galvanodynamic sweep experiment gave similar results like lsvs (see figure sm-12). at low current densities active dissolution takes place in mixed electrolytes and the potential increases linearly with increasing current density. this section of pcc fits to the pcc in pure nacl electrolyte. when a certain current density (depending on chloride mass transport) is reached the mechanism switches to transpassive dissolution which is indicated by an abrupt potential increase. from then on, the potential increases linearly with current density like the pcc in pure nano3 electrolyte. this is in agreement with the proposed mechanistic model. an example for switching between both dominating mechanisms by current density control is shown in figure 5. figure 5. in sequence (1) to (5) directly successive measured pccs with current density limits from 0 a cm-2 to 1 a cm-2 (1); 4.6 a cm-2 to 5.1 a cm-2 (2); 0 a cm-2 to 1 a cm-2 (3); 4.6 a cm-2 to 5.1 a cm-2 (4); 0.3 a cm-2 to 1 a cm-2 (5); complete pcc (6); θ = 0.5; rotation rate 3000 rpm; scan rate 51 ma cm-2 s-1. curve 1 starts at current density of 0 a cm-2 and active dissolution sets in immediately. a current density step to 4.6 a cm-2 leads to a change of the dominating mechanism to transpassive as expected (curve 2). this is indicated by an abrupt increase of potential. stepping back to 0 a cm-2 (curve 3) leads to a surprising effect. passivity of the surface does not break down and transpassive dissolution sets in. after a current density ramp in the range of transpassive dissolution (curve 4) a slight cathodic current density of 0.3 a cm-2 leads to an activation of the steel surface (curve 5). curve 6 shows a complete pcc. from the lsv experiments it is concluded that not only a sharp transition from active to transpassive anodic dissolution due to mass transfer rate of chloride anions occurs, but switching between the dominating mechanisms by potential and current density control is possible as well. flow channel experiments flow channel experiments permit a more realistic modelling of ecm experiments under various flow conditions using galvanostatic pulses. the anodic dissolution in pure nacl electroylte, in pure andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 49 nano3 electrolyte and in mixed chloride/nitrate electrolytes was investigated. it was assumed that flow conditions in flow channel are more turbulent than in rde experiments. the more turbulent flow causes a thinner diffusion layer and better mass transfer of chloride ions. therefore, the chloride-dominated mechanism can be maintained at higher current densities, or the chloridedominated mechanism at a given current density should be sustained by an electrolyte with a lower θ. this hypothesis was tested with relative low values of θ of 0.07, 0.17 and 0.33 by determining the current efficiencies according to equation 6. the dissolution valence z was set to 2. because of other probable reactions, such as the formation of trivalent iron and the formation of trivalent chromium species, the experimental results are useful only for relative comparisons between similar experiments of the same material. experiments were performed for pulse current densities j of 5, 10, 20 and 40 a cm-2 (figure 6). figure 6. current efficiencies η with γ = 0.625 ((a), 300 c), 0.5 ((b), 100 c) and 0.167 ((c), 100 c); relative chloride concentrations θ: 0 (1); 0.07 (2); 0.17 (3); 0.33 (4); 1 (5); j = 5 a cm-2, 10 a cm-2, 20 a cm-2 and 40 a cm-2; lines of all styles are guides to the eye. the current efficiencies in pure nacl electrolyte (curves 5) and in pure nano3 electrolyte (curves 1) correspond to literature values [11,14,15,20]. in pure nacl electrolyte η is about 100 %. decreasing η at higher j could be caused by additional formation of trivalent iron. in pure nano3 electrolyte it can be distinguished between passivation of the surface, transition zone and j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 50 transpassive dissolution. the region of the transition zone is shifted to higher current densities with lower γ. total experimental mass loss δmtot includes the erosion of electrochemically inert metal carbides. often the ratio δmtot/δmtheo is quoted as current efficiency that might exceed 100 %. from known carbon content and its compounds δmtot can be corrected by subtracting the mass of electrochemical inert components δmc in order to obtain the carbide-free weight loss δm by equation 10 (details in sm-3): tot inert corr theo theo 100 100 δm δm δm η % %. δm δm − = ⋅ = ⋅ (10) for example in pure nacl electrolyte with γ = 0.625 and 0.167 at 5 a cm-2 a η = δmtot/δmtheo of 112 % and 115 % are measured and yields a corrected ηcorr of 100 % and 102 %. there is still a trend of ηcorr being slightly larger than 100 % indicating a more complex dissolution mechanism for 100cr6. with γ = 0.625, the current efficiencies in the mixed electrolytes are rather uniform (curves 2 and 4, figure 6a). at 20 a cm-2 and 40 a cm-2 η is about 87 % and is approximately the same as in pure nano3 electrolyte. this is in agreement with the results of a microscopic inspection of the resulting surfaces at 40 a cm-2 with θ = 1, 0.33 and 0 (figure 7a-c). figure 7. optical and sem micrographs of the resulting surfaces with γ = 0.625. electrolyte flow direction in optical micrographs and in sem micrographs is from left to right. (a) j = 40 a cm-2; θ = 1. (b) j = 40 a cm-2; θ = 0.33. (c) j = 40 a cm-2; θ = 0. (b) j = 5 a cm-2; θ = 0.33. a very similar appearance of the surfaces is obtained in mixed electrolyte (θ = 0.33, figure 7b) and in pure nano3 electrolyte (figure 7c). the current efficiencies in the mixed electrolyte and in pure nano3 electrolyte (87 % and 84 %, respectively) and the surface roughnesses sa (0.79 μm and 0.74 μm, respectively) are approximately the same (figure 7b-c). moreover, the sem micrographs appear equal and are distinctly different from those obtained in pure nacl electrolyte (θ = 1, figure 7a). these are all indications of a nitrate anion-dominated dissolution mechanism at high current densities which is in line with the rde results. embedded small globular carbides can be seen clearly in the sem micrographs in figure 7a-c. the influence of additional turbulent flow due to the emerging edge can be seen in the optical micrographs, especially in figure 7c for θ = 0. the higher material removal at the edge of the samples can be explained by higher current density which can be calculated by secondary current distribution. applying 5 a cm-2 in mixed electrolyte with θ = 0.33 leads to a very porous, black surface structure with deep holes (figure 7d). the porosity is so high that sa cannot be measured with confocal microscopy. instead of a dominating chloride dissolution mechanism it is proposed that both mechanisms proceed simultaneously at different regions of the surface. the current efficiency of 57 % is an indication for that. active dissolution produces deep holes while the transition zone from passive to transpassive dissolution obtained in pure nano3 electrolyte produces the black surface film in between. this black surface andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 51 film could correspond to the surface films described by haisch in direct current experiments in pure nano3 [15]. described simultaneous dissolution by both mechanisms was not seen in rde experiments. these observations illustrate the importance of flow conditions for the electrochemical dissolution reaction and the turbulence increasing from rde to flow channel. with γ = 0.5 the influence of γ becomes apparent. for θ = 0.07 (curve 2, figure 6b) and 0.17 (curve 3, figure 6b) there are current efficiencies measured at 5 a cm-2 and 10 a cm-2 which indicates a simultaneous chloride-dominated and nitrate-dominated dissolution. the current efficiencies at 20 a cm-2 and 40 a cm-2 approach those in pure nano3 electrolyte. with θ = 0.33 (curve 4, figure 6b) a minor influence of chloride anions is obtained at 20 a cm-2 and 40 a cm-2 because of little higher current efficiencies than in pure nano3 electrolyte (93 % and 88 % compared to 82 % and 83 %). in this mixed electrolyte at 5 a cm-2 and 10 a cm-2 the anodic dissolution is dominated by chloride anions indicated by current efficiencies, surface roughnesses and the optical impression (figure 8a-d). figure 8. comparison of resulting surfaces with θ = 0.33 and 1 at 5 a cm-2 und 10 a cm-2; γ = 0.5. electrolyte flow direction in optical micrographs and in sem micrographs is from left to right. (a) j = 5 a cm-2, θ = 1. (b) j = 5 a cm-2, θ = 0.33. (c) j = 10 a cm-2, θ = 1. (d) j = 10 a cm-2, θ = 0.33. this result is in agreement with the proposed mechanism which is based on the idea of chloride-dominated dissolution mechanism at lower and nitrate-dominated dissolution mechanism at higher j. the more pronounced edge effect due to the additional flow effect at the emerging edge can be seen at the surface in mixed electrolyte with θ = 0.33 at 10 a cm-2 as well (figure 8d). here the surface has a completely different appearance. this is another hint for the importance of flow conditions. for the duty cycle γ of 0.167 the current efficiencies in the mixed electrolytes approaches η in pure nacl electrolyte (figure 6c). the current efficiencies with θ = 0.33 are equal to the ones in pure nacl electrolyte except for the current density of 40 a cm-2. at this current density, the current efficiency is η = 97 % in mixed electrolyte with θ = 0.33. this value is in between the values obtained in the pure chloride and nitrate electrolytes. also the surface quality obtained in the mixed electrolyte (figure 9e and g) appears similar to those obtained in pure nacl electrolyte (figure 9d and f) at 10 a cm-2 and 20 a cm-2. the sa values indicate that the chloride dissolution mechanism dominates. lower current densities show a slightly different behaviour. figure 9b shows the surface obtained at 5 a cm-2 in mixed electrolyte with θ = 0.33 and γ = 0.167. it is rougher than those obtained in pure nacl with the same γ (figure 9a) and in mixed electrolyte with the same mixing ratio and larger duty cycle of γ = 0.5 (figure 8b). a value sa > 1 μm indicates that under these specific conditions a higher roughness is obtained in the presence of both anions. perhaps the chloride ion flux towards the surface is insufficient with respect to competing nitrate anions. at higher current densities of 40 a cm-2 in a mixed electrolyte with θ = 0.33 (figure 9i) the surface show features found on surfaces obtained j. electrochem. sci. eng. 1(1) (2011) 39-52 100cr6 dissolution mechanisms 52 in both pure nacl and nano3 electrolytes (figures 9h and 9j). it is concluded that both mechanisms proceed simultaneously at the same regions of a surface (in contrast to the situation found in figure 7d). applying different duty cycles in series was tested with γ = 0.625 (q = 200 c), 0.5 (50 c) and 0.167 (50 c) at 5 a cm-2 with θ = 0.33 in order to assess the surface quality after each step. figure 9. comparison of resulting surfaces with γ = 0.167. electrolyte flow direction in optical micrographs and in sem micrographs is from left to right. (a) j = 5 a cm-2, θ = 1. (b) j = 5 a cm-2, θ = 0.33. (c) j = 5 a cm-2, θ = 0. (d) j = 10 a cm-2, θ = 1. (e) j = 10 a cm-2, θ = 0.33. (f) j = 20 a cm-2, θ = 1. (g) j = 20 a cm-2, θ = 0.33. (g) j = 40 a cm-2, θ = 1. (i) j = 40 a cm-2, θ = 0.33. (j) j = 40 a cm-2,θ = 0. figure 10. comparison of resulting surfaces after anodic dissolution in series with variation of duty cycle; j = 5 a cm-2 , θ = 0.33. (a) γ = 0.625; q = 200 c; (b) i. γ = 0.625; q = 200 c, ii. γ = 0.5; q = 50 c; (c) i. γ = 0.625; q = 200 c, ii. γ = 0.5; q = 50 c, iii. γ = 0.167; q = 50 c. electrolyte flow direction in optical micrographs and in sem micrographs is from left to right. in figure 10, three specimens are shown, one processed just with the first step (figure 10a), the second processed with the first two steps (figure 10b) and the third processed with all three steps andreas lesch et al. j. electrochem. sci. eng. 1(1) (2011) 39-52 doi: 10.5599/jese.2011.0004 53 (figure 10c). the conditions in figure 10a were identical to figure 7d except for the use of a lower charge (200 c instead of 300 c). because both dissolution mechanisms take place simultaneously, a black porous surface is observed (figure 10a). however, less material is removed in figure 10a than in figure 7b causing a less distinctive porosity. sa is still larger than the measurable range. the next step (γ = 0.5) in which the dissolution is dominated by chloride anions efficiently removes the black surface (figure 10b). only a few pits and an edge at transition from flat plateau to the rim of specimen remain. applying all three steps decreases the roughness in the last step (figure 10c). the last step in which chloride anions dominate the dissolution flattens the surface and a smooth transition to the rim of the specimen is achieved. a look at the sem micrograph shows that the surface is equal to the one in figure 9b with the same duty cycle and chloride anion ratio (γ = 0.167, θ = 0.33). conclusions the anodic dissolution of 100cr6 in chloride/nitrate mixed electrolytes was investigated. lsv at rde assuming mainly laminar flow conditions showed that either active or transpassive dissolution can be achieved by controllable mass transport conditions (bulk chloride concentration and rotation rate). this is indicated by an abrupt decrease in current density with increasing anodic potential. while active dissolution due to chloride-dominated mechanism takes place at lower current densities, transpassive dissolution due to nitrate-anion dominated mechanism is observed at higher current densities. by changing the potential or current density, it is possible to switch reversibly between both dissolution mechanisms. the following dissolution mechanism is proposed. if the flux of chloride ions relative to the current density is too small, the chloride anion mechanism cannot be maintained and transpassive dissolution due to nitrate-dominated dissolution commences. this transpassive dissolution is then stable in the observed current density ranges. the anodic dissolution is mass transport-controlled and depends on the anode/electrolyte interface layer. flow channel experiments applying galvanostatic pulses showed that under assumed turbulent flow conditions a separation of dominating dissolution mechanisms can be observed under specific conditions while both mechanisms may also proceed simultaneously under other conditions. there is not only an influence of anion ratio but also on current density, pulse parameters and flow conditions. acknowledgements: the technical assistance by ms. carmen pritsch is acknowledged. references [1] a. d. davydov, v. m. volgin in encyclopedia of electrochemistry (d.d. macdonalds, p. schmuki eds.) wiley, vol. 5, 2007pp. 809-854. 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[26] a. j. bard, l. r. faulkner, electrochemical methods: fundamentals and applications, john wiley & sons, new york, 2001, p. 347. © 2011 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) doi: 10.5599/jese.2011.0004 s1 j. electrochem. sci. eng. 1(1) (2011) s1-s2 open access : : issn 1847-9286 www.jese-online.org supplementary material to anodic dissolution of 100cr6 in nitrate/chloride mixed electrolytes andreas lesch, gunther wittstock, chris burger, benjamin walther and jürgen hackenberg j. electrochem. sci. eng. 1(1) (2011) 39-54; doi: 10.5599/jese.2011.0004 sm-1 photographs of formed films during lsv in mixed electrolyte at resting disc electrode figure sm-1. reaction products in mixed electrolytes at static anode; (a) green viscous film in section of “chloride curve“; (b) evolution of oxygen after passivation; (c) evolution of oxygen and yellow-green colored solution in section of “nitrate curve”. figure sm-1 shows photographs of the transition from active to passive dissolution. during active dissolution the assumed reaction products creep down as a green viscose film (figure sm-1a). after passivation, the evolution of oxygen breaks off the visible flakes of the black carbide-containing surface layer from the anode surface (figure si-1b) and transpassive dissolution can be observed by the formation of oxygen bubbles and yellow-brown colouration of the solution (figure sm-1c). sm-2 programmed current chronopotentiometry active dissolution in pure nacl electrolyte and transpassive dissolution in pure nano3 electrolyte are indicated by curves 1 and 7. the pccs in mixed electrolytes show active dissolution at low current densities. the transition from active to transpassive is indicated by the increase in potential. the current density at transition depends on the relative chloride anion concentration. http://www.jese-online.org/� s2 figure sm-2. pccs in 3 m nacl (1), 3 m nano3 (7) and in mixed electrolytes with relative chloride concentrations θ: 0.83 (2), 0.67 (3), 0.5 (4), 0.33 (5), 0.17 (6); rotation rate 3000 rpm; scan rate 50.9 ma cm-2 s-1. sm-3: correction of current efficiency of inactive carbides a correction of current efficiency considering the elementary carbon and the metal carbides which are assumed to be electrochemically inert can be done by using the equations sm-3-1 to sm-3-3. c,elementary fe3c fe2.5c inert tot c c ,elementary c,fe3c c,fe2.5c c,elementary c,elementary c,elementary m m m m m p p p p m m m   = ⋅ ⋅ ⋅ + ⋅ + ⋅     (1) inert tot tot 179.5457 151.6232 0.01 0.2 1 0.4 0.4 0.1123 12.0107 12.0107 m m m   = ⋅ ⋅ ⋅ + ⋅ + ⋅ = ⋅    (2) inert c,elementary fe3c fe2.5cm m m m= + + (3) minert – total mass of assumed inert carbon compounds in annealed to globular cementite 100cr6 mc,elementary – mass of elementary carbon mfe3c – mass of fe3c mfe2.5c – mass of fe2.5c mtot – experimental weight mass loss of annealed to globular cementite 100cr6 specimen pc – weight percentage of total carbon in 100cr6 pc,elementary – percentage of elementary carbon of total carbon mc,elementary – molar mass of carbon pc,fe3c – percentage of carbon, bound in fe3c, of total carbon mfe3c – molar mass of fe3c pc,fe2.5c – percentage of carbon, bound in of fe2.5c, of total carbon mfe2.5c – molar mass of fe2.5c elucidating mechanistic background of the origin and rates of peroxide formation in low temperature proton exchange fuel cells http://dx.doi.org/10.5599/jese.1659 753 j. electrochem. sci. eng. 13(5) (2023) 753-770; http://dx.doi.org/10.5599/jese.1659 open access : : issn 1847-9286 www.jese-online.org original scientific paper elucidating mechanistic background of the origin and rates of peroxide formation in low temperature proton exchange fuel cells ambrož kregar1,2, and tomaž katrašnik1 1university of ljubljana, faculty of mechanical engineering, aškerčeva cesta 6, ljubljana, 1000, slovenia 2university of ljubljana, faculty of education, kardeljeva ploščad 16, ljubljana, 1000, slovenia corresponding author:  ambroz.kregar@fs.uni-lj.si; tel.: +386-1-4771-512 received: january 6, 2023; accepted: july 3, 2023; published: july 19, 2023 abstract degradation of electrode-membrane assembly of the low-temperature hydrogen fuel cells represents one of the main obstacles in wider adoption of these clean and efficient electrochemical sources of electrical energy. chemical degradation of proton exchange membrane is initiated by hydrogen peroxide formation, which forms in the fuel cell as a byproduct to water in oxygen reduction reaction and decomposes to reactive radical species, damaging to the membrane chemical structure. depending on the operating conditions of the fuel cell, the source of hydrogen peroxide can be either cathode, anode, or, as we argue in the paper, also the pt particles in the membrane, which originate from the cathode catalyst dissolution, diffusion into the membrane and redeposition of pt ions inside the membrane. in the paper we propose a mathematical model of intertwined physical processes in membrane and catalyst layer, aimed at unifying the description of hydrogen peroxide formation throughout entire membrane-electrode assembly at any fuel cell operating conditions. the model results, compared to experimental data, indicate that pt particles inside the membrane can indeed be an important source of hydrogen peroxide in aged fuel cells. for a fresh fuel cell, numerical simulation using proposed model show that hydrogen peroxide can be formed at either cathode or anode, depending on the fuel cell operating condition, but with anode production being more prominent in standard fuel cell operating conditions. keywords fuel cell; membrane; degradation; hydrogen peroxide; modelling introduction the use of hydrogen-fueled low-temperature fuel cells with proton-exchange membrane (lt-pemfc) represent an important step in fulfilling the future goals of decarbonization of transport and energy sector. electricity is produced in the fuel cell by oxidation of hydrogen fuel in anode catalyst layer, resulting in protons which travel through proton-conducting polymer membrane to http://dx.doi.org/10.5599/jese.1659 http://dx.doi.org/10.5599/jese.1659 http://www.jese-online.org/ mailto:ambroz.kregar@fs.uni-lj.si j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 754 the cathode catalyst layer, where they are recombined with oxygen in reduction reaction. the resulting electric potential between anode and cathode can be used to extract useful work. since the reactants are supplied to the membrane-electrode assembly (mea) from external tanks, the size scaling of lt-pemfc is much better compared to batteries, which makes the fuel cells especially suitable for a heavy-duty transport applications, such as trucks and buses [1]. despite these clear advantages, lt-pemfcs still suffer from several drawbacks. among the most important ones is limited durability, caused by several degradation mechanisms of fuel cell components [2]. lt-pemfc catalyst layer is typically composed of nanoscopic pt particles, dispersed on a highly porous carbon matrix to provide both high specific surface area for electrochemical reactions and good diffusion properties for transport of reactants and products. electrochemical oxygen reduction in the cathode catalyst results in high local electric potential, which, combined with relatively high temperatures of about 80 °c can lead to oxidation and corrosion of both pt particles and carbon support, resulting in eventual loss of electrochemically active surface area [3-5]. proton-exchange membrane, usually made from perfluorinated polymer materials such as nafion, is also prone to degradation [6]. most notably, the hydrogen peroxide, formed as a byproduct of oxygen-reduction reaction (orr), can decompose in the presence of various metal ions into highly reactive radical species, which split chemical bonds in the membrane structure and result in loss of its proton conductivity and mechanical integrity [6,7]. while the basic electrochemical process of hydrogen peroxide formation is well understood [8], the specifics of its formation in fuel cell mea are still a topic of debate. since oxygen is one of the reactants in peroxide formation, the oxygen-rich cathode catalyst layer seems an obvious location of its origin. however, since hydrogen peroxide only forms when local electric potential between catalyst and adjacent ionomer (relative to rhe throughout the text) falls below 0.67 v compared to water, which is produced below 1.23 v [9], which is rarely encountered in cathode catalyst during normal fuel cell operation, the cathode peroxide formation might not be the main origin. anode catalyst layer, on the other hand, features low electric potential (≳ 0 v), but low concentration of oxygen, which can only arrive to the anode by means of diffusion through the membrane, limits the rate of peroxide production in anode catalyst layer. the third option is a formation of hydrogen peroxide inside the membrane [10]. in this case, the formation takes place on the surface of pt particles, formed in the membrane due to catalyst layer degradation. pt ions, resulting from dissolution of pt particles in cathode catalyst layer, diffuse into the membrane where they are reduced to crystallite pt particles, so-called pt band, when encountering the hydrogen, which slowly diffuses from anode catalyst layer towards the cathode [11]. raman spectroscopy measurements [12,13] of membrane chemical composition profile, published by ohma et al. [10], show significant membrane degradation at the location of pt band, which strongly indicates the formation of hydrogen peroxide inside the membrane and calls for better understanding of its relation to the formation rates in cathode and anode catalyst layer. we argue in this paper that pt band particles in the membrane can feature appropriate electric potential to support the formation of hydrogen peroxide from the diffusive oxygen and protons, abundant inside the membrane. since in situ measurements of spatial distribution of hydrogen peroxide concentration throughout mea are missing, it is difficult to conclude with high certainty which of the described peroxide formation processes is the most important. experimental measurements show a strong dependence of formation rate on fuel cell operating conditions, such as fuel cell voltage, temperature and humidity [9]. these effects are well described by several existing models of peroxide formation. the first mathematical model was proposed by sethuraman et al. [14], describing the peroxide a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 755 formation in both anode and cathode catalyst layer using tafel equation describing the effects of voltage, temperature and humidity. the model was further developed by chen and fuller, which added a description of hydrogen peroxide decomposition in fenton reaction [15], and in several papers by wong et al. [16-18], in which they include the description of peroxide transport through the membrane, its decomposition in fenton reaction and consequent chemical degradation. the model by futter [19] is further expanded by adding the effects of pressure on peroxide formation rate. aforementioned models describe the rate of peroxide production in cathode and anode catalyst layer in various fuel cell operation conditions with good precision, but, to our best knowledge, no attempts have thus far been made to model the hydrogen peroxide formation on pt band particles inside the membrane. the aim of this paper is therefore to propose a unified mathematical description of diffusion processes and electrochemical reactions involved in hydrogen peroxide production for all three components of mea: cathode, membrane, and anode. this is achieved by combining the existing models of hydrogen peroxide formation, applicable in anode and cathode catalyst layer [15] and applying them to the pt band particles inside the membrane [11]. the model thus allows direct comparison between peroxide formation rates in all there mea components and enables elucidating causal relations of peroxide formation phenomena and comparison of formation rates at different locations at different fuel cell operating conditions. experimental modelling procedure the proposed model considers three species involved in electrochemical hydrogen peroxide formation: hydrogen (h2) and oxygen (o2) as reactants and hydrogen peroxide (h2o2) as a product. to consistently describe electrochemical reactions in all three relevant sections of mea. cathode and anode layer are composed of a mixture of ionomer, highly porous carbon support on which pt nanoparticles are dispersed, and void space allowing the diffusion of gases. membrane is composed of ionomer in which electrically insulated pt band particles can reside, providing electrochemically active surface area. each of three modelled species (h2, o2 and h2o2) is described by a single onedimensional concentration field in direction perpendicular to the mea plane (x), describing the species concentration in the ionomer. the species concentration profiles are governed by a unified set of partial differential equations which apply throughout the entire modelling domain (cathode, membrane, and anode): ( ) ( ) ( ) 2 2 2h h h dif ec , , ,c x t c x t c x t t t t         = +                  (1) ( ) ( ) ( ) 2 2 2o o o dif ec , , ,c x t c x t c x t t t t         = +                  (2) ( ) ( ) ( ) ( ) 2 2 2 2 2 2 2 2h o h o h o h o dif ec fent , , , ,c x t c x t c x t c x t t t t t            = + +                         (3) describing three contributions to the dynamics: diffusion (dif), electrochemical reactions (ec) and fenton reaction (fent). each of the terms will be described in detail in the following sections. http://dx.doi.org/10.5599/jese.1659 j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 756 additionally, the model describes the local electric potential difference between pt catalyst particle surface and adjacent ionomer (electric double layer potential) upt(x,t): ( ) ( ) ( )pt pt pt ec cond , , ,u x t u x t u x t t t t         = +              (4) governed by electrochemical reactions and electron conduction (cond) through catalyst support structure. as for the treatment of chemical species, eqs. (1)-(3), the same set of equations is applied throughout the entire modelling domain, with the rates of processes determined by specific spatially dependent properties of the domain. the processes described by the model are schematically presented in figure 1. note that the size and volumetric density of pt particles in the membrane is not calculated by the model but taken as fixed initial condition. figure 1. schematic representation of modelled processes: concentrations of hydrogen, oxygen and hydrogen peroxide are governed by diffusion, fenton reaction, and electrochemical reactions between species, with rates determined by local electric potential upt diffusion model diffusion of species inside the membrane can be described by a diffusion equation with diffusion constant for each species di = di,ion, with i = h2, o2, h2o2. the description of diffusion process in anode and cathode is more challenging since the model needs to consider that species can diffuse through both ionomer (described by volume fraction εion) and void space (ε0) of the porous material. to address this challenge, we propose a diffusion equation with spatially dependent effective diffusion constant d̃i(x), describing parallel diffusion through both materials. to derive the mathematical expression d̃i(x), we first describe separately the species diffusion through each material. the molar flux through ionomer is described as ( ) ( ) ( ) ( )i,ion i,ioni,ion i,ion i,ion ion , , , c x t c x td j x t d x x x   = − = −   (5) where we consider the fact that the actual path through the ionomer x ̃ is longer than geometric path x by tortuosity factor τion: x ̃= τion(x) x. similar expression is used for void fraction equation with diffusion constants di,0: a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 757 ( ) ( ) ( ) ( )i,0 i,0i,0 i i,0 i,0 0 , , , c x t c x td h j x t d x x rt x   = − = −   (6) here we assume that species concentration in ionomer cion and in void c0 are always in equilibrium, determined by ionomer henry constant h: c0=h/rt cion [17], which is reasonable considering that the ionomer film in catalyst layer is thin. total flux per geometric area is obtained by summation of ionomer and void flux, weighted by their volumetric ratio: ( ) ( ) ( ) ( ) ( ) ( ) ( )i,ion i ion i,ion 0 i,0 i , , , , c x t j x t x j x t x j x t d x x  = + =  (7) with resulting effective diffusivity ( ) ( ) ( ) ( ) ( ) ion 0 i i i,ion i,0 ion 0 x x h d x d d x x rt  = + (8) which is similar to bruggeman expression deff = dε/τ. different relations between tortuosity and porosity are proposed in the literature [20], in our model we choose a simple relation τ = ε1-α with α = 1.5. to determine the rate of concentration change in ionomer we again take into account that species accommodate prescribed volume in void and ionomer and that both concentrations are in equilibrium: ( ) ( ) ( ) ( ) ( )i i idif ion 0 , ,1 = ic x t c x t d x ht x x x x rt                +    (9) in eq. (9) we write ci(x,t)= ci,ion(x,t) as all the concentrations in the reminder of the paper will refer to the concentration in ionomer phase. the same equation can be used to describe the transport of all three species i = h2, o2, h2o2 using their respective effective diffusion coefficient. the difference in transport properties between cathode, membrane, and anode are described by spatial distribution of ionomer εion(x) and void volume fraction ε0(x). electrochemical reactions hydrogen, oxygen, and hydrogen peroxide are all involved in electrochemical reactions on pt surface in cathode and anode catalyst layer, where the rates are determined by electric potential of the pt/c matrix. however, the reactions can also take place on the surface of pt particles in so-called pt band inside the membrane, which are electrically isolated from the catalyst layers [11]. the reactions in pt band follow the same principles as in the catalyst layer, but their rate needs to be determined by the local electric potential on pt particles, determined in turn by the rates of electrochemical reactions on their surface [11], which will be described in the next section. in the model, we consider 4 electrochemical reactions [17]. if potential difference between catalyst and ionomer is larger than 0 v, the hydrogen reacts hydrogen oxidation reaction (hor), producing protons [17]: h2  2h+ + 2e-, uhor = 0 v (10) this reaction is most prominent in the anode where hydrogen concentration is high. note that since electrons are deposited on catalyst, the reaction tends towards lowering the potential of the catalyst on which it takes place. http://dx.doi.org/10.5599/jese.1659 j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 758 oxygen is reduced on the catalyst surface in two similar reactions. 4-electron oxygen reduction reaction (orr4) results in water [17]: o2 + 4h+ + 4e 2h2o, uorr4 =1.23 v (11) and takes place when electric potential is lower than 1.23 v. 2-electron oxygen reduction reaction (orr2) is similar, but results in hydrogen peroxide formation [17]: o2 + 2h+ + 2e h2o, uorr2 =0.67 v (12) and occurs only when electric potential is lower than 0.67 v. since both reactions consume electrons and thus increase the electric potential on the catalyst, the resulting potential is typically closer to 1.23 v then 0.67 v, suppressing the formation of hydrogen peroxide. hydrogen peroxide can further react in peroxide reduction reaction (prr) [17]: h2o2 + 2h+ + 2e 2h2o, uprr =1.67 v (13) which takes place when electric potential is lower than 1.67 v. the rates of electrochemical reactions, listed above, are calculated using modified butler-volmer equations which take into account concentrations and electric potential on the pt particles, while they, in addition, consider also the diffusion limitations to species fluxes from bulk ionomer to the particle surface [11]. the equations for calculating the rates in [mol m-2 s-1] are [11,17]: ( ) ( ) ( )( ) ( ) ( )( ) ( ) ( )( ) hor pt hor hor pt hor + 2 hor pt hor 2 2 , 2 1 , hor h h hor 2 , pt hor h ref , , 1 t t t u x t u u x t u b b u x t u b k a x t e a e j x t r x k e d c    − − − − −    −     = + (14) ( ) ( ) ( ) ( )( ) ( )( ) ( ) ( )( ) orr4 pt orr4 orr4 pt orr4 2 2 orr4 pt orr4 2 4 1 , 4 , orr4 o h o orr4 4 , pt orr4 o ref , , 1 t t t u x t u u x t u b b u x t u b k a x t e a e j x t r x k e d c    − − − − −    −     = + (15) ( ) ( ) ( ) ( )( ) ( ) ( ) ( )( ) orr2 pt orr2 2 orr2 pt orr2 2 2 1 , orr2 o orr2 2 1 , pt orr2 o ref , , 1 t t u x t u b u x t u b k a x t e j x t r x k e d c   − − − − − − = + (16) ( ) ( ) ( )( ) ( ) ( )( ) 2 2 2 2 prr pt prr prr h o prr prr pt prrpt prr h o ref 2 , , sinh , 2 , 1 sinh t t u x t u k a x t b j x t u x t ur x k d c b   − = − + (17) the rates of reactions jr (r = hor, orr4, orr2, prr) are proportional to the reaction rate constants kr. species activities are calculated by dividing positionand time-dependent concentrations with the reference values: ah2(x,t) = ch2(x,t)/cref , ao2(x,t) = co2(x,t)/cref, ah2o2(x,t) = ch2o2(x,t)/cref. time dependance and spatial profile of electric potential between pt and adjacent ionomer upt(x,t) will be further explained in the next section. the size of pt particles rpt = rpt(x), affecting the limiting diffusion flux of species to pt surface, can in general evolve with time due to degradation of catalyst layer and growth of pt band in the membrane, but is assumed to be stationary in the proposed a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 759 model. the term bt = rt/f describes the temperature dependence, while the diffusion constants di for all species refer to their values in the ionomer. to relate the reaction rates to the changes in species concentrations, we need to consider the spatial distribution of pt surface on which the reaction can take place, and the volume in which the species can reside. we express these properties as volume specific pt surface density ( ) ( ) ( ) ( ) ( ) 2 pt pt pt ion 0 4 i r x x n x h x x rt   = + (18) where npt(x) is volumetric density of pt particles and effective volume modification is the same as for the case of diffusion equations. the spatial distribution of λpt(x) describes the differences in distribution of pt catalyst throughout the entire mea and thus allows the use of the same form of electrochemical reaction rates in all mea components. the rates of concentration changes can therefore be calculated for entire mea as ( ) ( ) ( )2 h pt hor ec , , c x t x j x t t    = −    (19) ( ) ( ) ( ) ( )2 o pt orr2 orr4 ec , , , c x t x j x t j x t t     = − +      (20) ( ) ( ) ( ) ( )2 2 h o pt orr2 prr ec , , , c x t x j x t j x t t     = −      (21) it should be noted that the model does not describe the dynamics of water and proton concentration in the mea, which are assumed to be constant and uniformly distributed. electric charging of pt surface the spatial distribution of electric potential on catalyst particles upt(x,t) is crucial for adequate modelling of hydrogen peroxide formation. as seen from reaction rate eq. (16), the formation of hydrogen peroxide is suppressed in the electric potential range between 0.67 and 1.23 v, where water production via orr4 eq. (15) is dominant. orr2 only becomes significant at potential below 0.67 v. since cathode and anode are electrically connected to the external load of the fuel cell, their electric potential can be controlled and measured by electric current through the fuel cell, making it possible to estimate which ec reactions will take place. it is assumed that potential of the pt band is determined by the rates of ec reactions on its surface, since particles inside the pt band are not easily accessible. therefore, potential of the pt is in turn determined by local concentrations of species in the membrane. reduction reactions, such as hor, produce electrons, which are deposited on pt particles and thus reduce the electric potential, while oxidation reactions consume electrons and consequently increase the potential. this process is modelled by equation [21] ( ) ( ) ( ) ( ) ( ) pt hor orr4 orr2 prr ec , -2 , 4 , 2 , 2 , dl u x t f j x t j x t j x t j x t t     = + + +      (22) where ζdl = cdl/s is a surface specific double layer capacitance of the pt surface [22]. in cathode and anode catalyst layer, the potential is also affected by electric conduction through carbon support structure, which causes the spatial distribution of potential to equilibrate to a constant value throughout the catalyst layer. this process is modelled by diffusion-like equation [21] http://dx.doi.org/10.5599/jese.1659 j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 760 ( ) ( ) ( ) ( ) ( )pt pt eff2 pt pt dlcond , ,1 4 u x t u x t x t x xr x n x         = −          (23) volumetric capacitance of catalyst layer is calculated as a product of pt surface capacitance and pt surface density. effective electric conductivity is determined from catalyst volumetric ratio εptc(x) and bulk carbon conductivity σc as σeff(x) = σc εptc(x)α. it should be noted that since the catalyst volumetric ratio inside the membrane is zero, electric conduction only affects the potential in the catalyst layers. fenton reactions the hydrogen peroxide produced in electrochemical reactions reacts with metal ions in a series of reactions, resulting in formation of radical species. this process is typically modelled as a set of fenton reactions with iron ions [6,17], which can be simplified to a single reaction, fe2+/3+ + 2h2o2 → fe2+/3+ + ho• + hoo• + h2o (24) since we are only interested in the current model with spatial distribution of hydrogen peroxide formation, but not in further chemical membrane degradation processes [6,7], the fenton-like consumption of peroxide is modelled by a simple equation ( ) ( )2 2 2 2 h o fent h o fent , , c x t k c x t t   =    (25) where kfent is related to the effective reaction rate of fenton reaction and to the concentration of relevant transition metal ions present in the ionomer, which are assumed to be uniformly dispersed through the mea. boundary conditions boundary conditions for concentration profiles are applied at the boundary between cathode and gas diffusion layer (gdl) at x = 0, and between anode and gdl at x = dmea. hydrogen and oxygen boundary conditions are determined by diffusive flux of species through the gas diffusion layer (gdl) with effective diffusivity d̃i,gdl = di,gdlhi/rt expressed in terms of ionomer concentration. ( ) ( ) ( ) ( )2 2 2 2 2 2 o o ,gdl o o ,ch o gdl0 , 0, ox c x t d rt d x c t c t x d h =    = −         (26) ( ) ( ) ( )2 2 2 2 h h ,gdl h h gdl0 , 0, x c x t d d x c t x d =   = −    (27) ( ) ( ) ( )2 2 2 2 2 2 2 2 h o h o ,gdl h o h o gdl0 , 0, x c x t d d x c t x d =   = −    (28) ( ) ( ) ( )2 2 2 2 mea o o ,gdl o mea gdl , , o x d c x t d d x c d t x d =   =    (29) ( ) ( ) ( ) ( )2 2 2 2 2 2 mea h h ,gdl h h mea h ,ch gdl h , , x d c x t d rt d x c d t c t x d h =    = −         (30) ( ) ( ) ( )2 2 2 2 2 2 2 2 mea h o h o ,gdl h o h o mea gdl , , x d c x t d d x c d t x d =   =    (31) a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 761 hydrogen concentration in anode gas feed channel ch2,ch(t) and oxygen in cathode gas feed channel is co2,ch(t). we assume that hydrogen concentration in cathode channel and oxygen concentration in anode channel is zero. for hydrogen peroxide we assume that concentration in both channels is zero [17]. boundary conditions for electric potential are determined from electric current density jel drawn from the fuel cell: ( ) ( )pt eff el 0 , x u x t x j x  =   =    (32) ( ) ( ) mea pt eff el , x d u x t x j x  =   =    (33) note that negative sign in boundary condition at x = 0 describes electric current flowing out of the cathode, while at x = dmea negative sign describes current flowing into the anode. experimental data the model was tested and verified on the experimental data, obtained from the paper by ohma et al. [10]. in their experiment, the durability of fuel cell membrane was related to the deposition of catalyst in the pt band. the degradation experiment was performed for 48 hours in open-circuit voltage (ocv) operation fuel cell at t = 90 °c with pure hydrogen and oxygen at ambient pressure as a fuel. as indication of membrane degradation, the raman spectrum of ionomer was measured at several locations across the aged membrane thickness. additionally, the size and density of pt particles in the membrane were measured using transmission electron microscopy. the aim of model verification is to determine whether the position of hydrogen peroxide production, determined by the proposed model, overlaps with the location of maximal membrane degradation, measured in the experiment. to reproduce the experimental conditions, the following model parameters need to be determined: spatial distribution of pt particle size rpt(x), pt particle density npt(x), void volume fraction ε0(x), ionomer volume fraction εion(x), temperature t and boundary conditions. the thickness of cathode and anode, used in experiment, are dcat = dcat = 10 μm. the membrane thickness changed from initial 50 to 33 μm after degradation. in the model, we used the latter value for easier comparison with the raman measurement, dmem = 33 μm. particle size and density distribution were determined separately for cathode (0 < x < dcat), membrane (dcat < x < dcat + dmem) and anode (dcat + dmem < x < dmea). for cathode and anode catalyst layer, rpt(x) and npt(x) were assumed to be constant and were determined from the data on catalyst properties, namely pt loading (ptload)= 0.35 mg/cm2 and electrochemical surface area esa = 80 m2/g (deduced from the type of catalyst, tec10e50e, tkk, [23]). particle size and density are calculated as [24]: pt pt 3 1.78 nm 1000 esa r  = = (34) 23 -3 3 load pt pt pt3 7.0×10 m 4 pt n r  = = (35) the paper [10] provides the detailed measurement of properties of pt particles in the membrane, such as size distribution and spatial distribution of particles above certain threshold. based on this data, the spatial distribution of pt band particle size rpt(x) and density npt(x) in the membrane, dcat < x < dcat + dmem was constructed. according to numerical study of pt band growth in the membrane by burlatsky et al., the volumetric density of particles in the membrane is determined by http://dx.doi.org/10.5599/jese.1659 j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 762 agglomeration dynamics during pt band formation and is homogeneous throughout the membrane [11]. based on estimated pt ion flux to the membrane from [10], constant distribution npt(x) = 2.08 × 1018 m-3 was assumed. size distribution rpt(x) was reconstructed from measured distribution of particles larger than 25 nm and was described in the model as a normal distribution ( ) ( ) a 2 2 band b nd2( pt pt,max ) x x r x r e  − − = (36) with mean value x̄band = dcat + 0.4 dmem, standard deviation band = 0.096 dmem and maximal size of particles rpt,max = 12.3 nm. spatial distribution of particle size and density for entire mea is plotted in figure 2. figure 2. spatial distribution of particle size rpt(x) and density npt(x) through the mea, used in the model (constructed based on data from [10]) ionomer volume fraction in the membrane was set to εion = 1, while in the catalyst layers, volumetric ratios were calculated from the data, provided in [10]. catalyst volumetric ratio was determined from pt loading and pt/c ratio (50:50), εptc = 0.191, and ionomer volumetric ratio was determined from nafion to carbon mass ratio (mnaf/mc = 0.9), εion = 0.199. since pure oxygen and hydrogen at ambient pressure and t = 90 °c were used in the experiment, their respective concentrations in the gas feed channels, used as boundary conditions, were calculated as co2,ch(t) = co2,ch(t) = p/rt = 33.6 mol m -3. boundary condition for electric potential is determined from ocv operation to be jel = 0. diffusion constant in gdl was assumed to be proportional to void diffusion constant. gdl thickness, used in experiment (20bc, sgl carbon), was dgdl = 235 μm and diffusivity was calculated from bulk diffusivity as di,gdl = q di,0 with q = 0.139 [25]. material parameters used in diffusion model were obtained from two papers by wong et. al [16,17] and are listed in table 1. table 1. diffusion model parameters species d / m2 s-1 reference species h / pa m3 mol-1 reference o2, gas phase 1.98×10-5 [17] o2 4.718×104 e-666/t [16] fit h2, gas phase 11.50×10-5 [17] h2 2.584×104 e170/t [16] h2o2, gas phase 1.88×10-5 [17] h2o2 6.830×107 e-7379/t [16] o2, ionomer phase 0.96×10-10 [17] h2, ionomer phase 2.08×10-10 [17] h2o2, ionomer phase 1.50×10-10 [17] a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 763 as explained in [26], the position of pt band in the membrane is determined by the values of diffusion and henry constants of oxygen and hydrogen. to ensure that values of this parameters are consistent with the measured position of pt band, henry constant of oxygen ho2 was reduced by a factor of 3.5 compared to the value, given in [16], designated by “fit” in table 1. this could be justified by effects of humidity on ho2 [27], which are not taken into account by the expression, provided in [16]. material parameters used in electrochemical reaction model were obtained from [17] and are listed in table 2. specific surface capacity of pt particles ζdl = 40 μf cm-2 was obtained from [22]. table 2. electrochemical reaction parameters reaction k / a m-2 reference reaction α reference hor 105 [17] hor 0.50 [17] orr2 40 [17] orr2 0.50 [17] orr4 50 [17] orr4 0.50 [17] prr 10-13 [15] prr 0.32 [17] the value of reaction rate constant for fenton reactions was calculated according to [6] as kfent = k0cfe2+/3+ (37) with k0 = a e-ea/rt = 845 m-1 s-1 at t = 363 k. iron ions concentration was calibrated to obtain best fit between measured degradation profile and modelled peroxide concentration, resulting in value cfe2+/3+ = 600 ppm, which is consistent with the values, reported in [6]. numerical simulations with the model parameters set up based on experimental data, three numerical simulations were performed using the model implemented in python programming language. differential equation eq. (1)-(4) governing the equations were solved on computational grid with 10 control volumes in cathode and anode catalyst layer and 33 control volumes in the membrane (δx = 1 μm) using scipy solver “solve_ivp” using “bdf” integration method [28]. in simulation a, the model simulated operation of the fuel cell, described in section experimental data, with already established and stationary pt band. since proposed model was not aimed to focus on chemical membrane degradation (this is covered in our previous publications [6,7]), but on the spatial distribution of hydrogen peroxide formation, the simulation time of 600 s was sufficient to achieve stationary mea conditions and extract the results, which are compared to the experimentally measured spatial distribution of chemical membrane degradation. after validation, the capabilities of the model were tested in two further simulations in fuel cell operating regimes where peroxide formation is expected to occur in either cathode or anode catalyst layer. as explained in introduction, anode peroxide formation is expected from oxygen diffusing through the membrane to the anode, so simulation b was performed with fuel cell in ocv operation with no pt band present in the membrane (rpt(x) = 0). cathode peroxide formation is expected at low electric potential on cathode, which is a result of high electric current, so simulation c was performed at sufficiently high electric current density to reproduce this behaviour: jel = 10 a cm-2. http://dx.doi.org/10.5599/jese.1659 j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 764 results and discussion simulation a with the first simulation, we want to verify that the model predicts the formation of hydrogen peroxide in the membrane, consistent with the observed membrane degradation. the spatial distribution of species concentrations co2(x), ch2(x), ch2o2(x) and electric potential upt(x) in stationary conditions are shown in figure 3. figure 3. spatial distribution of oxygen (red), hydrogen (blue), hydrogen peroxide (purple) and electric potential (green) in ocv fuel cell operation with aged membrane (simulation a). hydrogen peroxide forms mostly on pt band as a result of oxygen reduction on pt particles with low local electric potential due to high gas diffusivity in porous catalyst compared to the membrane, the concentrations of oxygen and hydrogen (red and blue line in figure 2, respectively) are uniform in the catalyst layer. different concentrations of reactants despite the same concentrations in channels are a result of different henry constants. concentrations then fall linearly in the membrane towards zero, indicating a flux of reactants towards the pt band, where oxygen and hydrogen are consumed in reactions orr2, orr4 and hor. the electric potential upt(x) (green line in figure 2) is determined by the rates of these reactions, eq. (22). left of the pt band, high concentration of oxygen results in orr reactions being more prominent than hor, leading to consumption of electrons and thus increasing the local electric potential to a value close to orr4 equilibrium 1.23 v, shifted by the concentration-dependent nernst term [29]. right of the pt band, high hydrogen concentration promotes hor reaction, resulting in deposition of electrons on pt particles, thus lowering their potential close to 0 v. at the location of oxygen-hydrogen mixing, the potential sharply transitions from one value to the other. this provides the conditions with sufficiently low electric potential (upt < 0.67 v) and sufficiently high oxygen concentration which result in the formation of hydrogen peroxide (purple line in figure 3). hydrogen peroxide thus produced diffuses into the rest of the mea, where it is consumed by peroxide reduction reaction and fenton reaction. the concentration of hydrogen peroxide at its peak is about ch2o2,max ≈ 0.12 mm, which is consistent with the data, reported in [6]. since fenton reaction rate is proportional to the ch2o2(x) and since the reactions between radical species and the membrane structure are fast [6], we argue that the degradation rate of the membrane is proportional to the hydrogen peroxide concentration [6]. to verify this claim, we plot in figure 4 the spatial profile of hydrogen peroxide concentration and spatial distribution of specific chemical bonds a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 765 in the membrane, measured by raman spectroscopy [10]. the changes in relative intensity between so bonds (red line in figure 3) and co bonds (orange line in figure 3), found in sidechains of nafion structure, compared to cf bonds, indicate the presence of chemical degradation phenomena. the spatial distribution of hydrogen peroxide concentration, predicted by the proposed model, coincides well with the observed degradation, confirming the validity of the modelling approach. figure 4. spatial distribution of hydrogen peroxide (purple), compared to the spatial distribution of specific membrane chemical bonds measured by raman spectroscopy [10] simulation b as demonstrated in simulation a, the reactions of oxygen and hydrogen on pt band particles play a crucial role in determining the rate and location of hydrogen peroxide formation. with the second simulation we want to explore the process of hydrogen peroxide formation in fresh mea, where no pt band particles are present in the membrane, in ocv conditions. the results of simulation for stationary conditions are shown in figure 5. figure 5. spatial distribution of oxygen (red), hydrogen (blue), hydrogen peroxide (purple) and electric potential (green) at ocv fuel cell operation with fresh membrane (no pt band, simulation b). hydrogen peroxide mostly forms at the anode from crossover oxygen flux through the membrane similarly to the results of simulation a, oxygen and hydrogen concentrations are constant in catalyst layers and change linearly through the membrane, but now gases diffuse not to the position of http://dx.doi.org/10.5599/jese.1659 j. electrochem. sci. eng. 13(5) (2023) 753-770 peroxide formation in lt-pemfc 766 pt band, but to their respective counter-electrode, where they immediately react due to large difference between equilibrium and local electric potential (green). in case of oxygen diffusing to the anode, this results in reaction at electric potential upt ≈ 0 v < uorr2, which leads to the formation of hydrogen peroxide. its concentration is somewhat lower compared to the case of pt band formation (ch2o2,max ≈ 7 μm), which can be explained by slower diffusion of oxygen to the anode due to longer diffusion path. note that since no pt particles are present in the membrane, the values of electric potential in this region (light green) are a residual of the model and play no role in electrochemical reactions. simulation c in the last simulation, we want to explore the conditions in which the peroxide formation would occur at the cathode catalyst layer. as explained in introduction, this would require sufficiently low electric potential at the cathode, which is in a fuel cell a result of high electric current density. in simulation c we again model a fresh membrane with no pt band and with external electric current density jel = 10 a/m2. note that this value is unrealistically high for an actual fuel cell, so the results should be taken only as a demonstration of the model capabilities with no direct quantitative value. the spatial distributions, resulting from simulation c, are shown in figure 6. figure 6. spatial distribution of oxygen (red), hydrogen (blue), hydrogen peroxide (purple) and electric potential (green) in fuel cell with fresh membrane (no pt band) operating at high electric current density (simulation c). hydrogen peroxide forms on both cathode and anode side, but with small overall concentration at the specified high current density, most of the oxygen is consumed in the cathode catalyst layer (note that oxygen and hydrogen concentration in figure 6 are not plotted in the same scale), resulting in the lowering of cathode electric potential. the hydrogen peroxide (purple) formation now takes place both in cathode and anode catalyst layer but is more prominent in cathode due to higher concentration of oxygen. note, however, that the overall hydrogen peroxide concentration (ch2o2,max ≈ 7 × 10 -2 μm) is much lower compared to simulation a or simulation b. the results of simulations a, b and c highlight the importance of both fuel cell operating conditions as well as its state of health in identifying the location and rate of hydrogen peroxide production in fuel cell mea. due to low electric potential, the anode is more prone to hydrogen peroxide formation, under the condition that oxygen is present in the anode, which might not be the case if the pt band is formed in the membrane. cathode, despite its abundance of oxygen, does a. kregar and t. katrašnik j. electrochem. sci. eng. 13(5) (2023) 753-770 http://dx.doi.org/10.5599/jese.1659 767 not seem to be an important source of hydrogen peroxide at moderate or normal current densities since high electric potential strongly promotes the reaction of oxygen to water (orr4). conclusion in the paper we present a one-dimensional model of coupled physical processes, contributing to the formation of hydrogen peroxide in the lt-pemfc membrane-electrode assembly. the model describes spatial distribution of oxygen, hydrogen, and hydrogen peroxide concentration as well as spatial distribution of local electric potential between catalyst particles and adjacent ionomer. the diffusion of chemical species is governed by a unified set of equations, considering the material composition and porosity of different mea components. consumption and production of species in electrochemical reactions across mea is described by a unified description of reaction rates in terms of contributions of local electric potential, available catalyst surface and diffusion limitations due to finite size of catalyst particles. electric potential of active catalyst particles is calculated by considering both external electric currents through to the catalyst layer and charging of double layer on particle surface due to electrochemical reactions on catalyst particles. the validity of the model was confirmed by successfully reproducing the spatial distribution of hydrogen peroxide concentration in ocv degradation experiment, which coincides well with experimentally determined spatial distribution of membrane degradation, measured by raman spectroscopy [10]. the results of this simulation reveal a rich interplay between intertwined physical processes incorporated in the presented model. diffusion of oxygen and hydrogen through the membrane and their electrochemical consumption at pt band results in non-trivial spatial distribution of local electric potential, which transitions from high value of upt ≈ 1.23 v in oxygen rich environment to low value of upt ≈ 0.0 v in hydrogen rich environment. the transitioning area inside the pt band thus features appropriate electric potential and sufficient oxygen concentration, suitable for hydrogen peroxide formation. further numerical simulations of fresh mea without pt band indicate that both position as well as rate of hydrogen peroxide formation depends strongly on fuel cell operating conditions via coupled dynamics of reactants spatial distribution and resulting electric potential on catalyst particles. due to its strong explanatory power, the proposed model elucidates causal relations of peroxide formation phenomena and comparison of formation rates at different locations at different fuel cell operating conditions. the model, therefore, represents a solid background for further theoretical exploration of dynamics of hydrogen peroxide formation in lt-pemfc systems. since the results of the model can easily be related to the experimentally observed phenomena via raman spectroscopy analysis of the membrane, the model also serves as useful tool in designing new experiments, exploring the detailed mechanisms of hydrogen peroxide formation, which might have not yet been sufficiently elaborated. after availability of more detailed experimental data and extensive model validation as well as calibration of its parameters, the proposed model can be characterized as a powerful toll for designing advanced fuel cell operation strategies and fuel cell designs, aimed at minimizing the degradation processes in the lt-pemfc, improving its performance, and increasing its lifetime. acknowledgements: this work has been partly conducted within the morelife project and has received funding from the fuel cells and hydrogen 2 joint undertaking (ju) under grant agreement no 101007170. the ju receives support from the european union’s horizon 2020 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article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1038/s41592-019-0686-2 https://doi.org/10.1021/ed071p493 https://creativecommons.org/licenses/by/4.0/) electrochemical investigation of the corrosion behavior of heat treated al-6si-0.5mg-xcu (x=0, 0.5 and 1) alloys doi: 10.5599/jese.2015.0070 1 j. electrochem. sci. eng. 5(1) (2015) 1-8; doi: 10.5599/jese.2015.0070 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical investigation of the corrosion behavior of heat treated al-6si-0.5mg-xcu (x=0, 0.5 and 1) alloys abul hossain, mohammed abdul gafur*, fahmida gulshan and abu syed wais kurny department of materials and metallurgical engineering, bangladesh university of engineering and technology, dhaka, bangladesh *pilot plant and process development centre (pp & pdc), bcsir laboratories, dhaka, bangladesh corresponding author: ah_buetmmesgfl@live.com; tel.: +88-01711243601 received: june 23, 2014; revised: december 31, 2014; published: march 15, 2015 abstract the corrosion behavior of heat treated al-6si-0.5mg-xcu (x=0, 0.5 and 1 wt %) alloys in 0.1 m nacl solution was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (eis) techniques. the potentiodynamic polarization curves reveal that 0.5 wt % cu and 1 wt % cu content alloys are less prone to corrosion than the cu free alloy. the eis test results also showed that corrosion resistance or charge transfer resistance (rct) increases with increasing cu content into al-6si-0.5mg alloy. maximum charge transfer resistance (rct) was obtained with the addition of 1 wt % cu and minimum rct value was for cu free al-6si-0.5mg alloy. due to addition of cu and thermal modification, the magnitude of open circuit potential (ocp), corrosion potential (ecorr) and pitting corrosion potential (epit) of al-6si-0.5mg alloy in nacl solution were shifted to the more noble direction. keywords al alloy; nyquist plot; corrosion rate; tafel plot; eis introduction aluminium and its alloys are considered to be highly corrosion resistant under the majority of service conditions [1]. the various grades of pure aluminum are the most resistant, followed closely by the al-mg and al-mn alloys. next in order are al-mg-si and al-si alloys. the alloys containing copper are the least resistant to corrosion [2]; but this can be improved by coating each side of the copper containing alloy with a thin layer of high purity aluminium, thus gaining a three http://www.jese-online.org/ mailto:ah_buetmmesgfl@live.com j. electrochem. sci. eng. 5(1) (2015) 1-8 corrosion behavior of heat treated al-si-mg(-cu) alloys 2 ply metal (alclad). this cladding acts as a mechanical shield and offers sacrificial protection [3]. when aluminum surfaces are exposed to atmosphere, a thin invisible oxide (al2o3) skin forms, which protects the metal from further corrosion in many environments [1]. this film protects the metal from further oxidation unless this coating is destroyed, and the material remains fully protected against corrosion [3]. the composition of an alloy and its thermal treatment are important do determine the susceptibility of the alloy to corrosion [4,5]. over the years a number of studies have been carried out to assess the effect of cu content and the distribution of second phase intermetallic particles on the corrosion behavior of al alloys. the distribution of cu in the microstructure affects the susceptibility to localized corrosion. intergranular corrosion (igc) is generally believed to be associated with cu containing grain boundary precipitates and the precipitates free zones (pfz) along grain boundaries [6-8]. in heat treatable al-si-mg(-cu) series alloys the susceptibility to localized corrosion (pitting and / or intergranular (igc)) and the extent of attack are mainly controlled by the type, amount and distribution of the precipitates which form in the alloy during any thermal or thermomechanical treatment performed during manufacturing processes [6-10]. depending on the composition of the alloy and parameters of the heat treatment process, these precipitates form in the bulk of the grain, or in the bulk as well as grain boundaries. as indicated by several authors, the precipitates formed by heat treatment in al-si-mg alloys containing cu are the θ (al2cu) q-phase (al4mg8si7cu2), β-phase (mg2si) and free si if si content in the alloy exceeds the mg2si stoichiometry [2-12]. the present study is an attempt to investigate the corrosion behavior of al-6si-0.5mg alloys containing 0.5 and 1 wt % cu in 0.1m nacl solution and to examine corroded surfaces by optical and scanning electron microscopy. experimental materials preparation: the al-6si-0.5mg-xcu(x= 0, 0.5 and 1) alloys were prepared by melting al-7si-0.3mg (a356) alloys and adding al and cu into the melt. the melting operation was carried out in a gas fired clay graphite crucible furnace and the alloys were cast in a permanent steel mould. after solidification the alloys were homogenised (500 o c for 24 hr), solution treated (540 o c for 2 hr) and finally artificially aged (225 o c for 1 hr). after heat treatment rectangular samples (30x10x5 mm) were prepared for metallographic observation and subsequent electrochemical test. deionized water and analytical reagent grade sodium chloride (nacl) were used for the preparation of 0.1 m solution. all measurements were carried out at room temperature. potentiodynamic polarization measurements: a computer-controlled gamry framework tm series g 300™ and series g 750™ potentiostat/galvanostat/zra were used for the electrochemical measurements. the potentiodynamic polarization studies were configured in cells, using threeelectrode assembly: a saturated calomel reference electrode, a platinum counter electrode and the sample in the form of coupons of exposed area of 0.50 cm 2 or 10 x 5 mm as working electrode. only one 10x5 mm surface was exposed to the test solution, the other surfaces being covered with teflon tape. the system was allowed to establish a steady-state open circuit potential (ocp). the potential range selected was -1 to +1v and measurements were made at a scan rate of 0.50 mv/s. the corrosion current (icorr), corrosion potential (ecorr), pitting corrosion potential (epit) and corrosion rate (mm/year) were calculated from tafel curve. the tests were carried out at room temperature in 0.1 m nacl solutions at a fixed and neutral ph value. the corroded samples were cleaned in distilled water and examined under optical light and scanning electron microscope. a. hossain at al. j. electrochem. sci. eng. 5(1) (2015) 1-8 doi: 10.5599/jese.2015.0070 3 electrochemical impedance measurements: as in potentiodynamic polarization test, three electrode cell arrangements were also used in electrochemical impedance measurements. rectangular samples (10 x 5 mm) were connected with copper wire and adopted as working electrode. eis tests were performed in 0.1m nacl solution at room temperature over a frequency range of 100 khz to 0.2 hz using a 5 mv amplitude sinusoidal voltage. the 10 x 5 mm sample surface was immersed in 0.1m nacl solution (corrosion medium). all the measurements were performed at the open circuit potential (ocp). the test cells were maintained at room temperature and the nacl solution was refreshed regularly during the whole test period. the impedance spectra were collected, fitting the experimental results to an equivalent circuit (ec) using the echem analyst tm data analysis software and evaluating the solution resistance (rs), polarization resistance or charge transfer resistance (rct) and double layer capacitance (cp) of the thermal treated alloys. results and discussion impedance measurements table 1 shows the electrochemical impedance spectroscopy (eis) test results. table 1. impedance test results alloy code alloy compositions rs / ω rct / kω cp / µf ocp, v vs. sce alloy-1 al-6si-0.5mg 40.37 15.57 1.259 -0.8454 alloy-2 al-6si-0.5mg-0.5cu 43.93 25.75 1.793 -0.7037 alloy-3 al-6si-0.5mg-1cu 44.08 27.13 3.219 -0.6534 ocp versus time behavior the open circuit potential (ocp) with exposure time of aged al-6si-0.5mg-xcu alloys in 0.1 m nacl solution is shown in table1. large fluctuations in open circuit potential for the alloys were seen during the time of 100 s exposure. after a period of exposure the ocp fluctuation decreased and reached steady state. the steady state ocp of cu free alloy (alloy-1) is -0.8454 v and it is the most negative ocp value among the alloys under investigation. the occurrence of a positive shift in ocp in the al-6si-0.5mg alloys containing 0.5 and 1.0 % cu indicates the existence of anodically controlled reaction. the ocp values mainly depend on the chemical compositions and thermal history of the alloys. the data obtained were modeled and the equivalent circuit that best fitted to the experimental data is shown in figure 1. rs represent the ohmic solution resistance of the electrolyte. rct and cp are the charge transfer resistance and electrical double layer capacitance respectively, which correspond to the faradaic process at the alloy/media interface. figure 1. electrical equivalent circuit used for fitting of the impedance data of al-6si 0.5mg-xcu alloys in 0.1m nacl solution. j. electrochem. sci. eng. 5(1) (2015) 1-8 corrosion behavior of heat treated al-si-mg(-cu) alloys 4 figure 2 shows the nyquist diagrams (suggested equivalent circuit model shown in figure 1) of the al-6si-0.5mg-xcu alloys in 0.1m nacl in de-mineralized (dm) water. in nyquist diagrams, the imaginary component of the impedance (z") against real part (z') is obtained in the form of capacitive-resistive semicircle for each sample. figure 3 shows the experimental eis results in bode magnitude diagram for al-6si-0.5mg-xcu alloys. bode plots show the total impedance behaviour against applied frequency. at high frequencies, only the very mobile ions in solution are excited so that the solution resistance (rs) can be assessed. at lower intermediate frequencies, capacitive charging of the solid-liquid interface occurs. the capacitive value cp can provide very important information about oxide properties when passivation or thicker oxides are formed on the surface. at low frequency, the capacitive charging disappears because the charge transfer of electrochemical reaction can occur and this measured value of the resistance corresponds directly to the corrosion rate. for this reason, this low frequency impedance value is referred to as polarization or charge transfer resistance (rct). figure 2. nyquist plots for the peak-aged alloys 1, 2 and 3 in 0.1m nacl solution. figure 3. bode plots for the peak-aged alloys 1, 2 and 3 in 0.1m nacl solution. a. hossain at al. j. electrochem. sci. eng. 5(1) (2015) 1-8 doi: 10.5599/jese.2015.0070 5 the solution resistance (rs) of the alloys varies from 40-44 ω (table 1) and these values are very similar to each other. so there are insignificant changes of rs values for the alloys during eis testing. the rs values are negligible with respect to rct and the electrolyte behaves as a good ionic conductor. impedance measurements showed that in 0.1m nacl solution, increasing cu in the al-6si-0.5mg alloys increases the charge transfer resistance (rct). for the cu free al-6si-0.5mg alloy (alloy-1), the charge transfer resistance (rct) value in 0.1m nacl solution is 15.57 kω, and this is increased to 25.75 and 27.13 kω with the addition of 0.5 and 1 wt % cu to the al-6si-0.5mg alloy respectively. the increase in the charge transfer resistance indicates an increase in the corrosion resistance of the alloys with cu addition. the double layer capacitance (cp) of the cu free al-6si-0.5 mg alloy (alloy-1) is 1.259 µf, which is the lowest value among the alloys investigated. the double layer capacitance of al-6si-0.5mg alloy increased with an increase in cu content and the maximum was found for alloy-3. potentiodynamic polarization measurements table 2 shows the potentiodynamic polarization test results obtained from the electrochemical tests. table 2. potentiodynamic polarization test results alloy code icorr / µa ecorr / mv epit / mv corrosion rate, mm/year alloy-1 6.300 -764 -480 5.287 alloy-2 5.640 -657 -408 4.732 alloy-3 2.950 -697 -370 2.474 potentiodynamic polarization curves of al-6si-0.5mg-xcu alloys in 0.1m nacl solution are shown in figure 4. anodic current density of al-6si-0.5mg-xcu alloys decreased with cu addition. this is caused by the slowing of the anodic reaction of al-6si-0.5mg-xcu alloy. figure 4. potentiodynamic polarization curves of peakaged alloys 1, 2 and 3 in 0.1m nacl solution. the addition of cu caused the formation of micro-galvanic cells in α-aluminum matrix. the different intermetallic compounds (like mg2si, al2cu etc.) can lead to the formation of microj. electrochem. sci. eng. 5(1) (2015) 1-8 corrosion behavior of heat treated al-si-mg(-cu) alloys 6 galvanic cells because of the difference of corrosion potential between intermetallics and αaluminum matrix. park [13] has also reported that the addition of cu increased the corrosion potential of a number of al–cu–si alloys. for the cu free al-6si-0.5mg alloy (alloy-1) corrosion potential is -764 mv, which is the highest negative potential among the alloys investigated. with increasing cu, the corrosion potential of the alloys shifted towards more positive values. pitting potential (epit) of all cu content alloys also shifted towards more positive values (from -480 mv to -370 mv). potentiodynamic tests showed that in 0.1m nacl solution, increasing cu in the al-6si0.5mg alloy decreases the corrosion current (icorr). for the cu free al-6si-0.5mg alloy (alloy-1), the corrosion current (icorr) value in 0.1m nacl solution is 6.3 µa, and this decreased to 5.640 and 2.950 µa with the addition of 0.5 and 1 wt % cu to the al-6si-0.5mg alloy respectively and the corresponding corrosion rate decreases for these alloys (alloy-2 = 4.732 mpy and alloy-3 = 2.474 mm/year). microstructural investigation the microstructure of some selected as-corroded samples was observed under olm and sem. there was evidence of corrosion products of intermetallic compounds in all the samples examined. besides, several pits were visible in all the samples examined. it is probable that the pits are formed by the intermetallics dropping out from the surface due to the dissolution of the surrounding matrix. however, it is also possible that the pits are caused by selective dissolution of the intermetallic/or particles of the second phase precipitates. osorio et al. [14] have demonstrated that in al-cu-si alloys a more finely and homogeneously distributed al2cu and needle-like si particles in the ternary eutectic mixture, tend to improve the corrosion resistance mainly due to the galvanic protection of both al2cu and si phases [14]. although it has also been reported [14,15] that fine si particles tends to decrease the corrosion resistance of binary al–si alloys when associated with the al2cu intermetallic phase, a better galvanic protection is provided for finer al–cu–si alloy microstructures. it was also reported [16] that the ternary eutectic mixture consisting of al + al2cu + si phases is nobler than the al-matrix and al-phase in the eutectic mixture [16]. consequently, the forms of corrosion in the studied al-6si-0.5mg-xcu alloys are slightly uniform and predominantly pitting corrosion as obtained by the olm and sem. samples were characterized by olm and sem following potentiodynamic polarization tests. the peakaged cu free alloy (alloy-1) exhibited pits on their surface (figure 5), which apparently had nucleated randomly. a b figure 5. (a)olm and (b) sem images show the damage surface morphology of as-corroded t6 aged alloy-1 in 0.1m nacl solution. a. hossain at al. j. electrochem. sci. eng. 5(1) (2015) 1-8 doi: 10.5599/jese.2015.0070 7 conversely, the exposed surface of the alloys exhibited a corrosion product with a rippled appearance covering the surface after polarization. all the optical micrographs (figures 5-6) also showed that there was no corrosion in the fragmented and modified al-si eutectics. a b figure 6. olm images of the damage surfaces morphology of as-corroded t6 aged (a) alloy-2 and (b) alloy-3 in 0.1m nacl solution. conclusions the following conclusions may be drawn from the above investigation: 1. the eis tests have shown that the additions of cu into al-si-mg alloy tend to increase the excellent corrosion resistance of al-si-mg alloy in nacl media. the corrosion resistance, rct value of the alloys shows a maximum at 1 wt % cu. 2. the linear polarization and tafel extrapolation plot show that the corrosion current (icorr) and corrosion rate (mm/year) decrease with increasing of cu content into al-6si-0.5mg alloy. the open circuit potential (ocp), corrosion potential (ecorr) and pitting corrosion potential (epit) in the nacl solution were shifted in the more noble direction due to cu additions into al-6si-0.5mg alloy. 3. consequently, the forms of corrosion in the studied al-6si-0.5mg-xcu alloys are pitting corrosion as obtained from the microstructures study with pits observations. references [1] m. g. fontana, n.d. greene, corrosion engineering, mcgraw-hill book company, new york, 1987, 8-29. 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[16] w. r. osório, l. c. peixoto, d. j. moutinho, l. g. gomes, i. l. ferreira, a. garcia. materials and design 32 (2011) 3832–3837. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {characterization of graphite-epoxy composite electrodes for free electrochemical detection of adenine and guanine in dna:} http://dx.doi.org/10.5599/jese.1005 247 j. electrochem. sci. eng. 11(4) (2021) 247-261; http://dx.doi.org/10.5599/jese.1005 open access : : issn 1847-9286 www.jese-online.org original scientific paper characterization of graphite-epoxy composite electrodes for free electrochemical detection of adenine and guanine in dna leodanis correa fajardo, abel ibrahim balbin tamayo and ana margarita esteva guas department of analytical chemistry, faculty of chemistry, university of havana, zapata s/n between g and carlitos aguirre, vedado, plaza de la revolución, cp 10400. havana, cuba corresponding author: ibrahim@fq.uh.cu received: may 18, 2021; accepted: september 19, 2021; published: october 1, 2021 abstract graphite-epoxy composites (gecs) are alternative construction materials for electrochemical sensors. for these materials, the electron transfer rate constant of some redox reaction depends additionally on the stoichiometric relationship between the insulating and conducting phases of the composite. in this work, the influence of different ratios of araldite/hardener/graphite on the electrochemical properties of gec electrodes is evaluated for the simultaneous determination of adenine and guanine in the single chain dna, using the square wave voltammetry technique. six gec electrodes were prepared with different ratios of components, and electrochemically characterized by cyclic voltammetry in the presence of ferri/ferrocyanide redox couple as a redox probe. gec electrodes that showed the best electrochemical responses of redox probe were characterized by thermogravimetric analysis (tga) and used for the simultaneous determination of free adenine and guanine in a solution, and dna oligonucleotides. the best results were obtained for gec electrodes containing twice higher volume of araldite resin with respect to the hardener. tga analysis revealed presence of 15-26 % of resin for these gec electrodes. the obtained results revealed potential application of these gec electrodes as dna sensors based on the oxidation signal of guanine. keywords electrochemical sensors; dna oligonucleotides; graphite-polymer; thermogravimetric analysis. introduction in recent years, the interest of scientific community in the development of electrochemical sensors as an alternative to dna analysis, has been increased. these devices constitute useful tools in obtaining reliable genetic information in an economical way and real time. due to advantages such as low cost, obtaining of sensitive signals, easy interpretation, short response times, capability http://dx.doi.org/10.5599/jese.1005 http://dx.doi.org/10.5599/jese.1005 http://www.jese-online.org/ mailto:ibrahim@fq.uh.cu j. electrochem. sci. eng. 11(4) (2021) 247-261 graphite-epoxy composite electrodes 248 of working with small volumes, there is also a possibility of their integration into portable devices to carry out measurements in situ [1]. in this aspect, the development of carbon composite electrodes with polymeric matrices has led to a significant progress in electroanalytical chemistry. advantages of polymer composite electrodes include the possibility of being easily modified, what improves their electrochemical properties, enhances their sensitivity and selectivity, and even converts them into specific materials [1,2]. these electrodes can be manufactured into different shapes and sizes, allowing easy adaptation to a wide variety of electronic configurations and application in different media. furthermore, they exhibit a surface that can be renewed with simple polishing without involving the loss of modifiers [3]. compared to pure conducting materials, polymer composites show better signal-to-noise ratio, since they behave like a microelectrode array, which generally contributes to lower detection limits [4,5]. compared to typical voltammetric sensors, polymer composite electrodes have a stable electrochemical response, lower ohmic resistance, and longer functional life [6,7]. there is a number of literature reports on the potential application of graphite-epoxy composite electrodes in the analysis of nitrogenous bases in dna. these electrodes showed higher sensitivity compared to glassy carbon electrodes and other composite materials [1,8]. the electrical properties of graphite-epoxy composite materials such as conductivity depend on the nature of each component, on the insulator/conductor proportions, and their distributions in the matrix of the material [9]. the electrical conductivity of these composites is determined by the connection of the conductive particles within the polymer matrix. therefore, the relative amounts of each component must be evaluated to achieve the optimal relationship [1,5]. until now, the investigations of epoxy-graphite composite electrodes have been focused mainly on their application as electrochemical sensors, leaving aside a study of their electrochemical properties. in some cases the influence of the percentage variation of graphite [10] or modifiers such as benzoic acid, stearic acid and graphene oxide [11-13], have been reported in describing electrochemical responses of graphite-epoxy composite electrodes. in the study of electrochemical properties of composite electrodes, one of the most widely used characterization techniques is cyclic voltammetry (cv) technique that allows an investigation of the redox behavior of an analyte at the electrode surface in a wide range of potentials. the information on the electrode processes, which is obtained in these cases is qualitative, and allows obtaining of characteristic parameters such as potential values of oxidation and reduction, and corresponding peak currents intensities. in addition, some conclusions can be derived about kinetic aspects of electron transfer at the electrode surface, and interfacial aspects such as adsorption of electroactive species on the electrode surface [14]. among other physicochemical characterization techniques, electrochemical impedance spectroscopy (eis) [15,16], scanning electron microscopy (sem) [17,18], differential scanning calorimetry (dsc) [19], thermogravimetric techniques (tg or dtg)[20,21], and spectroscopic techniques such as raman spectroscopy [22] were applied. the main aim of the present article is to evaluate the influence of araldite/hardener/graphite ratios on electrochemical properties of graphite-epoxy composite electrodes, applied for the simultaneous determination of oxidation signals of adenine and guanine of a single dna strand. experimental reagents all reagents used were of analytical quality, while deionized water with a conductivity of 1.26 s cm-1 was used. for the construction of electrodes, pure quality benzyl alcohol (uni-chem) l. correa fajardo et al. j. electrochem. sci. eng. 11(4) (2021) 247-261 http://dx.doi.org/10.5599/jese.1005 249 was used. araldite from beschleuniger dy 964 serva feinbiochemica, gmbh, hr hardener from ciba-geigy polymers, duxford, and fine graphite powder with a particle size of 50 μm from merk (germany) were supplied. the redox pair [fe(cn)6]3-/4of concentration 0.02 mol l-1 was prepared using potassium hexacyanoferrate(ii) trihydrate, and potassium hexacyanoferrate(iii) (merk, germany) in equimolar amounts in pbs buffer ph 6.9 from sigma aldrich. sodium hydrogen phosphate (99 %) and sodium dihydrogen phosphate dihydrate (99 %) from merk (germany) were used for the preparation of phosphate buffer (pb), ph 7 (supporting electrolyte) at 0.02 mol l-1. sodium hydroxide (pure for analysis > 99 %) was used. the adenine and guanine used were sigma grade with a minimum purity of 99 %. adenine solutions of concentrations between 10-60 mol l-1 were prepared using pb, ph 7. guanine was initially dissolved in sodium hydroxide at 0.01 mol l-1 and subsequently, solutions of concentrations between 10-60 mol l-1 were prepared using pbs, ph 7. likewise, mixed solutions of adenine and guanine were prepared. the single chain dna (ssdna) was the st 36ar oligonucleotide, 100 µmol l-1 from eurofins mwg operon corp. canada, with the sequence 5´-tgactcgttgtcgataccgacagc-3´, which was acquired from the center for protein studies, faculty of biology, university of havana. equipment the voltammetric method was developed using a palmsens potentiostat attached to a computer, managed by the pstrace software version 5.2, and operating in cyclic voltammetry and square wave voltammetry modes for voltammetric analyses. a three-electrode system was used, with graphiteepoxy composite working electrode(s), an auxiliary platinum electrode, and the reference ag/agcl (kcl, 3 mol l-1) electrode, from basic analytical systems (bas). thermogravimetric analysis (tga) was performed on a netzsch model 449 f3 jupiter thermal analyzer. deionized water was obtained from the water purifier, milli-q, hpw pure water system, heal-force (china). preparation of graphite-epoxy composite electrodes for the construction of graphite-epoxy composite (gec) electrodes, mixtures with the same mass of the conductive phase (graphite) and different ratios between the polymer (araldite) and hr hardener (diamine) in the insulating phase were prepared. replicas of six types of composites (geci to gec-vi) electrodes with different ratios of araldite/hardener/benzyl alcohol were constructed (table 1). benzyl alcohol was used to reduce the viscosity of the composite mixture. the theoretical percentages of graphite in the electrodes are also shown in table 1. table 1. epoxy resin/hardener/benzyl alcohol volume ratio and theoretical content of graphite in gec-i to gec-vi electrodes electrode volume, ml graphite content, % epoxy resin hardener benzyl alcohol gec-i 5.8 1 2 59.6 gec-ii 3.8 1 2 67.4 gec-iii 2.0 1 2 77.4 gec-iv 5.4 2 1 55.8 gec-v 3.6 2 1 62.5 gec-vi 2.0 2 1 71.0 the electrodes were constructed by introducing the composite mixture obtained in each case into glass capillaries of approximately 1 mm in diameter. for electronic contact, a copper wire was inserted. the prepared electrodes were heated for 48 hours at 40 oc in an oven to dry. later, the electrode surface was polished to perform the corresponding tests [13,22]. http://dx.doi.org/10.5599/jese.1005 j. electrochem. sci. eng. 11(4) (2021) 247-261 graphite-epoxy composite electrodes 250 experimental procedure for the electrochemical characterization of the constructed gec electrodes, cyclic voltammetry (cv) technique was applied. using a three-electrode system, the electrochemical response of the redox system [fe(cn)6]3-/4at prepared composite electrodes was evaluated. the analytical signal was obtained by applying a triangular potential sweep between -700 and 700 mv, with an increase of 4 mv, while scan rate (𝜈) was varied from 20-100 mv s-1. square wave voltammetry (swv) technique was used to determine free purines. the electrochemical signal was obtained by applying a potential sweep between 500 mv and 1600 mv, at the frequency of 10 hz, and a pulse width of 10 mv. to carry out this analysis, the electrodes were previously mechanically polished [13,22], electrochemically cleaned with several cycles of cyclic voltammetry (from 500 to 1200 mv, at 50 mv s-1) in 0.1 mol l-1 sodium hydroxide solution, and finally washed thoroughly with distilled water. for the electrochemical analysis of dna, gec electrodes were mechanically [13,22] and electrochemically cleaned as explained above. to detect the signs of oxidation of adenine and guanine in the oligonucleotide, the latter was heated in a water bath to the boiling temperature (t = 78 oc) in order to achieve elongation of dna oligonucleotide chains [23]. subsequently it was deposited on the electrode by means of wet adsorption, and a square wave voltammetry was performed in 0.02 mol l-1 pb, ph 7 under the aforementioned conditions. for the thermal analysis of graphite-epoxy composites by means of thermogravimetric analysis (tga), the thermograms were obtained in a temperature range of 25-1000 c, at a heating rate of 10 c min-1 and ar flow of 20 ml min-1. results and discussion electrochemical characterization of gec electrodes by [fe(cn)6] 3-/ 4redox system the reversibility of the redox system [fe(cn)6]3-/4was tested on the prepared gec electrodes by cyclic voltammetry (cv) technique. from cvs obtained, the influence of the potential scanning rate (𝜈) on the current intensity values and the potential difference between redox peaks (δe) were evaluated. figure 1 shows cyclic voltammograms (cvs) of gec-i to gec-vi electrodes, recorded at various potential scan rates. cvs of composite electrodes shown in fig. 1 are characteristic for the redox system [fe(cn)6]3-/4-. at negative potentials a peak with negative current (cathodic peak), corresponding to the reduction of fe(cn)63(ferricyanide) ions is observed and at higher potentials the anodic peak with positive current intensity, corresponding to the oxidation of fe(cn)64(ferrocyanide) is presented [24]. in all cases, an increase in the peak current intensity and slight increase in the potential difference between two current peaks can be observed by increasing the scan rate [14,24-26]. figure 2 shows the variation of potential difference between redox peaks (δe) with the scan rate for each gec electrode. for all gec electrodes, δe values increase more or less with the sweep rate and are generally higher than the values already reported in the literature for a reversible system [14,24-26]. this can be influenced by the variation in the composition of the graphite-epoxy composite and its interaction with possible molecular rearrangements during the electrode processes on the electrode surface, without bond breakage and mass transfer phenomena occurring [14,26]. for gec-i and gec-iv electrodes it is observed that δe values are the highest, and also show greater dependence on the potential sweep rate. this may be due to the fact that the applied l. correa fajardo et al. j. electrochem. sci. eng. 11(4) (2021) 247-261 http://dx.doi.org/10.5599/jese.1005 251 potential does not create necessary concentrations, according to the nernst equation at the electrode surface, and therefore an overpotential is generated. figure 1. cvs of gec-i to gec-vi electrodes in 0.02 mol l-1[fe(cn)6]3/ 4-for 0.02 mol l-1 pbs ph 6.9, at scan rates between 20-100 mv s-1 figure 2. variation of δe with potential scan rate (𝜈) for gec-i to gec-vi electrodes http://dx.doi.org/10.5599/jese.1005 j. electrochem. sci. eng. 11(4) (2021) 247-261 graphite-epoxy composite electrodes 252 in this aspect, the energy required to carry out the electrode processes is much higher than for other gec electrodes. this may be associated with an increase in the resistance of the composite, as a result of a higher content of the insulating phase (lower content of graphite) compared to other gec electrodes (table 1). in contrast, gec-iii, gec-v, gec-vi electrodes show lower δe values that are almost independent on the potential sweep rate. in this case, the behavior of the electrodes is characteristic for the systems that are closer to reversibility, indicating that oxidation-reduction process on the electrode surface occurs with less overpotential and energy cost [14]. gec-ii electrode showed an intermediate behavior, indicating less electrochemically active surface in this case. these results can be associated with three levels of graphite percentage (cf. table 1). the most dependent on the potential sweep rate are gec-i and gec iv electrodes, with percentages of graphite between 55-60 %. the electrodes which present the behavior closer to reversible are geciii and gec-vi electrodes, with percentages of graphite between 71-77 %. among the remaining gec-ii and gec-v electrodes with percentages of graphite around 65 % [10] only gc-ii electrode showed an intermediate behavior, while gec-v electrode, despite similar percentage of graphite, showed much better electrochemical response of [fe(cn)6]3-/4-redox pair. this may be associated with the influence of the araldite/hardener ratio on the curing of the compound [27] on its electrochemical properties. therefore, it can be assumed that when the araldite/hardener ratio is about 2:1 (table 1), better electrochemical properties of electrodes are achieved, which can be independent of the percentage of graphite in a narrow range. among here manufactured electrodes this araldite/hardener ratio was accomplished for gec-iii and gec-v electrodes. kinetic parameters of the redox system [fe(cn)6] 3/4 at gec electrodes to analyze the influence of the scan rate on the peak current intensity, it was taken into account that for a quasi-reversible processes controlled by diffusion, the peak current is defined according to the randles-ševčik equation [25,28], which is at 25 oc defined as: ip = 2.99×105 1/2 n3/2a d1/2 c 1/2 (1) in equation (1), ip is peak current intensity in a,  is electron transfer coefficient, n is the number of electrons exchanged, a is area of the electrode in cm2, d is diffusion coefficient of redox species in cm2 s-1, c is concentration of this species within the solution in mol cm-3 and  is the scan rate in v s-1. taking this equation into account, for diffusion-controlled processes, the peak current intensity would be proportional to the square root of the sweep rate. figure 3 shows the linear relationship of the anodic peak current (ip) as a function of the square root of the sweep rate for each gec electrode. in all cases, an increase in ip was observed with increasing potential sweep rate. in addition, since the intensity of the current is proportional to the flow towards the surface of the electrodes, it can be argued that this magnitude is small at low sweep rates and increases when the potential is swept at higher speeds [2]. from figure 3 it is observed that for all gec electrodes, the anodic peak current values were well adjusted to straight lines with correlation coefficients greater than 0.98. these results indicate that the electrode processes that occur on the surface of the electrodes are controlled by diffusion within the experimental error, and the time scale of the experiment. additionally, the constant term of the randles-ševčik equation [(2.99·105) α1/2 n3/2 a d1/2 c] was evaluated, as a slope of the linear regression between ip vs. 𝜈1/2, that is a measure of sensitivity of the electrode. parameters of linear regression for gec-i to cec-vi electrodes are summarized in table 2. l. correa fajardo et al. j. electrochem. sci. eng. 11(4) (2021) 247-261 http://dx.doi.org/10.5599/jese.1005 253 figure 3. linear regression of anodic ip as a function of 𝜈1/2 for gec-i to gec-vi electrodes table 2. parameters of linear regression of ip vs. 1/2 for gec-i to gec-vi electrodes. electrode slope r2 electrode slope r2 gec-i 58 ± 2 0.993 gec-iv 37 ± 3 0.982 gec-ii 99 ± 5 0.989 gec-v 131 ± 8 0.984 gec-iii 157 ± 6 0.995 gec-vi 148 ± 8 0.991 the results shown in table 2 reveal greater slope values for gec-iii and gec-vi electrodes, and to a lesser extent for gec-v electrode, which is indicative of greater sensitivity of these electrodes against the redox system [fe(cn)6]3-/4-. to identify the possible existence of coupled chemical reaction(s) during the electrode processes, the ratio of anodic (ipa) and cathodic (ipc) peak current intensities for each type of gec electrode was analyzed. for a reversible system without kinetic complications, the peak currents ipa and ipc are equal, i.e. the ratio ipa/ipc ≈ 1, and independent of the potential sweep rate and diffusion coefficients [25,26]. figure 4 shows that for all analyzed gec electrodes, the ratio between peak currents remains practically constant with the increase of potential sweep rate. additionally, for all electrodes it can be seen that at any , the peak current ratio takes value greater than unity, which may indicate possible coupled chemical reaction carried out during the charge transfer processes [26]. figure 4. peak current ratio (ipa/ipc) for gec-i to gec-vi electrodes as a function of the potential scan rate 𝜈 heterogeneous electron transfer rate constants (ko) were determined for each gec electrode. from δe values obtained in cv profiles, the values of the dimensionless kinetic parameter  were http://dx.doi.org/10.5599/jese.1005 j. electrochem. sci. eng. 11(4) (2021) 247-261 graphite-epoxy composite electrodes 254 estimated, taking into account the relationship between  and δep proposed by nicholson in 1965. ko values were determined according to the following equation [29]. 1/2 o o nf k d rt      =     (2) where do is diffusion coefficient of the oxidized species (cm2 s-1), n is the number of electrons transferred during the redox reaction,  is potential scanning rate (v s-1), f is faraday constant (96485 c mol-1), r is ideal gas constant (8.314 j mol-1k-1), and t is absolute temperature (298.15 k). during this analysis it was observed that gec-i, gec-ii, and gec-iv electrodes showed a high resistance to electron transfer, since for these cases, values of the constant ko are below the limit that can be determined by the nicholson method (< 4.6×10-3 cm s-1) [14]. on the other hand, the electrochemical reactions that occur on the surface of the remaining electrodes are carried out with appreciable rates, as shown by their ko values (gec-iii: (1.2 ± 0.7)×10-2; gec-v: (1.0 ± 0.4)×10-2; gec-vi: (6.6 ± ± 1.7)×10-3 cm s-1). for these latter electrodes, it could be observed that the values of the heterogeneous electron transfer constant are within the range of the quasi-reversible processes, considering the limiting scanning speeds (3×10-31/2  ko  2×10-7 1/2) [26]. gec-iii, gec-v and gec-vi electrodes showed the highest ko values, indicating that the kinetics of the electron transfer that occurs on the electrode surfaces during redox process is faster, and typical for a quasi-reversible system [24]. these results correspond to the higher percentage of graphite (conductive phase) of these electrodes with respect to the rest. in spite of somewhat lower graphite content, however, gec-v electrode showed ko value very similar to that of gec-iii electrode, which may be associated with the influence of the araldite/hardener ratio on the electrochemical properties of the electrode. individual and simultaneous determination of adenine and guanine at gec electrodes square wave voltammograms recorded for gec-iii, gec-v and gec-vi electrodes in adenine containing pb ph 7 solution, are shown in figure 5. for each gec electrode, well defined oxidation peak of high symmetry is observed at 950 mv. for gec-iii and gec-v electrodes, the increase of adenine concentration caused proportional increase in the current intensity of the oxidation peak for this molecule. the linear regression between the oxidation peak current and adenine concentration was significant for these two electrodes, with high values of correlation coefficients (gec-iii: r2= 0.993, gec-v: r2= 0.994). in contrast, gec-vi electrode is obviously less sensitive to changes in adenine concentration, since the linear regression between the current intensity of the oxidation peak and adenine concentration was not significant (r2 = 0.927). this result for gec-vi electrode can be influenced by the surface passivation that causes reduction of electrochemical activity of the surface, thus establish the sensitivity of the electrode to change in adenine concentration [30,31]. gec-iii and gec-v electrodes were used for the simultaneous determination of free adenine and guanine mixed in pb solution. figure 6 shows swvs for each type of gec electrode, where it is observed that in both cases, the increase in concentration caused proportional increase in the intensity of oxidation peaks for both molecules. it can be seen from figure 6 that adenine and guanine generate highly defined and symmetrical oxidation peaks, separated by δe of around 270 mv. almost no difference in the oxidation potential values for guanine and adenine between two gec electrodes is observed. l. correa fajardo et al. j. electrochem. sci. eng. 11(4) (2021) 247-261 http://dx.doi.org/10.5599/jese.1005 255 figure 5. swvs of different concentrations of adenine in 0.02 mol l-1pb (ph 7) for gec-iii, gec-v and gec-vi electrodes figure 6. swvs of different concentrations of mixed adenine and guanine solutions in 0.02 mol l-1 pb (ph 7) for gec-iii and gec-v electrodes compared to other electrodes already used for the development of dna sensors, the oxidation potentials of adenine (a) and guanine (g) at gec-iii and gec-v electrodes are somewhat lower than previously reported. thus, for glassy carbon electrodes modified with graphene and nafion, 820 mv for g and 1180 mv for a were obtained by differential pulse voltammetry [32]. for graphene electrodes modified with platinum nanoparticles, 830 mv for g and 1190 mv for a, were determined by linear scanning voltammetry [33]. for carbon composite electrodes and nanoparticles of graphene oxide modified with poly (l-cysteine), magnetite and dna (ds-dna/p(l-cys)/fe3o4 npsgo/cpe), 830 mv for g and 1120 mv for a were determined by differential pulse voltammetry [34]. in benzoic acid modified graphite-epoxy composite electrodes, 620 mv for g and 1260 mv for adenine, were obtained by square wave voltammetry [22]. all these results suggest that the purine bases showed the best electrochemical response at here built graphite-epoxy electrodes. http://dx.doi.org/10.5599/jese.1005 j. electrochem. sci. eng. 11(4) (2021) 247-261 graphite-epoxy composite electrodes 256 at gec-iii and gec-v electrodes, guanine oxidizes at lower potential than adenine, which indicates that the electrochemical oxidation processes for this molecule occurs with a lower overpotential and with lower energy consumption, which is in the correspondence with previous reports [31,35]. figure 7 represents the graphs of linear regressions between the values of current intensity of oxidation peaks and concentrations of adenine and guanine, respectively. for both gec electrodes, the peak current values are adjusted adequately to straight lines for both adenine and guanine, giving high correlation coefficient values. results of regression procedures for adenine and guanine are summarized in table 3. figure 7. linear regression between peak oxidation current and concentration of adenine and guanine for gec-iii and gec-v electrodes table 3. results of linear regression between ip and concentration of adenine and guanine at gec-iii and gec-v electrodes electrodes slope, a l mol-1 standard error, a l mol-1 intercept, a standard error, a equation adenine gec-iii 0.0116 0.0005 1.1×10-7 2×10-8 ip=0.0116c + 1.1×10-7 gec-v 0.0072 0.0002 6.0×10-8 7×10-9 ip=0.0072c + 6.0×10-8 guanine gec-iii 0.0067 0.0004 2×10-8 10-8 ip = 0.0067c + 2×10-8 gec-v 0.0060 0.0002 -1.1×10-8 8×10-9 ip = 0.0060c – 1.1×10-8 from the results reported in table 3, it appears that gec-iii electrode, having higher slope values is more sensitive to the determination of both molecules compared to gec-v electrode. these results indicate that the electrochemical oxidation processes at gec-iii electrode surface is carried out with faster electron transfer kinetics, what corresponds to the determined heterogeneous electron transfer rate constant (ko) value. as expected, both electrodes are found more sensitive to adenine determination, since this molecule is more electrochemically active and exchanges larger number of electrons during oxidation processes [30,31]. electro-oxidation of adenine and guanine in dna oligonucleotide purine bases were determined in the single-stranded dna sample (st 36ar oligonucleotide), using gec-iii and gec-v electrodes since they showed better electrochemical responses of the redox system [fe(cn)6]3-/4-, and better results in the simple determination of adenine (figure 5) and simultaneous determination of adenine and guanine (figure 6). figure 8 shows swvs of singlestranded dna in pb, ph 7.0 solution at gc-iii and gce-v electrodes. l. correa fajardo et al. j. electrochem. sci. eng. 11(4) (2021) 247-261 http://dx.doi.org/10.5599/jese.1005 257 figure 8. swvs of 100 mol l-1 single-stranded dna solution (st 36ar oligonucleotide) in 0.02 mol l-1 pb (ph 7) for gec-iii and gec-v electrodes it can be seen in figure 8 that for both gec electrodes, the presence of st 36ar oligonucleotide generates two very well defined and highly symmetrical oxidation signals, separated by δe of 280 mv. gec-iii electrode showed more intense oxidation peaks compared to gec-v electrode, as a consequence of the greater sensitivity of gec-iii electrode in detecting both nitrogenous bases, as already shown in figure 7 and table 3. as can be seen, δe that separates two oxidation signals in the oligonucleotide is very similar to that obtained for free purines (approximately 270 mv in figure 6). this was used as a criterion to identify that the signal at 960 mv is due to the oxidation of guanine, while the signal at 1240 mv to adenine oxidation. furthermore, in this potential range, the main oxidation signals are obtained to determine single dna strands. [30,31]. as shown in figure 9, the potentials at which the purine bases in the st 36ar oligonucleotide are oxidized, are higher than oxidation potentials provided when these nitrogenous bases are in their free form [30,31]. figure 9. swvs of 100 mol l-1 single-stranded dna solution and 10 mol l-1 mixed solution of adenine and guanine in 0.02 mol l-1 pb (ph 7) for gec-iii and gec-v electrodes the higher oxidation potentials of adenine and guanine in the oligonucleotide compared to the oxidation potentials of free purine bases is associated with the difficulty of oxidation of these nitrogenous bases when they are committed within a single dna chain. this may be associated with steric impediments due to phosphate groups in the dna chain and possible interactions that can occur http://dx.doi.org/10.5599/jese.1005 j. electrochem. sci. eng. 11(4) (2021) 247-261 graphite-epoxy composite electrodes 258 between complementary bases in the dna chain, causing it to fold and at the distance that separates the purine bases and the electroactive surface of the electrode [30,31]. all these molecular factors can cause an overpotential to be generated to achieve oxidation of the purine bases. thermogravimetric analysis (tga) of gec-iii, gec-v and gec-vi electrodes the electrodes that showed better electrochemical responses (gec-iii, gec-v, gec-vi) for ferri/ferrocyanide couple, were characterized by tga. tga was performed, in order to evaluate the composition and analyze the stoichiometry of the curing reaction between the epoxy resin and diamine (hardener). figure 10 shows tg and dtg curves obtained. t / oc figure 10. thermogravimetric analysis of gec-iii, gec-v and gec-vi electrodes the thermograms obtained for three chosen gec electrodes showed fairly similar behavior. two losses of mass were generally detected, where the first loss of mass was identified between 100-270 c, corresponding to decomposition of the hardener that remained unreacted. the second loss of mass took place around 270-480 c, corresponding to thermal decomposition of the epoxy resin. the decomposition temperature intervals shown in this analysis correspond to those obtained evaluating the components separately, as is already reported in the literature [36,37]. in compositions of three analyzed gec electrodes, an excess of hardener was detected, as expected according to the stoichiometry of the reaction, because the amount of substance of the equivalent of the epoxy resin initially used for the construction of the electrodes, was greater than the amount of substance of the equivalent of the hardener. from tga results summarized in table 4, it can be seen that gec-iii and gec-vi electrodes show very similar composition. the experimental values obtained from the percentage of residue are close to the added graphite values. the difference can be associated to the residual carbon product of the decomposition of the excess hardener and the resin. the greater percentage of excess hardener detected for gec-vi electrode can explain its low surface electrochemical activity in the simple determination of adenine. the lower sensitivity of gec-v electrode in the determination of purines with respect to gec-iii electrode, may be associated with its higher content of insulating phase, which causes lower surface reactivity to the redox process of the nitrogenous bases of dna. r e si d u a l w e ig h t lo ss , % (d m /d t ) / % o c -1 l. correa fajardo et al. j. electrochem. sci. eng. 11(4) (2021) 247-261 http://dx.doi.org/10.5599/jese.1005 259 table 4. composition obtained from tga data for gec-iii, gec-v and gec-vi electrodes electrode content, wt.% hardener resin residue gec-iii 7.2 15.2 77.6 gec-v 6.2 26.1 67.7 gec-vi 8.1 13.7 78.2 this analysis shows that percentages of resin (insulating phase) between 15-26% ensure adequate electrochemical response of the graphite-epoxy composite in the determination of nitrogenous bases in dna. conclusions graphite epoxycomposite (gec) electrodes with different epoxy resin/hardener/graphite ratios were prepared as sensors for adenine and guanine detection in dna. the analysis of cyclic voltammetry experiments performed using the system [fe(cn)6]3-/4-as a redox probe showed a quasi-reversible behavior, i.e. the redox reaction controlled by diffusion, for all prepared gec electrodes. more detailed kinetic analysis showed, however, that more reversible response of redox probe is indicated at gec electrodes having twice higher volume of araldite resin with respect to the hardener. the results of thermogravimetric analysis showed that gec electrodes with best electrochemical response to redox probe, and best results in detection of free purine bases in solution and singlestranded dna, possess 15-26 % of insulating phase (epoxy resin). these gce electrodes were successfully applied for the analysis of adenine, mixture of free adenine and guanine in the solution and single-stranded dna. the obtained results may serve for 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distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1005 https://doi.org/10.1016/j.ab.2004.06.021 https://doi.org/10.1016/j.procbio.2010.07.004 https://doi.org/10.1016/j.electacta.2014.06.163 https://doi.org/10.1016/j.bios.2017.11.002 https://doi.org/10.1016/j.electacta.2012.11.014 https://doi.org/10.48317/imist.prsm/morjchem-v4i1.3696 http://www.ijias.issr-journals.org/authid.php?id=1672 https://creativecommons.org/licenses/by/4.0/) {evaluation of the electrochemical performance of ag containing aao layers after extended exposure to a model corrosive medium} http://dx.doi.org/10.5599/jese.820 317 j. electrochem. sci. eng. 10(4) (2020) 317-334; http://dx.doi.org/10.5599/jese.820 open access: issn 1847-9286 www.jese-online.org original scientific paper evaluation of the electrochemical performance of ag containing aao layers after extended exposure to a model corrosive medium stephan kozhukharov1,, christian girginov2, denitsa kiradzhiyska3, aleksander tsanev4 and georgy avdeev5 1lamar laboratory for advanced materials research, university of chemical technology and metallurgy, sofia, bulgaria 2department of physical chemistry, university of chemical technology and metallurgy, sofia, bulgaria 3medical university plovdiv, department of chemical sciences, faculty of pharmacy, plovdiv, bulgaria 4institute of general and inorganic chemistry, bulgarian academy of sciences, sofia, bulgaria 5institute of physical chemistry, laboratory of x-ray diffraction methods, bulgarian academy of sciences, sofia, bulgaria corresponding author: stephko1980@abv.bg; tel.: +35 9899837282 received: march 25, 2020; revised: may 30, 2020; accepted: may 31, 2020 abstract the coating procedure appears to be an indispensable finishing stage in the production of al based industrial products, engineering facilities and equipment. for this reason, there is an ever-increasing interest towards the elaboration of reliable corrosion protective layers with apparent coverage, adhesion, and barrier properties. in this sense, both the formation of anodized aluminum oxide (aao) layer and its further modification with silver enable the elaboration of advanced (al-o-ag) films with extended beneficial characteristics. the present research activities are aimed at the determination of the corrosion protective properties of electrochemically synthesized al-o-ag layers on the technically pure aa1050 alloy. the structures and compositions of the obtained al-o-ag layers were characterized by x-ray diffractometry (xrd) and x-ray photoelectron spectroscopy (xps). the research activities were accomplished by means of two independent electrochemical characterization methods: electrochemical impedance spectroscopy (eis) and potentiodynamic scanning (pds). the electrochemical measurements were performed after 24, 168 and 672 hours of exposure to 3.5 % nacl solution used as a model corrosive medium (mcm), in order to determine the barrier properties and durability of the elaborated al-o-ag layers. the analysis of the obtained results has undoubtedly shown that the proposed electrochemical al-o-ag layer formation can successfully be used for the creation of self-standing layers with apparent corrosion protective properties. besides, al-o-ag system can be used as a basis for http://dx.doi.org/10.5599/jese.820 http://dx.doi.org/10.5599/jese.820 http://www.jese-online.org/ mailto:stephko1980@abv.bg j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 318 development of efficient protective layers suitable for application in biologically contaminated media. keywords al-o-ag layers, porous structures, compositions, protective abilities. introduction the widest field of use of the low doped al alloys, such as aa1050 is the packaging of various nutrition products and soft drinks [1-4]. however, reliable packaging requires additional surface protection of al-based materials, prior to being in contact with whatever nutrition product. this requirement arises for prevention of al-ions resorption from such packaging materials [5-7], which endangers human health [8,9]. anodization is a simple and reliable method for the formation of highly-ordered and self-organized porous oxides [10-14]. this method enables the formation of reliable, large-scale, protective oxide layers using relatively simple equipment. besides, the resulting anodized aluminum oxide (aao) possesses remarkable corrosion protective abilities after extended periods of exposure to model corrosive media [15,16]. the incorporation of silver is an efficient method for the further extension of the corrosion protective properties of the already formed aao layers because ag-compounds prevent the occurrence of microbially influenced corrosion (mic). mic is an electrochemical process where micro-organisms can initiate, facilitate or accelerate corrosion reactions through interaction among the components of the metal-solution-microorganisms system [17]. according to recent research, mic can appear in various media [18,19], including water supply and irrigation pipe systems [19], marine environments [20], in both tropic [20] and arctic [21] conditions, also in fuel tanks [22,23], etc. figure 1 presents a schematic illustration of localized mic on aluminum alloy, according to the concepts described in [14]. figure 1. schematic presentation of microbial tubercle causing localized mic according [14] the beneficial properties of silver and its ions are not limited only to their confirmed bactericide properties, recently widely investigated [24–40]. materials, containing this element possess also definitively catalytic [41–43] and photocatalytic [44–57] properties. furthermore, bu et al. [58] have proposed even hydrogen production by ag2o based photocatalyst. the specific combination of the benefits provided by anodization and the further deposition of silver applied to the widely used low-doped aluminum alloy has inspired the present research on the surface characteristics and corrosion protective abilities of the resulting al-o-ag films in comparison to unmodified aao layers and bare al samples. in this sense, the present study is an s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 319 extension of previous systematic research works [59,60] devoted to such electrochemically synthesized al-o-ag layers. the aim of the present paper is to clarify the structural and compositional features of the electrochemically synthesized al-o-ag films on the conventional aa1050 alloy and to define their corrosion protective capabilities up to 672 hours of exposure to 3.5 % nacl solution (serving as mcm). the clarification of these aspects should further extend the investigations already performed on the proposed electrochemically synthesized al-o-ag layer. experimental surface treatments and films formation all experimental procedures were performed on three sets of samples, represented by two al aa1050 coupons. the metallic samples underwent the surface treatment procedures, described in table 1. table 1. description of samples and film preparation conditions sample groups surface treatment procedures references preliminary treatments aao layers preliminary treatments anodization al-o-ag layers preliminary treatments anodization silver deposition preliminary treatment: this step included annealing, degreasing in ccl4 and final washing with double-distilled water, followed by brightening in solution, containing cro3 (20 g dm-3) and 85 % h3po4 (15 cm3 dm-3), at 85 °c. these procedures were finished by further cleaning with tap and double-distilled water and subsequent drying at room temperature. anodization: this process was performed in 15 wt% h2so4 at constant current density (15×10-3 a cm-2) at 20 °с, for 50 min. two of these samples were set apart and denominated as aao layers. finally, all anodized specimens were washed with tap and double-distilled water and dried at room temperature. silver deposition: this final stage was carried out by ac-polarization (sinusoidal, 50 hz) at a nominal voltage of 20 v in an electrolyte, composed of agno3 (1.51 g dm-3) and h3bo3 (45 g dm-3), at 20 °с. the resulting specimens were designated as al-o-ag film samples. finally, these samples were also cleaned and dried under the above-mentioned conditions. comparative test procedures the already treated samples underwent surface analyses by means of x-ray diffractometry (xrd), x-ray photoelectron spectroscopy (xps) and also electrochemical analyses after extended periods of exposure to chloride containing mcm. x-ray diffractometry: this analysis was performed with a philips pw 1050 vertical automatic diffractometer, equipped with a secondary graphite monochromator, operating with cukα radiation and scintillation counter. the diffraction curves were recorded in angular interval from 10 to 80 angular degrees 2θ with step 0.04° and 1 sec exposure. xps analysis: xps measurements were carried out on preliminary scraped al-o-ag layer by axis supra electron-spectrometer (kratos analytical ltd.) using achromatic alkα radiation with a photon energy of 1486.6 ev. aiming for more detailed spectra, the emission current for the spectra of ag3d and ag mnn was set to 25 ma and the resolution was 40, while for the wide scan the emission current was 15 ma and the resolution was 160. the energy calibration was performed by http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 320 normalizing the c1s line of adsorbed adventitious hydrocarbons to 284.6 ev. the binding energies (be) were determined with an accuracy of ± 0.1 ev. the changes in composition and chemical surrounding in the depth of the films were determined monitoring the areas and binding energies of c1s, o1s, ag3d and al2p photoelectron peaks. using the commercial data-processing software of kratos analytical ltd. the concentrations of the different chemical elements (in atomic %) were calculated by normalizing the areas of the photoelectron peaks to their relative sensitivity factors. electrochemical analysis: these measurements were performed employing two analytical electrochemical methods, after 24, 168 and 672 hours of exposure to 3.5 % nacl solution as model corrosive medium (mcm). these electrochemical techniques were electrochemical impedance spectroscopy (eis) and potentiodynamic scanning (pds). the eis spectra acquisitions were performed using a three-electrode configuration connected to autolab pg-stat 30, equipped with a frequency response analyzer (fra-2 module). the electrochemical cells containing 100 ml of mcm, provided radial concentric positioning of the electrodes. all measurements were performed against an ag/agcl/3m kcl reference electrode positioned 10 mm above the corrosion test areas (cta) of the specimens. the electrochemical cell was completed with a cylindrical pt-mesh mounted around the reference electrode. the cta of the samples (figure 2) was determined to be 2 cm2. figure 2. images of investigated samples after extended periods of exposure to 3.5 % nacl model corrosive medium the eis spectra were acquired in the frequency range from 10 khz to 0.01 hz, distributed in 50 measurement points with suitable amplitudes of the excitation signal against the open circuit potential (ocp), in order to acquire readable spectra. in this study, these amplitudes were 10 mv for the referent samples and 80 mv for the electrochemically treated ones, respectively. the application of different amplitudes was imposed by the need for overcoming the remarkable resistance of the aao and al-o-ag layers, resulted from the dielectric properties inherent for the basic al2o3 matrix. on the other hand, this amplitude value was too high for the reference samples, and causes polarization of the electrode, resulting in deformed impedance spectra and electrochemically enforced excessive corrosion. s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 321 the ocp values were determined directly prior to the respective eis measurements. the subsequent potentiodynamic curves were acquired after the eis spectra. the pds were obtained from -30 to +500 mv against ocp with a voltage sweep rate of 5 mv s-1. since previous works [15,16], dedicated to anodization of technically pure aa1050 alloy, have shown that the electrochemical behavior of the samples reveals satisfying repeatability, the measurements were performed twice. results and discussion x-ray diffractometry the xrd spectra did not reveal any distinguishable features among the different samples. only characteristic peaks for al metallic substrates were registered for all types of samples. however, at low angles, between 10 and 30 angular degrees, a more intensive noise signal was registered for aao and al-o-ag layers, which was clearer in the former case (figure 3). 2 / o figure 3. superposition of xrd spectra acquired from samples of each investigated set these features reveal that both aao and al-o-ag layers possess definitely amorphous structures and no crystalline phase was formed. the flatter noisy plateau in the case of al-o-ag layer compared to the one of the aao layer, is probably related to the shielding of aao pore walls and bottoms by rather thin films of ag deposits. this explication seems more plausible than any suggestion related to alterations of the thickness of the basic porous aao matrix, promoted by the electrochemical al-o-ag layer formation. indeed, previous research has shown that the thickness of formed aao layer under such conditions is about 20 μm [61,62] and the ac-assisted silver deposition does not result in alteration of the oxide layer thickness [59,60]. consequently, the lower xrd plateau in the case of alo-ag is probably a result of the quenching of the x-ray signal, reflected by the porous aao matrix. the chemical composition of the al-o-ag system and oxidation states of its components were further investigated by means of xps. xps analysis in a previous study [60], we have not succeeded to detect any silver traces on the surface of the al-o-ag system, because ag deposition proceeds on the pore bottoms and walls, the present sample was preliminary scraped by 1200 grid sandpaper prior to xps analysis. this approach has enabled to http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 322 detect and analyze the silver deposits inside the pores of the oxide film. the xps patterns acquired from the combined al-o-ag layer are shown in figure 4. binding energy, ev binding energy, ev binding energy, ev figure 4. integral and convoluted xps spectra acquired from al-o-ag film the detailed ag3d spectrum possessed low intensity, being rather noisy due to the low content of this element inside the oxide layer (0.4 at%). the further deconvolution of this spectrum has revealed that this element is in the form of ag+ ions, since the main peak of ag3d5/2 is positioned at 368.0 ev, and the spin-orbital splitting is 6 ev, being typical for the ionic state of silver. some negligible traces of metallic ag0 were detected, as well. the auger spectrum of this element [63] also confirms the ionic state of this element. the spectrum noise has imposed further linear fitting. as a result, it was established that at both peaks associated with ag m5n45n45 and ag m4n45n45, the auger electrons are situated at 352.4 ev and 359.9 ev, respectively. the values of the modified auger electron parameters for these peaks are 720.4 and 727.9 ev, respectively. consequently, silver deposition proceeds with oxidation of silver. following the basic concepts [64-69], ag oxidation probably proceeds with the inward o2ion migration from the electrolyte, across the surface of aao pore bottoms and walls. this oxidation is a result of the applied potential during the negative semi-period of the ac-signal, during the electrochemical ag-deposition (i.e. when the samples play the role of cathodes). the positive semi-periods promote anodic polarizations, which in turn, result in oxygen evolution, providing oxygen enrichment near the electrode surface (eq. 1). s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 323 6h2o → 4h3o+ + o2 + 4e− (1) these excessive amounts of dissolved oxygen near the al-o-ag layer oxidize the adjacent ag+ ions from the electrolyte (eq. 2) and intensify the above-mentioned migration of o2ions, which react directly with ag+ ions, entered inside the pores. 4ag+ + 4e− + o2 → 2ag2o (2) any amount of metallic ag already deposited on aao pores surfaces undergoes additional electrochemical (eq. 3) and/or chemical (eq. 4) oxidation: 2ag + o2→ ag2o + 2e− (3) 4ag + o2 → 2ag2o (4) namely, the mahogany color of the obtained al-o-ag layers, shown in figure 2, is the obvious evidence for the oxidized form of silver, proposed in previous studies [70]. the peak in the spectrum of the main metallic component al, which composes al-o-ag layer is situated at 74.1 ev. this fact shows that aluminum is only in the form of al2o3 (eqs. 5 and 6). this oxide is predominantly formed during anodization, where the aao layer grows in both inward and outward directions at the metal/oxide (eq. 5) and oxide/electrolyte (eq. 6) interface: 2al0 + 3o2→ al2o3 + 6e(5) 4al3+ + 3o2 → 2al2o3 + 12e(6) in conclusion, it can be inferred that the combined al-o-ag layer is composed by amorphous al2o3 with ag2o deposits on the pore bottoms and walls. the oxidized state of silver deposits is evinced by the deconvoluted ag 3d spectrum and the respective auger spectra analyses. the quantitative analysis of the element content of al-o-ag system has shown that the main elements composing the film are accompanied by alloying elements (i.e. zn) and traces of the electrolytes used during the sample treatments (s, n, cl, cr and p), as shown in table 2. table 2. elemental composition of al-o-ag established by xps analysis content, at% o ag al zn s n cl cr p others 78.60 0.40 15.60 0.07 1.40 1.80 1.00 1.00 0.10 0.03 table 2 also shows that although the layer is composed almost entirely of amorphous al2o3, the oxygen content is higher than the stoichiometric. this finding can be explained with entrapped water in the pores, which is retained by capillary forces. this is in accordance to previously published research [62], where it was established that anodization of this alloy results in definitely hydrophilic surface. the rather low silver content is a result of the low efficiency of the electrochemical silver deposition that was already established in another investigation [61], where the key reason for the low ag deposition efficiency is ascribed to various electrochemical reactions which consume the applied electric charge. zn occurrence in al-o-ag layer is an obvious result of cationic currents passing from the metallic substrate to the growing oxide layer during anodization and also during the anodic semi-periods of the subsequent ac-polarization, as proposed by charlesby [68]. this element was previously detected [62], and it was suggested that zn content is a result of either zn2+ ions migration from the metallic substrate to the growing oxide layer, or of direct inclusion of intermetallics during the inward growth of aao at the metal/oxide interface. the sulfur content originates from the sulfuric acid used for the sample anodization. the phenomenon of sulfur inclusion in aao layers is described by wernick et al. [71] and thompson et http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 324 al. [72], and it was later confirmed at anodization of high doped [59,60] and low doped [61,62] aluminum alloys. the rest detected elements originate from the solutions used for sample treatment and the formation of al-o-ag system. thus, preliminary treatment supplies p and cr from the brightening solution (containing h3po4 and cro3) and probably from the basic alloy composition, cl originates from the degreasing by ccl4, whereas n originates from agno3 electrolyte used for silver deposition. electrochemical measurements because of the predominantly electrochemical inherence of the corrosion processes, the electrochemical analytical methods are most suitable for the determination of corrosion mitigation rate. following the concepts described in previous studies [73], the corrosion protective properties can simply be divided into barrier ability and durability. the former can be described as rate of hindering of the access of corrosive species to the metallic surface, whereas the latter corresponds to the aptitude of the surface layer to keep its barrier properties after extended exposure to a corrosive medium. the results acquired via eis and pds after 24, 168 and 672 hours of exposure to 3.5 % nacl model corrosive medium are discussed below. eis results in order to obtain reliable results, the experiments were performed on two samples of each group. the impedance spectra acquired after 24 hours of exposure have shown quite different shapes (figure 5), obviously requiring different model equivalent circuits (mec) for numerical data acquisition. the most remarkable difference between the reference and anodized samples is that the nyquist plots of the former reveal semi-circles, whereas the latter are in form of slightly curved (for aao) and straight (for al-o-ag) lines. furthermore, after 168 hours of exposure, the reference samples show a slight increase of the semi-circle accompanied by disappearance of the low frequency tail. the comparison of impedance spectra in figure 5 with the photographs in figure 2 reveals that the reference samples have already corroded even before the initial 24 h period of exposure to the model corrosive medium. indeed, the shapes of the spectra for these samples do not suffer considerable change, although the obvious impact of the corrosion on the sample surfaces shown in fig. 2. the weak changes in the nyquist plots are concordant with the negligible differences between the average diagrams in the respective bode plots. thus, for the referent samples, only a slight displacement of the phase shift maximum towards lower frequencies after 168 h of exposure is observed. the obvious difference between the nyquist plots for the referent and other groups of aao and al-o-ag samples reveals that the formed oxide layers possess almost pure capacitive character, originated from the dense barrier layer under the pores. this capacitive behavior is even further enhanced after ag deposition. on the other hand, the difference between aao and al-o-ag layers is more evident in the respective bode plots. the (log|z|/log f) curves of the referent samples reach 4.5, at 0.01 hz, corresponding to the impedance magnitude |z| of 50 kω cm2, while for both types of anodized specimens, this value is higher than 10 mω cm2. this remarkable impedance value reveals the significant barrier properties possessed by both aao and al-o-ag layers. although, both types of layers reveal typical capacitance spectra after 24 hours of exposure, aao layers reveal a decay of phase shift (φ / log f)-curve at the high frequency domain after 672 hours of exposure. consequently, the silver containing layers are more durable and keep their obvious barrier properties during the entire exposure period. their distinct durability is a result of the deposition of silver oxide on the pore bottoms and walls. these notable differences in the shapes of the impedance spectra originate from the respective surface film properties and require different s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 325 mecs for the extraction of numerical data (tables 3 and 4). all used mecs (figure 6) are composed of consecutive rc unit connections. figure 5. eis spectra acquired after 24, 168 and 672 hours of exposure of the investigated samples to the model corrosive medium, represented in nyquist (a) and bode (b) plots in some of mecs, the capacitances were replaced with constant phase elements (cpe) in the cases where the maxima of the phase shift (φ / log f)-curve does not reach -90°. this indicates an incomplete capacitance, revealing occurrence of defects in the surface oxide layer. in the present case, these oxide layer imperfections are related to its amorphous structure, which predetermines http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 326 variations in the density of the film. in both cases of aao and al-o-ag layers, the oxide and electric double layers reveal pure capacitance, because the formed films possess remarkable thickness of about 20 µm, established in previous works [59,60,62]. in addition, according to the literature [11,12,64–66], these layers are composed of a thick porous layer and a dense barrier layer underneath. namely, this dense layer predetermines the capacitive character of the whole aao film. in the case of al-o-ag layers, a diffusion constant phase element (cpediff) is added which most probably is related to the hindering of the transport of interstitial ions and point defects through the dense barrier layer. thus, the appearance of this additional element reveals that the presence of silver obviously influences the characteristics of this underlayer. the influence of this process lies in the fact that it is performed in ac-regime. figure 6. model equivalent circuits used for the eis spectra fitting for the (a) references, (b) aao and (c) al-o-ag layers (rmcm – resistance of the model corrosive medium; roxy –resistance of mcm penetrated inside the oxide layers, whose resistances are assigned as raao and ral-o-ag, respectively; rct – charge transfer reactions resistance, reflecting the electrochemical corrosion reactions under the oxide layer defects; r(oxy + ct) – resistance of the mcm penetrated through the oxide layer defects and of the charge transfer related to the electrochemical corrosion reactions between the penetrated mcm and the metallic surface domains under the oxide layer defects; cpe(oxy+edl) – constant phase element comprising the capacitance of the oxide layer and the electric double layer between the penetrated mcm and the bare metal surface under the defects. cpediff – constant phase element related to the obstruction of the diffusion of mcm components across the oxide layers defects (in the case of al-o-ag layer, the mentioned defects are under the pore bottoms); caao and cal-o-ag – capacitances, originated from the insulating properties of the respective aao and al-o-ag films; cedl capacitance of the electric double layer, between the bare metallic surface, under the defects and the mcm, penetrated across them) thus, the cathodic semi-periods cause partial cathodic dissolution, which results in localized alkalization, especially in the pores (eq. 7). this reaction leads to a more defective dense underlayer, due to oxide dissolution (eq. 8). 2h2o + o2 + 4e→ 4oh(7) al2o3 + 3h2o + 2oh→ 2al(oh)4(8) s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 327 the polarization during the anodic semi-periods promotes oxidation processes, which result in a partial obstruction of the already formed defects in the dense sublayer. thus, the free al(oh)4ions and the deposited ag domains convert to insoluble oxides (eqs. 9–11). o2 + 4e→ 2o2(9) 2al(oh)4+ 2o2→ al2o3 + 2h2o (10) 4ag + 2o2 → 2ag2o (11) from recent works devoted to the electrochemical ag2o formation in alkaline electrolytes [74– 77], it can be inferred that the localized alkalization inside aao pores during the electrochemical silver deposition also contributes to the formation of ag2o. the numerical data obtained by fitting of the acquired eis spectra after 24 hours of exposure are summarized in table 3. table 3. numerical values of mec elements acquired after fitting of eis spectra obtained after 24 of exposure to mcm referent samples after 24 h of exposure element sample 1 sample 2 average values rmcm / ω cm2 cpeoxy+edl / sn (ω cm2)-1 n roxy+ct / ω cm2 cpediff / sn (ω cm2)-1 n 10.74 ± 0.09 (16.98 ± 0.12)×10-6 0.97 ± 0.01 (26.38 ± 0.83)×103 (87.75 ± 42.02)×10-5 0.63 ± 0.01 11.29 ± 0.09 (11.91 ± 0.11)×10-6 0.95 ± 0.01 (40.40 ± 1.06)×103 (70.25 ± 31.97)×10-5 0.65 ± 0.01 11.02 ± 0.09 (14.45 ± 0.12)×10-6 0.96 ± 0.01 (33.39 ± 0.95)×103 (79.00 ± 37.00)×10-5 0.64 ± 0.01 aao-layers after 24 h of exposure rmcm / ω cm2 caao / f cm-2 raao / ω cm2 cedl / f cm-2 rct / ω cm2 12.61 ± 1.01 (1.33 ± 0.03)×10-6 (74.60 ± 12.73)×106 (4.65 ± 0.42)×10-6 (35.10 ± 9.44)×103 12.90 ± 1.22 (1.39 ± 0.03)×10-6 (52.90 ± 13.14)×106 (4.26 ± 0.41)×10-6 (43.40 ± 12.56)×103 12.76 ± 1.12 (1.39 ± 0.03)×10-6 (63.75 ± 12.94)×106 (4.46 ± 0.42)×10-6 (39.25 ± 11.00)×103 al-o-ag layers after 24 h of exposure rmcm / ω cm2 cal-o-ag / f cm-2 ral-o-ag / ω cm2 cedl / f cm-2 rct / ω cm2 cpediff / sn(ω cm2)-1 n 9.58 ± 0.59 (3.08 ± 0.29)×10-6 2189.40 ± 35.97 (12.74 ± 0.19)×10-6 (13.35 ± 0.34)×103 (60.75 ± 0.18)×10-8 0.95 ± 0.01 6.38 ± 0.37 (4.58 ± 0.33)×10-6 2263.00 ± 37.77 (10.47 ± 1.39)×10-6 (16.48 ± 1.86)×103 (62.80 ± 0.17)×10-8 0.93 ± 0.01 7.98 ± 0.48 (3.83 ± 0.31)×10-6 2226.00 ± 36.87 (11.61 ± 0.79)×10-6 (14.92 ± 1.10)×103 (61.78 ± 0.18)×10-8 0.94 ± 0.01 the data of the capacitance elements (including cpes) ascribed to the oxide films and the electric double layers are in the range of 1 to 17 µf cm-2 for all samples, revealing that the dense sublayers of both aao and al-o-ag are with similar thicknesses to those of the native oxide layers of the referent samples. however, cpediff of al-o-ag layers are with three entire orders of magnitude lower than those of the referent samples. this fact shows that the diffusion of corrosive species from mcm (like cl-, oh ions, dissolved o2, etc.) through the al-o-ag layer is much more suppressed than this across the native oxide layers of the referent electrodes. the absence of cpediff in the case of aao layers suggests that the barrier layer underneath the aao pores is sparse, due to its amorphous structure. indeed, the xrd analysis has revealed amorphous alumina, which has a density of between 2.66 and 3.40 g cm-3 http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 328 [78,79], lower than this of the crystalline corundum. hence, the barrier layer does not completely prevent the access of corrosive species like clions towards the metallic surface. nevertheless, the average value of cpediff for al-o-ag layers is about 0.62 µf cm-2 resembling great capacitive reactance. this fact is a result of the sealing of the pore bottoms and walls by ag2o, which strongly hinders the access of mcm ions to the metallic surface (across the dense aao sublayer). however, due to the semiconductor properties of ag2o established by talukdar et al. [80] the formed layer of silver oxide deposits also does not completely hinder the charge diffusion between the sealed pore bottoms and the metallic surface of the substrate. that is why, the exponential multiplier (n) of the cpediff element for al-o-ag systems does not reach unit, although it is close to this value. at the end of the exposure periods (i.e. 168 h for the references and 672 h for the treated samples), the eis spectra did not change in shape significantly (figure 4). the numeric data acquired by fitting of eis spectra at the end of the exposure periods are summarized in table 4. table 4. numerical values of mec elements acquired after fitting of eis spectra obtained at the end of the exposure to mcm referent samples after 168 h of exposure element sample 1 sample 2 average values rmcm / ω cm2 cpeoxy+edl / sn (ω cm2)-1 n roxy+ct / ω cm2 cpediffl / sn (ω cm2)-1 n 8.12 ± 0.15 (36.56 ± 0.62)×10-6 0.90 ± 0.01 (39.96 ± 0.27)×103 (25.48 ± 1.55)×10-4 0.73 ± 0.01 11.16 ± 0.06 (18.78 ± 0.18)×10-6 0.98 ± 0.02 (42.80 ± 0.64)×103 (25.78 ± 1.52)×10-4 0.75 ± 0.01 9.64 ± 0.11 (27.67 ± 0.40)×10-6 0.94 ± 0.02 (41.38 ± 0. 46)×103 (25.63 ± 1.54)×10-4 0.74 ± 0.01 aao layers after 672 h of exposure rmcm / ω cm2 caao / f cm-2 raao / ω cm2 cedl / f cm-2 rct / ω cm2 34.00 ± 3.55 (1.18 ± 0.04)×10-6 (63.30 ± 34.27)×106 (139.90 ± 26.57)×10-9 (1.15 ± 0.14)×103 23.00 ± 3.26 (1.22 ± 0.06)×10-6 (57.00 ± 27.91)×106 (140.40 ± 21.66)×10-9 (0.97 ± 0.12)×103 33.50 ± 3.44 (1.20 ± 0.05)×10-6 (60.20 ± 31.09)×106 (140.15 ± 24.12)×10-9 (1.06 ± 0.13)×103 al-o-ag layers after 672 h of exposure rmcm / ω cm2 cal-o-ag / f cm-2 ral-o-ag / ω cm2 cedl / f cm-2 rct / ω cm2 cpediff / sn (ω cm2)-1 n 29.57 ± 0.35 (0.40 ± 0.02)×10-6 629.00 ± 1.11 (1.94 ± 0.10)×10-6 (9.50 ± 0.06)×103 (63.45 ± 0.50)×10-8 0.93 ± 0.01 22.71 ± 0.43 (0.76 ± 0.04)×10-6 637.30 ± 1.80 (2.16 ± 0.16)×10-6 (4.50 ± 0.05)×103 (63.19 ± 0.20)×10-8 0.93 ± 0.01 26.14 ± 0.39 (0.58 ± 0.03)×10-6 633.20 ± 1.46 (2.10 ± 0.16)×10-6 (7.00 ± 0.06)×103 (63.32 ± 0.35)×10-8 0.93 ± 0.01 the comparison of the eis data from tables 3 and 4 reveals that the samples did not suffer serious changes during their exposure to the 3.5 % nacl solution. in the case of the referent samples, the lack of notable changes in the values of the impedance elements suggests that the corrosion process has begun even before the initial measurements after 24h of exposure. thus, only cpediff has changed its value from 7.90×10-4 to 25.63×10-4 sn (ω cm2)-1, revealing intensification of the perforation of the native oxide layer by clions. however, the exponential multiplier (n) slightly increased its value, showing that after 168 hours of exposure, clions penetration is already slightly s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 329 hindered by polyoxoaluminate corrosion products, as proposed elsewhere [73]. the values of all the rest capacitance and resistance elements of the reference electrodes remain almost the same, showing the steady corrosion of these samples in 3.5 % nacl solution. even 672 hours of exposure did not change the impedance parameters of the obtained aao and al-o-ag layers. in the case of aao layers, only the charge transfer resistance (rct) has dropped from 39.25 kω cm2 to 1.06 kω cm2. nevertheless, the electric double layer capacitance (cedl) has changed from 4.46 to 0.14 μf cm-2, showing rise of the respective capacitive reactance most likely due to the obstruction of the defects in the dense sublayer by corrosion products. furthermore, probably during the exposure of the anodized samples, clions have reached the metallic surface, but the resulting corrosion products have already sealed the oxide layer, decreasing its electric capacity. the weakest changes in the impedance parameters were registered for al-o-ag layers. the difference of all capacitance and resistance values was between two and four folds, and only the oxide layer parameters (cal-o-ag and ral-o-ag) exceeded a half order of magnitude. the charge transfer resistance value detected after 672 hours of exposure was only twice lower than the initially registered one (from 14.92 to 7.00 kω cm2). the explication for this fact is related to the sealing of the pore bottoms and walls by the ag2o deposits, which hinders the access of clions to the substrate surface across the porous layer. consequently, the corrosive species from mcm can reach the surface of the metallic substrate only through the oxide layer. hence, these species would have to overcome about 20 μm film thickness. the retention of the very low cpediff values of about 0.63 µf cm-2 and high, near to unit value of n for the entire 672 hours of exposure, confirm the suggestion for the considerable hindering of mcm ions reaching the metallic surface. potentiodynamic scanning (pds) results the images in figure 7 reveal a great difference between the referent and the rest samples. this fact is an obvious consequence of the anodization process, which results in the growth of a thick alumina layer. in addition, the potentiodynamic curves show that the reference samples suffer intensive uniform corrosion, without any signs of localized corrosion. the absence of localized corrosion suggests that their surfaces are rather uniform, due to the relatively high purity of the basic substrate material (i.e. 99.5 % aluminum). for comparison, the curves of both aao and al-o-ag samples have almost horizontal anodic branches, showing almost complete passivation of the metallic substrates by the respective oxide layers. the deposition of ag2o did not show distinguishable contribution to the barrier ability of the resulting combined al-o-ag films, compared to the aao ones, due to the low ag-deposition efficiency, established in previous works [70]. nevertheless, these films should be more durable than the anodized only layers, because of the pore bottoms and walls obstruction by the ag2o deposits, commented in the previous paragraph. in addition, the slight shifting of the corrosion potential (ecorr) values, and the relatively lower current densities of the curves acquired from al-o-ag films, compared to those of the aao layers reveal the superior protective characteristics of the former ones. the tafel slope analysis has uncovered additional information regarding the polarization resistance (rp) and ecorr acquired from the potentiodynamic scanning curves. the average values (av) were determined with the deviations, predetermined by the difference between the higher (hv) and the lower (lv) modulus of rp or ecorr values. http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 330 figure 7. potentiodynamic curves acquired after 24 hours (a) and at the end (b) of the exposure of investigated samples to mcm hv + lv hv lv av= ± 2 2 (12) the numerical values, extracted from the tafel slope analysis have also revealed that both aao and al-o-ag layers render three orders of magnitude higher rp values (in the order of 15 mω cm2), compared to those of the reference samples (about 10 kω cm2), as shown in table 5. table 5. numerical values of corrosion potential (ecorr) and polarization resistance (rp), acquired from tafel slope analysis of pds curves obtained after 24 hours of exposure to mcm measure sample 1 sample 2 average values reference samples after 24 hours of exposure ecorr / mv vs. ag/agcl rp / kω cm2 -709 11.13 -715 6.96 -712 ± 3 9.05 ± 2.09 aao layers after 24 hours of exposure ecorr / mv vs. ag/agcl rp / kω cm2 -485 13.48×103 -381 17.88×103 -433 ± 52 (15.68 ± 2.20)×103 al-o-ag layers after 24 hours of exposure ecorr / mv vs. ag/agcl rp / kω cm2 -207 16.97×103 -226 14.13×103 -217 ± 10 (15.55 ± 1.42)×103 the anodization and silver deposition also result in remarkable shifting of ecorr in positive direction, from -712 mv for the references, to -217 mv for the al-o-ag layers. this ecorr shifting is an undoubted consequence of the modifications of the surface chemical composition, described in the previous paragraphs. the larger deviation between the aao layers of ± 52 mv is probably due to the access of different mcm species (like cl-, ohions, dissolved o2, etc.) through the pore bottoms to the surface of the metallic substrate. such a distinguishable access registered for the aao layers is a result of the amorphous structure of the investigated oxide films. the final potentiodynamic scans acquired after 168 hours for the references and 672 hours of exposure for the rest groups have shown almost the same values (table 6) like the initial ones (table 5). s. kozhukharov et al. j. electrochem. sci. eng. 10(4) (2020) 317-334 http://dx.doi.org/10.5599/jese.820 331 table 6. numerical values of corrosion potential (ecorr) and the polarization resistance (rp), acquired after tafel slope analysis of pds curves obtained at the end of exposure to mcm measure sample 1 sample 2 average values reference samples after 168 hours of exposure ecorr / mv vs. ag/agcl rp / kω cm2 -755 11.41 -833 8.70 -794 ± 39 10.05 ± 1.36 aao layers after 672 hours of exposure ecorr /mv vs. ag/agcl rp /kω cm2 -291 15.63 x 103 -306 15.94 x 103 -299 ± 8 (15.79 ± 0.15) x 103 al-o-ag layers after 672 hours of exposure ecorr /mv vs. ag/agcl rp /kω cm2 -253 16.51 x 103 -235 14.09 x 103 -244 ± 9 (15.30 ± 1.21) x 103 this fact reveals that the samples did not suffer distinguishable changes during their exposure to mcm. in the case of the reference samples, it can be inferred that the corrosion process has begun even before the initial 24 hours of exposure. in the cases of aao and al-o-ag films, the layers have kept their barrier ability, showing remarkable durability. the unique differences between the data in tables 5 and 6 are related to the ecorr values of the aao layers. after 672 hours of exposure, their values are displaced in a positive direction from about -433 mv to around -299 mv, probably due to obstruction of aao layers by keggin-like polyoxoaluminate corrosion products, as proposed in [73]. this suggestion also explains disappearance of the difference between ecorr values for aao samples (from 52 to only 8 mv). the comparison of figure 7a and figure 7b reveals that pds curves of aao and al-o-ag layers have exchanged their positions after 672 of exposure. this fact is confirmed by the data in tables 5 and 6, as well. these data reveal that aao layers have shifted their ecorr values with 134 mv in positive direction, whereas for al-o-ag layers the shift is by only -27 mv in the opposite direction. these facts show that al-o-ag system is more stable and does not change its chemical composition during the entire exposure period. the considerable potential shift in the case of aao towards nobler values is probably due to the already commented obstruction of aao pores by polyoxoaluminate products. the difference between this potential shift for the respective oxide layers is an additional evidence for the remarkable durability of al-o-ag layer since its pore walls and bottoms are sealed by ag2o. conclusions the present study is devoted to the structural and compositional characterization of aa1050 aluminum substrates after formation of thick oxide layers by means of anodization (aao) and the subsequent electrochemical modification by silver deposition (al-o-ag). the used combination of analytical techniques xrd, xps, and electrochemical tests after extended times of exposure to model corrosive medium, have led to several main conclusions. xrd patterns have shown completely amorphous structures of both aao layer types, with and without silver. xps analysis of the chemical composition of al-o-ag layers has revealed that this film is composed of al2o3 with traces of intermetallics originating from the metallic substrate and the used treatment solutions. their content was even higher than this of silver. the most important conclusion from xps analysis is that the deposited silver is in oxidized form. furthermore, the determined oxygen content showed entrapped water in the layer pores. http://dx.doi.org/10.5599/jese.820 j. electrochem. sci. eng. 10(4) (2020) 317-334 performance of ag containing aao layers 332 the electrochemical measurements via eis and pds were performed after different periods of exposure to 3.5 % nacl solution. this approach has enabled to determine the corrosion protective properties of the formed aao and al-o-ag films. thus, after 24 hours of exposure, both aao and al-oag layers have revealed significant barrier ability against the access of the corrosive medium to the metallic substrates, compared to the reference samples. the impedance spectra, acquired from the formed films resembled this of ideal capacitor, demonstrating their apparent insulating properties. the impedance spectra acquired after 672 hours of exposure were rather similar to those of the initial spectra (i.e. obtained after 24 hours). this fact showed that both types aao and al-o-ag layers possess remarkable durability, keeping their barrier abilities after large times of exposure to the model corrosive medium. the obviously different shapes of eis spectra between the references and the treated samples were successfully fitted to different model equivalent circuits. the respective fitting procedures have enabled to acquire numerical data regarding all capacitances and resistances, composing the total impedances of the samples. the resulting data have shown that all impedance components keep their values for the entire exposure period. besides, it was established that ag2o deposits on the pore bottoms and walls efficiently suppress the access of corrosive species to the substrate surface. their presence enables supplemental extension of the oxide layer durability. the pds analyses have confirmed the inferences done for the eis spectra. the polarization resistances of both aao and al-o-ag layers had three orders of magnitude higher values, compared to those of the references. besides, these values remained unchanged for the entire 672-hour period of exposure to the model corrosion medium. summarizing all the conclusions done for the respective analyses, it can be concluded that the electrochemically synthesized al-o-ag film on aa1050 alloy is composed mainly of amorphous al2o3 with low silver content in form of ag2o deposits on aao pore surface. besides, the films possess distinguishable barrier ability and durability, determined under exposure to 3.5 % nacl solution. acknowledgements: the authors appreciate the financial support from the bulgarian national research fund, via contract no: kп–06–н29/1. references [1] m. lamberti, f. escher, food reviews international 23 (2007) 407– 433. 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[80] m. i. talukdar, e. h. baker, solid state communications 7(2) (1969) 309–310. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://creativecommons.org/licenses/by/4.0/) doi: 10.5599/jese.2012.0011 53 j. electrochem. sci. eng. 2(2) (2012) 53-66; doi: 10.5599/jese.2012.0011 open access : : issn 1847-9286 www.jese-online.org feature article electrochemical engineering its appearance, evolution and present status. approaching an anniversary. velizar stanković department of metallurgical engineering, technical faculty bor, university of belgrade, serbia e-mail: vstankovic@tf.bor.ac.rs received: april 10, 2012; published: june 18, 2012 the goal of electrochemical engineering is to achieve in a quantitative way an optimal cell design n. ibl abstract through this story an attempt has been made to present a chronology of electrochemical engineering from its appearance as an individual science, via its growth throughout the five decades of its existence as an individual science, until today. the collaboration and linkage of electrochemical engineering with other disciplines has also been discussed in this essay. the role and duties of electrochemical engineering in the 21st century have been touched upon to the extent that it was possible. keywords electrochemical engineering, electrochemical reactors, cell productivity, education in electrochemical engineering 1. circumstances of the emergence of electrochemical engineering as a new scientific discipline the term “electrochemical engineering“ first appeared in the literature as a novel title of the fourth edition of c.l. mantel’s book industrial electrochemistry [1], published in 1960 (the first edition was published in 1931 and later editions in 1940 and 1950). just looking through its contents, one can easily realize that this book mainly describes the electrochemical technologies for production of some particular electrochemical products. although there was an attempt to group and systematise the exposed technologies in separate chapters, such as: basic electrochemical relationships and laws; molten salt electrochemistry, metal electrowinning and electrorefining; gases production; corrosion and electroplating, a big gap still remained between j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 54 the title and the contents. this book, along with others relating to industrial electrochemistry from around that time, demonstrated that, at the end of the ‘50s, the number of electrochemistry technologies had expanded to the extent that it was not possible even to describe each of them in a textbook without a lot of repetition in describing each technology. this made books more and more voluminous, making it troublesome for students, who had to learn each technology for the production of every particular electrochemical product. this was one of the reasons, maybe not the most important but still a very serious one, for starting to consider establishing a new discipline in a similar manner to what had been done when chemical engineering appeared at the end of the 19th and the very beginning of the 20th century firstly through unit operations and afterwards through transport phenomena and reaction engineering. hence, at the end of the ‘50s and in the early ‘60s, chemical engineering had already developed its capabilities to treat unit operations and processes in a way that interconnects physical and physico-chemical phenomena with chemical processes to chemical technology. at that time (the end of the ‘50s), it became evident that similar transformations to those that had occurred in chemical technology had to be implemented in industrial electrochemistry to get more engineering and a more scientific approach to improving the existing technologies and establishing new ones. by then, almost all that had been achieved in improvements to any electrochemical technology was due to the art of engineering rather than to an applied science. 2. interconnection between chemical engineering and electrochemistry pioneering works linking the two sciences during the ‘50s a close interconnection between electrochemistry and chemical engineering was established, on a theoretical level, through the penetration of electrochemistry into chemical engineering and vice versa. during that period, firstly c.s. lin with his co-workers [2], and shortly afterwards tobias, eisenberg and wilke [3] considered, both experimentally and theoretically, the mass transfer phenomenon of ions coming from the bulk electrolyte to the electrode surface by molecular diffusion, migration and convection. their considerations resulted in the well-known equation connecting limiting current density with the other variables affecting it: =l l bi k zfc (1) where: il is limiting current density; kl mass transfer coefficient; z number of exchanged electrons in electrochemical reaction; f – faraday’s constant; cb – bulk concentration of reacting ions, all given in arbitrary units. the published papers in which an electrochemical method of limiting current measurement was used to evaluate the mass transfer coefficient at convective mass transport forged a strong connection between electrochemistry and chemical engineering. the few early works [2,3] relating to local limiting current measurements had opened new opportunities – employing electrochemical methods for evaluation either local or overall, as well as instantaneous or timeaveraged transport characteristics of flowing liquid systems, e.g. to quantify mass, heat and momentum transfer features. in this way, methods of adsorption, dissolution, or sublimation used till then for evaluation of the mass transfer coefficient, were replaced by a much faster and more reliable electrochemical method of the limiting current measurement and computation of a mass transfer coefficient from experimentally recorded polarization curves, using eq. (1). this method was widely exploited during the ‘60s and particularly the ’70s, until today, for studying the mass transport rate in different electrochemical model-systems. the electrochemical method of limiting v. stanković j. electrochem. sci. eng. 2(2) (2012) 53-66 doi: 10.5599/jese.2012.0011 55 current measurement, in its early or advanced form, contributed greatly towards providing evidence about the hydrodynamic image of flowing systems. thanks to the chilton-colburn analogy widely used at that time in transport phenomena, data obtained for local or overall mass transfer coefficients were then employed to estimate heat transfer as well as momentum transfer in similar flowing systems. indeed, these works confirmed the great influence of flowing conditions in electrochemical reaction rate via mass transport in electrochemical reactors, opening up a new research area for creating new, or improving the existing electrochemical cells. in that period (the middle of the ‘60s), the method of conductivity measurement was also introduced in chemical engineering for local or overall porosity measurement in liquid fixed or fluidised beds and later for a gas hold-up quantification in gas-liquid systems, regardless of whether gas is dispersed in a liquid phase, or generated therein due to a chemical or electrochemical reaction. this close collaboration between electrochemistry and chemical engineering also had a reverse direction – bringing benefits for electrochemistry. the marker pulse technique, a method widely used in chemical engineering for axial and radial dispersion measurement of a tracer in a reaction chamber, was adopted and modified at the end of the ‘60s, to be used for investigation of ionic species distribution, generated in a flowing electrolyte in electrochemical reaction systems. at that time, terms like “plug flow reactor” or “perfectly mixed reactor” were reserved for chemical and/or catalytic reactors, but not yet used for electrochemical cells. mass transfer in a cell when changing the hydrodynamic conditions was investigated at that time as a method for mass transfer enhancement when a diffusion-controlled electrochemical reaction takes place. the use of dimensional analysis and similarity theory adapted for and applied in chemical engineering, was at that time introduced in electrochemical studies in order to minimize the possible combinations of operating variables that have to be tested in the research and development of new electrochemical processes, new electrochemical reactors, or new technologies. this was very important in the scaling of laboratory results to the pilot-plant and then from pilot data to the industrial scale. the experimental data obtained were then expressed in the form of dimensionless relationships, either as: sh = f(re, gr, sc, fo, γ1, …γn) (2) or, based on the chilton-colburn analogy, as: ffr/2 =jh = jd = f(re), (3) describing mass, heat and momentum transfer. equations like these had been introduced as a tool in the consideration of diffusion-controlled electrochemical reaction systems and published at that time in the literature. in equations (2) and (3) sh is sherwood number; re – reynolds number; gr – grashof number; sc – schmidt number; fo – fourier number; γi – dimensionless groups dictated by geometric similarity; ffr – fanning friction factor; jd, jh – mass and heat transfer factor, respectively. moreover, new similarity criteria, like the wagner number (wa), relating particularly to the nature of electrochemical reaction systems, were derived and introduced in dimensionless expressions similar to equations (2) and (3). obviously, both sciences – electrochemistry and chemical engineering profited from their mutual collaboration during the ‘50s and ‘60s. the main benefit from this interrelation between electrochemistry and chemical engineering was the appearance of a new discipline – named electrochemical engineering. j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 56 3. establishing electrochemical engineering on the scientific scene in the early ‘60s, as a result of the interrelation between chemical engineering and electrochemistry, numerous qualitatively new papers appeared in the relevant literature (mainly in chemical engineering rather than in electrochemical journals), founding the conditions for the appearance of a new multi-disciplinary science – electrochemical engineering. c. wagner, v.g. levich, n. ibl, c. tobias, p. le goff, and j. newman with their collaborators, as well as some others not mentioned here due to limited space, contributed a lot to the establishment and early development of the new discipline. these scientists may be considered as the founders of electrochemical engineering a separate scientific discipline born from the mutual influence of chemical engineering and electrochemistry. wagner first introduced the name to this discipline, publishing a paper in the second volume of advances in electrochemistry and electrochemical engineering in 1962, entitled “the scope of electrochemical engineering” [4]. in that article, wagner makes a clear distinction between electrochemical engineering science and electrochemical engineering technology. in the middle of the 20th century there were not so many electrochemical technologies. from this time-distance, we might suppose that improvements of these technologies, starting from their appearance, occurred little by little through decades, more by intuition or by trial and error than by a scientific approach to basic phenomena affecting features of these processes. the contribution of electrochemical engineering, in its early stage (at the beginning of the ‘60s), to the design of industrial electrochemical cells and processes was very modest almost negligible. mass transfer considerations in electrochemical reactors had an academic rather than an engineering approach. this was mainly because there was no mass transfer limitation when the process operated with electrolytes having high concentrations of reacting ions, as was mostly the case with industrial electrochemical processes from that period. current density distribution was not of major importance in those cases and for the side by side electrode configuration that was mainly used in electrochemical cells from that time. the operating current density was not limited by diffusion of reacting ions, but more by the cell voltage and consequently the specific energy consumption than by mass transport limitations or irregular current distribution. at that time, electrochemical reaction engineering was a term of no significance for industrial practice. it was enough to find empirically the cell volume and inter-electrode distance needed to estimate the highest degree of conversion and the highest current efficiency at the lowest cell voltage. not much attention was paid to enhancing the cell productivity by changing either the hydrodynamics conditions in a cell or the cell voltage in order to get optimal operating energy conditions. a great step towards new cell design and its optimisation was made when the pioneering work of jottrand and grunhard [5] appeared in 1962, concerning the mass transfer enhancement in a tubular cell with a fluidised bed of inert particles in the inter-electrode space. very soon after, numerous articles on the same subject but with different aims were published in the literature, providing evidence on how many times various inert turbulence promoters (itp), placed in interelectrode space in a cell, enhance the limiting current density due to an increase in the mass transfer coefficient. besides higher cell productivity, a smoother electrode surface was obtained in the case of either metal deposition or dissolution, so that these types of cells were further developed with the aim of employing them in electroplating, electro-polishing, electroforming and other similar processes of electrochemical surface treatment, but also for metal removal from various sources. several years after this series of articles, a new type of cell with expanded mesh v. stanković j. electrochem. sci. eng. 2(2) (2012) 53-66 doi: 10.5599/jese.2012.0011 57 electrodes, in which mass transport is facilitated by a fluidised bed of glass beads, was developed, and later marketed by bewt engineers, uk, as a “chemelec cell” for the recovery of metals from wastewaters and industrial effluents. at that period (the second half of the ‘60s), a new electrochemical reactor also appeared, named the three-dimensional electrode cell (tde-cell). the first articles on this subject were published at the end of the ‘60s and early ‘70s by english scientists from newcastle and southampton universities and shortly afterwards by french scientists from the university of nancy, as well as by german scientists from dechema institute. two revolutionary things, with a strong influence on further electrochemical engineering development and electrochemistry technology, also happened in the field of materials engineering during the ‘60s: • dimensionally stable anodes (dsa®) were introduced in 1968 in industrial practice, providing many advantages over the existing electrodes at that time, first of all a lower cell voltage, thus making a great contribution in energy saving, and better anti-corrosive features, thus having a longer lifetime; • a co-polymer, having ionic features (ionomer) was synthesised and patented in 1966, called nafion and, due to its ability to conduct protons, it was considered and applied first as a proton exchange membrane (pem) in fuel cells and later in many other electrochemical technology areas (chlor-alkali electrolysis, water electrolysis, metal electrowinning, electroplating, electrodialysis, sensors and battery fabrication, ...), mainly for dividing the anode from the cathode chamber in various electrochemical reactors. these two discoveries had a great influence on the philosophy of thinking of scientists and engineers, opening up new frontiers in the design and development of electrochemical cells and processes. 4. growth (expansion) of electrochemical engineering as a science and art from the ‘70s till the end of the ‘90s through these three decades of electrochemical engineering growth, several directions were profiled and developed, mainly simultaneously. these directions are: • electrochemical reactors – development, modelling and optimisation; • electrochemical processes – improvements of the existing ones, and research and development of new ones; • interconnection of electrochemical engineering with other sciences, in some cases forming new scientific sub-disciplines; • publishing affairs – education of new generations in the electrochemical engineering community. 4.1. electrochemical reactors research and development during the ‘70s and ‘80s electrochemical engineering had a very fruitful and progressive period contributing to electrochemical reactor engineering development, which resulted in new electrochemical reactors, as well as new electrochemical processes that appeared and found their place in electrochemical technology. one can say that the ‘70s and ‘80s were the peak of the electrochemical reactor research and development, when a lot of new cells appeared. some of them were developed on an industrial scale and marketed in that period, as: • “chemelec” cells (already mentioned) for metal removal from rinse and wastewaters; j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 58 • filter press cells for multipurpose use, either in monoor bipolar mode (applied in water electrolysis, in electrowinning, in electro-organic synthesis, known as ici cell, or su cell); • swiss-roll cells employed in the electro-organic synthesis of vitamin c in the period when anodic oxidation had still to play a role; • pump cells; • capillary-gap monoor bipolar cells, also for multipurpose usage; • rotating cylinder and rotating drum cells, both used for silver removal from spent photo chemicals; • tde cells either as a fixed bed cell, such as an en-viro cell, or as a fluidised bed cell for heavy metal ions removal from various industrial effluents; • cylindrical cells with coaxial electrodes configuration and tde anodes used in the nalco process; • bipolar trickle bed cells for electro-organic synthesis, as well as some others that can be found in the relevant literature that have found their use for all the aforementioned purposes [6]. particular attention in that period was paid to the tde cells with fluidised bed cathode and enviro cells, considered as a new very promising and very powerful electrochemical reactor. very quickly after the first papers were published on the subject of tde cells, a pilot-plant for the fluidised bed cell was constructed by cjb developments uk in 1972, and afterwards the first industrial plant was built in 1978 in germany by akzo zout chemie netherlands, only to be closed a few years later, having exhibited certain disadvantages [7]. later, during the ‘90s, thanks to the appearance of advanced conductive materials with a very developed surface area and high porosity, a new generation of tde cells, including the poro-cell, reno-cell, and retech-cell, was developed and marketed. although these cells were originally investigated and developed mainly for metal recuperation from various industrial waste solutions containing a low level of metal ions, sometimes a few p.p.m. only, they were later modified and used for electro-generation of different oxidants used for oxidation of different organic or inorganic compounds. the theoretical knowledge and laboratory experience in electrochemical reactor engineering accumulated in that period contributed significantly to producing an optimal version of some of these cells on an industrial scale. in fact, scientists working in electrochemical reactor engineering were faced with the problem of how to explore in the best way a simple relationship connecting the cell productivity with other variables affecting it. cell productivity per unit of installed volume and unit of time (termed also as space-time yield [8]) is defined by the basic, well-known equation: e l b 1 d ( ) d m k amc t v t η α= (4) where: ηe is the current efficiency; α = i/il – ratio between operating and limiting current density; a = a/v – specific surface area (working electrode surface per unit of cell volume); kl mass transfer coefficient; m – molar mass; cb(t) – concentration of metal ions in the bulk. it follows from eq. (4) that, for a given and constant bulk concentration cb(t), as is usually the case in effluent and wastewater treatments, there are three possibilities for an increase in the limiting current density as a measure of the maximum cell productivity: a. to increase the mass transfer coefficient; b. to increase the specific surface area of the working electrode; or c. to increase both the mass transfer and specific electrode surface area in the same cell. v. stanković j. electrochem. sci. eng. 2(2) (2012) 53-66 doi: 10.5599/jese.2012.0011 59 a: mass transfer around an electrode can be enhanced by making and keeping forced convection in a cell. this can be carried out by: 1. changing the electrolyte flow conditions around working electrodes by either pumping or stirring, or using an ultrasound source; rotating, shaking or vibrating a working electrode, or in some other way; 2. introducing different itps in the inter-electrode space that will locally change electrolyte flow close to the electrode surface; 3. bubbling gas around the electrodes. all three methods of mass transfer enhancement were extensively investigated and the contribution of each of them was quantified by scientists through the ’60s and particularly the ‘70s. b: specific electrode surface (a) area can be increased by: 1. decreasing the inter-electrode distance; 2. using either dispersive or porous conductive material connected with a current feeder as a working electrode (tde). c: a cell with a tde fulfils both requirements – it has a high specific surface area and, at the same time, it has good mass transport properties independently, whether electrolyte flows by, or through, the tde. this aspect of enhancing the productivity of a tde cell particularly occupied the attention of scientists at that time, resulting in many papers relating to this issue. table 1 presents an approximate specific surface area and corresponding maximum space-time yield (productivity) for different cells as an illustration supporting the statements made in the previous text, but also to demonstrate an appreciable progression in designing a cell of high performances. table 1 specific surface area and maximum productivity of some particular cells compared under similar operating conditions [7] type of cell specific surface area, m-1 productivity at α =1, mol m-3 h-1 conventional cell 7.5 0.14 filter press cell 30 – 170 0.56 – 3.17 capillary gap cell 100 – 500 1.9 – 9.33 fixed bed cell 1000 – 10 000 18.65 – 186.5 fluidized bed cell 1000 – 10 000 37.3 – 186.5 rotating drum cell 50 – 5000 9.32 – 93.3 it is important to emphasise that, when the bulk concentration in eq. (4) is time-dependent (metal removal, for example), the current efficiency ηe is also a complex and time-dependent variable, depending, among other things, on the galvanostatic or potentiostatic mode of operation. numerous papers on the modelling and optimization of electrochemical reactors were published at that time and later, in order to achieve the best working conditions, i.e., the highest cell productivity at optimum energy consumption for not only the cells considered in table 1 but also for others. these papers made a great contribution to the development of electrochemical reactor theory and engineering practice. j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 60 4.2. establishing new and improving the existing electrochemical processes thanks to the achievements in developing the new electrochemical reactors that are the core of every electrochemical process, great success was attained in developing new processes and technologies and in improving the existing ones. i. a particularly significant contribution was made in electro-organic synthesis, where various organic compounds were produced electrochemically on an industrial scale, such as: tetraethyl lead, sorbitol/mannitol, glyoxylic acid, methoxy-benzylalcohol, adiponitrile, methoxy-benzaldehyde, methylethylketone, propiolic acid, anthraquinone, naphthoquinone and others. many other organic products are under research and development on a pilot plant scale and many more are still on a bench scale and will be commercialized soon. ii. in electrochemical metal surface treatment, comprising electro-coating, electro-polishing, electro-sinking and grinding, many new electrochemical tools and new electrolytes were developed and commercialised. great success was achieved in electroforming, where numerous micro-products appeared, and various micro-fabrication methods were developed and marketed. micro-reactor technology was one of the more propulsive areas, and very promising for the electrochemical production of some special chemicals in very small amounts. in that period – the early ‘70s – the so-called liga process had already been developed and employed, spreading its role into many areas of present day techniques and technologies: in micro-mechanics, micro-acoustics, micro-optics, in information technologies, in biotechnologies, biomedicine, and many other areas. iii. the principle of the fuel cell (fc) was first described by w. r. grove in 1839. since then, many efforts have been made and many types of fcs have been considered for producing energy at low or high temperatures using mainly hydrogen as a fuel but also natural gas and coal gas, in a mixture with hydrogen or alone. the greatest improvements in development of proton exchange membrane (pem) fcs capacity was achieved during the ‘90s. an almost 25fold increase in output power was achieved in that period, starting from 70 kw m-3 in 1989 and reaching 1.8 mw m-3 in 1997. this leap was a consequence of the increased interest of the automotive industry, backed with adequate financial support, and was due also to new catalytic materials and cells development. iv. in parallel with fc developments, electrochemical engineering contributed a lot to developing new and improving the existing water electrolysis and hydrogen production technologies. the improvements of alkaline water electrolysis technology consist mainly in improving the electrolytic cells to have an increased specific surface area; using advanced materials as separators has led to a reduced inter-electrode gap, thus reducing specific cell resistance. this, coupled with an increased operating temperature, as well as the introduction of new catalytic materials, makes the whole process more energy/economy viable. besides these improvements, new technologies were developed in that period, based on nafion membranes offering qualitatively new approach to hydrogen/oxygen production as well as water steam splitting on oxide ceramic. v. several substantial improvements were made at that time in chlor-alkaline electrolysis in that period (from the ‘70s till the end of the ’90). the most important was substitution of graphite with dsa, thus prolonging the anode life-time many times and preserving lower chlorine evolution overpotential. the other improvements were similar to those applied in water electrolysis, and relate to decreasing the inter-electrode space of electrolysers; mono v. stanković j. electrochem. sci. eng. 2(2) (2012) 53-66 doi: 10.5599/jese.2012.0011 61 or bi-polar mode of operation; and substitution of amalgam process by diaphragm and membrane processes, thus making the whole technology more environmentally sustainable. 4.3. electrochemical engineering and other disciplines the period between the ‘70s and the end of the past century was also characterized by the penetration of electrochemical engineering into or interconnection with other disciplines. here we will discuss only those that i consider to be the most important. the others will only be touched upon. extractive metallurgy electrochemical engineering has particularly contributed a great deal to the development of hydrometallurgy, which also made its own very substantial progress during the ‘60s and on, of the past century. now, electrowinning is usually the last link in the technology chain for production of various metals from ores by leaching followed by solvent extraction/ion exchange and an electrowinning pathway. as a response to a significant growth in the theoretical knowledge and its experimental confirmation, many attempts were also made in that period to improve the existing or establish new purely electrochemical or metallurgical processes, as well as processes linked with other adjoining disciplines. we will mention here only those processes and technologies relating mainly to metallurgical engineering and environmental processes the fields i am more familiar with: • copper electrowinning from pregnant leach solutions, namely, the leaching-solvent extraction-electrowinning (l-sx-ew) process of copper production, as a qualitatively new technology was introduced on an industrial scale at the end of the ‘60s. as the major cost component of this technology is the electrowinning step, which requires 8 to 10 times more power than electrorefining, shortly afterwards many attempts were made with the aim of making electrowinning less expensive and more efficient. one approach was an attempt to replace the anodic reaction of o2 evolution by so2 oxidation at copper or zinc electrowinning, which would be a great step in making the process more economically acceptable than it is now. so far, it is still at a development level. another interesting attempt was to carry out the electrowinning from an electrolyte containing cuprous instead of cupric ions, thus cutting significantly the energy costs. another interesting process is electrolytic production of metals from corresponding sulphide mattes cast and served as anodes in electrorefining plants. so far, only in nickel metallurgy has such an approach been successfully implemented. • the improvements made in extractive metallurgy of basic heavy metals were implemented in some other technologies, in which electrowinning is the final step; • replacing the cementation processes by direct electrowinning in zinc electrolyte purification from other metal ions using advanced cells based on tde, prior to the zinc electrowinning; • recuperation of metals from spent electroplating baths, from spent solutions in electronics, from spent pickling solutions in the metal working industry, from mother solutions in salt production and from some other similar effluents that are produced in metallurgy or the inorganic chemical industry; • electrolytic metal powder production is nowadays an important segment, providing starting materials for sinter metallurgy, as well as in electronics, in metallic pastes production and for many other products; j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 62 • applying different electrochemical coating and surface finishing methods leads to minimising the corrosion effects in wastage of materials, to which electrochemical engineering contributed greatly in the past and still currently does. • organic chemistry and organic chemical technology (electrochemical reactor design for massive production; micro-reactors design and implementation for production of very hazardous chemicals). environmental protection in that period (‘70s to ‘90s), particular attention was given to environmental pollution and environmental protection, due to an increasing demand for environmentally sustainable chemical processes. as an answer to this demand, electrochemical engineering opened a new direction in the treatment of different industrial effluents. coming into this important and broad area, electrochemical engineering started to play a key role in different aspects of pollution abatement, in both aqueous and gaseous phase. electrochemical methods and devices were developed and employed for wastewater purification, either for removal of hazardous heavy metal ions from them or for cyanide or chromate destruction; for direct anodic destruction of organic species that must be destroyed prior to releasing such wastewaters into the recipient; for “in situ” water disinfection for human usage or for special purposes. nowadays, also thanks to the development of advanced materials, boron-doped diamond (bdd) anode is an industrial reality in drinking and wastewater treatment processes, and particularly useful for anodic destruction of those organic pollutants from wastewaters that can only be decomposed into less hazardous or even inert compounds at higher overpotentials with no water electrolysis [9]. electrochemical engineering has found its place also in off-gas cleaning technologies for either direct electrochemical or indirect (using a proper electrochemically generated red/ox mediator) conversion of harmful gaseous off-gas stream constituents such as: so2, h2s, nox, ammonia, cl2, amines, hydrocarbons and other organic constituents, previously absorbed by a proper absorbing solution, from off-gas streams. so, for example, different variants were considered and developed for so2 removal from flue gases and its oxidation by an electro-generated oxidant (ispra-mark 13), using a proper redox pair. a similar concept was also considered for nox and so2 removal from gaseous mixture via absorption and oxidation of absorbed species by ce4+ ions electrochemically generated in a separate electrochemical reactor. an interesting and promising approach to direct gas purification from organic and nitrogen compounds is the electrochemical catalyst promotion, known also as nemca. by polarising a catalyst layer deposited onto solid electrolyte, its catalytic activity increases 10 to 100 times. so far this method is still at the laboratory and bench scale. soil remediation is another area where electrochemical engineering has found itself contributing significantly to resolving the problem of the purification of soil polluted by previous industrial activities. recycling technologies following the concept of “zero discharge effluent technology”, electrochemical engineering found its place in different recycling technologies, as an ultimate stage, for reclaiming and recovery of useful or harmful metals from different waste devices such as: spent batteries, printed boards and other electronic scrap –the amount of which grows ever greater; for metals removal from spent electroplating baths, as already discussed; from spent pickling solutions; from mother liquors; from spent photo-chemicals, different rinsing waters and other similar liquid sources. v. stanković j. electrochem. sci. eng. 2(2) (2012) 53-66 doi: 10.5599/jese.2012.0011 63 significant results were achieved in the recycling of acids and alkalis by splitting corresponding salts from large volume solutions produced in chemical industry or in metallurgy by means of electrodialysis or a combination of membrane and ion-exchange resin techniques assisted by electrochemical regeneration of loaded resin. wastewater purification and recycling in a process is in many cases more economically viable than to treat it to a purity level tolerable to be discharged into a recipient. besides removing metal ions or destroying organic molecules, new electrochemically assisted techniques were developed and included in technologies for removal of fine suspended particles from rinse waters such as: electro-coagulation and electro-flotation, both efficiently employed on an industrial scale. some of the above-mentioned processes or technological innovations were developed and marketed, while others remained only as an attempt on a laboratory or pilot-plant scale waiting for further research, or were completely abandoned. other areas electrochemical engineering also penetrated the medical sciences, contributing to developing new medical tools, new devices, new indicators, new drugs, new compatible materials for orthopaedic purposes, for example, and many others. in biochemistry, thanks to close collaboration with electrochemistry and electrochemical engineering, a new sub-discipline has appeared recently, named bio-electrochemistry, where a lot of new projects have been driven in developing new alternative sources for green energy production, such as bio-fuel cells; for promoting or inhibiting bacterial activity and its proliferation; for bioelectrocatalysis and for similar purposes using small currents to manage the bacterial life and features. obviously the three decades of the past century we have considered were very productive for electrochemical engineering science, as well as for electrochemical engineering technology. 4.5. education of new generations of electrochemical engineers due to a significant increase of papers relating to electrochemical engineering subjects, a strong need appeared, at the end of the ‘60s, to establish a new, specialized journal for publishing such articles. so, the first issue of the journal of applied electrochemistry was published in 1970 [10]. in 1974, based on many theoretical and experimental contributions in electrochemical engineering, electrochemical reactor design, written by d.j. picket, was published as the first book on this subject. later, several books with the keyword electrochemical engineering in their titles appeared, with the aim of spreading the basic principles of the new science among younger scientists and students studying chemical and metallurgical engineering. some of the writers of these textbooks, who significantly contributed to the development of electrochemical engineering, are listed here chronologically as their books appeared: f. coeuret and a. stork (1984), i. roushar (1986), e. heitz and g. kreysa (1987), t. fahidy (1988), k. scott (1991), f. c. walsh (1993), h. wendt and g. kreysa (1999). at that time (the early ‘80s) the first scientific symposium entitled electrochemical engineering was held at loughborough university uk, as a response to the increased interest in presenting the research results amongst a specialised group of scientists. after this first attempt, the next symposia were held every three years, always at loughborough. these events were a good indicator for identifying major areas of research interest and key directions which electrochemical engineering had moved towards, in the period between two events. j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 64 in the meantime, the eewp (electrochemical engineering working party) was constituted as a branch of the efce (european federation of chemical engineers). following a successful tradition of organizing electrochemical engineering symposia, the eewp took over this duty from loughborough university, and the 1st european symposium on electrochemical engineering was organized soon after in bad-soden, germany, in 1990. the 9th was held in 2011 in patras, greece. besides this symposium, held every three years and having a european character, similar symposia are organized in the frame of world meetings such as ise annual meetings and chisa congresses, allowing the gathering of scientists from other parts of the world to present their results. also, taking care of the young scientific population interested in coming into electrochemical engineering science, the european summer school on electrochemical engineering was held for the first time in toulouse, france in summer 1995 and thereafter in ferrara, italy; patras, greece; then in palić, serbia and most recently in almagro, spain, in 2009. 5. electrochemical engineering at the end of the first decade of 21st century electrochemical engineering has extended far beyond the limits which were drawn by the early works in the ‘60s when the main theme was the mass transport of ions to or from an electrode. one can still occasionally find in the literature papers concerning this subject, but this is rather a reflection on the works done during the ‘60s to ‘80s. such papers are mainly directed towards the corrosion process induced by monoor two-phase flows. the published relationships at that time on mass transfer in electrochemical systems can today cover any particular technical problem. also, current density distribution is something that can be routinely solved employing modern computational techniques and reliable numerical methods. with these tools, modelling of any electrochemical reaction system with monoor two-phase flow of electrolyte and a dispersed gas or solid phase in a cell with or without separator is now possible and something that can be successfully used in everyday practice. in these circumstances, what does electrochemical engineering at the end of the first decade of the new century mean? what is the role of the electrochemical engineer in following the established principles and in finding challenging new subjects and directions for further upgrading electrochemical engineering science? it may sound too declarative, but the following text could be an answer to the above questions: “it is the responsibility of the electrochemical engineer in industry to simultaneously manage electrical consumption and chemical production. he or she must apply relevant scientific and engineering principles to design, construct, and operate a process in an economical, safe, and environmentally conscious manner. improved understanding of scientific principles and the application of new materials can lead to more efficient cell designs and processes. the constant evolution of technology provides challenging and rewarding careers to engineers and scientists in a range of disciplines. among the considerations that influence cell efficiency and lower consumption are temperature, spacing between electrodes, electrode material, electrolyte consumption, cell size, source of raw material, and production rate. clearly, engineering skill is required for understanding these effects and for achieving optimum production conditions”. from “industrial electrolysis and electrochemical engineering” by m. grotheer, r. alkire and r. varian [10]. v. stanković j. electrochem. sci. eng. 2(2) (2012) 53-66 doi: 10.5599/jese.2012.0011 65 6. new challenges and frontiers in electrochemical engineering [11] according to the last sentence from the cited text, there will be a great period for further development of electrochemical engineering. indeed, one should expect the further influence of electrochemical engineering on improvements of existing electrochemical technologies, and development and establishment of new ones that are still on the laboratory scale. some of them have already been mentioned in the previous sections. in the past, electrochemical engineering faced much more towards electrochemistry and less towards chemical engineering. closer collaboration with the second science has to be established in the future, particularly in solving problems arising in cell scaling and optimisation, and particularly those with three-dimensional electrodes (their implementation in different separation/purification processes is not as expected), as well as those with membrane processes. knowledge about transport processes across membranes, for example, is not complete yet, in spite of the fact that many industrial membrane processes exist. how to design and construct more efficient cells and large processes for electrowinning of copper to work at lower specific energy consumption? few new hydrometallurgical processes are at present under development. the question is when they will be marketed. each of them has metal electrowinning as the last stage in the whole technology chain. the contribution of electrochemical engineering to research and development of these processes could be extremely valuable. how to design cells and the process for electrorefining of metals to improve their purity? in this view, fundamental principles of thermodynamics, kinetics, hydrodynamics, mass transport and potential and current distribution have to be transformed into engineering concepts in order to achieve economical operation and high quality products, lessening impurities into deposit as much as is permitted. the contribution of electrochemical engineering to the research and development of these processes could be very valuable. how to make fuel cells less expensive for wider applications? this is mainly connected with the kind and amount of materials to be built in, but also with fuel cells design and their scaling, with fuel production and storage and with many other things, which electrochemical engineering may contribute towards. how to make fuel cells capable of operating and surviving in extreme conditions? what has electrochemical engineering still to do in the pollution abatement area? how to achieve a zero discharge of pollutants in both gaseous and liquid exit streams? beside improvement of the existing technologies, a problem with the existing relationships, describing kinetics and thermodynamics, and with their validity at concentrations approaching zero, needs to be reconsidered. this is a great challenge on the theoretical level. many opportunities exist in areas a few of which are listed below:  wider application of bdd electrode in wastewater treatment can be expected, which will allow a successful anodic destruction of hazardous organics from wastes.  electrokinetic soil remediation is an emerging technology that has attracted increased interest among scientists and officials because of its great potential.  it can be expected that electrochemical promotion of catalyst (nemca effect) should emerge from the laboratory scale, turning a new page in exhaust gas purification. j. electrochem. sci. eng. 2(2) (2012) 53-66 electrochemical engineering anniversary 66  the appearance of room temperature ionic liquids (rtil) opens new opportunities where electrochemical engineering has to find its place and role in establishing new processes based on a new approach. how can electrochemical engineering contribute in materials recovery, waste minimisation and recycling? this problem is an area where electrochemical engineering is already involved, but it is also an area where there are a lot of still unsolved problems. in particular, this relates to the recycling of electronic devices and scrap and the recovery of valuable materials. what could bioelectrochemistry and bioelectrochemical engineering, as sub-disciplines undergoing very fast development, do to establish new electrochemical processes, products and cells for different purposes? what more can electrochemical engineering do in and for nano-technologies? what should be next? where then is the limit? i do not know. do you? in the end, what does electrochemical engineering mean nowadays compared to the definition given below the title, provided almost half a century ago? do we have to redefine it according to the current state? acknowledgement the author would like to express his gratitude to f. c. walsh, g. kelsall and f. lapicque for their suggestions, help and useful critiques in composing this paper. references [1] c.l. mantell, electrochemical engineering, mcgraw hill, new york, usa, 1960 [2] c.s. lin, e.b. denton, n.s. gaskil, g.l. putnam, ind. eng. chem., 43 (9) (1951) 2136–2143 [3] c.w. tobias, m. eisenberg, and c.r. wilke, j. electrochem. soc. 99(12) (1952) 359c-365c [4] c. wagner, in advances in electrochemistry and electrochemical engineering, vol 2, p. delahey and c.w. tobias, eds., wiley interscience, new york, usa, 1962 [5] r. jotrand, f. grunchard, 3rd congr. eur. fed. chem. eng., proceedings, london, uk ,1962, p. 211 [6] h. wendt, g. kreysa, electrochemical engineering, springer-verlag, berlin, germany, 1999 [7] v. stankovic, chem. biochem. eng. q. 21(1) (2007) 33-45 [8] k. yuettner, u. galla, b. schmieder, electrochim. acta 45 (2000) 2575-2594 [9] c. comninellis, g. chem; electrochemistry for the environment, springer, new york london, 2010 [10] d. inman; editorial statement, j. app. electrochem.; 1(1) (1971) 1 [11] m. grotheer, r. alkire, r. varian; the electrochemical society’s interface 15(1) (2006) 52-54 [12] f. lapicque; chemical engineering research and design, 82(a12) (2004) 1571-1574 © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 25%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error 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/hrv (za stvaranje adobe pdf dokumenata najpogodnijih za visokokvalitetni ispis prije tiskanja koristite ove postavke. stvoreni pdf dokumenti mogu se otvoriti acrobat i adobe reader 5.0 i kasnijim verzijama.) /hun 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [4000 4000] /pagesize [612.000 792.000] >> setpagedevice p-toluene sulfonic acid doped polyaniline carbon nanotube com-posites: synthesis via different routes and modified properties doi: 10.5599/jese.2013.0029 47 j. electrochem. sci. eng. 3(2) (2013) 47-56; doi: 10.5599/jese.2013.0029 open access : : issn 1847-9286 www.jese-online.org original scientific paper p-toluene sulfonic acid doped polyaniline carbon nanotube composites: synthesis via different routes and modified properties ashok k. sharma and yashpal sharma* department of materials science & nanotechnology, d.c.r university of science & technology, murthal-131 039 (haryana) india *department of chemistry, g. j. university of science & technology, hisar-125001, india  corresponding author: e-mail: aksharma210@gmail.com received: october 8, 2012; revised: december 25, 2012; published: : april 19, 2013 abstract composites of polyaniline and carbon nanotube (cnt) were prepared by in-situ chemical polymerization method using various aniline concentrations in the initial polymerization solution with p-toluene sulfonic acid (pts) as secondary dopant and mechanical mixing of the pani and cnt using different weight ratios of pani and cnts. the structural characterizations of the composites were done by fourier transform infrared (ftir) and ultra violet visible spectroscopy (uv-visible). scanning electron microscopy (sem) was used to characterize the surface morphology of the composites. it was found that the composites prepared by in-situ chemical polymerization had smoother surface morphology in comparison to the composites obtained by mechanical mixing. the capacitive studies reveal that the in-situ composite has synergistic effect and the specific capacitance of the composite calculated from cyclic voltammogram (cv) was 385.1 f/g. thermal studies indicate that the composites are stable as compared to pani alone showing that the cnt contributes towards thermal stability in the pani-cnt composites. keywords cyclic voltammetry; tga; scanning electron microscopy; composites; conducting polymer; specific capacitance; chemical polymerization; doping, nanomaterials. introduction due to exceptional electrical, electronic and mechanical properties, conducting polymers are also popular as synthetic metals. various conducting polymers such as polyaniline, polyacetylene, polypyrrole and polythiophene have been the subject of intensive research because of their application in different fields such as optoelectronic and display devices, and as active electrode materials in primary and secondary batteries, supercapacitors, tissue engineering, drug delivery etc [1,2]. http://www.jese-online.org/ mailto:aksharma210@gmail.com j. electrochem. sci. eng. 3(2) (2013) 47-56 polyaniline carbon nanotube composites 48 carbon nanotubes after their discovery by iijima and ichihashi [3] have been the field of great interest because of their unique electronic, mechanical and magnetic properties [4]. owing to their unique properties there are varieties of applications of cnt which include supercapacitors and batteries, sensors, biological applications like bone tissue engineering and many more [5]. recently, composites of the cnt and conducting polymers were a matter of immense research because the composite from these two important materials produce the synergistic effect and produce better applications than the individual materials [6-9]. cnt acts as good filler in the polymer composite compared to other carbon materials because of their high surface area. various conducting polymers have been investigated to be used in the composite preparation but out of these pani acts as promising material because of its various properties like air stability, flexibility, light weight, high conductivity, ease of synthesis by chemical and electrochemical method and good processability [10,11]. recently, pani was investigated in the synthesis of panicnt composites by using mwnt [7,9], swnt [8] and carboxylated cnt [6,12]. the composite preparation from the pani and cnt produces a hybrid material having great mechanical strength, high capacitance properties, electrical properties and good processability [13-16]. there are mainly two methods to prepare such composites: chemical oxidative polymerization method and the electrochemical method. the electrochemical method has limitations in terms of mass production while chemical method gives a high yield and is cheaper [17]. doping plays an important role in the conducting polymers. doping is the process which converts the neutral polymer backbone to a charged π conjugated system. this allows electrons to flow through π conjugated system of the conducting polymers due to the formation of conduction bands. various dopant are used for doping of polyaniline which include p-toluene sulphonic acid (pts), dodecyl benzenesulphonic acid (dbsa), camphor sulphonic acid (csa), polyvinyl sulfonic acid (pva) etc. [18]. however, hcl itself acts as a dopant for pani but using a secondary dopant along with hcl enhance the electronic properties of the material and also the processability. this study describes the synthesis of some pani-cnt composites using p-toluene sulfonic acid (pts) as secondary dopant along with hcl by means of chemical oxidative polymerization method and by mechanical mixing of the pts doped pani and cnt in the same proportions as used in the in-situ chemical oxidative polymerization. the synergistic capacitive effect of the composite was also investigated. the difference in the morphology and thermal behaviour of the composites prepared by both the methods were also investigated. experimental materials mwnt (99 %) was purchased from nanostructured and amorphous materials inc. having outer diameter 20-40 nm and length of 1-2 µm. aniline (s.d. fine-chem. ltd, 99.5 %) was vacuum distilled prior to use. pvdf, mw vaco carbon and p-toluene sulfonic acid (pts) sodium salt (95 %) was purchased from sigma-aldrich. all other chemicals like hydrochloric acid (35.4 %), ammonium peroxydisulfate (aps) (98 %), ethanol (99.9 %), sulphuric acid (98 %), activated carbon and n, n-dimethyl formamide (dmf) were obtained from s.d. fine chem. ltd. and used as received without further purification. all chemicals were of analytical grade. solutions were prepared in deionized water. a. k. sharma et al. j. electrochem. sci. eng. 3(2) (2013) 47-56 doi: 10.5599/jese.2013.0029 49 preparation of pani-cnt composites by in-situ chemical polymerization for the preparation of pani-cnt composites by in-situ chemical oxidative polymerization method 0.5 g of cnt was dissolved in 1m hcl and then ultrasonicated for 2 hours in order to disperse cnt bundles. different amounts of aniline monomer (0.5, 1.5, 2.5, 3.5 and 4.5 ml yielding cnt: aniline ratios of 1:1, 1:3, 1:5, 1:7 and 1:9, wt/v) was dissolved in 1 m hcl and 0.2 m pts. this content was transferred to the flask containing ultrasonicated cnt. again the mixture (cnt+aniline) was put to ultrasonication for 2 hours, to incorporate aniline monomer in the cnt matrix. a freshly prepared pre-cooled ammonium peroxydisulphate (1 m) solution in 1 m hcl was added drop wise to the cnt-aniline mixture. the reaction mixture was stirred for 6 hours at 0-5 o c. appearance of greenish black color indicated the formation of the pani-cnt composite. the resulting solution was filtered and washed with distilled water and methanol several times to remove the unreacted oxidant and oligomers if any. the product was dried in vacuum oven at 60 o c overnight. pani was also prepared as above where 2.5 ml of aniline was dissolved in the initial polymerization solution. herein, the pani-cnt composites with different ratios of cnt and aniline monomer will be designated as ratios of cnt: aniline monomer given in parentheses: cpc1 (1:1), cpc2 (1:3), cpc3 (1:5), cpc4 (1:7), cpc5 (1:9). figure 1 is the schematic representation of the in-situ chemical oxidative polymerization method. figure 1. schematic representation of the in-situ chemical oxidative polymerization method preparation of pani-cnt composites by mechanical mixing method for the preparation of pani-cnt composites by mechanical mixing method, pani was prepared by the in-situ chemical route. the as such prepared pts doped pani was mixed with cnt in different ratios (cnt:pani ratios were 1:1, 1:3, 1:5, 1:7 and 1:9) in dmf solvent. the mixture was ultrasonicated for 2-3 hours to disperse the cnt bundles and for better incorporation of the cnt in the polymer matrix. after sonication the mixture was magnetically stirred for 6-7 hours. finally, the mixture was filtered and washed several times with distilled water and methanol. the panicnt composites were collected by drying the powder in vacuum oven at 60 o c. herein, the panicnt composites prepared by mechanical mixing method with different ratios of cnt and pani will be designated as ratios of cnt: pani given in parentheses cpm1 (1:1), cpm2 (1:3), cpm3 (1:5), cpm4 (1:7), cpm5 (1:9). j. electrochem. sci. eng. 3(2) (2013) 47-56 polyaniline carbon nanotube composites 50 preparation of electrode for cyclic voltammetry the pani-cnt composite prepared by in-situ chemical oxidative polymerization method was mixed with 10 wt. % of activated carbon and 5 wt. % of mw vaco carbon as conductor and 5 wt. % of binder (pvdf) in n,n-dimethyl formamide (dmf) solvent. the mixture was ultrasonicated for 2 hours and then stirred to form slurry. the slurry was brush coated on 2×2 cm 2 of graphite sheet as a current collector. characterization the chemical structure of the composite was characterized by using fourier transform infrared spectrophotometer (shimadzu ir affinity-i 8000 ft-ir spectrophotometer). interaction between pani and cnt was investigated by means of uv-visible spectroscopic technique (varian cary5000 spectrophotometer). the scanning electron microscopy (sem, jeol-jsm-5600lv) was used to examine the surface morphology and average thickness of deposited pani over the surface of cnt. the capacitive property of the pani-cnt composite electrodes was examined by cyclic voltammetry using three-electrode cell system with a reference electrode (saturated calomel electrode) and a counter electrode (pt) in 1 m h2so4 solutions as electrolyte. cyclic voltammetry measurements were carried out by potentiostat/galvanostat (ch instruments 600c series). the geometric surface area of the working electrode was 2×2 cm 2 . cyclic voltammograms were recorded in the voltage windows 0-0.8 v at a scan rate of 2 mv/s. thermal analyses of the cnt, pani and panicnt composites was done by using thermogravimetric analyzer (universal v4.7a ta instruments). results and discussion ftir study the ftir spectrum of the pani-cnt composite is given in figure 2. the spectrum shows all bands of pani. the band at 1448.58 and 1581.16 cm -1 may be assigned to the benzoid and quinoid ring vibrations, respectively. the band at 1294.11 cm -1 is due to the conducting form of the pani indicating that the pani exists in the conducting emeraldine salt form. the bands at 1100.79 and 785.68 cm -1 may be assigned to the aromatic c-h in plane and aromatic c-h out of plane bending vibrations, respectively [19]. a broad peak at 3402.13 cm -1 is due to the n-h stretching vibration of the aromatic amine. the bands at 1068.92 and 674.19 cm -1 represent the symmetric stretching of the so3 group [18]. the bands are in agreement with the theoretical predictions and confirm the deposition of pani over the surface of the cnt i.e. formation of pani-cnt composite using pts as secondary dopant [20,21]. uv-visible spectra uv-visible spectra of the cnt, pani and the pani-cnt composite were recorded in n,n-dimethylformamide (dmf) solvent in the wavelength range from 300 to 800 nm. figure 3 shows the uv-visible spectra of cnt, pani and the pani-cnt composite. the cnt has no characteristic peak in this region. pani has its characteristic peak at 540 nm. for the pani-cnt composite the pani shows its characteristic peak at 541 nm showing the protonated doped state of the polyaniline indicating the deposition of pani over the cnt surface and formation of composite [22,23]. a. k. sharma et al. j. electrochem. sci. eng. 3(2) (2013) 47-56 doi: 10.5599/jese.2013.0029 51 figure 2. ftir spectrum of the pani-cnt composite figure 3. uv-visible spectra of the cnt, pani and pani-cnt composite surface morphology surface morphology of the composites was studied by scanning electron microscopy (sem). figure 4a-c shows the sem micrographs of the cnt, cpc5 and cpm5 respectively. cnt shows smooth surface morphology with average diameter of ≈20-40 nm. comparing figure 4b with the sem micrograph of cnt (figure 4a) it could be concluded that the polyaniline becomes deposited over the surface of the cnt indicating the formation of the pani-cnt composite. pani-cnt composite shows that the average thickness of deposited pani over the surface of cnt is ≈200-230 nm. comparing figure 4c with the sem micrographs of cnt (figure 4a) and the j. electrochem. sci. eng. 3(2) (2013) 47-56 polyaniline carbon nanotube composites 52 pani-cnt composite (figure 4b), it can be said that the composite prepared through mechanical mixing method shows defective and irregular surface morphology which may be caused due to mixing of the two components for long period of time. however, it it is not easy to distinguish the cnt bundles in the composite which is clearly seen in case of pani-cnt composite prepared by insitu chemical oxidative polymerization method. in the case of the in-situ polymerization method, the pani wrapped the cnt surface in a regular manner using each cnt bundle as can be seen in figure 4b but in mechanical mixing method the pani deposition takes place instead of wrapping over the cnt in irregular fashion. figure 4. sem micrographs of (a) cnt, (b) cpc5 and (c) cpm5 respectively capacitive study the capacitive characteristic of the cnt, pani and the pani-cnt composite prepared by in-situ chemical oxidative polymerization was investigated by means of cyclic voltammetry. the capacitance was calculated from the obtained cyclic voltammogram by dividing the average current with scan rate while the specific capacitance was calculated by dividing the capacitance by the weight of active material. it can be concluded from figure 5 that cnt behaves like an electric double layer showing almost rectangularly shaped cyclic voltammogram and is almost mirror image with respect to zero current line. for the pani and pani-cnt composite the redox peaks are indicative of the higher currents flowing in the pani-cnt as compared to the pani alone and can be attributed to the porous structure of the composite in which cnt provides a larger surface area for aniline to polymerize a. k. sharma et al. j. electrochem. sci. eng. 3(2) (2013) 47-56 doi: 10.5599/jese.2013.0029 53 upon. the specific capacitance of cnt, pani and pani-cnt composite calculated from cyclic voltammograms was 31.56 f/g, 314.07 f/g and 385.1 f/g, respectively. it can be explained as the synergy contribution from the pani and the cnt because of better electrolyte access in the active material and improved contact between the current collector and the active material resulting in decreased overall internal resistance in the electrode and better capacitance value [24-27]. figure 5. cyclic voltammogram of cnt, pani and the pani-cnt composite thermal analysis thermogravimetric analysis (tga) of the cnt, pani and the cnt-pani composites were carried out under nitrogen atmosphere in order to investigate the % mass loss with respect to temperature. figures 6a and 6b show the tgas of the cnt, pani and cnt-pani composites prepared by chemical and mechanical mixing method, respectively. from figure 6a it can be inferred that the composites show very good thermal stability as compared to the pani, which can be attributed to the presence of cnt in the original polymer matrix [22]. cnt has highest thermal stability and does not show any decomposition in the temperature range of 30 to 580 o c. the polyaniline on the other hand exhibited a slight decrease in the weight at 114 o c which can be assigned to the loss of water [28]. up to temperatures of 340 o c approximately 15 % of the weight is lost. above this temperature pani showed regular decrease in the weight due to thermal degradation of the polymer. the order of thermal stability of various pani-cnt composites, prepared chemically is cpc1 > cpc5 > cpc4 > cpc3 > cpc2 indicating increased thermal stability of the composite as aniline content increases in the initial polymerization solution. highest thermal stability of cpc1 may be assigned to 1:1 aniline to cnt ratio and hence the lesser amount of pani. therefore, cnt in this composite played a dominant role in promoting the thermal stability. in case of higher aniline content the thermal stability is somewhat independent to the cnt content and can be attributed to the layer by layer degradation of the deposited pani over cnt surface. j. electrochem. sci. eng. 3(2) (2013) 47-56 polyaniline carbon nanotube composites 54 figure 6. tga curves of (a) cnt, pani and pani-cnt composites prepared by in-situ chemical oxidative polymerization method and (b) pani-cnt composites prepared by mechanical mixing method table 1. comparison of % weight loss of two kinds of composites at 380 and 600 o c composite weight loss at 380 o c, % weight loss at 600 o c, % cpc1 13 48 cpc2 19 64 cpc3 18 59 cpc4 15 56 cpc5 13 48 cpm1 19 28 cpm2 50 67 cpm3 41 55 cpm4 33 44 cpm5 20 30 a. k. sharma et al. j. electrochem. sci. eng. 3(2) (2013) 47-56 doi: 10.5599/jese.2013.0029 55 a similar trend was obtained for the mechanically mixed composites. for comparison purpose the weight losses of the composites at temperature 380 o c and 600 o c are given in the table i. the results revealed that the composites prepared by chemical method have higher thermal stability compared to composites by mechanical mixing at the temperature range of 380 o c. this can be attributed to the layer by layer deposition of the pani over the cnt surface in case of chemically prepared composites which can also be seen in the sem micrograph, whereas no such layered structure is obtained in the mechanically mixed composites. the other reason behind this is the smooth surface morphology of the chemically prepared composites due to separation of the cnt bundles. however, at temperature of 600 o c the reverse order of thermal stability was seen because of presence of higher pani content in the mechanically mixed composites [29]. another reason is that in the case of mechanically mixed composites there is a lower chance of separation of cnt bundles and hence the pani gets deposited over thick cnt bundles resulting in the enhanced stability at higher temperature. conclusions the composites of the pani-cnt have been successfully synthesized chemically and by mechanical mixing of the pani and cnt. composites prepared by chemical polymerization have smooth surface morphology indicative of layer by layer deposition of the pani over cnt substrate. whereas the composites prepared by mechanical mixing appear to have irregular deposition of pani. the thermal stability of the composite is better than the pani indicating the contribution of cnt towards the thermal stability of the composites. the composite exhibited higher value of specific capacitance than the cnt and pani alone and is indicative of the synergy. however, cnt did not contributed much to the capacitive value but acts as a good substrate for pani deposition. acknowledgements: authors are thankful to the university grants commission, new delhi (india) for providing financial assistance in the form of major research project. references [1] j.w. schultze, h. karabulut, electrochim. acta 50 (2005) 1739–1745. 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[29] a.m. showkat, k.p. lee, a.i. gopalan, s.h. kim, s.h. choi, s.h. sohn, j. appl. polym. sci. 101 (2006) 3721–3729. © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ synthesis of ammonia from water and nitrogen using a compo-site cathode based on la0.6ba0.4fe0.8cu0.2o3-δ-ce0.8gd0.18ca0.02o2-δ http://dx.doi.org/10.5599/jese.1793 589 j. electrochem. sci. eng. 13(4) (2023) 589-590 http://dx.doi.org/10.5599/jese.1973 open access : : issn 1847-9286 www.jese-online.org editorial battery technologies: lithium & beyond prayag biswal schlumberger ltd. usa pb542@cornell.edu published: july 11, 2023 global efforts to mitigate climate change are causing a transition from non-renewable energy resources (fossil fuels) to renewable energy resources (wind, solar, hydroelectricity, geothermal). this energy transition to sustainably meet the world’s growing needs for electricity, heating, cooling, and power for transport is widely considered to be one of the biggest challenges facing humanity in this century. the transition is enabled by improvements in generation and storage technologies critical to harvesting inherently intermittent renewable energy. moreover, growing needs for smaller, lighter, more powerful portable electronic devices and more powerful electric vehicles suitable for long-range transportation have further fostered the demand for dispatchable and efficient electrical energy storage. these have catalyzed rapid development and commercialization of high-energy and lightweight rechargeable batteries, primarily based on lithium. however, lithiumenabled rechargeable batteries are plagued with challenges such as uncontrolled surface/interface (low safety), sluggish transport & reaction kinetics (slow charging), & relatively rare abundance of the metal (high cost). moving beyond lithium necessitates the development of safe & fast-charging rechargeable batteries based on relatively abundant metals (i.e. na, zn, al, fe, etc.). this special issue discusses (a) the genesis and current state of lithium-ion batteries bridging chemistry to commercial applications (le, a general introduction to lithium-ion batteries: from the first concept to the top six commercials and beyond) (b) material chemistries that enable nextgeneration lithium-ion electrodes of high performance and energy densities (pavitra et al., brief review on carbon derivatives based ternary metal oxide composite electrode materials for lithiumion batteries) (c) fundamental understanding of anode and cathode reaction pathways that dictate lithium ion-battery performance (h. dasari, e. eisenbraun, predicting the effect of silicon electrode design parameters on thermal performance of a lithium-ion battery and m. sakai, cathode reaction models for braga-goodenough na-ferrocene and li-mno2 rechargeable batteries) (d) electrochemically inactive precursors and coatings that can enhance the performance of lithium electrodes and therein the battery performance across rate capability and longevity (c. cai et al., impact of carbon coating processing using sucrose for thick binder-free titanium niobium oxide lithium-ion battery anode) and (e) battery material processing and doping techniques that enhance the energy density and cyclability of the lithium-ion battery k. elong et al., annealing effect on structural and http://dx.doi.org/10.5599/jese.1793 http://dx.doi.org/10.5599/jese.1973 http://www.jese-online.org/ mailto:pb542@cornell.edu j. electrochem. sci. eng. 13(4) (2023) 589-590 editorial 590 electrochemical performance of ti-doped lini1/3mn1/3co1/3o2 cathode materials). in summary, this issue aims to provide the readers with a fundamental understanding of lithium battery technology, from material chemistry to commercial applications, that enables the readers to endeavor in their discovery of next-generation batteries. this issue is a tribute to peter faguy (1956 2022), a true scientist, a teacher, and a friend who led translational r&d programs to commercialize battery technology for electric vehicles. he was proud and determined to develop the future of renewable energy for the entire world. many people think he has done this; if not, it was only a matter of time. ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://creativecommons.org/licenses/by/4.0/) {recovery of nickel from spent nicd batteries by regular and ultrasonic leaching followed by electrodeposition} http://dx.doi.org/10.5599/jese.617 41 j. electrochem. sci. eng. 10(1) (2020) 41-47; http://dx.doi.org/10.5599/jese.617 open access: issn 1847-9286 www.jese-online.org original scientific paper recovery of nickel from spent nicd batteries by regular and ultrasonic leaching followed by electrodeposition elumalai selvamani nivetha, ramanujam saravanathamizhan department of chemical engineering, a.c.tech campus, anna university, chennai-600 025, india corresponding author: thamizhan79@rediffmail.com, rsthamizhan@gmail.com, tel: +91 44 22359237 received: september 6, 2018; revised: july 17, 2019; accepted: july 17, 2019 abstract this study focuses on the hydrometallurgical route of separating ni metal from spent nickel cadmium (nicd) batteries. the comparison of separation of nickel metal with the assistance of ultrasonic leaching and regular leaching was performed. sulphuric acid (solvent) was used as the lixiviant to leach out the nickel metal ions from the spent battery and the parameters affecting the leaching with and without ultrasonication were optimized. the major parameters affecting the leaching process are volume of solvent, concentration of lixiviant and leaching time. the electrodeposition of nickel metal ions from ultrasonically assisted and regular leaching was carried out at an optimized current density of 8 a dm-2, contact time of 4 hours and the concentration of lixiviant of 5 m. it was observed that the recovery efficiency for ultrasonically assisted leaching followed by electrodeposition is 98.5 %, while in regular leaching followed by electrodeposition is 90.8 %. keywords electrodeposition; leaching; spent battery; ultrasonication introduction rechargeable batteries such as nickel cadmium (nicd), nickel metal hydride (nimh) and lithium ion batteries are commonly used as the power sources in electronic devices like mobile phones, computers, digital cameras, and hybrid electric vehicles [1]. generally, batteries are complex systems containing large number of different materials inside a small volume. all materials in a battery contribute to environmental pollution when discarded [2,3]. in nicd battery, the anode is nickel oxy-hydroxide, the cathode is cadmium and the electrolyte is koh. the nickel content of the cathodic part is 69.26 %, which represents a significant amount to be recovered. there are two major techniques carried out in the recovery of metals, namely the hydrometallurgical process and the pyrometallurgical process. hydrometallurgical process includes solvent extraction, acid or base leaching, sequential leaching along with electrodeposition or selective precipitation, bio-leaching and ultrasonic assisted leaching [4-8]. http://dx.doi.org/10.5599/jese.617 http://dx.doi.org/10.5599/jese.617 http://www.jese-online.org/ mailto:thamizhan79@rediffmail.com mailto:rsthamizhan@gmail.com j. electrochem. sci. eng. 10(1) (2020) 41-47 recovery of nickel from spent nicd batteries 42 innocenzi and veglio [9] described two sequential leaching steps followed by selective precipitation of rare earths from nimh batteries. leaching was carried out with sulphuric acid as the lixiviant and three major parameters were studied, i.e. concentration of sulphuric acid, temperature and concentration of citric acid. they showed that citric acid promotes efficient leaching of metals acting as a reducing agent. the selective precipitation of metals was carried out using 5 m naoh. rare earths were precipitated as sulphates and it was observed that 99 % of rare earths were recovered after selective precipitation. tanong et al. [10] demonstrated a hydrometallurgical route to recover metals from a mixture of various spent battery samples. box-behnken design was used to optimize various factors that influence the metal removal process. eight leaching reagents were used to leach the metals at various concentrations and temperatures and sulphuric acid was found the most effective. it is reported that to recover the maximum amount of metal, the optimum values are 1 m sulphuric acid concentration, fixed solid/liquid ratio of 18 % w/w and 30 min of leaching time. one of the key technologies in the recovery of metals prevailing in the current scenario is ultrasonically assisted leaching. the resulting small particle size would increase the surface area and promote a higher rate of diffusion of solvent through convective motion [11,12]. xin et al. [13] have reported the leaching kinetics of zn with other metals using ultrasound and evaluated effects of major factors such as temperature, sulphuric acid concentration, solid/liquid ratio and ultrasound power. it was reported that 80 % of zn was recovered using ultrasonic leaching. li et al., [14] studied recovery of co and li from spent lithium-ion batteries using ultrasound assisted leaching. the author reported that more than 96 % co and 100 % li were recovered using ultrasound assisted citric acid leaching. a combined technique of electro-assisted leaching and electrodeposition of metals from spent nicd batteries was investigated by hazottee et al. [15]. in this investigation expanded platinized titanium was used as anode, and an aluminium plate as cathode. a cell voltage in the range of 6-10 v led to a highly selective deposition of cadmium from nickel and cobalt. tudela et al. [16] studied the ultrasonic effect on electrodeposition of nickel and the effect of ultrasonic power on the characteristics of the thin coating. the ultrasonic assisted electrodeposition was carried out in the ultrasonicator bath with an operating frequency of 33 khz. it was observed that the application of ultrasound led to the change in orientation of the ni deposit. santos et al. [17] studied chemical and electrochemical recycling methods for ni, co, zn and mn from the positive electrode of spent nimh batteries. the objective of the present study is to recover nickel from spent nicd batteries using ultrasonic leaching followed by electrodeposition. nicd batteries were manually dismantled to identify the various components and the cathode was characterized using atomic absorption spectroscopy (aas). the major factors that affect the leaching and electrodeposition such as concentration of sulphuric acid, amount of solvent required, leaching time and current densities were optimized. selective electrodeposition of ni was performed at a constant cell potential of 2.6 v. materials and methods dismantling of spent batteries four samples of nicd rechargeable battery with specifications of 700 mah, aa size and 1.2 v were collected from various electronic devices and toys. the spent batteries were manually dismantled using a saw and knife to identify various components present. these components presented in wt.% were: a paper insulation (15 wt.%), black mass (cathode) (17 wt.%), anode (29.23 wt.%), cathode cap and metal outer casing (34.54 wt.%), washer and a plastic outer cover (3.95 wt.%). material balance was done to account for the amount of each component present. e. s. nivetha and r. saravanathamizhan j. electrochem. sci. eng. 10(1) (2020) 41-47 http://dx.doi.org/10.5599/jese.617 43 characterization of cathode part the cathode part of the spent battery was characterized with the aid of aas. the metal components present in the cathode part and their wt.% contents are shown in the table 1. obviously, ni is present at about 70 wt.% of the total cathode part of the sample. table 1. characterization of cathode part of the spent nicd battery sample no metal component content, wt.% 1 ni 69.26 2 cd 22.14 3 co 0.62 4 fe 0.61 5 zn 0.49 6 mg 0.002 7 other metals 6.82 leaching of black mass in the bath type ultrasonic generator a standard electricity line of 50 hz ac supply is converted to an ultrasonic frequency through in-built piezoelectric transducer. the sonicator generates a constant frequency of 33 khz for the constant power input. it consists from a rectangular bath type with cooling water circulation to maintain the temperate in the bath. about 0.5 g of the cathode part of battery sample was taken for the leaching. in regular leaching, the mixture of sulfuric acid and battery powder was kept suspended with the aid of magnetic stirrer. the leaching was performed with different volumes of sulfuric acid as the lixiviant under various operating conditions. regular leaching and ultrasonic assisted leaching were carried out for 4 hours at various concentrations of sulfuric acid (3, 4, 5, 6 and 7 m). electrodeposition of nickel electrodepostion was carried out using stainless steel electrodes as anode and cathode, each having dimensions of 12.5 × 6 cm in a setup shown schematically in figure 1. figure 1. schematic representation of electrode position setup: 1-regulated power supply, 2-glass vessel with leachate solution, 3-anode, 4-cathode and 5-magnetic stirrer the electrodes were kept 2 cm apart. the electrolysis was carried out with the leachate from the regular and ultrasonically assisted leaching as the electrolyte at a current density of 8 a dm-2 through a time period of 4 hours. http://dx.doi.org/10.5599/jese.617 j. electrochem. sci. eng. 10(1) (2020) 41-47 recovery of nickel from spent nicd batteries 44 the recovery efficiency is calculated by the equation (1): recovery, % = ((c0-ct) /c0)  100 (1) in eq. (1), c0 is initial concentration of nickel (ppm) and ct is concentration of nickel (ppm) present in leachate after time (t). results and discussion leaching the effect of sulfuric acid concentration on regular and ultrasonically assisted leaching is shown in figure 2. the increase in acid concentration increases the dissolved concentration of nickel metal ions in the medium. this is because higher is the concentration of sulfuric acid, higher will be the generation of protons by acid dissociation, facilitating thus production of metal ions to the solution. the maximum dissolution concentration of nickel metal ions was achieved at about 5 m of sulfuric acid. by keeping the concentration constant at 5 m, the leaching was performed at various time intervals from one hour to 5 hours at room temperature (32 °c) as shown in figure 3. the increase in the time period increases the contact time of the solution and the battery sample so that the dissolution takes place effectively. the maximum dissolution concentration of nickel metal ions is obtained for a time period of 4 hours. ch2so4 / m figure 2. effect of sulfuric acid concentration on ni dissolution. solvent volume 60 ml, leaching time 4 h as shown in figure 4, the amount of required lixiviant is obviously a major factor that affects the dissolution of nickel metal ions. initially 15 ml of sulfuric acid was taken to leach out the metal ion which was sufficient enough to submerge the battery sample. doubling the sulfuric acid produced an increase in the concentration of nickel ions, and maximum is attained at 60 ml of sulfuric acid, which was used in the remaining work. d is so lu ti o n c o n ce n tr a ti o n o f n i m e ta l io n s, p p m e. s. nivetha and r. saravanathamizhan j. electrochem. sci. eng. 10(1) (2020) 41-47 http://dx.doi.org/10.5599/jese.617 45 leaching time, h figure 3. effect of leaching time on ni dissolution. sulfuric acid concentration 5 m, solvent volume 60 ml. amount of solvent, ml figure 4. effect of solvent volume on ni dissolution. sulfuric acid concentration 5 m, leaching time 4h comparing of dissolution concentrations of nickel metal ions at regular and ultrasonically assisted leaching shows that ultrasonically assisted leaching of spent battery enhanced the recovery of nickel metal ions. this can be due to its advantages such as the local temperature and pressure produced during the cavitation, improved convective motion and provided energy, what all facilitate dissolution of metals. the major reason for incorporation of ultrasounds during leaching is the cavitation, since the ultrasound causes formation of microscopic bubbles and subsequent implosion what reduces the size of particles. the reduced particle size provides larger surface area for an improved diffusion to occur. ultrasonic leaching followed by electrodeposition of metal the leachate from regular and ultrasonically assisted leaching was used to recover nickel metal by electrodeposition. the current density was varied from 2 a dm-2 to 8 a dm-2. the effect of an d is so lu ti o n c o n ce n tr a ti o n o f n i m e ta l io n s, p p m d is so lu ti o n c o n ce n tr a ti o n o f n i m e ta l io n s, p p m http://dx.doi.org/10.5599/jese.617 j. electrochem. sci. eng. 10(1) (2020) 41-47 recovery of nickel from spent nicd batteries 46 increase in current density on metal recovery is shown in figure 5. the sample of electrolyte was analyzed with the aid of aas to identify the concentration of nickel metal ions. the concentration of nickel metal ions decreases as the electrolysis is performed, thus indicating the deposition of nickel on the surface of cathode. maximum recovery efficiency is observed at the current density of 8 a dm-2. j / a dm-2 figure 5. effect of current density on percentage recovery of ni of regular leaching leachate. sulfuric acid concentration 5 m; solvent volume 60 ml; leaching time 4 h; electrolysis time 4 h the effect of electrolysis time on percentage recovery of ni is shown in figure 6. the current density was maintained at 8 a dm-2 for the maximum recovery of nickel. it is observed from figure 6 that as the electrolysis time increases, ni recovery increases up to 4 h, beyond which no further recovery of ni is observed. the recovery efficiency with ultrasonically assisted leaching and electrodeposition is 98.50 % compared to regular leaching at 90.80 %. it is concluded that the ultrasonically assisted leaching followed by the electrodeposition has higher recovery efficiency than the regular leaching. time, h figure 6. effect of electrolysis time on percentage recovery of ni. sulfuric acid concentration 5 m, solvent volume 60 ml, leaching time 4 h; current density 8 a dm-2 r e co v e ry e ff ic ie n cy , % r e co v e ry e ff ic ie n cy , % e. s. nivetha and r. saravanathamizhan j. electrochem. sci. eng. 10(1) (2020) 41-47 http://dx.doi.org/10.5599/jese.617 47 conclusion this research is focused on the effects of process conditions of the regular and ultrasonically assisted leaching of ni metal ions from spent nicd batteries. the parameters describing process conditions, i.e. concentration of lixiviant, amount of lixiviant and leaching time were optimized. for both types of leaching, the optimized values were 5 m for the concentration of sulfuric acid, 60 ml for the amount of lixiviant, and 4 h for leaching time. the incorporation of ultrasound at 33 khz during the leaching process showed a certain advantage due to size reduction of particles, what resulted in more effective diffusion. the electrodeposition of nickel metal from ultrasonically assisted leachates showed the maximum recovery of nickel metal of 98.50 %, what is higher than 90.80 % found for electrodeposition of nickel from regular leaching. references [1] l. e. o. c rodrigues, m. b. mansur, journal of power sources 195 (2010) 3735-3741. [2] c. c. b. martha de souza, d. corrêa de oliviera, j. a. s. tenório, journal of power sources 103 (2001) 120–126. [3] l. c. ferracin, a. e. chácon-sanhueza, r. a. davoglio, l. o. rocha, d. j. caffeu, a. r. fontanetti, r. c. rocha filho, s. r. biaggio, n. bocchi, hydrometallurgy 65 (2002) 137–144. [4] d. a. bertuol, a. m. bernardes, j. a. s. tenório, journal of power sources 193 (2009) 914-923. [5] i. c. nnorom, o. osibanjo, international journal of environmental science and technology 6 (2009) 641-650. [6] y. pranolo, w. zhang, c. y. cheng, hydrometallurgy 102 (2010) 37-42. [7] p. meshram, b. d. pandey, t. r. mankhand, hydrometallurgy 158 (2015) 172-179. [8] r. oza, n. shah, s. patel, society of chemical industries 86 (2011) 1276-1281. [9] v. innocenzi, f.veglio, journal of power sources 211 (2012) 184-191. [10] k. tanong, l. coudert, g. mercier, j.-f. blais, journal of environmental management 181 (2016) 95-107. [11] m. marafi, a. stanislaus, industrial engineering chemistry research 50 (2011) 9495-9501. [12] b. avvaru, s. b. roy, s. chowdhury, k. n. hareendran, a. b. pandit, industrial engineering chemistry research 45 (2006) 7639-7648. [13] x. wang, c. srinivasakannan, x.-h. duan, j.-h peng, d.-j., yang, s.-h. ju, separation and purification technology 115 (2013) 66–72. [14] l .li, l.zhai, x. zhang, j. lu, r. chen, f. wu, k. amine, journal of power sources 262 (2014) 380-385. [15] c. hazotte, e. meux, n. leclerc, f. lapicque, chemical engineering and processing 96 (2015) 83-93. [16] i. tudela, y. zhang, m. pal, i. kerr, a. j. cobley, surface and coatings technology 276 (2015) 89–105. [17] v. e. o. santos, v. g. celante, m. f. f. lelis, m. b. j. g. freitas, journal of power sources 218 (2012) 435-444. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.617 http://creativecommons.org/licenses/by/4.0/) {cyclic voltammetry: a simple method for determining contents of total and free iron ions in sodium ferric gluconate complex} http://dx.doi.org/10.5599/jese.749 281 j. electrochem. sci. eng. 10(3) (2020) 281-291; http://dx.doi.org/10.5599/jese.749 open access: issn 1847-9286 www.jese-online.org original scientific paper cyclic voltammetry: a simple method for determining contents of total and free iron ions in sodium ferric gluconate complex hong liu, qiao yu1, yixuan ma2, baoyu liu1 and hongchun pan1 1college of pharmaceutical sciences, southwest university, chongqing, china 2affiliated hospital of qinghai university corresponding author: lhphch@126.com; tel.:13368261386; received: november 1, 2019; revised: april 3, 2020; accepted: april 4, 2020 abstract sodium ferric gluconate complex (sfgc) is the third generation of iron supplement of polysaccharide iron (iii) complex (pic). for evaluation of technological level and application value of the prepared sfgc, it would be of great significance to determine the iron content in sfgc in a simple but effective way. this paper introduces the cyclic voltammetry (cv) method for determination of iron content in sfgc. under established optimal experimental conditions, the content of free iron ions can be directly scanned and calculated, while the total iron content can also be determined by completely acidifying sfgc into fe3+ ions. after optimizing and screening, the optimal scanning conditions are determined as ph 3 and 0.05 v/s of the scanning rate. prior cv measurements, 0.4 v of the enrichment potential, and 3 min of the enrichment time are found optimal. it has also been verified that co32ions present in the solution show little interference in the system within the experimental range of investigation. the contents of free fe3+, fe2+ ions and the total iron determined after acidhydrolysis of sfgc can be accurately calculated according to the corresponding linear relationships between peak currents and iron concentrations. in this paper, the repeatability and accuracy of the method are verified, and its feasibility as a convenient and effective method to determine the iron supplements is confirmed. keywords: iron content; free iron ions content; iron supplements introduction the iron-deficiency anemia is currently the most common type of anemia, which widely exists in many countries around the world, especially in the developing countries [1,2]. by inadequate iron intake, the synthesis of hemoglobin in human’s body will be affected, resulting in inadequate oxygen supply to cells and tissues [1,3]. iron-deficiency anemia has usually been treated by iron supplements. development of iron supplements is carried out through three stages, where the third generation, namely the macromolecular compound iron supplement, is a new type of iron complex http://dx.doi.org/10.5599/jese.749 http://dx.doi.org/10.5599/jese.749 http://www.jese-online.org/ file:///d:/dokumenti/dropbox/03_jese/0_web/articles/olf/doc/lhphch@126.com j. electrochem. sci. eng. 10(3) (2020) 281-291 determining contents of iron ions 282 formed by ferric ions and the carrier of organic macromolecular complex [4]. compared with previous two generations, the third one has higher iron content and better solubility, because iron in the complex does not exist as ions. hence iron supplements of this new generation cause little digestive side effects and among them, the saccharide-iron complexes are under wide research work [5]. in addition to being the iron supplement, the saccharide itself has many biological activities, and is beneficial to the body by various synergistic effects after being absorbed. therefore, the saccharide-iron (iii) complex is a promising type of nutritional iron supplement [6,7]. gluconic acid sodium salt (d-sg)(figure 1) is used as a molecular shell to effectively adsorb the metal iron ions in the solution, forming a stable iron complex. the complex is called sodium ferric gluconate complex (sfgc), having possible structure shown in figure 2. this iron supplement is highly stable and has no toxic side effects. figure 1. molecular structure of d-sg figure 2. proposed structure of sfgc the iron content is one of the most important indicators to analyze the complexation process and technological results of sodium ferric gluconate complex (sfgc) preparation [8-10]. since less content of free iron ions can reduce irritation in the digestive tract, high content of complex iron increases a status of the product. therefore, it is important to determine the contents of free and total iron in sfgc rapidly and accurately. so far, the determination of iron content mainly includes the methods such as potassium dichromate solution titration, phenanthroline spectrophotometry, atomic absorption spectrometry, etc. [11,12]. however, the titration technique not only consumes plenty of time and chemicals, but also produces a great deal of pollutants because of the utilization of toxic potassium dichromate and mercuric chloride in the operation [13]. the spectrophotometry requires a pre-treated sample with a complicated operation and has low sensitivity and poor accuracy [14]. although the atomic absorption spectrophotometry does not require a pretreatment process, the operation is simple and fast and accuracy is high, the equipment is expensive, and difficult to popularize [15]. the most important is that all above mentioned methods cannot accurately distinguish and separately determine the contents of free iron ions and total iron in the complex when they both exist in the system. this brings difficulties to evaluation of the complexation process and outcome of the iron complex system. therefore, it would be of great importance to develop an accurate, rapid, convenient and costefficient method for respective determinations of free iron ions and total iron in the complex. in this paper, a classical electrochemical research method [16,17], that is cyclic voltammetry technique is adopted. it allows free fe3+ and fe2+ ions in the sample solution to form a reversible redox system and generate current peaks by corresponding redox reactions. based on these redox reactions, the content of free iron ions in sfgc can be determined directly, while the total iron content can be determined after conversion of sfgc into free fe3+ ions by acid-hydrolysis. acidhydrolysis conditions, ph, potential scanning rate and some other experimental parameters are h. liu et al. j. electrochem. sci. eng. 10(3) (2020) 281-291 http://dx.doi.org/10.5599/jese.749 283 optimized in this paper, and the methodological verification is carried out in order to obtain a practical and efficient detection method. experimental reagents and materials d-sodium gluconate (d-sg) was purchased from chengdu kelong chemical co., ltd., sichuan province, china. ultrapure water (18.25 mω cm) was used throughout the experiment. all other reagents and chemicals employed in this experiment were of analytical grade. apparatus the electrochemical measurements of cyclic voltammetry (cv) were performed with a chi610e electrochemical workstation (chenhua co. ltd., shanghai, china). it contains a conventional threeelectrode system comprised of platinum as the auxiliary electrode, saturated calomel electrode as the reference electrode and 3-mm-diameter glassy carbon (gc) as the working electrode. the ph measurements were assayed with a fe20 fiveeasytm ph meter (mettler toledo, shanghai, china). all measurements were carried out at indoor temperature. preparation of sfgc 40 ml of 1.18 mol/l na2co3 solution was added to another 40 ml solution which contained 10 g d-sg. afterwards, the solution containing (60 ml) 1.25 mol/l of fecl3·6h2o was added dropwise under continuous stirring. the synthesis of the complex was performed at ph 11 (adjusted by adding 5 mol/l naoh). the mixture was heated at 100 °c at least 3 h in the oil bath, and then filtered and cooled to room temperature. the complex was precipitated by 0.2 l of ethanol. after 4 h of standing, the ethanol was decanted, and the precipitate was centrifuged at 3000 r/min for 10 min. the supernatant was decanted, and the precipitate was dissolved in 0.1 l redistilled water. dialysis of the solution was used to remove unbound ions (cl-, na+, fe3+). finally, the complex was washed successively from the dialyzed solution by ethanol and acetone and dried for 3 h in vacuum after decantation. thus, the sodium ferric gluconate complex (sfgc) was obtained as a black powder. preparation of fe standard solutions and acid hydrolysis of sfgc 0.01 mol/l fe3+ standard solution: 0.1351 g of fecl3·6h2o was dissolved in ultrapure water, mixed and diluted to the required concentration (50 ml). after that, the samples of standard solution were diluted to different concentrations (0.009, 0.008, 0.007, 0.006, 0.005 mol/l, 50 ml). 0.05 mol/l fe2+ standard solution: 0.4970 g of fecl2·12h2o was dissolved in ultrapure water, mixed and rapidly diluted to the required concentration (50 ml). after that, the samples of standard solution were diluted to different concentrations (0.045, 0.040, 0.035, 0.030, 0.025 mol/l, 50 ml). as fe2+ ions are easily oxidized into fe3+ ions, all prepared fe2+ solutions should be used right after preparation. sfgc acid-hydrolysis solution: 0.2500 g of sfgc was dissolved in ultrapure water, mixed and diluted to 5 g/l (50 ml). hydrochloric acid solutions (2 ml) with different concentrations (1, 3, 5, 7 mol/l) were slowly added to 4 ml of 5 g/l sfgc solution in 25 ml beaker, respectively. after 5 min stirring, the solution was treated by acid-hydrolysis method for 5 min at room temperature (rt), water bath (20 °c) and refrigerator (4 °c), respectively. test method to improve reproducibility and sensitivity of peak currents in cv measurements, the surface of the working gc electrode was polished successively with 1, 0.3, and 0.05 μm alumina powders on a http://dx.doi.org/10.5599/jese.749 j. electrochem. sci. eng. 10(3) (2020) 281-291 determining contents of iron ions 284 nylon polishing cloth to achieve the hydrophobic property, and then washed for five minutes in an ultrasonic bath of nitrate, ethanol and water respectively [18]. it is worth noting that the k3fe(n)6kcl standard solution should be scanned for the electrode testing purpose by cyclic voltammetry, where the potential difference of anode and cathode peaks of 60-80 mv is the main criteria of well electrode working [19]. test solutions were deoxygenated before measurements by purging with nitrogen for 5 min. after that, a three-electrode was placed, and then the system was rested for 2 min. experimental settings included the scan rate of 0.05 v/s, and the number of cycles was 6. cyclic voltammograms were measured within the potential range of +1.0 to -1.0 v, where peak currents are recorded [20]. standard calibration curves are plotted with fe3+ or fe2+ concentrations at the abscissa, and the oxidation peak currents at the ordinate axis, respectively. results and discussion characteristic peak scanning of free iron ions and sfgc sfgc has chelated iron properties, and the presence of free iron ions will reduce stability of the composite product [21]. therefore, the content of free iron ions in the solution is an important indicator for determining the complex iron [8,9]. standard solutions of 0.01 mol/l fe3+ and fe2+ were scanned by cv and electrochemical characteristic peaks are found consistent with those reported in the literature [22,23] (figure 3a,b). however, no electrochemical response indicated by absence of characteristic peaks of free iron ions is observed in the scanning of 1 g/l sfgc solution (figure 3c). hence, it is indicated that there are no free iron ions, or the content of free iron ions in sfgc solution is lower than the detection limit. figure 3. characteristic peaks recorded by cv (0.05 v/s) at gc electrode in standard solutions of (a) fe3+ ions, (b) fe2+ ions, (c) sfgc. determination of free iron ions in sfgc with cv technique, the corresponding electrochemical characteristic peaks can be obtained by scanning free fe3+ and fe2+ ions under certain conditions. in experiments, the detection conditions should be optimized in terms of ph, potential scanning rate, as well as enrichment potential and enrichment time applied prior cv measurements. additionally, with free fe3+ and fe2+ ions added to sfgc solution in the cyclic voltammetry scanning, the signal response showed a linear relationship h. liu et al. j. electrochem. sci. eng. 10(3) (2020) 281-291 http://dx.doi.org/10.5599/jese.749 285 with the added concentration, suggesting that the determination of free iron ions would not be affected by the chelated iron. therefore, the content of free iron ions can be detected and calculated by this method. optimization of detection conditions determination of optimal ph the value of ph has important impact for the studied system. as the acidity will affect the hydrolysis and h+ can inhibit the hydrolysis of fe3+ [24], keeping the system stable for a certain period of time for easy measurement [19]. ph was adjusted to 1, 2, 3, 4, 5 and 6 respectively, for cyclic voltammetry scanning (figure 4). at ph 1­3, the peak widths and peak currents changed little, indicating that the electrical signal within this ph range is relatively stable, what is beneficial for analytical determination. since at ph 3, the electrical signal showed the strongest response and the highest sensitivity, the optimal ph value was selected to 3. figure 4. cv curves (0.05 v/s) of gc electrode in diluted fe3+ solution at ph (1-6). determination of optimal scanning rate cyclic voltammetry curves were recorded for fe3+ standard solution diluted multiple times, within the scanning potential range of +1.0 to -0.4 v, at scanning rates of 0.01 0.09 v/s (figure 5). it is seen in figure 5 that as the scanning rate increases, the peak current also increases. at the same time, the oxidation peak potential is shifted positively, but the reduction peak potential is shifted negatively. when the scanning rate is too low, the electrical signal is weak and the curve peak is not so obvious, which is not conducive to the detection of fe3+. nevertheless, when the scanning rate is too high, the electrical signal is strong, and fe3+ characteristic peak becomes deformed. therefore, the optimal scan rate is chosen to be 0.05 v/s. http://dx.doi.org/10.5599/jese.749 j. electrochem. sci. eng. 10(3) (2020) 281-291 determining contents of iron ions 286 figure 5. cv curves of gc electrode in diluted fe3+ solution (ph 3) at different scanning rates (0.010.09 v/s) determination of optimal accumulation potential and time fe3+ standard solution was taken to investigate the effect of accumulation potential applied prior cv measurements on the peak current (figure 6 a). according to the results, the peak current did not change with the positive shift of the potential when the accumulation potential was changed from 0.8 v to -0.2 v, indicating that the reduction degree of iron ions on the electrode surface is practically the same. hence, 0.4 v was selected as the optimal accumulation potential. under this potential, the effect of accumulation time (10-240 s) on the peak current was investigated (figure 6b). cvs in figure 6 b suggest that with the extension of accumulation time, more iron ions are precipitated and enrichment is more complete. at this time, fe3+ concentration around the electrode increases, and the peak current also increases. 3 min later, however, the peak current did not change much, demonstrating that adsorption reached saturation. therefore, 3 min were selected as the best accumulation time. figure 6. cv curves of gc electrode in standard fe3+ solution under (a) different accumulation potentials (0.8 to -0.2 v) and (b) different accumulation times (10 to 240 s) h. liu et al. j. electrochem. sci. eng. 10(3) (2020) 281-291 http://dx.doi.org/10.5599/jese.749 287 method validation effect of interfering ions since sfgc solution contains co32ions, na2co3 was added into 0.001 mol/l fe3+ standard solution in order to simulate sfgc solution and to study effect of these ions to determination results [25]. na2co3 concentrations of 0, 0.001, 0.01 and 0.1 mol/l were added respectively in the standard solution of fe3+ ions, and measured cvs are shown in figure 7. no change or different peak is observed in figure 7. by comparing the peak current values, rsd was found less than ±2 %, which suggests that co32ions do not interfere to the determination of iron ions. figure 7. influence of interfering co32ions on cvs of gc electrode in standard fe3+ solution for (a) three different concentrations of na2co3, (b) fe3+ standard solution with three different concentrations of na2co3 accuracy on the basis of the current experiment, fe3+ standard solutions of 80, 100 and 120 % concentrations were respectively added into 1 g/l sfgc solution. after iron content was determined, the measured value was compared with the theoretical value to calculate the recovery rate. the results showed that recovery rates were 100.2, 99.8 and 98.6 %, and rsds were 3.02, 2.06 and 2.36 %, respectively. the same method was repeated by adding fe2+ standard solutions. the results showed that the recovery rates of those three concentrations were 99.1, 99.8 and 100.6 % and rsds were 1.98, 1.56 and 2.63 %, respectively. stability and repeatability in order to investigate stability of the system and repeatability of measured data, six cyclic voltammetry curves in standard fe3+ and fe2+ solutions were repeatedly measured according to the experimental conditions determined above [26]. rsd of the peak current was calculated to be 0.9 1.6 %, and the peak shape of each curve was good. therefore, this test method has high reproducibility and high stability, and can be applied in practice. limit of detection (lod) and limit of quantitation (loq) when the concentration of fe3+ is greater than 5.0×10-3 mol/l (loq), the oxidation peak area does not change with the concentration increase, indicating that fe3+ ions are adsorbed on the electrode surface. lod was calculated to be about 1.0 x 10-3 mol/l at snr of 3 times. in the same way, lod of fe2+ was determined as 8.9×10-3 mol/l and loq was 2.5×10-2 mol/l. http://dx.doi.org/10.5599/jese.749 j. electrochem. sci. eng. 10(3) (2020) 281-291 determining contents of iron ions 288 linearity and range gradient of standard solution concentrations of fe3+ (0.005-0.01 mol/l) and fe2+ (0.025-0.05 mol/l) were added to 1 g/l sfgc solution, respectively and their cvs are scanned under the optimal experimental conditions (figures 8 and 9). linear regression equations for two standard fe3+ and fe2+ solutions could be then obtained: y = 12.85667 + 2786x, r2 = 0.9961 (fe3+) and y = 42.32+703.8x, r2 = 0.9924 (fe2+). the detection limit was low, reaching mm level. thereupon, it could be reasoned that iron in the complex would not affect the interference of free iron ions and that the content of free iron ions in any sfgc sample could be calculated according to the standard curve equation. figure 8. cvs of gc electrode in sfgc solution containing standard fe3+ solutions (0.005-0.01 mol/l) and calibration plot (line is linear fit) figure 9. cvs of gc electrode in sfgc solution containing standard fe2+ solutions (0.025-0.05 mol/l) and calibration plot (line is linear fit). determination of the total iron content in sfgc the cyclic voltammetry was employed to scan 1 g/l sfgc solution and 1 mol/l d-sodium gluconate solution. in both solutions, however, no characteristic peak of free iron ions is observed (figure 10a and b). nevertheless, the characteristic electrochemical behavior can be observed after the acid-hydrolysis of sfgc solution (figure 10c). these results showed that stability of the h. liu et al. j. electrochem. sci. eng. 10(3) (2020) 281-291 http://dx.doi.org/10.5599/jese.749 289 complexed iron is high because dsodium gluconate serves as a molecular shell to effectively adsorb the metal iron ions in the solution, forming a stable complex. therefore, the iron core content is difficult to be determined directly by the electrochemical scanning [27]. when the sample was aciddecomposed, the complexed iron is completely converted into free fe3+ ions, and the electrochemical redox peak appeared in the potential range of -0.4-1.0 v. the ratio of anode to cathode peak currents (ipc/ipa) is 1.026, what is close to the theoretical value of 1 for the reversible reaction. this means that the redox reaction of iron ions is a reversible process. figure 10. comparison of cyclic voltammograms of (a) sfgc solution before acid-hydrolysis, (b) d-sg solution, (c) sfgc solution after acid-hydrolysis 0.01 m fe3+ standard solution, 5 g/l sfgc solution and their mixture were acidolyzed at the same time. after the cyclic voltammetry scanning (figure 11), it can be seen that the characteristic peaks of fe3+ are similar in shape, but with some horizontal stretching. the results showed that the complex iron can be completely converted to fe3+ ions by acid-hydrolysis in sfgc solution. figure 11. verification of fe3+ characteristic peaks in solutions of (a) fecl3, (b) fecl3 and sfgc mixture, (c) sfgc http://dx.doi.org/10.5599/jese.749 j. electrochem. sci. eng. 10(3) (2020) 281-291 determining contents of iron ions 290 optimization of acid hydrolysis conditions the electrochemical scanning results of reduction peak currents (ipc) in sfgc solutions after acidhydrolysis at different concentrations of hydrochloric acid solution and different temperatures are compared in table 1. it is shown that iron core in the complexing iron could be completely released and converted into free fe3+ ions by acid-hydrolysis with 5 mol/ml hydrochloric acid solution for 5 min. the temperature has small effect on the acid-hydrolysis process, which can be carried out directly at room temperature. table 1. determination of acid-hydrolysis conditions hcl concentration, moll/l temperature, ℃ 1 3 5 7 rt 4 20 acid­hydrolysis 5 min ­ ipca / μa 9.85 10.79 11.04 11.12 11.04 10.99 11.08 acid­hydrolysis 10 min ­ ipca / μa 9.85 10.82 11.11 11.09 11.09 11.03 11.07 aipc : reduction peak current linearity range standard solutions of fe3+ ions after acid-hydrolysis at different concentrations were scanned under optimal conditions (figure 12a). within the range of 0.01 0.04 mol/l, the peak currents presented a good linear relationship with fe3+ concentration (y = 2.27714+836x), r2 = 0.9957 (figure 12b). therefore, the total iron content in any sfgc sample solution can be obtained using this linear fitted equation. figure 12. (a) cv curves for different concentrations of standard fe3+ solution after acid-hydrolysis of sfgc, (b) reduction peak current vs. concentration of fe3+ application three sample solutions with concentration of 0.1 g/l were prepared by sfgc containing 10 mg iron, and the content of free iron in the samples was determined by cyclic voltammetry under the optimal conditions. further, acid-hydrolysis was carried out before the sample was measured. the total iron content in sfgc was calculated by the linear regression equation, and rsd value was then acquired (table 2). table 2. determination of free iron ions and total iron content in sfgc. sample number free iron total amount of iron founda, mg relative to labeled, % rsd, % 1 --b 9.89 98.9 0.75 2 -10.16 101.6 1.65 3 -10.08 100.8 1.07 amean value of 6 parallel measurements; bno free iron ions were detected h. liu et al. j. electrochem. sci. eng. 10(3) (2020) 281-291 http://dx.doi.org/10.5599/jese.749 291 conclusions in this paper, the contents of free iron ions and total iron in the system were detected by the cyclic voltammetry scanning with glassy carbon electrode. the optimized experimental conditions were defined as solution ph 3, potential scanning rate 0.05 v/s. before cv measurements, enrichment potential 0.4 v and enrichment time 3 min were found optimal. free fe3+ ions were measured in the range of 0.005-0.01 mol/l, while free fe2+ ions were measured in the range of 0.025-0.05 mol/l. the peak current was linearly correlated with the concentration of iron ions, and the results showed that the method is stable and reproducible. the chelated iron can be completely converted to fe3+ ions by acid-hydrolysis under 5 mol/l hydrochloric acid solution for 5 min, and then calculated according to the linear relationship. compared with the previous methods for determination of iron content, here proposed electrochemical detection method can determine the contents of free iron ions and total iron in the system. also, the proposed method has the advantages of simple operation, short response time, good sensitivity, and accurate stable results. this method can also be used for determination of other complexes of iron. acknowledgements: the investigations were carried out with the equipment of administrative office of laboratory and equipment, southwest university. references [1] x. l. zhan, f. s. wang, j. r. wang, j. wang, medical journal of qilu 5 (2008) 467-468. 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[27] d. merli, a. profumo, c. dossi, journal of pharmaceutical analysis 2 (2012) 450-453. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {modeling the performance of direct carbon solid oxide fuel cell -anode supported configuration} http://dx.doi.org/10.5599/jese.933 115 j. electrochem. sci. eng. 11(2) (2021) 115-128; http://dx.doi.org/10.5599/jese.933 open access: issn 1847-9286 www.jese-online.org original scientific paper modeling the performance of direct carbon solid oxide fuel cell anode supported configuration sanjeev raj, sakthi gnanasundaram and balaji krishnamurthy department of chemical engineering, bits pilani, hyderabad 500078, india corresponding author: balaji.krishb1@gmail.com received: november 17, 2020; revised: january 26, 2021; accepted: january 26, 2021 abstract a mathematical model is developed to study the performance of a direct carbon solid oxide fuel cell system (dc-sofc). simulation results indicate that in the anode supported configuration, anode design parameters (porosity, tortuosity and anode thickness) play very important role in the performance of dc-sofc, presented as the polarization curve. the effect of ag content in anode electrode is found to play a significant role in the performance of the dc-sofc. the effect of operating parameters, namely pressure and temperature, on the overpotentials (concentration, activation and ohmic) are studied. the concentration profiles of gases (co2 and co) as a function of operating current density across the anode electrode is studied. model results are compared with experimental data and found to compare well. keywords overpotential, activation, direct carbon, anode design parameters, current, potential introduction direct carbon solid oxide fuel cells (dc-sofcs) have received a lot of attention in the recent years. in dc-sofc, solid carbon is placed in the anode chamber and chemically oxidized by co2 gas to form co. the carbon monoxide then moves to the anode triple phase boundary (tpb) where it reacts to produce co2 [1]. in the case of anode supported configuration, the thickness of the anode dominates, while in the electrolyte supported configuration, the thickness of the electrolyte dominates. xu et al. [1] have shown that the anode supported dc-sofc offers a much better performance than electrolyte supported dc-sofc, owing to the reduced ohmic loss and only slightly increased concentration loss. very little modeling work exists in the sphere of dc-sofc. xu et al. [1] have published a paper studying the effect of the distance between carbon and anode on the performance of dc-sofc. the authors also compare the performance of anode supported dc-sofc and electrolyte supported dchttp://dx.doi.org/10.5599/jese.933 http://dx.doi.org/10.5599/jese.933 http://www.jese-online.org/ mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 116 sofc. in another paper [2], these authors have also studied the effect of applied voltage and operating temperature on the performance of dc-sofc. yang et al. [3] have developed a mathematical model to study the performance of a direct carbon solid oxide fuel cell. the authors postulated that a higher operating temperature of the fuel cell and a smaller distance between the anode and carbon enhance the performance of dc-sofc. these researchers [4] have also developed a mathematical model to study the performance of dc-sofc by adding water by carbon gasification in dc-sofc. zhang et al. [5] have evaluated the effect of operating parameters on dc-sofc performance. bai et al. [6] have assembled and tested a direct carbon solid oxide fuel cell stack and achieved significant peak power density. gür et al. [7] have experimentally verified the reaction mechanism of dc-sofc by comparing its performance with sofc running on pure co. alexander et al. [8] have proposed a steam carbon air fuel cell and was able to achieve efficiency of 78 % for hydrogen and electricity co-generation. sandip das [9] has used the dusty gas model to develop the analytical expression for the concentration overpotential of dc-sofc. in our previous work [10], the variation of concentration profiles of co and co2 across the anode electrode was studied as a function of design parameters of the cathode. in order to contribute to the already existing work in understanding the performance of dc-sofc, the performance and operating overpotentials of dc-sofc as a function of anode design parameters have thoroughly been studied in the present paper. modeling the schematic of the anode supported dc-sofc is shown in figure 1. figure 1. schematic of anode supported dc-sofc the activated carbon in the anode chamber reacts with co2 to form co by the reverse boudouard reaction: c + co2 → 2co (1) co diffuses to the anode electrode (triple phase boundary) where it reacts with the oxygen anions from the cathode: 2co + 2o2-→ 2co2 + 4e(2) at the cathode electrode, the oxygen anions are generated: o2 + 4e-→2o2(3) s. raj et al. j. electrochem. sci. eng. 11(2) (2021) 115-128 http://dx.doi.org/10.5599/jese.933 117 the thickness of the anode, electrolyte and cathode are 201, 20 and 20 µm (under anode supported configuration) [1]. the tubular dc-sofc has silver-gadolinium doped ceria composite (ag-gdc) as the anode and cathode electrodes, and ytria-stabilized zirconia (ysz) as the electrolyte. the main focus of our work is to study the effect of anode design parameters, namely anode porosity, anode tortuosity, anode electrode thickness, and ag content of the anode electrode on the performance of dc-sofc. none of the previous models have done this. model assumptions a) all the gases involved in the system are assumed to be incompressible and ideal. b) the system is assumed to be at steady state. c) the system is assumed to operate under isothermal conditions. d) the electrodes are assumed to be porous and the reactions are assumed to happen on the surface and across the porous anode. e) the model assumes that reaction (2) primarily drives the electrochemical reaction and is the current generation reaction. model equations 2d numerical model incorporates the chemical reaction model, the electrochemical reaction model and mass transfer model. reaction (1) is the chemical reaction which occurs in the anode chamber, and reaction (2) is the electrochemical reaction which happens at tpb of the anode electrode. from the schematic drawn in figure 1, it can be discerned that co/co2 flows in the anode, while air is supplied to the cathode. oxygen molecules diffuse to the cathode where they are reduced to oxygen anions according to the reaction (3). the oxygen ions travel through the electrolyte to tpb where they react with co to form co2 according to the reaction (2). thus, the overall reaction can be defined as: 2co + o2→ 2co2 (4) actual voltage the difference between the thermodynamic potentials of the electrode reactions determines the reversible cell voltage, which is also known as the electromotive force (emf). this depends on gas composition and temperature at the electrodes. the sofc cell potential (e) can be calculated using the nernst equation [1,2]: 2 2 0.5 co o 0 co2 p prt e e ln f p = + (5) where pi is the partial pressure of component i and e0 is the open circuit voltage. for a cell, the actual voltage is always lower than the open-circuit voltage because of some internal resistances and overpotential losses. output voltage of sofc can be calculated using: conc,a conc,c act,a act,c ohm – – – – –v e     = (6) where ηconc,a, ηconc,c, ηact,a, ηact,c, ηohm are concentration overpotential of anode, concentration overpotential of cathode, activation overpotential of anode, activation overpotential of cathode and ohmic overpotential, respectively. http://dx.doi.org/10.5599/jese.933 j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 118 activation overpotential activation overpotential, also known as surface overpotential, is the potential difference above equilibrium potential required to overcome the activation energy of any cell reaction to generate a desired amount of current. activation overpotential can be calculated using [5]: 2 1 act,i a,i a,i a,i sinh ln 1 2 2 2 rt j rt j j f j f j j  −        = = + +              (7) where ja,i is electrode exchange current density of component i. current density at anode and cathode can be calculated using [5]: ( ) ( ) 2 g p p s co co act,a a,a a 2 2 2 ref ref s p g 72 exp 1 1 l d d d p p e j p p rtd d l     − + −   =    − − (8) ( ) ( ) 2 0.25 g p p s o act,c a,c c 2 2 2 ref s p g 72 exp 1 1 l d d d p e j p rtd d l     − + −     =       − − (9) where pref represents reference pressure and are current densities at anode and cathode respectively, lg is the ratio of the grain contact neck to the grain size which is suffered by the limit of geometry and transport characteristics, eact is the activation energy level at the anode or cathode, pref is the reference pressure,  is porosity of electrodes, while dp and ds are the pore size (diameter of a pore) and grain size (diameter of a grain), respectively. concentration overpotential concentration overpotential can be calculated using [1,2]: 2 2 l co co conc,a l co co ln 2 p prt f p p    =       (10) 2 2 o conc,c l o ln 4 prt f p    =       (11) where pi represents partial pressure of component i at anode and pli represents partial pressure of component i at tpb. partial pressure can be expressed in terms of mole fractions, resulting in: ( ) ( ) ( ) ( ) a b conc,a a b 0 ln 2 0 x x lrt f x l x    =      (12) ( ) ( ) e e 12 1m a e e e e 1m 2m 12 1m a ee e 1 2m0 1m 2m 1 1 0 1 1 1 1 1 11 1 1 2 e m d d x d d d d x z jrt z d dfp d d + − + + = +   +− +    (13) x0(z) = 1 xa(z) (14) s. raj et al. j. electrochem. sci. eng. 11(2) (2021) 115-128 http://dx.doi.org/10.5599/jese.933 119 where z represents anode thickness, xa represents mole fraction of co, xb represents mole fraction of co2. xi (0) and xi (z) represent mole fraction of component i at anode and tpb respectively. here, deim is knudsen coefficient and p0 is the total pressure at anode. knudsen diffusion coefficient can be calculated using [9]: e 0 im ij i 8 3 m d rt d d      =     (15) where d0 represents diameter of pore dij and mi represent effective binary diffusion coefficient and molecular weight of species i. dij can be calculated using [9]: ( ) ( ) 1/2 7 1.75 i j ij 2 1 1 3 3 i j 1 1 10 t m m d p v v −   +     =   +      (16) where  and  corresponds to porosity and tortuosity of the medium, p is represented in atm, t in k, mi and mj are the molecular weights of species i and j respectively, vi is the sum of the diffusion volume of component i and dij is represented in m2/s. the effective binary diffusion coefficient is related to the binary diffusion coefficient by the following equation [9]: e ij ij d d     =     (17) where 𝜀 and 𝜏 correspond to porosity and tortuosity of the medium. ohmic overpotential the ohmic overpotential is given by: ohm = jrohm (18) now, rohm can be represented as: anode cathode elec ohm anode cathode elec r       = + + (19) where  represents thickness and σ represents conductivity. results and discussion figure 2 shows the performance of dc-sofc for three different values of anode tortuosity (1,3 and 5). tortuosity is an intrinsic property of a porous material, usually defined as the ratio of the actual flow path length to the straight distance between the ends of the flow path. inherently, it indicates the mass transfer limitation in a given reaction (the ability of the reactant to get to the reactant surface). figure 2 shows that dc-sofc shows the best performance at tortuosity value of 1. the analysis of the polarization curve in figure 2 indicates that the difference in performance among three different curves is the highest at higher current densities, indicating that mass transport limitations play a major role in the performance of dc-sofc. figure 3 shows the variation of the performance of dc-sofc with different values of anode porosity. the figure shows that the best performance is seen with anode porosity of 0.7. again, the effect of anode porosity is seen to be the highest at high current density values, showing the effect http://dx.doi.org/10.5599/jese.933 j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 120 of mass transport limitations on the reaction (2). the list of parameters used in these simulations is shown in table 1. figure 2. performance of dc-sofc under different anode tortuosity values figure 3. performance of dc-sofc under different anode porosity values figure 4 shows the effect of anode electrode thickness on the performance of dc-sofc. the figure shows that with increasing anode electrode thickness, the performance of the dc-sofc decreases showing the effect of ohmic resistance on the performance of dc-sofc. since the reactions are supposed to take place on the surface of tpb and in the interior of the anode electrode, increasing the anode electrode thickness should increase the performance of dc-sofc owing to the increased surface area for the reaction (2). at the same time, however, the ohmic resistance generated by the thickness of the anode electrode reduces the performance of dc-sofc, and the overall performance is seen to be reduced. s. raj et al. j. electrochem. sci. eng. 11(2) (2021) 115-128 http://dx.doi.org/10.5599/jese.933 121 figure 4. performance of dc-sofc under different anode thicknesses figure 5 shows the effect of the composition of the anode electrode on the performance of dcsofc. it is seen that when the volume fraction of ag in ag-gdc composite electrode increases, the performance of dc-sofc also increases, showing the beneficial effect of electronic conductivity of ag on the performance of dc-sofc. the electronic conductivity of ag and ionic conductivity of gdc are key factors in determining the transport of electrons and ions through the anode electrode. a higher content of ag in the anode electrode makes transport of electrons through the anode electrode easier, which leads to better fuel cell performance. figure 6 shows the effect of cell current density on the concentration profiles of co and co2 across the anode. the cell current in this case is principally determined by the reaction (2). thus, a higher current density will indicate more reaction of co and more production of carbon dioxide. figure 5. performance of dc-sofc under varying volume fractions of ag in ag-gdc anode figure 6 shows that a higher concentration of co2 and lower concentration of co is seen at tpb with increasing current density (concentration profiles are shown across the anode electrode). thus the concentration of co is seen the highest at the edge of the anode electrode and the concentration of co2 is seen to be the highest at tpb (anode electrolyte interphase). http://dx.doi.org/10.5599/jese.933 j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 122 figure 6. concentrations profiles of gases (co and co2) across the anode at varying current densities figures 7 and 8 show the effects of pressure and temperature on the performance of dc-sofc. figure 7 shows that the performance of dc-sofc with operating pressure (1, 2 and 3 atm). the figure shows that the performance of dc-sofc increases with operating pressure, particularly at high current density. this indicates that a higher operating pressure facilitates the reversed boudouard reaction kinetics producing more co in the process. it also indicates that higher operating pressure facilitates mass transfer in the anode electrode. this leads to higher current density in the mass transport limited reaction. figure 8 shows the effect of temperature on the performance of dc-sofc. the performance is seen to increase with increasing temperature. this again is due to the faster boudouard reaction kinetics (reverse reaction) at a higher temperature in addition to the increase in the electrochemical kinetics and the ionic conductivity of the oxygen ions through the anode electrode. an analysis of the effect of pressure and temperature on the overpotentials is given in the next section. figure 7. performance of dc-sofc as a function of operating pressure s. raj et al. j. electrochem. sci. eng. 11(2) (2021) 115-128 http://dx.doi.org/10.5599/jese.933 123 figure 8. performance of dc-sofc as a function of operating temperature effect of operating parameters on overpotentials activation overpotential inherently the activation overpotential indicates the resistance to the kinetics of the reaction taking place on the electrodes. the activation overpotential is derived from the butler-volmer equation. the values of the activation overpotential depends on several factors like operating pressure, temperature, gas composition, electrode porosity, etc. figures 9a and 9b indicate the effect of temperature and operating pressure on the activation overpotential for the reaction (2) occurring within the anode electrode. figure 9a shows that temperature has a significant effect on the activation overpotential. the activation overpotential (resistance to the kinetics of the reaction) is the lowest at the temperature of 850 c, showing the effect of temperature on the reversed boudouard reaction (1) (chemical reaction) and reaction (2) (electrochemical reaction). figure 9b shows the effect of pressure on the activation overpotential. the figure shows that an increasing pressure supports an increasing reaction rate both for the reversed boudouard reaction (1) and reaction (2). a b figure 9. activation overpotential vs. current density for a various temperatures; b various pressures http://dx.doi.org/10.5599/jese.933 j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 124 concentration overpotential the concentration overpotential is caused by the resistance to the transport of reactant species approaching tpb and the product species leaving tpb. figures 10a and 10b show the effect of temperature and pressure on the concentration overpotential. figure 10a shows that with increasing temperature, the resistance to transport of species to and away from tpb decreases, showing the effect of diffusion (diffusion increases with increasing temperature). thus, we see that the concentration overpotential is the lowest at 850 c. figure 10b shows that increasing pressure conditions reduces the concentration overpotential required for the reactions. it shows that at 3 atm, the resistance to the mass transport of the reactant and product species is the lowest, showing the effect of pressure on the diffusion, i.e., mass transport of the species. however, the effect of temperature and pressure on the concentration overpotential seems to be less significant than for the activation overpotential. a b figure 10. concentration overpotential vs. current density for a various temperatures; b various operating pressures s. raj et al. j. electrochem. sci. eng. 11(2) (2021) 115-128 http://dx.doi.org/10.5599/jese.933 125 ohmic overpotential the ohmic overpotential is mainly caused by the resistance to conduction of ions through the electrolyte and electrons through the electrodes. the conductivity of the electrolyte is usually a function of the operating temperature. figure 11 shows that the operating temperature plays a major role in reducing the ohmic overpotential in the system. the least resistance to the transport of ions through the anode electrode is seen at a temperature of 850 c. the temperature is seen to play a significant role in reducing the resistance to the conduction of ions through the electrolyte and electrodes. figure 12 shows the comparison of simulation results with experimental data [5]. the parameters used for simulations of curves in figure 12 are shown in table 2. simulation results presented in figure 12 are found to compare very well with experimental data. figure 11. ohmic overpotential vs. current density for various temperatures figure 12. comparison of modeling predictions with experimental data [5] http://dx.doi.org/10.5599/jese.933 j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 126 table 1. model parameters used for calculations of graphs presented in figures 2-10 [1]. parameter value cathode tortuosity 3 anode tortuosity 3 o2 exchange current density (io2), a m -2 2000 co exchange current density (ico), a m -2 2250 co charge transfer coefficient (αco) 0.5 o2 charge transfer coefficient (αo2) 0.5 pressure in anode chamber, atm 1 distance between anode chamber and electrode, µm 59 pressure in cathode chamber, atm 1 cathode gas composition air temperature, k 1123 diameter of pore, µm 1 ionic conductivity of gdc, s m-1 5322.92 6.66071100 10 t t − ionic conductivity of ysz, s m-1 10300 4 3.34 10 te − electronic conductivity of ag, s m-1 8 1.59 0.0038 0.1134 e t − porosity of cathode 0.46 porosity of anode 0.46 electrode volume fraction of gdc 0.21 electrode volume fraction of ag 0.79 table 2. model parameters for model and experimental [5] fits shown in figure 12. parameter value cathode tortuosity 5.4 anode tortuosity 5.4 o2 exchange current density (io2), a m -2 3640 co exchange current density (ico), a m -2 3492 co charge transfer coefficient (αco) 0.5 o2 charge transfer coefficient (αo2) 0.5 pressure in anode chamber, atm 1 pressure in cathode chamber, atm 1 cathode gas composition air temperature, k 1123 diameter of pore, µm 3 ionic conductivity of ysz, s m-1 10300 4 3.34 10 te − electronic conductivity of cathode, s m-1 8.4 103 electronic conductivity of anode, s m-1 8.0 103 porosity of cathode 0.48 porosity of anode 0.48 electrode volume fraction of gdc 0.21 electrode volume fraction of ni-ysz 0.5 electrode volume fraction of ysz-lsm 0.5 s. raj et al. j. electrochem. sci. eng. 11(2) (2021) 115-128 http://dx.doi.org/10.5599/jese.933 127 conclusions a mathematical model is developed to study the performance (displayed as cell polarization curve) of a carbon assisted solid oxide fuel cell (dc-sofc). the model postulates that different anode design parameters, i.e. anode porosity, anode tortuosity, anode electrode thickness and ag concentration in the anode electrode play a key role in the performance of dc-sofc. the effect of operational pressure and temperature on the activation, concentration and ohmic overpotentials is studied. the temperature and pressure are found to have a significant impact on the activation and concentration overpotentials. ohmic overpotential is found to be affected significantly by temperature. model results are compared with experimental data taken from the literature and found to compare well. nomenclature v / v operating potential e / v equilibrium nernst potential e0 / v standard potential r = 8.314 j mol-1 k-1 universal gas constant t / k temperature f = 96,485 c mol-1 faraday constant pi / atm bulk partial pressure of species pil / atm l ocal partial pressure of species i at tpb tpb triple phase boundary xi mole fraction of species i deij / m2 s-1 effective binary diffusion coefficient deim / m2 s-1 knudsen coefficient p0 or p / atm total pressure at anode pref reference pressure mi / kg kmol -1 molecular weight of species i do / m diameter of pore pc / atm operating pressure at cathode vi diffusion volume of component i j / a m -2 current density jo, a/c / a m-2 exchange current density at i (anode/cathode) rohm /  internal resistance of the cell l ratio of the length of the grain contact neck to the grain size eact,a / j mol -1 activation energy of anode eact,c / j mol -1 activation energy of cathode ds / m grain size greek letters ε porosity of medium  tortuosity of medium ηconc,a / v anode concentration potential ηconc,c / v cathode concentration potential ηact,a / v anode activation potential ηact,c / v cathode activation potential ηohm / v ohmic overpotential anode / m anode thickness cathiode / m cathode thickness elec / m electrolyte thickness σi / s m-1 conductivity at i (anode/cathode/electrolyte) a / a m coefficient for exchange current density of anode c / a m coefficient for exchange current density of cathode anode / m anode thickness cathode / m cathode thickness http://dx.doi.org/10.5599/jese.933 j. electrochem. sci. eng. 11(2) (2021) 115-128 performance of direct carbon solid oxide fuel cell 128 acknowledgement: the authors would like to thank bits pilani, hyderabad for support in writing this article. references [1] h. xu, b. chen, j. liu, n. meng, applied energy 178 (2016) 353-362 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[8] b. r. alexander, r. e. mitchell, t. m. gür, journal of the electrochemical society 159 (2012) f810f814 https://doi.org/10.1149/1.3560475. [9] s. k. das, journal of electrochemical energy conversion and storage 17 (2020) 031017 https://doi.org/10.1149/2.032212jes. [10] r. sanjeev, g. sakthi, b. krishnamurthy, ionics 27 (2020) 729-741 https://doi.org/10.1007/s11581020-03834-9. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.apenergy.2016.06.064 https://doi.org/10.1016/j.apenergy.2016.06.064 https://doi.org/10.1016/j.jpowsour.2018.02.033 https://doi.org/10.1016/j.enconman.2018.05.100 https://doi.org/10.1016/j.ijhydene.2017.05.075 https://doi.org/10.1016/j.ijhydene.2013.04.107 https://doi.org/10.1016/j.ijhydene.2011.04.171 https://doi.org/10.1149/1.3560475 https://doi.org/10.1149/1.3560475 https://doi.org/10.1149/2.032212jes https://doi.org/10.1007/s11581-020-03834-9 https://doi.org/10.1007/s11581-020-03834-9 https://creativecommons.org/licenses/by/4.0/) perspective on the mechanism of mass transport-induced (tip-growing) li dendrite formation by comparing conventional liquid organic solvent with solid polymer-based electrolytes http://dx.doi.org/10.5599/jese.1724 715 j. electrochem. sci. eng. 13(5) (2023) 715-724; http://dx.doi.org/10.5599/jese.1724 open access : : issn 1847-9286 www.jese-online.org original scientific paper perspective on the mechanism of mass transport-induced (tip-growing) li dendrite formation by comparing conventional liquid organic solvent with solid polymer-based electrolytes lukas stolz1, martin winter1,2 and johannes kasnatscheew1, 1 meet battery research center, institute of physical chemistry, university of münster, corrensstraße 46, 48149 münster, germany 2 helmholtz-institute münster, iek-12, forschungszentrum jülich gmbh, corrensstraße 46, 48149 münster, germany corresponding author: j.kasnatscheew@uni-muenster.de received: february 27, 2023; accepted: july 31, 2023; published: august 9, 2023 abstract a major challenge of li metal electrodes is the growth of high surface area lithium during li deposition with a variety of possible shapes and growing mechanisms. they are reactive and lead to active lithium losses, electrolyte depletion and safety concerns due to a potential risk of short-circuits and thermal runaway. this work focuses on the mechanism of tipgrowing li dendrite as a particular high surface area lithium morphology. its formation mechanism is well-known and is triggered during concentration polarization, i.e. during mass (li+) transport limitations, which has been thoroughly investigated in literature with liquid electrolytes. this work aims to give a stimulating perspective on this formation mechanism by considering solid polymer electrolytes. the in-here shown absence of the characteristic “voltage noise” immediately after complete concentration polarization, being an indicator for tip-growing dendritic growth, rules out the occurrence of the particular tipgrowing morphology for solid polymer electrolytes under the specific electrochemical conditions. the generally poorer kinetics of solid polymer electrolytes compared to liquid electrolytes imply lower limiting currents, i.e. lower currents to realize complete concentration polarization. hence, this longer-lasting li-deposition times in solid polymer electrolytes are assumed to prevent tip-growing mechanism via timely enabling solid electrolyte interphase formation on fresh li deposits, while, as stated in previous literature, in liquid electrolytes, li dendrite tip-growth process is faster than solid electrolyte interphase formation kinetics. it can be reasonably concluded that tip-growing li dendrites are in general practically unlikely for both, (i) the lower conducting electrolytes like solid polymer electrolytes due to enabling solid electrolyte interphase formation and (ii) good-conducting electrolytes like liquids due to an impractically high current required for concentration polarization. keywords mass transport limitation; li metal battery; concentration polarization; liquid electrolyte http://dx.doi.org/10.5599/jese.1724 http://dx.doi.org/10.5599/jese.1724 http://www.jese-online.org/ mailto:j.kasnatscheew@uni-muenster.de j. electrochem. sci. eng. 13(5) (2023) 715-724 mass transport-induced li dendrite formation 716 introduction li metal-based rechargeable batteries (lmbs) gain enhanced attention in battery r&d owing to higher theoretical specific and volumetric energies compared to conventional li ion batteries (libs) [1-6]. however, the hazardous formation of high surface area lithium (hsal) morphologies, e.g. mossy and dendritic li, limits their application [2,4,7-9]. hsal is reactive and leads to capacity losses, electrolyte depletion [2,9,10], as well as a potential safety risks via, e.g., short circuits, which can be typically detected via an electrochemical voltage noise response,[11-14] also in li ion batteries [8,15-18]. many research efforts have been conducted towards mechanistic understanding of hsal formation and growth, in particular in regard to the morphology of deposited li [7,8,19-23]. a well-known hsal-type is the tip-growing li dendrite which grows immediate after complete concentration polarization, i.e. after complete limitation of (li+) mass transport towards li metal electrode and can be electrochemically indicated via a voltage noise behavior [24-26]. in that scenario, dendrite growth relates with complete li+ depletion at the li|electrolyte interface resulting in tip-growing li dendrite propagation. it can be described as a process counteracting complete concentration polarization, leading to an over-limiting current [24-27]. this mechanism has been derived and studied for liquid organic solvent-based electrolytes, where, due to the good kinetic aspects, harsh electrochemical conditions are required, e.g. elevated currents and/or long inter-electrode distances to realize complete concentration polarization [26]. in this work, the aim is to give a perspective on mechanistical validation and understanding of the li-growth mechanism with the generally poorer ion-conducting solid polymer-based electrolytes [2,28-31] under complete concentration polarization conditions, first introduced by brissot et al. [32]. discussion complete concentration polarization: mass transport limitation in battery electrolytes when applying a moderate current in li metal batteries, extra li+ is provided and consumed at the anode|electrolyte and cathode|electrolyte interface, respectively [26,33,34]. as a result, a li salt concentration gradient emerges throughout the electrolyte and reaches a steady-state after a certain time while evoking li+ transport via diffusion (fick’s laws). at kinetically harsher conditions, e.g. higher currents, low temperature and/or large electrolyte thickness, the applied current can force continuous growth in concentration gradient up to the maximum, i.e. complete li+ depletion at the cathode|electrolyte interface [33-36]. this ‘complete concentration polarization’ obeys sand`s equation and its vertical-type polarization (overvoltage) starts at a defined sand time [26,34]. the limiting current, defining the threshold for complete concentration polarization, is a characteristic value of dic electrolytes and depends on li+ diffusion coefficient, li+ salt concentration and electrolyte thickness [33-35]. these relations can be simply validated in symmetric li||li cells as depicted in figure 1. a strategy to counteract this issue relies on the use of electrolytes with high li+ transference numbers (immobile anions), e.g. single-ion conducting (sic) electrolytes [1,37-41], that counteract the concentration polarization by enhanced migration for reasons of charge neutrality. the impact of sics on li deposition is discussed later. l. stolz et al. j. electrochem. sci. eng. 13(5) (2023) 715-724 http://dx.doi.org/10.5599/jese.1724 717 figure 1. schematic illustration of the complete concentration polarization seen at a characteristic sand time, which proceeds when applied current exceeds the limiting current (redrawn from [37]) complete concentration polarization: the onset of dendritic li deposition in liquid electrolyte-based batteries in the case of good ion conducting liquid electrolytes, e.g. 1 m lipf6 in a mixture of ethylene carbonate (ec) and dimethyl carbonate (dmc) [2,42-44], concentration polarization can only be observed beyond practical operating conditions, e.g. under impractically high currents and/or large electrolyte thickness. bai et al. [26] could visually observe complete concentration polarization via the onset of dendritic li deposition in symmetric li||li cells inside glass capillaries with an effective electrolyte thickness of about 7 mm in this model cell compared to a practical separator thickness of 0.02 mm in real cells as depicted in figure 2 [26]. it is demonstrated, that the onset of complete concentration polarization marks the transition between mossy and dendritic li deposition [26]. the latter is regarded as deposition morphology occurring during over-limiting current phenomena, which circumvents li+ depletion and is observable by a noisy voltage response immediately after the sand time (figures 2a and 2 b) [26]. it has been reported, that mossy li deposition is a root-growing phenomenon, where li plating occurs preferably below the protective solid electrolyte interphase (sei), while dendritic li deposition proceeds tip-growing preferably above the sei (figure 2c) due to higher growing rate compared sei formation rate, as stated in literature [24,26]. time, s figure 2. (a) voltage as a function of time for an over-limiting current in glass capillary-based li||li cell with liquid electrolyte (1m lipf6 in ec/dmc), derived from bai et al.[26] (b) visual observation of the onset of complete concentration polarization (ii) and immediate transition from mossy (root-growing) to dendritic (tip-growing) li deposition (iii), derived from bai et al.[26] (c) schematic illustration of tip-growing (dendritic) and root-growing (mossy) li deposition, which among others differs in the presence of sei on the high surface are lithium (hsal), depending on hsal growth kinetics relative to sei formation kinetics.[24,26] (redrawn from [26]) v o lt a g e , v http://dx.doi.org/10.5599/jese.1724 j. electrochem. sci. eng. 13(5) (2023) 715-724 mass transport-induced li dendrite formation 718 no impact of complete concentration polarization on dendritic li deposition in solid polymer electrolytes (spes) because of considerably poorer kinetic transport properties (e.g. lower diffusion coefficients), thus lower limiting currents, observing the concentration polarization in spes is practically easier than in liquid electrolytes and can be investigated in conventional cell configuration with conventional inter-electrode distances, e.g. coin cells [34,45]. interestingly, contrary to the observations derived by bai et al. for liquid organic electrolytes, the spe, i.e. solid poly(ethylene) oxide (peo)/lithium bis(trifluoromethanesulfonyl)imide (litfsi), within li||li cells proceeds without the characteristic voltage noise immediately after concentration polarization, as shown figure 3a [32-37]. it can be concluded that a direct transition from mossy li towards tip-growing dendritic li is absent with this type of electrolyte. a reason might be an interplay/competition between the rate of li deposition and rate of sei growth, which has been suggested by bai et al. [26] for liquid electrolytes (figure 3b) [24,26]. a requirement for dendritic (tip-growing) li deposition is a higher rate of deposition (according to the applied current rate) compared to the rate of sei growth, as otherwise the sei would passivate the metallic li deposits and lead to probably mossy (root-growing) li morphologies, as stated in literature [24]. hence, it is speculated that the tip-growing dendritic li deposition might be less favored with spes as a consequence of rather lower limiting current, i.e. longer lasting deposition times. time, h figure 3. (a) voltage as a function of time for over-limiting currents in liquid electrolyte (bai et al. [26]) vs. spe-based li||li cells. complete concentration polarization initiates an onset of dendritic (tip-growing) li deposition in liquid electrolytes, seen as voltage noise, while such voltage response is missing with spe. missing voltage noise points to absence of dendritic li in spe [26,34], (b) which is assumed to relate with lower rates (lower limiting currents), which realize sufficient time for sei growth on the li deposits, consequently leading to different hsal shapes (redrawn from [26]) interestingly, brissot et al. [32] detect a change in li plating morphology at high currents, suggesting dendritic li deposition, though, it is not observed in the first mass transport limiting polarization, i.e. first cycle (as seen in liquids), but only after subsequent cycling. this is probably caused by local defects in the sei. a theoretical possible difference might be also related with electroconvection [24,27,46]. where overpotentials required for tip-based dendritic growth might be significantly higher compared to liquid-based systems due to structural rigidity of spes. however, practically they are not observable in voltage ranges up to 10 v, which is significantly higher than voltage ranges in conventional batteries (<5 v); thus relativizing its practical relevance. v o lt a g e , v l. stolz et al. j. electrochem. sci. eng. 13(5) (2023) 715-724 http://dx.doi.org/10.5599/jese.1724 719 time, h figure 4. (a) voltage as a function of time at moderate areal current (200 µa g-1) of dicand sic-spe-based symmetric li||li cells, both leading to short-circuits.[37] (b) as schematically illustrated, short circuits are likely caused by inhomogeneous mossy li depositions and growth throughout the spe after several hours still, short-circuits based on inhomogeneous li deposition are still relevant in spes and happen also below the limiting currents, i.e. below onset of complete mass transport limitations/concentration polarizations [11-14]. though, tip-growing dendritic li is practically unlikely, other hsal morphologies like mossy li deposition still play a major role and also can lead to short circuits via penetration as they obviously cannot be mechanically suppressed by the flexible spe membranes as depicted in figure 4 for sic and dic electrolytes in symmetric li||li cells. the insufficient suppression in hsal penetration/short-circuits of specompared to liquid electrolyte-based battery cells is likely due to rather poor mechanical stability [12,14] combined with lack in homogeneity (e.g. salt distribution), which consequently enhance the risk of inhomogeneous plating [11], thus the growth throughout the spe-based membrane [13]. oxidative spe decomposition is unlikely, as the li+ depletion occurs at the negative electrode; also, no signs of decomposition (e.g. potential plateau) are observable, as would be the case during electrochemical decomposition [14,47,48]. figure 5. schematic comparison of li interface in contact with liquidand solid electrolyte at mass-transport limiting (i.e., li+ depleted) conditions. the tip-growing li dendrite in liquid electroyltes, seen by characteristic voltage noise, is absent in solid electrolytes v o lt a g e , v http://dx.doi.org/10.5599/jese.1724 j. electrochem. sci. eng. 13(5) (2023) 715-724 mass transport-induced li dendrite formation 720 conclusion and future perspective lithium metal electrodes can potentially raise the energy density and specific energy compared to li ion batteries, but suffer from growth of reactive hsal during charge/discharge cycling. drawbacks like electrolyte depletion, capacity losses, safety issues and short-circuits require intense r&d efforts. in literature, a complete li+ (mass) transport limitation, that is, complete concentration polarization is linked to the onset of a particular hsal shape, i.e. dendritic (tip-growing) li deposition and has been thoroughly investigated in li||li cells with liquid organic solvent-based electrolytes. this hsal shape is indicated by the characteristic over-limiting current and seen via voltage noise [24,26]. interestingly, with spes this particular voltage noise is absent during the complete concentration polarization, pointing to absence of dendritic tip-growing li growth under the applied specific electrochemical conditions [33-35,37], as summarized in figure 5. given the poorer ion transport properties of spes compared to liquid electrolytes [28,31,49], the considerably lower limiting current/rate in spes [33,34] is suggested to provide sufficient time for formation of sei on freshly deposited li, thus being a crucial difference to the literature known situation in liquid electrolytes, where the rate of li tip-growing is faster than the rate of sei formation [24,26]. still, dendritic li deposition has been observed in spe-based systems as discussed by brissot et al. [32], but only after subsequent cycling, probably due to defects in the sei [32,50], in other words, not observed in the initial charge as is seen for liquid-based electrolytes. it can be concluded, that tip-growing dendritic morphology is generally unlikely in li metal batteries with both, (i) good-conducting electrolytes (e.g. liquid) as impractically high currents and/or inter-electrode distances are necessary to achieve complete concentration polarization [26], or (ii) poor-conducting electrolytes (e.g. spes) as the rate of tip-growing mechanism is slower than the sei-formation rate, as also stated in literature [24]. still, short-circuits, most likely due to mossy li penetration, are still an issue and observed for dual ion conducting (dic)and single ion conducting (sic)-spe-based li battery cells [37]. these insights suggest that developing electrolytes with higher cation transference number is not expedient for suppression of the particular tip-growing li dendrite. other factors like mechanical stability of spe and/or homogeneity aspects of li deposition, have likely a stronger effect [11]. acknowledgements: financial support from the german federal ministry for education and research (bmbf) within the project, meet hi-end iii (03xp0258a) as part of the excellbattmat cluster is gratefully acknowledged. references [1] j. janek, w.g. zeier, a solid future for battery development, nature energy 1 (2016) 16141. https://doi.org/10.1038/nenergy.2016.141 [2] x. wu, k. pan, m. jia, y. ren, h. he, l. zhang, s. zhang, electrolyte for lithium protection: from liquid to solid, green energy & environment 4 (2019) 360-374. https://doi.org/10.1016/j.gee.2019.05.003 [3] m.s. whittingham, lithium batteries and cathode materials, chemical reviews 104 (2004) 4271-4301. https://doi.org/10.1021/cr020731c [4] x.-b. cheng, r. zhang, c.-z. zhao, q. zhang, toward safe lithium metal anode in rechargeable batteries: a review, chemical reviews 117 (2017) 10403-10473. https://doi.org/10.1021/acs.chemrev.7b00115 [5] x. zhang, y. yang, z. zhou, towards practical lithium-metal anodes, chemical society reviews 49 (2020) 3040-3071. https://doi.org/10.1039/c9cs00838a https://doi.org/10.1038/nenergy.2016.141 https://doi.org/10.1016/j.gee.2019.05.003 https://doi.org/10.1021/cr020731c https://doi.org/10.1021/acs.chemrev.7b00115 https://doi.org/10.1039/c9cs00838a l. stolz et al. j. electrochem. sci. eng. 13(5) (2023) 715-724 http://dx.doi.org/10.5599/jese.1724 721 [6] q. liu, c. du, b. shen, p. zuo, x. cheng, y. ma, g. yin, y. gao, understanding undesirable anode lithium plating issues in lithium-ion batteries, rsc advances 6 (2016) 88683-88700. https://doi.org/10.1039/c6ra19482f [7] l. frenck, g.k. sethi, j. a. maslyn, n. p. balsara, factors that control the formation of dendrites and other morphologies on lithium metal anodes, frontiers in energy research 7 (2019) 115. https://doi.org/10.3389/fenrg.2019.00115 [8] t. waldmann, b.-i. hogg, m. wohlfahrt-mehrens, li plating as unwanted side reaction in commercial li-ion cells, journal of power sources 384 (2018) 107-124. https://doi.org/10.1016/j.jpowsour.2018.02.063. 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electrochemical science and engineering is dedicated to the 6. european summer school on electrochemical engineering (essee6) held in zadar, croatia, september 16-21, 2012. as a triennial event the european summer school of electrochemical engineering aims to raise awareness of the importance of electrochemical engineering in the various aspects of technological applications as well as to help students and engineers to successfully meet the challenges they encounter in their careers. thanks to the participation of world-recognized professor and teachers, a warm and stimulating atmosphere was created in zadar, enabling students and other participants to acquire basic and advanced knowledge and skills of electrochemical engineering theory and practice. close to 70 students, mostly from european countries but also from other parts of the world, participated in essee6. most of the participants exhibited their current work in the form of poster presentation which attracted a great interest among professors and other students. the posters showed a content of high scientific level and revealed how much efforts students had to made in pursuing their work and career. the topics of their work covered almost all fields in the contemporary electrochemical engineering including electrochemical energy storage and conversion, electrochemical engineering in the environmental protection, corrosion engineering, industrial electrochemistry and electrochemical reactor design. in this issue of the journal of electrochemical science and engineering a small selection of four papers representing students’ work is published. one review and three original scientific papers give a general overview of the scientific content of the summer school and prove high scientific and engineering achievements of the authors. i wish authors and all other participants of essee6 a successful and brilliant career and a significant contribution to the field of electrochemical engineering. zoran mandić http://www.jese-online.org/ outstanding inhibitive effect of colchicine on aluminium alloy 6061 corrosion doi:10.5599/jese.185 197 j. electrochem. sci. eng. 5(3) (2015) 197-208; doi: 10.5599/jese.185 open access : : issn 1847-9286 www.jese-online.org original scientific paper outstanding inhibitive effect of colchicine on aluminium alloy 6061 corrosion mudigere krishnegowda pavithra, thimmappa venkatarangaiah venkatesha, mudigere krishnegowda punith kumar*, nanjanagudu subba rao anantha department of chemistry, kuvempu university, shankaraghatta-577451, shimoga, karnataka, india *department of materials engineering, indian institute of science, bangalore-560012, karnataka, india corresponding author: drtvvenkatesha@yahoo.co.uk;tel: +91-9448855079 received: may 5, 2015; revised: august 29, 2015; accepted: september 14,2015 abstract the corrosion protection ability of colchicine (cc) on aluminium alloy 6061 (aa6061) in 3.5% nacl medium was examined by potentiodynamic polarization, electrochemical impedance, and chronoamperometric techniques. about 99 % of protection efficiency was achieved by 2 mm concentration of cc in 3.5% nacl solution.the adsorption of cc on aa6061 surface obeys langmuir isotherm by following both physisorption and chemisorption mechanism. variation in the surface morphology of inhibited and uninhibited metal samples was examined by scanning electron microscopy. keywords corrosion inhibitor; aluminium alloy; potentiodynamic polarization; electrochemical impedance spectroscopy, adsorption isotherm introduction aluminium alloy 6061 (aa6061) has a remarkable technological importance and extensive applications in industries and machinaries due to low cost, light weight, high thermal and good corrosion resistance properties [1]. the corrosion resistance behaviour develops from the ability of metal to form a natural oxide film on its surface [2,3]. however, the localized corrosion takes place in an aggressive chloride medium and it leads to the breakdown of passive layer and resulted in pit formation [4,5]. thus, chemical inhibitors are widely employed to minimize the corrosion of aa6061 in chloride media. numerous organic compounds having n, s, and o atoms along with π http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 5(3) (2015) 197-208 inhibitive effect of colchicine on al alloy corrosion 198 electrons have been studied as proficient corrosion inhibitors for aluminium and its alloys in 3.5 % nacl solution [6-10]. but, now a day, the drug molecules find much attention in the field of corrosion inhibition. this is due to the strong chemical activity, low toxicity and negligible negative impact of drug molecules on the environment [1113]. the drug molecules inhibits metallic corrosion by the process of adsorption and the magnitude of adsorption depends on nature of the metal, mode of adsorption, chemical structure of the inhibitor, and type of corrosion media [14]. as a result colchicine (cc) drug molecule which is an oral anti-inflammatory agent used for the treatment of gout, familial mediterranean fever and pericarditis [15,16] has been choosen based on its structural considerations. cc contains one nitrogen and six oxygen atoms along with large number of π-electrons in the six and seven membered ring system which may induce the adsorption behaviour of molecule on the metal surface. meanwhile, according to literature study, there is no report found on the corrosion inhibition studies of cc in 3.5 % nacl medium. hence in the present investigation, the anticorrosive ability of cc on aa6061 in 3.5 % nacl solution has been explored by electrochemical techniques and surface analysis. experimental preparation of metal samples. the surface of aa6061 (bhandari metal house, k.r. market, bangalore, india)having composition 0.25 % cu, 1.0 % mg, 0.60 % si, 0.20 % cr and remainder being al was polished with different grades of emery papers (grade no. 400, 600, 800, 1000, and 1200). afterwards, the polished specimen was immersed in 10 % naoh solution for 30 sec, degreased with acetone and rinsed by millipore water, dried and stored in desiccator. the aa6061 specimens of 1 cm 2 area (exposed) with a 5 cm long stem isolated with araldite resin were used for electrochemical measurements. preparation of solutions corrosive medium was prepared from ar grade nacl by using millipore water. the millipore water was obtained from elix 3 milli-pore system (resistivity greater than 18 mω cm at 25 o c). cc was obtained from ramdev chemicals india pvt. ltd., mumbai and its structure is given in figure 1. the different concentrations (1.0, 1.5, 2.0 mm) of inhibitor solutions were prepared by dissolving specified amount of cc in 3.5 % nacl solution. all the experiments were carried out in static condition and 3.5 % nacl solution exhibit ph 6 and the same ph has been maintained for each concentration of inhibitor solutions. each experiment was carried out in triplicate, and the average values are reported. figure 1. the structure of cc [16] m. k.pavithra at al. j. electrochem. sci. eng. 5(3) (2015) 197-208 doi:10.5599/jese.185 199 electrochemical measurements the electrochemical measurements were conducted in a three electrode conventional glass cell using chi 660c electrochemical analyzer (ch instruments, austin, usa). an aa6061 specimen (of 1 cm 2 area), a platinum wire, and a ag/agcl electrode were used as working, auxiliary, and reference electrodes for all the experiments. prior to each potentiodynamic polarization and electrochemical impedance spectroscopic measurements (eis), a stabilization period of 30 min was allowed to establish a steady state open circuit potential (ocp). potentiodynamic polarization measurements the potentiodynamic polarization measurements were carried out over a potential range of − 200 mv to +200 mv at ocp with a scan rate of 0.5 mv s -1 . the corrosion parameters such as corrosion potential (ecorr), corrosion current density (icorr), and anodic (βa), and cathodic (βc) tafel slopes were generated from the software installed in the instrument. the inhibition efficiency ηt / % was evaluated from icorr values using the following expression: o corr corr t o corr / % 100 i i i     (1) where i o corr and icorr are the corrosion current densities without and with cc, respectively. electrochemical impedance measurements the impedance measurements were carried at ocp in the frequency range 1 mhz to 100 khz with 5 mv sine wave as the excitation signal. impedance data were analyzed using zsimp-win 3.21 software. the percentage efficiency ηz / % was evaluated from total resistance (rt) values using the following equation o t t t / % 100z r r r     (2) where rt o and rt are the total resistance (where rt is the sum of r1 and r2) in the absence and presence of cc, respectively. chronoamperometric measurements the chronoamperometric experiments were carried out by polarizing the working electrode anodically at – 0.68 v (ag/agcl) for 600 s in 3.5 % nacl solution. surface morphological studies the surface morphology of aa6061 samples after immersion in corrosive medium in the absence and presence of inhibitor was analyzed using scanning electron microscope (jeol, jsm 6400, jeol datum shanghai co. ltd., shanghai, prc). results and discussion potentiodynamic polarization (pdp) measurements the potentiodynamic polarization curves obtained for aa6061 in 3.5 % nacl solution with or without cc are presented in figure 2. it is evident from the figure that, both the cathodic and anodic curves are shifted towards lower current density in inhibited solution compared to nacl j. electrochem. sci. eng. 5(3) (2015) 197-208 inhibitive effect of colchicine on al alloy corrosion 200 solution. as a result it can be considered that both oxygen reduction and al dissolution process are limited by cc and thus it acts as a mixed type inhibitor. the corrosion kinetic data obtained from potentiodynamic polarization curves for aa6061 in 3.5 % nacl having different concentrations of cc are tabulated in table 1. it can be visualized from the table that by increasing the concentration of cc there exist a slight variation in both βa and βc values and ecorr values shifted towards more positive direction. this infers the formation of strongly adsorbed cc film on aa6061 surface. figure 2. potentiodynamic polarization curves obtained for aa6061 in 3.5 % nacl in the absence and presence of cc. usually, in 3.5 % nacl media, aluminium experience pitting corrosion because of the presence of destructive chloride ions. hence, aa6061 dipped in 3.5% nacl shows high icorr value. but the icorr values get reduced in presence of cc. this specifies that in inhibited solution the cc molecules adsorb on the al surface and decreases the aggressiveness of cl ─ ions as well as protect the surface from being pitted. this suggests that cc inhibits the corrosion of aa6061 in 3.5 % nacl solution and its efficiency increases with the increase of its content in the solution. table 1. potentiodynamic polarization parameters obtained for aa6061 in 3.5 % nacl in the absence and presence of cc c / mm ̶ ecorr/ v ̶ βc /mv dec -1 βa/ mv dec -1 icorr / µa cm -2 ηt / % blank 1.236 10.063 2.308 162.5 1.0 1.160 11.516 2.876 25.08 85 1.5 1.007 11.367 2.805 20.17 88 2.0 0.698 21.923 2.417 0.6728 99 on the other hand, it is apparent from the figure 2 that a wide passive region can be witnessed in the anodic curve obtained for 2 mm cc. this may be attributed to the formation of stable inhibitor film on the oxide layer which results in 99 % protection efficiency. by reviewing these results, it can be concluded that cc behaves as a potential corrosion inhibitor for aa6061 by inhibiting the corrosion of aluminium by blocking the active sites of the metal surface. m. k.pavithra at al. j. electrochem. sci. eng. 5(3) (2015) 197-208 doi:10.5599/jese.185 201 electrochemical impedance spectroscopy (eis) eis analysis was carried out to get information on the corrosion and inhibition mechanism of cc. the electrochemical impedance diagrams in the form of nyquist and bode plots obtained for aa6061 in 3.5 % nacl in the absence and presence of cc are given in figures 3 and 4, respectively. the eis results were simulated by an electrical equivalent circuit shown in the inset of figure 3. in this circuit, rs represents the solution resistance, r1 is the charge transfer resistance corresponding to the corrosion reaction at the al/solution interface, r2 represents the polarization resistance, which reflects the protective property of the film, cpe1 and cpe2 represents the constant phase elements (cpe) as a substitute for the double-layer capacitance (cdl). the impedance of cpe is defined as   -n1 cpez q j   (3) where q is the cpe constant, ω is the angular frequency, j 2 = −1 is the imaginary number and n is the cpe exponent which gives details about the degree of surface inhomogeneity resulting from surface roughness, inhibitor adsorption, porous layer formation etc. figure 3. nyquist plots for aa6061 in 3.5 % nacl in the absence and presence of different concentrations of cc. two capacitive loops were observed in nyquist plots. the first capacitive loop at higher frequencies is related to the sum of charge transfer resistance and accumulation resistance (some free molecules or ions or corrosion products), while the second capacitive loop at lower frequencies corresponds to the film resistance. the diameter of these capacitive loops gets increased with increasing cc concentration and signifies the adsorption of inhibitor molecules on the metal surface. the adsorption of inhibitor molecules at the aa6061/nacl solution interface results in an increase of polarization resistance and reduces the corrosion rate of aa6061 [17]. j. electrochem. sci. eng. 5(3) (2015) 197-208 inhibitive effect of colchicine on al alloy corrosion 202 table 2. electrochemical impedance parameters obtained for aa6061 in 3.5 % nacl in the absence and presence of different concentrations of cc c / mm rs / ω cm 2 q1 / μω -1 s n cm -2 n1 r1 / ω cm 2 q2 μω -1 s n cm -2 n2 r2 / ω cm 2 ηz / % blank 10.12 171.8 0.865 87.12 12060 0.811 442.5 1.0 10.68 71.05 0.913 483.6 3562 0.912 2116 80 1.5 6.934 155.2 0.827 1427 2322 0.920 3597 89 2.0 6.891 24.47 0.936 9259 309.4 1.000 13620 98 data from table 2 reveal that increasing the concentration of cc in chloride media leads to an increase in resistance value and decrease in cpe values. the decrease in q1 and q2 values may be due to the decrease in local dielectric constant and/or an increase in the electrical double layer thickness [18]. this infers cc acts via adsorption at the aa6061/nacl solution interface. meanwhile the increase in r1 and r2 values point out that the amount of cc molecules adsorbed on the metal surface increases and the adsorbed inhibitor molecules forms a protective film on the electrode surface and consequently become a barrier to hinder the mass and charge transfer, resulting in an increase in the protection efficiency. figure 4. bode plots for aa6061 in 3.5 % nacl in the absence and presence of different concentrations of cc. the bode diagrams (figure 4) that was plotted using the same experimental data in the nyquist format. in the lower frequency range, impedance modulus (zmod) gives information regarding the corrosion resistance behavior of metal samples. in figure 4, zmod values increases with increasing cc concentration compared to uninhibited solution and it reveals the better corrosion inhibition performance of cc in nacl medium. compared to uninhibited solution, the new phase shift is observed in the inhibited solutions. this means that the formation of inhibitor film changes the electrode interfacial structure and results in an extra time constant [19]. hence two time constants can be evidenced in figure 4 and this indicate that there are two major electrochemical kinetic processes on the electrode surface: the high frequency part is due to the adsorption of the cc molecule and the formation of the inhibitor film; the second time constant at medium frequency is due to the electrochemical corrosion process. moreover, the inhibition effect is more remarkable m. k.pavithra at al. j. electrochem. sci. eng. 5(3) (2015) 197-208 doi:10.5599/jese.185 203 and the middle frequency phase becomes loftier and wider at higher concentration [19] of cc (2 mm). these suggest the formation of compact, stable protective film of cc on aa6061 surface in 3.5 % nacl solution. chronoamperometric (ca) measurements the chronoamperometric experiments were performed so as to get information on the corrosion behaviour of aa6061 in 3.5 % nacl solution in the absence and presence of cc at less negative potentials.the experiments were carried out by polarizing the working electrode anodically at – 680 mv (vs. ag/agcl) for 600 s [20]. the current density values were obtained during the electrooxidation of aa6061 in 3.5 % nacl solution in the absence and presence of different concentrations of cc and the chronoamperometric curves are illustrated in figure 5. figure 5. chronoamperometric curves obtained for aa6061 in 3.5 % nacl in the absence and presence of different concentrations of cc. the absolute current of aa6061 in uninhibited solution is comparatively higher than inhibited solution. the addition of cc in to nacl solution results in a reduction in the absolute current of aa6061 from the first moment till the end of the measurement. besides, a significant decrease in the current was observed by increasing the concentration of cc. this supposes that cc improves the anticorrosive nature of aa6061 in 3.5 % nacl solution and hence it can be considered as a potential corrosion inhibitor. adsorption isotherm the anticorrosive ability of metals can be improved by the adsorption of organic molecules and ions at the metallic surface. the molecular structure, electronic properties and distribution of charge in inhibitor molecules, the nature and surface charge of the metal, and the type of corrosive medium [21] have great influence on the process of adsorption of inhibitors on metal surface. in order to gain information on the adsorption behaviour of cc on aa6061 surface, the data obtained from the pdp and eis techniques were tested with several adsorption isotherms. many attempts were made to fit experimental data with adsorption isotherms like langmuir, flory-huggins, temkin, and freundlich isotherms [22-24] (equations 4-7). j. electrochem. sci. eng. 5(3) (2015) 197-208 inhibitive effect of colchicine on al alloy corrosion 204 langmuir adsorption isotherm: ads 1c c k   , (4) flory – huggins adsorption isotherm:  adslog log log 1k a c           , (5) temkin adsorption isotherm: ads 1 lnk c f   , (6) freundlich adsorption isotherm: adslog log logk n c   , (7) where c is the inhibitor concentration, is the degree of surface coverage defined as η/100 and kads is the adsorptive equilibrium constant, f is the factor of energetic inhomogeneity, and the parameter a is the number of water molecules replaced by inhibitor molecules on the metal surface. the experimental data were fitted to langmuir, flory–huggins, temkin, and freundlich isotherms and the corresponding curves are illustrated in figure 6. in the present investigation, the best fit was obtained with langmuir adsorption isotherm and the plot of c/against c clearly indicates that the linear regression coefficient (r 2 ) and the slope values are very close to 1. this reveals that the adsorption of cc on aa6061 obeys the langmuir adsorption isotherm. figure 6. adsorption isotherm plots obtained for the cc on the surface of aa6061 in 3.5 % nacl (a) langmuir, (b) flory-huggins, (c) temkin and (d) freundlich isotherms m. k.pavithra at al. j. electrochem. sci. eng. 5(3) (2015) 197-208 doi:10.5599/jese.185 205 the equilibrium constant, kads is related to the standard gibbs free energy of adsorption δg o ads by the relation: o ads ads 1 exp 55.5 g k rt        (8) where r is the universal gas constant, t is the thermodynamic temperature and 55.5 is the molar concentration of water in the solution. generally, the values of δg o ads around  20 kj mol -1 or lower are consistent with physisorption and those around  40 kj mol -1 or higher involves chemisorption [25]. in the present work, the values of δg o ads obtained for the adsorption of cc from pdp and eis analysis are  29.93 and  29.18 kj mol -1 , respectively. this ensures that the adsorption of cc on aa6061 involves both physisorption and chemisorptions. meanwhile, the negative value of δg o ads suggests the spontaneity of the adsorption of the cc molecules and the stability of the adsorbed layer on aa6061 surface. surface morphological studies figure 7 shows the surface morphology of aa6061 samples after immersion in 3.5 % nacl in the absence and presence of 2 mm cc. it can be clearly seen from the figure 7a that aa6061 surface is strongly damaged with numerous pits in the absence of cc due to dissolution of metal in nacl medium. but a smoother surface free from pits can be observed in the figure 7b due to the presence of cc which results in the formation of protective layer on aa6061. this signifies that cc hinders the dissolution of al and thereby it reduces the pittig corrosion of aa6061 in 3.5 % nacl. figure 7. sem micrographs of aa6061 immersed in (a) 3.5 % nacl and b) 3.5 % nacl + 2.0 mm cc mechanism of corrosion inhibition aluminium has more electronegative potential value and hence, it undergoes oxidation very easily in an oxidizing media. this phenomenon leads to the formation of oxide layer (al2o3) on its surface by both cathodic and anodic reactions [26]. the mechanism of formation of oxide film on aa6061 surface is shown in figure 8a. j. electrochem. sci. eng. 5(3) (2015) 197-208 inhibitive effect of colchicine on al alloy corrosion 206 figure 8. mechanism of aa6061 corrosion and its inhibition by cc in 3.5 % nacl. the presence of oxide layer on metal surface induces corrosion resistance property. however, in 3.5 % nacl medium, aluminium undergoes pitting corrosion which results in the depletion of oxide layer. the mechanism of pitting corrosion is depicted in figure 8b. in the presence of 3.5 % nacl media, the stability of the oxide layer decreases due to the presence of aggressive chloride ions [7]. the aggressive chloride ions penetrates through the oxide film and breakdown the aluminium oxide formed on the metal surface [27]. then the adsorbed chloride ions reacts with al 3+ in the oxide lattice and results in the formation of oxychloride complexes, al(oh)2cl2 ─ [28-31]. the oxychloride complex reduces the stability of oxide film and intensifies the dissolution rate of aluminium [20]. m. k.pavithra at al. j. electrochem. sci. eng. 5(3) (2015) 197-208 doi:10.5599/jese.185 207 on the other hand, all the experimental results reveal that the corrosion of aa6061 can be minimized in presence of cc. the electrochemical studies disclose that the inhibition performance of cc occurs through the process of electrostatic interaction. the mechanism of corrosion inhibition is given in figure 8c. the inhibitor cc get adsorbed on metal surface by donor acceptor interactions between free electron pairs of four oxygen atoms present in the methoxy group and π electrons of the phenyl ring and the vacant orbital of aluminium atom. according to the quantum chemical studies carried out by our research group, nitrogen and oxygen atoms, and πelectrons in the phenyl ring and cycloheptatrienone ring, are the main active sites of adsorption of cc on metal surface [32]. as a result, cc gets adsorbed on aa6061 surface through these active sites and it prevents the formation of oxychloride complexes and inhibits metal dissolution reaction. conclusions the pitting corrosion of aa6061 can be minimized in the presence of cc in 3.5 % nacl solution. cc 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[32] m. k. pavithra, t. v. venkatesha, m. k. punith kumar, n. s. anantha, res. chem.intermed. doi 10.1007/s11164-015-2158-3, (2015) http://link.springer.com/article/10.1007/s11164015-2158-3 © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://link.springer.com/article/10.1007/s11164-014-1701-y http://link.springer.com/article/10.1007/s11164-014-1701-y http://creativecommons.org/licenses/by/4.0/ electrochemical strategies for detection of diazinon: http://dx.doi.org/10.5599/jese.1379 1041 j. electrochem. sci. eng. 12(6) (2022) 1041-1059; http://dx.doi.org/10.5599/jese.1379 open access : : issn 1847-9286 www.jese-online.org review electrochemical strategies for detection of diazinon azadeh lohrasbi nejad department of agricultural biotechnology, shahid bahonar university of kerman, kerman, iran corresponding author: a.lohrasbi@uk.ac.ir received: may 15, 2022; accepted: july 30, 2022; published: september 17, 2022 abstract diazinon (dzn) was first registered as an insecticide in the u.s. however, it was categorized in the limited group of pesticides due to its high toxicity for birds, aquatic animals, and humans. like other organophosphorus pesticides, this compound exhibits inhibitory effects on the acetylcholinesterase enzyme. the inhibition of the enzyme leads to the accumulation of acetylcholine and causes the death of insects. dzn is considered a toxic compound for humans due to its high adsorption via skin and inhalation, which leads to the emergence of different symptoms of toxicity. when dzn is used for plants, the compound residues in crops enter the food chain bringing about different health problems. moreover, the compound is easily washed by surface water and enters the groundwater. its entrance into aquatic environments can negatively affect a wide range of non-targeted organisms. thus, researchers seek to find fast and precise methods for the analysis of dzn. the electrochemical method for recognizing the compound in real samples is preferable to other analytical methods. because this method can be used without spending time preparing the sample, it is simple, fast, and cost-effective. since such determinations may be made by using electrochemical sensors and biosensors, numerous researchers have developed such sensors for dzn detection, and different sensitive materials were used in order to improve the selectivity, sensitivity, and detection limit. the present study aims to present the main progress and performance characteristics of electrochemical sensors and biosensors used to detect dzn, as it is reported in a number of relevant scientific papers published mainly in the last decade. keywords pesticide; electrochemistry; modified electrodes introduction pesticides are used worldwide to protect agricultural crops and avoid plant damage caused by pests. organophosphates are the most extensive and diverse class of pesticides [1]. the organophosphorus pesticides (opps) and organic ester derivatives of phosphorus are widely used as insecticides in agriculture [2,3]. diazinon (dzn) with the scientific name o,o-diethyl-o-(2isopropyl4-methyl-6-pyrimidinyl)-o,o-diethyl-o-(2-isopropyl-4-methyl-6-pyrimidinyl)-phosphorothioate is http://dx.doi.org/10.5599/jese.1379 http://dx.doi.org/10.5599/jese.1379 http://www.jese-online.org/ mailto:a.lohrasbi@uk.ac.ir j. electrochem. sci. eng. 12(6) (2022) 1041-1059 electrochemical strategies for detection of diazinon 1042 widely used as an insecticide in agriculture. dzn is ranked as the most common organophosphorus pesticide globally after malathion [4-6]. although dzn is a thiophosphoric ester, it shares a common mechanism of toxicity with other organophosphorus insecticides like chlorpyrifos, malathion and parathion [7]. dzn inhibits acetylcholinesterase (ache) and prevents acetylcholine (ach) hydrolysis in cholinergic synapses and neuromuscular junctions. therefore, it leads to abnormal accumulation of ach in the nervous system [8]. since that pesticide is chemically stable, its improper application can result in environmental contamination [9] and ultimately threaten human health [10,11]. the compound belongs to the contact insecticide group and kills the insects by altering typical neurotransmitters in their nervous system [7]. u.s. environmental protection agency (epa) canceled all residential uses of dzn in 2004. nowadays, it is only allowed to be used in agricultural environments [12]. dzn is relatively stable and mobile and cannot be absorbed by soil [13,14]. microbial degradation in the soil is the most important process for dzn removal from the environment [7]. conducted studies demonstrated that half-lives of dzn in soil were 21 to 103 days depending on the type of soil [15]. a compound called diazoxon is formed by dzn hydrolysis and is rapidly hydrolyzed to oxypyrimidine, which is more mobile in the environment than the original compound [7]. in addition, dzn can be decomposed by the photolysis process. in this case, the halflife of this compound on the soil surface is estimated to be between 17.3 to 37.4 hours [14]. dzn and its metabolites can be contaminated the groundwater considering their stability and mobility [13]. in the atmosphere, dzn is transformed into diazoxon, which is a more powerful ache inhibitor compared to dzn, with a half-life of 4 hours [16]. low levels of dzn applied outdoors can be carried indoors through airflow, dust, soil, and pets exposed to the compound [17]. the main problem of dzn application is that it can be absorbed by plant roots in soil [18]. the compound can degrade in leafy vegetables, forage crops, and grass with half-lives ranging from 2 to 14 days. however, low temperature and high oil content cause the half-life of dzn to increase in plants [18]. pyrimidinol, as well as hydroxypyrimidinol, are the compounds resulting from dzn hydrolysis in plants. since diazoxon can also be found in plants, the measured pyrimidinol may be a product of diazoxon hydrolysis rather than dzn hydrolysis [19]. dzn poses relatively high toxicity to vertebrates. once entering the body, dzn is decomposed to diazoxon by oxidation which is more toxic than dzn and mainly inhibits acetylcholinesterase (ache) [20]. the oxidation of dzn to diazoxon is carried out by the liver microsomal enzyme and requires o2 and nadph. dzn can also be decomposed through oxidation in the liver. both reactions are likely to be catalyzed nonspecifically by the same mixed-function oxidase. hydrolases further decompose dzn in the microsomal and other intracellular functions inside the liver. mammals metabolize diazoxon with an estimated half-life of 2 to 6 weeks. insects lack the hydrolysis step, and hence, the toxic substance is rapidly accumulated in their bodies. the toxic compound can enter the body through skin contact, feeding, and inhalation [21]. the signs and symptoms of toxicity by dzn are similar to other toxins inhibiting ache. depending on the exposure period, different symptoms emerge, including colic, diarrhea or vomiting, vertigo, headaches, miosis, bradycardia, sudden drop in blood pressure, convulsion, and apnea after several minutes to hours of exposure [22,23]. since dzn is lipid-soluble, it has the potential to be stored in fat tissues so that it can heighten delayed toxicity. it is important to detect and control organophosphorus chemicals regarding their toxicity and adverse effects on human life. so far, various in vitro methods have been proposed to detect those compounds, such as highperformance liquid chromatography (hplc) [24,25], liquid chromatography coupled with mass spectroscopy (lc-ms) [26], gas chromatography-mass spectrometry (gc-ms) [27], capillary gas https://wikivisually.com/wiki/acetylcholinesterase_inhibitor a. l. nejad j. electrochem. sci. eng. 12(6) (2022) 1041-1059 http://dx.doi.org/10.5599/jese.1379 1043 chromatography [28], electrochemical analysis [29-32], and colorimetric method [33]. however, the above methods suffer from several inherent flaws. for example, despite their high precision, instrumental analysis methods are usually time-consuming and expensive. some methods require complicated isolation processes, careful washing steps, and reagent addition. the colorimetric assays are not sensitive enough to detect pesticide traces [34]. however, electrochemical analysis has many advantages, including simplicity, repeatability, good stability, high sensitivity, and no need to sample preparation steps compared to other detection techniques mentioned above [35-49]. a drawback of this method is that it suffers from weak selectivity, which is resolved by modifying the electrode surface to improve the sensitivity and selectivity of electrochemical sensors and avoid surface fouling [50-63]. chemical modification of electrodes is a quite useful technique for detecting organophosphorus toxins [64]. such technologies are appropriate alternatives to increase the electron transfer rate in electrochemical sensors [65-70]. nanotechnology has been considered a technology of general use, being common in almost all technological sectors. this is due to the fact that the interactions between different materials when they are at the nanoscale (10−9 m), are able to generate new properties where unique phenomena may occur, different from those observed at the macroscopic scale, thus giving rise to the possibility of new applications for these materials [71-90]. various materials such as carbon-based nanomaterials, metal oxides, metals complex, polymers, and biological compounds can be used to modify electrode surfaces [91-107]. this work aims to explain the current developments in electrochemical sensors and biosensors for monitoring dzn. development of electrochemical dzn sensors electrochemical sensors are the most rapidly growing group of chemical sensors among presently available systems for practical applications. the most common electrochemical system used for laboratory study is made up of a reference electrode, counter electrode, and a working electrode (sensing electrode) immersed in an electrolyte which measures the potential and current generated from oxidative or reductive reactions. the working principle behind electrochemical sensors involves the generation of electrical signals from the analyte of interest, which is affected by the concentration of the analyte [108-112]. dzn is an electrochemically-active molecule, but its direct determination is difficult because of the weak response of dzn in conventional electrochemical sensors. different types of electrode modifications for electrochemical methods have been developed for the determination of dzn because they possess high sensitivity, short analysis time, good handling convenience and low cost. this section introduced examples of developed electrochemical dzn sensors. then, table 1 tabulates information about the electrochemical sensor, electrochemical method, limit of detection (lod), and linear range, which have been reported by various works. in 2003, a simple and easy electrochemical method was used by erdogdu for the determination of dzn. in this method, the surface of the glassy carbon electrode was coated with nafionperfluorinated ion-exchange powder (ncgce), and square-wave voltammetry (swv) was used for the quantitative estimation. the author demonstrated that the peak current slightly changed in the potential range from 0 to 0.35 v because dzn could not be reduced in this potential range. however, a sudden increase was observed in the peak current at 0.40 v. the effect of sw response was studied in the mentioned conditions because the peak current for dzn obtained in swv depended on http://dx.doi.org/10.5599/jese.1379 j. electrochem. sci. eng. 12(6) (2022) 1041-1059 electrochemical strategies for detection of diazinon 1044 different parameters such as sw amplitude, sw frequency, and step height. hence, modulation amplitude, 25 mv; modulation frequency, 14 hz; modulation step, 4 mv s-1 were considered optimal conditions for dzn detection. the results of the erdogdu study showed that combining the swv and electrode surface modification with nafion film worked excellently for dzn detection. thus, the proposed voltammetric procedure for dzn detection could be useful in many different applications. in this method, the minimum lod for diazinon was reported to be 75 nm. the results of 20 successive measurements showed a coefficient of variation of 1.9 % for ncgc electrodes. therefore, the modified electrode possessed a good reproducibility surface [113]. arvand et al. [114].used the differential pulse voltammetric (dpv) methods to investigate the electrochemical behavior of dzn at carbon paste electrode modified with tris(ethylenediamine) cobalt(ii) iodide (co-cpme). cobalt complex showed an anodic peak at 620 mv (vs. ag/agcl, in kno3 0.1 m as supporting electrolyte) at co-cpme. in the presence of dzn, anodic peak intensity increased with increasing concentration of dzn that confirming the electrocatalytic activity of cobalt complex for oxidation of dzn (ec’ mechanism). under optimized conditions, a linear calibration curve for diazinon was obtained in the range from 0.05 to 27.0 µg/l with a detection limit 0.015 nm (3sb/m). applications of the modified electrode to the determination of dzn in different water samples were also tested. the results showed a very good precision (rsd < 0.04 %) and a very stable voltammetric response towards dzn [114]. in a study by motaharian et al. an electrochemical sensor based on molecularly imprinted polymer (mip) nanoparticles for selective and sensitive determination of dzn pesticides was developed. the nanoparticles of dzn imprinted polymer were synthesized by suspension polymerization and then used to modify carbon paste electrode composition to prepare the sensor. cyclic voltammetry (cv) and swv methods were applied for electrochemical measurements. the obtained results showed that the carbon paste electrode modified by mip nanoparticles (nano-mip-cpe) has a much higher adsorption ability for dzn than the cpe-based non-imprinted polymer nanoparticles (nanonip-cpe). under optimized extraction and analysis conditions, the proposed sensor exhibited excellent sensitivity (95.08 μa l μmol-1) for dzn with two linear ranges of 2.5 to 100 nmol l-1 (r2 = 0.9971) and 0.1 to 2.0 mol l-1 (r2 = 0.9832) and also a detection limit of 0.79 nmol l-1. the sensor was successfully applied for the determination of dzn in well water and apple fruit samples with recovery values in the range of 92.53–100.86 % [115]. ghodsi and rafati described a voltammetric sensor for dzn pesticide determination based on dzn reduction on glassy carbon electrode surface modified with multi-walled carbon nanotubes covered by tio2 nanoparticles (mwcnts/tio2nps/gce). voltammetric investigations were carried out by cv, linear sweep voltammetry (lsv), dpv and swv. mwcnts/tio2nps nanocomposite was characterized with scanning electron microscopy (sem), x-ray diffraction (xrd) and energy dispersive x-ray analysis (edx) techniques. the mwcnts/tio2nps nanocomposite showed suitable synergic electrocatalytic properties in dzn reduction, which resulted in the completely sensitive determination of dzn in the laboratory and real samples, including city piped water and agricultural well water. linear range, lod and loq obtained by mwcnts/tio2nps/gce sensor were 11.0 to 8360.0 nm, 3.0 nm and 10.0 nm successively. also, the sensor was successfully examined for dzn determination in real water samples, including agricultural well water and city piped water and obtained results showed acceptable recovery amounts [116]. khadem et al. introduced the composition of carbon paste electrodes modified with mip and mwcnts as the easy-to-use electrochemical sensor (mwcnts-mip/cpe) for selective and sensitive determination of dzn pesticide in environmental and biological samples. the mwcnts enhance the a. l. nejad j. electrochem. sci. eng. 12(6) (2022) 1041-1059 http://dx.doi.org/10.5599/jese.1379 1045 sensor responses due to increasing of the electrode surface area, as well as improving the electron transfer between the electrode and the supporting electrolyte. the presence of mips can greatly increase the selectivity of the electrode. instrumental parameters affecting the square wave voltammetric response were adjusted to obtain the highest current intensity. the obtained linear range was 0.5 nm to 1.0 m. the detection limit of the sensor was 0.13 nm and the relative standard deviation for analysis of the target molecule by the proposed sensor was 2.87 % [117]. akyuz and koca constructed a mimic enzymeless electrochemical sensor (m-eless-es) based on the terminal alkynyl substituted manganese phthalocyanine (mnpc-ta) and 4-azido polyaniline (n3pani) hybrid and tested as a sensitive and selective pesticide sensor. during the construction of ito/mnpc-ta/n3-pani electrode, mnpc-ta was firstly deposited on indium tin oxide coated glass substrate (ito) with langmuir blodgett (lb) technique. then 4-azidoaniline (n3-ani) was bonded to the terminal alkynyl substituents of mnpc-ta (ito/mnpc-ta/n3-ani) with click chemistry (cc), and finally, ani groups of the solid mnpcta/n3-ani hybrid film was electropolymerized on ito surface to form ito/mnpc-ta/n3-pani electrode. the structure of the modified electrode was characterized by swv, xrd, ft-ir and sem analyses. the result of the analyses indicated successful construction of ito/mnpc-ta/n3-pani electrode. ito/mnpcta/n3-pani electrode was tested as a potential sensor for various pesticides and the results showed that it was used as a sensitive sensor for the dzn with lod of 0.062 µmol dm-3 and a wide linear range [118]. accordingly, zahirifar et al. employed a carbon paste electrode modified with multi-walled carbon nanotubes (mwcnt/cpe) to detect dzn in solutions and real samples (fruit juices). the obtained results showed that cnts increased the sensor response due to their large surface area, improved electrocatalytic activity, and enhanced electron-transfer rate between electrode and electrolyte. the mwcnt/cpe electrode could recognize dzn in fruit samples with a desirable lod with little interference of other compounds. because the thickness of cnt can affect the electrode response, zahirifar et al. recorded the amount of cnt used to make an optimized cnt/cpe electrode and its effect on the peak current of dzn. the results showed that the peak current rose as the quantity of cnt increased. the maximum peak current level was observed when the amount of cnt increased up to 10 %. however, any amount higher than 10 % negatively affected the sensor response in dzn detection. the researchers introduced the sensor as a proper candidate for the recognition of dzn in foodstuffs regarding its ability to detect dzn in the concentration range of 0.1 to 10 nm by the designed electrode. the limit of detection for dzn in this sensor was determined by the dpv technique, and it was found to be 0.45 nm [119]. tadayon and jahromi constructed a sensitive and selective electrochemical sensor based on bimetallic gold and palladium nanoparticles coated on a mixture of reduced graphene oxide and multiwall carbon nanotube nanocomposite modified glassy carbon electrode (au–pd/rgo– –mwcnts/gce)for determination of an organophosphorus insecticide, dzn. the effect of various parameters includes ph of the solution, scan rate, accumulation time and potential, and instrumental parameter was optimized. under optimized conditions, the proposed sensor showed that the stripping oxidation peak current of dzn was linearly proportional to its concentrations in an appropriate range of 0.009 to 11.3 µm with a lod and loq of 0.002 and 0.009 µm, respectively. finally, the au–pd/rgo–mwcnts/gce was successfully used for measurement of dzn in water, apple and cucumber samples recoveries ranging from 96 to 105 % [120]. ghiasi et al. employed the composition of graphene quantum dots (gqds), chitosan (cs), and nickel molybdate nanocomposites (nimoo4, nmo) for the modification of activated glassy carbon electrode (nmo/gqds/cs/gceox) as an electrochemical sensor for dzn determination (figure 1). http://dx.doi.org/10.5599/jese.1379 j. electrochem. sci. eng. 12(6) (2022) 1041-1059 electrochemical strategies for detection of diazinon 1046 the nmo/gqds/cs/gceox for precise and swift analysis of dzn in cucumber and tomato as real samples has been effectively used as a sensitive voltammetric sensor. under optimized conditions, the constructed sensor illustrated a linear range between 0.1 to 330.0 μm and a lod of 30.0 nm using differential pulse voltammetry methods [121]. figure 1. electrochemical sensor based on modified glassy carbon electrode with graphene quantum dots, chitosan and nickel molybdate nanocomposites for the determination of diazinon [121] topsoy et al. described a new simple, sensitive, and easy method to manufacture voltammetric sensor based on mwcnt-screen-printed electrode modification with poly(ε-caprolactone)/chitosan (pcl/chs) electrospun nanofibers (pcl/chs nanofiber/mwcnt-spe). the images of unmodified and modified electrodes in dzn detection are shown in figure 2. figure 2. the schematic diagram of bare mwcnt-spe and pcl/chs nanofiber-modified mwcnt-spe [122] a. l. nejad j. electrochem. sci. eng. 12(6) (2022) 1041-1059 http://dx.doi.org/10.5599/jese.1379 1047 the effects of different parameters such as coating thickness, analyte concentration, ph value of the buffer solution, and scan rate on the electrochemical behavior of unmodified and modified screenprinted carbon electrodes were investigated via cv, dpv, and electrochemical impedance spectroscopy (eis) techniques. the spinning time, ph value and scan rates were optimized at 6 min, 5.25, and 50 mv s-1, respecttively. the prepared pcl/chs nanofiber/mwcnt-spe displayed good electrochemical performances with a wide detection range of 3.0–100.0 nm in 0.1 m acetate buffer solution (ph 5.25), and a detection limit of 2.888 nm, respectively. meanwhile, the sensor was successfully applied for the determination of dzn in the tomato juice sample with recovery values in the range of 93.27–108.30 % [122]. porto et al. presented, for the first time, a fast and highly sensitive electrochemical method for the determination of three organophosphorus compounds (ops), dzn, malathion (mlt), and chlorpyrifos (clpf), using a modified pyrolytic graphite electrode (pge) coupled to batch injection analysis system with multiple pulse amperometric detection (bia–mpa). the pge was modified by a nanocomposite based on functionalized carbon nanotubes and silver nanoparticles (cntfagnp/pge). the ops samples were directly analyzed on the modified working electrode surface by bia-mpa system in britton-robinson (br) buffer 0.15 mol l− 1 at ph 6.0. the mpa detection of dzn, mlt and clpf was performed using two potential pulses, which were sequentially applied on modified pge at − 1.3 v (100 ms) and +0.8 v (100 ms) for selective determination of these three ops and working electrode cleaning, respectively. under optimized conditions, the sensor presented a linear range of 0.1–20.0 μm for dzn. the lod and loq of 0.35 and 1.18 μm for dzn were obtained. also, cntf-agnp/pge sensor exhibited high sensitivity of 0.068 ma μm for dzn detection. furthermore, the bia-mpa system provided an analytical frequency of 71 determinations per hour for the direct determination of these ops in water and food samples. the modified pge coupled to bia-mpa system showed high stability of electrochemical response for ops detection with a relative standard deviation (rsd) of 1.60 % (n = 20). the addition-recovery studies of the proposed method were carried out in tap water, orange juice, and apple fruit real samples, which showed suitable recovery values between 77 and 124 % [123]. table 1. features of various electrochemical sensors for the analysis of dzn. electrochemical sensor electrochemical method lod, nm linear range ref. ncgce swv 75.0 0-5.0 µm [113] co-cpme dpv 0.015 0.05-27.0 µg / l [114] nano-mip-cpe swv 0.79 2.5-2000.0 nm [115] mwcnts/tio2nps/gce cv, swv 3.0 11.0-8360.0 nm [116] mwcnts-mip/cpe swv 0.13 0.5-1000.0 nm [117] ito/mnpcta/n3-pani electrode swv 62 [118] mwcnt/cpe dpv 0.45 0.1-10.0 nm [119] au–pd/rgo–mwcnts/gce swasv 2.0 0.009-11.3 µm [120] nmo/gqds/cs/gceox dpv 30.0 0.1-330.0 μm [121] pcl/chs nanofiber/mwcnt-spe dpv 2.888 3.0-100.0 nm [122] cntf-agnp/pge bia-mpa 354.0 0.1-20.0 μm [123] development of electrochemical dzn biosensors biosensor-related research has experienced explosive growth over the last two decades. a biosensor is generally defined as an analytical device that converts a biological response into a quantifiable and processable signal [124]. in most biosensors developed so far, electrochemical detection has been used in one way or another, i.e., amperometric, voltammetric, and impedimetric http://dx.doi.org/10.5599/jese.1379 https://www.sciencedirect.com/science/article/abs/pii/s1878818119307194#! j. electrochem. sci. eng. 12(6) (2022) 1041-1059 electrochemical strategies for detection of diazinon 1048 detection. the basic principle for electrochemical biosensors is that chemical reactions between the immobilized biomolecule and target analyte produce or consume electrons which cause some change in the measurable electrical properties of the solution, such as electric current, conductance, and potential [125-128]. electrochemical biosensors, as a subclass of biological sensors, consist of a biological sensing element and an electrochemical transducer. the recognition element (enzymes, antibodies, dna/rna, proteins or other biomolecules) reacts selectively with the target analyte, and as a result, an electrical signal is produced and then transmitted via the transducer to the signal processor [129-132]. a bioreceptor allows the binding between the specific analyte of interest with the sensing surface for measurement with minimum intervention from other components in a complex mixture. so, parallel development in the immobilization of bioreceptors through robust attachment methods like electrodeposition and nanoparticle-bound entities at the working electrode interface is a significant step in the improved application of biosensors in various analytes analysis. the selection of robust and suitable immobilization methodology and precise selection of the bioreceptor molecule plays a great part in the better specificity, selectivity, and affinity for their target analytes in electrochemical biosensing. this section introduced examples of developed electrochemical dzn biosensors. then, table 2 tabulates information about electrochemical biosensor, electrochemical methods, limit of detection (lod), and linear range, which have been reported by various works. somerset et al. constructed a biosensor for organophosphate and carbamate detection with a gold electrode coated with a mercaptobenzothiazole (mbt) self-assembled monolayer (sam) and a polyaniline derivative poly(o-methoxyaniline, poma) or poly(2,5-dimethoxyaniline, pdma) polymer film in the presence of polystyrene sulfonic acid (pssa), on which ache was immobilized (au/mbt/poma-pssa/ache or au/mbt/pdma-pssa/ache biosensors). the pesticide biosensors were applied in the aqueous phase detection of dzn and carbofuran pesticides using osteryoung swv and dpv at low frequencies. between 85 and 94 % inhibition of the au/mbt/pdma-pssa/ache and au/mbt/poma-pssa/ache biosensors, respectively, by 1.19 ppb of these neurotoxins attests to their potency. both au/mbt/pdma-pssa/ache and au/mbt/pomapssa/ache biosensors exhibited low detection limits, which were calculated using the inhibition methodology. the au/mbt/poma-pssa/ache biosensor exhibited lower detection limits of 0.07 ppb for dzn and 0.06 ppb for carbofuran than the au/mbt/pdma-pssa/ache sensor system that had detection limits values of 0.14 ppb for dzn and 0.11 ppb for carbofuran. the average sensitivity of the pesticide biosensor systems is 4.2 µa/ppb [133]. in 2007, albuquerque and ferreira used acetyl cellulose–graphite composite film modified with cobalt phthalocyanine (copc) to modify the electrode surface (copc-cgce). first, graphite powder was mixed homogeneously with copc. then, acetyl cellulose was applied to the mixture. after homogenization, the final composite was applied to polycarbonate support. tyrosinase and bovine serum albumin (bsa) were then immobilized on the modified electrode's surface (tyr-bsa-copc-cgce) by adding glutaraldehyde. the authors used the biosensor to measure organophosphorus and carbamates concentration. first, the immobilized tyrosinase enzyme activity was evaluated by measuring the quantity of catechol consumed or o-quinone formed in the reaction (figure 3). they monitored the increased concentration of o-quinone produced or the reduced concentration of the consumed catechol at a potential of -0.20 and +0.60 v, respectively. after stabilizing the background current, all amperometric measurements were carried out with the biosensor at a constant potential of -0.20 v versus ag/agcl as the counter and reference electrode. to detect pesticides using a biosensor containing tyrosinase, they measured the enzyme activity in two steps using a solution a. l. nejad j. electrochem. sci. eng. 12(6) (2022) 1041-1059 http://dx.doi.org/10.5599/jese.1379 1049 containing catechol. in the first step, the biosensor was submerged in a solution containing catechol. the enzyme on the surface of the electrode converted the catechol to o-quinone, and then o-quinone produced was measured at −0.20 v after 2 minutes. in the second step, the biosensor was submerged in the catechol solution after adding a given amount of pesticide, and the biosensor activity was evaluated after 2 minutes. the results show that methyl parathion and carbofuran can lead to a competitive inhibition process of the enzyme, while diazinon and carbaryl act as mixed inhibitors. linear relationships were found for methyl parathion (6.0–100.0 ppb), dzn (19.0 to 50.0 ppb), carbofuran (5.0–90.0 ppb) and carbaryl (10.0–50.0 ppb). analysis of natural river water samples spiked with 30 ppb of each pesticide showed recoveries between 92.50 and 98.50 % and relative standard deviations of 2 % [134]. figure 3. principle of the enzymatic and electrochemical coupled reactions for the detection of catechol as substrate [134] somerset et al. modified a gold disc electrode with a mercaptobenzothiazole selfassembled monolayer prior to polyaniline electropolymerization, followed by ache immobilization and polyvinyl acetate coating in creating a thick film electrode (au/mbt/pani/ache/pvac) for sensitive organophosphorous pesticide detection. the dual role of polyaniline shows electrocatalytic activity towards thiocholine and serves as an immobilization matrix for the ache as an enzyme, and that of polyvinyl acetate as a binder in this thick film electrode is demonstrated. relatively low detection limits were obtained for the au/mbt/pani/ache/pvac pesticide bioelectrodes using the inhibition methodology and an incubation time of 20 min. the values for the detection limits were 0.147 ppb for dzn and 0.172 ppb for fenthion in acetone-saline phosphate buffer solution [135]. zehani et al. developed two novel impedimetric biosensors for highly sensitive and rapid quantitative detection of dzn in an aqueous medium using two types of lipase, from candida rugosa (microbial source) (crl) and porcine pancreas (animal source) (ppl) immobilized on the functionnalized gold electrode (crl immobilized gold electrode, ppl immobilized gold electrode). lipase is characterized to specifically catalyze the hydrolysis of ester functions leading to the transformation of dzn into diethyl phosphorothioic acid (detp) and 2-isopropyl-4-methyl-6-hydroxypyrimidine (imhp). the developed biosensors both presented a wide range of linearity up to 50 μm with a detection limit of 10 nm for crl immobilized gold electrode biosensors and 0.1 μm for ppl immobilized gold electrode biosensor. a comparative study was carried out between the two biosensors and the results showed higher efficiency of crl immobilized gold electrode biosensor. http://dx.doi.org/10.5599/jese.1379 j. electrochem. sci. eng. 12(6) (2022) 1041-1059 electrochemical strategies for detection of diazinon 1050 moreover, it presented good accuracy and reproducibility and had very good storage and multipleuse stability for 25 days when stored at 4 °c [136]. arvand and dehsaraei constructed a chemically modified glassy carbon electrode by graphene oxide (go), functionalized double-strand dna (ds-dna) and gold nanorods (gnrs). interaction between oxygenated groups of go and gnrs with amine-thiol groups of ds-dna was used to construct a sandwich-modified electrode named gnrs/ds-dna/go/gce. then, they examined the response of the gnrs/ds-dna/go/gce to the sensing of dzn. to find the effects of modifiers on dzn detection, cv and electrochemical impedance spectroscopy were used in each modification step. the ph and scan rate were optimized at 6.0 and 0.1 v s−1, respectively. the dynamic range of gnrs/ds-dna/go/gce in dzn determination was studied by amperometry with the linear concentration range of 1.9 to 56 m and a detection limit of 0.19 m. finally, the application of modified electrode was evaluated on two polluted river water samples with 98.5 to 101 % recovery [137]. pajooheshpour et al. employed an enzyme-less electrochemical biosensor based on a glassy carbon electrode modified with bovine serum albumin (bsa) templated au-pt bimetallic nanoclusters and graphene nanoribbons (gnrs) (au-pt@bsagnrs/gce) for the determination of dzn. the synthesized novel sensing layer not only demonstrated attractive structural features but was known to possess excellent electrochemical characteristics, which enabled the development of a reliable, sensitive, robust and selective electrochemical sensor for dzn. the electrochemical properties of the biosensor were investigated by cv, square wave anodic stripping voltammetry (swasv) and electrochemical impedance spectroscopy. the results showed that au-pt@bsagnrs/gce significantly catalyzes the oxidation and reduction of dzn during electrochemical detection. the linear ranges of dzn were between 0.01 to 10.0 and 10.0 to 170.0 µm, with a detection limit of 0.002 µm. the results indicate the excellent capability of the method for the detection of dzn in real samples in comparison with the standard method [138]. hassani et al. developed a novel label-free electrochemical thiolated aptasensor immobilized on gold nanoparticle-modified screen-printed gold electrodes (aptamer/ au np/ spges) in order to detect minimum levels of dzn in biological samples with high sensitivity and fast performance, as illustrated in figure 4. figure 4. schematic construction of the designed aptasensor for detection of diazinon [139] electrochemical impedance spectroscopy and cv were used to characterize the electrochemical properties of the novel aptasensor. electrochemical detection was carried out through dpv in a. l. nejad j. electrochem. sci. eng. 12(6) (2022) 1041-1059 http://dx.doi.org/10.5599/jese.1379 1051 [fe(cn)6]3-/4solution. fluctuation of the current was examined in the dzn concentration range of 0.1 to 1000.0 nm. according to the results, the designed aptasensor has exhibited the minimum limit of detection for dzn (0.0169 nm) [139]. zare et al. developed an electrochemical biosensor for the detection of dzn. for this purpose, they modified the surface of a glassy carbon electrode with mwcnts and poly-l-lysine to immobilize a double-strain dna (ds-dna) on the surface of the electrode (dna/pll/mwcnts/gce). the interaction of dzn with ds-dna was transduced via electrochemical impedance and uv–vis spectroscopy in the first step. the results revealed an interaction between dzn and ds-dna. l-lysine as a polycation and small-sized mwcnts provides a surface with positive charges and a high surface area for the immobilization of ds-dna. this interaction leads to reduced interfacial charge-transfer resistance (rct). the difference in the rct before and after the interaction is considered a suitable signal for dzn detection. the proposed biosensor has a low detection limit (0.3 nm), a wide linear dynamic range (0.001‒100.0 µm), and high selectivity for the determination of dzn. finally, the performance of the biosensor for detecting dzn is verified in real samples such as river water, agricultural wastewater, lettuce juice, and tomato juice [140]. khosropour et al. [141] synthesized the vanadium disulfide quantum dots (vs2qds) by a facile hydrothermal method and doped them on the graphene nanoplatelets/carboxylated multiwalled carbon nanotubes (gnp/cmwcnts) as a new group of the nanocomposite. they incubated the vs2qds-gnp/cmwcnts nanocomposite on a glassy carbon electrode with the dzn binding aptamer (dzba) through electrostatic interaction (gce/vs2qdsgnp/cmwcnts/dzba). the gce/vs2qdsgnp/cmwcnts/dzba was used for the low detection of dzn by monitoring the oxidation of [fe(cn)6]3-/4as the redox probe. the characterizations of the modified electrode were performed by several electrochemical methods, including cv, dpv, and eis. the dzba selectively adsorbs dzn on the modified electrode, leading to a decrease and increase in the current of dpv and charge transfer resistance (rct), respectively, as analytical signals. the developed electrochemical aptasensor at the optimal conditions has a low lod equal to 1.1×10-5 and 2.0×10-6 nm with wide dynamic ranges of 5.0×10-5-10 nm and 10-5-10 nm for dpv and eis calibration curves, respectively. moreover, the analytical approach was further utilized for dzn determination in human serum, zayandehrood river water, soil, apple, and lettuce samples and the gce/vs2qds-gnp/cmwcnts/dzba/bsa aptasensing strategy exhibited a recovery rate from 97.0 to 107.0 % [141]. table 2. features of various electrochemical biosensors for the analysis of dzn electrochemical sensor electrochemical method lod linear range ref. au/mbt/pdma-pssa/ache swv 0.14 ppb [133] au/mbt/pomapssa/ache dpv 0.07 ppb tyr-bsa-copc-cgce amperometric 19.0–50.0 ppb [134] au/mbt/pani/ache/pvac amperometric 0.147 ppb [135] crl immobilized gold electrode impedimetric 10.0 nm 0.01-50.0 µm [136] ppl immobilized gold electrode 0.1 µm 0.1-50.0 µm gnrs/ds-dna/go/gce amperometric 190.0 nm 1.9 56.0 m [137] au-pt@bsagnrs/gce swasv 0.002 µm 0.01-170.0 µm [138] aptamer/ au np/ spges dpv 0.0169 nm 0.1-1000.0 nm [139] dna/pll/mwcnts/gce impedimetric 0.3 nm 0.001-100.0 µm [140] gce/vs2qdsgnp/cmwcnts/dzba/bsa impedimetric 2.0×10-1 nm 10-5-10.0 nm [141] dpv 1.1 ×10-14 m 5.0×10-5-10.0 nm http://dx.doi.org/10.5599/jese.1379 j. electrochem. sci. eng. 12(6) (2022) 1041-1059 electrochemical strategies for detection of diazinon 1052 conclusion diazinon is a pesticide belonging to the organophosphorus pesticides class, which is widely used in agriculture to control pests in soil, vegetables, and fruits. residues of pesticides can enter the food chain and cause many problems. dzn is considered one of the most important contaminants, and its amount in the environment, waters, and crops should be managed due to its toxicity and long halflife. thus, we need analytical methods to be able to detect the amount of that compound quickly. the electrochemical technique has many advantages for environmental research compared to other analytical techniques; for example, it does not require sample preparation steps and is fast, simple, cost-effective, and non-destructive. the limit of detection obtained by this technique possesses good sensitivity to the target molecules and environmental applications. in addition to sensitivity, the technique's specificity in recognizing the target molecule is critical. the challenging problems associated with promoting specificity and selectivity make the researchers seek new substances to modify electrodes and develop new electrochemical sensors. thus, different modifying methods have been used to recognize dzn, which are different in terms of the type of the working electrode, the applied technique, and the method of modifying the surface. from the above discussion, it is evident that nanomaterials, such as cnts, graphene, metal nanoparticles,etc., are promising materials for fabricating electrochemical sensors by signal amplification, thereby improving the sensitivity of the assay. nanotechnology is one of the likely areas to show encouraging prospects for developing sensors for the detection of dzn by overcoming the shortcomings of currently-available analytical procedures. although each method can detect dzn, it seems that the techniques based on biological components such as dna and enzymes have the lowest lod for recognition of dzn compared to other techniques. however, biological receptors allow for specific recognition of the target molecules and are widely used in the detection, bioassays, and chemical sensors. apart from these advantages, the real challenges for the future are those of good electrode materials, miniaturization and of measurements in as close to real-time as possible. another trend for future research on electrochemical sensors is to develop them for in vivo analysis and continuous testing as well as for in vitro testing. sensor arrays for detecting multianalyte will be required and the densities of arrays for more complete and rapid information need to increase. microfluidic sensor systems, capable of expanding sizes of arrays while reducing sample volume, as well as non-invasive biosensors, will revolutionize sensor techniques and technology. references [1] r. osterauer, h. kohler, aquatic toxicology 86 (2008) 485-494. https://doi.org/10.1016/ j.aquatox.2007.12.013 [2] g. k. sidhu, s. singh, v. kumar, d. s. dhanjal, s. datta, j. singh, critical reviews in environmental science and technology 49 (2019) 1135-1187. https://doi.org/10.1080/10 643389.2019.1565554 [3] q. long, h. li, y. zhang, s. yao, biosensors & bioelectronics 68 (2015) 168-174. https://doi.org/10.1016/j.bios.2014.12.046 [4] a. salehzadeh, r. abbasalipourkabir, b. shisheian, a. rafaat, a. nikkhah, t. rezaii, drug and chemical toxicology 42 (2019) 280-285. https://doi.org/10.1080/01480545.2018.14 49852 [5] d. g. karpouzas, k. b. advances in microbial physiology 51 (2006) 185-122. https://doi.org/10.1016/s0065-2911(06)51003-3 https://doi.org/10.1016/j.aquatox.2007.12.013 https://doi.org/10.1016/j.aquatox.2007.12.013 https://doi.org/‌10.1080/10‌643389.2019.1565554 https://doi.org/‌10.1080/10‌643389.2019.1565554 https://doi.org/10.1016/j.bios.2014.12.046 https://doi.org/10.1080/01480545.2018.1449852 https://doi.org/10.1080/01480545.2018.1449852 https://doi.org/10.1016/s0065-2911(06)51003-3 a. l. nejad j. electrochem. sci. eng. 12(6) (2022) 1041-1059 http://dx.doi.org/10.5599/jese.1379 1053 [6] national pesticide information center, diazinon, technical fact sheet. http://npic.orst.edu/ factsheets/archive/diazinontech.html (accessed september 17, 2022) [7] environmental risk assessment for diazinon, environmental protection agency, office of prevention, pesticides and toxic substances, office of pesticide programs, u.s. government printing office: washington dc, usa, 2000. 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https://doi.org/10.1007/s13738-019-01741-z [140] h. khosropour, b. rezaei, p. rezaei, a. a. ensafi, analytica chimica acta 1111 (2020) 92102. https://doi.org/10.1016/j.aca.2020.03.047 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1379 https://doi.org/10.3390/s80314000 https://doi.org/10.3390/s22062238 https://doi.org/10.3390/ijms222313138 https://doi.org/10.1016/j.bios.2021.113252 https://doi.org/10.1016/j.bios.2021.113252 https://doi.org/10.1016/j.bios.2020.112214 https://doi.org/10.1016/j.bios.2016.03.068 https://doi.org/10.1002%2fjcla.20500 https://doi.org/10.1016/j.chemosphere.2022.135515 https://doi.org/10.3390/s20113256 https://doi.org/10.1080/00032710600713834 https://doi.org/10.1016/j.aca.2007.06.013 https://doi.org/10.1080/03601230701229288 https://doi.org/10.3389/fchem.2014.00044 https://doi.org/10.1007/s11581-017-2373-6 https://doi.org/10.1007/s11581-017-2373-6 https://doi.org/10.1016/j.snb.2018.08.014 https://doi.org/10.1016/j.snb.2018.08.014 https://doi.org/10.1016/j.bios.2018.08.041 https://doi.org/10.1016/j.bios.2018.08.041 https://doi.org/10.1007/s13738-019-01741-z https://doi.org/10.1016/j.aca.2020.03.047 https://creativecommons.org/licenses/by/4.0/) @article{lohrasbinejad2022, author = {lohrasbi‑nejad, azadeh}, journal = {journal of electrochemical science and engineering}, title = {{electrochemical strategies for detection of diazinon:}}, year = {2022}, issn = {1847-9286}, month = {sep}, number = {6}, pages = {1041--1059}, volume = {12}, abstract = {diazinon (dzn) was first registered as an insecticide in the u.s. however, it was categorized in the limited group of pesticides due to its high toxicity for birds, aquatic animals, and humans. like other organophosphorus pesticides, this compound exhibits inhibitory effects on the acetylcholinesterase enzyme. the inhibition of the enzyme leads to the accumulation of acetylcholine and causes the death of insects. dzn is considered a toxic compound for humans due to its high adsorption via skin and inhalation, which leads to the emergence of different symptoms of toxicity. when dzn is used for plants, the compound residues in crops enter the food chain bringing about different health problems. moreover, the compound is easily washed by surface water and enters the groundwater. its entrance into aquatic envi­ronments can negatively affect a wide range of non-targeted organisms. thus, rese­archers seek to find fast and precise methods for the analysis of dzn. the electro­chemical method for recognizing the compound in real samples is preferable to other analytical methods. because this method can be used without spending time preparing the sample, it is simple, fast, and cost-effective. since such determinations may be made by using electro­chemical sensors and biosensors, numerous researchers have developed such sensors for dzn detection, and different sensitive materials were used in order to improve the selecti­vity, sensitivity, and detection limit. the present study aims to present the main progress and performance characteristics of electrochemical sensors and biosensors used to detect dzn, as it is reported in a number of relevant scientific papers published mainly in the last decade.}, doi = {10.5599/jese.1379}, file = {:d\:/onedrive/mendeley desktop/lohrasbi‑nejad 2022 electrochemical strategies for detection of diazinon(2).pdf:pdf;:www/jese_v12_no6_1041-1059.pdf:pdf}, keywords = {pesticide, electrochemistry, modified electrodes}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1379}, } didanosine determination in diluted alkaline electrolyte by adsorptive stripping voltammetry at the mercury film electrode doi: 10.5599/jese.2012.0018 133 j. electrochem. sci. eng. 2 (2012) 133-142; doi: 10.5599/jese.2012.0018 open access : : issn 1847-9286 www.jese-online.org original scientific paper didanosine determination in diluted alkaline electrolyte by adsorptive stripping voltammetry at the mercury film electrode percio augusto mardini farias, arnaldo aguiar castro*, ana isa perez cordoves* department of chemistry, pontifícia universidade católica do rio de janeiro, 22451-900, rio de janeiro, rj, brazil *fac. química, química analítica, universidad de la habana, cuba corresponding author: e-mail: pfarias@puc-rio.br; tel.: ++55 21 35271317 received: march 27, 2012; published: august 30, 2012 abstract a stripping method for the determination of didanosine at the submicromolar concentration levels is described. the method is based on controlled adsorptive accumulation of didanosine at thin-film mercury electrode followed by a linear cyclic scan voltammetry measurement of the surface species. optimum experimental conditions include a naoh solution of 2.0×10-3 mol l-1 (supporting electrolyte), an accumulation potential of -0.20 v, and a scan rate of 100 mv s-1. the response of didanosine is linear over the concentration range 0.01 – 0.10 ppm. for an accumulation time of 7 minutes, the detection limit was found to be 0.43 ppb (1.0×10-9 mol l-1). the more convenient relation to measuring the didanosine in presence of the metals ions, efavirenz, acyclovir, nevirapine, indinavir, nelfinavir, saquinavir, lamivudine and zidovudine were also investigated. the utility of the method is demonstrated by the presence of didanosine together with hypoxanthine, atp or dna keywords didanosine determination; metals ions; antiretroviral drugs; hypoxanthine; atp; dna; thin-film mercury electrode; linear cyclic scan stripping voltammetry. introduction didanosine (figure 1) is a nucleoside analogue of adenosine. it differs from other nucleoside analogues, because it does not have any of the regular bases, instead it has hypoxanthine attached to the sugar ring. within the cell, didanosine is phosphorylated to the active metabolite of dideoxyadenosine triphosphate (ddatp) by cellular enzymes. like other anti-hiv nucleoside j. electrochem. sci. eng. 2(3) (2012) 133-142 didanosine determination by stripping voltammetry 134 analogs, it acts as a chain terminator by incorporation and inhibits viral reverse transcriptase by competing with natural datp. [1-3]. figure 1. didanosine; [9-[(2r,5s)-5-(hydroxymethyl)oxolan-2-yl]-6,9-dihydro-3h-purin-6-one] several chromatographic [4-11] analytical methods have been developed for the determination of didanosine but only one electroanalytical work was reported [12]. with the recent advances in capabilities of the adsorptive stripping voltammetry, new methodologies have been developed for adenine, thymine, guanine, atp, dna and antiretroviral drugs using alkaline solution with low ionic strength as the supporting electrolyte [13-20]. the present work reports a new stripping voltammetric procedure for trace detection of didanosine based on its adsorption at the thin film mercury electrode. the advantages, instrumental parameters, and possible limitations of this procedure are also explained in this paper. furthermore, the effects of a wide range of potentially interfering compounds such as efavirenz, acyclovir, nevirapine, indinavir, nelfinavir, saquinavir, lamivudine, zidovudine, some metal ions, hypoxanthine, atp and dna were examined. experimental linear cyclic voltammograms were obtained with an eg&g par model 384-b polarographic analyser (princeton applied research, princeton, nj, usa), equipped with an external cell and a houston ametek-dmp-40 series digital plotter. the working electrode was a glassy carbon electrode (gce, 3.0 mm diameter, basbioanalytical systems, inc., west lafayette, indiana 47906, usa) with thin-film mercury, an ag/agcl reference electrode with vycor tip and a platinum auxiliary electrode. a magnetic stirrer and stirring bar (nalgene cat. no. 6600-0010, rochester, ny, usa) provided convective transport during the accumulation step. stock solutions of 10-2 mol l-1 hg(no3)2 were initially prepared by dissolution of 0.4 g of mercury (ii) nitrate in 100 ml of an acidified milli-q water (5 % of hno3). the thin mercury film was formed using a glassy carbon electrode (gce, bas) initially polished with alumina (pk-4, bas) and then mounted with the help of a teflon holder in a voltammetric cell provided with an ag/agcl reference electrode, a platinum auxiliary electrode, and containing 1 ml of stock mercury(ii) nitrate solution, 1 ml of 10-1 mol l-1 potassium nitrate solution and 8 ml of purified water. the solution was purged with nitrogen for 240 s to eliminate the oxygen initially present. mercury plating was carried over for 5 min at a cell of -0.9 v. after checking that the electrode was plated properly the set of electrodes was rinsed with water and a new clean cell containing the analyte solution was fitted. each thin mercury film formed on glassy carbon electrode is stable during two hours [18-20]. water purified in a milli-q purification system (millipore, billerica, ma, usa) was used for all dilutions and sample preparations. all chemicals were analytical reagent grade. didanosine p. a. mardini farias et al. j. electrochem. sci. eng. 2(3) (2012) 133-142 doi: 10.5599/jese.2012.0018 135 standard was used as received by the instituto nacional de controle de qualidade em saúde brazil (incqs – lot l1, 100.2%). stock solutions of 1000 ppm were prepared dissolving 50 mg of the target reagent didanosine into 5 ml of 2 mol l-1 naoh and water until a volume of 50 ml was reached. diluted didanosine solutions of 100 or 10 ppm were prepared daily using 5 ml of 1000 or 100 ppm didanosine into water until 50 ml was reached. stock solutions of acyclovir were prepared in accordance with didanosine procedure. stock solutions of 1000 ppm of other hiv drugs were prepared dissolving 50 mg of the target reagent into 5 ml of 2 mol l-1 naoh, 5 ml of ethylic alcohol and water until a volume of 50 ml was reached. a 1000 ppm copper and other metals ions stock solutions (atomic absorption standard solution, sigma-aldrich brasil ltda.) were used, and diluted as required for standard additions. sigma chemicals (sigma-aldrich brasil ltda., são paulo-sp, brasil) stock solutions of 1000 ppm were prepared by dissolving 25 mg of the target reagent hypoxanthine plus solid naoh with 25 ml of water (to achieve a final concentration of 0.1 mol l-1 naoh). stock solutions of 1000 ppm of adenosine 5’-triphosphate, disodium salt hydrate (atp) was prepared by dissolving 10 mg of the target reagent in 2 ml of diluted perchloric acid (10-1 mol l-1) with an subsequent solution heating at 70 oc during 30 seconds. after heating, the sample was cooled down, and diluted to 10 ml with water. single-stranded calf thymus dna (cat. no. d-8899; lot 43h67951) was used as received from sigma. a 500 μg dna ml-1 stock solution (around 5 mg 10 ml-1; lyophilized powder containing 63% dna) was prepared according to atp procedure. the solutions were stored in the dark at 4 oc. a known volume (10 ml) of the supporting electrolyte solution (2.0×10-3 mol l-1 sodium hydroxide) was added to the voltammetric cell and degassed with nitrogen for 8 min (and for 60 seconds before each adsorptive stripping cycle). initially, the condition potential (usually -0.9 v) was applied to the electrode for a selected time (usually 60 s). the conditioning potential is used for cleaning the surface of the thin mercury electrode. afterwards the initial potential (usually -0.20 v) was applied to the electrode with a selected time (usually 240 s), while the solution was slowly stirred. the stirring was then stopped, and after 30 s, the voltammogram was recorded by applying a negative-going potential scan. the scan (usually at 100 mv s-1) was terminated at -0.90 v, and the adsorptive stripping cycle was repeated with the same thin-film mercury. after the background stripping voltammograms had been obtained, aliquots of the didanosine standards were introduced. the entire procedure was automated, as controlled by 384-b polarographic analyser. throughout this operation, nitrogen was passed over the solution surface. all data were obtained at room temperature (25 oc). results and discussion figure 2 compares differential-pulse, linear-scan and linear cyclic adsorptive stripping voltammograms for 0.05 ppm didanosine in a 2.0×10-3 mol l-1 naoh solution, after 120 seconds preconcentration with stirring at -0.20 v. a mercury film was use as work electrode. after equilibrium time of 30 s the differential pulse (a), linear (b) or cyclic cathodic voltammogram (c) were recorded at 25 (a) or 100 (b,c) mv s-1. both scan modes offer excellent signal-to-background characteristics but linear scan offers higher current peak and greater speed, and is recommended for the determination of didanosine. the linear cyclic stripping mode yield well-defined peak, with half-width (b½) of 130 mv, and was used throughout the further study. the didanosine cyclic cathodic peak of 2407 na (ip) appears at -0.56v (ep). no anodic peaks were observed in the first scan. j. electrochem. sci. eng. 2(3) (2012) 133-142 didanosine determination by stripping voltammetry 136 figure 2. differential-pulse (a), linear-scan (b) and adsorptive cyclic (c) voltammograms of 0.05 ppm didanosine in 2.0 x 10-3 mol l-1 naoh. conditioning time, 60 s at -0.90 v; accumulation time, 120 s at -0.20 v with stirring; amplitude pulse, 50 mv (a); scan rate, 25 (a) and 100 (b,c) mv s-1; thin-film mercury electrode several other chemical and instrumental parameters such as supporting electrolyte, ph, accumulation potential and time, and scan rate, which directly affect the didanosine adsorptive stripping peak response, were optimized. the adsorption properties of the didanosine can vary with the composition of the supporting electrolyte. various electrolytes, e.g. briton-robinson buffer, phosphate buffer and naoh solution, were evaluated as suitable media for the adsorptive stripping measurement of didanosine. best results (with respect to signal enhancement and reproducibility) were obtained in naoh electrolyte. this alkaline medium was employed throughout this study. the adsorptive stripping signal of didanosine depends on the sample ph. figure 3a, shows the dependence of the didanosine peak current on the solution ph (from 2 to 12). no response to didanosine was observed in solutions more acidic than ph 7. increasing ph from 8 to 11 resulted in a rapid increase in the didanosine peak current. however, the stability of didanosine peak in aqueous solutions decreases with ph above of 11. accordingly, the solution of approximately ph 11 was used throughout to satisfy the sensitivity and stability requirements. the effect of accumulation potential was observed from +0.05 to -0.40 v and the highest didanosine peak currents were observed at the accumulation potential of -0.20 v (figure 3b). the peak current (ip) for the surface-adsorbed didanosine is directly proportional to the scan rate (υ) (figure 3c). a plot of ip vs. υ was linear (correlation coefficient, 0.998), with a slope of 54.4, over the 10-200 mv s-1. the dependence of the linear cyclic current peak with the pre-concentration time was verified. for longer accumulation time, more didanosine is adsorbed and higher peaks current appears. such time-dependent profiles represent the corresponding adsorption isotherms because the peak current depends on the amount adsorbed. initially the current increases linearly (0-180 s), and then drops at longer accumulation times. the resulting plot of peak current vs. accumulation time (0-180 s) show a slope of 36.03 na s-1 and a correlation coefficient of 0.996. figure 4 shows two didanosine voltamogramms with accumulation time of 120 (a) and 240 (b) seconds. with 240 s of pre-concentration time the peak current for 1.0 ppm of didanosine was about 2.1 times larger than the corresponding peak obtained with a 120 s response. p. a. mardini farias et al. j. electrochem. sci. eng. 2(3) (2012) 133-142 doi: 10.5599/jese.2012.0018 137 figure 3. a effect of ph on the current peak height of the linear cyclic adsorptive stripping voltammograms of 1.0 ppm of didanosine in solution. condition time, 60 s at -0.90 v; accumulation time 240 s at -0.30 v; final potential, -0.9 v; scan rate, 50 mv s-1; equilibrium time, 30 s; thin-film mercury electrode (5 min at -0.9 v). b effect of accumulation potential on the current peak height of the linear cyclic adsorptive stripping voltammograms of 0.05 ppm didanosine in 2.0×10-3 mol l-1 naoh. accumulation time, 120 s; scan rate, 100 mv s-1. other conditions same as 3a. c effect of scan rate on the current peak height of the linear cyclic voltammograms of 1.0 ppm didanosine in 2.0×10-3 mol l-1 naoh. accculutation time, 240 s at -0.20 v. other conditions same as 3a. j. electrochem. sci. eng. 2(3) (2012) 133-142 didanosine determination by stripping voltammetry 138 figure 4. increasing current effects of accumulation time at -0.20 v on the linear cyclic adsorptive stripping voltammograms of 1.0 ppm of didanosine in 2.0 x 10-3 mol l-1 naoh. the (a) and (b) curve is relative at accumulation time of 120 and 240 s. conditioning time, 60 s at -0.90 v; scan rate, 100 mv s-1 ; equilibrium time, 30 s; thin-film mercury electrode (5 min at -.9 v). the effective pre-concentration (accumulation time) associated with the adsorption process results in significant lowering of the detection limit compared to the corresponding solution measurements. a detection limit of 0.43 ppb (1.0×10-9 mol l-1) was estimated from quantification of 0.01 ppm after 7 min accumulation (s/n = 2). thus, 4 ng could be detected in the 10 ml of solution used. the reproducibility was estimated by ten successive measurements in stirred 0.05 ppm didanosine solution (other conditions: supporting electrolyte, 0.002 mol l-1 naoh; conditioning time, 60 s at -0.9 v; accumulation time, 180 s at 0.2 v; final potential, -0.9 v; scan rate, 100 mv s-1; equilibrium time, 30 s and thin-film mercury electrode). the mean peak current was 3255 na with a range of 3111 3481 na and a relative standard deviation of 2.6 %. such precision compares similarly with those reported for other compounds measured by adsorptive stripping analysis [1320]. the ep and the b1/2 remain the same at -0.56 v and 130 mv, respectively. figure 5 shows voltammograms for increasing didanosine concentration (a: 0.005, b: 0.030 and c: 0.050 ppm) after 300 s accumulation. well-defined stripping peaks (at -0.56 v) were observed from 0.005 to 0.050 didanosine concentration range. the resulting plot of peak current versus concentration is linear (slope 22550 na ppm-1; correlation coefficient, 0.992). such linearity prevails as long as linear isotherm conditions (low surface coverage) exist. a separate experiment was performed to test linearity at high concentration range resulting also in well-defined stripping peaks between 0.2 and 0.4 ppm didanosine (accumulation time, 180 s at -0.3 v; scan rate, 50 mv s-1; final p. a. mardini farias et al. j. electrochem. sci. eng. 2(3) (2012) 133-142 doi: 10.5599/jese.2012.0018 139 potential, -0.9 v; other conditions same as in figure 5). the resulting plot of peak current versus concentration also showed linearity (slope 751 na ppm-1; correlation coefficient, 0.997). figure 5. linear cyclic scan adsorptive stripping voltammograms obtained after increasing the didanosine concentration in a solution of 2.0 x 10-3 mol l-1 naoh (with 1% v/v of ethylic alcohol). the (a), (b) and (c) curve is relative at 0.005, 0.030 and 0.050 ppm of didanosine concentration. accumulation time, 300 s at -0.20 v. condition time, 60 s at -0.9 v. scan rate, 100 mv s-1. final potential at -0.9 v. equilibrium time, 30 s. thin-film mercury electrode (5 min at -0.9 v). practical applications of differential pulse adsorptive stripping analysis may be affected by interferences due to the presence of ions and/or surface active compounds. with respect to the surface reaction, double layer charge or direct interactions among these substances may inhibit or aid in the accumulation of the analyte. measurements of 0.05 ppm didanosine were not affected by addition of up to 0.01 cobalt(ii); up to 0.06 ppm nickel(ii) and up to 0.10 ppm cadmium(ii), lead(ii), zinc(ii), copper(ii) and iron(iii). using higher concentration a perceptible negative shift of didanosine peak was observed by presence of nickel (ii) or cobalt (ii) indicating a possible formation of nior co-didanosine complex. more dramatic interference was observed with cobalt(ii). the addition of 0.03 ppm cobalt(ii) resulted in a 5-fold increase of didanosine peak height. the metal ions chosen to be used in this study were based on their importance to human life. cu, fe, co and zn are essential while pb and cd are considered toxic to human life, since ni is a genetic modifier. preliminary studies were carried out for the determination of didanosine in presence of others antiretroviral drugs routinely used for the treatment of a human immunodeficiency virus (hiv): efavirenz, acyclovir, nevirapine, indinavir, nelfinavir, saquinavir, lamivudine and zidovudine. these antiretroviral drugs were chosen because they are produced in brazil and have low costs. measurements of 0.05 ppm didanosine were not affected by addition of up to 0.01 ppm of lamivudine or zidovudine; up to 0.03 ppm of neviparine; up to 0.04 ppm of indinavir; up to 0.08 ppm of nelfinavir; up to 0.10 ppm of acyclovir and saquinavir. the more dramatic interference was observed with efavirenz. the addition of 0.03 ppm efavirenz resulted in j. electrochem. sci. eng. 2(3) (2012) 133-142 didanosine determination by stripping voltammetry 140 a twofold increase in didanosine peak height. a linear scan adsorptive stripping voltammogram for 0.1 ppm of free efavirenz in a 2×10-3 mol l-1 naoh solution showed a cathodic peak at -0.36v. a perceptible negative shift of didanosine peak was observed by presence of nelfinavir. figure 6 illustrates the method’s suitability for the determination of didanosine through linear cyclic adsorptive stripping voltammetry in a synthetic sample contained contain several antiretroviral drugs (efavirenz, nevirapine, didanosine, lamivudine, zidovudine and acyclovir; all with 2.0 ppm of concentration). four successive standard additions to sample resulted in wellshaped adsorptive stripping peaks. the didanosine peak in the original sample (curve a) can, thus, be quantified based on the resulting standard addition plot (also show in figure 6). because of the inherent sensitivity of the method, short (60 s) accumulation times could be used. five consecutive analysis of sample yielded an average value of 2.5 ppm with standard deviation of the 0.4 ppm. this is in “relative” agreement with the didanosine value (2.0 ppm). figure 6. illustration of didanosine determination in a synthetic sample containing several antiretroviral drugs (efavirenz, nevirapine, didanosine, lamivudine, zidovudine and acyclovir; all with 2 ppm of concentration) by linear cyclic adsorptive stripping voltammetry. supporting electrolyte, 10 ml of 2.0×10-3 mol l-1 naoh. a addition of 40 μl of synthetic sample; b -0.025 ppm and c 0.20 ppm addition of standard didanosine. condition time, 60 s at -0.9 v. accumulation time, 60 s at -0.30 v. final potential, -0.9 v. scan rate, 100 mv s-1. equilibrium time, 30 s. thin-film mercury electrode (5 min at -0.9 v). preliminary studies were also carried out for the determination of didanosine in the presence of hypoxanthine, atp and dna. the didanosine is a nucleoside analogue of adenosine [1-3]. the current measurements of 0.05 ppm didanosine (other conditions: supporting electrolyte, p. a. mardini farias et al. j. electrochem. sci. eng. 2(3) (2012) 133-142 doi: 10.5599/jese.2012.0018 141 2×10-3 mol l-1 naoh; condition time, 60 s at -0.9 v; accumulation time, 120 s at -0.2 v; final potential, -1.10 v; scan rate, 100 mv s-1; the equilibrium time, 30 s and thin-film mercury electrode) were not affected by the addition of up to 0.10 ppm of dna. in another experiment, after the addition of 0.10 ppm of dna and copper(ii) in 0.05 ppm of didanosine (with a potential peak at -0.56 v), a cu(ii) anodic and dna cathodic peaks were observed at -0.20 and -0.75 v, respectively (figure 7). figure 7. linear cyclic pulse adsorptive stripping voltammogram of 0.05 ppm didanosine (a) in 0.10 ppm copper(ii) and dna presence (b) in a solution of 2.0 x 10-3 mol l-1 naoh. (a) represent the didanosine, (b) the dna and (c) the copper (ii) peak. accumulation time, 120 s at 0.20 v. condition time, 60 s at -0.9 v. scan rate, 100 mv s-1. final potential at -1.1 v. equilibrium time, 30 s. thin-film mercury electrode (5 min at -0.9 v). the current measurements of 0.05 ppm didanosine (other conditions: same as dna experiment) were not affected by addition of up to 0.10 ppm of atp. in another experiment, after the addition of 0.10 ppm of atp and copper(ii) in 0.05 ppm of didanosine (with a potential peak at -0.52 v), a cu(ii) anodic and atp cathodic peak were observed in -0.15 and -0.75 v, respectively. the current measurements of 0.05 ppm didanosine (final potential, -1.20 v; other conditions the same as dna experiment were not affected by addition of up to 0.10 ppm of hypoxanthine. in another experiment, after the addition of 0.10 ppm of hypoxanthine and copper(ii) in 0.05 ppm of didanosine (with a potential peak at -0.52 v), a cu(ii) anodic and hypoxanthine cathodic peaks were observed in -0.30 and -1.10 v, respectively. without the didanosine presence, the hypoxanthine, atp and dna also showed similar behavior in presence of copper(ii) [13-20]. conclusions an effective means for the determination of trace levels of didanosine has been described. the use of a diluted alkaline electrolyte provided a sensitive and selective adsorptive stripping voltammetric method for didanosine determination. interference studies indicate the possibility of the determination of didanosine in presence of others antiretroviral drugs for the treatment of a human immunodeficiency virus (hiv), like acyclovir, nevirapine, indinavir, nelfinavir and j. electrochem. sci. eng. 2(3) (2012) 133-142 didanosine determination by stripping voltammetry 142 saquinavir. in particular, this approach offers similar efficiency to chromatographic methods. the didanosine peak increases in the presence of nickel(ii) or cobalt(ii) indicating a possible formation of ni or co-didanosine complex. the didanosine with the dna peak are separated by 0.19 v and with the atp by 0.23 v. also, further studies using diluted alkaline solution as supporting electrolyte and film mercury electrode modified in situ by metallic ions can be used for detection of other drugs, and dna-intercalating dyes, as well as amino-acids, peptides and proteins determinations. acknowledgements: the authors gratefully acknowledge the capes-brazil and mes-cuba for their support of this work. in addition, we thank dr. katia cristina leandro of fundação oswaldo cruz for generously supplying the sample of didanosine. references [1] e. de clercq, j. clin. virol. 30 (2004) 115-133. [2] e. de clercq, int. j. biochem. cell b. 36 (2004) 1800-1822. [3] sweetman, in: s.c. sweetman, editor, martindale the complete drug reference (35 th ed.), pharmaceutical press (2007). [4] r.d.e. estrela, m.c. salvadori, r.s.l. raices, g. suarez-kurtz, j.mass. spectrom. 38 (2003) 378-385. [5] y.huang, e. zurlinden, e. lin, x.h. li, j. tokumoto, j. golden, a. murr, j. engstrom, j. conte, j. chromatogr.b 799 (2004) 51-61. [6] a.m.c. de oliveira, t.c.r. lowen, l.m. cabral, e.m. dos santos, c.r. rodrigues, h.c. castro, t.c. dos santos, j.pharmaceut. biomed. 38 (2005) 751-756. [7] c.p.w.g.m. verweij-van wissen, r.e. aarnoutse, d.m. burger, j. chromatogr.b 816 (2005) 121-129. [8] t.n. clark, c.a. white, m.g. bartlett, biomed. chromatogr. 20 (2006) 605-611. [9] a. s. kumari, k. prakash, k. e. v. nagoji, m. e. b. rao, asian j. chem. 19 (2007) 2633-2636. [10] s. notari, c. mancone, t. alonzi, m. tripodi, p. narciso, p. ascenzi, j. chromatogr.b 863 (2008) 249-257. [11] c.v. kumar, d. anantantahakumar, k. madhuri, j.v.l.n.s. rao, v.l.n. seshagiri, asian j. chem. 23 (2011) 583-586. [12] k.i. ozoemena, r.i. stefan-van staden, t. nyokong, electroanal. 21 (2009) 1651-1654. [13] p.a.m. farias, a. de l.r. wagener, a.a. castro, anal. let. 34 (2001) 2125-2140. [14] p.a.m. farias, a. de l.r. wagener, a.a. castro, anal. let. 34 (2001) 1295-1310. [15] p.a.m. farias, a. de l.r. wagener, a.a. junqueira, a.a. castro, anal. let.. 40 (2007) 17791790. [16] p.a.m. farias, a.a. castro, wagener, a.de l.r., a.a. junqueira, electroanal. 19 (2007) 12071212. [17] p.a.m. farias, k.c. leandro, j.c. moreira, anal. let.. 43 (2010) 1951-1957. [18] a.a.castro, m.v.n. de souza, n.a. rey, p.a.m. farias, j. braz.chem. soc. 22 (2011) 16621668. [19] a.i.p. cordoves, p.a.m. farias, curr. pharm. anal. 7 (2011) 71-78. [20] a.a. castro, r.q. aucelio, n.a. rey, e.m. miguel, p.a.m. farias, comb. chem. high t.scr. 14 (2011) 22-27. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) << /ascii85encodepages false /allowtransparency false /autopositionepsfiles true /autorotatepages /none /binding /left /calgrayprofile (dot gain 20%) /calrgbprofile (srgb iec61966-2.1) /calcmykprofile (u.s. web coated \050swop\051 v2) /srgbprofile (srgb iec61966-2.1) /cannotembedfontpolicy /error /compatibilitylevel 1.4 /compressobjects /tags /compresspages true /convertimagestoindexed true /passthroughjpegimages true /createjobticket false /defaultrenderingintent /default /detectblends true /detectcurves 0.0000 /colorconversionstrategy /cmyk /dothumbnails false /embedallfonts true /embedopentype false /parseiccprofilesincomments true /embedjoboptions 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can be opened with acrobat and adobe reader 5.0 and later.) >> /namespace [ (adobe) (common) (1.0) ] /othernamespaces [ << /asreaderspreads false /cropimagestoframes true /errorcontrol /warnandcontinue /flattenerignorespreadoverrides false /includeguidesgrids false /includenonprinting false /includeslug false /namespace [ (adobe) (indesign) (4.0) ] /omitplacedbitmaps false /omitplacedeps false /omitplacedpdf false /simulateoverprint /legacy >> << /addbleedmarks false /addcolorbars false /addcropmarks false /addpageinfo false /addregmarks false /convertcolors /converttocmyk /destinationprofilename () /destinationprofileselector /documentcmyk /downsample16bitimages true /flattenerpreset << /presetselector /mediumresolution >> /formelements false /generatestructure false /includebookmarks false /includehyperlinks false /includeinteractive false /includelayers false /includeprofiles false /multimediahandling /useobjectsettings /namespace [ (adobe) (creativesuite) (2.0) ] /pdfxoutputintentprofileselector /documentcmyk /preserveediting true /untaggedcmykhandling /leaveuntagged /untaggedrgbhandling /usedocumentprofile /usedocumentbleed false >> ] >> setdistillerparams << /hwresolution [2400 2400] /pagesize [612.000 792.000] >> setpagedevice {electrochemical analysis of indigo carmine using polyarginine modified carbon paste electrode} http://dx.doi.org/10.5599/jese.953 87 j. electrochem. sci. eng. 11(2) (2021) 87-96; http://dx.doi.org/10.5599/jese.953 open access: issn 1847-9286 www.jese-online.org original scientific paper electrochemical analysis of indigo carmine using polyarginine modified carbon paste electrode d’souza s. edwin1,2, jamballi g. manjunatha1,, chenthattil raril1, tigari girish1, doddarasinakere k. ravishankar3 and huligerepura j. arpitha4 1department of chemistry, fmkmc college, madikeri, constituent college of mangalore university, karnataka, india 2department of chemistry, st. philomena college, puttur, karnataka, india 3department of chemistry, sri. mahadeshwara govt. first grade college, kollegal, chamarajanagar, karnataka, india 4department of physics, sri adichunchanagiri first grade college, channarayapatna, india corresponding author: manju1853@gmail.com; tel.: +9108272228334 received: january 13, 2021; revised: march 18, 2021; accepted: march 29, 2021 abstract suitable electrocatalytic method is established for the selective determination of indigo carmine (ic) at polyarginine modified carbon paste electrode (pamcpe). surface morphological study of bare carbon paste electrode (cpe) and pampce is done by field emission scanning electron microscopy (fesem). the influence of different parameters such as ic concentration, solution ph and potential scan rate on the electrode responses is studied using cyclic and differential pulse voltammetry techniques. the prepared pamcpe shows better electrochemical response towards ic than cpe. no interference is noticed at simultaneous presence of ic and riboflavin (rf) in the solution. the electrocatalytic current of ic at pampce is varied linearly with its concentration in two separate ranges, from 2×10-7 to 10-6 m, and 1.5×10-6 to 3.5×10-6 m. limits of detection (lod) and quantification (loq) are determined as 2.53×10-8 and 8.43×10-8 m, respectively. the developed pamcpe is showing successful reproducibility and stability. it is also found sensitive and reliable for trace amounts of ic in some real water and food samples. since preparation of pamcpe sensor is simple and easy, it could become a part of the standard method for determination of ic in real samples. keywords carbon paste; electropolymerization; food dye; real samples; cyclic voltammetry; differential pulse voltammetry http://dx.doi.org/10.5599/jese.953 http://dx.doi.org/10.5599/jese.953 http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 11(2) (2021) 87-96 analysis of indigo carmine 88 introduction dyes are the colored compounds of either synthetic or natural origin. synthetic dyes are used in various industries like printing textiles, paint, cosmetics, pulp, and even in food industry [1]. their degradation products are highly poisonous and dangerous to all living beings [2]. indigo carmine (ic) (5,5′ indigodisulphonic acid sodium salt) is one such dyes used in cosmetic industry [3], food industry and in coloring of polyester fibers [4,5]. it is toxic in nature and can cause diarrhea and vomiting in human beings [6]. in analytical chemistry ic is used as redox indicator [7], food colorant, and in biology as microscopic stain [8]. the textile industry releases waste-water containing significant amounts of non–biodegradable dyes [9,10]. these dyes are carcinogenic and their removal from the industrial effluents is of the major environmental concern [11]. ic is also utilized in medical diagnosis apart from its use as an additive in pharmaceutical tablets and capsules [12]. presence of ic in environment and real samples has to be traced, because ic ingestion can affect the liver, central nervous system, kidneys and eyes [13,14]. therefore, it is proposed obligatory to identify the concentration of ic in a variety of natural samples. hence, a susceptible and selective procedure for determination of ic would be of immense significance. variety of investigative methods have been selected for the identification of synthetic dyes, such as high-performance liquid chromatography with uv-vis, spectrophotometry, stripping voltammetry capillary electrophoresis, uv-dad or ms detectors [15-19]. recently, the cyclic voltammetric technique was introduced for procuring information regarding the electrochemical reactions qualitatively and quantitatively. cv provides a lot of information concerning thermodynamics and kinetics of redox reactions [20-25]. different electrodes were already applied to explore the performance of ic [26-27]. among them, carbon paste electrode in its unmodified or modified form is frequently used to establish the performance of several chemical compounds, including electrochemical behavior of ic [28-30]. the purpose of this work is to deploy a better and easier voltammetric method for the assessment of ic. by using polymer modified carbon paste electrode, a direct, perceptive, prompt and entirely validated method, based on the voltametric response and oxidation mechanism of ic is intended to be derived. the oxidation mechanism of ic is shown in scheme 1. scheme 1. structure and oxidation reaction mechanism of ic experimental reagents arginine and indigo carmine were purchased from molychem, mumbai, india. graphite (150 mesh, loss on drying = 0.5 % at 120 oc, residue on ignition = 1 % at 800 oc), monosodium e. s. d’souza et al. j. electrochem. sci. eng. 11(2) (2021) 87-96 http://dx.doi.org/10.5599/jese.953 89 dihydrogen phosphate, disodium hydrogen phosphate, silicone oil (kinematic viscosity = 1000 cs, refractive index = 1.4035) were obtained from nice chemicals, cochin, india. stock solution of ic (25×10-5 m) and arginine (25×10-3 m) were made ready in double distilled water. standard solutions of 0.1 m monosodium dihydrogen phosphate and 0.1 m disodium hydrogen phosphate were mixed in intended proportion to get 0.1 m phosphate buffer solution (pbs). instrumentation electrochemical analyzer chi-6038e (usa) was used to perform all electrochemical experiments. electrochemical cell was the three – electrode system, equipped with either cpe or pamcpe as the working electrode, platinum wire as the auxiliary electrode, and standard calomel electrode (sce) as the reference electrode. field emission scanning electron microscopy (fesem) from dst – purse laboratory, mangalore university, was used to investigate morphology of electrode surfaces. all measurements were done at the laboratory temperature. preparation of bare carbon paste electrode (cpe) carbon paste was developed by mixing the graphite powder and a binder (silicone oil) in the proportion of 60:40 (w/w) using a mortar and pestle, until a consistent paste is formed. a segment of the paste was crammed firmly into the cavity of a teflon tube having 3 mm inner diameter. the electrode surface was polished using a tissue paper to get a smoother surface. the electrical contact was established by connecting a copper wire to the paste end of the tube. the electrode surface of thus formed carbon paste electrode (cpe) was revived for every measurement. the surface of cpe was modified by polyarginine forming pamcpe, which was prepared according to the electrochemical polymerization procedure described below. results and discussion preparation of polyarginine modified carbon paste electrode arginine solution of 10-3 m was placed in electrochemical cell containing 0.1 m pbs of ph 5.7. the polymer film on cpe was developed by electrochemical polymerization of arginine using cyclic voltammetry in the potential range from -0.25 to 1.5 v, for 10 cycles at the sweep rate of 0.1 v s-1. the cyclic voltammograms produced are presented in figure 1. prior further experiments, the formed polyarginine modified electrode (pamcpe) was rinsed with distilled water to remove the unreacted monomer. figure 1. cvs of electrochemical polymerization of 10-3 m arginine on cpe in 0.1 m pbs, ph 5.7, in 10 cycles with sweep rate 0.1 v s-1 http://dx.doi.org/10.5599/jese.953 j. electrochem. sci. eng. 11(2) (2021) 87-96 analysis of indigo carmine 90 surface characterization of cpe and pamcpe the surface morphological investigations of cpe and pamcpe were assessed by field emission scanning electron microscopy (fesem). in figure 2, the surface morphological examination outcomes of bare (figure 1a) and polymer modified electrode (figure 1b) are indicated. the image of cpe shows an irregular arrangement of graphite layers on its surface, whereas the image of pamcpe is more compact and contains more uniform arrangement of graphite layers on its surface, which is due to presence of deposited polyarginine. it may be concluded that bare and polymer modified electrodes possess rather different surface morphology, what might have some influence on their electrochemical responses. figure 2. fesem images of (a) cpe and (b) pamcpe electrochemical behavior of ic the electrochemical activity of ic at both electrodes was investigated through cyclic voltammetry (cv) and differential pulse voltammetry (dpv) techniques in 0.1 m pbs, ph 6.0. differential pulse and cyclic voltammograms recorded for 10-5 m ic during the scrutiny of cpe and pamcpe are represented in figure 3. figure 3a exhibits oxidation peaks attained during dpv investigation of ic activity at cpe and pamcpe. in the potential range from 0.25 to 0.50 v, well defined peaks are detected for both electrodes, but the peak current response detected for the polymer modified carbon paste electrode is larger than the current of the bare electrode. enhanced peak current and shift of anodic peak potential epa towards less positive value, suggest some electrocatalytic action of pamcpe surface to ic oxidation. a b figure 3. redox activity of 10-5 m ic at cpe and pamcpe in 0.1 m pbs, ph 6.0, recorded by: (a) dpv at 0.05 v s-1 and (b) cv at 0.1 v s-1 e. s. d’souza et al. j. electrochem. sci. eng. 11(2) (2021) 87-96 http://dx.doi.org/10.5599/jese.953 91 cv responses of ic at cpe and pamcpe are represented in figure 3b. within the potential from 0.20 to 0.60 v, small redox peaks are observed for cpe at about 0.4 v. for pamcpe, however, sensitive and distinct oxidation and reduction peaks appear at 0.394 and 0.361 v, respectively. difference in these two peak potential values, δep = 0.33 v, indicates that the redox process of ic is reversible in nature, involving transfer of two electrons (scheme 1). effect of solution ph the role of electrolyte ph on the oxidation of 10-5 m ic at pamcpe in 0.1 m buffer solution was examined by cv technique at the scan rate of 0.1 vs-1. influence of variation of ph in the range from 5.5 to 8.0 upon anodic peak currents of cvs is indicated in figure 4a. the highest oxidation peak of ic is observed at ph 6.0, being more or less reduced at other ph values. therefore, in continuation of this study, ph 6.0 was chosen for further experiments with ic analyte. the relation between the oxidation potential (epa) and ph of the solution is shown in figure 4b. from the distinct linear part formed between ph 5.5 to 7.0, the linear regression equation epa / v = 0.56 0.029 ph (r = 0.99) was derived. if the linearity, however, was presumed for the whole ph range (5.5-8.0), the linear regression relationship is defined as epa / v = 0.71526 – 0.053 ph, but with lower squared correlation (r = 0.958). the obtained slope of 0.053 v ph-1 is extremely close to the theoretical value of 0.058 v ph-1, characteristic for equal number of electrons and protons in the redox process, what is in accordance with scheme 1. by presenting ipa values vs. ph in figure 4c, it is suggested again that ph 6.0 is the most constructive for the efficient and reliable determination of ic. figure 4. (a) cvs (0.1 v s-1) of ic (10-5 m) at pamcpe in 0.1 m pbs of different ph values (5.5-8.0) (b) epa vs. ph; (c) ipa vs. ph http://dx.doi.org/10.5599/jese.953 j. electrochem. sci. eng. 11(2) (2021) 87-96 analysis of indigo carmine 92 effect of scan rate a lot of information concerning the electrochemical process of analyte at an electrode surface can generally be derived from the relation between peak current and scan rate in cvs. the electrochemical reaction of 10-5 m ic at pamcpe was followed at variable scan rates (0.1, 0.125, 0.150, 0.175, 0.200, and 0.250 v s-1), and the obtained cyclic voltammograms are revealed in figure 5a. figure 5b shows the linear relation obtained when ipa is presented against the scan rate the linear regression equation, ipa / a = 1.15571 + 13.25714  / v s-1 (r = 0.998), was developed for the scan rate range of 0.1 to 0.250 v s-1. such linear relation between ipa and  reveals that the reaction of ic happening at pamcpe is the adsorption restrained process. potential, v / v s-1 figure 5. (a) cvs of ic (10-5 m) at pamcpe in 0.1 m pbs, ph 6.0 at different scan rates (0.1 0.250 v s-1); (b) anodic peak current vs. scan rate simultaneous determination of ic and rf the electrochemical behavior of ic that is present along with rf in pbs solution, was examined by means of dpv. possible interference of two substances and a capability for separate determinations of ic and rf at both electrodes were examined. figure 6 shows dpvs of pamcpe and cpe in the solution mixture containing ic (10-5 m) and rf (10-4 m) in 0.1 m pbs, ph 6.0. two separate distinct peaks with much higher current responses are obtained at the polymer modified electrode compared to bare electrode. it is obvious that pamcpe enables separate estimation of two analytes without any major interference between them. figure 6. dpvs (0.05 vs-1) of the solution containing ic (10-5 m) and rf (10-4 m) in 0.1 m pbs, ph 6.0 at cpe (curve a) and pamcpe (curve b) i p a /  a e. s. d’souza et al. j. electrochem. sci. eng. 11(2) (2021) 87-96 http://dx.doi.org/10.5599/jese.953 93 calibration curve and detection limit the correlation of the oxidation peak current value with ic concentration was investigated through dpv at 0.05 v/s, and the result of this investigation is plotted in figure 7. the oxidation peak current value presented versus ic concentration gives two linear segments, from 2×10-7 to 10-6 m (green line) and 1.5×10-6 m to 3.5×10-6 m (red line), respectively. corresponding linear regression equations are: ipa / a = 4.886×10–7 + 1.413 cic / m (r = 0.9952) and ipa / a = 1.56×10–6 + 0.106 cic / m (r= 0.9952). here, the first linear segment from 2×10-7 to 10-6 m was considered for determination of limit of detection (lod) and limit of quantification (loq). lod and loq values were calculated from respective relations 3σ/m and 10σ/m [31], where σ is the standard deviation obtained by 5 dpv measurements in blank solution (buffer without analyte) and m is slope of the calibration curve. the calculated values of loq and lod for ic at pamcpe are 8.43×10-8 and 2.53×10-8 m, respectively. cic / m figure 7. calibration plot for determination of ic at pamcpe in 0.1 m pbs, ph 6.0 at 0.05 v s-1 in table 1, the obtained detection limit and linear range of calibration plot for determination of ic at pamcpe are compared with corresponding data taken from some related literature works [3236]. table 1. comparison of linear range and limit of detection for determination of ic produced by dpv at pamcpe with literature data on some other techniques technique linear range of concentration, moll-1 lod, moll-1 reference resonance rayleigh scattering 2×10-6 – 32×10-6 2.4×10-8 [32] spectrophotometry 1.7×10-6 – 39×10-6 2.5×10-8 [33] cyclic voltammetry 2×10-6 – 6×10-5 1.1×10-7 [34] cyclic voltammetry 8×10-6 – 1.3×10-4 2.7×10-7 [35] differential pulse voltammetry 5.0×10-6 – 50×10-6 3.6×10-8 [36] differential pulse voltammetry 2×10-7 – 1×10-6 2.5×10-8 this work stability and reproducibility cv technique was applied to sense the reproducibility of ic (10-5 m) response at pamcpe in 0.1 m pbs, ph 6.0 with the scan rate of 0.1 vs-1. by maintaining the same solution and changing the http://dx.doi.org/10.5599/jese.953 j. electrochem. sci. eng. 11(2) (2021) 87-96 analysis of indigo carmine 94 electrode surface at each time, the re-appearance of ic response at pamcpe is studied for 5 consecutive series. the relative standard deviation (rsd) for reproducibility of anodic peak current response of ic was found to be 2.51 %. in order to detect stability of the electrode towards ic (10-5 m) sensing, the electrode is made to run for 20 consecutive measurements, stability can be evaluated by means of percentage degradation method, eq. (1). pn pl degradation, % = 100 i i (1) where, ipn is peak current of ic during the last cv cycle and ipl is the peak current of the first cv cycle. even after 20 cv cycles, it was found that about 84 % of the original current is regained. both these testing data represent that pamcpe possesses an outstanding reproducibility and stability towards ic oxidation. analytical application of pamcpe in real sample ic analysis in order to assess possible application of the prepared polymer modified electrodes for determination of ic in real water samples, tap water and laboratory waste-water samples were chosen. during this analysis, different concentrations of ic were added to 5 ml water sample and a supporting electrolyte is used to carry out the experiment. standard addition system was used to identify the quantity and concentration of ic in these samples. the outcome showed that pamcpe possesses the recovery capacity of the authentic sample in the range of 96.3 to 99 %. to study the applicability of the designed electrode for the presence of ic in a food sample, we selected a candy crush chocolate, purchased from the nearby marketplace. the standard addition method was also adopted to investigate the sample recovery. the recovery ranged from 91.2 % to 99 % for ic. the recovery statistics of the sample represents that the designed electrode is of high sensitiveness and has enormous potential for determination of trace amounts of ic. conclusions carbon paste electrode (cpe) was successfully modified with polyarginine, forming pamcpe electrochemical sensor for indigo carmine (ic) dye. cyclic and differential pulse voltammetry (cv and dpv) techniques were applied, showing that in comparison with bare cpe, the polymer modified electrode exhibits excellent electrochemical response of ic oxidation. surfaces of both electrodes were characterized by fesem. more compact surface structure observed for pamcpe can be due to the presence of deposited polymer film. anodic peak current values of pamcpe presented against ic concentration showed a linear two-slope plot. in the linear region from 2×10-7 to 10-6 m, loq and lod values were determined as 84.3 and 25.3 nm, respectively. under optimized experimental conditions, cv and dpv measurements of pamcpe have revealed high sensitivity, selectivity, reproducibility, and reliability towards ic determination. when applied to real water samples (tap) and food stuffs (candy crush chocolate), pamcpe showed an acceptable outcome. it can be concluded that the designed pamcpe sensor is easy to prepare, economical, steady, readily renewable and exhibits good catalytic activity towards ic oxidation. acknowledgements: we are grateful to acknowledge the financial support from the vgst, bangalore under the research project no. ksteps/vgst-kfist (l1) 2016-2017/grd-559/20172018/126/333, 21/11/2017. e. s. d’souza et al. j. electrochem. sci. eng. 11(2) (2021) 87-96 http://dx.doi.org/10.5599/jese.953 95 references [1] t. robinson, g. mcmullan, r. merchant, p. nigam, bioresource technology 77(3) (2001) 247-255 https://doi.org/10.1016/s0960-8524(0,0)00080-8. 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[36] n. hareesha, j. g. manjunatha, b. m. amrutha, m. m. charithra, n. prinith subbaiah, journal of electronic materials 50(3) (2021) 1230-1238 https://doi.org/10.1007/s11664-020-08616-7. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://www.jmaterenvironsci.com/document/vol10/vol10_n6/53-jmes-raril-2019.pdf https://www.sid.ir/fileserver/je/55002820181108.pdf https://doi.org/10.1039/d0ra03672b https://doi.org/10.17721/moca.2019.216-223 https://doi.org/10.1080/20014091076785 https://doi.org/10.1002/bio.3085 https://doi.org/10.1016/s0003-2670(99)00215-9 https://doi.org/10.1016/j.jfda.2017.05.002 https://doi.org/10.26479/2019.0501.68 https://doi.org/10.1007/s11664-020-08616-7 https://creativecommons.org/licenses/by/4.0/) a wse2@poly(3,4-ethylenedioxythiophene) nanocomposite-based electrochemical sensor for simultaneous detection of dopamine and uric acid: http://dx.doi.org/10.5599/jese.1375 1251 j. electrochem. sci. eng. 12(6) (2022) 1251-1259; http://dx.doi.org/10.5599/jese.1375 open access : : issn 1847-9286 www.jese-online.org original scientific paper a wse2@poly(3,4-ethylenedioxythiophene) nanocompositebased electrochemical sensor for simultaneous detection of dopamine and uric acid yasin tangal1, deniz coban2 and sadik cogal3, 1burdur mehmet akif ersoy university, the graduate school of natural and applied sciences, department of chemistry, 15030, burdur, turkey 2burdur mehmet akif ersoy university, the graduate school of natural and applied sciences, department of material technology engineering, 15030, burdur, turkey 3burdur mehmet akif ersoy university, faculty of arts and science, department of chemistry, 15030, burdur, turkey corresponding author: sadik_cogal@yahoo.com; tel.: +90-248-213-3060; fax: +90-248-213-3099 received: may 13, 2022; accepted: june 24, 2022; published: july 25, 2022 abstract in the present work, a nanocomposite of two-dimensional wse2 nanosheets with poly (3,4-ethylenedioxythiophene (wse2@pedot) was prepared by facile hydrothermal method and characterized in terms of structural and morphological analyses. this nanocomposite was used to modify glassy carbon electrode for the construction of an electrochemical sensing platform for simultaneous determination of dopamine (da) and uric acid (ua) in the presence of ascorbic acid (aa). it was found that the incorporation of pedot into wse2 nanosheets exhibited enhanced electrochemical behaviors and electro¬catalytic activity against da and ua. using differential pulse voltammetry (dpv) measurements, the wse2@pedot modified electrode displayed wide linear detection ranges of 16 to 466 µm for da and 20 to 582.5 µm for ua. the electrode also exhibited high selectivity against da and ua in the presence of major interference of ascorbic acid and other interferent substances. keywords tungsten diselenide; conducting polymer; modified electrode; selective sensing; biological compounds introduction dopamine (da) and uric acid (ua) are two important biological compounds that play important roles in human physiological functions [1]. abnormal concentrations of these molecules in biological fluids lead to many diseases, including schizophrenia, alzheimer, parkinson’s disease, drug addiction, hyperuricemia, chronic renal sickness, etc. [2-4]. therefore, early analyses of these molecules are highly important to deal with the concerned conditions. recently, electrochemical methods have http://dx.doi.org/10.5599/jese.1375 http://dx.doi.org/10.5599/jese.1375 http://www.jese-online.org/ mailto:sadik_cogal@yahoo.com j. electrochem. sci. eng. 12(6) (2022) 1251-1259 wse2@pedot nanocomposite based sensor 1252 become attractive for the determination of da and ua [5,6]. however, these two electroactive molecules have close oxidation potentials, which limits their selective detection in biological samples on traditional electrodes. therefore, tremendous research efforts have been directed toward the development of novel electrodes based on various materials. among these, two-dimensional (2d) nanomaterials such as graphene and its inorganic analogues have attracted significant interest in electrochemical applications because of their unique chemical and physical properties [7]. for example, transition metal dichalcogenides (tmds) have been widely investigated as electrode materials for electrochemical sensing platforms due to their high electrocatalytic properties, chemical stability, low cost and facile synthesis [8-10]. wse2, an important member of tmds, has received special attention in electrochemical applications [11,12]. however, wse2 tends to stack easily due to high interactions between its layers, which decrease the number of electroactive sites and electrochemical performance. therefore, various strategies have been applied to improve the electrochemical performance to decrease the number of layers and enhance the number of active sites. among the alternative strategies, the production of wse2 in the presence of conductive supports is an effective way to reduce the number of layers by preventing layer stacking. conducting polymers such as poly(3,4-ethylenedioxythiophene) (pedot) can be incorporated with tmd materials to prepare composite electrode materials [13,14]. pedot possesses high conductivity and electrochemical stability and shows high potential to be applied to an electrode material [15]. pedotlike supporting materials not only prevent the stacking of wse2 layers but also increase the active sites of wse2. in this work, wse2@pedot composite was prepared via a hydrothermal method to enhance the electrochemical properties of wse2. the as-prepared composite was coated on a glassy carbon electrode (gce) for electrochemical detection of da and ua. experimental materials: tungstic acid (h2wo4) (aldrich, 99 %), selenium (se) powder (sigma-aldrich, 99.0 %), sodium borohydride (nabh4) (sigma-aldrich, ≥98 %), 3,4-ethylenedioxythiophene (edot) (sigmaaldrich, 97 %), iron (iii) chloride (fecl3) (carlo erba), potassium chloride (sigma-aldrich), potassium ferricyanide(iii) (k3fe(cn)6) (sigma-aldrich, 99 %), potassium hexacyanoferrate(ii) trihydrate (k4[fe(cn)6]∙3h2o) (merck), sodium phosphate monobasic dihydrate (nah2po4∙2h2o) (sigmaaldrich), sodium phosphate dibasic dihydrate (na2hpo4∙2h2o) (sigma-aldrich) and dimethylformamide (dmf) (merck) were purchased and used as received. synthesis of pedot: pedot was synthesized via the oxidative chemical polymerization method. 100 µl of monomer edot was dissolved in 50 ml deionized water (diw) and placed in an ice bath. in another flask, 422.9 mg ammonium persulfate (aps) (edot:aps molar ratio is 1:2) was dissolved in 25 ml diw and then transferred slowly to monomer solution, keeping the temperature of the solution between 0-5 °c. the reaction mixture was stirred for 48 h at room temperature. the obtained black powder was filtered, washed with diw and ethanol several times and finally dried at 60 °c for 24 h. synthesis of wse2@pedot the preparation of wse2@pedot was carried out as follows: first, 0.1 g of as-prepared pedot was ultrasonically dispersed in 30 ml of dmf for 60 min. then, 0.32 g se powder and 0.1 g nabh4 as a reducing agent were added and stirred for 60 min to obtain good dispersion. later, 0.49 g h2wo4 was slowly added to this dispersion and stirred for an additional 30 min. the final dispersion was transferred to a 45 ml teflon-lined stainless steel autoclave reactor and heated at 200 °c for 12 h. after being cooled to room temperature, the product was filtered and washed with dw at y. tangal et al. j. electrochem. sci. eng. 12(6) (2022) 1251-1259 http://dx.doi.org/10.5599/jese.1375 1253 least three times and finally dried at 60 °c. the obtained powder was annealed at 500 °c for 3 h under n2 flow to enhance the crystallinity. the synthesis of wse2 as control was the same as that of wse2@pedot, except pedot was not added to the reaction mixture. electrode modification before coating, the cleaning process was applied to polish the gce surface by using 0.3 and 0.05 µm alumina powder. gce was then sonicated in dw and ethanol to remove any adsorbed particles on its surface. on the other hand, well-dispersed electrode ink was prepared by sonication of wse2@pedot in dmf for 1 h to obtain a concentration of 5 mg ml-1 wse2@pedot. later, 10 μl from this ink was drop-coated on gce and left to dry in an open atmosphere. finally, wse2@pedot coated-gce was carefully rinsed with 0.1 m phosphate buffer solution (pbs) before experiments. for the control, gce was also modified with pristine wse2 in the same way described above for gce/wse2@pedot. electrochemical measurements electrochemical experiments were conducted via ivium compactstat model potentiostat. threeelectrode configuration was used for electrochemical works, and in addition to gce as a working electrode, pt wire was used as a counter electrode and ag/agcl as a reference electrode. characterization the crystal structure of the as-prepared wse2@pedot was determined through x-ray diffraction (xrd) using bruker/d8 advance diffractometer. morphological characterization was conducted using scanning electron microscopy (fei model: quanta 400f) and transmission electron microscopy (fei tecnai g2 spirit biotwin model). figure 1. schematic representation of synthetic procedure and electrochemical detection results and discussion materials characterization wse2@pedot composite was successfully obtained via hydrothermal reaction, as schematically represented in figure 1. in this synthesis procedure, metallic se particles were initially reduced by a http://dx.doi.org/10.5599/jese.1375 j. electrochem. sci. eng. 12(6) (2022) 1251-1259 wse2@pedot nanocomposite based sensor 1254 reducing agent of nabh4 to produce se2ions. the se2ions were then reacted with w cations, forming wse2 nanosheets embedded in pedot polymeric structure. the structure of the wse2@pedot composite was evaluated by x-ray diffraction (xrd). figure 2 shows the xrd patterns of the wse2@pedot composite as well as pristine wse2 nanosheets. xrd pattern of wse2 displays major diffraction peaks (planes) at 13.3o (002), 31.9o (100), 37.6o (103), 47.3o (105), 55.9o (110), 65.8o (108) and 70.3o (203), which are in good agreement with the wellknown hexagonal wse2 structure (jcpds no. 38-1388) [16]. on the other hand, the wse2@pedot composite exhibited diffraction peaks at 13.2, 23.0, 31.4, 32.8, 37.8, 40.6, 47.3, 53.3, 55.9, 65.7, 69.4 and 74.2°, in which the major diffraction peaks of wse2 hexagonal structure are well matched. moreover, the major diffraction peaks are relatively stronger, and additional diffraction peaks are also observed in the wse2@pedot composite, suggesting the successful combination of wse2 nanosheets and conducting polymer. 2 / o figure 2. xrd pattern of wse2 nanosheets and wse2@pedot composite the surface morphology of the wse2@pedot composite was investigated using sem and tem. the sem images (figure 3a-b) of pristine wse2 and wse2@pedot indicate that nanosheets incorporated flower-like structures were obtained by hydrothermal treatment. however, the wse2@pedot possesses looser, dispersed and thinner nanosheets due to the incorporation of pedot. this difference is more obvious in the tem images, as shown in figure 3c-d. thus, the resulting composite can have more active sites for analyte adsorption, leading to an improved electrochemical signal. 2 / o figure 3. (a,b) sem and (c,d) tem images of wse2 (left) and wse2@pedot (right) y. tangal et al. j. electrochem. sci. eng. 12(6) (2022) 1251-1259 http://dx.doi.org/10.5599/jese.1375 1255 electrochemical studies to assess the electrochemical behavior, modified electrodes were investigated via cyclic voltammetry (cv) in 0.1 m kcl containing 5 mm [fe(cn)6]3-/4-, conducted between -0.3 to 0.6 v at 50 mv s-1 of scan rate (). figure 4(a) shows cvs of bare gce, gce-wse2 and gce-wse2@pedot in this solution. it is obvious that the wse2@pedot composite-modified electrode exhibited better electrochemical kinetics against ferri/ferrocyanide couple than bare gce and gce-wse2. wse2@pedot displays 99.1 / -106.1 µa of anodic/cathodic peak currents, while the bare gce and gce-wse2 show 51.2 / -60.1 µa and 75.01 / -86.3 µa, respectively. the enhanced electrochemical kinetics of gce-wse2@pedot could be attributed to the enhanced number of electroactive sites and higher conductivity due to the incorporation of conductive pedot in wse2 nanosheets. to estimate the electrochemically active surface area of the gce-wse2@pedot electrode, cv measurements at various scan rates were performed in 0.1 m kcl containing 5 mm [fe(cn)6]3-/4 (figure 4b). the inset figure shows that the anodic peak currents increased linearly with the square root of scan rates (1/2). therefore, the electrochemically active surface area (a / cm2) was calculated using randles-ševčik equation (1) [17]: ipa = (2.65 x 105) n3/2ad1/2c1/2 (1) where d is the diffusion coefficient, which is 7.610-6 cm2 s-1 for [fe(cn)6]3-/4-. the electrochemical surface area of the gce-wse2@pedot electrode was estimated to be 0.13 cm2, which is higher than 0.063 cm2 estimated for bare gce. in order to determine the electrocatalytic activities of bare, wse2 and wse2@pedot modified electrodes toward a binary mixture of da and ua, the cv and dpv measurements were performed in 0.1 m pbs solution containing 0.19 mm da and 0.28 mm ua. figure 4c shows cvs of different electrodes performed in the potential range between -0.2 to +0.6 v, at 50 mv/s of scan rate. it is clearly seen that the current response of bare gce is very low, which does not provide a sensitive sensing system for the determination of da and ua. on the other hand, as it is expected by modifying the gce electrode, the current response was improved and the highest activity was obtained with the electrode modified with wse2@pedot composite. electrocatalytic activities of electrodes were further investigated using the dpv technique, which is known as a more sensitive electrochemical method. figure 4d shows that oxidation peak positions and their current responses are more obvious than was observed in cv measurements. therefore, the dpv technique was chosen for the simultaneous determination of da and ua. moreover, from the cv and dpv curves, it is obviously seen that the current response of ua was greatly increased. this enhancement of the wse2@pedot modified electrode toward ua is due to the presence of pedot conducting polymer. at the working ph, da has a cationic character and ua exists as an anionic form, which leads to enhanced electrostatic interaction between ua and cationic pedot [18]. therefore, different interactions of da and ua with the wse2@pedot composite result in peak separation of da and ua. cyclic voltammetry was also used to evaluate the effect of various scan rates on the electrocatalytic behaviors of da and ua on the wse2@pedot electrode and to understand the reaction kinetics on its surface. figure 5(a) displays cvs of da (0.28 mm) and ua (0.37 mm) in 0.1 m pbs at varied scan rates from 25 to 250 mv/s. figure 5b shows the corresponding oxidation peak currents, which are proportional to the scan rate, indicating an adsorption-controlled process on wse2@pedot coated electrode [19]. http://dx.doi.org/10.5599/jese.1375 j. electrochem. sci. eng. 12(6) (2022) 1251-1259 wse2@pedot nanocomposite based sensor 1256 figure 4. (a) cvs of gce, gce-wse2 and gce-wse2@pedot in 0.1 m kcl containing 5 mm [fe(cn)6] 3-/4-; (b) cvs of gce-wse2@pedot in 0.1 m kcl containing 5 mm [fe(cn)6] 3-/4at different scan rates; inset figure shows anodic peak currents vs. square roots of scan rates; (c) cvs of gce, gce-wse2 and gce-wse2@pedot in 0.1 m pbs solution containing 0.19 mm da and 0.28 mm ua; (d) dpvs of gce-wse2 and gcewse2@pedot in 0.1 m pbs solution containing 0.19 mm da and 0.28 mm ua figure 5. (a) cvs of gce-wse2@pedot in 0.1 m pbs (ph 7.0) containing 1.5 mm aa, 0.36 mm da and 0.55 mm ua at different scan rates from 25-25 mv/s; (b) linear plots of anodic peak currents of da and ua vs. scan rate the simultaneous determination of da and ua on gce-wse2@pedot was performed using dpv. figure 6a displays the dpvs for increasing da and ua concentrations in 0.1 m pbs. it is obvious that oxidation peak positions of da and ua maintain the peak-to-peak potential separation by gradually increasing their concentrations. figure 6(b-c) displays calibration curves of da and ua and linear detection ranges of 16 to 466 µm for da and 20 to 582.5 µm for ua, were determined. the linear regression equations with corresponding regression coefficients were also placed in figure 6b, c. furthermore, considering the slopes of the regression equations, lods were evaluated to be 8 µm for da and 14 µm ua, respectively. y. tangal et al. j. electrochem. sci. eng. 12(6) (2022) 1251-1259 http://dx.doi.org/10.5599/jese.1375 1257 figure 6. (a) dpvs of gce-wse2@pedot in 0.1 m pbs (ph 7.0) containing various concentrations of da and ua; calibration plots of (b) da and (c) ua selectivity, reproducibility and stability of the sensor the selectivity of the gce-wse2@pedot electrode was investigated through dpv technique. the major interference molecule is ascorbic acid, found in a much higher concentration than da and ua in biological fluids. figure 7a shows dpvs of gce-wse2@pedot in 0.1 m pbs solution containing 0.38 mm da, 0.38 mm ua and 1.0 mm aa and other interference substances (e.g., nacl, kcl, nano3, mgso4, glucose and citric acid). figure 7. (a) dpvs of gce-wse2@pedot in 0.1 m pbs (ph 6.0) containing 1.0 mm interfering substances in presence of 0.38 mm da and 0.38 mm ua; (b) stability test it is clearly seen that adding interfering substances in much higher concentrations to the mixed da and ua solution does not have an obvious change in their peak currents, indicating high selectivity of http://dx.doi.org/10.5599/jese.1375 j. electrochem. sci. eng. 12(6) (2022) 1251-1259 wse2@pedot nanocomposite based sensor 1258 the sensor. it is also worth noting that the electrode modified with wse2@pedot inhibits the oxidation of aa, which has a close oxidation potential to da and ua. the stability test of the gcewse2@pedot electrode was also investigated using the dpv technique. the stability data was collected by obtaining the peak current values of da and ua for fifteen days and given in figure 7b, indicating an acceptable change compared with the initial currents. conclusions wse2@pedot composite was successfully prepared via a 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yin, s. wang, y. zhao, h. wu, h. mei, analytical and bioanalytical chemistry 412 (2020) 2433-2441. https://doi.org/10.1007/s00216-020-02455-5 [19] h. sun, j. chao, x. zuo, s. su, x. liu, l. yuwen, c. fan, l. wang, rsc advances 4(52) (2014) 27625-27629. https://doi.org/10.1039/c4ra04046e ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1375 https://doi.org/10.1016/j.aca.2018.02.080 https://doi.org/10.1021/acsanm.1c00594 https://doi.org/10.1016/j.microc.2022.107188 https://doi.org/10.1007/s00216-020-02455-5 https://doi.org/10.1039/c4ra04046e https://creativecommons.org/licenses/by/4.0/) @article{tangal2022, author = {tangal, yasin and coban, deniz and cogal, sadik}, journal = {journal of electrochemical science and engineering}, title = {{a wse2@poly(3,4-ethylenedioxythiophene) nanocomposite-based electrochemical sensor for simultaneous detection of dopamine and uric acid:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {6}, pages = {1251--1259}, volume = {12}, abstract = {in the present work, a nanocomposite of two-dimensional wse2 nanosheets with poly-(3,4‑ethylenedioxythiophene (wse2@pedot) was prepared by facile hydrothermal method and characterized in terms of structural and morphological analyses. this nano­composite was used to modify glassy carbon electrode for the construction of an electrochemical sen­sing platform for simultaneous determination of dopamine (da) and uric acid (ua) in the presence of ascorbic acid (aa). it was found that the incorporation of pedot into wse2 nano­sheets exhibited enhanced electrochemical behaviors and electro¬catalytic activity against da and ua. using differential pulse voltammetry (dpv) measurements, the wse2@pedot modified electrode displayed wide linear detection ranges of 16 to 466 µm for da and 20 to 582.5 µm for ua. the electrode also exhibited high selectivity against da and ua in the presence of major interference of ascorbic acid and other interferent substances.}, doi = {10.5599/jese.1375}, file = {:d\:/onedrive/mendeley desktop/tangal, coban, cogal 2022 a wse2@poly(3,4-ethylenedioxythiophene) nanocomposite-based electrochemical sensor for simultaneous detect.pdf:pdf;:17_jese_1375_1251-1259.docx:word_new;:17_jese_1375_1251-1259.docx:word_new;:www/jese_v12_no6_1251-1259.pdf:pdf}, keywords = {tungsten diselenide, biological compounds, conducting polymer, modified electrode, selective sensing}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1375}, } electrokinetic removal of heavy metals from soil doi: 10.5599/jese.2015.0055 47 j. electrochem. sci. eng. 5(1) (2015) 47-65; doi: 10.5599/jese.2015.0055 open access: issn 1847-9286 www.jese-online.org original scientific paper electrokinetic removal of heavy metals from soil puvvadi venkata sivapullaiah, bangalore sriakantappa nagendra prakash* and belagumba nagahanumantharao suma* department of civil engineering, indian institute of science, bangalore – 560 012, india *department of civil engineering, uvce, jnana bharathi campus, bangalore university, bangalore -560 087, india e-mail: siva@civil.iisc.ernet.in, phone: +91-80-22932672; fax: +91-80-23600404 received: april 3, 2014; revised: may 29, 2014; published: march 15, 2015 abstract removal of heavy metal ions from soils by electrokinetic treatment has several advantages. the extent of removal, however, is both soil specific and ion specific. the conditions to be maintained have to be established based on laboratory studies. with a view to maximize the removal of metal ions the trends of removal of heavy metal ions such as iron, nickel and cadmium form a natural indian kaolinitic red earth during different conditions maintained in the electrokinetic extraction process are studied. a laboratory electrokinetic extraction apparatus was assembled for this purpose. attempts are also made to elucidate the mechanism of removal of the metal ions from soil. the composition of the flushing fluid, voltage and duration of extraction are varied. while dilute acetic acid has been used to neutralize the alkalinity that develops at the cathode, edta solution has been used to desorb heavy metals from clay surface. generally the extent of removal was proportional to the osmotic flow. nickel and cadmium are more effectively removed than iron. the percentage removal of ni is generally proportional to the osmotic flow but shows sensitivity to the ph of the system. there is an optimum voltage for removal of metal ions from soil. the removal of iron was negligible under different conditions studied. keywords electric; clays; metal ions; removal; sorption introduction heavy metal toxicity occurs far more often and people are exposed to toxic metals on a day-today basis in our environment. military activities are one of the primary contributors to metals contaminated soil problems. military operations such as small arms training, electroplating and metal finishing operations, explosive and propellant manufacturing and use, and the use of lead based paint at military facilities, have resulted in vast tracts of land being contaminated with http://www.jese-online.org/ mailto:siva@civil.iisc.ernet.in j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 48 metals. whether heavy metal poisoning comes either from these or from cooking utensils, deodorants, pesticides, etc., all have a devastating effect on the human body. both soils and ground water are often contaminated with heavy metals ions. the existing methods to removal of these metals, particularly from fine grained soils are not often successful or are not cost effective. electrokinetic remediation has been used to remediate soil and ground water [1]. electrokinetic remediation of soils offers several advantages over the other remediation methods that are in widespread use today. these include: (i) it is an in-situ process that is 50-90% less expensive than the currently available metals remediation technologies, such as soil washing, pump and treat, and excavation, which are ex-situ methods; (ii) it is extremely effective in finegrained low permeability soils where other techniques, such as pump and treat, are not feasible. this is due to the fact that the contaminants are transported under charged electrical fields and not hydraulic gradients. (iii) it is suitable for heterogeneous materials such as soils and pressuredriven flushing processes invariably channel the fluid through the largest size pores, leaving the smaller ones nearly untouched. in low-permeability soils, the electro-osmotically driven flow is insensitive to pore size, which allows a rather uniform flow distribution and is applicable equally to coarse and fine-grained soils. electrokinetic extraction of contaminants electrokinetic remediation is an in-situ process in which an electrical field is created in a soil matrix by applying a low-voltage direct current (dc) to electrodes placed in the soil. as a result of the application of this electric field, heavy metal contaminants may be mobilized, concentrated at the electrodes, and extracted from the soil. the mass transport processes induced during electrokinetic treatment are primarily electroosmosis of water, ion-migration and electrophoretic transport of colloids. the rate and efficacy of these processes are often found to depend upon the mineral and chemical composition of the soil and soil water, and the type, age, distribution, and concentration of the contamination. an excellent review of electrokinetic flow processes in soils is given by yeung [2]. for practical purposes, electro-osmotic fluid volume flow rate is described as analogous to darcy’s law. q = ke ie a (1) where q = fluid volume flow rate m 3 /s; ke = coefficient of electro-osmotic conductivity, m 2 /v s ie = hydraulic gradient of electro-osmotic flow, m a = total cross-sectional area perpendicular to the direction of fluid flow, m 2 . the values of coefficient electroosmotic conductivity of different soils lie in the narrow range of 110 -9 to 110 -8 m 2 / v s. therefore, an electric field is a much more effective force in driving fluid through fine-grained soils of low hydraulic conductivity than a hydraulic gradient. during an electrokinetic extraction process, the applied dc electric field can thus drive an effective electroosmotic advection of contaminants through the soil and/or inject enhancement agents into the contaminated soil. ionic migration or electromigration is the movement of charged chemical species relative to the movement of pore fluid. anions (negatively charged ions) are moved towards the anode (positive electrode) and cations (positively charged ions) are moved towards the cathode (negative electrode). the alkali metals and alkali earth metals such as na, k, cs and sr, ca tend to remain ionic under a wide range of ph and redox potential values. therefore they are expected to p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 49 electromigrate and be extracted from soil readily unless they become preferentially sorbed onto solid surfaces and clay interstices. under ideal conditions, the predominant cation and its accompanying anion may be caused to separate efficiently by electromigration only, for which little or no electroosmotic water advection may be necessary. the ionic mobilities of ions in free dilute solutions, i.e., the velocities of the ions under the influence of a unit electric field, are in the range of 110 -8 to 110 -7 m 2 / v s. combining the mechanisms of electroosmotic advection and ionic migration results in the applicability of electrokinetic extraction. past experience with electrochemical treatment of contaminated porous media has shown that the process is most effective when the transported substances are ionic; surface charged or in the form of small micelles with little drags resistance. the effect of ionic migration of anions may be diminished by that of electro osmotic advection. however the direction of electro-osmotic flow may reverse during a prolonged application of a dc electric field across fine-grained soils. the phenomenon of reverse electro-osmotic flow cannot be described by (1) and is not well understood. most experimental results obtained to date indicate that ionic migration is the dominant. factors governing electro-kinetic removal there are many practical aspects of the technology that needs to be considered carefully before the technology can be successfully implemented in the field. soil type this technology can be successfully applied to clayey to fine sandy soils. it appears that soil type does not pose any significant limitation on the technology. however, contaminant transport rates and efficiencies depend heavily on soil type and environmental variables. soils of high water content, high degree of saturation, and low activity provide the most favorable conditions for transport of contaminants by electro-osmotic advection and ionic migration. however, soils of high activity, such as illite, montmorillonite, and impure kaolinite, exhibit high acid/base buffer capacity and require excessive acid and/or enhancement agents to disrobe and solubilize contaminants sorbed on the soil particle surface before they can be transported through the subsurface and removed. moreover the high sorbtive capacity of the clayey soil for contaminant would further aggravate the problem by retarding contaminant transport. the effects of the soil mineralogy on removal of chromium from soils by electrokinetics were also investigated by [3]. their results indicate that the presence of carbonate and hematite can adversely impact the process. values of hydraulic conductivity in different types of soils within a heterogeneous deposit can vary many orders of magnitude. for a contaminated soil deposit containing interlayer of sand and clay, typical values of hydraulic conductivity of these strata are 110 -4 and 110 -8 m / s, respectively. however, the values of electric conductivity of these soils are still within the order of magnitude. as a result the electric field strengths in the different soil layers will be similar when the externally electric potential is applied across the deposit. as the coefficient of electro-osmotic conductivity is insensitive to soil type, the electro -osmotic fluid volume flow rates in different soil layers will thus be very similar. complete removal of those metals that possess complex aqueous and electrochemistry and tendency for speciation and forming hydroxide complexes is particularly difficult under variable ph and redox conditions. the technique has been successful to remove >90 % of heavy metals (arsenic, cadmium, cobalt, chromium, copper, mercury, nickel, manganese, molybdenum, lead, antimony, and zinc) from clay, peat and argillaceous sand [4]; to remove spiked lead from kaolinite j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 50 [5,6]; to remove 85-95 % of the original concentrations of cadmium, cobalt, nickel, and strontium from laboratory samples prepared from georgia kaolinite, na-montmorillonite, and sandmontmorillonite mixture [7]; to remove cadmium from saturated kaolinite [8]. contaminant type and concentrations available experimental data indicate that removal of heavy metals, radio-nuclides, and selected organics by electrokinetics is feasible. focus is given for the removal of metals in this study. it is anticipated that pollutant, such as pbo, may dissolve and advance through the soil. the process helps in migration of different contaminants in the soil simultaneously. therefore, the type of contaminant does not pose a significant limitation on the technology, provided the contaminant does not exist in the sorbed phase on the soil particle surface or as precipitates in the soil pore. however a high concentration of ions in the pore fluid will increase the electrical conductivity of soil and thus reduce the efficiency of electro-osmotic fluid flow. more the strength of the electric field applied may have to be reduced to prevent excessive power consumption and heat generation during the process. nonetheless the concentration of the contaminant does not pose any insurmountable hurdle to the application of the process. ph gradient across the electrokinetic cell an electric field causes electrolysis of water and the ph value will drop at the anode and it will increase at the cathode as the produced ions by electrolysis move corresponding to their charge to the electrode of opposite polarity, causing a ph gradient in the soil. when an electric field is applied to wet soil, the soil ph undergoes transient and spatial variation due to decomposition of water, which in turn affect soil surface properties such as cation exchange capacity, ion (cation and anion) adsorption capacity, and magnitude and sign of the ξ potential. similarly, the electric gradient does not remain constant in time and space due to the changing resistivity and redistribution of the charges in the soil; and oxidation/reduction state of the soil, contaminants and also the aqueous solution near the electrodes. rødsand et al. [9] demonstrated the use of acetic acid to depolarize the cathode reaction and an ion-selective membrane to halt the hydroxyl migrating from the cathode into the soil and enhances electrokinetic extraction of lead while the membrane extraction technique does not enhance the technique as expected. voltage and current levels the electric current densities used in most studies are in the order of a few tens of ma /cm 2 . although a high current intensity can generate more acid and increase the rate of transport to facilitate the contaminant removal process, it increases power consumption tremendously, as power consumption is directly proportional to the square of electric current. an electric current density in the range of 1 10 a / m 2 has been demonstrated to be the most efficient for the process. however appropriate selection of electric current density and electric field strength depends on the electrochemical properties of the soil to be treated, in particular the electric conductivity. the higher the electric conductivity of the soil, higher will be the required electric current density needed to maintain the required electric field strength. electric field strength of the order of 50 v / m can be used as an initial estimate for the process. an optimum electric current density or electric field strength should be selected based on soil properties, electrode spacing, and time requirements of the process. p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 51 effluent chemistry and enhancement scheme contaminants can exist in different chemical forms in the subsurface depending on environmental conditions. they can exist as solid precipitates, dissolved solutes in the soil pore fluid and/or bonded species on organic matters in the soil. among these different forms, only dissolved solutes are mobile and removable by electrokinetic extraction and many other remediation technologies. transformation processes of the contaminant between different chemical forms are contaminant specific, reversible, and dependent on environmental conditions. nonetheless, most contaminants can be transformed to their dissolved forms. the acidic environment generated at the anode aids in de-sorption and dissolution of metal contaminants from the soil particle surface. however, the basic environment generated at the cathode can hinder the removal of metal contaminants from the soil. in some cases it is necessary to inject reagents into the soil to enhance solubility and transport of metal contaminants. in an acid environment, some metal ions exist as anionic complexes. both chemical forms are soluble and thus can be extracted from the soil by the process. it is thus clear that the success of an electrokinetic extraction of contaminants from polluted soils depends on the physical and chemical properties of the soil, the contaminant, and the operating parameters of the extraction system. it is necessary to understand the relative influences of various mechanisms such as osmotic flow induced and the applicability of various enhancement schemes and conditions to be maintained during extraction. materials and methods soil used red earth used in this study was obtained from indian institute of science, campus bangalore. the soil was collected by open excavation from a depth of one meter from the natural ground. the soil was dried and passed the is sieve size of 425 m. the soil so obtained has clay content of 35 %. the clay content consisted predominantly of kaolinitic mineral. properties of red earth are summarized in table 1. table 1. properties of red earth property red earth specific gravity 2.73 liquid limit, % 34.9 plastic limit,% 17.7 shrinkage limit, % 13.6 max dry density, g/cm 3 1.79 optimum moisture content, % 16.23 preparations of metal ion solutions the chemicals, cadmium acetate ((ch3coo)2cd·2h2o), nickel nitrate (ni(no3)2×6h2o), ferric chloride (fecl3), acetic acid (ch3cooh) solution, hydrochloric acid (hcl), used in this study. all these solution were prepared by adding required quantity of chemicals in deionised water. standard 100 ppm solutions of cadmium, nickel and iron were prepared by dissolving 2.372 g, 4.955 g and 2.904 g respectively in distilled water and make it to 1000 ml. the solutions were j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 52 acidified with a drop of hcl. the concentrations of the collected fluid were measured using atomic absorption spectrophotometer after standardization. laboratory electrokinetic extraction cell as the experimental apparatus required for this study program was not readily available in india from conventional geotechnical testing equipment, a new apparatus designed, fabricated and assembled at the geotechnical laboratory, department of civil engineering. indian institute of science, bangalore, india has been used. the electrokinetic cell was fabricated using nylon material for the body, which is a nonconductor of electricity, corrosive resistant, not affected by acid or alkali. the electrokinetic cell consists of two end caps and a specimen cylinder made up of nylon. the test sample was 80 mm in diameter and 300 mm long. ten electrical measurement nodes have been installed on the specimen cylinder at 25 mm intervals so that the electrical voltage distribution along the sample can be monitored continuously during the test. the end caps house the graphite plate electrodes, inflow and outflow tubings. as porous graphite is extremely expensive and fabrication of porous graphite electrodes is very labor intensive, normal grade graphite plates were used in this study. holes of 1 mm diameter were drilled through the graphite plate electrode to facilitate water transport during electro-osmosis. details of electrokinetic cell are shown in fig. 1 the required electrical circuit has been designed, connected and assembled. the circuit is designed to facilitate future automation of the experiment by means of computerized data acquisition system. provision has been made to pass water through the soil compacted in the cell. stand anode overflow gas vent line anode assembly anode reservoir sample standpower supply value gasvent pores stone anode compartment anode electrode contaminated soil8cm filter paper 30 cm to power supply / multimeter overall test setup value gasvent pores stone cathode compartment out-let cathode electrode filter paper contaminated soil stand anode overflow gas vent line cathode assembly cathode reservoir meter fig. 1. electrokinetic extraction apparatus. p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 53 electro-kinetic extraction the procedure involves the following sequence, 1. sample preparation, 2. variation of flow through red earth with and without contaminants during different phases of extraction, 3. calculation of electro osmotic permeability, and 4. estimation of percent removal of metal ions. the oven dried soil samples were mixed with required amount of water so as to bring it to 110 % of optimum moisture content (omc) which is about 17.93 %. these samples were kept one day in the desiccators to get uniform moisture content. to prepare contaminated soils with cadmium, nickel and iron solutions of cadmium acetate, nickel nitrate and ferric chloride containing the required amount of chemicals to bring it to 250 mg/kg, 500 mg/kg and 1000 mg/kg of soil mass respectively have been added. the soil samples were then compacted in the cell to bring it to 110 % of maximum dry density on wet of optimum on the compaction curve. the soil is divided into three equal parts by weight and then each part is emplaced into the specimen cylinder and compacted one by one using a screw jack to ensure uniform compaction for the entire specimen. after compaction, the perforated electrodes were covered with the filter papers and the end caps were closed. fluid in-let and out let tubes were connected to the cell. initially, the soil specimen was saturated by maintaining a hydraulic head using self-compensating device and then the head removed. the electrodes were then connected to a constant voltage power supply through the sample and flow is measured at regular intervals. the inlet and outlet were kept at same level such that the hydraulic head difference between them was zero. then a dc voltage was passed through the cell. the change in the flow, ph and electro osmotic permeability on application of the dc voltage in the soil with and without any contaminant has been studied. after studying the effect of higher voltage the effect of the addition of acetic acid in presence of dc current on the flow, ph and electro osmotic permeability have been studied. electrokinetic treatment was continued for a specific amount of time. the concentration of the contaminants in the out flow at the cathode induced by electro osmosis was measured using atomic absorption spectrophotometer. the percent removal of contaminants has been calculated. results and discussion electro-kinetic extraction of cd, ni, and fe (iii) ions the removal of metal ions by induced osmotic flow and ion migration has been studied. the effect of increased voltage on osmotic flow and ion migration has been studied. attempt has been made to understand the utility of passing acetic acid to control ph and the role of edta to desorb and remove contaminant has been studied. osmotic flow through soil with and without contaminant decontamination of fine grained soil by soil washing is largely inhabited because of their low hydraulic conductivity. one of the important processes by which the decontamination of soils by electro kinetic extraction process is achieved by enhanced flow through fine grained soils on application of voltage across the soil by electro osmosis. osmotic induces flow through soil even without hydraulic gradient. the extent of osmotic flow through the soil depends on several factors such as type of soil, voltage applied, the type and amount of contaminants present in the soil. the j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 54 effect of increased voltage across the soil on osmotic flow of fluid through the soil without any contaminants and with known amount of contaminants have been studied application of voltage across the soil increases the ph at the cathode and decreases the ph at the anode due to electrolysis of water. increase in the oh ion concentration near the cathode causes high negative value of zeta potential. this in turn increases the flow. for soils containing heavy metal ions the increase in the ph at cathode precipitates ions as hydroxides and reduces the flow rate. the ph at which the ion starts precipitating varies from ion to ion. the effect of this process on the rate of flow through the soil containing different types and levels of contaminant has been monitored. the efficiency of passage of dilute acetic acid solution during electro kinetic extraction test after noticing the reduction in flow rate in increasing the flow rate has been studied. increased acidity causes the zeta potential to become less negative. flow rates have been observed to decrease due to this. however as long as the concentration of ions in the solution is sufficient, electromigration continues even when electro-osmosis has declined. the effect of increase in the voltage while passing of acetic acid to flow rate is monitored. application of voltage (5 v) induced flow initially. increase in voltage (10 v) improves flow by almost 30 times as shown in fig. 2. prolonged application with enhanced voltage (20 v) doesn’t influence the flow, as it is almost 0. passing of acetic acid revives flow but the ph remains on the basic front, may be due to deposition of oh ions at the cathode. further increase in voltage (30 v) along with acetic acid compliments the flow marginally due to neutralization of oh ions at cathode. increase in the voltage to 45 v along with acetic acid further enhances the flow considerably and observed to be the maximum during the monitor. however, prolonged application of higher voltage doesn’t improve the flow any further, due to increase in ph at the cathode as in fig. 3. fig. 2. electrokinetic flow through uncontaminated soil p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 55 fig. 3. variation of ph of the effluent from uncontaminated soil based on the osmotic flow generated, the osmotic permeability of the soil has been calculated. the electro osmotic permeability of red earth without contaminant is in the range of 2.78±10 -6 to 4.0×10 -5 cm 2 / v s as shown in fig. 4. at 5 v electro osmotic permeability was 2.78×10 -6 cm 2 / v s. increase in the voltage to 10v enhances electro osmotic permeability to 4.0×10 -5 cm 2 / v s being the maximum rate of flow. increase in voltage to 20 v reduced electro osmotic permeability. further increase in voltage with acetic acid revived electro osmotic permeability to 1.33×10 -5 cm 2 / v s. however increase in voltage further (60 v) and prolong application reduced electro osmotic permeability as shown in fig. 4. maximum electro osmotic permeability of red earth without contaminant is about 4.0×10 -5 cm 2 / v s and is observed at 0.33 v / cm. effect of nickel ions on the osmotic flow through the soil to measure the osmotic flow through the soil no hydraulic head was maintained and the flow did not occur. significant increase in flow was observed on application of initial voltage (5 v) as shown in fig. 5. subsequent reduction in flow and increase in ph (fig. 6), has been attributed to the precipitation of oh ions at the cathode. further enhancement of voltage (10 v) doesn’t affect initially but increases the flow on addition of acetic acid at cathode side followed by the sudden reduction in ph. prolonged application of higher voltages (20, 30, 45 and 60 v) doesn’t improve the flow any further, due to increase in ph at the cathode as shown in fig 5. j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 56 fig. 4. electro-osmotic permeability in uncontaminated soil fig. 5. electrokinetic flow through soil contaminated with nickel ions p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 57 the range of electro osmotic permeability with nickel as contaminant in red earth is 0 to 3.55×10 -5 cm 2 / v s as shown in fig. 7. a maximum electro osmotic permeability of 3.55×10 -5 cm2 / v s was observed at 5volts. induction of further voltage (10 v) reduces the electro osmotic permeability by almost 3 times and further increments in the voltage (20, 30, 45, 60 v) gradually reduce the electro osmotic permeability and minimum value (0 cm 2 / v s) at 60 v. the electro osmotic permeability of soil with nickel as contaminant is observed to be maximum value of 3.55×10 -5 cm 2 / v s, even with a low applied voltage of 0.167 v / cm only. fig. 6. variation of ph of the effluent from soil contaminated with nickel ions fig.7. electro-osmotic permeability of soil contaminated with nickel ions j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 58 effect of cadmium ions on the osmotic flow through the soil application of voltage (5 v) induced the flow initially with gradual reduction along with time as shown in fig. 8. increase of voltage (10 v) doesn’t compliment the flow even with addition of acetic acid as shown in fig. 8, may be due to consequent precipitation of cadmium hydroxide, further increase in voltage doesn’t improve the flow, but with addition of acetic acid the flow enhancement was significant. with increase in ph (12.18) the flow reduces in fig. 9, even with further applications of prolonged voltages. fig. 8. electrokinetic flow through soil contaminated with cadmium ions fig. 9. variation of ph of the effluent from soil contaminated with cadmium ions p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 59 the electro osmotic permeability of red earth with cadmium as contaminant is in the range of 0 to 2.7×10 -5 cm 2 / v s , fig. 10. the electro-osmotic permeability was 2.7×10 -5 cm 2 / v s at a voltage of 5 v with a maximum flow rate. may be due to the precipitation of cadmium hydroxide at cathode the electro osmotic permeability and rate of flow were zero at 10 v. fig. 10. electro-osmotic permeability of soil contaminated with cadmium ions a marginal increase was observed with increase in voltage to 20 v. further increase in voltage with acetic acid resulted in gradual decrease in electro osmotic permeability. presence of cadmium as contaminant doesn’t increase the electro osmotic permeability. maximum electro osmotic permeability (2.7×10 -5 cm 2 / v s) is observed at 0.167 v / cm. effect of ferric ions on the osmotic flow through the soil a sudden initial flow was observed on application of 5 volts initially and it subsequent reduction along with time as in fig. 11. no change was observed when the voltage was increased to 10 v along with addition of acetic acid. increase in the flow was seen on addition of acetic acid at higher voltage of 20 v, subsequent increase in voltage to 30 v doesn’t enhance the flow (fig. 11), may be due to the low ionic mobility of iron. during the whole process the ph doesn’t vary substantially as shown in fig. 12. j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 60 fig. 11. electrokinetic flow through soil contaminated with ferric ions fig. 12. variation of ph of the effluent from soil contaminated with ferric ions p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 61 the range of electro osmotic permeability of red earth with iron as contaminant is in the range of 6×10 -7 to 1.1×10 -5 cm 2 / v s, as shown in fig. 13. a maximum electro osmotic permeability of 1.1×10 -5 cm 2 / v s was monitored at 5 volt as shown in fig. 13. and a sudden decrease of electro osmotic permeability was observed at an applied voltage of 10 v. it subsequently increased with higher voltage of 20 v. the electro osmotic permeability remained at constant rate with further increase in the voltages as shown in fig. 13. the maximum electro osmotic permeability of 1×10 -5 cm 2 / v s is observed at 0.167 v / cm. fig.13. electro-osmotic permeability of soil contaminated with ferric ions electrokinetic extraction of metal ions the extent of removal of a metal ion from any soil depends on the mobility of the ion and its affinity for the soil clay. the lower is the ionic mobility and the higher is the affinity, the lower is its removal from the soil. the removal also depends upon the solubility product of the species and the ph of the system. as the solubility product at any ph increases their solubility product, the ion is precipitated and hence not removed. the effectiveness of acetic acid to lower the ph and enhance their removal as well as the efficiency of edta solutions to enhance desorption of the ion from the soil and hence to increase its removal however during each of these phases the processes might not have been completed. only the removal trends with different fluids such as water, acetic acid and edta solution have been studied. with each fluid the applied voltage is varied. also the percent removal of the species with respect to increase in osmotic flow has been compared. how the selected contaminants respond to the removal by electrokinetics with respect to osmotic flow generated at different acidic and alkaline conditions existing during electrokinetic process are discussed in detail. j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 62 removal of nickel from red earth it can be seen from fig. 14 that the amount of removed ni is almost proportional to the quantity of the flow generated during different phases of electrokinetic process while passing acetic acid and water. this shows that the removal is essentially due to osmotic flow. however, prolonged application of 10 v, while increasing the flow does not proportionally enhance the removal of ni. this might be due to precipitation of ni at enhanced ph. increasing the voltage and passing acetic acid greatly help to remove the ni contamination. this confirms that ni is actually precipitated and by passing acetic the rate of removal of ni is greater than the increased flow due to sudden dissolution of precipitated ni. thus passing of acetic acid not only reduces ph at cathode but also enhances the electro-osmotic flow as well as desorption of ni from the surface of clay particles. with a small increase in voltage from 10 v to 20 v, brings about maximum removal of about 35 %. fig. 14. removal trends of nickel ions from soil by electrokinetic process as seen from fig. 14 further increase in voltage (20 30 v) increases osmotic flow and amount of removed cd is also proportional to electro osmotic flow. further, prolonged application of voltage beyond 30 v the rate of flow decreases and tends to be more or less constant as result of increasing ph at the cathode. thus, prolonged application of voltage beyond 30v is not beneficial in extraction of nickel form red earth. p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 63 removal of cadmium from red earth removal of cd from red earth is compared with the quantity of flow that is generated at different voltages and by passing water or dilute acetic acid. it can be seen from fig. 15 that the amount of removed cd is almost directly proportional to the quantity of flow generated. the variations in the rate of flow at different conditions that exist during the electrokinetic extraction are discussed earlier. there is no phase during which the removal of cd is greater than the rate of flow. this indicates that either cd is not precipitated or the precipitated cd is not dissolved at ph obtained by passing acetic acid. this might be due to the low solubility product of cadmium hydroxide. this indicates that desorbing of cd from soil is not a major limitation for removal by electrokinetics. during different conditions of removal the maximum removal of cd is about 16 %. the amount of removed cd can be enhanced if the rate of flow is maintained for longer periods. the osmotic flow itself is greatly increased by increasing the applied voltage to 20 v along with acetic acid. increasing the applied voltage beyond 30 v is not beneficial in enhancing the rate flow. thus, for enhance removal of cadmium passing of about 20 v for longer periods is advisable. fig. 15. removal trends of cadmium ions from soil by electrokinetic process removal of iron from red earth percent removal of iron compared to the quantity of flow generated is not proportional at different voltages applied as well as passing of acetic acid solution and water during the electrokinetic extraction process as seen from fig. 16 due to immediate precipitation as its hydroxide as well as the ion being strongly adsorbed to the clay particle. the solubility product of j. electrochem. sci. eng. 5(1) (2015) 47-65 removal of heavy metals from soil 64 iron hydroxide is very low and hence is not dissolved and removed. also, fe might not have been removed due to its strong adsorption to the clay. however, with increase in voltage to 60 v i.e., at 2 mv / cm it is observed that enhanced voltage desorbs the ions from clay surface and increase in % removal of the contaminant. thus, increase in voltage for longer durations might help to enhance its removal. however this method is not very effective for decontamination of iron from soil. fig. 16. removal trends of ferric ions from soil by electrokinetic process conclusions 1. osmotic permeability of soil is not significantly affected by the presence of ionic metal contaminants. the variation of osmotic permeability with different contaminants is in the order of soil without contaminant > ni > cd > fe. 2. the rate of flow increases it is not proportional to the applied voltage. presence of fe reduces the electro osmotic permeability. the maximum osmotic permeability is obtained 0.167 m v / s. presence of ionic contaminants reduces the applied voltage required to induce maximum osmotic permeability, though the maximum osmotic permeability induced itself doesn’t increase. passing of acetic acid doesn’t enhance the electro osmotic permeability though the ph of the system is controlled. 3. the increase in ph with application of voltage across the contaminated soil increases with ni and cd as the contaminants are significantly more. 4. the amount of removed ni is generally proportional to the osmotic flow but shows more sensitivity to the ph of the system than to the osmotic flow in case of nickel. p. v. sivapullaiah et al. j. electrochem. sci. eng. 5(1) (2015) 47-65 doi: 10.5599/jese.2015.0055 65 5. the amount of removed cd contaminant is almost proportional to the osmotic flow generated in case of cadmium. 6. the amount of removed fe is not proportional to the quantity of osmotic flow generated either by adsorption of the contaminant or if the removed contaminant is precipitated. 7. the higher is the solubility product and lower the affinity of the contaminant the higher is the decontamination of ionic metal contaminants.. references [1] a. n. alshawabkeh, y. b. acar, j. environ. sci. heal. a 27 (1992) 1835-1861. 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[9] t. rødsand, b. acar yalcin, b. gijs, publikasjon-norges geotekniske institute 195 (1996) 1518-1535. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {electrochemical behavior of tantalum in potassium hydroxide solutions} doi:10.5599/jese.537 291 j. electrochem. sci. eng. 8(4) (2018) 291-301; doi: http://dx.doi.org/10.5599/jese.537 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behavior of tantalum in potassium hydroxide solutions irena mickova faculty of technology and metallurgy, university “ss. cyril and methodius” rudjer boskovic 16, 1000 skopje, republic of macedonia e-mail: mickova@tmf.ukim.edu.mk received: april 16, 2018; revised: may 5, 2018; accepted: may 10, 2018 abstract the electrochemical behavior of tantalum in various concentrations of koh solutions (0.1 m 10 m), was investigated using the evolution of the open circuit potential in time, cyclic voltammetry and ellipsometric measurements. depending on koh concentrations, the open circuit potential measurements have shown three distinct behaviors concerning oxide film formation on the electrode surface and its dissolution. the cyclic voltammetry measurements were performed in various potential ranges, from -1.4 to 8 v, different concentrations of koh solutions (0.110 m) and sweep rates ranging from 0.005 v/s to 1 v/s. in the passive region, very stable passive films were formed, which reduction has not been possible during cathodic polarization even at highly concentrated koh solutions. in the trans-passive region, the very strong peak at 1.65 v was monitored, which nature and chemical composition is still not well known. keywords ta; koh; open circuit potentials; cyclic voltammetry; ellipsometry introduction tantalum is known as a refractory metal because its melting point of 2996 °c is greater than that of platinum 1750 °c. in high temperature processes, tantalum is frequently used as a substitute of platinum and gold, which are more expensive. tantalum also belongs to the group of valve metals because it develops a stable passivating layer spontaneously, when contacting air and/or water solutions. the resistance of valve metals against corrosion originates from the property of the passive film to regenerate sufficiently after an eventual breakdown [1-4]. among the valve metals, ta is known to be the most resistant metal, what is due to the quickly and cohesively formed passivating layer of the insulating amorphous ta2o5. also, ta is the most corrosion resistant metal in common use today. http://dx.doi.org/10.5599/jese.537 http://www.jese-online.org/ mailto:mickova@tmf.ukim.edu.mk j. electrochem. sci. eng. 8(4) (2018) 291-301 tantalum in potassium hydroxide solutions 292 in 21st century, the primary use of ta comes from its oxide ta2o5 which has received a considerable attention as capacitors in the field of micro-electronics and integrated microtechnologies [5]. ta2o5 thin films can be deposited by several methods, such as sputtering, anodic oxidation and chemical vapor deposition. in electrochemistry, the anodic oxidation of ta and formation of ta2o5 films was the subject of numerous investigations concerned with the growth of thin films in different acidic and neutral solutions [6-9]. so far, the passive and corrosion behavior of tantalum has been studied using several electrochemical techniques such as chronoamperometry [10], coulometry [11], linear polarization [12], cyclic voltammetry [13], galvanostatic polarization [14], electrochemical impedance spectroscopy (eis), capacitance and photocurrent measurements [15-17]. despite extensive literature data about the passive or corrosion behavior of ta, studies concerned with the achievement of passivity in the presence of higher concentrations of alkaline media were scarcely carried out [18,19] and thus, data on the behavior of ta in strong alkaline media are very limited [20]. on the other hand, there still remain a number of questions concerning the influence of the forming electrolyte and its concentration on the stability of passive behavior of ta/ta2ox-electrolyte systems which have not yet been addressed. the aim of this work is to present a more systematic study for enlarging the already existing domain of the influence of various parameters on the electrochemical behavior of ta in koh solutions. by investigating the evolution of open circuit potential (ocp) over time, in combination with in-situ ellipsometric measurements and cyclic voltammetry (cv) in larger concentration range of koh solution and larger potential region in (cv) measurements, it is expected to obtain more information concerning formation and stability of passive films formed on ta surfaces. experimental electrodes massive cylindrical ta rods (alfa aesar, johnson matthey company), with purity of 99.95 % and diameter of 6.4 mm, were cut in discs with thickness of 3 mm which served as the working electrodes. the discs were embedded into teflon tubing holder of appropriate internal diameter, leaving the one basis with surface area of 0.32 cm2 to be exposed to electrolyte solution. before each experiment, the electrode surfaces were mechanically polished using successively various grades metallographic emery papers (from 400 to 1200). after each experiment, the electrodes were mechanically re-polished and prepared for next measurements as described above. the counter electrode was a pt spiral wire and the reference electrode was a saturated calomel electrode (sce), having the contact provided through a bridge with a luggin capillary filled with the test solution. all measured potentials are referred to the sce. electrolytic optical cell for simultaneous electrochemical and in-situ ellipsometric measurements, the special cell of the pyrex glass vessel was used. it was equipped with three cylindrical compartments for the working, counter and reference electrode, as well as an inlet and an outlet for bubbling inert gas. prior to each experiment, the electrolyte in the cell was de-aerated by flowing argon gas through a fritted bubbler for at least 30 min before the run. solutions all experiments were carried out in koh solutions with concentrations of 0.1, 0.5, 1.0, 2, 5 and 10 m, prepared from koh crystals p.a. merck and triply distilled water. after each experiment, the electrolyte in the cell was exchanged in order to avoid any eventually formed soluble species. i. mickova j. electrochem. sci. eng. 8(4) (2018) 291-301 doi:10.5599/jese.537 293 apparatus the electrochemical measurements were carried out using autolab (model 128n) interfaced with pc. for ellipsometric measurements, the equipment was thin film ellipsometer, rudolph research, type 43603-200 working at wavelength of  = 546.1 nm and at an incidence angle of  = 70o. more details on simultaneous electrochemical and in-situ ellipsometric measurements have already been described in our previous works on ti and nb [21]. results and discussion when ta is handled in air, the electrode surface is always covered spontaneously by a natural oxide film. for a polished electrode, the film thickness tended to increase with increasing of exposure time and the film thickening was faster during initial 3 hours. it can be found in literature that the predominant constituent, about 88 mol % of natural oxide film, is ta2o5 formed as the outer layer. the remaining, a smaller fraction of about 12 mol %, is the inner sub-layer of the film that is primarily tantalum monoxide tao [22]. evolution in time of the open circuit potential (ocp) for ocp measurements, after mechanical polishing, ta electrodes were quickly transferred into electrolytic cell with desired concentration of koh, taking the immersion moment as zero time (figure 1a). simultaneously with ocp measurements, in-situ ellipsometric measurements were also performed (figure 1b). the ellipsometric measurements showed that at time zero, the thickness of the oxide film spontaneously formed in atmospheric conditions is between 1 and 1.5 nm. three distinct behaviors of ta, depending on koh concentration, can be noticed in fig. 1: (i) thickening of natural oxide film with augmentation of electrode passivity, (ii) a continuation of film thickening followed by its dissolution and (iii) only dissolution of natural oxide film. in 0.1 and 0.5 m koh solutions, the ocp shifted continuously to more positive values with time, illustrating the self-passivation processes on ta electrode with thickening of the air-formed natural oxide film. the healing and further thickening of this film will occur until the steady-state conditions are achieved. the relative thickness d of passive film on the primarily formed natural oxide film increases from about 1.2 nm in 0.1 m koh and from about 0.8 nm in 0.5 m koh. the possible anodic reaction during the passivation and thickening of the natural oxide film is oxidation of the sub-layer of tao into ta2o5: 2 tao + 6 oh ta2o5 + 3 h2o + 6 e(1) according to figure 1a, the ocp of ta-solution system shows a parabolic variation with exposure time for 0.1 m and 0.5 m koh solutions, which may indicate the thickening of the oxide film. after 1500 min of exposure, the steady state conditions are attained. in 1.0 m koh solution the ocp variation in time is also parabolic, but steady-state conditions are reached faster, approximately after 220 min. in 2 m koh solution, the process of passivation and thickening of oxide film begins first and the ocp is shifted in positive direction forming a broad peak and then it slightly shifts in the reverse potential direction until the steady-state is reached. the broadening of the curve indicates that after longer exposure time, the dissolution of the film begins to be slightly predominant process in comparison with the thickening of oxide film. this is confirmed by ellipsometric measurements where d firstly increases for about 0.4 nm and then decreases for about 0.2 nm. in 5 m koh solution, the ocp value shortly increases in a positive direction forming the maximum at -910 mv. formation of this maximum is associated with thickening of natural oxide film and its subsequent decrease for about d = 0.5 nm, indicating a process of film dissolution where the natural oxide film is also partly dissolved. from the shapes of the ocp vs. time curves for 2 m and 5 m koh, it is evident j. electrochem. sci. eng. 8(4) (2018) 291-301 tantalum in potassium hydroxide solutions 294 that the process of dissolution is faster in 5 m koh than in 2 m koh. in 10 m koh the ocp is shifted only in a cathodic direction, indicating that the oxide film is subject of continuous dissolution. a. robin [20] has already investigated evaluation of the ocp of ta as a function of exposure time in 5 – 30 % naoh solutions at room temperature and 50 and 75 °c. here obtained ocp vs. time curves are similar to ocp curves presented by this author for lower and higher concentrations of alkali solutions and shorter exposure time. time, min time, min fig. 1 variation with exposure time of (a) ocp for ta polished electrode in solution with different koh concentrations, (b) film thickness determined by simultaneous ellipsometric in-situ measurements. koh concentrations: 1 – 0.1 m, 2 0.5 m, 3 1 m, 4 2 m, 5 – 5 m and 6 10 m koh it can be noticed in fig. 1 that for medium hydroxide concentrations, the ocp curves change their shapes for longer exposure time, indicating thus evolution of two distinct processes: thickening and dissolution of oxide film. the dissolution mechanism in higher concentrations of naoh proposed in [20], should be also adequate for dissolution of the oxide film in higher concentrations of koh: 3 ta2o5 + 8 koh  k8ta6o19 + 4 h2o (2) a subsequent anodic reaction may be the dissolution of ta metallic substrate with precipitation of potassium isopolytantalate at steady-state conditions: 6 ta + 8 koh +30 oh-k8ta6o19 + 19 h2o + 30 e(3) cyclic voltammetry (cv) cyclic voltammetry (cv) studies on ta were performed by potential scanning in various potential ranges (from -1.4 to 8 v) in different concentrations of koh solutions and scan rates ranging from 0.001 to 1v/s. typical cyclic voltammograms of ta in koh solution are shown in figure 2. the potential scan was initiated at -0.85 v, what is more negative potential than ocp (-0.6 v in fig. 1). in the first positive scan, a narrow active region followed by the passivation transition was observed in figure 2. in the active region, where the anodic current peak at -0.65 v is formed, the dissolution of metal substrate proceeding by an anodic process is described as: ta ta5+ + 5e(4) dissolution rate of the process (4) is slower and the current of the active peak is quite smaller than were noticed for the anodic processes carried out under same conditions for the same sized electrodes of other valve metals ti and nb [23-25]. the electrode dissolution in the active region is followed by diffusion of ta+5 ions, undergoing through the pores of oxide film from the metal substrate to the electrolyte. 0 400 800 1200 1600 -1.5 -1.2 -0.9 -0.6 -0.3 p o te n ti a l / v v s (s c e ) time / min 1 2 3 4 5 6 a 0 400 800 1200 1600 0.8 1.2 1.6 2.0 2.4 t h ic kn e ss , n m time, min 1 2 3 4 5 6 b t h ic kn e ss , n m p o te n ti a l, v v s. s c e i. mickova j. electrochem. sci. eng. 8(4) (2018) 291-301 doi:10.5599/jese.537 295 fig. 2. typical cyclic voltammograms of ta electrode recorded in 1.0 m koh solution at 10 mv/s: 1 – first cycle, 2 – second cycle for more positive potentials than -0.65 v, the anodic current begins to decrease forming a passive region. in the passive region, the metal dissolution by reaction (4) is hindered and the thickness of anodic oxide film on ta grows according to the following reaction: 2 ta + 10 ohta2o5 + 5 h2o + 10 e(5) reaction (5) preferentially takes place when anodic current appears in the first positive cv scan [12]. the passive region extends over a potential range from approximately -0.55 v to 0.65 v and then the current begins to increase quickly, leading to a trans-passive region. in the first reverse potential scan, the current decays gradually without any peak observed in the cv curve, indicating that the anodic oxide film formed in the first positive scan remained stable. in the second and next subsequent potential scans, large current plateaus with values of current close to zero for both forward and reverse scans were recorded. it is evident that after the first positive scan, the ta electrode remained passive in the whole investigated potential region. the passive film formed in the first positive potential scan blocked all possible redox reactions and further dissolution of ta, minimizing thus transfer of za 5+ions into the electrolyte. effect of potential scanning range in order to get more valuable information about possible redox reactions on the ta electrode in various concentrations of koh solution, the potential scanning range was enlarged from -1.4 v to 8 v. at the initial cathodic potential of -1.4 v, a considerable cathodic current was observed due to the evolution of hydrogen on the electrode surface. a part of hydrogen which is generated on the ta electrode is adsorbed on the electrode surface as h atoms. when their amount becomes larger, the hydrogen will be liberated by electrochemical desorption forming h2 gas (heyrovski step) [23]: h2o + hads + e h2 + oh(6) the total hydrogen evolution reaction can be described as: 2 h2o + 2e h2 + oh(7) figure 3 presents cvs recorded at shorter potential range up to -0.5 v, just after apparition of anodic peak in the passive region where smaller thickness of the passive film is formed. j. electrochem. sci. eng. 8(4) (2018) 291-301 tantalum in potassium hydroxide solutions 296 fig. 3. two cycles of cyclic voltammograms of ta electrode recorded in 10 m koh at 2 mv/s: 1 – first cycle, 2 – second cycle at slow scan rates as 2 mv/s, cvs in figure 3 showed an active region with relatively big current oscillations observed as a result of high ohmic resistance of the ta surface. the slow scan rate was applied specially for the reason that after finishing of the activation process, the electrode surface would have, in the reverse of cycle 1, enough long time for electrochemical dissolution of very thin passive layer formed. since in the anodic scan of cycle 2, the apparition of reactivation peak is missing, it can be concluded that even in the very high concentration of koh, there is no appearance of dissolution of passive films. the same has already been observed for nb and ti [25,26]. after the first cycle, the electrode was kept at -1.4 v for 30 min and the second cycle was recorded to verify if the passive film formed in the first cycle could be reduced at -1.4 v. however, in the forward scan of the second cycle, only a current plateau with a small increase of the current at the end of the anodic potential is observed, indicating that the activation process has not been fully finished in the first cycle. from these results, it can be concluded that ta is more resistant in strong alkaline solution than ti and nb [27-29]. using a number of cv experiments it was approved that at the beginning of cv measurements, various cathodic currents generated by the initially applied cathodic potential, influenced negligibly to the shape of cv curves. therefore, our further measurements started at the cathodic potentials where small currents were recorded at the beginning of measurements, avoiding thus hydrogen evolution and bubbles rolls across electrode surface. figure 4 presents cv curves recorded for a larger range of anodic potentials up to 8 v. figure 4a shows that in the trans-passive region of potentials, the strong peak appears at 1.65 v for which a nature and chemical composition are still unknown, in spite of some theoretical explications that have already been given [30,31]. it is known that abnormal increase of current density in the transpassive region under some specific conditions such as applied potential, ph of the electrolyte, presence of active anions etc., is often associated with the local breakdown of the passive film. the process of the local breakdown represents the metal dissolution with the rate depending on the chemical energy stored in the system. there are spontaneous local increases of the film conductivity, which is sometimes higher by many orders of magnitude. in that a case, the passive film can be simulated as dielectric medium of a capacitor which is short circuited by the local breakdown. one of the explication of the current peak located at 1.65 v could be a localized dissolution of ta associated with the local breakdown process in the passive film which provokes an increase of the anodic current. if the film breakdown is associated with the film repair and its repassivation, the current begins to decrease forming a current peak. -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -20 -10 0 10 20 30 40 c u rr e n t,  a potential, v vs. sce 1 2 i. mickova j. electrochem. sci. eng. 8(4) (2018) 291-301 doi:10.5599/jese.537 297 figure 4b shows sequences of cv measurements where the final anodic potential on ta in 0.5 m koh was gradually increased in each next cycle. all subsequent cvs started from and returned to the same cathodic potential of -1.4 v. single difference in these measurements is only the limit of anodic potential. the progress of the film thickness growth with anodic potential may take place only if the actual applied potential exceeds the maximum of potential value attained in the previous cycle. for each sequence, the film thickness grows in the first forward scan and has constant value in the reverse scan. for each next sequence, in the first forward scan the film thickness has constant value up to the end of previous cycle and then continues to grow up to the final anodic potential. it is evident that by gradually increasing final anodic potential value, the passive film grows again. fig. 4 cyclic voltammograms of ta electrode recorded in 0.5 m koh at 50 mv/s: (a) first cycle, (b) multiple cycles at gradual increasing of final anodic potential the shapes of the sequences recorded in figure 4b represent just copy segments of the corresponding voltammogram recorded from the starting potential at -1.4 v to the final potential at 8 v at once (fig. 4a). the experiments shown in figure 4b confirm the barrier properties of anodic oxide film built on the ta surface. they also confirm that the localized dissolution of ta by local breakdown process which is associated with repair of the film and its re-passivation is only possible for the film thickness built at the anodic potential higher than 1.65 v. this can be concluded because in reverse scans, the reactivation peak at 1.65 v does not exist, as in the forward scan. using photoelectrochemical measurements, it has already been shown that the current peak around 1.65 v of abnormal intensity appears under the influence of an applied voltage, corresponds to the energy of the band-gap and can be related to the transition from semi-conducting to dielectric behavior of the film [32]. this current peak with abnormal intensity was also investigated by silva et al. [33]. using polarization curves and capacity measurements, as well as atomic force imaging microscopy (afm), these authors concluded that appearance of the peak at 1.65 v is generated only from the relationship between surface state and film growth. cavigliasso et al., have investigated electrochemical passivation of ta electrode in various electrolytes by cv measurements in the potential range from 0 to 8 v [15]. for naoh and also for some acid solutions, the peak at 1.65 v was missing. ta electrodes used by these authors, however, were firstly mechanically and then chemically polished in the mixture containing hf, hno3 and h2so4, which is different from the way of preparation applied in this work. one can therefore conclude that surface preparation of ta plays an important role on the profile of the recorded cv curves. -2 0 2 4 6 8 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 c u rr e n t, m a potential, v vs. sce 1 cycle 2 cycle 3 cycle 4 cycle 1 2 3 4 b j. electrochem. sci. eng. 8(4) (2018) 291-301 tantalum in potassium hydroxide solutions 298 effect of koh concentration the effect of koh concentration (c) on the peak potential (ep) and peak current (ip) of anodic peak was investigated for scan rates from 5 mv/s to 1 v/s. the potential sweep was initiated at -1.4 v, where at the beginning small cathodic currents for all investigated concentrations of koh were observed (figure 5). potential, v vs. sce fig. 5. cyclic voltammograms of ta electrode recorded in various concentrations of koh solutions at 0.1 v/s figure 5 shows that for increased koh concentration, the height of the maximum anodic current (ip) is noticeable increased, while its corresponding primary passivation potential (ep) is shifted towards more negative potential values, indicating enhancing of the rate of ta dissolution. from figure 5, figures 6 and 7 were constructed presenting plots of ep and ip on logarithm of koh concentration. fig. 6. dependence of primary passivation potential ep on the logarithm of koh concentration at 0.1 v/s fig. 7. dependence of primary anodic current ip on the logarithm of koh concentration at 0.1 v/s figure 6 shows linear dependence with the slope of dep /dlog c = -0.244 v/dec. an obvious negative shift of ep is observed at increasing koh concentration. according to figure 7, the -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 -1.5 -1.0 -0.5 0.0 0.2 0.4 0.6 c u rr e n t / m a potential / v (sce) 5 6 cu rr e n t / m a potential / v vs (sce) 1 0.1 m koh 2 0.5 m koh 3 1 m koh 4 2 m koh 5 5m koh 6 10 m koh 1 2 3 4 -1.0 -0.5 0.0 0.5 1.0 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 e p / v v s . s c e log (c / m) -1.0 -0.5 0.0 0.5 1.0 0.60 0.65 0.70 0.75 0.80 0.85 lo g ( i p / m a ) log (c / m) c u rr e n t, m a c u rr e n t, m a potential, v vs. sce i. mickova j. electrochem. sci. eng. 8(4) (2018) 291-301 doi:10.5599/jese.537 299 dependence of primary anodic current ip on the concentration of koh is linear up to 5 m koh and then shifts to higher value of ip, indicating that for 10 m koh dissolution of ta rapidly increases. effect of potential sweep rate the effect of the potential sweep rate ( ) on potentiodynamic polarization responses for ta electrode in 1 m koh is shown in figure 8. the recorded voltammograms confirm the strong influence of sweep rate to the dissolution of ta and the growth rate of the oxide film formed on ta. the maximal anodic currents at the primary passivation potential are noticeable increased with increasing the sweep rate. a b fig. 8. cyclic voltammograms of ta recorded in 1m koh; (a) for sweep rate from 0.005 to 0.07 v/s, (b) for sweep rate from 0.1 to 1 v/s on the other hand, the primary passivation potential ep is slightly shifted to more positive potentials as suggested in figure 9 by the linear fit of ep versus log () with a slope of 0.093 v/dec. relatively low value of the slope in comparison with other valve metals indicates very low dissolution rate of ta. the plots of primary anodic currents ip against the square root of sweep rate are shown in figure 10. fig. 9. dependence of primary passivation potential of ta electrode in 1 m koh on logarithm of the sweep rate fig. 10. dependence of primary current of ta electrode in 1 m koh on on logarithm of the sweep rate the square root of the sweep rate -2.5 -2.0 -1.5 -1.0 -0.5 0.0 -0.80 -0.75 -0.70 -0.65 -0.60 e p / v v s . s c e log (/ v s -1 ) 0.0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 i p / m a  1/2 / (v/s) 1/2 j. electrochem. sci. eng. 8(4) (2018) 291-301 tantalum in potassium hydroxide solutions 300 for 1 m koh, the plot is not a straight line passing through origin. similar shapes of ip vs. 1/2 plots to this presented in figure 10, were also obtained for ta in 0.1 10 m koh solutions, suggesting that formation/dissolution of passive films on ta electrode is not a purely diffusion-controlled process. conclusions • the ocp potential-time profiles revealed three distinct behaviors of ta depending on koh solution concentrations: (i) thickening of oxide film with enhancement of electrode passivity for 0.1, 0.5 and 1.0 m koh solutions, (ii) thickening of the oxide film followed by its dissolution in 2 and 5 m koh solutions and (iii) only dissolution of oxide film in 10 m koh. • the cyclic voltammetry profiles in all investigated concentration of koh solutions revealed the active/passive transition; the formed passive films are stable and do not undergo any reduction processes in the reverse scan. • for larger anodic potential up to 8 v, the anodic peak was recorded at 1.65 v which is located in trans-passive region. the origin of this peak was explained by local breakdown of passive film and dissolution of metal support. the film breakdown was associated with re-passivation and reparation of the film, so the current has begun to decrease forming the current peak. • the effect of koh concentration on the cyclic voltammetry profiles has shown that with the increase of koh concentration the value of peak current ip was 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characterization of lamno3 nanocrystals and graphene oxide: fabrication of graphene oxide–lamno3 sensor for simultaneous electrochemical determination of hydroquinone and catechol} http://dx.doi.org/10.5599/jese.634 255 j. electrochem. sci. eng. 9(4) (2019) 255-267; http://dx.doi.org/10.5599/jese.634 open access: issn 1847-9286 www.jese-online.org original scientific paper synthesis and characterization of lamno3 nanocrystals and graphene oxide: fabrication of graphene oxide–lamno3 sensor for simultaneous electrochemical determination of hydroquinone and catechol sedighe akbari1, mohammad mehdi foroughi1,, hadi hassani nadiki1 and shohreh jahani2 1department department of chemistry, kerman branch, islamic azad university, kerman, iran 2nanobioelectrochemistry research center, bam university of medical sciences, bam, iran corresponding author: mohammadmehdi869@yahoo.com; tel. +98 34331321750 received: november 1, 2018; revised: april 7, 2019; accepted: may 9, 2019 abstract for the first time, a new method for preparation of graphene oxide-lamno3 (go-lamno3) nanocomposite as a material of electrochemical sensor for simultaneous determination of catechol (ct) and hydroquinone (hq) is developed. lamno3 nanoparticles have been characterized by fourier-transform infrared spectroscopy (ft-ir), scanning electron microscopy (sem), x-ray diffraction (xrd), transmission electron microscopy (tem) and energy dispersive x-ray analysis (edx) technique. due to the excellent catalytic activity, enhanced electrical conductivity and high surface area, the simultaneous determination of hq and ct with two well-defined peaks has been achieved at the go-lamno3 modified electrode. comparing with unmodified electrodes, the oxidation currents of hq and ct increased remarkably. also, the result exhibited a great decrease in anodic overpotential resulting in about 150 mv negative shift of potential. the catalytic peak current values are found linearly dependent on the hq and ct concentrations in the range of 0.5–433.3 and 0.5–460.0 μm with sensitivity of 0.0719 and 0.0712 μa μm-1, respectively. the detection limits for hq and ct are determined as 0.06 and 0.05 μm, respectively. keywords catechol; hydroquinone; graphene oxide; lamno3 nanoparticles; voltammetry introduction phenolic materials compounds are widely used in a broad class of industries, such as coal mining, cosmetics, paint, pharmaceutical preparation and polymer [1]. during the manufacturing and application process of these compounds, some of them are unintentionally released into environment, http://dx.doi.org/10.5599/jese.634 http://dx.doi.org/10.5599/jese.634 http://www.jese-online.org/ j. electrochem. sci. eng. 9(4) (2019) 255-267 fabrication of graphene–lamno3 sensor 256 polluting thus ground waters and rivers. catechol (ct) and hydroquinone (hq) are two most important isomers of phenolic materials which are announced as environmental contaminants by the european union and the us environmental protection agency [2]. simultaneous determination of hq and ct is very important for environmental analysis because they coexist in environmental samples as environmental contaminants of high toxicity and too difficult to degrade [3]. ct has also been found in cigarette smoke and researches have demonstrated that it induces damage to dna and can cause cancer [4,5]. hence, it is essential to develop a sensitive, simple, rapid and cheap analytical method for determination of dihydroxybenzene isomers. so far, many analytical methods have been reported for their determination, such as capillary zone electrophoresis, liquid chromatography, synchronous fluorescence, gas chromatography/mass spectrometry, chemiluminescence, and ph based-flow injection analysis [6-12]. electrochemical methods have some attractive advantages such as simple and fast response, low maintenance cost, excellent selectivity and high sensitivity. however, a serious obstacle is that the oxidation peak potentials of the isomers are too close at a bare electrode which causes overlapping of voltammetric responses, making their discrimination very difficult [13-17]. a chemically modified electrode is a superior approach to solve the peak separation problem by applying a modifier [18-24]. hence, a few modified electrodes have already been reported to determine hq and ct such as gce modified by carbon nanotubes (cnt), poly-amid sulfonic acid-cnts, graphene– chitosan composite, carbon nanotubes/poly (3-methylthiophene), penicillamine), zn/al layered double hydroxide film, or gold-graphene [25-31]. graphene (gr) nano sheets have a great potential for the applications in the development of electrochemical sensors and biosensors. gr is used as a support material to improve the electrochemical reactivity of molecules on the surface of modified electrode, what is due to its superior mechanical, electrical, thermal, and optical properties [32]. on the other side, its low energy storage capacity greatly hinders its extensive use. to overcome these shortcomings, many researches have been focused on the synthesis of graphene-based inorganic composites. recently, the synthesis procedures of new composites of graphene with inorganic particles, such as metal, metal oxide, metal hydroxide, or metal sulfide particles, have been reported [33-38]. due to presence of graphene and inorganic particles, the fabricated electrochemical sensors display exceptional improvement in electrical properties and functionality for different types of composites. among inorganic particles, perovskite nanocrystals having electrically active structure and convenient magnetic and dielectric properties have been broadly studied and employed in the development of electrochemical gas sensors, solid fuel cells and many of electrochemical catalytic processes [39-44]. to the best of our knowledge, no study has already been reported for the electroanalysis and simultaneous determination of hq and ct using gce modified by go-lamno3. for the first time, we have described the preparation of a new, go-lamno3 modified gce for the electroanalysis and determination of hq and ct. in this paper, we have evaluated analytical performance of this sensor for simultaneous determination of hq and ct by the voltammetry technique. experimental reagents and solutions manganese sulphate, lanthanum chloride and oleic acid were purchased from merck and used as received. the hq and ct were purchased from sigma-aldrich (sigma-aldrich, usa) and used as received. a 1.010−2 mol l−1 hq and ct solution was prepared daily by dissolving appropriate amounts of hq and ct in water and the solution was diluted to 100 ml with distilled deionized water. the solution was kept in a refrigerator in dark. more dilute solutions were prepared by serial dilution of sedighe akbari et al. j. electrochem. sci. eng. 9(4) (2019) 255-267 http://dx.doi.org/10.5599/jese.634 257 phosphate buffer solutions. phosphate buffer solution (pbs, 0.1 mol l−1) was prepared by mixing the stock solution of 0.1 mol l−1 nah2po4 and 0.1 mol l−1 na2hpo4, and the ph was adjusted by hcl or naoh. all other materials used were of analytical reagent grade and all solutions were prepared with double distilled deionized water. all chemicals were used without additional purification. apparatus ft-ir spectra were recorded as kbr disks on a jasco ft/ir-460 plus instrument. x-ray powder diffraction (xrd) analysis was conducted on a philips analytical pc-apd x-ray diffractometer with graphite monochromatic cu k radiation ( 1, 1 = 1.54056 å, 2,  2 = 1.54439 å) to verify the formation of products. surface analysis was done using a low vacuum jsm, 6380 lv scanning electron microscope (sem) after coating the samples with a thin layer of gold by magnetron sputtering. energy-dispersive x-ray spectrometry (edx) is a significant nondestructive analytical tool typically applied for the chemical composition analysis. electrochemical determinations were done with a sama 500 electroanalyser (sama research center, iran) controlled by a personal computer. the three-electrode electrochemical cell system consisted of glassy carbon working electrode (gce, modified or unmodified), a saturated calomel reference electrode (sce) and a pt wire electrode as the auxiliary electrode. all electrochemical determinations were carried out under a pure nitrogen atmosphere at room temperature. synthesis of perovskite nanocrystal in order to synthesize lanthanum manganese oxide nanocrystals, manganese sulphate and lanthanum chloride were chosen as starting materials. solutions of manganese sulphate (10 ml, 0.1 m) and lanthanum chloride (10 ml, 0.1 m) were prepared and mixed together. oleic acid (2 ml) was added into the pink solution. the ph of solution was adjusted to 7-8 by dropwise addition of naoh (1.5 m) in the stirred solution. after complete precipitation, it was nebulized in the ultrasonic at 60 °c for 30 min. the precipitate was centrifuged for 15 min at (3000 rpm), washed with deionized distilled water and dried at 100 °c for 8 h. the achieved product was calcinated at 1100 °c for 6 h to let the sample self-ignite and burn off the impurity organic compound in the crystal. electrochemical synthesis of graphene oxide the graphene oxide nano sheets were prepared by the electrochemical functionalization of graphite. in a typical synthesis, 10 ml 1-octyl-3-methyl-imidazolium hexafluorophosphate ([c8mim]+[pf6]-) and 20 ml water were mixed and used as the electrolyte. a static potential of 2 v was applied between two graphite rods (6.0 cm distance between two rods) for 2 h and then potential increased to 6 v for 6 h. after corrosion for 6 h at room temperature, a black precipitate of go was obtained at the bottom of the reactor. the black precipitate was separated and washed with ethanol and then sonicated for 30 min. the product was dried for 3 h in an oven at 70 °c. preparation of gce and modified electrode prior to the surface modification, gce was carefully polished with 0.3 µm and 0.05 µm alumina slurries to obtain a mirror-like surface. after sonication in water and ethanol successively for 20 s, the electrode was rinsed with water, and then dried under an infrared lamp. the gce/go-lamno3 was prepared by casting 4 µl of go-lamno3 suspension (0.01 g lamno3 + 0.005 g go + 50 µl chitosan 1 % + 0.95 ml h2o) on the surface of cleaned gce. the solvent was then evaporated under an infrared heat lamp. as controls, gce/go and gce/ lamno3 were also fabricated with similar procedure by replacing go-lamno3 hybrid materials with go or lamno3, respectively. http://dx.doi.org/10.5599/jese.634 j. electrochem. sci. eng. 9(4) (2019) 255-267 fabrication of graphene–lamno3 sensor 258 results and discussion characterization of lamno3 nanocrystals the ft-ir spectra of the pure oleic acid (a), precursor (b) and lamno3 nanoparticles after calcination (c) are shown in figure 1(a). in curve (a), two sharp bands at 2925 and 2854 cm-1 are attributed to the asymmetric ch2 stretch and the symmetric ch2 stretch, respectively. the intense peak at 1710 cm-1 was derived from the existence of the c=o stretch and the band at 1285 cm-1 exhibits the presence of the c–o stretch. in the curve (b), the asymmetric ch2 stretch and the symmetric ch2 are shifted to 2922 and 2853 cm-1, respectively. the surfactant molecules in the adsorbed state were subjected to the field of the solid surface. as a result, the characteristic bands shifted to a lower frequency region which indicates that the hydrocarbon chains in the monolayer surrounding the precursor are in a closed-packed, crystalline state. all these bands disappeared when the precursor was calcinated at 800 °c, but two strong bands appear in the range of 400-600 cm-1 related to metal-o and o-metal-o vibration of lamno3 perovskite (fig. 1(a) (c)). figure 1. (a) ft-ir spectra of (a) oleic acid, (b) precursor before calcination and (c) the product after calcination, (b) xrd pattern of the product, (c) sem image of lamno3 (d) edx of nanocrystals xrd pattern of lamno3 sample prepared via the ultrasonic-assisted co-precipitation process at 1100 °c is shown in figure 1(b). for lamno3 nanoparticles all diffraction peaks can be directly sedighe akbari et al. j. electrochem. sci. eng. 9(4) (2019) 255-267 http://dx.doi.org/10.5599/jese.634 259 indexed to a hexagonal phase of lamno3 according to the jcpds no. 23-0484 and no additional peaks for other impurities can be detected, indicating high purity of prepared sample. furthermore, the narrow sharp peaks suggest that lamno3 nanoparticles are well crystallized. the size distribution of lamno3 nanoparticles has been estimated from the xrd spectra using the debye– scherrer equation [45]: c cos k d    = where d is the crystallite size, k is the so-called shape factor which usually takes a value about 0.9,  is the wavelength of radiation,  is the corrected full width at half maximum (fwhm), and  is diffraction angle. the crystallite size of lamno3 was estimated as 54 nm at 1100 °c. the morphologies of lamno3 nanoparticles were studied by sem. as can be seen from figure 1(c), lamno3 crystal is made from spherical particles with average size of 56 nm. the purity of the nanocrystalline product was evaluated by edx (figure 1(d)), which showed that the sample was merely composed of la and mn. characterization of go and go-lamno3 nanocomposite tem image shown in figure 2a proves the formation of very thin go. the flake-like go shows a very stable nature under irradiation of electron beam. very thin and featureless sections are likely to be few layers of go, suggesting that the electrochemical method is slight, but strong enough to break down the van der waals interaction and make go layers by fractional exfoliation of the graphite surface. as expected, tem image of go-lamno3 presented in figure 2b shows well dispersed combinations of lamno3 nanoparticles on go when lamno3 and go are mixed together, which may cause many conducting channels between the electrode and electrolyte for electron transfer. figure 2. tem image of (a) go and (b) go-lamno3. voltammetric behavior of mixed hq and ct electrochemical behavior of mixed components of hq and ct (166.0 μm of each) in 0.1 m pbs of ph 5 was carefully investigated at the surfaces of bare gce (bgce), gce/lamno3 and gce/golamno3 using cyclic voltammetry. bgce electrode showed a weak and broad oxidation peak for a mixture of hq and ct at 0.33 v (curve a in figure 3) suggesting slow electron transfer kinetics. the http://dx.doi.org/10.5599/jese.634 j. electrochem. sci. eng. 9(4) (2019) 255-267 fabrication of graphene–lamno3 sensor 260 oxidation peaks of hq and ct are merged with a very low peak current. in contrast, the gce/lamno3 modified electrode showed two well-defined and sharp oxidation peaks for hq and ct at 0.11 and 0.21 v vs. sce, and two corresponding reduction peaks. it can be seen from curve b in figure 3 that the oxidation peak current for hq and ct at gce/lamno3 is several times larger than that of the unmodified electrode (bgce) because of catalytic property of lamno3 that acts as a promoter by increasing the rate of electron transfer. the potential values of peaks shifted to less positive potentials comparing with bgce. after addition of go to the modifier composition, peak current values increased (curve c in figure 3) due to high surface area and conductivity of go. the separation of oxidation peak potential of hq-ct was 0.1 v which is high enough for the simultaneous determination of two isomers. figure 3. cvs recorded in pbs (0.1 m ph 5) in presence of hq and ct (166.0 µm each) at (a) bgce, (b) gce/lamno3, (c) gce/go-lamno3 electrodes and in absence of hq and ct (d). scan rate: 100 mv s−1. effect of ph on the oxidation of hq and ct the acidity of electrolyte has a significant influence on the hq and ct electro-oxidation because protons take part in the electrode reaction. the effect of ph on gce/go-lamno3 current signal was carefully investigated by cyclic voltammetry using 0.1 m buffer solutions at ph ranging from 2 to 6. the results are shown in figure 4(a). obviously, the peak current values of hq and ct increased with an increase of the solution ph until ph reached 5 and then decreased. it can be seen from figure 4(b) that the highest peak current value is for both compounds obtained at ph 5. also, as ph of the medium was gradually increased, peak potentials for the oxidation of hq and ct are shifted towards less positive values, showing that protons have taken part in the electrode processes. since phosphate buffer solution at ph 5 gave the best response in terms of peak current value, peak shape and negative shift, it was selected as the optimal ph for further studies. plot of ep vs. ph for hq and ct in the working ph range is shown in figure 4(c). ep values of two compounds showed linear relationship with ph of the buffer solution according to the following equations: sedighe akbari et al. j. electrochem. sci. eng. 9(4) (2019) 255-267 http://dx.doi.org/10.5599/jese.634 261 ct: ep / v = −0.06 ph + 0.506 (r2= 0.999) (1) hq: ep / v = −0.063 ph + 0.41 (r2= 0.9963) (2) the observed slopes of 0.063 and 0.06 mv/ph for hq and ct are close to the anticipated nernstian value for two-electron, two-proton electrochemical reaction [46]. therefore, it can be concluded that equal number of electrons and protons are involved in both electrode reactions. the presumed hq and ct oxidation mechanisms are presented in scheme 1. scheme 1. probable oxidation mechanism for hq and ct figure 4. (a) effect of ph on the peak separation and peak current for the oxidation of hq and ct (100.0 µm each); ph 2 ̶6. scan rate: 100 mvs−1. (b) plots of peak currents vs. ph. (c) plots of peak potentials vs. ph. http://dx.doi.org/10.5599/jese.634 j. electrochem. sci. eng. 9(4) (2019) 255-267 fabrication of graphene–lamno3 sensor 262 influence of scan rate on electrochemical behavior of hq and ct the influence of the scan rate on the oxidation peak currents of hq and ct was investigated on the go-lamno3 modified gce by cyclic voltammetry. as can be seen in figure 5(a), the peak current values continuously increase with the increase of scan rate. as shown in figure 5(b), the current is directly proportional to the square root of the scan rate in the range of 10–1000 mv s−1, which powerfully proposed that the redox reactions of hq and ct are diffusion controlled. also, the results show clearly that the peak separation of hq and ct is satisfactory for their separate analysis at higher scan rates. it is additionally observed in figure 5 that with increasing of the scan rate, potential of the oxidation peak shifted positively, while potential of the reduction peak shifted negatively. this pointed to a kinetic limitation existing at higher scan rates in the course of the reaction between the gce/golamno3 composite and hq and ct. due to our study, the scan rate of 100 mv s−1 has to be ultimately chosen to reach the best efficiency for peak currents and peak separation. figure 5. (a) cvs of gce/go-lamno3 electrode in ph 5 in the presence of hq and ct (100.0 µm of each) at various scan rates (from inner to outer curve): 10, 25, 50, 100, 150, 200, 250, 300, 500, 700 and 1000 mv s−1. (b) plots of oxidation and reduction peak currents vs. υ1/2. interference studies as dihydroxybenzene isomers usually coexist in real samples, it is very important to study the interference of each other for the selective detection of the single one species. in each experiment, the concentration of one species was changed, while the concentration of the other one was kept sedighe akbari et al. j. electrochem. sci. eng. 9(4) (2019) 255-267 http://dx.doi.org/10.5599/jese.634 263 constant. the results are shown in figure 6. it can be seen from figure 6(a) that the oxidation peak current of hq increased with an increase in the concentration of hq, while the peak current for the oxidation of ct remained constant. the same is seen in figure 6(c) for the voltammetric peak corresponding to the oxidation of ct. this peak increased linearly with the increase of ct concentration, whereas the peak current for the oxidation of hq remained constant. the results presented in figure 6 (b and d) show that peak currents are linearly proportional to the concentration of hq and ct, respectively. the fact that by changing concentration of one species, the peak current value of the other species did not change, indicates that oxidations of hq and ct at go-lamno3 modified gce take place independently. interference of some other species on determination of hq and ct was investigated by the addition of interfering species (foreign compounds) into a solution containing 40.0 µm of hq and ct and the effects on the electrode recovery are shown in table 1. it is clear that all kinds and amounts of interfering species (expressed as ratios between foreign compounds and hq and ct) did not cause effects on currents being higher than ±5 %. figure 6. dpv at the gce/go–lamno3 electrode in pbs (0.1 m ph 5) (a) containing ct (166.6 µm) and different concentrations of hq (from inner to outer): 0.0, 6.6, 13.3, 33.3, 53.3, 86.6, 116.0, 170.0, 216.6, 250.0, 283.3, 316.0 and 350.0 µm. (c) hq (233.3 µm) and different concentrations of ct (from inner to outer): 0.0, 6.6, 13.3, 33.3, 53.3, 86.6, 116.0, 170.0, 225.0, 275.0, 310.0 and 350.0 µm. plots of peak current vs. concentration of (b) hq and (d) ct. http://dx.doi.org/10.5599/jese.634 j. electrochem. sci. eng. 9(4) (2019) 255-267 fabrication of graphene–lamno3 sensor 264 table 1. effect of interferences on determination of hq (40.0 µm) and ct (40.0 µm). foreign compound molar ratio of foreign compound/hq and ct recovery, % k+ (12 mm) 300.0 98.2 ca2+ (8 mm) 200.0 97.3 zn2+ (12 mm) 300.0 96.0 fe2+ (10 mm) 250.0 98.0 cl(12 mm) 300.0 99.4 no3(16 mm) 400.0 98.0 ascorbic acid (8 mm) 200.0 95.7 urea (12 mm) 300.0 96.9 simultaneous determination of hq and ct differential pulse voltammetry (dpv) was performed to investigate the relationship between the peak current value and concentration of hq and ct. as shown in figure 7, dpv curves showed two well distinguished oxidation peaks. electrocatalytic peak currents of hq and ct oxidation at the surface of gce/go-lamno3 are linearly dependent on hq and ct concentrations over the range of 0.5–433.3 and 0.5–460.0 μm, respectively. detection limit was obtained 0.06 and 0.05 μm for hq and ct, respectively. for 7 successive determinations of 100.0 μm of hq and ct, the relative standard deviations were 2.2 % and 1.9 %, respectively. these results show that the proposed electrode can be used effectively for the simultaneous determination of hq and ct. comparison of here obtained results using gce/go-lamno3 electrode with the results obtained using other modified electrodes is presented in table 2. figure 7. (a) dpv of the mixtures of hq and ct at the gce/go–lamno3 electrode in pbs (0.1 m ph 5) at the scan rate of 100 mv s−1. concentrations from inner to outer of curves: (0.0, 0.5, 1.5, 2.0, 3.0, 6.6, 13.3, 20.0, 30.0, 40.0, 55.0, 66.6, 85.0, 100.0, 133.3, 166.6, 200.0, 233.3, 266.6, 280.0, 316.6, 330.0, 350.0, 400.0, 433.3 and 460.0). (b) and (c): plots of oxidation peak current vs. concentration. sedighe akbari et al. j. electrochem. sci. eng. 9(4) (2019) 255-267 http://dx.doi.org/10.5599/jese.634 265 table 2. performance comparison of gce/go-lamno3 electrode for the simultaneous determination of hq and ct with other modified electrodes. modifier linear range, µm detection limit, µm ref hq ct hq ct pt–mno2 3-447 15-4481 [26] mwnts-p3mta/gce 0.5-150 0.5-150 0.05 0.05 [28] gold-graphene 1–100 1–100 0.2 0.15 [31] rgob–mwcnt 8–391 5.5–540 2.6 1.8 [47] carbon nanoparticle chitosan 0.8-100 0.8-100 0.2 0.2 [48] imidazolium ionic liquid 1-500 1-400 0.4 0.17 [49] nb2o5 0.8-500 39.8-980 1.6 0.8 [50] aminated glassy carbon 5-260 5-260 0.2 0.2 [51] graphene oxide-lamno3 0.5-433.3 0.5-460.0 0.06 0.05 this work apoly (3-methylthiophene); breduced graphene oxide real sample analysis in order to evaluate the analytical applicability of the developed method for simultaneous determination of hq and ct, local tap and mineral waters were tested. the amounts of hq and ct in the tap (kerman drinking water, kerman, iran) and mineral (damavand mineral water co., iran) water samples were determined. the reliability of the method was checked by the analysis of the samples spiked with the known amount of hq and ct. the results are listed in table 3. the recoveries were 95.4–102.5 % for hq and ct. therefore, a capability of the proposed electrode for the simultaneous determination of hq and ct is clearly confirmed. table 3. determination of hq and ct in water samples using gce/go-lamno3 (n = 5). sample analyte amount, µm recovery, % detected added founda tap waterb hq ct ndc 20 30 20 30 19.4±0.8 28.6±1.0 20.5±0.8 29.2±1.2 97.0 95.4 102.5 97.4 mineral waterd hq ct ndc ndc 30 30 28.8±1.3 29.1±1.0 96.0 97.0 amean ± standard deviation for n = 5; bkerman drinking water, kerman, iran; cnot detected; ddamavand mineral water co., iran; stability of modified electrode long term stability of modified electrode, gce/go–lamno3, was assessed for a period of three weeks, when the modified electrode was stored at atmosphere conditions. dpvs showed that there were no significant variations in regard to hq and ct oxidation peak potential values, with the exception of a drop less than 2.8 % and 2.3 % in comparison to the primary response. the adjusted electrode oxidation antifouling capacity pertaining to hq and ct and relevant oxidation byproducts were examined via dpv evaluation. conclusions for the first time, we have demonstrated an effective approach to construct of gce/go-lamno3 sensor and its application for simultaneous determination of hq and ct. compared with the bare gce, a large peak-to-peak separation between ct and hq, and the significant increase of peak http://dx.doi.org/10.5599/jese.634 j. electrochem. sci. eng. 9(4) (2019) 255-267 fabrication of graphene–lamno3 sensor 266 current values were observed for gce/go-lamno3, which clearly demonstrated that go-lamno3 could be used as an efficient promoter to enhance the kinetics of the electrochemical process of hq and ct. the optimization of the experimental conditions for differential pulse voltammetry yielded a detection limit for hq and ct of 0.06 and 0.05 μm, respectively. these values are comparable or even better than those already described in the literature. in addition, the presented sensor was applied for the simultaneous determination of hq and ct in water samples with satisfactory results. acknowledgment: this project was supported by islamic azad university, kerman branch. references [1] s. l. mu, biosensors and bioelectronics 21 (2006) 1237-1243. 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[51] x. wang, m. xi, m. guo, f. sheng, g. xiao, s. wu, s. uchiyama, h. matsuura, analyst 141 (2016) 10771082. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.634 http://creativecommons.org/licenses/by/4.0/) {electrochemical measurements of ascorbic acid based on graphite screen printed electrode modified with la3+/co3o4 nanocubes transducer} http://dx.doi.org/10.5599/jese.643 197 j. electrochem. sci. eng. 9(3) (2019) 197-206; http://dx.doi.org/10.5599/jese.643 open access: issn 1847-9286 www.jese-online.org original scientific paper electrochemical measurements of ascorbic acid based on graphite screen printed electrode modified with la3+/co3o4 nanocubes transducer mohammad reza aflatoonian1,2,, somayeh tajik1,, behnaz aflatoonian3, hadi beitollahi4 1research center for tropical and infectious diseases, kerman university of medical sciences, kerman, iran 2leishmaniasis research center, kerman university of medical sciences, kerman, iran 3neuroscience research center, kerman university of medical sciences, kerman, iran 4environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran corresponding authors: m.aflatoonian97@gmail.com; tel.: +98 3426226613; fax: +98 3426226617; mtajik_s1365@yahoo.co received: december 4, 2018; revised: february 23, 2019; accepted: february 26, 2019 abstract electrochemical characterization of ascorbic acid oxidation on a graphite screen printed electrode (spe) modified with la3+/co3o4 nanocubes is performed by applying cyclic voltammetry (cv), chronoamperometry (cha), and differential pulse voltammetry (dpv) techniques. synthesized la3+/co3o4 nanocubes for spe modification, la3+/co3o4/spe, enhance the ascorbic acid electrooxidation kinetics by reducing the anodic overpotential. excellent la3+/co3o4/spe electrochemical properties provide sensitive ascorbic acid voltammetric determination with low detection limit, good stability and quick response towards electrooxidation of ascorbic acid as compared to bare spe. under optimized conditions, dpv current demonstrates a linear response for ascorbic acid over a concentration range of 1.0 to 900.0 m with a correlation coefficient of 0.9997, and detection limit (lod) (s/n = 3) = 0.3 m. the proposed procedure offers a potential application for producing the sensor with good repeatability. also, fast response of fabricated sensor can allow a real-time analysis of real samples. keywords ascorbic acid; electrochemical characterization, la3+/co3o4 nanocubes; graphite screen printed electrode; voltammetry http://dx.doi.org/10.5599/jese.643 http://dx.doi.org/10.5599/jese.643 http://www.jese-online.org/ mailto:m.aflatoonian97@gmail.com mailto:m.aflatoonian97@gmail.com mailto:mtajik_s1365@yahoo.co j. electrochem. sci. eng. 9(3) (2019) 197-206 electrochemical measurements of ascorbic acid 198 introduction ascorbic acid is a water soluble vitamin (vitamin c), widely present in many biological systems, fruits and vegetables. beyond its function in collagen formation, ascorbic acid is known to increase absorption of inorganic iron, to have essential roles in the metabolism of folic acid, some amino acid and hormones, and to act as an antioxidant 1-4. deficiency of ascorbic acid will cause scurvy disease 2-5. it is administered in the treatment of many disorders, including alzheimer's disease, atherosclerosis, cancer, and infertility 3-6. in the light of these facts, determination of low concentrations of ascorbic acid without any interference from other biomolecules is a very important subject in pharmacology fields 5-8. numerous approaches such as fluorescence 9, uv spectroscopy 10, chemiluminescence 11, electrophoresis 12, and liquid chromatography 13 have been reported for the determination of ascorbic acid. these methods suffer from the lack of specificity and are prone to interferences with other reducing agents in the sample. moreover, some complications in chromatography performances were detected, such as selecting a proper column or a mobile phase with liquid chromatographic methods, costly instruments, long response time, difficult procedure, and low detection ability. since ascorbic acid is an electroactive compound, electrochemical techniques for its detection has received a considerable interest 14-16. electrochemical methods came to attention due to their high sensitivity, easy operation and low cost 17-23. nowadays, a variety of nanoparticles with numerous properties, such as high speed, minor size, smaller distances for electrons to travel, lower power, and lower voltages, made important advances in the field of nanotechnology. thus, the utilization of nanomaterials such as metal nanoparticles, metal oxide nanoparticles, magnetic nanomaterials, carbon materials, quantum dots and metallophthalocyanines enhanced the electrochemical signals of biocatalytic events occurring at the electrode/electrolyte interface 24,25. along with nanoscience and nanotechnology development, there were numerous trials to employ innovative nanomaterials in constructing modified electrodes 26-28. nanostructured metal oxides were also investigated in recent years as electrochemical sensor materials, showing promising results owing to several benefits of the materials such as high surface area, workability, and increasing synthetic accessibility 29-32. currently, the screen printed electrode (spe) has become an interesting and widely applied electrode material. spes have already been successfully used as electrochemical sensors for various researches due to their simplicity and minimum sample preparation. in addition, the main benefit of spe is based on its single use, after which it is discarded. however, the limitation of spe is small surface area of the working electrode leading to the lack of sensitivity. to overcome this problem, the electrode modification is necessary 33,34. in this study, la3+/co3o4 nanocubes modified spe has been exploited for ascorbic acid sensing and the analytical performance of the suggested sensor for ascorbic acid determination in real samples is evaluated. experimental chemicals and apparatus electrochemical experiments were performed using an autolab potentiostat/ galvanostat (pgstat 302n, eco chemie, the netherlands). general purpose electrochemical system software was used to control the system. the screen printed electrode (dropsens, drp-c110, spain) contains three main parts which are a graphite counter electrode, a silver pseudo-reference electrode, and a graphite working electrode. the ph values were measured by a metrohm 710 ph meter. the mohammad reza aflatoonian et al. j. electrochem. sci. eng. 9(3) (2019) 197-206 http://dx.doi.org/10.5599/jese.643 199 morphology of the products was observed using a philips xl30 scanning electron microscope (sem). prior to sem analysis, samples were loaded on a gold film prepared by an scd005 bal-tec sputter coater. xrd studies were conducted on a rigaku d/max 2500 v instrument with a graphite monochromator and a cu target. the ft-ir spectra were prepared by use of a bruck equinox 55 spectrophotometer and the kbr pellet of samples were used. all reagents were purchased from merck (darmstadt, germany) in analytical grade. buffer solutions within the range of 2.0–9.0 were prepared using orthophosphoric acid and its salts. synthesis of la3+/co3o4 nanocubes in a typical synthesis, 25 ml of ammonia solution (28-30 %, reagent chemicals) was first mixed with ultra-pure water (1/1, v/v). then, 0.05 g of poly(vinylpyrrolidone) (pvp; m.w. 58,000, reagent chemicals) was dissolved in the solution. after shaking for a few minutes, 0.364 g of cobalt nitrate hexahydrate (co(no3)26h2o) and 0.043 g of lanthanum nitrate hexahydrate (la(no3)26h2o) were added into the reaction medium. after sonication, a browny transparent solution was obtained, which was then transferred into 100 ml teflon-lined stainless steel autoclave. the autoclave was kept in an electric oven at 180 °c for 6 h, after which the autoclave was taken out and cooled naturally to room temperature. after that, the black precipitate was harvested and washed with ultra-pure water several times via centrifugation. the product was then dried at 60 °c overnight, before being calcined at 300 °c for 2 h in air to be converted into la3+/co3o4 nanocubes. preparation of the electrode in order to prepare the modified electrode, the surface of bare spe was coated by a thin film of la3+/co3o4 nanocubes as follows: in a simple procedure, 1 mg la3+/co3o4 nanocubes were dispersed in 1 ml aqueous solution by ultrasonication for 15 min. then, surface of spe working electrode was treated with 2 µl of the prepared suspension. after drying at room temperature, the modified electrode was ready to use. preparation of real samples one milliliter of a vitamin c ampoule (darou pakhsh company, iran, contained 1 mg/ml of ascorbic acid) was diluted to 10 ml with 0.1 m pbs (ph 7.0); then, different volume of the diluted solution was transferred into each of a series of 25 ml volumetric flasks and diluted to the mark with pbs. the ascorbic acid content was analyzed by the proposed method using the standard addition method. five vitamin c tablets (labeled 250 mg per tablet, osvah pharmaceutical company, iran) were grinding. then, the tablet solution was prepared by dissolving 400 mg of the powder in 25 ml water by ultrasonication. after that, different volume of the diluted solution was transferred into a 25 ml volumetric flask and diluted to the mark with pbs (ph 7.0). the ascorbic acid content was analyzed by the proposed method using the standard addition method. five effervescent tablets of vitamin c (labeled 500 mg per tablet, hakim pharmaceutical company, iran) were grinded completely. after that 500 mg of the powder was dissolved in 25 ml water using ultrasonication and different volume of the diluted solution was transferred into a 25 ml volumetric flask and diluted to the mark with pbs (ph 7.0). the ascorbic acid content was analyzed by the proposed method using the standard addition method. http://dx.doi.org/10.5599/jese.643 j. electrochem. sci. eng. 9(3) (2019) 197-206 electrochemical measurements of ascorbic acid 200 result and discussion nanostructures characterization figure 1 displays the ft-ir spectrum of la3+/co3o4 nanocubes sample in the frequency range of 400–4000 cm-1. the ft-ir spectrum of la3+/co3o4 nanocubes showed strong vibrational bands in the lower frequency regions (about 400-600 cm-1), ascribable to the vibration of metal–o. the absorption seen at ~3389 cm-1 is thought to be due to the symmetric vibration of the -oh groups of the absorbed h2o molecules. the peak at ~1632 cm-1 corresponds to o-h groups, again related to adsorbed h2o molecules 34. fig. 1. ft-ir spectrum of la3+/co3o4 nanocubes. the main characteristic diffraction peaks of la3+/co3o4 nanocubes are consistent with the standard patterns of co3o4 (jcpds card no. 71-0816) with cubic spinel phase, suggesting that the doping of la does not change the backbone of pristine co3o4 (figure 2). by increase of la content in co3o4, the characteristic peak at 2 =37.3° becomes weaker and slightly shifted to small-angle reflections. the introduction of la ions with f-block electronic and large atomic radiuses into the co3o4 grain boundary, causes the loss of atomic degree of ordering of co3o4, thereby contributing to the limited growth of the grain, grain refinement and decrease of crystallinity. 2 / o fig. 2. xrd pattern of la3+/co3o4 nanocubes the broadness of diffraction peaks suggests the nano-sized nature of the product and the average crystallite size (t) was calculated as 67.0 nm using the debye–scherrer formula t = 0.9  /  cos , where λ is the wavelength of the x-ray radiation (1.54056 å for cu lamp),  is the diffraction angle and  is the full width at half-maximum (fwhm) [35]. mohammad reza aflatoonian et al. j. electrochem. sci. eng. 9(3) (2019) 197-206 http://dx.doi.org/10.5599/jese.643 201 the morphology of the product was examined by sem. figure 3 depicts sem pictures of la3+/co3o4 nanoparticles. it can be observed from figure 3, that nanoparticles which are nanocubes are less than 70 nm. fig. 3. sem images of la3+-doped co3o4 nanocubes for a) high concentration, b) low concentration electrochemical profile of the ascorbic acid on the la3+/co3o4/spe in order to investigate the electrochemical behaviour of ascorbic acid and obtain the accurate results, it is essential to determine the optimized ph value since its electrooxidation is ph dependent process. for this purpose, the experiments were performed using the modified electrode at various ph values ranging from 2.0–9.0 (figure 4). fig. 4. plot of ip vs. ph (2.0-9.0) in 0.1 m pbs in the presence of 900.0 μm ascorbic acid at the scan rate 50 mv s-1. according to figure 4, the best results for electro-oxidation of ascorbic acid are achieved at ph 7.0. figure 5 presents cyclic voltammograms (cvs) obtained for la3+/co3o4/spe (curve a) and unmodified spe (curve b) in the presence of 900.0 μm ascorbic acid. comparing cv results of modified and bare spe revealed that the maximum oxidation of ascorbic acid observed at 300 mv for la3+/co3o4/spe, is about 70 mv more negative for unmodified spe. when oxidation current peak values of ascorbic acid at la3+/co3o4/spe (1 µa) and bare spe (0.5 µa) are compared, an extensive (about 100 %) enhancement for la3+/co3o4/spe relative to the bare spe is observed. in other words, the results clearly indicate that la3+/co3o4 nanocubes improve the ascorbic acid oxidation signal. http://dx.doi.org/10.5599/jese.643 j. electrochem. sci. eng. 9(3) (2019) 197-206 electrochemical measurements of ascorbic acid 202 fig. 5. cvs of (a) la3+/co3o4/spe and (b) unmodified spe in 0.1 m pbs (ph 7.0) in the presence of 900.0 μm ascorbic acid at the scan rate 50 mv s -1. effect of potential scan rate as shown in figure 6, investigation of the effect of potential scan rates on the oxidation currents of ascorbic acid demonstrated that increasing of the scan rate results in increased oxidation peak current values (ip). the observed linear relation between ip and the square root of the potential scan rate (ν1/2) indicates that oxidation of ascorbic acid is a diffusion-controlled process. fig. 6. cvs of la3+/co3o4/spe in 0.1 m pbs (ph 7.0) containing 800.0 µm ascorbic acid at various scan rates (ν). numbers 1-19 correspond to 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900 and 1000 mv s-1, respectively. inset: variation of anodic peak current vs. ν1/2 chronoamperometric analysis the analysis of chronoamperometry (cha) for ascorbic acid samples was performed by use of la3+/co3o4/spe vs. ag/agcl/kcl (3.0 m) at 0.35 v. cha results obtained for different concentrations of ascorbic acid samples in pbs (ph 7.0) are depicted in figure 7. the cottrell equation for mohammad reza aflatoonian et al. j. electrochem. sci. eng. 9(3) (2019) 197-206 http://dx.doi.org/10.5599/jese.643 203 chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is expressed as follows 36: i =nfad1/2cbπ-1/2t-1/2 where d is diffusion coefficient, cm2 s-1, and cb refers to the bulk concentration, mol cm−3. experimental results of i vs. t−1/2 are plotted in figure 7a as the best fitted curves for different concentrations of ascorbic acid. the resulting slopes of the straight lines in figure 7a were then plotted against the concentration of ascorbic acid in figure 7b. finally, regarding the resulting slope and cottrell equation, the mean value of d was determined as 1.010-6 cm2 s-1. this value is comparable with the values reported in some previous works (2.210–6 cm2 s-1 [1] and 5.6610–6 cm2 s-1 [37]). fig. 7. chas of la3+/co3o4/spe in 0.1 m pbs (ph 7.0) for different concentrations of ascorbic acid. numbers 1–4 correspond to 0.1, 0.5, 1.0 and 1.5 mm of ascorbic acid. insets: (a) plots of i vs. t-1/2 obtained from chas 1–4. (b) plot of straight lines slope against ascorbic acid concentration calibration curve the modified electrode (la3+/co3o4/spe) was applied as the working electrode in the concentration range of ascorbic acid in 0.1 m pbs and subdued to differential pulse voltammetry (dpv) since this technique in analytical applications offers advantages of enhanced sensitivity and better efficiency (figure 8). according to the results, a linear relationship exists between the peak currents and concentrations of ascorbic acid within the concentration range of 1.0-900.0 µm with the correlation coefficient of 0.9997. the detection limit of 0.3 µm was obtained. interference study the influence of various substances as compounds potentially interfering with the determination of ascorbic acid was studied in the presence of 50.0 µm ascorbic acid. the tolerance limit was defined as the maximum concentration of the interfering substance that caused an error less than ±5 % in the determination of ascorbic acid. http://dx.doi.org/10.5599/jese.643 j. electrochem. sci. eng. 9(3) (2019) 197-206 electrochemical measurements of ascorbic acid 204 fig. 8. dpvs of la3+/co3o4/spe in 0.1 m pbs (ph 7.0) containing different concentrations of ascorbic acid. numbers 1-20 correspond to 1.0-900.0 μm of ascorbic acid. inset: plot of the peak current as a function of ascorbic acid concentration. according to the results, uric acid, l-serine, l-phenylalanine, ethanol, benzoic acid, isoproterenol, methanol, serotonin, l-asparagine, acetaminophen, nadh, saccharose, urea, glucose, l-lysine, l-glycine, carbidopa, caffeine, levodopa, fructose, l-threonine, lactose, l-histidine, l-proline, s2−, mg2+, so42−, nh4+, f− and al3+ did not show interference in the determination of ascorbic acid. analysis of real samples to assess the applicability of this modified sensor for the determination of ascorbic acid in vitamin c ampoule, vitamin c tablet and vitamin c effervescent tablet, the proposed method was tested in water samples using the described procedure. in this process, the standard addition method was employed, and the results are given in table 1. table 1. determination of ascorbic acid in vitamin c ampoule, vitamin c tablet and vitamin c effervescent tablet sample concentrations, μm (n=5) recovery, % rsd, % spiked found vitamin c ampoule 0 10.0 3.4 5.0 14.8 98.7 1.7 10.0 20.4 102.0 2.1 15.0 24.5 98.0 1.5 20.0 30.9 103.0 2.9 vitamin c tablet 0 5.0 2.4 10.0 15.2 101.3 1.7 15.0 20.1 100.5 3.4 20.0 24.3 97.2 2.1 25.0 30.6 102.0 3.2 vitamin c effervescent tablet 0 5.0 2.8 10.0 15.3 102.0 3.1 15.0 19.8 99.0 2.9 20.0 24.3 97.2 2.6 25.0 30.5 101.7 1.6 mohammad reza aflatoonian et al. j. electrochem. sci. eng. 9(3) (2019) 197-206 http://dx.doi.org/10.5599/jese.643 205 according to the results, the recovery of ascorbic acid is satisfactory and the reproducibility of the results is approved by the mean relative standard deviation (rsd.). the repeatability and stability of la3+/co3o4/spe the long-term stability of the la3+/co3o4/spe was evaluated over 3-week period. after the modified electrode was stored for 3 weeks in atmosphere at room temperature, the experiments were performed again. according to cyclic voltammograms, the peak potential for ascorbic acid oxidation remained unchanged and a decrement of less than 2.2 % compared with initial response was observed. the antifouling properties of the modified electrode towards ascorbic acid oxidation and its oxidation products were studied by recording cvs. voltammograms were recorded in the presence of ascorbic acid after cycling the potential 20 times at a scan rate of 50 mv s-1. results demonstrated that peak potentials remained unchanged and the currents decreased by less than 2.1 %. according to the results, application of modified la3+/co3o4/spe provides increased sensitivity and decreased fouling effect of the analyte and its oxidation product. conclusion la3+/co3o4 nanocubes-spe demonstrated an outstanding behavior toward ascorbic acid oxidation in the aqueous pbs (ph 7.0) solution. our investigation exhibited that in the case of the unmodified spe, the voltammogram of ascorbic acid displays just a small hump peak, but after modification of the electrode with la3+/co3o4 nanocubes, the oxidation peak currents of ascorbic acid are significantly enhanced. this technique offers a number of advantages compared to the other published electrochemical methods, such as simplicity in the preparation of the modified electrode and its high selectivity. the modified electrode represented appropriate performance in detecting ascorbic acid and revealed excellent stability and reproducibility. by dpv determination, the detection limit of ascorbic acid was estimated as 0.3 µm. based on the electrochemical oxidation, the quantitative determination of ascorbic acid in real samples was developed by a simple, rapid, selective and sensitive dpv technique. these properties indicate that the la3+/co3o4 nanocubes modified 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[37] h. beitollahi, m. mazloum ardakani, h. naeimi, b. ganjipour, journal of solid state electrochemistry 13 (2009) 353-363. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) original scientific paper http://dx.doi.org/10.5599/jese.1891 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1891 open access : : issn 1847-9286 www.jese-online.org original scientific paper effect of sintering temperature and time on corrosion characteristics of aluminum matrix composites sahib m. mahdi and lubna ghalib materials engineering department, mustansiriyah university, bab al-muadham 46049, baghdadiraq corresponding author: lubnaghalib81@uomustansiriyah.edu.iq received: may 15, 2023; accepted: july 21, 2023; published: july 26, 2023 abstract aluminum matrix composites outperform traditional alloys in terms of mechanical properties and corrosion resistance. silicon carbide is the main reinforcing material in aluminum-based composites that have developed rapidly in recent years. in this investigation, aluminum-silicon carbide composites were prepared through powder metallurgy with 10 and 20 wt.% of silicon carbide reinforcement. the influence of the sic content, sintering temperature, and sintering time on the corrosion behavior of prepared al-sic composites in 0.5 m hydrochloride acid solution was assessed. the surface microstructure was characterized by scanning electron microscopy with energy dispersive analysis and x-rays. the experimental results demonstrated how the sintering parameters can affect the corrosion characteristics of sintered samples. the electrochemical analysis curves showed that when increasing the sintering temperature and sintering time, there is a possibility of self-repair of damaged passive film on the surface while further reinforcing of the composite with silicon carbide prevents penetration of chloride ions. the sem images and eds analysis of composite surfaces after being corroded in 0.5 m hcl revealed that increasing the sintering temperature and prolonging the sintering time reduce the pitting corrosion of composites. keywords aluminum-based composite; silicon carbide; powder metallurgy; sintering parameters introduction the demand for composite materials has become essential for modern technologies because of already-reached improvements in their physical and mechanical properties. recently, metal matrix composites (mmc), i.e., metals reinforced with some fibers or particles, have also been advanced. aluminum matrix composites have lightweight properties, high environmental resistance, high specific strength and stiffness, good wear resistance, high thermal and electrical conductivity, low density, high toughness, and high corrosion resistance. these excellent properties encourage further research and development for further improvement of properties in order to extend their http://dx.doi.org/10.5599/jese.1891 http://dx.doi.org/10.5599/jese.1891 http://www.jese-online.org/ mailto:lubnaghalib81@uomustansiriyah.edu.iq j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion characteristics of al matrix composites 2 applications [1-5]. currently, aluminum matrix composites have extensive applications in the aerospace, military, and car industries [6,7]. ceramic particles are the most commonly employed materials for reinforcing aluminum matrices like sic, si3n4 [8], al2o3, and tic [9]. al-sic composites have lately received a lot of interest [10]. silicon carbide (sic) remains one of the most commonly used reinforcements in the production of aluminum-based composites due to its excellent resistance to acids and alkalis. the dispersion of sic substituents inside the matrix materials is influenced by several characteristics, including the molten matrix flow and deformation, the technique of incorporation grains, and their interaction throughout the admixture process [11]. the main disadvantage of al-sic is that it is highly susceptible to corrosion due to the influence of intermetallic compounds on the resistance of aluminum matrix to surface degradation. the defects in the protective aluminum oxide film, discontinuities, inclusions, and increased regions of corrosion may be due to the reinforcement of sic. the resulting microstructural heterogeneity makes al-sic more susceptible to corrosion due to interfacial problems and galvanic impact [12]. processing parameters have a considerable effect on the composite microstructure and as a result, on its corrosion behavior [11,13]. the differences in sintering parameters of al-sic can directly affect its microstructure and properties. previous research on the modification of sintering parameters in the metal matrix composite fabrication process led to improved corrosion resistance. guo et al. [14] examined the effect of sintering temperature on the corrosion behavior of ti-24nb4zr-7.9sn alloy made by powder metallurgy and found that alloys sintered at higher temperatures have excellent corrosion resistance due to increased density and decreased porosity. in contrast, alloys sintered at low temperatures have poor corrosion resistance due to decreased density and porosity. gupa et al. [15] evaluated the effect of sintering parameters on the corrosion resistance of fe-al2o3 matrix composite fabricated by powder metallurgy. the authors found that the effect of acid attack was more intense for the sample sintered at a lower temperature and for a shorter time than for the sample sintered at a higher temperature and for a longer time. veerappan et al. [16] investigated the impact of the sintering temperature and sic content varied from 0 to 12 wt.% on the mechanical characteristics and corrosion resistance of ni-co-cr-sic composites. it was found that the mechanical characteristics and corrosion resistance of ni-co-cr-sic composites were improved by increasing the sintering temperature and sic content. azaath et al. [17] investigated the effect of silicon carbide and sintering temperature on the results of compression and hardness tests, microstructure, and corrosion behavior of the cu-20al-4ni composite synthesized by the powder metallurgy route. they found that the corrosion resistance of composites sintered at higher temperatures was greater than that of composites sintered at lower temperatures. it was concluded that both the sintering temperature and sic reinforcement improve corrosion resistance. from the literature survey, it was found that the sintering process parameters of metal matrix composites sintered by powder metallurgy have a significant effect on corrosion resistance. corrosion resistance is a very important parameter in determining the potential use of these materials. reinforced sic in the aluminum matrix improves the mechanical properties of composites, but corrosion resistance depends on the fabrication conditions [18]. to our knowledge, there is no publication on the effect of sintering parameters on the corrosion resistance of aluminum composites reinforced with sic particles. in the present work, the effects of sintering temperature and sintering time on the corrosion properties of al-sic metal matrix composites synthesized by powder metallurgy techniques are experimentally investigated. s. m. mahdi and l. ghalib j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1891 3 experimental the matrix material was aluminum (al) powder of purity 99.5 %, while the reinforcement was silicon carbide. the powders were precisely weighed to obtain the proper stoichiometric composition of al-10sic and al-20sic. the powder was combined in a tube according to the specified weight ratio using an electric parallel mixer for two hours at 70 rpm. double-action tool steel dies with a diameter of 11 mm and a height of 30 mm were used to compress the composite powder, which was pressed under a pressure of 146 mpa. figure 1 depicts a highly vacuumed tubular furnace where the compacts were sintered at 500 and 550 °c for 2 and 3 hours, respectively. cylindrical shapes of sintered samples were removed from the vacuum atmosphere tube furnace, as illustrated in figure 2, and allowed to cool to room temperature. figure 1. vacuumed furnace system figure 2. sintered al-10sic and al-20sic composites the composite sample was cold-mounted in an epoxy resin, following a similar procedure described elsewhere [19]. the exposed flat surface of the mounted part was polished using emery papers of different grades, from 320 to 1000 grits. the corrosion studies were performed at room temperature using a gamry electrochemical cell with three electrodes connected to a gamry instrument potentiostat/galvanostat with a gamry framework system. a saturated calomel electrode (sce) was used as a reference electrode, a platinum electrode as an auxiliary electrode, and al-sic samples as working electrodes with a cross-sectional area of 0.95 cm2 exposed to the solution. the measurements of potentiodynamic polarization were performed by dipping the working electrode in 0.5 m hcl solution for 1000 s to ensure steady-state condition. the potential was scanned at a scan rate of 2.5 mv s-1 from -0.25 to +0.25 v vs. sce. phase analysis of corroded surfaces after the electrochemical experiment in 0.5 m hcl solution was examined by recording x-ray diffraction (xrd) patterns. energy dispersive spectroscopy (eds) was used for targeted analysis of the sample surface. scanning electron microscopy (sem) provided a characterization of the morphology of corroded surfaces. results and discussion cyclic potentiodynamic polarization measurements cyclic potentiodynamic polarization experiments were performed to report corrosion parameters determined by evaluating the influence of sintering temperature and sintering time on corrosion properties of al-sic composite in 0.5 m hydrochloric acid solution. it is essential to consider both the sintering temperature and the sintering time. higher sintering temperatures can http://dx.doi.org/10.5599/jese.1891 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion characteristics of al matrix composites 4 result in faster densification with less sintering time, but the coarsening rate will rise. when the sintering temperature is lower, the sintering time must be extended to allow diffusion and densification [20]. grain growth may be possible with a longer sintering time [21]. the main controlling factor in the sintering process is the sintering temperature [22]. figures 3 and 4 show cyclic potentiodynamic polarization curves of al-10sic composites sintered at different temperatures and times and exposed to 0.5 m hcl solutions at room temperature (25 °c). figure 3 illustrates the influence of sintering temperature on the cyclic potentiodynamic curve for a 10 wt.% sic composite. the composites were sintered at 500 and 550 °c with a fixed sintering time of 2 hours. the cyclic polarization curve for the composite sintered at 500 °c for 2 hours shows an active dissolution in the anodic branch with no transition to passivation. an incomplete positive hysteresis loop with a higher current in the reverse scan than the forward scan resulted from reversing the potential in the backward direction. this pointed toward the inability of the oxide passive film to re-passivate or cause pit initiation in 0.5 m hcl solution. this observation agrees with the study of loto and solomon [23], who observed a deterioration of passive film and its inability to self-repair due to the significant effect of chloride concentration on the pitting corrosion of aluminum. increasing the sintering temperature to 550 °c remarkably reduced the values of anodic current to develop an oxide film, which is indicated by the large passive region, while the cathodic current has only slightly increased. moreover, the corrosion potential value was shifted to a more positive direction, with no indication of pitting corrosion. the more positive corrosion potential value is an indication of higher corrosion resistance under testing conditions, as reported by guo et al. [14]. this proves that increasing the sintering temperature greatly decreases the extent of uniform corrosion. there is a complete negative hysteresis loop, with a current in the reverse scan being lower than the current in the forward scan, and an anodic nose also exists. the initiation of pitting corrosion and re-passivation of the composite has been suggested to occur at a higher sintering temperature. the area of the hysteresis loop also increased with increasing the sintering temperature, demonstrating that the greater the disruption of the passive film, the more difficult it is to restore, as esmailzadeh et al. [24] reported. the repassivation potential (erp) of al-10sic sintered at 550 °c for 2 hours is -705.1 mv, i.e., nobler than the corrosion potential of composite, which means that no pitting corrosion exists at the potential between them. figure 3. cyclic potentiodynamic polarization curves (0.5 m hcl, 2.5 mv s-1) for 10 wt.% sic composite sintered at 500 and 550 °c for 2 hours s. m. mahdi and l. ghalib j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1891 5 figure 4 presents cyclic potentiodynamic polarization curves of 10 wt.% sic sintered at 500 °c for 2 and 3 hours in 0.5 m hcl solution. it is observed from figure 4 that both composites displayed relatively similar polarization trends with increasing potential, except for a small difference in the anodic branch. from the cyclic polarization curve, it can be observed that the hysteresis loop for both curves was positive and incomplete, which confirmed that there was no pitting corrosion and indicated that the oxide passive film is unable to re-passivate in 0.5 m hcl solution. figure 4. cyclic potentiodynamic polarization curves (0.5 m hcl, 2.5 mv s-1) for 10 wt.% sic composite sintered at 500 °c for 2 and 3 hours the cyclic potentiodynamic results obtained for 20 wt.% sic composites sintered at 500 and 550 °c for 2 hours are presented in figure 5. both cyclic polarization curves display positive hysteresis as the sintering temperature increases, revealing the breakdown of the passive film. figure 5. cyclic potentiodynamic polarization curves (0.5 m hcl, 2.5 mv s-1) for 20 wt.% sic composite sintered at 500 °c and 550 °c for 2 hours however, the sample at 550 °c crossed the anodic arm at a protection potential of -787.0 mv, as anticipated, indicating the tendency of the oxide passive film to re-passivate and pit initiation. the size http://dx.doi.org/10.5599/jese.1891 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion characteristics of al matrix composites 6 of the hysteresis loop decreases when the sintering temperature of the samples is raised, which again is in agreement with esmailzadeh et al. [24]. the smaller the hysteresis loop, the less disruption of the passive film possible to restore. when increasing the sintering temperature from 500 to 550 °c, the cathodic branch shifts towards a higher current, indicating that the cathodic reaction is increased by the addition of sic reinforcement at a higher sintering temperature. figure 6 depicts the cyclic potentiodynamic curves of 20 wt.% sic composites sintered at 500 °c for 2 and 3 hours. the curves in figure 6 indicate shifts of both cathodic and anodic branches toward lower current values when increasing the sintering time from 2 to 3 hours. this means the sample with increasing sic content, prepared at a higher sintering time, shows less corrosion current. this is due to the good dispersion of sic particles in the al matrix at this sintering temperature compared to samples at a lower sintering time. the hysteresis loop of al-20sic composite sintered at 500 °c for 2 hours is positive and incomplete when compared to the composite sintered for 3 hours, which is positive and complete, with the repassivation potential erp = -775.8 mv vs. sce. the value of repassivation potential is nobler than corrosion potential, indicating the stoppage of propagation pits and the formation of a stable oxide film. figure 6. cyclic potentiodynamic polarization curves (0.5 m hcl, 2.5 mv s-1) for 20 wt.% sic composite sintered at 500 °c for 2 and 3 hours table 1 presents corrosion parameter values determined from potentiodynamic polarization curves (figures 3-6), including the values of cathodic tafel slope (c), anodic tafel slope (a), corrosion potential (ecorr), corrosion current (icorr), and corrosion rate (cr). the corrosion rate was calculated using the following equation: w corr0.00327cr e i da = (1) where cr is corrosion rate (mm year-1), ew is the equivalent weight of composite in gram, icorr is corrosion current obtained from tafel slope (µa), d is the density of composite (g cm-3), a is electrode area (cm2). here, the composite density was calculated based on the weight content and density of both compounds (al and sic) in the composite. the corrosion results listed in error! reference source not found. clearly show that aluminum matrix composite with 90 wt.% of aluminum displays higher resistance to pitting corrosion due to the strong protective oxide formation. however, the presence of sic improves the strength of the s. m. mahdi and l. ghalib j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1891 7 composite, but it also leads to the development of intermetallic particles, second phases, and precipitates, which inevitably incorporate micro-galvanic cells throughout the composite surface. table 1. corrosion parameters determined from polarization curves for al-sic composites in 0.5 m hcl sample sintering conditions a / mv decade-1 c mv / decade-1 icorr / a ecorr / mv cr, mm year -1 al-10sic 500 °c (2 h) 17.10 221.5 534.0 -814.0 8.66 500 °c (3 h) 17.50 132.9 164.0 -777.0 2.57 550 °c (2 h) 29.80 218.5 472.0 -744.0 7.67 al-20sic 500 °c (2 h) 47.50 403.0 1100 -811.0 17.92 500 °c (3 h) 152.1 724.0 645.0 -823.0 10.47 550 °c (2 h) 23.00 237.6 2.050 -777.0 32.33 x-ray diffraction analysis the x-ray diffraction (xrd) patterns of al-20sic composites processed by powder metallurgy after corrosion in 0.5 m hcl solution are illustrated in figure 7. for al-20sic composite samples fabricated at different sintering temperatures and times, four high-intensity peaks and many lowintensity peaks are displayed in xrd spectra. aluminium phases are shown by four strong diffraction peaks at 2 values of 38.45, 44.71, 65.09, and 78.23°, while low-intensity peaks correspond to sic and intermetallic compounds formation during sintering. these peaks can be used to prove their presence in fabricated aluminum-metal composites. generally, figure 7 shows xrd patterns of composites that are similar to each other and are dominated by the aluminum and sic peaks. the small oxide peaks were seen in the xrd analysis of al-sic composites sintered at 500 and 550 °c for 2 and 3 hours, indicating al2o3 formation on the surface. there is not much change in the shift of peaks related to al and sic, but the intensity of peaks increases while increasing the sintering temperature gradually. it is found that the intensity of al is higher in the composite sintered at 550 °c in comparison to the composite sintered at 500 °c for the same sintering time. the reason for this is due to the high temperature of diffusion. the porosity of the composite was expected to decrease significantly when the sintering temperature was increased and the contact among the powder particles was enhanced, which ultimately increased densification. 2 / ° figure 7. xrd patterns of sample with 20 wt.% sic at different sintering conditions after exposure to 0.5 m hcl solution http://dx.doi.org/10.5599/jese.1891 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion characteristics of al matrix composites 8 scanning electron microscopy analysis scanning electron microscopy analysis was conducted to determine if pitting corrosion occurs after electrochemical testing and to determine the influence of sintering temperature and sintering time on the surface morphology of composites sintered at 500 and 550 °c for 2 and 3 hours, respectively. the presence, morphology, and distribution of components on the corroded surface were examined using scanning electron microscopy and energy dispersive spectroscopy, as shown in figures 8-11. figure 8 shows the surface morphology of al20sic composite sintered at 500 °c for 2 hours after corrosion in 0.5 m hcl solution. the microstructure shows the presence of numerous cracks formed in the passive oxide layer, which correspond to trans-passivation as well as the presence of some small pores due to the attack of the aggressive chloride solution. the elemental analysis revealed that the film on the composite contains significant amounts of oxygen and aluminum, which confirms the formation of aluminum oxide film on the surface of a composite, but this film contains a large number of cracks. according to the eds analysis, the presence of the high amount of silicon on the surface of the composite indicates that the silicon carbide grains were not completely covered by the oxide film, which helps to minimize porosity in the composite, as was investigated by pech-canul and makhlouf [25]. figure 8. sem micrograph and eds analysis of 20 wt.% sic composite sintered at 500 °c for 2 hours after exposure to 0.5 m hcl figure 9 demonstrates the development of microstructure with the rise of the sintering temperature. it is seen that with increasing sintering temperature, a stable passive layer of aluminum oxide is formed, which is more homogeneous in thickness and free of cracks due to higher diffusion rates. this was clearly confirmed by the polarization curve behavior with repassivation potential in figure 5. the increased sintering temperature causes a slight reduction in the oxygen content of the composite, which was identified by selected field eds analysis, while the content of aluminum is high. this means the formed alumina layer continues on the surface, increasing the corrosion rate. a similar observation was discussed in a recent study by aruna and arivukkarasan [26]. from sem examination in figures 8 and 9, the sample sintered at 550 °c improves the homogenous distribution of reinforcement particles and reduces pore formation considerably. this can be explained by the fact that when the sintering temperature rises, high atom diffusion occurs, which leads to a reduction in the pores and the possibility of the oxide film re-passivation. previous s. m. mahdi and l. ghalib j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1891 9 work by rahimian et al. [6] found that samples had greater strength against acid attacks when the bonding strength between the particles increased with rising sintering temperatures. figure 9. sem micrograph and eds analysis of 20 wt.% sic composite sintered at 550 °c for 2 hours after exposure to 0.5 m hcl figure 10 shows the morphology of the specimen of 20 wt.% sic sintered at 550 °c for 3 hours after the corrosion test. eds analysis at 550 °c for 3 hours showed a decrease in the amount of aluminum to around 38.1 wt.% with a high content of oxygen in the corrosion products, demonstrating the formation of a thick oxide film over the composite surface. the formation of a stable oxide film on the composite surface helps to reduce the corrosion rate, as shown in table 1. after all, it can be concluded that the specimen of 20 wt.% sic sintered at 550 °c for 3 hours has significantly improved corrosion resistance due to the complete reaction and the optimum amount of aluminum oxide formed. increasing the sintering time from 2 to 3 hours has a slight effect on the corrosion rate. this might indicate that the disappearance of the small-sized porosity is affected by increasing the sintering time, but higher sintering temperatures are also required to significantly reduce the size of the large pores, which enhances corrosion resistance and prevents the formation of pits. figure 10. sem micrograph and eds analysis of 20 wt.% sic composite sintered at 550 °c for 3 hours after exposure to 0.5 m hcl http://dx.doi.org/10.5599/jese.1891 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion characteristics of al matrix composites 10 figure 11 shows the sem image and eds analysis of al-10sic composite sintered at 550 c for 2 hours. a wide spread of sic in the al matrix along with micropores indicates severe pitting corrosion. this is due to a porous oxide film formed with the possibility of re-passivation. a low amount of oxygen is found during the elemental analysis, which reveals that the surface developed a very thin oxide layer. this is well supported by eds analysis, which found a high amount of carbon. despite the fact that a 20 wt.% sic composite has a higher corrosion rate than a 10 wt.% sic composite, it is found that increasing sintering temperature and sintering time reduces the number of pits and forms a stable passive film in the same composite. figure 11. sem micrograph and eds analysis of 10 wt.% sic composite sintered at 550 °c for 2 hours after exposure to 0.5 m hcl conclusions the effect of sintering temperature and sintering time on the corrosion behavior of al-sic composites fabricated by powder metallurgy and exposed to 0.5 m hcl solution was studied through electrochemical testing and surface analysis techniques. the corrosion behavior was studied for two composites reinforced with a 10 and 20 wt.% content of sic, sintered at 500 and 550 °c, for 2 and 3 hours, respectively. the effect of sintering temperature was found to be the most significant, followed by the sintering time and sic content. the highest values of corrosion resistance for al-sic composites were obtained at the sintering temperature of 550 °c and a sintering time of 3 hours for both sic contents (10 and 20 wt.%) in the composites. there is a direct relationship between the formation of pitting corrosion and the sintering temperature and time in a way that increasing the sintering temperature and time enhances the pitting corrosion resistance. xrd analysis revealed the presence of al with a high peak intensity and sic with a low peak intensity. a small peak for intermetallic compounds indicates the development of an oxide film on the surface, and this fact was corroborated by eds analysis. the sem study of the surface of sintered samples after corrosion in 0.5m hcl revealed that for samples with an increased weight content of sic content, the corrosion rate increased with the occurrence of pitting corrosion. these corrosion pits, however, were found smaller and 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(https://creativecommons.org/licenses/by/4.0/) https://www.jmaterenvironsci.com/document/vol6/vol6_n1/19-jmes-1006-2014-gupta.pdf https://www.jmaterenvironsci.com/document/vol6/vol6_n1/19-jmes-1006-2014-gupta.pdf https://doi.org/10.1007/s12633-020-00455-9 https://doi.org/10.1007/s12633-021-01363-2 https://doi.org/10.1023/a:1004767113052 https://doi.org/10.1007/s10904-020-01863-5 https://doi.org/10.1201/9781315274126 https://doi.org/10.1088/2053-1591/ac286b https://doi.org/10.1134/s207020511805026x https://doi.org/10.1023/a:1009421727757 https://www.ripublication.com/ijaer20/ijaerv15n1_01.pdf https://creativecommons.org/licenses/by/4.0/) {pencil graphite electrodes as bioanodes for enzymatic glucose biofuel cell} http://dx.doi.org/10.5599/jese.807 385 j. electrochem. sci. eng. 10(4) (2020) 385-398; http://dx.doi.org/10.5599/jese.807 open access: issn 1847-9286 www.jese-online.org original scientific paper pencil graphite electrodes as bioanodes for enzymatic glucose biofuel cell madhavi bandapati1, sanket goel2 and balaji krishnamurthy1, 1department of chemical engineering, birla institute of technology and science (bits) pilani, hyderabad campus, hyderabad, india 2mems and microfluidics lab, department of electrical and electronics engineering, birla institute of technology and science (bits) pilani, hyderabad campus, hyderabad, india corresponding author: balaji@hyderabad.bits-pilani.ac.in, balaji.krishb1@gmail.com received: march 9, 2020; revised: may 19, 2020; accepted: may 26, 2020 abstract this study investigates the performance of pencil graphite (pg) electrodes to identify the grade of pencil most suitable as bioanode for enzymatic glucose biofuel cell. pencils of h, 3h, 5h and b grades are selected for this study. the surfaces of different grade pgs are modified with carboxylic acid functionalized multi walled carbon nanotubes (cooh-mwcnt/pg), followed by immobilization with glucose oxidase (gox) to fabricate the respecttive bioanodes (gox/cooh-mwcnt/pg). morphological and electrochemical characterizations are carried out using scanning electron microscopy, electrochemical impedance spectroscopy, cyclic voltammetry and energy dispersive x-ray spectroscopy. all tested pg electrodes exhibited positive results with variable response characteristics towards glucose oxidation reaction. b-grade pg bioanode is found to have the highest coverage of the deposited nanobiocomposite with the fastest electron transfer rate. the half-cell electrode assembly with this grade of pg recorded the highest current density of 4.25 ma cm-2 at physiological glucose conditions (5 mm glucose, ph 7.0). enzymatic glucose biofuel cell assembled with b-grade pg bioanode and platinum cathode generated an open circuit potential of 149 mv and maximum power density of 0.789 µw cm−2 from 5 mm glucose at ambient conditions (25 ± 3◦c). the results obtained for b-grade pg bioanode are comparable to those of conventional carbon and glassy carbon electrodes, thus demonstrating its applicability to enzymatic glucose biofuel cells. keywords pencil graphite electrode; bioanode; glucose oxidase; multiwalled carbon nanotubes; enzymatic glucose biofuel cell. http://dx.doi.org/10.5599/jese.807 http://dx.doi.org/10.5599/jese.807 http://www.jese-online.org/ mailto:balaji@hyderabad.bits-pilani.ac.in mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 10(4) (2020) 385-398 graphite electrodes as bioanodes 386 introduction enzymatic glucose biofuel cells (egbfcs) employ enzymes as catalysts which exhibit extremely specific electro-catalytic activities for biochemical and biological reactions [1]. these enzymes have relatively short life-span and redox centers are often buried deep inside the protein/glycoprotein shell, requiring thus pragmatic selection of electrode materials and fabrication of electrodes [2]. to establish efficient electron transfer between enzyme active sites and electrode, the enzyme must be supported with appropriate solid support, i.e. conducting support matrix, with adequate materials for binding and encapsulation, and with transport mediators [3]. to maximize the current output of the enzymatic redox reactions, a variety of materials are being explored as electrode substrates, including functionalized/conductive electroactive polymers, biocompatible composites, composites of organometallic compounds and transition metal-based complexes, hydro-gel and sol–gel materials, nanomaterials such as nano-metal oxides and composites [4,5]. among low cost electrode substrates, pencil graphite (pg) electrodes have already established an active area of research. aoki et al. have reported that the nature of pg influences the voltammetric and electroanalytical response of the analyte [6]. david et al. showed that just like other carbon-based working electrodes, pg presents high chemical and mechanical stability and large working potential window (-0.8 v to 0.6 v vs. sce in acidic and alkaline solutions) [7]. karuiki et al. have shown that the electron transfer rate of hb grade pg is similar to glassy carbon electrode [8]. tavare et al. have found out that most pgs have electrical resistance less than 5 ω and hence could be considered as electrode materials [9]. typically, pg comprises clay and graphite. increasing graphite content increases softness of pencil, while increasing the clay content adds hardness [10]. accordingly, different grades of pg ranging from 8b (softest) to 9h (hardest), have already been studied for various electroanalytical applications [11,12]. among different grade pgs (hb, h, f, 2h, 4h and 6h), wang et al. have shown that hb grade pencil generated the most favorable response for stripping voltammetric analysis of rna. the authors have also shown that 6h pencil displays the most favorable signal to background ratio in the electrochemical detection of deoxyribonucleic acid (dna) [13,14]. alipour et al. and ozcan et al. have shown that electrochemically modified pg can be used for detection and determination of morphine and uric acid concentration in blood streams [15,16]. kara et al. have used pg for electrochemical investigation of dna sequences present in herpes viruses [17]. several papers have reported the results of comparison between performance of pg and other electrodes with respect to electroanalysis of different compounds [18]. rubianes et al. have compared the performance of several graphite-based electrodes including pg for amperometric determination of dopamine [19]. several authors have also postulated low background current as one of the most important features of pg [20-22]. thus, use of pg electrodes for several electroanalytical applications including biosensors, is well known and well established in the literature. table 1 shows the summary of basic electroanalytical data for few surface modified pg electrodes. contrary to biosensors, however, egbfcs need to produce high potential and current. to enhance the overall performance of egbfc, enzyme activity and loading need to be improved through several electrode surface modification techniques. also, the output can be multiplied by cell stacking [3]. application of pg in egbfc requires exclusive studies, encouraging thus novel research in this area. in the pioneering work concerning the application of pg in egbfc, our recent research has already established 5h-grade pg as the optimal biocathode. m. bandapati et al. j. electrochem. sci. eng. 10(4) (2020) 385-398 http://dx.doi.org/10.5599/jese.807 387 table 1. electroanalytical data of pencil graphite (pg) electrode for various analytes pg based electrode analyte linear range detection limit ref. carmoisine–cu(ii) complex/polyaniline/pg cu (ii) 5×10-6 1×10-1 m 2×10-6 m [23] poly(4-vinyl pyridine)/pg cd (ii) 1×10-7 1× 10-1 m 2.51×10-8 m [24] diphenylcarbazide/polyaniline/pg cr (iv) 1×10-6 1×10-1 m 8×10-7 m [25] platinum nanoparticle/pg h2o2 10 -110 µm 3.6 µm [26] gold nanoparticles/pg n2h4 0.05 -1000 mol l-1 42 nmol l-1 [27] gold nanoparticles/pg uric acid 0.10.6 mm [28] gold nanoparticle/carbon paste/ pg glucose 0.033.4 mm 14.2 µm [29] single walled carbon nanotubes /pg p53 gene 3.2-8.0 µm 0.88 µm [30] multiwalled carbon nanotubes/pg buprenorphine 1.0 109.0 pm 0.6 pm [31] gold nanoparticles/chitosan/pg deferiprone 0.005-1000 µm 5 nm [32] copper hexacyanoferrate nanostructures (cuhcf/pg) l-cysteine 1-13 µm 0.13 µm [33] swcnts-chitosan/pg vitamin b12 5.0 100.0 nm 0.89 nm [34] swcnts-chitosan/pg dna 10 80 µg ml-1 13.2 µg ml-1 [35] mwcnt/pg methadone 0.115 µm 0.087 µm [36] in addition, it was shown that pg electrodes of 5h and b grades set as biocathode and bioanode yield the highest full cell performance [37,38]. in continuation of this effort, it would be important to find which pg grade is best suited for bioanode. accordingly, the objective of the present research work is to evaluate the performance of various grades (h, 3h, 5h and b) of pg in order to identify the grade best suited for bioanode in egbfc. the surface of these grades of pg were modified first by dip coating of carboxylic acid functionalized multiwalled carbon nanotubes (cooh-mwcnt/pg) and then by covalent immobilization of glucose oxidase (gox) enzyme, forming gox/cooh-mwcnt/pg electrode. the relative performances of fabricated pg bioanodes are studied for their efficiency towards oxidation of glucose. for this study, a mediated electron transfer (met) is employed using p-benzoquinone as a redox mediator to improve the rate of electron transfer between enzyme and electrode. in anodic enzyme glucose oxidase, the redox active centers are deeply buried in the protein structure making direct electron transfer very difficult [39]. half-cell and full cell performances of the optimized pencil grade bioanode are evaluated to validate its application to egbfc. experimental chemicals analytical grade chemicals and materials were procured from sigma–aldrich and used as received. they include dibasic sodium phosphate (na2hpo4), monobasic sodium phosphate (nah2po4), sodium hydroxide (naoh), hydrogen chloride (hcl), 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (edc), acetone, potassium ferricyanide (k3[fe(cn)6), dimethyl sulfoxide (dmso), d-(+)-glucose, n-hydroxy succamide (nhs), p-benzoquinone (pbq), carboxylic acid functionalized multiwalled carbon nanotubes, (cooh-mwcnt) with average diameter and length 9.5 nm and 1.5 μm. glucose oxidase (gox) from aspergillus niger was used as biocatalyst for glucose oxidation reaction. it was stored in lyophilized powder at -20 oc. enzyme solution was freshly prepared before each experiment. pencil graphite of h, 3h, 5h and b of apsara brand, india, with 1 cm length and 2 mm diameter, were purchased locally. equipment electrochemical measurements were performed using autolab potentiostat/galvanostat pgstat 302n model (metrohm, netherlands) with nova 1.11 version software, biologic model sp-150. the http://dx.doi.org/10.5599/jese.807 j. electrochem. sci. eng. 10(4) (2020) 385-398 graphite electrodes as bioanodes 388 morphological and elemental composition studies of fabricated pg based bioanodes were performed using scanning electron microscopy performed by apreo sem with an energy dispersive x-ray (edx) instrument. procedures all experiments were carried out at room temperature (25 ± 3 °c) and all solutions were made from milli-q grade deionized water having specific resistivity 18.2 mω cm. electrolyte and electrode material a solution of 0.1 m phosphate buffer was employed as electrolyte for carrying out electrochemical measurements. appropriate proportions of 0.2 m na2hpo4 and 0.2 m nah2po4 were added and the ph was fine tuned to 7.0 by either acid (1.0 m hcl) or base (1.0 m naoh) solutions. the pencil leads were extracted by peeling the wooden shell using a razor blade. all peeled pencil leads were washed with hcl (0.5 m) for 2 min in ultrasonication bath followed with acetone wash for 2 min using sonicator. the cleaned pencil leads were dried at 90 °c for 2 h in hot air oven to remove moisture content. the pencil leads prepared in this manner were then subjected to surface modification to obtain the pencil graphite electrodes. surface modification of pg dip coating technique was employed to coat cooh-mwcnt on the surface of cleaned pg. 1.0 mg ml-1 dry cooh-mwcnt was added to dmso and dispersed using a sonicator. the cleaned pencils were dipped in this mwcnt dispersion to ensure proper wetting of pg surface and this was followed by drying to evaporate solvent. for effective adsorption of mwcnt on the surface of pg, the process of wetting followed by drying was repeated four times. enzyme immobilization on electrode initially, the activation of carboxylic groups was carried out by immersing cooh-mwcnt/pg in edc/nhs (1:1) solution for 2 h. thereafter, gox, the most commonly used enzyme for oxidation of glucose [40], was covalently immobilized on cooh-mwcnt/pg. 7.0 mg of gox enzyme mixed in 1.0 ml of pbs (0.1 m, ph 7.0) was cast on the surface of cooh-mwcnt/pg and then allowed to dry for 4 h at ambient temperature in proper ventilation. here, gox enzyme becomes covalently associated to activated cooh groups in mwcnt by amide bond through bioconjugation technique. later, the prepared gox/cooh-mwcnt/pg was rinsed in pbs to remove unattached enzymes. electrochemical characterization cyclic voltammetry (cv) measurements were performed in three-electrode cell assembly comprising respective grade of gox/cooh-mwcnt/pg as the working electrode, platinum wire as the counter electrode and ag/agcl (sat. kcl) (metrohm) as the reference electrode. the potential was swept between -0.6 v to 0.6 v (vs. ag/agcl) with scan rates varied from 10 to 50 mv s-1. it was observed that 10 mv s-1 was the optimum scan rate for all tested grades of pg, and therefore, this scan rate was maintained for all experiments. all measurements were made with regard to the geometrical surface area (0.66 cm2) of prepared bioanodes. to measure effective electrode surface area (esa), cv was repeated for the same electrode system in 4 mm k3[fe(cn)6] dissolved in 0.1 m kno3 in a voltage range -0.2 – 0.8 v for a series of scan rates ranging from 20 to 100 mv s-1 [41]. electrochemical impedance spectroscopy was performed over the frequency range of 10 khz to 10 mhz, at an amplitude of 5 mv, resulting in nyquist plots (-z” vs. z’) of gox/cooh--mwcnt/pg [42]. m. bandapati et al. j. electrochem. sci. eng. 10(4) (2020) 385-398 http://dx.doi.org/10.5599/jese.807 389 all experiments were repeated with three sets of electrodes fabricated for each grade of pencil graphite to ensure the effectiveness of the fabrication procedure and to evaluate the consistency and reproducibility of the results obtained. half-cell and full cell assembly with modified pg bioanode half-cell measurements were carried out using a conventional three-electrode system of 25 ml volume capacity comprising the prepared gox/cooh-mwcnt/pg (modification described above) as the working electrode, platinum wire as the counter electrode and ag/agcl (sat. kcl) as the reference electrode. the solution of 0.1 m, ph 7.0 pbs containing 5 mm glucose as redox species was used as the electrolyte. to promote met, 2 mm p-benzoquinone (pbq) was added to the electrolyte as a redox mediator [43]. the electrolyte was purged with n2 gas for 15 min before each experiment to avoid the interference of oxygen to glucose oxidation reaction at anode. experiments carried out on the said grades of pencils at various scan rates showed that maximum performance was obtained at scan rate of 10 mv s-1. accordingly, this scan rate was considered as the optimized parameter for carrying out rest of the experiments. an assembly of enzymatic glucose biofuel cell was used to test gox/cooh-mwcnt composite modified pg based bioanode using biologic-150 potentiostat (biologic science instruments, france). egbfc comprises of 10 ml single compartment membrane-less chamber with gox/cooh-mwcnt composite modified pg as bioanode and platinum wire as biocathode. the cell is fed with 0.1 m, ph 7.0 pbs containing 5 mm glucose as anodic fuel and ambient air for cathode. pbq and abts were used as anode and cathode redox mediators, respectively. any half-cell and full cell set up was configured separately for each tested grade of pg bioanode and the measurements were recorded individually. polarization studies were conducted to evaluate the voltage and power output generated by egbfc. the maximum power output of assembled pg based egbfc at each known resistance was calculated by multiplying stabilized current and potential. the full cell performance parameters such as current and power densities were calculated based on the geometrical surface area of the electrode. the real images of the experimental set up are shown in figure 1. measurements effective surface area of electrode effective surface area (esa) of cooh-mwcnt/pg electrode was evaluated using randles–ševčik equation describing the effect of scan rate on peak current in cyclic voltammetry experiments [44]: ip = 2.69105n3/2d1/2cav1/2 (1) in eq. (1), ip is peak current, n represents the number of electrons transferred during redox reaction, d (cm2 s-1) represents diffusion coefficient of reaction species, ν is the sweep or scan rate (v s-1), c is concentration (m) of reaction species, and a symbolizes the effective surface area of electrode (cm2). in present experiments, 4 mm k3[fe(cn)6] was applied generating fe(cn)6-3/fe(cn)6-4 redox reaction and d was stated as 6.70×10−6 cm2 s-1. peak currents (ip) were measured at scan rates varying from 20-100 mv s-1 using cv and the slope (k) of the obtained linear expression for ip vs. ν½ was obtained. the slope is then substituted into the following equation to express esa (a) [41]: a = k/(2.69105n3/2d1/2) (2) http://dx.doi.org/10.5599/jese.807 j. electrochem. sci. eng. 10(4) (2020) 385-398 graphite electrodes as bioanodes 390 figure 1. real images of stepwise process of pg electrode fabrication: (a) pencil graphite of different grades; (b) peeled and pretreated pg; (c) mwcnt modified pg; (d) enzyme immobilization; (e) pg based bioanode half-cell performance; (f) pg based bioanode full cell egbfc; (g) storage of fabricated pg bioanode electron transfer kinetics electron transfer reaction rate (etr) constant the laviron equation [45] was employed to measure etr constant (γ) of each grade of fabricated pg based bioanode:        −  = − + − − −    p 1 δ log log 1 1 log log 2 3 ( )nf ert ( ) ( ) nfv . rt (3) in eq. (3), r stands for gas constant (8.314 j k-1 mol-1), ν for sweep rate (0.1 v s-1), α is transfer coefficient, t is temperature, k; f is faraday constant (96485 a s mol-1), n is the number of transferred electrons (n=2) and ∆ep represents potential difference (v) of cathodic and anodic redox peaks. surface enzyme density the surface concentration of immobilized enzyme in mol cm-2 was evaluated using cv measurements and brown–anson model [46]: = 2 2 p 4 av i n f rt (4) in eq. (4), a represents the electrode geometrical surface area (0.66 cm2), and  stands for the enzyme concentration on mwcnt nanocomposite of the fabricated bioanode surface. other symbols have their already explained meaning. results and discussion surface morphology analysis using scanning electron microscope the structure and morphology of fabricated pg bioanodes (cooh-mwcnt/pg, gox/coohmwcnt/pg) were studied using scanning electron microscopy (sem). sem images with 500 nm magnification taken during various stages of fabrication, are shown in figure 2. different grades of pg have exhibited different morphological characteristics due to the variation in their composition. m. bandapati et al. j. electrochem. sci. eng. 10(4) (2020) 385-398 http://dx.doi.org/10.5599/jese.807 391 b a b h 3h 5h figure 2. sem micrographs of fabricated (a) cooh-mwcnt/pg and (b) gox/coohmwcnt/pg of b, h, 3h, and 5h grades figure 2a shows sem images of cooh-mwcnt/pg with a network of mwcnts distributed on the surfaces of pg of various grades. from sem images, it can be seen that a relatively high density of mwcnt carpet is observed on b-grade pg when compared to other grades, with noticeably large http://dx.doi.org/10.5599/jese.807 j. electrochem. sci. eng. 10(4) (2020) 385-398 graphite electrodes as bioanodes 392 quantities of attachment of mwcnt to pg surface. this can be attributed to the porous network structure of b-grade pencil which remains undisturbed by the small amount of binder content. in the case of other grades of pencils, however, the increasing binder content resulted in the blockage of redox sites of the surface functional groups, resulting in sparse deposition as can be seen in sem images of other pg electrodes. it is assumed that by dip-coating of mwcnt, an increased electrode surface for anchoring of enzymes is provided with improved accessibility of redox species towards immobilized enzymes [47]. figure 2b shows sem images of gox/cooh-mwcnt/pg after gox immobilization. it is evident that b-grade pg has distinct morphology with relatively high gox coverage due to greater enzyme loading than at other grades pg. it can be inferred that b-grade pg with its dense deposition of mwcnt matrix with nano-wired structure provides a relatively large surface area that shows affinity towards gox enzyme, resulting in greater immobilization of gox on the electrode surface. surface morphology analysis using energy dispersive x-ray spectroscopy in addition to sem, energy dispersive x-ray spectroscopy (edx) analysis was employed to confirm the presence of immobilized gox enzyme on modified pg electrode surface. figure 3 displays carbon (c) and oxygen (o) contents before and after gox casting on mwcnt matrix of different grades pg. before the immobilization of gox, the unmodified pg showed low oxygen and higher carbon content. after gox immobilization, however, the oxygen content is increased, and the carbon content reduced. this observation confirms the association of gox enzyme layer on the surface of modified pg electrode. relative to other tested grades, higher difference of oxygen and carbon contents before and after gox immobilization was recorded for b-grade pg, indicating the highest enzyme immobilization among all tested grades of pencils. it is also evident that the oxygen to carbon ratio for all tested grades of pg is ranging from 13 to 30 % (o/c ratio for b grade pg 13 %), what is close to the o/c ratio of polished glassy carbon electrode (8-15 %) [18]. pencil graphite sample figure 3. edx characterization of carbon and oxygen contents of modified and unmodified pg based bioanode (um-unmodified; m-modified). electrode characterization using electron transfer kinetics the electrochemical reduction/oxidation of [fe(cn)6]-3/[fe(cn)6]-4 redox couple was used to compute the kinetics of electron transfer at modified pg electrodes (cooh-mwcnt/pg). the cv 0 20 40 60 80 100 b h 3h 5h c a r b o n a n d o x y g e n c o n te n t, w t% pencil graphite sample um carbon m carbon um oxygen m oxygen um o/c (%) c a rb o n a n d o xy g e n o n te n t, w t% m. bandapati et al. j. electrochem. sci. eng. 10(4) (2020) 385-398 http://dx.doi.org/10.5599/jese.807 393 curves for cooh-mwcnt/pg tested in 4.0 mm [fe(cn)6]-3 dissolved in 100 mm kno3 within the voltage range of -0.2 to 0.8 v at 10 mv s-1, are shown in figure 4. applied potential, v vs. ag/agcl figure 4. cyclic voltammograms for cooh-mwcnt/pg electrode of various grades in 0.1 m kno3 containing 4.0 mm k3fe(cn)6 at 10 mv s−1 scan rate. the highest peak currents are observed for b-grade pencil and the ratio of cathodic (ipc) and anodic (ipa) peak currents is close to 1.0, indicating reversible electron transfer kinetics [48]. also, this grade of pg bioanode showed less anodic and cathodic peak potential difference (∆ep) (which is a basis for electrochemical reversibility [46]), indicating that the heterogeneous pseudo reversible electron transfer process is present [40]. the respective peak current densities and ∆ep values of all tested pg grades are summarized in table 2. effective electrode surface area comparison electrochemical reaction of [fe(cn)6]-4/[fe(cn)6]-3 redox couple was also used for evaluation of effective electrode surface area (esa) of modified pg electrodes (cooh-mwcnt/pg) of various grades. resulting cv curves of tested cooh-mwcnt/pg electrodes showed well defined redox peaks for the scan rate range 10-100 mvs-1 and the linear expression was observed for peak currents plotted vs. square root of scan rates. using randles–ševčik equation defined by eq. (1), the values of esa were calculated using eq. (2) for all grades of cooh-mwcnt/pg and listed in table 2. the highest esa was obtained for b-grade cooh-mwcnt/pg, indicating high deposition of mwcnt as can also be observed from sem images. table 2. summary of electrochemical parameters for [fe(cn)6]-4/[fe(cn)6]-3 redox reaction at coohmwcnt/pg of various grades pencil grade ipa / μa cm-2 ipc /ipa ∆ep / mv average value of esa, cm2 sd (n=3) standard error, % (n=3) b 1013 1.06 71 0.34 0.01 1 h 968 1.37 100 0.28 0.03 1 3h 896 1.25 103 0.30 0.03 1 5h 900 1.04 152 0.33 0.03 1% electrochemical characterization using electrochemical impedance spectroscopy electrochemical impedance spectroscopy (eis) was opted to characterize the interface properties of fabricated pg based bioanodes during their interaction with pbs electrolyte containing 5 mm -100 -80 -60 -40 -20 0 20 40 60 80 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 c u r r e n t d e n si ty , m a c m -2 applied potential, v vs. agagcl b h 3h 5h c u rr e n t d e n si ty , m a c m -2 http://dx.doi.org/10.5599/jese.807 j. electrochem. sci. eng. 10(4) (2020) 385-398 graphite electrodes as bioanodes 394 glucose. in general, eis curves reveal semicircles and straight lines at high and low frequencies, respectively. the diameter of the semicircle corresponds to the charge transfer resistance (rct) of the redox reaction, whilst the straight line represents the superiority of mass diffusion control effect over the charge transfer process [42]. figure 5 shows electrochemical impedance spectra (nyquist plots) of gox/cooh-mwcnt/pg recorded at 0 v vs. ag/agcl in pbs (0.1 m, ph 7.0) containing 5 mm glucose and 2 mm pbq as a mediator. it can be observed from impedance spectra in figure 5 that gox/cooh-mwcnt/pg electrodes have undergone diffusion controlled redox reaction indicated by straight lines at low frequencies. at high frequencies, however, well defined semicircles corresponding to interfacial charge transfer resistance coupled to double-layer capacitance are noticed for gox/coohmwcnt/pg of all tested grades. the evaluated rct values are summarized in table 3, showing variations between 2 to 8 ω cm2. it can also be inferred that b-grade gox/cooh-mwcnt/pg offered the lowest resistance to electron transfer among all tested pg bioanodes. this may be attributed to the deposition of dense conducting matrix of mwcnt on pg surface. larger value of rct for 3h-grade pg may be due to large amount of clay content, as also seen by edx analysis, which offers more resistance for electron transfer. figure 5. nyquist plots of gox/cooh-mwcnt/pg bioanodes fabricated from various grades of pg in 0.1 m ph 7.0 pbs containing 5 mm glucose and 2 mm pbq. frequency range: 10 khz -10 mhz; amplitude: 5 mv electrochemical characterization of gox/cooh-mwcnt/pg electrodes using cv biocatalytic activity the biocatalytic activity of gox/cooh-mwcnt/pg based bioanodes towards glucose oxidation was also tested using cv. all the experiments were carried out in n2 gas saturated 0.1 m, ph 7.0 pbs containing 5 mm glucose at the sweep rate of 10 mvs-1. the resulting cv curves are presented in figure 6, showing well defined oxidation peaks at potentials between 200 to 300 mv for all grades of pg bioanodes. varying current densities are observed with the highest peak current density of 4.25 ma cm-2 recorded at peak potential of 253 mv for b-grade bioanode. this may be due to the enhanced surface area attained by increasing amount of mwcnt deposition as can be seen from sem images and high graphite content [18]. 0 2 4 8 10 12 14 16 18 20 -z " / ω c m 2 z' / ω cm2 b h 3h 5h m. bandapati et al. j. electrochem. sci. eng. 10(4) (2020) 385-398 http://dx.doi.org/10.5599/jese.807 395 applied potential, v vs. ag/agcl figure 6. cvs of gox/cooh-mwcnt/pg bioanodes of different grades in 0.1 m pbs ph 7.0, containing 5 mm glucose, recorded at 10 mvs-1. immobilized gox enzyme kinetics using the same cv studies, anodic peak currents were plotted against a series of scan rates for each fabricated pg bioanode. a linear raise in anodic peak current with increase in sweep rate was observed. using laviron (eq. (3)) and brown–anson equation (eq. (4)), the electron transfer rate (etr) constant () and surface concentration () of deposited nanobiocomposite for all tested pgs were determined and listed in table 3. the values of etr constants for all tested grades of pencil leads are varied from 2.0 to 5.4 s-1, and the highest surface concentration of enzymes biocomposite is observed for b-grade bioanode showing the fastest etr (5.39 s-1), what agrees with sem results. table 3. electrochemical characteristics of gox/cooh-mwcnt/pg bioanodes pencil grade rct / ω cm2 ipa / μacm-2  / s-1  / mol cm-2×10-10 b 2 4250 5.39 2.42 h 5 163 1.96 1.79 3h 8 3086 2.62 1.82 5h 3 3910 3.87 2.27 biofuel cell performance of gox/cooh-mwcnt modified pg bioanode to validate the application of fabricated bioanodes for egbfc, a prototype membrane-less enzymatic glucose biofuel cell was assembled using gox/cooh-mwcnt/pg as bioanode and platinum wire as cathode. both electrodes were immersed in pbs (0.1 m, ph 7.0) contained in 10 ml beaker at atmospheric conditions. figure 7a shows the real image of assembled membrane-less enzymatic glucose biofuel cell. it can be clearly observed from figure 7a, that pg-based bioanode of b-grade in pbs (0.1 m, ph 7.0) containing 5 mm glucose is well-suited for further miniaturization. figure 7b presents current density (i) and power density (p) of egbfc (i vs. p and i vs. v curves) as a function of operation cell voltage taken at room temperature (25 ± 3◦c). the current density was calculated with respect to the electrochemical active surface area (esa) of bioanode. figure 7b shows that open circuit potential (ocp) and maximum current density of egbfc are 149 ± 3 mv and 38 ± 0.7 µa cm−2, respectively. further, a maximum power density of 0.789 µw cm−2 can be attained at 141 mv. this value is very close to that of anodes made from carbon cloth [49,50] and even greater than those of anodes based on reduced graphene oxide (rgo/mwcnt) based carbon [51]. this proves the applicability of the fabricated bioanode to egbfc. at the same time, the value of -3 -2 -1 0 1 2 3 4 5 -1 -0.5 0 0.5 1 c u r r e n t d e n si ty , µ a c m -2 applied potential, v vs. agagcl h 3h b 5h without glucose c u rr e n t d e n si ty ,  a c m -2 http://dx.doi.org/10.5599/jese.807 j. electrochem. sci. eng. 10(4) (2020) 385-398 graphite electrodes as bioanodes 396 maximum power density is somewhat smaller than for carbon or mwcnt based wired enzyme bioanodes [52-54]. however, the unique advantages of pg like low cost, quick surface renewal and ease of modification and miniaturization, off-set its lower performance levels, maintaining thus applicability of pg to miniaturized bioelectronic systems. a b current density, a cm-2 figure 7. a) real image of the assembled single chamber membrane-less egbfc with an optimized grade pg as bioanode and platinum wire as biocathode in contact with 5 mm glucose dissolved in 0.1 m, ph 7.0 pbs containing 3 mm abts and 3 mm pbq as anodic and cathodic mediators and under air saturated conditions. b) potential and power density vs. current density plots of egbfc containing optimized pg bioanode obtained by polarization studies at 25 ± 3 °c. conclusions this work demonstrates the application of modified pencil graphite (pg) of different grades as suitable bioanodes for enzymatic glucose biofuel cell (egbfc). morphological characterization of tested pg of grades h, 3h, 5h, and b have indicated some variations in enzyme immobilization on the surface of pg, based on their composition and surface structure. the tested pg bioanodes exhibited positive responses towards glucose oxidation reaction with the fastest electron transfer rate recorded for b-grade pg. the half-cell assembly with this pg grade bioanode showed the highest current density of 4.25 ma cm-2 under physiological glucose concentration conditions. the obtained results reveal that physical and electrochemical characteristics of b-grade pg bioanode are comparable to those of conventional carbon and glassy carbon electrode substrates. the membrane-less enzymatic glucose biofuel cell assembled with b-grade pg bioanode showed ocp at 149 mv, maximum power density of 0.789 µw cm−2 and current density of 38 ± 0.7 µa cm−2 at 141 mv voltage, respectively. since these values are very close to those of anodes made from carbon cloth modified with mwcnt and even greater than those of anodes based on rgo/mwcnt based carbon, the applicability of the fabricated bioanode to egbfc becomes clear. the performance can be further optimized by subjecting pg to additional surface modifications with nanomaterials and conductive polymers. alternate enzyme immobilizing techniques can also be explored to enhance enzyme reaction kinetics. the performance is also likely to vary by introduction of pg based biocathode to realize fully assembled egbfc using pg electrodes. once p o w e r d e n si ty ,  w c m -2 potential power densuty p o te n ti a l, v m. bandapati et al. j. electrochem. sci. eng. 10(4) (2020) 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[54] a. kumar, s. sharma, l. m. pandey, p. chandra, materials science for energy technologies 1 (2018) 3848. ©2020 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {review on electrochemistry, a guide for newcomers by helmut baumg{\"{a}}rtel:} http://dx.doi.org/10.5599/jese.972 217 j. electrochem. sci. eng. 11(3) (2021) 217-219; http://dx.doi.org/10.5599/jese.972 open access: issn 1847-9286 www.jese-online.org book review review on electrochemistry, a guide for newcomers by helmut baumgärtel abdul hasib kazipur government mansur ali college, kazipur-6710, sirajgonj, bangladesh 82abdulhasib@gmail.com received: february 25, 2021; accepted: may 19, 2021; published: may 23, 2021 electrochemistry a guide for newcomers by helmut baumgärtel walter de gruyter gmbh, berlin/boston, oct. 2019 hardcover: 230 pages isbn 978-3-11-044340-0 e-isbn (pdf) 978-3-11-043739-3 e-isbn (epub) 978-3-11-043554-2 in his nobel lecture 2001, k. berry sharpless said: “when i started doing chemistry, i did it the way i fished – for the excitement, the discovery, the adventure, for going after the most elusive catch imaginable in uncharted seas” [1]. true, in electrochemistry, a subject considered by many as an apparently boring and complex one, there are the “seas”electron-sea in metals, electrolytic solutions and the “fishes”moving electrons, ions, ionophores and so on. then, this masterpiece by professor helmut baumgärtel may be cited as a good example amongst a few of the same genre that would retain the readers of a deeper subject in an engaging way like fishing in the “electron-sea”. the book as are paraphrased from p. 1 and p. 2 is written in an “up-to-date, understandable and systematic way” (p. 1) following all through pedagogical style. it is aimed primarily for those who “are not familiar with electrochemistry but are interested in it” (preface). they may be engineers, physicists, schoolteachers making use of electrochemistry in their respective professions or any interested individual making a way to electrochemistry through the “self-study” (p. 1). and this book is dedicated to you if you are one of the amongst those who “want to know the story behind energy storage as reported in daily newspapers” (preface). in its content organization, it may be compared to some other books [2] with respect to that there are only three core chapters, total absence of chapter-end practice questions, and only a http://dx.doi.org/10.5599/jese.972 http://dx.doi.org/10.5599/jese.972 http://www.jese-online.org/ mailto:82abdulhasib@gmail.com j. electrochem. sci. eng. 11(3) (2021) 217-219 book review 218 limited well-chosen worked-out mathematical examples. all these must be taken to be granted as per the declaration in the preface, “this book is not a textbook in the general sense”. thereby, in lieu of arranging the contents according to the traditional choice of most student-friendly text books, the author tries to construct a conceptual framework about the much complex topics that he expects to be discerned by his readers. and, it is one of the goals of this book (p. 2) that is, to psychup the newcomers to deal with the arising confusions in their psyche. in the chapter two “fundamentals”, the author has begun with a simple reaction of zinc rod dipped into a copper sulphate solution describing, in a simple story-telling manner, various situations and conditions that has resulted in establishing an electrochemical cell (p. 12). after that, the discussion culminated in the various types of cells and one might easily be intrigued along the way by the step-by-step approach in developing an experimentally verified scientific account. then, the thermodynamic aspects of equilibrium potential have been enumerated with complex mathematical derivations. the equations and derivations connected with the debye-hückel model, double layer models, current-overpotential relationship, the tafel behavior of the overpotential, mass transfer by diffusion all these to name a few, might not be so easy to catch thoroughly. in such instances, for a deeper understanding the author suggests: “it is necessary to study all the details……….” (p. 2). additionally, most of such derivations lack detail stepwise work-out. skipping the middle lines in complex derivations is perhaps a reasonable choice by the author who either has constraint to curb the size of his booklet as mentioned in many occasions (p. 110, p. 158, p. 165) or has left the trifles for us to be dealt with. however, any ambitious reader may be well satiated with appendices “a” to “g” where many complex calculations have been carried out in details. indeed, much of the mathematical portions of the chapter two has comprised topics that are likely wellsuited only for the advanced readers. a thorough illustration of a “unique principle” to study an electro-chemical system, that is; “we send a controlled input into the system and measure the response of that system” (p. 111) is the core focus of the chapter three, “practical work in electrochemistry”. hence, different types of currenttime curves, current-potential curves, cyclic voltammograms with respect to various experimental methods have predominately occupied the major parts of this chapter. a beginner might strive to cope with the subtle differences amongst the reversible and irreversible cyclic voltammetry (p. 132), the ce, ec and ece mechanisms of various chemical reactions (p. 139) and so forth, yet if explored, this chapter would certainly provide the must-have ideas about different electrochemical set-up and the methods to investigate into them. the last chapter “applications of electrochemistry” would appeal the readers most as there are the topics either we face those in our day-to-day experiences from various batteries to corrosion or, from routine laboratory course work to large scale industrial production we are to deal with. for it is seen a topic be started very occasionally with the conventional approach to define that at first, it seems that the author has a deliberate choice of scattering the so called memorisable statements elsewhere in the text e.g., the most conventional definition pertaining to electrochemistry appears in the last line of the first chapter (p. 9). the likewise occurrences happen all through the book whereupon any avid reader should ferret out the so-called definitions and/or terminologies. however, scholarly literature suggests that there is an utmost significance of making formal definitions through learning processes not only in the child learners but also in the scientists [3]. the thermodynamic aspects of electrochemistry have been whenever possible favorably spaced throughout the book. owing to this, any newcomer may find some basic chemical aspects like detail reactions of numerous types of cells, cell emf determination, salt bridge, etc. to be overshadowed. a. hasib j. electrochem. sci. eng. 11(3) (2021) 217-219 http://dx.doi.org/10.5599/jese.972 219 albeit they have been dealt with either, this kind of favor is much expected from the author who is a veteran professor of physical chemistry in an eminent university in berlin, germany. the use of the boxes with “twofold intension” as is claimed in the introduction (p. 2) is much appreciated but, throughout the book those useful boxes are only five in numbers: such a paradoxical happening is beyond our expectation. if there were more such boxes, they would undoubtedly have helped the readers more. there are a few mathematical worked out problems but no numerical exercises that can easily be substantiated from the modern teaching-learning perspectives for example, “chemistry teaching is polluted and ultimately compromised in its goal by numerical exercises…… such elementary mathematics do not give a clue as to the real understanding of chemical notions by one’s students” [4] and our present author is well aware of that. let’s be prepared for an excursion guided by the author who, by his briefings like “the next step of our experimental excursion is to connect the electrodes via a low resistance line……….” (p. 13) would for sure rejuvenate the young minds. such an approach, “to instruct into mainstream chemistry and to lead students into excursions outside of mainstream chemistry” is corroborated, amongst many by laszlo [5]. now, it’s time to start the experimental excursion in the way of electrochemistry and this book would certainly guide the beginners the right way! and for the libraries, it would certainly be a valuable one. references [1] wikiquote, https://en.wikiquote.org/wiki/k._barry_sharpless (date accessed, 15 feb. 2021). [2] c. m. a. brett, a. m. oliveira-brett, electrochemistry: principles, methods, and applications, oxford university press, oxford, uk, 1993. isbn 0 19 855389 7 (hardback). [3] m. a. iris, b. e. litowitz, m. w. evens, international journal of lexicography 1(3) (1988) 238-252 https://doi.org/10.1093/ijl/1.3.238. [4] k. j. smith, p. a. metz. journal of chemical education 73(3) (1996) 233 https://doi.org/10.1021/ed073p233. [5] p. laszlo, science and education 22 (2013) 1669-1706 https://doi.org/10.1007/s11191-011-9408-6. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.972 https://en.wikiquote.org/wiki/main_page https://en.wikiquote.org/wiki/k._barry_sharpless https://doi.org/10.1093/ijl/1.3.238 https://doi.org/10.1021/ed073p233 https://doi.org/10.1007/s11191-011-9408-6 https://creativecommons.org/licenses/by/4.0/) {a preliminary study into the effect of oxide chemistry on the bonding mechanism of cold-sprayed titanium dioxide coatings on sus316 stainless steel substrate:} http://dx.doi.org/10.5599/jese.1423 579 j. electrochem. sci. eng. 12(4) (2022) 579-591; http://dx.doi.org/10.5599/jese.1423 open access : : issn 1847-9286 www.jese-online.org original scientific paper a preliminary study into the effect of oxide chemistry on the bonding mechanism of cold-sprayed titanium dioxide coatings on sus316 stainless steel substrate noor irinah binti omar1,, suhana binti mohamed2,yusliza binti yusuf1, toibah binti abdul rahim3, zaleha binti mustafa3, syahriza binti ismail3, ilyani akmar binti abu bakar4 and santirraprahkash selvamani5 1faculty of mechanical and manufacturing engineering technology, universiti teknikal malaysia melaka,hang tuah jaya, 76100 durian tunggal, melaka, malaysia 2faculty of business management, universiti teknologi mara johor, kampus pasir gudang, jalan purnama, bandar seri alam, 81750, masai, malaysia 3faculty of manufacturing engineering, universiti teknikal malaysia melaka, hang tuah jaya, 76100 durian tunggal, melaka ,malaysia 4school of civil engineering, college of engineering, universiti teknologi mara, 40450, shah alam, selangor, malaysia 5department of mechanical engineering, toyohashi university of technology, 1-1 tempaku-cho, toyohashi, aichi 441-8580, japan corresponding author:  nooririnah@utem.edu.my; tel.: +60123661044; fax: 606-2701064 received: june 22, 2022; accepted: july 25, 2022; published: august 10, 2022 abstract current attention has focused on the preparation of thick ceramic coating of nanostructured materials as feedstock material using the thermal spray process. the cold spray method has appeared as a promising process to form ceramic nanostructured coating without significantly changing the microstructure of the initial feedstock materials due to its low processing temperature. however, deposition of ceramic powders by cold spray is not easy due to the brittle characteristics of the material. in this study, tio2 coatings were deposited on unannealed stainless steel substrates and substrates that were annealed from room temperature to 700 °c prior to spraying. the adhesion strength was evaluated to investigate the bonding mechanism. the influence of the remaining surface oxide layer of chromium oxide, cr2o3, which is thermodynamically preferred for stainless steel, on the bonding mechanism involved was investigated. the results showed that by increasing the annealing substrate temperature of stainless steel, the adhesion strength of the coatings (thicker oxide) is also increased. as a result, the bonding between the cold-sprayed tio2 particle and the steel substrate is given by the chemical bonding of an inter-oxide reaction. keywords tio2 coatings, chromium oxide, adhesion strength, bonding mechanism. http://dx.doi.org/10.5599/jese.1423 http://dx.doi.org/10.5599/jese.1423 http://www.jese-online.org/ mailto:nooririnah@utem.edu.my j. electrochem. sci. eng. 12(4) (2022) 579-591 cold-sprayed tio2 coatings on sus316 stainless steel 580 introduction cold spraying is a solid state process in which powder feedstock in the range of 5 to 40 nm in the powder feeder is supersonically accelerated in the de-laval part of the nozzle and then passed through the nozzle to form a coating on a substrate [1]. when the particle velocity exceeds the critical velocity, the plastic deformation of both, the sprayed metallic particle and the metallic substrate, removes the surface oxide layers, exposing newly formed surfaces. the metallic bond is formed on the newly formed surface, and cold spray deposition may follow. for formation jetting, the contact between the particles and the substrate is extruded. material flow at the interface is defined as shear instability for bonding formation [2-4]. this solid powder deposition approach can reduce oxidation and other negative impacts on coating characteristics induced by the hightemperature procedure [5-7]. it is well known that cold spraying of ceramic materials can be difficult because cold spraying requires plastic deformation of the feedstock particles for adhesion to the substrate. the difficulty lies in making hard and brittle ceramic materials such as tio2 deform plastically. however, previous studies reported the possibility of cold spraying thick pure tio2, but the bonding mechanism of cold sprayed tio2 is not fully understood yet [8]. few researchers have proposed chemical bonding as a bonding mechanism in the cold spray process, especially in the cases of ceramic cold spray like tio2, where a tio2 coating was produced from recombination of broken up crystallite links during the cold spray process, which was initiated by the porous structure of the agglomerated powders [9]. they proposed that chemical bonding may allow thick coatings to be deposited by the cold spray process. the agglomerated powder in nano-scale primary particles containing nano porosity was fractured, leaving an unstable surface with a dangling bond structure. to reobtain a stable surface, the fractured particles decoupled and formed a surface with improved stability, which led to the bonding of the newly impacting particles and building up the coating [9]. salim et al. [10] prepared 400 µm and 150 µm thick tio2 coatings on metal and tile, respectively. it was observed that adhesion strength changed little with spraying parameters. this indicated that mechanical interlocking was not the main bonding mechanism in this case, so was the substrate shear instability. it was discovered that both the hardness and the oxidizability of the substrates affected the adhesion strength of tio2 coatings. the adhesion strength of tio2 coatings could be improved by changing the surface chemistry of the substrates. it was proposed that the chemical or physical bonding mechanism was the main bonding mechanism of ceramic coating. tem images proved the existence of chemical bonding between tio2 particles. preheating could increase the oxidizability of the substrate, thus deteriorating the adhesion strength of coatings. gardon et al. [11] reported that the mechanism responsible for the deposition of tio2 on the stainless-steel substrate in the cold-spraying process is the chemical bonding between particles and substrate. they have shown that the previous layer of titanium sub-oxide prepares the substrate with the appropriate surface roughness needed for the deposition of tio2 particles on the substrate surface. from the previous results and discussion above, it is clear that cold spray coating of tio2 is not easy to be obtained, but not impossible. also, there is still considerable uncertainty concerning the bonding mechanism involved for pure tio2 powder metal substrates, especially focusing on the mechanism of chemical bonding. this paper is a preliminary study to study the effect of oxide chemistry on the bonding mechanism of cold sprayed titanium dioxide coating on stainless steel, sus316. n. i. binti omar et al. j. electrochem. sci. eng12(4) (2022) 579-591 http://dx.doi.org/10.5599/jese.1423 581 experimental materials we employed agglomerated tio2 powder (wp0097, tayca corporation, tokyo, japan) as a feedstock with a pure anatase crystalline structure and an average particle size of around 7.55 nm, as shown in figure 1. the substrate is stainless steel, sus316. figure 1. sem image of tio2 powder cold spray process all the coating attempts were carried out with cold spraying equipment and a de-laval 24tc nozzle (cgt kinetiks 4000; cold gas technology, ampfing, germany). at 500 °c working temperature, nitrogen was employed as the process gas. the pressures of 3 mpa, with a process traverse speed of 10 mm/s and a spray distance of 20 mm were utilized. characterizations strength testing specimen areas of  2510 mm were utilised to assess the coatings adhesive strength, which was supplied as the fracture load amount recorded by a universal testing machine, autograph ags-j series 10 kn, shimadzu, japan, in compliance with japan industrial standard h 8402. the adhesion strength of each spraying condition using a means of five specimens was tested. on the fracture coating surface, edx was used. coatings evaluation the cross-sectional microstructure of the tio2 coating on annealed substrates was analyzed via a scanning electron microscope (sem: jsm-6390, jeol, tokyo, japan). micro vickers hardness the hardness investigation was hv 0.1 with a test force on the cross-section of 98.07 mn at a dwell time of 10 s. oxide composition of the substrate and tio2 coating interface evaluation by xps x-ray photoelectron spectrometer (xps) was used to investigate the effect of the atomic and chemical composition of the substrate oxide layer on the adhesion strength of agglomerated tio2 coating. in addition, the chemical composition of the tio2 coating and the substrate interface was http://dx.doi.org/10.5599/jese.1423 j. electrochem. sci. eng. 12(4) (2022) 579-591 cold-sprayed tio2 coatings on sus316 stainless steel 582 also investigated by xps to identify if a change in chemical composition was present after coating formation by the cold gas spray method. table 1 shows the conditions of the xps analysis for substrate oxide analysis. tables 2 and 3 show the xps analysis conditions for tio2 coating and substrate interface analysis. the peak position was corrected with the c1s peak set at 285 ev. when pre-sputtering to clean the surface was performed, the sample surface was reduced and the measurement results were affected, so xps analysis was performed without pre-sputtering. table 1. xps parameters (substrate oxide layer analysis) measured regions fe 2p, o 1s, cr 2p, mn 3d measured x-ray output energy, w 10 probe diameter, μm 50 time per step, ms 30 pass energy, ev 140 number of cycle 30 table 2. xps parameters (tio2 / substrate interface wide scan) measured regions, ev 0-1100 measured x-ray output energy, w 10 probe diameter, μm 50 time per step, ms 5.2 pass energy, ev 140 number of cycle 5 table 3. xps parameters (tio2 / substrate interface narrow scan) measured regions fe 2p, cr 2p measured x-ray output energy, w 10 probe diameter, μm 50 time per step, ms 5.2 pass energy, ev 55 number of cycle 5 results and discussion coating adhesion strength figure 2 shows the coating tensile strength of tio2 on left x-axis and vickers hardness on right xaxis for annealed stainless steel, sus 316 from room temperature to 700 °c annealed. the result shows an increasing trend in the coating adhesion strength from 1.00 mpa to 1.88 mpa as the annealed substrate temperature increases from room temperature to 700 °c, but substrate hardness shows an opposite trend. figure 3 shows the image of the fractured surface of the substrate and tio2 coating after the tensile strength test was conducted. this image confirms that the fracture was an interfacial fracture of substrate and coating, proving thus that a strong, cohesive bond among tio2 particles has been achieved during the coating formation process. it was observed that adhesion strength changed little with the change of spraying parameters. this indicated that mechanical interlocking was not the main bonding mechanism, in this case, so was the substrates n. i. binti omar et al. j. electrochem. sci. eng12(4) (2022) 579-591 http://dx.doi.org/10.5599/jese.1423 583 shear instability. it was discovered that both the hardness and the oxidizability of the substrates affected the adhesion strength of tio2 coatings. the adhesion strength of tio2 coatings could be improved by changing the surface chemistry of the substrates. it was proposed that the chemical or physical bonding mechanism was the main bonding mechanism of ceramic coating. in order to confirm the surface chemistry factor, substrate analysis will be discussed further in this paper. figure 2. adhesion strength (◼) of the titanium dioxide coating on annealed sus316 and substrate hardness (__) for different annealed substrate temperature room temperature 300 oc 500 oc 700 oc figure 3. fractured surface of substrate and tio2 coating after tensile strength test cross-sectional observation of substrate and tio2 coating the cross-sectional microstructure of substrate and tio2 coating observed under sem is shown in figures 4 (a) and (b). a b figure 4. cross-sectional microstructure of sus316 and tio2 coating: (a) room temperature (b) 700 °c annealed substrates treated at room temperature and 700 °c were chosen for observation to clarify the interface adhesion between substrates which yielded the lowest and highest adhesion strength. coating with a thickness in the range of 200 to 300 μm was measured, showing that the critical 0 50 100 150 200 250 300 0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80 2.00 rt 300 500 700 m ic ro h a rd n e ss , h v a d h e si o n s tr e n g th , m p a temperature, °c (a) http://dx.doi.org/10.5599/jese.1423 j. electrochem. sci. eng. 12(4) (2022) 579-591 cold-sprayed tio2 coatings on sus316 stainless steel 584 velocity of particles was reached during the spraying process. meanwhile, the interface area of both types of substrates treated at room temperature and 700 °c shows good adhesion with visibly fewer cracks. this shows that the 700 °c annealing process minimizes the interface adhesion between the substrate and tio2 coating. depth profile analysis of oxide layer the depth analysis of the oxide layer by x-ray photoelectron spectroscopy for sus 316 substrate is shown in figures 5 (a) to (d) for the samples treated from room temperature to 700 °c. sio2 equivalent depth refers to the sputter etching rate to measure depth profiling measurement in this experiment. the sputter etching rate is generally estimated from an elemental depth profile of a standard sample with a known thickness, such as a sio2 film. a b c d figure 5. depth profile analysis of stainless steel, sus316: (a) room temperature (b) 300 °c annealed (c) 500 °c annealed and (d) 700 °c annealed this shows that the stainless steel oxide layer thickens as the substrate annealing temperature rises. the surface of the substrate plays an important role in achieving high adhesion strength of the cold sprayed coatings. this is because the adhesive strength of the coating is specifically determined by the bonding of the first layer of tio2 particles with the outermost surface of the substrate. when metal is used as a substrate, the outermost surface will always be composed of a passive oxide layer with a few nanometers of thickness. since the agglomerated tio2 particles will break down during contact with the surface of the substrate [9], it can be assumed that the first layer of contact between particles and substrate will be achieved in an area that is a few nanometers deep from the top surface of the oxide layer. the atomic composition of iron on the outermost surface of the oxide layer increases slightly as the annealing temperature increases. however, since the increment is small, it can be considered that this slight change has not affected the increase of the adhesion strength. the atomic composition of oxygen in the deepest part of the oxide layer increases significantly as the annealing temperature increases. this shows that the oxide layer of stainless steel grows thicker as the annealing temperature of the substrate increases. the increase in oxide thickness might have played n. i. binti omar et al. j. electrochem. sci. eng12(4) (2022) 579-591 http://dx.doi.org/10.5599/jese.1423 585 a role in the increasing trend of adhesion strength because the substrate oxide layer and feedstock powder, tio2, are made of oxide material. the behaviour of the chromium oxide is seen to have changed the most as the annealing temperature increases. in sus 316, the atomic composition of chromium at the outermost surface of the oxide layer decreases to near depletion as the annealing temperature increases from room temperature to 500 °c. this phenomenon has been reported in other research [12] known as chromium depletion. however, as the annealing temperature is raised to 700 °c, the amount of chromium at the outermost surface for sus 316 increases significantly. this sudden change can be considered a factor affecting the increases in adhesion strength. evaluation of the chemical state of iron and chromium in the oxide layer by x-ray photoelectron spectroscopy figure 6 (a) shows the peak position of iron, approximately at 706.7 ev. it was prominently present in the oxide layer for the room temperature substrate. this concludes that the oxide layer was thin for the not annealed substrate. at annealing temperatures of 300 and 500 °c, the peak position of magnetite fe3o4 (fe2+) at approximately 709.3 ev was detected at the outermost surface of the sus 316 substrate. below 570 °c, the wüstite state of iron oxide (fe1-xo, x = 0.84-0.95) is unstable, and the oxidation of iron directly results in magnetite or ferrite [8]. at the highest annealing temperature of 700 °c, the oxide layer consists of a two-layered microstructure with a thin outer layer of hematite fe2o3 and a secondary layer of magnetite fe3o4 (fe2+) at 710.7 ev [13]. figure 6. xps spectra of (a) iron and (b) chromium for annealed sus316 figure 6 (b) shows the peak of chromium at 574 ev, which was prominently present in the oxide layer for the room temperature substrates of sus 316. the peak position of chromium oxide at 576 ev was absent at the outermost layer of the oxide for both substrates at annealing temperatures of 300 and 500 °c. this shows that the chromium depletion phenomenon occurred at these temperatures [12]. however, at an annealing temperature of 700 °c, the peak position of chromium oxide, cr2o3, is present throughout the outermost and intermediate surface of the oxide layer for sus 316 substrates. this indicates that one of the major components of the oxide layer at 700 °c is chromium oxide, cr2o3. evaluation of oxide composition at the interface of coating by x-ray photoelectron spectroscopy figure 7 shows the schematic image of the evaluation of oxide composition at the interface by xps. the wide scan analysis of the coating interface indicates the presence of peak positions for titanium, oxygen, carbon and sodium. the traces of iron and chromium were not detected through the wide scan analysis, as shown in figure 8 (a). the narrow scan analysis shows that iron elemental peak was absent at the coating interface, but the chromium elemental peak was detected at trace amounts. http://dx.doi.org/10.5599/jese.1423 j. electrochem. sci. eng. 12(4) (2022) 579-591 cold-sprayed tio2 coatings on sus316 stainless steel 586 the peak position (577 ev) indicates that chromium oxohydroxyde, crooh, was present, as shown in figures 8(b) and 8(c). figure 7. schematic image of evaluation of oxide composition at the interface a binding energy, ev b c binding energy, ev binding energy, ev figure 8. xps spectra (a) wide scan, (b) narrow scan for iron and (c) narrow scan for chromium the chemical state of chromium changed from cr2o3 to crooh after the cold spray coating process. it can be assumed that this change in chemical state has affected the increase in adhesion strength. in te n si ty , a .u . in te n si ty , a .u . n. i. binti omar et al. j. electrochem. sci. eng12(4) (2022) 579-591 http://dx.doi.org/10.5599/jese.1423 587 however, the trace amount of chromium oxohydroxyde detected might have been the product of the reaction between chromium oxide, cr2o3, present at the outermost surface of the oxide layer and water molecules present in the atmosphere during the cold spray process. the thickness of the oxide layer on the substrate surface increased as the temperature of the substrate increased during annealing. oxide analysis was performed to confirm this situation. the atomic concentration of oxygen in the oxide film was raised significantly as the temperature of the annealing material rose from room temperature to 700 °c. this indicates that as the annealing temperature rises, the oxide layer of sus316 substrate becomes thicker. the literature indicates that bonding in the cold spray is governed by mechanical interlocking and metallurgical bonding. severe plastic deformation at the interface, owing to the high energy impacts of the particles, breaks the native oxide layers of the interacting surfaces. the disruption of oxide layers facilitates the penetration of particles into the substrate, which leads to the jet formation and mechanical interlocking [14]. ichikawa et al. [15] investigated the effect of substrate conditions on the deposition process, finding out that the initial oxide film thickness has a substantial effect on the deposition efficiency. yin et al. [16] claimed that in addition to the substrate hardness, the particle velocity and the spray angle can influence the deformation behaviour and final state of the oxide film. moreover, the results indicate that the formation of a fresh metal surface followed by immediate contact with the mating material, which can be a metal or oxide, is a necessary condition to form intimate bonding in a kinetic spraying process. many researchers have observed adhesion between metal coatings and smooth ceramic substrates. this suggests that mechanical interlocking is not necessary for adhesion and a physico-chemical interaction at the metal/ceramic interface significantly contributes to adhesion [17-19]. our findings revealed that as the oxide thickness on the sus316 substrate increased due to the annealing process, the coating adhesion strength increased steadily. this trend seems reversed from the mechanical or metallurgical bonding mechanism, which requires an oxide-free surface to form intimate contact between particle/substrate and provide good adhesion bonding. chemical oxide analysis was conducted to further understand how oxide thickness plays a role in the bonding mechanism. the chemical composition of the oxide layer for hard metals substrate is depicted earlier in this paper and the annealed 700 °c sus316 oxide consists of a mixture of fe2o3 and cr2o3 in the outermost layer. the ellingham-richardson diagram [20], as shown in figure 9 below, is utilized to show the standard free energy of formation for oxides as a function of temperature and equilibrium oxygen partial pressure at 200 °c as indicated by the red-dotted line and blue arrow for iron, chromium, and titanium. the gas temperature was 500 °c and referring to previous work in our lab, 200 °c was the temperature on the substrate when the coating was formed. gibbs equation helps us predict reactions' spontaneity directly based on and entropy values. thus, when the reaction is exothermic, the enthalpy of the system is negative, making gibbs free energy negative. hence, we can say that the reaction will proceed in the forward direction due to a positive value of the equilibrium constant. this law can also be scaled for two different reactions in a system. the overall reaction (combination of two reactions) will occur only if the net δg (sum of δgs of both reactions) of the two possible reactions is negative [21]. based on figure 9, we can arrange metal oxide for substrates involved as follows: stability = fe2o3 < cr2o3<tio2 http://dx.doi.org/10.5599/jese.1423 j. electrochem. sci. eng. 12(4) (2022) 579-591 cold-sprayed tio2 coatings on sus316 stainless steel 588 the oxidation process of metal generally depends on its thermodynamics and kinetics. the relationship between the standard gibbs free energy change, δgo, and temperature t [22] is presented by equations (1) and (2): δgo(fe2o3) = −543.349 j/mol + 167.4 (j/mol k)t (1) δgo(cr2o3) = −746.844 j/mol + 173.2 (j/mol k)t (2) δgo will be less than 0 when t changes within a certain range, indicating that the reaction favors the generation of oxides. moreover, δgo for cr2o3 is always more negative than δgo (fe2o3) at the same t, implying the greater affinity of oxygen to chromium. therefore, the cr2o3 formation is thermodynamically preferred on the sus316 substrate. meanwhile, for the fe atom, although the thermodynamics of its oxidation process is not dominant, it is the most mobile atom in the oxide film [16]. therefore, the fe2o3 formation is controlled more by kinetics than thermodynamics. figure 9. ellingham-richardson diagram for selected elements [20]. reproduced with permission from ref. [20], copyright (2018), university of cambridge the mechanisms of cold spraying of metal particles on ceramics or glass involve more factors, such as the chemical properties of impact particles and substrates. song et al. [18] used cold spraying of al particles on a glass substrate. the interlayer with a thickness of 80 nm between impacted al particles and glass substrate was composed of an amorphous phase and nanocrystalline grains with significant na enrichment. the layer is considered to form as a result of the reaction between the impacted al and the glass substrate, which took place due to the temporal high temperatures during impact, which produced liquid phases at the interface. it is believed that the high affinity of al with oxygen in the glass substrate enabled the relatively strong adhesion n. i. binti omar et al. j. electrochem. sci. eng12(4) (2022) 579-591 http://dx.doi.org/10.5599/jese.1423 589 observed. our coating adhesion strength on hard metals substrate is in good agreement with the report of song et al. [18]. tio2 has a characteristic structure of bridging oxygen atom rows along the [001] direction, sixfold-coordinated (6c or 6f) ti atoms below the bridging oxygen and five-fold-coordinated (5c or 5f) ti atoms in troughs between bridging oxygen rows, as shown in figure 10. the bridging oxygen vacancy (obr) is a common defect on tio2. such oxygen vacancies can be created by annealing the surface at a high temperature or bombarding with electrons [23]. for each obr, two excess electrons remain on the surface. consequently, tio2 becomes rather reactive to the dissociation of electrophilic adsorbates such as o2 and h2o. hence, it is important to understand the electronic structure of the defective tio2 surfaces. figure 10. side view of the tio2 surface with one oxygen vacancy and oh group. o atoms are represented by red spheres and ti atoms by grey spheres. reproduced with permission from ref. [23], copyright (2009), elsevier kim et al. [17] used kinetic spraying of single titanium particles on mirrored steel substrates. they showed that some portion of a thin amorphous oxide remained between the particle-substrate interface and even a severe plastic deformation was associated with the impacts of the particles onto the substrate. the remaining oxide provided a bond between a particle or particle–substrate. it was observed that adhesion strength changed little with spraying parameters. this indicated that mechanical interlocking was not the main bonding mechanism, in this case, so was the substrates’ shear instability. it was discovered that both the hardness and the oxidizability of the substrates affected the adhesion strength of tio2 coatings. the adhesion strength of tio2 coatings could be improved by changing the surface chemistry of the substrates. it was proposed that the chemical or physical bonding mechanism was the main bonding mechanism of ceramic coating. tem images proved the existence of chemical bonding between tio2 particles. by the way, preheating could increase the oxidizability of the substrate, thus deteriorating the adhesion strength of coatings. the result seems to be in agreement with the chemical bonding mechanism. yamada et al. [9] reported that the agglomerated powder of tio2 in nano-scale primary particles, which also contained nanoporosity, was fractured, leaving an unstable surface with a dangling bond structure. the fractured particles decoupled and formed a surface with improved stability to reobtain http://dx.doi.org/10.5599/jese.1423 j. electrochem. sci. eng. 12(4) (2022) 579-591 cold-sprayed tio2 coatings on sus316 stainless steel 590 a stable surface, which led to the bonding of newly impacting particles and coating building [4]. when high-velocity cold sprayed tio2 particle impacted on annealed stainless steel that already consists of oxide mixture cr2o3+fe2o3 on the top surface, inter oxide reaction occurs. tio2 particles that already have oxygen vacancy will break at collision and bond again to give cohesion and adhesion bonding by inter oxide reaction tio2-cr2o3 from chromium oxide on sus316 that is thermodynamically preferred compared to fe2o3. conclusions coating adhesion strength on stainless steel showed an increasing trend as the substrate annealing temperature increased (thicker oxide). the primary bonding mechanism of tio2 particle and sus316 substrate is chemical bonding by inter-oxide bonding, which is provided by a chemical reaction of tio2oh-, where ohis provided by chromium oxide, cr2o3, which is thermodynamically preferred on sus316. however, further precise research with the usage of pure chromium substrates is needed to clarify the change in the chemical states of elements after the cold spray process. acknowledgements: we acknowledge the interface & surface fabrication lab, toyohashi university of technology, majlis amanah rakyat, mara and universiti teknikal malaysia melaka, utem for noor irinah’s scholarship. references [1] a. papyrin, v. kosarev, s. klinkov, a. alkhimov, v. m. fomin, cold spray technology, elsevier, amsterdam, the netherlands, 2006, p. 70-96. isbn: 9780080465487. https://www.elsevier.com/books/cold-spray-technology/papyrin/978-0-08-045155-8 [2] h. assadi, f. gärtner, t. stoltenhoff, h. kreye, acta materialia 51(15) (2003) 4379-4394. https://doi.org/10.1016/s1359-6454(03)00274-x [3] t. schmidt, f. gartner, h. assadi, h. kreye, acta materialia 54(3) (2003) 729-742. https://doi.org/10.1016/j.actama.2005.10.005 [4] t. schmidt, h. assadi, f. gärtner, h. richter, t. stoltenhoff, h. kreye, t. klassen, journal of thermal spray technology 18 (2009) 794-808. https://doi.org/10.1007/s11666-009-9357-7 [5] f. gärtner, t. stoltenhoff, t. schmidt, h. kreye, journal of thermal spray technology 15 (2006) 223-231. https://doi.org/10.1361/105996306x108110 [6] t. h. van steenkiste, j. r. smith, r. e. teets, j. j. moleski, d. w. gorkiewicz, r. p. tison, d. r. marantz, k. a. kowalsky, w. l. riggs ii, p. h. zajchowski, b. pilsner, r. c. mccune, k. j. barnett, surface coating technology 111(1) (1999) 62-71. https://doi.org/10.1016/s02578972(98)00709-9 [7] r. ghelichi , m. gualiagno, journal frattura ed integrità strutturale 8 (2009) 30-44. https://doi.org/10.3221/igf-esis.08.03 [8] m. yamada, h. isago, h. nakano, m. fukumoto, journal of thermal spray technology 19 (2010) 1218-1223. https://doi.org/10.1007/s11666-010-9520-1 [9] m. yamada, h. isago, k. shima, h. nakano, m. fukumoto, in: thermal spray: global solutions to future application, proceedings of the international thermal spray conference, 2010, singapore, b. r. marple, a. agarwal, m. m. hyland, y.-c. lau, c.-j. li, r. s. lima, g. montavon (eds.), springer, 2011, p. 172-176. isbn-13: 978-1493951901 [10] n. t. salim, m. yamada, h. nakano, k. shima, h. isago, m. fukumoto, surface and coating technology 206(2-3) (2011) 366-371. https://doi.org/10.1016/j.surfcoat.2011.07.030 [11] m. gardon, c. fernández-rodríguez, d. garzón sousa, j. m. doña-rodríguez, s. dosta, g. cano, j. m. guilemany, journal of thermal spray technology 23 (2014) 1135-1140. https://doi.org/10.1007/s11666-014-0087-0 https://www.elsevier.com/books/cold-spray-technology/papyrin/978-0-08-045155-8 https://doi.org/10.1016/s1359-6454(03)00274-x https://doi.org/10.1016/j.actama.2005.10.005 https://doi.org/10.1007/s11666-009-9357-7 https://doi.org/10.1361/105996306x108110 https://doi.org/10.1016/s0257-8972(98)00709-9 https://doi.org/10.1016/s0257-8972(98)00709-9 https://doi.org/10.3221/igf-esis.08.03 https://doi.org/10.1007/s11666-010-9520-1 https://doi.org/10.1016/j.surfcoat.2011.07.030 https://doi.org/10.1007/s11666-014-0087-0 n. i. binti omar et al. j. electrochem. sci. eng12(4) (2022) 579-591 http://dx.doi.org/10.5599/jese.1423 591 [12] k. nomura, y. ujihira, journal of materials science 25 (1990) 1745-1750. https://doi.org/10.1007/bf01045379 [13] x.yu, j. zhou, intechopen, chapter 4, 2017, 61-73. https://doi.org/10.5772/66211 y. ichikawa, r. tokoro, k. ogawa, proceedings of the international thermal spray conference, itsc 2018, pp. 238–241. isbn 9781627081603. 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[20] p. straton, international heat treatment and surface engineering 7(2) (2013) 70-73. https://doi.org/10.1179/1749514813z.00000000053 [21] g. c. allen, j. m. dyke, s. j. harris, a. morris, oxidation of metals 29 (1988) 391-408. https://doi.org/10.1007/bf00666841 [22] l-m. liu, p. crawford, p. hu, progress in surface science 84(5-6) (2009) 155-176. https://doi.org/10.1016/j.progsurf.2009.01.002 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1423 https://doi.org/10.1007/bf01045379 https://doi.org/10.5772/66211 https://doi.org/10.1007/s11666-015-0299-y https://doi.org/10.1016/j.apsusc.2012.07.036 https://doi.org/10.1016/j.scriptamat.2010.03.061 https://doi.org/10.1088/0022-3727/46/19/195301 https://doi.org/10.1007/s11666-014-0200-4 https://www.doitpoms.ac.uk/tlplib/recycling-metals/ellingham.php https://doi.org/10.1179/1749514813z.00000000053 https://doi.org/10.1007/bf00666841 https://doi.org/10.1016/j.progsurf.2009.01.002 https://creativecommons.org/licenses/by/4.0/) @article{irinah2022, author = {irinah, noor and omar, binti and {binti mohamed}, suhana and yusuf, yusliza binti and binti, toibah and rahim, abdul and {binti mustafa}, zaleha and ismail, syahriza binti and binti, ilyani akmar and bakar, abu and selvamani, santirraprahkash}, journal = {journal of electrochemical science and engineering}, title = {{a preliminary study into the effect of oxide chemistry on the bonding mechanism of cold-sprayed titanium dioxide coatings on sus316 stainless steel substrate:}}, year = {2022}, issn = {1847-9286}, month = {aug}, number = {4}, pages = {579--591}, volume = {12}, abstract = {current attention has focused on the preparation of thick ceramic coating of nano­structured materials as feedstock material using the thermal spray process. the cold spray method has appeared as a promising process to form ceramic nanostructured coating without significantly changing the microstructure of the initial feedstock materials due to its low processing temperature. however, deposition of ceramic powders by cold spray is not easy due to the brittle characteristics of the material. in this study, tio2 coatings were deposited on unannealed stainless steel substrates and substrates that were annealed from room temperature to 700 °c prior to spraying. the adhesion strength was evaluated to investigate the bonding mechanism. the influence of the remaining surface oxide layer of chromium oxide, cr2o3, which is thermodynamically preferred for stainless steel, on the bonding mechanism involved was investigated. the results showed that by increasing the annealing substrate temperature of stainless steel, the adhesion strength of the coatings (thicker oxide) is also increased. as a result, the bonding between the cold-sprayed tio2 particle and the steel substrate is given by the chemical bonding of an inter-oxide reaction.}, doi = {10.5599/jese.1423}, file = {:01_jese_1423.pdf:pdf}, keywords = {tio2 coatings, adhesion strength, bonding mechanism, chromium oxide}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1423}, } removal of nickel from ni(ii)-nh3-so2-co2-h2o system by electrocoagulation, sedimentation and filtration processes http://dx.doi.org/10.5599/jese.1376 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; https://dx.doi.org/10.5599/jese.1376 open access : : issn 1847-9286 www.jese-online.orghttp://www.jese-online.org/ original scientific paper removal of nickel from ni(ii)-nh3-so2-co2-h2o system by electrocoagulation, sedimentation and filtration processes armando rojas vargas1,2,, maría elena magaña haynes3, crispin sánchez guillen3 forat yasir aljaberi4, 1empresa de servicios técnicos de computación, comunicaciones y electrónica "rafael fausto orejón forment", holguín, cuba 2universidad de holguín “oscar lucero moya”, holguín, cuba 3centro de investigaciones del níquel “alberto fernández monte de oca”, holguín, cuba 4chemical engineering department, chemical engineering department, college of engineering, al-muthanna university, al-muthanna, iraq corresponding authors: arojas@eros.moa.minem.cu; furat_yasir@yahoo.com received: may 5, 2022; accepted: june 24, 2022; published: july xx, 2022 abstract the nickel removal by electrocoagulation of ni(ii)-nh3-co2-so2-h2o system was studied in a batch reactor of 50 l useful volume, with stirring and two pairs of aluminum electrodes. the operating parameters were nickel concentration between 255 and 342 mg l-1, current density of 11.0 and 16.6 ma cm-2, ph 8.34±0.06, mean temperature 58.4±3.9 °c and retention time of 50 min. the maximum nickel removal was 99.7 % at 11.0 ma cm-2, specific energy consumption 16.86 kwh kg1 of al3+, 2.438 kwh kg-1 of ni and the adsorption capacity 5819 mg ni g-1 of al3+. the precipitate contained a nickel content of 37.2 % and a true density of 2720 kg m-3, hydrotalcite-like structure layered double hydroxides. the unit area of sedimentation was between 0.25 and 1.96 m2 t-1 day, at a density from 971 to 1019 kg m-1 and 53±4 °c. a model for predicting the specific cake resistance was estimated as a function of pressure drop and suspension concentration at 44.45 kpa and 59.52 kg m-3, resulting in the value of 6.47±107 m kg-1. the average cake humidity was 88 % base humid. keywords electrocoagulation, kinetics, nickel removal, layered double hydroxide introduction in ammoniacal carbonate leaching technology for the selective extraction of nickel and cobalt from the lateritic ore in punta-gorda, cuba, a liquid effluent in the distillation process is obtained with several species in the solution of ni(ii)-nh3-co2-so2-h2o system [1]. the concentration of nickel (ni2+) is between 128 and 1000 mg l-1, ammonia (nh3) 120 920 mg l-1, sulfur (s) 1.23-3.54 g l-1, anions thiosulfate (s2o32-) 1.18-3.85 g l-1 and sulfate (so42-) 2.90-4.90 g l-1. furthermore, in this liquid effluent, the turbidity is smaller than 20 ntu, the colour is from 15 to 100 pt/co, the total hardness http://dx.doi.org/10.5599/jese.1376 https://dx.doi.org/10.5599/jese.1376 http://www.jese-online.org/ http://www.jese-online.org/ mailto:arojas@eros.moa.minem.cu mailto:furat_yasir@yahoo.com j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 2 60 115 mg l-1, ph 7.38 8.62 and the flowed effluent reaches a temperature between 70 and 85 °c [2-5]. in a previous study, the possibility of removing dissolved nickel from this effluent by electrocoagulation (ce) was evaluated [6]. electrocoagulation is an effective process for removing dissolved heavy metals presented in wastewaters of different industries. it is based on the destabilization of suspended, emulsified, or dissolved compounds in an electrolytic cell. the electric current flowing through the electrodes provides the electromotive force that causes a series of chemical reactions, resulting in the stability of polluting molecules. these molecules move through the applied electric field to form ionized components, electrolyzed, hydrolyzed, or free radicals being hydrophobic that adhere to the walls of the electrode, precipitate, or float, facilitating their removal by some secondary separation method [6-9]. the main parameters from the operating point of view are ph, current density (j), retention time (t) and temperature (t). to increase conductivity and energy efficiency, electrolytes such as na2so4, k2so4 and nacl have been fed. for nickel removal, an efficiency between 93 and 100 % has already been reported [6,9-12]. vargas et al. [6] investigated the nickel removal from the ni(ii)-nh3-co2-so2-h2o system by electrocoagulation. using a complete experimental design and optimizing a multi-response procedure, the most favorable conditions of 95 % efficiency were obtained at 9.8 ma cm-2, 60 °c, ph 8.65 and 40 min of electrolysis. then, the investigation was carried out on a bench scale in a reactor with 25 l of useful capacity and a concentration of dissolved nickel between 300 and 562 mg l-1 [12]. the process efficiency was found between 97.7 and 99.7 %, at a specific energy consumption between 1.709 and 2.342 kwh kg-1 of al3+, or 0.7436 and 1.4007 kwh kg-1 of ni, and adsorption capacity 3342 to 7264 mg ni g-1 of al3+ [12]. thickening is the process of separating parts of the liquid of a suspension in such a way as to obtain a denser product and a flow of pure liquid. the objective of the process may be focused on obtaining a thicker pulp, or on recovering the liquid of a suspension. the first case is referred to as thickening and the second as clarification. the mechanism of thickening is sedimentation under the force of gravity. the best-known methods for sizing industrial settler tanks are coe and clevenger´s method [13], kynch theory of batch sedimentation [14], talmage and fitch´s method [15], and wilhelm and naide´s method [16]. according to stokes' law, particle size, the density difference between the dispersed phase and dispersion media, and fluid viscosity are the primary factors to consider in a sedimentation process. however, particle concentration, shape, surface charge and rheological properties of nonnewtonian fluids can also have a significant influence [17-20]. as the initial suspension concentration increases, the settling velocity becomes reduced due to an increase in collisions and interferences between particles [14,17,18]. also, the settling velocity of discrete particles and sludges increases with increasing temperature due to the decrease in kinematic viscosity under the given sediment concentration [19,20]. the influence of temperature differentials in the settling tank may produce short-circuiting, cooler influent produces a bottom density current, while in the cases of a warmer influent, the density current is along the surface. in both cases, there is an increase in the concentration of suspended solids [20]. in regards to filtration, it is the unit operation in which a mixture of solid liquid (suspension or slurry) or solid gas is forced through a porous medium in which the solids are entrapped. the filtration experiments include the determination of the specific cake resistance to filtration and filter a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 3 medium resistance. the specific resistance is always used as a measure of the difficulty of fluid permeation through the filter [21-24]. an increase in suspension concentration at the filter inlet causes a decrease in the specific resistance and results in the reduction of energy consumption. otherwise, when filtering suspension with low initial concentrations is fed, the ability of fine particles to migrate into the filter medium is greater and reduces fluid flow rates through them [21-23]. pressure drop provides the driving force, in which slurry will be pressurized to flow through the filter medium. as pressure drop across the cake increase, the specific resistance augments as a power-law relationship [23,24]. an increase in the filtration pressure causes an increase in filtration flux, the filtration velocity, and at the same time results in reduction of cake porosity on the surface of the filter medium. the reduction in the porosity is caused by greater compression forces and results in an increase in the specific resistance, but for the larger size of a particle, more porous voids are presented in the cake layer [21,24]. the purpose of this research was to evaluate the electrocoagulation electric parameters, i.e., specific energy consumption and current density for the nickel removal from the aqueous dissolution of the ni(ii)-nh3-co2-so2-h2o industrial system in a batch reactor, and to determine for the secondary separation the unitary area of sedimentation, the specific cake resistance and filtrate humidity. experimental electrocoagulation reactor the electrocoagulation process was made in a complete mixed batch reactor. the body is metallic and cylindrical in shape with a plain base, diameter (d = 400 mm) and useful volume (v = 50 l) (figure 1a). figure 1. (a) reactor of electrocoagulation; (b) factors of form the reactor was equipped with a dual impeller mounted on the same shaft axis, comprised of sixbladed rushton turbines (tr) and six concave-bladed disc turbines (cd-6). the separation distance between the impellers (c) was 200 mm and the rotating speed at 100 rpm. the factors of form were designed at standard conditions, as for impellent diameter (d), baffles wide (b), palettes wide (w) and impellent–base separation (a) according to the ratio: d/d = 3, h/d = 3, b/d = 0.1, w/d = 0.2, http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 4 a/d = 1. the baffle – reactor separation (f/d) was 1/50, and baffle–base separation l/d = 1/10 (figure 1b) [12]. also, the reactor was prepared with 2 couples of electrodes, wide 125 mm, long 190 mm, useful area of 0.095 m2, 8 mm distance between electrodes and the following chemical composition: 98.98 % al, 0.5 % mg, 0.33 % fe and 0.114 % si. a direct current (dc) power supply bca-5kt was used with the characteristics: input voltage 220 v, output voltage 0 to 150 v and output current 0 to 20 a. the electric parameters were measured using a multimeter unite-t (ut-208) (figure 2). figure 2. installation of electrodes (1 to 4), point to electric current measurement (m-1, m-2), and voltage between (1-2) and (3-4) operation parameters of electrocoagulation the electrocoagulation parameters were determined considering the results of previous studies on a bank scale with 25 l of useful volume [12]. the consumption of aluminium electrode (we / g) was estimated starting from the initial nickel concentration (g l-1), the adsorption capacity at equilibrium (qe / mg g-1), and the useful volume of the reactor (v = 50 l). as for current intensity (i / a), using the faraday's law, eq. (1), and the electric current efficiency (𝜂), eq. (2), (see figure 3) [6-12]. figure 3. algorithm to scaling electrical parameters =f imt w zf (1) cni2+/ mg l-1 qe / mg g-1 v / l t / s  sec, kwh kg-1 j / ma cm-2 we = f(cni2+; qe ; v) i = f(we; t; ) n = f(we; cni2+; sec) ap = f(j; i) end a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 5  = f ew w (2) where z = 3 is a number of electrons, molecular weight (m) for aluminum is 26.98 g mol-1, the faraday´s constant, f = 96485.33 c mol-1, t / s is electrocoagulation time, and wf / g) is the amount of al dissolved according to faraday's law. the electric potency (n / kw) was selected at the maximum value with respect to the specific energy consumption (sec, kwh kg-1) for each initial nickel concentration evaluated. regarding the effective area of electrodes (ap), it was estimated starting from the current intensity (i / a) and the current density (j / ma cm2). the specific energy consumption (sec) was calculated by eq. (3):  =sec 3600 zfu m (3) where u / v is electrical voltage and sec, kwh kg-1 al3+ is the specific energy consumption. consequently, the electrocoagulation experiments consisted of determining the nickel removal (x / %), the consumption of aluminium electrodes (we / g), the electric current efficiency (𝜂), and the specific energy consumption (sec), at an electric current intensity of 10 and 15 a, electrolysis time (t =50 min, ph 8.50.1 and temperature of 60 °c). a sufficient quantity of suspension was produced through three experimental runs to make the sedimentation and filtration experiments. synthetic liquor (s-1) and effluent liquor (e-2, e-3) were fed to the reactor. the synthetic liquor was prepared using basic nickel carbonate, ammonium hydroxide, sulfuric acid, and distilled water. the effluent liquor was sampled in the distillation columns discharge at the production plant in punta-gorda, cuba (table 1). table 1. characterization of liquors fed to electrocoagulation liquor concentration, g l-1 ni nh3 co2 s s-1 0.255 1.59 1.03 0.66 e-2 0.342 0.31 0.10 2.38 e-3 0.269 0.49 0.19 1.95 nickel removing the nickel removing (x / %) and the adsorption capacity (qt/mg g-1) or amount of adsorbate adsorbed per adsorbent unit were determined by eqs. (4) and (5): − = = 0 0 100 100 t c c x x c (4) ( )= −0 f t t v q c c w (5) where x is the converted fraction, c0 / mg l-1 is the initial concentration of nickel, ct / mg l-1 is the concentration of nickel in the liquid phase in each time interval and v / l is the volume. when the duration of the process is long enough, qt is constant and determines the charge or adsorption capacity (qe / mg l-1) corresponding to the concentration at equilibrium (ce / mg l-1). http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 6 kinetic model avrami's fractional kinetic model describes the change in volume (or mass) of a crystal as a function of crystallization time, although it has been assumed as an empirical model for the analysis of adsorption kinetic data. it is represented by differential eq. (6), integrated eq. (7) and linearized eq. (8) forms [25-28]: ( )−= −n 1av e d d nt t q k t q q t (6) ( )( )−− av av e= 1 e n k t tq q (7)    − − = +       av av e ln ln 1 ln lnt av q n k n t q (8) where, kav / min-1 is the kinetic constant or global constant, nav is the fractional reaction order, qe and qt are the amount (mg g-1) adsorbed in the equilibrium and at time t from the start of the process, respectively. the elovich’s model, which was firstly proposed by roginsky and zeldovich, can be expressed mathematically by eq. (9) and assuming α t >> 1, a linear form is obtained as eq. (10) [29-31]:  = d e d tqt q t (9) ( )   = + 1 1 ln lntq t (10) where, α (mg g-1 min-1) is the initial adsorption rate, and  (g mg-1) is a constant related to the extension of surface coverage. bangham’s equation was used to evaluate adsorbate pore diffusion activities, if the adsorption is pore-diffusion controlled, eq. (11) [32,33].        = +      −   0 b 2 0 ln ln ln t c k w t w v c q v (11) where, c0 / mg l-1 is the initial concentration of nickel (adsorbate), w / g is the weight of the adsorbent, v / l is the volume of the solution, kb / ml g-1∙l and  < 1 are constants. the weber and morris´s model suggests that intra-particle diffusion is involved in the adsorption process, or two or more steps govern the adsorption process. however, if a linear graph is obtained and the plot passes through the origin, then intra-particle diffusion is said to be the sole rate-limiting step, eqs. (12)-(13) [30,31]: − = 0.5 d d 1 d 2 tq k t t (12) = +dtq k t c (13) where kd /mg g-1 min-1/2 is the rate constant for intra-particle diffusion, c / mg g-1 is a constant associated with the thickness of the boundary layer (a higher value corresponds to a greater effect on the limiting boundary layer). a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 7 the conversion time model was proposed by rojas-vargas [12]. the model relates the conversion time (t) as a function of the converted fraction (x), the initial concentration (g l-1) of nickel, sulfur, carbon dioxide and aluminum mass, eqs. (14) and (15). ( ) ( ) ( ) ( )        =  − −  + − −  + − − + −               2 21 1 1 2 32 5 3 3 ni s co al1 1 5 ln 1 1 1 1 e f g h t a x b x c x d x x c c c c (14) where mechanisms coefficients are ascribed to: a external diffusion resistance; b nucleation; c chemical reaction and d autocatalytic effect by the roginskii-shultz equation. the constants e, f, g are the exponents of the initial concentration of nickel, sulfur and carbon dioxide, respectively. the faraday low was used to determine the aluminum mass and transformation of eq. (14) results in eq. (15): ( ) ( ) ( ) ( )        =  − −  + − −  + − − + −               2 21 1 1 2 32 5 3 31 1 5 ln 1 1 1 1 n e f g ni s co alt a x b x c x d x x c c c k (15) where, the coefficient kal depends on the electrical current fed to the process. the exponent of the conversion time (n) has a positive numerical value, it reflects the favorable effect and the high dependence that the process has on the formation of aluminum hydroxide, [al(oh)3], the main flocculating agent and formation of aggregates due to the high interfacial area. unitary area of sedimentation the talmadge-fitch and coe-clevenger methods were used to obtain the unitary area of sedimentation at da ensity of the slurry (ρp) 971 1019 kg m-3 anda temperature 534 °c [13,15]. the talmadge-fitch geometric method of thickener design consist of drawing the tangent to the free-fall zone (r1) and to the compression zone (r2) in the settling curve; using the bisector (r3) to the angle formed at the interception of (r1) and (r2) the critical point (pc) is obtained. then, drawing the tangent (r4) to the critical point perpendicular to (r3) and the asymptote (r5) to the depth hu corresponding to the final concentration of the sludge, the thickening time (tu) is obtained (figure 4). figure 4. talmadge and fitch thickener design method so, the free settling velocity (vf / m h-1), the critical settling velocity (vc / m h-1), and the unit area of sedimentation (ua / m2 day t-1) are calculated, eqs. (16)-(18). = =f d d h v m t (16) http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 8 = cc c h v t (17) = 5 u 0 0 10 144 t ua h c (18) where tu / min is the thickening time required to reach a wanted concentration of sludge, h0 / mm is the initial height of the pulp in the test tube, c0 / kg m-3 is the initial concentration of solids. on the other hand, the coe-clevenger method requires that pulps of various concentrations between the thickener feed and underflow be prepared. the method consists of determining the critical sedimentation time (tc / min) and carrying out extractions of the supernatant liquor from the test tubes at the height (he / mm) corresponding to several predetermined volumes (200, 300… to 750 ml) at a critical time (tc). then the suspended solids concentration, the settling velocity (vs / m∙h1) and the unit area (ua) are calculated, eqs. (19), (20). = 5 u 0 0 10 144 t ua h c (19)       = −         0 f l s 1000 24 l l ua s s v (20) where, vs / m h-1) is the settling velocity, l / s (kg kg-1) is the initial (0) and final (f) liquid-solid relationship, and l / kg m-3 is the liquid density. constant pressure filtration the filtration tests were carried out by the constant pressure method [34]. the sludge specific resistance to filtration (α / m kg-1), resistance (rm / kg-1) of filter medium and filtrate humidity (y / %) ware determined. the sludge was prepared in volumes of 150, 200, 250, 300 and 400 ml and heated in a water bath at 60 °c. before being poured into the büchner funnel, it was shaken to avoid sedimentation influences. the filtration pressure drop (p) was 28.0 and 44.0 kpa and suspension concentration (cs) 25.0 and 60.0 kg solid per m3 pure liquid. the area (af) was 1.60810-4 m2. filter paper for quantitative analysis, chmlab f1004, with pores size 0.02-0.025 mm, was used as a membrane. the common model for constant pressure filtration is represented by eq. (21):   = +  m2 f f2 tct v r v a pa p (21) where  / pa s is the liquid viscosity, and ct / kg solid per m3 liquid filtrated is the concentration. the synthesis product was dried in a vacuum oven at 60 °c during 74 h. the humidity was calculated base humid, eq. (22). − = h dh h m m y m (22) where, mh / g is the humid mass, md / g is the dry mass, and yh is the humidity base humid. chemical analysis the chemical analyses were performed applying volumetric, gravimetric, potentiometric, atomic absorption spectrophotometry (aas) and x-ray diffraction methods. a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 9 the ph measurements were performed at 25 °c, using a ph meter philips pw-9420, 115-230 v, 50–60 hz, precision 0.001 ph / °c. metals determinations were made using a spectrophotometer sp-9. the electrode characterization was realized with a spark optical emission spectrometer (oes) model gs 1000-ii. x-ray diffraction (xrd), x'pert3 equipment, cu anode lamp (cukα radiation) and wavelength 0.15405 nm, constant scanning at a measurement interval of (2 ) between 4.0042 and 79.9962 with a step of 0.0080 measured every 5 min. the potential difference is 45 kv and the current 40 ma, calibration external with silicon standard. the qualitative analysis of phases was performed using the panalytical highscore program. results and discussion nickel removal by electrocoagulation electrocoagulation (ec) was carried out with synthetic liquor (s-1) and effluent liquor (e-2, e-3) of the industrial process. the nickel concentration was of 28947 mg l-1, ammonia 0.780.69 g l-1, dioxide of carbon 0.440.51 g l-1, sulfur 1.660.90 g l-1 and ph 8.340.06. the average operating temperature was 58.43.9 ºc (see table 1). the maximum nickel removal was 99.7 % (e-2) at an electric current of 10.52.6 a, but with synthetic liquor (e-1), it was 94.1 % at 10.40.6 a, which is due to the favorable effect of the ionic species in the effluent liquor of the industrial process, on the removal efficiency (figure 5) [12]. figure 5. nickel removal by electrocoagulation from liquor synthetic (s) and effluent (e) regarding the experiment (e-3) at 15.72.8 a, the nickel removal was 98.9 %, which suggest that the electric current of 10.5 a (11.0 ma cm-2) is appropriate to the process. in agreement with faraday’s law, increasing the current electric leads to a higher al3+, ohproduction, and electrocoagulant formation, but in turn, it increases operating costs and undesirable processes, such as heat generation and an excessive oxygen evolution reaction. in all the experiments, the formation of abundant foam around and between the electrodes was observed, suggesting a need for the evacuation and treatment of the foam in a continuous industrial system. the behavior of the electric parameters the electric current diminished slightly with the reaction time from 10.97 to 9.77 a and so, to correct this tendency, at 40 min, it was increased up to 11.5 a (figure 6). the voltage increased from 1.68 v to 2.0 v, and after 40 min to 2.56 v when the electric current was corrected (figure 6 b). these trends in the electric parameters are due to the decrease of the http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 10 suspension conductivity directly proportional to the reduction in the nickel concentration and of the anions [sxoyz-] and [co32-] and also due to the increase of deposits on electrodes that growth the resistance to the passage of the electric current [9]. the increases in electric current with two pairs of electrodes of different areas, 0.06 m2 and 0.0952 m2, were evaluated (see figure 7). the greater the area, the lower the voltage required to generate the operating current. this is due to the fact that when the electrode area (ae) increases, the resistance (r) to the passage of electrons between the electrodes under the effect of electric field decreases, that is, the opposition to the charge flow decreases, eq. (23). according to ohm's law, the voltage (u) decreases proportionally with the reduction of the resistance for a fixed value of the current flow (i) that circulates through the closed electrical circuit, eq. (24). figure 6. (a) electric current and (b) voltage during the electrocoagulation process (s-1), at 60 c and 10.97 ma cm-2, total useful area of electrodes 0.095 m2 = e e l r a (23) = u r i (24) where, r / ω is electric resistance, e /ω∙m-1 is the resistivity of the at a given temperature, l is longitude of the driver, and ae is area of the traverse section of the driver. figure 7. variation of the electric current versus voltage for different electrode areas (s-1) however, in the experiment with effluent liquor (e-2), greater resistance to the passage of electrons between the aluminum electrodes was observed from the initial minutes of electrocoagulation (figure 8). measurements on the m-1 side showed higher electrical current than on the a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 11 m-2 side, which may be associated with more intense electrolytic reactions at electrode surfaces, the nucleation and the growth of deposits (see figure 2). the electric current that displayed the output of the power supply bca-5kt remained approximately constant equal to 10.20.5 a, and equal to the average of the measurements in m-1 and m-2. in this experiment, the voltage was controlled at irregular intervals to avoid the electric current diminishing and resulting in 12.900.54 a (m-1) and 7.730.44 a (m-2). regarding the process carried out at 16.6 ma cm-2, a similar trend in the electrical parameters was obtained as those represented in figure 8. the measurements on the m-1 side obtained an average of 18.420.44 a while on the m-2 side of 13.060.44 a. the voltage was 7.070.53 v (1-2) and 7.080.52 v (3-4). on the other hand, the current output of the power supply remained approximately constant equal to 15.630.55 a. figure 8. (a) electric current and (b) voltage during electrocoagulation process (e-2), at 60 c and 11.03 ma cm-2, total useful area of electrodes 0.095 m2 implementing a polarity switching in which the current direction in the reactor is reversed at regular intervals, and the addition of aggressive ions decreases deposit formation and electrode passivation, increases the solution conductivity and, therefore, the resistance to the passage of electric current is lower [10]. on the other hand, solar photovoltaic energy is an environmentally friendly alternative and could reduce the operation cost of the electrocoagulation process [11,35]. regarding the electric current efficiency (), it was 130.54.9 %. the specific energy consumption was estimated for a remaining dissolved nickel concentration of 15 mg l-1, resulting in 16.86 kwh per kg al3+ and 2.44 kwh per kg ni2+ for the experiment e-2. the adsorption capacity (qe) at the maximum removal efficiency was between 3022 and 4117 mg ni per g al3+ (table 2). table 2. electric and equilibrium parameters liquor cni / mg l-1 j / ma cm-2 ce / mg l-1 x / % 𝜂 / % sec, kwh kg-1 qe / mg ni per g al ni al s-1 255 11.0 15.0 94.1 130 1.46 4.60 4114 e-2 342 11.0 15.0 95.6 127 2.44 16.86 5918 e-3 269 16.6 15.0 97.0 134 4.40 16.82 3022 kinetic analysis kinetic models were fitted in the order: avrami > bangham > elovich. the best fit model was chosen by comparing the determination coefficients r2 values (table 3). http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 12 avrami's fractional kinetic model better describes nickel removal by electrocoagulation. it refers to the nucleation, growth orientation of crystallites [36] and possible changes in the adsorption mechanism [32], i.e., diffusion, surface interaction with random nucleation, or both [28]. elovich's model assumes that the active sites of the adsorbent are heterogeneous and exhibit different activation energies. it also describes multiple reaction mechanisms such as adsorbate molecules transport in the solution phase (bulk diffusion) and the surface diffusion. in this last mechanism, the adsorbate molecules are transported from the bulk liquid phase to the adsorbent external surface through a hydrodynamic boundary film [31]. as the current density increases, the constants related to the rate of adsorption () and the surface coverage () also increase. bangham's model was suitably fitted and suggests the influence of external mass transfer followed by pore diffusion. on the other hand, the intra-particle diffusion model showed a linear graph (s-1), but the plot did not pass through the origin, the constant associated with the thickness of the boundary layer resulted in the negative (c<0) (s-1, e-2) and the determination coefficient r2 was low (e-2, s-3), validating that adsorbate pore diffusion is not rate-controlling step [31,32]. table 3. kinetic model parameters liquor s-1 e-2 e-3 cni / mg l-1 255 342 269 j / ma cm-2 11.0 11.0 16.6 avrami kav / min-1 0.036 0.071 0.068 nav 2.436 1.41 1.453 qe / mg g-1 4117 5819 3022 r2 95.10 99.04 97.30 elovich α/ 10-2 mg·g-1·min-1 2.79 7.72 4.19  / 104 g mg-1 5.48 4.65 9,07 r2 93.25 97.26 93.38 bangham kb / ml g-1 l 1.19 20.16 16.27 α 2.34 1.40 1,316 r2 96.85 99.04 95.49 intra-particle diffusion kd / 10-2 mg g-1 min-1/2 8.78 9.80 4.33 c /10-2 mg g-1 -2.14 -4.73 2.29 r2 98.23 92.30 80.08 the conversion time kinetic model (ctv) presented the best quality of adjustment, as for the largest coefficient of determination r2 of 99.31 % and adjusted r2 of 98.99 % (table 4). table 4. parameters of the conversion time kinetic model (ctv), liquors s-1 and e-2, 11 ma cm-2 n1 a2 b2 c4 d5 e6 f7 g8 kal9 0.3138 0.6032 0.4091 1.1919 2.6276 0.2497 -0.7414 -0.2994 0.1422 1time constant; 2extern diffusion; 3nucleation; 4chemical reaction; 5autocatalysis; 6ni constant; 7s constant; 8co2 constant; 9al constant ctv model suggests the simultaneous contribution of the resistance of the mechanisms: external diffusion (a), nucleation (b), the chemical reaction (c) and a possible autocatalytic effect (d). according to the model coefficients, the process is probably under the control of the chemical a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 13 reaction. the contaminants (s, co2) perform a favorable effect over the nickel removal. the oxysulfur ions [sxoyz-] and carbonate [co32-] ions are attracted by electrostatic forces to balance charges and adsorbed on the active centers of the coordination surface; furthermore, those can be displaced by other competing ions in the solution (exchange adsorption) due to the interactions between the ions on the charged surface and in the diffuse layer around the surface, and the nickel removal by the formation of ni-al/ldh is promoted [12]. synthesis product characterization the synthesis product was characterized by atomic absorption spectrometry (aas) and x-ray diffraction (xrd). the precipitate had a nickel concentration between 30.86 and 38.26 %, aluminum from 6.4 to 7.2 %, sulfur 6.19 to 6.36 %, as well as cobalt and magnesium in lower concentrations (table 5). the true density, defined as the quotient of mass over the volume of a sample without considering pores in the material, was from 2377 to 2720 kg m-3. on the contrary, the apparent density that includes pores and water was 600-640 kg m-3. table 5. characterization of the synthesized product regarding xrd analysis, the largest diffraction peaks were obtained at 2 bragg angles of 10.0980.085, 22.0050.085, 35.0340.085 and 61.0700.085°, which are assigned to the crystalline planes, according to the miller indices (hkl): (003), (006), (012), (110) respectively, are also of interest at 72.2200.12° (019). these diffraction peaks are indexed on a hexagonal system with rhombohedral symmetry, special group r-3m (polytype of three layers), hydrotalcite-like structure layered double hydroxides. the presence of 0kl peaks anticipates the presence of stacked layers (figure 9) (jcpds file 15-0087) [37-41]. figure 9. x-ray diffraction (xrd) of the synthesized product the xrd pattern also showed phase impurities by comparison of the characteristic reflection pattern to a reference library of samples, such as nickel hydroxide-oxyhydroxide (ni(oh)2 niooh), nickel aluminum oxide hydrate (ni5al4o11·18h2o) and aluminum oxide hydrate (5al2o3·h2o). liquor concentration, wt.% density, kg m-3 ni co mg al s true apparent s-1 38.26 0.689 0.136 6.4 6.36 2377 620 e-2 37.21 0.059 1.300 6.6 6.24 2720 640 e-3 30.86 0.062 1.390 7.2 6.19 2570 600 http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 14 from the xrd analysis, the spacing (dhkl) of the ldhs, the crystal lattice parameters (a and c) and the crystallite size (dhkl) were determined (table 6). table 6. lattice parameters and size of ni/al-ldh crystallites sample spacing, nm cell parameters, nm v / nm3 crystallite size, nm d003 d006 d012 d110 a = b c d d003 s-1 0.8853 0.4039 0.2561 0.1513 0.303 0.244 0.193 12.2 13.0 e-1 0.8693 0.4021 0.2556 0.1515 0.303 0.242 0.192 8.5 13.3 the separation of the planes (dhkl) of the ldh, also called spacing, basal distance, or thickness of the interlayer gallery, was calculated using bragg's law and is similar to the compounds synthesized by coprecipitation reported in the literature: [ni/al-so42-] (0.801 ≤ d003 ≤ 0.859 nm) and [ni/al-no3] (0.782 ≤ d003 ≤ 0.876 nm) [42,43,44] and [ni/al-co32-] (0.740 ≤ d003 ≤ 0.763 nm) [38]. the variation in the basal distance is due to the variation in the amount (intercalation degree) and type of anions (atom size and valence) in the ldh interlayer. parameters a and c were calculated using the relationship between the spacing (dhkl) in the planes (hkl): (003), (006), (012), (110) and the lattice parameters (a, b and c) for the hexagonal crystal system (b = c). the data were adjusted using the statgraphic centurion xvii software in the nonlinear regression option. the average values of the lattice parameters ( standard deviation) were: a = b = 0.303 nm and c equal to 0.2430.014 nm, with a fit quality greater than 99.9 %, confirming that it is a hexagonal crystalline system. the parameter a is equivalent to the average distance between the center of adjacent cations in the lattice, and c is the basal axis, which is related to the distance between neighboring atoms and the interlayer distance. the parameters have been reported for the compounds obtained by coprecipitation: [ni/al-so42-]-ldh values of a = 0.303 nm and c = 0.2405 nm; and for [ni/al-co32-]-ldh in the following ranges: 0.302 ≤ a ≤ 0.308 nm and 0.222 ≤ c ≤ 0.2405 nm [8,41,43,44]. the unit cell volume (v = 0.866 a2c) was 0.1920.0007 nm3, similar to other [co32-]-ldh obtained by co-precipitation such as [zn/al] 0.189 nm3, [ni/al] 0.1876 nm3 and [mg/al] 0.180 nm3. on the other hand, by the sol-gel method it was lower [ni/al] 0.148-0.163 nm3. the crystallite size (dhkl) was calculated using the scherrer equation and the mean size (d) by the williason-hall "ssp" method, those reached lower values than other ldhs synthesized by coprecipitation, but those were similar to the ldhs obtained by the sol-gel technique [ni/al-co32-] (2.69 ≤ d003 ≤ 8.11 nm) [38,45,46]. analysis of batch settling test batch settling tests at initial solids concentration ranging from 94 to 168 kg m-3, density of the slurry (𝜌𝑝) 971 to 1019 kg m -3 and temperature 534 °c were carried out. the height of the interface between the settled solids and the clear supernatant liquor was plotted as a function of time at different solids contents in figure 10. then, the free settling velocity (vf / m h-1), critical velocity (vc / m h-1) and the unit area of sedimentation (ua / m2 day t-1) were determined through the talmadge and fitch´s method. as shown in figure 11a, increasing the solids content from 120 to 168 kg m-3 decreased the free settling velocity to 0.22 m h-1 because increasing the concentration of particles suspended in a fluid increases the density and viscosity of the suspension, as well as collisions and interference between particles. also, according to the takacs´s equation, vf decreased when the sludge concentration tended to zero. similar behavior of the critical settling velocity was obtained (figure 11b) [20,47,48]. a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 15 figure 10. effect of solids content on settling curve (e-2) the unit area (ua / m2 day t-1) was obtained at different solids concentrations (figure 11c). for the effluent liquor (e-2 and e-3), a tendency was observed to decrease the unit area with the increase in electric current density, which presupposes the formation of larger flocs. figure 11. interaction of solids concentration with: (a) free settling velocity (vf), (b) critical settling velocity (vc), (c) unit area (ua). the temperature of 534 °c furthermore, using the coe and clevenger´s formula, the unit area (ua), the settling velocity (vs) and the suspended solids concentration (sp) were determined at 55 °c and different solids concentrations (c0) (table 7). an increase in suspended solids was observed with increasing settling velocity and dilution of the suspension (table 7), and the process possibly requires feeding flocculants. at a current density of 15.7 ma cm-2 suspended solids were not obtained. http://dx.doi.org/10.5599/jese.1376 j. electrochem. sci. eng. 00(0) (2022) 000-000 removal of nickel from by electrocoagulation 16 table 7. results of settling tests by coe and clevenger´s formula (s-1) c0 / kg m-3 128 vs / m h-1 0.60 0.90 1.20 1.50 1.56 ua / m2 day t-1 0.338 0.261 0.198 0.167 0.122 sp / mg l-1 31 39 40 40 44 109 vs / m h-1 0.60 0.90 1.20 1.50 1.68 ua / m2 day t-1 0.308 0.236 0.173 0.102 0.099 sp / mg l-1 27 62 83 101 111 the density at 24 hours was 106520 kg m-3, for a liquid-solid ratio 3.520.52 kg kg-1 and fraction by weight of solids 0.2240.025 kg kg-1, the pulp is not compact and easily detached from the test tube. analysis of the filtration and dye the filtration resistances were determined at 60 °c for the method of constant pressure. the suspension density was 101511 kg m-3, ph 8.690.10 and the cake density 10633 kg m-3. in all the experiments, the filtered volume (v) vs. time (t) followed a potential function (figure 12a), and the general filtration equation for constant pressure in the plot of t v-1 vs. v (figure 12b) followed a linear function with high quality of fit, which is an indicator of goodness of fit for the observations. to the values of the independent variables planned, the specific cake resistance resulted in 10.565.94 107 m kg-1 and the filter medium resistance 1.620.96 108 kg-1. figure 12. filtration at 44.45 kpa and cs 59.52 kg m-3 (e-2): (a) volume of filtrate versus time; (b) filtration general model adjustment a model was obtained to predict the specific cake resistance as a function of pressure drop between 27.7 and 44.4 kpa and suspension concentration of 25.0 to 60.4 kg solid per m3 pure liquid. at a pressure drop 44.45 kpa and cs 59.52 kg m-3, the specific cake resistance resulted in 6.470.09107 m kg-1, eq. (25).  = 1.329×106 (p)2.95069 cs-1.80727 (25) the coefficient of determination r2 was 99.34 % and adjusted r2 99.16 %, which refers to the goodness of fit of a model. the cake humidity was 890.8 % humid base (yh). conclusions the maximum nickel removal by electrocoagulation of the ni(ii)-nh3-so2-co2-h2o system was 99.7 % at 11.0 ma cm-2 with effluent liquor, the specific energy consumption 16.86 kwh per kg al3+, 2.438 kwh kg-1 of precipitated ni and the adsorption capacity 5819 mg ni per g al3+. y = 1.0548x + 0.1992 r2 = 0.9971 y = 26.111x0.5309 r2 = 0.9993 a. r. vargas et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1376 17 the electric current diminished by approximately 10 % with the reaction time, and the voltage increased a 15 % due to the formation of deposits on the electrode's surface and the removal of ionic species from de liquor. a greater area of electrodes, a lower voltage is required to generate the operating current. the kinetic analysis suggested that the process is determined by the simultaneous contribution of the resistance: external diffusion, nucleation, the control of the chemical reaction and a possible autocatalytic effect. the synthesis product had a nickel concentration of 37.2 %, cobalt 0.059 %, magnesium 1.3 %, aluminum 6.6 %, true density of 2720 kg m-3 and apparent density 640 kg m-3, hydrotalcite-like structure layered double hydroxides, and phases impurities nickel oxide hydroxide, nickel aluminum oxide hydrate and aluminum oxide hydrate. the unit area of sedimentation was between 0.252 and 1.965 m2 t-1 day at a density of slurry from 971 to 1019 kg m-1 and a temperature of 534 °c. as the solids content increased, the free settling velocity and the critical settling velocity decreased and diminished when the sludge concentration tended to zero. the specific cake resistance to filtration was proportional to the pressure drop and inversely proportional to the suspension concentration. a model was obtained to predict this resistance at 44.45 kpa and 59.52 kg solid per m3 pure liquid, which resulted in 6.47107 m kg-1. finally, the average cake humidity was 88 %. acknowledgements: thanks to the nickel sectoral program "comprehensive use of natural resources", for granting financing for the execution of the project. references [1] a. r. vargas, m. e. m. haynes, acta chimica slovenica 67 (2020) 1239-1249. http://dx.doi.org/10.17344/acsi.2020.6147 [2] a. r. vargas, m. e. m. haynes, a. r. riverón, revista de metalurgia 55 (2019) e149. https://doi.org/10.3989/revmetalm.149 [3] m. e. m. haynes, a. r. vargas, revista tecnología química 33 (2013) 200-205. https://tecnologiaquimica.uo.edu.cu/index.php/tq/article/view/401 [4] m. e. m. haynes, revista tecnología química 36 (2016) 27-36. https://tecnologiaquimica.uo.edu.cu/index.php/tq/article/view/595 [5] s. r. rivas, g. m. vuelta, r. a. rodríguez, revista tecnología química 36 (2016) 145-153. https://tecnologiaquimica.uo.edu.cu/index.php/tq/article/view/1118 [6] a. r. vargas, a.r. riverón, m. p. medina, revista tecnología química 40 (2020) 363-382. https://tecnologiaquimica.uo.edu.cu/index.php/tq/article/view/5155 [7] f. y. aljaberi, s. a. ahmed, h. f. makki, heliyon 6 (2020) e03988. https://doi.org/10.1016/j.heliyon.2020.e03988 [8] f. y. aljaberi, chemical engineering and processing process intensification 174 (2022) 108864. https://doi.org/10.1016/j.cep.2022.108864 [9] n. beyazit, international journal of electrochemical science 9 (2014) 43154330. 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1376 http://dx.doi.org/10.1155/2011/646409 http://dx.doi.org/10.1155/2011/646409 http://dx.doi.org/10.1016/j.materresbull.2013.01.030 http://dx.doi.org/10.21577/0103-5053.20170093 http://dx.doi.org/10.9767/bcrec.12.1.460.1-13 https://doi.org/10.1007/s11356-018-3257-7 https://doi.org/10.1007/bf02708384 https://doi.org/10.2166/wrd.2016.041 https://doi.org/10.48442/imist.prsm/jasi-v2i1-3.10033 https://doi.org/10.1016/j.mtphys.2020.100292 http://dx.doi.org/10.4236/wjnse.2014.41004 https://doi.org/10.1016/0043-1354(91)90066-y https://doi.org/10.1051/e3sconf/20171700003 https://creativecommons.org/licenses/by/4.0/) reduced graphene oxide as efficient carbon support for http://dx.doi.org/10.5599/jese.1643 771 j. electrochem. sci. eng. 13(5) (2023) 771-782; http://dx.doi.org/10.5599/jese.1643 open access : : issn 1847-9286 www.jese-online.org original scientific paper reduced graphene oxide as efficient carbon support for pd-based ethanol oxidation catalysts in alkaline media sigrid wolf1,, michaela roschger1, boštjan genorio2, nejc hodnik3, matija gatalo3, francisco ruiz-zepeda3 and viktor hacker1 1institute of chemical engineering and environmental technology, graz university of technology, inffeldgasse 25/c, 8010 graz, austria 2faculty of chemistry and chemical technology, university of ljubljana, večna pot 113, 1000 ljubljana, slovenia 3department of materials chemistry, national institute of chemistry, hajdrihova 19, 1000 ljubljana, slovenia corresponding author: sigrid.wolf@tugraz.at received: december 21, 2022; accepted: january 24, 2023; published: february 8, 2023 abstract the sluggish kinetics of the ethanol oxidation reaction (eor) and the related development of low-cost, highly active and stable anode catalysts still remains the major challenge in alkaline direct ethanol fuel cells (adefcs). in this respect, we synthesized a pdnibi nanocatalyst on reduced graphene oxide (rgo) via a facile synthesis method. the prepared composite catalyst was physicochemically characterized by sem, stem, edx, icp-oes and xrd to analyze the morphology, particle distribution and size, elemental composition and structure. the electrochemical activity and stability towards eor in alkaline media were examined using the thin-film rotating disk electrode technique. the results reveal well-dispersed and strongly anchored nanoparticles on the rgo support, providing abundant active sites. the pdnibi/rgo presents a higher eor activity and stability compared to a commercial pd/c ascribed to a high ecsa and synergistic effects between pd, ni and bi and the rgo material. these findings suggest pdnibi/rgo as a promising anode catalyst in adefc applications. keywords synergistic effects; electrochemical active surface area; rotating disk electrode; alkaline direct ethanol fuel cell introduction fuel cells have emerged as one of the most promising renewable energy technologies of the future, as they convert chemical energy directly into electrical energy [1,2]. alkaline direct ethanol fuel cells (adefcs) have gained significance among the various fuel cell types, especially for portable and transportation applications, as they have a high theoretical energy density (8 kwh kg-1), they http://dx.doi.org/10.5599/jese.1643 http://dx.doi.org/10.5599/jese.1643 http://www.jese-online.org/ mailto:sigrid.wolf@tugraz.at j. electrochem. sci. eng. 13(5) (2023) 771-782 pdnibi/rgo as eor catalyst in alkaline media 772 are environmentally friendly, and ethanol can be easily handled compared to other fuels such as hydrogen [3,4]. ethanol has proven significant advantages over methanol as it can be produced from agricultural products, is non-toxic and features a lower crossover from the anode to the cathode [4,5]. however, one of the major problems of adefcs remains the development of cost-efficient, highly active and stable electrocatalysts for the ethanol oxidation reaction (eor) at the anode. pdbased catalysts are currently reported to be most suitable and show better eor performance in alkaline media than their pt counterparts [2,6]. the c-c bond of ethanol is difficult to break and the eor does not proceed via a complete oxidation (equation (1)) with the formation of co2 and 12 e-, but via incomplete oxidation to acetate and 4 e(equation (2)) [1,4]. complete oxidation: ch3ch2oh + 12 oh→ 2 co2 + 9 h2o + 12 e(1) incomplete oxidation: ch3ch2oh + 5 oh→ ch3coo+ 4 h2o + 4 e(2) to overcome this problem, alloying pd with oxophilic co-catalyst elements such as ni, bi, cd or co is very effective for increasing the eor activity and stability due to enhanced -ohadsorption while reducing costs [7-11]. deposition of the metal nanoparticles on a suitable support material offers another possibility to increase the efficiency of such catalysts, as it features the capability to control the size, distribution, morphology and stability of the immobilized particles. carbon powders such as carbon black, carbon nanotubes or graphene are commonly used as support [4,8,11]. among these carbon materials, reduced graphene oxide (rgo) has proven to be particularly suitable for this purpose, as its unique sp2-hybridized carbon structure provides properties such as a large specific surface area (ssa), high thermal and chemical stability, and excellent electronic conductivity. usually rgo still retains important oxygen-containing functionalities (e.g. epoxy, hydroxyl, and carboxyl groups), causing strong c-o-metal bridges that enhance the stability and prevent agglomeration of the nanoparticles [4,6,8,12]. krishna et al. [13] have described, for example, that a pd@nixb/rgo nanocomposite exhibited improved catalytic activity and stability compared with the unsupported nanoparticles. another study by alfi et al. [9] also showed that a pd-cd/rgo catalyst outperforms the commercial pd/c in terms of eor performance. a combination of adding oxophilic co-catalysts to the pd active material and the deposition on a suitable support material such as rgo therefore appears to be a promising approach for the development of highly active and stable eor catalysts. based on the above findings, in this work we synthesized a composite catalyst of pdnibi nanoparticles on rgo for the first time and investigated the electrochemical activity and stability towards eor in alkaline media. the pdnibi/rgo catalyst was comprehensively characterized by various physicochemical techniques, which confirmed the successful deposition of the metal particles on the rgo support. finally, the electrochemical rotating disk electrode (rde) experiments revealed a promising catalytic eor performance of the prepared nanocomposite attributed to synergistic effects between pd, ni and bi and the rgo support material. experimental materials the following chemicals were used for material preparation and electrochemical tests without further purification: graphene oxide (go) precursor (timrex ks44) was acquired from imerys (bodio, switzerland), sodium borohydride (nabh4, 97 %) was delivered by alfa aesar (haverhill, ma, usa) and sodium hydroxide (naoh, ≥98 %, acs, pellets) was purchased from honeywell fluka (charlotte, nc, usa). hydrazine hydrate (n2h4h2o, reagent grade), palladium chloride (pdcl2, anhydrous, 59-60 % pd basis), nickel(ii) nitrate hexahydrate (ni(no3)26h2o, 99 % trace metal basis), bismuth(iii) chloride s. wolf et al. j. electrochem. sci. eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 773 (bicl3, reagent grade, ≥98 %) and potassium hydroxide (koh, 1.0 m fixanal 1 l ampoule) were purchased from sigma-aldrich (darmstadt, germany). hydrochloric acid (hcl, rotipuran®37 % fuming, p.a., acs, iso), ethanol (etoh, 99.9 % p.a.) and isopropyl alcohol (2-propanol, ≥99.9 %, uv/ir-grade) were supplied by carl roth (karlsruhe, germany). nafiontm solution (5 wt.% in h2o) from quintech (göppingen, germany) and an alumina suspension (al2o3, 0.05 µm particle size) from masterprep® bühler (lake bluff, il, usa) were used. all solutions were prepared with ultrapure water (barnstead nanopurewater purification system, 18 mω cm). a commercial pd/c (40 wt.%) from fuel cell store (college station, tx, usa) was used as a benchmark for the electrochemical results. catalyst preparation the preparation of rgo was carried out using a previously published chemical reduction method [5,14]. first, the go precursor and ultrapure water were mixed in a 2 l-round bottom flask. the dispersion was slowly heated to 100 °c under reflux and stirred at 550 rpm using an oil bath and a ptfe magnetic stir bar. hydrazine hydrate was then slowly added to perform the reduction process at 105 °c for 24 h. after successful reduction, indicated by a color change from brown to black, filtration of the hot reaction mixture and intensive washing with hot ultrapure water and ethanol was performed. the rgo was obtained by drying at air (24 h) and subsequently under vacuum at 80 °c (overnight) and was used as catalyst support material without further purification. the pdnibi/rgo (40 wt.% of metal and 60 wt.% of carbon support) composite catalyst was synthesized according to the modified instant reduction method [7,15]. the as-prepared rgo was dispersed in ultrapure water under a nitrogen atmosphere and ice cooling using an ultrasonic probe (hilscher, up440s). afterwards, the following three metal precursor salt solutions were added to the rgo dispersion: pdcl2 and ni(no3)26h2o were both mixed with ultrapure water and 1 m hcl and bicl3 was dissolved in ultrapure water and a few drops of hcl (concentrated). the ph was then adjusted to 10, an aqueous solution of nabh4 (in ultrapure water and 1 m naoh) was slowly added and the reduction was carried out at 60 °c for 4 h. finally, the precipitate was filtered, washed with ultrapure water and dried at 40 °c for 24 h. characterization the morphology and particle size of the metal nanoparticles were studied in a probe cs-corrected jeol arm 200 cf scanning transmission electron microscope (stem) system equipped with an sdd jeol centuria energy-dispersive x-ray (edx) spectrometer at 80 kv. for the sample preparation, the catalyst was dispersed in ethanol, dropped on a cu-grid and dried. scanning electron microscopy (sem) was carried out on a zeiss ultra+ field emission scanning electron microscope at 1 kv with a secondary electron detector to further analyze the catalyst morphology. a conductive carbon tape on an al stub served as a sample holder. edx spectroscopy was performed at 7 kv within the sem using an sdd x-max 50 edx spectrometer to determine the elemental composition. metal concentration was additionally examined by inductively coupled plasma optical emission spectrometry (icp-oes) on an acros sop system by spectro using detection wavelengths of 223.061 (bi), 231.604 (ni) and 340.458 (pd). the sample preparation was as follows: microwave-assisted pressurized acid digestion on a multiwave 3000 of the sample (10 mg) in 7 ml of concentrated hno3, 0.2 ml hclo4 and 0.2 ml hf (40 %) was done at 195 °c and 1500 w for 25 min. the crystal structure of the composite catalyst was characterized by x-ray diffractometry (xrd). the measurement was conducted on a panalytical x’pert pro mpd diffractometer with a fully opened x’celerator detector using a cu kα1 radiation (λ = 0.15406 nm) within a 2 range from 20 to 60° and a step size of 0.034° step per 100 s. the sample was placed on a zero-background si holder. http://dx.doi.org/10.5599/jese.1643 j. electrochem. sci. eng. 13(5) (2023) 771-782 pdnibi/rgo as eor catalyst in alkaline media 774 electrochemical measurements electrochemical characterization of the pd-based composite catalysts was performed in a threeelectrode cell by using a rotating disk electrode (rde) and a workstation from pine research instrumentation. a glassy carbon (gc) disk electrode (afe5t0gc) coated with a thin film of the catalyst material, a platinized titanium rod (bank elektronik intelligent controls gmbh) and a reversible hydrogen electrode (rhe, hydroflex®, gaskatel) were used as working, counter and reference electrode, respectively. the gc disk was polished with an al2o3 suspension (0.5 µm) and rinsed with ultrapure water before the catalyst layer was applied. 10 µl of an ultrasonically dispersed (30 min) ink containing 8.1 mg catalyst powder, 1.75 ml 2-propanol, 0.737 ml ultrapure water and 13 µl of a nafion solution (5 wt.%) were pipetted onto the gc disk (0.196 cm-2) and dried at 700 rpm for 1 h to form a thin-film with a final pd loading of 56 µg cm-2. measurements were carried out using a reference 600tm potentiostat/galvanostat/zra and software from gamry instruments was utilized for data analysis. a constant temperature of 30 °c and de-aerated (n2-purging for 30 min) electrolyte solution was used for all experiments. first, cyclic voltammograms (cvs) in 1 m koh at 50 mv s-1 (until a reproducible cv curve was obtained) for cleaning purpose and at 10 mv s-1 for the analysis of the redox processes were recorded in a potential range from 0.05 to 1.50 v vs. rhe. then, cvs at a scan rate of 10 mv s-1 were conducted from 0.05 to 1.20 v vs. rhe in 1 m koh (base cvs) and a mixture of 1 m koh / 1 m etoh to evaluate the electrochemical active surface area (ecsa) and the eor activity of the pd-based nanocatalysts, respectively. for each measurement (except cleaning cvs) three cycles were performed and the third was used for investigation. the cvs for the eor were corrected with the base cvs to exclude the redox processes not linked to eor. chronoamperometry (ca) tests were carried out in the ethanol-containing electrolyte solution (1 m koh / 1 m etoh) at a potential of 0.83 v vs. rhe for 3600 s to examine the catalytic eor stability. results and discussion structural, morphological and elemental characteristics of pdnibi/rgo comprehensive physicochemical characterization of the prepared pdnibi/rgo composite was performed to analyze the morphology, particle size and distribution of the nanoparticles, the elemental composition and the crystal structure. figure 1 shows the stem, sem and edx elemental mapping images of the catalyst. the stem images in figure 1a-c and 1e-g indicate metal nanoparticles anchored on the rgo sheets and the sem image of pdnibi/rgo (figure 1d) evidences the two-dimensional wrinkled sheet morphology of the sample [2,6,9]. the particles are uniformly and well dispersed and only a few agglomerates are noticed. the remaining oxygen-containing functional groups, such as hydroxyl (-oh) of the rgo support, are crucial for holding the metal particles by strong c-o-m bridges and act as a capping agent to prevent strong agglomeration and ensure the formation of very small particles [13]. therefore, a large specific surface area and abundant active sites are provided. the particle size distribution (psd) is shown in the inset of figure 1f. the statistical analysis results in an average particle diameter of 2.6 nm. from the edx elemental mapping (figure 1h), the presence of pd, ni and bi and the homogeneous distribution of the elements can be proved. the elemental composition of pdnibi/rgo determined by edx and icp-ms is given in table 1. s. wolf et al. j. electrochem. sci. eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 775 figure 1. (a,b,c,e,f,g) bf and adf stem (with psd inset), (d) sem and (h) edx elemental mapping images of pdnibi/rgo table 1. elemental composition of pdnibi/rgo derived by edx or icp-oes compared with calculated values material content, wt.% c o pd ni bi calculated 60.0 33.9 2.2 3.9 edx 46.3 12.0 34.5 2.5 3.2 icp-oes 27.7 2.0 3.7 the ratio between support and the active metal material is approx. 60:40. it can also be observed that the support material is composed of c (46.3 wt.%) and o (12.0 wt.%), which confirms the presence of the oxygen-containing functional groups within the rgo. the active material (40 wt.%) is composed of pd, ni and bi and is well comparable with the calculated values. this results in an atomic proportion of approx. 85 wt.% pd, 10 wt.% ni and 5 wt.% bi (calculated from the active material only). the crystal structure of the pdnibi/rgo composite was analyzed by xrd. the xrd pattern of the prepared material and comparison with graphite (icsd #18838) and pd (icsd #64922) standard data is shown in figure 2. figure 2. xrd pattern of pdnibi/rgo with pd and graphite icsd standard data http://dx.doi.org/10.5599/jese.1643 j. electrochem. sci. eng. 13(5) (2023) 771-782 pdnibi/rgo as eor catalyst in alkaline media 776 the broad peak at a 2 value of 25.5° can be related to the rgo support material and belongs to the (002) plane of graphite sp2 carbon structure, signifying the successful synthesis of rgo [8,9,13,16]. the peaks at 2 values of 39.7 and 43.9° indexed to the (111) and (200) facets, respectively, can be assigned to the face-centered cubic crystalline structure of pd [8,11]. the shift to lower angles is caused by lattice expansion due to nanosized particles and alloying with ni and bi [7,13,15]. the nanocrystalline nature is further indicated by the broad peak shape. the crystallite size of the nanoparticles can be estimated by using the scherrer equation: d = kλ b cos  (3) where d is the crystallite size (nm), k is the shape factor for spherical particles (0.9), λ refers to the x-ray wavelength (0.154 nm), b is the full width at half maximum of the (111) plane and  is the half angle [16,17]. the estimated crystallite size was calculated to be approx. 3.8 nm and is in the same range as observed from the tem analysis. electrochemical performance of the eor catalysts the electrochemical properties of the pdnibi/rgo catalyst were investigated by cv and ca measurements and compared with a commercial pd/c catalyst. the most important results, such as the ecsa and the parameters for the eor activity and stability, are listed in table 2. table 2. electrochemical results of pdnibi/rgo compared with a commercial pd/c catalyst ecsaa, cm2 mg-1 eonsetb / v vs. rhe jfc / ma mg-1 jbc / ma mg-1 jdd / % pdnibi/rgo 455.4 0.234 2390 1942 85 pd/c 410 0.266 2018 2150 87 aelectrochemical active surface area; bonset potential of the ethanol oxidation at 0.02 ma; cpeak current density of forward and backward scan; dcurrent density decrease after 3600 s. cvs in de-aerated 1 m koh electrolyte solution at 10 mv s-1 were recorded in a potential range of 0.05 to 1.50 v vs. rhe and 0.05 to 1.20 v vs. rhe to investigate the oxidation and reduction processes of the materials and to determine the ecsa. the voltammograms are shown in figure 3. figure 3. cv curves in n2-saturated 1 m koh electrolyte solution at a scan rate of 10 mv s-1 of (a) pdnibi/rgo in a potential range of 0.05 1.50 v vs. rhe and (b) pdnibi/rgo and a commercial pd/c in a potential range of 0.05 1.20 v vs. rhe with ecsa evaluation (horizontal segments indicate region for reduction charge determination) the cvs of pdnibi/rgo present that oxidation of bi to bi2o3 in alkaline media is observed in the anodic scan at approx. 0.9 v vs. rhe [18], whereas the oxidation of pd to pdo is a broad peak, which s. wolf et al. j. electrochem. sci. eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 777 is not clearly visible due to overlapping [11]. the peaks between 1.2 to 1.5 v vs. rhe describe the oxidation of ni(oh)2 to niooh and the corresponding reduction of niooh to ni(oh)2 (figure 3a) [19]. the peaks in the cathodic scan between 0.9 v and 0.6 v vs. rhe are associated with bi oxide reduction and the reduction of pdo to pd [7]. a comparison with the cv (figure 3b) of the commercial pd/c shows that the hydrogen ad/absorption peaks for the pdnibi/rgo are suppressed, which can be attributed to the presence of bi [18,20,21]. consequently, the ecsa was estimated from the pdo reduction peak by using equation (4): ecsa= qpd qpd * ( 1 cl ) ( 1 agc ) (4) where qpd is the determined reduction charge of the pdo to pd (received using echem analysttm software-gamry for baseline correction and integration of the region indicated by horizontal segments), q*pd is the theoretical pdo to pd reduction charge (405 µc cm-2), cl is the catalyst loading (56 µgpd cm-2) and agc is the gc electrode area (0.196 cm-2) [22-24]. pdnibi/rgo shows a large ecsa of 455 cm2 mg-1 due to good distribution and small particle size (determined by stem) as well as the overall high surface area provided by the rgo support [11]. the commercial pd/c catalyst exhibits an ecsa of 410 cm2 mg-1. a high ecsa is very important for ensuring high catalytic activity, as it ensures abundant active sites for the ethanol oxidation reaction [25]. the activity of the pdnibi/rgo catalyst towards eor was investigated in a mixture of de-aerated 1 m koh and 1 m etoh electrolyte solution at a scan rate of 10 mv s-1 in a potential range of 0.05 to 1.20 v vs. rhe and was compared with the commercial pd/c catalyst. the cv curves in figure 4 show two characteristic oxidation peaks, one in the forward and one in the backward scan. figure 4. cv curves of pdnibi/rgo in n2-saturated 1 m koh electrolyte solution containing 1 m etoh at a scan rate of 10 mv s-1 compared with the commercial pd/c the forward peak is attributed to the primary oxidation of ethanol according to the generally accepted eor reaction mechanism of pd-based catalysts in alkaline media (equations (5) to (8)) as described by zhang et al. [19] and several other studies [2,26-28]: pd + ch3ch2oh ⇆ pd – (ch3ch2oh)ads (5) pd – (ch3ch2oh)ads + 3oh⇆ pd – (ch3co)ads + 3h2o + 3e (6) pd – (ch3co)ads + pd – ohads ⇆ pd – ch3cooh + pd (7) pd – ch3cooh + oh⇆ pd + ch3coo+ h2o (8) http://dx.doi.org/10.5599/jese.1643 j. electrochem. sci. eng. 13(5) (2023) 771-782 pdnibi/rgo as eor catalyst in alkaline media 778 in the eor process, ethanol adsorption and further dissociation (equations (5) and (6)) proceed rapidly in the lower potential region of the anodic scan and the carbonaceous intermediates such as ch3coads will strongly adsorb on the pd surface. as pd begins to adsorb ohwith increased potential, the oxidative removal of the adsorbed carbonaceous species (ascribed as the rate-determining step) happens and the current increases (equations (7) and (8)) [2,19,26,29]. at higher potential, the current rapidly drops when inactive pdo is formed. the backward peak results from the oxidation of freshly adsorbed ethanol molecules after the reduction of the pdo and, thus, recovery of the active sites [8,15,19]. the most important parameters for the activity of an eor catalyst are the onset potential (eonset), and the maximum peak current density in the forward and backward scan (jf and jb). pdnibi/rgo exhibits an eonset of 0.234 v vs. rhe and a jf and jb of 2390 ma mg-1 and 1942 ma mg-1, respectively. these results are compatible with the literature, e.g., chowdhury et al. [8] described that a jf of 2223 ma mg-1 was obtained with a pdnip/n-rgo. the eor activity of pdnibi/rgo is higher compared to the commercial pd/c, which is partly caused by the large ecsa generated by the well-distributed, nanosized particles on the rgo support providing abundant active sites [25]. to evaluate the specific activity of the electrocatalysts, the jf values were normalized by the ecsa. it was found that the pdnibi/rgo (5.25 ma cm-2) still outperforms the commercial pd/c (4.92 ma cm-2). therefore, it can be concluded that not only the higher ecsa but also the presence of the oxophilic elements ni and bi additionally ensures the enhancement of the eor activity, as it has also been shown in other studies in the literature [20,22,24,29]. ni and bi are not active for the eor in the applied potential range, but they have the capability to generate ohads at the surface at lower potentials due to their oxophilic character. the oxidative desorption of the intermediate carbonaceous species (equation (7)), which is the rate-determining step of the eor, can thus be facilitated, leading to lower onset potentials and higher current densities [8,15,29,30]. the catalytic stability of the pdnibi/rgo and commercial pd/c catalysts towards eor is determined by performing ca tests in de-aerated 1 m koh and 1 m etoh solution for 3600 s. the curves are shown in figure 5. figure 5. ca measurement of pdnibi/rgo and the commercial pd/c in n2-saturated 1 m koh electrolyte solution containing 1 m etoh at 0.83 v vs. rhe the current density decrease of eor catalysts with time can generally be attributed to the poisoning of the active sites on the catalyst metal surface by the adsorption of carbonaceous intermediates (e.g., ch3coads) during ethanol oxidation, as also described in the literature s. wolf et al. j. electrochem. sci. eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 779 [8,13,16,29,31]. lović et al. [29] have shown, for example, by recording cvs after the ca, that although the activity is initially lower than before the ca, the loss can be recovered with further cycling. this indicates the stability of the catalyst surface composition and the regeneration of the active sites by the removal of co-like adsorbates. the results of figure 5 reveal that pdnibi/rgo and the commercial pd/c display a different decrease in current density. both catalysts exhibit a sharp decay at the beginning, however, the pdnibi/rgo can then stabilize at a pseudo-steady state condition due to a gradual self-cleaning process [13,29]. the poisonous intermediates can be oxidatively removed by the generation of ohads, favored by the presence of oxophilic elements (equation (7)) [8,31]. in comparison, the current density of the pd/c steadily decreases with time as the catalyst is gradually poisoned [8,19]. therefore, the pdnibi/rgo composite shows a slightly higher remaining current density after 3600 s than the commercial pd/c. in summary, the combination of pd, ni and bi with rgo and their synergistic effects proves to be an effective method and highlights the pdnibi/rgo catalyst as a promising catalyst with high activity and stability for eor. conclusions in this study, pdnibi nanoparticles were successfully anchored on a reduced graphene oxide support using the modified instant reduction method. the physicochemical analyses have shown a two-dimensional wrinkled sheet morphology characteristic for graphene-based materials and uniformly and well-distributed metal particles with an average diameter of 2.6 nm on the carbon support caused by strong c-o-m bridges due to the remaining oxygen functionalities of the rgo. the electrochemical tests revealed that the pdnibi/rgo composite outperformed commercial pd/c in terms of eor activity and stability with an ecsa of 455 cm2 mg-1, an onset potential of 0.234 v vs. rhe, a maximum peak current density of 2390 ma mg-1 and a current density decrease of 85 % after 3600 s. this enhancement of the electrocatalytic activity could be attributed to the generation of abundant active sites due to the rgo support and the presence of the oxophilic ni and bi elements. based on these physicochemical and electrochemical results, pdnibi/rgo is highlighted as a promising anode catalyst in alkaline direct ethanol fuel cell applications. acknowledgements: the financial support from the austrian science fund (fwf) [grant number i 3871-n37] in the frame of the project »graphene oxide based meas for the direct ethanol fuel cell« and the slovenian research agency (arrs) through the research programms p1-0175, p20423, p2-0393 and i0-0003 and the bilateral research funding projects n2-0087, nc-0007, nc-0016, n2-0155 and n2-0257 are gratefully acknowledged. special thanks go to norbert kienzl for performing the icp-oes measurements. conflicts of interest: the authors declare no conflict of interest. references [1] l. an, t. s. zhao, y. s. li, 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782 [31] j. y. c. ribeiro, r. s. da silva, g. r. salazar-banda, k. i. b. eguiluz, pd-ni-b/c nanocatalysts for electrochemical oxidation of ethanol in alkaline media, catalysis research 3 (2023) 005. https://doi.org/10.21926/cr.2301005 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.21926/cr.2301005 https://creativecommons.org/licenses/by/4.0/) {the synthesis of porous indium phosphide with nickel oxide crystallites on the surface:} http://dx.doi.org/10.5599/jese.1301 593 j. electrochem. sci. eng. 12(4) (2022) 593-601; http://dx.doi.org/10.5599/jese.1301 open access : : issn 1847-9286 www.jese-online.org original scientific paper synthesis of porous indium phosphide with nickel oxide crystallites on the surface yana suchikova, ihor bohdanov1, sergii kovachov1, andriy lazarenko1, iryna bardus1, alma dauletbekova2, inesh kenzhina3 and anatoli i. popov2,4 1berdyansk state pedagogical university, berdyansk, ukraine 2l. n. gumilyov eurasian national university, nur-sultan 010008, kazakhstan 3kazakh-british technical university, almaty 050000, kazakhstan 4institute of solid state physics, university of latvia, riga lv-1063, latvia corresponding author: yanasuchikova@gmail.com; tel.: +38(066)338-78-64 received: february 15, 2022; accepted: june 26, 2022; published: july 6, 2022 abstract in this paper, the technology of synthesis of crystallites and nanocrystallites of nickel oxide on the surface of indium phosphide is described. this technology consists of two stages. in the first stage, porous indium phosphide is formed on the surface of a single crystal of indium phosphide. the formation of such a porous layer provides better adhesion to the surface of the sample. the second stage involves the preparation of the solution that contains nickel ions, application of this solution to the surface of porous indium phosphide, followed by annealing. as a result, nio/nic2o4∙2h2o/por -inp/mono-inp structure was formed. surface morphological parameters were obtained using scanning electron microscopy and edx-analysis of chemical composition. chemical analysis confirmed the partial formation of nickel oxide from nickel oxalate layer by thermal annealing. using scanning electron microscopy, it has been established that the crystallites have a large scatter in diameter, but they may be divided into three characteristic groups: macro-; mesoand nanocrystallites. such structures may find prospects for application in electrochemical capacitors and lithium-ion batteries. further research is needed for methodology improvement to obtain structures with predetermined controlled properties. keywords nickel oxalate; annealing; electrochemical etching; coating; edx spectrum introduction metal oxide semiconductors have recently been widely researched due to the prospects of their application as sensors, lasers, and in a wide range of photon detection and electronic devices, solar energy, etc. [1,2]. among such types of materials, mgo [3], al2o3 [4], knbo3 and linbo3 [5,6], tio2 [7,8], zno [9] and sno2 [10] are the most studied. thus, zinc oxide was grown using the epitaxial method on znse substrate, as researched in paper [11]. in another paper [12], the authors demonstrated the prospects for the application of sno2 to create solar cells. http://dx.doi.org/10.5599/jese.1301 http://dx.doi.org/10.5599/jese.1301 http://www.jese-online.org/ mailto:yanasuchikova@gmail.com j. electrochem. sci. eng. 12(4) (2022) 593-601 synthesis of porous inp with nio crystallites 594 at present, the great and close attention of many research groups is focused on new materials such as ga2o3 [13,14], znga2o4 [15] and baga2o4 [16]. among the top prospect materials is also nio due to its chemical stability and unique electrochemical properties. nickel oxide has a band gap in the range of 3.6 4.0 ev that offers a great value for photovoltaic devices, supercapacitors, sensors, etc. [17,18]. nickel oxide nanostructures, in particular nanotubes, nanoparticles and nanowires, offer even a greater value [19]. the widely used methods of obtaining nanoparticles and nanostructures of nickel oxide are methods of precipitation, dehydration and oxidation by steam-based etching [20]. however, these methods are usually quite complex and expensive and require special equipment. therefore, at present, there is a necessity for development of simple and inexpensive methods of nickel oxide synthesis. another problem in the growth of oxide semiconductors is the choice of substrate. the fact is that crystal lattices mismatch, while the chemical stability of many semiconductors prevents the adhesion to the substrates, significantly affecting the quality of the resulting layers. in this paper, we research the growth of nickel oxalate (nic2o4∙2h2o) and subsequently crystals of nickel oxide (nio) on indium phosphide (inp) substrate using a two-stage technology. experimental the structures were formed in two stages. the first stage is the formation of a porous pattern on the surface of monocrystal indium phosphide. this step is necessary for better adhesion to the monocrystalline indium phosphide (mono-inp) substrate of nickel-containing compounds. as it is known, porous layers have fewer dislocations under the surface, which hinders the excess stress [21]. therefore, such porous layers are considered to be ‘soft’ substrates, and they are becoming more popular [22]. the second stage is the deposition of nickel oxalate and synthesis of nickel oxide crystallites on the porous surface of inp (por-inp). formation of porous layer on mono-inp the first stage was intended to form pores on the surface of indium phosphide. monocrystalline high-alloy (s) indium phosphide samples of n-type conductivity with surface orientation (111) were used. before the experiment, the samples were cleaned, degreased in acetic solution, and dried in a stream of oxygen. the samples were then electrochemically etched in hydrochloric acid solution in a standard twoelectrode electrolytic cell with a platinum cathode. the aqueous-alcoholic solution (c2h5oh : h2o = = 1:2) of hydrochloric acid with an acid concentration of 7 % was used as the electrolyte. a few drops of bromic acid were added to the solution. it should be noted that this electrolyte composition is not sufficiently selective for the (111) surface of monocrystalline n-inp. such choice of electrolyte is conditional on the fact that the task was not to obtain a dense porous layer in this experiment [23,24]. the main purpose was to ensure surface roughness by etching the surface, as well as bulk defects to remove excess crystal stress. etching was carried out using the constant voltage of 5 v for 5 min. in the course of etching, a large number of bubbles were observed on the semiconductor wafer. bubbles shall be the result of oxygen evolution. as a rule, gas bubbles cause disruption of the oxides locally formed on the surface of the semiconductor. in order to reduce the effect of bubble generation, the electrolyte solution was periodically stirred with the use of a mechanical stirrer. following the etching, the samples were dried in a stream of oxygen for 2 h, washed in deionized water and kept outdoors for 3 days. the y. suchikova et al. j. electrochem. sci. eng. 12(4) (2022) 593-601 http://dx.doi.org/10.5599/jese.1301 595 samples became dark gray and lost their full gloss. this provides evidence of texturing of the crystal surface and formation of the extended macroroughness. formation of nickel particles on the surface of por-inp for the purpose of nio formation, the two-stage method was used, which consisted of the hydrothermal method and annealing. all substances used in this experiment had a high purity class and did not require additional purification. at the first stage, five portions (mass) of nicl2∙6h2o were dissolved in a mixture containing two portions of ethylene glycol and one portion of deionized water. the resulting mixture was heated in a beaker to 40 °с. then, it was stirred with a magnetic stirrer for 20 min. 1 g of na2c2o4 was added to the resulting solution. the resulting suspension was stirred with a magnetic stirrer for 15 min. the purpose of this step was to obtain ni+ ions and their homogeneous dispersion in the solution. the resulting suspension was transferred to porous indium phosphide. the samples were heated in a muffle furnace at 230 °с for 3 hours. then the samples were cooled to room temperature. following the experiment, the samples were air-dried for 3 months. the morphology of the obtained structures was researched using seo-sem inspect s50-b scanning electron microscope. the chemical composition was researched by edx method. the calculation by surface points and mapping technology was applied. results and discussion figure 1 shows the morphology of the n-inp porous surface following electrochemical etching. image analysis provides evidence of two types of etching pits, the occurrence of which is conditional on two different mechanisms of electrochemical crystal dissolution. the first type – massive etched areas that shall be the result of etching at the surface defect location areas. the size of such etching pits in the transverse diameter is 5 μm. it can be seen that such areas are characterized only by surface etching etching pits do not advance into the depth of the sample, and they grow mainly on the crystal surface. figure 1. sem image of the morphology of por-inp porous surface http://dx.doi.org/10.5599/jese.1301 j. electrochem. sci. eng. 12(4) (2022) 593-601 synthesis of porous inp with nio crystallites 596 the second type – so-called ‘seeding pore formation’is characterized by spontaneous removal of atoms from the crystal surface. such etching may lead to disruption of the stoichiometric composition of the material as a result of uneven etching of phosphorus and indium lattices. these etching pits form an assembly of small pores on the surface. the average pore diameter varies from 70 to 200 nm. the pores grow to the deep of the crystal in the form of thin parallel channels. the surface porosity is -below 15 %, which characterizes such a structure as a low porosity structure. as a result of etching, the sample stoichiometry was significantly disrupted (figure 2 and table 1). almost the same concentrations (in atomic units) of indium and phosphorus may be observed. carbon is also on the surface, it appeared due to the interaction of the sample surface with the electrolyte. in the etching of semiconductors in acid solution, the interphase layers are formed during constant potential superposition at the semiconductor/electrolyte interface (the gouy-chapman layer, the helmholtz layer). the result shall also be the formation of adsorbed species, which may subsequently form stable compounds. most likely, the presence of carbon is conditional on this mechanism. figure 2. edx spectrum of por-inp synthesized by electrochemical etching on mono-inp substrate. table 1. elemental composition of por-inp surface by atomic and mass units element line type content, mass% content, at.% p k series 20.37 40.64 in l series 76.05 40.92 c k series 3.58 18.44 total 100.00 100.00 it should be noted that carbon concentration is quite low (3.58 mass.% and 18.44 at.%, respectively). therefore, it can be assumed that these compounds will not have a critical effect on the properties of the resulting structure. on the other hand, in order to overcome this phenomenon in the course of electrochemical etching, it is necessary to stir the electrolyte, such stirring will prevent the formation of bubbles near the crystal surface and will allow forming of a clean structure without impurities. figure 3 shows the crystal surface following precipitation of nickel-containing suspension and annealing in a muffle furnace. the formation of a solid thin film with crystallites on the surface may be observed. the morphology of the resulting structures demonstrates spherical crystallites of different diameters that tend to agglomerate. y. suchikova et al. j. electrochem. sci. eng. 12(4) (2022) 593-601 http://dx.doi.org/10.5599/jese.1301 597 figure 3. sem image of por-inp surface with the nickel-containing surface layer figure 4 shows edx mapping of the formed structure surface. it can be concluded that the structure has atoms of indium, phosphorus, oxygen and nickel on the surface. chlorine and carbon are also present in low concentrations. it can be seen that phosphorus and indium are allocated on the surface almost evenly. these elements are not observed only in the points of concentration of crystallites. the predominant points of oxygen and nickel concentration are crystallites, but they may be observed on the entire surface. chlorine and carbon are present in small inclusions on the crystal surface. figure 4. energy dispersive x-ray (edx) mapping analysis of the sample based on the data obtained (fig. 4), it can be assumed that the structure consists of a dense layer of nickel oxalate dihydrate (nic2o4∙2h2o). nickel oxalate dihydrate is known to be one of the most important precursors for the synthesis of nickel oxide and pure nickel [25]. for this purpose, nickel oxalate is calcined and dehydrated, resulting in nio powder [26]. in our case, we used the application of nic2o4∙2h2o solution on the por-inp substrate, resulting in the formation of oxalate containing layer with nio inclusions (crystallites). our results are in good agreement with the data previously obtained [27,28]. note that a similar mechanism of synthesis of nio nanocrystallites with http://dx.doi.org/10.5599/jese.1301 j. electrochem. sci. eng. 12(4) (2022) 593-601 synthesis of porous inp with nio crystallites 598 nic2o4∙2h2o is demonstrated in the paper [28], with the fundamental difference that the oxalate layer was transferred to the oxide layer within 24 hours. figure 5 (a i) demonstrates the crystallites formed on the film surface. it can be seen that the crystallites tend to agglomerate (fig. 5 a, b, f). some of them have open ‘windows,’ which is evidence of the evaporation of water from them in the course of thermal annealing (b, c, d). crystallites have a very large scatter in diameter from 100 nm to 10 μm. in particular, we can distinguish three characteristic dimensions: [1] massive crystallites, that are the agglomerates of smaller ones, having a size of 10 20 μm (fig. 5 b, e, f); [2] medium crystallites, the size of which is in the range 1 5 μm (fig. 5 a, d, e); they have a characteristic spherical shape or torus shape; [3] small crystallites of nanometer size from 100 to 200 nm (fig. 5 c, e); such crystallites pack densely on the surface, and they are allocated in the form of islands. the presence of various types of structures shall be the evidence of evaporation and crystallization process incompleteness. in the event of longer processing periods, the islands will be ‘dried out’ and crystallized. this argument is also supported by the fact that the crystallites are in the form of nanowires in certain points, it is a characteristic of nickel oxide [29]. figure 5 (g, h, i) demonstrates the wire structures are formed. their thickness is 0.5 -1 µm, and length 23 µm. thus, a multilayer structure was formed, it contained 4 basic layers (from bottom to top) (figure 6): [1] a layer of monocrystal inp virgin crystal, basic ‘working’ substrate; [2] a porous inp layer that was formed by electrochemical etching on mono-inp in order to make the surface rough and to etch defects that are the sources of excessive stress; [3] a thin layer of nickel oxalate, nic2o4∙2h2o, that was formed on the por-inp surface by application of the solution and annealing; [4] an upper layer consisting of nio crystallites and nanocrystallites that were formed during partial annealing of the nickel oxalate layer. figure 5. sem images of nio crystallites and nanocrystallites y. suchikova et al. j. electrochem. sci. eng. 12(4) (2022) 593-601 http://dx.doi.org/10.5599/jese.1301 599 the nickel oxalate layer transformation is evidenced by cracking in some parts of the surface (figure 7). due to the aqueous phase drying, it can be seen that the layer becomes brittle, cracked and separated from the surface of porous indium phosphide in these parts. such behavior is typical and predictable. if annealing is continued for longer, the layer will be separated from the surface. the main problem is the preservation of nio crystallites on the inp surface. the formation of porinp on mono-inp allows one to solve this problem partially. figure 6. schematic image of nio/nic2o4∙2h2o/por-inp/mono-inp structure figure 7. sem images of nio/nic2o4∙2h2o/por-inp/mono-inp structure surface as already mentioned, nickel oxide is a very promising material. thus, nio nanoparticles are widely used as anodes for lithium-ion batteries [30], they are also promising for use in electrochemical supercapacitors [31]. their use as sensitized dyes and as photocathodes have been reported [32]. electrochemical, magnetic and antibacterial properties are also striking [33]. therefore, further development of the methods for obtaining nickel oxide nanoparticles is necessary. it should be noted that the first steps of the simple synthesis of crystallites and nickel oxide nanoparticles on porous semiconductor substrates were made in the presented research. the technology requires further improvement, particularly in the removal of the oxalate layer from substrate surface and development of mechanisms that will allow the synthesis of more uniform particles in size, well adhered to the substrate. http://dx.doi.org/10.5599/jese.1301 j. electrochem. sci. eng. 12(4) (2022) 593-601 synthesis of porous inp with nio crystallites 600 conclusions the article demonstrates the simple protocol of synthesis of the nic2o4∙2h2o layer with nio crystallites. the structures were formed on mono-inp substrate that was pre-etched in order to form rough porous layer on the surface. it is demonstrated that the two-stage technology of electrochemical etching of the substrate with subsequent application of nickel oxalate on the surface and annealing allows the formation of a multilayer structure of nio/nic2o4∙2h2o/porinp/mono-inp. provided, however, that nio is present on the surface in the form of crystallites and nanoparticles. such structure may find striking applications in modern electronic instrumentation. further steps should be taken in order to control the shape and the size of nanostructures, which is a common strategy of modern nanophase materials science, and to make decisions on optimization of the productivity and efficiency due to structural dependent properties. acknowledgements: y. suchikova, s. kovachov, i. bohdanov, і. bardus show their appreciation to the ministry of 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andriy and bardus, iryna and dauletbekova, alma and kenzhina, inesh and popov, anatoli i.}, journal = {journal of electrochemical science and engineering}, title = {{the synthesis of porous indium phosphide with nickel oxide crystallites on the surface:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {4}, pages = {593--601}, volume = {12}, abstract = {in this paper, the technology of synthesis of crystallites and nanocrystallites of nickel oxide on the surface of indium phosphide is described. this technology consists of two stages. in the first stage, porous indium phosphide is formed on the surface of a single crystal of indium phosphide. the formation of such a porous layer provides better adhesion to the surface of the sample. the second stage involves the preparation of the solution that contains nickel ions, application of this solution to the surface of porous indium phosphide, followed by annealing. as a result, nio/nic2o4∙2h2o/por--inp/mono-inp structure was formed. surface morphological parameters were obtained using scanning electron microscopy and edx-analysis of chemical composition. chemical analysis confirmed the partial formation of nickel oxide from nickel oxalate layer by thermal annealing. using scanning electron microscopy, it has been established that the crystallites have a large scatter in diameter, but they may be divided into three characteristic groups: macro-; mesoand nano­crystallites. such structures may find prospects for application in electrochemical capacitors and lithium-ion batteries. further research is needed for methodology improvement to obtain structures with predetermined controlled properties.}, doi = {10.5599/jese.1301}, file = {:d\:/onedrive/mendeley desktop/suchikova et al. 2022 the synthesis of porous indium phosphide with nickel oxide crystallites on the surface.pdf:pdf;:02_jese_1301.pdf:pdf}, keywords = {edx spectrum, annealing, coating, electrochemical etching, nickel oxalate}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1301}, } electrode configurations study for alkaline direct ethanol fuel cells http://dx.doi.org/10.5599/jese.1623 783 j. electrochem. sci. eng. 13(5) (2023) 783-793; http://dx.doi.org/10.5599/jese.1623 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrode configurations study for alkaline direct ethanol fuel cells michaela roschger1,, sigrid wolf1, andreas billiani1, selestina gorgieva2, boštjan genorio3 and viktor hacker1 1institute of chemical engineering and environmental technology, graz university of technology, inffeldgasse 25/c, 8010 graz, austria 2faculty of mechanical engineering, university of maribor, smetanova ulica 17, 2000 maribor, slovenia 3faculty of chemistry and chemical technology, university of ljubljana, večna pot 113, 1000 ljubljana, slovenia corresponding author: michaela.roschger@tugraz.at received: november 29, 2022; accepted: march 24, 2023; published: march 28, 2023 abstract the direct electrochemical conversion of ethanol, a sustainable fuel, is an alternative sustainable technology of the future. in this study, membrane electrode assemblies with different electrode configurations for an alkaline direct ethanol fuel cell were fabricated and tested in a fuel cell device. the configurations include a catalyst-coated substrate (ccs), a catalyst-coated membrane (ccm), and a mixture of these two fabrication options. two different anion exchange membranes were used to perform a comprehensive analysis. the fabricated ccss and ccms were characterized with single cell measurements, electrochemical impedance spectroscopy and scanning electron microscopy. in addition, the swelling behavior of the membranes in alkaline solution was investigated in order to obtain information for ccm production. the results of the experimental electrochemical tests show that the ccs approach provides higher power densities (42.4 mw cm-2) than the others, regardless of the membrane type. keywords membrane electrode assembly; alkaline swelling; single cell tests; anion exchange membrane introduction alternative, sustainable, and environmentally friendly technologies are receiving a tremendous amount of attention due to the growing environmental awareness of the human population as a result of global warming. energy conversion devices such as fuel cells represent promising alternatives to fossil fuel-powered devices and can ensure a significant contribution to the http://dx.doi.org/10.5599/jese.1623 http://dx.doi.org/10.5599/jese.1623 http://www.jese-online.org/ mailto:michaela.roschger@tugraz.at j. electrochem. sci. eng. 13(5) (2023) 783-793 electrode configurations study for alkaline defc 784 production of energy from renewable sources. the most heavily researched fuel cell today is the proton-exchange membrane fuel cell (pemfc), but the anion exchange membrane fuel cell (aemfc) is also regaining importance. these two types differ in terms of the electrolyte or membrane used: the pemfc uses a cation exchange membrane, and the aemfc uses an anion exchange membrane (aem) [1,2]. another important difference is that aemfc can use inexpensive, non-noble metal catalysts, while pemfc must use the costly and scarce catalyst platinum [3-6]. a special form of aemfc is the alkaline direct ethanol fuel cell (defc), shown in figure 1. this fuel cell allows the use of ethanol as a fuel, which has advantages associated with its renewability, liquid state (convenient handling and storage), low toxicity, and high energy density (8 kwh kg-1) [1,5,6]. figure 1. working principle (reactants + products) of an alkaline defc (cl: catalyst layer; gdl: gas diffusion layer, mea: membrane electrode assembly) however, the use of this fuel also presents challenges, such as the fact that ethanol is incompletely converted on the anode side. no catalysts have been identified that fully perform the ethanol oxidation reaction (eor). instead of splitting ethanol completely to co2 and 12 e-, acetate (acetic acid) or acetaldehyde and thereby less electrons are formed. the research on eor catalysts is focused on ptor pd-based catalysts, whereby pd catalysts show four times higher performance than pt catalysts in alkaline media. the combination of less expensive ad-atoms like ag, ru, co, fe or ni with pd-catalysts to produce bior tri-metallic catalysts can further enhance the activity of the catalysts [5,7-11]. in aemfcs, e.g. commercial aems based on synthetic polymers with quaternary ammonium groups (fumatech, solvay, japan tokuyama corporation) and membranes with polysaccharide-based biomaterials like chitosan (cs) can be used [4]. the use of aems in alkaline fuel cells compared to liquid-electrolyte systems exhibit a lower fuel crossover rate and an improved co2 tolerance [2]. however, these can swell in aqueous solutions and become brittle under dry storage conditions after undergoing anionic conversion in koh; and must therefore kept hydrated afterwards [2]. moreover, there are other challenges to cope with aem utilization, such as the thermal and mechanical stability, and the ionic conductivity [2,5,12]. in order to increase the ionic conductivity inside the cell and to improve the eor performance, an alkaline solution (koh) is added to the fuel [2,4,5,13]. the addition of koh, and therefore ohions, in the aemfc also has a conflicting adverse effect, which is long term degradation, by nucleophilic attack or hoffman elimination reaction [2]. furthermore, the addition of alkaline solution, unfortunately has also a big drawback in the alkaline defc: possible produced co2 m. roschger et al. j. electrochem. sci. eng. 13(5) (2023) 783-793 http://dx.doi.org/10.5599/jese.1623 785 can react with the ohions and form carbonate or bicarbonates and further carbonate salts (in case of koh: k2co3). the produced carbonate salts can block the pores of the catalyst layer and precipitate on the membrane [1,7]. despite these disadvantages, the addition of alkaline solution increases the power output of alkaline fuel cells; the advantages predominate over the disadvantages at least in short term [1]. due to the use of the liquid fuel, the gas diffusion layer (gdl) requirements on the anode side of the cell differ from those on the cathode side (hydrophilic vs. hydrophobic). the gdl should provide the best possible diffusion medium for ethanol, and the literature shows that the use of a carbon cloth as a gdl is ideal [14,15]. the electrodes (i.e., the catalyst layer (cl) and gdl) and the membrane together form the membrane electrode assembly (mea), which represents the heart of a fuel cell. the mea can be manufactured with one of this two basic methods, namely, using catalyst-coated substrates (ccs) or catalyst-coated membranes (ccm). with the first option, the catalyst ink is applied to the gdl substrate, and the resulting ccss or gas diffusion electrodes (gdes) are connected to the membrane. with the second option, the ink is applied to the membrane, and the resulting ccms are connected together with the blank gdls [16-18]. several ways to apply the catalyst ink have been reported, such as blading, spraying, or printing, whereby spraying has found to be suitable on a laboratory scale [16-20]. there are studies on the production methods of the whole mea related to acidic defc, but nearly no studies for the alkaline defc. song et al. [18]. proved that the catalyst layer containing nafion solution can be delaminated in the acidic defc as a result of nafion® 115 membrane swelling in ethanol solution. moreno-jiménez et al. [17] showed that a combination of the inner and outer catalyst layer (i.e. on gdl and membrane: nafion® 117) in the acidic defc positively effects cell performance, due to lower ohmic resistances. however, in the acidic defc, no additional koh is added; therefore, the conditions differ from those in the alkaline defc. several studies have been performed on fabrication optimization for alkaline electrolysis with anion exchange membranes. for example, plevová ei. [21] developed a membrane and ionomer for the alkaline water electrolyzer which can be used to produce with a computer-controlled ultrasonic dispersion deposition method ccms. ito et al. [20] discovered that the ccm-cathode and ccs-anode is the most appropriate configuration when using commercial membranes. considering this background, the aim of this study was to investigate the influence of the electrode configuration in the mea on the performance of the liquid feed alkaline defc, since the properties of this type of fuel cell (liquid koh and aems) are challenging. three different electrode configurations with both commercially available and custom-made aems were used in this work: (a) configuration a with a gde-based mea, (b) configuration b with a ccm-based mea, and (c) configuration c with a mixture of ccm and gde. the produced meas were characterized electrochemically in a single cell. furthermore, the morphological features of the meas were characterized with scanning electron microscopy (sem). in addition, the swelling behavior of the membranes in the alkaline solutions used was investigated. experimental materials the following materials and chemicals were used without further purification: ethanol (etoh, 99.9 % p.a.), potassium hydroxide (koh ≥ 85 %, p.a., pellets) and 2-propanol (99.9 % p.a.) were purchased by carl roth (karlsruhe, germany). carbon cloth (elat hydrophilic plain cloth, 0.406 mm thick) and carbon paper (sigracet 29 bc, 0.235 mm thick) were delivered from fuel cell store (college http://dx.doi.org/10.5599/jese.1623 j. electrochem. sci. eng. 13(5) (2023) 783-793 electrode configurations study for alkaline defc 786 station, tx, usa). fumasep® faa-3-50 (anion-exchange membrane, non-reinforced, 50 µm) and nafion™ solution (ns-5, pfsa 5%, ) were purchased from fumatech (bietigheim-bissingen, germany) and quintech (göppingen, germany), respectively. a cs/n-rgonrs anion exchange membrane (0.07 % n-rgonrs) that was previously developed was used [4]. an ag-mnxoy/c cathode catalyst, as well as a pdnibi/c anode catalyst, both consisting of 30 wt.% catalytic active material on the support material vulcan, as previously reported were utilized [8,11]. ultrapure water (18 mω cm, barnstead nanopure-water purification system (dubuque, ia, usa)) was used for the preparation of the solutions and inks. mea configurations the utilized spray-coated mea configurations are shown in figure 2. configurations consisting of a ccs + ccs (configuration a), a ccm + ccm (configuration b), and a mixture of these two (ccmanode + + ccscathode = configuration c) were tested. the membranes selected comprised two different aems, the cs/n-rgonrs [4] and the commercial fumasep® faa-3-50 membrane. configuration a and b were tested with both membranes, whereas configuration c was only tested with the cs/n-rgonrs membrane. the meas were obtained by assembling the individual parts together depending on the configuration. figure 2. schematics of different electrode configurations used for mea production (anode right and cathode left) (a) configuration a, (b) configuration b and (c) configuration c ink formulation and preparation the catalyst ink for the spray-coating process and thus the preparation of ccss or ccms consisted of the respective catalyst (ag-mnxoy/c for cathode and pdnibi/c for anode), isopropanol, water and nafion-ionomer (binder) solution, whereby the catalyst mass concentration was 4 mg ml-1. nafion ionomer was used because commercial anion exchange ionomers still have low thermal and chemical stability [5], and the addition of the aqueous fuel or humidified oxygen enables nafion to be used [22]. the ratio of water to isopropanol was 7:3 and the amount of ionomer/binder in the solution was 30 wt.% of the catalyst mass. all produced electrodes featured an i/c (ionomer/carbon) ratio of 0.6. to obtain a homogeneous dispersion, the mixture was ultrasonicated using an ultrasonicbath. coating conditions the ccss were prepared by spraying the catalyst inks on the gdls, whereby the carbon cloth was utilized at the anode and carbon paper on the cathode, using an ultrasonic sonotech exactacoat op3 spray coater (sonotek corporation, usa) with a 120 khz nozzle. the gdls were fixed on a porous ptfe substrate, which was tempered to 80 °c, using a template to spray a surface area of 4 cm2. the nozzle was continuously moved back and forth and from left to right and vice versa in staggered serpentine m. roschger et al. j. electrochem. sci. eng. 13(5) (2023) 783-793 http://dx.doi.org/10.5599/jese.1623 787 lines at a constant height of approximately 6 cm during the spraying process in an automated manner in order to obtain a homogeneous, uniform coating. an anodic active material loading of 0.5 mg cm-2 and a cathodic active material loading of 0.25 mg cm-2 were applied [8]. the same procedure was used to produce the ccms, but the dry membrane was coated instead. to prevent the membrane from wrinkling during the spray process, it was additionally fixed by vacuum suction. anionic conversion and koh doping of the membranes the pure membranes, as well as the ccms, were pretreated in de-aerated 1 m koh for 24 h [4]. commercial aems are normally stored in the stable cl-/brform; therefore, they require anionic conversion to be present in the ohform. a special fixture was used to clamp the ccms. the membrane cannot be converted or doped with koh before applying the catalyst with the spray coater to it, since it would dry out and become brittle [2]. another configuration b variant of the cs/n-rgonrs membrane (configuration bii) was produced, which was incorporated dry into the cell without koh doping of the membrane outside the cell; however, the membrane was doped inside, to see if fixation with the flow fields had an influence on the results. for this purpose, the test rig [13] was adapted by purging with n2 (1 ml min-1) through the cathode side and passing a 1 m koh solution (0.3 ml min-1) through the anode side for 24 h. the low flow rates were selected to optimally mimic the koh doping of the other configuration b variant for comparison. membrane characteristics in alkaline (ethanol) solution the swelling rate of the two aems was evaluated in 1 m koh at rt after 24 h (swelling during anionic conversion and doping with koh) and afterwards after 1 h and 24 h in a mixture of 1 m koh and 1 m etoh solution. therefore, the in-plane and through-plane dimensions were measured at the given time intervals and the change calculated. single cell tests all single cell tests of the prepared meas were conducted with a im6ex potentiostat combined with a pp240 power potentiostat (zahner-elektrik gmbh & co. kg, germany) and utilizing a selfdesigned cell [13]. in these tests, after heating up the cell and purging with highly basic solution and oxygen, the current was ramped up from small (0.4 ma cm-2) to high current densities using a step time of 30s, and the cell voltage was recorded until a cell potential of 0.180 v was reached. these performance tests were conducted with continuous supply of a liquid mixture of 1 m koh and 1 m etoh at a flow rate of 5 ml min-1 at the anode side and with humidified oxygen (60 % rh) at a flow rate of 25 ml min-1 at the cathode side of the cell. the measurements were performed at 80 °c. in addition to the polarization curves, electrochemical impedance spectra (eis) of configuration a and configuration c (both cs/n-rgonrs membrane) at 440 ma (amplitude: 10 %) between 50 khz and 0.1 hz were recorded after 5 min stabilization time. for fitting and evaluation, the equivalent circuit model from previous work [13] and zview® software (scribner associates inc., southern pines, nc, usa) was applied. scanning electron microscopy the sem images of the catalyst layers on the different membranes and electrodes were recorded with a zeiss ultra plus using inlens and se2 detectors at 2 kv or 5 kv at wd 5.5 mm by adhering the samples to an aluminum holder with conductive carbon tape. http://dx.doi.org/10.5599/jese.1623 j. electrochem. sci. eng. 13(5) (2023) 783-793 electrode configurations study for alkaline defc 788 results and discussion in this section, the results of the membrane swelling characteristics (in koh and a mixture of koh and etoh) are discussed first. the obtained behavior characteristics of the membranes were used to declare the influence of the mea production process on the performance in the single cell. eis measurements were applied to point out the impact of the electrode configuration on resistance. moreover, specific photos and sem images were shown to support the results. membrane characteristics in alkaline (ethanol) solution the swelling behavior of membranes is an important issue for the production and performance of ccms, due to possible delamination of the catalyst layer [18]. in figure 3 a, the in-plane and throughplane swelling in 1 m koh after 24 h, thus the swelling during anionic conditioning or koh doping, can be seen. the custom-made cs/n-rgonrs aem undergoes an in-plane change of 15 %, whereas the through-plane swelling of 78 % is very high. the faa-3-50 membrane behaves in exactly the opposite way, changing in-plane by 26 % and through-plane only by 2 %. however, this means that both membranes change significantly in size (cs/n-rgonrs changes considerably more) over the measured time period, resulting in a thickness of 55.7 ± 0.9 µm for the faa-3-50 and 57.0 ± 0.8 µm for the cs/n-rgonrs. for the investigation of the membrane swelling behavior during the measurement, the size change in 1 m koh and 1 m etoh was studied (figure 3 b). the remarkable result obtained is that the through-plane swelling behavior is constant for both membranes between 1 h and 24 h. in other words, both membranes swell in thickness to a certain degree (cs/n-rgonrs: 11 % and faa-3-50: 4 %) and then remain unchanged. however, both membranes enlarge in-plane over time, but the faa-3-50 (1h: 8 % and 24 h: 25 %) more than the cs/n-rgonrs (1h: 0 % and 24 h: 2 %). thus, the swelling behavior of the membranes is comparable to the swelling behavior in 1 m koh: the cs/nrgonrs swells more through-plane, and faa-3-50 more in-plane. the total size change of the membranes after 24 h in 1 m koh and 24 h in 1 m koh and 1 m etoh results in an in-plane swelling of 18 and 57 % for the cs/n-rgonrs and the faa-3-50, respectively and a through-plane swelling of 98 and 5 % for the cs/n-rgonrs and the faa-3-50, respectively. figure 3. membrane swelling in (a) 1 m koh solution after 24 h and (b) in a mixture of 1 m etoh and 1 m koh solution single cell tests the influence of the electrode configuration in the mea on the performance in the single cell can be seen in figure 4. m. roschger et al. j. electrochem. sci. eng. 13(5) (2023) 783-793 http://dx.doi.org/10.5599/jese.1623 789 figure 4. polarization and power density curves of the electrode configurations for the different membranes (a) cs/n-rgonrs, (b) fumasep® faa-3-50, and (c) bar chart of the power densities obtained for all measurements the configuration a approach yielded the highest power densities for both membranes used, whereby configuration b always performed worse. however, the difference between the values obtained for configuration a and configuration b for the tested membranes is considerable. the performance of configuration b is only one-fifth of the configuration a performance with the cs/n-rgonrs aem (9.2 mw cm-2 vs. 42.4 mw cm-2). the performance seen with the commercially available fumasep® faa-3-50 aem, meanwhile, was only one-fourth of the latter (6.8 mw cm-2 vs. 28.0 mw cm-2). a clear trend is visible, i.e. that the power losses are lower for the commercial membrane due to their swelling behavior during anionic conversion, as shown before. the high through-plane swelling of the cs/n-rgonrs aem resulted in large cracks in the catalyst layer, if the catalyst layer was sprayed on the membrane. smaller cracks were also seen within the faa-3-50 membrane, due to the in-plane swelling. however, the maximum power density values are still lower than those of the custom-made membrane. the performance of the configuration bii variant (where the doping with koh of the membrane was performed inside of the cell) of the cs/n-rgonrs aem was even lower (8.0 mw cm-2) than that of configuration b. this is due to the fact that the configuration bii type did not undergo full koh doping. only the active surface (where the catalyst layer is located) of the mea was in contact with koh at the anode (through the flow field). the whole membrane of the configuration b variant, however, was completely exposed to koh. configuration c resulted in much better performance (24.7 mw cm-2) than configuration b, but worse (one-half) than configuration a. for the investigation of this power loss, configuration a and configuration c were analyzed with eis (figure 5) to determine their resistances. the electrolyte http://dx.doi.org/10.5599/jese.1623 j. electrochem. sci. eng. 13(5) (2023) 783-793 electrode configurations study for alkaline defc 790 resistance rel (membrane) is nearly the same for both configurations, but a little bit higher for configuration c. the anodic ionomer resistance rion,a, the charge transfer resistance rct,a and the mass transfer resistance rmt and the resulting overall resistance rges increased for configuration c. therefore, the interface or connection of gde and membrane in comparison to gdl and ccm is preferable, due to lower ohmic resistance, as a result of lower ionic repulsion between the sulfonated groups of the ionomer and the hydroxide anions. possible presence of water at the interface between the membrane and the gde favors hydroxide transport [23]. figure 5. electrochemical impedance spectra of (a) configuration a, (b) configuration c, and (c) bar chart of the resistances of the cs/n-rgonrs aem measurements thus, the double ccs configuration had a clear advantage among both membranes which are anionic converted or koh doped in the alkaline defc for cell measurement. the more ccs configurations were replaced by ccm, the worse the performance was due to higher resistances. visual investigation of the different electrode configurations the different electrode configurations of the cs/n-rgonrs membrane were visually investigated before and after the single cell tests and the results are illustrated in photos. in figure 6 a the configuration b of the cs/n-rgonrs aem can be seen after 24 h koh doping. figure 6. photographs of the different electrode configurations before and after single cell testing (a) configuration b before, (b) configuration b after, (c) configuration bii after, (d) configuration a after, (e) configuration a anodic gde after, and (f) configuration a cathodic gde after m. roschger et al. j. electrochem. sci. eng. 13(5) (2023) 783-793 http://dx.doi.org/10.5599/jese.1623 791 after immersion in koh, cracks can be seen in the catalyst layer, due to the swelling of the cs/n-rgonrs aem, which could not be prevented [4], as determined before. these are even more pronounced after the cell measurement and delamination of the catalyst layer from the membrane can be seen in figure 6 b (the visible crack in the membrane occurred after the measurement during cell disassembly by taking off from the gasket). the cubic pattern exterior the active mea area is due to the gasket used inside the cell to prevent leakage. however, it can be ruled out that the appearance of the larger and more pronounced cracks is due to the cell operation with the mixture of koh and etoh and membrane swelling in this solution, since no cracks are visible in figure 6 c of configuration bii. the pre-damage of immersion in koh is the reason for these large cracks in configuration b. configuration bii was koh doped within the cell under fixation of the flow field plates. in other words, the fixation counteracts swelling within the cell, regardless of whether koh or a mixture of etoh and koh are used, since the catalyst layer is not damaged. this means, in turn, that we assume that delamination of the catalyst layer is caused by the membrane swelling behavior rather than by possible swelling of the ionomer/binder in the catalyst layer. these observations can also be seen in figure 6 d-f. the membrane without catalyst layer is flat and planar and shows no visible swelling where the ccs was located (due to the fixation with the flow fields) and the catalyst layers on the substrates are not delaminated. scanning electron microscopy the same observations could be made by performing sem analyse of the catalyst layers on the membranes and substrates. in figure 7 a and d no delamination of the catalyst layer from the gdls can be seen. the marked cracks (blue arrows) in the catalyst layer result from the structure of the carbon paper, as shown in our previous work [11]. configuration b can be seen in figure 7 b and e, where a crack has formed in the catalyst layer on the left-hand side of the image. in contrast, as previously described, no cracks are visible in the catalyst layers of configuration bii (figure 7 c and f). figure 7. sem images of the catalyst layers after single cell testing (a and d) configuration a anode and cathode, (b and e) configuration b anode and cathode and (c and f) configuration bii anode and cathode the photographic evidence indicates that aem swelling and pretreatment is a major problem in the production of meas with ccms [2]. whereby the ccm variant is extremely useful, as a direct connection can be formed by applying the catalyst layer and the anion exchange ionomer on it, as shown in literature [20,21]. however, therefore research is needed to reduce membrane swelling during anionic conversion or doping with koh and to develop chemically and thermally stable anion exchange ionomers [2]. http://dx.doi.org/10.5599/jese.1623 j. electrochem. sci. eng. 13(5) (2023) 783-793 electrode configurations study for alkaline defc 792 conclusions the influence of the manufacturing method of the mea with different membranes (commercially available and custom-made aems) could be successfully demonstrated. due to the inevitable swelling of the aems during anionic conversion or koh doping, the catalyst layer cracks; therefore, the ccm manufacturing method is currently not recommended for the tested aems in the alkaline defc using nafion ionomer in the catalyst layer. regardless of the membrane used, higher power densities could be achieved with the ccs fabrication variant on both sides (anode and cathode) than with the ccm variant. the cell resistance increases when the catalyst layer is applied to the membrane, which reduces the performance, due to the ionic repulsion between the sulfonated groups of the ionomer and the hydroxide anions. acknowledgements: the authors acknowledge the financial support from the austrian science fund (fwf, grant number i 3871-n37) and slovenian research agency (arrs, grant number: n2-0087, p20118, and p1-0175) as part of the project »graphene oxide based meas for the direct ethanol fuel cell«. we also thank azra 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https://doi.org/10.1016/j.jpowsour.2004.08.011 https://doi.org/10.1016/j.ijhydene.2022.03.043 https://doi.org/10.1007/s10800-018-1159-5 https://doi.org/10.1016/j.jpowsour.2022.231476 https://doi.org/10.1021/acs.energyfuels.2c02951 https://doi.org/10.1016/j.jpowsour.2005.10.019 https://creativecommons.org/licenses/by/4.0/) the performance of heteroatom-doped carbon nanotubes synthesized via a hydrothermal method on the oxygen reduction reaction and specific capacitance http://dx.doi.org/10.5599/jese.1664 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1697 open access : : issn 1847-9286 www.jese-online.org original scientific paper the performance of heteroatom-doped carbon nanotubes synthesized via a hydrothermal method on the oxygen reduction reaction and specific capacitance tienah h. h. elagib1,2,, nassereldeen a. kabbashi1, md zahangir alam1, ma’an f. al-khatib1, mohamed e. s. mirghani1 and elwathig a. m. hassan2 1kulliyyah of engineering, department of chemical engineering and sustainability, international islamic university malaysia, jalan gombak 53100, kuala lumpur, malaysia 2department of materials engineering, faculty of industrial engineering and technology, university of gezira, wad madani, sudan corresponding author: tienahhussain25024@gmail.com received: february 7, 2023; accepted: april 16, 2023; published: may xx, 2023 abstract due to the increasing demand for electrochemical energy storage, various novel electrode and catalysis materials for supercapacitors and rechargeable batteries have developed over the last decade. the structure and characteristics of these catalyst materials have a major effect on the device's performance. in order to lower the costs associated with electrochemical systems, electrochemical systems, metal-free catalysis materials can be employed. in this study, metal-free catalysts composed of nitrogen (n) and sulfur (s) dualdoped multi-walled carbon nanotubes were synthesized using a straightforward and costeffective single-step hydrothermal method. carbon nanotubes served as the carbon source, while l-cysteine amino acid and thiourea acted as doping elements. as a result of the physicochemical characterization, many defects and a porous structure were noted, along with the successful insertion of nitrogen and sulfur into the carbon nanotube was confirmed. according to the cyclic voltammetry tests for the dual-doped samples in alkaline conditions, the d-cnt2 catalyst exhibited onset potentials of -0.30 v higher than the -0.37 v observed for the d-cnt3 catalyst. this indicates enhanced oxygen–reduction reaction due to the synergistic effects of the heteroatoms in the structure and the presence of chemically active sites. moreover, the outstanding specific capacitance of the d-cnt2 catalyst (214.12 f g-1 at scanning rates of 1 mv s-1) reflects the effective porosity of the proposed catalyst. these findings highlight the potential of n/s dual–doped carbon nanotubes for electrocatalytic applications, contributing to efficient energy conversion. keywords metal-free catalyst; electrochemical energy storage; catalytic activity; porous structure http://dx.doi.org/10.5599/jese.1664 http://dx.doi.org/10.5599/jese.1697 http://www.jese-online.org/ mailto:tienahhussain25024@gmail.com j. electrochem. sci. eng. 00(0) (2023) 000-000 heteroatom-doped carbon nanotubes 2 introduction in recent years, fostering a sustainable society has been a focus [1-3]. developing renewable energy is important in achieving sustainable development and reducing pollution from fossil fuels [4-6]. the creation of clean energy storage and generation with low costs and great efficiency has garnered a lot of interest [7-9]. traditional energy supplies, according to the exxon mobil paper titled 2018 outlook for energy: a view of 2040, will play an irreplaceable part in our future daily lives. despite efficiency increases between 2016 and 2040, it expected worldwide energy consumption to increase by approximately 25 %, while they expected global electricity demand to climb by 60 %. thus, building green electro-based energy systems is urgently needed to support economic progress and accomplish the paris agreement's climate targets, and it surely bears enormous market opportunities [10]. presently, vital research is being focused on advanced technologies for electrochemical energy storage and conversion systems such as electrochemical capacitors, regenerative fuel cells, and rechargeable batteries (including na-ion batteries, li–sulfur batteries, li-ion batteries, and k-ion batteries [11]), including electrocatalysis due to their flexible capacities [12], eco-friendly [13-15], remarkable energy conversion ability, and sustainability [16,17]. electrochemical processes, like electrocatalysis, are crucial to electrochemical energy storage and conversion systems [18]. during discharging, such energy systems undergo an oxygen reduction reaction (orr) on their negative electrode [19]. it is important to rationally design highly active [20], durable, and low-cost electrocatalyst for effective orr as it represents a critical reaction in energy conversion systems [21]. although a variety of factors influence the performance of energy-related devices, the structure and characteristics of the materials utilized heavily influence the overall performance [22]. in fact, orr is slow in an acidic medium and requires noble catalysts such as pt to occur; however, orr has a lower over-potential in an alkaline medium than that in an acidic medium, allowing the use of noble-free catalysts [23]. to meet the needs of industry and advanced energy systems, the advancement of cost-effective metal-free electrocatalytic materials is of great interest [24]. in this direction, numerous non-precious orr electrocatalysts, such as transition metal chalcogenides [25], transition metal nitrogen-containing complexes, and metal-free doped carbon materials [26], have been explored and developed [27]. it is broadly acknowledged that doping nanostructured carbons with heteroatoms such as boron, nitrogen (n), phosphorus, and sulfur (s) improves oxygen-reaction activities significantly [28]. for instance, j. h. kim et al. [29] used chemical vapor deposition to create sulfur-doped carbon nanotubes (s-cnts) using dimethyl disulfide as the sulfur source. at a high discharge current density of 100 ma cm−2, the proposed material exhibited excellent specific capacitance of 120.2 f g−1. the doping effect was responsible for the superior electrochemical performance of s-cnts. as earlier reported in the literature, the doped heteroatoms can modify the spin-charge densities, causing charge redistribution in the carbon matrix [30]. the s element, in particular, increases the positive charge density and spin density of adjacent carbon atoms, promoting oxygen molecular adsorption on the carbon matrix and conferring carbon materials with effective orr catalytic activity [31,32]. the creation of c-n structures, such as graphitic-n and pyridinic-n, can aid in electron transfer during the orr process [31]. previous research has shown that incorporating heteroatoms improves the distribution and preserves the electrical conductivity of nanostructured carbons and, thus, catalytic activity. the doped material also has long-term durability, good stability [33], and exceptional poison resistance during the orr process [34,35]. raji atchudan and co-workers [36], for example, synthesized nitrogen-doped porous carbon, which showed a high specific capacity (160 f g-1) with a good cycling stability of 95 % and a high rate performance of 52 % retention from 0.5 to 5.0 a g-1 in a three-electrode system in an aqueous 1 m t. h. h. elagib et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1664 3 h2so4 electrolyte. to the best of our knowledge, dual or even triple heteroatom doping treatments have been employed to improve the orr catalytic performance of carbon-based nanostructures [37,38]. y. zheng and colleagues [39] used a simple and straightforward template-free technique to fabricate nitrogen/sulfur dual-doped hollow mesoporous carbon nanospheres for orr electrocatalysis. the unique hollow spherical and mesoporous structure formed in situ from an intricately engineered covalent triazine framework via a thermally initiated hollowing pathway. the material that resulted exhibited high n and s contents, large specific surface areas, and excellent electrochemical performance [39]. generally speaking, carbon materials have been designed for effective energy conversion and storage due to their unique physicochemical properties, such as high electronic conductivity, large surface area, and adjustable porous structure [22]. in this direction, cnts have sparked considerable interest in the fields of electrocatalysis, biosensors, and energy storage due to their amazing features, which include exceptional electronic conductivity, great thermal stability, and good mechanical characteristics [40]. herein, cnts, l-cysteine (l-cys), and thiourea (tu) were utilized for dual-doping with a hydrothermal treatment for electrocatalysis. although n and s dual-doped cnts have been extensively studied, little attention has been paid to the study of cnts treated with amino acids-cothiourea for electrochemical catalysis. therefore, we investigated the structural change caused by inserting dual heteroatoms into the surface of cnts, as well as the electrochemical performance in terms of the orr. for comparison, investigated heteroatom doping of oxidized cnts to test the hypothesis that heteroatom-doped cnt catalysts functionalized with selective oxygenic groups (cooh or oh) can improve intrinsic reactivity. neat cnts, doped cnt prepared at 4 h, and doped (cnt and oxidized cnts (ocnts)) prepared at 16 h are denoted as cnt0, d-cnt1, d-cnt2, and d-cnt3, respectively. experimental materials multiwall carbon nanotubes (mwcnts, ≥98 % purity) were provided by sigma aldrich. l-cys (≥99 % purity) was purchased from beijing solarbio science & technology. thiourea (>98 % purity), n, n-dimethyleformamide dmf, nano3 (>98 % purity), and h2so4 (98 %) were supplied by r&m chemicals. kmno4 (>99 % purity) and hydrogen peroxide were provided by chemiz sdn. bhd., and bendosen laboratory chemicals, respectively. oxidation of cnts typically, 0.1 g of cnts with 0.1 g of nano3 were dispersed in 60 ml h2so4 (78 %) and placed in the sonicated for around 40 minutes at 11 °c in an ice bath, after which 0.2 g of kmno4 was added to the dispersion portion wise and sonicated for 2 h at (35 to 45 °c). then 20 ml of 30 wt.% h2o2 was progressively added, followed by 300 ml of deionized water. finally, the cnts were filtered, washed, and dried. hydrothermal synthesis typically, 0.2 g of cnts were dispersed in 100 ml deionized water before being placed in an ultrasonic bath for 2 h at room temperature (solution a). then after, 0.15 g of l-sys and t-u (weight ratio 2:1) was added to 30 ml of solution a and then sonicated for 30 min (solution b). solution b was placed into a teflon-autoclave reactor for hydrothermal reaction and in an oven for the http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 00(0) (2023) 000-000 heteroatom-doped carbon nanotubes 4 appropriate reaction time at 160 °c. the reactor was then allowed to cool naturally before the cnts were filtered, washed, and freeze-dried. the same method was used to treat the oxidized cnts. measurements x-ray diffractometer (xrd) (d2 phaser – bruker, germany)was used to measure phase patterns of d-cnts. cu kα radiation, cu anode, and a scanning rate of 10° min-1 were used for measurements. raman spectroscopy (renishaw invia, germany) was used to identify the final chemical structure of modified cnts. to further reveal the chemical structure, a fourier transfer infrared (ftir) spectrometer (nicolet is50 ft-ir, usa) was used to characterize the functional group attached to doped cnts. analyses were performed by using the atr method over a spectral range of 4000 to 500 cm˗1. differential scanning calorimetry (dsc) analysis was performed by using a high-temperature dsc (mettler toledo, switzerland) instrument. about 0.5 mg of each sample was heated in an aluminum crucible with a heating rate of 10 °c min-1 from 30 to 450 °c. the morphology and eds elemental analysis of doped-cnts were studied using scanning electron microscopy (sem, jsm-5600lv, japan). thermo scientific elemental analyzer (flash smart chns, italy) was employed to measure the element content inserted in the modified cnts. about 3 mg of dried samples were used and the atomic percentage of nitrogen, carbon, hydrogen, and sulfur were determined. orr of modified samples was carried out on a potentiostat (auto-lab pgstat302n, netherlands). results and discussion xrd phase evaluation xrd patterns for nitrogen-co-sulfur-doped cnts are shown in figure 1. the xrd patterns show major peaks at around 2 ≈ 25 and 42°. these peaks are assigned to the hexagonal graphite structures (0 0 2) and (1 0 0), respectively [41]. obviously, when compared to untreated samples, the intensity of reflection of all doped cnts decreased, which may be attributable to the insertion of doping elements such as n and s. the substitution of a heteroatom (n or s) in the cnts generates defect sites and disrupts the carbon lattice; these findings demonstrated their less crystalline nature [42]. the raman spectra support this result, as seen in figure 2. 10 20 30 40 50 60 70 cnt0 in te n si ty , a -u . 2  / ° d-cnt1 d-cnt2 d-cnt3 0 0 2 25.6 1 0 0 42.7 53.150 0 4 t. h. h. elagib et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1664 5 figure 1. xrd pattern for the sample of (a) neat cnts, (b) d-cnt1, (c) d-cnt2, (d) d-cnt3. figure 2. raman spectra for the sample of (a) neat cnts, (b) d-cnt1, (c) d-cnt2, (d) d-cnt3 in particular, d-cnt2 and d-cnt3 exhibited lower intensity for both diffraction peaks than d-cnt1, which could be attributable to a longer doping time. furthermore, the (100) graphitic plane diffraction peak appears to have vanished in sample d-cnt3, possibly because oxidizing cnts provided more sites for chemical reactions. cnts can, in fact, be functionalized in an oxidative environment, therefore cnts with oh, =co, o, cooh functionalization are more effective for subsequent reactions [43]. raman spectroscopy analysis raman spectroscopy was used to explore further the sulfur-co-nitrogen functionalization of cnts (figure 3). two distinct peaks centered at 1350 and 1580 cm-1 are identified, corresponding to the disordered carbon (d-band) and the ordered graphitic carbon (g-band). the presence of a d-band with a high intensity indicates the existence of several defects and non-graphitic carbon in the doped cnts sample, which may have resulted from heteroatoms doping [44]. as can be seen, sample dcnt2 seems to have the maximum intensity when compared to the other samples. furthermore, as shown in figure s1, a corresponding endothermic peak on the dsc curve was identified, which is associated with the degradation/decomposition of the carbon framework. when compared to doped cnt, neat cnt has a lower decomposition temperature. the heat of the reaction rises when doping elements are introduced. these results can be a sign of interaction between cnt and heteroatoms. some time ago, it was pointed out that doping with heteroatoms can create chemically active regions, which can boost electrocatalytic activity [44]. the g-band of d-cnt1, dcnt2, and d-cnt3 is obviously shifted to a lower frequency by about 10 cm−1. in the literature, we observed a similar redshift behavior in sulfur-doped carbon nanotubes, indicating the electron delocalization effect caused by π-π interactions between the electron-donor doping precursors and cnt [35]. overall, the xrd and raman spectroscopy results suggest the nanocomposite is doped with n and s successfully by our one-step hydrothermal method and that variable doping levels are realized by varying the doping time [45]. in addition, clear evidence and additional conformation of nitrogen and sulfur incorporated into carbon nanotubes are provided in figure s2. 500 1000 1500 2000 2500 3000 cnt0 in te n si ty , a -u . raman shift, cm -1 d-cnt1 d-cnt2 d-cnt3 http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 00(0) (2023) 000-000 heteroatom-doped carbon nanotubes 6 furthermore, the d/g peak intensity ratios (id/ig) of d-cnt1, d-cnt2, and d-cnt3 are 1.75, 1.43, and 1.30, respectively. due to the fact that doping precursors wrapped around the surface of cnts and covered the partial defect sites, the id/ig ratio was slightly reduced [35]. sem morphological observation figures 3 show sem images of doped-cnts. as can be observed, d-cnt2 and d-cnt3 samples contain more clearly irregular nanoparticles, figure 3b and 3c. this finding indicated their porous nature and high surface area [41]. several corners and edges are visible on the surfaces of d-cnt2 and d-cnt3, which could become active sites of oxygen adsorption in the orr process. d-cnt2 has a significant number of pores in its morphology and appears to have a considerably looser porous structure, which helps to improve its orr activity [46]. to the best of our knowledge, the presence of porous carbons, which give an enhanced accessible surface area for electrolyte ions, is principally responsible for the improved performance of the working electrode [44]. furthermore, in samples d-cnt2 and d-cnt3, the particles appear densely stacked and agglomerated, implying that agglomeration occurred when amino acids were employed [22,47]. the overlapping and agglomeration behavior leads to forming complex three-dimensional stacked structures [34]. figure 3 − sem images with different magnifications of (a) neat cnts, (b) d-cnt1, (c) d-cnt2, (d) d-cnt3 elemental analysis the data from the (chns) analysis are shown in table 1 to confirm the incorporation of doping elements into cnts’ surface. as we can see, the d-cnt3 sample has a slightly lower c content than the other samples, but it has the highest n and s content of 9.38 and 15.20 %, respectively. this is most likely because of the significant doping amount of n and s atoms caused by oxidation of the cnt prior to the doping process, which is consistent with the xrd results. the result of eds color mapping (figure s3) further describes the heteroatoms doping of cnts. as seen the d-cnt3 has a higher s content than d-cnt2. nitrogen is not detected by sem-edx, presumably because nitrogen has a very faint response, making detection unreliable for most materials [48]. furthermore, eds may detect major elements with concentrations greater than 10 wt.% [49]. t. h. h. elagib et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1664 7 table 1. elemental content for the doped sample. sample content, wt.% nitrogen carbon hydrogen sulfur cnt0 0 88.41 0.102 0.105 d-cnt1 0.572 77.11 0.893 2.744 d-cnt2 1.428 75.695 4.722 5.389 d-cnt3 9.378 41.167 1.872 15.203 cyclic voltammetry – cv test the catalytic activity for oxygen reduction reaction of the doped cnts was measured using cyclic voltammetry (cv) with a three-electrode system (working electrode, counter electrode, and reference electrode) in 1.0 m koh aqueous electrolyte with potential between − 0.6 and 0.1 v at a scanning rate of 1 and 10 mv s-1. the working electrode was prepared without the addition of any binding materials. the following procedure was used to prepare all working electrodes; 4 mg of dcnts were dispersed in 500 µl alcohol and sonicated for 30 minutes. 1 to 2 µl of dispersion was loaded to the electrode’s surface and dried. typical cyclic voltammogram curves of doped cnts cathode/catalyst for the first two or three cycles are displayed in figure 4. on the cv curves of the d-cnt2 and d-cnt3 samples, a significant orr peak was observed. d-cnt2 and d-cnt3 have orr onset potentials of −0.30 and −0.37 v, respectively, while the reduction (cathodic) peaks are at −0.26 and −0.35 v, respectively. the reduction peak of dcnt2 and dcnt3 marginally changed to −0.24 and −0.39 v, respectively, when the scanning rate was increased to 10 mv s-1. these findings demonstrated that d-cnt2 catalysts have stronger orr catalytic performance than d-cnt3 catalysts. furthermore, the higher onset potential of d-cnt2 than d-cnt3 implies that the orr occurs more easily on its surface [34]. figure 4. cyclic voltammogram variation of specific capacitance with altering scan rates of the proposed catalysts (a) d-cnt2, scan rate 1 mv s-1; (b) (d-cnt2, scan rate 10 mv s-1; (c) d-cnt3, scan rate 1 mv s-1; (d) d-cnt3, scan rate 0.010 v s-1 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 c u r r e n t, m a potential, v cycle 1 cycle 2 cycle 3 reductive scan oxadative scan (a) (b) (c) (d) c u r r e n t, m a potential, v cycle 2 cycle 3 oxadative scan reductive scan c u r r e n t, m a potential, v cycle 1 cycle 2 cycle 3 oxadative scan reductive scan c u r r e n t, m a potential, v cylcle 1 cycle 2 cycle 3reductive scan oxadative scan http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 00(0) (2023) 000-000 heteroatom-doped carbon nanotubes 8 because of the large number of holes and edges, the dcnt2 electrode has increased activity and a positively shifted onset potential compared to the dcnt3 electrode [50]. to our knowledge, doping with heteroatom can improve the electron-transfer kinetics and consequently the catalytic activity [34]. besides, due to a unique electron distribution, the synergistic effect of dual-doping with two dopant elements of different electro-negativities can enhance catalytic activity [51]. although d-cnt3 contains more n and s than d-cnt2, the latter demonstrated higher orr catalytic activity, which could be attributed to oxygen-containing functional groups being primarily eliminated below 400 °c [34]. as a result, the reduced oxygen functional group of d-cnt3 during the hydrothermal reaction slightly inhibits orr activity. according to the raman analysis, another impact could be the wrapping of doping precursors on the surface of ocnts and the coverage of partial defect sites. additionally, because heteroatom doping can alter surface activity via conjugation between a lone pair of electrons and the π-system of the carbon lattice, it has the potential to generate further pseudo-capacitance improved electrocatalytic activity [22]. using the cv data, the following equation was used to calculate the variation of specific capacitance with changing scan rates [52]: p 2 12 ( ) a c mk e e = − (1) where; a is the cv loop's enclosed area and m is the mass of material at the working electrode, k is the scanning rate, and (e2 – e1) is the potential window (i.e., the entire voltage range through which the electrode system was tested for electrochemical behavior). table 2 compares the specific capacitance of each doped sample. as shown, cp is noticeably higher when the scanning rate is low (1 mv s-1) and higher in d-cnt2 samples than in d-cnt3 samples, reflecting their effective porosity. to the best of our knowledge, determining specific capacitance at differing scan rates is an essential step in understanding the inherent properties of any electrode material, particularly porosity influences ionic diffusion [52]. the analysis suggests that the dual-doped cnt can be used as an electrocatalyst, and d-cnt2 appears more capacitive than the other sample. table 2. specific capacity cp for two differing scanning rates dual–doped samples scan rate, mv s-1 specific capacity, f g-1 d-cnt2 1 214.12 10 91.14 d-cnt3 1 197.70 10 124.18 conclusions the nitrogen and sulfur dual-doped carbon materials were successfully synthesized with various morphological aspects through a hydrothermal method using l-cysteine and thiourea as precursors. after freeze drying, the resulting d-cnt2 and d-cnt3 exhibited a porous structure. the examination of elemental content proved the existence of sulfur and nitrogen functional groups in cnts after dual heteroatoms doping. the finding of the electrochemical characterization of the proposed catalysts demonstrates the great orr activity, which can be attributed to the synergistic effect of dual-doping with dopant elements of different electro-negativities, in addition to the presence of numerous active sites and effective porosity. besides, heteroatom doping generated the electrochemical capacitance characteristics. the proposed d-cnt2 catalyst achieved stronger orr catalytic performance and higher specific capacitance at a lower scanning rate than d-cnt3 catalysts. t. h. h. elagib et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1664 9 acknowledgements: the authors gratefully acknowledge the support and funding provided by the postdoctoral scholarship research program of the islamic development bank (isdb), ksa (grant no. 600040856) and the project of prof. md zahangir 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1664 https://doi.org/10.1016/j.mee.2020.111400 https://creativecommons.org/licenses/by/4.0/) modeling the velocity profiles in vanadium redox flow batteries-serpentine flow field http://dx.doi.org/10.5599/jese.1660 553 j. electrochem. sci. eng. 13(3) (2023) 553-561; http://dx.doi.org/10.5599/jese.1660 open access : : issn 1847-9286 www.jese-online.org original scientific paper performance of magnetized tool in electrochemical micromachining on scrapped alloy wheel matrix composite venugopal palaniswamy1,  and thanigaivelan rajasekaran2 1department of mechanical engineering, muthayammal college of engineering, rasipuram 637408, india 2akt memorial college of engineering and technology, kallakurichi, tamil nadu606 213, india corresponding author:  tvelan10@gmail.com received: january 7, 2023; accepted: march 9, 2023; published: march 13, 2023 abstract hybrid machining processes play an important role in traditional and non-traditional machining processes. adding the magnetic field effect in electrochemical micromachining (ecmm) improves the machining efficiency and accuracy of the micro-hole machined. the process parameters, namely, voltage, duty cycle and electrolyte concentration were used as input parameters, while machining rate (mr) and overcut (oc) are performance measures considered in this research. the workpiece was a scrapped alloy wheel matrix fabricated with a stir-casting process reinforced with alumina (al2o3). the edax and sem image study was performed to understand the composition and surface quality of the machined workpiece, respectively. the micro-hole without magnetic field effect was also conducted to understand the possible advantage of the magnetic field applied in ecmm. keywords aluminum metal matrix composite; magnetic field; machining rate (mr); machining overcut (oc) introduction the industrial usage of aluminum metal matrix composites (al-mmcs) is increasing day by day due to their unique properties compared to alloy materials. al-mmcs have several advantageous properties, such as a high strength-/stiffness-to-weight ratio, high resistance to fatigue, excellent wear, corrosion, and temperature resistance. al-mmcs are widely used in the aerospace, defense, and automotive industries, where they must be machined to the correct size and shape. the hybrid mmc is more difficult to machine than traditional materials, primarily because of the increased hardness and strength provided by the reinforced particles. in such a case, electrochemical machining (ecm), working with tool and workpiece electrodes, electrolyte and voltage supply, could be efficiently applied 1,2. http://dx.doi.org/10.5599/jese.1660 http://dx.doi.org/10.5599/jese.1660 http://www.jese-online.org/ mailto:tvelan10@gmail.com j. electrochem. sci. eng. 13(3) (2023) 553-561 magnetized tool in electrochemical micromachining 554 ao et al. [1] have considered a new-generation ecm process for machining sic particle-reinforced al-mmcs. the study revealed that a higher voltage supports a large depth in a workpiece. the surface roughness relies on electrolyte choice and voltage type and level. ibrahim et al. [2] have machined al-mmcs through electrochemical micromachining (ecmm) and optimized ecmm parameters. the optimized combination for radial overcut (oc) was 10 g l-1 of electrolyte concentration, voltage of 10 v, and inter-electrode gap of 0.3 mm. also, the electrolyte concentration of 30 g l-1, voltage of 18 v, and inter-electrode gap of 0.2 mm were found as the optimal parametric combination for the material removal rate (mrr). prakash et al. [3] have varied voltage, electrolyte concentration, duty cycle, and tool feed rate to optimize mr, radial oc, surface roughness and wear on tool electrode. the obtained results coincide with the response surface method result for a surface roughness value of 0.4 μm. kunar et al. [4] have reused a textured tool in ecmm and documented that the electrolyte concentration of 10 g l-1, pulsed frequency of 190 khz produces high quality micro circular pattern with higher machining accuracy. arul et al. [5] have used an al-mmc electrode to machine copper workpiece and found that the al-mmc electrode can be used for producing lesser oc holes. maniraj et al. [6] have developed a furnace slag-reinforced al-mmc sand machine using ecmm and studied the effect of process parameters on machinging rate (mr) and oc. l18 mixedlevel orthogonal array was used for executing the experiments. it was found that the composition of ground granulated blast furnace slag (ggbs) plays a major role in affecting the machining performance. in another paper, maniraj et al. [7] studied the ecmm performance with a heated electrode on mr, conicity factor (cf) and oc. they reported that the heated electrode improves the mr by 88.37 %, reduces the oc by 37.03 % and reduces the cf by 33.33 % compared to the ordinary electrode. generally, machinability studies using ecmm on metal matrix composites are sparse. moreover, the use of a magnetized tool in ecmm on scrapped alloy wheel matrix composites is of the first kind [8-13]. in this research, the ecmm input parameters are varied and the effect of magnetized tool electrode on mr and oc is studied. moreover, the composition of the casted al-mmc is studied using edax and surface quality studies are performed using the sem technique. experimental figure 1 shows the experimental setup used for the ecmm process. the setup consisted of a pulsed power supply unit and machine structure. figure 1. ecmm machine v. palaniswamy and t. rajasekaran j. electrochem. sci. eng. 13(3) (2023) 553-561 http://dx.doi.org/10.5599/jese.1660 555 the machine structure consists of 3 angle plates and one vertical plate. the stepper motor is housed on one angle and the remaining two angle plates support the tool feeding arrangement. the tool-feeding arrangement consists of a sliding rod and lead screw. the tool holder will slide up and down in the sliding rod powered by a stepper motor. the stepper motor is programmed to rotate at 6° per pulse. the electrode gap is the gap between the tool electrode face and the workpiece surface, which is controlled by stepper motor movement. the tool electrode holder is made up of copper material and a negative power supply is provided to the tool through the tool holder. the commercially available ten numbers of hollow n 52 grade permanent neodymium magnets were used for producing the magnetic flux. these magnets are attached to the tool holder, as depicted in figure 2. figure 2. sketch of permanent magnet with the tool the stainless-steel electrode of diameter 480 µm is used as the tool electrode. the chemical composition of the electrode is 58.80 % nickel, 12.32 % tungsten, and 11.19 % aluminum with chromium, iron and cobalt [14].the electrode is coated with epoxy to prevent stray current effects [15]. the electrolyte was prepared using sodium nitrate (nano3) salt mixed with distilled water. different concentrations of nano3 brine solution are prepared (15-35 g l-1) and used for experiments. the electrolyte is circulated to the machining through a pump and filter arrangement. the debris produced during the machining process was filtered using micro-level filters. the workpiece was prepared using the scrapped alloy wheel and aluminum oxide. honda car scrapped wheels of the chemical compositions copper, silicon, iron, zinc, tin, lead, titanium and aluminum of 0.09, 11.13, 2.92, 0.06, 0.19, 0.29, 1.70 and 83.62 wt.%, respectively, were used as a base metal. the scrapped alloy wheel was melted in a stir-casting setup and aluminum oxide was used as reinforcement. the developed al-mmc was cut into sheets having a thickness of 300 µm for ecmm. the content of the elements listed in table 1 shows the quantitative composition of the workpiece. comparing the composition of casted workpiece and scrapped alloy wheel, it appears that in the casted workpiece, aluminum is reduced to 30.36 wt.%, while oxygen is newly present with 13.87 wt.%. table 1. composition of the workpiece element content, wt % content, at.% error, % c 8.51 16.61 16.33 o 13.87 20.33 9.59 na 1.54 1.57 10.9 mg 5.00 4.83 5.92 al 30.36 26.37 4.42 si 31.88 26.61 5.85 s 0.04 0.03 18.12 cr 0.85 0.38 14.54 fe 4.64 1.95 5.11 ni 3.30 1.32 7.27 http://dx.doi.org/10.5599/jese.1660 j. electrochem. sci. eng. 13(3) (2023) 553-561 magnetized tool in electrochemical micromachining 556 the electrochemical cell is formed by stainless steel and workpiece electrodes submerged in the electrolyte solution. by applying a voltage difference between the stainless steel electrode (cathode) and workpiece (anode), the electrolysis process is ensured. during the electrolysis process, metal ions from al-mmc workpiece anode dissolve and move towards the bulk electrolyte and the stainless-steel cathode. at the same time, negative ions from the electrolyte are attracted by the workpiece anode. the hydrogen gas bubbles are released at the cathode by water breakdown during electrolysis. produced ohions combine with metal ions and form insoluble metal hydroxides, precipitating and removing metal ions from the system. the formation of metal hydroxides is a chemical process occurring out of electrodes and does not affect the total current flowing between electrodes. for experiments, the input variables, namely voltage, electrolyte concentration and duty cycle along with permanent magnet field effect, are investigated on mr and oc. during machining, the frequency was kept constant at 50 hz. the total time was calculated from the frequency of 50 hz, equal to 20 milliseconds, and the duty cycle is the ratio of pulse on time to the total time. table 2 shows the experiment parameters and output performance. during the machining process, the time taken for machining a through hole is noted in seconds. table 2. experiment levels and output performance s.no voltage, v duty cycle, % electrolyte concentration, g l-1 mr, µms-1 oc, µm 1 6 90 35 0.1111 800 2 7 90 35 0.1190 840 3 8 90 35 0.1315 900 4 9 90 35 0.1562 940 5 10 90 35 0.2500 970 6 10 50 35 0.1041 820 7 10 60 35 0.1282 860 8 10 70 35 0.1111 910 9 10 80 35 0.1851 950 10 10 90 35 0.2380 990 11 10 90 15 0.1428 870 12 10 90 20 0.1562 890 13 10 90 25 0.1785 910 14 10 90 30 0.2000 930 15 10 90 35 0.2777 940 the thickness of the workpiece sheet was measured using a micrometer and recorded in m. the mr is calculated by dividing the thickness of the workpiece sheet by the machining time. the oc is calculated by finding the difference between a tool and a machined hole diameter. the electrode diameter is measured in m using a micrometer and the machined hole diameter is using the optical microscope image data. results and discussion effect of voltage along with magnetic field on mr and oc figure 3 shows the effect of voltage and magnetic field on the machining rate and mr and oc. it is obvious from figure 4 that mr and or increase with the voltage increase. the mr increases firstly slowly from 6 to 8 v and then steeply from 8 to 9 v. the drastic increase in the mr is observed from 9 to 10 v. the increase in voltage enhances the current density required for machining, which is attributed to the increasing trend of mr. introducing a magnet creates a lorentz force, a mixture of v. palaniswamy and t. rajasekaran j. electrochem. sci. eng. 13(3) (2023) 553-561 http://dx.doi.org/10.5599/jese.1660 557 electric and magnetic force, which act on charged particles. the arrangement of a magnet on the tool electrode induces a lorentz force, resulting in increased mobility of ions and improved dissolutions process. in ecmm, the material removal takes place due to the dissolution of ions from the anode and with the aided magnetic force, the mr increases. the drastic increase in mr for higher voltage is due to the fact that at a higher voltage level, the current density required for machining is higher and along with lorentz force, higher mr is observed. as seen in figure 3, oc tends to increase with the voltage but in a weaker manner than mr. during ecmm, the removal of dissolute from the electrolyte is necessary for efficient machining. normally in ecmm, the oc produced is three times the electrode diameter and oc observed in al-mmcs is higher compared to alloys [8-10]. the oc of the micro-hole mainly depends on the current density, improper evacuation of debris, bridging of the electrode by debris and inadequate tool tip insulation. the presence of the magnetic field effect lines up the debris and dispels it from the machining zone, which can be attributed to lesser oc. figure 4 shows the micro-hole machined using the magnetic field effect with less delaminated areas and circularity. figure 3. mr and oc vs.voltage figure 4. micro-hole on al-mmcwith magnetic effect 0 200 400 600 800 1000 1200 0,00 0,05 0,10 0,15 0,20 0,25 0,30 5 6 7 8 9 10 11 12 o v e rc u t, µ m m a ch in in g r a te , µ m s1 voltage, v machining rate overcut http://dx.doi.org/10.5599/jese.1660 j. electrochem. sci. eng. 13(3) (2023) 553-561 magnetized tool in electrochemical micromachining 558 effect of duty cycle with magnetic effect on mr and oc figure 5 shows the effect of the duty cycle on mr and oc. an increase in the duty cycle increases the mr since an increase in the duty cycle enhances the pulse on the current required for machining. the long-duration availability of pulse on current can be attributed to higher mr. for the duty cycle of 50 to 70 %, the increasing and decreasing trend in mr is observed. the mr increases significantly for higher duty cycles (70 to 90 %), which is caused by the fact that the current density is higher at higher duty cycles. this phenomenon increases the mr along with the magnetic field effect. the rate of change of oc is more with the duty cycle and the presence of a magnetic field reduces the oc irrespective of faster dissolution due to the prolonged pulse time on time. figure 6 shows the micro-hole with lesser oc, micro cracks and delaminated surfaces. figure 5. effect of duty cycle on mr and oc figure 6. microhole machined at 10 v, 90 % and 35 g l-1 0 200 400 600 800 1000 1200 0,00 0,05 0,10 0,15 0,20 0,25 0,30 40 50 60 70 80 90 100 o v e rc u t, µ m m a ch in in g r a te , µ m s1 duty cycle, % machining rate overcut v. palaniswamy and t. rajasekaran j. electrochem. sci. eng. 13(3) (2023) 553-561 http://dx.doi.org/10.5599/jese.1660 559 effect of electrolyte concentration on mr and oc mr and or increase with an increase of electrolyte concentration, which is evident from figure 7. the rate of change in mr is less for concentrations up to 30 g l-1, and then increases drastically for higher electrolyte concentrations. at higher electrolyte concentrations, more ions are available for machining, and the presence of lorentz force accelerates the machining performance. the presence of lorentz force induces magneto-hydrodynamic convection in the electrolyte. figure 7. mr and oc vs. electrolyte concentration this phenomenon slightly increases the temperature of the electrolyte and electrolyte circulation in the machining zone. moreover, the presence of a magnetic field alters the trajectory motion of charged ions. this motion of ions induces the stirring effect in the electrolyte. with ethe ffect of the magnetic field, the charged ions will move along spiral lines and more ions reach the machining zone, as shown in figure 8 [11]. higher growth of mr is observed for the electrolyte concentration of 30 to 35 g l-1. the rate of change of oc, however, is high for the electrolyte concentration of 15 to 30 g l-1. although the increase in electrolyte concentration results in much debris due to a higher dissolution rate, the presence of a magnetic field effect orients the debris away from the machining zone [12,13]. figure 9 shows the micro-hole machined on al-mmcs with lesser micro-cracks and stray current affected zone. few pores are witnessed due to the detachment of micro reinforcement particles during the dissolution process. figure 10 shows the micro-hole machined without a magnet and more debris attachment; delaminated surfaces and micro pits are observed on the micro-hole's circumference. figure 8. sketch of ion movement in magnetic field [11] 850 875 900 925 950 975 0,00 0,05 0,10 0,15 0,20 0,25 0,30 0,35 10 15 20 25 30 35 40 o v e rc u t, µ m m a ch in in g r a te .µ m s1 electrolyte concentration, g l-1 machining rate overcut http://dx.doi.org/10.5599/jese.1660 j. electrochem. sci. eng. 13(3) (2023) 553-561 magnetized tool in electrochemical micromachining 560 figure 9. micro-hole machined at 10 v, 90 % and 25 g l-1 figure 10. micro-hole machined without magnet field at 10 v, 90 % and 25 g l-1 conclusions the scrapped alloy wheel matrix with al2o3 reinforcement is produced and tested for the presence of elements. the ecmm experiments were planned using varying one parameter at a time with a permanent magnetic field effect. the mr increased rapidly for voltage levels of 9 to 10 v, while less rate of change of oc is observed for the voltage range of 6 to 10 v. duty cycle range of 70 to 90 %, along with the magnetic field effect, shows higher mr and high rate of change of oc. the electrolyte concentration of 30 to 35 g l-1 produces higher mr, while a high rate of change of oc is observed for the electrolyte concentration range of 15 to 30 g l-1. moreover, the micro-hole produced has lesser oc, delaminated area, stray current affected zone, micro cracks and micro-pits. the micro-hole machined without a magnetic field resulted in more debris attachment on the circumference of the 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license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1660 http://dx.doi.org/10.21533/pen.v10i3.3006 https://doi.org/10.1007/s12633-020-00434-0 https://doi.org/10.2298/ciceq210501036a https://doi.org/10.1007/s12666-019-01772-3 https://doi.org/10.1080/10426914.2019.1655153 https://doi.org/10.1080/10426914.2020.1750630 https://doi.org/10.1016/j.matpr.2020.03.190 https://doi.org/10.20964/2020.02.10 https://doi.org/10.1016/j.ces.2004.07.060 https://doi.org/10.1007/s12046-020-01493-0 https://doi.org/10.1007/s11581-022-04686-1 https://doi.org/10.2298/ciceq220731027p https://creativecommons.org/licenses/by/4.0/) an oxygen reduction reaction electrocatalyst tuned for hydrogen peroxide generation based on a pseudo-graphite doped with graphitic nitrogen https://dx.doi.org/10.5599/jese.1407 1009 j. electrochem. sci. eng. 12(5) (2022) 1009-1023; https://dx.doi.org/10.5599/jese.1407 open access : : issn 1847-9286 www.jese-online.org original scientific paper an oxygen reduction reaction electrocatalyst tuned for hydrogen peroxide generation based on a pseudo-graphite doped with graphitic nitrogen kailash hamal1, dipak koirala1, jeremy may1, forrest dalbec1, nolan nicholas2 and i. francis cheng1, 1department of chemistry, university of idaho, 875 perimeter dr, ms 2343, moscow, id 83844, usa 2nano lab, 22 bedford st. waltham, ma, 02453, usa corresponding author: ifcheng@uidaho.edu; tel: (208) 885-6387 received: june 14, 2022; accepted: august 26, 2022; published: september 5, 2022 abstract the carbon material, guitar (pseudo-graphite from the university of idaho thermolyzed asphalt reaction) can be doped with nitrogen in two prevalent forms. in a previous study n(py)-guitar had a predominance of pyridinic and pyrrolic moieties with no graphitic nitrogen. in this study n(g)-guitar contains a 9.7 % n atomic abundance, with that fraction consisting of 72.3 % graphitic, 23.7 % pyridinic, and 0 % pyrrolic nitrogen. the two materials allow for the examination of hypotheses regarding the importance of the three different nitrogen moieties in the oxygen reduction reaction (orr). in the previous investigation, the lack of graphitic nitrogen of n(py)-guitar gave a preferred pathway of 4eorr to h2o. in this investigation, n(g)-guitar gave a 2eorr pathway to h2o2. this was elucidated by current efficiency and hydrodynamic voltammetry studies. the high predominance of graphitic nitrogen confirms the hypothesis regarding 2evs. 4eorr pathways with n-doped carbon materials. n(g)-guitar was also evaluated for parasitic pathways for h2o2 production. at -0.95 v vs. ag/agcl the combination of current efficiency for h2o2 is 96 % in 0.05 m na2so4 with a production rate of 4.9 mg cm-2 h-1, is the highest reported in the literature. this indicates possibilities for water purification and treatment applications, which require 10 to 250 mg l-1, depending on conditions. keywords hydrogen peroxide; oxygen reduction reaction; electrocatalysis; n-doped carbon introduction hydrogen peroxide (h2o2) is a relatively eco-friendly and widely used oxidant in the water purification, chemical, and medical industries [1,2]. it is a greener oxidizing agent than chlorine in that it leaves no halogenated by-products such as trihalomethanes and haloacetic acids and is https://dx.doi.org/10.5599/jese.1407 https://dx.doi.org/10.5599/jese.1407 http://www.jese-online.org/ mailto:ifcheng@uidaho.edu j. electrochem. sci. eng. 12(5) (2022) 1009-1023 oxygen reduction reaction electrocatalyst 1010 effective over a wider ph range [3,4]. a partial barrier to wider implementation is that the predominant method of h2o2 production, the anthraquinone process, is highly energy-intensive and requires large quantities of extraction solvents [2,3]. furthermore, transportation issues also hinder its widespread application in water treatment. onsite generation of h2o2 through the electrochemical process offers minimization of environmental impact as it can be conducted under aqueous and ambient conditions with no hazardous materials or waste [5-7]. this process proceeds by a 2eelectrochemical process outlined in equation 1. o2 + 2h+ + 2e→ h2o2 e0 = 0.695 v, e0’ (ph 7) = 0.282 v (1) the challenges in the electrochemical production of h2o2 are improving slow kinetics and minimizing competing reactions. slow electrode kinetics gives significant overpotentials for equation 1 that require electrode potentials from -0.5 to -1.5 v vs. ag/agcl [8,9]. at these potentials, three other parasitic electrochemical reactions are possible. these include the 4eoxygen reduction reactions (orr) to water (equation 2), the hydrogen evolution reaction (her), (equation 3), and the 2ereduction of h2o2 to h2o (equation 4) [8-10]. o2 + 4h+ + 4e→ 2h2o e0 = 1.23 v, e0’ (ph 7) = 0.817 v (2) 2h+ + 2e→ h2 e0 = 0.00, e0’ (ph 7) = -0.413 v (3) h2o2 + 2h+ + 2e→ 2h2o e0 = 1.763 v, e0’ (ph 7) = 1.35 v (4) it is therefore incumbent that any electrocatalyst for equation 1 must be selective for this process. this has been noted by several investigators [9,11]. for this contribution, we demonstrate an electrocatalyst that is preferential for the reaction in equation 1 based on guitar (graphite from the university of idaho thermolyzed asphalt reaction). this carbon material consists of an 85/15 mole ratio of sp2/sp3 carbon with a crystallite grain size of 1.5 nm [12,13]. the interlayer d-spacing of 350 pm is slightly expanded over graphite’s 335 pm. it is a pseudo-graphite in that it has visual and microscopic features that resemble graphite but has differing properties. distinguishing characteristics over graphite and other carbon allotropes is that guitar has fast heterogeneous electron transfer (het) kinetics with the fe(cn)64-/3probe at the basal and edge planes while maintaining excellent corrosion resistance [12-14]. the 3-volt aqueous potential window is among the largest reported. guitar is grown using a chemical vapor deposition process, which allows for the fabrication of electrodes ranging from the nanoto macro-sizes, depending on the substrate [12,15,16]. a significant consideration is the tunability of the electrocatalyst for performance and selectively between the reactions of equations 1-4. nitrogen doping of carbon materials is associated with the enhancement of electrocatalysis for orr [17,18]. the c-n bond is believed to facilitate the adsorption of o2 [19,20]. predominate forms of n in sp2 carbon lattices are pyridinic, pyrrolic, noxides, and graphitic moieties shown in figure 1. figure 1. nitrogen moieties in the graphite lattice k. hamal et al. j. electrochem. sci. eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1011 in computational studies, graphitic n is hypothesized to drive the 2eprocess of equation 1 [21]. however, experimental investigations have only been conducted on mixed nitrogen moiety carbon materials, therefore definitive experimental evidence has yet to be gathered [21-23]. pyridinic and pyrrolic moieties have been associated with the 4etransfer orr (equation 2) useful in fuel cell applications [18]. the n-oxide moiety is not associated with any specific orr pathway. in a previous investigation, a specific nitrogen doping method led to a guitar variant with a 0.9 % total nitrogen atomic abundance giving fractions of 0 % graphitic n, 46.0 % pyridinic, 41.3 % pyrrolic, and 12.4 % n-oxide. this electrocatalyst, n(py)-guitar, proved to be durable and efficient for the 4etransfer orr to h2o [15]. an alternative doping method produced a nitrogen-doped pseudo-graphite which contains 9.7 % n [13]. this material, called n(g)-guitar, contains graphitic n as the predominant nitrogen moiety [13]. it is noteworthy that guitar is the only known carbon material that can be selectively synthesized for predominantly pyridinic/pyrrolic or with graphitic nitrogen moieties. this provides an opportunity to examine the role of these functionalities in the pathway outlined in equations 1 and 2. experimental materials and chemicals kfd graphite felt (slg carbon company, st. marys, pa, usa) was used as received. the reagents in this investigation were compressed ar(g) and o2(g) (>99.5 %, oxarc, wa, usa), soybean oil (walmart), paraffin (gulf wax), sulfuric acid (96.3 %, j.t baker chemical co, phillipsburg, nj, usa). potassium chloride was obtained from fisher scientific (waltham, nj, usa), and potassium ferricyanide was obtained from acros organics (morris plains, nj, usa). sodium sulfate (emd chemicals, germany), 30 % h2o2 (fisher scientific, belgium), and ethanol (99.5 %, pharmaco, ct, usa) were used without further purification. all aqueous solutions were prepared with deionized water passed through an activated carbon purification cartridge (barnstead, model d8922, dubuque, ia). electrode fabrication and electrochemical setup n(g)-guitar samples were synthesized using the method described in a previous study [13]. in short, acetonitrile vapors were mixed with n2 gas and directed into a muffled tube furnace (heated to 900 °c) containing the substrate, for 25 minutes. quartz wafers, kfd graphite felts, and ketjen black were used as substrates. working electrode areas were isolated, as described in the previous study [16]. this was conducted by covering the copper alligator clip to the potentiostat with paraffin wax so as to avoid corrosion of this ohmic contact. complete wetting of the electrode was conducted by washing with isopropanol followed by deionized water prior to use [24]. all electrochemical studies were conducted in a three-electrode setup cell with ag/agcl/3m kcl reference electrode using either a bioanalytical system cv-50 w (west lafayette, in, usa) or a gamry instruments interface 1000 potentiostat. the cell design is shown in figure 2. the cell consisted of anodic and cathodic chambers with a separator membrane composed of cellulose acetate on cellulose paper [25]. this design allows for the formation of h2o2 via reaction 1 without consumption at the anode through reaction 4. hydrodynamic experiments were carried out with a pine instrument glassy carbon rotating electrode (rde) (grove city, pa, usa). for that study, n(g)-guitar was deposited on ketjen black particles and drop-cast onto the rde disks. this follows a procedure developed in a previous investigation [15]. https://dx.doi.org/10.5599/jese.1407 j. electrochem. sci. eng. 12(5) (2022) 1009-1023 oxygen reduction reaction electrocatalyst 1012 figure 2. electrochemical cell design for h2o2 generation h2o2 analysis this was conducted by a colorimetric method [26]. potassium titanium oxide oxalate dihydrate, (sigma-aldrich) forms an aqueous complex with h2o2, which has an optical absorbance maximum at 400 nm. a beer’s law-based calibration curve was established with h2o2 (pharmaco, ct, usa) standardized with kmno4 (j.t. baker chemical co, phillipsburg, nj, usa) and sodium oxalate (fisher scientific, waltham, nj, usa). results and discussion material characterization summary this material was fully characterized in a previous study [13]. in short, the x-ray photoelectron spectrograph (xps) of n(g)-guitar flakes indicated nitrogen, oxygen, and carbon content of 9.7, 3.9 and 86.4 %, respectively (see table 1). the deconvolved nitrogen xps peak revealed 23.7 % pyridinic n (398.2 ev), 60.5 % graphitic n-center (400.98 ev), 11.8 % graphitic n-valley (403.0 ev), and 4 % noxides (405.3 ev). this material was notably devoid of pyrrolic nitrogen. raman spectrographic analysis of n(g)-guitar exhibited an id/ig ratio of 1.66, indicating more defects from the inclusion of nitrogen relative to pristine guitar (id/ig of 1.15). the xrd analysis of n(g)-guitar indicated a similar degree of nano-crystallinity as pristine guitar (3.3 nm vs. 2.9 nm grain size) [13]. the composition of n(py)-guitar is also described in table 1. most noticeable is the lack of graphitic n in this electrocatalyst. table 1. content of total atomic nitrogen and deconvolved nitrogen moiety of n(py)-guitar and n(g)-guitar, determined via xps content of total atomic nitrogen, % deconvolved nitrogen moiety, % graphitic pyrrolic pyridinic n-oxides reference n(py)-guitar 0.9 0.0 41.3 46.0 12.4 [15] n(g)-guitar (used in this study) 9.7 72.3 0.0 23.7 4.0 [13] formation of the electrocatalyst and surface area the n(g)-guitar electrocatalyst was grown onto a kfd graphite felt substrate using chemical vapor deposition (cvd), as described in a previous study [13]. sem micrographs of bare kfd fiber and n(g)-guitar/kfd are shown in figure 3. the fiber diameter of kfd (7.4 ± 0.9 µm (n=10)) is within k. hamal et al. j. electrochem. sci. eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1013 the range reported in the literature [16,27]. deposition of n(g)-guitar increases this diameter to 10.04 ± 0.9 µm (n = 10). the surface area of 62.2 cm2 per cm2 of geometric area for n(g)-guitar/kfd was calculated assuming the electrode comprises cylindrical fibers [16,24]. this value agrees with cyclic voltammetric (cv) studies of fe(cn)64-/3using the randles-ševčik equation (equation 5) below, where ip, n, a, c (10-6 mol cm-3), d (7.26×10−6 cm2 s-1), and v are peak current, number of electrons transferred, area, concentration, diffusion coefficient and potential sweep rate, respectively [16]. ip = 268,600 n3/2ac (dv)1/2 (5) figure 4a shows the overlayed cv plots at multiple scan rates. a plot of ip vs. v1/2 in figure 4b indicates semi-infinite linear diffusion with the slope indicating an electrochemically active surface area (ecsa) of 59.0 cm2 per cm2 of geometric area. ecsa for guitar/kfd and bare kfd are 50.6 and 61.5 cm2 per cm2 of geometric area, respectively [16]. figure 3. a) scanning electron micrographs (sem) micrographs of as obtained kfd felt. b) n(g)-guitar coated kfd. the average diameter kfd fiber is 7.4± 0.9 µm (n = 10) and for n(g)-guitar/kfd is 10.0 ± 0.7 µm (n = 10). these measurements were estimated with imagej software potential, mv vs. ag/agcl v1/2 / (v / s)1/2 figure 4. a) cyclic voltammograms (cv) obtained at various scan rates of 1 mm fe(cn)6 3-/4 in 1 m kcl. (b) cathodic peak current (ip) vs. square root of scan rate for the estimation of the electrochemically active surface area (ecsa) from the randles-ševčik equation. the best fit line indicates semi-infinite linear diffusion cyclic voltammetric studies the electrocatalysts were examined for orr activity by cv in 0.05 m na2so4 at 50 mv s-1 under o2 and ar purged conditions, as shown in figure 5. under o2 purge, guitar/kfd exhibits two peak potentials (ep’s) of -0.65 and -1.10 v. this electrolyte is a wastewater surrogate often used in literature [28,29]. based on literature, this is an indication of the 2eprocess orr through equation (1) https://dx.doi.org/10.5599/jese.1407 j. electrochem. sci. eng. 12(5) (2022) 1009-1023 oxygen reduction reaction electrocatalyst 1014 and the subsequent consumption of h2o2 through the reaction of equation 4 [30-32]. in the case of n(g)-guitar/kfd, there is only a single ep at -0.77 v, which qualitatively indicates 2etransfer giving h2o2 in the orr of equation (1), without consumption of h2o2 through equation 4, this will be examined in detail below. also notable is the relatively large peak current (ip) of 7.0 ma cm-2at 50 mv s-1, which is the highest reported in the literature (0.74.5 ma cm-2) [30-33]. the ip based on ecsa is 0.12 ma cm-2. the cvs for both electrode materials under ar purge indicate an overpotential of about 1 v for the her (equation 3). figure 5. cyclic voltammograms at 50 mv/s in 0.05 m na2so4 under o2 and ar saturated conditions for n(g)guitar/kfd and kfd electrodes. the peak potentials (ep) for guitar/kfd are -0.65 and -1.10 v and for n(g)-guitar is -0.77 v. the hydrogen evolution reaction (her) for the scans is also indicated h2o2 production is the major pathway for the n(g)-guitar electrocatalyst the electrocatalyst was evaluated for generation of h2o2 by controlled potential electrolysis (cpe) at potentials ranging from -0.35 to -1.50 v vs. ag/agcl for 30 min in o2 saturated 0.05 m na2so4(aq). the saturation is ensured by bubbling o2 across the electrode surface, which also aids in mass transport. the n(g)-guitar cathode ecsa is 260 cm2, corresponding to a geometric area of 4.4 cm2. control electrodes of the same geometric areas include guitar/kfd (esca 223 cm2) and kfd (ecsa 270 cm2). selected current-time profiles with the n(g)-guitar/kfd cathode are shown in figure 6a. all chronoamperograms remained at a steady state during the electrolysis for each potential. this is an indication of constant mass transport of o2 to the electrode and stable electrocatalyst performance. figure 6c shows the time profile for h2o2 concentration at the applied potential of -1.50 v for n(g)-guitar/kfd. data was obtained by taking 0.5 ml aliquots every 5 min for h2o2 analysis. this system obeys zero-order kinetics with the observed linear increase in h2o2 concentration in figure 6c. these results show the typical behavior in comparison to other electrochemical h2o2 generation studies in the literature [34,35]. the performance of the control electrodes, guitar/kfd and kfd, are also shown in figure 6c. as can be observed through the slopes, the n-doped material is a much more efficient electrocatalyst than either of the control electrodes. figure 6b illustrates the effect of driving potential on the rates of h2o2 production (µmol cm-2 h-1). the n(g)-guitar/kfd electrode increased the h2o2 generation rate proportionally with driving potential from the range -0.35 to -1.50 v. both control electrodes decreased in production rate beyond -0.95 v. it is surprising that guitar/kfd is much less efficient than bare kfd. this may be from the consumption of h2o2 k. hamal et al. j. electrochem. sci. eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1015 through reaction 4 and may be observed in the 2nd cv wave at ep -1.1 v for guitar/kfd in figure 5. further evidence is presented in the current efficiency studies below. figure 6. (a) current-time curves recorded in o2 saturated 0.05 m na2so4 at various constant electrode potentials. (b) amount of h2o2 generated during constant potential analysis and normalized for geometric area. (c) concentration of measured h2o2 with time. (d) plot of current efficiency for h2o2 production at different applied potential under o2 purge, the controlled potential electrolysis gave ph shifts from ph 7.0 to approximately ph 12. the final ph is expected from the reactions of equations 1, 2 and 4. a pourbaix diagram in figure 7 illustrates that this shift in ph does not affect the spontaneity of reactions electrode potentials from -0.35 v to -1.50 v vs. ag/agcl. the current efficiency (ceh₂o₂/o₂) for h2o2 production was examined for each of three electrode systems of this study. this is described in equation 6 where n is a number of electrons transferred in the balanced half-reaction, f is faraday’s constant, v is a volume of the cell, and i is current [36]. 2 2 2 2 2 h o h o /o 0 100 d t nfvc ce i t =  (6) figure 6d demonstrates that n(g)-guitar/kfd maintains excellent ce h₂o₂/o₂ at all potentials. at the highest driving potential of -1.5 v, it has a ce h₂o₂/o₂ of 88 %, at -1.2 v it is 95 % (also see table 2, column d). the ce h₂o₂/o₂ for both kfd and guitar/kfd are notably lower, at 38 % and 29 %, respectively at -1.5 v vs. ag/agcl. this shows that both control electrodes increase h2o2 production rates from -0.35 to 0.95 v with noticeable drops at higher driving potentials. this may be from the consumption of h2o2 via reaction 4 and the parasitic her. these issues do not affect n(g)-guitar/kfd performance, as the h2o2 production rate increases with increasing cathodic potential. https://dx.doi.org/10.5599/jese.1407 j. electrochem. sci. eng. 12(5) (2022) 1009-1023 oxygen reduction reaction electrocatalyst 1016 figure 7. pourbaix diagram for the reactions in equations 1-4. the shaded region are the ph and potential conditions of this study figure 6d demonstrates that n(g)-guitar/kfd maintains excellent ceh₂o₂/o₂ at all potentials. at the highest driving potential of -1.5 v, it has a ceh₂o₂/o₂ of 88 %, at -1.2 v it is 95 % (also see table 2, column d). the ceh₂o₂/o₂ for both kfd and guitar/kfd are notably lower, at 38 and 29 %, respecttively at 1.5 v vs. ag/agcl. this shows that both control electrodes increase h2o2 production rates from -0.35 to 0.95 v with noticeable drops at higher driving potentials. this may be from the consumption of h2o2 via reaction 4 and the parasitic her. these issues do not affect n(g)-guitar/kfd performance, as the h2o2 production rate increases with increasing cathodic potential. in general, n(g)-guitar operates at a higher ce relative to literature electrocatalysts. figure 8 illustrates the competitiveness of this electrocatalyst at the various applied potentials relative to the literature on non-metal electrocatalysts. also included are the control surfaces of guitar and bare kfd at their maximum ce and production rate. figure 8. plot of h2o2 production rate and current efficiency for the n(g)-guitar orr electrocatalyst. the applied potentials are indicated. the highest rates of h2o2 production for guitar and kfd are included along with literature electrocatalysts [34,35,37-49] k. hamal et al. j. electrochem. sci. eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1017 this n-doped pseudo-graphite electrode operates at 95 to 100 % ce h₂o₂/o₂ at potentials ranging from -0.35 to -1.2 v (figure 6d), while literature electrodes are at 45 to 95 % at similar potentials at lower h2o2 production rates [34,35,37-39]. this gives n(g)-guitar the ability to effectively operate at higher driving potentials without a loss in ce. at the highest potential of this study, -1.50 v vs. ag/agcl, this electrocatalyst operates at 88 % ceh₂o₂/o₂ with a production rate of 250 µmol h-1 cm-2 (8.5 mg h-1 cm-2). this is the highest ce reported for this relatively high rate. higher production rates have been reported in the literature, but they typically come at the cost of a lower ceh₂o₂/o₂ [38]. considering these performance metrics (ceh₂o₂/o₂ and h2o2 production rate), n(g)-guitar has the best combination of high current efficiency and h2o2 production rate [34,35,37,38]. the hydrogen evolution reaction is a minor pathway for the electrocatalyst the high ceh₂o₂/o₂ indicates that the major pathway for the electrocatalyst is through reaction 1. the extent of the her as a competing pathway is examined under the conditions for the 2eorr. previous studies of guitar and its variants showed that these materials are kinetically slow for her. generally, the overpotential is 1 to 1.5 v and is among the highest of all electrode materials [12,13]. the n(g)-guitar/kfd cathode was examined for the her by chronoamperometry in ar-purged 0.05 m na2so4 at -0.95, -1.20 and -1.50 v vs. ag/agcl. figure 9 shows the respective chronoamperograms for this study. in the absence of o2 the charge from those currents is proportional to total h2 production. these charges are summarized in table 2, column a. when compared the charge obtained under saturated o2 conditions in figure 6a and summarized in table 2 column b, the her is a minor component of overall cathodic current ranging from 0.2 % at -0.95 v to 7.8 % at -1.5 v (table 2 column c). figure 9. chronoamperograms recorded in ar purged 0.05m na2so4. the potentials and steady-state currents for the n(g)-guitar/kfd electrodes are described in the diagram assessment of the contribution of the 4eorr pathway to the overall cathodic current: rotating disk electrode studies the contribution of the 4eorr (equation 2) to the overall cathodic current was estimated through the measurement of n, the number of electrons transferred in the half-reaction. contributions to orr through the reaction in equation 2 occur with n > 2. to assess this characteristic, rotating disk electrode (rde) experiments were conducted. the value, n is obtained through the koutecky-levich (k-l) relationship in equation 7. symbols j, jk, and ω are the current density, https://dx.doi.org/10.5599/jese.1407 j. electrochem. sci. eng. 12(5) (2022) 1009-1023 oxygen reduction reaction electrocatalyst 1018 kinetic current density, and angular velocity, respectively. in the k-l equation, b is 0.62nfν1/6 co₂do₂ 2/3, where f is faraday’s constant, ν is the kinematic viscosity of the electrolyte (0.01 cm2 s-1), co₂ is the bulk concentration of oxygen (1.20×10 -6 mol cm-3) and do₂ is the diffusion coefficient of o2 in 0.050 m na2so4 (1.90× 10-5 cm2 s-1) [15,36]. 1/2 k 1 1 1 j j b = + (7) for the rde study, the graphitic nitrogen-rich n(g)-guitar electrocatalysts were deposited on ketjen black particles as described in the experimental and previous studies [15]. in previous investigations, pristine guitar had n = 2.6 to 2.8 and a variant of an n-doped guitar (n´-guitar) which had a predominance of pyridinic and pyrrolic moieties, was found to have n = 3.6 to 3.7 [15]. the background corrected rde linear sweep voltammograms (5 mv s-1) from 400 to 2400 rpm are shown in figure 10a. between the potential range of -0.7 to -1.5 v vs. ag/agcl, this electrocatalyst was found to have n = 1.99 (see figure 10b). this, along with ce data of section 3.3, strongly indicate that this electrocatalyst has a strong preference for reaction in equation 1, with reaction 2 being a negligible pathway. figure 10. a) rotating disk electrode (rde) linear sweep voltammograms recorded at different rotating rates in 0.05 m na2so4. b) the number of electrons transferred from the koutecky-levich equation over the potential range of -0.7 to -1.5 v. the average electrons transferred (n) over the entire potential range with one standard deviation is shown other pathways that lead to the loss of h2o2 the pathways that lead to the loss of h2o2 produced through the 2eorr are the electroreduction of h2o2 giving h2o (equation 4) and disproportionation through the reaction in equation 8 [10]. 2h2o2 → o2 + 2h2o (8) the two reactions cannot be distinguished from one another and therefore are treated together. equation 2, the 4eorr, can also be considered in this analysis. the contribution of these three pathways that lead to the loss of h2o2 outlined in equations 2, 4, and 8 can be inferred from equation 9, and summarized in table 2, column e. pathways for h2o2 consumption, % = 100 – ceh₂ – ceh₂o₂/o₂ (9) the three values in column e are not statistically different from each other, it is possible that the disappearance of h2o2 is invariant to the applied potentials. k. hamal et al. j. electrochem. sci. eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1019 table 2. breakdown of major pathways during oxygen electrolysis in 0.05 m na2so4 (aq.) for 30 minutes with n = 3. one standard deviation interval is included potential, v vs. ag/agcl a b c d e charge obtained under ar purge (fig, 6), c charge obtained under o2 purge (fig. 5a), c current efficiency for h2 production via equation 3, % current efficiency for h2o2 production via equation 6, % pathway for h2o2 degradation pathways (equations 2, 4 and 7), % -0.95 0.17 ± 0.04 87.4 ± 5.3 0.2 ± 0.06 96 .0 ± 1.0 3.8 ± 1.0 -1.20 1.42 ± 0.10 100.0 ± 4.5 1.4 ± 0.2 95.0 ± 2.0 3.6 ± 2.0 -1.50 10.5 ± 0.5 134.0 ± 3.6 7.8 ± 0.5 87.8 ± 1.8 4.4 ± 1.8 the n(g)-guitar electrocatalyst is stable the electrocatalyst stability is demonstrated in the sequence of experiments outlined in figure 11. the electrode was subjected to a constant potential of -1.50 v for five, 30-minute cycles under o2 purged conditions. the h2o2 production rate (figure 11a) and current efficiencies (figure 11b) indicate no degradation in catalyst performance. figure 11. stress tests of the n(g)-guitar electrode for h2o2 production. average values and one standard deviation are indicated on each graph. a) rates of hydrogen peroxide generation at -1.5 v of applied potential in 0.05 m na2so4. b) current efficiencies as calculated using equation 6 summary and conclusions the ability to selectively dope guitar with different nitrogen moieties is unique to this pseudographitic material. our previous form of nitrogen-doped material, n(py)-guitar, contains 0.9 % nitrogen by atomic abundance with the division in those species as 0.0 % graphitic-n, 46.0 % pyridinic, 41.6 % pyrrolic, and 12.4 % n-oxide [15]. that electrocatalyst gave an orr pathway through a 4e mechanism giving h2o (equation 2). in this study, another form of nitrogen-doped pseudo-graphite, n(g)-guitar, has an atomic nitrogen abundance of 9.7 %. those n species have a division of 72.3 % graphitic n, 23.7 % pyridinic, 0.0 % pyrrolic, and 4.0 % n-oxide [13]. based on the rotating disk electrode and chronoamperometric studies, n(g)-guitar shows a strong preference for the 2eroute to h2o2 production (equation 1). this result agrees with the literature and indicates the importance of graphitic n for this pathway [21,23]. while n(g)-guitar has a relatively high abundance of pyridinic nitrogen (23.7 %), it was observed that this moiety did not significantly contribute to the 4epathway. this runs contra to literature hypotheses [18]. however, this result must be viewed in the context that guitar is a pseudo-graphite with a plethora of structural defects. the preference for the 2eorr pathway along with significant overpotentials for the her (equation 3) and h2o2 consumption (equation 4) gives an electrocatalyst with the highest combination of h2o2 production and current efficiency in literature. the h2o2 production rate from figure 6c is 106 mg l-1 h-1 cm-2 at -1.50 v and https://dx.doi.org/10.5599/jese.1407 j. electrochem. sci. eng. 12(5) (2022) 1009-1023 oxygen reduction reaction electrocatalyst 1020 65 mg l-1 h-1 cm-2 at -0.95 v. this indicates applications in water treatment and wastewater remediation, which require 17-68 mg l-1 h2o2 in conjunction with uv light, and 50-250 mg l-1 h2o2 for fenton reaction processes [50,51]. it also can be used directly as a disinfectant which requires 10-100 mg l-1 of h2o2 [52]. it should be expected that higher h2o2 production rates can be obtained with optimized cell and high surface area electrode configurations. references 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paper exploration of the effect of zn-mgo-upp coating on hardness, corrosion resistance and microstructure properties of mild steel itopa godwin akande1,, ojo sunday issac fayomi1, bassey jonah akpan1, olajide abraham aogo2,3 and patrick nwanne onwordi1 1department of mechanical engineering, bells university of technology, p.m.b. 1015, ota, ogun state, nigeria 2department of research and development unit, standard connections limited, nigeria 3department of mechanical engineering, university of ibadan, ibadan, oyo state, nigeria corresponding author: igakande@bellsuniversity.edu.ng; tel.: +8035604733 received: february 24, 2022; accepted: april 3, 2022; published: may 4, 2022 abstract this paper investigated the effect of unripe plantain peel (upp) nanoparticles reinforced zn-mgo composite coating on the hardness, anti-corrosion and microstructure properties of mild steel. the anti-corrosion characteristics of the coatings were examined using the potentiodynamic polarization method, employing 3.65 % nacl solution as the test medium. the hardness of the coatings was studied employing the brinell hardness technique, while the microstructure characteristics were examined using xrd and sem/eds. the results of the study revealed that the as-received mild steel sample exhibited the corrosion rate and hardness value of 8.6272 mm year-1 and 136.8 kgf mm-2, respectively, while the zn-mgo coated mild steel sample exhibited a corrosion rate and hardness value of 3.6362 mm year-1 and 42.5 kgf mm-2, respectively. the optimal performing zn-mgo-upp coated mild steel sample (sample coated with 20 g l-1 of mgo and 6 g l-1 of upp) exhibited a corrosion rate and hardness value of 0.8317 mm year-1 and 245.8 kgf mm-2, respectively. the corrosion rate and hardness value of the zn-20mgo-6upp coated mild steel sample indicated that the upp nanoparticles further improved the passivating and strengthening ability of zn-mgo coating. moreover, the xrd profile of the coatings possessed high intensities, which indicated that the coatings exhibit microstructural and chemical homogeneity, high stability and good texture. it was observed on the sem micrographs that the zn-mgo-upp coating exhibited a more refined microstructure compared to the zn-mgo coating, indicating the grain refining tendency of the upp nanoparticles. the eds further indicated the presence of essential and dispersion strengthening elements in the coatings. keywords sem; xrd; passivation; composite; homogeneity http://dx.doi.org/10.5599/jese.1311 http://dx.doi.org/10.5599/jese.1311 http://www.jese-online.org/ mailto:igakande@bellsuniversity.edu.ng j. electrochem. sci. eng. 12(5) (2022) 829-840 effect of zn-mgo-upp coating on steel properties 830 introduction the deterioration and operational failure of mild steels have been studied extensively due to their continued relevance in the manufacturing and construction industries [1]. as one of the leading engineering materials, the ready availability and low cost of mild steel have engendered their choice and usage for several engineering applications [2,3]. in spite of the continuous usage of many grades of mild steel, corrosion degradation on exposure to environmental contaminants within a short time of application has reportedly been a challenge [4,5]. it has also been reported that while the corrosion degradation of mild steel occurs rapidly in applications such as petrochemicals, marine and automotive, the corroded segment ultimately exhibits poor microstructure and diminished mechanical properties [6,7], which has led to the failure and total breakdown of structures [8]. besides failure and total breakdown consequences, corrosion degradation and loss of mechanical properties have resulted in undesired circumstances such as higher maintenance cost, loss and contamination of products, plant shutdown, expensive overdesign and reduction in production efficiency [9,10]. even though numerous materials have been employed as surface protectors for mild steel, a number of these materials reportedly failed too early, while other seemingly effective materials were regarded inappropriate and toxic to the environment of application [11,12]. for instance, aluminium oxide (al2o3) as a protective barrier is limited by its dissolution and instability in a few corrosive media, while titanium oxide (tio2) has provided more reliable protection due to its chemical stability [13]. however, the improvement of mechanical characteristics and corrosion resistance of mild steel has been accomplished via the combined electrodeposition of al2o3 and some metals such as nickel (ni), zinc (zn) and copper (cu) [14,15]. the surface of mild steel and other metals have been successfully coated through the electrodeposition process [16], which has been reported as one of the most broadly used techniques [17]. coatings produced through the electrodeposition method have also been confirmed durable for metal protection in material engineering over the years [18]. with the aid of electrodeposition certain essential properties have been incorporated into the surface of materials while still retaining their valuable characteristics [19,20]. the deposition of thin film on mild steel through electrodeposition has provided isolation for the metal surface against the surrounded corrosive environments. the thin film may, on occasion, act like galvanized steel where the film oxidizes or corrodes in preference to the metal beneath [21-23]. more so, environment-friendly materials such as by-products of fruit and agricultural wastes have been incorporated into some alloys for their reinforcement. for instance, included in al6063 were nanosized carbonized chicken bone particles to enhance its microstructural characteristics, corrosion resistance and mechanical properties [24]. moreover, concrete was strengthened using unripe plantain peel ash. the strengthening effect of the ash could be ascribed to the existence of compounds such as al2o3, cao, sio2, fe2o3 and mgo in the ash [25]. in this present research, unripe plantain peel (upp) particles were added to zn-mgo coating, and the effect of upp on the corrosion, hardness and microstructural properties of zn-mgo coating were examined. the effect of zn-mgo-upp coating on the properties of mild steel was also investigated. the upp nanoparticles were included with the aim of enhancing the eco-friendliness of the coating. experimental source of mild steel and deposit materials table 1 indicates the percentage weight of constituents in the mild steel plate used, procured in ogun state, nigeria. the chemical composition of the mild steel was determined using an x-ray i. g. akande et al. j. electrochem. sci. eng. 12(5) (2022) 829-840 http://dx.doi.org/10.5599/jese.1311 831 fluorescence spectroscopy, through which 99.145 % iron was observed. the major deposit material (zinc bar) was also purchased in nigeria, ogun state. the zinc bar was also observed to contain 99.5 % zinc. moreover, the other deposit materials, such as the nano-size unripe plantain peel (upp), magnesium oxide (mgo) and reagents, were likewise sourced in ogun state. table 1. chemical composition of mild steel element c si s ni mn al p fe content, wt. % 0.152 0.184 0.032 0.006 0.461 0.008 0.012 99.145 preparation of materials prior to coating the as-received mild steel plate was cut to samples of dimension 60 × 30 × 2 mm and polished with emery papers of different grades to remove corrosive scales and oxide layers on the surface of the samples. the zinc bars (anodes) of dimension 80×40×20 mm were also rinsed in distilled water and cleaned to get rid of some surface impurities. the upp nanoparticles were produced from unripe plantain peels. the peels were dried and ground to a nano-size form of about 25 μm. the surfaces of the polished mild steel samples were cleaned by immersion in 0.01 m na2co3 for 10 s and pickled in 10 % hcl at room temperature of 25.4 °c for 10 s in preparation for the electrodeposition. preparation of coating bath four coating baths were prepared in beakers containing 1000 litres of deionized water after a preliminary run of experiments. the weighed particles and reagents were poured into the beakers and stirred vigorously to dissolve. for the zn-mgo-upp baths, the mass concentration of mgo was held constant at 20 g l-1, while the concentration of the upp nanoparticles was varied (2, 4 and 6 g l1). the zn-mgo bath was prepared without upp nanoparticles. table 2 shows the bath formulation for the coatings. table 2. bath composition of the coatings coating bath constituents mass concentration, g l-1 zno 20 upp 0,2,4,6 boric acid 10 thiourea 10 na2so4 5 k2so4 5 znso4 15 mgso4 20 zncl 20 electrodeposition process the coating of the steel samples through electrodeposition was accomplished in accordance with the astm a53m standard. before the initiation of the coating, the developed bath was heated to 45 °c and kept constant. the zinc anodes were inserted in the bath 5 cm apart and connected to the positive terminal of the rectifier, while a mild steel sample (cathode) was connected to the negative terminal of the rectifier. the electrolyte in the bath was stirred constantly at the rate of 200 rpm during the deposition process to reduce the particle agglomeration and aid the particles electrophoresis (migration of ions to the cathode). the deposition was achieved at the current density of 1.5 a cm-2, a voltage of 2.5 v and a time of 20 minutes. table 3 indicates the process parameters employed for the http://dx.doi.org/10.5599/jese.1311 j. electrochem. sci. eng. 12(5) (2022) 829-840 effect of zn-mgo-upp coating on steel properties 832 deposition, which were arrived at after some preliminary experimental runs. the coatings developed, whose cross-sections are shown in figure 1, were thereafter dried in natural air and sectioned to 15×10×2 mm for the desired characterizations. the nomenclatures of the coated steels are as follows: zn-20mgo (zinc deposit with 20 g of magnesium oxide), zn-20mgo-2upp (zinc deposit with 20 g of magnesium oxide and 2 g of unripe plantain peel particles), zn-20mgo-4upp (zinc deposit with 20 g of magnesium oxide and 4 g of unripe plantain peel particles) and zn-20mgo-6upp2upp (zinc deposit with 20 g of magnesium oxide and 6 g of unripe plantain peel particles). figure 1. cross-sections of coated samples table 3. process parameters of deposition parameters value ph 4.8 cell voltage, v 2.5 current density, a cm-2 1.5 time, min 20 stirring rate, rpm 200 temperature, oc 45 characterization of samples the thickness of the developed coatings was examined using a mini thickness gauge with ± 0.05 accuracy. the thickness of each coating was examined in three different portions on the samples, and the average thickness was calculated. the corrosion properties of the coatings were investigated in a three-electrode system (reference, counter and working electrodes) containing simulated seawater (3.65 % nacl solution) following the astm g102 standard. the reference electrode is the silver/silver chloride, the counter electrode is a graphite rod, while the working electrode is the uncoated or the coated steel sample embedded in epoxy. using an auto lab pgstat 101 and a computer system on which nova 2.1.2 software was installed, the polarization curves were generated at a scan rate of 0.005 v s-1, between -1.5 and 1.5 v versus the open circuit potential (ocp) for 1 hour at a room temperature of 25.3 oc. the corrosion rate (cr) and polarization resistance (pr) of the samples were estimated using eqs. (1) and (2), respectively.  = corr eq0.00327 j w cr (1) = + a c corr a c 2.303 ( ) b b pr j b b (2) where ρ / g cm-3 is density of material, weq is equivalent weight and jcorr is current density. the corrosion rate values were obtained in mm year-1. i. g. akande et al. j. electrochem. sci. eng. 12(5) (2022) 829-840 http://dx.doi.org/10.5599/jese.1311 833 moreover, the microstructure of the coatings was studied using xrd (rigaku d/max-lllc model) and sem/eds (joel jsm-7600f model), while the hardness of the as-received and coated samples was examined in accordance with astm a833 standard using the eseway dvrb-m modelled brinell hardness machine. this hardness measuring machine has a load-bearing capacity ranging from 2.5 to 187.5 kgf. the test samples were subjected to a load (p) of 30 kgf for 30 seconds using a hardened indenting steel ball of diameter (d = 10 mm). the indentations diameter (d) was measured with a mini microscope. the brinell hardness values of the materials were then evaluated using eq. (3). ( ) = − − 2 2 2p bhv d d d d (3) results and discussion weight gained by samples and thickness of the coatings table 4 shows the weight gained by the coated samples. the zn-20mgo coating was observed to possess the largest weight gain of 0.38 g, while the zn-20mgo-2upp coating exhibited the least. similarly, the coating thickness of the samples indicated in figure 2 conforms to the weight gained. the zn-20mgo coating was observed to possess the largest thickness of 389.5 μm, while the zn20mgo-2upp coating exhibited the least thickness of 113 μm. though, the thickness of coatings could influence their stability [26]. table 4. weight gained by the coated samples sample weight before coating, g weight after coating, g weight gained, g as-received 29.93 zn-20mgo 30.03 30.41 0.38 zn-20mgo-2upp 29.49 29.56 0.07 zn-20mgo-4upp 29.52 29.68 0.16 zn-20mgo-6upp 29.63 29.72 0.09 figure 2. thickness of the coatings coating thickness could sometimes be proportional to the adhesive strength and other outstanding coating properties [27]. however, the thickest coating might, in some cases, not be the best coating due to the probable existence of internal defects or microvoids [28]. the notably low thickness possessed by the zn-mgo-upp coatings could be that the upp nanoparticles hindered the growth of the grains in the coated matrix, thus producing thin films of good quality, as asserted by islam et al. [29]. http://dx.doi.org/10.5599/jese.1311 j. electrochem. sci. eng. 12(5) (2022) 829-840 effect of zn-mgo-upp coating on steel properties 834 corrosion characteristics of the as-received and coated samples table 5 and figure 3 show the polarization data and polarization curves, respectively. the zn-20mgo coated sample exhibited better corrosion resistance than the as-received. while the as-received sample possessed the corrosion rate (cr) of 8.6272 mm year-1, corrosion current density, jcorr of 0.74245 ma cm-2 and polarization resistance (pr) of 86.45 ω cm2, the zn-20mgo coated sample exhibited a lower cr of 3.6362 mm year-1, low jcorr of 0.31293 ma cm-2 and high pr of 195.98 ω cm2. this indicated the zn-20mgo coated sample provided an inhibitive effect against the entrance of the nacl solution into the active sites of the steel [30,31]. the zn-mgo-upp coatings were observed to exhibit superior corrosion resistance compared to the zn-20mgo coatings. relative to the entire samples, the zn-20mgo-6upp coated sample exhibited the lowest cr and jcorr of 0.8317 mm year-1 and 0.071578 ma cm-2, respectively. this indicated that the zn-20mgo-6upp coating provided the best corrosion resistance against the dilapidating effect of the corrosive ions from the 3.65 % nacl solution. the reason for this could be the doping effect of the upp nanoparticles, filling the nano-holes in mgo coating and providing supportive covering against the diffusion of the chloride to the anodic and cathodic sites of the steel [32,33]. table 5. polarization data for the as received and coated samples sample ecorr / v jcorr / ma cm-2 cr / mm year-1 pr / ω cm2 as-received -0.7991 0.742 8.6272 86.45 zn-20mgo -0.6785 0.313 3.6362 195.98 zn-20mgo-2upp -0.7986 0.187 2.1742 198.9 zn-20mgo-4upp -1.3235 0.106 1.2344 274.51 zn-20mgo-6upp -0.7994 0.072 0.8317 712.91 moreover, figure 3 further indicated that the zn-mgo coating acted predominantly as an anodic corrosion protective coating, while the zn-20mgo-2upp and zn-20mgo-6upp coatings acted predominantly as a mixed-type inhibitor or mixed corrosion protective coating [34]. figure 3. polarization curves for the as-received and coated samples however, the zn-20mgo-4upp coating behaved predominantly as cathodic corrosion protective coating, as revealed by the negative shift in its polarization curve with respect to those of the asreceived sample [35]. also, figure 4 shows the ocp versus time curves of the samples, which indicated that the samples exhibited stable state potentials between 40 and 120 seconds of immersion [36]. i. g. akande et al. j. electrochem. sci. eng. 12(5) (2022) 829-840 http://dx.doi.org/10.5599/jese.1311 835 figure 4. time changes of open circuit potential values for the as-received and coated samples microstructure properties of the coatings some microstructural properties of the zn-20mgo and zn-20mgo-6upp coated steel are shown in figures 5-8. the sem image of the zn-20mgo coated sample in figure 5 revealed that the sample exhibited refined surface morphology. the coated surface exhibited a flake like structure with few cavities. conversely, the sem image of the zn-20mgo-6upp coated sample in figure 6 exhibited more refined grains with nodule structure. the grains and grain boundaries observed on the surface of the zn-20mgo-6upp coated sample were smaller than those on the zn-20mgo coated sample. this could be a reason for better corrosion resistance characteristics of the zn-20mgo-6upp coated sample. thus, the small grains and grain boundaries could have prevented the possibility of the grain boundaries to act as crevices for attack by corrosives [37,38]. therefore, from the examination of sem image of the zn-20mgo coated sample, it is expected to exhibit inferior mechanical and corrosion resistance characteristics compared to the zn-20mgo-6upp coated sample since microstructure behaviour of materials affects other major properties such as mechanical, corrosion and thermal resistance properties [39]. this shows the unripe plantain peel (upp) particles exhibit grain refining characteristics. more so, the eds of the zn-20mgo coated sample in figure 5 revealed elements such as c, si, o, mg, na and fe. however, the eds of the zn-20mgo-6upp coating revealed a larger percentage weight of carbon (c) compared to the carbon content in the zn-20mgo coating. calcium (ca) was also revealed to be present on the surface of the zn-20mgo-6upp coating, which is not found on the zn-20mgo coating. the larger percentage weight of carbon and the ca in zn20mgo-6upp coating could have been deposited from the unripe plantain peel (upp) particles. furthermore, the xrd profile of the zn-20mgo and zn-20mgo-6upp coated steel shown in figures 7 and 8, respectively, indicated the predominant crystallographic phases in both coatings are mgo, feo and sio. the highest peak intensity phase in the zn-20mgo coating is the crystal of mgo, which was observed at the intensity of 510 a.u (2 is about 26°). in the zn-20mgo-6upp coating, the highest peak intensity phase (a crystal of mgo) was lower; at the intensity of 490 a.u. (2 is about 42°). although it has been reported that the higher the peak intensity of phases in a coating, the more the stability of the coating and the less the microstructure lattice defects [40]. however, there are indications that the upp nanoparticles could probably have suppressed the growth of mgo in the matrix of zn-20mgo-6upp coating to refine the grains [41-43]. hence, the peak could also contain some out of sight elements that are not captured with the crystal. more so, http://dx.doi.org/10.5599/jese.1311 j. electrochem. sci. eng. 12(5) (2022) 829-840 effect of zn-mgo-upp coating on steel properties 836 the zn-20mgo-6upp coating also has a fairly high peak intensity of mgsio2, mgfeo2 and fesio2 crystallographic phases, which could be beneficial to the mechanical and corrosion-resistant properties of the coating. these high peak intensities revealed that the coatings exhibited microstructural and chemical homogeneity, good texture and high stability [44]. energy, kev figure 5. sem/eds images of zn-20mgo coated steel energy, kev figure 6. sem/eds images of zn-20mgo-6upp coated steel figure 7. xrd profile of zn-20mgo coated steel i. g. akande et al. j. electrochem. sci. eng. 12(5) (2022) 829-840 http://dx.doi.org/10.5599/jese.1311 837 figure 8. xrd profile of zn-20mgo-6upp coated steel brinell hardness of samples the brinell hardness of the as-received and coated samples is shown in figure 9. the coated samples exhibited notably high hardness values compared to the as-received samples. this shows that the coatings adhered firmly to the mild steel [45]. the strong adhesion could have resulted from the right choice of the process parameters, such as temperature, which aids the total dissolution of the coating constituents to form formidable layers at the steel-coating interface [46,47]. however, the entire zn-mgo-upp coated steel exhibited superior hardness to the mgo coated steel. this indicated that the upp (unripe plantain peel) nanoparticles provided some grain boundaries strengthening effect, minimizing slip and dislocation of the grains contained in the coatings [48,49]. while the as-received sample exhibited the least hardness of 136.8 kgf mm-2, the zn-20mgo-6upp coated sample exhibited the highest hardness value of 245.8 kgf mm-2. this signified that the zn20mgo-6upp coating provided the most impervious laminating covering against the indenting force. moreover, the high upp nanoparticles could also have significantly reinforced the grain boundaries against slip and dislocation of the grains, thus strengthening the coating appreciably [50]. figure 9. hardness of the as-received and coated samples http://dx.doi.org/10.5599/jese.1311 j. electrochem. sci. eng. 12(5) (2022) 829-840 effect of zn-mgo-upp coating on steel properties 838 conclusions i) from this work, the zn-mgo coating was observed to exhibit the highest thickness compared to the other coatings. the thickness of the zn-mgo coating was 389.5 μm. ii) more so, the uncoated mild steel sample exhibited the hardness value and corrosion rate of 136.8 kgf mm-2 and 8.6272 mm year-1, respectively, while the zn-mgo coated mild steel exhibited hardness and corrosion rate of 242.5 kgf mm-2 and 3.6362 mm year-1, respectively. this indicated that the zn-mgo coating strengthened the steel to some level and also offered some passivating effects in the test medium. iii) the optimal performing zn-mgo-upp coated steel (sample coated with 20 g l-1 of mgo and 6 g l-1 upp) exhibited the hardness and corrosion rate of 245.8 kgf mm-2 and 0.8317 mm year-1, respectively. this indicated that the upp particles further strengthened the zn-mgo coating, and also enhanced the passivating ability of the coating. iv) furthermore, the sem micrographs indicated that the zn-mgo-upp coating exhibited a more refined microstructure compared to the zn-mgo coating. this shows that the upp nanoparticles exhibited a grain refining tendency. the xrd profile of the coatings also indicated the 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https://doi.org/10.1016/j.carbon. 2019.12.029 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1007/s11665-021-05616-4 https://doi.org/10.1007/s11665-021-05616-4 https://doi.org/10.1007/s11668-021-01146-2 https://doi.org/10.1016/j.jmbbm.2018.05.030 https://doi.org/10.1016/j.actamat.2020.01.026 https://doi.org/10.1016/j.jmst.2021.08.002 https://doi.org/10.1016/j.surfcoat.2016.10.026 https://doi.org/10.1007/s10854-020-05040-9 https://doi.org/10.1149/2.0191902jes https://doi.org/10.1016/j.dt.2018.04.008 https://doi.org/10.1016/j.matdes.2015.11.027 https://doi.org/10.1149/2.0371803jes https://doi.org/10.1016/j.jallcom.2019.02.223 https://doi.org/10.1016/j.jallcom.2019.05.208 https://doi.org/10.1016/‌j.carbon.‌2019.12.029 https://doi.org/10.1016/‌j.carbon.‌2019.12.029 https://creativecommons.org/licenses/by/4.0/) corrosion behavior of ni content hypoeutectic al-si alloy doi:10.5599/jese.174 173 j. electrochem. sci. eng. x(y) (2015) 173-179; doi: 10.5599/jese.174 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical corrosion behavior of ni-containing hypoeutectic al-si alloy abul hossain, fahmida gulshan, abu syed wais kurny department of materials and metallurgical engineering, bangladesh university of engineering and technology, dhaka, bangladesh corresponding author: ah_buetmmesgfl@live.com; tel: +88-01711243601 received: march 8, 2015; revised: may 23, 2015; accepted: october 17, 2015 abstract electrochemical corrosion characteristics of the thermally treated 2 wt % ni-containing al-6si-0.5mg alloy were studied in nacl solutions. the corrosion behavior of thermally treated (t6) al-6si-0.5mg (-2ni) alloys in 0.1 m nacl solution was investigated by electrochemical potentiodynamic polarization technique consisting of linear polarization method using the fit of tafel plot and electrochemical impedance spectroscopy (eis) techniques. generally, linear polarization experiments revealed a decrease of the corrosion rate at thermal treated al-6si-0.5mg-2ni alloy. the eis test results showed that there is no significant change in charge transfer resistance (rct) after addition of ni to al-6si-0.5mg alloy. the magnitude of the positive shift in the open circuit potential (ocp), corrosion potential (ecorr) and pitting corrosion potential (epit) increased with the addition of ni to al-6si-0.5mg alloy. the forms of corrosion in the studied al-6si-0.5mg alloy (except al-6si-0.5mg-2ni alloy) are pitting corrosion as obtained from the scanning electron microscopy (sem) study. keywords al-si alloy; corrosion; potentiodynamic; impedance; sem introduction foundry aluminium alloys based on the al-si system are widely used in the automobile field since they provide excellent fluidity and castability, good resistance to corrosion and mechanical properties [1-4]. corrosion is the dissolution of a metal into an aqueous environment where the metal atoms dissolve as ions. since corrosion is always a function of the environmental conditions, control in many cases is important in order to prevent the contact between metal and the surrounding environment. the resistance of aluminum against corrosion in aqueous media can be http://www.jese-online.org/ mailto:ah_buetmmesgfl@live.com j. electrochem. sci. eng. 5(3) (2015) 173-179 corrosion of ni content hypoeutectic al-si alloy 174 attributed to a rapidly formed surface oxide film [5]. therefore, aluminum has been known to exhibit widely different electrochemical properties in different aqueous electrolytes [6]. aluminium and its alloys are considered to be highly corrosion resistant under the majority of service conditions [7]. the various grades of pure aluminum are the most resistant, followed closely by the al-mg and al-mn alloys. next in order are al-mg-si and al-si alloys. the alloys containing copper are the least resistant to corrosion [8] but this can be improved by coating each side of the copper containing alloy with a thin layer of high purity aluminium, thus gaining a three ply metal (alclad). this cladding acts as a mechanical shield and offers sacrificial protection[9]. when aluminum surfaces are exposed to atmosphere, a thin invisible oxide (al2o3) layer forms, which protects the metal from further corrosion in many environments [7]. this film protects the metal from further oxidation unless this coating is destroyed, and the material remains fully protected against corrosion [9]. the composition of an alloy and its thermal treatment are important do determine the susceptibility of the alloy to corrosion [10-11]. in ni-containing alloys the eutectic phase e consists of more or less soft ‘eutectic al (αe)’ and hard ‘eutectic si and al3ni [12]. when ni is added to the al-si system, the eutectic transformation is characterized by a simultaneous formation of eutectic si and al3ni and consequently, eutectic si and al3ni form a geometrically entangled system. during the course of a solution treatment eutectic al3ni does not significantly change its shape, as can be metallographically observed. furthermore, in the presence of ni-aluminides the loss of interconnectivity of eutectic si is significantly reduced [13-14]. in solution treated al-si-mg alloys containing up to 2% ni the positive effect of ni is much more prominent, especially with respect to the yield strength. while ni-containing alloys the contiguity of the ‘eutectic si and al3ni’ is more or less preserved due to the presence of ni-aluminides, the spheroidization of the eutectic si in the ni-free alloy (alsi12mg) results in its reduced contiguity and thus reduced strength [13-14]. new demanding applications are developed continuously. to be able to make the alloys competitive in future applications, assess to powerful tools and methods for materials or/ properties development is essential. the interest in this work for 2 wt% ni addition to al-6si0.5mg alloy comes from the possibility to improve the corrosion resistance in the t6 temper. in this study, electrochemical measurements were recorded to characterize the electrochemical behavior of the al-6si-0.5mg (-2ni) alloys. experimental materials preparation: the al-6si-0.5mg (-2ni) alloys were prepared by melting al-7si-0.3 mg (a356) alloys and adding al, ni and mg into the melt. the melting operation was carried out in a gas fired clay graphite crucible furnace and the alloys were cast in a permanent steel mould. after solidification the alloys were homogenised (500 °c for 24 h), solution treated (540 °c for 2 h) and finally artificially aged (225 °c for 1 h). after heat treatment rectangular samples (30×10×5 mm) were prepared for metallographic observation and subsequent electrochemical test. deionized water and analytical reagent grade sodium chloride (nacl) were used for the preparation of 0.1 m solution. all measurements were carried out at room temperature. potentiodynamic polarization test: a computer-controlled gamry framework tm series g 300™ and series g 750™ potentiostat/ galvanostat/zra was used for the electrochemical measurements. the potentiodynamic polarization studies were configured in cells, using three-electrode assembly: a saturated calomel reference electrode, a platinum counter electrode and the sample a. hossain at al. j. electrochem. sci. eng. 5(3) (2015) 173-179 doi:10.5599/jese.174 175 in the form of coupons of exposed area of 0.50 cm 2 or 10×5 mm as working electrode. only one 10×5 mm surface was exposed to the test solution, the other surfaces being covered with teflon tape. the system was allowed (100 s) to establish a steady-state open circuit potential (ocp). the potential range selected was -1 to +1 v and measurements were made at a scan rate of 0.50 mv/s. the corrosion current (icorr), corrosion potential (ecorr), pitting corrosion potential (epit) and corrosion rate (mpy) were calculated from tafel curve. the tests were carried out at room temperature in solutions containing 0.1m of nacl at a fixed and neutral ph value. the corroded samples were cleaned in distilled water and examined under scanning electron microscope. electrochemical impedance spectroscopy (eis) test: as in potentiodynamic polarization test, three electrode cell arrangements were also used in electrochemical impedance measurements. rectangular samples (10×5 mm) were connected with copper wire and adopted as working electrode. eis tests were performed in 0.1 m nacl solution at room temperature over a frequency range of 100 khz to 0.2 hz using a 5 mv amplitude sinusoidal voltage. the 10×5 mm sample surface was immersed in 0.1 m nacl solution (electrolyte). all the measurements were performed at the open circuit potential (ocp). the test cells were maintained at room temperature and the nacl solution was refreshed regularly during the whole test period. the impedance spectra were collected, fitting the experimental results to an equivalent circuit (ec) using the echem analyst tm data analysis software and evaluating the solution resistance (rs), polarization resistance or charge transfer resistance (rct) and double layer capacitance (cp) of the thermal treated alloys. results and discussion table 1 shows the electrochemical impedance spectroscopy (eis) test results. table 1. impedance test results alloy compositions rs / ω rct / kω cp / µf ocp, v vs.sce al-6si-0.5mg 40.37 15.57 1.259 -0.8454 al-6si-0.5mg-2ni 40.53 14.44 1.645 -0.6814 table 2 shows the potentiodynamic polarization test results obtained from the electrochemical tests. table 2. potentiodynamic polarization test results alloy compositions icorr / µa ecorr / mv epit / mv corrosion rate, mm/year al-6si-0.5mg 6.30 -764 -480 5.287 al-6si-0.5mg-2ni 2.54 -720 -426 2.132 electrochemical corrosion characteristics electrochemical impedance spectroscopy the open circuit potential (ocp) with exposure time of aged al-6si-0.5mg (-2ni) alloys in 0.1 m nacl solution is shown in table 1. large fluctuations in open circuit potential for the alloys were seen initially but, after a period of exposure the ocp fluctuation decreased and reached steady state. the steady state ocp of ni free alloy (al-6si-0.5mg) is -0.8454v and the al-6si-0.5mg2ni alloy -0.6814 v. the ocp values mainly depend on the chemical compositions and thermal history of the alloys. j. electrochem. sci. eng. 5(3) (2015) 173-179 corrosion of ni content hypoeutectic al-si alloy 176 the data obtained were modeled and the equivalent circuit that best fitted to the experimental data is shown in figure 1. rs represent the ohmic solution resistance of the electrolyte. rct and cp are the charge transfer resistance and electrical double layer capacitance respectively, which correspond to the faradaic process at the alloy/media interface. figure 2 shows the nyquist diagrams (suggested equivalent circuit model shown in figure 1) of the al-6si-0.5mg (-2ni) alloys in 0.1m nacl in de-ionized water. in nyquist diagrams, the imaginary component of the impedance (z") against real part (z') is obtained in the form of capacitive-resistive semicircle for each sample. figure 1. electrical equivalent circuit used for fitting of the impedance data of al-6si 0.5mg (-2ni) alloys figure 2. nyquist plots for the al-6si-0.5mg (-2ni) alloys figure 3 shows the experimental eis results in bode magnitude diagram for al-6si-0.5mg (-2ni) alloys. bode plots show the total impedance behaviour against applied frequency. at high frequencies, only the very mobile ions in solution are excited so that the solution resistance (rs) can be assessed. at lower intermediate frequencies, capacitive charging of the solid-liquid interface occurs. the capacitive value cp can provide very important information about oxide properties when passivation or thicker oxides are formed on the surface. at low frequency, the capacitive charging disappears because the charge transfer of electrochemical reaction can occur and this measured value of the resistance corresponds directly to the corrosion rate. for this reason, this low frequency impedance value is referred to as polarization or charge transfer resistance (rct). a. hossain at al. j. electrochem. sci. eng. 5(3) (2015) 173-179 doi:10.5599/jese.174 177 the solution resistances (rs) of the alloys (table. 1) are very similar to each other. so there are insignificant changes of rs values for the alloys during eis testing. the rs values are negligible with respect to rct and the electrolyte behaves as a good ionic conductor. impedance measurements showed that in 0.1m nacl solution, for the ni free al-6si-0.5mg alloy, the charge transfer resistance (rct) value in 0.1m nacl solution is 15.57 kω, and this is slightly decreased to 14.44 kω with the addition of 2 wt % ni to the al-6si-0.5mg alloy. the decrease in the charge transfer resistance indicates a decrease in the corrosion resistance of the alloy with ni addition. the double layer capacitance (cp) of the ni free al-6si-0.5mg alloy is 1.259 µf, which is the lower value between the alloys investigated. the double layer capacitance (1.645 µf) of al-6si-0.5mg alloy increased with an addition of 2 wt % ni. figure 3. bode plots for the al-6si-0.5mg (-2ni) alloys potentiodynamic polarization potentiodynamic polarization curves of al-6si-0.5mg (-2ni) alloys are shown in figure 4. anodic current density of al-6si-0.5mg alloy increased with ni addition. the addition of ni caused the formation of micro-galvanic cells in α-al matrix. figure 4. potentiodynamic polarization curves for the thermal aged al-6si-0.5mg (-2ni) alloys j. electrochem. sci. eng. 5(3) (2015) 173-179 corrosion of ni content hypoeutectic al-si alloy 178 the different intermetallic compounds (like mg2si, al3ni etc.) can lead to the formation of micro-galvanic cells because of the difference of corrosion potential between intermetallics and αaluminum matrix. with the addition of 2wt% ni, the corrosion potential of the alloy shifted towards more positive value (-720mv). pitting corrosion potential (epit) of the ni content alloys also shifted towards more positive values (from -480mv to -426mv). potentiodynamic tests showed that in 0.1 m nacl solution, addition of ni in the al-6si-0.5mg alloy decreases the corrosion current (icorr). for the ni free al-6si-0.5mg alloy, the corrosion current (icorr) value in 0.1 m nacl solution is 6.3µa, and this decreased to 2.54 µa with the addition of 2 wt % ni to the al-6si-0.5mg alloy and the corresponding corrosion rate decreases for the alloy (al-6si-0.5mg-2ni alloy = 2.132 mm/year). microstructural investigation the microstructures of some selected as-corroded samples were observed under sem. several pits were visible in the ni free sample examined. it is probable that the pits are formed by the intermetallics dropping out from the surface due to the dissolution of the surrounding matrix. however, it is also possible that the pits are caused by selective dissolution of the intermetallic/or particles of the second phase precipitates. figure 5. sem secondary electron image of the damaged surface morphology of as-corroded al-6si-0.5mg alloy. figure 6. sem secondary electron image of the damaged surface morphology of al-6si-0.5mg-2ni alloy. a. hossain at al. j. electrochem. sci. eng. 5(3) (2015) 173-179 doi:10.5599/jese.174 179 no severe pitting corrosion found in the studied al-6si-0.5mg-2ni alloy, examined by the sem. all surfaces were characterized by sem following potentiodynamic polarization tests. the thermal treated ni free alloy (al-6si-0.5mg) exhibited pits on their surface (figure 5), which apparently had nucleated randomly. conversely, the exposed surface of the alloy exhibited a corrosion product with covering the surface after polarization. all the micrographs (figures 5-6) also showed that there was no corrosion in the fragmented and modified al-si eutectics and ni containing eutectics (al3ni). conclusions the eis tests have shown that the additions of 2 wt % ni into al-6si-0.5mg alloy tend to slightly decrease the corrosion resistance of al-6si-0.5mg alloy. the linear polarization and tafel extrapolation plot showed that the corrosion current (icorr) and corrosion rate (mm/year) decreased with the addition of 2 wt % ni into al-6si-0.5mg alloy. the open circuit potential (ocp), corrosion potential (ecorr) and pitting corrosion potential (epit) were shifted in the more noble direction due to ni additions into al-6si-0.5mg alloy. no severe pits found in the studied al-6si0.5mg-2ni alloy, examined by the sem. references [1] p. leo, e. cerri, metallurgical science and technology 21 (2003) 27-31. 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[12] z. asghar, g. requena, f. kubel, materials science and engineering a 527(2010) 5691-5698. [13] f. stadlerl, h. antrekowitsch, w. fragner, h. kaufmann, p. j. uggowitzer, materials science forum 690 (2011) 274-277. [14] f. stadlerl, h. antrekowitsch, w. fragner, h. kaufmann, p. j. uggowitzer, international journal of cast metals research 25(4) (2012) 215-224. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ impact of carbon coating processing using sucrose for thick binder-free titanium niobium oxide lithium-ion battery anode http://dx.doi.org/10.5599/jese.1655 641 j. electrochem. sci. eng. 12(4) (2023) 641-658; http://dx.doi.org/10.5599/jese.1655 open access : : issn 1847-9286 www.jese-online.org original scientific paper impact of carbon coating processing using sucrose for thick binder-free titanium niobium oxide lithium-ion battery anode chen cai, patrick mccormack, ziyang nie, gary m. koenig jr. department of chemical engineering, university of virginia, 102 engineers way, charlottesville, 22904-4741, va, usa corresponding author: gary.koenig@virginia.edu; tel.: +1-434-982-2714; fax: +1-434-982-2658 received: december 30, 2022; accepted: may 4, 2023; published: may 21, 2023 abstract lithium-ion batteries are increasingly important for providing energy storage solutions. in the drive to improve the energy density at the cell level, optimizing the electrode architecture is crucial in addition to researching new materials. binder-free (bf) electrodes include porous pellets only containing battery electroactive materials. these electrodes can provide advantages with regard to mechanical stability and alleviated ion transport limitations relative to composite approaches for very thick and energy-dense electrodes. however, the absence of conductive additives often limits suitable material candidates for bf battery electrodes. tinb2o7 (tno) is a promising bf electrode material from a gravimetric and volumetric capacity standpoint, but phase pure tno has relatively low electronic conductivity. herein, a sucrose precursor coating method for tno materials was implemented to process the tno materials into bf electrodes. the sucrose served as a source to generate carbon in the electrodes, where the carbon coating resulted in an increase in rate capability, discharge voltage, and cycle life. keywords electronic conductivity; mechanical stability; rate capability; discharge voltage; cycle life introduction stimulated by rapid improvements in portable electronics energy storage solutions, batteries have drawn extensive attention. lithium-ion (li-ion) batteries have dominated the portable electronics market for years [1,2]. towards further improvements in li-ion battery energy and power density, efforts have been directed towards both new materials and electrode architecture improvements [3,4]. increasing electrode thickness is a straightforward route to improve cell-level energy density; however, with conventional composite electrodes, increased thickness results in very restricted ion transport, limiting power density and rate capability [5-9]. delamination for composite electrodes from the current collector at increasing thickness is also a consideration [10,11]. several approaches have been investigated to mitigate ionic transport limitations originating from the polymer http://dx.doi.org/10.5599/jese.1655 http://dx.doi.org/10.5599/jese.1655 http://www.jese-online.org/ mailto:gary.koenig@virginia.edu j. electrochem. sci. eng. 13(4) (2023) 641-658 carbon coating for tinb2o7 li-ion battery anode 642 binder and conductive additives which occupy the interstitial microstructure in conventional composite electrodes [12-14]. an alternative electrode architecture to mitigate some of the challenges of processing/cycling thick electrodes is binder-free electrodes (referred to as bf electrodes in this paper, and also “sintered electrodes”, or “all active material (aam) electrodes” in some prior works with similar processing), which is composed of only electroactive material and electrolyte-laden pores/voids [15-17]. bf electrodes are mechanically strengthened using a mild sintering treatment after hydraulic compression of the electroactive material powder [18-25]. these electrodes have reported thicknesses well over a half millimeter and areal loadings exceeding 150 mg cm-2 [18-24]. in the absence of conductive additives, electron transport through the electrode matrix must occur via the electroactive material itself rather than a percolated network of conductive carbon [18]. prior research has shown that when the electronic conductivity of the electrode matrix is greater than 1 s m-1, it no longer limits the electrochemical cell performance for typical li-ion electrolyte ion transport properties [8]. in a previous report from our group, bf full cells using licoo2 (lco) as cathode and li4ti5o12 (lto) as anode were successfully charged/discharged over many cycles [19]. although before cycling, the electronic conductivity of the materials in the lto/lco cell can be as low as 10-5 s m-1, over 95 % of the lithiation range of the electroactive materials during cycling have electronic conductivities exceeding 10 s m-1, an electronic conductivity magnitude where the electrochemical charge/discharge rate would be expected to be limited by ionic transport limitations over electronic transport resistances [20-22,26-28]. despite the excellent electronic conductivity and stability of lto, the volumetric and gravimetric energy densities are still relatively low [22,29]. an important note here is that the very common carbon-based li-ion anodes, such as graphite, cannot be used in the bf electrode architecture. this is because (i) at such high loading, even at relatively low c-rate high areal current density results, which readily induces lithium plating [30]; (ii) the 13 % lattice volume change is too great to be accommodated in bf electrodes without the addition of elastic polymer [31]; and (iii) with increased thickness, the solid electrolyte interface (sei) formation can clog pores, exacerbating ion transport restrictions [32]. to improve the energy density, an alternative anode material, tinb2o7 (tno), will be investigated herein. tno has relatively high gravimetric and volumetric capacity but relatively low electronic conductivity (especially relative to lto), which was expected to be a challenge to implement tno within the bf electrode architecture. tno does have an increasing electronic conductivity with lithiation of the material [33-38]; however, the increase has still been reported to be well below the ionic conductivity of typical li-ion electrolytes [39], suggesting that matrix electronic conductivity would still be expected to limit the electrochemical outcomes of cells with tno electrodes. there are different strategies to improve or overcome the inherent electronic conductivity limitations of electroactive materials. one route is to dope the electroactive material with elements that result in increased electronic conductivity, and this method has even been applied to bf electrodes, an example being the material limn2o4 (lmo) [18]. there are limits to the electronic conductivity improvements via doping methods, and secondary phases can result in too high of a doping level [40,41]. another option is to coat the electroactive material with metal oxides, carbon, and other materials, and this strategy has been attributed to improving the rate capability of low electronic conductivity electroactive materials such as olivine-phase lifepo4 and layered-phase lini0.6co0.2mn0.2o2 [42-44]. among coatings, carbon-based coating of electroactive materials has been broadly reported for improved interfacial stability and electronic conductivity [42,45-50]. sucrose is a common carbon source that also can serve as a particle binder material [46,51]. herein, we propose a facile method to fabricate carbon-coated bf electrodes using sucrose both as a c. cai et al. j. electrochem. sci. eng. 13(4) (2023) 641-658 http://dx.doi.org/10.5599/jese.1655 643 sacrificial binder necessary for bf electrode processing and as a carbon source, where the carbon coating process does not require an extra step for treating the electroactive material powder or the bf porous pellet relative to processing the electrodes without the coating. the carbon coating processing targeted in this report has a schematic illustration in figure 1. the electroactive material, tno, was initially synthesized via a sol-gel route. the tno powder was then coated with sucrose via mixing with an aqueous solution containing dissolved sucrose and drying of the solvent. the sucrose-coated powder was hydraulically pressed into a porous pellet. finally, the pellet was heated in an inert atmosphere which converted the sucrose to carbon and resulted in a porous bf pellet of tno with remaining residual carbon from the sucrose source. figure 1. schematic of the process for fabricating carbon-coated bf tno electrodes experimental electroactive material powder synthesis tno electroactive material powder was synthesized using a sol-gel method. briefly, 0.5 m of niobium chloride (nbcl5, sigma-aldrich) and 0.25 m of titanium isopropoxide (ti(oc3h7)4, sigmaaldrich) were dissolved in 40 ml ethanol at 40 °c and stirred at 300 rpm to facilitate dissolution and mixing. the solution was dried at room temperature overnight in air, then redissolved in 40 ml deionized (di) water and dried at 80 °c overnight in air within a fume hood. the resulting precursor was then heated to 700 °c and held at that temperature for 2 hours, where the ramp rates for heating and cooling were both 1 °c min-1. http://dx.doi.org/10.5599/jese.1655 j. electrochem. sci. eng. 13(4) (2023) 641-658 carbon coating for tinb2o7 li-ion battery anode 644 lco electroactive material powder was synthesized via precipitation of a precursor followed by a solid-state reaction [25]. 0.2 m of cobalt sulfate heptahydrate (coso47h2o, acros organics) and 0.2 m of sodium oxalate (na2c2o4, fisher) were separately dissolved into 400 ml of di water each and heated to 60 °c. the solutions were mixed via pouring all at once, and the mixture was stirred at 300 rpm. the reaction was allowed to proceed for 30 min before collection of the precipitate via vacuum filtration. the filter cake was rinsed with 1.6 l of di water. the resulting powder cake was dried at 80 °c overnight in air and blended with lithium carbonate (li2co3, fisher chemical) with a molar ratio of 1.05:1 li:co using a mortar and a pestle by hand for 10 min. the blended powder was then heated at a ramp rate of 1 °c min-1 to 800 °c without a hold at the top temperature and then allowed to cool to room temperature without control over the cooling rate. material characterization thermogravimetric analysis (tga) was conducted using a ta instruments q50. heating initiated from room temperature to 800 °c at a ramp rate of 10 °c min-1 in air atmosphere. scanning electron micrographs (sems) were collected using a fei quantum 650. powder x-ray diffraction (xrd) was conducted using a panalytical x'pert prompd. the electronic conductivity of materials was measured using a direct current (dc) technique. the as-prepared bf pellet electrodes were coated with silver paste (sigma-aldrich) by hand on each flat side of the porous disc. the coated pellets were sandwiched between stainless steel and compressed by a clamp. a constant voltage at 10 mv was applied and the current was recorded using a gamry reference 600 to measure the dc resistance necessary to calculate the pellet material's electronic conductivity. bf electrode fabrication during the fabrication of bf tno electrodes, 4 concentrations of sucrose binder were prepared in this work. the sucrose contents were 0.0, 2.5, 5.0, and 7.5 % by mass (denoted by 0.0 , 2.5, 5.0 and 7.5 % sucrose). the content indicates the mass percent of sucrose relative to the total combined mass of sucrose and tno. for the 0.0 % sucrose sample, 1 g of tno powder was blended with 2 ml of 1 wt.% polyvinyl butyral (pvb) solution (pfaltz & bauer) in ethanol using a mortar and pestle by hand until dry. for the other three samples containing sucrose, a total mass of 1 g consisting of tno and sucrose powder was blended with 0.5 ml of di water and 1.5 ml of ethanol using a mortar and a pestle by hand until dry. for all cases, the coated powder was next loaded into a circular pellet die (carver) with a diameter of 1.3 cm, followed by pressing at 420 mpa for 2 min using a hydraulic press (carver). the pressed pellet was sintered in an argon atmosphere at 800 °c for 1 hour at ramp-up and down rates of 2 °c min-1. the as-prepared pellets had thicknesses ranging between 580 and 610 µm (measured using digital calipers), areal loadings of 142-148 mg cm-2, and geometric pore/void fraction of 0.43-0.45 (calculated assuming the crystal density to be 4.3 g cm-3) [52]. in addition, thinner bf electrodes were fabricated to facilitate cycle life testing, with loadings of 18 to 20 mg cm-2 and thicknesses ranging between 70 and 80 µm. note that carbon-coated (and carbon-free) tno powders were also used in composite electrodes, and for tno powders used for composite electrodes, all processing steps were the same as for bf electrodes except that there was no hydraulic compression of the powder. for bf lco electrodes, 1 g of lco powder was blended with 2 ml of 1 wt.% pvb solution in ethanol using a mortar and a pestle by hand until dry. the pressing procedure was the same as above for tno materials. the pressed pellet was sintered in air at 600 °c for 1 hour at both rampup and down rates of 1 °c min-1. the as-prepared pellets had measured thicknesses ranging between c. cai et al. j. electrochem. sci. eng. 13(4) (2023) 641-658 http://dx.doi.org/10.5599/jese.1655 645 810 and 850 µm, areal loadings of 268 to 273 mg cm-2, and geometric pore/void fractions of 0.32 to 0.34 (calculated assuming the crystal density to be 5.0 g cm-3) [22]. thinner bf lco electrodes were also fabricated to facilitate cycle life testing at 37 to 38 mg cm-2, with thicknesses of 120 µm. detailed characterization of bf lco electrodes can be found elsewhere for materials fabricated using the same procedures [19-21]. electrochemical evaluation for composite electrode cells, the tno powder was mixed with acetylene carbon black (cb, alfa aesar) and 3.33 wt.% polyvinyl pyrrolidone (pvp, sigma aldrich, 360 kda molecular weight) in ethanol solution with a mass ratio tno:pvp:cb of 8:1:1. the resulting slurry was coated onto an aluminum foil current collector using a doctor blade, which resulted in final areal active material loadings measured for punched electrodes to range between 2.0-3.5 mg cm-2 after drying. circular electrodes with an area of 1.33 cm2 were punched by hand using a die and transferred into a glove box filled with argon. punched lithium metal foil was used as anodes and celgard 2325 was the separator, and the electrodes were assembled into electrochemical cells using 2032-type coin cell parts, with assembly all in the glove box. the electrolyte was 1.2 m lipf6 in 3:7 ethylene carbonate:ethyl methyl carbonate (gotion). the as-prepared cells were evaluated using a multichannel battery cycler (maccor) between 1.0-2.5 v at c/20, where 387.6 mah g-1 was assumed to be the tno theoretical capacity used for adjusting the currents for the c rate determination [46]. for li/tno cells, cycling starts on a discharge. for bf electrode cells, the as-prepared bf cathode (lco pellet) and bf anode (tno pellet) were attached to the bottom plate and the spacer of the 2032-type coin cell, respectively, with a custom carbon paste applied between the bf electrode and stainless steel to reduce contact resistance. the custom carbon paste was applied as a homogenous slurry consisting of 4.76 wt.% of cb, 4.76 wt.% of pvp, and 90.48 wt.% of ethanol. pellets adhered to the cell components with the paste were then dried in a vacuum at 80 °c for 20 min before transferring into the glove box. it is noted that the carbon paste amount was less than 1 mg cm-2, which was <1 % of the mass of the thicker bf electrodes. 2032type coin cells were assembled with a single spacer attached to the anode, as the cathode was directly attached to the bottom piece. the same electrolyte was used for the composite electrodes described above. glass fiber (fisher, type g6 circles) was used as a separator. although glass fiber has a relatively large thickness compared to porous polymer options, this separator was used to minimize the possibility of a short between the relatively rough ceramic surfaces of the pellets. while not quantitatively confirmed or estimated, the glass fiber would be expected to compress during cell assembly resulting in a lower thickness. the compression on porous polymer separators has been previously speculated to increase ion transport limitations in similarly constructed cells [23]. the bf cells were cycled between 1.0-3.2 v, and as a reference point for c rates, 1.69 ma cm-2 corresponded to a rate of c/20. for thinner bf electrode cells, the cells were cycled between 1.0 and 3.2 v at 2 ma cm-2 (c/2) for 100 cycles. electrochemical impedance spectroscopy (eis) was conducted on the cycled cell using a gamry reference 600. results and discussion electrochemical evaluation the initial sol-gel synthesized powder with the targeted tno composition had broad peaks in collected xrd patterns, suggesting relatively low crystallinity. the xrd and relevant reference patterns for titanium niobate phases can be found in supplementary material, figure s-1. the broad peaks may http://dx.doi.org/10.5599/jese.1655 j. electrochem. sci. eng. 13(4) (2023) 641-658 carbon coating for tinb2o7 li-ion battery anode 646 have partly been due to the relatively low calcination temperature and were consistent with previous reports [53-55]. explicit assignment of each individual peak was challenging to affirm unambiguously; however, there were tixnbyoz phases that could be associated with all peaks [56,57]. sem images of the initial sol-gel synthesized powder can be found in supplementary material, figure s-2. these micrographs suggested the morphology of the material was secondary aggregates that had length dimensions of approximately 20 μm, while the primary particles that formed the larger aggregates were less than 1 μm in length (with many primary particles in the size range of 100 nm). figure 2a shows the estimated mass percentage of carbon on the tno materials with the different amounts of sucrose added before hydraulic compression and thermal treatment in an inert atmosphere. figure 2. (a) theoretical maximum carbon content (blue line) and actual (black dots) carbon content determined from tga for different initial added sucrose content to the tno material before thermal treatment. (b) electronic conductivities of as-prepared pellet (black) and phase pure literature value (blue) [33] the measured carbon mass percentage was estimated from the mass change during tga experiments at 800 °c relative to room temperature, and the more detailed tga data can be found in supplementary material, figure s-3. carbon loss/oxidation was observed to occur between 400 and 500 °c, consistent with previous reports [54,58]. as the sucrose content added to the powder increased, the relative mass content of the carbon coating increased as well. an additional note is that based on the elemental c content of sucrose, theoretically, for the mass of sucrose, if all that c was converted to a carbon coating 42 wt.% of the initial sucrose mass would be expected to remain. the blue line in figure 2a depicts the amount of carbon expected if all the c in the sucrose was converted to a carbon coating. as can be seen, the carbon content for all samples was below these c. cai et al. j. electrochem. sci. eng. 13(4) (2023) 641-658 http://dx.doi.org/10.5599/jese.1655 647 theoretical values, suggesting loss of carbon to other processes, such as reduction of the tno material at the interface. it is also noted that the difference between the theoretical maximum carbon and carbon determined from tga increased as the sucrose added increased, suggesting less efficiency of sucrose conversion to a carbon coating as the sucrose content was increased. the dc electronic conductivity measured using tno porous pellets can be seen in figure 2b. all tno materials exhibited higher electronic conductivity than reported for phase pure tinb2o7 from the literature [33]. even with the 0.0 % sucrose sample, the conductivity was 5 orders of magnitude higher, and this higher conductivity was suspected to result in part from the presence of the tinbo4 phase (consistent with xrd results discussed in the next paragraph), which has a much higher electronic conductivity reported at 150 s m-1 [59]. additionally, there may have been some reduction of the tinb2o7 phase, which can have a large impact on the electronic conductivity of this material [33]. for the materials processed with sucrose and subsequently coated with carbon, the electronic conductivity also increased as the carbon coating content increased. the final measured conductivities for the materials with the highest carbon content were over 1 s m-1. xrd patterns for the tno pellets after hydraulic compression and heating in an argon atmosphere can be found in figure 3. all four amounts of sucrose addition resulted in similar xrd patterns. the major phase was assigned as tinb2o7, a monoclinic layered structure with a space group of c2/m [46]. an impurity phase was also noted after thermal treatment, attributed to tinbo4, suggesting a partial reduction of some nb5+ to nb4+ in the pellet [59-61]. the peaks consistent with arising due to a tinbo4 impurity are marked in figure 3. it is noted that to the naked eye, visually, the 0.0 % sucrose exhibited dark blue color, while the carbon-coated samples were all dark grey, consistent with previous literature [61]. figure 3. xrd patterns for 0.0 (green), 2.5 (orange), 5.0 (red) and 7.5 % sucrose (purple) there was an additional small impurity peak at a 2 position of 25°. explicit assignment of this peak was difficult, although likely candidates could be other nb-enriched oxide phases such as tinb24o62, ti2nb14o39, and ti2nb10o29 based on bulk stoichiometry [57,62-64]. since the bulk molar ratio of ti:nb from sol-gel synthesis was 1:2, the presence of tinbo4 suggested the presence of one or multiple of these nb-enriched phases. http://dx.doi.org/10.5599/jese.1655 j. electrochem. sci. eng. 13(4) (2023) 641-658 carbon coating for tinb2o7 li-ion battery anode 648 sem images of the 0.0, 2.5, 5.0 and 7.5 % sucrose can be found in supplementary material, figure s-4. all samples had a similar appearance. many of the primary particle sizes were still on the order of 100 nm, as was the case for the powder before hydraulic compression and thermal treatment. the retention of the primary particle size was also consistent with the mild sintering process. these primary particles were part of larger aggregates 10-20 µm in size. many of the aggregates appeared flattened, which likely resulted from the hydraulic compression processing step. electrochemical evaluation initial electrochemical analysis on the tno materials was conducted using composite electrodes paired with li metal anodes. the first charge and discharge cycles of all materials in composite half cells paired with li anodes can be found in supplementary material, figure s-5. as the sucrose content added to the tno before heat treatment increased, the gravimetric capacity of the resulting tno electroactive material decreased. the decrease in gravimetric capacity was likely due to increasing inactive carbon content in the powder and increasing relative amounts of tinbo4 phase. the voltage where tinbo4 has electrochemical capacity is between 1.0 and 2.0 v and thus overlaps with tno; however, tinbo4 has a reported capacity of 200 mah g-1 and thus increase in its content would lower the overall material gravimetric capacity relative to tno [60]. next, electrochemical characterization was done of cells containing the tno bf electrodes directly. the tno materials were used as bf anodes paired with lco bf cathodes in a full-cell configuration. bf electrodes were not evaluated in half cells due to the limitations of using lithium metal in such high areal capacity systems, where each cycle would have a significant amount of plating and stripping that make the li electrode the limiting factor [19]. as for bf cells paired with lco, when looking at the discharge voltage profiles (figure 4), at a relatively slow rate of c/20, the 0.0 % sucrose had the largest capacity as expected because at low rates the resistance due to that material having the lowest matrix electronic conductivity did not impact the electrochemical capacity. at higher rates of c/10 and c/5, the 2.5 % sucrose had the highest capacity among all materials evaluated. this outcome was not explained simply by improvements in the electronic conductivity of the electrode matrix. for such a thick bf electrode, the cell would be expected to be limited by ion transport through the electrolyte-laden electrode microstructure at such rates due to li depletion in the cathode region [20,22]. as the sucrose content increased, the voltage decreased with a steeper slope near the later discharge region (e.g.,>150 mah g-1 anode) at both c/10 and c/5. this outcome was attributed to ion transport in the microstructure being the limiting process once there was sufficient carbon coverage to overcome the matrix electronic conductivity resistance in the tno electrode. any excess carbon after achieving the electronic conductivity improvement would be expected to further impede ion transport in the interstitial regions between electroactive material particles, and excess carbon can also negatively impact the interfacial resistance of the electroactive materials [8]. the discharge capacities resulting from the full rate capability evaluation of the bf tno anodes paired with bf lco cathodes can be found in supplementary material, figure s-6. while the rate capability analysis above reflected the retention of electrochemical capacity, the average discharge voltage was another outcome that changed for the different tno electrode cells. the average discharge voltage as a function of the discharge c rate for the tno bf anode cells can be found in figure 5. at the lowest discharge rate of c/20, all cells had a similar average discharge voltage of 2.17 v. as the c-rates increased, for the 0.0 % sucrose, the average discharge voltage dropped substantially and was 1.64 v at c/5. as the sucrose content increased, the drop in average discharge voltage was mitigated with the 7.5 % sucrose tno anode cell achieving 1.96 v at c/5. c. cai et al. j. electrochem. sci. eng. 13(4) (2023) 641-658 http://dx.doi.org/10.5599/jese.1655 649 figure 4. voltage profiles of bf electrodes with 0.0 (green), 2.5 (orange), 5.0 (red), and 7.5 % sucrose (purple) in full cells paired with lco bf cathodes. lco mass loading was 1.9 times greater than tno electrodes to ensure an anode-limited cell. the charge and discharge rates were the same and corresponded to (a) c/20, (b) c/10, and (c) c/5. c/20 corresponded to a current density of 1.7 ma cm-2. all charge and discharge cycles used the same voltage window of 1.0-3.2 v (cell). note that in the plots, the orange curve generally overlaps with the red and is thus difficult to distinguish figure 5. average discharge potential of 0.0 (green), 2.5 (orange), 5.0 (red) and 7.5 % sucrose (purple) as the tno bf cells paired with bf lco cathodes were cycled at different c rates. all charge and discharge cycles used the same rates, *last five cycles when the charge/discharge rate was reduced back to c/20. c/20 corresponded to 1.7 ma cm-2 http://dx.doi.org/10.5599/jese.1655 j. electrochem. sci. eng. 13(4) (2023) 641-658 carbon coating for tinb2o7 li-ion battery anode 650 this outcome suggested the electronic conductivity improvement of the additional carbon helped to increase the average discharge voltage, although the ion transport limitations and li+ depletion in the electrolyte likely resulted in the reverse trend for the total capacity at an increasing discharge rate. the average voltage for the last five cycles of the rate capability experiment, where the rate was adjusted back to c/20, also increased as the sucrose content during tno processing was increased. δdq/dv peak shift analysis according to the butler-volmer equation [65,66], for most li-ion battery electroactive materials, the current has an exponential dependence on the overpotential if only considering the interfacial kinetics. thus, in the ideal case, galvanic charge and discharge of thin composite electrode batteries with low loadings would often primarily reflect the interfacial kinetics at different c rates. dq/dv plots show the voltage regions corresponding to the delivery of electrochemical capacity. an additional use of dq/dv is the δdq/dv calculated for shifts in the peak positions between the charge and discharge cycle at the same current density (shift in voltage location of peaks in differential capacity), where the shift is the δdq/dv values as a function of current densities. for batteries with very thick electrodes, such as the bf electrode cells evaluated herein, the overpotential contribution from interfacial compared to ionic and/or electronic overpotential is much smaller, according to previous analysis [7]. for such thick electrodes, the electronic and ionic conductivity of the solid electrode and liquid electrolyte phase can be reflected in the δdq/dv peak position where the baseline is the dq/dv peak at low cycling rates, and the δdq/dv is the shift in the peak locations at increasing c rates. for increasing c rates in a cell where the overpotential is primarily due to electronic and/or ionic conductivity through the matrix, it would be expected that the δdq/dv peak positions would shift linearly as a function of increasing current density (or c-rate for identically processed electrodes/cells). similar analysis was previously reported for indirect evaluation of the impact of the electronic conductivities of bf limn2o4-type (lmo) materials with different dopants and lithiation states [18]. the prior analysis of lmo materials demonstrated that dq/dv not only serves well to analyze the redox potentials in thin composite electrodes with negligible electronic and ionic overpotentials but also to analyze the electronic and ionic conductivities in thick bf electrodes with relatively low contributions from interfacial overpotentials to the overall cell overpotential [7]. when comparing electroactive materials with different compositions/dopants/treatments, using δdq/dv peak positions can exclude the impact of the difference in open circuit voltage (ocv). in addition, with thicker electrodes where lithium depletion, in general, will occur, the peak dq/dv position well represents the voltage where the electrochemical reactions occur most favorably well before lithium depletion in the cathode region. the li+ depletion likely resulted in a more dramatic decrease in voltage, resulting in lower capacity for the tno bf electrodes with the highest sucrose content during processing. plots of the δdq/dv peak positions as a function of areal current densities for all cells with bf tno anodes can be found in supplementary material, figure s-7. as expected for cells with ionic or electronic electrode conductivity being the primary contributors to overpotential, all cells had a linear dependence between the δdq/dv peak positions and current density. as the sucrose content increased, the slope decreased, with these results summarized in figure 6. in contrast to transmission line models or models often assumed in pseudo-two-dimensional simulations [22,65,67], where the current was distributed between the solid electrode and liquid electrolyte across the c. cai et al. j. electrochem. sci. eng. 13(4) (2023) 641-658 http://dx.doi.org/10.5599/jese.1655 651 electrode depth, an overall conductivity (effectively a combination of both electronic conductivity of the electrode and ionic conductivity of electrolyte) can be calculated as in eq. 1:  = a-1lr-1 (1) where σ is the overall conductivity, a is the electrode area, l is the total battery thickness including anode, cathode, and separator, and r is the resistance [68]. furthermore, we approximate the slope of the δdq/dv peak position with a unit of v cm2 ma-1 as in eq. 2: 2k = ar (2) where k is the slope. combining both equations, the approximate overall conductivity can be calculated as in eq. 3:  = 2k-1l (3) the overall conductivities of all tno bf anode cells are shown in figure 6, as the sucrose content increased, the overall conductivity increased and matched the electronic conductivity trend measured for the tno bf anode pellets. note that for the 2.5 % sucrose content and higher, the overall conductivity approached and exceeded the ionic conductivity of the electrolyte used, which further supported that the rate capabilities of 2.5 % sucrose and higher sucrose content tno bf anode cells were not limited by the electronic conductivities of the electrode matrix. figure 6. slope of δdq/dv peak position (black) and overall conductivity (blue) of bf electrode with 0.0, 2.5, 5.0 and 7.5 % sucrose. lines added to guide the eye cycle life evaluation was conducted using cells with the relatively thinner bf electrodes at c/2 (2 ma cm-2), and discharge capacity and average discharge voltage can be found in figure 7. in the first cycle, discharge capacity reached 224, 227, 229, and 235 mah g-1 anode, and average discharge voltages were 2.285, 2.262, 2.296, and 2.302 for 0.0, 2.5, 5.0 and 7.5 % sucrose. the increasing trend in capacity/voltage was consistent with improvements in the electronic conductivity of the tno material electrode matrix. at the end of 100 cycles, all cells had similar capacity retentions above 60 %. however, the 5.0 % sucrose had the best retention of average discharge voltage, over 70 mv higher than that of 0.0 % sucrose. when examining the nyquist plots (figure 8) extracted from eis on the cells after cycling, the charge transfer resistance of the 5.0 % sucrose was also significantly lower than the 0.0 % sucrose cell. this outcome suggested the carbon coating improved interfacial stability, contributing to improved voltage retention. sem images of the surfaces of 0.0 % sucrose and 5.0 % sucrose anodes after cycling can be found in supplementary material, figure s-8. http://dx.doi.org/10.5599/jese.1655 j. electrochem. sci. eng. 13(4) (2023) 641-658 carbon coating for tinb2o7 li-ion battery anode 652 extensive cracks on the electrode are seen on both bf electrodes. such observation could originate from the large volume change upon cycling for the tno materials [29,69,70], which supported the similar capacity fade observed among the bf electrodes with varying amounts of carbon coating. figure 7. (a) discharge capacity and (b) average discharge voltage during cycle life evaluation of cells with thinner bf electrodes for 0.0 % sucrose (green), 2.5 % sucrose (orange), 5.0 % sucrose (red), and 7.5 % sucrose (purple). charge and discharge cycling was at the same rate of c/2 (2 ma cm-2) within a voltage window of 1-3.2 v (cell) figure 8. nyquist plot extracted from eis conducted on the thinner bf electrode cells for 0.0 (green), 2.5 (orange), 5.0 (red) and 7.5 % sucrose (purple) at the conclusion of 100 charge/discharge cycles future considerations the electronic conductivity improvement from carbon coating was beneficial for electrochemical energy storage outcomes for tno materials, but only up until a certain threshold of carbon added. more generally, many coating materials, including carbon, do not participate in the electrochemical battery reaction and sometimes exacerbate the interfacial kinetics if the coating layer is too thick [71,72]. in a thick electrode architecture, another disadvantage of excess carbon material is that the excess material can potentially take up volume within the electroactive particle interstitial space of the electrode. additional material within the pore/void volume laden with electrolyte will decrease effective ionic transport properties through the electrode [8,73]. as mentioned earlier, so long as the effective electrode electronic conductivity exceeds approximately 1 s m-1, or roughly the ionic conductivity of the electrolyte (lipf6 in carbonates) [8,39], the matrix electronic conductivity of the bf electrode no longer limits the capacity of the cell. in the case of higher conductivity electrolytes such as those containing lithium bis(fluorosulfonyl)imide [7,24,74], the threshold for electronic conductivity would be increased to match a value similar in conductivity of the relevant electrolyte. c. cai et al. j. electrochem. sci. eng. 13(4) (2023) 641-658 http://dx.doi.org/10.5599/jese.1655 653 the carbon coating improved interfacial stability but had little impact on limiting capacity loss during extended cycling. the capacity loss may have been due to volume change during cycling which induced electrode fracturing, which would be consistent with the bulk tno material properties during lithiation/delithiation. bf electrode particle morphology control and particle encapsulation to mitigate the negative impacts of volume change may provide future opportunities to improve the cycle life of bf electrodes [75-77]. conclusions tinb2o7 was successfully synthesized using a sol-gel method. sucrose was incorporated into the processing of the tno material into porous bf electrodes, where the sucrose played a dual role of the binder during powder processing and carbon source for generating carbon during thermal treatment. as the relative weight fraction of sucrose was increased, the relative amount of carbon in the electrode and the resulting initial measured electrode matrix electronic conductivity also 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(https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.compchemeng.2011.05.007 https://doi.org/10.1149/1945-7111/ac1892 https://doi.org/10.1016/j.jpowsour.2008.12.032 https://doi.org/10.1149/1945-7111/acb403 https://doi.org/10.1039/c4ee00508b https://doi.org/10.1016/j.jpowsour.2008.09.053 https://doi.org/10.1016/j.jallcom.2010.07.215 https://doi.org/10.1021/acsami.7b17771 https://doi.org/10.1149/2.0101414jes https://doi.org/10.1002/eem2.12136 https://doi.org/10.1039/c5qi00247h https://doi.org/10.1002/aenm.201200389 https://creativecommons.org/licenses/by/4.0/) a generalized mathematical model to understand the capacity fading in lithium ion batteries: effects of solvent and lithium transport doi:10.5599/jese.197 181 j. electrochem. sci. eng. 5(3) (2015) 181-195; doi: 10.5599/jese.197 open access : : issn 1847-9286 www.jese-online.org original scientific paper a generalized mathematical model to understand the capacity fading in lithium ion batteries effects of solvent and lithium transport abhishek deshpande, saksham phul, balaji krishnamurthy department of chemical engineering, bits pilani, hyderabad 500078, india corresponding author: balaji.krishb1@gmail.com; tel: +91-040-66303552; fax:+91-040-66303998 received: july 2, 2015; revised: october 10, 2015; accepted: october 10, 2015 abstract a general mathematical model to study capacity fading in lithium ion batteries is developed. the model assumes that the formation of the solid electrolyte interphase (sei) layer is the primary reason behind the capacity fading in lithium ion batteries. previous models have assumed that either the solvent or the lithium plays a key role in the film formation reaction which drives the capacity fading in lithium ion batteries. the current model postulates that the solvent species and lithium ions could play a limiting role in the capacity fade in a lithium ion battery. the model studies the concentration profiles of the solvent species and lithium ions at the electrode/film interphase as a function of diffusion and migration parameters. model predictions are found to fit experimental data very well. keywords kinetics; solid electrolyte interphase; diffusion; migration 1. introduction lithium ion batteries are currently the main focus of research in the field of battery technology. however, the main problems with the lithium ion battery technology remains the capacity fading which occurs in the battery. several mathematical models 1-15 currently exist which study the capacity fading in lithium ion batteries. most of these models assume that the formation of the solid electrolyte interphase layer (sei) on the anode during storage and the initial charging discharging cycles is the principal reason behind the capacity fading in lithium ion batteries. some of these models 6-8,15 assume that the decomposition of the solvent is the principal reason behind the formation of the sei layer. they assume that the solvent species controls the rate http://www.jese-online.org/ mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 182 determining step in the film formation reaction and the lithium ions do not play a major role in the process because they are in abundance. a few models 10 suggest that the transport of electrons and interstitials dominate the film formation process. some models have tried to study capacity fading in lithium ion batteries as a function of film formation on the cathode of the battery 16,17. however, none of the above existing models study the capacity fading in a lithium ion battery by trying to understand the transport of both the solvent species and lithium ions through the sei layer on the anode. we have tried to understand this in the present model. 2. model development figure 1 shows the schematic of the formation of the sei layer. the solvent and lithium ions are assumed to move from the film/electrolyte interphase to the film/electrode interphase. the motion of the solvent species is only because of diffusion while the motion of the lithium ions is due to diffusion and migration (due to the potential drop across the sei layer and the lithium ions being charged). the following reaction from agubra 18 is proposed to be the principal reaction behind the formation of the sei layer: (ch2o)2co(s) + 2e + 2li + → li2co3 + c2h4(g) (a) here, ethylene carbonate is considered to be the solvent species and lithium carbonate is considered to be the product. figure 1. schematic showing the transport of lithium ions and solvent species in the sei layer of a lithium ion battery. model assumptions a) the sei layer is considered to be homogenous in composition. it is considered to be a solid phase. b) the growth of the sei layer is considered to be a one dimensional phenomena on the anode. the sei formation reaction is assumed to occur only at the electrode/film interphase. c) the sei layer is assumed to comprise only of li2co3 formed as a result of the reaction between ethylene carbonate/dimethyl carbonate with the lithium ions. lipf6 is assumed to be the salt used in the reaction. a. deshpande at al. j. electrochem. sci. eng. 5(3) (2015) 181-195 doi:10.5599/jese.197 183 d) the capacity fading in the battery is assumed to be solely because of the formation of the sei layer on the anode of a lithium ion battery. e) the model assumes that both the concentration of lithium ions and solvent ions at the electrode/film interphase play a role in the film formation reaction. the lithium ions are assumed to move from the film/electrolyte interphase to the electrode/film interphase by diffusion and migration while the solvent ions are assumed to move from the film/electrolyte interphase to the electrode/film interphase only by diffusion (since the solvent species are not charged). f) the effects of film dissolution and acid attack on the film is considered to be negligible. the sei formation reaction is considered to proceed with a second order rate constant k (since both the lithium and solvent species play a key role). g) the model assumes that the formation of the sei layer on the anode is the principal reason for capacity fading in lithium ion batteries. the capacity losses as a result of film formation on the cathode is assumed to be negligible. h) the model assumes that the electrode/film interphase is the moving boundary and the film/electrolyte interphase remains stationary. i) the value of the potential at the electrode/film interphase is assumed to zero. this in essence means that the model assumes that while the potential drop across the sei layer is a major factor in the transport of lithium ions, it does not play a major role in the reaction at the electrode/film interphase. the potential drop across the sei layer is assumed not to vary with time and decreases linearly from the film/electrolyte interphase to the electrode/film interphase. we follow our earlier work 19 and assume that the diffusivity and mobility of lithium ions are related by the nernst einstein equation. j) in accordance with the model of battaglia and newman 20, the model assumes that an initial film thickness for computational purposes. the concentrations of the solvent species and lithium ions are assumed to a constant across the very thin film at initial time. k) the concentration change in the electrolyte as a result of solvent consumption in the sei layer is considered to be negligible. 3. transport equations the differential mass balance for a material species k in a film phase is given by k k k c n r t      (1) dilute solution theory specifies the mass flux ni as i k k k k k k kn z u fc c d c c v     (2) as the sum of the migration, diffusion and convective contributions. since the convective contribution is zero for both lithium ions and solvent molecules in this system, the transport equations for the solvent molecules in this particular system can be written as (assuming that the homogenous reaction in the film is zero) 2 s s s 2 c c d t z      (3) since the only mode of transport of solvent molecules is through diffusion. from equation (2) and relating the diffusivity of lithium ions with the mobility by nernst einstein’s equation 19, we can write the following equation to define the flux of lithium ions j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 184 * i i i i i cv n d z c l z         (4) where v*refers to the dimensionless potential drop across the sei layer and l refers to the thickness of the sei layer at any given time. inserting equation (4) into (1), gives us the governing equation for the transport and material balance of lithium ions (assuming that the homogenous reaction within the film is zero and the transport by convection is also zero) 2 * li li li li li2 c c cv d yd t z l z          (5) v* refers to the dimensionless potential drop across the sei layer. the stoichiometric relation between the consumption of solvent molecules and lithium ions is taken into account in formulating these equations. the boundary conditions for solving equations (3) and (5) are given as follows: at the film/electrolyte interphase (z = 0), the initial concentration of cs is assumed to be co mol/m 3 which is a defined and known concentration. the initial concentration of lithium at the film/electrolyte interphase at z = 0 is also assumed to be the same as the solvent concentration and hence is taken as co mol/m 3 . at the electrode/film interphase (z=l(t)), the boundary conditions are defined by the rates of reactions for the particular species. thus the boundary conditions can be defined as at z = 0 cs = c0 mol/m 3 (for all t) (6) at z = 0 cli = c0 mol/m 3 (for all t) (7) at z = l(t) s s li s l( ) c ad kc c v z     (8) at z = l(t) li li li s l(2 ) c ad kc c v z     (9) at t = 0 ds = cli = c0(for all z) (10) where a is the cross sectional area of the reaction zone(for the formation of the sei layer) and vl is the volume of the reaction zone. equations 8 and 9 equate the diffusive fluxes at the interphases with the reaction rate of the film formation. equations 3, 5, 6, 7, 8, 9 and 10 can be written in the following dimensionless forms for the concentration profiles of the solvent molecules and lithium ions. 2 so so 2* *2 c c d t z       (11) 2 *lio lio lio 1 1* * * c c c d yd v t z z          (12) at z* = 0 cso = cli0 = 1 (for all t*) (13) at z* = 1 so 2 so lio * c d c c z        (14) at z* = 1 lio 1 so lio2 * c d c c z        (15) a. deshpande at al. j. electrochem. sci. eng. 5(3) (2015) 181-195 doi:10.5599/jese.197 185 at t* = 0 cso = cli0 = 1 (for all z*) (16) the definition of the different variables and dimensionless groups are given in tables 1 and 2. equations 11-16, are the dimensionless equations and boundary conditions to be solved for evaluating the concentration profiles of the solvent and lithium species at the electrode/film interphase. further the fractional capacity loss in the battery is given by the equation, dimensionless fractional capacity loss = (clioi – cliot)/clioi where clioi is the dimensionless initial concentration of lithium ions at the electrode/film interphase, cliot is the dimensionless concentration of lithium ions at the electrode/film interphase at any given time. by calculating the cliot value from the code, we are able to calculate the loss of lithium ions at any given time which relates to the lithium consumed in the reaction forming the sei layer. using the equation for fractional capacity loss, we find out the capacity loss in the battery. table 1. characteristic quantities used for dimensional analysis in the model dimensionless variable dimensional variable characteristic quantity z* z l initial film thickness cso, clio cs,cli co initial concentration of lithium at electrolyte/film interphase v v rt/f t* t 1/kco table 2. interpretation of dimensionless groups used in model dimensionless groups interpretation d1 = dli/kcol 2 the rate of diffusion coefficient of lithium to the rate constant of the sei formation reaction. d2 =ds/kcol 2 the ratio of diffusion coefficient of solvent species to the rate constant of the sei formation reaction. v* = fv/rt) dimensionless voltage 4. numerical solution equations 11 to 16 were solved numerically using matlab© with its predefined function called the ‘pdepe solver’ which solves initial-boundary value problems for systems of partial differential equations in one space variable z and time t. the pdepe solver converts the pdes (partial differential equations) to odes (ordinary differential equations) using a second-order accurate spatial discretization based on a fixed set of user-specified nodes. in this work, a set of equations consisting of two coupled partial differential equations with dependent boundary conditions which represent the solvent and lithium concentrations were solved for the concentration profiles at the electrode/film interphase using the pdepe solver. for the accuracy of solution, a fine mesh was created by dividing the film layer thickness into 100 spacesteps and similarly dividing the total time into 100 timesteps. this ensured uniformity in all the calculations. equations 11-16 were solved using matlab for the defined geometry for the concentration profiles of the solvent and lithium ions. 5. results and discussion figure 2 shows the variation of dimensionless lithium concentration at the electrode/film interphase as a function of dimensionless time with varying values of d1 and constant values of d2. j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 186 since the capacity loss of the battery is expressed in terms of lithium concentration, we have varied the value of d1 and kept d2 a constant. we wish to emphasize here that d1 and d2 are dependent on l, the characteristic length. however, we have assumed the characteristic length to be the initial value of film thickness used for computational purposes (0.1 nm). this essentially means that d1 and d2 do not vary with time. the potential drop across the sei layer is kept at 0.0 v for this simulation (this in effect means that the lithium ions are driven only by diffusion and not by migration). d1 is varied to find the effect of the diffusivity of lithium ions on the concentration of the lithium ions at the electrode/film interphase and the reaction at the interphase. simulations are run for 3 different values of d1. assuming that the values of the reaction constant k is kept constant and co is constant, this means that the only parameters which is varying is the diffusion coefficient of lithium. literature postulates that the diffusion coefficient of lithium through crystalline solids varies anywhere between 10 -7 cm 2 /s to 10 -15 cm 2 /s 21,22. figure 2 shows that with increasing values of the diffusivity of the lithium ions, there is a defined change in the concentration of lithium ions at the electrode/film interphase. the higher the value of the diffusion coefficient of lithium ions, the higher the concentration of lithium ions at the electrode/film interphase. figure 2. variation of dimensionless lithium ions at the electrode/film interphase as a function of varying d1 values. d2 = 1 and potential drop across the sei layer is 0. this indicates that though increasing diffusivity of the lithium ions increases the reaction rate at the electrode/film interphase, there is an increase in the accumulation of lithium ions at the electrode/film interphase which is reflected in the results. figure 3 shows the concentration profile of the solvent species for varying values of d1 and a constant value of d2. with higher values of d1, the reaction rate of the lithium ions at the electrode/film interphase increases. this results in additional consumption of lithium ions and solvent molecules. however, with d2 being maintained constant, the diffusion coefficient of solvent molecules remains a constant leading to a constant flux of solvent molecules to the electrode/film interphase. this leads to decreasing values of the solvent concentration at the electrode/film interphase with increasing values of d1. a. deshpande at al. j. electrochem. sci. eng. 5(3) (2015) 181-195 doi:10.5599/jese.197 187 figure 3. variation of dimensionless solvent concentration at the electrode/film interphase as a function of varying d1 values. d2 = 1 and v = 0. figure 4 shows the variation of dimensionless concentration of lithium ions at the electrode/film interphase as a function of varying values of d1 and constant value of d2, but with a potential drop of 0.2 v applied across the sei layer. while the concentration profiles are found to follow a similar pattern as figure 2, it is seen that the consumption of lithium ions is more when the potential drop is applied across the sei layer. the potential drop seems to be increasing the transport of lithium ions from the film/electrolyte interphase to the electrode/film interphase through migration effects. this increases the reaction rate for the sei film formation which in turn increases the consumption of lithium ions at the interphase. a comparison of figure 2 with figure 4 for constant values of d1 indicates this. to further study the effect of potential drop across the sei layer on the film formation process, we study the concentration of lithium ions at the electrode/film interphase for three different values of voltages 0.0, 0.1 and 0.2 for constant values of d1 and d2. figure 5 shows this graph. the dimensionless lithium concentration is seen to change with changing values of applied potential across the sei layer. it is seen that potential drops across the sei layer ranging from 0.1 to 0.2 v causes additional consumption of lithium ions at the interphase. this in essence is because of the additional transport of lithium ions from the film/electrolyte interphase to the electrode/film interphase caused by the migration effect due to the potential drop across the sei layer. we wish to indicate that very little experimental data exists in literature which actually studies the potential drop across the sei layer. since the sei film thickness used in our experimental fits are of the order of 10 -300 nm, we have used a maximum potential drop of 0.2 v across the sei layer. however, while several experimentalists have looked at the onset potential for the formation of the sei layer, no data exists which actually measures the potential drop across the sei layer. j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 188 figure 4. graph showing the variation of dimensionless lithium concentration at the electrode/film interphase as a function of varying d1 values. d2 = 1 and v = 0.2. figure 5. effect of potential drop across the sei layer on the concentration of lithium ions at the electrode/film interphase. figure 6 shows the concentration of the solvent species at the electrode/film interphase at constant value of d1 and varying values of d2, at v = 0. comparing figure 6 and figure 7 (solvent concentration at the electrode/film interphase for v = 0.2) it is seen that there is very little difference in the solvent concentration as a result of the potential drop across the sei layer. this shows that while the potential drop across the sei layer plays a major role in the transport of lithium ions, it plays very little role in the transport of solvent species across the sei layer. however, the increase in the reaction rate of lithium ions caused by the potential drop across the sei layer can cause additional consumption of the solvent species. a. deshpande at al. j. electrochem. sci. eng. 5(3) (2015) 181-195 doi:10.5599/jese.197 189 figure 6. variation of dimensionless solvent species at the electrode/film interphase with varying d2 values, v = 0.0. figure 7. variation of dimensionless solvent species at the electrode/film interphase with varying d2 values, v = 0.2. figure 8 shows the comparison of modeling results for capacity fade with experimental data for the he 12 type cells. . since the capacity fade is directly related to the fraction of lithium ions, we have kept d2 constant for these simulations (10 5 ). the potential drop across the sei layer for these simulations is kept at 0.2 v. the values of d1 are varied to fit modeling results with experimental data. it is seen that higher capacity fade is seen at lower values of d1. thickness of the sei layer can j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 190 be found from capacity loss curves with the equation given in our earlier work 6. table 3 shows the list of parameters used for fitting simulation results with experimental data. figure 8. simulation fit with experimental data for he type cells 12 table 3. parameters used for fitting simulation results with experimental data parameters range of values co 10 3 10 5 mol/m 3 k 10 -7 m 3 /mol s 10 -14 m 3 /mol s l 0.1 nm dli 10 -12 m 2 /s 10 -22 m 2 /s ds 10 -19 m 2 /s 10 -28 m 2 /s figure 9 shows the comparison of modeling results with experimental data for the mp type cells 12. simulation results are run at four different temperatures-15, 30, 45 and 60 °c. d2 values are maintained a constant at 10 5 for these simulations. while modeling results are seen to fit very well with experimental data, it is seen that the highest capacity fading is seen at the lowest values of d1. since d1 = dli/kcol 2 ) and since the values of co and l remain a constant, the only varying parameters with temperature are dli and k. the diffusion coefficient of lithium ions is seen to vary with temperature according to the eyring’s equation d = do e -ed/rt and the rate constant is seen to vary with temperature according to the arrhenius equation k = ao e -ea/rt . hence, both figures 8 and 9 indicate that highest capacity fade is seen at higher values of the rate constant indicating that the rate of increase of the rate constant k with temperature is much higher than the rate of increase of diffusion coefficient with temperature. this in turn indicates that the activation energy barrier for the kinetic reaction is lower than the diffusion process implying that the kinetics of the film formation reaction is a much faster process than the diffusion. the results confirm the observations from previous researchers 15. a. deshpande at al. j. electrochem. sci. eng. 5(3) (2015) 181-195 doi:10.5599/jese.197 191 figure 9. simulation fit with experimental data for mp type cells 12 figure 10 and 11 shows the comparison of modeling results with experimental data for cells studied by grolleu et al. 23 and prada et al. 24. as seen previously, modeling results are seen to fit very well with experimental data. figures 12,13 and 14 show the comparison of modeling results with experimental data from saft dd 25 , yardney cells 25 and boryann liaw et al. 26. modeling fits yield the same results-the activation energy barrier for the kinetic rate constant is much lower than that of the diffusion coefficient. figure 10. graph showing mmodeling comparison with experimental data obtained from grolleu et al. 23. j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 192 figure 11. modeling comparison with experimental data obtained from prada et al. 24. figure 12. modeling comparison with experimental data obtained from saft dd 25 cells. a. deshpande at al. j. electrochem. sci. eng. 5(3) (2015) 181-195 doi:10.5599/jese.197 193 figure 13. modeling comparison with experimental data obtained from yardney type cells25 figure 14. modeling comparison with experimental data obtained from boryann et al. 26 6. conclusions a mathematical model is developed to study the capacity fading in lithium ion batteries. unlike previous models, the model assumes that the capacity fading in lithium ion batteries is a function of solvent and lithium transport. transport equations are developed to understand the concentration profiles of lithium ions and solvent species at the electrode/film interphase. model results j. electrochem. sci. eng. 5(3) (2015) 181-195 mathematical model of capacity fading in li ion batteries 194 indicate that while the diffusion coefficient of lithium ions and the diffusion coefficient of solvent species play a role in enhancing the film formation reaction at the electrode/film interphase, the potential drop across the sei layer significantly enhances the transport of lithium ions across the sei layer and hence the reaction rate at the electrode/film interphase. however, the potential drop across the sei layer is not seen to affect the transport of solvent species across the sei layer. model results are found to predicate experimental data very well. future work would include studying the effects of the potential drop on the reaction rate at the electrode/film interphase. acknowledgements: we acknowledge bits pilani, hyderabad, india for supporting us during the course of this work. list of symbols ni flux of species, m 2 s -1 f charge on 1 mole of electron coulombs ∇∅ potential gradient, v m -1 di diffusivity of species i, m 2 s -1 dli diffusion coefficient of lithium ions, m 2 s -1 ds diffusion coefficient of solvent ions, m 2 s -1 z distance across the sei film, m zi valence of species, i ui ionic mobility of species i, m 2 v -1 s -1 ci concentration of the species, mol m -3 v convective velocity of species, m s -1 v potential drop across sei layer (this is also represented by v in the graphs), v l sei layer thickness, nm t time, s y stoichiometric coefficient k second order rate constant of the reaction, l mol -1 s -1 co dimensional initial concentration of lithium ions, mol m -3 cli dimensional concentration of lithium ions, mol m -3 cs dimensional concentration of solvent species, mol m -3 clio dimensionless concentration of lithium ions cso dimensionless concentration of solvent species t* dimensionless time z* dimensionless distance between electrodes a cross section area of the electrode/film interphase where film formation happens, m 2 vl volume of the electrode/interphase region where film formation happens, m 3 references: [1] y. s. wang, j. t. lee, electrochim. acta 142 (2014) 34-42. 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[24] e. prada, j. barnard, f. huet, j. electrochem. soc. 160 (2013) a 616-a 628. [25] b. v. ratnakumar, m. c.smar, l. d. whitcanack, ecs trans. 25(36) (2010) 297-305. [26] b. liaw, p. roth, r. jungst, d .doughty, j. power sources 119 (2003) 874-882. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ 404 not found {poly (dl-valine) electro-polymerized carbon nanotube paste sensor for determination of antihistamine drug cetirizine} http://dx.doi.org/10.5599/jese.934 27 j. electrochem. sci. eng. 11(1) (2021) 27-38; http://dx.doi.org/10.5599/jese.934 open access: issn 1847-9286 www.jese-online.org original scientific paper poly (dl-valine) electro-polymerized carbon nanotube paste sensor for determination of antihistamine drug cetirizine tigari girish1, jamballi g. manjunatha1,, pemmatte a. pushpanjali1, nambudumada s. prinith1, doddarasinakere k. ravishankar2 and gurumallappa siddaraju3 1department of chemistry, fmkmc college, madikeri, mangalore university constituent college, karnataka, india 2department of chemistry, sri. mahadeshwara government first grade college, kollegal, chamarajanagar, karnataka, india 3 department of chemistry, jss college for women, chamarajanagar, karnataka, india corresponding author: manju1853@gmail.com; tel: +91-0827222833 received: november 18, 2020; revised: december 11, 2020; accepted: december 12, 2020 abstract poly (dl-valine) modified multiwalled carbon nanotube paste sensor (pvlmcntps) was prepared by electro-polymerization route. pvlmcntps and bare multiwalled carbon nanotube paste sensor (bmcntps) morphologies and sensing properties for cetirizine (ctz) were confirmed through a field emission scanning electron microscope (fe-sem) and electrochemical studies, respectively. in contrast to bmcntps, pvlmcntps surface composite creates an electrocatalytic impact on the oxidation of ctz. pvlmcntps properties were optimized using parameters such as accumulation time, number of polymerization cycles, solution ph, and scan rate. the optimized pvlmcntps was applied for the determination of cetirizine in 0.1 m phosphate buffer solution (pbs) of ph 7.0, using cyclic voltammetry (cv). it is shown that pvlmcntps provides analytical linearity from 2.0 to 80 µm, with detection limit of 0.11 µm for ctz determination. pvlmcntps was found highly selective for ctz in presence of some interfering organic molecules. the stable and selective pvlmcntps was applied for ctz determination in pharmaceutical pills with satisfactory results. keywords polymer modified sensor; drug analysis; cyclic voltammetry introduction cetirizine (ctz) is an antihistamine drug which is usually prescribed for allergic symptoms like running nose, itching, hives, watery eyes, body ache, etc. it acts as an allergy reliever by blocking http://dx.doi.org/10.5599/jese.934 http://dx.doi.org/10.5599/jese.934 http://www.jese-online.org/ mailto:manju1853@gmail.com j. electrochem. sci. eng. 11(1) (2021) 27-38 polymer modified sensor 28 histamine which is produced during allergic reactions in the human body. the overdose of ctz can trigger many adverse effects such as stomach pain, tiredness, vomiting, diarrhea, drowsiness, etc., while in serious conditions, one may experience breathlessness [1-5]. therefore, it is significant to develop the analytical tool for determination of ctz drug. drug testing and analysis are important in the quality and quantity control of pharmaceutical utilities, and therefore, fast, simple and effective analytical methods are required for the analysis of drug molecules [6]. there are many methods previously used for ctz determination such as electrophoresis [7], high performance liquid chromatography [8], spectrophotometry [9], hplc/ms [10], spectrofluorimetry [11], and electrochemical methods [12,13]. all these methods, except electrochemical methods, have some drawbacks like time consuming, requirements for more organic solvents and reagents, and experts for handling, and also inclusion of complicated and expensive procedures [14]. nowadays, the field of interest has been focused on electrochemical devices and their applications, almost due to their benefits like low cost, simple sample preparations, low consumption of reagents and analytes, fast testing time, and portability with high sensitivity [15-20]. looking from the aspect of materials for electrochemical sensors, one must mention multiwalled carbon nanotubes (mwcnts). mwcnt is the widely used allotropy of carbon material for different applications including sensors, supercapacitors, drug delivery platforms, electronic chips, fuel cells, energy storage devices, additives in polymers, flat panel display, charge-discharge tubes, etc. [21-24]. to further improve their usefulness, mwcnts are frequently modified. the high utility of modified mwcnts is due to their enhanced electrical, thermal, mechanical, electronic, and stability properties [25-27]. different modification techniques such as surfactant immobilization, electropolymerization, etc. have already been reported. modification of a sensing electrode is usually performed to improve the sensing properties of electrodes [28-35]. in the present work, bare multiwalled carbon nanotube paste sensor (bmcntps) is coated with poly (dl-valine) through electro-polymerization. the formed poly (dl-valine) multiwalled carbon nanotube sensor (pvlmcntps) is used as a sensing surface for the determination of ctz in real samples. experimental instruments model chi-6038 potentiostat (procured from usa) connected with three-electrode arrangement and a computer was used for all voltammetric measurements. the three-electrode system included a working electrode (pvlmcntps/bmcntps), a counter electrode (platinum wire) and a reference electrode (hg/hg2cl2). all measurements were performed at laboratory temperature (27 ± 2 °c). chemicals and preparations riboflavin (rf), paracetamol (pc), and dl-valine (vl) were procured from molychem, india. cetirizine (ctz) is procured from tci japan. multiwalled carbon nanotubes were received from sisco laboratories ltd, mumbai, india. the remaining chemicals were of ar grade. ctz, rf, pc (10-4 m) standard solutions were prepared using distilled water. phosphate buffers standards of different ph were prepared by mixing na2hpo4 (0.1 m) and nah2po4 (0.1 m). sensors preparation bmcntps material was prepared by physical mixing of multiwalled carbon nanotubes and silicone oil in a proportion of 60:40 for 20 min. the obtained uniform paste was filled into the cavity of teflon rod pre-inserted with copper wire for electrical connection. surface of electrode was t. girish et al. j. electrochem. sci. eng. 11(1) (2021) 27-38 http://dx.doi.org/10.5599/jese.934 29 polished with a tissue to provide uniform surface and to avoid disturbance in electrochemical measurements. results and discussion preparation of pvlmcntps through electro-polymerization electrochemical polymerization on bmcntps was performed by the potential scanning in pbs (ph 5.5, 0.1 m) containing 1.0 mm dl-valine in the potential window -0.2 to 1.7 v with a scan rate of 0.1 v/s (figure 1a). as the number of scanning cycles passed, cv curves sloped down at positive potentials. this shows the formation of a polymer layer on electrode surface. the number of polymerization scans can affect the electrode properties, and so it was optimized by varying cycles from 5-20 as shown in figure 1b. five cv cycles were fine-tuned for ctz sensing, while after 5 cycles the cv shape and peak current values declined. the structure of electro-generated poly (dl-valine) is shown in figure 1c. a b c figure 1. (a) electrochemical polymerization of vl (1.0 mm) at bmcntp surface in 0.1 pbs, ph 5.5 with scan rate 0.1 v/s; (b) peak current values of ctz response against the number of polymerization cycles; (c) structure of electro-generated poly (dl-valine) film on bmcntp electrode surface. surface characterization of sensors figure 2 presents fesem images of bmcntps and pvlmcntps surfaces, where rather different surface textures can be clearly seen. whereas bmcntps shows arrangement of carbon layers with nanotubes on the surface, pvlmcntps provides a porous and fibrous morphology due to poly (dl-valine) layer growth on bmcntps. http://dx.doi.org/10.5599/jese.934 j. electrochem. sci. eng. 11(1) (2021) 27-38 polymer modified sensor 30 figure 2. fesem images of (a) bmcntps and (b) pvlmcntps. electrochemical characterization and calculation of effective surface area electrochemical characterizations of bmcntps and pvlmcntps were performed in 0.1 m kcl containing 2.0 mm [fe(cn)6]3-/4redox system using cv at 0.1 v s-1 scan rate, as depicted in figure 3. bmcntps surface interaction with [fe(cn)6]3-/4gives poor current signals with higher separation of peak potential values (δep). at the other side, pvlmcntps gives higher current response with lower δep upon interaction with fe(cn)6]3-/4-. the surface area of bmcntps and pvlmcntps was determined using randles-ševčik formula [36]: ip = kn3/2ad1/2c0 1/2 (1) here, k is the constant equal to 2.69×105 (c mol-1 v-1/2), ip is peak current (a), n is the number of transferred electrons, a is effective surface area (cm2), d is diffusion coefficient (cm2 s-1),  is voltage sweep rate (vs-1), and c0 is concentration of [fe(cn)6]3-/4in mol cm-3. using n = 1 and d = 7.6×10-6 cm2/s for [fe(cn)6]3-/4-, the calculated effective surface area of bmcntps and pvlmcntps was 0.034 and 0.13 cm2, respectively for 2 mol cm-3 [fe(cn)6]3-/4-. figure 3. cv responses (0.1 v/s) of bmcntps (curve a) and pvlmcntps (curve b) in 0.1 m kcl containing 2 mm [fe(cn)6] 3-/4-. electrochemical oxidation of ctz cv behavior of ctz (0.1 mm) was tested at bmcntps and pvlmcntps in pbs (0.1 m, ph 7.0) at the sweep rate 0.1 v s-1, as exhibited in figure 4. the curve a in figure 4 represents cv of pvlmcntps for blank solution. both bmcntps (curve b) and pvlmcntps (curve c) detect ctz oxidation. bmcntps recognizes ctz at the oxidation potential of 0.89 v with poor current response, while t. girish et al. j. electrochem. sci. eng. 11(1) (2021) 27-38 http://dx.doi.org/10.5599/jese.934 31 pvlmcntps gives two superior signals at 0.765 v and 0.888 v. at pvlmcntps, the current signal is five times higher and over-potential is reduced by 15.1 % compared to bmcntps. since cv of ctz does not show a peak in the reverse scan, the ctz process is considered irreversible. figure 4. cv responses (0.1 v/s) of ctz (0.1 mm) in 0.1 m pbs of ph 7.0 at pvlmcntps (curve c) and bmcntps (curve b) against blank solution at pvlmcntps (curve a). effect of accumulation time accumulation of analyte on electrode surface influences the electrode performance. the effect of accumulation time (0-60 s) on ctz (0.1 mm) detection at pvlmcntps was analyzed in 0.1 m pbs, ph 7.0, as shown in figure 5. the maximum current signal is achieved at 10 s, whereas for further increases of accumulation time, peak currents were decreased. this is probably due saturation in accumulation of analyte on the sensor surface. hence, 10 s of accumulation was optimized for further measurements. time, s figure 5. effect of accumulation of time on ctz (0.1 mm) detection at pvlmcntps in 0.1 m pbs, ph 7.0 at 0.1 v/s scan rate. http://dx.doi.org/10.5599/jese.934 j. electrochem. sci. eng. 11(1) (2021) 27-38 polymer modified sensor 32 sweep rate effect influence of the potential scan rate was carried out in order to elucidate the kinetics of the process at the electrode-analyte interface. cvs as functions of sweep rate (0.100-0.250 v/s) were recorded for ctz oxidation in 0.1 m pbs, ph 7.0, as shown in figure 6a. the peak current values of ctz oxidation are increased with the change in the scan rate, while peak potential values are shifted towards more positive side. the plots of peak current vs. square root of the scan rate (figure 6b), and log ipa against log scan rate (figure 6c) give clear evidence for diffusion process as per following equations: ip = 4.47×10-6 +1.3467×10-4 1/2 r=0.99 (2) log ip=0.56 log  + 2.12 r=0.99 (3) the number of electrons involved in the electrochemical oxidation of ctz is calculated using the following equation [37]:   = + − 0 pa ln 1 rt e e ( )nf (4) where n is number of electrons involved, r is gas constant (8.314 j k−1mol−1), t is temperature, f is faraday’s constant (96,485 c mol-1) and  is coefficient of charge transfer, assumed to be 0.5 for irreversible behaviour [38]. the number of electrons transferred for ctz electro-oxidation was found to be 1.027 ≈ 1. the electron transfer mechanism of ctz oxidation is sketched in scheme 1. a b c figure 6. (a) cv responses at different sweep rates (0.100-0.250 v/s) for ctz (0.1 mm) at pvlmcntps in 0.1 m pbs, ph 7.0; (b) plot of peak current vs. 1/2; (c) plot of log ipa vs. log v. t. girish et al. j. electrochem. sci. eng. 11(1) (2021) 27-38 http://dx.doi.org/10.5599/jese.934 33 scheme 1. electron transfer mechanism of ctz oxidation. selection of ph according to the given mechanism of ctz oxidation, the electrochemical sensing is strongly affected by ph of the supporting electrolyte. the effect of ph changes from 5.5 to 7.5 on the sensing of ctz (0.1 mm) in 0.1 m pbs at pvlmcntps is represented in figure 7a. obviously, the maximum anodic peak current (ipa) of ctz oxidation was obtained at ph 7.0. hence, pbs with ph 7.0 was chosen for further experimentation. as shown in figure 7b, the plot of peak potential (epa) against buffer ph is found linear according to the following equation: epa = 1.126 +0.051 ph (r = 0.99) (5) a b figure 7. (a) cv responses (0.1 v/s) of ctz (0.1 mm) at pvlmcntps in 0.1 m pbs of different ph (5.5-7.5); (b) graphical plot of peak potential vs. ph. http://dx.doi.org/10.5599/jese.934 j. electrochem. sci. eng. 11(1) (2021) 27-38 polymer modified sensor 34 from the linear plot in figure 7b, the slope was estimated to be 0.051, which is near the theoretical slope of 0.059. this strongly suggests that the electrochemical oxidation of ctz at pvlmcntps involves equal number of electrons and protons. construction of calibration plot in order to determine the analytical performance of sensor, the effect of different concentrations of ctz were studied by cv and shown in figure 8a. as presented in figure 8b, peak currents increased linearly with increase of the concentration of ctz in the range 2.0-80 µm. the linearity of curve is described as: ip / a = 9.68×10-6 + 0.223 m with r = 0.99. the detection limit (lod) and limit of quantification (loq) values were found to be 0.11 µm (3 s/n) and 0.372 µm (10 s/n) [39], with sensitivity of 0.223 a/m. a b figure 8. (a) cv responses (0.1 v/s) for ctz concentrations from 2.0-80 µm at pvlmcntps in 0.1 m pbs, ph 7.0; (b) calibration plot of ctz. table 1 presents a comparison of detection limit (lod) values for ctz determination of here proposed pvlmcntps with already existing sensors described in the literature [40-44]. it is clear that the obtained result for pvlmcntps is comparable with previous literature results. table 1. comparison of obtained lod values for ctz determination at pvlmcntps with literature data on other electrochemical sensors. electrochemical sensor lod, µm method of analysis reference glassy carbon electrode modified with multiwalled carbon nanotubes 0.071 cyclic voltammetry [40] glassy carbon electrode 4.5 differential pulse voltammetry [41] carbon black / glassy carbon electrode 0.4 square-wave adsorptive anodic stripping voltammetry [42] pre-treated graphite pencil 0.16 square wave voltammetry [43] multi-walled carbon nanotube and platinum nanoparticles nanocomposite modified carbon paste electrode 0.058 adsorptive stripping differential pulse voltammetry [44] pvlmcntps 0.118 cyclic voltammetry present work resolution of ctz in presence of riboflavin (rf) and paracetamol (pc) cv responses showing resolution of oxidation peaks for 0.1 mm of ctz, rf and pc at bmcntps (dashed line) and pvlmcntps (solid line) are depicted in figure 9. the bmcntps shows oxidation t. girish et al. j. electrochem. sci. eng. 11(1) (2021) 27-38 http://dx.doi.org/10.5599/jese.934 35 peaks for ctz, rf and pc at 0.913, -0.592 and 0.456 v with low current responses. pvlmcntps, however, exhibits enhanced current signals, showing clear peak separation for ctz, rf and pc at 0.771, -0.472 and 0.332 v, respectively. this ultimately shows that analysis of ctz in the mixture is more feasible at pvlmcntps than bmcntps. figure 9. cv responses (0.1 v/s) showing simultaneous separation of rf, pc and ctz (0.1 mm) oxidation peaks at pvlmcntps (solid line) and bmcntps (dashed line) in 0.1 m pbs, ph 7.0. stability and reproducibility reproducibility was tested at three newly formed pvlmcntps, where the results of ctz determination were obtained with relative standard deviation (rsd) of 1.24 %. this rsd result shows good reproducibility of the prepared sensor. the stability of the proposed sensor was examined by keeping the sensor in a dry place for 24 hours prior ctz detection. even after 1 day of standing, the sensor provided 93.15 % current retention for ctz determination. this clarifies good storage stability of the proposed pvlmcntps. selectivity to test reliability of the proposed pvlmcntps, the binding selectivity for 0.1 mm ctz detection in presence of various organic molecules such as ascorbic acid (aa), glucose (gu), riboflavin (rf), paracetamol (pc), ciprofloxacin (cf), oxalic acid (oa), anthrone (an), rhodamine (rh) (0.1 mm) was tested in 0.1 m pbs using cv technique. figure 10 shows the change of peak potential (ep) value of ctz oxidation with addition of 0.1 mm of each organic molecule. the results in figure 10 suggest no significant interference of these organic molecules for ctz detection, confirming thus the specificity of pvlmcntps. figure 10. graphical representation of effect of different organic molecules for detection of ctz at pvlmcntps http://dx.doi.org/10.5599/jese.934 j. electrochem. sci. eng. 11(1) (2021) 27-38 polymer modified sensor 36 ctz tablet sample analysis to examine the practicability of the proposed strategy, pvlmcntps was utilized to detect ctz concentration in ctz tablets. ctz tablets were firstly powdered uniformly using mortar and then 0.1 mm ctz solution was prepared using tablet powder. the analysis is performed in 0.1 m pbs, ph 7.0 using the standard addition method. the results are given in table 2, showing that ctz recovery in the pharmaceutical product is obtained between 90-102.4 %. table 2. real sample ctz analysis and recovery at pvlmcntps. sample amount of ctz added, µm amount of ctz found, µm recovery, % ctz 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[44] p. k. kalambate, a. k. srivastava, sensors and actuators b: chemical 233 (2016) 237-248 https://doi.org/10.1016/j.snb.2016.04.063. ©2021 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.jelechem.2017.11.013 https://doi.org/10.1007/%1fs00706-019-2384-2 https://doi.org/10.1007/%1fs00706-019-2384-2 https://creativecommons.org/licenses/by/4.0/) study of corrosion inhibition effect of expired ibuprofen drug on copper in sulfuric acid solution http://dx.doi.org/10.5599/jese.1867 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1867 open access : : issn 1847-9286 www.jese-online.org original scientific paper corrosion inhibition effect of expired ibuprofen drug on copper in sulfuric acid solution said el harrari, ayoub salim, driss takky and youssef naimi laboratory of physical chemistry of materials, faculty of sciences ben m’sick, hassan ii university of casablanca, morocco corresponding author: said.elharrari@gmail.com received: may 2, 2023; accepted: july 31, 2023; published: august 7, 2023 abstract the application of copper as a material in various fields is widely recognized. however, in acidic environments, the electrical and mechanical properties of copper undergo negative alterations, resulting in its dissolution. to protect copper from degradation, the most effective approach is to employ inhibitors. hence, in this paper, the expired ibuprofen drug has been investigated as a corrosion inhibitor for copper in 0.5 m h2so4, employing weight loss and electrochemical tests. compared with the pharmaceutical products used by other researchers in this field, the results showed that ibuprofen is highly effective in protecting copper from corrosion. it was noted that the inhibitory efficacy of ibuprofen increases with concentration. in addition, it was found that its adsorption follows langmuir isotherm. keywords expired drugs; copper corrosion; inhibition efficiency; weight loss; electrochemical tests introduction copper is utilized as a conductor in the electronics sector because it is thermally and electrically conductive. copper can dissolve under certain situations [1], which has detrimental effects on metal characteristics and may result in large financial losses [2]. therefore, researchers have created a variety of corrosion inhibitors to protect metals and alloys against corroding by acids like sulphuric acid. these blockers are either organic or inorganic compounds [3,4]. in comparison to organic chemicals, particularly azole compounds, inorganic compounds have lower inhibitory effectiveness [5,6]. many of these substances are poisonous to the environment and therefore, research has been focused on developing waste disposal methods and corrosion inhibitors that are acceptable to the environment. pharmacological substances [7] are potentially environmentally friendly corrosion inhibitors for copper, according to numerous studies and investigations [8], and pharmaceuticals that are out-of-date or flawed could also be used as conventional inhibitors. green corrosion inhibitors are natural substances that find application not only in pharmaceutical production [9] but also in plant extraction, which helps to mitigate costs while ensuring the continued availability of http://dx.doi.org/10.5599/jese.1867 http://www.jese-online.org/ mailto:said.elharrari@gmail.com j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion inhibition effect of expired ibuprofen drug 2 traditional medicines [10]. about 90 % of the medicine's active ingredient remains stable after expiration for a long time [11]. drugs are released into the environment through household garbage [12] and utilizing outdated medications as potential corrosion inhibitors may help lessen environmental pollution. according to studies [13], the pharmaceutical industries are also in charge of getting rid of old medications [14]. because of this, unused drugs should be carefully eliminated using techniques including photochemical and biodegradation [15,16]. furthermore, the potential for reusing the medications in corrosion testing would lead to reduced environmental damage from their application. ibuprofen c13h18o2, (rs)-2-(4-(2-methylpropyl) phenyl) propanoic acid is an antiinflammatory drug widely used in the treatment of muscle pain in rheumatic diseases. in this paper, the question of whether or not the expired ibuprofen could be used for the effective corrosion inhibition of copper in a sulfuric acid solution (h2so4), should be clearly answered. experimental materials and solution preparation the metal used in this study was copper, with 99 % purity. the metal was covered with epoxy resin to leave a contact surface of 0.5 cm2. the sample was abraded with emery paper (600, 800, 1200, 1500 grad grit) and cleaned with distilled water before each experiment. 0.5 m h2so4 was prepared using 97 % h2so4 solution of sigma-aldrich. solutions of ibuprofen inhibitor (scheme 1) were prepared by dissolving the required amount of ibuprofen powder in h2so4 solution. to create solutions with decreasing concentrations, the solution with the greatest concentration of 5 mm was diluted to 1, 0.5 and 0.1 mm. in our survey, the expired ibuprofen (stored at a local moroccan pharmaceutical company's warehouse) was used in the form of drug powder made from ibuprofen tablets of 200 mg. it is known that ibuprofen tablets contain not only ibuprofen compound but also up to 15 % of non-active ingredients such as lactose, starch, microcrystalline cellulose, croscarmellose sodium, colloidal silicon dioxide, polyethylene glycol (peg) and dyes. this ultimately means that “true” concentrations of ibuprofen were correspondingly lower than declared above. scheme 1: chemical structure of ibuprofen c13h18o2 electrochemical techniques the three-electrode cell and a potentiostat device type pgz 100, in conjunction with the required volta master software, were used to carry out electrochemical measurements. a copper electrode with an exposed surface of 0.031 cm2 served as the working electrode, while a standard calomel electrode and a platinum wire as reference and auxiliary electrodes, respectively. the copper electrode was cleaned with distilled water and dried before each measurement. the polarization measurements were recorded in the potential range between -600 mv and 600 mv with a sweep rate of 1 mv/s after a stabilization time of 40 minutes. s. el harrari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1867 3 electrochemical impedance measurements (eis) were carried out at the constant ocp in the frequency range of 100 khz to 10 mhz, under potentiostatic conditions using the alternating signal with the amplitude of 10 mv peak-to-peak. the values of electrochemical parameters were extracted using ec-lab software. weight loss measurements copper samples of size 75×13×3 mm were used in the weight loss experiments. these samples were immersed in 0.5 m h2so4 sulfuric acid solutions in the absence and presence of 5 mm ibuprofen for 10 days at room temperature. each sample was washed with distilled water and then weighed on a pa214 analytical balance with a weighing accuracy of 0.0001 g. analysis of copper surfaces by scanning electron microscopy in order to confirm the protective properties of ibuprofen, the surfaces of copper samples subjected previously to different acid solutions for 10 days were analyzed using a hirox sh4000m scanning electron microscope. the copper samples were prepared for weight loss measurements. results and discussion polarization potentiometric measurements the potentiodynamic polarization curves for copper in 0.5 m h2so4 without and with the addition of an expired ibuprofen inhibitor are shown in figure 1. figure 1. potentiodynamic polarization curves of copper in 0.5 m h2so4 solution without and with the addition of ibuprofen it is evident that the corrosion current density (jcorr) decreased in the presence of the expired drug. the corrosion potential (ecorr) of the inhibited media is shifted in the positive direction compared to the ecorr of the blank solution. the change in ecorr in the inhibited solutions is more than 85 mv, compared to the value of ecorr in the blank solution, which suggests that ibuprofen can be classified as an anodic type inhibitor [17]. on the other side, parallel cathodic tafel lines show that the presence of an ibuprofen inhibitor has little impact on the cathodic reaction [18]. the data for corrosion potentials as well as corrosion current densities, anodic (ba) and cathodic (bc) tafel slopes, and inhibition efficiencies (ie), are presented in table 1. -600 -400 -200 0 200 400 600 -6 -5 -4 -3 -2 -1 0 1 2 3 lo g j / m a c m -2 potential, mv blank h2so4 + 10 -4 inh h2so4 + 5*10 -4 inh h2so4 + 10 -3 inh h2so4 + 5*10 -3 inh lo g ( j / m a c m -2 ) http://dx.doi.org/10.5599/jese.1867 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion inhibition effect of expired ibuprofen drug 4 table 1 . electrochemical parameters of copper corrosion in 0.5m h2so4 solution without and with addition of different concentrations of ibuprofen inhibitor concentration, mm ecorr / mv jcorr /μa cm-2) ba / mv decade-1 bc / mv decade-1 ie, % 0.0 -309 171.40 93.4 -83.9 -- 0.1 -84 30.56 167.8 -241.9 82.17 0.5 -117 24.59 117.6 -219.4 85.65 1.0 -244.6 13.80 219.0 -89.2 91.95 5.0 -235.5 8.89 136.6 -81.4 94.81 according to table 1, the addition of the inhibitor changed the values of ba and bc as a result of the inhibitor molecules adhering to the metal surface to form a protective layer [19]. also, adding ibuprofen to sulphuric acid caused a drop in current density values, and this behavior continuously takes place as ibuprofen concentration rises. this demonstrates that under these circumstances, ibuprofen can shield copper surface. the inhibitor efficiency (ie, %) values of the expired ibuprofen were determined by eq. (1) [20]: ie = ((jcorr−jcorr(inh)) / jcorr) 100 (1) where jcorr and jcorr(inh) are corrosion current densities in the absence and presence of the inhibitor, respectively. it is clearly seen in table 1 that the inhibitor concentration affects inhibitor efficiency, which is connected to the adsorption of inhibitor molecules on the copper surface. furthermore, ibuprofen was found to be an efficient inhibitor for copper corrosion in the studied medium as the inhibition effectiveness reached even 94.81 % at 5 mm. electrochemical impedance spectroscopy electrochemical impedance spectroscopy experiments were carried out to further examine the impact of ibuprofen on copper corrosion behavior. the outcomes are displayed in figure 2 in the form of nyquist plots (-zi vs. zr). by analyzing the nyquist diagrams in figure 2, it is evident that when the inhibitor concentration rises, so does the diameter of an obtained semicircle, suggesting a decrease in the corrosion rate of the cu sample [21]. additionally, a small contribution of the warburg impedance is seen at the lowest frequencies, indicating diffusion processes, such as diffusion of soluble copper species or dissolved oxygen on the copper surface [22]. figure 2: nyquist plots for copper in 0.5m h2so4 in the absence and presence of different concentrations of ibuprofen 0 2000 4000 6000 8000 10000 12000 14000 0 2000 4000 6000 8000 10000 12000 14000 -z i / w c m 2 zr / w cm 2 blank h2so4 + 10 -4 inh h2so4 + 5*10 -4 inh h2so4 + 10 -3 inh h2so4 + 5*10 -3 inh https://www.nature.com/articles/s41598-019-51299-2#tab1 s. el harrari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1867 5 the ec-lab software program and the equivalent circuit shown in figure 3 were used to fit the experimental data, where rs is the solution resistance, rf is the resistance of the protective inhibitor film formed on the copper surface, rct is the charge transfer resistance of corrosion, while qf and qdl represent constant phase elements (cpe). cpes are put instead of pure capacitors, cf, for the film capacitance and cdl for the capacitance of the double layer. w is the warburg (diffusion) impedance and n is the deflection parameter. the parameters cf and cdl were calculated according to the equations (2) and (3): cf = (qfrf1-n1)1/n1 (2) cdl = (qdlrct1-n2)1/n2 (3) the calculated capacitance values and other impedance parameter values obtained by curve fitting of the electric equivalent circuit (eec) in figure 3 to impedance plots in figure 2 are listed in table 2 for different concentrations of ibuprofen. figure 3: equivalent electrical circuit for copper in sulphuric acid solution in the absence and presence of different concentrations of ibuprofen table 2. electrochemical impedance parameters for copper in 0.5 m h2so4 solution without and with ibuprofen added inhibitor concentration, m rs / ω cm2 cf / f cm−2 n1 rf / ω cm2 cdl / f cm−2 n2 rct / ω cm2 w0 / ω−1 cm−2 s0.5 rp = rs + rct / ω cm2 ie, % blank 1.192 0.52×10-3 0.72 399.6 7.21×10-3 0.53 221.4 0.38 621 -- 10−4 7.522 1.49×10-6 0.97 241.1 13.98×10-6 0.57 7 561 / 7 802 92.04 5×10−4 9.475 2.64v10-6 0.93 733.7 16.85×10-6 0.49 9 675 / 10 409 94.03 10−3 7.260 1.97×10-6 0.96 256.6 13.75×10-6 0.53 11 965 / 12 222 94.91 5×10−3 9.772 2.78×10-6 0.93 989 16.16×10-6 0.48 12 107 / 13 096 95.25 according to the results presented in table 2, the increased values of rct in the presence of the ibuprofen inhibitor indicate slower rates of copper corrosion, probably due to the increase in surface homogeneity due to inhibitor adsorption. furthermore, as the inhibitor concentration increased, the values of cf and cdl decreased. this is related to the adsorption of inhibitor molecules on the copper surface, which reduces the surface area of copper exposed to aggressive ions. according to the obtained results, it is assumed that the copper surface is uniformly coated. the following equation was used to compute the inhibitory efficiency: ie = [(rp – rp0) / rp] 100 (4) where rp0 is the polarization resistance of the copper electrode in the blank solution and rp is the polarization resistance of the studied medium in the presence of the inhibitor. note that polarization resistance rp is defined as rp = rf + rct. rp values are also listed in table 2. it can be seen that the computed ie values from eis shown in table 2 and from potentiodynamic polarization shown in table 1 are in relatively good agreement. http://dx.doi.org/10.5599/jese.1867 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion inhibition effect of expired ibuprofen drug 6 more about the interactions at the adsorbate/substrate interface may be obtained by the analysis of adsorption isotherms [23]. various adsorption isotherms were tested, and it was shown that the langmuir adsorption isotherm, with a linear regression coefficient (r2) equal to unity and a slope approaching unity, is the best representative for the inhibitor adsorption (figure 4). figure 4. langmuir adsorption isotherm for the adsorption of ibuprofen on copper surface equation (5) yields the langmuir isotherm formula: inh inh ads 1c c k = + (5) where  is the surface fraction covered by adsorbed species, and kads is the equilibrium adsorption constant (m-1). by using figure 5 and eq. (5), the adsorption equilibrium constant was determined as kads = 1.897 mm-1. adsorption free energy change (g0ads) value was calculated using equation (6): g0ads = -rt ln (55.4 kads) (6) herein, r is the universal gas constant, and t is the absolute temperature. the computed adsorption free energy value is g0ads= -40 kj mol-1, which means the process is spontaneous. since the value of g is -40 kj mol-1, we can say that the adsorption of ibuprofen on the copper surface takes place via physicochemical adsorption [24]. figure 5. weight loss curves for copper in 0.5 m h2so4 solution without and with addition of ibuprofen; ⎯ copper in h2so4, ⎯ copper in h2so4 with 5 mm of inhibitor 0.000 0.001 0.002 0.003 0.004 0.005 0.000 0.001 0.002 0.003 0.004 0.005 0.006 c  -1 / m c / m 22.615 22.620 22.625 22.630 22.635 22.640 22.645 22.650 1 2 3 4 5 6 7 8 9 10 11 m a ss o f co p p e r, g number of day c  1 / m s. el harrari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1867 7 weight loss measurements figure 5 shows a decrease of the copper mass during immersion in 0.5 m h2so4 without and with 0.5 mm of ibuprofen inhibitor. it is obvious, that the mass of the copper is less decreased once the inhibitor is added to the sulfuric acid [25]. in the absence of the inhibitor, the gravimetric test of copper immersed in sulfuric acid reveals a significant weight loss due to the aggression of the acid, but with the addition of 5 mm ibuprofen, the metal undergoes a much slower attack. surface characterization by sem sem was used to characterize the surfaces of copper coupons subjected to 0.5 m h2so4 in the presence and absence of the highest concentration of ibuprofen (5 mm). the coupons were submerged in solutions for 10 days at room temperature, as for weight loss measurements in figure 5, and the resulting sem micrographs are shown in figure 6. in contrast to the pitting and cracking that were found in the solution without the inhibitor, it has been noticed that the copper surface is smoother in the presence of the inhibitor. this can be the effect of ibuprofen forming a thick film on the metal surface. 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environment: insights from experimental and computational studies, environmental science and pollution research (2023). https://doi.org/10.1007/s11356-02327582-1 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1867 https://www.sciencedirect.com/journal/materials-and-design https://www.sciencedirect.com/journal/materials-and-design/vol/228/suppl/c https://doi.org/10.1016/j.matdes.2023.11‌1869 https://doi.org/10.1016/j.matdes.2023.11‌1869 https://doi.org/10.1016/‌j.scp.2022.100780 https://doi.org/10.1016/‌j.scp.2022.100780 https://doi.org/10.33263/briac116.143441‌4358 https://doi.org/10.33263/briac116.143441‌4358 http://dx.doi.org/10.1016/j.corsci.2014.02.016 https://www.sciencedirect.com/author/7403744157/qunjie-xu https://www.sciencedirect.com/journal/composites-part-b-engineering https://www.sciencedirect.com/journal/composites-part-b-engineering/vol/228/suppl/c https://doi.org/10.1016/j.compositesb.2021.109427 https://doi.org/10.1016/j.surfin.2020.100692 https://doi.org/10.1007/s11356-023-27582-1 https://doi.org/10.1007/s11356-023-27582-1 https://creativecommons.org/licenses/by/4.0/) {revised poubaix diagrams for the vanadium – water system} http://dx.doi.org/10.5599/jese.620 75 j. electrochem. sci. eng. 9(2) (2019) 75-84; http://dx.doi.org/10.5599/jese.620 open access: issn 1847-9286 www.jese-online.org original scientific paper revised pourbaix diagrams for the vanadium – water system igor povar1,2,, oxana spinu2, inga zinicovscaia1,2,3, boris pintilie2, stefano ubaldini4 1institute for nuclear research, joliot-curie str., 6, 141980, dubna, russia 2institute of chemistry, 3 academiei str., md 2028, chisinau, republic of moldova 3horia hulubei national institute for r&d in physics and nuclear engineering, 30 reactorului str. mg-6, bucharest magurele, romania 4institute of environmental geology and geoengineering of the italian national research council, rome, italy, stefano.ubaldini@igag.cnr.it corresponding author: ipovar@yahoo.ca; tel.: +373-22-73-97-36; fax: +373-22-73-97-36 received: september 27, 2018; accepted: january 3, 2019 abstract the forms of occurrence of vanadium metal are determined by the major chemical reactions in the aquatic environment such as hydrolysis, oxidation, reduction, and precipitation. depending on ph, potential and total concentration of inorganic ions and organic ligands, vanadium compounds may undergo various transformations to produce a whole range of chemical forms in aqueous systems. in this paper, a novel approach has been applied for calculating potential−ph (pourbaix) diagrams, based on the developed thermodynamic analysis of chemical equilibria in the v–h2o system. on the basis of currently revised thermodynamic data for v(iii), v(iv) and v(v) hydrolysis and original thermodynamic and graphical approach used, the repartition of their soluble and insoluble chemical species has been investigated. by means of δg–ph diagrams, the areas of thermodynamic stability of v(iv) and v(v) hydroxides have been established for a number of analytical concentrations of vanadium in heterogeneous mixtures. the obtained results, based on the thermodynamic analysis and graphic design of calculated data, are in good agreement with available experimental data. keywords potential−ph diagrams, thermodynamic analysis, thermodynamic stability, vanadium species introduction vanadium oxides (v-o) have been widely studied during the last three decades [1–12], mostly due to their broad applications in catalysis and microelectronics. vanadium is a transition metal with an elevated melting temperature and good corrosion resistance vs. different acids and alkalis at low temperatures. some alloys containing vanadium are extensively used in car constructions, http://dx.doi.org/10.5599/jese.620 http://dx.doi.org/10.5599/jese.620 http://www.jese-online.org/ mailto:stefano.ubaldini@igag.cnr.it mailto:ipovar@yahoo.ca j. electrochem. sci. eng. 9(2) (2019) 75-84 poubaix diagrams for the v – h2o system 76 electronics industry, nuclear reactor components and defense technology [13]. moreover, vanadium catalyst is one of the best catalysts used in chemistry and is also extensively used in the industrial production of sulfuric acid and petroleum refining [14]. in addition, vanadium is used in batteries, electrochemical and fuel cells [15] and ceramic industry [13]. however, the discharge of vanadium and its compounds in environment becomes one of the major causes of pollution. for vanadium, a simultaneous presence of several species in different degrees of oxidation is characteristic and therefore, a number of compounds in the v–o system may coexist. this includes oxides v2o5, vo2, v2o3, and vo, where vanadium is present in oxidation states +5, +4, +3, and +2, respectively. v(v) species are very toxic and their removal from water has been extensively investigated [16]. also, v(iv) and v(v) species have been defined as particularly toxic in [17]. many investigations show that toxicity of vanadium and its effect on health depends on its forms and their properties. therefore, the knowledge of vanadium speciation and its removal from wastewaters is the essential issue in environmental protection. at the same time, in addition to simple ions, existence of ions containing different number of oxygens in oxyanions is possible. the electron transfer reactions between oxyanions in the aqueous solution are carried out with the participation of hydrogen ions. consequently, under these conditions, the potential of the electrode would depend on the solution ph. this statement is also true for redox reactions involving solid phases [18]. in order to create the complete picture of thermodynamic properties of elemental compounds in different valence states both in aqueous solution and in solid phases, the potential-ph diagrams, so-called pourbaix diagrams, have to be used. these diagrams show thermodynamic properties of chemical elements, depending on the ph and their total concentrations in a solution. pourbaix diagram is a graphic representation of equilibria in heterogeneous systems containing a solid phase and aqueous solution. with increase in the number of components, polynuclear species appear and calculations of chemical and electrochemical equilibria become considerably complicated. the construction of pourbaix diagrams in these cases is usually made by calculating potential−ph dependency for various forms of oxidation-reduction pairs [19-21]. this approach sometimes leads to a linear dependence of chemical and electrochemical equilibria on ph [22]. in these cases, large number of unnecessary lines on the diagram diminishes the amount of information. the behavior of vanadium in solutions depends on the vanadium concentration, potential and ph value. vanadium extraction and its removal from wastewaters have been widely studied in order to decrease vanadium contaminant and employ vanadium sources [23,24]. despite of large number of experimental data, however, scarce information on thermodynamic properties of vanadium species has been published so far. the original approach of this paper consists in using the thermodynamic method developed by the authors [25] and consistent thermodynamic data for constructions of pourbaix diagrams for vanadium. theoretical for constructing pourbaix diagrams for the vanadium-water system, the standard gibbs energies of formation (gf0) of the vanadium solid and soluble species shown in table 1 have been used. the areas of thermodynamic stability of solid phases (oxides), depending on ph and total concentration of vanadium in the solution, have been initially determined. for this purpose, the gibbs energy variation of the interaction of the oxide with the aqueous solution components has been calculated. igor povar et al. j. electrochem. sci. eng. 9(2) (2019) 75-84 http://dx.doi.org/10.5599/jese.620 77 table 1. standard gibbs energies of formation for vanadium species at 298.15 k species gf0 / kj mol-1 source species gf0 / kj mol-1 source v2+ -218.0 [26] h2vo4-1023.0 [31] voh+ -467.0 [27] v2o74-1722.2 [31] vo(s) -402.6 [28] hv2o73-1778.8 [31] v3+ -242.7 [29] h2v2o72-1824.5 [31] voh2+ -418.2 [27] v4o136-3330.1 [31] v2o3(s) -1316.1 [28] hv4o135-3381.5 [31] vo2+ -447.3 [29] v4o124-3200.2 [31] hvo2+ -652.1 [30] v5o155-3999.5 [31] h2v2o42-1330.9 [30] v10o2867680.9 [31] hv2o5-1491.4 [30] hv10o285-7715.1 [31] v2o4(s) -1316.1 [28] h2v10o284-7736.5 [31] vo2+ -588.3 [29] h3v10o283-7745.8 [31] hvo42-977.8 [31] v2o5(s) -1421.2 [28] for the vanadium oxide (v) in aqueous solution, the following dissolution reaction can be written as: ½v2o5(s) + h+ =vo2+ + ½h2o (1) the gibbs energy variation (gr) of this reaction is described by the equation of the reaction isotherm: 2 + 0 r h δ =δ + ln vo r c g g rt c + (2) where gf0 is the variation of the standard gibbs energy of the reaction (1): 0 0 0 0 r f 2 f 2 f 2 5(s) 1 1 δ =δ (vo ,298.15k) δ (h o,298.15k) δ (v o ,298.15k) 2 2 g g g g+ + − (3) equation (1), however, characterizes the equilibrium in the system v2o5(s) aqueous solution only in a narrow range of ph because of various hydrolysis reactions possible in the solution which are listed in table 2. equilibrium constants (ki) of these reactions are calculated based on the data of table 1, using the well-known thermodynamic relation: 0 i i δ log 2.3 g k rt = − (4) table 2. possible hydrolysis reactions and their equilibrium constants hydrolysis reaction log k vo2+ + 2h2o = h2vo4+ 2h+ logk1 = -6.96 vo2+ + 2h2o = hvo42+ 3h+ logk2 = -14.88 vo2+ + 2h2o = vo43+ 4h+ logk3 = -29.14 2vo2+ + 3h2o = v2o74+ 6h+ logk4 = -29.09 2vo2+ + 3h2o = hv2o73+ 5h+ logk5 = -19.17 2vo2+ + 3h2o = h2v2o72+ 4h+ logk6 = -11.15 4vo2+ + 5h2o = v4o136+ 10h+ logk7 = -36.72 4vo2+ + 5h2o =hv4o135+9h+ logk8 = -27.62 4vo2+ + 4h2o = v4o124+ 8h+ logk9 = -17.84 5vo2+ + 5h2o = v5o155+ 10h+ logk10 = -22.42 10vo2+ +8h2o = v10o286+ 16h+ logk11 = -17.47 10vo2+ + 8h2o = hv10o285+ 15h+ logk12 = -11.47 10vo2+ + 8h2o =h2v10o284+ 14h+ logk13 = -7.73 10vo2+ + 8h2o = h3v10o283+ 13h+ logk14 = -6.10 http://dx.doi.org/10.5599/jese.620 j. electrochem. sci. eng. 9(2) (2019) 75-84 poubaix diagrams for the v – h2o system 78 evidence of the influence of hydrolysis reactions on the equilibrium reaction (1) leads to the following expression for δgr [25]: 2 0 r h vvo δ =δ ln ln + lnrg g rt rt c rt c + (5) in eq. (5), cv denotes the total concentration of v(v) in solution, while vo2+ is the coefficient which takes into account contributions of hydrolysis reactions listed in table 2, and is described by the expression αvo2+ = 1 + k1ch 2 + k2ch -3 + k3ch-4 + 2cvk4ch-6 + 2cvk5ch-5 + 2cvk6ch-4 + 4cv3k7ch-10 + 4cv3k8ch-9 + + 4cv3k9ch-8 +5cv4k10ch-10 + 10cv9k11ch-16 + 10cv9k12ch-15 + 10cv9k13ch-14 + 10cv9k14ch-13 where cvo2 + is the equilibrium concentration of the vo2+ ion, determined for a given ph value under the mass balance conditions: + i j n 2 2 v hvo vo vo i j n =c jc c  += (6) the calculations can be performed by well-known numerical methods using the software for real solutions of the system of nonlinear equations. results and discussion results of calculations of the δgr−ph dependence using eq. (5) for several cv values are shown in figure 1. within the used thermodynamic approach, the solid phase is stable for δgr < 0, and dissolves for δgr > 0. therefore, the condition δgr = 0 corresponds to the equilibrium of the solid phase with aqueous solution. one can see from figure 1 that vanadium oxide (v) is stable in a narrow range of low ph values, while for cv < 10-2 mol l-1, v2o5(s) does not precipitate at all. thermodynamic stability areas of other vanadium oxides are determined analogously. figure 1. dependence of δgr on ph for reaction (1), taking into account hydrolysis reactions and cv: 1 1 mol l-1; 2 10-1 mol l-1; 3 10-3 mol l1; 4 -10-5 mol l-1 0 2 4 6 8 10 12 0 2 4 6 8 4 3 2 1  g r/ 2 .3 r t ph igor povar et al. j. electrochem. sci. eng. 9(2) (2019) 75-84 http://dx.doi.org/10.5599/jese.620 79 within the ph range where the solid phase (oxide) is thermodynamically unstable (dissolves), it is necessary to determine the areas of predominance of the species in solution. in the presence of polynuclear species, it is useful to calculate the partial molar fractions of the species as functions of ph and cv. for vanadium compounds (v) for example, partial molar fractions are determined in the following way [32]: i j n i j n hvo ijn hvo i j n jc f jc =  (7) the calculation results of these functions for v(v) compounds and cv = 10-3 mol l-1 as functions of ph are shown in figure 2. based on these calculations, it is easy to determine the thermodynamic stability of vanadium species in the solution as a function of ph and cv. figure 2. partial molar fractions of vanadium (v) species for cv = 10-3 mol l-1 2 4 6 8 10 12 14 0.0 0.2 0.4 0.6 0.8 1.0 13 12 11 1098 7 6 5 4 3 2 1 f i ph 1 vo 2 + 2 h 2 v 10 o 28 43 hv 10 o 28 54 v 4 o 12 45 h 2 vo 4 6 h 3 v 10 o 28 37 h 2 v 2 o 7 28 v 10 o 28 69 v 5 o 15 510 hv 2 o 7 311 hvo 4 212 v 2 o 7 413 vo 4 32 4 6 8 10 12 14 0.0 0.2 0.4 0.6 0.8 1.0 13 12 11 1098 7 6 5 4 3 2 1 f i ph 1 vo 2 + 2 h 2 v 10 o 28 43 hv 10 o 28 54 v 4 o 12 45 h 2 vo 4 6 h 3 v 10 o 28 37 h 2 v 2 o 7 28 v 10 o 28 69 v 5 o 15 510 hv 2 o 7 311 hvo 4 212 v 2 o 7 413 vo 4 3f i f i http://dx.doi.org/10.5599/jese.620 j. electrochem. sci. eng. 9(2) (2019) 75-84 poubaix diagrams for the v – h2o system 80 by establishing the thermodynamic stability areas of both solid and soluble phases, one can calculate the ph dependence of δgr or e for different redox pairs. here, v(v) → v(iv) reduction as a function of ph for cv(v) = cv(iv) = 10-3 mol l-1 is examined as the example. the areas of thermodynamic stability of v(v) and v(iv) species under these conditions are shown in table 3. table 3. ph areas of thermodynamic stability of v(v) and v(iv) species for cv(v) = cv(iv) =10-3 mol l-1 v(v) ph area v(iv) ph area vo2+ 0  2.7 vo2+ 0 ≤ 3.7 h2v10o2842.7 ≤ 3.8 v2o4(s) 3.7 ≤ 8.2 hv10o2853.8 ≤ 5.5 hv2o58.2 ≤ 14.0 v4o1245.5 ≤ 7.9 h2vo47.9 ≤ 8.0 hvo428.0 ≤ 14.0 therefore, the reduction reaction of v(v) to v(iv) versus ph can be described by following eight equations: vo2+ + 2h+ + e= vo2+ + h2o 0 ≤ ph ≤ 2.7 (1/10)h2v10o284+ (17/5)h+ + e= vo2+ + (9/5)h2o 2.7 ≤ ph ≤ 3.7 (1/10)h2v10o284+ (7/5)h+ + e= (1/2)v2o4(s) + (4/5)h2o 3.7 ≤ ph ≤ 3.8 (1/10)hv10o285+ (3/2)h+ + e= (1/2)v2o4(s) + (4/5)h2o 3.8 ≤ ph ≤ 5 (1/4)v4o124+ 2h+ + e= (1/2)v2o4(s) + h2o 5.5 ≤ ph ≤ 7.9 h2vo4+ 2h+ + e= (1/2)v2o4(s) + 2h2o 7.9 ≤ ph ≤ 8.0 hvo42+ 3h+ + e= (1/2)v2o4(s) + 2h2o 8.0 ≤ ph ≤ 8.2 hvo42+ (5/2)h+ + e= (1/2)hv2o5+ (3/2)h2o 8.2 ≤ ph ≤ 14.0 on the basis of these data, the v(v) − v(iv) redox potentials (ei) are described by means of nernst equations for (cv(v) = cv(iv) = 10-3 mol l-1): e1 = e10 (rt/f) ln ch-2 e2 = e20 (rt/f) ln ch-17.5cv-9/10 e3 = e30 (rt/f) ln ch-7.5cv-1/10 e4 = e40 (rt/f) ln ch-3/2cv-1/10 e5 = e50 (rt/f) ln ch-2cv-1/4 e6 = e60 (rt/f) ln ch-2cv-1 e7 = e70 (rt/f) ln ch-3cv-1 e8 = e80 (rt/f) ln ch-5/2cv-0.5 where ei0 denotes standard redox potential. final potential−ph diagrams for vanadium species at total concentrations of all soluble species equal to 1, 10-3 and 10-5 mol l-1 are shown in figure 3. as one can see, the presence of polynuclear species in the system makes impossible the presentation of potential − ph diagrams for different cv in the single figure. this is explained by the fact that variation of cv changes not only thermodynamic stability areas, but also the composition of predominant species. thus, with the decrease of v(v) concentration, the stability of polynuclear species is diminished. at the same time, at low values of cv, vanadium oxide (v) becomes thermodynamically unstable. igor povar et al. j. electrochem. sci. eng. 9(2) (2019) 75-84 http://dx.doi.org/10.5599/jese.620 81 figure 3. potential−ph diagrams for vanadium species and cv: a 1 mol l-1, b 10-3 mol l-1, c 10-5 mol l-1 0 2 4 6 8 10 12 14 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 a 1 v 2 v 2+ 3 vo (s) 4 v 2 o 3(s) 5 v 3+ 6 vo 2+ 7 v 2 o 4(s) 8 v 2 o 5(s) 9 h 2 v 10 o 28 410 hv 10 o 28 511 v 10 o 28 612 v 4 o 12 413 v 4 o 13 614 v 2 o 7 414 1312 11 1098 7 6 5 4 3 2 1 e / v ph 0 2 4 6 8 10 12 14 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 19 1 v 2 v 2+ 3 vo (s) 4 v 2 o 3(s) 5 v 3+ 6 vo 2+ 7 v 2 o 4(s) 9 h 2 v 10 o 28 4 10 hv 10 o 28 5 12 v 4 o 12 415 voh 2+ 16 hv 2 o 5 17 vo 2 + 18 h 2 vo 4 19 hvo 4 218 1210917 16 7 6 15 5 4 3 2 1 b e / v ph 0 2 4 6 8 10 12 14 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.8 1.2 1 v 2 v 2+ 3 vo (s) 4 v 2 o 3(s) 5 v 3+ 6 vo 2+ 7 v 2 o 4(s) 15 voh 2+ 16 hv 2 o 5 17 vo 2 + 18 h 2 vo 4 19hvo 4 220 voh + 191817 16 7 6 4 15 5 3 20 2 1 c e / v ph e / v e / v e / v http://dx.doi.org/10.5599/jese.620 j. electrochem. sci. eng. 9(2) (2019) 75-84 poubaix diagrams for the v – h2o system 82 a particular feature of vanadium chemistry is that v(iv) species in alkaline medium are simultaneously oxidized and reduced to v(v) and v(iii) species. in the diagram, this is manifested by the fact that the range of potentials where v(iv) species are thermodynamically stable becomes narrowed with the ph increase. in calculating potential−ph diagrams in the general case, e (ph) (or δgr (ph)) functions are approximated by a system of linear−discontinued functions. in the presence of mononuclear species in the system, this approximation is quite justified [33,34]. it is interesting to elucidate whether this approximation can be justified in the presence of polynuclear species, because in such a case, areas of predominance of species in solution are insufficiently distinct. thus, for cv = 10-3 mol l-1 and ph 6, v(v) → v(iv) transformation within the linear approximation is described by the following equation: ¼v4o124+ 2h+ +e = ½v2o4(s) +h2o (8) at ph 6 value, v4o124ions predominate, but as shown in figure 2, other species are also present in the solution. the calculations show that partial molar fractions in this particular case are: 224 54 2 4 2 2 7 4 12 5 15hvo h vo h v o v o =0.01, =0.33, =0.13, 0.48, =0.05 v o f f f f f− = therefore, at ph 6 and cv = 10-3 mol l-1, the transformation v(v) → v(iv) is more correctly described by the following system of equations: hvo42+ 3h+ + e= (1/2)v2o4(s) + 2h2o 1 % h2vo4+ 2h+ + e= (1/2) v2o4(s) + 2h2o 33 % (1/2)h2v2o72+ 2h+ + e= (1/2) v2o4(s) + (3/2)h2o 13 % (1/4)v4o124+ 2h+ + e= (1/2)v2o4(s) + h2o 48 % (1/5)v5o155+ 2h+ + e= (1/2)v2o4(s) + h2o 5 % where next to each equation, the contribution of the respective equilibrium reaction is presented in fractions, %. the results of the v(v) v(iv) pair calculation in a small ph range for cv = 10-3 mol l-1, obtained by both the exact equation and linear approximation, are shown in table 4. the linear approximation approach was developed in [35]. in above mentioned example, instead of all the equilibria (9), the most predominant equilibrium with the 48 % contribution is taken into account. table 4. ph dependence of the electrode potential of the pair vanadium (v)/vanadium (iv) for cv = 110-3 mol l-1; 1 – exact calculation; 2 – calculation in linear approximation. ph 4 5 6 7 8 1 e / v 0.411 0.322 0.219 0.100 -0.004 2 e / v 0.418 0.329 0.233 0.115 0.007 conclusions 1. on the basis of thermodynamic data, the area of thermodynamic stability of vanadium chemical species as a function of ph for each degree of oxidation has been determined. 2. system of electrochemical equations for electrode reactions between chemical species in different degrees of oxidation and overlapped predominance areas has been derived. also, the dependence δgr or e on ph is calculated for different redox pairs. 3. depending on ph, pe (-log e), and total concentration of inorganic ligands, vanadium compounds may undergo various transformations to produce a whole range of chemical species in solution. igor povar et al. j. electrochem. sci. eng. 9(2) (2019) 75-84 http://dx.doi.org/10.5599/jese.620 83 e—ph diagrams of the v-h2o systems have been constructed, showing well agreement with previously reported experimental data. 4. it has been proved that the calculation of the e (ph) in linear approximation by a system of linear discontinued functions in 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[34] v. sjoberg, k. todd, l. sartz, s. karlsson, 11th international mine water association congress – mine water – managing the challenges, aachen, germany, 2011, 481-484. [35] i. fishtik, i. povar, koordinatsionnaya khimiya 17 (1991) 17-20. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://dx.doi.org/10.1016/j.hydromet.2015.11.014 https://doi.org/10.5599/jese.229 http://creativecommons.org/licenses/by/4.0/) surface engineering and performance of biomaterials: editorial http://dx.doi.org/10.5599/jese.1698 1 j. electrochem. sci. eng. 13(1) (2023) 1-3; http://dx.doi.org/10.5599/jese.1698 open access : : issn 1847-9286 www.jese-online.org editorial surface engineering and performance of biomaterials hitesh vasudev and chander prakash division of research and development, lovely professional university, phagwara 144411, india corresponding author: hiteshvasudev@yahoo.in published: february 15, 2023 this special issue highlights original research papers or review articles that discuss the current state-of-the-art surface engineering of biomaterials, particularly implants and biomedical devices. hydroxyapatite is a commonly used material for biomedical implants due to its resemblance with bone materials. plates, screws, pins, and artificial joints are only some bone fixation devices that often use 316l stainless steel. the behaviour of hydroxyapatite powder on ss316l has been mentioned to understand the adhesion of hydroxyapatite powder with ss316l, its bioactivity behaviour and mechanical properties. the plasma spray technique is used to deposit the hydroxyapatite-based coatings on the biomedical implants [1]. there is some issue involved with the deposition of this material, which leads to the deterioration of desired phases. the consequences related to the plasma spraying deposition have been discussed in detail and the various methods such as doping of other phases in hydroxypatite-based coatings have been analysed [2]. the recent advancements in biomaterials lead to product development using cold spray additive manufacturing, which has been discussed in the paper with future prospects [3]. tricalcium phosphate (tcp) has many advantages in biomedical applications, especially in teeth and bones, and therefore many researchers focused on enhancing the properties of this material by different methods [4]. the relationship modelling for surface finish for laser-based additive manufacturing has been described in detail with experimental analysis [5]. in another study related to biomedical applications, biological tests were performed to check the performance of the developed composites, which showed significant improvement in the biological aspect [6]. the importance of aluminum matrix composites has also been studied for their performance. the wear rate was evaluated by adding reinforcement and the effect of the addition of reinforcement was studied [7]. in another study, the yttrium stabilized zirconia-based coatings were deposited by plasma spray method. the coatings were reinforced with different oxides to increase the melting point of the coatings and thereby reduce the porosity[8]. the applications of biopolymer coatings in biomedical engineering were presented in review articles, wherein the various types of biopolymer coatings material and methods were discussed [9]. the use of expert systems in biomedical science has also been included in this special issue. the dental implants and industry was considered in the paper in detail and it can help the readers to indulge themselves into this advanced field of biomaterials [10]. http://dx.doi.org/10.5599/jese.1698 http://dx.doi.org/10.5599/jese.1698 http://www.jese-online.org/ mailto:hiteshvasudev@yahoo.in j. electrochem. sci. eng. 13(1) (2023) 1-3 editorial 2 a machining is an important step of manufacturing and the biomedical implants are very sensitive materials due to its brittleness and the effect of machining of bio-implants have been discussed in the study [11]. the recent advancement in the processing and fabrication of material is microwave route. the same has been adopted for the fabrication of materials on bio-medical steels. the paper describes the fundamental principle of microwave processing, and its development of bio-medical implant claddings using this technique [12]. the corrosion cracking is an important area in bio-medical implants. mg alloys based bio implants are commonly used as biomaterials and their performance analysis has been presented in a review and various failure analysis approaches have been discussed [13]. references [1] j. singh, j. p. singh, s. kumar, h. s. gill, short review on hydroxyapatite powder coating for ss 316l, journal of electrochemical science and engineering 13(1) (2023) 25-39. https://doi.org/10.5599/jese.1611 [2] a. mehta, g. singh, consequences of hydroxyapatite doping using plasma spray to implant biomaterials, journal of electrochemical science and engineering 13(1) (2023) 5-23. https://doi.org/10.5599/jese.1614 [3] g. prashar, h. vasudev, understanding cold spray technology for hydroxyapatite deposition, journal of electrochemical science and engineering 13(1) (2023) 41-62. https://doi.org/10.5599/jese.1424 [4] h. a. abdulaah, a. m. al-ghaban, r. a. anaee, a. a. khadom, m. m. kadhim, ceriumtricalcium phosphate coating for 316l stainless steel in simulated human fluid: experimental, biological, theoretical, and electrochemical investigations, journal of electrochemical science and engineering 13(1) (2023) 115-126. https://doi.org/10.5599/jese.1257 [5] s. jawade, g. kakandikar, relationship modelling for surface finish for laser-based additive manufacturing, journal of electrochemical science and engineering 13(1) (2023) 127-135. https://doi.org/10.5599/jese.1286 [6] s. kundu, l. thakur, microhardness and biological behavior of az91d-nhap surface composite for bio-implants, journal of electrochemical science and engineering. 13(1) (2023) 137-147 https://doi.org/10.5599/jese.1316 [7] a. s. channi, h. s. bains, j. s. grewal, v. s. chidambranathan, r. kumar, tool wear rate during electrical discharge machining for aluminium metal matrix composite prepared by squeeze casting: a prospect as a biomaterial, journal of electrochemical science and engineering 13(1) (2023) 149-162. https://doi.org/10.5599/jese.1391 [8] v. v. s. miryala, h. vasudev, mechanical and microstructural characterization of ysz/al2o3/ceo2 plasma sprayed coatings, journal of electrochemical science and engineering 13(1) (2023) 163-172. https://doi.org/10.5599/jese.1431 [9] j. singh, s. singh, r. gill, applications of biopolymer coatings in biomedical engineering, journal of electrochemical science and engineering 13(1) (2023) 63-81. https://doi.org/10.5599/jese.1460 [10] j. singh, s. singh, a. verma, artificial intelligence in use of zro2 material in biomedical science, journal of electrochemical science and engineering 13(1) (2023) 93-97. https://doi.org/10.5599/jese.1498 [11] g. singh, r. singh, j. gul, machinability behavior of human implant materials, journal of electrochemical science and engineering. 13(1) (2023) 99-114. https://doi.org/10.5599/jese.1514 https://doi.org/10.5599/jese.1611 https://doi.org/10.5599/jese.1614 https://doi.org/10.5599/jese.1424 https://doi.org/10.5599/jese.1257 https://doi.org/10.5599/jese.1286 https://doi.org/10.5599/jese.1316 https://doi.org/10.5599/jese.1391 https://doi.org/10.5599/jese.1431 https://doi.org/10.5599/jese.1460 https://doi.org/10.5599/jese.1498 https://doi.org/10.5599/jese.1514 h. vasudev and c. parkash j. electrochem. sci. eng. 13(1) (2023) 1-3 http://dx.doi.org/10.5599/jese.1698 3 [12] g. singh, a. mehta, a. bansal, electrochemical behaviour and biocompatibility of claddings developed using microwave route, journal of electrochemical science and engineering 13(1) (2022) 173-192. https://doi.org/10.5599/jese.1604 [13] j. singh, y. sharma, corrosion cracking in mg alloys based bioimplants, journal of electrochemical science and engineering 13(1) (2023) 193-214. http://dx.doi.org/10.5599/jese.1636 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1698 https://doi.org/10.5599/jese.1604 http://dx.doi.org/10.5599/jese.1636 https://creativecommons.org/licenses/by/4.0/) @article{vasudev2023, author = {vasudev, hitesh and prakash, chander}, journal = {journal of electrochemical science and engineering}, title = {{surface engineering and performance of biomaterials: editorial}}, year = {2023}, issn = {1847-9286}, month = {feb}, number = {1}, pages = {1--3}, volume = {13}, abstract = {this special issue highlights original research papers or review articles that discuss the current state-of-the-art surface engineering of biomaterials, particularly implants and biomedical devices. hydroxyapatite is a commonly used material for biomedical implants due to its resemblance with bone materials. plates, screws, pins, and artificial joints are only some bone fixation devices that often use 316l stainless steel. the behaviour of hydroxyapatite powder on ss316l has been mentioned to understand the adhesion of hydroxyapatite powder with ss316l, its bioactivity behaviour and mechanical properties.}, doi = {10.5599/jese.1698}, file = {:d\:/onedrive/mendeley desktop/vasudev, prakash 2023 surface engineering and performance of biomaterials editorial.pdf:pdf;:jese_v13_no1_1-3.pdf:pdf}, keywords = {biomaterials}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1698}, } {principles of the express method for controlling interelectrode space condition during wire electrochemical processing} http://dx.doi.org/10.5599/jese.660 269 j. electrochem. sci. eng. 9(4) (2019) 269-280; http://dx.doi.org/10.5599/jese.660 open access: issn 1847-9286 www.jese-online.org original scientific paper principles of the express method for controlling interelectrode space condition during wire electrochemical processing vasyl osypenko, oleksandr plakhotnyi and oleksii timchenko cherkasy state technological university, cherkasy, 18006, ukraine corresponding author: o.plakhotny@chdtu.edu.ua received: february 3, 2019; revised: june 26, 2019; accepted: june 27, 2019 abstract in the practical implementation of the sequential wire electrical discharge machining – pulsed electrochemical machining (wedm – pecm) technology and in order to perform high quality electrochemical processing, there is a need for the real-time operational control of electrical parameters of inter-electrode space and corresponding adaptive correction of amplitude-frequency power supply parameters (afpsp). in the context presented by the authors, a mathematical apparatus and an algorithm of operational galvanostatic mode monitoring of anode dissolution using wire electrode-tool are proposed. this will allow adaptive adjustment of afpsp to ensure controlled passage of electrochemical reactions and significantly increase process stability, dissolved surface layer thickness uniformity along entire electrode tool movement trajectory and resulting surface quality. keywords wire electrical discharge machining; pulsed electrochemical machining; current and voltage waveforms; process monitoring; surface finishing. introduction wire electrical discharge machining (wedm) technology fully satisfies the requirements of modern production in terms of miniaturization and precision. however, modern industry and especially instrumental production require electrical discharge wire cutting machines to form surfaces with roughness ra < 0.1 microns. obtaining such surface parameters and getting rid of a structurally modified heat affected zone (haz) using only material removal by a highly concentrated source of heat having a spark discharge is extremely costly and difficult. in modern machines of the world leader in the field of electrical discharge nanotechnology, sodick co. ltd, a mirror-like smoothing of steel blank surfaces of up to 0.08 microns is achieved in 12 passes with a change of working environment. this leads to an unacceptable cost increase of machined parts. http://dx.doi.org/10.5599/jese.660 http://dx.doi.org/10.5599/jese.660 http://www.jese-online.org/ mailto:o.plakhotny@chdtu.edu.ua j. electrochem. sci. eng. 9(4) (2019) 269-280 interelectrode space condition 270 pulse electrochemical machining (pecm) principally allows obtaining surfaces with roughness ra < 0.1 microns with the absence of a structurally altered layer on the workpiece surface. however, implementation of an efficient pecm process by a mobile wire tool electrode (wte) following to the wedm technology scheme is currently not sufficiently researched and is challenging in scientific and technical terms. the solution requires complex experimental and theoretical studies, as well as mathematical and physical modelling of basic processes that determine the nature of the electrochemical dissolution of the anode surface obtained by wedm. literature review and problem statement for wedm, the express methods of preliminary determination of power supply parameters and inter-electrode space (ies) condition monitoring in real time are already developed and successfully used on serial machines. the ies resistance is controlled by the hardware. afpsp and machine drives feed rate have adaptive control. more sophisticated modern schemes are implemented through the continuous supply of low-power diagnostic impulses to ies. by analysing the response, a decision is made to switch on a more powerful source of pulsed technological current, which in fact, carries the electro-discharge destruction of material [1,2]. for wire pecm, such control methods have not been developed. the time of periodic process of double layer charging to the overvoltage activation and its discharge time are very important for nanosecond pulsed electrochemical processing [3-5]. cylindrical wire electrode causes significant current density distribution on the anode surface. therefore, even a slight change in electrode potential leads to a significant change in current density and accordingly, to dissolution localization. with a longer duration of microsecond pulses, a diffusion processes becomes a limiting factor in ies. accordingly, calculation of stationary and transition components of diffusion current, which depend on main parameters of each technological pecm scheme, must be preceded by the choice of current amplitude [6]. for configuration of flat anode and cylindrical cathode with stream electrolyte flow, it is problematic to determine diffusion layer thickness, which depends on velocities distribution of electrolyte in near-anode zone. consequently, calculated mathematical models of electrochemical processes in ies are complex, require usage of computer-aided design software and significant computational time [7]. these mathematical models are adequate for preliminary calculation of pecm processes parameters but are not suitable for operational monitoring and adaptive real-time correction. therefore, there is an urgent need for the creation of algorithms and schemes suitable for practical implementation in the existing technological equipment. the proposed approach is based on the determination of magnitude and dynamics change of ies electrical parameters and as a result, process nature of electrochemical dissolution. these can be obtained from analysis of response oscillograms using equivalent electrical circuit substitution scheme. a similar approach was used in [8], using a complex mathematical apparatus not suitable for operative quantitative analysis of response oscillograms. in the subsequent work [9] only a qualitative analysis of oscillograms was conducted. proper choice of afpsp according to pecm scheme, can significantly improve surface quality, machining accuracy, material removal rate and overall process controllability [10,11]. the purpose is to increase the technological characteristics of high-performance combined methods of current-conducting materials machining with the help of further development of mathematical apparatus of operational monitoring and adaptive efficiency correction of galvanostatic pecm mode. vasyl osypenko et al. j. electrochem. sci. eng. 9(4) (2019) 269-280 http://dx.doi.org/10.5599/jese.660 271 experimental equipment and methods technological scheme when using the pecm scheme of unipolar current pulses with moving wte as a cathode (figure 1), developed by the authors and implemented in particular experimental technological equipment, the process of anodic dissolution of workpiece material significantly depends on the magnitude and growth dynamics of anode polarization and relaxation of its potential during the pause between current pulses. figure 1. machining scheme of sequential wire edm and wire pecm replacement of the partition boundary “anode – electrolyte” for equivalent electrical circuit (randles circuit) is carried out and described in figure 2 [8,12]. the choice of circuit should be based on electrical properties of ies. technological parameters during machining process determine dynamics change of electrical properties of ies. electrolyte flow speed (renewal rate) influences re – electrolyte resistance. change of ieg value, a size of interacting surface while moving on intricatecontoured trajectory affects capacity. wte feed rate along trajectory alters dissolution conditions of edm-affected surface layers and influences the electrical properties of ies. a b figure 2. a current pulse parameters; b electrical equivalent scheme for anode-electrolyte boundary interface of pulse ecm (pecm): re – electrolyte resistance; c – double layer capacity; rf – faraday resistance methods for obtaining oscillograms of anode current pulses are well worked out and have a relatively simple scheme implementation (figure 3) [8,13]. research of physical and technological parameters of wire cutting electrical discharge machining and subsequent electrochemical processing by a wire electrode was carried out on seld-02 (wire-cut http://dx.doi.org/10.5599/jese.660 j. electrochem. sci. eng. 9(4) (2019) 269-280 interelectrode space condition 272 electrical discharge machine on linear motors, ukraine). seld-02 machine has linear motors, granite guides and gas-lubricated supports. this provides minimal displacement error. a b figure 3. schemes of measurements: a flat electrodes, b flat anode – cylindrical (wire) cathode. 1 – anode, 2 – cathode, 3 – reference electrode (platinum), 4 – electrolyte, 5 – bridge, 6 – driving pulse generator, 7 – current source, 8 – current transducer, 9 – oscillograph, 10 – computer, software, 11 – computer numerical control unit, 12 – positioner experimental conditions: anode – stamping steel din x155crvmo12-1, cathode – 0.2 mm diameter cobra cut b wire (agie, switzerland), cuzn37 hard brass, 1 m nacl electrolyte at temperature of 28 °c which was delivered as a jet from the upper chamber of the machine at pressure of 5104 pa. height of the part is 11 mm. the machine-tool provided wire electrode movement with a given speed along the trajectory at a given distance (ieg) from the machined surface. conductivity of the electrolyte was controlled by the oakton tds-5/con-5 with a special electrode which expands measurement scale (cell factor k = 10). polarization-time and current-time dependencies were recorded by digital atten ads1000 dual-channel oscillograph and transmitted to computer in a mathematical software package for processing. preliminary testing of the proposed method for monitoring ies was carried out in studies of transient processes in electrochemical system. it had fixed flat electrodes and the process was vasyl osypenko et al. j. electrochem. sci. eng. 9(4) (2019) 269-280 http://dx.doi.org/10.5599/jese.660 273 conducted by the passing of a sequence of rectangular current pulses. electrodes 2012.51.5 mm were immersed in a bath with immovable 1 m nacl electrolyte, so the electrodes interaction area was 2.5 cm2. anode material – stamping steel din x155crvmo12-1, cathode – brass сuzn37. mathematical method for calculating parameters of an electric circuit loaded by microsecond current pulses as is known, current i(t) and voltage u(t) at the ends of an electrical circuit element with active resistance r and capacity c are bound by formula [14] = ( ) ( ),u t r i t   = +     1 0 0 1 ( ) ( )du t i t t q c (1) where q0 – initial charge on condenser covers. using the operational method [14], we introduce an integral transform of laplace’s "operational current", →( ) ( )i t i p and "operational voltage", →( ) ( )u t u p , where p is a complex number frequency parameter. then, relation (1) will be converted into operational formulas = ,u ri = 1 u i cp (2) if we consider q0 = 0. formulas (2) combine into "operational ohm’s law " =u zi (3) where z is “operational resistance” or impedance, which in the case of active resistance and capacitance has the following form: = , r z r = 1 c z cp (4) considering rules for adding parallel and serial impedances, we obtain the expression for the overall circuit (figure 2b) impedance, = + +1 f e f r z r cpr (5) and the operational equation, respectively:   = +  + 1 f e f r u r i cpr (6) similarly, it is possible to formulate the operational equation for more complex electric substitution schemes taking into account additional components of inter-electrode space, such as cathode double electrical layer capacitance, resistance of passivating films that can be formed on surface of electrodes, etc. also, in the substitution scheme, inductivities can be introduced if response oscillograms taken from ies clearly indicate oscillatory attenuation signals and model must reproduce them. it is important to substantiate the conformity of each element of electrical circuit and way of its connection to the real processes occurring in ies during electrochemical machining. to construct a mathematical description of the passage of a rectangular current pulse through a model of an electrochemical cell (figure 2b), we first consider the problem of dc current i0 activation at time t = 0. http://dx.doi.org/10.5599/jese.660 https://en.wikipedia.org/wiki/complex_number j. electrochem. sci. eng. 9(4) (2019) 269-280 interelectrode space condition 274 = 0 ( ) ( ),i t i t   =   0, if 0, ( ) 1, if 0; t t t ( )t – heaviside function. operational current is defined as i = i0/p, while the operational voltage according to eqs. (3) and (6) is defined as:   = +  +  0 1 f e f ir u r cpr p (7) we perform the transition operation to the original ( ) ( )u t u p started using the conclusion of the second expansion theorem [14], according to formula = = → + 1 ( ) (0) ( ) ( ) (0) '( ) k k l p tk k k k a p a a p e pb p b p b p (8) where pk – kth equation root of b(p) = 0. in our case, using eq. (7) we have only one root, p1 = 1/crf. then using eq. (8) → + − 1 0 0 ( ) ( ) ( ) p t e f f a p r r i r i e pb p u(t) becomes defined as: = + − 1 0 ( ) [ (1 )] p t e f u t i r r e (9) next, to get response function to the rectangular shape current pulse passage,   = − − 0 0 ( ) ( ) ( ) i i t i t i t , where  i is pulse duration, the lag theorem [14] was used and we get:     − = + − − + − −1 1 ( ) 0( ) {[ (1 )] ( ) [ (1 )] ( )} ip t p t e f e f iu t i r r e t r r e t (10) here it is assumed that electrochemical cell parameters re, rf, c have not changed during pulse duration. figure 4. sequence of current pulses with arbitrary parameters of duration and on-off time for a series of current pulses with an arbitrary time duration and on-off time parameters (figure 4), it is suitable to use the following form of the response function       − − − − − = + − − + − − + + + − − − + − − + + + − − − + − − + + 1 2 1 3 2 4 3 5 4 6 5 ( ) 0 1 1 2 2 1 ( ) ( ) 3 3 2 4 4 3 ( ) ( ) 5 5 4 6 6 5 ( ) {[ (1 )] ( ) [ (1 )] ( ) [ (1 )] ( ) [ (1 )] ( ) [ (1 )] ( ) [ (1 )] ( ) ...}, p t p t t e f e f p t t p t t e f e f p t t p t t e f e f u t i r r e t r r e t t r r e t t r r e t t r r e t t r r e t t (11) where according to the trend of parameters changing from re1, rf1, c1 to re2, rf2, c2, then to re3, rf3, c3, etc, it is possible to clearly observe dynamics of ies condition indicators for an electrochemical cell during pulsed current workload in the galvanostatic mode. vasyl osypenko et al. j. electrochem. sci. eng. 9(4) (2019) 269-280 http://dx.doi.org/10.5599/jese.660 275 results and discussion flat electrodes figure 5(a) presents the results of dynamic measurements of anode polarization at given amplitude-time parameters of current. it should be noted that amplitude of current pulses and interaction area of electrodes in this experiment do not correspond to the operating modes of current density of electrochemical dissolution using wire wedm + pecm technology. duration and amplitude of pulse current, area of interaction of electrodes, ieg value were selected in such a way to maximize manifestation of ies resistance and capacity in anodic polarization curves. in addition, oscillograms were recorded at the beginning of dissolution process in order to minimize influence of external factors such as electrochemical reaction products saturation (slag) and ies gas pollution or formation of various films on electrodes, which had not yet appeared in a short time. a b figure 5. oscillograms and their mathematical processing: a rectangular current pulses (red curve); increase and decrease of anode polarization (blue curve); b approximation by response function of areas with transient processes then, the inverse mathematical problem was solved using the experimentally obtained plot of polarization-time dependence, and selecting coefficients in the response function (eq. (10)), i.e. parameters re, rf, c of the electrical equivalent substitution scheme of an electrochemical cell. polarization growth area (figure 5b) was approximated by the least squares method using function (9), which is a response function when passing the front edge of current pulse through an electrochemical cell. the decrease of area which corresponds to the back edge of the switch-off of current pulse, was approximated by the function represented in the second term eq. (10). according to oscillogram processing results (figure 5b), it was found that at the moment of passing the forward front of current pulse, ies parameters had following values: re = 0.12 ohm, rf = 0.33 ohm, c = 1.2·10-4 f. at the moment of passing rear front, these values were: re = 0.12 ohm, rf = 0.4 ohm, c = 3.9·10-4 f. estimated total ies resistance, r, according to the ohm’s law   =r s (12) using values of electrolyte conductivity,  = 8.61 s m-1, electrodes interaction area, s = 2.5·10-4 m2 and ieg,  = 0.8·10-3 m, was 0.37 ohm. specific capacity of double electric layer i.e. capacity according to the equivalent interaction area of electrodes is 0.48 f/m2 and 1.6 f/m2 in the first and second case, respectively. according to the literature data, specific capacity values should be within 0.2-0.4 f/m2 [15, helmholtz model, 0.6-0.8 f/m2] [15, or 0.4-0.5 f/m2 [16, measured at the boundary between mercury anode and 1м nacl solution at 25 °с, 0<u<0.4 v. that is, the order of experimentally determined resistance values re + rf (values are adding for serial connection) http://dx.doi.org/10.5599/jese.660 j. electrochem. sci. eng. 9(4) (2019) 269-280 interelectrode space condition 276 coincides with theoretical calculation. the found specific capacity value along the pulse forward front is located in well-known range. the rear edge determined capacity, however, is not correct (similar to [8]), so it is obvious that the electrical equivalent scheme needs to be clarified. with an increase of the current amplitude up to 9 a and a decrease in of ieg value down to 0.6 mm, ies is filled with sludge over time due to reactions products. this is noticeable when analysing oscillograms (figure 6). at the beginning of experiment, electrochemical cell specific capacities are 0.22 f/m2 and 0.76 f/m2 for forward and rear pulse fronts, respectively. after 2 minutes of the anode dissolution, a significant amount of sludge is observed visually in ies, and from analysing oscillograms it follows that the corresponding capacities increased to 0.49 f/m2 and 1.2 f/m2 and resistance decreased. consequently, ies is filled by sludge, caused by insufficient rate of stationary electrolyte renewal that is clearly visible on the oscillograms and can be quantified by their mathematical analysis. figure 6. oscillograms, approximated by response function, when working with a larger current amplitude 9a: voltage at the beginning of experiment – medium graph; voltage after 2 minutes of polluted ies – lower graph with a variation of the ieg and afpsp parameters, other characteristic cases of polarization change dynamics of electrodes were experimentally obtained. oscillograms in figure 7 are mathematically processed in the following way. on the first pulse, we obtain resistance and capacitance parameters of elements and using eq. (11) further polarization change is predicted for several subsequent pulses (red curve). in the first case (figure 7a), an unsuccessful choice of afpsp for experimental conditions led to increasing residual polarization. in another example (figure 7b) there is an increase in polarization and a simultaneous change in ies conditions. both cases are unacceptable and require an immediate correction of afpsp for normalization of the anode dissolution process in electrochemical cell with flat electrodes. a novel pulsed-power supply method allows to accelerate depolarization and to improve the machining accuracy of pecm [17]. vasyl osypenko et al. j. electrochem. sci. eng. 9(4) (2019) 269-280 http://dx.doi.org/10.5599/jese.660 277 a b figure 7. oscillograms with (a) increasing residual polarization, (b) increasing polarization and changes in ieg characteristics flat anode – cylindrical cathode for polarization oscillograms with flat anode and wire cathode analysis it is incorrect to use value of electrodes interaction area s in eq. (12). to obtain more accurate quantitative results of oscillograms analysis, the following mathematical description of electric field distribution for the calculation scheme of flat anode – cylindrical cathode (figure 8) [18,19] must be applied. figure 8. scheme for electrochemical machining of flat anode by a wire cathode if cathode potential is u(t), then field strength distribution on anode surface is determined by the formula = = + 2 2 2 ( ) 1 ( , ) 0 ln y au t e x t y p x a (13) where = − 2 2 a h r , p = (a-)/(a+). field strength component ex is insignificant due to smallness. http://dx.doi.org/10.5599/jese.660 j. electrochem. sci. eng. 9(4) (2019) 269-280 interelectrode space condition 278 amplitude current in circuit can be described by the following equation     = = = a a 0 2 d 2 0 ln y u i e l x l y p (14) where  is specific electrolyte conductivity, and l is the part height. then ies resistance filled with electrolyte is   = ln 2 p r l (15) or by analogy with eq. (12)  = s a r a (16) where, as = 2al/|ln p|i.e. equivalent interaction area for cylindrical cathode and flat anode, similar to the interaction area of electrodes s for flat electrodes. the area of electrodes interaction, which in turn determines electrical capacity of electrochemical cell, has much less calculated value for a wire electrode than for a pair of flat electrodes. therefore, transient processes of increasing and decreasing polarization under pulsed current are visually less obvious. figure 9 depicts an oscillogram of anode polarization at ieg of 0.5 mm, amplitude current ia = 2.34 a and voltage ua = 11 v. figure 9. oscillogram of anode polarization during electrochemical machining of flat anode by a wire cathode. figure 10 shows areas of voltage growth and decrease of oscillogram in figure 9, approximated by the response function (eq. 10). the experimental value of resistance is 4.7 ohm, while the theoretically calculated value according to eq. (15) is 4.33 ohm. the difference between the experimental and theoretical results is 7.9 % and may be explained by the appearance of partial gas pollution of ies as a result of electrochemical reaction, which leads to a decrease in electrolyte conductivity. following the results of the response function approximation of forward pulse front, the following values were obtained: re = 3.74 ohm, rf = 0.89 ohm, c = 8.3·10-6 f. specific capacity, i.e. the capacity of electrochemical cell according to the equivalent interaction area of electrodes, is 0.5 f/m2. data of the voltage drop area approximation asserts that specific capacity reached 0.45 f/m2. with the prolongation of response function according to eq. (11) with parameters obtained for several subsequent pulses and comparison with the oscillogram in figure 9, it was established that electrochemical process proceeds normally, without significant changes in ies condition and without accumulation of residual polarization. that is, afpsp for given conditions is selected correctly, providing necessary metal removal rate and avoiding a process of entering passivation zone. vasyl osypenko et al. j. electrochem. sci. eng. 9(4) (2019) 269-280 http://dx.doi.org/10.5599/jese.660 279 a b figure 10. voltage oscillogram (blue curve) approximated by response function (red curve) (a) area of voltage increase on forward pulse front; (b) area of voltage decrease on rear pulse front. following the results of the response function approximation of forward pulse front, the following values were obtained: re = 3.74 ohm, rf = 0.89 ohm, c = 8.3·10-6 f. specific capacity, i.e. the capacity of electrochemical cell according to the equivalent interaction area of electrodes, is 0.5 f/m2. data of the voltage drop area approximation asserts that specific capacity reached 0.45 f/m2. with the prolongation of response function according to eq. (11) with parameters obtained for several subsequent pulses and comparison with the oscillogram in figure 9, it was established that electrochemical process proceeds normally, without significant changes in ies condition and without accumulation of residual polarization. that is, afpsp for given conditions is selected correctly, providing necessary metal removal rate and avoiding a process of entering passivation zone. according to faraday’s law we obtain an expression for determining material depth dissolution distribution on anode surface due to the action of one current pulse   = + 2 2 0 2 1 ( ) ( ) ln t v k a h x u t dt p x a (17) where kv is coefficient of electrochemical machinability of anode material;  is current efficiency coefficient; t is pulse period; u(t) is response function (10) obtained by approximating the corresponding voltage oscillogram section during current pulse passage. if the process of electrochemical dissolution is stable as in the considered example, then according to eq. (17) it is possible to calculate the thickness of the removed surface layer, taking into account the velocity of the wire electrode and current pulse frequency. in the case of ecm parameters deviation from normal reaction, oscillogram analysis will indicate unacceptable changes. refusal to apply corresponding operational measures for correction of afpsp will result in a significant reduction of current efficiency coefficient. this is approved by numerous experimental evidences in practical implementation of wire ecm technology. approximation of oscillogram signals in this work was carried out using a mathematical software package, which is quite convenient for research, but is not fundamentally necessary. computational experiments have shown that sufficient accuracy in this case can provide an approximation of 3-5 points of the curve (filtered from digital noise). according to the least squares deviation method, this means that in order to find unknown coefficients, it is necessary to solve a system of 3-5 linear algebraic equations, which solution can be represented in an analytical form. that is, calculations are reduced to normal multiplication and addition operations that can be performed in real time on http://dx.doi.org/10.5599/jese.660 j. electrochem. sci. eng. 9(4) (2019) 269-280 interelectrode space condition 280 microprocessor elements used in modern computer numerical control equipment for controlling and managing machine-tools and technological systems. conclusions 1. the concept of operational monitoring, forecasting and adaptive correction of afpsp of ecm using mobile wire electrode is substantiated in order to improve accuracy and quality of surface shaping using the latest combined technology of sequential wedm and ecm. 2. based on experimental data, a mathematical model was developed that adequately describes the increase and decrease of polarization dynamics of anode in electrochemical machining using wire electrode, with a series of unipolar rectangular current pulses. 3. the solution of inverse mathematical problem is proposed which, based on the experimentally obtained plot of the response polarization-time dependence, allows determination of re, rf, c parameters of the substituted electrical equivalent circuit scheme of an electrochemical cell. references [1] v. і. osypenko, s. p. poliakov, d. о. stupak, r. і. savisko ua 10153 мpк 7в23н1/02 (2005). [2] y. jiang, w. zhao, x. xi, l. gu, x. kang, international journal of advanced manufacturing technology 61(1-4) (2012) 171-183. [3] r. schuster, v. kirchner, p. allongue, g. ertl, science 289(5476) (2000) 98-101. [4] s. skoczypiec, a. ruszaj, precision engineering 38(3) (2014) 680-690. [5] s. skoczypiec, international journal of advanced manufacturing technology 87(1-4) (2016) 177-187. [6] c. zhao, l. xu, journal of electrochemical science and engineering 8(4) (2018) 321-330. [7] j. a. kenney, g. s. hwang , nanotechnology 16(7) (2005) s309. [8] s. galanin, elektrokhimicheskaya obrabotka metallov i splavov mikrosekundnymi impul’sami toka (electrochemical treatment of metals and alloys by microsecond current pulses), kostrom. gos. tekh. univ., kostroma, russia, 2001, p.118 (in russian). [9] s. galanin, i. kalinnikov, surface engineering and applied electrochemistry 44(5) (2008) 359-366. [10] m. mithu, g. fantoni, j. ciampi, international journal of advanced manufacturing technology 55(912) (2011) 921-933. [11] g. wollenberg, h. schulze, h. trautmann, g. kappmeyer, proceeedings of the 15th isem, (2007) 335338. [12] o. weber, a. rebschlager, p. steuer, d. bahre, proceedings of the european comsol conference, rotterdam, 2013. [13] j. sun, e. taylor, r. srinivasan, journal of materials processing technology 108(3) (2001) 356-368. [14] m. a. lavrent'ev, b. v. shabat, methods of the theory of function of complex variable, nauka, moscow, 1987, p.544 (in russian). [15] v. s. bagotsky, fundamentals of electrochemistry. john wiley & sons, inc.: hoboken, nj, 2005, p.752. [16] j. newman, k.e. thomas-alyea, electrochemical systems, john wiley & sons, ny, 2012, p.672. [17] f. han, w. chen, w. ying, procedia cirp 68 (2018) 493-498. [18] v. p. zhitnikov, a. n. zaytsev, impul’snaya elektrokhimicheskaya razmernaya obrabotka: monografiya (pulse electrochemical machining: monograph), mashynostroenye, moscow, 2008, p. 413 (in russian). [19] v. i. osipenko, a. p. plakhotny, a. yu. denisenko, pratsi odeskoho politekhnichnoho universytetu 1(43) (2014) 55-60. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) a review of liquid metal anode solid oxide fuel cells doi: 10.5599/jese.2013.0032 91 j. electrochem. sci. eng. 3(3) (2013) 91-105; doi: 10.5599/jese.2013.0032 open access : : issn 1847-9286 www.jese-online.org review a review of liquid metal anode solid oxide fuel cells aliya toleuova* , **, vladimir yufit***, stefaan simons* , ****, william c. maskell* , ***, daniel j. l. brett* ,  *electrochemical innovation lab, department of chemical engineering, university college london, torrington place, london wc1e 7je, uk **school of engineering, nazarbayev university, 53 kabanbay batyr ave, astana 010000, republic of kazakhstan ***department of earth science and engineering, imperial college london, london sw7 2az, uk ****international energy policy institute, ucl australia, 220 victoria square, adelaide, australia corresponding author: e-mail: d.brett@ucl.ac.uk; tel.: +44(0)207 679 3310; fax: +44(0)207 383 2348 received: december 31, 2012; published: june 12, 2013 abstract this review discusses recent advances in a solid oxide fuel cell (sofc) variant that uses liquid metal electrodes (anodes) with the advantage of greater fuel tolerance and the ability to operate on solid fuel. key features of the approach are discussed along with the technological and research challenges that need to be overcome for scale-up and commercialisation. keywords solid oxide fuel cell; liquid metal anode; direct carbon fuel cell. introduction the world’s growing energy demands have led to increasing environmental and resource availability concerns. in this regard, stabilization of increasing anthropogenic co2 emissions is one of the most urgent issues associated with global climate change. according to the international energy agency (iea), fossil fuels will remain the primary energy sources, representing more than 75% of the overall growth in energy use from 2007 to 2030 [1]. moreover, carbon-free energy technologies are unable to meet global demands in the foreseeable future [2]. therefore, continued use of fossil fuels in a ‘low-carbon economy’ necessitates the reduction of co2 emissions by improving process efficiency. notwithstanding the reduction in greenhouse gas emissions brought about by high efficiency, fuel cells generate carbon dioxide free from diluting nitrogen which can be sequestered at lower cost compared with the exhaust products from most electricity generation based upon combustion of fossil fuels. http://www.jese-online.org/ mailto:d.brett@ucl.ac.uk j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 92 while coal remains the most available and abundant fuel source around the world (60% of all global sources), highly efficient fuel technologies are required to minimize the severe environmental impact of electric energy production from coal [3]. in this regard, several studies [4–7] have presented novel concepts for the direct electrochemical oxidation of carbon (coal or biomass) for highly efficient electricity generation using liquid metal electrode solid oxide fuel cells (lme sofcs). this new class of fuel cell offers the advantage of high efficiency with the benefits of greater fuel tolerance, the concept being particularly well suited to the direct use of solid fuel. high temperature solid oxide fuel cells (sofc) are highly efficient electrochemical energy conversion devices capable of converting the chemical energy stored in gaseous fuels (including hydrogen, alkanes, alcohols, alkenes, alkynes, ketones, etc.) into electrical power [8]. the operational efficiency of such systems is higher than conventional heat engines since the process is electrochemical and not constrained by the carnot efficiency limitation. typical efficiencies obtained from sofcs are in the range of 50-60% (lhv). the technology is highly scalable with systems in the hundreds of kilowatts demonstrated. the sofc is increasing attention as a clean and efficient power-generating technology. however, durability and performance degradation due to impurities contained in hydrocarbon fuels, such as sulfur and other trace elements, continue to be a concern [4]. contaminants may react with the anode materials via various mechanisms to decrease electrochemical reaction rates by increasing charge transfer resistance and result in mechanical failure of materials [9]. in order to mitigate the influence of fuel impurities on cell degradation, the supplied fuel should undergo pretreatment using a variety of adsorbents and filters [10]. alternatively, substantial improvements in anode materials are necessary to make the anode more tolerant of fuel contaminants. one approach to achieving this is to use a liquid metal anode solid oxide fuel cell (lma sofc). this allows operation on a variety of carbonaceous fuels – gaseous or solid, while remaining highly tolerant towards fuel contaminants. some of the earliest work on liquid metal electrodes was reported by cell tech power, who used liquid tin as their anode material of choice [5,6,11,12] and achieved a power density of 170 mw cm −2 on hydrogen and jp-8 fuel [4]. the projected efficiency of such a system operating on coal is 61 % [13]. however, lma sofc technology is not yet well-understood. the nature of the cells makes experimental investigation challenging and the processes occurring in the melt are difficult to study in situ. there is a certain inconsistency between published studies in terms of the experimental arrangement and little is known about the mechanism of fuel oxidation within the liquid metal media. being a comparatively novel system, lma sofc technology is some way from being fully commercialized due to technical challenges unique to this class of fuel cell. the objective of this review paper is therefore to provide a summary of emerging lma sofc technologies, discuss the advantages of the approach and set out the research and technological challenges that need to be addressed and overcome. background to fuel cells fuel cells are electrochemical energy conversion devices that convert chemical energy in fuel directly into electricity (and heat) without involving the process of combustion. the technology is highly efficient, can be applied to a range of fuels (depending upon the type of fuel cell), quiet in operation (the fuel cell itself has no moving parts) and scalable from mw to mw. as such, fuel a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 93 cells are considered as one of the most promising technological solutions for sustainable power generation. fuel cells can be used in a broad range of applications, including: transportation, residential combined heat and power (chp), large scale distributed power generation and battery replacement uses. fuel cells come in a range of architectures and material sets, but central to all is the electrolyte onto which there is attached the anode and cathode electrodes on each side. figure 1 illustrates the simplistic operation of a fuel cell (using the sofc as an example). the cell is constructed from two porous electrodes separated by an electrolyte. oxygen or air is supplied to the cathode (also referred to as the ‘air electrode’) where electrochemical reduction of the oxygen takes place to form oxide ions (o 2) that migrate through the electrolyte, to the anode (also referred to as the ‘fuel electrode’), and oxidize the fuel (hydrogen in this case) releasing water, heat and electrons that travel around an external circuit to the cathode and do useful work. figure 1. schematic operation of an individual fuel cell (taking a solid oxide fuel cell as an example). different types of fuel cells fuel cell types vary from one to another in operating parameters and technical characteristics (e.g. power density, efficiency, etc.). however, the fundamental feature of the fuel cell which is different, and indeed names the fuel cell type, is the electrolyte. there are at least five main fuel cell types: phosphoric acid fuel cell (pafc), proton exchange membrane fuel cell (pemfc), alkaline fuel cell (afc), molten carbonate fuel cell (mcfc) and solid oxide fuel cell (sofc). in the first two types, hydrogen ions (protons) flow through the electrolyte from anode to cathode to react with oxygen, and produce water; whereas in the last three types (alkaline, molten and solid oxide fuel cells) anions (oh , co3 2, o 2) migrate through the electrolyte from cathode to anode to react with fuel and similarly produce water (figure 2). fuel cell electrolytes are electronically insulating but ionically conducting, allowing certain types of ions to transport through them. j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 94 figure 2. flow of ions through different fuel cell electrolyte types. fuel cell applications the most common types of fuel cell systems and their main characteristics, including fuel efficiency, operating temperature, lifetime, etc. are summarised in table 1. the type of fuel cell and the range of operating temperatures are primarily related to the electrolyte material. hence, the first three types (afc, pafc, and pemfc) are considered as low temperature fuel cells, and the other two types (mcfc and sofc) require high temperature. low temperature fuel cells have the advantage of rapid start-up time but necessitate precious metal electrocatalysts and high purity hydrogen as fuel. table 1. technical characteristics of the five main fuel cell types (1 [14]; 2 [15]; 3 -[16]) f u e l c e ll t y p e o p e ra ti n g te m p e ra tu re 1 ,2 ,c 0 p o w e r d e n si ty 1 ,2 , m w / cm 2 t y p ic a l st a ck s iz e 3 f u e l e ff ic ie n cy 2 , % c o t o le ra n ce 1 li fe ti m e 2 × 1 0 0 0 , h r applications 2,3 ; capital cost 2 , $/kw advantages 3 disadvantages 3 pafc 150-200 150-300 400 kw 100 kw module 55 poison (<1%) > 40 distributed power; 3000 $/kw higher temperature enables chp; -increased tolerance to fuel impurities pt catalyst; long start up time; low current and power; pemfc 50-100 300-1000 (350 2 ) < 1-100kw 45-60 poison (<50ppm) > 40 backup power; portable power; distributed generation; transportation; specialty vehicles; >200 $/kw solid electrolyte reduces corrosion & electrolyte management problems; low temperature; quick start-up expensive catalysts; sensitive to fuel impurities; low temperature waste heat afc 90-100 150-400 10-100 kw 40-60 poison (<50ppm) > 10 military; space; >200 $/kw cathode reaction faster in alkaline electrolyte, leads to high performance; low cost components sensitive to co2 in fuel and air; electrolyte management mcfc 600-700 100-300 0.3 – 3 mw 300 kw module 60-65 fuel > 40 electric utility; distributed power generation; 1000 $/kw high efficiency; fuel flexibility; can use a variety of catalysts; suitable for chp; high temperature corrosion and breakdown of cell components; long start-up time; low power density sofc 800-1000 250-350 1kw-2mw 55-65 fuel > 40 baseload power generation; auxiliary power; electric utility; distributed generation; 1500 $/kw high efficiency; fuel flexibility; can use a variety of catalysts; solid electrolyte; suitable for chp&chhp; hybrid/gt cycle high temperature corrosion and breakdown of cell components; high temperature operation requires long start up time a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 95 liquid metal electrode electrochemical systems the generic construction of a lma sofc is illustrated in figure 3. the arrangement is ostensibly identical to a conventional sofc except that the anode is a molten metal. fuel is fed to the electrode and oxidized products are generated in the same way. the difference is that both gaseous and solid fuel can be fed directly and effectively converted with no pre-treatment stage. figure 3. operation of liquid metal electrode reactor in fuel cell and electrolyser mode. figure 3 also shows the potential for operation in electrolyser mode. this concept has the cell fed with co2 and steam (h2o) that are electrochemically converted to synthesis gas (a mixture of hydrogen and carbon monoxide), that can be used to make liquid fuel (i.e. via the fischer-tropsch process) or used in a fuel cell directly [17]. liquid metal anode sofcs the liquid metal electrode resides in a layer between the fuel chamber (gaseous or solid fuel) and the solid electrolyte (figure 4). the oxygen reduction reaction occurs at the cathode/electrolyte interface resulting in oxygen ions, which then migrate though the solid electrolyte, typically represented by yttria-stabilised zirconia (ysz), to the liquid metal anode. the o 2 ions react electrochemically with the liquid metal, generating metal oxide which is the active species for the oxidation of the fuel (gaseous or solid carbon and hydrogen in the diagram), producing carbon dioxide or water respectively. however, the exact mechanism occurring and the species involved in the liquid metal anode media are not well defined and depend upon the metal used. the molten metal blocks direct contact of electrolyte with gaseous impurities and hence reduces electrolyte degradation by inhibiting the reaction between contaminants and electrolyte [4]. moreover, the fuel contaminants can become a fuel source themselves as they undergo electrochemical oxidation. all of the electrolyte surface in contact with the anode is available to supply a flux of oxide ions for reaction there is no need to engineer a complex ceramic metal (cermet) electrode with optimized triple phase boundary (tpb) as is necessary in conventional sofcs [18,19]. j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 96 figure 4. schematic showing operation of lma sofc with (a) hydrogen and (b) solid carbon, as the fuel. operation of liquid metal anode sofcs the liquid metal anode acts as an intermediary for the oxidation of fuel [4]. taking a molten metal (m) and hydrogen as a fuel, the reactions at the lma-sofc anode follows the generic mechanism: m + xo 2  m[ox]m + 2xe reaction (1) xh2 + m[ox]m m + xh2o reaction (2) reaction (1) proceeds at the liquid anode-electrolyte interface, whereas reaction (2) occurs at the fuel-anode interface. the oxide ions (o 2) are delivered from the cathode via the electrolyte. [ox]m is a state of oxygen within the liquid metal anode. the state of oxygen in the metal varies with operating conditions, and hence ‘x’ is used to represent this uncertainty. when using tin, oxygen may form oxide or suboxide or remain dissolved in the melt [20]. the state of oxygen may vary, though it has no particular effect on the overall reaction, which is the sum of reactions (1) and (2): h2+ o 2 h2o+ 2e reaction (3) reaction (3) is applicable to any liquid metal anode sofc system including sn or cu [21]. thermodynamics of lma sofc operation the maximum electrical voltage generated by an electrochemical system can be determined via the standard equilibrium electrode potential (eeq), expressed by the nernst equation (1) as follows: 0 i i ilneq rt e e v a nf    equation (1) where ai and νi are the activities (fugacities for gases) and stoichiometric factors of the species i (the stoichiometric factors νi taking positive and negative values for oxidised and reduced species respectively), r is the molar gas constant, 8.314 j mol-1 k 1, t is absolute temperature and e o is the standard electrode (equilibrium) potential of the reaction that can be defined from the equation (2): 0 0 rg rte nf   equation (2) a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 97 where δg0 r is the gibbs free energy change of electrochemical reaction, n – number of electrons transferred during electrochemical reaction and f is faraday’s constant, 96,485 c mol -1 . figure 5 shows the standard equilibrium potentials of several metal-metal oxide couples as a function of temperature. the data for the figure was generated using the chemical reaction and equilibrium software hsc chemistry 6.1 (outotec, finland). it can be seen that the cell voltage is expected to decrease with increasing temperature, as a result of the negative entropy change associated with the reactions. however, the voltage obtained under operating conditions (with a net current flowing) is difficult to predict. in addition to the usual loss mechanisms associated with reaction kinetics, ohmic and mass transport limitations, cell voltage will depend on the actual reaction at the anode: whether it is directly with dissolved fuel or the oxidation of the metal to oxide (electrochemical reaction) followed by oxidation of the hydrogen by the oxide (chemical reaction). if the reaction is directly with the fuel then conventional fuel cell reaction ocvs will hold, which are generally higher than those for the reaction with the metal. therefore, from a thermodynamic perspective, it may be preferable for the metal to have a low affinity for oxygen and a high solubility of fuel and oxygen. figure 5.effect of temperature on standard equilibrium cell potentials for various metal oxidation reactions. battery effect the use of lmas can have an additional positive impact on fuel cell performance by creating the so-called ‘battery’ buffering effect. it is well known that sofcs (similar to other fuel cells) can be detrimentally affected by power demand surges during operation, since such surges can cause fuel starvation, which in turn may cause irreversible structural changes at the electrodes. the presence of a certain amount of lma acts as a metal-oxygen battery, where the metal itself is oxidised to provide sufficient load [4,5,7]. the oxidised metal can be reduced again by the fuel fed into the anode when normal load is resumed. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 500 700 900 1100 1300 1500 s ta n d a rd e q u il ib ri u m p o te n ti a l / v temperature / k j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 98 liquid tin anode sofc liquid tin anode sofcs (lta sofc) have been demonstrated with a variety of fuels including gaseous, solid and liquid carbonaceous fuels that did not require any processing, reforming or pretreatment to remove sulfur. the power density reported was >160 mw cm -2 with hydrogen, and 120 mw cm -2 with jp-8 fuel[20]. operated at 900 c, a liquid tin layer of 6 mm was exposed to humidified h2 gas (3 % h2o). from the analysis of transient characteristics time, the oxygen (oxide) diffusion coefficient of 1×10 -3 cm 2 s -1 was derived from test results. the value differs significantly from previously measured literature values [20]. oxide diffusivity is one of the most critical fundamental parameter that determines cell performance. inconsistencies in literature reported values call for further work to develop robust methods for the determination of this parameter. physical and chemical properties of tin abernathy et al. make the case for tin as an ideal metal for lma sofcs [4]. a comparison table (table 2) summarises basic physical properties and abundances of various metals that can be used as a liquid anode. the metal should have relatively low melting point (simplifying operational conditions) and high boiling point. tin has a low vapour pressure (9.87×10 -4 pa at 1000 c) and hence, low volatility [12], i.e. the anode is not vaporised into the fuel exit stream during operation. being technically easier to operate with, sn remains the most commonly used anode material for such la sofc systems, though some researchers are seeking better anode materials and suggest the use of metal alloys based upon tin. such factors as metal abundances in the earth’s crust, annual production and associated prices also have to be taken into consideration. research studies so far have shown results working with tin, copper and bismuth. other metals may not be suitable due to hazards (sb), high price (ag) or low abundance (in). table 2. common metals properties and abundances, prices (adapted from [4]) metal tm / c tb /, c price*, $/kg abundance in earth’s crust, ppm reported experimental studies using metal and/or metal alloys aluminum 660 2520 1.7 83,000 antimony 631 1587 5.0 0.20 [22], [23] bismuth 271 1564 16.3 0.063 [21], [24–27] cadmium 321 767 2.7 0.10 copper 1085 2563 5.2 79 [21] indium 157 2073 390 0.05 [23] lead 328 1750 1.7 7.9 pb [23];sn-pb alloy-[25]; sn-pb-bi alloy-[25], tin 232 2603 13.9 2.5 [4–6], [11–13], [20], [24], [26–28] silver 962 2163 471 0.079 zinc 420 907 1.7 79 *the metal prices are taken from us geological survey minerals information team, mineral commodity summaries 2010 the following table (table 3) shows the change of thermophysical properties of tin at a range of temperatures, including the solubility of oxygen, hydrogen and sulfur. electrical resistivity of tin at 1000  c has a reasonable value of 67.1 µω cm compared to the resistivity of nickel (53.1 µω cm) used in conventional ceramic metal (cermet) sofc anodes [12]. a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 99 table 3. thermophysical properties of tin (adapted from [4]) property temperature, c value surface tension 232 5.44 mn cm -1 viscosity 232 1.85 mn s m -2 expansion on melting 232 2.3 % density 800 6580 kg m -3 1000 6480 kg m -3 resistivity 800 62.1 µω cm 1000 67.1 µω cm gas solubility-oxygen 536 0.000018 wt % 750 0.0049 wt % gas solubilityhydrogen 1000 0.04 cm 3 h2 / 100 g sn 1300 0.36 cm 3 h2 / 100 g sn gas solubility sulfur 800 4.5 wt% 1000 8.0 wt% the density of tin and nickel at room temperature are 7.28 g cm -3 and 8.90 g cm -3 respectively. this makes tin an even more preferable metal on the basis of weight [12]. high solubility of sulfur in the melt is an attractive property of tin, allowing ‘dirty’ fuels to be used without sulfur removal or fuel reforming. fuels investigated various fuels have been studied for lta sofcs including biodiesel [5], military fuels, such as jp-8 [6,11,13], gasoline, methane, etc. all these studies confirm the lta sofc fuel flexibility. figure 6 summarises measured efficiency of lta sofc operated on different fuels [6]. figure 6. measured efficiency of lta sofc over the range of fuels [6] electrochemical characterisation of lta sofcs the first studies on lta sofc developed by cell tech power llc were reported in 1998 [12] and subsequently mainly focused on applying lta sofc for distributed power generation using natural gas. the next step was to test alternative fuels such as waste plastics and military logistic fuels (e.g. jp-8) applicable for portable applications, followed by studies where biomass and coal are j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 100 considered to be directly converted in lta sofc. tao et al. [12] initially developed the gen 3.0 cell and stack and demonstrated a power density of 40 mw cm -2 with jp-8 fuel. cell performance was significantly improved in the next generation cell gen 3.1, which was lighter by a factor of four. the operational power density was 120 mw cm -2 with jp-8 fuel that contained 1400 ppm of sulphur [12]. this study confirmed the operation of lta sofc without fuel desulfurization or preprocessing. to overcome existing drawbacks of the previous cell design (gen 3.0) an alumina porous separator was introduced to increase fuel mass transport to liquid tin. prior to testing with jp-8, gen 3.1 was tested with hydrogen to ensure proper cell operation (figure 7 (left)). figure 7. power curves for tin based cells operating on hydrogen and jp-8 (left); durability test on jp-8 (v, w) (right) [12] at equal operating conditions the cell fed with jp-8 showed 70-80% of the cell performance with hydrogen: the maximum current density for hydrogen was 316 ma cm -2 and 220 ma cm -2 for jp-8, corresponding to a maximum power density of 153 mw cm -2 and 120 mw cm -2 respectively. in terms of durability, gen 3.1 with jp-8 demonstrated an average efficiency of 41.3% at 76 mw cm -2 for 1 hour. the whole durability test is illustrated in figure 7 (right). the next generation of cells aims to reach >200 mw cm -2 power density with the military fuels. to achieve this a more detailed evaluation of separator properties such as porosity and stability for long-term operation is still needed [12]. it has been pointed out that interactions between the liquid tin anode and fuel have not been systematically studied and that better modelling and a fundamental study is needed. direct conversion of biodiesel has been demonstrated using the lta sofc [5]. in an approach similar to that adopted for jp-8, a tubular lta sofc cell was tested for 4.5 h with biodiesel (b100) prepared from virgin and waste cooking oils [5]. peak power and current density values of 117 mw cm -2 and 217 ma cm -2 and overall cell efficiency of 40% were reported. the results of the study also verify the fuel flexibility of the lta socf system. an interesting approach of using a separate electrochemical reactor coupled to an external chemical reactor with liquid tin anode sofc was recently proposed by colet lagrille et al. [26]. the system oxidizes carbon directly in fuel cell mode and produces h2 in the electrolyser mode. the schematic of two separate reactors is illustrated in figure 8. the approach has the advantage of allowing the ‘combustion’ (fuel oxidation) process and the electrochemical oxidation of the melt to be optimized in two separate reactors; and it also improves mass transport losses. the disadvantage is that pumping of liquid metal between the reactors is necessary. a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 101 figure 8. (a) schematic of a lma sofc coupled to an external combustion reactor, (b) schematic of a lma solid oxide electrolyser (soe) coupled to an external combustion reactor [26] this study also reports a 2d model of a micro-tubular hollow fiber fuel cell to predict distribution of oxygen concentration within the liquid tin anode [26]. the diffusion coefficient of oxygen in tin was assumed, the authors highlighted the need for accurate determination of this parameter for reliable model prediction. alternative metals and metal-alloy anodes for lme sofc other metals and metal alloys have been demonstrated as interesting anode materials capable of direct carbon oxidation in novel sofcs. as summarised earlier in table 2, the most common liquid metal anodes are made of: tin, bismuth, antimony, lead, copper, as well as indium and silver. liquid copper anode sofcs successful performance of lme direct carbon fuel cell (dcfc) has been demonstrated using a molten copper anode operating at 1373 k [21]. the overall conversion efficiency measured at maximum power density was 62.5 %. design of this system was different to those discussed earlier in the way that the cathodesupported ysz tube was immersed into liquid copper media. figure 9(a) shows the schematic presentation of the system [pt, cr cermet, cu-o (l) +c // (y2o3) zro2//la0.9sr0.1 (mn, fe, co)o3-δ o2 (air), pt]. the fuel (coke particles) was added via an alumina inlet tube and stirred thoroughly with a stirrer; the entire cell was suspended in a vertical furnace. the ocv of the system was measured to be 1.2 v, and generated a maximum power density of 1.7 w cm -2 (2.25 a cm -2 at 0.75 v) as shown in figure 9(b). despite good overall performance, liquid copper sofcs are still difficult to operate. the relatively high vapour pressure of copper, compared to tin and other metals, is a challenge for practical operation. in addition, copper anodes necessitate very high operating temperatures (m.p. = 1357.8 k), which inevitably complicates operation of the system. j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 102 figure 9: (a) schematic of liquid copper anode sofc, (b) polarisation and power density curves for liquid copper anode sofc[21] liquid antimony liquid antimony has been assessed as an anode for dcfcs [29]. operating at 973 k and employing a sc-stabilized zirconia (scsz) electrolyte, the system, incorporating an air electrode but with no input of fuel, has demonstrated an open-circuit voltage (ocv) of 0.75 v for the redox couple sb sb2o3. having an anode electrode resistance of 0.06 ω cm 2 , the operated power density was 350 mw cm -2 . as expected, the addition of sugar char immediately increased the cell voltage as the sb2o3 was reduced with fuel, preventing formation of a metal oxide layer on the electrolyte surface. scaling up of such liquid antimony dcfc technology may require an increase in the rate of reaction between fuels and antimony oxide, as some carbonaceous fuels (carbon black, rice starch) have not demonstrated any interaction with liquid sb [29]. liquid bismuth liquid bismuth has been reported to have an inferior performance to tin anodes [25]. the ocv was reported to be slightly lower than theoretical (440 mv): 424 mv with hydrogen supplied and 408 mv without [25]. being ion-conductive, bismuth oxide may solve the problem with oxide transport blockage through ysz-anode media [26]. however, operating at lower power densities, liquid bismuth sofcs require in-depth investigation and are not considered to be a feasible alternative to sofcs with a tin anode [21,22] . other metal anodes for lme sofcs indium (in), lead (pb), antimony (sb) [23] and silver (ag) [30] have also been examined in battery and fuel cell modes (if feasible) to test their anodic efficiency in lme sofcs. similar to tin, the performance of indium was adversely affected by formation of metal oxide at the electrolyte interface when higher current was drawn. this was not the case for pb and sb, where the melting points of their metal oxides were lower than the operating temperature. direct oxidation of coal has also been proposed to operate in a liquid iron anode sofc [31]. the application of a liquid silver anode for dcfcs has been experimentally explored by javadekar [30] who achieved an ocv of 1.12 v with charcoal. liquid silver has lower melting point and higher oxide solubility compared to other metals (e.g. copper) and therefore is advantageous. another approach was to use a ag-sb alloy, where the high solubility of oxide in silver might be combined with the low melting point of sb, to improve overall anode performance [30]. (b) a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 103 metal alloy anodes in an effort to optimize anode properties, various alloy systems have been investigated. the work involving the pure metal anodes (bismuth) mentioned above was developed further to explore anodes of liquid tin-lead and liquid tin-lead-bismuth alloys in battery mode followed by direct oxidation of coal [25]. for the tin-lead alloy, performance degradation (from 11.7 mw cm -2 to 10.1 mw cm -2 ) occurred within 13 hours, with an increase in performance to 14 mw cm -2 when coal was introduced. the addition of bismuth to tin-lead alloy decreased the overall anode effectiveness. uncertainties and technological challenges of lme sofc prior to commercialization of liquid metal anode solid oxide fuel cells, greater understanding of the electrochemical reactions, redox and transport processes within liquid metal electrodes is needed [20]. the following research challenges are particularly highlighted: the nature of oxidised species in liquid metal electrode media; transport of oxygen or oxidized species in lme sofc; reaction kinetics between oxide and fuel within liquid anode; kinetics of oxygen transfer at liquid metal anode electrolyte interface; oxygen (oxide) diffusivity / transport through liquid metal layer and solubility of oxide and fuels in the melt. as argued by abernathy et al. [4], improvement of oxygen transport within liquid metal media can be achieved by increasing oxygen solubility through alloying. wetting characteristics of liquid metal with the electrolyte and fuel oxidation kinetics need also to be considered. most of the work so far has been focused on the testing of single cells, which limits its further application. in this regard, it would be beneficial to model the performance of lme sofcs stacks and prototype based on derived design guidelines. in order to be able to model and later design practical lme sofc systems the information on oxygen solubility and transport in liquid metals is essential as it is a central process that may limit the performance of the fuel cell. fundamental investigation of these interactions within a liquid anode, as well as analysis of other physical metal properties with temperature (e.g. vapour pressure, melting point, surface tension, contact angle, etc.) will be beneficial in designing and building durable and stable fuel cells. other important factors requiring investigation include: aspects concerned with the introduction of solid fuel into the melt; optimization of thickness of the melt; prevention of unfavorable formation of metal oxide tending to result in blocking of the electrolyte; fundamental studies concerned with transportation of the oxide from the generation zone at the interface with the electrolyte to the reaction zone at the interface with the fuel (relevant aspects include melt thickness, diffusion of oxidised species and convection in the melt); effect and optimization (which may include surface structuring) of contact area between the liquid anode and electrolyte. engineering considerations include: matching of electrolyte composition to temperature of operation; choice of electrolyte thickness based upon optimization between mechanical properties and ionic conductivity; agitation of the melt; generation of suitable electrolyte surface structures; cell design; scale-up of laboratory cells to pilot and then to commercial plant. conclusions lma sofcs are a highly efficient option for direct electrochemical conversion of solid fuels into electrical energy. a range of possible metal anodes have been reviewed with tin the material on which most development work has been performed. despite numerous studies, the understanding j. electrochem. sci. eng. 3(3) (2013) 91-105 liquid metal anode solid oxide fuel cells 104 of the reaction kinetics at liquid metal-fuel and liquid metal-electrolyte interfaces, as well as oxygen diffusivity through the liquid metal layer needs further systematic investigation. acknowledgements: the authors wish to thank nazarbayev university and the government of republic of kazakhstan for the bolashak international scholarship for aliya toleuova and the epsrc supergen fuel cells programme (ep/g030995/1) for supporting brett’s research. references [1] international energy agency, world energy outlook 2010. 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[23] a. jayakumar, j. m. vohs, r. j. gorte, ind. eng.chem. res. 49 (2010) 10237–10241. a. toleuova et al. j. electrochem. sci. eng. 3(3) (2013) 91-105 doi: 10.5599/jese.2013.0032 105 [24] . ayakumar, s. ee, . horn s, . . ohs, r. . orte, j. electrochem. soc. 157 (2010) b365–b369. [25] m. labarbera, m. fedkin, s. lvov, ecs trans. 35 (2011) 2725–2734. [26] m. colet lagrille, u. doraswami, g. h. kelsall, ecs trans. 42 (2012) 137–148. [27] t. tao, m. t. koslowske, j. bentley, j. brodie, c. mackean, ecs trans. 41 (2012) 125–135. [28] s. c. rayman, m. t. koslowske, l. s. bateman, t. tao, r. e. white, ecs trans. 33 (2011) 93– 121. [29] . ayakumar, r. küngas, s. roy, . avadekar, d. . buttrey, . . ohs, r. . orte, energy environ. sci. 4 (2011) 4133–4137. [30] a. d. javadekar, ph.d. thesis ,university of delaware (2012). [31] i. v. yentekakis, p. g. debenedetti, b. costa, m. konsolakis, v. kiousis, ionics, 5 (1999) 460– 471. © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {local electrochemical deposition of copper from sulfate solution:} http://dx.doi.org/10.5599/jese.1352 557 j. electrochem. sci. eng. 12(3) (2022) 557-563; http://dx.doi.org/10.5599/jese.1352 open access : : issn 1847-9286 www.jese-online.org original scientific paper local electrochemical deposition of copper from sulfate solution georgii vasyliev, viktoria vorobyova, dmytro uschapovskiy and olga linucheva national technical university of ukraine “igor sikorsky kyiv polytechnic institute”, 37, prospect peremohy, kyiv-56, 03056, ukraine corresponding author: g.vasyliev@kpi.ua ; tel.: +38-096-924-9888; fax: +38-044-204-9773 received: april 20, 2022; accepted: may 26, 2022; published: june 7, 2022 abstract local electrochemical deposition is a type of electroplating used to plate metal locally or form metal objects using electrochemical principles at a short distance from the working electrode. in this work, deposition of the copper spot was modelled using comsol software and experimentally tested in copper sulfate electrolyte using soluble copper anode. the working capillary diameter was 4 mm and the interelectrode distance was 5 mm. the deposited copper of 100 µm thickness was investigated using the 3d-profilometry technique. the geometry of deposited metal was found to be in good accordance with the comsol model. the inclusions of anodic sludge were responsible for the surface inhomogeneity of the deposited copper. keywords local electrochemical deposition; additive manufacturing; copper plating; computer modelling; throwing power; anodic sludge introduction additive manufacturing (am) is an emerging trend in industry and materials science that provides simple manufacturing of complex objects and opens new approaches in the materials design and manufacturing process. the variety of am materials becomes wider every year simultaneously with the development of manufacturing techniques. metal am is a challenging task due to their high melting temperature. the most industrially developed technique is based on the local melting of metal powder with a laser or electron beam [1,2]. despite the ability to produce complex objects, the technique requires metal powder preparation and a high energy power source. electrochemical deposition can be an alternative to the local laser melting because of its low energy consumption (the process takes place at room temperature) and the object is not subjected to temperature influence. several electrochemical am techniques are known today. they can be divided into two main groups, depending on the deposition conditions: mask-based deposition and mask-less deposition [3]. in the former technique, the cathode is covered with the insulating mask covering areas where deposition should not occur. so, the metal deposits on uncovered surfaces [4]. another mask-based http://dx.doi.org/10.5599/jese.1352 http://dx.doi.org/10.5599/jese.1352 http://www.jese-online.org/ mailto:g.vasyliev@kpi.ua j. electrochem. sci. eng. 12(3) (2022) 557-563 local electrochemical deposition of copper 558 technique is instant masking plating. in this technique, the mask moves with an anode. the deposition starts only when the mask is attached to the surface. once the electrolyte is depleted, the depositions stop, the mask is lifted away, and the electrolyte is renewed [5]. electrochemical fabrication is a manufacturing technique where the deposition of structural material is alternated with the deposition of sacrificial metal, which plays the role of a mask [6]. once the deposition of the whole structure is completed, the sacrificial metal is etched to obtain the final structure. mask-less deposition techniques are carried out with no mask on the surface and the environment around the electrode is open. the first example of this group is localized electrochemical deposition (lecd), proposed in 1995 [7]. the technique is based on the deposition of metal in the localized area under the working electrode. to get a very localized electric field, an ultrafine inert anode tip and small interelectrode gap are used. further development of the proposed technique was focused on anode construction improvement. the anode was placed inside the dielectric capillary that provides an additional focus of the electric field [8-10]. jet electrodeposition is another lecd technique, where the impinging anodic electrolyte jet as anode tool to deposit metal selectively deposits metal on the cathode [11]. in meniscus-confined electroplating, the deposition is located in a meniscus-shaped electrolyte bridge between a narrow anode tip and cathode substrate [12-16]. the deposition rate is determined by the diffusion rate of ions and is very low. extremely small objects can be obtained using fluidic force microscope electrodeposition [17]. in this technique, the hollow afm cantilever with an aperture at the tip is used. during deposition, the cantilever serves as a working electrode and is filled with electrolyte. the flow of electrolyte from the probe is controlled by pressure regulation. once the probe is positioned, the deposition starts and ends when the deposited metal touches the tip. then the probe moves to the next position. comparing different electrochemical additive manufacturing techniques, it can be seen that the higher precision of manufacturing, the lower the deposition rate. further investigations are required to develop a highly productive ecam technique that is able to produce large-scale objects at commercially acceptable rates. in this work, the local electrochemical deposition of copper was performed using a computer model and compared to electrochemically deposited metal using 3d-profilometry. computer modelling the computer model of the local electrochemical deposition was built using comsol multiphysics 5.2 software. the scheme of the cell is given in figure 1a. the computer model gives the ability to predict the localization of the electric field and thus the location of the deposited metal. moreover, the model gives the expected profile of the deposited copper based on the electrical field distribution and deposition duration. the localization of deposition is highly dependent on the throwing power of the electrolyte. the letter one depends on the conductivity of the solution. the conductivity of the copper plating electrolyte depends on the cuso4 content, but in technological processes, the sulfuric acid is added. so, the addition of h2so4 increases the conductivity, which makes the distribution of deposited copper more uniform and decreases the voltage load on the plating bath. on the contrary, in this study copper deposition is localized, so the amount of acid should be reduced. to study the influence of h2so4 content on copper local deposition, the calculations were performed for several concentrations. the comsol electrochemical deposit module requires the value of polarizability of the cathodic electrochemical process. the polarizability di/de was determined experimentally (figure 2). g. vasyliev et al. j. electrochem. sci. eng. 12(3) (2022) 557-563 http://dx.doi.org/10.5599/jese.1352 559 galvanodynamic cathodic polarization curves were obtained using conventional three-electrode cell. the scan rate was 20 ma s-1. the plates of 11 cm2 manufactured from copper grade m0 served as working and auxiliary electrodes, and a saturated silver chloride electrode (ssce) was used as a reference one (eref = 0.2 v vs. nhe). the potential values on polarization curves are given vs. ssce. standard electrode potential of copper ecu2+/cu = +0.34 v vs. nhe or ecu2+/cu = +0.14 v vs. ssce, which agrees well with experimental data (figure 2). a b figure 1. scheme (a) and photo (b) of electrochemical cell: 1 – cathode; 2 – anode; 3 – polypropylene tube; 4 – electrolyte figure 2. cathodic polarization curves of copper deposition in the electrolyte containing 200 g/l cuso4 and sulfuric acid in different concentrations the limiting current density appeared to depend on the sulfuric acid content even in the same concentration of copper ions [18]. this can be explained by the influence of migration part of mass transfer. according to equation (1), the limiting current density, il, of copper reduction is dependent on the copper ion transport number: ( ) 2+ 2+ cu l cu 2 1 fdc i t  = − (1) where d – diffusion coefficient of copper ions, ccu2+ copper ions concentration, tcu2+ copper ions transport number,  diffusion layer thickness. 0 40 80 120 160 -0.50-0.40-0.30-0.20-0.100.000.100.20 i / m а cm -2 е / v 10 g/l acid 30 g/l acid 150 g/l acid di / de = -796 di / de = -516 di / de = -400 http://dx.doi.org/10.5599/jese.1352 j. electrochem. sci. eng. 12(3) (2022) 557-563 local electrochemical deposition of copper 560 the electrolyte solution contains the following ions: h+, cu2+, so42– and copper ions transport number can be determined by the equation (2): 2+ 2+ 2+ 2+ 2+ 22+ + 4 4 cu cu cu cu cu so so h h 2 2 2 2 c t c c c     + = + + + + + (2) where  cu2+, so42-, h+ are molar conductivity of copper, sulfate, and proton ions. due to the much higher molar conductivity of protons, the ion transport number of copper will decrease as acid concentration increases, so the limiting current density of copper deposition will decrease as well. the configuration of the cell is given in figure 3a. the diameter of the working nozzle was 4 mm, and the copper wire was placed inside it and worked as a soluble anode. the distance between the copper anode and the surface was 5 mm. the gap between the nozzle and the surface was 0.1 mm. the deposition duration was 70 min. the rest of the parameters are given in table 1. table 1. copper local deposition modelling parameters parameter study number 1 2 3 h2so4 content, g l-1 10 30 144  / s сm-1 0.050 0.125 0.433 (di / de) / ma cm-2 v-1 796 516 400 φext / v 0.82 0.55 0.49 the results of modelling are given in figure 3b. the growth profiles show that the copper was deposited locally in the form of a cylinder, with a thickness of 100 µm. however, the curvature of the side surface depends on the acid concentration. the closest profile was obtained in the lower tested acid concentration of 10 g l-1. the increase of h2so4 content to 150 g l-1 makes the deposited copper spot nearly 2 times wider. so, for experimental testing, the electrolyte with 200 g l-1 cuso4 and 10 g l-1 h2so4 was chosen. a b figure 3. current density distribution in the interelectrode area of the cell (a) and the growth profiles of copper (b), modelled for a 0.1 mm gap between the nozzle and the surface in 200 g/l sulfate electrolyte and different sulfuric acid concentrations experimental the deposition of copper was performed in the same electrochemical cell, consisting of two electrodes (figure 1b). the same cell configuration was used for computer modelling. the copper 0 20 40 60 80 100 120 0.146 0.148 0.150 0.152 0.154 δ / μ m x / m 10 g/l acid 30 g/l acid 150 g/l acid surface: electrolyte current density, a m-2 x / mm y / m m g. vasyliev et al. j. electrochem. sci. eng. 12(3) (2022) 557-563 http://dx.doi.org/10.5599/jese.1352 561 plate was placed at the bottom of the vessel, filled with electrolyte, and attached to the negative pole of the current source. the counter electrode was made from copper wire and placed inside the polypropylene tube of 50.5 mm. the tube was oriented in the center of the copper plate and the distance between the plate and the pipe was 5 mm. this arrangement was maintained during the deposition process. the electrolyte used in the study contained 200 g l-1 cuso4 and 10 g l-1 h2so4. working current density was determined from polarization curves and composed 65 ma cm-2. the height of the deposited copper was set to 100 m, so the calculated deposition time was 70 min. the electrolysis was performed at ambient temperature. the characterization of the deposited copper was performed with the 3d-profilometry technique. the sensitive piezoelectric sensor with a vertical resolution of 250 nm/dot and horizontal resolution of 625 nm dot-1, scanned the area of 77 mm2 around the deposited copper spot. the obtained data were collected using a pc and surfare software was used to get a 3d image of the surface. the height of the deposited copper, surface roughness and precision of deposition was determined and compared to the comsol model. results and discussion copper deposition is an electrochemical reduction process which undergoes according to equation (3): cu2+ + 2e= cu (3) the rate of copper deposition is determined by the rate of copper supply to the surface, so the working current density was determined experimentally. figure 2 shows the polarization dependence of copper that was used to determine the working current density. the equilibrium potential of copper in the electrolyte was 0.1 v vs. ssce. limiting current density was found to be 120 ma cm-2. however, limiting current densities are never applied for metal deposition due to the formation of low-quality deposits with powder inclusions. the working current density was selected to be half of the limiting – 65 ma cm-2, which gives the deposition rate of 85 µm h-1. deposited copper is shown in figure 4. it can be seen that the surface of the deposited metal is not smooth and dark inclusions can be observed. figure 4. locally deposited copper on the surface of copper plate during 3d-profilometry the measured profile of the deposited copper and the comparison of the modeled and experimentally deposited copper are given in figure 5. the profile of the deposited copper shows that the average height agrees well with the comsol computer model. http://dx.doi.org/10.5599/jese.1352 j. electrochem. sci. eng. 12(3) (2022) 557-563 local electrochemical deposition of copper 562 a b a b figure 5. 3d-vizualization of the deposited copper (a) and comparison of deposited, modelled and target profiles of copper (b) the side surface of the profile obtained in the experiment is even more straight than in model. however, some areas on the top of the deposited copper demonstrate the presence of peaks, which makes the surface rough. these peaks result from anodic sludge inclusion in the deposits and deposition of copper dendrites. the lower the sulfate acid content in the electrolyte, the lower is copper deposition overvoltage that leads to the formation of dendrites. also, the dendrite formation may be caused by monovalent copper ions reproportion described by equation (4). cu+ ions are formed by staged copper anodic dissolution. this process is known to stimulate the increase of roughness and dendrites formation. 2cu+ = cu + cu2+ (4) in the experiment, the soluble anode was used, and the sludge was able to detach from the anode and fall on the surface, where it was grown in the copper. to avoid the inclusions of anodic sludge in the deposited metal and reproportion of copper in further studies, the insoluble anode will be used, and copper overvoltage will be increased using organic additives. conclusions comsol computer model is adequate and can be used to select preferable electrolyte conductivity, determine interelectrode gap and predict current distribution during local electrochemical deposition of metals. in the present study the conductivity was 50 ms cm-1, ieg 5 mm. the deposited copper profile was found in good agreement with the model. the thickness of the metal layer was 0.1 mm, and the diameter 4 mm, which agrees with the model. the inclusions of the anodic sludge and dendrites formation were responsible for the surface roughness of the deposited 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https://doi.org/10.26153/tsw/590 https://doi.org/10.1109/mhs.1995.494221 https://doi.org/10.1007/s00170-020-05799-5 https://doi.org/10.1007/s10800-017-1124-8 https://doi.org/10.1063/1.4973622 https://doi.org/10.4028/www.scientific.net/kem.667.259 https://doi.org/10.1126/science.1190496 https://doi.org/10.1002/smll.201500177 https://doi.org/10.1002/adma.201705107 https://doi.org/10.3390/cryst10040257 https://doi.org/10.1007/s00542-019-04665-z https://doi.org/10.3390/mi11010006 https://doi.10.15587/1729-4061.2014.30660 https://creativecommons.org/licenses/by/4.0/) {optimization of electrochemical parameters in microbial fuel cell system based on fuzzy-pid and cmac neural network} http://dx.doi.org/10.5599/jese.636 135 j. electrochem. sci. eng. 9(2) (2019) 135-142; http://dx.doi.org/10.5599/jese.636 open access: issn 1847-9286 www.jese-online.org original scientific paper optimization of electrochemical parameters in microbial fuel cell system based on fuzzy-pid and cmac neural network chengcai fu1,2, fengying ma1, 1school of electrical engineering and automation, qilu university of technology, shandong academy of sciences, jinan 250353, china 2school of mechanical electronic & information engineering, china university of mining & technology, beijing 100083, china corresponding author: e-mail mafengy@163.com received: november 13, 2018; revised: november 29, 2018; accepted: december 5, 2018 abstract due to the extensive application prospects on wastewater treatment and new energy development, microbial fuel cells (mfcs) have gained more and more attention by many scholars all over the world. the bioelectrochemical reaction in mfc system is highly complex, serious nonlinear and time-delay dynamic process, in which the optimal control of electrochemical parameters is still a considerable challenge. a new optimal control scheme for mfc system which combines proportional integral derivative (pid) controller with parameters fuzzy optional algorithm and cerebellar model articulation controller (cmac) neural network was proposed. the simulation results demonstrate that the proposed control scheme has rapider response, better control effect and stronger anti-interference ability than fuzzy pid controller by taking constant voltage output of mfc under the different load disturbances as example. keywords microbial fuel cells, electrochemical parameters, fuzzy optional algorithm, cmac neural network introduction environment and energy are two major problems that influence the sustainable development of human beings [1]. bioelectrochemistry systems (bes), especially microbial fuel cells (mfcs), are a newly arising technology that could generate electricity from organic matters, which caused widespread attention of scholars due to their potential applications in environmental purification and solving the energy problem [2]. the current research of mfcs is mainly focused in electricigens, electrode materials, stack electricity production, wastewater treatment and biosensors [3-7]. however, the energy generated http://dx.doi.org/10.5599/jese.636 http://dx.doi.org/10.5599/jese.636 http://www.jese-online.org/ mailto:mafengy@163.com j. electrochem. sci. eng. 9(2) (2019) 135-142 microbial fuel cell system 136 from mfcs is unstable because of the complex impact of the system configuration and electrochemical parameters such as concentration of the solution, voltage, power, dissolved oxygen, temperature, etc. it is obvious that optimal control of electrochemical parameters plays an important role in the aspect of improving the performance of mfcs. based on the research and development of a simple and feasible mathematical model [8], a suitable controller can be designed to maintain the desired output of mfc under different load demands. however, the traditional proportion integration differentiation (pid) controller can hardly control the complex process, owing to the time-varying disturbance, strong coupling, nonlinearity and strict operating conditions of mfcs [9]. with the development of artificial intelligence (ai) technology, the artificial neural network is used successfully in control of the complicated process [10,11]. the approximation ability of cerebellar model articulation controller (cmac) based on highspeed local learning and generalization ability is found superior to other neural networks which are more suitable for nonlinear real-time control in complex dynamic environment [12]. this paper investigates a cmac-pid optimal control method for two-chamber microbial fuel cell by combining cmac and fuzzy pid control. the nonlinear relationship between input and output can be operated by the concept and actual mapping of cmac [12,13]. this method showed advantages not only of high precision and anti-interference of pid, but also has fast response speed and high precision control performance. in the rest of paper, a brief description of the mathematical model for twochamber microbial fuel cell is given in the section 2, while the cmac-pid intelligent control optimization algorithm is proposed in the section 3. in the section 4, the voltage control of mfcs under disturbance is taken as an example to prove the superiority of this method. finally, some conclusions and predictive prospect are given at end of this paper. dynamic model of two-chamber mfc a typical two-chamber mfc consists of an anode, cathode, proton exchange membrane and electrical circuit. in the anode chamber, organic matters (eg. acetate in wastewater) are oxidized and biodegraded by microorganisms, releasing electrons and protons. electrons flow to cathode through an electrical circuit and react with protons and electron acceptors in the cathode chamber (figure 1). equations of electrochemical reactions occurring at anode and cathode are described as follows [8]: anode: (ch2o)2 + 2h2o → 2co2 + 8h+ + 8e (1) cathode: o2 + 4e+ 2h2o → 4oh(2) the anode chamber operates under anaerobic conditions, what is one of necessary conditions for the operation of mfc. therefore, the anodic reaction rate (r1) can be described by the monod type-equation, which is depicted as 0 ac 1 1 a ac ac exp cαf r k η x rt k c   =   +  (3) where k10 is the rate constant of anodic reaction at standard conditions,  is the corresponding charge-transfer coefficient, f is the faraday constant, r is the gas constant, t is the operating temperature, a is the anodic polarization overpotential, kac is the half rate constant for acetate, while cac and x represent acetate and biomass concentrations in the anode chamber. chengcai fu and fengying ma j. electrochem. sci. eng. 9(2) (2019) 135-142 http://dx.doi.org/10.5599/jese.636 137 figure 1. schematic diagram of typical two-chamber mfc system the bulter-volmer expression is used to characterize the electrochemical reaction at the cathode, where the cathode reaction rate (r2) is described as 0 2 2 2 o 2 2 o o 1 exp c c( )f r k rt k c −  =   +    (4) where k20 is the rate constant of cathode reaction at standard conditions,  is cathodic charge transfer coefficient, c is cathodic polarization overpotential, while co2 and ko2 are concentration and half velocity rate constant for dissolved oxygen, respectively. anode and cathode chambers of the mfc can be regarded as a continuous reactor [11]. four mass balance equations in the anode chamber are expressed as: inac a a ac ac m 1 d v ( ) d c q c c a r t = − (5) 2 m 12 2 co in a a co co d v ( )+2 d c q c c a r t = − (6) 1 inh a a h h mv ( ) 8 d dc q c c a r t = + (7) in a a m ac 1 a dec x dx v d x x q a y r v k x t f  − = + −    (8) in above equations, subscript a and superscript in denote anode and flow of feed, respectively. v, q and am denote the volume, flow rate and membrane area, while fx, yac and kdec denote the reciprocal of wash-out fraction, bacterial yield and decay constant for acetate utilizers, respectively. similarly, in the cathode compartment, three mass balance equations are represented as 2 2 2 o in c c o o m 2 d v ( ) d c q c c a r t = + (9) inoh c c oh oh m 2 d v ( )-4 d c q c c a r t = (10) inm c c m m m m d v = ( )+ d c q c c a n t (11) where the subscripts c stands for cathode, nm represents the flow of m+ through the proton exchange membrane, which can be calculated as follows nm = 3600icell/f (12) the charge balances at the anode and cathode are given by http://dx.doi.org/10.5599/jese.636 j. electrochem. sci. eng. 9(2) (2019) 135-142 microbial fuel cell system 138 ca(da/dt) = 3600 icell – 8fr1 (anode) (13) cc(dc/dt) = 3600 icell – 4fr2 (cathode) (14) where icell is current density of mfc, while ca and cc represent the capacitance of anode and cathode, respectively. the output voltage of a single mfc is expressed by the following equation cellm cell o a c cell m aq v = dd v i k k   + +      (15) where vo is the open-circuit voltage, dm, dcell, while km and kaq represent proton film thickness, electrode distance, conductivity of membrane and electrical conductivity of solution, respectively. the main parameters of mfc system are shown in table 1. table 1. operating parameter values of acetate mfc model symbol description value unit f faraday constant 96485.4 c mol-1 r gas constant 8.3144 j mol-1 k-1 t temperature 303 k dm thickness of membrane 1.778e-4 m dcell distance between anode and cathode 0.022 m km conductivity of membrane 17 ω-1 m-1 kaq electrical conductivity of solution 5 ω-1 m-1 ca capacitance of anode 400 f m-2 cc capacitance of cathode 500 f m-2 va volume of anode compartment 5.5e-5 m3 vc volume of cathode compartment 5.5e-5 m3 am area of membrane 5e-4 m2 yac bacterial yield 0.05 dimensionless kdec decay constant for acetate utilizers 8.33e-4 h-1 fx reciprocal of wash-out fraction 10 dimensionless qa flow rate of fuel feed to anode 2.25e-5 m3 h-1 qc flow rate feeding to cathode compartment 1.11e-3 m3 h-1 cacin concentration of acetate in the influent of anode 1.56 mol m-3 cco2 in concentration of co2 in the influent of anode 0 mol m-3 xin concentration of bacteria in the influent of anode 0 mol m-3 chin concentration of h+ in the influent of anode 0 mol.m-3 co2 in concentration of dissolved o2 in the influent of cathode 0.3125 mol m-3 cmin concentration of m+ in the influent of cathode 0 mol m-3 cohin concentration of oh− in the influent of cathode 0 mol m-3 vo open-circuit voltage of mfc 0.77 v k10 rate constant of anodic reaction at standard conditions 0.207 mol m-2 h-1 k20 rate constant of cathode reaction at standard conditions 3.288e-5 mol m-2 h-1 kac half velocity rate constant for acetate 0.592 mol m-3 ko2 half velocity rate constant for dissolved oxygen 0.04 mol m -3  anodic charge transfer coefficients 0.051 dimensionless  cathodic charge transfer coefficients 0.663 dimensionless design an cmac-pid control system a cmac neural network controller was used as feed-forward controller to increase the response speed, while an intelligent pid controller based on fuzzy algorithm was used as feedback controller to ensure system stability and restrain disorder. the structure of cmac-pid controller is shown in figure 2. chengcai fu and fengying ma j. electrochem. sci. eng. 9(2) (2019) 135-142 http://dx.doi.org/10.5599/jese.636 139 quantization address mapping cmac memory cmac function calculation pid controller learning algorithm mfc system + + + _ vcell* vcell un up u(k) e de/dt fuzzy inference kp ki kd ec figure 2. structure of cmac-pid control for two-chamber mfc system tutor learning algorithms are applied in cmac neural network which includes input layer, middle layers and output layer [14,15]. the nonlinear mapping of input layer and adaptive mapping for weights of output layer are determined by the designer separately between adjacent layers. the basic function of middle layers is connected with the output layer by adjusting weight parameter based on gradient descent algorithm. the design of cmac neural network mainly includes the division of input space, the realization of nonlinear mapping from input to output layer and the learning algorithm for weight of output layer [12]. here the reference voltage vcell* is used as an input to cmac. at the end of each control cycle, the output un(k) of cmac is calculated and compared with the final control output u(k) of the system, and amends the weight to enter the learning process. the difference between input and output of cmac is little gradually, then the total control output of the microbial fuel cell system is only generated by cmac [13,14]. three parameters kp, ki, kd of the pid controller are real-time optimized by the fuzzy control algorithm according to the tracking error e and the change in error ec to enhance the anti-disturb ability of the system [10]. the fuzzy sets for e(k), ec(k) are {nb, nm, ns, z, ps, pm, pb}. the fuzzy domain for e(k), ec(k) are [-1,1]. the ultimate control goal of mfc is make the error between the output voltage vcell and the given output voltage vcell* minimum. therefore, the control algorithm can be described as: n i i cell cell i 1 ( )= ( ) *( )= ( )( 1) c u k w e k v v ec k e k e k = = −  p p i d 0 ( )= ( )+ ( )+ ( ) k j u k k e k k e j k ec k =  (16) u(k)=un(k)+up(k) where i is the binary vector, c is the generalization parameter of cmac network, wi is the weight of the i-th storage unit, un(k) and up(k) are the output of cmac and pid controller, respectively, kp = kp0 + kp; ki = ki0 + ki; kd = kd0 +kd. adjustment indicators of cmac are expressed as: 2 n( ( )( ))( ) 2 u k u k e k c = p i ( )( ) ( )== u ke k w k w c      (17) w(k) = w(k-1) + w(k) + (w(k) – w(k-1)) where  is the network learning rate, and the ranges from 0 to 1,  is an inertia quantity,   (0,1). setting w = 0 at the beginning of the system, by this time, un = 0, u = up, and the system is only controlled by pid, and then, the control signal up(k) from the pid controller gradually approach zero http://dx.doi.org/10.5599/jese.636 j. electrochem. sci. eng. 9(2) (2019) 135-142 microbial fuel cell system 140 and the control signal un(k) from cmac controller is gradually closing in on the final control output signal u(k) along with incremental learning of cmac. simulation and results cmac and pid concurrent control for mfc system is realized by matlab/simulink. as shown in figure 3, the feed flow of the anode chamber will directly affect the output voltage of mfc system, so the optimization of the voltage could be achieved by controlling the flow of the anode feed. time, h figure 3. impact of different anode feed rates on the output voltage of mfc for the control optimization algorithm proposed in section 3, the time of sampling was t = 1 ms. parameters of cmac neural network are n = 100, c = 5, η = 0.1 and α = 0.5, the initial parameters of the pid controller are kp = 2.5 e-4, ki = 4.08 e-5 and kd = 3 e-4. the dynamic parameters of pid controller are real time optimized by a fuzzy control algorithm. we set the reference voltage to 0.5 v and switch the resistance from 500 to 400 ω as an external disturbance within the time of 100 h. the simulation results of intelligent control based on cmacpid, independent fuzzy-pid and without controller (uncontrol) are shown in figures 4-6 in comparison to the reference voltage. time, h figure 4. simulation results of cmac-pid intelligent control 0 50 100 150 200 250 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 time(h) v o u t( v ) 95 100 105 110 0.46 0.48 0.5 reference cmac-pid v o lt a g e , v v o lt a g e , v chengcai fu and fengying ma j. electrochem. sci. eng. 9(2) (2019) 135-142 http://dx.doi.org/10.5599/jese.636 141 time, h figure 5. simulation results of fuzzy-pid control time, h figure 6. simulation results without control it can be seen from simulations presented in figures 4-6, that for cmac-pid intelligent optimization control, the output voltage of mfc is adjusted within shorter time, with faster response speed and higher robustness than for the independent fuzzy-pid control or uncontrolled case. also, for this last case, it seems to be very difficult to maintain stable output and reach the reference value after long period of operation in the presence of external disturbances, as shown in table 2. table 2. comparison of three control methods for mfc system control method rise time, h recovery time under interference, h cmac-pid 21.8 5.3 fuzzy-pid 32.3 11.2 uncontrol 64.9 - 0 50 100 150 200 250 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 time(h) v o u t( v ) reference fuzzy-pid 0 50 100 150 200 250 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 time(h) v o u t( v ) reference uncontrol v o lt a g e , v v o lt a g e , v http://dx.doi.org/10.5599/jese.636 j. electrochem. sci. eng. 9(2) (2019) 135-142 microbial fuel cell system 142 conclusion optimal control strategy for electrochemical parameters of two-chamber mfc has been developed by combining fuzzy pid and cmac neural network, which provides an alternative to the parameter optimization for biochemical reaction process showing serious nonlinearity and hysteresis in the cell. the simulation results have validated the proposed optimal algorithm. in addition, this control strategy can be applied to the control of various parameters such as dissolved oxygen, ph, temperature and other factors that affect the performance of microbial fuel cell due to simple control rules, fast convergence speed, few calculations and strong adaptability. acknowledgements: this work was supported by the independent innovation technology project of ji'nan institute (201401210), statistical research project of shandong province in 2016 (kt16131), and the open fund of key lab pulp and paper science technology (kf201418). references [1] k. s. chirag, b. n. yagnik, research journal of biotechnology 8 (2013) 84-90. [2] m. h. zhou, h. y.wang, j. d. hassett, t. y. gu, journal of chemical technology & biotechnology 88 (2013) 508-518. [3] j. m. luo, j. yang, h. h. he, j. tao, z. li, m. wang, m. h. zhou, bioresource technology 139 (2013) 141148. [4] w. l.yang, k. y. kim, p. e. saikaly, b. e. logan, energy & environmental science 5 (2017) 1025-1033. [5] l. pablo, s. andrew, g. john, l. ioannis, physical chemistry chemical physics pccp 15 (2013) 2278-2281. [6] a. gil-carrera, a. escapa, r. moreno, r. moran, journal of environmental management 122 (2013) 17. [7] a. schievano, a. colombo, m. grattieri, s. p. trasatti, a. liberale, p. tremolada, c. pino, p. cristiani, journal of power sources 340 (2017) 80-88. [8] y. zeng, , y. f.choo, b. h. kim, p. wu, journal of power sources 195 (2010) 79-89. [9] l. fan, c. li and k. boshnakov, pakistan journal of pharmaceutical sciences 27 (2014) 685-690. [10] m. x. yan, l. p. fan, international journal of electrochemical science 8 (2013) 3321-3332. [11] a. m. an, y. l. liu, h. s. zhang, c. d. zheng, j. fu, journal of chemical engineering 68 (2017) 1090-1098. [12] j. k. liu,advanced pid control and matlab simulation, electronic industry press, beijing, china,2007, p.183. [13] c. m. lin, h. y. li, nonlinear analysis-real world applications 14 (2013) 206-223. [14] d. z. xu, n. ji, y. tang, international journal of energy and power 4 (2015) 92-97. [15] c. y. jia, x. y. shan, y. c. cui, t. bai, f. j. cui, journal of iron and steel research 20 (2013) 17-22. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {advanced electrochemical sensors based on the functional carbon materials} http://dx.doi.org/10.5599/jese.1302 1 j. electrochem. sci. eng. 12(1) (2022) 1-2; http://dx.doi.org/10.5599/jese.1302 open access : : issn 1847-9286 www.jese-online.org editorial advanced electrochemical sensors based on the functional carbon materials hadi beitollahi environment department, institute of science and high technology and environmental sciences, graduate university of advanced technology, kerman, iran; ; email: h.beitollahi@yahoo.com there is a growing need for fast, robust, and reliable methods for the selective determination of organic compounds in various matrices of importance for human health and well-being. searching through the recent scientific literature revealed the ever-increasing trend of a number of papers published annually on different topics related to electrochemical sensors [1]. as the electrochemical techniques are well established and easy to use, the scientific interest is focused on emerging applications such as non-invasive point-of-care (poc) devices and disposable wearable sensors. there are many different concepts on how to approach the sensor design, which often includes tailoring the surfaces by functional molecules of various physico-chemical properties, size, and compositions. the versatility of approaches for the sensor design offers practically unlimited possibilities for the researchers and analytical chemists to direct the development of the sensors toward more accurate and non-disposable devices useful in essential fields of interest such as human health and environmental protection. the present special issue of the journal of electrochemical science and engineering aimed to give a short overview of the various state-of-the-art technologies for the implementation of different forms of carbon nanomaterials such as carbon nanotubes, graphene derivatives, carbon nano-dots, active carbon, screen-printed carbon electrode and others to design novel electrochemical sensors. the special issue attracted great interest and there are 18 papers selected for publishing. in the review by manjunatha et al. [2], the new developments of carbon-based materials for the determination of hormones are presented. in another review, the possibility of using an electrochemical biosensor in the diagnosis of covid-19 and thus in contributing to the fast and accurate diagnostic practices during covid pandemic was thoroughly analysed [3]. the covid-19 analysis was also addressed in the paper by sari et al. [4]. most of the papers applied modifications of screen-printed carbon electrodes to prepare selective sensors for various organic compounds. graphene, graphene oxide or carbon nanotubes and their derivatives were mainly used, such as nitrogen-doped graphene [5], cu2o/fe2o3 modified reduced graphene oxide [6], graphene oxide modified with fe2o3 nanoparticles [7], multi-wall carbon nanotubes [8]. graphene quantum dots were used to modify carbon paste electrode for the selective determination of levodopa [9]. screen-printed carbon electrode were also modified with http://dx.doi.org/10.5599/jese.1302 http://dx.doi.org/10.5599/jese.1302 http://www.jese-online.org/ mailto:h.beitollahi@yahoo.com j. electrochem. sci. eng. 12(1) (2022) 1-2 editorial 2 nafion/tio2 for the selective determination of olopatadine [10] and with cu complexes for the simultaneous determination of dopamine and uric acid [11]. modified carbon paste electrodes were also used in the determination of nickel contamination [12] and glutathione [13]. analysis of ciprofloxacin in blood was achieved with the construction of the selective sensor based on the iron-doped graphitic carbon nitride [14]. in the paper by pereira et al. [15], the development of multiwalled carbon nanotube ink was described for its application in electrochemical sensors, and in the paper by araujo et al. [16], graphene papers were functionalized to increase the efficiency of enzyme immobilization. references [1] j. bobacka, journal of solid state electrochemistry 24 (2020) 2039–2040. https://doi.org/ 10.1007/s10008-020-04700-4 [2] j. g. manjunatha, g. tigari, h. nagarajappa, n. s. prinith, journal of electrochemical science and engineering 12(1) (2022) 3-23. https://doi.org/10.5599/jese.1094 [3] r. k. satvekar, journal of electrochemical science and engineering 12(1) (2022) 25-35. https://doi.org/10.5599/jese.1116 [4] a. k. sari, y. w. hartati, s. gaffar, i. anshori, d. hidayat, h. l. wiraswati, journal of electrochemical science and engineering 12(1) (2022) 217-. https://doi.org/10.5599/jese.1206 [5] s. esfandiari baghbamidi, journal of electrochemical science and engineering 12(1) (2022) 37-45. https://doi.org/10.5599/jese.1103 [6] f. irannezhad, j. seyed-yazdi, s. h. hekmatara, journal of electrochemical science and engineering 12(1) (2022) 45-55. https://doi.org/10.5599/jese.1101 [7] h. tashakkorian, b. aflatoonian, p. mohammadzadeh jahani, m. r. aflatoonian, journal of electrochemical science and engineering 12(1) (2022) 69-77. https://doi.org/10.5599/ jese.1145 [8] s. a. ahmadi, s. z. mohammadi, m. jafari, p. mohammadzadeh jahani, r. mashayekh, journal of electrochemical science and engineering 12(1) (2022) 125-133. https://doi.org/ 10.5599/jese.1141 [9] p. mohammadzadeh jahani, journal of electrochemical science and engineering 12(1) (2022) 79-88. https://doi.org/10.5599/jese.1133. [10] m. mehmandoust, a. mehmandoust, n. erk, journal of electrochemical science and engineering 12(1) (2022) 89-101. https://doi.org/10.5599/jese.1117 [11] h. beitollahi, s. tajik, m. r. aflatoonian, a. makarem, journal of electrochemical science and engineering 12(1) (2022) 197-206. https://doi.org/10.5599/jese.1231 [12] k. pokpas, n. jahed, p. bezuidenhout, s. smith, k. land, e. iwuoha, journal of electrochemical science and engineering 12(1) (2022) 151-162. https://doi.org/10.5599/jese.1173 [13] h. beitollahi, s. tajik, m. r. aflatooniand, a. makarem, journal of electrochemical science and engineering 12(1) (2022) 207-215. https://doi.org/10.5599/jese.1230 [14] h. s. vedhavathi, b. p. sanjay, m. basavaraju, b. s. madhukar, n. k. swamy, journal of electrochemical science and engineering 12(1) (2022) 57-68. https://doi.org/10.5599/jese.1112 [15] a. e. f. oliveira, a. c. pereira, l. f. ferreira, journal of electrochemical science and engineering 12(1) (2022) 103-124. https://doi.org/10.5599/jese.1134 [16] j. luzardo, d. aguiar, a. silva, s. oliveira, b. archanjo, r. simão, j. araujo, journal of electrochemical science and engineering 12(1) (2022) 135-149. https://doi.org/10.5599/ jese.1099 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1007/s10008-020-04700-4 https://doi.org/10.1007/s10008-020-04700-4 https://doi.org/10.5599/jese.1094 https://doi.org/10.5599/jese.1116 https://doi.org/10.5599/jese.1206 https://doi.org/10.5599/jese.1103 https://doi.org/10.5599/jese.1101 https://doi.org/10.5599/jese.1145 https://doi.org/10.5599/jese.1145 https://doi.org/10.5599/jese.1141 https://doi.org/10.5599/jese.1141 https://doi.org/10.5599/jese.1133. https://doi.org/10.5599/jese.1117 https://doi.org/10.5599/jese.1231 https://doi.org/10.5599/jese.1173 https://doi.org/10.5599/jese.1230 https://doi.org/10.5599/jese.1112 https://doi.org/10.5599/jese.1134 https://doi.org/10.5599/jese.1099 https://doi.org/10.5599/jese.1099 https://creativecommons.org/licenses/by/4.0/) synthesis of vanadium oxide nanoplates for electrochemical detection of amaranth in food samples: http://dx.doi.org/10.5599/jese.1394 1153 j. electrochem. sci. eng. 12(6) (2022) 1153-1163; http://dx.doi.org/10.5599/jese.1394 open access : : issn 1847-9286 www.jese-online.orghttp://www.jese-online.org/ original scientific paper synthesis of vanadium oxide nanoplates for electrochemical detection of amaranth in food samples reza zaimbashi1,2, ali mostafavi1, and tayebeh shamspur1 1department of chemistry, shahid bahonar university of kerman, kerman, iran 2young researchers society, shahid bahonar university of kerman, kerman, iran corresponding author: amostafavi@uk.ac.ir; tel/ fax: +98 3433257433 received: may 31, 2022; accepted: july 12, 2022; published: october 6, 2022 abstract amaranth dye is an organic compound largely used in the food and beverage industries with potential toxicity effects on humans. in this paper, a new electrochemical sensor used for the determination of amaranth in foods was reported, where a kind of v2o5 nanoplates (v2o5-nps) was employed as electrode modifying materials. the v2o5 nanoplates modified electrode enhanced its electrochemical signal obviously in the determination of amaranth in foods and exhibited a wider linear response ranging from 0.1-270.0 µm with a low detection limit of 0.04 ± 0.001 µm (3sb/m). this work offers a new route in developing new electrochemical sensors for the determination of colorant additives and other hazardous components in foods. keywords azo dyes; artificial food dyes; differential pulse voltammetry; screen printed electrode introduction the rapid development of the food industry leads to an increasing number of specific products with a certain shape, colour, taste, smell, texture, etc. thus, different food additives such as presservatives, sweeteners, thickeners, and colouring agents are used to improve the organoleptic properties of the food. dyes play a special role here, as the food quality and taste are often associated with its colour. colorants have been used throughout history, from the ancient ages to the present days. in ancient times, natural colorants were used to make cave paintings and some of these can still be found in the altamira cave (spain) and lascaux cave (france). dyes are classified into natural and synthetic. natural dyes are extracted from plant and animal sources by physical methods. with the great development of science and technology, synthetic dyes are the foundation of various industries, including food, textile, pharmaceuticals, paper, leather, and cosmetics owing to their versatile colours, easy preparation, and low costs. synthetic dyes can be classified by their chemical structures. the groups of atoms deciding the dye colours are called http://dx.doi.org/10.5599/jese.1394 http://dx.doi.org/10.5599/jese.1394 http://www.jese-online.org/ http://www.jese-online.org/ mailto:amostafavi@uk.ac.ir j. electrochem. sci. eng. 12(6) (2022) 1153-1163 vanadium oxide nanoplate for detection of amaranth 1154 chromophores and auxochromes. the chromophores include azo (-n=n-), carbonyl (-c=o), nitro (-no2), methine (-ch =) and quinoid groups [1,2]. azo dyes form the largest group of artificial food dyes. synthetic azo dyes such as amaranth, tartrazine, sunset yellow, brilliant blue etc., are used in most food items to make them more attractive. when compared with many natural dyes, these offer harmful effects on the human body, and many of them are suspected carcinogens too. the proposed study is the determination of water-soluble synthetic food colourant amaranth, (trisodium(4e)-3-oxo-4-[(4-sulfonato-1naphthyl) hydrazono] naphthalene-2,7-disulfonate), using voltammetric techniques. amaranth has been extensively used in food, textiles, pharmaceuticals, etc. due to its attractive dark red to purple colour .therefore, it is very important and indispensable to develop a sensitive, rapid analytical method for the determination of amaranth in diverse food products [3-6]. there have been numerous investigations to achieve techniques for effective and green detection of azo dyes [7-9]. the electrochemical methods are showing great attention in studying analytes redox behaviour, and a high interest in electrochemical methods giving a fast response, low cost, high sensitivity, and selectivity exists [10-15]. voltammetry is a technique utilized for the analysis of compounds that undergo oxidation or reduction. in the voltammetry methods normally using electrodes such as glassy carbon electrodes (gce), carbon paste electrodes (cpe), screen printed electrodes (spe), along with some modifiers [16-22]. different modifiers will show different electrochemical activities [23-28]. spes are designed to analyse low sample volumes and include three electrodes (working, reference and counter electrode) in the same device. spes are mass produced at low cost and are thus disposable. considering such remarkable advantages, spes have received widespread acceptance in fields of analytical chemistry, including food analysis [29]. in recent years, interests have focused on the use of nanosized materials in various fields [3038]. nanomaterials are applied in the fabrication of electrochemical sensors due to an increased surface area of the electrodes, facile electron transfers and decreased surface fouling [39-44]. the vanadium oxide (v2o5), as n-type semi-conducting metal oxide, has impressive features like optical bandgap energy (2.3 ev), thermo-electric potentials, appreciable chemical, and thermal stability. accordingly, the v2o5 has been used for different purposes like electrochemical sensors and lithiumion battery [45-47]. therefore, the current work aimed to employ the stripping voltammetric technique using a screen-printed electrode modified with v2o5 nanoplates for sensitive, selctive and accurate detection of amaranth in real specimens. experimental chemicals and apparatus the electrochemical measurements were performed with an autolab potentiostat/galvanostat (pgstat 302n; eco chemie: the netherlands). moreover, general purpose electrochemical system (gpes) software has been used to control the experimental condition. notably, the spe (dropsens; drp-110: spain) involved 3 major sections of the graphite counter electrode, a graphite working electrode, as well as a silver pseudo-reference electrode. finally, we employed a metrohm 710 phmeter to measure the ph. all reagents had an analytical grade. these products have been purchased from merck (darmstadt; germany). r. zaimbashi et al. j. electrochem. sci. eng. 12(6) (2022) 1153-1163 http://dx.doi.org/10.5599/jese.1394 1155 orthophosphoric acid was utilized to freshly prepare all needed phosphate buffer solutions (pbss), and sodium hydroxide was responsible for adjusting the desired ph values (the ph range between 2.0 and 9.0). v2o5 nanoplates synthesis according to a typical production protocol, the ammonium metavanadate (0.5 g, amv, from sigma-aldrich) was dispersed in deionized water (50 ml) while magnetic stirring. the solution ph value was dropwise adjusted to about 2 using nitric acid while stirring continuously at 323 k. the evaporation of water during stirring resulted in an orange-coloured gel of vanadium complex that was then dissolved in methanol (20 ml) in a vial and subsequently allowed to age at an ambient temperature for 24 hours. the change in the gel colour from light to dark orange occurred when it was left in methanol. afterward, the resultant product was washed several times with deionized water prior to the next testing. preparing the electrode the v2o5 nanoplate stock solution in the aqueous solution (1 ml) was prepared through the dispersion of v2o5 nanoplates (1 mg) under ultra-sonication for half an hour, whereas 5 µl aliquot of v2o5 nanoplates was cast on carbon working electrode, followed by the solvent evaporation at the room temperature. the surface areas of v2o5-np/spe and the unmodified spe were obtained by cyclic voltammetry (cv) using 1 mm k3fe(cn)6 at various scan rates. using the randles-sevcik equation for v2o5-np/spe, the electrode surface was found to be 0.096 cm2 which was about 3.1 times greater than unmodified spe. the brief preparation process of the v2o5-np/spe for the determination of amaranth is shown in scheme 1. scheme 1. the schematic preparation of v2o5-np/spe for determination of amaranth e / mv i /  a http://dx.doi.org/10.5599/jese.1394 j. electrochem. sci. eng. 12(6) (2022) 1153-1163 vanadium oxide nanoplate for detection of amaranth 1156 result and discussion structure and morphology the morphology and structure of the prepared sample were then investigated by field-emission scanning electron microscopy (fe-sem) (figure 1). it is observed that v2o5 nanoparticles with platelike morphology have grown well. the thicknesses of the v2o5 nanoplates are around 26 nm. figure 1. the fe-sem images of v2o5 nanoplates at different magnifications the elemental analysis from the energy dispersive x-ray spectroscopy (edx) measurement is presented in figure 2. the eds spectrum shows peaks corresponding to v (73.6 wt.%) and o (26.4 wt.%) elements, thereby confirming the successful formation of the v2o5 nanoplates without any impurities. also, to investigate the distribution of elements in the v2o5 nanoplates, the elemental mapping images of v2o5 are shown in figure 3. in te n si ty , a .u . energy, kev element content, wt.%  v 73.6 1.3 o 16.4 1.3 figure 2. the edx spectrum of v2o5 nanoplates r. zaimbashi et al. j. electrochem. sci. eng. 12(6) (2022) 1153-1163 http://dx.doi.org/10.5599/jese.1394 1157 figure 3. the elemental mapping images of v2o5 nanoplates electrochemical response of amaranth at different electrodes the effect ph value of electrolyte solution was investigated by differential pulse voltammograms (dpv) in 0.1 m phosphate buffer solution (pbs) at the ph range from 2.0 to 9.0 containing 40.0 μm amaranth on the v2o5-nps/spe surface. the oxidation peaks current of amaranth reached a maximum value at ph 7.0, and therefore pbs with ph 7.0 was chosen as the optimum ph to detect amaranth. figure 4 displays the linear sweep voltammograms (lsv) of the amaranth at unmodified spe (curve b) and v2o5-nps/spe (curve a), with the same concentration of 200.0 μm in 0.1 m pbs (ph 7.0). figure 4. linear sweep voltammograms curves of unmodified spe (curve b) and v2o5-np/spe (curve a) in 0.1 m pbs containing 80.0 μm amaranth; scan rate: 50 mv s-1 the anodic peak potential for the oxidation of amaranth at v2o5-nps/spe (curve a) is about 740 mv compared with 790 mv, for that on the unmodified spe (curve b). similarly, when the oxidation of http://dx.doi.org/10.5599/jese.1394 j. electrochem. sci. eng. 12(6) (2022) 1153-1163 vanadium oxide nanoplate for detection of amaranth 1158 amaranth at the v2o5-nps/spe (curve a) and unmodified spe (curve b) are compared, an extensive enhancement of the anodic peak current at v2o5-nps/spe, relative to the value obtained at the unmodified spe (curve b), is observed. in other words, the results clearly indicate that the v2o5-nps improve amaranth oxidation. effect of scan rate on the determination of amaranth at v2o5-nps/spe the influence of the scan rate (ʋ) on the peak currents (ipa) of amaranth at v2o5-nps/spe was investigated by lsv. figure 5 shows the voltammetric response of 80.0 μm amaranth at v2o5-nps/spe at different scan rates in the range of 10 to 300 mv s-1. the oxidation peak current of amaranth increases linearly with increasing scan rate. linear regression equation was obtained from the plot ipa and vs. 1/2 (square root of scan rate) as follows; ipa = 1.4242 1/2 – 1.7969 (r2 = 0.9991) for the oxidation process, which indicates that the reaction of amaranth at v2o5-nps/spe is diffusion controlled. figure 5. linear sweep voltammograms of v2o5-np/spe in 0.1 m pbs (ph 7.0) containing 80.0 µm amaranth at various scan rates; 1-7 correspond to 10, 25, 50, 75, 100, 200 and 300 mv s-1, respectively. inset: variation of anodic peak current vs. ν1/2 chronoamperometric analysis chronoamperometric measurements of amaranth at v2o5-nps/spe were carried out by setting the working electrode potential at 0.78 v for the various concentrations of amaranth in 0.1 m pbs (ph 7.0) (figure 6). for an electroactive material (amaranth in this case) with a diffusion coefficient of d, the current observed for the electrochemical reaction at the mass transport limited condition is described by the cottrell equation. experimental plots of i vs. t−1/2 were employed, with the best fits for different concentrations of amaranth (figure 6a). the slopes of the resulting straight lines were then plotted vs. amaranth concentration (figure 6b). from the resulting slope and cottrell equation, the mean value of the d was found to be 3.3×10-6 cm2 s-1. y = 1.4242x + 1.7969 r2 = 0.9991 r. zaimbashi et al. j. electrochem. sci. eng. 12(6) (2022) 1153-1163 http://dx.doi.org/10.5599/jese.1394 1159 t / s figure 6. chronoamperograms obtained at v2o5-np/spe in 0.1 m pbs at ph of 7.0 for different concentrations of amaranth (1-4 refer to: 0.1, 0.5, 1.1 and 1.5 mm). a the i plot versus t-1/2 observed by chronoamperograms 1-4; b the slope plot of the straight line vs. concentration of amaranth calibration curve because dpv commonly has a higher sensitivity than cv, the dpv technique was applied for the quantitative detection of amaranth. figure 7 shows the differential pulse voltammograms of amaranth at various concentrations using v2o5-nps/spe (step potential = 0.01 v and pulse amplitude = 0.025 v). as seen, the oxidation peak currents of amaranth enhance gradually by increasing its concentration. figure 7. dpvs of v2o5-np/spe in 0.1 m (ph 7.0) containing different concentrations of amaranth. numbers 1–10 correspond to to 0.1, 2.0, 7.0, 15.0, 30.0, 45.0, 70.0, 100.0, 200.0 and 270.0 μm of amaranth. inset: plot of the electrocatalytic peak current as a function of amaranth concentration in the range of 0.1-270.0 μm y = 0.071x + 0.8511 r2 = 0.9995 camaranth dye / m camaranth day / mm y = 6.1181x + 1.598 r2 = 0.999 s lo p e ,  a s -1 /2 http://dx.doi.org/10.5599/jese.1394 j. electrochem. sci. eng. 12(6) (2022) 1153-1163 vanadium oxide nanoplate for detection of amaranth 1160 the oxidation peak currents (ipa) show a good linear relationship with the concentrations of amaranth ranging from 0.1 m to 270.0 μm. the linear equation is ipa = 0.071camaranth + 0.8511 (r2 = 0.9995) (figure 7 (inset)). also, the detection limit, cm, of amaranth was obtained using equation (1) cm = 3sb / m (1) in the equation, m is the slope of the calibration plot (0.071 µa. µm-1) and sb is the standard deviation of the blank response which is obtained from 20 replicate measurements of the blank solution. the limit of detection (lod) was estimated to be 0.04±0.001 μm. in addition, table 1 shows that the v2o5-nps/spe can compete with other sensors for the determination of amaranth. table 1. linear range and lod obtained at the v2o5-np/spe for the determination of amaranth compared with other sensors. electrochemical sensor method linear range, µm lod, µm ref. multiwalled carbon nanotube/gold electrodes differential pulse voltammetry 1.0–10.0 0.068 [48] graphene/tio2-ag based composites/gold electrodes linear sweep voltammetry 0.3-100.0 0.1 [49] ni–mo-mof/screen printed graphite electrodes differential pulse voltammetry 0.15–500.0 0.05 [50] co3o4-ceo2/graphene nanocomposite modified electrode differential pulse voltammetry 2.0–96.0 0.1591 [51] v2o5 nanoplate/ screen printed electrodes differential pulse voltammetry 0.1-270.0 0.04 this work v2o5-nps/spe repeatability and stability the v2o5-nps/spe stability was tested by keeping the new sensor in the pbs at the ph value of 7.0 for 15 days, and then the cyclic voltammograms (cvs) were consequently recorded in the solution consisting of amaranth (20.0 µm) for the comparison with the cvs recorded before submersion. the peak amaranth oxidation was not altered, whereas the current was reduced in the signals by 2.8 % compared to the initial responses. this means satisfactory stability of v2o5-nps/spe and antifouling activity of the modified spe for the amaranth oxidation. additionally, the resultant products for the modified spe were measured by the cv in the absence and presence of amaranth. at last, the cvs were recorded in the presence of amaranth u after 15 potential cycles at 50 mv s-1, such that the currents were decreased up to >2.6 % but the peak potential did not alter. analytical application the determination of amaranth in real samples such as apple juice and water samples was performed using v2o5-nps/spe sensor. the concentration values of amaranth were calculated by the standard addition method. the results are summarized in table 2, the recovery is between 96.4 and 104.3 %, and the relative standard deviations (rsds) are all less than or equal to 3.4 %. the experimental results confirmed that the v2o5-nps/spe sensor has a great potential for analytical application. table 2. determining amaranth in real samples by using v2o5-np/spe (n=5). sample c / μm recovery, % rsd, % spiked found apple juice 4.5 4.6 102.2 2.7 5.5 5.3 96.4 3.4 6.5 6.4 98.5 1.8 7.5 7.6 101.3 2.2 r. zaimbashi et al. j. electrochem. sci. eng. 12(6) (2022) 1153-1163 http://dx.doi.org/10.5599/jese.1394 1161 sample c / μm recovery, % rsd, % spiked found tap water 5.0 4.9 98.0 3.2 6.0 6.1 101.7 2.1 7.0 7.3 104.3 3.0 8.0 7.9 98.7 1.9 conclusions the voltammetric behavior of synthetic food colorant, amaranth, was studied using a v2o5-np/spe in 0.1 m pbs of ph 7.0. a well-defined oxidation peak was obtained for amaranth at 0.730 v with the modified electrode. the diffusion-controlled 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https://doi.org/10.1016/j.matchemphys.2017.07.022 https://doi.org/10.1007/s12161-013-9676-7 http://dx.doi.org/10.1149/2.0101808jes https://doi.org/10.1007/s11694-021-01027-0 http://dx.doi.org/10.1149/1945-7111/abe3a3 https://creativecommons.org/licenses/by/4.0/) @article{zaeimbashi2022, author = {zaeimbashi, reza and mostafavi, ali and shamspur, tayebeh}, journal = {journal of electrochemical science and engineering}, title = {{synthesis of vanadium oxide nanoplates for electrochemical detection of amaranth in food samples:}}, year = {2022}, issn = {1847-9286}, month = {oct}, number = {6}, pages = {1153--1163}, volume = {12}, abstract = {amaranth dye is an organic compound largely used in the food and beverage industries with potential toxicity effects on humans. in this paper, a new electrochemical sensor used for the determination of amaranth in foods was reported, where a kind of v2o5 nanoplates (v2o5-nps) was employed as electrode modifying materials. the v2o5 nanoplates modified electrode enhanced its electrochemical signal obviously in the determination of amaranth in foods and exhibited a wider linear response ranging from 0.1-270.0 µm with a low detection limit of 0.04 ± 0.001 µm (3sb/m). this work offers a new route in developing new electrochemical sensors for the determination of colorant additives and other hazardous components in foods.}, doi = {10.5599/jese.1394}, file = {:d\:/onedrive/mendeley desktop/zaeimbashi, mostafavi, shamspur 2022 synthesis of vanadium oxide nanoplates for electrochemical detection of amaranth in food sample.pdf:pdf;:www/jese_v12_no6_1153-1163.pdf:pdf}, keywords = {azo dyes, artificial food dyes, differential pulse voltammetry, screen printed electrode}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1394}, } {limiting current density in electrochemical micromachining} doi:10.5599/jese.586 321 j. electrochem. sci. eng. 8(4) (2018) 321-330; doi: http://dx.doi.org/10.5599/jese.586 open access : : issn 1847-9286 www.jese-online.org original scientific paper limiting current density in electrochemical micromachining chuanjun zhao, lizhong xu college of mechanical engineering of yanshan university, qinhuangdao, hebei, china corresponding author e-mail: xlz@ysu.edu.cn; tel.: +86-131 7196 5996 received: february 7, 2018; revised: april 17, 2018; accepted: april 17, 2018 abstract in this paper, a method of calculating the diffusion current density on the electric pole in the electrochemical micromachining is proposed. changes of the diffusion current with system factors are investigated from which a method of increasing the diffusion current density on the electric pole under small clearance between the hole and the tool is proposed. this is the electrochemical micromachining method using vibrating tool electrode. using this method, an etching experiment on a steel plate is performed which shows that a reasonable machining speed and high machining accuracy are simultaneously obtained. keywords vibration tool electrode; machining accuracy; diffusion current introduction micro-electromechanical systems (mems) are independent intelligent systems with an internal structure in the order of micron or even nanometer. their performance mainly depends on micro manufacturing technology [1-3]. electrochemical micromachining has a good development potential in the micromachining field of materials because the material transfers on the ion scales during the electrochemical micromachining and the metal ion scales are smaller than 10 % of nanometer [4-6]. the conventional electrochemical machining has a shortcoming in machining localization, what limits its micromachining ability. in order to improve the machining localization of the electrochemical micromachining, a lot of work has already been done [7-11]. among them, the use of the ultra-short voltage pulses made an important breakthrough in which ultra-short voltage pulses of nanosecond duration were used and the machining localization of the electrochemical micromachining is increased significantly [12]. for this technique, a lot of research was completed [13-16]. improved size and shape of the electrode, together with the reduced duration of the ultrashort pulse were used to increase micromachining precision of this technique [17-19]. the http://www.jese-online.org/ mailto:xlz@ysu.edu.cn http://dx.doi.org/10.5599/jese.586 j. electrochem. sci. eng. 8(4) (2018) 321-330 current density in electrochemical micromachining 322 simulation approaches and computational models for the pulse electrochemical machining were also reported [20]. as the pulse width of the current is further reduced, the clearance between the hole and the tool becomes smaller and smaller which causes a decrease of the diffusion current density on the electric pole. this decrease can cause obvious decrease of the machining speed and even result in electrolytic product deposition and short circuit. therefore, the diffusion current density on the electric pole and its change with system factors should be investigated. the way for increasing the diffusion current density under small clearance between the hole and the tool should be find out. here, we proposed a method of calculating the diffusion current density on the electric pole in the electrochemical micromachining and derive the corresponding equations. based on derived equations, changes of the diffusion current with system factors are investigated. from these results, a method of increasing the diffusion current density on the electric pole under small clearance between the hole and the tool is proposed. using this method, an etching experiment on the steel plate is performed in order to illustrate the method. experimental here, an electrochemical micromachining system is developed (see figure 1). it consists of machine tool, control system and the exciting part of tool electrode vibration. the exciting part of vibration includes tool electrode fixture, exciting beam of vibration and piezo drive power supply. by the converse piezoelectric effect of piezoelectric ceramic piece, a beam on which piezoelectric ceramic pieces are adhered is excited to vibrate vertically, driving the tool electrode to vibrate vertically. figure 1. electrochemical micromachining system using vibrating tool electrode by the electrochemical micromachining system, we performed etching experiments on a steel plate 0.2 mm-thick. for this experiment, a cylindrical w wire of 30 μm in diameter was used as a tool. the electrolyte was 0.01 m nano3. static voltage of 3.5 v was applied between two electrodes. the exciting frequency of the vibration-exciting power supply was 125 hz and the exciting voltage amplitude was 100 v. the vibrating amplitude of the tool electrode was 5 μm which means that a tool motion velocity of 2.5 mm s-1 is obtained during the machining. it took 30 min to etch through the steel plate. besides it, an etching experiment on the steel plate was also performed by electrochemical micromachining system without the vibrating tool electrode. results and discussion it has already been known that “in the small gap in front of the tool face, limited diffusion lowers the effective etching rate to a value significantly lower than expected from the high local overpotential with simple tafel behavior” [12]. so, it can be concluded that diffusion or mass c. zhao and l. xu j. electrochem. sci. eng. 8(4) (2018) 321-330 doi:10.5599/jese.586 323 transport on the anode governs the rate of surface finishing in electrochemical micro machining experiments. as the gap in front of the tool face decreases, limited diffusion will lower the effective etching rate which is dependent on the limiting diffusion current id. figure 2 shows a model for calculating limiting diffusion current in the gap in front of the tool face. here, x is coordinate perpendicular to in tool pole axis; y is coordinate in tool pole axis; d0 is gap in front of the tool face; r0 is radius of the tool pole. figure 2. model of limiting diffusion current in the gap in front of the tool face the diffusion equation in the gap can be approximately defined as 2 2 2 2 c c c d t x y            (1) where c is the electrolyte concentration, d is the diffusion coefficient and t is time. for steady-state, c / t = 0, letting c = (x) f(y) and substituting it into eq. (1), yields '' '' 2( ) ( ) ( ) ( ) x f y x f y       (2) where  is a constant. from eq. (2), one can obtain following equations: '' 2 ( ) ( ) 0f y f y  (3) '' 2 ( ) ( ) 0x x    (4) solution of eq. (3) gives 1 2 ( ) sin cosf y c y c y   (5) the boundary conditions that c = 0 at y = 0 and c / y = 0 at y = d0 (here d0 is the distance between the tool face and workpiece) correspond to following conditions: f(0) = 0 and f ’(d0) = 0 from above equations and taking 0 2d    and 0 y y d , one can obtain the following equation: 0 ( ) sin sin 2 2 f y y y d         (6) solution of eq. (4) gives 3 4( ) x x x c e c e       (7) the boundary conditions that c=c0 at x=  and c=0 at x=0 correspond to following conditions: () = 1 and (0) = 0 j. electrochem. sci. eng. 8(4) (2018) 321-330 current density in electrochemical micromachining 324 from above equations and assuming 0 x x d , one can obtain the following equation: 02 2( ) 1 1 x x d x e e          (8) thus, the electrolyte concentration is defined as: 2 0 ( , ) (1 )sin 2 x c x y c e y     (9) from eq. (9), the following equation results 0 0 0 2 0 0 sin 2 2 r d x r c c e y x d        (10) where r0is the radius of the tool pole. thus, the diffusion flow rate is defined as 0 0 0 1 2 0 0 0 0 0 2 2 r d t x r c f d r d dy r dc e x           (11) and the stationary current density on the electric pole is defined as 0 0 0 02 22 0 0 0 2 / 2 r r d d s t co nfdc i nff r e i e r          (12) where 0 0 4 co nfdc i r  i.e., the current density on the disc electric pole at d0 = , defined in the cylindrical coordinate system. it shows that correction coefficient is 2  when the current density on the electric pole is calculated by the cartesian coordinate system. for transient state ( 0c t   ), letting ( ) ( ) ( )c q t x f y and after substituting it into eq.(1), one can obtain: '' '' ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ) q t x f y x f y dq t x f y         (13) from eq. (13), following equations are obtained: ( ) ( ) 0q t dq t  (14) '' '' 2( ) ( ) ( ) ( ) ( ) f y x x f y x          (15) from eq. (15), one can obtain eq. (3) together with '' 2 ( ) ( ) 0dx x    (16) where 2 2 d    . solution of eq. (16) can be written as: ( ) x x x e ce       (17) by substitution of boundary conditions (0) 0  and 0 ( ) 1r  into eq. (17) and assuming d  1/2r0, one can obtain: 0 02 2 0 ( ) 2sh 2 x x r r x x e e r            (18) c. zhao and l. xu j. electrochem. sci. eng. 8(4) (2018) 321-330 doi:10.5599/jese.586 325 2 2 2 2 0 0 1 1 4 d d r                  (19) solution of eq. (14) gives 5 6( ) dt q t c e c    (20) from initial conditions ( ) 0q   and 0 (0)q c , it follows that 0 ( ) dt q t c e   (21) thus, the electrolyte concentration is defined as: 0 0 ( , ) 2 sin sh 2 2 dt x c x y c e y r              (22) the transient diffusion current density on the electric pole is defined as: 2 t t 0 co 0 1 / ch 2 dt i nff r i e r      (23) thus, the total diffusion current density on the electric pole is defined as: 0 02 0 d s t co 0 1 ch 2 2 d r dtd i i i i e e r                (24) after correction, the current density on the electric pole is: 0 02 0 d s t co 0 0.7 d r dtd i i i i e e r                (25) as the motion velocity of the electric pole is considered, the diffusion equation in the gap can be approximately defined as 2 2 2 2 y c c c c d v t x y y               (26) where vy is the motion velocity of the electric pole. for stable state ( 0c t   ), letting ( ) ( )c x f y and substituting it into eq.(26), yields ' '''' 2 ( ) ( )( ) ( ) ( ) y v f y df yx x df y       (27) from eq. (27), ones can obtain eq. (4) together with: '' ' 2 ( ) ( ) ( ) 0 y v f y f y f y d    (28) solution of eq. (28) is 2( ) sin yv y d d f y e y (29) where 2 1 d     , 2 y v d     , 0 2d    . thus, the electrolyte concentration is defined as: j. electrochem. sci. eng. 8(4) (2018) 321-330 current density in electrochemical micromachining 326 2 2 0 ( , ) 1 sin yv x y d d c x y c e e y          (30) from eq. (30), it follows that 0 0 0 2 2 0 0 sin 2 y r v y d d d x r c c e e y x d         (31) thus, the diffusion flow rate is defined as 0 0 0 0 1 2 2 0 0 0 0 0 0 2 2 sin( ) y r v d d d t d x r c f d r d dy r dc e e d x              (32) where 2 0 tg 1 y d d v            . the current density on the electric pole is defined as: 0 0 0 0 0 02 22 0 2 2 s 0 d 0 d 0 0 2 / sin( ) sin( ) 2 y y r rv v d d d dd d t co nfdc i nff r e e d i e e d r                (33) for transient state, the diffusion current density on the electric pole is already defined by eq. (23). thus, the total diffusion current density on the electric pole is defined as: 0 0 02 2 d s t co d 0 0 sin( ) 0.7 y r v d d dtdi i i i e e d e r                   (34) equations (23), (24), (25), (33) and (34) derived in this paper are utilized for analyzing diffusion current density on the electric pole for electrochemical micromachining. the parameters of the numerical example are shown in table 1. figures 3-5 present calculated diffusion current density as a function of time for different system parameters. table 1. parameters of the numerical example d/ cm2 s-1 n f / c mol-1 c0 / mol 10-5 2 96500 10-2 following points could be summarized from figures 3-5: 1) in electrochemical micromachining, the diffusion current on the electric pole includes two parts: steady-state current and transient state current, i.e. id = is + it. at initial condition, the transient state current, it, is large and drops quickly to zero with time, while the steady-state current, is, does not change with time. the total diffusion current, id, is near to the it, initially and then near to the is. as the radius of the electric pole tool drops, it drops quicker and the is becomes larger than it, making id almost equal to is . 2) as the ratio of the clearance d0 to the radius of the electric pole tool r0 drops, the total diffusion current drops quickly. the ratio d0/r0 determines a distance between the tool pole and the workpiece. a small ratio d0/r0 means a small distance between the tool pole and the workpiece. the total diffusion current, id, is lower and drops faster with decreasing distance between the tool pole and the workpiece. as the radius of the electric pole tool drops, the total diffusion current becomes higher and also drops faster with decreasing the ratio d0/r0. this is because both stable state current and transient state current drops quickly with decreasing the ratio d0/r0. (a) (b) c. zhao and l. xu j. electrochem. sci. eng. 8(4) (2018) 321-330 doi:10.5599/jese.586 327 figure 3. diffusion current density (is, it and id) for d0/r0=0.5 and (a) r0=5 μm, (b) r0=2 μm. (a) (b) figure 4. total diffusion current density for different d0/r0 and (a) r0=5 μm, (b) r0=2 μm. 3) when the motion velocity, vy, of the tool electrode is considered, the stable state current changes with the velocity, but the transient state current does not change with it. if vy increased from 0 to 2 mm s-1, is increased from 2 to 26 ma cm-2. here, the steady-state current is much higher than the transient state current density, is it, even at the initial time, and so the total diffusion current density, id, is determined mainly by the steady-state current. as the radius of the electric pole tool drops, id becomes large, and the effects of the velocity vy on the diffusion current become weak. however, the total diffusion current still grows obviously with increasing vy. all these results suggest that the motion velocity of the tool electrode can increase the total diffusion current on the electrode by which the machining speed and machining accuracy of the electrochemical micromachining can be increased. based on above mentioned results, a new electrochemical micromachining method is proposed. it is the electrochemical micromachining using vibrating tool electrode which can increase the diffusion current density on the electric pole under small clearance between the hole and the tool. by the electrochemical micromachining system, we performed etching experiments shown in figure 6. it took 30 min to etch through the steel plate. as seen in figure. 6(a), the diameter of the etched hole is about 108 μm. for comparison purpose, the etching experiment on a steel plate 0.2 mm-thick performed by electrochemical micromachining system without vibrating tool electrode is presented in figure 6(b). the micromachining was not completed because of short circuit. as seen in figure 6(b), the diameter of the hole etched is much larger than that obtained j. electrochem. sci. eng. 8(4) (2018) 321-330 current density in electrochemical micromachining 328 with the vibrating tool electrode and accumulation of the electrolytic product occurs, causing the short circuit between the tool and a work piece. (a) (b) (c) (d) figure 5. diffusion current density, (is, it and id) for d0/r0=0.5 at different vy and (a) r0=5 μm, (b) r0=5 μm, (c) r0=2 μm, (d) r0=2 μm. (a) (b) (c) figure 6. holes etched by electrochemical micromachining system: (a) vibrating tool electrode, (b) normal tool electrode, (c) tool pole. present results show that a reasonable machining speed and high machining accuracy is obtained simultaneously by the electrochemical micromachining technique using vibrating tool electrode, illustrating thus the effectiveness of our method and correctness of our theories. because the vibrating tool electrode can increase the diffusion current density on the electric pole under small clearance between the hole and the tool, machining localization and machining speed can simultaneously be increased. besides it, effects of the main vibrating parameters on the machining accuracy are investigated for the electrochemical micromachining technique using vibrating tool electrode. in figure 7, the c. zhao and l. xu j. electrochem. sci. eng. 8(4) (2018) 321-330 doi:10.5599/jese.586 329 machining resolution dc, estimated by measuring the tool diameter and the hole diameter [(final hole diameter – tool diameter)/2] is presented as a function of vibrating frequency of the tool electrode, f, and vibrating amplitude of the tool pole, az. (a) (b) figure 7. machining resolution as a function of: (a) frequency change (az=5 μm), (b) amplitude change (f =108 hz) figure 7(a) shows that when the vibration frequency of the tool electrode increases, the machining resolution improves significantly, attains an optimum machining resolution at 108 hz, and then becomes obviously bad. the same effect appears in figure 7(b), showing the effect of the vibrating amplitude increase and an optimum machining resolution attained at 5 μm. the resultant micro holes machined at the optimum f = 108 hz for different az values are shown in figure 8. (a) (b) (c) (d) (e) (f) figure 8. micro holes machined at f = 108 hz for different vibration amplitudes: (a) 3 μm, (b) 4 μm, (c) 5 μm, (d) 6 μm, (e) 7 μm, (f) 8 μm. the obtained results suggest that the machining accuracy for the electrochemical micromachining technique using vibrating tool electrode increases with vibrating frequency and amplitude, but the optimum vibration parameters exist, above which the machining accuracy becomes bad. this probably happens because the electrochemical reaction time increases and the j. electrochem. sci. eng. 8(4) (2018) 321-330 current density in electrochemical micromachining 330 depolarization time reduces when the vibrating frequency and amplitude is larger than the optimum values. conclusions in this paper, a method of calculating the diffusion current density on the electric pole in the electrochemical micromachining is proposed. changes of the diffusion current calculated with the system factors are investigated. based on these results, a method of increasing the diffusion current density on the electric pole under small clearance between the hole and the tool is proposed. this is an electrochemical micromachining method using a vibrating tool electrode. using the method, an etching experiment on a steel plate is performed which showed that a reasonable machining speed and high machining accuracy can be obtained simultaneously. acknowledgments: this project is supported by key basic research foundation in hebei province of china (13961701d). references [1] k. p. rajurkar, g. levy, a. malshe, annals of the cirp 55 (2006) 634-666 [2] d. t. pham, s. s. dimov, s .bigot, a. lvanov, k.popov, journal of materials processing technology 149(1-3) (2004) 50-57 [3] b. bhttacharyya, b. doloi, p. s. sridhar, journal of materials technology 113(1) (2001) 201-305 [4] l. xu, y. pan, c. zhao, scientific reports 6 (2016) 31778 [5] k. k. saxena, j. qian, d. reynaerts, international journal of machine tools and manufacture 127(2) (2018) 28-56 [6] s. skoczyppiec, international journal of advanced manufacturing technology 87(1-4) (2016) 177-187 [7] m. sen, h. s. shan, international journal of machine tools and manufacture 45(2) (2005) 137-152 [8] m. takashi, journal of micromechanics and microengineering 14 (2004) 76-80 [9] o. zinger,p. e. chauvy, d. landolt, journal of the electrochemical society 150(11) (2003) b495-b503 [10] s. r. a, giar, g. e. bridges, journal of the electrochemical society 147(2) (2000) 586-591 [11] v. rathod, b. doloi, b. bhsttacharyya, international journal of advanced manufacturing technology 92 (1-4) (2017) 505-518 [12] r. schuster, v. kirchiner, p. allongue, g.ertl, science 289 (5476) (2000) 98-101 [13] r. schuster, chemphyschem 8 (2007) 34-39 [14] j. j. maurer joseph, j. l. hudson, s. e. fick, t.p.moffat, g.a.shaw, electrochemical and solid-state letters 15(2) (2012) d8-d10 [15] l. xu, c. zhao, electrochimica acta 248 (2017) 75-78 [16] s. s. anasane, b.bhattacharyya, international journal of advanced manufacturing technology 86(5-8) (2016) 2147-2160 [17] y. wang, y. zeng, n. qu, d.zhu, international journal of advanced manufacturing technology 84 (5-8) (2016) 851-859 [18] y. liu, h. cai, h. li, journal of manufacturing processes 17 (2015) 162-170 [19] x. qi, x. fang, d. zhu, international journal of refractory metals & hard materials 71 (2018) 307-314 [20] e. l. hotoiu, s.v. damme, c. albu, d. deconinck , a. demeter, j. deconinck, electrochimica acta 93 (2013) 8-16 ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) comparison of erosion performance of uncladded and http://dx.doi.org/10.5599/jese.1333 911 j. electrochem. sci. eng. 12(5) (2022) 911-922; http://dx.doi.org/10.5599/jese.1333 open access : : issn 1847-9286 www.jese-online.org original scientific paper comparison of erosion performance of uncladded and wc-based laser cladded ss304 and ss410 steels sarpreet singh1,, parlad kumar1and deepak kumar goyal2 1punjabi university, patiala, punjab, india 2i. k. gujral punjab technical university jalandhar, kapurthala, punjab, india corresponding author: singh.sarpreet9976@gmail.com received: march 28, 2021; accepted: june 3, 2022; published: august 19, 2022 abstract myriad hydro materials have been encountered with the severe attacks of eroded particles during the operation, leading to degradation of target material and economic loss. in this study, colmonoy-6+wc powders were deposited with the help of the laser cladding method on the bare ss304 and ss410 steel surfaces. distinct properties of cladded surfaces, such as mechanical as well as metallurgical properties, were investigated. the influence of slurry erosion parameters like angle of impact and impact velocity of eroded particles on the cladded as well as uncladded steel specimen was analysed. under all slurry erosion conditions, there was an escalation in slurry erosion resistance in the case of cladded steel as that of as-received steel specimens. in slurry erosion, the influence of the impact velocity of erodent particles is more against the impact angle. the scanning electron microscopy images of eroded uncoated steel specimens represent ductile behaviour along with the formation lip, ploughing etc., while eroded cladded steels exhibit brittle behaviour. keywords ni+wc based coatings; erosive wear; laser cladding introduction in this contemporary era, there is a plethora of manufacturing units that are facing the problem of erosive wear. owing to the influence of abrasive along with hard particles employed in the components, which are utilised in hydro machinery causing erosion, the reduction of the efficiency of turbine frequently occurs, which ultimately halts whole working operations [1-2]. there are myriad factors, for instance, the size and hardness of erodent particles, level of slurry concentration, the impact velocity of abrasive particles as well as the target surface characteristics that influence the erosive wear of elements utilised in hydro machinery [3-7]. turbines, propellers, pumps, and valves are the most commonly used components in fluid machinery. the equipment utilised in this machinery is generally made of cast iron, mild steel and stainless steel. these materials, on the other http://dx.doi.org/10.5599/jese.1333 http://dx.doi.org/10.5599/jese.1333 http://www.jese-online.org/ mailto:singh.sarpreet9976@gmail.com j. electrochem. sci. eng. 12(5) (2022) 911-922 erosion of ss304 and ss410 steels 912 hand, are much less resistant to erosive wear [8-9]. the development of novel erosion-resistant materials is important. various techniques, including heat treatment and surface coatings, have been explored to make the surface hard to the impact of erodent particles in order to reduce erosion wear. the laser cladding technique has numerous advantages over traditional approaches, including strong adhesion between the deposition powder and the substrate, reduced heat involvement, and surface deformation, which need to be cladded. due to significant advancements in technology along with science involved in lasers, material processing techniques using lasers [10,11] have been considered a favoured technique for the treatment of the surface. in comparison to conventional approaches, surface treatments done by laser processing techniques have shown improved surface hardening of the material, fatigue, reduction in the erosion and corrosion rate, as well as reduction of stress corrosion along with the elimination of cracks, which are most common during other surface treatments, of distinct alloys and metals [10,11]. savanth et al. [12] deposited the colmonoy-5 coatings on medium carbon steel by the laser cladding technique by varying the laser cladding parameters such as laser beam power and scanning speed. with the help of laser cladding, there was a strong metallurgical bond between the substrate and coating. the hardness of the coating has been increased with an increase in the laser cladding as that of the scanning speed. the sound cladding has been obtained and laser power had more influence on the hardness than of scanning speed. moskal et al. [13] analysed the characterization of the primary microstructure of laser-cladded nicraly coatings deposited on inconel 625 ni-based superalloy and 316l stainless steel. it was found that the dendritic microstructure was the result of rapid solidification due to differences in the chemical composition of dendritic and inter-dendritic areas. paul et al. [14] used the laser cladding approach and deposited two powders, namely, metco-41c and nicrsibc and coatings produced by this technique were found to be crack-free. yao et al. [15] produced tin/al composite coating and after the deposition of coating powders, there was a significant improvement in wear as well as the micro-hardness. the reason was an increase in the count of narrow and elongated dendrites in the cladded surfaces. thus, the laser cladding method as a surface treatment should be investigated to improve the erosion problem in hydro machinery components. on the paradoxical side, in the last few decades, plenty of researchers used distinct powders such as nickel-based, iron-based, tungsten carbide-based and cobalt-based powders in many industrial applications to enhance the characteristics of different materials [16]. nickel-based and wc powders have received the most interest as wear-resistant coatings [16]. nickel-based powders exhibit a number of desired features that make them feasible to use in a variety of prospective engineering purposes [17], including strong bonding strength, improved corrosion behaviour, and great resistance to wear owing to adhesive and abrasive mode of failure. in addition, wc-cladded coating offers a lot of potential by virtue of qualities such as high hardness, enhanced resistance to wear, coefficient of thermal expansion is low, and good plasticity [18-21]. in order to attain sound carbide composite coating, fishman et al. [22] used a laser cladding approach to manufacture wc coating on high-speed steel. the coatings, single layers produced with 40-50 percent wc, had hardness in the range of 1100–1200 hv9.81n. yang et al. [23] created laser clads on the target surface of aisi 1010 steel using a blend of wc/co powders and reported that the wear resistance was improved over the base material. hence, in the current study, colomonoy-6+wc coatings have been deposited on ss304 and ss410 steels using the technique of laser cladding. further, cladded steels are used for the analysis of micro-hardness, porosity, microstructure and slurry erosion. a comparison of the mass loss owing to the slurry of colmonoy-6+wc coated as well as uncladded s. singhet al. j. electrochem. sci. eng. 12(5) (2022) 911-922 http://dx.doi.org/10.5599/jese.1333 913 ss304 and ss410 steels was also conducted. furthermore, the mechanism behind the wear of eroded surfaces has been investigated. experimental materials the aim of the present study is to produce ni-based and wc coatings on the ss304 and ss410 steels and evaluate their performance against slurry erosion. samples of both above-mentioned steels were made. specimens of 25 mm diameter and 7 mm width were made from the steel rod of 25 mm. before the deposition of coatings, the surface of the target specimens was finished to remove the contaminants with the assistance of a surface grinder. table 1 and table 2 show the chemical composition of ss304 and ss410 steels as determined by spectroscopic analysis, as well as the astm standard composition [24,25]. the chemical composition of ss304 and ss410 steels, as determined by spectroscopy, is consistent with the astm standard. owing to the presence of chromium and nickel, steel provides resistance to wear and corrosion. table 1. the chemical composition of ss410 steel component content, wt.% cr c s mn p si fe astm 11.5 to 13.5 <0.15 <0.03 <1 <0.04 <1 balance untreated 11.857 0.1307 0.0249 0.9401 0.0319 0.9881 balance table 2. the chemical composition of ss304 steel content, wt.% components cr c s mn p si ni n fe astm 17.5 to 19.50 <0.03 <0.015 <0.045 <0.045 <1.0 8.00 to 10.50 .10 balance untreated 17.928 0.028 0.012 1.469 0.0319 0.9814 8.91 .095 balance laser cladding set up the process of deposition of coatings was done at m/s magod laser technologies, situated in pune, india, with the diode laser cladding setup. a diode laser (laserline, ldf 4.000, germany) system has a special feedings system, which is coaxial in function, a beam delivery system, and a five-axis workstation provides more flexibility. the cladding material on the substrate was colmonoy-6 +wc powder, and the properties of its elements [26,27] are listed in table 2. as illustrated in figure 1, a scanning electron microscope (sem) jeol, jsm6510lv has been utilized in order to investigate the structure and shape of the coating powder particles. the colmonoy-6 particles had a spherical shape, as shown in sem images, but the shape of wc particles was found to be elongated. prior to laser cladding, colmonoy-6 and wc powder were combined in an equivalent amount (50 wt.%) utilising a blending method. a double cone blender (ildcb001, innovative engineering work, india) was employed for optimum blending. powders were then put into the powder feeding mechanism. a diode laser with a coaxial powder feeding system was used to clad the steel specimens with a laser beam. the powders created from the double cone blender, which blended the colmonoy-6+wc powders, were fed into the powder feeding system's bin. the powder feed system allows for a consistent flow of powder in a stream with a diameter of 2 mm and a flow rate of powder ranging from 10-40 g min-1. the purpose of shielding and carrier gas is done by argon. earlier, the aim of removing all contaminants from the substrate's surface was done by sandblasting, which involved grit blasting the substrate with alumina (al2o3), and after the http://dx.doi.org/10.5599/jese.1333 j. electrochem. sci. eng. 12(5) (2022) 911-922 erosion of ss304 and ss410 steels 914 sandblasting, the surface was washed with de-ionized water along with acetone. there is a need for required surface roughness (7-9 µm) for producing the sound coating adhesion and this process was done through sandblasting [28]. as indicated in table 3, steels were cladded using the laser cladding technique at optimal levels of several process parameters, namely: scanning speed, laser power, and powder feed rate. it was discovered that the above-mentioned laser cladding process parameters (scanning speed to be 1700 mm min-1, laser power to be 2000 w, powder feed rate to be 30 g min-1) were optimal for producing sound coating and illustrated in table 4 [29]. table 3. properties of powders (tungsten carbide and colmonoy-6) name of powder density, g/cm3 melting point, °c hardness, rockwell c scale colmonoy-6 8.10 1030 56-61 tungsten carbide 10.0 2785-2830 88+ table 4. process parameters used during laser cladding process parameters range laser power, kw 2 scanning speed, mm min-1 1700 powder feed rate, g min-1 30 figure 1. sem micrograph of colmonoy-6+wc powder mechanical and material characterization of treated and untreated specimens for micro-hardness testing, the grit size of emery paper was from p100 to p1500 and used to polish the laser-clad and uncoated specimens. a vickers microhardness tester (rmht201, radical scientific equipment pvt. ltd., india) has been used to assess the changes in cross-sectional microhardness of the colmonoy-6+wc coatings under a 1 kg load and a 25-seconds dwell period. in addition, the space between two consecutive indentations was maintained at 0.1 mm. microhardness measurements were calculated as the average of twelve readings made in similar areas. sem analysis was utilised to study the splat distribution and porosity of the polished coated samples in cross-section. surface sem micrographs were utilized to analyse the vivid porosity of cladded materials surface using envision 3.0 series image analyser software (chennai metco private limited, india). furthermore, sem has been utilized to analyse the slurry erosion performance of laser cladded and uncoated specimens. s. singhet al. j. electrochem. sci. eng. 12(5) (2022) 911-922 http://dx.doi.org/10.5599/jese.1333 915 slurry preparation natural sand has been employed as striking material in the experimental investigation of slurry erosion, and its morphology is depicted in figure 2. the shape of sand particles is uneven, with sharp edges, as shown in the sem image. larger silt particle sizes over 800 are unable to access the hydraulic components due to the existence of sedimentation chambers and sand filters and are thus excluded for erosion testing. therefore, an average particle size of 350 µm is used for the current study. figure 2. sem image of erodent sand particles slurry erosion analysis slurry erosion experimentation was conducted at various variables using a specifically manufactured test rig, as depicted in figure 3 and the same had been utilized in previous work [29]. figure 3. schematic diagram of the slurry erosion test rig the influence of two factors, impact velocity of erodent particle and impact angle, were investigated. clad specimens were kept within the testing chamber in the specimen holder and had the capacity to adjust the orientation of the target surface in order to study the impact angle effect. a nozzle directs the combination of erodent particles and water in the testing chamber. a rotating vane pump is installed at the bottom, and one end of the pump is used to carry the slurry from the mixing chamber and directs the flow to the nozzle. there are valves built in the delivery pipe in order to http://dx.doi.org/10.5599/jese.1333 j. electrochem. sci. eng. 12(5) (2022) 911-922 erosion of ss304 and ss410 steels 916 control the amount and pressure of the slurry. the slurry concentration is prepared in the mixing chamber with a predefined sand particle size, as required by the test and for present experimentation, the slurry concentration is 35,000 ppm. in order to avoid the settlement of erodent particles at the bottom, a stirrer is used in the mixing chamber. the mass loss was calculated at regular intervals by a precision microelectronic balance (me36s, cubis laboratory balances) and the accuracy of this machine is 0.1 mg. the diameter of the nozzle and the distance between the nozzle and target surface were set at 5 mm and 2.8 cm, correspondingly. as per astm standard g-73 [30], erosive wear analysis has been done on uncladded and cladded steel specimens. moreover, the levels of the parameters mentioned above have been considered are illustrated in table 3. impact angles can be adjusted by moving the specimen holder around to examine the influence of impact angles of erodent particles. myriad investigations have determined that ductile materials exhibit the most erosion at 20-40o impact angles, while materials that are brittle in nature exhibit the most erosive wear at 90° [31-34]. as a result, impact angles of 30 and 90° were chosen for slurry erosion experimentation. most of the time, the relative velocity between the slurry and impeller varied between 10 and 35 m s-1 [35,36]. as a response, two levels of impact velocity (15 and 30 m s-1) were considered in the slurry erosion testing to imitate real-world conditions. in order to analyse the impact of both variables, full factorial was used, as represented in table 5. slurry erosion testing was done for one hour. table 6 represents slurry erosion testing using a full factorial array and these combinations have been used for the slurry erosion testing. according to the slurry erosion tests, the cladded specimens had a lower rate of erosion than uncladded specimens. after each hour of testing, the mass loss was computed, and in order to measure accurate measurement, the process of cleaning the specimens with acetone was done before the measurement of mass loss. sem images were used to examine the eroded specimens in order to understand the actual mechanisms that cause slurry erosion. table 5. different levels of slurry erosion parameters s. no. velocity, m s-1 impact angle, o 1 15 30 2 30 90 table 6. slurry erosion testing using a full factorial array velocity, m s-1 impact angle, o test 1 15 30 test 2 15 90 test 3 30 30 test 4 30 90 results and discussion characteristics of coating deposition figure 4 represents sem images of the cross-section surface of laser-clad ss304 and ss410 steels, respectively. from the cross-sectional sem images, the coating had a metallurgical connection between the coating and substrate. though there were fewer microvoids owing to the porosity of cladding materials, the coating seemed to have fewer defects. the value of porosity obtained for the coatings was in the range between 1.3 and 1.6 % (in both steels), and the same values were reported by different researchers with the deposition of the same cladded materials [21]. according to the s. singhet al. j. electrochem. sci. eng. 12(5) (2022) 911-922 http://dx.doi.org/10.5599/jese.1333 917 cross-section sem images, the thickness, which was higher than 1 mm, was observed for the coating. from the sem images, it was witnessed that carbides were evenly dispersed in colmonoy-6 pool. although most of the carbides were spherical in shape, on the surface, a few unevenly shaped carbides were detected. a b figure 4. (a) cross-sectional sem image of laser cladded ss410 steel and (b) cross-sectional sem image of laser cladded ss304 steel the hardness of the uncoated ss304, ss410 steel, and cladded steels was represented in figure 5. the value of hardness was measured at a distinct position on the cladding top surface with the application of 1 kg. the hardness of cladded surface was much higher than the substrate materials. the average hardness of ss304 and ss410 steel was noticed to be 152 and 178 hv9.81n, respectively. an increase in microhardness was seen after the cladding of ni+wc-based coatings on both steels, which could be related to the presence of dispersed carbides on the cladded surfaces [37]. the coating's micro-hardness ranged from 1009 to 1113 hv9.81n, with a mean value of 1074 hv9.81n in the case of ss304 steel and 1079 hv9.81n for ss410 steel. this represents that after the coating deposition, there is no remarkable impact on the base material and hardness, found to be almost the same in both cladded surfaces. steel specimen figure 5. micro-hardness of coated and uncoated steel specimens m ic ro h e rd n e ss , h v http://dx.doi.org/10.5599/jese.1333 j. electrochem. sci. eng. 12(5) (2022) 911-922 erosion of ss304 and ss410 steels 918 mechanism of slurry erosion experimental investigations of slurry erosion were carried out on the above-mentioned cladded material and uncladded steel specimens under various operating circumstances. as indicated in figure 6, the bar graph elucidates the information regarding the change in mass loss of cladded and uncladded specimens. when comparing uncoated ss410 and ss304 steels to colmonoy-6 + wccladded steel, it was discovered that the latter had stronger resistance to erosion. this could be due to the higher clad surface's hardness compared to uncladded steels. the maximum erosion was at the maximum impact velocity (30 m s-1). the reason for this, when the impact velocity increases then, there is also an increase in the kinetic energy of sand particles. consequently, maximum mass loss was noticed in all experiments. goyal et al. [38] carried out the research work on slurry erosion analysis of steel and it has been observed that impact velocity was the major contributor to slurry erosion and similar results have been reported by lopez et al. [39]. further, utmost erosion was observed at an angle of 30° and this represents the ductile failure and as the impact angle increases, this exhibits more resistance to erosion owing to the brittle mode of failure [40-42]. steel specimen figure 6. loss of mass after erosion of laser-cladded and uncoated ss304 and ss410 steel using 4 mm jet eroded surface analysis most of the time, the mechanism of removal of material under slurry erosion is affected by the properties of encountered surfaces of the material and working parameters in general [38]. plastic deformation, along with cutting, is used to erode the material from the target surface in the case of the ductile mode of wear [43]. fatigue failure occurs in brittle materials when energy is transferred to the target surface of the material by the repetitive influence of erodent particles [38]. furthermore, distinct erosion mechanisms exist for different operating situations, such as low plus high impact angles, and for the lower impact angles, ploughing, chip formation, and cutting are the mechanisms which are generally occurred on eroded surfaces [44]. cracks and craters formed by material erosion as platelets, on the other hand, are plausible erosion mechanisms for normal impact [45]. the subsequent paragraphs illustrate the erosion mechanisms of cladded and uncladded steel specimens. uncoated ss304 and ss410 steel after the polishing of as received steels before erosion causes some scratches on the surface. sem images of uncoated steels after the erosive wear occurred at using impact angle of 30° and m ic ro h e rd n e ss , h v s. singhet al. j. electrochem. sci. eng. 12(5) (2022) 911-922 http://dx.doi.org/10.5599/jese.1333 919 impact velocity of 30 m s-1, are shown in figure 7. maximum erosion has been witnessed in the case ss304 steel as that of ss410 steel. this is because of the higher hardness of ss410 steel against the ss304 steel. at this operating condition, slurry erosion was found to be highest as with a higher impact angle, the more mass removal rate was noticed, and the mode of failure in both cases was ductile. steel surface had some cracks and the same was represented in figure 7, which paved the way to the continuous and normal impact of erodent particles, while on the eroded surfaces of steel specimens, lip formation was seen, which could be related to the ductile mode of failure of the material under higher erodent particle velocity (30 m s-1) and low impact angle (30o) [45]. signatures of the ploughing and micro-cutting have been observed on the eroded surface of steel, as illustrated in sem pictures, may be caused by the same mechanism. further, it also has been witnessed that the wear resistance of ss410 is more owing to the higher hardness, and that is the reason there is less plastic deformation in comparison with ss304. a b figure 7. sem images of (a) uncoated ss304 steel and (b) uncoated ss410 steel after erosion coated ss304 and ss410 steel figure 8 shows sem images of cladded ss304 and ss410 steel after the maximal rate of wear at the velocity of 30 m s-1 and 90o impact angle. a b figure 8. sem images of (a) coated ss304 steel and (b) coated ss410 steel after erosion a significant number of cracks were created on the eroded cladded surface by dint of the continuous impact of erodent particles at 90°. after the deposition of cladded powder, a significant http://dx.doi.org/10.5599/jese.1333 j. electrochem. sci. eng. 12(5) (2022) 911-922 erosion of ss304 and ss410 steels 920 increase in wear resistance was noticed and the mode of failure was brittle because there was no formation of lip, ploughing and crater. the reason behind this mode of failure is owed to the impact angle of 90°. t. savanth et al. [12] evaluated the slurry erosion performance of colomonoy-5; it was found that under the same operating conditions, the mode failure was brittle, which was in accordance with the present research work. further, research work carried out by paul et al. [14] was on the slurry erosion performance of laser cladded 316 l steel and it was found that the mode of failure was brittle. moreover, the mode of failure was almost the same in the case of coated ss304 and ss410 steels and no significant effect of the base material was observed. conclusions • the laser cladding technique was used to successfully create colmonoy-6+ wc coatings on ss304 and ss410 steels under specified laser cladding process parameters: scanning speed, laser power and powder feed at 1700 mm/s, 2 kw and 30 g/min, respectively. • there was a significant improvement in hardness after the deposition of coating materials. the hardness of the cladded steels was found to be more than six times of the uncladded steel specimens. • mass loss was found to be less in the coated specimens than in the uncladded specimens. the reason behind this was the higher hardness of the deposited coating. the most significant factor for mass loss of coated and uncladded steel was found to be 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https://doi.org/10.1016/j.apsusc.2014.01.011 https://creativecommons.org/licenses/by/4.0/) short review on hydroxyapatite powder coating for ss 316l: review paper http://dx.doi.org/10.5599/jese.1611 25 j. electrochem. sci. eng. 13(1) (2022) 25-39; http://dx.doi.org/10.5599/jese.1611 open access : : issn 1847-9286 www.jese-online.org review paper short review on hydroxyapatite powder coating for ss 316l jashanpreet singh1,, jatinder pal singh2, satish kumar3 and harjot singh gill4 1university center for research & development, chandigarh university, mohali 140413, india 2school of mechanical engineering, lovely professional university, phagwara 144411, india 3department of mechanical engineering, national institute of technology, jamshedpur 831014, india 4department of mechanical engineering, chandigarh university, mohali 140413, india corresponding author: ijashanpreet@gmail.com; jashanpreet.t1764@cumail.in received: november 22, 2022; accepted: december 10, 2022; published: january 14, 2023 abstract medical implants and other biomaterials are used by millions of individuals all over the globe to restore lost bodily functions due to injury or illness. many of these implants fail after a short time or have difficulties, despite the fact that they play important roles in keeping a person's life safe or increasing the quality of their lives. it is the lack of biocompatibility that has proven to be the biggest downfall of biomaterials. investments in this industry may be made using a thin film of hydroxyapatite powder (hap) on stainless steel. plates, screws, pins, and artificial joints are only some fixation devices for bones that often use 316l stainless steel. however, due to its unique advantageous qualities such as super-elasticity and low-profile feature, thin film hap signals a high potential for use in compact new cardiac devices like the cardiovascular system and protecting stent grafts. keywords biomaterials; biocompatibility; implants; plasma spray; coatings introduction the bones, joints, and teeth that make up the human skeletal system may heal from a broad range of ailments with basic care. there has been a dramatic increase in the complexity of orthopedic injuries since the advent of industrialization. orthopedic surgery may be necessary to repair or replace a broken bone. above 500,000 joints are replaced each year in the usa alone, and thousands of patients have this procedure every year globally [1]. the average demand for complete implants is anticipated to rise by almost 30 % over the next decade and a half [2]. there will be an increase from 10.1 million to 13.9 million people living with osteoporosis between 2002 and 2020 [3]. as a result, ti-based alloys, stainless steel, and alumina/zirconia ceramics have been the only biomaterials employed in these implants up to this point [4]. bone cement, used for affixing joint prostheses, is a common component of such treatments. bone cement is used to fill bone and dental cavities using self-curing polymers, and they also serve as load distributors for implants. due to its bio-stability and high mechanical http://dx.doi.org/10.5599/jese.1611 http://dx.doi.org/10.5599/jese.1611 http://www.jese-online.org/ mailto:ijashanpreet@gmail.com mailto:jashanpreet.t1764@cumail.in j. electrochem. sci. eng. 13(1) (2022) 25-39 hap coating for ss 316l 26 qualities, poly(methyl methacrylate) is used in this sector since the 1960s [5]. it is essential to mimic the composition and structure of genuine bone to achieve features comparable to those of natural bone. in order to do this, it is necessary to study the anatomy of real bone [6]. coating is the technique used to improve the mechanical properties of a surface. this method is also used in biomedical applications where metal implants predominate. despite the fact that metal devices offer beneficial features, including higher strength, corrosion-resistant, and biocompatible properties within the human body, they are nonetheless not without their drawbacks. these gadgets still used metal and were rusted in varying conditions. because of its great stability and biocompatibility, hydroxyapatite (hap) is often used as a coating for metal implants [7]. it is wellknown that steel corrodes in moist settings like the human body. so, babu et al. [8] use a dip-coating technique to attempt to improve the bioactive coating of implants. ss316l steel was used as a base material with hap made of biphasic calcium phosphate for the experiments. before anything else, specimens of the substrate were heated for 15 minutes in a solution containing 20 % of hno3 at 600 °c to create nonreactive substrate surfaces. substrate specimens were then submerged in the hno3 based solution to start the coating. results showed that utilizing this approach at the right temperatures will strengthen the adherence of the coating to the base material. many scientists have tried utilizing ceramic powders as additives to coating solutions to form a protective layer on metals since thermally treating metals to decrease corrosion is still insufficient. choudhuri et al. [9] offered a novel method, cold spraying bio-ceramic coatings onto metals at temperatures below their melting points (metal melting point). the composite powders used in this research are composed of titanium and hap, with a proportion of hap as high as 30 %. the research also found that the ratio of hap deposition to hap in the coating solution decreased with time. the binding strength for the deposit was also similar to that of plasma-sprayed hap. the electrophoretic deposition (efd) often used in thin-film fabrication was used to apply hap coatings on ss316l surfaces in an investigation by fadli et al. [10]. three different voltage impacts of 40, 50, and 60 v on the characteristics of the coating layer were investigated. quantitative analysis of ca and p in hap coatings was also conducted. during the corrosion testing, the release of ni-ion from uncoated and coated materials was confirmed. in addition, the vickers microhardness and wear test were used to assess the coating's mechanical properties, and the results of these tests are shown below. the development of the hap layer was confirmed by the xrd of the coated sample. an increase in the number of pulses significantly enhanced the microstructure and growth of the hap film, as shown by sem. however, jafari et al. [11] used the electrophoretic deposition approach to coat synthetic hap on 316l for varying amounts of time (between 1 and 5 minutes) while maintaining a constant potential of 60 v. electrochemical studies were conducted on bare and hap-coated ss316l in biomimetic conditions. hap-coated samples display a more noble change in the corrosion potential and corrosion current density compared to uncoated samples regarding the corrosion properties derived using open circuit potential and potential dynamic polarization. rakngarm et al. [12] created a bio-active coating by employing a layer of hap on metal-made implants within the humanoid body to take advantage of the mechano-chemical characteristic of metal-made implants and the bio-medical compatibility of human body tissues for the hap surface. a literature survey indicates that compared to uncoated ss316l, the corrosion resistance of ss316l treated with a hap layer is much higher [13]. therefore, there is a need to study the mechano-chemical properties of the hap powder-coated ss 316l to protect it against corrosion and biodegradation. in this context, a literature review is carried out to assess the mechano-chemical characteristics of ss316l, hap, and hap-coated ss316l. j. singh et al. j. electrochem. sci. eng. 13(1) (2022) 25-39 http://dx.doi.org/10.5599/jese.1611 27 biomedical materials substrates among the many materials used in medicine, the stainless-steel type (ss 316l) is a popular choice for implants in orthopedic surgery because of its resistance to corrosion, strong mechanical capabilities, and low cost. hip and knee replacement pins, screws, and other orthopedic implants are often made of stainless steel. however, scientific trials reveal that the corrosion from ss316l implants results in the release of metal ions from implants into the body tissues, which might have adverse effects. also, the fact that corrosion failures have occurred while using devices made from these materials within the human body has paved the way for many studies to make these materials more biocompatible and corrosion-resistant. mechanical properties of ss 316l and bone were tested on the tensile testing machine. figures 1 and 2 illustrate the stress-strain diagram of ss316l and bone material, respectively. table 1 compares the mechanical properties of ss316l to the natural bone. in this work, the elemental composition of ss 316l was assessed by oxford instruments-manufactured foundry master spectrometer (uedem, germany). table 2 shows an elemental analysis range for ss316l. the chemical composition for ss316l shows good agreement with the literature data [14-18]. figure 1. stress-strain diagram of ss 316l table 1. various mechanical properties of bone and ss 316l no. properties 316l bone 1 elastic modulus, gpa 210 10-30 2 tensile stress, mpa 485 70-15 3 tensile yield stress, mpa 170 120 table 2. elemental composition for ss 316l element c mn si co p s cr mo ni fe content, wt.% 0.005 1.02 0.135 1.69 0.083 0.005 16.60 1.69 10.35 balance http://dx.doi.org/10.5599/jese.1611 j. electrochem. sci. eng. 13(1) (2022) 25-39 hap coating for ss 316l 28 figure 2. stress-strain diagram of bone material hydroxyapatite: structure and properties a ceramic material known for its medical applications is hydroxyapatite [19]. the bone material is composed of apatites which act as packing for ca, mg, p, and na to strengthen the skeleton. mineral hap apatites (ca10(po4)6(oh)2) and brushite (cahpo42h2o) have structural similarities with biological apatites. there are two stable calcium phosphate phases in bodily fluid and temperature: the brushite phase at (ph<4.2) and the hap phase at (ph>4.2). crystals of hydroxyapatite have a hexagonal, rhombic prism shape. hydroxyapatite has the lattice configuration (a = 0.9432 nm) and (c = 0.6881 nm), as given in figure 3. corners of the base plane are hotspots for producing hydroxyl ions (oh-). ions are arranged to be parallel to the c-axis of the crystals and transverse to the base plane of the crystals at every 0.344 nm (1/2 of the unit cell). figure 3. simplified crystal structure of hydroxyapatite [21]. (cc by 4.0 attribution) thus, about sixty percent of the ca-ions in the crystal lattice are linked to the oh ions. hap has a density of 3.219 g cm-3 [20]. hap utilized in bones, hip replacement, and dental metal implants are generating around $2.3 billion in yearly sales, which is expected to rise. the success of implants is contingent on many aspects, including those related to biocompatibility and mechanical features. j. singh et al. j. electrochem. sci. eng. 13(1) (2022) 25-39 http://dx.doi.org/10.5599/jese.1611 29 owing to corrosion concerns, the use of metals within the human body has been minimal. morphologically, the hap powder seems blocky and irregular in shape, as presented in figure 4. figure 4. sem image of hydroxyapatite powder [22] (cc by 4.0 attribution) coatings surface engineering plays a crucial role in protecting biomaterials used in implants. it protects against corrosion, biodegradation, and erosion [18,23-38]. various coating processes are used to enhance the surface of engineering and biomedical materials [39-42]. electroplating, electroless plating, hot dipping [43-49], vapor deposition techniques (physical [50-56], chemical [57-59] and electrochemical [60]), thermal spraying [61-63], claddings [64-75], plasma spraying [76-83], and electrophoretic deposition are all methods used to deposit coatings onto metals (epd). as seen in figure 4, the size of hap powder depicts that larger as well as smaller particles were present. moreover, the particle melting and layer formation phenomenon depends upon the coating technology as well as particle properties. therefore, the hap can be deposited to different materials in different thicknesses by following the different procedures, as illustrated in figure 5. figure 6(a) presents the sem images of hap coating on ti6al4v. figure 6(b) presents the xrd of the hap electrodeposited coating, which shows the presence of different crystals, namely stoichiometric hap and -tricalcium phosphate (-tcp) in the as-coated surface. natural bone in its mature form, bone is a biphasic composite. collagen, a kind of protein, and hap, a mineral phase that does not undergo chemical reactions, are its primary components. it is difficult to determine which phase is the primary load-bearer; maybe crystal serves as a filler to harden the collagen by limiting its mobility when stressed. collagen is just 1 % as rigid as the mineral. thus, it can't be relied upon to support bending or compressive forces. only collagen contributes to the bone's fibrous structure. hap is twice as powerful and significantly more rigid (table 3). bone development involves inserting tiny platelets of hap into predetermined pores in collagen, creating a composite of around two-thirds inorganic. because of this, the fiber comparison only http://dx.doi.org/10.5599/jese.1611 j. electrochem. sci. eng. 13(1) (2022) 25-39 hap coating for ss 316l 30 works if it is considered a composite itself, reinforced with platelets because the bone is not homogenous even at the morphological level [85]. figure 5. substrate temperature versus thickness of coatings deposited by different processes [84] (cc by 4.0 attribution) figure 6. (a) sem and (b) xrd of hydroxyapatite [22] (cc by 4.0 attribution) j. singh et al. j. electrochem. sci. eng. 13(1) (2022) 25-39 http://dx.doi.org/10.5599/jese.1611 31 table 3. material properties of the bone material [86] no. feature value 1 hap ultimate strain 0.001 2 hap strength, gpa 0.1 3 collagen’s strength, mpa 50 4 hap stiffness, gpa 130 5 stress acting on collagen for 0.001 strain, mpa 1 figure 7 illustrates that bone has a complicated hierarchical structure that has an effect on its mechanical performance. the magnitude and direction of the stresses that bones can withstand and how these stresses are transmitted to the rest of the body are both controlled by the density and configuration of mineralized platelets. there are two primary ways in which collagen fibrils may be arranged: firstly, randomly (woven bone), and secondly, inlayers with desired orientation (lamellar bone). primary lamellar bone consists of stacked layers of lamellae in a single plane, whereas secondary lamellar bone, haversian bone, and woven bone all form cylindrical stacks of lamellae (laminar bone). in compact bone, you may find each kind [85]. an osteon is a fundamental unit of the skeletal structure. a hollow tube, approximately 200 m in diameter and 1-2 cm in length, serves as a weight-bearing component and a blood artery. osteon walls are constructed using a coiled pattern of fine fibers (i.e. helical) of collagen interrupted by neighboring platelets of hydroxyapatite. as a consequence, compressive loads may be successfully carried by a small column, preventing buckling. however, its primary purpose is not durability [85]. figure 7. (a) hierarchical structure of osseous tissue, (b) cylindrical haversian systems or osteons, (c) cells are covered in a cluster of cell membrane receptors that react to particular binding sites, and (d) well-organized nano-structure of the surrounding extracellular matrix [91]. (cc by 4.0 attribution) http://dx.doi.org/10.5599/jese.1611 j. electrochem. sci. eng. 13(1) (2022) 25-39 hap coating for ss 316l 32 conventional orthopedic sir john charnley's groundbreaking research is essential to the current generation of implant designs [87]. the metallic component (named femoral) may be made of ss316l metallic alloys, such as cobalt-chromium-molybdenum, cobalt-nickel-chromium-molybdenum, or titanium-aluminumvanadium alloys, while the uhmw (ultra-high molecular weight polyethylene) covering the femoral head may be manufactured of synthetic materials (figure 8). pmma bone cement is used to secure the devices in place. the current methods are temporary. as a result, 10 to 20 % of implants need replacement after 10 years, and some may require replacement even sooner, within the first 5 years [88]. such short lifespans may be attributed to loosening, erosion-corrosion, the action of uneven loading, and irritation in the body tissues. charnley [89] suggested that such types of implants can be used only for aged patients due to short life expectancy because of the very short lifetime for these implants owing to inadequate cement technology. a second characteristic that might interfere with implant function is stress shielding. load misallocation at the bone-implant contact is a cause of implant dislocation or loosening [90]. this issue is present in all currently used metal implant components. figure 8. model of an implanted total hip prosthesis [92]. (cc by 4.0 attribution) conclusion and future prospective in conclusion, metals may be used in a wide variety of contexts. and one of these sectors is medicine, which benefits from traits like strong strength, among many others. however, there is another side to metals: they tend to corrode when used in harsh environments like the one found within the human body. although hap has long been used to promote bone regeneration by conduction or as a scaffold for filling defects, recent advances in our understanding of its osteoinductive characteristic indicate that it may soon play an even more important role in fostering new bone growth. previously unimaginable clinical applications for hap coatings are now a reality because of advances in technology. as a key component in the formation of calcified tissues, nanostructured calcium apatite is crucial. the nanostructured material may mimic the fundamental structure of bone and other calcification tissues by attaching biological molecules like proteins, and it can also synthesize controlled structures of apatite. novel nanostructured biomimetic and biocompatible materials are needed for the regeneration process. composites containing nanostructured bio-ceramic particles have the potential to increase the durability of injectable and controlled-setting synthetic bone implants and bone cements. the potential of nanotechnology for bone regeneration is the subject of j. singh et al. j. electrochem. sci. eng. 13(1) (2022) 25-39 http://dx.doi.org/10.5599/jese.1611 33 intensive study in the future. additionally, nanostructured coatings have shown great promise in many other industrial applications. therefore, hap-based nanostructured coatings can be a topic of interest among researchers in the future. the coating is only one method that may protect these metals against corrosion or reduce their effects to an acceptable level. additionally, as time went on, improvements were made to the coating process, and new coating 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1611 http://www.ncbi.nlm.nih.gov/pmc/articles/pmc2502711/ https://doi.org/10.1302/0301-620x.79b2.7165 https://doi.org/10.1016/j.arth.2003.12.081 https://doi.org/10.1126/science.1106587 https://doi.org/10.3390/polym14204308 https://creativecommons.org/licenses/by/4.0/) @article{singh2023e, author = {singh, jashanpreet and singh, jatinder pal and kumar, satish and gill, harjot singh}, journal = {journal of electrochemical science and engineering}, title = {{short review on hydroxyapatite powder coating for ss 316l: review paper}}, year = {2023}, issn = {1847-9286}, month = {jan}, number = {1}, pages = {25--39}, volume = {13}, abstract = {medical implants and other biomaterials are used by millions of individuals all over the globe to restore lost bodily functions due to injury or illness. many of these implants fail after a short time or have difficulties, despite the fact that they play important roles in keeping a person's life safe or increasing the quality of their lives. it is the lack of biocompatibility that has proven to be the biggest downfall of biomaterials. investments in this industry may be made using a thin film of hydroxyapatite powder (hap) on stainless steel. plates, screws, pins, and artificial joints are only some fixation devices for bones that often use 316l stainless steel. however, due to its unique advan­tageous qualities such as super-elasticity and low-profile feature, thin film hap signals a high potential for use in compact new cardiac devices like the cardiovascular system and protecting stent grafts.}, doi = {10.5599/jese.1611}, file = {:d\:/onedrive/mendeley desktop/singh et al. 2023 short review on hydroxyapatite powder coating for ss 316l review paper.pdf:pdf;:jese_v13_no1_25-39.pdf:pdf}, keywords = {biomaterials,biocompatibility,coatings,implants,plasma spray}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1611}, } electrochemical determination of an antitumour platinum(iv) complex: trans-[ptcl2(oh)2(dimethylamine)(isopropylamine)]. application to biological samples doi: 10.5599/jese.2013.0031 81 j. electrochem. sci. eng. 3(2) (2013) 81-89; doi: 10.5599/jese.2013.0031 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical determination of an antitumour platinum(iv) complex: trans-[ptcl2(oh)2(dimethylamine)(isopropylamine)]. application to biological samples carmen s. hernandez domínguez and pedro hernández departamento de química analítica y análisis instrumental, facultad de ciencias, universidad autónoma de madrid, cantoblanco, 28029 madrid, spain  corresponding author: e-mail: carmen.hernandez.dominguez@gmail.com; tel: +34 914974988; fax: +34 914974931 received: november 4, 2012; revised: february 22, 2013; published: april 19, 2013 abstract a differential pulse voltammetry (dpv) method has been applied for the first time for determination of trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)]. to this end, all chemical and instrumental variables affecting the determination of trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)] were optimized. from studies of the mechanisms governing the electrochemical response of trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)], it was concluded that it is an electrochemically irreversible system with a reduction under diffusion control, with a reduction potential of -425 mv. under optimal conditions, the variation in the analytical signal (ip) with trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)] concentration is linear in the 0.8 µg ml -1 to 20 µg ml -1 range, with an lod of 97 ng ml -1 and a loq of 323 ng ml -1 , rsd = 1.58 % and er = 0.83 %. the optimized method was applied to the determination of trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)] in biological fluids, human urine and synthetic urine. keywords trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)]; differential pulse voltammetry; biological fluids; human urine. 1. introduction the use of cis-diamminedichloroplatinum(ii), known as cisplatin or cis-ddp, in cancer chemotherapy has made a major impact on the observed response rates of some tumour types, such as testicular or ovarian carcinoma [1]. however, cisplatin has two major drawbacks: 1) severe toxicity that includes nephrotoxicity, neurotoxicity and ototoxicity and 2) the acquisition or presence of resistance to the drug [2]. because tumour resistance to cisplatin limits its efficacy, http://www.jese-online.org/ mailto:carmen.hernandez.dominguez@gmail.com j. electrochem. sci. eng. 3(2) (2013) 81-89 antitumour pt(iv) complex: application to biological samples 82 there is an urgent need to discover new platinum complexes capable of overcoming cisplatin resistance. figure 1. structure and molecular weight of the platinum complex, transpt[cl2(oh)2(dimethylamine)(isopropylamine)] (compound 1), used in this article because enhanced removal of cisplatin-dna adducts has been reported as one of the main causes of cell resistance to cisplatin, there is general consensus that this particular resistance mechanism may be circumvented by platinum complexes that bind to dna by a different mechanism than cisplatin [3,4]. one platinum compound that possesses dna-binding properties distinct from those of cisplatin is its trans isomer transplatin, trans-diamminedichloroplatinum(ii), or trans-ddp (fig. 1). in fact, it has been found that cis-ddp mainly forms 1,2 intrastrand crosslinks on dna, whereas the main adducts of trans-ddp are 1,3 intrastrand cross-links [2]. unfortunately, early structure-activity relationship studies showed that trans-ddp and other transpt complexes were inactive as antitumour drugs [5]. however, in 1989, farrell et al. [6] reported the first cytostatic trans-pt(ii) complexes. in recent years, several classes of biologically active trans-platinum complexes have been reported [7]. among these unusual classes of platinum drugs, it has recently been found that trans-pt(ii)cl2 complexed with an asymmetric set of aliphatic amines is able to circumvent cisplatin resistance. it is known that resistance to cisplatin is multifactorial and includes three main mechanisms: decreased cellular accumulation of cisplatin, increased cytoplasmatic detoxification (through increased cytoplasmic levels of glutathione and metallothioneins), and increased dna repair/tolerance of platinum-dna adducts[8]. it has also been postulated that alterations in the apoptotic cell death pathway may constitute a fourth mechanism of cisplatin resistance[9]. the group of carmen navarro-ranninger [10] has reported that the trans-platinum(ii) complex with mixed aliphatic amines, trans-pt[cl2(oh)2(dimethylamine)(isopropylamine)] (fig.1) circumvents cisplatin resistance in cells that overexpress oncogenes. in addition, compound 1 is also able to circumvent resistance to cisplatin in a2780cisr ovarian tumour cells, which exhibit resistance through a combination of the three mechanisms mentioned above [11]. moreover, they observed that circumvention of cisplatin resistance by compound 1 is associated with a higher level of apoptosis induction relative to cis-ddp [12,13]. also of interest was the observation that in a2780cisr cells there is a correlation between the dna interstrand cross-linking efficiency of compound 1 and its ability to induce apoptosis [13]. on these grounds, navarro-ranninger et al. [10] reported the cytotoxic activity of the transpt[cl2(oh)2(dimethylamine)(isopropylamine)] (tpt complex) (fig. 1) in pairs of cisplatin-sensitive and-resistant human ovarian tumour cell lines. the resistant cell lines showed acquired resistance to cisplatin and were selected with regard to the three major mechanisms of resistance to the drug. in addition, these cell lines have been used previously to identify novel platinum complexes capable of circumventing cisplatin resistance [14-16]. because cellular and molecular pharmacology studies are essential to understanding the relationships between structure and c. s. h. domínguez et al. j. electrochem. sci. eng. 3(2) (2013) 81-89 doi: 10.5599/jese.2013.0031 83 anticancer properties, navarro-ranninger et al. [10] have compared the cellular accumulation, dna binding, interstrand cross-linking efficiency, apoptosis induction, and binding to serum albumin and plasma proteins of the trans-pt(iv) complex (compound 1) with that of its corresponding trans-pt(ii) analog. finally, the in vivo antitumour activity of compound 1 has been evaluated in mice bearing tumour xenografts. in this article, a new electrochemical method to determine tpt complex in biological fluids is developed. this paper makes a study of the electrochemical behaviour of this compound by differential pulse voltammetry (dpv) aimed at obtaining the optimal conditions for its determination in biological fluids and establishing its formation constants. the analysis of biological fluids presents a special relevance because small changes in the concentration of its components are thought to be correlated with several neurological or metabolic disorders [14]. 2. experimental reagents: tpt complex [10] was provided by carmen navarro-ranninger. the purity of the analyte was 98 %. this high purity is because the complex of platinum(iv) was a synthetic product, provided by the group of navarro-ranninger triple distilled mercury was used as working electrode. buffer solutions prepared with acetic acid, phosphoric acid and borate were adjusted to the desired ph values with sodium hydroxide. unless otherwise stated, all other reagents were of analytical grade and were used as received. ultrapure water, obtained from millipore milliq system, was used in the preparation of buffers and solutions and to clean the working electrode of possible substances adsorbed on the gold electrode surface. all solutions were prepared just prior to use, preserved at 4 °c and protected from light. apparatus: electrochemical measurements were performed using a bioanalytical system (bas) epsilon mercury stand workstation, an ag/agcl/kcl 3m reference electrode, and a platinum wire as counter electrode. for ph adjustment, a ph-meter methrom c831 was employed as well as a circulation ultrathermostat frigiterm-10 (p-selecta, spain) to control the temperature. procedure: tpt complex stock solution (500 µg ml -1 ) was prepared with ultrapure water and stored at 4±1 c preserved in aluminium foil covered bottles to avoid photodegradation. prior to analysis, sample bottles were allowed to equilibrate to room temperature (rt). sample preparation: the tpt complex dissolved readily and was further diluted to desired concentrations using 0.5 m acetate buffer solution. the samples used for the implementation of the method described above included a synthetic urine sample prepared in the laboratory and a sample of real human urine. the composition of the synthetic urine is presented in table 1 [17]. table 1. synthetic urine composition nacl kcl caco3 mgcl2.6h2o h2so4 (98 %) hcl (conc) nh4h2po4 c 5.08 g l -1 2.86 g l -1 0.31 g l -1 0.42 g l -1 0.67 ml l -1 8.7 ml l -1 3.09 g l -1 processing of the human urine sample consisted of: 1) doping of the sample with an aliquot of tpt complex (10 µg ml -1 ). this value was chosen because, at this time, there are no published references devoted to the use of these compounds in humans; j. electrochem. sci. eng. 3(2) (2013) 81-89 antitumour pt(iv) complex: application to biological samples 84 2) protein precipitation with 0.1 m hclo4 ; 3) digestion of the precipitate for a weekend (48 hours); 4) filtering of the sample. the filters used in the development of this process were cellulose membrane filters with a pore size of 0.45 m and 5) adjust the ph of the sample to the appropriate value. 3. results and discussion the electrochemical study of the tpt complex was performed by dpv and square wave voltammetry in order to determine the best conditions for its analysis. 3. 1. influence of ph and ionic strength. in order to select the optimum ph, a ph range between ph 2 and ph 12 was employed. the buffers used were phosphate buffer for ph 2, 3, 6, 7, 8, 10, 11 and 12; acetic acid buffer for ph 4 and 5; and borate buffer for ph 9. the concentration of all the buffers was 0.5 m. in figure 2 the variation in peak intensity with respect to ph (fig. 2a) and the variation in peak potential versus ph (fig. 2b) are represented. this figure shows that the greatest intensity was reached at ph 4 (fig. 2a). by representing the variation in peak potential with respect to ph (fig 2b) three straight segments with clear variations in slope are apparent, indicating that the compound under study has two dissociation constants, which correspond to the values of k1 = 10 -4.2 , k2 = 10 -7.7 . this fact can be corroborated by observing the slopes in the ep vs. ph graph (fig. 2b) figure 2. a) variation in dpv peak intensity at different ph values. b) variation in peak potentials. dissociation constants were calculated from the variations in the slopes. measurement conditions: ionic strength = 0.05 m; tpt complex concentration = 5 μg ml -1 ; pulse amplitude = 50 mv; scan rate = 10 mv s -1 ep = -2ph 426; r 2 = 1 (1) ep = -35.2 ph 287.4; r 2 = 0.995 (2) ep = -211.5 ph + 1072.5; r 2 = 0.992 (3) from the standpoint of electrochemistry, ph 4 was chosen for further study because it is the value that provided the greater intensity. the ionic strength necessary to achieve the greatest possible sensitivity in the electrochemical reaction was also studied. for this study we used solutions of acetic acid-acetate buffer (ph 4) in the concentration range between 5×10 -2 and 2.0 m, and reached a maximum intensity at 5x10 -2 m. this concentration was used for further studies. c. s. h. domínguez et al. j. electrochem. sci. eng. 3(2) (2013) 81-89 doi: 10.5599/jese.2013.0031 85 3. 2. instrumental variables once the study of the chemical variables was completed, we proceeded to the optimization of instrumental variables, both by dpv and square wave voltammetry (swv). in both techniques the values of ph 4 and ionic strength 5×10 -2 m, determined above, and the analyte concentration of 10 μg ml -1 were kept constant. 3. 2. 1. influence of pulse amplitude by changing this variable using differential pulse voltammetry, it appeared that the peak current increased linearly according to the equation: i = 0.0015 ∆e +0.0003 (4) using a scan rate of 10 mv s -1 throughout the study. however, at the same time, the width of the half peak increased, so it was necessary to compromise between sensitivity and selectivity. this occurred at a value of pulse amplitude 50 mv. the peak potential was modified according to the equation: ep = 0.4338 ∆e 466.88 (5) to calculate the number of electrons exchanged during the reduction a dc voltammetry was carried out, and compared with the findings obtained with dpv (figure 3). figure 3. comparison of dc and dp polarograms. measurement conditions: ph 4; ionic strength = 0.05 m; tpt complex concentration = 5 μg ml -1 ; scan rate = 10 mv s -1 the number of electrons exchanged was found to be two using the following equation [18]: ep = e1/2 ± ∆e/ne (6) where ep is the peak potential calculated at ph 4 in dpv, e1/2 is the potential calculated from dc voltammetry and δe is the pulse applied, at a peak potential of -426 mv and -409 mv half-wave. therefore, the reduction process can be written as: pt(iv) + 2e ⇔ pt(ii) (7) the square-wave behaviour is similar to that observed with dpv, the peak current increasing linearly with pulse amplitude according to the equation: i = 0.0035 ∆e + 0.32 (8) j. electrochem. sci. eng. 3(2) (2013) 81-89 antitumour pt(iv) complex: application to biological samples 86 at a frequency of 25 hz and a step of 2 mv, which promotes an effective scan rate of 50 mv s -1 . when performing these studies by using square wave voltammetry it is presented the same situation of commitment that in differential pulse voltammetry, in other words, by increasing the amplitude of pulses produces a substantial increase in the half-peak width the variation in peak potential follows the equation ep = 0.85 ∆e 457.02 (9) 3. 2. 2. influence of scan rate as in previous studies, the ph, ionic strength and analyte concentration (10 μg ml -1 ) were held constant. when the scan rate was modified at dpv and the pulse amplitude remained constant (δe = 50 mv), the variation in peak current was linear with scan rate according the equation: i = 0.0058 v 0.5 + 0.32 (10) however, the peak potential does not remain constant, indicating that the electrochemical reduction is an irreversible process controlled by diffusion. a study of this variable in swv was carried out by changing the frequency and step, but at a constant pulse amplitude (δe = 50 mv). the results were that in modifying the frequency and the step, the peak intensity followed a linear trend fitting the square root of each of the two variables. in addition, the peak potential shifts as well, indicating that the reduction process is irreversible and under diffusion control. as in previous studies, half-peak width increases, selecting as the most appropriate conditions a frequency of 50 hz and 2 mv step, representing a scan rate of 100 mv s -1 . the results obtained (fig. 4) are consistent with those found earlier; the peak intensity fits linearly to the square root of the scan rate by: i = 0.012 v 0.5 + 0.12 (11) figure 4. cyclic voltammograms obtained at different scan rates. measurement conditions: ph 4; ionic strength = 0.05 m; tpt complex concentration = 5 μg ml -1 pulse amplitude = 50 mv c. s. h. domínguez et al. j. electrochem. sci. eng. 3(2) (2013) 81-89 doi: 10.5599/jese.2013.0031 87 and the peak potential also changes linearly as: ep = -2.06 ∆e 487.78 (12) this confirms the irreversibility of the process controlled by diffusion. the reduction process was also studied by cyclic voltammetry in order to contrast the results obtained in earlier techniques. the voltammograms obtained clearly demonstrate that the process is irreversible and under diffusion control. 3. 2. 3. calibration curve and limit of detection after choosing the most appropriate measurement conditions for determining the compound under study, the influence of the analyte concentration was studied using the following conditions (table 2) table 2. conditions for calibration curve ph ∆e / mv v / mv s -1 ionic strength, m differential pulse polarography 4 50 10 0.05 square wave polarography 4 50 100* 0.05 *frequency = 50 hz, scan increment = 2 mv the analytical performance of dpv method developed for tpt complex determination was evaluated. from the voltammograms recorded for increasing amounts of tpt complex using the optimized parameters described above, it was observed that the peak current increased with the analyte concentration over a large range (0.8 µg ml -1 to 20 µg ml -1 ) with good linearity according to i = 0.015 c 0.0053; r 2 = 0.998 with an er = 0.83 % and an rsd of 1.58 % (n = 10 at the 10 µg ml -1 level). the minimum detectable amount was 97 ng ml -1 , while a concentration of 323 ng ml -1 was calculated as the detection limit (fig. 5). we proceeded in the same manner when swv was carried out. in this case, square-wave voltammograms were recorded for increasing amounts of tpt complex under the optimal conditions found for its determination. a linear response was found in the 0.1 µg ml -1 to 20 µg ml -1 range according to i = 0.63c 0.0158; r 2 =0.9962. the sensitivity of the method was inferred from the lod(3σ)=105 ng ml -1 and loq(10 σ)=393 ng ml -1 values. on the other hand, the rsd and er values of 2.10 % and 1.72 %, respectively (n=10 at the 18 µg ml -1 level) indicated the accuracy and reproducibility of the proposed method. 3. 3. analytical application prior to using the method for determinations of tpt complex in urine, we conducted a study of the inorganic salts present in a synthetic urine prepared in the laboratory with the composition detailed in table 1 and doped with 10 μg l -1 tpt [17]. to perform this experiment the standard addition method was used, obtaining the equation, i = 0.0012c 0.023 with a correlation coefficient of r 2 = 0.993. given the suitability of the results, we proceeded to determine the tpt complex in human urine samples. the following procedure was used in pretreating the sample and the dpv. the sample was doped with an aliquot of 10 μg ml -1 tpt complex, then proteins were precipitated with 1 m hclo4. the precipitate was digested for 48 hours, filtered and the filtrate j. electrochem. sci. eng. 3(2) (2013) 81-89 antitumour pt(iv) complex: application to biological samples 88 was then brought to ph 4 with a 0.05 m buffer solution and standard measurement by dpv was performed. figure 5 shows the voltammograms obtained; the inset shows the reciprocal plot of i versus [tpt complex], showing the excellent fit of the experimental data to the equation i = 0.21c + 1.98, with a correlation coefficient of r 2 =0.997. when the data obtained were extrapolated into the equation of the calibration curve, the concentration of tpt complex obtained was 9.800.05 µg ml -1 the procedure followed in the electrochemical study of the synthetic urine sample was the same as in human urine. the equation obtained when the standard addition was carried out was i = 0.0034c + 0.0023, with a correlation coefficient of r 2 =0.9903. by extrapolating the obtained data, the concentration of the tpt complex was 9.75 0.05 µg ml -1 . the difference between the obtained data for human urine and synthetic urine could be because the matrix of the human urine sample is much more complex than the synthetic urine matrix. just as in the calibration section, the differences between the slopes are due to the determination conditions, being that determination of the tpt complex was performed using dpv and swv, and each of these electrochemical techniques has optimum conditions. figure 5. calibration curve for dpv in human urine. measurement conditions: ph 4; ionic strength = 0.05 m; pulse amplitude = 50 mv; scan rate = 10 mvs -1 4. conclusions a sensitive differential pulse voltammetry (dpv) method to analyse tpt complex has been applied and the mechanisms governing the electrochemical response have been elucidated. the proposed method can be applied to the determination of the studied pt(iv)-complex in biological fluids, with the inherent advantages of electrochemical techniques, i.e., greater economy and easy instrumentation with high accuracy and reproducibility, as shown from the obtained results. c. s. h. domínguez et al. j. electrochem. sci. eng. 3(2) (2013) 81-89 doi: 10.5599/jese.2013.0031 89 acknowledgements: the authors thank carmen navarro for providing the platinum complex and micinn for their economic support. references [1] e. wong, c.m. giandomenido, chem. rev. 99 (1999) 2451-2466. 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[18] j.m. pingarrón, p. sánchez batanero química electroanalítica: fundamentos y aplicaciones, editorial síntesis, madrid, 2003. © 2013 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://creativecommons.org/licenses/by/3.0/ {simultaneous determination of trace levels of cd(ii) and pb(ii) in tap water samples by anodic stripping voltammetry with 2-mercaptobenzothiazole modified electrode} http://dx.doi.org/10.5599/jese.650 231 j. electrochem. sci. eng. 9(4) (2019) 231-242; http://dx.doi.org/10.5599/jese.650 open access: issn 1847-9286 www.jese-online.org original scientific paper simultaneous determination of trace levels of cd(ii) and pb(ii) in tap water samples by anodic stripping voltammetry with 2-mercaptobenzothiazole modified electrode sophy phlay1, weena aemaeg tapachai2, supunnee duangthong1, puchong worattananurak1 and pipat chooto1, 1analytical chemistry division, department of chemistry, faculty of science, prince of songkla university, hatyai, songkhla, 90112, thailand 2inorganic chemistry division, department of chemistry, faculty of science, prince of songkla university, hatyai, songkhla, 90112, thailand corresponding author: pipat.c@psu.ac.th received: december 12, 2018; revised: may 7, 2019; accepted: may 9, 2019 abstract glassy carbon electrode (gce) modified by 2-mercaptobenzothiazole (mbt), mesoporous silica (meso) and bismuth was developed to determine cd(ii) and pb(ii) simultaneously by square wave anodic stripping voltammetry (swasv). in-situ preparation was found to work best in optimum conditions of acetate buffer ph 6, accumulation potential of -1.1 v, deposition time of 300 s and scan rate of 200 mv/s. sw peaks revealed the linear range of 5-50 µg/l cd(ii) and 5-50 µg/l pb(ii). lod and loq for cd(ii) and pb(ii) were determined as 0.56, 0.80, 1.87 and 2.66 µg/l, respectively. the interaction of metals with bismuth and mbt, as well as higher surface area due to mesoporous silica, support beneficial performance of the modified electrode. insignificant interferences from other regularly present metal ions were found. with srm1640 standard, the swasv results are found comparable to those obtained by inductive coupled plasma optical emission spectrometry (icp-oes). the method was used to analyze the metals in tap water by standard addition method with the satisfactory recovery of 100.7 % for cd(ii) and 100.8 % for pb(ii). keywords modified electrodes; 2-mercaptobenzothiazole; square wave anodic stripping voltammetry introduction among heavy metals with high potential in contamination of water for daily consumptions are cd(ii) and pb(ii). at present, the contamination problems from both metals are still found in a number of areas all over the world. the main source of cd(ii) is from discarded batteries, whereas http://dx.doi.org/10.5599/jese.650 http://dx.doi.org/10.5599/jese.650 http://www.jese-online.org/ mailto:pipat.c@psu.ac.th j. electrochem. sci. eng. 9(4) (2019) 231-242 determination of cd(ii) and pb(ii) traces 232 pb(ii) can contaminate water via dyes, paints, pipes and solders. it has been known that these metals can occur together and cause more damage especially to the brain [1,2]. the limits set by usepa are 0.005 mg/l for cd(ii) and 0.015 mg/l for pb(ii) [3]. the techniques which are cost-effective, fast, simple and sensitive are therefore required in testing water samples. even though there are a number of possibilities, a reference technique that can be applied for simultaneous determination of pb(ii) and cd(ii) in problematic areas around the world is still a matter of challenge. traditional methods with high sensitivity have usually been used in laboratory conditions for the detection of pb(ii) and cd(ii), such as uv–vis spectrophotometry [4], fluorescence method [5], ion-selective electrode [6], graphite furnace atomic absorption spectrometry [7], atomic emission spectroscopy [8,9] and inductively coupled plasma mass spectrometry [10]. however, these techniques have the drawbacks of being time-consuming, complex, expensive and not suitable for onsite analysis. with electrode modification by organic compounds, polymers and nanomaterials, electrochemical methods, especially anodic stripping voltammetry (asv), can provide some advantages including speed, accuracy, sensitivity, selectivity, reproducibility and stability [11]. boron doped diamond electrode (bdd) [12] was found to work well with pb(ii) and its modification with 4aminomethyl benzoic acid provided figures of merits for cd(ii) analysis [13]. for simultaneous determination of cd(ii) and pb(ii), the most widely investigated is the use of bismuth which forms alloys with both metals [14,15]. satisfactory lod values were especially obtained when bi was coupled with various materials such as carbon nanotubes in order to facilitate formation of larger surface areas. a variety of compounds such as graphene [16], co3o4 [17], polymers [18,19] and crown ether [20,21] were also found to be successful in pre-concentrating both metal ions. organic ligands with electron rich atoms including phytic acid oxygen [22,23], phenolic oxygen and nitrogen [24], cysteine sulfur [25,26] and lysine nitrogen [27] were found to implement better analytical performances for simultaneous determination of cd(ii) and pb(ii). mbt is an alternative ligand containing nitrogen and sulfur [28] which was used in the extraction of both metals before analysis by flame atomic absorption spectroscopy (faas) [29], as a biosensor for pesticides [30] and in forming polymeric structure to accommodate more complexing sites [31]. this paper reports an investigation of using mbt modified electrode for the first time to analyze pb(ii) and cd(ii) simultaneously by an asv technique. the method is optimized, verified and then applied to the real samples. experimental reagents and samples all reagents were used as received without any further treatment. cd(ii) and pb(ii) standards were prepared in-house from their nitrate salts as an atomic spectroscopy standard solution. tetrabutylammonium hexafluorophosphate with the purity of  98.0 % and bismuth (iii) nitrate pentahydrate were obtained from fluka, whereas 2-mercaptobenzothiazole and mesoporous silica were from sigma-aldrich. other reagents and metal salts for interference studies were of the highest purity commercially available. all subsequent solutions were prepared with deionized water (resistivity not less than 18 mω cm, elga water purification system, england) and purged by nitrogen gas (99.99 %) for 2 min before use. laboratory glassware was cleaned with 10 % (v/v) nitric acid solution and then rinsed with deionized water. cd(ii) and pb(ii) standard solutions were prepared by diluting the respective stock solutions with the high purity deionized water and stored in polyethylene bottles before use. sophy phlay et al. j. electrochem. sci. eng. 9(4) (2019) 231-242 http://dx.doi.org/10.5599/jese.650 233 nist (national institute of standards and technology) srm 1640 composed of natural fresh water from clear creek, colorado usa, was used as a reference with the certified value of 22.79 ± 0.96 µg/l cd(ii) and 27.89 ± 0.14 µg/l pb(ii). tap water samples were collected in different spots within the city of hatyai, songkhla, thailand, digested by mixing the aliquot of 500 ml with 2 ml of concentrated hno3 and 2 ml of kno3, put in cleaned polyethylene bottles and kept at 4 °c before analysis. instrumentation a powerlab 2/20 with potentiostat (adinstrument, australia) and echem software was used for cyclic voltammetry (cv) and square wave anodic stripping voltammetry (swasv), whereas metrohm autolab pgstat 302n with nova software was used for eis measurements. an ag/agcl, 3 m kcl reference electrode (metrohm), a platinum counter electrode (metrohm) and a modified glassy carbon electrode (windsor scientific ltd., uk) with an inner diameter of 3 mm put in 50 ml cell were used for electrochemical measurements. all voltages were reported versus ag/agcl reference electrode. ph was measured by ph meter model 510 (eutech instruments, usa) and inductively coupled plasma optical emission spectrometer (icp-oes) optima 4300 dv (perkin-elmer, usa) was used for the comparison of methods. preparation of modified electrode and starting procedure the modified electrodes with different compositions and different orders of mixing were tested to select the one with the best sensitivity enhancement. differential pulse (dp) and square wave (sw) results, as well as electrodes modified by in-situ and ex-situ bi deposition were also compared. the following starting procedure was designed for further optimization: three electrodes were put in 50 ml cell containing 50 ml of 0.1 m acetate buffer ph 6. then, 0.1 g/l cd(ii) and pb(ii), 200 g/l bi(iii), 1000 mg/l meso and 1000 mg/l mbt were added into solution. the metals were then deposited under the initial conditions of -1.4 v for 300 s with stirring. after the equilibration time of 30 s, swasv potential was scanned from -1.0 to -0.2 v with the scan rate of 250 mv/s, frequency of 50 hz, amplitude of 75 mv and potential step of 160 mv to obtain the stripping current signals for analysis. optimization was conducted by varying a number of parameters (vide infra) for the best analytical performance to be used in real sample analysis. results and discussion electrode modification and voltammograms of cd(ii) and pb(ii) a number of modification experiments has been conducted to figure out the most suitable materials and methods. ex-situ bi deposition was found to result in a lower current value and unsymmetrical peak shape. as shown in figure 1, the best sensitivity was obtained for in-situ modified gce in the order of bi + meso + mbt. the results reflect the role of meso in increasing surface areas and sites, and role of bi and mbt in alloy forming and complexation, respectively. within the potential range of -1.0 to -0.2 v applied after the starting procedure, two well defined stripping peaks at bi + meso + mbt modified electrode were observed at -0.72 v for cd(ii) and -0.58 v for pb(ii), and the enhancement of stripping currents can be clearly seen for both metal ions. this kind of electrode modification was therefore used for further optimization, verification and analysis. http://dx.doi.org/10.5599/jese.650 j. electrochem. sci. eng. 9(4) (2019) 231-242 determination of cd(ii) and pb(ii) traces 234 figure 1. comparison of stripping currents of 0.1 mg/l cd(ii) and pb(ii) at differently modified electrodes in conditions of 0.1 m acetate buffer ph 6, deposition potential -1.4 v, deposition time 300 s, equilibration time 30 s and concentrations of meso 1000 mg/l, mbt 1000 mg/l and bi 0.2 mg/l. cv and eis for electrode characterizations figure 2 shows the cyclic voltammetry (cv) results of differently modified gces in acetate buffer solution containing ru(nh3)63+ (a) and fe(cn)64(b). it is clear that each modification step of gce supports the electron transfer well for both inner and outer spheres by maintaining reversibility and increasing the current. figure 2. cyclic voltammetry for bare gce, bi/gce, meso-bi/gce and bi-meso-mbt/gce, with scan rate 100 mv/s in 0.1 m acetate buffer containing 2.5 mm ru(nh3)63+ (a) and fe(cn)64(b). sophy phlay et al. j. electrochem. sci. eng. 9(4) (2019) 231-242 http://dx.doi.org/10.5599/jese.650 235 the electron transfer capacities of modified electrodes were characterized by electrochemical impedance spectroscopy (eis). less curvature in an impedance spectrum is well known to represent less resistance to electron transfer. therefore, figure 3 reveals that addition of each modifying agent facilitates the electron transfer exhibiting less resistance, and the effect is highest after addition of mbt. z‛ /  figure 3. impedance spectra of the bare gce, bi/gce, bi-meso/gce, bi-meso-mbt/gce. optimization comparison of swasv and dpasv swasv and dpasv parameters are summarized in table 1. comparison of corresponding stripping currents is presented in figure 4, showing higher current enhancement in the case of swasv. hence, swasv technique is selected for the following experiments. table 1. swasv and dpasv parameters parameter swasv dpasv accumulation step deposition potential deposition time equilibration time measuring step frequency step potential amplitude -1000 mv 400 s 30 s 50 hz 160 mv 75 mv -1000 mv 800 s 30 s 100 mv 75 mv figure 4. comparison of stripping currents of 0.1 mg/l cd(ii) and pb(ii) from swasv and dpasv. -z ‛‛ /  http://dx.doi.org/10.5599/jese.650 j. electrochem. sci. eng. 9(4) (2019) 231-242 determination of cd(ii) and pb(ii) traces 236 effect of ph within the experimental ph range of 1 to 7, the maximum peak current for both metals was obtained at ph 6 as shown in figure 5(a) and this value was used for the next investigations. ph 6 is suitable for the formation of sulfide anions of mbt to form complexes with metal ions corresponding with pka of 6.93 [32]. if ph is lower than 6, protonation causes the formation of sulfhydryl groups which make complex formation more difficult. at ph higher than 6, the metals are probably susceptible to form hydroxides. deposition potential the influence of deposition potentials was investigated over the potential range of -0.1 to -1.5 v. as shown in figure 5(b), the current firstly increased steeply up to -1.1 v and then became almost constant. this is due to the greater extent of metal accumulation until the potential was high enough for deposition of both metals. beyond -1.4 v, the current started to drop, possibly because greater thickness slows down the mass transfer and higher negative potential is susceptible to side reactions. the highest current for both metals is found at -1.1 v which was fixed for metal electrodeposition for the following study. deposition time the deposition time was varied from 100 to 500 s. as shown in figure 5(c), the current values gradually increased with time up to 300 s because greater accumulation of bismuth facilitated formation of alloys until surface saturation was reached. however, the current dropped before it went up again, reflecting that certain time is needed for alloy rearrangement. the highest peak current was found at 300 s and this was used for further optimizations and applications. effect of bi concentration the concentration of bi was varied from 100 to 500 g/l. figure 5(d) reveals firstly a normal trend of increasing current which is due to the increase of film thickness and then the current decreased because greater thickness can inhibit the mass transfer during the stripping step [33]. the concentration of 200 g/l provided the greatest peak current for both metals and was therefore selected for the following experiments. effect of meso concentration as shown in figure 5(e), similar trend is obtained when meso concentrations were changed from 100 to 600 µg/l. similar to previous explanation of the effect of bi concentration, the excess of meso can result in less current due to the obstruction of mass transfer. the concentration of 300 µg/l showing the greatest current was chosen for further investigations. effect of mbt concentration as shown in figure 5(f), within the studied range of 100 to 600 µg/l, the stripping current increased with mbt concentration up to 100 µg/l and then decreased for both metals. this is due to the fact that high concentration of mbt could block the mass transfer of metal ions at electrodeposition sites. the mbt concentration of 200 µg/l was therefore selected for further experiments. scan rate the stripping scan rate was changed from 50 to 300 mv/s. the stripping peak height was found to increase with the scan rate from 50 to 250 mv/s as shown in figure 6. under the criteria of peak shapes, 250 mv/s is chosen for the further study. for equilibration time, step potential and pulse amplitude, the optimization value based on the greater current was selected. sophy phlay et al. j. electrochem. sci. eng. 9(4) (2019) 231-242 http://dx.doi.org/10.5599/jese.650 237 figure 5. effects of ph (a), deposition potential (b), deposition time (c), bi concentration (d), meso concentration (e) and mbt concentration (f) on stripping peak currents of 20 μg/l cd(ii) and pb(ii). all aforementioned optimized parameters are summarized in table 2 and used in further experiments. table 2. summary of optimized operating conditions parameter studied range optimum value deposition potential, v deposition time, s ph bi concentration, µg/l mbt concentration, µg/l meso concentration, µg/l equilibration time, s step potential, mv pulse amplitude, mv scan rate, mv / s -0.1 – -1.5 100 – 500 3 – 7 100 – 500 100 – 600 100 – 600 10 – 50 1 – 20 25 – 80 50 – 300 -1.1 300 6 200 200 300 30 5 75 250 http://dx.doi.org/10.5599/jese.650 j. electrochem. sci. eng. 9(4) (2019) 231-242 determination of cd(ii) and pb(ii) traces 238 figure 6. effect of scan rate on stripping currents for concentrations of pb(ii) and cd(ii) 0.1 mg/l, bi 200 µg/l, mbt 200 µg/l, meso 300 µg/l. analytical performance swasv was used for simultaneous determination of cd(ii) and pb(ii) with the modified electrode bi-meso-mbt/gce performed under optimized conditions to obtain current signals. the results for certain typical concentrations of cd(ii) and pb(ii) are shown in figure 7 and the corresponding calibration curves are presented in figure 8. the linearity in the range of 5-50 µg/l is observed for both metals with the correlation coefficient of 0.9978 for cd(ii) and 0.9960 for pb(ii), respectively. the linear regression equations of ip = 0.0142x + 0.0372 (ip: µa, x: µg/l) for cd(ii) and ip = 0.0113x 0.0699 for pb(ii) are defined. the limits of detection are found to be 0.56 µg/l for cd(ii) and 0.80 µg/l for pb(ii) by 3n/m, where n is the standard deviation of replicate (n=10) responses of 5 µg/l of both metals taken as blank and m is the slope of the calibration curve. the limits of quantification, loq, defined as 10 n/m, are determined as 1.87 µg/l for cd(ii) and 1.66 µg/l for pb(ii). the relative standard deviations were 2.97 % for cd(ii) and 2.04 % for pb(ii) with repetitive determinations (n=10) of 20.0 µg/l. all prove that the proposed method is satisfactorily reproducible and reliable for simultaneous determination of cd(ii) and pb(ii) at trace level and can be applied to real samples. figure 7. typical swasv voltammograms of water samples after spiking with 5 (as a blank) to 50 µg/l of both cd(ii) and pb(ii) standard solutions. conditions: accumulation potential -1.1 v, accumulation time 300 s, acetate buffer solution ph 6, and scan rate 250 mv/s. sophy phlay et al. j. electrochem. sci. eng. 9(4) (2019) 231-242 http://dx.doi.org/10.5599/jese.650 239 figure 8. calibration curves for cd(ii) and pb(ii) obtained by the proposed method. effect of other ions due to the capability of mbt to coordinate with a number of metal ions, the level of interference both from each other and other metal ions was investigated by the developed method under optimized conditions and the results are shown in table 3. with the increase of pb(ii) concentration, no significant interference was observed for cd(ii) peak current. other ions including ca(ii), mg(ii), zn(ii), mn(ii), fe(ii), cu(ii) and al(iii) at 1000 µg/l were found to provide not high contributions for both metals. the most interfering ions here if present at high concentration are cu(ii) and co(ii) which normally can be masked by using a suitable and effective complexing agent such as ferrocyanide before analysis. table 3. interference study of the stripping current measurements of 20 μg/l cd(ii) and pb(ii) at bi/meso-mbt/gce in the absence and presence of interfering metal ions interference contribution, %a (ip (cd(ii)) = 100 %) contribution, %a (ip (pb(ii)) = 100 %) cu(ii) zn(ii) mg(ii) ca(ii) al(iii) mn(ii) co(ii) fe(iii) ni(ii) -47.17 -16.68 37.49 0.48 2.34 13.36 -34.68 5.29 -4.37 -21.78 -13.44 14.98 -3.68 -7.63 0.65 -40.14 7.51 -5.87 acontribution = [(ip with interferent ip without interferent) / ip without interferent]  100 certified reference material determination, method comparison and real sample analysis the proposed method was applied to a certified reference material, natural water srm 1640 from the national institute of standards and technology (nist), usa. as shown in table 4, satisfactory recoveries, of 98.02 % for cd(ii) and 97.74 % for pb(ii) were obtained. the proposed method was then used in the analysis of cd(ii) and pb(ii) in tap water samples collected from 11 sites in hatyai city. typical results of 5th and 6th regions are compared with icpoes results and summarized in table 5, reflecting good agreement between here proposed and standard methods. http://dx.doi.org/10.5599/jese.650 j. electrochem. sci. eng. 9(4) (2019) 231-242 determination of cd(ii) and pb(ii) traces 240 table 4. recovery of cd(ii) and pb(ii) for certified reference material determination concentration, µg/l error, % recovery, % certified determineda cd(ii) pb(ii) cd(ii) pb(ii) cd(ii) pb(ii) cd(ii) pb(ii) 22.79±0.96 27.89±0.14 22.34±0.010 (rsd = 2.97 %) 27.26±0.005 (rsd = 2.04 %) 1.97 2.25 98.02 97.74 a mean ± standard deviation (n = 5) table 5. determination of cd(ii) and pb(ii) in tap water samples (n = 4) sample concentration of spiked solution, µg/l concentration from the proposed method, µg/lb recovery, % concentration from icp-oes, µg/lb difference, %d cd(ii) pb(ii) cd(ii) pb(ii) cd(ii) pb(ii) cd(ii) pb(ii) tap water 5a 0 ndc nd nd nd nd nd 5 5.11±0.003 5.12±0.005 5.10±0.1 5.30±0.12 1.00 3.45 10 10.19±0.010 10.20±0.007 100.8 100.8 10.15±0.12 10.50±0.20 0.4 2.9 20 20.40±0.012 20.35±0.010 101.4 101.1 20.35±0.021 20.50±0.25 0.25 0.7 30 30.50±0.022 30.47±0.016 101.3 101.2 30.40±0.023 30.60±0.19 0.33 0.42 tap water 6a 0 nd nd nd nd nd nd 5 5.16±0.004 5.15±0.006 5.10±0.02 5.03±0.400 1.17 2.36 10 10.23±0.006 10.22±0.01 100.7 100.7 10.11±0.100 10.5±0.120 1.18 2.70 20 20.36±0.012 20.33±0.01 101 100.9 20.01±0.002 20.04±0.05 0.15 0.14 30 30.5±0.012 30.40±0.01 101.1 100.8 30.40±0.020 30.20±0.06 0.32 0.66 awater sample from 5th and 6th regions were selected for standard addition test; bmean ± standard deviation (n = 4); cnot detected; ddifference of the concentration from the proposed method and that from icp-oes for real sample analysis with water from the 1st, 5th and 6th regions used as typical samples, the recoveries values were found to be 100.7-101.4 % for cd(ii) and 100.8-101.2 % for pb(ii) as shown in table 6. table 6. recovery test for the proposed method using tap water samples (n = 3) sample concentration of spiked solution, µg/l concentration found, µg/l recovery, % tap water 1st region cd(ii) pb(ii) cd(ii) pb(ii) 0 nd nd 5 5.15±0.07 5.19±0.067 10 10.23±0.002 10.25±0.008 100.8 100.6 20 20.31±0.015 20.27±0.011 100.8 100.4 30 30.35±0.011 30.36±0.015 100.4 100.5 tap water 5th region 0 nd nd 5 5.11±0.003 5.12±0.005 10 10.19±0.01 10.20±0.007 100.8 100.8 20 20.40±0.012 20.35±0.010 101.4 101.1 30 30.50±0.022 30.47±0.016 101.3 101.2 tap water 6th region 0 nd nd 5 5.16±0.004 5.15±0.006 10 10.23±0.006 10.22±0.01 100.7 100.7 20 20.36±0.012 20.33±0.01 101.0 100.9 30 30.50±0.012 30.40±0.01 101.1 100.8 mean ± s.d. (n = 3); nd: not detected as shown in table 7, the results obtained with here proposed method are comparable with the results obtained by other anodic stripping voltammetric based methods. it is clear, however, that sophy phlay et al. j. electrochem. sci. eng. 9(4) (2019) 231-242 http://dx.doi.org/10.5599/jese.650 241 here proposed method has the advantage of wider linear range, lower detection limits and greater simplicity. the only disadvantage could be a little longer deposition time, which can be adjusted according to the required accuracy. other very recent methods without using bismuth are also included (entry 1-6) as references to show that here proposed method is reasonably satisfactory. table 7. comparison of the proposed method for determination of cd(ii) and pb(ii) in water sample with other recent anodic stripping voltammetric methods entry electrodes method deposition time, s linear range of concentration, µg/l lod, µg/l ref cd(ii) pb(ii) cd(ii) pb(ii) 1 hmgfe-edg/g swasv 180 11.2 – 207 11.2 – 207 1.22 0.304 [34] 2 st pani nts swasv 600 0.207 – 24.84 1.12 – 19.04 0.02 0.03 [35] 3 ca/rgo/gce swasv 1500 0.0207 – 2.07 0.112 – 13.44 0.004 0.002 [36] 4 nafion/cls/pgr/gce swasv 140 10.35 – 1035 5.60 – 560 2.06 0.336 [37] 5 go/k-car/l-cys/gce swasv 120 1.03 – 10.35 0.56 – 5.60 0.12 0.12 [38] 6 ncqds-go dpasv 300 10.35 – 20.7 0.112 – 11200 1.16 7.4 [39] 7 in situ bi/graphite/epoxy swasv 120 41.4 – 352 56 – 280 14.5 5.60 [40] 8 bi-meso-mbt/gce swasv 300 5 50 5 – 50 0.56 0.80 this work hmgfe-edh/g: hierarchical mgfe-layered double hydroxide microsphere graphene composite st pani nts: size-tunable polyaniline nanotube-modified electrode ca/rgo/gce: calixarene functionalized reduced graphene oxide cls/pgr: calcium lignosulphonate functionalized porous graphene nanocomposite go/-car/l-cys/gce: graphene oxide -carrageenan l-cysteine nanocomposite ncqds-go: n-doped carbon quantum dots graphene oxide hybrid in situ bi/graphite/epoxy: in situ bismuth film on graphite dispersed in epoxy resin bi-meso-mbt/gce: bismuth mesoporous silica 2-mercaptobenzothiazole modified gce conclusions gce was modified with bi, meso and mbt and applied to simultaneous determination of cd(ii) and pb(ii) in trace levels, using square wave anodic stripping voltammetry (swasv). the method provides satisfactory advantages of speed, simplicity, sensitivity and selectivity. good recoveries of cd(ii) (100.7 %) and pb(ii) (100.8 %) were obtained for the real sample, along with lod of 0.56 µg/l for cd(ii) and 0.80 µg/l for pb(ii), respectively. the method can be used satisfactorily for the analysis of tap water in local areas and the results were found in good agreement with those obtained by icp-oes. references [1] v. karri, m. schuhmacher, v. kumar, environmental toxicology and pharmacology 48 (2015) 203213. 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[40] k. pokpas, s. zbeda, n. jahed, n. mohamed, p. g. baker, e. i. iwuoha, international journal of electrochemical science 9 (2014) 736-759. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) file:///c:/users/userx/appdata/local/temp/chemicalland21.com http://chemicalland21.com/specialtychem/perchem/2-mercaptobenzothiazole.htm http://chemicalland21.com/specialtychem/perchem/2-mercaptobenzothiazole.htm http://creativecommons.org/licenses/by/4.0/) joint event: 8th regional symposium on electrochemistry of south-east europe (rse-see 8) and 9th kurt schwabe symposium http://dx.doi.org/10.5599/jese.1989 713 j. electrochem. sci. eng. 13(5) (2023) 713-714; http://dx.doi.org/10.5599/jese.1989 open access : : issn 1847-9286 www.jese-online.org editorial joint event: 8th regional symposium on electrochemistry of south-east europe (rse-see 8) and 9th kurt schwabe symposium bernhard gollas1, and viktor hacker2 1institute for chemistry and technology of materials, graz university of technology, stremayrgasse 9, 8010 graz, austria 2institute of chemical engineering and environmental technology, graz university of technology, inffeldgasse 25/c, 8010 graz, austria corresponding author: bernhard.gollas@tugraz.at received: july 18, 2023; accepted: july 20, 2023; published: august 9, 2023 after a one-year delay caused by the covid-19 pandemic, the 8th regional symposium on electrochemistry of south-east europe was held jointly with the 9th kurt schwabe symposium from july 11-15, 2022 at graz university of technology in austria. this special edition of the jese contains a collection of articles presented at this meeting. the 5-day event (including monday’s satellite student symposium) organized by the association of south-east european electrochemists (aseee) featured 5 plenaries, 15 keynotes, 71 contributed talks and 38 posters and was attended by 152 scientists and researchers from 23 countries. the scientific program is available in the book of abstracts and consisted of the following sessions: • batteries & electrochemical energy storage • electrochemical energy conversion & catalysis • electrochemical processing & engineering • corrosion science & corrosion protection • organic electrochemistry & bioelectrochemistry • electroanalytical chemistry & sensors • general topics & physical electrochemistry • kurt schwabe symposium the chairs of this symposium placed special emphasis on the participation of young scientists from southeastern europe and more than 2 years of online-only conferences, this opportunity for a face-to-face meeting was gladly accepted. it had already become clear during the previous events in this symposium series how important personal professional exchange in this region is for the progress of the traditionally strong electrochemistry in southeastern europe. against the background of the current difficult situation, the participation of a researcher from the ukraine can also be considered a success. http://dx.doi.org/10.5599/jese.1989 http://dx.doi.org/10.5599/jese.1989 http://www.jese-online.org/ mailto:bernhard.gollas@tugraz.at https://www.aseee.eu/ https://openlib.tugraz.at/download.php?id=62cbd9c17e110&location=browse https://openlib.tugraz.at/download.php?id=62cbd9c17e110&location=browse j. electrochem. sci. eng. 13(5) (2023) 713-714 rse-see 8 and 9th kurt schwabe symposium 714 rse-see 8 dinner party in the nikola tesla laboratory of graz university of technology. as guest editors of this special issue, we would like to thank everyone involved for their contributions and support. this applies in particular to the authors and reviewers of the articles, whose interaction is essential for the professional quality. we would be pleased, if the readers of the jese enjoy the articles in this special issue and we hope, if they offer both insights and stimuli for discussion. ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://creativecommons.org/licenses/by/4.0/) high-velocity air fuel coatings for steel for erosion-resistant applications http://dx.doi.org/10.5599/jese.1369 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1369 open access : : issn 1847-9286 www.jese-online.orghttp://www.jese-online.org/ review paper high-velocity air fuel coatings for steel for erosion-resistant applications yasemin yıldıran avcu1, mert guney2, and egemen avcu1, 1department of mechanical engineering, natural and applied sciences, kocaeli university, kocaeli 41001, turkey 2school of engineering and digital sciences, department of civil and environmental engineering, the environment & resource efficiency cluster (erec), nazarbayev university, nur-sultan 010000, kazakhstan corresponding author: mert.guney@nu.edu.kz; avcuegemen@gmail.com received: april 30, 2022; accepted: july 1, 2022; published: july 28, 2022 abstract high-velocity air fuel (hvaf) coating processes have advantages over conventional highvelocity oxygen fuel (hvof) processes, resulting in coatings with superior properties. the present review first provides a concise overview of hvaf coatings, highlighting their advantages over hvof coatings. then, the fundamentals of solid particle, slurry, and cavitation erosion are briefly introduced. finally, the performance of hvaf coatings for erosion-resistant applications is discussed in detail. the emerging research consistently reports hvaf-coatings having higher erosion resistance than hvof-coatings, which is attributed to their elevated hardness and density and improved microstructural features that inhibit the surface damages caused by erosion. the dominant wear mechanisms are mainly functions of particle impact angle. for instance, the removal of the binder phase at high impact angles causes the accumulation of plastic strain on hard particles (e.g., wc particles) in the matrix, forming micro-cracks between the hard particles and the matrix, eventually decreasing the erosion resistance of hvaf coatings. the binder phase of hvafcoatings significantly affects erosion resistance, primarily due to their inherent mechanical properties and bearing capacity of hard particles. optimizing spraying parameters to tailor the microstructural characteristics of these coatings appears to be the key to enhancing their erosion resistance. the relationship between microstructural features and erosion mechanisms needs to be clarified to process coatings with tailored microstructural features for erosion-resistant applications. keywords cavitation; high-velocity oxygen fuel; slurry erosion; solid particle erosion; thermal barrier coatings; hvof; hvaf http://dx.doi.org/10.5599/jese.1369 http://dx.doi.org/10.5599/jese.1369 http://www.jese-online.org/ http://www.jese-online.org/ mailto:mert.guney@nu.edu.kz mailto:avcuegemen@gmail.com j. electrochem. sci. eng. 00(0) (2022) 000-000 high-velocity air fuel coatings for steel 2 brief introduction to hvaf coatings high-velocity air fuel (hvaf) is a relatively new thermal spray process compared to more established ones like high-velocity oxygen fuel (hvof). hvaf has recently attracted interest for applications involving the deposition of wear-resistant alloys and cermet coatings [1]. cermet-, iron-, and nickel-based coatings have primarily been deposited on substrates (the majority of which are steel) for solid particle erosion-, slurry erosion-, and cavitation erosion-resistant applications such as hydro turbines, circulating fluidized bed combustors, and boilers. the hvaf process (figure 1) involves several steps: first, a gaseous fuel (propane, propylene, or natural gas) and compressed air are mixed in a premixing chamber and then ignited at the front of a ceramic plate. secondly, the reaction in the combustion chamber produces high-temperature and high-pressure gas, heating and accelerating the powder injected into the process via n2 gas. lastly, the gas flow is further accelerated to supersonic speeds through a divergent nozzle [2], depositing molten and semi-molten powder particles onto the substrate and providing a protective coating [3]. figure 1. schematic presentation of the hvaf process. reprinted with permission from reference [2]. copyright (©) 2019 springer nature. numerous studies so far have reported the advantages of hvaf over hvof, such as higher deposition rate and efficiency [1], higher jet velocity [4,5], and lower spray/flame temperature [4,5]. using compressed air instead of o2 helps to reduce flame and particle temperatures [6], assisting in maintaining raw material properties via inhibiting fine particle oxidation during spraying [4]. furthermore, particles can reach higher velocities in hvaf compared to hvof (up to 1,000 m/s), resulting in the deposition of very dense coatings [6,7]. consequently, dense, highly cohesive coatings with low porosity, and superior wear performance can be deposited via hvaf [4,6,8]. therefore, hvaf-coatings tend to exhibit higher mechanical properties (i.e., fracture toughness, elasticity modulus, hardness, and compressive strength) than hvof-coatings [5,9] due to the abovementioned advantages. as hvaf-coatings have received increased attention in parallel to the development of thermal spray processes utilizing lower particle temperatures and higher particle velocities [10], there is an expanding effort to develop erosion-resistant hvaf-coatings, which comprises the subject of the present review. here, the development of erosion-resistant hvaf-coatings in terms of their erosion properties is evaluated and discussed, while their microstructural and mechanical properties are briefly presented. brief introduction to erosion erosion could be defined as the progressive material removal from a target surface due to the repeated impact of solid particles carried by gaseous or liquid media [11,12]. it can be utilized to tailor surface properties (e.g., grit and sandblasting [13]) or machine engineering materials (e.g., y. y. avcu et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1369 3 abrasive water jet cutting [14]) [15]. solid particle and slurry erosion are also important industrial problems that cause wear, roughening, and degradation of engineering components, which may eventually lead to their premature failure in engineering applications [15,16]. various engineering applications, including aerospace (e.g., steam and jet turbine components, helicopter rotor blades) and power plant applications (e.g., fluidized bed combustion systems, pipelines and valves carrying particulate matter in mining and oil transportation [17]) are typically vulnerable to erosion [15,16]. any erosion-related failure of engineering components in these applications may result in safety concerns, financial overburden due to expensive repairing costs and production outages, and possible harmful environmental effects such as oil leakage [17]. therefore, the erosion behavior of materials used in these applications must be clearly understood to improve their longevity and prevent erosion-related failures. recently, there has been an interest in understanding the erosion behavior of newly developed erosion-resistant coatings, which is usually a complex phenomenon. the present section briefly explains the type of erosion that occurs on hvaf-coatings. it also describes the fundamentals of erosion along with the methods that are used to investigate the erosion behavior of materials. the literature generally classifies erosion under four different groups/systems: (1) solid particle erosion, (2) slurry erosion, (3) cavitation erosion, and (4) water droplet erosion (figure 2). in solid particle erosion, wear occurs due to the repeated impact of solid particles carried and accelerated by pressurized air/gas, whereas in slurry erosion, particles are carried and accelerated by a liquid medium. slurry erosion can be carried out via two distinct methods/devices: jet type and rotating type equipment. in cavitation erosion, wear occurs due to the repeated impact of cavitation bubbles under high-turbulent fluid flow [18]. finally, water droplet erosion (not covered in the present review) occurs via progressive material removal from a surface due to the repeated impact of droplets [5]. figure 2. schematics of different erosion test rigs. a) solid particle erosion. reprinted with permission from reference [19], © 2015 sage publications. b) slurry erosion (rotating type). reprinted with permission from reference [20], © 2015 elsevier. c) water droplet erosion. reprinted with permission from reference [21], © 2015 elsevier. d) cavitation erosion. reprinted with permission from reference [18], © 2016 springer nature http://dx.doi.org/10.5599/jese.1369 j. electrochem. sci. eng. 00(0) (2022) 000-000 high-velocity air fuel coatings for steel 4 the wear mechanisms causing erosion damage differ as a function of numerous parameters, such as particle properties, target material, and impingement conditions. after performing the erosion tests described above, the erosion rates, surface topography, morphology, hardness, and surface and cross-sectional microstructures of eroded samples could be examined via scanning electron microscopy, non-contact profilometry, and microhardness testing. the emerging literature on the solid particle, slurry, and cavitation erosion of hvaf-coatings is discussed below. hvaf coatings for erosion-resistant applications hvaf coatings against solid particle erosion table 1 summarizes studies on understanding solid particle erosion behavior of hvaf-coatings along with utilized hvaf and solid particle erosion parameters. matthews et al. [22] reported the first study revealing the solid particle erosion behavior of hvafand hvof-sprayed carbide-based coatings (cr3c2–25nicr) using an air jet erosion rig and sem (table 1). hvaf-coatings exhibited better erosion resistance than hvof-coatings, which was attributed to the variation of erosion mechanisms depending on the coatings microstructure. similarly, hvaf-sprayed fecrnimobsic amorphous coatings showed higher erosion resistance than hvof-sprayed, which was attributed to the higher hardness of hvaf-coatings compared to hvof (956 ± 56 hv0.1 vs. 821 ± 72 hv0.1) [23]. however, baiamonte et al. [24] reported similar erosion behavior of hvaf-sprayed carbide-based cermet coatings and hvof-sprayed coatings in terms of erosion rates, volume loss, and observed erosion mechanisms as a function of impingement angle. table 1. hvaf coatings developed against solid particle erosion reference coating hvaf parameters solid particle erosion parameters [22] cr3c2-25nicr substrate: mild steel fuel: kerosene fuel consumption: 23 l / h oxidant: o2 feeding gas: nitrogen powder feed rate: 35 g / min air jet erosion rig (astm g76) erodent: alumina erodent diameter: 20 µm impact angle: 90 o erodent velocity: 150 m / s standoff distance: 25 mm [19] wc-10co-4cr substrate: 0cr13ni5mo fuel: propane combustion gas: o2 feeding gas: n2 powder feed rate: 50-150 g / min coating thickness: 180-450 µm air jet erosion rig (astm g76-83) erodent: alumina erodent diameter: 48-58 µm erodent mass feed rate: 5 g / min impact angle: 15 o, 45 o, 75 o, 90 o erodent velocity: 90 m / s standoff distance: 10 mm [25] wc-10co-4cr substrate: stainless steel fuel: propane combustion gas: o2 feeding gas: n2 powder feed rate: 50-150 g/min coating thickness: 180-450 µm air jet erosion rig (astm g76-83) erodent: alumina erodent diameter: 48-58 µm erodent mass feed rate: 5 g / min impact angle: 15 o, 90 o erodent velocity: 30 m / s standoff distance: 10 mm [26] 86wc-10cr-4co substrate: ca6nm hydro turbine steel standoff distance: 300 mm feed rate: 200 g / min deposition rate: 20 µm / pass coating thickness: 341 µm air jet erosion rig (astm g76) erodent: alumina average size: 50 µm impact angle: 30˚, 60˚, 90˚ time: 10 min erodent discharge: 2 g / min erodent velocity: 35, 70 m / s nozzle diameter: 1.5 mm standoff distance: 10 mm y. y. avcu et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1369 5 reference coating hvaf parameters solid particle erosion parameters [23] fecrnimobsic substrate: low alloyed mo steel fuel: propane combustion gas: o2 feeding gas: n2 powder feed rate: 100 g / min coating thickness: 300 µm air jet erosion rig (astm g76-07) air pressure: 10 kpa erodent: alumina erodent diameter: 50 µm erodent feed rate: 5 g / min impact angle: 90˚ erodent velocity: 40 m / s standoff distance: 10 mm [24] cr3c2-10nicr cr3c2-feni cr3c2-wc-ma substrate: carbon steel, nozzle diameter:15 mm, air: 690 kpa fuel: 690 kpa powder feed rate: 100-120 g / min coating thickness: 180-200 µm air jet erosion rig (astm g76) erodent: alumina (d50=50) angle: 30 o, 90o time: 10 min erodent feed rate: 5 g / min erodent velocity: 30 m / s [6] wc-nimocrfeco, wc-fenicrmocu, wc-fecral substrate: high-strength low-alloy steel fuel: propane feeding gas: nitrogen powder feed rate: 200 g / min coating thickness: 250 µm air jet erosion rig (astm g76) erodent: alumina erodent diameter: 50 µm erodent feed rate: 2 g / min impact angle: 90˚ erodent velocity: 30-70 m / s standoff distance: 10 mm li et al. [19] showed that erosion and deformation mechanisms (e.g., crack formation and propagation) of hvaf-sprayed wc-co-cr coatings were strongly related to the coating microstructure. the variation of particle impingement angle affected the dominant erosion mechanism: microcutting was active at low impingement angles (<30°), whereas spallation was dominant at higher angles (<75°). lian et al. [25] presented similar results where the erosion mechanisms were highly dependent on the microstructure of hvaf-sprayed wc-co-cr coatings. briefly, coatings with a denser and more uniform microstructure have exhibited higher erosion resistance. the removal of co-cr binder was dominant at lower impingement angles, whereas severe plastic deformation and fracturing occurred at higher angles, causing an increased erosion rate at 90° impingement angle compared to those at lower angles. hamilton et al. [26] schematically presented the material removal mechanisms at 90° impingement angle and showed an increased erosion rate at normal impact angles. briefly, the removal of the binder phase at high impact angles causes the accumulation of plastic strain on wc particles in the matrix, resulting in the formation of micro-cracks between the wc particles and the matrix (figure 3). then, loose wc particles could easily be removed from the surface due to erodent particle impact. figure 3. schematic of material removal mechanisms at high impingement angles during solid particle erosion of hvaf-coatings. reprinted with permission from reference [26] © 2017 world scientific publishing http://dx.doi.org/10.5599/jese.1369 j. electrochem. sci. eng. 00(0) (2022) 000-000 high-velocity air fuel coatings for steel 6 torkashvand et al. [6] also studied the solid particle erosion behavior of hvaf-sprayed wc-based coatings with binders with no or limited content of co. the eroded coatings (wc-nimocrfeco, wcfenicrmocu, and wc-fecral) exhibited comparable wear mechanisms; indentation, ploughing, and shoveling wear mechanisms are depicted in figure 4(a) and subsurface cracks and chipping mechanisms are shown in figure 4(b). due to a combination of deep ductile grooving (cutting, ploughing, and indentation (figure 4(d)) and subsurface brittle fracture, severe erosive wear occurred for the wc-nimocrfeco coating compared to other coatings. to sum up, the replacement of co binders with environment-friendly binders appears promising for preserving the erosion resistance of hvaf-coatings [6] (figure 4(c)). figure 4. a) surface morphology after erosion, b) cross-sectional morphology after erosion tests, c) mass loss and erosion rate of the coatings, d) material removal mechanisms. reprinted with permission from reference [6], © 2022 elsevier hvaf coatings against slurry erosion table 2 summarizes studies on understanding the slurry erosion behavior of hvaf-coatings along with utilized hvaf and slurry erosion parameters. liu et al. [27] examined the effect of nano-wcy. y. avcu et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1369 7 12co powder reinforcement in hvaf-sprayed wc-10co-4cr coatings and showed that powder reinforcement improved erosion resistance and hardness compared to that of unreinforced coating, suggesting the use of nano-wc-12co powder rather than micro-sized wc particles. wang et al. [28] deposited wc-10co-4cr coatings on low-carbon steel substrates using hvof and hvaf and then examined their slurry erosion resistance, aiming to develop erosion-resistant coatings for hydroturbine components. it was shown that hvaf-coatings exhibited better erosion performance along with higher hardness and fracture toughness. micro-cutting and micro-ploughing were the primary erosion mechanisms at low impingement angles, whereas micro-cutting and micro-chiseling were active at high impingement angles, causing the removal of co-cr binder and pulling of wc particles within the microstructure. besides, erodent particle size also affects the dominant erosion mechanisms. figure 5 schematically depicts the active erosion mechanisms as a function of erodent size. table 2. hvaf coatings developed against slurry erosion. reference coating hvaf parameters slurry erosion parameters [27] nano-wc-12co powder addition in wc-10co-4cr substrate: aisi 304 stainless steel fuel gas: propane carrier gas: nitrogen standoff distance: 150 mm powder feed rate: 60 g / min coating thickness: 400 µm jet-type slurry erosion rig (astm g-73) erodent: quartz erodent size: -400 to +650 µm standoff distance: 35 mm erodent concentration: 300 g 10 l water velocity: 46 m / s slurry flow: 468 l / min impact angle: 30o [29] wc-12co, crc-25nicr substrate: stainless steel matrix 0cr13ni5mo coating thickness: 350-400 µm rotating type slurry erosion rig (silt erosion) circular velocity: 15.5 m / s silt concentration: 14 kg / l (ds50=38 µm) [28] wc-10co-4cr substrate: low carbon steel fuel gas: propane standoff distance: 150 mm powder feed rate: 80 g / min coating thickness: 300 µm jet-type slurry erosion rig (custom-made) erodent: sio2 erodent size: 40-70 µm concentration: 1:10 (sand/water) standoff distance: 25 mm feed rate: 150 g / min impact angle: 30o, 90o pressure: 0.2 mpa duration: 720 min [30] fecrmomnwbcsi substrate: 316l ss fuel pressure: 70 psi air pressure: 82 psi standoff distance: 180 mm powder feed rate: 3 g / min coating thickness: 250 µm jet-type erosion testing rig erodent: silica erodent size: 75-150 µm concentration: water +2 wt.% silica jet velocity: 20 m / s nozzle diameter: 3 mm standoff distance: 5 mm [31] wc-10co-4cr substrate: ss410 fuel gas: propane 83.8 psig standoff distance: 7 in spray particle velocity: 1010, 960, 895 m / s deposit thickness per pass: 28 µm jet-type erosion testing rig impact angle: 15˚, 30˚, 45˚, 60˚, 90˚ jet velocity: 20, 27, 36 m / s slurry concentration: 2 wt.% [32] wc-10co-4cr cr3c2-25nicr substrate: low carbon steel rotating type slurry erosion rig: (1) water+33 wt.% 0.1-0.6 mm quarts (2) water+33 wt.% 2-3 mm quarts time: 4×20 min erodent velocity: 16 m/s centrifugal erosion tester erodent: 0.1-0.6 mm quartz erodent velocity: 80 m / s http://dx.doi.org/10.5599/jese.1369 j. electrochem. sci. eng. 00(0) (2022) 000-000 high-velocity air fuel coatings for steel 8 figure 5. slurry erosion mechanism of carbide coatings as a function of erodent particle size a) fine quartz particles (0.1–0.6 mm) and b) coarse quartz particles (2–3mm): 1) removal of the soft metal matrix via selective erosion, 2) removal of weak coating area near-surface, 3) removal of the coating with low carbide content via selective erosion, 4) wear of weak splat interface, 5) micro-ploughing of coating by large particle, 6) lip formation due to plastic deformation, 7) fractured quartz fragment embedment and 8) carbide cracking. reprinted with permission from reference [33], © 2019 elsevier. kumar et al. [31] presented similar results, where the erosion rate of hvaf-sprayed wc-cocr was 2.26 times higher than hvof-coatings due to improved hardness, adhesion, and density of the hvaf-coatings. the erosion mechanisms were micro-cutting and micro-ploughing at low impingement angles and brittle cracking at high angles, in line with the results of wang et al. [28]. similar findings were reported in another study [30], where hvaf-sprayed fecrmomnwbcsi amorphous metallic coatings exhibited better erosion resistance than hvof-coatings, which is attributed to superior microstructural and mechanical features of hvaf-coatings. matikainen et al. [33] compared the slurry erosion behavior of hvafand hvof-sprayed wc10co4cr and cr3c2-25nicr coatings and showed that hvaf was capable of producing slurry erosionresistant coatings (figure 6(a)). all surfaces in figure 6 exhibited significant ploughing of material because of the repeated impacts of quartz particles. figures 6(b) and 6(c) depict fine primary wc particles bulging from the surface along with some embedded quartz fragments indicated by white arrows (1). a lower microhardness revealed more distinct ploughing marks and localized material extrusion marked by green arrows (2). in figures 6(c) and 6(d), the majority of the wear was caused by the ploughing of the material by large quartz particles, which resulted in the formation of a lip of extruded material (2). the higher kinetic energy of the quartz particles caused an increase in the number of surface fragments (1) [33]. figure 6. a) volume losses of coatings after slurry erosion, and worn surface of; b) wc-10co4cr (porous), c) wc-10co4cr (dense), d) cr3c2-25nicr (porous), e) cr3c2-25nicr (dense). numbers indicate embedded quartz fragments (1), lip formation (2), carbide-rich clusters (3), carbide cracking (4) and coating cracking (5). reprinted with permission from reference [33], © 2019 elsevier. y. y. avcu et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1369 9 hvaf coatings against cavitation erosion table 3 summarizes studies on understanding cavitation erosion behavior of hvaf-coatings along with utilized hvaf and cavitation erosion parameters. the cavitation erosion resistance of hvafsprayed 86wc-10co4cr [18], wc-10co4cr, wc-cocr [10], cr3c2-25nicr [34], fecrmnsini, fecrmnsib [35], cr3c2–50nicrmonb, cr3c2–25nicr, and cr3c2–37wc–18nicocr [36] coatings were superior to hvof-coatings due to their lower porosity, higher hardness, higher toughness [18], and lower oxides content [35]. table 3. hvaf coatings against cavitation erosion reference coating hvaf parameters cavitation erosion parameters [18] 86wc-10cr-4co substrate: ss410 air: 90.3 psig propane: 83.8 psig standoff distance: 7 in spray particle velocity: 1010, 960, 895 m / s deposit thickness per pass: 28 µm commercial ultrasonic processor frequency: 20 khz solution: tap water [34] wc-10co-4cr cr3c2-25nicr substrate: s235 structural steel fuel gas: propane standoff distance: 300 mm powder feed rate: 200 g / min (wc-10co4cr) powder feed rate: 150 g / min (cr3c2-25nicr) coating thickness: 300 µm the alternative in-direct test method (astm g32-16) frequency: 20 khz duration: 12 h [33] wc-10co-4cr cr3c2-25nicr substrate: s235 steel the alternative in-direct test method (astm g32-16) frequency: 20 khz solution: de-ionized water duration: 8 h for coatings, 12 h for reference materials [35] fecrmnsini fecrmnsib substrate: aisi/sae 1020 fuel gas: propane standoff distance: 300 mm powder feed rate: 90 g / min the vibratory ultrasonic cavitation equipment indirect test method (astm g32-16) frequency: 20 khz solution: distilled water [36] cr3c2–25nicr cr3c2-37wc18nicocr cr3c2-50nicrmonb substrate: s235 steel fuel gas: propane standoff distance: 300 mm powder feed rate: 130-200 g / min coating thickness: 300 µm the alternative in-direct test method (astm g32-16) frequency: 20 khz solution: de-ionized water duration: 6 h [37] nicrmonb nicrbsi substrate: steel coating thickness: 350-400 µm ultrasonic vibration method frequency: 20 khz solution: tap water (12 v) duration: 330 min [10] wc-10co-4cr substrate: s355 steel fuel gas: propane standoff distance: 250 mm powder feed rate: 95 g / min the vibratory ultrasonic cavitation equipment indirect test method (astm g32-16) frequency: 20 khz duration: 6 h [9] wc-10co-4cr wc-20crc-7ni substrate: aisi 1040 steel fuel gas: propane carrier gas: nitrogen standoff distance: 180 mm powder feed rate: 200 g / min coating thickness per pass: 40 µm ultrasonic vibration method frequency: 20 khz solution: tap water (ph 7.5, 8.5 v) duration: 330 min [7] wc10co4cr wc20crc7ni substrate: structural steel coating thickness: 400 µm ultrasonic vibration method frequency: 20 khz time: 330 min solution: tap water (ph 7.5, 8.5 v) duration: 330 min hvaf-sprayed nicrmonb coatings also exhibited promising cavitation resistance due to superior microstructural and mechanical features [37]. varis et al. [10] proposed that high compressive stresses of hvaf-coatings inhibit the fatigue crack formation and propagation along the microstructure consisting of lamellae interfaces, thus improving the cavitation resistance of the coating. according to korobov et al. [9], the fine structure of wc-crc-ni coating increases the specific surface area of carbide particles and, consequently, the required crack propagation energy. http://dx.doi.org/10.5599/jese.1369 j. electrochem. sci. eng. 00(0) (2022) 000-000 high-velocity air fuel coatings for steel 10 in turn, this improves their resistance to chipping and consequently, to cavitation compared to the coarser wc-cocr coating (figure 7). in the case of the wc-crc-ni coating, the hardening of the matrix caused by the dissolution of cr is accompanied by an increase in the matrix's plasticity, which prevents carbides from detaching from the matrix [9]. figure 7. a) volume loss during cavitation test, sem images of worn surfaces and subsurfaces of coatings; b) wc-cocr, c) wc-crc-ni. reprinted with permission from reference [9], © 2021 springer nature kumar et al. [18] reported that spraying parameters (e.g., particle velocity) affected the mechanical properties of hvaf-sprayed tungsten carbide coatings (e.g., fracture toughness), influencing the resultant cavitation resistance. the coating microstructure (i.e., carbide content) was shown to affect the toughness and cavitation resistance [36]. the microstructural features of hvaf-coatings (e.g., pores and oxides) were the other key parameters affecting the cavitation resistance [35]. commonly, the formation of deep and wide craters has been demonstrated on the surfaces of cavitation-eroded hvaf-coatings [7]. furthermore, the repeated impact of cavitation bubbles has caused the formation of fatigue micro-cracks and severe plastic deformation within the microstructure. this leads to the removal of material from the surface [33]. removal of material occurs through crack propagation due to the continuous implosion of cavitation bubbles [33]. summary of hvaf coatings for erosion-resistant applications so far, wccocr-, cr3c2-, nicr-, and fecr-based coatings have been deposited on steel substrates for solid particle erosion-, slurry erosion-, and cavitation erosion-resistant applications. table 4 provides a comparison of erosion-resistant hvaf coatings by listing their thickness, composition, and hardness, as well as active wear mechanisms and final remarks. table 4. erosion resistant hvaf coatings: a summary of compositions, hardness, thickness, wear mechanisms, and final remarks coating coating composition coating properties erosion tests wear mechanisms final remarks w c c o c rb a se d wc-10co-4cr [7,9,10,18, 19,28,25,31-34] 86wc-10cr-4co [18,26] wc-nimocrfeco [6] wc-fenicrmocu [6] wc-12co [29] wc-20crc-7ni [7,9] thickness: 180-450 µm hardness: wc-10co-4cr (850±90 1170±18 hv0.5) 86wc-10cr-4co (1290±30 1473±40 hv0.3) wc-nimocrfeco & wcfenicrmocu (1100 1300 hv0.3) wc-12co (1160 hv) wc-20crc-7ni (950±60 1160±90 hv0.5) • solid particle erosion • slurry erosion • cavitation erosion severe plastic deformation, indentation, ploughing, shoveling, subsurface cracks and chipping mechanisms, and brittle fracture. • hvaf coatings are harder, tougher, and more cavitationresistant than hvof coatings. • porosity and hardness affect cavitation resistance. • hvaf-coatings binder phase affects erosion resistance. y. y. avcu et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1369 11 coating coating composition coating properties erosion tests wear mechanisms final remarks c r 3 c 2 -b a se d cr3c2-25nicr [22,32-34,36] cr3c2-10nicr [24] cr3c2-feni [24] cr3c2-50nicrmonb [36] cr3c2-37wc-18nicocr [36] cr3c2-wc-ma [24] thickness: 180-300 µm hardness: cr3c2-25nicr (920±51-1002±69 hv0.3) cr3c2-10nicr (1080 hv0.1) cr3c2-feni (965 hv0.1) cr3c2-50nicrmonb (885±58 hv0.3) cr3c2-37wc-18nicocr (1104±118 hv0.3) cr3c2-wc-ma (1261 hv0.1) • solid particle erosion • slurry erosion • cavitation erosion cutting and ploughing are the main wear mechanisms in solid particle erosion. cavitation erosion causes coating fatigue wear, plastic deformation, and cracks. • due to their lower porosity, higher hardness, and higher toughness, hvaf coatings resist cavitation better than hvof coatings. • hvaf produced slurryresistant coatings. • denser hvaf coatings resist slurry erosion better. n ic rb a se d nicrmonb [37] nicrbsi [37] thickness: 350-400 µm hardness: nicrmonb (640±60 hv0,05) nicrbsi (910±80 hv0,05) • cavitation erosion not reported. • nicrmonb coatings show higher cavitation resistance than nicrbsi coatings. f e c rb a se d fecrnimobsic [23] fecrmomnwbcsi [30] fecrmnsini [35] fecrmnsib [35] thickness: 250-300 µm hardness: fecrnimobsic (956±56 hv0.1) fecrmomnwbcsi (1433 hv0.1) fecrmnsib (557±40 hv0.3) fecrmnsini (487±26 hv0.3) • solid particle erosion • slurry erosion • cavitation erosion electrochemical and active corrosion mechanisms. • high hardness and compactness of hvaf coatings improve erosion–corrosion resistance. • lower porosity and oxide content in hvaf-coatings improve cavitation and corrosion resistance. the following section provides key findings from the emerging literature on the development of erosion-resistant high velocity-air fuel (hvaf) coatings. conclusions the present review critically reviews the emerging literature on the development of erosionresistant high velocity-air fuel (hvaf) coatings. the performance of hvaf-coatings against solid particle erosion, slurry erosion, and cavitation erosion as a function of microstructural features, compositions, and mechanical properties is discussed. the dominant erosive wear mechanisms causing degradation of these coatings are addressed by providing eroded surface morphologies of the coatings subjected to different erosive wear conditions. • so far, numerous cermet-, iron-, and nickel-based hvaf-sprayed coatings have been successfully deposited on steel substrates. these are promising for numerous engineering applications, including hydro turbines, circulating fluidized bed combustors, and boiler applications, due to their high solid particle, slurry, and/or cavitation erosion resistance. • due to enhanced microstructural and mechanical features (i.e., dense microstructure and high hardness), the erosion resistance of hvaf-coatings is superior to that of hvof-coatings. optimizing spraying parameters to tailor the microstructural characteristics of these coatings appears to be the key to enhancing their erosion resistance. • clarifying the dominant erosion mechanisms under various erosion conditions is crucial for understanding how these recently developed coatings degrade over time. the binder phase of hvaf-coatings has been shown to significantly affect erosion resistance, primarily due to its inherent mechanical properties and bearing capacity of the hard particles (e.g., wc particles). • to further improve the erosion resistance of hvaf coatings, the dominant erosion mechanisms and their progression through the coating's microstructure are the primary phenomena that http://dx.doi.org/10.5599/jese.1369 j. electrochem. sci. eng. 00(0) (2022) 000-000 high-velocity air fuel coatings for steel 12 need to be revealed. these dominant wear mechanisms are mainly altered as a function of particle impact angle, where high impact angles cause more severe damage due to matrix microcracking, resulting in matrix and removal of hard particles from the microstructure. • the relationship between microstructural features and erosion mechanisms has yet to be clarified to process coatings with tailored microstructural features for erosion-resistant applications. more widespread implementation of advanced microstructural and mechanical characterization methods (e.g., feg-sem, nanoindentation) is required to understand the microstructural features of these coatings and their effects on the mechanical and erosion behavior. in addition, optimizing spraying parameters (e.g., velocity) seems key to processing 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.4028/www.scientific.net/ssp.299.893 https://creativecommons.org/licenses/by/4.0/) electrodeposition of zinc-nickel alloys from ethylene glycol-based electrolytes in presence of additives for corrosion protection http://dx.doi.org/10.5599/jese.1895 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1895 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrodeposition of zinc-nickel alloys from ethylene glycol-based electrolytes in presence of additives for corrosion protection negar fouladvari, gozde firtin, buse kahyaoglu, luca nobili and roberto bernasconi dipartimento di chimica, materiali e ingegneria chimica giulio natta, via mancinelli 7, 20131, milano, italy corresponding author: roberto.bernasconi@polimi.it received: may 19, 2023; accepted: july 17, 2023; published: july 27, 2023 abstract in the present work the electrodeposition of zinc-nickel alloys with 15-20 wt.% nickel from non-aqueous solutions based on ethylene glycol is investigated. potentiostatic deposition conditions are used, which are found to offer optimal coating quality and superior control over composition. in addition, ammonium chloride is evaluated as additive to partially suppress nickel incorporation into the deposit and to enhance layer quality. layers composition, surface morphology of the deposits and their anticorrosive properties are investigated. the electrochemical characteristics of the zn-ni electrolytes are studied using cyclic voltammetry measurements. from the phase composition point view, x-ray diffraction results confirm that a metastable γ phase is present in the as deposited zn-ni alloys with nickel content 16-18 wt.%. corrosion tests show that the barrier behaviour against corrosion of zn-ni films electrodeposited from the nh4cl containing bath is superior in comparison to layers plated from an additive-free bath. the use of the additive enlarges the grains and provides a compact surface structure, which upskills the anticorrosive behaviour of the deposit. keywords plating, non-aqueous solvent, ammonium chloride, anticorrosion coating introduction zinc has a number of characteristics that make it a well-suited corrosion protective coating for iron and steel products. zinc excellent corrosion resistance in most environments accounts for its successful use as a protective sacrificial coating on a variety of products and in many exposure conditions [1]. there are different ways to apply zinc coatings on steel-based products such as hotdip galvanizing, sherardizing, thermal spraying or electroplating and each of them has unique features. among the protective methods, electrodeposition provides a smooth and hard surface, http://dx.doi.org/10.5599/jese.1895 http://dx.doi.org/10.5599/jese.1895 http://www.jese-online.org/ mailto:roberto.bernasconi@polimi.it j. electrochem. sci. eng. 00(0) (2023) 000-000 electrodeposition of zinc-nickel alloys 2 good bonding between the deposited layer and metal substrate, easy control of the coating thickness and the possibility to deposit metallic alloys [2]. the latter point is of particular interest, since it is well-established that zinc alloys can provide better protection than pure zinc; therefore, zinc alloys have a great practical importance in the field of coating technology [3,4]. according to the available literature, electrodeposited zn-co [5], zn-ni [6], and fe-zn [6] can improve the corrosion resistance properties in comparison to pure zinc layers. currently, zn-ni alloy coatings have become an important and eco-friendly alternative to toxic cadmium coatings [7,8]. among the possible film compositions, zinc-rich stoichiometry is one of the most exploited due to the low corrosion potential (ecorr) of the γ-phase (9-15 wt.% ni) that makes it an ideal sacrificial layer in case the substrate is exposed to the environment [2]. generally, aqueous solutions of a simple salt or a complex salt type are preferred in various industrial applications for the electrodeposition of zn-ni alloys [9-13]. the major reasons for carrying out electrodeposition in non-aqueous electrolytes (such as conventional organic solvents, ionic liquids and molten salts [14]) are the low vapor pressure and the wide electrochemical window of these media [15,16]. these characteristics make them safe to use for electrodeposition and stable over time. water content is an important parameter in the case of electrodeposition from nonaqueous solutions, since it considerably influences the electrochemical behaviour of the metallic ions present in the bath, the potential window of the electrolyte and its conductivity/viscosity. in the case of deep eutectic solvents, water contamination can be beneficial or detrimental according to the metal plated [17]. in general, the research on alternative electrolytes is an attractive research topic and new non-aqueous solutions have been developed to apply electroplating processes to metal surfaces [15,16,18,19]. indeed, non-aqueous electrolytes have been employed to successfully deposit a large variety of zn-based alloys: zn-fe [20], zn-mn [21], zn-co [22] and zn-sn [23]. waterfree solutions have been widely employed also to deposit zn-ni [24,25]. one of the aspects of the electrodeposition from non-aqueous electrolytes that has received limited attention in the last years is the use of additives to improve the quality of the deposits. additives are routinely employed in industrial plating from aqueous solutions to control the morphology of the layers, to prevent pitting or to enhance levelling [26]. however, it is evident that the mechanism of action for additives in ionic liquids is very different to that in aqueous systems. as such, it is probable that additives like brighteners and levellers will be unique to each ionic liquid/metal system studied. in general, looking for additives suitable for electrodeposition in nonaqueous solvents is a matter of major interest in modern electrochemistry. the use of additives has been evaluated, for example, in the case of ni deposition from choline chloride/ethylene glycol [27,28], for zn plating from choline chloride/ethylene glycol and choline chloride/urea [29] or for co deposition from chloride/ethylene glycol [30]. the use of additives has been investigated also for the deposition of alloys. alesary et al. [31], for example, evaluated the effect of some additives on the deposition of zn-sn in choline chloride/ethylene glycol. fashu et al. [32], in turn, studied the effect of nh4cl and edta on the deposition of zn-ni in choline chloride/ethylene glycol. in particular, they observed that ammonium chloride influenced the amount of ni co-deposited. moreover, the morphology, composition, nucleation, and roughness of the zn alloy deposits were significantly influenced when the electrodeposition was performed in electrolytes containing this additive. in the present work, the use of ammonium chloride as additive for the electrodeposition of zincnickel alloy on steel substrate has been investigated. the choice of using nh4cl was inspired by the interesting results obtained by fashu et al. [32] in the choline chloride/ethylene glycol system. plating was carried out from a non-aqueous solution for the deposition of zn-ni based on the use of n. fouladvari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1895 ethylene glycol (eg) as solvent [24]. the electroplating of zn-ni alloys was performed using zinc chloride, nickel chloride as metal sources and ammonium chloride as additive to enhance coating quality and uniformity. the effect of nh4cl addition was investigated from the electrochemical, morphological and corrosion resistance point of view. experimental zinc-nickel alloy was electrochemically deposited on a steel substrate from an organic solution. the bath consisted of anhydrous ethylene glycol (sigma aldrich; purity 99.8 %; water content ≤ 0.003 %), nickel (ii) chloride hexahydrate (sigma aldrich; purity ≥ 98 %), zinc (ii) chloride (sigma aldrich; purity ≥ 98 %) and ammonium chloride (sigma aldrich; purity ≥ 99.5 %), which were employed as received. the salts were dissolved in eg according to the following quantities: 0.4 m zncl2, 0.05 m nicl2∙4h2o and 0.6 m nh4cl. the resulting electrolytes were used to perform zn-ni deposition. solutions were maintained in temperature (60 °c) by using a thermal jacket during solution preparation, electrochemical characterization and electrodeposition. annealing at 400 °c for 1h in inert atmosphere (n2) has been done for some selected samples. a standard threeelectrode cell setup, with an amel 2550 potentiostat/galvanostat, was employed for the electrochemical characterization. during the cyclic voltammetry (cv), platinum wires were used as working, counter and reference electrodes [33]. without stirring, a 20 mv s-1 scan rate was applied. in order to perform potentiostatic deposition tests, an amel 2550 potentiostat/galvanostat was used. in this case, mild steel plates were used as working electrodes and platinum wires as reference and counter electrodes. oxides were removed from the steel surface by immersing it in acetone and etching it in a 20 wt.% hcl solution. cathodic efficiencies in the 60-70 % range were observed for ni contents ranging from 16 to 18 wt.%. potentiodynamic polarization tests were done in a 3.5 wt.% nacl aqueous solution. a 1 mv s-1 scan rate was employed, without stirring. sem was carried out employing a zeiss evo 50 ep setup. xrd was done by a philips pw1830 (kα1cu = 15.4058 nm) setup. in order to measure the films composition and their thickness by x-rays fluorescence spectroscopy (xrf), a fischerscope x-ray xan was employed. conductivity was measured using an amel model 2131 conductivity meter, while viscosity was evaluated using a haake viscotester vt5r rotational viscometer. electrolytes water content was determined via karl fischer titration with a metrohm 870 kf titrino plus. repeatability of both deposition experiments and characterization was proofed by repeating the tests at least twice. atomic force microscopy (afm) was carried out using a ntmdt solver pro (in contact mode). glow discharge optical emission spectroscopy (gdoes) was performed using a spectruma gda 750 analyzer. results and discussion electrolyte water content the global amount of moisture present in the electrolyte is an important parameter in the case of electrodeposition from non-aqueous solutions. a relevant percentage of water can considerably influence the electrochemical behaviour of the bath [17] or lead to the co-deposition of a significant level of impurities [34]. the amount of water dissolved in the ammonium chloride free bath has already been determined in our previous work [24]: 1.42 ± 0.02 wt.%. in the present work, however, a relevant amount of ammonium chloride was added to the electrolyte (potentially dragging in more water). for this reason, the amount of water dissolved in the additivated version of the zn-ni plating bath based on eg was determined. the karl-fischer measurements carried out evidenced a global http://dx.doi.org/10.5599/jese.1895 j. electrochem. sci. eng. 00(0) (2023) 000-000 electrodeposition of zinc-nickel alloys 4 water content equal to 1.63 ± 0.03 wt.%. this value is similar to the one observed without nh4cl, suggesting a limited impact of its addition on overall water content. electrolytes physical characterization one of the most important physical properties that must be evaluated, in the case of electrodeposition from non-aqueous electrolytes, is viscosity. indeed, while aqueous electrolytes are normally characterized by values of viscosity close to that of water (1 cp), water-free baths can present values significantly higher. these high levels of viscosity can strongly influence the electrodeposition process, limiting ionic mobility. consequently, viscosity was evaluated for pure eg, for the additive-free solution and for the electrolyte with ammonium chloride as an additive (figure 1). 1000 t-1 / k-1 figure 1. viscosity data for pure eg and for the two zn-ni electrolytes (with and without nh4cl) temperature dependence of viscosity was modelled by arrhenius law [35], which correlates the viscosity and temperature according to equation 1. e rt t e     = . (1) where eη is the activation energy, η is the limit viscosity. it appears evident in figure 1 that salt addition (nh4cl) to pure ethylene glycol increases the viscosity of the solution. conversely, temperature decreases viscosity due to the increase in ions mobility. measured viscosities are considerably higher than water, which may constitute a disadvantage for deposition from ethylene glycol-based solutions. high viscosities, indeed, limit the ionic diffusion in the solution [36] and result into lower conductivity [37]. nevertheless, the viscosity of eg based solutions is significantly lower than most non aqueous based solutions [35] and, in general, compatible with electroplating. besides viscosity, also conductivity is a fundamental parameter for an electrolyte intended for electrodeposition. the conductivity of a solution is generally proportional primarily to the concentration and to the type of ions present. temperature is another important parameter that greatly affects conductivity. generally, ions mobility in an electrolyte increases proportionally to the temperature [38]. figure 2 reports the data obtained from the conductivity measurements performed on pure eg, on the additive-free solution and on the nh4cl additivated electrolyte. pure ethylene glycol is characterized, as expected, by a very low conductivity, due to the absence of dissociated species. the conductivity of pure ethylene glycol strongly increases with the addition of zncl2 and nicl2∙6h2o. these two salts readily dissolve and dissociate in ethylene glycol and they yield ions able to efficiently conduct electricity. moreover, the addition of nh4cl further increases the conductivity of the zn-ni electrolyte to higher values at different temperatures as shown in figure 2. n. fouladvari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1895 1000 t-1 / k-1 figure 2. conductivity data for pure eg and for the two zn-ni electrolytes (with and without nh4cl) the effect of temperature on conductivity is evident for all the electrolytes, with higher temperatures resulting in higher conductivities. recorded conductivity values, even though considerably lower than those typical of deep eutectic solvents [35], are compatible with electroplating processes. deposition was performed at 60 °c exactly to take advantage of the higher ionic mobility, higher conductivity and lower viscosity. electrochemical characterization the electrochemical properties of the zn-ni plating solutions, both with and without the additive, were studied by cyclic voltammetry (figure 3). the electrochemical behaviour of pure eg was acquired for comparison. figure 3. cyclic voltammetry for pure eg and for the two zn-ni electrolytes (with and without nh4cl); scan rate = 10 mv s-1 as expectable, the voltammetry of pure eg, due to its very low conductivity, is visible as a flat line. for what concerns the non-additivated zn-ni, two clear reduction peaks for the two metals were detected [24]. a neat separation between the reduction potentials of zn (znred) and ni (nired) could be clearly observed both with and without the additive. the first was observed around -1.25 v vs. pt, while the latter was observed around -0.6 v vs. pt. the nired potential was not substantially changed by the addition of nh4cl, while the znred potential was slightly shifted towards more positive values (-1.2 v vs. p). in addition, the current density observed at potentials lower than znred http://dx.doi.org/10.5599/jese.1895 j. electrochem. sci. eng. 00(0) (2023) 000-000 electrodeposition of zinc-nickel alloys 6 was considerably increased by the addition of nh4cl, suggesting a higher partial current for zn deposition in presence of the additive. considering these two effects, nh4cl was expected to reduce the concentration of ni in the deposits. finally, at negative potentials, a sharp increase in current density was always observed. this could be related with hydrogen evolution at the cathodic side due to the breakdown of the hydration water introduced in the electrolyte with the metallic salts. considering the reduction potentials observed, potentials lower than -1.2 v were expected to allow zn-ni co-deposition. for this reason, they were considered for subsequent deposition tests. for what concerns the anodic side of the voltammogram, a superimposition of different anodic peaks was observed (niox/znox). these correspond to the dissolution of zn and ni from different phases present in the zn −ni alloy deposited during the cathodic scan. potentiostatic deposition tests following the electrochemical characterization of the electrolyte, potentiostatic co-deposition tests were carried out (figure 4). figure 4. composition vs. potential for the nh4cl-free and for the nh4cl additivated electrolytes (deposition time = 30 min) as expectable, decreasing deposition potentials resulted in the co-deposition of lower amounts of ni [24]. the presence of nh4cl induced, in line with the behaviour observed during the voltammetric tests, a decrease in the quantity of co-deposited ni. at -1200 mv vs. pt the variation observed was 13.57 wt.%, at -1500 mv vs. pt it was -8.11 wt.% and at -1800 mv vs. pt it was -5.05 wt.%. the thickness of the resulting deposits is generally correlated to their ni content and, for this reason, it was determined via xrf. figure 5a shows the result obtained. the alloys characterized by the highest zn contents were also characterized by the most relevant growth rate. the presence of ammonium chloride in the bath not only decreased ni codeposition, but also significantly increased the thickness of the coatings. at -1200 mv vs. pt the increase observed was 77.78 %, at -1500 mv vs. pt it was 63.07 % and at -1800 mv vs. pt it was 75.96 %. the dependence of thickness on deposition time was evaluated as well (figure 5b). it is possible to observe a non-linearity in the growth rate at high deposition times and a delayed start (probably as a consequence of an unfavorable nucleation kinetic). by adding ammonium chloride, the deposited layer thickness slightly increased. n i co n te n t in t h e c o a ti n g , w t. % n. fouladvari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1895 figure 5. thickness vs. potential at a constant deposition time of 30 min (a) and thickness vs. time at a constant deposition potential of -1800 mv vs. pt (b) for the nh4cl-free and for the nh4cl additivated electrolytes morphological characterization the morphology of the zn-ni layers deposited at -1800 mv vs. pt was analyzed. figure 6a depicts the sem morphology without ammonium chloride, while figure 6c shows the sem image of the layer plated with nh4cl. figures 6b and 6d show the corresponding afm images. a mild cracking is evident in both cases. the typical nodular morphology of an electrodeposited layer is clearly visible in both cases, but the layer deposited in absence of nh4cl looks more cracked and rougher. figure 6. sem (a) and afm (b) morphology of zn-ni layer deposited without nh4cl; sem (c) and afm (d) morphology of zn-ni layer deposited with nh4cl atomic force microscopy (afm) was performed to observe the topography and roughness characteristic of zn-ni coatings both with and without ammonium chloride as shown in figure 6b and 6d. the sample deposited from the 0.4 m zncl2 + 0.05 m nicl2 ∙ 6h2o + 0.6 m nh4cl solution allowed smoother znni coatings. the surface roughness ra was calculated for the sample with additive and it resulted equal to 13.52 nm for the 20 × 20 μm area shown in figure 6d. in contrast, the sample deposited from the 0.4 m zncl2 + 0.05 m nicl2 ∙ 6h2o solution had a rougher surface. the measured surface roughness ra resulted equal to 68.39 nm for the 20 × 20 μm area shown in figure 6b. http://dx.doi.org/10.5599/jese.1895 j. electrochem. sci. eng. 00(0) (2023) 000-000 electrodeposition of zinc-nickel alloys 8 the distribution of the two elements, nickel and zinc, along the thickness of the deposited coatings was evaluated using gdoes (figure 7). data obtained are qualitative due to the impossibility to calibrate the etch rate of the plasma in the gdoes system. figure 7. gdoes characterization of two zn-ni layers, deposited without (a) and with (b) nh4cl despite their qualitative nature, the data obtained are valuable in order to carry out some considerations. according to figures 7a and 7b, the composition of the layer was found to be not uniform along the thickness. the ni concentration showed a gradient moving from the surface of the coating (depth 0) to the interface with fe. comparing the deposit plated without ammonium chloride (figure 7a) with the one plated with ammonium chloride (figure 7b), it can be seen that the latter was more uniform. indeed, without the additive, zn content increased by 20.48 % between the interface with fe and the outer surface. if this value is normalized with respect to the thickness of the layer measured via xrf (9.04 microns), the concentration gradient is 2.27 % μm-1. with the additive, zn increased by 22.83 %, but the thickness was higher. by normalizing with respect to the xrf thickness, the value of 1.69 % μm-1 can be obtained. meanwhile, in both cases, the level of carbon contamination was found to be negligible. phase composition of the deposited layers the phase composition of the layer plated at -1800 mv vs. pt was analysed using xrd (figure 8). two coatings, annealed at 400 °c and non-annealed, were analysed to compare the effect of annealing. 2 / ° figure 8. xrd characterization of a zn-ni layer deposited with nh4cl before and after annealing at 400 °c for 1 h (a = fe substrate; b = zn-ni peaks before annealing; c = zn-ni peaks after annealing; d = secondary phases) n. fouladvari et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1895 from the x-ray diffractogram registered before annealing (figure 8), some reflections typical of the cubic 𝛾 phase are noticeable (marked as b in the graphs). however, these do not totally correspond to the peaks expected for the fully ordered 𝛾 phase. this behaviour is indicative of the fact that the ionic liquid-based electrolyte deposits a metastable 𝛾 phase with short range order, which can be obtained only by electrochemical methods. this behaviour has been observed in water based electrolytes [39] and also in the same eg based electrolyte here described in absence of nh4cl [24]. in particular, the peak at 34.95° is substantially difficult to explain without taking into consideration the presence of a metastable phase. in addition, some high-intensity reflections expectable for the ni5zn21 phase are not present [24,39]. the phase composition obtained in presence of nh4cl is therefore similar to the case without additive [24]. the heat treatment process, carried out at 400 °c, significantly altered the phase composition of the alloy. the resulting diffraction features were found to totally match the theoretical spectrum of the stable 𝛾 phase. by comparing the x-ray diffraction patterns before and after annealing, the intensity of the 𝛾 phase peaks increases. moreover, the peaks look sharper with respect to the case without additive [24]. this is probably indicative of a larger crystallite size. to support this statement, the mean crystallite size for the layer obtained in presence of nh4cl was calculated using the scherrer equation on two intense peaks (corresponding to the 330/411 and 552/633 reflections). this resulted equal to 32.36 ± 2.03 nm, whereas the non-additivated solution yielded a mean crystallite size equal to 11.06 ± 1.84 nm [24]. corrosion performances the corrosion behaviour of the electrodeposited zn-ni coatings was examined for four different as-plated samples (figure 9). the linear polarization curves of the zn-ni alloys were recorded and compared with those obtained for the steel substrate in order to estimate the corrosion protection. in addition, a commercial zn-ni layer (electrodeposited from the glovel 800 bath, an alkaline zn-ni formulation by glomax) was analysed for comparison. the corrosion potential of both commercial and eg-based zn-ni alloys is more negative compared to steel (-440 mv) and a little nobler than zinc (which has corrosion potentials not higher than -1000 mv in 3.65 % wt. nacl solutions [40]) due to their nickel content. during corrosion, zn dissolves first as sacrificially and leaves a layer enriched with ni or corrosion products of the two metals [41]. this layer can act as a physical barrier to further corrosion and it shows that zn-ni coatings can offer protection for ferrous-based substrates [42]. 10-5 10-4 10-3 0.01 0.10 1.00 10.00 current density, ma cm-2 figure 9. linear polarization tests for the iron substrate, for a reference commercial zn-ni layer and for the two layers plated in presence and in absence of nh4cl http://dx.doi.org/10.5599/jese.1895 j. electrochem. sci. eng. 00(0) (2023) 000-000 electrodeposition of zinc-nickel alloys 10 table 1 reports the corrosion data measured for the samples represented in figure 9. table 1. corrosion data for the different samples analyzed sample ecorr / mv vs. ag/agcl icorr / a cm-2 fe substrate 402 7.5 commercial zn-ni (glovel 800) 826 11.8 zn-ni from eg without nh4cl (0.4 m zncl2 + 0.05 m nicl2∙6h2o) 852 27.1 zn-ni from eg with nh4cl (0.4 m zncl2 + 0.05 m nicl2∙6h2o + 0.6 m nh4cl) 772 3.7 eg solutions without additive showed corrosion properties similar to commercial ones. the tests showed that corrosion resistance was slightly improved by the use of nh4cl, even if the content of ni slightly decreased. this effect is probably due to the increased grain size resulting from the deposition of zn-ni in presence of nh4cl. moreover, the addition of nh4cl resulted in a more positive corrosion potential. corrosion current density (icorr) decreased after the addition of nh4cl with respect to non additivated solutions. thus, zn-ni alloys electrodeposited from nh4cl presented improved corrosion resistance. in general, zn-ni alloys obtained from eg were characterized by corrosion rates lower than pure zn [40], which makes them suitable for zn replacement as corrosion protection layers. conclusions in the present work, zn-ni electrodeposition on steel was successfully performed from an ethylene glycol-based bath in presence of ammonium chloride as additive. cyclic voltammetry, viscosity and conductivity measurements allowed to analyze the characteristics of the eg-based znni plating bath with and without addition of ammonium chloride. cyclic voltammetry clearly evidenced the presence of reductions features for both ni and zn. the presence of ammonium chloride shifted the reduction peak of zn towards more positive values and increased the cathodic currents for zn plating. these two aspects resulted into an increase in the amount of co-deposited zn, which was punctually detected by performing potentiostatic deposition tests. the morphology of the resulting layers was studied using sem and afm, evidencing a beneficial effect of nh4cl on the roughness and on the cracking level of the coatings. regarding the phase composition of the znni layers, x-ray diffraction spectroscopy highlighted the phases and intermetallics in the deposited layers. evidence of metastable γ phase formation, in analogy with what observed in the existing literature for the additive free bath, was observed. finally, potentiodynamic polarization tests were carried out to investigate the corrosion behaviour of the deposits. zinc dissolved first as sacrificial material and the rest of the layer, which was enriched with nickel or corrosion products of the two metals, acted as a physical barrier. this barrier became stronger and more stable with the addition of nh4cl. layers deposited in the presence of nh4cl presented the best corrosion properties, probably as a consequence of their comparatively larger grain size. the 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https://www.researchgate.net/profile/virgil-constantin-2/publication/269035448_density_viscosity_and_electrical_conductivity_of_three_choline_chloride_based_ionic_liquids/links/547d9ccd0cf285ad5b08a86d/density-viscosity-and-electrical-conductivity-of-three-choline-chloride-based-ionic-liquids.pdf https://www.researchgate.net/profile/virgil-constantin-2/publication/269035448_density_viscosity_and_electrical_conductivity_of_three_choline_chloride_based_ionic_liquids/links/547d9ccd0cf285ad5b08a86d/density-viscosity-and-electrical-conductivity-of-three-choline-chloride-based-ionic-liquids.pdf https://www.researchgate.net/profile/virgil-constantin-2/publication/269035448_density_viscosity_and_electrical_conductivity_of_three_choline_chloride_based_ionic_liquids/links/547d9ccd0cf285ad5b08a86d/density-viscosity-and-electrical-conductivity-of-three-choline-chloride-based-ionic-liquids.pdf https://www.researchgate.net/profile/virgil-constantin-2/publication/269035448_density_viscosity_and_electrical_conductivity_of_three_choline_chloride_based_ionic_liquids/links/547d9ccd0cf285ad5b08a86d/density-viscosity-and-electrical-conductivity-of-three-choline-chloride-based-ionic-liquids.pdf https://doi.org/10.1021/jp046355c https://doi.org/10.5772/62621 https://doi.org/10.1002/0471473332.ch23 https://doi.org/10.1179/0020296714z.000000000209 https://doi.org/10.1016/s1452-3981(23)19502-2 https://doi.org/10.1016/s1452-3981(23)19502-2 https://doi.org/10.1007/s10008-021-04898-x https://doi.org/10.1016/j.surfcoat.2009.02.129 https://creativecommons.org/licenses/by/4.0/) reduced graphene oxide as efficient carbon support for http://dx.doi.org/10.5599/jese.1595 725 j. electrochem. sci. eng. 13(5) (2023) 725-737; http://dx.doi.org/10.5599/jese.1595 open access : : issn 1847-9286 www.jese-online.org original scientific paper vanadium oxide poly(3,4-ethylenedioxythiophene) cathodes for zinc-ion batteries: effect of synthesis temperature* filipp s. volkov, svetlana n. eliseeva, mikhail a. kamenskii, alexey i. volkov, elena g. tolstopjatova and veniamin v. kondratiev saint petersburg state university, institute of chemistry, 7/9 universitetskaya nab., saint petersburg 199034, russia corresponding author: vkondratiev@mail.ru; tel.: +7-812-4286900; fax: +7-812-4286900 received: november 14, 2022; accepted: february 8, 2023; published: march 27, 2023 abstract vanadium oxide composites with conducting polymer poly(3,4-ethylenedioxythiophene) (pedot) were obtained by one-step microwave-assisted hydrothermal synthesis at two different temperatures: 120 and 170 °c (denoted as v-120 and v-170, respectively). the structure and composition of the obtained samples were characterized by scanning electron microscopy (sem), x-ray diffraction (xrd) analysis, x-ray photoelectron spectroscopy (xps), and thermogravimetric (tg) analysis. the detailed study of the electrochemical properties of the composites as cathodes of aqueous zinc-ion battery was performed cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) at different current densities and by electrochemical impedance spectroscopy (eis). it was shown that v-120 demonstrated excellent electrochemical performance in the 0.3 to 1.4 v vs. zn/zn2+ potential range reaching specific capacities up to 390 ma∙h∙g−1 at 0.3 a∙g−1 with excellent capacity stability after 1000 charge-discharge cycles. its functional parameters were found to be much better than those of the electrodes based on the v-170 composite obtained at a higher temperature. the effect of the synthesis temperature on the electrochemical properties is discussed in terms of the crystallographic, compositional, and thermogravimetric properties of the samples. keywords aqueous zinc-ion battery, hydrothermal synthesis, temperature of synthesis, electrochemical performance, structure introduction the development of novel energy storage systems is at the forefront of electrochemical research. among diverse types of metal-ion batteries, lithium-ion batteries are universally applied in various fields: electric vehicles, portable electronic devices, etc. [1,2]. however, a major problem of these *dedicated to 300th anniversary of saint petersburg state university http://dx.doi.org/10.5599/jese.1595 http://dx.doi.org/10.5599/jese.1595 http://www.jese-online.org/ mailto:vkondratiev@mail.ru j. electrochem. sci. eng. 13(5) (2023) 725-737 vanadium oxide pedot cathodes for zinc-ion batteries 726 systems is unsafety due to the presence of flammable components such as electrolyte solvent or high flammability of lithium [3]. another issue is the high cost of lithium compounds due to its low abundance. to solve these problems, other alkali-ion or multivalent metal-ion batteries such as mgion, ca-ion, zn-ion, or al-ion have been proposed [4-7]. among them, aqueous zinc-ion batteries (azibs) have attracted increasing attention due to high gravimetric (820 ma∙h∙g−1) and volumetric (5855 ma∙h∙cm−3) capacities of zinc metal anode, high safety, environmental friendliness, and low cost of zinc salts [8,9]. the main problem in any metal-ion battery is the proper choice of cathode material. for azibs, vanadium oxides are considered as an attractive cathode due to their high reversibility towards zn2+ ion intercalation, low cost and high availability, easily scalable synthesis procedures. vanadium pentoxide v2o5 has the highest theoretical capacity value of 589 ma∙h∙g−1 per two-electron transfer [10], while most reported experimental results are ca. 300 ma∙h∙g−1 [11,12]. nevertheless, the activation process for highly crystalline materials [13,14] restricts the development of commercial vanadiumbased azibs. the activation process hinders the initial performance of the cells and results in their low cycling stability due to vanadium dissolution during both charging/discharging and storage [11,15]. to overcome these drawbacks, several solutions can be applied, e.g. nanostructuring of the materials, introduction of conductive additives or coatings, and substitution of vanadium ions or inclusion of different metal ions into the crystal structure of the material via pre-intercalation [16]. first, synthesis conditions can affect the crystal lattice and, in particular, the amount of crystallized water in the molecule. water molecules can enter the crystal lattice and then be replaced by zinc ions during cycling [17-19]. it was established that the presence of water in v6o13 or v2o5·nh2o allows to decrease the interaction energy of zinc ions with the crystal lattice, which promotes the intercalation processes [20,21]. this can be related to the increase in interlayer spacing after water insertion [19]. instead of carbon-modified composites which only have enhanced electronic conductivity, the use of conducting polymers allows to increase both electronic and ionic conductivities of the composite materials. the introduction of conducting polymers also improves the zinc ions diffusion within the crystal lattice because of the formation of highly porous structures and the weakening of the coulombic interactions between zinc ions and oxygen anions in the lattice, which was established for v2o5/polypyrrole composite [22]. furthermore, chains or blocks of conducting polymers can enter the crystal lattice and enlarge the interlayer spacing which has been confirmed for the composites with polyaniline and its derivatives [23,24]. the use of poly(3,4-ethylenedioxythiophene) (pedot) enhances the electrochemical properties of vanadium oxide[25,26], ammonium vanadateand sodium vanadate-based electrode materials [27,28]. the incorporation of conducting polymers is usually done by two simple methods: i) in-situ chemical approach with partial reduction of v2o5 in the presence of monomer, which acts as a reductant in a reaction with solid oxidant, i.e. vanadium oxide species [29-31]; ii) electrochemical polymerization of a monomer to create a 3d or binder-free electrode on the conductive substrate [25]. the first method can be easily modified to obtain vanadium oxide with given morphology and polymer content. however, the effect of synthesis conditions, e.g. temperature, on the structural, morphological, and electrochemical properties of vanadium oxide with conducting polymer composite cathodes in azibs has rarely been investigated and requires further study. in this work, the structure, composition, and electrochemical performance of vanadium oxidebased composites with pedot obtained at two different temperatures of microwave-assisted synthesis have been studied for the first time. it was previously reported that such a modification method resulted in superior electrochemical performance [32,33]. the effect of the temperature of f. s. volkov et al. j. electrochem. sci. eng. 13(5) (2023) 725-737 http://dx.doi.org/10.5599/jese.1595 727 hydrothermal synthesis on the functional properties of the cathodes for aqueous zinc-ion battery was evaluated by different electrochemical techniques. the results obtained are discussed in terms of crystallographic, compositional, and thermogravimetric analyses of the synthesized products and functional properties in cathodes of azibs. experimental synthesis of composites the composite materials were synthesized by soft reduction of commercial v2o5 (nevareaktiv, russia) by edot (97 %, sigma-aldrich, usa) under microwave irradiation according to the procedure adopted from [34]. the identical mixtures, consisting of 1.1 mmol of v2o5 and 0.42 mmol of edot in aqueous solution, were placed in a pyrex reactor (35 ml). the synthesis was carried out at temperatures of 120 and 170 °c at 0.421 mpa for 1 h at 290 w microwave power level in a lab microwave system discover sp (cem corp., usa). the upper limit of the temperature of 170 °c was selected as a higher temperature at which the decomposition of pedot did not proceed. the products were designated as v-120 and v-170. then, the obtained dark blue powders were washed with water and ethanol, and dried at 80 °с for 2 h. characterization methods the powders of the initial v2o5 and the composites were characterized by x-ray diffraction (xrd, d8 discover, bruker-axs, germany) using cu-kα radiation. scanning electron microscopy (sem, supra 40vp, carl zeiss, germany) and energy dispersive x-ray analysis (edx, inca x-act, oxford instruments, germany) were performed on the v2o5 and the composite powders. after extraction of the electrodes from the cells, they were carefully rinsed with di water and dried in air. thermogravimetric analysis (tga) in the 30 to 600 °c temperature range at a heating rate of 10 °c∙min−1 in air (netzsch tg 209 f1 librа, germany) was performed for the v-120 and v-170 powder samples. the x-ray photoelectron spectrometer escalab 250xi (thermo scientific, usa) equipped with an al kα source (photon energy of 1486.6 ev) was used to study the elemental states in the v-120 and v-170 powders, as well as in the v-120 electrode material after electrochemical studies. for comparison, in some cases, we also analyzed data for pristine v2o5 powder. electrochemical characterization electrode materials were prepared by mixing v-120 or v-170 with carbon black and polyvinylidene fluoride (pvdf) in a 70:20:10 weight ratio in n-methylpyrrolidone. the resulting viscous slurry was applied onto the titanium foil (current collector), vacuum-dried at 60 °c and roll-pressed. the mass loading of the electroactive material was 1.5 2.0 mg∙cm−2. coin cells cr2032 were assembled vs. zn anode with aqueous 3 mol∙dm−3 znso4 (lenreactiv, russia) as electrolyte and whatman® glass fiber gf/a as separator. the potentials in this work are referred to the zn/zn2+ redox couple potential. the electrochemical performance of the electrodes was studied using galvanostatic charge/discharge (gcd) and cyclic voltammetry (cv) in the 0.3 1.4 v potential range at 25±2 °c. charge/discharge tests were done using a ct-4008 battery testing system (neware co., china) in the 0.3 to 20 a∙g−1 current range, cv measurements were carried out on a biologic bcs-805 potentiostat (france) at a scan rate of 0.1 to 1.0 mv∙s−1. electrochemical impedance spectroscopy (eis) measurements were conducted in the 10 khz-0.1 hz frequency range at 1.4 v potential with 10 mv rms amplitude in a three-electrode electrochemical cell using a biologic bcs-805 potentiostat (france). http://dx.doi.org/10.5599/jese.1595 j. electrochem. sci. eng. 13(5) (2023) 725-737 vanadium oxide pedot cathodes for zinc-ion batteries 728 results and discussion the phase composition of the samples was determined by matching the xrd patterns (figure 1) with the iccd data. the initial powder peaks are well-resolved and correspond to the orthorhombic v2o5 (iccd #01-077-2418). for the v-120 composite the xrd peaks match well with those of monoclinic v10o24∙12h2o (icdd #00-025-1006). the peaks are broadened, indicating low crystallinity [32]. the most intense peak of the (0,0,2) plane at 6.48° allows to estimate the interlayer distance in v10o24∙12h2o as 1.36 nm. the indexable bands in the xrd pattern of sample v-170 correspond predominantly to those of monoclinic vo2 (icdd #01-084-3056). 2 / ° figure 1. xrd patterns of the powders of the initial v2o5 and composites v-120, v-170 the xps measurements were performed, and they showed the characteristic peaks of v, o, s, c elements. the xps spectra of v 2p (figure 2a) for both composites show the changes that occur with the vanadium oxide upon interaction with edot at different temperatures. both v-120 and v-170 v 2p spectra contain the peaks at 524.4 and 526.0 ev for v 2p3/2 are related to v5+ and v4+, respecttively, which is consistent with available literature data [35]. the v 2p3/2-v 2p1/2 splitting value is 7.4 ev. the fwhm values for v 2p3/2 are ca. 1.4 ev, while the v 2p1/2 peaks are broadened, as the fwhm values are 2.57 ev, which is a result of the coster-kronig effect [36]. figure 2. xps spectra of v-120 and v-170 samples in (a) o 1s and v 2p, (b) c 1s, and (c) s 2p regions there are three o 1s peaks present for both v-120 and v-170 samples (figure 2a): the first one at 529.9 ev is typical [35] for oxygen in oxides and agrees well with the v 2p peaks. the second peak at a higher binding energy of 530.6-531.2 ev is related to the defective sites in the oxide crystal [37] and might overlap with c=o [38]. the third one at 532.3-532.9 ev we assign to the o-ether carbon atom signal due to the presence of pedot in the sample [39]. the trapped water can also provide this oxygen peak [40]. since the oxidative polymerization occurs when the composite is formed by 10 20 30 40 50 60 70 (2 0 0 ) (1 1 1 ) (3 1 0 ) (0 1 1 ) (3 0 1 ) (1 1 0 ) (1 0 1 ) (0 0 1 ) precursor (v2o5) — iccd #01-077-2418 in te n si ty , a .u . (0 0 2 ) (3 1 0 ) (4 0 2 ) (4 0 6 ) (2 0 2 ) (0 0 4 ) (5 1 0 ) (5 1 8 ) (4 0 1 6 ) (1 1 0 ) (0 0 6 ) v-120 (v10o24·12h2o) — iccd #00-025-1006 2q / ° (4 2 -1 ) (0 2 0 ) (6 0 1 ) (4 0 0 ) (3 1 0 ) (1 1 0 ) v-170 (vo2) — iccd #01-084-3056 c-o v-o odefects v5+ v 4+ (a) v -1 2 0 o 1s v 2p 535 530 525 520 515 in te n si ty , a .u . v -1 7 0 eb / ev c-c/c=c c 1s(b) v -1 2 0 291 288 285 282 in te n si ty , a .u . c-o c-s v -1 7 0 eb / ev s-o c-s s 2p(c) v -1 2 0 171 168 165 162 v -1 7 0 eb / ev in te n si ty , a .u . f. s. volkov et al. j. electrochem. sci. eng. 13(5) (2023) 725-737 http://dx.doi.org/10.5599/jese.1595 729 the interaction of the initial v2o5 powder and the edot monomer, the reduction of the initial vanadium species should occur. from the xps data, the v5+/v4+ ratio for the composites can be estimated to be 3.5:1 for v-120 and 1.9:1 for v-170. the xps spectra of c 1s and s 2p (figures 2b and 2c) further confirm the presence of pedot [41,42]. specifically, the c 1s spectra contain three peaks: the most intense one at 283.9 ev is related to c-c/c=c, the peak at 285.2 ev is related to the c-s bond, and the one at 287.3 ev is for c-o, which is consistent with the o 1s spectra. similar to o 1s, the intensity of the c 1s peak, which we attribute to the c-o bond, increases as the temperature of the synthesis increases. the s 2p spectra contain the peaks typical for the presence of s-o (166.4 and 168.8 ev) and c-s bonds (163.1 and 164.4 ev) [39] in the sample, which, along with the increase in the signal-to-noise ratio with increasing synthesis temperature, further confirms the formation of the v2o5/pedot composite. the morphology of the samples was examined by sem (figures 3a and 3b). the v-120 sample consists of wavy layers with an average layer thickness of 0.6 to 1.0 μm. the v-170 sample consists of smaller needle-like crystals forming larger agglomerates. the edx mapping (figures 3c and 3d) for the s and v elements confirms the uniformity of their distribution and thus the rather complete coverage of v-120 and v-170 with pedot. t / °c 2 / ° figure 3. sem images of v-120 (a, c), and v-170 (b, d). sem image with the corresponding edx mapping of v and s elements for v-120 (c) and v-170 (d) samples, tga curves for v-120 and v-170 samples (e), and xrd pattern of the v-120 sample after tga (f) 0 500 80 100 30 60 90 v-120 −10.3% −2.4% −6.9% r e si d u a l w e ig h t lo ss , % t / °c v-170 −4.6% +6.0% +2.1% post-tga v-120 iccd # 01-077-2418 in te n si ty , a .u . 2q / ° (f)(e) http://dx.doi.org/10.5599/jese.1595 j. electrochem. sci. eng. 13(5) (2023) 725-737 vanadium oxide pedot cathodes for zinc-ion batteries 730 thermogravimetric analysis in air (figure 3f) allowed the determination of the pedot content in the v-120 and v-170 composites. the loss of moisture causes the initial weight decrease, which passes through two stages: the evaporation of free water, followed by the loss of interlayer water [40]. this is followed by the decomposition of pedot, with the most intense evaporation starting at 240 °c. the decomposition is completed at 400 °c, and then the vanadium species in the v-120 and v-170 samples are oxidized to pristine orthorhombic v2o5, with oxygen uptake. this is confirmed by an xrd spectrum of the sample after thermogravimetric analysis (figure 3g). as indicated by the tga data, the pedot content in the composites was quite different. pedot constituted 2.4 % of the total weight in the v-170 sample, and 6.9 % in the case of v-120 sample. electrochemical performance the cv of the v-120 sample (figure 4a) contains two pairs of peaks at 0.76/0.63 and 1.09/0.96 v from the start. the same is true for v-170 (figure 4b), for which the cv response contains two pairs of peaks at 0.73/0.59 v and 1.10/0.93 v. in both cases, the change in the positions of the peaks within ten cycles is insignificant. this contrasts with the evolution of the cv shape of the initial v2o5 which showed a drastic transformation of the current response during the ten cycles and structural transformations within the material structure [32]. during the same period, the more negative anodic peak in the v-170 voltammogram slightly decreases in intensity, while the more positive peak increases in intensity. meanwhile, the cathodic peaks remain stable. for the v-120 electrode, there is no reorganization of the peaks in either intensity or position. such behavior may be caused by v-120 having a more stable layered structure with large interlayer spacing, which allows for rapid zn2+ (de)insertion. figure 4. cv curves of the initial ten cycles for the coin-cells with v-120 (a), v-170 (b) at a scan rate of 0.1 mv∙s−1 the electrochemical performance of the electrode materials was also investigated by galvanostatic charge/discharge (figure 5). the c-rate capability in asymmetric mode (with charge current of 0.3 a∙g−1 and discharge current in the range of 0.3 to 20 a∙g−1) was studied for both electrodes (figure 5a). for v-120, the discharge specific capacity values are 390, 357, 326, 274, 192, 137 and 101 ma∙h∙g−1 at current densities of 0.3, 1, 2, 5, 10, 15 and 20 a∙g−1. for v-170, the capacity values are 265, 207, 163, 108, 79, 60 and 46 ma∙h∙g−1 at the same current densities. the specific capacity values of v-120 exceed those of v-170 at every current density. v-170 provides similar specific capacity values to v2o5 at low current densities, yet v-170 prevails over v2o5 at high current densities, likely due to the facilitated kinetics of zn2+ (de)intercalation [32]. f. s. volkov et al. j. electrochem. sci. eng. 13(5) (2023) 725-737 http://dx.doi.org/10.5599/jese.1595 731 cycle life studies in symmetric charge/discharge mode at 2 a∙g−1 (figures 5b and 5c) show that the initial specific capacity of 215 ma∙h∙g−1 of the v-120 sample increases continuously and reaches the maximum of 310 ma∙h∙g−1 at the 175th cycle. the capacity then decreases until the final value of 228 ma∙h∙g−1 is recorded. the capacity retention from the first cycle was 73.5 % of the maximum value. the specific capacity of v-170 is initially 215 ma∙h∙g−1. this value decreases continuously and at the 1000th cycle only 100 ma∙h∙g−1 remains, which is 46.5 % of the initial capacity. the cyclic stability of the materials in the symmetrical gcd mode at 5 a∙g−1 is similar. initially the v-120 sample provides the specific capacity value of 150 ma∙h∙g−1. the capacity continues to increase until the 370th cycle, where the material provides 225 ma∙h∙g−1. the next 630 cycles show a gradual decrease to 170 ma∙h∙g−1. again, the capacity retention exceeds 100 % of the initial value, yet the final capacity is 75.5 % of the maximum value. the initial specific capacity of 221 ma∙h∙g−1 for v-170 falls rapidly, and the 1000th cycle has the capacity of only 26 ma∙h∙g−1, which is 11.7 % of the initial value. the shapes of the gcd curves are typical of v2o5-based electrode materials. for v-120 and v-170 electrodes (figures 5d and 5e), there is a pair of skewed intercalation-related plateaus in both the charge and discharge curves, although they diminish as the current rate increases. at high current densities, there is an almostlinear e(q) dependence, indicating a significant contribution from capacitive processes. figure 5. c-rate performance and coulombic efficiency of v-120 and v-170 electrodes at currents from 0.3 a∙g−1 to 20 a∙g−1 (a); cycling performance and coulombic efficiency of electrodes v-120 (b) and v-170 (c) at currents of 2 a∙g−1 and 5 a∙g−1; charge/discharge curves for v-120 (d) and v-170 (e) from 0.3 a∙g−1 to 20 a∙g−1 (the dashed line is the return to 0.3 a∙g−1 after all other currents) to estimate the contribution of diffusion-controlled and capacitive-controlled currents to the total measured currents, we performed cv measurements in the 0.3-1.4 v range at the scan rates from 0.1 to 1 mv∙s−1 (figures 6a and b). the shape of the cvs at different scan rates was similar for all electrode types, with a slight potential shift. the dependence of the peak current (i) on the scan rate (v) can be represented by (1): 0 500 1000 0 200 400 0 500 1000 0 200 400 0 40 0 200 400 0 200 400 0,3 0,6 0,9 1,2 1,5 0 200 400 0,3 0,6 0,9 1,2 1,5 5 a g−1 2 a g−1 q / m a ·h ·g − 1 v-120 50 100 5 a g−1 2 a g−1 q / m a ·h ·g − 1 cycle v-170 50 100 c o u lo m b ic e ff ic ie n cy , % a·g−10.3 20151052 1 0.3 q / m a ·h ·g − 1 cycle v-120 v-170 (a) (b) (c) v-120 e / v v s. z n /z n 2 + q / ma h g−1 (d) a g−1: 0.3 1 2 5 10 15 20 0.3 v-170 q / ma h g−1 e / v v s. z n /z n 2 + (e)1.5 1.2 0.9 0.6 0.3 1.5 1.2 0.9 0.6 0.3 http://dx.doi.org/10.5599/jese.1595 j. electrochem. sci. eng. 13(5) (2023) 725-737 vanadium oxide pedot cathodes for zinc-ion batteries 732 i = ab (1) or, in logarithmic form (2): log i = b log + log a (2) here the values a and b are constants. the constant b is usually in the 0.5 to 1.0 range, where the proximity to either 0.5 or 1.0 formally means that the current is controlled either by diffusion of the ions, or by capacitive processes, respectively. analysis of the rate dependence of the cv peaks (figure 6с and 6d) for different materials shows that two redox peak pairs are dissimilar in their current contributions. this means that the currents of the process in the less positive potential region are diffusion-controlled, whereas capacitive control is prevalent for the redox processes at more positive potentials. the b-values for sample v120 show this. we observed a significant contribution of capacitive processes over all redox peaks, with b = 0.85/0.88 for the less positive peak pair and b = 0.95/0.81 for the more positive one, which is usually typical of materials intended for use in supercapacitors [43]. the behavior of v-170 electrode is like that of the v2o5 electrode, as the more negative peak pair has b values of 0.61/0.72, i.e. the currents are diffusion-controlled, and the more positive has 0.90/0.80 values, indicating pseudocapacitive control. the predominant capacitive control of the current in the case of v-120 agrees well with the results reported for other v2o5-polymer composites [26,44-47]. figure 6. cv curves for v-120 (a), v-170 (b) with different scan rates and log i vs. log  curves of cathodic and anodic peaks of cv curves (c, d) 0.6 1.2 −3 0 3 0.6 1.2 −3 0 3 0.0 0.5 1.0 −1 0 0.0 0.5 1.0 −1 0 i / m a e / v vs. zn/zn2+ d2d1 c1 v-120 c2 a) i / m a e / v vs. zn/zn2+ v-170 d2 d1 c1 c2 b) − lo g ( i / m a ) −log ( v / mv·s−1) bc1=0.85 bd1=0.81 bc2=0.95 bd2=0.88 v-120c) − lo g ( i / m a ) −log ( v / mv·s−1) bc1=0.60 bd1=0.72 bc2=0.90 bd2=0.80 v-170d) f. s. volkov et al. j. electrochem. sci. eng. 13(5) (2023) 725-737 http://dx.doi.org/10.5599/jese.1595 733 the kinetics of the electrode processes was investigated by eis in three-electrode cells with 3 mol∙dm−3 znso4 electrolyte solution. the impedance spectra (figure 7a) were recorded at 1.4 v after 10 gcd cycles at 0.1 a∙g−1. in the recorded spectra, the determined rct values were 2.2 ω for v-120 and 9.8 ω for v-170. consistent with the reported enhanced capacity values, the v-120 electrode also has a lower rct value, which explains the facilitation of charge transport and hence can increase the capacity values of this material. the presence of regions of 45° slope in the nyquist plots for both samples indicate semi-infinite diffusion, modeled by warburg impedance. this allows us to estimate warburg constant values (σw) from the slopes of parallel linear sections in zre,−zim vs. ω−0.5 plots (figure 7 b and 7c). for v-120 and v-170, σw equals 1.2 ω∙s−0.5 and 1.7 ω∙s−0.5, respectively. a slight difference shows that the apparent diffusion coefficients should also be within one order of magnitude, with v-120 having a slight advantage over the v-170 material. figure 7. nyquist plots of eis spectra of v-120 and v-170 electrodes in 10 khz to 100 mhz range at 1.4 v (a), zre,−zim vs. ω−0.5 plots for v-120 (b) and v-170 (c) electrodes at 1.4 v with a linear fit of warburg region, i.e. corresponding to the 45°-phase areas in nyquist plots conclusions the vanadium oxide/poly(3,4-ethylenedioxythiophene) composites (v-120 and v-170) were successfully synthesized by a facile one-step microwave-assisted hydrothermal synthesis in two temperature modes at 120 and 170 °c, and their performance as cathodes in aqueous zinc-ion batteries was compared. the obtained composites demonstrate excellent electrochemical performance, reaching specific capacities of up to 390 ma∙h∙g−1 at 0.3 a∙g−1 (v-120) and maintaining the value of specific capacity 101 ma∙h∙g−1 at a high current density of 20 a∙g−1. in contrast to the pristine v2o5-based cathode, the activation process usually observed for such materials was negligible for both v-120 and v-170 materials. the electrochemical properties of the composites based on vanadium oxide and pedot are therefore sensitive to the synthesis temperature. this is related to the difference in the results of the syntheses depending on the conditions: the composite samples synthesized at 170 °c contain both less structural water and pedot (2.4 vs. 6.9 % for v-120), which are responsible for the interlayer spacing and hence for the “lubrication” of zn2+ diffusion. this could also result in lower http://dx.doi.org/10.5599/jese.1595 j. electrochem. sci. eng. 13(5) (2023) 725-737 vanadium oxide pedot cathodes for zinc-ion batteries 734 electronic and ionic conductivities of the composites, and thus a deterioration of the properties of the v-170 sample. acknowledgements: the financial support from rfbr (grant № 21-53-53012) is gratefully acknowledged. the authors would like to thank the centre for x-ray diffraction studies, the interdisciplinary resource centre for nanotechnology, the centre for physical methods of surface investigation, the thermogravimetric and calorimetric research centre, the chemical analysis and materials research centre and the centre for optical and laser materials research of the research park 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https://doi.org/10.1021/acsaem.1c00573 https://doi.org/10.1016/j.ensm.2021.04.004 https://creativecommons.org/licenses/by/4.0/) cathode reaction models for braga-goodenough na-ferrocene and li-mno2 rechargeable batteries http://dx.doi.org/10.5599/jese.1704 687 j. electrochem. sci. eng. 13(4) (2023) 687-711; http://dx.doi.org/10.5599/jese.1704 open access : : issn 1847-9286 www.jese-online.org original scientific paper cathode reaction models for braga-goodenough na-ferrocene and li-mno2 rechargeable batteries masanori sakai tenjinbayashicho 1225-144, hitachiota-shi, ibaraki 3130049, japan corresponding author: sakaimasanori144@gmail.com; tel: +81-80-7179-3084; fax: +81-294-73-1880 received: february 13, 2023; accepted: may 4, 2023; published: may 24, 2023 abstract braga-goodenough all-solid-state na-fc and li-mno2 batteries demonstrate deposition of na and li on the cathode during discharge. these reaction mechanisms were investigated in light of the generalized charge neutrality level and the experimental results of braga et al., and two new types of mechanisms were proposed. the na-fc mechanism is represented by a multi-step c[(ce)cc]n mechanism where c is the chemical step, e is the electrochemical step, c is the catalytic (ce) step, and n denotes the number of [(ce)cc] part cycles. the nth cycle corresponds to n moles of na and li deposition. for li-mno2, two mechanisms were considered. one is the c[(ce)cc]n mechanism which is the same as nafc, and the other is the c[2(ce)cc]n mechanism, which involves two consecutive (ce)c steps. in the c step of (ce)c of both mechanisms, fc and mno2 reduce na+(sf) and li+(sf) (sf surface states) to deposit na and li, respectively, which are intramolecular charge transfer reactions within the adsorbed molecules. fc and mno2 are oxidized to intermediates immediately reduced to fc and mno2 by their anodes in the subsequent e step. based on these mechanisms, these batteries' discharge capacity and cathode alkali metal deposition were examined in detail. keywords alkali metal battery; all-solid-state battery; li deposition; na deposition; ferrocene; manganese dioxide list of symbols fc ferrocene, fe(c5h5)2 na-fc sodium-ferrocene all-solid-state rechargeable battery li-mno2 lithium-manganese dioxide all-solid-state rechargeable battery li-s lithium-sulfur all-solid-state rechargeable battery gcnl generalized charge neutrality level cnl charge neutrality level na+glna-glass solid electrolyte of na2.99ba0.005o1+xcl1-2x li+glli-glass solid electrolyte of li2.99ba0.005o1+xcl1-2x (sf) surface states http://dx.doi.org/10.5599/jese.1704 http://dx.doi.org/10.5599/jese.1704 http://www.jese-online.org/ mailto:sakaimasanori144@gmail.com j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 688 na+(sf) na+ in surface states of na2.99ba0.005o1+xcl1-2x li+(sf) li+ in surface states of li2.99ba0.005o1+xcl1-2x c chemical reaction e electrochemical reaction c in c[(ce)cc]n catalytic reaction steps c in c[2(ce)cc]n catalytic reaction steps n in c[(ce)cc]n number of cycles of the [(ce)cc] part n in c[2(ce)cc]n number of cycles of the [2(ce)cc] part 2 in c[2(ce)cc]n two consecutive (ce)c homo highest occupied molecular orbital lumo lowest unoccupied molecular orbital fcna+(sf) (ad) adsorbed molecule between fc and na+(sf) mno2li+(sf) (ad) adsorbed molecule between mno2 and li+(sf) for c[(ce)cc]n mno2[2li+(sf) (ad)] adsorbed molecule between mno2 and 2li+(sf) for c[2(ce)cc]n high-k high relative permittivity |tvb-cb’| transfer energy between the valence band orbital of fc and the conduction band orbital of na+(sf) or transfer energy between valence band orbital of mno2 and conduction band orbital of li+(sf) |tvb’-cb| transfer energy between the valence band orbital of na+gland the conduction band orbital of fc or transfer energy between the valence band orbital of li+gland the conduction band orbital of mno2 ef fermi level ecb energy level of the conduction band bottom for fc or mno2 evb energy level of the valence band top for fc or mno2 e’cb energy level of the conduction band bottom for na+glor li+gl e’vb energy level of the valence band top for na+glor li+gl e’g band gap, e’cb e’vb, for na+glor li+gl dvb density of states of the valence band for fc or mno2 dcb density of states of the conduction band for fc or mno2 d’vb density of states of the valence band for na+glor li+gl d’cb density of states of the conduction band for na+glor li+gl  (|tvb-cb’|2/|tvb’-cb|2) (dvb/dcb) (dvb/dcb)fc (dvb/dcb) for fc (dvb/dcb)mno2 (dvb/dcb) for mno2 x0 closest distance of na+(sf) to fc or li+(sf) to mno2 p, p-value stoichiometric coefficient, number of na+(sf) or li+(sf) available for adsorption (pfc)max maximum number of na+(sf) adsorption onto fc (pmno2)max maximum number of li + (sf) adsorption onto mno2 (sfc/sna+) the ratio of fc total surface area for 2x0 extended unit crystal lattice to the circle area in na+ radius (smno2/sli+) the ratio of mno2 total surface area for 2x0 extended unit crystal lattice to the circle area in li+ radius fs fraction ratio of the area actually occupied by na+(sf) or li+(sf) on adsorption surface fp 1(porosity, %, of positive active materials)/100 nfc number of fc in the unit crystal lattice nmn number of mn in the unit crystal lattice f the faraday constant (mfc)eff effective molality of fc (mfc)mes measured molality of fc (mmno2)eff effective molality of mno2 (mmno2)mes measured molality of mno2  utilization coefficient fc fc utilization coefficient, (mfc)eff/(mfc)mes mno2 mno2 utilization coefficient, (mmno2)eff/(mmno2)mes q number of cubic or rectangular unit blocks to examine  (qmax)(na-fc) measured maximum discharge capacity of na-fc positive electrode (qmax)(li-mno2) measured maximum discharge capacity of li-mno2 positive electrode m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 689 introduction the specific capacity 3860 mah/g of li is more than 10 times higher than that of lithium-ion battery positive electrodes, and alkali metal rechargeable batteries are expected to be the nextgeneration energy device. in order to take advantage of the high specific capacity of alkali metals, it is necessary to develop cathode-active materials with high specific capacity. sulfur, which has a specific capacity of 1670 mah/g, more than six times that of lithium-ion positive electrodes, is one of the candidates for future cathode active materials [1]. in 2017, braga et al. presented three types of batteries of li-s, na-fc, and li-mno2, which employed newly developed glass solid electrolytes （li2.99ba0.005o1+xcl1-2x or na2.99ba0.005o1+xcl1-2x） [2]. in all of these batteries, alkali metal deposition in the positive active materials during battery discharge was observed, which was a difficult battery phenomenon to understand [3]. among these three types of batteries, li-s was reported in the most detail, and its capacity was over 10 times the theoretical capacity of s8 in the positive active materials. the emergence of these batteries was considered the forerunner of a paradigm-shifting battery phenomenon that was not an extension of conventional battery development concepts. these battery phenomena should reaffirm their importance and attractiveness in the future development of all-solid-state rechargeable batteries with alkali metal anodes. needless to say, a detailed reaction mechanism for these battery phenomena is essential for their proper design as practical batteries. however, at the time, an electrochemical reaction mechanism relating to these battery phenomena was unknown [2-6]. one of the main reasons for the lack of understanding of the cathode reactions in bragagoodenough batteries seems to be the lack of analysis from a cross-disciplinary perspective, such as chemistry, electrochemistry, solid-state physics, and heterojunction physics [7]. different fields of expertise generally have different perspectives of analysis. however, as the same battery reactions are observed, the results of the analyses of the reaction mechanisms examined from the perspective of the respective disciplines are considered to contain elements that complement each other. therefore, it is considered that there are common elements that allow results on reaction mechanisms from the viewpoints of chemistry and electrochemistry to share rationality with results on reaction mechanisms from the viewpoints of solid-state and heterojunction physics [8]. the alkali metal deposition reaction is usually written as li+ + e → li, taking the lithium deposition reaction as an example. however, it is clear that this reaction does not proceed in the cathode potential range [3]. for alkali metal deposition to occur at the cathodes as spontaneous reactions, the electron energy levels of the na+gland li+glalkali metal ions in the positive active materials must be lower than those of the alkali metal ions in the anode active materials. in other words, the condition for δg < 0 with regard to the gibbs’ free energy change of the reaction systems is that the electron energy levels of the alkali metal ions in the positive active materials must be lower than those of the alkali metal ions in the anode active materials. that is, the na+gland li+glalkali metal ions in the positive active materials and the alkali metal ions in the anode active materials are chemically different. on the basis of the experimental results of braga et al. [2], the electron energy levels of alkali metal ions in the na+gland li+glsurface states in the positive active materials are considered to be lowered to the electron energy levels of fc and mno2, the potential-determining materials of the positive electrodes. adsorption as chemisorption is a possible cause of this significant change in the electron energy levels [9-12]. the major difference between the active materials of the cathodes and those of the anodes is the presence of fc and mno2 in the positive active materials and the absence of those in the negative active materials. therefore, na+gland li+glalkali metal ions in the positive active http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 690 materials are considered to be in a state where molecular orbital contact is possible through solid/solid contact with fc and mno2, and the adsorption by this molecular orbital contact is expected to lower the electron energy levels of the alkali metal ions to the electron energy level region of fc and mno2. na+gland li+glare insulators [2], fc and mno2 are semiconductors [13,14]. from the perspective of solid-state and heterojunction physics, these solid/solid contact interfaces are regions such as schottky barriers and band discontinuities. in recent years, the theory of generalized charge neutrality level, gcnl, [15-25] (see appendix), has made it possible to analyze the electron energy levels of these solid/solid contact interfaces in consideration of orbital hybridization. the cathode reaction mechanisms of na-fc and li-mno2 proposed in this paper are based on these backgrounds and disclose the deposition mechanisms of alkali metals at the positive electrodes. in the previous paper [8], li-s was first focused on, and its electrochemical reaction mechanism was investigated by employing gcnl. in this paper, the electrochemical reaction mechanisms of the other two batteries of na-fc and li-mno2 were also investigated by employing gcnl on the basis of the facts from the experiments of braga et al. [2], and two new types of mechanisms for na-fc and li-mno2 were proposed. li-mno2 shows two possible mechanisms. one is the same mechanism as na-fc, where na+(sf) and li+(sf) adsorb one-to-one onto fc and mno2 in the positive active materials, respectively. another mechanism of li-mno2 is the simultaneous adsorption of two li+(sf) onto mno2. in both reaction mechanisms for na-fc and li-mno2, the deposition of alkali metals is caused by intramolecular electron transfer reactions within adsorbed molecules formed by orbital hybridization at heterojunction interfaces. these mechanisms proposed in this paper reflect the relationship between orbital hybridization at heterojunction interfaces and intramolecular electron transfer reactions, taking account of the gcnl analysis and its consistency with the facts from the experiments of braga et al. [2]. the two mechanisms of li-mno2 were also analyzed using gcnl to determine which mechanism is the thermodynamically dominant. gcnl has served to provide significant criteria for examining these reaction mechanisms in terms of solid-state and heterojunction physics. these reaction mechanisms are analyzed in a manner complementary to chemistry, electrochemistry, solid-state physics, and heterojunction physics. on the basis of these mechanisms, the maximum alkali metal deposition and maximum discharge capacity of na-fc and li-mno2 positive electrodes were investigated and found to be determined by the effective number of contacts that allow orbital hybridization between na+(sf) and fc and li+(sf) and mno2 and the effective molality of fc and mno2 in the positive active materials. the discharge capacity of these batteries is expected to be about 14 times that of the na-fc cathode and about 17 times that of the li-mno2 cathode under the conditions considered herein. in this paper, these reaction mechanisms are presented in detail, and based on these mechanisms, the cathode alkali metal deposition and discharge capacity of na-fc and li-mno2 are investigated in detail. experimental in this paper, all experimental results for na-fc and li-mno2 are taken from the data and descriptions in braga et al. [2]. the reaction mechanisms of these batteries are essentially based on the facts [2] that na and li are deposited on the positive electrode during the discharge of na-fc and li-mno2, respectively. m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 691 results and discussion a c[(ce)cc]n model for na-fc and li-mno2 figure 1 shows the braga-goodenough na-fc and li-mno2 battery diagram. these positive active materials consist of na+glpowder, fc powder, carbon black powder, and cu current collector for na-fc, and li+glpowder, mno2 powder, carbon black powder, and cu current collector for li-mno2. at the interfaces between the potential-determining materials fc and mno2 and these glass solid electrolyte powders, na for na-fc and li for li-mno2 are deposited in the positive active materials during discharge [2]. figure 1. diagrams of positive electrode active materials reflecting the c[(ec)cc]n mechanism on the bragagoodenough na-fc and li-mno2 battery under open circuit and discharge conditions in ordinary battery terminology, for na-fc and li-mno2, the cathode corresponds to the positive electrode, and the anode corresponds to the negative electrode. in this paper, however, since the phenomenon of alkali metal deposition on the cathode during battery discharge is confined to bragagoodenough batteries, these terms are often used together to clarify the battery phenomenon. in the case of na-fc, one type of reaction mechanism of the positive electrode is considered. on the other hand, in the case of li-mno2, two types of mechanisms are considered to be possible. one is the one-to-one adsorption of li+(sf) onto mno2, which is the same case as na+(sf) onto fc. the other is that two li+(sf) adsorb simultaneously onto mno2. in this section, the case of the same mechanism in na-fc and li-mno2 is examined, using mainly the notation in na-fc. the other mechanism for limno2 will be discussed in the next section. eqs. (1) to (10) are the elementary reactions of na-fc cathode discharge. the first step, eq. (1) shows the adsorption equilibrium reaction. the left side of eq. (1) shows that pna+(sf) for one fc will occupy the position where orbital hybridization is possible due to the contact between fc and na+gl-. the right side of eq. (1) indicates that among these pna+(sf), one na+(sf) with the highest probability of orbital hybridization at the interface with fc forms the adsorbed molecule of fc[na+(sf) (ad)](p-1)(na+(sf)). the remaining (p-1)na+(sf) are in a waiting state because fc is already used for one adsorption bond. here, the stoichiometric coefficient p is an integer, p ≥ 1. in this paper, terms related to adsorption, such as adsorption equilibrium reactions, are based on orbital hybridization at the heterojunction interfaces. http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 692 fc is a stable metallocene with fe(ii) as the center and cyclopentadienyl as the upper and lower sandwich structure. fc has fe(ii) 3dz2, 3dxy, and 3dx2-y2 orbitals in the valence band and 3dxz and 3dyz orbitals in the conduction band [13]. fc+/fc shows a typical reversible voltammogram in organic solvents, and its redox center is at fe(ii). this is a one-electron redox reaction on the 3d orbital, and eq. (1) is also considered to involve the 3d orbital. assuming that eqs. (2), (3) and (4) are a set of reactions related to na deposition, the discharge reaction proceeds by repeating this set of reactions with the p-value decreasing by one after reaction eq. (5). if the number of cycles of this set of reactions is n, the nth na deposition occurs in the nth cycle. then at the end of the nth cycle, the amount of discharge electricity and na deposition of the positive electrode are nf (c) and n moles, respectively, where n is an integer n ≤ p. since the first and the second set of reactions correspond to eqs. (2) to (4) and eqs. (5) to (7), respectively, the following eqs. (8) to (10) correspond to the nth set of reactions. fc + pna+(sf)  fc [na+(sf) (ad)] (p-1)(na+(sf)) (1) fc [na+(sf) (ad)] (p-1)(na+(sf)) → fc+ na (p-1)(na+(sf)) (2) fc+ na (p-1)(na+(sf)) + e → fc na (p-1)(na+(sf)) (3) fc na (p-1)(na+(sf))  fc [na+(sf) (ad)] na (p-2)(na+(sf)) (4) fc [na+(sf) (ad)] na (p-2)(na+(sf)) → fc+ 2na (p-2)(na+(sf)) (5) fc+ 2na (p-2)(na+(sf)) + e → fc 2na (p-2)(na+(sf)) (6) fc 2na (p-2)(na+(sf))  fc [na+(sf) (ad)] 2na (p-3)(na+(sf)) (7) fc [na+(sf) (ad)] (n-1)na (p-n)(na+(sf) ) → fc+ nna (p-n)(na+(sf) ) (8) fc+ nna (p-n)(na+(sf) ) + e → fc nna (p-n)(na+(sf) ) (9) fc nna (p-n)(na+(sf) )  fc [na+(sf) (ad)] nna (p-n-1)(na+(sf) ) (10) eq. (2) is the intramolecular electron transfer reaction of the adsorbed molecule. in eq. (2), fc reduces [na+(sf) (ad)] in the fc[na+(sf)(ad)](p-1)(na+(sf)) in the one-electron transfer reaction and is itself oxidized to deposit na as fc+na(p-1)(na+(sf)). eq. (3) is the electrochemical reaction in which fc+ of the fc+na(p-1)(na+(sf)) generated in eq. (2) receives one electron from the na negative electrode and immediately returns to the original fc to form fcna(p-1)(na+(sf)). eq. (4) is the adsorption equilibrium reaction that produces the new adsorbed molecule fc[na+(sf)(ad)]na(p-2)(na+(sf)) from fcna(p-1)(na+(sf)) produced in eq. (3). at this time, one na+(sf) with the highest probability of orbital hybridization from (p-1)(na+(sf)) forms the new adsorption bond orbital of fc[na+(sf)(ad)]na(p-2)(na+(sf)). in this paper, this mechanism is denoted as c[(ce)cc]n, where c denotes the chemical reaction step, and e denotes the electrochemical reaction step. the c in (ce)cc denotes that the (ce)c is catalytic reaction steps. here, the intramolecular electron transfer reaction of the adsorbed molecule is denoted as a chemical reaction because it does not appear as an external current. the catalytic function is performed by fc for na-fc. the n in c[(ce)cc]n indicates that the set of reactions of this [(ce)cc] part will cycle n times. therefore, for n = p, the cycle is maximized. under n = p conditions, the [(ce)cc] cycle process terminates at the (ce)c, and the subsequent c, the adsorption equilibrium reaction, disappears because all pna+(sf) on fc are discharged and reduced to pna and there is no na+(sf) available for adsorption on fc. the step of the deposition of na corresponds to the step c in the (ce)c. the intramolecular electron transfer of the adsorbed molecule in eqs. (2), (5) and (8) results in fc becoming fc+, which is ascribed to the one-electron transfer from the d orbital of the valence band of fe(ii) of fc to na+(sf)(ad). the open circuit voltage of na-fc is about 2.3 volts as read from the m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 693 charge-discharge cycle data [2], and the significant figures would be two digits at most. as described above, fc+ is immediately reduced and returned to fc in the subsequent e step by eqs. (3), (6), (9) and fc consequently undergo no apparent change. in eqs. (1) to (10), the currents observed externally correspond to the currents in these electrochemical reactions eqs. (3), (6) and (9). on discharge, fc contacts the electron energy of the na negative electrode, whose electron level is about 2.3 ev higher than the fermi level of fc, via the current collectors. that is, fc is in an electrochemical reaction condition where the overpotential is greatly superimposed on the negative side, and fc+ is immediately reduced to fc, with a large reaction rate expected in this e step. the catalytic reactions of fc regarding the na deposition in the positive active materials correspond to be the second c and the subsequent e steps in the c[(ce)cc]n mechanism. the intramolecular electron transfer reaction corresponding to the second c in the c[(ce)cc]n mechanism will be further discussed together with the case of li-mno2 in the section of the orbital hybridization and charge transfer at heterojunction interfaces in terms of gcnl. the total reaction equation for the (ce)c part at n = p, i.e. the maximum number of cycles, is the sum of eqs. (8) and (9), resulting in the following eq. (11), and the c[(ce)cc]n mechanism terminates at the e step of eq. (9), as described above. here, at n = p, the terms of fc[na+(sf)(ad)](n-1)na (p-n)(na+(sf)), fc+nna(p-n)(na+(sf)), and fc nna(p-n)(na+(sf)) in eqs. (8) and (9) become fc[na+(sf)(ad)] (p-1)na, fc+pna, and fc pna, respectively. fc [na+(sf) (ad)] (p-1)na + e → fc pna (11) in comparing eqs. (8), (9), and (11), it is clear that the elementary reactions of fc as a catalyst correspond to eqs. (8) and (9) in the (ce)c part of the c[(ce)cc]n mechanism. eq. (12) is the overall reaction of the positive electrode, which is obtained from the sum of eqs. (1) through (10). fc + pna+(sf) + ne → fc [na+(sf) (ad)] nna (p-n-1)(na+(sf) ) (12) eq. (12) involves the general case for n < p. at n = p, eq. (12) demonstrates the maximum number of cycles, as described above, and eq. (10) disappears, so that the overall reaction for the positive electrode is eq. (13) from the sum of eqs. (1) through (9). eq. (13) is also obtained from eq. (12) as n = p. fc + pna+(sf) + pe → fc pna (13) in the battery overall reaction, the na negative electrode reactions, which are paired with the overall reaction of the positive electrode in eq. (12), are eqs. (14) and (15) as follows: nna → nna+ + ne (14) nna+  nna+(sf) (15) the overall reaction of the na anode is eq. (16) from the sum of eqs. (14) and (15) as follows: nna → nna+(sf) + ne (16) therefore, the overall battery reaction of na-fc is eq. (17) from the sum of eqs. (12) and (16) as follows: fc + (p-n)na+(sf) + nna → fc [na+(sf) (ad)] nna (p-n-1)(na+(sf) ) (17) when n = p, eq. (17) is equal to eq. (18), which is also obtained from the sum of eq. (13) for the positive electrode and eq. (16) for the negative electrode. fc + pna → fc pna (18) the charging reaction of this c[(ce)cc]n mechanism corresponds to the reverse reaction of this mechanism. under charge-discharge cycle conditions, it is unlikely that na+(sf) and li+(sf) after http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 694 charging are the same as before discharging. it is natural to assume that the stoichiometric coefficient p-value appearing in eq. (1) can change from cycle to cycle. regarding na-fc charge-discharge cycle tests [2], the data are demonstrated to 200 cycles, corresponding to 33.3 days. 100th cycle discharge and charge voltages overlap when compared to those of the 200th cycle, and there is no degradation of charge-discharge characteristics between 100 and 200 cycles. these charge-discharge cycle characteristics of na-fc are considered to be ascribed to the sustainability of the c[(ce)cc]n mechanism during charge-discharge cycling. in the case of li-mno2, the notations of eqs. (1) to (18) of the c[(ce)cc]n mechanism for li-mno2 can be expressed by changing the notations from na-fc to li-mno2, where fc is replaced by mno2, intermediate fc+ by intermediate mn(o2)+, na by li, and na+(sf)(ad) by li+(sf)(ad). eq. (1) is the adsorption equilibrium reaction resulting from the contact between mno2 and li+gl-. eqs. (1) to (10) relate to the adsorption equilibrium reactions of one li+(sf) to mno2. the valence band of mno2 is composed of oxygen o2-, and its conduction band is composed of mn(iv) [20]. mno2 involves two o22p orbitals per mn, and eq. (1) for li-mno2 shows that one li+(sf) is capable of adsorbing one of these o22p orbitals. mno2 becomes mn(o2)+ by the intramolecular electron transfer of the adsorbed molecule in eqs. (2), (5), and (8). mn(o2)+ does not appear in the overall reaction equations of the positive electrode as fc+ does. mn(o2)+ is immediately reduced to mno2 for the same reason as fc+, according to eqs. (3), (6), and (9): the li anode potential is about 3 volts lower than the limno2 cathode because the open circuit voltage [2] of li-mno2 is about 3.0 volts. the notation mn(o2)+ is considered to be appropriate for eqs. (2), (5) and (8) in the sense that the oxygen anion 2o2in mno2 is the side that donates one electron to li+(sf). the catalytic reactions of mno2 on li deposition correspond to be the second c and the subsequent e steps, i.e. (ce)c steps, in the c[(ce)cc]n mechanism as well as na-fc. eq. (19) from eq. (12) is the overall reaction of the positive electrode of li-mno2, and eq. (20) from eq. (17) is the overall battery reaction of li-mno2. mno2 + pli+(sf) + ne → mno2 [li+(sf) (ad)] nli (p-n-1)(li+(sf)) (19) mno2 + (p-n)li+(sf) + nli → mno2 [li+(sf) (ad)] nli (p-n-1)(li+(sf)) (20) at n = p for the maximum number of cycles, the overall reaction of the positive electrode is eq. (21) from eq. (13) or eq. (19). the battery overall reaction is eq. (22) from eq. (18) or eq. (20) as n = p. mno2 + pli+(sf) + pe → mno2 pli (21) mno2 + pli → mno2 pli (22) not only fc but also mn(iv) in mno2 is not changed in this mechanism, which is also supported by the charge-discharge cycle tests, which were conducted under various charge and discharge time conditions for up to 270 cycles for a total of 112.5 days [2]. in these tests, no degradation of chargedischarge characteristics was observed until the test was terminated on day 112.5. if mn(iii) is formed, like a primary battery of lithium manganese dioxide, on discharge, the charge-discharge cycle characteristics will gradually deteriorate, and cycle tests will not continue. in respect of the electron energy levels of adsorbed particles, in general, they are not the same as those of isolated ones [9-12]. in the c[(ce)cc]n mechanism, na+(sf) forms the adsorption bond with fc which significantly lowers na+(sf) electron energy level. the electron energy level of na+(sf) decreases from the lumo level of na+(sf) to the electron energy level region of fc as na+(sf) approaches fc, while gradually broadening its level width [9,12]. figure 2 shows that the electron energy levels of na+(sf) vary significantly with distance from fc. the relationship between mno2 and li+(sf) can be drawn similarly. the electron energy level width m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 695 reaches its maximum at a distance of x0 from fc where orbital hybridization is possible [12]. at the same time, the adsorption energy curve, δgad, becomes the most stable energy state [9,12]. therefore, the position of pna+(sf) in eq. (1) is in this x0 region. the x0 on the x-axis in figure 2 is considered to be around 200 pm (0.2 nm) level, as shown in the following consideration. this value x0 is one of the essential points in determining the maximum number of cycles of the deposition of alkali metals in the positive active materials and the maximum discharge capacity of na-fc and limno2 positive electrodes, as will be described in other sections. figure 2. variation of electron energy levels of na+(sf) adsorbates at various distances from the adsorbent fc surface. as na+(sf) approaches fc, the electron energy levels of na+(sf) spread out due to its interaction with fc, having the widest probability density at x0 of the gad minimum. δgad = adsorption energy curve of na+(sf) approaching fc, w = probability density of electron energy states, x = distance to na+(sf), and x0 = distance to adsorbed na+(sf) (ad) in the solution-based electric double layer, the inner helmholtz layer is 200 pm and the outer helmholtz layer is 300 pm [21]. the inner helmholtz layer is the region of inner-sphere electron transfer, where reaction species are desolated, and the orbital hybridization and adsorption behavior between reaction species and electrode materials become apparent. therefore, the inner-sphere electron transfer is clearly distinguished from the electron transfer reaction in the outer helmholtz layer, i.e. outer-sphere electron transfer, where orbital hybridization between reaction species and electrode materials is not easy perceptible. the x0 shown in figure 2 is considered equivalent to the position of this inner-sphere electron transfer, which is around 200 pm for na-fc and li-mno2. the y-axis scale is estimated based on the homo level of -4.8 ev for isolated fc and the open circuit voltage of ca. 2.3 volts for na-fc. in figure 2, the shaded region below the ef indicates the overlap of the electron energy distributions of the current collectors, fc, and na+(sf)(ad). the fermi level, ef, of the cu and carbon black current collectors, coincides with that of fcna+(sf)(ad) and demonstrates the potential of the positive electrode of na-fc under open circuit equilibrium conditions. http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 696 with regard to the surface states, na+(sf) and li+(sf) in the c[(ce)cc]n mechanism are denoted with the intention that the electron energy levels of these surface states are different from those of the internal na+ of na+gland li+ of li+gl-. it is clear that na+(sf) and li+(sf) levels at the na+gland li+gl surfaces, rather than within them, are directly involved in the battery reactions of na-fc and li-mno2. in general, in ionic-bonded oxides, the metal ion constitutes the conduction band [20], and the oxygen ion constitutes the valence band [20], even though some of these oxides are partially covalent. the surface states of a metal ion in a metal oxide are basically the electron level of the metal ion, which should be at the level of the conduction band, but whose energy is reduced due to surface effects and shifted into the band gap [22]. na+gland li+glare amorphous [2,4-6], and their band structures differ from crystalline ones. in the amorphous case, the conduction band bottom ecb in the crystalline case corresponds to the diffuseband-tail states, a distribution of density of states that decays slowly toward the low energy side [23]. amorphous structures do not seem to inhibit the c[(ce)cc]n mechanism. rather, it affects the c[(ce)cc]n mechanism. diffuse-band-tail states may be considered to have the effect of dispersing and increasing the surface states in the region below the lumo level shown in figure 2. amorphous structures of na+gland li+glwould enhance the opportunity for orbital hybridization contacts between fc and na+(sf) and mno2 and li+(sf) at lower energy levels in the c[(ce)cc]n mechanism. a c[2(ce)cc]n model for li-mno2 in the case of li-mno2, the reaction mechanism of the simultaneous adsorption of two li+(sf) onto mno2 is discussed in this section. the corresponding reaction equations are given in eqs. (23) to (38) below. this reaction mechanism is denoted as c[2(ce)cc]n in this paper. the 2 in the 2(ce)c part means that two consecutive (ce)c steps are involved, and the c in the 2(ce)c part denotes that it is the catalytic reaction part as in the case of the c[(ce)cc]n mechanism. the group of reaction equations between the long horizontal lines corresponds to the [2(ce)cc] part in c[2(ce)cc]n. the n in c[2(ce)cc]n indicates that the set of reactions of this [2(ce)cc] part will cycle n times. it is sufficient to elucidate this mechanism when p is even because when p is odd, adding the c[(ce)cc]n mechanism with p = 1 will complete the c[2(ce)cc]n mechanism. in the following, p-values are treated as even numbers. mno2 + pli+(sf)  mno2 [2li+(sf) (ad)] (p-2)(li+(sf)) (23) mno2 [2li+(sf) (ad)] (p-2)(li+(sf)) → mn(o2)+ [li+(sf) (ad)] li (p-2)(li+(sf)) (24) mn(o2)+ [li+(sf) (ad)] li (p-2)(li+(sf)) + e → mno2 [li+(sf) (ad)] li (p-2)(li+(sf)) (25) mno2 [li+(sf) (ad)] li (p-2)(li+(sf)) → mn(o2)+ 2li (p-2)(li+(sf)) (26) mn(o2)+ 2li (p-2)(li+(sf)) + e → mno2 2li (p-2)(li+(sf)) (27) mno2 2li (p-2)(li+(sf))  mno2 [2li+(sf) (ad)] 2li (p-4)(li+(sf)) (28) mno2 [2li+(sf) (ad)] 2li (p-4)(li+(sf)) → mn(o2)+ [li+(sf) (ad)] 3li (p-4)(li+(sf)) (29) mn(o2)+ [li+(sf) (ad)] 3li (p-4)(li+(sf)) + e → mno2 [li+(sf) (ad)] 3li (p-4)(li+(sf)) (30) mno2 [li+(sf) (ad)] 3li (p-4)(li+(sf)) → mn(o2)+ 4li (p-4)(li+(sf)) (31) mn(o2)+ 4li (p-4)(li+(sf)) + e → mno2 4li (p-4)(li+(sf)) (32) mno2 4li (p-4)(li+(sf))  mno2 [2li+(sf) (ad)] 4li (p-6)(li+(sf)) (33) eq. (23) is the first step, which shows that two li+(sf) of the pli+(sf) in the region of adsorption position x0 shown in figure 2 are adsorbed simultaneously, and the adsorption equilibrium is m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 697 established. eq. (24) is the adsorption intramolecular one electron transfer from o2in the valence band of mno2 to one of 2li+(sf)(ad), where the mno2 side becomes mn(o2)+, and one li is deposited from 2li+(sf) (ad). this electron transfer does not take current externally. eq. (25) is the e step in which mn(o2)+ produced by the intramolecular electron transfer is immediately reduced to mno2 under the potential of the li anode, as described in the c[(ce)cc]n mechanism. eq. (26) is again the intramolecular electron transfer reaction. so far, one of the two li+(sf) in the adsorbed molecule formed in eq. (23) has already been reduced to li. eq. (26) is the reaction in which the remaining li+(sf) adsorbed molecule is reduced again by the intramolecular electron transfer, resulting in li deposition. eq. (27) is equivalent to eq. (25), which is the electrochemical reduction of mn(o2)+ generated by the intramolecular electron transfer reaction in eq. (26). eq. (28) is again the adsorption equilibrium reaction involving two new adsorptions of li+(sf). assuming that eqs. (24) to (28) are a set of reactions related to li deposition, the discharge electricity and li deposition amount to 2f (c) and 2li, respectively, in this set of reactions. after eq. (29), this set of reactions is repeated, and the discharge reaction proceeds with the p-value (p ≥ 2) decreasing by two. in the case of p = 2 corresponding to the number of cycle n = 1, the cycle does not continue and terminates with eq. (27). the group n = 1 is from eq. (24) up to eq. (28), and the group n = 2 is from eq. (29) up to eq. (33). same as above for n = 1, when the cycle terminates with n = 2, eq. (33), which is the third c in the group n = 2 disappears as in the case of the c[(ce)cc]n mechanism. the group of reaction equations between short lines and between long and short lines corresponds to the 2(ce)c part in c[2(ce)cc]n. that is the pair of eqs. (24) and (25) and the pair of eqs. (29) and (30) correspond to the first (ce)c part in 2(ce)c, while the pair of eqs. (26) and (27) and the pair of eqs. (31) and (32) correspond to the second (ce)c part in 2(ce)c. the first c in c[2(ce)cc]n is eq. (23), and the third c corresponds to eqs. (28) and (33). since eqs. (24) to (28) correspond to cycle n = 1 and eqs. (29) to (33) correspond to cycle n = 2, the reaction equation set for cycle n is the following eqs. (34) to (38). mno2 [2li+(sf) (ad)] (2n-2)li (p-2n)(li+(sf)) → mn(o2)+ [li+(sf)(ad)] (2n-1)li (p-2n)(li+(sf)) (34) mn(o2)+ [li+(sf)(ad)] (2n-1)li (p-2n)(li+(sf)) + e → mno2 [li+(sf)(ad)] (2n-1)li (p-2n)(li+(sf)) (35) mno2 [li+(sf)(ad)] (2n-1)li (p-2n)(li+(sf)) → mn(o2)+ 2nli (p-2n)(li+(sf)) (36) mn(o2)+ 2nli (p-2n)(li+(sf)) + e → mno2 2nli (p-2n)(li+(sf)) (37) mno2 2nli (p-2n)(li+(sf))  mno2 [2li+(sf) (ad)] 2nli (p-2n-2)(li+(sf)) (38) the overall reaction of the positive electrode is eq. (39) derived by the sum of eqs. (23) through (38). the battery's overall reaction, including the negative electrode reactions, is derived by the same procedure as the c[(ce)cc]n mechanism. mno2 + pli+(sf) + 2ne → mno2 [2li+(sf) (ad)] 2nli (p-2n-2)(li+(sf)) (39) the maximum number of cycles is when n = p/2, and all pli+(sf) is used for discharge. the reaction terminates with eq. (37), and eq. (40) of this overall reaction of the positive electrode is obtained from the sum of the reactions in eqs. (23) to (37), obtained from eq. (39) as n = p/2. mno2 + pli+(sf) + pe → mno2 pli (40) at n = p/2, the terms of 2nli (p-2n)(li+(sf)) in eq. (37) and [2li+(sf) (ad)] 2nli (p-2n-2)(li+(sf)) in eq. (39) are equal to pli. eq. (40) becomes eq. (41) in the case of cycle n = 1 corresponding to p = 2. mno2 + 2li+(sf) + 2e → mno2 2li (41) when p is odd, pli+(sf) decrease by two, and the positive electrode reaction terminates once at the c[2(ce)cc]n mechanism. consequently, one li+(sf) remains as mno2li+(sf)(ad). this further discharge reaction of mno2li+(sf)(ad) proceeds by the c[(ce)cc]n mechanism. in eqs. (1) to (3) in the http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 698 c[(ce)cc]n mechanism, replacing fc with mno2, the intermediate fc+ with the intermediate mn(o2)+, na with li, and na+(sf)(ad) with li+(sf)(ad), the overall reaction for the remaining one li+(sf) becomes the following eq. (42). mno2 + li+(sf) + e → mno2 li (42) therefore, when p is odd, the amount of the positive electrode discharge electricity and li deposition correspond to the addition of 1f (c) and 1 mole to the amount of the discharge electricity of 2nf (c) and 2n moles of li deposition when p is even in the c[2(ce)cc]n mechanism. eq. (21) in the c[(ce)cc]n mechanism is consistent with eq. (41) when p = 2 and with the combined result of eqs. (41) and (42) when p = 3. therefore, regardless of whether p is odd or even, the discharge result of the c[2(ce)cc]n mechanism is consistent with that for n = p in the c[(ce)cc]n mechanism. as a result, the discharge results for li-mno2 are the same for both c[2(ce)cc]n and c[(ce)cc]n mechanisms, although their elementary reactions are different. orbital hybridization and charge transfer at heterojunction interfaces in terms of gcnl in this section, interfacial reactions at heterojunction interfaces are examined from the viewpoint of heterojunction physics. in the previous paper [7], the theory of the generalized charge neutrality level, gcnl, was applied to analyze the reaction mechanism of the braga-goodenough li-s battery (see appendix). gcnl is also applied here to analyze the reaction mechanisms of the c[(ce)cc]n and c[2(ce)cc]n. the main points of this section are as follows: 1. intramolecular electron transfer appearing in the mechanisms is from which orbital to which orbital the electron transfer occurs. this investigation is to reveal the possibility of alkali metal deposition in gcnl view. 2. whether adsorption processes and intramolecular electron transfer reactions by orbital hybridization are compatible with gcnl analysis. 3. which of the two reaction mechanisms in li-mno2 is more likely to proceed thermodynamically in terms of gcnl. this symbol gcnl has two meanings [8]: one is the theory itself and the other is the charge neutrality level itself evaluated by gcnl. initially, gcnl views on the directions of electron transfer within adsorbed molecules are shown. gcnl shows two possible directions [15-19] of electron transfer at a heterojunction interface as follows: with respect to na-fc, figure 3 shows the possibilities for the directions of electron transfer based on gcnl. two possibilities for the electron transfer within adsorbed molecules fcna+(sf)(ad) are shown here. one is the electron transfer from the valence band of fc to the na+(sf) conduction band of na+gl-. the other is the electron transfer from the o2and clvalence bands of na+glto the fc conduction band. in the former case, it is consistent with the experimental fact of braga et al. [2] that na is deposited. therefore, the electron transfer within adsorbed molecules fcna+(sf)(ad) is from the valence band of fc to the conduction band of na+(sf), as shown by the bold line in figure 3. as described in the c[(ce)cc]n mechanism, the valence band and conduction band of fc correspond to those of the splitting of the d-orbitals of fe(ii) at the center of fc in the ligand field [13]. na+gland li+glconduction bands contain the frontier orbitals of na+ and li+ cations, and their valence bands comprise the frontier orbitals of o2and clin the glass electrolytes [20]. with respect to li-mno2, figure 4 shows the possibilities regarding the directions of the electron transfer based on gcnl. that is, as in the case of na-fc, there are two possibilities of intramolecular m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 699 electron transfer in adsorbed molecules mno2li+(sf)(ad). one is from o2-, responsible for the valence band of mno2, to li+(sf), responsible for the conduction band of li+gl-. the other is from o2and cl-, which are responsible for the valence band of li+gl-, to mn(iv), responsible for the conduction band of mno2. figure 3. two possibilities of orbital hybridization in the theory of gcnl between fc and na+gl-. the inset arrows show the direction of possible electron transfer at their interfaces. d’cb and d’vb respectively correspond to the density of states of the conduction band and valence band of na+gl-, and dcb and dvb respectively correspond to the density of states of the conduction band and valence band of fc figure 4. two possibilities of orbital hybridization in the theory of gcnl between mno2 and li+gl-. the inset arrows show the direction of possible electron transfer at their interfaces. d’cb and d’vb respectively correspond to the density of states of the conduction band and valence band of li+gl-, and dcb and dvb respectively correspond to the density of states of the conduction band and valence band of mno2 in the former case, it is consistent with the experimental fact of braga et al. [2] that li is deposited. therefore, the intramolecular electron transfer in adsorbed molecules of mno2li+(sf) (ad) and mno2[2li+(sf) (ad)] is in the direction from o2to li+(sf), as shown by the bold line in figure 4. from figures 3 and 4, the deposition of na and li in the c[(ce)cc]n and c[2(ce)cc]n mechanisms is consistent with the results predicted in terms of gcnl. therefore, in respect of intramolecular electron transfer appearing in both mechanisms, electron transfer occurs from fc to na+(sf) and from mno2 to li+(sf), which are consistent with the gcnl views. na+gland li+glare insulators [2]. fc is a semiconductor [13], and mno2 is also a semiconductor [14] with a narrow band gap. in the case of na-fc and li-mno2, the application of gcnl corresponds to the case of semiconductor-insulator interfaces. in this case, gcnl is given by eq. (43): gcnl = e’vb + e’g|tvb’-cb|2dcbd’vb/(|tvb’-cb|2dcbd’vb + |tvb-cb’|2dvbd’cb) (43) where e’vb, e’g, d’vb, and d’cb are the level of the valence band top, the band gap, the density of states of the valence bands, and the density of states of the conduction bands of na+glor li+gl-, respectively; dvb and dcb are the density of states of the valence bands and the density of states of the conduction bands of fc or mno2, respectively; |tvb’-cb| is the transfer energy between valence band orbitals of http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 700 na+glor li+gland conduction band orbitals of fc or mno2; |tvb-cb’| is the transfer energy between valence band orbitals of fc or mno2 and conduction band orbitals of na+glor li+gl-. the terms of |tvb’-cb|2dcbd’vb and |tvb-cb’|2dvbd’cb correspond to effective bond strengths at the heterojunction interface [17]. although na+gland li+glare amorphous structures, their band structures are expressed in terms of ordinary terminology because there is no obstacle in the gcnl analyses of these reaction mechanisms. the theory gcnl is built on an orbital hybridization between the unoccupied and occupied states of two materials at an interface [15-19]. in this theory, the charge transfer between fc and na+gl and mno2 and li+glare realized by orbital hybridization between their unoccupied and occupied states. their electron transfer is confined to just interfaces between fc and na+gland mno2 and li+glbecause na+gland li+gldemonstrate greater than 8ev band gap [2,4-6]. eq. (43) is obtained by δρ = 0 in eq. (44), which was derived by applying the second-order perturbation theory of quantum mechanics [17,24]. δρ means transfer charges from fc or mno2 to the insulator and proportional to the equation’s right-hand side. at δρ = 0 in eq. (44), the ef corresponds to the fermi level and the charge neutrality level for fc after contacting na+(sf) and is identical to gcnl of eq. (43). these relationships are the same for li-mno2. δρ ∝ dvbd’cb|tvb-cb’|2/(e’cb ef) dcbd’vb|tvb’-cb|2/(ef e’vb) (44) where e’cb corresponds to the conduction band bottom of na+glor li+glin this study. assuming dvb = dcb and |tvb-cb’|2 = |tvb’-cb|2 in (43), gcnl is consistent with the conventional charge neutrality level, cnl, expressed in eq. (45), and this cnl is capable of demonstrating neither the effects of orbital hybridization nor those of the adsorption between fc and na+(sf) and mno2 and li+(sf) [15-9,25]. cnl = e’vb + e’g d’vb /(d’vb + d’cb) (45) in order to evaluate the effect of orbital hybridization, the magnitude of the second term on the right-hand side of eq. (43) will be compared with eq. (45) to evaluate the difference in electron energy levels at heterojunction interfaces with and without orbital hybridization. in this study, let eq. (43), which is gcnl, be transformed into eq. (46) as follows: gcnl = e’vb + e’g d’vb /(d’vb +  d’cb) (46) where  = (|tvb-cb’|2/|tvb’-cb|2)(dvb/dcb). in comparing eq. (46) and eq. (45), gcnl < cnl corresponds to  > 1, and gcnl > cnl corresponds to  < 1. therefore, the relationship between cnl and gcnl can be confirmed by examining . therefore, depending on whether  is greater than or less than 1, the difference in electron energy levels at heterojunction interfaces with and without orbital hybridization can be evaluated. at first, |tvb-cb’|2/|tvb’-cb|2 in  is examined. from the results shown in figure 3, in the case of nafc, the electron transfer based on gcnl is from the valence band of fc to the conduction of na+(sf). in the case of li-mno2, figure 4 shows that the electron transfer based on gcnl is from the valence band of mno2 to the conduction of li+(sf). therefore, orbital hybridization is strong between these bands, so that, for both na-fc and li-mno2, |tvb-cb’|2 ≫ |tvb’-cb|2, i.e., |tvb-cb’|2/|tvb’-cb|2 ≫ 1. next, consider (dvb/dcb) in . in the following (dvb/dcb) calculations, the density of states was represented by the number of states of orbitals in the energy levels actually relating to these battery reactions [26]. in the case of fc in na-fc, the density of states for the valence band, dvb, corresponds to the three orbitals 3dz2, 3dxy, and 3dx2-y2 of fe(ii) [13] located at the fc center. the number of states in each orbital is 2, and thus the total is 2x3 = 6. on the other hand, the density of states for the conduction band, dcb, corresponds to the two orbitals 3dxz and 3dyz of fe(ii) [13], which is 2x2 = 4. m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 701 therefore, in the case of fc, taking account of these dvb = 6 and dcb = 4, (dvb/dcb)fc > 1. therefore,  >> 1 for the interface between na+(sf) and fc, taking account of |tvb-cb’|2/|tvb’-cb| >> 1 and (dvb/dcb)fc > 1. therefore, in the case of na-fc, clearly  >> 1, and clearly gcnl < cnl. the electron energy level at the heterojunction interface with orbital hybridization is obviously lower than without orbital hybridization. therefore, this finding indicates that when fc and na+(sf) are in contact in the positive active materials, the formation of adsorbed molecules fcna+(sf)(ad) and the associated intramolecular electron transfer due to orbital hybridization is more thermodynamically stable than when no orbital hybridization occurs. under na-fc open circuit conditions, at the fc and na+(sf) heterojunction interface, intramolecular electron transfer forms a positive dipole on the fc side, lowering the electron energy level of the fc side and pushing up that of the na+(sf) side relatively, resulting in equilibrium at gcnl where the electron energy levels of both fc and na+(sf) sides are balanced. this dipole state is retained under open circuit conditions. once a discharge state appears, the dipole at this heterojunction interface once disappears due to the electrochemical reaction of the e step following the interfacial intramolecular electron transfer c step in the (ce)c part of the c[(ce)cc]n mechanism. that is, as described in the former sections, under na-fc discharge conditions, the na+(sf) side is consequently reduced to na by fc side, fc itself is oxidized to the fc+ intermediate, which is immediately reduced to fc under the potential of the na anode, and these reactions are repeated n times in the c[(ce)cc]n mechanism. in the case of li-mno2, two mechanisms were demonstrated as possibilities in the previous sections. one is the same c[(ce)cc]n mechanism as na-fc, and the other is the c[2(ce)cc]n mechanism. in respect of the term of |tvb-cb’|2/|tvb’-cb|2 in  in eq. (46), |tvb-cb’|2/|tvb’-cb|2 >> 1 for both na-fc and li-mno2, as described above. therefore, for li-mno2, the term of (dvb/dcb)mno2 in  in eq.(46) has to be examined in order to confirm whether  for li-mno2 is greater than or less than 1. at first, the c[(ce)cc]n mechanism for li-mno2 is examined, where li+(sf) is adsorbed one-to-one against mno2, i.e., the formation of adsorbed molecules mno2li+(sf)(ad). as shown in figure 4, in this case, dvb corresponds to the 2p orbitals of 2o2per one li+(sf), so that the number of states of the 2p orbitals is 2(23) = 12. dcb corresponds to the 3d orbitals of mn(iv), which is 25 = 10. thus, (dvb/dcb)mno2 > 1 for the c[(ce)cc]n mechanism. therefore,  >> 1 for the heterojunction interface between li+(sf) and mno2 also results in gcnl < cnl. in the c[(ce)cc]n mechanism, this result, gcnl < cnl, indicates that when mno2 and li+(sf) are in contact with the positive active materials, the formation of adsorbed molecules mno2li+(sf)(ad) and the associated intramolecular electron transfer due to orbital hybridization is also more thermodynamically stable than when no orbital hybridization occurs. in the case of the c[2(ce)cc]n mechanism, the adsorption of two li+(sf) onto mno2 occurs simultaneously, i.e., the formation of adsorbed molecules mno2[2li+(sf)(ad)]. in this case, dvb corresponds to the 2p orbitals of 2o2for two li+(sf), so that the number of states of the 2p orbitals per one li+(sf) is 1(23) = 6. on the other hand, dcb corresponds to the 3d orbitals of mn(iv), which is ten, as in the case of the c[(ce)cc]n mechanism. therefore, (dvb/dcb)mno2 < 1 for the c[2(ce)cc]n mechanism. taking account of |tvb-cb’|2/|tvb’-cb|2 >>1 and this (dvb/dcb)mno2 < 1, it could be considered that  >1 for the interface between 2li+(sf) and mno2, which also results in gcnl < cnl. consequently, the relationship between gcnl and cnl for li-mno2 is gcnl < cnl for both c[(ce)cc]n and c[2(ce)cc]n mechanisms. http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 702 as examined above, in the c[(ce)cc]n mechanism, gcnl < cnl is clear, but in the c[2(ce)cc]n mechanism, gcnl < cnl is not as clear as that for the c[(ce)cc]n mechanism. however, even for the c[2(ce)cc]n mechanism, this finding, gcnl < cnl, is reasonable to assume that when mno2 and 2li+(sf) are in contact in the positive active materials, the formation of adsorbed molecules mno2 [2li+(sf)(ad)] by orbital hybridization and the associated intramolecular electron transfer are thermodynamically stable than when no orbital hybridization occurs. as in the case of na-fc, under li-mno2 open circuit conditions, at the mno2 and li+(sf) heterojunction interface, intramolecular electron transfer forms a positive dipole on the mno2 side, lowering the electron energy level of the mno2 side and pushing up the energy level of the li+(sf) side relatively, resulting in equilibrium at gcnl where the electron energy levels of both mno2 and li+(sf) sides are balanced. this dipole state is retained under open circuit conditions. once a discharge state appears, as in the case of na-fc, the dipole at this heterojunction interface once disappears due to the electrochemical reaction of the e step following the interfacial intramolecular electron transfer c step in the (ce)c part of the c[(ce)cc]n mechanism and the 2(ce)c part of the c[2(ce)cc]n mechanism. that is, as described in the former sections, under li-mno2 discharge conditions, the li+(sf) side is consequently reduced to li by the mno2 side, mno2 itself is oxidized to the mn(o2)+ intermediate, which is immediately reduced to mno2 under the potential of the li anode, and these reactions are repeated n times in the mechanisms. at the end of this section, we examine which mechanism, c[(ce)cc]n or c[2(ce)cc]n, is more thermodynamically favorable for li-mno2. as shown above, (dvb/dcb)mno2 < 1 in the c[2(ce)cc]n mechanism. on the contrary, (dvb/dcb)mno2 > 1 in the c[(ce)cc]n mechanism. the value of |tvb-cb’|2/|tvb’-cb|2 is common as |tvb-cb’|2/|tvb’-cb|2 >> 1 in both mechanisms. therefore,  for c[(ce)cc]n is obviously bigger than that for c[2(ce)cc]n, which means that gcnl for c[(ce)cc]n is lower than that for c[2(ce)cc]n from eq. (46). therefore, the c[(ce)cc]n mechanism for li-mno2 is considered to be more thermodynamically stable than the c[2(ce)cc]n mechanism. the c[2(ce)cc]n mechanism requires two li+(sf) to form adsorption orbitals simultaneously, which seems to be less probable than the c[(ce)cc]n mechanism. this intuitive view is compatible with the results of the gcnl analysis. taking these findings into consideration, although gcnl indicates that the c[2(ce)cc]n mechanism is still possible, the c[(ce)cc]n mechanism is considered to be dominant in the li-mno2 cathode reactions. limitations of the alkali metal deposition cycle in mechanism-based stoichiometry, the maximum number of cycles of alkali metal deposition at the positive electrodes per mole of fc and mno2 is n = p for c[(ce)cc]n and n = p/2 for c[2(ce)cc]n, as described in the previous section. with regard to this p-value, this specific value per actual effective mole of fc and mno2 will be considered in this section. in order to do this, (mfc)eff and (mmno2)eff are introduced, where (mfc)eff and (mmno2)eff are the effective molalities (mol/kg) of fc and mno2 in the positive active materials, respectively. in general, the activity of a reactive species does not numerically match its concentration determined in the reagent preparation stage. since the utilization coefficient is used as a parameter for active material utilization in practical batteries, in the same way, the utilization coefficient  is introduced here. in practice, fc = (mfc)eff/(mfc)mes for na-fc, and mno2 = (mmno2)eff/(mmno2)mes for li-mno2, where (mfc)mes and (mmno2)mes are defined as the molalities of fc and mno2 determined at the preparation stage of the positive active materials, respectively. taking account of fc and mno2, (mfc)eff = fc(mfc)mes and (mmno2)eff = mno2 (mmno2)mes. therefore, to determine the actual maximum number of cycles of the m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 703 alkali metal deposition at the positive electrodes, not only the specific p-value but also fc for (mfc)eff and mno2 for (mmno2)eff have to be determined. first, the procedure to determine the specific p-value is examined, assuming that both (mfc)eff and (mmno2)eff are fixed at 1 mol/kg. that is, the specific p-value, i.e. the maximum number of cycles of alkali metal deposition, demonstrated in this section corresponds to the case for (mfc)eff = (mmno2)eff = = 1 mol/kg. the determination procedures of fc for (mfc)eff and mno2 for (mmno2)eff will be examined in detail in the next section, and the maximum discharge capacity of na-fc and li-mno2 positive electrodes will be examined, taking account of the p-values corresponding to (mfc)eff = (mmno2)eff = 1 mol/kg determined in this section. in the case of li-mno2, gcnl studies indicate that the c[(ce)cc]n mechanism is more thermodynamically favorable than the c[2(ce)cc]n mechanism, and the discharge results are the same for both li-mno2 mechanisms. therefore, in the following, the c[(ce)cc]n mechanism will be used to describe the reaction mechanism for na-fc and li-mno2. the number of na+(sf) and li+(sf) available for adsorption onto fc and mno2 in the positive active materials, i.e. those in contact at x0 position in figure 2, is the p-value itself appearing in the c[(ce)cc]n mechanism. as discussed in the previous section, the closest distance of na+(sf) and li+(sf) to fc and mno2 was considered to be ca. 200pm. therefore, it is necessary to examine the p-value at this x0 position. in this section, this p-value will be evaluated by steps 1-5 as follows: 1. from the size of the unit crystal lattice of fc and mno2, consider a new size lattice with axes that are stretched by 200 pm each to the left, right, top, and bottom of each axis of each unit crystal lattice for a total of 400 pm larger for each axis. 2. find the total surface area of the new lattice size. find how many times this total surface area corresponds to the area of the circle, taking into account of radii, r, of na+(sf) and li+(sf), respectively, expressed as (total surface area)/r2, for fc as (sfc/sna+) and for mno2 as (smno2/sli+). 3. find the number of fc and mn involved in the unit crystal lattice and denote it as nfc and nmn, respectively. 4. introduce a factor of fs for the area factor, which is the fraction of the total surface area of this large new lattice that is actually occupied by na+(sf) and li+(sf) because the surface of the new lattice of fc and mno2 will be in common contact with the glass solid electrolyte as well as carbon black. 5. introduce a factor fp relating to the porosity of the positive active materials, where fp = 1(porosity of positive active materials)/100. from 1-5 above, the maximum p-value per effective molality of 1 mol/kg of fc, (pfc)max, and that per effective molality of 1 mol/kg of mno2, (pmno2)max, are given by the following eqs. (47) and (48), respectively. (pfc)max = fs fp (sfc/sna+)/nfc (47) (pmno2)max = fs fp (smno2/sli+)/nmn (48) figure 5 shows a schematic of na+(sf) around the fc unit crystal lattice and na deposited on discharge. the projected plane of the solid line is the bc plane of the unit crystal lattice of fc. the fc crystal is the monoclinic system, with crystal axes a = 1050, b = 763, c = 595 pm and axis angles  =  = 90° and  = 121°. the  angle corresponds to the angle between the a and c axes. the fc number involved in the unit crystal lattice is nfc = 2. this data can be referred to elsewhere. the dashed line corresponds to the solid line extended 200 pm to the left and right of the b axis and 200 pm above and below the c axis. na+(sf) and na tangent to the dashed line are on the ab and ac planes. other na+(sf) and na are projected onto the bc plane. http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 704 figure 5. an image of fc monoclinic bc plane projection and na+(sf) and deposited na. circles and dark ones correspond to adsorbable na+(sf) and na deposited in each discharge cycle. circle radii reflect those of 186 na and 99 na+ in comparison with the fc monoclinic system axes of b = 763 and c = 595 pm for the unit cell. x0 is 200 pm, which is the closest distance of na+(sf) to the unit cell in figure 5, na+(sf) and na total 10. thus, in this case, the p-value in eq. (1) is at most 10. two of these are shown depositing as na on discharge. the first cycle shows na deposition corresponding to cycle number n = 1 and the second cycle to n = 2. here, 8 na+(sf) are left, therefore when all of these are discharged, the number of cycles of na deposition will reach a maximum at 10. although there is no specific information on the amounts of fc, mno2, carbon black, na+gl powder, and li+glpowder in the preparation of the positive active materials [2], realistic estimates for fs and fp for the evaluation of (pfc)max are to be employed. based on the above step 2, (sfc/sna+) is directly calculated as [2(14501163)+2(1163x995)+2(11631450sin59°)]/992 = 277.6. the radius of na+(sf) is replaced with 99 pm of na+. radius data on na+ can be referred to elsewhere. assuming that fs for na+(sf) is 0.5 and fp is 0.5, (pfc)max is 0.50.5277.6/2 = 34.7 from eq. (47) with nfc as 2. these values of fs = 0.5 and fp = 0.5 are considered realistic range values. rounding down to the decimal point, (pfc)max = 34. therefore, in the case of na-fc, under the above conditions, the maximum number of reaction cycles n corresponding to n moles of na deposition in the c[(ce)cc]n mechanism is 34 per effective molality of 1 mol/kg of fc. figure 6 shows a projection schematic of li+(sf) around the -mno2 unit crystal lattice and li deposited by partial discharge reactions. the crystal structure of -mno2 is the tetragonal system with crystal axes a = 440, b = 440, and c = 290 pm, the most compact and simplest of all mno2 crystal structures. the number of mno2 in the unit crystal lattice is nmn = 2. these data can be referred to elsewhere. the projection plane is the ab plane, and the shape of the solid line is similar to the ab plane of the unit crystal lattice of -mno2. the dashed line corresponds to the solid line extended 200 pm above and below the a-axis and 200 pm to the left and right of the b-axis. li+(sf) and li tangent to the dashed line are on the ac and bc planes. the other li+(sf) and li are imaged as projected on the ab plane. in figure 6, li+(sf) and li total 10. therefore, in this case, the p-value in eq. (1) is at most 10. two of these are li deposited in the discharge reaction. in figure 6, cycle 1 shows li deposition corresponding to cycle number n = 1 and cycle 2 to n = 2. since 8 li+(sf) are left, if these are all discharged, the number of li deposition cycles will be a maximum at 10. m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 705 figure 6. an image of -mno2 tetragonal ab plane projection and li+(sf) and deposited li. circles and dark ones correspond to adsorbable li+(sf) and li deposited in each discharge cycle. circle radii reflect those of 152 li and 59 li+ in comparison with the -mno2 tetragonal system axes of a = 440 and b = 440 pm for the unit cell. x0 is 200 pm, which is the closest distance of li+(sf) to the unit cell in examining (pmno2)max using eq. (48), as in the case of na-fc, estimates for fs and fp are used. based on the above step 2, (smno2/sli+) is directly calculated as [28402+4(840690)]/59 2 = 342. the radius of li+(sf) is replaced with 59 pm of li+. radius data on li+ can be referred to elsewhere. assuming that fs for li+(sf) is 0.5 and fp is 0.5, (pmno2)max = 0.5x0.5x342/2 = 42.8 from eq. (48) with nmn as 2. as described above, fs = 0.5 and fp = 0.5 are considered realistic range values. therefore, in the case of li-mno2, under the above conditions, the maximum number of reaction cycles n corresponding to n moles of li deposition in the c[(ce)cc]n mechanism is 42 per effective molality of 1 mol/kg of mno2 when the decimal point of (pmno2)max = 42.8 is rounded down to 42. in the above discussions, fs and fp are factors that obviously depend on the conditions for the preparation of the positive active materials. therefore, (pfc)max and (pmno2)max are sensitive to fs and fp. when a 0.1 increase in fs from 0.5 to 0.6 results in (pfc)max = 41.6 from 34.7 and (pmno2)max = 51.3 from 42.8, care should be taken in discussing the first digit of the above values of the factors. mno2 has several types of crystal systems. in the systems, both -mno2 and -mno2 are used in batteries, and -mno2 has long been known to have better battery properties [27]. it was not clear in the paper [2] which type of mno2 was used, but -mno2 [28] seems to have been employed. mno2 contains two mno2 in the unit crystal lattice and has the highest electrical conductivity of all mno2 crystal systems due to the shortest mn-mn distance and small band gap [14].  -mno2 is a large lattice size containing four mno2, and its crystal structure is more complex than that of -mno2. the unit crystal lattice size of -mno2 is the smallest among the various mno2 crystal systems, and the crystal lattice surface area of -mno2 per mn is smaller than that of -mno2 per mn [27], which (smno2/sli+)/nmn is smaller for -mno2 than for -mno2. therefore, the maximum number of cycles (pmno2)max is smaller for -mno2 than for -mno2. braga et al. use the expression “self-charging,” [5,6,28], which in the c[(ce)cc]n mechanism would correspond to an increase in the p-value. an increase in the p-value leads to an increase in discharge capacity. as described in the c[(ce)cc]n mechanism, under charge-discharge cycle conditions, it is naturally considered that the p-value appearing in eq. (1) changes from cycle to cycle http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 706 as the charge-discharge cycle progress. it is considered that an increase in the effective density of na+(sf) for na-fc and li+(sf) for li-mno2 after charging corresponds to being capable of increasing the p-value. discharge capacity in this section, the pragmatic maximum discharge capacity of na-fc and li-mno2 cathodes will be examined on the basis of (pfc)max in eq. (47) and (pmno2)max in eq. (48). in order to do this, it is necessary to determine the utilization coefficients fc and mno2 to determine (mfc)eff and (mmno2)eff, as described in the previous section. taking account of fc mno2 (pfc)max, and (pmno2)max, the actual maximum discharge capacity of na-fc and li-mno2 cathodes are directly obtained from the following eqs. (49) and (50), respectively. (qmax)(na-fc) = (fc)[(mfc)mes][(pfc)max]f (c) (49) (qmax)(li-mno2) = (mno2)[(mmno2)mes][(pmno2)max]f (c) (50) where (qmax)(na-fc) and (qmax)(li-mno2) represent the pragmatic maximum discharge capacity of na-fc and li-mno2 positive electrodes, respectively; (fc)[(mfc)mes] = (mfc)eff and (mno2)[(mmno2)mes] = (mmno2)eff. it is apparent that even though 1 mol/kg of fc and mno2 is prepared at the preparation stage of the positive active materials, 1 mol/kg of fc and mno2 is not dispersed as a single unit crystal lattice in the positive active materials, but they are present in a state of clusters. therefore, the unit crystal lattice of fc and mno2 inside clusters cannot participate in the adsorption equilibrium reactions. therefore, obviously, fc < 1 and mno2 < 1 as in the case of ordinary batteries. in order to estimate these practical values for fc and mno2, simple cubic and rectangular blocks were considered. that is, each of these blocks was assumed as a unit crystal lattice, and the outer surface area of a block formed in one piece by several blocks was compared to the sum of the surface areas of each unit block. in this case,  can be expressed as  = (outer surface area of block assembly)/(sum of surface areas of each individual unit block), which can be basically equivalent to (mfc)eff/(mfc)mes and (mmno2)eff/(mmno2)mes as defined in the previous section. the results showed that the rate of decrease in  with respect to the rate of increase in the number of unit blocks is the largest when these unit blocks are connected by the same number of unit blocks on the xyz axis and assembled into a shape similar to that of the unit block. on the other hand, if the blocks are connected in only one of the xyz axis, the rate of decrease in  is the smallest relative to the rate of increase in the number of unit blocks. in the former case,  can be simplified to  = 1/q for the homomorphism unified by q cubes or q rectangles where q is the number of cubic or rectangular unit blocks: e.g. for q = 2,  = 0.5; for q = 5,  = 0.2; for q = 10,  = 0.1. in the latter case, the relationship between  and the number of blocks is not simple:  = 0.83 for a sequence of 2,  = 0.73 for a sequence of 5, and  = 0.70 for a sequence of 10, with an asymptote around 0.67. in practice,  is considered to be present between these two extreme conditions. when  = 0.1 from the former case and  = 0.73 from the latter case are taken into account, a pragmatic value of  is considered to be around in the middle of the range between  = 0.73 and 0.1, which seems to be around  = 0.4. when fc = mno2 = 0.4 and (mfc)mes = (mmno2)mes = 1 mol/kg, (fc)(mfc)mes = ( mno2)(mmno2)mes = = 0.4 mol/kg for (mfc)eff and (mmno2)eff. in these cases, from eqs. (49) and (50), the maximum discharge capacity of the na-fc positive electrode is (0.434)f = 13.6f (c), and that of the li-mno2 positive electrode is (0.442)f = 16.8f (c). under the above conditions, the discharge capacity of m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 707 the na-fc positive electrode is about 14f (c), which corresponds to 14 cycles of na deposition corresponding to 14 moles of na. in the case of li-mno2, the discharge capacity of the positive electrode is about 17f (c), which corresponds to 17 cycles of li deposition corresponding to 17 moles of li. in respect of the positive active materials for na-fc and li-mno2, in accordance with the c[(ce)cc]n and c[2(ce)cc]n mechanisms and eqs. (47) to (50), both fc and na+glcorrespond to those for na-fc, and both mno2 and li+glcorrespond to those for li-mno2. as shown in eqs. (49) and (50), (qmax)(na-fc) and (qmax)(li-mno2) are proportional to the amount of fc and mno2 in terms of (fc)[(mfc)mes] and (mno2)[(mmno2)mes], respectively. that is, fc and mno2 obviously play the role of active materials. fc and mno2, on the other hand, act as catalysts, as indicated in the previous sections. therefore, preparing the amount of fc and mno2 is considered equivalent to preparing the amount of catalyst. na+gland li+glaffect (pfc)max in eq. (47) and (pmno2)max in eq. (48), respectively, and each of (pfc)max and (pmno2)max consists of mainly three parameters. on the other hand, with regard to fc and mno2, the relationships between (mfc)mes and (mfc)eff and (mmno2)mes and (mmno2)eff are simple as controlling parameters of the discharge capacity because these pairs of parameters are connected in terms of only one parameter of . therefore, it is considered that fc and mno2 are simpler and more straightforward than na+gland li+glas controlling parameters for the positive electrode capacity. taking account of these findings, it seems simple and appropriate to consider the capacity of the positive electrode represented by (mfc)mes and fc for na-fc and (mmno2)mes and mno2 for mno2. as with conventional batteries, if the capacity of na and li negative electrodes is sufficient above that of the positive active materials, the discharge capacity of na-fc and li-mno2 will be determined by each capacity of the positive electrodes, and vice versa. however, the capacity balance between the cathodes and anodes of na-fc and li-mno2 is considered very specific in this respect, as is the braga-goodenough li-s [2,8]. in order to balance the discharge capacity between the anode and cathode, when fc = mno2 = 0.4 and (mfc)mes = (mmno2)mes = 1 mol/kg, the capacity of the na anode of na-fc is required to be at least 14 times higher than that of the cathode, and the capacity of the li anode of li-mno2 is required to be at least 17 times higher than that of the cathode. that is, for fc and mno2 with an effective molality of 0.4 mol/kg, the na anode capacity of na-fc and the li anode capacity of li-mno2 need an effective molality equivalent to at least 14 and 17 times the capacity of the cathode, respectively. according to braga et al. [2], the discharge capacity of na-fc and li-mno2 was controlled by the na and li anodes. although this fact [2] is simply considered to be ascribed to a lack of the capacity of alkali metal anodes, taking account of the capacity balance between the anodes and cathodes of na-fc and li-mno2 in terms of the mechanisms as described in this paper, the battery capacity determination of na-fc and li-mno2 by the alkali metal anodes is considered to be plausible and comprehensible. based on the above findings, na-fc and li-mno2, including braga-goodenough li-s, appear to be absolutely different from the conventional concept of cathode capacity limits for alkali metal anodes. optimization of the positive active materials of the braga-goodenough rechargeable batteries was not performed [2]. if it had been done, the performance of these batteries would have been much enhanced. orthogonal array experiments are necessary to obtain the actual maximum capacity of the cathodes because multiple factors are involved with respect to active materials, as shown in eqs. (47)-(50). possible control factors other than eqs. (47)-(50) in orthogonal array experiments may include the reagent manufacturer, the powder mesh number, the pressure on the active materials, and so on. http://dx.doi.org/10.5599/jese.1704 j. electrochem. sci. eng. 13(4) (2023) 687-711 braga-goodenough na-ferrocene and li-mno2 reactions 708 conclusions concerning the cathode deposition of alkali metals during discharge in na-fc and li-mno2, two new types of the c[(ce)cc]n and c[2(ce)cc]n mechanisms were proposed. these reaction mechanisms were obtained by the results of complementary analyses between the chemical and electrochemical viewpoints and the solid-state physics and heterojunction physics, gcnl, viewpoints on the solid/solid contact interfaces. gcnl, the theory of generalized charge neutrality level, has provided critical decision criteria for examining these reaction mechanisms with regard to orbital hybridization and intramolecular electron transfer, which are directly related to the adsorption and alkali metal deposition in these mechanisms. the na-fc mechanism corresponds to a multi-step c[(ce)cc]n mechanism where c is the chemical reaction, e is the electrochemical reaction, c is the catalytic (ce) step, and n is the n-cycle of the [(ce)cc] step. the nth cycle corresponds to n moles of na or li deposition. in the case of li-mno2, two mechanisms were considered. one is the c[(ce)cc]n mechanism, which is the same as na-fc, and the other is the c[2(ce)cc]n mechanism, in which the (ce) of the catalytic reaction 2(ce)c is two consecutive steps. in the c[(ce)cc]n mechanism, na+(sf) and li+(sf) form one-to-one adsorption onto fc and mno2, respectively. on the other hand, in the c[2(ce)cc]n mechanism of li-mno2, two li+(sf) absorb simultaneously onto mno2. although the elementary reactions in the two mechanisms of limno2 were different, the same results were obtained for the discharge capacity and deposited li content. in the two mechanisms of li-mno2, gcnl analysis indicated that the c[(ce)cc]n mechanism lowered the charge neutrality level more than the c[2(ce)cc]n mechanism, indicating that the c[(ce)cc]n mechanism is dominant and more thermodynamically stable than the c[2(ce)cc]n mechanism. in this regard, the c[(ce)cc]n mechanism is considered to be representative of these cathode reactions. in both mechanisms, the first c and the third c are the steps in the formation of adsorption equilibrium. in the second c step corresponding to the c in the (ce) part, fc and mno2 reduce na+(sf) and li+(sf) to deposit na and li, respectively, which correspond to intramolecular charge transfer reactions within adsorbed molecules during discharge. fc and mno2 are themselves oxidized to fc+ and mn(o2)+ intermediates, respectively, which are immediately reduced to fc and mno2 under alkali metal anode potentials in the subsequent e step. fc and mno2 act as catalysts in these (ce)c and 2(ce)c steps because they are not consequently changed on discharge. in the case of na-fc, the orbital hybridization involving adsorption occurs between the conduction band orbital of na+(sf) and the valence band orbital of fc. in the case of li-mno2, this orbital hybridization occurs between the conduction band orbital of li+(sf) and the valence band orbital of 2o2in mno2. the direction of intramolecular electron transfer is from the valence band orbital of fc to the conduction band orbital of na+(sf) for na-fc and from the valence band orbital of 2o2in mno2 to the conduction band orbital of li+(sf) for li-mno2. the maximum alkali metal deposition and maximum discharge capacity of na-fc and li-mno2 cathodes were evaluated on the basis of these mechanisms and found to be essentially determined by the effective number of na+(sf) and li+(sf) contacts that can hybridize orbitals with fc and mno2 and the effective molality of fc and mno2 in the positive active materials. algorithms for determining the effective contact numbers of na+(sf) and li+(sf) and the effective molality of fc and mno2 were examined. to determine the effective contact numbers of na+(sf) and li+(sf), it was necessary to determine the closest distance of these ions to the fc and mno2 unit crystal lattices and introduce parameters such as the area fraction of these ions in relation to the total surface area around the fc and mno2 unit crystal lattices at the closest distance being 200 pm. to determine the m. sakai j. electrochem. sci. eng. 13(4) (2023) 687-711 http://dx.doi.org/10.5599/jese.1704 709 effective molality of fc and mno2, it was necessary to introduce their utilization coefficients in the positive active materials. as a result employing realistic values of these parameters, when the effective molality of fc and mno2 was 0.4 mol/kg, the number of alkali metal deposition cycles of the cathodes in the c[(ce)cc]n mechanism was about 14 for na-fc and about 17 for li-mno2, and the deposition of alkali metals amounted to about 14 moles na for na-fc and about 17 moles li for li-mno2, and cathode discharge capacities corresponding to these were about 14f (c) for na-fc and about 17f (c) for li-mno2. in this respect, the anode capacities of na and li to balance these cathode capacities were more than 14f (c) for the na anode capacity of na-fc and more 17f (c) for the li anode capacity of li-mno2. therefore, for fc and mno2 with an effective molality of 0.4 mol/kg, the na anode of na-fc and the li anode of li-mno2 needed to have an effective molality equivalent to at least 14 and 17 times the capacity of the cathode, respectively. on the basis of these c[(ce)cc]n and c[2(ce)cc]n mechanisms, the self-charging phenomenon described by braga et al. [5,6,28] was considered to be attributed to an increase in the effective number of na+(sf) and li+(sf) contacts that can hybridize orbitals with fc and mno2 during chargedischarge cycling. conflict of interest: this paper is free of conflict of interest. acknowledgements: the author thanks takahiro hidaka for helpful discussions on ligand field chemistry. appendix charge neutrality level in heterojunction the concept of charge neutrality level appeared more than half a century ago [29,30], but was only observed experimentally in 2000 [31,32]. the charge neutrality level can be regarded as the effective ef on the semiconductor and insulator side relative to the schottky barrier, and charge transfer takes place to match the ef of the metal with the charge neutrality level of the semiconductor and insulator at the interface [17]. the theory of generalized charge neutrality level, gcnl, emerged around 2005 [15-19]. gcnl in a heterojunction is characterized by the energy and density of states of the bands of the materials on both sides and by orbital hybridization at their contact interfaces [15-19]. on the other hand, the conventional cnl consists of the energy and density of states of the bands for a one-sided material [25]. cnl cannot handle orbital hybridization at the contact interface and the associated charge transfer. in recent years, problems such as tunneling leakage currents have arisen due to the increasing integration of electronic devices into nano-devices, and high-k insulators have attracted attention as a material for solving these problems. hfo2, a candidate for the high-k insulators, exhibited behavior that was difficult to resolve with conventional understanding of heterojunction interfaces, and this gcnl succeeded in clarifying this schottky barrier behavior [15-19]. gcnl can be extended from the schottky barrier in metals/insulators to the evaluation of semiconductor/insulator and semiconductor/semiconductor band discontinuities [15-19]. 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[32] s. hara, surface science 494 (2001) 2805-2810. https://doi.org/10.1016/s00396028(01)01596-5 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1704 https://dx.doi.org/%2010.1016/0921-4526(93)90175-6 https://dx.doi.org/10.1103/physrevb.35.6182 https://doi.org/10.5796/denka.21.367 https://dx.doi.org/10.1103/physrev.71.717 https://dx.doi.org/10.1063/1.1702952 https://dx.doi.org/10.1380/jsssj.21.791 https://doi.org/10.1016/s0039-6028(01)01596-5 https://doi.org/10.1016/s0039-6028(01)01596-5 https://creativecommons.org/licenses/by/4.0/) {determination of 4-nitrophenol using moo3 loaded glassy carbon electrode via electrochemical sensing approach:} http://dx.doi.org/10.5599/jese.956 143 j. electrochem. sci. eng. 11(3) (2021) 143-159; http://dx.doi.org/10.5599/jese.956 open access: issn 1847-9286 www.jese-online.org original scientific paper determination of 4-nitrophenol using moo3 loaded glassy carbon electrode via electrochemical sensing approach bhagyashri b. kamble1, kalyanrao m. garadkar2, kirankumar k. sharma3, pravin kamble3, shivaji n. tayade2 and balu d. ajalkar1, 1shivraj college, gadhinglaj, shivaji university, kolhapur maharashtra, india 416502 2department of chemistry, shivaji university, kolhapur, maharashtra, india 416004 3school of nanoscience and technology, shivaji university, kolhapur, maharashtra, india 416004 corresponding author: bajalkar@rediffmail.com; tel.: +919404255274 received: january 27, 2021; revised: april 14, 2021; accepted: april 18, 2021; published may 24, 2021 abstract in order to raise possible ways of moo3 synthesis and improve its existing applications, moo3 nanomaterial was successfully synthesized through the solvo-hydrothermal route by utilizing a mixture of ionic liquid (1-butyl-3-methylimidazolium bromide) as a solvent, and water as co-solvent in 1:1 ratio. the morphology and structural parameters of il-assisted moo3 product were examined by x-ray diffraction (xrd), fourier-transform infrared spectroscopy (ft-ir), and scanning electron microscopy (sem). additionally, the surface wettability and particle size distribution were inspected using the contact angle and dynamic light scattering (dls) analysis. glassy carbon electrode (gce) surface was then modified by il-assisted moo3. the formed il-moo3/gce was employed as an electrochemical sensor for determination of 4-nitrophenol (4-np), which is very toxic and important pollutant. the redox behavior of 4-np at the surface of il-moo3/gce was investigated by cyclic voltammetry (cv) and differential pulse voltammetry (dpv) techniques. limits of detection (lod) and limits of quantification (loq) determined from cv were found to be 6.76 and 22.5 m, while from dpv recordings, 5.41 and 18.0 m are found. the obtained results clearly reveal possible application of moo3 for selective and sensitive sensing of 4-np. the decorated electrode was successfully employed for determination of 4-np in the river water real samples. keywords molybdenium oxide nanoparticles; ionic liquid; solvo-hydrothermal synthesis; nitrophenol; voltammetry techniques http://dx.doi.org/10.5599/jese.956 http://dx.doi.org/10.5599/jese.956 http://www.jese-online.org/ mailto:bajalkar@rediffmail.com j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 144 introduction environmental crisis across the globe has been increasing day by day due to numerous causes such as global warming, ozone depletion, and the greenhouse effect. high contribution to the extensive environmental pollution is caused by the exponential growth of industrialization, which frequently uses different hazardous starting materials. a good example of hazardous materials is 4-nitrophenol (4-np) that has been used as a starting material in pesticide production and manufacturing of drugs and dyes. consequently, 4-np has been frequently found in wastewaters from pharmaceutical, paint, and petrochemical industries. environmental exposure and distribution of 4-np in air, water, residue, and soil were found out to be 0.006, 94.6, 4.44 and 0.95 %, respectively. these data suggest that an enormous amount of 4-np is released in the environment, particularly water, in which even after degradation, it retains for almost 2.8 to 13.7 days [1,2]. consequently, 4-np is considered as the major pollutant by epa (environmental protection agency), usa. several studies showed that 4-np is potentially genotoxic and considered as drug impurity with the threshold limit value (tlv) of 4 mg/day for humans, which is relatively higher than the reference dose [3]. therefore 4-np is considered as a hazardous compound which leads to unfavorable effects on human health such as nausea, cyanosis, etc. 4-np is highly stable and soluble in water and therefore, purification of water by conventional methods such as adsorption, extraction, and nanofiltration is a challenging task since these methods involve formation of toxic byproducts [4]. because of these negative impacts, the need for detection and monitoring of concentrations of 4-np has become the priority of researchers. it has generally been considered essential to develop a relatively inexpensive and simple method for rapid detection of 4-np in an aqueous solution, with high sensitivity and selectivity. different techniques have been utilized for determination of 4-np, such as spectrophotometric method [5,6] high-performance liquid chromatography [7,8] high-performance capillary electrophoresis [9], and gas chromatography [10]. the main disadvantages of these methods, however, are complexity, cost, and time-consuming. nowadays, electrochemical methods have established significant attention due to their immense advantages such as good sensitivity, fast response, and simple operation [11,12]. it has already been observed, however, that electrochemical detection of 4-np at bare electrodes is seriously suffering from high over-potential, interference issues, and low sensitivity. therefore, electrode surfaces were usually chemically modified, and extensively used to develop highly efficient electrochemical sensors for determination of 4-np. in the past decade, numerous studies on the use of electrodes chemically modified with metal nanoparticles [13,14] metal oxide nanoparticles [15], graphene oxide [16], graphene-gold nanoparticles and their nano-composites have been reported to detect and measure 4-np. among these materials, graphene and its derivatives have been preferably studied in electrochemical sensing due to their superior properties such as two-dimensional layered structure, low charge transfer resistance, and wide electrochemical potential window [17]. however, graphene consists of zero band gaps that limit its diverse applications. during the past decade, various metal oxides (e.g. mn, cu, co, ti, mo, and w) and their electrode materials have turn out to be an essential part of the research field concerned primarily to avoiding environmental and energy crises via degrading environmental organic pollutants and green energy production [18]. it is interesting to note, however, that two-dimensional structure of moo3, is analogous to graphene, and therefore, it could also be considered as a promising material in sensing applications, due to high surface area, thermal stability, superior mobility, and high electrochemical performances compared to other metal oxides [19]. b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 145 moo3 is made up of moo6 layers, and each layer is held together by strong covalent bonds and weak van der waals force with a wide band-gap (3.0 ev) [20]. these structural properties of moo3 can offer a diverse approach in the field of electrochemical sensing. thus, moo3 has been studied for efficient bio-sensing (glucose and dopamine) due to its structure and bonding between mo-o, leading to the intercalation of analyte. various stoichiometric and reduced forms of molybdenum oxide (moox) have been explored in dye degradation and other applications such as li-ion batteries and supercapacitors [21]. consequently, moo3 is the most studied, environment-friendly, low-cost electrode material in the energy storage field, what is due to its significant multiple oxidation states, which could promote redox reaction of electrode. therefore, moo3 is generally considered as an appreciable electro-active compound [22,23]. various synthetic routes of moo3 have already been studied, such as chemical bath deposition [24], sol-gel [25] and hydrothermal methods [26] among which, the hydrothermal is the simplest and cheapest method. in this work, the hydrothermal route is explored in preparation of moo3 using ionic liquid (il). il has been considered as a designer solvent due to its organized structure with extended hydrogen bonds, which exhibit exclusive interaction with metal nanoparticles. therefore, many efforts have already been reported to study the interaction between some ionic liquids and metal nanoparticles [27,28]. herein, we report a facile synthesis of moo3 using il, [bmim]+[br]-, as co-solvent to achieve different morphology and better performance of moo3. il assisted synthesized moo3 was then used to modify gce and prepare il-moo3/gce, which was subsequently tested as an electrochemical sensor for determination of 4-np. for the electrochemical sensing analysis, cyclic voltammetry (cv) and differential pulse voltammetry (dpv) techniques were employed. experimental chemicals 4-nitrophenol (≥90 %), n-butyl bromide (c4h9br) and toluene (c6h5.ch3) (ar) (≥99 %) were procured from thomas baker chemicals, mumbai, india, while 1-methylimidazole (c4h6n2) (≥99 %) was obtained from spectrochem pvt. ltd. mumbai, india. ammonium heptamolybdate tetrahydrate (ahm) (99 % pure) (nh4)6mo7o24 4h2o), concentrated nitric acid (hno3) (≥99 %), absolute ethanol (≥99 %), dihydrogen potassium orthophosphate (kh2po4) (≥99 %), and dipotassium orthophosphate (k2hpo4) (≥99 %) were obtained from s d fine-chem. ltd., mumbai, india. synthesis of ionic liquid assisted moo3 moo3 was synthesized by solvo-hydrothermal route using a mixture of il, [bmim]+ [br]-, and triple distilled water (tdw) in 1:1 ratio as reported in our previous work [29]. ammonium heptamolybdate (0.03 m) was initially dissolved in a mixture of il. after dissolution, the concentrated hno3 was added to the same beaker with strong stirring. the above solution was moved into teflon-lined stainless-steel autoclave containing a previously cleaned glass substrate. the hydrothermal reaction was carried out at the temperature of 180 °c for 6 h. after completion of the reaction, the autoclave was naturally cooled to room temperature. deposited moo3 thin film was washed few times with tdw, ethanol, and dried at 60 °c. annealing process of dried moo3 thin film was carried out at 350 °c for 2 h. the annealed moo3 was used for fabrication of gce for electrochemical analysis. the mechanism of reaction is represented by equations 1-4 and the experimental setup is shown in scheme 1. [bmim]+ [br]+ h2o → il mixture (1) il mixture + [(nh4)6mo7o24.4h2o] → 6[bmim]+ [mo7o24]-6 + 6nh4br (2) http://dx.doi.org/10.5599/jese.956 j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 146 6[bmim]+ [mo7o24]-6 + 6nh4br + 6hno3 → 6[bmim]+ [br]+ 6nh4no3 + [h6mo7o24] (3) [h6mo7o24] → 7moo3 + 3h2o (4) scheme 1. hydrothermal reaction setup for il-assisted synthesis of moo3 thin film preparation of modified electrode il assisted moo3 film was dispersed in ethanol (2 mg ml−1) with the aid of ultrasonication for 30 minutes. gce surface was cleaned by polishing with al2o3 (0.05 μm) using a polishing cloth, then washed several times with distilled water, ultrasonicated for 5 minutes, and finally dried. 2 μl of moo3/ethanol dispersion was drop coated on pre-cleaned gce, until gce surface was exclusively covered by moo3 particles. to provide stability of deposited moo3 particles, nafion 2 %, perfluoro-sulfonated polymer was added. the active surface area of the modified gce was determined using 1 mm ferrocene, a standard redox couple in 0.1m kcl as the supporting electrolyte at room temperature, and randles– ševčik equation. for a reversible redox process, the peak current is expressed as ip = 2.69 ×105 n3/2adapp1/2coν1/2 (5) where ip represents anodic peak current, n = 1 is number of electrons transferred, a is active surface area of electrode, dapp = 0.16×10-6 cm2 s-1 is diffusion coefficient of ferrocene [30], ν is potential scan rate and co is bulk concentration of ferrocene (1 mm). the active surface area of the modified electrode is determined as 0.54 cm2, which is about 7 times greater than the geometric area (0.0707 cm2) of bare gce. the roughness factor was found to be 7.22, when calculated from the ratio of active surface area to the geometric area of the working electrode. characterizations powder x-ray diffraction (xrd) was recorded using (thermo arl x’tra with cukα irradiation, λ = 0.154056 nm). the scanning electron microscopy (sem) observation was performed with hitachi su-70 (japan). fourier transform infrared (ft-ir) spectroscopy analysis was performed using ft-ir -6600 spectrometer (bruker company), the contact angle determination was performed using holmarc opto mechanics with 10 microliter droplet, and dls analysis was employed using litesizer-500. electrochemical experiments the redox behavior of 4-np was examined using cyclic voltammetry (cv), differential pulse voltammetry (dpv) techniques and electrochemical workstation (pgatat302n metrohm auotolab). a threeelectrode system was employed, where either bare or modified gce served as the working electrode (geometric area = 0.07 cm2), a platinum wire as the counter electrode, and ag/agcl-3 m kcl as the reference electrode. cyclic voltammetry was recorded in the potential range from −0.2 to 0.6 v, with 4-np in phosphate buffer solution of ph 5.8. a differential pulse voltammetry was conducted in the b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 147 range of −0.2 to 0.6 v, with modulated amplitude 0.025 v, step width of 0.005 v, and interval time of 0.5 s. electrochemical impedance spectroscopy (eis) measurements were performed in 0.1 m kcl solution containing 1 mm ferrocene with frequencies ranging from 105 to 0.1 hz, and ac amplitude of the applied wave potential of 10 mv. eis measurements of bare and modified electrodes were performed at 0.15 v. results and discussion structural studies of il-moo3 figure 1 depicts highly intense and spiky diffraction peaks at 2 = 12.60, 23.26, 25.70, 27·34, 33.89, 38·96, 46.06, 49.18, 52.80, 58.66 and 64.60ο, corresponding to the planes (020), (110), (021), (111), (150), (002) (211), (081) and (062), indicating orthorhombic crystal structure. all xrd peaks were recognized as moo3 peaks from jcpds card 76-1003 [31]. the intense and sharp peaks from xrd imply that the prepared material is of crystalline nature. the calculated lattice constant of annealed moo3 was found as: a = 3.94 å, b=14.01 å and c = 3.70 å, which were in good agreement with the jcpds card for orthorhombic structure. the crystallite size and shape in finely divided powdered solid often help to associate many physical properties of the system. the average crystallite size of 62.8 nm was anticipated from debye-scherrer’s equation    = 0.9 cos d (6) where d is average crystallite size,  = wavelength,  = diffraction angle, and  = fwhm (full width half maximum). ftir study of il-moo3 figure 2 shows ft-ir spectrum of as-synthesized il-moo3. different peaks appear at 617 to 3392 cm-1. the bending vibration mode of mo-o-mo appears at 617 cm-1, while vibrations of mo-omo of mo6+ are attributed by 870 cm-1 [32]. figure 1. xrd pattern of il-moo3 figure 2. ft-ir spectrum of il-moo3 the characteristic peak of mo-o-mo is slightly shifted to a higher wavelength, what indicates the peculiar structure of moo3 obtained in the presence of il. the absorption band at 992 cm-1 is assigned to the presence of a mo=o bond [33]. therefore, the absorption peaks below 1000 cm-1 are associated to the interatomic vibration of metal and oxygen. the peak appearing at 1492 cm-1 and 2852-2922 cm-1 are associated with imidazole h-c-c and h-c-n bending mode [34]. the http://dx.doi.org/10.5599/jese.956 j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 148 characteristic absorption band of the hydrogen-bonded hydroxyl group (-oh) appears at 3392 cm-1 (ν-oh) and 1612 cm-1 (δ-oh). therefore, ft-ir results show the successful formation of il-moo3. morphology of il-moo3 (sem) figure 3 shows sem images of il-moo3, revealing an aggregated, irregular and to some extent, a flower-like morphology. figure 3. sem images of il-moo3 at different resolutions from (a) to (d) it is well known that the cationic part of il [bmim]+[br]-, can form micelles in the aqueous solution which further promote aggregation [35]. during formation of moo3, il is consumed as co-solvent, and the resulting increase in the steric hindrance and growing alkyl chain lengths limits diffusion of mo6+ ions and hence, the aggregated particles are developed. il moiety consists of hydrogen bond and free ions, which induce electrostatic interaction and van der waals force into metal nanoparticles. il typically interacts with the surface of metal nanoparticles with either cations or anions to produce diversity in morphology. it is also well known that the anion dominant interaction shows flower shape morphology [36]. scheme 2 indicates that bromine anions prefer to adsorb on the surface of metal oxide nanoparticles along with the imidazole moiety to form hydrogen bond, leading to enhancement of electrostatic interaction to induce flower-like morphology. scheme 2. ionic liquid [bmim]+[br]and metal oxide surface interaction b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 149 contact angle measurement of il-moo3 the measurement of the contact angle of il-moo3 on the surface of the glass substrate was performed using sessile drop method. the contact angle is a major factor for characterization of surface wettability and also, measurements of phase interface on the solid surface. if the contact angle is smaller than 90°, the solid surface is considered hydrophilic and if the contact angle is larger than 90°, the surface is referred as hydrophobic [37]. as shown in figure 4, the contact angle of water at the surface of il-moo3 is 67.3ο. since the contact angle is less than 90ο, there is an increase in the hydrophilicity of il-moo3, what can influence the buffer solution. as a result, il-moo3 surface shows compatibility for immobilization of analyte [38]. therefore, the surface wettability of il-moo3 is relevant for applications such as sensors and biosensors. figure 4. wetabillity test of il-moo3 particle size distribution and zeta potential dynamic light scattering (dls) analysis was performed for further assessment of particle size distribution. figure 5(a) shows the particle size distribution of il-moo3 nanoparticles investigated by the laser diffraction method and dls technique. the obtained results revealed that the particle size distribution of il-moo3 ranges from 20 to 150 nm with the mean particle size of 65 nm. this suggests a broad distribution range of oxide nanoparticles size, and gives the average particle size of il-moo3. figure 5. (a) particle size distribution vs. particle diameter and (b) zeta potential of il-moo3 in various solvent systems figure 5(b) depicts zeta potential in different solvent systems, which is indicative for stability of il-moo3. as shown in figure 5(b), il-moo3 nanoparticles possess high stability in the phosphate buffer solution (pbs) with the highest negative zeta potential of -260 mv, as compared to -202 mv for ethanol. the lowest stability is observed in water (-3.8 mv). these results suggest that the prehttp://dx.doi.org/10.5599/jese.956 j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 150 sence of il increases stability of moo3, possibly due to electrostatic interactions of il and moo3 [39]. agglomeration of il-moo3 in the water system leads to low stability of dispersion, what is one of the factors for the observed low zeta potential magnitude of -3.8 mv. electroanalysis of 4-np at modified electrode cv and dpv techniques were employed to investigate the electrochemical behavior of 4-np at the surface of il-moo3/gce. figure 6(a) shows cyclic voltammograms of blank phosphate buffer solution (ph 5.8), and the same solution containing 37 m 4-np. no faradic activity at il-moo3/gce occurred in the blank solution, but the current was increased significantly when phosphate buffer solution contained 37 m of 4-np. three distinct anodic and cathodic current peaks can be observed, where the first redox pair (ox1/red1) is associated with epa at 0.05 v, and epc at 0.008 v, with peak separation of 42 mv. the second redox pair (ox2/red2) shows the oxidation peak at 0.27 v, and reduction peak at 0.18 v, with peak separation of 90 mv, while the third redox pair (ox3/red3) is associated with epa at 0.49 v, and epc at 0.36 v, with 130 mv peak to peak separation. figure 6. voltammetric responses of il-moo3/gce in phosphate buffer solution (ph 5.8) without (blank) or with 37 m 4-np recorded at 50 mv/s by (a) cv and (b) dpv similarly, dpvs shown in figure 6(b) also reveal poor faradic current response at il-moo3/gce in the blank solution, and considerable enhancement of faradic current in the presence of 37 m of 4np. well-defined anodic potentials (eox1= 0.06 v, eox2 = 0.27 v and eox3 =0.4 v) of three redox peaks can be noticed in dpv of il-moo3/gce in phosphate buffer solution (ph 5.8) contining 4-np. such strong enhancement of cv and dpv current values, suggests higher electrocatalytic activity of ilmoo3 modified gce towards 4-np. this can be attributed to larger surface area and bonding between mo-o of il-moo3 which facilitates the electron transfer significantly. moreover, the effective interaction between il and metal oxide nanoparticles can promote the electro-catalytic activity of 4-np [40]. therefore, il-moo3 manifests excellent properties for electrochemical sensing of 4-np. according to literature, organic molecules usually undergo multiple electron transfer reaction with successive steps and formation of intermediates. in such multi-electron transfer, cv consists of distinct redox peaks and waves that are well separated. interaction of il-moo3 and 4-np cv scanning in the positive direction (-0.2 to 0.6 v) induces oxidation of 4-np, and as a result, moo3(vi) gets reduced to mo(v) in the form of moo2(oh), due to chemisorbed (oh-) anions at the interfacial electrode surface [41-43]. therefore, superior interaction between chemisorbed ohand π b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 151 effect of 4-np enhances oxidation of 4-np. when cv was scanned in the reverse direction toward negative potentials, the negative charge gets collected at the surface region of il-moo3/gce, which increased reduction of 4-np. this is due to the availability of adsorption sites on the surface of il-moo3 which shows high electro-catalytic effect of 4-np. effect of varying concentration of 4-np sensing performance of il-moo3/gce was studied by cyclic voltammetry with diverse concentrations of 4-np (7.48 73.1 m). figure 7(a) reveals that peak currents increased with increase of the concentration of 4-np. figure 7(b) depicts linear relation between anodic peak current of ox3 and concentration of 4-np within a wide linear range (7.48 to 73.1 m). the linear regression equation was defined as: ip = 2×10-6 + 0.106c (r2=0.99). the detection limit (lod) and quantification limit (loq) were calculated using following equations: lod = 2sb / k (7) loq = 10sb / k (8) where, sb is standard deviation and k is slope of the calibration plot [29]. lod and loq values were determined as 6.76 and 22.5 m, respectively. figure 7. effect of increasing concentration of 4-np (7.48-73.1m) in phosphate buffer solution (ph 5.8) on voltametric response of il-moo3/gce, recorded by (a) cv at 50 mv/s; b) linear variation of ox3 peak current against concentration; (c) dpv at 50 mv/s; (d) linear variation of ox2 peak current against concentration similarly, figure 7(c) shows dpv responses of 4-np in the concentration range from 7.48 to 73.1 m. three diverse oxidation peaks can be observed (ascribed as ox1, ox2 and ox3), which all increase with increase of the concentration of 4-np. this suggests that il-moo3/gce exhibits higher catalytic activity at higher concentrations of 4-np. the highest peak current was observed for ox2 and selected for construction of the calibration plot shown in figure 7(d). the calibration plot http://dx.doi.org/10.5599/jese.956 j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 152 depicted in figure 7(d) shows linearity between peak current and concentration of 4-np, and linear regression was found to be: ip = 7.42×10-6 + 0.230c (r2=0.99) lod and loq determined from dpv were calculated as 5.41 and 18.0 m, respectively. furthermore, the sensitivity of il-moo3/gce was calculated using the following equation: sensitivity = m / a (9) where m is slope of calibration plot and a is active surface area [44]. the calculated sensitivity from cv and dpv was 0.212 and 0.461 a l mol-1 cm-2, respectively. table 1 compares analytical performances (lod and linear range of 4-np) obtained for il-moo3/gce and some other modified electrodes using different voltametric techniques. by comparing data listed in table 1, it is clear that il-moo3/gce possesses comparable or even better analytical performances towards 4-np determination than other modified electrodes [45-48]. table 1. comparison of analytical performances of various modified electrodes for 4-np determination using voltametric techniques ser. no electrode method linear range of c4-np, µm lod, µm reference 1 gns-fepc/gce cv 100 -700 10.00 [45] 2 zg-2/gce, zno/gce cv 10 -1000 13.00 [46] 3 nano-au /gce lsv 10 -1000 8.00 [47] 4 pt/inorganic organic coatings swv 30–90 10.28 [48] 5 cnt/spe dpv 25 – 620 1.3 [49] 6 pd-ga/rgo cv 2-80 9×10-9 [50] 7 rgo–hnt–agnps cv 0.1 363.9 0.0486 [51] 8 mos2/gce dpv 4-0.02 0.01 [52] 9 il-moo3/gce cv 7.48 73.1 6.76 present work 10 il-moo3/gce dpv 7.48 73.1 5.41 present work gns-fepcgraphene nanosheets decorated iron phthalocyanine; zg-zinc glycerolate nano-gel, cntcarbon nanotubes, spescreen printed electrode; pd-ga/rgo pd nanospheres decorated reduced graphene oxide; rgo–hnt–agnps reduced graphene oxide-halloysite nanotubes-silver nanoparticles. among modified electrodes reported in table 1, there are few carbon-based electrodes (cnt and gns) [45,49], which are generally considered as superior candidates for electrochemical sensing due to high surface area and wide potential window. however, the major drawback of the carbon-based sensor is necessity for its purification and synthetic protocol which may affect sensitivity of analyte. in comparison to electrodes modified by noble metals, as well as carbon nanotubes modified electrodes that were already tested for determination of 4-np [45,50], here developed il-moo3/gce system is inexpensive and hg-free material, that can be applied in rather uncomplicated and facile detection method. the linear dynamic range and limit of detection are found within the biological range of 4-np, what is a prominent advantage of the present method. table 2 represents the results of comparison of lod values for 4-np determination obtained in this work and by some usual (not electrochemical) analytical methods. table 2. comparison of lod values for some analytical methods used for 4-np determination analytical method lod, μm reference colorimetry 6.30 [53] hplc 0.021 [54] fluorometry 0.2 [55] colorimetry 1.28 [56] spectrophotometry 0.057 [57] cv 6.76 this work dpv 5.41 this work b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 153 from data in table 2, it is clear that electrochemical sensing techniques used in the present work show moderate sensitivity compared to other, but rather expensive analytical methods. it seems, however, that a flower shape moo3 possesses significant catalytic activity towards 4-np. therefore, it could be expected that by varying synthesis parameters such as temperature and time, a diverse morphology of il-moo3 could be obtained, resulting eventually in even better catalytic response towards 4-np. generally, this work represents the facile synthetic strategy of moo3 that could be a basis for its further development and application in the electrochemical analysis of other potential organic pollutants. effect of varying scan rate influence of the scan rate on 4-np redox peaks was also examined within the scan rate range of 10100 mv/s. it can be seen in figure 8(a) that all redox peak currents increase with increasing scan rate. figure 8. (a) effect of increasing scan rate (10-100 mv/s) on cv of il-moo3/gce in phosphate buffer solution (ph 5.8) containing 73.1 m of 4-np. inset: log of anodic peak (ox1) current vs. log of scan rate; (b) anodic (oxidation) and cathodic (reduction) peak currents (ox1/red1) vs. scan rate figure 8(b) represents linear correlations of anodic and cathodic peak currents (ox1/red1) with respect to the scan rate. linear regression equations were found out as: ipa (μa) = 70 + 0.09 (r2 = 0.9914), and ipc (μa) = 80 – 0.2 (r2=0.9968). good linearity between peak current and scan rate suggests that surface adsorption process of 4-np is occurring at il-moo3/gce surface [58]. furthermore, the inset of figure 8(a) shows the linear plot of log peak current versus log scan rate, with the slope value 1, indicating adsorption-controlled process [58-59]. this confirms that adsorption of 4-np is energetically favorable at il-moo3/gce. the amount of adsorbed 4-np at il-moo3/gce was calculated as 7.8010-11 mol cm-2 using the following formula [60]:  = q / nfa (10) where,  is amount of adsorbed analyte in mol cm-2, n is the number of transferred electrons, q is total charge, f is faraday’s constant, and a is surface area of electrode in cm2. the relatively high amount of adsorbed 4-np signifies high surface area possessed by il-moo3/gce. effect of ph the impact of ph of phosphate buffer solution on the electrode reaction of 73.1 m of 4-np was recorded at ph 5.8, 7.4 and 8.3 (acidic, neutral and basic media) at the favored scan rate of 50 mvs-1. figure 9(a) depicts no faradic activity at ph 8.3, but slightly increased currents are observed at ph 7.4. http://dx.doi.org/10.5599/jese.956 j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 154 drastic enhancement of faradic current, however, was observed in ph 5.8, what implies that redox behavior of 4-np is ph dependent. figure 9(b) shows the relationship between peak current and ph, in which the utmost current peak is reached at ph 5.8. this indicates that the protic environment of the solution promotes the electron transfer process over the surface of the electrode [60]. in a highly alkaline medium, the electrostatic repulsion between 4-np anions and hydroxyl groups significantly increases, and as a result, the barrier to remove 4-np is increased. therefore, 4-np becomes less reactive in the basic buffer system, while in the acidic media degradation of 4-np is feasible. figure 9(c) represents that the anodic peak potential (epa) is linearly decreased as ph was increased from 5.8 to 8.3. the corresponding linear regression equation was found out to be: epa = 0.47 ph + 0.61 (r2=0.99). the slope obtained from this equation is nearly equal to the nernstian value of 58.5 mv/ph, signifying that the electrochemical reaction of 4-np carries equal number of protons and electrons [60], i.e., two electrons and two protons in the present case. also, from the negative slope, it is clear that the electron transfer process is facilitated by protonation. hence from the above experimental observations ph 5.8 was opted out for further analysis. figure 9. (a) cvs (50 mv/s) of il-moo3/gce in phosphate buffer solution containing 73.1 m 4-np at different ph (5.8-8.3); (b) oxidation peak current vs. ph; (c) oxidation peak potential (epa) vs. ph electrochemical impedance spectroscopy (eis) eis is an efficient tool to monitor kinetic and interfacial properties of modified electrodes. curves (a) and (b) in figure 10 represent nyquist plots of bare gce and il-moo3/gce in 0.1m kcl solution containing ferrocene redox probe. the frequency range varied from 105 to 0.1 hz, and ac amplitude of 10 mv was applied to the system. the working potential of eis measurements was 0.15 v. b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 155 nyquist plots in figure 10 show that both electrodes exhibit semicircular parts in high-frequency region, followed by straight lines at lower frequencies. this is characteristic electrode impedance response when electron transfer kinetics is controlled by diffusion process. it is also well known in impedance data analysis that the diameter of semicircular part in a nyquist plot denotes charge transfer resistance (rct) of redox reaction, while straight line of about -/4 slope denotes diffusion impedance (w) of reaction species (ferrocene here). figure 10. nyquist plots of bare gce and il-moo3/gce in 0.1 m kcl and 1 mm ferrocene according to figure 10, bare gce showed higher semicircular part, indicating a higher rct value (78.7 ) than il-moo3/gce (38.6 ). this suggests that bare gce possesses higher resistance to the electron transfer, while il-moo3/gce shows smaller resistance to electron transfer at the electrode interface. the apparent electron transfer rate constant kapp was calculated using the equation (11) [61]: = app 2 2 ct rt k n f r ac (11) where rct is charge transfer resistance, c is concentration of redox probe (1 mm ferrocene), n = 1 is the number of transferred electrons, a is electrode surface area, while r, t and f have their usual meaning. the values of kapp and rct are given in table 3. increase of kapp values and decrease of rct values when compared to bare gce imply an improvement of electron transfer at il-moo3/gce, suggesting its increased activity toward redox reaction. table 3. charge transfer resistance (rct) and kapp values for bare gce and il-moo3/gce in 0.1 m kcl and 1 mm ferrocene electrode rct /  kapp / 103 cm s-1 bare gce 78.7 6.26 ilmoo3/gce 36.8 13.4 reproducibility and stability reproducibility was investigated by repetitive measurements using different il-moo3/gce to estimate the sensing performance of 73.1 mm of 4-np in favorable conditions. the obtained rsd value was less than 2 %, suggesting superior reproducibility of the designed sensor. stability of the modified electrode was tested for successive 25 cycles. before analysis, the electrode was kept in a dry condition for 30 days. the percentage degradation was calculated using eq. (12) [62, 63] degradation, % = (ipl / ipi) 100 (12) http://dx.doi.org/10.5599/jese.956 j. electrochem. sci. eng. 11(3) (2021) 143-159 determination of 4-nitrophenol 156 where, ipl and ipi denote last and initial anodic peak current, respectively. the calculated degradation was 94 %, what indicates that il-moo3/gce shows satisfactory reproducibility and stability. interference study figure 11 shows selectivity property of the modified sensor. it was evaluated using cv by inspecting the effect of possible interferents such as aniline and resorcinol (20 µm of each) in 0.2 m phosphate buffer solution (ph 5.8) containing 20 µm 4-np. since, aniline and resorcinol along with 4-np are commonly used as starting materials in various industries, just these two organic pollutants in combination with 4-np were selected for the interference study at il-moo3/gce. it is obvious from figure 11, that the redox peaks around 0.26 v and 0.48 which were used for the determination of 4-np remained unaffected as 20 µm aniline and resorcinol were added. in fact, no redox activity was observed for aniline and resorcinol looking separately. this result suggests that the designed sensor exhibit outstanding selectivity towards 4-np. figure 11. interference study of 4-np with aniline and resorcinol application of il-moo3/gce in real water samples determination of 4-np in water samples from krishna and panchganga rivers using il-moo3/gce was performed, and results are shown in table 4. a recovery study was carried out using the standard addition method. finally, differential pulse voltammogram was recorded, showing a clear signal of 4np oxidation. the recovery percentage was calculated using eq. (13) [29]: recovery, % = (c4-np found / c4-np added) 100 (13) the recovery was nearly 100 and 98 %, respectively, which indicates that il-moo3 modified gce sensor has good efficiency and sensitivity for 4-np determination. table 4. determination of 4-np in river water samples sample amount of 4-np, μm recovery, % added amount of 4np found amount of 4-np 1 14.5 14.5 100 2 15.3 15.0 98.0 3 16.6 16.3 98.1 standard deviation 0.092 b. b. kamble et al. j. electrochem. sci. eng. 11(3) (2021) 143-159 http://dx.doi.org/10.5599/jese.956 157 conclusions within green approach toward the synthesis of metal oxide nanomaterials, ionic liquid [bmim]+[br]was utilized for the successful synthesis of moo3. the obtained thin film was dispersed in ethanol and il-moo3 nanoparticles were subsequently used to modify gce surface. the modified electrode, il-moo3/gce, was tested as a sensor for the important organic pollutant, 4-np, promising results were obtained, showing il-moo3/gce as the efficient sensor for 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https://doi.org/10.1016/j.microc.2020.104945 https://doi.org/10.1039/c5ra06286a https://doi.org/10.1007/s10800-011-0281-4 https://doi.org/10.1016/j.electacta.2008.07.051 https://doi.org/10.1007/s00604-011-0628-x https://doi.org/10.1016/j.jhazmat.2017.03.015 https://doi.org/10.1039/d0qi00006j https://doi.org/10.1007/s11581-018-2538-y https://doi.org/10.1007/s00604-019-3259-2 https://doi.org/10.15171/ps.2019.9 https://doi.org/10.1016/j.talanta.2018.02.114 https://doi.org/10.1016/j.saa.2016.08.043 https://doi.org/10.1134/s1061934819060066 https://doi.org/10.1016/j.jelechem.2020.114533 http://dx.doi.org/10.3390/nano9030429 https://doi.org/10.1007/s13738-020-01876-4 http://dx.doi.org/10.5599/jese.717 https://creativecommons.org/licenses/by/4.0/) electrochemical immunosensor for the determination of beta casein doi: 10.5599/jese.2015.0074 17 j. electrochem. sci. eng. 5(1) (2015) 17-24; doi: 10.5599/jese.2015.0074 open access : : issn 1847-9286 www.jese-online.org original scientific paper effect of electropolishing on vacuum furnace design sutanwi lahiri, girish kumar sahu, biswaranjan dikshit, radhelal bhardwaj, ashwini dixit, ranjna kalra, kiran thakur, kamalesh dasgupta, asoka kumar das and lalit mohan gantayet* laser and plasma technology division, bhabha atomic research centre, mumbai-400085, india *beam technology development group, bhabha atomic research centre, mumbai-400085, india corresponding author: e-mail: sutanwi@barc.gov.in; tel.: +91-22-25592975 received: august 16, 2014; revised: february 28, 2015; published: march 15, 2015 abstract the use of thermal shields of materials having low emissivity in vacuum furnaces is wellknown. however, the surface condition of the heat shields is one of the most important factors governing their efficiency as radiation resistances. the emissivity of the thermal shields dictates the power rating of the heaters in furnace design. the unpolished materials used in the heater tests showed poor performance leading to loss of a significant percentage of the input power. the present work deals with the refurbishment of the radiation heat shields used in a furnace for heating graphite structure. the effect of refurbishment of the heat shields by the buffing and subsequently electropolishing was found to improve the performance of the shields as heat reflectors. the composition of the electrolyte was chosen in such a way that the large shields of mo, inconel and ss can be polished using the same reagents in different ratios. the present work deals with the development of a standard electropolishing procedure for large metallic sheets and subsequently qualifying them by roughness and emissivity measurements. the improvement noted in the shielding efficiency of the furnace in the subsequent runs is also discussed here. keywords thermal shields; etching; emissivity introduction the dominant mode of heat transfer in vacuum furnaces is radiation. the use of radiation heat shields to reduce the power loss from the hearth of a furnace is well known [1-4]. a set of heat shields made of various materials and assembled with minimum inter-shield separation, increases http://www.jese-online.org/ mailto:sutanwi@barc.gov.in j. electrochem. sci. eng. 5(1) (2015) 17-24 effect of electropolishing on vacuum furnace design 18 18 the net thermal resistance of the set. the most important factor governing the performance of each heat shield in the pack is the emissivity of both the surfaces, which, in turn, depends on their surface condition [1]. the vacuum furnace used in this study, has a pack of nine heat-shields as shown in figure 1. the pack of shields is designed to: (a) support heaters and (b) enclose the graphite hearth except a few view-ports. the innermost shield facing the hearth is of molybdenum and the next three shields were inconel-600 and the rest ss-304l. the mo heater wires are laid on the innermost shield facing the graphite structure. the power input to the heaters is used to maintain the graphite structure at a temperature of 1500 k. the heat shields help in maintaining the hot zone at 1500 k without increasing the temperature of the chamber wall beyond 400 k. the vacuum chamber is maintained at a vacuum of the order of 10 -5 mbar. the primary objective of the experiment was to maintain the graphite hearth at a temperature of 1500 k. however, experiments with the unpolished heat shields as received from the fabricator failed to achieve the target temperature. heat was being lost from the furnace primarily due to the high emissivity of the shields. the graphite structure could reach a temperature of 1200 k at 124 kw. on extrapolation, the required power to achieve 1500k was 320kw. on further analysis, the presence of moo and moc was detected on the surface of the innermost shield. carbon was present on all surfaces of the heat shields. the oxidation and the carburization is a result of the outgassed species from the graphite hearth. the data from the residual gas analyser (rga) showed the presence of methane and carbon in significant quantities during the experiment. the presence of carbon and the formation of carbide resulted in an increase in their emissivity and therefore, loss of input power. the unpolished surface provides a larger area for carbon deposition and carbide formation and hence acts as a catalyst in the deterioration of its emissivity. figure 1. schematic diagram of the furnace the effective emissivity or reflectivity is a strong function of the surface finish [12-15]. as the temperature of the shield increases, the surface roughness needs to be significantly less than the peak wavelength of the heat radiation at working temperature to obtain high values of reflectivity. this calls for a highly polished surface. s. lahiri et al. j. electrochem. sci. eng. 5(1) (20xx) 17-24 doi: 10.5599/jese.2015.0074 19 electropolishing is essentially the process of anodically smoothening of a surface in a suitable electrolyte. it removes the microscopic roughness on the surface, eliminating imperfections which trap and contain contaminants, in a more controlled manner compared to the other forms of chemical cleaning. hence, electropolishing was chosen as the method of refurbishment of these shields. in short, the object to be electropolished is immersed in an electrolyte of an optimized composition and subjected to direct current [8]. experimental a flowchart summarizing the experimental procedure is given in figure 2. it may be noted here that the range of average roughness values that electropolishing can achieve is 0.8 to 0.1 microns [18]. therefore, 0.2 microns was selected for the roughness check. figure 2: flowchart of the experimental procedure selection of the bath constituents there is a wide spectrum of electrolytes used in industrial application for electropolishing. the stainless steel has established electropolishing procedures with sulfuric acid and phosphoric acid [17]. however, some of the electrolytes reported for molybdenum and inconel (like potassium dichromate, etc) are too hazardous to be handled on a large scale [6-7]. moreover, it is convenient selection of bath constituents buffing roughness measurement roughness measurement emissivity measurement electropolishing is roughness ra < 0.2 µm ? no yes j. electrochem. sci. eng. 5(1) (2015) 17-24 effect of electropolishing on vacuum furnace design 20 20 to use the same bath constituents for polishing all the radiation heat shields by changing the ratio of the acids, instead of using separate baths and chemicals for separate materials. since the process was to be carried out on large shields of 2000×500 mm area, a few trials were carried out on small samples to determine the optimum electropolishing conditions using the industrial solvents like sulfuric acid and phosphoric acid. pretreatment poor base metal conditions can result in less than optimum electropolished finishes. these flaws are revealed by electropolishing. hence suitable pretreatment is required. in view of this, the shields were buffed with buffing wheel of grades 60 and 120. after the mechanical polishing and removal of burrs and sharp edges, the samples were washed in running tap water. the micrographs of the samples were obtained. its roughness was measured and recorded. electropolishing an electrolytic bath of dimensions 1500×1500×1000 mm was filled with a given composition of electrolyte. the copper plates of dimensions 400×200×2 mm was used as cathode. the voltage was set at a given value and the surface of the specimen (200×80×1 mm) was inspected after an interval of 3 minutes using a magnifying glass. the distance between anode and cathode was 800 mm. the current, voltage and time were noted. temperature of the bath was monitored and found to rise to a steady state maximum temperature of 45 °c. it may be noted that there was no external agitation in the bath. after the specimen was removed from the bath, it was put under running tap water and dried in air for half an hour. emissivity measurement one of the objectives of heat shield cleaning is to reduce the emissivity so that the heat loss due to absorption in the heat shields is reduced. the samples of mo, inconel and ss 304 sheets used in the heater runs and those obtained after electropolishing were made into filaments of suitable size. the bare wires of chromel and alumel are spot-welded on the filament and used as a thermocouple for measuring temperature as shown in figure 3. the transmissivity of the glass window is known. hence the difference in the temperature readings of the thermocouple (tactual) was compared to the temperature measured by the pyrometer (tmeasured) and the emissivity value ε of the filament was determined at 0.65 μm wavelength. the following expression was used for estimating emissivity [4]: measured actual 1 1 ln( ) 1.4388t t     . a b figure 3. (a) filaments made out of the heat shields (b) hot filament as viewed by the pyrometer s. lahiri et al. j. electrochem. sci. eng. 5(1) (20xx) 17-24 doi: 10.5599/jese.2015.0074 21 figure 4. typical surface profile and evaluation length roughness measurement a token of area 50×50mm was made from each material and roughness was measured before and after electropolishing. the specimen is placed on a paper placed on a vibration less table. one edge of the specimen is aligned to the stand of a calibrated roughness meter and the stylus was used to scan the surface over a tracing length of 17.5 mm. the tracing length consists of pre-travel lv, post travel ln, and the evaluation length lm. the evaluation length (in this case, 12.5 mm) consists of n (n = 5) number of sampling lengths (2.5 mm). the surface profile was recorded and analysed. a typical surface profile is shown in figure 4. results and discussion condition before electropolishing it is observed that the surface condition of the virgin heat shields brought from the mill were not good enough to offer low value of emissivity. the imperfections acted as contamination traps and led to the deterioration of emissivity as the power loss from the hot zone was increased. the micrographs of the unpolished heat shields are shown in figure 5. a b c figure 5. micrographs of the virgin heat-shields: a) stainless steel b) inconel c) mo electropolishing the optimum electrolytic conditions for molybdenum, inconel and steel are given in table 1. several trials were carried out to determine the optimum conditions for electropolishing for each material. the ratio of the phosphoric acid and sulfuric acid was changed to determine the optimum composition for each material. it may be noted that the voltage across the electrodes was varied during the process of optimization. it was found that, the composition reported by zamin et al. [16] was unsuitable for mo sheets. the blue patches of molybdenum trioxide developed after electropolishing mo was attributed to the oxidizing action of the sulfuric acid and hence the electropolishing was carried in orthophosphoric acid only. traversing length, l evaluation length l l l l l j. electrochem. sci. eng. 5(1) (2015) 17-24 effect of electropolishing on vacuum furnace design 22 22 table 1. optimum condition for electropolishing parameter molybdenum inconel ss electrolyte h3po4 (85%) h3po4 (85%) and h2so4 (5:1) h3po4 (85%) and h2so4 (4:1) voltage, v 7 6 6 time, min 6-10 5-8 5-8 observation shining surface shining surface shining surface condition after electropolishing it is observed that the micrographs of the electropolished heat shields show improved surface condition of all the three types of samples. figure 6 presents a comparison of the micrographs showing the improvement caused by the exercise. stainless steel inconel molybdenum buffing electrolysis h3po4 (85%) and h2so4 (4:1) h3po4 (85%) and h2so4 (5:1) h3po4 (85%) rz = 1.177 μm rz = 1.551 μm rz = 0.605 μm figure 6. micrographs of the electropolished heat-shields improvements in emissivity and roughness as discussed earlier, emissivity of the surface is the most important parameter governing the performance of the shields. since the roughness of the surface affects the emissivity values, an improvement of the surface profile was observed after the exercise. table 2 gives the values of the surface roughness while table 3 summarizes the improvement in emissivity caused by electropolishing. the roughness values of the buffed samples before and after electropolishing were compared to determine the etch rates. the etch rate of molybdenum was found to be the fastest and that of ss was the slowest in their respective electrolytic baths. this encouraged us to carry out similar polishing on the large shields used in the industrial furnace. the dimensions of the large heat shields vary from 500×500×1 mm to 2000×2000×1 mm. s. lahiri et al. j. electrochem. sci. eng. 5(1) (20xx) 17-24 doi: 10.5599/jese.2015.0074 23 figure 7 shows the refurbished heat shields. the heat-shields are cleaned and polished to a roughness less than 2 μm. the effect of electropolishing was demonstrated in the performance of the heat shield in the subsequent heater test. the graphite structure was found to achieve the desired temperature at a much lower power compared to the previous test due to the improvement in the emissivity values (table 4). hence the objective of the refurbishment of the heat shields was fulfilled. table 2. roughness values of the samples specimen roughness before electro-polishing rz2 / μm roughness after electro-polishing rz2 / μm reduction in roughness rz1 / rz2 etch rate, (mg/cm 2 ) / min ss-316 10.410 1.177 8.84 1.054 inconel 8.162 1.551 5.26 1.116 mo 9.380 0.605 15.5 1.800 table 3. emissivity values of the samples before and after refurbishment material emissivity at 0.65 μm before after ss-304 0.82 0.31 inconel 0.78 0.5 mo 0.7 0.27 table 4. heater power requirement parameter heat shield before refurbishment heat shield after refurbishment heater power, kw 124 320 124 210 temperature of graphite, k 1200 1523 1346 1546 figure 7. electropolished heat shields conclusion the present study gives a simple route for electropolishing of mo and inconel on a large scale using the commonly available laboratory chemicals. the results obtained in the trials were further 300 mm 300 mm j. electrochem. sci. eng. 5(1) (2015) 17-24 effect of electropolishing on vacuum furnace design 24 24 analyzed and the roughness and emissivity of the surface was measured to qualify the polish. from table 2, it is evident that the reduction in roughness has been the highest for mo, followed by ss and inconel. a good improvement of emissivity of the shields was observed. since surface roughness, among other factors, influences the emissivity of a material and catalysis the surface reactions, a reduction in power fed to the heaters. electropolishing of heat shields reduces the heater power requirement of the furnace and therefore plays a significant role in the design and economics of vacuum furnaces. the power requirement came down by 33 % from the earlier 320 to 210 kw. acknowledgments: the authors would like to thank dr d. das, chemistry division for his advice. the authors also thank dr v. k. mago for his valuable suggestions. references [1] w. espe, materials of high vacuum technology, vol. 1: metals and metalloids, pergamon press, new york, usa, 1966, p.360. [2] f. p. incropera,, d. p. dewitt, t. l. bergman, a. s. lavine, fundamentals of heat and mass transfer, john wiley & sons, hoboken, new jersey, usa, 2007, p. 245. [3] j. p. holman, heat transfer, mcgraw-hill, new york, usa, 2009, p. 256. [4] j. r. howell, r. siegel, m. p. menguc, thermal radiation heat transfer, crc press, new york, usa, 2010, p. 250. [5] c. afonso, j. matos, int. j. refrig. 29 (2006) 1144-1151. [6] c. l. faust, metal finish. 80 (1982) 21–25. [7] k. b. hensel, metal finish. 87 (1989) 89-96. [8] d. a. jones, principles and prevention of corrosion, macmillan publishing company, new york, ny, 1992, p 35-39. [9] j. mendez, r akolkar, t. andryushchenko, u. landau, j. electrochem. soc., 155(1) (2008) d27d34. [10] bing du, ian ivarsuni, j. electrochem. soc., 151(6) (2004) c375-c378. [11] r. p. allen, h. w. arrowsmith, w. c. budke, proc. 70th aiche annual meeting, new york, usa, november 13-17, 1977, paper 109c. [12] c. wen, i. mudawar, int. j. heat mass tran. 48 (2005) 1316-1329. [13] c. wen, i. mudawar, int. j. heat mass tran. 49 (2006) 4279-4289. [14] c. wen, i. mudawar, int. j. heat mass tran. 47 (2004) 3591-3605. [15] s. agababov, t. vysokikh, temperature 8 (1970) 770-773. [16] m. zamin, p mayer, m k murthy, j. electrochem. soc. 124(10) (1977) 1557-1562. [17] s. habibzadeh, l. li, d. shum-tim, e. c. davis, s. omanovic, corros. sci. 87 (2014) 89-100. [18] e. p. degamo, j. t. black, r. a. kohser, materials and processes in manufacturing, john wiley & sons, hoboken, new jersey, usa, 2003, p.430. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {char of tagetes erecta (african marigold) flower as a potential electrode material for supercapacitors:} http://dx.doi.org/10.5599/jese.1381 787 j. electrochem. sci. eng. 12(4) (2022) 787-797; http://dx.doi.org/10.5599/jese.1381 open access : : issn 1847-9286 www.jese-online.orghttp://www.jese-online.org/ original scientific paper char of tagetes erecta (african marigold) flower as a potential electrode material for supercapacitors venkata naga kanaka suresh kumar nersu1,, bhujanga rao annepu1, subhakaran singh rajaputra2 and satya srinivasa babu patcha3 1department of instrument technology, andhra university college of engineering (a), visakhapatnam, andhra pradesh 530003, india 2centre for advanced energy studies, koneru lakshmaiah education foundation, vaddeswaram, ap, india 3center for flexible electronics, department of electronics and communication engineering, koneru lakshmaiah education foundation, vaddeswaram, ap, india corresponding author: sureshk834@gmail.com received: may 16, 2022; accepted: july 9, 2022; published: july 25, 2022 abstract a char of tagetes erecta flowers (tfc) was derived through simple thermal decomposition of tagetes erecta flowers (tf). physico-chemical properties of as-prepared tfc were evaluated using xrd, fesem, ftir, tga, n2 adsorption-desorption isotherm analysis and water contact angle measurements. the practicality and applicability of tfc as promising electrode material in supercapacitors (scs) were evaluated in full-cell configuration by performing electrochemical characterizations like cv, gcd and eis on a lab-scale tfc-based symmetric sc. tfc exhibited a remarkable specific capacitance of 118.4 f g-1 at a constant current density of 0.2 a g-1 and a specific energy of 4.1 wh kg-1 at specific power of 0.1 kw kg-1. tfc showed excellent cyclic stability by retaining 92 % of its initial capacitance even after 6000 gcd cycles at 2 a g-1. the superior capacitive behaviour and cyclic stability of tfc could be attributed to its good wettability towards water. this excellent supercapacitive performance of tfc establishes it as a potential floral waste-derived carbon-based electrode material for scs. keywords biochar; flowers waste; gel polymer electrolyte; electric double-layer capacitor (edlc); carbon cloth; hydrophilicity introduction in india, nearly 300,000 hectares are under floriculture producing nearly 3 million tons of flowers annually (as per national horticulture board of india statistics of 2018-2019). flowers like rose, marigold (tagetes erecta), jasmine, hibiscus, etc., are most commonly used in the preparation of garlands, decoration of religious sites and statues during festivals, as well as offerings during http://dx.doi.org/10.5599/jese.1381 http://dx.doi.org/10.5599/jese.1381 http://www.jese-online.org/ http://www.jese-online.org/ mailto:sureshk834@gmail.com j. electrochem. sci. eng. 12(4) (2022) 787-797 char of tagetes erecta flower as supercapacitors 788 weddings and other ceremonies [1]. after serving their purpose, these flowers are left unused and are usually dumped in soil or disposed into water bodies [2]. daily, around 40 % of flowers produced in india are left unsold, generating a huge amount of floral waste [3]. disposing floral waste in open landfills would attract microorganisms that degrade it, thereby releasing harmful gases like methane, carbon dioxide and ammonia, developing a foul smell, and promoting greenhouse emissions [4]. in india, floral waste is dumped in rivers and other water bodies resulting in serious water pollution [5,6]. disposal of flower waste is a major concern to the environment and there is a need for novel methods to convert floral waste into value-added products [3]. currently, flower waste is being utilized for the preparation of compost, eco-friendly incense sticks, soaps, as well as conditioners in lawn dressing [7]. eco-friendly energy storage devices are of utmost importance in the current scenario as alternatives to storing energy harvested from renewable energy sources [8]. electric double-layer capacitors (edlcs) are a type of supercapacitors (scs) that store charge in the form of an electric double layer (edl) at the electrode-electrolyte juncture and are familiar for their superior charge-discharge capability and long cyclic life [9]. carbon-based materials are often used as electrode materials in edlcs and commercially available edlcs use activated carbon as electrode material [10]. recently, the development of low-cost and eco-friendly electrode materials such as biological waste-derived carbon-based materials has gained a lot of attention from scientific communities worldwide [11]. carbons obtained from bamboo [12], sunflower stems [13], lotus stems [14], rice straw [15], wheat straw [16], corn stalks [17], cotton stalks [18], and sugar cane bagasse [19] were already tested as electrode materials in scs. wood sawdust [20] and wood [21] were also used to derive carbons and tested as electrode materials in scs. carbons obtained from hemp fibers [22], bamboo fibers [23], jute fibers [24], lotus leaves [25], eucalyptus leaves [26], ficus religiosa leaves [27] and pine leaves [28] were tested for their potential as electrode materials in scs. rice husk [11] and shells of coconuts [29], oil palm kernel [30], and macadamia nuts [31] were used as precursors to derive carbon-based electrode materials for scs. biological waste of animal origin like scales of fish [32], hair of human beings [33], shells of shrimps [34] and chicken eggs [35] and chicken feathers [36] were used to derive carbon-based electrode materials for scs. in the past, flowers like cherry blossom [37], rose [38] and catkins of willow [39-42] were used to derive carbon-based materials and applied as electrode materials in scs. flowers of tagetes erecta (tf), also known as african or mexican marigold, are commonly used for decoration, extraction of carotenoids and essential oils, adding colour to food items and medicinal purposes [43]. in india, around 3910 hectares of area is under marigold cultivation with an average yield of 22.4 tons per hectare [44], and producing nearly 87,000 tons of tf per year. disposal of the resulting floral waste requires strategies like the production of bioethanol from tf waste, etc. [2]. gupta et al. [45] reported the synthesis of graphene quantum dots from tf and tested its potential as electrode material in scs in a three-electrode configuration. in the present work, char of tagetes erecta flower (tfc) was obtained by facile thermal decomposition of tf excluding activation steps making the procedure eco-friendly. physico-chemical properties of tfc were investigated. a tfc-based sc (full-cell) was constructed using tfc, hydrothermally reduced carbon cloth (cchy), graphene incorporated sulfonated polyvinyl alcohol (spva-hrg-0.5) hydrogel, and whatman® filter paper as electrode material, current collector, electrolyte and porous separator, respectively. supercapacitive performance of the constructed cell was investigated using electrochemical techniques like cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical impedance spectroscopy (eis). v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 789 experimental materials tf waste was collected from nearby temples of andhra pradesh, india. carbon cloth was procured from avcarb, usa. multiwalled carbon nanotubes (mwcnts) were procured from ad nano technologies, india. sulfuric acid (h2so4) (98 %), hydrogen peroxide (h2o2) (30 % w/v), graphite (<20 m), sodium nitrate (nano3), polyvinylidine difluoride (pvdf) (mw 320,000), ethanol (99.9 %), sodium hydroxide (naoh), polyvinyl alcohol (pva) (mw 1,25,000), nitric acid (hno3), isopropanol (ipa), hydrochloric acid (hcl) (35-38 %), n-methyl-2-pyrrolidone (nmp) and potassium permanganate (kmno4) were used as received. whatman® qualitative filter paper was procured locally and used as a separator during the fabrication of tfc-based sc. deionized (di) water was utilized throughout the synthesis. preparation of tfc collected tf were washed with di water and dried at room temperature (rt). trimmed petals of tf were placed in a crucible and covered with aluminum foil in which tiny holes were made for the elimination of volatile compounds while thermal decomposition. tf petals were heated for 3 h at 350 °c to obtain tfc (figure 1). figure 1. preparation of tfc through thermal decomposition of tf preparation of cchy cchy was prepared from commercially acquired carbon cloth following the synthesis procedure reported elsewhere [46]. carbon cloth was dropped into a 2:1 mixture (30 ml) of h2so4 and hno3, and kmno4 (3 g) was added to the mixture while agitating at rt. later, di water (100 ml) was added to the mixture and stirred for 3 h continuously, followed by the addition of 30 % h2o2 (5-10 ml), leading to the formation of a transparent solution. the obtained oxidized carbon cloth was cleaned many times with di water and later transferred into a di water-filled polytetrafluoroethylene (ptfe) lined autoclave and heated for 14 h at 180 °c to obtain cchy. collected cchy was dried and later used as a current collector in the fabrication of tfc-based sc. http://dx.doi.org/10.5599/jese.1381 j. electrochem. sci. eng. 12(4) (2022) 787-797 char of tagetes erecta flower as supercapacitors 790 preparation of ngpe hydrothermally reduced graphene oxide (hrg) was synthesized following the procedure reported elsewhere [47]. graphite (2 g) was dispersed into a mixture of h2so4 (98 ml) and nano3 (2 g) and agitated for 4 h continuously by preserving the suspension temperature under 5 °c using an ice bath. to this suspension, kmno4 (12 g) was slowly added and stirred for 2 days continuously at rt. the temperature of the suspension was further decreased to <5 °c using an ice bath and then di water (184 ml) was added to the suspension and stirred for another 2 h. finally, 30 % h2o2 was slowly added to the suspension while stirring until the suspension color changed yellow, signifying graphene oxide (go) formation. the yellow-coloured suspension was kept a side to allow sedimentation of go. later, the supernatant solution was decanted to obtain go precipitate, which was washed and centrifuged multiple times using 1 m hcl and di water until color of go precipitate turned black. finally, the go precipitate was cleansed using ethanol and later centrifuged and vacuum dried for 12 h at 70 °c to obtain go films, which were further powdered to obtain go. later, go (0.2 g) was dispersed into di water (200 ml) and transferred into a ptfe-lined autoclave while maintaining ph at 11 using naoh pellets. the autoclave was heated at 180 °c for 14 h and the collected hrg was vacuum dried at 60 °c for 12 h to acquire hrg. ngpe was prepared by incorporating hrg into sulfonated pva hydrogel to obtain hrg incorporated sulfonated pva hydrogel (spva-hrg-0.5) following the procedure reported elsewhere [48]. to di water (4 ml), h2so4 (270 l) was added and heated up to 80 °c followed by the addition of pva (0.5 g) and stirred until a clear sulfonated pva (spva) hydrogel was obtained. finely powdered hrg (25 mg) was dispersed in ipa (10 ml) and this dispersion was slowly added into spva hydrogel while stirring at 80 °c for 0.5 h to obtain spva-hrg-0.5, where 0.5 represents the wt.% of hrg in spva hydrogel. the as-prepared ngpe was used as an electrolyte in the fabrication of tfcbased sc. characterization studies the structural properties of tf and tfc were analyzed utilizing x-ray diffraction (xrd) technique (rigaku miniflex 600). the functional groups present in tf and tfc were determined using fouriertransform infrared (ftir) spectroscopy (cary 630). field emission scanning electron microscopy (fesem) (fei-quanta feg 200f) was used to investigate the surface morphology of tfc. the surface area and porosity profile of tfc was evaluated using a surface area and porosity analyser (quantachrome nova 2200e). thermogravimetric analysis (tga) (simultaneous thermal analyser sta 7200, hitachi htg) of tfc was carried out under n2 atmosphere up to 800 °c at a heating rate of 20 °c min-1. a pellet of tfc (diameter = 1.2 cm) was prepared to perform water contact angle measurement (kyowa dm-501) through the sessile drop technique. a lab-scale tfc-based symmetric sc was fabricated by sandwiching tfc-coated cchy on either side of a spva-hrg-0.5 electrolyte soaked whatman® filter paper. the supercapacitive behaviour of tfc-based symmetric sc (full-cell) was tested using an electrochemical workstation (parstat pmc 2000a) by performing cv, gcd and eis techniques. tfc, mwcnts and pvdf were dispersed in nmp in the ratio of 80:10:10 to obtain an electrode ink, which was later coated over two cchy (area of each cchy = 0.8 cm2) and heated at 120 °c for 15 min under vacuum, resulting in tfc coated cchy (tfc-cchy) with a tfc loading of 1 mg cm-2. a full cell was fabricated by sandwiching a pair of tfc-cchy on either side of a spva-hrg-0.5 gel polymer electrolyte soaked whatman® filter paper. v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 791 results and discussion physico-chemical characterizations figure 2 (a) illustrates the diffraction patterns of tf and tfc showing characteristic broad peaks, indicating the presence of amorphous carbon and the peaks in tfc are less sharp compared to tf, indicating an increase in its amorphous nature after thermal decomposition [49,50]. figure 2. (a) x-ray diffractograms and (b) ftir spectra of tf and tfc, respectively figure 2(b) illustrates the ftir spectra of tf and tfc. both tf and tfc showed broad peaks around 3274 and 3271 cm-1 resulting from vibrational stretching o-h bonds of hydroxyl (-oh) groups [45]. the peaks observed around 2918 and 2857 cm-1 of tf and 2919 and 2854 cm-1 of tfc could result from vibrational stretching of c-h bonds of -ch groups [45]. the peaks observed around 1600 cm-1 in both tf and tfc, resulted from vibrational stretching of c=c bonds [45]. the peaks observed around 1017 and 1018 cm-1 in ftir spectra of tf and tfc, respectively, could have resulted from vibrational stretching of c-n bonds [3,45]. figure 3 (a and b) represents fesem images of tfc at different magnifications showing layered morphology with voids in between, which could promote access for electrolyte ions into deeper regions of tfc, thereby improving edl formation [51]. figure 3. (a) and (b) fesem images depicting surface morphology of tfc http://dx.doi.org/10.5599/jese.1381 j. electrochem. sci. eng. 12(4) (2022) 787-797 char of tagetes erecta flower as supercapacitors 792 figure 4 (a) illustrates the tga curve of tfc in which a weight loss of around 6 % was observed when heated up to 100 °c, which could be associated with water [52]. tfc lost 65 % of its weight when heated from 100 to 800 °c, which could be assigned to the degradation of lignin structures and removal of volatiles [53]. figure 4 (b) illustrates the water contact angle of tfc with a contact angle of 52°, determining its hydrophilic property, which could improve the formation of edl [54]. this hydrophilic nature of tfc could be attributed to the presence of -oh groups, as confirmed from ftir data [45]. figure 5(a) depicts the n2 adsorption-desorption isotherm of tfc and the brunauer–emmett–teller (bet) surface area of tfc was calculated to be around 17 m2 g-1. figure 5(b) represents pore size distribution in tfc, where dv₀ and d represent differential pore volume and pore diameter, respectively, while the average pore diameter was found to be around 2.8 nm. figure 4. (a) tga curve and (b) contact angle measurement of tfc figure 5. (a) n2 adsorption-desorption isotherm and (b) pore size distribution in tfc electrochemical characterizations full cell studies the supercapacitive behaviour of the fabricated tfc-based full-cell was tested using techniques like cv, gcd and eis. figure 6(a) illustrates the cv curves of tfc at different scan rates confirming superior capacitive behaviour with good reversibility and rate probability [55]. figure 6 (b) v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 793 represents gcd curves of tfc at diverse constant current densities. the specific capacitance (cs), specific energy (es) and specific power (ps) of tfc-based symmetric sc were calculated using equations (1-3) [56]: s 2 i t c m v  =  (1) 2 s s 8 c v e  = (2) s s e p t =  (3) where i, v, t and m represent constant discharge current, discharge voltage, discharge time and mass of tfc (0.8 mg) on each electrode, respectively. figure 6 (c) illustrates the ragone plot of tfc. at 0.2 a g-1 constant current density, tfc exhibited exceptional cs of 118.4 f g-1. at a ps of 0.1 kw kg-1, tfc exhibited an es of 4.1 wh kg-1. table 1 represents the experimental data obtained from gcd studies of tfc, while table 2 illustrates the comparison of tfc’s supercapacitive performance with carbon-based materials derived from some other flowers. table 1. experimental data obtained from gcd of tfc in full-cell studies current density, a g-1 ir drop, v discharge time, s specific capacitance, f g-1 0.2 0.028 148 118.4 0.5 0.062 55 110 1 0.108 26 104 2 0.195 12 96 5 0.56 4.5 90 table 2. comparison of tfc supercapacitive performance with carbon-based materials derived from flowers precursor activating agent measurement specific capacitance, f g-1 reference cherry blossom petals ― 2-electrode 154 at 10 mv s-1 [37] rose flower koh 3-electrode 208 at 0.5 a g-1 [38] willow catkin ― 3-electrode 251 at 0.5 a g-1 [39] willow catkin koh 3-electrode 292 at 1 a g-1 [40] willow catkin koh 3-electrode 298 at 0.5 a g-1 [41] willow catkin koh 3-electrode 340 at 0.1 a g-1 [42] palmyra palm flowers koh 3-electrode 155 at 1 a g-1 [57] tf naoh 3-electrode 200 at 2 a g-1 [45] tf ― 2-electrode 118.4 at 0.2 a g-1 this work the cyclic stability of tfc was investigated by performing 6000 gcd cycles at a constant current density of 2 a g-1. figure 7 (a and b) represents the cv and gcd curves of tfc before and after cycling for 6000 cycles. figure 7 (c) illustrates the cyclic stability of tfc for 6000 gcd cycles, where tfc retained 92 % of its initial capacitance. eis studies were conducted at 0 v, using an alternating signal of 5 mv amplitude and the frequency range from 100 khz to 0.1 hz. figure 7 (d) illustrates nyquist plots of tfc before and after cycling for 6000 gcd cycles, which indicates a slight increase in cell impedance after cycling. http://dx.doi.org/10.5599/jese.1381 j. electrochem. sci. eng. 12(4) (2022) 787-797 char of tagetes erecta flower as supercapacitors 794 figure 7. (a) cv curves of tfc at 50 mv s-1 and (b) gcd curves of tfc at 1 a g-1, before and after cycling; (c) cyclic stability of tfc for 6000 gcd cycles at 2 a g-1; (d) nyquist plots of tfc before and after cycling; inset image shows the magnified image of the high frequency region of nyquist plots conclusions tf waste was used as a precursor to obtain tfc, a flower waste-derived carbon-based material through thermal decomposition and its physico-chemical properties were investigated. the practicality and applicability of tfc as potential electrode material in edlcs was determined by evaluating supercapacitive behaviour of fabricated tfc-based sc using techniques like cv, gcd and eis. at 0.2 a g-1 constant current density, tfc exhibited an exceptional specific capacitance of 118.4 f g-1. at a ps of 0.1 kw kg-1, tfc exhibited an es of 4.1 wh kg-1. tfc showed superior cyclic stability by preserving 92 % of its original capacitance in spite of 6000 gcd cycles. the layered morphology of tfc and presence of voids, as confirmed from fesem analysis, promote efficient access of electrolyte ions into deeper regions of tfc, thereby enhancing edl formation. water contact angle measurements of tfc confirmed its hydrophilic nature, which could be attributed to the presence of -oh groups in tfc as confirmed from ftir analysis. this hydrophilic nature of tfc could have contributed to the superior supercapacitive behaviour of tfc, establishing it as a potential flowerwaste derived carbon-based electrode material for scs. acknowledgements: the authors are grateful to er. koneru satyanarayana garu, hon’ble president, koneru lakshmaiah education foundation (deemed to be university) for providing infrastructure to carry out this work. the authors thank prof. y. anjaneyulu, director, caes, klef and v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(4) (2022) 787-797 http://dx.doi.org/10.5599/jese.1381 795 dr. k. naga mahesh, nanosol energy pvt ltd, hyderabad for their support. the 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(2022) 545-556. https://doi.org/10.5599/jese.1314 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1381 https://doi.org/10.1016/j.ijhydene.2021.09.094 https://doi.org/10.1115/1.4051143 https://doi.org/10.1007/s11581-021-04144-4 https://doi.org/10.5599/jese.1031 https://doi.org/10.1590/s1517-70762007000300009 https://doi.org/10.3390/molecules25010096 https://doi.org/10.1016/j.jclepro.2020.120822 https://doi.org/10.1016/j.wasman.2018.04.041 https://doi.org/10.1016/j.ecoleng.2015.01.011 https://doi.org/10.1016/j.ijheatmasstransfer.2018.12.134 https://doi.org/10.1039/c7cs00505a https://doi.org/10.1039/c3ta15046a https://doi.org/10.5599/jese.1314 https://creativecommons.org/licenses/by/4.0/) @article{nersu2022, author = {nersu, venkata naga kanaka suresh kumar and annepu, bhujanga rao and rajaputra, subhakaran singh and patcha, satya srinivasa babu}, journal = {journal of electrochemical science and engineering}, title = {{char of tagetes erecta (african marigold) flower as a potential electrode material for supercapacitors:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {4}, pages = {787--797}, volume = {12}, abstract = {a char of tagetes erecta flowers (tfc) was derived through simple thermal decompo­sition of tagetes erecta flowers (tf). physico-chemical properties of as-prepared tfc were evaluated using xrd, fesem, ftir, tga, n2 adsorption-desorption isotherm analysis and water contact angle measurements. the practicality and applicability of tfc as promising electrode material in supercapacitors (scs) were evaluated in full-cell configuration by performing electrochemical characterizations like cv, gcd and eis on a lab-scale tfc-based symmetric sc. tfc exhibited a remarkable specific capacitance of 118.4 f g-1 at a constant current density of 0.2 a g-1 and a specific energy of 4.1 wh kg-1 at specific power of 0.1 kw kg-1. tfc showed excellent cyclic stability by retaining 92 % of its initial capacitance even after 6000 gcd cycles at 2 a g-1. the superior capacitive behaviour and cyclic stability of tfc could be attributed to its good wettability towards water. this excellent supercapacitive performance of tfc estab­lishes it as a potential floral waste-derived carbon-based electrode material for scs.}, doi = {10.5599/jese.1381}, file = {:14_jese_1381.pdf:pdf}, keywords = {biochar, carbon cloth, electric double layer capacitor (edlc), flower waste, gel polymer electrolyte, hydrophilicity}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1381}, } phenol removal by electro-fenton process using a 3d electrode with iron foam as particles and carbon fibre modified with graphene http://dx.doi.org/10.5599/jese.1806 537 j. electrochem. sci. eng. 13(3) (2023) 537-551; http://dx.doi.org/10.5599/jese.1806 open access : : issn 1847-9286 www.jese-online.org original scientific paper phenol removal by electro-fenton process using a 3d electrode with iron foam as particles and carbon fibre modified with graphene hind h. thwaini and rasha h. salman department of chemical engineering, college of engineering, university of baghdad, baghdad, iraq corresponding authors: hind.abd2107m@coeng.uobaghdad.edu.iq; tel.: +964-772-453-0704 received: april 3, 2023; accepted: june 2, 2023; published: june 21, 2023 abstract the 3d electro-fenton technique is, due to its high efficiency, one of the technologies suggested to eliminate organic pollutants in wastewater. the type of particle electrode used in the 3d electro-fenton process is one of the most crucial variables because of its effect on the formation of reactive species and the source of iron ions. the electrolytic cell in the current study consisted of graphite as an anode, carbon fiber (cf) modified with graphene as a cathode, and iron foam particles as a third electrode. a response surface methodology (rsm) approach was used to optimize the 3d electro-fenton process. the rsm results revealed that the quadratic model has a high r2 of 99.05 %. at 4 g l-1 iron foam particles, time of 5 h, and 1 g of graphene, the maximum efficiency of phenol removal of 92.58 % and chemical oxygen demand (cod) of 89.33 % were achieved with 32.976 kwh kg-1 phenol of consumed power. based on the analysis of variance (anova) results, the time has the highest impact on phenol removal efficiency, followed by iron foam and graphene dosage. in the present study, the 3d electro-fenton technique with iron foam partials and carbon fiber modified with graphene was detected as a great choice for removing phenol from aqueous solutions due to its high efficiency, formation of highly reactive species, with excellent iron ions source electrode. keywords wastewater treatment; organic pollutants; removal efficiency; 3d-electrode system; iron foam; response surface methodology; analysis of variance introduction phenol is one of the most significant bio-recalcitrant pollutants found in numerous chemical and biochemical industries. it is created during the operation of manufacturing facilities for resins, coke, pharmaceuticals, olive groves, and other food-related businesses [1,2]. pesticides, herbicides, dye molecules, phenolic compounds, antibiotics, medicines, and surfactants are a few examples of organic pollutants that are typically exceedingly difficult to break down [3]. phenolic compounds are http://dx.doi.org/10.5599/jese.1806 http://dx.doi.org/10.5599/jese.1806 http://www.jese-online.org/ mailto:hind.abd2107m@coeng.uobaghdad.edu.iq j. electrochem. sci. eng. 13(3) (2023) 537-551 phenol removal by electro-fenton process 538 among the substances that should be the most concerned due to their tendency to persist in the environment for long periods and their potentially dangerous effects [4]. the presence of phenol in drinking water and irrigation poses substantial health hazards to people as a possible carcinogen, even at low dosages [5]. the health of people and all other living things are highly impacted by water quality. therefore, surface and groundwater contamination by harmful or persistent contaminants became a significant issue [6,7]. several aromatic and aliphatic intermediates that are more toxic than phenol are produced during the electrochemical oxidation of phenol [8]. since it is a clean process that produces high concentrations of hydroxyl radicals (oh•) which can be used to oxidize, degrade, and mineralize a variety of organic compounds, the electro-fenton process has been identified as a particularly attractive technology. it is a promising and emerging advanced oxidation process (aops) [9,10]. the use of transition metals as heterogeneous catalysts will prevent the production of iron sludge and increase the range of potential catalysts that may be employed in aops [11]. the proposed heterogeneous electro-fenton system exhibited high efficiency, stability and economy [12]. the electro-fenton method has many advantages; it is an appealing technology for wastewater treatment because it does not create secondary pollutants. the power used is clean and pollutionfree, it uses no hazardous chemicals, which make this method to be an environmentally benign procedure [13,14]. electro-fenton's fundamental idea is summarized in equation (1), where the oxidation of fe2+ to fe3+ makes it easier for h2o2 to become the highly oxidizing oh•. the oh• radical is a strong, non-selective oxidant that takes part in the degradation of organic contaminants [15]. fe2+ and h2o2 are necessary for oh• to be continuously produced [16]. if fe3+ ions do not form hydroxide precipitates in the solution phase, the fe2+ ions can be created by reducing the fe3+ at the cathode (equation (2)), providing a nearly constant supply of fe2+. the h2o2 is produced by the twoelectron reduction of dissolved oxygen, as shown in equation (3) [17]. the rate constant (k) for each reaction is illustrated as follows [18]: fe2+ + h2o2 + h+ → fe3+ + oh• + h2o; k = 55.0 m-1 s-1 (1) fe3+ + e→ fe2+; k = 0.1 s-1 (2) o2 + 2h+ + 2e→ h2o2; k = 5.9 nm s-1 (3) due to the comparatively easy handling of the reagents needed for this process and the efficient performance that can be obtained at a low cost, the electro-fenton process has been widely used in wastewater treatment [19]. furthermore, no hazards are associated with the handling, storage, or transportation of h2o2 [20]. the electro-fenton process is affected by several factors, including the initial contaminant concentration, ph, current intensity, and reagent dosage [21]. carbonaceous electrodes are often employed as anodes in wastewater treatment due to their large surface area per volume [22]. to enhance the efficiency of electrochemical processes for eliminating organic pollutants, the selection of cathode material is considered an essential factor. the good characteristics of carbon fiber (cf), like high specific surface and low cost, make it a good choice as a cathode for h2o2 generation. the modification of carbon electrodes with graphene was confirmed to exhibit better performance in producing h2o2 [23]. the modified electrode demonstrated higher electron-transfer ability than the raw carbon felt electrode since the redox current and charge-transfer resistance are increased [24]. among sophisticated oxidation methods, the three-dimensional (3d) electrode reactor has received attention for efficiently degrading organic pollutants due to its high efficiency, ease of operation, and environmental compatibility, among other factors [25]. in comparison to standard two-dimensional (2d) electrode technology, the electrocatalytic property of 3d particle electrodes h. h. thwaini and r. h. salman j. electrochem. sci. eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 539 is the core technology of the 3d electrode reactor [26]. in terms of current efficiency, energy consumption (ec), and pollutant removal efficiency, the 3d electrode technology outperforms the 2d electrode technology. the overall 3d electrode system offers a large effective contact area as well as a large number of micro-electrolysis cells, and the electrochemical reaction takes place not only on the surface of the main electrode but also on the surface of each particle electrode [27,28]. on the other hand, using iron foam has two benefits. iron foam functions as a 3d particle to offer active sites and increase reaction efficiency and supplies the reaction with fe2+, which reacts with h2o2 to create oh• that oxidizes phenol [29]. the goal of the current study is to detect the enhancement of phenol removal efficiency by utilizing cf modified with graphene as cathode and iron foam as the third electrode in a 3d electrofenton system and optimizing results using box-behnken design. experimental materials all aqueous solutions were prepared using distilled water. all reagents were of analytical grade and no additional purification was needed. the chemicals were: phenol crystals (c6h5oh, 99.5 % purity, alpha chemical reagent company, india), sdfcl or ferrous sulfate heptahydrate feso4.7h2o (cdh company, india), sodium sulfates (na2so4, purity ≥99.0 %, sdfcl), and sulfuric acid (h2so4, with a concentration of 98 %, sigma-aldrich). cf (99.5 % carbon content, china) was used as a cathode that has a 0.111 mm thickness and a 12k yarn size purchased from jiaxing acg composites co. ltd. (china) and iron foam was purchased from xiamen top new energy technology (china) with porosity of 110 (pores per cm) experimental methods the electro-fenton reaction was conducted in 1 l cylindrical beaker. the batch reactor contained 150 mg l-1 of a phenol solution and three electrodes that were fully submerged in the solution. a cathode was made of carbon fiber felt modified with graphene (186 cm), and the anode was a graphite plate (1860.5 cm). iron foam particles (111 cm) served as a third electrode and the distance between anode and cathode was 3 cm. the iron foam was washed with 0.1 m h2so4 and deionized water in sequence to remove the oxide layer on its surface. to enhance the amount of dissolved oxygen, the solution was aerated with (aco-001 electromagnetic, china) for 20 minutes before the reaction and continued until the completion of the experiment at a rate of 10 l h-1. before each experiment, a ph value of 3 was achieved by adding 0.1 m h2so4 acid to the solution, and the ph value was measured with a ph meter of hanna (romania). to achieve the desired concentration of iron ions, they were released in a specific amount from iron foam particles into the solution. to keep the ionic strength constant and increase the medium conductivity, 0.05 m of na2so4 was added to the solution. electrolytes improve the effectiveness of electrodes and processes, such as increasing the conversion of oxygen to hydrogen peroxide at the cathode surface due to an acceleration in the speed of ion exchange and the electrostatic resistance between electrodes and ions [30]. the electrodes were connected to a dc power supply type (uni-t, utp3315pe) which operated at a constant current density of 4 ma cm-2. a digital voltmeter (flowtech, china) was used to detect voltage and current values. before the ph correction, a first sample of 10 ml of aqueous solution with phenol was obtained. before analysis, periodic samples of treated solution were filtered through 0.45 µm filter papers to eliminate any suspended contaminants. the temperature throughout the 3d electro-fenton process was kept constant at 27 ±1 °c. samples http://dx.doi.org/10.5599/jese.1806 j. electrochem. sci. eng. 13(3) (2023) 537-551 phenol removal by electro-fenton process 540 were taken and analyzed to determine cod and phenol concentration by the rd125, lovibond, and uv-9200 spectrometers, respectively. a schematic drawing of the 3d electro-fenton system is shown in figure 1. the efficiency of phenol removal was evaluated using equation (4) [31]. rephenol = [(c1-c2)/c1]100 (4) where rephenol / % is phenol removal efficiency, c1 and c2 / mg l-1 represent the original and final phenol concentrations, respectively. the energy consumption (ec / kwh kg-1 phenol) that defines the amount of energy required to digest one kilogram of phenol was estimated by equation (5) [31]: ec = 1000iut/δcphenolv (5) where i / a is the operating current intensity, u is voltage, v / l is the volume of the solution, t / h is the electrolysis time, and δcphenol / mg l-1 is the difference in experimental phenol concentrations. figure 1. schematic drawing of the electro-fenton system: 1) power supply, 2) multimeter, 3) magnetic stirrer, 4) iron particles, 5) cathode, 6) anode, 7) flow meter and 8) pump carbon fiber modification before each process, the commercial cf used as one of the working electrodes (cathode) was cut into an 186 cm2 rectangle piece. this cf piece was activated for 30 minutes at 80 °c with 5 % hno3, cleaned, and stored in distilled water. the cf cathode was then modified using a certain amount of graphene. the slurry was created by combining 0.14 ml of polytetrafluoroethylene (ptfe), 3 ml of ethanol, and 2 ml of deionized water. using a brush, this mixture was applied to the two sides of the cf. the cf was then allowed to dry at ambient temperature before being calcined for 30 minutes at 360 °c [32,33]. characterization of electrodes graphite and cf modified with graphene were used as the anode and cathode, respectively. the crystallographic characterization of the graphite, graphene, pure cf, and modified cf was examined by x-ray diffraction (xrd) as shown in figure 2. the xrd device characteristics were model, xrd 6000, and shimadzu, japan. a scan speed of 5 degrees per minute was used with a 40 kv voltage and 30 ma current for the x-ray tube. a scanning electron microscope (sem) was employed to investigate the electrodes surfaces as shown in figure 3. energy dispersive x-ray spectroscopy (edx) h. h. thwaini and r. h. salman j. electrochem. sci. eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 541 data was also measured at a voltage of 25 kv for iron foam before and after the reaction, as shown in figure 4. figure 2 demonstrates that cf modified with graphene structure can be identified as the source of the sharp diffraction peak visible at 2 of 26.99°, corresponding to the diffraction line c (002). there is a weaker abrupt peak at 2  of 18.81° related to the ptfe as it is used in the modification process. the pure graphite shows a sharp and tight peak at 26.5°, corresponding to the diffraction line c (002) [34]. a b c d figure 2. xrd patterns of a graphite, b pure cf, c graphene and d cf modified with graphene by utilizing sem, the morphology of graphene on the cf was revealed. figure 3 displays the sem images of the pure and modified cathode. the graphene flakes were thin sheets, and they were all aligned on the cf surface. figure 3a illustrates the cf without modification, while figure 3b shows the transparent layer of graphene developed on the cf. a definite penetration of graphene between the cf threads can be detected in figures (3a and b) which indicates an increase in the probability of solution mixture penetration between the cf threads. this penetration of graphene between the cf threads increases the surface area, enhances electrode performance, and produces more h2o2. figures 3c and d show the structure of the third electrode (the iron foam). 0 10 20 30 40 50 60 0 2000 4000 6000 8000 10000 12000 in te n s it y 2 /° 0 10 20 30 40 50 60 0 200 400 600 800 1000 in te n s it y 2 /° 0 10 20 30 40 50 60 -200 0 200 400 600 800 1000 1200 1400 1600 in te n s it y 2 /° 0 10 20 30 40 50 60 0 200 400 600 800 1000 1200 1400 in te n s it y 2 /° http://dx.doi.org/10.5599/jese.1806 j. electrochem. sci. eng. 13(3) (2023) 537-551 phenol removal by electro-fenton process 542 figure 3. sem images of: a cf before modification, b to d cf after modification with graphene and iron foam before the electro-fenton process design of experiment with the aid of mathematical and statistical data collection techniques, including response surface methodology (rsm), it is feasible to ascertain the relationship between a process response and its contributing elements [35]. in order to detect the best conditions for higher phenol removal efficiency in the 3d e-fenton process, a box-behnken design (bbd) was employed. using preliminary data, this study used three main independent variables and three levels. iron particles, graphene dosage, and electrolysis time were selected as variables in the performed experiments. table 1 displays the experimental range and levels of independent variables. table 2 displays the boxbehnken design. table 1. variables and different levels of experimental design independent variable levels -1 0 1 x1 mass of iron particles, g 2 3 4 x2 mass of graphene, g 0.5 0.75 1 x3 time, h 3 4 5 the empirical quadratic polynomial model depicted by equation (6) can represent the mathematical relationship between independent factors and response [36,37]. y = a0 + aixi +  aii xi2 + aij xixj (6) h. h. thwaini and r. h. salman j. electrochem. sci. eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 543 table 2. design of experiments with the level of each factor using box-behnken designs, bk. = 1 run coded values real values x1 x2 x3 mass of iron particles, g x1 mass of graphene, g x2 time, h x3 1 -1 1 0 2 1.00 4 2 1 0 -1 4 0.75 3 3 0 -1 -1 3 0.50 3 4 -1 0 1 2 0.75 5 5 0 -1 1 3 0.50 5 6 0 1 1 3 1.00 5 7 1 1 0 4 1.00 4 8 -1 0 -1 2 0.75 3 9 0 0 0 3 0.75 4 10 0 1 -1 3 1.00 3 11 0 0 0 3 0.75 4 12 1 0 1 4 0.75 5 13 0 0 0 3 0.75 4 14 1 -1 0 4 0.50 4 15 -1 -1 0 2 0.50 4 where y represents the response (rephenol), i and j are the index numbers for independent variables, 𝑎0 is the intercept term, x1, x2 … xk are the process variables (independent variables) in coded form. ai is the first-order (linear) main effect, aii is the second-order main effect, and aij is the interaction effect [36,37]. the minitab-18 software was used to examine the results of phenol removal efficiency. iron foam characterization figure 4a and b shows the surface compositions of the iron before and after the reaction which were analyzed by edx. the results showed that the highest elemental peak is for the fe element with a content of 72.32% before the reaction, and there are other peaks for other elements with (co of 24.67 wt.% and c of 4.01 wt.%). the results show a reduction in the fe element to 69.32 wt.% after the reaction due to the release of iron ions needed for the reaction. figure 4. edx results for iron foam: (a) before reaction, (b) after reaction iron foam is characterized by high surface porosity, where fluid can move easily through the connections between porous structures, and it plays a vital role in the 3d electro-fenton system as a source of iron ions [29]. based on the images of sem for iron foam after the reaction, as illustrated in figures 5a and 5b, the structure was varied due to the iron ions releasing. http://dx.doi.org/10.5599/jese.1806 j. electrochem. sci. eng. 13(3) (2023) 537-551 phenol removal by electro-fenton process 544 figure 5. sem images of iron foam after reaction results and discussion statistical analysis the following quadratic model of the phenol removal efficiency in terms of real values of process parameters was obtained by evaluating the phenol removal effectiveness by the minitab-18 program. rephenol = -39.3 + 8.10x1 + 18.0x2 + 27.19x3 0.960(x1)2+ 20.1(x2)22.940(x3)2 9.90x1x2 + 2.794x1x30.40x2x3 (7) table 3 displays the experimental phenol removal efficiency (rephenol) and energy consumption. table 3. experimental results for phenol removal efficiency and power consumed obtained using the box-behnken design, bk. = 1 run mass, g time, h rephenol / % voltage, v ec / kwh kg-1 phenol iron particles graphene actual predicted 1 2 1.00 4 75.460 73.8532 3.98 31.012 2 4 0.75 3 62.290 60.5942 4.07 28.813 3 3 0.50 3 55.110 55.1990 4.25 34.009 4 2 0.75 5 70.216 71.9117 3.98 41.661 5 3 0.50 5 79.820 78.8905 4.25 39.134 6 3 1.00 5 87.220 87.1310 4.06 34.911 7 4 1.00 4 80.312 81.0782 4.09 29.944 8 2 0.75 3 53.330 54.0072 3.95 32.682 9 3 0.75 4 72.310 72.9467 4.25 43.559 10 3 1.00 3 62.910 63.8395 4.00 28.040 11 3 0.75 4 72.820 72.9467 4.25 34.317 12 4 0.75 5 90.350 89.6727 4.11 33.344 13 3 0.75 4 73.710 72.9467 4.23 33.743 14 4 0.50 4 75.980 77.5867 4.05 31.342 15 2 0.50 4 61.230 60.4637 3.94 37.845 the results show that the efficiency of phenol removal is in the range of 53.33 to 90.35 % and the specific energy consumption is in the range of 28.040 to 41.661 kwh kg-1 phenol. the impact of time on phenol removal was superior, as shown by the comparison of runs 2 and 12. thus, at the constant graphene dosage of 0.75 g and iron particles of 4 g, phenol removal increased from 62.29 to 90.35 %, making a difference of 28.06 % as time increased from 3 to 5h. also, based on this comparison, it is h. h. thwaini and r. h. salman j. electrochem. sci. eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 545 clear that increasing the electrolysis time from 3 to 5 h led to an increase in ec from 28.813 to 33.344 kwh kg-1 phenol. phenol removal efficiency increased from 70.216 to 90.35 % with a difference of 20.134 %, as shown in runs 4 and 12 results by increasing the iron particles dosage from 2 to 4 g. this indicates that increasing the dosage of iron particles has the second influence on phenol removal. also, by comparing the values of ec, it is obvious that by increasing the iron particles dosage from 2 to 4 g, the ec decreased from 41.661 to 33.344 kwh kg-1 phenol due to an increase in the removal of phenol by increasing the iron particles dosage. additionally, when the graphene dosage increased from 0.5 to 1 g as shown in runs 15 and 1, an increase in phenol removal from 61.23 to 75.46 % with a 14.23% difference was attained. also, based on this comparison, it is clear that increasing the graphene from 0.5 to 1g led to an increase in ec from 31.012 to 37.845 kwh kg-1 phenol due to a decrease in phenol removal. analysis of variance (anova) the influence of three variables (electrolysis time, graphene dosage, and amount of iron particles) and their interactions can be examined comprehensively by the anova analysis. anova was used to examine the model's significance. the results of anova are presented in table 4. the terms "sum of the square" (seq. ss), "adjusted sum of the square" (adj. ss), and "adjusted mean of the square" (adj. ms) are used to represent statistical terms. contr. denotes the contribution of each variable, and df represents the degree of freedom of the model and its parameters [38]. table 4 displays the low probability value (p value = 0.0001) and high f model value (57.87), these two values denote the significance of the obtained model [39]. the p-values lower than 0.05 specify that the terms of the model are significant [40]. the multiple regression models can be used to predict the effectiveness of phenol removal in the 3d electro-fenton reactor, as evidenced by the high value of r2 (0.9905). high values of adjusted r2 and predicted r2 confirmed that the model fit is sufficient for the regression of experimental data. electrolysis time had the highest significant effect on the phenol efficiency with a high f-value = 350.29 and contr. of 66.6 %, the iron particles had lower contr. = 17.89 %, and finally the graphene dosage had the lowest effect on the phenol removal efficiency with contr. of 8.60 %. table 4. anova tests in removing phenol by electro-fenton process source df seq. ss contr., % adj. ss adj. ms f-value p-value model 9 1641.09 99.05 1641.09 182.34 57.87 0.0001 linear 3 1542.60 93.10 1542.60 514.20 163.20 0.000 x1 1 296.41 17.89 296.41 296.41 94.08 0.000 x2 1 142.48 8.60 142.48 142.48 45.22 0.001 x3 1 1103.70 66.61 1103.70 1103.70 350.29 0.000 square 3 42.74 2.58 42.74 14.25 4.52 0.069 x1x2 1 2.63 0.16 3.40 3.40 1.08 0.346 x2x2 1 8.19 0.49 5.85 5.85 1.86 0.231 x3x2 1 31.92 1.93 31.92 31.92 10.13 0.024 2-way interaction 3 55.75 3.36 55.75 18.58 5.90 0.043 x1x2 1 24.49 1.48 24.49 24.49 7.77 0.039 x1x3 1 31.21 1.88 31.21 31.21 9.91 0.025 x2x3 1 0.04 0.00 0.04 0.04 0.01 0.915 error 5 15.75 0.95 15.75 3.15 lack-of-fit 3 14.75 0.89 14.75 4.92 9.79 0.094 pure error 2 1.00 0.06 1.00 0.50 total 14 1656.84 100.00 model summary s r2 / % r2 (adj.) / % press r2 (pred.) / % 1.77504 99.05 97.34 238.256 85.62 http://dx.doi.org/10.5599/jese.1806 j. electrochem. sci. eng. 13(3) (2023) 537-551 phenol removal by electro-fenton process 546 effect of studied factors rsm makes it feasible to identify the best available selection for the examined parameters and provides fundamental data on actual interactions between them. the contour and its related 3d surface plots can be investigated in order to more extensively understand the interaction between studied parameters on phenol reduction [41]. the effectiveness of phenol elimination was investigated by changing electrolysis time (3, 4 and 5 h) and amount of iron particles (2, 3 and 4 g) under constant graphene dosage, as shown in figure 6. the contour plot in figure 6a shows that high rephenol > 85 % is obtained over a narrow range of iron particles (3.54 g). according to the 3d surface figure 6b, when iron particulates increased from 2-4 g, the effectiveness of phenol removal increased progressively before essentially remaining constant as time increased to 5 h. the surface plot makes it obvious that at the time of 3 h, the efficiency of phenol removal dramatically dropped as the amount of iron particles dropped from 4 to 2 g. the effectiveness of phenol elimination increased linearly as the amount of iron particles raised to 4 g at a longer electrolysis time (5 h). so, the results showed that increasing the reaction time would improve the phenol removal efficiency. these results are in agreement with zhang et al. [42], who also demonstrated that an increase in the reaction time improves cod removal from the landfill leachate by the ef process, and also umar et al. [10], who described how the removal of landfill leachate is mostly dependent on h2o2 dose. the generation of oh• increased by increasing h2o2 in the electrolytic solution and the ferrous ions, which led to an increase in phenol removal efficiency [43]. figure 7 shows the contour and 3d surface plots obtained by changing the graphene dosage over a range of electrolysis times (3, 4 and 5 h). the highest phenol removal efficiency (85 %) was attained at an electrolysis time of 5 h and 1 g of graphene. graphene has a high surface area and strong conductivity, which boosts the electron transfer rate and enhances the formation of h2o2 from the oxygen reduction reaction that takes place on the cathode electrode (equation (3)) [44,45]. at modified cathodes, the effectiveness of phenol removal is increased. the rate of the fenton reaction and, consequently, the rate of homogenous oh• production in the medium are controlled by the yield of h2o2 generation [46,47]. so, the results approved that increasing graphene dosage leads to an increase in phenol removal efficiency. a b figure 6. effect of time and amount of iron particles on rephenol at hold value of graphene 0.75 g: (a) contour plot and (b) 3d surface plot 2 3 60 70 80 2 3 4 4 5 80 90 phenol re % h ,emit nori fo ssa ,particlem g rephenol / % h. h. thwaini and r. h. salman j. electrochem. sci. eng. 13(3) (2023) 537-551 http://dx.doi.org/10.5599/jese.1806 547 a b figure 7. effect of time and graphene dosage on rephenol at hold value of iron particles of 3 g: (a) contour plot and (b) 3d surface plot figure 8 shows the contour and 3d surface plots by changing iron particles (2, 3, and 4 g) over a range of graphene dosages (0.5, 0.75, and 1 g) at a constant electrolysis time of 4 h. the highest phenol removal efficiency (>85 %) was attained at iron dosage of 4 g, and 1 g of graphene. graphene is very appropriate for electrochemical methods, such as its application in the electro-fenton systems to generate h2o2 by the reduction of dissolved oxygen. using carbon or graphite felt coated with graphene as a support for cathodes is one of the most common approaches due to low cost, high surface area, excellent conductivity, and high porosity [32]. a b figure 8. effect of graphene dosage and amount of iron particles on rephenol at hold value of time 4h: (a) contour plot, and (b) 3d surface plot optimization of 3d electro-fenton process outlining the optimal values of parameters to maximize phenol removal efficiency is the primary objective of the optimization. there are five options for the target field of the variables: maximize, objective, minimize, within the range, and none. the maximum removal of phenol with a corresponding weight of 1.0 was chosen as the goal. the results of the optimal values are presented in table 5. two confirmation experiments were conducted under optimal values of operating variables to obtain the highest value of phenol removal efficiency and the corresponding value of cod. the average phenol removal efficiency attained at optimum conditions was 92.83 %, as shown 2 3 60 70 2 0.50 4 0.75 1.00 80 enol re %hp rg fo sam s g ,enehpa g ,elcitrap noassm of ir 0.50 0.75 60 70 80 3 1.00 4 5 80 90 e %r lonehp emit h , g ,enehparg fo mass rephenol / % rephenol / % http://dx.doi.org/10.5599/jese.1806 j. electrochem. sci. eng. 13(3) (2023) 537-551 phenol removal by electro-fenton process 548 in table 6, and the cod value at these conditions was 89.33 % with consumed energy of 32.976 kwh kg-1 phenol. previous study by zheng et al. [29], who utilized the 3d electrode with iron foam particles, gave 70.4 % of folic acid removal efficiency in 6 h, while in the present study, the modification of the cathode electrode increased the efficiency of the 3d electro-fenton process in removing 92.58 % of phenol in 5 h, what which signifies the enhancement obtained in the present electrolytic cell. table 5. optimal results of system parameters for the maximum elimination of phenol response goal lower target upper weight importance rephenol / % maximum 53.33 90.35 1 1 solution of parameters multiple response prediction mass of iron particles, g mass of graphene, g time, h rephenol / % fit standard error fit confidence interval 95 % prediction interval 95 % composite desirability 4 1 5 92.58 2.10 (87.19; 97.97) (85.51; 99.64) 1 table 6. confirmation tests for phenol removal efficiency and consumed energy run mass, g time, h voltage, v ec / kwh kg-1 phenol rephenol, % iron particles graphene actual average 1 4 1 5 4.15 33.076 92.66 92.835 2 4 1 5 4.16 32.876 93.01 conclusion 3d electro-fenton reactor for phenol elimination with the carbon fibers (cf) modified by graphene cathode and graphite anode was assessed in the presence of iron foam particle electrode. the response surface methodology (rsm) was employed to ascertain the impact of electrolysis time, graphene dosage, and iron foam dosage on the phenol removal efficiency and how these factors interact. the predicted multiple regression correlation showed a high value of r2 (0.9905), which confirmed that the model sufficiently fits the regression of experimental data. high f-values and low p-values indicated that all variables influence phenol removal efficiency. the highest phenol removal efficiency of 92.835 % was achieved at 5 h, iron foam dosage of 4 g, and graphene of 1 g. cf approved its effectiveness in the present study as one of the most favored cathodes for h2o2 generation. this is due to the high specific surface and low cost of cf, which 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https://doi.org/10.1016/‌j.jhazmat.2005.11.025 https://doi.org/10.1016/‌j.jhazmat.2005.11.025 https://doi.org/‌10.1016/j.watres.2014.12.022 https://doi.org/‌10.1016/j.watres.2014.12.022 https://doi.org/10.1016/j.apcatb.2018.05.079 https://doi.org/10.1016/j.apcatb.2018.05.079 https://doi.org/10.1016/j.apcatb.2014.09.074 https://doi.org/10.1016/j.electacta.2019.03.067 https://doi.org/10.1016/j.electacta.2017.11.048 https://creativecommons.org/licenses/by/4.0/) anodic hfo2 crossbar arrays for hydroxide-based memristive sensing in liquids http://dx.doi.org/10.5599/jese.1644 805 j. electrochem. sci. eng. 13(5) (2023) 805-815; http://dx.doi.org/10.5599/jese.1644 open access : : issn 1847-9286 www.jese-online.org original scientific paper anodic hfo2 crossbar arrays for hydroxide-based memristive sensing in liquids ivana zrinski1,, dominik knapic1, achim walter hassel,1,2 and andrei ionut mardare1, 1institute of chemical technology of inorganic materials, johannes kepler university linz, altenberger str. 69, 4040 linz, austria 2danube private university, steiner landstrasse 124, 3500 krems-stein, austria corresponding authors: ivana.zrinski@jku.at; andrei.mardare@jku.at received: december 22, 2022; accepted: march 7, 2023; published: april 10, 2023 abstract the development of miniaturized and portable sensing devices is crucial to meeting the high processing capacity demands of contemporary computing systems. hence, the conceptualization of memristive sensors for hydroxide-containing liquids is proposed in this study. metal-insulator-metal (mim) structures were formed on electrochemically anodized hf thin films with pt patterned as top electrodes. these mim memristive structures were integrated into a crossbar array, allowing the investigation of a high number of potential memristor sensors. the mim structures have demonstrated sensing possibilities in the detection of the hydroxyl ion in d-glucose, used as a standard solution. the sensing method was based on the resistive state ratio extracted from i-u sweeps measurements. analytical characterization of the memristor sensor was done based on the resistive state ratio in relation to different concentrations of a standard solution drop cast directly on the surface of the device. linearity was found for d-glucose concentrations ranging from 10 mm to 80 mm with a reasonable corresponding correlation factor (r2=0.96809). additionally, d-glucose incorporation in anodic oxide was studied by xps to investigate its effect on conductive filaments formation. a carbon bonded by a single covalent bond to oxygen (o-c-o) was detected, confirming the proposed sensing mechanism defined by the glucose penetrating the oxide/electrode interface. keywords memristors, anodic oxides, ultra-thin films, liquid detection, valve metals introduction enormous progress has been recently made in the field of complementary metal-oxidesemiconductor (cmos) technology [1-3]. with this respect, computing performance has leveled up with a demand for a large amount of data to be processed and stored. however, these computing systems rely on von neuman architectures based on separated processing and memory units [1,3,4]. http://dx.doi.org/10.5599/jese.1644 http://dx.doi.org/10.5599/jese.1644 http://www.jese-online.org/ mailto:ivana.zrinski@jku.at mailto:andrei.mardare@jku.at j. electrochem. sci. eng. 13(5) (2023) 805-815 anodic hfo2 crossbar arrays for memristive sensing 806 this architecture is inefficient in the sense that it considerably increases energy losses and limits processing speeds and scalability, thus defining an issue known as the von neuman bottleneck. clearly, a necessity for the development of miniaturized in-memory computing systems has emerged. as such, memristors are foreseen as an ideal replacement for conventional memories, being a nonvolatile type of memory [2,3]. furthermore, not only that memristors can be used as rerams, for neuromorphic networks, artificial synapses and image processing, but also in sensing applications [5-11]. the storage concept, based on the resistive switching between a high resistive state (hrs) and a low resistance state (lrs), can also be considered a sensing mechanism [12]. while describing the memory window and power consumption of memristors, the resistive state ratio (hrs/lrs) can also define sensing capacity. following the same analogy, the larger the resistive state ratio, the larger the sensing capability of the device [6]. additionally, memristors can be integrated into crossbar arrays due to their simple two-terminal structures, which can further increase their surface density [13-15]. at the same time, miniaturized sensing electronics are more desired than large conventional sensors that can be only utilized as separated units in computing systems [7]. valve metals have already demonstrated promising memristive behavior [16-22]. one commonly used memory storage material in memristive devices is hafnium dioxide (hfo2) [18-21]. besides its wide band gap, high refractive index, high melting point, and high chemical stability, it is also used as a high-κ gate dielectric in the semiconductor industry [23-27,27,28]. hence, there is a strong motivation for hfo2 to be used as a sensing material. until now, several gas or liquid sensors based on hfo2 have been investigated [7,29-31]. however, the sensing mechanism in memory applications still requires further investigation. memristive sensors have been studied for biomolecule detection or temperature probing in dry conditions [32-34]. furthermore, the development of devices for liquids with hydroxyl group sensing is vital due to the wide range of their possible biomedical applications [6,7,31]. finally, a sensing mechanism based on hfo2 anodic memristors has been demonstrated in the current work for several d-glucose concentrations, which was used as a standard. such sensing device has shown improved performance with respect to memristive-based sensors reported in the literature. experimental fabrication of memristors integrated into a crossbar array the bottom electrodes of metal-insulator-metal (mim) structured devices were produced by deposition by sputtering. the process was carried out in a high vacuum system with a base pressure in the range of 10-6 pa. the deposition of hf thin films onto pre-oxidized si wafers was conducted in ar atmosphere (5∙10-1 pa, 25° c) by applying a constant power to a high purity hf target (99.95 % demaco, the netherlands) [35,36]. the hf thin film was deposited through a pre-attached ni shadow mask (mecachimique, pierrelaye, france). this allowed patterning parallel hf lines, defining word lines or bottom electrodes (figure 1a). following that, these hf metallic lines were used as working electrodes during the electrochemical oxidation process in a classical three-electrode set containing hg/hg2so4/sat. k2so4 electrode (0 v vs. hg/hg2so4 = 0.640 v vs. she) as a reference electrode, and a commercial graphite foil, used as a counter electrode (0.5 mm thick, 99.8 % thermofisher, erlangen, germany). the resulting anodic oxide was grown potentiodynamically up to 8 v vs. she in 1 m phosphate buffer solution (pb), ph 7.0 (na2hpo4, nah2po4, merck, darmstadt, germany) [37]. finally, pt was also patterned through a pre-attached shadow mask in order to form i. zrinski et al. j. electrochem. sci. eng. 13(5) (2023) 805-815 http://dx.doi.org/10.5599/jese.1644 807 bit lines or top electrodes perpendicular to the bottom electrodes (figure 1a). in this way, two terminal mim structures are created at the intersections of the electrode lines, representing memristors as shown in figures 1b, 1c and 1d. electrical measurements electrical measurements referring to i-u sweeps, endurance and retention evaluation were performed in environmental conditions (22 °c, 55 % relative humidity) using a self-developed electrical setup. a gantry robot with xyz stages, microscope cameras and force sensing is described in previous studies. the system is connected to keithley 2450 sourcemeter unit parameter analyzer and controlled via labview software [35,36]. pure hf bottom electrodes were biased during i-u sweeping, while pt top electrode remained grounded during the measurements. electric contacts were established by contacting gold and stainless steel needles at the ends of metallic lines, thus the bottom and top electrodes, respectively. the contact force was controlled by a force sensor keeping it constant at 20±2 mn. hence, direct needle contact on top of each mim structure was avoided. this prevents the system from possible chemical reactions with the needle, mechanical damage such as breaking the thin film or any other undesirable effects. once the switching voltages were determined, the pulsed voltage stress (pvs) method was applied for device lifetime testing (endurance measurements) at a maximum of 260 hz. the resistance values were read at very low voltage values (20 mv), as in the case of retention measurements. in this case, devices were not switched from hrs to lrs state or vice versa, but hrs or lrs values were only read to test data retention. sensing characterization in order to test the sensing response of memristors, different concentrations of d-glucose were drop cast on the surface of a device. phosphate buffer solution (0.1 m, na2hpo4/nahpo4, ph 7.0) was used as a supporting electrolyte due to its matching properties to those liquids' presence in the human body. current-voltage (i-u) sweeps were recorded from -2 v to 2 v after adding aliquots of dglucose, freshly prepared on the day of the experiment. hence, a calibration curve was obtained by adding aliquots of a standard solution while performing i-u sweeps. all chemicals (d-c6h12o6, na2hpo4/nahpo4) were used as acquired (merck, germany) without any further purification. standard solutions were prepared with ultrapure water (arium mini, sartorius, 18 mω cm). microscopic and spectroscopic analysis the samples with memristive devices were thinned by site-specific focused ion beam technique (fib, helios nanolab 650i, fei). the thin lamella was cut from a carefully chosen memristor sample and thinned by ga ions until the final thickness was less than 100 nm at 30 kv with a zeiss 1540-xb. the microstructure studies were performed by conventional tem (a jeol jem-2200fs.) operating at 200 kv. a theta probe system (thermo scientific, uk) was applied for xps measurements, while the system was controlled by the avantage software package provided by the system manufacturer. the samples were analyzed with monochromatic al kα x-ray radiation with an energy of 1486.6 ev. the radiation is focused into 400μm diameter spots. survey spectra were acquired with a pass energy of 200 ev and a binding energy step of 1 ev. for the high-resolution spectra, a pass energy of 20 ev with an energy step of 0.05 ev was used. a dual flood gun was used for the charge compensation accumulated on the surface. the measured spectra were calibrated to the c 1s peak of the adventitious carbon, found to be at 285.0 ev. http://dx.doi.org/10.5599/jese.1644 j. electrochem. sci. eng. 13(5) (2023) 805-815 anodic hfo2 crossbar arrays for memristive sensing 808 figure 1. (a) anodic memristors integrated in crossbar array. (b) a top view of anodic memristors integrated in crossbar array. (c) a single memristive sensor device. (d) cross section of memristive sensor formed on hf thin flims imaged by tem results and discussion resistive switching of hfo2 memristor sensors the switching behavior of mim structured devices (figure 1c) has been studied by performing iu sweeps. the structures, assembled at the intersections of metallic lines (figure 1b), demonstrated typical bipolar memristive switching from -2 v to 2 v, as exemplified in figure 2a (black i-u curve). the switching was observed at several resistive levels tested by applying different current limitation ranges during the voltage sweeping. this behavior was already observed for anodic memristors grown on hf thin films in previous studies [35,36]. once the switching effect was confirmed, a sensing response of memristors was investigated by drop-casting d-glucose at the surface of the device while performing i-u sweeps. the sensing analysis is based on the resistive state ratio extracted from i-u sweeps. this is schematically described in figure 2a. hence, hrs and lrs were extracted for a selected reading voltage. as it can be seen in figure 2a, the resistive state ratio was lower for the i-u curve obtained by sweeping voltage without an analyte. by adding d-glucose, the resistive state ratio increased dramatically (figure 2a, red i-u curve). this demonstrates the sensing capabilities of the device, which are most likely linked to the dynamics of the accumulation zones beneath the top electrode [38]. electrolyte species containing hydroxyl groups diffuse on the surface and influence the position and size of these zones, directly influencing the redox cycle of the cfs during switching [36]. following that, a quantification of glucose was studied by selecting four different concentrations (10, 20, 40 and 80 mm of d glucose) to be drop cast at the surface of the sensor. in this case, typical i-u measurements were performed for each selected concentration one after the other (figure 2b). the slopes of i-u curves, recorded for different d-glucose concentrations, changed as illustrated in i. zrinski et al. j. electrochem. sci. eng. 13(5) (2023) 805-815 http://dx.doi.org/10.5599/jese.1644 809 figure 2b. it was observed that the resistive state ratios increased with the concentration of dglucose (figure 2). as noticed, the shape of i-u curves recorded for memristor sensors was mainly irregular. on the one hand, the respective curves of i-u sweeps recorded without the addition of standard solution exhibited a symmetric hysteresis loop (figure 2a, black curve), switching bipolarly in both positive and negative voltage values range. on the other hand, the regular switching trend was demonstrated only in the positive voltage range for i-u sweeps recorded for the memristor sensor with the addition of the d-glucose standard solution. the resistive state ratio in negative voltage switching range values was significantly lower (figure 2b). hence, only the resistive switching in the positive voltage values range was considered for the calibration curve. however, the i-u sweeps recorded for a number of memristors (5 up to 25 selected devices) demonstrated variations of device resistance values (considering both lrs and hrs values). accordingly, the resistive state ratios ranged from 10 up to a few orders of magnitude. once the sensing response was proven, the possibility of quantifying d-glucose was tested for all devices, showing either a low resistive state ratio or a high resistive state ratio. devices, which reached the resistive state ratio in the range of a few orders of magnitude, did not show a linear trend in resistive state ratio change with respect to added d-glucose concentration (figure 2c). hence, the sensing capabilities in the case of higher resistive state ratios were lower as compared to lower ratios, as explained further. increasing the concentration of glucose exponentially increased the current response. in other words, lrs values decreased while the resistive state ratio increased. eventually, memristors were irreversibly switched due to dielectric breakdown or possible permanent conductive filaments (cfs) disruption. it may be assumed that the stability of cfs, formed inside the active oxide, directly affects the performance of sensors [18,20,39]. therefore, there is a strong motivation to use the anodic hfo2 memristor concept for sensing applications assuming the stability of cfs. an improved switching performance of hfo2 memristors is explained by cfs pinning, which was confirmed for hfo2 grown in phosphate buffer [18]. the resistive state ratio increased linearly with respect to four different d-glucose concentrations for memristive devices that initially reached a resistive state ratio in the range of several ohms (figure 2b). this proves higher sensing capabilities in the case of lower resistive state ratios. this was already linked to cfs stability. permanent destruction of cfs is immediate for higher initial resistive state ratios due to higher current passing through in the low state. for this reason, these mim structures were considered for further optimization of experimental parameters. figure 2. (a) i-u sweeps recorded for memristor without analyte and (b) with the addition of d-glucose. (c) dependence of i-u sweeping response on the concentration of d-glucose (10 to 80 mm) http://dx.doi.org/10.5599/jese.1644 j. electrochem. sci. eng. 13(5) (2023) 805-815 anodic hfo2 crossbar arrays for memristive sensing 810 analytical performance of anodic hfo2 memristor sensor the analytical performance of sensors was studied based on at least five different devices that have shown stable switching voltage range from -2 to 2 v. during the voltage sweeping, the current limitation was set to a maximum of 1 ma. the resistive switching was observed in the low current range, reaching up to 100 µa. four different d-glucose concentrations were selected for quantification. as already described, i-u sweeps were recorded for each selected concentration (10, 20, 40 and 80 mm of d-glucose). resistive state ratio (hrs/lrs) values were extracted from these curves and shown with respect to reading voltage values (figure 3a). in order to avoid any possible error, hrs/lrs ratio values were determined by extracting lrs or hrs values always at the same reading voltage value. figure 3b and 3c present a calibration curve obtained by extracting hrs/lrs ratios from r-u sweeps with respect to d-glucose concentrations. in this case, resistive state ratio values were extracted by reading lrs and hrs values at a selected reading voltage value of 20 mv. additionally, resistive state ratio values for selected reading values, in the range from 0 to 2 v, are shown in figure 3d. error bars, shown in figure 3d, are determined empirically by the variations in extracted hrs/lrs values between different devices. figure 3. (a) r-u sweeps extracted for different d-glucose concentrations. (b), (c) calibration curve obtained in 0.1 m phosphate buffer for 10 mm -80 mm d-glucose. (d) resistive state ratios extracted from i-u sweeps with respect to different reading voltage values. (e) endurance assessment based on successive resistive switching. (f) retention assessment based on successive resistive readings linearity was found in the range from 10 mm to 80 mm with a corresponding correlation factor equal to r2 = 0.96809. moreover, the limit of detection (lod) and limit of quantification (loq) were determined based on five different calibration curves using the lowest concentration of d-glucose. while lod was estimated as 0.8 mm, loq was assessed as 2.6 mm. these lod and loq values are relatively high compared to the values of conventional sensors reported in the literature. however, the determination of analytical parameters was not reported for hfo2 anodic memristor-based sensors used for liquid detection. only several liquid sensors using this concept, mainly based on tio2, have been communicated until now [6,31,40]. hence, this work evidences an improved performance of such sensors regarding resistive state ratio but also enhanced quantification i. zrinski et al. j. electrochem. sci. eng. 13(5) (2023) 805-815 http://dx.doi.org/10.5599/jese.1644 811 possibilities. the development of memristive sensors integrated into crossbar arrays is relevant since a high number of devices can be studied at the same time. this is crucial for understanding the fundamental concept of miniaturized in-memory computing systems, possibly leading to their industrial implementation [7]. moreover, the reproducibility based on five different sensors was evaluated as 3.7 %, while repeatability was estimated as 2.3 % for five measured hrs/lrs values using the same device. this confirms the high repeatability and stability of mim structures, with no significant difference in hrs/lrs ratio recorded even if devices were used after a longer time. the extraction of analytical performance parameters is highly relevant for comparison, hence the future development of memristive sensors grown on different materials or in different geometry. for this reason, the sensitivity (s) of sensors was also evaluated by selecting two hrs/lrs ratio values from the calibration curve, for the corresponding concentrations (figure 3c, equation (1)). 2 1 2 1 2 1 hrs hrs lrs lrs s c c − = − (1) the sensitivity was calculated as 5.3 mm-1 , thus being higher as compared to tio2-based memristive sensors produced by sputtering [6]. finally, endurance and retention measurements were performed in order to confirm the stability of each sensor. this was done by successive resistive switching (followed by resistance evaluation) and successive resistive readings, respectively, and the results are presented in figures 3e and 3f. the device lifetime and data retention reached up to 105 cycles. a high resistive state suddenly becoming conductive is the event that is considered the end of the lifetime or data retention. this is comparable to previously reported hf-based anodic memristors [35,36]. xps analysis the composition of the oxide grown on hf thin films was examined by xps (figure 4). the sample taken for the xps analysis was drop cast with d-glucose and used as a mim structured memristive sensor. the samples analyzed were covered by pt top electrodes (as used for sensing purposes). in addition, depth profiling was performed to study the electrolyte species and glucose incorporation. in such a case, pt was sputtered away. the presence of c, o, hf and pt was found at the surface of the anodic oxide grown in pb covered by pt top electrode (figure 4a-4d). four chemical forms of c can be identified from c 1s high-resolution spectra (figure 4b). the peak at 285.0 ev corresponds to adventitious c related to the c–c/c–h carbon type. the second peak found at 289.2 ev can be assigned to a carboxylic functional group (o═c─o), whereas the peak at 286.5 ev (c-o/n) is most likely related to the rest of glucose as c-oh reacting with impurities found within the sample being also present as amine at 399.4 ev (n 1s) [41,42]. the impurities found at the surface by depth profiling were less than 1 at.%, reaching 1.4 at.%. oppositely, the peak at 288 ev (figure 4b) is related to carbon bonded with a single covalent bond to oxygen (o-c-o) [41,42]. this points towards glucose penetrating the oxide/electrode interface, confirming the sensing concept stated in previous sections [41,42]. moreover, two chemical forms of o can be noted based on the o 1s spectrum (figure 4d). these peaks (figure 4a and 4d) found at 532.8 and 531.3 ev are related to o from organic molecules (d-glucose) and o bound in oxides as hfo2, respectively [41-43]. the electrolyte species incorporation in anodic oxide can be recognized only as a presence of na+ (na 1s) at 1071.7 ev (figure 4a). the stoichiometric amount of o to hf within the same oxide is confirmed by a hf 4f7/2 peak at 16.9 ev and a hf 4f5/2 peak at 18.6 ev, representing hfo2. a single chemical form of hf (figure 4c) is http://dx.doi.org/10.5599/jese.1644 j. electrochem. sci. eng. 13(5) (2023) 805-815 anodic hfo2 crossbar arrays for memristive sensing 812 visible in figure 4d as hf 4f scan [41-43]. finally, the pt top electrode above the oxides is recognized as pt 4f5/2 and pt 4f7/2 doublet (figure 4a) at 73.9 and 70.6 ev, respectively [41-43]. hence, the sensing capabilities of hf memristive sensor are based on o species originating from d-glucose functional groups. likely, o species penetrating the electrode/oxide are contributing to the event of cfs formation. this is achieved by their movement throughout the oxide induced by applying an external electric field. this results in resistive state ratio value change upon adding the analyte solution, as previously discussed. figure 4. a) xps survey and b–d) high-resolution spectra of hf memristive sensor samples prepared in phosphate buffer conclusions this study proves the concept of a memristive sensor based on hydroxyl group influence on accumulation regions in the vicinity of the top electrode. the sensing mechanism relying on resistive state ratio has been proven as a tool for qualitative and quantitative liquid detection. however, further optimization of the sensor is vital. the performance of such devices is dependent on the size of electrodes, the reliability of the pattering mask or the scan rate applied during i-u sweeps performance. once the performance of each device is established, this will allow its functioning in a crossbar array, which is currently under study. additionally, a standard solution of d-glucose was selected due to -oh groups in its structure, thus allowing further investigation of oh-contained liquids. this is important for the investigation of the selectivity of the sensor, possibly leading to real sample analysis and implementation of memristor sensors in biomedical applications, consequently. conflicts of interest: the authors declare no conflict of interest. 1 binding energy, ev binding energy, ev 2 binding energy, ev binding energy, ev in te n si ty , cp s in te n si ty , cp s in te n si ty , cp s in te n si ty , cp s i. zrinski et al. j. electrochem. sci. eng. 13(5) (2023) 805-815 http://dx.doi.org/10.5599/jese.1644 813 acknowledgment: the financial support from the austrian science fund (fwf): p 32847-n and the experimental support (fib, tem and xps) of günter hesser, peter oberhumer and dr. jiri duchoslav from the zentrum für oberflächenund nanoanalytik (zona) at johannes kepler university linz are gratefully acknowledged. references [1] i. h. im, s. j. kim, h. w. jang, memristive devices for new computing paradigms, advanced intelligent systems 2 (2020) 2000105. https://doi.org/10.1002/aisy.202000105 [2] s. carrara, the birth of a new field: memristive sensors. a review, ieee sensors journal 21 (2021) 12370-12378. https://doi.org/10.1109/jsen.2020.3043305 [3] k. sun, j. chen, x. yan, the future of memristors: materials engineering and neural networks, advanced functional materials 31 (2021) 2006773. https://doi.org/10.1002/adfm.202006773 [4] a. sebastian, m. le gallo, r. khaddam-aljameh, e. eleftheriou, memory 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binding doi: 10.5599/jese.2012.0026 19 j. electrochem. sci. eng. 3(1) (2013) 19-27; doi: 10.5599/jese.2012.0026 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical investigations of unexplored anthraquinones and their dna binding afzal shah, abdur rauf , asad ullah, azeema munir, rumana qureshi, iftikhar ahmad, muhammad tahir soomro, zia-ur-rehman department of chemistry, quaid-i-azam university, 45320, islamabad, pakistan and national center of excellence in analytical chemistry, university of sindh, jamshoro, pakistan corresponding author: e-mail: afzals_qau@yahoo.com; tel.: +925190642110; fax: +925190642143 received: september 15, 2012; revised: november 2, 2012; published: november 19, 2012 abstract the redox behaviour of two potential anticancer anthraquinones, 9,10-anthraquinone and 2-chloromethyl-9,10-anthraquinone was investigated in a wide ph range. cyclic voltammetry based assay was developed for the assessment of the effect of medium, substituents, potential scan rate and number of scans on the voltammetric response of anthraquinones. the electrode reaction mechanism was suggested on the basis of cyclic and differential pulse voltammetric results. the effect of dna on anthraquinones was also probed at physiological ph which could lead to further investigation of possible citotoxic activity in vitro. the results revealed that anthraquinones interact with dna more strongly than the clinically used anticancer drug, epirubicin. keywords anthraquinones; redox mechanism; voltammetry; binding constant. introduction quinones are known to have a variety of biochemical and physiological functions. literature survey reveals that these compounds are endowed with antibacterial, antifungal, antiinflammatory, wound healing, analgesic, antipyretic, antimicrobial, antibiotic and antitumor activities [1,2]. some derivatives of quinones are common constituents of biologically important molecules such as vitamins k [3]. others, like mitoxantrone and pixantrone, have been reported to have appreciable antineoplastic activity [4]. it has been reported that the biological action of quinones is due to their electron transfer rates and redox potentials [5]. consequently, the knowledge of the redox properties of quinones derivatives seems conceivable for a better understanding of their biological action. http://www.jese-online.org/ mailto:afzals_qau@yahoo.com j. electrochem. sci. eng. 3(1) (2013) 19-27 anthraquinones and their dna binding 20 anthraquinones (aqs), an important class of tricyclic compounds, have drawn the utmost attention in medical field as anticancer drugs [6]. the main role of aqs in biological electron transport [7] and industrial processes as redox catalysts [8] have led to the extensive investigations of their electrochemical behavior [9,10]. quinone-hydroquinone redox complexes are widely used in voltammetric studies due to their robust electrochemistry. the electron transfer process of most of the quinones has been reported to occur through two one-electron steps via a mechanism involving semiquinone free radical [11]. thus, the study of electron transfer reactions is expected to provide an useful information about molecular structure [12]. literature survey reveals that in spite of the broad range activities of quinones the electrochemical redox mechanism and dna binding studies of biologically important aqs, 9,10-anthraquinone (aq) and 2-chloromethyl-9,10anthraquinone (cm), is an unexplored matter at physiological ph. to bridge this gap and to offer an explanation for the role of the selected aqs (scheme 1) in exercising anticancer effect their detailed electrochemistry has been carried out in a wide ph range. due to the existing resemblance between electrochemical and biological reactions it can be assumed that the redox mechanisms taking place at the electrode and in the body share similar principles [13]. by monitoring the variation in the electrochemical signal of the dna binding drug it is possible to propose the mechanism of the interaction and the nature of the complex formed. since aqs participate in several electron transfer processes in the cellular milieu, the present study is expected to provide valuable insights into molecular mechanisms of their action and designing specific dna-targeted drugs [14]. o o o o cl 9,10-anthraquinone (aq) 2-chloromethylanthraquinone (cm) scheme 1. structures of the chemicals used. experimental anthraquinones were purchased from sigma and used without further purification. 0.5 mm working solutions of aqs were prepared in 50 % ethanol and 50% buffer. all supporting electrolyte solutions (table 1) were prepared using analytical grade reagents. double stranded dna was extracted from chicken blood by falcon method [15,16]. the concentration of the dna stock solution (2.0 mm) was determined from uv absorbance at 260 nm using a molar extinction coefficient (ε) of 6600 m −1 cm −1 [17,18]. a ratio of absorbance at 260 and 280 nm of (a260/a280) > 1.8 indicated protein free dna [19]. doubly distilled water was used throughout. voltammetric experiments were performed using µautolab running with gpes 4.9 software, eco-chemie, the netherlands. a glassy carbon (gc) (a = 0.07 cm 2 ) was used as a working electrode, a pt wire served as a counter electrode and a saturated ag/agcl electrode was employed as the reference electrode. before each experiment the surface of gce was polished with alumina powder followed by thorough rinsing with distilled water. for reproducible experimental results the clean gc electrode was placed in the supporting electrolyte solution and a. shah et al. j. electrochem. sci. eng. 3(1) (2013) 19-27 doi: 10.5599/jese.2012.0026 21 various cyclic voltammograms were recorded until the achievement of a steady state baseline voltammogram. all the voltammetric experiments were conducted in a high purity argon atmosphere at room temperature (25±1°c). the ph measurements were carried out with a crison micro ph 2001 ph-meter with an ingold combined glass electrode. table 1. supporting electrolytes of 0.1 m ionic strength. ph composition ph composition 2.0 hcl + kcl 7.0 nah2po4 + na2hpo4 3.0 haco + naaco 8.0 nah2po4 + na2hpo4 4.0 haco + naaco 9.0 nh3 + nh4cl 5.0 haco + naaco 10.0 nh3 + nh4cl 6.0 nah2po4+na2hpo4 12.0 naoh + kcl results and discussion the cyclic voltammograms of 0.5 mm aq (fig. 1) obtained in 0.1 m acetate based supporting electrolyte (ph 3.0) registered a prominent cathodic peak 1c (more concentration sensitive) at 0.41 v followed by a small reduction peak 2c at -0.60 v. a single anodic peak 1a corresponding to oxidation of the reduction product of aq was recorded in the reverse scan. the ep1c-ep1c/2 of 60 mv (peak 1c) indicated reversible one electron transfer process as reported by the previous investigators [20]. however, the unequal anodic and cathodic peak currents pointed to the quasireversible nature of the overall electrochemical process. the decrease in the cathodic peak current of aq, as evident from 2 nd , 3 rd and 4 th voltammograms obtained under the same conditions, indicated the adsorption of the reduction product at the electrode surface. -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 1a 2c e / v vs. ag/agcl 4a 1 4 1c figure 1. cvs of 0.5 mm aq obtained in one experiment in a medium buffered at ph 3.0 using 0.1 m acetate based electrolyte (haco+naaco): 1 st , 2 nd , 3 rd and 4 th scans at 100 mv s -1 . the appearance of two cathodic peaks in the differential pulse voltammogram -dvp (shown in fig. 2) confirmed the cv result of two step reduction of aq. in non-aqueous media quinones undergo two reversible1e processes, but the results of our experiments indicate the appearance j. electrochem. sci. eng. 3(1) (2013) 19-27 anthraquinones and their dna binding 22 of one quasi reversible and another irreversible peak in a 50:50 mixture of ethanol and water. the width at the half peak height (w1/2) of 91 and 93 mv (close to the theoretical value of 90.4 mv) indicated the involvement of one electron in each reduction step. figure 2. first scan dpv of aq obtained in ph 3.0 at 5 mv s -1 ; pulse amplitude = 50 mv and pulse period = 200 ms for the assessment of the effect of ph, cvs of aq (fig. 3) were obtained in acidic, neutral and alkaline media. in contrast to acidic conditions, the reduction of aq at ph 7.0 was evidenced by a sharp single cathodic peak. the appearance of two broad anodic peaks in the reverse scan showed two step oxidation of the aq reduction product the cathodic peak width (epc-epc/2) of 34 mv (close to the theoretical value of 30 mv) indicated the transfer of two-electrons as reported by bard et al. [20]. the equal anodic and cathodic currents at ph 9.0, with peak potential difference of 75 mv can be attributed to the almost reversible nature of the overall redox process under these conditions [21]. the variation in voltammetric response indicated the strong dependency of the aq redox mechanism on the ph of the medium. with the increase in ph, the potential of 1c displaces to more negative values. this indicated the involvement of protons in the electroreduction of aq. two peaks were also noticed in the cv of cm at ph 4.5 (fig. 4) but at positions different from aq indicating the modulation of the electrophore by the substituent attached at the aromatic ring. the reduction potential of cm was also found to shift cathodically with the increase in ph indicating the involvement of protons during electron transfer processes. on the basis of results obtained from cv and dpv the mechanism shown in scheme 2 was suggested. -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 c b e / v vs. ag/agcl 5 a a figure 3. 1 st scan cvs of 0.5 mm aq obtained in ph (a) 4.0, (b) 7.0 and (c) 9.0 at 200 mv s -1 . 0 0.05 0.1 0.15 0.2 0.25 0.3 -1.2-1-0.8-0.6-0.4-0.20 e / v vs. ag/agcl i /  a a. shah et al. j. electrochem. sci. eng. 3(1) (2013) 19-27 doi: 10.5599/jese.2012.0026 23 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 10 a e / v vs. ag/agcl figure 4. 1 st scan cv of 0.5 mm cm obtained in ph 4.5 at 200 mv s -1 . in order to rapidly screen the selected compounds for their anticancer activities, a relatively simple cyclic voltammetric based assay was used without performing other elaborate assays. although easy to implement, this assay cannot be applied to electroinactive compounds in the potential window of the working electrode. in addition to it, the method is limited to compounds exerting their anticancer effect through direct interaction with dna. in spite of these obvious limitations the assay can differentiate the binding strength and modes of action of dna-binding compounds. as the redox behaviour of quinones is sensitive to their covalent or non covalent binding to other molecules, therefore, cyclic voltammetry was used for their dna binding studies. for cv titrations both the concentration and volume of anthraquinones were kept constant while varying the concentration of dna in solution. the cvs of anthraquinones in the presence of dna have been presented at physiological ph 4.5 (ph of lysosomes of the cell) as no report is available in literature on their dna-binding behavior under such conditions. + +e+h e+h aq aqh haqh (strongly acidic conditions) + + +2e+2h -e-h -e-h aq haqh aqh aq   (neutral conditions) aq q e a (alkaline conditions) scheme 2. proposed redox mechanism of aq typical cv behaviour of 0.5 mm aq at ph 4.5 and v = 200 mv s -1 in the absence and presence of 40, 100 and 120 nm dna has been shown in fig. 5. with the gradual addition of dna, peak 1c got diminished and peak 2c increased in height. the positive peak potential shift accompanied with the decrease in current of 1c is attributed to the disappearance of reactant, aq and its 1e , 1h + reduction product (haq) due to their intercalation into the stacked base pair domain of dna. the increase in current of peak 2c can be related to the greater current carrying ability of negatively charged aq in the presence of dna having anionic phosphate backbone. the cv of 0.5 mm cm in the presence of 40 nm dna (shown in fig. 6) showed quite different behavior from aq. the cathodic peak potential was shifted (80.3 mv) anodically while the anodic peak potential underwent a drift of 30.2 mv in the cathodic direction accompanied with the decrease in peak currents. j. electrochem. sci. eng. 3(1) (2013) 19-27 anthraquinones and their dna binding 24 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 e / v vs. ag/agcl 5 a a d figure 5. cvs of 0.5 mm aq in the (a) absence and presence of (b) 40, (c) 100 and (d) 120 nm dna at ph 4.5; v = 200 mv s -1 . -1.0 -0.8 -0.6 -0.4 -0.2 0.0 b e / v vs. ag/agcl 10 a a figure 6. cvs of 0.5 mm cm in the (a) absence and presence of (b) 40 nm dna at ph 4.5; v = 200 mv s -1 . in general the positive shift in peak potential is suggested for intercalation of the drug into the stacked base pair pockets of dna [22], while negative shift is related to electrostatic interaction of the drug with the anionic phosphate backbone of dna [23]. so, the obvious positive peak potential shift in the cathodic peak of cm can be attributed to the intercalation of cm into the base pair pockets of dna. the shift in the anodic direction further indicates that cm is easier to reduce in the presence of dna because its reduced form is more strongly bound to dna than its oxidized form (neutral form). the displacement of the anodic peak in the negative going direction can be linked with the electrostatic interaction of the reduction product of cm with anionic oxygen of dna. the interaction is expected to alter the dna replication machinery that may lead to the death of cancerous cells. for such a system, where both forms of the drug interact with dna, scheme 3 can be applied [24]. a. shah et al. j. electrochem. sci. eng. 3(1) (2013) 19-27 doi: 10.5599/jese.2012.0026 25 cm + e + h + ↔ hcm ef cm-dna + e + h + ↔ cm-dna eb scheme 3. general redox process of the free and dna bound aq. based upon the process discussed in scheme 3, the following equation is obtained [25] eb ef = 0.059 log (kred/kox) (1) where efand eb are the formal potentials of the free and dna-bound forms of cm respectively. for a shift of 60 mv caused by the addition of 40 nm dna into 0.5 mm cm a ratio of kox/kred was calculated as 0.096, which indicates 10.40 times stronger interaction of the reduced form of the drug with dna than its oxidized form. the substantial diminution in peak current can be attributed to the formation of slowly diffusing cm-dna supramolecular complex due to which the concentration of the free drug (mainly responsible for the transfer of current) is lowered. the dependence of the current function ip, on the scan rate, ν, is an important diagnostic criterion for establishing the type of mechanism. the peak currents of these compounds were found to vary linearly with the square root of the scan rate ( 1/2 ) as expected for diffusion-limited electrochemical process. the diffusion coefficient was determined by the following form of randles sevcik equation ipc = 2.99  10 5 n (cn) 1/2 a c do 1/2  1/2 (2) where n is the number of electrons transferred during the reduction, c is the cathodic charge transfer coefficient, a the electrode area in cm 2 , do the diffusion coefficient in cm 2 s -1 , c the concentration in mol cm -3 and  the scan rate in v s -1 [26]. taking n = 1 (as determined from w1/2 of dpv) and c = 0.73 (evaluated from ep-ep/2 = 47.7/n), the values of d were obtained from the plots of ipc vs.  1/2 at ph 4.5. the electroactive area (0.071 cm 2 ) of the working electrode was determined as described in literature [27]. from the measured slopes of 1.24  10 -5 and 2.17  10 -5 the diffusion coefficients of aq and cm were determined as 2.49  10 -6 and 7.58  10 -6 cm 2 s -1 . the general idea about diffusivity is that the molecule having smaller molecular mass should have greater diffusion coefficient but the results of our experiments showing the opposite trend disproved the common concept. the high d value of cm may be due to the presence of electronwithdrawing chlorine substituent group that will lower the electronic density at the electrophore thus increasing its affinity for reduction at the electrode surface. the lower diffusion coefficients of the dna bound aq (9.28  10 -7 ) and cm (1.42  10 -6 cm 2 /s) as compared to their free forms is suggestive of slowly diffusing supramolecular drug-dna adduct formation. heterogeneous electron transfer rate constant ( ο shk ) was evaluated by using nicholson equation [28] which is based on correlation between ep and ο shk through a dimensionless parameter  by following equation,   ο sh 1/2 o k ad     (3) j. electrochem. sci. eng. 3(1) (2013) 19-27 anthraquinones and their dna binding 26 where  = do/dr, a = nf/rt and  is the scan rate.  for different values of δep can be obtained from the table reported in literature [20]. from the knowledge of , ο shk values of aq and cm were calculated as 1.02 × 10 -3 and 8.51 × 10 -4 cm s -1 using equation (3). the ο shk values also characterized the reduction processes to be quasi-reversible in nature. based upon the decrease in peak current of aq and cm by the addition of different concentration of dna, ranging from 20 to 120 nm, the binding constant was calculated from the intercept of the plot of log (1/[dna]) versus log (i/(io-i)) [29] log (1/[dna]) = log k + log (i/(io-i)) (4) where k is the binding constant, io and i are the peak currents of the drug in the absence and presence of dna respectively. the linear fitting of the amperometric data yielded the binding constant of cm and aq as 2.14 × 10 6 and 1.50 × 10 6 m -1 indicating their high affinity with dna. the higher binding constant values than the k = 4.1 × 10 5 m -1 of the classical intercalator, proflavin and k = 3.4 × 10 4 m -1 of clinically used intercalating anticancer drug, epirubicin [30-32] suggested their preferred anticancer drug candidature. however, the clinical use of these drugs demands more research work on their side effects, reactions with fe(iii) in the blood and production of radicals. conclusions the present study shows that anthraquinones can be reduced at the glassy carbon electrode in a ph dependent diffusion controlled manner involving the same number of electrons and protons. the two cathodic peaks, noticed in the cv of cm at positions different from aq, indicated the modulation of the electrophore by the substituent attached at the aromatic ring. the potential anticancer aq and cm at physiological ph were found to interact with dna by intercalative mode as evidenced by the decrease in current intensity and positive peak potential shift. the diffusion coefficients of free anthraquinones were found greater than their dna-bound forms as expected. the binding constant varied in the sequence: kcm > kaq with values greater than the clinically used anticancer drugs. the investigation of the electrochemical behaviour of aq and cm has the potential of providing valuable insights into biological redox reactions of this class of molecules, resulting in a better understanding of the data described for biological systems and increasing the overall knowledge of anthraquinones physiological mechanism of action. acknowledgements: the authors gratefully acknowledge the funds provided by quaid-i-azam university and higher education commission islamabad, pakistan, for supporting this work. references [1] s. dettrakul, s. surerum, s. rajviroongit, p. kittakoop, j. nat. prod. 72 (2009) 861-865. 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[32] s. charak, d.k. jangir, g. tyagi, r. mehrotra, j. mol. struct. 1000 (2011) 150-154. © 2012 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/3.0/) http://www.ncbi.nlm.nih.gov/pubmed?term=%22khan%20am%22%5bauthor%5d http://www.ncbi.nlm.nih.gov/pubmed?term=%22ansari%20fl%22%5bauthor%5d http://www.ncbi.nlm.nih.gov/pubmed?term=%22nazar%20mf%22%5bauthor%5d http://www.ncbi.nlm.nih.gov/pubmed?term=%22shah%20ss%22%5bauthor%5d http://creativecommons.org/licenses/by/3.0/ 404 not found effect of nano yttria-stabilized zirconia on properties of ni-20cr composite coatings https://dx.doi.org/10.5599/jese.1319 901 j. electrochem. sci. eng. 12(5) (2022) 901-909; https://dx.doi.org/10.5599/jese.1319 open access : : issn 1847-9286 www.jese-online.org original scientific paper effect of nano yttria-stabilized zirconia on properties of ni-20cr composite coatings sukhjinder singh1,, khushdeep goyal1 and rakesh bhatia2 1department of mechanical engineering, punjabi university, patiala, india 2yadavindra department of engineering, punjabi university guru kashi campus, damdama sahib, india corresponding author: sukhjindermoga@gmail.com ; tel.: +91-9876785500 received: march 4, 2022; accepted: july 18, 2022; published: august 16, 2022 abstract in the present work, 5 and 10 wt.% yttria-stabilized zirconia (ysz) nanoparticles were reinforced in ni-20cr powder and deposited on boiler tube steel using a high-velocity oxy-fuel spraying process. the effect of ysz reinforcement on microhardness, surface roughness and porosity were investigated. the hardness was the highest for nanocomposite coating reinforced with 10 wt.% ysz and hardness was found to increase with a decrease in porosity. the coating microstructure and elements were characterized using field emission scanning electron microscopy (fe-sem) with an energy dispersive spectroscope (eds). the constituents of the coating were identified using x-ray diffractometer. it was found that the composite coating with 10 wt.% ysz reinforced nanocomposite coating has the highest microhardness, in the range of 1008-1055 hv. during the coating process, nano ysz particles were dispersed in the gaps between the micrometric ni-20cr particles, providing a better coating matrix than conventional ni-20cr. the ni-20cr with 10 wt.% of ysz nanoparticles showed better results in terms of mechanical and microstructural properties during the investigation. keywords thermal spray coatings; nanocomposites; porosity, hardness, microstructure; oxy-fuel; hvof introduction the desire to raise the power output of thermal power plants is the main drive for the inventions of novel materials due to increasing market competition. heat and corrosion-resistant coatings are among the novel materials used in these units [1,2]. the coatings are applied to the surface of boiler steels used in thermal plants in order to slow down their deterioration [2-4]. surface modifications of these components result from corrosion and erosion in a high-temperature environment. surface changes result in a loss of quality, which eventually leads to sudden premature failure [2,3]. the thermal spray coatings extend the life of these boiler steels in high-temperature environments significantly [4]. various thermal spraying techniques, such as flame spraying, plasma spraying, arc spraying, and high-velocity oxygen fuel (hvof) techniques, are used to coat the components of these power https://dx.doi.org/10.5599/jese.1319 https://dx.doi.org/10.5599/jese.1319 http://www.jese-online.org/ mailto:sukhjindermoga@gmail.com j. electrochem. sci. eng. 12(5) (2022) 901-909 effect of nano ysz on ni-20cr composite coatings 902 plants. due to the powder spraying technique, these coatings have a large number of pores, and corroding species attack the substrate steels through these pores [4-6]. with the development of an oxide layer, this attack induces alterations in the microstructure. the failure of thermal spray coatings has been linked to the severe development of oxide layers [7]. many researchers have discovered that changing the microstructure of these coatings can increase their corrosion resistance [8,9]. the microstructure of coatings is affected by various coating spraying procedures, as well as post-treatment of as-sprayed coatings [10]. using composing powders, some researchers have altered the microstructure of coatings. traditional powders were combined with nano-size ceramic powders, cermet, and rare earth oxides to create composite coatings. because of the possibilities of synthesizing a surface protection layer with unique physical-chemical properties often not achieved in conventional materials, nanostructured coatings composed of crystalline/amorphous nanophase mixtures have recently gained interest in developing corrosion resistance coatings [11]. nanostructured materials are a new type of engineering material with improved characteristics and a structural length scale ranging from 1 to 100 nm [12]. shaw et al. [13] investigated the relationship between microstructure and nanostructure properties, as well as those of nanostructured coatings and plasma spray settings. using reconstituted nanosized al2o3 and tio2 powder, plasma spray was used to create al2o3 – 13 wt.% tio2 coatings [14]. mohsen et al. [10] and saremi et al. [15] combined ysz powder with alumina to create a composite coating that reduced the thickness of the oxide layer and improved corrosion resistance at high temperatures. yugeswaran et al. [16] revealed that a number of interfaces are higher in composite coatings than in conventional coatings, which causes blocking of the micro-pores. this blocking causes a hindrance to the corrosion species and improves corrosion resistance. many authors have reported the development of ni-20cr coatings on steel alloys [17-23], but no researcher has deposited nano yttria-stabilized zirconia (y2o3/zro2) (ysz) reinforced composite coatings on boiler steels. therefore, there is a scope to develop new nano yttria-stabilized zirconia (ysz) reinforced (ni-20cr) nanocomposite coatings and subsequently deposit and investigate the microstructure, porosity, and microhardness of these newly developed composite coatings on boiler tube steel. in this research work, hvof sprayed 5 and 10-weight percent ysz-(ni-20cr) nanocoatings were developed and deposited on t22 boiler tube steel. the microstructure, porosity and microhardness of these newly developed composite coatings have been investigated. hvof thermal spraying technique was used in this research work because the coatings produced with the hvof method have high adhesive strength with the base material and also individual splats have high cohesive strength [24,25]. goyal et al. [26] have observed that this spraying process provides homogeneous coatings having a low value of porosity along with high hardness. experimental coating material to generate different coating powders, commercially available ni-20cr powder was blended with 5 and 10 wt.% ysz powder using low energy ball milling. 950 ni-20cr powder was mixed with 50 g ysz (y2o3/ zro2) powder to get the composite mixture. similarly, 900 g ni-20cr powder was mixed with 10 g ysz (y2o3/ zro2) powder. the composite powders were continuously rolled for four hours at a speed of 200 rpm. b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 901-909 http://dx.doi.org/10.5599/jese.1319 903 the t22 steel substrates were cut from the boiler tubes procured from guru gobind singh thermal power plant, ropar, india. the sample size was 22155 mm. the t22 substrates were polished with sic paper, and subsequently, grit was blasted with alumina powder of grit 45. the substrates were coated using high-velocity oxy-fuel spraying equipment at metallizing equipment co. pvt. ltd. jodhpur, india. the process parameters of the hvof spraying method are shown in table 1. during the spraying procedure, these process parameters were kept constant. table 1. hvof spraying process parameters oxygen flow rate, l min-1 260 fuel (acetylene) flow rate, l min-1 80 air-flow rate, l min-1 560 spray distance, mm 220 powder feed rate, g min-1 25 fuel pressure, kg cm-2 1.4 oxygen pressure, kg cm-2 2.40 air pressure, kg cm-2 3.50 mechanical properties, microstructural properties and phase characterization of coatings using a minitest-2000 thin film thickness gauge, the coating thickness was measured during the thermal spraying process. to expose the coating cross-section, the coated samples were sectioned, mounted in epoxy, and polished. with a load of 300 g, the microhardness of the coating was tested along the coating cross-section with a vickers hardness tester. the surface roughness of the uncoated and coated samples was measured with the help of a surface roughness tester (surftest sj310, mitutoyo). from sem micrographs of the cross-section, the porosity of the hvof sprayed ni20cr, 5 and 10 wt.% ysz reinforced ni-20cr nanocomposite coatings were assessed using an image analysis method employing leica image analyzer software. the pore area size is calculated using a computer-based porosity analysis technique that converts grey-level areas (pore areas) into a background different from the rest of the microstructure. after that, the computer system counts the number of pixels. the average of five porosity measurements was calculated for each type of coated specimen. fe-sem was used to examine the microstructure of ni-20cr, 5 and 10 wt.% ysz reinforced ni-20cr nanocomposite coatings. using an x-ray diffractometer, the phases in the conventional and cnt-enhanced coatings were discovered. to investigate the various phases, present in all types of samples, xrd analysis was carried out using panalytical x’pert pro x-ray diffractometer with the copper target (0.15419, 40 kv and 45 ma). phase identification was made with high score plus software. results and discussion the thickness, porosity, surface roughness and microhardness values of ni-20cr, 5 and 10 wt.% ysz reinforced ni-20cr nanocomposite coatings are shown in table 2. table 2. average coating thickness, porosity and surface roughness values for different coatings substrate coating type coating thickness, µm porosity, % average surface roughness, μm asme-sa213-t22 ni-20cr 252 1.89 3.98 5 wt.% ysz(ni-20cr) 255 1.67 3.32 10 wt.% ysz(ni-20cr) 253 1.38 2.86 http://dx.doi.org/10.5599/jese.1319 j. electrochem. sci. eng. 12(5) (2022) 901-909 effect of nano ysz on ni-20cr composite coatings 904 the average coating thickness for all the coatings was in the range of 250-255 µm. the average thickness of ni-20cr was measured as 255 µm, whereas the average thicknesses of 5 and 10 wt.% ysz reinforced ni-20cr nanocomposite coatings were 255 and 253 µm. the thermal spray coating is usually porous in nature, which affects coatings' properties. the porosity values of ni-20cr and ysz reinforced ni-20cr nanocomposite coatings were less than 2 %. the ni-20cr coating showed a porosity value of 1.89 %. the porosity value of 5 wt.% ysz(ni-20cr) was found to be 1.67 %. the composite coating with 10 wt.% ysz was found to be denser with 1.38 % porosity. with the addition of nano yttria-stabilized zirconia (y2o3/zro2), the porosity of nanocomposite coatings was significantly reduced. the surface roughness values for ni-20cr conventional coating, 5 wt.% ysz(ni-20cr) and 10 wt.% ysz(ni-20cr) nanocomposite coated samples were found to be 3.98, 3.32 and 2.86, respectively. better surface characteristics were observed for 10 wt.% ysz(ni-20cr) nanocomposite coating as compared to conventional ni-20cr coating as the surface roughness was decreased by the addition of ysz nanoparticles. the lower value of the porosity for 5 wt.% ysz(ni-20cr) nanocomposite coating is the main reason for the reduction in surface roughness. the higher degree of melting of nanoparticles reinforced powders causes a decrease in porosity with increasing nanoparticle concentration [24,25]. the nano yttria-stabilized zirconia distributed in the ni-20cr matrix can absorb more heat, resulting in improved powder particle melting. according to goyal et al. [25], accelerated melting is caused by the higher thermal conductivity of nanoparticles. the inflight temperature is affected by nanoparticle content and uniform distribution, which further affects heat transfer and results in a dense composite coating [25,26]. figure 1 shows the microhardness profiles for ni-20cr conventional coating, 5 wt.% ysz(ni-20cr) and 10 wt.% ysz(ni-20cr) nanocomposite coatings. the hardnesses of ni-20cr conventional coating, 5 wt.% ysz(ni-20cr) and 10 wt.% ysz(ni-20cr) nanocomposite coated specimens were found to be in the range of 712-759 hv, 912-964 hv, and 1008-1055 hv, respectively. the hardness values of nanocomposite coatings increased with an increase in nano yttria-stabilized zirconia content in the coating. the nano yttria-stabilized zirconia particles were able to increase the hardness values of the composite ni-20cr coatings. distance from the interface, m figure 1. microhardness profiles of ni-20cr conventional coating across the cross-section 0 200 400 600 800 1000 1200 -120 -80 -40 0 40 80 120 160 200 m ic ro h a rd n e ss ( v ic k e rs ) ni-20cr 5 wt.% ysz(ni20cr) 10 wt.% ysz(ni20cr) m ic ro h a rd n e s, v ic k e rs b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 901-909 http://dx.doi.org/10.5599/jese.1319 905 the hardnesses of ni-20cr conventional coating, 5 wt.% ysz(ni-20cr) and 10 wt.% ysz(ni-20cr) nanocomposite coated specimens were found to be in the range of 712-759, 912-964 and 1008-1055 hv, respectively. the hardness values of nanocomposite coatings increased with an increase in nano yttria-stabilized zirconia content in the coating. the nano yttria-stabilized zirconia particles were able to increase the hardness values of the composite ni-20cr coatings. the nano yttria-stabilized zirconia particles were able to fill the pores in the ni-20cr matrix and reduced the porosity of the coating, which caused an increase in hardness. the microhardness profiles clearly show that the hardness through the coating cross-section was found to be nearly uniform for all coated specimens. the increase in hardness has been attributed by porthina et al. [27] and kumar et al. [28] to the decrease in porosity. due to the closure of porosities, the inclusion of ysz nanoparticles increased the resistance to indentation. due to accelerated melting, nano ysz particles were able to fill the spaces between the ni-20cr particles, resulting in an increase in the hardness of composite coatings as ysz concentration increased. increased hardness of ysz reinforced composites is attributed to fracture suppression and improved dispersion hardening [29]. the effect of carbon-based nanoparticle reinforcement has been firmly substantiated by the increase in hardness of composite coatings, as described in the literature [30-32]. the x-ray diffraction spectra for all coatings are shown in figure 2. diffraction angle, o figure 2. xrd profiles of hvof sprayed coatings: (a) ni-20cr; (b) 5 wt.% ysz(ni-20cr); (c) 10 wt.% ysz(ni-20cr) xrd profile of ni-20cr coated t22 boiler tube steel sample shows ni as the main phase, along with traces of cr. the xrd profile of the ni-20cr coating reinforced with 5 and 10 wt.% ysz nanoparticles revealed that chromium and carbon are present as a major phase, and the presence in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1319 j. electrochem. sci. eng. 12(5) (2022) 901-909 effect of nano ysz on ni-20cr composite coatings 906 of nickel, yttrium, and zirconium was also observed as a minor phase. the identical phases were obtained for all coatings. because of the rapid cooling during the spraying process, the development of non-crystalline amorphous phases increases in cases of ysz reinforced nanocomposite coatings. the xrd patterns depicted that the hvof method can effectively limit the generation of decarburization phases because of the high velocity and relatively low temperature of the flame. figure 3 shows fe-sem micrographs obtained with energy dispersive x-ray spectroscopy analysis for hvof sprayed ni-20cr nanocomposite coatings on t22 boiler steel, as well as 5 and 10 wt.% ysz reinforced ni-20cr nanocomposite coatings. figure 3. fe-sem with energy dispersive spectroscopy analysis of hvof sprayed coatings: (a) ni-20cr; (b) 5 wt.% ysz(ni-20cr); (c) 10 wt.% ysz(ni-20cr) as illustrated in figure 3a, the microstructure of ni-20cr coating is dense, consisting of interlocking particles of regular shape. several oxide stringers may also be seen in the coating b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 901-909 http://dx.doi.org/10.5599/jese.1319 907 microstructure. as illustrated in figures 3b and 3c, the ysz nanoparticles diffused uniformly in the ni-20cr matrix. the use of ysz nanoparticles to strengthen the coating layer resulted in a dense and uniform layer. the microstructures demonstrate that in the composite coating, homogeneous coalescence of nano ysz has happened with the base ni-20cr matrix. energy dispersive spectroscopy analysis revealed that the elemental composition of the various coatings was comparable to that of the feedstock powder, as shown in figure 3. the presence of fe and mn in the ni-20cr conventional coating was found by eds analysis, which could be attributable to fe and mn migration from the substrate to the coating matrix due to porosity in conventional coating. diffusion of base elements through pores/voids in the coating matrix has also been reported by various authors [32-35]. reinforcement of nano ysz particles in cr3c2-25nicr coating filled the gaps, reducing porosity which prevents diffusion of base elements to the coating matrix, thereby increasing the microhardness of the coatings. conclusions the following conclusions are made from this experimental work: • the thicknesses of hvof sprayed ni-20cr, 5 and 10 wt.% ysz reinforced ni-20cr nanocomposite coatings were determined to be between 250 and 255 m. • as the amount of ysz in a nanocomposite coating increases, the porosity value lowers. the 1.38 % porosity of the 10 wt.% ysz-(ni-20cr) coating was revealed to be the lowest. the surface roughness values are improved as the porosity decreases. • the composite coating with 10 wt.% ysz reinforced nanocomposite coating showed the highest microhardness, which was in the range of 1008-1055 vickers. this could be due to nano ysz particles filling pores/voids in the coating matrix. • the quick cooling of the spraying procedure resulted in the formation of non-crystalline amorphous phases, as evidenced by the xrd spectra of all nanocomposite coatings. • the presence of fe and mn in the ni-20cr coating was detected by sem/eds analysis, which could be attributed to the diffusion of these elements through holes in the coating matrix. sem/eds investigation of ysz reinforced nanocomposite coatings revealed a homogeneous, dense coating surface with no base element diffusion due to nanoparticle filling of voids/pores. acknowledgement and conflict of interest statement: this research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors. the authors also declare that they have no 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distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1319 https://doi.org/10.1007/s40033-015-0074-8 https://doi.org/10.1021/la203056f https://doi.org/10.1007/s12613-019-1742-8 https://doi.org/10.1016/j.actamat.2007.10.038 https://doi.org/10.1016/j.wear.2005.02.031 https://doi.org/10.1080/02670844.2019.1662645 https://doi.org/10.1007/s41779-018-0237-9 https://creativecommons.org/licenses/by/4.0/) {fabrications of electrochemical sensors based on carbon paste electrode for vitamin detection in real samples:} http://dx.doi.org/10.5599/jese.1313 421 j. electrochem. sci. eng. 12(3) (2022) 421-430; https://dx.doi.org/10.5599/jese.1313 open access : : issn 1847-9286 www.jese-online.org review fabrications of electrochemical sensors based on carbon paste electrode for vitamin detection in real samples vaishnavi sharma and gururaj kudur jayaprakash laboratory of quantum electrochemistry, school of advanced chemical sciences, shoolini university, bajhol, himachal pradesh, 173229, india corresponding author: rajguru97@gmail.com; tel.: +91-953-876-2343 received: february 26, 2022; accepted: april 24, 2022; published: may 4, 2022 abstract this review article examines some advancements in electrochemical sensors for vitamin detection in the past few decades. vitamins are micronutrients found in natural foods essential for maintaining good health. most vitamins cannot be synthesized by a body and must be obtained externally from natural food. vitamins make a class of organic chemicals that shortage can cause various ailments and diseases, and consumption can become harmful if it exceeds the usually needed level. because of these factors, vitamin detection has become highly significant and sparked interest over the past few decades. the electrochemical sensors function on the concept of electrochemical activity of practically all vitamins. this implies that concentrations of vitamins in the electrolyte may be detected by measuring the amounts of current generated at certain potentials by their oxidation and reduction at the working electrode surface. voltammetric methods are superior to other methods because they are cheaper and show sharp sensitivity with faster analysis speed. the carbon-based electrodes, in particular carbon paste electrodes (cpe), have significant advantages like easier catalyst incorporation, surface renewability, and expanded potential windows with lower ohmic resistance. this review goes into detail about several electrochemical sensors involving cpe as the working electrode and its utilization to detect water and fat-soluble vitamins. keywords water-soluble vitamins; fat-soluble vitamins; redox reactions; voltammetry; modifiers introduction vitamins are a collection of complex chemical molecules that a human body needs in small amounts to maintain optimum health. in general, vitamins are not synthesized by the human body (apart from vitamins b3, d, and k) and must be obtained from the diet. vitamins aid in maintaining cells, tissue, and organ growth and act as an immune system booster. another important role of vitamins is to act as cofactors (co-enzymes) in enzymatic reactions [1-5]. table 1 provides the daily http://dx.doi.org/10.5599/jese.1313 https://dx.doi.org/10.5599/jese.1313 http://www.jese-online.org/ mailto:rajguru97@gmail.com j. electrochem. sci. eng. 12(3) (2022) 421-430 sensors based on cpe for vitamin detection 422 allowance of vitamins by normal male and female adults and the diseases caused due to their deficiency. table 1. required amounts of vitamins for adults (males and females) and related diseases due to their deficiency vitamin common name required daily amounts, mg deficiency diseases male female a retinol 0.90 0.70 night blindness b1 thiamine 1.2 1.1 beri-beri b2 riboflavin 1.3 1.1 ariboflavinosis b3 niacin, niacin amide 16 14 pellagra b5 pantothenic acid 5 5 paresthesia b6 pyridoxine 1.3 1.3 anemia b7 biotin 0.030 0.030 dermatitis, enteritis b9 folic acid 0.400 0.400 megaloblastic anemia b12 cobalamin 0.0024 0.0024 megaloblastic anemia c citric acid 90 75 scurvy d calciferol, ergosterol 0.015 0.020 rickets, osteomalacia e alfa-tocopherol 15 15 less fertility k menadione 0.120 0.090 non-clotting of blood vitamins also aid in converting food into energy, lowering the risk of sickness, and encouraging a healthy way of life. the human body requires enough vitamins and any deviation from the appropriate amount might indicate a problem. therefore, widespread research has been done to detect vitamins in the human body and real samples. depending on the solubility properties, vitamins can be divided into two broad categories, watersoluble vitamins, and fat-soluble vitamins. fat-soluble vitamins include a, d, e, and k. fat-soluble vitamins can be stored in adipose tissue. water-soluble vitamins, such as those of the b group and vitamin c, cannot be kept in human bodies since they travel with the water and are extracted through the urine, so they must be consumed regularly by our body. vitamins may be detected using a variety of techniques, including high-performance liquid chromatography, immuno-affinity columns, fluorescence, ultraviolet spectroscopy, and capillary electrophoresis. although these approaches offer many benefits, such as high selectivity and sensitivity, they have significant drawbacks, including long analysis time, difficult sample preparation, huge sample amounts, expensive apparatus, and the need for highly skilled workers. electrochemical sensors are viable alternatives to these technologies because of their simplicity, cost-effectiveness, high sensitivity, ease of downsizing, dependability, and repeatability. so far, electrochemical sensors have found widespread use in a variety of disciplines, including pharmaceutical, food, and clinical analyses [6-8]. electrochemical sensors record the concentration of analyte (here vitamins) in the electrolyte solution by measuring the current at the working electrode, which is based on the electrochemical redox (oxidation/reduction) process of an analyte. the current response is typically proportional to the analyte concentration. at present, amperometry and voltammetry are the most used electrochemical methods. cyclic voltammetry (cv) and linear scan voltammetry (lsv) are the two simplest voltammetry procedures. with the tuning of different parameters, square wave voltammetry (swv) and differential pulse voltammetry (dpv) are two approaches that can give improved sensitivity [1]. the selection of the working electrode is crucial in electrochemical analysis. carbon paste electrode (cpe) has several advantages over other typical carbon working electrodes, including ease v. sharma and g. k. jayaprakash j. electrochem. sci. eng. 12(3) (2022) 421-430 http://dx.doi.org/10.5599/jese.1313 423 of preparation, rapid surface renewal, consistent response, and, most importantly, decreased ohmic resistance. in this review, some advancements in vitamin detection accomplished by the utilization of cpe will be presented and discussed. electrochemical sensors for water-soluble vitamins water-soluble vitamins are commonly present in a variety of foods, including milk powder, drinks, and supplements. water-soluble vitamins cannot be kept in the human body for lengthy periods because they are promptly eliminated by the urine. insufficiency of water-soluble vitamins has a detrimental impact on human health and is frequently linked to illness. as a result, maintaining the proper operation of the human body needs a regular and sufficient dietary intake of watersoluble vitamins. electrochemical sensors based on the carbon paste electrodes are a typical tool for examining redox characteristics of water-soluble vitamins since each water-soluble vitamin is made up of distinct molecules that may transfer electrons in an aqueous electrolyte. electrochemical sensors for the detection of water-soluble vitamins have become quite popular over the past few decades. electrochemical sensors for vitamin b1 vitamin b1 (vb1) is one of eight vitamins that make up the vitamin b complex, a potent group of nutrients. it is extremely important for both physical and mental wellness and necessary for the human body’s regular metabolism, particularly in the neurological system. deficiency of vb1 is linked to several illnesses, including beriberi and neurological problems. it is important to note that vb1 are electrochemically active redox moieties and therefore, it is possible to detect them using voltammetric methods. the electrochemical response of vb1 in an alkaline medium at a carbon paste electrode (cpe) is discussed by j. oni et al. [2], who used cv for the determination of vb1 at a carbon paste electrode (cpe) modified with manganese phthalocyanine (mnpc). for the detection of vb1, a ph 10 buffer solution was selected. mnpc modified cpe showed catalytic activity towards the oxidation of vb1 by considerably increasing the oxidation current compared to the unmodified electrode. the detection limit for vb1 at the mnpc-cpe was 14.6 mol dm−3. the real sample analysis was done using commercial vb1 tablets [2]. for the detection of vb1, muppariqoh et al. [3] developed a cpe modified by a molecularly imprinted polymer (mip) based on polypyrrole. the authors also used cv for sensing purposes in potassium chloride (kcl) tris buffer supporting electrolyte of ph 10. the limit of detection (lod) was found to be 0.069 mmol l-1 [3]. p. k. brahman et al. [4] developed a simple methodology for the determination of vb1. the method is based on the interaction of vb1 with dna using a multiwalled carbon nanotube paste electrode (pmwcntpe) and the dpv method. under the optimum conditions, lod was found to be 0.44 mg ml−1 at ds-dna modified pmwcntpe. the analysis for serum, plasma, and urine samples suggested that the proposed electrode is promising for vb1 analysis in real samples [4]. electrochemical sensors for vitamin b2 vitamin b2 (vb2) (chemical name: 7,8-dimethyl-10-ribityl-isoalloxazine), also known as riboflavin, is a water-soluble vitamin found in several foods. vb2 plays an important part in human physiological activities like the breakdown of fats and carbohydrates, thereby playing a major role in maintaining the human body’s energy supply. unfortunately, the human body cannot synthesize vb2, and it must be received from the food from sources such as liver, cheese, milk, pork, eggs, wines, and tea. the http://dx.doi.org/10.5599/jese.1313 j. electrochem. sci. eng. 12(3) (2022) 421-430 sensors based on cpe for vitamin detection 424 deficiency of vb2 is linked to eye lesions and skin illnesses. it is possible to detect vb2 using voltammetric methods. e. mehmeti et al. [5] developed mno2 modified cpe (mno2mcpe) to sense vb2 using the dpv method. the mno2mcpe showed higher redox peak currents when compared to unmodified cpe because of the higher active surface area obtained from mno2. the found lod value was 15 nm. the authors also used the developed sensor to determine vb2 in pharmaceutical formulations [5]. a. nezamzadeh-ejhieh et al. [9] modified cpe with co2+ -y zeolite. using cv, the authors showed that the presence of co2+ ions lowers the oxidation potential (for around 200 mv) and increases the peak currents of vb2. which shows that co2+ ions improve cpe sensing properties for vb2 detection. the authors checked the analytical utility of the prepared sensor electrode for analysis of vb2 in pharmaceutical products with the fluorometric method evidence [9]. electrochemical sensors for vitamin b6 vitamin b6 (vb6) (pyridoxine) is included in a variety of products, including multivitamin supplements and enhanced foods. it is essential for both mental and physical well-being. also, vb6 is required for over 100 enzymes involved in protein metabolism and producing hemoglobin (which transports oxygen to tissues). m. f. s. teixeira et al. [10] have studied the redox behaviour of vb6 using cpe modified with n, n-ethylene-bis(salicylideneiminato)oxovanadium (iv) complex. the redox nature of the modified cpe and the electrooxidation of vb6 were analysed using cv. after the modification, cpe showed a lod value of 37 mol l−1. the concentrations of vb6 in commercial drugs were also tested by cross verifying results with the spectrophotometric method. obtained results were satisfactory and within an acceptable range of errors [10]. in another work, m. f. s teixeira et al. [11] modified cpe using copper (ii) hexacyanoferrate (iii) (cuhcf) for the determination of vb6 using cv. cuhcf modified cpe showed a lod value of 0.41 mol l-1. the authors tested the analytical utility of prepared electrodes by testing vb6 concentration in different pharmaceutical samples [11]. electrochemical sensors for vitamin b9 vitamin b9 (vb9) is also known as folate (folic acid), which is required to produce white and red blood cells in the bone marrow and produce rna and dna. it also plays a crucial role in the conversion of carbohydrates into energy. therefore, it is extremely critical to get enough folate, especially during periods of fast growth, such as infancy, puberty, and pregnancy. khaleghi et al. [12] developed a sensitive voltammetric sensor by modifying cpe with 1,3-dipropylimidazolium bromide (1,3-dibr) and zno/cnts nanocomposite, forming (1,3-dibr/zno/cnts/cpe as an electrocatalyst. the modified cpe showed a negative shift of oxidation potential for 95 mv when compared to cpe, and lod was found to be 0.05 μm. the proposed electrode was also tested to detect vb9 in real samples like (tablets and food samples) with a 95% confidence level [12]. s. cheraghi et al. [13] measured curcumin in the presence of vb9, using a new sensor, which was built with cadmium oxide nanoparticles-ionic liquid (1,3-dipropylimidazolium bromide as a binder) modified cpe (cdo-il-mcpe). it was found that curcumin and vb9 oxidation signals are separated, appearing at roughly 0.41 and 0.73 v vs. ag/agcl at ph 7.0, respectively. curcumin and vb9 concentrations were altered concurrently for this purpose, and swvs were measured. the sensitivity of the cdo-il-mcpe to curcumin in the absence and presence of vb9 was nearly identical, indicating that the oxidation processes of curcumin and vb9 on the sensor are independent of one another, allowing thus a simultaneous or independent determination of curcumin and vb9. v. sharma and g. k. jayaprakash j. electrochem. sci. eng. 12(3) (2022) 421-430 http://dx.doi.org/10.5599/jese.1313 425 electrochemical sensors for vitamin b12 vitamin b12 (vb12) is one of the significant vitamins of b family members, which is commonly known as cobalamin. vitamin b12 is the most massive and structurally complex of all vitamins. it is found naturally in animal products such as meat and eggs and can also be manufactured by bacterial fermentation synthesis. its presence in mammals will help a proper rbc synthesis and brain functions like nerve tissue health. when the concentration of vb12 is below a certain concentration, it may lead to severe symptoms like brain disorders. s. karastogianni and s. girousi [14] constructed the cpe modified with manganese complex (thiophene-2-carboxylic acid and triethanolamine as ligands) and used swv for the characterization of sensing properties of the modified cpe. the authors confirmed vb12 interaction with modified cpe by observing reduction peaks (positive shift of 48 mv). this unique method helps to confirm the presence of vb12 in an electrolyte solution. however, the authors did not report lod values of modified cpe towards cobalamin. later, the authors extended their procedure to detect vb12 in real samples like commercial drugs. the obtained values were in very good agreement with the prescribed concentration of cobalamin in tablets [14]. r. g. compton's research group also modified cpe with trans-1,2dibromocyclohexane for vb12 detection using swv [15]. a lod value of 0.85 nmol dm-3 was obtained at modified cpe and also, vb12 was successfully detected in pharmaceutical products and biological matrix media [15]. electrochemical sensors for vitamin c vitamin c (vc), commonly known as ascorbic acid, is required to repair, develop, and grow human tissues. vc is also engaged in a variety of human body activities, including collagen creation, iron absorption, immune system function, wound healing, cartilage, bone, and tooth maintenance. another important vc duty is to act as an antioxidant to protect healthy cell lines from free radicals, hazardous chemicals, and pollutants like cigarette smoke attacks. therefore, it is important to monitor vc concentrations in the human body. electrochemical voltammetric methods are very promising for detecting vc in real samples. here, some important works on developing sensors to detect vc in real samples using modified cpe will be presented [16-19]. g.k. jayaprakash et al. [16] used the cetylpyridinium modified cpe (cpmcpe) to sense vc using cv. first, the authors compared the effects of different cationic surfactants on the cpe and found that the active surface area of the electrode is higher when cpe is modified by cetylpyridinium surfactant. later, the authors compared the bare cpe and cpmcpe behaviour for the detection of vc in phosphate buffer solution. bare cpe showed a broad cv peak at 0.342 mv vs. sce, while cpmcpe showed a sharp oxidation peak at -0.084 mv vs. sce with an increased oxidation peak current value. therefore, the authors concluded that using a cetylpyridinium modifier of cpe is beneficial for the detection of vc. the authors also used the proposed electrode to detect the vc in urine samples without any pre-treatment and the proposed electrode detected vc with about 107 % recovery. the authors did not check the lod of cpmcpe for the detection of vc [16]. a. baghizadeh et al. [17] applied zro2 nanoparticle/ionic liquids modified cpe for the determination of vc in the presence of vb6. the formed zro2/nps/il/cpe detected vc in the presence of vb6 with the lod value of 0.009 μm. the authors performed the real sample analysis using tablets, urine, fruit juice, and injection, obtaining satisfactory results [17]. s. gheibi et al. [18] determined vc amounts using p-aminophenol modified carbon nanotubes paste electrode (cntpe) using the cv method. at the modified cpe electrode, apmcntpe, the oxidation peak potential of vc was shifted negatively, and therefore, the authors concluded that the proposed electrode is suitable for the detection of http://dx.doi.org/10.5599/jese.1313 j. electrochem. sci. eng. 12(3) (2022) 421-430 sensors based on cpe for vitamin detection 426 vc, showing lod value of 80 nm. the analytical application of the electrode was tested using fruit juices and fresh vegetable juice [18]. a summary of cpe-based electrochemical sensors developed for the detection of water-soluble vitamins (b1, b2, b6, b9, b12, and c) is presented in table 2. table 2. summary of cpe based electrochemical sensors for detection of water-soluble vitamins vitamin modifier method lod real sample analysis ref. vb1 mnpc cv 14.6 mmol dm−3 commercial vb1 tablets [2] vb1 polypyrrole cv 0.069 mmol l-1 na [3] vb1 ds-dna dpv 0.44 mg ml−1 serum, plasma, urine [4] vb2 mno2 mcpe dpv 15 nm pharmaceutical formulations [5] vb2 co2+y zeolite cv pharmaceutical products [9] vb2 poly-arginine dcv 93 nm human blood serum, b-complex capsule [20] vb6 n,n-ethylene-bis(salicylideneiminato) oxovanadium(iv) complex cv 37 mmol l−1 pharmaceutical formulations [10] vb6 (cuhcf) cv 0.41 mmol l-1 pharmaceutical formulations [11] vb9 (1,3-dibr) & (zno/cnts) nanocomposite cv 0.05 μm drugs [12] vb9 cadmium oxide nanoparticles-il swv 0.08 μm food samples [13] vb9 (pt:co/ rtil ) swv 40 nm food samples [14] vb12 manganese complex drugs [15] vb12 trans-1,2dibromocyclohexane swv 0.85 nmol dm-3 pharmaceutical products, plasma [15] vc cetylpyridinium cv - [16] vc zro2 nanoparticle/ionic liquids 0.009 μm tablet, urine, fruit juice, injection [17] vc p-aminophenol 80 nm food sample [18] electrochemical sensors for fat-soluble vitamins a, d, e, and k are fat-soluble vitamins and hence, they are absorbed in the lymph, transported in the blood via carrier proteins, and can be stored in the liver and fatty tissues. these vitamins do not dissolve in water and are present in foods containing fats. the human body absorbs these vitamins as dietary fats do. all these vitamins are physically similar in featuring a five-carbon isoprene segment as a fundamental structural unit of the molecule. electrochemical sensors for vitamin a vitamin a (va) is a group of unsaturated fat-soluble organic compounds called retinoids which include retinol, retinal, and retinyl esters. va is critical for vision and is involved in immune function, reproduction, and cellular communication. the deficiency of va may lead to many serious diseases like night blindness. in the human diet, va is available in two forms: preformed vitamin a (retinol and retinyl esters) found in animal food, including dairy products, fish, meat, etc. provitamin a is plant pigments that are consumed through vegetables and fruits. x. lv et al. [21] used swv for determining va concentrations in vegetables (cabbage) and drugs, utilizing a cpe modified with nano-alloy (pt:co) room temperature ionic liquid (pt:co/il). the v. sharma and g. k. jayaprakash j. electrochem. sci. eng. 12(3) (2022) 421-430 http://dx.doi.org/10.5599/jese.1313 427 prepared pt:co/il/cpe was compared with bare cpe (bcpe), ionic liquid (il) modified cpe, and pt:co modified cpe. based on the results, pt:co/il/cpe showed maximum sensitivity and electrochemical reversibility for the va detection in phosphate buffer solution (pbs), ph 9, as the supporting electrolyte. for pt:co/il/cpe, the highest oxidation peak current and the lowest oxidation peak potential were observed compared to other electrodes. therefore authors concluded that the modification of cpe with pt:co/il is beneficial for sensing va. the pt:co/il/cpe showed lod of 0.04 μm and the authors also analysed the electrode utility using cabbage and tablet specimens [21]. s. žabčíková et al. [22] used cv to investigate the oxidation mechanism of all-trans retinol and their esters in non-aqueous, aqueous organic mixture, and pure aqueous media. their most sensitive oxidation peak appearing at +0.8 v vs. ag/agcl was used for the development of a new direct voltammetric method based on dpv for the determination of retinol at carbon paste. the results showed that modification of cpe using 30 % (by mass) sodium dodecyl sulphate (sds) has optimal sensitivity. in comparison to the regularly used glassy carbon electrode (gce), the cpe/sds demonstrated remarkable progress in retinol electroanalysis, yielding a lod of 1.3 m [22]. electrochemical sensors for vitamin d3 also called cholecalciferol, vitamin d3 (vd3) is a fat-soluble secosteroid responsible for increasing the absorption of calcium, magnesium, and phosphate in the intestine and maintaining their level in blood and cardiovascular health and preventing infections. vd3 helps to strengthen bones and muscles, and its deficiency may lead to serious diseases like rickets, osteoporosis, osteomalacia, autoimmune disorders, cardiovascular disease, cancer, etc. a hybrid imprinted polymer-based electrochemical sensor was set up by kia et al. [23], which was found to be highly sensitive to vd3 and applied successfully in real samples. the modified carbon paste electrode was constructed by mixing graphite powder and n-eicosane (binder) with a molecularly imprinted polymer synthesized in the presence of multi-walled carbon nanotubes, forming mip/ mwcnt/ cpe. swv was employed to study the oxidation of vd3 using a mixture of perchloric acid and 70 % methanol as an electrolyte at ph 3. the linear range was observed between 0.05 and 7 m and lod was found to be 3×10-8 m [23]. electrochemical sensors for vitamin e vitamin e refers to a group of eight different compounds: α, β, γ, and δ tocopherols and corresponding four tocotrienols, among which αtocopherol is biologically most active as it is transported and used by the liver. it is one of the major antioxidants which protect cells, tissues, and organs from free radicals and maintains immune and neurological health. therefore, ve needs to be consumed in proper amounts through diet. sýs et al. [24] introduced a silicone oil-modified glassy carbon paste electrode (so/gcpe) for the detection of ve in portions of margarine and edible oils. to obtain the results with the highest efficiency, 60 % aqueous-acetone solution was used. square wave anodic stripping voltammetry (swasv) was carried out after ve accumulation on so/gcpe at ph 9.0, using hno3 as the supporting electrolyte. after 5 minutes of accumulation, the linear range was 0.5–40 mol l-1, with an lod of 0.1 mol l-1. after 15 minutes of accumulation, however, the linear range was 0.05–10 mol l-1 with an lod of 3.3 nmol l-1 [24]. electrochemical sensors for vitamin k vitamin k (vk) is a fat-soluble vitamin that may be found in two forms: vitamin k1 (vk1) also called phylloquinone and vitamin k2 (vk2) menaquinone. vk2 is found in plants and leafy green vegetables http://dx.doi.org/10.5599/jese.1313 j. electrochem. sci. eng. 12(3) (2022) 421-430 sensors based on cpe for vitamin detection 428 but also in cheeses, eggs, meat, and a variety of fermented foods. in blood coagulation and bone calcification, vk plays a critical function. vk insufficiency produces a drop in prothrombin and clotting factors in the blood, which causes hemorrhagic and profuse bleeding. for detecting vk1, j. p. hart et al. [24] built up carbon paste electrodes made with various kinds of graphite, and pasting agents were studied using linear sweep voltammetry (lsv). the vitamin was shown to accumulate strongly but dependent on the graphite type and pasting agent utilized. then using adsorptive stripping voltammetry (adsv), a carbon paste electrode comprising nujol-ultra carbon ultra superior purity graphite provided the maximum sensitivity where the optimal accumulation time was 15 minutes at the open circuit potential. to remove vitamin k1 from plasma before adsorptive stripping analysis, several techniques were tested. the vitamin levels in the blood were measured throughout these procedures. the best recovery (91 %) and lod of 180 ng ml–1 in the supporting electrolyte and 450 ng ml-1 in plasma were obtained utilizing a solvent extraction approach employing hexane and ethanol. however, by deproteinizing the plasma with ethanol and measuring the resultant supernatant directly, the analysis time might be cut in half (at the cost of some sensitivity) [25]. a summary of cpe-based electrochemical sensors developed to detect fat-soluble vitamins (a, d3, e, and k1) is given in table 3. table 3. summary of electrochemical sensors for detection of fat-soluble vitamins vitamin modifier method lod real sample analysis ref. va pt:co/il/cpe swv 0.04 μm [21] va sds/cpe dcv 1.3×10-6 m cabbage and drugs [22] vd3 mip/ mwcnt/ cpe swv 3 ×10-8 m [23] ve so/gcpe swasv 5 min: 10-7 mol l-1 15 min: 3.3×10-9 mol l-1 margarine and edible oils [24] vk1 lsw 180 ng ml–1 human blood and plasm [25] conclusion the advances in the field of electrochemistry have demonstrated good results on carbon paste electrodes (cpe) used as electrochemical sensors, which generated a great deal of attention. this is also due to exceptional features of cpe like cost-effectiveness, repeatability, dependability, and sensitivity. although there has been a significant improvement in electrochemical sensors used for vitamin detection, many elements, such as improving sensitivity, remain a hot issue for researchers. up to now, it is also not possible to identify the presence of both fat-soluble and water-soluble vitamins at the same time. this review attempted to thoroughly discuss several electrochemical approaches employing cpe for the detection of fatand water-soluble vitamins. the electrical conductivity, active surface area, and stability of the cpe are poor compared to the other electrodes for detecting fat and water-soluble vitamins. therefore, different types of modifiers such as redox mediators (mnpc, mno2, bis(salicylideneiminato)oxovanadium (iv) complex), nanomaterials (zro2, mwcnt), polymers, and surfactants (cpb, ctab, tx-100) were used to sense vitamins in real samples. after modifications, cpe showed excellent activity towards sensing vitamins in real samples like vegetables, oils, human serum, and drugs. acknowledgment: v.s. is very thankful to arundhati sharma, b. a. hons, semester-i, delhi university, india, for proofreading the article. v. sharma and g. k. jayaprakash j. electrochem. sci. eng. 12(3) (2022) 421-430 http://dx.doi.org/10.5599/jese.1313 429 references [1] l. huang, s. tian, w. zhao, k. liu, j. guo, talanta 222 (2021) 121645. https://doi.org/10.1016/j.talanta.2020.121645 [2] j. oni, p. westbroek, t. nyokong, electroanalysis 14(17) (2002) 1165-1168. https://doi.org/ 10.1002/1521-4109(200209)14:17%3c1165::aid-elan1165%3e3.0.co;2-s3. 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1109/bia48344.2019.8967459 https://doi.org/10.1016/j.foodchem.2017.02.068 https://doi.org/10.1039/an9891400933 https://creativecommons.org/licenses/by/4.0/) {cubic like comn2o4 nanostructures as advanced high-performance pseudocapacitive electrode:} http://dx.doi.org/10.5599/jese.1353 777 j. electrochem. sci. eng. 12(4) (2022) 777-786; http://dx.doi.org/10.5599/jese.1353 open access : : issn 1847-9286 www.jese-online.org original scientific paper cubic like comn2o4 nanostructures as advanced high-performance pseudocapacitive electrode puratchimani mani1, venkatachalam vellaikasi2,, xavier thankappan suryabai3, abraham rajasekar simon2 and thamizharasan kattaiyan2 1department of physics, as-salam college of engineering and technology, thirumangalakudi, aduthurai612 012. india 2department of physics, sir theagaraya college, chennai -600 025, india 3department of physics, centre for advanced material research, govt. college for women, thiruvananthapuram, kerala695014, india corresponding author: venkatcnst12@gmail.com received: april 24, 2022; accepted: june 21, 2022; published: july 18, 2022 abstract in this work, a synthesis of cubic-like comn2o4 uniform nanostructures with koh-naoh involved in the hydrothermal method has been reported. the crystal structure phase purity, functional groups, and morphology of the comn2o4 have been investigated by x-ray diffraction (xrd), field emission scanning electron microscopy (fe-sem), high-resolution transmission electron microscopy (hr-tem) analyses. the electrochemical behaviour of comn2o4 electroactive material has been examined for supercapacitors. the electrode displays excellent capacitive behaviour with superior electrochemical properties. the cubic-like morphology structure with enough free space is beneficial for improving electrochemical performance. the comn2o4 electrode exhibits a faradaic capacitance with the highest specific capacitance value of 762.4 f g-1 at a scan rate of 5 mv s-1. the coulombic efficiency of the comn2o4 electrode was found to be 91.2 % after 2000 charging-discharging cycles. the nanostructures of comn2o4 make a prominent contrib0ution to the excellent electrochemical performance of the prepared electrode. keywords hydrothermal method; faradaic capacitance; long-term stability; energy storage introduction in recent years, supercapacitors (scs) have been focused on energy storage applications due to their distinctive advantages such as low cost, high-energy density, rapid charge-discharge rates, and environmental benignity when compared to batteries. the choice of electrode materials plays an important role in determining sc behaviour. these factors have triggered a significant research interest in the development and design of new and superior electrode materials for supercapacitors. http://dx.doi.org/10.5599/jese.1353 http://dx.doi.org/10.5599/jese.1353 http://www.jese-online.org/ mailto:venkatcnst12@gmail.com j. electrochem. sci. eng. 12(4) (2022) 777-786 comn2o4 as pseudocapacitive electrode 778 among numerous available electrode materials, binary metal oxides with excellent electrochemical performance were found to be promising, effective, and scalable alternatives. in addition, binary metal oxides possess achievable oxidation states, high electrical conductivities, excellent electrochemical properties, and moreover, they are affordable and environmentally friendly when compared to single-component oxides. in general, transition metal oxides with multiple oxidation states undergoing some redox reactions were already being used as pseudocapacitive or faradaic materials [1-3]. a huge interest has been put towards the synthesis and development of metal-oxide nanostructures with excellent electrochemical properties. simple binary metal oxide electrode materials such as mno2 [4], co3o4 [5] and nio [6] have been widely studied as electrode materials for supercapacitor with excellent electrochemical behaviour. meanwhile, ternary metal oxides with different metal cations like cofe2o4, femn2o4, mnco2o4 and nife2o4 have also attracted attention because of their promising applications in the energy storage field [7-11]. the coupling of two metal species might increase oxidation-state-rich redox reactions crucial for pseudocapacitors and different combinations of cations during the charging-discharging process could provide great opportunities to manipulate the physical and chemical properties of electrodes. comn2o4 has already been considered a promising material for supercapacitors owing to its outstanding reversible capacity and widespread availability. the co2mno4 material is found to exhibit an excellent capacitive behavior because cobalt exhibits higher oxidation potential while manganese can transport more electrons [12]. for example, jiang et al. [13] reported the growth of cobalt-manganese composite oxide nanostructures on ni foam by hydrothermal method, displaying specific capacitance of even 840.2 f g-1 at a current density of 10 a g-1 and showing the excellent cycling performance. ren and his co-workers [14] have reported the solvothermal synthesized uniform and decentralized flower-like comn2o4 of hierarchical nanostructures that exhibited a specific capacitance of 188 f g-1 in na2so4 solution. metal oxide nanostructures as supercapacitor electrodes are fascinating owing to their large surface area and reduced particle size, which generates more interfacial active sites. various synthesis methods for the preparation of metal oxide nanostructures have been widely explored, including the co-precipitation technique, hydrothermal method, template-assisted synthesis, and electrochemical deposition. among these methods, hydrothermal is a powerful method to synthesize nanostructures, where the size, crystal structure, and morphology of the product materials can easily be controlled. herein, we have developed a facile method to prepare comn2o4 nanostructured material by the hydrothermal method, involving potassium and/or sodium hydroxide solution treatment. the prepared material has been characterized by some surface techniques, while its pseudocapacitive and stability properties are determined by cv and galvanostatic charging-discharging experiments. experimental material synthesis comn2o4 nanostructures were prepared using double-hydroxide treatment involved in the hydrothermal method. the material was synthesized as follows: initially, 9 g of mixed hydroxides solution was prepared by adding an equal molar ratio of potassium hydroxide (koh) and sodium hydroxide (naoh) molten pellets. to this solution, 1 m cobalt nitrate (co(no3)26h2o) and 2 m manganese nitrate (mn(no3)26h2o) solutions were added and then mixed thoroughly by stirring the solution for a few minutes to ensure the homogeneity. the solution was then transferred into an p. mani et al. j. electrochem. sci. eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 779 autoclave containing a teflon vessel and kept in the furnace for 24 hours at a constant temperature of 180 °c. after completion of the reaction, the autoclave containing solution was cooled down naturally. finally, the precipitate was washed several times by using water and ethanol to remove the impurities, dried, and annealed at 500 °c for 5 hours and collected for characterization. possible chemical reactions resulting in the formation of pure comn2o4 nanostructured material can be expressed by eqs. (1) (3): mn2+ + 2oh− → mn(oh)2 (1) co2++ 2oh−→ co(oh)2 (2) 2mn(oh)2 + co(oh)2 + 1/2o2 → comn2o4 + 3h2 (3) surface characterization the crystal structure of the comn2o4 material was studied by powder xrd (xrd, rigaku miniflux (ii)-c). the debye-scherer formula was used to evaluate the average crystallite size of the synthesized material. 0.9 cos d    = (4) where  is the x-ray wavelength,  is the full-width half-maximum,  is the bragg diffraction angle, and d is the crystallite size. fourier-transform infrared (ft-ir) study was performed using a perkin elmer spectrometer with kbr pellet-based samples in the frequency range between 4000 and 400 cm-1. morphology of the synthesized material was viewed by field emission scanning electron microscopy (fe-sem) and highresolution electron microscopy (hr-tem) using a hitachi h7650. electrochemical measurements and preparation of the electrode the electrochemical behaviour of comn2o4 nanostructures was characterized by cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical impedance spectroscopy (eis) analyses. these measurements were performed using a conventional three-electrode system with platinum wire as the counter electrode and ag/agcl/ sat. kcl as the reference electrode. for the preparation of working electrodes, the comn2o4 material was mixed with polyvinylidene difluoride and activated carbon added to the above mixture in order to achieve homogeneity. the prepared material was brush coated onto nickel foil (nf). the mass of the loaded active material ranged from 0.4 and 0.5 mg. all electrochemical experiments were conducted at room temperature. the specific capacitance value was calculated from cyclic voltammetry and galvanostatic charging/discharging curves. cyclic voltammetry was recorded to determine the charge-storage capacity within a potential window of 0.0 to 0.5 v (vs. ag/agcl sat. kcl) at various scan rates from 5 to 100 mv s-1 using chi (7081c) electrochemical workstation (usa). for testing charging and discharging properties, chronopotentiometry measurements at various constant currents (2-4 a g-1) were applied between 0.0 and 0.5 v (vs. ag/agcl sat. kcl). electrochemical impedance spectrum was also carried out at 0.5 v vs. ag/agcl sat. kcl, in the frequency region between 1 mhz and 1 hz. results and discussion xrd analysis the structure and phase purity of the prepared comn2o4 material after calcining were studied by x-ray powder diffraction, as shown in figure 1. all reflections could be indexed as body-centredhttp://dx.doi.org/10.5599/jese.1353 j. electrochem. sci. eng. 12(4) (2022) 777-786 comn2o4 as pseudocapacitive electrode 780 tetragonal (bct) phase (jcpds no. 77-0471) with a distorted spinel structure. the sharpening of the peaks is observed, which indicates the improvement in crystalline quality. no residues or contaminants have been detected in the xrd pattern, which indicates the complete transformation from co–mn-dh (double hydroxide) precursors to comn2o4 and the high phase purity of the sample. the mean crystallite size was calculated from the (211) peak with the scherrer formula given by eq. (4), and the size is found to be 34 nm for the comn2o4 sample [15,16] 2 / o figure 1. xrd patterns of comn2o4 nanostructured material ft-ir analysis ft-ir spectrum of the comn2o4 nanostructured material prepared by dh (double hydroxide) treatment is shown in figure 2. the peaks observed around 3415 (broad) and 2921 (small) cm-1 correspond to the stretching vibration of hydrogen-bonded hydroxyl groups. the shoulder peak at 1626 cm-1 corresponds to the bending mode of absorbed water molecules. the characteristic absorption small bands at around 621 cm-1 are attributed to the vibration of the tetrahedral oxygen environment (mainly cobalt oxide), while the peak observed at 493 cm-1 can be correlated to the vibration of the octahedral oxygen environment (mainly manganese oxide), which further demonstrate the formation of pure comn2o4 by annealing at 500 oc [17]. figure 2. ft-ir spectrum of comn2o4 nanostructured material t / % p. mani et al. j. electrochem. sci. eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 781 morphology analysis the morphology of the prepared comn2o4 material was investigated by fe-sem and hr-tem with different magnifications. figure 3(a) displays fe-sem images of comn2o4 prepared by double hydroxide solution treatment, while the corresponding hr-tem images are shown in figure 3 (b and c). clearly, the prepared comn2o4 is composed of disordered cubic-like nanostructure with small agglomerates and rough surfaces. in figure 3(d), the selected area diffraction (saed) pattern displays well-defined rings, representing the characteristic polycrystalline nature of comn2o4 and not amorphous. it is anticipated that this material can absorb and strongly retain electrolyte ions, ensuring sufficiently fast faradic reactions, specifically at high current densities. in addition, this kind of morphology can afford higher active sites and a specific surface area leading to high performance in the electrochemical energy storage system. figure 3. (a) fe-sem, (b, c) hr-tem images and (d) saed pattern of comn2o4 nanostructured material electrochemical properties cyclic voltammetry analysis cyclic voltammetry (cv) was used to study the charge storage capacity and charge storage mechanism of the comn2o4 electrode. the electrochemical performance of the comn2o4 electrode was tested using a three-electrode working system in 2 m koh electrolyte. the cv curves in the potential region from 0 to 0.5 v at various scan rates are shown in figure 4(a). for the as-obtained electrode, the shape of cv curves indicates a predominant pseudocapacitive behaviour. the current values were found to increase upon increasing the scan rate due to the fast electronic and ionic transport rates between nanostructured electrode and electrolyte. at low scan rates, cv curves exhibit nearly rectangular shapes, indicating no kinetic limitations and well stability of the http://dx.doi.org/10.5599/jese.1353 j. electrochem. sci. eng. 12(4) (2022) 777-786 comn2o4 as pseudocapacitive electrode 782 supercapacitor [18]. at higher scan rates, cv curves are slightly deviated from a rectangular-like response but still demonstrate capacitive behaviour at scan rates up to 100 mvs-1. this indicates the rapid current response to voltage reversal (small contact resistance) and reveals pseudocapacitive properties of the comn2o4 electrode. the probable charge storage mechanism may be described ba eq. (5): (comn2o4)surface + k+ + e− = [kcomn2o4]surface (5) according to the reaction described by eq. (5), energy storage is based on the adsorption/absorption of electroactive ions (k+) at the surface (or near-surface) electrode region, which is undergoing charge transfer and without bulk phase transformation. the specific capacitance values were calculated using eq. (6): ( ) c a s a c 1 d v v c iv v vm v v = −  (6) where  c a d v v iv v is the integral value during the cathodic scan, m is the mass of the active electrode material loaded on the substrate, 𝑣 is the scan rate, and va – vc is the working potential window range. based on figure 4(a) and eq. (6), specific capacitance values of comn2o4 electrode material were calculated to be 762.4, 601.35, 454.08, 292.39 and 256.91 f g-1, for 5 , 10, 25, 50 and 100 mv s-1, respectively. from the cv results, the modified electrode reveals the pseudocapacitive behaviour. the specific capacitance value of the electrode material decreased as the scan rate increased, which may be owing to the availability of cations near the surface of the electrode, which cannot be effectively utilized on the surface of the electroactive material. anyhow, the cv testing of the electrode under scan rate of 5 mv s-1 and obtained cs of 762.4 f g-1 suggests a potential use of such material in scs used in high power applications [19]. table 1 shows that here obtained specific capacitance of comn2o4 is higher than found in earlier reported studies and other ternary/binary transition metal oxides. table 1. comparison of the specific capacitance value of comn2o4 with earlier studies. material synthesis method specific capacitance, f g-1 electrolyte reference nico2o4 solvothermal 758 at 1 a g-1 2 m koh [20] v2o5/carbon electrospinning 150 at 1 ma 6 m koh [21] carbon/nickel electrospinning 103.8 at 0.5 a g-1 6 m koh [22] comn2o4 hydrothermal 140.6 mah g-1 at 1 ma cm-2 3 m koh [23] comn2o4 electrospinning 121 at 5 mv s-1 3 m naoh [24] comn2o4 hydrothermal 762.4 at 5 mv s-1 2 m koh present work charging-discharging analysis the charge/discharge analysis confirms the excellent reversibility characteristics with a nearly symmetric charging/discharging profile. galvanostatic charge-discharge analysis was carried out at different current densities of 1, 2, 3 and 4 a g-1 in the potential window range between 0 to 0.5 v, as shown in figure 4(b). at high current densities, the discharge curves are nearly symmetrical to the corresponding charge curves, which indicates a highly capacitive nature of the comn2o4 electrode and electrochemical reversibility of the underlying faradaic reaction. the nearly symmetrical charge/discharge profiles are in good agreement with almost rectangular cv curves. p. mani et al. j. electrochem. sci. eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 783 the apparent deviation from straight lines in the charge-discharge plots is obtained at low currents, indicating some inhibition of the faradic pseudocapacitive reaction of as-synthesized material [25]. also, the curves are highly symmetric at peak values, showing that the electrode has low internal resistance. furthermore, the specific capacitance values were calculated using eq. (7), z’ /  scan rate, mv s-1 figure 4. (a) cv curves of comn2o4 nanostructured material in 2 m koh at different scan rates; (b) gcd curves at different current densities; (c) nyquist plot at 0.5v; (d) variation of specific capacitance with scan rate; insert: variation of specific capacitance with current density applied; (e) coulombic efficiency during 2000 charge-discharge cycles recorded at a current density of 2 a g-1; insert: few long term cycles -z '' /  http://dx.doi.org/10.5599/jese.1353 j. electrochem. sci. eng. 12(4) (2022) 777-786 comn2o4 as pseudocapacitive electrode 784 d s it c m v =  (7) where, i is the constant current applied, td is discharge time, m is the mass of the electroactive material loaded, and v is the potential difference (0.5 v) [24]. specific capacitance values calculated from charging/discharging curves in figure 4(b) and eq. (7) are presented in the inset of figure 4(d). the specific capacitance values of the comn2o4 material calculated from gcd results concord with cv results. long-term cycle charge/discharge analysis was also carried out to evaluate the cyclic stability of the prepared comn2o4 electrode material. a constant current density of 2 a g−1 was applied to examine the comn2o4 electrode in 2 m koh electrolyte, and 2000 charging-discharging cycles were performed. the results are in the form of coulombic efficiency vs. cycle number presented in figure 4(e). the coulombic efficiency of the comn2o4 electrode was calculated using the relation,  = (td / tc) 100 (8) where,  is coulombic efficiency, td is the discharge time and tc is charging time derived from chargedischarge curves. figure 4(e) shows a gradual increase of the coulombic efficiency during the first few cycles, which indicates a certain electro-activation process of the electrode under given testing conditions (voltage range of 0 to 0.5 v and current density of 2 a g-1). after that, the coulombic efficiency slightly decreased with the cycling but retained 91.2 % of the initial efficiency after 2000 cycles. electrochemical impedance spectroscopy analysis the electrochemical impedance spectroscopy analysis is exploited to evaluate the electrochemical properties of comn2o4 material. figure 4(c) shows the nyquist plot of the comn2o4 electrode in 2 m koh, measured 0.5 v within the frequency region of 1 mhz to 1 hz. the plot reveals a distinct semicircle at high frequencies and a sloping straight line in the lower frequency region. the semicircle impedance response in the high-frequency region corresponds to the charge-transfer resistance at the electrode/electrolyte interface, and the straight line observed at low frequencies corresponds to the diffusion process of electroactive species (warburg impedance) [26]. the equivalent circuit diagram proposed to fit the impedance spectrum is shown in the inset of figure 4(c). the resistance (rs), which involves ohmic resistance of the active material and electrolyte, as well as the contact resistance at the interface between the active material and current collector, is obtained as the high-frequency intercept of the semicircle at the real impedance axis. faradaic interfacial charge-transfer resistance (rct) is associated with the diameter of the semicircle in the high-frequency range. the warburg impedance, which indicates the frequency dependence of ion diffusion [27], is probably related to the diffusion of k+ within the surface and near-surface layer according to the charge storage mechanism described by eq. (5). conclusion in summary, spinel comn2o4 nanostructure was successfully synthesized using facile hydrothermal route at 180 °c followed by annealing at 500 °c. the prepared material was characterized by xrd, ft-ir, fe-sem and hr-tem analyses, revealing a nanostructured material of polycrystalline nature. the electrochemical properties were investigated in 2 m koh aqueous electrolyte using cyclic voltammetry, charge-discharge, long-term cycle stability performance and electrochemical impedance spectroscopy measurements. the maximum specific capacitance of 762.4 f g-1 was obtained from cv measurements at 5 mv s-1, while about 600 f g-1 was obtained by galvanostatic charging-discharging at 1 a g-1. the material showed sufficient cycling stability over p. mani et al. j. electrochem. sci. eng. 12(4) (2022) 777-786 http://dx.doi.org/10.5599/jese.1353 785 2000 cycles. the results demonstrate that the spinel-comn2o4 nanostructure should be a promising electrode material for supercapacitor 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zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1177%2f155892501300800405 https://doi.org/10.1016/j.matlet.2018.11.100 https://doi.org/10.1016/j.jallcom.2016.09.005 https://doi.org/10.1016/j.est.2020.101483 https://doi.org/10.1016/j.electacta.2012.01.095 https://doi.org/10.1007/s11696-020-01448-z https://creativecommons.org/licenses/by/4.0/) @article{mani2022, author = {mani, puratchimani and vellaikasi, venkatachalam and suryabai, xavier thankappan and simon, abraham rajasekar and kattaiyan, thamizharasan}, journal = {journal of electrochemical science and engineering}, title = {{cubic like comn2o4 nanostructures as advanced high-performance pseudocapacitive electrode:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {4}, pages = {777--786}, volume = {12}, abstract = {in this work, a synthesis of cubic-like comn2o4 uniform nanostructures with koh-naoh involved in the hydrothermal method has been reported. the crystal structure phase purity, functional groups, and morphology of the comn2o4 have been investigated by x‑ray diffraction (xrd), field emission scanning electron microscopy (fe-sem), high-reso­lution transmission electron microscopy (hr-tem) analyses. the electrochemical beha­viour of comn2o4 electroactive material has been examined for supercapacitors. the elec­trode displays excellent capacitive behaviour with superior electrochemical properties. the cubic-like morphology structure with enough free space is beneficial for improving electrochemical performance. the comn2o4 electrode exhibits a faradaic capacitance with the highest specific capacitance value of 762.4 f g-1 at a scan rate of 5 mv s-1. the coulombic efficiency of the comn2o4 electrode was found to be 91.2 % after 2000 charging-discharging cycles. the nanostructures of comn2o4 make a prominent contr­ib0­ution to the excellent electrochemical performance of the prepared electrode.}, doi = {10.5599/jese.1353}, file = {:d\:/onedrive/mendeley desktop/mani et al. 2022 cubic like comn2o4 nanostructures as advanced high-performance pseudocapacitive electrode.pdf:pdf;:13_jese_1353.pdf:pdf}, keywords = {hydrothermal method, energy storage, faradaic capacitance, long, term stability}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1353}, } {chemical vapor deposited graphene-based quasi-solid-state ultrathin and flexible sodium-ion supercapacitor:} https://dx.doi.org/10.5599/jese.1411 799 j. electrochem. sci. eng. 12(4) (2022) 799-813; https://dx.doi.org/10.5599/jese.1411 open access : : issn 1847-9286 www.jese-online.org original scientific paper chemical vapor deposited graphene-based quasi-solid-state ultrathin and flexible sodium-ion supercapacitor mohammed saquib khan1†, preeti shakya1†, nikita bhardwaj2, deependra jhankal2, atul kumar sharma1, malay kumar banerjee3 and kanupriya sachdev1,2, 1materials research centre, malaviya national institute of technology jaipur, jln marg, jaipur 302017, india 2department of physics, malaviya national institute of technology jaipur, jln marg, jaipur 30201, india 3research chair, sgvu, jaipur,302017, india corresponding author: ksachdev.phy@mnit.ac.in; tel.: 0141-2713382 received: june 17, 2022; accepted: august 23, 2022; published: september 9, 2022 abstract flexible electronic devices find wide application in wearable electronics and foldable gadgets. this article reports chemical vapor deposited (cvd) few-layers graphene for a solid-state flexible supercapacitor device. raman spectroscopy analysis reveals up to five layers in the graphene samples. polyvinyl alcohol-na2so4 hydrogel membrane is used as a gel polymer electrolyte (gpe). 50 nm thick silver (ag) deposited on polyethylene terephthalate (pet) through e-beam deposition served as the flexible current collector for the device. galvanostatic charge-discharge (gcd) executed on the fabricated device to analyze its electrochemical performance yielded a specific areal capacitance of 15.3 mf cm-2 at 0.05 ma cm-2 current density. the obtained power density of the fabricated device is 0.53 µwh cm-2 at a power density of 25 µw cm-2. keywords gel polymer electrolyte; e-beam deposition; flexible electronics; lightweight supercapacitor; chemical vapor deposition; few-layer graphene introduction the technological advancement in wearable and bendable electronics has created the need for thin, lightweight, and flexible solid-state energy devices [1]. these electronic devices require energy storage, fast charging and discharging, and high energy density. among several energy storage devices, supercapacitors are the potential candidate owing to their advantages such as long cycle life, fast charge-discharge rates, high power performance and low maintenance cost [2-12]. carbon †mohammed saquib khan and preeti contributed equally to this work. https://dx.doi.org/10.5599/jese.1411 https://dx.doi.org/10.5599/jese.1411 http://www.jese-online.org/ mailto:ksachdev.phy@mnit.ac.in j. electrochem. sci. eng. 12(4) (2022) 799-813 vapor deposited graphene-based supercapacitor 800 based materials (graphene, carbon nanotube, activated carbon, carbon fibres, 3d carbon aerogel, etc.) play a crucial role in electrochemical energy storage credited to their exceptional properties such as a large specific surface area for electrochemical double layer capacitance, optimisable structures, adjustable compositions/components and tunable physiochemical properties [13]. graphene is a two-dimensional monolayer of graphite with sp2 hybridized carbon atoms arranged in a unique honeycomb structure, exhibiting high conductivity, excellent thermal stability, and excellent mechanical strength and showing the possibilities in multiple electronic applications [14,15]. it has been widely utilized in electrodes because it consists large theoretical surface area of 2630 m2 g-1, good chemical stability, excellent thermal stability, and its capability to facilitate the acquisition of electrons or the transfer of holes on its two-dimensional surface [16]. several synthesis methods are used for single-layer and multilayer graphene. compared to others, chemical vapor deposition (cvd) has been considered the most favourable method to fabricate graphene layers [17-19] because it yields superior electrical conductivity in comparison to chemically produced graphene [20,21]. the quality of synthesized graphene through cvd is adequate for sensitive electronic applications, making it ideal for electrochemical applications due to its large single crystal, easy synthesis of single/few-layer graphene, and transfer to another substrate [22,23]. on the other hand, graphene sheets are tremendously susceptible to restacking of multiple 2d layers, which adversely affects the mobility of ions and reduces electrochemically active sites [25,25]. holey graphene, which is graphene with a large number of holes, is an excellent solution to the restacking problem [26]. sodium sulphate has been widely used as a neutral electrolyte in na-ion energy storage devices owing to its eminent ionic conductivity (125 ms cm−1 in water) and environmental safety [27,28]. due to the scarcity of lithium resources worldwide, sodium-ion-based supercapacitors gained a lot of research attention for developing a less expensive and superior performing alternative compared to lithium-ion supercapacitors [29]. liquid electrolytes have been used in electrochemical energy storage due to their benefits, including high ionic conductivities and ability to make good contact with electrodes, but they have disadvantages, viz. corrosive, toxicity, leakage, combustion of organic electrolytes and demand for a huge packaging cost for flexible capacitors [30]. compared to them, gel polymer electrolytes (gpe) provide the advantage of flexibility, easy fabrication, broad potential window, stretchability, better adhesiveness, and high ionic conductivity [31]. polyvinyl alcohol (pva) is an excellent choice owing to its nontoxicity, biodegradability, cost-effectiveness, compatibility, fast film-forming property, high dielectric constant, and excellent chemical stability [32]. different flexible and non-flexible conducting substrate materials, like ti foil, ni foam, aluminium foil, carbon cloth, fabric coated with graphene, stainless steel foil, graphite foil, or carbon fibres, are used as current collector plates in supercapacitor applications [33]. metallic current collectors normally used in energy storage devices are not useful in lightweight and flexible devices because of their limitations of high resistance, low flexibility, and massive weight [34]. to address this, silvercoated polyethylene terephthalate (pet) substrate through electron beam deposition has been demonstrated as a prominent choice for the fabrication of ultrathin, lightweight, and flexible supercapacitor devices, which provides a wide range of temperature stability (≈ up to 250 °c) [35-36]. however, due to its insulating nature, it requires to be metalized for its utilization as a current collector. metallization on a flexible substrate shows excellent potential, allowing flexibility and high electrical conductivity. the choice of coating material includes metals such as ag, au, cu, and ni or carbon-based materials including graphene and carbon nanotubes (cnt). silver is a transition metal that offers several advantages over other metallic current collectors, including higher conductivity, m. s. khan et al. j. electrochem. sci. eng. 12(4) (2022) 799-813 https://dx.doi.org/10.5599/jese.1411 801 environment friendliness, and good electrochemical stability for supercapacitor application [37,38]. in comparison to the conventional metallic current collectors, silver-coated pet reduces the series resistance, overall weight, and manufacturing cost of the device and provides enhanced flexibility and better electrochemical performance [37]. in this communication, we report an ultrathin, flexible and eco-friendly silver deposited pet substrate-based supercapacitor device with cvd-grown few-layer holey graphene electrodes. pva – na2so4 quasi solid state hydrogel membrane was used as the separator-less electrolyte. parallel plate supercapacitor devices were fabricated by sandwiching na2so4 – pva hydrogel membranes as the quasi solid-state and flexible electrolyte. herein, we synthesized the holey few-layer graphene using the chemical vapour deposition method as resolution of the restacking problem in few-layer graphene, which facilitates the ionic movement of the electrolyte in the supercapacitor electrode that leads to the reduction of charge transfer resistance of the supercapacitor device. in addition, holes in the graphene layer give access to the inner layers for electrochemical energy storage, improving the supercapacitor's specific capacitance. nevertheless, it is believed that there are no reports till now on the investigations of an ultrathin and flexible supercapacitor device using cvdbased few-layer holey graphene on silver deposited pet (current collector) as the flexible electrodes in combination with na2so4 – pva hydrogel membranes as the quasi-solid-state electrolyte. experimental chemicals and materials copper foil (99.7 %) thickness ≈ 0.1 mm was procured from loba chemie pvt. ltd., mumbai, india. argon gas (99.999%), methane gas (99.999 %) and hydrogen gas (99.999 %) were purchased from ankur speciality gases & technologies, jaipur, india. ferric chloride (fecl3 99.99 %), iso-propanol alcohol (ipa 99.9 %), hydrochloric acid (hcl 35 %), acetone (99.9 %) and anisole (99.7 %) were purchased from merck life science private limited, darmstadt, germany. silver powder (8-10 µm) (99.9 %) was purchased from sigma aldrich, missouri, usa. polyethylene terephthalate (pet, thickness ≈0.1mm) was purchased from savita scientific and plastic products private limited, jaipur, india. poly-methyl methacrylate granules (pmma 98 %) molecular weight ≈15,000 were procured from himedia laboratories, thane, india. cvd growth of few-layer graphene the low-pressure chemical vapor deposition (lpcvd) technique was used to grow few-layer graphene (flg). after the cloth polishing and hcl treatment, the copper foil was annealed at 800 °c in argon and hydrogen gas at the rate of 100 and 50 standard cubic centimetres per minute (sccm), respectively, for 180 minutes. methane gas is used as the carbon source, and the hydrogen and methane gas flow rates were 25 and 50 sccm, respectively, during the growth phase. the pressure of the chamber was maintained at 20 kpa, and the growth time of graphene was 35 min at 950 °c (figure 1). after the growth phase, the furnace was cooled down to room temperature. e-beam deposition of silver on pet substrate e-beam evaporation technique was used to deposit silver (99.99 %) of 50 nm thickness on pet (polyethylene terephthalate, 0.1 mm) substrates. the substrate temperature was maintained at 90 °c, controlled with a precision ± 0.5 °c, and the pressure inside the chamber was 10-4 pa with a deposition rate of 0.5-1 nm s-1. the quartz balance technique was used to monitor film thickness https://dx.doi.org/10.5599/jese.1411 j. electrochem. sci. eng. 12(4) (2022) 799-813 vapor deposited graphene-based supercapacitor 802 during deposition. after thin film deposition, ag-pet substrates were thermally annealed at 120 °c for 60 min in an argon atmosphere. figure 1. process parameters for few-layer graphene growth through cvd technique (rate of increase in temperature was 10 °c per minute and chamber pressure was 20 kpa) transfer process of few-layer graphene on the desired substrate the cvd-grown graphene was transferred on the flexible silver/pet substrate through the copper etching technique, as reported in previous literature [39-42]. figure 2. schematic of flg transfer on the desired substrate the process begins with the spin coating of 10 % solution of pmma in anisole on the samples. ferric chloride solution (1 m) was used as a wet etchant to dissolve copper foil (thickness = 0.12 mm) and it takes approximately 15 – 20 minutes to completely dissolved (figure 2), giving the floating pmma-coated graphene sheets that are washed with di water. these graphene sheets were placed on the silver-coated pet substrate and heated at 110 °c for 5 minutes. at last, acetone is used to remove pmma coating, yielding only flg on the substrate. the transferred flg makes good adhesion with silver-coated pet substrate with appropriate uniformity. m. s. khan et al. j. electrochem. sci. eng. 12(4) (2022) 799-813 https://dx.doi.org/10.5599/jese.1411 803 results and discussion raman spectroscopy analysis in the raman spectroscopy of graphene (figure 3), the d band appearing at ≈1350 cm−1 arises due to out-of-plane vibrations attributed to the presence of structural defects [43]. the defects in few-layer graphene were quantified using the intensity ratio of the d-band to the g-band (graphitic band appearing at ≈1580 cm−1) in the raman spectrum (id/ig) [44]. the id/ig ratio of ≈0.25 indicates the presence of significant defect density in the synthesized samples [45]. figure 3. raman spectrum of chemical vapor deposited graphene the defects in graphene arise due to the in-plane cavities (during cvd growth), resulting in augmented edge formation in the graphene sheet. it is favorable for energy storage applications as it enhances the surface area, facile intercalation and deintercalation of electrolyte ions leading to better electrochemical performance [26]. it is well established from the previously reported research articles that the number of graphene layers in the sample can be estimated utilizing the normalized intensity ratio of the 2d-band (due to the second overtone of a different in-plane vibration) to the g-band (i2d/ig). if the i2d/ig ratio of greater than 2 is considered for monolayer graphene and i2d/ig ratio of 0.5 is considered for less than five-layer formation in graphene samples [46]. i2d/ig ratio in the graphene samples was found to be ≈0.5, indicating the formation of few-layer graphene. fesem analysis scanning electron microscopy images (shown in figure 4a) were obtained for chemical vapor deposited graphene on copper foil samples, using fei, nova nanosem 450. wrinkled micro-island formation of graphene can be easily visualized in the samples. these wrinkles increase electrochemically active surface area of graphene for charge storage [47]. at the same time, the https://dx.doi.org/10.5599/jese.1411 j. electrochem. sci. eng. 12(4) (2022) 799-813 vapor deposited graphene-based supercapacitor 804 island formation in graphene supports the swift movement of electrolyte ions inside the electrode material and the availability of inner layers of graphene for charge storage leading to enhanced electrochemical performance [48]. the eds mapping image of carbon (shown in figure 4c) shows the uniformity of graphene on the copper catalyst. the presence of oxygen in the sample may be due to the oxidation of copper foil in the ambient atmosphere. figure 4. a) fesem image of cvd deposited graphene on copper b) eds mapping image of carbon, oxygen and copper in cvd deposited graphene on copper c) eds mapping image of carbon in cvd deposited graphene on copper d) eds mapping image of oxygen in cvd deposited graphene on copper and e) eds mapping image of copper in cvd deposited graphene on copper tem analysis tem images (figure 5) of the cvd deposited graphene were acquired using tecnai g2 20 s-twin [fei] hrtem instrument. restacking in multilayer graphene is a well-known setback that originated due to the van der waals interactions among the graphene sheets. van der waals force is directly proportional to the surface area of sheet overlapping and inversely proportional to the square of the interlayer distance [49]. restacking in graphene can be evaded in perforated or holey graphene owing to the deficiency of sufficient carbon atoms resulting in the lack of enough van der waals forces. the reduced restacking and the existence of a large number of holes could result in a superior ionic approach to the surfaces, thus enhancing the available electrochemical surface area. the accessibility to a large number of perforations and increased interspacing between layers improve ionic diffusion and affluent mass transport [50]. this opens up a significant application of holey graphene in electrochemical energy storage. the tem images reveal few-layer perforated graphene films with wrinkles, multiple creasing, and perforations that arise due to the hydrogen annealing of the copper catalyst and hydrogen etching of graphene [19,51]. the perforation in the graphene films provides facile back-and-forth passage of anions and cations throughout the electrode material, whereas the wrinkles and multiple creasing in graphene will provide enhanced surface area, resulting in excellent electrochemical performance [52,53]. m. s. khan et al. j. electrochem. sci. eng. 12(4) (2022) 799-813 https://dx.doi.org/10.5599/jese.1411 805 figure 5. tem images of cvd deposited graphene transferred on the copper grid electrochemical characterizations and analysis to evaluate the merits of utilizing flexible electrodes in the device to form wearable and bendable electronics devices such as scs, the sc performance of graphene transferred silver-coated pet substrate as an electrode was investigated. the graphene nanosheet was uniformly aggregated onto a silver-coated pet substrate. the used (graphene-transferred silver-coated pet substrate) electrode has a coarse and creased surface with a large number of holes. for the ultrathin and flexible supercapacitor device fabrication, the few-layer graphene was transferred onto the silvercoated pet substrates, and 1m na2so4 soaked pva membranes were used as the separator-less quasi solid-state electrolyte. the cv curves illustrate the typical quasi-rectangular shape profiles at all scan rates as an indication of charge storage due to edlc or intercalation capacitance. the cyclic voltammetry (cv) profile of the fabricated supercapacitor device (figure 6a) was obtained at progressive scan rates (10-500 mv/s) and within the potential range of 0 – 1 v, no redox peaks were seen that accentuate no involvement of any faradic dominated reactions in the charge-storage kinetics of graphene electrode. galvanostatic charge-discharge (gcd) curves (figure 6b) at different current densities (0.050.5 ma cm-2) show a nearly triangular shape for the fabricated device. it indicates that the fabricated sc exhibit excellent capacitive behavior and high cyclic stability. the specific areal capacitance values for the fabricated scs were calculated based on the equation – s 4i t c a v  =  (1) where i is the discharge current, δt is the time of discharge, δv is the operating potential window, and a is the area of a single electrode [54]. the total area of the single electrode was 4 cm2. the specific areal capacitance of the fabricated flexible scs was obtained as 15.3 mf cm-2 at the current density of 0.05 ma cm-2. to the best of our knowledge, the reported capacitance value is superior in comparison to the previously reported cvd-based graphene supercapacitors [55-59]. the device exhibited good capacitance retention at high current densities (as shown in figure 6d) with respect to the specific capacitance at the current density of 0.05 ma cm-2. the specific capacitance retention of 93.6, 84.9 and 78.4 % was calculated at the current density of 0.1, 0.25 and 0.5 ma cm-2, respectively. the energy density values of 0.53, 0.50, 0.45 and 0.42 µwh cm-2 at a power density of 25, 50, 125 and 250 µw cm-2, respectively. in figure 6c, the cyclic stability and coulombic efficiency of the https://dx.doi.org/10.5599/jese.1411 j. electrochem. sci. eng. 12(4) (2022) 799-813 vapor deposited graphene-based supercapacitor 806 flexible device are shown as 87.73 and 97.03 %, respectively. this extraordinary electrochemical performance for both the power and energy densities of the flexible supercapacitor device indicates that the wrinkles and holey structure of the graphene and silver coated flexible pet substrate promotes a synergistic role in minimizing the internal resistance due to efficient intercalationdeintercalations of ions and intact electrode – current collector contact. figure 6. a) cv plot of the flexible device at multiple scan rates (10-500 mv s-1), b) gcd plot of the flexible device at various current densities (0.05 – 0.5 ma cm-2), c) capacitance retention and coulombic efficiency versus number of cycles plot of the fabricated flexible device, d) plot of specific capacitance versus current density of the fabricated sc device the eis measurement (figure 7a) was executed between 500 khz and 0.01 hz with an amplitude of 10 mv. in the case of an ideal capacitor, the phase shift angle of the impedance is perpendicular ( ≈90°). consequently, the nearly vertical impedance in the nyquist plots of these ultrathin and flexible sc devices validates their performance as ideal capacitors. from the definition of the traditional capacitor, the capacitor impedance at high and low frequencies tends to be minimum (zero) and infinity, respectively [60]. thus, at the higher frequency (500 khz), the impedance of the supercapacitor device decreases steeply and will act as a short circuit resulting in the charge flow through esr (equivalent circuit resistance) voluntarily and the phase angle comes to zero. this implies that the phase angle between voltage and current in an absolute resistor is zero. nevertheless, at smaller frequencies (≈10 hz), the capacitor impedance is sufficient enough to act as an open circuit. consequently, at lower frequencies, the vital charge flow occurs through rct (charge transfer resistance) rather than esr, therefore, rct will be the primary contributor to the device function [60]. the series and charge transfer resistances (rct were found to be 0.38 and 1.2 , respectively, indicating free ionic movement at the electrode-electrolyte interface resulting in overall low resistance of the device. m. s. khan et al. j. electrochem. sci. eng. 12(4) (2022) 799-813 https://dx.doi.org/10.5599/jese.1411 807 figure 7. a) nyquist plot of the sc device, b) phase angle based bode plot, c) real part of specific capacitance of sc device, d) imaginary part of specific capacitance of sc device. a supercapacitor impedance characteristics lie between an absolute resistor at phase angle 0° and an ideal capacitor at phase angle 90° [61]. the frequency is inversely proportional to capacitance, leading to almost zero impedance for a capacitor which results in an absolute resistor behavior at high frequencies [61]. physical parameters like morphology, porosity, the thickness of the electrode, etc., influence intermediate regions of frequencies that control the electrolyte ions diffusion at the electrode-electrolyte interface and inside the electrode material. the bode plot of the flexible supercapacitor is shown in figure 7b. phase magnitude denotes the phase angle between the charging current and the voltage. for ideal supercapacitors, the magnitude possesses a phase angle of -90°. from the bode plot (as shown in figure 7b), the phase angle decreases sharply at critical frequency. in our case, the critical frequency magnitude of 10 hz with a phase angle of  -80.8° was observed, indicating that this device is very close to the ideal supercapacitive behavior. for calculating the capacitive reactance of a capacitor from the bode plot, equation (2) [62] c 1 2 x fc = (2) where xc is capacitive reactance, and f denotes ac frequency. for the bode plot, the real capacitance was measured using equation (3) [62] ( ) ( ) ( ) '' ' 2 z c z     = (3) for the bode plot, the imaginary capacitance was measured using equation (4) [62] ( ) ( ) ( ) ' '' 2 z c z     = (4) where =2f, z’ is real impedance, and z’’ is imaginary impedance. for calculating the response time (0), equation (5) https://dx.doi.org/10.5599/jese.1411 j. electrochem. sci. eng. 12(4) (2022) 799-813 vapor deposited graphene-based supercapacitor 808 0 0 1 f  = (5) where, f0 is the frequency for which the imaginary part of the capacitance is maximum. figure 7c and figure 7d represent changes in the real (c’) and imaginary part (c’’) of capacitance versus the logarithm of frequency, respectively. according to the previous description, as frequency decreases, capacitance increases in the case of real capacitance and tends to be less frequencydependent. this relation shows characteristics of electrode and electrode-electrolyte interface [63] and implies that the fabricated device is frequency-independent which makes it a strong candidate for ac filtering applications demanding ultrathin and flexible devices. however, the imaginary part of the capacitance attains a maximum value corresponding to a specific value of frequency which gives the response time of the device. by using the above equation (equation 5), response time is calculated as 4 seconds for the flexible sc device, which indicates the device can undergo ultrafast charging-discharging cycles. figure 8. a) flexible supercapacitor device without bending (0°), b) flexible supercapacitor device with 45° bending, c) flexible supercapacitor device with 90° bending, d) flexible supercapacitor device with 180° bending, e) cv curves (at 50 mv s-1) of the flexible supercapacitor device at 0°, 45°, 90° and 180° bending, f) schematic of the flexible supercapacitor device table 1. comparison of electrochemical results of this study with previously reported works. no. electrode material electrolyte substrate csp / mf cm-2 energy density, nwh cm-2 power density, µwcm-2 ref. 1. cvd-graphene pva-h2so4 graphite paper 11.1 1.24 24.5 [64] 2. au/graphene h3po4-pva 0.0081 [65] 3. graphene/pdms pva-h2so4 0.00533 0.20 11.17 [66] 4. cvd graphene h3po4-pva pet 0.0124 0.47 70 [67] 5. cvd graphene bmim-pf4 stainless steel 0.29 [68] 6. graphene core sheath microfibers pva-h2so4 1.7 0.17 100 [69] 7. cnt/graphene fiber 1 m na2so4 au/pet 0.98 [70] 8. few-layer cvd graphene pva-na2so4 hydrogel membrane ag/pet 15.3 (at 0.05 ma cm-2) 0.53 25 this work m. s. khan et al. j. electrochem. sci. eng. 12(4) (2022) 799-813 https://dx.doi.org/10.5599/jese.1411 809 table 1 shows a comparative study of previously reported few-layer graphene materials employed to serve as the supercapacitor electrode material. the proposed device configuration successfully outperformed the results of the previously reported materials. graphite foil, stainless steel, and indium doped tin oxide (ito) have been widely used as the current collector for fabricating graphene electrodes, but due to their limited flexibility, heavy wight and relatively poor conductivity, there are not suitable choices for ultrathin and flexible supercapacitors [71,72]. this investigation offers a new approach to the fabrication of highly flexible and cost-effective silver deposited pet current collectors for supercapacitors. further, the utilization of pva-na2so4 hydrogel electrolyte in the supercapacitor device enhances the device performance in terms of energy density, power density and long cycle stability [65]. the flexible supercapacitor device demonstrated outstanding mechanical durability under several stress conditions, confirming its applicability in real applications. figure 8e shows the cyclic voltammetry of the quasi-solid-state supercapacitor device without bending and under various bending conditions. the area of the single electrode of the device was 4 cm2 and the bending was done in the middle region of the supercapacitor device. under static bending, the area of the cv curve slightly increases due to the stretching and compression of hydrogel membrane electrolyte and the increment predominantly occurs in the pseudocapacitive region. it can be clearly observed that the cv curves show insignificant change due to stable connections between the silver current collector, graphene layers and hydrogel membrane electrolyte under all the bending conditions (0 – 180°), representing the robust, flexible operation of the supercapacitor [73]. figure 8f shows a schematic diagram of the device, illustrating the lpcvd grown few-layer graphene transferred on e-beam deposited silver pet substrate (flexible current collector) sandwiched by a separator-less quasi-solidstate hydrogel membrane. conclusions to summarize, we have been successful in developing lightweight and flexible sc using chemical vapor deposited a few layers of holey graphene. the e-beam deposition of silver on pet substrates provides an excellent substitute for the metallic conductors and the use of polymer gel membranes in the device leads to the ultrathin and quasi-solid-state device architecture. the device shows extraordinary mechanical stability under various bending conditions. the fabricated flexible scs demonstrated an excellent specific areal capacitance of 15.3 mf cm-2 with an energy density and power density of 0.53 µwh cm-2 and 25 w cm-2, respectively, at the current density of 0.05 ma cm-2. the outstanding performance of the fabricated ultrathin and flexible sc can be explained as: 1) the holey structure of graphene allows rapid movement of ions in the electrode material and makes the inner graphene layers accessible for charge storage, 2) the wrinkles and multiple creasing in the graphene has improved the theoretical electrochemical surface area for charge storage, leading to enhanced sc performance, 3) significant defect density in the graphene samples permits the prompt transport of large sodium ions. conflict of interest: the authors express no conflict of interest. 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https://doi.org/10.1038/s41467-017-00550-3 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://dx.doi.org/10.5599/jese.1411 https://doi.org/10.1002/adma.201300132 https://doi.org/10.1039/c3nr00320e https://doi.org/10.1039/d1ta01291f https://doi.org/10.1021/am504695t https://doi.org/10.1038/s41467-017-00550-3 https://creativecommons.org/licenses/by/4.0/) @article{khan2022, author = {khan, mohammed saquib and shakya, preeti and bhardwaj, nikita and jhankal, deependra and sharma, atul kumar and banerjee, malay kumar and sachdev, kanupriya}, journal = {journal of electrochemical science and engineering}, title = {{chemical vapor deposited graphene-based quasi-solid-state ultrathin and flexible sodium-ion supercapacitor:}}, year = {2022}, issn = {1847-9286}, month = {sep}, number = {4}, pages = {799--813}, volume = {12}, abstract = {flexible electronic devices find wide application in wearable electronics and foldable gadgets. this article reports chemical vapor deposited (cvd) few-layers graphene for a solid-state flexible supercapacitor device. raman spectroscopy analysis reveals up to five layers in the graphene samples. polyvinyl alcohol-na2so4 hydrogel membrane is used as a gel polymer electrolyte (gpe). 50 nm thick silver (ag) deposited on polyethylene tere­phthalate (pet) through e-beam deposition served as the flexible current collector for the device. galvanostatic charge-discharge (gcd) executed on the fabricated device to ana­lyze its electrochemical performance yielded a specific areal capacitance of 15.3 mf cm-2 at 0.05 ma cm-2 current density. the obtained power density of the fabricated device is 0.53 µwh cm-2 at a power density of 25 µw cm-2.}, doi = {10.5599/jese.1411}, file = {:d\:/onedrive/mendeley desktop/khan et al. 2022 chemical vapor deposited graphene-based quasi-solid-state ultrathin and flexible sodium-ion supercapacitor.pdf:pdf;:15_jese_1411.pdf:pdf}, keywords = {e, gel polymer electrolyte, beam deposition, chemical vapor deposition, few, flexible electronics, layer graphene, light, weight supercapacitor}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1411}, } gama-fe2o3 silica-coated 2-(2-benzothiazolyl azo)-4-methoxyaniline for supercapacitive performance http://dx.doi.org/10.5599/jese.1657 521 j. electrochem. sci. eng. 13(3) (2023) 521-536; http://dx.doi.org/10.5599/jese.1657 open access : : issn 1847-9286 www.jese-online.org original scientific paper -fe2o3 silica-coated 2-(2-benzothiazolyl azo)-4-methoxyaniline for supercapacitive performance jinan mohammed mahmood1, zaid h. mahmoud1,, noor sabah al-obaidi 1 and ahmed m. rahima2 1chemistry department, college of science, diyala university, iraq 2chemistry department, college of science, mustansiriyah university, iraq corresponding author: zaidhameed_91@yahoo.com received: december 30, 2022; accepted: january 30, 2023; published: february 6, 2023 abstract magnetic -fe2o3@sio2 core-shell nanocomposite was prepared using stöber method and functionalized firstly by isopropenyloxytrimethylsilane as a coupling agent to enter active acetylacetone on the surface of nanoparticles, and after that by the synthesized azo dye ligand, 2-(2-benzothiazolyl azo)-4-methoxyaniline. in such a way, -fe2o3@sio2-azo dye hybrid nanocomposite was formed. the structure of the synthesized azo dye was evidenced by physical and chemical analysis using melting point, fourier-transform infrared spectroscopy (ft-ir), chns elemental analysis, proton nuclear magnetic resonance (hnmr) and gas chromatography mass spectrometry (gc-ms). the magnetic properties, structure, element composition and morphology characterization of prepared materials (-fe2o3, -fe2o3@sio2, and -fe2o3@sio2-azo dye) were investigated by vibrating sample magnetometer (vsm), x-ray diffraction (xrd), x-ray photoelectron spectroscopy (xps), transmission electron spectroscopy (tem) and field electron scanning electron microscopyenergy dispersive x-ray-mapping techniques. the electrochemical performance of synthesized -fe2o3, -fe2o3@sio2, and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline) electrodes were carried out using cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical impedance spectroscopy (eis). it was shown that the finally prepared -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline) hybrid nanocomposite electrode possesses good storage charge capability of 580 f g-1 at 1 a g-1. keywords hybrid nanocomposite; maghemite nanoparticles; azo dye ligand; supercapacitor electrode; coatings introduction supercapacitor (sc) is a storage energy device with long cycle life, high power density and fast recharge capability, but in spite of many advantages, scs still suffer from the low density of energy http://dx.doi.org/10.5599/jese.1657 http://dx.doi.org/10.5599/jese.1657 http://www.jese-online.org/ mailto:zaidhameed_91@yahoo.com j. electrochem. sci. eng. 13(3) (2023) 521-536 supercapacitive propeties of -fe2o3@sio2-azo dye hybrid 522 before they could have authentic electric applications [1,2]. sc is composed of two electrodes (anode and cathode), membrane and electrolyte solution. different metal oxides, sulfides [3], hydroxides [4], and metal complexes [5] have already been studied as electrode materials to improve the capacitance of sc. in order to attain high efficiency, two electrodes should store the energy or charges equally, which means that if one electrode stores charge more than the other, it will cause a difference in the charge balance of the electrodes of sc [6]. maghemite phase (-fe2o3) is a one of the auspicious electrode materials because of its natural abundance and large specific capacitance [7,8]. the electrochemical behavior of iron oxide [9] and iron oxide coated by sio2 have already been examined, but without reporting their supercapacitance or electrochemical performance [10-12]. however, their chemical stability, good theoretical capacitance and low cost have resulted in protracted studies targeted to improve the electrochemical properties of iron oxides. the studies reported that as high as 207 f g-1 specific capacitance for iron oxide prepared from ultrasonic method [13]. we believe that incorporating of -fe2o3 with ligand compounds nanocomposite may be an effective method to enhance electrochemical properties and performance [14,15]. so, the maghemite/ligand hybrid nanocomposite will be more committing electrode for supercapacitor than pure maghemite. in this paper, we report the synthesis of azo dye on the surface of silica-coated magnetic maghemite nanoparticles incorporated with 2-(2-benzothiazolyl azo)-4-methoxyaniline ligand as new nanomagnetic electrodes for supercapacitor applications. experimental materials in this work, the reagents and solvent were purchased from fisher scientific and sigma-aldrich with 98 % purity, while other chemical materials were supplied from fluka. all chemicals were used without purification. preparation of azo dye ligand a mixture (10 ml ethanol, 2 ml conc. hcl) has been used to melt of (0.335 g, 1 mm) of 2-aminobenzothiazole, and diazotized with 10 % solution of nano2 at 5 °c. the mixture was added gradually with stirring to cool ethanolic solution (0.307 g, 1 mm) of 4-methoxyaniline. after that, 25 ml of 1 m naoh solution was added to the colored mix and azo ligand was precipitated, filtrated and washed various times with (1:1) c2h5oh: h2o, and left to dry. the reaction is shown in scheme 1. some other physical characteristics of the synthesized azo-dye are summarized as follows. color: deep brown. m.p. = 300 °c >. 1h nmr (400 mhz, dmso-d6):  = 7.00-7.857 (m, 6h), 7.908 (s, 2h) (nh2), 3.426 (s, 3h) (och3). ir (cm-1):  (n-h) = 3387-3441,  (c=n) = 1604,  (c=c) = 1504,  (n=n) = 1535 and 1568, as,s ch3 = 1404 and 1387, 1309. max = 240, 354 and 412 nm. elemental analysis calculated: c 58.92 %, h 3.92 %, n 18.98 %, o 4.98 %, s 10.92 %. preparation of magnetic -fe2o3 nanoparticles potassium ferric oxalate k3[fe(c2o4)3]3h2o was used as a source for synthesis of -fe2o3 nanoparticles by photolysis method [16-18]. in a 125 w irradiation system, 1 g of ferric complex solution was irradiated for 30 min until a yellow precipitate of ferrous oxalate was formed. the precipitate was decanted, washed with distilled water and dried at 80 °c for 4 h. the dried precipitate was burned at 400 °c for 2 h. j. m. mahmood et al. j. electrochem. sci. eng. 13(3) (2023) 521-536 http://dx.doi.org/10.5599/jese.1657 523 scheme 1. preparation of azo ligand preparation of -fe2o3@sio2@iptms the preparation of -fe2o3@sio2@ consists of two steps: the first step consists of dispersing 0.5 g of the brown magnetic precipitate of -fe2o3 in the mixture of water and 2-propanol with a ratio of 1:10 with stirring for 2 h at 22 °c. the mixture of oleic acid (10 ml), ammonium solution (28 %, 5 ml), distilled water (10 ml) and 5 ml of tetraethyl orthosilicate (teos) was added to the dispersing mixture under continuous stirring. the product was magnetically isolated and dried at 80 °c for 2 h, and used in the next step. the second step was carried out by dispersing 1 g of -fe2o3@sio2 in 50 ml of 70 % ethanol solution by sonication process for 1 h. then 2 ml of (3-iodopropyl)trimethoxysilane (iptms) was added to the mixture under a continuous stirrer. finally, the product was magnetically isolated and dried at 80 °c for 2 h. preparation of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline firstly, (0.1 g) -fe2o3@sio2@acetylacetone (synthesized from refluxed -fe2o3@sio2@iptms with pentane-2,4-dione in the presence of sodium hydride) was dispersed in 10 ml mixture of water and ethanol with ratio (1:1) for 30 min. then, 0.2 g of 2-(2-benzothiazolyl azo)-4-methoxyaniline was added to the dispersion mixture and refluxed for 10 h. after that, the functionalized magnetic nanocomposite was separated by pieces of magnetic bar. finally, it was washed several times with acetone and distilled water and dried at 80 °c for 3 h. all processes and prepared compounds are summarized in scheme 2. electrochemical measurements the electrochemical cell contained three electrodes, i.e., 11 cm pieces of prepared samples as working electrodes, ag/agcl (saturated kcl) as the reference electrode and pt foil as the counter electrode. 1 m na2so4 was utilized as the electrolyte solution. the active material(s) (85 %), carbon black (10 %) and polyvinyl fluoride (5 %) were mixed by using a few drops of n-methyl-2-pyrrolidione solvent to obtain mixtures that were coated on the foil of graphite substrate (1 cm2) and dried in an oven for 10 h at 70 oc. the mass of active materials (-fe2o3, -fe2o3@sio2, and -fe2o3@sio2azo dye) on each of thus formed working electrodes was approximately 0.5 g. the three electrodes were utilized to examine the pesudocapacitive performance of prepared samples in terms of cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical impedance spectroscopy http://dx.doi.org/10.5599/jese.1657 j. electrochem. sci. eng. 13(3) (2023) 521-536 supercapacitive propeties of -fe2o3@sio2-azo dye hybrid 524 scheme 2. steps during preparation of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline results and discussion azo ligand characterization the 2-(2-benzothiazolyl azo)-4-methoxyaniline azo ligand was characterized using fragmentation electron effect. the results of hit mass spectroscopy (figure 1) illustrated that the molecular weight of the c14h12n4os ligand is 284 g mol-1. peak at m/z = 284 is assigned to [m]+ and correlated with the prepared azo ligand. set of peaks located at 210, 134, 77.2 and 35 are related to other fragmentation. the main fraction supported with cracking product is shown in scheme 3. m / z figure 1. mass spectrum of azo ligand 0 50 100 150 200 250 300 350 400 450 500 0 20 40 60 80 100 r e la ti v e a b u n d a n c e m/z 284 210 134 77.2 35 j. m. mahmood et al. j. electrochem. sci. eng. 13(3) (2023) 521-536 http://dx.doi.org/10.5599/jese.1657 525 scheme 3. fragmentation pattern of azo ligand magnetic properties the magnetic properties of prepared materials were investigated by vibrating sample magnetometry (vsm). the magnetization plots of -fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2benzothiazolyl azo)-4-methoxyaniline are shown in figure 2. the results show that the saturation magnetization is equal to 25-40 emu g-1 without any hysteresis loop. in addition, the results show quick ascension in magnetization curves without any coercivity and the samples show supermagnetic behavior at room temperature. these magnetic properties of prepared nanoparticles are pivotal in their applications since they prevent aggregation of particles and enable their re-dispersion in the reaction medium in the absence of a magnetic field [19]. -10000 -5000 0 5000 10000 -40 -30 -20 -10 0 10 20 30 40 m a g n e ti z a ti o n , e m u g -1 applied field, oe -fe2o3 -fe2o3/sio2 -fe2o3@sio2-ligand figure 2. vsm of prepared electrode materials http://dx.doi.org/10.5599/jese.1657 j. electrochem. sci. eng. 13(3) (2023) 521-536 supercapacitive propeties of -fe2o3@sio2-azo dye hybrid 526 structure characterization the structure properties of -fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline were evaluated by xrd and xps. the xrd of -fe2o3, -fe2o3@sio2, azo ligand 2-(2-benzothiazolyl azo)-4-methoxyaniline, and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline are presented in figure 3a-d, respectively. the results in figure 3a show diffraction peaks located at 30.1, 35.6, 43.2, 53.4, 57.1 and 62.5o, which correspond to face-centered cubic (220), (311), (400), (422), (511) and (440) crystalline planes, in agreement with (jcpds-39-1340). on the other side, the results (figure 3b) show a new diffraction peak centered at 22.8°, which corresponds to (110) plane of amorphous sio2. by comparing diffraction peaks of -fe2o3 and -fe2o3@sio2, it was found that the coated sio2 doesn’t change the structure of -fe2o3. as can be seen (figure 3d), the presence of sharp peaks confirms that the structure of -fe2o3@sio2 remains crystalline after being coated with a layer of azo ligand [20,21]. 2 / ° figure 3. xrd patterns of: (a) -fe2o3, (b) -fe2o3@sio2, (c) azo ligand, and (d) -fe2o3@sio2-azo ligand the chemical composition of -fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo) -4-methoxyaniline was characterized using xps. the xps of prepared compounds corresponding to c, n, o, si, s and fe energy levels are presented in figures 4a to 4f. the energy level of c (figure 4a) shows three peaks located at 283.5, 286.2 and 289.1 ev corresponding to c-c, c=n and c=c bonds, respectively, which confirm bonding c of acetylacetone and n of 2-(2-benzothiazolyl azo)-4-methoxyaniline ligand [22,23]. the spectrum energy level of n 1s is illustrated at figure 4b. the spectrum can be deconvoluted to 398.4, 399.5 and 401.2 ev, corresponding to [-c=n-], [-n=n-] and [-nh-], respectively [24]. at 531.8 ev, the energy level spectrum is shown (figure 4c), which is related to o 1s, confirming the presence of o2in the composite [25]. as shown in figure 4d, the characteristic peak of si 2p is clearly formed at 103.2 ev, which indicates the successful coating of sio2 around -fe2o3. 10 20 30 40 50 60 70 2/° (a) (220) (3 11 ) (400) (422) ( 51 1) (4 40 ) (311) (3 11 ) (b) sio2 (c) (d) ligand in te ns ity , a .u . j. m. mahmood et al. j. electrochem. sci. eng. 13(3) (2023) 521-536 http://dx.doi.org/10.5599/jese.1657 527 binding energy, ev figure 4. xps of: (a) c 1s, (b) n 1s, (c) o 1s, (d) si 2p, (e) s 2p, (f) fe 2p, -fe2o3@sio2-azo dye and (g) survey spectra of -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline 280 282 284 286 288 290 292 in te n s it y , a .u . binding energy, ev c-c c=n c=c (a) c 1s 398 399 400 401 402 403 in te n s it y , a .u . binding energy, ev c=nn=n -nhn 1s (b) 526 528 530 532 534 536 538 in te n s it y , a .u . binding energy, ev si-o-fe si-o-si si-o-h o 1s(c) 100 101 102 103 104 105 106 in te n s it y , a .u . binding energy, ev si 2p(d) 159 160 161 162 163 164 165 166 167 168 169 170 in te n s it y , a .u . binding energy, ev s 2p5/2 s 2p3/2 (e) 705 710 715 720 725 730 in te n s it y , a .u . binding energy, ev 2p 3/2 2p 1/2 fe 2p (f) 0 200 400 600 800 1000 1200 o 1s in te n s it y ( a .u .) binding energy (ev) o 1s si 2p si 2s fe 2p s 2p n 1s c 1s (g) http://dx.doi.org/10.5599/jese.1657 j. electrochem. sci. eng. 13(3) (2023) 521-536 supercapacitive propeties of -fe2o3@sio2-azo dye hybrid 528 when comparing the survey spectrum of -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline (figure 4e), a new peak centered at 145.67 ev is found, corresponding to level energy spectrum of s 2p. this indicates the successfully incorporated ligand with -fe2o3@sio2. on the other side, the survey spectrum of prepared compounds shows a small peak of fe, which approves the coating of fe3o4 by sio2. morphology characterization the morphology of -fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline were performed utilizing tem, fesem and edx-mapping. tem images of prepared compounds are shown in figures 5a to 5c. figure 5. tem images of (a) -fe2o3, (b) -fe2o3@sio2, (c)-fe2o3@sio2-azo ligand; fesem images of (d)-fe2o3, (e) -fe2o3@sio2, (f)-fe2o3@sio2-azo ligand, and saed images of: (g) -fe2o3, (h)-fe2o3@sio2-azo ligand j. m. mahmood et al. j. electrochem. sci. eng. 13(3) (2023) 521-536 http://dx.doi.org/10.5599/jese.1657 529 the results (figure 5a) clearly display agglomerates of spherical -fe2o3 nanoparticles with 11 nm grain size. tem images illustrate the core-shell system structure of -fe2o3@sio2 and confirm the perfect coating of -fe2o3 nanoparticles by sio2, as shown in figure 5b. on the other side, the results (figure 5c) illustrate the complete surrounding of 2-(2-benzothiazolyl azo)-4-methoxyaniline layer around -fe2o3@sio2, which indicates success in the preparation of the final compound. the microscope evaluation of -fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4methoxyaniline were investigated using fesem. the fesem images of prepared compounds are shown in figure 5d-f. the results (figure 5d) show that the magnetic -fe2o3 particles were formed in uniform shapes with apparent agglomeration due to their magnetic properties. added sio2 to magnetic -fe2o3 makes the shapes of catalyst nanoparticles spherical (figure 5e). the fesem images of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline show nanocomposite with a small agglomeration of particles, which may be back to fictionalized of -fe2o3 by 2-(2benzothiazolyl azo)-4-methoxyaniline layer. the results of fesem are in agreement with xrd and tem results. the lattice fingers of -fe2o3 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline are shown in figure 5g-h. the results show the highly crystalline nature of -fe2o3@sio2azo, as well as the well-organized dots and ring (figure 5h), prove the crystalline nature of synthesized material that could be beneficial in structure retention for long-term stability. on the other side, a large number of gleam spots (figure 5h) compared with pure -fe2o3 (figure 6g) elucidates the coating of -fe2o3 by sio2 [27,28]. the chemical composition of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline was investigated by element mapping patterns, while compositions of -fe2o3 and -fe2o3@sio2 were carried out by edx analysis. the eds spectrum result (figure 6) of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline shows detectable singles back to fe, o, si, s, n and s, as well as the weight percentage of detecting elements, suggesting that the nanocomposite was successfully prepared figure 6. eds spectrum of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline and mapping of fe, si, s, n, o, c the edx spectra of -fe2o3 and -fe2o3@sio2 are shown in figure 7. clear peaks related to fe and o without other peaks are evident in figure 7a, which indicates the successful preparation of pure ferric oxide. a new peak related to si can be detected from eds analysis after the coating of fe2o3 by sio2 (figure 7b). http://dx.doi.org/10.5599/jese.1657 j. electrochem. sci. eng. 13(3) (2023) 521-536 supercapacitive propeties of -fe2o3@sio2-azo dye hybrid 530 figure 7. edx spectra of: (a) -fe2o3 and (b) -fe2o3@sio2 bet analysis the specific area, mean size of particles and the volume of pores for -fe2o3 and -fe2o3@sio2-2 -(2-benzothiazolyl azo)-4-methoxyaniline were investigated using bet analysis, and the results are summarized in table 1. the results show that the surface area of -fe2o3 and -fe2o3 functionalized by sio2 and azo compound was 99.3 and 275.5 m2/g, respectively, meaning that the functionalized compound has higher surface area due to the presence of silica and azo dye and so, higher capacity for adsorption of species on the electrode surface. in addition, the results demonstrated that the mean pore size of -fe2o3 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline are 3.9 and 4.3 nm, respectively. according to the iupac categories, both prepared electrode materials can be classified into mesopores group [29,30]. table 1. nitrogen sorption characteristics of prepared electrode materials electrode material sbet / m2 g-1 pore volume, cm3/g average pore diameter, nm -fe2o3 99.3 2.66 3.9 -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline 275.5 4.9 4.3 electrochemical performance the electrochemical properties of -fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2-benzo thiazolyl azo)-4-methoxyaniline electrodes were investigated using cv, gcd and eis techniques. the specific capacitances of three electrodes in the presence 1 m na2so4 as electrolyte solution were calculated using cyclic voltammetry data. the cvs of prepared compounds were recorded at different scan rates and within 0-1.2 v potential window, as presented in figure 8. the cvs of -fe2o3, fe2o3@sio2 and -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline examined at 10 mv s-1 are shown in figure 8a. the results demonstrate redox peaks for -fe2o3@sio2-2-(2-benzothiazolyl azo)-4--methoxyaniline electrode. in addition, the cv result of -fe2o3@sio2 showed significant improvement when its surface was modified with 2-(2-benzothiazolyl azo)-4-methoxyaniline, and higher specific capacitance was observed. the cvs of -fe2o3@sio22-(2-benzothiazolyl azo)-4methoxyaniline at different scan rates are presented in figure 8b. the results show redox peaks until the highest scan rates with slight shifting, which indicates good electrode capability. on the other hand, the separation of current peaks raised with the rising scan rate because the redox peaks change with reversible reactions between the electrode and electrolyte solution [31]. the specific capacitance (csp) of prepared electrodes was evaluated by using equation )1) [32]: j. m. mahmood et al. j. electrochem. sci. eng. 13(3) (2023) 521-536 http://dx.doi.org/10.5599/jese.1657 531  =  sp 2 a c m v (1) where a is the area under plot,  / v s-1 scan rate, m / g is the active mass of the electrode, and v is a potential window. the results are shown in figure 8c. the results exhibit that the modified fe2o3@sio2 by azo ligand (2-(2-benzothiazolyl azo)-4-methoxyaniline) has higher specific capacitance than -fe2o3@sio2 and -fe2o3, because of the interaction between the ferric oxide and azo ligand, which makes the structure of electrode superior and easy for na2so4 electrolyte solution accessibility. the specific capacitance of prepared electrodes was also examined at different scan rates 10-100 mv s-1 and the results are also presented in figure 8c. the results show a reduction in specific capacitance values with increasing scan rate because of difficult redox transition at high scan rate value [33,34]. figure 8. cvs of (a) -fe2o3, -fe2o3@sio2 and -fe2o3@sio2 azo ligand electrodes at 10 mv s-1, and (b) -fe2o3@sio2-azo ligand electrode at different scan rates (10-100 mv s-1), (c) specific capacitance of three electrodes at different scan rates the gcd measurements of prepared compounds (-fe2o3, -fe2o3@sio2 and -fe2o3@sio2-2-(2 -benzothiazolyl azo)-4-methoxyaniline) were performed at different current densities (0-10 ma g-1) in the presence of 1 m na2so4 as the electrolyte solution. the results show that discharge times are decreased with increasing current density due to increased drop voltage. at different current densities, the specific capacitance value was calculated using the following equation [35]:  =  sp i t c m v (2) 0.00 0.25 0.50 0.75 1.00 -0.010 -0.005 0.000 0.005 0.010 c u rr e n t d e n s it y , a c m -2 potential, v ---fe2o3 ---fe2o3@sio2 ---fe2o3@sio2-azo (a) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -0.04 -0.02 0.00 0.02 0.04 c u rr e n t d e n s it y , a c m -2 potential, v (b) 0 10 20 30 40 50 60 70 80 90 100 110 0 100 200 300 400 500 600 s p e c if ic c a p a c ta n c e , f g -1 scan rate, mvs-1 fe2o3@sio2-ligand fe2o3@sio2 fe2o3 (c) http://dx.doi.org/10.5599/jese.1657 j. electrochem. sci. eng. 13(3) (2023) 521-536 supercapacitive propeties of -fe2o3@sio2-azo dye hybrid 532 where i is the current density in the gcd experiment, t is discharge time, m is the mass of active material (0.5 g) and v is the potential window. the specific capacitance values of prepared compounds at different current densities are summarized in table 2. table 2. specific capacitance values obtained by gcd measurements at different current densities current density, a g-1 specific capacitance, f g-1 electrodes -fe2o3 -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline 1 273 580 2.5 214 513 5 167 458 10 123 399 the results show that specific capacitance is promoted by incorporating 2-(2-benzothiazolyl azo)4-methoxyaniline with -fe2o3@sio2 compared with -fe2o3 electrode because of providing more active sites for transferring electrons inside the electrodes and many sites for storing energy. on the other hand, the results show a decrease in the specific capacitance with increasing current density because there is not enough time for ions to arrive at the surface of the electrode and work in electrochemical reactions [36-39]. the cycling stabilities of -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline and -fe2o3 electrodes are investigated by gcd measurements (figure 9) at 10 a/g current density for 1000 cycles. pure -fe2o3 electrode shows lower stability compared with the functionalized electrode due to its destruction via large current, discharge and circular charge [39]. after 750 cycles at 10 a g-1, the functionalized electrode exhibited higher stability compared with -fe2o3, with 92 % of initial capacitance remaining after 1000 cycles, which indicates a good role of the azo compound in the cycle stability of the electrode. figure 9. cycling stability of -fe2o3 and -fe2o3@sio2 -azo ligand electrodes from electrochemical measurements, our prepared hybrid nanocomposite shows a very good capability to store the charges and electric energy (580 f g-1) when comparing it with other polymeric and binary oxide compounds (table 3). it can be noted that the synthesized -fe2o3@sio2-2-(2 0 200 400 600 800 1000 40 50 60 70 80 90 100 c a p a c it a n c e r e te n ti o n , % number of cycles fe2o3@sio2-azo fe2o3 j. m. mahmood et al. j. electrochem. sci. eng. 13(3) (2023) 521-536 http://dx.doi.org/10.5599/jese.1657 533 -benzothiazolyl azo)-4-methoxyaniline) hybrid nanocomposite has interesting electrochemical properties already reported in the literature. table 3. specific capacitance for some iron oxide composites (current density = 1 a g-1) compounds specific capacitance, f g-1 reference iron oxide@sio2@polyfc 675 [40] porous-fe2o3@c nanowire 280 [41] iron oxide@sio2-bis(aminopyridine)-cu 263 [42] flexible iron oxide@c/mno2 306 [43] iron oxide/reduced graphene oxide 480 [44] -fe2o3@sio2-2-(2-benzothiazolyl azo)-4-methoxyaniline 580 this study conclusion stöber method was used to prepare -fe2o3@sio2 core-shell nanocomposite that was functionalized by 2-(2-benzothiazolyl azo)-4-methoxyaniline modified-mnps, and characterized by different techniques such as xrd, xps, tem, fesem-edx-mapping, while the electrochemical performance cv, gcd and eis. electrochemical characterization showed good cycling stability, good rate capability and high specific capacitance of 580 f g-1 at 1 a g-1 current density. after 100 cycles, the specific capacitance of -fe2o3@sio2-azo ligand electrodes is reduced to 92 % of retention capacitance. the good electrochemical 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srinivasan, h. zhang, h. h. hng, q. yan, achieving high specific charge capacitances in fe3o4/reduced graphene oxide nanocomposites, journal of materials chemistry 21 (2011) 3422-3427. https://doi.org/10.1039/c0jm03175e ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1002/er.4678 https://doi.org/10.1007/s41918-020-00063-6 https://doi.org/10.1186/s11671-020-03379-w https://doi.org/10.1186/s11671-021-03543-w https://doi.org/10.1038/s41598-022-15980-3 https://doi.org/10.1007/s10854-017-8192-8 https://doi.org/10.1016/j.electacta.2022.141663 https://doi.org/10.3390/nano8070487 http://doi.org/10.20964/2022.10.49 https://doi.org/10.1038/s41598-017-15535-x https://doi.org/10.1039/c0jm03175e https://creativecommons.org/licenses/by/4.0/) self-assembling nanomaterial-based peptide surface for target cell adhesion http://dx.doi.org/10.5599/jese.1664 491 j. electrochem. sci. eng. 13(3) (2023) 491-504; http://dx.doi.org/10.5599/jese.1664 open access : : issn 1847-9286 www.jese-online.org original scientific paper self-assembling nanomaterial-based peptide surface for target cell adhesion hasret turkmen izmir biomedicine and genome center, balcova 35340, izmir, turkey; department of biochemistry, ege university, izmir, turkey and job and vocational counselor, turkish employment agency, izmir, turkey corresponding author: hasret.turkmen@ibg.edu.tr received: january 11, 2023; accepted: april 7, 2023; published: april 18, 2023 abstract non-covalent modification of electrode surfaces with nanoparticle-based peptides does not change the chemical properties of the electrode but allows electrochemical measurement of cell adhesion. this study examines the effect of self-modified nanomaterial/peptide surfaces on cell adhesion. this adhesion to the surface is caused by the negative gibs free energy formed in the system because of the presence of -0h, sulfur, carbonyl, or reactive groups. a cheaper and more practical method for electrode surfaces targeting cell adhesion, which does not use heavy chemicals and edc/nhs chemistry, is used in this work. thanks to the bioactive materials immobilized on the screen-printed carbon electrode (spce) surface in a controlled manner and the surface chemistry offered by these materials, a biocompatible self-assembling nanomaterial-based peptide surface platform is created, and cell adhesion is measured by an electrochemical technique. after the characterization steps, electrochemical techniques created a calibration curve of the current value as a function of concentration for each cell line. the adhesion of the generated bioactive electrode surfaces to the selected cell lines was examined comparatively. keywords screen-printed carbon electrode (spce); gold nanoparticles, biofunctionalization; cell lines introduction non-covalent biofunctionalization of nanoparticles (nps) is a physical conjugation strategy that can be accomplished through electrostatic, hydrophobic and affinity interactions [1-3]. both electrostatic and hydrophobic interactions require fewer modification steps, providing a simple, fast, and costeffective way to biofunctionalized nanoparticles [4]. the adsorption of globular proteins on the negatively charged nps surface depends on electrostatic interactions. due to the repulsion between the surface and protein molecules, the maximal adsorption state occurs at the protein isoelectric point http://dx.doi.org/10.5599/jese.1664 http://dx.doi.org/10.5599/jese.1664 http://www.jese-online.org/ mailto:hasret.turkmen@ibg.edu.tr j. electrochem. sci. eng. 13(3) (2023) 491-504 self-assembling nanomaterial-based peptide surface 492 [5,6]. bioconjugation of nps with biomolecules through covalent interactions can be accomplished by a simple chemical reaction event through cysteine residues on the surface of biomolecules or using a bifunctional linker [7]. the easiest way to bind biomolecules to gold nanoparticles (aunps) is to create weak interactions (electrostatic, hydrophobic, and van der waals interactions) between the nps and the ligand. thus, proteins self-attach to nps by adsorption. the surface charge of nps can be enhanced by ligand functionalization, metal ion binding, or hydrophilic polymer coating. aunps can be easily coated with desired ligands by citrate coating, thiol, or amine ligands with higher binding affinity immediately after reduction or by ligand exchange [8-11]. the hydrophobicity, size, radius, and surface coatings of nps significantly affect np-cell interactions [12]. the main advantages of these materials are high surface area, improved signal-to-noise ratio, catalytic activity, higher selectivity, and convergent diffusion mode [13]. the therapeutic uses of cell-penetrating cationic, hydrophobic, or amphipathic peptides are common due to their cell targeting and transduction properties. cationic peptides such as arginine, lysine, or histidine act independently of the receptor by interacting electrostatically with negatively charged phosphates and sulfates on the plasma membrane, and the partial protonation of the amino groups of histidine makes the peptides in this group the most effective in carrier-cell interaction [14,15]. moreover, amino acids with aliphatic and aromatic hydrophobic side chains have a water solubility-reducing effect. peptides containing acidic and basic groups such as histidine and glutamic acid have been reported to have a water solubilityincreasing effect [16]. the thiol group, which acts as a reducing agent in vivo, breaks disulfide bonds in proteins to yield free cysteine groups [17,18]. glutathione (gsh) is involved in many biological processes, including metabolism, protein and dna synthesis, and maintenance of cells [17]. tang et al. developed an electrochemical immunosensor by adding cea antibodies (ceaabs) to gsh monolayer-modified aunps [19]. similarly, rusling et al. used gsh-conjugated aunps and magnetic nanoparticles (mnps) for the detection of psa [20]. sun et al. used an electrochemical desorption method to effectively release hepg2 tumor cells by cleaving au-s bonds at spge (surface-imprinted gold electrode) interfaces to selectively capture and isolate cancer cells [21]. han et al. reported that a layer of gsh self-assembles into gold-plated nps and serves as a binding site through hydrogen bond interactions [22]. since aunps are highly reactive toward the thiol group, they form au-s bonds with superior affinity [23]. in another study, it was reported that the adsorption of proteins with cysteine residues occurs through a single-point interaction of thiol-derived protein on the aunp surface, while adsorption of proteins without cysteine residues occurs through nonspecific multipoint binding of the protein [24]. it is known that the imidazole ring has a metallic bonding affinity [25]. dipeptide l-carnosine (car) has a broad biological activity and has antioxidant (antiglycation) effects, especially for carbohydrates and products on lipids (antioxidation) [26]. the presence of the β-alanine (βah) residue allows it to react directly with oxidized carbohydrates and lipids, while the histidine (h) residue has been reported to bind to transition metal ions [26]. another study reported the use of l-carnosine and aunps to image hela cells [27]. it has been reported that the conductivity of iron oxide nanoparticles is increased due to covalent binding and synthesis with l-carnosine, making it applicable in cell separation, diagnosis, and targeted drug delivery for cancer therapy [28]. it has been reported that carnosine inhibits cancer cell growth by up to 23 % and increases reactive oxygen species (ros) levels to 30 and 31 % in mdck and hela cells [29]. it has been reported that it inhibits the growth of cancerous cells and shows chaperone activity and anticancer activity by killing transformed cells [30]. also, it has been reported that the imidazole ring can bind to au in many ways [31]. insulin-like growth hormone (igf-1), which is very similar to insulin, is a basic peptide [32] consisting of 70 amino acids belonging to the tyrosine kinase receptor h. turkmen j. electrochem. sci. eng. 13(3) (2023) 491-504 http://dx.doi.org/10.5599/jese.1664 493 family, and having a molecular weight of about 7.5 kda. it is rich in cysteine and these proteins have important roles in carbohydrate metabolism [33]. studies have shown that igf-1 inhibits proliferation and apoptosis and is measured as a biomarker. it was shown that igf-1 stimulates il-8 expression in du-145 (prostate cancer cell line) cells by increasing the transcriptional activity of igf-1, which has an important role in cancer cells [34]. some studies have reported that the igf-1 system is also associated with diabetes and that insulin therapy causes changes in the igf system [35]. igf1 immunosensor was made by immobilizing igf-1-specific antibodies on gold nanoparticles prepared on the gold electrode surface [36]. igf-1 receptor (igf-1r) is overexpressed in lung cancer (a549) and cancer cell proliferation and contributes to malignant transformation and poor prognosis, especially in patients with lung cancer [37]. many studies have shown that igf-1 and ros play a role in the development and progression of various cancers. the already presented research concluded that cancer cells (hela, hepg2, and sw1116) and healthy cells (ncm-460) were treated with different concentrations of igf-1 and that, at other times, cancer cells produced large amounts of cytoplasmic ros but were not healthy cells. further mechanistic analyzes showed that igf-1 activates nfkb and nlrp3 inflammatory signaling in hela cells. systematic analysis showed that igf-1 activates nfkb and nlrp3 and activation depends on cytosolic rosand nadph oxidase 2 (nox2) [38]. in this study, aunps attached to the screen-printed carbon electrode (spce) surface by adsorption, and selected peptides (l-glutathione, l-carnosine, and insulin-like growth factor-1) were immobilized on the aunp-modified surface under optimum conditions. thus, nanoparticle-based peptide bioconjugates have formed an electroactive region on the electrode surface. thanks to this selfassembling binding strategy, non-specific adsorption of electroactive surfaces was prevented. characterization studies of aunp/peptide-modified scpe were carried out using electrochemical and spectroscopic methods to analyze the changes in the chemical and physical structure of the modified spce surface. after the addition of cancer (u87, a549 and hela) and healthy (vero) cell lines, the interactions of the immobilized electroactive surface and added cells were investigated using electrochemical techniques. a calibration curve of the current value as a function of the concentration of cells was generated for each cell line [39]. thus, the adhesion behavior of the selected cells to the modified surface was examined mutually. the concentration range in which the current difference was found is directly proportional to the concentration of the analyte on the electrode surface. thus, an important step was realized for the accurate detection of the target samples. the originality of this article is primarily based on the absence of similar studies in the literature on the ability to induce cell adhesion for insulin-like growth factor-1, which binds to the gold nanomaterial on the electrode surface. note that real-time measurement of point-of-care tests (pocts) through the functionalization of disposable modified electrode surfaces may be useful in their dissemination and diversification. experimental methods and reagents l-glutathione (gsh) reduced, l-carnosine (car) (1 mg/ml) and insulin-like growth factor-1 (igf-1; 10 mg/ml) were prepared in phosphate buffer (pbs), gold nanoparticles (40 nm diameter, od 1, stabilized suspension in 0.1 mm pbs, reactant free) were purchased from sigma-aldrich chemical company (st. louis, mo, usa.) and screen-printed carbon electrode (spce, drp-110, 4 mm diameter) were purchased from metrohmdropsens company. 5.0 mm solution of potassium hexacyanoferrate (hcf) (iii) (k3fe(cn)6) /potassium hexacyanoferrate (hcf) (ii) trihydrate (k4fe(cn)63h2o) redox couple was prepared in phosphate (ph 7.4) buffer containing 0.1 m kcl solution. sodium http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 13(3) (2023) 491-504 self-assembling nanomaterial-based peptide surface 494 phosphate buffer (50 mm, ph 7.4) and phosphate-buffered saline (pbs; ph 7.4, 1x) were used as the working buffer solutions. human lung carcinoma cell line (a549, atcc, usa), human brain glioblastoma cell line (u87 mg, atcc, usa), human cervical adenocarcinoma cell line (hela, atcc, usa), and african green monkey normal kidney cell line (vero, atcc, usa) were used in experiments. cells were cultured in a humidified medium at 37 °c in a 5 % co2 incubator atmosphere. culture medium eagle’s minimum essential medium (emem, m4655, merck, usa), supplemented with 10 vol.% fetal bovine serum (fbs, biowest, france), 1 mm sodium pyruvate (s8636, sigma, usa), 2 mm l-glutamine (k0283, biochrom, germany) and 100 units/ml penicillin and 100 µg/ml streptomycin (a2213, biochrom, germany). the culture medium was replaced with fresh medium every other day and cells were incubated until the desired cell number was reached. cells separated from each other in tissue culture dishes with trypsin-edta solution (0.05 % trypsin, 0.02 % edta, sigma-aldrich, usa). scanning electron microscopy (sem)/energy-dispersive x-ray spectroscopy (eds) and x-ray photoelectron spectroscopy (xps) analyses and cell culture studies were performed with standardized methods by ege university test and analysis laboratory application and research center (egematal). thermo scientific apreo s brand device was used for sem/eds technique and the thermo scientific k-alpha brand device was used for the xps technique. for differential pulse voltammetry (dpv) and cyclic voltammetry (cv) measurements, palmsensepotentiostat (palm instruments, houten, netherlands) ps trace 5.5 program and chi 6004 c (ch instruments incorporated, austin, tx, usa) devices were used to take eis measurements. no electrochemical signal change was observed when the prepared modified electrodes were stored in a closed container at 4 °c for 10 days. all electrochemical experiments were performed on the surface of a screen-printed carbon electrode (spce; dropsense drp110; overall dimensions: 3.41.00.05 cm) containing an ag/agcl reference electrode, a silver counter electrode and a carbon working electrode. the electrochemical cell placed on a ceramic substrate consists of a threeelectrode electrochemical system. all electrochemical experiments were performed in the presence of a redox probe (75 µl) dropped on the surface of the spce working electrode. in addition, using the solution prepared to drop on the surface of the spce working electrode can reduce the difficulty and cost of electrode fabrication. measurements were taken using the differential pulse voltammetry (dpv) technique and cyclic voltammetry (cv) technique in the 0.4 v 0.7 v potential range. the impedance (eis) measurement was performed at an interference frequency range of 100 khz  10 mhz and an alternating potential amplitude of 5 mv. by impedance data analysis, ohmic internal resistance, charge transfer internal resistance and diffusion internal resistance were obtained. all measurements were performed in 50 mm phosphate buffer (ph 7.4) containing 5.0 mm fe(cn)63−/4− and 0.1 m kcl solution. immobilization of spce surfaces before the spce modification process steps, the spce was dipped in distilled h2o, followed by activation. after washing the spce, aunp (2 µl) was evenly dropped onto the surface of the spce working electrode and left for 1 hour in a closed, dark environment. thus, the physical adsorption of aunps took place on the spce surface. after drying, the peptide solution (gsh, 4 µl) was dropped onto the electrode. the same procedures were applied to another spce surface, this time, the same processes were applied to the electrode surface for car (4 µl) and igf-1 (4 µl) instead of gsh. peptides immobilized to self-assemble on the aunp-modified spce surface were kept in a closed and dark environment overnight. hela, u87, vero, and a549 (prepared in pbs, cell lines 10 and 105 cell concentration, 5 µl) were dropped onto modified spce and incubated at 37 °c for 30 minutes. h. turkmen j. electrochem. sci. eng. 13(3) (2023) 491-504 http://dx.doi.org/10.5599/jese.1664 495 sem/eds and xps analyses were performed to obtain information about the morphology and chemical structure of the nanomaterial/peptide formed on the spce surface in egematal. measurements were taken using dpv, cv and eis techniques. linearity was determined by calculating the logarithm of the x (number of cells) and y (current difference) axes results for cell adhesion. results and discussion spectroscopic characterization x-ray photoelectron spectroscopy (xps) survey peaks to analyze the changes in the chemical and physical structure of the prepared bio-functional spce surfaces are given in figure 1 [39-43].to obtain information about the morphological conditions of the prepared bio-functional spce surfaces, sem images of prepared surfaces are given in figure 2 for blank spce surface, spce surface after aunp adsorption and spce surface after aunp/gsh immobilization, spce surface after aunp/car immobilization, and spce surface after aunp/igf-1 immobilization. figure 1. x-ray photoelectron spectroscopy (xps) survey peaks: a) aunp/gsh immobilization; s 2p peaks; b) aunp/car immobilization; c 1s peaks; c) aunp/igf-1 immobilization; n 1s peaks electrochemical characterizations potentials applied after functionalization on the electrode surfaces cause conformational changes in peptides due to electrostatic attraction [44]. the resulting biofunctional surfaces resist adhesion under negative potential while they act toward increasing adhesion under positive potential [44]. electrochemical characterization was performed to evaluate how the ohmic and charge transfer resistance changed. the electrochemical characterization of the modified surfaces is given in figure 3. in this section, cv, dpv and eis measurements were analyzed [45]. all these measurements are complementary to each other and present a modification of the surface. when dpv measurements of spce in the presence of fe(cn)63−/4− were examined, the peak current decreased from 27.08 to 19.05 a after aunp/gsh modification, from 29.04 to 25.20 a after aunp/car modification, and from 28.22 to 20.37 a after aunp/igf-1 modification of the electrode. nyquist diagrams of eis spectra of different modified electrode layers are shown in figure 3. the high to medium-frequency semicircular segments of each impedance spectrum are due to the fe(cn)63-/4electron transfer process. the linear portion of each impedance spectrum appearing at low frequencies is diffusion dependent. also, the fe(cn)63-/4(redox probe) on the electrode surface clarifies the electron transfer kinetics. an increase in the semicircular diameter indicates an increase in electron transfer resistance. the dpv results are in concordance with eis results. the obtained results of eis and dpv strongly support each other. these reductions of current in dpv measurements on the electrode surface after the modification processes are proof of successful immobilization on the surface. http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 13(3) (2023) 491-504 self-assembling nanomaterial-based peptide surface 496 figure 2. scanning electron microscope (sem) images (500 nm mag:150 000×): a) blank spce, b) spce surface after aunp adsorption, c) spce surface after aunp/gsh immobilization, d) spce surface after aunp/car immobilization, e) spce surface after aunp/igf-1 immobilization figure 3. characterization of modified spce surfaces by electrochemical (dpv, cv and eis) methods: a) aunp/gsh@spce, b) aunp/car@spce c) aunp/igf-1@spce. (black: blank spce; red: aunp/peptide) h. turkmen j. electrochem. sci. eng. 13(3) (2023) 491-504 http://dx.doi.org/10.5599/jese.1664 497 in cv measurements, a slight decrease in anodic and cathodic peaks was observed between the modified and blank electrodes. the decrease in peak current or an increase in the diameter of the semicircle revealed that electron transfer was inhibited. the inhibition of electron transfer is due to insufficient electrochemical communication between the electrode and the modified surface. also, the diffusion layer may thicken after immobilization. adhesion and linear detection range of selected cell lines after modifications of electrodes, cell adhesions were performed for the targeted cell lines under specified conditions, and a decrease in dpv peak currents was observed. dpv measurements were taken to determine the linear detection range for cancer (u87, a549 and hela) and healthy (vero) cell types after bio-functionalization of the electrode surface (spce) with aunp/gsh, aunp/car, and aunp/igf-1. for this purpose, cell numbers between 10 and 106 cells/ml were used, each cell type had at least 10 replicates [39]. the current value vs. cell concentration function is, for every modified electrode, which produced a linear response in some remarkable region of cell concentrations, shown in figure 4. standard calibration plots that resulted from differences in peak currents observed with increasing cell concentration (cell number per ml) are presented in the insets of figure 4, together with corresponding linear equations and correlation coefficients (r2), which are also collected in table 1. figure 4. cell adhesion for modified electrodes: a) aunp/gsh@spce linear standard graphic for the hela cell line; b) aunp/gsh@spce linear standard graphic for the u87 cell line; c) aunp/car@spce linear standard graphic for the a549 cell line; d) aunp/car@spce linear standard graphic for the hela cell line electroactive surfaces created using nanomaterial-based peptides to target cells offer researchers privileged opportunities. peptide-based surfaces support the attachment of cells to extracellular structures (extracellular matrix/ecm). the system must work for a molecule to have a high adsorption capacity. for the system to work, it must have a large surface area, or in other words, the free enthalpy of adsorption (∆h) and surface free entropy (∆s) always points in the negative direction. these interfaces between electrodes and modified electrode surfaces are widely used in biotechnology, therapy, diagnosis (diagnosis), and medical applications. in the created system, aunps were firstly attached to the spce surface that is negatively charged thanks to its carboxyl ends by adsorption and http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 13(3) (2023) 491-504 self-assembling nanomaterial-based peptide surface 498 then functionalized with gsh, car, and igf-1.gsh, at ph 7.4. thanks to the cysteine residues in its structure, gsh has a direct affinity and binding to gold nanoparticles. the system creates a nucleophile structure. since it has free radicals, the system reacts directly and non-enzymatically, and cysteines (reduces oxidation) directly affect the cellular redox potential. thanks to the imidazole group in its structure, car acts as a metal-chelating agent and can easily bind to metal nanoparticles. the affinity of the imidazole group in the car structure to the au metal came into play here and functioned as a chelating agent [25]. disulfide bonds between two cysteine residues form a cyclic region within the igf-1 molecule. therefore, it can serve as both a diagnostic biomarker and a therapeutic target. when the xps spectra are examined, the n1 nitrogen peak at 400 ev demonstrates the presence of a covalent peptide bond at the surface. it has been reported that all these data play an active role in cellular experiments, with the binding energy of aromatic carbons (c-c) 284.28 ev and the binding energy of c-oh bonds 284-285 ev in c 1s spectra and 286.38 ev binding energy c-o/n, 288.98 ev binding energy o-c=o shows the characteristic peaks [41,46]. n 1s, the binding energy of 400.18 evn-h, and the binding energy of 402.88 ev n-h show characteristic peaks. the binding peak of 404.68 ev is due to nitrites. it is also consistent with the data of other researchers [40]. the binding energy of 163.78 ev in s 2p indicates thiol-bound gold. the low binding energies may be due to a charge transfer or the formation of a gold-thiolate bond. it is frequently encountered in xps simplified sources where the binding energy of 163.78 ev is due to the thiol bond, and the binding energy of 169.38 ev is the metal sulfate bond, the gold-s bond. the xps data in figure 1a proves that gsh was successfully attached to spce surfaces with aunp, which agrees with previous studies and retains its chemical structure. when the xps spectra (figure 1b) survey of car is examined, it is seen that the n 1s nitrogen peak, which occurs at 400 ev, forms a covalent peptide bond on the surface, and this data plays an active role in cellular experiments. the c 1s component was 284.8 ev (c-c in aromatic rings), 286.38 ev (c-o), and 288.9 ev (o-c=o), while c-n (amine) and n-c=o (amide in the imidazole ring of carnosine) two new peaks appeared at 286.38 and 289.8 ev, respectively. it was concluded that the imidazole ring in the car structure made metallic bonds with aunp. the imidazole ring of carnosine is a cyclopentadiene structure with a non-contiguous ch group (n3 and n1 regions) replaced by n and nh, and it has been reported that it makes a strong bond with a lone electron pair from the n3 region with gold [31]. it has been reported that the imidazole ring can be attached to au in many ways, and complexes with a single au atom prefer two imidazole rings, while complexes with 2 or 3 au atoms prefer three imidazole rings [31]. xps spectra (figure 1c) showed that the nitrogen peak at 400 ev is characteristic. it has been reported that if an n1s peak below 400 ev occurs, or if these peaks did not occur, it could be said that igf-1 was bound by adsorption to the surface as an easily diffused unstable bioactive molecule [44]. therefore, it suggests that the cysteine residues in its structure are attached not by adsorption but by its affinity for aunp. it is a characteristic peak of 400 ev, and it has been stated that it is a peptide bond peak and is covalently bound to the surface [43]. morphological characterization and comparative sem images of the bifunctional surfaces formed with aunp/peptide on the spce surface and the empty spce surface are shown in figure 2. the particle diameters of the created spce surface are between 35.61 and 45.39 nm for the blank spce (figure 2a), while the diameters of particles after aunp adsorption were between 41.09 and 52.28 nm (figure 2b). the diameters of the spce surface particles after aunp/gsh immobilization were between 48.42 and 55.06 nm (figure 2c). after aunp/car immobilization, the diameter of the spce surface is between 47.52 and 53.90 nm (figure 2d), while after aunp/igf-1 immobilization, the diameter of particles is between 45.86 and 62.95 nm (figure 2e). generally, sem analysis showed h. turkmen j. electrochem. sci. eng. 13(3) (2023) 491-504 http://dx.doi.org/10.5599/jese.1664 499 that the surface particle diameter increased as the surface was successfully modified, showing the aggregate formation and homogeneous distribution of particles on the surface. differential pulse voltammetry (dpv) measurements were performed to examine the redox properties of modified spce electrodes, cv to show the electroactivity and modification of the formed surface, and eis measurements for the interfacial properties of molecules on the surfaces during immobilization were performed with at least 10 replications, respectively [47]. the compatibility of these different measurements with each other indicates that the surface has been modified (figure 3). looking at dpv currents, the diffusion layer has thickened, and studies have shown that the decrease in the oxidation peak current is due to a lack of establishing electrochemical communication between the electrode and the modified surface because of the inhibition of electron transfer [48,49]. the changes in the peak potentials between the blank spce and the modified electrode suggest that the surface was changed and the electron transfer rate on the surface was also changed, proving that the surface was successfully modified [50]. impedance (eis) studies have shown that electrode immobilization with inorganic nps, gold nanoparticles [51], or silver nanoparticles [52], significantly reduces charge transfer resistance. dpv measurements, cv measurements and eis measurements provide information as evidence for surface immobilization. the bioconjugate surface formed has been functionalized in a controlled manner. surface functional groups such as carboxyl (cooh) and hydroxyl (oh) form integrin bonds with cell ligands. hydroxyl and carboxyl groups on the surface potentially form hydrogen bonds with cell surface lipids and ions, and higher surface energy leads to the formation of much stronger cell-ligand bonds. also, the partial positive result of the o=c−n nitrogen atom is proven in xps data. the charge on the oxygen atom and the partial negative charge form a dipole and hydrogen bonds with the lower phase molecules, and cell-substrate interaction occurs. since van der waals and electrostatic interactions, and hence cell-surface interaction, affect the surface energy of the bioactive surface created, surface energy also affects cell adhesion. thanks to this interaction, any non-specific cell adsorption is prevented. for the adhesion of the selected cell lines, the linear detection range for cells was established by the change of peak current values observed for modified electrodes with respect to the log of the cell concentration (cell number/ml) obtained by dpv analysis (figure 4). differences in peak current values with increasing cell concentrations per ml were obtained as a function of the cell number selected on surfaces. when current difference versus log (cell/ml) graphs are plotted, the correlation coefficient (r2) was calculated in the linear regression analysis for modified surfaces (table 1). although the surfaces prepared, as seen in dpv measurements with various cell lines, provide cell adhesion, standard graphs were created to understand whether the standard graphs of data obtained from these surfaces were linear. for linearity, the difference of current values was taken on the y-axis, while the logarithms of cell concentrations were taken on the x-axis, and graphs were created (figure 4). vero was used as a control group in this study. all modified surfaces were used, and cell incubation was performed under the same conditions. between all modified electrodes treated here, however, some surfaces showed a linear response, but some did not (table 1). table 1. linear equations and correlation coefficients of calibration plots for selected cell lines modified spce hela cell line u87 cell line a549 cell line vero cell line aunp/gsh@spce r2=0.995 y=0.92x+0.586 r2=0.991 y=1.312x-0.99 aunp/igf-1@spce aunp/car@spce r2=0.972 y=4.353x-3.68 r2=0.996 y=0.363x+6.584 although the resistance of the bioconjugates formed on the spce surface to electron transfer causes its conductivity to decrease, its response to cell adhesion indicates that it maintains its http://dx.doi.org/10.5599/jese.1664 j. electrochem. sci. eng. 13(3) (2023) 491-504 self-assembling nanomaterial-based peptide surface 500 conductivity. thanks to the equations of the calibration graphs, the cell concentrations were increased per ml and the change in the resistance applied by the cell membrane to the biofunctional surface was observed. thanks to the linear detection equations of the calibration charts, the targeted cells can be accurately detected. for aunp/gsh@spce, a linear range of up to 10 to 104 cells/ml was obtained in hela and u87 cell lines. although adhesion was observed for vero and a549 cell lines, a linear range could not be obtained. for aunp/car@spce, a linear range of 10 to 104 cells/ml was obtained for a549 and 100 to 105 cells/ml for hela cell lines. although adhesion was observed for vero and u87 cell lines, no linear range could be obtained. for aunp/igf-1@spce, adhesion was obtained for all selected cells but not a linear range. this may be caused by steric hindrance on the electrode surface due to its high molecular weight. igf-1 can be used as a biomarker that detects cancer cells, optimization studies are carried out thanks to its excellent nature and the bioconjugate it forms with aunp. conclusion in this study, conductive-bifunctional surfaces that can be used for cell adhesion are prepared. it was shown that successful electrode surface modification could eventually result from a cancer diagnostic sensor for detecting selective cancer cells. the surfaces of spce were modified with aunp/gsh, aunp/car, and aunp/igf-1 and used as a basis for targeted cancer diagnosis sensors. thanks to the topographical interaction between gold nanostructures and peptide imprinted surfaces, microscale concave structures created a suitable surface area for the adhesion of target cells. therefore, this study could be considered a pioneer work in recognizing specific cell surfaces. this work demonstrates that the aunp/peptide monolayer mounted on the spce surface promotes cell adhesion. the gold nanoparticles were assembled on the spce, which increases the electrode surface, makes covalent bonds with peptides, and enhances cell adhesion properties. functional interfaces of selected aunp/peptides indicate cell adhesion even if they are not rgd peptides or combinations. however, it is unclear whether the adhesion to the cell is due to the combination of carbon material, aunp, or the selected peptide. in the future direction, the study allows it to be used in point-of-care (poc) studies. the results of this study demonstrate that nanomaterial-based peptides are important in cell targeting and cell differentiation. the success of the disposable np/peptide electrode surface, which is created with a simple method without heavy chemicals and at a lower cost, in the study allows it to be used for biomedical applications. in the future, it may pave the way for personalized sensor applications iot-connected to detect cancer cell lines. in addition, we will integrate blockchain technology, a decentralized technology, into sensor-iot-based architectures for the traceability and security of the obtained data. acknowledgements: the author would like to thank the editors and anonymous reviewers for providing insightful suggestions and comments to improve the quality of the research paper. also, i would like to thank my professors, figen zihnioglu, suna timur, and dilek 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https://doi.org/10.1021/am5016099 https://doi.org/10.1149/2.1191810jes https://doi.org/10.1039/c8nj02325e https://creativecommons.org/licenses/by/4.0/) experimental determination of optimal clamping torque for doi:10.5599/jese.198 9 j. electrochem. sci. eng. 6(1) (2016) 9-16; doi: 10.5599/jese.198 open access : : issn 1847-9286 www.jese-online.org original scientific paper experimental determination of optimal clamping torque for ab-pem fuel cell noor ul hassan* , ** ,  murat kilic***, emin okumus***, bahadir tunaboylu** , ***, ali murat soydan* *nanotechnology research center, gebze technical university, cayirova, kocaeli 41420, turkey **department of industrial and system engineering, istanbul sehir university, istanbul, 34662, turkey ***tubitak marmara arastirma merkezi (mam), baris mah., dr. zeki acar cd no:1, gebze merkez/kocaeli, turkey corresponding author: noormct@hotmail.com received: july 6, 2015; revised: march 11, 2016; accepted: april 18, 2016 abstract polymer electrolyte membrane (pem) fuel cell is an electrochemical device producing electricity by the reaction of hydrogen and oxygen without combustion. pem fuel cell stack is provided with an appropriate clamping torque to prevent leakage of reactant gases and to minimize the contact resistance between gas diffusion media (gdl) and bipolar plates. gdl porous structure and gas permeability is directly affected by the compaction pressure which, consequently, drastically change the fuel cell performance. various efforts were made to determine the optimal compaction pressure and pressure distributions through simulations and experimentation. lower compaction pressure results in increase of contact resistance and also chances of leakage. on the other hand, higher compaction pressure decreases the contact resistance but also narrows down the diffusion path for mass transfer from gas channels to the catalyst layers, consequently, lowering cell performance. the optimal cell performance is related to the gasket thickness and compression pressure on gdl. every stack has a unique assembly pressure due to differences in fuel cell components material and stack design. therefore, there is still need to determine the optimal torque value for getting the optimal cell performance. this study has been carried out in continuation of development of air breathing pem fuel cell for small unmanned aerial vehicle (uav) application. compaction pressure at minimum contact resistance was determined and clamping torque value was calculated accordingly. single cell performance tests were performed at five different clamping torque values i.e 0.5, 1.0, 1.5, 2.0 and 2.5 n m, for achieving http://www.jese-online.org/ mailto:noormct@hotmail.com j. electrochem. sci. eng. 6(1) (2016) 9-16 clamping torque for ab-pem fuel cell 10 optimal cell performance. clamping pressure distribution tests were also performed at these torque values to verify uniform pressure distribution at optimal torque value. experimental and theoretical results were compared for making inferences about optimal cell performance. a clamping torque value of 1.5 n m was determined experimentally to be the best for getting optimal performance as well as uniform pressure distribution for this specific fuel cell. keywords air breathing pem fuel cell; stack clamping torque; compaction pressure; contact resistance; optimal cell performance 1. introduction due to the growing concerns on the depletion of petroleum based energy resources and climate change, fuel cell technologies have received much attention in recent years owing to their high efficiencies and low emissions. fuel cells, which are classified according to the electrolyte employed, are electrochemical devices that directly convert chemical energy stored in fuels such as hydrogen, to electrical energy. its efficiency can reach as high as 60 % in electrical energy conversion and overall 80 % in co-generation of electrical and thermal energies with more than ninety percent reduction in major pollutants [1]. the stack design and cell assembly can significantly affect the performance of fuel cells. adequate contact pressure is needed to hold the fuel cell stack components together to prevent leaking of the reactants between the layers and minimize the contact resistance between layers. the clamping force is equal to the force required to compress the gasket, fuel cell layers, and internal force. the assembly pressure affects the characteristics of the contact interfaces between components due to thin dimensions and the low mechanical strength of fuel cell layers versus the gaskets, bipolar plates, and end plates. the most important goal in the stack design and assembly is to achieve appropriate and uniform pressure distribution. if inadequate or non-uniform assembly pressure is used, there will be stack sealing problems, such as fuel leakage, internal combustion, and unacceptable contact resistance. too much pressure may damage the fuel cell layers, resulting in a broken porous structure and a blockage of the gas diffusion passage. in both cases, it will decrease the cell performance as well as stimulating degradation. every stack has a unique assembly pressure due to differences in fuel cell materials and stack design. several studies were carried out to determine optimal clamping force in order to achieve optimal fuel cell performance. bates et al performed simulation of a single cell and 16-cell fuel cell stack at various clamping pressures. they performed experimental testing of clamping pressure effects on a 16-cell stack by placing a thin pressure-sensitive film between gdl and bipolar plate. they applied clamping pressure using various loads, durations, and two types of gdl resulting in detailed 3d plots of stress and deformation [2]. chang et al. [3] measured the electro-physical properties of the gas diffusion layer (gdl) such as porosity, gas permeability, electrical resistance and thickness using a special designed test rig under various clamping pressure levels. they developed correlations for the gas permeability of the gdl in terms of the clamping pressure. moreover, they also measured contact resistance between the gdl and the bipolar (graphite) plate under various clamping pressures. they showed that increasing the clamping pressure reduces the interfacial resistance between the bipolar plate and the gdl that enhances the electrochemical performance of a pem fuel cell. on the other hand, at the high clamping pressure levels, increasing the clamping pressure not only reduces the ohmic resistance but also narrows n. hassan et al. j. electrochem. sci. eng. 6(1) (2016) 9-16 doi:10.5599/jese.198 11 down the diffusion path for mass transfer from gas channels to the catalyst layers [3]. changes in pem fuel cell performance have been reported as a function of compression pressure resulting from torque applied on bolts [4]. authors used three different types of gdls and reported that, the optimum is related to the gasket thickness and the measured compression pressure on the diffusion layer. wang et al. [5] studied the effect of internal pressure distribution on the performance of a pem fuel cell. they designed a pressurized endplate. they used pressure sensitive films to measure the pressure distribution for both conventional and newly designed end plates and achieved improved cell performance with newly designed end plate [5]. zhou et al. [6] studied the effect of clamping force on the performance of pem fuel cell with inter-digitated gas distributors considering the interfacial contact resistance, the non-uniform porosity distribution of the gas diffusion layer (gdl) and the gdl deformation. they reported that there exists an optimal clamping force to obtain the highest power density for the pemfc with the inter-digitated gas distributors [6]. finite element analysis (fea) procedures were established for a pem single cell with point stack assembly method [7]. yu et al. [8] designed a new asymmetric composite sandwich end plate made of carbon fiber reinforced composite and glass fiber reinforced composite with a pre-curvature generated by the residual thermal deformation, which yields the required pressure distribution in the stack when the end plates are fastened by the clamping device [8]. liu et al. developed a methodology based on fea model and monte carlo simulation to investigate the effect of dimensional error of the metallic bipolar plate (bpp) on the pressure distribution of gas diffusion layer (gdl) [9]. lin et al. reported the effect of gas diffusion layer compression on the performance in a proton exchange membrane fuel cell [10]. zhou et al. reported the effect of non-uniformity of the contact pressure distribution on the electrical contact resistance in pem fuel cells. for a given clamping force, the minimum electrical contact resistance is expected by making the pressure distribution as uniform as possible [11]. avasarala et al. [12] studied the effect of surface roughness of composite bipolar plates on the contact resistance of a proton exchange membrane fuel cell. they observed that most of this contact resistance is governed by electrical properties of the interface layer between the contacting surfaces [12]. wen et al. [13] carried out an experimenttal study of clamping effects on the performance of a single proton exchange membrane fuel cell and a 10-cell stack. they found that the uniformity of the contact pressure distribution, the ohmic resistance and the mass transport limit current, had highly linear correlations with the mean contact pressure [13]. montanini et al. measured the clamping pressure distribution in polymer electrolyte fuel cells using piezo-resistive sensor arrays and digital image correlation techniques [14]. xing et al. [15] reported a three-dimensional model to investigate the effect of assembly clamping pressure on the gdl properties and thus on the performance of pem fuel cells, and to determine the optimum clamping pressures when the cell is operated under different operating voltages. they suggested that the optimum clamping pressures increase when the operating voltage increases [15]. 1. 1. clamping torque calculation theory clamping torque on the bolts can be calculated from the following equation [17]: tt = fclamp kb db / nb fclamp = pc a where, tt is the tightening torque in n m, fclamp is the clamping force in n, kb is the friction coefficient, db is the bolt nominal diameter in m, and nb is the number of bolts. fclamp is a function of clamping pressure pc and cell active area a. colleen spiegel et al. [18], also considered other j. electrochem. sci. eng. 6(1) (2016) 9-16 clamping torque for ab-pem fuel cell 12 important dimensions for calculating clamping torque i.e. material properties of cell components and bolts, geometry of holes, stiffness of the components, stack thickness, and contact resistance between bpp and gdl etc. the torque value obtained by these calculations give us range of clamping torque to be applied, but still there is need to find optimum torque value which will give optimum cell performance. a clamping torque value of 1.2 n m was calculated for our fuel cell stack. this study focused experimental procedure for obtaining optimum clamping torque. the effects of clamping force can be summarized in the following chart (figure 1). figure 1. effects of increasing assembly pressure 1. 2. contact resistance and clamping pressure the contact resistance between bpp and gdl can be deduced as measured by various authors in literature [19,20]. the governing equation for calculation of contact resistance is given below; rcontact = (res1 – res2 – rbpp – rgdl )/ 2 where, res1 and res2 are measured resistances from setup 1 and setup 2, respectively as shown in the figure 2. rbpp is the bulk resistance of graphite bpp and rgdl is the bulk resistance of gdl. rbpp and rgdl are calculated according to their bulk resistivity. contact resistance was observed to be minimum at a value of 140 n/m 2 . a b figure 2. a schematic diagram of the experimental apparatus used for contact resistance measurement: (a) setup 1 (b) setup 2 2. experimental 2. 1. materials, method and assembly graphite bipolar plates with double serpentine flow channels for anode and parallel flow channels for cathode side were prepared by machining. commercially available nafion 212 increase assembly force decrease pore volume improve contact between bpp and gdl improve contact between fibers in gdl decrease gas permeability, change water removal path lower electrical and thermal contact resistance increase electrical and thermal bulk resistivity compaction force insulation insulation compaction force copper plate bpp gdl copper plate r bpp gdl compaction force insulation insulation compaction force copper plate bpp copper plate r bpp n. hassan et al. j. electrochem. sci. eng. 6(1) (2016) 9-16 doi:10.5599/jese.198 13 membrane was used for our fuel cell. custom made meas were prepared with pt loading of 0.6 mg cm -2 , for both anode and cathode side. gasket thickness was calculated based on the thicknesses of membrane and gdl. silicon gasket with a thickness of 0.125 mm was used. polymer (ptfe) end plates were used for providing sufficient contact force between the components. a total quantity of eight m4×20 bolts was used to clamp the cell components. copper sheets with a thickness of 0.8 mm were used as current collectors. several meas were prepared at the same conditions to test designed fuel cell. the active areas of the mea was 20 cm 2 (10×2 cm). other components were also prepared as per designed dimensions with acceptable properties as defined by department of energy (doe). finally, a single cell was assembled for testing at different clamping torque values. 2. 2. contact resistance measurement different experimental setups have been used by authors to measure the contact resistance [19,20]. we used the similar procedures to measure the contact resistance. bpp specimen was inserted between two carbon papers (gdl) and all sand-witched between copper plate current collectors. a series of compression pressures from 0.5 to 3 mpa were applied and the corresponding contact resistances were measured. under each clamping pressure, the contact resistance measurements were repeated four times to obtain the average values. 2. 3. pressure distribution pressure distribution measurement method as used by several authors [7,13,16] has been followed for our experiment. to get the pressure distribution, tests were conducted on our hardware with eight bolts at five different applied torque values, from 0.5 to 2.5 n m with an increment of 0.5 n m. a rectangular piece of film was cut with the dimensions similar to bpp used and was inserted between the membrane electrode assembly (mea) and gas diffusion layer (gdl). firstly, 0.5 n m torque was applied on each bolt of the cell structure in a certain sequence. after approximately five minutes, the bolts were loosened and the film was taken out. same procedure was repeated with other torque values and pressure distribution was observed. 2. 3. polarization and power curves the experiments were hold at 60 °c while hydrogen and oxygen were purged fully humidified at 50 °c. the gas flow rates were kept constant, both for the anode and cathode and were 0.2 l min -1 and 0.4 l min -1 , respectively. the meas were conditioned before recording data. polarization curves were obtained while decreasing the potential slowly by withdrawing current. polarization data was recorded after six continuous cycles. 3. results and discussion 3. 1. pressure distribution results as the pressure was applied, microcapsules on the pressure sensitive film were broken and a color-forming material was released and absorbed on the film. the color (red) intensity of the film is directly related to the amount of applied pressure. the greater the pressure, the more intense the color. after waiting another five minutes, the fuel cell was opened, and the pressure distribution was observable by the color (red) density pattern formed on the film. it is observed that pressure distribution was uniform at a clamping torque of 1.5 n m compared to other torque values. figure 3 below, shows the experimental results obtained at 0.5 and 1.5 n m torque values, respectively. j. electrochem. sci. eng. 6(1) (2016) 9-16 clamping torque for ab-pem fuel cell 14 a b figure 3: pressure distribution results, (a) torque applied 0.5 n m, (b) torque applied 1.5 n m 3. 2. single cell performance upon obtaining assembly pressure effects on mass transfer resistance and contact resistance above, the overall pem fuel cell performance was obtained experimentally, as shown in figure 4. a b figure 4. experimental results: a polarization at different clamping torque values, b power curves at different torque values. 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 v o lt a g e , v current density, a/cm2 0.5 n m 1.0 n m 1.5 n m 2.0 n m 2.5 n m 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 p o w e r, w current density, a/cm2 0.5 n m 1.0 n m 1.5 n m 2.0 n m 2.5 n m n. hassan et al. j. electrochem. sci. eng. 6(1) (2016) 9-16 doi:10.5599/jese.198 15 the fuel cell performance lowers at low clamping torque values. as we increase the clamping torque, the cell performance increases and again starts decreasing after a certain torque value. however, an optimum assembly pressure exists within the lower pressure region. this optimal value is the result of the competing effects of contact resistance and mass transfer resistance. when the assembly pressure is very low, contact resistance could be high and reverse the effect of low mass transfer resistance. obviously, an assembly pressure near this optimum value is preferred. furthermore, if surface parameters of the bpp change, especially when the surface standard deviation becomes larger, the contact resistance tends to be higher. the optimum assembly pressure could then shift to a higher level, which can be achieved in practical assembly processes. for a lower value of assembly torque (0.5 n m), the compression pressure is not sufficient to compress gdl to gasket level and cause fuel leakage as well as a poor contact between gdl and bipolar plate. on the other hand, higher assembly torque (2.5 n m) leads to smaller porosity, imposing more impedance to gas transfer. thus, the current density generated under the land area is less and has more variation in its distribution resulting in lower cell performance. the optimum clamping torque (1.5 n m) is a negotiation between contact resistance and mass transfer due to gdl compression giving the best cell performance. experimental results obtained at different clamping torque values are shown in the figure 4. 4. conclusion in order to study the effects of assembly pressure on the performance of pem fuel cells, theoretical calculations were made for calculating the clamping torque value. experimental procedures were carried out for obtaining the optimized clamping torque giving optimal performance. fuel cell performance has been observed by polarization and power curves. it has been observed that assembly pressure has noteworthy effects on pem fuel cell performance. in a broader view, low assembly pressure causes fuel leakage problems and an increase in contact resistance between bipolar plates and gdl, consequently decreasing fuel cell performance. whereas, high assembly pressure increases mass transfer resistance. current density decreases significantly with an increase of assembly pressure. however, by incorporating the competing effects of electrical contact resistance, the overall performance first increases and then decreases with the increase of assembly pressure. there exists an optimum assembly pressure, in the lowerpressure region, at which fuel cell performance is optimum. acknowledgment: we are thankful to dr. osman okur and dr. suha yazici who provided us financial as well as equipment support to carry out this study at tubitak mam (energy center – hydrogen and fuel cell laboratory) gebze, turkey. references [1] d. papageorgopoulos, an introduction to the 2010 fuel cell pre-solicitation workshop in doe fuel cell pre-solicitation workshop. department of energy, lakewood, colorado; march 16, 2010. 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[20] l. zhang, y. liu, h. song, s. wang, y. zhou, s. j. hu, journal of power sources 162 (2006) 1165-1171. © 2016 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ fe3o4/go nanocomposite modified glassy carbon electrode as a novel voltammetric sensor for determination of bisphenol a http://dx.doi.org/10.5599/jese.1541 815 j. electrochem. sci. eng. 12(5) (2022) 815-817; http://dx.doi.org/10.5599/jese.1541 open access : : issn 1847-9286 www.jese-online.org editorial surface coatings: latest developments in the protection of steels from corrosion and erosion khushdeep goyal department of mechanical engineering, punjabi university, patiala, india email: khushgoyal@yahoo.com material conservation has become an important concept due to global competition. one of the major limitations of steel components is their degradation due to corrosion or erosion. it has been identified as a serious problem for many aggressive environment applications, such as gas turbines, hydraulic components, boilers, internal combustion engines, aviation automotive and mining equipment, chemical process plants, etc. conventional steels used in these aggressive environments are not able to sustain corrosion and erosion problems. many researchers have tried to minimize the effect of corrosion and erosion with design modifications. some researchers have developed fe, ni, cr and co-based alloys for good resistance to erosion and corrosion. significant research work has been done to reduce corrosion and erosion of steel by applying protective coatings. these coatings can be classified into three groups, i.e. diffusion coatings, thermal barrier coatings and overlay coatings. there has been an important development in thermal spray technologies, which develops surface coatings with high corrosion and erosion resistance properties. this special issue entitled “surface coatings: latest developments in the protection of steels from corrosion and erosion” focuses on significant advancements and developments in coating technologies in the protection of steels from corrosion and erosion in different operative environments. this special issue intends to cover original research and critical review articles on recent advances in various techniques to combat corrosion and erosion of steels. novel nano yttria-stabilized zirconia (ysz) reinforced cr3c2-25nicr composite coatings were prepared and successfully deposited on asme-sa213-t-22 (t22) boiler tube steel substrates using high-velocity oxy-fuel (hvof) thermal spraying method by singh et al. [1]. akande et al. [2] investigated the effect of unripe plantain peel (upp) nanoparticles reinforced zn-mgo composite coating on the hardness, anti-corrosion and microstructure properties of mild steel. the studies [3] have shown that protective coatings deposited by thermal spray methods are successful in controlling the wear and enhancing the service life of steels. the radio frequency (rf) magnetron sputtering process was used to develop boron nitride thin films on 316l stainless steel by singh et al. [4]. the researchers [5] deposited inconel and micro and nano wc-12co powders on aisi 4140 carbon steel by high-velocity oxy-fuel (hvof) coating and followed by laser surface modification. kumar et al. [6] http://dx.doi.org/10.5599/jese.1541 http://dx.doi.org/10.5599/jese.1541 http://www.jese-online.org/ mailto:khushgoyal@yahoo.com j. electrochem. sci. eng. 12(5) (2022) 815-817 surface coatings 816 fabricated carbon nanotubes (cnts) reinforced zircon¬nium yttrium coatings on boiler tube steel and also investigated the microstructural and mechanical properties of these coatings. abradable coatings are essentially sealing materials and are deposited by thermal spray techniques [7]. in the research work [8], 5 and 10 wt.% yttria-stabilized zirconia (ysz) nanoparticles were reinforced in ni20cr powder and deposited on boiler tube steel using a high-velocity oxy-fuel spraying process. colmonoy-6+wc powders were deposited with the help of the laser cladding method on the bare ss304 and ss410 steel surfaces by the authors [9]. the effect of the nozzle distance and pulse number parameters was investigated on the modification process by ozbek et al. [10]. in the experimental work [11], carbon nanotubes-reinforced alumina-titania coatings have been developed and corrosion resistance of the newly developed coatings has been evaluated in a boiler environment. references [1] s. singh, k. goyal, r. bhatia, mechanical and microstructural properties of yttria-stabilized zirconia reinforced cr3c2-25nicr thermal spray coatings on steel alloy, journal of electrochemical science and engineering 12(5) (2022) 819-828. https://doi.org/10.5599/jese.1278 [2] i. g. akande, o. s. i. fayomi, b. j. akpan, o. a. aogo, p. n. onwordi, exploration of the effect of zn-mgo-upp coating on hardness, corrosion resistance and microstructure properties of mild steel, journal of electrochemical science and engineering 12(5) (2022) 829-840. https://doi.org/10.5599/jese.1311 [3] d. dhand, p. kumar, j. s. grewal, wear behaviour and microstructural characteristics of cold sprayed nickel-alumina coatings on boiler steel, journal of electrochemical science and engineering 12(5) (2022) 841-849. https://doi.org/10.5599/jese.1270 [4] m. singh, h. vasudev, m. singh, surface protection of ss-316l with boron nitride based thin films using radio frequency magnetron sputtering technique, journal of electrochemical science and engineering 12(5) (2022) 851-863. https://doi.org/10.5599/jese.1247 [5] s. sivaranjan, a. joshi, k. c. palani, r. padmanabhan, j. t. stokes, corrosion and wear protection of aisi 4140 carbon steel using a laser-modified high-velocity oxygen fuel thermal sprayed coatings, journal of electrochemical science and engineering 12(5) (2022) 865-876. https://doi.org/10.5599/jese.1320 [6] s. kumar, r. bhatia, h. singh, r. l. virdi, microstructural and mechanical properties of cntreinforced zro2-y2o3 coated boiler tube steel t-91 journal of electrochemical science and engineering 12(5) (2022) 877-888. https://doi.org/10.5599/jese.1228 [7] b. malvi, m. roy, elevated temperature erosion of abradable seal coating journal of electrochemical science and engineering 12(5) (2022) 889-899. https://doi.org/10.5599/jese.1388. [8] s. singh, k. goyal, r. bhatia, effect of nano yttria-stabilized zirconia on properties of ni-20cr composite coatings, journal of electrochemical science and engineering 12(5) (2022) 901909. https://doi.org/10.5599/jese.1319 [9] s. singh, p. kumar, d.k. goyal, comparison of erosion performance of uncladded and wcbased laser cladded ss304 and ss410 steels, journal of electrochemical science and engineering 12(5) (2022) 911-922. https://doi.org/10.5599/jese.1333 [10] y. y. özbek, a. s. demirkiran, the effect of process parameters on microstructure and corrosion behavior of aisi 4140 steel modified by pulse plasma treatment, journal of electrochemical science and engineering 12(5) (2022) 935-935. https://doi.org/10.5599/jese.1259 https://doi.org/10.5599/jese.1278 https://doi.org/10.5599/jese.1311 https://doi.org/10.5599/jese.1270 https://doi.org/10.5599/jese.1247 https://doi.org/10.5599/jese.1320 https://doi.org/10.5599/jese.1228 https://doi.org/10.5599/jese.1388 https://doi.org/10.5599/jese.1319 https://doi.org/10.5599/jese.1333 https://doi.org/10.5599/jese.1259 editorial j. electrochem. sci. eng. 12(5) (2022) 815-817 http://dx.doi.org/10.5599/jese.1541 817 [11] r. goyal, k. goyal, development of cnt reinforced al2o3-tio2 coatings for boiler tubes to improve hot corrosion resistance, journal of electrochemical science and engineering 12(5) (2022) 937-945. https://doi.org/10.5599/jese.1291 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1541 https://doi.org/10.5599/jese.1291 https://creativecommons.org/licenses/by/4.0/) electrochemical detection of folic acid using a modified screen printed electrode: http://dx.doi.org/10.5599/jese.1360 1111 j. electrochem. sci. eng. 12(6) (2022) 1111-1120; http://dx.doi.org/10.5599/jese.1360 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical detection of folic acid using a modified screen printed electrode sayed zia mohammadi1, farideh mousazadeh2,  and maryam mohammadhasani-pour1 1department of chemistry, payame nour university, tehran, iran 2school of medicine, bam university of medical sciences , bam, iran corresponding author: faridehmousazadeh1398@gmail.com received: april 27, 2022; accepted: june 4, 2022; published: july 27, 2022 abstract in this work, an electrochemical sensor was established for the detection of folic acid based on ni-btc (btc = benzene-1,3,5-tricarboxylic acid) metal-organic framework (mof) modified screen-printed electrode (spe). electrochemical techniques: cyclic voltammetry (cv), differential pulse voltammetry (dpv), linear sweep voltammetry (lsv) and chronoamperemetry (cha) were used for the detection of folic acid at ni-btc mof modified spe. the results indicate that the asprepared sensor has a good electrocatalytic effect on the detection of folic acid. this electrochemical sensor showed a dynamic linear response range from 0.08 to 635.0 µm and the detection limit was estimated to be 0.03±0.001 µm. moreover, the feasibility of ni-btc mof/spe sensor to detect folic acid in real samples was also evaluated by the standard addition method. keywords electrochemical sensor; ni-btc metal-organic framework, voltammetry introduction folic acid is the synthetic form of folate, vitamin b9, found in vitamin tablets and fortified foods, which acts as a coenzyme in one-carbon transfer reactions in the various human metabolic pathways [1,2]. perhaps the most important role of folic acid is purine and pyrimidine nucleotide biosynthesis, necessary for the synthesis and replication of deoxyribonucleic acid (dna) and thus differrentiation of cells. it is also essential for the synthesis of methionine from homocysteine [3]. promotion of the formation of red blood cells and thus prevention of anemia is another advantageous role of folic acid [4]. folic acid has an important effect on the normal growth and development of the fetus, preventing congenital malformations, specifically spina bifida and anencephaly, referred to as neural tube defects (ntds) [5]. its deficiency may carry potential risks such as the development of cancer, megaloblastic anemia, cardiovascular disease, alzheimer’s disease, and psychiatric disorders [6-10]. in this sense, monitoring folic acid and its metabolites is highly desirable. different analytical procedures are currently available for the determination of folic acid, as high-performance liquid http://dx.doi.org/10.5599/jese.1360 http://dx.doi.org/10.5599/jese.1360 http://www.jese-online.org/ mailto:faridehmousazadeh1398@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1111-1120 detection of folic acid using a modified spe 1112 chromatography (hplc) [11], ultra-violet (uv) spectrophotometric [12], flow injection luminescence [13] and capillary electrophoresis [14]. these methods are time-consuming, expensive and require complicated pre-treatment steps. on the other hand, the electrochemical methods are a very promising way for the determination of folic acid and other compounds due to easy fabrication, low cost and rapid analysis compared to other analytical methods [15-30]. since the chemical analysis has recently focused on the design of portable, disposable, and lowcost instruments, screen-printed electrodes (spes) have become a quite widespread substitute for classic electrodes in electrochemical analytical measurements [31, 32]. spe based on threeelectrode systems is the best option to develop an electrochemical sensor with high sensitivity and selectivity [33-35]. in recent years, modified electrodes have had many applications in various fields, including sensing, electrocatalysis, fuel cells, batteries and supercapacitors [36-48]. chemical modification of electrodes with various materials such as nanomaterials, single molecular, multi-molecular, ionic, and polymeric components provides electrocatalytic properties in terms of increasing active surface area and accelerating electron transfer kinetics, which is beneficial for electrochemical sensing applications [49-61]. metal-organic frameworks (mofs) constructed by metal ions as nodes and organic ligands as linkers are a new class of nano-sized polymeric and crystalline material. due to their high porosity, structural diversity and open metal sites, mofs possess tunable functionalities, adsorption affinity, and high surface area [62-69]. mofs were demonstrated as a novel material to modify the electrode for the electrochemical application because of the electrochemical activity of the metal ions and the well-ordered porous skeleton [70-72]. nickel-based mofs such as ni (btc) have been broadly used in different applications due to the low cost and natural abundance of nickel [73]. in the present work, an electrochemical sensor with high sensitivity for measuring folic acid was developed based on the ni-btc modified spe. compared with unmodified spe, the ni-btc modified spe exhibited excellent electrochemical activity for the determination of folic acid. this sensor has been employed to measure folic acid in various real samples. experimental apparatus and chemicals all electrochemical measurements were performed on a pgstat302n potentiostat/galvanostat autolab and controlled with the general purpose electrochemical system (gpes) software. all experiments were carried out within a conventional three-electrode cell. the spes were purchased from dropsens (spain) and consisted of a graphite working electrode, graphite counter electrode and ag pseudo reference electrode. solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). folic acid and other chemicals used were analytical grade and were purchased from merck. synthesis of ni-btc mof for the synthesis of ni-btc mof, 1.10 g of ni(no3)2·6h2o and 0.44 g of btc were added to methanol. the prepared mixture was stirred for 90 min at room temperature and then transferred to a teflon-lined stainless steel autoclave and heated at 150 °c for 24 h. after the autoclave reached room temperature, the precipitate was separated by centrifuge and washed several times using methanol. finally, the collected product (ni-btc mof) was dried in an oven at 70 °c for 10h. the field emission-scanning electron microscopy (fe-sem) image of the ni-btc mof is shown in figure 1. s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1111-1120 http://dx.doi.org/10.5599/jese.1360 1113 figure 1. fe-sem image of ni-btc mof preparation of ni-btc mof/spe for modification of spe by ni-btc mof, 1 mg of ni-btc was dispersed in 1 ml of deionized water using an ultra-sonication bath for 20 min to get a homogeneous solution of 1 mg 1 ml-1 of ni-btc. then, 3 µl of ni-btc mof suspension was drop cast on the surface of unmodified spe and dried at room temperature to achieve ni-btc mof/spe. the surface area of ni-btc mof/spe and the bare spe were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula for ni-btc mof/spe, the electrode surface was found to be 0.116 cm2 which was about 3.8 times greater than bare cpe. results and discussion electrochemical response of folic acid at different electrodes the effect ph value of electrolyte solution was investigated by dpv in 0.1 m phosphate buffer solution (pbs) at the ph range from 2.0 to 9.0 containing 50.0 μm folic acid on the ni-btc mof/spe surface. the oxidation peak current of folic acid reached a maximum value at ph 7.0, and therefore pbs with ph 7.0 was chosen as the optimum ph to detect folic acid. figure 2 displays the cyclic voltammograms of the folic acid at unmodified spe (curve a) and nibtc mof/spe (curve b), with the same concentration of 200.0 μm in 0.1 m pbs (ph 7.0). the anodic peak potential for the oxidation of folic acid at ni-btc mof/spe (curve b) is about 590 mv compared with 740 mv, for that on the unmodified spge (curve a). similarly, when the oxidation of folic acid at the ni-btc mof/spe (curve b) and unmodified spe (curve a) are compared, an extensive enhancement of the anodic peak current at ni-btc mof/spe, relative to the value obtained at the unmodified spe (curve b), is observed. in other words, the results clearly indicate that the ni-btc mof improves folic acid oxidation. http://dx.doi.org/10.5599/jese.1360 j. electrochem. sci. eng. 12(6) (2022) 1111-1120 detection of folic acid using a modified spe 1114 figure 2. cv curves of unmodified spe (curve a) and ni-btc mof/spe (curve b) in 0.1 m pbs containing 200.0 μm folic acid; scan rate: 50 mv s-1. influence of the scan rate on the results the voltammograms obtained using lsv were recorded at various scan rates to see the differences in the peak potential and current in 0.1 m pbs at ph 7.0 (100.0 μm folic acid) (figure 3a). the scan rates were changed in the interval  = 10-300 mv s-1. the voltammograms show that, with increasing scan rate, the peak current permanently increases, and the peak potential moves to more positive values. as can be seen in figure 3b, the equation between the peak current and the square root of the scan rate for oxidation peak is given by equation (1): ipa (folic acid) = 1.2055  ½ 1.9976 (r2 = 0.9996) (1) the linearity of ipa vs.  ½ graphs specified that the reaction is diffusion-controlled process. figure 3. a) lsv curves of 100.0 μm folic acid in 0.1 m pbs (ph 7.0) at a scan rate of 10 to 300 mv s-1 at ni-btc mof/spe (1-6 refers to 10, 30, 70, 100, 200, and 300 mv s-1). b) plot of the square root of the scan rate vs. the oxidation peak current of folic acid in order to obtain some information on the rate-determining step, we drew a tafel plot (figure 4b) using the data from the rising part of the current-voltage curve recorded at a low scan rate of 10 mv s-1 y = 1.2055x – 1.9976 r2 = 0.9996 s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1111-1120 http://dx.doi.org/10.5599/jese.1360 1115 (figure 4a) for 100.0 μm folic acid. the linearity of the e versus log i plot implies the intervention of the kinetics of the electrode process. the slope of this plot can be used to estimate the number of electrons transferred in the rate-determining step. according to figure 4b, the tafel slope for the linear part of the plot was estimated to be equal to 0.1598 v. the value of the tafel slope indicates that one-electron transfer process is the rate-limiting step, assuming a transfer coefficient () of about 0.63. figure 4. a) lsv response for 100.0 μm folic acid with 10 mvs-1 scan rate; b) tafel plot derived from the rising part or the corresponding voltammogram chronoamperometric analysis chronoamperometric measurements of folic acid at ni-btc mof/spe were carried out by setting the working electrode potential at 0.65 v for the various concentrations of folic acid in 0.1 m pbs (ph 7.0) (figure 5a). figure 5. a) the chronoamperograms obtained at ni-btc mof/spe in 0.1 m pbs at ph of 7.0 for different concentrations of folic acid (1-5 refers to: 0.1, 0.3, 0.5, 0.75, and 1.0 mm). b) the i plot versus t-1/2 observed by chronoamperograms 1-5. c) the slope plot of the straight line vs. concentration of folic acid y = 0.1598x + 0.4616 r2 = 0.9995 http://dx.doi.org/10.5599/jese.1360 j. electrochem. sci. eng. 12(6) (2022) 1111-1120 detection of folic acid using a modified spe 1116 for an electroactive material (folic acid in this case) with a diffusion coefficient d, the current observed for the electrochemical reaction at the mass transport limited condition is described by the cottrell equation [74]. experimental plots of i vs. t−1/2 were employed, with the best fits for different concentrations of folic acid (figure 5b). the slopes of the resulting straight lines were then plotted vs. folic acid concentration (figure 5c). from the resulting slope and cottrell equation, the mean value of the d was found to be 1.85×10-5 cm2 s-1. calibration curve, linear range and detection limit figure 6a shows the electrochemical detection of folic acid by using dpv at various concentrations using ni-btc mof/spe (step potential = 0.01 v and pulse amplitude = 0.025 v). as seen in figure 6b, the oxidation peak current of folic acid linear with a folic acid concentration in the concentration range of 0.08 – 635.0 µm. the regression equation of the calibration graph is given by eq. (2): ipa (folic acid) = 0.0529 cfolic acid + 1.4675 (r2 = 0.9996) (2) the limit of detection value was calculated to be 0.03±0.001 μm. it is better than some recent report in determination of folic acid using spe [75]. figure 6. dpv responses of folic acid on ni-btc mof/spe at different folic acid concentrations (1-13 refer to: 0.08, 1.0, 7.5, 15.0, 50.0, 75.0, 100.0, 200.0, 300.0, 400.0, 500.0, 600.0, and 635.0 μm) in 0.1 m pbs (ph 7.0). inset: the relationship between the oxidation peak currents and folic acid concentration analytical application the determination of folic acid in real samples such as folic acid tablets and urine was performed using ni-btc mof/spe sensor. the concentration values of folic acid were calculated by the standard addition method. the results are summarized in table 1, the recovery is between 96.0 and 102.7 %, and the relative standard deviations (rsds) are all less than or equal to 3.3 %. the experimental results confirmed that the ni-btc mof/spe sensor has a great potential for analytical application. y = 0.0529x + 1.4675 r2 = 0.9996 s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1111-1120 http://dx.doi.org/10.5599/jese.1360 1117 table 1. determining folic acid in real samples by using ni-btc mof/spe (n=5) sample concentration, μm recovery, % rsd, % spiked found folic acid tablet 0 4.0 3.3 2.5 6.4 98.5 1.9 3.5 7.7 102.7 2.4 urine 0 5.0 5.1 102.0 2.2 7.5 7.2 96.0 2.9 conclusion in this work, we developed a ni-btc mof modified spe as an electrochemical sensing platform for the detection of folic acid. according to voltammetric results, the prepared sensor showed an excellent performance toward folic acid oxidation. under an optimized condition, the prepared electrochemical sensor displayed a low detection limit of 0.03±0.001 μm and a broad linear range of 0.08 to 635.0 μm for folic acid. furthermore, the ni-btc mof/spe was also tested for its ability to detect folic acid in real samples giving excellent recoveries. 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https://doi.org/10.1016/j.microc.2022.107261 https://doi.org/10.1016/j.microc.2022.107261 https://doi.org/10.1016/j.mssp.2022.106500 https://doi.org/10.33945/sami/ecc.2020.5.7 https://creativecommons.org/licenses/by/4.0/) @article{mohammadi2022, author = {mohammadi, sayed zia and mousazadeh, farideh and mohammadhasani-pour, maryam}, journal = {journal of electrochemical science and engineering}, title = {{electrochemical detection of folic acid using a modified screen printed electrode:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {6}, pages = {1111--1120}, volume = {12}, abstract = {in this work, an electrochemical sensor was established for the detection of folic acid based on ni-btc (btc = benzene-1,3,5-tricarboxylic acid) metal-organic framework (mof) modified screen-printed electrode (spe). electrochemical techniques: cyclic voltammetry (cv), differential pulse voltammetry (dpv), linear sweep voltammetry (lsv) and chrono­ampere­metry (cha) were used for the detection of folic acid at ni-btc mof modified spe. the results indicate that the as-prepared sensor has a good electrocatalytic effect on the detection of folic acid. this electro­chemical sensor showed a dynamic linear response range from 0.08 to 635.0 µm and the detec­tion limit was estimated to be 0.03±0.001 µm. moreover, the feasibility of ni-btc mof/spe sensor to detect folic acid in real samples was also evaluated by the standard addition method.}, doi = {10.5599/jese.1360}, file = {:d\:/onedrive/mendeley desktop/mohammadi, mousazadeh, mohammadhasani-pour 2022 electrochemical detection of folic acid using a modified screen printed electrode.pdf:pdf;:05_jese_1360_1111-1120.docx:word_new;:www/jese_v12_no6_1111-1120.pdf:pdf}, keywords = {btc metal, electrochemical sensor, ni, organic framework, voltammetry}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1360}, } an electrochemical sensor for detection of vanillin in food samples using cufe2o4 nanoparticles/ionic liquids modified carbon paste electrode: http://dx.doi.org/10.5599/jese.1395 1193 j. electrochem. sci. eng. 12(6) (2022) 1193-1203; http://dx.doi.org/10.5599/jese.1395 open access : : issn 1847-9286 www.jese-online.org original scientific paper an electrochemical sensor for detection of vanillin in food samples using cufe2o4 nanoparticles/ionic liquids modified carbon paste electrode afsaneh hajializadeh department of natural resources, sirjan branch, islamic azad university, sirjan, iran corresponding author: hajalizadeh.813@gmail.com received: january 1, 2022; accepted: may 24, 2022; published: september 8, 2022 abstract a highly selective electrochemical sensor modified with cufe2o4 nanoparticles and the ionic liquid was constructed for the detection of vanillin. the sensor could recognize vanillin from its analogs and possible coexistent substances. the response peak current and vanillin concentration showed a good linear relationship in the range of 0.01 300.0 μm, with a sensitivity of 0.0923 μa μm-1. the detection limit was 0.008 μm (s/n = 3). besides, the reproducibility and stability measurements were also evaluated. it was applied to the determination of vanillin in real samples with satisfactory results. keywords voltammetric sensor; cufe2o4 nanoparticles; vanillin; modified electrode introduction vanillin (c8h8o3), (4-hydroxy-3methoxybenzaldehyde) is a phenolic aldehyde derived from vanilla bean e bean or pod of the tropical vanilla plant that contains vitamin-b, potassium, calcium, thiamin, riboflavin, and iron [1,2]. vanillin has been an indispensable smell additive in most beverages and desserts such as candies, biscuits, cakes, pudding, chocolate, ice creams, and wine which attract a large number of people, especially at young ages. in mm range of concentration (more than 75 mg/body weight), vanillin has toxic effects and can lead to dangerous consequences to consumers because it can affect the liver and kidney functions, producing headaches, nausea and vomiting. due to the side effects of vanillin, it is forbidden in infant food [3-5]. therefore, the quantitative determination of vanillin in foods and beverages has significant importance, especially for growing children. several determination methods of vanillin were introduced, including chromatography, spectroscopy and capillary electrophoresis [6-12]. most of these methods provide very effective information for identification, quantification and selectivity, but they are time-consuming and complicated in sample pretreatment processes and thus require highly sophisticated instruments. in recent years, electroanalytical methods have been used determination of target species as an alternative analytical method, thanks to their low cost, fast response, simple operation, high http://dx.doi.org/10.5599/jese.1395 http://dx.doi.org/10.5599/jese.1395 http://www.jese-online.org/ mailto:hajalizadeh.813@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1193-1203 electrochemical sensor for detection of vanillin in food 1194 sensitivity and online detection ability [13-38]. in addition, because the vanillin molecules can be oxidized, the usage of electrochemical sensors is an ideal technique for the detection of vanillin. however, the determination of vanillin on the bare solid electrode is a relatively high overpotential with low reproducibility is a major problem. this is due to the contamination effect that causes poor selectivity and sensitivity. to overcome these obstacles, the modification of the electrode surface is an effective way [39-41]. electrochemical studies of the intrinsic properties of ionic liquids (ils) have also been reported and the electrochemical window of ionic liquids can be as wide as 4.5 v compared with 1.2 v in most aqueous electrolytes. properties such as non-flammability, high ionic conductivity, electrochemical and thermal stability of ionic liquids make them ideal electrolytes in electrochemical devices like in batteries, capacitors, fuel cells, photovoltaics, actuators and electrochemical sensors [42-46]. nanostructured materials have been the subject of scientific research because of their unique electrical, thermal, chemical and physical properties. nanomaterials and especially metal nanoparticles (nps) of a variety of shapes, sizes and compositions are changing nowadays in electrochemical measurement [47-62]. in this research, cufe2o4 nanoparticles (cufe2o4-nps) were prepared using co-precipitation reaction. the electrochemical behavior of vanillin at a modified and unmodified carbon paste electrode (cpe) was investigated. the obtained results show the advantage of cufe2o4-nps/ils/cpe to the bare carbon paste electrode in terms of better reversibility and higher sensitivity. the proposed voltammetric sensor is highly selective and sensitive enough for the determination of vanillin in the real samples. experimental apparatus and chemicals all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab consisting of a traditional three-electrode system: a bare or modified cpe as the working electrode, an ag/agcl as the reference electrode and a pt wire as a counter electrode. solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). vanillin and other chemicals used were analytical grade and were purchased from merck. ionic liquid (n-hexyl-3-methylimidazolium hexafluoro phosphate) was purchased from sigma aldrich co. cufe2o4 nanoparticles were synthesized in our laboratory and a typical sem is shown in figure 1. figure 1. sem image of cufe2o4 nanoparticles a. hajializadeh j. electrochem. sci. eng. 12(6) (2022) 1193-1203 http://dx.doi.org/10.5599/jese.1395 1195 preparation of cufe2o4-nps/ils/cpe cufe2o4-nps/ils/cpe was prepared by mixing 0.04 g of cufe2o4-nps with 0.96 g graphite powder and approximately 0.8 ml of ionic liquids with a mortar and pestle. the paste was then packed into the end of a glass tube (3.4 mm inner diameter and 15 cm long). a copper wire inserted into the carbon paste provided the electrical contact. for comparison, ionic liquids/carbon paste electrode (il/cpe) in the absence of cufe2o4-nps, (cufe2o4-nps/cpe) consistent of cufe2o4-nps powder and paraffin oil, and bare carbon paste electrode (cpe) consisting of graphite powder and paraffin oil were also prepared in the same way. the surface areas of the cufe2o4-nps/ils/cpe and the unmodified spe were obtained by cyclic voltammetry using 1 mm k3fe(cn)6 at various scan rates. using the randles–sevcik equation for cufe2o4-nps/ils/cpe, the electrode surface was found to be 0.373 cm2 which was about 4.14 times greater than un-modified spe. results and discussion electrochemical behavior of vanillin at the surface of various electrodes the mechanism of the vanillin oxidation on cufe2o4-nps/ils/cpe is based on the relationship between the oxidation potential and ph of the supporting electrolyte. the effect of the electrolyte ph on the oxidation of 40.0 μm vanillin was investigated at cufe2o4-nps/ils/cpe using differential pulse voltammetry (dpv) measurements in the phosphate buffer solution (pbs) in the ph range from 2.0 to 9.0. according to the results, the oxidation peak current of vanillin depends on the ph value and increases with increasing ph until it reaches the maximum at ph 7.0, and then decreases with higher ph values. the optimized ph corresponding to the higher peak current was 7.0, indicating that protons are involved in the reaction of vanillin oxidation. the electrochemical behavior of vanillin was investigated by differential linear sweep voltammetry (lsv). the differential pulse voltammograms obtained using the bare cpe (curve d), ils/cpe (curve c), cufe2o4-nps/cpe (curve b) and cufe2o4-nps/ils/cpe (curve a) in 0.1 m pbs (ph 7.0) in the presence of 65.0 μm vanillin are shown in figure 2. figure 2. the linear sweep voltammograms of (a) cufe2o4-nps/ils/cpe, (b) cufe2o4-nps/cpe, (c) ils/cpe and (d) bare cpe in 0.1 m pbs (ph 7.0) in the presence of 150.0 μm vanillin at the scan rate 50 mvs-1 http://dx.doi.org/10.5599/jese.1395 j. electrochem. sci. eng. 12(6) (2022) 1193-1203 electrochemical sensor for detection of vanillin in food 1196 on a bare cpe, an irreversible signal with a low oxidation current of 2.9 μa was obtained with a peak potential of 680 mv. after modifying the electrode with ils/cpe, and cufe2o4-nps/cpe, the peak current increased up to 6.5 and 10.6 μa, respectively. in contrast, cufe2o4-nps/ils/cpe exhibited an enhanced sharp anodic peak current (ipa = 14.9 μa) at a much lower overpotential ep = 550 mv. these results confirmed that the cufe2o4-nps/ils improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of vanillin. effect of scan rate on the determination of vanillin at cufe2o4-nps/ils/cpe the influence of the scan rate () on the peak currents (ipa) of vanillin atcufe2o4-nps/ils/cpe was investigated by lsv. figure 3 shows the voltammetric response of 150.0 μm vanillin at cufe2o4-nps/ils/cpe at different scan rates in the range of 10 to 300 mv/s. the oxidation peak current of vanillin increases linearly with increasing scan rate. linear regression equation was obtained from the plot ipa vs. 1/2) as follows: ipa / μa = 1.2723 1/2 / mv1/2 s-1/2 + 5.9454 (r2 = 0.9998) for the oxidation process, which indicates that the reaction of vanillin at cufe2o4-nps/ils/cpe is diffusion controlled. figure 3. linear sweep voltammograms of cufe2o4-nps/ils/cpe in 0.1 m pbs (ph 7.0) containing 150.0 μm vanillin at various scan rates; 1-5 correspond to 10, 50, 100, 200 and 300 mv s-1, respectively. inset: variation of anodic peak current vs. 1/2 a. hajializadeh j. electrochem. sci. eng. 12(6) (2022) 1193-1203 http://dx.doi.org/10.5599/jese.1395 1197 in addition, the plot of log i vs. e (tafel plot) is linear, having the following regression equation: log i = 0.2429 e+0.1997 (r2 = 0.9988) (figure 4). assuming that nα=1, the value of α was calculated as 0.76. figure 4. tafel plot for cufe2o4-nps/ils/cpe in 0.1 m pbs (ph 7.0) in the presence of 150.0 µm vanillin and scan rate 10 mv s−1 chronoamperometric analysis the analysis of chronoamperometry for vanillin samples was performed by use ofcufe2o4 -nps/ils/cpe vs. ag/agcl/kcl (3.0 m) at 0.6 v. chronoamperometric results of different concentrations of vanillin in pbs (ph 7.0) are demonstrated in figure 5. http://dx.doi.org/10.5599/jese.1395 j. electrochem. sci. eng. 12(6) (2022) 1193-1203 electrochemical sensor for detection of vanillin in food 1198 figure 5. chronoamperograms obtained at cufe2o4-nps/ils/cpe in 0.1 m pbs (ph 7.0) for different concentrations of vanillin; 1-4 correspond to 0.1, 0.45, 0.75 and 1.2 mm of vanillin. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-4. (b) plot of the slope of the straight lines against vanillin concentration the cottrell equation for the chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is as in equation (1): i = nfad1/2cbπ-1/2t-1/2 (1) where d represents the diffusion coefficient (cm2 s-1), and cb is the applied bulk concentration (mol cm-3). experimental results of i vs. t-1/2 were plotted in figure 5a, with the best fits for different concentrations of vanillin. the resulted slopes corresponding to straight lines in figure 5a, were then plotted against the concentration of vanillin (figure 5b). the mean value of d was determined to be 2.3×10-5 cm2 s-1 according to the resulting slope and cottrell equation. calibration curve because dpv commonly has a higher sensitivity than cyclic voltammetry technology, the dpv technique was applied for the quantitative detection of vanillin. figure 6 shows the differential pulse voltammograms of vanillin at various concentrations using cufe2o4-nps/ils/cpe (step potential of 0.01 v and pulse amplitude of 0.025 v). as seen, the oxidation peak currents of vanillin enhance gradually by increasing its concentration. the oxidation peak currents (ipa) show a good linear relationship with the concentrations of vanillin ranging from 0.01 m to 300.0 μm. the linear equation is ipa / μa = 0.0923cvanillin / μm + 1.3481 (r2 = 0.9993) (figure 6 (inset)). also, the limit of detection, cm, of vanillin was calculated using the equation (2): cm=3sb/m (2) where m is the slope of the calibration plot (0.0923 μa / μm) and sb is the standard deviation of the blank response obtained from 15 replicate measurements of the blank solution. the detection limit for the determination of vanillin using this method 0.008 μm was obtained. a. hajializadeh j. electrochem. sci. eng. 12(6) (2022) 1193-1203 http://dx.doi.org/10.5599/jese.1395 1199 figure 6. dpvs of cufe2o4-nps/ils/cpe in 0.1 m (ph 7.0) containing different concentrations of vanillin. 1–11 correspond to 0.01, 1.0, 5.0, 15.0, 35.0, 65.0, 90.0, 110.0, 150.0, 200.0 and 300.0 µm of vanillin. inset: plot of the electrocatalytic peak current as a function of cvanillin in the range of 0.01-300.0 µm. the performance of this sensor is compared with some of the recently reported electrodes for vanillin quantification (see table 1) [63-66]. table 1. comparison of the sensing performances toward the detection of vanillin between the existing modified electrodes and the proposed cufe2o4-nps/ils/cpe electrochemical sensor method linear range lod ref. anodically pre-treated boron-doped diamond electrode square-wave voltammetry 3.3-9.8 μm 0.167 μm [63] nitrogen-doped graphene/carbon nanotubes/glassy carbon electrode square-wave voltammetry 0.01-10.0 μm 0.0033 μm [64] cationic cetylpyridium bromide /carbon nanofibers/ glassy carbon electrode dpv 0.5-750.0 μmol dm-3 0.14 μmol dm-3 [65] cellulose diacetate/au–ag nanoparticles/ glassy carbon electrode amperometry 0.2-50.0 μmol dm-3 0.04 μmol dm-3 [66] cufe2o4-nps/ils/cpe dpv 0.01–300.0 μm 0.008 μm this work stability and reproducibility of the modified electrode the long-term stability test of the cufe2o4-nps/ils/cpe sensor using dpv was performed at room temperature. the results exhibited that the peak current of 40.0 μm vanillin at the cufe2o4nps/ils/cpe stayed at 94.9 % of its primary current after 7 days, 92.4 % after 14 days, and 90.7 % after 21 days, indicating the superior long-term stability of the proposed sensor. for reproducibility, cufe2o4-nps/ils/cpe was stored for 7, 14, and 21 days at room temperature. voltammetry measurements were performed in pbs (0.1 m, ph 7.0) containing 40.0 µm vanillin. http://dx.doi.org/10.5599/jese.1395 j. electrochem. sci. eng. 12(6) (2022) 1193-1203 electrochemical sensor for detection of vanillin in food 1200 the results showed that cufe2o4-nps/ils/cpe response did not change with time. hence, the developed electrode provides good reproducibility for the determination of vanillin. analysis of real samples the real samples for the analysis were prepared and quantified by dpv method. the developed sensor was applied to detect vanillin in chocolate and coffee milk samples. the results are summarized in table 2. each measurement was repeated five times. the recovery and relative standard deviation (rsd) values confirmed that the cufe2o4-nps/ils/cpe sensor has great potential for analytical application. table 2. application of cufe2o4-nps/ils/cpe for determination of vanillin in real samples (n = 3) sample concentration, μm recovery, % rsd, % spiked found chocolate 0.0 2.7 3.2 2.0 4.6 97.9 1.9 4.0 6.9 102.9 2.8 coffee milk 0.0 4.0 1.8 1.0 5.1 102.0 2.3 3.0 6.8 97.1 3.3 conclusion a sensitive electrochemical sensor for the detection of vanillin was developed based on cufe2o4 nanoparticles and ionic liquid. cufe2o4 nanoparticles can provide a larger specific surface and excellent electrical conductivity for the sensor. this sensitive electrochemical sensor 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creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1395 https://doi.org/10.1007/s11694-021-01201-4 https://doi.org/10.1016/j.bios.2016.07.086 https://doi.org/10.22034/chemm.2020.113657 https://doi.org/10.22034/chemm.2020.113388 https://doi.org/10.1016/j.fct.2022.112864 https://doi.org/10.33945/sami/chemm.2020.1.1 https://doi.org/10.1016/j.fct.2022.112961 https://dx.doi.org/10.22034/ajca.2020.252025.1215 https://doi.org/10.22034/chemm.2020.109975 https://doi.org/10.1039/c8nj05581e https://doi.org/10.1016/j.foodchem.2013.04.085 https://doi.org/10.1016/j.aca.2014.05.010 https://doi.org/10.1007/s00706-015-1559-8 https://doi.org/10.1016/j.snb.2010.04.031 https://creativecommons.org/licenses/by/4.0/) @article{hajializadeh2022, author = {hajializadeh, afsaneh}, journal = {journal of electrochemical science and engineering}, title = {{an electrochemical sensor for detection of vanillin in food samples using cufe2o4 nanoparticles/ionic liquids modified carbon paste electrode:}}, year = {2022}, issn = {1847-9286}, month = {sep}, number = {6}, pages = {1193--1203}, volume = {12}, abstract = {a highly selective electrochemical sensor modified with cufe2o4 nanoparticles and the ionic liquid was constructed for the detection of vanillin. the sensor could recognize vanillin from its analogs and possible coexistent substances. the response peak current and vanillin concen­tration showed a good linear relationship in the range of 0.01 300.0 $\mu$m, with a sensitivity of 0.0923 $\mu$a $\mu$m-1. the detection limit was 0.008 $\mu$m (s/n = 3). besides, the reproducibility and stability measurements were also evaluated. it was applied to the determination of vanillin in real samples with satisfactory results.}, doi = {10.5599/jese.1395}, file = {:d\:/onedrive/mendeley desktop/hajializadeh 2022 an electrochemical sensor for detection of vanillin in food samples using cufe2o4 nanoparticlesionic liquids modif.pdf:pdf;:www/jese_v12_no6_1193-1203.pdf:pdf;:www/jese_v12_no6_1193-1203.pdf:pdf;:www/jese_v12_no6_1193-1203.pdf:pdf}, keywords = {cufe2o4 nanoparticles, voltammetric sensor, modified electrode, vanillin}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1395}, } feasibility study of amperometric and electrochemical quartz crystal microbalance measurements for in situ state of charge monitoring in vanadium flow batteries http://dx.doi.org/10.5599/jese.1699 739 j. electrochem. sci. eng. 13(5) (2023) 739-751; http://dx.doi.org/10.5599/jese.1699 open access : : issn 1847-9286 www.jese-online.org short communication feasibility study of amperometric and electrochemical quartz crystal microbalance measurements for in situ state of charge monitoring in vanadium flow batteries claudia weidlich1,, felix lulay1,2, matthias wieland1 1dechema-forschungsinstitut, theodor-heuss-allee 25, 60486 frankfurt, germany 2centre for electrochemical and surface technology, viktor-kaplan-strasse 2, 2700 wr. neustadt, austria corresponding author: claudia.weidlich@dechema.de; tel.: +49 69 7564 633; fax: +49 69 7564 388 received: february 13, 2023; accepted: may 14, 2023; published: may 31, 2023 abstract for an efficient flow battery operation, knowledge of the state of charge of the battery is essential. monitoring the state of charge of both half cells is advantageous concerning a timely detection of crossover processes. we present the first results for amperometric and electrochemical quartz crystal microbalance measurements in a vanadium flow battery test setup. by validating with half cell potential measurements as well as ex situ titration we investigated the applicability of both electrochemical methods for an in situ half cell state of charge monitoring. keywords crossover; half cell; electrolyte properties introduction to overcome the challenges to mitigate climate change, the storage of renewable energies is essential. flow batteries (fb) are most suitable to store wind or solar energy due to the characteristic independent scalability of energy and capacity [1,2]. of all fb chemistries, all-vanadium flow batteries (vfb) are most common and commercially prevailed. they contain vanadium electrolyte in the negative half cell (nhc) as well as the positive half cell (phc) [3]. the charging (from left to right) and discharging (from right to left) processes of the nhc and phc are characterised by reactions (1) and (2): v3+ + e– ⇄ v2+ (nhc) (1) vo2+ + h2o ⇄ vo2+ + e– + 2h+ (phc) (2) during charging, v(iii) ions are reduced to v(ii) in the nhc while the oxidation of v(iv) ions to v(v) occurs in the phc. during discharging v(iii) ions and v(iv) ions are formed in the nhc and phc, respectively. the crossing of vanadium species through the ion exchange membrane, which separates http://dx.doi.org/10.5599/jese.1699 http://dx.doi.org/10.5599/jese.1699 http://www.jese-online.org/ mailto:claudia.weidlich@dechema.de j. electrochem. sci. eng. 13(5) (2023) 739-751 charge monitoring in vanadium flow batteries 740 the phc from the nhc, does not lead to a contamination of the half cell electrolytes since both half cells contain vanadium. however, the battery half cells are suffering from self discharge. furthermore, the half cell electrolytes may undergo other reversible and also irreversible changes as e.g. concentration changes due to osmotic processes, overoxidation and electrolysis of the electrolyte [4-8]. therefore, knowledge of the state of charge (soc) of the battery is necessary for an efficient operation of the vfb. in commercial vfb the so called reference cell monitoring is widely used [4-9] but this monitoring method is limited to the determination of the soc of the full cell electrolyte and does not give information about electrolyte crossover and self discharge of the battery half cells. monitoring soc of each half cell separately is much more suitable than reference cell monitoring for a timely detection of crossover and other processes which alter the half cell electrolytes. since, next to oxidation state of the electroactive vanadium species, several electrolyte properties such as concentration, conductivity, ph, density, viscosity and absorbance may change during charge and discharge, numerous measurement methods are suitable [1] for an in situ detection of these changes. monitoring the redox potential of the half cells using reference electrodes [4,5,9-11] as well as using uv/vis spectroscopy [12-15] to monitor the oxidation state and concentration of the vanadium ions the half cell electrolytes have been widely investigated. in addition to that, density measurements [10], viscosity measurements [16] and distribution of relaxation times (drt) analysis applied on electrochemical impedance spectroscopy (eis) data [17] have been implemented to investigate the vfb electrolyte in situ. amperometric measurements might be suitable for a novel in situ half cell monitoring in vfb. this method has already been studied for ex situ and in situ monitoring using microelectrodes in organic fb [3,18-20]. various potentials have been applied and the ratio of resulting oxidation and reduction currents has been plotted vs. the ratio of the concentration of the oxidized and reduced species in the electrolyte. an outstanding advantage of this approach is the independence on temperature, electrolyte composition and concentration [18]. for vfb, the suitability of amperometric measurements using working electrodes coated with porous diffusion layers for an ex situ determination of the soc, has been shown [2] and also an electro catalytic metal electrode for estimating the soc of the positive electrolyte has been patented [21]. also, the electrochemical quartz crystal microbalance (eqcm) might be suitable for soc monitoring. an eqcm consists of a quartz crystal in contact with electrodes on both sides of the quartz which stimulate oscillation. the electrode facing the electrolyte is also contacted as working electrode to a potentiostat. simultaneously with the changes in oscillation frequency of the quartz crystal, electrochemical reactions can be analysed at the electrode [22]. eqcm has been already used for density and viscosity measurements [23,24] and battery monitoring in general [25] as well as for measuring density and viscosity in lead acid batteries in particular [26]. eqcm therefore provides the possibility to investigate two different electrolyte parameters as redox potential and viscosity at the same time within the same setup [25]. we are investigating these two different electrochemical methods according to their feasibility for soc monitoring in vfb. experimental the battery test setup for this study contains the battery cell and a tank for each battery half cell. the electrolyte is pumped with 1.5 – 2.1 l/h electrolyte flow using a membrane pump (prominent, delta opto drive). the battery cell with an electrode area of 10 cm2 has been manufactured by the machine shop at the dechema-forschungsinstitut and is equipped with a f10100 membrane c. weidlich et al. j. electrochem. sci. eng. 13(5) (2023) 739-751 http://dx.doi.org/10.5599/jese.1699 741 (fumatech bwt gmbh), gfd 4.6 ea electrode felts (sgl carbon) and sp4369 bipolar plates (schunk kohlenstofftechnik gmbh). commercial vanadium electrolyte (gfe metalle und materialien gmbh) with a vanadium concentration of 1.6 m and acid concentration of 2 m is used. for charge and discharge experiments volumes between 60 and 150 ml electrolyte are filled in each tank. the battery cycling is performed under argon with charge/discharge currents of 50 or 100 ma cm-2 and voltage criteria of +1.65v (charged) and +0.8 v (discharged) using a gamry reference 3000ae potentiostat/galvanostat. the components and parameters described have been utilized both for the amperometric and eqcm measurements. additionally, each half cell electrolyte circuit is equipped with a flow cell to measure open circuit potential (ocp) of the half cell electrolyte. both flow cells are connected by an electrolyte bypass to the battery test setup and the electrolyte is pumped through the bypass using a ism796 peristaltic pump (ismatec). a glassy carbon (gc) rod with 2 mm diameter (sigradur d, hochtemperatur-werkstoffe gmbh) and a hg/hg2so4 reference electrode (xylem analytics germany sales gmbh & co. kg) in a luggin capillary (filled with 2 m h2so4) are mounted in each flow cell to measure ocp. figure 1 provides a scheme for the test set up with cells for ocp (insertion in the middle), amperometric (insertion right) and eqcm (insertion left) measurements. the electrochemical cells used for amperometric and eqcm measurements are described below. figure 1. scheme for vfb test setup with electrochemical cells for in situ amperometric measurements in the positive half cell (phc) and in situ electrochemical quartz crystal microbalance (eqcm) measurements in the negative half cell (nhc). open circuit potential (ocp) measurements. cells equipped with electrodes: working (we), reference (re) and counter (ce) ex situ calibration of the mentioned monitoring methods has been done using potentiodynamic titration (titrando 888, deutsche metrohm gmbh & co. kg) with 0.1 m ce(so4)2 (carl roth gmbh & co. kg). as sensor, a platinum indicator electrode has been utilized for all measurements (combined ipt ring electrode, deutsche metrohm gmbh & co. kg). the equivalence points have been calculated with tiamo v2.5 (deutsche metrohm gmbh & co. kg). the concentration of v4+ has been determined by titration using ce(so4)2. afterwards feso4heptahydrate has been added until the potential dropped below 550 mv relative to the sensor phc positive half cell v2+/v3+ v 4+/v5+ nhc negative half cell bypass eqcm cell bypass amperometry cell bypass ocp cell bypass ocp cell pumps ce we rewe rece re we quartz http://dx.doi.org/10.5599/jese.1699 j. electrochem. sci. eng. 13(5) (2023) 739-751 charge monitoring in vanadium flow batteries 742 electrode to ensure that v5+ is reduced quantitatively. therewith, the total concentration of vanadium in the positive electroyte has been titrated using ce(so4)2. v5+ concentration has been calculated as the difference of the total vanadium concentration and v4+ concentration. the soc has been determined as the proportion of v5+ on the total vanadium concentration. amperometric measurements are tested (using a gamry reference 3000ae potentiostat/galvanostat) concerning their feasibility for soc monitoring at vfb and several setup configurations have been applied: a. ex situ three-electrode setup using a carbon microelectrode as working electrode (carbon fibre with 33µm diameter, mce micro carbon fiber electrode, als co., ltd.), hg/hg2so4 reference electrode (xylem analytics germany sales gmbh & co. kg) in a luggin capillary (filled with 2 m h2so4) and a glassy carbon rod counter electrode with 2 mm diameter (sigradur d, hochtemperatur-werkstoffe gmbh), all electrodes installed in a small glass cell (20 ml cell volume). the battery has been operated with 100 ml electrolyte in the nhc tank and 60 ml in the phc tank to ensure full charging and discharging of the phc electrolyte. during battery charge and discharge at several soc, 1 ml electrolyte has been withdrawn from the tank of the phc, added to the amperometric cell subsequently mixed with 2 ml ultrapure h2o. another 0.5 ml electrolyte has been withdrawn from the tank to determine the soc by titration (see paragraph above). for the camperometric measurement, a reduction potential of (-1.2 v vs. hg/hg2so4) has been applied to the microelectrode for 30 s and then an oxidation potential of (+1.0 v vs. hg/hg2so4) has been applied for 30 s. reduction and oxidation have been applied in an alternating manner several times (see fig. 4). all amperometric measurements have been conducted under argon protection. b. in situ three-electrode setup using a carbon microelectrode as working electrode (carbon fiber with 33 µm diameter, als co.,ltd.), hg/hg2so4 reference electrode (xylem analytics germany sales gmbh & co. kg) in a luggin capillary (filled with 2 m h2so4) and a glassy carbon rod as counter electrode with 2 mm diameter (sigradur d, hochtemperatur-werkstoffe gmbh). the amperometric cell has been connected to the electrolyte bypass with two hoses, one for filling and one for emptying the amperometric cell. in order to carry out the amperometric measurement, the cell has been filled up to a level which was constant for all measurements (7 ml) and convection has been avoided. a reduction potential of (-1.2 v vs. hg/hg2so4) has been applied for 30 s and an oxidation potential of (+1.0 v vs. hg/hg2so4) has been applied for 30 s. after the amperometric measurement, 0.5 ml electrolyte has been withdrawn from the cell for soc determination via titration. afterwards, the electrolyte in the amperometric cell has been pumped back through the bypass to the electrolyte circuit. this “stop and flow” methode for electrolyte sampling in the amperometric cell has also been used for setup c and d. c. in situ three-electrode setup using a glassy carbon rod as working electrode with 2 mm diameter (sigradur d, hochtemperatur-werkstoffe gmbh), hg/hg2so4 reference electrode (xylem analytics germany sales gmbh & co. kg) in a luggin capillary (filled with 2 m h2so4) and a glassy carbon rod counter electrode with 2 mm diameter (sigradur d, hochtemperatur-werkstoffe gmbh). the setup and the measurement are described in b. a reduction potential of (-0,7 v vs. hg/hg2so4) has been applied for 30 s and an oxidation potential of (+0.9 v vs. hg/hg2so4) has been applied for 30 s. d. in situ two-electrode setup using a glassy carbon rod as working electrode and a second glassy carbon rod counter electrode with 2 mm diameter (sigradur d, hochtemperatur-werkstoffe c. weidlich et al. j. electrochem. sci. eng. 13(5) (2023) 739-751 http://dx.doi.org/10.5599/jese.1699 743 gmbh). the setup and the measurement are described in b. a reduction potential of (-1.0 v) has been applied for 30 s and an oxidation potential of (+0.7 v) has been applied for 30 s. the electrochemical quartz crystal microbalance (eqcm) consists of a commercially available cell case (deutsche metrohm gmbh & co. kg) in which the quartz crystal is inserted. a carbon plate has been installed as a counter electrode (ce). the cell head has been manufactured to work as a flow cell. electrolyte inlet and outlet and the luggin capillary with hg/hg2so4 reference electrode (xylem analytics germany sales gmbh & co. kg) have been connected to the cell head. after assembling the cell, it has been connected to the electrolyte bypass circuit and filled from the bottom and emptied from the top in order to avoid accumulation of bubbles on the electrodes. the eqcm cell has been connected to the bypass electrolyte of the nhc and the oscillation frequency of the crystal and the potential between the carbon coated crystal and hg/hg2so4 re (ocp) have been monitored during battery cycling. for controlling the eqcm experiment, a pgstat302n potentiostat (deutsche metrohm gmbh & co. kg) and gold coated quartz crystals with a thin layer of carbon evaporated onto one side (6 mhz resonance frequency, renlux crystals ltd.) serving as working electrode in the eqcm cell have been utilised. carbon has been applied as coating material to mimic the carbon electrodes used in vfb. the battery cell has been cycled between 0.8 and 1.65 v with a current density of 50 ma cm-2 during the eqcm experiments. results and discussion the vfb in the test setup has been charged and discharged using the parameters described above. figure 2 shows exemplary operation of the battery cell respectively the resulting potentials during charging and discharging with 50 ma cm-2. the vfb is running stable, and the charge/discharge cycles are reproducible. since the battery has been operated with an excess of nhc electrolyte to ensure full charging and discharging of the phc electrolyte, a slightly increasing duration of the cycles can be observed which is due to crossover of nhc electrolyte through the cation exchange membrane and therefore capacity increase in the phc. figure 2. cycling of the vfb test setup (10 cm2 battery cell) with 50 ma cm-2 during cycling of the battery cell, the open circuit potential (ocp) has been monitored for both half cells using flow cells equipped each with a gc rod and a hg/hg2so4 reference electrode. figure 3 shows the soc calculated from ocp measured in the positive half cell (phc) (grey curve) in http://dx.doi.org/10.5599/jese.1699 j. electrochem. sci. eng. 13(5) (2023) 739-751 charge monitoring in vanadium flow batteries 744 comparison to the soc calculated from the titration of samples taken from phc electrolyte during the amperometric measurements. the soc calculated from in situ ocp measurements in the phc is in good accordance with the soc determined by titration (black crosses). figure 3. state of charge (soc) calculated from open circuit potential (ocp) measurements in the positive half cell (phc) and soc calculated from titration (black crosses) the titration samples have been taken from the phc and analysed directly after several amperometric meaurements. the results for these measurements are described below and are shown in figures 4 and 5. amperometric measurements at first, the three-electrode setup using a carbon microelectrode (a) has been tested ex situ to ensure a convection-free measurement at the microelectrode. figure 4. ex situ amperometric measurements at the positive half cell with three-electrode setup using a microelectrode figure 4 shows that for the amperometric experiments the reduction current resulting from polarisation with -1.2 v increases with increasing soc whereas the oxidation current resulting from polarisation with +1.0 v decreases with increasing soc. since in the phc the share of vanadium ions in the charged state (v5+) increases with soc, the reduction current increases because the amount of reducible vanadium ions increase. vice versa, the oxidation current decreases with increasing soc c. weidlich et al. j. electrochem. sci. eng. 13(5) (2023) 739-751 http://dx.doi.org/10.5599/jese.1699 745 since the amount of oxidisable v4+ ions decrease. the currents measured for the reduction and oxidation steps are reproducible. the average of the last 10 seconds of the measured currents is referred to as oxidation/reduction current and the ratio ln(iox/ired) is calculated according to [3,18]. the ln for the ratio of the measured oxidation and reduction current is supposed to linearly depend on the ln for the concentration of the oxidised and reduced vanadium ions in the electrolyte [18]. according to theory, an equal share of oxidised and reduced species (ln(cox/cred) = 0) should result in equal oxidation and reduction currents (ln(iox/ired) = 0). plotting the raw data, it has been observed that all data points are shifted towards higher ln(cox/cred) values, which has also been observed for all amperometric measurements. because of that, all measured currents have been corrected by the y-intercept of the linear regressions calculated for each set of oxidative and reductive currents, respectively, according to [18]. nevertheless, the slope of the regression line is expected to be -1 for all experiments. within the following the ln(iox/ired) vs. ln(cox/cred) plots for the corrected currents are shown and discussed. the plot of ln(iox/ired) as determined by amperometric measurements vs. ln(cox/cred) as determined by titration (as explained in the experimental section) is shown in figure 5. for easier understanding, the soc values calculated from titration results are added. figure 5. ln(iox/ired) vs. ln(cox/cred) plot for ex situ amperometric measurements at the positive half cell with three-electrode setup using a microelectrode figure 5 reveals that ln(iox/ired) linearly depends on ln(cox/cred) with a slope of nearly -1. to investigate the applicability of this setup for in situ monitoring, the amperometric cell was connected to the phc electrolyte circuit and operated as described in b. in figure 6, the results for the in situ three-electrode setup using a carbon microelectrode (b) are shown and the soc calculated from titration is added. the results are in good accordance to the results for the ex situ three-electrode setup using the microelectrode. the regression line again shows a slope of nearly -1. since a microelectrode might be susceptible for interferences when applied in a technical vfb system, an in situ three-electrode setup using a gc rod (c) working electrode has been tested in order to simplify and to optimise the amperometric setup regarding maintenance and cost efficiency. figure 7 shows that, using a gc rod working electrode, ln(iox/ired) also results in a linear dependency on ln(cox/cred) with a slope of nearly -1. comparing figures 4, 5 and 6, differences in the scaling of the axis for ln(iox/ired) and ln(cox/cred) are due to different soc ranges in which the experiments have been done and samples have been taken. http://dx.doi.org/10.5599/jese.1699 j. electrochem. sci. eng. 13(5) (2023) 739-751 charge monitoring in vanadium flow batteries 746 figure 6. ln(iox/ired) vs. ln(cox/cred) plot for in situ amperometric measurements at the positive half cell with three-electrode setup using a microelectrode figure 7. ln(iox/ired) vs. ln(cox/cred) plot for in situ amperometric measurements at the positive half cell with three-electrode setup using a glassy carbon rod working electrode for comparison, in figure 8 the results for (iox/ired) are plotted vs.(cox/cred) to reveal an exponential dependency and the necessity to plot the natural logarithms to yield a linear calibration. figure 8. iox/ired vs. cox/cred plot for in situ situ amperometric measurements at the positive half cell with three-electrode setup using a glassy carbon rod c. weidlich et al. j. electrochem. sci. eng. 13(5) (2023) 739-751 http://dx.doi.org/10.5599/jese.1699 747 for further simplification and cost reduction, the reference electrode has been omitted and the results for amperometric measurements with an in situ two-electrode setup using a gc rod (d) working electrode are shown in figure 9. the results reveal that also for this simplified setup the results are in good accordance to the results gained for the three-electrode setup using a microelectrode. the results for ln(iox/ired) still show a linear dependency on ln(cox/cred) but with a slope which deviates slightly stronger from -1. this might be caused by a potential shift at the glassy carbon rods during the experiment. figure 9. ln(iox/ired) vs. ln(cox/cred) plot for in situ amperometric measurements at the positive half cell with 2-electrode setup using glassy carbon rods in addition, we also have been testing the electrochemical quartz crystal microbalance (eqcm) concerning its applicability for soc monitoring. the quartz crystal coated with a carbon layer as working electrode on one side can be utilized to measure ocp in the electrolyte circuit and furthermore measuring the oscillation frequency may allow the investigation of electrolyte properties as density and viscosity at the same time respectively within the same setup. to check the applicability, an eqcm cell has been mounted in a bypass of the nhc electrolyte circuit of the vfb test setup and the battery cell has been charged and discharged. figure 10 (a) shows that the results for ocp measured at the carbon coated quartz crystal and ocp measured at the gc rod in the flow cell are comparable and in good accordance. figure 10 (b) shows the frequency of the crystal which has been measured simultaneously and also in a currentless state. the results reveal that changes of the frequency of the oscillating quartz crystal depend on the soc of the electrolyte. high frequencies which are expected to be measured for low electrolyte densities are observed for high ocp respectively high soc of the nhc. this is in good accordance to results published for vfb electrolyte densities in dependence on soc [10]. for the charged state of the nhc, the density of the electrolyte is low which results in high frequencies whereas the higher electrolyte density for the discharged negative half cell electrolyte results in lower frequencies. the frequency shift from cycle to cycle, which can be observed predominantly for the charged nhc, might indicate crossover processes. since the method is temperature dependent, temperature has been also monitored and plotted in figure 10 (b) but the observed frequency changes are not caused by changes in the electrolyte temperature. the feasibility of eqcm measurements to identify crossover of different electrolyte components (active vanadium species, sulfuric acid, water) will be investigated further. http://dx.doi.org/10.5599/jese.1699 j. electrochem. sci. eng. 13(5) (2023) 739-751 charge monitoring in vanadium flow batteries 748 figure 10. (a) negative half cell: open circuit potential (ocp) measured in an eqcm cell compared to ocp measured in a flow cell at a glassy carbon rod. (b) oscillation frequency of the quartz crystal changes in dependence on soc, temperature of the negative half cell electrolyte is also shown conclusions based on our first results for amperometric measurements in the electrolyte of a positive half cell of a vanadium flow battery laboratory test set up, we conclude this method is applicable for the in situ determination of the state of charge (soc) of the positive half cell (phc) electrolyte in vanadium flow batteries. a linear dependency of the current ratio (ratio of the measured oxidation and reduction currents, ln(iox/ired)) on the concentration ratio (ratio of the concentration of the oxidised and reduced vanadium ions, ln(cox/cred)) in the phc has been found for several test setups and electrodes. a three-electrode setup using a carbon microelectrode as well as a glassy carbon (gc) rod and also a simplified two-electrode setup using two gc rods have been successfully applied for state of charge (soc) monitoring in the phc electrolyte. additionally, soc monitoring of negative half cell electrolytes using amperometric measurements will be investigated in the future. for all tested setups the choice of suitable oxidation and reduction potentials is essential for an accurate soc determination. in addition to that, the ratio of diffusion coefficients and the long-term stability of the electrodes will also be considered in future research. since all shown results have been gained in situ but with a stop and flow sampling of the electrolyte to ensure convection free amperometric c. weidlich et al. j. electrochem. sci. eng. 13(5) (2023) 739-751 http://dx.doi.org/10.5599/jese.1699 749 measurements, we will further investigate and develop a flow cell and a polarisation routine which allows an amperometric monitoring of the soc in a flow-through mode. since this method is simple concerning technical equipment and maintenance, we predict an applicability for field use in commercial vanadium flow batteries. summarizing the electrochemical quartz crystal microbalance (eqcm) results, we conclude that the redox potential respectively the open circuit potential of the negative half cell electrolyte can be monitored using a carbon coated quartz crystal. furthermore, the oscillation frequency of this quartz crystal depends on the state of charge (soc) of the electrolyte and therewith offers a new opportunity for soc monitoring. as far as we know, an application of eqcm measurements for in situ soc monitoring has not been published yet and we will further investigate the suitability of this technique for soc determination as well as crossover identification and monitoring of different electrolyte components for the negative as well as the positive half cell of vanadium flow batteries. also, the sensitivity to changes in temperature and concentration of the electrolyte will be investigated as well as the stability of the electrodes. due to the complexity of the eqcm setup and its susceptibility 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distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1699 https://doi.org/10.1063/1.4963298 https://doi.org/10.1016/j.mechatronics.2019.04.005 https://doi.org/10.1039/d1cs00629k https://doi.org/10.3390/s120810604 https://creativecommons.org/licenses/by/4.0/) voltammetric studies on mercury behavior in different aqueous solutions for further development of a warning system designed for environmental monitoring doi: 10.5599/jese.2014.0068 177 j. electrochem. sci. eng. 4(4) (2014) 177-186; doi: 10.5599/jese.2014.0068 open access : : issn 1847-9286 www.jese-online.org original scientific paper voltammetric studies on mercury behavior in different aqueous solutions for further development of a warning system designed for environmental monitoring paul-cristinel verestiuc, igor cretescu* , , oana-maria tucaliuc, iuliana-gabriela breaban and gheorghe nemtoi** faculty of geography and geology, al. i. cuza university of iasi, 20 a. carol i bd., iasi, 700505, romania *faculty of chemical engineering and environmental protection, gheorghe asachi technical university of iasi, 73, d. mangeron street, iasi, 700050, romania **faculty of chemistry, al. i. cuza university of iasi, 11, carol i bd., iasi, 700506, romania corresponding author: e-mail: icre@tuiasi.ro; tel.: +40-741-914-342. received: october 5, 2014; revised: october 31, 2014; published: december 6, 2014 abstract this article presents some results concerning the electrochemical detection of mercury in different aqueous solutions, using the following electrodes: platinum-disk electrode (pde), carbon paste electrode (cpe) and glass carbon electrode (gce). using the voltammetric technique applied on the above mentioned electrodes, the experimental conditions were established in order to obtain the maximum current peaks, in terms of the best analytical characteristics for mercury analyses. the dependence equations of cathodic current intensity on the scan rate were established in the case of mercury ion discharge in each prepared solution of 0.984 mm hgcl2 in different electrolyte background: 0.1 m kcl, 0.1 m h2so4 and 0.9 % nacl. among the three investigated electrodes, the carbon paste electrode presented the highest detection sensitivity toward mercury ions in the aqueous solution. it was observed that, at a low scanning rate, the ph had an insignificant influence over the current peak intensity; however, the quantification of this influence was achieved using a quadratic polynomial equation, which could prevent the errors in mercury detection in case of industrial waste stream ph changes. the calibration curves for mercury in 0.9 % nacl solution and in the tap water respectively were carried out. keywords cathodic linear voltammetry; platinum electrode; carbon paste electrode; glass carbon electrode; mercuric ion; flow electrochemical cell. http://www.jese-online.org/ mailto:icre@tuiasi.ro j. electrochem. sci. eng. 4(4) (2014) 177-186 mercury behavior in system for environmental monitoring 178 178 introduction the ubiquitous presence of mercury in the environment is due to both natural geological activities as well as due to increasing anthropogenic pollution. because of its unique electronic configuration, mercury behaves similarly to noble gas elements, but the physical and chemical properties of mercury such as high surface tension, high specific gravity, low electrical resistance, and a constant volume of expansion over the entire temperature range in liquid state can rapidly transform this element into a hazardous air pollutant [1,2]. the water framework directive (2000/60/ec) classified mercury as a priority hazardous substance, establishing that from 2015, no more mercury from production processes can be discharged [3, 4]. mercury exists in a large number of forms, i.e. as “elemental” hg(0), monovalent or divalent mercury hg(i) and hg(ii), and in inorganic and organic compounds. metallic mercury hg(0) and most mercury compounds present high toxicity, acting as a bioaccumulative neurotoxin [5,6]. mercury ions are strongly adsorbed by soils or sediments in acid medium and are slowly desorbed, due to the content of clay minerals and/or organic matter, which are responsible for its behavior. the reaction products resulting from the methylation of inorganic mercury forms impose a significant risk to humans and wildlife due to tendency to accumulate in the food chain, and their ability to act as neurotoxins [6,7]. exposure to various forms of mercury will harm human health. moderate and repeated exposure to organic forms (lower than a few mg m -3 hg, but higher than 0.05 mg m -3 hg) causes symptoms of poisoning such as: lack of coordination of movement, impairment of peripheral vision, speech, hearing or walking as well as muscle weakness. inhaled or physical contact with inorganic forms cause: tremors, emotional or neuromuscular changes, insomnia, headaches, disturbances in sensation, changes in nerve responses, and performance deficits on tests of cognitive function. with prolonged or high concentration exposure, kidney effects, respiratory failure and death may occur [8,9]. mercuric chloride (hgcl2) is used as a depolarizer in electric batteries and as a reactant in organic synthesis and analytical chemistry [10]. the presence of this element in different environmental components could be considered as harmful to human health well environmentally dangerous due to the mercury content. taking into consideration all the above mentioned aspects, the detection of mercuric ion has become a priority for environmental safety and human health. the determination of trace amounts of mercury, has led to some analytical problems because it can be found in several chemical forms [11]. for an accurate determination of mercury at trace and ultra-trace levels, analytical methods with high sensitivity and selectivity are needed. there are a number of analytical methods for mercury detection which require expensive instruments, well-controlled experimental conditions, sample preparation and relatively large sample volumes [12]. electrochemical detection of trace metals offers important advantages, such as remarkable sensitivity, inherent miniaturization and portability, remote monitoring and decentralized measurements, low cost and compatibility with turbid samples [13,14]. therefore, electrochemical methods are less costly and require no sophisticated equipment. chemically modified electrodes have received increasing attention, which has led to improvements in the sensitivity and selectivity of electrochemical analysis techniques in the recent decades. the determination of hg(ii) ions using chemically modified electrodes has been investigated recently, using plants [14], gold film [15], polymer films [16] or organic compounds with chelating groups [17]. p.-c. verestiuc et al. j. electrochem. sci. eng. 4(4) (2014) 177-186 doi: 10.5599/jese.2014.0068 179 the behavior of electrodes has also been studied using modified a carbon paste electrode [18], modified glass carbon electrode [17] or modified gold nanoelectrode ensembles [19]. a relatively recent review of electrochemical sensors and detectors has been done by bakker [20], characterizing this area of research as being one of the most fruitful and interdisciplinary areas of research in analytical chemistry. a recent paper by pujol [21] analyzed the actual state of art concerning the sensors and devices for heavy metals detection in water, but the mercury detection from environmental samples was not studied in details. for this reason in this study it was investigated the cathodic discharge of the mercuric ion on different type of electrodes such as: platinum-disk electrode, carbon paste electrode and glass carbon electrode, in respectively different aqueous solutions (0.1 m kcl, 0.1 m h2so4, 0.9 % nacl) by the voltammetric method in order to find the most suitable conditions for analytical purpose. experimental in order to simulate the wastewater from the industrial stream, aqueous solutions of hgcl2 in 0.1 m h2so4 at a ph value of 0.99, 0.9 % nacl at a ph value of 5.66 and 0.1 m kcl at a ph value of 7.43 was prepared, using analytical purity reagents and double distilled water. the investigation of mercury behavior in these solutions was carried out using voltammetric measurements [22-26] with the above mentioned electrodes (gce, cpe and pde) individually. these electrodes play the role of working electrode (we) in a flow electrochemical cell (as is presented in fig. 1) through which the samples from the industrial stream are passed using a peristaltic pump. a saturated calomel electrode (sce) was used as the reference electrode (re), while a platinum electrode was used as the counter (auxiliary) electrode (ce) at the temperature of 25 °c. also, ph and temperature sensors were located in the electrochemical cell in order to provide corrections in the case of possible changes in the above mentioned parameters which should be kept constant at the same values as were used during the calibration procedure. figure 1. experimental setup for study of the voltammetric detection of mercury in aqueous solution the ph corrections were achieved using software (the ph dependence of the current peak was determined by a simple equation), while the temperature corrections were first achieved by the thermostatic system for sample processing and finally by the software corrections for the fine ph adjustments. j. electrochem. sci. eng. 4(4) (2014) 177-186 mercury behavior in system for environmental monitoring 180 180 the voltammograms were recorded using the potentiostat voltalab 32 (radiometer analytical) [27-29] after stopping the flow of liquid samples through the electrochemical cell (using a solenoid mini-valve) and nitrogen bubbling in the investigated solutions, in order to remove the dissolved oxygen. the investigated electrodes were used as purchased from radiometer analytical, except for the carbon paste electrode, which was prepared according to methods previously presented in the literature [30-31]. the electrode surfaces were: 3.14·10 -2 cm 2 for pde; 7.07 10 -2 cm 2 for gce and 19.63 10 -2 cm 2 for cpe. results and discussion in figures 2, 3 and 4, the cathodic linear voltammograms are presented, corresponding to the reduction of hg 2+ on the pde (φ = 2mm) at different scanning rates in different aqueous media. figure 2. linear cathodic voltammograms on pde in 0.1 m h2so4 aqueous solution recorded at different scanning rates figure 3. linear cathodic voltammograms on pde in 0.1 m kcl aqueous solution recorded at different scanning rates figure 4. linear cathodic voltammograms on pde in 0.9 % nacl aqueous solution recorded at different scanning rates. figure 5. linear cathodic voltammograms of mercury ion discharge on pde in different solutions at constant scanning rate of 50 mv s -1 p.-c. verestiuc et al. j. electrochem. sci. eng. 4(4) (2014) 177-186 doi: 10.5599/jese.2014.0068 181 concerning the electrochemical behavior of mercuric ions in the three different aqueous solutions shown in figures 2-4, it was observed that there was a simultaneous increase in pde sensitivity with the scanning rate, at a concentration of 0.984 mm, as was expected. this behavior leads to the following dependence equations for the cathodic peaks, which express the discharge of the mercuric ion on the platinum electrode, in all investigated solutions: a. 0.1 m h2so4: i = -2.114 – 0.058 v + 3.474×10 -5 v 2 (r = 0.99952) (1) b. 0.1 m kcl: i = -4.285 – 0.072 v + 2.168 ×10 -5 v 2 (r = 0.99757) (2) c. physiological serum (0.9 % nacl): i =-6.815 0.140v 6.611×10 -5 v 2 (r = 0.99827) (3) where: i = intensity current (ma), v = scanning rate (mv s -1 ), r = correlation coefficient according to the experimental results, it was noted that the highest sensitivity of pde was obtained in physiological serum, followed by the solution prepared based on sulfuric acid and potassium chloride, respectively (figure 5). figure 6. linear cathodic voltammograms on cge in 0.1 m h2so4 aqueous solution recorded at different scanning rates. figure 7. linear cathodic voltammograms on cge in 0.1 m kcl aqueous solution recorded at different scanning rates figure 8. linear cathodic voltammograms on cge in 0.9 % nacl aqueous solution recorded at different scanning rates figure 9. linear cathodic voltammograms of mercury ion discharge on gce in different aqueous solution at constant scanning rate of 50 mv/s j. electrochem. sci. eng. 4(4) (2014) 177-186 mercury behavior in system for environmental monitoring 182 182 figures 6-8 present the cathodic linear voltammograms for each of the aqueous solutions recorded on the gce at different scanning rates. in this case, the following dependence equations for the cathodic peak were established, which express the discharge of the mercuric ion (0.984 mm) on the platinum disc electrode, in all three investigated solutions: a. 0.1 m h2so4: i = -5.7573 0.06488+1.3139×10 -4 v 2 (r = 0.99404) (4) b. 0.1 m kcl: i = -11.895 0.239 v + 1.079×10 -4 v 2 (r = 0.998) (5) c. physiological serum (0.9 % nacl): i = -3.745 0.347 v + 1.355×10 -4 v 2 (r = 0.99918) (6) where: i = current intensity (ma), v = scanning rate (mv s -1 ), r = correlation coefficient in accordance with the figure 9, the sensitivity of the gce toward the mercuric ion was highest in kcl aqueous solution and lowest in sulfuric acid. the entire cathodic process of mercuric ion discharge on the cpe in all of the three investigated solutions is presented in detail in linear voltammograms (figures 10-12). figure 10. linear cathodic voltammograms on cpe in 0.1 m h2so4 aqueous solution recorded at different scanning rates. figure 11. linear cathodic voltammograms on cpe in 0.1 m kcl aqueous solution recorded at different scanning rates figure 12. linear cathodic voltammograms on cpe in 0.9 % nacl aqueous solution recorded at different scanning rates figure 13. linear cathodic voltammograms of mercury ion discharge on cpe in different solutions at constant scanning rate of 50mv s -1 p.-c. verestiuc et al. j. electrochem. sci. eng. 4(4) (2014) 177-186 doi: 10.5599/jese.2014.0068 183 data obtained using the cpe revealed the same behavior as with the gce regarding higher sensitivity, as follows: 0.1 m kcl, physiological serum and 0.1 m h2so4 (figure 13). the measurements performed on the cpe, in each investigated solution led to the following equations: a. 0.1m h2so4: i = -38.137 1.282 v + 9.944×10 -4 v 2 (r = 0.99581) (7) b. 0.1 m kcl: i = -80.923 2.168 v + 1.27×10 -3 v 2 (r = 0.99808) (8) c. physiological serum (0.9 % nacl): i = -34.086 3.25 v + 2.23×10 -3 v 2 (r=0.99867) (9) where: i = current intensity (ma), v = scanning rate (mv s -1 ), r = correlation coefficient these equations point out the dependence of the current intensity (corresponding to the discharge of mercuric ions in aqueous solutions at a concentration of 0.984 mm hgcl2) on scanning rates. increasing the scanning rate during investigations into mercuric ion discharge for each studied electrode in the aqueous solutions led to a change in the cathodic potential values, which were shifted to negative values, suggesting a quasi-reversible process. among the electrodes used in this study, the carbon paste electrode was the most sensitive with respect to the cathodic discharge of mercuric ions in all the investigated aqueous solutions. taking into consideration this aspect, the ph dependence of the current peak (figure 14) and the calibration curves (figure 15) in the nacl solution and tap water, respectively, were presented only for the cpe. figure 14. influence of solution ph on the cathodic peak on cpe in 0.9 % nacl aqueous solution recorded at two scanning rates figure 15. calibration curves for determination of hgcl2 using cpe in 0.9 % nacl aqueous solution and tap water, respectively as presented in figure 14, a low ph dependence of the current peak was observed for a scanning rate of 50 mv s -1 , but if this value was multiplied by three fold, the dependence became more evident, especially in the acidic region. these equations are presented as follows: i50 = -0.12847 0.09001 ph + 0.00774 ph 2 (10) i150 = -0.10381 0.0407 ph + 0.00419 ph 2 (11) j. electrochem. sci. eng. 4(4) (2014) 177-186 mercury behavior in system for environmental monitoring 184 184 the calibration curves presented in figure 15 pointed out an approximately linear dependence for both calibration curves, which are separated into two regions. however, these calibration curves could be very well fitted following quadratic polynomial equations: x = -lg[hgcl2] inacl = -242.414 + 257.620 x 102.678 x 2 + 18.175 x 3 1.205 x 4 (12) itap water = -121.207 + 128.825 x -51.339 x 2 +9.087 x 3 0.602 x 4 (13) the decreased sensitivity of the voltammetric method for mercury detection in real samples (tap water provided from the iasi drinking water treatment plant) was evident, but the analytical signal was sufficiently high to be discriminated from the global noise determined in such a complex matrix. the response limit of the concentration peak for hg (ii) detection was in the range of 10 -5 m to 10 -3 m. the optimal operating conditions were established after a period of three minutes of preconcentration. conclusions the following conclusions can be drawn for the voltammetric studies regarding the behavior of mercuric ions in the presence of different supporting electrolytes in aqueous solutions using different electrodes:  the cathodic discharge of mercury ions from hgcl2 aqueous solutions in different electrolyte support media such as 0.1 m kcl, physiological serum (0.9 % nacl), and 0.1 m h2so4, using different electrode materials (platinum, glass carbon and carbon paste) was evidenced by a relevant current peak, whose height (in terms of the analytical method sensitivity) is dependent on the potential scanning rate;  the cathodic discharge of mercuric ions, in all investigated aqueous solutions, was a slow and quasi-reversible process, on all three of the investigated electrode materials;  the best electrode was the carbon paste electrode, in terms of the sensitivity toward mercuric ions, in concordance with the equations describing the dependence between the cathodic peak intensity and the potential scanning rate. it was pointed out that the cpe presented the highest detection sensitivity toward mercuric ions in an aqueous solution, working in a wide ph range and having a short response time. besides these aspects, the cpe is very easily prepared and has good reproducibility and repeatability for mercury analysis, which recommend it for this analytical application;  based on the results, a miniaturized voltammetric flow cell will be developed as an integrated component of the equipment used for mercury detection/monitoring in industrial waste streams. based on the mercury detection capacity, a warning system will be designed to avoid mercury discharge into surface waters in the case of accidentally increasing the mercury concentration.  due to the increased capacity for mercury detection by this proposed monitoring system, based on a voltammetric method and an electrochemical flow cell, its versatility could be extended using some auxiliary equipment to other environmental components such as air and soil.  at low scanning rate, the ph had an insignificant influence over the current peak intensity in the case of the cpe in 0.9 % nacl solution. p.-c. verestiuc et al. j. electrochem. sci. eng. 4(4) (2014) 177-186 doi: 10.5599/jese.2014.0068 185  in order to avoid errors in mercury detection in cases of industrial stream ph changes, the quantification of the ph influence was achieved using a quadratic polynomial equation.  the calibration curves for mercury in a 0.9 % nacl solution 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https://www.google.ro/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&uact=8&ved=0cb8qfjaa&url=http%3a%2f%2fwww.sciencedirect.com%2fscience%2fjournal%2f00032670%2f48&ei=9xrovl6tk8hoygpm2yh4dw&usg=afqjcnfklntpeht2eb8w8mupg8t1bcuweg&sig2=neyomwx_sfuuibqmdal3aq&bvm=bv.79142246,d.bgq j. electrochem. sci. eng. 4(4) (2014) 177-186 mercury behavior in system for environmental monitoring 186 186 [23] d. m. gordin, t. gloriant, g. nemtoi, r. chelariu, n. aelenei, a. guillou, d. ansel, materials letters 59 (23) (2005) 2936-2941 [24] d. mareci, c. bocanu, g. nemtoi, d. aelenei, journal of the serbian chemical society. 70 (6) (2005) 891-897 [25] g. nemtoi, a. ciomaga, t. lupascu, revue roumaine de chimie 57 (9-10) (2012) 837-841 [26] a. v. sandu, a. ciomaga, g. nemtoi, c. bejenariu, i. sandu, microscopy research and technique 75(12) ( 2012) 1711-1716 [27] g. nemtoi, h. chiriac, o. dragoş, m.o. apostu, d. lutic, acta chemica iasi 17 (2) (2009) 151168 [28] g. nemtoi, m. s. secula, i. cretescu, s. petrescu, revue roumaine de chimie 58 (12) (2007) 655-659 [29] g. nemtoi, d.i. cuciurean, revista de chimie 53 (2) (2002) 146-149 [30] s. i. m. zayed , h. a. m. arida, international journal of electrochemical science 8 (2013) 1340 – 1348 [31] j. zima, i. svancara, j. barek, k. vytras, critical reviews in analytical chemistry 39 (2009) 204–227 © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ electrochemical oxidation of amoxicillin in its pharmaceutical formulation at boron doped diamond (bdd) electrode doi:10.5599/jese.186 129 j. electrochem. sci. eng. 5(2) (2015) 129-143; doi: 10.5599/jese.186 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical oxidation of amoxicillin in its pharmaceutical formulation at boron doped diamond (bdd) electrode gnamba corneil quand-même, appia foffié thiéry auguste, loba evelyne marie hélène, sanogo ibrahima*, ouattara lassine  laboratoire de chimie physique, ufr ssmt, université félix houphouët-boigny de cocody, abidjan, 22 bp 582 abidjan 22, côte d’ivoire *centre hospitalier universitaire (chu) de treichville, abidjan, 01 bp v3 abidjan 01, côte d’ivoire corresponding author: ouatlassine@yahoo.fr ; tel: +22502143382 received: may 13, 2015; revised: july 25, 2015; published: august 26, 2015 abstract in this work, voltammetric and electrolysis experiments have been carried out on a conductive boron doped diamond (bdd) electrode in a solution containing amoxicillin in its pharmaceutical formulation. the physical characterization of the bdd surface by scanning electron microscopy (sem) reveals a polycrystalline structure with grain sizes ranging between 0.3 and 0.6 µm. with raman spectroscopy, bdd surface is composed of diamond type carbon (csp 3 ) and graphitic type carbon (csp 2 ). the xps survey of the bdd surface has revealed the presence of c 1s and o 1s. the deconvolution of the c1s spectrum showed that the bdd surface chemical bonds were composed by c-c and c-h. the ferri/ferrocyanide redox couple showed a quasi reversible behavior on bdd and bdd showed a quasi metallic properties with a good electrical contact between the diamond coating and the silicono substrate. the electrochemical characterization of the bdd electrode in sulfuric acid electrolyte showed a wide potential window of 2.74 v. the oxidation of amoxicillin showed an irreversible anodic wave on the voltammogram in the domain of water stability indicating a direct oxidation of amoxicillin at bdd surface. the treatment of amoxicillin in the synthetic wastewaters under various constant current densities 20, 50, 100, 135 ma cm -2 on bdd showed that amoxicillin is highly reduced under 100 ma cm -2 reaching 92 % of the chemical oxygen demand (cod) removal after 5 h of electrolysis. investigation performed in perchloric acid as supporting electrolyte led to 87 % of cod removal after 5 h of electrolysis. mineralization of amoxicillin occurs on bdd and the cod removal was higher in sulfuric acid than in perchloric acid owing to the involvement of the in-situ formed persulfate and perchlorate to the degradation process mainly in the bulk of the solution. the instantaneous current efficiency (ice) presents an exponential decay indicating that the process was limited by diffusion. the specific energy consumed after 5 h of the amoxicillin electrolysis was 0.096 kwh cod -1 and 0.035 kwh cod -1 in sulfuric acid and in perchloric acid respectively. keywords boron doped diamond, amoxicillin, electrolysis, anodic oxidation, hospital wastewater http://www.jese-online.org/ mailto:ouatlassine@yahoo.fr j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 130 introduction antibiotics, one of the most widely used and prescribed pharmaceuticals, are applied for killing microbes [1-3]. recently the wide occurrence of antibiotics in groundwater raised concerns about potential adverse on human health and aquatic ecosystem [4-7]. in côte d’ivoire, the consumption of antibiotic per year is huge especially for hospitalized patients. the occurrence of antibiotics in natural water is due to the lack of effective treatment of the wastewaters containing such medicines. that is the case of the teaching hospitals of côte d’ivoire where the wastewater treatment plant built in the former days precisely in the last four decades do not operate anymore. thus, the hospital wastewaters are directly rejected in the environment without treatment. the presence of antibiotics in the aquatic environment even at very low concentration levels can promote the growth of antibiotic resistant bacteria or pathogens [8-10]. antiobotics are found to be resistant to biological degradation processes, escaping almost intact from conventional wastewater treatment plants. among antibiotics, amoxicillin, one of the most used drug worldwide, has been detected in surface waters and showed a resistance to biological treatment [11,12]. so a promishing approach to the remediation of wastewaters contaminated with high contents of this antibiotic is the application of innovative techniques such advanced oxidation processes [13-16]. advanced oxidation processes are characterized by the production of extremely reactive and unselective hydroxyl radicals which is able to oxidize and mineralize almost all organic compounds to co2 and water. as advanced oxidation process, ozonation has been used for amoxicillin oxidation and that resulted in a very low mineralization degree [17]. with photofenton methods, stable intermediates are formed and they further undergo difficult mineralization [18]. among these advanced oxidation processes to be used for drugs removal, electrochemical advanced oxidation processes (eaopr) are very attractive for wastewater decontamination without a need of addition of toxic chemical reagents and also without producing dangerous wastes. among eaopr, anodic oxidation is the most effective technique. it consists in the destruction of pollutants by hydroxyl radicals generated from water oxidation at the surface of the anode [19,20]. in such a way, oxide anodes like tin dioxide have successfully been used for amoxicillin oxidation but less than 80 % mineralization is found after 24h of electrolysis [21]. thus, great attention for wastewater treatment was paid on the use of boron doped diamond (bdd) electrodes which have been found to be effective for various organic compound oxidation [22,23]. in fact, the use of boron doped diamond is based on its physical and chemical properties such as surface inertness with low adsorption capability and surface corrosion stability. boron doped diamond (bdd) thin films are usually prepared for water remediation since they produce very high amount of weakly physisorbed hydroxyl radicals. due to these properties, it has been used for electrosynthesis, electroanalysis, and electrochemical combustion [23]. boron doped diamond has been successfully used for real and synthetic industrial wastewater treatment [2325]. in the aim of contributing to the treatment of the hospital wastewater in côte d’ivoire especially in the teaching hospitals, the investigation of the electrochemical treatment of a simulated wastewater of amoxicillin in its pharmaceutical formulation has been carried out on boron doped diamond. in this work, voltammetric and electrolysis techniques have been employed in conditions where several experimental parameters have been varied experimental boron doped diamond (bdd) electrodes were prepared by hot-filament chemical vapor deposition (hf-cvd) on low resistivity (1-3 mω cm) p-si wafers (siltronix, diameter 10 cm, g. c. quand-meme et al. j. electrochem. sci. eng. 5(2) (2015) 129-143 doi:10.5599/jese.186 131 thickness 0.5 mm). the process gas was a mixture of 1 % ch4 in h2 containing trimethylboron. film growth occurred at a rate of 0.24 μm h -1 . the film thickness was about 1 μm. more details concerning the preparation of bdd electrodes are given elsewhere [26,27]. the scanning electron microscopy (sem) images of the bdd electrode were taken with a jeol ljms-6300-f instrument. the raman spectrum of the bdd electrode was obtained at room temperature with a renishaw rm 1000 raman spectrometer. x-ray photoelectron spectrum of the diamond film was recorded with a kratos axis-ultra spectrometer with a monochromatic al kα x-ray source operated at 15 kv with pass energy of 20 ev. the carbon 1s spectrum has been deconvoluted using the casaxps computer program. the voltammetric measurements were performed in a three-electrodes electrochemical cell using a voltalab pgp 201 (voltamaster 1 as software). the counter electrode (ce) was a platinum wire and the reference electrode (re) was a saturated calomel electrode (sce). to overcome the potential ohmic drop, the reference electrode was mounted in a luggin capillary and placed close to the working electrode by a distance of 1 mm. the apparent exposed area of the working electrode was 1 cm 2 . for the exhaustive electrolysis, a batch reactor was used. the synthetic amoxicillin wastewater was fed to the reactor by a peristaltic pump with a flow rate of 2.08 ml s -1 . the exposed electrode surface area with the solution was about 16 cm 2 . the chemicals used in this work composed of h2so4 (fluka), hclo4 (fluka), k3fe(cn)6 (fluka), k4fe(cn)6 (fluka) and amoxicillin tablets (from a pharmacy in abidjan). all the chemicals were used as received without any further treatment for the experiment. all the solutions used in the current work were prepared with distilled water. all the electrochemical experiments were made at ambient temperature of 25 °c. results and discussion physical characterization of the boron doped diamond (bdd) electrode figure 1 shows the scanning electron microscopic image of the boron doped diamond electrode. figure 1. micrograph of boron doped diamond the image indicates that bdd presents a polycrystalline structure [28]. the morphological grains sizes are ranging between 0.3 and 0.6 µm. the grains are heavily twinned. at the bottom of j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 132 the diamond grains, a relative dark space was observed especially at grains boundary which can be related probably to graphitic carbon (csp 2 ) formed during the bdd preparation. figure 2 shows the raman spectrum of the bdd sample excited by the 514.5 nm laser. a narrow peak at 500 cm -1 corresponding to silicon signal was observed. a narrow peak at 1332 cm -1 is characteristic of the diamond (csp 3 ) crystal signal and a broad band centered at 1550 cm -1 corresponds to the non-diamond carbon impurities (csp 2 ). that finding indicates that the graphitic type carbon is present on the boron doped electrode probably in the diamond grains boundaries. figure 2. raman spectroscopy spectrum of the bdd electrode figure 3: xps spectrum of the as prepared bdd electrode. inset : deconvoluted xps c 1s of bdd figure 3 shows the xps spectrum of the bdd film. that survey presents the same trend as that found in literature for other bdd electrodes [29,30]. the elemental composition of the bdd surface has been given. the spectrum is dominated by a sharp peak located at 285 ev g. c. quand-meme et al. j. electrochem. sci. eng. 5(2) (2015) 129-143 doi:10.5599/jese.186 133 corresponding to c 1s [29,30]. a small peak related to o 1s has been observed at 532.5 ev. the ratio of o/c is about 0.209 indicating that there is a very low amount of oxygen at the surface of the as grown bdd electrode. the surface chemical bonds have been checked through the deconvolution of c 1s spectra. the obtained result is inserted in figure 3. the deconvolution of the parent c 1s spectrum led to two peaks. one located at 284.2 ev and the other one at 284.7 ev corresponding to the sharp and the broad peak presented respectively in the c 1s spectrum (inset of figure 3). the main peak located at 248.2 ev could be assigned to c-c bonds and that located at 284.7 ev could be assigned to c-h bonds [29,30]. cyclic voltammetry figure 4 shows the voltammetric i-e curves of boron doped diamond in 100 mm (equimolar) ferri/ferrocyanide ([fe(cn)6 ] 3/[fe(cn)6] 4) in 0.1 m koh at various potential scan rates ranging between 120 mv min -1 and 480 mv min -1 . in this figure, the observed anodic and a cathodic peaks correspond to the oxidation and reduction of the redox couple, respectively. the observed peaks are typical for such redox couple on various good conductors electrodes [31,32]. the increase in the potential scan rates leads to an increase of the peak current values. the separation between the potential of the anodic and cathodic current peaks, ep, increases slightly with the increase of the potential scan rates investigated and amounts to 276 mv for the scan rate of 120 mv min -1 i.e. 2 mv s -1 which is the lowest observed ep value determined in this work. the value is higher than the 60 mv expected for the fast and reversible system. this could either be due to a slow electrontransfer reaction at the bdd electrode/solution interface or possibly due to the intrinsic semiconducting properties of the diamond. moreover, the absolute values of the ratios of the anodic current peak density over the cathodic current peak densities are about |jpa/jpc| = 1.046±0.021 which is an indication of the reversibility of the electrode process. from the curves, the current of the peaks were plotted against the square root of the potential scan rates and a linear trend was obtained for both the anodic and cathodic peaks. the slopes of the straigth lines were almost the same in both cases and were about (25.7 ma cm -2 s 1/2 v -1/2 , r 2 = 0.9994) and (25.5 ma cm -2 s 1/2 v -1/2 , r 2 = 0.998) for the anodic and the cathodic straigth lines respectively. the linear relationship between the response current and the square root of the potential scan rate indicates that the redox kinetics at the boron-doped diamond electrode is a planar diffusioncontrolled process. the overall obtained results indicated eventually that the redox couple behaves in a quasireversible manner at the boron doped diamond and also demonstrated the formation of a good electric contact between the substrate in silicon and the diamond coating [31,32]. figure 5 gives the voltammetric curves recorded on bdd for various concentrations of the redox couple at 480 mv min -1 . the height of the peak current increases as the concentration of the redox couple increases. plotting the cathodic and the anodic peak current against the redox couple’s concentration, a linear evolution is obtained. for both the cathodic and anodic peak current investigated, straigth lines lead to the same slope 2.10 -2 ma cm -2 mm -1 , r 2 =0.99. the current results confirm the fact that the boron doped diamond in use behaves as a quasi-metallic material with the tendancy to be irreversible towards the ferri/ferrocyanide redox couple compared to metallic electrodes such as platinum electrode. that metallic behavior of the bdd can be related to the content of boron in the bdd’s lattice and also to the involvement in the overall processes of all the boron doped diamond surface components such as diamond (csp 3 ) and non-diamond carbon (csp 2 ) at the diamond grain j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 134 boundaries [33]. finally, the boron doped diamond used in this work showed an acceptable electrical conductivity that can ensure electrochemical measurements and can be used for pharmaceutical product investigation. figure 4 : cyclic voltammetric curves of ferri/ferrocyanide on boron doped diamond electrode at various potential scan rates (120 mv min -1 -480 mv min -1 ), c = 100 mm , t = 25°c, ce: pt, re: sce, in the inset: evolution of current peak against square root of potential scan rates figure 5 : cyclic voltammetric curves of ferri/ferrocyanide on boron doped diamond electrode at various redox couple concentration at 480 mv min -1 , t = 25 °c, ce: pt, re: sce, in the inset : evolution of current peak against redox couple concentration figure 6 presents the voltammetric curves recorded on boron doped diamond at a low potential scan rate (480 mv min -1 ) in sulfuric acid free or containing 1 g l -1 amoxicillin. in absence g. c. quand-meme et al. j. electrochem. sci. eng. 5(2) (2015) 129-143 doi:10.5599/jese.186 135 of amoxicillin, the obtained voltammogram showed a wide potential window of e=2.74 v between the onset of the potentials of the oxygen evolution reaction (oer) and the hydrogen evolution reaction (her). close to the onset of the oer, an oxidation wave is observed. according to literature, that wave could be related to the oxidation of surface redox species such as csp 2 especially surface quinone functional group [34]. figure 6. cyclic voltammetry curves recorded on boron doped diamond electrodes in several concentration of amoxicillin containing sulfuric acid electrolyte at 480 mv/min, ce: pt, t= 25 °c, re: sce. inset: plot of current density of amoxicillin oxidation wave versus amoxicillin concentration at 1.9 v in the presence of amoxicillin, change in the voltammogram is observed and the current starts to increase at about 1.2 v vs. sce followed by an irreversible anodic wave at about 1.8 v vs. sce. the observed wave is characteristic of the anodic oxidation of amoxicillin. moreover, as the concentration of the amoxicillin increases, the height of the wave increases too. this results evidenced a direct oxidation of amoxicillin in the electrochemical window of water stability at bdd. the oxidation of amoxicillin at high potential close to oer could have been catalyzed by csp 2 presents at the bdd’s grain boundaries. in the inset of figure 6, the current recorded at 1.9 v vs. sce on the wave was plotted against the concentration of the amoxicillin. one observes a linear relationship of the current versus concentration (i-c) curve with a slope of 1.7 ma cm -2 l g -1 , r 2 = 0.99. successive scans have been performed at 480 mv min -1 on the bdd electrode and the results are presented in figure 7. one observed well superimposed curves in the anodic potential domain while a decrease of the cathodic current in absolute values leading to the increase of the onset of the hydrogen evolution reaction potential was found. this finding could indicate the formation of a polymeric film at the investigated electrode’s surface which, during the backwards of the potential scan in the negative direction, inhibited the her. varying the potential scan rates, figure 8 was obtained. the oxidation wave of amoxicillin increases with the potential scan rate. moreover, plotting the current recorded at 1.9 v vs. sce against the square root of the potential scan rates, a linear relationship was obtained indicating that the amoxicillin oxidation process was diffusion controlled. the obtained straight line has a slope of 17 ma cm -2 s 1/2 v -1/2 , r 2 = 0.9996. j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 136 figure 7. cyclic voltammetry curves recorded on boron doped diamond electrodes in several successive scans in 1 g l -1 of amoxicillin containing 0.1m sulfuric acide electrolyte at 480 mv min -1 , ce: pt, t= 25°c, re: sce. figure 8. cyclic voltammetry curves recorded on boron doped diamond electrodes in 1 g l -1 of amoxicillin containing 0.1 m sulfuric acid electrolyte at several potential scan rates, ce: pt, t= 25 °c, re: sce. in the inset : evolution of currents density recorded at 1.9 v against the square root of the potential scan rates bulk electrolysis for the bulk electrolysis of the amoxicillin in sulfuric acid, an undivided reactor has been used under galvanostatic regime. the system worked under a batch operation mode. the simulated wastewater was fed into the electrochemical reactor at a flow rate of 2.08 ml s -1 . the mass transfer coefficient determined using the ferri/ferrocyanide redox couple was 2.36×10 -7 m s -1 . the calculated initial current density using equation (1) [35] is about 6.0 ma cm -2 . g. c. quand-meme et al. j. electrochem. sci. eng. 5(2) (2015) 129-143 doi:10.5599/jese.186 137 ilim = 4 f kd a cod (1) with f = 96500c, kd is the mass transfer coefficient, a is the electrode surface area in the contact with the solution, cod is the chemical oxygen demand. according to the work of michaud and coworkers [35], two main regimes can be reached depending on the applied current compared to the initial limiting current. if i < ilim, a galvanostatic process governs the overall oxidation process i.e. limited by charge transfer reaction. if i < ilim, the oxidation process is mass transfer controlled i.e. limited by diffusion. in the current case, the imposed current densities ranging from 20-135 ma cm -2 are higher than the initial limiting current density. the obtained results are presented in figure 9. it appears that the normalized cod (equation 2) decreases with time for the investigated current densities. that decrease follows an exponential trend like what has been described eleswhere [35] when applied current density was higher than the initial limited current density. the process is under mass transport control. the abatment rate of the chemical oxygen demand (cod) increases from 20 ma cm -2 to 100 ma cm -2 and then decreases from 100 ma cm -2 to 135 ma cm -2 . that observation is highlighted in figure 10 where the cod abatement rate determined after 4 h was plotted against the applied current densities. * 0 cod normalized cod cod 100 x cod t  (2) cod0 and codt are the chemical oxygen demands at time t = 0 and t ≠ 0 respectively. figure 9. electrolysis of 1 g l -1 amoxicillin on boron doped diamond electrodes in 0.1 m sulfuric acid electrolyte at several current densities. anode: bdd, cathode: titanium plate, t = 25 °c. q = 2.08 ml s -1 in fact, as the current increases, the reaction of the mineralization of amoxicillin is faster. however, the trend of the observed cod evolution on bdd clearly shows that the oxidation process is under diffusion control for which a very high current attaining 135 ma cm -2 could lower the degradation yield by the occurrence of side reaction which could be attributed mainly to oer. thus, bdd appears to be a high efficient electrode for the oxidation of the investigated antibiotic. j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 138 according to various reports in that field, bdd was found to promote the mineralization of organics with an efficiency only limited by mass transport control. figure 10: cod removal after 4 h of electrolyis of 1 g l -1 amoxicillin on boron doped diamond electrodes in 0.1 m sulfuric acid electrolyte at several current densities. anode: bdd, cathode: titanium plate, t=25°c, q=2.08 ml s -1 figure 11. electrolysis of 1 g l -1 amoxicillin on boron doped diamond electrodes in 0.1 m sulfuric acid electrolyte under a current density of 100 ma cm 2 , anode: bdd, cathode: titanium plate, t = 25 °c, q = 2.08 ml s -1 cod0 = 1322 ppm, inset: a: plot of ice versus time, b: plot of ln(cod) versus time figure 11 shows the evolution of cod during the electrochemical oxidation of amoxicillin at constant current density of 100 ma cm -2 . in the inset the plot of the instantaneous current efficiency (ice, equation 3) versus the electrolysis time (t) was depicted (figure 11a). in figure 11b (inset of figure 11) a curve resulting from the evolution of the logarithm of the chemical oxygen demand versus the electrolysis time was presented. it appears that the ice is less than 100 % and a b g. c. quand-meme et al. j. electrochem. sci. eng. 5(2) (2015) 129-143 doi:10.5599/jese.186 139 decreases exponentially with time. such behaviour is in accordance with the model developed by michaud and coworkers [35]. cod cod ice 4 t t tfv i t     (3) where codt and codt+t are the cods at times t and t+t (in mol(o2) dm -3 ) respectively, i is the current, a; f is the faraday constant (96500 c) and v is the volume of the electrolyte (dm 3 ) moreover the decrease in ice indicated that under these working conditions, although amoxicillin mineralization occurs completely, oxygen evolution reaction, which is an undesired parallel reaction, takes place in a high extent that it leads to a decrease in the value of ice during the oxidation of amoxocillin with time. in figure 11, one observed that the decrease of cod with time follows an exponential trend. and the curve of the logarithm of cod with time is linear. the slope of the straigth line is 0.5258 s -1 , r 2 =0.998). from those results, the decay of cod during amoxicilin oxidation indicates that the reaction is a pseudo first order with the oxidation rate constant of 0.5258 s -1 . figure 12. electrolysis of 1 g l -1 amoxicillin on boron doped diamond electrodes in two supporting electrolytes (0.1 m h2so4 and 0.1 m hclo4) at 100 ma cm -2 . anode: bdd, cathode: titanium plate, t= 25°c. q = 2.08 ml s -1 . figure 12 presents the curves resulting from the electrolysis of amoxicillin in sulfuric acid and in perchloric acid solution under 100 ma cm -2 . in both acid solutions, cod decreases with time. two curves follow an exponential decay and linear relationship of the logarithm of cod with time was obtained. the decrease of cod seems to be more rapid in sulfuric acid than in perchloric acid owing to fact that after 5 h of electrolysis, cod removal is about 87 % and 92 % in perchloric acid and in sulfuric acid, respectively. in both acid solutions, a pseudo first order reaction occurs and the cod removal rate constant determined were 0.5258 s -1 and 0.4304 s -1 in sulfuric acid and in perchloric acid, respectively. the observed difference of the rate of the cod removal in both solutions could result from the differen types of reactive species produced in the solution during j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 140 the electrolysis of amoxicillin. besides oh* production, peroxyde is produced in perchloric acid solution while in sulfuric acid solution persulfate is produced. those oxidants contribute in extending the oxidation of the organic from the vicinity of the electrode surface to the bulk of the solution. the rapid decay of cod in sulfuric acid compared to perchloric acid could be linked to the oxidative performance of persulfate which is higher than that of peroxyde. during the electrolysis, the absorbance of the samples withdrawn from the simulated wastewater tank was recorded. the result is shown in figure 13. one observed in figure 13 that during the first 1 h, an increase of the absorbance is observed. that could be due to the production of intermediates which absorb at the same wavelength as amoxicillin. as the electrolysis is still running, a decrease of the absorbance is observed until total degradation is obtained after 5 h of electrolysis. in the course of the experiment, the colour of the solution was followed. the colour of the solution became yellow as soon as the electrolysis of amoxicillin started in the current experimental conditions. the intensity of the solution colour increased until the electrolysis time reaches 2 h. after 2h of electrolysis, a decrease in the yellow colour intensity was observed and that occured until the end of the electrolysis i.e. 5 h when the solution became colourless indicating that a complete oxidation of amoxicillin. in fact, the appearance of the yellow colour during the electrolysis could be an indication of the production of intermediates which further undergo oxidation (removal) owing to the desappearence of the observed colour. figure 13. absorption spectra of amoxicillin samples during amoxicillin electrolysis at 100 ma cm -2 . the energy consumption was estimated after the electrolysis. during the degradation of amoxicillin, the cell potential was followed and it remains almost constant either in sulfuric acid or in perchloric acid. the cell potentials were about 14.6 ± 1.0 v and 5.8 ± 1.0 v in sulfuric acid and in perchloric acid solution respectively. the specific energy consumed during the electrolysis of amoxicillin was about 0.096 kwh cod -1 and 0.035 kwh cod -1 in sulfuric acid and in perchloric acid respectively. the cod being expressed in g l -1 . g. c. quand-meme et al. j. electrochem. sci. eng. 5(2) (2015) 129-143 doi:10.5599/jese.186 141 figure 14. photo of the solution withdrawn from the wastewater tank during electrolysis (from left to rigth : t =0 h; 1h; 2h; 3h; 4h; 5h) at the end, bacl2 was added to the solution that was withdrawn after 5h in case of perchloric acid, and positive reaction occurs indicating the release of so4 2− in the solution. finally, complete mineralization of amoxicillin occured in the current experimental conditions. the overall reaction could be: c16h19n3o5s + 40 h2o  16co2 + 3no3 + so4 2 + 99h + + 94e conclusion from this work, it appears that the boron doped diamond electrode (bdd) have a polycrystalline structure. its surface is composed of grains with sizes between 0.3 and 0.6 µm. besides diamond (csp 3 ) crystal, non-diamond carbon impurities (csp 2 ) i.e. graphitic type carbon are found on the bdd surface. as grown boron doped diamond surface chemical bonds are composed of c-c and c-h. the electrochemical characterization of bdd with the ferri/ferrocyanide redox couple showed that the used boron doped diamond electrode is an electrical conducting electrode and a good electrical contact is formed between the silicon substrate and the diamond coating. in sulfuric acid used as a supporting electrolyte, the voltammetric investigation showed a wide potential window of 2.74 v of water stability on bdd. the electrochemical oxidation of amoxicillin using cyclic voltammetric techniques indicated that a direct oxidation of amoxicillin occured in the electrochemical window of water stability on bdd. that oxidation was catalyzed by the non-diamond carbon impurities such as csp 2 presents at the bdd’s grain boundaries. for the bulk electrolysis of the amoxicillin in sulfuric acid under galvanostatic regime in a batch system, amoxicillin undergoes degradation reaching the highest chemical oxygen removal (92 %) under a current density of 100 ma cm -2 in sulfuric acid used as supporting electrolyte. the cod decrease follows a pseudo first order reaction with the cod removal rate constant of 0.5258 s -1 . using perchloric acid as supporting electrolyte, 87 % as cod removal yield and 0.4304 s -1 as cod removal rate constant were obtained under 100 ma cm -2 . the rapid decay of cod in sulfuric acid compared to perchloric acid was linked to the oxidative performance of in situ formed oxidant such as persulfate and peroxyde. the specific energy consumed were about 0.096 kwh cod -1 and 0.035 kwh cod -1 in sulfuric acid and in perchloric acid respectively after 5 h of electrolysis. complete mineralization of amoxicillin was achieved after 5 hours of electrolysis and a release of minerals such as so4 −2 was observed. acknowledgements: we greatly thank the swiss national funds for its financial support that allowed this work to be carried out. our team has received part of the grant iz01z0_146919 for j. electrochem. sci. eng. 5(2) (2015) 129-143 oxidation of amoxicillin at bdd electrode 142 that work. we thank prof. christos comninellis for supplying us the boron doped diamond electrodes. we also thank prof. bakayoko-ly ramata, the president of the university felix houphouët-boigny for her help in the realization of that work. references [1] k. g. koura, a. garcia, b. todoégnon, p. deloron, m. cot, j.-f. faucher, acta tropica 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[35] l. gherardini, p. a. michaud, m. panizza, c. comninellis, n. vatistas, journal of the electrochemical society 148(6) (2001) d78-d82. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) 0 http://creativecommons.org/licenses/by/4.0/ phenolic compounds removal from mimosa tannin model water and olive mill wastewater by energy-efficient electrocoagulation process doi: 10.5599/jese.2014.0066 215 j. electrochem. sci. eng. 4(4) (2014) 215-225; doi: 10.5599/jese.2014.0066 open access : : issn 1847-9286 www.jese-online.org original scientific paper phenolic compounds removal from mimosa tannin model water and olive mill wastewater by energy-efficient electrocoagulation process marijana kraljić roković, mario čubrić and ozren wittine faculty of chemical engineering and technology, university of zagreb, marulićev trg 19, croatia corresponding author e-mail: mkralj@fkit.hr; tel.: +-385-1-4597112; fax: +-385-1-4597139 received: august 1, 2014; revised: august 26, 2014; published: december 6, 2014 abstract the objective of this work was to study the influence of nacl concentration, time, and current density on the removal efficiency of phenolic compounds by electrocoagulation process, as well as to compare the specific energy consumption (sec) of these processes under different experimental conditions. electrocoagulation was carried out on two different samples of water: model water of mimosa tannin and olive mill wastewater (omw). low carbon steel electrodes were used in the experiments. the properties of the treated effluent were determined using uv/vis spectroscopy and by measuring total organic carbon (toc). percentage of removal increased with time, current density, and nacl concentration. sec value increased with increased time and current density but it was decreased significantly by nacl additions (0-29 g l -1 ). it was found that electrocoagulation treatment of effluents containing phenolic compounds involves complex formation between ferrous/ferric and phenolic compounds present in treated effluent, which has significant impact on the efficiency of the process. keywords complexation; nacl; low carbon steel, uv/vis spectroscopy, total organic carbon (toc). introduction high amounts of waste water are generated in the mediterranean area each year during the short periods of the olive oil production process, from october to december. its direct disposal in nature is not acceptable since it contains dark colour, organic materials, and emulsion oils. additionally, such waters contain phenolic compounds that have a negative impact on vegetation http://www.jese-online.org/ mailto:mkralj@fkit.hr j. electrochem. sci. eng. 4(4) (2014) 215-225 phenolic compounds removal by electrocoagulation 216 and microorganisms, and therefore must be treated in order to remove organic and toxic pollutants. the amount of these compounds is usually in the range of 5-25 g l -1 depending on climate, variety of olive fruit, cultivation, ripeness at harvest, as well as on extraction process [1,2]. different methods have been developed for olive mill wastewater (omw) treatment such as biological treatment [3-5], physico-chemical treatments [6], photocatalytic oxidation [7,8], electrooxidation [9], electrocoagulation [10-13], and a number of combined treatments [14,15]. phenolic compounds extracted from omws could possibly be utilized in the cosmetic, pharmaceutical, or food industries [16], or even as potential source of natural dyes [17]. however, none of treatments used were completely satisfying. electrocoagulation is a well-known remediation technique that can be used alone [10-18] or in combination with other techniques [19]. it is a simple, effective, and low cost process, easily adaptable to other systems. furthermore, in remote areas without electricity, it could be directly powered by a photovoltaic system in order to achieve a self-sustainable unit [20-22]. the goal of this work was to study the influence of nacl concentration, current density and time on the removal efficiency of phenolic compounds by the electrocoagulation process, as well as to compare specific energy consumption (sec) of the processes under different experimental conditions. electrocoagulation was carried out for two different samples of water: model water of mimosa tannin and olive mill wastewater (omw). mimosa tannin is a phenolic compound similar to those phenolic compounds present in omw but it is somewhat more complex. in this work it was used in order to examine the influence of pure phenolic compounds on the removal efficiency of the electrocoagulation process. after detailed analysis of the results obtained in model water, experiments were also carried out in omw. electrocoagulation processes are usually carried out without or with only small amounts of nacl to avoid its presence in discharge water. however, since most olive oil production plants are situated on the mediterranean coastline, additions of nacl and the disposal of the treated effluent containing nacl into the sea is acceptable. additionally, it is well known that the amount of salt is the crucial parameter for energy consumption in the electrolysis process, and for the feasibility of this technique. experimental all the chemicals used in this research were of analytical grade. mimosa tannin (mimosa central co-operative ltd., south africa) solutions were prepared using bi-distilled water with the addition of an appropriate amount of nacl. the olive mill wastewater (omw) used in this study was obtained from a local olive oil manufacturer (croatia). it was stored in an open puddle for two months before the sampling. prior to the experiments the omw was filtrated to remove suspended solids. conductivities and ph values were measured using a conductivity meter (oakton pcd650) and a ph meter (radiometer, phm 220). at the end of the electrocoagulation process of the mimosa tannin model water, the formed colloids were left during the night to settle and afterwards the sludge was separated from the effluent on a buchner funnel using a water aspirator. the sludge was dried in open air for three days and then the mass of the sludge was determined. treated effluents were analysed by different techniques (ph meter, conductivity meter, uv/vis spectroscopy, and toc). before the uv/vis spectroscopy and toc analysis were carried out, the solutions was centrifuged using 9000 rpm (hettich universal, mikro 12-24 centrifuge). m. kraljić roković et al. j. electrochem. sci. eng. 4(4) (2014) 215-225 doi: 10.5599/jese.2014.0066 217 at the end of electrocoagulation process of the omw, the water was immediately filtrated in a buchner funnel and analysed using the same techniques as in the case of mimosa tannin solution. the phenolic compounds concentration was measured at the wavelength corresponding to the maximum absorbance using a spectrophotometer (ocean optics 200, uv light source analytical instrument systems inc., model d 1000 ce) connected to a computer, in 1 cm path-length cells. the equation used to calculate the phenol removal efficiency in the experiments was: 0 0 / % = 100r     (1) where γ0 and γ are defined as the concentration before and after electrocoagulation process. a similar equation was used to calculate total organic carbon (toc) removal efficiency (rtoc / %). the toc measurements were done on a shimadzu analyser toc v-csn using the npoc method. the electrocoagulation experiments were carried out in a glassy electrolytic cell with dimensions of 7 x 7 x 6 cm. parallel plate electrodes were immersed in the cell with a working electrode situated between two counter electrodes in order to achieve a good current and potential distribution, and a uniform electrode dissolution. low carbon steel (composition: 0.06 % c, 0.015 % p, 0.008 % s, 0.007 % si and 0.35 % mn) was used for all electrodes. before the experiment the electrodes were polished using 600 grit emery paper, they were washed with bidistilled water, and finally, with ethanol. the working electrode had a total immersed area of 10 cm 2 , and the counter electrodes had a total immersed area of 20 cm 2 . the distance between the electrodes was 1 cm. before the experiment, the appropriate amount of nacl was dissolved in the treated solutions. during the experiment constant stirring speeds of 600 rpm and dc power supply (iskra, ma 4165; 1.5 a; 25 v) were used. all the experiments were carried out at room temperature (23±1 o c). results and discussions treatment of the model water containing mimosa tannin the mimosa tannin (γ = 1 g l -1 ) solution prepared in 0.5 mol dm -3 nacl has ph 5.1 and κ = 38.3 ms cm -1 . in order to find out the removal efficiency of mimosa tannin over different durations, the process was carried out for 5, 15, and 35 minutes using current densities of 10 ma cm -2 . the characteristic voltage of this process was 1.25 v. the influence of the duration of the experiment on the electrocoagulation efficiency is presented in table 1. within the 5 min removal period removal efficiency reached 92.2 %, while further treatment resulted only in its slight increase (35 min, 96.7 %). energy consumption during the experiment increased proportionally with time since the current density and voltage were constant throughout the experiments. to explain the mechanism of the electrocoagulation process one must consider the reactions occurring on both electrodes. during the anodic process iron is oxidized and dissolved as fe 2+ . under the experimental conditions used in this work (ph = 5.1) it does not undergo hydrolysis. however, in aerated conditions it is further oxidized by dissolved oxygen to fe 3+ , which is susceptible to hydrolysis, resulting in different aqua and hydroxyl complexes, such as fe(oh) 2+ , fe(oh) + , fe(oh)3 under acid conditions, or fe(oh)6 3 and fe(oh)4 under alkaline conditions [23]. since the ph values registered before and after electrocoagulation process varied from 5-7, j. electrochem. sci. eng. 4(4) (2014) 215-225 phenolic compounds removal by electrocoagulation 218 positively charged or neutral particles were expected under the given experimental conditions. furthermore, negative ions present in water (in the case of nacl addition it is cl ) will surround a positive charge, forming a diffuse layer which makes the particle neutrally charged. these surface properties make the particles unstable and agglomeration takes place. as a result of agglomeration, particles form flocks precipitating or floating by the bubbles of hydrogen (figure 1.). the stability of the particles and their agglomeration depends on the type and concentration of ions in the solution. the formed flocks can effectively remove pollutants by adsorption or enmeshment in a precipitate. the aim of this work was to remove mimosa tannin from model water. it is well known from the literature that dissolved iron in the presence of tannin forms ferrous/ferric tannates. these reactions are pretty complex since both ions, ferrous and ferric, can participate in the reaction, and in addition there is also a possibility of fe 3+ reduction by mimosa tannins. according to the author’s knowledge, the formation of the complex during phenolic compounds removal by electrocoagulation technique was not considered before, although it might be of a great importance for its progress. it can influence the amount of ferrous/ferric ions required for the coagulation, since fe 2+ /fe 3+ are consumed by the complex formation. additionally, it can also impact the mechanism of the reaction because generated complex will be involved in the adsorption or enmeshment by formed flocks instead of mimosa tannin. it could also be important for the electrode reaction kinetics since tannin inhibits dissolution of iron [24,25]. therefore it can be concluded that complex formation should not be ignored when considering the electrocoagulation process. figure 1. illustration of flocks precipitating due to gravity, and floating due to the bubbles of gas an important parameter of the electrocoagulation process is the effluent ph value at the end of the process. in these experiments ph values decreased for processes conducted over 5 min, while they increased for prolonged treatments (15 and 35 min) (table 1). the increase of ph values were caused due to the intensive hydrogen evolution at the cathode and the generation of oh ions. the generated oh ions were consumed during the hydrolysis of fe 3+ , but the overall reaction obviously resulted in the excess of oh ions. the final ph value depended on the equilibrium of each reaction in the process. additionally, the increase of ph value can be explained by mimosa tannin removal due to its acidic behaviour. m. kraljić roković et al. j. electrochem. sci. eng. 4(4) (2014) 215-225 doi: 10.5599/jese.2014.0066 219 table 1. results of treatments at different times (j = 10 ma cm -2 , u = 1.25 v, γ (nacl) = 29.22 mol g l -1 , κ = 38.3 ms cm -1 , ph = 5.1, γ0(tannin)=1 g l -1 , v = 0.1 l). t / min r / % sec/ kw h kg -1 m(precipitate) / g ph(after ec) 5 92.21 0.113 0.143 4.81 15 95.54 0.327 0.197 5.74 35 96.68 0.754 0.263 6.31 since the concentration of ferrous/ferric ions is dependent on current density, the efficiency of the process is dependent on current density as well. in this work the electrocoagulation process was conducted for 15 min using different current values (figures 2 and 3). when the current increased from 1 to 10 ma cm -2 , removal efficiency increased from 10 to 20 % in 0.58 g l -1 nacl, from 47 to 93 % in 5.84 g l -1 nacl, and from 58 to 96 % in 29.22 mol dm -3 nacl (figure 2). however, specific energy consumption per mass of mimosa tannin increased more significantly from 0.302 to 7.311 kw h kg -1 in 0.58 g l -1 nacl, from 0.034 to 0.469 kw h kg -1 in 5.84 g l -1 nacl, and from 0.021 to 0.327 kw h kg -1 in 29.22 g l -1 nacl (table 2). as evident, the highest removal efficiency was obtained at high current density. the obtained results also show that the highest removal efficiency was registered in the presence of high nacl concentrations, where the lowest specific energy consumption is required. it is supported by figure 4, where the colour of the solution suggests more difficult precipitation of flocks for small additions of nacl causing reduced removal efficiency. this is in accordance with theory that coagulation process depends on type and concentration of ions present in solution. it is obvious that optimal process conditions were obtained in the presence of high amounts of nacl. these results pointed out the importance of the nacl concentration as a key parameter for an efficient and low cost process. nacl’s presence is important because of the two effects: (a) it decreased the applied voltage and energy power demand [26] and (b) it changed the ionic strength that affected the coagulation process as evident from figures 2 and 4. the influence of ionic concentration and zeta potential on the electrocoagulation process was reported previously [27, 28]. figure 2. influence of nacl concentration and current density on removal efficiency. j. electrochem. sci. eng. 4(4) (2014) 215-225 phenolic compounds removal by electrocoagulation 220 the main factor influencing energy consumption is applied voltage. the overall voltage is dependent on equilibrium potential difference (er,k-er,a), anode and cathode over-potentials (ηa, ηk), and ohmic potential drop in the solution (ηir ) according to the equation (2):  er r,k r,a a k iru e e        (2) ohmic potential drop in the solution is dependent on cell configuration, electrode area (a / m 2 ), and the distance between electrodes (d / m), as well on the conductivity of solution (κ / s cm -1 ): ir d i a          (3) table 2. results of treatments at different nacl concentration and current density (t = 15 min, ph = 5.1, γ(tannin) = 1 g l -1 , v = 0.1 l). γ (nacl) / g l -1 j / ma cm -2 u / v sec / kw h kg -1 m(precipitate) / g 29.22 (κ = 38.3 ms cm -1 ) 10 1.25 0.327 0.197 5 0.95 0.146 0.134 1 0.50 0.021 0.103 5.84 (κ = 10.11 ms cm -1 ) 10 1.75 0.469 0.140 5 1.15 0.188 0.128 1 0.65 0.034 0.094 0.58 (κ = 0.98 ms cm -1 ) 10 5.75 7.311 0.080 5 3.65 3.061 0.044 1 1.25 0.302 0.038 figure 3. results of treatments at different current densities in the case of mimosa tannin: (a) 10 ma cm -2 ; (b) 5 ma cm -2 ; (c) 1 ma cm -2 (t = 15 min; 29.22 g l -1 nacl). figure 4. results of treatments at different nacl concentrations in the case of mimosa tannin: (a) 29.22 g l -1 nacl; (b) 5.84 g l -1 nacl; (c) 0.58 g l -1 nacl (j = 10 ma cm -2 ; t = 15). a a c b c b m. kraljić roković et al. j. electrochem. sci. eng. 4(4) (2014) 215-225 doi: 10.5599/jese.2014.0066 221 sec is dependent on current value (i), applied voltage (u) and time (t) and it is expressed as energy consumption per mass of removed tannin:     0 sec iut tannin tannin v    (4) where γ is the mass concentration of tannin or phenolic compound, v is volume of treated solution. another important parameter for sec is the distance between the electrodes, which in most of the previous reports ranged from 0.3-3.0 cm, while the distance in this work was 1 cm. a small distance is preferable to decrease potential drop, but the electrodes should be adequately separated in order to enable unhindered movement of flocks between them. conductivity of the solution i.e. salt concentration also plays important role for sec value and according to our knowledge its value was quite different in different reports. kobaya et al. [29] treated textile wastewater by electrocoagulation process and it was shown that an addition of nacl (κ = 1000-4000 s cm -1 ) did not influence process efficiency but energy consumption decreased with increased wastewater conductivity (2.2-0.75 kw h kg -1 (cod)). sengil et al. [30] have used electrocoagulation for decolourization of reactive red and it was found that small additions (0.5-2.0 g l -1 ) of nacl increase efficiency while further addition did not have any impact. the optimal conditions were found to be 2.3 g l -1 nacl and 4.54 kw h kg -1 (dye). b. k. nandi et al. [31] varied nacl concentration from 0.1-1.0 g l -1 and it was found that efficiency had increased from 97-100 % and energy consumption had decreased from 17-3 kw h kg -1 (fe). x. chen et al. [32] separated pollutants from restaurant wastewater by electrocoagulation process without the addition of nacl when energy consumption was in the range of 0.2-1.4 kw h m -3 depending on the solution conductivity that varied from 770-227 s cm -1 . the additions of nacl changed conductivity from 443-2850 s cm -1 and energy consumption as well, from 0.32-0.29 kw h m -3 ; however, it did not change the efficiency of the process. from the previous results it follows that nacl concentration can influence specific energy consumption drastically, which will have an impact on operating cost. however, the dependence of removal efficiency on nacl concentration is not completely clear and it depends on the type of pollutant and its concentration. the results of this paper confirm that nacl addition decreases specific energy consumption in accordance with the previous results. furthermore, it was shown that efficiency of the phenolic compound removal can also be improved considerably by the addition of nacl in the range from 0.58 g l -1 to 29.22 g l -1 . treatment of omw the starting omw solution had the following characteristics: ph 5.37, concentration of phenolic compounds, γ0 = 0.613 g l -1 (mimosa tannin equivalent), and the toc value was 1376 mg l -1 . these values were somewhat lower in comparison to the values frequently found in the literature [10-12]. this can be explained by the fact that the omw was kept in an open puddle for 2 months. during omw treatment by electrocoagulation process in previous investigations the solution as received was used or it was diluted with water. the conductivity of the pure omw sample was 11 ms cm -1 [10-12] and for diluted omw (1:5) conductivity was 3.6 s cm -1 [13]. the sec value obtained during the omw treatment was found to be 4 kw h kg -1 (cod) [13] or 20-300 kw h m -3 (volume of treated solution) [12], which were quite similar considering the characteristic cod j. electrochem. sci. eng. 4(4) (2014) 215-225 phenolic compounds removal by electrocoagulation 222 value for omw. also, it was shown that small additions of nacl improve removal efficiency while additions higher than 2 g l -1 decrease removal efficiency. energy consumption has decreased upon nacl addition. from the results of the treatment of mimosa tannin, the current density of 10 ma cm -2 was chosen for the omw treatment. the process was carried out with different additions of nacl from 0-20 g l -1 during 35 or 60 min. depending on nacl addition conductivity of the solutions was changed from the value similar to the previously reported values (2.3 s cm -1 ) to the value higher than previously reported (23.7 s cm -1 ). the sec value was changed from 8.5-1.6 kw h kg -1 (mass of phenolic compounds) during 35 min or it was changed from 8.2-2.6 kw h kg -1 (mass of phenolic compounds) during 60 min. similarly as in the case of model water, addition of nacl had positive impact on removal efficiency (figures 5 and 6) and energy consumption (table 3). furthermore, better efficiency was obtained by prolonged process time while sec was not increased significantly. at the end of the electrocoagulation process ph value was close to 7, which was acceptable for discharge, while conductivity increased only slightly. table 3. results of treatments at different nacl concentration and process times (j= 10 ma cm -2 . ph = 5.37. v= 0.1 l, γ0 = 0.613 g l -1 ). t/ min γ(nacl) / g l -1 u / v sec/ kw h kg -1 phafter ec κ(omw)before ec / ms cm -1 κ(omw)after ec / ms cm -1 0 3.8 8.492 6.65 2.32 3.43 35 5 1.9 2.677 6.85 8.23 9.48 10 1.5 1.998 6.88 13.77 14.75 20 1.4 1.604 6.9 23.72 24.79 0 3.9 8.226 6.7 2.32 3.26 60 5 2.1 4.26 6.81 5.99 6.55 10 1.6 2.941 6.94 10.12 10.98 20 1.5 2.562 6.98 20.44 21.75 figure 5. dependence of removal efficiency of phenolic compound in omw treated effluent on nacl concentration and process time. m. kraljić roković et al. j. electrochem. sci. eng. 4(4) (2014) 215-225 doi: 10.5599/jese.2014.0066 223 figure 6. dependence of removal efficiency of toc in omw treated effluent on nacl concentration and process time. from figures 5 and 6 it is evident that the removal of overall organic loading (toc) is lower (30 70 %) in comparison to phenolic compounds (40 90 %). better efficiency in the case of phenolic compounds could be the consequence of complex formation between ferrous/ferric and phenolic compounds present in the omw. it is also evident that removal efficiency of phenolic compounds in omw was lower compared to the removal efficiency of mimosa tannin, although longer times were used (figures 2 and 5). it is not surprising since this solution, apart from the phenolic compounds, contains some other constituents such as oil, sugar, and pulp suspension [2]. therefore, the capacity of produced sludge for phenolic compounds removal is reduced in the case of omw compared to the model water. conclusions the results obtained in this paper show that it is possible to obtain high removal efficiency of mimosa tannin and phenolic compounds from omw by electrocoagulation process. in the case of mimosa tannin, electrocoagulation was able to reduce the phenolic content up to 96 %, while in the case of omw electrocoagulation wasa able to reduce the phenolic content up to 92 %. the percentage of removal was increased with increased time, current density, and nacl concentration. apart from the increasing removal efficiency of the process, an improvement in energy demand was also obtained with the addition of nacl. therefore, it can be concluded that an addition of nacl can significantly improve the electrocoagulation process. furthermore, additions of nacl and the disposal of treated effluent containing nacl are acceptable for the production plants located close to the coast. at the end of process ph value was close to 7, which is acceptable for discharging. it was shown that complex formation between ferrous/ferric and phenolic compounds present in treated effluent could change the efficiency of the process. thus, due to the complexation, removal of phenolic compounds was higher in comparison to removal of overall organic loading (toc). it was also observed that the removal efficiency of mimosa tannin is higher compared to the removal efficiency of phenolic compounds from omw although longer times were used. it is explained by the decreased capacity of produced flocks for phenolic compounds removal, in the case of omw, due to the presence of other organic constituents. j. electrochem. sci. eng. 4(4) (2014) 215-225 phenolic compounds removal by electrocoagulation 224 acknowledgements: financial support by ministry of science, education and sports of republic of croatia (project 125-1252973-2576) is gratefully acknowledged. the authors express their gratitude to višnja pavić for providing mimosa tannin. references [1] http://www.agbiolab.com/files/agbiolab_polyphenols.pdf [2] f. federici, pomologia croatica 12 (2006) 15-27. 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[31] b. k. nandi, s. pateol, arabian journal of chemistry, in press, corrected proof, doi: 10.1016/j.arabjc.2013.11.032 [32] x. chen, g. chen, p. l. yue, separation and purification technology 19 (2000) 65-76. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ electrochemical immunosensor for the determination of beta casein doi: 10.5599/jese.2015.0072 9 j. electrochem. sci. eng. 5(1) (2015) 9-16; doi: 10.5599/jese.2015.0072 open access : : issn 1847-9286 www.jese-online.org short communication electrochemical immunosensor for the determination of β-casein judith molinari, carlos moina and gabriel ybarra unidad técnica nanomateriales, inti-procesos superficiales, instituto nacional de tecnología industrial, c.c. 157, b1650wab san martín, argentina corresponding author: e-mail: gabriel@inti.gov.ar; tel.: 011-4724-6333; fax: 011-4724-6343 received: october 6, 2014; revised: february 20, 2015; published: march 15, 2015 abstract an amperometric biosensor for the quantification of food allergens based on an inhibitory immunoassay is presented. as a proof of concept, the experimental conditions were optimized for the detection of β-casein in the 0-10 ppm range. eight electrochemical cells were integrated into a small-sized portable potentiostat controlled by a smartphone via bluetooth communication. the determination of β-casein in eight different samples can be measured with the electrochemical biosensor, which has the potential to be modified for the detection of multiple allergens. keywords allergen; casein; electrochemistry; elisa; immunoassay introduction food allergies are a worldwide growing concern due to its impact on food safety and public health. milk is one of the most common food allergens affecting infants, with prevalence roughly between 1 % and 15 % [1]. milk contains several proteins that can be potentially involved in allergic sensitization but only a few of them are recognized as being major allergens, mainly βlactoglobulin, caseins, and α-lactalbumin. there are several technical possibilities for the detection of milk allergenic proteins, targeting either the protein itself or dna fragments [schubert-ullrich et al. 2009]. elisa is the most commonly used method in food industry laboratories due to its high precision, low detection limits, and high specificity. however, elisa’s drawbacks include its timeconsuming procedure and the requirement of relatively expensive laboratory instrumentation. biosensors are compact analytical devices which employ specific biorecognition elements combined with state-of-the-art integrated electronics, and represent an inexpensive alternative to http://www.jese-online.org/ mailto:gabriel@inti.gov.ar j. electrochem. sci. eng. 5(1) (2015) 9-16 determination of β-casein 10 10 methods which require desktop instrumentation such the enzyme-linked immunosorbent assay (elisa) [3,4]. biosensors are characterized by their portability, ease of use and high degree of automation, ideally maintaining high quality analytical standards. although a considerable amount of work in the field of biosensors has been carried out in recent years, the use of electrochemical biosensors aimed at the detection of food allergens is rather scarce. a recent review presents an up-to-date outlook of the field [5]. competitive elisa biosensors have been developed for the detection of cyanobacterial toxins [6] and okadaic acid in shellfish [7]. most of the electrochemical biosensors for the detection of food allergens are based on transduction techniques such as differential pulse voltammetry and electrochemical impedance spectroscopy. low limits of detections (in the order of nm or ng ml -1 ) have been reported using these techniques [8,9]. however, these techniques require a rather complex electronic instrumentation. moreover, in many cases of practical interest the most useful range of concentration of food allergens lies in the order of 1-10 ppm [10,11]. here we present the preliminary results of an alternative method for the detection of β-casein in the order of a few ppm consisting in a portable immunosensor devised for the analysis of food allergens. it is based on antigen-antibody reactions between soluble and immobilized antigens. antigen-antibody complexes formed onto the electrode surface are detected with a redox enzyme-labeled secondary antibody. the enzymatic activity is amperometrically detected after the application of a potential step so that the associated electronics are simple and data processing is straightforward. moreover, bluetooth connectivity and a smartphone application make possible to carry out quantitative measurements with this portable, compact and easy-to-use device, without the need of computers. experimental n-hydroxysuccinimide (nhs), 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (edc), type i horseradish peroxidase (hrp) and β-casein from bovine milk were purchased from sigma–aldrich. rabbit polyclonal antibody anti-(bovine β-casein) was provided from the biochemistry and nutrition area, institute of food technology, agrobusiness research centre, national institute of agricultural technology. goat anti-rabbit igg (h + l)-hrp conjugate was purchased from biorad. phosphate buffer saline ph 7.2-7.5 (pbs) was prepared with 0.1 m nah2po4 (mallinckrodt) and 0.15 m nacl (biopack). phosphate buffer 0.1 m ph 7.0 was prepared with kh2po4 (merck). blocking buffer was prepared with 0.01 % polysorbate 20 (biopack), 1 % gelatin (merck) in 0.1 m buffer phosphate of ph 7.0. rinsing buffer was prepared with 0.05 % polysorbate 20 (biopack®) in 0.1 m buffer phosphate of ph 7.0. measurement buffer was prepared in pbs with 0.1 m kcl, 4 mm hydroquinone and 1.5 mm hydrogen peroxide (biopack). thick film carbon electrodes were printed onto α-al2o3 substrates by screen printing technology (fig. 1a). a commercial carbon paste (dupont bq242) and 96 % α-al2o3 substrates were employed. electrode layout was transferred by means of photolithography to a stainless steel mesh (200 wires per inch) with a negative photosensitive film (ulano cdf-4). carbon ink printing was performed with an ekra microtronic-ii printer, dried in box oven at 70 °c during 20 min. the electrodes were integrated in an electrochemical cell, constructed with poly(methylmethacrylate) (pmma) using a numeric control device from a cad layout (fig. 1c). the carbon electrodes were treated with an oxygen plasma to promote the formation of carboxylic groups on the surface. a diener plasma polymerization equipment was used under following conditions: an oxygen pressure of 1 mbar, a set temperature of 50 °c and a time of plasma treatment of 15 s. j. molinari et al. j. electrochem. sci. eng. 5(1) (2015) 9-16 doi: 10.5599/jese.2015.0072 11 a c b figure 1. (a) set of 8 pairs of working and counter electrodes screen printed on alumina; (b) electrochemical cells; (c) electrochemical platform comprehending electrochemical cells and electronic instrumentation connected to a smartphone via bluetooth. β-casein was immobilized onto the electrode surface using the carbodiimide method [12]. carboxylic groups formed on the carbon electrode after plasma treatment were treated with 100 µl of 0.1 m edc and 10 µl of 25 mm nhs for 30 minutes and, after rinsing, incubated for 2 hours with 50 µl of a 100 ppm β-casein solution in 0.1 m phosphate buffer of ph 7.0. after rinsing, the electrodes were incubated overnight at 4 °c in a wet chamber with the blocking buffer. finally, the electrodes were rinsed and were ready to be used. solutions of β-casein in pbs of different concentrations (0, 1, 5, 10, 15 and 20 ppm) and 1:10000 rabbit polyclonal antibody antibovine β-casein in blocking buffer solution (1:1) were pre-incubated and added to each electrode for 1 hour at 37 °c. rinsed electrodes were incubated with 1:2000 goat anti-rabbit igg conjugate blocking buffer solution in same conditions. potentials were measured and referred to in the text against an ag|agcl|0.1 m kcl reference electrode (ø 1 mm). electrochemical measurements were carried out at 25 °c in 50 µl of a pbs buffer of ph 7, 0.1 m kcl, 4 mm hydroquinone (redox mediator) and 1.5 mm h2o2, prepared from analytical grade reagents (merck) and mili-q water. amperometric measurements were carried out with a portable potentiostat nanopoc® controlled by a smartphone via bluetooth connection, schematically shown in fig. 1c. details on the electronic instrumentation are given elsewhere [13]. the working electrode potential was set at -280 mv and the resulting current was recorded during 60 s; this potential value is negative enough to produce the reduction of 1,4 benzoquinone under diffusion-controlled conditions [14]. the concentration of β-casein used for the immobilization, the dilutions of the primary and secondary antibodies and the incubation parameters were optimized by elisa based on a protocol developed by vitkova et al. [15]. the optimized values found for elisa were tested on the biosensor and the electrochemical signal was optimized again adjusting the dilution of the primary and secondary antibodies. the polystyrene microplates were filled with different concentration of β-casein (0, 2.5, 10 and 100 ppm) in carbonate-bicarbonate buffer of ph 9.6 (50 µl/well) and incubated overnight at 4 °c for absorption. the coated plates were washed three times with 0.01 m pbs containing 0.01 % polysorbate 20 (pbs-p20) to remove unbound antigens. each well was filled with 300 µl of blocking buffer (pbs-p20 containing 1 % gelatin) for 1 hour at room j. electrochem. sci. eng. 5(1) (2015) 9-16 determination of β-casein 12 12 temperature and was washed three times with pbs-p20. 55 µl of pbs-p20 containing different concentrations of β-casein (0, 0.1, 10, 100 ppm) and 55 µl of different dilutions in pbs-p20 1 % gelatin (1:10000, 1:128000, 1:1000000) of the primary antibody (rabbit polyclonal antibody antibovine β-casein) were pre-incubated during 1 h at 37 °c. aliquots (90 µl) from the content of each preincubated well were incubated with immobilized β-casein during 1 h at 37 °c. after washing three times with pbs-p20, 100 µl of the hrp conjugate secondary antibody (goat anti-rabbit igg (h + l)-hrp conjugate) diluted 1:5000 in pbs-p20 1 % gelatin were added to each well, incubated for 1 h at 37 °c, washed three times with pbs-p20. aliquots (50 µl/well) of 50 mm citrate/phosphate buffer of ph 3.2 containing 60 mm hydrogen peroxide and 50 µl/well tmb (3,3',5,5'tetramethylbenzidine 0.01 m) in 0.1 n hcl were incubated during 30 min at room temperature. the reaction was stopped with 50 µl/well of 2m h2so4. absorbance was measured at 450 nm. in brief, the tested values of concentration of β-casein used in the immobilization on polystyrene wells were 0, 2.5, 10 and 100 ppm. the used primary antibody dilutions were 1:10000, 1:128000 and 1:1000000 while the dilution of the secondary antibody was kept constant at 1:5000, following the recommendations given by the supplier. the best results for elisa were obtained when a solution of 100 ppm β-casein used for the immobilization on polystyrene, with 1:128000 and 1:5000 dilutions for the primary and secondary antibodies respectively. when the optimized values found for elisa were used in the electrochemical biosensor, it was found that higher antibody titers were necessary; consequently a 1:10000 and 1:2000 dilutions were used for the primary and secondary antibodies, respectively. results and discussion the biosensor presented here is a portable immunosensor devised for food allergen analysis. the allergen of choice was β-casein, one of the main milk proteins. this method is based in inhibitory antigen-antibody reactions between soluble and electrode surface immobilized antigen. the electrochemical signal detected, due to the enzymatic activity of hrp-conjugated secondary antibody, is inversely related to the β-casein concentration in the sample solutions. carbon screen printed electrodes were used because of their low cost and ease of preparation. the central working electrode has a diameter of 1 mm. a set of 8 pairs of working electrodes and counter electrodes were printed to fit an acrylic cell with a total volume of 50 µl. the carbon electrodes were treated with an oxygen plasma which is known to generate carboxylic groups in the carbon surface [16]. the carboxylic groups were used as anchoring sites for proteins, either hrp or β-casein, through the carbodiimide reaction. enzymatic electrodes in order to assess the performance of the chosen immobilization procedure, hrp enzymes were immobilized onto the plasma-treated carbon screen printed electrodes. fig. 2a shows the current transients obtained for concentrations of hydrogen peroxide in the 0-3 mm range. the measured current increases with increasing hydrogen peroxide concentration. a plot of the current measured at 60 s versus the hydrogen peroxide concentration is shown in fig. 2b. j. molinari et al. j. electrochem. sci. eng. 5(1) (2015) 9-16 doi: 10.5599/jese.2015.0072 13 figure 2. electrochemical detection of the enzymatic activity of hrp immobilized onto the carbon working electrode. (a) current transients obtained at an applied electrode potential of -0.280 v for h2o2 concentrations between 0 and 3 mm (from top to bottom: 3, 1.5, 0.75 and 0 mm); (b) dependence of the current measured at 60 s with the peroxide concentration. inset: lineweaver-burk plot. the measured current can be related to the rate of reaction. the catalyzed reduction of peroxide can be expressed as: h2o2 + h2q → 2h2o + bq (1) where h2q and bq stands for hydroquinone and 1,4-benzoquinone respectively. on the other hand, a fraction f of the total of the 1,4-benzoquinone produced in the catalytic reaction is reduced back into hydroquinone at the electrode surface: bq + 2 h + + 2 e → h2q (2) considering that the consumption of n moles of hydrogen peroxide will generate the circulation of a charge equal to 2ffn, where f is the faraday constant, it follows that the reaction rate (ν) of peroxide consumption is related to the current by the stoichiometry of the reactions and the collection factor f by the following equation: -1 / mol s 2 i v ff  (3) for enzymatic catalyzed processes, the relationship between the reaction rate and the concentration of the substrate ([h2o2]) is given by the michaelis-menten equation [28]: max 2 2 m 2 2 [h o ] [h o ] v v k   (4) where vmax is the maximum rate and km is the michalis-menten constant. the values of vmax and km can be obtained from the slope and the intercept of a linearized lineweaver-burk plot, 1/v vs. 1/[h2o2]: m max 2 2 max 1 1 1 [h o ] k v v v   (5) j. electrochem. sci. eng. 5(1) (2015) 9-16 determination of β-casein 14 14 assuming a collection efficiency of 100 % (i.e. f = 1), reproducible values of around 1.3 mm have be estimated for km from lineweaver-burk plots such as the one shown in the inset of fig. 2b. this value for km is not far from others reported for hrp immobilized onto surfaces (e.g, 0.65 mm in [17]). although the actual value of the collection factor is not known, the reproducibility of the kinetic curves for different electrodes suggests that a stable architecture of the enzymatic electrode is attained so that the collection factor has a constant value. in brief, these results show that the enzymes immobilized onto plasma-treated carbon electrodes exhibit a high catalytic activity, which can be explained in terms of a michales-menten kinetics [chang and tang 2014], and that the enzymatic electrode are suitable for quantitative measurements. figure 3. electrochemical determination of β-casein: (a) current-time curves obtained at an applied electrode potential of -0.280 v at a h2o2 concentration of 1.5 mm with different β-casein concentration in the sample (from top to bottom: 0, 1, 5, 10, 15 and 20 ppm, and blocking buffer), (b) dependence of the current measured at 60 s on the β-casein concentration. error bars were calculated as the standard deviation of three independent experiments. immunoenzimatic determination of β-casein the determination of β-casein is based on an inhibitory immunoassay. firstly, rabbit ig anti-βcasein antibodies were incubated with the sample and then transferred into the electrochemical cell which contained the electrode with immobilized β-casein. depending on the concentration of β-casein present in the sample, a quantity of free anti-β-casein antibodies could still be available to bond to the immobilized β-casein. those bonded antibodies are detected with a hrpconjugated secondary antibody. finally, the enzymatic activity is amperometrically detected by adding hydrogen peroxide and a suitable redox mediator at an electrode potential negative enough to produce the reduction of the redox mediator (fig. 3a). under these conditions, the concentration of β-casein could be inversely related to the measured current as shown in fig. 3b. most countries have not established regulations on the maximum allowed content of allergens in food. in this regard, the japanese food sanitation law, dated 2001 and amended in 2008, is one of the few available regulations, in which a maximum concentration of 10 ppm was established taking into account that traces of allergens can lead to allergic reactions [11]. considering the japanese regulation as a valid reference, the conditions were optimized to obtain a standard curve for the detection of β-casein in the 0-10 ppm range (fig. 3b). j. molinari et al. j. electrochem. sci. eng. 5(1) (2015) 9-16 doi: 10.5599/jese.2015.0072 15 this measurement range is useful for allergen monitoring in food industry activities and this portable equipment represent an important advantage as compared to commercial analytical methods in terms of costs and portability. conclusions the determination of food allergens is a subject of increasing concern. the development of an amperometric electrochemical biosensor associated with an inhibitory immunoassay presented in this work for the detection of food allergens has several advantages. the immunoassay has been optimized for the 0-10 ppm range, which is the most relevant according to health regulations. the chosen electrochemical transduction technique used in the presented biosensor was amperemetry, which is the least regarding in terms of electronic instrumentation and data treatment. furthermore, the set of 8-electrochemical cells has been designed to fit a portable potentiostat, which can be connected to a smartphone or tablet via bluetooth connectivity and can be used for analyzing different analytes at the same time. the biosensor has been designed for the determination of β-casein and can be properly modified for the detection of other allergens. acknowledgements: this work has been supported by the instituto nacional de tecnología industrial and by fs nano 2010/05 “nanopoc” granted by the ministry of science, technology and innovative production of the argentine republic. the authors thank mijal mass and mariano roberti from inti-cmnb for the design and preparation of thick film electrodes and the acrylic electrochemical cell, anahí medrano from inti-cmnb for plasma treatments, vanina ambrosi and gustavo polenta from the laboratorio de compuestos protéicos of the instituto nacional de tecnología agropecuaria for kindly providing proteic compounds and marcela pagano and estela faulkes from the immunoserology section of the instituto de investigaciones médicas “alfredo lanari”. references [1] a. fiocchi, j. brozek, h. schünemann, s. l. bahna, a. von berg, k. beyer, m. bozzola, j. bradsher, e. compalati, m. ebisawa, m. a. guzman, h. li, r. g. heine, p. keith, g. lack, m. landi, a. martelli, f. rancé, h. sampson, a. stein, l. terracciano, s. vieths., pediatr. allergy immunol. 21 (2010) 1-125. 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[16] e. de las heras, g. ybarra, i. bracera, p. corengia, surface modification by plasma-based processes in functional properties of biological surfaces: characterization and technological applications, e. favret, n. o. fuentes eds., world scientific publishing co., new jersey, u.s.a., 2009, p. 343-378 [17] q. chang, h. tang, molecules 19 (2014) 15768-15782. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ influence of polarization curve slope on the accuracy of local copper electrodeposition from sulphate electrolyte http://dx.doi.org/10.5599/jese.1899 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1899 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of polarization curve slope on the accuracy of local copper electrodeposition from sulphate electrolyte georgii vasyliev, viktoria vorobyova, dmytro uschapovskiy, mykhailo kotyk and olga linucheva national technical university of ukraine “igor sikorsky kyiv polytechnic institute”, 37, prospect peremohy, kyiv-56, 03056, ukraine corresponding authors: g.vasyliev@kpi.ua; tel.: +38-096-924-9888; fax: +38-044-204-9773 received: maz 22, 2023; accepted: june 24, 2023; published: june xx, 2023 abstract local electrochemical deposition is an emerging technique, used in the field of additive manufacturing. the advantage of electrochemical additive manufacturing lies in the utilization of room temperature electrolyte and permits to manufacture microscale objects with high precision. the increase in deposition current increases the deposition area, so measures are to be taken to focus the electric field. this work describes the influence of polarization curve slope on the accuracy of local deposition, both experimentally and by computer modelling. the copper was deposited using rotating anode on the surface of stainless steel from sulphate electrolyte. the influence of electrolyte composition on the accuracy of deposition was investigated. the profile of deposited parts was analyzed by profilometry and microscopy. the increased amount of sulfuric acid and presence of the additive in the electrolyte was shown to increase the accuracy of deposition by changing the slope of cathodic polarization curve from 320 to 1100 ma v–1cm–2). keywords additive manufacturing; 3d printing; copper electroplating; throwing power, profilometry introduction additive manufacturing (am) or 3d-printing is a manufacturing technique, rapidly developing in recent years. am utilizes a bottom-up approach, when the material is added layer by layer, in contrast to traditional manufacturing techniques, where the needed part is produced from blank though removing the excess material. the cost of 3d-printed parts decreases, with the increase of part complexity, due to lower consumption of material, despite subtractive production, where the more complex part is, the more operations is required for its production. thus, the research in the field of am is of extreme importance [1]. metal am remains one of the most demandable techniques, because metals remain on the first positions of the world most used materials. the existing metal am techniques require application http://dx.doi.org/10.5599/jese.1899 http://dx.doi.org/10.5599/jese.1899 http://www.jese-online.org/ mailto:g.vasyliev@kpi.ua j. electrochem. sci. eng. 00(0) (2023) 000-000 accuracy of local copper electrodeposition 2 of high energy devices for local metal melting: selective laser melting (slm) and electron beam melting (ebm) [2-5], and the precision of am is limited to the size of a metal powder (for laser or electron beam sintering). unlike them, electrochemical additive manufacturing (ecam) enables am production at room temperature by using the metal electrodeposition principles [6-7]. electrochemical deposition enables the use of the smallest species of a material – ions. other techniques use metal powders or wires, that are bigger sizes. thus, the main investigation direction is electrochemical additive manufacturing of micron and submicron scale objects. the application of fluidic force microscope (fluidfm) permits the production of pillars, needles, helix, connectors, and other objects of only few micrometers in size [8-12]. the technique is extremely precise; however, the deposition rate is slow. a faster approach for bigger objects is a meniscus confined deposition [13-18]. here the deposition is located in the small volume of electrolyte solution between the capillary and substate. the anode is placed inside the capillary and the substrate is polarized cathodically. this technique permits to print flat objects; however, they can be produced from different metals, that are deposited layer by layer. the deposition rate is faster than fluidfm technique, because the volume of electrolyte is larger, however, the growth rate is determined with the diffusion rate of metal ions in meniscus. even faster deposition rates can be achieved using electrodeposition from a large volume of electrolyte [14-26]. high deposition rate requires high concentration of metal ions, that makes an electrolyte conductive and electric field distributes easily around the anode. to localize the deposition region, masks are applied on the substate [27], or the size of the anode is miniaturized [28]. typical objects are printed by sequential deposition of the metal under anode until deposited layer touches the anode. after this, the anode is moved to another position or upwards and the process repeats. in this condition, the deposition is also limited by the diffusion rate of metal ions, but their concentration is much higher than in meniscus deposition. to achieve higher deposition rates the pulsed current is applied instead of constant. the aim of this work was to test the influence of copper plating electrolyte composition on the accuracy of local deposition in conditions of working electrode rotation. experimental local electrochemical deposition the electrodeposition process was organized in the electrochemical cell. the stainless-steel plate aisi 321 was placed on the bottom of the plastic vessel and was connected to the negative pole of current source. the stainless-steel was chosen to ensure separation of the printed part from the base after deposition. the working electrode was rotated with an electric motor, the rotation speed was 1 s-1. the working electrode was a cylinder-shaped platinum foil, connected to the positive pole of current source. the design of the laboratory set-up is given in figure 1. the stainless-steel substrate was polished with sic emery paper, degreased, and placed on the bottom of the vessel. the 1 l of electrolyte solution was poured into the vessel, the level of electrolyte was 20 mm above the substrate. the composition of the electrolyte is given in table 1. the anode was placed 5 mm above the substrate and started to rotate. the current was applied with a density of 40-50 ma cm-2, for the 60-460 min. after the deposition, the current was turned off, the solution was drained from the vessel. the deposited part in the form of the ring was cleaned with distilled water, removed from substrate, and investigated. the experimental conditions are provided in table 2. expected thickness was calculated by comsol software, based on faraday’s law. g. vasyliev et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1899 3 figure 1. the scheme of laboratory set-up: 1 platinum foil (anode); 2 stainless steel base (cathode); 3 polypropylene capillary; 4 electrolyte table 1. electrolyte composition component content, g l-1 electrolyte 1 electrolyte 2 cuso4·5h2o 200 200 h2so4 10 75 kcl 1 0.3 fe2(so4)3 10 gelatine 0.05 additive rubin t-200* 2 ml l-1 of rubin t200-a; 8 ml l-1 of rubin t200-g 2 ml l-1 of rubin t200-e [29] *leveling and brightness additive, kiesow oberflächenchemie gmbh & co. kg table 2. experimental conditions sample number electrolyte i / ma cm-2 expected thickness, m 1 1 11 20 2 1 20 40 3 2 40 50 4 2 50 50 5 2 40 200 6 2 50 200 local electrochemical deposition the geometry of printed objects was analyzed (figure 2). figure 2. the scheme of locally deposited metal characterization 1 2 3 4 ø4 mm 5 m m 1 mm + − 2 3 microscopy profilometry http://dx.doi.org/10.5599/jese.1899 j. electrochem. sci. eng. 00(0) (2023) 000-000 accuracy of local copper electrodeposition 4 for this purpose, the profilograms of the rings were measured in 4 positions (every 90°). the profilometry results were compared to the thickness measurements on the cross-sections of the printed rings. the sections of the rings were cut, sealed with epoxy resin, polished, and prepared for optical microscopy. the cross section of the printed rings was observed using iscope is.1053-plmi optical microscope (euromex microscopen bv). the accompanying software was used to conduct linear measurement. results and discussion copper deposition is an electrochemical process of cu2+ ion reduction on the surface of negatively charged substrate. the electrochemical reaction can be expressed by the following reaction: cu2+ + 2e− = cu (1) the rate of the reaction is expressed in terms of electric current that passes through the electrochemical cell and corresponds to the number of copper ions being reduced in certain time intervals. the deposited copper ions form a metal lattice. the dependence between the charge passed through the cell, and the mass of the deposited metal is expressed by the faradays law: m = mit / zf (2) where m /g is the mass of deposited metal; m / g mol-1 is atomic mass; i / a us current; t / s is time,; z is number of electrons required for ion reduction; f / c mol-1 is the faraday constant,. the dependence between the applied voltage and the deposition rate is expressed by the current-voltage dependence or the polarization curve. the typical polarization curve for copper deposition is given in figure 3. two regions can be determined in the curve. the first one shows the growing current-voltage dependence and is characterized by the slope of polarization curve di/de. the second region is a limiting current region, where deposition rate is independent of the applied potential. the limiting current, ilim, can be increased in the same electrolyte by applying electrode/electrolyte movement, see curves 1 and 2. figure 3. typical polarization curve for copper deposition: 1 stationary deposition; 2 relative movement of electrode and solution comsol multiphysics 4.3 (https://www.comsol.com/) [30] was used for computer modeling of the secondary distribution of the current density in the space between the electrodes (the working area of the 3d printer) and on the surface of the cathode. based on the built-in model of − e i (di/de)1 ilim2 (di/de)2 1 2 ilim1 https://www.comsol.com/ g. vasyliev et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1899 5 "long-term growth of copper deposits" [20], a model of the working area of the 3d printer was developed (figure 1). the used model can be described by the following equations. electric field distribution in the body of the electrodes: i = 0, i = v (3) electric field distribution in the volume of the electrolyte: i = 0, i = κ (4) on the surface of electrodes, the simplified linear dependence was used: i n = b  (5)  = v –  – e (6) where: i – the current density; σ and κ – electrical conductivities of electrode material and electrolyte, respecxtively; v – cell voltage; η – overvoltage; b – constant; n – the normal vector, pointing out of the domain;  – potential in the electrolyte; e0 – equilibrium electrode potential. the distribution of the current density in the electrolyte corresponds to the secondary one and depends both on the geometric parameters of the cell and characteristics of the electrode reaction, the polarization of the cathode and is generally described by the wagner criterion [30,31]. it is also assumed that the electrical conductivity of the solution in the interelectrode space is constant. in particular, one of the determining parameters according to the used model [30], is the derivative of the current density by overvoltage (di/de). this value is determined based on the polarization characteristics of the working electrodes of the used electrochemical system and corresponds to the slope of their linear sections. the polarization curves for the two tested electrolytes are given in figure 4. polarization parameters and electrolyte conductivity used in the computed model are given in table 3. figure 4. the stationary cathodic polarization curves of copper deposition: 1 electrolyte 1; 2 electrolyte 2. potentials are given vs. saturated silver chloride electrode (ssce) table 3. electrochemical parameters of the electrolyte electrolyte number σ / s сm-1 [32] (di/de) / ma v-1 cm-2 applied voltage, v 1 0.068 320 0.3-0.5 2 0.231 1100 0.2-0.5 the results of electrochemical manufacturing are given in figure 5. the copper deposits obtained from electrolyte 1 at a current density of 10-20 ma cm-2 were mostly fine-crystalline and had a light http://dx.doi.org/10.5599/jese.1899 j. electrochem. sci. eng. 00(0) (2023) 000-000 accuracy of local copper electrodeposition 6 color. at the same time, their thickness did not exceed 40 μm. the copper deposits obtained from electrolyte 2 at current densities of 40-50 ma cm-2 had a compact but coarser crystalline structure and a darker color. in particular, the formation of spherulitic deposits was observed at a deposit thickness of 200 μm, electrodeposited at a current density of 50 ma cm-2. the metallic copper is deposited in the form of rings, easily detached from the stainless-steel substrate. the morphology of the deposited rings was investigated using profilometry technique. the profilometry results are given in figure 6, along with the profile of deposited metal, calculated using the comsol multiphysics software. the height of the deposited copper agrees with the results of computer modelling. the modelled profile shows the typical shape of gaussian distribution. the shape of the real deposited copper profile differs from normal distribution. the profile of deposited copper has an uncommon shape, with the increased height at the edges. this is caused by the internal stress of copper deposits. as it can be seen from the deposited profile, this is tensile stress. the reason for tensile stress is the formation of crystal lattice defects, caused by the presence in the electrolyte of surface-active substances [33]. due to absorption and incorporation in metal structure of surface-active substances during electrodeposition the defects of crystal lattice appear, namely vacancies. the presence of vacancies leads to the deformation of metal structure. in this case of local electrodeposition, the electrolyte contains gelatin and iron (iii) sulphate, that cause vacancies incorporation in the lattice. 1 2 3 4 5 6 figure 5. copper rings manufactured by local electrodeposition using rotating anode while the thickness of deposited metal agrees well with the model, the thickness distribution under the anode is different. in particular, the area of metal deposition under the anode in computer model is wider than the experimental one. the following explanation may be suggested. the comsol software models the deposition profile using the constant polarizability of the process. at the same time, the real polarization curve (figure 4, curves 1, 2) has a constant slope value only in g. vasyliev et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1899 7 a short potential range. below this potential range, the slope differs, so practically no deposition may occur. that is what can be seen from figure 6. on the other hand, according to the theory of electrocrystallization [34] in the range of low current densities (the polarization resistance range), which is characterized by a linear current-potential dependance, metal deposition occurs in the singe crystal form. some parts of the surface are not suitable for deposition at all. moreover, it is worth mentioning that electrodeposition was performed on the surface of stainless steel. the base, made from metal different from the deposited one, also leads to the increase of phase overvoltage and, as a result, to the reduction of deposition rate in the low current density regions. the latter also suggests using stainless steel as a base metal for 3d electrochemical printing. figure 6. the profile of deposited copper parts: 1 averaged profilometry results; 2 computer simulation of deposited copper the thickness values measured in the profilometry test agree well with the results of crosssection microscopic investigation (figure 7). however, the microphotographs show no wrapped edges, so the thickness measurements were performed accurately. samples 1-5 show dense metal http://dx.doi.org/10.5599/jese.1899 j. electrochem. sci. eng. 00(0) (2023) 000-000 accuracy of local copper electrodeposition 8 structure on the cross-section, but sample 6 appeared to be fractured. this was caused by the application of high current density, 50 ma/cm2. this led to the formation of powder-like large copper crystals with low cohesion. so, current density above 40 ma/cm2 should not be recommended when objects higher than 50 m are electrodeposited. 1 2 3 4 5 6 figure 7. cross-section photographs of deposited copper rings conclusions 1. local electrochemical deposition makes it possible to form parts from aqueous solutions without the use of high energy. 2. it is possible to increase the accuracy of printing by choosing an electrolyte that has a narrow range of deposition potentials. when the polarizations 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[33] x. feng l, h. cao, h. yu, l. gaol, m. lil. study of internal stress on electroplating copper used in through silicon via filling, 12th international conference on electronic packaging technology & high density packaging, shanghai, china, 2011, p.1-4. http://dx.doi.org/10.1109/icept.2011.6067001 [34] r. winand, electrocrystallization-theory and applications, hydrometallurgy 29 (1992) 567-598. https://doi.org/10.1016/0304-386x(92)90033-v ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.procir.2017.12.056 https://doi.org/10.1149/1.1393237 https://doi.org/10.1088/0960-1317/18/10/105008 https://doi.org/10.1016/j.electacta.2009.11.002 https://doi.org/10.1115/1.4029022 https://doi.org/10.1016/j.procir.2017.12.053 http://dx.doi.org/10.5599/jese.1352 https://doi.org/10.1016/s0013-4686(97)00146-1 https://doi.org/10.1016/s0013-4686(97)00146-1 https://doi.org/10.3390/mi8030090 https://doi.org/10.1109/tpel.2010.2041557 http://dx.doi.org/10.1109/icept.2011.6067001 https://doi.org/10.1016/0304-386x(92)90033-v https://creativecommons.org/licenses/by/4.0/) optimization of the inhibition corrosion of carbon steel in an acidic medium by a novel eco-friendly inhibitor asphodelus ramosus using response surface methodology http://dx.doi.org/10.5599/jese.1628 469 j. electrochem. sci. eng. 13(3) (2023) 469-490; http://dx.doi.org/10.5599/jese.1628 open access : : issn 1847-9286 www.jese-online.org original scientific paper optimization of the inhibition corrosion of carbon steel in an acidic medium by a novel eco-friendly inhibitor asphodelus ramosus using response surface methodology narimane saigaa1,2, sabrina bouguessa1,2, wafia boukhedena2,3,, mohammed nacer1,2, ayoub nadji1,2 and abdelkrim gouasmia1,2 1laboratory of organic materials and heterochemistry, echahid cheikh larbi tebessi university tebessa, constantine road, 12002 tebessa, algeria 2department of science materials, echahid cheikh larbi tebessi university-tebessa constantine road, 12002 tebessa, algeria 3mines laboratory, echahid cheikh larbi tebessi university-tebessa, constantine road, 1200, tebessa, algeria corresponding author: wafia.boukhedena@univ-tebessa.dz; tel.: +213 7 71 64 25 62 received: december 3, 2022; accepted: february 27, 2023; published:: march 14, 2023 abstract ethyl acetate extract of asphodelus ramosus (arae) was examined as an anti-corrosion agent for carbon steel (cs) in 1 m hcl acid medium using different techniques, namely weight loss method, potentiodynamic polarization, and electrochemical impedance spectroscopy (eis) at various temperatures and inhibitor concentrations. an inhibition efficiency of 89.81 % was obtained by the weight loss method at the inhibitor concentration of 700 ppm at 293 k. increasing the temperature decreases the corrosion inhibition rate. potentiodynamic polarization results showed that the extract is adsorbed on cs surface according to the freundlich isotherm, while negative values of the standard free energy of adsorption (g0ads) suggested the physical spontaneity of the adsorption reaction. scanning electron microscopy (sem) and energy dispersive spectrometry (eds) analyses were performed to examine the surface morphology of inhibited and uninhibited cs samples. central composite design (ccd) based optimization was engaged to analyze factors and maximize inhibition efficiency by applying response surface methodology (rsm) using design-expert software. keywords plant extract; hydrochloric acid, response surface method (rsm) introduction corrosion is a natural electrochemical process that transforms metal into a more stable chemical form, such as metal oxides, sulfides, and chlorides [1]. steel and its important alloys are widely used as important construction materials in the chemical, petrochemical, thermal and nuclear industries. in all http://dx.doi.org/10.5599/jese.1628 http://dx.doi.org/10.5599/jese.1628 http://www.jese-online.org/ mailto:wafia.boukhedena@univ-tebessa.dz j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 470 these industrial facilities, hydrochloric acid has usually been used for descaling carbon steel (cs), which promotes the acceleration of metallic corrosion. this has a negative impact on the ecological balance and the economic field, particularly in terms of repair, replacement, and product losses [2,3]. due to all these difficulties, researchers have developed plans to mitigate corrosion and increase the life of infrastructure, machinery, metal devices, etc. the degradation of different metals can be reduced largely by using various methods of corrosion protection, such as material improvement, alloying, use of different types of coatings, environmental modification and application of corrosion inhibitors [4]. among corrosion inhibitors, synthetic compounds show good anti-corrosion effects, but most are highly toxic and harmful to health and the environment. this serious limitation turned research studies toward inhibitors obtained from several plant parts, including stems, seeds, fruits, stem bark, and bagasse [5]. as it is universally known, plants make a huge source of natural compounds, which have complex molecular structures with different chemical, biological and physical properties. these compounds are mainly used as they are eco-friendly, low-cost, effective, and abundant. due to these advantages, plant extracts are widely used in many applications, such as corrosion inhibition of metals and alloys [6]. these compounds contain heteroatoms such as n, s, and o and π electrons in their structure, which makes them good corrosion inhibitors in the acidic media by forming a protective layer on the surface of the metal [4,7-10]. in general, the inhibitory action of these compounds has been improved according to the following order: o < n < s < p [11]. much of scientists' attention has recently been directed toward the use and development of various sustainable inhibitors. numerous studies have looked into the efficacy of green plant-derived corrosion inhibitors in attempting to prevent metal corrosion in acidic environments. among them is ficus tikoua leaf extract, which has a 95.8 % effectiveness for carbon steel [12], azadirachta indica leaf extract, with an efficiency of 86.4 % [13], kiwi juice, with an efficiency of 96.1 % [14], phyllanthus amarus leaf extract with an efficiency of 95 % at 303 k [15], peach juice with an efficiency of 91 % [16], laurus nobilis leaf extract with an efficiency of 92 % [17], juniperus plants [18], origanum majorana extracts [19], and phyllanthus fraternus extracts [20]. in addition, xanthium strumarium extract showed an efficiency of 94.8 % for carbon steel in 1 m hcl at 10 g/l [21], while glycyrrhiza glabra leaves extract showed an efficiency of 88 % for mild steel in 1 m hcl at 800 ppm [22]. response surface methodology (rsm) has already been applied by various researchers to study and statistically analyze corrosion inhibition for metals in acidic environments. this strategy can be used to optimize experimental results achieved by weight loss measurements and is, therefore, essential to any research in this area. rsm offers the possibility to significantly reduce the number of experiments required to fully evaluate the performance of inhibitors, thus minimizing the cost and duration of the experiment [23-30]. in this work, a novel eco-friendly inhibitor extracted from the asphodelus ramosus plant has been investigated. the main interests of the survey of this plant stem from the fact that, on the one hand, this plant has never been studied as a corrosion inhibitor. on the other hand, phytochemical analysis carried out on asphodelus ramosus showed richness in flavonoids and acid phenol [31,32], while in the study conducted by chimona et al. [33], even thirty-eight secondary metabolites were detected in tepals of ephemeral flowers of asphodelus ramosus. therefore, the present study could also serve for further assessment of the properties of this useful plant. research in the corrosion area has been developed considerably in the last few years and is increasingly oriented towards the development of non-toxic, non-polluting, and stable organic molecules. also, plant extracts are commonly obtained by simple extraction processes and have good n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 471 inhibiting properties. in this work, weight loss measurements, electrochemical impedance spectroscopy, and potentiodynamic polarization are employed to explore the inhibitory influence of asphodelus ramosus ethyl acetate extract (arae) in 1 m hydrochloric acid medium of carbon steel. the rsm is used for modeling, analysis, experiment design, and processing parameters optimization. experimental materials in the corrosion inhibition study, carbon steel (16mncr5) with a chemical composition (0.19 wt.% c, 0.4 wt.% si, 1.3 wt.% mn, 0.035 wt.% s, 0.035 wt.% p, 0.8-1.1 wt.% cr and the rest fe) was used. specimens of 1×1×1 cm3 in size were prepared for weight loss measurements. for electrochemical measurements, specimens were incorporated in an epoxy resin, leaving an exhibition area of 1 cm2. the surface of specimens was pre-treated by mechanical grinding with 500, 800, 1000, 1200, and 2000 grit abrasive paper. then, specimens were washed with acetone, degreeased with distilled water, and dried at room temperature before being used in experiments. plant extracts in this study, we used plant tissue from the flowers and leaves of the plant asphodelus ramosus to produce three types, i.e., dichloromethane, ethyl acetate, and n-butanol extracts. the plant was cut into small pieces and then extracted by maceration in a hot methanol-water mixture (7/3: v/v) for 24 hours. this operation was repeated three times. the various fractions recovered are then combined and evaporated under reduced pressure at a temperature lower than 70 °c until a syrup residue is obtained. to remove chlorophyll, boiling water was added to the residue and stored at room temperature for one night. the filtered mixture was submitted to liquid-liquid extraction using several solvents separately in a sequence of increasing polarity, starting from dichloromethane, ethyl acetate, and n-butanol, where the organic phase is recovered for each solvent. the last solutions were evaporated to dryness using a rotary evaporator to obtain the desired extracts (paste form). experiments performed with all three prepared extracts showed that the ethyl acetate extract has significantly higher inhibitory power than the two other extracts. therefore, the ethyl acetate extract was chosen for presentation in this work. electrolyte and inhibitor solutions the aggressive solution used was 1 m hydrochloric acid prepared by dilution of 37 wt.% hcl (merck) with distilled water. to prepare 250 ml of a 700 ppm aqueous solution of the arae extract, the weight of the solute (arae) is determined from the relationship: c=(marae/msolution)×106. for an aqueous solution, and marae = (cmsolution)/106= 0.175 g. therefore, to prepare 250 ml of the aqueous solution of the arae extract of concentration 700 ppm, it is necessary to dissolve 0.175 g of arae in methanol (a small drop added with a pasteur pipette to dissolve the extract). a little distilled water is added, followed by the addition of 20.72 ml of concentrated hcl (37 wt.%) (to have 1 m hcl), and then the solution is completed to 250 ml with distilled water. the same protocol was followed for all other extract concentrations. weight loss measurements the determination of weight loss is the common method to calculate corrosion rates. the specimens were immersed for 3 h in 1.0 m hcl without and with the presence of different concentrations of asphodelus ramosus ethyl acetate extract (arae) at different temperatures http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 472 ranging from 293 to 323 k. to remove any oil and organic impurity, the coupons were degreased with acetone and finally washed with distilled water and dried in air. the accurate weight of each coupon was taken using an electronic weighing balance and the initial weight was recorded. the coupons were labeled in a manner to avoid any mix-up. the following equations were used to estimate the corrosion inhibition performance of the used inhibitor. w cr st  = (1) where w is the average weight loss (mg), s is the total area of carbon steel specimen (cm²), t is immersion time (h), and cr is the corrosion rate in mg cm-2 h-1. the inhibitory efficiency (iew), as well as the surface coverage , were determined using the following equations: 0 i w 0 100 cr cr ie cr = − (2) 0 i 0 cr cr cr  = − (3) where cr0 and cri represent the corrosion rate in the absence and presence of inhibitors, respectively. electrochemical measurements electrochemical measurements were realized using a voltalab-pgz 301 potentiostat controlled by a computer using voltamaster 4 software. the electrochemical cell used was a three-electrode pyrex glass cell with cs coupon as the working electrode (we), saturated calomel electrode (sce) as the reference electrode (re), and pt-plate as the counter electrode (ce). each potential was estimated relative to the submerged sce. before each measurement, a steady state of the system was obtained by immersing a freshly polished cs electrode in the test solution at the open circuit potential (eocp) for 30 minutes. the potentiodynamic polarization curves studied in the absence and presence of different inhibitor concentrations were obtained from the cathodic potential of -250 mv to the anodic potential of + 50 mv relative to eocp [34]. the inhibition efficiency (iep) values were derived from corrosion current density (jcorr) using the following equation: 0 arae corr corr p 0 corr 100ie j j j − = (4) where, jcorr0 and jcorrarae represent corrosion current density in the absence and presence of the inhibitor, respectively. electrochemical impedance spectroscopy (eis) measurements were carried out at the eocp in the frequency range of 100 khz to 10 mhz with a perturbation using 10 mv amplitude signal. the inhibition efficiency (ieeis) was calculated using the following formula: arae p p eis arae p 100 r r ie r − = (5) where rp and rparae are polarization resistance in the absence and presence of the inhibitor, respectively. scanning electron microscopy and energy dispersive spectroscopy (sem/eds) the influence of corrosion attack on the surface morphology of carbon steel after immersion in the corrosive solution (1 m hcl) with and without the addition of 700 ppm of arae at 293 k for 3 h was observed using sem/eds tescan vega 3 scanning electron microscope and bruker energy dispersive spectroscopy devices, by which surface morphology and chemical composition can be evaluated. n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 473 weight loss measurement using response surface methodology (rsm) the response surface method of the design-expert software was used to design experiments for the weight loss method. process variables such as inhibitor concentration and temperature were considered, while inhibition efficiency was the expected response of the study. several models in terms of coded factors have been proposed to provide predictions of the response for given levels of each factor. the 3-level (-1, 0, +1), two factors central composite design (ccd) was implemented in this study. this reduced the number of experiments to 13, including 8 factorial points and 5 central points. results and discussion weight loss measurements table 1 displays the results of carbon steel corrosion rate (cr) and inhibition efficiency (ie) in 1 m hcl medium in the absence and presence of various concentrations of arae after 3 hours of immersion in the temperature range of 293-323 k. according to data collected in table 1 and shown in figure 1, it can be seen that corrosion rate decreases, while the inhibitory efficiency ei increases with rising of the extract concentration. as the temperature increases, however, the inhibitory effectiveness falls down because the rising temperature tends to spread the extract from the surface of the carbon steel, decreasing the inhibitory efficiency. the optimal inhibition efficiency of 89.81 % was registered at the temperature of 293 k and 700 mg/l of inhibitor. this phenomenon is attributed to the coverage of the metal surface by the accumulation of a great number of molecules leading to the formation of an adsorbed film of extract (arae) which isolates the surface from the corrosive solution [35]. table 1. corrosion parameters derived from weight loss measurements of carbon steel in 1m hcl medium containing varying concentrations of arae at different temperatures t / k 293 303 313 323 c / ppm cr /mg cm-2 h-1 iew /% cr / mg cm-2 h-1 iew / % cr / mg cm-2 h-1 iew / % cr / mg cm-2 h-1 iew / % blank 0.0436 0.3658 0.7703 1.0408 100 0.0180 58.80 0.1636 55.27 0.3672 52.32 0.5230 49.75 300 0.0119 72.72 0.1111 80.26 0.2579 66.52 0.3782 63.66 500 0.0074 82.95 0.0722 69.63 0.1728 77.56 0.2642 74.62 700 0.0044 89.81 0.0476 86.98 0.1250 83.77 0.2018 80.62 (a) (b) figure 1. (a) corrosion rate of carbon steel in 1 m koh in dependence on the concetration of area extract at different temperatures; (b) inhibition efficiency of various concentrations of area extract at different temperatures adsorption isotherm essentially, the use of adsorption isotherms can provide basic information about the interaction between the inhibitor and the surface of carbon steel. furthermore, in-depth knowledge of the http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 474 adsorption character of inhibitors is vital for a proper understanding of the kinetics process. the adsorption of the inhibitor is attributed to the presence of heterocyclic compounds in the extract, which obstruct corrosion by forming a protective film layer that functions as a barrier, preventing the ingress of the corrodent onto the metal surface. in this study, langmuir, temkin, and freundlich isotherms were assessed. general forms of equations describing three adsorption isotherm models that are based on different assumptions were explored. to find the appropriate adsorption isotherm, the results of weight measurements were fitted to the aforementioned isotherms, and the correlation coefficient (r²) was used to choose the isotherm that best fits the experimental data. after the experimental study, the analysis of the results showed that the adsorption of arae on the surface of carbon steel obeys the freundlich adsorption isotherm model with a large r2 value of 0.9952. the linearized form of freundlich adsorption isotherm can be represented by the following equation: log  = log kads +  log cinh (6) where cinh is the inhibitor concentration,  is the fraction of the surface covered with inhibitor and kads is the adsorption equilibrium constant. kads of the inhibitor is determined by the intercept of the straight line obtained as the freundlich adsorption isotherm for the acetate extract (figure 2), which is used to evaluate the adsorption capacity of the inhibitor . figure 2. freundlich adsorption isotherm of arae on carbon steel in 1.0 m hcl at different temperatures thermodynamic parameters the value of the equilibrium constant of the adsorption process is related to the standard free energy change of adsorption (g0ads) by the following relation: g0ads = -rt ln(ch2okads) (7) where r is gas constant, t is absolute temperature, and ch2o is the concentration of water expressed in mg l-1 with an approximate value of 106. the thermodynamic parameters are collected in table 2. the negative values of g0ads indicate that the adsorption of the corrosion inhibitor on the metal surface is spontaneous. through these values, this adsorption can be defined as physical or chemical. g0ads values close to or less negative than -20 kj mol-1 correspond to electrostatic interactions between charged molecules and the metal (physical adsorption). those close to or more negative than -40 kj mol-1 indicate a charge transfer between organic molecules and the metal surface (chemisorption) [36-38]. g0ads values listed in table 2 are between -20 and -40 kj mol-1, which suggests a mixed type of physical and chemical adsorption. the mode of adsorption (physisorption and chemisorption) observed could be attributed to the fact that arae extract n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 475 contains many different chemical compounds, some of which can adsorb chemically and others physically [1,2]. the values of the standard adsorption enthalpy (h0ads) were obtained from van’t hoff equation: 0 0 ads ads 6 1 ln ln 10 s h k r rt    = + −    (8) the straight lines of ln kads as a function of 1/t of arae at different temperatures are shown in figure 3. this plot gave straight lines with a correlation coefficient close to unity. the slope is equal to -h0ads/rt, while the intersection of each straight line is equal to the constant ln(1/106) + s0ads/r, which allows the determination of s0ads. the estimated values of these parameters are given in table 2. figure 3. ln kads versus 1/ t for adsorption of arae on carbon steel in 1.0 m hcl the negative value of h0ads indicates the exothermic nature of the adsorption process of inhibitor molecules [3]. an exothermic adsorption process can be chemical, physical, or a mixture of both [37], whereas the endothermic process is attributed to chemisorption [38]. this process means the efficiency decreases as the temperature increases [3]. the positive sign of the adsorption entropy of arae can be explained by the adsorption of inhibitor on the carbon steel surface, which is produced via a quasi-substitution process between the inhibitor molecules in the aqueous phase [orgsol] and water molecules [h2oads] on the electrode surface. table 2. thermodynamic parameters of adsorption of arae on carbon steel surface in 1 m hcl at different temperatures thermodynamic parameters t / k kads / l mg-1 -g0ads / kj mol-1 -h0ads / kj mol-1 -s0ads / j mol-1 k-1 293 0.2146 -29.89 303 0.1923 -30.63 -8.53 72.92 313 0.1692 -31.31 323 0.1561 -32.10 activation parameters of the corrosion process with increasing temperature, most chemical and electrochemical reactions become faster. in general, the temperature has a great influence on the corrosion phenomenon, especially the corrosion rate, which leads to some changes in the action of inhibitors. the relationship between the corrosion rate and temperature is often expressed by the arrhenius equation, which allows calculating of the activation energy ea: ln cr = ln a ea / rt (9) 1 2 t-1 / k-1 3 ln ( k a d s / l m g -1 http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 476 where cr is corrosion rate, ea is the apparent activation energy, and a is arrhenius pre-exponential factor. figure 4 shows the arrhenius curves of ln cr versus 1/t for corrosion of carbon steel in 1 m hcl without and with different concentrations of arae. figure 4. arrhenius plots of ln(cr) vs. 1/t for carbon steel corrosion in 1 m hcl with and without arae inhibitor the thermodynamic parameters of activation, such as enthalpy of activation (ha) and entropy of activation (sa) were calculated using the transition equation: ( ) a aln ln s hrt cr nh rt rt      = + −        (10) where n is avogadro’s number (6.022×1023 mol−1) and h is planck’s constant (6.626×10-34 j s). figure 5 shows plots of ln (cr/t) against 1/t. the straight lines were obtained with a slope of (ha/r) and intercept of (ln(r/nh)+(sa/r)) from which the values of ha and sa were calculated. the values of the activation energy, the activation enthalpy, and the activation entropy in the absence and presence of the inhibitor (arae) are represented in table 3. many studies showed a decrease in activation energy ea in the presence of an inhibitor compared to the uninhibited solution, which could be interpreted as chemical adsorption (chemisorption) [39]. other studies reported an increase in activation energy, ea, which indicates physical adsorption. in our case, it is clear that ea is higher in the presence of the inhibitor than in its absence and increases with increasing inhibitor concentration [40]. figure 5. alternative arrhenius diagrams of ln (cr/t) versus 1/t for carbon steel corrosion in 1 m hcl with and without arae inhibitor n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 477 this increase of ea reflects the adsorption of arae extract molecules onto the cs substrate by electrostatic bonds (physisorption). as temperature rises, this kind of temperature-sensitive liaison (weak bonds) cannot effectively control corrosion. the positive sign of ha denotes that the adsorption of the extract on the surface of cs is an endothermic process and that the dissolution of steel is difficult [36]. the sa values in the presence of arae are positive and high compared to the blank solution, reflecting an increase in the disorder that occurs during the formation of the complex metal/adsorbed species [41]. table 3. thermodynamic activation parameters of carbon steel dissolution in 1 m hcl solution with and without various concentrations of arae inhibitor c / ppm ea / kj mol-1 ha / kj mol-1 sa / j mol-1 k-1 0 81.58 79.02 2.80 100 86.77 84.22 13.20 300 89.11 86.56 17.76 500 91.97 89.42 23.60 700 98.51 95.96 41.70 sem and eds results to assess the elemental composition of the surface before and after exposure to the corrosive media with and without arae inhibitor, the surface morphology of carbon steel before and after immersion in corrosive solution was investigated by sem analysis together with eds. sem-eds micrographs presented in figure 6 were recorded for (a) polished carbon steel, (b) after 3 h of immersion in 1.0 m hcl, and (c) after 3 h of immersion in 1.0 m hcl in the presence of 700 ppm arae. figure 6a shows the image of the abraded surface with some lines resulting from polishing. it can be seen that the carbon steel surface, after 3 hours of immersion in 1 m hcl (figure 6b), is heavily damaged and severely corroded. after immersion in the corrosive solution with 700 ppm of arae (figure 6c), the damages are reduced, and the external morphology appears softer. the metal surface has been remarkably improved due to the formation of the adsorbed protective layer, which prevents the aggressive attack of the electrolyte. table 4 presents mass percentages of various elements obtained by the energy dispersive spectroscopy (eds) of the carbon steel surface in 1 m hcl before and after immersion in the inhibited and uninhibited solution for 3 hours. figure 6a (right) indicates a greater percentage of iron compared to the steel submerged in the inhibited and uninhibited solutions, while figure 6c shows an increase in the peak of oxygen, carbon, and nitrogen when compared to figure 6b, which displays the eds spectra of the corroded steel in 1 m hcl alone. this demonstrates that the tested inhibitory molecules adhere to the metal surface. table 4. content of elements for each specimen obtained from eds analysis of carbon steel prior and after 3 h of immersion in corrosive solutions sample content, wt.% fe o c n s polished carbon steel 90.56 1.75 4.48 0.78 0.03 carbon steel in 1 m hcl 69.41 9.66 4.29 0.32 0.02 carbon steel in 1 m hcl + 700 ppm of arae 82.66 22.47 5.45 0.86 0.08 http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 478 a b c figure 6. sem micrographs (left) and eds spectra (right) of carbon steel: (a) prior immersion and after 3 hours of immersion in (b) 1.0 m hcl and (c) 1 m hcl containing 700 ppm arae inhibitor electrochemical experiments electrochemical impedance spectroscopy (eis) in an attempt to acquire more in-depth information on the phenomenon of corrosion inhibition of carbon steel in 1 m hcl without and in the presence of different concentrations of arae at 20 °c, eis measurements were performed. these measurements were carried out when the stability of cs electrode was reached after 30 min of a wait at the open circuit potential. figure 7 displays impedance spectra in the form of nyquist (zi vs. zr) and bode (log|z| and phase angle vs. log f) plots, respectively. n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 479 a b zr /  cm2 log (f / hz) figure 7. nyquist (a) and bode (b) impedance plots of carbon steel in 1 m hcl in the absence and presence of different concentrations of arae inhibitor at 293 k in nyquist plots, impedance spectra display slightly flattened semi-circles, with diameters increasing with the extract concentration rise. the same is seen from impedance magnitude, log |z|, values of bode plots that are at all frequencies higher for higher arae concentration. the rise of impedance could happen due to the adsorption of arae, which forms a resistant inhibiting film on the metal surface [23]. depending on the concentration, the adsorbed arae film has delayed the steel dissolution rate. the formation of a protective layer on the electrode surface may cause decreased capacity (i.e. thickness growth of the electrical double layer) with a rise in plant extract concentration [42]. the electronegative charge of heteroatoms present in the extract and the electropositive charge on the steel surface can both be used to explain this phenomenon [37]. in bode plots of figure 7b, decreased capacitance values are clearly seen by increased impedances of about -1 sloped lines appearing at f between 1000 and 10 hz, which denote dominant capacitive impedance response at these frequencies. on the other side, increased impedances of zero-slope lines dominant in bode plots at frequencyes (10 hz) indicate an increase in the resistive contribution with increased concentration of arae. in nyquist plots presented in figure 7a, this increase of resistive contributions at the lowest frequencies is seen as increased diameters of semi-circles, which can be observed for the increase of arae concentration. the eis graphs must be fitted with equivalent electrical circuits to thoroughly investigate the corrosion inhibition process. it is obvious from the impedance spectra in figure 7 that the steel/acid interface could be approximated by an equivalent circuit with a single time constant. figure 8 depicts such single time constant equivalent electrical circuit (eec) that should match experimental data. figure 8. electrical equivalent circuit used to fit measured impedance spectra the fact that eec in figure 8 can well simulate experimental impedance spectra is shown in figure 9, where fitting results that best suit the experimental data for carbon steel in 1.0 m hcl in the presence of 700 ppm of arae, are presented as nyquist and bode plots, showing excellent agreement between experimental and fitted impedance spectra. eec in figure 8 is made up of the polarization resistance (rp), which is the total of all possibly present resistances (rp = rct + rd + rf + ra), and the solution resistance (rs) [23]. rct is the charge transfer resistance of metal dissolution, rd represents the resistance of the diffuse layer, ra is the 1 zr /  cm2 log (f / hz) 2 -z i /  c m 2 lo g ( z  /  c m 2 ) p h a se a n g le , ° lo g ( z  /  c m 2 ) -z i /  c m 2 http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 480 resistance of species accumulated at the metal/solution interface and rf is the resistance of the inhibitor film on the steel considered only in the presence of inhibitors. figure 9. best fitted impedance plots of carbon steel in 1 m hcl without and with 700 ppm arae inhibitor (a) nyquist diagrams and (b) and (c) bode diagrams the polarization resistance is in parallel with the constant phase element (rp//cpe), where cpe replaces the electrical double layer (cdl) capacitance. the increase of absolute impedance at low frequencies in bode plots confirmed the higher protection with the increasing of inhibitor concentration and the good performance of the inhibitor with a constant period for different concentrations of arae related to the adsorption of inhibitor on the carbon steel surface [21]. based on the following equation, the double-layer electrical capacity (cdl) for each inhibitor concentration was calculated according to: 1 dl p nnc r q−= (11) where n is the deviation parameter of the cpe: 0 ≤ n ≤ 1 and q is the magnitude of the cpe. the electrochemical impedance parameters values, including rp, q, n, rs, and ieeis, obtained by fitting the eec in figure 8 to impedance spectra in figures 7 and 9, are listed in table 5. the data in table 5 show that as inhibitor concentrations increase, rp values rise, but cdl values decrease. these findings imply that molecules adhere to the carbon steel surface, forming a barrier that prevents the carbon steel from dissolving into the hcl medium [43,44]. table 5. electrochemical impedance parameter values for carbon steel in 1m hcl containing different concentrations of arae at 293k extract cinh / ppm rs / ω cm2 rp / ω cm2 q / ω−1 sn cm−2 n cdl / µf cm-2 ieeis / % blank 0 1.837 64.38 ± 0.27 45.04×10-5 0.897 ± 0.54 299.9 / arae 100 1.665 158.6 ± 0.33 40.84×10-5 0.756 ± 0.51 168.8 59.41 300 2.729 268.4 ± 0.35 33.52×10-5 0.744 ± 0.50 146.4 76.01 500 3.677 385.1 ± 0.31 25.93×10-5 0.761 ± 0.50 125.8 83.28 700 3.015 567.7 ± 0.34 18.52×10-5 0.798 ± 0.50 104.7 88.66 (a) n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 481 potentiodynamic polarization measurements the potentiodynamic polarization curves were used to show the effect of the arae inhibitor on the dissolution of carbon steel on the anode side and the evolution of hydrogen on the cathode side. figure 10 shows the straight tafel curves for cs in 1 m hcl solution without and with the addition of different concentrations of arae at 293 k. the addition of inhibitors causes the appearance of nearly parallel cathodic branches of tafel. the parameters obtained from the fitting of the polarization curve, such as corrosion potential ecorr, cathodic and anodic tafel slopes (a, b) as well as corrosion current density (jcorr) and inhibition efficiency (ier), are presented in table 6. figure 10. polarization curves of carbon steel without and with presence of arae inhibitor in 0.1 m hcl at 293 k analysis of polarization curves in figure 10 and data in table 6 show a random change in slope values (a, c), and the change of ecorr in the absence and presence of the inhibitor. the change of ecorr in presence of inhibitor is less than 85 mv, what suggests the mixed type of inhibition [45]. the addition of inhibitor strongly affects the anodic and cathodic reactions [46] by decreasing anodic and cathodic current densities. on the other hand, with increasing extract concentration, the inhibition efficiency shows an increasing trend, which is consistent with the results of weight loss experiments. this increase in inhibition efficiency is the result of the strong interaction of the inhibitor with the metal surface, which covers active sites of the surface and causes the formation of a barrier layer that reduces the reactivity of the metal. table 6. fitting parameters of polarization curves of carbon steel in 1 m hcl without and with the presence of arae inhibitor at 293 k cinh / ppm -ecorr / mv vs. sce jcorr / ma cm-2 a / mv dec-1 -c / mv dec-1 iep / % 0 492.0 0.2927 107.4 117.4 100 477.6 0.1165 88.6 105.1 60.20 300 454.6 0.0758 87.9 135.6 74.10 500 436.9 0.0476 52.8 134.9 83.74 700 471.6 0.0290 58.4 119.5 90.09 these suggest that the inhibition actions are due to adsorption onto the steel surface, where adsorbed molecules mechanically protect the coated portion of the metal surface from corrosion action [47]. in addition, the arae extract has a good protective effect at 293 k, showing at 700 ppm the maximum value of 90.09 % (table 6). http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 482 inhibition efficiency of arae on carbon steel using response surface methodology (rsm) the generated experimental data were analysed using design-expert to obtain an analysis of variance (anova) at 95 % confidence interval. the factor levels with corresponding actual values are presented in table 7, while the experimental design matrix is presented in table 8 with actual and coded values. table 7. experimental range of independent variables variable symbol low level average level high level concentration, ppm a 100 (-1) 400 (0) 700 (+1) temperature, °c b 20 (-1) 35 (0) 50 (+1) table 8. rsm results of the corrosion inhibition of carbon steel in 1 m hcl by arae extract run order coded variables real factor values response a b c / ppm (a) t / °c (b) ei / % 1 0 0 400 35 68.09 2 0 0 400 35 67.81 3 0 0 400 35 68.05 4 -1 +1 100 50 49.75 5 0 +1 400 50 66.15 6 0 0 400 35 68.10 7 0 -1 400 20 76.48 8 0 0 400 35 67.77 9 -1 -1 100 20 58.80 10 -1 0 100 35 53.06 11 +1 +1 700 50 80.62 12 +1 0 700 35 84.13 13 +1 -1 700 20 89.81 in this study, a variety of mathematical models were put forth to illustrate how the best model was chosen for the description of inhibitory efficiency data of arae on carbon steel. linear and polynomial models were applied as two main sets of models. in terms of the response (y) and independent variables (xi…xn), these models are generally defined by equations (12) to (17): linear model: 0 1 1 n n i i i i i y a a x  = = = + +  (12) logarithmic – linear model: 0 1 1 log n n i i i i i y a a x  = = = + +  (13) linear–interaction effect model: 1 0 1 1 1 n n n i i i i j i i i i j n i y a a x a x x   = = = = = + + +   (14) logarithmic– interaction effect model: 1 0 1 1 1 log n n n i i i i j i i i i j n i y a a x a x x   = = = = = + + +   (15) the second-order polynomial models are: polynomial–interaction effect model 1 0 1 1 1 1 n n n n i i i i j i i i i i i j n i i y a a x a x x a x  = = = = = = + + + +    (16) logarithmic – polynomial – interaction effect model n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 483 1 0 1 1 1 1 log n n n n i i i i j i i i i i i j n i i y a a x a x x a x  = = = = = = + + + +    (17) in the present case, the response surface methodology (rsm) was employed to explore the interaction between the factors, i.e., inhibitor concentration and temperature. therefore, in eqns. (12-17), y is inhibition efficiency, xi are variables (x1 = a is inhibitor concentration, x2= b is the temperature), n is the number of variables, ε is the standard error, a0 and ai are constants [48]. equations (12) to (17) can be expanded and regression has been carried out to evaluate the coefficients of these equations. design-expert was used to estimate the coefficients. the expanded equations can be rewritten as presented in equations (12a) to (17a): y = ao + a1a + a2b + total (12a) log y = ao + a1a + a2b + total (13a) y= ao + a1a + a2b + a3ab + total (14a) log y= ao + a1a + a2b + a3ab + total (15a) y= ao + a1a + a2b + a3ab + a4a² + a5b² + total (16a) log y= ao + a1a + a2b + a3ab + a4a² + a5b² + total (17a) for a successful model with high predictive efficiency, the value of r2should be close to 1.0 [28], for adequate precision, the estimate of the signal-to-noise ratio should be greater than 4, while a model can be considered reasonable if its statistical measure of the relative variability coefficient (cv) does not exceed 15 % [49]. using design-expert software 10.0, the suggested models and numerical values of these coefficients were rewritten, equations (12b) to (17b): y = 69.12 + 15.49a 4.76b (12b) log y = 1.83 + 0.099a 0.03b (13b) y = 69.12 + 15.49a 4.76b -0.035ab (14b) log y = 1.83 + 0.099a 0.03b + 6.426×10-3 (15b) y = 68.27 +15.49a -4.76b -0.035ab -0.43a² + 2.29b² (16b) log y = 1.83 + 0.099a 0.03b + 6.426×10-3ab 0.014a² + 0.013b² (17b) total errors, adequate precision, coefficient of variation, and correlation coefficients were evaluated, as shown in table 9. table 9. total error, adequate precision, coefficient of variation, and correlation of suggested models equation r² adequate precision cv, % εtotal (12a) 0.9881 61.120 2.00 0.38 (13a) 0.9842 52.951 0.55 2.82×10-3 (14a) 0.9881 50.222 2.10 0.40 (15a) 0.9867 47.455 0.54 2.726×10-3 (16a) 0.9975 79.496 1.09 0.31 (17a) 0.9980 88.170 0.24 1.794×10-3 the analysis of variance (anova) (r², coefficient of variation, and adequate precision) of the different models revealed that the logarithmic-polynomial model has the most satisfactory r² correlation coefficient (eqn. 17a). according to table 9, this model is marked by a coefficient of variation (cv = 0.24 %), which means that the model is appropriate with a faithful estimation. the ratio of 88.170 % obtained indicates an adequate signal. therefore, the choice of the logarithmicpolynomial model is justified. http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 484 statistical analysis values of "prob > f" less than 0.05 indicate that the terms of the model are significant at a 95 % confidence level [48-49]. in this case, a, b, ab, a2, and b2 are significant terms in the model. the anova results related to the inhibitory efficacy response of the extract are summarized in table 10. table 10. anova results for the inhibition efficiency of arae on mild steel corrosion in hcl solution source sum of squares df mean square f value prob  f model 0.065 5 0.013 698.96 0.0001 significant a 0.059 1 0.059 3149.01 0.0001 b 5.550×10-3 1 5.550×10-3 297.42 0.0001 ab 1. 652×10-4 1 1. 652×10-4 8.85 0.0207 a² 5.229×10-4 1 5.229×10-4 28.02 0.0011 b² 4.974×10-4 1 4.974.10-4 26.65 0.0013 residual 1.306×10-4 7 1.866×10-5 lack of fit 1.264×10-4 3 4.214×10-5 40.00 0.0019 significant pure error 4.214×10-6 4 1.054×10-6 cor total 0.065 12 stdandard deviation 4.320×10-3 r20.9980 mean 1.83 adj r2 0.9966 cv, % 0.24 0.9863 press 8.927×10-4 adeq precision 88.170 the f-value (fisher ratio) and the p-value of the model denote the statistical significance of the model as a totality. the f-value must be greater than the p-value to assess whether the result is significant enough to reject the null hypothesis. in this analysis, the f-value of 698.96 is superior to the p-value of 0.0001. as a result, we can assert that this model is significant. the fit statistics for the response data exhibit an r2 value of 0.9980 and an adjusted r2 value of 0.9966. this indicates a good correlation between the experimental outcomes and those generated by the model. the predicted r2 of 0.9863 is in reasonable agreement with the adjusted r2 of 0.9966 since the difference is less than 0.2. this suggests that the experimental data obtained for the inhibition efficiency of the arae extract on carbon steel in 1 m hcl were statistically consistent and that the second-order logpolynomial model adopted was appropriate for modelling. the model in decoded form is given below by the equation: log eiw = 1.8417 + 4.02214×10-4c 6.77376×10-3t + 1.42791×10-6ct – 1.52891×10-7 c² + 5.96428 ×10-5t² (18) a normal probability of the residuals was performed to fully understand the nature of the fit. figure 11 shows that the fit of the regression data is close to a straight line. figure 11. normal plot of residuals n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 http://dx.doi.org/10.5599/jese.1628 485 this indicates that the hypothesis of the analysis is satisfied. in addition, the plots of the predicted values against the experimental values were used to ensure the predictability of the experimental results. figure 12 illustrates the predicted values of inhibition efficiency obtained from eq. (18) are in good agreement with the experimental values. figure 12. correlation between measured and predicted values of inhibitory efficiency the effect of the two factors (concentration of the arae extract and temperature) on the response (inhibition efficiency) is represented by 3d curves in figure 13. figure 13. 3d response surface plots for inhibition efficiency of area extract on mild steel in 0.1 m hcl: temperature vs. inhibitor concentration when time is maintained constant, the inhibition efficiency decreases with increasing temperature and extract concentration. the amount of heteroatoms responsible for the inhibition in the corrosive solution increases as the arae extract concentration increases. optimization and confirmation of the results an optimization study of the inhibitory efficacy of the arae extract on carbon steel in a 1 m hcl acid medium was conducted. its purpose is to predict the optimal conditions under which the maximum inhibition efficiency can be achieved. the experimental findings of the top 10 cases with 1 ieactual / % 2 ie p re d ic te d / % 1 ie w / % t / °c (b) c / ppm (a) http://dx.doi.org/10.5599/jese.1628 j. electrochem. sci. eng. 13(3) (2023) 469-490 eco-friendly corrosion inhibitor asphodelus ramosus 486 the maximum desirability are selected and reported in table 11. the confirmation of these results is illustrated in figure 14. the optimal conditions of the highest inhibitory efficiency of 90.52 % are reached for a temperature equal to 20 °c and a concentration of the extract of 700 ppm. table 11. ten best solutions for parameters influencing corrosion inhibition and inhibition efficiency for carbon steel in the absence and presence of arae inhibitor (desirability = 1.00) number c / ppm t / °c iew / % 1 2 3 4 5 6 7 8 9 10 699.83 694.72 695.10 695.68 698.00 693.16 700.00 699.30 696.34 688.47 20.79 20.45 20.03 20.72 20.21 20.15 20.20 20.93 20.60 20.20 89.96 89.96 90.27 89.82 90.28 90.10 90.52 89.84 89.93 89.85 figure 14. optimal conditions selected for parameters influencing corrosion inhibition of carbon steel in 0.1m hcl (concentration of area inhibitor and temperature) with their responses (inhibition efficiency) conclusion the effect of the extract (arae) on the corrosion inhibition of carbon steel (16mncr5) in 1 m hcl acid media was experimentally investigated and statistically analyzed using the response surface methodology (rsm) based on the composite-centered design (ccd). many mathematical models have been proposed to statistically analyze the inhibitory efficiency of the arae extract on the corrosion of carbon steel. polynomial models and linear models are the two groups of models. according to a comparison of the various models (r2, coefficient of variation, and appropriate accuracy), the logarithmic-polynomial model has the best r2 correlation coefficient and by a coefficient of variation (cv = 0.24 %), a ratio of 88.170 % was obtained and indicated an adequate signal. experimental results showed that both parameters, concentration and temperature, were significant in obtaining maximum inhibition efficiency. after optimization using rsm, the results revealed that the highest value of the inhibition efficiency is 90.52 % when the temperature is equal to 20 °c and the concentration of inhibitor 700 ppm. the results also showed that corrosion rate decreases, while the efficiency of inhibition increases with arae concentration but decreases with the rise of temperature. according to the thermodynamic analysis, the extract spontaneously adheres to the steel surface following the freundlich adsorption isotherm model. g0ads values obtained are between -20 and -40 kj mol-1 of arae, which suggests the mixed type of adsorption attained by physical and chemical interactions. potentiodynamic polarization experiments indicated that arae extract is a mixed-type inhibitor and the electrochemical impedance spectroscopy eis n. saigaa et al. j. electrochem. sci. eng. 13(3) (2023) 469-490 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https://www.tandfonline.com/author/kaabi%2c+ilhem https://www.tandfonline.com/author/douadi%2c+tahar https://www.tandfonline.com/author/daoud%2c+djamel https://www.tandfonline.com/author/issaadi%2c+saifi https://www.tandfonline.com/author/sibous%2c+lakhdar https://www.tandfonline.com/author/chafaa%2c+salah https://doi.org/10.1080/01694243.2020.1816777 https://doi.org/10.1016/j.hydromet.2013.06.018 https://doi.org/10.1016/j.hydromet.2013.06.018 https://doi.org/10.1016/j.hydromet.2013.06.018 https://doi.org/10.1016/j.hydromet.2013.06.018 https://creativecommons.org/licenses/by/4.0/) palmyra palm flower biomass-derived activated porous carbon and its application as a supercapacitor electrode http://dx.doi.org/10.5599/jese.1658 215 j. electrochem. sci. eng. 13(1) (2023) 215; http://dx.doi.org/10.5599/jese.1658 open access : : issn 1847-9286 www.jese-online.org corrigendum to palmyra palm flower biomass-derived activated porous carbon and its application as a supercapacitor electrode sofia jeniffer rajasekaran and vimala raghavan centre for nanotechnology research, vellore institute of technology, vellore, tamil nadu, india j. electrochem. sci. eng. 12(3) (2022) 545-556. http://dx.doi.org/10.5599/jese.1314 corresponding author: lvimala.r@vit.ac.in received: january 3, 2023; accepted: january 3, 2023; published: januaray 5, 2023 there was an unintentional mistake while selecting the sem images from the folder in which the images of several synthesized materials were saved. we, the authors deeply apologize for the mishap. the scanning electron microscopic image (figure 3 of the published article) was wrongly given. the updated correct images of figure 3 (a-c) and d are given below. the authors apologize for any inconvenience caused. figure 3. (a-c) sem images of ppf-700, ppf-800, and ppf-900 based activated carbon at magnification 20,000 x (d) edax analysis of ppf-900 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1658 http://dx.doi.org/10.5599/jese.1658 http://www.jese-online.org/ http://dx.doi.org/10.5599/jese.1314 mailto:lvimala.r@vit.ac.in https://creativecommons.org/licenses/by/4.0/) electrochemical immunosensor for the determination of beta casein doi: 10.5599/jese.2015.0073 25 j. electrochem. sci. eng. 5(1) (2015) 25-36; doi: 10.5599/jese.2015.0073 open access : : issn 1847-9286 www.jese-online.org short communication development of zn-sic composite coatings: electrochemical corrosion studies mudigere krishnegowda punith kumar, thimmappa venkatarangaiah venkatesha and mudigere krishnegowda pavithra department of chemistry, kuvempu university, shankaraghatta-577451, shimoga, karnataka, india corresponding author: e-mail: drtvvenkatesha@yahoo.co.uk; tel.: +91-9448855079 received: march 30, 2014; revised: february 27, 2015; published: march 15, 2015 abstract the zn-sic composite coatings were fabricated by using sulphate plating bath dispersed with 1, 2 and 3 g l -1 of 64.28 nm sic nanoparticles. appreciable influence on morphology and microstructure was observed in scanning electron microscopy, x-ray diffraction spectroscopy and texture co-efficient calculations for sic incorporated zinc coatings. the electrochemical corrosion behavior of zinc and zn-sic composite coatings was studied by potentiodynamic polarization and electrochemical impedance analysis. significant reduction in corrosion current and corrosion rate with increased charge transfer resistance was noticed for composite coatings. the sic incorporated zinc coatings shown improved micro-hardness property to pure zinc coating. the properties of zn-sic composite coatings were compared with that of pure zinc coating. keywords sic nanoparticles; electrodeposition; zn-sic composite coatings; corrosion introduction zinc has been utilized as a sacrificial layer to protect steel from corrosion. however, the life span of zinc coating is limited in aggressive environment. consequently, considerable efforts have been made to advance their corrosion resistance property along with other mechanical and physical properties. one of the possible solutions for this is the incorporation of inert particles (nano or micro sized) into a growing zn metal matrix during electrodeposition. the second phase particles incorporated metal coatings are called as metal matrix composites (mmcs) and these coatings are renowned for their unique functional properties such as corrosion resistance and wear, hardness, magnetic and semiconducting properties compare to pure metal coating [1,2]. http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 5(1) (2015) 25-36 development of zn-sic composite coatings 26 26 also co-deposition of second phase or inert particles with growing metal matrix is environmentally friendly compared to surface modifications using organic chelating agents and hazardous post plating treatment like chrome passivation [3]. the fabrication of mmcs can be achieved by various methods such as electrodeposition, electroless plating, hot dipping, chemical and physical vapor deposition, stir casting and plasma spraying techniques. among these techniques, electrodeposition has advantages like low cost, room-temperature operation, single step, good reproducibility and ecofriendly. the nanoparticles like tio2 [4], zro2 [5], ceo2 [6] and sio2 [1] were used to prepare zn metal matrix composites and their mechanical and electrochemical behavior have been studied. however, the silicon carbide (sic) nanoparticles are well known for their high chemical stability, high temperature resistance and strong heat impact resistance [7]. in literature, substantial reports are accessible on use of sic nanoparticles along with nickel matrix to improve mechanical and electrochemical properties of nickel deposits [8-10]. however, not many reports are available on the generation of zn-sic composite coatings. swiderska-sroda et al. [11] used sic nanoparticles to generate zn-sic composite coatings and analyzed their surface appearance and gabriella roventi et al. [12] studied the effect of sic incorporation on the morphology and micro hardness of zinc matrix of zn-sic composite coatings developed by acidic chloride bath. nevertheless, the influence of sic particles on the corrosion behavior of zinc deposit has not been explored. hence, the present work has been accomplished to investigate the effect of sic particles on the corrosion behavior of zinc deposit. the zn-sic composite coatings were generated by means of electrodeposition method and also their morphological and electrochemical behavior were studied. experimental deposition process zn and zn-sic composite coatings were generated on mild steel specimen from zinc plating bath given in the table 1, which contains separately 0, 1, 2 and 3 g l -1 of suspended sic (64.28 nm) nanoparticles. the plating solutions were mechanically agitated for 24 h by a magnetic stirrer to attain uniform dispersion of sic nanoparticles in plating bath. the electrodeposition process was carried out at current density of 0.04 a cm -2 [2] for 10 min with a solution stirring speed of 300 rpm. table 1. optimized plating bath composition and operating parameters used for zinc and zn-sic composite coating process bath constituents concentration, gl -1 deposit code operating parameters basic bath (bb) znso4 200 z anode: zinc plate cathode: mild steel plate current density: 0.04 a cm -2 plating time: 10 min ph 3.5 temperature: 27 ± 2 c na2so4 10 h3bo3 8 ctab 0.05 b1 sic nanoparticles +bb 1.0 zs1 b2 sic nanoparticles +bb 2.0 zs2 b3 sic nanoparticles +bb 3.0 zs3 surface characterization the surface morphology of the coating was investigated using joel jem 1200 ex ii scanning electron microscopy (sem) and the tio2 particle content in the coated film was determined by energy dispersive x-ray (edx) analysis coupled with sem. x-ray diffraction (xrd) analysis of electrodeposits was carried out using philips tw3710 x-ray diffractometer with cu kα radiation (λ = 0.1540 nm) working at 30 ma and 40 kv. m. k. punith kumar et al. j. electrochem. sci. eng. 5(1) (2015) 25-36 doi: 10.5599/jese.2015.0073 27 electrochemical corrosion studies the electrochemical corrosion studies were performed in a conventional three electrode glass cell by using chi 660c electrochemical work station (us make) at 27±2 °c. a saturated ag/agcl electrode and a platinum wire served as the reference and counter electrode respectively. the coated specimens were used as working electrode with 1 cm 2 exposure area and were immersed in the corrosive media for about 30 min before polarization and impedance measurements to ascertain the steady state potential or open circuit potential (ocp). for the corrosion studies, 3.5 % nacl solution was used as corrosive media. tafel curves for coatings were obtained by applying potential between -200 and +200 mv from their ocp value. electrochemical impedance spectroscopic (eis) behavior of coatings was measured at their ocp, and the sinusoidal signal amplitude of 5 mv was employed with an alternating current of frequency range from 100 khz to 10 mhz at 6 points per decade. the measured eis data were curve fitted and analyzed with the help of commercial zsimpwin 3.21 software to obtain impedance parameter. each experiment was repeated thrice to confirm the reproducibility. micro-hardness measurements the micro-hardness of the zn and zn-tio2 composite coatings were measured by vickers indenter using the instrument “clemex micro-hardness tester”, by forcing a diamond indenter having the geometry as vickers pyramid, applying the test load 100 g on the individual specimen. the indentation time was 30 s on each specimen surface for the individual load. results and discussion characterization and surface analysis the zinc plating bath solutions (table 1), each containing 0, 1, 2, and 3 g l −1 of sic respectively, were prepared and stirred for 24 h for the uniform dispersion of nanoparticles. the zn-sic composite coatings were obtained on mild steel using operating conditions given in table 1 and the deposits were named in different deposit code and are given in same table. the thickness of the zn and zn-sic mmcs was calculated by using the equation 1. the corresponding thickness of the deposits is 14.72, 15.24, 15.51 and 15.58 µm for z, zs1, zs2 and zs3 coatings, respectively. m t ad  (1) where m is the mass of the deposit, a is the area (cm 2 ) and d is the density of the coated metal. the presence of sic nanoparticles in the zinc matrix was tested by analyzing the edx spectra recorded on the zn-sic metal matrix composite coatings. the peak corresponds to si and c in the edx spectra (figure 1) recorded at different points on composite coating surface confirms the presence of sic nanoparticles in zinc matrix. the weight percent of silicon carbide nanoparticles in zn-sic mmcs was calculated from the given equation (equation 2) [12] using the percentage of zn and si values obtained from edx analysis. si si c si sic si zn si c si w w pa pa w w w w pa pa     (2) where pasi and pac are si and c atomic weights respectively and wsi and wzn are mass fractions obtained from edx data. j. electrochem. sci. eng. 5(1) (2015) 25-36 development of zn-sic composite coatings 28 28 figure 1. edx spectra of zn-sic composite coating zs2 weight percent of sic nanoparticles in the deposit is given in table 2. it can be observed from table 2 that the amount of sic nanoparticles in zinc matrix is slightly increased with an increase in the concentration of sic nanoparticles in the plating bath. about 0.4799 wt% of sic particles are embedded in the zinc matrix obtained from the bath solution containing of 2 g l -1 nano particles. figure 2 represents the sem micrographs of zinc and zn-sic mmcs. the randomly distributed, hexagonal zinc platelets without having any local defects were observed in zinc and zn-sic deposits but comparatively improved compactness and uniformity was noticed in composite coatings. table 2. weight percent of sic present in the composite coatings obtained from edx analysis. coating zs1 zs2 zs3 amount, wt% c k* 1.93 1.98 2.01 si k* 0.30 0.33 0.32 zn k* 97.77 97.69 97.67 total 100.00 100.00 100.00 sic 0.4361 0.4799 0.4655 *k shows the peaks generated by x-rays given off as electrons return to the k electron shell xrd patterns were obtained for pure zinc and zn–sic mmcs to know the influence of sic nanoparticles on the average crystallite size and orientation of zinc crystals, and are provided in figure 3. the peaks in diffraction pattern correspond to the zinc hexagonal crystal structure (jcpds card number 04-0831). the peak correspond to sic nanoparticles were not observed in diffraction patterns, this may be due to the little incorporation (≈0.45 %, from edx analysis) of sic nanoparticles in to zn matrix. the average grain size of the deposited films was calculated using the scherrer equation by considering the (101) plane peak broadening (equation 3) [13]. m. k. punith kumar et al. j. electrochem. sci. eng. 5(1) (2015) 25-36 doi: 10.5599/jese.2015.0073 29 cos k l     (3) where l average crystal size, k scherer constant, λ wavelength of scattering, full width half maxima and scattering angle. figure 2. sem micrographs of as deposited zinc and zn-sic composite coatings figure 3. xrd diffractograms obtained from zinc and zn-sic composite coatings j. electrochem. sci. eng. 5(1) (2015) 25-36 development of zn-sic composite coatings 30 30 the calculated average crystallite size using scherrer equation is 89.95, 94.69, 48.80 and 114.76 nm for z, zs1, zs2 and zs3 deposits, respectively. it can be seen that the inclusion of silicon carbide nanoparticles, significantly influence the grain size of deposit. the change in grain size of the mmcs can be related to the modification of the competition between nucleation and crystals growth in the presence of nanoparticles [4]. the incorporated sic nanoparticles impede the crystal growth by inhibiting the diffusion of adatoms towards the growing centers and which leads to fresh nucleation [4]. however, in the present work, decrease in average crystal size was observed for zs2 coating, i.e., for zinc coating, the average crystallite size is 89.95 nm but it is reduced to 48.80 nm for zs2. however in zs1 and zs3 coatings, the grain size is not reduced by the sic nanoparticles but they impart compactness to the mmcs. according to literature, the incorporation of second phase particles to growing metal matrix will alter the preferred orientation of the deposit [4,14]. texture co-efficient values are calculated by using equation 4 [15] for all the xrd profiles and the obtained texture co-efficient values are plotted in figure 4. ( )( ) ( ) / % 100 ( ) ( ) o c o i hkli hkl t hkl i hkl i hkl      (4) where, tc(hkl) is the texture coefficient; i(hkl) is the peak intensity of the zinc electrodeposits; ∑i(hkl) is the sum of intensities of the independent peaks and the index ‘o’ refers to the intensities for the standard zinc sample. figure 4. preferred orientation of crystallites in zinc and zn-sic composite coatings. the preferred orientation plot in figure 4 reveals that in the zinc deposit, orientation of the zinc crystals is dominated in the (100) plane, however it is changed to (101) plane after the incorporation of sic nanoparticles into the zinc matrix. the change in compactness, average crystallite size and texture co-efficient corresponds to znsic mmcs with respect to pure zinc coating reveals the significant influence of sic nanoparticles on the morphology and microstructure of the deposits. electrochemical corrosion analysis characterization studies confirm that the incorporation of sic particles to zinc matrix influence the morphology and microstructure of the deposits. and the electrochemical corrosion analysis m. k. punith kumar et al. j. electrochem. sci. eng. 5(1) (2015) 25-36 doi: 10.5599/jese.2015.0073 31 was carried out in order to compare the effect of morphological and microstructural changes on the quality of zn-sic mmcs with respect to the zinc coating. potentiodynamic polarization analysis polarization measurements were carried in order to examine the influence of sic nanoparticles on the corrosion behavior of the zinc deposits. the potentiodynamic polarization measurement was performed after 30 min immersion in 3.5 % nacl corrosive media. the potentiodynamic polarization curves or tafel plots were recorded for all the deposits in a potential range of ±200 mv from ocp of the corresponding deposit and are given in figure 5. the extrapolation on these curves resulted in the determination of corrosion parameters such as corrosion potential (ecorr), corrosion current (icorr), corrosion rate (cr), and anodic and cathodic slopes (a and c) and are tabulated in table 3. the decreased corrosion current and little drift in corrosion potential towards positive side for zs1, zs2 and zs3 reveal that the zn-sic composite coatings are more resistive towards corrosive media than pure zinc coating. in particular, 80 % decrease in corrosion rate is noticed for zs2 deposit. the change in anodic and cathodic tafel co-efficient values of zn-sic composite coatings compare to pure zinc coating indicates that the presence of sic nanoparticles in the zinc matrix influence the kinetics of both anodic and cathodic electrochemical reactions. figure 5. potentiodynamic polarization curves for zinc and zn-sic composite coatings measured with respect to sat. ag/agcl electrode in 3.5 % nacl solution. table 3. electrochemical kinetic parameters of the zinc and zn-sic composite coatings derived from tafel plots. specimen ecorr / v icorr / μa cm -2 c / mv dec -1 a / mv dec -1 corrosion rate, μg h -1 cm -2 z -1.047 8.505 280 35.72 10.06 zs1 -0.995 2.802 248 32.80 3.314 zs2 -0.989 2.215 182 32.92 2.620 zs3 -1.000 3.526 192 36.69 4.170 j. electrochem. sci. eng. 5(1) (2015) 25-36 development of zn-sic composite coatings 32 32 electrochemical impedance analysis the impedance measurement is the nondestructive and most informative method to assess the corrosion behavior of metal coating. in the present work, the impedance measurement was carried out for all coatings at their open circuit potential in the 100 khz to 10 mhz frequency range. initially working electrode was left for 30 min to attain equilibrium potential in 3.5 % nacl solution before eis measurement. the measured electrochemical impedance (eis) data are presented as nyquist and typical bode plots in figure 6(a) and figure 6(b) respectively. two capacitive loops or two time constants corresponds to corrosion process are observed in the nyquist and bode plots. the shape of the spectra is influenced by the electrochemical process at the surface and/or by the geometric factors of the electrode [16]. hence to obtain electrochemical parameters, the experimentally determined eis data were fitted with a suitable electrical equivalent circuit (eec) given in figure 7, with the help of the zsimpwin 3.21 software. to get appreciable fitting results the employed high frequency capacitance element (c) in eec was replaced by constant phase element (cpe). the impedance of cpe is defined as z(iω) = (q) -1 (iω) -n where q is the cpe constant, i is the imaginary unit, ω is the angular frequency (ω=2πf, f is the frequency) and ‘n’ is the cpe exponent (where ‘q’ and ‘n’ are frequency independent parameters). the value of ‘n’ lies between −1 and 1 (i.e., −1≤ n ≤1), for ideal capacitor n = 1, for ideal inductor n =−1, if n = 0 the cpe is ideal resistor [17, 18]. figure 6. impedance (a) nyquist and (b) bode plots for zinc and zn-sic composite coatings measured with respect to sat. ag/agcl electrode in 3.5% nacl solution. figure 7. electrical equivalent circuit used to simulate the recorded impedance nyquist plots. the contribution of each element in the used eec is as follows [19]. re is the electrolyte resistance that appeared between the reference electrode and the surface working electrode. the high frequency contribution (qcoat–rcoat) is ascribed to the dielectric m. k. punith kumar et al. j. electrochem. sci. eng. 5(1) (2015) 25-36 doi: 10.5599/jese.2015.0073 33 character qcoat of the coating that is reinforced by ionic conduction through its pores rcoat. the low frequency contribution is attributed to the double layer capacitance (cdl) at the electrolyte/coated surface interface at the bottom of the pores coupled with the charge transfer resistance (rct). the obtained corrosion parameters are tabulated in table 4. the polarization resistance was calculated by adding rcoat and rct of the corresponding deposits. the constant phase element is considered as capacitor because the value of cpe exponent ‘n≈1’. the extent of anticorrosive nature of the deposit can be analyzed by the resistance offered by the deposit, considering both coating resistance and charge transfer resistance. the polarization resistance exhibited by the pure zinc coating z is 494.8 ω cm 2 . the incorporation of sic particles significantly influenced on the polarization resistance of zn-mmcs coatings i e., 3570, 5795 and 2449 ω cm 2 for zs1, zs2 and zs3 coatings respectively. it should be noted that, sic incorporated zinc coatings exhibited more resistance for charge transfer with minimum double layer capacitance value when compared to pure zinc coating. the decrease in double layer capacitance value suggests that the electroactive behavior of the surface in corrosive environment is minimum and hence mmcs are much resistive towards external aggressive media. among all, zs2 deposit showed tenfold increase in rct value with decrease in cdl value to the same extent. the two capacitive loops which were observed in the nyquist plot are also observed in bode plots. the impedance modulus |z| vs. the frequency plot shows that impedance modulus value of the composite coatings are greater than that of z coating, and it is in accordance with the trend observed in polarization and nyquist plot analysis. figure 8. sem micrographs of zinc and zn-sic composite coatings recorded after potentiodynamic polarization measurement. j. electrochem. sci. eng. 5(1) (2015) 25-36 development of zn-sic composite coatings 34 34 4. electrochemical corrosion parameters obtained from eec simulation of impedance nyquist plots. specimen ccoat / µω 1 cm -2 s -n ncoat rcoat / ωcm 2 rct / ω cm 2 cdl / 10 -3 f cm -2 *rp / ω cm 2 z 29.14 0.817 419.6 75.2 6.968 494.8 zs1 16.38 0.864 2530 1040 0.340 3570 zs2 13.31 0.931 4188 1607 0.451 5795 zs3 6.784 0.946 1768 681 0.692 2449 *rp = rcoat + rct also the sem images of the deposits after polarization (figure 8) shows that the composite coating surface is less deteriorated and retained its compactness when compare to pure zinc coating. furthermore, the uniform corrosion process was observed on all deposit surfaces and it confirms the defect free nature of the zinc coatings. if deposit has defects then the pitting corrosion behavior is expected. it is already reported that, the inclusion of sic nanoparticles to zinc matrix will improve the mechanical properties of the deposit [12]. however, the results gathered from the present work confirm that the incorporation of sic nanoparticles also enhance the corrosion resistance property of the zinc coatings. the dissolution of metallic coating depends on the texture, morphology and chemical composition of the deposit [14,20]. under aggressive environment, the dissolution activities vary because of the difference in binding energy of atoms between the crystallographic planes. the deposits having the crystallographic planes with higher number of adjacent atoms requires higher energy to breakdown the atoms followed by their dissolution. therefore more energy is required to breakdown the close packed planes or low-index planes because they are associated with high bonding energy of the surface atoms [20]. all the coatings in particular zs2 exhibit maximum orientation in (100) and (101) planes. so all the coatings including pure zinc coating has to show high corrosion resistance. but better anticorrosion behavior was noticed for composite coatings in particular zs2, because since the different texture and grain boundary distribution of the coating also influences on the corrosion behavior of the coatings. as a result, we observed different corrosion resistance property for zinc coatings. micro-hardness analysis the measured vickers micro-hardness value for zn and zn-sic mmcs is given in figure 9. the given micro-hardness values were an average of 5 measurements performed on different locations at the centre portion of coated samples. the slightly increased micro-hardness value was observed for zn-sic mmcs compared to pure zinc coating, because of the minimum incorporation of sic nanoparticles in metal matrix. also, it can be observed that there is no significant difference in micro-hardness value between zs1, zs2 and zs3 coating. it may be due to the percentage of sic incorporation is comparatively equal in all the mmcs. however, mmcs exhibit better micro-hardness value compared to that of pure zinc coating. it can be supposed that nanoparticles co-deposited in the composite coating might act as barriers to impede the plastic deformation of zn matrix and hence enhance the micro hardness [21]. m. k. punith kumar et al. j. electrochem. sci. eng. 5(1) (2015) 25-36 doi: 10.5599/jese.2015.0073 35 figure 9.vickers micro-hardness value of zn and zn-sic mmcs. conclusions the zn-sic composite coatings were successfully generated on mild steel by electrodeposition method. the sem and xrd analysis demonstrate that, the incorporation of sic nanoparticles considerably change the morphology and microstructure of the deposits. the preferred orientation of the zinc crystallites in sic incorporated coatings is changed to (101) plane from (100) plane of the zinc crystallite in pure zinc coating. also uniform and compact nature was observed in zn-sic mmcs. the incorporation of sic nanoparticles in to zinc metal matrix improved the microhardness property of the deposit. the electrochemical corrosion studies reveal that zn-sic composite coatings have better corrosion resistance property than pure zinc coating. compare to pure zinc coating, 60 to 80% decrease in corrosion rate was observed for composite coatings; in particular, zs2 coating has significant resistance towards external aggressive media. the passive barrier behavior of uniformly distributed sic nanoparticles and morphological changes may responsible for the anticorrosive behavior of the zn-sic mmcs. acknowledgements: the authors thank kuvempu university karnataka, india for providing the laboratory facilities to bring about this work, and also the department of science and technology (dst), new delhi, government of india (goi) for providing financial support by major research project [project no. s.r/s3/me/014/2007]. references [1] tj. tuaweri, g. d. wilcox, surf. coat. technol. 200 (2006) 5921-5930. 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[21] l. m. chang, m. z. an, h. f. guo and s. y. shi, appl. surf. sci. 253 (2006) 2132-2137. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ a sensitive and simple electrochemical technique for detecting ascorbic acid content in pharmaceutical and biological compounds https://dx.doi.org/10.5599/jese.1388 889 j. electrochem. sci. eng. 12(5) (2022) 889-899; https://dx.doi.org/10.5599/jese.1388 open access : : issn 1847-9286 www.jese-online.org original scientific paper elevated temperature erosion of abradable seal coating bharti malvi1, and manish roy2 1national institute of technology, jamshedpur: 831014, india 2defence metallurgical research laboratory, po: kanchanbagh, hyderabad, 500 058, india corresponding author: bharti.malvi@gmail.com received: may 23, 2022; accepted: june 28, 2022; published: august 11, 2022 abstract abradable coatings are essentially sealing materials and are deposited by thermal spray techniques. the main function of these coatings is to control the clearance of the gas path of the gas turbine engines. the abradable coating prevents turbine blade damage by abrading itself when there is an offset or vibration during turbine operation. since the coating is meant to abrade, the preferred coating material is relatively softer than the turbine blade material. as these coatings are prone to solid particle erosion at high temperatures, the erosion response of these coatings at elevated temperatures needs to be investigated. in order to achieve this objective, mcraly boron nitride polymer coating was deposited employing an air plasma spraying technique on a ni-base alloy substrate. the important features of the microstructure and mechanical properties of the coating were examined, and the coating was subjected to erosion at various temperatures under different erosion conditions. the results indicate a ductile erosion behaviour for an abradable top coat. the erosion rate increases with the temperature of the coating. the detailed results of the investigation are presented, and the erosion mechanisms are studied. keywords abradable coating, high-temperature erosion, erosive wear, plasma spraying technique, mass-loss introduction deposition of abradable coating by thermal spraying has revolutionized the sealing technology of turbomachinery [1]. in order to ensure enhanced engine efficiency, the clearance between the static and the rotating parts should be minimized. abradable coatings are employed in turbomachinery to reduce leakage gaps between stationary and rotating parts and control the clearance between the blade tip and casing in the turbines. shaft labyrinth seal, shrouded blade labyrinth seal, and unshrouded labyrinth blade seal are three important labyrinth seals of gas turbine engines. abradable coatings employed for sealing turbomachinery should exhibit not only resistance to various high-temperature corrosions and high cohesive strength but also should be resistant to solid particle erosion. thermal spraying is an important technique for depositing various coatings [2,3]. abradable coatings are deposited using flame spray, atmospheric plasma spray (aps) technique, and highhttps://dx.doi.org/10.5599/jese.1388 https://dx.doi.org/10.5599/jese.1388 http://www.jese-online.org/ mailto:bharti.malvi@gmail.com j. electrochem. sci. eng. 12(5) (2022) 889-899 erosion of abradable seal coating 890 velocity oxy-fuel (hvof) technique. coatings such as aluminum polymer, aluminum boron nitride, and aluminum bronze polymers deposited by aps are used for relatively low-temperature applications. zirconia-based ceramic abradable coatings used for a temperature around 1100 °c are another variety of plasma sprayed coating. mcraly boron nitrite polymer coatings are developed and deposited by plasma spraying for intermediate temperatures ranging from 700 to 850 °c. this coating is harder than low-temperature coatings, and abradability is controlled by porosity content, while boron nitrite provides lubricating action. the quantity of porosities in this coating is controlled by post-coating heat treatment. abradable coatings are subjected to various types of degradation such as high-temperature corrosion [4], thermal shock [5], erosion at room and elevated temperature [6], etc. erosion is characterized by continuous material removal from eroding surfaces because of repeated impingement by erodent [6]. elevated temperature erosive wear is a result of simultaneous degradation due to the impact of solid particles and oxidation [7]. it is also an important degradation process for mcraly boron nitrite polymer abradable coatings. erosion of abradable coating is already reported in the literature by several investigators [8–10]. kumar et al. [11] studied erosion al-si polyester abradable coatings sprayed by hvof at 25, 200 and 400 °c temperatures and at 30, 60 and 75° impact angles. they found erosion rate is maximum at an intermediate impact angle. an investigation by siddiqui et al. [9] with conicraly with polyester and bn coating indicated that the average erosion rate is 0.22 (mm/s) (seconds/mil). maozhong et al. [8] examined erosion of coatings having 75 % ni +25 % graphite, 57 % al + 8 % si + 35 % graphite, 40 % al + 5.5 % si + 45.5 % graphite + 9 % organic binder and 40 % polyester + 60 % al–si alloy deposited by plasma spray method. they reported that the maximum erosion occurred at a 60o angle of impact. erosion occurs by tunneling via pores and non-metal phase, micro-cutting, and plowing at oblique impact. the particles impact extrudes and produces indentations and extruded lips. these lips work-harden and eventually fall off. maximum erosion rate at 60° impact angle is also reported by sharsar et al. [10] for al-si polyester powders sprayed on titanium substrates by hvof. the above discussion clearly brings out the fact that although erosion is an important degradation process for abradable coating, erosion of abradable coating, particularly at elevated temperatures, has not been investigated comprehensively. the influence of temperature, impact angle, and impact velocity on erosion rate has not been reported. the influence of eroding conditions on the erosion behavior of mcraly boron nitrite polymer coatings at various temperatures is not available in the literature. thus, the main objective of the proposed investigation is to evaluate the erosion response of mcraly boron nitrite polymer abradable coatings as a function of test conditions. experimental substrate and coating powder material ni-cr alloy is taken as substrate material for abradable coatings. the ni-cr alloy was cut into pieces of 30×30×5 mm dimensions with the help of edm. the specimens were subjected to sandblasting followed by ultrasonic cleaning in acetone prior to the coating deposition by plasma spraying. the characteristics of feedstock powder are listed in table 1. sem images showing the morphologies of the powders used for depositing bond coat and abradable coatings are shown in figure 1. the powders for the bond coats are of high purity, spherical and dense. in contrast, powders of the abradable layer are spheroidal with different morphologies. the bond coat was essentially a conicraly layer, and the composition of the bond coat was 38.5 wt.% co, 32wt.%ni, 21 wt.% cr, 8 wt.% al and 0.5 wt.% y. bond coat was about 100 m thick. b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 889-899 http://dx.doi.org/10.5599/jese.1388 891 table 1. feedstock powders used for abradable coating coating typeabradable coating purpose bond coat top coat coating thickness 100µm 500µm designation amdry 961 (nicraly) metco 2043 (conicraly bn polyester) chemical composition ni 22cr 10al 1.0y 30co 25ni 16cr 6al 0.3y 4bn 15polyester figure 1. sem images showing morphologies of the powders used for depositing bond and abradable coating deposition of coatings on substrates the abradable coatings were deposited on ni-cr alloy substrates using an aps unit. the test specimens were held in a jaw for coating deposition immediately after sandblasting. the 9 mb plasma gun was operated at 590 amp to deposit coating material in powder form. argon (carrier gas), argon (plasma arc), and hydrogen (secondary gas) were allowed to flow through the plasma gun. the plasma plume was formed by striking the arc with the gas molecules. huge thermal energy was released as plasma ions returned to the gaseous state. the coating powders were fed into a gas plume. the powders were melted in the plume and propelled towards the substrate material to form the coating. the air plasma spray parameters used to coat abradable layers are listed in table 2. table 2. plasma spray conditions used for deposition of powders plasma spray parameters amdry 961 (nicraly) metco 2043 (conicraly bn polyester) plasma arc current, a 450 450 arc voltage, v 61.3 61.3 plasma gas (ar) flow rate, m3s-1 63.3e-5 63.3e-538 secondary gas (h2) flow rate, m 3s-1 5.0e-5 5.0e-53 carrier gas (ar) flow rate, m3s-1 5.0e-5 5.0e-53 stirrer rotation, rotation min-1 30 30 torch to base distance (tbd), mm 100 100 erosion testing elevated temperature erosion was performed by employing a solid particle erosion test rig shown in figure 2 [12]. the test facility was fabricated in a defense metallurgical research laboratory. the rig can conduct erosion tests at temperatures up to 1073 k with the combination of a wide range of parameters affecting erosion. the rig is equipped with two furnaces for heating the specimen and the air. the temperatures of the specimen and the furnace were regulated with the help of a controller setup and thermocouples. a miniature conveyer belt was employed at the bottom of the particle feeding hopper to control the particle feed rate. the impact velocity is monitored by varying the incoming air pressure by a pressure valve. further detail of the test rig is available elsewhere [12]. http://dx.doi.org/10.5599/jese.1388 j. electrochem. sci. eng. 12(5) (2022) 889-899 erosion of abradable seal coating 892 figure 2. schematic diagram of high-temperature erosion rig impinging particles from the hopper were fed at a constant predetermined feed rate and were fluidized by dry compressed air. the particle-entrained air mixture was then accelerated through a nozzle of a diameter of 8 mm, and it was made to impinge on the test sample placed at a distance of 10 cm from the end of the nozzle. the impact angle was varied by changing the sample holders whose inclinations vary with respect to impacting air particle stream. air pressure and erodent feed rate were regulated by the valves provided. the rotating disc method was used to determine the velocity of the erodent particles [13]. ni substrates coated with an abradable layer were subjected to impingement by gas-laden particles at various temperatures. after exposure to a predetermined time, the samples were ultrasonically cleaned with acetone, dried, and then weighed in an electronic balance having precision up to 0.1 mg. erosion test conditions are summarized in table 3. an established test procedure was followed for each erosion test up to 673 k. the cleaned samples were weighed first and then subjected to erosion in the test rig for 5 minutes. the eroded specimen was weighed again to estimate the mass loss. this erosion is carried out 6 times or 30 minutes by weighing every 5 minutes. the ratio of mass loss suffered to the mass of the particles causing the mass loss gives the dimensionless erosion rate e. this procedure was repeated until a steady-state erosion rate was obtained. further details of test techniques are available elsewhere [14]. table 3. erosion test conditions test variable test conditions erodent sio2 erodent size 150 ± 50 µm erodent shape angular velocity low 57, high 93 m / s erodent feed rate 2.0 g / min angle of impingement 30 and 90o temperatures room temperature, 200, 400 and 600 °c sample size 30 ×30×5 mm exposure time 5 min b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 889-899 http://dx.doi.org/10.5599/jese.1388 893 examination of eroded surfaces eroded samples were examined under an environmental scanning electron microscope (esem) equipped with an energy dispersive spectroscopy (eds) facility operating at 20 kv after cleaning the samples ultrasonically. transverse sections of eroded samples were analyzed after polishing the cross-sections metallographycally. samples were embedded into resin using a hot mounting technique for metallographic preparation. the demolition of the porous structure during sample preparation due to grinding, cutting, and polishing, etc., was avoided by infiltrating the porous coating with epoxy adhesives. the porosity of the porous coating was measured by image analysis. results and discussion microstructure of abradable coating the xrd pattern of the abradable coating surface is illustrated in figure 3. in addition to h-bn and -co-ni-cr, the coating contains ’ni3al and nial. thus, there is no perceptible change in phases as a result of spraying. -co-ni-cr and ’ni3al phases exhibit high intensity. the as-sprayed microstructure of the abradable coating is shown in figure 4. the microstructure presented in figure 4 shows that the ductility agents such as a polymer which provides friability, and h-bn, which acts as a lubricant, are homogeneously distributed in the coating, and there is no segregation of these phases in the microstructure. 2 / o figure 3. x-ray diffraction pattern of the abradable coating figure 4. sem image showing the cross-section of the mcraly boron nitrite polymer abradable coating in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1388 j. electrochem. sci. eng. 12(5) (2022) 889-899 erosion of abradable seal coating 894 this segregation may be detrimental to the performance of the coatings. the microstructure was uniform in various regions of the sample and in different samples. the thickness of the nicraly bond coat and conicraly bn polyester topcoat are 100 µm and 650 µm approximately. to serve the purpose of abradability, the coating has a porous structure. the powders for their respective coatings were selected on the basis of their compatibility with the ni-cr substrate. the purpose of the abradable coating is to abrade off while rubbing, but at the same time, it should be erosion resistant. abradability and erosion resistance are two contradictory phenomena. hence, a compromise between them has to be made. in other words, abradability is governed by soft phases such as polyester and h-bn. in contrast, hard metallic phases such as -co-ni-cr, ’ni3al and nial determine erosion resistance. thus, the microstructure is optimized to ensure good abradable behavior and erosion resistance. elevated temperature erosion of the abradable coating variations of incremental erosion rate as a function of the cumulative mass of the erodent at ambient and elevated temperature are illustrated in figure 5 and figure 6, respectively. incremental erosion rate is presented under normal and oblique impact at two different impact velocities. under all conditions, a steady-state erosion rate is attained from an initial high value. the highest erosion rate is obtained as expected under normal impact at an impact velocity of 93 m/s. the erosion rate at a higher temperature is higher than at an ambient or lower temperature. cumulative mass of erodent, 103 kg figure 5. variations of incremental erosion rate as a function of the cumulative mass of the erodent at ambient temperature under normal and oblique impact at two different impact velocities the influence of test temperatures on the steady-state erosion rate of the abradable coating is presented in figure 7. the erosion response is ductile, signifying a higher erosion rate at oblique impact at lower impact velocity and all temperatures. in contrast, a brittle erosion behavior at lower test temperature and ductile erosion response at a higher temperature at high impact velocity can be noted. such difference in erosion behavior can be attributed to the deformation condition of the thermal sprayed coating during erosion [15] and the presence of porosities in abradable coatings, which contain around 12-18 % porosities. in cr e m e n ta l e ro si o n r a te , k g k g -1 b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 889-899 http://dx.doi.org/10.5599/jese.1388 895 cumulative mass of erodent, 103 kg figure 6. variations of incremental erosion rate as a function of the cumulative mass of the erodent at 673 k under normal and oblique impact at two different impact velocities it has been shown in earlier publications [16] that the strain rate associated with impact is directly proportional to the square root of impact velocity. thus, the strain rate at higher impact velocity is higher than that at lower impact velocity. it is also noted that the strength of metallic materials increases, and the ductility decreases with an increase in strain rate. this increase in strength and decrease in ductility is more pronounced for a porous material. thus, at low impact velocity, porous abradable mcraly boron nitrite polymer coatings exhibit brittle behavior and become ductile with an increase of strain rate at high impact velocity. temperature, k figure 7. the influence of test temperature on the steady-state erosion rate of the abradable coating the influence of impact velocity on erosion rate is characterized by velocity exponent n and is given by equation (1) er = kvn (1) s te a d y -s ta te e ro si o n r a te , k g k g -1 in cr e m e n ta l e ro si o n r a te , k g k g -1 http://dx.doi.org/10.5599/jese.1388 j. electrochem. sci. eng. 12(5) (2022) 889-899 erosion of abradable seal coating 896 where er is erosion rate, v is impact velocity, n is velocity exponent and k is a constant. the variation of velocity exponent n calculated for abradable coating as a function of test temperatures is furnished in figure 8. velocity exponents are between 2 to 3 and are generally indicative of ductile erosion response [17]. test temperature, k figure 8. the variation of velocity exponent n calculated for abradable coating as a function of test temperatures morphology of eroded surfaces figure 9a shows the morphology of the surface of worn abradable coating at a 30° impact angle, at 93 m/s impact velocity, and ambient conditions. material loss due to decohesion at the intersplat interface can be seen. no evidence of particle embodiment is noticed. such features can be considered good in the sense that the embodiment of blade materials will be minimized during the operation of turbines, and coatings can perform their sealing action. cracks were found to nucleate at the curled parts in the metallic matrix phase. the curled parts easily fall off under repeated impingement of impacting particles. this highlights the important role played by the micro-cutting and plowing due to the shear component of abrasive particles at a lower impact angle. figure 9. sem morphology of eroded coating tested at high velocity, oblique impact angle, and 673k temperature brittle chipping v e lo c it y e x p o n e n t b. malvi and m. roy j. electrochem. sci. eng. 12(5) (2022) 889-899 http://dx.doi.org/10.5599/jese.1388 897 energy, ev energy, ev figure 10. eds profile of eroded coating regions a and b in figure 9 tested at high velocity, normal impact angle, and 673k temperature the sem image of the eroded surface at oblique impact at high impact velocity and 673 k is presented in figure 9b. the presence of formed curled parts, micro-cutting, and plowing traces are evident. the eds profiles for the two regions marked in figure 9 are shown in figure 10, and this confirms the presence of an aluminum oxide scale on the surface. the presence of si in eds profiles in fig. 10a confirms the presence of si erodent particles embedded in the eroded surface, although si embodiment is marginal. this also brings out the possibility of the formation of a kind of composite layer on the eroded surfaces [18,19]. the transverse section of the eroded abradable coating surfaces is illustrated in figure 11. this also confirms inters plat decohesion as the main mechanism for materials loss, as noted in other thermal sprayed coatings [20]. this mechanism of materials removal from the thermal sprayed coating is different from what is normally observed for ductile materials, where the loss of material is generally caused by the formation of a lip and its subsequent fracture due to localization of deformation [21]. this mechanism is also different from brittle ceramic material, where material removal occurs by the formation of intersecting cone or radial cracks, nucleating from preexisting flaws once critical tensile stress is exceeded [22]. at this stage, it is pertinent to mention that the dimensionless erosion rate of conicraly bn polyester abradable topcoat is comparable to that of ysz thermal barrier coating (tbc) system under normal impact and lower than that of ysz tbc system under oblique impact [23]. the erosion rate of ysz thermal barrier coating (tbc) under normal and oblique impact were 1.62 and 0.57 g/ kg, respectively, at ambient conditions under high impact velocity. at lower impact velocity, the tbc system exhibits a lower erosion rate at all tested temperatures. the erosion rates of tbc at low impact velocity under normal and oblique are 0.36 and 0.20 mg/g respective at ambient conditions. the comparable erosion rates of polymer containing abradable coating [24] and ceramic thermal barrier coating can be attributed to the deformation condition during erosion [25]. it can be noted that during erosion, the target material is indented by hard angular particles, as it happens during the hardness test. this leads to a multi-axial stress state. the thickness of the coating is significantly higher than the plastic zone size beneath the erodent. thus, the plastic zone beneath the indenting erodent is totally confined. as a result, constrained plastic flow in the presence of hydrostatic compressive stresses can be realized. the estimated strain rates during erosion are in the range of 104 to 107 s-1, which is ultrahigh strain rates. the eroding material can deform to large strains substantially higher than the tensile fracture strains prior to fracture during erosion. it is also demonstrated that the deformation during erosion is adiabatic. these above facts together in te n si ty , a .u . in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1388 j. electrochem. sci. eng. 12(5) (2022) 889-899 erosion of abradable seal coating 898 contribute towards unexpected erosion rates of polymer containing abradable coating and ceramic thermal barrier coating. figure 11. cross-sectional sem micrographs of eroded abradable coating at different magnifications conclusions the erosion behavior of conicraly bn polyester abradable coating has been evaluated as a function of erosion conditions up to 673 k. main conclusions are: 1) the erosion rate of conicraly bn polyester abradable coating increases with an increase in temperature. 2) this coating exhibits ductile erosion response at elevated temperature and at high impact velocity. this coating, however, responds in a brittle way at ambient conditions under high impact velocity. 3) detachment at the intersplat adhesion boundary is responsible for material loss. 4) velocity exponent is the maximum at intermediate temperature. 5) embodiment of erodent on the eroded surface is hardly noticed. acknowledgments: authors are grateful to the director, dmrl hyderabad, for the opportunity to carry out this research and for permitting to publish this work. references 1. y. cui, m. 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attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1388 https://doi.org/10.1016/s0043-1648(01)00681-0 https://doi.org/10.1016/s0043-1648(01)00681-0 https://doi.org/10.5185/amlett.2014.588 http://www.ijrmet.com/vol4issue2/shekhar-sharsar.pdf http://www.ijrmet.com/vol4issue2/shekhar-sharsar.pdf https://www.researchgate.net/publication/285944502_wear_testing_-_an_assessment_of_test_methodologies https://doi.org/10.1016/0043-1648(75)90154-4 https://doi.org/10.1016/0043-1648(75)90154-4 https://doi.org/10.1007/s11661-001-0232-5 https://doi.org/10.1361/105996302770348790 https://doi.org/10.1016/0043-1648(83)90266-1 https://doi.org/10.1016/0043-1648(83)90266-1 https://doi.org/10.1179/mst.1995.11.8.791 https://doi.org/10.1016/s0043-1648(98)00139-2 https://doi.org/10.1088/0022-3727/39/6/r01 https://doi.org/10.1088/0022-3727/39/6/r01 https://doi.org/10.1007/978-3-7091-0101-8_1 https://doi.org/10.1016/0921-5093(93)90626-p https://doi.org/10.4018/978-1-4666-7489-9.ch007 https://doi.org/10.1007/s11666-021-01189-9 https://doi.org/10.1080/10402000903097411 https://doi.org/10.1016/j.matchar.2016.01.008 https://creativecommons.org/licenses/by/4.0/) electrochemical determination of amaranth in food samples by using modified electrode: http://dx.doi.org/10.5599/jese.1531 1165 j. electrochem. sci. eng. 12(6) (2022) 1165-1177; http://dx.doi.org/10.5599/jese.1531 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical determination of amaranth in food samples by using modified electrode sayed zia mohammadi1, yar-mohammad baghelani2, farideh mousazadeh3, shamsi rahimi1 and maryam mohammad-hassani1, 1department of chemistry, payame noor university, tehran, iran 2department of chemistry, faculty of sciences, ilam university, ilam, iran 3school of medicine, bam university of medical sciences, bam, iran corresponding authorс: mohammadhassanipourm@gmail.com received: september 23, 2022; accepted: october 5, 2022; published: november 4, 2022 abstract in this paper, a new electrochemical sensor was reported for the determination of amaranth in drink soft. in this sensor, activated carbon-co3o4 nanocomposite (ac-co3o4) was employed as electrode modifying material. high pores of the activated carbon favour an access of aramanth nolecules within the pores of the working electrode surface, and allow fast electron transfer that is beneficial for the electrochemical detecion process. thus, the electrochemical signal is obviously enhanced at ac-co3o4 modified electrode compared to bare carbon paste electrode, and exhibited a wide linear response ranging from 0.1-215 m with a low detection limit of 10.0 nm (based on 3sb/m). this work offers a new route in developing new electrochemical sensors for the determination of collorant additives and other hazard components in drink soft. keywords electrochemical sensor; carbon paste electrode; activated carbon-co3o4 nanocomposite introduction bright colors make foodstuffs and soft drinks more attractive and appealing [1]. therefore, various edible colorants, including natural colorants and artificial colorants are widely used in the food processing industry [2]. because of charming color uniformity, excellent water solubility, cheap production cost, low microbiological contamination as well as high stability to light, oxygen and ph, synthetic colorants especially azo dyes have been widely used in food industry to improve the appearance and color of food products [3]. amaranth, a typical synthetic azo colorant, has been extensively applied to give fascinating red color and make food more attractive and appealing. however, excessive intake of amaranth may cause adverse health effects such as dizziness, anxiety, allergic reaction, and even cancer [1,4-6]. therefore, many countries have strictly regulated the additive dosage of amaranth. for example, in http://dx.doi.org/10.5599/jese.1531 http://dx.doi.org/10.5599/jese.1531 http://www.jese-online.org/ mailto:mohammadhassanipourm@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1165-1177 electrochemical determination of amaranth 1166 both the usa and japan, the use of amaranth has been voluntarily restricted in foodstuffs and beverages [7] and in china, amaranth is permitted to be added into some foodstuffs [8]. the world health organization has set the acceptable daily intake of amaranth as 0.5 mg kg-1 [9]. therefore, the content of amaranth in foods must be severely controlled and the detection of amaranth in a rapid, sensitive, simple, and cost-effective manner is paramount for human health and food safety. recently, various methods such as thin layer chromatography [10], high-performance liquid chromatography mass spectrometry [11,12], liquid chromatography-tandem mass spectrometry [13,14], spectrophotometry [15,16] and electrochemical analysis [17-21] have been reported to determine amaranth. among those methods, electrochemical methods are more attractive than others because of their unique advantages such as low-cost, simple equipment and operation, quick response and ease of miniaturization [22-33]. various electrochemical sensors have been successsfully designed for detecting amaranth in foodstuffs and soft drinks [1,3,34-40]. the nanomaterials have excellent catalytic and electronic traits to convert biorecognition processes into electrochemical reaction, and also have the high loading receptor molecules to synergistically amplify target reaction, as well as have large surface area to increase mass transport. herein, we developed novel activated carbon-co3o4 nanocomposite-modified carbon paste electrode (ac-co3o4-cpe) for the sensitive detection of amaranth in foodstuffs. as far as we know, there was no report on the amaranth oxidation with the ac-co3o4-cpe. then, it was investigated for amaranth voltammetric detection in the real samples. experimental instruments and chemicals instruments utilized in this research are similar to the devices reported in the previous article [30]. amaranth, cobalt chloride hexahydrate, na2hpo4 and nah2po4 were sigma-aldrich (st. louis, usa) and the rest reagents with analytical grades were also sigma and thus applied with any additional treatments. moreover, we employed na2hpo4 and nah2po4 for preparing phosphate buffer (pbs) supporting electrolyte solution. notably, we used deionized water to prepare all solutions freshly. fabrication of ac/co3o4 nanostructure the ac-co3o4 nanocomposite has been synthesized according to the previous report [41]. briefly, five-gram cocl26h2o was poured into ethanol solution (40 ml) plus activated carbon (5 g), followed by adding 12.5 ml of hydrazine hydrates (50 % n2h4h2o) and 12.5 g of sodium hydroxide (naoh) as dropwise with vigorously stirring at an ambient temperature. then, the produced ac-co3o4 was collected, rinsed using the deionized water and the ethanol to exclude additional hydrazine. drying the ac-co3o4 has been performed in the oven for 4 hours at 90 °c. preparation of electrode 100 mg of ac-co3o4 nanocomposite and 900 mg graphite was mixed using a mortar for 10 min, and then 0.7 ml of paraffin was added to it and remixed for 10 min. then, the resultant paste inserted into a glass tube (length = 10 cm; inner diameter = 3.4 mm) and packaged and then put a copper wire in the electrode for establishing an electrical contact. after that, we pressed additional paste out of the glass tube and used a weighing paper to polish it for obtaining the surface. the resulting electrode is displayed with ac-co3o4-cpe. furthermore, the bare cpe without any ac-co3o4 nanocomposite was constructed for comparison. s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1165-1177 http://dx.doi.org/10.5599/jese.1531 1167 preparation process of the real samples food samples including grape soda and lemon soda were provided by a local supermarket. the liquid samples were transferred into beaker and degassed in an ultrasonic bath. then 1.0 ml of the samples was pipetted into the voltammetric cell, and diluted to 10 ml with 0.1 m pbs (ph 7). results and discussions characterization of ac/co3o4 the morphology of ac/co3o4 nanocomposite was examined by the scanning electron microscope (sem) (tescan, czech republic) images and the results was given in fig 1. as can be seen, the ac/co3o4 has an even nano-flower-shape morphology that is assembled through nano-sheets, thereby causing an elevation in the activated carbon surface area that shows a highly efficient surface area for electron transfer. figure 1. sem image of ac/co3o4 nanocomposite figure s-1 (given in the supplementary material) shows the fourier transform – infra red (ft-ir) (bruker, germany) spectra obtained for the functional groups of ac-co3o4 nanocomposite. as seen, the bands at 3416 and 3445 cm−1 correspond to o–h stretching related to the water molecule adsorption. the bands from 2850 to 2950 cm−1 stand for the c-h stretching, the bands at 1623 and 1703 cm−1 for carbonyl stretching vibration and carboxylic group [42]. the peak 1062 cm−1 stands for c–o stretching vibration. the bands at 1000 to 1100 cm−1 relate to c–o–c and c–o vibrations. the peak 2923 cm−1 corresponds to ch2 anti-symmetric and symmetric stretch vibrations. the peaks 435 and 564.47 cm−1 show respectively metal oxygen (m–o) vibration bond in octahedral and tetrahedral locations, confirming the existence of cobalt oxide on the ac-co3o4 surface [42]. xrd has been assessed using a diffractometer bruker d8 adance, having cu ka radiation. high angle x-ray diffractogram for the ac-co3o4 nanocomposite are shown in figure s-2. the diffraction peaks of 2 = 41.8, 44.4, 47.2, 51.9 and 76.0° (pdf# 15-0806) exhibit the typical metallic cobalt nanoparticles (nps) [42]. the amorphous structure of carbon is evident in a broad peak at 2 = 24° [42]. the calculation of particle size of cobalt nps in the structure of ac-co3o4 nanocomposite was performed in accordance with the diffraction peak (2 = 44.4°) using the scherrer equation and was 27.9 nm. http://dx.doi.org/10.5599/jese.1531 j. electrochem. sci. eng. 12(6) (2022) 1165-1177 electrochemical determination of amaranth 1168 the x-ray photoelectron spectroscopy (xps) (tescan, czech republic) was applied to chemically determine the composition of ac-co3o4 nanocomposite, whose spectra are shown in figure s-3. the findings confirm the presence of o, co, and c in the structure of ac-co3o4 nanocomposite. the weight ratios of c (60.50 %), co (28.72 %) and o (10.78 %) have been obtained having analyzed data quantitatively. electrochemical behaviour of amaranth at the ac-co3o4-cpe surface the cvs responses for the bare cpe and the ac-co3o4-cpe in 0.1 m pbs (ph 7.0) containing 100.0 μm amaranth were presented in figures 2a and 2b, respectively. as can be seen, potential of peak (ep) for amarant is 600 mv and 825 mv at the surface of ac-co3o4-cpe and the bare cpe, which reflects the capability of ac-co3o4 nanocomposite as good mediator. compared to the unmodified electrode, the ep is shifted by about 225 mv to negative values. on the other hand, compared to the unmodified electrode, the peak current (ip) in 100.0 μm amaranth solution (curve b) considerably elevated, which was caused by the probable electrocatalytic impacts of ac-co3o4 nanocomposite on the amaranth. figure 2. lsvs of the bare cpe (a) and the ac-co3o4-cpe (b) in the presence of 100.0 μm amaranth in 0.1 m pbs (ph of 7.0). scan rate = 50 mv s-1 in all cases in the following, optimizing the solution ph would be crucial to achieve electrocatalytic oxidation of amaranth. therefore, the dependence of electrochemical activity of amaranth on the ph value of the aqueous solution was also assessed. for this purpose, the cv used to investigate electrochemical activity of amaranth into 0.1 m pbs with different ph-values (3.0 < ph < 9.0) at the ac-co3o4-cpe surface (figure 3). the results showed that the electrochemical oxidation of amaranth at the acco3o4-cpe surface in neutral conditions is better than acidic or alkaline conditions. therefore, ph 7.0 was chosen as optimum ph for electrochemical oxidation of amaranth at the ac-co3o4-cpe surface. additionally, the obtained results showed that with increase of ph, the peak potential (ep) was shifted linearly to negative values (slope = 56.1 mv per unit of ph) that is close to the theoretical value (59.2 mv) and indicates that the number of protons and electrons involved in the oxidation process are equal [1,43,44]. with respect to these results, the possible oxidation mechanism of amaranth at the surface of ac-co3o4-cpe was proposed in scheme 1 that is in agreement with the proposed mechanism in the literature [1,43,44] and as can be seen, 2 electrons are involved in the oxidation of amaranth. s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1165-1177 http://dx.doi.org/10.5599/jese.1531 1169 figure 3. variation of peak current of ac-co3o4cpe in 0.1 m pbs containing 100.0 µm aramanth with different ph values (3 to 9) + 2h+ + 2e nao3s n n ho so3na so3na nao3s n n o o so3na scheme 1. electro-oxidation of amaranth at the surface of ac-co3o4-cpe in order to illustrate that the ac-co3o4 nanocomposite could increase the electrode surface area in comparison to the surface of bare cpe, real surface area of ac-co3o4-cpe and bare cpe were determined by the randles-sevcik (eq. 1) [45]: i = 2.69×105acd1/2n3/2ν1/2 (1) were a is the effective surface area of each electrode (cm2), n is the number of electrons taking part in charge transfer process, d is the diffusion coefficient of the analyte in the solution and c is the concentration of the analyte in the solution. the values of n and d for k3fe(cn)6 are 1 and 7.6 ×10−6 cm2 s−1 respectively. the results showed that the real surface area of ac-co3o4-cpe and bare cpe were 21.42 and 0.10 cm2, respectively. therefore, as expected, the surface area of ac-co3o4-cpe increases with the addition of ac-co3o4 nanocomposite to the matrix of the carbon paste. in the following, the effect of scanning speed on the peak current (ip) of amaranth was studied using linear sweep voltammetry (lsv) (figure 4). the observations showed that the higher the scan speed, the higher ip and the ep shifted towards higher positive potentials. as can be seen (inset of figure 4), the ip vs. the square root of the scanning rate ( 1/2) varied linearly, therefore, the oxidation process of amaranth at the surface of ac-co3o4-cpe is controlled by diffusion [45]. http://dx.doi.org/10.5599/jese.1531 j. electrochem. sci. eng. 12(6) (2022) 1165-1177 electrochemical determination of amaranth 1170 in the next step, the lsv obtained at the scan rate of 10 mv s-1 was used for draw the toefl diagram (figure 5). the ascending part of the voltammogram is called the tafel region that under the influence by the electron transfer kinetic between amaranth and the ac-co3o4-cpe. figure 4. lsvs of ac-co3o4-cpe in 0.1 m pbs (ph 7.0) containing 100.0 µm amaranth at various scan rates; numbers 1 6 correspond to 10, 25, 50, 100, 300 and 500 mv s-1, respectively; inset: changes in the anodic peak current versus square root of scan rate figure 5. lsv of ac-co3o4-cpe in 0.1 m pbs (ph 7.0) containing 100.0 µm amaranth at the scan rate of 10 mv s-1. the inset exhibits the lsv-derived tafel plot due to the slope of the tafel curve, which corresponds well to the involvement of 1 electron in the electrode rate-determining step [45], a charge transfer coefficient (α) of 0.6 was obtained for amaranth. chronoamperometric study in the following, to perform the chronoamperometric measurements of amaranth at the surface of the ac-co3o4-cpe, the working electrode potential was adjusted at 650 mv (figure 6). the current obtained from electrochemical reaction at the mass transport limited condition for electroactive material (in this case: amaranth), that having a diffusion coefficient d can be ascribed by cottrell equation (2) [45]: i = nfad1/2cbπ-1/2t-1/2 (2) where d imply the diffusion coefficient (cm2 s-1) and cb the bulk concentration (mm). chronoamperometric measurements of aramanth were performed in diverse concentrations of amaranth (0.1 m of pbs at ph 7.0). then, the plots of i versus t-1/2 were plotted (figure 6a). in the next step, the s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1165-1177 http://dx.doi.org/10.5599/jese.1531 1171 slopes of the obtained straight lines were plotted versus (figure 6b) concentration of amaranth. based on the final slope and the cottrell equation, the mean value of d for amaranth was calculated as 6.5×10-5 cm2 s-1. figure 6. chronoamperograms at the ac-co3o4-cpe in 0.1 m pbs (ph of 7.0) containing different concentrations of amaranth; 1 to 4 correspond to 0.1, 0.3, 0.6 and 1.0 mm of amaranth. inset: the plot of i versus t-1/2 obtained from the chronoamperograms 1–4 (a), the slope of straight lines against concentration of amaranth (b) analytical functions the obtained results show that the electrooxidation peak current of amaranth at the ac-co3o4-cpe surface could be used for measurement of amaranth in solution. since differential pulse voltammetry (dpv) has a higher sensitivity compared to other quantitative methods, just this method was used to investigate the linear range of the method. the dpv parameters were tested and the best currents were obtained by using (initial potential = 470 mv, end potential = 690 mv, potential = 0.01 v and pulse amplitude = 0.025 v). to perform this study, the ac-co3o4-cpe was placed in a series of amaranth solutions with different concentrations and peak current was measured. according to the data (figure 7), the oxidation current of amaranth at the ac-co3o4-cpe surface has a linear dependence on the amaranth concentration (at range 0.1 to 215.0 µm), with a correlation coefficient of 0.9994. finally, based on 3sb/m, a detection limit of 10 nm for amaranth was calculated. table 1 compares analytical function of the ac-co3o4-cpe with the other modified electrodes [1,3,18-20,34,43,44,46]. as seen in the table 1, the detection limit and the linear range obtained using the ac-co3o4-cpe is in the range of many reports or even better. the high sensitivity may be ascribed to the porous structure of the ac-co3o4 nanocomposite, which facilitates the transport of electroactive molecules. the low detection limit may be the result of the synergistic effect of the activated carbon and the co3o4 nps. http://dx.doi.org/10.5599/jese.1531 j. electrochem. sci. eng. 12(6) (2022) 1165-1177 electrochemical determination of amaranth 1172 figure 7. dpvs of the ac-co3o4-cpe in 0.1 m pbs (ph 7.0) containing different concentrations of amaranth (0.1, 2.5, 10.0, 30.0, 70.0, 100.0, 150.0 and 215.0 μm). inset: the peak current plots versus concentration of amaranth (0.1-215.0 μm) table 1. figure of merit of the ac-co3o4-cpe compared to other electrodes used in the measurement of amaranth method lod, nm linear range, µm ref. gce modified with electropolymerization of molecularly imprinted polypyrrole film on multiwalled carbon nanotube surface 0.4 0.007-17 [1] carbon paste electrode modified with alumina microfibers and accumulation 0.75 0.001-0.15 [3] expanded graphite paste electrode 36 0.08-4 [18] carbon paste electrode modified with nanostructured resorcinolformaldehyde carbonized polymers and accumulation 0.27 0.0005-0.1 [19] glassy carbon electrode (gce) modified with pd-doped polyelectrolyte functionalized graphene 7 0.1-9 [20] gce modified with molecularly imprinted electrochemical sensor based on pd-cu bimetallic alloy functionalized graphene 2 0.006-10 [34] gce modified with porous graphene material-graphene nanomeshes 0.7 0.005-1 [43] gce modified with manganese dioxide nanorods/electrochemically reduced graphene oxide nanocomposites 1 0.02-400 [44] gce modified with functionalized graphene oxide/chitosan/ionic liquid nanocomposite supported nanoporous gold 37.3 0.008-1.2 [46] ac-co3o4-cpe 10.0 0.1-215.0 this work repeatability, reproducibility and stability to evaluate the reproducibility of the ac-co3o4-cpe sensor, dpv was used. the calculated rsd for five measurements of 100 μm of amaranth at five different electrodes prepared in the same way was 3.4 %, which demonstrates reasonable reproducibility of the ac-co3o4-cpe sensor. also, the relative standard deviation (rsd) of six repeated determinations of 100.0 μm of amaranth with one electrode was 3.7 % which demonstrates reasonable repeatability of the ac-co3o4-cpe sensor. to assess stability of the ac-co3o4-cpe sensor, a modified electrode was stored for three weeks, and then used for amaranth determination. the obtained data showed that current response of the s. z. mohammadi et al. j. electrochem. sci. eng. 12(6) (2022) 1165-1177 http://dx.doi.org/10.5599/jese.1531 1173 ac-co3o4-cpe sensor to the amaranth after 3 weeks was 94.5 % of its initial value, which illustrates a very good sensor stability. interference effect in the analytical chemistry, the selectivity of sensor is a very important factor. for this assesses, the effect of some annoying species on the measurement of 50.0 μm amaranth was assessed by dpv under optimum conditions. the possible interfering species were selected in such a way that they could be present in the real samples. the obtained data showed that 50-fold lemon yellow, sunset yellow, rhodamine b, glucose, ascorbic acid and citric acid, show no disturbance in the measurement of amaranth and the change in ip due to interfere was less than 5 %. therefore, can be said that the electrode has a good selectivity. real-sample analysis artificial colors are often used as brightening agent to add in soft drinks for better appearance and taste, and therefore we chose soft drinks as real samples to determine amaranth. for this purpose, the applicability of the ac-co3o4-cpe in determining the true samples, the pre-prepared samples were transferred to electrochemical cell and the amount of amaranth was measured using dpv. the standard addition method was used to increase the accuracy of the measurement. the results were given in table 2 and showed that the recovery percentages were in the range of 98.2 to 103.4 %. also, the rsd percentages of the method were 3.6 % or less, which indicates the high reliability of the ac-co3o4-cpe in real samples. table 2. application of the ac-co3o4-cpe to detect amaranth in real samples (n = 5) sample camaranth / μm recovery, % rsd, % spiked* found** grape soda 0.21 0.56±0.03 --3.3 5.0 5.63±0.18 101.4 3.6 10.0 10.41±0.34 98.5 3.3 20.0 20.92±0.64 101.8 3.2 lemon soda --nd* ---- 5.0 5.17±0.21 103.4 3.5 10.0 10.32±0.38 103.2 3.2 20.0 19.64±0.66 98.2 3.3 *the added standard to the sample, **the amount of amaranth that was determined, ***not detected conclusions in this research, a new modified cpe was constructed for the measurement of amaranth. due to modification of the electrode surface using ac-co3o4-cpe nanocomposite, the electrode response to amaranth increased and thus the sensitivity of the measurement increased too. the ac-co3o4cpe was successfully applied for the measurement of amaranth in soft drinks samples. simplicity, portability and cost-effectiveness are some of the advantages of the developed ac-co3o4-cpe. conflict of interest authors declare that there is no conflict of interest related to publishing of the present work. references [1] y. wu, g. li, y. tian, j. feng, j. xiao, j. liu, x. liu, q. he, electropolymerization of molecularly imprinted polypyrrole film on multiwalled carbon nanotube surface for highly selective and stable determination of carcinogenic amaranth, journal of electroanalytical chemistry 895 (2021) 115494. https://doi.org/10.1016/j.jelechem.2021.115494 http://dx.doi.org/10.5599/jese.1531 https://doi.org/10.1016/j.jelechem.2021.115494 j. electrochem. sci. eng. 12(6) (2022) 1165-1177 electrochemical determination of 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y. yang, x. feng, c. gao, l. meng, x. shen, y. zhang, x. tang, an electrochemical method for determination of amaranth in drinks using functionalized graphene oxide/chitosan/ionic liquid nanocomposite supported nanoporous gold, food chemistry 367 (2022) 130727. https://doi.org/10.1016/j.foodchem.2021.130727 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1531 https://doi.org/10.1007/s13762-020-02767-0 https://doi.org/10.1016/j.jallcom.2020.154942 https://doi.org/10.1016/j.jelechem.2017.12.059 https://doi.org/10.1016/j.colsurfb.2018.09.005 https://doi.org/10.1016/j.foodchem.2021.130727 https://creativecommons.org/licenses/by/4.0/) @article{mohammadi2022b, author = {mohammadi, sayed zia and baghelani, yar-mohammad and mousazadeh, farideh and rahimi, shamsi and mohammad-hassani, maryam}, journal = {journal of electrochemical science and engineering}, title = {{electrochemical determination of amaranth in food samples by using modified electrode:}}, year = {2022}, issn = {1847-9286}, month = {nov}, number = {6}, pages = {1165--1177}, volume = {12}, abstract = {in this paper, a new electrochemical sensor was reported for the determination of amaranth in drink soft. in this sensor, activated carbon-co3o4 nanocomposite (ac-co3o4) was employ­ed as electrode modifying material. high pores of the activated carbon favour an access of aramanth nolecules within the pores of the working electrode surface, and allow fast electron transfer that is beneficial for the electrochemical detecion process. thus, the electrochemical signal is obviously enhanced at ac-co3o4 modified electrode compared to bare carbon paste electrode, and exhibited a wide linear response ranging from 0.1-215 mm with a low detection limit of 10.0 nm (based on 3sb/m). this work offers a new route in developing new electro­chemical sensors for the determination of collorant additives and other hazard components in drink soft.}, doi = {10.5599/jese.1531}, file = {:d\:/onedrive/mendeley desktop/mohammadi et al. 2022 electrochemical determination of amaranth in food samples by using modified electrode.pdf:pdf;:www/jese_v12_no6_1165-1177.pdf:pdf;:www/jese_v12_no6_1165-1177.pdf:pdf}, keywords = {co3o4 nanocomposite, electrochemical sensor, activated carbon, carbon paste electrode}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1531}, } non-toxic leguminous plant leaf extract as an effective corrosion inhibitor of uns s30403 in 1 m hcl http://dx.doi.org/10.5599/jese.1343 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1343 open access : : issn 1847-9286 www.jese-online.orghttp://www.jese-online.org/ original scientific paper non-toxic leguminous plant leaf extract as an effective corrosion inhibitor of uns s30403 in 1 m hcl okiemute dickson ofuyekpone1,, adeolu adesoji adediran2,3,, ochuko goodluck utu4, basil o. onyekpe5 and ufuoma g. unueroh5 1department of metallurgical engineering, delta state polytechnic, ogwashi-uku, delta state, nigeria 2department of mechanical engineering, landmark university, omu-aran, kwara state, nigeria 3department of mechanical engineering science, university of johannesburg, south africa 4department of welding and fabrication, delta state polytechnic, ogwashi-uku, delta state, nigeria 5department of mechanical engineering, university of benin, benin city, edo state, nigeria corresponding authorс: okedickbest@yahoo.com; dladesoji@gmail.com, tel.: +234-803-411-9210 received: april 11, 2022; accepted: september 23, 2022; published: october 19, 2022 abstract weight loss, polarization, and open circuit potential methods were used to investigate the corrosion inhibitory impact of centrosema pubescens leaf extract on 304l austenitic stainless steel uns s30403 in 1 m hydrochloric acid. this non-toxic extract behaves as a mixed-type inhibitor according to the polarization curves, thermodynamics and activation parameters. both the weight loss calculations and potentiodynamic polarization investigations showed that 1.2 g l-1 was the optimal concentration of the leaf extract. while the weight loss method gave inhibition efficiency of 86.84 and 75.00 % after 10 and 60 days of immersion at the optimum concentration, polarization studies revealed inhibition efficiencies of 93.08 and 98.66 % at 303 and 333 k, respectively. the extract molecules adhered to the uns s30403 surface according to langmuir adsorption isotherm. the presence of the protective film on the uns s30403 surface was confirmed by sem, edx, and xrd measurements. the inhibition performance of the leaf extract was noted to be a function of the extract concentration, immersion time and temperature. the ftir analysis indicated an interaction between austenitic stainless steel uns s30403 and the molecules of centrosema pubescens leaf extract. keywords stainless steel 304l; green inhibitor; inhibition efficiency; protective film introduction uns s30403 austenitic stainless steel (304l) has a wide range of applications in a variety of industries. usually, this type of stainless steel is protected by a highly protective film of chromium http://dx.doi.org/10.5599/jese.1343 http://dx.doi.org/10.5599/jese.1343 http://www.jese-online.org/ http://www.jese-online.org/ mailto:okedickbest@yahoo.com mailto:dladesoji@gmail.com j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 2 oxyhydroxides and possesses excellent corrosion resistance in a wide range of aggressive environments. however, the uns s30403 is easily attacked in acid solutions commonly used in industrial processes such as acid pickling, cooling systems, chemical and electrochemical etching, and oil well acidization. it is worth noting that corrosion never stops, but its extent and severity can be reduced to a safe level. corrosion control is a fundamental issue from an application standpoint, and it has been established that inhibitors, which act as barriers to lessen corrosion pitting assaults, are potent and should be employed [1-3]. corrosion inhibitors have recently gained popularity as anticorrosion methods for stainless steel protection due to their ease of use, low cost, and high efficiency. corrosion inhibitors are chemical substances added in small amounts to a corrosive environment. most inhibitors contain noxious and lethal compounds, resulting in a ban on their use due to negative effects on the environment and humans. the safety and environmental issues raised by corrosion inhibitors of inorganic origin in industries have always been a worldwide concern [4,5]. in the past few decades, scientists have focused on the development of organic and environmentally friendly inhibitors. nitrogen, sulfur, oxygen, and p-orbitals are present in the majority of organic compounds. these polar group structures can act as active centers, which are important in the adsorption of inhibitors onto the metal surface [6]. there have been numerous reports on the use of plant extracts as green metal inhibitors in acid solutions [7-12]. watermelon rind, seed, and peel [7], curcuma longa extract [8], aloe vera leaf extract [9], cucumis sativus peel extract [10], salvia hispanica [11], glycyrrhiza glabra leaf extract [12], stylosanthes gracilis leaf extract [13-15] and centrosema pubescens leaf extract [16] were investigated for their inhibition properties. nonetheless, more research on other plants that can be used as corrosion inhibitors in industrial applications is required. also, it is necessary to evaluate the inhibitive capacity of the aforementioned plant extracts in several corrosive environments. centrosema pubescens is a member of the fabaceae family, subfamily faboideae, and tribe phaseolae. it is also known as centro or butterfly pea and is a leguminous plant that can root at the nodes of trailing stems [16]. the goal of this study is to use various surface and electrochemical techniques to investigate the corrosion behavior of uns s30403 in 1 m hcl in the presence of centrosema pubescens leaf extract. the surface morphology of uns s30403 specimens was also investigated. experimental preparation of uns s30403 specimens specimens of austenitic stainless steel uns s30403 with a composition of: 18.37 wt.% cr, 8.01 wt.% ni, 1.00 mn, 0.39 wt.% si, 0.03 wt.% c, 0.003 wt.% s, 0.04 wt.% p and the remainder is fe, of dimensions 110.1 cm were used for various analyses. before each analysis, two surface ends of each specimen were abraded with emery papers 220, 320, 550, 800, 1000 and 1200 grits. for the electrochemical study, 1 cm2 surface area of the uns s30403 was exposed to the 1.0 m hcl solution, while the remaining piece of the electrode was secured by epoxy resin. leaf extract and electrolyte preparation only matured leaves of centrosema pubescens were collected, cleaned with water, and shadedried for seventeen days. after that, dried leaves were pulverized and soaked in acetone for 72 h. thereafter, the solution was filtered, and the filtrate was evaporated to remove the solvent from o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 3 the sample via a water bath. the prepared extract was then stored at 273 k in an airtight bottle until further use. the leaf extract was used to prepare different concentrations of the extract solution by dissolving different amounts of the extract into the aggressive solution of interest. the active ingredients in centrosema pubescens leaf extract were identified using gas chromatography/mass spectroscopy (gc/ms). the obtained results are presented in table 1. the electrolyte solution of 1.0 m hcl was made by diluting 36 percent analytical grade hydrochloric acid with double distilled water. for weight loss and electrochemical studies, the inhibitor concentrations were kept between 0.2 and 1.4 g l-1. table 1. active phytochemical ingredients found in the leaf extract of centrosema pubescens by gc/ms characterization (using acetone as solvent) name of phytochemical compound concentration, % molecular formula molecular weight, g mol-1 2-hexanol, 2-methyl6.74 c7h16o 116.2013 2-butanamine, (s)1.41 c4h11n 73.1368 oxirane, 2,2'-(1,4-butanediyl)bis20.66 c8h14o2 142.195 11-(2-cyclopenten-1-yl) undecanoic acid, (+)29.70 c16h28o2 252.398 1,2:4,5:9,10-triepoxydecane 2.91 c10h16o3 184.235 1-hexene, 6-bromo0.95 c6h11br 163.056 ethane,1,2-dicyclopropyl1.65 c8h14 110.1968 1,6-hexanediol 1.67 c6h14o2 118.17 acetamide, n-2-propynyl0.90 c5h7no 97.12 8-nonynoic acid 6.94 c9h14o2 154.209 7-oxabicyclo[4.1.0]heptane, 3-oxiranyl10.48 c9h16o2 156.2221 1-cyclohexyl-1-propyne 1.14 c9h14 122.2075 1,10-dichlorodecane 0.98 c10h20cl2 211.172 3-octen-1-ol, (z)0.81 c8h16o 128.2120 methyl-4,6-ethylidene-alpha-d-galactopyranoside 2.93 c9h16o6 220.221 melezitose 4.91 c18h32o16 504.4 1,3-bis-(2-cyclopropyl,2-methylcyclopropyl)-but-2-en-1-one 0.80 c18h26o 258.4 4a,5,6,7,8,8a,10,10a-octahydro-2h-1-oxa-9a-azaanthracen-9-one 1.61 c12h17no2 207.269 cyclohept-4-enecarboxylic acid 1.02 c8h12o2 140.18 sucrose 1.14 c12h22o11 342.3 1h-3a,7-methanoazulene, octahydro-1,4,9,9-tetramethyl1.52 c15h26 206.3669 weight loss measurement in this study, uns s30403 test specimens were firstly weighted and then fully immersed in 100 ml of 0.1 m hcl containing different concentrations of the leaf extract for 60 days (1440 hours). at each 10 days-interval, a test specimen was taken out from the tested solution, washed with distilled water, rinsed with acetone, dried, and re-weighed to determine the weight loss. the following equations were used to calculate the corrosion rate (cr) and inhibition efficiency (ie) [13 -15]: 87.6w cr dat = (1) a p a 100 cr cr ie cr − = (2) where w is weight loss in mg, d is density in g cm-3, a =area in cm2, t = exposure time in hours, and cra and crp are corrosion rates in the absence and presence of the inhibitor, respectively. http://dx.doi.org/10.5599/jese.1343 https://pubchem.ncbi.nlm.nih.gov/#query=c8h12o2 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 4 electrochemical measurements potentiodynamic polarization measurements were used to investigate the electrochemical behaviour of the uns s30403 austenitic stainless-steel samples in uninhibited and inhibited solutions. polarization experiments were performed with an autolab potentiostat (versastat 4) electrochemical system connected to a computer via a usb interface and the versa studio electrochemistry software package. polarization measurements were performed using a standard three-electrode pyrex glass cell at temperatures of 30 and 60 °c, respectively. a platinum rod served as the counter electrode in each case, while ag/agcl (3.5m kcl) served as the reference electrode, with a luggin probe placed close to the working electrode. all experiments were carried out in aerated stagnant solutions. a working electrode was completely immersed in the aggressive solution for one hour until a stable open circuit potential (ocp) value is obtained. the potentiodynamic polarization study was carried out using a linear sweep technique at the scan rate of 1 mv s-1, within the potential range of -0.25 to 0.25 v with respect to ocp. tafel plots of potential versus the log of current density were used to calculate the corrosion current density (jcorr) and corrosion potential (ecorr). the corrosion rate (cr), the degree of surface coverage () and the inhibition efficiency (ie) were calculated by eqs. (3-6) [6]: cr = 0.00328jcorreq / d (3) where jcorr = current density in a cm-2, d = density in g cm-3, eq = equivalent weight of stainless steel. the inhibition efficiency was evaluated from jcorr values using the equation (4): corr1 corr2 1 100 j ie j = − (4) where jcorr1 and jcorr2 are corrosion current densities in the absence and presence of inhibitor, respectively. in addition, the polarization resistance (rp) values were obtained from the measured jcorr values by applying the relationship (5)[6] p corr b r j = (5) where b is defined by eq. (6): c a c a2.303( ) b b b b b = + (6) where bc and ba represent cathodic and anodic tafel slopes. scanning electron microscopy (sem) vega3 tescan model scanning electron microscope was used to examine the morphology of test samples before and after immersion in 1.0 m hcl with and without the inhibitor. x-ray diffraction analysis the raw data was obtained using a rigaku miniflex 600 x-ray diffractometer with monochromatic cu kα radiation produced at k = 0.154443 nm, and the patterns of the phases/films on the metal surface were identified using panalytical xpert highscore in the absence and presence of the optimum concentration of centrosema pubescens leaf extract. the diffraction patterns were captured at room temperature of 25 °c in the angular range of 2 = 2 to 90°, with a step size of 2 = 0.020° and a scan step time of 1.0 s. in both cases, the crystalline phases formed on the uns s30403 stainless steel surface were identified using the icdd-pdf database. o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 5 ftir spectral analysis the ftir spectra were recorded using an agilent technologies cary 630 ftir to evaluate the composition of the corrosion product generated on the uns s30403 surface. results and discussion weight loss measurements the corrosion penetration rate and inhibition efficiency of uns s30403 stainless steel in 1.0 m hcl solution with and without the addition of centrosema pubescens leaf extract, measured by weight loss in 10 day-intervals during 60 days of immersion, were calculated using eqns. (1) and (2), and listed in table 2. table 2. corrosion rates (cr) and inhibition efficiencies (ie) obtained by weight loss measurements in 10 dayintervals after exposing uns s30403 specimens to 1.0 m hcl solution during 60 days without and with different concentrations of centrosema pubescens leaf extract results from table 2 are better visualized by the 3-d graph presented in figure 1, which depicts the variation of the corrosion rate of stainless steel in 1 m hcl solution caused by the change of leaf extract concentration, as well as immersion time. obviously, the corrosion rate was reduced by the addition of the centrosema pubescens leaf extract up to the concentration of 1.2 g l-1. the decrease in corrosion rate is a consequence of the increase in extract concentration, which implies that more extract molecules were adsorbed on the metal surface, providing higher surface coverage and suppressing both the anodic and cathodic reaction [17]. similarly, figure 2 depicts the relationship between inhibition efficiency, the concentration of centrosema pubescens leaf extract, and immersion time, revealing a strong influence of the leaf extract on the system. it can be seen that the inhibition efficiency was higher for a higher concentration of the leaf extract. this indicates that the compact barrier film was formed on the metal surface, effectively separating the surface from aggressive anionic species and stifling the redox reactions associated with the corrosion process. as shown in figure 2, at all immersion times, the inhibition efficiency of the leaf extract increases with its concentration. the highest ie value of 86.84 % was reached after 10 days of immersion at the optimum leaf extract concentration of 1.2 g l-1. the observed increase in inhibition efficiency with increasing extract concentration could be attributed to an increase in surface coverage caused by the adsorption of extract molecules onto the steel surface. this suggests that the corrosion inhibition performance of centrosema pubescens leaf extract is largely due to its ability to provide a strong adherent and stable film on the metal surface. this film inhibits penetration of chloride ion reacting species which initiate and sustain corrosion reactions, as well as diffusion of fe2+ ions from the electrode surface [18]. cleaf extract / g l-1 10th day 20th day 30th day 40th day 50th day 60th day cr / mm y-1 ie / % cr / mm y-1 ie / % cr / mm y-1 ie / % cr / mm y-1 ie / % cr / mm y-1 ie / % cr / mm y-1 ie / % blank 1.76 -1.23 -1.10 -1.10 -0.99 -0.93 - 0.2 1.07 39.47 0.84 32.08 0.82 25.35 0.87 20.21 0.83 16.82 0.80 14.17 0.4 0.93 47.37 0.63 49.06 0.63 42.25 0.67 38.30 0.67 32.71 0.63 31.67 0.6 0.70 60.53 0.46 62.26 0.50 54.93 0.53 51.06 0.52 47.66 0.63 44.17 0.8 0.56 68.42 0.39 67.92 0.37 66.20 0.37 65.96 0.34 65.42 0.47 55.00 1.0 0.42 76.32 0.30 75.47 0.31 71.83 0.30 72.34 0.28 71.96 0.24 70.00 1.2 0.23 86.84 0.21 83.02 0.25 77.46 0.24 77.66 0.24 75.70 0.80 75.00 1.4 1.21 31.58 1.00 18.87 0.93 15.49 0.93 14.89 0.87 12.15 0.93 9.17 http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 6 figure 1. 3-d surface plot showing the interaction between corrosion rate, inhibitor concentration and immersion time for centrosema pubescens leaf extract in 1.0 m hcl solution figure 2. 3-d surface plot showing the interaction between inhibition efficiency, inhibitor concentration and immersion time for centrosema pubescens leaf extract in 1.0 m hcl solution as also shown in figures 1 and 2 and data in table 2, the increase of the leaf extract concentration beyond 1.2 g l-1resulted in a sudden increase in corrosion rate and a significant decrease in inhibition efficiency. this implies that the leaf extract becomes harmful when present at higher than optimal concentration, possibly due to the desorption of inhibitor at the metal–electrolyte interface [18]. on the other hand, the corrosion rate and the inhibition efficiency were found to have an inverse relationship with immersion time. the observed decrease in corrosion rate with immersion design-expert® software factor coding: actual corrosion rate (mm/yr) design points above predicted value design points below predicted value 0.208969 1.76463 x1 = a: time x2 = b: inhibitor concentration 0 0.2 0.4 0.6 0.8 1 1.2 1.4 240 540 840 1140 1440 0 0.5 1 1.5 2 c o rr o s io n r a te , m m y e a r -1 time, hinhibitor concentration, g/l o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 7 time connotes that the reactivity of uns s30403 in the specified acidic solution decreased over time, whereas a decrease in inhibition efficiency can be attributed to desorption [16]. electrochemical measurements open circuit potential (ocp) versus leaf extract concentration plots of ocp values of the uns s30403 electrode against the concentration of the leaf extract in 1.0 m hcl, measured after 600 s of immersion at two temperatures (303 and 333 k), are shown in figure 3. the graph shows that adding centrosema pubescens leaf extract shifts the ocp value in a positive direction. generally, a shift of ocp value to more negative values indicates that metal is corroding, i.e., any protective coating established on the metal surface is going to dissolve. if the ocp value is shifted to the noble direction (as is happening here), it would mean that the metal surface is protected by the formation of a corrosion products film. it has also been demonstrated that adsorption of the leaf extract is more favored at higher temperatures, as the positive shift in ocp is greater at higher temperatures [19,20]. figure 3 ocp values of uns s30403 specimens in 1 m hcl solution versus concentration of centrosema pubescens leaf extract after 600 s of immersion at 303 and 333 k tafel polarization measurements plots of figure 4 depict the relationships between current density and potential for uns s30403 in 1.0 m hcl with various concentrations of centrosema pubescens leaf extract, measured at two temperatures. at both temperatures, figures 4 (a) and (b) illustrate a positive shift of corrosion potential (ecorr) toward a more passive region after the addition of the leaf extract into the systems. this shift in ecorr in a positive direction suggests that increasing the concentration of the extract in the test media increases the corrosion resistance of the uns s30403 [21]. typically, the changes in polarization curves observed after the addition of the inhibitor are used to classify inhibitors as either cathodic, anodic, or mixed. according to figure 4, both anodic and cathodic polarizations are influenced concurrently and to almost the same extent, indicating that centrosema pubescens leaf extract influences both anodic and cathodic reactions, i.e., hydrogen evolution and metal dissolution. http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 8 figure 4. polarization curves for uns s30403 specimen corrosion in 1.0 m hcl solution without (a) and with different concentrations, 0.2-1.4 g l-1 (b – h) of centrosema pubescens leaf extract at 30 °c, and without (a) and with different concentrations, 0.2-1.4 g l-1 (b – h) of centrosema pubescens leaf extract at 60 °c essentially, when the change in ecorr is greater than 85 mv, an inhibitor is classified as either anodic or cathodic, while lower values indicate a mixed-type inhibitor [22]. figure 4 shows that the greatest change of ecorr in the presence of the applied leaf extract is 72 mv at 30 °c and 59 mv at 60 °c, respectively. this suggests that the centrosema pubescens leaf extract acts as a mixed-type inhibitor [23,24]. values of corrosion parameters derived from potentiodynamic polarization curves in figure 4, i.e., corrosion potential, corrosion current density, cathodic and anodic tafel at two temperatures (303 and 333 k), are listed in table 3. in addition, table 3 lists the parameter values calculated by eqs. (36), i.e., corrosion rate, inhibition efficiency, polarization resistance, and surface coverage fraction. the fractional surface coverage by extract molecules ( ) was computed by dividing the inhibition efficiency defined by eq. (4) by 100. table 3. parameters of potentiodynamic polarization curves obtained during corrosion of alloy 304l in 1.0 m hcl containing diverse concentrations of centrosema pubescens leaf extract temperature cinhibitor / g l-1 ecorr / mv bc / mv dec-1 ba / mv dec-1 jcorr / ma cm-2 rp / ω cm2 cr / mm year-1 ie / %  303 k 0 -588 49 63 0.0405 296 0.470 -- 0.2 -583 65 62 0.0108 1281 0.125 73.42 0.7342 0.4 -574 57 60 0.0089 1426 0.103 78.01 0.7801 0.6 -569 58 56 0.0074 1667 0.086 81.67 0.8167 0.8 -586 53 50 0.0058 1913 0.068 85.57 0.8557 1 -577 60 57 0.0043 2980 0.049 89.48 0.8948 1.2 -516 61 59 0.0028 4651 0.033 93.08 0.9308 1.4 -541 60 495 0.0179 1300 0.208 55.73 0.5573 333 k 0 -432 121 43 0.4394 31 5.100 -- 0.2 -420 120 55 0.091 180 1.056 79.29 0.7929 0.4 -401 90 56 0.0564 266 0.655 87.16 0.8716 0.6 -395 84 77 0.0299 583 0.347 93.20 0.9320 0.8 -392 82 80 0.0151 1164 0.175 96.56 0.9656 1 -396 44 91 0.0101 1271 0.118 97.69 0.9769 1.2 -373 45 97 0.0059 2274 0.068 98.66 0.9866 1.4 -378 47 100 0.1449 96 1.682 67.02 0.6702 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 1e-12 1e-10 1e-08 0.000001 0.0001 0.01 e / v v s. a g /a g c l log (j /a cm-2) a b c d e f g h a b c d e f g h o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 9 according to data in table 3, at both temperatures (303 and 333 k), inhibition efficiency, degree of surface coverage, and polarization resistance values increased with an increase of leaf extract concentration up to the concentration of 1.2 g l-1. at the same time, the corrosion current density and corrosion rate varied inversely with leaf extract concentration, showing the lowest values at 1.2 g l-1. at each leaf extract concentration, as well as for blank solution, corrosion currents and corrosion rates are significantly lower at a lower temperature (303 k) than at a higher temperature (333 k), while the opposite is true for ie,  and rp values. thus, for the extract concentration of 1.2 g l-1, corrosion current density reached its lowest value of 0.0028 ma cm-2 at 30 °c and 0.00587 ma cm-2 at 60 °c, respectively. at the same time, the polarization resistance value increased considerably, from 296 (blank) to 4651  cm2 at 1.2 g l-1 of leaf extract at 30 °c and from 31 (blank) to 2274  cm2 at 1.2 g l-1 of leaf extract at 60 °c. these changes of jcorr and rp values can be connected with extract molecules that react with liberated iron (fe2+) ions to generate a layer of corrosion products, which forms a barrier between the metal and the environment. this barrier makes metal contact with corrosive solution ions more difficult, lowering thus the corrosion current density and increasing rp values. however, data in table 3 show that corrosion current density and corrosion rate increased at the leaf extract concentration higher than 2 g l-1. this can be explained by the inhibitor molecules with the same electric charge, and at high adsorbed quantities, they repel each other electrostatically. this causes the extract components to desorb from the uns s30403 surface, leaving the metal at desorption sites fully exposed to the corrosive solution. also, data in table 3 show that increasing the extract concentration up to 1.2 g l-1 increases its inhibition efficiency value, while higher concentrations reduce it. the fractional surface coverage by extract molecules behaves similarly to the extract inhibition efficiency. with the addition of 1.2 g l-1, the greatest ie values of 93.08 and 98.66 % were obtained at 30 and 60 °c, respectively. this significant improvement in the extract inhibition efficiency at a higher temperature, also shown in figure 5, suggests that more of the extract was strongly adsorbed at a higher temperature. figure 5. inhibition efficiency vs. concentration of centrosema pubescens leaf extract for uns s30403 in 1.0 m hcl at 303 and 333 k, determined by potentiodynamic polarization http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 10 thus, the inverse relationship that exists between polarization resistance and registered corrosion current density suggests that protective films with improved properties (as the temperature was increased from 30 to 60°) were formed. these films act as a barrier at the metal-solution interface, reducing the anodic dissolution of the uns s30403 and modifying and inhibiting the evolution of the h+ ion discharge [25,26]. as a result, the current findings show that for a relatively low concentration of 1.2 g l-1 of centrosema pubescens leaf extract, high inhibition efficiency is achieved and the corrosion current density value is substantially and significantly reduced. scanning electron microscopy (sem) figures 6(a-c) show sem micrographs of the uns s30403 surface in the as-received state and after ten days (240 hours) of immersion in the absence and presence of the optimum concentration (1.2 g l-1) of the centrosema pubescens leaf extract in 1.0 m hcl. a b c figure 6. sem images of uns s30403: as-received (a), and immersed for 240 h in 1.0m hcl in the absence (b), and presence (c) of the optimum leaf extract concentration (1.2 g l-1) it can be seen in figure 6a that the polished uns s30403 surface is smooth and free of pits but with obvious lines resulting from cutting during preparation. however, for the surface corroded for ten days in 1.0 m hcl without leaf extract, severe damage, with pits and cracks, appeared (figure 6b). normally, figure 6b shows an extremely rough and porous surface that corroded in the absence of extract. a great number of pits, micro pits, and highly corroded topography of uns s30403 stainless o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 11 steel coupons are visible as a result of corrosive effects of chloride ions, caused by the breakdown of the passive coating of chromium oxide. the rate of re-passivation of the steel is faster than the rate of diffusion of aggressive anions through cracks of the passive film. the adsorbed oxygen species and chromium cations responsible for passivation are displaced by chloride ions which diffuse from bulk solution into the oxide/liquid interface of the steel surface, whereas fe2+ ions diffuse into the solution. this results in the quick creation of voids, which form and proliferate in specified locations such as defect and flaw regions, inclusions, and pitting-prone areas in general. when the uns s30403 specimen surface is exposed to the optimum concentration of the centrosema pubescens extract in the 1.0 m hcl acid solution (figure 6c), a stable protective layer is formed on the steel surface, which acts as a barrier to charge and mass transfer. as seen in figure 6c, this resulted in a sharp modification in the morphology of the uns s30403 surface compared to figure 6b and, consequently, improved properties on the surface of the stainless steel [27]. energy dispersive x-ray analysis (edx) the compositional information of the surface of the uns s30403 sample in 1.0 m hcl in the absence and presence of the centrosema pubescens leaf extract was obtained using energy dispersive x-ray analysis. figures 7(a) – (c) show edx spectra obtained for the specimens in the asreceived state and specimens left for 10 days in 0.1 m hcl in the absence and presence of the optimum concentration of the leaf extract. a energy, kev b energy, kev c energy, kev figure 7. eds spectra of uns s30403 surface: (a) as-received; after 10 days of immersion in (b) blank 1.0m hcl solution and (c) 1.0m hcl with 1.2 g l-1 centrosema pubescens leaf extract high peaks of fe, mn, cr, and ni in the eds spectrum of the as-received specimen shown in figure 7a confirm that the adsorbent is stainless steel of 300 family. a conspicuous and visible presence of peaks of chloride ions can be seen in the eds spectrum of figure 7b, obtained after immersing the http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 12 uns s30403 coupons in 1.0m hcl without adding the extract. the chloride ion peak was, however, noticeably reduced after the introduction of centrosema pubescens leaf extract at the optimum concentration of 1.2 g l-1 (figure 7c). this observation attests the inhibitory potential of the leaf extract in reducing the rate of corrosion in hcl solutions [27,28]. adsorption isotherm the adsorption of an extract at the metal/solution interface is thought to be related to its performance as a corrosion inhibitor in acidic solutions. to ensure the effective adsorption of an extract on a metal surface, the reciprocity force between metal and extract must be greater than the synergistic force between metal and water molecules [29]. as a result, the adsorption isotherm can be used to understand corrosion inhibition processes. despite the existence of several adsorption isotherms, the magnitudes of the coefficient of determination values (correlation coefficients) were used as indices for selecting the langmuir isotherm for the corrosion adsorption processes of centrosema pubescens leaf extract on the uns s30403 surface in 1.0 m hcl solution. the fit to the langmuir isotherm was determined by plotting c/ versus c according to eq. (7): ads 1c c k = + (7) where kads is adsorption/desorption equilibrium constant (l g-1),  is surface coverage, and c is the concentration of inhibitor (g l-1). the langmuir isotherm plots at 303 and 333 k are depicted in figure 8, while the corresponding values of slope and intercept of straight lines are, together with calculated kads, shown in table 4. it is worth noting that the basic assumptions of the langmuir adsorption isotherm [30] are as follows: i. the metal surface is homogeneous; ii. the adsorbate is adsorbed specifically, with each adsorbed species occupying only one surface site; iii. adsorbed compounds do not diffuse on the surface, and iv. the standard adsorption-free energy is unaffected by the degree of coverage. figure 8. langmuir adsorption isotherms for centrosema pubescens leaf extract (0.2– 1.2 g l-1) on uns s30403 stainless steel surface in 1.0 m hcl solution at 303 and 333 k table 4. langmuir adsorption isotherm and thermodynamic parameters for adsorption of centrosema pubescens leaf extract on uns s30403 stainless steel surface in 1m hcl temperature, k slope intercept kads / l g-1 r2 goads / kj mol-1 303 1.0141 0.1004 9.96 0.9959 -23.20 333 1.0139 0.018 55.56 0.9998 30.25 o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 13 as seen in table 4, straight lines with coefficients of determination r2 = 0.9959 and 0.9998 at 303 k and 333 k, respectively, were obtained from the langmuir isotherm equation, but the slopes are not exactly equal to unity. these slopes refute the langmuir adsorption isotherm postulation of monolayer adsorption of inhibitor molecules on the adsorbent surface, despite r2 values being acceptable for the langmuir isotherm. this discrepancy could be due to the limitations inherent in using the langmuir adsorption isotherm to fit data from typical metal inhibition processes; for example, monolayer inhibitor adsorption is only possible on homogeneous metal surfaces and without adsorbed molecules diffusion at any temperature [31]. anyhow, it is impossible to fully accept these assumptions even for a typical acid inhibition system like the one under consideration because foreign ions and interactions in the same medium are possible. also, some adsorption processes may be potential dependent and selective in terms of the types of charges on the metal surface. according to [31], another physical feature of the adsorption isotherm must be considered to explain these digressions as they affect the adsorption mechanism. as a result, the langmuir adsorption isotherm can be mathematically modified to account for the dimensionless separation constant, kl, as shown in equation (8). l ads 1 1 k k c = + (8) where kl is the dimensionless separation factor of the extract adsorption, kads is the adsorption/desorption equilibrium constant and c = concentration of the extract. the calculated values of kl for various concentrations of centrosema pubescens leaf extract at 303 and 333 k for uns s30403 in 1 m hcl are shown in table 5. table 5. values of dimensionless separation factor, kl, for varied concentrations of centrosema pubescens leaf extract at 303k and 333k for 304l stainless steel corrosion in 1 m hcl cleaf extract / g l-1 kl 303 k 333 k 0.2 0.4 0.6 0.8 1.0 1.2 1.4 mean 0.334 0.201 0.143 0.112 0.091 0.077 0.067 0.146 0.083 0.043 0.029 0.022 0.018 0.015 0.012 0.032 in general, for values of kl ˃ 1, adsorption is unfavorable and inconsistent with the langmuir adsorption isotherm). for kl ˂ 1, adsorption is favorable and experimental data fit the langmuir adsorption isotherm), while kl = 1 and kl = 0 connote that the adsorption is linear and irreversible, respectively. table 5 shows that all kl values for all inhibitor concentrations are less than unity, indicating that the adsorption process is well aligned with the langmuir isotherm. furthermore, the inhibitive potency of centrosema pubescens leaf extract is more favorable at higher temperatures, based on the estimated mean value of kl for each temperature [31]. for the adsorption process, the adsorption/desorption equilibrium constant (kads) is related to the free energy of adsorption, δgoads, through the eq. (9) [32]: o ads ads solvent 1 exp g k c rt − = (9) http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 14 where r is molar gas constant (j mol-1), t is the temperature (k), csolvent is the concentration of water in solution (g l-1), kads is the adsorption/desorption equilibrium constant (l g-1) obtained from the intercept of straight line in figure 8, and δgoads is free energy change of adsorption (kj mol-1). it is understandable that the unit of csolvent is a function of that of kads. as pointed out in table 4, the unit of kads is l g − 1, and so the unit of c solvent is g l-1 with a value of approximately 103 [33]. the outcome of calculations displayed in table 4 indicates that the values of δg0ads calculated by eq. (8) are negative, which clearly indicates spontaneous adsorption of the extract molecules. higher kads value obtained at higher temperatures suggests that adsorption is more favored at higher temperatures [30,33]. accordingly, the obtained values of δg0ads showed that a mixed mode of adsorption is occurring. this assertion is because the calculated values of δg0ads listed in table 4 reveal values of -23.20 and -30.25 kj mol -1 at 303 k and 333 k, respectively. usually, δg0ads values of 20 kj mol-1 or less are associated with electrostatic interaction between charged organic molecules and charged metal surface (physisorption), while those of 40 kj mol -1 or higher are associated with charge sharing or transfer from the organic molecules to the metal surface to form a coordinate type of bond (chemisorption) [6,32]. the observed combined mode of adsorption (physisorption and chemisorption) could be attributed to the fact that centrosema pubescens leaf extract contains a variety of chemical compounds (cf. table 1), where depending on the temperature, some can be adsorbed chemically and others physically [34]. xrd analysis figure 9 shows x-ray diffraction (xrd) patterns of uns s30403 stainless steel surfaces from both the extract-free and extract-present acid test solutions. for a better understanding of corrosion processes, phase compositions (as opposed to elemental compositions) must be described. a 2 / ° b 2 / ° figure 9. xrd spectra showing phases formed on uns s30403 surface in 1.0m hcl: a) blank solution; b) in the presence of 1.2 g l-1 centrosema pubescens leaf extract information on phase compositions is useful for both explaining the corrosion process and locating the source of corrosion in a facility while also providing solutions to the problem [35]. figure 9a shows the presence of fe2o3, fe2(co)9 and feocl, typical of a corroding system [36]. the addition o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 15 of the leaf extract of centrosema pubescens to the corrosive system helped to drastically minimize the corrosive effect of chloride ions in the hcl solution, as the xrd patterns of the surface in figure 9b showed no peak for fe2o3, fe2(co)9 and feocl. this indicates that a protective film capable of stifling the corrosion reaction is developed on the surface of uns s30403 stainless steel [36]. thermodynamic and activation parameters influence of temperature on corrosion of uns s30403 in 1.0 m hcl solution without and in the presence of various concentrations of the centrosema pubescens leaf extract was investigated using the logarithmic form of arrhenius equation (10) [20]: a1 1 1 2 1 1 log 2.303 ecr cr r t t   = −    (10) where cr1 and cr2 are corrosion rates (mm year-1) at temperatures t1 and t2, ea is the apparent activation energy for the reaction (kj mol− 1), and r is the molar gas constant (8.314 j k-1 mol-1). the second column of table 6 lists the values of activation energy, ea, calculated using eqn. (10). the values of ea obtained for systems containing varying concentrations of the investigated leaf extract are lower than in the blank solution. this finding, combined with the fact that inhibition efficiency increases with increasing temperature, suggests that the centrosema pubescens leaf extract is possibly chemisorbed on the uns s30403 surface [32]. table 6. activation and thermodynamic parameters of centrosema pubescens leaf extract on uns s30403 surface in 1.0 m hcl solution cinhibitor / g l-1 ea / kj mol-1 h* / kj mol-1 h = ea rt / kj mol-1 s* / kj mol-1 qads / kj mol -1 0 66.69 63.47 64.17 -62.26 - 0.2 59.71 56.55 57.19 -96.09 9.13 0.4 51.64 48.55 49.12 -124.08 18.15 0.6 39.00 36.01 36.46 -166.99 31.41 0.8 26.57 23.71 24.05 -185.53 43.50 1 24.23 21.39 21.71 -219.84 44.92 1.2 20.70 17.90 18.18 -234.85 47.62 1.4 58.46 55.31 55.94 -95.97 13.39 when explaining the relationship between the temperature dependence of extract inhibition efficiency and the activation energy in the presence of the extract, it is important to remember that: (a) for extracts showing decreasing inhibition efficiency as temperature rises, the magnitude of activation energy (ea) is higher for inhibited systems than uninhibited systems; (b) for extracts showing constant inhibition efficiency regardless of temperature, the magnitude of activation energy (ea) does not change in the presence or absence of inhibitor; (c) for extracts with a temperature-dependent inhibition efficiency, the activation energy (ea) in inhibited systems is lower than in extract-free media [37–42]. the observed decrease in ea of corrosion with increasing extract concentration up to 1.2 g l-1 (table 6), could be due to a shift in the net corrosion reaction from the uncovered part of the metal surface to one that directly involves adsorbed sites [40]. an alternative form of the arrhenius equation is the transition state equation (11) [38]: a aexp exp s hrt cr nh r rt     = (11) where δsa* / j mol-1 k-1change of entropy for activation, δha* / j mol-1 change of enthalpy for activation. http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 16 according to eq. (11), the graph of ln (jcorr/t) vs. 1/t would give a straight line with a slope equal to –δha* / r and intercept equal to [ln (r/nh) + (δsa*/r)]. thus, δha* and δsa* can be calculated from the transition-state equation. the δha* was verified using equation (11) [32]: ea ha = rt (12) the values calculated from eq. (11) compares favorably to rt value, which is 2.52 kj mol-1 at 303 k. in addition to ea, table 6 summarizes the calculated values of δha* and δsa*. it seems that values of ea and δha are similar in magnitude in the extract-free medium and also when the extract is added to the medium. both values, however, are lowering with an increase of extract leaf concentration. this finding strongly suggests that the energy barrier of the corrosion reaction was lowered in the presence of the extract without affecting the metal depletion process [41]. furthermore, the positive sign of the activation enthalpy, δha*, as shown in table 6, indicates that the dissolution mechanism of uns s30403 is endothermic in nature and slow [32,37]. in addition, the observed activation entropy, δsa*, values in the absence and presence of centrosema pubescens leaf extract is negative, implying that the rate-determining step for the activated complex is the association rather than the dissociation step [6,41]. the entropy of activation, δsa*, decreased as the extract was introduced into the system (table 6), implying that the adsorption of the extract onto the adsorbent surface was accompanied by a decrease in entropy, which is the driving force for the adsorption of the leaf extract onto the uns s30403 austenitic stainless steel surface [42]. the heat of adsorption (qads) of centrosema pubescens leaf extract on the adsorbent surface was applied as a confirmatory test using the equation (13) [43]: 2 1 1 2 ads 2 1 2 1 2.303 log log 1 1 t t q t t       = −  − − −  (13) where 1 and  2 are surface coverages at two temperatures, t1 and t2, and r is the molar gas constant. the last column of table 6 shows the heat of adsorption values, qads, which were calculated using eqn. (12) and ranged from 9.13 to 47.62 kj mol-1. these results are all positive, demonstrating that the adsorption of centrosema pubescens leaf extract onto the surface of uns s30403 austenitic stainless steel in 0.1 m hcl solution is endothermic in nature [32,41-42]. mechanism of corrosion inhibition organic inhibitors are typically adsorbed on metal surfaces to control the rate of metal corrosion by disrupting either the anodic or cathodic reaction or both. the chemical structure of the inhibitor, the nature of the metal surface, the interaction between organic molecules and the metallic surface, and the type of corrosive medium influence the inhibitor adsorption process [13,16]. the leaf extract of centrosema pubescens significantly controls the rate of attack of the corrosive electrolyte on the uns s30403 austenitic stainless steel by disrupting the anodic and cathodic reactions, as shown by the potentiodynamic polarization parameters (table 3). because of the complex phytochemical constituents of centrosema pubescens leaf extract (table 1), it is difficult to attribute the protective effects to the adsorption of any single constituent, especially because some of these phytochemicals, such as tannins, organic and amino acids, alkaloids, proteins, flavonoids, and organic pigments, as well as their acid hydrolysis products, have been shown effective against acid corrosion of metals [22,30-32,37]. however, more research is being done to define the key active phytochemical ingredients that are responsible for the inhibitory activity of extracts. in an acid solution, for example, the hydroxyl groups in these molecules become polarized, causing the oxygen atoms to o. d. ofuyekpone et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1343 17 become electron donors. as a result, hydroxyl groups may be responsible for the metal electrostatic attraction [44,45]. when these organic molecules are adsorbed onto the metal surface, they produce protective coatings with inhibitive capabilities, according to many researchers [3031,37,40]. the extract active components could bind to the metal-aggressive solution interface in one of four ways [46-47]: (i) electrostatic interactions of protonated molecules with already adsorbed anions, and (ii) donor-acceptor interactions between π -electrons of aromatic rings and vacant d-orbitals of surface iron atoms, (iii) interaction between unshared electron pairs of hetero atoms and vacant d-orbitals of surface iron atoms, and (iv) a combination of the mechanisms described. ftir spectral analysis ftir spectra of pure centrosema pubescens leaf extract and leaf extract adsorbed on uns s30403 stainless steel surface are shown in figure 10. figure 10. ftir spectra of (a) pure centrosema pubescens leaf extract and (b) surface film formed by adsorption of centrosema pubescens leaf extract on uns s30403 it is obvious from figure 10 that after adsorption on the uns s30403 stainless steel surface, there are some changes in the ftir spectrum of the centrosema pubescens extract. the o-h peak, which appears at 3265.1 cm−1, has shifted to 3250.2 cm−1 with reduced intensity. the c≡c peak, which appears at 2105.9 cm−1, has completely disappeared. also, the x=c=y peak, which appears at 1982.9 cm−1, has disappeared. meanwhile, the n-h bend peak, which appears at 1636.3 cm−1 still remains but with reduced intensity. the peaks of c-o and c-n became visible at 1189.0 and 1051.1 cm−1, respectively. the changes in the absorption pattern of these bands confirm the involvement of bonds in the adsorption of the extract components onto the stainless-steel surface. these shifts further indicate that there is an interaction between uns s30403 austenitic stainless steel and the centrosema pubescens leaf extract molecules [27,28,48]. http://dx.doi.org/10.5599/jese.1343 j. electrochem. sci. eng. 00(0) (2022) 000-000 leguminous plant leaf extract as corrosion inhibitor 18 conclusions using the weight loss method, potentiodynamic polarization measurements, and morphological characterization, the corrosion inhibition effectiveness of centrosema pubescens leaf extract on corrosion of uns 30403 in 1.0m hcl solution was investigated. the main conclusions can be summarized as follows: • the inhibition efficiency of the centrosema pubescens leaf extract increases with increasing extract concentration and temperature, reaching maximum values of 93.08 and 98.66 % at 30 and 60 °c, respectively. also, inhibition efficiency decreases with immersion time, reaching values of 86.84 and 75.00 % after 10 and 60 days at the optimal leaf extract concentration of 1.2 g l-1. • the thermodynamic investigation revealed that the leaf extract is adsorbed spontaneously on the uns 30403 stainless steel surface, following the langmuir adsorption isotherm model. the gibbs free energy of adsorption (gads) is found negative, indicating a significant interaction between leaf extract molecules and uns 30403 stainless steel surface. • according to activation, thermodynamic, and adsorption parameters, evaluated on the basis of potentiodynamic polarization measurements, the extract functions as a mixed-type inhibitor, and the adsorption of the active moieties in centrosema pubescens leaf extract on the uns s30403 surface is endothermic and occurs through mixed adsorption modes. • the presence of a protective coating on the surface of the uns s30403 stainless steel was confirmed by morphological and surface characterizations using sem, eds, xrd, and ftir. it was found specimens in hcl solution containing optimum concentration of inhibitor extract showed less pitting corrosion occurrences, indicating that the extract of centrosema pubescens leaf was able to provide enough protection. declaration of competing interest the authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 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abdelfatah, zaštita materijala 60(1) (2019) 3-17. http://dx.doi.org/10.5937/zasmat1901003f ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1080/17518253.2011.625980 https://doi.org/10.1016/j.jiec.2013.12.002 https://doi.org/10.1016/j.corsci.2012.07.017 https://doi.org/10.1016/j.corsci.2012.05.008 http://dx.doi.org/10.1002/9781118520994 http://dx.doi.org/10.1016/j.ejpe.2012.06.002 https://doi.org/10.1016/j.matchemphys.2005.09.020 https://doi.org/10.1016/0013-4686(81)85107-9 http://dx.doi.org/10.1016/j.corsci.2009.05.017 http://dx.doi.org/10.5006/0010-9312-23.8.252 http://dx.doi.org/10.5006/0010-9312-23.8.252 http://www.electrochemsci.org/papers/vol8/80709201.pdf http://dx.doi.org/10.4061/2011/157576 file:///c:/users/user/downloads/adsorption_and_inhibitive_properties_of_‌ethanol_ex.pdf file:///c:/users/user/downloads/adsorption_and_inhibitive_properties_of_‌ethanol_ex.pdf https://doi.org/10.1007/s40735-019-0280-2 https://doi.org/10.1016/j.arabjc.2014.12.024 https://doi.org/10.17675/2305-6894-2016-5-3-4 http://www.electrochemsci.org/papers/vol7/71009423.pdf http://dx.doi.org/10.5937/zasmat1901003f https://creativecommons.org/licenses/by/4.0/) jese manuscript doi: 10.5599/jese.2014.0067 1 j. electrochem. sci. eng. 4(4) (2014) 187-201; doi: 10.5599/jese.2014.0067 open access : : issn 1847-9286 www.jese-online.org original scientific paper use of hydrous titanium dioxide as potential sorbent for the removal of manganese from water ramakrishnan kamaraj, pandian ganesan and subramanyan vasudevan csir-central electrochemical research institute, karaikudi 630 006, india corresponding author e-mail: vasudevan65@gmail.com; tel.: +91-4565-241278 received: october 4, 2014; revised: october 30, 2014; published: mmmm dd, yyyy abstract this research article deals with an electrosynthesis of hydrous titanium dioxide by anodic dissolution of titanium sacrificial anodes and their application for the adsorption of manganese from aqueous solution. titanium sheet was used as the sacrificial anode and galvanized iron sheet was used as the cathode. the optimization of different experimental parameters like initial ion concentration, current density, ph, temperature, etc., on the removal efficiency of manganese was carried out. the maximum removal efficiency of 97.55 % was achieved at a current density of 0.08 a dm -2 and ph of 7.0. the langmuir, freundlich and redlich peterson isotherm models were applied to describe the equilibrium isotherms and the isotherm constants were determined. the adsorption of manganese preferably followed the langmuir adsorption isotherm. the adsorption kinetics was modelled by firstand secondorder rate models and the adsorption kinetic studies showed that the adsorption of manganese was best described using the secondorder kinetic model. thermodynamic parameters indicate that the adsorption of manganese on hydrous titanium dioxide was feasible, spontaneous and exothermic. keywords: titanium dioxide; manganese; adsorption; thermodynamics; isotherm; kinetics. introduction manganese (mn) is a naturally occurring element found in the air, soil, and water. it can exist in seven oxidation states ranging from -2 to +7. it is rarely found in its elemental state and is therefore a component of over 100 minerals and exists mainly as oxides, carbonates, and silicates. its most common mineral is pyrolusite (mno2). in ground water, manganese is a common contaminant and its presence is due to leaching processes and varies widely depending on the rock type. also, manganese has a variety of applications such as in ceramics, metallurgical http://www.jese-online.org/ mailto:vasudevan65@gmail.com j. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 188 188 processes, mining, dry cell batteries, pigments and paints which all can be the sources of underground pollution [1]. in addition to the disposal of untreated discharge from above the applications into water, another major source of pollution of the manganese is burning of coal and oil [2]. manganese is an essential metal for the human system and many enzymes are activated by manganese. the manganese contaminant in ground water affects the intelligent quotient (iq) of children. intake of higher concentrations of manganese causes neuro toxic disease like parkinsonism and manganese psychosis, an irreversible neurological disorder [3–5]. the prolonged over intake potentially affects the central nervous system, lungs, also causes diseases of disturbed speech called prognosis, also cause bronchitis and pneumonia [6,7]. the world health organization (who) prescribed the permissible limit for the manganese in the ground water is 0.05 mg l -1 . for this reason, there is great interest in the development of environmentally clean methods to destroy such compounds in aqueous medium for avoiding their dangerous accumulation in the aquatic environment. because of its high solubility over a wide ph range, toxicity and non-degradable nature, it is notoriously difficult to remove manganese from contaminated water [8,9]. hence the researchers in the world have carried out significant work on their removal from aqueous solutions and industrial effluents [10-16]. the usual method for removing toxic metals from water include electrodialysis, chemical coagulation, reverse osmosis, co-precipitation, complexation, solvent extraction, ion exchange, electrochemical treatment and adsorption. physical methods like ion exchange, reverse osmosis and electrodialysis have proven to be either too expensive or inefficient to remove manganese from water. at present, chemical treatments are not used due to disadvantages like high costs of maintenance, problems of sludge handling and its disposal, and neutralization of effluent. in this scenario, the electrochemical technologies have received great attention for the prevention of pollution problems, as reported in several reviews [17-19]. in the recent decade, electrodissolution process, where the coagulants generated in-situ, has been increasingly used in the world for treating the industrial wastewater, ground water and surface water and many studies conducted to optimize this process for specific problems [19]. the sacrificial anodic electrodes, commonly consisting of iron and aluminum, are used to continuously supply metallic ions as the source of coagulants, which can hydrolyze near the anode to form a series of metallic hydroxides capable of destabilizing dispersed particles. this process generates large quantities of iron and aluminum salt coagulated sludge, which inhibits efficient water treatment. from the generated coagulant, nothing may be recovered or reused, and require further incineration and landfill treatment. furthermore, the appearance of dissolved iron in aquatic suspensions can lead to visual, odor and taste problems resulting from later growth of iron bacteria [20]. even aluminum salts are suspected to be harmful to human and living things [21]. consequently, a coagulant that is safer and produces more reusable coagulated sludge could offer a novel solution to many environmental and economic problems associated with sludge handling. however, reports on novel electrodes materials remain very scarce in the literature for the generation of reusable and environmentally friendly coagulant. removal of metal contaminants by the chemically synthesized, different forms of, titanium dioxide was widely reported [22-25] and was similar to that of the most widely used iron and aluminum salt flocculation. furthermore, long-term toxicological studies have not found titanium salt in water to have any adverse effects. all the above factors suggest that the titanium salt can be used as an alternative coagulant [26]. in this investigation, titanium was used instead of iron and aluminum as a novel alternative sacrificial anode, and the removal of manganese from water by titanium-based electrocoagulation r. kamaraj et al. j. electrochem. sci. eng. 4(4) (2015) 187-201 doi: 10.5599/jese.2014.0067 189 was investigated. to optimize the maximum removal efficiency of manganese, different parameters like current density, ph, and temperature, inter electrode distance and co-existing ions were studied. in doing so, the equilibrium adsorption behavior is analyzed by fitting models of langmuir, freundlich and redlich peterson. the adsorption kinetics was modeled by firstand secondorder rate models. activation energy is evaluated to study the nature of adsorption. experimental chemicals manganese nitrate [mn(no3)2] of analytical grade was purchased from merck. hydrochloric acid (hcl) and sodium hydroxide (naoh) used for ph adjustment were of analytical grade from merck. sodium chloride (nacl) used for better conductivity of electrolyte of analytical grade from merck. sodium phosphate, sodium silicate, sodium carbonate and sodium fluoride used as coexisting ions were of analytical grade and purchased from merck. electrolytic system and electrolysis the experiments were carried out in a monopolar batch reactor using 1000 ml plexiglas vessel that was fitted with a polycarbonate cell cover with slots to introduce the electrodes, ph sensor, a thermometer and the electrolytes. titanium (alfa aesar, uk) of surface area (0.02 m 2 ) acted as the anode. the cathode was galvanized iron (commercial grade, india) sheets of the same size as the anode is placed at an inter-electrode distance of 3 cm. the temperature of the electrolyte was controlled to the desired value with a variation of ± 2 k by adjusting the rate of flow of thermostatically controlled water through an external glass-cooling spiral. a regulated direct current (dc) was supplied from a rectifier (10 a, 0-25 v; aplab model). the required concentration of manganese was prepared using milli q water. in all the experiments 3 g l -1 of sodium chloride was used for better conductivity. the solution volume of 900 ml was used for each experiment as the electrolyte. the ph of the electrolyte was adjusted and measured initially and during the electrolysis by a ph meter (dkk-toc, japan). the ph was adjusted using either 0.1 m naoh or 0.1 m hcl as necessary. after adjusting the initial solution ph to the desired value (3 to 9), the current density was set. the solution was stirred at 250 rpm to ensure good mixing and transport of reactants. temperature studies were carried at varying temperature (323-343 k) to determine the type of reaction. analytical procedures the concentration of manganese was determined using uv-visible spectrophotometer with manganese kits (merck, pharo 300, germany). the sem image of titanium dioxide was analyzed with a scanning electron microscope (sem) made by hitachi (model s-3000h). the constituents of the titanium dioxide were analyzed by x-ray fluorescence (xrf) made by horiba (model xgt2700). the fourier transform infrared spectrum of titanium dioxide was obtained using nexus 670 ftir spectrometer (thermo electron corporation, usa) and x-ray diffraction (xrd) patterns of titanium dioxide was analyzed using an x’per pro x-ray diffractometer (panalytical, usa). tga of titanium dioxide was carried out in the thermal analyzer (ta instruments; model sdt q600). the concentration of carbonate, silicate, and phosphate were determined using uv-visible spectrophotometer with respective standard ion kits supplied by merck (merck, pharo 300). j. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 190 190 results and discussion effect of current density on the removal efficiency the current density is one of the prominent factors which strongly influence the performance of electrodissolution process. the current density not only determines the coagulant dosage and bubble production rate but also the size and growth of the flocks, which can influence the treatment efficiency. therefore, the effect of current density on the removal of manganese was investigated. applying a constant current to the titanium effectively dissolved ti according to ti→ti 2+ →tio 2+ . tio 2+ combined easily with oh − to form tio2·h2o or ti(oh)4. ti(oh)4 is unstable substance, which changes gradually into tio2·h2o by dehydration. the reaction equations were, tio 2+ + 6oh + 2e → ti(oh)4 + h2 + 3o 2 (1) and tio 2+ + 2oh → tio2 . h2o (2) and at the cathode the following reaction is taking place, 2h2o + 2e → h2 (g) + 2oh (3) the amount of manganese removal depends upon the quantity of adsorbent (hydrous titanium dioxide) generated, which is related to the time and current density [27]. the amount of adsorbent was determined from faraday’s law. with the increase in current density the amount of hydrous titanium dioxide generation also increases. to investigate the effect of current density on the manganese removal, a series of experiments were carried out by solutions containing a constant pollutants loading of 2 mg l -1 , at a ph 7.0, with current density being varied from 0.02 to 0.1 a dm -2 . the removal efficiencies are 60.25, 82.54, 90.10, 97.55 and 97.90 % for 0.02, 0.04, 0.06, 0.08 and 0.1 a dm -2 respectively. from the results, it was found that very small raise in removal efficiency was observed for current densities 0.08 and 0.1 a dm -2 . hence, the further experiments were carried out at 0.08 a dm -2 . effect of ph on the removal efficiency it is believed that the initial ph is an important operating factor influencing the performance of electrodissolution process. to explain this effect, a series of experiments were carried out using 2 mg l -1 of manganese containing solutions, by adjusting the initial ph in the interval from 3 to 9. the removal efficiencies for the ph 3, 4, 5, 6, 7, 8 and 9 are 60, 79, 82.5, 95.50, 97.55, 97.56 and 97.65 % respectively. it is well know that the titanium dioxide adsorption is ph dependent. at acidic ph it is positively charged while at alkaline ph it negatively charged. mostly point of zero charge for titanium dioxide is approximately ph 6 to 8 [28,29]. the results agreed well with earlier results from the literature. further experiments were carried out at ph 7. effect of electrolyte concentration in order to evaluate the effect of initial concentration of manganese, experiments were conducted with varying initial concentration from 0.25-2.0 mg l -1 . figure 1 shows that the uptake of manganese (mg g -1 ) increased with increase in manganese concentration and remained nearly constant after equilibrium time. the equilibrium time was found to be 180 min for all concentration studied. the amount of manganese adsorbed (qe) increased from 0.248 to 1.982 mg g -1 as the concentration increased from 0.25-2.0 mg l -1 . from the figure 1 it is found that the plots r. kamaraj et al. j. electrochem. sci. eng. 4(4) (2015) 187-201 doi: 10.5599/jese.2014.0067 191 are single, smooth and continuous curves leading to saturation, suggesting the possible monolayer coverage to manganese on the surface of the adsorbent. 0 100 200 300 400 500 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 0.25 mg / l 0.5 mg / l 1.0 mg / l 1.5 mg / l 2.0 mg / l q e / m g g 1 time, min figure 1. effect of time and initial concentration of manganese for the adsorption on hydrous titanium dioxide, ph 7.0, t = 303k. effect of competing ions carbonate effect of carbonate on manganese removal was evaluated by increasing the carbonate concentration from 0 to 250 mg l -1 in the electrolyte. the removal efficiencies are 97.55, 95.3, 72.8, 50.7, 38, and 19 % for the carbonate concentration of 0, 2, 5, 65, 150 and 250 mg l -1 , respectively. from the results it is found that the removal efficiency of the manganese is not affected by the presence of carbonate below 2 mg l -1 . significant reduction in removal efficiency was observed above 5 mg l -1 of carbonate concentration is due to the passivation of anode resulting, the hindering of the dissolution process of anode [30]. phosphate the concentration of phosphate ion was increased from 0 to 50 mg l -1 , the contaminant range of phosphate in the ground water. the removal efficiency for manganese was 97.55, 91.3, 60.7, 35.5 and 29.2 % for 0, 2, 5, 25 and 50 mg l -1 of phosphate ion, respectively. there was no change in removal efficiency of manganese below 2 mg l -1 of phosphate in the water. at higher concentrations (at and above 5 mg/l) of phosphate, the removal efficiency decreased drastically. this was due to the preferential adsorption of phosphate over manganese as the concentration of phosphate increased. j. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 192 192 arsenic the concentration of arsenic was gradually increased from 0 to 5 mg l -1 . from the results it was found that the efficiency decrease for manganese was 97.55, 90.7, 78.5, 68.6 and 44.6 % by increasing the concentration of arsenate from 0, 0.2, 0.5, 2.5 and 5.0 mg l -1 , respectively. this was due to the preferential adsorption of arsenic over manganese as the concentration of arsenate increases. so, when arsenic ions are present in the water to be treated arsenic ions compete greatly with manganese ions for the binding sites. silicate from the results it is found that no significant change in manganese removal was observed, when the silicate concentration was increased from 0 to 2 mg l -1 . the respective efficiencies for 0, 2, 5, 10 and 15 mg l -1 of silicate are 97.55, 80.2, 72.4, 51.6 and 43.8 %. in addition to preferential adsorption, silicate can interact with titanium dioxide to form soluble and highly dispersed colloids that are not removed by normal filtration [30]. adsorption kinetic modeling the kinetic studies predict the progress of adsorption; however, the determination of the adsorption mechanism is also important for design purposes. in this research investigation, first and second order kinetic models were tested at different concentration (0.25 to 2.0 mg l -1 ) at a current density of 0.08 a dm -2 . first order kinetic model the first order kinetic model is generally expressed as follows [31], dqt/dt = k1 (qe-qt) (4) where qe / mg g -1 and qt / mg g -1 are the adsorption capacities at equilibrium and at time t / min respectively, and k1/ min -1 is a rate constant of first order adsorption. the integrated form of the above equation with the boundary conditions t = 0 to t = t and qt = 0 to qt = qt is rearranged to obtain the following time dependence function, log(qe-qt) = log qe – k1t / 2.303 (5) the experimental data were analyzed initially with first order model. the plot of log (qe-qt) vs. t should give the linear relationship from which k1 and qe can be determined by the slope and intercept, respectively eq. (5). the computed results are presented in table 1. the results show that the theoretical qe (cal) value doesn’t agree to the experimental qe (exp) values at all concentrations studied with poor correlation coefficient. this result indicated that the adsorption system do not follow a first-order reaction. so, further the experimental data were fitted with second order model. second order kinetic model the second order kinetic model is expressed as [32], dqt/dt = k2 (qe-qt) 2 (6) the integrated form of eq. (6) with the boundary condition t = 0 to t = t and qt = 0 to qt = qt is, 1/(qe-qt) = 1/qe+ k2t (7) eq. (7) can be rearranged and linearized as, r. kamaraj et al. j. electrochem. sci. eng. 4(4) (2015) 187-201 doi: 10.5599/jese.2014.0067 193 t/qt = 1/k2 qe 2 + t/qe (8) where, qe / mg g -1 and qt / mg g -1 are the amount of manganese adsorbed on hydrous titanium dioxide at equilibrium and at time t / min, respectively, and k2 is the rate constant for the second order kinetic model. table 1 comparison between the experimental and calculated qe values at different concentrations in first order and second order adsorption kinetics at a current density of 0.08 a dm -2 . c / mg l -1 qe / mg g -1 (exp) pseudo first order adsorption pseudo second order adsorption qe / mg g -1 (cal) k1 / min -1 r 2 qe / mg g -1 (cal) k2 / (g mg -1 ) min -1 r 2 0.25 0.50 1.0 1.5 2.0 0.248 0.463 0.951 1.470 1.982 35.78 22.84 1.543 1.337 5.780 0.0244 0.0212 0.0168 0.0124 0.0178 0.0069 0.0204 0.6154 0.3462 0.3425 0.248 0.461 0.950 1.470 1.980 1.6715 0.1013 0.1071 0.0874 0.0343 0.992 0.991 0.995 0.986 0.987 the kinetic data were fitted to the second order model eq. (8). the equilibrium adsorption capacity, qe (cal) and k2 were determined from the slope and intercept of plot of t/qt versus t and are compiled in table 1. figure 2 shows the plot of t/qt versus t for manganese adsorption and the plots were found to be linear. the theoretical qe (cal) value also agreed very well with the experimental qe value, indicating the pseudo second-order kinetics. in addition, the correlation coefficient for the second-order kinetic model was 0.99, which suggest the applicability of this kinetic equation and the second-order nature of the sorption process of manganese on hydrous titanium dioxide. 50 100 150 200 250 300 350 400 450 0 200 400 600 800 1000 1200 1400 1600 1800 (t /q t) / m in ( m g /g )1 time, min 0.25 mg / l 0.5 mg / l 1.0 mg / l 1.5 mg / l 2.0 mg / l figure 2. second-order kinetic model plots for adsorption of manganese at different concentrations, ph of the electrolyte: 7.0, temperature: 303 k, current density: 0.08 a dm -2 the computed results obtained from first order and second order models were depicted in table 1. from the tables, it was found that the correlation coefficient values are in the order of j. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 194 194 second order > first order. this indicates that the adsorption follows the second order model. further, the calculated qe values well agree with the experimental qe values for second order kinetics model. these results indicate that the second-order kinetic model can be applied suitably to predict the manganese adsorption process onto hydrous titanium dioxide. isotherm modeling in order to explain the mechanism of the adsorption process, it is important to establish the most appropriate correlation for the equilibrium curves. in this study, three adsorption isotherms viz., freundlich, langmuir and redlich isotherm models were applied to establish the relationship between the amounts of manganese adsorbed onto the hydrous titanium hydroxide and its equilibrium concentration in the electrolyte containing contaminant ions. freundlich isotherm the freundlich adsorption isotherm typically fits the experimental data over a wide range of concentrations. this empirical model includes considerations of surface heterogeneity and exponential distribution of the active sites and their energies. the isotherm is adopted to describe reversible adsorption and is not restricted to monolayer formation. the linearised in logarithmic form and the freundlich constants can be expressed as [33], log qe = log kf + n log ce (9) where, kf is the freundlich constant related to adsorption capacity, n is the energy or intensity of adsorption, ce is the equilibrium concentration of manganese (mg l -1 ). in testing the isotherm, the manganese concentration used was 0.25 to 2.0 mg l -1 , current density of 0.08 a dm -2 and at an initial ph 7. the adsorption data is plotted as log qe versus log ce by equation (9) should result in a straight line with slope n and intercept kf. the intercept and the slope are indicators of adsorption capacity and adsorption intensity, respectively. the value of n falling in the range of 1-10 indicates favorable sorption. freundlich constant (kf) and n values were listed in table 2. from the analysis of the results it is found that the freundlich plots fit only satisfactorily with the experimental data obtained in the present study which is shown in the figure. 3 (a). table 2 constant parameters and correlation coefficient for different adsorption isotherm models for manganese adsorption at 0.252.0 mg l -1 at a current density of 0.08 a dm -2 isotherm parameters concentration of mn, mg l -1 0.25 0.5 1.5 1.5 2.0 langmuir qm / mg g -1 0.2383 0.4597 0.9564 1.4616 1.9491 b / l mg -1 0.1113 0.1102 0.1099 0.1014 0.0948 r 2 0.9943 0.9987 0.9954 0.9962 0.9991 rl 0.9729 0.9479 0.8998 0.8569 0.8179 freundlich kf / mg g -1 0.5803 0.5512 0.5174 0.4897 0.4613 n / l mg-1) 2.1786 2.0257 1.9457 1.8798 1.7259 r 2 0.9812 0.9789 0.9881 0.9836 0.9820 redlich peterson kf / l g -1 0.9978 0.9981 0.9968 0.9990 0.9891  0.9764 0.9854 0.9817 0.9897 0.9789 ar / l mmol -(1-1/β ) 27.412 28.417 29.648 30.568 32.516 r. kamaraj et al. j. electrochem. sci. eng. 4(4) (2015) 187-201 doi: 10.5599/jese.2014.0067 195 log (ce / mg l -1 ) ce -1 / g mg -1 figure 3. (a) frendlich plot (log qe vs log ce) for adsorption of manganese, ph of the electrolyte: 7.0, current density: 0.08 a dm -2 , concentration: 2.0 mg l-1, (b) langmuir plot (1/qe vs. 1/ce) for adsorption of manganese, ph of the electrolyte: 7.0, current density: 0.08 a dm -2 , concentration: 2.0 mg l -1 langmuir isotherm this model assumes a monolayer deposition on a surface with a finite number of identical sites. it is well known that the langmuir equation is valid for a homogeneous surface. the linearized form of langmuir adsorption isotherm model is [34], ce/qe=1/qmb+ce/qm (10) where, qe is amount adsorbed at equilibrium, ce is the equilibrium concentration, qm is the langmuir constant representing maximum monolayer adsorption capacity and b is the langmuir constant related to energy of adsorption. the essential characteristics of the langmuir isotherm can be expressed as the dimensionless constant rl. rl=1 / (1+bco) (11) where rl is the equilibrium constant it indicates the type of adsorption, b, is the langmuir constant. co is various concentration of manganese solution. the rl values between 0 and 1 indicate the favorable adsorption. langmuir isotherm was tested from eq. (10). the plots of 1/qe as a function of 1/ce for the adsorption of manganese on hydrous titanium dioxide are shown in figure 3 (b). the plots were found linear with good correlation coefficients (>0.99) indicating the applicability of langmuir model in the present study. the values of monolayer capacity (qm) and langmuir constant (b) is presented in table 2. the values of qm calculated by the langmuir isotherm were all close to experimental values at given experimental conditions. these facts suggest that manganese is adsorbed in the form of monolayer coverage on the surface of the adsorbent. the dimensionless constant rl was calculated from eq.(11). the rl values were found to be between 0 and 1 for all the concentration of manganese studied. the correlation co-efficient values of langmuir and freundlich isotherm models are presented in table 2. redlich peterson isotherm it is a three parameter hybrid isotherm. it is having features of both the langmuir and frendlich isotherms. this model has linear dependence in the numerator component and exponential component in denominator of non-linear form [35,36]. lo g ( q e / m g g -1 ) (t / q e ) / m in ( m g / g ) 1 j. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 196 196 β er ef e 1 ca ck q   (12) where qe / mmol g -1 is the solid-phase sorbate concentration at equilibrium, ce / mmol l -1 is the concentration of adsorbate in equilibrium with liquid phase, kf / l g -1 and ar / l mmol -(1-1/β) are the redlich-peterson isotherm constants, and  is the exponent, which lies between 1 and 0. if the  tends to 0 then the adsorption follows the frendlich isotherm and if the  value tends to one it fits with the langmuir isotherm. in order to verify our investigation regarding the monolayer or multilayer adsorption, the linear form of redlich-peterson is used. it is little bit complicated compared to the other isotherms. the strategy to find these parameters is based in the maximization of correlation coefficients (r 2 ) from the linear fit to the data. in this way the kf values are modified until obtain the best fit of the data. the linear form of the equation for this model is ln (kf(ce/qe-1) = ln ar +  ln ce (13) plotting of ln (kf(ce/qe-1) vs. ln ce by the eq. (13) gives the redlich peterson equation. this isotherm is a three parameters isotherm in which kf values are indirectly obtained by plotting the graph with maximum correlation coefficient by justifying the values of kf. by that kf,  and ar are in the table 2 for all concentrations. here the  values are above the 0.95. so the adsorption favors langmuir isotherm rather than frendlich isotherm. adsorption thermodynamics to understand the effect of temperature on adsorption process, thermodynamic parameters should be determined at various temperatures. the energy of activation for adsorption of manganese can be determined by the second order rate constant is expressed in arrhenius form [37], ln k2 = ln ko e/rt (14) where ko is the constant of the equation (g mg -1 ) min -1 ), e is the energy of activation (j mol -1 ), r is the gas constant (8.314 j mol -1 k -1 ) and t is the temperature (k). figure 4(a) shows that the rate constants vary with temperature according to eq.(14) giving an activation energy of 21.01 kj mol -1 for manganese from the slope of the fitted equation. the free energy change is obtained using the following relationship ∆g = -rt ln kc (15) where ∆g is the free energy (kj mol -1 ), kc is the equilibrium constant, r is the universal gas constant and t is the temperature in k. the values of kc and ∆g are presented in table 3. the negative value of ∆g indicates the spontaneous nature of adsorption. other thermodynamic parameters such as entropy change (δs) and enthalpy change (δh) were determined using the van’t hoff equation: rt h r s k    cl n (16) the enthalpy change (∆h = -60.57 j mol -1 ) and entropy change (∆s = -0.047 j mol -1 k -1 ) were obtained from the slopes and intercepts of the van't hoff linear plots of ln kc versus 1/t (figure. 4(b)) eq.(16). negative value of enthalpy change (∆h) indicates that the adsorption process is exothermic in nature, and the negative value of change in internal energy (∆g) show the spontaneous adsorption of manganese on the adsorbent. negative values of entropy change show r. kamaraj et al. j. electrochem. sci. eng. 4(4) (2015) 187-201 doi: 10.5599/jese.2014.0067 197 the increased randomness of the solution interface which gains heat from the surroundings during adsorption of manganese on the adsorbent [38] (table 3). negative enthalpy and negative entropy shows that the adsorption is more favorable at low temperature. this is due to the decrement of pore size as temperature increases. t -1 / k -1 t -1 / k -1 figure 4. (a) plot of log k2 vs. t -1 ; (b) plot of ln kc vs. t -1 : ph 7.0; j = 0.08 a dm -2 , c = 2 mg l -1 the enthalpy change (∆h = -60.57 j mol -1 ) and entropy change (∆s = -0.047 j mol -1 k -1 ) were obtained from the slopes and intercepts of the van't hoff linear plots of ln kc versus 1/t (figure. 4(b)) eq.(16). negative value of enthalpy change (∆h) indicates that the adsorption process is exothermic in nature, and the negative value of change in internal energy (∆g) show the spontaneous adsorption of manganese on the adsorbent. negative values of entropy change show the increased randomness of the solution interface which gains heat from the surroundings during adsorption of manganese on the adsorbent [38] (table 3). negative enthalpy and negative entropy shows that the adsorption is more favorable at low temperature. this is due to the decrement of pore size as temperature increases. table 3 thermodynamics parameters for adsorption of manganese. temperature, k kc ∆g o / kj mol -1 ∆h o / jmol -1 ∆s o / j mol -1 k -1 323 333 343 1.0433 1.0406 1.0382 -0.0464 -0.0450 -0.0436 -60.57 -0.0470 table 4. comparison between the experimental and calculated qe values at different temperatures in first and second order adsorption kinetics of manganese: c = 2.0 mg l -1 , ph 7.0, j = 0.08 a dm -2 t / k qe / mg g -1 (exp) first order adsorption second order adsorption qe / mg g -1 (cal) k1 / min -1 r 2 qe / mg g -1 (cal) k2 / (g mg -1 ) min -1 r 2 323 333 343 2.048 2.160 2.165 1.112 0.987 0.874 -0.0061 -0.0068 -0.0071 0.5430 0.4569 0.4037 2.01 2.13 2.14 0.0320 0.0095 0.0097 0.990 0.986 0.991 using lagergren rate equation, pseudo second order rate constants and correlation co-efficient were calculated for different temperatures (323-343 k). the calculated qe values obtained from the second order kinetics agrees with the experimental qe values better than the first order lo g ( q e / m g g -1 ) ln ( k 2 / ( g m g -1 ) m in -1 ) j. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 198 198 kinetics model, indicating adsorption following second order kinetics. table 4 depicts the computed results obtained from pseudo first and pseudo second order kinetic models. characterization of hydrous titanium dioxide xrf studies the contents of titanium dioxide were analyzed by xrf. the titanium dioxide sample was dried in a drying chamber at 100 °c were ground in the agate mortar. as shown in table 5, 95.2 % of the sample, by weight, was titanium dioxide. sodium chloride was came from the electrolyte, and calcium and barium elements came from the impurity in the titanium electrode. xrd studies the crystal structure of hydrous titanium dioxide nanoparticles was analyzed by x-ray powder diffractometer operating with cukα radiation source filtered with a graphite monochromator. figure. 5(a) shows the x-ray diffraction pattern of hydrous titanium dioxide nanoparticles. from the figure it is found that, most diffraction peaks belong to the anatase phase (jcpds card number 73-1764), and minor peaks from the brookite phase (jcpds card number 76-1936) could also be observed. the crystallite size d was determined from the broadening of corresponding strongest x-ray diffraction peaks by using scherrer's formula [39]:   cos 9.0 d (17) where d is the crystalline size, λ is the average wavelength of the x-ray radiation (λ = 1.5418 å),  is the line-width at half-maximum peak position, and  is the diffracting angle (2 = 25.4°). the average crystallite size of the hydrous titanium dioxide is 4.3-8.4 nm. ft-ir spectrum the hydrous titanium dioxide was analyzed using ftir and results are presented in figure 5(b). the strong peak at 3381 cm −1 is attributed to the stretching vibrations of surface and interlayer water molecules and hydroxyl groups. this is related to the formation of hydrogen bonds of interlayer water with guest anions as well as with hydroxide groups of layers. at 1630.18 cm -1 , there is a strong adsorption peak for hydroxyl bending vibration belonging to physically adsorbed h2o. one small adsorption peak could also be identified at 1371.37 cm -1 , which represents the coordinated hydroxyl groups. these observations demonstrate that these hydrous titanium dioxide nanoparticles have high adsorption capacities to h2o and hydroxyl groups exist on their surfaces [40]. sem and edax analysis figure 5(c) shows the sem images of hydrous titanium dioxide. the sem images show different size, shape and dimension and these nanoparticles are aggregated into micro-sized particles. the volume median diameter value of these nanoparticles in distilled water was determined at approximately 9.8 nm by the dynamic light scattering technique, which is in accordance with the sem observation. this type of aggregation of nanoparticles is beneficial to their removal from aqueous environment after the treatment process. energy-dispersive analysis of x-rays was used to analyze the elemental constituents of titanium dioxide generated during the electro dissolution process and the results are presented in figure 5(d). the figure indicates that the titanium dioxide was composed mainly of ti an o, which affirms that the titanium dioxide was generated by anodic dissolution. r. kamaraj et al. j. electrochem. sci. eng. 4(4) (2015) 187-201 doi: 10.5599/jese.2014.0067 199 a b wavenummber, cm -1 c d figure 5. (a) x-ray diffraction pattern of hydrous tio2, (b) ftir pattern of hydrous tio2, (c) sem image of the hydrous tio2, (d) edax image of the hydrous tio2 tga analysis tga analysis (figure not shown) of hydrous titanium dioxide was carried out. from the results we found that the weight loss of 13.0 % where observed when the samples were heated from the 32 800°c. the entire range will be divided into three stages viz., first, second and third stage. in the first stage (32 – 122 °c) weight loss (6.9%) could be attributed to the elimination of physically absorbed water. in the second stage (122 to 438 °c) weight loss (6.0 %) could be contributed to the loss of surface hydroxyl groups. in the third stage (438 to 800 °c) no exothermic peak was observed and the weight loss is around 0.1 %. conclusions the maximum removal efficiency of 97.55 % was achieved with titanium as sacrificial anode at a current density of 0.08 a dm -2 , ph 7.0. the results indicate that the hydrous titanium dioxide, by electro-dissolution of sacrificial anodes, efficiently adsorbs the manganese from water. hence this process can be used as an effective process for the removal of manganese contaminated water resources. the results indicate that, the second-order kinetic model accurately described the adsorption kinetics. the adsorption mechanism was found to be chemisorption and the ratej. electrochem. sci. eng. 4(4) (2015) 187-201 tio2 as sorbent for the removal of mn from water 200 200 limiting step was mainly surface adsorption. the langmuir isotherm showed a better fit than the freundlich and redlich isotherms, thus, indicating the applicability of monolayer coverage of manganese on hydrous titanium dioxide. the thermodynamic parameters like δg, δh and δs were determined. their values indicated that the adsorption process was favorable, spontaneous, and exothermic in nature. as the temperature increased δg became less negative, indicating a stronger driving force, resulting in a greater adsorption capacity at higher temperatures. the negative value of δh confirmed that the process was exothermic. negative values of entropy change show the increased randomness of the solution interface which gains heat from the surroundings during adsorption of manganese on the adsorbent. edax analysis confirmed that manganese was adsorbed on to the hydrous titanium dioxide. acknowledgments: the authors wish to express their gratitude to dr. vijayamohanan k. pillai, director, csir-central electrochemical research institute, karaikudi to publish this article. references [1] y. h. chang, k. h. hsieh, f. c. chang, journal of applied polymer science 112 (2009) 2445– 2454. 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[38] l. d. rio, j. aberg, r. renner, o. dahiaten , v. vedral, nature 474 (2011) 61-63 [39] c. s. barrett, t. b. massalski, structure of metals, third edition, mc graw-hill, newyork, 1966 [40] s. debnath, u. c. ghosh, desalination 273 (2011) 330-342. © 2014 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ zno/1-hexyl-3-methylimidazolium chloride paste electrode, highly sensitive lorazepam sensor http://dx.doi.org/10.5599/jese.1577 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1577 open access : : issn 1847-9286 www.jese-online.org original scientific paper zno/1-hexyl-3-methylimidazolium chloride paste electrode, highly sensitive lorazepam sensor seddigheh chenarani1, mahmoud ebrahimi1, vahid arabali2 and safar ali beyramabadi2 1 department of chemistry, mashhad branch, islamic azad university, mashhad, iran 2department of chemistry, sari branch, islamic azad university, sari, iran india corresponding author: m.ebrahimi@mshdiau.ac.ir; tel.: +98-9155095175h; fax: +98-51-3663-0781 received: october 30, 2022; accepted: december 5, 2022; published: january 18, 2022 abstract the measurement of pharmaceutical compounds in biological fluids is considered an effective way to evaluate their effectiveness. on the other hand, lorazepam is a drug with good efficiency in treatment and some side effects, which measurement is very important. in this study, the zno nanoparticle was synthesized as an electrocatalyst by chemical precipitation method. in continuous a simple modification on paste electrode (pe) by zno nanoparticle (zno-nps) and 1-hexyl-3-methylimidazolium chloride (hmimcl) was made and new sensor was used for sensing of lorazepam. the hmimcl/zno-nps/pe showed catalytic behavior on oxidation signal of lorazepam and improved its signal about 2.17 times compared to unmodified pe. on the other hand, oxidation signal of lorazepam was reduced about 110 mv at surface of hmimcl/zno-nps/pe compare to unmodified pe that confirm accelerating the electron exchange process after modification of sensor by hmimcl and zno-nps as powerful catalysts. the hmimcl/zno-nps/pe was used for monitoring of lorazepam in water and injection samples and results showed recovery data 98.5 to 103.5 % that are acceptable for a new sensor. keywords pharmaceutical sensor; modified sensor; nano-catalyst; ionic liquid introduction medicines, as one of the most important substances used in human life, have many positive and negative effects, which have attracted the attention of many researchers [1,2]. although drugs play an important role in the treatment of diseases, the harmful effects of some of them on the body as well as the harmful effects on the environment have caused many studies to be done on this group of compounds [3]. measuring medicinal compounds is one of the most important studies in this field and can provide a lot of information during patient treatment or its role in environmental pollution [4,5]. in between of analytical methods in sensing of pharmaceutical compounds [6-10], electrochemical http://dx.doi.org/10.5599/jese.1577 http://dx.doi.org/10.5599/jese.1577 http://www.jese-online.org/ mailto:m.ebrahimi@mshdiau.ac.ir j. electrochem. sci. eng. 00(0) (2023) 000-000 highly sensitive lorazepam sensor 2 methods showed more attentions due to many advantage such as easy modification for sensitive sensing, low cost and wide range applications [11]. the lorazepam is one of famous medicines that used to treat anxiety [12]. there are several reports of adverse effects of lorazepam use such as dizziness, tiredness and weakness [13]. therefore, monitoring of this medicine is so important in during treatment [14]. due to high over-voltage and low redox signal of lorazepam, the sensing of it is so hard with usual electrochemical sensors [11]. for overcoming to this problem, modification and amplification of electrochemical sensors is necessary [15]. modification of electrodes is one of the proven solutions to increase the sensitivity and selectivity of electrochemical sensors [16-21]. different types of mediators such as polymers, nanomaterials, ionic liquids, mof etc. were suggested for modification and amplifycation of electrochemical sensors in recent years [22-30]. nanotechnologies opened a new approach in science and improve many of chemical and physical properties of materials [31-36]. with this way, nanomaterials were selected as first choice in different branch of science [37-42]. due to high electrical conductivity of some nanomaterials such as metal nanoparticles and carbon-based nanomaterials, they were used for modification of electrochemical sensors [43-45]. on the other hand, ionic liquids showed good advantage as binder for fabrication of paste electrode and improved electrical conductivity of modified paste electrodes [46-51]. in this research work, the hmimcl/zno-nps/pe was fabricated and used as new approach for monitoring of lorazepam with good limit of detection compared to previous suggested electrochemical sensors. the hmimcl/zno-nps/pe showed acceptable analytical data in sensing of lorazepam in real samples with recovery data 98.5 103.5%. modification of electrode improved oxidation current of lorazepam about 2.17 times and reduced oxidation potential of it about 110 mv compare to unmodified electrode. experimental materials and instruments zinc nitrate hexahydrate and sodium hydroxide were purchased from merck company and used for synthesis of zno nanoparticle. 1-hexyl-3-methylimidazolium chloride, graphite powder, paraffin oil and diethyl ether were purchased from sigma-aldrich company and used for fabrication of paste electrode. phosphoric acid purchased from merck and used for preparation phosphate buffer solution. the i v signals were recorded by vertex – ivium (potentiostat/galvanostat) connect with hmimcl/ /zno-nps/pe as working electrode, ag/agcl as reference electrode and pt wire as counter electrode. synthesis of zno nanoparticle the 100 ml zinc nitrate hexahydrate (0.5 m) was stirred in erlenmeyer flask for 15 min and then 100 ml sodium hydroxide (0.5 m) added dropwise and stirred form 30 min. white precipitate of zinc hydroxide washed for 10 times by distilled water and then dried ate 100 °c for 16 h. the white powder was calcinated at 250 °c for 4 h and zno nano-powder was obtained. fabrication of hmimcl/zno-nps/pe for fabrication of hmimcl/zno-nps/pe; 90 mg zno-nps + 910 mg graphite powder was mixed in mortar and pestle and 15 ml diethyl ether was add on it. after evaporation of diethyl ether, the paraffin oil + hmimcl with ratio of (7:3 vol.%) were used as binders and sample hand mixed for 1 h. the hmimcl/zno-nps paste was added i the end of glass tube for fabrication of hmimcl/znonps/pe in the presence of copper wire. j. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1577 3 results and discussion electrochemical investigations the electrochemical behavior of lorazepam was investigated in the ph range 5.0 to 7.0 and results showed in figure 1. figure 1. current-ph diagram for sensing of 300 µm lorazepam using hmimcl/zno-nps/pe as electroanalytical sensor (n=4) as can be seen, with increasing in ph from 5.0 to 7.0, the oxidation signal of 300 µm lorazepam increased and then decreased. therefore, this ph was selected as optimum condition for monitoring of lorazepam using hmimcl/zno-nps/pe as electroanalytical sensor. the oxidation signal of 200 µm lorazepam was recorded at surface of pe (figure 2a), zno-nps/pe (figure 2b), hmimcl/pe (figure 2c) and hmimcl/zno-nps/pe (figure 2d), respectively. figure 2. linear sweep voltammograms of 200 µm lorazepam at surface of pe (a), zno-nps/pe (b), hmimcl/pe (c) and hmimcl/zno-nps/pe (d); ph 7.0 the oxidation currents 10.85 µa, 16.53 µa, 18.55 µa and 23.6 µa were detected for monitoring of 200 µm lorazepam at surface of pe, zno-nps/pe, hmimcl/pe and hmimcl/zno-nps/pe, respecttively. as can be seen, with moving pe to hmimcl/zno-nps/pe, the oxidation current of lorazepam http://dx.doi.org/10.5599/jese.1577 j. electrochem. sci. eng. 00(0) (2023) 000-000 highly sensitive lorazepam sensor 4 increased due to high electrical conductivity and synergic effect of two conductive mediators. this result confirms synergic effect of two mediators after modification of pe and fabrication of a highly sensitive voltammetric sensor to monitoring of lorazepam. the linear sweep voltammogarms of 300 µm lorazepam at surface of hmimcl/zno-nps/pe and in the scan rate range 10 to 100 mv/s were recorded and signals are presence in figure 3 inset. figure 3. current vs. 1/2 plot for oxidation of 300 µm lorazepam at surface of hmimcl/zno-nps/pe. linear sweep voltammogarms of 300 µm lorazepam at scan rates; a) 10; b) 20; c) 40; c) 70 and e) 100 mv / s (n=4) as can be seen, a linear relation between oxidation signals of 300 µm lorazepam and 1/2 with equation i = 2.5907 1/2 + 3.4097 (r2 = 0.9916) that confirm diffusion process [52-55] to lorazepam oxidation at surface of hmimcl/zno-nps/pe. on the other hand, positive shift in oxidation signal of lorazepam with increase in scan rate confirm kinetic limitation in redox reaction of this drug. the tafel plot relative to oxidation of 300 µm lorazepam at scan rate 10 mv/s showed in figure 4. using tafel equation and tafel slope, the value of α was calculated 0.33, that confirms irreversible behavior to redox reaction of lorazepam. figure 4. tafel plot of 300 µm lorazepam at surface of hmimcl/zno-nps/pewith scan rate 10 mv/s analytical parameters the linear dynamic range (ldr) and limit of detection (lod) of proposed system for sensing of lorazepam was investigated by square wave voltammetric methods (figure 4 inset). j. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1577 5 figure 4. current – concentration curve for monitoring of lorazepam using hmimcl/zno-nps/pe. inset) square wave voltammograms of lorazepam at surface of hmimcl/zno-nps/pe in the concentration of 1) 0.5; 2) 25; 3) 40; 4) 70; 5) 100; 6) 125; 7) 170; 8) 200 and 9) 250 µm (n=4) a linear relation in the concentration range 0.5 to 250 µm with equation i = 0.0844c + 0.8738 (r2 = = 0.9926) was detected for sensing of lorazepam using hmimcl/zno-nps/pe as electro-analytical sensor. the detection limit 0.1 µm was reported for sensing of lorazepam using hmimcl/zno-nps/pe in this study. stability, selectivity and real sample analysis investigation the stability of hmimcl/zno-nps/pe for monitoring of 200 µm lorazepam was investigated in period time 70 days. results showed in figure 5 and confirm that hmimcl/zno-nps/pe has good stability for sensing of lorazepam in 2 months. figure 5. current – days diagram for oxidation of 200 µm lorazepam at surface of hmimcl/zno-nps/pe on the other hand, selectivity of hmimcl/zno-nps/pe in sensing of 15 µm lorazepam was investigated and results with acceptable error 5 % reported in table 1. as can be seen in table 1, there are any important interference for monitoring of 15 µm lorazepam using hmimcl/zno-nps/pe and this sensor showed good selectivity in monitoring of lorazepam. in the final step, ability of hmimcl/zno-nps/pe in monitoring of lorazepam in water, dextrose saline and injection samples were checked, and results reported in table 2. as can be seen, the http://dx.doi.org/10.5599/jese.1577 j. electrochem. sci. eng. 00(0) (2023) 000-000 highly sensitive lorazepam sensor 6 hmimcl/zno-nps/pe detected lorazepam with recovery range 98.5 to 103.5 % that are acceptable values for a new sensor. table 1. interference study results for monitoring 15 µm lorazepam using hmimcl/zno-nps/pe as sensor species tolerant limits (wsubstance/wlorazepam) cl−, br−, ca2+, k+, na+ 1000 glucose 700 starch saturation table 2. application of hmimcl/zno-nps/pe for sensing of lorazepam in real sample sample c / µm recovery, % added expected founded water ----<lod -- 10.00 10.00 10.35±0.54 103.5 injection --2.00 2.03±0.05 -- 10.00 12.00 11.82±0.45 98.5 dextrose saline ----<lod -- 10.00 10.00 10.31±0.51 103.1 conclusions in this study, a new and simple analytical plan was described for monitoring of lorazepam in aqueous solution. the suggested sensor (hmimcl/zno-nps/pe in this case) showed good selectivity for monitoring of lorazepam. the ph 7.0 was selected as optimum condition in voltammetric analysis. in addition, hmimcl/zno-nps/pe was successfully used to monitoring of lorazepam in the concentration range 0.5 to 250 µm with detection limit 0.1 µm. the modification of pe by hmimcl and zno-nps improved oxidation signal of lorazepam about 2.17 times compared to unmodified pe. on the other hand, no specific interference for monitoring of lorazepam has been reported at 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1007/s11244-021-01541-x https://doi.org/10.1007/s11244-021-01541-x https://doi.org/10.1007/s11581-012-0832-7 https://doi.org/10.1016/j.snb.2014.08.037 https://doi.org/10.1002/dta.232 https://doi.org/10.2174/1573411016999201022141930 https://doi.org/10.1016/j.fct.2022.113362 https://doi.org/10.1007/s11694-020-00585-z https://doi.org/10.1002/elan.201100465 https://doi.org/10.1002/elan.201400013 https://doi.org/10.1039/c1ay05031a https://doi.org/10.1016/j.jpba.2022.114657 https://creativecommons.org/licenses/by/4.0/) synthesis of ammonia from water and nitrogen using a compo-site cathode based on la0.6ba0.4fe0.8cu0.2o3-δ-ce0.8gd0.18ca0.02o2-δ http://dx.doi.org/10.5599/jese.1535 s1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1535 open access : : issn 1847-9286 www.jese-online.org original scientific paper synthesis of ammonia from water and nitrogen using a composite cathode based on la0.6ba0.4fe0.8cu0.2o3-δ-ce0.8gd0.18ca0.02o2-δ ibrahim a. amar department of chemistry, faculty of sciences, sebha university, sebha, libya corresponding authorс: ibr.amar@sebhau.edu.ly received: september 30, 2022; accepted: november 5, 2022; published: november 8, 2022 abstract carbon-free electrochemical synthesis of ammonia is a promising technology for co2 emission reduction. this study aims to explore the electrocatalytic activity of a-site badoped perovskite cathode catalyst (la0.6ba0.4fe0.8cu0.2o3-δ, lbfcu) for ammonia synthesis from water and nitrogen. lbfcu was prepared via the sol-gel method using combined edta-citrate complexing agents and characterized by x-ray diffraction (xrd) and scanning electron microscope (sem). ammonia was successfully synthesised from water and nitrogen under atmospheric pressure, and lbfcu mixed with ce0.8gd0.18ca0.02o2-δ (cgdc) was used as a cathode. when a voltage was applied to the cell containing cgdccarbonate composite solid electrolyte, ammonia formation was observed at 375, 400, 425 and 450 °c. at 400 °c and 1.4 v, the maximum rate of ammonia production was achieved at 4.0×10-11 mol s-1 cm-2, which corresponds to faradaic efficiency of  0.06 % at the current density of 19 ma cm-2. according to the findings, the synthesis of ammonia directly from water and nitrogen may be considered a promising green synthesis technology. keywords ammonia production; electrosynthesis; electrocatalyst; perovskite oxide; oxidecarbonate composite electrolyte introduction ammonia is one of the most produced chemicals worldwide, widely used as a chemical feedstock for the production of reactive nitrogen compounds such as urea, ammonium nitrate, nitric acid, synthetic fibers, pharmaceuticals, resins, dyes, plastics and ammonium sulphate. the global production of ammonia reached 160 million tons in 2019 [1,2]. since its development in the early 1900s, the haber-bosch process has been applied for industrial production of ammonia, converting hydrogen and nitrogen gas into ammonia at high temperature (500 °c) and high pressure (15 to 30 mpa), using fossil fuels (natural gases or coal) as energy sources, which now becomes a big challenge when moving toward renewable energy transition. the steam reforming step to generate hydrogen gas causes serious carbon emissions as it consumes approximately 84 % of the energy http://dx.doi.org/10.5599/jese.1535 http://dx.doi.org/10.5599/jese.1535 http://www.jese-online.org/ mailto:ibr.amar@sebhau.edu.ly j. electrochem. sci. eng. 00(0) (2022) 000-000 synthesis of ammonia from water and nitrogen 2 required for the ammonia industry [3-6]. more than 300 million metric tons of co2 are generated per year from the ammonia production industry [7]. it is urgent to find an environmentally friendly process for the ammonia industry. electrochemical synthesis of ammonia appears more and more attractive since renewable electricity is the energy source instead of fossil fuels [8-11]. different catalysts have been developed for nitrogen reduction [12-15]. however, there are still challenges for the electrochemical process to be applied in the industry [16]. a number of review papers have been published to discuss the problems and strategies in the electrochemical synthesis of ammonia [15,17-19]. solid-state electrochemical synthesis of ammonia has been reported [8,20-22]. h2 was used as a precursor for ammonia synthesis in these reports, despite the problems associated with its production, purification, transportation and storage [6,23]. ammonia synthesis directly from h2o and n2 will bypass the hydrogen production stage, which can help reduce the carbon emission in h2 production when fossil fuel is used as energy resources [24]. it has been demonstrated that ammonia can be synthesised from h2o and n2 in electrolytic cells based on proton (h+) or oxygen ion (o2−) conducting electrolytes and different working electrodes (cathodes) [25-28]. the principle of the electrochemical synthesis of ammonia from h2o and n2 using oxygen-ion, and conducting electrolytes is presented in eqs. (1-3) [25]. at the cathode, 3h2o + n2 + 6e→ 3o2+ 2nh3 (1) at the anode, oxygen ions are converted to gaseous oxygen, 3o2→ 3/2o2 + 6e(2) the overall reaction is: 3h2o + n2 → -2nh3 + 3/2o2 (3) perovskite-type oxides are generally noted as abo3, where a is a rare-earth or alkaline earth element (e.g., la, sr, ba) and b is a transition metal element (e.g., fe, cr, co). these oxides are lowcost, easily synthesized, with high thermal stability and good catalytic activity [28-30]. perovskitebased catalysts have been applied in solid oxide fuel cells (sofcs) [31,32], solid oxide steam electrolysis cells (soecs) [33,34], direct carbon fuel cells (dcfcs) [35], and electrochemical synthesis of ammonia [28,36-38]. in the literature, it has been reported that the catalytic activities of perovskite oxides were improved by the a-site doping strategy [39-41]. by a-site doping, oxygen vacancies will be generated [39]. it was also reported that oxygen vacancies at the cathode improve the catalytic activity of ammonia synthesis [42]. in the previous study [43], a-site sr-doped perovskite cathode la0.6sr0.4fe0.8cu0.2o3-δ (lsfcu) was successfully employed for the electrochemical synthesis of ammonia from water and nitrogen. alkaline earth elements, such as barium, were found to be an efficient promoters for ammonia synthesis catalysts [44-47]. it was reported that ba-promoted iron-cobalt alloys and ba-promoted cobalt catalysts supported on magnesium-lanthanum mixed oxide catalysts showed significantly higher activity for ammonia synthesis [46,47]. based on this discussion, it is anticipated that a-site ba-substituted perovskite oxide with the form la0.6ba0.4fe0.8cu0.2o3-δ (lbfcu) may have good electrocatalytic activity for ammonia synthesis. to the best of our knowledge, the electrosynthesis of ammonia using lbfcu as an electrocatalyst (cathode) has not been reported yet. thus, in this paper electrocatalyst activity of lbfcu for ammonia synthesis from water and nitrogen is explored. i. a. amar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 3 experimental materials synthesis la0.6ba0.4fe0.8cu0.2o3-δ (lbfcu) was synthesised via a combined edta-citrate complexing sol-gel process [48]. lanthanum oxide (la2o3, alfa aesar, 99 %), barium nitrate (ba(no3)2, alfa aesar, 99 %) (fe(no3)3·9h2o, alfa aesar, 98 %) and cobalt nitrate (co(no3)2·6h2o, sigma aldrich, 98 %) were used. la2o3 was dissolved in diluted nitric acid to form a lanthanum nitrate solution. calculated amounts of ba(no3)2, fe(no3)3·9h2o and co(no3)2·6h2o were dissolved in deionised water, and then added to the lanthanum nitrate solution. citric acid and edta (ethylene-diamine-tetra-acetic acid) were added as complexing agents, with a ratio of citric acid: edta: metal cations of 1.5:1:1. dilute ammonia solution was added to adjust the ph to  6. on a hot plate, water in the mixture was gradually vaporized under heating and stirring. the resultant powder was ground and subsequently calcined in air at 900 °c for 2 h, at 5 °c min-1 heating/cooling rate, to obtain singlephase lbfcu. sm0.5sr0.5coo3-δ (ssco) catalyst, and gd and ca co-doped ceria ce0.8gd0.18ca0.02o2-δ (cgdc) powders were also synthesised via combined edta-citrate complexing sol-gel process. the composite electrolyte was prepared by mixing cgdc and ternary carbonate ((li/na/k)2co3) at a weight ratio of 70:30, as described elsewhere [27]. materials characterization x-ray diffraction (xrd) data were collected at room temperature using a panalytical x'pert pro diffractometer with ni-filtered cukα radiation ( = 0.15405 nm), using 40 kv and 40 ma, fitted with a x'celerator detector. absolute scans were recorded in 2 ranges 5-100°, at a step size of 0.0167°. the crystallite size (d) was estimated using sherrer's equation as follows:    = 0.9 cos d (4) where  is the x-ray wavelength,  bragg’s diffraction angle and  is the full width at half maximum (fwhm). the microstructures of the prepared catalyst and the cross-sectional area of the single cell were examined using a hitachi su6600 scanning electron microscope (sem). thermo-gravimetry and differential scanning calorimetry (tga/dsc) analyses were performed using a stanton redcroft sta/tgh series sta 1500, operating through a rheometric scientific system interface controlled by the software rsi orchestrator. the thermal behaviour of lbfcu ash was studied in flowing air at a flow rate of 50 ml min-1. the thermal behaviour of lbfcu (cathode) was investigated in n2 atmosphere from room temperature up to 500 °c at 10 °c min-1 heating/cooling rate. fabrication of the single cell for ammonia synthesis a tri-layer single cell was fabricated through the one-step dry-pressing method. the anode consisted of ssco, cgdc, and starch as a pore former, at a weight ratio of 61:26:13. the electrolyte was cgdc/(li/na/k)2co3 (70 : 30 wt.%). the cathode consisted of lbfcu, cgdc and starch, with a weight ratio of 61:26:13. the anode, electrolyte and cathode were fed into the die, layer by layer, with the aid of a sieve to ensure uniform powder distribution, and then uniaxially pressed at 121 mpa. the freshly made pellet was sintered in air at 700 °c for 2 h, at 2 °c min-1 heating/cooling rate. the active surface area of the cathode was 0.785 cm2. the silver paste was painted in a grid pattern on the electrode surface as a current collector. ag wires were used as output terminals. http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 synthesis of ammonia from water and nitrogen 4 ammonia synthesis the fabricated cell was sealed into a self-designed double-chamber reactor using ceramic paste (aremco, ceramabond 552). the electrolytic cell was constructed: air, ssco-cgdc | cgdc-carbonate | lbfcu-cgdc, 3 % h2o-n2. the cathode chamber was fed with 3 % h2o-n2 (boc). the water vapour (3 % h2o) was supplied to the cathode chamber by bubbling n2 through liquid water at 25 °c. the anode was exposed to air. the voltage was applied by a solartron 1287a electrochemical interface controlled by the software corrware/corrview. a constant voltage was applied to the cell. ammonia synthesised at the cathode chamber was absorbed in 20 ml diluted hcl (0.01 mol l-1). the concentration of nh4+ in the absorbed solution was analysed using ion selective electrode, ise (thermo scientific orion star a214). the rate of ammonia formation and faradaic efficiency were calculated as follows [49,50]: = + 4 3 + 4 nh nh nh c v r m ta (5) = 3 nh3 faradaic efficiency 100 fr j (6) where rnh3 is the rate of ammonia formation (mol s -1 cm-2), cnh4+ is the concentration of nh4 + (g ml1), v is the volume of hcl solution (20 ml), mnh4+ is the molar mass nh4 + (18 g mol-1), t / s is ammonia collection time, a is the cathode geometric surface area (0.785 cm2), f is the faradaic constant and j is the generated current density, ma cm-2, (taking into consideration the cathode geometric surface area). ac impedance spectroscopy (is) measurements were performed using a schlumberger solartron si 1250 analyser, coupled with a si 1287 electrochemical interface controlled by z-plot/z-view software. the in-situ ac impedance spectra of the electrolytic cell under open circuit conditions were recorded with 100 mv ac excitation signal amplitude over the frequency range from 65 khz to 0.01 hz. results and discussion thermal and xrd analysis thermal analysis (tga-dsc) of the corresponding ash indicates that the reaction completes at about 830 °c (figure 1a), therefore 900 °c was chosen as a firing temperature for the preparation of la0.6ba0.4fe0.8cu0.2o3-δ (lbfcu). when the ash was calcined in air at 900 °c for 2 h, single-phase perovskite oxide la0.6ba0.4fe0.8cu0.2o3-δ was obtained, and its x-ray diffraction pattern shows a typical cubic perovskite oxide structure. therefore, the space group pm-3m (221) was used as the starting model for rietveld refinement [51]. wyckoff sites assigned to la (and ba), fe (and cu) and o were 1a, 1b and 3d, respectively. rietveld refinements were carried out using the general structure analysis system (gsas) [52]. the experimental data, calculated profile and the difference between experimental and calculated profiles for la0.6ba0.4fe0.8cu0.2o3-δ are shown in figure 2. the final refined structural data and thermal factors are listed in table 1. the lattice parameters are given in table 2. the crystallite size of lbfcu is 32.30 nm, as estimated from sherrer's equation (4). figure 1b shows tga-dsc curves of lbfcu in the n2 atmosphere from room temperature up to 500 °c. from room temperature to 150 °c, weight losses of about 0.6 and 0.8 % were observed at 80 and 140 °c, corresponding to the likely losses of absorbed water and other gases. from 150 to 500 °c, a 1 % weight loss was observed, possibly due to the loss of lattice oxygen from the i. a. amar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 5 la0.6ba0.4fe0.8cu0.2o3-δ structure. figure 1b shows no obvious thermal effects in the dsc curve, indicating no phase transition or sample decomposition and no reaction between lbfcu and n2 in the measured temperature range. figure 2b is the xrd pattern of the lbfcu catalyst after thermal analysis in the n2 atmosphere. la0.6ba0.4fe0.8cu0.2o3-δ retains the same perovskite structure, indicating its thermal stability in n2. the refined lattice parameters are listed in tables 1 and 2. after thermal measurements, cell volume expanded from 0.06066 (1) to 0.06094 (1) nm3, indicating that the transition elements located at b-sites might have been slightly reduced. figure 1. (a) tga-dsc curves up to 500 °c of (a) lbfcu ash; (b) lbfcu catalyst in nitrogen a b figure 2. rietveld refinement plot of the xrd data of a: lbfcu calcined in air at 900 °c for 2 h and b: lbfcu after sta analysis in n2 at 500 °c. experimental (red), calculated (green) and the difference between experimental and and calculated profiles (purple) table 1. atomic positions and thermal factors after refinement for la0.6ba0.4fe0.8cu0.2o3-δ. atom x y z occupancy uiso × 100 lbfcu asha lbfcu catalystb la 0 0 0 0.6 1.50 (1) 1.48 (1) ba 0 0 0 0.4 1.50 (1) 1.48 (1) fe 0.5 0.5 0.5 0.8 1.69 (1) l.67 (1) cu 0.5 0.5 0.5 0.2 1.69 (1) l.67 (1) o 0.5 0.5 0 1 3.71 (1) 4.18 (1) albfcu ash calcined in air at 900 °c for 2 h and blbfcu catalyst after sta measurment in n2 at 500 °c http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 synthesis of ammonia from water and nitrogen 6 table 2. refinement parameters for la0.6ba0.4fe0.8cu0.2o3-δ. parameters lbfcu ash fired in air at 900 °c for 2 h lbfcu catalyst after sta analysis in n2 at 500 °c crystal system cubic cubic space group pm-3m (221) pm-3m (221) a = b=c / nm 0.39292 (2) 0.39351 v / nm3 0.06066 (1) 0.06094 (1) ρth / g cm-3 6.670 6.640 rp / % 3.80 5.95 wrp / % 4.96 7.76 χ2 1.310 1.191 sem analysis figure 3a shows the sem image of lbfcu powder calcined in air at 900 °c for 2 h, showing a microporous structure with fine agglomerated particles of different sizes and shapes. figure 3b is an sem micrograph of the cross-sectional area of a single cell (before testing) sintered in air at 700 °c for 2 h. the cell consists of an ssco-cgdc anode, cgdc-(li/na/k)2co3 electrolyte and lbfcucgdc cathode. figure 3b suggests a good adhesion between the dense electrolyte layer and both porous electrodes, indicating good contacts between them, and that cgdc-carbonate composite is compatible with both ssco-cgdc and lbfcu-cgdc. figure 3. sem images of (a) lbfcu calcined in air at 900 °c; (b) cross-sectional area of the single cell before the ammonia synthesis synthesis of ammonia at different temperatures figure 4 is the electrolytic cell performance during ammonia synthesis at 1.4 v over a period of 30 min operated at different temperatures (375-450 °c). the performance is stable under all operating temperatures. the current densities increase with the increase of operating temperature, a maximum value of about 33 ma cm-2 was achieved at 450 °c. the increase of current density at higher temperatures could be attributed to the increase of the ionic conductivity of electrolyte and the decrease of electrode polarisation resistance [20]. in-situ ac impedance spectra of the electrolytic cell under open circuit conditions at different temperatures (375-450 °c) are shown in figure 5. the impedance spectra consist of one rather small and depressed semicircle at high frequencies and an almost straight line at low frequencies, suggesting at least two electrode processes. figure 5 also shows that the ohmic resistance (rohm) of electrolyte, seen as the high-frequency intersection of the depressed semicircle with the real i. a. amar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 7 impedance (z’) axis, decreased significantly with increased temperature and reached the lowest value of 5.86 ω at 450 °c (inset of figure 5). another words, the ionic conductivity of the electrolyte increased at higher temperatures. when the cell operating temperature is increased, there was also a significant decrease in the polarisation resistance (rp), seen as a diameter of a depressed semicircle, with the lowest value of 0.92 ω attained at 450 °c, due to the higher activity of both composite electrodes (cathode and anode) at higher temperature [27]. figure 4. electrolytic cell performance stability at 1.4 v and 375-450 °c. the electrolytic cell: air, sscocgdc|cgdc-carbonate|lbfcu-cgdc, 3% h2o-n2 figure 5. impedance spectra of the electrolytic cell containing lbfcu-cgdc cathode in cgdc-carbonate electrolyte, under open circuit conditions, at different temperatures (375-450 °c) the rate of ammonia formation was investigated at different temperatures from 375 to 450 °c, with a constant applied voltage of 1.4 v, as shown in figure 6. the maximum ammonia formation rate of 4.0×10-11 mol s-1 cm-2 was obtained at 400 °c, with the generated current density of 19 ma cm-2 (figure 4) and a faraday efficiency of 0.06 % (figure 6). this ammonia formation rate is comparable with that of a-site sr-doped (la0.6sr0.4fe0.8cu0.2o3-δ, lsfcu) perovskite cathode (5.0×10-11 mol s-1 cm-2 at 400 °c) [43]. this increase in the ammonia production rate from 375 to 400 °c could be ascribed to http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 synthesis of ammonia from water and nitrogen 8 the higher oxygen-ion conductivity of the composite electrolyte [20]. however, when the operating temperature was further increased, the ammonia formation decreased significantly and reached the lowest value at 450 °c. this could be attributed to ammonia decomposition at higher tempera tures [21,27]. figure 6. dependence of the rate of ammonia formation on the operating temperature synthesis of ammonia at different applied voltages to investigate the effect of the applied potential on the formation rate of ammonia, the cell operating temperature at a constant value (400 °c) and different voltage from 1.2 to 1.8 v was applied. figure 7 shows the cell performance during 30 min of ammonia synthesis processes at different applied voltages (1.2-1.8 v) at 400 °c. the electrolytic cell performances were stable. the current density increased with increased applied voltage from 1.2 to 1.4 v, indicating that more oxygen ions were transported through the electrolyte to the anode. however, when the applied voltage was increased above 1.4 v, the current decreased, with the lowest value obtained at 1.8 v, which means the transportation of o2− through the electrolyte to the anode becomes more and more difficult at voltages above 1.4 v. this could be explained by the blocking effect of li+, na+ and k+ ions. under the electric field, these positively charged ions will move and accumulate at the cathode/electrolyte interface to form a layer that will partially block the transfer of oxygen ions (o2−) and result in lower current densities [27,53]. figure 7. electrolytic cell performance stability at 400 °c and 1.2-1.8 v. the electrolytic cell: air, sscocgdc|cgdc-carbonate|lbfcu-cgdc, 3% h2o-n2 i. a. amar j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 9 the formation rates of ammonia and faradaic efficiency at different voltages are presented in figure 8. the same trend for current densities was observed for formation rates and faradaic efficiencies. from 1.2 to 1.4 v, the formation rate of ammonia increased with the increase of the applied voltage and reached a maximum value of 4.0×10-11 mol s-1 cm-2 at 1.4 v, with a corresponding current density of 19 ma cm-2 and a faradaic efficiency of 0.06 %. when the applied voltage was further increased above 1.4 v, the ammonia production rate decreased significantly and reached its lowest value of 1.0×10-11 mol s-1 cm-2 at 1.8 v, corresponding to a current density of 8.68 ma cm-2 and a faradaic efficiency of 0.03 %. low ammonia formation rates with corresponding low faradaic efficiencies at higher voltages indicate that more than one process occurs at the cathode, and hydrogen evolution might be the predominant [26,49,54]. the formation rate of ammonia achieved using lbfcu is still higher than that reported using a ru-based catalyst and water and nitrogen as precursors to synthesise ammonia (3.75 ×10-13 mol s-1 cm-2 at 650 °c) [25]. this could be due to the higher ionic conductivity of the cgdc-carbonate composite electrolyte compared to ysz and the low operating temperature applied in that research (400 °c) that reduced associated ammonia decomposition. figure 8. dependence of the rate of ammonia formation on the applied voltage at 400 °c conclusions in summary, the electrocatalytic activity of a-site ba-substituted perovskite cathode (la0.6ba0.4fe0.8cu0.2o3-δ, lbfcu) for ammonia synthesis from water and nitrogen was successfully explored. the xrd results demonstrated that a single-phase perovskite oxide (lbfcu) with a cubic structure was obtained. ammonia was successfully synthesised from water and nitrogen under atmospheric pressure using lbfcu-ce0.8gd0.18ca0.02o2-δ (cgdc) composite as a cathode, and cgdccarbonate as composite solid electrolyte. the maximum rate of ammonia production was found to be 4.0×10-11 mol s-1 cm-2 at 400 °c and 1.4 v. the results indicated that a lower operating temperature is favourable to reducing ammonia decomposition, which at the same time, requests higher electrolyte conductivity and higher catalytic activity of the cathode. the results revealed that the catalytic activity of the proposed cathode catalyst (lbfcu) towards ammonia synthesis formation is comparable to that of a-site sr-substituted perovskite cathode (la0.6sr0.4fe0.8cu0.2o3-δ). according to http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 synthesis of ammonia from water and nitrogen 10 the findings, the prepared ba-containing electrocatalyst (lbfcu) may be considered as a promising candidate material for carbon-free ammonia synthesis. acknowledgements: the author would like to thank professor shanwen tao from the university of warwick in coventry, uk, for providing the ammonia synthesis equipment. the author would also like to thank dr. rong lan of coventry university in coventry, uk, for her assistance and encouragement. the authors would also like to thank strathclyde university's department of pure and applied chemistry in glasgow, uk, for the xrd and sem analysis. conflict of interest authors declare that there is no conflict of interest related to publishing 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[53] r. lan, s. tao, electrochemical synthesis of ammonia directly from air and water using a li+/h+/nh4+ mixed conducting electrolyte, rsc advances 3 (2013) 18016-18021. https://doi.org/10.1039/c3ra43432j [54] v. kordali, g. kyriacou, c. lambrou, electrochemical synthesis of ammonia at atmospheric pressure and low temperature in a solid polymer electrolyte cell, chemical communications (2000) 1673-1674. https://doi.org/10.1039/b004885m ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1535 https://doi.org/10.1149/2.021406jes https://doi.org/10.1016/j.jcat.2007.03.023 https://doi.org/10.1016/j.jcat.2009.04.024 https://doi.org/10.1039/d1ra01584b https://doi.org/10.1016/s0021-9517(02)00182-3 https://doi.org/10.1016/j.jpowsour.2010.11.017 https://doi.org/10.1016/j.ssi.2019.115010 https://doi.org/10.1016/j.jclepro.2019.119525 https://doi.org/10.1016/0022-4596(95)80040-v https://doi.org/10.1039/c3ra43432j https://doi.org/10.1039/b004885m https://creativecommons.org/licenses/by/4.0/) {cerium-based conversion films developed on lial-layered double hydroxide coatings for corrosion protection of aa7075 aluminum alloys:} http://dx.doi.org/10.5599/jese.1305 703 j. electrochem. sci. eng. 12(4) (2022) 703-719; http://dx.doi.org/10.5599/jese.1305 open access : : issn 1847-9286 www.jese-online.org original scientific paper cerium-based conversion films developed on lial-layered double hydroxide coatings for corrosion protection of aa7075 aluminum alloys aurora petica1, adrian cristian manea1, geanina mihai1, ioana nicola2, victoria ceara2 and liana anicai1, 1center of surface science and nanotechnology, university politehnica of bucharest, splaiul independentei 313, 060042 bucharest, romania 2icpe-sa, splaiul unirii 313, 030138, bucharest, romania corresponding author: lanicai@itcnet.ro; tel.: +40 21 4029100; fax: +40 21 3181001 received: febrzary 18, 2022; accepted: june 18, 2022; published: july 18, 2022 abstract the paper presents several experimental results regarding the influence of various procedures applied to incorporate ce species in hydrotalcite–type chemical conversion coatings on the overall corrosion performance for aa 7075 aluminum alloy as a metallic substrate. two routes were envisaged: (i) chemical incorporation of ce by immersion in cerium nitrate solutions and (ii) electrochemical deposition of hydrophobic ce based layer involving ethanolic solutions of stearic acid and cerium nitrate. the chemical route involving immersion in cerium nitrate solutions led to the incorporation of 2.46 – 6.84 wt.% ce in the composition of the porous conversion layer. the electrochemical process facilitated the formation of a hydrophobic cerium stearate layer on porous hydrotalcite conversion coating showing water contact angles of about 132o and a higher ce content incorporated of 6.55 – 9.73 wt.%. the corrosion performance of the ce-based conversion coatings is also discussed. keywords conversion coatings; chromium-free conversion coatings; aluminum alloys; corrosion behavior; hydrophobic layers introduction the development of alternative surface pretreatments and pigmented coatings on al and its alloys to eliminate the health hazards and toxicity of soluble hexavalent chromium has been pursued for many years. especially in the aeronautical industry, the replacement or elimination of hexavalent chromium has been and still is a major challenge [1-5]. chromate-containing conversion coatings (ccs) are among the most developed and large-scale applied to protect al alloys with superior performance. http://dx.doi.org/10.5599/jese.1305 http://dx.doi.org/10.5599/jese.1305 http://www.jese-online.org/ mailto:lanicai@itcnet.ro j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 704 whilst many approaches have been attempted to achieve acceptable alternatives, relatively few systems have demonstrated similar effectiveness as chromate-containing systems [6,7]. a variety of investigations have been performed to find suitable candidates to replace chromates. ideally, these compounds should have a similar performance as chromates, i.e., providing corrosion protection to the substrate and serving as a reservoir of corrosion inhibitor, which has the ability to be released from the coating when damages occur and heal the defect by the formation of a protective or passivation layer. this leaching of soluble chemical species is the main corrosioninhibiting strategy used in corrosion-inhibiting coating technology for the protection of highstrength al alloys [8,9]. rare earth compounds are among the most promising options for replacing chromate conversion coatings on al [10,11]. such compounds have been found to act as cathodic inhibitors, including ce, sm, la, nd, pr, y, gd and tb. cerium compounds are generally the most active and the corrosion resistance usually follows the sequence: ce ˃ nd ˃ pr ˃ y ˃ la. with the exception of la, the other rare earth agents are effectively ruled out of widespread commercial use due to cost considerations. however, la-containing coating not only is less active than ce one but also shows poor adhesion as well. thus, usually, the term „rare earth” is to be considered to mean cerium [10 and included refs.]. cerium-based conversion coatings can be applied to al alloys by a large range of procedures, including electrolytic deposition, spray, swabbing and immersion. immersion processes may be either (i) unaccelerated or (ii) accelerated thermally or chemically [10,12,13 and included refs.]. most of the investigations have been focused on immersion processes because both electrodeposition and spray deposition were found to still have several hurdles facing them before being considered for a mainstream application. the use of rare-earth compounds as corrosion inhibitors has been proposed for the first time by hinton and his co-workers in australia [14,15], who involved ce salts in protecting al-based substrates, thus providing excellent resistance to localized corrosion. hughes et al. [16] investigated the generation of ceand mocontaining cc coatings on al alloy by a multistep process. in spite of excellent corrosion performance, the proposed procedures are unsuitable for a large-scale application due to the long period required (more than 24 h). therefore, for any practical purpose, an accelerated process should be used. usually, the deposition of cerium oxide/hydroxide-based coatings occurs at local cathodic sites, where the ph is increased, depending on the operation parameters, which should be rigorously controlled because they influence both the formation kinetics and their final protective characteristics [10,13,17]. application of post-coating seals is also recommendable [10,18]. the application of thinner coatings demands new strategies to extend their lifetime and improve their effectiveness. new synthesis routes and procedures have been reported in order to improve corrosion protection of al alloys while benefitting from the characteristics ce-based compounds may provide. under these circumstances, combinations of ce-based conversion coatings with layered double hydroxides (ldhs) or various types of sol-gel films have also been reported. ldh is a hydrotalcite-like compound, which is typically composed of positively-charged mixed metal mii-miii hydroxide layers and interlayers occupied by anions (ay-) and water molecules [19,20]. the pioneering work of buchheit et al. [21] showed that a coating consisting in a hydrotalcite (ht) layer (formed by immersion in a bath containing a mixture of lithium, potassium, and sodium salts), sealed by contact with ce solutions, exhibits self-healing behaviour probably because ce is introduced into the ht as a soluble highoxidation-state species. ldhs have often been employed as containers for corrosion inhibitors such as a. petica et al. j. electrochem. sci. eng. 12(4) (2022) 703-719 http://dx.doi.org/10.5599/jese.1305 705 rare earth ions to provide active corrosion protection for the metal substrates. in addition, visser et al. [22,23] recently showed that li salts are also of interest as an alternative corrosion inhibitor to protect al alloys after earlier reports of buchheit et al. [24,25] regarding the passivity of al in alkaline lithium salt solutions. it has also been shown that the lithium-leaching coatings also suppressed localized corrosion and provided additional corrosion resistance on more corrosion-resistant aluminium alloys such as aa5083‐h111 and aa6014‐t4, by the formation of a protective layer in the defect area and preventing local corrosion processes despite the different intrinsic electrochemical activity of the alloys [26]. fernandes et al. [27] showed that hydrotalcite (denoted ht-htc) based coatings formed onto aa 6061 alloy involving lithium nitrate solutions showed an increased corrosion performance compared to those formed in lithium carbonate ones. moreover, ce incorporation in the ht-htc layer further enhanced the metallic substrate's corrosion resistance. recently, zhang et al. [28] reported the synthesis of ce-doped zn-al ldhs and their incorporation into a hybrid sol-gel protective coating for aa2024. the sol-gel coating modified with ce-doped zn-al ldhs exhibited higher corrosion protection behavior compared with both unmodified and ce-undoped ldhs containing coatings. this proved the applicability of ce-doped ldhs in delaying coating degradation and their potential application as nanocontainers of corrosion inhibitors in self-healing coatings. chen et al. [29] prepared double-doped ldh films on aa2024, using ce(iii) and vanadate species as dopants through a two-step procedure involving: (i) the in-situ growth of ce-doped znal ldh with urea hydrolysis followed by (ii) the intercalation of v2o74by the anion-exchange process. the performed eis (electrochemical impedance spectroscopy) investigations involving the immersion in 0.05 m nacl solution for 15 days showed that the double incorporation of ce(iii) and v2o74significantly enhances the corrosion performance, exhibiting high impedance values of ≈106 ω cm2 order. however, the proposed procedure suffers from a quite long synthesis procedure (24 h for the first step followed by 2 h for the second one). iqbal and fedel [30] applied ce-doped mgal ldhs directly on the anodized aluminum surface and on the hot-water sealed anodized aluminum specimens at 80 °c for 18 h. according to eis investigations during immersion in 0.1 m nacl solution for 1200 h, it was found that the addition of ce ions facilitated improved corrosion protection, mainly when the growth of cemgal-ldhs has been developed on the anodized layer. to get more information on the role of trivalent cerium species present in ce-doped mgal ldhs, fedel and zampiccoli [31] developed the conversion layers directly on the 6082 aluminum alloy surface at 80 °c for preparation periods between 1 and 9 h. the analysis of the data acquired before and after 400 h of continuous immersion in the 0.1 m nacl, suggested a corrosion inhibition mechanism based on the dissolution and re-precipitation of ce compounds. in order to enhance the corrosion protection of aluminum alloys and inspired by nature, the development of highly hydrophobic/superhydrophobic surfaces has also been considered an attractive strategy. in particular, the specific morphology of ldh-based conversion layers might facilitate superhydrophobic characteristics upon modification with low-surface-energy chemicals [32]. in this regard, yang et al. [33] reported the preparation of li-al ldh coating on aa2099-t83 al-cu-li alloy, followed by 1h, 1h, 2h, 2hperfluorodecyl trimethoxysilane (pfdtms) modification. the modified layer showed superhydrophobic properties with a water contact angle of about 155° and superior corrosion performance materialized by a corrosion current decrease of two orders of magnitude. iqbal et al. [34] prepared superhydrophobic ce-doped mgal ldhs directly on the anodic surface of aa6082 alloy through modification by pfdts (1h, 1h, 2h, 2h perfluorododecyl trichlorosilane) through anion exchange reaction, exhibiting high water contact angle of about 156°, improved corrosion resistance and self-cleaning properties. while some superhydrophobic surface layers on http://dx.doi.org/10.5599/jese.1305 j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 706 various aluminum and aluminum alloys have been developed as an efficient barrier against corrosion, they mostly involve the use of fluorinated chemicals, which negatively affect the environment and human health. in this context, the use of fatty acids, including myristic acid (ch3-(ch2)12-cooh), stearic acid (ch3-(ch2)16-cooh), or decanoic acid (ch3–(ch2)8–cooh) could represent an attractive more environmentally friendly route [35]. iqbal et al. [36] used 0.01 m sodium stearate solution at 50 °c for 5 h to develop the superhydrophobic structure over ce doped mgal ldh grown directly on an anodized aa6082 surface, which exhibited a water contact angle of 155°. it has been shown that the obtained layers presented superior corrosion resistance characteristics in 0.1 m nacl, also benefitting from the synergic effect of cerium inhibitors and superhydrophobic surfaces. quite recently, zhang et al. [37,38] proposed an easier and quicker approach to fabricating superhydrophobic surfaces directly on pure al substrate, involving a one-step electrodeposition process and ethanolic solutions of myristic acid or stearic acid and cerium nitrate under constant voltage conditions. the electrochemically prepared superhydrophobic surfaces showed greatly enhanced corrosion resistance and good lasting quality under air exposure and 3.5 wt.% nacl immersion. considering all mentioned above, the present paper intends to explore the influence of various procedures applied to incorporate ce species in hydrotalcite–type chemical conversion coatings, on the overall corrosion performance, for aa 7075 aluminum alloy as a metallic substrate. two routes were envisaged: (i) chemical immersion in cerium nitrate solutions and (ii) electrochemical deposition of hydrophobic ce-based layer involving ethanolic solutions of stearic acid and cerium nitrate. in this context, it is worth mentioning that, to the best of our knowledge, no hydrophobic coating was reported on ht layers or aa 7075 aluminum alloy involving electrolytes based on ce salts and stearic acid. experimental in order to perform experiments, unpolished specimens of aa 7075 alloy (chemical composition shown in table 1) have been cut in 35×70×10 mm coupons and used as metallic substrate. table 1. chemical composition of aa 7075 content, wt.% si fe cu mn mg cr zn ti other al max. 0.40 max. 0.50 1.2-2.0 max. 0.30 2.1-2.9 0.18-0.28 5.1-6.1 max. 0.2 max. 0.15 rest before any process, the specimens have been firstly degreased in acetone for 30–60 s to remove surface contaminations. then, a chemical alkaline degreasing step was applied using an aqueous solution of 30–50 g/l na2co3∙10h2o + 30–50 g/l na3po4∙12h2o + 5 g/l naoh, for 5 min. at 60–80 °c, followed by a chemical desmutting in hno3 1:1 (vol.) solution containing 0.37 % cecl3 [39] for 30–60 s at room temperature, water rinsing and drying. the specimens have been rinsed in deionized water between each step, too. the hydrotalcite type conversion coatings (denoted ht) have been developed by immersion at a constant temperature of 90±5 °c for 20 min. in solutions containing a mixture of lithium and potassium salts, whose detailed composition is presented in table 2. the ht-coated surfaces were rinsed with distilled water and then subjected to the ce species incorporation step, either (i) by immersion in 0.05 m ce(no3)3∙6h2o solution for 20 min. (specimens denoted ht-ce) or (ii) by electrochemical deposition of hydrophobic ce-based layer in ethanolic solutions of 0.1 m ce(no3)3∙6h2o + 0.1 m stearic acid at a constant voltage (udc) of 20 v for 20 min., at room temperature (specimens a. petica et al. j. electrochem. sci. eng. 12(4) (2022) 703-719 http://dx.doi.org/10.5599/jese.1305 707 denoted ht-ce-s). in this case, a two-electrode cell configuration has been used, where platinized titanium plates acted as anode. after preparation, the specimens were rinsed with ethanol and dried using an air blower. table 2. chemical composition and applied operating parameters during conversion coatings preparation conversion type solution composition operating parameters ht 6.9 g l-1 lino3; 28.3 g l-1 kno3 2.4 g l-1 lioh; ph 12 12.5 t = 20 min t = 90 ± 5 oc ht-ce 0.05 m ce(no3)3∙6h2o t = 20 min t = 23 ± 2 oc ht-ce-s 0.1 m ce(no3)3∙6h2o 0.1 m stearic acid in ethanol udc(const) = 20 v t = 20 min; t 23 ± 2 oc scanning electron microscopy (sem) (hitachi su 8230, hitachi high-tech corporation, tokyo, japan) and energy dispersive x-ray spectroscopy (edx) (oxford instruments, oxford, uk) were used to analyze the surface morphology and the composition of the prepared conversion films. the investigation of the phase composition and structure of the prepared films was performed by x-ray diffractometry (xrd) (high resolution smartlab x-ray diffractometer rigaku, tokyo, japan 9 kw, with rotating anode) using cukα radiation ( = 0.15406 nm) at room temperature, in the 2 range of 5–90 degree. to get information about the surface composition, the fourier transform infrared (ftir) spectrometry (spectrum two ft-ir instrument equipped with universal diamond/krs-5 atr component, perkin elmer, usa) was used and the spectra were collected from 4000 to 450 cm−1. the surface wettability characteristics of the investigated specimens have been determined by measurements of the contact angles (cas) using a cam 100 compact angle meter at room temperature (24±2 °c). an average value of at least five measurements performed on different areas of the specimens was considered. the hydrophilic ( < 90°) or hydrophobic ( > 90°) character of the prepared surfaces has been assessed using deionized water as liquid. the assessment of corrosion behavior of the ht-ce and ht-ce-s conversion coatings has been performed involving accelerated laboratory tests consisting of continuous immersion in aerated 0.5 m nacl aqueous solution at 25 °c for 360 h, with intermediary visual examinations. minimum 3 pieces of each conversion coating type (70×35 mm) were subjected to an immersion test. potentiodynamic polarization curves at a sweeping rate of 1 mv s−1 and electrochemical impedance spectra (eis) at open-circuit potential using 0.5 m nacl solution were recorded against ag/agcl reference electrode and using a pt counter electrode. for both electrochemical investigations, the geometrical surface of the working electrode was 0.196 cm2. eis spectra, recorded with 10 mv a.c. voltage within 100 khz to 100 mhz frequency range, have been processed using zview 2.4 software from scribner association inc., derek johnson. all electrochemical investigations have been performed using a parstat 4000 potentiostat controlled with versastudio software. results and discussion in order to develop hydrotalcite type conversion coatings (ht), aa 7075 specimens have been immersed in conversion solution for 30 min. and the open circuit potential (uoc) during the time has been monitored to evidentiate the different stages of the formation as illustrated in figure 1 (black line). a sudden shift of the uoc occurs during the first 15 s, up to -1.04 v, followed by a further displacement in the cathodic direction up to -1.11 v for about 10 min. of exposure. this is related to http://dx.doi.org/10.5599/jese.1305 j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 708 the metal dissolution in the strong alkaline solution, associated with h2 evolution from the surface. then, uoc started to increase as the conversion film forms, up to a quite stable value, assigned to thickening of the coating and gas evolution ceased. this gradual increase suggests a slow but continuous formation of the layer on the alloy surface. the general shape of uoc recording is relatively similar to those reported in [23,40] for aa2024-t3 and pure al substrates. figure 1. evolution of open circuit potential vs. conversion time for ht (black line) and ht-ce (blue line) during coatings formation during the next ce species incorporation step (see figure 1 blue line), a shift of uoc towards more positive values in time is noticed, assigned to the formation of a second protective layer. according to [41-43 and included references], the development of the cerium-based film on aluminum and its alloys using solutions of simple cerium salts with no presence of an oxidizing agent could occur through the participation of the dissolved oxygen in the solution. therefore, the reduction of oxygen occurs (eq. 1), leading to ohions formation [41,43]: o2 + 2 h2o + 4 e→ 4 oh(1) further, the reaction of ce (iii) species with hydroxide ions could lead to the formation of protective cerium oxide (eq. 2) that covers the surface, also facilitated by the alkaline ph of ht coating [43]: 2 ce3+ + 4 oh+ o2 + 2 e→ 2 ceo2 + 2 h2o (2) sem micrographs of the developed ht-ce coating onto aa 7075 alloy are presented in figure 2, at various magnifications. figure 2. sem micrographs at different magnifications in the case of ht-ce layer developed onto aa 7075 alloy:(a) 3000×; (b) 25000× and (c) 50000× 0 200 400 600 800 1000 1200 1400 1600 1800 2000 -1.10 -1.05 -1.00 -0.95 -0.90 u o c / v v s . a g /a g c l re f. time , s -0.6 -0.5 -0.4 -0.3 u o c / v v s . a g /a g c l re f. ht ht-ce a. petica et al. j. electrochem. sci. eng. 12(4) (2022) 703-719 http://dx.doi.org/10.5599/jese.1305 709 a porous morphology could be noticed, entirely covering the metallic surface. at higher magnification (figure 2b and 2c), a nest-like structure is revealed, composed of curved blade-like formations of about 10–20 nm in width and a length of 200-300 nm. besides the presence of aluminum, oxygen, as well as of mg, and cu as alloying elements, edx analysis evidenced the presence of 2.46 –6.84 wt.% ce in the composition of ht-ce layer. figure 3 presents an example of the recorded sem micrographs at various magnifications for the ht-ce-s coatings obtained after the electrochemical step in cerium-stearic acid ethanolic solution, applying a constant voltage of 20 v for 20 min. figure 3. sem micrographs at different magnifications in the case of ht-ce-s layer developed onto aa 7075 alloy: (a) 5000×; (b) 25000× and (c) 50000× as shown in figure 3, a better profiled curved platelet structure developed perpendicular to the substrate is observed, also exhibiting a certain level of disorder. traditionally, electrochemical deposition could be seen as a process occurring at the atomic/molecular level so that the formed layer entirely takes the three-dimensional shape of the substrate with very high accuracy [44]. therefore, the applied electrochemical step on the already formed ht coating led to the formation of a quite similar morphology (not shown here), however, with much more well-defined features. according to edx measurements, the ce content within the ht-ce-s layer was higher as compared to ht-ce ones, in the range 6.55–9.73 wt.%, suggesting an enhanced cerium ions insertion after the electrodeposition process. in addition, the presence of carbon, aluminum, oxygen, as well as of mg and cu as alloying elements has also been identified. the c/ce atomic ratio calculated from edx analysis was found to be between 49.5 (c/ce = 42.56:0.86) and 52.77 (c/ce = 27.44:0.52), which is relatively close to the theoretical value in cerium stearate, ce (ch3(ch2)16coo)3, i.e., c/ce = 54:1. in the employed cerium nitrate solution, ce3+ ions resulted from dissociated cerium nitrate hexahydrate, moving toward the cathode on applying the constant voltage. near the cathode, ce3+ ions react with stearate ions available as a result of the dissociation reaction of stearic acid, forming cerium stearate over the porous ht surface. the reduction of hydrogen ions at the cathode surface also occurs, leading to the formation and release of gaseous hydrogen, evidenced by the presence of bubbles. consequently, the following reactions can be formulated: ch3(ch2)16cooh ↔ ch3(ch2)16coo+ h+ (3) ce3+ + 3 ch3(ch2)16coo→ ce (ch3(ch2)16coo-)3 (4) 2 h+ + 2 e→ h2↑ (5) figure 4 presents the xrd patterns of ht-ce and ht-ce-s films developed on aa 7075 alloy. the characteristic peaks located at 38.4, 44.7, 64.9 and 78° were assigned to the al phase of aa 7075 alloy [45]. in addition, peaks occurring at around 10 and 20° suggest the formation of ldh http://dx.doi.org/10.5599/jese.1305 j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 710 intercalated with no3[28,29,45-48]. however, slight shifts of the diffraction peaks ascribed to (003) and (006) reflections suggest the cerium species intercalated in the hydrotalcite structure. figure 4. xrd patterns of: (a) ht-ce and (b) ht-ce-s films developed on 7075 al alloy the peak related to (003) reflection is displaced from 11.64 to 10.51°, while the peak corresponding to (006) reflection is repositioned from 23.40 to 19.76°. based on the bragg’s equation, the d-spacing value of the (003) reflection increased from 0.760 nm to 0.841 nm and the d-spacing value of the (006) reflection enlarged from 0.3801 nm to 0.4489 nm. the low intensity of (003) and (006) reflections suggests a decline of the ldh crystallinity. as similar xrd patterns were recorded both for ht-ce and ht-ce-s films, it could be considered that the type of procedure, i.e., either chemical or electrochemical, does not produce a significant change in the layer’s structure. ft-ir spectra of ht-ce and ht-ce-s films are presented in figure 5. figure 5. ftir/atr spectra related to (a) ht-ce and (b) ht-ce-s layers onto aa 7075 alloy the spectrum of the ht-ce layer (see figure 5a) exhibits a broad shoulder centered at about 3430 cm-1, assigned to o-h stretching vibration of the ldhs layer as well as to interlayer water molecules 0 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90 in te n s it y , a .u . 2 / o in te n s it y , a .u . a l (1 1 1 ) a l (2 0 0 ) a l (2 2 0 ) a l (3 1 1 ) (a) in te n s it y , a .u . (b) in te n s it y , a .u . a l (1 1 1 ) a l (2 0 0 ) a l (2 2 0 ) a l (3 1 1 ) 0 10 20 30 l d h ( 0 0 3 ) l d h ( 0 0 6 ) 0 10 20 30 l d h ( 0 0 3 ) l d h ( 0 0 6 ) 4000 3500 3000 2500 2000 1500 1000 500 4000 3500 3000 2500 2000 1500 1000 500 t / % ht-ce (a) wavenumber, cm-1 1630 530 1360 3430 t / % 700 ht-ce-s (b) 730 1360 530 3430 wave number / cm -1 3000 2900 2800 2700 t / % 2918 2849 a. petica et al. j. electrochem. sci. eng. 12(4) (2022) 703-719 http://dx.doi.org/10.5599/jese.1305 711 bonded to lial-oh groups [48,49]. the low-resolution adsorption peak is located at about 1630 cm1 was ascribed to the bending mode of water molecules, indicating a small amount of interlayer water in the lial-based ldh [50]. the adsorption band present at about 1360 cm-1 corresponds to the no3anions stretching vibration. the peaks located between 720 cm-1 and 530 cm-1 can be assigned to m=o and m-oh (m = al, li) lattice vibrations in ldh layers [48]. after electrochemical modification involving stearic acid-cerium nitrate ethanolic solutions, two additional peaks located at 2918 cm-1 and 2849 cm-1 are evidenced (see figure 5b), related to stearic acid and attributed to (-ch-) asymmetric and symmetric stretch vibration, respectively [38,51]. the above-mentioned results associated with the presence of ce and of the determined c/ce atomic ratio, as evidenced from edx analysis, suggest the formation of a cerium stearate (ce [(ch3(ch2)16coo]3) on the ht formed layer on aa 7075 alloy during electrochemical step under employed operation conditions. the surface wettability of the ht-ce and ht-ce-s layers has been assessed by water contact angle (ca) measurements. figure 6 comparatively shows the determined water cas of the bare 7075 alloy after surface preparation treatment and the two investigated conversion coatings, both initially and after continuous immersion in 0.5 m nacl solution for 360 h. as illustrated in figure 6, the bare 7075 alloys exhibited hydrophilic characteristics, with ca values around 31°. the presence of ht conversion layer decreased the ca up to ≈11° (not shown here), probably due to the presence of the hydrophilic mixed oxy-hydroxide. specimen type figure 6. contact angles of bare 7075 alloy after surface pretreatment, of ht-ce and ht-ce-s coatings: asprepared (dark blue) and after 360 h of continuous immersion in 0.5 m nacl (light blue) the chemical cerium insertion to produce ht-ce coatings does not produce a significant ca modification, the developed surface showing ca values of ≈24.5°, as the porous structure of pristine ht is dominant. on the contrary, the use of the electrodeposition step at constant voltage significantly modified the water ca, whose values are about 132°, suggesting hydrophobic characteristics. consequently, the resulted surface shows water repellency, leading to a potential enhancement of corrosion performance. after the continuous immersion test, the conversion coatings retained their initial characteristics, i.e. ht-ce-s layers retained their hydrophobicity and ht-ce remained hydrophilic. to evaluate the corrosion performance of the developed cerium containing conversion coatings, potentiodynamic polarization curves and electrochemical impedance spectra at open circuit c o n ta ct a n g le , o http://dx.doi.org/10.5599/jese.1305 j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 712 potential in a free-aerated 0.5 m nacl solution at room temperature have been recorded after various immersion periods. typical polarization plots in semilogarithmic coordinates corresponding to ht-ce and ht-ce-s conversion layers are presented in figure 7. for comparison, the same experiments have been carried out for untreated 7075 alloy. figure 7. polarization curves in semilogarithmic coordinates of bare 7075 alloy, ht-ce and ht-ce-s conversion layers in 0.5 m nacl for the initial and final moments of the continuous immersion test (25 °c, 1 mv s-1) as shown in figure 7, the curves recorded at the initial moment of immersion for ht-ce and htce-s conversion layers display a slight shift of the corrosion potential towards the electropositive direction, with about 60 and 80 mv, respectively, as compared to untreated alloy. no passive film was formed on a bare 7075 alloy electrode and metal dissolution could be noticed by the shape of the anodic branch of the polarization curve. a shift of the cathodic branch of the polarization curve to lower current densities could be noticed in the case of cerium-based conversion layers, more significant in the case of the hydrophobic ht-ces layer. this is due to the cathodic inhibition that cerium species may provide. as the ce content in ht-ce-s film was found to be usually higher as compared to the values detected in ht-ce one, this behavior is much more pronounced in the case of ht-ce-s coating. the tafel curves after 360 h of conditioning showed lower currents compared to the initial moment as a result of the protective action of ce species in the case of the ht-ce layer and also associated with its hydrophobic nature in the case of ht-ce-s one. moreover, as the layers exhibit certain porosity, as sem micrographs presented earlier evidenced, the additional formation of an aluminum mixed oxide/hydroxide passive film could be possible, which is a usual phenomenon in the case of immersion in aerated solutions. this adds supplementary protection, as well. table 3. values of corrosion parameters determined from polarization curves for bare 7075, ht-ce and ht-ce-s conversion coatings immersed in 0.5 m nacl substrate type initial after 360 h ecorr / v vs. ag/agcl jcorr / µa cm-2 ecorr / v vs. ag/agcl jcorr / µa cm-2 7075 untreated -0.601 11.6 -0.757 15.6 ht-ce -0.654 8.52 -0.716 5.23 ht-ce-s -0.634 2.01 -0.644 1.45 the corrosion current density, jcorr, and corrosion potential, ecorr, were determined by tafel extrapolation of both linear portions of cathodic and anodic polarization curves and their values for the investigated specimens are summarized in table 3. 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 7075 untreated ht-ce ht-ce-s i / a cm-2 e / v v s . a g /a g c l initial 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 7075 untreated ht-ce ht-ce-s i / a cm-2 e / v v s . a g /a g c l 360 h a. petica et al. j. electrochem. sci. eng. 12(4) (2022) 703-719 http://dx.doi.org/10.5599/jese.1305 713 as shown in table 3, ce-based conversion layers significantly improve the aluminum alloy's corrosion performance. thus, on the initial moment of immersion, values of corrosion current of about 11 µa cm-2 for bare alloy have been determined, which decreased up to ≈8.5 µa cm-2 and to ≈2 µa cm-2 for ht-ce and ht-ce-s, respectively. after 360 h of exposure, lower corrosion currents were evidenced in the case of ce-based conversion coatings, also associated with a positive shift of the corrosion potential. the increase of the jcorr in the case of the bare alloy is evidenced as a result of the continuation of the metal dissolution process. in addition, the lowest jcorr could be observed in the case of the hydrophobic ht-ce-s layer, which could be attributed to the synergic action of the presence of cerium species and of the hydrophobic nature of the coating, also associated with the presence of an aluminum mixed oxide/hydroxide passive film. the electrochemical impedance spectra of ht-ce and ht-ce-s conversion layers recorded at open circuit potential (ocp) in 0.5 m nacl solution for various immersion periods are presented in figs 8 and 9 as nyquist and bode plots. the proposed electrical equivalent circuits (eecs) to describe the corrosion behavior of ht-ce and ht-ce-s conversion films are shown in figure 10. a b 0 10000 20000 30000 40000 50000 0 10000 20000 30000 40000 50000 initial 24 h 48 h 96 h 240 h 360 h ht-ce -z im / w zre / w 10-1 100 101 102 103 104 105 101 102 103 104 105 106 initial 24 h 48 h 96 h 240 h 360 h f / hz ½ z ½ / w ht-ce 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 p h a s e a n g le , o figure 8. nyquist (a) and bode (b) plots for ht-ce conversion coating in 0.5 m nacl at open circuit potential after various continuous immersion periods (solid lines are the fit to the measured points using the electrical equivalent circuits shown in figure 10; swe = 0.196 cm 2) a b figure 9. nyquist (a) and bode (b) plots for ht-ce-s conversion coating in 0.5 m nacl at open circuit potential after various continuous immersion periods (solid lines are the fit to the measured points using the equivalent circuit shown in figure 10; swe = 0.196 cm 2) 0 20000 40000 60000 80000 100000 0 20000 40000 60000 80000 100000 initial 24 h 48 h 96 h 240 h 360 h -z im / w zre / w ht-ce-s 10-1 100 101 102 103 104 105 102 103 104 105 106 f / hz ½ z ½ / w initial 24 h 48 h 96 h 240 h 360 h 0 -20 -40 -60 -80 ht-ce-s p h a s e a n g le , o http://dx.doi.org/10.5599/jese.1305 j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 714 all nyquist diagrams show a semicircle arc in the relative high-frequency range. the diameter of the semicircles is usually related to the film resistance and may be correlated to the rate of corrosion: the larger the resistance, the lower the rate of corrosion. in the case of ht-ce conversion layers, two times constants are evidenced in the bode plot (see figure 8) from the initial moment of immersion up to the end of exposure, attributed to the porous ht layer possibly filled with cerium ionic species as well as to the corrosion process at the metallic substrate/ht interface [48,49,52]. aa7075 / ht-ce-s; 24, 48, 96, 240 and 360 h of immersion aa7075 / ht-ce, initial moment of immersion aa7075 / ht-ce, 24, 48, 96, 240 and 360 h of immersion aa7075 / ht-ce-s, initial moment of immersion figure 10. the electrical equivalent circuits used to fit the impedance spectra (figs. 8 and 9) recorded during 360 h of immersion in 0.5 m nacl solution. the significance of circuit components: rsol-ohmic resistance of solution; cdl-double layer capacitance; rct-charge transfer resistance; c(f) and r(f) – capacitance and ohmic resistance of the formed film for the ht-ce-s coated specimens, two time constants are noticed in the bode plot from figure 9. the time constant located in the high-frequency region could be related to the electrodeposited cerium stearate, while the second one from the lower frequency could be attributed to the porous ht layer. the values of the phase angle at the high frequency increase up to 240 h of exposure being also associated with higher impedance modulus, as an indicator of the barrier properties of the electrodeposited layer. this could be attributed to the deposition of cerium oxide/hydroxide at the external part of the ht porous structure, suggesting possible active corrosion protection. furthermore, up to 360 h of immersion, the phase angle value decreased both at the high-frequency and at the lower-frequency regions due to the damage of the electrodeposited layer containing cerium oxide/hydroxide and possibly due to the corrosion process as a result of the possible infiltration of water and chloride ions through the pores. in order to fit the experimental electrochemical impedance data and get more quantitative information on the corrosion performance of the investigated ce-based conversion layers, several electrical equivalent circuits have been tested and those showing the lowest chi-squared (i.e. 2 in the range 3×10-4 ÷ 6×10-3) and minimum error have been selected, as it was previously detailed in figure 10. during the fitting of the experimental data we used constant phase elements (cpes) instead of capacitances [28,48,53] in order to model more accurately the nonideal capacitive behavior of the electrode/electrolyte interface. rsol represents the ohmic resistance of the solution. c(f) and r(f) represent the capacitance and resistance of the formed conversion layer, which might be filled by cerium species, while cdl and rct stand for the double layer capacitance and charge transfer resistance at metal substrate. the evolution of r(f) and rct against the immersion period for ht-ce and ht-ce-s layers is shown in figure 11(a) and figure 11(b). r(f) shows a quite similar trend for both investigated conversion coatings. however, it should be noticed that r(f) values of ht-ce-s layers are about one order of magnitude higher than ht-ce ones. a. petica et al. j. electrochem. sci. eng. 12(4) (2022) 703-719 http://dx.doi.org/10.5599/jese.1305 715 a b c figure 11. evolution of: r(f) and rct for ht-ce (a) and ht-ce-s (b) as well as of rtotal (c) for the ce-based conversion layers against immersion duration in 0.5 m nacl correspondingly, rct values of ht-ce-s layers are larger than those of ht-ce ones. the rct values of ht-ce coated alloy increased gradually up to 96 h, then remained almost constant up to 240 h, suggesting certain corrosion protection during immersion. after 360 h of exposure, a slight decrease of rct occurs, suggesting the ingress of chloride ions and damaging the integrity of the layer. the rct values of ht-ce-s coated alloy are significantly larger as compared to ht-ce ones and increase faster. this could be attributed to the synergic effect of the higher content of cerium species which may enhance the active corrosion protection and of the non-wetting characteristics of the layer. indeed, on removing the specimens from the aggressive solution, the visual examination of ht-ce-s coated alloy specimens showed no surface modifications, while ht-ce coated alloy samples evidenced a number of corrosion spots. as different electrochemical parameters have been used to analyze the processes occurring at the conversion layer/metal interfaces, the total resistance rtotal was determined and its evolution against the immersion time is illustrated in figure 11c. rtotal includes the resistance of the conversion layer and the charge transfer resistance, i.e., rtotal = r(f) + rct and could represent a useful tool to assess the corrosion performance. as shown in figure 11c, the ht-ce-s conversion layers provide the best corrosion protection, evidenced by rtotal values of about two orders of magnitude higher than ht-ce ones. in addition, it can be noticed that rtotal progressively increased to 96 h and kept the relatively high values up to 240 h, indicating possible active corrosion protection provided by the cerium species as stearate. moreover, the occurrence of the hydrophobic characteristics prevents the chloride ions ingress. 0 24 48 72 96 120144168192216240264288312336360 100 101 102 103 104 105 r(f) rctht-ce immersion period, h r (f ), r c t / w c m 2 0 24 48 72 96 120144168192216240264288312336360 101 102 103 104 105 106 r (f ), r c t / w c m 2 r(f) rct ht-ce-s immersion period, h 0 24 48 72 96 120144168192216240264288312336360 102 103 104 105 106 r to ta l / w c m 2 immersion period, h ht-ce-s ht-ce http://dx.doi.org/10.5599/jese.1305 j. electrochem. sci. eng. 12(4) (2022) 703-719 ce-based films for corrosion protection 716 conclusions as a result of the performed investigations, several procedures applied to incorporate ce species in hydrotalcite–type chemical conversion coatings have been proposed and applied on aa 7075 aluminum alloy and their impact on the overall corrosion performance has been assessed. the chemical route involving immersion in cerium nitrate solutions produced a porous morphology, entirely covering the metallic surface, exhibiting hydrophilic characteristics. the edx analysis evidenced a content of 2.46 – 6.84 wt.% ce in the composition of ht-ce layer. the electrochemical process involving ethanolic solutions of stearic acid and cerium nitrate and an applied constant voltage led to the formation of a hydrophobic cerium stearate conversion layer on aporous ht conversion coating. a better profiled curved platelet structure developed perpendicular to the substrate was evidenced, also exhibiting a certain level of disorder. based on edx measurements, the ce content within the ht-ce-s layer was found to be higher, in the range 6.55 9.73 wt.%. the calculated c/ce atomic ratio was between 49.5 and 52.77, very close to the theoretical value in cerium stearate. the electrochemically produced ht-ce-s layers exhibited hydrophobic characteristics, materialized by water contact angles of about 132°. based on preliminary experimental results, the electrochemically prepared ht-ce-s conversion layers onto aa7075 alloy showed improved corrosion performance compared to chemically produced ht-ce ones, materialized in corrosion current densities of 1-2 µa cm-2 and total resistances of 340-385 kw cm2. this behavior could be attributed to the synergic effect of the higher content of cerium species which may enhance the active corrosion protection and the hydrophobic characteristics of the conversion layer. 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[48] k. lin, x. luo, x. pan, c. zhang, y. liu, applied surface science 463 (2019) 1085-1096. https://doi.org/10.1016/j.apsusc.2018.09.034 [49] c. zhang, x. luo, x. pan, l. liao, x. wu, y. liu, applied surface science 394 (2017) 275-281. https://doi.org/10.1016/j.apsusc.2016.10.034 [50] h. wang, g. fan, c. zheng, x. xiang, f. li, industrial & engineering chemical research, 49 (2010) 2759-2767. https://doi.org/10.1021/ie901519h [51] f. di franco, a. zaffora, p. vassallo, m. santamaria, journal of the electrochemical society 168 (2021) 101502. https://doi.org/10.1149/1945-7111/ac29df [52] m. a. iqbal, l. sun, a. t. barrett, m. fedel, coatings 10 (2020) 428. https://doi.org/10.3390/coatings10040428 [53] s. abirami, t. bharathidasan, s. sathiyanarayanan, corrosion 77(10) (2021) 1080-1099. https://doi.org/10.5006/3799 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1305 https://doi.org/10.1016/j.apsusc.2018.09.034 https://doi.org/10.1016/j.apsusc.2016.10.034 https://doi.org/10.1021/ie901519h https://doi.org/10.1149/1945-7111/ac29df https://doi.org/10.3390/coatings10040428 https://doi.org/10.5006/3799 https://creativecommons.org/licenses/by/4.0/) @article{petica2022, author = {petica, aurora and manea, adrian cristian and mihai, geanina and nicola, ioana and ceara, victoria and anicai, liana}, journal = {journal of electrochemical science and engineering}, title = {{cerium-based conversion films developed on lial-layered double hydroxide coatings for corrosion protection of aa7075 aluminum alloys:}}, year = {2022}, issn = {1847-9286}, month = {jul}, number = {4}, pages = {703--719}, volume = {12}, abstract = {the paper presents several experimental results regarding the influence of various procedures applied to incorporate ce species in hydrotalcite–type chemical conver­sion coatings on the overall corrosion performance for aa 7075 aluminum alloy as a metallic substrate. two routes were envisaged: (i) chemical incorporation of ce by immersion in cerium nitrate solutions and (ii) electrochemical deposition of hydro­phobic ce based layer involving ethanolic solutions of stearic acid and cerium nitrate. the chemical route involving immersion in cerium nitrate solutions led to the incorpo­ration of 2.46 – 6.84 wt.% ce in the composition of the porous conversion layer. the electrochemical process facilitated the formation of a hydrophobic cerium stearate layer on porous hydrotalcite conversion coating showing water contact angles of about 132o and a higher ce content incorporated of 6.55 – 9.73 wt.%. the corrosion performance of the ce-based conversion coatings is also discussed.}, doi = {10.5599/jese.1305}, file = {:d\:/onedrive/mendeley desktop/petica et al. 2022 cerium-based conversion films developed on lial-layered double hydroxide coatings for corrosion protection of aa7.pdf:pdf;:09_jese_1305.pdf:pdf}, keywords = {conversion coatings, aluminum alloys, chromium, corrosion behavior, free conversion coatings, hydrophobic layers}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1305}, } {electrochemical behaviour and voltammetric determination of p-phenylenediamine at carbon paste electrode} doi:10.5599/jese.386 111 j. electrochem. sci. eng. 7(3) (2017) 111-118; doi: http://dx.doi.org/10.5599/jese.386 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behaviour and voltammetric determination of p-phenylenediamine at carbon paste electrode abdelaziz ait sidi mou, aicha ouarzane, mama el rhazi laboratory of physical chemistry and bioorganic chemistry mohammedia (fstm), morocco corresponding author: e-mail: aitsidimou_aziz@yahoo.fr; tel.: +212-0675-251421 received: april 18, 2017; revised: june 26, 2017; accepted: july 26, 2017 abstract an electrochemical method was developed for voltammetric determination of p-phenylenediamine using a carbon paste electrode (cpe). the electrode exhibited the highest electrocatalytic activity toward oxidation and reduction of p-phenylenediamine in the phosphate buffer, ph 7. after optimizing the experimental conditions and square wave voltammetry parameters, a linear current response toward concentration of pphenylenediamine was obtained in the range of 0.12–3.00 µm with detection limit of 0.071 µm. the proposed procedure was successfully applied for determination of the total p-phenylenediamine content in the takeout extract sample. keywords p-phenylenediamine; electroanalysis; voltammetry introduction p-phenylenediamine (p-pd) is an aromatic diamine and derivative of the aniline with the chemical structure presented in figure 1. it is commonly used in the manufacture of dyestuffs, as hair dye and as adjuvant of henna [1,2], in several industries, dyeing furs, photochemical processes, cosmetic and fabrication of household goods [3,4]. toxic effects of p-pd, however, have also been reported by several authors [5-8]. many analytical methods, including gas chromatography coupled mass spectrometry (gc-ms) [9,10], gas chromatography (gc) [11], and high-performance liquid chromatography (hplc) [2] have already been reported for the determination of p-pd. although chromatographic methods are sensitive and reliable, they have some disadvantages such as being time and labour consuming, expensive and demanding for sample pre-treatments and qualified personnel. promising alternatives in this regard have been found in the electroanalytical methods which can offer high sensitivity, rapid response, easy operation and low cost [12]. http://dx.doi.org/10.5599/jese.386 http://www.jese-online.org/ j. electrochem. sci. eng. 7(3) (2017) 111-118 determination of p-phenylenediamine 112 though the electrochemical properties of p-pd have been investigated at glassy carbon [13,14] and carbon paste electrodes [15-18], most of these studies were mainly concerned on the mechanism of p-pd polymerization. although several studies on the detection of some similar compounds in medical plants [19,20] and drugs [21,22] were already published, the direct electroanalytical determination of p-pd on carbon paste electrode has not been reported previously. in this paper, we report on the electrochemical properties of p-pd on the surface of carbon paste electrode and this electrode will also be applied for the determination of p-pd content in the plant of takeout. figure 1. chemical structure of p-phenylenediamine experimental apparatus electrochemical experiments were carried out with an autolab (metrohm-autolab, utrecht netherlands) pgstat302n potentiostat/galvanostat controlled by gpes 4.9 software. a threeelectrode electrochemical cell was employed for all electrochemical measurements. carbon paste electrode (4 mm diameter) served as the working electrode, a platinum plate as the counter electrode, and ag/agcl (3 mol/l kcl) as the reference electrode, respectively. the ph of the buffer solutions was measured using a fisher scientific accumet ab15 basic ph-meter. reagents all chemicals used were of analytical grade. p p-pd and graphite were purchased from sigma and fluka, respectively. sodium hydroxide, naoh, na2hpo42h2o and nah2po42h2o powders were purchased from riedel-de haën. all aqueous solutions were prepared in distilled water. the ph was adjusted with naoh. a p-pd stock solution of 1 mm was prepared by dissolving p-pd in the phosphate buffer solution (pbs) before use in measurements. preparation of the carbon paste electrode the carbon paste electrode (cpe) was prepared by mixing 1 g of graphite powder and 0.3 ml of paraffin oil using a mortar and pestle until a homogenous paste was obtained. the paste was then incorporated into the electrode cavity (4 mm diameter) and polished by a smooth paper. prior each measurement, the electrode surface was renewed. sample preparation takeout plant (tamarix aphylla) samples were obtained from zagora region, morocco. the samples were firstly grounded by a crusher, then 1 g of the powder was weighed, dissolved in 50 ml of ethanol, and placed in a mounting reflux for 2 h. after filtration, the solvent was evaporated by a rotavapor and an amount of the filtrate (108 mg) was dissolved directly in 100 ml of 0.1 m phosphate buffer solution (pbs), ph 7.0 for the final determination of the p-pd content. nh 2 nh 2 a. ait sidi mou et al. j. electrochem. sci. eng. 7(3) (2017) 111-118 doi:10.5599/jese.386 113 analytical procedure the electrochemical cell containing 1 mm of p-pd dissolved in 0.1 m pbs, ph 7.0 was prepared. after that, the cpe electrode was placed in the test solution and the cyclic voltammetry was applied between – 0.2 and 0.8 v at a scan rate of 50 mv s-1. the parameters of swv of all measurements were, step potential 5 mv, amplitude 50 mv and frequency 50 hz. the calibration curve of p-pd concentrations was illustrated by swv in the range of 0.12–3.0 µm under the optimized conditions, and then used for directly determination to the amount of p-pd in the sample of takeout extracts. results and discussion electrochemical behaviour of p-phenylenediamine the electrochemical behaviour of 1 mm p-pd on a cpe in the 0.1 m, ph 7.0 pbs was investigated using cyclic voltammetry at a scan rate of 50 mv s−1. whilst not any response is observed for the cpe in the blank solution (fig. 2a), a pair of well-defined redox waves are observed in presence of 1 mm p-pd in 0.1 m, ph 7 pbs (fig. 2b). the anodic, epa, and cathodic, epc, potentials are situated at epa = 0.10 v and epc = 0.03 v, respectively. in general, presence of a peak in the reverse potential scan indicates reversibility of the oxidation process on the surface of the electrode. figure 2. cyclic voltammograms of cpe recorded at 50 mv s−1 in blank (a) and for 1 mm p-pd in 0.1 m, ph 7.0 pbs (b). the oxidation mechanism of p-phenylenediamine on the electrode surface the redox reaction of p-pd belongs to a class of two-electron and two-proton processes [23]. possible oxidation mechanism of p-pd is shown in the scheme 1. after formation of radical cation from the amino group, electrons are delocalized over the cycle forming another cation from the second amino group, while by elimination of two protons, the oxidized p-pd is formed. the mesomeric effect of nitrogen atom plays an important role, and electron density increases at π-bond and hence oxidation becomes easy. j. electrochem. sci. eng. 7(3) (2017) 111-118 determination of p-phenylenediamine 114 scheme 1. the oxidation mechanism of p-phenylenediamine effect of buffer ph the effect of ph of the pbs on the electrochemical behaviour of p-pd was studied in the ph range of 3.0–10.0. fig. 3 shows that as ph value of the solution was increased, the redox peak is shifted negatively what indicates involvement of protons in the redox reaction [24]. depending on a ph ranging from 3 to 7, the formal potential (eo) for the anodic and cathodic peaks changed linearly. the redox current values increased from ph 3.0 and reached a maximum at ph 7.0 (curve e in fig. 3). therefore, the pbs, ph 7.0 was selected as the optimum working solution which will offer a higher peak resolution of p-pd and relatively higher current response. -0.2 0.0 0.2 0.4 0.6 0.8 -20 0 20 40 60 80 g f e d c b a i / µ a e / v vs. ag/agcl figure 3. cyclic voltammograms of cpe recorded at 50 mv s−1 for 1 mm p-pd in pbs having ph 310: a 3, b 4, c 5, d -d 6, e 7, f 8, g 10. effect of potential scan rate fig. 4 shows the cyclic voltammograms of p-pd at cpe in the pbs, ph 7.0, recorded at the scan rates varied from 20 to 400 mv s-1. as is seen in the inset of fig. 4, the redox peak currents at the cpe in the solution containing p-pd increased linearly with the scan rate in the range from 50 to 250 mv s−1, what indicates the adsorption-controlled redox process. the linear regression equations can be defined as: ipa / µa = 71.76 + 0.36 υ (mv s −1) (r2 = 0.995) ipc / µa = 18.30  0.37 υ (mv s −1) (r2 = 0.996) n h 2 n h 2 n h n h n h 2 n h + +2e+ 2h+ a. ait sidi mou et al. j. electrochem. sci. eng. 7(3) (2017) 111-118 doi:10.5599/jese.386 115 -0.2 0.0 0.2 0.4 0.6 0.8 -100 -50 0 50 100 150 200 50 100 150 200 250 80 90 100 110 120 130 140 150 160 170 i p a / µ a v/ mv s -1 50 100 150 200 250 -80 -70 -60 -50 -40 -30 -20 -10 0 v/ mv s -1 i p c / µ a 400 mv s -1 20 mv s -1 i / µ a e / v vs. ag/ agcl figure 4. cyclic voltammograms of cpe for 1 mm p-pd in pbs, ph 7.0, recorded at scan rates (υ) of 20, 50, 80, 100, 150, 200, 250, 300, 350 and 400 mv s−1. inset: plots of peak currents (ipa and ipc) vs. scan rate (υ) in the range from 50 to 250 mv s−1. effect of the square wave voltammetry (swv) parameters and accumulation conditions the effect of the accumulation potential in swv measurements was investigated between 0.2 v and 0.1 v at the accumulation time of 30 s. as shown in fig. 5a, the peak current values decreased significantly when the accumulation potential was shifted to potentials more positive than 0.1 v, because the p-pd adsorbed on the electrode becomes oxidized. therefore, 0.1 v was used as the accumulation potential. similarly, the effect of accumulation time was studied in the 30–240 s range at the accumulation potential of 0.1 v. fig. 5b shows firstly a rapid increase in peak current values with increase of the accumulation time, then a maximum is reached at 120 s and after that, the constant values obtained indicate the saturation of the electrode surface by adsorbed layer. therefore, 120 s was used as the proper accumulation time. -0.20 -0.15 -0.10 -0.05 0.00 0.05 0.10 200 250 300 350 400 450 500 550 600(a) i p a / µ a e acc / v 0 50 100 150 200 250 560 580 600 620 640 660 (b) t acc / s i p a / µ a figure 5. a effect of accumulation potential (0.2 v to 0.1 v) to ipa values of swv responses measured for the accumulation time of 30 s; b effect of accumulation time (30 s to 240 s) to ipa values of swv responses measured at the accumulation potential of – 0.1 v. calibration curve the relationship between the peak current and concentration of p-pd was studied at the cpe by using swv under the optimized conditions. fig. 6 shows that the peak current values were linearly υ / mv s-1 υ / mv s-1 i p a /  a i p c /  a j. electrochem. sci. eng. 7(3) (2017) 111-118 determination of p-phenylenediamine 116 related to the concentration of p-pd in the range of 0.12–3.0 µm. the linear regression equation was defined as: ipa / µa = 0.181 + 0.241 c / µm, with the correlation coefficient of 0.999. the detection limit calculated as (3σ/p) was 0.071 µm, where σ and p are the standard deviation of the blank and the slope of the calibration graph, respectively. the relative standard deviation (rsd) for repetitive measurements of 1.5 µm p-pd was found to be 3.49 % (five replicates), suggesting that the electrode demonstrated excellent repeatability. -0.2 0.0 0.2 0.4 0.6 0.8 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 r 2 = 0,999 i pa / µa= 0,181+ 0,241 c ppd / µm i p a / µ a c p-pd / µm 0.12 m 3 m i p a / µ a e / v vs. ag/agcl figure 6. swv responses of cpe in 0.1 m pbs, ph 7.0 and various concentrations of p-pd in the range 0.12–3.0 µm. inset: linear calibration curve of peak current (ipa) versus p-pd concentration interference study to evaluate possible interference of some foreign species on the determination of p-pd, a systematic study was carried out. data listed in table 1 show the p-pd peak current ratios in the presence and absence of respective interferents. it seems that presence of 1.5 µm mg2+, k+, na+ and zn2+ did not affect the determination of p-pd. on the other hand, presence of 1.5 µm cu2+ showed a slight interference. in general, despite the high redox potential used for p-pd detection, the interference study showed promising results for its selective determination. table 1. effect of some foreign species on the peak current response of 0.3 µm p-pd in pbs, ph 7 at a fivefold concentration of interferent cations. interfering compound peak current ratio mg2+ zn2+ k+ na+ cu2+ 1.013 0.760 1.050 1.050 1.330 analytical application the cpe was used to determine the amount of p-pd in the takeout extracts. the results are shown in table 2. recovery measurements were made by adding known amounts of the standard p-pd solution to the previously analyzed samples. the recovery for determination of p-pd was 110 %. the recovery indicated that the accuracy of the proposed method is acceptable. i p a /  a 0.0 1.0 2.0 3.0 cp-pd / m 1.0 0.8 0.6 0.4 0.2 cppd / m ipa / ma = 0.181 + 0.241 cp-pd / mm r2 = 0.999 a. ait sidi mou et al. j. electrochem. sci. eng. 7(3) (2017) 111-118 doi:10.5599/jese.386 117 following the sample preparation, measurements were performed similarly to those of standard p-pd solutions. the total p-pd contents were determined by using the former calibration curve equation. the total measured p-pd content of the takeout sample (w/w) was determined as 1 %. table 2. determination of p-pd in plant takeout extract sample. amount of p-dp, µm recovery, % sample detected added expected found takaout extract 0.54 1.5 2.04 2.25 110 conclusions in this paper, a simple, rapid, and sensitive determination of p-phenylenediamine in the phosphate buffer solution, ph 7.0 using the carbon paste electrode was proposed. a pair of welldefined redox peaks was obtained at the electrode. under optimized conditions, the electrode showed excellent performance in terms of detection limit, linearity range, selectivity, repeatability, and recovery values. the proposed method was successfully applied for the determination of the total content of p-phenylenediamine in the plant of takeout sample. references [1] a. ababou, k. ababou, a. mosadik, c. lazreq, a. sbihi, 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[24] q. zheng, p. yang, h. xu, j. liu, l. jin, journal of environmental sciences 24(9) (2012) 1717– 1722. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) substantial enhancement in the anticorrosivity of aluminium alloy 6061 by doxycycline hydrochloride drug doi:10.5599/jese.181 115 j. electrochem. sci. eng. 5(2) (2015) 115-127; doi: 10.5599/jese.181 open access : : issn 1847-9286 www.jese-online.org original scientific paper substantial enhancement in the anticorrosivity of aluminium alloy 6061 by doxycycline hydrochloride drug mudigere krishnegowda pavithra, thimmappa venkatarangaiah venkatesha, mudigere krishnegowda punith kumar* and nanjanagudu subba rao anantha department of chemistry, kuvempu university, shankaraghatta-577451, shimoga, karnataka, india *department of materials engineering, indian institute of science, bangalore-560012, karnataka, india corresponding author: drtvvenkatesha@yahoo.co.uk;tel: +91-9448855079 received: march19, 2015; revised: april 18, 2015; published: august 26, 2015 abstract the significant anticorrosive property of the antibiotic drug doxycycline hydrochloride (dch) was investigated by electrochemical techniques such as potentiodynamic polarization, electrochemical impedance and chronoamperometry. dch inhibited the pitting corrosion of aluminium alloy 6061 (aa6061) in 3.5 % nacl media with 90 % efficiency. the adsorption of dch on aa6061 conform modified langmuir isotherm by means of comprehensive adsorption. quantum chemical calculations were evaluated to ascertain the active sites of dch molecule responsible for adsorption and to support the experimental findings. keywords corrosion inhibitor; aluminium alloy; potentiodynamic polarization; electrochemical impedance spectroscopy, adsorption isotherm. introduction aluminium alloy 6061(aa6061) is an important alloy of aluminium used in a wide range of industrial applications owing to their low cost, light weight, high thermal and good corrosion resistance properties [1]. the corrosion resistance property arises from the ability of aa6061 to form a natural oxide film on its surface [2,3]. in aggressive chloride media, localized corrosion can occur and it leads to the breakdown of the passive layer and pit formation [4,5]. the protection of aa6061 and its oxide films from corrosive chloride attack can be done by using chemical inhibitors. http://www.jese-online.org/ mailto:drtvvenkatesha@yahoo.co.uk j. electrochem. sci. eng. 5(2) (2015) 115-127 anticorrosivity of aluminium alloy 6061 116 organic compounds containing heteroatoms are more efficient corrosion inhibitors for aluminium in 3.5% nacl media [6-10]. but the development of drugs as inhibitors for metallic corrosion has drawn much attention in the field of corrosion science because the drugs have strong chemical activity, low toxicity and negligible negative impacts on the environment. hence in the present investigation doxycycline hydrochloride (dch), which is a tetracycline antibiotic drug useful for the treatment of many infections, has been selected. dch is used in the treatment of chronic prostatitis, sinusitis, leptospirosis, pelvic inflammatory disease and respiratory tract infections [1112]. the choice of dch is based on its structural considerations such as it contains large number of π – electrons along with the hetero atoms like two nitrogen and eight oxygen atoms. in this study, the inhibition performance of dch molecule on the corrosion of aa6061 in 3.5% nacl solution has been examined by electrochemical techniques and also quantum chemical calculations were evaluated by ab initio method to confer theoretical support to the experimental findings. experimental materials the surface of aa6061 (bhandari metal house, k.r. market, bangalore, india) having composition 0.25 % cu, 1.0 % mg, 0.60 % si, 0.20 % cr and remainder being al was polished with different grades of emery papers (grade no. 400, 600, 800, 1000, and 1200). afterwards, the polished specimen was immersed in 10 % naoh aqueous solutions for 30 sec, degreased with acetone and rinsed by millipore water, dried and stored in desiccator. the aa6061 specimens of 1 cm 2 area (exposed) with a 5 cm long stem isolated with araldite resin were used for electrochemical measurements. the ar grade nacl was used to prepare 3.5 % nacl solution by using millipore water. the millipore water was obtained from elix 3 milli-pore system (resistivity greater than 18 mω cm at 25 o c). dch inhibitor compound was obtained from ramdev chemicals india pvt. ltd., mumbai and its structure is given in figure 1. the different concentrations (0.5, 1.0, 1.5, 2.0 mm) of inhibitor solutions were prepared by dissolving specified amount of inhibitor in 3.5% nacl solution. the ph of 3.5 % nacl solution is 6 and all the experiments were carried out at the same ph even in the presence of dch. figure 1. the structure of dch [13] methods electrochemical measurements were conducted in a conventional glass cell using the chi 660c electrochemical analyzer (ch instruments, austin, tx, usa) at 302 k. aa6061 specimen (of 1 cm 2 area), a platinum electrode, and a ag/agcl electrode were used as working, auxiliary, and reference electrodes, respectively. prior to each electrochemical measurement, a stabilization period of 30 min was allowed to establish a steady state open circuit potential (ocp). each m. k. pavithra et al. j. electrochem. sci. eng. 5(2) (2015) 115-127 doi:10.5599/jese.181 117 experiment was carried out in triplicate and the average values of corrosion parameters are reported. the potentiodynamic polarization measurements were carried out over a potential range of −200 mv to +200 mv at ocp with a scan rate of 0.5 mv s -1 . the impedance measurements were carried at ocp in the frequency range 1 mhz to 100 khz with 5 mv sine wave as the excitation signal. impedance data were analyzed using zsimp-win 3.21 software. for aa6061, the chronoamperometric curves were obtained by polarizing the working electrode anodically at -0.68 v (ag/agcl) for 600 sec in 3.5 % nacl solution. the surface morphology of the metal samples after immersion in corrosive medium in the absence and presence of 1.5 mm inhibitor was analyzed using scanning electron microscopy (jeol, jsm 6400, jeol datum shanghai co. ltd., shanghai). quantum chemical calculations were performed with complete geometry optimization using standard hyperchem, release 8.0 software (hypercube, inc. gmbh austria). geometrical structure of inhibitor molecules was optimized by ab initio method using hartree–fock level with 3-21 g basis set. the polak–rieberre algorithm which is fast and accurate has been used for computation. the energy parameters in the form of root mean square gradient were kept at 0.1 kcal mol -1 å -1 . results and discussion open-circuit potential (ocp) measurements the change in the ocp as a function of immersion time for aa6061 in the absence and presence of different concentrations of dch in 3.5 % nacl is given in figure 2. figure 2.(a) ocp curves obtained for aa6061 in the absence and presence of different concentrations of dch in 3.5 % nacl and (b) close up view of figure (a). in the beginning, the potential of each solution get slightly varied and became constant after few minutes of immersion. these constant potentials correspond to the free corrosion potential of aluminium in each solution. compared to uninhibited solution, the ocp shifted towards more positive values in the presence of dch. the shift in ocp in presence of dch indicates that the corrosion of aluminium gets decreased to a greater extent in presence of dch and hence it acts as a better inhibitor. j. electrochem. sci. eng. 5(2) (2015) 115-127 anticorrosivity of aluminium alloy 6061 118 potentiodynamic polarization (pdp) measurements the potentiodynamic polarization curves of aa6061 in 3.5 % nacl solution in the absence and presence of different concentrations of dch are presented in figure 3. the corrosion kinetic parameters such as corrosion potential (ecorr), corrosion current density (icorr), and anodic (a), and cathodic (c) tafel slopes were generated from the software installed in the instrument. the percentage inhibition efficiency ηt / % was computed from icorr values using the following expression;   100%    o corr corr o corr t i ii  (1) where i o corr and icorr are the corrosion current densities without and with dch, respectively. the corrosion kinetic data obtained from potentiodynamic polarization curves for aa6061 in 3.5 % nacl having different concentrations of dch are depicted in table 1. figure 3. potentiodynamic polarization curves obtained for aa6061 in 3.5% nacl in the absence and presence of dch it can be seen from the figure 3 that both the anodic and cathodic branches of the curves shifted towards lower current densities in inhibited solution as compared to uninhibited solution. this indicates that oxygen reduction and aluminium dissolution process are limited by dch which acts as a mixed type inhibitor by inhibiting the corrosion of metal surface. table 1.potentiodynamic polarization parameters obtained for aa6061 in 3.5% nacl in the absence and presence of dch c / mm – ecorr /mv – c /mv dec -1 /mv dec -1 icorr/µa cm -2 ηt/% blank 1129±2.4 8.725 3.089 68.71±2.4 0.5 633±2.6 2.168 17.807 15.16±1.9 78 1.0 659±1.1 3.941 16.901 12.09±1.1 83 1.5 715±1.2 5.401 10.682 6.13±1.9 91 2.0 648±0.9 4.565 21.147 8.39±1.3 88 it can be visualized from table 1 that enhancing the concentrations of dch up to 1.5 mm improves the values of both a and c, and ecorr values are shifted to more negative direction in the presence of different concentrations of inhibitor compound. this can be attributed to the formation of strongly adsorbed film on the metal surface where the adsorbed dch molecules reduce the aggressiveness of cl ─ ions and protect the surface from being pitted. meanwhile, the m. k. pavithra et al. j. electrochem. sci. eng. 5(2) (2015) 115-127 doi:10.5599/jese.181 119 protection efficiency increases up to 1.5 mm concentration of dch and further increase in the concentration of inhibitor leads to decrease in efficiency. the highest efficiency at 1.5 mm concentration of dch may be resulted from the maximum surface coverage. however, above this concentration, there may be possibility of interaction between unadsorbed and adsorbed dch molecules which favors desorption [14]. consequently, the gradual decrease in efficiency was noticed in chloride media containing dch greater than 1.5 mm. this suggests that dch is fairly an effective inhibitor at 1.5 mm concentration for aa6061 in 3.5 % nacl medium. electrochemical impedance spectroscopy (eis) eis studies have been carried out to get information on the corrosion and inhibition mechanism. the nyquist plots obtained for aluminium in 3.5 % nacl in the absence and presence of dch are given in figure 4. the impedance behaviour of aa6061 surface was simulated by an equivalent circuit shown in the inset of figure 4. in the circuit, rs represents the solution resistance, r1 is the charge transfer resistance corresponding to the corrosion reaction at the al/solution interface, r2 represents the polarization resistance, which reflects the protective property of the film, cpe1 and cpe2 represents the constant phase elements (cpe) as a substitute for the double-layer capacitance (cdl). the impedance of cpe is defined as   -n1 cpez q j   (2) where q is the cpe constant, ω is the angular frequency, j 2 = −1 is the imaginary number and n is the cpe exponent which gives details about the degree of surface inhomogeneity resulting from surface roughness, inhibitor adsorption, porous layer formation etc. figure 4. electrochemical impedance plots obtained for aa6061 in 3.5% nacl in the absence and presence of different concentrations of dch the inhibition efficiency ηz / % was evaluated from total resistance (rt) values using the following equation j. electrochem. sci. eng. 5(2) (2015) 115-127 anticorrosivity of aluminium alloy 6061 120 o t t z t / % 100 r r r     (3) where rt o and rt are the total resistance (where rt is the sum of r1 and r2) in the absence and presence of dch, respectively. the obtained electrochemical impedance parameters are depicted in table 2. as the concentration of dch increases the diameter of the capacitive loop also increases. but the diameter of the capacitive loop is large at 1.5 mm suggesting that the film formed by the adsorption of dch molecules on the al surface become dense and exhibit better protection efficiency. meanwhile, the diameter of the capacitive loop gets decreased when the concentration is 2 mm. moreover, each nyquist plot is composed of two loops and it suggests that there are two major electrochemical kinetic processes on the electrode surface. the high frequency part is due to the adsorption of dch molecule which leads to the formation of inhibitor film and the second loop at lower frequency is due to the electrochemical corrosion process [15]. table 2. impedance parameters obtained for aa6061 in 3.5 % nacl in the absence and presence of different concentrations of dch c /mm rs/ω cm 2 q1/μω -1 s n cm -2 n1 r1/ω cm 2 q2/μω -1 s n cm -2 n2 r2/ω cm 2 ηz/ % blank 7.358 130.3 0.91 76.98±2.7 11320 1 381.5±8.6 0.5 6.975 56.53 0.92 392.9±3.9 6863 0.88 1501±5.8 76 1.0 18.80 131.7 0.83 521.9±5.5 3294 0.85 2991±5.3 87 1.5 11.07 47.75 0.92 757.4±3.8 2130 0.93 3857±8.2 90 2.0 6.933 155.6 0.83 143.5±3.1 2340 1 2337±8.3 88 it is evident from the table 2 that the presence of dch in chloride media leads to an increase in the resistance and decrease in cpe values up to 1.5 mm concentration. the decrease in q values may be due to the decrease in local dielectric constant and/or an increase in the electrical double layer thickness [16]. this infers dch acts via adsorption at the metal/solution interface. meanwhile, the increase in r1 and r2 values suggest that the amount of dch molecules adsorbed on the al surface increases and the adsorbed inhibitor molecules forms a protective film on the electrode surface and consequently hampers the dissolution of aluminium, resulting in an increase in the protection efficiency. on the other hand, the value of r decreases and q increases above 1.5 mm concentration which may be due to the desorption of inhibitor molecules at higher concentration above 1.5 mm. hence, the gradual decrease in protection efficiency was noticed in 3.5 % nacl media containing dch greater than 1.5 mm. also, the factor n1 values had changed a little; which means that the corrosion reaction on the aluminium surface was inhibited by the absorbed inhibitor film. by reviewing these results it can be considered that dch is relatively an effective inhibitor at 1.5 mm concentration for aa6061 in 3.5 % nacl medium. chronoamperometric measurements potentiostatic current-time experiments were carried out in order to gain information on the corrosion and corrosion inhibition of aa6061 in nacl solution in the absence and presence of dch at less negative potentials. the chronoamperometric curves obtained at ̶ 680 mv vs. ag/agcl [7] for aa6061 electrode after its immersion for 600 sec in 3.5 % nacl solution are shown in figure 5. m. k. pavithra et al. j. electrochem. sci. eng. 5(2) (2015) 115-127 doi:10.5599/jese.181 121 figure 5. chronoamperometric curves obtained for aa6061 in 3.5% nacl in the absenceand presence of dch. the absolute current of aa6061 gets decreased from the initial moment till the end of the measurement in the presence of dch. further, pronounced decreasing of current was obtained with increasing the dch concentration up to 1.5 mm. this infers that dch inhibited the dissolution of aa6061 in chloride solution and shows better protection efficiency as an inhibitor at 1.5 mm concentration. adsorption isotherm the fundamental information dealing with the interactions between the inhibitor molecule and the metal surface can be obtained from the adsorption isotherm [17]. the data obtained from the polarization and impedance techniques were tested with several adsorption isotherms and the langmuir isotherm was found fit well with the experimental data. the langmuir adsorption isotherm equation is given by: ads 1c c k   (4) where c is the inhibitor concentration and kads is the equilibrium constant of the inhibitor adsorption process and  is the degree of surface coverage defined as η/100 . the plot of  versus c is given in figure 6(a). langmuir model assumes monolayer adsorption which occurs only at limited number of sites which are indistinguishable and equivalent. there is no tangential interaction and steric hindrance between the adsorbed molecules even on adjacent sites [18,19]. langmuir adsorption is graphically represented by a plateau which is an equilibrium saturation point. at this point, no further adsorption takes place once the surface of the adsorbent is completely occupied by a monolayer of inhibitor [20,21]. however, plateau is not observed clearly in figure 6(a) which may be attributed to the occupancy of more or less typical adsorption site by the adsorbed molecule at the metal solution interface. meanwhile, the divergence from pure monolayer adsorption may be due to the interactions between adsorbate species on the metal surface as well as changes in the adsorption heat with increasing surface coverage [22] and these aspects were not taken into consideration in derivation of the langmuir isotherm. accordingly, langmuir adsorption isotherm cannot be applied to study the adsorption behaviour j. electrochem. sci. eng. 5(2) (2015) 115-127 anticorrosivity of aluminium alloy 6061 122 of dch and hence, it is better to use the modified langmuir adsorption isotherm [23,24] given by the equation ads c n nc k   (5) where n (0 <n>1) is the heterogeneity character of the surface. the plot of c/versus c is given in figure 6(b) and it clearly reveals that the linear correlation coefficients (r 2 ) are almost equal to unity and the slopes are very close to 1. this presumes that the adsorption of dch on aa6061 alloy follows the modified langmuir adsorption isotherm. the equilibrium constant, kads is related to the standard gibbs free energy of adsorption δg o ads by the relation (6) o ads ads 1 exp 55.5 g k rt        (6) where r is the universal gas constant, t is the thermodynamic temperature and 55.5 is the molar concentration of water in the solution expressed in mol l -1 . figure 6. (a) plot of concentration of dch versus surface coverage and (b) langmuir isotherm plot obtained for the adsorption of dch on the surface of aa6061 in 3.5 % nacl.[symbols □ and ● represent the data points of pdp and eis studies respectively, connected with trendlines] it is generally accepted that for the values of –δg o ads up to 20 kj mol -1 are consistent with physisorption, while those around 40 kj mol -1 or higher are associated with chemisorption [25]. physisorption is due to electrostatic attractive forces between the inhibiting organic ions or dipoles and the electrically charged surface of the metal. chemisorption is due to interaction between unshared electron pairs or π electrons of the adsorbate with the metal in order to form a coordinate type of bond. in the present investigation, the –δg o ads value obtained from both pdp and eis techniques are 33.04 and 33.31 kj mol -1 , respectively, and it ensures that the adsorption of dch on aa6061 involves physisorption as well as chemisorption. in the meantime, the negative value of δg o ads signifies the spontaneous adsorption of dch molecules on aa6061 surface. surface morphological studies the sem micrographs of aa6061 immersed in 3.5 % nacl in the absence and presence of 1.5 mm dch are shown in figure 7. the surface of aa6061 was severely damaged with large number of pits in 3.5 % nacl solution. whereas, a less damaged surface with reduced pits were observed on the surface of aa6061 in presence of dch in 3.5 % nacl solution. as a result, it can be m. k. pavithra et al. j. electrochem. sci. eng. 5(2) (2015) 115-127 doi:10.5599/jese.181 123 deduced that dch forms a protective adsorbed layer on metal surface and it hampers the corrosion of aa6061 in 3.5 % nacl medium. figure 7. sem micrographs of aa6061 immersed in (a) 3.5 % nacl and (b) 3.5% nacl +1.5 mm dch quantum chemical studies to study the effect of molecular structure on inhibition efficiency, quantum chemical calculations were performed using the ab initio method [26], and all calculations were carried out with the help of complete geometry optimization using 3-21g basis set. the optimized structure and the highest occupied molecular orbital (homo) and the lowest unoccupied molecular orbital (lumo) of dch molecule are shown in figures 8 and 9. figure 8. optimized structure of dch molecule. the energies of the frontier molecular orbitals such as ehomo and elumo are significant parameters for the prediction of the reactivity of a chemical species. ehomo is often associated with the electron-donating ability of a molecule. high ehomo values indicate that the molecule has a tendency to donate electrons to a metal with unoccupied molecular orbitals. a lower value of elumo indicates an easier acceptance of electrons from a metal surface [27]. the gap between the lumo and homo energy levels of inhibitor molecules is another important parameter. low absolute values of the energy band gap (∆e= elumo ehomo) means good protection efficiency. j. electrochem. sci. eng. 5(2) (2015) 115-127 anticorrosivity of aluminium alloy 6061 124 figure 9. distribution of (a) homo and (b) lumo in dch molecule. from the figure 9, it can be observed that the homo and lumo orbitals are distributed on the phenyl ring and the oxygen atoms present in its environs. hence, it can be considered that the oxygen atoms of the hydroxyl groups and carbonyl groups which are in the vicinity of phenyl ring and the π-electrons are the active sites of adsorption of dch on metal surface. some quantum chemical parameters such as ionisation potential (i), electron affinity (a), absolute electronegativity (χ), global hardness (η), and global softness (σ) were calculated by the following relations; i = ̶ ehomo (7) a= ̶ elumo (8) 2 i a    (9) 2 i a    (10) 1    (11) the calculated quantum chemical data are tabulated in table 3. the high inhibition efficiency of a molecule can be attributed to the high value of dipole moment and low values of ∆e. the results of the high dipole moment and the low energy gap indicate that electron transfer from dch to the surface takes place during adsorption on the aa6061 surface. the adsorption centres of organic molecules can also be approximated by net atomic charges in the molecule. the net atomic charges of dch molecule are given in figure 10. it has been reported that as the charge of the adsorbed centre become more negative, the atom more easily donates its electron to the unoccupied orbital of the metal [28]. hence by visualizing the figure 10, it can be considered that oxygen atoms and the π-electrons present in the dch molecule are the active centres of adsorption. (b)(a) m. k. pavithra et al. j. electrochem. sci. eng. 5(2) (2015) 115-127 doi:10.5599/jese.181 125 table 3. quantum chemical parameters of dch quantum chemical parameters ehomo / ev ̶ 8.041 elumo/ ev 2.291 ∆e / ev 10.332 χ / ev 2.875 η / ev 5.166 σ / ev -1 0.194 µ/ d 4.516 total energy, kcal mol -1 ̶ 970248.9 figure 10. the net atomic charges of dch molecule. mechanism of inhibition based on the experimental results, the following mechanism has been proposed for the corrosion behaviour of aa6061 in 3.5 % nacl in presence of dch molecules. in 3.5 % nacl, the cathodic reduction of oxygen takes place on the surface of aluminium and leads to the formation of oxide film by anodic reaction. both the cathodic and anodic reactions result in the formation of oxide film are as follows [7,29-31]; h2os + ½o2 + e ─  ohads + oh ─ (12) ohads + e ─  oh ─ (13) alsads + 3oh ─  al(oh)3,ads + 3e ─ (14) al(oh)3,ads al2o33h2o (15) j. electrochem. sci. eng. 5(2) (2015) 115-127 anticorrosivity of aluminium alloy 6061 126 the aluminium undergoes pitting corrosion in nacl solution due to the presence of aggressive chloride ions and it leads to the breakdown of aluminium oxide formed on the metal surface [7]. this phenomenon can be explained by the following reactions; al al 3+ + 3e ─ (16) al 3+ + 4cl ─  alcl4 ─ (17) or al 3+ + 2cl ─ + 2oh ─  aloh2cl2 ─ (18) many of the researchers have explained the mechanism of pitting corrosion in different ways [32-35]. some of the authors propose that the alcl4 ─ formed within the pits get diffuses in to the bulk solution and leads to the pitting corrosion. however, other researchers emphasize that the adsorbed chloride ions reacts with al 3+ ions in the oxide lattice and results in the formation of oxychloride complexes, al(oh)2cl2 ─ . this complex reduces the stability of oxide film and enhances the dissolution rate of aluminium. the electrochemical results disclose that the dch molecules interact with the surface of aa6061 through the process comprehensive adsorption mechanism. thus, it can be considered that the electrostatic interaction occurs between the inhibitor molecules and charged metal surface. also, the quantum chemical calculations reveal that the oxygen atoms of hydroxyl groups and carbonyl groups which are in the vicinity of phenyl ring and the π-electrons present in dch molecule are the active sites responsible for the process of adsorption. consequently, the inhibitor get adsorbed on the surface of aa6061 through these active sites and it prevents the formation of al(oh)2cl2 ─ , on the oxide film. as a result, dch decreases the aggressiveness of chloride ions and protects the metal surface from pitting corrosion. conclusions the molecule dch is an efficient corrosion inhibitor for aa6061 in 3.5% nacl medium. the electrochemical studies reveal that dch hampers aa6061 corrosion through the process of adsorption. in nacl medium, the mere adsorption of dch obeys modified langmuir adsorption isotherm and follows both physisorption and chemisorption mechanism. quantum chemical studies disclose that the hetero atoms and the π – electrons present in dch molecules are the active sites of adsorption which are responsible for the inhibition behaviour. acknowledgement: the authors are grateful to the authorities of department of chemistry, kuvempu university, karnataka, india for providing lab facilities. authors also thank department of science and technology, new delhi, govt. of india [dst: project sanction no. 100/ifd/1924/20082009 dated 2.07.2008] for providing instrumental facilities. the authors are also gratified to university grant commission, new delhi, govt. of india [ugc grant ref. 41-231/2012(sr) dated 16.07.2012] for providing “hyperchem software” facility. references [1] r. banerjee, ranjana, s.s. panja, m.m. nandi, ind. j. chem. tech. 18 (2011) 309 -313. 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[35] n. sato, corros. sci. 37(1995) 1947 – 1967. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.drugbank.ca/unearth/q?utf8=%e2%9c%93&query=doxycycline+hydrochloride&searcher=drugs&button http://www.drugbank.ca/unearth/q?utf8=%e2%9c%93&query=doxycycline+hydrochloride&searcher=drugs&button http://link.springer.com/article/10.1007%2fs11164-014-1561-5 http://creativecommons.org/licenses/by/4.0/ corrosion and wear protection of aisi 4140 carbon steel using a laser modified high speed oxide fuel thermal sputtered coatings http://dx.doi.org/10.5599/jese.1320 865 j. electrochem. sci. eng. 12(5) (2022) 865-876; http://dx.doi.org/10.5599/jese.1320 open access : : issn 1847-9286 www.jese-online.org original scientific paper corrosion and wear protection of aisi 4140 carbon steel using a laser-modified high-velocity oxygen fuel thermal sprayed coatings shanmugasundaram sivarajan1, , adwait joshi1, karthikeyan palani2, raghupathy padmanabhan1 and joseph stokes3 1vellore institute of technology, vandalur kelambakkam road, chennai, pin 600127, india 2saveetha engineering college, saveetha nagar, thandalam, chennai, pin 602105, india 3dublin city university – glasnevin campus dublin 9 d09 na55, ireland corresponding author: sivarajan.s@vit.ac.in; tel.: 919841617961 received: march 4, 2022; accepted: june 1, 2022; published: july 21, 2022 abstract inconel and micro and nano wc-12co powders were deposited on aisi 4140 carbon steel by high-velocity oxy fuel (hvof) coating and followed by laser surface modification. laser power and scan speed were varied at different levels. microstructure and microhardness were investigated. nanocoatings performed better than microcoatings. nanostructured wc powder coatings exhibited greater hardness compared to microstructured powder coating. when the laser power is increased to 170 w, a small cellular dendrite microstructure through multiphase solidification is formed due to the difference in thermal properties of inconel 625 and wc particles. adequate laser power and low scan speed were preferred to produce a high-quality coating. from the electrochemical corrosion test results, it was observed that the corrosion rate of laser-modified hvof sprayed coating is lower than the carbon steel sample. this shows that the inconel sprayed by laser-modified hvof coating enhanced the corrosion resistance of the substrate steel material. the porosity percentage was higher for all the samples when laser scan speed was increased. keywords inconel-625; tungsten carbide; wc-12co introduction aisi 4140 carbon steel is extensively used in petrochemical and other industrial applications. however, this steel cannot be used in an aggressive corrosive atmosphere due to its poor corrosion resistance. many coating techniques like physical vapor deposition (pvd), chemical vapor deposition cvd, thermally sprayed coatings, and laser surface modification can be employed to improve the corrosion resistance of carbon steel. machine component degradation occurs in petrochemical and related industries due to wear and tribo-corrosion. a material deterioration is troublesome to http://dx.doi.org/10.5599/jese.1320 http://dx.doi.org/10.5599/jese.1320 http://www.jese-online.org/ mailto:sivarajan.s@vit.ac.in j. electrochem. sci. eng. 12(5) (2022) 865-876 aisi 4140 carbon steel corrosion protection 866 production and plant workers and expensive to prevent. many researchers worldwide have tried to combat material degradation by developing new advanced materials. to minimize material degradation, protective coatings produced by the electrolytic hard chromium process are not recommended due to environmental issues. moreover, water plays a crucial role in hard chrome plating; today, its depletion limits its use [1]. pvd and cvd coatings are not preferred to combat material degradation due to inferior properties and high application costs [2]. thermal spraying is an extensively used coating technique to protect materials from wear and corrosive atmosphere [3-14]. however, this process induces changes to the coating powder, resulting in defective coatings. additionally, the oxide formation during thermal spraying can affect the efficiency of coatings in extreme and aggressive surroundings [15]. the adhesive strength between coating and substrate is also not very good in thermal sprayed coatings. hence, it is unsuitable for protecting the surface of materials in severe conditions. laser surface modification can be considered a probable technique to minimize the flaws of the coatings. laser surface modification via heat treatment offers advantages like accurately controlled dimensions, and least heat-affected zones, producing no thermal effects on the substrate. the laser surface modification can also be employed for parts with complex shapes. furthermore, automation can be easily achieved in laser processing. laser processing as a post-treatment process for high-velocity oxy fuel (hvof) coatings to enhance corrosion resistance has been reported by many researchers [16-23]. nanostructured materials are of great interest because of their superior properties, which differ from bulk materials. nanostructured wc-12co powder is used to enhance for wear resistance of various metallic surfaces. nevertheless, a spray of nano-sized powder is difficult owing to its low weight. inconel 625 coatings are widely used to improve the corrosion resistance of steel. inconel 625 coatings find many applications in petrochemical and power generation industries to protect the equipment operating in severe conditions. the influence of laser heating on the hardness of wc-cocr hvof sprayed coating was examined [24]. the average value of the microhardness of the coating increased after laser heating from 1161.7 hv to 1579.8 hv. furthermore, the microhardness is improved with reduced porosity of the coating. coatings produced by a laser-assisted spraying process; also called laser hybrid spraying, where thermal spraying and laser melting processes were combined to get denser coatings of different ni-base materials on low carbon steel substrate were reported [25]. laser-hybrid inconel 625 coatings showed poor corrosion behavior due to cracks and high iron dilution from the substrate. the influence of laser melting on the mechanical properties of inconel 625 and inconel 625 with wc as mmc hvof sprayed on 316l steel substrate was investigated [26]. it was observed that wear resistance increases by increasing the percentage of wc in the coating material. the residual stress profile and room temperature wear behavior of wc-co coatings deposited by kinetic metallization were characterized [27]. the sem imaging and etched optical micrographs showed significant plastic deformation and particle penetration at the interface due to the high energy impact of feedstock particles. the wear resistance measured by the pin-on-disk method showed a wear performance comparable to conventionally sprayed wc-co coatings. the laser cladding technology, a modern joining technique to fulfill the requirements of many industrial applications, is reviewed [28]. the purpose is to facilitate researchers by providing comprehensive knowledge about this technology to encourage them to explore it in future research and innovation efforts. it was reported that shorter spray distance and higher fuel flow increased the particle velocity during hvof spraying of femncrsi coatings [29]. higher particle velocity showed higher compressive residual stress. the lower powder s. sivarajan et al. j. electrochem. sci. eng. 12(5) (2022) 865-876 http://dx.doi.org/10.5599/jese.1320 867 feed rate promoted an increase in compressive residual stress. cavitation wear resistance is higher for higher compressive residual stress. the performance of two coating processes to combat the water droplet impingement erosion (wdie) phenomenon was compared [30]. high-velocity air fuel (hvaf) and high-velocity oxygen fuel (hvof) processes are used to spray wc-10co-4cr powder on ti-6al-4v. at 250 m/s and 300 m/s, gradual damage occurs due to accumulated impact and jetting, where hvaf outperformed hvof coating. hvof's lower performance is attributable to the formation of an unwanted brittle w2c phase. suspension-high velocity oxygen fuel spraying (s-hvof) and suspension plasma spraying (sps) were applied to produce al2o3 coatings using homemade and commercially available al2o3 water-based liquid feedstocks containing 40 wt.% of sub-micrometer-sized raw powders [31]. the influence of feedstock characteristics and spray process conditions on the mechanical and tribological properties was analyzed. furthermore, the corrosion behavior of superalloys used in turbines is studied and reviewed [32]. grey cast iron (ci) substrate was coated with inconel718 based composite coating with a high-velocity oxy-fuel technique [33]. the coating with 10 wt.% al2o3 content exhibited the maximum corrosion resistance. in thermal power plants, protection of materials surface from degradation is crucial since it leads to plant inefficiency [34]. the role of thermal spray coatings has been investigated to protect different steel grades exposed to such degrading conditions at high temperatures in coal-based power plants. the bimodal composite coatings developed with inconel and al2o3 protect the underlying substrate due to their hardness and fracture toughness [35]. this paper aims to investigate the viability of laser remelting of the hvof spraying process to coat nano-sized wc-12 co mixed with inconel 625 on steel substrates. in addition, the effect of laser parameters on these coatings will be evaluated by microstructure and microhardness measurements. experimental coating powders to protect the aisi carbon steel from corrosion and wear, three different powders were used for deposition using hvof thermally sprayed coating. the first was inconel-625 (diamalloy 1005). the chemical composition of diamalloy 1005 is 66.5 wt.% ni, 21.5 wt.% cr, 8.5.wt.% mo, 3 wt.% fe and 0.5.wt.% co. tungsten carbide wc-12co (diamalloy 2004sintered) was also used to improve wear resistance. the chemical compositions for (diamalloy 2004) are 88 wt.% wc and 12 wt.% co. a nanostructure superfine wc-12co infralloy tm s7412 was used in the composite mixture to compare against conventional coatings. its chemical composition is as follows: 88 wt.% wc and 12 wt.% co. the substrate material used is made up of aisi 4140 carbon steel. in each sample, two of the three powders were mixed to various compositions, as shown in table 1. the coating powders were deposited on aisi carbon steel using an a1050 djh automated hvof gun spray device. grit blasting was performed on these aisi carbon steel samples with the help of honite powder particles. moisture was removed from steel samples by heating them to 110 °c. the parameters were selected based on the powder composition to avoid decomposition. laser equipment and processing parameters laser surface modification was carried out on hvof sprayed steel samples using a pulsed laser system. the co2 rofin laser has a power rating of 1.5 kw. compressed argon was provided coaxially as a shielding gas. the prf (pulse repetition frequency) was kept constant at 400 hz. the focal point position was kept at –5 mm. in the first set of six experiments, the laser power was varied at six http://dx.doi.org/10.5599/jese.1320 j. electrochem. sci. eng. 12(5) (2022) 865-876 aisi 4140 carbon steel corrosion protection 868 levels at a constant scan speed of 1000 mm min-1. laser power levels used were 20, 55, 105,170, 240, and 290 w. in the second set of experiments, the laser scanning speed varied at six levels at a constant laser power of 55 w. scanning speed levels were 750, 1000, 1250, 1500, and 2000 mm min1. in addition, the scanning electron micrographs were taken. table 1. composition of powders in sample sample content, wt.% wc-12co (nano) wc-12co (micro) inconel 625 (micro) s1 25 0 75 s2 50 0 50 s3 75 0 25 s4 0 75 25 s5 0 50 50 s6 0 25 75 mechanical characterization of coatings the hardness of the coatings was measured by vickers microhardness testing machine manufactured by shimadzu (model: hmv-g). the resolution was 0.01 μm, and the load used was 100 g. the duration of the load applied was 10 seconds. indentation was made on different parts of the heat-affected zone and the shape of indentation observed was of the rhombus. the length of both the diagonals was measured and based on that measurement vickers hardness number was directly calculated and graphs plotted. three sets of readings were taken to ensure consistency in results. electrochemical corrosion tests the tafel tests of laser-modified hvof sprayed sample and carbon steel sample were performed in salt solution at 36.5 °c using a potentiostat (interface 1010, gamry instruments) by exposing 0.375 cm2 area of the samples. the corrosion cell consists of a saturated calomel electrode (sce) and a platinum wire as reference and counter electrodes. tafel plots were created by polarizing the specimen about 0.3 v anodically and cathodically with reference to open circuit potential (ocp) at a scan rate of 0.5 mv s-1 after an initial delay of 60 minutes. after the measurements were obtained, the tafel data were analyzed by curve fitting and equivalent circuit modeling using gamry echem analyst software. porosity measurement assuming that resolution limits are considered, porosity within a microstructure can be easily detected by image analysis due to the high degree of contrast between the dark pores (voids) and the more highly reflective coating material. in this study, the sem images were used to check the porosity using imagej software. the areas of porosity were selected in the sem image and the percentage of porosity was analyzed and processed by imagej software results and discussion the detailed microstructure analysis of hvof sprayed wc-12co-inconel-625 coatings exhibits spat-like layered morphologies due to the re-solidification of molten and semi-molten powder particles. during hvof spraying, wc-12co-inconel 625 coatings under different powder compositions contain three distinct zones: fully, partially, and unmelted core. the formation of molten, semi-molten and unmelted zones is due to variation in melting temperature of wc-12co and inconel particles relative to flame temperature. the formation of three separate zones was confirmed by s. sivarajan et al. j. electrochem. sci. eng. 12(5) (2022) 865-876 http://dx.doi.org/10.5599/jese.1320 869 previous research work [36]. in addition, defects such as pores, micro defects and cracks are found in most hvof coatings. figure 1. sem image of laser-modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power = 55 w; laser scan speed = 1000 mm min-1 microstructured powder figure 2. sem image of laser-modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power = 55 w; laser scan speed = 1500 mm min-1 microstructured powder partially molten and fully molten particles of hvof coatings were modified by laser with a reduction in cracks and porosity. the minimization of discrete splats and porosity by laser modification will improve the wear resistance. figure 1 shows the sem image of laser-modified hvof coating with laser power of 55 watts and a scan speed of 1000 mm min-1. it can be seen that the pores present in the hvof coating are minimized to a larger extent in the laser-modified zone. also, the pore size is considerably reduced compared to those present elsewhere. the occurrence of porosity is minimal in the laser-treated region compared to other regions, as can be observed in figure 2. the sem image (figure 2) also shows that the diffusion of coating constituents into the substrate is less, which has helped maintain the original properties of coatings. improved material consolidation is found to be more effective through laser. figure 3 shows the sem image of the coatings in which splats, pores, and cracks are visible in the microstructure. this is because the laser power is too low to melt wc and inconel particles with nanostructured powder. figure 4 shows an sem image of coating in which a fine-grained homogenous poly phase microstructure is developed due to low laser scan speed while maintaining low laser power (55 w). when the laser power is increased to 170 w, a small cellular dendrite microstructure through multiphase solidification is formed due to the difference in thermal properties of inconel 625 and wc particles, as observed in figure 5. the surface is made up of coating material and the substrate, leading to a differential cooling rate; hence, the cell size and orientation vary in the coating [37-38]. figure 6 shows the sem image in which large pores are seen due to higher scan speed. when the scan speed and power are high, the time of contact is less and there is less time for the gases and bubbles formed during the coating powder reactions to escape, resulting in pores in the coating remelting zone. a dendrite microstructure is observed at higher magnification in the sem image due to multiphase solidification, as shown in figure 7. http://dx.doi.org/10.5599/jese.1320 j. electrochem. sci. eng. 12(5) (2022) 865-876 aisi 4140 carbon steel corrosion protection 870 figure 3. sem image of laser modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power 55 w laser scan speed = 1250 mm min-1 nanostructured powder figure 4. sem image of laser modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power 55 w laser scan speed = 1000 mm min-1 nanostructured powder figure 5. sem image of laser modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power 170 w. laser scan speed = 1500 mm min-1. nanostructured powder figure 6. sem image of laser modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power 170 w. laser scan speed 1750 mm min-1. micro structured powder the influence of laser scan speed on the microhardness of laser melted high-velocity oxy fuel coated samples is shown in figure 8. the first three coated samples (nano-sized powder coatings) showed higher hardness than the remaining three coated samples (micro-sized powder coatings). the maximum hardness with nano powder coatings is hv1199 and with micro-sized powder coatings, it is hv775. nano powder coatings exhibited better melting leading to the dense coating of higher microhardness. when the laser scan speed was between 1250 to 2000 mm min-1, proper powder melting and consolidation took place, resulting in uniform high microhardness in all three cases. the s. sivarajan et al. j. electrochem. sci. eng. 12(5) (2022) 865-876 http://dx.doi.org/10.5599/jese.1320 871 influence of laser power on the microhardness of laser melted high-velocity oxy fuel coated samples is shown in figure 9. when laser power is increased, the micro-hardness of coated samples improves. an increase in laser power combined with low scan speed ensures proper melting and less porosity, leading to high hardness. it was observed that coatings produced by spraying nanopowders show higher hardness than coatings produced by micro-sized powders. figure 7. sem image of laser modified hvof thermally sprayed inconel 625 and wc-12co coatings; laser power = 170 w. laser scan speed = 1750 mm min-1. nanostructured powder figure 8. variation of microhardness of coatings with laser scan speed; laser power = 55 w figure 9. variation of microhardness of coating with laser power; laser scan speed = 1000 mm min-1 figure 10. microscopic pictures of laser modified hvof sprayed wc-12co-inconel coatings. sample s1; laser power = 55 w; laser scan speed: (a) 750 (b) 1000 (c) 1250 (d) 1500 (e) 1750 (f) 2000 mm min-1 0 200 400 600 800 1000 1200 1400 700 1200 1700 2200 m ic ro h a rd n e ss , h v laser speed, mm min --1 s1 s2 s3 s4 s5 s6 0 200 400 600 800 1000 1200 1400 0 200 400 m ic ro h a rd n e ss , h v laser power, w s1 s3 s3 s4 s5 s6 http://dx.doi.org/10.5599/jese.1320 j. electrochem. sci. eng. 12(5) (2022) 865-876 aisi 4140 carbon steel corrosion protection 872 figure 11. microscopic pictures of laser-modified hvof sprayed wc-12co-inconel coatings; sample s2; laser power = 55 w.; laser scan speed: (a) 750 (b) 1000 (c) 1500 (d) 1750 (e) 2000 mm min-1 figure 12. microscopic pictures of laser-modified hvof sprayed wc-12co-inconel coatings; sample s3; laser power = 55 w.; laser scan speed: (a) 750 (b) 1000 (c) 1250 (d) 1500 (e) 1750 mm min-1 figure 13. microscopic pictures of laser-modified hvof sprayed wc-12co-inconel coatings; sample s; laser power = 55 w; laser scan speed: a) 750 (b) 1000 (c) 1250 (d) 1500 (f) 2000 mm min-1 figure 14. microscopic pictures of laser-modified hvof sprayed wc-12co-inconel coatings, sample s5; laser power = 55 w; laser scan speed: (a) 750 (b) 1000 (c) 1250 (d) 1500 mm min-1 from the electrochemical corrosion test results (table 2 and figure 17), it was observed that the laser-modified hvof sprayed coating exhibited lower corrosion current density when compared with the plain carbon steel sample. the corrosion rate of laser-modified hvof sprayed coating is also lower than that of the carbon steel sample. this shows that the inconel sprayed by lasermodified hvof coating has enhanced the corrosion resistance of the substrate steel material. s. sivarajan et al. j. electrochem. sci. eng. 12(5) (2022) 865-876 http://dx.doi.org/10.5599/jese.1320 873 figure 15. microscopic pictures of laser-modified hvof sprayed wc-12co-inconel coatings.; sample s6; laser power 55 w; laser scan speed: (a) 750 (b) 1000 (c) 1250 (d) 1500 (e) 1750 (f) 2000 mm min-1 figure 16. microscopic pictures of laser-modified hvof sprayed wc-12co-inconel coatings; sample s1; laser scan speed: 1000 mm min-1; laser power: (a) 20 (b) 55 (c) 105 (d) 170 (e) 240 (f) 290 w figure 17. tafel curve of laser modified hvof sprayed coating and carbon steel substrate during the corrosion tests table 2. electrochemical corrosion tests results laser-modified hvof sprayed sample carbon steel sample icorr / a cm-2 22.10 808.0 ecorr / mv -110.0 -27.00 corrosion rate, mm year-1 0.26035 9.53262 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 -10 -8 -6 -4 -2 0 p o te n ti a l, v v s. s c e log (i / a cm-2) carbon steel sample laser modifies hvof sprayed coating http://dx.doi.org/10.5599/jese.1320 j. electrochem. sci. eng. 12(5) (2022) 865-876 aisi 4140 carbon steel corrosion protection 874 table 3. porosity measurement results sample laser power, w laser scan speed, mm min-1 porosity, % sample laser power, w laser scan speed, mm min-1 porosity, % s1 55 750 1.300 s4 55 750 1.951 s1 55 1000 1.780 s4 55 1000 2.130 s1 55 1250 2.186 s4 55 1250 2.550 s1 55 1500 2.753 s4 55 1500 2.980 s1 55 1750 3.020 s4 55 1750 3.921 s1 55 2000 4.124 s4 55 2000 5.128 s1 20 1000 2.110 s4 20 1000 3.021 s1 55 1000 2.770 s4 55 1000 2.982 s1 105 1000 1.991 s4 105 1000 2.100 s1 170 1000 2.670 s4 170 1000 2.990 s1 240 1000 3.121 s4 240 1000 3.050 s1 290 1000 2.770 s4 290 1000 3.100 s2 55 750 2.100 s5 55 750 2.020 s2 55 1000 2.320 s5 55 1000 2.760 s2 55 1250 2.952 s5 55 1250 3.320 s2 55 1500 2.380 s5 55 1500 3.641 s2 55 1750 3.456 s5 55 1750 4.124 s2 55 2000 3.950 s5 55 2000 4.985 s2 20 1000 2.220 s5 20 1000 2.650 s2 55 1000 2.780 s5 55 1000 2.550 s2 105 1000 2.640 s5 105 1000 2.450 s2 170 1000 2.561 s5 170 1000 3.050 s2 240 1000 1.792 s5 240 1000 2.760 s2 290 1000 2.650 s5 290 1000 3.110 s3 55 750 1.890 s6 55 750 2.123 s3 55 1000 1.720 s6 55 1000 2.520 s3 55 1250 2.456 s6 55 1250 3.820 s3 55 1500 2.670 s6 55 1500 3.621 s3 55 1750 3.222 s6 55 1750 4.980 s3 55 2000 3.992 s6 55 2000 5.320 s3 20 1000 2.051 s6 20 1000 2.990 s3 55 1000 2.221 s6 55 1000 3.290 s3 105 1000 2.781 s6 105 1000 2.870 s3 170 1000 2.450 s6 170 1000 2.560 s3 240 1000 2.680 s6 240 1000 2.740 s3 290 1000 2.730 s6 290 1000 2.690 the porosity measurement is made using imagej software [39]. the average porosity of nano samples (s1, s2, s3) is 2.59 % and the average porosity is 3.13 %. in all six samples, the porosity increased with an increase in laser scan speed. when the scan speed and power are high, the time of contact is less and there is less time for the gases and bubbles formed during the coating powder reactions to escape and this has resulted in the occurrence of pores in the coating remelting zone. conclusions in conclusion, wc powder particle size, laser power and laser scan speed were varied, and its effects on the microhardness and microstructure developed in the coating were examined. microhardness measurements performed on laser-modified hvof coatings displayed constantly greater values for coatings produced by nanopowder compared to coatings made by micro powder. when the laser power is increased to 170 w, a small cellular dendrite microstructure through multiphase solidification is formed due to the difference in thermal properties of inconel 625 and wc particles. s. sivarajan et al. j. electrochem. sci. eng. 12(5) (2022) 865-876 http://dx.doi.org/10.5599/jese.1320 875 adequate laser power and low scan speed will be preferred to produce a coating with high quality. the electrochemical corrosion test results showed that the corrosion rate of laser-modified hvof sprayed coating is lower than the carbon steel sample. this shows that the inconel sprayed by lasermodified hvof coating has enhanced the corrosion resistance of the substrate steel material. the porosity percentage is higher for all the samples when laser scan speed increases. therefore, for aisi 4140 carbon steel, to improve the wear and corrosion resistance, laser-modified hvof nanosized wc-12co –inconel-625 coatings show the promising result. acknowledgements: the authors would like to express their sincere gratitude 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.jmatprotec.2007.11.232 https://doi.org/10.1016/j.optlastec.2012.02.011 https://doi.org/10.1016/j.jmatprotec.2006.08.026 https://doi.org/10.5767/anurs.cmat.100101.en.019m https://doi.org/10.1016/j.surfcoat.2012.03.018 https://doi.org/10.1016/j.surfcoat.2006.06.035 https://doi.org/10.1016/j.jmatprotec.2008.07.027 https://doi.org/10.1016/j.matchar.2008.01.003 https://doi.org/10.1016/j.surfcoat.2006.04.042 https://doi.org/10.1016/j.surfcoat.2007.01.032 https://doi.org/10.1016/j.surfcoat.2021.127359 https://doi.org/10.1016/j.optlastec.2020.106619 https://doi.org/10.1016/j.surfcoat.2017.07.073 https://doi.org/10.1016/j.wear.2021.203904 https://doi.org/10.1016/j.surfcoat.2020.126463 https://doi.org/10.1016/j.matpr.2020.02.226 https://doi.org/10.1142/s0218625x22500172 https://doi.org/10.1515/corrrev-2020-0043 https://doi.org/10.1016/j.surfcoat.2022.128450 https://doi.org/10.1007/s11003-006-0047-z https://doi.org/10.1016/j.jmatprotec.2012.07.012 https://doi.org/10.1016/j.jmatprotec.2005.02.091 https://imagej.net/software/imagej/ https://creativecommons.org/licenses/by/4.0/) predicting the effect of silicon electrode design parameters on thermal performance of a lithium-ion battery http://dx.doi.org/10.5599/jese.1558 659 j. electrochem. sci. eng. 13(4) (2023) 659-672; http://dx.doi.org/10.5599/jese.1558 open access : : issn 1847-9286 www.jese-online.org original scientific paper predicting the effect of silicon electrode design parameters on thermal performance of a lithium-ion battery harika dasari and eric eisenbraun suny polytechnic institute, college of nanoscale science and engineering, albany, ny 12203, usa corresponding author: dasarih@sunypoly.edu received: october 21, 2022; accepted: january 12, 2023; published: february 20, 2023 abstract the present study models the role of electrode structural characteristics on the thermal behaviour of lithium-ion batteries. preliminary modelling runs have employed a 1d lithium-ion battery, coupled to a two-dimensional axisymmetric model using silicon as the battery anode material. the two models are coupled by the heat generated and the average temperature. our study is focused on the silicon anode particle sizes, and it is observed that silicon anodes with nano sized particles reduced the heating of the battery under charge/discharge cycles when compared to anodes with larger particles. these results are discussed in context of the relationship between particle size and thermal transport properties in the electrode. keywords particle size, lithium nickel manganese cobalt oxide, heat generation, separator introduction lithium-ion batteries, which possess relatively high energy and power densities, are useful in many fields including electric vehicles and aerospace applications. electric vehicles require high energy and power density to ensure both maximum mileage and maximum travel speed. a trade-off exists between energy and power density in batteries and most research is focused on the simultaneous improvement of both characteristics [1,2]. while graphite is widely used as an anode material in lithium-ion batteries, silicon (si) can be considered as an electrode material since it possesses good electrochemical properties and high theoretical, volume specific capacity as shown in table 1. however, si is susceptible to failure mechanisms like large volume changes during lithiation, decomposition of the solid electrolyte interface (sei) layer, and low conductivity; consequently, capacity fade is also observed [3,4]. employing nanostructured silicon particles as the active material for the negative electrode is one of the best ways to overcome the above-mentioned problems. such structures reduce mechanical stresses which cause volume changes. in our previous research work, we observed that negative electrode particle size in nanometers improved capacity when compared to particle size in micrometers [5]. http://dx.doi.org/10.5599/jese.1558 http://dx.doi.org/10.5599/jese.1558 http://www.jese-online.org/ mailto:dasarih@sunypoly.edu j. electrochem. sci. eng. 13(4) (2023) 659-672 thermal performance of a lithium-ion battery 660 table 1. properties of graphite and silicon anodes [6-7] anode material graphite silicon lithiated phase lic6 li4,4 si theoretical specific capacity, mah g-1 372 4200 theoretical volume capacity, mah cm-3 837 9786 volume change, % 12 320 apart from studies of capacity fade, it is also important to understand the thermal behavior of nanostructured silicon particles. in general, the heat generated in a battery is classified as reversible heat and irreversible heat. while reversible heat originates from the entropy changes of the active materials of the electrode, irreversible heat is created from other processes which occur in the battery [8]. there are two sources of irreversible heat: polarization heat, which is formed due to the overpotential, and ohmic heat, resulting from the resistance due to the movement of li+ during the electrochemical reaction process [8]. the electrode is the most important component of a battery, as it affects the capacity, power density, and energy density, and defines the quantity and speed of lithium storage within the battery. therefore, approaches like selecting appropriate materials, e.g., using different mixed cathode materials, altering the ratio of active to inactive materials, or optimizing battery design parameters of the electrodes such as particle size, electrode thickness, volume fraction, and other factors are typically considered for electrode design [8-18]. the most widely used separators are comprised of a polymer matrix embedded in the electrolyte solution. this electrolyte solution is a liquid with salts dissolved in water or organic solvents. poly (vinylidene fluoride-co-hexafluoropropylene) p(vdf-hfp) exhibits high polarity, good thermal and mechanical properties, and wettability by organic solvents. they are chemically stable and inert and possess porosity that is tailorable even through binary and ternary solvent or nonsolvent systems, therefore, we have used it as the separator material for our current model [19]. to quantify the impact of specific design variables on cell heat generation and electric performance in lithium-ion batteries, parametric studies were carried out using higher fidelity models. a coupled electrochemical-thermal model was developed by wu et al. [20] and bin huang et al. [21] to study the impact of electrode thickness and particle size on heat generation rate and battery performance [20] in cylindrical batteries [21]. a new modeling framework was employed by madsen et al. [22] to study the effect of design parameters like particle sizes on heat generation and battery performance. c-rate is a unit used to measure the rate at which a battery is fully charged or discharged relative to its capacity. at higher c-rates, smaller size electrode particles lead to a higher battery capacity in charge and discharge cycles. larger particle sizes and higher c-rates also increase overpotential in a battery [23]. these studies quantified the impact of various parameters in electrodes on heat generation of the battery where the effect of particle size was not monotonic across the discharge rates and the larger particle sizes increased heat generation and potential sweep rate. increasing the volume fraction of electrolyte in the electrodes will result in a large amount of li+ intercalation and deintercalation in the electrode, subsequently increasing the capacity fade, internal resistance, and polarization, resulting in a larger gradient of electrochemical properties and a higher rate of heat generation. hence, we chose the optimized volume fraction value that we obtained in our former work as 0.45 for the current model [5]. though the electrochemical behavior of silicon is well studied, most of the work is focused on halfcells with li metal as a counter electrode which cannot address the long-term cycling performance of silicon anodes [24]. the current work is focused on electrochemical modeling where we studied the h. dasari and e. eisenbraun j. electrochem. sci. eng. 13(4) (2023) 659-672 http://dx.doi.org/10.5599/jese.1558 661 effects of different particle sizes of the silicon electrode on the temperatures of the battery. for this purpose, we have used a full-cell configuration in our simulations where we have used a lithium nickel manganese cobalt oxide (nmc) cathode. in our previous work [5] nmc cathode has shown an improved relative capacity with a silicon anode, so we have used the same cathode in this work to understand its behavior with a silicon anode. a cylindrical structure was modeled to understand electronic current conduction in the electrodes and ionic charge transport in the electrodes and electrolyte/separator. methods model development the current cylindrical 2d thermal model employs heat transfer across a solid interface and has axial symmetry. it uses silicon as the negative electrode, lini1/3mn1/3co1/3o2 (nmc 111) as the positive electrode, and lipf6 in 2:1 ec: dmc and poly (vinylidene fluoride-co-hexafluoropropylene) p(vdf-hfp) as the separator. in a spirally wound battery, heat conduction in the spiral direction is not considered and wound sheets are modeled as one active battery material domain [25] as shown in fig 1. figure 1. layered spiral geometry of a cylindrical battery [25] model equations the cylindrical battery geometry (9 mm radius, 65 mm height) consists of three domains (shown in fig. 2): battery canister (steel, 0.25 mm thick), active battery material domain (wound sheets of cell material, 55 m), mandrel (isolator around which the battery cell sheets are wound, 2 mm radius). the initial state of charge of the battery is 20 %. the active material of the battery consists of several battery cells wound spirally into a cylinder. the current model has higher thermal conductivity along the battery sheets, which is in the cylinder length direction, and lower in the normal direction to the sheets, the radial direction [25]. the thermal conductivity in the radial direction, kt,r is calculated by eq. (1): i t,r i t,i l k l k =   (1) http://dx.doi.org/10.5599/jese.1558 j. electrochem. sci. eng. 13(4) (2023) 659-672 thermal performance of a lithium-ion battery 662 figure 2. cross-sectional cutaway view of the geometry of thermal model battery [25] thermal conductivity in the cylinder length direction, kt,ang is determined by eq. (2): i t,i t,ang i l k k l =  (2) the density of the active battery material is calculated by eq. (3): i i batt i l l   =   (3) similarly, the heat capacity of the active material is as given in eq. (4): i i p,batt i lc c l =   (4) governing equations, boundary conditions, and parameters used in the modeling are taken from the comsol material library [25]. material dependent thermal parameters are from published research works [26-28]. tables 2 and 3 describe the dimensions of the electrode, separator, and thermal parameters of the battery components. we have listed the various particle sizes considered in our model as case studies in table 4. table 2. electrochemical parameters of the model [25] parameter symbol nmc silicon separator solid phase li-diffusivity negative, m2 s-1 ds_neg 3.9 10-14 solid phase li-diffusivity positive, m2 s-1 ds_pos 10-13 max solid phase concentration negative, mol m-3 csmax_neg 278000 max solid phase concentration positive, mol m-3 csmax_pos 49000 volume fraction of the active material s,1, s,2 0.30 0.45 electrolyte salt concentration, mol m-3 c0 2000 particle size, nm rp varied varied reaction rate coefficient negative, m2 s-1 kneg 2 10-11 reaction rate coefficient positive, m2 s-1 kpos 2 10-11 positive electrode density, kg m-3 pos 2328.5 negative electrode density, kg m-3 neg 1347.33 length of the negative electrode, m lneg 55 length of the positive electrode, m lpos 55 length of separator, m lsep 30 charge transfer coefficient α 0.5 0.5 bruggeman coefficient  1.5 1.5 transference number t+ 0.363 specific surface area as,i eq. (9) eq. (9) average molar activity coefficient f 0 h. dasari and e. eisenbraun j. electrochem. sci. eng. 13(4) (2023) 659-672 http://dx.doi.org/10.5599/jese.1558 663 table 3. thermal parameters of the model thermal parameters symbol values positive current collector density, kg m-3 pos_cc 2770 negative current collector density, kg m-3 neg_cc 8933 separator density, kg m-3 sep 1008.98 positive electrode heat capacity, j kg-1 k-1 cpos 1270 negative electrode heat capacity, j kg-1 k-1 [26,27] cneg 680 positive current collector heat capacity, j kg-1 k-1 cpos_cc 875 negative current collector heat capacity, j kg-1 k-1 cneg_cc 385 separator heat capacity csep 1978.16 negative electrode thermal conductivity, w mk-1 [28] kt_neg 156 positive electrode thermal conductivity, w mk-1 kt_pos 1.58 table 4. test cases used for simulations case study rp / m nmc silicon case 1 8.0 12.5 case 2 8.0 0.100 case 3 0.100 2.000 case 4 0.100 0.100 results and discussion the electrochemical-thermal model was resolved using comsol multiphysics version 5.6 software. fig. 3 shows the temperature distribution in the battery for a silicon electrode particle radius of 12.5 m and nmc electrode particles 8 m for case study 1. the temperature time graphs in fig. 4 show that the peak temperature change in the battery over 2000 seconds charging and discharging cycle is 13 k. the temperature-time graphs show slope changes up to 1500 seconds at various points of time representing charging/discharging in the battery. the battery is charged until 300 seconds, followed by discharge until 600 seconds then charged again until 900 seconds and discharged until 1200 seconds, and then it is charged until 1500 seconds. at 1500 seconds, an open circuit voltage (or when the battery is disconnected) results in a fall in the temperature. the individual curves represent the maximum, minimum, and mean temperatures of the battery. similarly, fig. 5 shows the temperature distribution of the battery where the silicon electrode particle radius is 100 nm and nmc electrode particles 8 m for case study 2. fig. 6 shows the peak temperature change is 11.5 k in the battery for silicon electrode particle radius is 100 nm and nmc electrode particles 8 μm. fig. 7 and fig. 9 show the temperature distributions in the battery for silicon electrode particle radii of size 2000 nm and 100 nm, with constant particle sizes of 100 nm of nmc respectively for case study 3 and case study 4. similarly, fig. 8 and fig. 10 show that when the silicon electrode particle radii are 2000 nm and 100 nm, the peak temperature changes in the battery were 8.8 and 7.5 k, respectively. the increasing temperatures during charging/discharging were due to ohmic heating and entropy changes in the battery [25]. the differences between the maximum and minimum temperatures would not exceed 3 k using the current thermal modeling. entropy effects or reversible heating contributes to the heating rate difference between the charge and discharge in the battery. the temperature change was higher for larger (micron scale) particle sizes than the nanometersized particles. hence, nanoparticles reduce the temperature variations in a battery when compared to micro-sized particles. these results are summarized in table 4. http://dx.doi.org/10.5599/jese.1558 j. electrochem. sci. eng. 13(4) (2023) 659-672 thermal performance of a lithium-ion battery 664 time = 0 s surface temperature, k time = 300 s surface temperature, k time = 900 s surface temperature, k time = 1950 s surface temperature, k figure 3. simulated comparison of temperature distribution in the battery at different time periods when the radius of the negative electrode particles is 12.5 μm and nmc electrode particles 8 m (case 1) figure 4. temperature vs. time when the radius of the negative electrode particles is 12.5 μm and nmc electrode particles 8 m (case 1) 0 2 4 6 8 10 12 14 0 500 1000 1500 2000 2500 δ t / k time, s mean temperature max. temperature min. temperature h. dasari and e. eisenbraun j. electrochem. sci. eng. 13(4) (2023) 659-672 http://dx.doi.org/10.5599/jese.1558 665 time = 0 s surface temperature, k time = 300 s surface temperature, k time = 900 s surface temperature, k time = 1950 s surface temperature, k figure 5. simulated comparison of temperature distribution in the battery at different time periods when the radius of the negative electrode particles is 100 nm and nmc electrode particles 8 μm (case 2) figure 6. temperature vs. time when the radius of the negative electrode particles is 100 nm and nmc electrode particles 8 μm (case 2) 0 2 4 6 8 10 12 14 0 500 1000 1500 2000 2500 δ t / k time, s mean temperature max. temperature min. temperature http://dx.doi.org/10.5599/jese.1558 j. electrochem. sci. eng. 13(4) (2023) 659-672 thermal performance of a lithium-ion battery 666 time = 0 s surface temperature, k time = 300 s surface temperature, k time = 900 s surface temperature, k time = 1950 s surface temperature, k figure 7. simulated comparison of temperature distribution in the battery at different time periods when the radius of the negative electrode particles is 2000 nm (case 3) figure 8. temperature vs. time when the radius of the negative electrode particles is 2000 nm (case 3) 0 1 2 3 4 5 6 7 8 9 10 0 500 1000 1500 2000 2500 δ t / k time, s mean temperature max. temperature min. temperature h. dasari and e. eisenbraun j. electrochem. sci. eng. 13(4) (2023) 659-672 http://dx.doi.org/10.5599/jese.1558 667 time = 0 s surface temperature, k time = 300 s surface temperature, k time = 900 s surface temperature, k time = 1950 s surface temperature, k figure 9. simulated comparison of temperature distribution in the battery at different time periods when the radius of the negative electrode particles is 100 nm (case 4) figure 10. temperature vs. time when the radius of the negative electrode particles is 100nm (case 4) 0 1 2 3 4 5 6 7 8 0 500 1000 1500 2000 2500 δ t / k time, s mean temperature max. temperature min. temperature http://dx.doi.org/10.5599/jese.1558 j. electrochem. sci. eng. 13(4) (2023) 659-672 thermal performance of a lithium-ion battery 668 table 4. simulation results of the test cases case study rp / μm peak temperature, k nmc silicon case 1 8.0 12.5 13 case 2 8.0 0.100 11.5 case 3 0.100 2.000 8.8 case 4 0.100 0.100 7.5 heat generation the heat generation rate is dependent on the battery chemistry and kinetics. these factors influence heat generation, which greatly varies with the battery design. the battery chemistry is well defined for a battery system with a defined range of operating conditions. battery design parameters and operating conditions define the kinetics of the battery system. the following equations are involved in battery kinetics. the energy conservation in the battery occurs according to eq. (5) [8]: 2 2 2 p 2 2 2x y z t t t t c q t x y z         = + + +     (5) where,  / kg m-3 = density cp / j kg-1 k-1 = heat capacity  / w m-1 k-1 = thermal conductivity q = qrev + qirrev; qirrev = qohm + qact where, q / w m-3 = heat generation rate qrev / w m-3 = reversible heat qirrev / w m-3 = irreversible heat qohm / w m-3 = ohmic heat qact / w m-3 = polarization heat ohmic heat is created by the resistance to the flow of electrons in the solid phase and lithium ions in the electrolyte phase. ohmic heat is calculated by eq. (6) [8,29]: qohm = -iss ill (6) where s is electric potential which is the potential energy difference per unit charge between two points in an electric field and l is the electrolyte potential is the potential of electrochemical species in a solution. is = -ss (7) where s is the electrical conductivity. polarization heat is dependent on the sizes of the active material particles and the overpotential generated in the battery. it increases with the particle sizes of the electrodes, and it is calculated by eq. (8): qact = asiloc (8) where as is specific interfacial area of the porous electrode or specific surface area, eq. (9) as = 3s / r (9) r / m = radius distance variable of the solid particles  = overpotential. butler volmer equation is used for the local current density calculation, eqs. (10) and (11): h. dasari and e. eisenbraun j. electrochem. sci. eng. 13(4) (2023) 659-672 http://dx.doi.org/10.5599/jese.1558 669 a loc 0 cf f rt rti i e e      = −     (10) ( ) a a c a l 0 s,max s s l,eff c c c i fk k c c c c    = − (11) where a and c are the anode and cathode transfer coefficients and ka and kc are the reaction rate constants for the anode and cathode. reversible heat in the electrodes is caused due to entropy changes and is as given in eq. (12): rew s loc u q a i t t  =  (12) where, u / t is potential deviation of the electrodes resulting from entropy changes. the charge and mass conservation in the solution phase happen according to eqs. (13) to (16): ( )ll l l 2 ln 1 1 ln ln l rt f i t c f c    +   = −  + + −     (13) ( )tot l ll l l l i q c r t d c f t  + +  − = +    (14) il = itot + ql (15) charge conservation in solid phase, eq. (16): is = -itot + qs (16) mass balance of li in the particles is determined by eq. (17): ( )s s s c d c t  =    (17) where cs is the concentration of li in solid phase, ds is solid phase li diffusion coefficient. some of the temperature-dependent parameters include electrolyte conductivity, the diffusion coefficient of electrolyte, and the potential deviation of the electrodes [8,29]. these parameters influence the temperature versus time graphs in each case study, leading to discontinuities in the temperature curves. as shown in the above formulas, polarization heat and reversible heat are dependent on the size of the active electrode particles. decreasing the particle sizes decreases polarization heat and reversible heat in the battery. a reduction in particle size leads to an increase in the specific surface area. specific surface area is one of the contributing factors in the reduction of solid phase diffusion polarization. it was observed that, as the particle size decreased, the heat generation in the battery decreased. the reason behind the phenomenon is that the particle size influences the solid phase diffusion polarization [8]. when the electrode active material particle sizes are smaller, they provide shorter distances for insertion and de-insertion. consequently, the larger specific surface area of smaller particles ensures lower surface current density which leads to a decrease in solid phase diffusion polarization [8]. most of the published research works were focused on how electrode particle sizes effect overpotential, capacity and the heat generation rate [23]. our work estimated the temperature change with respect to time and different particle sizes of the negative electrode. conclusion our current full cell model found that si anodes with nano-sized particles reduce the temperature of the battery compared to larger particles in li-ion batteries. the primary reason for this observed behavior is believed to be that li+ ions take less time to diffuse across smaller particles and hence lead to a smaller increase in the battery temperature. reduction in the diffusion length of lithiumhttp://dx.doi.org/10.5599/jese.1558 j. electrochem. sci. eng. 13(4) (2023) 659-672 thermal performance of a lithium-ion battery 670 ion in the insertion or extraction process by nanoparticles enhances the rate performance of the electrode material, thus increasing the capacity and overall performance of lithium-ion batteries. list of symbols and abbreviations 𝑎𝑠 / m 2 m-3 specific surface area il / a m-2 electronic current density in the solid phase ql electrolyte current source itot the sum of all electrochemical current sources cl / mol m-3 electrolyte salt concentration f average molar activity coefficient qs current source term rl total li+ source term in the electrolyte r / m radius distance variable of the solid particles iloc / a m-2 local current density i0 / a m-2 exchange current density t / k battery temperature r gas constant, 8.314 j mol-1 k-1 l / s m-1 electronic conductivity of solid phase s / s m-1 ionic 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https://doi.org/10.1115/1.4032012 https://doi.org/10.1002/aenm.201700715 http://www.comsol.com/ http://www.thermophysics.ru/pdf_doc/autoplay/docs/collectionofmanuscripts/ectp2005paper139.pdf http://www.thermophysics.ru/pdf_doc/autoplay/docs/collectionofmanuscripts/ectp2005paper139.pdf https://doi.org/10.1023/a:1017562709942 https://doi.org/10.1002/9780470068328.app7 https://doi.org/10.1016/j.apm.2018.04.001 https://creativecommons.org/licenses/by/4.0/) electrochemical sensing of caffeic acid antioxidant in wine samples using carbon paste electrode amplified with cdo/swcnts http://dx.doi.org/10.5599/jese.1645 825 j. electrochem. sci. eng. 13(5) (2023) 825-838; http://dx.doi.org/10.5599/jese.1645 open access : : issn 1847-9286 www.jese-online.org technical report development of sensor nodes and sensors for smart farming moritz schlagmann1,2,, jos balendonck3, thomas otto2,4, michelle brandao silva de assis5,6, michael mertig5,6 and stefan hess1 1infineon technologies ag, am campeon 1-15, 85579 neubiberg, germany 2technische universität chemnitz, reichenhainer str. 70, 09107 chemnitz, germany 3wageningen university & research, droevendaalsesteeg 1, 6708 wageningen, the netherlands 4fraunhofer enas, technologie campus 3, 09126 chemnitz, germany 5kurt-schwabe-institut für messund sensortechnik meinsberg e.v., 04736 waldheim, germany 6physikalische chemie, technische universität dresden, 01062 dresden, germany corresponding author:  moritz.schlagmann@infineon.com; tel.: +49-89-234-39048 received: december 23, 2022; accepted: june 12, 2023; published: july 12, 2023 abstract the world population is continuously increasing. smart farming is required to keep up with this development by producing more food in a sustainable way. in many new sensor solution developments, the results of the sensor itself is at the target, but the whole solution fails to meet the requirements of the agriculture sensing use cases: the developments suffer from singular approaches with a constricted view solely on the sensor, which might be exchangeable. in this article, we present a holistic approach that can help to overcome these challenges. this approach considers the whole use case, from sense, compute, and connect to power. the approach is discussed with the example of the plantar project, where we develop a soil nitrate sensor and a new leaf wetness and microclimate sensor for application in a greenhouse. the resulting sensor is integrated into a sensor node and compared to a state-of-the-art system. the work shows what is needed to assess the best tradeoffs for agriculture use cases based on a horticulture application. keywords agriculture; sensor systems; leaf wetness sensor; nitrate sensor; plantar; sustainability introduction holistic sensor system development ramping up a new sensor use case is a long process that does not rely solely on the sensor. therefore, a holistic approach is needed to find an optimal tradeoff, for example, between sensor node lifetime and measurement rate for the need of the specific use cases. new results of advances in sensor development, material improvements [1], new algorithms [2], and the next generation of http://dx.doi.org/10.5599/jese.1645 http://dx.doi.org/10.5599/jese.1645 http://www.jese-online.org/ mailto:moritz.schlagmann@infineon.com j. electrochem. sci. eng. 13(5) (2023) 825-838 sensors for smart farming 826 wireless communication protocols [3] are published frequently. however, the development and integration of a whole use case solution that takes all aspects into account has not been reported yet. with the so-called holistic sensor system development approach, all system levels are considered, from sensing over connection and computation to power. a cross-functional team has to work together, from the sensor and the integrated circuit (ic) technology to the software, including layers of data analytics. where a single component can not be optimized without the consideration of the remaining components from the whole sensor system. within the sensor system, the network links the sensor nodes to the user and builds the data foundation for smart farming. a sensor network, for example, a monitoring system of a high-tech greenhouse, can consist of several sensor nodes. the sensor node has four main tasks sensing, computing, communication, and powering, visualized in figure 1. the sensor digitalizes real-world influences. the computing part is for data management, analytics, and security. power for energy supply and communication connects the sensor node with the user. these nodes can differ by the combination of sensors or other components. they can be as small as 1x1 mm2 if optimized for size or even attached to birds to track their annual migration, as well as agriculture projects [4-6]. however, commercializing this sensor and sensor nodes is challenging and depends on finding the best tradeoffs with a holistic approach. in the following sections, we take a closer look at each component of a sensor node. figure 1. sensor with four main tasks: sensing, computing, connecting, and powering sensor system development for smart farming for example, the eu-funded research project plantar follows the holistic approach described above, covering the complete value chain. the sensor developers contribute new sensing methods, like the leaf wetness and nitrate sensor. the technology partners provide biodegradable integration techniques, heterogeneously integrated materials, communication solutions, and artificial intelligence. platform owners with direct customer access deploy the sensor solutions with their expertise to demonstrate and apply the sensor use case. figure 2. shows all the partners and their contributions. the variety of partners is beneficial to tackle the challenges from all perspectives. on the other hand, this approach raises additional challenges because multiple stakeholders and developers must align for co-development and co-optimization. the cross-functional project team needs a shared understanding of the challenges of the specific use case within reliable partners in an ecosystem over the whole value chain. m. schlagmann et al. j. electrochem. sci. eng. 13(5) (2023) 825-838 http://dx.doi.org/10.5599/jese.1645 827 figure 2. holistic sensor development with partners covering the whole value chain. in celadon green on the left side are the sensor developers. marked with green circles are the sensors presented at the 9th kurtschwabe-symposium. in pink red are the technology and knowledge providers. in green on the right side are the platform owners nevertheless, it is essential for a successful use case solution to apply a holistic approach. exemplary for the interaction between the levels is that the sensor hardware depends on whether edge artificial intelligence is used. edge artificial intelligence is the latest trend to implement artificial intelligence optimized for low-power devices, like microcontrollers. therefore, the microcontroller has to be capable of performing these computations. another dependency is data analysis, whether directly at the sensor, at the edge of a microcontroller, or in the cloud. the last level is connectivity and security. these dependencies converge together in the sensor node. as the sensor node is an integral part of the sensor system. sensor nodes sensing sensing environmental factors is a broad domain with various applications. excellent facilities for sensor development are based in europe. for example, micro-electrical and mechanical system (mems) microphones are widely used in smartphones and mems pressure sensors in automotive applications. the potential for sensor usage in agriculture is abundant and the number of connected devices will continue to grow [7,8]. the world population is growing, while the available resources are limited. the european commission tackled these challenges with the farm-to-fork strategy to reduce the use of chemical pesticides by 50 % and fertilizer by 20 % until 2030. the goal is to have sustainable food production. the agricultural sector shows an annual growth rate of 21.2 % in global spending on enterprise internet of things (iot) technology. that makes it one of the sectors with the highest spending increase in the iot market [9]. reviews on this topic emphasize the benefit of recent iot developments [10,11]. http://dx.doi.org/10.5599/jese.1645 j. electrochem. sci. eng. 13(5) (2023) 825-838 sensors for smart farming 828 this is expected, as sensors are the key element to smart farming. they provide information about plants, conditions, time and place and enable growers to apply the required amount of raw materials at the right place and at the right time. the use cases for sensor nodes identified in the eu-funded plantar project vary greatly depending on the deployment area. figure 3 shows the three identified application domains, namely, arable farming, non-arable farming, and indoor farming. table 1 gives each domain its characteristic requirements. figure 3. the three identified use cases groups for smart farming. non-arable farming, orchids and ornamental trees in cities. indoor farming, like cucumber in greenhouses. arable farming, like wheat on open fields table 1. uses cases for smart farming deployment area non-arable farming indoor farming arable farming example ornamental trees, plant nursery cucumber, tomato, lettuce grain, pulse, oil seed sensor density low medium high low medium price medium medium ultra low low biodegradability no priority no priority advantage sense 1 leaf wetness sensing sensors translate a stimulus into a digital value. in recent years, the complexity of sensors has been highly increasing. the trend is going to low-power, heterogeneous sensor systems. application experts provide an understanding of the stimulus of the sensor use case. typical definitions are: what kind of stimulus, what is causing the stimulus, what sensitivity is necessary, and how rapidly or slow is it changing [12]. the challenge is to find the best tradeoff between general sensor performance, power consumption and cost efficiency by assessing the use case in a holistic way and understanding the stimulus in detail. for example, leaf wetness and microclimate sensing are used for integrated pest management because moisture and temperature can be linked to how diseases spread [13,14]. the duration of the presence of liquid water, temperature and relative humidity on the crop surface is measured. the main sources of moisture are rain, fog, overhead irrigation or dew. in this case, the stimulus is moisture, and the change is in the order of minutes. with miniaturized and cost-efficient sensors, more sensors can be deployed in the greenhouse and measure the microclimate at the leaf. this matters because temperature and relative humidity within the canopy can substantially differ from the ambient air in the greenhouse [15]. furthermore, in greenhouses, there can be a large spatial variability of temperature and relative humidity, which can cause local outbreaks of diseases [16]. state-of-the-art sensors use resistive or capacitive measurement principles to detect moisture, as shown in figure 4. these bulky sensors mimic the properties of leaves. with special coatings, the m. schlagmann et al. j. electrochem. sci. eng. 13(5) (2023) 825-838 http://dx.doi.org/10.5599/jese.1645 829 sensors try to have the same radiation absorption and, thus, the same thermal properties. the goal is the same condensation and evaporation rate as the real leaf [17,18]. figure 4. state of the leaf wetness sensor. left: capacitive sensor (metergroup) [17]; right: resistive sensor (spectrum) [18] the leaf wetness sensor developed in the plantar project uses a capacitive measurement principle. this mems sensor is miniaturized to be attached to the real leaf and is intended to have very similar properties to the leaf. by miniaturization, it is possible to measure directly at the leaf. the sensor is coated with biocompatible parylene c. the coating serves as a protective layer and as a sensitive layer. underneath the sensitive layer are the sensing electrodes. the combination of the sensitive material and copper electrodes are the sensitive capacitors, as shown in figure 5. the microclimate parameters change the dielectric properties of the sensitive layer. the sensing and reference capacitors are arranged in a wheatstone bridge. reference capacitors are independent of the changes in the sensitive layer. an application-specific integrated circuit (asic) provides an excitation voltage and converts the cross voltage of the bridge, caused by the capacitance change, to a digital value [19]. figure 5. left: schematic sketch of the sensor's electrode configuration with electric field lines and sensitive layer (orange). right: schematic of the capacitor arrangement this digital data is then available for further processing. this sensor asic combination was mounted three times in a ceramic package in order to test three sensors in parallel. for the characterization under laboratory conditions, a 3d gas box was designed and various humidity levels were set to calibrate the sensors, shown in figure 2 of [19]. in addition to the biocompatible materials, standard materials were tested to compare their sensitivity. the results, shown in figure 4 and table 1 of [19], proved the working principle of the biocompatible sensor. after the characterization in the laboratory, the sensor is ready to be tested in the greenhouse environment. the sensor configuration is adapted in serval steps for the test. the sensor on a printed circuit board is shown in figure 6. http://dx.doi.org/10.5599/jese.1645 j. electrochem. sci. eng. 13(5) (2023) 825-838 sensors for smart farming 830 figure 6. leaf wetness sensor on a pilea peperomioides leaf sense 2 nitrate sensing the request for mobile, decentralized usable chemical analysis methods for the determination of nitrate content as one important parameter of agriculture and the environment is permanently growing. the main reason for this is the intensive fertilization in farming, resulting in high nitrate concentrations in the soil as well as surface and groundwater bodies. in many groundwater samples, the legal limit of 50 mg/l is exceeded. the methods for soil nitrate determination currently on the market have, among others, disadvantages in terms of sample handling, energy, time consumption, and costs. ion chromatography is one example. as a highly precise method, it is commonly used to determine the nitrate content in environmental samples. however, ion chromatography cannot be used in field measurements it is cost-intensive and needs well-trained personnel to run the measurements. other state-of-the-art methods, like photometric cuvette tests, are simpler to run, but in most cases, they are lagging sensitivity and accuracy. moreover, they may require the use of toxic chemicals or special preparation of the samples. potentiometric ion analysis using electrochemical sensors has shown to be suited for nitrate determination on site. however, commercially available nitrateselective electrodes are still cost-intensive and, even more important, contain glass parts and fluid inner electrolytes. the latter makes their handling under field conditions difficult. to the best of our knowledge, no sensor system is available that allows long-term, autarkic on-site monitoring of nitrate, combined with intelligent data acquisition and iot-based transfer. to overcome these difficulties, within plantar ion-selective nitrate sensors based on graphite electrodes in all-solid-state configuration are developed (figure 7). each sensor has a working electrode based on graphite and a reference electrode based on silver/silver chloride, both in an allsolid-state configuration. they are made by screen-printing technology and contain neither fluidic electrolytes nor glassy components. the modification of these electrodes determines the sensor performance in terms of nitrate detection, stability, and reproducibility. focusing on these aspects, the graphite electrode modified with polypyrrole as conducting polymer and a final ion selective membrane layer with complexing compounds showed promising results. conductive polymers are well known in the literature as solid contact materials due to their electrochemical properties. their electrical conductivity is related to the formation of charge carriers in the polymer backbone, either by oxidation or reduction. moreover, the presence of an additional charge requires the uptake of m. schlagmann et al. j. electrochem. sci. eng. 13(5) (2023) 825-838 http://dx.doi.org/10.5599/jese.1645 831 ions, e.g., from a solution, to maintain electroneutrality. thus, the effect also makes it an ion exchange matrix. the coating with the polymer polypyrrole was carried out by electropolymerization by cyclic voltammetry from the monomer pyrrole. for the ion-selective membrane, the nitrate complexing compound tridodecylmethylammonium nitrate was immobilized equally in a polyvinyl chloride membrane (pvc). this membrane is responsible for selective and reversible forms of complexes with the desired analyte from a sample. the ion-selective membrane was applied by drop-casting method on top of the polypyrrole layer [20]. figure 7. schematic view of the screen-printed, potentiometric sensor the working electrode and reference electrode are built on one planar alumina substrate (50x15x0.63 mm) used as the sensor body, which allows better miniaturization of the sensor. the production of this intrinsically robust sensor is cost-efficient and it features another interesting property. the overall mass of one sensor is only about 3 g, and it consists of more than 99 % of bio-inert materials, i.e., it shows already in the current state a very high degree of biocompatibility. figure 8 shows an example of the potentiometric response of the sensor to ammonium nitrate (nh4no3) standard solutions in the range between 10-5 to 1 mol/l, and the corresponding calibration curve. the sensor shows a linear nernst behavior over 5 orders of magnitude. thus, it can potentially be used for nitrate detection in water (c 50 mg/l) as well as in soil with higher nitrate concentrations in the range of 1 g/l. thorough materials’ design of the working electrode has led to sensors that allow for reproducible nitrate detection in real samples over periods of months [20]. t / min log (c / mol l-1) figure 8. left: sensor response to nh4no3 standard solutions. right: linear calibration curve e / m v v s. a g /a g c l m o d if . e / m v v s. a g /a g c l m o d if . http://dx.doi.org/10.5599/jese.1645 j. electrochem. sci. eng. 13(5) (2023) 825-838 sensors for smart farming 832 in this investigation, the process of conditioning, calibration, and nitrate detection of the samples was performed with a working electrode sensor modified with polypyrrole and graphite. the process was repeated at different times and with the same sensor. this experiment also showed that the sensor can be repeatedly dried out and reconditioned, still detecting the correct nitrate concentration. moreover, it turned out that the working electrode is mechanically stable enough to position the sensor directly into the soil and determine the nitrate content as long as the soil is wet. in figure 9, an example of that is given. it shows the result of an experiment where an electrochemical sensor could be inserted directly into a wet soul sample without any extra protection on the working electrode membranes and detect stable voltage signals over time after being conditioned and calibrated in standard nitrate solutions. the results are summarized in table 2. reasonable agreement is observed with the nitrate concentration value added to the soul samples (0.01 mol/l) and with the reference measurements from the ion chromatography of the supernatant of this centrifuged sample. t / min figure 9. measurement in a wet soil sample. the soil was poured with 0.01 mol/l of nitrate solution. the sensor measurement was directly done in the soil (sludge). for reference measurements, the sludge was centrifuged, and the supernatant was analyzed by using the potentiometric sensor and by ic. left: photograph of the sample. right: potentiometric response of the sensor: calibration in standard solutions of known concentrations, followed by measurements in sludge and after centrifugation table 2. measurement results in a wet soil sample. measured by nitrate content in sludge in supernatant of centrifuged sludge mg/l mol/l mg/l mol/l potentiometric sensor 859.1 0.013 981.9 0.015 ion chromatography 844.63 0.014 these investigations suggest that it is advantageous for the interpretation of the measured nitrate values to combine the potentiometric measurements with the determination of soil moisture and temperature. at this point, the holistic approach of sensor system development proves to be the true one, easily realized. compute in analogy to humans, a sensor node needs a brain to compute, process and store information. within sensor nodes, microcontrollers or microprocessors perform these tasks. these ics consume e / m v v s. a g /a g c l m o d if . m. schlagmann et al. j. electrochem. sci. eng. 13(5) (2023) 825-838 http://dx.doi.org/10.5599/jese.1645 833 a significant part of the available energy budget, strongly related to the sensor node lifetime. highperformance tasks, like security encryption, data processing, and artificial intelligence, are energyconsuming. optimization techniques can increase energy efficiency, e.g. power-mode variations, batching, software and computation level adjustments [21]. the cross-functional team needs to find the best tradeoffs between computational power, storage size, security capabilities, availability of analog and digital interfaces and energy consumption. for this, they need to assess the data size and measured values, whether the analysis is performed on edge or in the cloud, redundancy, and possible sensor combinations. for example, e-noses for pathogen detection generate a complex data set from which the relevant features and measurement points need to be extracted for further analysis. this can be done on the edge, at the sensor, and only relevant findings are communicated. in contrast, a leaf wetness sensor measures only one value, but several sensors are needed to cover a full canopy. therefore, less computational power is needed, but more sensor nodes must be linked over the cloud. the digital values are read-out from the asic and calculated based on calibration. this value is then stored and ready to be sent to the system. connect the sensor node communicates the measured and processed data either to other nodes or to a gateway. a gateway allows the connection from a low-power wide area network to another network, for example, an internet cloud server. recent developments result in a broad variety of wireless communication solutions with different strengths and weaknesses. the solutions differ greatly in the range, data rate, topology and more [22]. for example, low power wide area networks (lpwan) are comparable in lorawan, sigfox and narrow band iot (nb-iot). for example, sigfox and lorawan operate in the unlicensed frequency range of the industrial, scientific, and medical (ism) band, while nb-iot is in the licensed frequency range of the lte frequency bands. this is an important consideration for the possible interference in the unlicensed band, where any device can send and receive. whereas, in the licensed band, a telecommunication provider is responsible for the quality of service. [23] it is on the developers to decide which hardware best fits. depending on the use case, the best tradeoffs have to be found between communication range, data bandwidth (how much data can be sent), data rate and latency, cost of ownership, reliability and power consumption. for this decision, several points have to be assessed like available wireless technology, system architecture, measurement and reaction rate, as well as available infrastructure. for example, in greenhouses as compared to fields for arable farming range can drastically differ. sensor nodes with a range of less than 300 m can be useless in large open fields but work perfectly fine in greenhouses. depending on the sensor system, the data bandwidth can be only a few digital values of temperature and humidity or pictures to determine ripening of fruits. depending on the stimulus, latency is important, whether to receive immediate or delayed feedback from the sensor node. in remote areas, cellular connections are not always available, depending on the cost of ownership to setup and reliability, a wireless network can be setup and maintained by a provider or a farmer. finally, yet importantly, wireless data can be very power-consuming. in general, more bandwidth costs more energy. moreover, sending data over a longer range takes more power output. balancing the factors of bandwidth, range, and latency can be specifically challenging. http://dx.doi.org/10.5599/jese.1645 j. electrochem. sci. eng. 13(5) (2023) 825-838 sensors for smart farming 834 power the backbone of the sensor node is the power source. for long-term monitoring and measurements, sufficient power must be stored and reliably retrieved. for energy provisioning, power management must monitor and control energy storage, like batteries, and potentially energy harvesting, e.g., solar panels. the best fit for energy storage depends on various factors, like energy and power density, battery discharge, energy capacity, operating temperature, and rechargeability. typical non-rechargeable batterie compositions are alkaline (alk), lithium manganese dioxide (lmd), and rechargeable compositions are lithium polymer (lipo) [24]. an energy harvester can ease maintenance because the sensor node does not need battery exchange services and saves the resources of non-reusable power storage production. different energy sources and their combination are under investigation, like solar, wind, radio frequency, or vibrations [25]. for co-optimizing the sensor use case, the best tradeoffs have to be selected for the power source, power storage and/or energy harvesting, voltage level, maximum current and power density. with energy-aware optimization of data processing, encryption, and wireless communication, like “think before you talk” and “race to sleep”, significant improvements are possible [24]. the best tradeoffs are found when the following is assessed: the available infrastructure and use case environment, the components of the sense-compute-connect solution and the desired sensor node lifetime, one harvesting season or several years. for example, a biodegradable sensor node can remain on the arable field, so the battery needs to last for one growing season. one data transmission per day is sufficient. therefore, a biodegradable battery can be the best choice. this mostly biodegradable sensor node is planned within the plantar project. whereas non-biodegradable, more expensive sensor nodes are expected to last longer, as in the use case of indoor farming in greenhouses. demonstration of sensor nodes for smart farming for the next step towards the demonstration of the plantar sensor node in indoor farming, a first demonstrator for the greenhouse application is developed, as shown in figure 10. figure 10. left: commercial datalogger (campbell cr1000x) for state-of-the-art leaf wetness sensors in the red frame. in the blue frame is the current compute, connect and power components of the new leaf wetness sensor. right: sens element of the leaf wetness sensor attached to a cucumber leaf m. schlagmann et al. j. electrochem. sci. eng. 13(5) (2023) 825-838 http://dx.doi.org/10.5599/jese.1645 835 this intermediate step allows the gathering of data without the constraints of highly optimized solutions, like low data rates and power restrictions. as the sensing component, the sensor was mounted and wire-bonded to a printed circuit board (pcb) and coated with parylene c. an additional 3d printed housing ensures protection and constant distance to the leaf. for communication and to power the sensor, we connected the sensor to a raspberry pi. the raspberry pi is a single-board computer widely used for research projects. the performance, connectivity, ease of use and modularity make it a good fit for rapid prototyping. power was retrieved from a wall plug. communication was established with a wired connection. a python program was programmed to initialize the sensor node and collect data. the sensor values were transmitted to an online database. with this configuration, a rapid demonstrator, the parameters of this use case were recorded for further investigations. the sensor was deployed and tested in a venlo-type greenhouse compartment at the wageningen university and research center in bleiswijk, the netherlands. in this study, cucumber was used as a crop. the large, sturdy cucumber leaves are suitable for this application. the grower’s goal is to maximize crop growth, but the climate, especially high air humidity or longer periods of leaf wetness, may affect production due to disease outbreaks. in greenhouses, several parameters are monitored and controlled, like temperature, humidity, radiation, co2 level and opening angles of windows. the goal is to maximize plant growth and development. while the climate is also affecting the development and activities of diseases. the leaf wetness sensor data could be used additionally to the existing sensors to warn in case of local nonoptimal climate conditions that could invoke outbreaks of diseases. figure 11 shows an example of the recorded data for one week. in the top graph, the leaf wetness sensor response left y-axis, and the relative humidity on the right y-axis, are plotted. date and time, h figure 11. collected greenhouse data. top: leaf wetness sensor and relative humidity from central climate control. center: accumulated moisture time of state-of-the-art sensor, reset every day. bottom: window opening of windward and leeward side w in d o w o p e n in g , % d u ra ti o n , m in le a f w e ln e ss s e n so r, a .u r e la ti v e h u m id it y, % http://dx.doi.org/10.5599/jese.1645 j. electrochem. sci. eng. 13(5) (2023) 825-838 sensors for smart farming 836 the middle graph shows the response of the state-of-the-art leaf wetness sensor [17]. once the sensor detects moisture, the minutes are counted until the sensor is dried up. this value resets every day. for example, in the middle of 21.10.22 moisture is detected. the minutes are counted up over 100 minutes until the moisture evaporates. the duration of wetness is then constant until it is reset to zero the next day. in order to check the correlation with other events in the greenhouse, the bottom graph shows the window opening in percent of the windward and leeward windows. conclusion and outlook developing solely a sensor is not sufficient to solve a use case challenge. the development of a whole sensor solution takes years and is not only relying on the sensor alone. the goal is to have a wide impact with the innovative sensor use case. a holistic approach must tackle the use case challenges with partners over the whole value chain by co-development and co-optimization. this raises skills and organizational challenges for the cross-functional teams between partners and ecosystems. however, with this approach, the risk can be mitigated, and the best tradeoffs between the discussed solutions for each component can be identified for smart farming. with the results of this first trial, we determine the sensing capabilities under relevant conditions and the needed data rate. once algorithms are developed to extract the important features of the recorded data, the needed computational power can be assessed. accordingly, to the sensor response, the communication rate is selected for the best tradeoff between latency and power consumption. with these results at hand, the power components for power storage or power harvesting can be selected. the miniaturized potentiometric nitrate sensor showed from the beginning accurate results for real samples. further investigations demonstrated that the sensor can be used for nitrate determination over months, even after extreme conditions for an electrochemical sensor. the robustness of the electrodes allowed them to be inserted directly into wet soil and measure consistent nitrate values. all these features make this potentiometric sensor promising for versatile monitoring, easy 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bradai, s. khriji, g. bouattour, d. el houssaini, m. ben ammar, s. naifar, a. bouhamed, f. derbel, c. viehweger, energy-aware system design for autonomous wireless sensor nodes, sensors 21 (2021) 548. https://doi.org/10.3390/s21020548 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.5162/15dss2021/p4.4 https://doi.org/10.1109/wf-iot48130.2020.9221219 https://doi.org/10.1109/wf-iot48130.2020.9221219 https://doi.org/10.1016/j.jnca.2019.102459 https://doi.org/10.1109/percomw.2018.8480255 https://doi.org/10.3390/s21030913 https://doi.org/10.3390/s21020548 https://creativecommons.org/licenses/by/4.0/) lignin-based porous junction for silver-silver chloride reference electrodes http://dx.doi.org/10.5599/jese.1520 817 j. electrochem. sci. eng. 13(5) (2023) 817-824; http://dx.doi.org/10.5599/jese.1520 open access : : issn 1847-9286 www.jese-online.org original scientific paper lignin-based porous junction for silver-silver chloride reference electrodes stefan breitenbach1,2, dominik knapic2, christoph unterweger1,, christian fuerst1 and achim walter hassel2 1wood k plus – kompetenzzentrum holz gmbh, altenberger strasse 69, 4040 linz, austria 2institute of chemical technology of inorganic materials (tim), johannes kepler university linz, altenberger strasse 69, 4040 linz, austria corresponding author: c.unterweger@wood-kplus.at; tel.: +43 732 2468 6758 received: september 14, 2022; accepted: december 30, 2022; published: january 30, 2023 abstract carbonized lignin powder was used as a salt bridge for a silver-silver chloride reference electrode. this easy-to-prepare reference electrode exhibited excellent stability in saturated potassium chloride solution. in addition, the electrochemical impedance spectra showed that the prepared reference electrode is stable in acidic, neutral, and basic aqueous solutions (ph 1 12) and has similar impedances to its glass frit equivalent. keywords porous carbon material, carbonization, impedance spectroscopy introduction a reference electrode with its constant equilibrium potential provides the reference point for each electrochemical measurement. by far the most commonly used reference electrode is the silver-silver chloride electrode, due to its advantageous characteristics like low cost, high stability, reproducibility of potential and electrochemical reversibility [1]. this electrode consists most often of a silver wire coated with silver chloride in a plastic or glass tube with a diaphragm at the end. here, the diaphragm serves as a salt bridge between the reference solution of the reference electrode and the electrolyte of the system under investigation. commercially available are junctions made of glass [2-3], polyethylene [4], polytetrafluoroethylene [5-6], aluminium oxide [7], or as agar-based gel electrolyte [8-10]. the most common is the use of glass frits, as they have excellent chemical resistance in many solvents. however, they cannot be used in either strongly alkaline or fluorine-containing solutions. the same applies to diaphragms made of aluminium oxide [11]. porous teflon and polyethylene frits can be used here, but the larger pores in these materials can cause the electrolyte solution to become contaminated with the reference solution [12] and their hydrophobicity makes wetting of the frits difficult [13]. gelled materials, like agar-gel, on the other http://dx.doi.org/10.5599/jese.1520 http://dx.doi.org/10.5599/jese.1520 http://www.jese-online.org/ mailto:c.unterweger@wood-kplus.at j. electrochem. sci. eng. 13(5) (2023) 817-824 lignin-based porous junction for ag|agcl reference electrodes 818 hand, might suffer from biofouling [13] and cannot be used at temperatures above 40 °c because they decompose thermally [10]. carbon materials are ideally suited for use in electrochemical systems due to various advantageous properties like high chemical and thermal resistance, good conductivity, controllable porosity etc. [14-17], with bio-based carbons offering additional benefits regarding low cost and sustainability aspects [18-19]. however, it is difficult to produce dimensionally stable carbon materials without the use of large amounts of additives that would affect the electrochemical measurement when using the carbon material as a salt bridge for a reference electrode. lignin is the second most abundant biopolymer after cellulose [20] and available in large quantities with an annual production of 70 million tons, however, it is almost completely burnt for energy recovery [21]. due to the high carbon yield and the fact that it is a renewable raw material, lignin is a promising precursor for the production of carbon materials. it has been used for carbon fibers [22-26], carbon nanofibers [22,27,28] or particulate carbons [29-32]. as lignin partially melts during carbonization [14], these materials usually require a stabilization step or very low heating rates during carbonization in order to avoid structural deformation during thermal treatment [28,33]. however, this also offers the opportunity for preparation of porous structural bodies from lignin. as partial melting does not have to be avoided, in fact it is required for the formation of stable bodies, the conversion to carbons can be done at industrially viable process conditions. previous work testing carbon materials as diaphragms for reference electrodes have used commercial carbon materials such as drawing charcoal or pencil lead [11,34], the composition of which is not known. in addition, these materials cannot be optimized to meet the desired requirements, since it is difficult to optimize the carbon structure, especially the formed pore structure, after the carbonization step. based on the mentioned drawbacks of frequently used glass frits, the promising results using carbon materials and the possibility to easily prepare bio-based porous carbon bodies, in this work, lignin powder is carbonized and investigated as a salt bridge for a silver-silver chloride reference electrode. the electrochemical performance is tested in different solutions and compared with a commercial glass frit. experimental all solutions were prepared from reagent-grade chemicals and ultrapure water. the softwood kraft lignin indulin at (ingevity, usa) was used without any further purification. a commercial ag|agcl|sat. kcl reference electrode (6.0726.110, metrohm ag, switzerland) was used for the comparative investigations. for the preparation of porous junction, 50 mg of the kraft lignin indulin at was placed in a hole with a diameter of 6 mm, which was previously drilled into a graphite plate. in this cast, the lignin was carbonized at 850 °c with a heating rate of 1 °c min-1 in nitrogen atmosphere. the sample was kept isothermal at the target temperature for 30 min before cooling. silver chloride was deposited electrochemically on a silver wire (diameter 1.0 mm, purity 99.9 %) in 1 m hcl. the electrode was first cleaned and roughened by cycling 10 times between 0.5 v (against standard hydrogen electrode (she)) and -0.1 v (she) with a scan rate of 10 mv s-1. afterwards, silver chloride was deposited by polarizing to 0.1 v (she) for 120 s and finally to 0.3 v (she) for 600 s. after rinsing with deionized water and drying in air the wire was ready for use. the coated silver wire was placed in a polyurethane (pu) housing and closed at the top with a lid equipped with a connector (figure 1). either the carbonized lignin or a glass frit (4 nm pore size, s. breitenbach et al. j. electrochem. sci. eng. 13(5) (2023) 817-824 http://dx.doi.org/10.5599/jese.1520 819 gamry instruments inc., usa) was attached to the bottom of the pu housing as a porous junction using a heat-shrinkable tube. saturated kcl solution was used as the electrolyte. the morphology of the carbonized lignin sample was investigated using the scanning electron microscope phenom prox (thermo fisher scientific inc., usa). the used acceleration voltage was 15 kv and a backscattered electron detector was employed. figure 1. schematic representation of the reference electrode used stability measurements on the constructed reference electrode were performed in a 2-electrode setup against the commercial reference electrode transiently in saturated kcl for 200 ks (≈55.6 h). electrochemical impedance spectroscopy (eis) was initially performed at the open circuit potential of the electrode in saturated kcl in a 2-electrode setup. for other solutions, a 3-electrode setup was used with graphite as the counter electrode and the commercial ag|agcl|sat.-kcl electrode as the reference. an amplitude of 10 mv was always used. the potentiostat vertex.one (ivium technologies bv, the netherlands) was used for all electrochemical measurements. results and discussion carbonization of the lignin powder in the cast resulted in a molded body with a diameter of about 5 mm and a height of about 7 mm. the morphology of the sample was examined by sem (figure 2). figure 2. sem images of the carbonized lignin; the black box in (a) represents the zoom shown in (b) the sem images show that the carbon body is highly porous. presumably, the lignin particles melt during carbonization and crosslink, which leads to the structure of the carbon body. the conversion of lignin to carbon occurs with the formation of volatile gases [35], which contribute to the formation of high porosity. in this way, pores are created that range from a few nanometers to several hundred micrometers in diameter. http://dx.doi.org/10.5599/jese.1520 j. electrochem. sci. eng. 13(5) (2023) 817-824 lignin-based porous junction for ag|agcl reference electrodes 820 to characterize the fabricated reference electrode with the carbon diaphragm, its open circuit potential was measured against a commercial ag|agcl reference electrode in saturated kcl for 200 ks (≈55.6 h), with the open circuit potential of the cell showing just minor variation in the range of -0.5 – 1 mv. these negligible oscillations in the measurement process can be attributed to the temperature of the laboratory. otherwise, the potential is very stable over the observed time period. electrochemical impedance spectra were first recorded using the reference electrode with the carbonized lignin, then with the glass frit as a salt bridge in a 2-electrode setup against the commercial reference electrode in saturated kcl. the bode plot (figure 3) shows both the magnitude of the impedance and the phase angle in the frequency domain. the reference electrode with the carbonized lignin as a salt bridge shows a higher impedance than the electrode with glass frit over the entire observed frequency range. it achieved an impedance of 21.5 – 18.7 kω, while an impedance of 20.0 – 17.5 kω was reached using glass. the higher impedance of the carbon frit can probably be attributed to its greater thickness. the phase shift is very similar for both electrodes and does not exceed -2°. these only minor differences, show that the carbonized lignin is well suited for use as a diaphragm instead of glass frits in ag|agcl reference electrodes. figure 3. bode plot of eis measurements on the ag|agcl reference electrode with carbonized lignin (black) or glass frit (red) as the junction. the measurement was carried out against a commercial ag|agcl reference electrode in sat. kcl the constructed reference electrode was investigated with both, the carbonized lignin and the glass frit as a salt bridge in different solutions (0.1 m h2so4, 0.1 m citrate buffer, 0.1 m na2so4, and 0.01 m koh) against a graphite rod with a commercial ag|agcl|sat. kcl reference electrode (figure 4). in all solutions, a decrease in impedance with increasing frequency is observed, regardless of the junction. significantly higher impedances are achieved with the carbon membrane than with the glass frit in all the solutions investigated. the phase shift of the diaphragm made of carbonized lignin is slightly higher than that of the glass frit, but does not exceed -7.5°. in the case of the alkaline 0.01 m koh solution, both junctions show a small phase shift of up to -4°. with longer measurements in the alkaline solution, the glass frit would dissolve [16], which shows a clear advantage for the use of carbon materials as salt bridge for reference electrodes. comparing the values of the impedance in figure 3 and figure 4, large differences can be spotted. in figure 3, eis was measured in a 2-electrode setup in which the impedance of the glass frit or carbonized lignin were measured against a commercial reference electrode. in figure 4, a 3-electrode setup involving additionally a graphite rod as counter electrode was used. in the 2-electrode setup, in 10-2 10-1 100 101 102 103 15 16 17 18 19 20 21 22 23 24 25 |z | / k w f / hz carb. lignin glass frit -5 -4 -3 -2 -1 0 f / ° s. breitenbach et al. j. electrochem. sci. eng. 13(5) (2023) 817-824 http://dx.doi.org/10.5599/jese.1520 821 which a commercial reference electrode is both a reference and a counter electrode, the resistance values are higher, because the frit in a reference electrode has larger impedance than the carbon rod counter electrode in the 3-electrode setup. therefore, larger impedance is measured [36,37]. figure 4. bode plot of eis measurements on the ag|agcl reference electrode with carbonized lignin (black) or glass frit (red) as the junction. the measurement was performed against a graphite rod with a commercial ag|agcl reference electrode in 0.1 m h2so4 (a), 0.1 m citrate buffer (b), 0.1 m na2so4 (c), and 0.01 m koh (d) conclusions an ag|agcl reference electrode was successfully prepared with carbonized lignin as a salt bridge instead of a glass frit. this reference electrode exhibits excellent stability in saturated kcl. in acidic to weakly basic solutions, it exhibits similar behavior to the equivalent with glass frit. the carbon salt bridge has the advantage that it is very inexpensive to produce and sustainable due to the use of a bio-based waste product as precursor material. in addition, the carbon salt bridge can presumably be used in systems in which a glass frit cannot be used, such as in strong alkaline solutions. the porosity of the carbon can be easily adjusted by the carbonization process and thus tailored and optimized by an additional activation. based on the reported results, the lignin-based porous carbon junction might be a sustainable, low-cost alternative to currently used glass frits showing already similar performance. however, possible advantages like usability in strong alkaline media need further investigations. optimization of the carbon with respect to the pore structure and the use of the reference electrode in a larger variety of measurement conditions will be the subject of 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industrial & engineering chemistry product research and development 18 (1979) 127–130. https://doi.org/10.1021/i360070a010 [36] m. e. orazem, b. tribollet, electrochemical impedance spectroscopy 2nd edition, john wiley & sons inc., hoboken, usa, 2017. isbn: 978-1-119-34092-8 [37] a. lasia, electrochemical impedance spectroscopy and its applications, springer, new york, usa, 2014. isbn: 978-1-4614-8933-7 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.3390/polym8060205 https://doi.org/10.1016/j.jelechem.2020.113872 https://doi.org/10.1021/i360070a010 https://creativecommons.org/licenses/by/4.0/) consequences of hydroxyapatite doping using plasma spray to implant biomaterials: review paper http://dx.doi.org/10.5599/jese.1614 5 j. electrochem. sci. eng. 13(1) (2023) 5-23; http://dx.doi.org/10.5599/jese.1614 open access : : issn 1847-9286 www.jese-online.org review paper consequences of hydroxyapatite doping using plasma spray to implant biomaterials amrinder mehta1 and gurbhej singh2, 1division of research and development, lovely professional university, phagwara 144411, india 2amritsar group of colleges, amritsar 143001, india corresponding author: gurbhejsingh612@gmail.com received: november 22, 2022; accepted: december 10, 2022; published: january 18, 2023 abstract hydroxyapatite (hap) is still one of the most common bioactive coatings used on metal implants in orthopaedics due to its biocompatibility. the application of hap to metallic implants can be accomplished using a variety of processes. plasma spray (ps) coating stands out as the method of choice due to its dependability, affordability, and ability to protect metal surfaces against rust and wear. the use of hap in medicine has been limited due to the material's unfavorable mechanical characteristics, such as brittleness, a lack of fracture toughness, and inadequate tensile strength. in addition, the remodeling durations of hapcovered implants are significantly longer, the rate of osseointegration is significantly lower, and no antimicrobial actions or features are present in these implants. the mechanical and biological properties of hap have been improved by applying various approaches, all of which fall under the category of surface modification tactics. dopants are one of those strategies that are extremely successful at changing the characteristics and using them in hap is one of those methods. as a result, this review study aims to consolidate data on implant hap coating using the plasma spray approach and assess the benefits and problems associated with employing this method. in addition, the paper addresses how altering the structural, chemical, and mechanical features of hap can assist in overcoming these limitations. in conclusion, it explains how the incorporation of entering the hap structure can change the features that, when coated using the plasma spraying approach, alter the functionality of the implant. keywords natural bone; strong bonding; orthopaedic fields; bioactive coating introduction fractures of the bone are a common complication of bone-related diseases in individuals worldwide, which often necessitates bone transplantation. in addition, the world population increases. as a result, the contention that there is a growing demand for prosthetic implants to treat bone-related difficulties is well-founded. the implants are frequently utilized in orthopaedic and maxillofacial http://dx.doi.org/10.5599/jese.1614 http://dx.doi.org/10.5599/jese.1614 http://www.jese-online.org/ mailto:gurbhejsingh612@gmail.com j. electrochem. sci. eng. 13(1) (2023) 5-23 hydroxyapatite doping using plasma spray 6 reconstructive procedures. as a result of the growing demand for the implants described above, there is also a requirement for the development of novel implants that are exceptionally biocompatible and feature a high level of functionality [1-3]. metallic materials are the most common choice for implant construction because of the exceptional mechanical properties they possess for the hard tissues found in the human body. metallic implants, on the other hand, provide a challenge of biocompatibility. metallic implants also have a number of drawbacks, including a high rate of corrosion, a deficiency in anti-infection properties, and a tendency to become loose due to the creation of potentially harmful wear debris particles. as a result, one of the most significant problems associated with metallic implants is an early failure. thus, it is essential for the implant's continued viability to improve its features while simultaneously decreasing its negative sides. one strategy for improving the characteristics of implants and extending their lives is to cover them in bioactive material [4-6]. the hydroxyapatite (hap) material that is coated on biomedical implants is by far the superior choice when compared to the other bioactive materials that are used. it has a significant versatility in clinical settings throughout the orthopaedics areas. the fact that calcium (ca)/phosphorrous (p) molar ratio is both physically and chemically comparable to that of natural bones is the primary advantage of this material. in addition, implants with a hap covering have excellent biocompatibility, conductivity, quicker implant attachment, and a strong relationship with living bone [7]. figure 1. various potential uses for hap biomaterials as in figure 1, hap is frequently used as a covering material for implants. this is done so that artificial cement-free bone can be replaced in implants. metallic implants with excellent biocompatibility and bioactivity also have good strength and high ductility after being coated with hap. this is because hap has a high molecular weight. the use of hap as a material for covering metallic implants can be accomplished using a wide variety of processes, such as plasma spraying, oxy-fuel high-velocity combustion spraying, dip coating, ion-beam sputter coating, and electrophoretic deposition. ca-p coatings with better adhesion have been employed recently, primarily on magnesiumbased alloys. these coatings were primarily inspired by organic molecules. the plasma spray coating application s of hap biomaterial s carnial prosthesis dental implants surgical guide interbody fusionn cage knee and hip joints replacement a. mehta and g. singh j. electrochem. sci. eng. 13(1) (2023) 5-23 http://dx.doi.org/10.5599/jese.1614 7 process is one of the many available options, and it stands out thanks to its benefits over other approaches. for example, the electrophoretic deposition technique can only be used for the deposition of immaterial substances. the high-velocity process for oxy-fuel is limited because of the high cost of the necessary equipment, the restricted powder size, installation requirements, and application complexity. in a manner analogous to this, ion-beam sputter coating is limited in its capabilities due to the limited size of the ion beam, slow deposit, and restricted bombardment target [8-10]. in most cases, thin and thick coatings are placed on bioimplants in order to protect them from corrosion and erosion [11-14]. both corrosion and erosion have a significant negative impact on the longevity and tenacity of the various materials [15-30]. various surface protection techniques are used commercially, such as claddings [31-43], electrochemical vapor deposition (evd) [44], chemical vapor deposition (cvd) [45-48], plasma vacuum deposition (pvd) [13,42,49-54], hvof/hvaf [17,18,25,28-30,55-63], plasma spray (ps) [64-66], sputtering [13,60,67-74], dip coatings [46,67,7579] etc. when it comes to biomedical applications, thin coatings are often favoured over thick coatings. thick coatings might be problematic [72,74]. the use of a dip coating can be problematic due to inadequate mechanical qualities such as adhesive strength. the benefits and drawbacks of utilizing these tactics have been elaborated upon elsewhere [79]. when everything is taken into consideration, the plasma spray coating stands out as the most dependable and most dependable practical, low-cost method of shielding metal from wear and corrosion. in addition to these benefits, it has a smaller porosity, a surface free of fractures, improved mechanical properties, and minimal contaminants. in addition, it increased biocompatibility, which led to a quicker bone-implant attachment and better adhesion between them. this was made possible by the chemical and structural similarity between metal implants and bone minerals, as well as the implants' superior surface quality. consequently, the method of plasma spraying is commonly used to cover the surface of orthopaedic implants, and it is also feasible from a business perspective. this was made possible by the chemical and structural similarity between metal implants and bone minerals, as well as the implants' superior surface quality. the use of hap as an implant covering is limited due to the material's unfavorable mechanical characteristics, such as brittleness and a lack of fracture toughness. despite the fact that hap is an excellent material to use, its application in medicine is restricted. in addition, the remodeling time for hap-coated implants is longer, the rate of osseointegration is lower, and there are no antimicrobial actions or features present in these implants [80-82]. because of these and other constraints, hap-coated implants' biomechanical properties need to be altered before they can be used for long-term applications. it is imperative that the hap material's phase, morphological, structural, and compositional features be accurately regulated to further enhance the osteoblast cells' bio-functionality when they are used as a coating material on the surface of the implant. this is a result of the fact that the surface features of the material can have a significant influence on the way in which the cell and the coating layer interact with one another. numerous strategies have been implemented as surface modification techniques in the quest to enhance the hap's mechanical and biological properties. doping ions into the hap is one of these approaches, and it is one of the more successful methods for modifying the properties of the hap. this review paper will assemble essential material in order to analyze the benefits and downsides of biomedical implants with the hap (ps) method. specifically, this paper will focus on biomedical implants [83-85]. overcoming limitations by altering the structural, chemical, and mechanical properties of hap is also discussed. in the end, it explains how the incorporation of doping single, binary and multi-ion hap has the potential to change the properties that, after being coated by plasma spraying, alter the functionality of the implant. http://dx.doi.org/10.5599/jese.1614 j. electrochem. sci. eng. 13(1) (2023) 5-23 hydroxyapatite doping using plasma spray 8 hydroxyapatite plasma spray coating as a result of its many benefits, plasma-spray-coated hap on metallic implants has found broad application in orthopaedics and dentistry. using a plasma flame generated by an electrical discharge between electrodes, the plasma spraying approach's primary goal is to elevate hap particles' temperature to very high levels. soon after being deposited, the molten hap particles cool rapidly to create a coating layer on the surface of the substrate. after the coating cools, it has been shown that partial apatite breakdown occurs, which may be easily characterised by the thermal stability of apatite, and other secondary phases formation. in the vast majority of instances, this has been the case. the use of hap plasma spray coating on metallic biomedical implants has been the subject of extensive research. coatings of hap and tetra calcium phosphate (tcp) were sprayed onto a titanium substrate using a plasma spraying technique for biomedical applications, and it was found that plasma-sprayed composite coatings with tunable solubility can be obtained for the orthopaedic and dental applications by selecting a specific powder composition and then undergoing a subsequent thermal treatment [86,87]. this was discovered through the application of the method for biomedical use. when they applied a ps coating of simply hap and 80al2o3-20tio2 to a ti6al4v substrate improved reinforcing the most advantageous characteristics of a coating. this was discovered when they found that the coating could be advantageous for the implants. because of the reinforcement, it was also discovered that undesired phases such as tetra calcium phosphate (ttcp) and tri-calcium phosphate were present. these phases could be harmful because of their quick breakdown in the environment of the body; thus, their presence was noted. the outcome of applying hap and hap-silicon oxide coatings deposited by using plasma spraying on the stainless steel is significant in biomedical applications. their research showed that the hap-coated substrate had a lower corrosion resistance than the composite coating but a higher resistance than the untreated substrate [88,89]. according to reports, the porous structure of the hap coating makes the corrosion worse. the plasma-sprayed hap and hap-ti composite coatings on ti6al4v substrate were demonstrated to have bioactive qualities when subjected to an eight-week immersion in a simulated bodily fluid (sbf) solution. they came to the conclusion that the layer had been through two unique stages: first, it had undergone a process of disintegration, and then, on top of that, an apatite crystal precipitate similar to the bone had evolved. in addition, they found that the coating's mechanical strength decreased as the soaking time rose, owing to the coating's propensity to become detached from the substrate as a result of a lack of adhesive strength in the coating. they claimed that composite coatings performed better in terms of mechanical strength than pure hap coatings, indicating enhanced long-term stability in a physiological environment [90,91]. their reasoning can be found here. in order to improve the ps hap bonding strength, it is necessary to first make a composite out of titanium and then coat it on the surface of substrates made of titanium alloy. they stated that including ti in the coating resulted in a considerable improvement in the coating's bonding strength while preserving the bioactivity of the hap coating. to look into the mechanical and microstructural properties of coatings made of hap, ysz, and ti6al4v that were sprayed using a plasma, they claimed that in comparison to ps coated hap, where tcp and cao phases were present in trace amounts, but other phases such as tcp and ttcp were absent, composite coating demonstrated superior levels of mechanical strength. the melting of particles, the rate at which they cooled, recrystallization, re-hydroxylation, dehydroxylation, and the efficiency with which they deposited material during plasma spraying were all related to phase and structural changes, as indicated by studies. according to studies that investigated the connection between spraying parameters, the rate of dissolution, and the surface attributes of hap coatings. a. mehta and g. singh j. electrochem. sci. eng. 13(1) (2023) 5-23 http://dx.doi.org/10.5599/jese.1614 9 increasing the ps power distance led to a gradual deterioration in the phase purity and crystallinity of hap. coatings sprayed at higher intensities and for longer distances showed significantly more particle melting, the researchers found. the surfaces of the coatings exhibited this behavior [92-94]. coatings sprayed with less force were also found to have a crystalline hap structure. coatings sprayed at higher intensity, however, displayed a pattern reminiscent of bone apatite. this was the case regardless of the type of coating. researchers have hypothesized that plasma spraying may be used to produce hap coatings that are dense, adhesive, and bioactive. restriction to hap coating in order to stimulate bone formation and speed up the healing process, hap is a popular implant coating. the use of polysulfone hap-coated biomedical implants attracted a lot of interest and controversy during the past few years. even though reports have suggested that hap-coated implants attach more rapidly and thoroughly and promote greater bone development, the longevity of these implants is still in question. this chaotic behavior is brought on as a result of the thermal disintegration of hap that occurs during the plasma spraying process. this process tends to bring to light a variety of observable downsides regarding the coating's chemical, physical, and biological properties [95-97]. numerous investigations have been conducted on the possibility of utilizing hapcoated metal implants to reconstruct human bone pieces that have been lost or shattered. however, due to weak resistance to wear and corrosion, the implants that are now in use have been shown to deteriorate over the course of several years. the presence of porosity in a physiological environment has a detrimental effect on the corrosion resistance of hap-coated implants. this is the case even when the porosity is quite small. it has been reported that the plasma spraying method causes hap to degrade, which results in the formation of phases such as calcium oxide, tricalcium phosphate (tcp), tetra-calcium phosphate (ttcp), and others. the dissolution creates a poor coating-metal connection, the discharge of particles, and delamination as a result of the loss of cohesive strength [97-99]. the hap coating utilized in medical applications slows the process of osseointegration. however, it does not possess any antibacterial actions or features. to make hap practical for use, its mechanical strength and bioactivity must be enhanced. the potential is limited because it has a low mechanical interfacial strength and takes a relatively long period to reconstruct. implications of doping to overcome constraints the performance of hap coating is affected by its components, including its composition and structure, as well as its physical, biological and mechanical properties, which prevent synthetic hap from being used as an implant for artificial bone in load-bearing orthopaedic applications. these characteristics limit the implantation of synthetic hap. as a result, it is absolutely necessary to enhance these properties for implants to be used for a wide variety of load-bearing applications for an extended time. several approaches have been utilized to achieve this level of excellence. the development of ion-doped changed hap is one way that can be utilized. recent research has focused a lot of attention on ions because their substantial contribution to the production of artificial bone with superior bioactivity and mechanical characteristics has garnered a lot of recent attention. in addition to calcium and phosphorus, it is general knowledge that hard tissues such as bone, enamel, and dentin contain a great deal of other trace elements as well [99-101]. other structural similarities between bone and ion-doped apatite that served as an inspiration for the development of this material are also present. it is possible to incorporate either cationic or anionic substituents into a hap structure. substituted hydroxyapatite (sha) has a reduced solubility in a physiological environment due to the effects of these dopants on hap's morphology, crystallinity, http://dx.doi.org/10.5599/jese.1614 j. electrochem. sci. eng. 13(1) (2023) 5-23 hydroxyapatite doping using plasma spray 10 and lattice properties. in other words, this is due to the existence of these dopants. a safer and more effective method of enhancing the coating's tissue ingrowths and angiogenesis properties can be provided by integrating dopant ions into the hap structure. it was found that the biological reaction to natural hap with a non-stoichiometric structure is greater than that to stoichiometric hap. the presence of extra ions and a lower ca/p (1.5) molar ratio in natural hap led researchers to this conclusion [102,103]. it has been proven in both in vitro and in vivo experiments that doping with ions boosts material’s mechanical characteristics and accelerates bone formation and resorption. the initial rate of degradation, the mineralization property, and, most importantly, the biological activity of hap are all affected by the introduction of dopants due to the wide range of crystal defects and changes in surface charge introduced throughout the doping process. effects of single ion doped hap as shown in figure 2, single ionic doping can change the parameters of the hap structure. these changes are dependent on the quantity and length of the dopant. as a direct consequence of this, a great number of research projects focusing on the production of single ion-doped hap have been carried out. when it came to the synthesis and subsequent structural characterization, it was determined that single ion doping in the hap structure was the simplest option [104,105]. as a coating material for biomedical implants, we will explore the effects of single ion doping on hap structure in the following sections. particular attention will be paid to coatings applied using plasma spraying when this method is most applicable. figure 2. ionic doping potential hap doped with sr it was found that strontium, one of the many different types of metallic dopant ions, was especially useful in improving synthetic hap's biological and structural properties. it is beneficial on the treatment of osteoporosis and is most commonly found in regions with high metabolic activity. in these regions, sr can be found in the bone mineral phase. in vitro studies show that it increases the number of osteoblast cells while simultaneously decreasing the activity of osteoclasts. in addition, it has been found that sr can reduce the amount of bone resorption that occurs while simultaneously increasing the number of preosteoblastic cells that multiply, encouraging a. mehta and g. singh j. electrochem. sci. eng. 13(1) (2023) 5-23 http://dx.doi.org/10.5599/jese.1614 11 osteogenesis, osteoblast differentiation, and the development of new bone. when utilized as a dopant to hap, the ion with the charge of sr2+ can replace or substitute ca2+ ion for the whole composition range. to fully realize the effect of sr on the hap structure during ps deposition, numerous studies have been conducted to investigate the effect that different sr doping concentrations have on the structure of hap [106,107] the level of cell adhesion increased with the strontium content. sr doping was shown to gradually decrease the crystallinity. prodan et al. [108] used to create sr doped hap powder with 15 mol.% sr and reported the decreased crystallinity with a concentration more than 2 mol.% of sr. in addition, it was observed that the tcp phase was present in the latter. further, it has been found that higher sr concentrations contributed to the stabilization of the hap phase with the removal tcp phase. the higher sr concentration, which was above 15 weight percent, led to the destabilization of the hap structure, which resulted in the formation of tcp as a secondary phase. they came to the conclusion that single-phase hap could include as much as 1.5 % strontium. the inclusion of these other phases causes apatite to become destabilized, which leads to a loss in crystallinity and crystallite size. the decrease in crystallinity and crystallite size may be induced by an increased quantity of strontium incorporation. because of the low strontium content, both the crystallinity and the crystallite size of hap were found to be diminished, whereas an increase in strontium concentration was discovered to cause an increase in both of these features. when strontium-doped hap was implanted into bone fractures, the procedure encouraged the formation of new bone, indicating that osseointegration had been successful. during the in vitro study, the used cells derived from a fetal osteoblast cell line known as hfob. they found that sr-hap-coated surfaces had superior initial cell adhesion and greater cell proliferation compared to hap-coated surfaces. one possible explanation is the effect strontium has on the rate of early differentiation as well as the activity of alp [109,110]. they concluded that the incorporation of sr into hap resulted in a significant enhancement of the material. recent studies have demonstrated that strontium affects the mineral metabolism that occurs during bone regeneration. specifically, strontium promotes the formation and proliferation of osteoblasts while suppressing the activity of osteoclasts in the case of strontium-doped hap coating. because sr-hap is significantly more bioactive and has improved solubility compared to hap, its application in vivo is necessary. the results of all of these tests point to the same conclusion: sr incorporation fosters cell adhesion and cell growth. in addition, the study found that doping the coating with strontium increased its bioactivity without diminishing its mechanical properties, making it an excellent candidate for implant coating. mg-infused hap magnesium (mg), the fourth most prevalent trace element in mammalian bodies, is essential for the catalytic activity of over one hundred different enzymes. it is a large trace ion found in bones and teeth. it does this by quickening the activity of osteoblasts, particularly in the early phases of osteogenesis, which speeds up bone metabolism. a deficiency in magnesium ions can cause problems with bone formation and metabolism, a loss of bone density, and an inhibition of osteoblast activity, all of which contribute to an increased risk of bone fracture [111,112]. since hap with magnesium doping has the potential to be used in a variety of biomedical applications, a significant amount of study has been conducted on the topic. to explore the effect of different concentrations of the mg2+ ion on the mg-hap coating applied to the ti alloy substrate. they discovered that it had a greater level of biocompatibility when the magnesium concentration reached 2 percent by weight. however, because the coating has a greater specific surface area and http://dx.doi.org/10.5599/jese.1614 j. electrochem. sci. eng. 13(1) (2023) 5-23 hydroxyapatite doping using plasma spray 12 smaller crystallite size, it is better adapted to receive proteins or drugs that induce bone formation. this is because the coating has a higher specific surface area. when compared to stoichiometric hap granules that are commercially available, it has been observed that osteoconductivity and solubility are improved when mg-doped granules are implanted in rabbit femoral bone defects [113,114]. despite the beneficial effects that the mg-doped hap coating by ps method on biomedical implants has, there are not many writers working on it. zn-modified hap zinc (zn), a trace metal found in bone minerals, plays a critical role in the regulation of the immune system and the development of bone. in addition, it can stimulate the metabolism of nucleic acids, bone growth, cell division, enzyme activities, the maintenance of membrane structures, and, most importantly, the calcification of diseased conditions. it can prevent bone loss and the absorption of bone by osteoclasts in vivo. it also has the potential to improve bone formation and metabolism in addition to its corrosion resistance activities. when more than 10 weight percent of zinc is doped into hap, an increase in the amorphous apatite phase occurs. by introducing zn into the structure, grain size can be decreased while at the same time increasing hap's solubility [115,116]. it is possible to replace ca with zn at a concentration of 20 mol% so that the cation can penetrate the apatite lattice rather than remain on the surface. zn can increase the formation of new bone and a reduction in bone resorption by decreasing osteoclastic activity. a zndoped hap coating was applied to the ti rods, and an effective rise in fibroblast cell proliferation was found after its application. in addition, a number of experiments have demonstrated that increasing the amount of zinc in hap increases the material's mechanical properties [117]. in addition, hap possesses antibacterial and anti-inflammatory qualities, all of which lend credence to the idea that it could be utilized as a material for bone restoration cell differentiation and adherence on surfaces treated with zn-doped hap. the research demonstrated that doping hap led to increased bone protein gene expression and greater levels of both alp and osteogenic activity. the rabbit tibia model was used to carry out in vivo testing. they discovered that zn-doped hap had significantly improved biocompatibility as well as osteoconductivity [118]. numerous studies revealing better hap osseointegration and physiologic responses have been published on plasma spray zn-doped hap coatings. as a result, a wide variety of opportunities for applications in biomedicine are available. however, the powder is subjected to high temperatures throughout the process [119,120]. however, up to now, the applications of zn-doped hap, such as its optoelectrical properties, anti-cancerous activity, and ability to operate as a corrosion-resistant coating, have not being utilized to their full potential. effects of binary doped hap this section describes the physical, mechanical, biological, cytotoxic, and antibacterial properties of the coated binary-doped hap doping with binary or multiple ions can affect various physical properties, such as the phase, crystallinity, mechanical properties, and lattice parameters of a material [121]. after being coated in doped hap, the physical features of biomedical implants, such as phase and surface porosity, change [122]. this is because the plasma temperature, interactions with the substrate and spraying circumstances all play a role in this transformation. doping results on electrochemical and mechanical properties it is not possible to employ dense pure hap as a replacement for hard tissue due to the material's inadequate mechanical strength, which includes inadequate fracture toughness and bending a. mehta and g. singh j. electrochem. sci. eng. 13(1) (2023) 5-23 http://dx.doi.org/10.5599/jese.1614 13 strength. since its mechanical properties can be enhanced by adding dopants, hap is often treated with them. coatings of hap were strengthened using a wide variety of materials, including sio2, cap, zno, and sr, to enhance bio-implant corrosion resistance and mechanical properties. this was done to meet regulatory requirements. the implants' corrosion resistance and biodegradability are also improved [123,124]. zno is added to hap to improve its corrosion resistance and mechanical properties for use in bio-implants. these properties include surface hardness, surface roughness, and surface wettability. it is a novel inorganic substance with a wide range of applications and a consistent set of chemical and physical properties. it plays an essential part in the maturation of bone as well as the increase of a material's resistance to corrosion. a limited amount of research is being conducted on ti13nb13zr alloys with a plasma-sprayed hap/zno reinforced coating for potential use in the biomedical industry [125-127]. in light of this, the surface features of the ti13nb13zr alloy and the electrochemical corrosion behavior was investigated. the ecorr of coated samples was significantly higher compared to the ecorr of naked titanium alloy samples. the coated samples appear to be less corrosive and exhibit a continual increase in the amount of zno in hap. as a result of increased resistance to corrosion, a more robust attachment between the implant and the tissue develops [128]. it has the additional effect of causing the damaged tissue to heal rapidly and in a short amount of time. electrochemical experiment revealed that the ti13nb13zr alloy was better protected against corrosion by hap/zno-reinforced coatings than by pure hap coatings. this was the case regardless of the kind of coating used. the icorr value declined even more when reinforcement in the form of zno was added to the hap, and the sample coated with (hap+12 % zno) had the smallest icorr value [129,130]. in prior experiments, specimens with hap/zno coatings displayed substantially stronger corrosion resistance than the naked alloy. it has been hypothesized that the ions that are generated due to the corrosion of metallic implants may affect cell metabolism. the difference in surface roughness between coatings made of pure hap and coatings made of hap reinforced with zno may have been a factor in the reduction in icorr value, which indicates an improvement in corrosion resistance [131]. the degree of roughness of a surface has a substantial impact on the corrosive behavior. when compared to a surface with a finer texture, a surface with a coarser texture is more likely to develop pits. with implants manufactured of ti13nb13zr better corrosion resistance was achieved, hence preserving their mechanical strength [132,133]. the current research demonstrated that zno reinforcement boosted the ability of the hap coating to withstand corrosion, which would boost long-term survival and osseointegration. on the other hand, surface hardness is an essential mechanical property frequently linked to the rate of surface deterioration in implants. this association exists because surface hardness is a measure of how well an implant resists wear. the microhardness of a surface also has a substantial impact on the material's ability to resist corrosion. conclusion and prospective future a significant amount of research has been done on plasma spray (ps) coating of hap on metallic implants because of hap’s outstanding bioactivity in physiological environments. the current study findings lead to the following deductions: 1. the plasma spraying leads to the formation of tcp phase in the developed coating surface. the apatite formation is slow in such cases due to the presence of unwanted phases present on the surface. http://dx.doi.org/10.5599/jese.1614 j. electrochem. sci. eng. 13(1) (2023) 5-23 hydroxyapatite doping using plasma spray 14 2. the reduction in the undesired phase formation can be achieved by using dopants, which increases the crystallinity of the coating with the optimum concentration. 3. the other techniques such as hvof and cold spray has limitations such as high cost of the tools required, the limited powder size, the realising, and the product complexities in the development of bio-implant 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https://creativecommons.org/licenses/by/4.0/) @article{mehta2023, author = {mehta, amrinder and singh, gurbhej}, journal = {journal of electrochemical science and engineering}, title = {{consequences of hydroxyapatite doping using plasma spray to implant biomaterials: review paper}}, year = {2023}, issn = {1847-9286}, month = {jan}, number = {1}, pages = {5--23}, volume = {13}, abstract = {hydroxyapatite (hap) is still one of the most common bioactive coatings used on metal implants in orthopaedics due to its biocompatibility. the application of hap to metallic implants can be accomplished using a variety of processes. plasma spray (ps) coating stands out as the method of choice due to its dependability, affordability, and ability to protect metal surfaces against rust and wear. the use of hap in medicine has been limited due to the material's unfavorable mechanical characteristics, such as brittleness, a lack of fracture toughness, and inadequate tensile strength. in addition, the remodeling durations of hap-covered implants are significantly longer, the rate of osseointegration is significantly lower, and no antimicrobial actions or features are present in these implants. the mechanical and biological properties of hap have been improved by applying various approaches, all of which fall under the category of surface modification tactics. dopants are one of those strategies that are extremely successful at changing the characteristics and using them in hap is one of those methods. as a result, this review study aims to consolidate data on implant hap coating using the plasma spray approach and assess the benefits and problems associated with employing this method. in addition, the paper addresses how altering the structural, chemical, and mechanical features of hap can assist in overcoming these limitations. in conclusion, it explains how the incorporation of entering the hap structure can change the features that, when coated using the plasma spraying approach, alter the functionality of the implant.}, doi = {10.5599/jese.1614}, file = {:d\:/onedrive/mendeley desktop/mehta, singh 2023 consequences of hydroxyapatite doping using plasma spray to implant biomaterials review paper.pdf:pdf;:jese_v13_no1_5-23.pdf:pdf}, keywords = {natural bone,bioactive coating,orthopaedic fields,strong bonding}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1614}, } {palmyra palm flower biomass-derived activated porous carbon and its application as a supercapacitor electrode:} http://dx.doi.org/10.5599/jese.1314 545 j. electrochem. sci. eng. 12(3) (2022) 545-556; http://dx.doi.org/10.5599/jese.1314 open access : : issn 1847-9286 www.jese-online.org original scientific paper palmyra palm flower biomass-derived activated porous carbon and its application as a supercapacitor electrode sofia jeniffer rajasekaran and vimala raghavan centre for nanotechnology research, vellore institute of technology, vellore, tamil nadu, india corresponding author: lvimala.r@vit.ac.in received: february 26, 2022; accepted: may 20, 2022; published: may 25, 2022 abstract due to its abundant availability, eco-friendliness, and high sustainability, biomass-derived activated carbon has captured more attention in recent years. in this study, activated carbon was derived from palmyra palm flowers (ppf) using a conventional chemical activation process and carbonization at different ambient temperatures, viz. 700, 800, and 900 °c. the carbonized ppf was chemically activated using 1 wt.% potassium hydroxide to increase the microporosity and specific surface. the experimental data were analyzed using x-ray diffractometer (xrd), scanning electron microscopy (sem), energy-dispersive x-ray spectrometer (edx), raman spectroscopy and fourier transform infrared spectroscopy (ft-ir). the nitrogen adsorption/desorption isotherm curve for activated carbon synthesized at the activation temperature of 900 °c indicated type iv with a hysteresis loop associated with mesopores formation and a specific surface area of 950 m2g-1. the supercapacitor electrodes made with ppf-derived carbon were evaluated for their electrochemical performance by electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge measurements. in the aqueous electrolyte (3 m koh), electrochemical experiments showed that ppf-900 electrode has a specific capacitance of 155 f g-1 at 1 a g-1 and significant cyclic stability (97.3 % capacitance retention over 5000 cycles at 10 a g-1), while energy and power densities were estimated as 15.1 wh kg-1 and 100.6 w kg-1. this study suggests that biowaste products could be transformed into activated carbon materials to improve the performance of energy storage materials, and it adheres to the 'waste to treasure' principle. keywords biowaste products; chemical activation; potassium hydroxide; electrochemical measurements; specific capacitance; energy storage introduction there has been an urgent need in recent years for energy storage devices with quick charging/discharging, high energy density, and long-term cyclability [1]. based on their charge storage mechanisms, electrochemical capacitors are categorized into two types, viz. electrochemical double-layer capacitors (edlcs) and pseudocapacitors. the edlc has distinct advantages such as rapid charging time, operational safety, pulse power supply, and outstanding service life [2]. it stores the charge electrostatically at the electro-active material surface via reversible ion adsorption and desorption (a http://dx.doi.org/10.5599/jese.1314 http://dx.doi.org/10.5599/jese.1314 http://www.jese-online.org/ mailto:lvimala.r@vit.ac.in j. electrochem. sci. eng. 12(3) (2022) 545-556 ppf biomass-derived activated porous carbon 546 non-faradaic process). in comparison, the primary charge storage mechanism in pseudocapacitors is a quick and reversible redox reaction (faradaic process). owing to its excellent cycle stability, greater power capability, inexpensive, simple manufacture, and nontoxicity, the main commercially available supercapacitors, i.e., edlcs, utilize several carbon-based materials as active electrode materials [3]. supercapacitors are the substitutional power sources for conventional batteries for applications including energy harvesting systems, electrical vehicles, and portable electronic devices. hence, there is increasing attention towards research in new electrode materials with high performance, high stability, cost-effectiveness, and eco-friendliness. activated carbon with abundant pore structures is researched extensively for energy storage devices, particularly supercapacitors, over the last ten years [4]. researchers have begun to focus on newly activated carbon precursors, which are of relatively low cost, chemically stable, highly electrically conductive, of high specific surface area, and well-built hierarchical pore structure [5]. because of their renewable nature, accessibility, and availability, biomass wastes are currently being employed as precursors for the preparation of activated carbon. agro-industrial by-products and residues can be transformed into activated carbon. being a precursor for producing activated carbon, almost any carbon-containing material can be utilized [6]. several studies have considered the possibility of synthesizing activated carbon from biowaste products like chestnut shell [7], xanthoceras sorbifolia seed coats [8], tea seed shell [9], pine sawdust [10], dead neem leaves [11], rice husk [12,13], pomelo peels [14], etc. in general, physical and chemical activation procedures can convert these raw materials to activated carbon. the use of activation gas, such as co2, steam, or a combination of gases, is followed to prepare the activated carbon by a physical activation process. during the process of chemical activation, the carbon precursor is combined with chemicals like koh, naoh, h3po4, h2so4 and zncl2. koh was reported as the most effective agent for activation, able to suppress tar formation, reduce the temperature of the activating reaction, and expedite the exclusion of non-carbon components, apart from boosting the pyrolysis efficiency [15]. koh-treated activated carbons are prepared by combusting activated carbon precursors in an oxygen-free environment. they have wider pore sizes and volumes and higher specific surface area than activated carbon precursors that have not been koh treated. the activated carbon precursor and the carbon products acquired by potassium hydroxide chemical activation may differ significantly in surface topography compared to physical activation approaches such as co2 and steam. the specific surface area and characteristic pore structure of carbon materials are essential for electrochemical energy storage applications. to improve the capacitive ability, surface functional groups combining nitrogen, oxygen, and sulphur can produce pseudocapacitive behavior and promote wettability. as a result, new and low-cost, environmentally sustainable biowaste products with the role of facilitating economic development for energy and environmental domains have attracted a great deal of attention. liu et al. [16] used koh-based chemical activation to create activated carbon from soybean pod biomass as electrode materials for supercapacitors. the carbon exhibited good electrochemical properties, with a specific capacitance of 321 f g-1 at 1 a g-1 current density attributed to the high specific surface area (2245 m2 g-1), distribution of pore sizes, and structural features rich in oxygen and nitrogen [16]. taking this into consideration, this work is focused on the investigation of the palmyra palm flowers (ppf) based activated carbon. palmyra palm (borassus flabellifer l.) is a well-known member of the arecaceae family with its origin from india, the philippines, laos, bangladesh, malaysia, thailand, cambodia, burma, sri lanka, vietnam, and indonesia. it is discarded as a waste material that gets degraded without being used effectively. because of its low cost and abundant availability, it could be a feasible biological renewable resource and a promising carbon precursor for obtaining activated carbon. s. j. rajasekaran and v. raghavan j. electrochem. sci. eng. 12(3) (2022) 545-556 http://dx.doi.org/10.5599/jese.1314 547 experimental materials and chemicals the palmyra palm flowers were collected on a local agricultural farm in vellore district, tamil nadu, india. the collected palm flowers, which were dry, dark brown and of stick-like elongated structures, were cut into small pieces. these pieces were thoroughly washed with di water to eliminate dirt particles and dried for 24 hours at 80 oc before being crushed into powder. the powdered samples were stored in a desiccator for further study. the carbon (graphitized carbon black, 99.95 %) and polyvinylidene fluoride (pvdf), and carbon foil (0.2 mm thick, 99.8 %) were procured from alfa aesar. all solvents, including potassium hydroxide (koh), n-methyl-2-pyrrolidone (nmp), and ethanol (ch3ch2oh, 99 %), were procured from sigma aldrich. hydrochloric acid (30 %) was procured from sd fine-chem limited, india. throughout the experiments, deionized water (di) was used. preparation of porous activated carbon the powdered palmyra palm flowers were carbonized for 3 hours at 400 oc. the heating rate was 5 °c min-1 under a nitrogen atmosphere before being cooled to ambient temperature. after carbonization, the palmyra palm flowers were mixed with potassium hydroxide at wt.% of 1:1. to produce activated carbon at three different temperatures, viz. 700, 800, and 900 °c, the mixture was subjected to activation in a nitrogen atmosphere for 1 hour at a rate of 5 °c min-1. the obtained activated carbon samples were allowed to reach room temperature and washed with hydrochloric acid solution and distilled water to maintain a neutral ph. finally, the washed activated carbon samples were kept overnight in a hot oven at 60 oc for drying and designated as ppf-x. here, x indicates different activation temperatures (700, 800, and 900 oc). the steps followed in the synthesis procedure of palmyra palm flower-derived activated carbon are depicted in scheme 1. scheme 1. schematic of the synthesis of palmyra palm flower-derived activated carbon characterization the crystallinity of porous activated carbon was investigated using powder x-ray diffraction (bruker d8 advance diffractometer, germany) with cu ka (15.406 nm) radiation. raman spectra http://dx.doi.org/10.5599/jese.1314 j. electrochem. sci. eng. 12(3) (2022) 545-556 ppf biomass-derived activated porous carbon 548 were analyzed using a horiba jobin yvon labram aramis excited at 633 nm. the morphology and elements of the sample were evaluated using sem and energy dispersive x-ray analysis (edx), zeiss neon 40 sem microscope. the surface functional groups of the synthesized ppf-derived activated carbon were characterized using a fourier transform infrared (ft-ir) spectrometer (bruker, tensor 27 spectrometer). n2 adsorption/desorption isotherms were measured (beishide 3h-2000ps2) to determine the surface area, pore size, and pore volume of the prepared activated carbon samples. electrochemical measurements electrochemical analyses were carried out on a chi 660c electrochemical workstation, with cyclic voltammetry, charge-discharge, and electrochemical impedance spectroscopy performed in 3m koh aqueous solution as the electrolyte. platinum foil and ag/agcl (saturated kcl) served as the counter and reference electrodes in the three-electrode system. the active materials (85 %), polyvinylidene fluoride (5 %), and carbon black (10 %) were mixed in n-methyl-2-pyrrolidone solvent (few drops) to get the active electrode ink and further employed as the working electrode. the obtained mixture was coated on a graphite foil substrate (1 cm2) and subjected to drying in an oven at 80 °c for 12 hours. the coated electrode was used as a supercapacitor electrode and tested with various tests to analyze its efficiency. the active material mass loading for each electrode was approximately 0.002 g. cyclic voltammetry and galvanostatic charge/discharge (gcd) measurements were carried out at 0 to 0.4 v (potential window) at different scan rates and current densities. the cv measurements were performed in a three-electrode cell system, and the specific capacitance is calculated using the equation (1): s 2 a c mk v =  (1) where cs is specific capacitance (f g-1), a is the area of cv curve, m is total mass of active material (g), k is scan rate and ∆v is potential window (v). equation (2) was used to calculate the specific capacitance of the supercapacitor electrode from charge/discharge curves: s i t c m v  =  (2) where i is imposed constant current (a), and t is discharge time (s). the energy density (ed) and power density (pd) values were calculated from galvanostatic charge/discharge measurements according to: 2 d s 1 2 e c v=  (3) d d e p t =  (4) electrochemical impedance spectroscopic analyses of ppf electrodes were recorded at 0.3 v in the frequency range of 100 khz to 0.1 hz, with an amplitude of 0.005 v of ac voltage. impedance (z) was expressed as the real (z') and imaginary (-z") components. results and discussion figure 1a depicts the x-ray diffraction patterns of the activated carbon prepared at different temperatures. all three activated porous carbon samples didn’t show any sharp peaks, indicating that they are weakly crystallized. less intense peaks at 23 and 42° correspond to graphitic carbon planes (002) and (100), indicating the presence of amorphous carbon. the broad diffraction peak s. j. rajasekaran and v. raghavan j. electrochem. sci. eng. 12(3) (2022) 545-556 http://dx.doi.org/10.5599/jese.1314 549 with low peak intensity centered at 2 = 23° could be indexed as highly disordered graphite [17]. the reduced intensity of the diffraction peak with koh activation is attributed to the turbostratic carbon structure in ppf with randomly oriented graphene layers. raman spectra further confirmed the presence of a graphitic phase as observed in figure 1b, where the d-band represents the disordered carbon layer structure and defects in the carbon material, whereas g-band represents the vibration of sp2 hybridized carbon atoms in the graphite sheet structure [18]. it denotes the carbon phases of diamond (d) and graphite (g) at 1353 and 1597 cm-1, respectively. the term id/ig (the ratio of d band to g band intensity) is commonly used to describe the degree of carbon disorder. the intensity of the g band is slightly greater than the intensity of the d band, indicating that the prepared samples contain more structurally ordered graphite. the higher id/ig value of the ppf900 indicated that the ppf-900 had more defective (amorphous) carbon structures than other synthesized at the other two temperatures, which was induced by koh activation. 2 / o raman shift, cm-1 figure 1. (a) xrd patterns and (b) raman spectra of ppf-700, ppf-800, and ppf-900 activated carbon the values of id/ig (the intensity ratio of d band to g band) varied with activation temperature, with ppf-900 having the highest id/ig value of 0.89 and ppf-700 and ppf-800 gave id/ig values of 0.86 and 0.82, respectively, indicating that the defect structures in sample ppf-900 are more disordered. as the activation temperature was raised, the id/ig ratio gradually increased, implying that the order of the graphitic structure was destructed and that high-temperature activation increased the presence of defects in ppf-900. results showed that a significant koh ratio (1:1) inhibits graphitization and increases the disorder of the material microstructure, which is consistent with the xrd results. to further evaluate the properties of the surface and chemical composition of the carbonized sample, viz. ppf-700, ppf-800, and ppf-900, the materials were investigated using ftir (figure 2). all carbon samples prepared from palmyra palm flowers have functional groups on their surfaces, confirming the presence of oxygen. o–h stretching vibrations were assigned to broadband in the region of 3157 cm-1 [19]. the presence of o–h and carboxyl groups has been figured out to improve electrode wettability and electrochemical performance. c=n stretching vibrations are assigned for the peak at 2085 cm-1 [20]. c-c vibration in aromatic rings and c-o stretching could be assigned to the 1521 cm-1 and 1062 cm-1 bands, respectively [21,22]. the presence of surface functional groups in activated carbon derived from ppf could be attributed to potassium hydroxide activation. in te n si ty , a .u . in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1314 j. electrochem. sci. eng. 12(3) (2022) 545-556 ppf biomass-derived activated porous carbon 550 wavenumber, cm-1 figure 2. ftir spectra of ppf-700, ppf-800, and ppf-900 activated carbon figure 3(a-c) shows sem images of activated porous carbon samples synthesized at three temperatures and the respective edax. after carbonization, all samples exhibited a significant porous structure, which is necessary for active ion transport and higher surface area, and thereby the efficient storage of charge. the surfaces of ppf-700, ppf-800, and ppf-900 confirm the formation of disordered carbon structure due to the release of volatile compounds during the carbonization procedure. figure 3. (a-c) sem images of ppf-700, ppf-800, and ppf-900 based activated carbon at magnification 20,000  (d) edax analysis of ppf-900 the chemical treatment created large pores, which aided in the activation of the carbon particle internal surfaces. for supercapacitor electrodes, the porous structure of activated carbon creates an ideal surface environment for ion diffusion during the charge-discharge process [23]. furthermore, edax was used to examine the elements of the palmyra palm flower-derived activated carbon. the edax spectrum of ppf-900 activated carbon presented in figure 3d shows that it t ra n sm it ta n ce , % in te n si ty , a .u . energy, kev s. j. rajasekaran and v. raghavan j. electrochem. sci. eng. 12(3) (2022) 545-556 http://dx.doi.org/10.5599/jese.1314 551 consists of 88.81 % of carbon weight and 9.65 % of oxygen weight. the high carbon content indicates that most of the ppf-900's got volatilized, and non-carbon content was removed during the activation process, resulting in much higher carbon content. the nitrogen adsorption/desorption isotherms from bet isotherms are given in figure 4a. the palmyra palm flower-derived carbon isotherm at ppf-900 is classified as type iv because it is horizontal and parallel throughout the (p/p0) range. the internal surface area is represented by the bet surface area, which is generally in two forms, such as micropores and mesopores. the micropores surface area identifies the percentage of microporous activated carbon [24]. the ppf900 gave a surface area of 950 m2 g-1 as a result of different types of pores formed during chemical activation. the steep growth at low pressure and narrow hysteresis loop indicates the presence of micropores. an increased activation temperature has increased specific surface area and pore volume. as an activator, koh significantly affects pore development [25]. increasing the carbonization temperature to 900 oc causes the precursor to release more volatile compounds, resulting in an increased pore development and the formation of new pores. further, as observed from the pore size distribution curve (figure 4b), the average pore size was calculated to be 1.7 nm, revealing that the obtained activated carbon exhibits a microporous structure. p/p0 pore diameter, nm figure 4. (a) n2 adsorption-desorption isotherms (stp standard temperature and pressure); (b) pore size distribution of ppf-900 the porous structures were obtained through the chemical etching reactions during the koh activation process and as a result, micropores are the most potential sites where charges are held at electrolyte/electrode double-layer interfaces. in brief, the specific surface area of ppf-900 and hierarchical micropore distribution can promote electrolyte ion penetration and diffusion and provide significant surface area that is electrochemically active for double-layer charging/discharging. electrochemical investigations the cv and gcd curves of ppf-700, ppf-800, and ppf-900 samples were evaluated in a threeelectrode system using 3 m koh as the electrolyte, in order to analyze the influence of activation temperature on the electrochemical performance of the electrode material derived from palmyra palm flowers. all cv curves in the voltage range of 0 0.4 v have an approximately quasi-rectangular voltammogram shape, as given in figure 5a, indicating good charge propagation without significant redox peaks. the rate of ion transport/diffusion in carbon material can be determined by the shape of the cv curve [26]. among three ppf electrodes activated at different temperatures, the ppf-900 electrode has the largest area as observed from the cv curve, indicating higher specific capacitance q u a n ti ty a d so rb e d , cm 3 g -1 ( s t p ) (d v / d d ) / cm 3 g -1 n m -1 http://dx.doi.org/10.5599/jese.1314 j. electrochem. sci. eng. 12(3) (2022) 545-556 ppf biomass-derived activated porous carbon 552 than for other electrodes. specific capacitance values calculated using eq. (1) for ppf-700, ppf-800, and ppf-900 electrodes were 13, 29 and 76 f g-1, respectively. the shape of the cv curves changes as the activation temperature rises, resulting in an enhancement in the cv curve area and current density. despite a high scan rate (100 mv s–1), the ppf-900 maintained symmetric and rectangular cv curves. this could be due to the electrode material surface area, large microporosity, and low internal resistance. ppf-900 has the largest integrated area and the longest charge-discharge time, indicating the highest specific capacitance. figure 5b depicts the gcd curves of electrodes with active material formed at various temperatures, measured at a current density of 1 a g-1. the curves are linear and unsymmetrical, confirming that ppf-700, ppf-800, and ppf-900 electrodes are almost reversible. the slope of the electrode discharge curve decreased slightly as the activation temperature increased, indicating an increase in specific capacitance. figure 5c shows specific capacitance for ppf-700, ppf-800, and ppf-900, calculated from the gcd curves using eq. (2). it is clear that ppf900 exhibited higher specific capacitance than ppf-700 and ppf-800 at all current density values applied (1-10 a g-1). at a current density of 1 a g-1, the specific capacitances of ppf-700, ppf-800, and ppf-900 are 100, 138 and 155 f g-1, respectively. the specific capacitance values obtained by gcd were higher than that obtained by cvs, which might be attributed to different current densities (1-10 a g-1 in gcd and 0.01-0.02 a g-1 in cv) in two different techniques. supercapacitors are power sources operating at higher currents, and in order to check the performance and stability of prepared electrodes, gcd measurements were performed at higher current densities [27]. eis results of all three electrodes, developed from activated carbons synthesized at different temperatures, are shown in figure 5d. at high frequencies, the nyquist plot is a depressed semicircle, followed by a more or less visible sloping line in the high to the middle-frequency region and a prominent inclined vertical tail at low frequencies [28]. such a shape of nyquist plots is characteristic of the impedance response of highly porous capacitive electrodes, where an inclined vertical line at low frequencies is directly attributed to a non-ideal capacitive response of electrode material [29]. the semicircle response at higher frequencies can be associated with surface properties of porous active material, charge transfer of some possible pseudocapacitive reaction (such of surface functionalities), and/or contact impedance generated between active material and current collector. the impedance seen as a linear sloping line at high to medium frequencies is characteristic for diffusion of electrolyte ions within a porous material. resistive components of all these impedances contribute to the internal electrode resistance value, which is generally determined by the electronic resistance of activated carbon particles, electronic resistance of the current collector to the active material and ionic resistance of electrolyte distributed within the pores. in a nyquist plot, internal electrode resistance can be roughly defined by two impedance curve intercepts at z′ axis, i.e., of the semicircle at high frequencies and the capacitive line at low frequencies [30,31]. the vertical line represents the non-faradaic, i.e., purely capacitive impedance of a porous activated carbon electrode, where specific capacitance in f/g can be determined from z’’ value at some low frequency where z’’  z’, according to: cs = (2fz’’m)-1 (5) in eq. (5), z” is the imaginary part of impedance () measured at the frequency f (hz), and m is the mass of active electrode material (g). from eis, the specific capacitance calculated for ppf-700, ppf-800, and ppf-900 at f = 0.1 hz, are 6.9, 7.8 and 10 f g-1, respectively. due to the steady-state in eis measurements, the values of cs are rather small. although at f= 0.1 hz, the condition z’’  z’ was not strictly fulfilled (figure 5d), the order of cs values can be considered reliable. thus, the s. j. rajasekaran and v. raghavan j. electrochem. sci. eng. 12(3) (2022) 545-556 http://dx.doi.org/10.5599/jese.1314 553 higher specific capacitance of ppf-900 could be attributed to higher porosity and thereby higher surface area and the improved electrolyte transport in the electrodes. potential, v time,s current density, a g-1 z’ /  figure 5. electrochemical performance of ppf-700, ppf-800, and ppf-900 activated carbon electrodes: (a) cyclic voltammetry at scan rate of 100 mv s-1; (b) galvanostatic charge-discharge curves at current density of 1 a g-1; (c) specific capacitance at different current densities; (d) nyquist plots measured at 0.3 v in the frequency range from 100 mhz to 0.1 hz in 3 m koh figure 6a shows cv curves of the ppf-900 sample at scan rates ranging from 10 to 100 mv s-1, while figure 6b shows gcd curves of ppf-900 at various current densities ranging from 1 to 10 a g-1. furthermore, as current density increases, charge-discharge time decreases significantly because electrolyte ions have much time to enter and diffuse into the porous structures at reduced current densities [32]. a typical double-layer response is characterized by triangular shapes with excellent symmetry. the discharge time of ppf-900 was significantly longer than that of ppf-700 and ppf-800 activated carbon at high current densities, indicating that the ppf-900 has a significantly greater chargestorage ability, which is in accordance with cv experiment results. due to the high surface area, well-distributed micropores, and thick pore walls in ppf-900, it delivered a capacitance of 120 f g-1 at 10 a g-1, indicating good stability in electrochemical studies. as given in figure 6c, the ragone plot calculated from gcd experiments showed that ppf-900 symmetric supercapacitor would have a maximum energy density of 15.1 w h kg-1 and retained power density of 100.6 w kg-1 in 3 m koh electrolyte, indicating that the as-prepared electrodes have an excellent rate capability. s p e ci fi c ca p a ci ta n ce , f g -1 c u rr e n t d e n si ty , a g -1 -z “ /  p o te n ti a l, v http://dx.doi.org/10.5599/jese.1314 j. electrochem. sci. eng. 12(3) (2022) 545-556 ppf biomass-derived activated porous carbon 554 potential, v time,s power density, w kg-1 cycle number figure 6. electrochemical characteristics of ppf-900: (a) cyclic voltammograms at scan rates from 10 to 100 mv s-1; (b) gcd curves at different current densities (1 to 10 a g-1); (c) ragone plot of ppf-900; (d) cyclic stability of the electrode at 10 a g-1 for 5000 cycles the capacitive performance of ppf-900 was considerably higher than that of other activated carbon materials derived from biomass (table 1). table 1. comparison of the capacitive performance of supercapacitor electrode of biomass-derived porous activated carbon from different precursors with the reported work biomass activating agent specific capacitance, f g-1 current density, a g-1 electrolyte reference red cedar hno3 115 0.5 h2so4 [33] rotten carrot zncl2 135 0.5 koh [34] cassava peel waste koh 153 10.0 h2so4 [26] sago waste koh 64 0.1 h2so4 [35] banana fiber zncl2 74 0.1 na2so4 [36] coffee shells zncl2 150 0.5 koh [37] rice husk koh 80 0.1 koh [12] tobacco waste koh 148 0.5 koh [38] waste tire h3po4 106 1.0 koh [39] palmyra palm flower koh 155 1.0 koh this work the long cycling stability of ppf-900 was tested using gcd at a current density of 10 a g-1, as given in figure 6d. the capacitance remained at 97.3 % after 5000 cycles despite a slight change in the shapes of the charge and discharge curves. the capacitance value was observed to increase in the first 500 cycles and thereafter decreased, owing to an increase in wettability of the electrode material over a precise number of cycles. these findings confirmed that the electrode based on ppf-900 was highly reversible and stable. thus, palmyra palm flower activated at 900 °c exhibited good electrochemical performance, which can be attributed to high specific surface area facilitating the development of e n e rg y d e n si ty , w h k g -1 c u re n t d e n si ty , a g -1 c a p a ci ta n ce r e te n ti o n , % t im e , s s. j. rajasekaran and v. raghavan j. electrochem. sci. eng. 12(3) (2022) 545-556 http://dx.doi.org/10.5599/jese.1314 555 electrochemical double layers and appropriate pore size that facilitates quick ion transport, resulting in an effective improvement in the charge-discharge process. conclusions in this work, porous activated carbon was developed by chemically activating palmyra palm flowers in koh and subsequent carbonization at different temperatures for perceiving the effect of temperature on the intrinsic properties of the obtained activated carbon. palmyra palm flowerderived activated carbon demonstrated good edlc behavior in 3 m koh electrolytes, with high specific capacitance and remarkable reversibility. the porous carbons activated at 900 oc, among other activation temperatures, have a high specific surface area of 950 m2 g-1 with a maximum specific capacitance of 155 f g-1 at 1 a g-1. furthermore, the electrodes have an energy density of 15.1 wh kg-1, a power density of 100.6 w kg-1, and stable cycle life of over 5000 at 10 a g-1. these results demonstrated the interesting nature of using biomass waste to develop cost-effective electrode materials for supercapacitors with high performance. references [1] x. fan, b. liu, j. liu, j. ding, x. han, y. deng, x. lv, y. xie, b. chen, transactions of tianjin university 26(2) (2020) 92-103. https://doi.org/10.1007/s12209-019-00231-w. 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manivannan, n. wu, acs sustainable chemistry & engineering 2(7) (2014) 1592-1598. https://doi.org/10.1021/sc500336h ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1002/slct.201800609 https://doi.org/10.1002/slct.201800609 https://doi.org/10.1016/j.carbon.2018.03.079 https://doi.org/10.1016/j.polymer.2011.12.054 http://dx.doi.org/10.20964/2018.06.09 https://www.nature.com/articles/s41598-019-55794-4 https://doi.org/10.3390/en11123410 https://doi.org/10.1007/s11356-018-3789-x https://doi.org/10.1007/s11771-010-0609-y https://doi.org/10.1016/j.rinp.2016.09.012 https://doi.org/10.1016/j.rinp.2016.09.012 https://doi.org/10.1039/c9ra06501f https://doi.org/10.1039/c9ra06501f https://doi.org/10.1016/j.biortech.2009.12.123 https://doi.org/10.1002/elan.201300262 https://doi.org/10.3390/ma13051179 https://doi.org/10.1039/c4ra05425c http://dx.doi.org/10.1149/1.2799683 https://doi.org/10.5599/jese.536 https://doi.org/10.1038/s41528-020-00093-6 https://doi.org/10.1016/j.electacta.2013.09.121 https://doi.org/10.1016/j.jscs.2018.03.002 http://dx.doi.org/10.2174/1874088x01004010117 https://doi.org/10.1021/jp067009t https://doi.org/10.1016/j.matchemphys.2008.11.010 https://doi.org/10.1016/s1872-5805(17)60140-9 https://doi.org/10.1021/sc500336h https://creativecommons.org/licenses/by/4.0/) flower-like mos2 screen-printed electrode based sensor for the sensitive detection of sunset yellow fcf in food samples: http://dx.doi.org/10.5599/jese.1413 1099 j. electrochem. sci. eng. 12(6) (2022) 1099-1109; http://dx.doi.org/10.5599/jese.1413 open access : : issn 1847-9286 www.jese-online.org original scientific paper flower-like mos2 screen-printed electrode based sensor for the sensitive detection of sunset yellow fcf in food samples peyman mohammadzadeh jahani school of medicine, bam university of medical sciences, bam, iran corresponding author: peymanjahani1234@gmail.com received: june 19, 2022; accepted: july 1, 2022; published: july 23, 2022 abstract in this study, synthesis and electrochemical sensor application of flower-like mos2 for sunset yellow fcf sensing were evaluated. linear sweep voltammetry (lsv), chronoamperometry and differential pulse voltammetry (dpv) were used to determine flower-like mos2 electrochemical sensor performances, which had lod of 0.04 μm in the linear working range of 0.1–150.0 μm calculated from dpv. the sensor was successfully applied for the determination of sunset yellow fcf in real samples. keywords electrochemical sensor; modified electrode; voltammetric determination introduction over the past centuries, food and beverage products used different colors to maintain nutriational and health-related advantages. these compounds actively participate in various biological processes. nowadays, color additives are commonly applied in a variety of foods, including dairy products, beverages, cereals, snack foods and ice creams, to make them more attractive and appetitive. among various types of food additives, food colorants are played an important role in foodstuff due to their physical appearance and consumer acceptance. food colorants are categorized into natural and synthetic dyes. however, synthetic food dyes are widely used in the food industry compared to natural colorants due to higher light, oxygen, and ph stability. in addition, they provide a uniform color and a strong tinting ability. synthetic color additives are organic pigments using artificial synthesis methods and are generally made from coal tar from aniline dyes as raw material [1]. among the most used dyes by the food industry are those that contain an azo group (–n=n–), including the sunset yellow (sy), the bordeaux red, and the tartrazine yellow. these dyes are extensively used in cereals, candies, dairy products, jellies, ice creams, fillings, liqueurs, powdered juices, soft drinks, and yogurts [2-4]. sunset yellow (sy), also known as evening yellow, e110 or edible yellow 3, is one of the most used azo dyes. it has an orange color and is used in a great number of fruit products, like sodas, juices, candies, and ice creams. usually, it is the only http://dx.doi.org/10.5599/jese.1413 http://dx.doi.org/10.5599/jese.1413 http://www.jese-online.org/ mailto:peymanjahani1234@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1099-1109 mos2 spe for detection of sunset yellow 1100 artificial dye present in soft orange beverages. sy also has a large use in the pharmaceutical industry and in cosmetics. nevertheless, it also causes some side effects in humans, and its consumption has been related to renal failure and hepatocellular damage [5-7]. until now, different methods such as high-performance liquid chromatography, near-infrared spectroscopy and interval random forest, surface-enhanced raman spectroscopy and fluorescence spectroscopy have been reported for the determination of sunset yellow [8-11]. in addition, voltammetry was also used to determine sunset yellow dye based on their electrochemical activity [12-14]. electrochemical sensor systems do not require particular expertise or expensive apparatus for operation. as a result, miniaturization of these systems for in-situ analysis of ecological contaminants is enabled. the advent of screen-printed electrodes (spes) has prospects for direct electrochemical mechanisms outside centralized laboratories. features like rapid response, mass production, low power requirement, and compactness have reinforced spes as ideal transducers for environmental assays. further, the sample volume required for an spe is as low as 50 μl. additionally, the surface of the spes’ working electrode may be readily modified to improve the sensibility, lowering the detection limit, and increasing the selectivity of the electrochemical methods [15,16]. the chemical modification of inert substrate electrodes offers significant advantages in the design and development of electrochemical sensors [17-30]. in operations, the redox-active sites shuttle electrons between a solution of the analyte and the substrate electrodes often along with a significant reduction of the activation overpotential. a further advantage of chemically modified electrodes is that they are less prone to surface fouling and oxide formation compared to inert substrate electrodes [31-45]. the development of nanoscience and nanotechnology has allowed attempts to apply different nanomaterials in different scientific fields [46-51], including the fabrication of chemically modified electrodes [52-56]. in recent years, various nanomaterials have been used singly or in composite form for the modification of electrodes [57-69]. mos2 obtained by different preparation methods have different morphology, such as 2d-layered structures, nanorods, nanotubes and three-dimensional structures. among them, flower-like mos2 has a unique three-dimensional structure. because of its large specific surface area, ultra-short transmission path, high interfacial transfer, and excellent catalytic activity as a matrix, it is widely used in energy storage, catalysis, sensing and other fields. mos2 can increase electrocatalytic activity. molybdenum disulfide (mos2), which is a two-dimensional (2d) transition metal dichalcogenides (tmds), has gained immense attention over the past few years. as a semiconductor with an indirect energy band of 1.29 ev, mos2 has a typical 2d layered structure, and each layer is composed of two sulfur atoms and a molybdenum (mo) atom, forming an s–mo–s sandwich structure. mos2 shows tunable electric properties, excellent electrocatalytic activity, electrochemiluminescent (ecl) signal amplification, photochemical reactivity, fluorescence quenching ability, and adsorption performance and hence is a candidate for catalysis, biology, medicine, sensor monitoring, energy storage and conversion [70, 71]. the aim of this study was to synthesize flower-like mos2 (fl-mos2) to modify the screen-printed electrodes for the detection of sunset yellow fcf food dye. experimental apparatus and chemicals all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab. screen-printed electrodes (spe) (dropsens; drp-110: spain) consisted of a graphite counter electrode, a graphite working electrode, and a silver pseudo-reference electrode. solution ph values were determined using a 713 ph meter combined with glass electrode (metrohm, switzerp. m. jahani j. electrochem. sci. eng. 12(6) (2022) 1099-1109 http://dx.doi.org/10.5599/jese.1413 1101 land). sunset yellow and other chemicals used were of analytical grade and were purchased from merck. flower-like mos2 was synthesized in our laboratory [72]. a typical sem is shown in figure 1. figure 1. sem image of flower-like mos2 preparation of fl-mos2/spe first, 1 mg of prepared fl-mos2 was added into an aqueous solution (1 ml), followed by sonication for 30 min to give a homogeneous solution. then, 4 μl of fl-mos2 was dispersed on the surface of spe dropwise. following the solvent's evaporation, the sensor's surface was washed several times with deionized water to clean free modifier molecules and subsequently air-dried. the obtained electrode was noted as fl-mos2/spe. the surface areas of the fl-mos2/spe and the bare spe were obtained by cyclic voltammetry using 1 mm k3fe(cn)6 at various scan rates. using the randles–ševčik equation for fl-mos2/spe, the electrode surface was found to be 0.11 cm2 which was about 3.5 times greater than bare spe. results and discussion electrochemical behavior of sunset yellow at the surface of various electrodes the effect of the electrolyte ph on the oxidation of 60.0 μm sunset yellow was investigated at fl-mos2/spe using dpv measurements in the phosphate buffer solution (pbs) in the ph range from 2.0 to 9.0. according to the results, the oxidation peak current of sunset yellow depends on the ph value and increases with increasing ph until it reaches the maximum at ph 7.0, and then decreases with higher ph values. the optimized ph corresponding to the higher peak current was 7.0, indicating that protons are involved in the reaction of sunset yellow oxidation. this result indicates the oxidation mechanism of sunset yellow involving electron transfers coupled with proton exchange. it is well-known that the hydroxyl group of sunset yellow undergoes a 1e-/1h+ oxidation mechanism at the surface of reported electrodes. the electrochemical behavior of sunset yellow was investigated by lsv. the linear sweep voltammetry was obtained using the bare spe (curve b) and fl-mos2/spe (curve a) in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm sunset yellow are shown in figure 2. on a bare spe, a signal with a low oxidation current of 3.3 μa was obtained with a peak potential of 760 mv. in contrast, http://dx.doi.org/10.5599/jese.1413 j. electrochem. sci. eng. 12(6) (2022) 1099-1109 mos2 spe for detection of sunset yellow 1102 fl-mos2/spe exhibited an enhanced sharp anodic peak current (ipa = 14.0 μa) at much lower overpotential ep = 740 mv. these results confirmed that the fl-mos2/spe improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of sunset yellow. figure 2. linear sweep voltammograms of (a) fl-mos2/spe and (b) bare spe in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm sunset yellow at the scan rate 50 mvs-1 effect of scan rate on the determination of sunset yellow at fl-mos2/spe the influence of the scan rate () on the peak currents (ipa) of sunset yellow at fl-mos2/spe was investigated by lsv (figure 3). figure 3. linear sweep voltammograms of fl-mos2/spe in 0.1 m pbs (ph 7.0) containing 50.0 μm sunset yellow at various scan rates; 1-6 correspond to 5, 15, 45, 70, 100 and 200 mv s-1, respectively figure 4 shows the voltammetric response of 50.0 μm sunset yellow at fl-mos2/spe at different scan rates in the range of 5 to 200 mv/s. the oxidation peak current of sunset yellow increases p. m. jahani j. electrochem. sci. eng. 12(6) (2022) 1099-1109 http://dx.doi.org/10.5599/jese.1413 1103 linearly with increasing scan rate. the linear regression equation was obtained from the plot ipa and vs. 1/2 (square root of scan rate) as follows; ipa = 1.9165ʋ1/2 0.0611 (r2 = 0.9996) for the oxidation process, which indicates that the reaction of sunset yellow at fl-mos2/spe is diffusion controlled. figure 4. plot of anodic peak current vs. 1/2 at different scan rates in the range of 5 to 200 mv/s. chronoamperometric analysis the analysis of chronoamperometry for sunset yellow samples was performed by use of flmos2/spe at 0.77 v. the chronoamperometric results of different concentrations of sunset yellow in pbs (ph 7.0) are demonstrated in figure 5. the cottrell equation for the chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is as in eq. (1) i = nfad1/2cbπ-1/2t-1/2 (1) figure 5. chronoamperograms obtained at fl-mos2/spe in 0.1 m pbs (ph 7.0) for different concentration of sunset yellow. numbers 1-4 correspond to 0.1, 0.75, 1.1 and 1.5 mm of sunset yellow where d is diffusion coefficient (cm2 s-1), and cb is the applied bulk concentration (mol cm-3). experimental results of i vs. t-1/2 were plotted in figure 6a, with the best fits for different concentrations of sunset yellow. the resulting slopes corresponding to straight lines in figure 6a, http://dx.doi.org/10.5599/jese.1413 j. electrochem. sci. eng. 12(6) (2022) 1099-1109 mos2 spe for detection of sunset yellow 1104 were then plotted against the concentration of sunset yellow (figure 6b). the mean value of d was determined to be 1.1×10-5 cm2/s according to the resulting slope and cottrell equation. figure 6. (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-4, (b) plot of the slope of the straight lines against sunset yellow concentration (0.1-1.5 mm) calibration curve because dpv commonly has a higher sensitivity than cyclic voltammetry technology, the dpv technique was applied for the quantitative detection of sunset yellow. figure 7 shows the differential pulse voltammograms of sunset yellow at various concentrations using fl-mos2/spe (step potential = 0.01 v and pulse amplitude = 0.025 v). figure 7. dpvs of fl-mos2/spe in 0.1 m (ph 7.0) containing different concentrations of sunset yellow. numbers 1–7 correspond to 0.1, 2.5, 10.0, 30.0, 70.0, 100.0 and 150.0 µm of sunset yellow as seen, the oxidation peak currents of sunset yellow enhance gradually by increasing its concentration. the oxidation peak currents show a good linear relationship with the concentrations of p. m. jahani j. electrochem. sci. eng. 12(6) (2022) 1099-1109 http://dx.doi.org/10.5599/jese.1413 1105 sunset yellow ranging from 0.1 m to 150.0 μm. the linear equation is ipa = 0.1315csunset yellow + + 1.0472 (r2 = 0.9994) (figure 8). also, the limit of detection (lod) was estimated to be 0.04 μm. csunset yellow / m figure 8. plot of the electrocatalytic peak current as a function of sunset yellow concentration in the range of 0.1-150.0 µm the performance of this sensor is compared with some of the recently reported electrodes for sunset yellow quantification (see table 1) [73-76]. table 1. comparison the sensing performances toward the detection of sunset yellow between the existing modified electrodes and the proposed fl-mos2/spe electrochemical sensor method linear range, µm lod, µm ref. poly(l-phenylalanine)-modified glassy carbon electrode dpv 0.04–14.0 0.04 [73] graphene oxide decorated with silver nanoparticles–molecular imprinted polymers modified glassy carbon electrode lsv 0.1-12.0 0.02 [74] gold nanoparticles modified carbon paste electrode dpv 0.1-2.0 0.03 [75] multi-walled carbon nanotubes modified glassy carbon electrode dpv 0.55-7.0 0.12 [76] fl-mos2/spe dpv 0.1–150.0 0.04 this work stability and repeatability the long-term stability test of the fl-mos2/spe sensor using dpv was performed at room temperature. the results exhibited that the peak current of 35.0 μm sunset yellow at the flmos2/spe stayed at 93.2 % of its primary current after 7 days, 90.8 % after 14 days, and 89.97 % after 21 days, indicating the superior long-term stability of the proposed sensor. the oxidation of sunset yellow (35.0 μm) on the same fl-mos2/spe sensor was performed by ten repeated voltammetric measurements, the results of which confirmed the superior repeatability of the fabricated sensor with the relative standard deviation (rsd) of 2.9 %. http://dx.doi.org/10.5599/jese.1413 j. electrochem. sci. eng. 12(6) (2022) 1099-1109 mos2 spe for detection of sunset yellow 1106 analysis of real samples the real samples for the analysis were prepared and quantified by dpv method. the developed sensor was applied to detect sunset yellow in orange and apple juice samples. the results are summarized in table 2. each measurement was repeated five times. the recovery and relative standard deviation (rsd) values confirmed that the fl-mos2/spe sensor has a great potential for analytical application. table 2. the application of fl-mos2/spe for the determination of sunset yellow in real samples (n=3) sample concentration, µm recovery, % rsd, % spiked found apple juice 0.0 3.0 3.4 2.0 1.9 3.0 2.2 orange juice 0.0 2.5 2.1 3.0 3.0 4.0 2.8 conclusion in the present study, sensing of sunset yellow fcf was attempted and implemented successfully with flower-like mos2 (fl-mos2) modified screen-printed electrodes. the morphology of flower-like mos2 was characterized by sem. using linear sweep voltammetry and differential pulse voltammetry, the electrochemical oxidative sensing of sunset yellow fcf was investigated. the fl-mos2/spe showed a linear response of 0.1 μm to 150.0 μm 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{12}, abstract = {in this study, synthesis and electrochemical sensor application of flower-like mos2 for sunset yellow fcf sensing were evaluated. linear sweep voltammetry (lsv), chrono­am­perometry and differential pulse voltammetry (dpv) were used to determine flower-like mos2 electrochemical sensor performances, which had lod of 0.04 $\mu$m in the linear working range of 0.1–150.0 $\mu$m calculated from dpv. the sensor was successfully applied for the determination of sunset yellow fcf in real samples.}, doi = {10.5599/jese.1413}, file = {:d\:/onedrive/mendeley desktop/jahani 2022 flower-like mos2 screen-printed electrode based sensor for the sensitive detection of sunset yellow fcf in food samples.pdf:pdf;:www/jese_v12_no6_1099-1109.pdf:pdf}, keywords = {electrochemical sensor, modified electrode, voltammetric determination}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1413}, } hybrid polymer inclusion membrane as anion exchange membrane for recovering pd2+ ions in electrogenerative process http://dx.doi.org/10.5599/jese.1501 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1501 open access : : issn 1847-9286 www.jese-online.org original scientific paper hybrid polymer inclusion membrane as anion exchange membrane for recovering pd2+ ions in electrogenerative process syed fariq fathullah syed yaacob, nadia mansor, syaza atikah nizar, ayo olasupo, norita mohamed and faiz bukhari mohd suah green analytical chemistry laboratory, school of chemical sciences, universiti sains malaysia, 11800 minden, pulau pinang, malaysia corresponding authors: syedfariq@usm.my; fsuah@usm.my received: august 23, 2022; accepted: november 36, 2022; published: december 8, 2022 abstract a novel non-plasticized nano-porous hybrid inorganic-organic polymer inclusion membrane (pim) was synthesized, characterized, and evaluated as an anion exchange membrane for application in electrogenerative processes to recover pd2+ ions. ionic liquids 1-ethyl-3-methylimidazolium chloride (emim-cl) and 1-butyl-3-methylimidazolium chloride (bmim-cl) were used as the carrier molecules in the polymeric network of pim to enhance anion exchange process. this hybrid anion exchange membrane also consists of a polymeric matrix of non-plasticized cellulose triacetate modified by incorporating an inorganic material (silane) prepared by the sol-gel route. different parameters affecting the ion transport performance efficiency, i.e., the composition of the membrane, type of ionic liquid (carrier molecule) and ion–exchange capacity, were investigated and optimized. in the electrogenerative process, the results revealed that the prepared pim yields better recovery results for recovering pd2+ ions from its chloride solution compared to the commercial anion exchange membrane neosepta® am-01, with a full recovery of 100 mg/l pd2+ ions in 30 min. this preliminary study shows that the prepared low-cost hybrid anion exchange membrane pim can act as an inexpensive material suitable for the rapid and efficient recovery of pd2+ ions from an aqueous solution. keywords hybrid inorganic-organic membrane; chloride solution; ionic liquids introduction an electrogenerative process is a spontaneous electrochemical process that reduces (more noble) metal ions to elemental metal at the cathode [1]. as in a galvanic system, the energy associated with chemical reactions is converted into electrical energy or direct current electricity as a by-product. although electrogenerative systems and fuel systems use catalytic electrodes, the cell components differ in their primary function [2]. the emphasis on chemical processing in electrohttp://dx.doi.org/10.5599/jese.1501 http://dx.doi.org/10.5599/jese.1501 http://www.jese-online.org/ mailto:syedfariq@usm.my mailto:fsuah@usm.my j. electrochem. sci. eng. 00(0) (2022) 000-000 hybrid polymer inclusion membrane 2 generative processes results in special considerations for these types of systems, such as product recovery [3]. in general, the setup for an electrogenerative process consists of two electrodes (cathode and anode), electrolytes (catholyte and anolyte), an ion exchange membrane, and an external cable to complete the circuit [4]. the electrogenerative process is driven by an oxidation-reduction reaction. the anode side is where oxidation occurs with the substance losing electrons, while reduction occurs on the cathode side, which gains electrons (e.g., the oxidant pd2+ gains electrons to become pd0). the standard reduction potential for the half-cell reaction shows an overall positive cell potential, resulting in the spontaneous deposition of the reduced metal (pd0) onto the cathode. the effect of pd deposition on the membrane was further investigated. anode: zn2+ + 2e-⇌ zn e°ox = -0.763 v vs. she (1) cathode: pd2++ 2e-⇌ pd e°red = +0.981 v vs. she (2) overall: zn + pd2+ ⇌ zn2++ pd e°cell = +1.744 v vs. she (3) ion exchange membranes are semi-permeable membranes composed of ionic head groups attached to polymer matrices [5]. depending on the type of ion permitted to cross the membrane layer, they can be classified as anion exchange membranes, cation exchange membranes or bipolar membranes [6,7]. for example, anion exchange membranes contain positively charged head groups in the membrane, which permit the passage of anions while repelling cations [6]. by exploiting the selective nature of ion exchange membranes, anion and cation exchange membranes can be adapted for various applications. commercially available ion exchange membranes are primarily found in water treatment applications such as desalination or high-purity water production in food and beverage, as well as in pharmaceuticals, semiconductors, power generation, fuel cells, electrodialysis, donnan analysis, electrolysis and many more [4-15]. one type of ion exchange membrane is a hybrid inorganic-organic ion exchange membrane. inorganic–organic composite materials are increasingly important due to their extraordinary properties within a single molecular composite, which arise from the synergism between the individual component properties [16]. a sol-gel method can be used to prepare this hybrid membrane by adding a silane into the organic membrane. the purpose of adding silane to the membrane is to provide beneficial impacts to the organic polymeric membrane (cellulose triacetate). in particular, adding silane will assist the migration of ions across the anion-exchange membrane, resulting in better mass transport due to the formation of hydrated microchannels. these materials have gained much interest due to the transformation in their properties, such as mechanical, thermal, electrical, and magnetic properties, compared to pure organic polymers or inorganic materials [17-20]. in these composites, organic materials offer structural flexibility, convenient processing, tunable electronic properties, photoconductivity, efficient luminescence, and the potential for semiconducting and metallic behaviour. inorganic compounds provide the potential for high carrier mobilities, bandgap tunability, a range of magnetic and dielectric properties, and thermal and mechanical stability. in addition to combining distinct characteristics, new or enhanced phenomena can also arise due to the interface formed between the organic and inorganic components [21,22]. to date, only commercially available anion exchange membranes, namely neosepta® am-01 and neosepta® amx, have been utilized in an electrogenerative process [23,24]. however, the commercially available anion exchange membranes are costly, produced for general (non-specific) applications, and have low dimensional ability and high solute diffusion rates. using polymer inclusion membranes (pims) as anion exchange membranes in electrogenerative processes was already proposed with remarkable results [25]. to further improve the performance of pims, a nons. f. f. syed yaacob et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1501 3 plasticized nano-porous hybrid inorganic-organic anion exchange pim containing an ionic liquid has been introduced in this study. this study aims to prepare and characterize the novel nano-porous hybrid inorganic-organic anion exchange pim and evaluate its performance as an anion exchange membrane in the electrogenerative process for recovering pd2+ ions from a chloride solution. experimental chemicals and materials for hybrid pim preparation, tetraethyl orthosilicate (teos) and dichlorodimethylsilane (dcdms) from sigma aldrich, uk, were used to obtain the silane mixture. trioctylmethylammonium chloride (aliquat 336), 1-ethyl-3-methylimidazolium chloride (emim-cl) and 1-butyl-3-methylimidazolium chloride (bmim-cl) from sigma aldrich (united kingdom) were used as a carrier in the membrane, and cellulose triacetate (cta) (acros, malaysia) was used as organic support to provide the polymeric mechanical strength. the commercial anion exchange membrane was neosepta am-01 (tokuyama corp., japan). chemicals such as palladium(ii) chloride, chloroform and diethyl ether were obtained from qrec chemicals. the electrogenerative batch cell design was the same as in our previous work [1], with each compartment 5.0×4.5×6.5 cm. the cathode used in this experiment was reticulated vitreous carbon (rvc) (the electrosynthesis co., usa) with dimensions 2.5×1.0×0.6 cm, pre-treated with concentrated h2so4. zinc foil (99 % purity) (2.5×1.0×0.05 cm) was used as the anode and polished with sandpaper before rinsing with distilled water and ethanol. the membrane was cut into dimensions that fit the cell and filled with water before use. all experiments were run for 4 h at room temperature. silane mixture preparation a multistage procedure prepared the inorganic part of the membrane consisting of a silane mixture. 4 ml of dichlorodimethylsilane (dcdms) was dissolved in 10 ml of diethyl ether, and then 1.5 ml of water was added in aliquots of 0.5 ml every 20 min and mixed for 1 h in an iced water bath. later, both immiscible phases were separated, and 1 ml of tetraethylorthosilicate (teos) was added to the extracted organic phase to obtain a cross-linked network. finally, the homogenous solgel mixture was kept in the water bath for 5 days at 50 °c. hybrid pim membrane preparation for membrane samples preparation, the organic part of the membrane was prepared by dissolving different amounts of cellulose triacetate (cta) and carrier in chloroform. the obtained silane mixture (0.03 g) was added to the organic solution, and all components were vigorously stirred to mix them homogeneously. the resulting solution was poured into a petri dish and allowed to evaporate for over 24 h at room temperature. the polymer inclusion membrane (pim) was later cured in an oven at 80 °c for 20 h and stored in a sealed bag. the process is shown in figure 1. hybrid membrane characterization the thickness of the membrane and its surface morphology were measured with an electronic digital micrometer and corroborated with scanning electron microscopy (sem) (quanta 650, usa). the membrane functional groups were identified using fourier transform infrared (ft-ir) spectrometer (perkin elmer model system 2000, usa). measurement was scanned in the frequency range of 400 to 4000 cm-1 and analysed using commercial software. http://dx.doi.org/10.5599/jese.1501 j. electrochem. sci. eng. 00(0) (2022) 000-000 hybrid polymer inclusion membrane 4 figure 1. schematized procedure for preparation of nano-porous hybrid inorganic-organic anion exchange pim (dcdms: dichlorodimethylsilane, dee: diethyl ether, teos: tetraethoxysilane, cta: cellulose triacetate, il: ionic liquid) electrogenerative experiment the optimum condition of pd2+ ions recovery was studied using a batch cell, similar to what was reported by mansor et al. [23]. the batch cell configuration was used for each experiment at room temperature (figure 2). during the electrogenerative experiment, electrodes with attached copper collectors were linked to a multimeter to measure the potential of electrodes, as well as the flow of electric current. in order to compare the differences in the concentration of pd2+ left in the solution after being deposited on the cathode, 500 μl of the catholyte solution was initially removed, and this procedure was repeated frequently every 30 minutes during the tests. at the same time, every withdrawn aliquot was substituted with 500 μl of distilled water to ensure the same volume of both electrolytes. prior to analysis, the aliquot samples were diluted with distilled water to a volume of 25 ml in 60 ml plastic bottles. figure 2. experimental setup of the batch cell s. f. f. syed yaacob et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1501 5 atomic absorption spectroscopy, aas (perkin elmer analyst 200 model, usa) with a wavelength of 247.6 nm was then used to analyze pd2+ recovery concentrations every 30 minutes over the 6-hour experiment. a blank and various pd2+ standard solutions at concentrations of 1, 2, 3, 4, 5, and 10 mg l-1 were examined for calibration curve analysis. the calculation of the recovery in % is presented in equation (4). 0 0 pd recovery = 100t c c c − (4) where co is the initial and ct is the pd concentration, mg l-1, at time t. polarization of the membrane a voltmeter measured the potential of the rvc cathode against the reference electrode (saturated calomel electrode, sce) (hanna instruments, malaysia). initially, the cell was connected to a resistance box loaded with a maximum resistance of 9.99 mω and was decreased gradually until the minimum of 1 ω. the system was allowed to reach a steady state for 3 min after each change in the resistance value. later, the cathode potential and current readings were recorded, and the cathodic polarization curves were plotted. results and discussion membrane characterization membrane composition for the preliminary studies using different types of ils (figure 3), the pim composition was prepared by mixing 0.10 g of cta and 0.08 g of il, followed by 0.03 g of silane. it is shown in table 1 that among three tested hybrid membranes (pim1-pim3), the excellent recovery of pd2+ ions (100 mg/l) is obtained in the presence of bmim-cl (pim2). the presence of extra cl− ions in the hybrid membrane enhances the mass transfer process, which subsequently increases the recovery of pd2+ ions [26]. further studies were conducted using different quantities of bmim-cl due to its direct relationship with transport efficiency and results are shown in table 2. figure 3. different types of ionic liquids (ils) used in this study table 1. hybrid pim compositions membrane mass, g content, wt.% recovery, % cta il silanes blank 0.1 pim 1: il (aliquat 336) 0.1 0.08 0.03 48:38:14 84.0 pim 2: il (bmim-cl) 0.1 0.08 0.03 48:38:14 99.9 pim 3: il (emim-cl) 0.1 0.08 0.03 48:38:14 93.0 http://dx.doi.org/10.5599/jese.1501 j. electrochem. sci. eng. 00(0) (2022) 000-000 hybrid polymer inclusion membrane 6 table 2. optimization of bmim-cl concentration for hybrid pim membrane mass, g content, wt.% cta bmim-cl silanes blank 0.10 i 0.10 0.08 0.03 48:38:14 ii 0.14 0.14 0.03 37:52:11 iii 0.12 0.12 0.03 44.5:44.5:11 iv 0.12 0.12 0.05 41.5:41.5:17 it was observed that when the excess transporting agent was added (> 0.10 g), a sticky and fragile membrane unsuitable for transport was formed. similarly, adding too much silane (> 0.05 g) resulted in a crusty membrane surface and a non-homogeneous film. in addition, experimental results showed that it is necessary to cover the petri dish with filter paper to slow the evaporation of the solvent and promote even drying, producing a clear, transparent, and homogeneous film. identification of membrane spectroscopic composition in this study, atr-ftir analysis was performed to confirm the presence of the main components of the pim membrane by identifying the most important peaks (figure 4 and table 3). figure 4. atr-ftir spectra of different isolated components of hybrid membrane: (a) silanes, (b) cellulose triacetate (cta) and (c) hybrid membrane (pim 2) s. f. f. syed yaacob et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1501 7 table 3: values of wavenumbers and types of molecular vibrations for each constituent material and hybrid membrane materials wavenumber, cm-1 type of molecular structure cta 3365–3079 o – h 2944 c – h 1721 c = o stretching 1203 1025 c – o – c silanes 1258 (si –)ch3 1021 si – o 801 si – c pim 2 3230–3578 quarternary amine 2969  c – h 1751 c = o 1258 (si –)ch3 1021 si – o 801 si – c as shown in figure 4(a), the atr-ftir spectrum of the silane mixture shows an absorption band at 801 cm−1 corresponding to the si–c stretching in si(ch3)2. the absorption bands observed at 1258 cm−1 and 1021 cm−1 correspond to the symmetrical deformation vibration of (si–)ch3 and si–o stretching, respectively. figure 4(b) shows that the absorption band located around 1721 cm−1 corresponds to stretching vibrations of the carbonyl group of cta. the firm peaks at 1203 and 1025 cm−1 reflect the stretching modes of c–o–c symmetric and asymmetric bonds of cta. meanwhile, less intense bands seen at 2944 cm−1 are attributed to c–h bonds, and the absorption for o–h bonds stretching modes are observed in the region of 3365–3079 cm−1. figure 4(c) shows the expected bands related to the primary products described above: si–c, (si–)ch3, and si–o peaks from silane mixtures, and the c=o stretching vibration at 1751 cm−1 due to the presence of cta. the peak at 2969 cm−1 is the aliphatic asymmetric and symmetric (c–h) stretching vibration due to the methyl groups in bmim-cl. a broad peak in 3230–3578 cm−1 corresponds to quaternary amine salt formation with chlorine. based on the membrane spectrum, there are no signs of covalent bond formation between cta, bmim-cl and silane. this implies only weak interactions between membrane components, such as hydrophobic, van der waals and/or hydrogen bonds. morphological properties of pims figure 5 shows sem images of all prepared pims, i.e., cta, cta–aliquat 336–silane, cta–bmim -cl–silane, and cta–emim-cl–silane. these images are presented as cross-sections (i) and surface sections (ii). as shown in figure 5a, the surface of the cta membrane appears dense and non-porous, whereas the pim membranes shown in figures 5b to 5d possess a porous structure. the membrane cross sections in figures 5b and 5c show estimated membrane thicknesses of 20 to 30 µm, which is very suitable for mass transport. this is because as the membrane thickness increases, the metal ion transport decreases due to the membrane resistance [27]. higher porosity, with larger pores than the others, are observed for the membrane containing ionic liquid bmim-cl (pim 2) (figures 5c and 5e). this suggests that the larger pore size may increase the rate of ion movement across the membrane. http://dx.doi.org/10.5599/jese.1501 j. electrochem. sci. eng. 00(0) (2022) 000-000 hybrid polymer inclusion membrane 8 a b c d e figure 5. sem (i) cross section and surface (ii) images of: (a) cta; (b) pim 1 (ctaa336-silane); (c) pim 2 (cta-bmim-cl-silane); (d) pim 3 (cta-emim-cl-silane); (e) pim 3-higher magnification. compositions are given in table 1 optimization of pd2+ ions recovery process effect of different types of ils the efficiency of different ils for the recovery of pd2+ ions is illustrated in figure 6. figure 5. percent of recovery of 100 mg/l pd2+ in 0.2.m nacl vs. time with different types of ionic liquids used in pims as shown in table 1, using bmim-cl as the ion-exchanger (carrier) in the hybrid pim resulted in the highest recovery of pd2+ ions with 99.9 % recovery, followed by emim-cl with 93.0 %, and aliquat s. f. f. syed yaacob et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1501 9 336 (84.0%). the efficiency of these ils as ion exchangers depends on numerous factors, including the anolyte ph, the catholyte concentration, the amount of carrier used, the type of contaminants, membrane morphology, the molecular weight of the carrier, and others [23,24]. however, for comparison, the investigation was conducted under identical experimental conditions. the lower recovery of pd2+ ions when aliquat 336 was used could be attributed to the surface morphology of the aliquat 336-based membrane and the molecular weight of the carrier. according to the sem image in figure 5b, the aliquat 336-based membrane shows a smooth surface with no apparent porosity compared to bmim-cl, which has a large pore size (figures 5b and 5e). furthermore, the molecular weight of aliquat 336 is higher than that of bmim-cl and emim-cl. thus, due to the lack of porosity, it might be difficult for the carrier to permeate the interface of the pim during ion exchange with the anionic moiety of the anolyte. hence, subsequent experiments were conducted using bmim-cl as the il of choice. effect of different pim compositions the fastest recovery of pd2+ ions was achieved using a pim with a composition of cta 0.1 g, bmim-cl 0.08 g, silane 0.03 g, where 99.9 % recovery was achieved in 30 min (figure 7). from visual observation, this pim composition formed a transparent film, which was smoother and more homogeneous than other pims (table 4). it has been reported that high silane concentrations can affect the membrane appearance, resulting in a crusty layer and non-homogeneous characteristics [27]. a similar result was observed for the pim with a composition of cta 0.12 g, bmim-cl 0.12 g, and silane 0.05 g. hence, the pim composition cta 0.1 g: bmim-cl 0.08 g: silane 0.03 g is preferred and chosen for additional analysis. figure 7. percent of recovery of 100 mg/l pd2+ in 0.2.m nacl vs. time for different compositions of pim (cta-bmim-cl silane) table 4. pd recovery and visual observation of different compositions in cta-bmim-silane membrane mass, g (cta : bmim : silane) 0.10 : 0.08 : 0.03 0.12 : 0.14 : 0.03 0.12 : 0.12 : 0.05 0.20 : 0.00 : 0.03 time (99.9 %), min 30 120 analysis not conducted >30 visual http://dx.doi.org/10.5599/jese.1501 j. electrochem. sci. eng. 00(0) (2022) 000-000 hybrid polymer inclusion membrane 10 effect of silane in pim composition figure 8 shows the effect of silane presence on the recovery of pd2+ ion. it can be seen that the introduction of silane into the membrane matrix slightly increases the pd2+ ion recovery rate. complete recovery of pd2+ ions was achieved after 30 min using hybrid pims containing silane. the presence of silane in the pim matrix could enhance the migration of ions and resulting mass transport due to the formation of hydrated microchannels and higher porosity [25,28]. therefore, the hybrid pim containing silane was selected for this study. figure 6. effect of silane in pim on recovery of 100 mg/l of pd2+ in 0.2 m nacl. pim composition: 0.1 g cta, 0.08 g bmim-cl, 0.03 g silane effect of different types of membranes three different types of membranes were utilized in this electrogenerative study, i.e., pim consisting of 0.1 g cta, 0.08 g bmim-cl, 0.03 g silane, neosepta am-01 anion exchange membrane and pure cta membrane. the initial study showed the cell voltage generated during full recovery of pd2+ ions (after 30 min of the experiment) using different membranes as the anion exchange membrane was 510.20 mv for pim, 495.3 mv for neosepta am-01 membrane, and 30.6 mv for cta membrane. a high voltage potential value indicates a high deposition of pd2+ ions onto the cathode [25]. this manifests that the presence of bmim-cl (which contains chloride ions) plays an essential part in expediting the ion transfer process, which gives a high potential reading and, subsequently, a rapid recovery time [25]. figure 9 shows similar results for pim and neosepta am-01 membrane and much worse results for the cta membrane. the highest pd2+ ions recovery after 30 min was achieved by pim (99.9 %), followed by neosepta am-01 membrane (96.0 %). in contrast, a pure cta membrane used as an ion exchange membrane could not fully recover pd2+ ions, even after 240 minutes, when only up to 60 % of pd2+ ions are recovered. this result proves that using hybrid pim consisting of silane can increase the recovery rate of pd2+ ions. moreover, the presence of ionic liquid (bmim-cl) led to this result. the ionic liquid's role is to facilitate the movement of the ions across the membrane due to the presence of clions [23]. additionally, silane provides additional strength to pim, where the hydrated micro channels are formed from the mixture of hydrophilic and charged groups. these microchannels become the passage for the ions to transfer across the membrane [29]. the prepared pim and neosepta am-01 membrane reusability study was also conducted (60 min of recovery time). after five cycles of recovery test, it was found that pim could maintain the s. f. f. syed yaacob et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1501 11 recovery performance with only 4 % of decreased performance while neosepta am-01 membrane has lost up to 8 % of recovery performance. these results also confirmed the suitability of pim to be used as an anion exchange membrane. figure 7. influence of different types of the membrane towards the recovery of 100 mg/l of pd2+ in 0.2 m nacl polarization study a polarization study was conducted to investigate the deviation of the electrochemical process from equilibrium due to the passing of electric current through the electrogenerative reactor. it can be seen in figure 10, different types of anion exchange membranes produce current densities of 5.3 ma cm-2 (neosepta am-01 membrane) and 4.6 ma cm-2 (hybrid pim) in the short-circuited condition when voltage is 0.0 v. it is desirable to have high current densities in the electrogenerative process because it will provide faster kinetics for the deposition process [23]. the rvc shows less polarization in the range of 0 to 0.45 ma cm-2 for both membranes (hybrid pim and neosepta am01 membrane), which suggests a great flow of electrons between the cathode (rvc) and anode (zinc foil). these results show that the polarization performance of the hybrid pim used in this study is on par with the commercially available membrane. figure 8. polarization curves of rvc cathode using different types of ion exchange membrane http://dx.doi.org/10.5599/jese.1501 j. electrochem. sci. eng. 00(0) (2022) 000-000 hybrid polymer inclusion membrane 12 the kinetic study of the mass-controlled process for different types of membranes was also performed. based on figure 11, the concentration [ct/c0] decreased as the time increased. moreover, it was noticed that the concentration of pd2+ ions decreased from initial values to less than 0.1 mg/l after 30 min with the hybrid pim used as the anion exchange membrane. this shows that hybrid pim is the most suitable anion exchange membrane for recovering pd2+ ions in this study. figure 11. normalized concentration [ct/c0] vs time between hybrid pim and commercial membrane neosepta for deposition of 100 mg/l of pd2+ in 0.2 m nacl conclusions in this study, a series of hybrid pim membranes containing cta, silane mixture and different ionic liquids has been successfully prepared and used as an anion exchange membrane in the electrogenerative process of pd ions. the electrogenerative process showed that hybrid pim made up of cta, bmim-cl and silane mixture with respective compositions of 0.10, 0.08 and 0.03 g, performs well as an anion exchange membrane. furthermore, compared to a commercially available anion exchange membrane, this hybrid pim achieves a full percentage recovery of pd2+ ions in a shorter time (30 min). thus, hybrid membranes can potentially 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(ii) transport, journal of molecular liquids 301 (2020) 112457. https://doi.org/10.1016/j.molliq.2020.112457 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.jhazmat.2022.129162 https://doi.org/10.1016/j.cej.2022.137083 https://doi.org/10.1016/j.compositesb.2019.106927 https://doi.org/10.1016/j.jpowsour.2017.07.117 https://doi.org/10.1016/j.jcis.2022.04.082 https://doi.org/10.1002/jctb.6878 https://doi.org/10.1016/j.seppur.2017.03.032 https://doi.org/10.1149/2.0101807jes https://doi.org/10.1016/j.jelechem.2019.02.014 https://doi.org/10.1016/j.memsci.2005.06.008 https://doi.org/10.1016/j.memsci.2006.11.049 https://doi.org/10.1016/j.molliq.2020.112457 https://creativecommons.org/licenses/by/4.0/) {electrochemical behavior of electrodeposited pd and pdni coatings for the ethanol oxidation reaction in alkaline solution} doi:10.5599/jese.445 39 j. electrochem. sci. eng. 8(1) (2018) 39-47; doi: http://dx.doi.org/10.5599/jese.445 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical behaviour of electrodeposited pd and pdni coatings for the ethanol oxidation reaction in alkaline solution jelena d. lović, vladimir d. jović1 ictm, department of electrochemistry, university of belgrade, njegoševa 12, 11000, belgrade, serbia 1institute for multidisciplinary research, university of belgrade, kneza višeslava 1, 11030, belgrade, serbia corresponding authors e-mail: jlovic@tmf.bg.ac.rs; tel.: +381-11-337-0389; fax: +381-11-337-0389 received: september 19, 2017; accepted: october 25, 2017 abstract electrodeposited pd and pdni coating samples were tested for ethanol oxidation reaction (eor) in alkaline solution using cyclic voltammetry (cv), chronoamperometric (ca) and quasi steady-state measurements. all alloy samples showed higher current densities for the eor than pure pd coating. the current density increased with increasing the amount of pd in the pdni coating and the most active one was found to be pd0.74ni0.26. based on ca measurements a pseudo-steady state is achieved after 1500 s showing that pd0.74ni0.26 is more efficient and poisoning more tolerant than other investigated coatings. upon the end of the current-time transient, the investigated catalysts were subjected to the potential cycling showing the ability to recover activity loss implying the surface composition stability of binary coatings. keywords palladium; nickel; stability introduction among the different types of fuel cells, direct alcohol fuel cells possess besides high energy densities, low pollutant emissions and low operating temperatures (60–100 °c) [1-3]. alkaline direct ethanol fuel cells (adafcs) is recognized as promising power source because ethanol has higher energy density and lower toxicity compared to methanol [4]. anode catalysts with high catalytic activities as well as good poison resistances are of great significance to the commercialization of adafcs. for alcohol oxidations, pd-based nanocatalysts are superior to pt-based catalysts in alkaline media and are widely used as anode catalysts in adafcs [1,5]. http://dx.doi.org/10.5599/jese.445 http://www.jese-online.org/ mailto:jlovic@tmf.bg.ac.rs j. electrochem. sci. eng. 8(1) (2018) 39-47 electrodeposited pd and pdni coatings 40 for the sake of commercialization, the activity and stability of pure pd needs improvement which could be achieved by combining pd with other metals or metal oxides. it has been reported that modifications of the pd nanocatalyst by metals such as ni [6-8], au [9,10], cu [10,11], ag [10,12], co [10, 13 ] effectively improve the catalytic performances of the pd nanocatalyst toward alcohol oxidations in alkaline media. nickel has been used to modify anode catalysts due to its high electrochemical stability in alkaline media and the low cost. nickel-modified pd electrodes have been reported as catalysts with excellent performances in ethanol oxidation reaction (eor) such as the activity, the low overpotentials and the improved poison resistances. examined parameters of eor such as onset potential reaction and long-term stability show that among a series of graphene (g)-supported nixpd100−x binary alloyed catalysts, ni50pd50/g catalyst exhibits 60 mv lower onset potential compare to ni0pd100/g catalysts and current density approximately 8, 4, and 1.7 times superior than that of ni75pd25, ni0pd100/g, and ni25pd75/g catalysts, respectively [6]. the mass activity of the pd83ni17 hollow nanospheres aerogel is 5.6-fold higher than that of the commercial pd/c catalyst [14] while the mass activity of porous bimetallic pdni catalyst is 3.5 times higher compared to the commercial pd/c [15]. it is revealed that the onset potential is 80 mv lower and the peak current is about 3 times higher for ethanol oxidation using multi-walled carbon nanotubes (mwcnt) catalysts with pd1ni1.5 compared to those of pd/mwcnts due to the small particle size and high crystallinity of binary catalyst [16] although it can be found that binary pd3.7ni1 nanocatalyst with ultra-low loading of metals immobilized on mwcnt exhibits anodic current density over 11 times higher than on the pd/mwcnt [17]. the electro-catalytic activity of the carbon nanofibers (cnf) supported pd–ni nanoparticles prepared by chemical reduction with nabh4 was examined for eor show that the onset potential was 200 mv lower and the peak current density 4 times higher compared to that for pd/c as a result of the uniform distribution of metal nanoparticles on the cnf support while the significant increase in reaction kinetics was achieve by raising the temperature to 60 °c [7]. negatively shifted onset potential and doubled peak current density in potentiodynamic measurements for core-shell ni-pd/c compared to pd/c was found [18], while high activity and excellent stability during continuously cycling were found for pd supported on ni foam [19,20]. it was proposed that the ligand and strain effects contribute the enhanced activity of pd in the presence of ni. namely the insertion of smaller ni atoms into the pd crystal lattice causes a contraction of the pd lattice. this causes a downshift of the pd d-band center and consequently leads to weaker bonding with adsorbates such as poisoning intermediates in the eor. besides, pd has a higher ionization energy than ni and because of that ni atoms become positively charged, facilitating the formation of oxides on ni [21,22]. generated ohad species of ni participate in the oxidative desorption of intermediates in the eor enhancing the activity of binary pdni catalysts. the electrochemical methods for the preparation of metal alloy nanoparticles are extensively developed topic in materials science. in order to increase the activity and durability physical properties such as composition of the alloys, structure and the size of nanoparticles are carefully selected. most of the synthesis methods use organic surfactants, capping agents or high temperatures, consequently heating or cleaning treatment are necessary and therefore, the catalytic activity can be affected by undesirable adsorbed species. nevertheless, among the various methods, electrodeposition is recognized as a simple and versatile method to prepare bimetallic surfaces. several methods for pdni alloy electrodeposition can be find in the literature such as: doublepotential step electrodeposition technique from the solution containing niso4, h2pdcl4 and na2so4 [23]; cathodic deposition from the solution containing pdcl2, h2so4, nh4cl and nicl2 using hydrogen j. d. lović and v. d. jović j. electrochem. sci. eng. 8(1) (2017) 39-47 doi:10.5599/jese.445 41 dynamic bubble template producing three-dimensional hierarchical pores of intercomnected dendrite walls [ 24 ]; electrodeposition in the presence of complexing agent, ethylenediamine [25,26]; electrodeposition from a ionic liquid as electrolyte [16,27]; physical vapour deposition which was used to obtain palladium-modified nickel foam material [19]. the aim of this work is to investigate the electrochemical behavior of electrodeposited pd and pdni coatings for the eor in 1 m naoh at room temperature. an attempt is made to estimate the difference in catalytic activity of binary coatings in comparison with the pd coating toward eor, to test the stability of pdni coatings and their ability to recover activity loss. in addition, kinetic and mass transport properties of the electrodeposited coatings are examined. experimental all experiments were carried out with an voltalab pgz 402 (radiometer analytical, lyon, france) at room temperature in three compartment electrochemical glass cells with pt wire as the counter electrode and saturated calomel electrode (sce) as the reference electrode. a mirror-like polished gold rotating disk electrode (d = 5 mm) prepared as described elsewhere [28] served as working electrode. all the solutions used were prepared with high purity uv water (millipore, 18.2 mω cm resistivity) and p.a. grade chemicals (merck). the electrolytes were purged with purified nitrogen prior to each experiment. electrodeposition of pdni coating samples was achieved galvanostatically on the rotating au disc electrode from the plating bath composed of 0.01 m pdcl2 + 0.6 m nicl2 + 2 m nh4cl while pure pd was electrodeposited from the bath containing 0.05 m pdcl2 + 2 m nh4cl. all the conditions are the same as it was described in [29]. the electrochemically active surface area (ecsa) was estimated from the charge corresponding to the pd-oxide reduction peak in 1 m naoh by dividing obtained charge with the charge of 420 µc cm-2 (corresponding to the monolayer of pd-oxide), in accordance with the previous reports [6,8,22]. the presented results are corrected for the ecsa. behavior of electrodeposited pd and pdni samples during the investigation of the eor was recorded in the solution containing 1 m c2h5oh + 1 m naoh by using cv and polarization measurements at 1000 rpm. for current density–time responses of eor the potential was stepped from 800 mv to -400 mv. results and discussion electrochemical characterization of the electrodeposited samples the cvs of the investigated binary coatings and pure pd coating in 1 m naoh are displayed in fig. 1. anodic linear sweep voltammetry (alsv) analysis and the energy dispersive x-ray spectroscopy (eds) were used for the determination of the alloy coatings composition giving following compositions: pd0.74ni0.26, pd0.50ni0.50 and pd0.28ni0.72 [29]. the cvs presented in fig. 1 clearly indicate that the current densities for hydrogen adsorption-absorption/desorption and oxide formation and reduction are higher for pdni samples than those recorded on the cv for pure pd coating. reduction of pd-oxide on all catalysts creates a well-defined peak, but at different potentials. on pdni coatings the peak maximum is shifted towards more negative potentials compared to pure pd coating, which indicates a stronger adsorption of oxide species on the surface of bimetallic coatings due to the presence of ni [8]. it was shown that the formation of a monolayer of the -ni(oh)2 occurred in the potential region between -1000 mv and -700 mv, while at potentials more positive than 200 mv further oxidation of ni(ii) species into niooh occurs [30]. j. electrochem. sci. eng. 8(1) (2018) 39-47 electrodeposited pd and pdni coatings 42 hence, the formation and the reduction of ni(oh)2, as well as partial formation of the niooh increase the anodic current density at potentials more positive than 100 mv as is presented in fig. 1. also, these processes occurred simultaneously with the formation and reduction of pd-oxide. figure 1. cvs recorded with the scan rate 50 mv s−1, 1000 rpm on pd and pdni coatings in 1 m naoh. the eor the activity of the pd0.74ni0.26, pd0.50ni0.50, pd0.28ni0.72 and pd coatings for the eor in alkaline medium was investigated by the cv (fig. 2). considering electrochemical behavior of pure pd during the eor it was suggests that the carbonaceous intermediates can be strongly adsorbed on the pd surface blocking the activity in the forward scan up to  -650 mv [31,32]. since the pd begins to adsorb oh species in the region of hydrogen adsorption [23], strongly adsorbed carbonaceous species can be oxidized, causing the increase of current density of the forward peak. surface oxide formation block further adsorption of reactive species leading to decrease of current density of the forward peak. in the backward scan, the reduction of surface oxides enables ethanol adsorption at the free pd surface, so that eor current densities in the backward peak ascend. the peak in the backward scan could be assigned to the elimination of carbonaceous species that are not completely oxidized in the forward scan [23]. among the cvs for the eor at pdni coating samples presented in fig. 2, the current density increased with the increase of pd content up to 74 at.%. also, it seems that the small amount of ni is sufficient to shift the reaction onset potential to more negative values since eor on pd0.74ni0.26 starts 50 mv earlier compared to the other binary coatings and 100 mv compared to the pd coating. it was demonstrated based on the ratio of the forward and the backward peak current density that the alloy surfaces were less poisoned than pure pd coating [29]. among them the pd0.74ni0.26 coating is the most poisoning tolerant. the role of ni in binary coatings can be rationalized in following way: since ni itself is not active for the eor at the potentials relevant for j. d. lović and v. d. jović j. electrochem. sci. eng. 8(1) (2017) 39-47 doi:10.5599/jese.445 43 the practical application [33] the presence of ni in the pdni alloy enhance the eor by increasing the presence of oh species at the electrode surface [6,8,17] leading to the shift of onset potential to more negative values and the increase of the current density, as observed in fig. 2. also, the presented results reveal better utilization of pd on the surface of pd0.74ni0.26 coating due to appropriate surface morphology since more active sites are accessible to the eor. it can be pointed out that improved catalytic activity of investigated binary coatings can be achieved through the optimization of the ni content and appropriate surface morphology. figure 2. cvs recorded with the scan rate 50 mv s−1, 1000 rpm on pd and pdni coatings in 1 m naoh + 1 m c2h5oh. considering the literature, it can be stated that the best ratio of pd and ni for the eor vary, depending on several factors as was explained in [29] whereby excess ni decrease the activity for eor due to the active surface blocking [6,35]. it can be found that carbon supported pdni catalyst with atomic ratio of 40:60 synthesized by the simultaneous reduction method using nabh4 as reductant exhibits 2 times higher activity and better stability than does the pd/c catalyst [34]. also the optimized carbon supported pdni catalyst synthesized through a modified solution phase-based nanocapsule method, with atomic ratio of 44:56 represents the promising anode catalyst for alkaline defcs giving 180 mv more negative eor onset potential and the 33 times higher exchange current density than pd/c [35]. graphene supported pdni catalyst prepared by chemical reduction method, with atomic ratio of 50:50 showed the lower onset potential on cv and better long-term stability on amperometric measurements of eor in a series of investigated binary alloyed nixpd100-x/g catalysts [6]. chronoamperometric technique is an effective method to evaluate the electrocatalytic activity and stability of catalyst materials. figure 3 shows the typical current density–time responses of three pdni coatings and pure pd coating for the eor at e = -400 mv. the oxidation current densities rapidly decrease in first 100 s, likely due to the formation of intermediates and poisoning species j. electrochem. sci. eng. 8(1) (2018) 39-47 electrodeposited pd and pdni coatings 44 during the eor. with the time, a pseudo-steady state is achieved. after a polarization of 1500 s, the investigated electrodes reached their steady-state current densities which are shown in the inset of fig. 3 with respect to pd content (at %). it can be seen that the current density of the eor on pd0.74ni0.26 electrode is higher than those on other electrodes, as found above in the cv measurements. figure 3. current density–time responses recorded on pd and pdni coatings in 1 m naoh + 1 m c2h5oh at e = 400 mv. 1000 rpm. inset: current densities after 1500 s vs. at % pd. upon the end of the current-time transient, the investigated catalysts were subjected to the potential cycling. figure 4 depicts the 1. and 10. cycles of pd0.74ni0.26 electrode as the representative ones. this pdni coating shows reduced current densities at the beginning of cycling after ca with further increase of the activity during the cycling. the ability to recover activity loss demonstrates surface composition stability of investigated binary coatings. hence, pdni coatings showed enhanced electrocatalytic activity towards eor which nominates this type of catalyst for possible practical application. for the purpose of comparing the kinetic and mass transport properties of the investigated electrodes during ethanol oxidation, tafel polarization analysis and the relation between the peak current density and the scan rate were provided. the tafel plots are showed in fig. 5. the slope of 140 mv dec-1 for pd0.74ni0.26 is obtained for the potentials up to –600 mv, while 160 mv dec-1 for pd0.50ni0.50, pd0.28ni0.72 and pd are tafel slops also obtained at the beginning of the peak in potential window up to  -550 mv. lower tafel slope indicate faster eor charge-transfer kinetics. the results of the tafel polarization study corroborate the findings of cv. nearly the same values of tafel plots were obtained on graphene supported nipd binary catalysts [6], binary composite films of pd and ni on multiwalled carbon nanotubes (mwcnt) [36], carbon supported pdni nanoparticles [35,37] and rather higher on pdni nanoparticles supported on sulfonated mwcnt [38]. figure 6 shows the cvs obtained at pd0.74ni0.26 catalyst in 1 m naoh +1.0 m c2h5oh solution at different scan rates. the relation between the peak current density obtained from forward cv scan and v0.5 of cv is shown in the inset. it can be seen that the oxidation potential and peak current density for ethanol oxidation become larger with the increasing of scan rate. j. d. lović and v. d. jović j. electrochem. sci. eng. 8(1) (2017) 39-47 doi:10.5599/jese.445 45 figure 4. cvs (1. and 10. cycle) recorded on pd0.74ni0.26 after ca in 1 m naoh + 1 m c2h5oh. scan rate 50 mv s−1, rpm = 1000. figure 5. tafel responses recorded on pd and pdni coatings in 1 m naoh + 1 m c2h5oh. scan rate 1 mv s−1, rpm = 1000. the peak current densities in the forward scan are linearly proportional to the square root of scan rates (fig. 6a). close inspection of fig. 6a depict nonzero intercept of the peak current at zero scan rate suggesting that it not is a pure diffusion controlled process. nonzero intercept could suggest the involvement of some type of surface interactions in examine reaction. additionally, the peak potential in the forward scan (epa), increase with the increase of v, and a linear dependency can be obtained between epa and ln v, as shown in fig. 6b. the same dependency was obtained on j. electrochem. sci. eng. 8(1) (2018) 39-47 electrodeposited pd and pdni coatings 46 pure pd coating and binary coatings with higher content of ni indicating that the oxidation of ethanol is an irreversible electrode process [6]. figure 6. cvs on pd0.74ni0.26 coating in 1 m koh+ 1 m c2h5oh at various scan rates. insets: plots current densities in the forward scan vs. v0.5 (a) and peak potentials in forward scan vs. ln of scan rates (b). conclusions in summary, the coating samples obtained by simultaneous electrodeposition were tested for the eor using cv, ca and quasi-steady state measurements and compare to pure pd coating. the most active one was found to be pd0.74ni0.26 exhibiting also negatively shifted onset potential for the eor. kinetic and mass transport properties reveal lower tafel slope obtained on pd0.74ni0.26 indicating faster eor charge-transfer kinetics while the eor is not a pure diffusion controlled process. furthermore, it was shown that eor is an irreversible electrode process on all examined coatings. it was pointing out that pd0.74ni0.26 coating is more efficient and the more poisoning tolerant than the other investigated coatings. also, the ability to recover activity loss confirms the surface composition stability of investigated binary coatings. acknowledgements: this work was financially supported by the ministry of education, 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[38] t. ramulifhoa, k. i. ozoemenaa, r. m. modibedia, c. j. jafta, m. k. mathe, electrochimica acta 59 (2012) 310-320. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {large scale model predictions on the effect of gdl thermal conductivity and porosity on pem fuel cell performance} doi:10.5599/jese.413 223 j. electrochem. sci. eng. 7(4) (2017) 223-235; doi: http://dx.doi.org/10.5599/jese.413 open access : : issn 1847-9286 www.jese-online.org original scientific paper large scale model predictions on the effect of gdl thermal conductivity and porosity on pem fuel cell performance obaid ur rehman1,, amber fishan zafar2 1chemical engineering department, university of karachi, karachi, pakistan 2automotive and marine engineering department, ned university of engineering & technology, karachi, pakistan corresponding authors e-mail: orehman@uok.edu.pk; tel. +92-321-2536798 received: july 25, 2017; revised: september 24, 2017; accepted: september 25, 2017 abstract the performance of proton exchange membrane (pem) fuel cell majorly relies on properties of gas diffusion layer (gdl) which supports heat and mass transfer across the membrane electrode assembly. a novel approach is adopted in this work to analyze the activity of gdl during fuel cell operation on a large-scale model. the model with mesh size of 1.3 million computational cells for 50 cm2 active area was simulated by parallel computing technique via computer cluster. grid independence study showed less than 5% deviation in criterion parameter as mesh size was increased to 1.8 million cells. good approximation was achieved as model was validated with the experimental data for pt loading of 1 mg cm-2. the results showed that gdl with higher thermal conductivity prevented pem from drying and led to improved protonic conduction. gdl with higher porosity enhanced the reaction but resulted in low output voltage which demonstrated the effect of contact resistance. in addition, reduced porosity under the rib regions was significant which resulted in lower gas diffusion and heat and water accumulation. keywords computational fluid dynamics; parallel computing; experimental validation introduction the impact of immense utilization of fossil fuels in the automotive and power generation sector has drawn enormous damage on environment. the development of renewable and clean energy resources has been a key topic of researchers in past few decades. among other energy conversion devices fuel cells have been widely recognized as a promising clean energy resource due to their high energy density and conversion efficiency [1,2]. pemfc has emerged as a substitute for internal combustion engines in automotive sector as it requires low operating temperatures and offers quick http://dx.doi.org/10.5599/jese.413 http://www.jese-online.org/ mailto:orehman@uok.edu.pk j. electrochem. sci. eng. 7(4) (2017) 223-235 thermal conductivity and porosity on pem fuel cell 224 startup. though yet the fuel cells have not been fully commercialized due to high manufacturing cost and lack of infrastructure for hydrogen storage and its utilization, but still it has a great potential to replace conventional energy conversion systems. the main components of pemfc are represented in figure 1. air is injected from cathode side of the cell while hydrogen gas from anode. at the surface of anode side catalyst layer (cl), hydrogen oxidation reaction (hor) occurs as shown in the reactions below. simultaneously, the oxygen reduction reaction (orr) occurs at the cathode side cl. the hydrogen ions (protons) from anode travel through the pem and reach the cathode catalyst where they react with oxygen to produce water. product heat is released as the reaction is exothermic. figure 1. schematic of pem fuel cell the performance of pemfc is highly dependent upon the thermal and transport characteristics of the porous cl and gdl [3]. gdls are employed for distribution of reactant gases to the reaction sites of the catalysts which increase the diffusion capacity and enhance reaction rate. they also provide the pathway for removal of water and heat from cl to gas flow channels which in turn limit the concentration overpotential. to achieve high current densities, the gdl must be porous and allow for the flow of both water and reactant gases. it must also be thermally and electrically conductive for the flow of product heat and electric current in both in-plane and through plane directions[4]. water droplets form at low operating temperature in fuel cell which block pores of gdl and reduce gas diffusivity and number of reaction sites at cl [5]. on the other hand, high water content (h2o/so-3 ratio or ) in pem promotes proton conductivity [6]. this trade-off between water content and reaction rate should be taken into consideration for an efficient operation of pemfc. favorable characteristics of gdl and cl are required for improved gas diffusivity and thermal management in the cell. various studies have been done experimentally to analyze the physical and thermal effects of gdls on pemfc performance [7-10]. when compared with the experimental procedures numerical simulation provides relatively agile methods to design and analyze complex systems and it also offers access to diversified results. currently various numerical models of pemfc are available in the published literature covering the o. rehman et al. j. electrochem. sci. eng. 7(4) (2017) 223-235 doi:10.5599/jese.413 225 transport phenomena and electrochemical kinetics [11-21]. to analyze the performance of pemfc at different operating conditions and design configurations various simulations have been produced but very few provide analysis of full gdl on large-scale model. zhang et al. [22] investigated the effect of porosity of cathode side gdl on catalyst potential distribution and pressure drop along flow channels. they noticed that at porosity of 0.6 the potential was maximal but as the value of porosity was increased potential dropped, which indicated that the contact resistance impeded the transfer of current through gdl. inamuddin et al. [23] studied a single channel, three dimensional model of pemfc to evaluate its performance at different gdl porosities and thickness. they noticed a gradual increase in current density with gdl porosity. however, performance at porosity greater than 0.7 was not estimated which may indicate its limiting value. khazaee et al. [24] developed a three dimensional model to investigate the effect gdl and membrane characteristics on performance of annular pemfc. their results suggested that high gdl porosity was not favorable at high current densities as it led to increased contact resistance. they also suggested high thermal conductivity of gdl to prevent pem from drying. alhazmi et al. [25] developed a 11-channel, three dimensional model to estimate the performance of pemfc at different in-plane and through plane gdl thermal conductivities. improved power density was observed for high gdl thermal conductivities which corresponded to low pem temperature. low temperature operations were favorable for low electrical and protonic resistance in gdl and pem respectively. fadzillah, nee and rosli [26] simulated a two dimensional model to investigate the distribution of oxygen on cathode side gdls with different porosities and thicknesses. it was observed that porosity of gdl played a key role to facilitate the reactant to reach more reaction sites which resulted in improved pemfc performance. maslan et al. [27] developed a three dimensional single channel model to predict the performance of pemfc with respect to gdl properties. effect of porosity and ptfe content of gdl was analyzed. their results showed that at low porosity the concentration overpotential dominated the pemfc performance because the water droplets were trapped in gdl pores which resulted in reduced reaction sites. in this paper a simulation work is presented for a full cell model using commercial code of ansys fluent® with parallel computing technique. effects of gdl thermal conductivity and porosity on pemfc performance were investigated. cell performance was analyzed with relation to water content and temperature across pem. oxygen concentration and reaction heat production at gdlcatalyst interface was also examined to observe the impact of gdl porosity. model description the governing transport phenomena and reaction kinetics of pemfc has been modelled in numerous works. some good reviews on the model of pemfc can be found in [28] and [29]. the geometrical and mathematical models are described in subsequent paragraphs. geometric model the full cell geometry with 50 cm2 of active area and 45-channel serpentine flow design was developed using gambit pre-processor. the model consists of seven layers as shown by the schematic in figure 1. the geometry was meshed by hexahedral scheme as shown in figure 2. the whole geometry comprised approximately 1.3 million computational cells. the photograph of the flow channels is presented in figure 3. the geometrical dimensions are presented in table 1. j. electrochem. sci. eng. 7(4) (2017) 223-235 thermal conductivity and porosity on pem fuel cell 226 figure 2. meshed geometry of pemfc figure 3. engraved serpentine gas flow channels in bipolar plate table 1. model physical properties dimension length, mm channel height 1 channel length 70 channel width 0.75 rib width 0.82 gdl thickness 0.19 catalyst layer thickness 0.01 membrane thickness 0.0508 current collector thickness 0.25 mathematical model governing equations a multiphase mathematical model was employed in present work. the main reaction at cathode side takes place at triple phase boundary. hydrogen ions formed at anode cl travel to cathode cl through pem where they are combined with oxygen gas to form water. conservation equations in a finite volume method the basic equation for a conservation of a general property φ over a control volume in a steady state problem is given as [30]. vsa)n(γau)n(ρ cv v aa ddd    (1) where n represents the vector normal to a differential surface da and γ is a diffusion coefficient. the left-hand side gives the convective flux and right-hand side gives the diffusive flux plus the generation or consumption of the property. the continuity equation for the model is given as     msuερ t ρε     (2) where ρ is the density, ε is the porosity, u  is the fluid velocity vector and sm denotes the mass production term. o. rehman et al. j. electrochem. sci. eng. 7(4) (2017) 223-235 doi:10.5599/jese.413 227 the momentum conservation equation is given as follows       usuεμpεuuερ t uρε      (3) where p represents the pressure, and su is the force per unit volume. for the species conservation following equation is employed       kk eff kk k scdcuερ t εc     (4) where ck is the concentration of species and eff kd represents the diffusivity, which is given for porous material by bruggeman equation as follows k 1.5 m eff k dεd  (5) the species production term sk is related to electrochemical reaction in the fuel cell which is given in the following form j kw, k r nf m s  (6) where k shows the specie, n is the electron transfer number, and subscript j used for anode or cathode. energy conservation equation is given as follows       h eff stkhuερ t ρεh     (7) keff is the effective thermal conductivity of porous material. the energy source (sh) is a sum of different source terms such as heat of reaction, ohmic loss and electric work and latent heat of evaporation for water, and can be given as lohm 2 catan,catan,reacth hriηr-h=s  (8) the equation for conservation of charge is given as   0e eff e  φsφσ (9) where eφ is the charge potential for membrane or solid phase, eff eσ is the ionic conductivity, φs is the source term which depends on exchange current density (a/cm2). the dependence of exchange current density on reactant concentration can be expressed by butler-volmer’s [13] equation. simplified form of which known as tafel equation is given below                                       rtfηα γ rtfηα γ φ eii eii s / ref2 2ref catcat / ref2 2ref anan catcat cat anan an o o h h (10,11) where iref is the reference exchange current density, γ is the concentration dependence factor, α is the transfer coefficient, f is faraday constant, and η is the overpotential. pem properties pem proton conductivity ( memσ ) is related to membrane water content () and temperature by following equation           teλσ 1 303 1 1268 mem 0.3260.514 (12) j. electrochem. sci. eng. 7(4) (2017) 223-235 thermal conductivity and porosity on pem fuel cell 228 the membrane water transport by osmotic drag is given by the following relation [11] )(2d drag iαjw  (13) the osmotic drag coefficient ( dα ) for proton conductivity of membrane also depends upon λ and is related by following equation 22 2.5 λ αd  (14) the back diffusion flux of water molecules through membrane can be related to λ as λdm m ρ jw  loh m mdiff 2 (15) where mρ and mm are density and equivalent weight of dry membrane. the diffusion coefficient, ld , is represented by the following equation 632 1 303 1 2416 l 10)0.0006710.02640.332.563          λλλ(ed t (16) water content of membrane was estimated using equation [11] as follows       1for1)1.4(14 1for3639.8517.180.043 32 aa aaaa λ (17) where a is water activity and is defined mathematically as: sat wv p p a  (18) the vapor pressure can be related to molar fraction and total pressure as follows: pp ohwv 2x (19) model parameters and boundary conditions the simulations were carried out at steady state and non-isothermal conditions. the reaction parameters were set at 1 atm and 353 k (80°c). output voltages were calculated at a fixed current density of 0.6 a/cm2. flow rate for hydrogen gas was set at 0.4 slpm (9.477×10-7 kg/s) and for air at 1.26 slpm (2.82×10-5 kg/s). both streams were entered with 100 % relative humidity. the model was calibrated in order to generate comparable solutions with the experimental results by tuning the reference exchange current density. the model parameters are presented in table 2. table 2. model parameters quantity value anode reference current density [31] 100 a m-2 anode reference molar concentration [31] 0.04 kmol m-3 anode concentration exponent [17] 0.5 anode exchange coefficient [32] 0.5 cathode reference exchange current density 0.00035 a m-2 cathode reference molar concentration [31] 0.04 kmol m-3 cathode concentration exponent [17] 1 cathode exchange coefficient 0.6 open circuit voltage 1.05 v hydrogen reference diffusivity [31] 1.1028×10-4 m2 s-1 oxygen reference diffusivity [31] 3.2348×10-5 m2 s-1 water reference diffusivity [31] 7.65×10-5 m2 /s s-1 nitrogen reference diffusivity [31] 3×10-5 m2 s-1 o. rehman et al. j. electrochem. sci. eng. 7(4) (2017) 223-235 doi:10.5599/jese.413 229 material properties in order to solely recognize the effect of thermal conductivity and porosity of gdl its other properties were taken identical in all solutions. isotropic model was adopted for specifying the values of viscous and electrical resistances. the properties of materials used in this model are given in table 3. table 3. material properties material property value nafion density 1968 kg m-3 specific heat capacity 4188 j kg-1 k-1 thermal conductivity, dry at 65°c 0.12 w m-1 k-1 equivalent weight 1100 kg kmol-1 toray carbon bulk density 440 kg m-3 specific heat capacity 685 j kg-1 k-1 electrical conductivity 1250 -1 m-1 viscous resistance 1.02×1011 m-2 platinum, pt thermal conductivity at 60°c 73 w m-1 k-1 carbon support (vulcan xc 72) bulk density 264 kg m-3 specific heat capacity 685 j kg-1 k-1 thermal conductivity 7.63 w m-1 k-1 electrical conductivity 400 -1m-1 graphite plate density 1990 kg m-3 specific heat capacity 710 j kg-1 k-1 thermal conductivity 117 w m-1 k-1 electrical conductivity 92600  -1m-1 solver specification a finite volume based fluent® solver was implemented to solve the governing equations. the large scale computational domain was handled by parallel processing technique. the urfs are employed to control the solution of highly coupled equations. to achieve convergence the urfs were tuned to an optimum value in order to lower the residuals for each variable without large oscillations. about 700 iterations were performed to achieve converged solutions. stopping criteria was set at a residual value of 1×10-6 for the equation of continuity and 1×10-5 for other variables (i.e. potential fields, water content, species outlet mole fractions etc.) which took more time to converge than the scaled residuals. results and discussion grid independence study to make sure that the large-scale model is independent of the meshing criteria the model was meshed with three different sizes as sown in table 4. the current densities were calculated for each mesh size at a fixed potential of 0.67 v. about 8.5 % deviation in the criterion parameter was found when mesh size was increased from 1 to 1.3 million cells. the deviation reduced to about 4.4% when mesh size was further increased to 1.8 million cells. the study showed that the deviation diminished as the mesh became finer. due to high computational load the grid size with 1.3 million computational cells was selected for all the calculations. j. electrochem. sci. eng. 7(4) (2017) 223-235 thermal conductivity and porosity on pem fuel cell 230 table 4. average current densities for different mesh sizes at 0.67 v mesh cells average cd, a cm-2 fine 1858725 0.425 medium 1352475 0.408 coarse 1099350 0.375 experimental validation the simulation results were validated with experimental data by comparing the polarization curves, as shown in figure 4. in-house experiments were carried out to generate a polarization curve. a standard test pem fuel cell with 50 cm2 active area, developed by electro chem inc. was used for experiments at 50°c temperature and atmospheric pressure. toray carbon (tp 60) material was used as gdl and the catalyst layer was made with pt nanoparticles on carbon substrate (0.2 mg pt/mg vulcan xc 72) with loading of 1 mg cm-2. the measurement of current density was carried out by galvanostatic control. fuel cell testing system (fcts) was employed for data acquisition. constant gas flow rates were chosen throughout the experiments to achieve a minimum stoichiometry of 1.5 and 2 for hydrogen and oxygen at 0.8 a cm-2. figure 4. polarization and power density curves at 50 °c and 1 atm a good agreement was achieved as the numerical results followed the experimental curve as shown in figure 4. however, at high current densities above 0.4 a cm-2 the curve for numerical solution deviated due to inadequacy of the model to reproduce the actual behavior of pemfc. at high current density, the concentration overpotential dominate the cell performance as water droplets blocks the diffusion pathways for reactants to reach reaction sites as depicted by the experimental curve. however, the discrepancies at high current density could be minimized by decreasing the urfs for saturation source term and membrane water content but the number of iterations would be very high and would result in stalling of convergence. o. rehman et al. j. electrochem. sci. eng. 7(4) (2017) 223-235 doi:10.5599/jese.413 231 effect of gdl thermal conductivity commercially available data [32] for four types of carbon fiber paper used as gdl was incorporated namely toray carbon, e-tek, spectracarb and sigarcet. the effect of thermal conductivity of gdl on the water content and temperature of pem is illustrated by contours shown in figure 5 and 6 which represent the iso-surface at the middle of pem. figure 5 shows the overall decrease in water content () of pem with thermal conductivity of gdl. the gradual increase of water content (figure 5) along the channel clearly shows the saturation of the reactant gas with product water generated by orr which is removed through gdl. pem drying is evident in figure 5 as gdl thermal conductivity is decreased from 1.7 to 0.16 w / m k. the drying phenomenon is related to high temperature operation as shown in figure 6. the effect of gdl thermal conductivity on pem temperature is distinctively revealed in figure 6. pem temperature increased as gdl thermal conductivity is reduced from 1.7 to 0.16 w / m k. moreover, a low temperature profile is eminent in both figures making an interdigitated pattern in the regions under the rib where water accumulates due to low gas flow. the temperature of membrane plays a significant role in the pemfc performance. high temperature causes drying and consequently lowers the proton conductivity ( memσ ). moreover, enhanced proton conductivity of pem effectively reduced the ohmic overpotential which resulted in high output voltages as depicted in figurer 7 which shows the calculated output voltages for each gdl at fixed current density of 0.6 a cm-2. figure 5. contours of pem water content () at gdl thermal conductivity of (a) 1.7 w/m k; (b) 1.4 w / m k; (c) 0.6 w / m k; (d) 0.16 w / m k h20/so-3 x / mm x / mm x / mm x / mm x / mm x / mm j. electrochem. sci. eng. 7(4) (2017) 223-235 thermal conductivity and porosity on pem fuel cell 232 figure 6. temperature, k contours of pem at gdl thermal conductivity of (a) 1.7 w / m k; (b) 1.4 w / m k; (c) 0.6 w / m k; (d) 0.16 w / m k figure 7. output voltages at different gdl thermal conductivities figure 8. output voltages at different gdl porosities effect of gdl porosity among other functions of gdl the distribution of reactant gases to the active surface area of cl is very critical. the diffusivity of gases highly depends upon the porosity of material as shown in equation 5. a highly porous gdl will lead to better transport of reactant gases. on the other hand, high porosity promotes contact resistance between gdl and bipolar plate which reduces electrical x / mm x / mm x / mm x / mm o. rehman et al. j. electrochem. sci. eng. 7(4) (2017) 223-235 doi:10.5599/jese.413 233 conductivity [33]. therefore, an optimum porosity is desired for efficient process. figure 8 shows the output voltages that are generated by the simulations at different gdl porosities obtained from commercially available data. the results depict that the output voltage was increased as the porosity of gdl was lowered from 0.88 to 0.63 due to improved electrical conductivity which indicate reduced contact resistance. the effect of gdl porosity on reaction rate at gdl-catalyst interface on cathode side of pemfc is shown in figures 9 and 10. the gdl was treated as a single domain with uniform porosity. a subtle change is noticeable in both contours of figure 9 which represents mole fraction of oxygen that increased from 0.1 to 0.22 along the channel. the results showed that oxygen was diffused at a higher rate in gdl with 0.88 porosity (figure 9(a)) than with 0.63 porosity (figure 9(b)). furthermore, pathways of gas to cl are obstructed in the regions under the rib which exhibit the accumulation of water. figure 9. oxygen mole fraction at cathode side gdl-catalyst interface at gdl porosity of (a) 0.88; (b) 0.63 the heat generated in the orr is illustrated in figure 10. a considerable change in both diagrams 10(a) and 10(b) is noticed which signifies the dependence of orr rate on gdl porosity. product heat was increased along the channel corresponding to the same phenomenon represented in figure 9. heat accumulated under rib while in the channel it was swept away by the gas stream. this suggests that an optimum porosity of gdl is highly decisive in efficient performance of pemfc. x / mm x / mm j. electrochem. sci. eng. 7(4) (2017) 223-235 thermal conductivity and porosity on pem fuel cell 234 figure 10. reaction heat source (w/cm3) at gdl-catalyst interface at gdl porosity of (a) 0.88; (b) 0.63 conclusions this study provides a numerical investigation of the effect of thermal and transport properties of gdl on the performance of pemfc. the mathematical model was validated with experimental results by the comparison of the polarization and power density curves. contours of different isosurfaces were presented to show the impact of thermal conductivity and porosity of gdl on the electrochemical behavior of pemfc. the results provide a substantial basis in understanding the actual phenomenon occurred inside the complex system of fuel cell. it is reported that higher thermal conductivity of gdl led to improved proton conductivity of pem by maintaining low temperatures. it is also found that higher porosity of gdl promoted the reactant gas transport to cl but at the same time raised contact resistance which resulted in lower electrical conductivity. the compressive force in the cell also affected the performance by impeding the gas flow through porous gdl. these results suggest that the optimum gdl characteristics and compressive force is required for maximum efficiency of pemfc. further study can be done by incorporating other gdl properties like gas permeability, thickness, electrical conductivity and interface resistance between the layers adjacent to gdl. x / mm x / mm o. rehman et al. j. electrochem. sci. eng. 7(4) (2017) 223-235 doi:10.5599/jese.413 235 references [1] m. f. b. sunden, transport phenomena in fuel cells (developments in heat transfer). wit press, southampton, uk, 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[33] y.-x. huang, c.-h. cheng, x.-d. wang, j.-y. jang, energy 35(12) (2010) 4786–4794. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) duplex electroless ni-p/ni-cu-p coatings: preparation, evaluation of microhardness, friction, wear, and corrosion performance http://dx.doi.org/10.5599/jese.1359 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1359 open access : : issn 1847-9286 www.jese-online.org original scientific paper novel electrochemical sensing platform for detection of hydrazine based on modified screen-printed graphite electrode farideh mousazadeh1, sayed zia mohammadi2,, maryam mohammadhasani-pour2 1school of medicine, bam university of medical sciences, bam, iran 2department of chemistry, payame nour university, tehran, iran corresponding authorс: szmohammadi@yahoo.com received: april 26, 2022; accepted: july 29, 2022; published: october 25, 2022 abstract the current work aimed to fabricate a screen-printed graphite electrode (spge) modified by mno2 nanorods (mno2 nrs) for sensing hydrazine. thus, a facile protocol was adopted to construct the mno2 nanorods that were subsequently applied to modify the spge surface directly. as-synthesized mno2 nrs/spge sensor exhibited a strong sensing behavior towards the hydrazine, with a large peak current and small oxidation potential. this electrochemical sensor in the optimized conditions to detect the hydrazine possessed a low detection limit (0.02 μm), a broad linear dynamic range (0.05–275.0 μm) and an admirable sensitivity (0.0625 μa μm-1). the sensor applicability was practically estimated in real water samples, which revealed successful recovery values. keywords mno2 nanorods; electrochemical sensor; hydrazine diffusion coefficient introduction hydrazine, a vital chemical reagent, has attracted widespread research interest due to its industrial application and poisonousness. it is a robust reducing agent with various applications in the production of pesticides and medicine [1-3]. it is also a chemical deoxidizer with broad applications as an oxygen scavenger in water boilers [4]. hydrazine is a key raw material in the construction of rockets and explosives [5,6]. although hydrazine plays a great role in human production and life, it is easy to be absorbed by living organisms. the persistent contact with this agent may be associated with complications, such as some disturbances in the reproductive system, central nervous system, liver, kidneys and lungs [7-9]. according to the u.s. environmental protection agency (us epa), hydrazine is positioned in a class of possibly carcinogenic to humans, with a recommended threshold limit value (tlv) of less than 10 ppb [10]. accordingly, it is substantial to achieve a practical and efficient sensing protocol for the determination of this agent in various media. in this regard, different analytical and instrumental http://dx.doi.org/10.5599/jese.1359 http://dx.doi.org/10.5599/jese.1359 http://www.jese-online.org/ mailto:szmohammadi@yahoo.com j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensing platform for detection of hydrazine 2 protocols have been designed to detect hydrazine so far, such as gas chromatography, capillary electrophoresis, chemiluminescence, high-performance liquid chromatography, liquid chromatography, colorimetry, and spectrophotometry [11-17]. despite the unique benefits of each of these techniques, they typically require precision instrumentation and length of time. among these, electrochemical approaches have been concerned by many researchers owing to their peculiar traits, which are simplicity, rapidity, affordability and short response time [18-34]. voltammetric measurements typically use economical manners to detect analytes by recording variations in response current over alteration in electrode potential [35-42]. the recent development of electrochemical approaches has been greatly simplified by screenprinted electrodes (spes) due to their high sensitivity, functionality and versatility [43-45]. the main performance features of a sensor are all gathered in spes, including cost-effectiveness, minimal sample preparation, ease of operation, high speed, small size, limited background, and comfortable surface modification. there is always a need for further research to develop electrode materials to improve the selectivity and sensitivity of electrochemical sensors [46-51]. many advances in nanotechnology have been made in diverse fields in recent years [52-75], which have led to the introduction of highly efficient sensing platforms. types of nano-scale materials have so far been identified with distinct physicochemical properties that can be employed in the electrochemical sensors to detect various analytes exhibiting admirable results [76-82]. a popular oxide material is manganese dioxide (mno2), whose behavior can be enhanced by changing its morphology and surface area. mno2 is a polymorph owing to an octahedral [mno6] spatial arrangement. nano-sized mno2 exhibits commendable benefits due to a larger surface-tovolume ratio and further reactive surface for electrochemical reactions. the diverse application of this substance in electrochemistry and sensor fabrication can be attributed to the simple reduction of mno2 to mn2o3 and mno and, at the proper potential, the re-oxidation to mno2 as a catalytic circle for electrochemical detection [83-87]. the current work aimed to fabricate a new screen-printed graphite electrode (spge) supported by mno2 nanorods (mno2 nrs/spge) for sensitively sensing hydrazine. the sensor applicability was tested in real water samples, the results of which revealed successful recovery values. experimental chemicals and instrumentations all materials with analytical grades applied throughout this work were supplied from aldrich and merck. electrochemical experiments were recorded using a pgstat-302n autolab potentiostat/galvanostat (eco chemie, the netherlands). the control of all experiments was carried out by a general purpose electrochemical system (gpes) software. the spges were purchased from dropsens (spain) and consisted of an ag pseudo-reference electrode, graphite axillary electrode, and graphite working electrode. all ph values were measured by a digital metrohm 710 ph meter. synthesis of mno2 nanorods the mno2 nrs were obtained by dissolving kmno4 (0.316 g) in deionized water (30 ml) while vigorously stirring, followed by the addition of 3 m hcl (1.4 ml) under vigorous stirring for another half hour. then, the solution was placed in a 50-ml teflon-lined autoclave at 160 °c for six hours. next, the products were cooled down to room temperature and subsequently centrifuged and thoroughly rinsed with ethanol and deionized water to clean any impurities, followed by drying at 60 °c for 12 h. f. mousazadeh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1359 3 preparation of mno2 nrs/spge first, 1 mg of prepared mno2 nanorods was added into an aqueous solution (1 ml), followed by sonication for 30 min to give a homogeneous solution. then, 4 μl of mno2 nrs was dispersed on the surface of spge dropwise. following the solvent's evaporation, the sensor's surface was washed several times with deionized water to clean free modifier molecules and subsequently air-dried. the obtained electrode was noted as mno2 nrs/spge. results and discussion characterization of mno2 nanorods figure 1 illustrates the fe-sem images captured for the as-fabricated mno2 nrs, and observing them confirmed rod-shaped mno2 nanorods with a thickness ranging from 15 to 25 nm and a length of about 3 µm. the mno2 nrs showed an almost uniform size distribution. figure 1. the fesem images of synthesized mno2 nanorods electrochemical response of hydrazine at different electrodes the differential pulse voltammetry (dpv) method was recruited to study the effect ph value of electrolyte solution in different ph values (2.0-9.0) in the presence of 40.0 μm of hydrazine in phosphate buffer solution (0.1 m pbs) on the mno2 nrs/spge surface. the peak current of hydrazine oxidation was maximum at the ph value of 7.0, thereby selecting this value as the optimum ph in the hydrazine detection. figure 2 shows the application of the cyclic voltammetry (cv) method to evaluate the electrochemical behavior of 200.0 μm hydrazine at different electrodes (unmodified spge, and mno2 nrs/spge) in pbs (0.1 m, ph 7.0) at the scan rate of 50 mv/s. based on the results, there was an oxidation peak on the surfaces of the electrodes, but no reduction peak, highlighting an irreversible electrochemical response of hydrazine on the electrodes. a relatively wide and weak peak current (ipa) of hydrazine oxidation was found on the unmodified spge (at 1000 mv with 3.0 μa), which reveals that the electrochemical oxidation does not happen spontaneously due to high activation overpotential. the hydrazine ipa on mno2 nrs/spge, when compared with unmodified spge, displayed further elevation to 13.0 μa, meaning an increase up to 4.3 times that on the unmodified spge. in addition, hydrazine oxidation occurred at a lower potential than unmodified spge. http://dx.doi.org/10.5599/jese.1359 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensing platform for detection of hydrazine 4 e / v figure 2. cv curves of unmodified spge (curve a), and mno2 nrs/spge (curve b) in 0.1 m pbs containing 200.0 μm hydrazine; scan rate: 50 mv s-1 effect of the scan rate (ʋ) on the results the influence of various scan rates between 10 and 400 mv/s on the anodic peak currents for hydrazine (100.0 μm) was studied using the mno2 nrs/spge (figure 3). the regression equation was ipa (hydrazine) = 1.7036 ½ 4.2319 (r2=0.9994) (figure 3, inset). this result indicates that the oxidation process is controlled by diffusion. further, there was a shift in the oxidation peak potential of hydrazine toward a more positive potential by increasing the scan rates. e / mv figure 3. cv curves of 100.0 μm hydrazine in 0.1 m pbs (ph 7.0) at a scan rate of 10 to 400 mv s-1 at mno2 nrs/spge (a-h refers to 10, 25, 50, 75, 100, 200.0, 300.0 and 400.0 mv s-1) inset: plot of the square root of the scan rate vs. the oxidation peak current of hydrazine i /  a i /  a i /  a 1/2 / mv1/2 s-1/2 y = 1.7036x – 4.2319 r2 = 0.9994 f. mousazadeh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1359 5 to study the rate-determining step as shown in figure 4, the data of the rising part of the currentvoltage curve obtained at 10 mv/s scan rate were applied to draw a tafel plot for 100.0 μm of hydrazine. the linearity of the e versus log i plot, implies the intervention of the kinetics of the electrode process. the slope of this plot was utilized to estimate the number of electrons transferred in the rate-determining step. figure 4 shows the tafel slope of 0.2153 v for the linear section of the plot, which means the rate-limiting step of one-electron transfer with a transfer coefficient of α = 0.72. e / mv figure 4. cv response for 100.0 μm hydrazine with 10 mvs-1 scan rate and the inset is the tafel plot derived from the rising part or the corresponding voltammogram chronoamperometric analysis chronoamperometric determinations for different concentrations of hydrazine on the surface of mno2 nrs/spge were measured by adjusting the potential of the working electrode at 0.94 v in pbs (0.1 m, ph 7.0), see figure 5. for an electroactive material (hydrazine in this case) with a diffusion coefficient of d, the current observed for the electrochemical reaction at the mass transport limited condition is described by the cottrell equation [88]. as shown in figure 5a, i versus t−1/2 plots were used with the optimal fit for various hydrazine concentrations. we drew the slopes from straight lines against different concentrations of hydrazine, see figure 5b. according to the cottrell equation and obtained slope, the mean d value was 2.7×10−5 cm2/s. y = 0.2153x + 0.6494 r2 = 0.9991 i /  a i / a e / v http://dx.doi.org/10.5599/jese.1359 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensing platform for detection of hydrazine 6 t / s figure 5. the chronoamperograms obtained at mno2 nrs/spge in 0.1 m pbs at ph of 7.0 for different concentrations of hydrazine. noted that a–d related to 0.1, 0.5, 1.3, and 2.0 mm of hydrazine inset a: the i plot versus t-1/2 observed by chronoamperograms a to d. inset b: slope plot of the straight line vs. concentration of hydrazine calibration curve, linear range and detection limit mno2 nrs/spge sensor was used to electrochemically detect different hydrazine concentrations (figure 6). a gradual elevation was observed for the peak currents of hydrazine oxidation by raising its concentrations, which means an advanced performance of our sensor in the electrocatalytic oxidation of hydrazine. oxidation peak currents of hydrazine versus chydrazine (figure 6, inset) showed a wide linear range from 0.05 to 275.0 μm. the detection limit (lod=3σ/s; where σ is the standard deviation of blank response, and s is the slope of the calibration curve with a linear range of concentrations of the analyte) was calculated to be 0.02 μm. interference study the effect of some interference species on the determination of hydrazine was studied. the results show that the interfering effects of glucose, sucrose, urea, uric acid, na+, cl-, no3-, pb2+, and ag+ on the anodic peak current of hydrazine is less than 5%. hence, the mno2 nrs/spge has a superior selectivity for hydrazine. i /  a i /  a y = 17.693x + 7.7016 r2 = 0.9991 t-½ / s--½ chydrazine / mm f. mousazadeh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1359 7 e / v figure 6. dpv responses of hydrazine on mno2 nrs/spge at different hydrazine concentrations (a-m refers to: 0.05, 3.0, 10.0, 20.0, 30.0, 60.0, 100.0, 130.0, 160.0, 200.0, 225.0, 250.0 and 275.0 μm) in 0.1 m pbs (ph 7.0). inset: the relationship between the oxidation peak currents and [hydrazine] analytical application the detection of hydrazine in the water samples (drinking water and tap water) was performed using mno2 nrs/spge sensor. the concentration values of hydrazine were calculated via the method of standard addition. attained findings are summarized in table 1, the recovery is between 96.7 and 102.5 %, and the relative standard deviations are all less than or equal to 3.0%. the experimental results confirmed that the mno2 nrs/spge sensor has great potential for analytical application. table 1. determining hydrazine in water samples through mno2 nrs/spge. (n=3). sample c / μm recovery, % rsd, % spiked found drinking water 0 4.0 4.1 102.5 3.2 6.0 5.8 96.7 2.4 tap water 0 5.0 4.9 98.0 1.9 7.0 7.1 101.4 3.0 chydrazine / mm i /  a y = 0.0625x + 1.2841 r2 = 0.999 i /  a http://dx.doi.org/10.5599/jese.1359 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensing platform for detection of hydrazine 8 conclusion the present work utilized an ultra-facile protocol to construct mno2 nanorods-modified spge (mno2 nrs/spge) for the electrochemical determination of hydrazine. according to cv findings, the as-fabricated sensor exhibited an electrocatalytic performance compared with the unmodified spge for the oxidation of hydrazine. the linear current response to the hydrazine level was between 0.05 and 275.0 μm, and the limit of detection was 0.02 μm with a sensitivity of 0.0625 μa μm-1. the diffusion coefficient for hydrazine using mno2 nrs/spge, 2.7×10−5 cm2 s-1, was obtained. the developed sensor applicability was practically tested 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https://doi.org/10.1007/s40097-020-00362-w https://dx.doi.org/10.22034/ajgc/2020.2.10 https://doi.org/10.1016/j.electacta.2017.01.007 https://doi.org/10.1016/j.snb.2013.05.076 https://doi.org/10.1016/j.electacta.2013.10.116 https://doi.org/10.1016/j.electacta.2012.11.071 https://creativecommons.org/licenses/by/4.0/) a sensitive sensor based on graphitic carbon nitride/1-butyl-3-methylimidazolium hexafluorophosphate ionic liquids modified carbon paste electrode for sensing of carmoisine dye in food samples: https://dx.doi.org/10.5599/jese.1396 1143 j. electrochem. sci. eng. 12(6) (2022) 1143-1152; https://dx.doi.org/10.5599/jese.1396 open access : : issn 1847-9286 www.jese-online.org original scientific paper a sensitive sensor based on graphitic carbon nitride/1-butyl-3-methylimidazolium hexafluorophosphate ionic liquids modified carbon paste electrode for sensing of carmoisine dye in food samples sayed zia mohammadi1 and farideh mousazadeh2, 1department of chemistry, payame nour university, tehran, iran 2school of medicine, bam university of medical sciences, bam, iran corresponding author: faridehmousazadeh1398@gmail.com received: june 4, 2022; accepted: june 20, 2022; published: august 26, 2022 abstract the electrochemical sensors for carmoisine detection were explored and applied in the field of food safety because of the striking merits of fast detection speed, high sensitivity and easy operation. herein, we developed a high-performance electrochemical carmoisine sensor based on g-c3n4 and ionic liquids (ils), prepared by a one-pot hydrothermal synthesis method. most importantly, the sensing performances of electrochemical carmoisine sensors based on g-c3n4 (graphitic carbon nitride)/il (1-butyl-3-methylimidazolium hexafluorophosphate) are better than the bare carbon paste electrode (cpe) sensors. the detection limit of this sensor (g-c3n4/ils/cpe) determined via differential pulse voltammetry was 0.1 µm over a wide linear range of 0.4–125.0 µm. keywords electrochemical sensors; differential pulse voltammetry; food analysis; chemically modified electrodes introduction color additives were used by the ancient egyptians. historians estimated that colored food appeared by 1500 b.c. now, food manufacturers deem color as a vital criterion for food choice; thus, synthetic colorants are now being widely used due to their coloring properties, uniformity, stability, low cost, and availability in many hues, which overall increase the esthetic quality (mpountoukas below the acceptable daily intake (adi) limit, the intake of artificial colorants seems to be safe. however, consumption of high concentrations of these artificial colorants may result in many ailments, especially in children, due to their low body weights. children are the major consumers of colored food in developing countries, and thus, they are at high risk because they often exceed prescribed adis [1,2]. carmoisine dye is a synthetic azo food dye broadly used in food industries to get on red color products. because this dye is not carcinogenic and toxic, it is utilized as a coloring agent in https://dx.doi.org/10.5599/jese.1396 https://dx.doi.org/10.5599/jese.1396 http://www.jese-online.org/ mailto:faridehmousazadeh1398@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1143-1152 sensor based on g-c3n4/ionic liquids modified cpe 1144 confectionery, ice cream, gelatin desserts drinks, medicine and cosmetics. however, at high doses of carmoisine, the vital organs in the human body, such as the liver and kidneys, can be damaged and the gens can be changed from the mediated reaction of this dye with dna. in europe, carmoisine is authorized for use in certain foods and beverages, such as cheeses, dried fruit, and some alcoholic beverages, and is permitted for use as an excipient in medications. therefore, the determination of this type of food additive is very important in food products [3-5]. until now, different methods such as chromatography, spectroscopy and liquid-liquid microextraction have been reported for the determination of carmoisine dye [6-10]. in addition, voltammetry was also used to determine carmoisine dye based on its electrochemical activity [11,12]. electrochemical sensors showed many advantages for the analysis of food compounds due to their fast response. high oxidation/reduction over-potential and low sensitivity of electroactive compounds at the surface of unmodified electrodes are major problems for trace level analysis [1320]. therefore, modified electrodes were suggested as highly powerful tools for trace-level analysis of electroactive compounds. according to the previously published papers, room temperature ionic liquids and nanomaterials can improve the sensitivity of electrochemical sensors for the analysis of electroactive materials [21-35]. room temperature ionic liquids (rtils) are an exciting class of solvent, receiving much attention in recent years as a replacement for conventional solvents in many applications. as their name suggests, ionic liquids are salts that melt below 100 °c and room temperature ionic liquids exist in the liquid state at room temperature (25 °c). they are typically comprised of a bulky organic cation (e.g., imidazolium, tetraalkylammonium or pyrrolidinium) paired with an inorganic/organic anion (e.g., tetrafluoroborate, hexafluorophosphate, bis(triflurormethylsulfonyl)imide). rtils possess several archetypal properties such as low volatility, high physical and chemical stability, wide electrochemical windows, intrinsic conductivity, wide liquid range, high polarity and the ability to dissolve a wide range of compounds. in addition to the use of rtils in applications such as catalysis, ‘green’ chemistry, organic reactivity, analytical methods, biocatalysis and enzymes and applications in the chemical industry, they have also become recognized as ideal alternative electrolytes for use in many electrochemical devices, such as actuators, capacitors, batteries, fuel cells, solar cells and sensors [36-38]. nanomaterials have become the most popular material due to their unique properties of low cost, large surface area, good biocompatibility and distinct catalytic activity [39]. many researchers have been studying nanomaterials for electrochemical sensors [40-47]. intensive research on g-c3n4 has been conducted since its appearance. with amazing properties of high chemical and thermal stability, medium energy gap (2.7 ev), ideal electronic structure, non-toxic, low density and high conductivity, gc3n4 has become one of the hottest materials for modifying electrochemical sensors [48-50]. therefore, in the present study, we fabricated a highly sensitive voltammetric sensor based on a carbon paste electrode modified with g-c3n4 and ionic liquids for the analysis of carmoisine in food samples. the fabricated sensor showed good electrical conductivity compared to the unmodified electrode and improved the quality of the carbon paste electrode for analysis of carmoisine in food samples. experimental apparatus and chemicals all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab consisting of a traditional three-electrode system: a bare or modified cpe as the s. z. mohammadi and f. mousazadeh j. electrochem. sci. eng. 12(6) (2022) 1143-1152 http://dx.doi.org/10.5599/jese.1396 1145 working electrode, an ag/agcl as the reference electrode and a pt wire as a counter electrode. solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). carmoisine and other chemicals used were analytical grade and were purchased from merck. ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate) was purchased from sigma aldrich co. graphitic carbon nitride nanostructures were synthesized in our laboratory and a typical sem is shown in figure 1. figure 1. fe-sem image of g-c3n4 preparation of g-c3n4/ils/cpe g-c3n4/ils/cpe was prepared by mixing 0.04 g of g-c3n4 with 0.96 g graphite powder and approximately 0.8 ml of ionic liquids with a mortar and pestle. the paste was then packed into the end of a glass tube (3.4 mm internal diameter and 15 cm long). a copper wire inserted into the carbon paste provided the electrical contact. for comparison, ionic liquids/carbon paste electrode (il/cpe) in the absence of g-c3n4, (g-c3n4/cpe) consistent with g-c3n4 powder and paraffin oil, and bare carbon paste electrode consisting of graphite powder and paraffin oil were also prepared in the same way. the surface area of g-c3n4/ils/cpe and the bare cpe were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula for g-c3n4/ils/cpe, the electrode surface was found 0.288 cm2 which was about 3.2 times greater than bare cpe. results and discussion electrochemical behavior of carmoisine at the surface of various electrodes the effect of the electrolyte ph on the oxidation of 35.0 μm carmoisine was investigated at g-c3n4/ils/cpe using differential pulse voltammetry (dpv) measurements in the phosphatebuffered solution (pbs) in the ph range from 2.0 to 9.0. according to the results, the oxidation peak current of carmoisine depends on the ph value and increases with increasing ph until it reaches the maximum at ph 7.0, and then decreases with higher ph values. the optimized ph corresponding to http://dx.doi.org/10.5599/jese.1396 j. electrochem. sci. eng. 12(6) (2022) 1143-1152 sensor based on g-c3n4/ionic liquids modified cpe 1146 the higher peak current was 7.0, indicating that protons are involved in the reaction of carmoisine oxidation. the electrochemical behavior of carmoisine was investigated by differential pulse voltammetry. the differential pulse voltammograms obtained using the bare cpe (curve d), ils/cpe (curve c), g-c3n4/cpe (curve b) and g-c3n4/ils/cpe (curve a) in 0.1 m pbs (ph 7.0) in the presence of 60.0 μm carmoisine are shown in figure 2. on a bare cpe, an irreversible signal with a low oxidation current of 2.5 μa was obtained with a peak potential of 740 mv. after modifying the electrode with ils/cpe, and g-c3n4/cpe, the peak current increased up to 6.06 and 8.93 μa, respectively. in contrast, g-c3n4/ils/cpe exhibited an enhanced sharp anodic peak current (ipa = 12.0 μa) at a much lower overpotential ep = 450 mv. these results confirmed that the g-c3n4/ils improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of carmoisine. e / mv vs. ag/agcl/kcl figure 2. differential pulse voltammograms of (a) g-c3n4/ils/cpe, (b) g-c3n4 /cpe, (c) ils/cpe and (d) bare cpe in 0.1 m pbs (ph 7.0) in the presence of 60.0 μm carmoisine at the scan rate 50 mv s-1 effect of scan rate on the determination of carmoisine at g-c3n4/ils/cpe the influence of the scan rate () on the peak currents (ipa) of carmoisine at g-c3n4/ils/cpe was investigated by linear sweep voltammetry (lsv). figure 3 shows the voltammetric response of 35.0 μm carmoisine at g-c3n4/ils/cpe at different scan rates in the range of 5 to 400 mv s-1. the oxidation peak current of carmoisine increases linearly with increasing scan rate. a linear regression equation was obtained from the plot ipa and vs. 1/2 (square root of scan rate) as follows; ipa = 1.5769 ½ – 1.4736 (r2 = 0.9995) for the oxidation process, which indicates that the reaction of carmoisine at gc3n4/ils/cpe is diffusion controlled. s. z. mohammadi and f. mousazadeh j. electrochem. sci. eng. 12(6) (2022) 1143-1152 http://dx.doi.org/10.5599/jese.1396 1147 e / mv vs. ag/agcl/kcl figure 3. linear sweep voltammograms of g-c3n4/ils/cpe in 0.1 m pbs (ph 7.0) containing 35.0 μm carmoisine at various scan rates; 1-8 correspond to 5, 15, 45, 75, 100, 200, 300 and 400 mv s-1, respectively. inset: variation of anodic peak current vs. ν1/2 in addition, the plot of log i vs. e (tafel plot) is linear, having the following regression equation: log i = 0.1743e + 0.3498 (r2=0.9945) (figure 4). assuming that nα=1, the value of α was calculated as 0.71. e / mv vs. ag/agcl/kcl figure 4. tafel plot for g-c3n4/ils/cpe in 0.1 m pbs (ph 7.0) in the presence of 35.0 µm carmoisine and scan rate 10 mv s−1 http://dx.doi.org/10.5599/jese.1396 j. electrochem. sci. eng. 12(6) (2022) 1143-1152 sensor based on g-c3n4/ionic liquids modified cpe 1148 chronoamperometric analysis the analysis of chronoamperometry for carmoisine samples was performed using g-c3n4/ils/ /cpe vs. ag/agcl/kcl (3.0 m) at 0.5 v. the chronoamperometric results of different concentrations of carmoisine in pbs (ph 7.0) are demonstrated in figure 5. figure 5. chronoamperograms obtained at g-c3n4/ils/cpe in 0.1 m pbs (ph 7.0) for different concentration of carmoisine. the 1-4 correspond to 0.1, 0.3, 0.6 and 1.0 mm of carmoisine. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms 1-4. (b) a plot of the slope of the straight lines against carmoisine concentration the cottrell equation for chronoamperemetric analysis of electroactive moieties under mass transfer limited conditions is as in equation (1): i = nfad1/2cbπ-1/2t-1/2 (1) where d represents the diffusion coefficient (cm2 s-1), and cb is the applied bulk concentration (mol cm-3). experimental results of i vs. t-1/2 were plotted in figure 5a, with the best fits for different concentrations of carmoisine. the resulting slopes corresponding to straight lines in figure 5a, were then plotted against the concentration of carmoisine (figure 5b). the mean value of d was determined to be 7.2×10-5 cm2/s according to the resulting slope and cottrell equation. calibration curve because dpv commonly has a higher sensitivity than cyclic voltammetry technology, the dpv technique was applied for the quantitative detection of carmoisine. figure 6 shows the differential pulse voltammograms of carmoisine at various concentrations using g-c3n4/ils/cpe. as seen, the oxidation peak currents of carmoisine enhance gradually by increasing its concentration. the oxidation peak currents (ipa) show a good linear relationship with the concentrations of carmoisine ranging from 0.4 m to 125.0 μm. the linear equation is ipa = 0.1914ccarmoisine + 0.8746 (r = 0.9992) (fig. 6 (inset)). also, the detection limit, cm, of carmoisine was obtained using the equation (2): cm = 3sb / m (2) s. z. mohammadi and f. mousazadeh j. electrochem. sci. eng. 12(6) (2022) 1143-1152 http://dx.doi.org/10.5599/jese.1396 1149 e / mv vs. ag/agcl/kcl figure 6. dpvs of g-c3n4/ils/cpe in 0.1 m (ph 7.0) containing different concentrations of carmoisine. numbers 1–10 correspond to 0.4, 2.5, 7.5, 15.0, 25.0, 40.0, 60.0, 80.0, 100.0 and 125.0 µm of carmoisine. inset: plot of the electrocatalytic peak current as a function of carmoisine concentration in the range of 0.4-125.0 µm in the above equation, m is the slope of the calibration plot (0.1914 μa.μm-1) and sb is the standard deviation of the blank response obtained from 20 replicate measurements of the blank solution. the detection limit is 0.1 μm. stability, repeatability, and reproducibility the long-term stability test of the g-c3n4/ils/cpe using dpv was performed at room temperature. the results exhibited that the peak current of 50.0 μm carmoisine at the g-c3n4/ils/cpe stayed at 93.7 % of its primary current after 7 days, 92.9 % after 14 days, and 89.1 % after 21 days, indicating the superior long-term stability of the proposed sensor. the oxidation of carmoisine (50.0 μm) on the same g-c3n4/ils/cpe electrode was performed by ten repeated voltammetric measurements, the results of which confirmed the superior repeatability of a fabricated sensor with the relative standard deviation of 3.2 %. similar conditions were provided for the carmoisine of the response currents of carmoisine (50.0 μm) on five g-c3n4/ils/cpe, the results of which confirmed the excellent reproducibility of a fabricated sensor with the rsd of value 3.8 %. analysis of real samples the real samples for the analysis were prepared and quantified by the dpv method. the developed sensor was applied to detect carmoisine in lemon juice and powdered juice samples. the results are summarized in table 1. each measurement was repeated three times. the recovery and http://dx.doi.org/10.5599/jese.1396 j. electrochem. sci. eng. 12(6) (2022) 1143-1152 sensor based on g-c3n4/ionic liquids modified cpe 1150 relative standard deviation (rsd) values confirmed that the g-c3n4/ils/cpe sensor has a great potential for analytical application. table 1. the application of g-c3n4/ils/cpe for determination of carmoisine in real samples (n=3) sample concentretion, μm recovery, % rsd, % spiked found lemon juice 5.0 4.9 98.0 3.1 7.0 7.2 102.9 1.9 powdered juice 5.5 5.6 101.8 2.2 7.5 7.3 97.3 3.0 conclusion a facile electrochemical sensor for carmoisine detection was designed and constructed by using 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https://doi.org/10.1016/j.fct.2022.112907 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.5599/jese.956 https://doi.org/10.5599/jese.1112 https://doi.org/10.1016/j.aca.2007.12.011 https://doi.org/10.1016/j.bios.2010.08.064 https://doi.org/10.1016/j.foodchem.2019.01.143 https://doi.org/10.5599/admet.1172 https://doi.org/10.22034/chemm.2020.113388 https://doi.org/10.1016/j.bios.2016.07.086 https://doi.org/10.22034/chemm.2021.118774 https://doi.org/10.1016/j.molliq.2019.01.081 https://doi.org/10.22034/chemm.2021.125035 https://doi.org/10.1039/c8nj05581e https://doi.org/10.1016/j.jelechem.2018.01.034 https://doi.org/10.22034/chemm.2020.113657 https://doi.org/10.1016/j.carbon.2018.01.008 https://doi.org/10.1016/j.apt.2020.06.024 https://doi.org/10.1016/j.fct.2022.112907 https://creativecommons.org/licenses/by/4.0/) @article{ziamohammadi2022, author = {{zia mohammadi}, sayed and mousazadeh, farideh}, journal = {journal of electrochemical science and engineering}, title = {{a sensitive sensor based on graphitic carbon nitride/1-butyl-3-methylimidazolium hexafluorophosphate ionic liquids modified carbon paste electrode for sensing of carmoisine dye in food samples:}}, year = {2022}, issn = {1847-9286}, month = {aug}, number = {6}, pages = {1143--1152}, volume = {12}, abstract = {the electrochemical sensors for carmoisine detection were explored and applied in the field of food safety because of the striking merits of fast detection speed, high sensitivity and easy operation. herein, we de­veloped a high-performance electrochemical carmoisine sensor based on g-c3n4 and ionic liquids (ils), prepared by a one-pot hydrothermal synthesis method. most importantly, the sensing per­formances of electrochemical carmoisine sensors based on g-c3n4 (graphitic carbon nitride)/il (1-butyl-3-methylimidazolium hexafluorophosphate) are better than the bare carbon paste electrode (cpe) sensors. the detection limit of this sensor (g-c3n4/ils/cpe) de­ter­mined via differential pulse voltammetry was 0.1 µm over a wide linear range of 0.4–125.0 µm.}, doi = {10.5599/jese.1396}, file = {:d\:/onedrive/mendeley desktop/zia mohammadi, mousazadeh 2022 a sensitive sensor based on graphitic carbon nitride1-butyl-3-methylimidazolium hexafluorophosphate i.pdf:pdf;:08_jese_1396_1143-1152.docx:word_new;:www/jese_v12_no6_1143-1152.pdf:pdf;:www/jese_v12_no6_1143-1152.pdf:pdf}, keywords = {electrochemical sensors, chemically modified electrodes, differential pulse voltammetry, food analysis}, publisher = {international association of physical chemists (iapc)}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1396}, } conducting polymer functionalized graphene-based electrochemical sensors for sensing pollutants in water http://dx.doi.org/10.5599/jese.1506 251 j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 open access : : issn 1847-9286 www.jese-online.org review paper conducting polymer functionalized graphene-based electrochemical sensors for sensing pollutants in water siti nur akmar mohd yazid1,2,, aina adriana che adnan1, illyas md isa1,2, mohamad idris saidin1,2, mohamad syahrizal ahmad1,2 and chin suk fun3 1department of chemistry, faculty of science and mathematics, universiti pendidikan sultan idris, 35900 tanjong malim, perak, malaysia 2nanotechnology research centre, faculty of science and mathematics, universiti pendidikan sultan idris, 35900 tanjong malim, perak, malaysia 3department of chemistry, faculty of resource science and technology, universiti malaysia sarawak, 96000 kota samarahan, sarawak, malaysia corresponding author: snakmar@fsmt.upsi.edu.my received: october 10, 2022; accepted: december 29, 2022; published: january 10, 2023 abstract recent trends in electrochemical sensors based on conducting polymer functionalized graphene for the detection of pollutants in water are highlighted in this review. graphene has been the subject of a lot of scientific research to be composited with conducting polymers. researchers are interested in graphene and its variants because they have a lot of good qualities, like good electrical and mechanical properties and very high surface area. with this review, we intend to arouse interest in the important topic of graphene and conducting polymer nanocomposite that is making significant advances in electrochemical sensing, especially for sensing pollutants in water. keywords composites, voltammetry; sensing properties; water pollution introduction water is necessary for all living things to survive, although different species use it for different purposes. for example, water is an essential component of respiration for all oxygen-dependent species and serves as a solvent, temperature buffer, metabolic product, and habitat. however, water pollution has become one of the most pressing environmental problems of our time due to increasing industrialization and urbanization. figure 1 shows the sources of emerging water pollution that may be of concern. the introduction of toxic substances into water bodies (e.g., lakes, rivers, and oceans) results in water pollution, where the substances either dissolve, float, or accumulate and affect water quality. not only is this harmful to aquatic ecosystems, but the pollutants that leach into groundwater also contaminate domestic water supplies, where water is http://dx.doi.org/10.5599/jese.1506 http://dx.doi.org/10.5599/jese.1506 http://www.jese-online.org/ mailto:snakmar@fsmt.upsi.edu.my j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 252 used for a variety of activities, including drinking. therefore, it is important to limit human exposure to contaminants through proper water quality monitoring. figure 1. sources of emerging water contaminants that could be of concern world health organization (who) has continuously revised the guideline values for all water quality measures since 1983 (table 1) [1]. the hazardous substances that accumulate in the human body through water are degraded only slowly. therefore, limiting human exposure to toxins is critical and should be achieved through comprehensive water quality monitoring. detection of water contaminants has traditionally been performed with hanging mercury drop electrodes or mercury-modified electrodes, as replacement of the mercury drop is sufficient to restore a pristine surface [2]. however, due to new regulations and the high toxicity of mercury, new alternatives for its replacement are being investigated. there are some reports of work using gold nanoparticles, microelectrodes, and also cathode stripping potential to detect heavy metals. unfortunately, it is difficult to detect water pollutants using these methods [3,4]. table 1. maximum permissible concentrations for water quality testing according to who [1] parameter maximum permissible concentration antimony 20 ppb arsenic 10 ppb benzene 10 ppb chloramines 3 ppm chlorine 5 ppm chromium 50 ppb (total) copper 2 ppm fluoride 1.5 ppm lead 10 ppb mercury 6 ppb (inorganic) nitrate 50 ppm nitrite 3 ppm selenium 40 ppb toluene 0.7 ppm uranium 30 ppb recently, the electrochemical approach to water contaminant analysis has been gaining popularity due to its ease of use, low cost, rapid analysis, and high sensitivity [5,6]. in the past, electrochemical sensors have been used to monitor oxygen concentration in the industry since the 1950s, when they were first introduced [7]. at that time, the monitoring of hazardous gases and s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 253 fuels in confined spaces was mandated by labour legislation, which led to an explosion in research on electrochemical sensors with high selectivity. leland c. clark [8] introduced an oxygen sensor that consisted of an oxygen-permeable membrane separating two electrodes in a cell, producing a current that was inversely proportional to the concentration of oxygen in the sample. since the invention of clark's electrochemical oxygen sensor, a variety of other sensors have followed. despite their commercial success, clark's oxygen sensors produce an unstable electrical current reading due to contamination or corrosion of the working electrode surface, which requires frequent recalibration of the oxygen analyzers, making them unsuitable for routine use [8]. in addition, conventional electrochemical sensors usually require a catalytic enzyme to detect impurities. with an enzyme sensor, one can achieve higher sensitivity, selectivity, and stability [9], but it also has some limitations, including enzyme denaturation and limited stability, and a difficult purification process due to weak immobilization of the enzyme on electrodes [10]. therefore, scientists are constantly looking for ways to improve them, for example, by developing enzyme-free sensors. non-enzymatic sensors for the detection of water contaminants that do not involve enzymes seem to be viable alternatives, but they still need significant improvements in sensitivity and selectivity before being used commercially. this is why the development of non-enzymatic sensors is so important. therefore, scientists are investigating the aspect of more sensitive materials with novel morphologies while maintaining all the performance criteria of the sensor. while the flexibility of polymers makes them attractive for a variety of applications, they are often undesirable for conductive applications because most polymers are insulating. in terms of their electrical and optical properties, organic polymers have comparable characteristics to inorganic semiconductors. the most important and distinctive feature of a conducting polymer is alternating single and conjugated double bonds along the main chain, with the highly delocalized, polarized, and electron-dense π-bonds providing electrical and optical properties [11]. in 1977, shrikawa, heeger, and macdiarmid [12] first proposed that conjugated polymers are tailored to transmit electricity. their study involved the removal of an electron from a delocalized bonding configuration by a halogen dopant, resulting in the formation of a hole. an electron then jumps to an adjacent site and fills the hole, creating another and allowing a charge to flow along the polymer chain. the structural properties of some conducting polymers are shown in figure 2, including poly(3,4-ethylenedioxythiophene) (pedot), polyacetylene (pac), polypyrrole (ppy), polythiophene, polyaniline (pani), poly(pphenylenevinylene) (ppv), polyethylene (pe) and polypropylene (pp). carter et al. [13] developed nanocomposites of elastomer-reinforced polymers with exfoliated layered silicate fillers as early as 1950, and a study published over forty years later found that mechanical properties of a clay-filled nylon-6 matrix were improved [14]. this triggered a great interest in nanocomposites both in academia and industry. recently, there has been growing interest in functionalizing conductive polymers with nanofiller-based materials to provide new or improved properties for sensor development [15,16]. the polymer matrix filled with a variety of nanofillers, such as spherical nanoparticles, is the most basic nanocomposite system, while all kinds of anisotropy-inducing fillers, such as graphene, graphene oxide, and other graphene derivatives find their way into polymer nanocomposites. a study by marsden et al. [17] proposed conductive composites prepared by infusing the polymer matrix with graphene. when the filler loading approaches the percolation threshold (pc), the conductivity of the composite increases rapidly and the conductivity plot as a function of loading takes a typical s-shape indicating three different states: insulating, permeable and conductive. http://dx.doi.org/10.5599/jese.1506 j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 254 figure 2. structural properties of numerous conducting polymers thus, the polymer remains insulating at minimum loading. a conductive network forms when the filler content reaches the percolation threshold. a robust network forms and the conductivity saturates with increasing loading [17]. conducting polymer-functionalized graphene nanocomposites are more promising to improve the properties of conducting polymers, and the use of graphene and its derivatives with excellent properties has attracted the attention of researchers [18-20]. novoselov et al. discovered graphene in 2004 [21], and it is a honeycomb-shaped, two-dimensional (2-d), and atomically dense material composed of sp2 carbon atoms. compared to carbon nanotubes, polymer molecules can easily reach the surface of 2-d graphene. as a result, more filler material can be incorporated into the graphene matrix and the final conductivity values increase. table 2 shows someof other important physicochemical properties of graphene. table 2. some of the important physicochemical properties of graphene physiochemical properties estimated value reference large specific surface area 2630 m2g-1 [22] excellent electrical conductivity 1738 siemens/m [23] strong mechanical strength young’s modulus 1100 gpa [24] good thermal conductivity 5000 w m‐1 k‐1 [25] mobility of charge carriers 200000 cm2 v-1 s‐1 [26] ease of functionalization π-π interaction and electrostatic interaction [27] band gap zero [28] the carbon atoms in graphene have a total of six electrons, two of which are in the inner shell, while four electrons are scattered in the outer shell with chemical bonding orbitals 1s22s22p2 (figure 3a). the two electrons in the 1s core orbital of the honeycomb crystal structure remain in this orbital, while the remaining electrons form sp2 bonds with three other orbitals and one pz orbital and also hybridize with each other (figure 3b). the free electron is held in the pz orbital, which is located above the plane and is thus responsible for the formation of the π-bond [29]. it is this overlap of p electrons that gives graphene its unique electronic properties, which are controlled by the bonding and counter-bonding of atoms (figure 3c). in addition, the honeycomb structure of graphene can also be characterized as a triangular lattice, with each unit cell containing two atoms a and b as studied by castro neto et al. [30]. the sublattices a and b are connected by basis vectors in graphene (a1, a2). s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 255 the dirac points are two unequal sites labeled k and k' at the transition of the brillouin zone of graphene. when the valence and conduction bands cross in space, the dirac point is reached [30]. figure 3. (a-c) fundamental property of graphene's bonding (with ©permission from [29]) analysis of the available literature shows that the surface of graphene can be easily functionalized while maintaining the quality of electron transport. this is of great importance in the field of water quality detection, as it allows selectivity towards the analyte of interest [31-34]. the use of graphenebased materials as nanofillers in polymers has the potential to take advantage of the beneficial properties of graphene but also to reduce the cost and speed up the production process of the composite. it is also possible that the polymer matrix, on the other hand, may have attractive properties such as functional groups or molecular imprinting that are of importance. this review article provides an overview of recent research and methods for the construction of electrochemical sensors based on conductive polymer-functionalized graphene for the detection of water pollutants over the past seven years. for the reader's convenience, the detection mechanisms of electrochemical sensors based on conductive polymer-functionalized graphene materials are briefly explained. then, a series of methods for preparing nanocomposite materials of graphene and conducting polymers, including covalent and noncovalent functionalization, are discussed. nanocomposites of graphene and conducting polymers have been shown to improve the performance of electrochemical sensors in a variety of target samples. figure 4 illustrates how the number of publications dealing with conductive polymer-functionalized graphene electrochemical sensors has increased from 706 in 2016 to 2,483 in 2021 and 2,470 in 2022 (sciencedirect), showing the increasing importance of these nanocomposites. table 3 provides a list of review articles on conductive polymers and carbon-based sensors for the detection of contaminants in water. figure 4. histogram illustrating the annual number of scientific articles published in the last seven years (research published on march 18, 2022 in ‘science direct’ with graphene-conducting polymer composite electrochemical sensor) http://dx.doi.org/10.5599/jese.1506 j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 256 table 3. list of review papers on conducting polymer and graphene-based electrochemical sensors for sensing pollutants in water year of publication title main emphasis ref. 2021 a review of nanocomposite-modified electrochemical sensors for water quality monitoring surface modification of electrodes [33] 2021 electrochemical sensors based on conducting polymers for the aqueous detection of biologically relevant molecules conducting polymer [34] 2020 review—graphene-based water quality sensors. graphene [31] 2020 nanocomposites (conducting polymer and nanoparticles) based electrochemical biosensor for the detection of environment pollutant: its issues and challenges. conducting polymer and nanoparticles [32] 2020 recent trends of micro and nanostructured conducting polymers in health and environmental applications nanostructured conducting polymers [35] 2019 application of graphene-based materials for detection of nitrate and nitrite in water a review graphene-based nanocomposites [36] 2019 polymer-based electrochemical sensing platform for heavy metal ions detection-a critical review polymers modified electrodes [37] 2018 composites based on conducting polymers and carbon nanomaterials for heavy metal ions sensing (review) organic conducting polymers and carbon nanotubes [38] a thorough analysis of the existing literature reveals a gap in knowledge regarding the combination of graphene and conductive polymer as an electrochemical nanocomposite for sensing. according to studies, the combination of graphene and conducting polymers can improve the selectivity and other important sensing parameters of electrochemical sensors, which could be due to various factors such as synergistic and geometric effects. in this article, we first review the fabrication and sensing performance of electrochemical graphene sensors functionalized with conducting polymers, which are used to detect pollutants in water. detection mechanism of electrochemical sensors based on conductive polymer functionalized graphene nanocomposites a sensor is a device that detects and responds to signals. this signal must be some type of energy, such as heat, light, motion, or electrochemical reaction. a sensor will convert one or more of these signals as soon as it detects them [39]. electrochemical sensors are gaining popularity among chemical sensors because they use an electrode as a transducer element in the presence of an analyte. electrochemical sensor systems offer analytical techniques that are fast, accurate, selective, sensitive, and easy to use. these sensor systems are effective and suitable for detecting and monitoring contaminants in environmental samples because electrochemical analysis requires only a small amount or volume of sample, and no preparatory steps. in electrochemical reactions, both current and voltage play a role, as well as potential and resistance. there are three types of electrochemical sensors: potentiometers, amperometers, and conductometers [40]. table 4 lists some of the most common electrochemical processes and electrical signals they produce. at the potentiometric sensor interface, the electrode or membrane potential is recorded, and information about the composition of a sample can be obtained from the potential difference between the two electrodes. in an amperometric sensor, electroactive species can be oxidized or reduced by applying a voltage between the reference and working electrodes and recording the resulting current. conductometric sensors, on the other hand, are used to measure conductivity at various frequencies. coulometry and capacitive approaches, for example, derive their results from electric current and resistance [41]. s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 257 table 4. electrochemical methods, electrical properties, and units of measurement electrochemical methods monitored electrical property potentiometry potential difference, v conductometry resistance, ω amperometry and voltammetry current as a function of applied potential, a coulometry (q) current as a function of time (q = is), a·s capacitance (c) potential load (c = q / v), f when the chemical reactions of a target species occur on electrodes, electrochemical sensors convert the chemical reactions into electrical signals. electrochemical sensors consist of two or three electrodes, depending on the device. in a three-electrode cell, as shown in figure 5, each electrode serves a specific purpose [42]. the reaction of interest takes place at the working electrode, which in this case, involves the reference electrode. materials for the working electrode, such as platinum, gold, and/or carbon, have a significant impact on the performance of an amperemetric sensor. the reference electrode is responsible for maintaining a constant potential throughout the amperometric experiment [43]. silver/silver chloride (ag/agcl) is the most practical reference electrode and is commonly used in an electrochemical sensor. the potential of the reference electrode is stable, reproducible, and can be calculated thermodynamically. the maximum temperature at which this type of electrode can function is about 130 °c [44]. the current flow is provided by the counter electrode, which has the opposite sign of the working electrode. the counter electrode is reduced when the working electrode undergoes oxidation and vice versa. however, this does not affect the response of the working electrode. the charge flowing through the counter electrode is always the same as at the working electrode [45]. platinum is commonly used as a counter electrode for electrochemical sensors due to its good conductivity. in addition, platinum typically releases hydrogen or oxygen as a counter electrode in an aqueous system. this hydrogen bubbling does not lead to the dissolution of species in the electrolyte that could affect the working electrode [46]. table 5 shows the list of conductive polymer functionalized graphene and their derivatives together with their performances in electrochemical sensors for the detection of pollutants in water. figure 5. typical electrochemical measurement setup with three electrodes, a working electrode, a reference electrode, and a counter electrode for (a) square wave anodic strip voltammetry (swasv), (b) electrochemical impedance spectroscopy (eis), (c) amperometric, and (d) cyclic voltammetry (cv) http://dx.doi.org/10.5599/jese.1506 j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 258 table 5. list of graphene functionalized with conducting polymers and its derivatives and their performances in electrochemical sensors for detection of pollutants in water electrode matrix detection technique linear range, µm detection limit, µm analyte ref. pgo/ppy dpsv 1 to 100 0.05 cd2+ [47] 3dgo-py10 swasv 5 to 400 3.6 cd2+ [48] pani@go/ag 105 to 102 10 cd2+ [49] popd/ergo/gce swasv 10-3 to 50 × 10-3 0.13 × 10-3 cd2+ [50] fe3o4-go-pani 0.01 to 0.15 3.6 × 10-3 cd2+ [51] rgo-pani-hbr dpv 0.01 to 0.18 0.01 to 0.23 7.3 × 10-3 6.5 × 10-3 cd2+ pb2+ [52] pa/ppy/go dpv 5 to 150 2.13 0.41 cd2+ pb2+ [53] rgo/ala/pani swasv 0.1-8 × 10-5 3 × 10-5 4.5 × 10-5 6.3 ×10-5 cd2+ pb2+ cu2+ [54] pani-go-aptes lsv 0.01 to 0.4 0.0053 pb2+ [55] ppy-rgo swasv 5 × 10-3 to 0.75 4.7 × 10-5 pb2+ [56] ppy-rgo swasv 5 ×10-4 to 0.45 3 ×10-4 pb2+ cd2+ cu2+ hg2+ [57] pgmgpe cv 0.02 to 0.045 and 0.05 to 0.0012 0.015 to 0.0015 0.8 6.6 pb2+ hg2+ [58] polypatt-go/gce cc 1.0 10000.0 (ppb) 0.08 (ppb) 0.20 0.05 0.09 0.10 cd2+ pb2+ zn2+ cu2+ hg2+ [59] edta_pani/go dpsv 1 to 30 (ppb) 0.612 (ppb) hg2+ [60] pedot/go/gce dpsv 0.01 3.0 2.78 × 10⁻3 hg2+ [61] p-np-mip-pani/go cv 6 × 10−5-14 × 10−5 2 × 10⁻⁵ p-nitrophenol [62] gce-pani-g-itn dpv 0.03 × 10−3 (ipa ) 0.01 × 10−4 (ipc ) 0.0052 0.0024 4-nitrophenol [63] go-pani-aunps amperometric 0.5 240 and 240 258 0.17 nitrite [64] gce/rgo/ppy/nr cv 5 × 103 2 × 105 275 nitrite [65] cg/ppy/cs/gce dpv 0.2 1000 0.02 nitrite [66] rgo/agnps/poly(pyy) amperometric 0.1-1000 0.012 nitrite [67] agnps@pani/rgo dpv 2.7 − 24.4 1.0 − 28.2 0.077 0.056 sulfite nitrite [68] gonrs + pedot:pss amperometric 0.05 − 16.55 0.05 − 16.55 0.05 − 16.55 0.05 − 16.55 0.041 0.030 0.011 0.018 ascorbic acid uric acid dopamine nitrite [69] agnps@ppy/rgo dpv 0.6 − 6.6 0.019 0.021 hydrazine nitrite [70] gr-pani current-potential 10 − 200 5.880 urea [71] song et al. [47] described the concept of the cd(ii) sensor. first, cd2+ ions in an aqueous solution were simultaneously absorbed by the porous go/ppy electrode and the carboxyl group graphene oxide. second, cd2+ was reduced to cd under stripping voltage on the electrode surface. after reduction, cadmium was finally oxidized to cd2+ via carboxyl groups, pgo/ppy and ppy by differential pulse voltammetry. a heavy metal sensor based on the pgo/ppy material was developed and tested for its performance. however, the authors did not test the stability of the sensor of their method in s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 259 real water. guo et al. [48] studied the interaction of pyrrole with graphene oxide using different characterization approaches to better understand the role of the doped pyrrole. they discovered that the reaction ratio of pyrrole and graphene oxide is crucial to improve the effect of pyrrole. it is believed that pyrrole can be used not only as a nitrogen supplier but also as a reducing agent and regulator to change the composition and structure of graphene aerogels, which will affect the electrochemical sensing performance for cd2+ ions. eltayeb et al. [49] prepared a nanocomposite of go functionalized with pani and doped with silver to improve its thermal and electrical properties. by oxidizing it with simple persulfate while it was in solution, a very simple sol-gel method was used to make it. the go was used as a template, while aniline was polymerized on the surface of go by oxidation. then, ag nanoparticles were attached. the change in dc conductivity properties with temperature was also investigated. this showed the range between the metallic and conductive regions, but the sensor did not have a very small sensing range. the authors did not investigate how sensitive their sensor was. aboobakri et al. [51] also used pani in their work. to increase the extraction capacity and selectivity for cd2+ ions, they laboriously incorporated magnetite nanoparticles into graphene oxide pani nanocomposite (mgopa). since centrifugation and filtration are time-consuming processes that could be skipped, the magnetic property of the adsorbent is an essential feature that significantly increases the throughput of the method. the novel adsorbent has low toxicity and is inexpensive. it has been shown to have a high adsorption capacity with good recoveries. the total time for sample extraction and pre-concentration is less than 25 minutes, showing fast adsorption-elution kinetics for cd2+ ions. although the sensor was used as it was fabricated, the interference effect was not investigated by the authors. dai et al. [53] investigated the simultaneous measurement of cd2+ and pb2+ samples using a phytic acid functionalized (ppy)/graphene oxide (go) modified electrode (pa/ppy/go). from the electrochemical performance, they found that the electrode modified with ppy/go nanocomposite had no detection signal for heavy ions, most likely due to the coverage of the go nanosheets by the ppy. oxidation current peaks of cd(ii) and pb(ii) ions at modified electrodes pa/go and pa/ppy/go occurred at 0.88 v and 0.64 v, respectively. the enhancement of the signals of the heavy ion stripping peaks on pa/ppy/go may be due to two factors. first, the large surface area of go and the better electrical conductivity of ppy may make it easier for electrons to move during the detection process. second, the presence of functional groups with significant negative charges on pa and go is advantageous for increasing heavy metal ion adsorption. alruwais et al. [55] used wet chemistry to create polyaniline (pani), graphene oxide (go), and 3aminopropyltriethoxylsilane (aptes). in the linear range from 0.01 to 0.4 m, the lowest measurable lead concentration was 0.0053 m. the measured sensitivity was 165.71 a cm2 m-1 and the response time was less than 20 s. even though metal ions and electroactive species might have interfered, the panigo-aptes current response to pb2+ was stable. raril and manjunatha [58] described the invention of an electrochemical sensor based on polyglycine-modified graphene (pgmgpe) for the determination of hg2+ and pb2+ ions by cyclic voltammetry (cv). the electrochemical behavior of heavy metal ions on bare and modified electrodes was studied, and it was discovered that the modified electrode improves the redox signal of these two heavy metal ions. hashemi et al. [63] reinforced glassy carbon electrodes with decorated graphene oxide nanoflakes with interconnected, porous, and high-density patterns of iron tungsten nitride (go-itn) for the detection of 4-nitrophenol in aqueous environments. they suggested that modification of pani with hybrid 2d http://dx.doi.org/10.5599/jese.1506 j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 260 go-itn nanoflocks improves the conductivity/sensitivity of the created sensor by promoting protonation and active functional groups in the final platforms. rong et al. [56] and suvina et al. [57] used ppy to fabricate their pb2+ detecting sensors. rong et al. [56] synthesised the ppy-rgo nanocomposite without difficulty by electrochemical polymerization and electrochemical reduction. anodic stripping voltammetry was used to study the response of the electrode modified with the nanocomposite and the corresponding parameters were set. they discovered that the introduction of nanosheets of reduced graphene oxide (rgo) not only increased the effective area of the nanocomposite, but also increased the rate of electron transfer at the interface. combined with the selective recognition of ppy, this porous three-dimensional ppyrgo nanocomposite exhibited high pb2+ selectivity. on the other hand, suvina et al. [57] investigated the synthesis of hydrogels from the ppy and rgo for the simultaneous detection of four different metal ions, i.e., cd2+, pb2+, cu2+ and hg2+. a ppy-rgo hydrogel composite was prepared by in situ polymerization of pyrrole in the presence of an rgo dispersion. the authors discovered that the ppyrgo hydrogel composite has a surface area of 21.48 m2/g, which is more than three and a half times that of ppy (5.89 m2/g). mahadik et al. [60] reported the electrochemical detection of hg2+ using a pani/go composite modified with ethylenediaminetetraacetic acid (edta). electrochemical detection of mercury ions was performed using differential pulse stripping voltammetry (dpsv). the edta-modified pani/go electrode showed excellent response to hg2+ from 30 ppb to 1 ppb. however, the authors did not address the effects of interference even though the sensor was used in its original form. zuo et al. [61] also found a way to determine hg2+ using poly(3,4-ethylenedioxythiophene) nanorod/graphene oxide nanocomposite modified glassy carbon electrode (pedot/go/gce). a simple liquid-liquid interfacial polymerization method was used to propose a pedot/go nanocomposite. the results showed that pedot with a structure like nanorods adhered to the surface of go nanosheets, which could improve the electrically active sites of the nanocomposite. dpsv was used to find out how much hg2+ was present at low concentrations on pedot/go/gce. under the best conditions, there was a good linear relationship between the peak currents and the concentration of hg2+ ions in the range of 10.0 nm 3.0 mm. with a signal-to-noise ratio of 3, the minimum amount that could be found was estimated to be 2.78 nm. saadati et al. [62] and hashem et al. [63] used pani in their studies for the detection of nitrophenol. saadati et al. [62] developed an electrochemical sensor for the detection of pnitrophenol (p-np) based on a molecularly imprinted polymer (mip) on go. the sensor host molecule and initiator, aniline and ammonium persulfate, were precipitation polymerized to create the sensor. the variable functional groups (i.e., hydroxyl, carboxyl, and epoxide groups), high surface area, outstanding mechanical properties, and good compatibility with polymers make go a suitable material for the synthesis of nanocomposites. in the presence of related phenol chemicals, such as 4-chlorophenol, 4-bromophenol, 2,4-dinitrophenol, and 2-nitrophenol, the sensor demonstrated high selectivity for nitrophenol. hashemi et al. [63] improved the polymer structure and sensitivity of pani by reinforcing it with decorated go nanoflocks with an interconnected, porous and dense pattern of iron tungsten nitride (go-itn) for the detection of nitrophenol in aqueous media. they reported that the surface of a gce electrode was modified with a pani -based nanocomposite containing 50 wt.% go -itn and then subjected to various studies. in addition to improved protonation, the prepared nanocomposite showed superior sensitivity with a detection limit, quantification limit and sensitivity of 5.2 nm, 18.2 nm, and 253.08 a cm-2 m-1 for the oxidation peak current, and 2.4 nm, 7.1 nm and 354.92 a cm-2 m-1 for the reduction peak current, respectively. s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 261 umar and nasar [65] developed a sensor (gce/rgo/ppy/nr) based on a nanocomposite of reduced graphene oxide (rgo) and ppy immobilized on a glassy carbon electrode (gce) by nitrate reductase (nr). acidic in situ oxidative polymerization of pyrrole in the presence of rgo was used to create the conductive nanocomposite (rgo/ppy). direct electron transport from nitrate to the electrode was realized by nitrate reductase on the gce/rgo/ppy. nitrate reductase is an enzyme that helps electrons to move directly from the active site of the rgo/ppy electrode surface. this causes the current density to go up to 4.24 ma cm-2. however, the linear range of their study was not very broad. ppy was also used by xiao et al. [66] in their study to detect nitrite. an electrochemical sensor based on n-carboxyl graphene (cg), ppy and a gce nanocomposite modified with chitosan (cs) was prepared using differential pulse voltammetry (dpv). the combination of ppy and cg provides a large surface area and numerous active sites and increases the charge transfer between the electrode and the reagent. at the same time, the many amino groups in cs attract negatively charged nitrite, which makes it easier for nitrite to build up on the surface of the electrode. some researchers have also used ppy for the detection of hydrazine and nitrite. kaladavi et al. [70] generated agnps@ppy/rgo composite using the interfacial polymerization technique employing an organic solvent dispersion of pyrrole as the go reducing agent. an aqueous solution of silver nitrate was utilized as a precursor for ag nanoparticles and oxidizing agent to commence the polymerization reaction at the interface. the nanocomposite film electrode was used for the electrochemical oxidation of inorganic pollutants, hydrazine and nitrite, and their oxidation potentials in the phosphate buffer were +0.53 and +0.76 v (vs. ag/agcl). significantly, the electrochemical sensor was synthesized by means of interfacial polymerization without the use of any powerful oxidizing chemical, such as ammonium persulfate. the believability of the proposed system as an electrochemical sensor is supported by the absence of interference from specific inorganic contaminants and other biomolecules. fabrication of conducting polymer-functionalized graphene nanocomposites the fabrication of graphene hybrid nanocomposites to modify materials is one of the highlights of graphene research. it is obvious that graphene, as a component of polymer nanocomposites leads to better material properties than conventional fillers. a high degree of graphene dispersion in a matrix solvent is required for better integration between graphene and polymer material particles. graphene oxide is the most commonly used dispersant in water for graphene dispersion. it is possible to produce large amounts of graphene oxide by exfoliating graphite under highly acidic and oxidative conditions, resulting in a high concentration of hydroxyl, epoxy, and carboxyl groups in the resulting material [72]. moreover, both covalent and noncovalent functionalization can help to improve the graphene dispersibility in conducting polymers, increasing their interfacial contacts and enabling graphene dispersion in a polymer matrix. covalent functionalization approach chemical modification, also known as covalent functionalization, is a straightforward technique for forming covalent bonds between graphene oxide or graphene in conducting polymers. two unique approaches are widely used for the covalent modification of graphene with a conducting polymer, i.e., either by connecting end-functionalized polymer chains to the solid substrate, which is referred to as 'grafting on', or by polymerization from the surface where the grafting process can occur, which is also referred to as 'grafting off' [73]. in short, the 'grafting on' preserves the graphite lattice by connecting polymer chains and functional groups to the graphene oxide edge. as shown by the significantly reduced dispersed phase size and high current values, tan et al. [74] discovered http://dx.doi.org/10.5599/jese.1506 j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 262 that conductive polymers grafted onto modified, reduced graphene oxide significantly increase electrical conductivity and compatibility. vallés et al. [75] prepared graphene with a polymer functionalized covalently bonded polymer by coupling a diazonium addition reaction to produce -nh2-terminated nh2-gnps with an amidation between the nh2-graphene nanoplatelets (nh2-gnps) and poly(methyl methacrylate) (pmma) chains to produce pmma-grafted nh2-gnps. the polymer grafted onto chemically reduced graphene oxide sheets by emulsion polymerization improves the dispersion and contact between the sheets and the matrix, resulting in effective stress transfer. however, current research in the grafting and subsequent fabrication of conducting polymer and graphene composites is limited due to difficulties in matching the unique surface chemistry of graphene oxide with the specific functions of polymer chains. in addition, the selectivity of the chemical reactions between the active sites on the prefabricated polymers and go cannot be controlled, which is essential to consider. as a result, the final structure of the composite is not clear [76]. in the 'grafting of' process, a polymer backbone functionalized with initiation sites serves as a macroinitiator from which the side chains are subsequently prepared [77]. this method demonstrates the advantages of making grafted polymer brushes that are both thick and wellcontrolled. most graphene-polymer composites have been prepared by the grafting method based primarily on the controlled free radical live polymerization, radical transferred atom polymerization (atrp) [78-81], reversible chain transfer polymerization (raft) [82-86], radical polymerization with a single electron transfer (set-lrp) [87,88], polymerization by ring opening [89-91], polymerization by free radicals [92,93], grafting by electrochemical methods [94], polymerization process triggered by ultrasound [95-97], and polymerization mediated by copper and triggered by light [98]. park et al. [99] presented the 'grafting of' covalent graphenepani nanocomposites. the perfluorophenyl nitrene generated by photochemical or thermal activation of pfpa reacts with graphene to form a covalent adduct. in this study, the aniline derivative pfpa-ani was used to add aniline groups to graphene and found to be effective. in situ polymerization of aniline was used to prepare pani, which was then grafted onto the graphene surface. by functionalizing graphene nanoplatelets with poly(methyl methacrylate) (pmma), rubio et al. [100] contrasted the 'grafting from' and 'grafting to' approaches. according to their results, grafting rates were much higher for ’grafting from’ (44.6 to 126.5 %) than for ‘grafting to’ (12.6 to 20.3 %). ‘grafting to’ products resulted in higher absolute grafting rates and improved dispersibility in acetone as monomer supply increased, while ‘grafting to’ products due to steric effects resulted in lower absolute grafting rates and a decreasing trend as polymer molecular weight increased. noncovalent functionalization approach the noncovalent functionalization of graphene can improve its dispersion without affecting its structure or electronic network, which means that crucial properties such as electrical conductivity and mechanical strength are not affected [101]. in this process, the surface of graphene is modified by the physical absorption of modifiers on the surface of the material to improve its dispersibility in various solvents. these modifiers are usually substances that have the potential to develop different interactions with graphene layers, such as π-π, cation-π, and anion-π interactions [102]. to understand the energetic and geometric significance of π-interactions, studies were conducted in which apatiga et al. [103] discovered that polymers with aromatic rings in their structure are more compatible with graphene nanoparticles to maximize conductivity by inducing a π-π-stacking effect that increases the transport of electrons from graphene to the polymer and vice versa. noncovalent s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 263 interactions are present in polymers with linear architecture and are mainly affected by the ch /πstacking. even though electrons are not accelerated as in aromatic polymers, electronic mobility is still promoted in these materials. it has been discovered that the use of surfactants can be an effective approach to improving the mechanical and thermal properties of polymer nanocomposites. examples of surfactants such as sodium dodecylbenzene sulfonate (sdbs) [104-106], sodium dodecyl sulfate (sds) [107,108], and cetyltrimethylammonium bromide (ctab) [109] are among the most commonly used noncovalent modifiers for graphene-filled polymer composites. stankovich et al. [110] proposed that the adsorption of the surfactant polymer onto the graphene sheets should be due to the π-π interaction between the pss benzene rings and the graphene sheets, which reduces the interactions between the graphene sheets and thus prevents aggregation [110]. the use of pedot:pss was demonstrated by nardes et al. [111] and hakimi et al. [112] due to its large electrical conductivity, high transparency, excellent processability and low redox potential. unfortunately, the process is difficult because pss-modified graphene can only be dispersed in water. as an alternative to pss-modified graphene, solution coagulation can be used to combine water-insoluble polymers with pss-modified graphene [113]. noncovalent graphene-polymer nanocomposites can be synthesized by other methods, such as solution mixing, melting, and in situ polymerization [114]. the final characteristics and properties of the nanocomposites are affected by the particular method. the solution mixing method is widely used and particularly effective in the synthesis of polymer-graphene nanocomposites. the chemically or thermally reduced graphene oxide can be well dispersed in organic solvents due to oxygen-containing groups, which are not removed from the graphene oxide by this technique [115]. three processes are involved in the mixing of the solution. ultrasonic dispersion of the graphene fillers in a suitable solvent is the first step. the next step is the addition of polymer to an ultrasonically prepared premix of graphene and solvent. the entire mixture is then vortexed to ensure that the graphene is well dispersed in the polymer matrix. the solvent must then be removed in the final stage by evaporation, filtration, precipitation or distillation. this step determines the quality of the polymer-graphene nanocomposite produced on a solid support, as well as the effectiveness of the overall solution mixing process and solvent compatibility. for example, lego et al. [116] have developed a solution mixing approach for the preparation of polycarbonate/graphene composite pellets from 1,3-dioxolane-based dispersion that serves a dual function by acting as a dispersant for the graphene flakes and simultaneously dissolving the polycarbonate to obtain the final polymer composite. the addition of graphene flakes to a polymer increases the mechanical and thermal properties of the polymer as well as its electrical conductivity. the approach of iqbal et al. [117] to improve the dispersion of graphene in pe by mixing the polymer with oxidized pe (ope) was successful. nanocomposites of graphene with pe and graphene with ope blends were prepared by solvent mixing. compared to pure pe nanocomposites (1 and 0.29 vol.%), they discovered improved dispersion of graphene in pe/ope blends, significantly reducing percolation in both rheological (0.3 vol.%) and electrical (0.13 vol.%) measurements, indicating improved dispersion and exfoliation of graphene sheets in the blends compared to pure pe nanocomposites. unfavourably, large amounts of organic solvents were required for the preparation of the blended solutions during industrial production, which pollutes the environment. the use of organic solvents also led to a significant increase in production costs. therefore, this process is less frequently used for production on an industrial scale compared to the melt blending process. compared to solution mixing, melt mixing is more practical, clean, and adaptable because no solvent is required for the process of conductive polymer with graphene fillings [118]. thus, melt http://dx.doi.org/10.5599/jese.1506 j. electrochem. sci. eng. 13(2) (2023) 251-274 graphene-based electrochemical sensors 264 blending is not only safe for the environment but also straightforward and cost-effective. many polymer manufacturers prefer this technology for commercial production. however, care must be taken during the mixing process to prevent the polymer from disintegrating due to the high temperatures required to produce a homogeneous blend [119]. according to tu et al. the electrical conductivity of graphene-filled pe/pp blends was affected by the processing sequence during melt blending [120]. in their study, they fabricated graphene-filled composites from high-density pe and pp by melt compounding and compared the electrical conductivity of the composites fabricated in three different processing sequences. it was found that the graphene, pe and pp processing sequences affect the graphene localization, affecting the observed electrical conductivity of the composites. graziano et al. [121] compared high-density pe-functionalized graphene enhanced by the melt mixing method with the solvent reaction method. for comparison, high-density pe-reduced functionalized graphene oxide composites were prepared by solvent reaction, while melt blending and solvent reaction were used for high-density pe-reduced functionalized graphene oxide composites. high-density pe-functionalized graphene has better mechanical properties than highdensity pe and high-density pe-functionalized graphene oxide. this is because graphene was well distributed in the polymer matrix and had strong interactions with the polymer, allowing stresses to be transferred more easily, heat to be distributed more uniformly, and crystals to grow faster [121]. melt compounding is more flexible, less expensive, and less environmentally harmful than the solvent reaction because no solvents or chemicals are needed. the performance improvements of the nanocomposite (prepared by melt compounding) were identical to those of the nanocomposite (prepared by solvent reaction) except for the mechanical properties [121]. another approach to the preparation of conductive polymer-functionalized graphene is extremely useful, and known as in situ polymerization. the main advantage of this method is that it allows the polymerization of monomers located inside and outside the graphene interlayers. this leads to polymer nanocomposites with delaminated graphene platelets at the nanoscale. however, the problem of lack of distribution of graphene fillers in this process remains, leading to a lower polymerization rate in the later phases [122]. in a study by qi et al. [123], it was found that the lamellar thickness and crystallinity of nanocomposites, as well as their crystallization temperature, are increased by the heterogeneous nucleation effect of graphene. the thermal stability of the composites was improved by the ability of graphene to absorb free radicals generated during the decomposition of high-density pe. a good interfacial polarization effect in the materials was also demonstrated by an increase in the dielectric constant and conductivity of the composites when the graphene concentration was increased. based on spherical and sandwich graphene/sio2 supports, su et al. prepared a high molecular weight pe/graphene nanocomposite using in situ polymerization method [124]. the porous spherical graphene support offers more advantages for this technology than conventional methods and could preserve the intrinsic graphene performance in nanocomposites. when the ziegler-natta (z-n) catalyst is loaded onto the support, the active components adhere to the surface and inside the pores. when ethylene molecules contact the active sites, a regular and well-organized chain of molecular bonds is formed on the surface and inside the catalytic sites. cheng et al. [125] proposed a noncovalent functionalization approach to prepare graphene pani nanocomposites to improve the electrical conductivity, mechanical and electromagnetic shielding properties of the polyimide composite film. the polymerization of graphene in the pani matrix helps to distribute the graphene sheets uniformly in the matrix. due to the van der waals forces, the graphene sheets tend to clump together during the stirring process without pani and π-π noncovalent interactions between the s.n.a. mohd yazid et al. j. electrochem. sci. eng. 13(2) (2023) 251-274 http://dx.doi.org/10.5599/jese.1506 265 graphene sheets, resulting in an irregular and discontinuous network. the graphene sheets were welded together by noncovalent bonding between adjacent graphene sheets, avoiding aggregation of graphene sheets in organic solvents. once the polymer matrix is thermally imidized, electrons can move more efficiently through the graphene pani network. the high conductivity of graphene is not affected by the presence of noncovalent interactions in this mixture of graphene and pani [125]. conclusions and perspectives this review attempts to provide an overview of current trends in conductive polymer functionalized graphene-based electrochemical sensors for the detection of contaminants in water. electrochemical sensors are very promising because they are portable, fast, inexpensive, have good stability, sensitivity and selectivity, and are also suitable for in situ measurements. conducting polymers are extensively studied for their superior properties, including tunable electrical properties, optical and high mechanical properties, easy synthesis and effortless fabrication, and high environmental stability compared to conventional inorganic materials. although conducting polymers have many limitations in their original form, these are overcome by functionalization with other materials. graphene has recently attracted much attention due to its potential as a conductive filler for polymer matrix composites. the synergistic effects of conductive polymer and graphene composites can be successfully used to overcome the limitations of each material. in this review, it has been suggested that methods for preparing graphene functionalized with conducting polymers include covalent and noncovalent functionalization. both have a number of advantages and disadvantages that need to be considered. we compared graphene functionalized with conducting polymers and its derivatives and their performances in electrochemical sensors for the detection of contaminants in water in terms of detection technique, linear range, detection limit, and target analytes. among different electrochemical techniques, swasv, dpasv, cv, dpv, current potential and amperometry were used for the detection of water pollutants with conductive polymer functionalized graphene-based sensors. these results spark interest and illuminate key areas where graphene and conductive polymer nanocomposites are 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open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1515/secm-2017-0199 https://doi.org/10.3390/ma13030528 https://doi.org/10.1007/s00289-016-1774-4 https://doi.org/10.1515/polyeng-2018-0106 https://doi.org/10.1166/sam.2011.1136 https://doi.org/10.1088/2053-1591/ab102b https://doi.org/10.1186/s11671-018-2515-4 https://doi.org/10.1039/c9ra08026k https://creativecommons.org/licenses/by/4.0/) comparative corrosion behavior of au50-ag25-pd25 and ni88.6-cr11.4 alloys utilized in dental applications http://dx.doi.org/10.5599/jese.1931 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1931 open access : : issn 1847-9286 www.jese-online.org original scientific paper comparative corrosion behavior of au50-ag25-pd25 and ni88.6-cr11.4 alloys utilized in dental applications irfan liaquat1, amer bashir ziya1, warda mushtaq1, athar ibrahim2, urva malik2, gao qilong3 and muhammad danial4, 1institute of physics, bahauddin zakariya university, multan-60800, pakistan 2department of metallurgical engineering and materials science, university of engineering and technology, lahore, pakistan 3school of physical and microelectronics zhengzhou university, zhengzhou 450001, china 4department of physics, university of education, lahore, pakistan corresponding author: irfanliaquat099@gmail.com received: june 8, 2023; accepted: july 26, 2023; published: july xx, 2023 abstract the electrochemical behaviour of alloys (au50-ag25-pd25 and ni88.6-cr11.4) was studied in fusayama's artificial saliva at ph 6.5 and 37 °c by using open circuit potential, electrochemical impedance spectroscopy, and potentiodynamic polarization measurements. electrochemical impedance spectroscopy results were simulated with an equivalent electrical circuit. after immersion in artificial saliva, surface characterization of samples was done using scanning electron microscopy connected with energy-dispersive spectroscopy. all obtained results revealed that au50-ag25-pd25 alloy is much more resistive than ni88.6-cr11.4 and can be recommended for the effective treatment of patients with dental prosthetics that have metal frameworks. keywords dental alloys; gold-silver-palladium; nickel-chromium; corrosion; electrochemical tests; surface morphology introduction in the history of dental alloys, precious and non-precious metals such as au, ni, cr, co, ag, and pd have often been preferred because of their favorable mechanical as well as biological properties [1]. a special preference in research is reserved for gold (au) alloys because of their high mechanical effects, low corrosion rate, and superb biocompatibility [2]. nevertheless, o'brian and german [3] suggested that the formation of a second phase would increase the corrosion rate of au-ag-cu ternary alloys. german et al. [4] suggested that nobility determines the corrosion resistance of gold-based alloys, but alloying with cu induces silver segregation, causing a lower corrosion rate. some researchers, however, have used non-precious dental alloys instead of gold http://dx.doi.org/10.5599/jese.1931 http://dx.doi.org/10.5599/jese.1931 http://www.jese-online.org/ mailto:irfanliaquat099@gmail.com j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion of alloys utilized in dental applications 2 alloys to reduce costs [5-8]. ni–cr alloys are also utilized as a substitute for noble dental alloys due to their poor cost and low corrosion rate [9]. however, the biological material of ni-based alloys in a mouth environment causes several problems owing to allergic reactions to ni ions. by adding different elements, the microstructure of ni-based alloys and their properties can be modified. thus, cr is commonly used to form a barrier, such that microstructures containing a cr-rich oxide layer prevent the release of ni ions from alloys. in general, ni-cr casting alloy shows a homogeneous structure but low corrosion resistance. if a small amount of be element is added to a ni-based casting alloy, the structure becomes non-homogenous, but corrosion resistance increases [10-12]. the corrosion resistance of ni-cr alloys depends on their composition and the formation of a passive oxide film. experimental studies revealed that alloys with 16 to 27 % cr form a protective oxide film on the surface and show a low corrosion rate [13]. commonly, the purpose of the protective oxide film is to act as an obstacle to the flow of electrons within the metal and the electrolyte [14]. the objective of this research is to compare the decomposition rate of precious au50-ag25-pd25 alloy with that of non-precious ni88.6-cr11.4 alloy by using electrochemical tests and surface characterization techniques to see their usefulness as dental alloys. throughout the text, au50-ag25pd25 alloy is referred to as saap, while ni88.6-cr11.4 alloy as snc. experimental specimens the conventional arc-melting method was used to prepare the samples saap and snc with the inert atmosphere of pure argon (ar). the alloys’ constituents were weighed and washed with alcohol in an ultrasonic bath and dried in open air prior to the arc melting procedure. the alloy ingot obtained after melting was re-melted 10 times to ensure the homogeneity of alloy components [15]. samples were cropped into the circular form of diameter of 10 mm, thickness of 2 mm and the exposed surface area was 0.79 cm2. before electrochemical testing, the alloys were polished with a carborundum sheet and then bathed with an ultrasonic bath for 10 min in acetone to remove impurities from the surface. one side of both samples was soldered with a copper wire in order to make an electrical connection with the potentiostat [16-18]. test solution fusayama artificial saliva was selected as an electrolyte with the following composition: 0.35 g l-1 nacl, 0.35 g l-1 kcl, 0.0906 g l-1 cacl2h2o, 0.70 g l-1 nah2po42h2o, 0.05 g l-1 na2s9h2o and 1.0 g l-1 urea. these constituents were soluble in purified water, and the ph value was adjusted up to 6.5 by using lactic acid [13,14,19]. corrosion monitoring electrochemical tests were performed using a gamry 3000 potentiostat and a three-electrode setup. the alloy samples served as working electrode(s), while a standard calomel electrode (sce) was used as the reference electrode and a graphite rod as the counter electrode. samples were stabilized for 3600 s under open circuit conditions and open circuit potential (ocp) values were measured as a function of time. electrochemical impedance spectroscopy (eis) was performed in the frequency range of 100000 to 0.01 hz with 5 mv sinusoidal voltage. the data were fitted by gamry echem analyst software, version 5.30. the results of eis are shown through nyquist plots and bode plots. the potentiodynamic polarization curves were obtained by scanning the potential from -1 to +1 v with respect to sce at a scanning rate of 1 mv s-1, corrosion potential (ecorr), corrosion current (icorr), i. liaquat et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1931 3 anodic tafel slope (a), and cathodic tafel slope (c) were determined by using extrapolation method [20,21]. corrosion current densities and corrosion rates were calculated from icorr values. surface analysis the surface analysis of testing samples was carried out through scanning electron microscopy (sem), linked with energy dispersive spectra (eds) for the purpose of detecting different elements. the protective oxide film formed on the surfaces of the sample was estimated by images obtained by sem [18]. results and discussion open circuit potential (ocp) in corrosion studies, ocp is a crucial parameter that determines the stability of alloy samples in the electrolyte. the behaviour of the oxide film on the surface of the sample is directly related to the variations observed in ocp. the ocp was measured as a function of time for both samples in artificial saliva, and the results are presented in figure 1 [13]. figure 1. open circuit potential vs. time trends of saap and snc samples in artificial saliva the experimental results demonstrate a clear increasing trend in ocp values for both samples as a function of time. however, figure 1 shows that the stability time of the snc sample was 600 s and that of saap sample was 3600 s, suggesting the different behaviour of the two samples. the shift in ocp values towards the positive side indicates the formation of a protective layer for both samples, which is consistent with previous studies [13,16]. for the saap sample, the open circuit potential is shifting to more positive values than for the snc sample during the entire exposure time, which can be explained by an oxide layer at the sample-electrolyte interface, which significantly protects the sample against the aggressiveness of chloride ions from the electrolyte, and results by a low tendency towards corrosion [22,23]. electrochemical impedance spectroscopy (eis) the electrochemical impedance spectroscopy was performed to investigate the corrosion resistance of alloys in artificial saliva at 37 °c. the results of eis were graphically described by nyquist and bode plots shown in figure 2. nyquist plots (zimag vs. zreal) of saap and snc samples shown in figure 2a have similar shapes, with semicircles covering almost all frequency scales. since chargetransfer resistance is related to the semicircle diameter, an increase in the radius of a semicircle http://dx.doi.org/10.5599/jese.1931 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion of alloys utilized in dental applications 4 suggests higher rct and, thus, a more protective oxide film formed on a sample. figure 2a shows that snc has a considerably smaller diameter than the saap sample, which suggests that the protection of the oxide film formed on the saap sample is stronger than on the snc sample [13,19]. bode plots (log modulus z and phase angle vs. log of frequency) give more details about the capacitive, inductive, and resistive behaviors of samples at various ranges of frequencies. bode magnitude plots presented in figure 2b show that at all frequencies, the saap sample possesses a greater modulus of impedance than snc, thus showing a better protective behaviour of oxide film. furthermore, it was observed that at the highest frequencies, log z showed a zero-sloping line characteristic for the electrolyte solution resistive response. at other frequencies, the impedance modulus exhibited a near -1 sloping line, suggesting domination of capacitive impedance response in the impedance spectrum. the corresponding bode phase angle responses in figure 2b show three different features, i.e., 0° at the highest frequencies, then an increase to higher (negative) values, attaining a maximum close to -68 and -52° for saap and snc samples respectively, and decrease back toward 0° in the low-frequency region. such bode plots (figure 2b) and single semicircle responses in the nyquist plot (figure 2a) are typical for a corrosion process showing resistance rct and double layer and/or oxide film capacitive response. as stated above, higher semicircle diameter and higher impedance modulus values observed for saap indicate that the saap sample has stronger corrosion resistance compared to the snc sample [24]. (a) (b) zreal /  f / hz (c) figure 2. eis results for saap and snc samples: (a) nyquist plots; (b) bode plots; (c) equivalent electrical circuit (eec) model -z im g /  i. liaquat et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1931 5 the most suitable equivalent electrical circuit (eec) for fitting eis spectra of samples was the randles model shown in figure 2c. in eec, rs is a solution resistance, rct is a charge transfer resistance and cpe is a constant phase element ascribed to oxide film/double-layer capacity. the double-layer capacitor (cdl) was replaced in the randles equivalent circuit with the constant phase element (cpe) to ensure a more accurate fit. it was calculated by equation (1). cdl = (yo rct1-n)1/n (1) in the above equation, y0 is the magnitude of the constant phase element (cpe), and n is the exponent, usually in the order of 0 to 1 and provides information regarding the roughness of the surface of samples. if the value of n is −1, 0, and 1, the cpe represents an inductor, resistor, and capacitor, respectively. in this study, n values are between 0.79 and 0.85. the decrease in the value of cdl for the saap sample compared to the snc sample (table 1) could be attributed to increasing the thickness of the protective layer, which suggests a higher corrosion resistance. eis results are given in table 1. a sample with a higher rct value exhibits a lower corrosion rate because of the lower release rate of ions into the fluid. the rct value of the saap sample was higher than that of snc because of the development of a more protective oxide film on its surface, which shows a higher corrosion resistance [14,20,25-27]. table 1. eis parameters of samples saap and snc sample rct / kω cm2 rs / ω cm2 n yo / 10-5 sn  -1cm2 cdl / f cm -2 saap 229.8 314.8 0.785 1.26 8.29 snc 145.7 129.9 0.84 3.78 12.77 potentiodynamic polarization the corrosion behavior of samples was investigated using potentiodynamic polarization curves shown in figure 3. by extrapolating method, the corrosion potential (ecorr / v), corrosion current density (jcorr / μa cm-2), and the anodic/cathodic (a, c / v decade-1) parameters might be calculated. the corresponding corrosion rate was calculated using equation 2 [28]. all calculated parameters are given in table 2. cr = ki (jcorr / ρ) ew (2) where cr is corrosion rate in mm year-1, ki= 3.27×10−3, ρ is density in g cm-3, and ew is the equivalent weight of sample material. (a) (b) figure 3. (a) potentiodynamic polarization curve for saap and snc samples, (b) corrosion rate http://dx.doi.org/10.5599/jese.1931 j. electrochem. sci. eng. 00(0) (2023) 000-000 corrosion of alloys utilized in dental applications 6 a low value of jcorr represents the formation of a strong oxide layer on metallic surfaces that offers a higher value of corrosion resistance by yielding a lower susceptibility to corrosion mechanisms. it is seen in table 2 that snc possesses a higher value of jcorr which suggests a smaller value of corrosion resistance by yielding a higher value of corrosion rate compared to the saap sample. in terms of corrosion potential (ecorr), a more electropositive value shows that a more protective oxide layer has developed on the surface of the sample and indicates a lower corrosion rate. based on this standard, the saap sample with a higher ecorr value shows a lower corrosion rate than snc as shown in figure 3b. [29]. table 2. potentiodynamic polarization parameters for saap and snc samples sample jcorr / μa cm-2 ecorr / mv a / mv dec-1 c / mv dec-1 cr, mm year -1 saap 1.32 -190 321 412 0.034 snc 2.82 -325 240 320 0.162 surface characterization of corroded alloys the surface morphology of both samples was studied by scanning electron microscope (sem) after performing corrosion testing in artificial saliva. smaller pores and fewer dark areas could be seen on the saap sample compared to snc, indicating the development of a more protective oxide film and stronger corrosion resistance. on the other hand, continuous propagation of pores and breaking of the oxide film indicate a steady decrease in corrosion resistance. sem micrographs in figure 4 reveal the minor-sized pores, major-sized pores, isolated pores, and correlated porous structures. in our study, the average size of the major pores of the samples was measured through image j software and determined as 13 μm and 60 μm for saap and snc alloys, respectively [30,31]. (a) (b) figure 4. sem images at magnification of 100 000 of: (a) saap sample; b) snc sample compositional analysis with eds eds analysis was performed to reveal the chemical composition of the samples. the results obtained are given in table 3. the presence of oxygen on the surface of the investigated alloys leads to the formation of a protective oxide layer, which protects the samples from corrosion attacks. in the case of the saap sample, a protective oxide layer of ag2o is formed, but in the case of snc, oxygen reacts with chromium to form a chromium oxide (cr2o3) layer. as shown in figure 5, the higher amount of oxygen content and the formation of oxides on the surface of the saap sample offers strong corrosion resistance compared to snc [32]. a high amount of carbon is present in the composition of the sample saap, it may be coming from the solder wires. i. liaquat et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1931 7 table 3. results of compositional analysis of sample surfaces by eds sample figure 5. histogram showing the amount of oxygen detected on the surface of saap and snc samples conclusions in this study, ni88.6-cr11.4 and au50-ag25-pd25 samples were immersed in an artificial saliva solution and the corrosion resistance was investigated by electrochemical techniques. the formation of a protective oxide layer was examined by ocp, tafel plots, and eis spectra. the obtained results were found complementary and in accordance with the results of sem and edxs analysis. • ocp values for saap alloy were higher compared to snc alloy, suggesting an increase in passive oxide thickness. • the electrochemical impedance spectroscopy showed that the rct value was higher and cdl was smaller for saap alloy as compared to snc alloy. • the polarization curves represented that the ecorr shifted to more noble values and the jcorr decreased significantly for saap alloy as compared to snc alloy. • the scanning electron microscopy analysis showed that saap alloy has smaller pores (13 m of average size) and a less dark area compared to snc alloy (60 m of average size), indicating the development of a more protective oxide film on the surface of saap alloy. • the energy dispersive spectroscopy analysis reveals that alloy saap contains a higher amount of oxygen as compared to alloy snc, suggesting the development of a more protective oxide layer on the surface of saap alloy. • alloy saap has better corrosion resistance compared to snc alloy and can be recommended for the favourable treatment of patients with dental prostheses. acknowledgements: one of the authors (i.l) acknowledges dr. amer bashir ziya from the institute of physics, bahauddin zakariya university, multan, for funding and supporting. references [1] f. golgovici, m. prodana, f. ionascu, g. demetrescu, a comparative electrochemical and morphological investigation on the behavior of nicr and cocr dental alloys at various temperatures, metals 11(2) 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https://doi.org/‌10.1007/s12034-010-0075-z https://doi.org/‌10.1007/s12034-010-0075-z https://doi.org/10.3390/ma14237185 https://doi.org/10.1007/s12666-020-01904-0 https://doi.org/10.3390/met8030188 https://doi.org/10.3390/ma14164635 https://creativecommons.org/licenses/by/4.0/) a general introduction to lithium-ion batteries: http://dx.doi.org/10.5599/jese.1544 591 j. electrochem. sci. eng. 13(4) (2023) 591-604; http://dx.doi.org/10.5599/jese.1544 open access : : issn 1847-9286 www.jese-online.org review paper a general introduction to lithium-ion batteries: from the first concept to the top six commercials and beyond phuoc-anh le institute of sustainability science, vietnam-japan university, vietnam national university-hanoi, hanoi, vietnam and faculty of textile science and technology, shinshu university, 3-15-1 tokida, ueda, nagano 386-0018, japan corresponding author: lephuocanh86@vnu.edu.vn received: october 14, 2022; accepted: january 24, 2023; published: february 8, 2023 abstract the birth of lithium-ion batteries (libs) is not a breakthrough scientific discovery overnight but a successor and continuous development of scientists for a long time based on the previous generation of electrochemical batteries. the development of libs and succeeding generations of batteries, however, is highly hopeful given the broad commercialization of libs during the previous ten years. intensified research is required to create next-generation libs with drastically better performance, including enhanced energy density, charging rate, lifespan, stability, and safety, in order to fulfill the rising demand for energy storage. research into libs and next-generation is currently in an explosive phase with the goal of overcoming the significant challenge posed by conventional libs that can keep up with the rapidly evolving needs of the electronics, mechanical, and automation industries, particularly electric vehicles. in this case, this tutorial review might offer a broad overview of libs as well as an optimistic look toward the upcoming generation. keywords energy storage; secondary batteries; cathode; anode; electrochemistry introduction one of the most technologically sophisticated rechargeable batteries on the market, lithium-ion batteries, has attracted a lot of attention and is now the standard mobile power source for portable electronic devices, which are widely used in various fields [1,2]. electric and hybrid vehicles, which demand next-generation libs with not only high power, high capacity, high charging rate, and long life, but also significantly enhanced safety performance and low cost, are another significant expanding market for libs [3,4]. rechargeable batteries are being used more frequently as a result of the rise in portable consumer electronics that run on batteries. additionally, it is anticipated that over the course of the projection period, demand for lithium-ion batteries will rise due to rising global sales of electric vehicles and the presence of market participants in the nation [5-8]. in order http://dx.doi.org/10.5599/jese.1544 http://dx.doi.org/10.5599/jese.1544 http://www.jese-online.org/ mailto:lephuocanh86@vnu.edu.vn j. electrochem. sci. eng. 13(4) (2023) 591-604 introduction to lithium-ion batteries 592 to meet the high performance and power density demands of gadgets, battery technology is constantly developing. in the upcoming years, electric and hybrid electric cars are anticipated to be the major consumers of lithium-ion batteries. over the forecast period, rising fossil fuel prices and increased public awareness of the advantages offered by battery-powered vehicles are expected to support the growth of the automotive application category [9-10]. before the advent of lithium-ion batteries, most electronic devices and portable compact devices used the first generation of dry batteries (primary generation). these are the first generations of batteries, such as lead-acid, zinc-carbon and nickel-cadmium (the first alkaline battery) batteries with low capacity, just enough for use in small compact devices [11-14]. for equipment that requires a larger power supply for the purpose of supporting voltage or creating a starting source, such as gasoline cars, generators, and mechanical machine tools, it is preferable to use batteries belonging to the second generation of nickel metal hydride batteries, such as nickel-iron (nife), nickelcadmium (nicd), and nickel metal hydride batteries (nimh) [13,14]. the period of the 1980s was the time of the explosion of rechargeable alkaline batteries with their presence in all areas of life, from the tv remote controls to portable music players, even egg beaters in the kitchen [15,16]. however, with low energy density, short lifetime due to leakage of a corrosive liquid that damages the electrodes, flammable when recharging, and high internal resistance that reduces the output devices, they have promoted scientific research and developed new generations of batteries. in the 1990s, the appearance of the first type of lithium battery (1913 by gilbert n. lewis), which led to the commercialization of libs in the 1970s, forever changed the game [17-22]. currently, libs have appeared in a variety of fields, particularly consumer electronics, especially portable devices such as remote controls, cell phones, laptop computers, digital cameras, portable healthcare devices, mechanical power tools, etc. [23,24]. also included are electric vehicles (electric cars, motorcycles, scooters, bicycles, and advanced wheelchairs) and stationary energy storage [25,26]. it is not an exaggeration to say that our lives today would be impossible to imagine without the use of batteries. furthermore, libs are seen as an alternative to gasoline and energy supply solutions, particularly in the current automotive industry revolution [27]. electric cars powered by libs have emerged as a development trend for the 2030s [28-30]. the goal of this review is to present a comprehensive picture of the introduction of lithium-ion technologies and several distinct types of libs. in addition, the discussion covers new developments and technological problems relating to the next-generation secondary battery systems. also included is statistical data about the present and upcoming industry demand. the adventure of lithium-ion batteries from the first concept to the 6 top commercial types the majority of portable compact electronics and electronic devices used the first generation (primary generation) of dry batteries prior to the development of lithium-ion batteries. these are the initial iterations of low-capacity batteries, such as lead-acid, zinc-carbon, and nickel-cadmium (nicd) batteries. batteries from the second generation of rechargeable alkaline batteries, including nickel-iron (nife), nickel-cadmium (nicd), and nickel metal hydride (nimh), are recommended for equipment that needs a larger power supply to support voltage or create a starting source, such as gasoline cars, generators, and mechanical machine tools [31,32]. rechargeable alkaline batteries became extremely popular in the 1980s and have been found everywhere until today [33]. however, the safety, hazard, and recycling problems of rechargeable alkaline batteries require the development of the next generation to overcome the previous barrier. p.-a. le j. electrochem. sci. eng. 13(4) (2023) 591-604 http://dx.doi.org/10.5599/jese.1544 593 the adventure of lithium-ion batteries began in 1965 when nasa tried to create a new generation of batteries using cuf2/lithium electrodes [34]. according to the flow of time (figure 1), in the 1970s, m. stanley whittingham introduced tis2 material as an electrode that showed high performance, but its high cost and toxic emissions of h2s were a great barrier to commercializetion [35,36]. then came the period of the 1980s–1990s, which was a golden time in the development history of lithium-ion batteries with announcements that changed the history of energy storage components; new electrode materials such as licoo2 and graphite leading to the first commercialization of lithium-ion batteries for portable electronic devices [37-38]. until the 2000s, lithium-ion batteries were developed and commercialized massively with many different types, such as limncoo2, limno2, lifepo4, etc. thus, three scientists michael stanley whittingham, john bannister goodenough, and akira yoshino were awarded the nobel prize in chemistry in 2019 for their pioneering contributions to lithium-ion battery research. figure 1. a general developing timeline of lithium-ion batteries from the 1960s to the 2030s lithium-ion batteries are classified in the industry based on their cathode chemical composition. currently, some most popular commercial lithium-ion batteries can be listed, such as lithium cobalt oxide (lco), lithium nickel oxide (lno), lithium manganese oxide (lmo), lithium nickel manganese cobalt oxide (nmc), lithium iron phosphate (lfp), lithium nickel cobalt aluminum oxide (nca), lithium titanate (lto) [39-43]. lithium-ion battery definition and operation principle the first battery was invented in 1800 by an italian physicist named alessandro volta. in an electrochemical reaction, electrons are moved from one material (referred to as an electrode) to another through an electric current. in general, a battery is a chemical device that turns chemical energy into direct current (dc) electric energy through an electrochemical reaction after storing electrical energy in the form of chemicals. throughout the 19th century, the first-generation battery was the only choice and held the dominant position. typical batteries for this era of dominance include lead acid, zinc-carbon, alkaline, nickel-cadmium, and the first generation of nickel metal hydride rechargeable batteries [44-46]. in fact, there has been more than five decades since people used first-generation batteries such as lead-acid, zinc-carbon, and alkaline batteries. however, with the rapid development of electronic, semiconductor, mechanical, and especially information technologies in the early 20th century, the old battery generations seemed overloaded with more advanced applications that required durability and convenience. cost-effective batteries such as zinc-carbon and nickel oxyhydroxide batteries are non-rechargeable and short-life, while lead-acid batteries are toxic, and alkaline http://dx.doi.org/10.5599/jese.1544 j. electrochem. sci. eng. 13(4) (2023) 591-604 introduction to lithium-ion batteries 594 batteries are unstable and leak electrolyte easily after a period of use, also known as the phenomenon of potassium compound leakage [47,48]. the first generation of rechargeable batteries, the nickel–cadmium battery, had a negative impact on the environment when using cadmium chemicals, which is a toxic heavy metal. the most stable battery in terms of price and performance in this first generation is the nickel-metal hydride batteries, which is rechargeable and offers high energy density. however, nickel-metal hydride batteries also have some major problems, such as self-discharge, memory effect, and relativity short cycle [48-50]. due to their low cost and simple production procedure, several types of batteries, such as lead-acid, zinc-carbon, and alkaline, have been utilized for basic and simple activities in portable electronic devices such as remote controls, children's toys, etc. however, recycling these batteries is a major undertaking. because of these factors, as well as the rapid advancement of science and technology toward the end of the nineteenth and beginning of the twentieth centuries, a new generation of batteries with high energy density, rechargeable, long lifespan, and high-temperature operation was required. that is why lithium batteries have stepped out of the stage curtain and changed the game. one typical lithium-ion battery cell is made by five main components: an anode, a cathode, a separator, an electrolyte, and two current collectors (positive and negative) (figure 2). the anode and cathode store lithium, while the separator creates a separate layer between the two electrodes, and the migration of lithium ions from the anode to the cathode depends on whether the battery is charging or discharging. herein, the mobility of the lithium ions in the anode generates free electrons, resulting in a charge on the positive current collector. after that, the electrical current passes from the current collector to the negative current collector through a powered device. the separator keeps electrons from flowing within the battery. figure 2. the working principle of one lithium-ion battery cell is depicted schematically rechargeable lithium-ion batteries operate on the basis of lithium's small atomic radius, light atomic weight, and low reductive potential, which give li batteries an advantage in terms of high energy density. electrode materials may be classified into three main categories based on the way that lithium reacts with them: conversion, intercalation, and others (figure 3) [51-55]. the charge/discharge process in conversion intercalation electrodes, such as lmo, lco typically involves a conversion reaction and is always accompanied by phase transitions, which increases the specific capacity but also leads additional unstability issues like structural collapse due to lithium dendrites, p.-a. le j. electrochem. sci. eng. 13(4) (2023) 591-604 http://dx.doi.org/10.5599/jese.1544 595 increasing temperature, growth of solid electrolyte interface (sei) on the anode, formation of electrolyte oxidation (eo) at the cathode leading poor reversibility. in contrast, intercalation electrode materials are frequently used in commercialized cells due to increase in durability. in the intercalation electrode, li ions intercalate and deintercalate reversibly, producing steady cycling but a decreased specific capacity (figure 3b) [54,55]. a b figure 3. general working principle of battery for (a) battery based on the conversion reaction, and (b) battery based on the intercalation reaction different chemistries of lithium-ion batteries the names of the various lithium battery types are derived from their active materials. the chemistry, cost, size, weight, capacity, and many other characteristics differ between different types of lithium ion batteries. until now, six commercial types have been widely used in a variety of fields, particularly electronic technology and electric vehicles, as well as electricity storage for solar cells and wind power. lithium cobalt oxide (licoo2) lco cobalt oxide cathode with a layered structure and a graphite anode make up lithium cobalt oxide batteries, commonly known as lithium-ion cobalt batteries. lithium ions migrate in two directions: during the discharge process, the lithium ions move from graphite to layer cathode and during the charge process, the lithium ions move from layer cathode to graphite anode. but because of changes in the solid electrolyte interface and anode thickness, the graphite electrode also reduces battery life (table 1) [39,41,56]. table 1. lco type characterizations, licoo2 cathode and graphite anode commercial time 1991 voltage, v 3.6 with working voltage window of 3.0 to 4.2 v per cell energy density, wh kg-1 150 – 200 typical charge (c-rate) 0.7–1 c. charge current above 1 c shortens battery life. discharge (c-rate) 1 c. discharge current above 1 c shortens battery life. cycle life, cycles 500 1000 thermal runaway, °c 150 applications consumer electronics (cell phones, laptops, tablets, etc.) http://dx.doi.org/10.5599/jese.1544 j. electrochem. sci. eng. 13(4) (2023) 591-604 introduction to lithium-ion batteries 596 the lco type has a short life expectancy, poor thermal stability, and constrained load capabilities. the graphite anode of the lco type herein limits the cycle life due to a changing solid electrolyte interface (sei), anode thickening, and lithium plating [56]. lithium manganese oxide (limn2o4) lmo lithium manganese oxide, also known as lithium manganate, is a spinel-structured cathode material that can be discharged at high rates. this structure produces a three-dimensional structure that enhances thermal stability and safety while enhancing ion flow, decreasing internal resistance, and increasing current handling [57]. although lithium manganese oxide batteries have a lower cycle and lifespan than other lithium-ion battery types, they are known for their great temperature stability (table 2) [58]. table 2. lmo type characterizations, limn2o4 cathode and graphite anode commercial time 1996 voltage, v 3.7 with working voltage window of 3.0 4.2 v per cell energy density, wh kg-1 100 – 150 typical charge (c-rate) 0.7–1 c with maximum until 3 c, and charges to 4.2 v discharge (c-rate) 1 c with maximum until 10 c for special versions cycle life, cycles 300–700 thermal runaway, °c 250 applications power tools, medical devices, some kind of electric vehicles lithium nickel manganese cobalt oxide (linimncoo2) nmc the most popular variety of libs, the nmc battery type, combines the benefits of nickel, manganese, and cobalt cathode. nickel and manganese together form the basis of this specific sort of lithium-ion battery. while manganese has low internal resistance but high specific energy, nickel is noted for having high specific energy but poor stability. the right combination of both metals in a cell can produce the necessary balance of both metals' qualities (table 3) [42,59]. nickel, manganese, and cobalt are frequently combined in equal amounts to form the cathode combination, or 1-1-1. by lowering the cobalt concentration, this combination keeps some important performance criteria while lowering costs. although nmc battery types offer a long lifespan and good energy options, they are also quite expensive and less safe than other lithium-ion battery types [59]. table 3. nmc type characterizations, linimncoo2 cathode and graphite anode commercial time 2008 voltage, v 3.6 and 3.7 with working voltage window of 3.0 4.2 v per cell energy density, wh kg-1 150–220 charge (c-rate) only from 0.7 to 1 c discharge (c-rate) 1c cycle life, cycles 1000–2000 thermal runaway, °c 210 applications power tools, medical devices and electric vehicles lithium iron phosphate (lifepo4) lpo lithium iron phosphate batteries, based on nanoscale phosphate cathode material, have excellent electrochemical performance with low resistance [60]. the highest current rating, p.-a. le j. electrochem. sci. eng. 13(4) (2023) 591-604 http://dx.doi.org/10.5599/jese.1544 597 prolonged cycle life, better thermal stability, increased safety, and higher abuse tolerance are the most significant benefits (table 4) [39,41,60]. when considering their lengthy battery lives, the lpo type is frequently the most affordable choice. on the other hand, compared to other lithium battery types, the lpo has a lower voltage and, therefore, less energy. table 4. lpo-type characterizations, lifepo4 cathode and graphite anode commercial time 1996 voltage, v 3.2 with working voltage window of 2.5 3.65 v per cell energy density, wh kg-1 90–120 charge (c-rate) 1 c discharge (c-rate) 1 c cycle life, cycles 2000 thermal runaway, °c 270 applications power tools lithium nickel cobalt aluminum oxide (linicoalo2) nca lithium nickel cobalt aluminum oxide is a cathode material used in lithium-ion batteries that provide highly thermally stable, long lifecycle, good specific power, and high specific energy device (table 5) [42,61]. aluminum doping stabilizes the thermal and charge transfer resistance of lithiumnickel cobalt oxide. due to research for the car industry, this kind is rarely used in consumer applications. the nca battery type has a high cost and low safety compared to other lithium-ion battery types, despite its high capability and endurance [61]. table 5. nca type characterizations, linicoalo2 cathode and graphite anode commercial time 1999 voltage, v 3.6 with working voltage window of 3.0 4.2 v per cell energy density, wh kg-1 200-260 charge (c-rate) 0.7 c discharge (c-rate) 1 c typical cycle life, cycles 500 thermal runaway, °c 150 applications automotive industry lithium titanate (li2o–tio2 composition line) lto the rechargeable battery type known as lithium titanate or lithium-titanium-oxide batteries has the benefit of a quicker charging time than other types but the limitation of having a significantly lower energy density (table 6) [39,51,62]. table 6. lto type characterizations, lmc or nmc cathode and li2tio3 anode commercial time 2008 voltage, v 2.4 with working voltage window of 1.8 2.85 v per cell energy density, wh kg-1 50–80 charge (c-rate) 1 c typical with maximum until 5 c discharge (c-rate) from 1 c to maximum 10 c cycle life, cycles 3000–7000 thermal runaway, °c 100 applications automotive industry, electric power train http://dx.doi.org/10.5599/jese.1544 j. electrochem. sci. eng. 13(4) (2023) 591-604 introduction to lithium-ion batteries 598 in this type of battery, lithium-titanium oxide replaces graphite in the anode and forms a spinel structure, whereas the cathode might be lithium manganese oxide (lmo) or lithium nickel manganese cobalt oxide (nmc) [62]. introduction of lithium-ion batteries commercial form lithium-ion batteries are available in a range of forms, including cylindrical cell, prismatic cell, and pouch cell, depending on the intended use and device design (figure 4). although there are obviously distinct forms for various applications, the same essential concepts are employed. figure 4. three main types of lithium-ion batteries: (a) cylindrical cells, (b) prismatic cells, and (c) pouch cells lithium cylindrical cell batteries a lithium cylindrical battery is a cell packed and rolled with thin flat electrodes inside a rigid cylinder. batteries are commonly made from a sequence of these cylindrical cells since they are widely employed in a range of applications. the three standard sizes are currently 18×65 mm, 21×50 mm, and 26×65 mm. because they are compact and spherical, lithium cylindrical cells have the benefit of being stackable in a variety of devices. additionally, this type may cost-effectively employ the preceding battery's manufacturing method. the drawbacks of this form are that it is heavy and underpowered (table 7) [63]. lithium prismatic cell batteries introduced in the early 1990s, lithium prismatic cell batteries have a higher power density than cylindrical cell batteries. they are generally flat or rectangular in shape and are frequently employed to power electronic devices and automobiles. they also provide superior performance in colder climates and are less vulnerable to vibration damage, so they are an excellent choice for building large battery packs for energy storage [64]. however, they have the disadvantage of being more expensive to manufacture than cylindrical cells. currently, lithium prismatic cell batteries are predominantly used in cell phones, laptops, portable medical tools, and other portable electronic devices (table 7) [63,64]. lithium pouch cell batteries lithium pouch cells were first commercialized in 1995, with the construction of conductive foil tabs welded to the electrode and sealed to the pouch to convey the positive and negative terminals to the outside. because it has the maximum capacity among battery packs, the pouch cell makes the most effective use of space. lithium-ion pouch cell batteries are popular due to their lightweight, simple, and flexible architecture. however, this format uses a polymer bag cover, which is less durable and mechanically strong in comparison with a cylindrical cell using a metal cover. p.-a. le j. electrochem. sci. eng. 13(4) (2023) 591-604 http://dx.doi.org/10.5599/jese.1544 599 furthermore, the gas is released by the expansion or swelling of pouch cells over a long period of charge-discharge, resulting in a shorter lifespan and making it easily flammable. temperature and humidity are also big factors that have an effect strongly on pouch cells. despite some disadvantages, lithium pouch cell batteries are one of the most popular formats and are available in a variety of sizes and thicknesses depending on the applications (table 7) [62,65]. table 7. comparison of three main lithium-ion batteries format type main advantages main disadvantages cylindrical cell • low-cost due to using the preceding battery's manufacturing method. • mechanical stability due to cylindrical shape with metal protected cover. • large scale systems using many small cells will easily dissipate heat because of the space between the cylindrical cells. • cylindrical cells have a low packing density due to the round cross-section of the cell, preventing full use of the available space. • increase the weight and complexity of large scale cell (using a large number of small cells). prismatic cell • optimal utilization of battery packs and easily increases the size. • not swelling after long time working. • higher energy density in comparison with the other types. • expensive to manufacture. • the interior electrodes are easily prone to expansion and contraction, which might result in deformation and an internal short circuit. pouch cell • lowest weight with lowest thickness • easily fitting with the limitation available space in a product. • poor heat dissipation ability is easily affected by high temperatures. • decreasing lifespan due to gas release during long cycle charge-discharge. the current lithium-ion battery technology almost achieves maximum energy density. the great variety of cell designs and chemistries available allows for fine-tuning of characteristics like rapid charge-discharge and a wide temperature working window. additionally, lithium-ion batteries exhibit benefits, including very low self-discharge, an extremely long lifespan, and cycling capabilities that can generally withstand thousands of charging and discharging cycles. lithium-ion battery technology is anticipated to hit an energy limit during the next few years using existing materials and cell designs. however, relatively recent research into novel materials should be able to overcome current constraints, which can store more lithium in positive and negative electrodes. additionally, the rarity and importance of raw materials are taken into consideration with these novel compounds. future beyond lithium-ion batteries although li-ion battery technology has been around for 50 years, it is regrettably getting close to its theoretical limitations and isn't keeping up with advances in electronic technology, which call for longer lifespans, higher energy densities, and lower prices. although the search for a better battery has always been continuous, there is now a sense of urgency since battery shortages are stifling not only consumer devices but also the clean energy sector's efforts to develop electric vehicles and associated technologies. some of the next battery generations are considered to aim at alternative lithium-ion batteries in the near future, such as lithium-sulfur batteries, sodium-ion batteries, metalair batteries, and solid-state batteries. http://dx.doi.org/10.5599/jese.1544 j. electrochem. sci. eng. 13(4) (2023) 591-604 introduction to lithium-ion batteries 600 lithium-sulfur batteries lithium-sulfur batteries are rechargeable types like lithium-ion batteries in which there are no host structures inside: the lithium anode is consumed during the discharge process, and sulfur is converted into a variety of chemical compounds; this process is reversed while charging. due to the use of sulfur in the positive electrode, lithium-sulfur batteries have the benefit of being lighter and having a plentiful precursor. following the theoretical simulation, the lithium-sulfur batteries have nearly four times greater energy density than lithium-ion batteries. however, lithium-sulfur batteries face some major limitations of high cost and decreasing lifetime due to the charging process in one lithium-sulfur cell making a chemical deposition on the electrodes [66]. currently, various kinds of solid-state electrolytes have been studied for combination in lithium-sulfur batteries in order to overcome this chemical deposition problem. sodium-ion batteries sodium-ion batteries are also one type of rechargeable battery as lithium-ion batteries where replacing lithium electrode and li+ ions with sodium electrodes and na+ ions electrolyte as the charge carriers. sodium-ion batteries have much attraction due to large natural sodium resources, which decrease the price and lower the average cost per kilowatt hour of capacity. the idea of using hard carbon anode and sodium cathode brings the advantages of cost-effectiveness, leading to scaled production. however, sodium-ion batteries need to improve some properties, such as energy density, short cycle life and incomplete industrial chain [67]. metal-air batteries like lithium-ion batteries, metal-air batteries are secondary generation cells. the positive electrode (external cathode of ambient air) in metal-air batteries is made of carbon and covered with various precious metals to react with oxygen. metal anodes (the negative electrode) are constructed using a variety of metals, including zinc, aluminum, magnesium, and lithium. theoretically having a substantially higher energy density than lithium-ion batteries, metal-air batteries are commonly promoted as a next-generation electrochemical energy storage option for grid energy storage or electric vehicle applications [68]. however, because of difficulties with the metal anode, air cathode, and also aqueous electrolyte, they have not reached their full potential. before metalair batteries can be used on a large scale and become a practical reality, these issues must be appropriately addressed. solid-state batteries solid-state batteries are interesting novel energy storage devices using solid electrolytes instead of aqueous and gel polymer electrolytes, which avoid electrolyte leak and explosion [69]. therefore, there is no need for safety-related components, which frees up more space. currently, solid-state batteries have been focused strongly and firstly commercialized for electric vehicles as testing products. with higher energy density and larger cells in the same space as lithium-ion batteries, solid-state batteries may take a piece of the global market in the near future. the solid battery is in intensive research and very small-scale testing for the purpose of being widely developed in the future. currently, solid batteries still have some major problems that need to be solved, such as high production costs, high resistance in the contact area between the electrode and the electrolyte, and the phenomenon of dendrite in the type of lithium solid-state battery [70]. p.-a. le j. electrochem. sci. eng. 13(4) (2023) 591-604 http://dx.doi.org/10.5599/jese.1544 601 conclusions in summary, this review provided an overview of the development of lithium-ion batteries from the first concept to the six most popular types, with their advantages and limitations. additionally, the development of next-generation lithium-ion batteries is also covered, with a favorable outlook on their future in the domains of rigid energy storage and electric vehicles. nowadays, batteries have always been a crucial component of various designs, but notably in electrical technologies and grids for energy storage. research has been ongoing for years in order to boost the energy density and longevity of batteries following the rise in portable electronic equipment, from handheld gadgets to industrial measuring instruments, which has created a need for higher energy densities. battery capacities frequently take center stage in technological advancements while researchers have been attempting to improve battery technology. however, electronic technology continues to grow more rapidly and needs more energy and power than ever before. as the electronic industry has grown, lithium-ion batteries have developed ever-higher energy densities. the operational time of a battery is proportional to its energy density. to reach better densities, battery industry experts have constantly modified the chemistries and designs of the technology. currently, there are various types of novel materials have been studied as the new electrode and new electrolyte aiming to not only reduce the cost but also increase the energy density and expand the lifespan. acknowledgements: this work belongs to series of research on batteries and recycling technology that has been led by dr. phuoc-anh le. the authors wish to acknowledge the support given by the vietnam-japan university, vietnam and shinshu university, japan. references [1] d. deng, energy science and engineering 3(5) (2015) 385-418. https://doi.org/10.1002/ese3.95 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https://doi.org/10.1002/aenm.202102448 https://doi.org/10.1149/1945-7111/ac4e11 https://doi.org/10.1021/cr500062v https://doi.org/10.1021/acsenergylett.0c02181 https://doi.org/10.1016/j.matt.2019.05.008 https://doi.org/10.1038/s41578-019-0165-5 https://doi.org/10.1016/j.susmat.2021.e00297 https://creativecommons.org/licenses/by/4.0/) fe3o4/go nanocomposite modified glassy carbon electrode as a novel voltammetric sensor for determination of bisphenol a http://dx.doi.org/10.5599/jese.1521 577 j. electrochem. sci. eng. 12(4) (2022) 577-578; http://dx.doi.org/10.5599/jese.1521 open access : : issn 1847-9286 www.jese-online.org editorial advances in processing and characterization of conversion coatings jelena bajat faculty of technology and metallurgy, university of belgrade, karnegijeva 4, 11120 belgrade, serbia email: jela@tmf.bg.ac.rs there is an ongoing demand for metal surface properties improvement. new films with altered physical and chemical properties that provide the substrates with new functionalities are increasingly examined and widely disseminated. one of the broadly used traditional surface modifications is conversion coatings (cc), formed in situ on a metal surface by the chemical or electrochemical reaction of the metal substrate with anions in the solution. new synthesis routes and performances of ccs are emerging with requests spanning from better appearance, increased adhesion, hardness, wear or lubricity, and corrosion protection. the present special issue of the journal of electrochemical science and engineering presents contributions on various kinds of non-toxic conversion coatings on different metal substrates: steel, stainless steel, zinc, copper, aluminum, and the broad range of their properties and applications. different functions require surface science and surface engineering at a molecular level and are related to the chemical, phase and morphological nature of their surfaces. the effect of stainless steel annealing on the adhesion of tio2 spray coating was discussed in [1]. another novel approach to enhancing adhesion was proposed in [2], by a two-step synthesis of porous indium phosphide with nickel oxide crystallites, as a potential material in modern electronic instrumentation. the influence of ultrasound intensities on hydrodynamic conditions in copper coatings electrodeposition was analyzed in [3]. besides pt, as an inert electrode, the thiophene synthesis by electropolymerization was successfully attained also on various oxidizable metals: ti, ni and stainless steel, via soluble oligomers of thiophene, as described in [4]. superhydrophobicity, showing strong water-repellency, is an important property in many applications. the production of metal textured surfaces with uniform superhydrophobic properties using laser texturing was presented in [5]. in the research in [6], a high-velocity oxy-fuel thermal spraying technique was applied to deposition wc-10co-4cr coatings reinforced with yttria-stabilized zirconia (y2o3/zro2; ysz) nanoparticles on steel turbine. the second group of articles presented in this special issue is related to new achievements in tailoring ccs for corrosion protection. the sacrificial znfe alloy coating electrodeposited on steel was further modified by a hybrid sol-gel silane coating [7], while electrodeposited zn coating on steel was modified by electroless cerium oxide layer deposition and additional sealing in boiling http://dx.doi.org/10.5599/jese.1521 http://dx.doi.org/10.5599/jese.1521 http://www.jese-online.org/ mailto:jela@tmf.bg.ac.rs j. electrochem. sci. eng. 12(4) (2022) 577-578 conversion coatings 578 water [8]. ce was also used in [9] as a dopant in hydrotalcite-type protective chemical conversion coatings on aluminium alloy aa 7075. finally, the effect of tannic acid embedded in polystyrene coatings either directly or within mesoporous silica-nano-containers on the corrosion protection of steel and zinc substrates was discussed in [10]. references [1] n. i. b. omar, s. b. mohamed, y. b. yusuf, t. b. a. rahim, z. b. mustafa, s. b. ismail, i. a. b. a. bakar, s. selvamani, a preliminary study into the effect of oxide chemistry on the bonding mechanism of cold-sprayed titanium dioxide coatings on sus316 stainless steel substrate, journal of electrochemical science and engineering 12(4) (2022) 579-591. https://doi.org/10.5599/jese.1423 [2] y. suchikova, i. bohdanov, s. kovachov, a. lazarenko, i. bardus, a. dauletbekova, i. kenzhina, a. i. popov, the synthesis of porous indium phosphide with nickel oxide crystallites on the surface, journal of electrochemical science and engineering 12(4) (2022) 593-601. https://doi.org/10.5599/jese.1301 [3] i. mladenović, j. lamovec, d. vasiljević radović, v. radojević, n. d. nikolić, influence of intensity of ultrasound on morphology and hardness of copper coatings obtained by electrodeposition, journal of electrochemical science and engineering 12(4) (2022) 603615. https://doi.org/10.5599/jese.1290 [4] m. bouabdallaoui, z. aouzal, a. q. el guerraf, m. bazzaoui, e. a. bazzaoui, electrodeposition of strictly alpha,alpha’-polythiophene chains on oxidizable metals in acidic concentrated aqueous media, journal of electrochemical science and engineering 12(4) (2022) 617-637. https://doi.org/10.5599/jese.1201 [5] v. kumar, r. verma, h. k. bairwa, fabrication of superhydrophobic surfaces by laser surface texturing and autoxidation, journal of electrochemical science and engineering 12(4) (2022) 639-649. https://doi.org/10.5599/jese.1260 [6] r. kumar, d. bhandari, k. goyal, mechanical and microstructural characterization of yttriastabilized zirconia (y2o3/zro2; ysz) nanoparticles reinforced wc-10co-4cr coated turbine steel, journal of electrochemical science and engineering 12(4) (2022) 651-666. https://doi.org/10.5599/jese.1190 [7] c. arrighi, y. paint, c. savall, j. creus, m. olivier, improvement of the corrosion resistance of electrodeposited zn-fe by sol-gel conversion films, journal of electrochemical science and engineering 12(4) (2022) 667-683. https://doi.org/10.5599/jese.1282 [8] s. v. kozhukharov, c. girginov, n. boshkova, a. tzanev, impact of the final thermal sealing of combined zinc/cerium oxide protective coating primers formed on low carbon steel, journal of electrochemical science and engineering 12(4) (2022) 685-701. https://doi.org/10.5599/jese.1297 [9] a. petica, a. c. manea, g. mihai, i. nicola, v. ceara, l. anicai, cerium-based conversion films developed on lial-layered double hydroxide coatings for corrosion protection of aa7075 aluminum alloys, journal of electrochemical science and engineering 12(4) (2022) 703-719. https://doi.org/10.5599/jese.1305 [10] j. both, g. s. szabo, g. katona, l. m. muresan, anti-corrosive polystyrene coatings modified with tannic acid on zinc and steel substrates, journal of electrochemical science and engineering 12(4) (2022) 721-730. https://doi.org/10.5599/jese.1293 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.5599/jese.1423 https://doi.org/10.5599/jese.1301 https://doi.org/10.5599/jese.1290 https://doi.org/10.5599/jese.1201 https://doi.org/10.5599/jese.1260 https://doi.org/10.5599/jese.1190 https://doi.org/10.5599/jese.1282 https://doi.org/10.5599/jese.1297 https://doi.org/10.5599/jese.1305 https://doi.org/10.5599/jese.1293 https://creativecommons.org/licenses/by/4.0/) {electrochemical oscillation of vanadium ions in anolyte} doi:10.5599/jese.406 139 j. electrochem. sci. eng. 7(3) (2017) 139-144;doi: http://dx.doi.org/10.5599/jese.406 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical oscillation of vanadium ions in anolyte hao peng1,2,, zuohua liu2, changyuan tao2, 1college of chemistry and chemical engineering, yangtze normal university, chongqing, china; 2college of chemistry and chemical engineering, chongqing university, chongqing, china; corresponding authors e-mail: cqupenghao@126.com; liuzuohua@cqu.edu.cn; taocy@cqu.edu.cn received: july 11, 2017; revised: july 27, 2017; accepted: july 27, 2017 abstract periodic electrochemical oscillation of the anolyte was reported for the first time in a simulated charging process of the vanadium redox flow batteries. the electrochemical oscillation could be explained in terms of the competition between the growth and the chemical dissolution of v2o5 film. also, the oscillation phenomenon was possible to regular extra power consumption. the results of this paper might enable new methods to improve the charge efficiency and energy saving for vanadium redox flow batteries. keywords vanadyl sulphate; oscillation phenomenon; redox flow battery introduction vanadium is an important metal used for manufacturing iron, steel non-ferrous metals, and petrochemicals because of its excellent physicochemical properties [1-4]. it is applied in energy storage [5]. the all-vanadium redox flow batteries (vrfbs) proposed by skyllas-kazacos et al. as an efficient energy-storage systems [6] have attracted attention for their long life time, flexible design, high efficiency and fast response time [7]. the main components of vrfb are electrolytes, membranes [8-10] and electrodes [11-13]. the reaction that occurs at the positive electrode of the vrfb during charge and discharge process is defined as follows: 2   charg e+ + + 2 2 discharg e vo h o e vo + 2h (1) the electrical energy is transferred to the chemical energy during the charge process and stored in the electrolytes. the charge process performed in flowing systems is driven far from equilibrium. calculations based on prigogine’s dissipative structure equilibrium indicated that the kinetics of electrochemical system far from equilibrium is much more complicated than that near the equilibrium [14,15]. http://dx.doi.org/10.5599/jese.406 http://www.jese-online.org/ mailto:cqupenghao@126.com mailto:liuzuohua@cqu.edu.cn j. electrochem. sci. eng. 7(3) (2017) 139-144 oscillation of vanadium ions 140 electrochemical oscillation is an interesting self-organizing phenomenon in which the current or potential periodically fluctuates in constant external electric field [16,17]. oscillations existed in systems far from equilibrium and are interesting phenomena of theoretical significance. there are, however, only few studies that reported influence of oscillation on the electrolytic efficiency of a practical electrolysis process. in this work, we have reported the periodic electrochemical oscillation of the anolyte in a simulated charging process of the vrfb. a connection between the electrochemical oscillation phenomenon and the reaction mechanism was also investigated. experimental the experimental process was nearly the same as presented before [10]. the anolyte was composed of voso4 (varied from 0.5 m to 1.5 m) and h2so4 (2 m), and the catholyte was h2so4 (2 m) with the same volume. the anolyte and catholyte were separated with a nafion membrane. the anode and cathode used in the experiments were platinum slices (1.52 cm) in which no other unexpected side reaction occurred except water oxidation. they were subjected to ultrasonic cleaning in de-ionized water for 30 min to remove residues before testing. electrochemical tests were carried out on chi660d electrochemical workstation (shanghai chenhua, china). all chemicals were of analytical grade and de-ionized water was used for all experimental solution. results and discussion to explore electrochemical oscillation in a simulated charging process of the vrfbs, electrochemical tests were carried out on chi660d electrochemical workstation with the anolyte of 1 m voso4 + 2 m h2so4, and the catholyte of 2 m h2so4. as shown in figure 1a, the period of electrochemical oscillation emerged during the charging process. the potential increased quickly at the beginning, and then the periodic electrochemical oscillation emerged. after then, the oscillation period slowly decreased, while the amplitude increased within the initial 500 s and then decreased what is shown in figure 1b. figure 1. oscillation phenomenon at j = 0.07 a cm-2 a) the potential-time curve; b) time dependence of oscillation period and amplitude the effect of the charge current density on the threshold potential value scope and oscillation features of electrochemical oscillation are shown in figure 2. before the test, all electrodes were subjected to ultrasonic cleaning in de-ionized water for 30 min to remove residues. when charge current density < 0.0067 a cm-2, no electrochemical oscillation was observed. in the range of 0.01 0.063 a cm-2 (boxed in figure 3a), the electrochemical oscillation was observed. the results h. peng et al. j. electrochem. sci. eng. 7(3) (2017) 139-144 doi:10.5599/jese.406 141 of chronopotentiometry with current ramp experiments are used to judge the appearance of electrochemical oscillation. it is shown in figure 3b that induction period decreased with the increasing of charge current, which suggested that the electrochemical oscillation was easy to occur at high charge current within the effective range. in addition, the oscillation period was also decreased with charge current and the trend was consistent with the induction period. figure 2. the potential-time curve at different charge current densities figure 3. a) chronopotentiometry with current ramp results; b) oscillation and induction periods in dependence on charge current density if no electrochemical oscillation occurred, the average power consumption (pa) was usually calculated using the product of potential and current described by equation (2): pa = v i (2) if electrochemical oscillation occurred, the calculation of pa was more complicated. at constant current condition, the calculation of average power consumption could be calculated through the integration method shown by equation (3) [17]: j. electrochem. sci. eng. 7(3) (2017) 139-144 oscillation of vanadium ions 142 )aver baseline baseline= + ( d /p v i i v v t t (3) the term i(v-vbaseline)dt/t is denoted as the alternating potential part of power consumption (pe). the results shown in figure 1 indicated that the potential have increased at a constant current, what resulted in an extra power and energy consumption. the effect of concentration of voso4 on oscillation was also investigated, and the results are shown in figure 4. the electrochemical oscillation did not occur at low concentration (figure 4a), while at increased concentrations, electrochemical oscillations occurred (figure 4b-d). these results indicated that the electrochemical oscillation was not a result of the o2 coverage and release but a special reaction in the anolyte. figure 4. the potential-time curve in different anolyte at j= 0.07 acm-2: a) 0.5 m voso4+2 m h2so4; b) 0.75 m voso4+2 m h2so4; c) 1.0 m voso4+2 m h2so4; d) 1.5 m voso4+2 m h2so4 in order to investigate the reaction mechanism of the electrochemical oscillation, the solvent was also changed. as is shown in figure 5a, no electrochemical oscillation phenomenon occurred in the water solution and red film, probably v2o5, is produced around the anode (figure 5c). as the solvent was changed to h2so4, the electrochemical oscillation was observed (figure 5b) and the red film disappeared (figure 5d). during the charging process described by equation (1), the vo2+ was oxidized to vo2+ and in water solution hydrolyzed to form v2o5 film according to the equation (4). this v2o5film becomes absorbed on the anode in the water solution. in the h2so4 solution, however, the v2o5 is dissolved due to the presence of large amount of h+ what can be described by equation (5). 2vo2+ + h2o  v2o5 + 2h+ (4) v2o5 + 2h+ 2vo2+ + h2o (5) h. peng et al. j. electrochem. sci. eng. 7(3) (2017) 139-144 doi:10.5599/jese.406 143 figure 5. the potential-time curve in different anolytes at j= 0.07 acm-2: a) 1 m voso4+ water; b) 1 m voso4+ h2so4 c) anode in the water solution; d) anode in h2so4 solution from all results and discussion mentioned above, the electrochemical oscillation could be explained in terms of the competition between the growth and the chemical dissolution of v2o5 film [18,19]. conclusions in summary, the periodic electrochemical oscillation was first observed in the charging process of anolyte in simulated vrfb. the chronopotentiometry with current ramp results could be used to judge the appearance of electrochemical oscillation. the electrochemical oscillation could be explained in terms of the competition between the growth and the chemical dissolution of v2o5 film in the h2so4 solution. it was possible to regular the extra power consumption resulted by the electrochemical oscillation. this work might provide new focus on the charging process of the vrfbs and guide for new methods in energy saving. acknowledgements this work was supported by the nation 973 projects of china (no. most2011cb933300), the natural science foundation of china (no. 51274261). references [1] h. peng, z. liu, c. tao, journal of research and development 4 (2016) 139 [2] h. peng, , z. liu, c. tao, journal of environmental chemical engineering 3 (2015) 1252-1257 [3] z. liu, a. nueraihemaiti, m. chen, j. du, x. fan, c. tao, hydrometallurgy155 (2015) 56-60 [4] h. peng, z. liu, c. tao, energy and fuels 30 (2016) 7802-7807 [5] c. ding, h. zhang, x. li, t. liu, f. xing, the journal of physical chemistry letters 4 (2013) 1281-94 [6] e. sum, m. rychcik, m. skyllas-kazacos, journal of power sources 16 (1985) 85-95 [7] j. kim, j.-d. jeon, s.-y. kwak, electrochemistry communications 38 (2014) 68-70 [8] d. chen, m. a. hickner, e. agar, e. c. kumbur, electrochemistry communications 26 (2013) 37-40 [9] o. ryota, m. hisayoshi, u. mikito, electrochemistry communications70 (2016) 5-7 [10] h. peng, z. liu, c. tao, 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ślepski, k. darowicki, m. orlik, journal of solid state electrochemistry 15 (2011) 2311-2320 [19] m. t. gorzkowski, m. orlik, journal of solid state electrochemistry15 (2011) 2321-2330 ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) cathode reaction models for braga-goodenough na-ferrocene and li-mno2 rechargeable batteries http://dx.doi.org/10.5599/jese.1882 1 dj. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1882 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical and microbial treatment of bromophenol blue dye in aqueous solution john o. anyanwu, kanayo l. oguzie, toochukwu e. ogbulie, christogonus o. akalezi, and emeka e. oguzi  africa centre of excellence in future energies and electrochemical systems (ace-fuels), federal university of technology owerri, pmb 1526, owerri, imo state, nigeria corresponding author:  emeka.oguzie@futo.edu.ng; tel. +2348037026581 received: may 8, 2023; accepted: july 2, 2023; published: july 15, 2023 abstract bromophenol blue (bb) is an important triphenylmethane derivative widely used in research and industry as a dye or colorant for silk, leather, and drugs. bb has a high proclivity for water contamination because of its high solubility and stability, causing damage to living organisms even in low quantities. this study assessed the feasibility of electrochemical and biological for the decolorization of bb using graphite-copper electrode and indigenous microorganisms, respectively. it was found that effectiveness of electrochemical treatment improves with higher electrolysis time, current density, and electrolyte concentration, drops with rising ph, and shows a nonlinear pattern with temperature. the microbial strains identified as acinetobacter baumaniu, serratia marcescens, aspergillus niger, aspergillus flavus, bacillus megaterium, rhizopus stolonifer, and bacillus subtilis performed poorly, irrespective of the dye/mineral salt ratio. the electrochemical technique was much more effective for decolorizing bromophenol blue dye-contaminated water. the computational results showed clearly that bromine atoms are the most susceptible sites for attack by oxidizing species and, thus, the onset of bb-decolorization. keywords wastewater treatment; dye removal efficiency; quantum chemical calculations; graphite electrodes; dye pollution; environmental remediation introduction dyes are an important class of pollutants, and once they reach the water bodies, remediation becomes difficult due to their synthetic origins and complicated molecular structures. dyecontaining wastewater are steadily generated by textile industries [1]. bromophenol blue dye (bb) is an important triphenylmethane derivative widely used in research and industry as a dye/colorant for silk, leather, and drugs, as well as a laboratory indicator, biological stain, and color maker to keep track of the polyacrylamide gel electrophoresis processes and http://dx.doi.org/10.5599/jese.1882 http://dx.doi.org/10.5599/jese.1882 http://www.jese-online.org/ mailto:emeka.oguzie@futo.edu.ng j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 2 agarose gel electrophoresis. bb has a high proclivity for water contamination because of its high solubility and stability, causing damage to living organisms even in low quantities [2]. the molecular structure of bb is shown in figure 1. figure 1. molecular structure of bromophenol blue dye since bb is labeled highly toxic and persistent, several remediation technologies have been investigated for its removal from aqueous systems. these technologies include adsorption [2], photo-catalysis [3], electrochemical treatment [4], and biotic and biological treatment [5]. it is interesting to note that over 50 % of the literature on bb decolorization is focused on photo-catalysis and other light-induced methods. although photocatalytic oxidation is a fast and effective method for dye decolorization, it has the disadvantage of being expensive and requires harsh operating conditions, elaborate setup, and management procedures [6]. electrochemical degradation has been employed in wastewater treatment in recent years because of its high efficiency, low cost, and low formation of secondary pollutants [7]. rong et al. [8] have experimented with the electrochemical degradation of bb using boron-doped diamond (bdd) electrodes with positive results, while zhang et al. [9] assessed the role of sulfate electrolytes on electrochemical decolorization of bb in bdd anode cells zhao et al. [10] reported 96.9 % electrochemical decolorization efficiency for bb achieved using pbo2-zro2 composite electrodes. composite electrodes like bdd and pbo2-zro2 represent the new trend in contaminant electrochemical degradation technologies due to their superior degradation performance and remarkable current efficiencies [11]. conversely, such composite electrode materials often fail to serve as durable electrodes in sizeable electrooxidation operations and are prohibitively expensive for industrial-scale applications [12]. hence, dimensionally durable and low-cost electrodes are still required for long-term, large-scale applications [13]. many studies have been conducted on using carbon-based electrode materials for the effective electrochemical decolorization of dyes in aqueous solutions. zakaria et al. [14] investigated the electrochemical oxidation of reactive orange 16 using carbon-based electrodes from biomass, obtaining a decolorization efficiency of up to 98.5 %. with graphite material used as electrodes, oguzie et al. [15] investigated the electrochemical decolorization of dispersed blue-1 dye in an aqueous solution achieving a decolorization efficiency of 96 %. luan et al. [16] employed carbon nanotubes for electrochemical oxidation of methylene blue and the reduction of carbon dioxide to obtain valuable products. goren et al. [17] studied the electrochemical removal of methylene blue and reported up to 99.9 % removal efficiency using flexible graphite electrodes with operation costs as low as 0.012 $/m3. microbial treatment (bioremediation), involving the breakdown of complex organic substances into less toxic smaller molecules by means of living organisms, is another inexpensive and benign option for color removal from aqueous systems. microorganisms are able to degrade synthetic dyes j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 3 via contaminant adsorption on the microbial biomass or biodegradation via enzyme production [18]. potadar et al. [19] successfully employed bacterial isolates of pseudomonas sp. and salmonella sp. from water samples for the treatment of effluents contaminated with bb. chmelova and ondrejovic [20] employed lacasse produced by ceriporiopsis subvermispora to degrade bb, achieving decolorization efficiency of more than 89.8% with a mediator, and 49.8 % without a mediator. singh and singh [21] successfully degraded bb with aspergillus flavus and trichoderma harzianum fungi. this study assesses the feasibility of electrochemical and biological methods for the decolorization of bb using graphite/copper electrodes and indigenous microorganisms, respectively. the influence of different process variables on the efficiency of the electrochemical and biological processes is assessed. quantum chemical computations are also carried out to identify the sites where decolorization is initiated on the dye molecule. experimental chemicals and reagents bromophenol blue dye, macconkey agar, sabouraud dextrose agar and nutrient agar were obtained from aldrich, while other reagents like sulphuric acid and sodium hydroxide were obtained from sinopharm, china. all reagents were used as sourced without additional purification. preparation of dye-polluted soil one gram of bb dye was diluted in 1 l of distilled water to make 1000 ppm of bb dye solution. a soil sample was collected on a farm beside the sops extension building at the federal university of technology, owerri, using a soil auger. the soil auger was inserted 30 cm below the soil, which was used to obtain 4.5 kg of soil. one liter of 1000 ppm of bb dye solution was then mixed with the soil sample. preparation of culture media macconkey agar: fifty-two grams of mac conkey agar powder were dispersed in one liter of deionized water. allowed to soak for 10 minutes before swirling to mix and sterilizing by autoclaving at 121 °c for 15 minutes. after cooling to 47 °c, the solution was mixed and poured into petri dishes. sabouraud dextrose agar: sixty-five grams of the powder was dissolved in 1000 ml distilled water. the mixture was boiled to dissolve the medium completely at 121 °c for 15 min and cooled to 45 to 50 °c. then it was properly mixed and dispersed. nutrient agar: twenty-eight grams of nutrient agar powder was dissolved in 1 l of distilled water, swirled to mix properly and dissolved completely. the solution was sterilized by autoclaving at 121 °c for 15 min. the liquid was then poured into a petri dish and allowed to solidify. preparation of mineral salt the media composition used for this study is a mineral salt medium (broth) with the following composition: 1.0 g/l nh4no3; 0.2 g/l mnso47h2o; 1.0 g/l k2hpo4; 0.1 g/l cacl22h2o; 0.2 g/l kcl; 1.0 g/l feso46h2o; 0.1 g/l znso47h2o; 1.0 g/l mnso4 and 1.0 g/l yeast extract which was dissolved in 1 l of sterile water and autoclaved at 121 °c for 15 minutes. microorganism isolation and identification a ten-fold serial dilution of bb dye-polluted soil was carried out as described by cheesebrough [22]. this dilution process was carried out to a diluent factor of 10 (d10). thereafter, samples from diluent d5, d7, and d9 were inoculated on the three different media in triplicate. the microhttp://dx.doi.org/10.5599/jese.1882 j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 4 organisms were then subcultured by streaking on a sterile nutrient agar plate to obtain pure culture for microbial identification and characterization following a series of biochemical tests as described in [22] and [23]. microbial decolorization experiments mineral salt medium (50 ml) and 100 ml of 50 mg/l bb dye solution were mixed in a conical flask. a loopful of each isolated microbial strain was separately inoculated into the mixture. the setup was allowed to stand for 9 weeks. about 4 ml of the mixture is aspirated once a week, centrifuged at 4000 rpm for 15 min, and sent for analysis. the procedure was followed for 100 ml of mineral salt and 50 ml of bb dye solution. electrochemical decolorization experiments electrochemical decolorization experiments for bb were conducted in a 500 ml capacity electrolytic cell equipped with a magnetic stirrer. an anode of graphite with dimensions of 60×25×2.5 mm and a cathode of copper of the same size were used. both electrodes were connected to a digital dc power supply (model mch-k305d) with specific voltage and current settings. the electrolytic bath was prepared by mixing 490 ml of 30 mg/l of bb dye solution with 10 ml of supporting electrolyte. the ph of the solution was adjusted using 1 m h2so4 or 0.1 m naoh as needed. a ph meter (phs-3c) was used to determine the desired ph values (3, 7, 9, 11). the mixed bb solution was then added to the electrolytic cell, and the electrodes were submerged in the solution. the electrolysis process was initiated by applying the programmed current density and 25 v settings on the dc power supply. during the electrolysis process, samples were collected at 10minute intervals for up to 70 minutes. the collected samples were then subjected to analysis using spectrophotometry. the experimental variables considered in the study included the initial solution ph (3, 7, 9, 11), supporting electrolyte type (kcl and nacl) of concentrations (1.00, 0.50, 0.10, 0.05 m), temperature (301, 311, 321 and 331 k), and current density (15 75 ma/cm2). each experiment was conducted in triplicate to ensure statistical reliability. analytical procedure the solutions aspirated from the electrolysis cell and microbial treatment sample bottles at intervals were monitored using a shimadzu uv-3600 plus uv-vis-nir spectrophotometer, over a wavelength range of 200-800 nm. the peak at the maximum wavelength (λmax = 590 nm) was especially monitored for dye solutions before and after electrolysis as an indicator for bb decolorization. the full wavelength scan enabled visualization of other absorption peaks associated with bb degradation intermediates. the bb removal efficiency was calculated as follows: 0 0 de 100t a a a − = (1) where a0 is the initial absorbance of the bb solution before electrolysis and at is the absorbance of bb at the given time t. quantum chemical computations quantum chemical computations were undertaken using the dmol3 package of material studio software (biovia materials studio academic research suite (product no: 5cb-lur)). j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 5 results and discussion identification of microorganism biochemical tests and morphological characterization of different isolates were used in identifying the microbial strains. the code and associated names of the isolated strains shown in table 1 reveal s a total of 7 distinct isolates from soil contaminated with bb dye. this implies that the dye served as a source of nutrients for the seven isolates but was toxic to other species in the soil ecosystem, due to the known toxicity of triphenylmethane dyes. a similar result has been reported by other authors, for instance, serratia marcescens [24], aspergillus sp [25], bacillus subtilis [26], bacillus megaterium [27], rhizopus stolonifera [28], eubacterium sp [18], acinetobacter baumaniu [29] have all been reported to show some efficacy in decolorization of textile dyes. table 1. isolated microbial strains from the polluted soil sample sample name of strain organism type bb1 acinetobacter baumaniu bacteria bb2 serratia marcescens bacteria bb3 bacillus subtilis bacteria bb4 bacillus megaterium bacteria bb5 aspergillus niger fungus bb6 aspergillus flavus fungus bb7 rhizopus stolonifera fungus microbial decolorization of bb to investigate the bb decolorization efficiency of the isolated micro-organisms, 50 ppm of the dye-contaminated water with the mineral broth was inoculated with the different identified strains, respectively. the results are shown in figure 2. figure 2. biodecolorization efficiency of bb by different microbial strains: bb1 acinetobacter baumaniu; bb2 serratia marcescens; bb3 bacillus subtillis; bb4 bacillus megaterium; bb5 aspergillus niger; bb6 aspergillus flavus; bb7 rhizopus stolonifer the identified microbial strains were found to have varying abilities for decolorizing bb dye since the efficacy of microbial decolorization depends on the adaptability and activity of individual microorganisms. figure 2 shows that serratia marcescens (bb2) achieved the highest decolorizing http://dx.doi.org/10.5599/jese.1882 j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 6 effect for all time intervals, while bacillus subtilis (bb3) achieved the lowest. decolorization efficiency for all strains increased with time. dye decolorization can occur in two ways, i.e., by biosorption on microbial biomass or bio-decolorization of dyes by microbial cells [30]. dye adsorption can occur on both growing/living and dead microbial cells. biosorption preserves the original structure of the dyes, as the dye does not undergo degradation. on the other hand, biodegradation destroys the original dye structure, degrading it into smaller molecules and moieties, occasionally accomplishing complete mineralization into co2 and h2o. effect of dye and mineral salt on microbial decolorization of bromophenol blue the effect of dye and mineral salt on the decolorization efficiency of bb was investigated by performing decolorization experiments using two different volume ratios of dye and broth solution. figure 3 shows the effect of dye and mineral salt volume ratio on microbial decolorization of bb. figure 3. effect of dye/mineral salt volume ratio on biodecolorization efficiency: bb1 acinetobacter baumaniu; bb2 serratia marcescens; bb3 bacillus subtillis; bb4 bacillus megaterium; bb5 aspergillus niger; bb6 aspergillus flavus; bb7 rhizopus stolonifer figure 3 shows that acinetobacter baumaniu (bb1), serratia marcescens (bb2), aspergillus niger (bb5), aspergillus flavus (bb6), and rhizopus stolonifer (bb7) achieved higher decolorization efficiency for 1:2 dye to broth volume ratio, while bacillus subtilis (bb3) and bacillus megaterium (bb4) performed poorly irrespective of the dye/mineral salt ratio. the presence of dangerous levels of dyes may affect the growth of microorganisms, i.e., the ability to decolorize colors. the dye class governs its structural properties and influences how much a dye gets decolored. the amount of nitrogen and glucose in the medium can also influence dye decolorization by altering the capacity of bacteria to create enzymes. this is consistent with our findings that ratio 1:2 outperforms ratio 2:1. electrochemical decolorization of bb electrochemical techniques have been found effective for the treatment of organic pollutants in wastewater. since organic pollutants do not readily yield electroactive species that can respond to an applied electric field, electrolyte salts that readily dissociate into electroactive species are added to the solution to enhance electrical conductivity. other factors influencing electrochemical treatment techniques include solution ph, initial contaminant concentration, applied current and temperature. j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 7 effect of supporting electrolyte on electrochemical decolorization of bb supporting electrolytes, especially chlorides, play a crucial role in electrolysis processes, especially those involving organic species, which are very poorly electroactive and have low conductivity. therefore, supporting electrolytes are added to enhance ionic conductivity and decrease solution resistance. experiments were carried out to ascertain how different concentrations of kcl and nacl applied as supporting electrolytes, influence the electrochemical decolorization of bb. the obtained results are illustrated in figure 4a for kcl and figure 4b for nacl. parameters held constant for the experimental runs include bb concentration, temperature, current density, and voltage. figure 4. effect of kcl (a) and nacl (b) as supporting electrolytes on bb decolorization efficiency (dye concentration = 30 mg/l, ph 7, temperature = 301 k, voltage = 25 v, current density = 75 ma/cm2) both plots show that decolorization efficiency increased steadily with supporting electrolyte concentration, attaining optimum values of 99.9 and 99.7 % in 1 m kcl and 1 m nacl, respectively, suggesting that both salts exerted identical effects. however, the results also show that the kcl system attained optimal efficiency within 30 min, whereas the nacl took 50 min. this implies that kcl is a better-supporting electrolyte, yielding more rapid decolorization. moreover, values of 55.9 and 28.8 % were recorded at the lowest electrolyte concentration (0.05 m) for kcl and nacl, respectively. also, 0.5 m kcl achieved close to optimal decolorization, albeit at a much longer time interval (60 min). this is because kcl has a larger electronegativity difference, k atoms are larger than na atoms, and k is more electropositive than na. as a result, kcl is a more ionic substance than nacl. it should be noted that too high a concentration of the supporting electrolyte may create adverse consequences. for instance, high concentrations of chloride salts could promote the formation of unwanted organochlorides [31], as well as high concentrations of hypochlorous acid, which can promote corrosion of some electrode materials. so, one has to make a trade-off between speed and efficiency in deploying higher or lower salt concentrations. effect of initial ph of bb solution on electrochemical decolorization efficiency the efficacy of the electrochemical decolorization processes is significantly influenced by electrolyte solution ph. the ph dependence of bb decolorization efficiency as a function of time is plotted in figure 5. the variation of decolorization efficiency with initial solution ph presents an interesting trend within the time interval of the experiments. although generally, the decolorization efficiency decreased as ph increased from acidic via neutral to alkaline over the available time interval, almost optimum efficiency is attained at each ph value, at different time intervals (30 min at ph 3, 40 min at ph 7, 50 min at ph 9 and 70 min at ph 11). this implies that just the solution ph influences mostly the kinetics of the decolorization process. http://dx.doi.org/10.5599/jese.1882 j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 8 figure 5. effect of initial ph of electrolyte solution on bb decolorization efficiency (dye concentration = 30 mg/l, current density = 75 ma/cm2, temperature = 301 k, voltage = 25 v, electrolyte = 1 m kcl) ph impacts various properties, including chemical states of solution species and solvent molecules, electrostatic interactions at the electrode/solution interphase, the kind of active oxidizing species generated during the reaction, etc., making thus any interpretation difficult. the processes of oxygen generation and oxidative destruction of organic molecules on the anode are always competing in the anodic oxidation system. the speed of oxygen production at the anode surface rises as the ph of the solution increases. this reduces the oxygen evolution potential, delays organic pollutant transport to the anode, and so reduces the rate of organic matter oxidation at the anode electrode [4]. on the other hand, when the environment is acidic, the electrochemical conditions for organic pollutant degradation at the anode surface get better as the oxygen evolution process slows [32]. additionally, the conductivity of the solution diminishes under alkaline circumstances due to a lack of appropriate supporting electrolytes, which further reduces the effectiveness of organic pollutant decomposition [33]. this supports higher degradation efficiency in the acidic than in an alkaline medium [34]. therefore, ph 3 was found as the ideal condition for bb decolorization. in order to avoid strong acid conditions, further experiments were performed at ph 7, which according to figures 1 and 5 give acceptable decolorization efficiency results. effect of current density on electrochemical decolorization of bb the correlation between bb degrading efficiency and current densities from 15 to 75 ma/cm2 is shown in figure 6. time, min figure 6. effect of current density on decolorization efficiency of bb (dye concentration = 30 mg/l, temperature = 301 k, voltage = 25 v, electrolyte = 1 m kcl, ph 7) j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 9 the plots follow the expected trend of increased decolorization efficiency with an increase of current density over the entire time interval, optimal efficiency and kinetics attained at the highest studied current density of 75 ma/cm2. again, very appreciable decolorization efficiencies were obtained at all current densities, albeit at different rates. current density has been recognized as one of the most essential parameters controlling the anodic oxidation process, capable of regulating the rate of electrochemical degradation. increasing the current density over a constant period improves the efficiency of electrochemical degradation by boosting electron flow between the anode and cathode and increasing electrochemical transformations as well as contaminant oxidation at the anode [34]. using a higher current density in the electrochemical cell accelerates electron transfer kinetics at the interface and prevents the accumulation of electrolysis intermediates that could deactivate electroactive sites on the anode. a much higher current density, on the other hand, may increase side reactions associated with oxygen generation at the anode electrode, which interferes with organic matter oxidation and, eventually, reduces the efficacy of pollutant oxidation. it is as well noteworthy that high current densities will increase power requirements and overall operational costs [1]. effect of temperature on electrochemical decolorization of bb experiments were undertaken at 301 to 331 k to determine the optimal temperature for the decolorization of bb solutions. the obtained results are shown in figure 7. although there is clear evidence that the decolorization rate increased with an increase in temperature, the trend was established over the entire time interval following the order 301 < 331 < 321 < 311 k. figure 7. effect of temperature on decolorization efficiency of bb (dye concentration = 30 mg/l, voltage = 25 v, electrolyte = 1 m kcl, ph 7, current density = 75 ma/cm2) nonetheless, decolorization at all temperatures ultimately attained near-optimal efficiency but at different reaction times. superior decolorization kinetics is attained with the system at 321 k, attaining higher optimal efficiency values after 30 min, while the 331 k system attained slightly lower values at the same time. such inconsistencies in the influence of temperature on the electrochemical degradation of organic contaminants arise because high temperatures can either enhance the oxidizing power of the highly oxidizing species that degrade the contaminants or hinder the generation of oxidizing chloride species or both [35]. the prevailing effect would depend on the nature of the organic compound and the environment under study. http://dx.doi.org/10.5599/jese.1882 j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 10 electrochemical degradation profile of bb time-dependent full wavelength scans were obtained for bb solutions as electrolysis progressed. by monitoring the intensity of the characteristic absorption peak of the bb chromophore at max = 590 nm, as a function of electrolysis time, it is possible to ascertain the efficacy of the electrochemical decolorization process using graphite and copper electrodes. figure 8 illustrates the full wavelength absorption spectra for bb after electrolysis for time intervals ranging from 0 to 70 min. the plots show that the intensity of the bb absorption peak at 590 nm was markedly diminished after electrolysis treatment for 10 min and decreased steadily thereafter until it completely disappeared after about 30 min. this is clear evidence that the electrolysis cell employing a copper cathode and a graphite anode very effectively degrades bb and achieves complete mineralization of the dye contaminant. the appearance of new peaks between 380 nm and 430 nm indicates the presence of less conjugated reaction intermediates, which are subsequently degraded with time. as observed by jović et al. [35], the features of bb breakdown on anodes include quick mineralization and little aromatic intermediate buildup. figure 8. full wavelength uv-visible absorption spectra showing time-dependent electrochemical decolorization behavior of bb (dye concentration = 30 mg/l, voltage = 25 v, electrolyte = 1 m kcl, ph 7, current density = 75 ma/cm2) electrochemical decolorization kinetics the electrochemical decolorization process is often facilitated by in-situ generation of strongly oxidizing species formed via electrochemical transformation of the supporting electrolyte(s) added to enhance solution conductivity. for instance, chloride ions from chloride salts undergo reactions with hydroxyl radicals in the vicinity of the anode to form hypochlorous acid (hocl), and hypochlorite ions (ocl-) that act as the main oxidizing agent in oxidative degradation [36], equations (2) to (4): cl− + ho• → clo− + h+ + e (2) cl− + ho•→ hclo + e (3) hclo → clo+ h+ (4) the generated oxidizing species then initiate the attack at certain susceptible sites, mostly electron-rich centers on the contaminant structure. the most convenient way to monitor dye j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 11 decolorization kinetics is to track the gradual decrease in dye concentration as a function of time. figure 9 shows the kinetic plots for bb decolorization at t = 301 k, derived from the first-order kinetic equation (5): ln(a/a0) = −kt (5) where a0 = initial bb absorbance, a = bb absorbance at time t, and k = rate constant for electrochemical decolorization for bb, which is determined from the slope of the kinetic plot. figure 9. first-order kinetic plot for electrochemical decolorization of bb the good linear fit of the plot obtained means that the electrochemical decolorization of bb from our experiments obeyed the first-order kinetics law. energy consumption by determining the amount of electrical energy (kwh) used during the electrochemical decolorization of bb, the energy consumption related to this process may be determined using the equations (6) and (7): sol pt e v = (6) where: p = iv (7) p / w is the electrochemical cell electrical output, t / h is the electrolysis's, vsol / dm3 is the amount of dye solution electrolyzed, i / a is the current, and v / v is voltage. table 2 highlights the process of energy consumption for bb decolorization, calculated at different treatment times: 30, 50, and 70 min and currents of 1 to 5 a, which makes it possible to calculate how much energy is used to treat each cubic meter of bb contaminated water, according to the revised nigeria electricity tariff, which has recently been increased to 0.09 usd/kwh. for a given current value, energy consumption increased with treatment time and for a given treatment time, energy consumption increased with current. so, achieving optimal efficiency of 99 %, at the peak current value (5 a) after 30 mins will consume 125 kwh/m3 of energy, with a total cost of 11.53 usd. whereas achieving similar decolorization efficiency at 4 a takes 50 min and will consume 166.67 kwh/m3 at a total cost of 15.32 usd. http://dx.doi.org/10.5599/jese.1882 j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 12 table 2. calculated energy requirements for electrochemical treatment of bb solution at different currents and treatment time at 301 k current, a voltage, v time, min energy consumption, kwh m-3 1 25 0.50 25.00 0.83 41.67 1.17 58.33 2 25 0.50 50.00 0.83 83.33 1.17 116.67 3 25 0.50 75.00 0.83 125.00 1.17 175.00 4 25 0.50 100.00 0.83 166.67 1.17 233.33 5 25 0.50 125.00 0.83 208.33 1.17 291.67 electrochemical decolorization initiation mechanism to determine the mechanism of bb decolorization, at least theoretically, dft computations were carried out to model the electronic structure and local reactivity of the dye molecule to enable the identification of the active sites for the oxidative attack. the hirshfeld population analysis [37], the perdew-wang (pw) local correlation, limited spin polarization on the dnd basis set, and the dmol3 software (materials studio) were all employed in the computations. the detailed computational protocol is reported elsewhere [15]. figure 10 depicts the optimized structure, highest occupied molecular orbital (homo), fukui function, and lowest unoccupied molecular orbital (lumo) of bb. (a) (b) (c) (d) (e) figure 10. electronic properties of bb: (a) optimized structure; (b) homo; (c) lumo; (d) fukui f0; (e) fukui f the corresponding quantum chemical parameters are given in table 3. the homo regions, which measure the propensity of the molecule to donate electrons, lie around the bromine-bearing nuclei, while the electrophilic fukui functions depicting the sites most prone to attack by electron-seeking species coincide with the locations of the electronegative bromine atoms. the electronic structures can be further assessed by considering basic molecular reactivity indicators within the framework of the hard and soft acids and bases (hsab) theory [37]. the computed quantum chemical parameters like the homo and lumo energies, chemical hardness (or softness), homo/lumo energy gap, etc., are key structure-activity indicators relating electronic structure to chemical reactivity. j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 13 table 3. calculated quantum chemical parameters for bb chemical parameters ehomo */ ev -6.099 elumo*/ ev -2.546 energy gap (∆egap= (elumo – ehomo)), ev 3.553 electrochemical potential (μ = -x), ev -4.323 ionization potential (i), ev 6.099 electron affinity (ae)./ ev 2.546 electronegativity (x = (i + a) / 2), ev 4.323 chemical hardness (η = (i a)) / 2, ev 1.777 chemical softness (s = 1 / η), ev 0.563 electrophilicity (ω = μ2 / 2η), ev 5.258 ∆e back donation (-η / 4), ev -0.444 table 4 presents fukui indices (fi) for electrophilic (f-) and radical (f0) attack, which allows the determination of the reactivity order of the various functional groups in the bb molecule with respect to strongly oxidizing species and radicals. the results show clearly that the bromine atoms (especially br (23) and br (24)) represent the most susceptible sites for attack by both radical (f0) and electrophilic (f-) species and represent the most probable sites for attack and onset of decolorization of bb. table 4. fukui indices for the electrophilic and radical attack on bb atom no ff0 atom no ff0 c (1) 0.023 0.017 br (21) 0.027 0.024 c (2) 0.012 0.012 br (22) 0.068 0.044 c (3) 0.015 0.008 br (23) 0.082 0.050 c (4) 0.015 0.012 br (24) 0.147 0.082 c (5) 0.012 0.012 o (25) 0.091 0.050 c (6) 0.022 0.016 o (26) 0.007 0.015 c (7) 0.005 0.004 s (27) 0.005 0.024 c (8) 0.052 0.027 o (28) 0.011 0.030 c (9) 0.031 0.022 o (29) 0.009 0.028 c (10) 0.051 0.030 h (30) 0.008 0.008 c (11) 0.065 0.038 h (31) 0.010 0.008 c (12) 0.036 0.023 h (32) 0.017 0.012 c (13) 0.037 0.021 h (33) 0.019 0.012 c (14) 0.008 0.062 h (34) 0.005 0.027 c (15) 0.011 0.036 h (35) 0.005 0.019 c (16) 0.008 0.052 h (36) 0.005 0.024 c (17) 0.005 0.039 h (37) 0.005 0.018 c (18) 0.001 0.019 h (38) 0.009 0.006 c (19) 0.005 0.036 h (39) 0.027 0.015 o (20) 0.030 0.019 in order to obtain some insights into the nature and stabilities of bb degradation intermediates reported by [38] and [39], further quantum chemical computations were performed to model the electronic structures of the molecules and determine their theoretical enthalpies of formation. this is because intermediate products with more negative δhf values would be more stable and less prone to undergo side reactions, and vice versa. figure 11 shows the electronic structures of bb and the major degradation intermediates and corresponding enthalpies of formation. http://dx.doi.org/10.5599/jese.1882 j. electrochem. sci. eng. 00(0) (2023) 000-000 treatment of bromophenol blue dye in aqueous solution 14 comp. molecular structure total electron density homo orbital ehomo / ev lumo orbital elumo / ev δif / j mol-1 bb -6.099 -2.546 -110.3 bb-1 -6.097 -3.556 -137.0 bb-2 -5.893 -2.324 -130.0 bb-3 -6.023 -1.852 -16.2 bb-4 -5.750 -2.471 -182.1 bb-5 -4.916 -2.494 -128.0 bb-6 -6.126 -2.734 -124.3 bb-7 -7.131 -3.227 -181.5 bb--8 -6.378 -2.829 -111.1 figure 11. electronic structures of bb and the major degradation intermediates and corresponding enthalpies of formation j. o. anyanwu et al. j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1882 15 the enthalpy of the formation of a compound can be used as a measure of its relative stability compared to its constituent elements. if the enthalpy value is negative, then it is an exothermic compound and its elements could theoretically be made to react and form the compound. conversely, a positive enthalpy of formation value indicates that the compound is relatively unstable compared to its elements and that its formation would require energy. however, this does not necessarily mean that the elements cannot be made to react and form the compound directly. conclusion in this study, electrochemical and microbial techniques were employed in the decolorization of bromophenol blue dye-contaminated water. seven microorganisms that could decolor bb dye, especially at lower dye concentrations, were isolated. the electrochemical technique using graphite anode and copper cathode was found more effective for the decolorization of bb that followed the first-order reaction kinetics. a decolorization efficiency of 99 % was reached after 30 min with increasing current density up to 75 a/cm2. the highest decolorization efficiency was observed at ph 3, and lowest at ph 11. decolorization efficiency showed a nonlinear relationship with temperature rise. the computational results showed clearly that bromine atoms are the most susceptible sites for attack by oxidizing species and, thus, the onset of bb decolorization. conflict of interest: all authors declare no conflict of interest. acknowledgements: the authors gratefully acknowledge financial support from the world bankfunded second africa higher education centres of excellence for development impact (ace impact) project – p169064, ida no 6510-ng. references [1] k. h. kazm, s. t. najim, study kinetic reaction and removal of indigo carmine dye in aqueous solutions by direct 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.desal.2006.09.026 https://doi.org/10.1039/d1ra04533d https://creativecommons.org/licenses/by/4.0/) one-pot synthesis of crystalline structure: nickel-iron phosphide and selenide for hydrogen production in alkaline water splitting http://dx.doi.org/10.5599/jese.1721 575 j. electrochem. sci. eng. 13(3) (2023) 575-588; http://dx.doi.org/10.5599/jese.1721 open access : : issn 1847-9286 www.jese-online.org original scientific paper one-pot synthesis of crystalline structure: nickel-iron phosphide and selenide for hydrogen production in alkaline water splitting mukhtiar ahmed1,, abdul hanan2, muhammad nazim lakhan2,, altaf hussain 3,4 irfan ali soomro5, binguang niu1, yang yang1 1state key laboratory of multiphase complex systems, institute of process engineering, chinese academy of sciences, university of chinese academy of sciences, beijing 100190, china 2key laboratory of superlight material and surface technology, ministry of education, college of materials science and chemical engineering, harbin engineering university, harbin 150001, china 3state key laboratory of electroanalytical chemistry, changchun institute of applied chemistry, chinese academy of sciences, changchun 130000, china 4university of science and technology of china, hefei, anhui, china 5college of chemical engineering, beijing university of chemical technology, beijing 100190, china corresponding authors: ahmedmukhtiar461@gmail.com; nazimlakhan@gmail.com received: february 21, 2023; accepted: june 22, 2023; published: june 27, 2023 abstract electrocatalytically active nanocomposites play a vital role in energy generation, conversion, and storage technologies. transition metal-based catalysts such as nickel and iron and their pnictide (phosphide), and chalcogenide (selenide) compounds exhibit good activity for hydrogen evolution reaction (her) in the alkaline environment. in this study, transition metals-based catalysts (ni-p-se, fe-p-se, and ni-fe-p-se) solutions were prepared using a simple one-pot method. prepared solutions were deposited on ni foam, and different characterization techniques were used to determine the composition, structure, and morphology of as-prepared catalysts. furthermore, it was found that ni-fe-p-se as a cathode material showed better her performance compared to other investigated materials with the overpotential value of 316 mv at 10 ma cm-2 current density and 89 mv dec-1 tafel slope value. the stability tests of the as-prepared catalyst confirmed that the synergistic effect between various elements enhances the electrocatalytic performance for up to 24 hours, providing a fair, stable nature of ni-fe-p-se based sample. keywords electrocatalysis; hydrogen evolution reaction; transition metal phosphide; transition metal selenide introduction developed nations have made a commitment to switching over immediately to environmentally friendly electrochemical energy. renewable energy sources like wind and solar energy are crucial in http://dx.doi.org/10.5599/jese.1721 http://dx.doi.org/10.5599/jese.1721 http://www.jese-online.org/ mailto:ahmedmukhtiar461@gmail.com mailto:nazimlakhan@gmail.com j. electrochem. sci. eng. 13(3) (2023) 575-588 one-pot synthesis of crystalline structure 576 order to reduce the use of fossil fuels and environmental degradation. electrochemical water splitting (ews) is one of the most promising strategies that can be used to address energy issues due to its purity, safety, and ease of use [1,2]. ews includes two half-cell reactions, such as hydrogen evolution reaction (her) and oxygen evolution reaction (oer). both the anodic and cathodic reactions result in the creation of hydrogen and oxygen, respectively [3,4]. in addition, several studies have focused not only on the production of new catalysts but also on the discovery of new affordable fuels. in this regard, particular attention has recently been paid to hydrogen fuel, which has been suggested as the secondary energy source for the future [5-7]. among noble metals, platinum-based catalysts are the most active for her. however, the high price and resource shortage of noble metal electrocatalysts limit their application in water splitting application. to address this problem, scientists began to explore earth-rich transition metals (tms) to replace precious metal-based catalysts. transition metal phosphide and selenide (tmp/tmse) have got a lot of interest due to their flexibility in a variety of applications, such as catalysts, batteries, and supercapacitors [8-10]. there have been several efforts to produce a less expensive composite material that can replace noble metals for hydrogen production. according to previous studies, tmp/tmse have been synthesized through one-pot processes resulting in highly active catalysts [11-14]. nickel (ni) and iron (fe) tm alloys were also considered efficient materials owing to their exceptional properties, such as hardness and conductivity. therefore, preparing ni and fe composites with p and se could be a valuable approach for hydrogen production. for example, pei et al. [15] developed a series of highly active and inexpensive co–ni–p films by a one-step constant current density electrodeposition method. these films were demonstrated to be efficient bifunctional catalysts for both h2 and o2 evolution (her and oer), while deposition time was deemed the crucial factor governing electrochemical performance. in particular, it requires −103 mv overpotential for her and 340 mv for oer to achieve the current density of 10 ma cm–2, corresponding tafel slopes of 33 and 67 mv dec-1, respectively. xu et al. [16] fabricated the fe–co– p multi-heterostructure arrays consisting of cop, co2p and fep, built for water splitting by a selfsacrificial template method. such multi-heterostructures adjust the local electronic structure and then improve the adsorption of reaction intermediates on active sites, which was further proved through density functional theory (dft) calculations. xing et al. [17] fabricated a unique 2d fese2/cose nanosheet structure synthesized by hydrothermal and selenization processes. in an alkaline solution, fese2/cose nanosheet electrode exhibited a low overpotential of 73 mv at 10 ma cm–2 for the her. kwak et al. [18] reported the synthesis of cose2 and nise2 nanocrystals (ncs) as excellent bifunctional catalysts for the simultaneous generation of h2 and o2 in water-splitting reactions. nise2 ncs exhibited superior electrocatalytic efficiency in oer, with a tafel slope of 38 mv dec-1 (1 m koh) and her with 44 mv dec-1 (0.5 m h2so4). one-pot method is considered advantageous over traditional multi-step synthesis methods, as it reduces the number of reaction steps and the amount of waste generated. it also has the potential for scalability and lower cost [19,20]. this novel one-pot method is offering inherent advantages such as step economy, operational simplicity, and synthetic efficiency. herein, ni and fe-based p and se composites such as ni-p-se, fe-p-se, and ni-fe-p-se were prepared by using the one-pot synthesis method and ni-foam (nf) was used as a substrate. the asprepared composites were firstly characterized physically by different techniques, including xrd, sem, eds, and hrtem, for their crystallographic analysis, surface features, elemental presence and deeper morphological aspects, respectively. after that, the as-prepared materials were analysed for her response in an alkaline medium. the obtained physicochemical and electrochemical outcomes https://www.sciencedirect.com/topics/materials-science/nanosheet https://www.sciencedirect.com/topics/materials-science/hydrogen-evolution https://pubs.acs.org/action/dosearch?field1=contrib&text1=in+hye++kwak m. ahmed et al. j. electrochem. sci. eng. 13(3) (2023) 575-588 http://dx.doi.org/10.5599/jese.1721 577 demonstrated that the ni-fe-p-se could be considered as a prominent electrocatalyst for her in the alkaline medium. experimental chemicals the experimental procedure involved the use of ni foam (nf), which was supplied by sigmaaldrich. the ni foam used had a high purity of 99.9 % and a specific surface area ranging from 0.5 to 3 m2 g-1. additionally, the foam had a pore size of around 100-200 micrometres, commonly used in electrochemical applications. the 2-mercaptoethanol with a purity of 93.4 % was provided by tianjin guangfu fine chemical research institute. selenium (se) powder with a purity of 97 % was purchased from sinopharm chemical reagents co. ltd. sodium dihydrogen phosphate (nah2po4) was purchased from tianjin yaohua chemical reagents co. ltd, and nickel(ii) nitrate ni(no3)26h2o was obtained from sinopharm chemical reagents co. ltd. all chemicals used in the synthesis process were of analytical grade. the chemicals were added to the reaction without further treatment, ensuring that the experimental conditions were consistent and reliable. preparation of ni-p-se, fe-p-se and ni-fe-p-se materials in a typical experimental procedure, commercially purchased chemicals were taken into a mixture with ni(no3)2·6h2o (1.45 g), fe(no3)3 (1.40 g), nah2po2 (2.00 g), and selenium (se) (1.2 g) were mixed to prepare different solutions. ni-p-se, fe-p-se and ni-fe-p-se were prepared, dissolved separately in 35 ml distilled water (dw), and stirred for one hour to form clear solutions. later, the obtained solutions were centrifuged and washed several times with dw and ethanol for 10 minutes. the centrifuged products, such as ni-p-se, fe-p-se and ni-fe-p-se, were collected and dried in a vacuum oven overnight at 70 °c. finally, the dried products were placed in a furnace for annealing under a nitrogen atmosphere. the furnace was heated at 400 °c for 3 hours with a heating rate of 5°c min-1. after cooling down the samples (ni-p-se, fe-p-se and ni-fe-p-se) at room temperature, the asobtained powder samples were collected and used for further characterizations and electrochemical analysis. the schematic procedure of samples preparation is depicted in figure 1. figure 1. schematic illustration of the step-by-step synthesis process of investigated materials structural characterizations the x-ray diffraction (xrd) patterns were observed by using x-ray diffractometer (xrd, rigaku ttr-iii) operated at 40 kv/150 ma with high-intensity cu ka radiation (λ = 0.15406 nm). transmission http://dx.doi.org/10.5599/jese.1721 j. electrochem. sci. eng. 13(3) (2023) 575-588 one-pot synthesis of crystalline structure 578 electron microscopy (tem, tecnai-g220s-twin, feitem) was used to check the samples' topography and interplanar analysis at 120 kv an accelerating voltage. the corresponding d-spacing value has been calculated through image-j software (via hrtem images). the morphology and structure of the samples were characterized by scanning electron microscopy (sem, jsm-6480a, joel) run at 20 kv an accelerating voltage and used to determine the shape and structure of the deposited materials on ni-foam. for deposition, a dispersed solution of binder and the as-prepared catalyst was dispersed on nf via drop casting. eds (energy dispersive x-ray spectroscopy) mapping attached with sem was used for the confirmation of present elements. this information helped us to optimize their synthesis methods and improve the properties of the resulting materials. electrocatalytic measurements electrocatalytic measurements of her were conducted by using the traditional three-electrode workstation (chi760e) bought from shanghai chenhua instrument co. ltd. a platinum rod electrode was used as the counter electrode, while a silver silver-chloride (ag/agcl) filled with 3.0 m kcl served as the reference electrode. the pt rod electrode is oft en used as a counter electrode because it is inert and does not participate in the chemical reactions taking place in the electrochemical cell. as-prepared samples were deposited separately on nf (1×1cm) pieces and used as working electrodes. in 1.0 m koh, all potentials were calibrated to the reversible hydrogen electrode (rhe) using the nernst equation and all electrochemical experiments were performed at room temperature. first, 5 mg of each catalyst solution dissolved in 1 ml of dw and 50 ml of 5 % nafion solution (as a binder) were individually added for catalyst ink preparation. then, the catalyst ink was properly dispersed using an ultrasonic bath for 20 minutes. later, 10 ml of dispersed solution was coated by drop casting on nf foam and dried at ro om temperature. the loaded mass of each catalyst was about 0.2 mg cm-2. the electrocatalytic activity for her was investigated in 1.0 m koh through the linear sweep voltammetry (lsv) at 5 mv s-1 scan rate. koh is a strong base and can provide a high-conductivity solution, making it a good choice for many electrochemical applications, including her [21,22]. all potentials measured by ag/agcl reference electrode (eag/agcl) were converted to the potential of reversible hydrogen electrode (erhe) using equation (1): erhe = eag/agcl + 0.059ph + e°ag/agcl (1) where e°ag/agcl is equal to 0.21 v. the electrode overpotential (ƞ) in the her region is defined as the difference between erhe and standard equilibrium potential for h+/h2 (0.00 v), resulting in ƞ = erhe. the tafel slope, on the other hand, was computed using the tafel equation (2). ƞ = b log j + a (2) where the overpotential is ƞ, the tafel slope is b, and the current density is j. the current-time curve is required for assessing the performance of electrocatalyst for her because it gives essential information on the catalyst efficiency, kinetics, and long-term stability. based on a particular current level, a current-time curve displays the interrupting time of an overcurrent device, all tests were carried out in an electrocatalysis cell. preliminary studies revealed that a little adjustment in v had no discernible effect on the current-time curves in the cell. in other words, the small adjustments did not affect the current-time curves in the cell. if significant modifications are made, the current-time curves are likely to be altered [23]. the proposed mechanism of her on ni-fe-p-se is presented in figure 2. m. ahmed et al. j. electrochem. sci. eng. 13(3) (2023) 575-588 http://dx.doi.org/10.5599/jese.1721 579 figure 2. proposed mechanism of her on ni-fe-p-se coated on nf in alkaline media results and discussion physicochemical analysis the x-ray diffraction has been applied to analyse the crystallographic characteristics of various materials, including ni-p-se, fe-p-se, and ni-fe-p-se, and results are shown in figure 3. 2 / ° figure 3. xrd patterns of corresponding electrocatalytic materials: (a) ni-p-se, (b) fe-p-se, (c) ni-fe-p-se figure 3a exhibits the ni-p-se with a number of diffraction patterns at two theta angles like 26.3, 31.8, 46.5, 49.8, 54.2, and 61.8°, which correspond to the crystallographic planes (100), (101), (110), (111), (003) and (013), having a jcpds no. 01-083-2439 which resembles the se presence [24]. two peaks that have been observed at around 44.1 and 51.3° correspond to (111) and (200) in te n si ty , a .u . http://dx.doi.org/10.5599/jese.1721 j. electrochem. sci. eng. 13(3) (2023) 575-588 one-pot synthesis of crystalline structure 580 crystallographic planes and strongly justify the existence of ni within the as-prepared composite material [25,26]. fe-p-se diffraction patterns can be seen in figure 3b, revealing the presence of se with additional peaks for iron (fe) at 44.3 and 65.1° that are attributed to the (110) and (200) crystal planes, having a jcpds card no. 03-065-4899, which is entirely supported by existing research [27,28]. x-ray patterns of ni-fe-p-se shown in figure 3c resemble the existence of ni, fe, and se elements. such findings through xrd analysis confirmed the successful preparation of ni-fe-p-se composition, and this combination of three different elements was already shown to promote the synergistic effect toward her performance [29,30]. hrtem is one of the most advanced analytical imaging measurement techniques to distinguish the size and shape of as-prepared samples [31]. in figure 4a-4c, the images perfectly reflect the detailed morphology of the as-prepared material with the scale bars of 500, 200 and 10 nm, respecttively. the hrtem image for the synthesized material ni-fe-p-se (figure 4) reveals an uneven surface and different domains resulting from the tight aggregation of nanoparticles with a wellresolved lattice fringes space assigned to the plane of ni-fe-p-se [32]. figures 4a and 4b illustrate the different magnifications of the samples. the lattice fringe, along with their planner distance measurement, is depicted in figure 4c, and its fast fourier transform (fft) conversion can be found on the right side of the image. the d-spacing value for as-prepared ni-fe-p-se is calculated as 0.221 nm, which is well-matched with recently reported research [33]. figure 4. (a-b) hrtem images of ni-fe-p-se at different magnifications; (c) lattice fringes and corresponding fft images for d-spacing calculation distance, nm m. ahmed et al. j. electrochem. sci. eng. 13(3) (2023) 575-588 http://dx.doi.org/10.5599/jese.1721 581 figure 5 presents the elemental mapping result of ni-fe-p-se to demonstrate the uniform distribution of ni, fe, p, se, c, n, and o, which confirms the uniform growth of the ni-fe-p-se composite. figure 5. elemental mapping of as-synthesized material (ni-fe-p-se) different morphology of electrocatalytic materials, as well as chemical composition, were studied via sem. average morphologies of the as-obtained ni-p-se, fe-p-se and ni-fe-p-se were observed at different magnifications, as shown in figure 6. it is observed that the ni-p-se shows the morphology on nf with its desired quantity, as depicted in figure 6a. figure 6b reveals the morphological aspects of fe-p-se, which is grown on nf as well. it can be seen that fe-p-se has an agglomerated lumpy structure. in figure 6c, the morphology of ni-fe-p-se can be seen, which shows a higher amount of prepared material on nf, with varied surface features than both previous samples of nip-se and fe-p-se, respectively. through the sem images, we have observed agglomerated surface features of various elements within the as-prepared ni-fe-p-se structure as a potential composite for electrochemical water splitting application [34,35]. http://dx.doi.org/10.5599/jese.1721 j. electrochem. sci. eng. 13(3) (2023) 575-588 one-pot synthesis of crystalline structure 582 figure 6. sem images; (a) ni-p-se, (b) fe-p-se, and (c) ni-fe-p-se electrochemical analysis the electrocatalytic her performance on ni-p-se, fe-p-se, and ni-fe-p-se was initially investigated in an alkaline electrolyte (1.0 m koh) using lsv. figure 7a exhibits the lsv curves of three proposed electrocatalysts with reference to a pt-based electrocatalyst. it can be seen that the constructed nife-p-se has an activity for her with a lower overpotential value of 316 mv at 10 ma cm-2 as compared to other electrocatalysts (ni-p-se and fe-p-se) with overpotential values of 385 and 458 mv, respectively. tafel slope has always been a crucial analysis tool in her reaction kinetics for the determination of the rate-determining step [36-38]. the tafel slope value of ni-fe-p-se was calculated as 89 mv dec-1, which is significantly lower than for the other two prepared electrocatalysts, computed as 162 and 138 mv dec-1 for ni-p-se and fe-p-se, respectively. based on the tafel slope values, it should be discussed what her mechanism is in question and which reaction step is ratedetermining in each case (tafel, volmer or heyrovsky). the her mechanism for tafel slopes for tms involves the electrochemical reduction of protons to hydrogen gas on the surface of a transition metal catalyst. there are two categories of the her mechanism: volmer-tafel and heyrovsky mechanisms. tafel slope has been expressed as the logarithmic derivative of the current density with respect to the applied potential. figure 7 b shows the computed tafel slope values for ni-p-se, fe-p-se, and ni-fe-p-se, respectively. moreover, the stable and durable nature of an electrocatalyst is essential toward its practical usage [39]. therefore, the stability of the as-prepared electrocatalyst (ni-fe-p-se) was analysed through chronopotentiometry technique for 24 hours at the current density of 10 ma cm-2, as shown in figure 8a. it is observed that the ni-fe-p-se has a strong, stable nature in terms of potential value, which showed a negligible change with time. furthermore, durability is another important factor for the electrocatalyst, especially in h2 production through electrocatalysis [40]. the stability of ni-fe-p-se is tested through chronopotentiometry at 10 ma cm-2 (figure 8b), and durability before and after chronopotentiometry test, m. ahmed et al. j. electrochem. sci. eng. 13(3) (2023) 575-588 http://dx.doi.org/10.5599/jese.1721 583 as demonstrated in figure 8c. the material (ni-fe-p-se) showed a very durable behaviour for electrochemical her in an alkaline medium without any major change in lsv curves. such stable and durable behaviour of ni-fe-p-se material supports this composite as a potential candidate for active her response. figure 7. (a) linear sweep voltammetry curves of ni-p-se, fe-p-se, ni-fe-p-se, and pt (as reference), (b) corresponding tafel slopes figure 8. (a) overpotential values at 10 ma cm-2 of various electrocatalysts (b) chronopotentiometry test of ni-fe-p-se for 24 hours at a current density of 10 ma cm-2, (c) durability test of the as-prepared ni-fe-p-se before and after chronopotentiometry test http://dx.doi.org/10.5599/jese.1721 j. electrochem. sci. eng. 13(3) (2023) 575-588 one-pot synthesis of crystalline structure 584 the corresponding overpotential values at 10 ma cm-2 for every electrocatalyst material are shown in the form of histogram, as depicted in figure 8a. the lower overpotential value, lower tafel slope, and stable nature of ni-fe-p-se have proven this composite as a comparable transition metal-based electrocatalyst appropriate for electrochemical applications in alkaline water splitting [41,42]. figure 9 demonstrates the functioning of a current-time curve. chemical reaction kinetics, diffusion processes, and adsorption are all studied using current-time analysis. this approach entails applying a quick shift in electric potential to the electrode and then observing the resulting current as a function of time [43,44]. it is shown that the greater the current, the straight the time, and its activity may preserve the efficiency of the device [44,45]. the corresponding current-time values of ni-p-se, fe-p-se, and ni-fe-p-se can be seen in figures 9a-9c). using the parameters listed in table 1, the current-time curves were created. after 600 to 1200 s of the water-splitting reaction, no corrosion on the electrodes could be seen in the plots. table 1. experimental values for amperometry at sensitivity of 10-6 reading initial v run time, s 1 1.0 60 2 0.8 600 3 0.5 1200 figure 9. current-time curve for: (a) ni-p-se, (b) fe-p-se, (c) ni-fe-p-se conclusion ni-p-se, fe-p-se, and ni-fe-p-se solutions were effectively synthesized via a facile one-pot method. after the corresponding treatment, the prepared materials were deposited on the nickel foam by drop casting method. the xrd analysis of samples confirmed the crystallinity of different materials, including ni-p-se, fe-p-se, and ni-fe-p-se. sem showed the surface structure of these m. ahmed et al. j. electrochem. sci. eng. 13(3) (2023) 575-588 http://dx.doi.org/10.5599/jese.1721 585 materials with some modifications between them, while eds confirmed the presence of various elements within as-prepared materials. according to the electrochemical analysis, ni-fe-p-se exhibited much better catalytic activity for her as compared with ni-p-se and fe-p-se, showing the overpotential value of 316 mv at 10 ma cm-2, and 89 mv dec-1 value of tafel slope, respectively. the prepared electrocatalyst is stable for 24 hours at a current density value of 10 ma cm-2. such electrochemical features, including lower overpotential value, lower tafel slope and high stability, suggest that this composed material based on ni, fe, p and se is a robust electrocatalyst for h2 production in alkaline media and energy conversion applications. acknowledgments: we express our gratitude and thanks to the financial support of the national natural science foundation of china (nsfc 51402065 and 51603053), the natural science foundation of heilongjiang province (lh2019e025), the foundation research fund of central university and harbin applied technology research and development project (3072019cf1003), national defence industrial technology development program (jcky2016604c006), and the national key research and development program of china (2016yfe0202700). we also express gratitude and thanks to the national key research and development program of china, grant no. 2016yff0102601, the "strategic priority research program" of the institute of process engineering, chinese academy of sciences. references [1] h. el alami, j. creus, x. feaugas. thermodynamic parameters evolution versus plastic strain during her on nickel in sulphuric acid, electrochimica acta 52(12) (2007) 4004-4014. https://doi.org/10.1016/j.electacta.2006.11.029 [2] a. hanan, m. n. 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https://doi.org/10.1039/d0ee02485f https://doi.org/10.1007/s41918-022-00136-8 https://doi.org/10.1002/smll.202002212 https://doi.org/10.1016/j.ijhydene.2021.04.122 https://creativecommons.org/licenses/by/4.0/) {platinum utilization in proton exchange membrane fuel cell and direct methanol fuel cell review} http://dx.doi.org/10.5599/jese.665 281 j. electrochem. sci. eng. 9(4) (2019) 281-310; http://dx.doi.org/10.5599/jese.665 open access: issn 1847-9286 www.jese-online.org review platinum utilization in proton exchange membrane fuel cell and direct methanol fuel cell madhavi bandapati1, sanket goel2 and balaji krishnamurthy1, 1department of chemical engineering, bits-pilani, hyderabad campus, hyderabad-500078, india 2department of electrical and electronics engineering, bits-pilani, hyderabad campus, hyderabad500078, india corresponding author: balaji@hyderabad.bits-pilani.ac.in; balaji.krishb1@gmail.com; tel.: +91-040-663-03552 received: february 15, 2018; revised: march 22, 2019; accepted: march 25, 2019 abstract proton exchange membrane fuel cell (pemfc) and direct methanol fuel cell (dmfc) are increasingly used as substitutes to conventional energy systems. their compact design, high energy density and efficient energy-conversion offer several advantages over existing energy systems with potential for use in a variety of applications. however, performance, robustness and cost are the key challenges to overcome before fuel cells can be commercialized. even though the use of platinum (pt) and platinum group metal (pgm) alloy catalysts provide higher performance and durability, they are at the same time the largest cost components which need to be addressed. this paper reviews different approaches adopted to enhance pt utilization such as reducing pt loading, decreasing pt particle size, developing pt free metallic alloy catalyst, improving pt dispersion, developing membrane electrode assembly (mea) fabrication methods, increasing mass-transport at the electrode surface and modifying the catalyst support materials. finally, the performance optimization efforts for pt utilization are summarized with insights into probable directions of future research in this area. keywords electrochemical active surface area; hydrogen adsorption; co stripping; catalyst particle size; catalyst loading; catalyst dispersion; catalyst support introduction over the last two decades, proton exchange membrane fuel cell (pemfc) and direct methanol fuel cell (dmfc) have been extensively studied and are emerging as potential systems as they provide clean energy and are commercially viable [1]. they possess a series of highly advantages features such as low operating temperature (<100 °c), high efficiency, sustainability at high current http://dx.doi.org/10.5599/jese.665 http://dx.doi.org/10.5599/jese.665 http://www.jese-online.org/ mailto:balaji@hyderabad.bits-pilani.ac.in mailto:balaji.krishb1@gmail.com j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 282 density, development of catalyst material for hydrogen generation, 10-100 fold reduction of the catalyst loading, multiple options for membrane electrode assembly (mea) fabrication, manufacturing and synthesis process for gas diffusion layer (gdl) and membranes, development in thermal and water management, compactness, potential for low volume and weight, long stack life, fast start-up and suitability for discontinuous operation [2-4]. in the recent years, leveraging these advantages, automobile and fuel cell (fc) manufacturers are moving towards commercialization from a demonstration phase. this is carried out through development of suitable catalysts, introduction of zero emission vehicles and development of prototype vehicles that have adopted fc systems. however, several technological challenges and cost reduction still need to be overcome. the primary approaches towards addressing these challenges include reducing the precious metal catalyst loading with controlled particle deposition through improved electrode preparation methods, optimizing the mea, enhancing the tolerance of mea to high temperature to promote higher degrees of electro-catalysis, exploring non-platinum based electrode materials [5-7]. platinum (pt) or pt-alloys such as ptru, ptco, ptni, ptfe, ptv, ptmn, ptcr are the most suitable electrocatalysts for pemfcs and dmfcs as they have low overpotential values, high catalytic activities and also ability to tolerate the harsh acidic surroundings inside fc. pt is a less abundant and expensive noble metal that contributes as much as 50 % (even at 1.7 % of pt) to the total cost of fuel cell. in addition, pt utilization is quite low with only 25-35 % utilized for electroactivity [8-10]. the limited supply and high cost of pt necessitate its optimal utilization and reduction in pt loading. table 1 illustrates the status and long term goals set by the u.s. department of energy for pt utilization with pt loading targets. as a result, optimizing pt utilization in order to reduce cost has been the primary focus of much research in this field [11-13]. table 1. platinum metal content status and targets [13] characteristics 2011 status targets 2017 2020 platinum group metal (pgm) total content (both electrodes), g kw-1 0.19 0.125 0.125 pgm total loading, mg m-2 0.15 0.125 0.125 loss in initial catalytic activity, wt% 48 40 40 electrocatalyst support stability, wt% 10 10 10 mass activity, a mgpt-1 0.24 0.44 0.44 non-pt catalyst activity per volume of supported catalyst, a cm-3 60 300 300 in order to improve pt utilization efficiency, the electrocatalysts need to be developed with the reduced pt loading and increased pt active sites. the active research approaches explored towards enhancement of effective pt utilization in pemfc and dmfc electrodes are grouped in following categories [14,15]: • reducing the electrocatalysts loading in fuel cell electrodes. • developing novel nano-structured thin-film pt by decreasing the electrocatalysts nanoparticle size • reducing pt dependence by developing metallic alloys and pt-free electrocatalysts. • improving electrocatalysts dispersion by using novel fabrication methods. • developing mea fabrication methods to enable better catalyst dispersion and utilization. • using new techniques to increase mass-transport at the fuel cell electrode surface. • improving the performance of carbonaceous electrocatalysts support and exploring novel noncarbonaceous electrocatalysts support materials. madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 283 although pt is the catalyst commonly used for both anode and cathode in pemfcs, the oxygen reduction reaction (orr) at cathode is more complex with sluggish reaction kinetics, thus requiring higher pt loading, typically several times higher than that of anode. in addition, the hydrogen oxidation reaction (hor) on the dmfc anode catalyst suffers from high over potential. this has triggered researchers to explore optimization of pt catalyst including the synthesis of catalysts that are more active and less expensive than pt [16,17]. this paper focuses on various approaches, already taken by researchers to optimize pt utilization for cathode of pemfc and both electrodes of dmfc to reduce the cost and improve the performance and durability of pemfcs and dmfcs. catalyst utilization can also be represented in terms of mass activity and specific activity. as catalyst utilization is a surface phenomenon [18], the progress made in this area has been presented by considering electrochemical active surface area (ecsa), as the benchmark parameter across various research approaches over a period of time. the calculation procedure of this parameter is briefly described in the following section. in addition, fuel cell performance can also be increased by increasing specific activity of the catalyst by alloying or shaping. specific activity (sa) is the ratio of the kinetic current to the real surface area of the catalyst. nevertheless, in the case of nanostructures the real surface area is significantly greater than geometric and can be determined using co stripping [21]. however, discussing specific activity of the catalyst is beyond the scope of this review. measuring platinum utilization efficiency platinum utilization (ptut) is the amount of exposed pt atoms available for catalytic reaction among those dispersed on the electrode. as catalyst utilization is a surface phenomenon, the surface and the specific surface area are considered as critical factors for measurement. here, electrochemical active surface area (ecsa) is considered as a measure of pt utilization. a comparison of ecsa with geometrical specific surface area (gsa) tells the number of electrochemically active pt surface atoms. therefore, the ratio of ecsa to gsa is considered as a measure of ptut [19,20]. ptut, % = [ecsa/gsa] × 100 (1) electrochemical active surface area (ecsa) ecsa is a measure of the number of electroactive pt sites on the surface of the catalyst on which reaction occurs and is sometimes called the intrinsic pt catalyst surface area. numerous methods are available in the literature to calculate ecsa of pt catalysts. in the context of this review, two primary methods, h-adsorption (ecsah) and co stripping (ecsaco) used for measuring ecsa are explained below [21]. h-adsorption (ecsah) ecsah of pt catalyst, reported in m2gpt-1 units, is determined conventionally from hydrogen under-potential deposition (hupd) reaction, which can be expressed as [22]: pt + h+ + e⇄pt-hads (2) figure 1 shows a typical hydrogen electrosorption/desorption voltammetric profile for the polycrystalline pt electrode in 0.5 m h2so4. characteristic peaks between 0.0 and ~ 0.2 v are attributed to the atomic hydrogen desorption/adsorption at pt surface and ecsah can be determined by: ecsah = (100 qh) / (qh(ref) m) (3) http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 284 in eq. (3), qh / μc is the charge calculated by integrating the area under the cyclic voltammetry (cv) curve (figure 1) within the potential range of hydrogen desorption, qh(ref) is the charge corresponding to formation of the monolayer of adsorbed hydrogen on the polycrystalline pt given as 210 µc cm-2, and m is the mass of pt loading (mg) that indicates the amount of pt at the working electrode [23,24]. figure 1. typical hydrogen adsorption/desorption voltammetry profile for polycrystalline pt electrode in 0.5 m h2so4 at room temperature at 100 mvs-1 [21] when electrodes are fabricated with the polymer electrolyte membrane (pem), however, all pt active sites are not available for electrochemical reaction. this may be due to an improper contact of pt particles with the solid polymer electrolyte or to electrical insulation between pt particles caused by a thin film of the electrically non-conducting polymer electrolyte. in such case, the ecsa measured by cv in solid polymer electrolyte is denoted as ecsamea which is called as driven – cell mode [25]. this value may be smaller than ecsa. the ratio of ecsa and ecsamea is termed as mea platinum utilization [81]. co stripping the most commonly used anode catalyst for dmfc is platinum-ruthenium (ptru). the ecsa and performance of the prepared ptru anode catalysts for dmfc are measured by means of co stripping method. pre-adsorbed carbon monoxide (coad) oxidation can be calculated using co adsorption removal voltammogram (figure 2) at a particular scan rate. gaseous co can be purged into the cell to permit complete co adsorption onto the electrocatalyst while maintaining a steady voltage. the surplus co can be removed by nitrogen gas [26]. the following reactions occur on the platinum surface for methanol oxidation: ch3oh + pt → coad-pt + 4h+ + 4e (4a) pt +coad-pt + h2o → coad-pt + h+ + ohad-pt + e→ 2pt + 2h+ + co2 + 2e (4b) for ptru, the equation for methanol oxidation is as follows: ch3oh + ptru → coad-ptru + 4e+ 4h+ (5a) coad-ptru + h2o → coad-ptru-ohad + e+ h+ → ptru + co2 + 2e-+ 2h+ (5b) madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 285 figure 2. co stripping current (red line) recorded at 500 mvs-1 scan rate for polycrystalline pt electrode in 0.5 m h2so4 at 298 k together with a blank curve (black line). co adsorption at 0.29 v vs. rhe for 20 min [21] as the charge of the co oxidation peak is taken as two electrons per co molecule, the resulting charge is approximately double of that corresponding to the hydrogen-desorption peak. ecsaco can be calculated using the following equation: ecsaco = (100 qco) / (qco(ref) m) (6) in eq. (6), qco / μc is the charge calculated by integrating the area under co desorption peak, qco(ref) is the charge needed for oxidation of the monolayer of adsorbed co on the platinum surface, given as 420 μccm-2, and m is the mass of pt loading (mg) indicating the amount of platinum on the working electrode. co stripping gives realistic ecsa on pt-alloys, whereas esca based on hupd is usually lower [27]. optimization approaches for pemfc cathode some approaches to optimizing pt utilization in pemfc cathode are described below: reducing pt loading and decreasing pt particle size typically, the performance of a catalyst layer (cl) has proportional relationship with the catalyst loading. however, in the case of platinum, there is a challenge of enhancing cl performance while decreasing pt loading. in order to achieve this, reducing of particle size and improving pt dispersion on the catalyst support have been explored. as was already reported, the effective particle size of pt based catalyst is between 2 nm and 4 nm [28]. since the percentage of exposed pt decreases with increase of pt particle size, only 20-50 % of total pt is available for catalytic reaction, what is far below ideal dispersion. martin et al. [29] attempted to optimize electrode performance at significantly low pt loading (0.012 mg cm-2) obtained by electrospray method. by optimizing control parameters (nafion® content, flow rate for electro-spraying catalyst ink and pressure for hot pressing mea) they could achieve high http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 286 ptut of 20 kw g-1 at 3.4 bar, a 70 °c. this is relatively high ptut when compared to the previous of 3.3 kw g-1 [30]. later on, the same team reported [31] further increase of ptut to 30-35 kw g-1 at low pt loading of 0.01 mg cm-2, achieved by utilizing additional benefits of a microporous layer of carbon nanotubes coated on gdl as the cathode catalyst support. the authors also observed that pt utilization remained unchanged for nafion® content in the range of 30-50 %. wang et al. [32] fabricated mea with low pt loading of 0.022 mg cm-2 at the cathode side by simultaneously using electrospray and electrospinning (e/e) dispersion techniques, where they introduced nafion® and pt using two separate needles to produce nafion® nanofibers and platinum nanoparticles (ptnp). using this technique, they could regulate the size of fibers and loading of pt catalyst, what resulted in improved ecsa with excellent pt utilization of 42 kw g-1 for h2/o2 electrode at 0.022 mg cm-2 pt cathode loading. alia et al. [33] developed a novel electrocatalyst using pt/pt alloy nanotubes (ptnt) and carried out voltammetry measurements for every 6000 cycles to investigate durability and orr activity. they found that the electrode with cathode loading of 0.04 mg cm-2 showed highest ecsa of 23.9 m2 g-1 with 127 % ptut (eq. 1) for which electrode porosity was stated as the reason. su et al. [34] combined ultrasonic spray coating technique and catalyst coated substrate (ccs) to achieve low pt loaded gas diffusion electrodes (gde). four electrode samples with varying pt loading (between 0.138 and 1.208 mg cm-2) were studied at 160 °c (air and h2 flow rate 250 and 100 cm3 min-1), while maintaining atmospheric conditions. as shown in figure 3, they observed that current density values increased with decrease in pt loading of cathode electrode and found that the electrode having 0.712 mg cm-2 pt loading showed the highest current density values due to the highest ecsa. therefore, low pt loading electrode means thin cl, when most of the catalyst is concentrated in the interface between cl and electrolyte membrane, resulting in higher pt utilization. figure 3. cv profiles of gdes with different catalyst loadings. reproduced from [34] with permission from elsevier nabil et al. [35] electrochemically characterized the novel niobium carbide/carbon nano tubular porous structure (nbc) as the catalyst support for pemfc electrodes. supports were prepared by electrospinning and functionalized with pt nano particles. the orr activity and catalyst stability madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 287 showed less ecsa for pt/nbc/c (43 m2 g-1) than for geometrically calculated surface area (90 m2 g-1). the less conductive nb2o5 was stated as a reason for lower ptut. also, 31 % retention in ecsa was observed for pt/nbc/c after 10,000 cycles when compared with commercial pt/c (only 5 % retention). the greater stability is attributed to the corrosion resistance of one dimensional nbc nanostructures. the same group [36] using the same technique synthesized nanotubes with suitable properties as the catalyst support for pemfc cathode. 3 mm pt particles were deposited on nbc nanotubes. from electrochemical characterization, they observed less ecsa (43 m2 g-1) for 30 % pt/nbc than for 50 % pt/c. also, better stability was found for the novel support than commercial 50 % pt/c, when ecsa of both catalysts were equal. summarized results for the above category are given in table 2. table 2. reducing pt loading and decreasing particle size (pemfc cathode) method / technique / support pt loading, mg cm-2 ecsa, m2 g-1 ptut, % reference electrospray 0.012 - [29] carbon microporous layer (cmpl) 0.01 - [31] electrospinning/electrospray (e/e) 0.022 93.9 [32] porous ptnt 40 23.9 127 [33] ultra sonic spray coating technique 0.713 28.38 -[34] electrospinning (pt/nb/c) 0.2 43 [35] electrospinning (pt/nb/c) 30 % pt 43 [36] improving pt dispersion dispersion of pt on different carbon supports has been explored using various methods such as impregnation-reduction, ion exchange (ie), micro emulsion-based synthesis, etc. the supports used may include vulcan, multiwalled carbon nanotubes (mwcnt), vulcan xc-72r, carbon aerogel (ca), bp2000, activated carbon, carbon cryogel and others. the objective of dispersion is to reduce average pt particle size and hence, to increase the catalytic surface area [37]. bayrakceken et al. [38] deposited pt nanoparticles of size 1-2 nm uniformly distributed on carbon supports (pt/vxr, pt/mwcnt, pt/bp2000) using super critical carbon dioxide deposition technique (figure 4). a b c figure 4. hrtem images of pt particles on different carbon supports (a) pt/vxr (b) pt/mwcnt and (c) pt/bp2000. reproduced from [38] with permission from elsevier they achieved nearly three times higher ecsa (173 m2 g-1 for pt/vxr) with the reduced pt loading when compared to a commercial catalyst (57 m2 g-1). the authors assumed that even for smaller pt particles, the hydrophobic nature of carbon support makes pt particles inaccessible for electrolytes. http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 288 here, the highest ptut of 93 % was observed for pt/mwcnt with ecsa of 130 m2 g-1. the observed differences were due to variation in structure of carbon supports. fang et al. [39] deposited ptnts on vulcan xc-72 (vc) carbon black (cb) by employing the homogeneous deposition technique with ethylene glycol (eg) (reducing agent) using polyol process (hd-eg). they found higher ecsa of 78 m2 g-1 with high pt utilization of 78 % when compared with the microwave (eg-mw) assisted polyol approach or conventional sodium borohydride (nabh4) reduction process. this highest pt utilization (60 wt% on vc by hd-eg) was attributed to the relatively small ptnp with improved particle dispersion creating more active sites for the anticipated reactions. mroz et al. [40] used the pulse laser deposition (pld) method for depositing catalyst films directly on nafion® membranes and gdl and achieved accurately controlled low catalyst loadings with greater uniformity. under various pressure ranges, electrodes were prepared and studied by varying thickness of the catalyst film between 0.09 and 3.47 nm, in proportion to the laser pulses between 75 and 3000. at low cell voltage, the results revealed higher current densities and less transport resistance of reagents when compared to cells with pt catalyst placed on the nafion® membrane. in the case of high cell voltages, low currents were generated due to less active surface area of pt available for reactions. the authors stated that the efficiency can be further increased by growing the contact area between electrolyte and catalyst and also by enhancing the reaction zone to advance the passage of reagents. rost et al. [41] deposited pt particles on carbon nanofibers (cnf) by employing the pulse plating technique (ppt). they observed 3.1 times higher ecsa of 148.6 cm2 mg-1 with cnf functionalization at 120 w for 10 min. the creation of the functional groups on fibrous surface of the nanoparticles and graphene (gr) is stated as the reason for smaller and homogenous dispersion of catalyst. huang et al. [42] used the impregnation method to synthesize highly dispersed pt catalyst with addition of centyltrimethylammonium bromide (ctab) as dispersant at 300 °c. they prepared cl samples with different pt loadings and observed higher ecsa and high ptut for the catalyst sample with 4.12mg cm-2 dispersant concentration. the improved results were attributed to lypophilic alkyl and hydrophilic amine groups of ctab. lv et al. [43] synthesized the novel carbon mixed carbide-based catalyst support (sic/c) to improve electrochemical performance and catalytic activity of pemfc cathode. they deposited pt catalyst on nano sic/c particles and observed homogenous dispersion of pt particles on nano-sic particles with evidently increased ecsa (48 m2 g-1) for pt/sic/c when compared to pt/sic (13 m2 g-1). the addition of carbon to pt/sic and uniform dispersion of pt nano particles on the novel support were attributed to the increased ptut in orr activity of pemfc electrode. summarized results for the above category are given in table 3. table 3. improving pt dispersion (pemfc cathode) dispersion echnique dispersant catalyst support pt loading ecsa, m2 g-1 ptut, % ref. scco2 pt/vxr pt/cnt pt/bp2000 9.0 wt% 9.9 wt% 47.5 wt% 173 130 102 74 93 36 [38] hg-ed cb 0.2 mg cm-2 78 78 [39] pld cb 1.24 µg cm-2 [40] ppt pt/cnf 1 mg cm-2 148.6 [41] ctab ctab: 0.096 mg cm-2 65.97 63.53 [42] eg reduction method pt/sic/c 20 % 48 [43] madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 289 structural modification of catalyst layer catalyst layer needs to be designed in order to ensure effective movement of electrons, protons, reactant gas, and water through a porous medium. structural modification of cl aims to improve mass transport, decrease interfacial resistance between catalyst layer and gas diffusion layer, enhance catalyst utilization, and reduce pt loading [44]. su et al. [45] replaced the conventional double catalyst layer (dcl) with a novel double carbon layer (ndcl) cathode with high pt content for inner layer and low pt content for outer layer. five cathodes (ndcl-1 to ndcl-5) with different pt distribution ratio of inner and outer layers were prepared (table 4). from cv (figure 5), better ecsa for the novel dcl than single catalyst layered (scl) and conventional dcl catalysts can be observed. the highest ecsa of 81.4 m2 g-1 was recorded for ndcl2 having the same inner and outer layer thicknesses, which was attributed to the reduced mass transport resistance. further, the reduced catalyst and nafion® content in outer layer was proved beneficial for orr and water removal. table 4. five dcl cathodes with different inner and outer pt distribution ratios [45] novel double catalyst layer cathode ndcl-1 ndcl-2 ndcl-3 ndcl-4 ndcl-5 pt distribution (inner : outer) 9:1 8:2 7:3 6:4 5:5 lout/lin (estimated value)a 0.44 1 1.71 2.78 3.91 alout: outer catalyst layer thickness, lin: inner catalyst layer thickness. figure 5. cvs for mea with single, conventional double and novel double catalyst layered cathode structures. reproduced from [45] with permission from elsevier (esca is marked as easa on the graph qiu et al. [46] developed a novel cathode structure (ncs) having double catalyst layer with reduced pt loadings of 0.23 mg cm-2 and 0.11 mg cm-2 for inner and outer cl, respectively (figure 6). the ncs was provided with hydrophobic and hydrophilic regions. it was observed that rich small pores of the outer layer provided efficient electrochemical active surface with the reduced gdl interfacial resistance, improving thus mass transfer. the compact structure of the inner layer http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 290 provided more active catalyst sites, resulting in improved migration of protons towards the membrane. the results showed 50 % pt utilization for the developed ncs which was 28 % higher than that of traditional catalyst structure (tcs). figure 6. schematic of novel cathode structure (ncs). reproduced from [46] with permission from elsevier avcioglu et al. [47] made an attempt to explain the significance of cl in water management since cl impacts water balance, while water flooding leads to mass transfer limitation at high current densities. to enhance durability performance and to ensure efficient thermal and water management in the components of pemfc, the authors constructed a mesoporous hydrophobic channeled catalyst layer by addition of poly(tetraflouroethylene) (ptfe) nanoparticles and employing a two-step catalyst preparation method. by electrochemical characterization of three different types of catalyst viz., pt/c, pt/c-nafion®, pt/c-nafion®/ptfe, it was found that addition of ptfe to the catalyst diminished pt agglomeration, but reduced ecsa and ptut of pt/c-nafion®/ptfe. it was concluded that higher ecsa (77 m2 g-1) was observed for pt/c catalyst because of increased total catalyst surface area. the ecsa (70 m2 g-1) of pt catalyst with nafion® (pt/nafion®) was slightly reduced, but ptut was improved due to formation of pt agglomerates by nafion® addition. the ecsa of pt/nafion®/ptfe, however, was only 45 m2 g-1 and ptut was 56 % with enhanced diffusion process in the catalyst layer. disruption of tpb, active sites blockage and increased average pore diameter were stated as the consequences of ptfe addition. in order to overcome kinetic loss occurring at low pt loading, breitwieser et al. [48] employed a direct membrane deposition technique (dmd) in mea fabrication process, wherein the conventionally free-standing membrane was replaced by two 8–15 μm thin ionomer layers with low pt loading of 0.102mg cm-2 on anode and 0.029 mg cm-2 on cathode, respectively. at 300 kpaabs of total pressure and under oxygen atmosphere, the dmd fuel cell yielded a maximum power density of 2.56 w cm-2 with very high ptut of about 88 kw g-1. this value was 2.3 fold higher than that of a commercial nafion n-211 membrane reference fuel cell. the high ptut efficiency was attributed to the novel dmd fabrication technique, which favored lower ionic resistance and improved power density in the high current density range. to reduce pt loading, kaplan et al. [49] developed pt alloys and core-shell structures. they synthesized two carbon supported pt-surface enriched nano sized (pt-sens) catalysts and performed characteristic studies with a partial pt shell and low-cost ruthenium (ru) and iridium (ir) madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 291 metal cores. instead pt loading of 48 mg cm-2 for commercial pt catalyst, they used pt loading of 14 mg cm-2 for pt/ir/xc72 and 19.5 mg cm-2 for pt/ru/xc72 prepared electrodes. ecsa of 29 m2 g-1 was obtained for pt/ru/xc72 catalyst using co-stripping method, while 26 m2 g-1 for pt/ir/xc72 and 48 m2 g-1 for 50 % wt pt/c were obtained by hupd method. it was concluded that pt-surface-enriched structure increased availability of pt particles for the catalytic reactions. chen et al. [50] designed a new gradient cathode with appropriate pt/c ratio gradient direction (70 wt% inner side and 40 wt% outer sides) and nafion® gradient span (33 wt% inner side and 23 wt% outer sides). this improved both pt utilization and mass transfer, thus significantly improving the performance. especially under low humidity, the performance of appropriate gradient mea was 135.7 % higher than the mea with scl cathode at 20 % relative humidity. it also provided better water management, improved proton conductivity in cathode, and showed lower ohmic resistance and charge transfer resistance. summarized results for the above category are given in table 5. table 5. structural modification of catalyst layer (pemfc cathode). novel cathode structure pt loading ecsa, m2 g-1 ptut, % references ndcl 0.2mg cm-2 81.4 -[45] ncs 0.4mg cm-2 35.42 50.56 [46] pt/c pt/c-nafion® pt/c-ptfe 21 wt% 13 wt% 8 wt% 77 70 45 75 99 56 [47] dmd 29 µg cm-2 88 kw gpt-1 [48] pt/ru/xc72 pt/ir/xc72 19.5 µg cm-2 14 µg cm-2 29 26 - - [49] pt/c &nafion® ratio 200 µg cm-2 8.4 -[50] optimization of catalyst parameters high pt utilization can be achieved by optimizing various catalyst layer parameters such as nafion® content, pt loading, teflon content, catalyst layer and carbon support thicknesses, etc. [51]. cheng et al. [52] investigated the effect of nafion® and ptfe on ptut for two catalyst layers: a) etek electrode/gde prepared using immersion method where pt is impregnated with nafion® and b) thin film catalyst layer (tfcl) prepared by blending nafion® and pt catalyst. ptut of 77.8 % and 45.4 % were observed for the first and the second catalyst layer, respectively. it was concluded that while the catalyst particles are provided with sufficient passage ways for the transfer of protons in the first case, the thin film catalyst layer experienced lack of conduction because of the blockage of passage ways by the nafion® solid. antolini et al. [53] investigated the effect of nafion® loading on orr at pemfc electrode. they observed that the electrode exhibited poor electrolytic conductivity at low nafion® loading and that ecsa increased with increasing content of nafion® (figure 7). the introduction of nafion® is stated to improve the active surface area, resulting in better transport of protons through the electrolyte. sasikumar et al. [54] investigated the dependence of nafion® content on pt loading. electrodes were prepared with different pt loadings (0.5, 0.25, 0.1 mg cm-2) and different nafion® ionomer contents (20, 30, 40, 50, 60 wt%). the highest ptut (52 %) was obtained at 40 wt% nafion® content for pt content of 0.25mg cm-2. the authors concluded that nafion® content must be determined by pt loading and the fabrication technique used. the electrode performance was found to improve with enhancement of nafion® content up to 40 %, but further increase in nafion® content reduced the performance. the reasons attributed to this effect are blockage of pt sites, decreased gas http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 292 permeability and augmented mass transport polarization. the increased distance for gas permeation, diffusion and migration are stated as reasons for transport limitation. figure 7. ecsa (marked as esa on the graph) variation with nafion® loading in the cl (20 wt% ptfe/c on dl, 0.2mg cm-2 of pt in cl). reproduced from [53] with permission from elsevier jiang et al. [55] studied the effect of thickness of carbon supported pt catalyst layer in orr using a thin film rotating disc electrode. various catalyst layer thicknesses (0.25, 0.5, 1.0, 2.0, 4.0, 8.0 mm) were examined. the highest ptut (77.9 %) was observed for 8.0 mm film. it was concluded that ecsa tends to increase with increasing thickness of the catalyst layer, showing more activity towards orr accordingly. bayrakceken et al. [56] used super critical co2 deposition to prepare vulcan supported pt catalyst (pt/vulcan) and studied the effect of thermal reduction. samples with pt loading of 9 %, 15 %, 35 % were produced and ecsa were investigated using cv. the order of ecsa was found as follows: pt/vulcan 35 %  pt/vulcan 15 %  pt/vulcan 9 %. although ecsa for pt/vulcan 15 % was found higher than for pt/vulcan 35 %, the ptut was lower. this difference is, however, quite small (58 % vs. 62 %) and is not considered significant related to the uncertainty in both the calculated and the measured average pt particle sizes. the difference in ecsa between pt/vulcan 15 % and 35 % catalysts is attributed to the differences in the pt particle sizes, since ecsa pt values differ by a factor of approximately 1.5. using plasma dc-magnetron sputtering technique, khan et al. [57] studied the conical nanopillar structured pt catalyst sputtered on the cathode with argon pressures of 0.05, 0.1, 0.5 mbar and at pt loading range of 0.05 0.2 mg cm-2. they observed high ecsa for the catalyst nanopillar deposited at 0.5 mbar at low pt loading of 0.05 mg cm-2. it was concluded that the deposition of uniform conical nanopillars on the electrode at high pressure improved catalytic activity and led to higher ptut. slavcheva et al. [58] studied the effect of sputtering parameters (film thickness, argon pressure, dcsputtering power) on thin films deposited by dc-sputtering technique. at low sputtering power (120– 130 nm thick pt films, 6 mtorr argon pressure, 100 w dc power), they found a stable and crystalline catalyst film with thickness between 120-130 nm, with large ecsa and ptut. it was concluded that ecsa increased with increase of film thickness and improved the electrochemical activity towards orr. madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 293 wang et al. [59] studied the influence of ptfe on electro spun/electro sprayed (e/e) mea. they introduced only 1 wt% ptfe to the process which resulted in ptut of 0.076 g kw-1 at only 0.094 mg cm-2 pt loading. the improvement was attributed to enhanced hydrophobicity of the nanofibers which increases mass transfer and overall performance. speder et al. [60] studied the effect of pt/carbon ratio on degradation of pemfc catalyst. they used a colloidal synthesis approach for pt/vulcan and pt/ketjenblack catalysts using accelerated stress test (ast) in half cells. it was concluded that the loss in ecsa due to ast treatment depends on carbon support and pt loading. zhao et al. [61] investigated the impact of pore size of fdu-15 (two-dimensional hexagonal ordered mesoporous carbon (omc)) supported pt catalyst by adopting a soft-template method. samples with four different pore sizes ranging from 4.0 nm to 8.1 nm were prepared by the addition of pore expanding agents. by impregnation method, pt catalysts were prepared using fdu-15 samples as catalyst supports and reducing agent, eg. cv and linear sweep voltammetry (lsv) measurements illustrated that catalyst with a pore size of 6.5 nm showed the highest ecsa (70.2 m2 g-1). it was also observed that pt particle size is reduced with increase in pore size up to 6.5 nm and remained unchanged with further increase in pore size. kim et al. [62] examined the effect of diffusing solvents with varying main chain mobility of nafion® to demonstrate large influence on nafion® agglomerates, morphology and other properties such as proton conductivity and water uptake. the mea synthesized from the dimethyl sulfoxide based isopropyl alcohol (ipa) and n-methyl-2-pyrrolidone (nmp)-based catalyst inks presented ecsa of 40.9, 43.0 and 52.5 m2 g-1, respectively. the introduction of diffusing solvents resulted in better diffusion of nafion® into pt/c aggregates, bringing more pt particles in contact with nafion® thereby increasing the catalytic activity at reduced current density. ruengkit et al. [63] investigated the influence of gdl properties (type of material (carbon paper (cp), carbon cloth (cc)), thickness, ptfe content and presence of micro porous layer (mpl)) on ecsa of pt electrodeposited layer and fuel cell performance. pt catalyst was electrodeposited by dc current onto gdl, with cc and cp of different thicknesses and hydrophobicity, and with and without presence of mpl. despite their morphological differences, relatively similar ecsa (~120 m2 g-1 pt) were obtained, while better performance for cc at higher current densities is due to its lower resistance and higher porosity. pt deposited on a very thin gdl (90 μm) or gdl having high ptfe content (≥30 %) or containing mpl, consist of non-uniform particles distributed unevenly on the gdl surfaces, resulting in low ecsa (≤120 m2 g-1 pt) and fc performances (i≤210 ma cm-2). pt electrodeposition on the cc having low ptfe content (10 %) produced small and uniform pt particles distributed evenly throughout the gdl surfaces resulting in highest ecsa (193 m2 g-1 pt). the same phenomenon was also observed for the two-layered gdl, which is attributed to the absence of the hydrophobic sublayer in the posttreatment of the gdl. it was concluded that selection of gdl as the fuel cell electrode is crucial when pt catalyst is prepared by electrodeposition. egetenmeyer et al. [64] investigated electrodeposition of cathode catalyst layer using the plasma process. at different catalyst loadings and plasma durations, nanostructured pt and pt3co cathodes were prepared with varying quantity of ionomer and its distribution in the cathode catalyst layer (ccl). there was an increase of ecsa in comparison with the untreated substrate. ecsa increased with increase in the plasma treatment duration up to 23.1 m2 g-1 (highest) after 15 min plasma treatment, which is indicative of the deposition of smaller pt particles on the electrode. summarized results for the above category are given in table 6. http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 294 table 6. optimization of catalyst parameters (pemfc cathode) parameter/variable/method pt loading, mg cm-2 ecsa, m2 g-1 ptut, % ref. tfcl gde 0.4 63.6 - 45.4 77.8 [52] nafion® loading 0.2 115 -[53] optimum nafion® content 0.25 648 cm2 52 [54] thickness of cl 0.256 91.1 77.9 [55] thermal reduction(scco2) 0.028 173 74 [56] dc-sputtering parameters 0.05 106.10 -[57] dc-magnetron sputtering parameters 47.09 cm2 93.62 [58] effect of ptfe on electro spun /electro sprayed mea 0.094 81 [59] platinum loading 14 126.5 [60] pore size on omc/fdc-15 2 70.2 [61] solvent effect (nmp) 0.2 52.2 [62] effect of gdl thickness 0.25 193 [63] plasma process 0.3 23.1 [64] improving the performance of pt support materials the selection of appropriate support materials plays a key role in enhancing the overall performance of pemfc and dmfc. characteristics of good support material include presence of surface functional groups that promote catalyst-support interaction, mesoporous structure which improves triple phase boundary (tpb), good electrical conductivity and electrochemical stability, high surface area and corrosion resistance. typically, conductive and porous membranes are known to possess above mentioned features and are therefore preferred as support materials for pt catalyst [44]. existing research reveals that use of carbon-based support with various structural and morphological properties improved stability and electrocatalytic activity of catalyst. in addition, mass transfer and electronic conductivity in cl are also found to be enhanced at carbon supports. some of the novel carbon-based support materials investigated so far are carbon nanofibers (cnf), ordered mesoporous carbons, carbon aerogels, carbon nanohorns, carbon nanocoils and carbon nanotubes (cnt) [44]. in this connection, zhao et al. [65] demonstrated that nearly 100 % of pt can be exposed to catalytic activity by using gold nanoparticles as pt catalyst support (aupt). ecsa measurements were carried out on vulcan xc-72 cb samples by varying pt and au ratio. prominent enhancement of ecsa and pt utilization was observed when pt/au ratio becomes lower than 0.2. further decrease of pt/au ratio to 0.05 or less, makes ecsa and ptut to increase up to 100 %. this approach ensured uniform dispersion of pt particles and prevented pocketing in the micropores of supported material, making thus pt particles efficient for orr. kongkanand et al. [66] and orfanidi et al. [67] elucidated use of carbon nanotubes (swcnt) and (mwcnt) as pt catalyst supports. pt particles supported on swcnt showed lower ecsa (17.8 m2 g-1) than the commercial carbon support (33.5 m2 g-1) with 10 mv shift in the onset potential for orr. they inferred that the commercially available pt has better distribution of pt nanoparticles than the one developed in the lab with swcnt. pt supported on mwcnt, however, showed improved ecsa of 78.4 m2 g-1 with 84 % ptut. higher ecsa and better catalytic utilization were attributed to uniform dispersion and narrow particle size distribution (2.5 to 4 nm). instead of using pure swcnt, zhu et al. [68] used a mixture of swcnt and carbon nanofibers (cnf) at 1:3 (sf13) and 1:5 (sf15) ratios (bucky paper) to prepare a novel catalyst. they observed reduced ecsa due to larger pt particle size (6 nm) than of commercial catalyst support (2 nm). however, ptut madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 295 of 88 % and 84 % for sf13, sf15 were observed which was ascribed to availability of the catalyst particles at electrolytic and electronic pathways of the bucky paper electrodes. kou et al. [69] investigated the properties of functionalized graphene sheets (fgs) as the cathode catalyst support, mostly because of higher conductivity, high surface area and low manufacturing cost with unique graphitized basal plane structure. results revealed higher initial ecsa (109 m2 g-1) with improved stability when compared to the commercial catalyst (e-tek, 75 m2 g-1). this was attributed to smaller pt particle size of 2 nm and less extent of pt nanoparticles agglomeration on fgs. salernitano et al. [70] used the controlled plasma improved chemical vapour deposition (cvd) technique to deposit pt nanofibers on a support made from carbon nano fibers (cnf). electrodes were prepared with different structures of cnf, by changing the orientation of graphene layers in relation to the fiber axis. this resulted in availability of large number of active sites for catalyst nanoparticles. due to the controlled morphology grown of graphite paper, they found higher ecsa (225 m2 g-1) with improved ptut at reduced pt loading (0.018 mg cm-2). huang et al. [71] used conductive titanium oxide as pt catalyst support and investigated pt/tio2 durability and stability using in-house potential cyclic experiments. although initially, the pt/tio2 (31.2 m2 g-1) showed less ecsa than pt/c (56.4 m2 g-1), it continued to remain stable even after 4000 cycles with minimum loss in ecsa. on the contrary, pt/c showed no activity after 2000 potential cycles owing to pt dissolution, carbon corrosion and pt particles sintering. ecsa was calculated after 2500 cycles and observed larger ecsa for pt/tio2 than pt/c catalyst. song et al. [72] developed the novel compound carbon supported pt catalyst (pt/ccs) by introducing ultra-fine porous carbon fibre (upcf) powder to commercial pt catalyst as a second carbon support. partially supported pt catalyst jm-40 catalyst with particle size 3.4 nm was combined with upcf. using cv measurements, it was observed that pt/ccs-20 showed the highest ptut with ecsa 71.9 m2 g-1. sanli et al. [73] used reduced graphene oxide (rgo) and carbon black hybrid material as pt support and fabricated a novel cathode structure for pemfc. using electrospray technique, they synthesized platinum/reduced graphene oxide (pt/r-go) electrocatalyst and platinum/reduced graphene oxide/vulcan xc-72, pt/r-go/vxc-72, hybrid electrocatalyst with 50 wt% r-go. comparing the results with air-sprayed electrode, it was observed that pt supported on the hybrid support (pt/rgo/vulcan xc-72) showed higher ptut efficiency with relatively low pt loading. this enhancement of the electro-sprayed hybrid electrode was credited to facile proton and oxygen transport to the catalytic sites, what resulted from electrode morphology and hybrid support structure. thus, efficient diffusion of reactant gases to the catalytic sites was achieved. singh et al. [74] used carbon aerogel which was synthesized by impregnating pt nano particles using the microwave assisted polyol process. performance and stability were investigated, and results compared with those for commercially available pt/c (jm-20) having the same pt loading. ecsa of 73 and 56 m2 g-1 for pt/ca and commercial jm-20, respectively were obtained. it was concluded that good utilization ratio of 82 % could be due to the conducting and mesoporous ca support which has significant role in improving pt activity and pt utilization. lori et al. [75] employed the modified polymer-assisted deposition (mpad) technique and synthesized molybdenum carbide (mo2c) nano-crystallites without free carbon and used them as the catalyst support, showing ecsa of 28.4 m2 g-1. the reason for low ecsa was attributed to formation of aggregates during deposition, making pt particles inaccessible for reactions. ignaszak et al. [76] studied titanium carbide (tic) and core shelled tic@tio2 material as catalyst supports for pt and pt3pd alloy (pt/tic, pt3pd/tic, pt3pd/tic@tio2) for orr activities. they observed http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 296 close values of ecsa (40, 41 and 37 m2 g-1) for all three catalysts with no significant difference in electrochemical features. however, pt3pd/tic@tio2 retained its activity after 500 cycles, while significant loss of activity was noticed for pt/tic and pt3pd/tic. to overcome the drawbacks of carbon supports of pemfc electrodes, lobato et al. [77] prepared meas with two non-carbonaceous catalyst supports, sic and sictic, for pemfc cathode side and compared the electrochemical performance with carbonaceous support. they observed that catalyst supported on novel materials (pt/sic, pt/sictic) showed excellent electrochemical stability towards environmental corrosion, what was attributed to higher catalyst particle size. also, at the end of 400 cycles, less ecsa degradation of 19.7 % for pt/sic and 17.01 for pt/sictic was observed when compared to commercial (31.09 %) and homemade (29.82 %) catalysts. in an effort to find better alternative to carbon support for pemfc catalyst, shahgaldi et al. [78] synthesized a novel nano composite, titanium encapsulated in carbon nanosperes and carbon titanium core shell structures as cathode catalyst supports, using simple and less costly hydrothermal method. they prepared 10 % and 20 % tio2 encapsulated carbon (ec-10, ec-20) and core shell structures (cs-10, cs-20). the electrochemical study revealed that 10 % tio2 samples (ec10, cs-10) showed higher ecsa (414 and 440 m2 g-1) than ec-20, cs-20 (280 and 180 m2 g-1), suggesting the effect of tio2 amount on the active surface area. using a new synthesis method, borchardt et al. [79] prepared pt metal containing silicon carbide derived carbon (cdc-pt). they reported high ecsa (66 m2 g-1) for the synthesized catalyst than conventional pt/c catalyst and concluded that the deposition of pt nanoparticle onto carbon matrix provided long term stability with superior corrosion stability. summarized results for the above category are given in table 7. table 7. carbonaceous and non-carbonaceous electrocatalysts support materials (pemfc cathode) catalyst support pt loading,mg cm-2 ecsa, m2 g-1 ptut, % ref. gold nanoparticles(aupts) 0.012 234.1 99.5 [65] single wall carbon nanotubes(swcnt) 0.014 17.8 -[66] multiwalled carbon nanotubes(mwcnt) 0.005 78.4 84 [67] buckypaper 0.09, 0.14 43.3, 39.2 88, 84 [68] functionalized graphene sheets(fgs) 20 wt% 109 -[69] carbonnanofibers (cnf) 0.3 225 -[70] titanium oxide 0.4 31.2 -[71] ultra porous carbon fiber (upcf) 0.35 71.9 87.2 [72] pt/rgo/cb (h75) (hybrid support) 0.5 102 [73] carbon aerogel(pt/ca) 0.4 73 [74] molybdenum carbide(pt/mo2c) 0.096 28.4 [75] tic, tic@tio2(pt-tic, pt3pd-tic-tio2) 0.048 40, 36 [76] sic, sictic (pt/sic, pt/sictic) 0.6 13.36, 13.20 [77] tio2, c-tio2 (pt/tio2,pt/c-tio2) 0.1 414, 440 [78] si –cdc (silicon carbide derived carbon) 12 wt% 66 [79] developing pt metallic alloy and pt-free catalysts pt metal is alloyed with other elements to decrease the overall cost of electrodes. several studies have been performed to obtain nearly equivalent or better catalytic activity through the introduction of pt based binary systems such as pt-ni, pt-fe, and pt-co by varying the proportion of pt and the alloying metal. in addition, the performance of some pt free catalysts has also been explored [80,81]. madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 297 using simple chemical reduction method, wu et al. [82] prepared 20 wt% pt and nickel (ni) catalyst with xc-72 conducting furnace black (ptni/xc-72) and graphene (ptni/graphene) as carbon supports. from results of chemical characterization, they found that ecsa of catalyst samples are in the following order: pt/xc-72  ptni/graphene  ptni/xc-72. owing to their special electrical properties, ptni/xc-72 and ptni/graphene electrodes showed better catalytic activity towards oxygen reduction when compared with pt/xc-72. ptni/graphene showed better performance than ptni/xc-72. due to enhanced catalyst carbon interaction and high graphite component, graphene has the potential to provide much higher durability than xc-72. sahoo et al. [83] synthesized doped porous carbon (dpc) with nitrogen and sulphur in a simple and green one-pot process from ionic liquid as the sole source precursor and used as the non-pt cathode catalyst in pemfc. the results of half-cell and full cell studies were compared with pt decorated dpc (pt/dpc). the ecsa for pt/dpc and commercial pt/c after ignoring the role of electric double layer formed between electrode and electrolyte, were 46.88 and 112.34 m2 g-1 with pt utilization efficiency 98.2 % and 60.2 %, respectively. the even distribution of pt nanoparticles over high surface area and porous support of doped carbon material is stated as a reason for 2.5 times higher ecsa for pt/dpc when compared to commercial pt/c. wang et al. [84] synthesized a variety of pt–ni alloy catalysts by potentiostatic electrodeposition. they use an orthogonal array to optimize deposition parameters. particularly, the electrochemical performance of pt–ni alloy catalysts was optimized by dealloying treatment experiment conducted at 0.35 v, 20 min at 50 °c. the results indicated that the deposition time has the most significant impact on the performance of pt–ni alloy catalysts. the prepared sample (pt-ni) showed the highest ecsa value of 81.7 m2 g-1 with the catalyst loading of 0.329 mg cm-2 (pt loading of 0.3 mg cm-2, ni loading of 0.029 mg cm-2). pt–ni catalysts were dealloyed by rapid potential cycling, which resulted in a significant refinement of morphology and enhancement of electrocatalytic activity. the sample with low ni atomic ratio was not suitable for the rapid dealloying treatment. pt–cu (platinum copper alloy) has been studied as a catalytically active material with high surface area, porous platinum structure and hydrogenation promoting property [85]. bele et al. [86] demonstrated the application of stable and highly active ptcu3 nanoparticles on the carbon support. the pt-skin ptcu3 alloy catalyst was fabricated with core shell nanoparticles having intermetallic ordered shell at a disordered core that is tightly embedded into a carbon support. consistent ecsa of about 45 m2 g-1 is reported with utilization much higher than those already reported in the literature for 3 nm pt nanoparticles on high surface area carbon supported electrodes. the thermal annealing enables pt-skin and intermetallic orderings, which are favorable for enhanced activity. the stability is attributed to the firm embedding of pt-cu into a heterogeneous carbon matrix caused by the partial oxidation and xerogelation. mani et al. [87] proposed the technique of surface dealloying as an effective strategy to modify the surface electrocatalaytic properties of platinum. the copper (cu) surface atoms are dealloyed from pt25cu75 alloy precursor compound by electrochemical dissolution (voltammetric dealloying to prepare the active catalyst phase). this novel catalyst yielded an ecsa of 75 m2 g-1. hong et al. [88] fabricated a novel electrode, pd/c@ptskin (c-u), with electrospun structure by using electrospinning and under potential deposition (upd) techniques. by in situ deposition of pt on the surface of pd-np in the electrospun pd/c catalyst layer, they observed that almost all pt shells become accessible for reactant gases and tpb is optimized with pt loading of 19 mg cm-2. the c-u electrode delivered higher peak power density (0.62 w cm-2) than conventional electrode (0.55 w cm-2) at pt loading of 100 mg cm-2. owing to the nearly 5-fold pt loading, the conventional electrode showed http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 298 better performance than c-u electrode at low current density, however at high current density there was a significant loss of voltage. this was due to the increased oxygen transfer resistance close to pt surface. summarized results for the above category are given in table 8. table 8. developing pt metallic alloy and pt-free catalysts pt metallic alloy & pt free metal pt loading, mg cm-2 ecsa, m2 g-1 ptut, % references ptni/xc-72 0.2 93 [82] pt/dpc 0.2 112 98 [83] ptni alloy 0.3 81 [84] pt-cu3 0.025 45 [86] pt25cu75 0.169 75 [87] c-u electrode 330 92.3 [88] optimization approaches for dmfc cathode some approaches for optimizing pt utilization in dmfc cathode are described below. reducing pt loading and decreasing pt particle size wang et al. [89] observed that addition of naoh to aqueous nafion® solution suppressed aggregation of large particles and led to smaller agglomerate particle size with uniform distribution of pt elements in the cathode catalyst layer. further, they obtained improved ecsa (13.94 m2 g-1) and better performance in dmfc. to enhance pt utilization, you et al. [90] adopted multi step reduction, pt/c-nr (n = number of reduction steps), to synthesize pt catalyst with ultra-high pt loading up to 85 %. cv results revealed that pt/c-2r catalyst exhibited higher ptut of 90 % when compared to pt/c-1r (60 %) with the same pt loading. although pt particles size appears to be larger for pt/c-1r than pt/c-nr, the stacked microstructure in pt/c-1r is stated to favor higher ptut. wang et al. [91] investigated electrochemical oxidation of methanol and reduction of oxygen on the commercial pt/c catalyst. high catalytic current was observed for orr with ecsa of 41 m2 g-1. to improve dmfc performance, pu et al. [92] utilized fabricated pt nanorod assemblies in double layered cathode with a catalyst loading of 1.0 and 2.0 mg cm-2 and observed significant increase in ecsa (35.5 m2 g-1) and catalyst utilization. the improvement in performance is attributed to the reduction of charge transfer resistance in dmfc, enabling more efficient pt loading and improving performance of mea. the increased ecsa is attributed to the reduced size of pt particles, loose construction of pt nanorods and embedding of nanorods in the catalyst layer. optimization of catalyst parameters krishnamurthy et al. [93] studied the performance of dmfc by varying the content of nafion® in anode cl while keeping it constant on cathode. the optimized results showed the highest catalyst utilization of 65 % at 0.8 nafion® to carbon (n/c) ratio. further increase of nafion® content was reported to result in pore blocking of the catalyst layer which leads to decrease in performance of dmfc. krishnamurthy et al. [94] studied the influence of ptfe content of anode and cathode cls on dmfc performance. they examined 5, 10, 20, 30 % ptfe content in cathode cl with anode having constant ptfe content of 20 %. they observed the highest catalyst utilization of 65.54 % for 20 % ptfe and concluded that 20 % ptfe content in the cathode catalyst layer resulted in the maximal catalyst utilization. they also concluded that further increase in ptfe content to 30 % leads to the madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 299 blocking of pores of cl, thus reducing oxygen transportation and water removal. the same authors [95] also studied the catalyst utilization by varying nafion® content. 20 % pt/c, 40 % pt/c and platinum black were used as cathode catalysts and the nafion® content was optimized for each. based on cv results, the highest value of catalyst utilization of 65 % for 33 % constant nafion® content with 40 % pt/c catalyst layer was obtained. no significant influence of nafion® content on platinum black catalyst was also observed. matar et al. [96] investigated the effect of thickness of cathode catalyst on dmfc performance. they considered 8.4 µm thickness layers as standard/base thickness for both anode and cathode with same pt loadings. two additional cathode catalyst layers with 19 and 44 µm were examined. the authors observed decrease of ecsa as thickness of cathode catalyst layer increased and found higher ecsa of 282.88 cm2 mg-1 for the standard/base case (8.4 µm). using citrate stabilized method, jiang et al. [97] prepared 60 % pt/c catalyst by varying carbon supports and reductants (pt/xc-72, pt/bp2000, pt/xc-72r). they observed that pt/bp2000 support resulted in higher ecsa (66.46 m2 g-1) with improved catalyst utilization. the difference in performance was attributed to the addition of chloroplatinic acid solution to citrate which contributed to formation of small pt particles corresponding to large specific surface area with high ecsa and catalytic activity. moore et al. [98] investigated catalytic activity of dmfc in terms of morphology of porous cathode catalyst substrate. they synthesized three electrocatalysts, i.e. pt/c-celatum (a macro porous carbon substrate), pt/cmk-3 (a mesoporous carbon substrate), and pt/vulcan xc-72 (a micro porous carbon substrate). after cv studies, they observed major variation between active surface area and total surface area for nafion® impregnated sample (pt/c-celatum). it was concluded that the difference is due to reduced contact between nafion® electrolyte and catalyst surface, blockage of the pores, and poor accessibility of protons between crystalline platinum and carbon substrate, which all led to low pt utilization. zhao et al. [99] employed an electrochemical technique to investigate the influence of nafion® ionomer content (10, 20, 30, 40, and 50 wt%) catalyst layers on the cathode performance. they found the highest ecsa (27 m2 g-1) and catalyst utilization (40 %) at 30 wt% nafion® content with less difference between the highest and lowest ecsa and concluded that nafion® content has less effect on ecsa and ptut in 10-50 wt% range. figure 8 illustrates the effect of nafion® content on ecsa and pt catalyst utilization. figure 8. effect of nafion® content on pt utilization and ecsa (marked as esa on the graph). reproduced from [99] with permission from elsevier http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 300 using various novel catalyst support materials wang et al. [100] fabricated dmfc cathode using high surface area swcnt (pt/swcnt) to increase the triple phase boundary sites. pt/swcnt was prepared using ion exchange (ie) method and the performance with borohydride (bh) synthesis method was compared. they observed improved pt utilization and concluded that entire catalyst particles were loaded on swcnt support, being well dispersed (by ie method) and electrochemically active. chen et al. [101] used microwave digestion modified carbon nanotubes (cnt) on cc as the catalyst support. larger ecsa than the others were found, which is attributed to higher pt dispersion and uniform distribution of pt nanoparticles. yaldagard et al. [102] studied the performance of pt electrocatalyst supported on the protonated polyaniline (pani) coated tungsten carbide (wc) nano composite and compared the results with the commercial pt/c electrocatalyst. high pt utilization of 77 % compared to the commercial one was found, and this improvement was attributed to the synergic effect between pt nanoparticles and wc. jafri et al. [103] synthesized nitrogen doped graphene to support pt catalyst. they used mwcnt spacer to remove face to face agglomeration, what cleaned graphene sheets. two types of samples, pt/nga800+mwcnt, pt/ng180+mwcnt, were prepared using thermal solid state and hydrothermal methods and compared the results with samples without mwcnt spacer (pt/nga800, pt/ng180). the authors observed 10 % improvement in ecsa and 55 and 54 % pt utilization and concluded that the addition of mwcnt spacer provided good accessibility to the catalyst active site and nitrogen doped cnt, what resulted in decomposition of reactive intermediates like h2o2 into o2 during orr. algere et al. [104] investigated sulfurized carbon xerogels (s-cxg) as the support material for pt. the highest catalytic activity was observed for pt/cxg-3s (3 wt% sulphur content) because of right balance of high ecsa (59.0 m2 g-1) and good intrinsic activity. it was found that pt/cxg-3s exhibited better catalytic activity than pt/cxg without sulfurization or vulcan. tesfu et al. [105] investigated the use of mesoporous structured hollow graphitized carbon spheres (hgs) as dmfc cathode catalyst support material in 100-150 °c temperature range. pt/hgs catalyst showed improved ecsa and double layer capacity values which is attributed to higher accessibility of pt particles. however, large decrease in ecsa (60 % compared to 14 % for pt/vulcan) was observed at thermal treatment beyond 850 °c. they inferred that this does not correlate with increased particle size and concluded that during thermal treatment, the decomposition of hgs carbon structure resulted in the blockage of pores, making pt active sites inaccessible to the electrolyte. developing pt metallic alloy and pt-free catalysts hosseini et al. [106] synthesized ni/pt/c electrocatalyst using sodium borohydride as the reducing agent and sodium dodecyl sulphate (sds) as a structure directing agent. this electrode exhibited improved catalytic activity than nipt/c catalyst. ecsa of 41 % (ni/pt/c) and 16.8 m2 g-1 (nipt/c), respectively, were recorded. core shell structure of the prepared catalyst was stated as the reason for the increased ecsa and current density. summarized results for the above category are given in table 9. madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 301 table 9. platinum utilization for dmfc cathode gde/structure/support/membrane/ parameters pt loading, mg cm-2 ecsa, m2 g-1 ptut, % ref. reducing platinum loading and decreasing pt size effect of nafion® ionomer aggregation 6.3 13.94 [89] multi reduction method pt(76)/c-1r /pt(76)/c-2r 2.0 60.2 /65.3 68 / 90 [90] ptnt 41 [91] double layered catalyst based mea 1.0, 2.0 35.5, 34.3 [92] optimization of catalyst parameters nafion® to carbon (n/c) ratio 2.0 6 [93] effect of polytetrafluoroethylene(ptfe) 2.0 57.0 65.54 [94] effect of nafion® content 40 wt% 67.03 65.12 [95] cathode catalyst layer thickness 1 28.3 [96] citrate-stabilized method with different reductants and carbon supports (pt/bp2000) 2.8 66.46 [97] effect of the cathode catalyst substrate morphology (pt/vulcan xc-72) 1 33.4 25.1 % [98] influence of ionomer content(30 % nafion®) 2 28 40 % [99] using various novel catalyst support materials pt/swcnt 11.8 wt% 129.6 cm2 90 % [100] pt/mwcnt 0.3 23.9 cm2 25.1 % [101] pt/pani/wc/c 4 20.46 77 % [102] nitrogen doped graphene with mwcnt spacer (pt/ng1800+mwcnt) 0.42 47 55 % [103] carbon xerogels (cxg) 20 wt% 59.0 [104] hgs 2 [105] optimization approaches for dmfc anode some approaches to optimizing pt utilization in dmfc anode are given below. optimization of catalyst parameters liang et al. [107] electrochemically deposited pt nanowires into partial layer of nano nafion® membrane and formed pt nanowire network. cv tests showed large ecsa for pt nafion® integrated electrodes than conventional e-tek electrode. initial results revealed higher ecsa for pt-nafion® integrated electrode and a low rate of methanol. utilization of methanol during the discharge of cell and increased membrane tortuosity are attributed as reasons for enhancement of the catalytic activity. scibioh et al. [108] explored the ionomer coating (1, 2, 5 wt%) on ketjen black ec 300jd (ec), vulcan xc-72r (vc) and black pearls 2000c (bp) carbon supports with different surface area and pore characteristics. they observed better performance for ec5 (ketjen black ec 300jd coated with 5 wt% nafion®) due to extension of tpb, resulting from the improved interaction between catalyst particles and ionomer. pu et al. [109] employed a thermal imprint technology to prepare passive dmfc using nafion® 115 membrane on the anode side. the anode catalyst utilization was found to be improved with improved ecsa, which is credited to the additionally increased roughness factor of membrane. they concluded that catalyst utilization at the anode can be improved by using patterned membranes h4, h6, h10, with pore depths of 4, 6, 10 µm and roughness factors 2.7, 4.6, 5.4 and observed the highest ecsa for h10 membrane. kaplan et al. [110] studied platinum utilization on methanol-oxidation surface by varying pt:ru surface composition. carbon-supported catalysts with iridium-nickel cores and varying atomic-ratio platinum ruthenium shells (pt25ru75 to pt80ru20) were synthesized. the ecsa values obtained for the studied catalysts were 50 m2 gptru-1 for the commercial catalyst, 35 m2gptruir-1 for pt13ru22, 28 m2 gptruir-1 http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 302 for pt16ru14, 27 m2 gptruir-1 for pt23ru9, and 27 m2 gptruir-1 for pt28ru7. the highest mass activity was obtained for pt13ru22 despite its 23 % lower surface activity compared to pt16ru14. this was explained by better pt utilization of pt13ru22 compared to pt16ru14. this result demonstrated higher platinum utilization of the core shell structure and possible reduction of pt loading with use of core–shell catalysts. the authors stated that further improvement can be achieved by enhancing pt utilization with the highest surface activity and optimal ptru ratio. this can be done by reducing pt wt% while adjusting ruthenium wt% accordingly, in order to maintain the favorable surface pt/ru ratio. lin et al. [111] used a newly developed active screen plasma (asp) technique to examine the effect of temperature and treatment duration on the growth of pt nanowires on the gdl surface. they observed that starting from 100 °c temperature onwards, ecsa increased expressively, showing a maximum at 150 °c, and decreased before further increase of temperatures between 180 and 210 °c. the ecsa for samples treated at 150 °c was observed to be 65 % higher when compared with untreated one, indicating the influence of temperature on pt nanowire growth. alcaide et al. [112] studied the influence of solvent on the catalyst inks for active layer manufacturing in dmfc applications. two types of solvents, 2-propanol (ipa) and n-butyl acetate (nba), have been used. the ecsa determined by co stripping were 52.4 and 47.3 m2 g-1, i.e. about 11 % greater ecsa was found for nba than ipa. results indicate that the ecsa of cl formulated with nba was higher than that formulated with ipa. this clearly indicates that secondary pores in the catalyst formulated with nba make accessible a greater surface area of the catalyst. using various novel catalyst support materials han et al. [113] tested the performance of dmfc anode with three different support materials. they prepared pt-ru electrocatalyst with acetylene black, vulcan xc-72 and ketjin black ec-300j as carbon supports. they found small particle size, good dispersion and high methanol oxidation activity for ketjin black carbon support with 80 wt% pt content, thus leading to higher ecsa and better pt utilization. ptru supported on acetylene black has the lowest activity because of large clusters of nanoparticles caused by agglomeration. in contrast, pt-ru supported on ketjen black having better dispersion, exhibited three-fold increase in catalytic activity when compared to p-ru/acetylene black. sun et al. [114] synthesized omc-ft, omc-f ordered mesoporous carbon with varying pore framework. higher ecsa and good stability was observed for omc-ft/pt (62.96 m2 g-1) than omcsf/pt (54.36 m2 g-1). uniform dispersion of pt nanoparticles resulted from high surface area and increased meso-porosity lead to high pt utilization. to avoid the waste of noble catalysts, carmo et al. [115] used nitric acid incorporated carbon black support to prepare pt/ru electrocatalyst (ptru/c-hno3). they observed better nano particle distribution on the catalyst support with high affinity and more exposed area of ru towards species containing oxygen, leading to better ecsa of 104 m2 g-1 with better pt utilization. luo et al. [116] prepared a pt-nafion® high performance catalyst carbon support functionalized by perflourosulfonic acid and compared the results with pt/o-c and pt-nafion®/c catalysts. they observed 30 % higher ecsa of 104.9 mg cm-2 for pt-nafion®/c catalyst when compared to pt/o-c. the presence of nafion® leads to higher dispersion of pt nanoparticles and increased ecsa. liu et al. [117] used multi walled carbon nanotubes doped with phosphorous (p-mwcnt) as the catalyst support and prepared pt/p-mwcnt electrode for dmfc. improved pt particles dispersion with increased intrinsic activity resulting in high pt utilization was observed. this was attributed to change of charge density of p-mwcnt, leading to better interfacial interaction between surfaces of pt and p-mwcnt. madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 303 sun et al. [118] designed and synthesized 1d open pt structures with porous surface. using polyelectrolyte-modified feooh nanorods as a template, they prepared hollow pt nanotubes (ptnt). employing the electrostatic attraction-based layer-by-layer assembly process, the self-supporting porous electrode was synthesized. ecsa of 19.6 m2 g-1 was observed for this electrode when compared to 16.8 m2 g-1 of pt black catalyst. in spite of similar structure and composition of these electrodes, the increased ecsa was attributed to 1d porous structure of the prepared electrode. using chemical interfacial method, gharibi et al. [119] prepared a composite of polyaniline (pani) fiber doped with para-toluenesulfonic acid (ptsa) and fabricated the vulcan-polyaniline composite, c-pani-ptsa. pt electrocatalyst was deposited on the composite with different ratios (10, 15, 20, 25, 30 wt%), optimized and compared with pt/c electrocatalyst. they found high ecsa (120.5 m2 g1) for pt/c-20 % pani-ptsa electrocatalyst. c-pani-ptsa composite exhibited better dispersion of pt particles resulting in increased ecsa. for depositing pt nanoparticles, li et al. [120] prepared a hybrid support tixsn1-xo2-c via thermal method with substitution of ti4+ by sn4+ in tio2 lattice and observed improved pt utilization with increased ecsa (68.9 m2 g-1) compared to the commercial catalyst. patel et al. [121] used the complexed sol-gel (csg) process to synthesize pt/ru/tin anode catalyst with titanium nitride (tin) as the support. higher ecsa of 59 m2 g-1 than the commercial catalyst was observed with uniform dispersion of pt/ru on the support. it was stated that the csg process resulted in generation of the nanocrystalline pt/ru on the support. zhang et al. [122] used nitrogen-doped carbon nanotubes (cnx-nts) as the catalyst support material. they were prepared by carbonization of ppy-nt which were synthesized using methyl orange as a structure-guiding agent. ecsa values observed for these catalysts are 44.4 m2 g-1 (pt/cnx-nts-600), 55.9 m2 g-1 (pt/cnx-nts-700), 94.0 m2 g-1 (pt/cnx-nts-800) and 71.2 m2 g-1 (pt/cnxnts-900). the highest catalytic activity of pt/cnx-nts-800 catalyst is attributed to the optimal proportion of n-5, n-6 and n-q. gao et al. [123] used graphene/vulcan xc-72 carbon as the catalyst support and prepared platinum/graphene/vulcan xc-72 (pt/gr-c) catalyst. graphene oxide (go) and h2ptcl6 were simultaneously reduced in ethylene glycol. then, through ultra-sonication in aqueous solution, low-cost vulcan xc-72 carbon was incorporated between graphene sheets. ecsa values of 37.9 m2 g-1 (pt/c), 41.8 m2 g-1 (pt/gr) and 50.5 m2 g-1 (pt/gr-c), respectively were obtained. the best result for pt/gr-c was attributed to the inhibition effect of vulcan xc-72 carbon sphere on the overlap of graphene and smaller particle size. to increase the methanol oxidation activity of pt catalyst, jeon et al. [124] developed pt/wc/c catalyst and analyzed electrochemical properties. co-stripping studies revealed ecsa of 3.8 m2 g-1 for pt/wc/c catalyst and 5.9 m2 g-1 for the conventional pt/c. higher ecsa (7.5 m2 g-1) was recorded for pt/wc/c and not much different ecsa was observed for pt/c (5.6 m2 g-1) when hydrogen adsorption method was used. difference in ecsa observed by two methods was due to h+ spill-over at the synthesized pt/wc/c catalyst. further, from co stripping studies the reduced potential of 0.68 v for the synthesized catalyst pt/wc/c than pt/c (0.8 v) was found, indicating improved pt/wc/c activity towards co electro-oxidation. developing pt metallic alloy and pt-free catalysts giorgi et al. [125] deposited ptau bimetallic nanoparticles on carbon nano fibers (cnf) directly grown on graphite paper and observed three times increase in the performance than for the commercial carbon support electrode having pt utilization of 11 %. the augmented catalytic action of this http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 304 electrode when compared to ptru catalyst was attributed to increased utilization of catalyst particles, which leads to superior ability towards methanol oxidation. the specific morphology of platelet cnf reduces amalgamation and regulates a robust foundation of catalyst nanoparticles on the substrate. in addition, the surface of the nanofibers is encompassed by functional groups which enable better oxidation, leading to the reduction of carbon-based species like co that are accumulated on the electrode. thereby pt poisoning during cell operation is reduced and stability is enhanced. jang et al. [126] deposited pt-au alloy nanoparticles on graphene to prepare three-dimensional graphene electrodes (3d-gr/ptau). using the spraying and evaporation technique, pt-au alloy is deposited on colloidal graphene by employing the aerosol process. the ecsa (92 to 325 m2 g-1) values were found to increase with decrease in graphene oxide concentration (0.5 to 0.1 wt%). the highest ecsa of 325 m2 g-1 obtained for this electrode was relatively higher than for two-dimensional graphene electrode (2d-gr/ptau) and commercially available pt/c catalyst. this is attributed to increased active reaction sites, resulting in greater ability towards methanol oxidation. chang et al. [127-130] demonstrated anode catalyst for dmfc methanol electro oxidation using ni2p, cop promoters. they synthesized pt-ni2p, ptru-ni2p, pt-cop and ptru-cop by co-depositing ni2p, cop promoters with pt and ptru on carbon support and optimized the electrodes by varying the amount of promoters (10, 20 30, 40 and 50 %). from h-adsorption and co stripping measurements, they observed maximum ecsa for 30 % loading promoter for pt-ni2p, pt-cop catalyst and 40 % loading of promoter for ptru-ni2p and ptru-cop, respectively. the cv results are shown in figure 9. the enhanced results were attributed to electron transfer between catalyst promoters and pt, ptru catalyst. the ecsa results are summarized in table 10. table 10. platinum utilization analysis for dmfc anode gde/structure/support/membrane/parameters pt loading, mg cm-2 ecsa, m2 g-1 ref. nano-nafion® membrane 4 11.5 [107] ionomer coating (5 wt%) 40 wt% 63.0 [108] roughness factor(rf) 1.0 45.8 [109] atomic ratio 0.7 35 [110] plasma treatment conditions 25.8 [111] effect of solvent: 0.8 52.4 [112] p-ru/ketjen blank 5 45.63 [113] ordered mesoporous carbon 20 wt% 62..96 [114] nitric acid functionalized cb/vulcan xc72r-hno3 20 wt% 104 [115] nafion® functionalized cb (pt/nafion®-c) 20 mg 104.9 [116] phosphorous doped cnt(pt/p-mwcnt) 78.9 [117] 1d open pt structures with porous surface(ptnts) 0.028 19.6 [118] pt/c-pani-ptsa 0.1 120 [119] sno2-tio2 1 68.9 [120] nanostructures tin 0.3 59 [121] pt/cnx-nts-800 5.3 wt% 94.0 [122] pt/gr-c 0.019 50.5 [123] pt/wc/c 3.8 [124] platelet carbon nano fiber grown on graphite paper. 0.011 59 [125] 3d-gr/ptau 20 wt% 325 [126] pt-ni2p-30 % 5 69 [127] ptru-ni2p-40 % 2 113 [128] pt-cop-30 % 0.3 81 [129] ptru-cop-40 % 2 116 [130] madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 305 a b c d figure 9. cyclic voltammograms of (a) pt-ni2p (b) ptru-cop/c (c) pt-cop/c (d) ptru-ni2p for varying amount of promoters (10, 20, 30, 40 and 50 %). conclusion this review has summarized various approaches adopted by researches over the past two decades to enhance pt utilization efficiency and thereby reduce cost and address various other technical barriers for pemfc’s and dmfc’s to become commercially viable. it is established that catalysis is a surface phenomenon on electrodes, where catalyst activity is largely governed by the dispersion of particles, particle size and the exposed percentage of particles available in the catalyst layer. for this reason, and in order to reduce pt loading, it is essential that efforts must be made to use all pt particles during reaction. reducing particle size can therefore be seen as the primary focus of research so far in the area of catalyst utilization. it is well known that the exposed percentage of the catalyst available for catalytic activity is the reciprocal of particle size. this means that reducing the particle size to below 1 nm levels may lead to 100 % pt utilization. however, research revealed that the porous structure of the carbon support with the pore size at approximately 2 nm is a hindrance. that is, reduction of pt particle size below 2 nm lowers its specific activity and results in the pocketing of the particles inside the porous structure of the carbon support, rendering them inaccessible for catalyst reaction. further, the behavior of metal particles smaller than 1 nm display greater interaction with the carbon support in deviation of their normal conventional behavior thereby reducing their participation in electroactivity. http://dx.doi.org/10.5599/jese.665 j. electrochem. sci. eng. 9(4) (2019) 281-310 platinum utilization in fuel cell 306 nevertheless, the recent progress in catalyst utilization research has brought down the effective particle size of platinum-based catalyst to about 2-4 nm range and the percentage of exposed pt particles to the range of 20-50 %. this has been achieved through the following means: • increasing of tpb, where electrolyte, electrode and fuel gas come into contact with each other, by optimizing the composition of the catalyst layer and catalyst distribution. • attainment of reduced mass transport losses by controlling pore structure and hydrophobicity especially at high current density of cl to provide more electrochemical reaction area with improved gas and liquid transport pathways. • reducing of the interfacial resistance by tailoring cl and gdl, and between cl and membrane interfacial structures • designing novel methods for ordered cl preparation. as it was established that reducing the particle size may not result in optimizing catalyst utilization, other viable alternatives need to be explored. in addition to the use of pt and pt alloy as a catalyst, efforts are also being carried out to investigate the application of non-pt alloys as catalysts to reduce the overall cost of the fuel cell. also, it is worth mentioning here that recent research indicates stability or the life span of pemfc as the most important factor, irrespective of pt loading per area. this means that as long as high pt loading contributes to increase in stability, life span and the overall effectiveness of the fuel cell, a reduction of pt loading should not be prioritized. nevertheless, development of pt electrocatalyst with low pt loading will continue to stay in the focus of active research. nomenclature 1d one dimensional asp active screen plasma ast accelerated stress test aupt gold nanoparticles supporting pt bh borohydride bp black pearl ca carbon aerogels cb carbon black ccl cathode catalyst layer ccs catalyst coated substrate cc carbon cloth ccs-20 compound carbon support cl catalyst layer cmk a family of mesoporous material cmpl carbon microporous layer cnf carbon nanofibers cnt carbon nanotubes coad adsorbed carbon monoxide cp carbon paper ctab centyl trimethyl ammonia bromide cv cyclic voltammetry cvd chemical vapor deposition cxg carbon xerogels dcl double catalyst layer dc direct current dmfc direct methanol fuel cell dpc doped porous carbon e/e electrospinning and electrospraying hd-eg homogeneous deposition of ethylene glycol hgs hollow graphitized carbon spheres ie ion-exchange ipa isopropyl alcohol lsv linear sweep voltammetry mea membrane electrode assembly mpad modified polymer-assisted deposition mpl microporous layer mwcnt multiwalled carbon nanotubes n/c nafion® carbon ratio nabh4 sodium borohydride ncs novel cathode structure ndcl novel double catalyst layer nmp n-methyl-2-pyrrolidoone omc ordered mesoporous carbon orr oxygen reduction reaction pemfc proton exchange membrane fuel cell pgm platinum group metal pld pulse laser deposition technique ppt pulse plating technique ptut platinum utilization ptfe poly tetra flouro ethylene ptnp platinum nanoparticles ptnt platinum nanotubes ptsa para sulfonic acid rgo reduced graphene oxide scco2 super critical carbon dioxide scl single catalyst layer madhavi bandapati et al. j. electrochem. sci. eng. 9(4) (2019) 281-310 http://dx.doi.org/10.5599/jese.665 307 ecsa electrochemical active surface area eg ethylene glycol fgs functionalized graphene sheets gde gas diffusion electrode gdl gas diffusion layer go graphene oxide gr graphene gsa geometrical specific surface area sds sodium dodecyl sulphate sens surface enriched nanosized swcnt single walled carbon nanotubes tcs traditional catalyst structure tfcl thin film catalyst layer tpb triple phase boundary upcf ultrafine porous carbon fibre hupd hydrogen under potential deposition references [1] c. berger, handbook of fuel cell technology, prentice-hall, nj, united states, 1968. 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[130] l. feng, k. li, j. chang, c. liu, w. xing, nano energy 15 (2015) 462-9. ©2019 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons. org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {fabrication possibilities and characterisation of chalcogenide glass-based sensors for bromide determination:} http://dx.doi.org/10.5599/jese.1325 475 j. electrochem. sci. eng. 12(3) (2022) 475-484; http://dx.doi.org/10.5599/jese.1325 open access : : issn 1847-9286 www.jese-online.org original scientific paper fabrication possibilities and characterisation of chalcogenide glass-based sensors for bromide determination ute enseleit1,, claudia feller2, uwe partsch2, werner schneider3 and winfried vonau1 1kurt-schwabe-institut für messund sensortechnik meinsberg e.v., waldheim, germany 2fraunhofer institut für keramische technologien und systeme, dresden, germany 3viimagic gmbh, dresden, germany corresponding author: ute.enseleit@ksi-meinsberg.de received: march 9, 2022; accepted: may 16, 2022; published: may 27, 2022 abstract the fabrication and characterisation of potentiometric chalcogenide glass (cg) electrodes with selectivity for bromide in aqueous media are reported. the focus is on the production of chalcogenide glasses of the material system agbr ag2s as2s3 and their investigation by means of a series of physico-chemical methods, as well as on their fabrication with the aid of precision mechanical manufacturing and assembly techniques to form usable sensors and their testing. in addition to the production of conventional rod electrodes, it will be examined whether it is also possible to realize planar electrodes using thick-film technology. keywords potentiometric sensor; chalcogenide glasses; bromide selective rod-shaped electrode; ion selective solid-state electrode introduction potentiometry is often used as an analytical method for the quantitative determination of halide ions, which requires the availability of so-called ion-selective electrodes (ise). these have been available for some time in the form of electrodes of the 2nd kind based on electrode membranes of homogeneous and heterogeneous compacts or single crystals of poorly soluble salts [1]. in the case of electrodes sensitive to bromide ions, the membrane usually consists of a moulded body formed by a mixture of ag2s and agbr. the practically usable measuring range here is between 5 mol/l and 0.1 mol/l br-. in the literature [2], potentiometric electrodes based on glassy crystalline membranes of the composition agbr ag2s as2s3 usable for brdetermination are also described, which analytical properties are said to be superior in terms of nernst response, as well as resistance to acids, strong oxidizing agents and corrosive media. to produce the chalcogenide glass electrodes, discs of up to 3 mm thickness were made from the reaction product obtained in evacuated quartz ampoules in a http://dx.doi.org/10.5599/jese.1325 http://dx.doi.org/10.5599/jese.1325 http://www.jese-online.org/ mailto:ute.enseleit@ksi-meinsberg.de j. electrochem. sci. eng. 12(3) (2022) 475-484 chalcogenide sensors for bromide determination 476 thermal manufacturing process, polished on one side and silver-plated on the other side by vacuum evaporation before the functional element was installed in a pvc pipe to produce the final sensor. it is well known that due to the chemical composition required for functionality in chemosensors and the associated high brittleness, the processing of chalcogenide glasses (cgs) for electroanalytical purposes is not trivial in contrast to those glasses used for optical applications [3]. for example, the reaction products, which are in lumpy form after completion of the melting process, removal from the quartz ampoules and quenching in cold water, cannot usually be further processed into electrode membranes by sawing in larger quantities, as described in the abovementioned publication. the pieces are available in different sizes and shapes and the application of mechanical processing methods very often leads to their pulverization. therefore, the available pieces are often only coarsely crushed or used for further processing in their already existing form. this essentially involves making contact between the ion-selective chalcogenide glass and an electrical conductor by using electrically conductive adhesives, such as gold conductive lacquer [4], but also the use of conductive polymers [5]. most of the sensors manufactured to date are used for the analysis of metallic cations [6]. some of them are also commercially available [7]. in addition to improving analytical performance (lower detection limits, cross-sensitivities, ...), current research goals in the field addressed here primarily consist of optimizing sensor production. in this context, approaches to solutions are discussed in the scientific literature with regard to the use of pulsed laser deposition (pld) [8] and thick-film technology [9,10], although none of the methods described has yet found broad acceptance due to numerous shortcomings. for example, there are sometimes problems with the formation of droplets inherent in the system when using pld. only thermally stable glass (paste) material that can be compacted during sintering while maintaining stoichiometry is suitable for the thick-film process. thermal effects due to decomposition, crystallization and phase transformation of the chalcogenide glass may only occur above the sintering temperature, which does not apply, for example, to the production of thick-film electrodes selective for lead and iodide ions [11,12]. the present work focuses on finding an optimized chalcogenide glass composition for the production of electrodes selective for bromide ions and an optimized fabrication process for this. for the latter task, the focus will be on improving the classical precision manufacturing process and examining the possibility of using thick-film technology. the pld method, on the other hand, is very cost-intensive, time-consuming and hardly suitable for mass production from the perspective. in addition to evaluating the various fabrication options, the manufactured sensors will be characterized chemically and analytically. experimental glassmaking for the production of the cg, agbr, ag2s and as2s3 powders were used, each with a purity of >99.99 wt.%, purchased from the companies alfa aesar gmbh co kg and sigma aldrich chemie gmbh. cg glass systems of different stoichiometries (figure 1, table 1) were melted in order to find the optimal glass composition for the production of bromide-sensitive sensors. from the literature [2,13] it was found that the bromide cg electrodes exhibit anion sensitivity when the composition is in a narrow concentration range of 55-65 mol % agbr and when the ag2s to as2s3 ratio varies from 1:1 to 2:1. u. enseleit et al. j. electrochem. sci. eng. 12(3) (2022) 475-484 http://dx.doi.org/10.5599/jese.1325 477 as2s3 content, mol.% figure 1. phase diagram agbr ag2s as2s3 table 1. glass composition in mol % signature content, mol % agbr ag2s as2s3 ● 60 20 20 ○ 60 24 16 □ 65 15 20 n 55 25 20 « 65 20 15 ® 60 15 25 p 60 30 10 the melting of the sensor material of the compositions, according to table 1 was carried out in a cylindrical quartz ampoule. this was evacuated (<10 bar) and hermetically sealed in a reaction chamber filled with nitrogen. the powder-containing body was transferred to a quartz glass reactor, which was then placed in a nabertherm tube furnace (b180, maximum temperature: 1300 °c). a stirrer was used for the slow movement of the ampoule during the heating phase with 100 to 110 °c/h up to a temperature of 980 °c and a melting process duration of 20 h. the stirrer was used for the heating of the ampoule. immediately after the melting process, the ampoule was quenched in cold water and destroyed to collect the glass. the smaller pieces were used for orientational examination by the following methods: scanning electron microscopy (sem) with energy dispersive x-ray spectrometer (edx) using the quanta 200 measuring system (fei company, nl) coupled with an energy dispersive x-ray spectrometer qanttax qx2 (röntec company), x-ray diffraction (xrd), using the d8 advance instrument from bruker, and differential thermal analysis (dta) and thermogravimetry (tg) with the sta 449 c instrument from netzsch-gerätebau gmbh. the sintering behaviour of the cg powder was http://dx.doi.org/10.5599/jese.1325 j. electrochem. sci. eng. 12(3) (2022) 475-484 chalcogenide sensors for bromide determination 478 investigated with the thermomechanical analyzer tma 402 f1 hyperion, also from netzschgerätebau gmbh, in order to determine whether the cg is suitable in perspective for the realization of cg ises with the aid of thick-film (tf) technology. for this purpose, cylindrical powder compacts were produced and measured by means of uniaxial pressing. for the determination of resistivity, large chalcogenide glass pieces (ø = 8 mm, length = 2 cm) were cast with two-component adhesive epoxy 4439 (kager gmbh) and cut into approximately 1 to 2 mm thick slices using an accutom 50 diamond saw from struers. for impedance measurements (gamry electrochemical measurement system of the company gamry instruments, inc., usa), leakage leads consisting of silver wire ( = 0.3 mm) were attached with the aid of silver adhesive to both sides of the surfaces of the cut pieces that were fully contacted with silver conductive lacquer. a resistivity of 1.1 to 1.35 mω was determined from recorded ground diagrams in the frequency range from 10 mhz to 100 khz, the thickness and area of the fused bodies. electrode fabrication from a series of cg melts for the detection of bromide ions, sample structures, so-called rod electrodes, were prepared for initial evaluations of the electrochemical properties. for this purpose, glass fragments were electrically contacted, metallographically prepared and converted into a measurable structure. the starting point for the production of the rod electrodes are cg fragments of various sizes and shapes (figure 2). figure 2. chalcogenide glass fragments the greatest difficulty in using the cg fragments arose from their extremely inhomogeneous shape and extreme brittleness. fragments had to be selected that could be sufficiently contacted electrically and had a shape that was largely suitable for subsequent metallographic preparation. the electrically conductive polymer-based adhesives known from microelectronic packaging were not suitable for this due to their chemical and physical properties. instead, adhesives with strong bonding properties and favorable curing characteristics were used. the 2-component adhesive cw 2400 from chemtronics proved itself. contrary to the usual assembly methods/bonding technologies, this was only done manually, without dispensing technologies and special annealing conditions. the use of a soldering lug as an aid for contacting the cg fragments has proven itself. the contacted glass fragments with the copper or silver (ø = 0.35 mm) electrical connecting wire were then inserted and embedded in a suitable glass tube to be transformed into the shape and homogeneity required for the subsequent measurements by means of metallographic preparation (embedding, grinding and polishing of the cg surface). epofix from the company struers proved to be a suitable embedding medium. the aim was to achieve the largest possible and most homogeneous glass ground surface without cracks and inhomogeneities (figure 3). u. enseleit et al. j. electrochem. sci. eng. 12(3) (2022) 475-484 http://dx.doi.org/10.5599/jese.1325 479 figure 3. overall structure of a cg electrode, diameter 12 mm, length 9 cm (left), glass surface (right) potentiometric measurements the determination of the response times and slopes of the electrodes was carried out in calibration solutions in the measuring range from 10 to 10-3 mm brwith 0.1 m kno3 in each case to adjust the ionic strength. the solutions were prepared from a 0.1 m kbr stock solution according to the dilution method. the potassium bromide standard in purity of >99.99 wt. % was purchased from sigma aldrich chemie gmbh. an ag/agcl, sat. kcl reference electrode (re) was used as a reference electrode. stirring the measuring solution led to higher sensitivities, as this avoided saturation and enrichment effects on the surface of the electrode. a salt bridge filled with 0.1 m kno3 was used to separate the reference electrode from the measuring solution and to prevent precipitation, e.g., of silver chloride. the lm 3000 measuring system from sensortechnik meinsberg gmbh (xylem analytics germany sales gmbh & co. kg) served as the measuring device for electrode potential measurement and data recording. conditioning of the cg electrodes was performed in 10 or 1 mm potassium bromide calibration solutions over a period of 30 min. the cross-sensitivity of the glasses was determined in the calibration solutions by adding 1 mm interfering anions (cl-, so42-, scn-, and i-) according to the mixed solution method recommended by iupac (international union of pure and applied chemistry). the determination of the selectivity coefficients of a brsensitive electrode based on the material system 60agbr 24ag2s 16as2s3 was carried out in a measurement range 10 to 10-4 mm kbr, 0.1 m kno3 with the addition of interfering ions in a constant concentration of 1 mm in each case. we added kcl, ki, kscn and k2so4. for the calculations, concentrations instead of activities were considered because we adjusted the ionic strength of the solutions with 0.1 m kno3. results and discussion scanning electron micrographs at 20000 magnification of the cg of composition 60agbr 24ag2s 16as2s3 (mol %) in figure 4, reveal the formation of butterfly-shaped crystalline formations on the surface of the selective powder material. the cg shows a tendency to recrystallize due to the grinding of the bulk material in the vibrating disc mill and planetary ball mill. figure 5 shows x-ray diffraction patterns of powder samples of a cg of the above composition. the reflection line shows visible peaks of the compounds agbr. this clearly shows the glassycrystalline character of the material. this state seems to cause brsensitivity and thus usability as a functional element as an anion-sensitive material. in work on the sensitivity of chalcogenide glassycrystalline materials based on silver bromide, it has been reported that the ises have far better analytical properties in terms of the measured electrode potentials and the resulting nernst slope than those sensors based on conventional fully crystalline agbr ag2s or agbr membranes [2,12]. http://dx.doi.org/10.5599/jese.1325 j. electrochem. sci. eng. 12(3) (2022) 475-484 chalcogenide sensors for bromide determination 480 figure 4. scanning electron micrographs of agbr ag2s as2s3 cg (se-secondary electron detector) however, in the pulverization of cg necessary for the preparation of tf pastes, the occurrence of undesirable partial recrystallization is not conducive. however, the solidified cgs show high chemical stability in aqueous solutions, comparable to completely amorphous glasses. 2 / o figure 5. x-ray diffraction diagram of brsensitive cg powder with agbr peaks the thermoanalytical behaviour of agbr ag2s as2s3 cg powder was studied under a nitrogen atmosphere using equipment from netzsch-gerätebau gmbh. figure 6 shows the measured curves of tg and dta. according to the tg curve, the glass is thermally stable up to about 260 °c. at higher temperatures, there is a significant loss of mass caused by thermal decomposition. in the dta curve, exothermic signals at 190 and 250 °c indicate phase formation. figure 6. mass change (tg signal) and heat flow (dta signal) of agbr ag2s as2s3 cg (( d ( q ) / d t) / m 0 ) / (m w / m g ) u. enseleit et al. j. electrochem. sci. eng. 12(3) (2022) 475-484 http://dx.doi.org/10.5599/jese.1325 481 another endothermic signal at 310 °c indicates thermal decomposition or phase transformation. the results of the thermal analysis show that the glass is not thermally stable at temperatures above 260 °c due to thermal decomposition. the sintering behaviour of agbr ag2s as2s3 cg powder was also investigated (figure 7). the sintering curve of a powder compact was measured with thermomechanical analysis tma 402 f1 hyperion from netzsch-gerätebau gmbh. the glass begins to sinter homogeneously at 225 °c. after a shrinkage of 8 % at 305 °c, the thermal decomposition of the glass dominates. the decomposition causes the sintered body to bubble, which is indicated by the rise of the shrinkage curve. above 330 °c, shrinkage increases again due to the evaporation of the decomposition products. thus, the thermal decomposition of the bromide glass influences its sintering. the glass cannot be sufficiently compressed and is therefore unsuitable for the thick-film process. figure 7. linear shrinkage vs. temperature of agbr ag2s as2s3 cg (l = length of the measurement sample, lo = length of the sample at the beginning of the measurement all configured ises, with chalcogenide glasses of the stoichiometric composition given in table 1, were subjected to potentiometric measurement to determine the most optimal glass composition in terms of their electrode properties. here, the cg sensors of the composition 60 agbr 24 ag2s 16 as2s3 (mol %) showed the highest sensitivity to bromide ions (figure 8). the response time t90 of the electrode was determined to be 0.5 to 1 min. figure 8. calibration curves of bromide electrodes of different compositions the response of a bromide ise in a range of 10 to 10-3 mm kbr, 0.1 m kno3 measured on different days is shown in figure 9. (d l / l 0 ) 1 0 0 / % http://dx.doi.org/10.5599/jese.1325 j. electrochem. sci. eng. 12(3) (2022) 475-484 chalcogenide sensors for bromide determination 482 figure 9. calibration curves of a bromide ion-sensitive electrode of the material system 60agbr 24ag2s 16as2s3 (mol %) the linear measuring range of the electrode extends in a range from 10 m to 0.01 mm kbr, 0.1 m kno3. the slope of the measuring electrode in this range was 53 ± 5 mv/pbr-. this value is slightly lower than the index of 59 mv/pbrfor monovalent ions given according to the nernst equation. even after one year, no decrease in the slope can be observed. the detection limit is about 0.1 m kbr. at even lower concentrations, the electrode does not show any potential changes. a great advantage is the dry storage of the ises. however, it is necessary that the sensor is conditioned and calibrated before measurement. the temperature dependence of the electrode voltage of a br--sensitive ise based on the material system 60agbr 24ag2s 16as2s3 (mol %) vs. ag/agcl, sat. kcl reference electrode is shown in figure 10. for a precise determination of the bromide concentration, temperature compensation is necessary since the electrode signal increases by approx. 1 to 2 mv at a temperature increase of 1 °c. figure 10. temperature dependence of the measurement signal of a cg bromide electrode the calibration curves without interfering ions addition and each with a constant interfering ions concentration of a bromide-cg-ise of the composition 60agbr 24ag2s 16as2s3 (mol %) are shown in figure 11. the determination of the selectivity coefficients (table 2) was carried out in the measuring range from 10 to 10-5 mm kbr, 0.1 m kno3 of the measuring ion. iodide ions interfere with the measurements or have a strong influence on the electrode potential and were therefore not included in the determination of the selectivity coefficient. thus, extreme errors or inaccuracies can occur in the presence of iions in the measuring solution. this circumstance should be taken u. enseleit et al. j. electrochem. sci. eng. 12(3) (2022) 475-484 http://dx.doi.org/10.5599/jese.1325 483 into account for practical measurements. furthermore, all chalcogenide glasses of the agbr ag2s as2s3 system show a high sensitivity to silver ions. figure 11. determination of the selectivity coefficient of the agbr ag2s as2s3 cg electrode table 2. selectivity coefficients of a brcg with the mixed solution method (primary ion bragainst the interfering ion) interfering ion (1 mm) selectivity coefficient so427.9×10-4 cl1.1×10-2 scn3.9×10-2 i for practically-relevant applications of the sensors, the applicable ph range is another important parameter. figure 12 shows that the measurement signals of a br-sensitive rod electrode remain stable in the ph range from ph 2 to 12, independent of ph changes in the test solution. measurements were made in 10 and 0.1 mm concentrations of bromide, with the ionic strength being constant at 0.1 m kno3. the respective ph value of the test solution was adjusted using potassium hydroxide solution or nitric acid. figure 12. ph dependency of the measuring signal conclusions a series of chalcogenide glasses for the determination of bromide in aqueous solutions of different stoichiometric compositions were melted and characterized using various physicohttp://dx.doi.org/10.5599/jese.1325 j. electrochem. sci. eng. 12(3) (2022) 475-484 chalcogenide sensors for bromide determination 484 chemical methods. the focus was on the material system agbr ag2s as2s3, whereby the cg glass of the composition 60agbr 24ag2s 16as2s3 (mol %) showed the most optimal results in terms of nernst behaviour, signal stability and reproducibility of sensitivities. by improving the classical precision manufacturing process, it was possible to configure measurable ises in rod form by means of an optimal metallographic preparation and the use of new processing materials. it was found that due to its thermal decomposition behaviour in the sintered region of the glass, the agbr ag2s as2s3 cg material is not suitable for processing into a glass paste and screen printing and firing it onto a ceramic substrate to produce a dense glass membrane using the thick film method. however, it is the crystalline-glassy character that seems to be responsible for the ionic sensitivity of the cg. acknowledgements: the presented work is based on a project supported by the federal ministry of economic affairs and energy (bmwi) through the german federation of industrial research; associations "otto von guericke" e.v. (aif) within the program "zentrales innovationsprogramm mittelstand" (zim)-cooperation, grant number zf 4088707aw9. both institutes are co-financed by tax funds on the basis of the budget passed by the saxon state parliament. references [1] k. camman, h. galster, das arbeiten mit ionenselektiven elektroden: eine einführung für praktiker, springer, heidelberg, 2013. isbn-13: 978-3642648274. [2] y. g. vlasov, l .n. moskvin, e. a. bychkov, d. v. golikov, analyst 14 (1989) 185-190. https://doi.org/10.1039/an9891400185 [3] h.-g. krolla, m. winkler-trudewig, w. schumann, infrarotdurchlässiges chalkogenidglas. eur. pat. appl. 0217 195a1 (b1), april 8, 1987. [4] c. könig, entwicklung und charakterisierung einer chalkogenidglaselektrode zur selektiven detektion von eisen(iii)ionen. phd thesis, frankfurt am main, 1993. [5] s. pollrich, chalkogenidgläser als ionensensitive membranen für potenziometrische sensoren. diploma thesis, hochschule mittweida, 2004. https://www.hsb.hsmittweida.de/primo-bibliotheksportal/ [6] c. t. baker, i. trachtenberg, journal of the electrochemical society 118 (1971) 571-576. [7] sensor systems, llc, scientific and development company, st. petersburg, russia, 2020. http://www.sensorsystems.spb.ru [8] j. p. kloock, y. g. mourzina, y. ermolenko, t. doll, j. schubert, m. j. schöning, sensors 4 (2004) 156-162. http://www.mdpi.net/sensors [9] u. enseleit, c. feller, u. partsch, w. vonau, current topics in analytical chemistry 13 (2021) 23-33. http://www.researchtrends.net [10] c. feller, u. partsch, journal of sensors and sensor systems 10 (2021) 83-91. https://doi.org/10.5194/jsss-10-83-2021 [11] u. enseleit, m. berthold, c. feller, u. partsch, s. körner, w. vonau, sensors & transducers 219 (2018) 1-8. http://www.sensorsportal.com [12] u. enseleit, c. feller, u. partsch, w. vonau, journal of solid state electrochemistry 25 (8) (2021) 2293-2300. https://doi.org/10.1007/s10008-021-05000-1 [13] d. v. golikov, y. g. vlasov, e. a. bychkov, l. n. moskvin, zhurnal analiticheskoi khimii 41 (1986) 1635-1640. ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1039/an9891400185 https://www.hsb.hs-mittweida.de/primo-bibliotheksportal/ https://www.hsb.hs-mittweida.de/primo-bibliotheksportal/ http://www.sensorsystems.spb.ru/ http://www.mdpi.net/sensors http://www.researchtrends.net/ https://doi.org/10.5194/jsss-10-83-2021 http://www.sensorsportal.com/ https://doi.org/10.1007/s10008-021-05000-1 https://creativecommons.org/licenses/by/4.0/) editorial : : j. electrochem. sci. eng. 4(4) (2014 j. electrochem. sci. eng. 4(4) (2014) open access : : issn 1847-9286 www.jese-online.org editorial special issue on new achievements and methodologies of electrochemistry and electrochemical engineering in the environmental protection and pollution control during the last decades, many applications of electrochemistry and electrochemical engineering have arisen for the characterization and remediation of environmental problems. as a result, nowadays this subject has become one of the most interesting areas of research in applied electrochemistry, with hundreds of papers published every year and many applications already available in the market. this special issue contains sixteen very valuable contributions on these topics, written by highly recognized authors and covering the most relevant areas of interest within the topic. environmental monitoring is a matter of the major importance because it helps to prevent and remediate pollution with the development of novel warning detection systems. for this reason, the first sets of contributions are related to characterization of environmental issues with electrochemical methods and it contains valuable information about new tools for the characterization of organics, heavy metals and sulphur. treatment of industrial wastes is one of the more stimulating environmental applications nowadays. water is extensively used in industry not only as a heat exchanger fluid or a cleaning agent, but also for the production of many chemicals. as a consequence, significant volumes of wastewater are produced every day in our industries and they get into the environment after their treatment with technologies which are not always completely effective. an electrochemicallybased solution to this problem is faced in this special issue with exciting contributions on electrolysis, electro-fenton oxidation and electrocoagulation of wastewater, in which technologies for the efficient removal of dyes, persistent chemicals and inorganic pollutants are evaluated. finally, the last set of papers included in this special issue focusses on soil remediation and bioelectrochemical treatments. electrokinetic soil remediation (eksr) is one of the most motivating topics of research for electrochemical and environmental engineering in our time. many applications are currently working at the full scale and in this issue, an authoritative review is included, in which the fundamentals and applications of the technology are clearly described. http://www.jese-online.org/ j. electrochem. sci. eng. 4(4) (2014) editorial to conclude, trying to save energy, one of the more exciting and innovative areas of research is the production of electricity from bio-electrochemical processes. research on this topic is still at a very early stage but results are promising and the concept of producing energy directly from waste is an out breaking idea as it is explained in the last contribution of this special issue. as a conclusion, this special issue is a very good summary of the most exciting research on electrochemistry and electrochemical engineering in the environmental protection and pollution control and, for sure, it will become a reference for many researchers in the near future. manuel andrés rodrigo rodrigo influence of various metallic oxides on the kinetic of the oxygen evolution reaction on platinum electrodes doi:10.5599/jese.157 79 j. electrochem. sci. eng. 5(2) (2015) 79-91; doi: 10.5599/jese.157 open access : : issn 1847-9286 www.jese-online.org original scientific paper influence of various metallic oxides on the kinetic of the oxygen evolution reaction on platinum electrodes kambire ollo, pohan lemeyonouin aliou guillaume, appia foffié thiéry auguste, gnamba corneil quand-meme, kondro konan honoré, ouattara lassiné laboratoire de chimie physique, ufr ssmt, université félix houphouët-boigny de cocody, abidjan, 22 bp 582 abidjan 22, côte d’ivoire corresponding author: ouatlassine@yahoo.fr ; tel: +22502143382 received: december 23, 2014; revised: april 13, 2015; published: august 26, 2015 abstract pt, 50pt-50ruo2 and 50pt-50iro2 electrodes were prepared on titanium (ti) substrate by thermal decomposition techniques. the micrographs of 50pt-50ruo2 and 50pt-50iro2 have revealed that their surfaces are rough with cracked structures in contrast to platinum which exhibits smooth, compact and homogeneous surface. the richer the electrode surface in platinum, thinner is the crack size and also more compact is the electrode surface. the electrodes have also been characterized electrochemically by cyclic voltammetry in acidic (hclo4) and alkaline (koh) electrolytes. these characterizations showed that the surface of the 50pt-50ruo2 and 50pt-50iro2 electrodes were composed of platinum and metal dioxide active sites. the tafel slope obtained on pt, 50pt-50ruo2 and 50pt-50iro2 for the oxygen evolution reaction (oer) were respectively 120, 90 and 44 mv dec -1 in acid electrolyte. in the alkaline electrolyte, they were 119, 87 and 42 mv dec -1 on pt, 50pt-50ruo2 and 50pt-50iro2 electrodes, respectively, indicating that for the prepared electrodes, tafel slopes are the same in acidic and in alkaline media. moreover, in acidic and in alkaline media, the kinetic of the oxygen evolution reaction was faster on 50pt-50ruo2 and 50pt-50iro2 than pt owing to a synergetic effect of pt and the oxides. that additional effect of the surface component 50pt-50ruo2 and 50pt-50iro2 electrodes let them possess higher electrocatalytic activity towards oer than pt in the two media. though the kinetic of the oxygen evolution reaction is practically the same in acidic and alkaline media for all the electrodes, oer occurred at lower overpotential in alkaline electrolyte than in acidic electrolyte on the prepared electrodes. keywords platinum, ruthenium dioxide, iridium dioxide, tafel slope, cyclic voltammetry, electrocatalytic activity, oxygen evolution reaction http://www.jese-online.org/ mailto:ouatlassine@yahoo.fr j. electrochem. sci. eng. 5(2) (2015) 79-91 kinetic of the oxygen evolution reaction 80 introduction oxygen evolution reaction (oer) has been extensively studied in electrochemistry because of its importance in many fields such as fuel cells, wastewater treatment, chlorine evolution, corrosion, water electrolysis [1-7]. oer reaction kinetics and mechanism aspects drew a great attention and still continue to be investigated. because of the use of electrochemistry to solve environment pollution problems [8-10], a particular interest was directed towards the search of better electrocatalysts that could lead to a decrease of the oer overpotential. in the above mentioned field, platinum played an important role because of its good electronic and corrosion resistance properties. although platinum is known to be one of the best electrocatalyst in acidic electrolytes, its high cost requires its replacement with new materials. used for the oxidation of organics such as methanol and ethanol, platinum undergoes electrode fouling consequently reducing its electrooxidation activities. since it is known that oer depends mainly on chemical and structural properties of electrode materials surface, our interest was focused on platinum and transition metal oxide modified platinum electrodes. indeed, since the discovery of dimensionnally stable anodes (dsas) by beer around 1970, a lot of research efforts were made on such kind of electrodes in relation to a variety of processes has been carried out including oer. it has been shown that among the dsas, iridium dioxide and ruthenium dioxide exhibit good oer performance. in prior investigation in our team, it has been shown that combining 50 % molar ratio of pt and metallic oxides led to higher activity for organic oxidations [11]. the aim of the present study was to investigate the electrocatalytic activity and the kinetics of the oer on the same kind of electrode i.e. platinum modified with various transition metal oxides. experimental the electrodes used in the following work were all prepared in our laboratory with appropriate metallic precursors. the coating precursors were prepared from h2ptcl6,6h2o (fluka), rucl3,×h2o (fluka) and h2ircl6,6h2o (fluka). all the precursors were dissolved in pure isopropanol (fluka) used as solvent. the commercial products were used as received without any further treatment. the titanium substrates on which the electrode films were deposited have the following dimension 1.6 × 1.6 × 0.5 cm. the surface of each substrate was sandblasted to ensure good adhesion of the deposit on it. after, sandblasting, all the substrates were washed vigourously in water and then in isopropanol to clean their surface from residual sands. the substrates were then dried in an oven at 80 c and weighed. after that, the precursor was applied by a painting procedure on the titanium (ti) substrate then put in an oven for 15 min at 80 °c in air to allow the solvent evaporation. then after, it is put in a furnace at 400 °c for 15 min in air to allow the decomposition of the precursor. theses steps were repeated until the desired mass of the coating is reached. a final decomposition of 1 h was done at 400 °c. the electrodes prepared by this technique are platinum (pt), platinum-iridium dioxide (50pt-50iro2) and platinum-ruthenium dioxide (50pt-50ruo2) electrodes. for the combined electrodes, a mixture of 50 % molar ratio of the corresponding precursor was used. the deposit loading was about 5 g m -2 on each titanium substrate. the physical characterization of the electrodes were performed using a scanning electronic microscopy (sem, zeiss, supra 40vp) device. the voltammetric measurements were performed under either 1 mv s -1 or 100 mv s -1 on the prepared electrodes in a three-electrode electrochemical cell using an autolab pgstat 20 (ecochemie). the counter electrode (ce) was a platinum wire and the reference electrode (re) was k. ollo et al. j. electrochem. sci. eng. 5(2) (2015) 79-91 doi:10.5599/jese.157 81 a saturated calomel electrode (sce). all the potential were reported against she according to the following relation: e / v vs. she = e / v vs. sce + 0.25. to decrease the contribution of ohmic losses, the reference electrode was mounted in a luggin capillary and placed close to the working electrode by a distance of 1 mm. in fact, ohmic drop is known to increase the value of the tafel slope and careful attention was to be paid on it during experiment. in order to reduce totally the residual ohmic drop, the polarization curve should be corrected according to method described elsewhere [12-15]. a brief description of the correction is given as followed. the overpotential η / v observed in the experiment can be given by η / v=a + b lnj+jr (1) where a / v is the tafel constant, b / v dec -1 is the tafel slope, j / a cm -2 is the current density and r /  cm -2 is the total area-specific uncompensated resistance of the system, which is assumed to be constant. the derivative of equation (1) with respect to current density gives equation (2) from which b and r can be easily obtained by plotting dη / dj as a function of 1/j d d b r j j    (2) the estimation of r allows correcting the experimental overpotential by substracting the ohmic drop jr according to equation (3) ηcorr = η – jr (3) where ηcorr stands for the corrected overpotential during the calculations, the derivatives dη⁄dj was replaced by their finite elements δη⁄δj estimated from each pair of consecutive experimental points. the apparent exposed area of the working electrode was 1 cm 2 . for the electrochemical investigations hclo4 (fluka), koh (fluka) were used as received for the preparation of the solutions employed in that work with distilled water. all the experiments were made at ambient temperature of 25 °c. results and discussion physical characterization of the prepared electrodes in figure 1, the micrographs of the prepared electrodes were presented. it appears that the surface was totally covered by the deposit. the surface of the platinum electrode seems to be smooth, compact and almost homogeneous (figure 1a). the surface of 50pt-50ruo2 (figure 1b) is rough and seems to be compact as that of platinum but cracks were observed like those of dsa’s [16,17]. such finding indicated that the electrode surface of the 50pt-50ruo2 electrode is composed by platinum and ruthenium dioxide. in figure 1c, the prepared 50pt-50iro2 electrode is shown and its surface presents almost the same characteristic as 50pt-50ruo2 but in this case, the electrode surface is highly rough with more cracks. the size of the cracks are thinner as generally observed at such magnification for pure dsas possibly due to the presence of platinum which contributes to render the combined electrode surface to have compact structures. j. electrochem. sci. eng. 5(2) (2015) 79-91 kinetic of the oxygen evolution reaction 82 a b c figure 1: scanning electron micrographs of pt(a), 50pt-50ruo2(b) and 50pt-50iro2(c) electrochemical characterization of the electrodes in perchloric acid medium cyclic voltammetric measurements were performed on a platinum electrode in perchloric acid 1 m under a scan rate of 100 mv s -1 and the results were depicted in figure 2a. the figure showed the usual feature generally observed with polycrystalline platinum in acidic media [18]. well defined adsorption and desorption peaks located in the potential domain between 0 and 0.4 v k. ollo et al. j. electrochem. sci. eng. 5(2) (2015) 79-91 doi:10.5599/jese.157 83 were shown. the double layer region characterized by an almost zero current in the forward potential scan followed by oxide formation domain (0.74 v-1.51 v) and an oxide reduction peak at 0.72 v (backward potential scan) were also presented. a rapid increase in the current related to oer started at 1.51 v. in figures 2b and 2c, the voltammograms resulted from the investigation made on the platinum modified electrodes were presented. for such investigation, 50pt-50iro2 and 50pt-50ruo2 have been used. as a general response, the modified electrodes led to voltammograms resembling to that obtained with pure platinum electrode. that result indicated that the surface of the modified electrodes is partly composed of platinum clusters. the voltammetric charges of the combined electrodes are higher than that of platinum. that is due to the presence of metal oxides on the electrode surface. the voltammogram of the 50pt-50ruo2 electrode presents a feature that bears more resemblance to that of platinum than the voltammogram of 50pt-50iro2 compared to that of platinum. that result indicated that ruo2 has more electronic and structure affinity with platinum than iro2 towards platinum. it can also be deduced from those results that 50pt-50ruo2 surface is composed of much platinum clusters due to its migration from the inside of the coating to its surface during the thermal decomposition of the precursor. but in case of 50pt-50iro2, a hindering effect of iro2 on platinum migration occurs that results in lower spread of platinum on such composite electrode surface. although this observation is made, presence of metal oxide and platinum on the electrode surface showed synergetic effect i.e. increasing in active sites than platinum used alone. in the high potential domain, using oxygen evolution as a probe reaction to investigate the electrocatalytic activity of the prepared electrodes, it appeared that the onset of the potential at the increase in current are 1.43 v; 1.41 v and 1.51 v on 50pt-50ruo2, 50pt-50iro2 and pt respectively. from these results, it appears that 50pt-50iro2 and 50pt-50ruo2 are more electrocatalytic for oer than pure platinum owing to their low overpotential for oer than pure platinum electrode. for the prepared composite electrodes 50pt-50iro2 appeared to be more catalytic for oer than 50pt-50ruo2. such result is in accordance with the above obtained result and is explained by the fact that the surface of 50pt-50ruo2 is mostly composed by platinum than on 50pt-50iro2 electrode. indeed, mixing metallic oxides such as transition metal oxide led to an increase in the electrocatalytic behavior of platinum and/or the prepared electrode towards oxygen evolution reaction. plotting log j versus the potential e, straight lines were obtained in the tafel region as shown in figure 3. the obtained tafel slopes are presented in table 1. it is worth noting that tafel slopes are an indicator of electrocatalytic activity. in fact, the lower the tafel slope the faster the kinetics of the reaction and the more active the electrocatalyst is. from table 1, it could be noticed that pure platinum exhibits a tafel slope of 120 mv dec -1 which is higher than those obtained on the composite electrodes. the tafel slope of 50pt-50ruo2 is higher than that of 50pt-50iro2. the exchange current densities have been determined (table 1) and the obtained values indicated that the system o2/h2o is kinetically slow on all the investigated electrodes. meanwhile, from these findings, regarding the tafel slopes, it appears that oer is more rapid on 50pt-50iro2 than on 50pt-50ruo2 and sluggish on pure platinum electrode. these results are in accordance with the above obtained results where the onset of the potential of oer was concerned. in fact, as the surface of the electrode is dominated by platinum clusters in the following order 50pt-50iro2 < 50pt-50ruo2 < pt as shown by voltammetric investigation in acidic medium, an increase in the tafel slope is observed. j. electrochem. sci. eng. 5(2) (2015) 79-91 kinetic of the oxygen evolution reaction 84 figure 2: cyclic voltammograms of pt (a), 50pt-50ruo2 (b) and 50pt-50iro2 (c) in 1 m hclo4 at 100 mv s -1 , ce: platinum, t = 25°c table 1: tafel slopes and exchange current densities of various electrodes in 1 m hclo4; v =1 mv s -1 ; ce : platinum; t = 25 °c electrodes tafel slope / mv dec -1 exchange current density j0 10 4 / a cm -2 pt 120 0.728 50pt-50ruo2 90 4.375 50pt-50iro2 44 1.069 k. ollo et al. j. electrochem. sci. eng. 5(2) (2015) 79-91 doi:10.5599/jese.157 85 figure 3 : tafel line of pt (a), 50pt-50ruo2 (b) and 50pt-50iro2 (c) in perchloric acid 1 m ; v =1 mv s -1 ; ce : platinum; t = 25°c although the tafel slopes delineate the rapidity of the kinetics occurring on the electrode surface, it is also related to the rate determining step in the oer mechanism. the electrodes j. electrochem. sci. eng. 5(2) (2015) 79-91 kinetic of the oxygen evolution reaction 86 presented thus different rate determining steps. from the literature, a globally accepted oer mechanism is described as followed in acidic media [14, 19]. s + h2o  s-oh + h + + e (4) s-oh  so + h + + e (5) 2so  2s + o2 (6) where s stands for electrode active sites; oh and o represent adsorbed intermediates. if the step (4) is the rate determining step, current density associated with reaction (4) can be given as:    1 2 1 2h o 1 f j kf exp rt               (7) j current density, k constant of the kinetic of the reaction, γ total active sites,  1 and 2 : rate of adsorbed oh and o respectively, α charge transfer coefficient, η overpotential taking into account a second electron transfer taking place in step (5), the total faradic current density can be obtained through the following equation:    1 2 1 22 2 h o 1 f j j kf exp rt                (8) considering k as k=2kf[h2o]γ(1-1-2 ) (9) (8) becomes exp f j rt k         . (10) then ln10 ln10 log log rt rt j k f f      (11) the expression of the tafel slope is obtained by doing r a p,t,c ln 10 log| | rt b j f          (12) in the experimental condition ba= 120 mv dec -1 . when each of the steps (5) and (6) is considered as a rate determining step, the corresponding tafel slope would be 40 mv dec -1 and 15 mv dec -1 respectively. according to these results and regarding the tafel slopes of table 1, one can consider that the rate determining step occurring on the platinum electrode is related to the hydroxyl radical adsorption on the active sites of that electrode (step 4) since the tafel slope is 120 mv dec -1 . such a result has been found by many other authors on platinum in acidic media [20,21]. with 50pt50iro2, a tafel slope of 44 mv dec -1 was found. that result indicated that the rate determining step is given by equation (5) of the proposed mechanism. with 50pt-50ruo2, the obtained tafel slope is 90 mv dec -1 which is almost close to 120 mv dec -1 . rate determining step is similar to that k. ollo et al. j. electrochem. sci. eng. 5(2) (2015) 79-91 doi:10.5599/jese.157 87 obtained with pure platinum but the kinetic of the oer is higher on that electrode than platinum owing to either a mixed kinetic control proceeding via unstable adsorption species such as s-oh* (equations 13 and 14) [14,19] or the synergetic effect of the two surface components (pt and ruo2). s + h2o  s-oh* + h + +e (13) s-oh*  s-oh (14) electrochemical characterization of the prepared electrodes in alkaline electrolyte in the 0.5 m potassium hydroxide electrolyte, voltammograms have been recorded on platinum electrode. the obtained result is depicted in figure 4a. the trend of the curve is different to that of platinum in acidic medium. the voltammogram presents a rectangular shape in the potential window explored. figures 4b and 4c show the voltammograms recorded on 50pt-50iro2 and 50pt-50ruo2 electrodes. all the voltammograms showed almost the same feature. in the low potential domain, one observed superimposed voltammograms of platinum and 50pt-50ruo2 indicating that the surface of 50pt-50ruo2 electrodes is rich in platinum clusters. the onset of the oxygen evolution reaction potential decreased highly as platinum is associated with transition metal oxides (iro2 and ruo2). the obtained potential values are 0.67 v, 0.67 v and 0.78 v for 50pt-50iro2, 50pt-50ruo2 and pt respectively. it appears that adding transition metal oxide such as ruthenium dioxide and iridium dioxide to platinum leads to electrodes with a higher electrocatalytic activity towards oxygen evolution reaction than pure platinum electrodes. that finding is in accordance with the results that are obtained in acidic medium indicating that the enhancement of the electrocatalytic activity of the composite electrodes could be due to a synergetic effect of the two elements that exist on the electrode surface. stability in the voltammograms was observed in that medium. that is the sign that no other oxide growth of the electrodes surface happened. in both media, platinum modified with iro2 or ruo2 possesses the best electrocatalytic activity for oer than pure platinum. the tafel slopes of the electrodes used in this study have been determined in alkaline medium. the ohmic drop correction has been carried out like in acidic medium. figure 5 shows the measured tafel line of the electrodes. the obtained tafel slopes are consigned in table 2. it appears that the slope decreases in the following order pt>pt-ruo2>pt-iro2 indicating that the rate determining steps are not the same on all the electrodes. regarding the tafel slopes, 50pt50iro2 seemed to behave as the best electrocatalyst for oxygen evolution reaction. in contrast, pt presents the lowest electrocatalytic activity towards oxygen evolution reaction. coupling platinum and iridium or ruthenium oxides helps to increase oxygen evolution kinetics. the exchange current densities have been determined (table 2) and the obtained values indicated that the system o2/oh is kinetically low on all the investigated electrodes. it worth indicating that the same tafel slope was obtained on the prepared electrodes independently of the nature of the electrolyte. table 2. tafel slopes of the prepared electrodes in koh 0.5 m; v =1 mv s -1 ; ce : platinum ; re : she ; t = 25°c electrodes tafel slope, mv dec -1 exchange current density j0 10 4 / a cm -2 pt 119 0.186 50pt-50ruo2 87 0.140 50pt-50iro2 42 10.83 j. electrochem. sci. eng. 5(2) (2015) 79-91 kinetic of the oxygen evolution reaction 88 figure 4. cyclic voltammograms of pt (a), 50pt-50ruo2 (b) and 50pt-50iro2 (c) in koh 0.5 m at 100 mv s -1 , ce : platinum, t= 25°c k. ollo et al. j. electrochem. sci. eng. 5(2) (2015) 79-91 doi:10.5599/jese.157 89 figure 5. tafel line of pt (a), 50pt-50ruo2 (b) and 50pt-50iro2 (c) in koh 0.5 m; v = 1 mv s -1 ; ce : platinum, t = 25°c oxygen evolution reaction is known to be a complex reaction. it depends normally on many factors such as electrode materials, experimental condition, and electrode preparation conditions. j. electrochem. sci. eng. 5(2) (2015) 79-91 kinetic of the oxygen evolution reaction 90 the following oxygen evolution reaction mechanism that has received much agreement among researchers in alkaline electrolytes is proposed as followed [22-24]. s + oh  soh + e (15) soh + oh  so + h2o (16) so  so + e (17) 2so  2s + o2 (18) s represents the electrode active sites; oh, o and o represent the adsorbed intermediates. assuming step (15) of that mechanism as the rate determining step, the tafel slope determined theoretically was about 120 mv dec -1 . in the case that the rate determining step is supposed to be step (16), the tafel slope would be 60 mv dec -1 . tafel slopes are 40 mv dec -1 and 15 mv dec -1 in alkaline electrolyte if steps (17) and (18) were considered as rate determining steps respectively. considering the platinum electrode with a tafel slope of 119 mv dec -1 , the rate determining step in the oxygen evolution process on this electrode is step (15). with 50pt-50iro2 the tafel slope is about 42 mv dec -1 which is close to 40 mv dec -1 . thus, on this electrode, step (17) could be the rate determining step in the oxygen evolution reaction. for the 50pt-50ruo2 electrode, the tafel slope is 87 mv dec -1 . as has been observed in acidic medium, that value is almost close to 120 mv dec -1 indicating that platinum effect seems to dominate the totally surface effect. step (15) could be the rate determining step in the oer process. conclusion the micrographs of 50pt-50ruo2 and 50pt-50iro2 have revealed that their surfaces are rough with cracked structures. that of platinum was smooth, compact and homogeneous. the richer the electrode surface in platinum, thinner is the crack size and also more compact is the electrode surface. cyclic voltammetry investigations have revealed the presence of both iro2 or ruo2 and pt on the thermally prepared 50pt-50ruo2 and 50pt-50iro2 electrode surface. the addition of metallic dioxide to platinum leads to electrodes with higher electrocatalytic activity than platinum towards oxygen evolution used as probe reaction. that electrocatalytic enhancement is due to a synergetic effect of the surface component. all the prepared electrodes showed different rate determining steps. tafel slopes are found to be almost the same in acid and in alkaline electrolytes indicating the same rate determining steps for each electrode. acknowledgements: we greatly thank the swiss national funds for its financial support that allowed this work to be carried out. our team has received part of the grant iz01z0_146919 for that work. we also thank prof. cesar pulgarin at the swiss federal institute of lausanne (switzerland) and prof. ricardo torres at the university of antioquia (colombia) for their help in that work. references [1] g. b. tissot, a. anglada, p. dimitriou-christidis, l. rossi, j. s. arey, c. comninellis, electrochemistry communications 23 (2012) 48-51. 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[21] g. lodi, e. sivieri, a. de battisti, s. trasatti, journal of applied electrochemistry 8(2) (1978) 135-143 [22] c. eun-ok, k. young-uk, m. sun-il, bull. korean chemical society 18 (9) (1997) 972-976 [23] c. iwakura; k. fukuda, h. tamura, electrochimica acta 21 (1976) 501-508 [24] i. m. sadiek, a. m. mohammad, m. e. el-shakre, m. s. el-deab, international journal of hydrogen energy 37(1) (2012) 68-77 © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ {rice husk char as a potential electrode material for supercapacitors:} http://dx.doi.org/10.5599/jese.1310 451 j. electrochem. sci. eng. 12(3) (2022) 451-462; http://dx.doi.org/10.5599/jese.1310 open access : : issn 1847-9286 www.jese-online.org original scientific paper rice husk char as a potential electrode material for supercapacitors venkata naga kanaka suresh kumar nersu1,, bhujanga rao annepu1, satya srinivasa babu patcha2and subhakaran singh rajaputra3 1department of instrument technology, andhra university college of engineering (a), visakhapatnam, andhra pradesh 530003, india 2center for flexible electronics, department of electronics and communication engineering, koneru lakshmaiah education foundation, vaddeswaram, ap, india 3centre for advanced energy studies, koneru lakshmaiah education foundation, vaddeswaram, ap, india corresponding author: sureshk834@gmail.com received: february 22, 2022; accepted: march 28, 2022; published: april 4, 2022 abstract rice husk char (rhc), a carbon-based material, was obtained by thermal decomposition of rice husk (rh) biological waste. physicochemical properties of rhc were determined using xrd, ftir, fesem, tga, n2 adsorption-desorption studies and contact angle measurement. a lab-scale supercapacitor (sc) was fabricated using as-prepared rhc and its supercapacitive behaviour was investigated using techniques like cv, gcd and eis studies. each of rhc electrode showed a specific capacitance of 80.2 f g-1 at the constant charging/discharging current density of 0.05 a g-1. rhc exhibited 90 % retention of its initial capacitance even after 5000 gcd. the presence of amorphous sio2 in rhc could contribute to the excellent wettability of rhc towards the water, enhancing its effective surface area by improving access of electrolyte ions into rhc. this remarkable supercapacitive performance of biological waste-derived rhc demonstrates its potential as a cost effective and environmentally benign electrode material for aqueous electrolyte-based scs. keywords biochar; carbon-sio2 composite; superhydrophilicity; carbon cloth; nanocomposite gel polymer electrolyte; electric double layer capacitor (edlc) introduction recently, an exponential increase in demand for the design and development of novel ecofriendly energy storage systems has been witnessed for harvesting energy from renewable resources [1]. energy storage devices like supercapacitors (scs) are well known for their high specific power, exceptional charge-discharge capability and long life [2]. types of scs include electric doublehttp://dx.doi.org/10.5599/jese.1310 http://dx.doi.org/10.5599/jese.1310 http://www.jese-online.org/ mailto:sureshk834@gmail.com j. electrochem. sci. eng. 12(3) (2022) 451-462 rice husk char as material for supercapacitors 452 layer capacitors (edlcs), pseudocapacitors and hybrid scs. edlcs store charge in the electrochemical double layer (edl) and are familiar for their prolonged cycle life and outstanding chargedischarge capabilities [3]. activated carbon (ac), carbon black, graphene and carbon nanotubes (cnts) are the most abundant electrode materials for edlcs [4], and most of the commercially available edlcs use ac as electrode material [5]. recent advances in sc research were focused on exploring new resources for electrode materials that are economically feasible, abundant and derived from renewable precursors. therefore, extensive research was directed toward deriving carbons from biological wastes for application as electrode materials in scs [1]. carbons derived from sugar cane bagasse [6], bamboo [7], straw of rice [8] and wheat [9], stems of sunflower [10] and lotus [11], stalks of corn [12] and cotton [13] were already applied in scs as electrode materials. natural fibers obtained from bamboo [14], jute [15] and hemp [16] and leaves of pine [17], lotus [18], ficus religiosa [19] and eucalyptus [20] were also utilized to derive carbon-based electrode materials for sc application. seed shells like coconut shells [21], macadamia nut shells [22] and oil palm kernel shells [23] were also used to derive carbons and tested as electrode materials in scs. rice is the chief food source to around 50 % of the world population [24], and per annum, around 759.6 million tons of paddy is cultivated, while their de-husking results in around 150 million tons of rice husk (rh) [25]. rh disposal is a major concern to rice cultivators and millers [26], as burning or open dumping of rh pollutes the environment [27]. the main constituents of rh are cellulose, lignin, silica (sio2) and moisture [28,29]. de-siliconized porous carbons obtained from rh through a two-step carbonization-activation method have been extensively applied as electrode materials in scs [30]. firstly, rh is carbonized and later, the carbonized rh is activated using activating agents like koh [31], naoh [32], h3po4 [33] and zncl2 [34], to obtain porous carbons. few reported methods also include a pre-treatment step in which rh is leached using hydrogen fluoride (hf) before activation [35]. the use of such chemical agents consumes a major portion of the production cost and the release of chemical effluents causes harm to the environment, making these methods very expensive and not eco-friendly [36]. removal of volatile components from rh results in carbon-sio2 composite, which can be applied in energy storage [37]. in the present work, rice husk char (rhc), a carbon-sio2 composite, was derived by thermal decomposition of rh without the steps like leaching and activation, making the process simple, cost-effective and eco-friendly. the rhc obtained is composed of amorphous sio2 and amorphous carbon. the physicochemical properties of rhc were investigated. a lab-scale symmetric sc was fabricated using rhc as electrode material, hydrothermally reduced carbon cloth (cchy) as current collectors and graphene-based nanocomposite gel polymer electrolyte (ngpe). the supercapacitive behaviour of rhc in the full-cell configuration was evaluated using techniques like cyclic voltammetry (cv), galvanostatic charge-discharge (gcd) and electrochemical impedance spectroscopy (eis) to determine the practicality of rhc as a potential biological waste derived carbon-based electrode material for scs. experimental materials rh was acquired from a local rice mill located in india and used directly for the preparation of rhc. carbon cloth was obtained from avcarb, usa. propan-2-ol (isopropanol / ipa), graphite powder (particle size <20 µm), sodium hydroxide (naoh) pellets, ethanol 99.9 %, hydrogen peroxide (h2o2) (30 % w/v), polyvinylidene difluoride (pvdf) (homopolymer powder, m.w. ̴ 320,000), nmethyl-2-pyrrolidone (nmp), sulphuric acid (h2so4) 98 %, nitric acid (hno3), poly (vinyl alcohol) v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(3) (2022) 451-462 http://dx.doi.org/10.5599/jese.1310 453 (pva) (m.w. ̴ 125,000), hydrochloric acid (hcl) 35–38 %, potassium permanganate (kmno4) and sodium nitrate (nano3) were used as received. whatman® qualitative filter paper: grade 1 (diam. 125 mm) procured locally was used as a separator in the fabricated lab-scale sc. deionized (di) water was employed in the entire work. preparation of rhc rhc was obtained by thermal decomposition of rh (figure 1). untreated rh was placed in a silica crucible and covered with an aluminum foil. small perforations were made to remove volatile compounds during the heating and minimize the reaction with oxygen. rh was heated up to 500 °c at a rate of 5 °c min-1 to obtain black-colored charred flakes of rhc, which were later ground to obtain rhc powder. figure 1. preparation of rhc through thermal decomposition of rh preparation of cchy current collectors commercially obtained carbon cloth was modified to obtain cchy following the synthesis procedure reported elsewhere [38]. pieces of untreated carbon cloth were dropped into h2so4 and hno3 (2:1) mixture. to this mixture, kmno4 (3 g) was added and stirred. later, di water (100 ml) was added and stirred continuously for three hours. to this mixture, h2o2 was added until the reaction turned transparent, resulting in chemically oxidized carbon cloths. the collected oxidized carbon cloths were rinsed several times, transferred into a di water-filled ptfe lined autoclave, and heated at 180 °c for 14 h. then cchy were collected and dried under vacuum. the prepared cchy were used as flexible current collectors while testing rhc as electrode material in full-cell studies. preparation of graphene-based ngpe hydrothermally reduced graphene oxide (hrg) was derived from graphene oxide (go) following the synthesis procedure reported elsewhere [39]. go was synthesized following modified hummer’s method from graphite precursor. graphite (1 g) was dispersed in conc. h2so4 (50 ml) into which nano3 (1 g) was added and the suspension was placed in an ice bath and stirred for 4 h. later, kmno4 (6 g) was added to the suspension and agitated for 48 h continuously. the suspension was kept in an ice bath followed by the addition of di water (92 ml) and stirred for 2 h continuously. 30 % h2o2 (10 ml) was added to this suspension until the reaction turned yellow, resulting in go. the go suspension was centrifuged and the obtained precipitate was washed many times using 1 m hcl and di water to gain a black-colored neutral go suspension. finally, to obtain go, neutral go suspension was centrifuged and filtered under vacuum to obtain a precipitate of go, followed by http://dx.doi.org/10.5599/jese.1310 j. electrochem. sci. eng. 12(3) (2022) 451-462 rice husk char as material for supercapacitors 454 washing with ethanol, vacuum filtration and vacuum drying for 12 h at 70 °c. go (0.1 g) was ground into fine powder and dispersed into di water (100 ml) to obtain a dispersion of go which ph was maintained at 11 using naoh, and then transferred to a ptfe lined autoclave and heated for 14 h at 180 °c. obtained hrg was vacuum dried (12 h at 60 °c) and ground to fine powder. hrg (0.5 wt.%) incorporated sulfonated pva (spva-hrg-0.5) hydrogel electrolyte was prepared by synthesis procedure reported elsewhere [40]. pva (0.5 g) was added to a mixture of di water (4 ml) and h2so4 (270 µl) and stirred continuously for half an hour at 80 °c to obtain sulfonated pva hydrogel (spva). 25 mg of hrg was suspended in ipa (10 ml) through ultrasonication and added slowly into spva hydrogel while stirring at 80 °c for 30 min in order to obtain spva-hrg-0.5. the as-prepared spva-hrg0.5 was employed as an electrolyte while testing rhc as electrode material in full-cell studies. characterization studies the structural properties and functional groups were investigated through x-ray diffraction (xrd) (rigaku miniflex 600) and fourier-transform infrared (ftir) spectroscopy (cary 630) analyses, respecttively. the surface morphology was determined from fesem analysis (fei-quanta feg 200f). thermogravimetric analysis (tga) was carried out under n2 atmosphere up to 800 °c (20 °c min-1 heating rate) using a thermal analyzer (sta 7200, hitachi htg) simultaneously. surface area and porosity profile were established using surface area and porosity analyzer (quantachrome nova 2200e). the wettability towards the water was determined from contact angle measurements (kyowa dm-501). the electrochemical behaviour of rhc was evaluated in full-cell configuration using electrochemical workstation (parstat pmc 2000a) by performing cv, gcd and eis studies. an electrode ink, prepared by sonicating a mixture of rhc, mwcnts and pvdf (80:10:10) in n-methyl-2-pyrrolidone (nmp) for 1 h, was drop cast over two cchy (area of each cchy = 0.8 cm2) and vacuum dried for 15 min at 120 °c to obtain flexible electrodes with rhc loading of 1 mg cm-2. a full cell was fabricated by sandwiching spva-hrg-0.5 ngpe soaked whatman® filter paper with flexible electrodes on either side. results and discussion physicochemical characterizations figure 2(a) represents x-ray diffractograms of rh and rhc powders. both the rh and rhc diffractograms showed a characteristic broad peak of amorphous sio2 at 2 value around 22° [41]. the broad diffraction peak observed at 2 value of about 43o in both rh and rhc, indicates the presence of amorphous carbon [37]. figure 2(b) represents the ftir spectra of both rh and rhc. ftir spectrum of rh shows a widespread band around 3284 cm-1 resulting from stretching of o-h bonds of hydroxyl groups [42,43]. the small peaks observed around 2920 and 2852 cm-1 in the spectra of rh resulted from c-h stretching of -ch2 groups [44]. the thermal decomposition of rh to rhc has resulted in the removal of -oh and -ch2 groups, indicated by the absence of their characteristic peaks in ftir spectra of rhc. the peaks observed around 1717 and 1712 cm-1 in rh and rhc, respectively, resulted from the stretching of carbonyl (c=o) groups [44]. the peaks observed around 1635 and 1634 cm-1 in rh and rhc, respectively, resulted from c=c stretching [45]. the peaks observed around 1034 and 1078 cm-1 in rh and rhc, respectively, are due to asymmetric stretching of si-o-si [46]. the peaks observed around 795 and 787 cm-1 in rh and rhc, respectively, relate to symmetric stretching of si-o-si [46]. the intensities of bands related to sio2 in ftir spectra of rhc were reduced compared to rh due to clenching and the development of a sio2 network on heating [44,47]. v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(3) (2022) 451-462 http://dx.doi.org/10.5599/jese.1310 455 figure 2. (a) x-ray diffractograms and (b) ftir spectra of rh and rhc figure 3 represents fesem images of rhc at different magnifications. figures 3(a) and (b) depict the surface morphology of rhc composed of multiple irregular crevices, which could help in improving the surface area, thereby enhancing double layer (edl) capacitance [48]. figure 3. fesem images depicting surface morphology of rhc at different magnifications figure 4 represents the tga curve of rhc. in the tga curve, rhc lost around 8 % of its weight below 100 °c due to the elimination of water content [49]. from 110 to 800 °c, a weight loss of 43 % was observed due to the breakdown of volatile compounds and the remaining 49 %, even after 800 °c, could be associated to amorphous sio2 [29]. from tga, the wt.% of amorphous carbon and amorphous sio2 was determined to be around 43 and 49 %, respectively. a pellet of 1.2 cm diameter was prepared using rhc powder and water contact angle measurement was performed by sessile drop technique. during contact angle measurement of rhc, the tested water droplet penetrated rhc resulting in a water contact angle of 0°, determining its superhydrophilic nature [50]. the superior wettability of rhc towards water could be attributed to hydrophilic metal oxide, i.e., amorphous sio2 [51]. this superior wettability of rhc towards the water improves the access of spva-hrg-0.5 hydrogel electrolyte to deeper regions of rhc, thereby improving its supercapacitive behaviour by enhancing the edl charging/discharging response [52]. the electrode-electrolyte interaction creates a considerable influence on charge-discharge ability, cycle life, charge storage and power delivery of a sc [53]. http://dx.doi.org/10.5599/jese.1310 j. electrochem. sci. eng. 12(3) (2022) 451-462 rice husk char as material for supercapacitors 456 figure 4. tga curve of rhc figure 5(a) represents n2 adsorption-desorption isotherm of rhc. the calculated brunauer– emmett–teller (bet) surface area of rhc was around 38 m2 g-1. figure 5(b) represents the pore size distribution in rhc, where dv₀ and d represent differential pore volume and pore diameter, respectively. figure 5(b) indicated that rhc mostly contains nanopores with an average pore size of around 2.8 nm. the presence of mesopores enhances capacitive retention in rhc by widening pathways for the transport of electrolyte ions [32]. figure 5. (a) n2 adsorption-desorption isotherm and (b) pore size distribution in rhc electrochemical characterizations full-cell studies the supercapacitive behaviour of rhc was evaluated in an in-house fabricated lab-scale full-cell. the electrochemical behavior was determined by performing cv, gcd and eis studies. cv was performed within a voltage window of 0 to 1 v at scan rates varying from 10 to 100 mv s-1 (figure 6 (a)), which determined the excellent rate probability and reversibility of rhc [54]. gcd was performed within a potential window of 0 to 1 v at current densities ranging from 0.05 to 5 a g-1 (fig. 6 (b)). the specific capacitance (cs), specific energy (es), and specific power (ps) of a single capacitor plate were calculated from gcd studies using equations 1, 2 and 3, respectively [55]. v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(3) (2022) 451-462 http://dx.doi.org/10.5599/jese.1310 457 s 2 i t c m v  =  (1) 2 s s 8 c v e  = (2) s s e p t =  (3) where, m, i, δv and δt represent mass (g) of rhc (0.8 mg) on each electrode, constant discharge current (a), discharge voltage window (v), and discharge time (s), respectively. rhc showed cs in the range of 80.2 to 60 f g-1 at constant current densities varying from 0.05 to 5 a g-1, respectively. at 1 a g-1 current density, rhc showed cs of 66 f g-1. rhc exhibited specific energy of 2.2 wh kg-1 at a specific power of 1 kw kg-1. figure 6(c) represents the ragone plot obtained using calculated es and ps values. table 1 represents the experimental data obtained from gcd studies. table 2 represents data for the comparison of supercapacitive performance of rhc with carbon-based electrode materials from rh precursors. figure 6. (a) cv curves of rhc at multiple scan rates, (b) gcd curves of rhc at various current densities and (c) ragone plot of rhc cyclic stability of rhc-based flexible supercapacitor was tested for 5000 gcd cycles at a constant current density of 1 a g-1. figure 7(a) represents the cv curves of rhc at 50 mv s-1, before and after cycling for 5000 gcd cycles. figure 7(b) represents the gcd curves of rhc at 1 a g-1, before and after cycling for 5000 gcd cycles. as shown in figure 7(c), rhc retained 90 % of its initial capacitance during 5000 gcd cycles. eis was performed within a frequency range of 100 khz to 0.1 hz at an amplitude of http://dx.doi.org/10.5599/jese.1310 j. electrochem. sci. eng. 12(3) (2022) 451-462 rice husk char as material for supercapacitors 458 5 mv at 0 v. figure 7(d) represents the nyquist plots obtained from eis analysis of rhc before and after cycling for 5000 gcd cycles, showing a slight increase in cell impedance after cycling. table 1. experimental data obtained from gcd studies of rhc electrodes in full-cell configuration current density, a g-1 ir drop, v discharge time, s specific capacitance, f g-1 0.05 0.01 401 80.2 0.1 0.017 197 78.8 0.2 0.032 92 73.6 0.5 0.071 34 68 1 0.111 16.5 66 2 0.218 8 64 5 0.592 3 60 table 2. comparison of specific capacitances of carbon-based electrode materials derived from rh precursor activating agent specific capacitance, f g-1 electrolyte reference naoh 51.4 at 0.5 a g-1 6 m koh [32] koh 147 at 0.1 a g-1 6 m koh [56] h3po4 112 at 1 a g-1 1 m na2so4 [33] naoh 110.2 at 0.1 a g-1 6 m koh [36] ― 40.8 at 0.1 a g-1 6 m koh [36] ― 80.2 at 0.05 a g-1 spva-hrg-0.5 this work figure 7. (a) cv curves of rhc at 50 mv s-1 and (b) gcd curves of rhc at 1 a g-1, respectively, before and after cycling; (c) cyclic stability of rhc for 5000 gcd cycles at 1 a g-1 and (d) nyquist plots of rhc before and after cycling; inset image shows the magnified image of nyquist plots v. n. k. s. k. nersu et al. j. electrochem. sci. eng. 12(3) (2022) 451-462 http://dx.doi.org/10.5599/jese.1310 459 conclusions rhc obtained through thermal decomposition of rh was characterized using physicochemical techniques. fesem images confirmed the presence of crevices in rhc, which could enhance edl formation by providing access of the electrolyte to the porous surface. xrd and ftir analysis confirmed the presence of amorphous sio2 in rhc and the wt.% of sio2 and carbon in rhc were estimated to be around 49 and 43 %, respectively, from tga analysis. bet specific surface area and average pore diameter of rhc were found to be around 38 m2 g-1 and 2.8 nm, respectively. the superior wettability of rhc towards the water, as confirmed from contact angle measurements, could be attributed to the presence of sio2. the practicality and applicability of rhc as potential electrode material in edlcs were evaluated by fabricating a lab-scale sc and testing its supercapacitive performance using electrochemical techniques like cv, gcd and eis. in full-cell (2-electrode) configuration, rhc at each electrode exhibited a specific capacitance of 80.2 f g-1 at 0.05 a g-1. cyclic stability tests confirmed 90 % of capacitive retention in rhc even after 5000 gcd cycles at 1 a g-1. the superior wettability of rhc, which in turn improved access of aqueous electrolyte ions into pores of rhc, thereby increasing the effective surface area of rhc, resulting in better supercapacitive behaviour. current work emphasizes the potential of rhc as a biological waste derived carbon-based material as electrode material in aqueous electrolyte-based scs. acknowledgments: the authors are grateful to er. koneru 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electrochimica acta 192 (2016) 110-119. https://doi.org/10.1016/j.electacta.2016.01.140 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.est.2019.101072 https://doi.org/10.1039/c7cs00505a https://doi.org/10.1039/c3ta15046a https://doi.org/10.1016/j.electacta.2016.01.140 https://creativecommons.org/licenses/by/4.0/) direct electrochemical detection mechanism of ammonia in aqueous solution using cu-decorated si microelectrodes http://dx.doi.org/10.5599/jese.1843 1 j. electrochem. sci. eng. 00(0) (2023) 000-000; http://dx.doi.org/10.5599/jese.1843 open access : : issn 1847-9286 www.jese-online.org short communication direct electrochemical detection mechanism of ammonia in aqueous solution using cu-decorated si microelectrodes alfredo e. hench-cabrera and enrique quiroga-gonzález institute of physics, benemérita universidad autónoma de puebla, 72570 puebla, mexico corresponding author: equiroga@ieee.org; tel.: +52 222 2295610 received: april 7, 2023; accepted: july 18, 2023; published: july 23, 2023 abstract most of the reports on electrochemical ammonia detection with copper electrodes have been performed at ph 10 or higher. however, according to phase diagrams, no reactions take place between copper and ammonia under those conditions, qualifying such detection of ammonia as indirect. this short paper deals with the detection of ammonia concentration in the micromolar range through a direct mechanism at ph 9, using a cu-decorated microstructured si electrode. the reaction mechanism is thoroughly studied. keywords ammonia detection; electrochemical sensing; silicon nanowalls; copper nanoparticles; cu complex introduction ammonia (nh3) is a substance that can be found in aqueous systems and determining its presence and/or change in its concentration could be of vital importance to establish the state of a system. for example, low nh3 concentration in urine may be indicative of kidney disease [1], while high concentrations of nh3 in the blood of covid-19 patients may indicate pre-existing liver damage [2], and high concentrations of nh3 in saliva could be related to helicobacter pylori infection [3]. for this reason, many reports have been devoted to developing technology for detecting and determining the concentration of nh3 [4]. electrochemical methods have received special interest due to the possibility of miniaturizing the equipment used in this method [5], which would allow testing at the sampling site. among the electrochemical devices proposed to detect nh3 in an aqueous solution, ones quite attractive because of their performance are electrodes containing copper nanoparticles (cu nps) [6-8]. manufacturing cu-based materials is easy [9], which favors the optimization of sensors, while a high exposed surface area resulting from nanostructures confers a good sensitivity. also, one of the most important features is that cu is a common material, significantly reducing fabrication costs [9]. http://dx.doi.org/10.5599/jese.1843 http://dx.doi.org/10.5599/jese.1843 http://www.jese-online.org/ mailto:equiroga@ieee.org j. electrochem. sci. eng. 00(0) (2023) 000-000 electrochemical detection mechanism of ammonia 2 although cu nps-based electrodes can be used to detect nh3 in an aqueous solution, little research has been devoted to exploring this possibility [6-8]. this may be because it is still unclear which electrochemical reactions allow the detection of nh3 [6,7]. mayo’s group detected nh3 with a cu/graphite paste electrode using the cyclic voltammetry technique [8]. in that research, the change of current intensity corresponding to extreme values of potential of a voltammogram of an nh3-containing solution was related to a change in its concentration. this was attributed to the oxidation of the cuprous complex (formed by nh3 and cu+ ion), although the exact reaction is not shown [8]. in that report, the ph of the solution was not mentioned, which is important to establish the current species in the system. in more recent works, such as that of valentini’s group, ammonia has been detected with copper nanoparticles in a linear concentration range (3-100 m) [7]. the detection mechanism was attributed to the electrochemical oxidation of copper in the presence of ammonia. this work emphasized a need to oxidize the copper particles since the oxide layer favors this reaction, although the report did not show the chemical and/or electrochemical reactions in the system. on the other hand, yang’s group detected ammonia with copper particles with a limit of detection of 1.25 m [6]. the detection mechanism was attributed to the formation of a stable copper-ammonia complex by an electrodissolution process of copper. according to the reactions shown in that report, the electrochemical signal used for ammonia detection was attributed to the oxidation of copper to form the cupric hydroxide, which is chemically dissolved by ammonia to form the cupric complex. both yang’s and valentini’s groups mentioned that the competition between cu-nh3 and cu-oh reactions could be avoided if the ammonia solution ph was 10 [6,7]. according to phase diagrams for nh3 concentration 10-3 m [10], cu-ohreactions are the most favorable at that ph. this means that even when the studies of the aforementioned groups have been successful in quantifying ammonia, the voltammetric and thermodynamic data indicate that at ph 10 no electrochemical reactions occur between cu and ammonia [6,10]. thus, the detection of ammonia at ph 10 can be qualified as indirect detection. this situation, however, could cause a misinterpretation of measured data because other substances present in the fluids to be analyzed (e.g., sodium carbonate in saliva) can cause the same effect that ammonia has on the oxidation of copper to form cupric oxide [11]. in previous works, it was reported that at ph value of about 9 and nh3 concentration of 10-3 m, cu-nh3 species (such as [cu(nh3)2]+ and a cu(nh3)ohads layer) are formed electrochemically [10]. thereby in this short work, we are focused on the possibility of detecting ammonia at ph 9 through linear sweep voltammetry (lsv) employing an electrode made of copper nanoparticles deposited on silicon nanowalls (cu nps/si nw electrode). this new ammonia detection mechanism is based on the pourbaix diagram calculated for ammonia concentrations in the micromolar range and involves a direct interaction between cu and nh3 with cu nanoparticle-based electrodes. in addition, the effect of lsv scanning rate and ionic strength of the aqueous ammonia solution has been elucidated. experimental materials for electrode fabrication, the following reagents were used: single-side polished p-type si wafer with resistivity 15-25 ω∙cm and orientation (100), hydrofluoric acid (hf, 48 %, emsure), silver nitrate (agno3, 99.8 %, riedel-h�̈�rm), hydrogen peroxide (h2o2, 30 %, j. t. baker), deionized water (18.2 mω∙cm), polyethylene glycol (peg, aldrich chemistry), and copper sulphate pentahydrate (cuso4∙5h2o, j.t. a. e. hench-cabrera and e. quiroga-gonzález j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1843 3 baker). the following reagents were employed to execute the voltammetry experiments: naoh (sigma aldrich), 2 m nh3 solution in ethanol (sigma-aldrich), and deionized water (18.2 mω∙cm). preparation of cu nps/si nw electrode si nanowalls were prepared by metal-assisted chemical etching (mace), as previously reported by our group [12]. first, a rectangular piece of si wafer (12 cm) was rinsed with acetone in an ultrasonic bath. the si sample was then rinsed with deionized water and dried with n2. subsequently, the si sample was introduced into a solution composed of 25 ml agno3 0.005 m and 0.5 ml hf for 10 s at 30 °c to carry out the deposition of ag nps on the si surface. si sample was then placed in a solution of hf : h2o : h2o2 with a 4 : 35 : 1 v/v composition for 5 min at 30 °c to etch the wafer and form si nanowalls. based on previous experiments in our laboratory, under these synthesis conditions, the silicon nanowalls layer has a thickness of approximately 5 μm. the thickness of the si nanowalls must be less than 24 m since there are reports about diffusion limitation of aqueous electrolytes into nanostructures when the depth is around 24 μm [13]. for the cu nps chemical deposition [14], the si nanowalls sample was placed in a solution composed of 0.11875 g cuso4∙5h2o, 0.5 ml hf, 99.5 ml deionized water and 2 g peg for 5 min at 30 °c. then it was rinsed with deionized water and dried with n2 at room temperature. in this method, the fluoride ions (f-) react with si, causing si to release electrons that reduce cu2+ ions present on the si surface, thus generating the deposition of the copper nanoparticles on si (see eqs. 1 and 2) [14]. si + 6f→ sif62+ 4e(1) 2cu2+ + 4e→ 2cu (2) electrochemical detection linear sweep voltammetry experiments were performed in four different aqueous solutions: 0.01 mm nh3 solution (ph 9), 6 mm naoh solution (ph 10), 3 mm naoh solution (ph 9) and a 0.006 mm nh3 + 2 mm naoh solution (ph 9). naoh solution ph 10 and naoh solution ph 9 were used to distinguish the effect of ph on the electrochemical processes because, in the copper-ammonia-water system, the main reactions are associated with the interaction of copper and hydroxyl ions (oh-) to form copper oxides and hydroxides. on the other hand, the naoh solution ph 9 and the nh3 solution ph 9 were used to discern the electrochemical processes involving nh3 at this ph value and thus determine the peak of the voltammogram that allows the detection of nh3. finally, the nh3/naoh solution was used to study the effect of the supporting electrolyte on the peak voltammogram used for nh3 detection. electrochemical experiments were controlled by a potentiostat/galvanostat (zennium), interfaced with a pc running thalesxt software. an electrochemical setup composed of a teflon cell (as a container of the nh3 solution) and a three-electrode system was used to obtain voltammograms (figure 1). figure 1. diagram of the electrochemical setup used for the experiments. the figure shows the cross-section view of the teflon cell, having a container for the electrolyte of 0.78 cm3 http://dx.doi.org/10.5599/jese.1843 j. electrochem. sci. eng. 00(0) (2023) 000-000 electrochemical detection mechanism of ammonia 4 the counter electrode was a platinum wire (with a diameter of ca. 0.003 cm), and the potential of the working electrode was measured vs. the ag/agcl (sat. kcl) electrode (with a diameter of 0.2 cm). the cu nps/si nw electrode was employed as the working electrode. because of the design of the teflon cell, the surface of the working electrode in contact with the ammonia solution was 0.78 cm2 and the depth of the container was 1 cm. to perform lsv experiments, 0.5 ml of solution was measured and put directly into the teflon cell. subsequently, the lsv experiment was started. the cell was washed with water after measuring each solution. lsv was performed from -2.0 v to 2.0 v in naoh solution at ph 10 and from -1.5 v to 1.5 v in the rest of the solutions, at sweep rates of 2, 5 and 10 mv s-1. all potentials in this work were reported vs. the ag/agcl reference electrode. the intensity of the peaks in the voltammograms was measured by taking the intensity reading of the peak base (i1) and peak tip (i2) (figure 2) and then subtracting these values (eq. 3). peak intensity = i2 – i1 (3) figure 2. determination of lsv peak intensity. results and discussion figure 3 a-c shows linear sweep voltammograms (lsvs) of the cu nps/si nw electrode in naoh solutions, in which the effects of sweep rate and ph on the electrochemical reactions taking place in the cu/naoh system are analyzed. at ph 10 and 10 mv s-1 (figure 3a), three clear peaks are observed. based on the shape of the voltammogram, these peaks can be associated with the following oxidation processes: peak i is related to the oxidation of metallic copper (cu) to form cuprous oxide (cu2o) (see eq. 4 and 5) [15]. peak ii is attributed to the production of cupric oxide (cuo) from either oxidation of cu or cu2o (see eqs. 6-8) [15]. peak iii is assigned to the formation of soluble cu species (see eqs. 9-11) [15]. cu + oh→ cuoh + e(4) 2cuoh ↔ cu2o + h2o (5) cu + 2oh→ cu(oh)2 + 2e(6) cu2o + 2oh+ h2o → 2cu(oh)2 + 2e(7) cu(oh)2 ↔ cuo + h2o (8) cu + 3oh→ hcuo2+ h2o + 2e(9) cu(oh)2 + 2oh → cuo2 + 2h2o + e(10) 2cuo + 2oh→ cu2o3 + h2o + e(11) a. e. hench-cabrera and e. quiroga-gonzález j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1843 5 figure 3. linear sweep voltammogram of cu/naoh system (a) ph 10, sweep rate 10 mv s-1, (b) ph 9, sweep rate 10 mv s-1, (c) ph 9, sweep rate 5 mv s-1, (d) cu/nh3 system, ph 9, sweep rate 5 mv s-1 in the study of the effect of sweep rate on electrochemical reactions in the cu/naoh system (figures 3b and 3c), it is observed that peak iii re-appears when the sweep speed is reduced. on the other hand, peak ii is more intense than peak i at ph 10 compared with peaks i and ii at ph 9 (see table 1). this fact could be the evidence that at ph 10 two electrochemical reactions associated with peak ii are contributing to the intensity of this peak (see eqs. 6-8), while at ph 9 only one reaction is contributing to the intensity. it should be noted that these trends have been observed by other research groups [15]. figure 3d shows the voltammogram of the cu nps/si nw electrode in nh3 solution at ph 9 and a sweep rate of 5 mv s-1. in this voltammogram, two peaks are noted. according to previous work on nh3 detection in aqueous solution [6], these peaks could be associated with the oxidation of cu to form cu2o (see eqs. 4 and 5), while peak ii is assigned to two processes: the oxidation of cu and cu2o to produce cuo (see eqs. 6-8) [6]. in the comparison of the results of the cu/nh3 and cu/naoh systems, it is observed that in the cu/nh3 system, peak i increases its intensity by an order of magnitude compared to all the experiments of the cu/naoh system, while peak ii has approximately the same intensity in both systems at ph 9 (see table 1). table 1. experimental details and results produced by lsv analyses solution ph sweep rate, mv s-1 potential, v current intensity, ma peak i peak ii peak i peak ii naoh 10 10 0.0863 0.5047 0.00019 0.01660 naoh 9 5 0.0799 0.4703 0.00016 0.00036 nh3 9 5 0.0959 0.3519 0.00261 0.00034 nh3 9 2 0.1236 0.4549 0.01010 0.00028 nh3+naoh 9 2 0.0198 0.4554 0.00008 0.0003 nh3+naoh 9 5 0.0626 0.5419 0.00149 0.00016 http://dx.doi.org/10.5599/jese.1843 j. electrochem. sci. eng. 00(0) (2023) 000-000 electrochemical detection mechanism of ammonia 6 the significant increase in the intensity of peak i in the cu/nh3 system compared to the cu/naoh system could have two explanations: the first one is that this peak originated from more than one electrochemical reaction [10]. in addition to the formation of cu2o from the reaction between cu and ohions (see eqs. 4 and 5), reactions take place to form cu-nh3 complexes and a cu(nh3)ohads layer (eqs. 12 to 15), the latter being also reactive for the formation of cu2o (see eq. 14) [10]. this assumption is based on the phase diagram for nh3 concentrations of 10-3 m. in this diagram, at ph close to 9, cu-nh3 species are stable. cu + nh3 + h2o → cu(nh3)ohads + h+ + e(12) cu(nh3)ohads + nh3 ↔ [cu(nh3)2]+ + oh(13) cu + cu(nh3)ohads → cu2oads + nh3 + h+ + e(14) cu + 2nh3 → [cu(nh3)2]+ + e (15) another possibility could be that nh3 reacts chemically with cu2o after it is formed (eq. (16)) [16]. this fact favors electron transfer increasing the peak intensity, a similar effect to that reported by yang’s group [6]. cu2o + 4nh3 + 2h+ → 2[cu(nh3)2]+ + h2o (16) at ph 9 the intensity of peak ii is similar in the cu/nh3 and cu/naoh systems. from these results, it could be proposed that this peak originated only by reactions between cu and ohions (eqs. (6) to (8)), ruling out any direct or indirect interaction between nh3 and cu, as has been reported in the cu/nh3 system at ph 10 [6,7]. the effect of the voltage sweep rate on the electrochemical reactions of the cu/nh3 system was also analyzed (figure 4). it can be observed that peak i increases in intensity by one order of magnitude while peak ii maintains approximately the same intensity as the sweep rate is reduced from 5 to 2 mv s-1 (table 2). the reduction in the intensity of peak i as the sweep speed increases may indicate that the electrochemical reaction between cu and nh3 is diffusion controlled due to the low concentration of nh3. figure 4. voltammogram of cu/nh3 system at ph 9 and (a) 2 mv s-1 and (b) 5 mv s-1. the increase in the intensity of peak i was not observed in the cu/nh3 system at ph 10 [6] because increasing the concentration of ohions favors the formation of cuoh (see eq. 4) over the generation of cu(nh3)ohads layer (see eq. 12) [10]. figure 5 shows the voltammograms of the cu nps/si nw electrode in the nh3+naoh solution at ph 9, which has a higher ionic strength than nh3 solutions due to the presence of na+ ions. it is noted that as the sweep speed decreases, the intensity of peak i (associated with the formation of the cu(nh3)ohads layer) is reduced. this phenomenon may be caused by the presence of na+ cation in the solution since this cation is solvated by nh3 molecules [17], which generates that nh3 is part of a a. e. hench-cabrera and e. quiroga-gonzález j. electrochem. sci. eng. 00(0) (2023) 000-000 http://dx.doi.org/10.5599/jese.1843 7 complex whose mobility in the solution is lower compared to the mobility of the individual nh3 molecule due to the size difference [18]. this phenomenon was observed in previous reports in which carbonate and borax buffer solutions (both solutions containing na+ cations) were used as supporting electrolytes [8,9], but in these reports, the phenomenon was related only to chemical reactions between the nh3 molecules and the carbonate and borate anions. in this work, the supporting electrolyte (naoh) does not contain anions that can react with nh3 or ammonium ions nh4+, although the same phenomenon is observed. future research should analyze the use of supporting electrolytes that do not contain cations that form complexes with nh3 or are solvated by nh3. figure 5. voltammogram of cu nps /si nw electrode in nh3+naoh solution at ph 9 at (a) 2 mv s-1 and (b) 5 mv s-1 conclusions in this work, the electrochemical detection of ammonia, present in the micromolar concentration range in an aqueous solution at ph 9, was carried out using an electrode made of copper nanoparticles deposited on silicon nanowalls. at ph 9, the signal that could be used for detecting nh3 is associated with the electrochemical reactions between cu-nh3 to form cu-nh3 complexes and cu(nh3)ohads layer. this signal is more reliable than the one used in the usual detection of nh3 at ph 10, where signal increment is 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of preferential cation solvation in aqueous ammonia, journal of the chemical society, faraday transactions 2: molecular and chemical physics 84 (1988) 1779-1788. https://doi.org/10.1039/f29888401779 [18] c. h. hamann, a. hamnett, w. vielstich, electrochemistry, wiley-vch, weinheim, 2007. isbn: 978-3-527-31069-2 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1843 https://doi.org/10.1039/f29888401779 https://creativecommons.org/licenses/by/4.0/) synergistic of yeast saccharomyces cerevisiae and glucose oxidase enzyme as co-biocatalyst of enzymatic microbial fuel cell (emfc) in converting sugarcane bagasse extract into electricity http://dx.doi.org/10.5599/jese.1559 321 j. electrochem. sci. eng. 13(2) (2023) 321-332; http://dx.doi.org/10.5599/jese.1559 open access : : issn 1847-9286 www.jese-online.org original scientific paper synergistic of yeast saccharomyces cerevisiae and glucose oxidase enzyme as co-biocatalyst of enzymatic microbial fuel cell (emfc) in converting sugarcane bagasse extract into electricity marcelinus christwardana1,2,, j. joelianingsih3 and linda aliffia yoshi3 1department of chemistry, diponegoro university, jl. prof. sudarto sh, tembalang, semarang, central java 50275, indonesia 2master program of energy, school of postgraduate studies, diponegoro university, jl. imam bardjo sh, pleburan, semarang, central java 50241, indonesia 3department of chemical engineering, institut teknologi indonesia, jl. raya puspiptek serpong, south tangerang 15014, indonesia corresponding author: marcelinus@lecturer.undip.ac.id received: october 21, 2022; accepted: december 22, 2022; published: january 13, 2023 abstract the microbial fuel cell (mfc) is an ecologically friendly alternative energy source. due to the typically limited electron transfer in mfc systems, co-biocatalysts are necessary to enhance their performance. enzymes are used as co-biocatalysts due to their superior ability to generate energy, and the system is known as an enzymatic microbial fuel cell (emfc). one of the substrates that may be used is bagasse waste extracted from sugarcane. saccharomyces cerevisiae and the enzyme glucose oxidase (gox) serve as co-biocatalysts in the breakdown of sugarcane bagasse waste in this study, which uses single-chamber emfcs. in emfc using sugarcane bagasse waste extract employing s. cerevisiae biocatalyst and glucose oxidase enzyme co-biocatalyst, the open circuit voltage was 0.56 v and the maximum power density was 146.65 mw m-2, an increase of 10.4 times to mfcs that solely employed only yeast biocatalyst. in addition, the chemical oxygen demand (cod) reduction achieved by this technology is 75 %. in addition, the ph of sugarcane bagasse waste extract samples treated with saccharomyces cerevisiae yeast and gox enzyme decreased from 4.6 to 4.2. this research demonstrates that adding the co-biocatalyst gox enzyme may boost the performance of the traditional yeast mfc. keywords bioelectricity; fuel cells; enzymes; sustainable energy introduction indonesia is now struggling to deal with its energy sources. unfortunately, most energy sources rely on nonrenewable fossil fuels. numerous nations are attempting to reduce their reliance on http://dx.doi.org/10.5599/jese.1559 http://dx.doi.org/10.5599/jese.1559 http://www.jese-online.org/ mailto:marcelinus@lecturer.undip.ac.id j. electrochem. sci. eng. 13(2) (2023) 321-332 converting sugarcane bagasse extract into electricity 322 fossil fuels because of their detrimental impact on the environment and humans [1]. in an attempt to adopt the usage of new and renewable energy, a number of nations are accelerating their infrastructure construction. the usage of biomass and fuel cell technology are only two examples. microbial fuel cell (mfc) technology, which mixes biomass with fuel cell technology, is one kind of sustainable power technology. mfc techniques may lessen the electrical strain on fossil fuels, but they cannot replace electrical energy since the electrical power generated is still rather low. mfc is used as a source of electrical energy for nighttime lighting and as a power source for microcomputers, for instance. mfc is an electrochemical technology that uses the catalytic activity of live microorganisms to transfer chemical energy from organic components into electrical energy [2]. utilizing organic waste components, mfc is an alternative energy-generating technology that is ecologically beneficial and capable of reducing environmental pollution. the microbial fuel cell technique offers a number of special benefits, including its applicability to treatment via a low-concentration substrate at temperatures below 20 °c [3]. in addition, the quantity of electrical energy generated by mfc is small, hindering its use on a broad scale [4]. the selectivity of microorganisms to the substrate is also one of the limitations of mfc, which is impacted by the kind of waste used as the substrate [5]. sugarcane is a plant with considerable economic value and the primary raw material for sugarproducing industries [6]. around 35-40 % of the weight of milled sugarcane may be extracted as bagasse from a sugar refinery. indonesian sugarcane production reached 33.7 million tons in 2013 [7]. therefore, it can be determined that the average quantity of bagasse generated throughout that time period was 11 million tons. this amount is estimated to rise since sugar production in indonesia is projected to rise. as a waste, bagasse may be harmful to the environment and must be properly managed. bagasse may be used as fuel in the sugar industry, as well as in the production of composites, textiles, paper, and animal feed [8]. however, its use has not been optimal, thus the rise in bagasse's economic worth has not been realized, and there are still many great deals of bagasse that have not been employed. typically, unused bagasse waste is stacked around the mill or on the factory scale in the form of cube-shaped bagasse, generating an offensive odor issue that disturbs people [9,10]. because the effect of sugarcane bagasse waste management is unfavorable, a new approach is required. mfc is one answer to the issue. bagasse's glucose content may be used as a substrate for the mfc system. glucose oxidase (gox) is an enzyme that has been extensively developed for fuel cell applications due to its electrochemical catalytic activity under physiological settings [11,12]. gox has the capacity to convert glucose into gluconolactone and electrical energy, making it an excellent candidate as a co-biocatalyst in a biological fuel cell for energy production. christwardana et al. [13] researched the use of glucose oxidase and catalase as co-biocatalysts for the anode of an enzymatic fuel cell (efc). christwardana et al. [14] investigated glucose oxidase and peroxidase as co-biocathode catalysts in ebc. in biological fuel cells, the use of gox as a co-biocatalyst with microbes is now a distinct option kovačević et al. [15] explored the production of fructose and gluconic acid from sucrose using yeast cell walls expressing gox. as fuel cell biocatalysts, bahartan et al. [16] combined gox and glucoamylase enzymes shown on the yeast cell surface and compared them to unmodified yeast or pure enzymes. bahartan et al. [17] encapsulated yeast with gox on its surface in graphene oxide hydrogel as a co-biocatalyst fuel cell. co-biocatalyst yeast with gox has been studied for its electrochemical properties, but it has never been used in wastewater treatment. this may be investigated further so that enzymes and microorganisms can be integrated as co-biocatalysts to m. christwardana et al. j. electrochem. sci. eng. 13(2) (2023) 321-332 http://dx.doi.org/10.5599/jese.1559 323 minimize cod in the wastewater treatment process and turn them into electrical energy utilizing a biological fuel cell system. in this study, the yeast saccharomyces cerevisiae was paired with the enzyme glucose oxidase as a co-biocatalyst in the enzymatic microbial fuel cell (emfc) system for treating sugarcane bagasse waste extract. the inclusion of the glucose oxidase enzyme may facilitate the conversion of glucose in sugarcane bagasse waste extract into protons and electrons, hence enhancing the efficiency of glucose conversion into energy. the employment of enzymes as co-biocatalysts in emfc for waste or wastewater treatment applications has never been done previously, hence it may be considered a novelty in this study. the goals of this research are to (1) establish the function and impact of adding glucose oxidase enzyme to the yeast mfc to form an emfc system in degrading sugarcane bagasse waste extract into electrical energy and (2) assess the value of lowering cod waste that can be achieved using enzymatic microbial fuel cells. experimental materials bagasse is purchased from a seller of sugarcane ice in serpong, tangerang, indonesia. bagasse is composed of around 33 % lignin, 32 % hemicellulose, and 35 % cellulose [18]. as a separating membrane, nafion 212 (dupont, usa) was employed. lesaffre produced the yeast saccharomyces cerevisiae (lessafre, marcq-en-baroeul, france), and commercial glucose oxidase enzyme extracted from penicillium notatum (light yellow powder; activity 2800 unit/g) was bought from xi'an taicheng chem (china). bagasse waste extract preparation bagasse waste is cut into smaller pieces before being sun-dried for typically three days. this treatment may be utilized as raw material for the subsequent waste processing step. 15 g dry bagasse waste is mixed in 150 ml of distilled water [18] using a blender phillips hr2056 for 60 seconds. this procedure tries to dissolve glucose or other sugars from the tissue matrix of sugarcane into distilled water. the processed bagasse waste slurry is subsequently filtered using a cheesecloth to remove the cellulose component, while the permeate fraction may be used as an emfc substrate. analyzed using a glucose sensor gluco dr agm-2100 (gyeonggi-do, south korea), the sugarcane bagasse extract had 11900 ppm of glucose and 9975 ppm of cod. structure and operation of the emfc reactor in this experiment, three identical plexiglass cubic single-chamber reactors with an active volume of 28 ml were utilized, with plain carbon electrodes with a projected surface area of 7 cm2. in addition, nafion 117 (3 wt.% h2o2, 0.5m h2so4, and di water) was utilized as a proton diffusion area separator between the two electrodes [19,20]. the sugarcane bagasse extract was mixed with 14 mg/ml of commercial baker's yeast saccharomyces cerevisiae, used as a bio-catalyst, and incubated semi-aerobically in a system known as mfc [21]. copper wire was used to complete the circuit, after which 1000 ω of external resistance was applied. voltage and current density were measured during the incubation period while the system was run in batch mode at 27 °c for three days (72 hours) every cycle for three cycles. at each cycle, fresh substrates and biocatalysts were introduced into the system. for efc and emfc systems, the biocatalyst is replaced by the gox enzyme and goxyeast combination, respectively. the schematic diagram of mfc, efc, and emfc system can be shown in figure 1. http://dx.doi.org/10.5599/jese.1559 j. electrochem. sci. eng. 13(2) (2023) 321-332 converting sugarcane bagasse extract into electricity 324 figure 1. schematic illustration of a) mfc, b) efc, and c) emfc system for wastewater treatment electrochemical and cod analysis the output current and voltage density throughout the incubation procedure (372 hours) are the primary parameters used to assess mfc performance. this required the collection of closedcircuit voltage (ccv) with an external load set to 1000 ω. voltage and current are measured using a digital multimeter uni-t ut61e, followed by the calculation of current density, ma m-2, dividing the produced current by anodic surface area, m2. in addition, manual polarization was done to analyze mfc behavior, using a resistor configuration of elenco rs-500 that was set between 5 mω and 100 ω. the relative decrease in cell potential (rdcp) was used to determine the maximum sustainable power produced during operation, inspired by the relative decrease in anode potential (rdap). when doing polarization analysis, rdcp is evaluated using a graph between rdcp content vs. external resistance value, where the content of rdcp, %, is determined by the formula (vocv/vn)100, in which vocv is the voltage at an open circuit, and vn is the voltage when external resistance is applied. analyzing the cod and ph of anolyte using the lutron wa-2015, the performance in terms of treatment efficiency was measured. results and discussion mechanism of biocatalyst activity in mfc, efc, and emfc systems saccharomyces cerevisiae uses glucose from sugarcane bagasse waste extract as a carbon source for its development, producing protons and electrons in the process. in mfc, yeast absorbs sugarcane bagasse waste extract and converts it to pyruvate via glycolysis [22]. pyruvate is subsequently utilized in the tricarboxylic acid (tca) cycle, which involves numerous enzymes [23]. then the electron transfer chain (etc) process takes place, during which the redox reaction of nad+ m. christwardana et al. j. electrochem. sci. eng. 13(2) (2023) 321-332 http://dx.doi.org/10.5599/jese.1559 325 and fad to nadh and fadh2 occurs [24]. the protons and electrons that have been generated then leave the cell. however, yeast has a complex etc, requiring more time for electron transport. the reaction that happens in mfc with yeast biocatalyst is shown in equation (1): yeast + 6 12 6 2 2c h o +6h o 6co +14h +24e⎯⎯⎯→ (1) meanwhile, the efc process turns chemical energy into electricity by using the high selectivity of enzyme for glucose substrates. it is responsible for glucose oxidation, oxygen reduction, and fad/fadh2 redox processes. as glucose becomes accessible, it is oxidized to generate electrons, protons, and gluconolactone. the protons and electrons reacted with fad to form fadh2, and vice versa, while some protons and electrons will be used for o2 reduction to h2o2 [25]. without o2, gox cannot convert glucose into protons, electrons, and gluconolactone, as gox is an oxidoreductase enzyme. this gox enzyme biocatalyst has a high electron transfer rate, therefore it would generate a great deal of energy [26]. in equation (2), glucose undergoes an oxidation process with oxygen and the presence of gox enzyme biocatalyst to generate gluconolactone and h2o2. in equation (3), the redox process at the active site of gox, fad/fadh2 as co-enzyme, involves proton and electron activity. gox 2 2glucose + o gluconolaceton + h o⎯⎯→ (2) gox(fad) + 2h+ + 2e gox(fadh2) (3) the production of electrons in an emfc is a joint effort of yeast and enzymes. yeast first takes in glucose and uses glycolysis to turn it into pyruvate; the pyruvate is then used in the tca cycle, which concludes the multienzyme process. in turn, this activates nad+ and fad, which pass through the etc pathway. electricity is produced as a result of the following generation of protons and electrons. simultaneously, the gox enzyme co-biocatalyst oxidizes glucose, transforming it into protons, electrons, and gluconolactone. it is expected that the combination would increase electrical energy production from the emfc system due to the high electron transfer rate of gox. voltage measurement in mfc, efc, and emfc systems figure 2 represents the observed voltage in the mfc, efc, and emfc systems. at the 75th hour, the efc produces the maximum voltage with a value of 335 mv, followed by the emfc at the 81st hour with 295 mv. at the 96th hour, mfc generates the maximum voltage with a value of 80 mv. emfc produces the greatest average voltage value at 92.3 ± 2.51 mv, followed by efc at 36.6 ± 2.42 mv and mfc at 27.3 ± 1.19 mv. the yeast biocatalyst required more time per cycle than the enzyme and yeast-enzyme combination to achieve the maximum current density. this is possible since microbes are still in the lag phase of forming biofilms on the anode surface [27]. in contrast, the time needed when using enzyme biocatalysts is less than when using yeast because the gox enzyme has a faster electron transfer rate [28]. because the gox enzyme has a higher electron transfer rate, the yeast-enzyme combination achieves the maximum current density more quickly than yeast alone. in addition, the enzyme biocatalyst required a short time during the second and third cycles to provide the highest current density. enzymes perform their activities immediately and do not need an adaptation period, unlike yeast, since they may react directly with existing substrates to produce energy supported by co-enzyme fad/fadh2 activity [29]. in addition, the enzyme dissolves directly on the substrate, which brings the enzyme molecule, substrate, and electrode closer together and accelerates electron transfer [30]. regarding the usage of yeast-enzyme biocatalyst, the time necessary to generate the greatest current density in the second and third cycles was not too lengthy. this resulted from the formation of biofilm on the electrode surface during the second and third cycles, which increased electron transport to the anode. moreover, following the second http://dx.doi.org/10.5599/jese.1559 j. electrochem. sci. eng. 13(2) (2023) 321-332 converting sugarcane bagasse extract into electricity 326 substrate change (cycle 2), the resultant current density rose. however, after the third substrate change (cycle 3), the resulting current density declined significantly. this is because the biofilm layer is excessively thick, creating a large distance between the biofilm and the electrode, which hinders electron transmission. although the enzyme biocatalyst produced the maximum current density in the efc, the yeast-enzyme combination biocatalyst produced the highest average value in the emfc system. this is due to the fact that the addition of enzyme co-biocatalyst to yeast biocatalyst increases the electromotive force of electrons produced during the substrate conversion process. figure 2. voltage of mfc, efc, and emfc system during sugarcane bagasse waste extract incubation process of 216 h maximum power density analysis according to figure 3a, the open circuit voltage of mfc, efc, and emfc is 0.45, 0.53, and 0.56 v, respectively. the interesting thing about this polarization curve is the high activation or reaction rate losses. since the electrochemical reaction in the mfc is limited by the absorption reaction, the necessary activation energy will be larger (from 0.45 to 0.11 v). while reactions involving gox in efc and emfc are limited by surface reactions, the reaction rate losses are not as significant as mfc, with efc from 0.53 to 0.26 v and emfc from 0.56 to 0.28 v. from figure 3b, the maximum power density generated by emfc is 145.65 mw m-2 at a current density of 645.1 ma m-2. while the mfc generates a maximum power density of 14 mw m-2 at a current density of 200 ma m-2, the efc produces a maximum power density of 73.94 mw m-2 at a current density of 459.64 ma m-2. these power density results are consistent with the current density result presented in the previous sections. several factors influence the value of the electric power density, including the design, type, and surface area of the membrane, the electrodes, the substrate, and the biocatalyst [31,32]. using a catalyst to transform the substrate into electrical energy generates a certain quantity of electricity, as measured by this power density. because each catalyst has a unique performance, the employment of various catalysts results in distinct power densities [33]. additionally, the presence of molecules other than glucose on the substrate will limit the efficacy of the biofuel cell in converting it into electrical energy. m. christwardana et al. j. electrochem. sci. eng. 13(2) (2023) 321-332 http://dx.doi.org/10.5599/jese.1559 327 figure 3. a) polarization and b) power density curves of mfc, efc, and emfc system during sugarcane bagasse waste extract incubation process of 216 h table 1. comparison of emfc performance with others taken from the literature anode cathode biocatalyst substrate mpd, mwm-2 ref. carbon paper carbon paper saccharomyces cerevisiae glucose 3 [34] ammonia-treated carbon cloth carbon fiber enterobacter cloacae pure cellulose 5.4 [35] bow-shaped carbon fibers circular stainless-steel wire saccharomyces cerevisiae glucose 52 [36] carbon paper pt@carbon paper brevibacillus borstelensis sugarcane molasses 188.5 [37] carbon felt carbon felt rhizobium anhuiense glucose 4.93 [38] carbon felt carbon felt s. cerevisiae and enzim gox sugarcane bagasse extract 145.65 this work table 1 contains a variety of data on the maximum power density taken from some previously published studies. in comparison to prior study findings, it can be stated that this yeast-enzyme biocatalyst delivers an excellent maximum power density. the yeast-enzyme combination in the emfc system increases the conversion of organic matter in the substrate into electricity when two distinct biocatalysts are simultaneously converting the substrate. since emfcs can only generate a small quantity of electricity compared to conventional fuel cells, they will likely only be used as backup power for smaller units in industries, such as nighttime lighting for streets and buildings. a promising method, the emfc system, can be applied downstream of the sugar industry process that uses sugar cane as a raw material, i.e., before the bagasse waste is disposed of. a relative decrease in cell potential (rdcp), a novel method for evaluating the maximum sustainability of mfc based on the capacity to produce electricity, is a function of external resistance. as can be seen in figure 4a-b, the rdcp for the mfc, efc, and emfc systems were all around the same value, i.e., 500, 154, and 438 kω, respectively. furthermore, the findings show that the enzyme may lower rdcp resistance, demonstrating the enzyme's superiority to yeast cells in terms of conductivity. in both systems, the anode conditions have a voltage proportional to resistance, while the anode's increased inclination for oxidation offers more energy from the biocatalyst. this is notably linked to the biocatalyst activity, especially when the electron release is quicker, which adds to increased electricity and energy output. http://dx.doi.org/10.5599/jese.1559 j. electrochem. sci. eng. 13(2) (2023) 321-332 converting sugarcane bagasse extract into electricity 328 figure 4. relative decrease in cell potential (rdcp) of: a) mfc, b) efc, and c) emfc system cod removal analysis figure 5 demonstrates that the mfc's cod decreased by 27.95 % in the first cycle, 41.32 % cycle, and 19.19 % in the third cycle. the cod removal in the first, second, and third cycles of the efc experiment was 21.92, 69.34 and 25.01 %, respectively. and for the emfc system, the cod reduction values for the three consecutive cycles were 74.08, 75.04 and 72.46 %. using a yeastenzyme combination biocatalyst in the second cycle resulted in the maximum reduction of cod or elimination of cod by 75.04 %, as shown by these data. due to poor co-biocatalyst stability, the third cycle of emfc had a lower cod reduction than the second cycle. due to the synergistic effect of two kinds of biocatalysts that breakdown organic matter in the substrate of sugarcane bagasse extract, the cod reduction in emfc was higher than in mfc and efc. the yeast-enzyme combination allows the decomposition of organic matter by yeast through the metabolism of microorganisms and enzymes via their catalytic activity simultaneously, hence increasing the quantity of organic matter that may be degraded and transformed into electrical energy. as the amount of organic matter is degraded, the cod drop is likewise increasing. this study demonstrated a 75.04 % drop in cod reduction, which is more than the 36.36 % seen in earlier research [18]. there is a correlation between the drop in cod and the generation of power, such that the larger the decrease in cod, the greater the generation of electricity [39]. m. christwardana et al. j. electrochem. sci. eng. 13(2) (2023) 321-332 http://dx.doi.org/10.5599/jese.1559 329 figure 5. cod removal of sugarcane bagasse waste extract using mfc, efc, and emfc system in every incubation cycle ph analysis figure 6 demonstrates that the ph declined throughout the fermentation process in cycles one, two, and three and every variation of biocatalyst use. in the mfc system, the starting ph of the substrate was 4.6, while after 72 hours (cycle 1), it fell to 3.38. the ph was then reduced to 3.09 during the subsequent 144 hours (cycle 2) after a change in the substrate where the starting ph was 4. the process was repeated by introducing a new substrate with an initial ph of 4.26 and reducing it to 3.12 at the end of cycle 3. figure 6. decrease in ph of mfc, efc, and emfc system during incubation of 216 h the initial ph in the efc was 4.6 and after 72 hours, it dropped to 4.43. ph increased to 5.02 when substrates were introduced but decreased to 4.11 after fermentation. the substrate was switched to a fresh one with a ph of 5.22, and at the end of cycle 3, the ph declined to 4.05. the substrate ph in the first cycle emfc system was 4.6 at the beginning of the cycle and dropped to 4.2 after 72 hours of fermentation. at the start of the second cycle, the ph was 4.49, and it decreased to 4.14 at the end. the ph of the substrate turnover increased to 5.82 at the beginning of cycle 3 before falling http://dx.doi.org/10.5599/jese.1559 j. electrochem. sci. eng. 13(2) (2023) 321-332 converting sugarcane bagasse extract into electricity 330 to 4.2 at the end of fermentation. based on these results, it can be concluded that the ph continued to decrease during the fermentation process. the mfc has the lowest ph of the three types of biocatalyst use. as a consequence of microbial metabolism, malic acid, tartaric acid, citric acid, lactic acid, acetic acid, butyric acid, and propionic acid are produced during fermentation, resulting in a reduction in ph [40]. in addition, the efc experiences a decrease in ph because the rate of proton migration from the anode to the cathode across the membrane is diminished, causing an accumulation of protons [41]. conclusions this study reveals that the emfc system combined with the fermentation of sugarcane bagasse to produce electricity directly utilizing s. cerevisiae and the enzyme glucose oxidase as co-biocatalyst performs well. with an emfc system, 145.65 mw m-2 of power density was reached, or a larger 940 and 96 % than the mfc and efc systems, respectively. when compared to mfc and efc systems, this system achieves a cod elimination rate of 75.04 %, which is 1.8 and 1.1 times higher. yeast metabolism and/or enzyme activity produced acid throughout the incubation phase, which resulted in a drop in ph after the process. the synergistic effect of gox and yeast as co-biocatalysts in mfc systems is more impressive than in mfc and efc systems. the transfer rate of electrons is increased by the presence of gox in the 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commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1016/j.jpowsour.2017.03.109 https://doi.org/10.1016/j.bioelechem.2012.02.001 https://doi.org/10.1128/aem.02600-08 https://doi.org/10.1016/j.matpr.2021.01.327 https://doi.org/10.1016/j.energy.2019.04.087 https://doi.org/10.3390/bios12020113 https://doi.org/10.1016/j.biortech.2015.09.088 https://doi.org/10.1111/1567-1364.12161 https://doi.org/10.14710/ijred.5.2.107-112 https://doi.org/10.14710/ijred.5.2.107-112 https://creativecommons.org/licenses/by/4.0/) preparation and properties of electrodeposited ni-tio2 composite coating doi:10.5599/jese.135 37 j. electrochem. sci. eng. 5(1) (2015) 37-45; doi: 10.5599/jese.135 open access : : issn 1847-9286 www.jese-online.org original scientific paper preparation and properties of electrodeposited ni-tio2 composite coating sukhdev singh bhogal, vijay kumar*, sukhdeep singh dhami and bahadur singh pabla national institute of technical teachers and research, sector-26, chandigarh, india *punjab technical university campus, sas nagar (mohali)-punjab, india  corresponding author: sukhdev_bhogal@yahoo.com received: 4 november 2014; revised: 7 january 2015; published: march 15, 2015 abstract the mechanical properties of cutting tools, such as microhardness, corrosion resistance, and coating adhesiveness, directly affect the tool life and indirectly affect the component cost. in this paper, ni-tio2 composite coating was prepared through electrocodeposition in order to improve the mechanical properties of tungsten carbide cutting tools. the microhardness of the ni-tio2 composite layer was studied by varying the input current density (ma cm -2 ), ph value of the electrolyte, and particle concentration of tio2 in electrolyte bath. the microstructure and phase structure of the composite layer were investigated using atomic force microscopy , scanning electronic microscopy and x-ray diffraction. the surface morphology of the ni-tio2 coated layer shows fine-grained structures and higher microhardness at lower currents. the maximum microhardness of the coated layer, 1483 hv, is found at a current of 15 ma cm -2 and watts solution ph of 4.5. it is observed that with the increase of tio2 content, the microhardness of the coating also increases. keywords microhardness; electrocodeposition; ccomposite layers introduction tio2 is well known as a metal dioxide ceramic which has been utilized in many applications for its fine physical and chemical properties. nano-size tio2 particles are usually introduced into nickelbased composite coating to enhance its properties such as corrosion resistance, microhardness, and abrasiveness [1]. electrodeposition is one of the traditional processes used for the http://www.jese-online.org/ mailto:sukhdev_bhogal@yahoo.com j. electrochem. sci. eng. 5(1) (2015) 37-45 electrodeposited ni-tio2 composite coating 38 improvement of the surface. specially, it is used for the enhancement of coating properties such decorativeness, functionality, and electroforming of the coated surfaces. this method has advantages like low cost, low temperature, ease of use, and being a single-step process that does not require additional thermal treatment [2]. in the manufacturing industry, metal cutting tools are in high demand due to their greater sustainably in the cutting of hard materials, as tough conditions of high cutting force and high contact pressure between the workpiece and tool materials are found during machining. further cutting conditions and tool shape, make the tool condition to the severity and ultimately causes the earlier tool failure. mostly the cutting tools which are used have more ultimate resistance against the load and ability to limit the thermal and mechanical stresses during cutting of hard and difficult-to-cut materials. nowadays, highperformance cutting tool materials obtained with chemical vapour deposition (cvd) and physical vapour deposition (pvd) as well as other coated carbides like cermets, ceramics, cubic boron nitride, and diamond-coated tools are frequently used, but tungsten carbide (wc)-coated tools are also in use for metal-cutting applications due to their high strength and the fact that they can be manufactured economically with complicated geometries. protective coatings on cutting tools have been in use for a little more than three decades. the search for wear-resistant materials has since been redirected to encompass material properties other than just hardness. cutting performance is also dependent on both the cutting tool and the machine tool system, that is, on the tool material, cutting tool geometry, and hard coating [3]. in the early twenty century hard coatings were deposited by cvd and pvd by a magnetron sputtering process. magnetron sputtering is preferred to the cvd process due to the contamination that occurs in the latter [4]. single-layer coatings such as tin, tic, crn, dlc, ti-o2, and aln played an important role a few years ago, due to their mechanical and tribological properties [5]. nowadays, these coatings are being mixed, designed, and improved by creating hetero-structured thin films and multilayered coatings with periods in the range of nanometres, which is one of the advanced coating concepts more commonly investigated during the last two decades, together with nanocomposites [6,7]. the nanocomposite coatings range from 3 to 10 nm in size, have been selected from nitrides, carbides, borides, and oxides, and are embedded in an amorphous or crystalline matrix. the large volume of grain boundaries provides ductility through sliding of the grain boundary along grain/matrix interfaces. these coatings have sometimes been reported to have hardnesses of up to 100 gpa [8,9]. electrodeposition offers some unique advantages over other techniques such as improved appearance, corrosion resistance, and physiochemical properties of surfaces which find potential applications in the fabrication of advanced components for micro-/nano-electromechanical systems (mems/nems). moreover the initial setup cost for coating by electrodeposition is quite low compared to other conventional coating processes like cvd and pvd. moreover fine structure growth and easy control of the coating thickness up to fractions of a micrometre are possible via electrodeposition. the aim of this work is to evaluate the influence of ni-tio2 composite coatings deposited onto a wc cutting tool by the electrodeposition technique. ni-tio2 composite coating was employed to investigate the microhardness behaviour, adhesion to the substrate, and coating thickness with different currents at constant temperature and deposition time. experimental details ni-tio2 composites were electrodeposited from an organic free watts nickel electrolyte with suspended tio2 nanoparticles on the wc tool bits by electrodeposition. electrodeposited nickel s. bhogal at al. j. electrochem. sci. eng. 5(1) (2015) 37-45 doi:10.5599/jese.135 39 and composite ni-tio2coatings were prepared from watts solution. the composition of the plating solution, as well as the plating conditions, is given in table 1. all the chemicals used were of analytical grade with high purity and were purchased from merck. a gamry reference 3000 potentiostat was used for coating. the triangular-shaped cnc tool bit (wc) was mechanically polished with different grades of emery papers (between 800 and 4000) and finally ultrasonically cleaned in acetone before electroplating without any activation pretreatment. the substrates were rinsed with doubly distilled water and dried under air. a specially designed three-electrode electrodeposition cell was used for electrodeposition of ni-tio2, where the cutting tool was the working electrode, ag/agcl was the reference electrode, and pt wire acted as the counter electrode (anode). the distance between the ag/agcl reference electrode and the working electrode was 1 cm. the particle size of tio2 particles was about 50 nm. it was used without any pretreatment and was kept in a bath in suspension by continuous magnetic stirring at a rate of 210 rpm in order to maintain a uniform concentration of particles in the bulk solution. before starting the electroplating, the electrolyte was placed in an ultrasonic bath for 20 min to prevent agglomeration of tio2 particles. table 1. watts solution for ni/tio2 composite electrodeposits electrolyte (watts) solution composition concentration of niso46h2o, g l -1 concentration of nicl26h2o, g l -1 concentration of h3bo3, g l -1 concentration of tio2, g l -1 ph of electrolyte temperature, °c substrate average current density, ma cm -2 agitation type of current 300 45 40 6 4.4 ± 0.1 30 tungsten carbide bit 15, 30, 45 180 rpm dc the surface morphology of the ni-tio2 composite coating was determined by sem (jeol, jsm-6460lv) with energy dispersive x-ray analysis (eds) and afm. to determine the phase structure of ni–tio2 composite coatings, x-ray diffraction (xrd) analysis was performed on a pw 1710 diffractometer with cu-kα (λ = 1.54 a0) radiation. the operating target voltage was 40 kv and the tube current was 100 ma. using the scherrer equation, the average grain diameter could be calculated as follows: cos k d     (1) where k is the constant of the grains (k = 0.89), λ the wavelength, ω the standard full width at half maximum (fwhm), and  the bragg angle. vickers hardness was measured by applying a 0.5-kg force load for 10 seconds using a vickers hardness tester (mitutoyo). results and discussion the surface morphology and microstructure of the composite coating were investigated through atomic force microscopy (afm) and scanning electron microscopy (sem). the surface morphology and microstructure of the coating layer are discussed below. j. electrochem. sci. eng. 5(1) (2015) 37-45 electrodeposited ni-tio2 composite coating 40 surface morphology and microstructure the composite coating was investigated by afm. figure 1 shows the afm images of ni-tio2 composite coating surfaces (20 × 20 µm area) on a tungsten carbide tool tip. figures 1a and 1b show the surface after deposition of the ni-tio2 coating with 5 ma of current and a ph of the solution of 4.5 respectively. in order to compare the surface roughness (ra) of the coatings, afm data are used, and based on the experimental results, the ni-tio2 composite coating prepared at lower current i.e. at 15ma with ph 3.5 of solution (fig. 1a) shows a uniform and fine structure among the micro-regions, whereas the coating prepared with a lower current and ph 4.5 of the watts solution (fig. 1b) appears to be finer and shows a regular crystalline grain structure. as the ph of the bath influences the hydrogen evolution voltage the precipitation of basic inclusion of solution and decomposition of the hydrated ion is deposited more freely. thereby a finer and more uniform grain structure is obtained. (a) (b) figure 1. afm images of ni-tio2 composite coating: (a) current = 15 ma, temperature = 30 o c, and ph 3.5; (b) current = 15 ma, temperature = 30 o c, and ph 4.5 the xrd patterns of the ni-tio2 (fig. 2) composite coatings were detected by xrd to further confirm the existence of tio2 particles. scans were recorded for the range 2 = 20-80° with a scan step of 0.02°. the xrd patterns of the three types of ni-tio2 composite coatings are presented in fig. 2. figure 2. xrd pattern of ni-tio2 composite coating deposited at different conditions: (1) current = 15 ma and temperature = 30 °c; (2) current = 15 ma, temperature = 30 °c, ph 4.5 (3) 0.4 % tio2, current = 15 ma, temperature = 30 °c, ph 4.5 2 /degree s. bhogal at al. j. electrochem. sci. eng. 5(1) (2015) 37-45 doi:10.5599/jese.135 41 the figure shows that the composite coatings consist of ni phase and tio2 phase. for ni, the diffraction peaks at 44.54°. for tio2, the diffraction peaks at 51.96° and 75.51°. according to the xrd data, the average grain size of ni and tio2 can be calculated using eq. (1). the results are shown in table 2. the xrd results demonstrate the average grain diameters of ni and tio2 in the composite coating prepared by electrocodeposition. at lower current, that is, 15 ma, the average grain sizes of ni and tio2 are 60 and 19.65 nm, respectively, while at ph 4.5 the average grain sizes of the coating are 73.1 and 23.84 nm respectively. these results are consistent with the afm results. table 2. average grain size of ni and tio2 composite coatings type of coatings dni / nm dtio2 / nm (1) 60.0 19.65 (2) 73.1 23.84 (3) 51.3 25.70 mechanical properties of nitio2 coatings mechanical properties such as microhardness play an important role in tool life enhancement. the inclusion of nano-sized tio2 particles in the metal matrix deposit is dependent on parameters like ph concentration, current density, temperature, stirring rate, and so on. the microhardness of the samples was examined by applying a 0.5-kg force load using a vickers microhardness tester (mitutoyo). after coating deposition, six indentations were made at different places and the mean of these values was taken. the effect of the applied current, the ph value of the watts solution, and the percentage concentration of tio2 nanoparticles in solution by volume directly affected the microhardness of the composite coatings. the effects of these parameters are discussed below. effect of current on microhardness of ni -tio2 coatings ni-tio2 composite coating is deposited on the cutting tool bit by varying the applied current from 15 to 45 ma. the effect of the current density applied during the process is studied by measuring the microhardness of the composite coatings. the morphological characterization of as-prepared samples was done using a scanning electron microscope (jeol jsm 6510 lv) at an accelerating voltage of 20 kv as shown in figure 3. the change in structure of the composite layer is observed with changes in current density at constant time of deposition. fine close-grained structures are observed at low current as shown in figure 1. on examination of all coated layers, some fine cracks are found in the layer deposited at higher current, that is, at 45 ma. figure 3. sem photographs of ni/tio2 composite layers at currents of 15, 30, and 45 ma the microhardness of ni-tio2 composite coatings is a function of the grain structure developed during the process. the increase in microhardness of ni-tio2 composite coatings can be explained on the basis that tio2 nanoparticles are uniformly distributed in the ni matrix. further it is found j. electrochem. sci. eng. 5(1) (2015) 37-45 electrodeposited ni-tio2 composite coating 42 that growth of the ni grains, through its grain refining and dispersive strengthening effects, also affects the microhardness of coatings [13]. as shown in fig. 3, the sem results shows that at lower current, that is, at 15 ma, a fine closegrained structure is observed, whereas the course grain structure is observed at current intensities of 30 and 45 ma. the measurements show that the microhardness is influenced by the change in current density as shown in fig. 4. the measurements were carried out three times for each sample to confirm the reproducibility of the results, and the maximum indentation depth was of the order of 1 m. figure 4. effect of current on microhardness and grain size of composite layer at ph 3.5 and temperature of 30 °c. in nanocomposite coatings, it is found that dispersion strengthening is largely due to the uniform distribution of tio2 nanoparticles in the coatings. such particles possess higher density and fine size, and close and fine grains are obtained in the composite [14]. codeposition of fine tio2 nanoparticles in ni matrix restrained the growth of large ni grains, resulting in the finegrained structure of the composite layer. further dispersive strength obtained during the reinforcement phase acts as a barrier that retards the plastic deformation of the ni matrix, resulting in high microhardness of the composite coatings [15]. accordingly, higher microhardness and improved friction and wear properties are found in the hardened matrix. effect of ph value on microhardness of ni-tio2 coatings in electrodeposition, the ph value of the solution is a vital parameter to be controlled to maintain the process optimization. if the ph of the watts electrolyte is too high or too low due to chloride ion concentration, more oxygen will evolve and then hydroxyl ions will be discharged in preference to the dissolution of nickel. under this condition, the nickel anode becomes passive and the efficiency of anode dissolution is almost zero [16,17]. the ph value of the bath influences the hydrogen evolution voltage, the precipitation of basic inclusion, and the decomposition of the complex or hydrate from which the metal is deposited. in a complex bath, the ph may influence the equilibrium between various processes. when the anode is insoluble, oxygen evolution takes place at the anode: 2h2o →o2 + 4h + + 4e s. bhogal at al. j. electrochem. sci. eng. 5(1) (2015) 37-45 doi:10.5599/jese.135 43 on the other hand, hydrogen evolution at the cathode is accompanied by the production of hydroxide ions: 2h2o + 2e →2oh + h2 if the current efficiency is greater at the anode than at the cathode, the bath becomes more alkaline. if the electrode efficiencies are similar, the ph of the bath remains unchanged. hence a change in ph of a plating bath is a good indication of electrode efficiencies. since it is not possible to predict all the factors, the best ph range must be determined empirically for specific coating compositions. in this study, coating has been done at different ph concentrations of the watts solution of 3.5, 4.5, and 5.5. the effects of the quality of coatings were investigated using constant conditions of a current density of 15 ma, temperature of 50 °c, deposition time of 1200 s, and stirring rate of 210 rpm. figure 5 shows the microhardness of the composite layer at different ph values at a current of 15 ma. figure 5. effect of ph value on microhardness and grain size of composite layer at a current of 15ma and temperature of 30 °c the microhardness, which is mainly dependent on the grain size, is influenced by the ph value of the solution. figure 6 shows the sem cross-sectional image of ni-tio2 composite coatings produced at ph concentrations of 3.5, 4.5, and 5.5. figure 6. sem photomicrographs of ni/tio2composite layer at ph 4.5, 5.5, and 3.5, respectively. the micrographs shows a dark dispersion uniformly distributed all over the coatings and xrd analysis of these dispersions reveals the dark area to be rich in tio2 particles; however the j. electrochem. sci. eng. 5(1) (2015) 37-45 electrodeposited ni-tio2 composite coating 44 concentration of tio2 particles in the coating varied with changes in the ph of the electrolyte. the amount of tio2 particles is highest at ph 4.5. effect of tio2 particle concentration on the microhardness of composite coatings the incorporation of tio2 nanoparticles in the nickel matrix leads to the formation of finer-grained structures, which further decreases the residual tensile stresses and increases the microhardness and wear and corrosion resistance of the composite coatings [18]. during electrodeposition a coating of a thin layer of one metal is deposited on top of a different metal at different concentrations of tio2 nanoparticles in the solution in order to modify the surface properties. figure 7. effect of tio2 content on microhardness and grain size at current = 15 ma, ph = 4.5, and temperature = 30 °c. deposition was carried out by varying the concentration of ti-o2 nanoparticles in watts solution from 0.3 to 6 g l -1 at a lower current, that is, 15 ma, and a solution ph of 4.5. the microhardness of each composite coating was measured and the maximum microhardness was found at 0.4 g l -1 solution. the addition of more solution had a negligible effect on the coating as shown in fig. 7. when tio2 nanoparticles were added to the electrolytic bath of watts solution, the borders of the ni grains became more localized, resulting in a reduction in mean grain size compared to pure ni. the addition of tio2 nanoparticles to the nickel matrix changed the structure of the composite coatings. according to scherrer’s equation, with increasing content of particles in the coating, the ni grains decrease in size [13]. conclusions electrodeposition is a versatile route for depositing composite coatings of ni-tio2 on the surface of cutting tools. moreover the setup cost for electrodeposition coating is quite low compared to other conventional coating processes like cvd and pvd. in this work, a ni-tio2 nanocomposite coating was deposited on a wc cutting tool by the electrodeposition technique in order to improve the mechanical properties. the effects of the applied current, ph of the solution, and tio2 particle concentration were studied. the microstructure and phase structure of the composite s. bhogal at al. j. electrochem. sci. eng. 5(1) (2015) 37-45 doi:10.5599/jese.135 45 layer were investigated using atomic force microscopy, scanning electronic microscopy, and x-ray diffraction. it was found that the surface morphology of ni-tio2 coating was crack free at lower current densities; however some fine cracks appeared at high current densities, which means that ni-tio2 coatings are prone to cracking at higher current density. the microhardness of ni-tio2 coatings decreases due to increases in grain size and decreases in tungsten content. the highest microhardness value is obtained at a lower current, that is, 15 ma. the ph value of the electrolyte bath influences the hydrogen evolution voltage. the microhardness of the composite layer is influenced by the ph value of the solution. a higher microhardness value is obtained at ph 4.5 at lower current density. further addition of tio2 nanoparticles was carried out and the maximum microhardness of 1484 hv was found at 0.5 % tio2, ph 4.5, and a current of 15 ma. references [1] c. s. lin, c. y. lee, c. f. chang, surf. coat. technol. 200 (2006) 3690-3697. 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[18] s. spanou, e. a. pavlatou, n. spyrellis, proceedings of the 7 th international conference coatings in manufacturing engineering, chalkidiki, greece, 2008, p. 57-71. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ electrochemical determination of vitamin b6 in pharmaceutical and energy drink samples http://dx.doi.org/10.5599/jese.1574 297 j. electrochem. sci. eng. 13(2) (2023) 297-319; http://dx.doi.org/10.5599/jese.1674 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical determination of vitamin b6 in pharmaceutical and energy drink samples gizaw tesfaye, negussie negash and merid tessema department of chemistry, addis ababa university, p. o. box 1176, addis ababa, ethiopia corresponding author: gizawtes@gmail.com received: january 24, 2023; accepted: february 25, 2023; published: march 1, 2023 abstract a simple and low-cost electrochemical sensor based on poly(phenylalanine) and functionnalized multi-walled carbon nanotubes (f-mwcnts) modified glassy carbon electrode (gce) was developed for the determination of vitamin b6 (vb6). the surface morphology of modified glassy carbon electrodes was investigated with scanning electron microscopy (sem) and fourier transform infrared spectroscopy (ftir). the electrocatalytic activities of the bare and modified electrodes were investigated in the presence of ferri-ferrocyanide redox couple using cyclic voltammetry (cv) and electrochemical impedance spectroscopy (eis). the exchange current density (jo = 2462 µa cm-2) and electron transfer rate constant (ko = 0.002 cm s−1) were calculated using 5 mm k3[fe(cn)6]. the electrochemical activity of poly(phenylalanine)/f-mwcnt/gce towards vb6 oxidation was investigated using cv. parameters including the number of electrons transferred (n = 2), number of protons transferred (h+ = 2), electron transfer coefficient (α = 0.51) and surface concentration of vb6 ( = 0.24 nmol cm−2) were calculated. at the optimal experimental conditions, the oxidation peak current of vb6 measured by square wave voltammetry (swv) was found proportional to its concentration in two linear ranges of 0.5 to 20 µm and 20 to 200 µm with a low detection limit (lod) of 0.038 µm and limit of quantification (loq) of 0.125 µm. finally, the sensor was successfully used to determine vb6 in soft drink and pharmaceutical formulation samples. keywords glassy carbon electrode; electro-polymerization; poly(phenylalanine); f-mwcnts; pyridoxine; electrochemical sensor; real samples introduction vitamins are nutrients that are essential for normal cell development, energy generation and red blood cell production [1–3]. furthermore, they are important cofactors of many enzymes in different metabolic processes occurring in the body [3]. based on their solubility, vitamins are classified as water-soluble vitamins (vitamin b and vitamin c) and fat-soluble vitamins (vitamin a, vitamin d, http://dx.doi.org/10.5599/jese.1574 http://dx.doi.org/10.5599/jese.1674 http://www.jese-online.org/ mailto:gizawtes@gmail.com j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 298 vitamin e, and vitamin k) [1]. water-soluble vitamins are not stored in the body due to easy discharge through urine [2,4]. furthermore, they cannot be synthesized in the human body [3,5]. therefore, they must be obtained from dietary sources for normal cell growth. among the water-soluble vitamins, vb6 (pyridoxine) (figure 1a) plays an important role in body metabolism. vb6 is involved in amino acid metabolism, red cell production, hemoglobin synthesis, gene expression, enzymecatalyzed reactions and nervous and immune systems [1,5]. vb6 is found in many foods such as chickpeas, turkey, fish, starchy vegetables, potatoes, bananas, meats, organ meats, fortified cereals and whole grains [6,7]. the recommended dietary allowance (rda) for vb6 is 1.3 mg/day for adult males and females and even higher amounts are advised for males and females over the age of fifty years [6]. the normal range of vb6 in human blood is 5–50 μg l-1 [1]. the deficiency of vb6 causes colorectal cancer, skin problems, weakness, depression, convulsions, neurological diseases, anemia, hyperlipidemia, hypertension, obesity, cardiovascular diseases, mucous membranes and circulatory system problems [1,3]. however, excessive intake of vb6 for a long time leads to different allergic reactions such as rash, itching, severe dizziness and breathing [2]. it is vital to develop sensitive and selective methods for the quick determination and monitoring of vb6 in various matrices, such as food and pharmaceuticals, in light of its significance and potential negative effects. for the purpose of determining vb6, a number of analytical techniques, including liquid chromatography [8], gas chromatography-mass spectrometry [9], chemiluminescence [10], capillary zone electrophoresis [11] and spectrophotometry [12] have been described in the literature. many of these analytical techniques require tedious and time-consuming procedures and involve highly sophisticated instrumentation and toxic organic reagents [5,13,14]. due to great selectivity, sensitivity, quick response time, ease of use and low cost, electrochemical methods have received a lot of interest in electroanalysis [4,15]. furthermore, combined with portable detection equipment, electrochemical methods offer great promise for on-site environmental monitoring [16]. glassy carbon electrode (gce) has been widely used as a working electrode for sensor fabrication due to its resistance to chemical attack, easy modification, good reproducible surface, low porosity, high electrical conductivity, wide potential window and good mechanical stability. however, at bare electrode surfaces, vb6 shows weak or no electrochemical response. in order to increase the rate of electron transfer and lower the overpotential, it is required to change the electrode surface with appropriate modifiers [17]. various modified electrodes, such as gce modified with a hybrid of polydopamine and reduced graphene [18], multi-walled carbon nanotube and cobalt phthalocyanine (copc) modified pyrolytic graphite electrode [19], nio-carbon nanotubes (cnts) nanocomposite and 1-methyl-3-octylimidazolium hexafluorophosphate modified carbon paste electrodes [20], poly(arginine) modified carbon nanotube paste electrode [21], cobalt hexacyanoferrate modified carbon paste electrode [22] and gold nanostructures modified carbon paste electrode [23] have been fabricated for the electrochemical determination of vb6. however, all these electrodes suffer from tedious modification processes. therefore, it is important to develop low-cost and simple electrochemical method for the vb6 determination. recently, polymers have received much attention for electrode modification due to their biocompatibility, chemical stability, ease of synthesis, more active sites and low-cost processability [24,25]. the advantages of electrochemical polymerization over chemical polymerization for polymer preparation include homogeneous and stable polymer film formation on the electrode surface, ease of preparation, cost-effectiveness, strong adhesion of the polymer film to the electrode surface and simple control of film thickness by adjusting electrochemical parameters only [25]. because of their ease of processing, high electrocatalytic capacity, easy interaction with the target analyte via g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 299 hydrogen bonding or electrostatic interaction caused by the presence of amine and carboxyl groups, stable film forming ability, high biocompatibility and low cost, poly(amino acids) were used extensively as electrode modifiers [26]. different amino acids have been used to modify electrode surfaces for the electrochemical analysis of various analytes [27-31]. among the major amino acids, phenylalanine (figure 1b) is easily electropolymerized on the electrode surface to form poly(phenylalanine) [32]. a number of analytes have been studied electrochemically using electrodes modified with poly(phenylalanine) [33-35]. with respect to the intended analytes, the modified electrode demonstrated high electrocatalytic activity and stability. figure 1. chemical structures of (a) vb6 and (b) phenylalanine on the other hand, cnts modified electrodes have been widely used in electrochemical sensors due to their high electrical conductivity, biocompatibility, ease of functionalization, mechanical strength, high adsorption capacity, chemical stability and high surface area [26,36]. furthermore, carbon nanotubes are easily incorporated into other materials to produce synergistic effects that increase sensitivity to a target analyte. however, a significant obstacle to the current development of cnt-based devices is the limited solubility of cnt in most solvents. because of the strong - interactions between the aromatic rings, cnts tend to aggregate in aqueous solutions, making challenging to disperse them and use for development of electrochemical sensors. therefore, it is important to prepare hydrophilic surface cnts to overcome the dispersion problem. functionalization of cnt enhances solubility, processibility and interaction with other materials [37]. cnts can be covalently functionalized by oxidation with strong acids such as hno3, h2so4 or their mixtures [38]. oxidation of cnts results in open-ended nanotubes containing oxygenated functional groups such as carboxylic acid, ketone, alcohol and ester groups [38]. the presence of these oxygen-containing functional groups on cnt increases its interaction with a polymer during composite formation [38]. as described in many literature studies, the combination of carbon nanotube with polymer significantly improves the electrocatalytic activity of the modified surfaces compared to individual cnts or polymer-modified electrodes due to the synergistic effect of cnt and polymer [25,26]. in this study, taking into account the synergistic effect of f-mwcnt and polymer, we constructed an effective and low-cost electrochemical sensor for the determination of vb6 in food and pharmaceutical samples. the electro-polymerization of phenylalanine on carbon nanotubes is carried out using potentiodynamic polarization. the prepared electrode, poly(phenylalanine)/f-mwcnt/gce is characterized by fourier-transform ir (ftir), scanning electron microscopy (sem), cyclic voltammetry (cv) and electrochemical impedance spectroscopy (eis) techniques. the electrochemical behavior of vb6 at the modified electrode is investigated using cv, while its analytical determination is performed by square wave voltammetry (swv). the preparation procedures for the f-mwcnt and poly(phenylalanine) modified gce and sensing mechanism are illustrated in the graphical abstract. http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 300 experimental reagents and chemicals all chemicals used in this study were analytical grade and were used without any further purification: pyridoxine hydrochloride (vb6) (≥ 98 %), vitamin b1 (≥ 99 %), potassium nitrate (99 %), sodium sulfate (≥ 99 %) and ascorbic acid (≥ 99.7 %) were obtained from (bdh, england). vitamin b2 (≥ 98 %), l-phenylalanine (99 %), caffeine (99 %), tartaric acid (98 %), acetic acid (99.8 %), glucose (≥ 99.5 %), starch (ar grade), multi-walled carbon nanotube (mwcnt) (> 90 % carbon basis) synthesized by chemical vapor deposition, potassium ferricyanide (99 %), sucrose (≥ 99.5 %), lactose (≥ 99%), vitamin b12 (≥ 98 %), vitamin b9 (≥ 97 %), sodium hydroxide (98 %) and hydrochloric acid (37 %) were obtained from sigma-aldrich (usa). citric acid (99 %), sodium citrate (98 %) and dipotassium hydrogen phosphate (98 %) were obtained from research–lab. chem. industries (mumbai, india). potassium dihydrogen phosphate (98 %), magnesium chloride (99.5%), copper nitrate (98 %), iron(ii) nitrate (98 %), sodium carbonate (99.5 %) and sodium bicarbonate (99.7 %) were obtained from hopkin and williams ltd (england). potassium chloride (98 %) was purchased from riedel-de haën (france). boric acid (99.5 %) was obtained from carlo erba reagent spa (italy). 5 mm stock solution of vb6 was prepared by dissolving accurately weighed amounts of vb6 in distilled water. until use, the stock solution was kept in a refrigerator. a suitable quantity of phenylalanine was dissolved in phosphate buffer solution ph 8.0 to prepare a 2mm phenylalanine solution. acetate buffer solutions (abs) were prepared by mixing 0.1 m acetic acid and 0.1 m sodium acetate. 0.1 m naoh and 0.1 m hcl solution were used to adjust the ph of the solution. the stock solutions were diluted with abs ph 4.5 to prepare the working solutions. apparatus and instruments electrochemical experiments, including cv, swv and eis were carried out using chi 760d electrochemical workstation (ch instruments, usa). a three-electrode system consisting of bare gce or modified gce (geometric surface area of 0.07 cm2) as a working electrode, platinum wire as a counter electrode and ag/agcl (3 m kcl) as a reference electrode were used. the ph of the solution was measured using a ph meter (sension, sha snilu instruments co. ltd., china). the surface morphology and chemical structure of the modified electrodes were studied using sem (cx-200 coxem, korea) and ftir (spectrum 65 ft-ir (perkin elmer, usa) using kbr disk, respectively. real sample preparation beverage sample energy drink samples (predatory energy drink, ambo mineral water s.c. ethiopia) were obtained from a nearby supermarket in addis ababa, ethiopia, and kept in a refrigerator until analysis. 100 ml of the liquid sample was transferred into a beaker and degassed in an ultrasonic bath before voltammetric analysis. then 2.0 ml of the sample was diluted to 10 ml with 0.1 m abs ph 4.5 solution. the determination of vb6 in the sample was performed using the standard addition method. the samples were spiked with a stock solution of vb6 for the recovery test. pharmaceutical sample a commercial sample of the vitamin b complex (n-vit, ningbo shuangwei pharm.co., ltd., china, containing vitamins b1, b6 and vb12) was obtained from a local drugstore in addis ababa. five tablets containing 100 mg of vb6 were precisely weighed and ground into a fine powder. an amount equivalent to one tablet was taken in a 50 ml volumetric flask and dissolved in distilled water by g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 301 sonicating it for 10 min. then, the solution was filtered using whatman filter paper to obtain a clear filtrate and transferred to a 100 ml volumetric flask and the volume was made up to the mark with distilled water. to achieve a final concentration in the range of calibration curve, the sample solution was further diluted with abs ph 4.5. swv quantification of vb6 in the pharmaceutical formulation was carried out by the standard addition method. for the recovery test, the samples were spiked with various concentrations of the standard solution of vb6. functionalization of multi-walled carbon nanotubes functionalization of mwcnts was performed utilizing the acid oxidation method according to the procedure described in the literature with a minor modification [39]. briefly, 500 mg of pristine mwcnts was added to 100 ml of a concentrated solution of hno3 and h2so4 at a ratio 1:3 (v/v) in a 200 ml conical flask and heated at 80 °c with continuous starring for 5 h. the resulting mixture was diluted with 200 ml of distilled water and filtered. then, the mixture was thoroughly rinsed with distilled water until the ph of the filtrate reached 7.0. finally, the functionalized f-mwcnts were gathered and dried in an oven at 40 °c for 12 h. preparation of modified electrodes the gce was properly rinsed with double-distilled water after being carefully polished with 0.05 µm alumina slurry on the polishing pad. the electrode was then successively sonicated in distilled water and ethanol for five minutes to remove any remaining polishing agent from the electrode surface. f-mwcnt suspension was made by dispersing 5 mg of f-mwcnt in 5 ml of double-distilled water and sonicating the mixture for 30 minutes. to prepare f-mwcnt modified gce, 8 µl of an optimal amount of f-mwcnt suspension was drop cast onto the cleaned gce and allowed to dry for five minutes in the open air. then, electro-polymerization was carried out by cyclic voltammetry in the potential range −1.5 to 2.5 v at a scan rate of 0.05 v s−1 for 5 cycles in 0.1 m phosphate buffer solution ph 8.0 containing 2 mm phenylalanine. to remove unreacted monomers, the fabricated poly(phenylalanine)/f-mwcnt/gce was rinsed with double distilled water and dried for five minutes at room temperature. for comparison purposes, poly(phenylalanine)/gce and f-mwcnt were also prepared with the same procedures described above. results and discussion construction of poly(phenylalanine)/f-mwcnt/gce poly(phenylalanine) film was deposited on the surface of bare gce and f-mwcnt/gce by electrochemical polymerization of phenylalanine using 5 cycles of cv in the potential range −1.5 to 2.5 v at the scan rate of 0.05 v s−1 in 0.1 m phosphate buffer solution (ph 8) containing 2 mm phenylalanine monomer. the mechanism of electro-polymerization of phenylalanine wasalready reported [40]. the cyclic voltammograms (cvs) for the electro-polymerization of phenylalanine are displayed in figure 2. in the forward scan, the phenylalanine monomer oxidation peak was observed at 1.6 v, corresponding to the formation of monomer radical due to the oxidation of the amino functional group of phenylalanine. during the reverse scan, the cathodic peak at −0.50 v was observed, corresponding to phenylalanine reduction. the monomer cation radicals can form carbon-nitrogen linkages on the surface of the carbon electrode [28,30]. the deposition of the polymer film on the electrode surface is demonstrated by an increase in the anodic and cathodic peak currents with increasing cycle numbers [28]. after a few cycles, the anodic and cathodic peak currents remain almost stable, indicating that the formation of a polymer film has reached the level http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 302 of saturation. a blue coating was noticed on the electrode surface after washing with distilled water, indicating the formation of poly(phenylalanine) film [30]. scheme 1 describes the electropolymerization mechanism of phenylalanine. e / v vs. ag/agcl figure 2. cvs of electro-polymerization of 2mm phenylalanine in 0.1 m phosphate buffer solution (ph 8.0) on f-mwcnt/gce surface in the potential range −1.5 and 2.5 v for 5 cycles at a scan rate of 0.05 v s−1. inset: peak current of 200 µm vb6 versus the number of polymerization cycles scheme 1. mechanism of electro-polymerization of phenylalanine -2 -1 0 1 2 3 -500 0 500 1000 1500 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 i (m a ) number of polymerization cycle i / m a e / v ns. ag/agcl i / m a i / m a g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 303 the thickness of the polymer film has an impact on the electrocatalytic capability of the prepared electrode [41]. therefore, the effect of the number of cycles of electro-polymerization on the electrocatalytic activity of the prepared electrode was examined from 3 to 15 cycles. due to an increase in active sites at the electrode surface, the peak current of vb6 increased along with the number of cycles up to 5 cycles (inset of figure 2). after 5 cycles, the thickness of the polymer film increased and blocked the electron transfer between the electrode and vb6. thereby, 5 potential scans were chosen to electropolymerize phenylalanine on the electrode surface. electrochemical properties of bare and modified gce the electrochemical behavior of the bare gce and modified electrodes were examined by cv and eis in 0.1 m kcl with 5 mm [fe(cn)6]3–/4– taken as a redox probe. the cv responses of bare gce (curve a), f-mwcnt (curve b), poly(phenylalanine)/gce (curve c) and poly(phenylalanine)/fmwcnt/gce (curve d) are shown in figure 3. weak redox peaks and a broad voltammogram were observed for bare gce (curve a) in comparison to the modified electrodes. due to the high surface area and conductivity of f-mwcnt, the modification of bare gce with f-mwcnt enhanced the current response of [fe(cn)6]3–/4– and lowered the peak-to-peak potential separation. after electrodepositing poly(phenylalanine) on the gce, the redox peak current increased and peak-to-peak potential separation reduced because the high conductivity of polymer and more active sites made it easier for [fe(cn)6]3–/4– to reach the electrode surface. similar to this, after f-mwcnt/gce was modified with poly(phenylalanine), the redox peak currents increased further, and the peak-to-peak separation decreased because of the synergistic effect of f-mwcnt and poly(phenylalanine) for the electron transfer between [fe(cn)6]3–/4– and poly (phenylalanine)/f-mwcnt/gce. using randlesševčik equation, the electroactive surface areas of the unmodified gce and modified gce were estimated [27,42]: ip = (2.69×105) ad1/2z3/2n1/2c (1) where ip / a is peak current, z is a number of electrons involved in the reaction (z = 1), a / cm2 is electrode active surface area, d (7.6×10-6 cm2s−1) is diffusion coefficient of ferricyanide ions in 0.1 m kcl solution, c is the concentration of k3[fe(cn)6] (mmol cm−3) and n / v s−1 is scan rate. e / v vs. ag/agcl figure 3. cvs (n = 0.1 v s-1) of 5.0 mm [fe(cn)6]3−/4− in 0.1 m kcl at: bare gce (a), f-mwcnt/gce (b), poly (phenylalanine)/gce (c) and poly(phenylalanine)/f-mwcnt/gce (d) according to the scan rate analysis, the anodic and cathodic peak currents of all electrodes were found to be proportional to the square root of the scan rate in the range 0.025 to 0.4 v s−1. from the -0.25 0.00 0.25 0.50 0.75 -150 -75 0 75 150 i / m a e/ v ns. ag/agcl a b c d a i / m a http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 304 slopes of ip versus n 1/2, plots, the electroactive surface area was calculated for each electrode. the calculated values were 0.05, 0.08, 0.09 and 0.12 cm2 for bare gce, f-mwcnt/gce, poly(phenylalanine)/gce and poly(phenylalanine)/f-mwcnt/gce, respectively. electrochemical impedance spectroscopy (eis) was used to further examine electrochemical characteristics of unmodified and modified electrodes. important information about the kinetics of electron transfer at the electrodes can be provided by eis [43]. in nyquist plots, the semicircle diameter refers to the electron transfer resistance (rct) at the electrode surface [44]. figure 4a shows nyquist plots of eis for all electrodes in 0.1m kcl containing 5.0 mm [fe(cn)6] 3−/4− redox probe. in comparison to other modified electrodes, bare gce exhibited the highest charge transfer resistance (rct) and the lowest double-layer capacitance (cdl) value, as shown in table 1. it was evident that the f-mwcnt modified surface has higher conductivity and surface area than bare gce since the rct value of the f-mwcnt/gce was lower than that of bare gce. electrodeposition of poly(phenylalanine) on the surface of bare gce can also reduce the rct value by accelerating the electron transfer rate due to its high electrocatalytic ability. poly(phenylalanine)/f-mwcnt/gce exhibits the lowest rct and the highest cdl compared to all other electrodes. the high value of cdl at poly(phenylalanine)/f-mwcnt/gce indicates the increase in the surface area of the electrode after modification. thus, more ions transfer from the solution to the electrical double layer, increasing the electrical double layer capacitance. similarly, bode plots (figure 4b, c) revealed that mwcnt or poly(phenylalanine) modified electrode has lower charge transfer resistance (lower impedance at all frequencies) than bare gce. poly(phenylalanine)/f-mwcnt modified electrode exhibited the lowest charge transfer resistance. at very high frequencies where solution resistance (rs) dominates, the impedance values are almost similar for all electrodes. this suggests that modifiers did not have any significant impact on the solution resistance value. therefore, the combination of mwcnt and poly(phenylalanine) increases exclusively the electron transport between the electrode and [fe(cn)6]3−/4−. in light of this, the eis results confirm the cv results. furthermore, the exchange current density (jo) and electron transfer rate constant (ko) for the electrodes were computed from the eis data using the following equations [45]: 0 ct rt j zr af = (2) 0 2 ct rt k r acf = (3) where j0 / a cm-2 is the exchange current density, ko / cm s−1 is the electron transfer rate constant, r is the universal gas constant (8.314 j k−1 mol−1), t is the temperature (298 k), f is faraday constant (96485 c mol−1), rct / ω is the electron transfer resistance, a / cm2 is the electrode surface area and z is the number of electrons involved in the reaction (z = 1). the j0 and ko values obtained for the bare gce, f-mwcnt/gce, poly(phenylalanine)/gce and poly (phenylalanine)/f-mwcnt/gce are listed in table 1. table 1. electron transfer rate constant, exchange current density, charge transfer resistance and double layer capacitance for bare and modified gce electrode ko / cm s−1 j0 / µa cm-2 rct / ω cdl / µf gce 0.0034 1633 314.4 1.22 mwcnt/gce 0.0043 2100 152.8 1.62 poly(phenylalanine)/ gce .0046 2246 127 2.45 poly(phenylalanine) /f-mwcnt/gce 0.0051 2462 86.91 16.0 g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 305 z’ /  log (f / hz) log (f / hz) figure 4. (a) nyquist plots of 5.0 mm [fe(cn)6] 3−/4− in 0.1 m kcl at: bare gce (a), f-mwcnt/gce (b), poly (phenylalanine)/gce (c) and poly(phenylalanine)/f-mwcnt/gce (d). inset: randles equivalent electrical circuit used for fitting nyquist plots; rs is solution resistance, rct is charge transfer resistance, cdl is double layer capacitance, and zw is diffusion (warburg) impedance. (b) bode plots, log z vs. log frequency and (c) phase angle vs. log frequency at bare gce (a), f-mwcnt/gce (b), poly(phenylalanine)/gce (c) and poly(phenylalanine)/f-mwcnt/gce (d). frequency range of 100 khz−0.1 hz, applied potential of 0.15 v and amplitude of 0.005 v the findings demonstrated that greater j0 and ko values were achieved for the composite modified electrode, poly(phenylalanine)/f-mwcnt/gce, due to its large surface area and presence of additional functional groups that were active and facilitated the transport of electrons between the electrode and [fe(cn)6]3–/4– . it seems that the electron transfer process is simpler and quicker at the composite-modified electrode. surface characterization ftir was used to investigate the surface functional groups of functionalized mwcnt and the deposition of polymer film on the electrode surface. figure 5 shows ftir spectra of pristine mwcnt, f-mwcnt and poly (phenylalanine)/f-mwcnt. it could be seen that the f-mwcnt exhibited new bands compared to pristine mwcnt due to the attachment of new functional groups on their surface. the ftir spectrum of mwcnt showed weak peaks at 3430 and 1646 cm−1 corresponding to o–h stretching vibration of the c–oh, which can be introduced during their synthesis and conjugated c=c stretching vibration of pristine mwcnt, respectively. absorption bands at 2918 and 0 200 400 600 800 0 150 300 450 600 -z " ( o h m ) z' (ohm) c d a b a -3 -2 -1 0 1 2 3 4 5 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 lo g ( z /o h m ) log (frequency/hz) a b cd b -3 -2 -1 0 1 2 3 4 5 0 10 20 30 40 50 -p h a s e a n g le / d e g re e log (frequency / hz) a b c d c -z “ /  lo g ( z /  ) rs rct cdl zw  /  http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 306 2853 cm−1 are assigned to asymmetric and symmetric stretching vibrations of the c–h [46]. for fmwcnt, a strong absorption band observed at 3445 cm−1 is attributed to oh stretching vibration of the ─cooh groups [39,46]. the absorption bands at 2927 and 2862 cm–1 are assigned to asymmetric and symmetric stretching vibrations of the c–h bond in ch2 of f-mwcnt, respectively [46]. a weak band at 1749 cm−1 is assigned to the stretching vibration of c=o of the carboxyl group [39]. the absorption peak at about 1639 cm–1 is assigned to the c = c skeletal stretching vibration of fmwcnt. the band at 1447 cm−1 is assigned to c-h bending vibration of –ch2 [46]. the peak at 1380 cm−1 corresponds to o–h bending vibration [39]. the absorption band at about 1083 cm–1 can be ascribed to the c–o stretching vibrations of epoxy groups [46]. the increase in the intensity of o–h stretching vibration and formation of c=o and c–o functional groups after oxidation of pristine mwcnt indicate successful functionalization of mwcnt [39]. the structural properties of poly(phenylalanine) /f-mwcnt have also been investigated using ftir spectroscopy. the absorption band at 3443 cm−1 corresponds to o–h stretching of mwcnt and the n–h stretching of poly (phenylalanine) [47]. the peaks at 2922 and 2852 cm−1 are attributed to asymmetric and symmetric stretching vibrations of the c–h in ch2 [47]. the characteristic band at 1738 cm−1 is assigned to the stretching vibration of c=o of the carboxylic group. the band at 1634 cm−1 is attributed to the c=o stretching vibration of the amide group on the poly (phenylalanine) [47]. the band at 1465 cm−1 is attributed to c–h bending vibration of the ch2 groups and phenyl ring stretching. the band observed at 1383 cm−1 is assigned to o–h bending vibration and phenyl ring stretching [39]. the band at 1226 cm−1 is due to c–n stretching of secondary amine. the peak observed at 1081 cm−1 is assigned to c–o stretching of the alkoxy group and c–h bending in the phenyl ring [46]. the results obtained from the ftir spectrum indicated the successful functionalization of mwcnt and deposition of the polymer film. wavenumber, cm-1 figure 5. ftir spectra of mwcnt, f-mwcnt and poly(phenylalanine) /f-mwcnt the morphologies of bare gce, f-mwcnt/gce, poly(phenylalanine)/gce and poly(phenylalanine)/f-mwcnt/gce were investigated by sem. as shown in figure 6a, bare gce has a smooth surface structure. after the deposition of f-mwcnt on gce, porous and thread-like structures of fmwcnt were observed (figure 6b). figure 6c shows a loose and rough surface of poly (phenylalanine) film on gce. after the deposition of poly(phenylalanine) on f-mwcnt (figure 6d), a dense and tight surface structure of poly(phenylalanine) film is observed on f-mwcnt, providing 0 1000 2000 3000 4000 5 10 15 20 25 30 1210 1220 1230 1240 1250 1260 1226 t ra n s m it tn c e / % wavenumber / cm-1 mwcnt f-mwcnt poly (phenylalanine) /f-mwcnt 3443 2922 2852 1738 1634 1465 1383 1081 t ra n sm it ta n ce , % g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 307 supplementary surface area for adsorption of vb6. the π-π interaction and hydrogen bonding between f-mwcnt and poly(phenylalanine) were strong enough to make the polymer film attached to f-mwcnt. this strong interaction between f-mwcnt and poly (phenylalanine) results in high stability of the composite material. the results obtained from the sem study reveal the successful deposition of f-mwcnt on bare gce and the uniform deposition of poly(phenylalanine) on fmwcnt. figure 6. sem images of bare gce (a), mwcnt/gce (b), poly (phenylalanine/gce (c) and poly(phenylalanine) /mwcnt/gce (d) cyclic voltammetric performance of vb6 at bare and modified gce the electrochemical behavior of vb6 at bare gce and modified gce was studied by cv at the scan rate of 0.1 v s−1 in abs ph 4.5. figure 7 shows cvs of 200 µm vb6 at gce (curve a), f-mwcnt/gce (curve b), poly(phenylalanine)/gce (curve c) and poly(phenylalanine)/f-mwcnt/gce (curve d). at the bare gce (curve a), vb6 shows a weak oxidation peak indicating slow electron transfer kinetics. compared to bare gce, an increase in peak current and decrease in peak potential are observed at f-mwcnt/gce due to the presence of oxygen-containing functional groups and high conductivity of f-mwcnt which facilitate the accumulation of vb6 and electron transfer rate at the surface of the electrode. at poly (phenylalanine)/gce, oxidation peaks higher than the bare gce and fmwcnt/gce were observed. the enhanced peak current observed at poly(phenylalanine)/fmwcnt/gce indicates the enhanced kinetics of the electrochemical reaction due to the synergistic effect of mwcnt and poly(phenylalanine), such as high conductivity and presence of functional groups, which facilitate the accumulation of vb6 through hydrogen bonding and π-π interaction (scheme 2). furthermore, the deposition of f-mwcnt and poly(phenylalanine) onto the gce provide more conducting pathways (more active sites) to ease the electron transfer between the surface of the electrode and the analyte. in the reverse scan, no cathodic peak was observed at all the electrodes, indicating that the electrochemical oxidation of vb6 is irreversible [22]. http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 308 e / v vs. ag/agcl figure 7. cvs (0.1 vs-1) of bare gce (a), f-mwcnt/gce (b), poly(phenylalanine)/gce (c), poly(phenylalanine) /f-mwcnt/gce (d) (background subtracted) in the presence of 200 µm vb6 in 0.1 m abs ph 4.5 after accumulation at 0.0 v for 120 s scheme 2. possible interaction of vb6 with poly(phenylalanine)/f-mwcnt modified electrode and its mechanism of electrooxidation the performance of the sensor was examined to determine the impact of the order of deposition of f-mwcnt and polymer. the electrocatalytic activity for the oxidation of vb6 was found to be higher when f-mwcnt is deposited on the gce before phenylalanine polymerization (poly(phenylalanine)/f-mwcnt/gce), as opposed to the case when phenylalanine polymerization was followed by f-mwcnt deposition (f-mwcnt/poly(phenylalanine)/gce). it is likely that the superior performance of poly(phenylalanine)/f-mwcnt/gce is caused by the increased surface area when f-mwcnt was applied first. this makes it easier to apply poly(phenylalanine) and increases the number of active sites on the electrode surface [48]. additionally, it was examined how well the poly(phenylalanine)/f-mwcnt/gce worked during successive measurements. the oxidation peak current was not considerably altered after 10 swv measurements of 200 µm vb6 solution, demonstrating high operational stability of the modified electrode. i / m a g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 309 optimization of experimental conditions effect of amount of f-mwcnt swv was used to examine how the amount of f-mwcnt affected the sensitivity of vb6. to get optimum amount of f-mwcnt, different amounts of f-mwcnt were dropped onto gce. increasing the amount of f-mwcnt up to 8 µl improved the current response of vb6. however, the sensitivity was reduced at volumes greater than 8 µl. this is explained by the thickening of the fmwcnt layer, which interferes with electron transfer and decreases electrocatalytic activity. therefore, 8 µl was selected as the optimum amount of f-mwcnt for the preparation of the sensor electrode. impact of solution ph cv measurements were made in the ph range of 3.0 to 7.0 to determine how the ph of abs affects the current response of 200 µm vb6 at poly(phenylalanine)/f-mwcnt/gce (figure 8a). the peak current of vb6 increased progressively from ph 3.0 to 4.5, as shown in figure 9b, and then fell gradually with increasing ph. from the fact that pka values of vb6 are pka1 = 5 and pka2 = 8.96 [4,49] while pkas of phenylalanine are pka1= 2.47 and pka2 = 9.13 [32], the increase in current response with ph from 3.0 to 4.5 might be due to the increase in electrostatic attraction between the positively charged vb6 (ph ˂ 5.0) and negatively charged polymer film (ph ˃ 2.47). the charge density on both vb6 and poly (phenylalanine) increases as the ph rises, which causes the electrostatic attraction between vb6 and poly (phenylalanine) to decrease. this reduces the accumulation of vb6 at the electrode surface. thus, 0.1 m abs ph 4.5 was chosen as the appropriate supporting electrolyte for subsequent investigations. furthermore, the ph of the solution also affects the oxidation peak potential of vb6. the anodic peak potential shifted negatively with the increasing ph of the solution, demonstrating that peak potential is dependent on solution ph. the oxidation peak potential of vb6 changed linearly with the solution ph in the range 3.0 to 7.0. the regression equation for the dependence of the peak potential on the ph is given by the equation (figure 8b): ep = −0.063 ph +1.18 (r2 = 0.994). the slope is close to the nernstian value of −59 mv, indicating the same number of protons and electrons are involved in the electrochemical oxidation of vb6, which is in agreement with previous works [4,5,20]. e / v vs. ag/agcl ph figure 8. (a) cvs of 200 µm vb6 at poly (phenylalanine)/f-mwcnt/gce in 0.1 m abs of various ph values (3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6, 6.5 and 7) at scan rate of 0.1 v s−1. (b) effect of ph on the peak current and peak potential values of vb6 0.2 0.4 0.6 0.8 1.0 1.2 1.4 -20 -10 0 10 20 30 40 50 60 i / m a e / v ns. ag/agcl a 37 3 4 5 6 7 4 6 8 10 12 14 ph i / m a b 0.75 0.80 0.85 0.90 0.95 1.00 1.05 e / v n s . a g /a g c l ep (v) = -0.063 ph + 1.18 r2 = 0.994i / m a i / m a e / v v s. a g /a g c l ep = -0-063 ph + 1.18 r2 = 0.994 http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 310 scan rate study by using cyclic voltammetry, the influence of scan rate on the oxidation peak current and peak potential of 200 m vb6 in abs ph 4.5 at the poly(phenylalanine)/f-mwcnt/gce was investigated in order to determine the electrochemical reaction mechanism of vb6 at the modified electrode (figure 9a). as shown in figure 9b, the oxidation peak current is proportional to the scan rate in the range 0.025 to 0.3 v s−1. according to regression equations ipa = 112.6 + 1.08 (r2 = 0.998), the electrooxidation of vb6 at the surface of the modified electrode is adsorption controlled [15]. the relationship between the logarithm of peak current (log i) and the logarithm of scan rate (log υ) in the range 0.025 to 0.3 v s−1 was further investigated. as shown in figure 9c, there is good linearity between log i and log υ. the corresponding regression equations is: log ipa = 0.89 log n + 1.98 (r2 = 0.996) with a slope close to the theoretical value of 1, confirming the electrochemical oxidation of vb6 at poly (phenylalanine)/f-mwcnt/gce as a surface-controlled process. moreover, the influence of scan rate on the peak potential of vb6 was studied. with increasing scan rate, the oxidation peak potential of vb6 shifted to a more positive value, indicating that the electrooxidation of vb6 at poly(phenylalanine)/ /f-mwcnt/gce is irreversible [20]. the linear regression equations for the variation of the oxidation peak potential (ep) with the logarithm of scan rate (log υ) in the range 0.025 to 0.3 v s−1 is given by the equation: epa = 0.052 log n 0.93, r2 = 0.996 (figure 9d). according to laviron’s equation [50,51], the relationship between ep and log ν for an irreversible electrode reaction is given by: p 2.303 2.303 log log o rt rtk rt e e zf zf zf n    = + + (4) where eo is the formal potential, t is the temperature (298 k), α is the transfer coefficient, z is the number of electrons transferred, n is scan rate, f is faraday’s constant (96,480 c mol−1), r is the universal gas constant (8.314 j mol−1 k−1), and k is the heterogeneous electron-transfer rate constant. from the plot of ep versus log n, the slope 2.303rt/ αzf equals 0.052. using this value, αn was calculated to be 1.12. for an irreversible electrode reaction [51], α can be given as: pa pa/2 47.7 e e  = − (5) where epa/2 is the potential where the current is half the peak value. thus, from eq. (5) α was found to be 0.51. additionally, it was determined that 2.2 (2) electrons were involved. thus, the oxidation of vb6 requires two electrons [5,18]. from the relationship between epa and ph, an equal number of electrons and protons are transferred during the electrooxidation of vb6. therefore, the electrode reaction mechanism for vb6 oxidation at poly(phenylalanine)/f-mwcnt/gce involves two electrons and two protons. furthermore, it is possible to determine the surface concentration of vb6 at poly (phenylalanine)/f-mwcnt/gce from the slope of the linear plot of oxidation peak current vs. v using the following equation [52]: 2 2 pa 4 z f a i rt  n = (6) where  / mol cm-2 is the surface concentration of electroactive species, a / cm2 is the electrode surface area and z, f, n, r, and t are as described in eq. (4). the surface coverage of vb6 at the poly(phenylalanine)/f-mwcnt/gce was calculated to be  = 0.24 nmol cm-2. g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 311 i / m a i / m a e / v vs. ag/agcl n / v s-1 lo g ( i / m a ) e p / v log (n / v s-1) log (n / v s-1) figure 9. (a) cvs of 200 µm vb6 at poly (phenylalanine)/f-mwcnt/gce in abs ph 4.5 at scan rates of 0.025, 0.05,0.075, 0.1, 0.125, 0.15, 0.175, 0.2, 0.25, 0.3, 0.350, 0.4 v s−1. (b) plot of the oxidation peak current versus scan rate (n). (c) variation of the logarithm of the oxidation peak current (log i) with the logarithm of scan rate (log n). (d) variation of the oxidation peak potential (ep) with the logarithm of scan rate (log n) determination of vb6 at poly(phenylalanine)/f-mwcnt/gce square wave voltammetry (swv) determination of vb6 was performed under the optimized experimental conditions in abs ph 4.5 in the potential range 0 to 1.2 v. according to figure 10a, the peak currents increased linearly from 0.5 to 200 m of vb6 concentration. i / m a i / m a e / v vs. ag/agcl c / mm figure 10. (a) swvs for different concentrations of vb6: 0.5, 0.8, 1.5, 2, 4, 6, 10, 15, 20, 30, 40, 60, 80, 100, 140, 160, 180 and 200 µm in 0.1 m abs ph 4.5. (b) plot of the peak current versus concentration of vb6 at poly(phenylalanine)/f-mwcnt/gce. conditions: eacc, 0.0 v; tacc, 120 s. swv parameters: frequency = 35 hz; step potential = 8 mv and pulse amplitude = 40 mv 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -40 -20 0 20 40 60 80 i / m a e / v ns. ag/agcl 0.025 v s− 1 0.3 v s−1 a 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0 5 10 15 20 25 30 35 40 i / m a scan rate, n / v s−1 ipa = 112.6 n + 1.08 r2 = 0.998 b -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 0.4 0.6 0.8 1.0 1.2 1.4 1.6 lo g i / m a log n/ vs−1 log ipa = 0.89 log n + 1.98 r2 = 0.996 c -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 0.84 0.85 0.86 0.87 0.88 0.89 0.90 0.91 e p / v log n / v s−1 epa = 0.052 log n + 0.93 r2 = 0.996 d 0.4 0.6 0.8 1.0 1.2 -10 0 10 20 30 i / m a e / v ns. ag/agcl 0.5 mm 200 mm a 0 50 100 150 200 0 5 10 15 20 25 30 i / m a concentration / mm i = 0.093 c + 9.11 r2 = 0.995 i = 0.504 c + 0.259 r2 = 0.995 b ipa =112.6 n + 1.08 r2 = 0.998 ipa = 0.052 log n + 0.93 r2 = 0.996 ipa = 0.89 log n + 1.98 r2 = 0.996 i = 0.093 c + 9.11 r2 = 0.995 i = 0.504 c + 0.259 r2 = 0.995 http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 312 two linear ranges could be seen on the vb6 calibration curve. the first linear range is 0.5 to 20 µm and the second is 20 to 200 µm with regression equations: i = 0.504 c + 0.259, r2 = 0.995 and i = 0.093 c + 9.11, r2 = 0.995, respectively (figure 10b). error bars are added to the graph using the standard deviation of triplicate measurements of each data point. lod and loq limits were calculated from the oxidation peak current of vb6 using the following equations: lod = 3s/m and loq = 10s/m, where s is the standard deviation of the oxidation peak currents (n = 3) of the lowest concentration in the linear range and m is the slope of the calibration curve. the calculated lod and loq values are 0.038 and 0.125 µm, respectively. furthermore, the linear range and detection limit obtained in this study were compared with the results of previously reported electrochemical methods for the determination of vb6 (table 2). the findings suggest that the proposed sensor exhibits a better linear range and detection limit than most of the reported sensors. the proposed sensor is also inexpensive and easy to make. table 2. comparison of reported analytical data for vb6 determination obtained by sensor electrodes modified electrode technique concentration linear range, µm lod, µm ref. ap-doped/pge ldpv 0.5–300 0.065 [4] b44-dabp/gce dpv 15.7–2210 4.7 [5] caunps/poap/pge dpv 5–200 0.30 [1] dpmg/fcnt/ce dpv 100–800 9.4 [53] eau-cuo/mwcnts/gce dpv 0.79–18.4 0.15 [54] fcrhcf/gce cv 1.33–13.2 0.035 [55] gnizcb-gce dpv 0.3–5.9 0.09 [49] hmwcnt/copc/pge dpv 10–400 0.5 [19] icohcf/mcpe swv 5–26 0.17 [22] jn,s-gqd-cs/ gce swasv 0.1–18 0.03 [56] kepads swv 200–2000 29.5 [57] poly(phenylalanine)/f-mwcnt/gce swv 0.5–20 and 20–200 0.038 this work aphosphorus-doped pencil graphite electrode; b4,4’-diamino benzophenone modified glassy carbon electrode; cgold nanoparticles and non-conducting polymeric film of o-aminophenol; dpoly(methylene green) (pmg)and functionalized carbon nanotubes (fcnt) modified graphite composite electrode (ce); ecopper oxide (shell) nanoparticles-mwcnt; fchromium(iii) hexacyanoferrate(ii); gni-zeolite/carbon black modified glassy carbon electrode; hmultiwalled carbon nanotube (mwcnt and cobalt phthalocyanine (copc) modified pyrolytic graphite electrode; icobalthexacyanoferrate modified carbon paste electrode; ldifferential pulse voltammetry; jnitrogen and sulfur co-doped graphene quantum dot-modified glassy carbon electrode; kelectrochemical paper-based analytical devices repeatability and stability of poly(phenylalanine)/f-mwcnt/gce to evaluate the repeatability of the prepared sensor electrode for determining 20 µm vb6, 10 consecutive measurements of 20 µm vb6 were performed using the same electrode. the relative standard deviation (rsd) of the current response was 2.38 %, indicating that the prepared electrode can be used for several measurements to determine vb6. the electrode fabrication repeatability for vb6 determination was evaluated by measuring the peak current of 20 µm with four different electrodes prepared under identical conditions. with an rsd of 2.16 %, the outcome demonstrates that all electrodes have essentially identical responses, showing good repeatability of the electrode preparation process. in order to determine the poly (phenylalanine)/f-mwcnt/gce long-term stability, the current responses to 20 m vb6 were monitored weekly over one month. the electrode used every week is kept at 4 °c in the refrigerator. the synthesized poly(phenylalanine)/fmwcnt/gce was found to have effective long-term stability, as evidenced by the observation that the oxidation current response dropped by 6.3 % at the end of the month. robustness study the robustness of the developed method was examined by studying the effect of small variations of some experimental parameters, such as ph (4.5 ± 0.1), accumulation potential (0.0 ± 0.02 v) and accumulation time (120 ± 2 s) on the recovery of vb6. as shown in table 3, the recovery for vb6 g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 313 under all studied conditions is in the range of 96.1 – 103.4 %, indicating that small variation of the studied experimental parameters has an insignificant effect on the current response of vb6. therefore, the proposed swv method has good robustness and is reliable for the quantification of vb6. table 3. robustness study for the voltammetric method used for determination of 20 µm vb6 (n=3) parameters mean, µm ± rsd, % recovery, % optimum values (ph = 4.5, eacc = 0.0 v and tacc = 120 s) 20.68 ± 1.97 103.4 ph (variation) 4.4 19.42 ± 3.6 97.1 4.6 19.35 ± 1.2 96.8 accumulation potential (variation) 0.02 v 19.48 ± 2.63 97.4 -0.02 v 19.22 ± 2.9 96.1 accumulation time (variation) 118 s 19.42 ± 1.58 97.1 122 s 19.55 ± 3.8 97.8 effect of interferents the impact of potential interfering compounds on the determination of 20 µm vb6 was examined by swv under optimal conditions. except for folic acid (fa), the data demonstrated that none of these interferants at poly(phenylalanine)/f-mwcnt/gce exhibited a voltammetric response. as can be seen from figure 11, the response of vb6 was unaffected by 200− fold excess concentrations of ca2+, k+, mg2+, cu2+, fe2+, no3−, co32− and hco32− , 100− fold excess concentrations of ascorbic acid (aa), citric acid (ca), sodium citrate (naca), tartaric acid (ta), glucose, fructose, starch, sucrose, caffeine, vitamin b1 (vb1), vitamin b2 (vb2) and vitamin b12 (vb12). interferent figure 11. change in peak current response in the presence of different interferents for 20 µm vb6 however, fa exhibited two oxidation peaks close to that of vb6. in the presence of folic acid, even at a concentration ratio of 1:1, the peak current of vb6 was reduced and the potential shifted to a positive value. therefore, fa must be separated before the voltammetric measurement of vb6. here, however, the examined tablet and beverage samples did not have any folic acid. additionally, the standard addition method was applied for the quantification of vb6 to minimize the interference s ig n a l ch a n g e , % http://dx.doi.org/10.5599/jese.1574 j. electrochem. sci. eng. 13(2) (2023) 297-319 determination of vitamin b6 314 effect. as a result, the proposed electrode can be successfully utilized to measure vb6 in pharmaceutical and beverage samples. real sample analysis the developed method was applied for the determination of vb6 in commercially available pharmaceutical tablets (vitamin b-complex containing vb1, vb6 and vb12) and an energy drink (predatory energy drink). the samples were prepared and diluted to the appropriate concentration with the supporting electrolyte. swv was used to record signals at poly(phenylalanine)/fmwcnt/gce and concentrations of vb6 were determined by the standard addition method to minimize the matrix effect. as shown in table 4, the result obtained for vb6 using the proposed method was found to be 95 mg vb6/tablet, which is in good agreement with the labelled value of 100 mg vb6/tablet (relative error 5 %). similarly, the value of vb6 in the energy drink was found to be 0.29 mg/100ml, which is close to the labelled amount (0.3 mg/100 ml) (relative error of 3.3 %). furthermore, recovery studies were carried out by the standard addition method to validate the accuracy and precision of the developed sensor and the results are shown in table 5. the recovery values varied from 90.0 to 100.4 %, suggesting that the developed method is reliable and effective for the determination of vb6 in vitamin b-complex and energy drinks without the effect of other constituents of the samples. furthermore, the low values of rsd (ranging from 0.4 to 4.0 %) indicate good precision of measurements. table 4. comparison of experimental results with labelled values sample labelled content experimental content ± sd relative error, % predatory energy drink 0.3 mg/100 ml 0.29 mg/100 ml ± 0.08 3.3 n-vit 100 mg/tablet 95 mg/tablet ± 3.1 5.0 table 5. recovery study conclusions in the present study, poly(phenylalanine) and f-mwcnt modified glassy carbon electrode was successfully prepared for the electrochemical determination of vb6 in pharmaceutical tablet and energy drink samples. the developed sensor exhibits high electrocatalytic activity towards the oxidation of vb6 due to the synergistic effect between poly(phenylalanine) and f-mwcnt. cv study showed that the electrode reaction of vb6 is the irreversible and adsorption-controlled process. under the optimized conditions, swv experiments showed peak currents that increased linearly with vb6 concentration in two linear ranges (0.5 to 20 and 20 to 200 µm). the detection limit (lod) was found to be 0.038 µm. the prepared electrode showed high selectivity, stability, repeatability and reproducibility. the acceptable recovery values obtained for vb6 during real samples analysis sample c / µm rsd, % n = 3 recovery, % added found n-vit 0 4.98 3.10 3 7.68 4.00 90.0 5 9.58 1.30 92.0 8 12.6 0.37 95.3 12 17.0 1.80 100.4 predatory energy drink 0 3.48 2.40 3 6.28 0.26 92.7 5 8.17 0.95 93.8 8 10.7 2.90 90.8 12 15.3 0.40 98.8 g. tesfaye et al. j. electrochem. sci. eng. 13(2) (2023) 297-319 http://dx.doi.org/10.5599/jese.1574 315 demonstrate that the proposed sensor can be effectively used for the determination of vb6 in pharmaceutical tablets and energy drinks with high accuracy. acknowledgment: the authors gratefully acknowledge oromia education bureau for the financial support. we are grateful to the department of chemistry, addis ababa university for providing laboratory facilities. conflicts of interest: the authors declare no conflict of interest. references [1] r. rejithamol, s. beena, electrochemical quantification of pyridoxine (vb6) in 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licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1574 https://doi.org/10.1590/s0103-50532009000300014 https://doi.org/10.1016/j.talanta.2022.123836 https://doi.org/10.1016/j.microc.2022.107588 https://creativecommons.org/licenses/by/4.0/) {nox and propene conversion on la0.85sr0.15mno3+d/ce0.9gd0.1o1.95 symmetrical cells} doi:10.5599/jese.419 153 j. electrochem. sci. eng. 7(4) (2017) 153-166; doi: http://dx.doi.org/10.5599/jese.419 open access : : issn 1847-9286 www.jese-online.org original scientific paper nox and propene conversion in la0.85sr0.15mno3+d/ce0.9gd0.1o1.95 symmetrical cells anja zarah friedberg, kent kammer hansen department of energy conversion and storage, technical university of denmark, dk-4000 roskilde, denmark corresponding authors e-mail: kkha@dtu.dk, anzf@topsoe.dk received: august 14, 2017; revised: september 23, 2017; accepted: october 10, 2017 abstract the catalytic electrochemical reduction of no with propene was investigated on la0.85sr0.15mno3+d/ce0.9gd0.1o1.95 symmetrical cells. the electrodes were infiltrated with bao and pt. the cells were catalytically active towards the selective catalytic reduction of nox with propene, but bao infiltration lowered the no conversion, probably because of active-site blocking on la0.85sr0.15mno3+d. pt infiltration enhanced the reduction of nox with propene. when a voltage was applied to the cell with bao infiltrated electrodes, the no conversion increased in absence and presence of propene in the feed gas and presence of 10 % o2. the addition of propene into the feed gas did not enhance the conversion of no when the electrodes were infiltrated with bao. when platinum was co-infiltrated with bao, the catalytic activity towards the reduction of no with propene was enhanced. however, almost no effect was observed when a voltage was applied. additionally, when the cells were infiltrated with pt, an electrochemical promotion was observed with respect to co2 formation. keywords no; propene; lsm; cgo introduction diesel engines exhibit better fuel economy than otto engines and are thus becoming more popular. otto engines run at a stoichiometric fuel/air ratio, whereas diesel engines operate with excess air. the exhaust contains toxic nox, which for engines running under a stoichiometric fuel/air ratio is efficiently removed by the so-called three-way catalytic converter (twc). a twc does not operate in the presence of excess oxygen; therefore, nox removal from a diesel engine is impossible with a twc [1]. when removing nox under net oxidizing conditions, selective catalytic reduction (scr) with ammonia/urea and nox storage and reduction (nsr) are the most mature technologies. http://dx.doi.org/10.5599/jese.419 http://www.jese-online.org/ mailto:kkha@dtu.dk j. electrochem. sci. eng. 7(4) (2017) 153-166 nox and propene conversion 154 the use of scr and nsr technologies is not straightforward. scr technology requires on-board storage of ammonia and an ammonia slip catalyst. for the nsr technology, sophisticated engine control is needed. in 1991, iwamoto and hamada [2] showed that nox was reduced with hydrocarbons via a process referred to as hc-scr method. this is a potentially attractive technology because hydrocarbons already exist in exhaust gas. the hc-scr method has been studied on precious metals such as pt [3–5], metal oxides [6] and ag [7,8]. it is also possible to reduce nox with hydrocarbons on zeolites [9–12] and perovskites [13–19]. the main issue with hc-scr is that the activity is very low. one way to address this issue is to electrochemically promote the reduction of nox with hydrocarbons. electrochemical promotion has been well described in the literature, see i.e. [20]. in some cases, the rate of the enhancement in a given reaction during cell polarization is larger than that predicted by faraday’s law. this effect is referred to as the non-faradaic electrochemical promotion of catalytic activity (nemca) or electrochemical promotion (ep), and is due to the back spillover of i.e. oxide anions to the electrodes [20]. one defines the rate enhancements factor, , as  = r/ro (1) where r is the reaction rate during polarization and r0 is the reaction rate at ocv. nox can be removed using an electrochemical reactor that is based on an oxide ion conducting electrolyte. the major drawback of the electrochemical reduction of nox is the simultaneous reduction of oxygen at the cathode, which leads to high power consumption. therefore, both the activity and selectivity are important. jayaweera et al. [21] investigated the reduction of no with co under net oxidizing conditions. using the perovskite la0.8sr0.2co0.9rh0.1o3- as an electrode, they showed that the reaction was electrochemically promoted under net oxidizing conditions. earlier studies focused on metal electrodes for nox reduction, see [20]. the combination of a storage compound (as used in nsr technology) and electrochemical promotion appears to be an attractive solution. traulsen et al. [22] investigated the use of bao as a storage compound in an all-ceramic electrochemical cell with la0.85sr0.15mno3/ce0.9gd0.1o1.95 electrodes (lsm15/cgo10) and found that no could be reduced to n2 with a conversion of 60 % at 400 °c in the presence of 10 % oxygen. additionally, perovskites have been suggested as substitutes for pt in nsr technology [23]. in this work, the influence of adding a reducing agent (propene) to the electrochemical reduction of no in the presence of a storage compound bao infiltrated in the cathode is studied. hydrocarbons are used as reducing agents in nsr technology while an engine is running with a rich fuel/air mixture. because pt is normally used as a catalyst for no oxidation in nsr technology, it is also investigated in this work. experimental symmetrical cell fabrication for the evaluation of electrochemical cells for no conversion with and without propene in the gas feed, symmetrical cells with lsm15-cgo10 electrodes, as well as with lsm15-cgo10 electrodes infiltrated with bao, pt, or bao and pt were used. each of these cells consists of two identical electrodes with a dense cgo10 electrolyte. lsm15 powder was purchased from haldor topsoe a/s, a. z. friedberg and k. k. hansen j. electrochem. sci. eng. 7(4) (2017) 153-166 doi:10.5599/jese.419 155 and cgo10 powder was a commercial product of rhodia. a dense 300-µm-thick sintered tape purchased from kerafol was used for cgo10 electrolyte. an electrode ink was prepared by mixing 50 wt% lsm15 and 50 wt% cgo10 powders with terpineol. the mixture was then milled on a planetary mill. next, an etylcellulose binder was added to the mixture, and the resulting ink was screen printed on the kerafol electrolyte tape. the cells were then sintered at 1050 °c for 2 h and laser cut to round cells with a diameter of 13 mm. the thickness of the electrodes was approximately 15 µm. gold was used as the current collector, sputtered using magnetron sputtering on top of the electrodes to generate a 50 nm porous layer. more details on the sputtering procedure can be found in [24]. the infiltration with bao was performed using a 0.32 m aqueous solution of ba(no3)2. the solution was prepared using ba(no3)2 from merck and millipore water with 1 wt% p123 (basf) as the surfactant. the cells were soaked in the solution, placed in a vacuum chamber at 0.1 mbar for 15 s and then heated to 700 °c. pt infiltration was conducted using a 34 mm solution of pt(nh3)4(no3)2 (sigma aldrich) in millipore water with 3 wt% of triton-x45 (sigma aldrich) as the surfactant. the infiltration procedure was as previously described, and the cell was dried for several hours at 80 °c before sintering at 300 °c to decompose both the pt complexes and surfactant. it was not possible to evaluate pt loading because the weight of the dense electrolyte was much higher than the weight gain obtained as a result of pt infiltration. test setup each symmetrical cell was placed in a quartz glass tube in atmospheric pressure setup, and the tube was placed inside a homemade furnace. a gas flow of 2 l/h was fed using brooks mass flow controllers. the composition of the gas feed was 1000 ppm no (air liquide; 1 % ±0.02 % no in ar) + 10 % o2 (air liquide; 20 % o2 ±2 % ar) in ar or 1000 ppm no (air liquide; 1 % ±0.02 % no in ar) + 1000 ppm c3h6 (air liquide; 1 % ±0.02 % c3h6 in ar) + 10 % o2 (air liquide; 20 % o2 ±2 % ar) in ar. the composition of the outlet gas was monitored and recorded by a chemiluminescence detector (model 42i hl, thermo scientific, usa) for no and no2 detection. the propene, co, co2 and n2o concentrations were monitored using an agilent 490 micro gas chromatograph connected to the reactor and equipped with porapak q and molecular sieve 5x columns and two thermal conductivity detectors. microstructure and electrochemical characterization the structure of the gold current collector layer was investigated with a hitachi tm 3000 microscope. a zeiss supra 35 scanning electron microscope was used to characterize the microstructure of the electrodes and evaluate the bao and pt infiltration. a gamry ref 600 potentiostat-galvanostat was used for the electrochemical measurements. the gamry was used for applying a voltage and for electrochemical impedance spectroscopy. the impedance spectra were measured in the frequency range from 878787 hz to 1.3 mhz with 6 points per decade and amplitude of 36 mv rms. catalytic and electrocatalytic conversion measurements the same testing procedure was applied for all the different cells. each cell was placed in the setup and heated to 300 °c in air. then, the gas feed was changed to 1000 ppm no + 10 % o2, and the pure catalytic conversion of no and propene was measured. j. electrochem. sci. eng. 7(4) (2017) 153-166 nox and propene conversion 156 the electrochemical measurements were conducted as follows: before and after polarization, impedance spectroscopy measurements were performed to check whether any degradation of the cell had occurred during testing. polarization measurements were performed for 30 min at +2 v vs. open circuit voltage (ocv). the cell was then maintained at the ocv for 30 min or until steady-state was reached, before a new test was performed. the temperature was then increased to 350 and 400 °c, and the procedure was repeated. afterwards, the gas feed was changed to 1000 ppm no + 1000 ppm c3h6 + 10 % o2, and the electrochemical tests were repeated. results structural characterization representative images of the gold current collector are shown in figure 1(a) and 1(b). as shown in the figure, the current collector layer is approximately 50 nm thick and is porous as needed. a b figure 1. microscope images of symmetrical lsm15-cgo10 cells: a) gold-sputtered, porous structure. top view; b) gold-sputtered, thin gold layer. side view in figure 2(a) and 2(b), the electrode structure with bao infiltration is shown. it is seen that before testing, the bao nanoparticles are distributed evenly on the lsm part of the electrodes (fig. 2(a)). a b figure 2. microscope images of symmetrical cells with bao infiltrated lsm15-cgo10 electrodes: a) before testing; b) after testing. after testing, the bao nanoparticle structure is fluffier, and the particles have agglomerated, which shows that bao nanoparticles are undergoing structural changes during testing (fig. 2(b)). a. z. friedberg and k. k. hansen j. electrochem. sci. eng. 7(4) (2017) 153-166 doi:10.5599/jese.419 157 the cell electrodes infiltrated with both, bao and pt appeared similar to those infiltrated with just bao. no pt nanoparticles were observed in the sem images. other studies [25] have shown that the presence of pt could not be confirmed with an eds measurement. this result was attributed to low pt loading and a small particle size. conversion measurements all cell electrodes showed activity for the catalytic removal of nox. figure 3(a) and 3(b) shows the percentage of nox removed in the presence and absence of propene in the gas feed. the four types of electrodes behaved very differently. at the ocv, the non-infiltrated lsm15-cgo10 electrodes were most active towards no removal when propene was present, and no conversion reached a maximum at 350 °c with a conversion degree of 8 %. when propene was absent in the gas feed, the activity decreased with increasing temperature and the no conversion decreased to zero at 400 °c. figure 3. percentage of nox conversion as a function of temperature in symmetrical cells with non-infiltrated (ni), bao-infiltrated and bao + pt-infiltrated lsm15-cgo10 electrodes, measured at the ocv (open symbols) and during + 2 v polarization (solid symbols) in: a) 1000 ppm no + 10 % o2 ; b) 1000 ppm no + 1000 ppm c3h6+ 10 % o2. the activity of electrodes that were infiltrated with bao decreased with increasing temperature in both atmospheres, being highest when no propene was present in the atmosphere. a conversion degree of up to 8 % was observed. the electrodes co-infiltrated with bao and pt revealed a higher conversion degree when propene was present. the results indicate that 27 % of nox was removed at 300 °c. the conversion for this cell showed highest decrease with increasing temperature. the catalytic activity of the electrodes that were infiltrated with only pt showed activity similar to that of the non-infiltrated electrode in no + o2 atmosphere. in presence of propene, however, the activity increased to almost the same level as that for the bao + pt infiltrated electrodes. during testing in the no + o2 atmosphere, no n2o was detected, and the nitrogen selectivity was 100 %. when propene was added to the gas feed, a small amount of n2o was detected. the cells containing pt showed the highest formation of n2o. n2o formation decreased with temperature as expected (n2o is stable only at lower temperatures). a positive effect of applying a voltage on the conversion of nox was observed for all types of cells under certain experimental conditions. the effect was most pronounced in the 1000 ppm no + 10 % o2 atmosphere (see figure 3(a) and 3(b), solid points). the selectivity to n2 was 100 % for the non-infiltrated and pt-infiltrated electrodes, but a small quantity of n2o was detected for the electrodes that contained bao. when propene was added to the feed gas, only a small polarization j. electrochem. sci. eng. 7(4) (2017) 153-166 nox and propene conversion 158 effect was observed (see table 1). the rate enhancement ratio with the polarization of the electrodes infiltrated with only bao was highest in the presence of propene, but the extra converted no was almost the same in both atmospheres. at 300 and 350 °c, the polarization effect on the cell with bao + pt infiltrated electrodes was low and less than 1 % extra no was removed. the activity enhancement under polarization of the electrodes with only pt and atmosphere without propene, was very similar to that of the bao + pt electrodes in presence of propene in gas feed, whereas it was much lower in the absence of propene. the amount of n2o formed during polarization was the same as that at ocv. thus, it might be argued that the nox converted by polarization generates n2, whereas the nox that was catalytically decomposed generates n2o. table 1. rate enhancement ratio of nox conversion for four types of lsm15-cgo10 electrodes t / oc  gas composition: 1000 ppm no + 10 % o2 1000 ppm no + 10 % o2 + 1000 ppm c3h6 bao 300 1.1 1.5 350 1.9 3.8 400 6.4 ∞ bao + pt 300 1.1 1 350 2.3 1 400 15.3 6 ni 300 1 1 350 3 1 400 ∞ 1 pt 300 1.3 1 350 4.3 1.3 400 13 1.6 in figure 4(a) and 4(b), the conversions of propene and selectivity to co2 for the cells with baoinfiltrated and non-infiltrated electrodes are plotted as a function of temperature. figure 4. temperature dependence of propene conversion (solid lines) and selectivity to co2 (dotted lines) in symmetrical cells measured at the ocv (open symbols) and during polarization (solid symbols) for: a) bao and bao + pt-infiltrated lsm15 cgo10 electrodes ; b) non-infiltrated lsm15 cgo10 electrodes. the cells with non-infiltrated electrodes revealed a high catalytic activity, with over 60 % conversion in one of the cells at 400 °c. the cells with bao-infiltrated electrodes showed a much lower catalytic activity towards the oxidation of propene with the highest value being just below 7 % at 400 °c. the cell with electrodes containing bao + pt exhibited a high catalytic conversion, where 95 % of the propene was removed at 300 °c. the conversion of propene decreased with a. z. friedberg and k. k. hansen j. electrochem. sci. eng. 7(4) (2017) 153-166 doi:10.5599/jese.419 159 increasing temperature in this type of cell. the cells with only pt infiltrated electrodes showed a 100 % conversion of propene, except at 300 °c, where only about 70 % was converted. all propene was oxidized to co2 since no co was detected. for the non-infiltrated electrodes, the selectivity to co2 increased with increasing temperature, whereas the selectivity peaked at approximately 350 °c for the bao-infiltrated electrodes. almost all propene was oxidized to co2 in the cell with bao + pt electrodes at 300 and 350 °c. the propene conversion increased to 74 % at 400 °c for the most active non-infiltrated electrodes when the cell was polarized. additionally, when the cell was polarized, it was possible to obtain only 11 % conversion at 400 °c for the cell with bao-infiltrated electrodes. for all the cells, the highest enhancement was obtained at 400 °c, and 84 % of the propene was removed at 400 °c in the cell with bao + pt infiltrated electrodes. electrochemical impedance spectroscopy all impedance spectra recorded at the ocv were fitted with equivalent circuits corresponding to three and four arcs appearing in the nyquist plots, depending on temperature and atmosphere. most of the arcs were best fitted by an ohmic resistance model in parallel with a constant phase element (cpe). some of the low-frequency arcs, however, could be fitted using a voigt element, as they were almost perfect semicircles. the admittance (y) of a cpe (denoted shortly as q) is given by: y = y0 (j)n (2) where y0 is a constant,  is the angular frequency, and n is an exponent. y0 and n values are estimated from the fitting of the selected equivalent circuit to the experimentally measured impedance data. the near equivalent capacitance, c, values were estimated using the following equation: c = (y0)1/n r (1-n)/n (3) the impedance spectra were fitted using elchemea analytical software [26]. all impedance spectra were kramers-kronig validated before fitting. in the propene-containing atmosphere, the low-frequency part of the impedance spectrum obtained for the cell with bao + pt-infiltrated electrodes did not reach steady-state and therefore it was not possible to be fitted. representative examples of impedance spectra and their spectral deconvolution are shown in figures 5 and 6 for the cells with non-infiltrated and bao infiltrated electrodes measured at 400 °c. in all nyquist plots, two high-frequency arcs, denoted arc i and arc ii, were observed. when propene was present in the gas feed, a small arc that could not be fitted was found in the highfrequency part of arc i at 300 °c. due to the high summit frequencies of arc i (see tables 2 and 3), part of this arc is often "dragged" down by the inductance below the real axis. this arc has an activation energy, ea, of approximately 1 ev, and c is independent of the atmosphere and temperature, with a value of 1×10-7  9×10-8 f/cm2. arc ii exhibits a low n value (0.44-0.54). for the cells with the bao-infiltrated and the non-infiltrated electrodes, c increased with temperature. this increase was steeper for the cell with bao-infiltrated electrodes, for which the capacitances were also higher than those for the cell with non-infiltrated electrodes. the resistance of this arc, that concerns the activation energy of the corresponding process, decreased when propene was present in the cells with non-infiltrated electrodes. no dependence on the presence of propene was observed for the resistance of this arc for the cells with bao-infiltrated electrodes. in all impedance spectra, the dominating arc, denoted as arc iii, was observed in the middle frequency range. the activation energy of the corresponding process increased in the presence of propene for the cells j. electrochem. sci. eng. 7(4) (2017) 153-166 nox and propene conversion 160 with non-infiltrated electrodes, whereas no dependence on presence of propene was observed for the cells with the bao-infiltrated electrodes. a b c figure 5. nyquist plots of the impedance spectra measured at 400 °c for symmetrical cells with: a) non-infiltrated lsm15 cgo10 electrodes in 1000 ppm no + 10 % o2; b) non-infiltrated lsm15 cgo10 electrodes in 1000 ppm no + 1000 ppm c3h6 + 10 % o2; c) bao-infiltrated lsm15 cgo10 electrodes in 1000 ppm no + 10 % o2. it was not possible to fit the high-middle to middle frequency region of the impedance spectrum of the cell with the bao + pt infiltrated electrodes with rq elements having the same n (cpe) value at all three temperatures. the fitting results were not improved even after addition of more elements. the n values for arc i and arc iv (the low-frequency arc) were the same as for the cell with bao infiltrated electrodes, but the n values of arc ii and arc iii changed with temperature. therefore, no activation energies or capacitance values could be calculated for arc ii and arc iii for this cell. instead, the total resistance of these two arcs was used to obtain activation energies of a. z. friedberg and k. k. hansen j. electrochem. sci. eng. 7(4) (2017) 153-166 doi:10.5599/jese.419 161 0.91 ev and 0.97 ev in the presence and absence of propene, respectively. a low-frequency arc (arc iv) with a high n value was observed in the impedance spectra of all cells. this arc appeared only in the no + o2 atmosphere for the cell with non-infiltrated electrodes, but in the cells with bao infiltrated electrodes, the arc was observed in both types of gas mixture. this arc was not observed at 300 °c. in all cases, the n value of the cpe was the highest at 350 °c. for the cell with noninfiltrated electrodes, the change in n value with increasing temperature was small, with n decreasing from 0.97 to 0.92. however, the n value was higher for the cell with bao infiltrated electrodes, especially in the presence of propene, where it decreased from 1 to 0.6. the behavior was the same in the bao + pt cells. the resistance corresponding to this arc was changed by only a small amount with increasing temperature. a b c figure 6. nyquist plots of the impedance spectra measured at 400 °c for symmetrical cells with: a) bao-infiltrated lsm15 cgo10 electrodes in 1000 ppm no + 1000 ppm c3h6 + 10 % o2; b) bao + pt-infiltrated lsm15 cgo10 electrodes in 1000 ppm no + 10 % o2; c) bao + pt-infiltrated lsm15 cgo10 electrodes in 1000 ppm no + 1000 ppm c3h6 + 10 % o2. j. electrochem. sci. eng. 7(4) (2017) 153-166 nox and propene conversion 162 table 2. characteristics of the processes corresponding to different arcs of the impedance spectra of the symmetrical cell with non-infiltrated lsm15-cgo10 electrodes, measured at 300-400 °c in the atmosphere containing 1000 ppm no +10 % o2 processes/arcs fsummit / hz c / f cm-2 characteristics i 4×103–9×105 ~10-7 independent of temperature and atmosphere. ea = 0.9 ± 0.06 ev independent of atmosphere. ii 17-661 4 × 10-6  3.5 × 10-5 increased with increasing temperature. higher values in the presence of propene. ea = 0.75-1 ev decreases when propene is present. resistance decreases with polarization. fsummit values are higher when propene is present. n = 0.44 – 0.54 iii 0.5-19 1.6 × 10-4 – 6 × 104 independent of temperature. higher values in the presence of propene. ea = 0.71-0.92 ev increases when propene is present. fsummit values are independent of atmosphere. iv high n value (n = 1) appears at 350 and 400 °c at ocv in no + o2. appears in the presence and absence of propene. resistance is independent of temperature. table 3. characteristics of the processes corresponding to different arcs of the impedance spectra of the symmetrical cell with bao-infiltrated lsm15-cgo10 electrodes, measured at 300-400 °c in the atmosphere containing 1000 ppm no + 1000 ppm c3h6 + 10 % o2. processes/arcs fsummit / hz c / f cm-2 characteristics i* 300090000 ~10-7 independent of temperature and atmosphere. ea = 0.94 ± 0.02 ev ii 3-65 5.4 × 10-5 2 × 10-4 increased with increasing temperature. higher values in absence of propene. ea ~0.95 ± 0.1 ev fsummit is higher when propene is present. n = 0.44 – 0.54 iii 0.2-2 9 × 10-4 – 1.6 × 104 increased with higher temperatures. higher values in presence of propene. ea = 0.81-0.93 ev increases when propene is present fsummit are independent of atmosphere. iv* 0.012-0.1 appears at 350 and 400 °c at the ocv n = 0.6 * including results for the bao + pt infiltrated electrode discussion conversion and the effect of propene when a voltage was applied to the cells with bao infiltrated electrodes, the nox conversion increased. the enhancement factor, , was highest in a propene atmosphere, but the amount of n2 formed was almost the same in the presence and absence of propene. a higher  observed in the propene-containing atmosphere can be attributed to the lower catalytic activity in this case. the lower catalytic activity in the propene-containing atmosphere suggests that the reduction of no was inhibited by propene when bao is used as a storage compound. however, the catalytic activity a. z. friedberg and k. k. hansen j. electrochem. sci. eng. 7(4) (2017) 153-166 doi:10.5599/jese.419 163 of the non-infiltrated electrodes was promoted in the presence of propene, which supports the findings of hansen et al. [13], who found that lsm is a catalyst for the scr of no with propene. it is well known that no2 is an intermediate in the reduction of no to nitrogen when using a storage compound. propene and no will then compete for the same active sites on lsm. when propene is present, fewer active sites are available for no oxidation, and bao will cover some of the active sites on lsm. this coverage could be the reason for the lower activity of bao infiltrated electrodes in the presence of propene. the catalytic reduction of no on the bao + pt infiltrated electrodes showed a dependence on temperature similar to the other tested electrodes, but nox conversion was much higher and much more dependent on temperature. for the cells with electrodes infiltrated with pt and bao, the higher conversion in the no + o2 atmosphere could be attributed to both, the removal via a storage compound and pt. in the presence of propene, the higher conversion in this cell at high temperatures could be attributed to the excellent oxidation ability of the pt catalyst. the positive effect of polarization in the propene-containing atmosphere was highest for the cell with baoinfiltrated electrodes at all three temperatures. however, the total nox conversion was low for this type of the cell but equally high as that for the cell with the bao + pt infiltrated electrodes when propene is present. the presence of platinum together with bao in electrodes provided a higher no conversion in the presence of propene at 300 and 350 °c, compared to electrodes with either pt or bao. this finding may indicate that pt enhances nitrate formation by forming no2, which is the step preceding nitrate formation [27]. the higher conversion could also be simply attributed to the sum of the reactions via bao and pt. the amount of n2o found for the cell with the bao + pt infiltrated electrodes was much higher than that formed in the cells with electrodes infiltrated only with bao, where almost nothing was formed when propene was present. the amount of n2o formed in the cell with bao + pt infiltrated electrodes was similar, but still higher than that formed in the cell with electrodes infiltrated only with pt. the reduction route via the storage compound is still not known for certain [28], but formation of n2o could suggest that the production from nitrate occurs via pt, which simply reduces it only to n2o in the presence of propene. the total conversion of nox in the presence of propene was higher on the non-infiltrated electrodes than in bao infiltrated ones. the reduction of no could occur via different routes, depending on the nature of the catalyst. the lsm behaved as a catalyst for the scr of no with propene, and this reaction was not significantly enhanced by polarization. in the cells with bao-infiltrated electrodes, the reduction could also proceed via nitrate formation. the formation of n2 in the presence and absence of propene suggests that nitrate formation was not coupled with propene oxidation. on the pt containing electrodes, the reduction could proceed via lsm or pt. in the presence of propene, polarization decreased nox conversion on the pt containing electrodes. this effect might be explained by the behavior of nox reduction on pt, which is known to peak as a function of temperature. this peak occurs at a lower temperature upon polarization [29], which means that if one is looking at only three temperatures, the peak could easily be missed, and the observed behavior can look like if the polarization has a negative effect. impedance analysis the polarization resistance of the cells with non-infiltrated electrodes is similar for the two tested cells, which indicates good reproducibility. in all impedance spectra obtained at 300 °c in the propene-containing gas, a small arc in the high-frequency range was observed. it was not possible to fit the arc, but it was clear that it overlapped with the larger high-frequency arc, referred as arc i. j. electrochem. sci. eng. 7(4) (2017) 153-166 nox and propene conversion 164 clearly, something occurred in the presence of propene in the atmosphere, which caused this response at high frequencies. at this point, it is not possible to determine the origin of this arc. the change in the activation energy corresponding to the resistance of the high-frequency arc (arc i) with the addition of propene can be explained by the presence of this small high-frequency arc. the capacitance and summit frequencies together with an ea close to 1 ev indicate that oxide ion transport across the interface of the electrode and electrolyte is responsible for this impedance response [30-32]. the near-equivalent capacitance of arc ii was higher in the cells containing bao infiltrated electrodes. in these cells, the value of c decreased when propene was introduced in the atmosphere. the opposite was observed for the cells with non-infiltrated electrodes, in which the presence of propene increased the capacitance. the higher value of c for the cell with bao-infiltrated electrodes can be associated with the presence of bao and better adsorption of no on bao sites compared to lsm sites. the capacitance value of the arc ii was close to capacitance values associated with oxygen adsorption and dissociation on the composite electrode [22]. the higher capacitance of the cells with bao-infiltrated electrodes could be related to the production of active oxygen species by reduction of nitrates. the decrease in c observed when propene is present in the cell with bao-infiltrated electrodes could indicate that propene interacts via the above mentioned reaction, which is supported by a decrease in no reduction in the presence of propene. in the cells with noninfiltrated electrodes, propene could interact with adsorbed oxygen, thereby forming more active oxygen species for dissociation on the electrode surface. the near-equivalent capacitance correlated well with the capacitance values related to oxygen adsorption and dissociation on the electrode surface reported in earlier findings from our group [22,33]. in the case of arc ii, the c value of the mf arc (arc iii) observed for the cells with bao infiltrated electrodes was higher than that for non-infiltrated electrodes. the increase of c and decrease of the resistance of this arc with increasing temperature indicates that the responsible process is related to the extension or broadening of the three-phase boundary [34]. the ea corresponding to this process increased for both cells with the non-infiltrated and bao-infiltrated electrodes, when propene was introduced in the gas feed. the values of ea are in good agreement with previous findings of our group [33,35], where the processes related to the corresponding arc were stated as adsorption, surface diffusion and transfer across the triple-phase boundary (tpb). the sensitivity of ea in the presence of propene was reasonable since different species will adsorb in two atmospheres, while propene is believed to interact with adsorbed oxygen [34]. for these reasons, the arc iii is ascribed to the triple-phase boundary that is related to adsorption, diffusion and/or charge transfer involving oxygen and nitric oxide species. the change in the n values for arc ii and arc iii observed for the cell with bao + pt infiltrated electrodes suggests that these arcs represent different processes at different temperatures. in this cell, propene conversion decreased with increasing temperature, opposite to what is expected for a platinum catalyst [29]. this result suggests that something changes in the structure of the cell that inhibited this oxidation reaction. one explanation could be that the bao nanoparticles which created a fluffier structure after testing covered the platinum nanoparticles, thereby inhibiting the reaction by lowering the number of active sites. this change in microstructure could also lead to a change in the impedance response. arcs ii and iii were related to the adsorption, dissociation, and diffusion of surface species at bao infiltrated and non-infiltrated electrodes. these processes were very dependent on the infiltrated materials. a. z. friedberg and k. k. hansen j. electrochem. sci. eng. 7(4) (2017) 153-166 doi:10.5599/jese.419 165 adsorption on bao is believed to proceed through no2 [22], whereas the chemisorption of no is expected on pt [29,36]. this means that when both, pt and bao nanoparticles are present, the adsorption and reduction of no can proceed via different pathways. the impedance response, shown as arc ii and arc iii, might correspond to a mixture of these processes. in some of the impedance spectra, a low-frequency arc was found, but not for the cells with noninfiltrated electrodes in the presence of propene. the size of this arc and the n value were difficult to be determined from the impedance spectra measured at the ocv. in view of the low activation energy related to this arc, the fact that it appears at low frequencies of impedance spectra of all cells regardless of impregnation material, is in good agreement with the gas diffusion impedance, as described by mogensen [37]. in both cells with bao and bao + pt infiltrated electrodes, the low-frequency parts of impedance spectra changed at 350 °c and 400 °c. the n value of the low-frequency arc was found to be approximately 0.9 in the absence of propene, but it decreased to approximately 0.7 in the presence of propene at 400 °c. it was not possible to create a better fit with additional arcs. the different n values of the arcs could represent two different processes at two different temperatures in the presence of propene in the gas phase. conclusions in this work, it was shown that the catalytic reduction of no by propene on la0.85sr0.15mno3/ce0.9gd0.1o1.95 (lsm 15/cgo10) electrodes in the presence of excess of oxygen was not electrochemically enhanced. however, the direct reduction of no was enhanced in the presence of excess oxygen. the impregnation of 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[37] s. primdahl, m. mogensen, journal of electrochemical society 146 (1999) 2827-2833. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.elchemea.com/ http://creativecommons.org/licenses/by/4.0/) {graphene-modified nickel foam electrode for cathodic degradation of nitrofuranzone: kinetics, transformation products and toxicity} doi:10.5599/jese.458 201 j. electrochem. sci. eng. 7(4) (2017) 201-212; doi: http://dx.doi.org/10.5599/jese.458 open access : : issn 1847-9286 www.jese-online.org original scientific paper graphene-modified nickel foam electrode for cathodic degradation of nitrofuranzone: kinetics, transformation products and toxicity ya ma1,2, yanhong tang1,, kai yin2, shenglian luo2, chengbin liu2, tian liu2, liming yang2,3 1college of materials science and engineering, hunan university, changsha 410082, p. r. china 2state key laboratory of chemo/biosensing and chemometrics, hunan university, changsha 410082, p. r. china 3key laboratory of jiangxi province for persistent pollutants control and resources recycle, nanchang hangkong university, nanchang 330063, p. r. china corresponding authors e-mail: tangyh@hnu.edu.cn received: october 10, 2017; revised: november 7, 2017; accepted: november 27, 2017 abstract simple, efficient, and durable electrodes are highly demanded for practical electrochemical process. in this study, a reduced graphene oxide modified nickel foam electrode (gr-ni foam) was facilely prepared via one-step cyclic voltammetry electrodeposition of graphene oxide suspension onto the ni foam. the electrochemical degradation of nitrofuranzone (nfz, a kind of typical antibiotics) was studied on the gr-ni foam cathode. the cyclic voltammetry and electrochemical impedance spectra analysis confirmed that presence of gr loading accelerated the electron transfer from the cathode surface to nfz. with the applied cathode potential of −1.25 v (vs. ag/agcl), the removal efficiency of nfz (c0 = 20 mg l−1) at the gr-ni foam electrode reached up to 99 % within 30 min, showing a higher reaction rate constant (0.1297 min−1) than 0.0870 min−1 at the pd-ni foam and 0.0186 min−1 at the ni foam electrode. it was also found that the ph, dissolved oxygen and nfz initial concentration have slight effect on nfz degradation at the gr-ni foam electrode. the reactions first occurred at nitro groups (-no2), unsaturated c=n bonds and n-n bonds to generate furan ring-containing products, and then these products were transformed into linear diamine products. the direct reduction by electrons was mainly responsible for nfz reduction at the gr-ni foam electrode. even after 18 cycles, the removal efficiency of nfz still reached up to 98 % within 1 h. in addition, the cathodic degradation process could eliminate the antibacterial activity of nfz. the gr-ni foam electrode would have a great potential in electrochemical process for treating wastewater containing furan antibiotics. keywords graphene; nickel foam; nitrofuranzone; electrochemical degradation http://dx.doi.org/10.5599/jese.458 http://www.jese-online.org/ mailto:tangyh@hnu.edu.cn j. electrochem. sci. eng. 7(4) (2017) 201-212 graphene-modified nickel foam electrode 202 introduction antibiotics discharged into the environments (e.g., soil and aquatic ecosystems) could increase antibiotic resistance of ecosystems [1-5], bringing about potential threats to aquatic life and even human health. nitrofuranzone (nfz) as a low-cost and high-efficiency broad spectrum of antimicrobial agents of nitrofurans is widely used for the treatment of gastrointestinal and dermatological infections in animal husbandry and aquaculture [6]. however, the overuse and misuse of them have led to adverse environmental effects and antibiotics resistance transfer to human pathogenic microbes [7-12]. because of the microbial resistance of antibiotics, it is very difficult to efficiently remove them using conventional methods (e. g., activated sludge) in wastewater treatment plants. as a result, the water containing nitrofuran antibiotics is discharged frequently into the environment [13-16]. thus, wastewater containing antibiotics contaminants must be decontaminated before it is discharged into the environment. a variety of methods including photolysis [17,18], adsorption [19-21], and advanced oxidation process [22, 23] have been developed to remove antibiotics. however, photolysis process greatly relies on adequate illumination or involve addition of catalysts, adsorption process is limited due to poor adsorption capacity towards low-concentration antibiotics, and advanced oxidation process involves catalysts or a great deal of reagents that lead to the secondary pollution. compared to these methods, electrochemical method has been regarded as one of the most effective technology due to its environmental-friendly and high-efficiency properties [24-26]. it is believed that electrochemical reduction process is governed and affected by atomic h* mediation produced on noble metal catalysts [27,28]. however, the limited mining capability and prohibitive cost of noble metals hinder their field application. furthermore, noble metal catalysts are easily poisoned in the presence of organics and electrochemical process [29,30]. to move the practical application of electrochemical process forward, avoidance of noble metal catalysts is necessary. as a result, developing high-performance electrode materials is crucial for electroreductive reactions in practice. in this work, we developed a graphene-modified ni foam electrode (gr-ni foam), which was facilely synthesized via one-step cyclic voltammetry electrodeposition technique. the as-prepared electrode has the following merits: (i) ni foam has the advantages including low cost, high stability and porous structure and (ii) graphene has excellent conductivity for fast electron transfer and good adsorption properties towards organic molecules. ability of this electrode to decompose nfz was tested using cathodic degradation process. the fate of nfz during the degradation process was investigated to uncover its destruction pathway. the toxicity of nfz and the degradation products was evaluated by antimicrobial property. an electrochemical system based on a simple and effective gr-ni foam electrode could potentially serve for treatment of wastewater containing antibiotics. experimental nitrofuranzone (nfz), citric acid, sodium citrate, palladium chloride (pdcl2), sodium sulfate (na2so4), sulfuric acid (h2so4) and tertiary butanol (t-buoh) were used as received from aladdin (shanghai, china). natural flake graphite with an average particle size of about 50 µm was purchased from shanghai carbon co., ltd. (shanghai, china), ni foam (pore density 100 ppi, thickness 1.5 mm) was purchased from shenzhen lifetion environmental protection equipment co., ltd (shenzhen china), and nafion 117 solution (5 wt%) was purchased from hesen co., ltd (shanghai, china). ultrapure water was used throughout all the experiments. y. ma et al. j. electrochem. sci. eng. 7(4) (2017) 201-212 doi:10.5599/jese.458 203 ni foam was first ultrasonically cleaned in acetone, hcl solution and ultrapure water, respectively. graphite oxide (go) suspension was synthesized from the flake graphite using modified hummers method [31]. the graphene modified ni foam (gr-ni foam) electrode was fabricated by cyclic voltammetric electrolysis of go suspension (0.3 g l-1) using an electrochemical workstation (chi 660c, chenhua co., ltd., shanghai, china) with a three-electrode system. ni foam was applied as the working electrode, pt foil as the counter electrode, and a saturated calomel electrode (sce) as the reference electrode, respectively. the potential scan was performed between −1.5 and 0.6 v at a rate of 50 mv s-1 for 80 cycles. as a control, the pd-loading ni foam (pd loading amount of 0.135 mg cm2) was prepared through one-step electrodeposition of a mixture solution of pdcl2 (1 mm) and h2so4 (5 mm) under magnetic stirring at −1.0 v for 60 s. finally, all the working electrodes were thoroughly washed with ultrapure water and dried at room temperature. the morphology and composition of as-prepared materials were characterized by field emission scanning electron microscope (sem, hitachi s-4800). the crystal structure was characterized using x-ray diffraction equipment (xrd, cu kα radiation, λ=1.54056 å). raman spectra were measured using a labram-010 raman spectrometer with a 632.8 nm laser. the pd content in pd-ni foam was determined by inductively coupled plasma-optical emission spectrometry (icp-oes, 5110icp, agilent). the cyclic voltammetry (cv), electrochemical impedance spectra (eis) and electric doublelayer capacitor (edlc) characterization were performed in a dual chamber equipped with chi 660c electrochemical workstation using a pt foil as the counter electrode and sce as the reference electrode. cv curves were recorded in the scan range between −1.2 and 0 v. the eis was recorded by applying an alternating current (ac) voltage with 5 mv amplitude in a frequency range 1000000.01 hz at the open circuit voltage. the edlc characterization was performed between −0.15 and −0.05 v. all the experiments were performed at room temperature. a dual-chamber reactor was used for the electroreduction of nfz, which was separated into a cathode cell (20 ml) and anode (20 ml) cell by a cation exchange membrane. the working area of the prepared electrode was 1×1 cm2. a platinum plate (1×1 cm2) was used as the counter electrode, and a silver chloride electrode (ag/agcl) was employed as the reference electrode. the counter electrode was put into the anode chamber, and the working electrode and reference electrode were inserted into the cathode chamber. a constant potential on the cathode was controlled using the amperometric i-t technique by setting up initial e with the electrochemical workstation (chi660c). the cathode chamber was magnetically stirred at a rate of 1000 rpm. before triggering reaction, the dissolved oxygen in the reactor was removed by bubbling n2 gas. the concentrations and the decomposed products of nfz were quantified using a high-performance liquid chromatography (hplc, hitachi l-2130, japan) with an l-2420 uv-vis detector. the mobile phase contained 50 % methanol and 50 % water at a rate of 1 ml min–1, the wavelength was 250 nm, and the column temperature was 30 °c. nfz transformation products were analyzed by a triple quad hplc-esi-tqms (aglient 1290/6460) and ltq-orbitrap velos pro (thermo fisher scientific). the positive mode was employed in electrospray ionization. to avoid the possible contamination of equipment, all samples were filtered through 0.22 µm filters before the detection. luminescent bacteria such as escherichia coli dh5α [24] (a typical gram-negative bacterium) was used to evaluate toxicity of substances. the cells of the strains were harvested at the end of their exponential growth phase. the separated cells were washed with a sterile phosphate buffer solution (pbs) (50 mm) and then inoculated into 24 ml of 3-fold-diluted luria−bertani (lb) medium. to evaluate this non-specific toxicity, the levels of bacteria bioluminescence (0.3 ml bacteria source) were measured in 6 ml solution of pbs blank, 20 mg l-1 nfz and degradation products, respectively. j. electrochem. sci. eng. 7(4) (2017) 201-212 graphene-modified nickel foam electrode 204 this system was loaded into a 50 ml conical flask and shaken in a shaking incubator with a shaking rate of 180 rpm at 30 °c. the absorbance of optical density at 600 nm (od600) could reflect the concentration of the living bacteria. results and discussion characterizations the sem images of ni foam and gr-ni foam are shown in figure 1(a-b). the surface of bare ni foam was smooth (figure 1a), while the surface became rough after depositing graphene sheets (figure 1b). on the basis of the analysis of raman spectroscopy (figure 1c), the two prominent bands observed in gr-ni foam around 1327 and 1586 cm−1 were assigned to the d and g bands of carbon, respectively, indicating that gr sheets were successfully deposited onto the ni foam substrate. the eis was applied to investigate the electrical properties of the gr-ni foam and ni foam cathodes. the typical nyquist plots are shown in figure 1d. the gr-ni foam had a smaller semicircle diameter than bare ni foam, indicating that the introduction of gr increased conductivity and promoted electron transfer [32,33]. additionally, cyclic voltammetry (cv) can determine the double layer capacitance (cdl) which is linearly proportional to effective active surface area [34,35]. the cv current responses for the ni foam and gr-ni foam electrodes in the region from –0.15 to –0.05 v vs. sce should be mostly due to the charging of the double layer (figure s1a and b). figure 1. sem images of (a) ni foam and (b) gr-ni foam, (c) raman spectra of ni foam and gr-ni foam electrodes, and (d) nyquist plots of ni foam and gr-ni foam electrodes in 0.1 m na2so4. the double layer capacitance was estimated by plotting the δj (ja-jc) at –0.10 v against the scan rate (figure s1c), and the gr-ni foam had a much larger double-layer capacitance than ni foam. the results strongly indicated that the loaded gr could increase the density of catalytically active sites. as a control, the pd-ni foam electrode was prepared through one-step electrodeposition of a pdcl2 solution using ni foam as the working electrode. the sem image showed that dendritic pd (c) (d) 1000 2000 3000 0 100 200 300 400 500 600 d+g2d in te n si ty , a .u . roman shift, cm -1 ni foam gr-ni foam d g (a) (b) 100 μm (a) (b) 1 μm 1 μm 100 μm 0 5 10 15 20 25 30 0 5 10 15 20 25 30 -z ''/ k  z'/k ni foam gr-ni foam y. ma et al. j. electrochem. sci. eng. 7(4) (2017) 201-212 doi:10.5599/jese.458 205 particles lay on the surface of ni foam (figure s2). the xrd result confirmed the existence of metallic pd (figure s3). the content of pd in pd-ni foam was 9.4 mg/g determined by icp-oes analysis. nfz degradation on the prepared electrodes the characteristic shape of cyclic voltammograms (cv) is a result of the potential which is dependent on the changes in surface concentrations of redox system and simultaneous diffusion processes [36]. the cv tests were applied for the reduction of nfz at different electrodes (figure 2a). in the presence of nfz, both, the ni foam and the gr-ni foam showed characteristic reduction peak of nfz at –0.95 v (peaks c1 and a2) [24,26,37]. in addition, there appeared another reduction peak of nfz at –0.60 v (peak a1) for the gr-ni foam. this result indicated that nfz reduction occurred on the gr-ni foam at a smaller potential than on the ni foam, which could probably be attributed to the reduction of overpotential due to introduction of gr. additionally, there was an oxidation peak at about –0.50 v. similar cv behaviors were observed during the reduction of nitroaromatics nitrobenzene and chloramphenicol [38-40] and attributed to the reversible reactions of nitroso and hydroxylamino intermediates. in contrast, the gr-ni foam did not show any current peak in the absence of nfz. figure 2. (a) cyclic voltammograms (vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz, ph = 4.28), (b) degradation efficiency, (c) degradation kinetics of nfz at −1.25 v vs. ag/agcl, and (d) adsorption of nfz without bias potential. the performances of different (ni foam, gr-ni foam, and pd-ni foam) electrodes for removal of nfz were determined under the same experimental condition (0.1 m na2so4 solution, ph 4.28, initial nfz concentration of 20 mg l−1, and applied potential of –1.25 v vs. ag/agcl) (figure 2b). only 53 % nfz was degraded on the bare ni foam electrode within 30 min, while the removal efficiency of nfz on the gr-ni foam reached up to 99 % what was higher than on the pd-ni foam (94 %) within 30 min. the results indicate that the gr-ni foam would seriously compete with noble metals (e.g. -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 -4 -2 0 2 4 6 c u rr e n t, m a potential, v nfz+gr-ni foam gr-ni foam nfz+ni foam a1 a2 c 1 (c) (a) (b) 0 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 c t/ c 0 time, min ni foam gr-ni foam pd-ni foam 0 10 20 30 40 50 0 1 2 3 4 5 6 k=0.0186 r 2 =0.9474 k=0.0870 r 2 =0.9953 ni foam gr-ni foam pd-ni foam -l n (c t/ c 0 ) time, min k=0.1297 r 2 =0.9808 (d) 0 5 10 15 20 25 30 0.0 0.2 0.4 0.6 0.8 1.0 c t/c 0 time, min ni foam gr-ni foam j. electrochem. sci. eng. 7(4) (2017) 201-212 graphene-modified nickel foam electrode 206 pd)–based electrocatalysts for electrocatalytic degradation of nitrofuran antibiotics. the degradation of nfz followed a pseudo-first order kinetic model, –ln(ct/c0) = kt + b, where t is the reaction time (min), k is the reaction rate constant (min−1), b is a constant, and ct and c0 are the nfz concentrations (μm) at times of t = t and t = 0, respectively. the reaction rate constants (k) of nfz were 0.1297 min−1 (coefficient r2 = 0.9808) for the gr-ni foam and 0.087 min−1 (r2 = 0.9953) for the pd-ni foam, which were 6.97 and 4.68 times higher than 0.0186 min−1 (r2 = 0.9474) for the ni foam, respectively (figure 2c). figure 2d showed that the adsorption basically did not contribute to the removal of nfz. effect of different parameters the influence of different experimental parameters such as cathode potential, initial concentration, solution ph and dissolved oxygen on electrochemical removal efficiency of nfz on the gr-ni foam was also investigated (figure 3). the degradation rate markedly increased with cathode potential from –0.35 to –1.25 v (vs. ag/agcl) (figure 3a). figure 3. effect of different experimental parameters on reduction of nfz on gr-ni foam cathode: (a) potential (0.1m na2so4, 20 mg l-1 nfz, ph 4.28), (b) dissolved oxygen (−1.25 v vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz, ph 4.28), (c) solution ph (−1.25 v vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz), and (d) initial nfz concentration (−1.25 v vs. ag/agcl, 0.1 m na2so4, ph 4.28). more negative cathode potential could produce more available electrons leading to higher efficiency in nfz reduction. nfz was nearly completely degraded at the applied potential of –1.25 v within 30 min. the nfz degradations were fitted to the pseudo-first reaction kinetics with rate constant k of 0.0060 min−1 (r2 = 0.9904), 0.0240 min−1 (r2 = 0.9996), 0.0728 min−1 (r2 = 0.9999) and 0.1297 min−1 (r2 = 0.9808) at –0.5, –0.75, –1.0 and –1.25 v, respectively (figure s4). 0 10 20 30 40 50 60 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 c t/ c 0 time, min 1.25 v 1.00 v 0.75 v 0.50 v 0.35 v (a) (b) 0 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 c t/ c 0 time, min 8.60 7.81 6.80 5.84 4.28 (c) (d) 0 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 c t/ c 0 time, min 20mg l -1 10mg l -1 5mg l -1 1mg l -1 0 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 c t/ c 0 time, min without n2 with n2 y. ma et al. j. electrochem. sci. eng. 7(4) (2017) 201-212 doi:10.5599/jese.458 207 it has already been reported that dissolved oxygen could compete with the contaminant for both the electrons and atomic h* on the cathode surface, suppressing thus the target contaminant reduction [27,41]. by purging the solution with n2, the dissolved oxygen was removed. the degradation of nfz on the gr-ni foam electrode was studied with and without n2 sparging (figure 3b). it was observed that the dissolved oxygen had slight effect on nfz degradation. in addition, nfz degradation was promoted by decreasing ph from 8.60 to 4.28 (figure 3c). the generation of atomic h* is favored in acid solutions, therefore acidic solution facilitates the reduction process by consuming h*[41-43]. even so, nfz degradation efficiency reached up to 83 % at ph 8.60 within 30 min, suggesting that the gr-ni foam electrode could be used in a wide range of ph for nfz degradation. also, the nfz degradation slightly increased with increasing initial concentration of nfz and the degradation efficiency still reached approximately 90 % within 30 min when the initial concentration of nfz was as low as 1 mg l-1 (figure 3d). it is clearly seen that the gr-ni foam electrode is feasible to work in practical wastewater with different ph, dissolved oxygen and nfz concentration. transformation pathway and proposed degradation mechanism of nfz the degradation products of nfz on different (ni foam, gr-ni foam and pd-ni foam) electrodes were measured using lc/ms. the extraction ion chromatogram (eic) for the detection of these products is shown in figure s5-s7. three furan ring-containing products and two linear chain terminal products were detected in the three electrode systems. the furan ring-containing products were (5-nitro-2-furyl) methanamine (nfm), (5-amino-2-furyl)methylene-hydrazinecarboxamide (amh) and (5-hydroxyamino-2-furyl)methyl-hydrazinecarboxamide (hmh), corresponding to m/z [m+na]+ of 165, 191 and 209, respectively. the linear chain terminal products were 5-aminopentanamide (apa) and 5-hydroxycadaverine (hoc), corresponding to m/z [m+na]+ of 139 and 141, respectively. the na+ was from na2so4 electrolyte. the transformation pathway of degradation products was illustrated in scheme 1. the reactions first occurred at nitro groups (-no2), unsaturated c=n bonds and n-n bonds to obtain hma, nfm and amh products, and then these products were transformed into linear apa products which were finally reduced to hoc. oo2n n h n nh2 o o o2n nh2 oh2n n h n nh2 o ohohn n h h n nh2 o h2n nh2 o h2n nh2 oh eee eeee(nfz, m/z = 198) (hmh + na+, m/z = 209) (nfm + na +, m/z = 165) (amh + na +, m/z = 191) (apa + na+, m/z = 139) (hoc + na+, m/z = 141) scheme 1. transformation pathway of degradation products it is believed that electrocatalytic reduction of organic pollutants involves direct electron transfer reduction and atomic h*-mediated reduction [41-43]. tertiary butanol (t-buoh) as a scavenger of h* [44] was used to study nfz degradation. the t-buoh addition (0, 2 and 5.0 mm) did not show effect on nfz reduction over bare ni foam, indicating a direct electron transfer reduction j. electrochem. sci. eng. 7(4) (2017) 201-212 graphene-modified nickel foam electrode 208 (figure 4a). in contrast, only at a high t-buoh concentration of 5.0 mm, the nfz reduction on gr-ni foam electrode was influenced (figure 4b). even so, the removal efficiency of nfz still achieved 88.7 % within 30 min at 5.0 mm t-buoh. it was reported that hydrogen produced in electrochemical process could be adsorbed on gr and subsequently activated to atomic h*[45-47]. it can be inferred that the produced atomic h* may be in part responsible for nfz reduction. the results indicated that direct electron transfer reduction played a crucial role in nfz reduction on the gr-ni foam. expectedly, a significant inhibition of nfz reduction was observed at the pd-ni foam in the presence of t-buoh (figure 4c). pd is well-known to have the unique property of activating protons to atomic h* and subsequently adsorbing atomic h*[48,49]. therefore, the reduction of nfz at the pd-ni foam mainly proceeded via h* mediation. figure 4. effect of t-buoh (0, 2 and 5 mm) on nfz reduction on (a) ni foam, (b) gr-ni foam, and (c) pd-ni foam electrodes (−1.25 v vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz, ph=4.28). the furan ring-containing products at different electrodes were further quantified using hplc with uv-vis detector (figure s8). the linear apa and hoc products could not be quantified using hplc due to lack of uv-vis absorption. the results are shown in figure 5. it was found that the transformation products were generated rapidly from the start on the gr-ni foam electrode, indicating fast reaction kinetics. after approximately 50 min, the concentrations of these products decreased, what is indicative of the further reduction to linear apa and hoc products. in addition, concentrations of the products increased monotonically on the ni foam electrode, suggesting a limited deep degradation to linear apa and hoc products. it was also seen that after 60 min, although the removal efficiency of nfz on the pd-ni foam electrode reached almost 100 % (figure 2b), the concentrations of the transformation products were lower than those for the ni foam and gr-ni foam electrodes. the results indicated that the furan ring-containing transformation products were more easily reduced to linear apa and hoc products using the pd-ni foam electrode with the aid of atomic h*. 4 y. ma et al. j. electrochem. sci. eng. 7(4) (2017) 201-212 doi:10.5599/jese.458 209 figure 5. nfz degradation product concentrations of (a) nfm, (b) amh, (c) hmh on ni foam, gr-ni foam and pd-ni foam electrodes (−1.25 v vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz, ph 4.28). toxicity assessment of degradation products e. coli dh5α (a typical gram-negative bacterium) luminescent bacteria is commonly used to evaluate the toxicity of substances. the antibacterial activity assay was performed using e. coli dh5α (figure 6a), which was reflected by the optical density of the bacterial strains at 600 nm (od600). the growth of e. coli dh5α was nearly completely inhibited in the presence of 20 mg l-1 nfz. in contrast, in the presence of terminal degradation products, the growth of e. coli dh5α was slightly inhibited and then returned to normalization after 10 h. generally, after the nitro group reduction, heterocyclic amine products were less toxic and considerably easier to be mineralized by aerobic bacteria than the nitro group containing antibiotics [50]. so, it can be stated that the electrochemical treatment of wastewater containing nfz using the gr-ni foam electrode is efficient to reduce the risk of adverse environmental effect. electrode durability the durability of electrodes is crucial for practical application of electrochemical process. to evaluate the durability of the gr-ni foam electrode, electrocatalytic reduction of nfz was performed after 18 cycles (each cycle time was 1 h) (figure 6b). significantly, in the 18th cycle, the removal efficiency of nfz still reached up to 98 %, being nearly the same as in the first cycle. the results indicated that the gr-ni foam electrode was very stable, showing a great potential of practical application. j. electrochem. sci. eng. 7(4) (2017) 201-212 graphene-modified nickel foam electrode 210 figure 6. (a) toxicity assessment of control, nfz (20 mg l-1) and degradation products (−1.25 v vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz, t = 1 h, ph 4.28). (b) durability of gr-ni foam electrode (−1.25 v vs. ag/agcl, 0.1 m na2so4, 20 mg l-1 nfz). conclusions an efficient gr-ni foam electrode was facilely prepared using one-step electrodeposition of go suspension onto the ni foam. by application of the gr-ni foam electrode as cathode, the efficient and rapid degradation of nfz was achieved in the electrochemical reduction process without using an expensive noble metal catalyst. the degradation rate of nfz on the gr-ni foam electrode was higher than that on the pd-ni foam electrode, and the degradation products showed much lower antibacterial activity than the nfz parent compound. because of its high effectiveness and stability, low cost, and environmental friendliness, the gr-ni foam electrode could play a beneficial role as a promising cathode in electrochemical process for efficient removal of nitrofuran antibiotics in water. acknowledgements: this work was supported by the national natural science foundation of china (51778218 and 51478171). references [1] j. davies, d. davies, microbiology and molecular biology reviews 74(3) (2010) 417-433. 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[50] b. a. donlon, e. razo-flores, g. lettinga, j. a. field, biotechnology and bioengineering 51(4) (1996) 439-449. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) predicting the effect of silicon electrode design parameters on thermal performance of a lithium-ion battery http://dx.doi.org/10.5599/jese.1547 795 j. electrochem. sci. eng. 13(5) (2023) 795-804; http://dx.doi.org/10.5599/jese.1547 open access : : issn 1847-9286 www.jese-online.org original scientific paper wolffia globosa as a biocatalyst in plant-based biofuel cells yolina hubenova1, 2,, eleonora hubenova1 and mario mitov3 1department of biochemistry and microbiology, plovdiv university “paisii hilendarski”, 24 tsar asen str., plovdiv, bulgaria 2department of electrocatalysis and electrocrystallization, institute of electrochemistry and energy systems “acad. e. budevski” – bulgarian academy of sciences, sofia, bulgaria 3innovative center for eco energy technologies, south-west university “neofit rilski”, blagoevgrad, bulgaria corresponding author: jolinahubenova@yahoo.com; tel.: +359 888592301 received: october 15, 2022; accepted: december 25, 2022; published: january 31, 2023 abstract the rootless duckweed wolffia globosa, not explored toward electrogenicity till now, is investigated as a putative biocatalyst in plant-based biofuel cells (p-bfc) for the electrical current generation and its basic metabolic changes during the polarization are depicted. after a short adaptation period, the open-circuit voltage of p-bfc, utilizing w. globosa as an anodic biocatalyst, reaches values of 630 mv. at a connected external resistor of 1 kω in the electric circuit, stable current densities of 170±10 ma m-2 are achieved. the electrical outputs depend on the anodic potential, reaching negative values of ca. -200 mv (vs. she). w. globosa produces an electrochemically active compound, acting as an electron shuttle. the polarization intensifies the w. globosa metabolism, expressed in a double increased glucose and starch content along with 1.82 times higher specific amylase activity of 70.0±2.8 u g-1 wet biomass in the organelle-enriched fractions of the explored as biocatalysts plants compared to the control. the results reveal that wolffia globosa can be utilized as a biocatalyst in p-bfc for simultaneous electricity generation and increased carbohydrate and protein content. keywords duckweed; polarization; metabolic activity; electricity generation introduction decarbonization is one of the main goals of the eu energy system, needing new technologies. as primary producers, plants are those that convert co2 into organic matter. recently, a new technology named plant-based biofuel cell (p-bfc), mimicking the processes imposed in nature, has been proposed as an alternative approach for intensified co2 capturing and utilization. the principles of p-bfc, based on the fact that specific bacteria in the rhizodeposits of plants can transfer electrons to the anode of the fuel cell located in the soil, were proved in 2008 [1,2]. it has been verified that by http://dx.doi.org/10.5599/jese.1547 http://dx.doi.org/10.5599/jese.1547 http://www.jese-online.org/ mailto:jolinahubenova@yahoo.com j. electrochem. sci. eng. 13(5) (2023) 795-804 wolffia globosa as a biocatalyst in biofuel cells 796 p-bfc, solar energy and co2 can be harvested for plant biomass production with a simultaneous electric current generation [3]. oryza sativa, spartina anglica, arundinella anomala, and glyceria maxima have been proven as suitable biocatalysts in p-bfcs [4]. as far as these plants cooperate with the soil microbes, the systems are called plant microbial fuel cells (p-mfc). the p-bfc technology was further developed by using aquatic plants as biocatalysts, which can directly communicate with the anode of the bioelectrochemical system (bes) [5,6]. in a previous study [5], we reported that higher aquatic plant lemna minuta duckweeds can be used as biocatalysts in direct photosynthetic plant fuel cells (dppfc) without the participation of electrogenic bacteria. the influence of abiotic factors such as temperature, humidity, and light intensity, as well as the day/night alternation, on the electrical outputs obtained by the bes, was established. it has been demonstrated that lemna minuta duckweed, autotrophically grown in plant fuel cell under natural sunlight illumination, can generate high power density (380±19 mw m-2), corresponding to 120±6 gj ha-1 year, which remains the highest achieved value reported in the literature [7,8]. under artificial light sources, p-bfc generated an order of magnitude lower current indicating the significance of the light-dependent processes implemented in the chloroplasts of these anodic biocatalysts. indeed, tracing the changes in the current during the day and night, higher electrical outputs were achieved during daylight hours. the energy generated by duckweed-based p-bfc so far is summarized and presented in table 1. table 1. summary of the generated current and achieved power by p-bfc carried out with different biocatalysts belonging to the lemnaceae family upon the anode under different grown conditions duckweed light source energy by pbfc e`0 / mv* proposed plant components participating in eet ref. lemna minuta natural sunlight 10 k lx predicted and achieved: 1.6 a m−2  400 mw m−2 (120 gj ha-1 year-1) −280 dependence on the day/night cycles [5] lemna minuta mixed lab light 1.5 k lx measured: 160 ma m−2 predicted by polarization curve: 250 ma m−2, 50 mw m−2 [5] lemna valdiviana mixed lab light 1.5 k lx 226 mа m−2 , predicted by polarization curve: 1.2 a m−2 and 140 mw m−2 −266 light-dependent processes of chloroplasts, respiration [6] lemna minor 60 k lx coherent light source, irradiation by optical fibers, with polarized monochromatic light: red ( = 650 nm, 0.059 w m−2); violet ( = 450 nm, 0.732 w m−2); visible non-polarized (1.5 w m−2) 17.5 ma m−2 13.0 ma m−2 15.5 ma m−2 cytochrome b6f , red lightabsorbing photosystems ps i and ps ii [9] wolffia globosa mixed lab light 1.5 k lx 170±10 ma m-2, predicted 1 a m-2, 90 mw m-2 −260 confirms the above this study *determined by cv, 10 mv s-1 besides lemna minuta [5], only two more duckweed species lemna valdiviana [6] and lemna minor [9] have been investigated for exoelectrogenic properties so far. although under artificial illumination these systems generate lower electrical current, the exploration of model aquatic plants in the laboratory conditions contributes to the improvement of the technology, especially when clarifying the mechanisms of extracellularly transferred electrons. the common feature of the plants belonging to the lemnaceae family is that they all have roots, which are in contact with the p-bfc anode and therefore are supposed to contribute to the electrical current generated by a direct y. hubenova et al. j. electrochem. sci. eng. 13(5) (2023) 795-804 http://dx.doi.org/10.5599/jese.1547 797 extracellular electron transfer (eet). depending on the illumination intensity and the period of cultivation, the response of the duckweeds to the fuel cell polarization has been also related to the membrane potential of the fronds/roots as well as to the secretion of plant endogenous mediators. the direct eet mechanism was further verified when lemna minor duckweeds have been grown in a p-bfc under total light source control conditions [9]. the plants were grown in fuel cells which were wrapped with black non-transparent folio with a black light-tight lid. polarized monochromatic light and, as a control light from the visible spectrum was diffused over the plants through optical fibers and the electrical current was recorded over time. of special interest were the p-bfcs which were irradiated with polarized monochromatic light with the precise wavelength of 650 nm (red light) and 450 nm (blue-violet light) [9]. the higher current values during the photoperiods and the analyses of the charge transfer by the electrochemical impedance spectroscopy suggested that a direct, photo-induced charge transfer between the duckweed and the fuel cell anode took place with the participation of the light-absorbing photosystems. although the plant’ chlorophyll ”a” absorbs at a maximum of about 450 nm wavelength, the red light is more effective because both photosystems (ps i and ps ii) absorb light of wavelengths in the red specter. when ps ii absorbs light, electrons in the chlorophyll reaction center are excited to a higher energy level. photo-excited electrons travel through the cytochrome b6f complex to photosystem i via an electron transport chain set in the thylakoid membrane [9]. in the present study, the rootless duckweed wolffia globosa, the smallest flowering plant worldwide, with free-floating fronds, is investigated for the first time as a biocatalyst in p-bfc regarding its capability of generating electricity. the influence of the polarization conditions on the plant's metabolism was examined in the organelle fractions of w. globosa obtained from duckweeds which are grown in p-bfc and compared to those of normally grown plants (control). experimental experimental setup the fuel cell construction consists of a transparent glassy body where the cathode and the anode have been placed (figure 1). a rectangular piece of carbon felt with dimensions 3.54.5 cm upon cork pontoons, served as an anode. figure 1. scheme of the experimental setup. the duckweed was grown under polarization conditions of the biofuel cell and compared with plants grown as a control without connection in an electrical circuit http://dx.doi.org/10.5599/jese.1547 j. electrochem. sci. eng. 13(5) (2023) 795-804 wolffia globosa as a biocatalyst in biofuel cells 798 1 g wolffia globosa has been grown in the fuel cell in 40 ml 67 mm phosphate buffer, ph 7.0, for enhanced conductivity, at open-air conditions. the losses from the evaporation and water uptake by the plants were daily compensated. mixed light (artificial and natural sunlight) was used as a light source to irradiate the plants. graphite granules (4.3 g total weight), placed into a 100 µm christean handy water filter as a separator, were used as an air cathode. identical p-bfcs have been developed and different experiments were carried out in triplicate – the first experimental setup consisted of p-bfcs operating across a 1 kω load resistor (a closed electrical circuit), the second as control samples (open circuit conditions). electrochemical measurements the voltage across the resistor was measured daily. the resistor was switched off and after 10 minutes of stabilization the open-circuit voltage (ocv), and the anodic potential were measured against ag/agcl (3 m kcl) reference electrode. the p-bfc was also characterized by polarization curves obtained by varying the external resistance from 100 kω to 10 ω by using a resistance decade box. the load resistances were changed through 2 units at each decade. the cell voltage, u, was recorded 5 min after switching a given resistance by using a digital multimeter dmm2700 (keithley instruments inc., us). the current was calculated according to the equation i / ma = u / r, where u/mv is the measured cell voltage, r / ω is the external resistance, and the current density according to j / ma m-2 = i / s, where s / m2 is the geometric area of the anode. the power density was calculated as p /mw m-2 = j u / 1000. the electrochemical activity of the exploited anolyte was explored by cyclic voltammetry (cv). the anolyte was filtered through a 0.2 μm microporous sterile filter. the cv was carried out in a three-electrode arrangement, consisting of a pt-wire as a working electrode, a platinized titanium mesh as a counter electrode, and ag/agcl as a reference electrode, with a scan rate of 10 mv/s. biochemical and enzymatic analyses fractionation 1 g of the used as a biocatalyst or control grown duckweeds was mechanically disintegrated for 10 min in 2 ml 67 mm phosphate buffer, ph 7.0. after centrifugation at 12,000 g for 10 minutes, the organelles (chloroplasts and mitochondria) have been harvested in the pellet fractions, re-suspended in 1 ml 67 mm phosphate buffer, ph 7.0, and used for biochemical and enzymatic analyses. these parameters have been also examined in the supernatant fractions and the anolyte/water samples. the electrolyte, respectively the buffer solution, is filtered through a sterile filter with a pore size of 0.25 μm by a syringe. proteins and sugars quantification the intracellular protein concentration was analyzed by the modified bradford method (merck) and estimated in mg ml-1 by using a bsa-calibration curve. the determination of reducing sugars (glucose equivalents) was carried out by using the dns method [10]. the iodine method of halick and keneaster was used for the determination of starch [11]. the method was modified as described in [5]. both glucose and starch concentrations in the samples were determined in mg ml-1 by using calibration curves. specific amylase activity the amylase activity was assayed by quantifying the reducing sugars (glucose equivalents) liberated from soluble starch using the method described by bernfeld [12]. one unit of enzyme y. hubenova et al. j. electrochem. sci. eng. 13(5) (2023) 795-804 http://dx.doi.org/10.5599/jese.1547 799 activity (u) is defined as the amount of enzyme required to release 1 μmol of glucose from soluble starch per minute. the determined glucose amount in the pellet fraction was subtracted from that estimated after the enzyme assay. the results are calculated and presented as a specific amylase activity (u g-1 wet biomass). quantification of inorganic phosphates the molybdenum blue method [13] was used for the quantitative determination of inorganic phosphates. the concentration is estimated as mm by using a calibration curve after subtracting the amount of the inorganic phosphate in the buffer. specific phytase activity phytase, myo-inositol hexakisphosphate phosphohydrolase (ec. 3.1.3.8), catalyzes the hydrolysis of phytic acid to inositol polyphosphates and free orthophosphoric acid. the reaction (400 l volume) was carried out by using 0.2 mm na-phytate in 0.1 m acetate buffer, ph 5.5, at 37 c for 60 min. the reaction was stopped by adding two parts of freshly prepared acetone: 5 n sulfuric acid: 10 mm ammonium molybdate in proportion 2:1:1. after mixing, 40 μl of 1.0 m citric acid was added to each tube. the orthophosphates released from phytate were determined by engelen's method [14]. the phytase activity is defined as 1 μmol inorganic orthophosphate released from the phytate per minute and presented as a specific phytase activity (u g-1 wet biomass). each biochemical or enzymatic parameter has been analyzed in triplicate and the results are presented as a mean value ± a standard deviation. results and discussion electrical and electrochemical properties of w. globosa-based p-bfc. at the beginning of the experiment, the open-circuit voltage (ocv) of w. globosa-based p-bfc was ca. 330 mv but after about six days of polarization the ocv increased and reached steady-state values of 615±15 mv over the rest of the period (figure 2a). as far as the cathodic potential was relatively constant (ca. +250 mv vs. ag/agcl) over time, the decreasing values of the anodic potential up to −400 mv vs. ag/agcl confirmed the role of this parameter and the contribution of the biological component to the current generation processes (figure 2b). 0 2 4 6 8 10 12 300 400 500 600 700 o c v / m v t / day a 0 2 4 6 8 10 12 -400 -300 -200 -100 0 e a n / m v v s . a g /a g c l t / day b figure 2. changes of (a) the open-circuit voltage, (b) the anodic potential of w. globosa p-bfc over time after stabilization of the ocv, the p-bfc was analyzed by polarization and power curves. the results, shown in figure 3a, reveal that the developed bioelectrochemical system can generate a current reaching density of 1 a m-2 and a maximal power density of ca. 90 mw m-2. the current o c v , m v e a n / m v v s. a g /a g c l http://dx.doi.org/10.5599/jese.1547 j. electrochem. sci. eng. 13(5) (2023) 795-804 wolffia globosa as a biocatalyst in biofuel cells 800 density was close to that previously established for lemna valdiviana based p-bfc [6]. the generated over time current by w. globosa p-bfc at a continuously switched external resistor in the circuit increased much faster than the ocv. after two days of operation, steady-state current values in the range of 170±10 ma m-2 were achieved (figure 3b). although the achieved current was lower than that predicted by the polarization and power curves, the estimated current densities were comparable with the current densities generated by the duckweed l. minuta, which has been grown in p-bfc under artificial illumination [5]. 0 0.2 0.4 0.6 0.8 1 0 100 200 300 400 500 600 700 0 20 40 60 80 100 u / m v j / a m -2 a p / m w m -2 0 2 4 6 8 10 12 0 50 100 150 200 j / m a m -2 t / day b figure 3. (a) polarization (u as a function of j) and power (p as a function of j) curves of w. globosa p-bfc on the 6th day of operation, (b) current density recorded during p-bfc operation across an external load of 1 kω till now, the technology of the p-bfc includes the utilization of duckweed species, spread onto the carbon felt surface, and being in permanent contact with the anode by their roots while swimming on the water surface. one of the main differences between the duckweeds of the genus lemna and that of wolffia is that the latter are rootless. to check the possible mechanism, by which this aquatic plant transfers electrons extracellularly to the anode, additional cyclic voltammetry (cv) of the spent anolyte was carried out. the cv results (figure 4) suggest that the grown in the biofuel cell w. globosa secretes an electrochemically active compound in the anolyte, which could contribute to the generated current acting as an endogenous electron transfer mediator (enm), shuttling electrons between the plants and the anode. this mediated electron transfer mechanism is in good agreement with previously proven enms for both aquatic plants [5,6] and yeast cells [15,16]. however, so far only a few enms have been identified from bacteria, which is the reason why they are collectively called "endogenous mediators" [17]. while shewanella oneidensis secretes flavin molecules, pseudomonas aeruginosa self-produces nitrogen-containing heterocyclic phenazine metabolites such as pyocyanin [18]. phenazines are derived from the shikimic acid pathway that is highly conserved in most organisms including plants [19]. chorismic acid serves as the phenazine branch point once the phenazine biosynthetic genes are expressed. the polarization conditions and the results obtained suggest that secondary metabolites are included in the duckweed response to polarization. the duckweed is capable of expressing phenolic compounds and flavonoids to maintain the redox state of the plant. comparing the cv of wolffia globosa with the cvs of other duckweed species (table 1), it is seen that the formal potential e`0 values of −260 mv vs. ag/agcl (−55 mv vs. she) are close to that of l. valdiviana and slightly shifted from that of l. minuta. the potential of the oxidation peak reaction was recorded at −114 mv vs. ag/agcl, corresponding to ca. 90 mv vs. she, while that of the reduction peak reaction was −404 vs. ag/agcl (ca. -200 mv vs. she). although the nature of the enm in duckweeds needs further clarification, the established redox potentials overlap that of the 2-cyanophenazine [20] suggesting that it could be y. hubenova et al. j. electrochem. sci. eng. 13(5) (2023) 795-804 http://dx.doi.org/10.5599/jese.1547 801 a phenazine derivate. the duckweeds grown normally as a control do not express redox activity (figure 4-inset), indicating the redox substance is produced only under polarization conditions. -1.2 -0.8 -0.4 0 0.4 -8 -6 -4 -2 0 2 4 -0,4 -0,2 0,0 0,2 0,4 0,6 -3 -2 -1 0 1 2 i / m a e / v (vs. ag/agcl) i / m a e / v vs. ag/agcl e / mv vs. ag/agcl figure 4. cyclic voltammogram of the w. globosa p-bfc –anolyte (phosphate buffer, ph 7.0) on the 12th day of operation. inset negative control toward polarization. scan rate: 10 mv s-1, the second scan in conclusion, w. globosa-based fuel cell gives high and relatively stable electrical outputs over time, which reveals that the smallest flowering plants w. globosa are promising biocatalysts for further development of the p-bfc biotechnology. although rootless, they are secreting redox-active compounds (enm), which shuttle electrons to the anode. influence of the polarization on the w. globosa biochemical parameters the extracellular electron transfer from living organisms to the anode is associated with metabolic changes caused by the applied polarization [6,15]. to examine how w. globosa responds to the polarization, the explored in biofuel cells duckweeds and those grown as a control have been collected and partially fractionated for receiving an organelle-enriched fraction. the basic biochemical parameters such as the intracellular content of proteins, glucose, starch, and inorganic phosphate have been studied. the enzymes catalyzing the degradation of the reserve carbohydrates and phosphates (in form of phytates) the amylase and phytase, respectively, have been analyzed as well. the results from these analyses are summarized in table 2. besides, these parameters have been also inspected in the supernatants obtained by centrifugation as well as in samples taken from the liquid media (anolyte and water) and presented as suppl. table s-1. table 2. the basic biochemical parameters and the specific activities of the enzymes responsible for the degradation of carbohydrates and phosphorus reserves in the enriched with chloroplasts and mitochondria fractions obtained from w. globosa grown under normal (control) and polarization conditions (p-bfc) organelles enriched fraction content, mg g-1 wet biomass activity, u g-1 wet biomass cpo43/ mm protein glucose starch amylase phytase w. globosa p-bfc 495.0±10.3 15.5±3.1 125.1±8.7 70.0±2.8 4.3±0.2 160.8±30 w. globosa control 392.5±20.2 7.8±2.0 73.5±3.5 38.5±1.9 2.8±0.1 22.4±3.3 about a 25 % increase in the total protein content of the organelle-enriched fraction has been registered under polarization conditions, while the protein content of the supernatant was twice lower than that of the control, indicating that the polarization influences processes implemented in the chloroplasts and mitochondria of the duckweeds. indeed, a much more sensitive change is e / mv vs. ag/agcl i / m a http://dx.doi.org/10.5599/jese.1547 j. electrochem. sci. eng. 13(5) (2023) 795-804 wolffia globosa as a biocatalyst in biofuel cells 802 established for the carbohydrate content – the twice higher quantity of reducing sugars and 1.7 times higher reserve carbohydrates in the form of starch of the same fraction. having in mind that the synthesis of glucose in plants takes place in the calvin cycle in the chloroplasts, it can be assumed that polarization affects the photosynthetic processes of the duckweeds. at the same time, peroxisomal photorespiratory enzymes have been reported to protect against stress [21]. for example serine: glyoxylate aminotransferase can contribute to the polarization stress response [21,22]. negligible amounts of starch and glucose were registered in the anolyte/water (suppl. table s 1), indicating that the plants are intact, the cellular membranes are impermeable, and do not allow the loss of valuable nutrients. at the same time, the specific amylase activity was 1,82 times higher than that of the control, indicating that not only more starch is produced under polarization, but also more reserve carbohydrates were hydrolyzed to cover the plant's energy demands. the results show that both catabolic and anabolic processes are enhanced in w. globosa grown as a biocatalyst under polarization. the higher content of inorganic phosphates as well as enhanced phytase activity supports the hypothesis about intensified metabolic processes. from the initial 67 mm phosphates, in which the control plants grew, one-third was established in the water (suppl. table s-1), showing that ca. 40 mm phosphates were sufficient to supply the plants with the required amount of phosphorus. 20 mm phosphates were determined in the organelle fraction of the control, while these values were extremely higher in the respective fraction of the plants grown under polarization. having in mind that the duckweeds contain inositol phosphates (phytates) as a reserve form for storing phosphorus [15,23] and to check whether the change in the number of inorganic phosphates in the duckweeds grown during polarization is associated with additional degradation of inositol phosphates, the phytase activity of the samples was also determined. the results show a 53 % increase in the specific phytase activity, suggesting, on the one hand, an explanation for the higher phosphate content, and on the other hand, that the phytate is synthesized and hydrolyzed simultaneously to a greater extent under polarization. the results for phytate degradation suggest that duckweeds can be used not only as food/feed resources and pharmaceuticals but as well as for phytoremediation [24]. conclusions wolffia globose duckweed can be grown in biofuel cells and is capable of generating an electrical current whose value is comparable with the current achieved by duckweed species belonging to the lemnaceae family. the electrons are transferred to the anode by secretion of an electrochemically active substance acting as an endogenous mediator of electron transfer. in addition, intensified metabolism in aquatic plants used as biocatalysts in plant-bfc is observed at polarization, expressed in increased photosynthetic properties, phosphorylation, and dephosphorylation processes. the increased content of proteins in 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the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1007/s10529-014-1571-9 https://doi.org/10.1016/j.bioelechem.2015.04.001 https://doi.org/10.1002/celc.202200918 https://doi.org/10.1007/978-981-10-7431-8_11 https://www.bing.com/ck/a?!&&p=34d0383764b05686jmltdhm9mty3mdyzmdqwmczpz3vpzd0xotvjyjm2ys00mtuwltywntqtmjrknc1hmta0ndbmnjyxotamaw5zawq9ntixma&ptn=3&hsh=3&fclid=195cb36a-4150-6054-24d4-a10440f66190&psq=appl+microbiol+biotechnol&u=a1ahr0chm6ly93d3cuc3byaw5nzxiuy29tl2pvdxjuywwvmjuzl3n1ym1pc3npb24tz3vpzgvsaw5lcw&ntb=1 https://www.bing.com/ck/a?!&&p=34d0383764b05686jmltdhm9mty3mdyzmdqwmczpz3vpzd0xotvjyjm2ys00mtuwltywntqtmjrknc1hmta0ndbmnjyxotamaw5zawq9ntixma&ptn=3&hsh=3&fclid=195cb36a-4150-6054-24d4-a10440f66190&psq=appl+microbiol+biotechnol&u=a1ahr0chm6ly93d3cuc3byaw5nzxiuy29tl2pvdxjuywwvmjuzl3n1ym1pc3npb24tz3vpzgvsaw5lcw&ntb=1 https://doi.org/10.1007/s00253-010-2509-3 https://doi.org/10.1007/s00253-010-2509-3 https://www.bing.com/ck/a?!&&p=3c197ce0959fbd23jmltdhm9mty3mdyzmdqwmczpz3vpzd0xotvjyjm2ys00mtuwltywntqtmjrknc1hmta0ndbmnjyxotamaw5zawq9nte5mw&ptn=3&hsh=3&fclid=195cb36a-4150-6054-24d4-a10440f66190&psq=j.+phys.+energy&u=a1ahr0chm6ly9wdwjsaxnoaw5nc3vwcg9ydc5pb3bzy2llbmnllmlvcc5vcmcvam91cm5hbhmvam91cm5hbc1vzi1wahlzawnzlwvuzxjnes9hym91dc1qb3vybmfslxboexnpy3mtzw5lcmd5lw&ntb=1 https://doi.org/10.1088/2515-7655/abebc8 https://doi.org/10.1007/s11240-012-0280-0 https://doi.org/10.1016/j.bioelechem.2017.03.003 https://doi.org/10.1002/yea.3027 https://doi.org/10.1186/s13765-021-00644-z https://creativecommons.org/licenses/by/4.0/) {benchmarking of electrolyte mass transport in next generation lithium batteries} doi:10.5599/jese.408 213 j. electrochem. sci. eng. 7(4) (2017) 213-221; doi: http://dx.doi.org/10.5599/jese.408 open access : : issn 1847-9286 www.jese-online.org original scientific paper benchmarking of electrolyte mass transport in next generation lithium batteries jonas lindberg, henrik lundgren, göran lindbergh, mårten behm applied electrochemistry, department of chemical engineering and technology, school of chemical science and engineering, kth royal institute of technology, se-100 44 stockholm, sweden corresponding authors e-mail: jonasl3@kth.se; tel.: +46-8-7908171 received: july 14, 2017; revised: august 15, 2017; accepted: august 15, 2017 abstract beyond conductivity and viscosity, little is often known about the mass transport properties of next generation lithium battery electrolytes, thus, making performance estimation uncertain when concentration gradients are present, as conductivity only describes performance in the absence of these gradients. this study experimentally measured the diffusion resistivity, originating from voltage loss due to a concentration gradient, together with the ohmic resistivity, obtained from ionic conductivity measurements, hence, evaluating electrolytes both with and without the presence of concentration gradients. under galvanostatic conditions, the concentration gradients, of all electrolytes examined, developed quickly and the diffusion resistivity rapidly dominated the ohmic resistivity. the electrolytes investigated consisted of lithium salt in: room temperature ionic liquids (rtil), rtil mixed organic carbonates, dimethyl sulfoxide (dmso), and a conventional li-ion battery electrolyte. at steady state the rtil electrolytes displayed a diffusion resistivity ~ 20 times greater than the ohmic resistivity. the dmsobased electrolyte showed mass transport properties similar to the conventional li-ion battery electrolyte. in conclusion, the results presented in this study show that the diffusion polarization must be considered in applications where high energy and power density are desired. keywords li-ion battery; li-o2 battery; room temperature ionic liquid; diffusion resistivity; electrolyte mass transport resistivity http://dx.doi.org/10.5599/jese.408 http://www.jese-online.org/ mailto:jonasl3@kth.se j. electrochem. sci. eng. 7(4) (2017) 213-221 electrolyte mass transport in lithium batteries 214 introduction lithium-ion batteries are found in nearly all portable electronics, and in technologies still to reach a wider commercial breakthrough such as hybrid, plug-in hybrid, and electric vehicles [1, 2]. in order to reach that breakthrough, batteries that can supply higher power densities are needed. the maximum power density a battery can deliver is limited by its electrolyte mass transport properties that describe the voltage losses associated with ion transport. these voltage losses (polarizations) cause the battery cell voltage to deviate from its equilibrium, i.e. the observed cell voltage is lower than the theoretical value during discharge and higher at charging, resulting in lowered energy efficiency, energy density, and power density of the battery. instantly, when a current is applied to the battery, a polarization directly proportional to the current appears, it follows ohm’s law and is thus referred to as ohmic polarization. in the electrolyte, the ohmic polarization is characterized by the ionic conductivity, κ, that measures the conductance in an electrolyte when no concentration gradients are present. gradually, as current passes, concentration gradients develop in the battery due to the diffusion and migration of the ions (convection, being the third way of ion transport, can be neglected in a battery using a porous separator). migration is the transport of ions in an electric field. it is described by the transport number, t +  and t −  for a cation and anion, respectively, describing the relative amount of current carried by each ion. since cations and anions will have opposing directions of migration, there will be a resistance to mass transport, as electroneutrality cannot be violated, resulting in a polarization. diffusion is the transport of species against a concentration gradient. it is characterized by the diffusion coefficient, d̃. as li-ion and post li-ion batteries keep developing there is an increase in the number of electrolytes used. the conventional li-ion battery electrolyte is typically composed of a li salt, such as lithium bis(trifluoromethulsulfonyl)imide (li-tfsi) or lithium hexafluorophosphate (li-pf6) , and an organic carbonate solvent mixture of for example ethylene carbonate (ec), propylene carbonate (pc), or diethyl carbonate (dec). these electrolytes are thermodynamically unstable against the negative electrodes and rely on their ability to form stable passivating films (solid electrolyte interfaces) in order to be used [3]. electrolytes based on room temperature ionic liquids (rtil) have a wide electrochemical stability window making them stable against a larger number of electrodes compared with conventional li-ion electrolytes [4]. rtils are also praised for their high conductivity and safety [5]. these properties also make rtil-based electrolytes possible candidates for lithiumoxygen (li-o2) batteries [6]. more frequently, electrolytes based on dimethyl sulfoxide (dmso) have been used in li-o2 batteries as conventional li-ion electrolytes have been proven unstable in the presence of o2 [7]. the distribution of the total polarization of a battery during a hybrid pulse power characterization test on a conventional li-ion battery with an organic carbonate electrolyte has been investigated [8]. it was found that the electrolyte polarization due to concentration gradients and the electrolyte ohmic polarization contributed to 15 % and 28 % of the total polarization, respectively. this means that 43 % of the total polarization was directly attributed to the electrolyte (remaining sources of polarization was attributed to contract resistance, diffusion in solid phase, ohmic resistance in solid phase, and activation overpotential). furthermore, when enough time had passed for the concentration gradient in the electrolyte to reach a steady state its contribution to the polarization was larger than that of the ohmic polarization [9]. to fully describe the time-dependent mass transport, and its associated polarizations, in a non-dilute and non-ideal electrolyte a model based on concentrated electrolyte theory needs to be set up, and the parameters therein, such as diffusion j. lindberg et al. j. electrochem. sci. eng. 7(4) (2017) 213-221 doi:10.5599/jese.408 215 coefficients, transport numbers, and thermodynamic factors need to be determined, in addition to the conductivity. the full set of those mass transport parameters has been determined for some electrolytes [9-14], but since the methods for such comprehensive characterizations are still quite complex and time consuming there is also a need for quick benchmarking methods that yield relevant performance metrics. little is known about the mass transport in dmso-based electrolytes. for rtils the most common electrolyte performance parameter found in the literature today is conductivity [15], although transport numbers have been determined [16], as well as self-diffusion coefficients obtained by nmr [17]. a problem with using parameters such as the above-mentioned ones for benchmarking is that they are intimately linked to the model that was used to define them. one consequence of this is that it makes it difficult to compare parameters for different types of electrolytes, for example a binary liquid electrolyte, a polymer gel electrolyte and an rtil. a further problem is that the parameters do not give direct information about the magnitude of electrolyte polarization for a certain current density, i.e. the translation of diffusion coefficients and transport numbers into overpotential is not straightforward. a way of benchmarking electrolytes that meet these requirements was suggested by nyman et al. [9] by introducing the concept of electrolyte mass transport resistivity (emtr)1  aim the purpose of this study is twofold: (i) to demonstrate the usefulness of electrolyte mass transport resistivity (emtr) as a method for benchmarking electrolyte mass transport limitations and (ii) to highlight the essential lithium battery electrolyte properties by a direct comparison of a set of candidate electrolytes for the next generation of li batteries. this is done by measuring the emtr of electrolytes based on rtils, organic carbonates, a mixture of rtil and organic carbonates, and dmso. little is known about the diffusive mass transport in rtil and dmso based electrolytes beyond conductivity, thus, making the use of conductivity as the sole figure of merit potentially misleading. therefore, the use of a method including concentration gradient polarization is well motivated. for all electrolytes, the time dependence of the electrolyte polarization was also examined. theory electrolyte mass transport resistivity electrolyte mass transport resistivity (emtr) was initially used as a figure of merit, to quantify polarizations with and without concentration gradients, for interpretation of results from full mass transport characterizations based on concentrated electrolyte theory [9,13,18]. it has been experimentally used alongside other techniques to evaluate the influence of flame retardants in liion battery electrolytes [19]. the definition of the emtr is shown in eq. 1, where it is expressed in terms of the parameters in the concentrated electrolyte model [9]. however, it was also suggested that it could be determined experimentally in a relatively simple way, by rewriting eq. 1 in the form of eq. 2, where the variables only depend on the experimentally measurable quantities conductivity and diffusion potential at steady state.     2 11 2 2 solvent solvent totsalt t/ x crt i cf c         (1) 1 in nyman et al. it was called normalized potential gradient. j. electrochem. sci. eng. 7(4) (2017) 213-221 electrolyte mass transport in lithium batteries 216   2 1 1s.s salt δ diff ,s .s . / x i if c          (2) in these equations  is the electrolyte potential2 , 𝑖 the current density, κ the conductivity, t +  the cationic transport number with respect to the solvent and d̃ the diffusion coefficient with respect to the thermodynamic driving force. diff,s.s. is the potential drop caused by concentration gradients at steady state in a cell of length l. the influence of the porosity and tortuosity of the separator is taken into account by multiplying with the porosity, ϵ, raised to the power of the bruggeman constant 𝛽. the first term on the right hand side of eq. 2 is the ohmic resistivity, the second term is called the diffusion resistivity3. note that when eq. 1 is rewritten as eq. 2 there is no longer a model dependence and eq. 2 can therefore be used to describe any electrolyte. further, the method is not limited to dilute and/or binary electrolytes. the diffusion polarization can be obtained in a relatively simple galvanostatic polarization experiment, and the conductivity is easily obtained by standard methods. by measuring both ohmic and diffusion resistivities the relative contribution of these two sources to the full steady-state electrolyte polarization is obtained. another way of understanding the emtr is that multiplied with a given current density it represents the corresponding electrolyte polarization in the electrolyte at steady state. experimental materials the salt used was li-bis(trifluoromethanesulfonyl)imide (li-tfsi, sigma-aldrich 99.95 % purity). the solvents used were dimethyl sulfoxide (dmso, sigmaaldrich, anhydrous), n-ethyl-n-methylimidazoleum-tfsi (emi-tfsi, solvionic, 99.9 % purity), n-propyl-n-methyl-pyrrolidinium-tfsi (pyr13-tfsi, solvionic, 99.9 % purity) and n-propyl-n-methyl-piperidinium-tfsi (pip13-tfsi, solvionic, 99.9 % purity). the organic carbonate electrolyte consisted of li-tfsi salt in ethylene carbonate:diethyl carbonate (ec:dec) with a 1:1 weight ratio (novolyte, battery grade). all chemicals were used as received. all rtil electrolytes contained 1.0 m li-tfsi, for the dmso and ec:dec electrolytes the li-tfsi content was also varied. the experiments were conducted at 25° c in an argon-filled glove box. ohmic resistivity experiment the conductivities were measured using a consort k912 conductometer with a sk21t microelectrode probe. the conductometer was calibrated outside of the glove box using potassium chloride standard solutions. the ohmic resistivity was then calculated by using the first term on the right-hand side of eq. 2. diffusion resistivity experiment in order to determine the diffusion resistivity a symmetrical test cell with two li-metal electrodes was assembled. to ensure a constant inter-electrode distance the electrodes were separated by a teflon spacer-ring with a thickness of 500 µm and an inner diameter of 6 mm. the electrolyte was 2 the electrolyte potential is defined and measured with a metal reference electrode of the same kind as the cation. the emtr is therefore also defined for that species. 3 note that this term describes both migration and diffusion, it contains both 𝑡+ and �̃�, and both these modes of transport will give rise to concentration gradients. the term “diffusion resistivity” is simply chosen to agree with previously used terminology. j. lindberg et al. j. electrochem. sci. eng. 7(4) (2017) 213-221 doi:10.5599/jese.408 217 soaked into a whatman gf/a glass micro fiber filter, placed inside the spacer-ring. the filter has a porosity of 0.9, and the bruggeman coefficient has previously been determined to 3.44 [9]. the diffusion potential (diff in eq. 2) was measured in the test cell directly after a galvanostatic polarization. a schematic of the experimental raw data is seen in figure 1. the evolution of the diffusion potential with polarization time could be followed by measuring the potential during current interrupts which were very short in relation to the time scale of the total polarization. typically, a galvanostatic current was applied for several minutes followed by a two seconds interrupt. when the measured diffusion potential no longer showed any change with polarization time the electrolyte concentration gradients was assumed to be at steady state. the steady state diffusion potential was inserted into the second term on the right-hand side of equation 2 in order to calculate the diffusion resistivity. the magnitude of the polarization current was chosen so that the potential directly after current switch-off was between 5 and 50 mv. the lower potential limit was chosen to give low noise contribution, and the upper limit was set to avoid dendrite formation. each electrolyte composition was tested in at least four test cells and each cell was polarized with at least four different currents giving potential drops at steady state spanning over 5 to 50 mv. figure 1. schematic of the galvanostatic polarization experiment raw data. at the start of the experiment a constant current is applied. when steady state is reached the current is turned off, the potential remaining at this point is caused by the concentration gradients in the electrolyte and corresponds to the diffusion potential, .diff this value is used in equation 2 in order to calculate the diffusion resistivity the measurements turned out to be sensitive to the surface of the li foil, giving less spread in results when pristine li foil was used. due to thermodynamic instability between the electrolyte and the li-metal electrodes, there might be mixed potential between the solvent reduction and the lithium oxidation, thus affecting the measured potential. since the mixed potential would be solvent dependent its influence would differ from solvent to solvent. however, if the concentration gradients was allowed to fully relax after the galvanostatic polarization the potential difference between the two li electrodes were in the order of µv for all electrolytes in this study, thus, indicating that the there was no significant influence of a mixed potential in this study. results and discussion by measuring the diffusion polarization at steady state, diff in a galvanostatic current-interrupt experiment and the conductivity, 𝜅,  using a conductivity meter, the diffusion and ohmic resistivity j. electrochem. sci. eng. 7(4) (2017) 213-221 electrolyte mass transport in lithium batteries 218 was calculated for each electrolyte according to equation 2. the calculated resistivities for the electrolytes examined at concentrations of 1 m li-tfsi are seen in table 1. table 1. the mean ohmic and diffusion resistivity of the various electrolytes tested containing 1 m li-tfsi. the diffusion resistivities are reported with their standard deviations that show that there was a variation of the measured diffusion resistivity between cells (variation between consecutive experiments in the same cell was smaller) ohmic resistivity,  m diffusion resistivity,  m ec:dec 1.5 3.0±0.8 dmso 1.0 4.6±1.6 emi-tfsi 3 91±9 pyr13-tfsi 10 220±10 pip13-tfsi 31 590±50 all electrolytes show a clear difference in the magnitude between ohmic and diffusion resistivity with diffusion resistivity being larger in all cases. specifically, in the electrolytes based on rtils the total polarization was dominated by the diffusion resistivity, whereas for ec:dec and dmso both ohmic and diffusion resistivity contributed more equally. for example, comparing emi-tfsi with ec:dec the ohmic resistivity was twice as high and the diffusion resistivity was  ≈30 times as high for emi-tfsi, thus stressing the importance of evaluating the diffusion resistivity alongside with ohmic resistivity. organic solvents and rtils are miscible; it is therefore possible to create rtilorganic mixed electrolytes that combine the rtil safety features with the lower viscosity of carbonate solvents. figure 2 displays the two right-hand side terms of eq. 2 separately for electrolytes consisting of 1 m li-tfsi with varying amount of emi-tfsi and ec:dec in the solvent. both the ohmic and the diffusion resistivity increased with emi-tfsi content, with the diffusion resistivity being far greater in magnitude at high emi-tfsi content. electrolytes based on ec:dec and dmso showed similar ohmic and diffusion resistivity. this shows that dmso is a suitable electrolyte for li-o2 batteries from a mass transport perspective, however, how suitable dmso is regarding other factors is still debatable. both ec:dec and dmso electrolytes were tested at several li-tfsi concentrations, see figure 3. the lowest emtr was observed at concentrations above 0.5 m li-tfsi. lower concentrations resulted in higher ohmic and diffusion resistivities. the spread in measurements was also larger at lower concentration. a resistivity vs. concentration graph was produced by nyman et al. from a full characterization of lipf6 in 3:7 ec:emc [9] displaying similar results, although the diffusion resistivity is higher in the present study, which might be due to a larger anion inhibiting mass transport. this confirms that the method applied here provides results equivalent to those obtained from complex full characterizations based on physical models. a reason that the rtils showed large emtr may be found by looking at the constituents of the rtil electrolytes. in the rtils in this study there are three ionic species present: li+, tfsi and a bulky cation (emi, pyr13 or pip13). for 1 m li-tfsi in emi-tfsi the coordination number is a little bit lower than two [20] suggesting that the predominant coordination complex is [li(tfsi)2]-. this would change the effective charge of the current-carrying specie from positive to negative i.e. changing the migration direction of li, thus making diffusion the only means of transport carrying li to the cathode. as the rtil content increases in a mixed electrolyte, so does the viscosity [5]. j. lindberg et al. j. electrochem. sci. eng. 7(4) (2017) 213-221 doi:10.5599/jese.408 219 figure 2. the effect on ohmic (□) and diffusion (∘) resistivity when emi-tfsi was added to ec:dec are shown in (a) and (b), respectively. all electrolytes contained 1 m li-tfsi figure 3. the diffusion (∘) and ohmic (□) resistivity at different litfsi concentrations in (a) ec:dec and (b) dmso measured at steady-state. the error bars show standard deviations however, explaining the variation in mass transport properties in terms of change in viscosity is not suitable since viscosity should affect the ohmic and diffusion resistivities equally and the results do not show this (see figure 2). an alternative explanation could be found if looking at the system in a molecular perspective. when dec is the only solvent present the li ion coordinates to dec in a [li(dec)3]+ complex [21]. in a mixed ec:emi-tfsi solvent the predominant solvation complex of li+ is [li(ec)4]+ down to a li:ec molar ratio of 1:4 [22,23]. at an emi-tfsi content higher than 50 vol.% in ec:dec there is not enough ec and dec to fully solvate all li ions. when the emi-tfsi content is increased there is a steep rise in diffusion resistivity suggesting that the solvation of the li ions is of key importance, see figure 2. rtils have promising safety features and addition of rtils to organic solvents increases safety. an addition of 40 60 vol.% emi-tfsi to 1 m lipf6 in ec:dec has been suggested as an optimal composition with no flammability, high conductivity and low viscosity [5]. however, this study shows that at 50 vol.% emi-tfsi there is a fourfold increase in diffusion resistivity compared to when no rtil was present. the time needed for the diffusion polarization to reach a steady state depends on the interelectrode distance, electrolyte, and the polarization current. in the galvanostatic polarization j. electrochem. sci. eng. 7(4) (2017) 213-221 electrolyte mass transport in lithium batteries 220 experiment, currents varying by one order of magnitude was needed in order to get diffusion polarizations within the desired range of 5 to 50 mv. therefore, the time needed to reach steady state also varied by one order of magnitude with larger currents needing more time. the diffusion polarization for the electrolytes based on ec:dec and dmso typically reached steady state within 10 to 20 minutes for polarizations of 10 mv, see figure 4a. for the rtils this time was typically 4 to 20 hours, see figure 4b. keep in mind that the test cell had an electrode distance of 500 µm and that if the electrode distance were to be reduced one order of magnitude, to better agree with a real battery, then the time would be reduced by two orders of magnitude (according to fick’s second law). this would correspond to 2-12 min for the electrolytes based on the rtils and 6-12 seconds for those based on ec:dec and dmso. further, the time needed for the diffusion resistivity to equal the ohmic resistivity would be typically five seconds for the rtil based electrolytes and 1-2 seconds for the electrolytes based on ec:dec and dmso. this indicates that losses due to concentration gradients in a li battery must be considered in most usage scenarios where energy efficiency and power density is important. polarization time, h polarization time, h figure 4. (a) the diffusion potential (-) for 1 m litfsi in ec:dec shown as a function of time. the current normalized ohmic potential is shown as a time-independent horizontal line. (b) the current-normalized diffusion potentials of emi-tfsi (−, black), pyr13-tfsi (·, blue) and pip13-tfsi (·−, red). the current-normalized ohmic potentials are shown as time-independent horizontal lines, emi-tfsi (−, black), pyr13-tfsi (·, blue) and pip13-tfsi (·−, red). all electrolytes contained 1 m li-tfsi conclusions in this study, both ohmic and diffusion voltage losses, presented as resistivities, were measured in a quick and convenient way to examine the mass transport in next generation li-battery electrolytes. the results highlight the importance of quantifying the diffusion together with the ohmic polarization when trying to estimate the total electrolyte polarization. when a conventional li-ion battery electrolyte was examined the results showed good agreement with ohmic and diffusion resistivities from a full characterization based on a physical model, thus, validating the method used in this study. electrolytes based on dimethyl sulfoxide (dmso) showed mass transport resistivities similar to those of the conventional li-ion electrolyte, making the dmso based electrolytes suitable for use in li-o2 batteries from a mass transport perspective. when examining electrolytes based on room temperature ionic liquids (rtil) the mass transport resistivities were 25-130. the diffusion resistivity at steady-state was  ~ 20 times greater j. lindberg et al. j. electrochem. sci. eng. 7(4) (2017) 213-221 doi:10.5599/jese.408 221 than the ohmic resistivity for 1 m li-tfsi in the rtils tested. this ratio was only about two for conventional li-ion battery electrolytes. for all the electrolytes examined, the concentration gradients developed quickly enough for the diffusion resistivity to become larger than the ohmic resistivity in short time. when a mixture of rtil and organic carbonates was used as solvent the diffusion resistivity was larger than when pure organic carbonates were used. adding more than 50 vol.% rtil drastically increased the diffusion resistivity. as concluding remarks, it can be said that the method employed here, measuring both the ohmic and diffusion resistivity, offers insight to the mass transport of a group of compounds where previously little has been known beyond conductivity and viscosity. acknowledgements: financial support from the swedish foundation for strategic research (ssf) and the swedish electromobility centre is gratefully acknowledged. references [1] e. j. cairns, p. albertus, annual review of chemical and biomolecular engineering 1 (2010) 299-320 [2] i. a. fulton, renewable and sustainable energy reviews 68 (2017) 685-692 [3] p. verma, p. maire, p. novák, electrochimica acta 55 (2010) 6332-6341 [4] m. galinski, a. lewandowski, i. stepniak, electrochimica acta 51 (2006) 5567-5580 [5] a. guerfi, m. dontigny, p. charest, m. petitclerc, m. lagace, a. vijh, k. zaghib, journal of power sources 195 (2010) 845-852 [6] c. j. allen, s mukerjee, e. j. plichta, m. a. hendrickson, k. m. abraham, journal of physical 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110-112 ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) understanding cold spray technology for hydroxyapatite deposition http://dx.doi.org/10.5599/jese.1636 193 j. electrochem. sci. eng. 13(1) (2023) 193-214; http://dx.doi.org/10.5599/jese.1636 open access : : issn 1847-9286 www.jese-online.org review paper corrosion cracking in mg alloys based bioimplants jatinder pal singh1, and yogita sharma2 1school of mechanical engineering, lovely professional university, phagwara 144411, india 2department of mechanical engineering, shaheed bhagat singh state technical campus, ferozepur 152004, india corresponding author: jatinderpal.25000@lpu.co.in received: december 10, 2022; accepted: january 28, 2023; published: february 19, 2023 abstract recently, magnesium alloys have garnered a lot of interest as a potentially useful material for applications involving biodegradable implants. cracking or fracture of metal-based implants under the combined action of corrosion and mechanical stresses, namely stress corrosion cracking (scc) is an obviously critical criterion before any new material might be deployed as implants. cracking or fracture of metal-based implants occurs under the simultaneous action of corrosion and mechanical stresses. this article gives a review of the existing literature on the scc of magnesium alloys in corrosive environments, including simulated body fluid and the accompanying fracture process. it also indicates the knowledge gap that exists in this area of research. in addition, a high-level review of the preventative measures that may be taken to avoid potential corrosion fatigue failures in magnesium alloys is provided. keywords biomaterials; coatings; biocompatibility; implants; magnesium implants introduction it is one of the most difficult tasks of our day to find solutions to the health problems faced by an ageing population. the usage of magnesium alloys to implant devices is emerging as an innovative method. the temporary implants are made out of classic materials that are still regularly used, such as stainless steel, ti-alloys, co-alloys, and other similar materials, it is usual practice to remove them by the use of a surgical procedure. this second operation results in more stress for patients and additional expenses, in addition to the potential for consequences such as patient morbidity and infection. mg boasts finest biocompatibilities with human body, making it one of the most desirable materials for use in metallurgical engineering. additionally, magnesium alloys have the highest level of mechanical agreement with bones. therefore, the compatibility of the alloys based on magnesium with human implants draws attention of researchers [1,2]. to be suitable for this application, the alloys must be resistant to cracking or fracture when subjected to both mechanical http://dx.doi.org/10.5599/jese.1636 http://dx.doi.org/10.5599/jese.1636 http://www.jese-online.org/ mailto:jatinderpal.25000@lpu.co.in j. electrochem. sci. eng. 13(1) (2023) 193-214 corrosion cracking in mg alloys based bioimplants 194 stresses as well as corrosive fluid. magnesium alloys as a result of loading in the human body is an important study area, such as: • when using implants made of conventional materials, one of the primary concerns [3] always involves the possibility of breakage caused by human bodily fluids. this kind of fracture of magnesium alloys used as bio-implants is a study field that is severely underexplored [4,5]. • recent research [6-9] has shown that simulated-body-fluid aided stress corrosion cracking (scc) may occur in magnesium alloys. • in-vivo testing of a few different magnesium alloys designed specifically for use as body implants [10-12] revealed that these particular alloys had the unique property of harmlessly melting away as shattered bone heals (figure 1). however, the resistance of alloys to human body fluid-assisted fracture is still a key problem, and it is a study area that has been woefully underexplored. the main aim of this article is to facilitate a specific study of corrosion fatigue (cf) and fracture processes of magnesium alloys when exposed to corrosive. figure 1. after six months of implantation, 2d histology slides of mg-10gd, mg-4y-3re, and mg-2ag that were stained with toluidine blue were examined. for every kind of screw, there is visible residual metal indicated by the red arrows in the black regions [18]. permissions under cc ny 4.0 international attribution mg alloys as bio-degradable orthopedic implants bio-degradable orthopedic implants are medical devices that are designed to be absorbed by the body over time. they are typically made from polymers that are designed to break down into smaller particles and be eliminated from the body. these implants are used for a variety of orthopedic applications, such as joint replacement, fracture repair, and tendon repair. they offer several advantages over traditional implants, such as reduced inflammation and scarring, faster recovery time, and reduced risk of infection. bio-degradable implants also reduce the need for revision surgery, as the implant j. p. singh and y. sharma j. electrochem. sci. eng. 13(1) (2023) 193-214 http://dx.doi.org/10.5599/jese.1636 195 will eventually break down and be eliminated from the body. magnesium has lower elasticity modulus and specific density i.e. 42-45 gpa and 1.74-2.0 g/cm3 respectively, both material properties of mg resembles with human bones [13,14]. the use of magnesium alloys in the orthopedics implants rapidly increase the attention in research [13-15]. as a result, in contrast to the implant materials used in past, the mg based biodegradable products are non-toxic to human body. in point of fact, it has been reported that the mg2+ ions released due to degradation helps in the healing and growth of tissues [16]. however any excess mg2+ is safely eliminated through the urine [15]. additionally, the price of mg alloys is significantly lower than the price of traditional implant alloys. in spite of the fact that magnesium and its alloys possess a number of characteristics that are highly desirable in human implants; the use of these materials is extremely uncommon. magnesium orthopedic implants are a type of implantable device used in orthopedic surgery to treat a variety of bone-related conditions. these implants are made from magnesium alloys, which are lightweight and biocompatible materials. they are designed to be used in place of traditional metal implants, such as titanium or stainless steel, due to their superior mechanical properties. magnesium implants are often used to repair fractures or to replace missing or damaged bones. they can also be used to support artificial joints and to treat osteoporosis-related bone fractures. additionally, they are being investigated for use in drug delivery and regenerative medicine applications. the most significant barrier is low corrosive nature of magnesium alloys in the physiological environment, which has a ph range from 7.4 to 7.6 [17]. mg alloys may witness loss of their mechanical integrity in physiological environment because of body fluids in the degradation process. this might occur before the tissues have adequate time to restore. despite the fact that these limitations have reduced use of magnesium based alloys especially for the permanent implants. however, still for temporary implants mg alloys are used due to their desirable characteristics. therefore, it is possible to make use of non-toxic elements based mg alloys in the human body. as a result, it is of the utmost importance to evaluate this type of fracture in the more recent magnesium alloys that have been developed specifically for use in body implants. corrosion of mg alloys magnesium orthopedic implants are biodegradable and are becoming increasingly popular for orthopaedic and trauma surgeries. however, their use is limited due to the potential for corrosion. magnesium is highly reactive and can corrode in the body, releasing toxic ions into the surrounding tissue. this can lead to local inflammation, infection, and tissue damage. additionally, the corroded magnesium can accumulate in the body, leading to systemic toxicity. to prevent corrosion, magnesium implants should be coated with a protective layer such as titanium, hydroxyapatite, or polymers. additionally, the implants should be designed to minimize the contact between the magnesium and the body fluids. even in a medium that is only moderately corrosive, such as sbf, magnesium alloys corrode at an alarming rate. magnesium corrosion always involves the release of a substantial amount of hydrogen. hydrogen gas should be avoided in large quantities because it can cause subcutaneous gas bubbles to form, which can cause tissue layers to separate and potentially impede blood flow [19-21]. it has been revealed from the in-vivo and in-vitro analysis that due to hbf pitting is observed on the alloys mainly derived from magnesium [22]. the existence of mg6zn3ca2 particle has also been reported that an alloy that was developed specifically for temporary implant applications, known as zx50 [12]. this finding was made possible by the study of the alloy. when considering the use of magnesium alloy as a possible implant material, it is essential that the material decay slowly and uniformly rather than experiencing any localized deterioration. the http://dx.doi.org/10.5599/jese.1636 j. electrochem. sci. eng. 13(1) (2023) 193-214 corrosion cracking in mg alloys based bioimplants 196 pits results into the formation of brittle surface as atomic hydrogen enters into the mg matrix due to bare and film free surface [5]. a newly developed zx10 mg alloy has showed remarkable resistance to pitting in addition to exhibiting the desirable characteristics of gradual and homogenous deterioration. this alloy does not produce any hydrogen bubbles and degrades in the intended manner. the zn concentration was decreased, which resulted in the creation of the (mg, zn)2ca phase. this phase is less noble than the mg-matrix, which led to improved degrading performance [23]. stress corrosion cracking and corrosion fatigue of mg alloys stress corrosion cracking (scc) is a form of corrosion that occurs when a material is subjected to both a tensile stress and a corrosive environment. it is a form of localized corrosion that is especially hazardous because it can occur without any external signs of corrosion, such as pitting or discoloration. scc can occur in metals such as stainless steel, aluminum alloys, and copper alloys, and is often more rapid than general corrosion. it is usually caused by a combination of chemical and mechanical factors, such as a combination of high tensile stress and a corrosive environment. scc is often encountered in industries such as oil and gas, petrochemical, and aerospace, and can lead to catastrophic failures of components and structures if not addressed properly. implant devices are subjected to significant mechanical loadings when they are in the presence of hbf. as an example, a spine may be subjected to a load that is more than 3500 n, while a cardiovascular stent is continually subjected to cyclic loading as a result of the beating of the heart [5,7]. the presence of cf and scc is a real possibility because to the dynamic stress that occurs inside the human body, in addition to the corrosive physiological environment. failures of this kind often take place at stresses that are far lower than the design stresses for an environment that does not cause corrosion. to prevent scc in biomaterials, it is important to use corrosion resistant materials, properly clean and sterilize the material, use proper design considerations to reduce stresses on the material, and properly coat the material to protect it from corrosion. additionally, material selection and design should be tailored to the specific environment the material will be exposed to, as different environments may have different levels of corrosive factors. this is due to the ductile nature of the material due to which it undergoes elongation before fracture in corrosive environment. there have been multiple examples of failures due to fatigue in the human body [24-26], despite the fact that conventional implants have excellent fatigue strength. these unexpected breakdowns of an implant may have major repercussions, such as the arduous process of removing the defective device and the excruciating irritation or inflammation of the tissues around the implant [27]. when it comes to the use of metallic implants, this particular aspect of high cycle fatigue (hcf) is of the utmost importance. the fatigue strength of various implant materials is compared with that of native bone after 107 cycles in corrosive bodily fluid. in light of the fact that magnesium alloys have a lower resistance to fatigue in comparison to traditional implant materials it is absolutely necessary to carry out a comprehensive categorization of the fatigue behavior of mg alloys before implantation. cf and scc will consider as a serious for implants made of mg alloys due to the following reasons: (a) sharp shapes of temporary implant devices, and (b) pitting of mg alloys in chloride solutions [28] also in in hbf [5-9]. implant devices made of mg alloys will have a tendency to have sharp contours in point of fact, it has been documented that magnesium alloys are vulnerable to the occurrence of scc in environments containing chlorides [29]. even though research on the cf of magnesium alloys is very scarce, there is a respectable amount of published information on the scc of these alloys in chloride solutions [29]. the mechanical and j. p. singh and y. sharma j. electrochem. sci. eng. 13(1) (2023) 193-214 http://dx.doi.org/10.5599/jese.1636 197 fractographic analysis depicts that investigations have shown that magnesium alloys are typically vulnerable to stress corrosion cracking (scc). this is a pressing necessity (i.e. resistance to cf and scc). it's possible that rare earth elements (re) alloys with magnesium will be appealing. on the other hand, there is a paucity of information about the cf and scc of alloys that include rare earth elements when exposed to chloride [29]. in addition, there is just one paper on the topic of cf investigations conducted on biocompatible magnesium alloys in bodily fluid [30]. the fatigue limit of the natural bone, conventional implants materials and mg alloys in physiological environments at 107 cycles were noticed. the fatigure strength ranges between 15-35 mpa for bones, 70-180 mpa for mg alloys, 140-230 mpa for stainless steel, 250-280 mpa for co-cr alloys and 280-710 mpa for tialloys, respectively [31-33]. corrosion fatigue of mg alloys corrosion fatigue is a type of fatigue failure in which a material suffers from both corrosion and fatigue. it occurs when repeated cyclic loading of a material leads to a combination of fatigue (mechanical) and corrosion (chemical) damage. in the case of biomaterials, corrosion fatigue is the process whereby repeated mechanical loading of a biomaterial leads to an increase in its susceptibility to corrosion in the presence of an electrolyte. commonly used biomaterials such as stainless steel and titanium alloys are highly susceptible to corrosion fatigue. the damage caused can result in decreased strength and increased risk of failure through fracture or fatigue. the most common methods of mitigation against corrosion fatigue in biomaterials involve surface treatments such as passivation, electrochemical treatments, and coatings. cracks caused by fatigue often begin at stress concentration sites, such as those that are developed during the manufacturing process [34]. a relatively recent study conducted testing on an alloy in air and a modified simulated bodily fluid hypothesized that inclusions and corrosion pits were the places where cracks first started to form (m-sbf) [35]. because of this, a significant decrease in fatigue strength was detected when the alloy was evaluated using the m-sbf method (figure 2). in addition, the nucleation and propagation of pits were found to be influenced by electrochemical circumstances as well as the amounts of applied stress. the researchers concluded that increasing the pitting resistance of mg alloys in m-sbf would enhance the cf life of these alloys. figure 2. s-n curves for az91-t6 mg alloy under corrosion fatigue at 150 °c [36]. permissions under cc ny 4.0 international attribution http://dx.doi.org/10.5599/jese.1636 j. electrochem. sci. eng. 13(1) (2023) 193-214 corrosion cracking in mg alloys based bioimplants 198 slip bands and twin limits are two more possible locations for the initiation of cracks [37,38]. because magnesium and alloys of mg undergoes twinning [38]. in corrosive settings, a sharp tip of the corrosion pit provides high stress intensity, and so acts as the starting location for fatigue cracks (figure 3). pits play a significant part in the acceleration of fatigue fracture propagation because they may strike the necessary balance among the electrochemical dissolution and cyclic stress [30] came to the conclusion that fatigue fractures developed from micropores when the alloys were tested in air. when the alloys were put through fatigue cracking tests in a corrosive environment, corrosion pits were found to be the cause. figure 3. sem images of fatigue crack initiation sites in az91-t6 mg alloy at 150 °c [36]. permissions under cc ny 4.0 international attribution the fatigue strength of a few different magnesium alloys is analyzed and compared in both air and many different corrosive conditions in figure 4. fatigue strenght, mpa figure 4. fatigue strength comparison of different mg alloys tested for 107 cycles in air and corrosive environments [30,42-46]. j. p. singh and y. sharma j. electrochem. sci. eng. 13(1) (2023) 193-214 http://dx.doi.org/10.5599/jese.1636 199 the most damaging effects are caused by aqueous solutions that include ions of chloride, phosphate, and nitrate. this is because these ions speed up the process of dissolving the corrosion layer of (mg(oh)2) [34]. for fatigue strengths in a select number of settings, however, the hydroxide layer's inherent instability—even under benign conditions is readily apparent. this is shown by the fact that certain environments have higher fatigue strengths than others (figure 4). corrosion has been shown to have a part in the beginning stages of fatigue cracking as well as their progression, which means that surroundings that are corrosive tend to have a negative impact on the fatigue life of a material. the negative difference effect, magnesium can produce hydrogen even at anodic potentials [39]. in film surface may be caused by the regional growth of a less embedded integrated due to the undesirable microstructure of the underpinning alloy that cause anodic reaction (pitting), and/or the practices will help of the physical loading. both of these factors can contribute to the deterioration of the exterior film's integrity [40,41]. it is commonly believed that a breach in the surface corrosion film provides a pathway for hydrogen to enter the matrix, and that a concentration of hydrogen in a specific area then accelerates the spread of the crack. the general public now agrees with this theory. they further believe that anodic dissolution plays a secondary role in the accelerated production of fatigue fractures, with hydrogen embrittlement playing the primary role [47-49]. figure 3 further demonstrates that fatigue crack occurring in az91d mg alloy is due to mechanochemical phenomenon. this is shown by the fact that fatigue cracking occurs at a faster rate. under anodic charging conditions, there is a larger propensity for cracking, which may be explained by the evolution of hydrogen and the contemporaneous pitting. on the other hand, when the material was subjected to cathodic charging conditions, the fatigue life was significantly extended. this was owing to the fact that pit depths under cathode circumstances were insufficient to cause cracking at low stress amplitudes. ripples and striations emerge on the cracked surface as a result of crack propagation during the second stage of fatigue. figure 5(a) displays a striated pattern that serves as an example. each striation is the result of a single cycle of stress, and it indicates the location of a fracture front that is progressing [48,49]. a comparison of fracture propagation rates in various conditions may be accomplished by measuring the inter-striation space between cracks. even though the production of striations is a telltale sign of stress, it is possible for some materials to fail due to fatigue even in the absence of striations [48,50]. striation may be difficult to see if the surface that was cracked has corroded, and if there is debris left behind from the products of corrosion. also, before attributing striations to cf, one should exercise care since comparable characteristics may be formed by scc of magnesium alloys, as can be shown in figure 5(a). this can be seen in figure 5(b). the striation is the result of crack-tip sharpening and blunting that occurs repeatedly under fatigue loading. this is in contrast to the scc characteristic of magnesium alloys, which can be entirely attributable to the hydrogen mechanism. it is possible that using fractography alone to differentiate between cf and scc is not the most straightforward course of action to take. http://dx.doi.org/10.5599/jese.1636 j. electrochem. sci. eng. 13(1) (2023) 193-214 corrosion cracking in mg alloys based bioimplants 200 figure 5. (a) fatigue striation and (b) parallel markings produced by stress corrosion cracking mechanism in az91 [36]. permissions under cc ny 4.0 international attribution hbf assisted cracking of mg alloys there are some studies on the critical temperature (cf) of magnesium alloys [51]. since these investigations are restricted to alloys that include aluminum (al), however, they are at best only vaguely applicable to applications involving body implants. in spite of the fact that it protects against corrosion and strengthens, aluminum is often believed to be poisonous to the human body [6]. because of this, the aluminum-zinc alloys that include magnesium (az-series) are often disregarded as potential candidates for use in biodegradable implant applications. these research used testing conditions that were vastly different from those encountered in vivo, including atypical frequencies and methods of loading, as well as artificial test environments and geometrically constrained samples [52]. this is another reason why the cf studies that have been reported are not very relevant. mode and frequency of loading when placed in a real body setting, implants may be subjected to complicated state of stresses at the same time, which leads to failures such as twisting and bending. the components of the bending are also involved in the failure mechanism [53]. on the other hand, the cf tests that are most typically used are conducted under straightforward uniaxial loading. it has been reported in the literature that cf testing of mg alloys [54-56] has usually carried out under load of fixed amplitude. therefore, it is necessary to perform testing using waveforms that are more representtative of real-world settings in order to simulate the actual loading conditions. even in the loading conditions, corrosion frequency is the dominant factor which controls the rate of corrosion. it's possible that a less range will provide ample time for corrosion, which would in turn encourage a synergistic interaction between corrosion and mechanical stress, which will, in a j. p. singh and y. sharma j. electrochem. sci. eng. 13(1) (2023) 193-214 http://dx.doi.org/10.5599/jese.1636 201 nutshell, speed up the development of fatigue cracks [26]. on the other hand, this pattern may not always hold true. according to rozali et al. [57] during the course of the investigation into the cf of az61 in nacl solution, it was discovered that the influence of frequency was more apparent when the dk was low as shown in table 1 [58,59]. in most cases, this frequency falls somewhere in the range of 1 to 3 hz. table 1. the noise figure for implants used in cardiac and bariatric practices [58,59]. s. no. implants frequency, hz activity 1 orthopedic 1-3 normal walking (vertical direction) 2 orthopedic 0.5-1.5 normal walking (lateral direction) 2 cardiovascular 0.8-2 0.5-1.5 normal heart chemistry of the corrosive environment although the effect of inorganic components on the deterioration of magnesium alloys has been studied and documented extensively [60-62]. however, the function of the organic components of real blood plasma, such as glycogen, organic molecules, and enzymes, is only recorded in a few studies. yamamoto et al. [61] studied that bio sorption promotes calcium breakdown longer progressive, but proteins make dissolution more rapid. xin et al. [62] indicated the creation of a bilayer as a consequence of the absorption of albumin on magnesium alloys, which further enhances the corrosion of the material. however, the effectiveness of this shield decreases gradually over the course of period [63]. however, there is no existing study on the potential function of endogenous substances in corrosionassisted cracking phenomena like cf, hence this information is currently unavailable. as a result, it is of the highest importance to provide the appropriate state of the environment for conducting biological crack testing. physical form and symmetry of the sample sharp and smooth curves are often seen in the devices used in implant procedures. while cf and scc cracks sometimes originate on clean metallic particles, pointed forms are frequently the sites where accelerated commencement occurs for the first time. the region of stress concentration may be found in fabricated components and devices like implants, for example, stents, screws, and plats (sharp contours). in addition, magnesium alloys are prone to pitting when exposed to chloride solutions, particularly hbf; pits are the most prevalent cause of corrosion fatigue (cf). this means that the cf and scc data that are typically frequently used for layout causes need to be acquired but use the samples that have well before sharp deformation; to put it differently, it could be completely essential to use before the samples. to study a biomaterial’s response to deformation, scientists use a variety of testing methods. these include tensile testing, compression testing, fatigue testing, and creep testing. each of these tests measures different aspects of the material’s properties. corrosion fatigue-stress corrosion cracking interaction it's possible for localized corrosion, such pitting, to offer starting sites for fatigue cracks. the crack tip may also experience localized corrosion, which can further accelerate crack growth. additionally, if the material is prone to scc, the fracture propagation rate may be further accelerated, as stated in [3]. thus, comprehending that cf and scc affect the crack progression (da/dn) in three circumstances is crucial. figure 6 shows how such stress distribution (kmax) and stress corrosion crack limit (kiscc) define these circumstances [64-66]. kiscc always exceeds kmax which measures fatigue. it is clear from looking at figure 6(a) that the corrosive liquid lowered the threshold for the fatigue fracture to start propagating (kmax). even when kmax exceeds kiscc, the surroundings still cause fatigue cracks, http://dx.doi.org/10.5599/jese.1636 j. electrochem. sci. eng. 13(1) (2023) 193-214 corrosion cracking in mg alloys based bioimplants 202 rendering this realistic cf performance. when it comes to the second kind (shown in figure 6(b), moderate fracture production levels have minimal environmental impact. the scc framework reaches a peak; however, when kmax is greater than kiscc, a superposition of the scc and cf mechanisms takes place. this suggests that the process is stress dependent. the third kind of behavior, seen in figure 6(c), is a mix of time-dependent and externally induced stress mechanisms. nevertheless, load ratio effects are not taken into consideration in this classification. recent research [67] has suggested developing a more comprehensive version of this classification. very little research has been done on the role that scc plays in the progression of cf cracks in magnesium alloys. figure 6. the genesis of a fracture as depicted in a schematic under a variety of conditions, including concurrent cyclic and tensile loads, in addition to an innocuous and hostile environment. reducing mg alloy transplant body-fluid-assisted corrosive failure cf and scc originate from just a dynamic relationship among surface properties and enhanced localized stress. the prevention of such cracking requires the implementation of strategies that are suited to address each of these elements or any combination of them. procedure of composites making and structure of metals this is necessary in order to use these alloys for implant applications. a few important alloying components are covered in this section. aluminum, which offers resistance to corrosion in addition to strengthening, is the primary alloying element in the most prevalent types of magnesium alloys (i.e. az series). however, due to the fact that aluminium is known to be harmful to the human body, alloys of magnesium that include aluminium. nutrients are an essential component of skeletal tissue, particularly in bone fragments [68]. ca is also necessary for the transmission of chemical signals across the human cytosystem [68]. additionally, the incorporation of ca into bone was discovered to be aided by the incorporation of mg. ca helps to reduce the grain size of magnesium alloys, which enhances both the mechanical characteristics and the corrosion resistance of the alloys. however, when present at a weight j. p. singh and y. sharma j. electrochem. sci. eng. 13(1) (2023) 193-214 http://dx.doi.org/10.5599/jese.1636 203 percentage of p 1 %, calcium causes the precipitation of magnesium calcium along grain boundaries, which results in embrittlement [68]. zinc (zn) in the amount of 15 mg per day is needed by the human body [69]. a solid solution of magnesium may be strengthened via alloying with zinc. however, if the percentage of p reaches 6.3 wt.%, zinc will begin to form secondary phases, which will again result in brittlement. rare earth elements, often known as res, are regarded not only be poisonous. [68], but they are also said to 'display anti-carcinogenic qualities [70]. this is the broad consensus among scientists. mg alloys with an adequate amount of re content may generate a surface film that is resistant to corrosion. for example, the elektron 31 alloy, which contains 2.3 % neodymium and 1.6 % gadolinium, produces a mixed oxide film. these intermetallics can cause localized corrosion as well as embrittlement. it has also been noted that adding a very little amount of re may increase the fatigue strength [71]. the function of alloying additives in cf of magnesium alloys in sbf has received very little research attention. refining the microstructure of mg alloys is a potent method that may significantly improve the materials' resistance to corrosion as well as fatigue, in addition to the positive benefits that can be achieved by appropriate alloying [72]. in addition to this, there are claims of improvements in both tiredness and corrosion characteristics of magnesium alloys that were produced by these techniques [73]. mechanical surface treatment it is well knowledge that the application of compressive pressures may enhance fatigue strength and life. according to zhang et al. [74], shot peening compressed residual stresses increased az80 alloy fatigue life. khan et al. [75] shot blasting also strengthens am60 alloy. roller burnishing az80's clean surface and residual stress improved cf endurance in solvent [75]. however, physical imperfections, such as quick zones, formed by such procedures may make cf cracks easier to begin and progress. biomaterials can be treated with various techniques to render them more biocompatible. this can include surface treatments such as chemical modification, deposition of thin films, and thermal treatments. chemical modification involves treating the surface of the biomaterial with a chemical agent such as an acid or alkali. this can be done to alter the surface energy, which can make the material more hydrophilic or hydrophobic. it can also be used to create a functionalized surface that can interact with biological molecules such as proteins or cells. thin film deposition involves depositing a thin protective layer of material onto the surface of the biomaterial. this can be done by different methods. this layer can be used to create a more inert surface that is resistant to corrosion and wear. thermal treatments such as plasma etching, annealing, and laser ablation can also be used to modify the surface of the biomaterial. these treatments can be used to alter the surface roughness and morphology of the material, which can affect its biocompatibility. biocompatible surface chemical treatment biocompatible surface chemical treatments involve the use of a variety of chemicals and processes to modify the surface properties of a material to make it more compatible with living tissue. these treatments can include deposition of proteins, polymers, and other molecules, as well as modification of the surface energy of the material through etching, plasma treatments, or other methods. the ultimate goal of these treatments is to create a surface that is non-toxic, non-fouling, and promotes cell adhesion and proliferation. the best approach to avoiding corrosion and highly corrosive cracking is to apply a surface coating. this is because magnesium has a high chemical http://dx.doi.org/10.5599/jese.1636 j. electrochem. sci. eng. 13(1) (2023) 193-214 corrosion cracking in mg alloys based bioimplants 204 reactivity, making it one of the most reactive elements. according to bhuiyan and colleagues [43] findings, many different coating techniques that are able to increase the fatigue life of magnesium alloys when exposed to corrosive conditions [76]. surface engineering methods provides a safeguard against the corrosion, biodegradation, and erosion [77-93]. various coating methods are employed to protect the surface of different biomedical materials [94-97]. spray pyrolisis deposition [98], electroplating [99], electrodeposition [100], hot dipping [98-104], physical vapor deposition (pvd) & chemical vapor deposition (cvd) [105-117], electrochemical vapor deposition (evd) [118], cold spray [119], thermal spraying [120-125], sputtering [126], claddings [64-75], plasma spraying [79,80,88,9193,127-150], and electrophoretic deposition are all methods used to deposit coatings onto metals (epd). however, the coatings that were used in these trials were not always biocompatible since the focus of the coatings was on addressing the cf resistance in settings that were not biological [88]. it has been observed that such coatings not only increase the implant's biocompatibility but also improve its resistance to corrosion. calcium orthophosphates were shown to increase the corrosion resistance and surface biocompatibility of magnesium-based metallic biomaterials, as reported by dorozhkin [151]. there are several findings that demonstrate an increase in corrosion resistance as a result of electro-deposited ca-p coatings on a variety of magnesium alloys [152]. it is possible that referring to a research by srinivasan and colleagues [153] on the impact of silica-based plasma electrolytic oxidation (peo) coating on the surface-corrosion-corrosion (scc) of magnesium alloy in a chloride solution is not completely out of place. although the peo coating made the alloy more resistant to general corrosion and pitting corrosion, it did not make a significant difference in the alloy's resistance to scc. when the az61 mg alloy was tested in chloride solution, a recent research reveals that peo coating resulted in about a 56 % drop in the fatigue strength of the material [154]. conclusion and future prospective in order for a material to be considered suitable for use as an implant, it must first satisfy a number of criteria, including those pertaining to its mechanical, electrochemical, and biological properties. alloys made of magnesium provide a favorable balance of ductility and strength. since magnesium is both biocompatible and biodegradable, it is a strong contender for use in applications involving temporary implants. when it comes to implants, however, the resistance to cracking might occur because of the synergistic action. however, the existing studies are mostly inappropriate for body implants for the reasons listed: (a) the alloys involved in the earlier research typically include al, which is known to be poisonous to humans; and (b) the lab testing variables adopted in the in vitro tests, such as the regularity and mode of loads, testing process, and specimen geometries, were distinct in the preclinical studies from those in the real in vivo execution of the tests. both issues may be traced back to the fact that the alloys used in the earlier research were, for the most part, generic. recent research has unequivocally shown that separate corrosion films form in in vivo circumstances as opposed to the in vitro conditions that are often used, with in vivo corrosion rates being much lower than in vitro rates. therefore, it is of the greatest priority to first ascertain the in vivo model, such as the solubility and maximum load spectral response, and then to conduct virtual crack experiments on the mg alloys that have been 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académica química, universidad autónoma del estado de hidalgo. carret. pachucatulancingo, km. 4.5, c. p. 42076. mineral de la reforma. hidalgo. méxico 2área académica de agronomía, universidad autónoma del estado de hidalgo. carret. pachucatulancingo, km. 4.5, c. p. 42076. mineral de la reforma. hidalgo. méxico corresponding author: jud_292003@yahoo.com.mx received: march 30, 2017; revised: june 6, 2017; accepted: june 6, 2017 abstract anode (al and fe) and cathode (graphite and ti/ruo2) materials have been tested for electrocoagulation (ec) and purification of the acid whey. the electroactive areas (ea) of electrodes were calculated by the double layer capacitance method. experiments were performed by cyclic voltammetry, chronoamperometry and polarization experiments. among cathodic materials, the ti/ruo2 electrode showed higher ea (2167 cm2) than graphite (1560 cm2). the fe anode was found more stable than al with greater charge transfer carried out in less time. correlation of these results with those obtained during preliminary tests confirmed high removals (79 %) in 8 h. for the al electrode, 24 h were required to achieve efficiency of 49 %. keywords electrocoagulation, electrode materials, acid whey, electroactive area, capacitance, tafel slopes introduction the electrodes are indispensable components of any electrochemical cell because just on their surface the processes of oxidation-reduction of the species occur together with other reactions carried out during electrolysis 1,2. when an electrocoagulation (ec) process is applied, the electrodes must be cost-effective and easy to handle. in addition, the electrodes used as sacrificial anodes must also possess great dissolution power. the most frequent materials used for ec processes of different effluents with good removal efficiencies are aluminum (al) and iron (fe), but other materials such as steel 3,4 have also been tested. http://www.jese-online.org/ mailto:jud_292003@yahoo.com.mx j. electrochem. sci. eng. 7(2) (2017) 89-101 electrocoagulation of whey acids 90 in the present study, different materials acting as anodes (al and fe) and cathodes (graphite and ti/ruo2) are tested for the ec and purification of residual acid whey. the evaluation is performed with respect to the electroactive electrode area and through polarization curves. methodology determination of electroactive area (ea) of electrodes determination of real electroactive area (ea) was done for al, fe, graphite (g) and ti/ruo2 electrodes. the cyclic scanning voltammetry technique, exploring the basi epsilon ec potentiostat 2.13 usb was used. experiments were performed in 0.1 m kcl solution acidified with hcl to maintain ph 4.7, what simulates the initial ph of the acid whey 5,6. the electroactive area of each electrode surface was calculated by the capacitance of the double layer, cdl, generated on the electrode in contact with an electrolyte 7. a glass beaker (2 l) containing 0.1 m kcl solutions was used as the electrochemical cell in which the electrodes were immersed (fig. 1). the electrodes were submerged to a height of 5 cm, exposing a geometric area of approximately 100 cm2 to the solution. the dimensions of electrodes were as follows: al (10.2  10  0.1 cm), fe (10.1  10  0.2 cm), g (8.2  10  1.2 cm) and ti/ruo2 mesh (10.5  10  0.1 cm). titanium (ti) mesh electrode (10  10  0.1 cm) and the saturated calomel electrode (sce) served as the counter and reference electrode, respectively. figure 1. glass cell containing ti mesh counter and sce reference electrodes used for determination of electroactive area of working electrodes (al, fe, g, ti/ruo2) cyclic voltammetry experiments were performed at different scanning rates, v = 10100 mv s-1 in a potential window of ± 10 mv around the equilibrium potential (e0). anodic (ia) and cathodic (ic) currents were obtained as electrochemical responses to the potential change. the capacitive current (icap) was calculated by means of equation (1), while cdl value was calculated as the slope of linear relation obtained by presenting icap vs. v. the ea, is obtained by means of equation (2), where cst is the normal capacitance taken as cst = 60 f cm-2 for roughened electrodes 7. a c cap 2 i i i   (1) f. prieto-garcía et al. j. electrochem. sci. eng. 7(2) (2017) 89-101 doi:10.5599/jese.381 91 dl st ea c c  (2) measurements of polarization curves with the polarization curves it is possible to determine the electrode of the highest oxidation capacity attained with consumption of the lowest energy. the potentioamperometric technique was explored using a sample of acid whey (30 ml) and geometrically smaller electrodes. the electrodes were submerged to a height of 2.0 cm and the exposed geometric areas were 6.96 cm2 (al), 8.78 cm2 (fe), 4.65 cm2 (g) and 8.6 cm2 (ti/ruo2). the real electrode areas were determined following the methodology described above. three-electrode glass cell was used and shown in figure 2a, where the working electrode is the g electrode, while ti mesh and sce served as the auxiliary and reference electrodes, respectively. figure 2b shows a three-electrode cell containing 30 ml of acid whey used for chronoamperometric experiments, performed by applying different overpotentials, η, defined by the equation (3) for 45 minutes. figure 2. three-electrode glass cell used for determination of electroactive area of small electrodes (g, ti-mesh and sce are the working, counter and reference electrodes) a). electrolytic cell containing 30 ml of acid whey for polarization measurements (al, ti-plate and sce are the working, counter and reference electrodes) b) constant stirring is maintained during the test to obtain the current (i) in response. the current density (j) is calculated by the equation (4). = eapplied e0 (3) j = i / ea (4) results and discussion determination of electroactive area (ea) of electrodes cyclic voltammograms of al, fe, g and ti/ruo2 electrodes in 0.1 m kcl recorded at different scan rates (v) are together with i vs. υ curves shown in figures 36. numerical data for icap determined by eq. (1) are collected in table 1. contrary to figures 3 and 4 that showed ordinary capacitive current responses of g and ti/ruo2 electrodes, in the case of al and fe electrodes presented in figures 5 and 6, the tendency is quite different. lack from usual linear icap vs. v response is particularly obvious for the fe electrode and is probably due to the dissolution of electrode in acid medium. a) b) j. electrochem. sci. eng. 7(2) (2017) 89-101 electrocoagulation of whey acids 92 e / v vs. sce e / v vs. sce figure 3. cyclic voltammograms of graphite electrode (g) in 0.1 m kcl, at different scanning rates (v = 0.01 to 0.10 v s-1) (a). icap vs. v (b) e / v vs. sce v / v s-1 figure 4. cyclic voltammograms of ti/ruo2 electrode in 0.1 m kcl at different scanning rates (v= 0.01 to 0.10 v s-1) (a). icap vs. v (b) e / v vs. sce v / v s-1 figure 5. cyclic voltammograms of al electrode in 0.1 m kcl at different scanning rates (v = 0.01 to 0.10 v s-1) (a). icap vs. v (b) the electroactive area (ea) of the working electrode represents the number of active sites where the electron transfer takes place and is usually estimated as the real electrode area (areal) defined by eq. (2). ea can increase or decrease with respect to the geometric area, what is mainly due to the modification of the electro-active electrode surface. when an ec process is carried out, the anode (al or fe) undergoes oxidation reaction, which leads to the release of metal ions (sacrificial i / a i / a i / a i / a i / a i / a f. prieto-garcía et al. j. electrochem. sci. eng. 7(2) (2017) 89-101 doi:10.5599/jese.381 93 anode) and concomitant dissolution process 4,8-11. causing diminishing of the surface area. at the other hand, increase of the ea could also happen due to the porosity induced by the ec process. e / v vs. sce v / v s-1 figure 6. cyclic voltammograms of fe electrode in 0.1 m kcl at different scanning rates (v = 0.01 to 0.10 v s-1) (a). icap vs. v b) table 1. capacitive currents (icap) for different ec electrodes at different scanning rates v v / v s-1 icap / a g ti/ruo2 al fe 0.01 0.00209 0.00229 0.00003 0.0134 0.02 0.00318 0.00378 0.00005 0.0165 0.03 0.00434 0.00573 0.00007 0.0222 0.04 0.00538 0.00728 0.00009 0.0231 0.05 0.00623 0.00874 0.00011 0.0240 0.06 0.00718 0.01007 0.00013 0.0244 0.07 0.00799 0.01121 0.00015 0.0245 0.08 0.00899 0.01218 0.00017 0.0245 0.09 0.0098 0.01308 0.00019 0.0242 0.10 0.01055 0.01385 0.00021 0.0242 double-layer capacitance (cdl) and ea values, calculated using data in table 1 and eq. (2), are summarized in table 2. table 2. real area (ea) and capacitance (cdl) of different electrodes for the electrocoagulation process electrodes equation r2 cdl / f ea, cm2 ga*, cm2 roughness factor g y=0.0936x+0.0014 0.9967 0.0936 1,560 103.84 15.03 ti/ruo2 y=0.130x+0.0010 0.9821 0.130 2,167 107.05 20.24 al y=0.002x +10-5 1.0000 0.0020 33.3 104.02 0.32 fe y=0.101x +0.016 0.617 0.1010 1,689 105.02 16.1 ga* geometric area it is obvious from table 2 that ti/ruo2 electrode showed the largest ea (2,167 cm2), what is due to the ruo2 metal oxide that generates micro, meso and macropores, and so greater surface roughness and higher capacitance (cdl) values were obtained. at the same time, al electrode showed the smallest ea (33.3 cm2) because it is a smooth and polished metal plate that exhibits the lowest cdl among all electrodes. it has generally been stated that for cathodes, greater ea is generally i / a i / a j. electrochem. sci. eng. 7(2) (2017) 89-101 electrocoagulation of whey acids 94 beneficial. data in table 2 show that for ti/ruo2 and g electrodes, ea are 2,167 cm2 and 1,560 cm2, respectively. this makes the ti/ruo2 electrode to be better cathode under the perspective of electroactive areas. also, regarding the electroactive area, the fe electrode having ea of 1,689 cm2 seems better than the al electrode having 33.3 cm2. although being indicative, the value of ea, however, is not a definitive factor for the choice of the most proper electrodes for ec. choice of better arrangement of electrodes by means of polarization curves aluminum electrode chronoamperometric, i vs. t responses of the al electrode in acid way sample recorded during 45 min are presented in figures 7 and 8. similar behavior is observed at different potentials (eapp) applied between 0.650 and 1,050 mv. as eapp was increased, i was increased in the same way. however, at very high eapp (850 and 1,050 mv) the decomposition of the medium is already occurring, which leads to unnecessary energy consumption. figure 9 shows i vs. eapp responses for the al electrode (ea = 33.3 cm2) in the acid whey sample. it is obvious from fig. 9 that the limiting current (il) where diffusional control begins is reached at about 400 mv vs. sce and that diffusional currents are in a range of 0.039 and 0.042 a. despite application of higher voltages, there are no greater transformations of the species present in the medium, up to the potential suitable for secondary reactions like the electrolysis of the water. figure 10 shows the polarization curve, log j vs., of the al electrode. in this graph, it is observed that the limiting current is reached by diffusional control at overpotential of 200 mv. this suggests that from the results obtained previously during the ec with a removal efficiency of 79 % in 8 h, the energy required can be reduced (by decreasing  from 1.74 to 0.2 v) to obtain the same removal efficiency and possible reduction of time. time, s figure 7. chronoamperometric i vs. t responses of al electrode in acid whey sample recorded during 45 minutes at eapp between 650 mv and +1,050 mv 1 time, s i / a f. prieto-garcía et al. j. electrochem. sci. eng. 7(2) (2017) 89-101 doi:10.5599/jese.381 95 time, s figure 8. chronoamperometric i vs. t responses of al electrode recorded at eapp between 650 mv and 550 mv (zoom of fig. 7) e / mv figure 9. i vs. eapp of al electrode in acid whey sample at eapp between 650 mv and +650 mv figure 10. log j vs. dependence of al electrode in acid whey sample 1 time, s 1 2 -7 -6 -5 -4 -3 -2 -1 0 0 300 600 900 1200 1500 1800  / mv lo g ( j/ m a c m -2 ) i / a i / a j. electrochem. sci. eng. 7(2) (2017) 89-101 electrocoagulation of whey acids 96 iron electrode the chronoamperometric, i vs. t curves of the fe electrode in acid whey sample recorded during 45 min are shown in figures 11 and 12. almost constant current values observed with time at each eapp applied between 650 and 150 mv suggest that only capacitive currents are generated at the fe electrode in contact with the acid whey solution. when applying eapp between 0 to 100 mv, however, the first change in the oxidation state of fe° to fe2+ is observed with transfer of 2e coupled with the decomposition of the medium. at eapp between 100 to 450 mv, a second change in current is observed due to the electrochemical oxidation of fe2+ to fe3+, so that the oxidation of iron is considered to occur in two stages (eqs. 5 and 6): fe°  fe2+ + 2e (5) fe2+  fe3+ + 1e (6) finally, by applying eapp higher than 450 mv the electrolysis of the medium that is decomposition of the whey begins, what is accompanied by the significant current increase with time. i vs. eapp and log j vs.  dependences are presented in figures 13 and 14. here also, two different processes which are related to iron oxidation can be observed as in the chronoamperometry experiments. with these polarization curves it is verified that in the ec system, generation of fe2+ or fe3+ can be favored to improve the efficiency of the system in a shorter time and lower energy cost. time, s figure 11. chronoamperometric i vs. t responses of fe electrode in acid whey sample recorded during 45 minutes at eapp between 1050 mv and +1050 mv the above statement can be based on the pourbaix diagram for the thermodynamic states of the iron in aqueous solution, where when applying potentials within the range of 0.6 to 1.6 v vs. sce, the oxidation process occurs and fe2+ or fe3+ ions are released depending on the ph of the solution. for acid conditions, both species are present and the process of corrosion is facilitated. it should be analyzed in more detail, however, which reaction would favor the ec process for the whey solution 1 i, a i / a f. prieto-garcía et al. j. electrochem. sci. eng. 7(2) (2017) 89-101 doi:10.5599/jese.381 97 and formation of the iron oxy-hydroxides. thus, one would be able to establish the optimum limiting current according to the limiting reagent that controls the reaction. time, s figure 12. chronoamperometric i vs. t responses of fe electrode at eapp between 650 mv and 450 mv (zoom of fig. 11) e / mv vs. sce figure 13. i vs. eapp of fe electrode in acid whey sample at eapp between 650 mv and 850 mv the bibliographical reports on the ec processes using iron electrodes are contradictory. most ec studies have focused on the removal of contaminants by their co-precipitation with the solid(s) such are already formed precipitates of fe(oh)2(s) and fe(oh)3(s). also, the reactions reported in the literature showed a certain ambiguity in the proposed mechanisms. some studies 13-15 have reported that the oxidation of iron fe0 to fe2+ occurs in an electrolytic form and produces insoluble fe(oh)2(s) compounds by hydrolysis. other authors 16 pointed to the formation of fe(oh)2(s) without giving details of how it occurs. several studies 8,17,18 reported formation of fe2+ at the anode that is followed by oxidation to fe3+ due to dissolved oxygen (do) and rapid formation of fe(oh)3(s). the third mechanism reported 7,8,10 involves the single and fast electrolytic oxidation 1 i, a 1 e/mv vs. sce i / a i / a j. electrochem. sci. eng. 7(2) (2017) 89-101 electrocoagulation of whey acids 98 step of fe0 to fe3+, followed by hydrolysis to produce fe(oh)3(s). finally, some studies 16,19,20 have reported formation of fe(oh)3(s) as the final product during ec without specifying the reactions involved. according to 21, the third mechanism is questionable. these authors pointed out dependence of the kinetics of these reactions on the ph of the medium. they reported that at ph 7.5, approximately 80% of the fe2+ initially formed is oxidized to fe3+ in less than 10 minutes, while at ph 6.5, the same oxidation process takes about 300 minutes (5 h). the rapid increase in fe2+ oxidation rate with ph increase is consistent with other reports in the literature, where based on equations (7) and (8), it was shown that increase of ph increases the oxidation rate of fe2+ for about 100 times 2224.  / mv figure 14. log j vs. dependence of fe electrode in acid whey sample. 4fe2+ + o2 + 2h2o + 8oh ↔ 4fe(oh)3(s) (7) d[fe2+]/dt = k[fe2+] po2 [oh -]2 (8) in eq. (8), k is the rate constant, po2 is the oxygen partial pressure, while the molar concentrations of present species are represented in the brackets. a slight increase in the concentration of oh ions as observed during ec and corroborated with the increase of ph, has resulted in a greater concentration of the fe2+ species forming fe3+ by subsequent oxidation and fe(oh)3(s) by precipitation. since oh ions are consumed during the hydrolysis of fe3+, ph in the electrolytic cell decreases in a subsequent step that does not occur immediately. by the time elapsed between the production of fe2+ and its oxidation to fe3+ by the oxygen dissolved in the medium, a reaction that is much slower has been indicated. graphite electrode in figures 15 and 16, the chronoamperometric i vs. t responses of the g electrode recorded through 45 min are presented, showing the same behavior at different eapp applied. as eapp increases, i increased in the same way. at very negative eapp (1050 mv), however, it can be observed that the decomposition of the medium occurs. by presenting log j vs. dependence, fig. 17 suggests that the limiting current (il) is occurring at overpotential of 500 mv, where diffusional control begins. 1 2 3 4 5 6 7 8 9 10 lo g ( j / m a c m -2 ) f. prieto-garcía et al. j. electrochem. sci. eng. 7(2) (2017) 89-101 doi:10.5599/jese.381 99 time, s figure 15. chronoamperometric i vs. t responses of g electrode in acid whey sample recorded during 45 min at eapp between 70 mv and 1050 mv. time, s figure 16. chronoamperometric i vs. t responses of the g electrode at eapp between 12 mv and 250 mv (zoom of fig. 15)  / mv figure 17. log j vs.  dependence of g electrode in acid whey sample. i / a i / a lo g ( j /m a c m -2 ) j. electrochem. sci. eng. 7(2) (2017) 89-101 electrocoagulation of whey acids 100 selection of the anodic material for the electrocoagulation process. with the electrochemical information obtained so far, the tafel slopes for the electrodes acting as sacrificial anodes (al and fe) are compared in figure 18. to select the best electrode, the largest slope must be taken into account. the results for fe and al electrodes are different, for the al electrode, b = -54.55 mv dec-1 and for fe, b = -109.98 mv dec-1. in this respect, it is possible to define that the fe electrode is considered the most effective. figure 19 shows a better separation of the precipitate and purified liquid more transparent with the use of the fe electrode (figure 19 a). log (j / a cm-2) figure 18. tafel slopes of fe and al electrodes in acid whey samples figure 19. purification kinetics of acid whey with fe (a) and al (b) electrodes conclusions the iron electrode proved to be better anode than al for the ec process. this is due to its stable thermodynamic behavior in the range of applied overpotentials, which translates into a greater transfer of charge in a shorter time. by correlating these results with the results obtained during the preliminary tests, it can be confirmed that the fe electrode is better than the al electrode. with the fe electrode, greater removals (79 are obtained in less time (8 h)). on the other hand, with the al 1 2 3 log (i/a cm -2 )  /m v v s s c e a) b)  / m v v s. s c e f. prieto-garcía et al. j. electrochem. sci. eng. 7(2) (2017) 89-101 doi:10.5599/jese.381 101 electrode, the results of macroelectrolysis required more time (24 h) to achieve an efficiency of 49 %. based on the determinations of the electroactive area, the ti/ruo2 electrode (2,167 cm2) proved to be a better cathode than the graphite (1,560 cm2). references [1] m. piña, a. martín, c. a. gonzález, f. prieto, a. guevara, j. e. garcía. revista mexicana de ingeniería química 10(2) (2011) 257-271. 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[23] w. stumm, s. morgan, journal. aquatic chemistry; 1981. wiley-interscience, new york. [24] m. bond, s. kratsis and s. mitchell. journal analyst 122 (1997) 1147-1152. © 2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {flow batteries with solid energy boosters:} http://dx.doi.org/10.5599/jese.1363 731 j. electrochem. sci. eng. 12(4) (2022) 731-766; http://dx.doi.org/10.5599/jese.1363 open access : : issn 1847-9286 www.jese-online.org review flow batteries with solid energy boosters yuriy v. tolmachev1,2, and svetlana v. starodubceva3 1department of civil and environmental engineering, university of massachusetts, 01002 usa 2department of chemistry, lomonosov moscow state university, moscow 119991 russia 3dialog-kontrakt, moscow 119988 russia corresponding author: ruthenium2008@yahoo.com received: april 27, 2022; accepted: september 10, 2022; published: september 17, 2022 abstract adding solid electroactive materials as energy boosters to flow battery tanks provides, in principle, a path to electrical energy storing systems with simultaneously high specific energy and specific power that can solve the needs of both automotive and stationary energy storage markets. this work reviews the physical and chemical principles behind this new class of flow batteries, the history of this technology, and the reasons why it failed to reach the markets. keywords redox batteries, redox mediators, redox-targeted solids, redox-assisted flow batteries, redox -mediating fluid, solid electroactive materials introduction flow batteries are unique electric power sources that are rechargeable and allow for independent scaling (decoupling) of energy and power [1-9]. the latter feature allows for optimizing (runtime, cost, size, weight, etc.) of the system for a particular application, whereas most other energy storing systems (e.g. batteries with solid electroactive materials (seam), supercapacitors, flywheels) have a narrow range of the energy-to-power ratio (i.e., the characteristic runtime). figures 1a and 1b illustrate the difference between the two aforementioned battery types. on the top (figure 1a) is the most common type of battery that uses only solid electroactive materials (seams). for the sake of illustration, let’s assume that the discharged negative seam is lithium titanium phosphate and the discharged positive seam is lithium iron phosphate. then, the discharge reactions are (1)-(3): in a seam battery in the posode: fepo4 + li+ + e− = lifepo4 (1) in the negode: ½li3ti2(po4)3 − li+ − e− = ½liti2(po4)3 (2) in full cell: fepo4 + ½ li3ti2(po4)3 = ½ liti2(po4)3 + lifepo4 (3) http://dx.doi.org/10.5599/jese.1363 http://dx.doi.org/10.5599/jese.1363 http://www.jese-online.org/ mailto:ruthenium2008@yahoo.com j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 732 a b c d figure 1. schematic diagram of a a conventional battery (only one cell is shown) with solid electroactive materials (seams): green-dielectrics, yellow-electrolyte, bluenegode, red-posode. electronically conducting particles and binders are not shown; b a traditional redox flow battery (trfb) with one cell in a stack: negolyte fluids are blue, posolyte fluids are red, charged fluids are darker colors, discharged are lighter color; c a slurry-flow (semisold) redox flow battery (srfb; ); d a flow battery with solid energy boosters (seb y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 733 in figure 1a, the iron phosphate is shown in red, the lithium titanate is in blue, a nonelectroactive liquid electrolyte is in yellow, and a dielectric porous membrane and a cell casing are in green. both electrode reactions (1) and (2) are electrochemically reversible, and they are coupled via exchange of the same species (li+), thus their combination can produce a rechargeable battery. indeed, such a system has been demonstrated [10]. a full cell reaction in the discharge direction for such a seam battery is shown in eq. (3). let’s jump to the opposite situation, where all electroactive species are present only in liquid phases. figure 1b shows a redox flow battery, where all electroactive species are dissolved in water and the discharge reactions are [10]: in a flow battery on the posode: [fe(cn)6]3+ e= [fe(cn)6]4(4) on the negode: s2− e= ½s22 (5) in full cell: [fe(cn)6]3+ s2= [fe(cn)6]4+ ½s22 (6) a counter-cations, such as li+, na+ or k+, can be used for charge balance in half-reactions (4) and (5), and they are transferred through a cation-selective membrane (figure 1b). disadvantages of rfbs rfbs usually present five main disadvantages (d1-d5 below) compared to seam batteries, which explains why the former have not been able to capture even a single market niche. d1 heavier weight due to the presence of a large amount of non-electroactive solvent(s), the specific energy of flow batteries is usually lower than that of seam batteries. there are, however, several important exceptions, where highly soluble (or fluid in a pure state) multi-electron species are involved, such as zn-cl2 [11-17] and h2-libro3 [18,19] flow batteries. the weight problem is further aggravated by the use of mixed reagents, as described in d3 below. d2 chemical crossover since at least one electroactive species in flow batteries is present in a fluid state, and the separator is usually not perfectly selective, a chemical crossover between the posode and negode is normally expected in rfbs. in the example shown figure 1b, we intentionally selected a chemistry where electroactive species are anions because a durable cation-selective membrane such as nafion can slow down the internal self-discharge reactions (7)-(8) inside this battery: in solution: [fe(cn)6]3+ ½s22= [fe(cn)6]4+ s↓ (7) in solution: [fe(cn)6]3+ ½ s2= [fe(cn)6]4+ ½ s↓ (8) nevertheless, even ion-selective membranes have donnan’s exclusion limit on how much charge selectivity they can sustain. for example, nafion with an equivalent weight of 1200 g/mol h+ becomes noticeably permeable to chloride ions in contact with hcl solutions above 1 m concentration [20]. with respect to other anions, nafion and its analogues typically maintain charge selectivity also up to ca. 1 m [21-25] and they can slow down the parasitic (i.e., without generating the electric current on through the cell) reaction (6) in solution as well as other potential sidereactions. at higher concentrations of electroactive anions, however, the cross-over becomes noticeable. in the particular example of sulfide-ferricyanide redox battery (4) and (5), with the total dissolved sulfur concentration of 2.0 m (in 0.1 m lioh) and the total dissolved iron cyanide complex concentration of 0.3 m (also in 0.1 m lioh), sulfur deposits form on the positive (ferricyanide) side of a nafion 117 membrane [10]. http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 734 the solvent cross-over in the flow batteries is usually even more significant than the cross-over of the electroactive species. as nafion has no selectivity with respect to water permeation, even trivial pressure differences between the posolyte and negolyte during pumping result in water transfer from one flow circuit to another [26]. the cross-over of liquid-phase species is a common drawback of redox batteries. d3 energy and power dilution in flow batteries undergoing long-term cycling, the crossover is mitigated by starting with a premixed solution of discharged reagents and by occasional mixing and splitting the discharged posolyte and discharged negolyte during cycling. although, this approach does not work for the example in eq. (6) because this particular system suffers from the precipitation of sulfur in reactions (6) and (7), the use of premixed reagents is the standard practise for many flow battery chemistries, e.g., cr-fe [27]. the main drawback of premixing is the reduced concentrations of the electroactive species in the redox fluids. for example, while the aqueous solubilities of both crcl3 and fecl2 are both ca. 2.0 m at room temperature, the mixed solution has half of these concentrations, i.e., 1.0 m of crcl3 + 1.0 m of fecl2 [27]. of course, the iron species in the negolyte and chromium species in the posolyte do not participate in the energy storage cycle and serve as ballasts adding to the cost of energy since twice the amount of chemicals, solvent and storage tanks volume per ah is needed. halving the concentration of the electroactive species also results in ca. two-fold increase in the capital cost of power: since both electrode kineticsand mass-transport limited currents typically have a first-order dependence on the reagent concentrations, lowering the concentrations requires a compensatory increase in the cell area (i.e., a large power stack), if the same power rating and energy efficiency is required. a clever way to overcome the mixing problem in an rfb is by employing electroactive species with three or more oxidation states. we shall refer to such chemistries as ambipolar [28,29], i.e., capable of a redox process on both negode and posode. although numerous ambipolar rfb chemistries have been demonstrated in laboratories [30,31] (e.g. uranium [32], neptunium [33], all-chromium-edta [34], polyoxometallates [35,36], fullerene [37], borane clusters [38], and nitronyl nitroxide [39]), allvanadium redox flow battery (vrfb) invented in 1986 by maria skyllas-kazacos at the university of new south wales, australia [40,41] is the only ambipolar flow battery chemistry, that has been commercialized [42-51]. d4 incomplete utilization of reagents since it is not feasible to fully electrolyze the flowing reagent in a finite time, flow batteries are not cycled between 100 and 0 % state of charge (soc) but rather between 85 and 15 % (or even more narrow ranges) [52], which contributes to an increased cost of energy and a heavier weight. d5 inferior energy efficiency of rfbs the most significant and rarely discussed drawback of redox flow batteries is their lower energy efficiency compared to seam batteries [53]. for example, whereas lithium-ion batteries typically operate at better than 90 % cycle energy efficiency [54-56], the corresponding value for vrfbs is usually around 65 % [57,58]. for sufficiently thick electrodes commonly used in both libs and vrfbs, and when the electronic conductivity of the porous electrode is much larger than its ionic conductivity (a goal we try to attain), the electrode area-specific resistance (asr) is approximately equal to the product of the electrode thickness l and of its ionic conductivity σ [59]. y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 735 asr = σ l (9) in order to understand the reasons for the inferior energy efficiency of vrfbs compared to libs we need to look at the following factors: 1. vrfbs use acidic aqueous redox fluids with ca. 30-50 s/m ionic conductivity [60], whereas libs electrolytes have lipf6 in alkyl carbonate and ca. 0.25 s/m conductivity [61]. the electrolyte conductivity favors vrfbs in terms of better energy efficiency, but the useful figure of merit here is the cell area-specific resistance rather than the ionic conductivities of the electrolytes. in practice, the higher ionic conductivity of vrfb cannot overcome the drawbacks of the other two factors; 2. lithium-ion batteries have a higher (3.20 v at 50 % soc for lithium iron phosphate batteries)[62] open circuit voltage than vrfbs (1.35 v at 50 % soc) [63]. this, however, is a minor effect in practice because of the small parameter ratio (3.20 v / 1.35 v = 2.37); 3. the most important reason for the superior energy efficiency of libs is their low operating current density (ca. 1 ma/cm2) compared to vrfbs, which are usually operated 100 to500 ma/cm2 in order to reduce the capital cost of power (i.e., the size of the power stack) [53]. for details about the structure and manufacturing of modern libs and vrfbs, we refer the readers to the refs. [64] and [65], respectively. as a result of their design and operation differences, and despite their significantly lower area-specific resistance, vrfbs are inferior to libs in terms of their energy efficiency. in general, redox flow batteries have a lower energy efficiency than seam batteries. in fact, a 2016 german study concluded that for household-sized solar panel systems, the use of a lib for electric energy storage is a "better choice for all cases economically" than vrfb, primarily because of the better energy efficiency of libs [66]. advantages of rfbs so far, we have discussed the disadvantages of rfbs compared to seam batteries. these disadvantages explain why seam batteries dominate all energy storage markets currently. this brings the question: “why does the interest in flow batteries persist?” there are two market niches where flow batteries may be competitive: a1 electric vehicles some flow batteries are attractive for use in electric vehicles [67] because the transfer of charged reagents (two hoses) and discharged products (third hose) between the charging station and the vehicle may be accomplished quickly and similarly to gasoline transfer in a case of cars with internal combustion engines. unfortunately, not many rfb chemistries have high enough specific energy to assure a 300+ km driving range on a single charge. only if the mass fraction of non-electroactive solvent(s) in an rfb is reduced and multi-electron reagents are employed rfbs can achieve specific energies competitive with (or better than) lithium-ion batteries. besides cars, submarines and air electric vehicles are examples of such high-specific energy markets. in fact, the very first widely publicized flow battery was used to power an airship: on august 9, 1884, french inventor and military engineer charles renard flew the dirigible la france, powered by a zn-cl2 rfb [68,69]. zinc-chlorine [70,71] and zinc-bromine [70-93] flow batteries were investigated for use in fully electric vehicles (fevs) in the 1970s and 1980s, and so were, at different times, li-cl2 [87,94-97] and h2-libro3 [18,19,98-101] flow batteries. another type of high-specific energy flow battery is shown in figure 1c. it uses slurries instead of true liquids to store and transport energy within a flow battery. in the case of lithium-ion battery materials, the equivalent concentration of redox-electrons in a slurry can reach 24 m, and this is where the name of an mit http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 736 startup, which attempts to commercialize such technology, comes from [102-104]. 24 m is specifically interested in using slurry flow batteries for fully electric vehicles (fevs) [105]. in general, the equivalent electron concentration in lithium-ion battery intercalation materials varies between 20 and 24 m [106,107], whereas in all-liquid rfbs it is 1-2 m. a2 stationary energy storage the second market niche for rfbs is stationary energy storage (ses), where flow batteries may allow for a lower total cost of ownership in applications requiring long (e.g.,> 4 h) charge-discharge cycles [108]. let’s illustrate this with an example. figure 2. the capital costs of lithium iron phosphate (lfp) batteries (magenta) [109] and of vanadium redox flow (vrf) batteries (red, green, blue and violet)[110,111] the magenta line in figure 2 shows the cost of lithium-iron phosphate batteries approximated and extrapolated from the wholesale price data in ref. [109]. in our model, the cost of this seam battery scales proportionally with its nominal energy. the nominal power for seam batteries is also directly proportional to their energy, thus, one curve (magenta in figure 2) can represent batteries with a variable power energy rating. this is not the case for redox flow batteries (rfbs), where energy (the tanks) and power (the stack) ratings (as well as their costs) can be scaled independently from each other, as shown in figure 1b. for this reason, we use four cost-energy plots to illustrate the economics of vrfbs in figure 2: for 1 kw (red), 10 kw (green), 100 kw (blue) and 1 mw (violet) systems. in practice, the rfb’s cost advantage shows up only in systems with a design half-cycle runtime longer than 3-6 h. for example, in figure 2, the cost of vrfbs becomes lower than that of libs for the energy/power ratios over 7 h. we shall emphasize here that the cost and weight advantages of rfbs originate from their ability to scale their energy (tanks) and power (stack) independently, thus allowing for a cost, weight, runtime, etc. optimization depending on the application. for the sake of full disclosure, we shall note that: 10 2 10 3 10 4 10 5 10 6 10 7 10 8 102 10 3 10 4 105 10 6 107 s y st e m c o st , u s $ battery energy, wh cost of lfp batteries 420 $/kwh cost of vrf batteries e 350 $/kwh + p 800 $/kw 1 kw 10 kw 100 kw 1 mw y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 737 1. we were able to demonstrate in figure 2 the capital cost advantage of vrfb over libs, only when we assumed the vrfb’s capital cost of energy (350 $/kwh) on the lower end of the literature data [110]; 2. more sophisticated cost analysis methods (such as levelized cost of energy and net present value) yield more favorable outcomes for vrfbs, due to the longer cycle life of this technology [112 -117], but the calculated profit margin is nevertheless too low to justify investments into this technology, especially when risks and alternative investments are taken into account [118-124]; 3. our analysis does not take into account the effects of energy efficiency, which favors seam batteries due to their low operating current density, as discussed in our previous publication [53]. in order to properly account for energy efficiency, the cost of the energy storage system should be considered together with the cost of the input energy. solid energy boosters for redox flow batteries finally, let’s take a look at figure 1d. it shows the same stack as figure 1b and the same dissolved redox couples. what is different is the solid electroactive materials (seams) added to the energy tanks. these seams can exchange electrons with liquid phase redox-mediators, as shown in eqs. (10) and (11), as well as in figure 3. thus, the solid electroactive materials (seams), added to tanks with redox flow battery (rfb) fluid, serve as solid energy boosters (sebs). such systems are the subject of this review. another example of a redox flow battery with solid energy boosters (sebrfb) is shown in figure 11. in positive seam: fepo4 + li+ + [fe(cn)6]4= lifepo4 + [fe(cn)6]3(10) on the posode: [fe(cn)6]3+ e= [fe(cn)6]4(4) on the negode: s2− e= ½s22(5) in negative seam: ½li3ti2(po4)3 + ½s22= ½liti2(po4)3 + s2+ li+ (11) full cell: fepo4 + ½ li3ti2(po4)3 = ½ liti2(po4)3 + lifepo4 (3) potential, v vs. hg/hgo figure 3. cyclic voltammograms of (black) carbon electrode in 0.1 m lioh aq.; (blue) same as black with liti2(po4)3 on the carbon electrode; (green) same as black with li2s4 added; (red) same as black with lifepo4 on the carbon electrode; (magenta) same as black with k4fe(cn)6 added. digitized and replotted from data in ref. [10] -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -3 -2 -1 0 1 2 3 fe(cn)4-6 /fe(cn) 36 c u rr e n t d e n si ty , m a /c m 2 potential, v vs. hg/hgo 0.1m lioh liti2(po4)3 s2-/s2-2 lifepo4 c u rr e n t d e n si ty , m a c m -2 http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 738 redox flow batteries with solid energy boosters (seb-rfbs) offer several potential advantages over non-flow batteries with solid electroactive materials (see items 1-7 below) and over other redox flow batteries (see items 8-11 below) [7,125,126]: 1. seams that have too poor electric conductivity to be used in regular seam batteries can be used to store energy as solid energy boosters (sebs), as long as redox-fast soluble mediators are used to deliver this energy to the power stack [127]. even conventional seams with good electric conductivity can benefit from redox-targeting as their charge utilization improves [125,128,129]. 2. seb rfbs may allow for higher specific energy (lower weight and volume) than the seam batteries based on the same chemistry since the relative massand volumefractions of solvent(s), binders, conductors, current collectors, etc. in a seb battery are small or zero. for instance, the pure c6 licoo2 chemicals contain 157.7 ah/kg of reversible li+ charge, but an actual cylindrical (i.e., most densely packed) 18650 battery in 2017 had only 57.8 ah/kg or 37 % of the theoretical [130], apparently leaving room for an up to 2.7 times (= 1 / 0.37) improvement. in most commercial lithium-ion batteries, the mass dilution factor was between 4 and 5 (compare the theoretical values for rots-lib and actual values for seam li-ion in figure 4) [131]. the 3 to 5-fold gains in specific energy were considered significantly larger than any potential concurrent losses in the energy efficiency (see double voltage loss below), thus possibly justifying the preference of lithium-ion batteries of the seb-rfb type in electric vehicles. however, history took a somewhat different trajectory, and the specific energy of libs increased as well, reaching by 2022 ca. 70 % of the theoretical specific energy (see conclusions and outlook below). 3. for the same reason of eliminating/reducing coating/supporting materials and processes, the energy cost can be lower. 4. more uniform rates of solid-state reactions that reduce the stress and improve the durability of individual seam particles and of seam assemblies [125]. 5. the specific power of the system can be significantly increased [125]. 6. redox targeting assures better tolerance to overcharging and overdischarging [125]. 7. recycling used batteries is much easier [125]. 8. the specific energy and energy density of seb-rfbs are larger (3-15 times improvements have been demonstrated) [132] than those of no-boosters rfbs based on the same liquid-phase chemistry. 9. the seam layer on the electrode may inhibit undesirable reactions displayed by a bare current collector [133]. 10. soluble redox-couples and solvents that are prohibitively expensive for traditional rfbs can be used as mediators in seb-rfbs, because of the smaller amounts of the liquid electrolyte(s) that are required in the latter case. 11. due to their chemical decoupling of energy and power, seb-rfbs routinely achieve much larger area-specific power (asp) than other high-specific energy flow batteries, i.e., slurry [103, 104] and microemulsions [134,135] rfbs. 12. the main disadvantage of the seb-rfbs is the double voltage loss: for both negode and posode there is an overvoltage on the electrode as well as a voltage mismatch between the mediator and booster [132], while seam and traditional flow batteries have only one of these two types of overvoltages, as explained in supplementary material (thermodynamics of redox targeting). y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 739 figure 4. volumetric energy density versus specific energy for different flow battery systems. redrawn from the data in ref. [136] ed o – eb o / v figure 5. fraction of the booster’s charge accessible during 20-80 % soc cycling of a single redox mediator couple as a function of the standard redox potential mismatch (ed° -eb°) between the mediator (d) and the booster (b) for different values of the booster’s “interaction parameter”  shown in the inset as different colors. in all cases n = 1 it is worth examining the voltage loss/mismatch between a solid booster and a dissolved mediator in more details, since such a phenomenon exists neither in seam batteries nor in traditional flow batteries, and is unfamiliar to most readers. when one mediator couple per solid energy booster is used, this booster/mediator energy mismatch manifests itself not as voltage loss a c c e si b le s o c o f th e b u st e r http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 740 but as an incomplete charge utilization of the booster or the mediators or both. this is illustrated in figure 5, where the x axis shows the mismatch between the standard redox potential of booster b and of the dissolved d mediator. the y-axis shows the fraction of the booster’s ah capacity, which can exchange charge with the dissolved mediator. for the latter, we assumed a fast 1-electron reaction with a nernstian slope of rt/f, i.e., a thermal boltzman distribution of electronic density of states. for the booster, however, we assumed a broader distribution, defined by an “interaction parameter” 𝛽, as described in supplementary material (thermodynamics of redox targeting). as evident from figure 5, narrow boosters (with 𝛽 ≥ 1.5) can achiever over 85 % charge utilization, but only if a dissolved mediator with a perfect match with the standard redox potential of the booster is used. even a 50 mv potential mismatch would result in poor utilization of the booster capacity. on the other hand, a broad booster (see the curve with 𝛽 = 5 in figure 5) can tolerate a larger mismatch in standard potentials, but the booster’s charge utilization is low even under the best conditions. poor charge utilization of the booster’s capacity defeats the whole purpose of using a seb to increase the specific energy of a flow battery system. in most demonstrated systems, the problem of poor booster utilization is solved by using two dissolved mediating couples for each electrode: one has a redox potential more positive than the booster’s, and the other – more negative. the couples on the outer end of the boosters’ voltage window are used to charge the boosters, and the couples inside the boosters’ voltage window are used to discharge the boosters, as shown in figure 9 below. the use of two mediating couples for one booster solves the problem of poor utilization of the booster’s charge capacity, but it creates an additional source of voltage loss and thus reduces the charge-discharge energy efficiency. usually, this double voltage loss is mitigated using solid energy boosters (sebs) with a large voltage difference between them. however, the larger voltage difference requires using a solvent with a large voltage window, which are either non-aqueous solvents or water-in-salt systems. both suffer from poor ionic conductivity, which manifests itself in lower area-specific power. the inferior energy efficiency of seb-rfbs is a major factor against their use for stationary energy storage. however, rots-rfbs have been considered not only for stationary energy storage (where efficiency and cost are the decisive factors) but also for high specific-energy applications (such as the military), and thus we shall examine such applications in more details below. what are the appropriate figures of merit to compare seb-rfbs with seam-bs and regular rfbs? for electric vehicles, specific energy (wh/kg), i.e., the system weight, is the most important. for flow batteries, a great challenge has been lowering the cost of power. thus, in these cases, comparing area-specific power (asp) is apposite. however, peak asp is usually observed near a 50 % energy efficiency, which is too low for practical applications. for this reason, we chose to compare the asp at 0.750.5 = 86.6 % one-way efficiency, and only for discharge, since such data are usually more readily available than the data for charge. there are at least two ways to look at the energy storage system in figure 1d. one can start with an rfb shown in figure 1b and treat the seams added in figure 1d as solid energy boosters for the flow battery. in this view, the system in figure 1d increases the specific energy of the system in figure 1b (since the seams normally have a high ah/kg content than liquids with dissolved redox species) and reduces the cost of energy (if the $/ah cost of the boosters (seams) is lower than that of the dissolved mediators (deams). alternatively, one can start with a picture of a seam battery in figure 1a, remove non-electroactive solids from the electrode layers, move current collectors to a power stack, and add dissolved mediators to transfer the charge from the seams to the current y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 741 collector. in this case, one would refer to the seams as redox-targeted solids (rots). such different views partially explain the different terminology discussed below in relation to table 1. for the sake of completeness, we shall mention that redox-targeting has been applied to some non-flow batteries (i.e., where the liquid phase with soluble mediator(s) does not circulate), e.g., to lithium-sulfur batteries [137,138] and non-solid energy boosters, such as to o2 oxidant from the air in redox-mediated fuel cells [139,140]. this review focuses, nevertheless, on systems that employ both flowing reagents and solid energy boosters. qi and koenig proposed in 2017 a classification of redox flow batteries with solid-electroactive materials [7]. in addition to three kinds of slurry rfbs, they mention type iv: type i: flowing carbon as electrochemical reaction electrodes; type ii: flowing solid active materials with flowing carbon conducting network; type iii: flowing active material particles colliding on current collectors without carbon; type iv: targeted redox mediators as the power carriers with static solid active materials providing energy storage. the focus of this review would be classified as type iv. our search and data analysis methodologies are described in supplementary material (patent search and non-patent search). terminology table 1 shows what terminology was used to describe the subject of this review in different years. the overall breakdown is: 107 redox mediator; 73 redox-targeted solid (rots); 32 solid energy booster (seb); 11 chemical redox; 7 redox-assisted; 5 redox relay; 2 insoluble redox-active material. however, it is more instructive to analyse the change in the terminology usage shown in table 1. table 1. the number of different terms used to described rots/seb-rfbs in different years (the earliest priority year is used for patent families and the publication year for all other documents) year occurence per year chemical redox insoluble active material redox assisted redox mediator redox targeted solid redox relay solid energy booster 2006 1 2 2 2 2007 1 1 2008 2009 2010 2011 1 2012 2013 2 1 2014 6 3 2015 1 6 6 2016 1 13 6 1 2017 3 2 20 9 3 2018 1 2 16 8 1 3 2019 2 1 17 12 7 2020 2 1 11 10 1 6 2021 1 1 10 11 10 2022* 1 2 4 3 total 11 2 7 107 73 5 32 *the data for 2022 are incomplete at the time of writing. http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 742 one can see that the terms redox relay and chemical redox flow batteries are the oldest, but they never gained popularity. the term flow batteries with redox mediator(s) came second, and it was displaced by the term flow batteries with redox-targeted solids. currently, the term flow batteries with solid energy boosters is gaining popularity. the terms chemical redox flow batteries, redoxassisted flow batteries and flow batteries with insoluble active materials were used only a few times and quickly fell out of use because these terms are neither descriptive nor accurate. we believe the terms solid energy boosters and redox-targeted solids are the most appropriate, and we will use them preferably throughout this review. history of redox flow batteries with solid energy boosters a summary of patenting and publication activities in seb-rfb area is shown in table 2. it is worth noting that these trends follow the classical triz cycle of innovation, which alternates between fast growth and plateau stages [141], with the very first triz innovation cycle being completed nowadays. as a result, despite the short (ca. 15 years) history of seb-rfbs, it can be split into four time periods, as shown in figure 6 and tables 2 and 3: 2006-2010 proof of concept; 2011-2015 independent verifications; 2016-2018 explosive growth; 2019-2022 steady growth. we shall discuss each of these periods separately below. 2006 2008 2010 2012 2014 2016 2018 2020 2022 0 2 4 6 8 10 12 2016-2018 explosive growth 2011-2015 independent verifications 2006-2010 proof of concept n u m b e r o f d o c u m e n ts p e r y e a r year patent families journal articles other documents 2019-2022 steady growth figure 6. publications (patent families, journal articles and other) related to seb-rfbs vs priority or publication year y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 743 table 2. publications about seb-rfbs sorted by reference type year all by year patent families books and chapters conf. papers journal papers anywhere filed in us editorials original reviews 2006 3 1 1 1 2007 1 1 2008 0 2009 0 2010 0 2011 2 1 1 2012 0 2013 2 1 1 2014 9 5 3 1 2015 9 2 1 3 3 2016 26 8 7 5 6 2017 28 10 7 2 1 6 2 2018 28 12 9 1 5 1 2019 22 7 3 1 1 1 7 2 2020 15 5 1 6 3 2021 11 1 7 3 sum 156 52 33 3 3 2 43 20 table 3. counts of patent families (in red) and non-patent publications (in blue) about seb-rfbs sorted by inventors’/authors’ country* year p r c h in a e p o f in la n d f ra n ce g e rm a n y is ra e l ja p a n s .k o re a m a la y si a s in g a p o re s p a in s w e d e n s w it ze rl a n d u k u s a as priority office for patent applications 27 2006 1 1+1 1 2007 1 2008 2009 2010 2011 1 2012 2013 1 1 1 1 1 2014 1 1 3 2015 1 1+1 5 1 2016 6 1 6 1 1 2017 8 1+7 1 1+3 2018 1 1 7 1+6 1 2 2019 2 2 4 1 1+5 1 1 1 3 2020 5 4 1 3+1 2021 1+1 1 1 1 3 1 1 non-patent publications 7 3 1 1 1 1 1 40 1 1 6 10 patent families by inventors’ country 3 1 2 1 27 1 3 2 2 10 sum 10 2 3 1 3 1 28 2 1 43 1 1 8 2 20 *the “year” column for patents refers to the earliest priority date, and for all other documents to the publication date. the data for 2021 and after are incomplete due to patent publication delays (usually 18 months between priority and publication dates) [142]; a bubble plot version of this table, sorted by applicant, is provided in supplementary material (2022 patent thickets) http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 744 2006-2010: proof of concept the earliest documented evidence of the idea of flow batteries with solid energy boosters can be traced to patent application ib2006/053832 (publication number wo2007/116363)[143] filed on 2006-04-07 by high power lithium sa [143,144], a venture capital-funded start-up company in lausanne, switzerland. the goal of this work was to improve the utilization of lithium-ion posode materials (with an example of lifepo4) in lithium-ion batteries and to reduce the amount of conductive carbon additives at the same time. the idea of dissolved redox-mediators came about as an extension of the inventors’ earlier work on adsorbed redox-mediators (“molecular wires”) for a poor electronic conductor lifepo4 [145]. the inventors prepared an electrode layer consisting of 95 wt.% lifepo4 and 5 wt.% of pvdf without carbon additives (see figure 7). this electrode did not show noticeable electrochemical intercalation of lithium ions, as was expected from a poorly electronically conducting lifepo4. upon addition of dissolved 10-methylphenothiazine (or 2,2’-bipyridine derivative complexes of os or ru) to the posolyte, a significant increase (not shown) in the accessible charge was observed, suggesting the occurrence of reactions similar to (10) and (11). the inventors concluded that the addition of a dissolved mediator allowed for electrochemical polarization of the whole particle network by the current collector even though the lithium insertion material is electronically insulating and no carbon additive is used to promote conduction [146]. the inventors also reported an unexpected beneficial role of carbon nanotubes added to such a system with solid electroactive material and a dissolved redox-mediator. indeed, if one looks at equations (10) and (11), one may conclude that these are purely chemical processes that proceed via a direct transfer between a solid booster and a dissolved mediator. in this scenario, there is no room for catalysis by carbon nanotubes. however, if one adds reactions (4) and (5), which are electrochemical and allow for a spatial decoupling of the electron donor and the electron acceptor sites, the “catalytic effect” of the carbon nanotubes becomes understandable. figure 7. the principle of redox targeting an insulating electrode material such as lifepo4 by a freely diffusing molecular shuttle s. redrawn from a figure in ref. [147] the patent application ib2006/053832 [146] was filed by high power lithium (without naming epfl) directly with wipo as the first receiving office. although the practice of patent filing directly at wipo bypassing a national office is allowed by the patent cooperation treaty of 1970 per rule 19.1(a)(iii), it is rather rare in general. however, it is a common practice in switzerland because this country typically issues patents without substantial examination and leaves it up to the courts to decide the patent’s validity. using wipo’s headquarters as a filing office allows swiss inventors to get their patents examined by the swiss federal institute of intellectual property before issuance, thus affording such patents presumption of validity in swiss courts [148]. y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 745 the application eventually entered the national phase in austria (via the epo), india, pr china, usa, s. korea and japan, as shown in figure 2. the unusually large (for a small company and compared to the other patent applications from hpl) geographic scale implied that the company considered this invention as highly valuable commercially. nevertheless, while the applications were still pending, high power lithium (hpl) was acquired by new york stock exchange-listed dow inc., headquartered in midland, mi, usa. soon after, dow abandoned all three patent families that it acquired with hpl, including ib2006/053832 (figure 4), despite favorable reports from the patent examiners. during this period, the lausanne group published two journal communications [147,149] based on the results disclosed in the aforementioned [146] patent application. the angewandte chemie communication was focused on charge-discharge cycling of a poorly electronically conducting lifepo4 using either two dissolved mediators (both were os+2/+3 complexes with substituted 2,2’-bipyridine ligands) or one (10-methylphenothiazine, mptz) [149]. the journal of power sources communication added triphenylamine, phenoxazines and phenothiazines, known as overcharge protectors in lithiumion batteries [147]. after epfl, qing wang took a postdoctoral position at national renewable energy laboratory in colorado, usa, where he worked on different topics. the epfl team also did not immediately follow this work. thus, after the first proof-of-concept, the development of flow batteries with solid energy boosters came hiatus. 2011-2015: independent verifications in 2008, prof. qing wang started a tenure-track position at the department of materials science and engineering at the national university of singapore (nus). while most of his work at that time was related to dye-sensitized solar cells, his group published a few studies of seb-rfbs [106,136,150 -155]. they demonstrated redox-targeting of lifepo4 (using ferrocene couple on the low-potential side and 1,1’-dibromoferrocene couple on the high-potential side) [106] and of li0.5tio2 (using cobaltocene and decamethylcobaltocene couples) [151]. the two-flanking couple's approach [156,157] differed from the original approach, with one mediator introduced in the earlier work [126]. the difference between the two redox-targeting methods is discussed in supplementary material (thermodynamics of redox targeting). the nus team found that the slow electron transport inside poorly-conducting lifepo4 particles often limits the rate of (de)lithiation and that addition of carbon accelerates the process. during this period, some new research groups became interested in redox-targeting lithium battery materials. collaborators from bar-ilan university (israel) and technical university of munich (germany) demonstrated redox-targeting of sulfur using ferrocene derivatives [158,159]. a team from panasonic (japan) demonstrated redox targeting of lifepo4 with tetrathiafulvalene and of v2o5 with 9,10 – phenanthrenequinone [160]. collaborators from the massachusetts institute of technology (mit) and the university of california-oakland (uco) patented a battery where sulfur was reversibly redox-targeted using soluble benzoperyleneimide derivatives [161]. a team from sandia national lab (snl) filed a us patent application, where redox targeting polyoxometalates with soluble organic mediators was proposed along with prophetic-only examples [162]. neither the mit-uco [161], nor the snl [162] patents have been transferred to a manufacturer as of 2022-04. first review articles about flow batteries with solid energy boosters (redox-targeted solids) were also published during this period [136,155,163]. http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 746 2016-2018: explosive growth the most distinct feature of this period was aggressive patenting by panasonic (japan), shown as the red squares in figure 6 and data for japan in table 3. although it was limited only to japan, pr china and the usa, the number of filed patent families (21) was significant [164-184]. however, only one [182] of these applications had experimental data from a full cell and only two [178,179] from half-cells. all three of these applications were filed in 2018. in all previous years, panasonic reported prophetic examples only. during this period, prof. qing wang’s team at the national university of singapore (nus) was more active in journal publishing [10,127,128,185-200] (see table 3) than in patenting [201,202] (see the green line in figure 9), while their contribution to the development of the technology was much more substantial than that of panasonic’s. figure 8 shows a clever approach to redox-targeting lifepo4 (e°=3.45 v vs. li/li+) using the coexisting i3ˉ/3iˉ (3.15 v) and 2i3ˉ/3i2 (3.70 v) solution-phase redox couples. more recently, a swedish group used the same booster-mediator (iodine-triiodideiron phosphate) combination paired with sodium ”-alumina membrane and metallic na negode to demonstrate a 3 v battery [203]. regrettably, the high ohmic resistance of the ceramic membrane limited the cycling rate to 0.13 ma/cm2. c u rr e n t,  a potential, v vs. li/li+ figure 8. schematic diagram of redox-targeting of lifepo4 electrode using the triiodide/ /iodine/iodine triplet according to ref. [185] figure 9. cyclic voltammograms of f-dopped sno2-al2o3 electrode in 1 m litfsi in tegdme (blue) with lifepo4 coating on the electrode; (green) bare electrode with lithium iodide added to the electrolyte. the two waves correspond to the reactions of triiodide in fig. 15; (red) coated with lifepo4 and with lii in the electrolyte. digitized and redrawn from ref. [185] another group from the national university of singapore demonstrated redox targeting of lifepo4 using ferrocene and sym-dibromoferrocene [192]. they also reported that pre-lithiation of the membrane in such an anhydrous battery significantly improves its conductivity and reduces cross-over. prof. girault’s group at epfl, where qing wang performed the very first studies of sebs for rfbs [146,147,149], also became interested in seb-rfbs. they demonstrated the use of polyaniline (pani) as a solid energy booster (seb) coupled with fe3+/2+ dissolved mediator for the emeraldine-pernigraniline transition [204]. not surprisingly, the v4+ / v3+ couple was not effective as a redox-mediator for the leucoemeraldine-emeraldine transitions due to its well-known slow kinetics [205-207]. the 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 -80 -70 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 c u rr e n t /  a potential / v vs. li/li+ 5mm lii + lifepo4 lifepo4 5mm lii y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 747 addition of electronically conducting carbon black was found to increase the accessible charge of pani 3-fold, similar to what was reported earlier for other sebs [146,151]. a group from the nanjing university of information science and technology (pr china) demonstrated the use of ni and cr cyclopentadienes as well as of ethylviologen as redox-mediators for sulfur in li-s batteries [208,209]. several reviews touching seb-rfbs were published by qing wang’s group [10,187,193,195,199] and others [210]. 2019-2022: decline from the main players and increase from newcomers the most notable change during this period was a decline in the number of patent applications from panasonic (see the data for japan in table 3). since this decline started in 2018 (i.e., earlier than the 18-month publication delay prior to our patent search in 2022-03), it is caused by an actual decrease in the seb-rfb patenting activity by panasonic and not by a data availability artefact. neither panasonic’s own patent disclosures nor other publications explain the rationale behind the apparent withdrawal of the firm from the seb-rfb area. the number of non-patent publications from the national university of singapore (nus) and pr china also went somewhat during this period (see table 3). at the same time, there was a continuous growth of publications from the usa (see table 3) and the entrance of new research groups from multiple countries (germany, estonia, finland, france, s. korea, malaysia, spain, sweden, uk) albeit with only 1 or 2 publications. during this period, qing wang’s group at nus reported the use of redox-targeting in recycling spent lithium-ion batteries [211], and demonstrated nearly-quantitative utilization of seb in a non-aqueous sodium-ion battery (see figures 11 and 12). this group was also interested in developing rfbs with single molecule redox-targeting (smrt) [212] on both negode and posode: e.g., for lifepo4 in water using water-soluble ferrocene derivatives [213], for polyimide-nickel hexacyanoferrate [214], for alkaline nickel-metal hydride battery [215], and for two redox-transitions in prussian blue [216] for the more negative (3eˉ at 0.5 v vs. she) using ferri/ferrocyanide dissolved mediator (dm) and for the more positive (2eˉ at 1.2 v vs. she) using br2/brˉ dm (see figure 10) [217,218]. one of the most important works from nus during this period was the development of solid energy boosters for vrfbs [220]. the currently most popular version of vrfb chemistry, developed around 2012 at pacific northwest national laboratory in the usa [221,222], uses solutions comprising 2.5 m of dissolved vanadium species, 2.5 m of sulfate and 6 m of chloride. although this mixed sulfatechloride chemistry allowed for a ca. 3-fold increase in the vanadium concentration compared to the older sulfate-only chemistry, it still suffers from the precipitation of v3+ species at -5 °c and v2o5 at +50 °c [223]. the nus group wanted to add solid energy boosters (sebs) to the vanadium tanks so that the size, weight and, potentially, the operating cost of the battery would be reduced while allowing the use of the dissolved mediators at lower concentrations so that the precipitateon/clogging problems can be avoided. (vo)6[fe(cn)6]3 prussian blue analogue (pba) was discharged form of the posolyte’s seb. since it has a redox transition at the same potential as the vo2+/vo2+ transition, no other dissolved mediator was used. rde measurement found that the booster also served as an excellent electrocatalyst for the v5+/4+ redox couple in the solution (see figure 13). galvanostatic cycling solution (see figure 14) of a positive half-cell showed that the utilization of the pba booster was ca. 70 % [220]. although these results are encouraging, the durability of prussian blue analogues (pbas) in acidic solutions still needs to be demonstrated. http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 748 c u rr e n t,  a potential, v vs. na/na+ figure 11. schematic illustration of a redox targetingbased redox flow sodium-ion battery. the tanks are filled with naxv2(po4)3 granules. the equivalent sebs’ concentrations correspond to 14.7 m (molal, i.e. mol/kg of solvent) in the negolyte tank and to 7.4 m in the posolyte tank. 9-fluorenone in tegdme+dmso was used as a mediator in the posolyte and mptz in pc was used as a mediator in the negolyte. nafion+pvdf membrane. redrawn from ref. [219] figure 12. cyclic voltammograms of n-methylphenothiazine (mptz, in propylene carbonate) and 9-fluorenone (in glym 4 + dimethylsulfoxide mixture) and those of the solid material na4v2(po4)3. the scan rate is 0.1 v/s for redox molecules and 0.001 v/s for solid material. digitized and redrawn from ref. [219] figure 13. 5 mv/s cyclic voltammograms of a bare (black and green) and pba-modified (blue and red) glassy carbon disk electrode rotating 1600 rpm in 2 m h2so4 (black and blue lines) and in 2 m h2so4 + 5 mm vo2+ + + 5 mm vo2+ (green and red lines). digitized and redrawn from ref. [220] figure 14. (right) galvanostatic cycling at 30 ma·cm−2 of 0.60 m vo2+/vo2+ sulfate solution without (black) and with vanadium pba solid. negolyte is present in an excess. digitized and redrawn from ref. [220] 1.0 1.5 2.0 2.5 3.0 3.5 4.0 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 solid booster c u rr e n t / a .u . potential / v vs na/na+ na3v2(po4)3 na4v2(po4)3 na1v2(po4)3 o n s 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 -1.0 -0.5 0.0 0.5 1.0 c) 5 mm vo+2 + 5 mm vo 2+ on bare gc c u rr e n t d e n si ty , m a /c m 2 potential, v vs ag/agcl a) bare glassy carbon (gc) in 2 m h2so4 b) pba on gc d) 5 mm vo+2 + 5 mm vo 2+ on gc+pba 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.6 0.8 1.0 1.2 1.4 1.6 1.8 0.60 m vanadium solution e /v charge / ah vanadium prussian blue analog booster added y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 749 also notable during this period was a demonstration by a spanish group of redox targeting a wellknown seam material niooh/ni(oh)2 using a single ferri/ferrocyanide mediator couple in combination with several different negolytes (see fig. 15) [224]. fig. 16 shows experimental data [107] equivalent to our calculations in supplementary material (thermodynamics of redox targeting). using that terminology, we conclude from fig. 16 that niooh/ni(oh)2 is a supernernstian material, as expected for a two-phase electrochemical transition. in fig. 16, cycling ferri/ferrocyanide mediator in solution between 15 and 85 % state of charge (soc) results in a favorable cycling of a narrow niooh booster between 0 and 78 % soc, despite a ca. 80 mv mismatch (the exact value depends on the solution composition) in the standard redox potentials of the mediator and the booster. figure 15. operating principle of a solid niooh electrode coupled with ferrocyanide as a dissolved redox mediator. redrawn from a figure in ref. [224] state of charge figure 16. equilibrium potentials of the niooh booster and ferri/ferrocyanide mediator couple as functions of their state of charge. this figure is equivalent to fig. s-4-in the supplementary material (thermodynamics of redox targeting). replotted from the experimental data in ref. [107] 0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100 0.20 0.25 0.30 0.35 0.40 0.45 0.50 e / v v s h g /h g o state of charge dissolved mediator: fe(cn)3-/4-6 solid booster : niooh / ni(oh)2 85 % 15 % = 70 % 78 % 0 % = 78 % e / v v s. h g /h g o http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 750 in 2021-2022, several research groups undertook detailed studies of redox-targeting of lifepo4 booster with aqueous ferricyanide. the group of prof. emmanuel baudrin from université de picardie jules verne in france studied single-molecule redox targeting (smrt) of lifepo4 using ferri-/ferrocyanide couple, with e°’ tuned by the addition of dmso and licl [225]. the authors demonstrated that the total capacity of the posolyte can be doubled upon the addition of only 1 % of solid lfp by volume. it’s worth noting that to be able to achieve high flow rates and high seb utilization with a stable flow, they used porous lfp pellets prepared by spark plasma sintering rather than an lfp powder. to the best of our knowledge, the solid energy boosters in rfbs have not been used in a fluidized bed reactor. some interesting results came from fundamental studies as well. it was found that during chemical redox-targeting, the oxidation (delithiation) of lifepo4 was generally faster than the reduction (lithiation) of fepo4 [226]. also, the activation energy for the chemical pathways was higher than for electrochemical [226]. further studies are needed to find out if this effect is real, or an artefact of experiment/data analysis. what was clear, nevertheless, is that the redox-targeting reactions can be fast enough to experience mass-transport limitations in porous solids. in their following study, the university of virginia group improved their packed-bed reactor for the delithiation of ca. 60 nm lifepo4 particles, which eliminated mass-transport restrictions with just 0.3 m concentration of [fe(cn)6]3[226]. increasing the delithiation temperature from 22 to 40 °c did not have a significant effect on the reaction rate, suggesting that transport inside the booster may be rate-limiting. however, decreasing the delithiation temperature from 22 to 13 and 4 °c resulted in a reduced reaction rate, which could be due to reaction (10) becoming rate-limiting because of the slow diffusion of li+ in the solid phase. sanford, mcneil and their coworkers from the university of michigan demonstrated a general route to rots-rfbs based on the use of a dissolved redox-active monomer in solution (and of its corresponding polymer as a solid energy booster [227]. they found that ca. 50 % of the booster is utilized in their first non-optimized cell. among the most significant recent developments from pr china was a demonstration of zn-mno2 battery with a positive br2 posolyte mediator [228] and lithium-sulfur rotsrfb [209,229]. several reviews about seb-rfbs were also published during this period [230-233]. conclusions and outlook in the preceding discussion, we showed that flow batteries with solid energy boosters (sebs) had a turbulent history with peak activity in 2016-2018 and apparent abandonment of this technology by the main contributors thereafter. neither we nor the publications cited in this review discuss the reasons for the decline in the interest in this technology. we shall attempt to answer this question now. in disadvantage d5 we showed that redox flow batteries (rfbs) are inferior in the stationary energy storage (ses) market to batteries with solid electroactive materials (seam) in general (and to libs in particular) in terms of energy efficiency. we used lfp chemistry for this analysis because it is the lowest cost lib chemistry and because by 2022, it has been selected for both small and large stationary installations by most developers. the conclusions that we made for rfbs, in general, apply to sebrfbs as well, because the energy efficiency of the latter is worse than the energy efficiency of the former. thus, based on the energy efficiency considerations alone, one should reject the use of flow batteries (with or without solid energy boosters) in the stationary energy storage markets in favor of non-flow batteries with solid electroactive materials. one possible exception to this conclusion is market, where the cost of the input energy (and thus the energy efficiency) is less important than the y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 751 system’s durability and self-discharge. reserve power is an example of such a market, and it can be served well by traditional (without solid energy boosters) rfbs [53]. the question now is whether seb-libs are better than traditional (non-flow) libs for cars (and other types of vehicles), since longer driving range (i.e., battery’s system-level specific energy) is more important in this market, that battery’s energy efficiency [234,235]. figure 17 shows the historical progress of the specific energy of lithium-ion batteries. figure 17. specific energy of lithium-ion batteries (various formats and chemistries) vs. production year. references: 2015-crabtree [237], 2020-dominko [238] 2020-tesla [239] the red and blue triangles in figure 17 refer to the data from recent review articles, where neither the battery chemistry nor battery formats were differentiated. it is worth noting that in thirty years since their first commercial introduction libs specific energy increased by a factor of 3.75. in 2020 tesla, inc. reported [236] that the specific energy of panasonic’s 2170 format cylindrical cells used in model 3 is 260 wh/kg, i.e., 57 % of the value for pure chemicals (459 wh/kg), as shown in figure 17. if this chemistry is used in a seb-rfb, such system-level energy dilution (57 %) will be achieved only if the booster tank and the stack have over 75 % (i.e., 0.570.5) energy efficiency each, even if we do not account for the stack weight of the stack, redox fluids and pumps. this simple estimation shows that the contemporary traditional seam-libs surpassed the seb-libs in terms of specific energy at the system level. regarding the electric vehicles market, we conclude that any weight reduction gained due to the elimination of binders and current collectors when going from seam batteries to seb-rfbs, is likely to be negated by the reduced energy efficiency and by the addition of a power stack and related components. with both stationary and vehicle markets lost to traditional libs, redox flow batteries with solid energy boosters do not have a competitive market niche that can revive interest in this technology. 1990 1995 2000 2005 2010 2015 2020 0 100 200 300 400 500 s p e c if ic e n e rg y , w h /k g year 2015-crabtree 2020-dominko 2020-tesla lfp 2170 theoretical for c6 + lifepo4 theoretical for lic6 +3.125 li0.32ni0.8co0.15o2 s p e fi fi c e n e rg y , w h k g -1 http://dx.doi.org/10.5599/jese.1363 j. electrochem. sci. eng. 12(4) (2022) 731-766 flow batteries with solid energy boosters 752 abbreviations: asp area-specific power cv cyclic voltammogram or cyclic voltammetry deam dissolved electroactive material dm dissolved mediator dmso dimethylsulfoxide e° standard electrode potential epo european patent office eu european union fev fully electric vehicle gc glassy carbon hpl high power lithium, a swiss startup company ib international bureau of wipo ice internal combustion engine les liquid energy storage, i.e., traditional rfbs without solid energy boosters (sebs) lfp lithium iron phosphate lfpb lithium-iron phosphate seam battery (lfpb) lncmb lithium ion-nickel-cobalt-manganese oxide seam battery mit massachusetts institute of technology mptz 10-methylphenothiazine noss no stoichiometric solutes nus national university of singapore ocv open-circuit voltage pba prussian blue analogue pc propylene carbonate pct patent cooperation treaty of 1970 rfb redox flow battery rmf redox -mediating fluid rots redox-targeted solid, same as seb seam solid electroactive material seb solid energy booster, same as rots smrt single-molecule redox-targeting snl sandia national laboratories, new mexico, usa soc state of charge sql structured query language rfb redox flow battery rmf redox-mediating fluid rots redox-targeted solid(s), same as seb triz theory of solving inventive problems vrfb vanadium redox flow battery wipo world’s intellectual property organization uco university of california-oakland uk united kingdom of great britain and northern ireland usa united states of america acknowledgements: this work was supported in part by the russian science foundation (grant 1513-20038). y. v. tolmachev and s. v. starodubceva j. electrochem. sci. eng. 12(4) (2022) 731-766 http://dx.doi.org/10.5599/jese.1363 753 references [1] p. alotto, m. guarnieri, f. moro, redox flow batteries for the storage of renewable energy: a review, renewable and 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[239] m. stock, w. g. stock, intellectual property information. a case study of questel-orbit, information services and use 25 (2005) 163-180. https://doi.org/10.3233/isu-2005-253-404 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1002/chem.202200149 https://doi.org/10.1021/acs.nanolett.1c03319 https://doi.org/10.1021/acsenergylett.0c00611 https://doi.org/10.1088/1361-6463/ab3251 https://doi.org/10.1002/cssc.201903379 https://doi.org/10.1016/j.coelec.2021.100743 https://doi.org/10.1002/adma.202104562 https://arstechnica.com/cars/2021/10/tesla-made-1-6-billion-in-q3-is-switching-to-lfp-batteries-globally/ https://arstechnica.com/cars/2021/10/tesla-made-1-6-billion-in-q3-is-switching-to-lfp-batteries-globally/ https://www.caranddriver.com/news/a38414682/tesla-new-cheaper-battery/ https://www.caranddriver.com/news/a38414682/tesla-new-cheaper-battery/ https://www.reuters.com/article/us-tesla-batteries/teslas-musk-hints-of-battery-capacity-jump-ahead-of-industry-event-iduskbn25l0mc https://www.reuters.com/article/us-tesla-batteries/teslas-musk-hints-of-battery-capacity-jump-ahead-of-industry-event-iduskbn25l0mc https://doi.org/10.1557/mrs.2015.259 https://battery2030.eu/research/roadmap/ https://doi.org/10.3233/isu-2005-253-404 https://creativecommons.org/licenses/by/4.0/) @article{tolmachev2022, author = {tolmachev, yuriy v and starodubceva, svetlana v}, journal = {journal of electrochemical science and engineering}, title = {{flow batteries with solid energy boosters:}}, year = {2022}, issn = {1847-9286}, month = {sep}, number = {4}, pages = {731--766}, volume = {12}, abstract = {adding solid electroactive materials as energy boosters to flow battery tanks provides a path to electrical energy storing systems with unprecedently high specific energy and specific power, that can solve the needs of both automotive and stationary energy storage markets. this work reviews the physical and chemical principles behind this new class of flow batteries, the history of this technology, and the most promising research directions.}, doi = {10.5599/jese.1363}, file = {:d\:/onedrive/mendeley desktop/tolmachev, starodubceva 2022 flow batteries with solid energy boosters.pdf:pdf;:11_jese_1363.pdf:pdf}, keywords = {redox batteries, assisted flow batteries, mediating fluid, redox, redox mediators, solid electroactive materials, targeted solids}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1363}, } {simulation of square wave voltammetry of three electrode reactions coupled by two reversible chemical reactions} doi:10.5599/jese409 119 j. electrochem. sci. eng. 7(3) (2017) 119-129; doi: http://dx.doi.org/10.5599/jese.409 open access : : issn 1847-9286 www.jese-online.org original scientific paper simulation of square wave voltammetry of three electrode reactions coupled by two reversible chemical reactions milivoj lovrić divkovićeva 13, zagreb 10090, croatia mlovric@irb.hr received: july 15, 2017; accepted: august 7, 2017 abstract three fast and reversible electrode reactions that are connected by two reversible chemical reactions that are permanently in the equilibrium are analysed theoretically for square wave voltammetry. the dependence of peak potentials on the dimensionless equilibrium constants of chemical reactions is calculated. the influence of the basic thermodynamic parameters on the square wave voltammetric responses is analysed. keywords ecece mechanism; equilibrium conditions; voltammetry; theory introduction multistep electrode processes appear in electrochemistry of many organic compounds [1-9]. some of these mechanisms include chemical steps [10-15]. in this communication, a theory of three electrode reactions that are coupled by two reversible chemical reactions is developed for square wave voltammetry under equilibrium conditions. the acronym of this mechanism is ecece [16,17]. it was observed in reductions of dibromonitrobenzene [17] and benzoquinone dioxime [18-20] and oxidation of catechol in methanol [21,22]. theoretical models of ecece are developed for reversible electrode reactions and totally irreversible chemical reactions in cyclic voltammetry [16] and chronoamperometry [17,19]. the model it is assumed that the product of the first electrode reaction is the reactant of the first chemical reaction and that the product of the latter is the reactant of the second electrode reaction. also, it is assumed that the reactant and the product of the second chemical reaction connect the second and the third electrode reactions in the same way: a ↔ b+ + e (1) http://dx.doi.org/10.5599/jese.409 http://www.jese-online.org/ mailto:mlovric@irb.hr j. electrochem. sci. eng. 7(3) (2017) 119-129 simulation of square wave voltammetry 120 b+ + x↔ e (2) e ↔ f+ + e (3) f+ + y↔ g (4) g ↔ h+ + e (5) furthermore, it is assumed that the mass transport can be described by the stationary, planar, semiinfinite diffusion model and that the reagents xand yare present in great excess, so that the variation of their concentrations can be neglected. finally, it is assumed that both chemical reactions are permanently in the equilibrium. the model is based on the following differential equations: 𝜕𝑐a 𝜕𝑡⁄ = 𝐷 𝜕 2𝑐a 𝜕𝑥 2⁄ (6) 𝜕𝑐int1 𝜕𝑡⁄ = 𝐷 𝜕 2𝑐int1 𝜕𝑥 2⁄ (7) 𝜕𝑐int2 𝜕𝑡⁄ = 𝐷 𝜕 2𝑐int2 𝜕𝑥 2⁄ (8) 𝜕𝑐h 𝜕𝑡⁄ = 𝐷 𝜕 2𝑐h 𝜕𝑥 2⁄ (9) 𝑐int1 = 𝑐b + 𝑐e (10) 𝑐int2 = 𝑐f + 𝑐g (11) 𝑡 = 0, 𝑥 ≥ 0: 𝑐a = 𝑐a ∗ , 𝑐b = 𝑐e = 𝑐f = 𝑐g = 𝑐h = 0 𝑐x = 𝑐x ∗ , 𝑐y = 𝑐y ∗ (12) 𝑡 > 0, 𝑥 → ∞: 𝑐𝐴 → 𝑐a ∗ , 𝑐x → 𝑐x ∗ , 𝑐y → 𝑐y ∗ , 𝑐b → 0 𝑐e → 0, 𝑐f → 0, 𝑐g → 0, 𝑐h → 0 (13) 𝑥 = 0: 𝑐x = 𝑐x ∗ , 𝑐y = 𝑐y ∗ (14) 𝑐b,x=0 = 𝑐a,x=0 exp(𝐹(𝐸 − 𝐸1 0) 𝑅𝑇⁄ ) (15) 𝐾1𝑐𝑋 ∗ = ce,x=0 cb,x=0⁄ (16) 𝑐f,x=0 = 𝑐e,x=0 exp(𝐹(𝐸 − 𝐸2 0) 𝑅𝑇⁄ ) (17) 𝐾2𝑐y ∗ = 𝑐g,x=0 𝑐f,x=0⁄ (18) 𝑐h,x=0 = 𝑐g,x=0 exp(𝐹(𝐸 − 𝐸3 0) 𝑅𝑇⁄ ) (19) 𝐷(𝜕𝑐a 𝜕𝑥⁄ )x=0 = 𝐼1 𝐹𝑆⁄ (20) 𝐷(𝜕𝑐int1 𝜕𝑥⁄ )x=0 = (𝐼2 − 𝐼1) 𝐹𝑆⁄ (21) 𝐷(𝜕𝑐int2 𝜕𝑥⁄ )x=0 = (𝐼3 − 𝐼2) 𝐹𝑆⁄ (22) 𝐷(𝜕𝑐h 𝜕𝑥⁄ )x=0 = −𝐼3 𝐹𝑆⁄ (23) 𝑥 > 0: 𝐾1𝑐x ∗ = 𝑐e 𝑐b⁄ (24) 𝐾2𝑐y ∗ = 𝑐g 𝑐f⁄ (25) the meanings of symbols are reported in table 1. differential equations are solved by the numerical method [23]. the solution is the system of recursive formulae for the dimensionless current: 𝛷2,𝑚 = 𝑢5(𝑢1 + 𝑢2 − 𝑢3 − 𝑢4) −1 − ∑ 𝛷2,j𝑠m−j+1 m−1 j=1 (26) milivoj lovrić j. electrochem. sci. eng. 7(3) (2017) 119-129 doi:10.5599/jese.409 121 table 1. meanings of symbols 𝑐𝐴, 𝑐𝐵 , 𝑐𝐸 , 𝑐𝐹 , 𝑐𝐺 , 𝑐𝐻 concentrations of species a, b +, e, f+, g and h+ 𝑐𝐴 ∗, 𝑐𝑋 ∗ , 𝑐𝑌 ∗ concentrations of species a, xand yin the bulk of solution 𝐷 common diffusion coefficient 𝐸 electrode potential 𝐸1 0, 𝐸2 0, 𝐸3 0 standard potentials of the first, the second and the third electrode reactions 𝑑𝐸 potential step in square wave voltammetry 𝐸𝑆𝑊 square wave amplitude 𝐸𝑝 peak potential 𝐹 faraday constant 𝑓 square wave frequency 𝐼1, 𝐼2, 𝐼3 currents of the first, the second and the third electron transfers 𝐾1, 𝐾2 equilibrium constants of the first and the second chemical reactions 𝑅 gass constant 𝑆 electrode surface area 𝑇 temperature 𝑡 time 𝛷1,m = 𝑢6 + 𝑢7 ∑ 𝛷2,j𝑠m−j+1 − ∑ 𝛷1,j𝑠m−j+1 m−1 j=1 m j=1 (27) 𝛷3,𝑚 = 𝑢8 ∑ φ2,j𝑠m−j+1 − ∑ φ3,j𝑠m−j+1 m−1 j=1 m j=1 (28) 𝛷k = 𝐼k(𝐹𝑆𝑐a ∗ )−1(𝐷𝑓)−1 2⁄ (29) 𝑠p = √𝑝 − √𝑝 − 1 (30) m = 1, 2, 3 …. (31) 𝑢1 = (1 + 𝐾2𝑐y ∗ )−1 (32) 𝑢2 = 𝐾1𝑐x ∗ (1 + 𝐾1𝑐x ∗ )−1 exp(𝐹(𝐸 − 𝐸2 0) 𝑅𝑇⁄ ) (33) 𝑢3 = 𝑢2[1 + (1 + 𝐾1𝑐x ∗ ) exp(𝐹(𝐸 − 𝐸1 0) 𝑅𝑇⁄ )]−1 (34) 𝑢4 = 𝑢1𝑧3(1 + 𝑧3) −1 (35) 𝑧3 = 𝐾2𝑐y ∗ 𝑢1 exp(𝐹(𝐸 − 𝐸3 0) 𝑅𝑇⁄ ) (36) 𝑢5 = 5√𝜋 2⁄ 𝑢2𝑧1(1 + 𝑧1) −1 (37) 𝑧1 = (1 + 𝐾1𝑐x ∗ ) exp(𝐹(𝐸 − 𝐸1 0) 𝑅𝑇⁄ ) (38) 𝑢6 = 5√𝜋 2⁄ 𝑧1(1 + 𝑧1) −1 (39) 𝑢7 = (1 + 𝑧1) −1 (40) 𝑢8 = 𝑧3(1 + 𝑧3) −1 (41) the sum 𝛷 = 𝛷1 + 𝛷2 + 𝛷3 is reported as a function of electrode potential. results and discussion square wave voltammograms of the mechanism (1) – (5) under equilibrium conditions depend on the standard potentials of electron transfers and the dimensionless constants 𝐾1𝑐𝑋 ∗ and 𝐾2𝑐𝑌 ∗ of chemical reactions. an ideal case in which the concentrations of reagents xand ycan be changed experimentally from zero to the limit of solubility is investigated here. if 𝐸1 0 = 𝐸2 0 = 𝐸3 0, 𝐾1𝑐x ∗ = 1 and 𝐾2𝑐y ∗ = 1, the response is a single peak with the dimensionless net peak current δ𝛷p = 2.44 that j. electrochem. sci. eng. 7(3) (2017) 119-129 simulation of square wave voltammetry 122 appears at 𝐸p = 𝐸1 0. this is shown in fig. 1. the real peak current depends linearly on the square root of frequency, but the net peak potential is independent of frequency. if the concentration of y is reduced to zero, the third electrode reaction vanishes and the net peak current is diminished to 1.50 but the peak potential does not change. finally, if the concentrations of both xand yare set to zero, only the first electrode reaction occurs and the net peak current is 0.75 at the same peak potential. fig. 1 dimensionless square wave voltammograms of electrode reactions (1) – (5). 𝐸1 0 = 𝐸2 0 = 𝐸3 0, 𝑑𝐸 = 5 mv and 𝐸𝑆𝑊 = 50 mv. 𝐾1𝑐𝑋 ∗ = 1 and 𝐾2𝑐𝑌 ∗ = 1 (1), 𝐾1𝑐𝑋 ∗ = 1 and 𝐾2𝑐𝑌 ∗ = 0 (2) and 𝐾1𝑐𝑋 ∗ = 0 and 𝐾2𝑐𝑌 ∗ = 0 (3) fig. 2a shows that the variation of xconcentration may lead to the split response. if 𝐾1𝑐x ∗ = 300 and 𝐾2𝑐y ∗ = 1, two peaks appear at -0.135 v and 0 v vs. 𝐸1 0. the peak currents are 0.81 and 1.53, respectively, which means that the first electron is transferred at lower potential than the other two. if 𝐾1𝑐x ∗ = 1000 the first peak potential is -0.175 v vs. 𝐸1 0 and if 𝐾1𝑐x ∗ = 100 only a shoulder appears at about -0.13 v, both for 𝐾2𝑐y ∗ = 1. these indicate that the first chemical reaction consumes the first product b+ and decreases the formal potential of the first electrode reaction. the influence of yconcentration is shown in fig. 2b. if 𝐾2𝑐y ∗ = 300 the peak currents are 1.71 and 0.78 and the peak potentials are -0.145 v and -0.005 v vs. 𝐸1 0. as the second chemical reaction consumes the product of the second electrode reaction, the formal potentials of both the first and the second electron transfers are diminished and only the third transfer occurs at the standard potential. these explanations are confirmed by the relationships between net peak potentials and the logarithm of dimensionless constants 𝐾1𝑐x ∗ and 𝐾2𝑐y ∗ that are shown in fig. 3. if 𝐾2𝑐y ∗ = 1 and 𝐾1𝑐x ∗ > 300 the potential of the first peak is given by the formula: 𝐸p,1 − 𝐸1 0 = −2.3 (𝑅𝑇 𝐹⁄ ) log(𝐾1𝑐x ∗ ) + 0.005 v, which is predicted for an ec mechanism in the equilibrium [24]. the slope of the line 1 in fig. 3a is 0.020 v. it corresponds to the single peak responses. this slope can be explained by the theory of eee mechanism with unstable intermediates [3]: 𝐸p = [𝐸1 0 − (𝑅𝑇 𝐹⁄ ) ln(𝐾1𝑐x ∗ ) + 𝐸2 0 − (𝑅𝑇 𝐹⁄ ) ln(𝐾2𝑐y ∗ ) + 𝐸3 0 ] 3⁄ (42) 𝐸p = (𝐸1 0 + 𝐸2 0 + 𝐸3 0) 3⁄ − 2.3 (𝑅𝑇 3𝐹⁄ ) log(𝐾1𝑐x ∗ ) (43) milivoj lovrić j. electrochem. sci. eng. 7(3) (2017) 119-129 doi:10.5599/jese.409 123 a b fig. 2. square wave voltammograms of electrode reactions (1) – (5). net responses and their forward and backward components are shown. 𝐸1 0 = 𝐸2 0 = 𝐸3 0, 𝐾1𝑐𝑋 ∗ = 300 and 𝐾2𝑐𝑌 ∗ = 1 (a) and 300 (b). all other parameters are as in fig. 1 if 30 < 𝐾1𝑐x ∗ < 300 the main peak is preceded by the shoulder at about -0.13 v. the variation of the second dimensionless constant is shown in fig. 3b. the potential of the single peak is defined by eq. (42) if -1 < log(𝐾2𝑐y ∗ ) < 2. outside this range either the responses with shoulders or the split responses appear. if 𝐾2𝑐y ∗ < 0.005 the concentration of the third reactant g is much smaller than the concentration of the second product f+ and the formal potential of the third electrode reaction is higher than the third standard potential. at this point several special cases can be analyzed. the first one is the ece mechanism that appears if 𝐾2𝑐y ∗ = 0. fig. 4a shows the relationship between peak potentials and the logarithm of the first dimensionless constant. the slopes of straight lines 1, 2 and 3 are -0.03, -0.06 and -0.06 v, respectively. j. electrochem. sci. eng. 7(3) (2017) 119-129 simulation of square wave voltammetry 124 a b fig. 3. dependence of net peak potentials on the logarithm of dimensionless equilibrium constants of chemical reactions (2) and (4). 𝐸1 0 = 𝐸2 0 = 𝐸3 0 and 𝐾2𝑐𝑌 ∗ = 1 (a) and 𝐾1𝑐𝑋 ∗ = 1 (b). the lines are linear approximations the first slope corresponds to the average of the formal potentials of the first and the second electrode reaction: 𝐸p = [𝐸1 0 − (𝑅𝑇 𝐹⁄ ) ln(𝐾1𝑐x ∗ ) + 𝐸2 0] 2⁄ (44) 𝐸p = (𝐸1 0 + 𝐸2 0) 2⁄ − 2.3 (𝑅𝑇 2𝐹⁄ ) log(𝐾1𝑐x ∗ ) (45) the slopes of the second and the third straight lines correspond to either ec, or ce mechanism in the equilibrium [24]. this agrees with the previous calculations [15]. very similar relationships appear in the ecece mechanism in which the standard potential of the third electrode reaction is much higher than the other two standard potentials. this can be seen in fig. 4b. the slopes of straight lines in figs 4a and 4b are identical. milivoj lovrić j. electrochem. sci. eng. 7(3) (2017) 119-129 doi:10.5599/jese.409 125 a b fig. 4. dependence of net peak potentials on the logarithm of dimensionless equilibrium constant of the first chemical reaction. 𝐸1 0 = 𝐸2 0 = 𝐸3 0 and 𝐾2𝑐𝑌 ∗ = 0 (a) and 𝐸1 0 = 𝐸2 0, 𝐸3 0 − 𝐸1 0 = 0.5 v and 𝐾2𝑐𝑌 ∗ = 1 (b). the lines are linear approximations the second special case is the influence of the reagent yon the second and the third electrode reactions that are separated from the first electrode reaction. if the first standard potential is 400 mv lower than the other two standard potentials and if 𝐾1𝑐x ∗ = 1, the response consists of either two, or three peaks. this is shown in fig. 5a. the first peak is independent of 𝐾2𝑐y ∗ while the dependence of the second and the third peaks on the logarithm of 𝐾2𝑐y ∗ resembles the relationships of the ece mechanism. within a wide range -2 < log(𝐾2𝑐y ∗ ) < 2 the second peak satisfies the straight line 𝐸p,2 − 𝐸1 0 = -0.03 log (𝐾2𝑐y ∗ ) + 0.410 v. if 𝐾2𝑐y ∗ > 300 the second peak is defined by the equation 𝐸p,2 − 𝐸1 0 = -0.06 log(𝐾2𝑐y ∗ ) + 0.425 v and the third peak potential is equal to the third standard potential. if 𝐾2𝑐𝑌 ∗ < 0.001 the second peak tends to 0.42 v and the third peak approaches the asymptote 𝐸p,3 − 𝐸1 0 = -0.06 log(𝐾2𝑐y ∗ ) + 0.395 v. however, if the separation of the first electrode j. electrochem. sci. eng. 7(3) (2017) 119-129 simulation of square wave voltammetry 126 reaction from the other two is not high enough, the responses of the first and the second reactions may overlap under certain conditions. fig. 5b shows the relationships between peak potentials and the logarithm of 𝐾2𝑐y ∗ for the differences 𝐸2 0 − 𝐸1 0 = 0.2 v and 𝐸3 0 − 𝐸1 0 = 0.2 v. this figure differs from fig. 5a if 𝐾2𝑐y ∗ > 100. under the influence of high value of the second dimensionless constant, the formal potential of the second electrode reaction is diminished and approaches the first standard potential. the line 2 in fig. 5b is defined by the equation: 𝐸p,1 − 𝐸1 0 = -0.03 log (𝐾2𝑐y ∗ ) + + 0.1 v. this means that it satisfies the following relationship: 𝐸p = (𝐸1 0 + 𝐸2 0) 2⁄ − 2.3 (𝑅𝑇 2𝐹⁄ ) log(𝐾2𝑐y ∗ ) (46) a b fig. 5. dependence of net peak potentials on the logarithm of dimensionless equilibrium constant of the second chemical reaction. 𝐾1𝑐𝑋 ∗ = 1 and 𝐸2 0 − 𝐸1 0 = 0.4 v and 𝐸3 0 − 𝐸1 0 = 0.4 v (a) and 𝐸2 0 − 𝐸1 0 = 0.2 v and 𝐸3 0 − 𝐸1 0 = 0.2 v (b) milivoj lovrić j. electrochem. sci. eng. 7(3) (2017) 119-129 doi:10.5599/jese.409 127 these mixed responses can be formed only if the difference between formal potentials is smaller than 0.1 v [3]. if this condition is not satisfied, three peaks appear at all concentrations of the reagent y-. an example is shown in fig. 6. the straight lines 1 and 2 are given by the equations: 𝐸p,3 − 𝐸1 0 = -0.06 log(𝐾2𝑐y ∗ ) + 0.4 v and 𝐸p,2 − 𝐸1 0 = -0.06 log(𝐾2𝑐y ∗ ) + 0.32 v, respectively. they demonstrate that the second chemical reaction influences only the second electrode reaction if 𝐾2𝑐y ∗ > 10 and only the third electrode reaction if 𝐾2𝑐y ∗ < 0.1. fig. 6. dependence of net peak potentials on the logarithm of dimensionless equilibrium constant of the second chemical reaction. 𝐸2 0 − 𝐸1 0 = 0.3 v, 𝐸3 0 − 𝐸1 0 = 0.4 v and 𝐾1𝑐𝑋 0 = 1 the mixed response of the second and the third electrode reactions may split into two peaks if the second dimensionless equilibrium constant is bigger than the certain limiting value. fig. 7 shows the relationship between the logarithm of this critical constant and the difference between the third and the second standard potentials. it satisfies the following equation: log(𝐾2𝑐y ∗ )crit. = -17.4 (𝐸3 0 − 𝐸2 0) + 2.176 (47) the last special case considers the possibility that the reagents xand yare identical, so that their concentrations cannot be changed separately. in the simplest version of this mechanism it is assumed that 𝐾1 = 𝐾2. fig. 8 shows relationship between peak potentials and the logarithm of 𝐾1𝑐x ∗ under this condition. it can be compared to fig. 3. if 𝐾1𝑐x ∗ < 0.005 the first peak corresponds to the single electron transfer, while the second peak is a consequence of the transfer of two electrons. on the contrary, if 𝐾1𝑐x ∗ > 500 the first peak is twice as big as the second one. the slope of the straight line 1 is -0.040 v. this means that it satisfies eq. (42). considering that 𝐾2𝑐y ∗ = 𝐾1𝑐x ∗ one obtains: 𝐸p,1 = (𝐸1 0 + 𝐸2 0 + 𝐸3 0) 3⁄ − 2.3 (2𝑅𝑇 3𝐹⁄ ) log (𝐾1𝑐x ∗ ) (48) the slopes of straight lines 2 and 3 are -0.060 v. they both describe mixed responses that include two electrons. the first one is the cece mechanism and the second one is ecec mechanism: 𝐸p,2 = [𝐸2 0 − 2(𝑅𝑇 𝐹⁄ ) ln(𝐾1𝑐x ∗ ) + 𝐸3 0] 2⁄ (49) 𝐸p,2 = (𝐸2 0 + 𝐸3 0) 2⁄ − 2.3 (𝑅𝑇 𝐹⁄ ) log(𝐾1𝑐x ∗ ) (50) 𝐸p,1 = (𝐸1 0 + 𝐸2 0) 2⁄ − 2.3 (𝑅𝑇 𝐹⁄ ) log(𝐾1𝑐x ∗ ) (51) j. electrochem. sci. eng. 7(3) (2017) 119-129 simulation of square wave voltammetry 128 fig. 7. dependence of the critical value of the dimensionless equilibrium constant of the second chemical reaction on the difference between standard potentials of the third and the second electron transfer steps. 𝐾1𝑐𝑋 ∗ = 1 and 𝐸2 0 𝐸1 0 = 0.4 v fig. 8 dependence of net peak potentials on the logarithm of the common dimensionless equilibrium constant. 𝐸1 0 = 𝐸2 0 = 𝐸3 0 and 𝐾2𝑐𝑌 ∗ = 𝐾1𝑐𝑋 ∗ comparing to fig. 3, the response exhibits a single peak within rather narrow range -1 < log (𝐾1𝑐x ∗ ) < 1, conclusions the difference between the ecece and the simple eee mechanisms under equilibrium conditions is in the dependence of the former on the concentrations of the reagents of chemical reactions. by the variation of concentrations of these reagents, the potentials of peaks can be changed and the number of peaks can be increased under some conditions. depending on standard potentials of electrode reactions, several special cases of ecece mechanism can be distinguished. the first ece milivoj lovrić j. electrochem. sci. eng. 7(3) (2017) 119-129 doi:10.5599/jese.409 129 mechanism can be analyzed separately if the third standard potential is much higher than the other two. also, the second ece mechanism appears isolated if the first standard potential is much lower than the other two. the influence of dimensionless equilibrium constants of chemical reactions on the certain peak potential depends on the number of electrons that are transferred at this potential. if chemical reactions include the same reagent, the first two, or the last two electrons may be transferred according to the ecec, or cece mechanisms, respectively. references [1] a. molina, j. gonzalez, e. laborda, r. g. compton, int. j. electrochem. sci. 7 (2012) 5765 5778. [2] a. molina, e. laborda, j. m. gomez-gil, r. g. compton, j. solid state electrochem. 20 (2016) 3239 3253. [3] m. lovrić, š. komorsky-lovrić, j. electroanal. chem. 735 (2014) 90 94. [4] a. molina, c. serna, m. lopez-tenes, r. chicon, electrochem. commun. 2 (2000) 267 271. [5] r. gulaboski, v. markovski, z. jihe, j. solid state electrochem. 20 (2016) 3229 3238. [6] c. batchelor-mcauley, r. g. compton, j. electroanal. chem. 669 (2012) 73 81. [7] r. gulaboski, p. kokoškarova, s. mitrev, electrochim. acta 69 (2012) 86 96. [8] m. lopez-tenes, a. molina, c. serna, m. m. moreno, j. gonzalez, j. electroanal. chem. 603 (2007) 249 259. [9] a. molina, c. serna, m. lopez-tenes, m. m. moreno, j. electroanal. chem. 576 (2005) 9 19. [10] r. gulaboski, v. mirčeski, i. bogeski, m. hoth, j. solid state electrochem. 16 (2012) 2315 2328. [11] s. o. engblom, j. c. myland, k. b. oldham, anal. chem. 66 (1994) 3182 3187. [12] a. b. miles, r. g. compton, j. electroanal. chem. 499 (2001) 1 16. 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[23] l. k. bieniasz, modelling electroanalytical experiments by the integral equation method, springer, berlin, 2015. [24] f. scholz in: f. scholz (ed.), electroanalytical methods, springer, berlin, 2010, p. 11-31. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {carbon nanofiber modified with osmium based redox polymer for glucose sensing} doi:10.5599/jese.422 181 j. electrochem. sci. eng. 7(4) (2017) 181-191; doi: http://dx.doi.org/10.5599/jese.422 open access : : issn 1847-9286 www.jese-online.org original scientific paper carbon nanofiber modified with osmium based redox polymer for glucose sensing amos mugweru1,, reaz mahmud2, kartik ghosh2, adam k. wanekaya3 1department of chemistry and biochemistry, rowan university, glassboro, nj 08028, usa 2physics astronomy and materials science department, missouri state university, springfield, missouri 65897, usa 3chemistry department, missouri state university, springfield, missouri 65897, usa corresponding authors e-mail: mugweru@rowan.edu, tel.: +1-856-2565454; fax: +1-856-256-4478 received: august 22, 2017; revised: october 26, 2017; accepted: november 12, 2017 abstract electrochemical detection of glucose was performed on carbon nanofibers containing an osmium based redox polymer and using glucose oxidase enzyme. redox polymer assembled on the nanofibers provided a more stable support that preserved enzyme activity and promoted the electrical communication to the glassy carbon electrode. the morphologies, structures, and electrochemical behavior of the redox polymer modified nanofibers were characterized by scanning electron microscope, energy dispersive spectrometer and voltammetry. the glucose oxidase showed excellent communication with redox polymer as observed with the increased activity toward glucose. both cyclic voltammetry and amperometry showed a linear response with glucose concentration. the linear range for glucose determination was from 1 to 12 mm with a relatively high sensitivity of 0.20±0.01 μa mm−1 for glucose oxidase in carbon nanofibers and 0.10±0.01 μa mm−1 without carbon nanofibers. the apparent michaelis–menten constant (km) for glucose oxidase with carbon nanofibers was 0.99 mm. on the other hand, the km value for the glucose oxidase without the nanofibers was 4.90 mm. keywords cyclic voltammetry; amperometry; michaelis-menten kinetics; glucose sensors introduction regular monitoring of glucose concentration in the blood is a key in the diagnosis as well as treatment of diabetes. there is continuous interest in low cost, stable and sensitive biosensors for glucose. many glucose sensing platforms have been developed using glucose oxidase enzyme and various forms of redox mediators on electrodes [1-4]. glucose oxidase (gox) is the most popular enzyme extensively used in fabrication of electrochemical glucose biosensor [5] as well as in other http://dx.doi.org/10.5599/jese.422 http://www.jese-online.org/ mailto:mugweru@rowan.edu j. electrochem. sci. eng. 7(4) (2017) 181-191 carbon nanofiber for glucose sensing 182 bio-catalytic systems [6,7]. previously, osmium based redox polymer was used as a mediator entrapped together with gox hydrogels through the uv-initiated free radical cross-linking [1,2]. the advantage of using electrochemical methods with glucose oxidase enzyme in glucose sensors is the fact that the enzyme is very selective to glucose. like all enzymes, glucose oxidase stability is affected by the environment conditions such as ph and temperature [8]. incorporating conducting nanomaterials as a transducer material provides fast and accurate electron transfer at electrode surface. the combination of nanomaterials and conducting polymers has attracted remarkable attention for development of new immobilization matrices for enzymes. many glucose sensors are incorporating carbon nanotubes for immobilization of different electron transfer mediators [9,10]. carbon nanomaterials have excellent properties and unique structure for use in the study of direct electrochemistry of enzymes [11,12]. enzyme immobilization techniques are the key in improving its performance. recently, nanofibers have shown to be ideal in immobilization of enzymes [13,14]. carbon nanofibers made from nylon have been used for biosensing applications due to their excellent stability and biocompatibility [15,16]. nylon nanofibers have porous structure and have large surface area [17]. in general, the nanofibers are good materials for enzyme immobilization due to high loading resulted from high interfacial adhesion. direct electron transfer (det) between glucose oxidase enzyme and electrode surface would be ideal in glucose sensor. then, the electron transfer process is quite sluggish [18] and hence use of mediators is initiated. inclusion of mediators and carbon-based nanomaterials can greatly improve the electro-catalytic performance of these sensors. enzymes, including gox may denature upon absorption on the nanostructured surface. denaturation of the enzyme may result in decreased or total loss of function as a result of altered structure [19]. hybrid materials comprising of carbon nanofibers and redox polymer can induce a synergic effect of protecting the enzyme from the nanostructured surface, as well as providing a stable surface for immobilization onto a carbon surface. this can facilitate charge transfer from gox to redox polymer, as well as increase of conductivity of the composite polymeric film. nanofibers are promising nanomaterials as enzyme immobilization matrices [20]. in this work, poly[vinylpyridine os(bipyridine)2]cl (referred here as redox polymer) coupled with carbon nanofibers were used as materials in which gox was immobilized. new literature suggests that this redox polymer can also mediate electron transfer between cells and electrodes [21]. the osmium based redox polymer has well-established redox chemistry and has been used before in different matrices [22-24]. the os(ii)/(iii) redox potential is lowered when the osmium is complexed with ligands. we have previously used osmium redox polymers in construction of different biosensors [25,26]. the new material containing carbon nanofibers and osmium based redox polymers were used to fabricate a stable glucose sensor. experimental materials and reagents glucose oxidase (gox, ec 1.1.3.4, type x-s, 128 units/mg, solid from aspergillus niger), hexafluorophosphate, sodium dithionite, ether, n,n-dimethyl formamide, hydrochloric acid, ethyl alcohol, ethylene glycol, and acetonitrile were purchased from vwr (west chester, pa). pyrolytic graphite electrodes (area 0.07 cm2) were obtained from momentive performance materials quarts inc. the polishing alumina and 1 μm diamond polish were obtained from bioanalytical systems inc (west lafayette in). graphitized carbon nanofibers free from iron, poly(4-vinyl pyridine) and poly(ethylene glycol) di-acrylate (mw 575), were purchased from sigma-aldrich, while hexa-fluoroa. mugweru at al. j. electrochem. sci. eng. 7(4) (2017) 181-191 doi:10.5599/jese.422 183 phosphate, sodium dithionite, ether, n,n-dimethyl formamide, ammonium hexa-chloroosmate (iv) and 2-bromoethylamine hydro-bromide, were from alfa aesar. synthesis of redox polymer poly[vinylpyridine os(bipyridine)2]cl the poly-cationic redox polymer, poly[4-vinylpyridine os(bipyridine)2]cl (noted as poly-bipyoscl) was synthesized according to a procedure described previously [27-29]. os(bpy)2cl2 was synthesized according to a standard procedure with minor modifications [30]. briefly, bipyridine (1.44 g) and ammonium hexa-chloroosmate (iv) (2.0 g) were mixed in 100 ml ethylene glycol before refluxing for one hour. addition of a supersaturated solution of sodium dithionate to the reaction mixture precipitated os(bpy)2cl2. the product was repeatedly washed with water and finally with ether. in synthesis of poly[vinylpyridine os(bipyridine)2]cl, os(bpy)2cl2 (0.988 g) and poly(4-vinylpyridine) (0.860 g) were mixed and then heated to reflux under a nitrogen atmosphere to prevent oxidation. after two hours of reflux, the solution was cooled down to room temperature. 60 ml of dmf and 3.0 g of 2-bromoethylamine hydro-bromide were added and then stirred overnight at 50 °c. a crude polymer precipitate was formed by pouring the solution into rapidly stirred acetone. the soluble portion of the synthesized redox polymer was used in this work. the structure of this polymer is shown in figure 1. figure 1. structure of poly[vinylpyridine os(bipyridine)2]cl redox polymer, (poly-bipy-oscl) (a = 1, b = 4, c = 1) are based on initial reaction conditions electrochemical apparatus and preparation procedure cyclic voltammetry (cv) and amperometric techniques were carried out with a computer controlled electrochemical workstation (chi 660c, usa) with 98 % ohmic drop (ir) compensation. a three-electrode electrochemical cell was used for all electrochemical experiments. the working and the counter electrodes were obtained from bioanalytical systems inc (west lafayette, in). glassy carbon electrodes (0.07 cm2) were polished with 1 µm diamond polishing paste then ultra-sonicated in ethanol and distilled water successively for 1 min followed by rinsing in water. a pyrolytic graphite (pg) electrode functionalized with composite materials of carbon nanofibers, osmium based redox polymer and glucose oxidase was used as the working electrode. platinum wire was used as the counter electrode. ag/agcl, equipped with a glass tip, separated from the sample solution compartment by a salt-bridge containing kcl and terminating in a medium porosity glass frit, was used as the reference electrode. all electrochemical measurements were carried out at ambient conditions. the concentration of acetate buffers were 50 mm. a precursor solution 2 ml j. electrochem. sci. eng. 7(4) (2017) 181-191 carbon nanofiber for glucose sensing 184 of 10 mg/ml redox polymer and 0.5 ml of 50 mg/ml gox was made using the acetate buffer. this precursor solution was mixed with 0.5 g of carbon nanofibers (cnf) and then diluted to 10 ml with acetate buffer. after mixing well, 10 µl of this composite mixture was casted as dispersion on the electrode and left to dry at room temperature. using the electrode area of 0.07 cm2, we can estimate the polymer loading to be 0.3 mg/cm2, glucose oxidase was 0.36 mg/cm2 while the cnf loading was 7.1 mg/cm2. the electrode containing redox polymer, cnf and gox was later rinsed with water before use. the pictorial representation of the electrode preparation is shown in figure 2. figure 2. schematic illustration of the preparation of electrode containing carbon nanofiber redox polymer-glucose oxidase, pg-cnf-(poly-bipy-oscl)-gox results and discussion scanning electron microscopy and energy-dispersive x-ray spectroscopy the scanning electron microscope images were recorded using a fei quanta 200 scanning electron microscope with an oxford inca eds detector. figure 3 shows sem images of redox polymer with gox on cnf at different regions and different magnification. figure 3. sem images at different magnification of carbon nanofibers with redox polymer and glucose oxidase a. mugweru at al. j. electrochem. sci. eng. 7(4) (2017) 181-191 doi:10.5599/jese.422 185 the graphitized conical cnf were used as the anchor for both, the redox polymer and the gox. from figure 3 we can estimate the length of the longest nanofiber to about 5 µm. figure 3a shows the cnf functionalized with the redox polymer. the redox polymer is not uniformly distributed on the cnf, but it is making some cnf to be bound closer together. immobilization of the redox polymer on cnf results in formation of globular particles around the nanofibers (figure 3a). figures 3b-d show images at various magnifications of cnf with both, the redox polymer and the gox. it is obvious from these images, that the carbon nanofibers integrity was not affected. gox adhered well onto carbon nanofibers after immobilization. however, the adherence of gox to the nanofiber surface was not well-organized. figure 4 shows the energy-dispersive x-ray spectroscopy (edx) of composite materials of cnf and the osmium based redox polymer. the presence of the osmium signal indicates that the polymer was incorporated into the composite material. figure 4. edx spectrum of the redox polymer carbon nanofiber composite directly casted on the glassy carbon cyclic voltammetry figure 5 shows cyclic voltammograms of the redox polymer casted on a glassy carbon electrode with and without cnf. reversible pairs of oxidation-reduction peaks correspond to the os(ii)/os(iii) redox couple. figure 5. cyclic voltammograms of redox polymer on glassy carbon electrode recorded at 50 mv/s scan rate in 50 mm acetate buffer, ph 5.5: (blue) (poly-bipy-oscl); (red) cnf-(poly-bipy-oscl) j. electrochem. sci. eng. 7(4) (2017) 181-191 carbon nanofiber for glucose sensing 186 the redox polymer on cnf shows an enhanced signal compared with polymer electrode, what is seen as peak amplitudes that are about three times greater than those of the redox polymer by itself. however, incorporation of the redox polymer in the cnf resulted in a 0.05 v positive potential shift of the redox peak. the oxidation peak for redox polymer recorded at 50 mv/s scan rate occurred at about 0.44 v, while the reduction peak occurred at 0.34 v using the cnf. the oxidation peak for polymer by itself on the electrode surface appeared at 0.39 v while the reduction peak appeared at about 0.29 v. in earlier literature, the redox polymer trapped in polyethylene glycol gel still gave similar voltammograms but the peak current was unstable after continuous scanning [31,32]. for the redox polymer modified nanofibers, scanning several times over three weeks yielded the same peak current, indicating thus a good stability of redox polymer adsorbed on cnf. in presence of glucose the current using pg-cnf-(poly-bipy-oscl)-gox was found to increase (figure 6). presence of glucose caused the current increase. flavin adenine dinucleotide of glucose oxidase gox(fad) reacts with β-d-glucose to form a reduced form gox(fadh2) and gluconic acid and hydrogen peroxide. the reduced form of gox (fadh2) is in turn oxidized by the electrochemically generated os3+ form of the redox polymer, setting up a catalytic pathway which produces an enhanced oxidation peak. the electrons are transferred from the enzyme to the redox polymer, shuttled between the redox sites in self exchange reaction until being transferred to an electrode surface. the catalytic current produced is proportional to the glucose concentration. figure 6. (a) cyclic voltammograms of pg-cnf-(poly-bipy-oscl)-gox in 50 mm acetate buffer, ph 5.5, recorded at 50 mv s-1 scan rate after continuous addition of glucose; (b) calibration curve for glucose figure 6a shows the cyclic voltammograms of pg-cnf-(poly-bipy-oscl)-gox system recorded after continuous addition of glucose. the resultant voltammogram in presence of glucose shows a significant increase in the magnitude of the oxidation peak. figure 6b shows the observed peak current values plotted against concentration. the glucose concentration shows a linear relationship with the peak current up to about 12 mm of glucose. a plot of increase in the peak current versus the concentration of glucose yielded linear plot (y = 3.4 + 0.35x, r=0.996, n=10) with 0.20 ± 0.01 μa mm-1 sensitivity. glucose concentration higher than 12 mm did not yield a current in the linear region. the linear response obtained in the range of 1 to 12 mm is an improvement related to 0.05– 100 μm reported recently for comparable systems [33-35]. in the present system, presence of oxygen did not influence the rate of the reaction and hence the catalytic current is obtained. there was no difference in results obtained when oxygen was removed by bubbling the solution with nitrogen and blanketing the solution with nitrogen. a. mugweru at al. j. electrochem. sci. eng. 7(4) (2017) 181-191 doi:10.5599/jese.422 187 pg-cnf-(poly-bipy-oscl)-gox and pg-(poly-bipy-oscl)-gox systems were also compared using the cyclic voltammetry results. the oxidation peak currents of both systems were recorded with increased amount of glucose in the reaction cell. the catalytic currents obtained for both systems at different glucose concentrations are compared in figure 7. it is clear that presence of cnf in the system amplifies the catalytic current obtained using cyclic voltammetry. figure 7. comparison of glucose calibration curves for the electrode with nanofibers, pg-cnf-(poly-bipy-oscl)-gox and without nanofibers, pg-(poly-bipy-oscl)-gox amperometry figure 8a shows amperometric responses of both, pg-cnf-(poly-bipy-oscl)-gox and pg-(polybipy-oscl)-gox electrodes. each electrode was held at 0.35 v vs. ag/agcl and the current was continuously monitored as a function of time. for both systems, the amperometric responses showed a clear stepwise increase in the current value upon addition of glucose. however, the amperogram using pg-cnf-(poly-bipy-oscl)-gox shows a much higher current increase than pg(poly-bipy-oscl)-gox using the same amount of glucose. figure 8. (a) current versus time curve of pg-cnf-(poly-bipy-oscl)-gox and pg-(poly-bipy-oscl)-gox in 50 mm acetate buffer, ph 5.5, at 0.35 v vs. ag/agcl after continuous addition of glucose (b) comparison of current versus glucose concentration for pg-(poly-bipy-oscl)-gox and pg-cnf-(poly-bipy-oscl)-gox sensors. this could probably be due to a three-dimensional structure formed by anchoring the gox on cnf. this three-dimensional structure coupled with the large surface area of cnf resulted in the high sensitivity observed. glucose oxidase is comprised of two identical subunits. the two units have j. electrochem. sci. eng. 7(4) (2017) 181-191 carbon nanofiber for glucose sensing 188 two moles of flavin adenine dinucleotide (fad) which are located deep inside the enzyme [36]. for our sensor to provide immediate response, analytes have to move into the gox and electrons have to move into the redox polymer. the rapid response observed imply that this movement is not hindered at the same time as the cnf containing the redox polymer must remain in place. each step current observed represents an increase of 0.70 mm of glucose concentration. a plot of change of current versus the glucose concentration also yielded a linear plot (figure 8b). higher sensitivity of pg-cnf-(poly-bipy-oscl)-gox sensor compared to pg-(poly-bipy-oscl)-gox was observed. the sensitivity of electrodes with carbon nanofibers was 0.21 ± 0.01 μa mm-1, while the sensitivity of electrodes without the carbon nanofibers was 0.10 ± 0.01 μa mm-1. the use of cnf improves the sensitivity of this sensor by around fifty percent. the sensitivity of the current sensor is much lower compared with others recently reported [37,38]. for the system with cnf, the peak current plateaued at about 12 mm glucose concentration, while without cnf, the peak current plateaued at about 6 mm. glucose oxidase is known to have a short operational life due to the lack of stability. this is a major drawback in the construction of glucose biosensors [39]. to compare the enzymatic glucose metabolism in pg-cnf-(poly-bipy-oscl)-gox and pg-(poly-bipy-oscl)-gox systems, the michaelis– menten kinetics were determined and compared. based on the equations (1) and (2) and using the plots drawn in figure 9, imax and michaelis-menten constant (km) values for the gox were calculated for both systems. max cos ss cos = glu e m glu e i c i k c (1) m ss max max cos 1 1 1 = glu e k i i i c  (2) in equations (1) and (2), imax represents the maximum current achieved in the system, while the iss is the steady state current. figure 9 shows the lineweaver-burk plots of pg-cnf-(poly-bipy-oscl)-gox and pg-(poly-bipyoscl)-gox, respectively. the apparent km values with respect to glucose were estimated using the eq. (2) and plots in figure 9. these enzymatic kinetics studies showed that pg-cnf-(poly-bipy-oscl)-gox system had a km of 0.99 mm, while the calculated km value for pg-(poly-bipy-oscl)-gox was 4.90 mm. it is obvious that glucose oxidase enzyme catalytic property of pg-cnf-(poly-bipy-oscl)-gox is much higher than of pg-(poly-bipy-oscl)-gox enzymes. mass transfer limitations of glucose is the same in both electrodes and the only explanation for higher activity at cnf could be due to a loss of glucose oxidase activity in pg-(poly-bipy-oscl)-gox electrode. this might be due to the conformation change of the enzyme in this system. for most enzymes a favorable environment is essential for the optimum enzyme activity. our results indicate an extraordinary stability of the pg-cnf-(poly-bipyoscl)-gox system as compared to pg-(poly-bipy-oscl)-gox. the recent literature showed that km for glucose oxidase immobilized at other materials is equal to 2.84 mm [40]. here obtained value of the michaelis-menten constant of 0.99 mm is much lower than 5.20 mm for gox/gold-platinum alloy nanoparticles/carbon nanotubes/chitosan system [41] and also 1.42 mm for gox/pva-fe3o4/sn [42]. our experiment was performed in the acetate buffer of ph 5.5 which is slightly different from the optimum ph for gox activity. however, this ph is optimum for redox polymer mediator. a. mugweru at al. j. electrochem. sci. eng. 7(4) (2017) 181-191 doi:10.5599/jese.422 189 figure 9. lineweaver-burk plots for glucose oxidase (a) pg-cnf-(poly-bipy-oscl)-gox; (b) pg-(poly-bipy-oscl)-gox the ability of a sensor to discriminate other interfering species with similar properties to the target analyte is very important. oxidative species such as uric acid, fructose, sodium chloride and sucrose among others co-exist with glucose in human blood. the interference study was carried using amperometry by spiking 1.4 mm of each of the interfering agent in the electrolyte. the resulting amperogram for pg-cnf-(poly-bipy-oscl)-gox is shown in figure 10. figure 10. current versus time curve of pg-cnf-(poly-bipy-oscl)-gox in 50 mm acetate buffer, ph 5.5 at 0.35 v vs. ag/agcl after continuous addition of glucose, fructose, sodium chloride sucrose and uric acid the peak current obtained from glucose solution and interfering ions was noted and it proves that glucose oxidation current was dominant when compared to oxidation currents of uric acid, fructose, sucrose and sodium chloride. the small rise with addition of sodium chloride suggests the electrolyte solution may not have been properly buffered. in general, the pg-cnf-(poly-bipy-oscl)gox sensor shows high selectivity towards glucose. conclusions in this work, carbon nanofibers were used in conjunction with poly[vinylpyridine os(bipyridine)2]cl to prepare a sensor for glucose determination. both, the glucose oxidase and the 0 0.2 0.4 0.6 0.8 1 1.2 0 200 400 600 800 1000 1200 time, s glucose glucose fructose sucrose sucrose uric acid uric acid nacl nacl fructose j. electrochem. sci. eng. 7(4) (2017) 181-191 carbon nanofiber for glucose sensing 190 osmium based redox polymer adhered strongly onto the carbon nanofibers as observed from sem images. in this new system, glucose oxidase was found to be almost two fold active and more stable, using both cyclic voltammetry and amperometry techniques. the michaelis-menten constant (km) and steady state current (iss) values showed the improved gox performance in presence of carbon nanofibers, what is partly due to enhanced electron transfer from the glucose oxidase to the redox polymer. if properly optimized, the new material could be promising in improving glucose biosensors involving glucose oxidase enzymes. acknowledgements: the authors would like to thank the zahilis mazzochette and the department of chemistry and biochemistry, rowan university. references [1] a. mugweru, b. l. clark, m. v. pishko, electroanalysis 19 (2007) 453-458. 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[42] sanaeifar, m. rabiee, m. abdolrahim, m. tahriri, d. vashaee, l. tayebi, analytical biochemistry 519 (2017) 19-26. ©2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) {square wave and differential pulse voltammetric methods for the analysis of olivetol at gold electrode} doi:10.5599/jese.382 77 j. electrochem. sci. eng. 7(2) (2017) 77-88; doi: 10.5599/jese.382 open access : : issn 1847-9286 www.jese-online.org original scientific paper square wave and differential pulse voltammetric methods for the analysis of olivetol at gold electrode jyoti t. bagalkoti, vijay p. pattar and sharanappa t. nandibewoor p.g. department of studies in chemistry, karnatak university, dharwad 580 003, india corresponding author: stnandibewoor@yahoo.com; tel: +918362215286; fax: +918362747884 received: january 20, 2017; revised: february16, 2017; accepted: march 7, 2017 abstract few electrochemical techniques have been employed for the determination of the powerful antitumor agent olivetol (olv) in real samples at gold electrode. an intense and well pronounced oxidation peak was obtained at 1.04 v in the phosphate buffer ph 5.0 as the supporting electrolyte. the effects of ph and scan rate on the oxidation peak were studied. the electrochemical behavior of olv was investigated using cyclic voltammetric (cv), square wave voltammetric (swv) and differential pulse voltammetric (dpv) techniques. in dpv and swv, the gold electrode showed a good sensitivity for olv in a linear range of 0.1-1.5 µm and 0.1-1.3 µm and detection limits of 1.936×10-9 m and 4.754×10-9 m, respectively. a plausible mechanism involving an adsorption controlled oxidation reaction was deduced. the effect of various excipients was also studied and the method was successfully applied for the determination of olv in human biological samples. keywords olivetol, electrooxidation, voltammetric techniques, gold electrode introduction olivetol (olv) is one of the naturally occurring alkyl resorcinol and is also known as 5-pentylresorcinol or 5-pentyl-1,3-benzenediol. olv is benzendiol with an odd numbered carbon chain attached to 5th carbon of the benzene ring, as shown in scheme 1. scheme 1. structure of olivetol 1 2 3 5 6 4 http://www.jese-online.org/ mailto:stnandibewoor@yahoo.com j. electrochem. sci. eng. 7(2) (2017) 77-88 analysis of olivetol at gold electrode 78 olv shows an antibiotic behavior [1] and is known to reduce risks of cancer, cardiovascular disease, diabetes and other diseases associated with ageing. olv acts as an anti-tumor against kb cells that are reported to contain human papilloma viruses closely related to human genital, cervical and oropharyngeal cancers. the cytotoxic potency of olv is measured, as it produced a cd50 (concentration of a compound required for 50 % cytotoxic dose of cell growth) of 105.10 µm in 3(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2h-tetrazolium bromide (mtt) assay and 116.69 µm in sulforhodamine b protein (srb) assay [2]. several techniques like hplc [3], gas chromatography (gc) [4] and colorimetry [4,5] have already been used for the determination of alkyl resorcinols, but have proved, however, to be quite expensive and time consuming. therefore, a sensitive, accurate, cost effective and less time consuming technique is required. previously, we have studied the electrochemical property of olv using cyclic voltammetry and differential pulse voltammetry (dpv) with the help of pencil graphite (pge), glassy carbon (gce) and carbon paste electrodes (cpe) [6]. in continuation of our previous work on the determination of olv, another analytical method for the determination of olv is proposed in the present study. the objective of the study is to find suitable experimental conditions and to investigate the oxidation mechanism of olv on the gold electrode using square wave voltammetry (swv), differential pulse voltammetry (dpv) and cyclic voltammetric (cv) techniques. the gold electrode has been chosen because of its stability, wide potential window and fast rate of electron transfer. the method has been proved to be effective even at lower concentrations and also applied for the determination olv in human fluid samples and in investigation of pharmacokinetics. materials and methods reagents and chemicals 95 % pure olv was purchased from sigma-aldrich. 1.0 mm stock solution of olv was prepared using doubly distilled water and stored in the dark and cool place. 0.2 m phosphate buffer stock solutions were prepared within the ph range of 3.010.0 using 98.5 % pure h3po4, na2hpo4, na3po4 and kh2po4 [7,8] and double distilled water. authentic and analytical grade reagents and doubly distilled water were used in preparation of all solutions. instrumentation chi630 electrochemical analyzer (ch instruments inc., usa) was used in the present electrochemical study. the voltammetric experiments were carried out in a conventional threeelectrode system containing the working gold electrode, the platinum wire auxiliary electrode and the ag/agcl/kcl reference electrode. ph measurements were made using an elico li 120 ph meter (elico ltd., india). working electrode the working electrode used here is the au disk of 2 mm in diameter that was placed at the bottom of the three-electrode cell. since some diminishing of sensitivity of gold electrode due to poisoning of the electrode surface has frequently been observed with successive usage, a pretreatment of the electrode prior to every use is needed to acquire reproducible results. with this aim and prior to each experiment, the au working electrode was rinsed with double distilled water and polished to get a mirror-like finish using 0.3 m alumina powder on a smooth polishing cloth or polishing pads and rinsed again with double distilled water and dried. experiments were conducted at room temperature, 25 ± 0.2 oc. j. t. bagalkoti j. electrochem. sci. eng. 7(2) (2017) 77-88 doi:10.5599/jese.382 79 area of the working electrode the area of the gold working electrode was determined by variation of the scan rate in cyclic voltammetric experiments in 0.1 m kcl containing 0.001 m potassium ferricyanide. the effect of the scan rate on the peak current of the (fe(cn)63/4) redox reaction is given by the well-known randles–sevcik equation [9]: ipa = 0.4463 (f3/rt)1/2n3/2ad0 1/2c0 υ1/2 (1) in eq. (1), ipa is the anodic peak current, n represents the number electrons in the reaction,  is is the scan rate (v/s), a is the electrode area (cm2), co is the concentration of the probe molecule in the bulk solution (mol/cm3), while do is its diffusion coefficient (7.6 × 10−6 cm2 s−1 [9]). using eq. (1) and by knowing the constants, the surface area of gold electrode was calculated to be 0.399 cm2. experimental procedure before the beginning of the experiment, all three electrodes were rinsed using double distilled water and then connected to the electrochemical work station. ph was determined within a ph range of 310. the working parameters for cv experiments were chosen as: initial potential 0.4 v; final potential 1.6 v; scan rate 0.1 v/s; sensitivity 110-4 a/v. effect of the scan rate change was studied at ph 5.0 at the same working parameter values. the working parameters for swv experiments were chosen as: initial potential 0.4 v; final potential 0.9 v; sensitivity 110-5 a/v, while for dpv experiments the working parameters were chosen as: initial potential 0.4 v; final potential 1.4 v and sensitivity 110-5 a/v. all the working parameters were set and reviewed thoroughly before commencing the experiment. results and discussion cyclic voltammetric behavior of olivetol cyclic voltammetric behavior of olivetol (olv) at the gold electrode was studied in 0.2 m phosphate buffer, ph 5.0 solution as the supporting electrolyte. the cyclic voltammograms were measured for the bare gold electrode in either presence (fig. 1(a)) or absence (fig. 1(b)) of olv. fig. 1(a) shows one anodic peak at 1.041 v. on scanning in the negative direction, a reduction peak at about 0.5 v was observed as previously [10]. this peak can be ascribed to the reduction of gold oxides and will not be considered further. fig. 1. cyclic voltammogram of gold electrode in phosphate buffer ph 5.0 solution, recorded at scan rate of 0.1 v s-1 in: (a) presence of 0.1 mm olv; (b) absence of olv potential (v) c u rr en t 1 0 5 (a ) (a) (b) c u r r e n t /1 0 -5 a potential / vpotential, v c u rr e n t, 1 0 -5 a j. electrochem. sci. eng. 7(2) (2017) 77-88 analysis of olivetol at gold electrode 80 in cv experiments, the voltammograms obtained from the first run were always considered. this was due to apparent decrease in oxidative peak current values with successive sweeps which is usually ascribed to adsorption of oxidative products over the electrode surface [11]. effect of variation of ph effect of ph change on the electrooxidation of olv at gold electrode is shown in fig. 2. it can be observed in fig. 2(a) that the peak potential values are displaced towards more negative values as the ph was increased from 3.0 to 10.0 (curves (i-viii)). the highest peak current was observed for ph 5.0 and so all further experiments were carried at this ph value. the graph of epa versus ph drawn in fig. 2(b) showed a linear relationship with a slope of 58 mvph-1 and its linear regression equation is given by: epa = 1.299 − 0.058 ph (r2 = 0.874). the slope value of 58 mv ph-1 is close to the theoretical value of 59 mv ph-1 [12,13], indicating transfer of equal number of protons and electrons involved in the electrochemical reaction. fig. 2(a). cyclic voltammograms of gold electrode for 0.1 mm olv in the phosphate buffer solution recorded at scan rate of 0.1 vs-1 and ph 3.0-10.0 (i-viii) fig. 2(b). peak potential epa vs. ph dependence 1 0 -5 a (i) (viii) ph 5.0 potential, v c u rr e n t, 1 0 -5 a e p a / v 2.0 1.5 1.0 0.5 0.0 0 5 10 ph j. t. bagalkoti j. electrochem. sci. eng. 7(2) (2017) 77-88 doi:10.5599/jese.382 81 influence of scan rate effects of variation of the scan rate over domain of 0.1 – 0.5 v/s on cvs of gold electrode in presence of 1.0 mm olv are shown in fig. 3. the shapes of the voltammograms are obviously dependent on the applied scan rates. when the scan rates were varied in the ascending order (from 0.1-0.5 v/s), there was a steady increase in peak current values, showing a sensitivity of 1.0  10-4 a/v (fig. 3(a)). fig. 3. (a). cyclic voltammograms of gold electrode in phosphate buffer, ph 5.0 solution containing 0.1 mm olv at different scan rates (0.1-0.5 vs-1) the peak current versus scan rate drawn in fig. 3(b) showed a linear dependence, indicating adsorption regime. the straight line equation is given by: ipa = 12.73  + 0.858 (r2=0.999). fig 3(b). peak current vs. scan rate dependence from the graph of log of the ipa vs. log of the scan rate given in fig. 3(c), a straight line was obtained with a slope value of 0.760. this value is relatively close to 1.0 that is the theoretical value for the adsorption controlled electrode process. the obtained equation is given by: log ipa = 0.760 log  + 1.075 (r2=0.995). 1 0 -5 a (ix) (i) potential, v c u rr e n t, 1 0 -5 a i p a / 1 0 -5 a 0.0 0.2 0.4 0.6  / v s-1 j. electrochem. sci. eng. 7(2) (2017) 77-88 analysis of olivetol at gold electrode 82 fig. 3(c) log of peak current vs. log of scan rate dependence two more graphs showing epa vs.  (fig. 3(d)) and epa vs. log  (fig. 3(e)) were also drawn, which gave slope values of 0.225 and 0.137, respectively. with the reference to these graphs, it was observed that with the increase of the scan rate, the oxidation potential shifted towards more positive values. the dependence of ep on log log  is expressed as: epa = 0.137 log  + 1.242. fig 3(d). peak potential (epa) vs. scan rate (). (e) peak potential (epa) vs. log of scan rate (log ). in order to study the adsorption-controlled irreversible process at the electrode, laviron [14] suggested the following equation 0 0 pa 2.303 2.303 log log rt krt rt e e nf nf nf                      (2) in eq. (2), eo is the formal redox potential, r is the universal gas constant (8.314 j mol-1 k-1), t is room temperature (298.15 k), α is electron transfer coefficient, n is the number of transferred electrons, f is the faraday constant (96 480 c mol-1 ), ko is the standard rate constant of the surface reaction and  is the scan rate. from the experimental results obtained for the linear graph shown in fig 3(e), the value of αn was determined from its slope (m = 0.137). from the eq. (2) we have: -1.5 -1.0 -0.5 0.0 log / v s-1) lo g (ip a / 1 0 -5 a ) (d) 1.5 1.3 1.1 0.9 1.2 1.0 0.8 0.6 0.4 0.2 0.0 e p a / v 0 -0.2 -0.4 0.6  / v s-1 -1.5 -1.0 -0.5 0.0 log / v s-1) e p a / v 1.3 1.2 1.1 j. t. bagalkoti j. electrochem. sci. eng. 7(2) (2017) 77-88 doi:10.5599/jese.382 83 2.303rt m nf  (3) α was calculated from the bard and faulkner’s [15] equation: pa pa/2 47.7 /mv e e    (4) in eq. (4), epa/2 is the potential measured at the half of the peak current value and hence α = 0.206. the calculated number of electrons was 2.095 ≈ 2. by extending the line in the graph of epa vs.  to the intercept where υ =0 [16], we get e0= 1.096 v which was used to calculate the ko value using eq. (2): 0 0 2.303 log rt rtk c e nf nf              (5) in eq. (5), c is the intercept value obtained from the graph of epa vs. log v. from the above equation, it was possible to determine the value of ko as 1.95  102 s-1. mechanism from the previous results, the number of electrons involved in the reaction was found to be equal to the number of protons and calculated to be equal to 2. based on the earlier literature [6], a probable mechanism was proposed according to the scheme 2. scheme 2. probable mechanism for the electrooxidation of olv calibration curve and reproducibility swv and dpv voltammograms recorded between 400850 mv at various concentrations of olv are presented in fig. 4. fig 4. (a) square wave voltammograms of gold electrode recorded at various concentrations of olv (i-viii): 0.1, 0.3, 0.5, 0.7, 0.9, 1.1, 1.3, 1.5 µm; (b) differential pulse voltammograms of gold electrode recorded at various concentrations of olv (i-viii): 0.1, 0.3, 0.5, 0.7, 1.0, 1.3, 1.5, 1.7 µm c u rr en t 1 0 7 (a ) potential (v) c u rr en t 1 0 7 (a ) potential (v) (a) (b) (i) (i) (viii) (viii) (viii) (viii) (i) (i) c u rr en t 1 0 7 (a ) potential (v) c u rr en t 1 0 7 (a ) potential (v) (a) (b) (i) (i) (viii) (viii) (viii) (viii) (i) (i) c u rr en t 1 0 7 (a ) potential (v) c u rr en t 1 0 7 (a ) potential (v) (a) (b) (i) (i) (viii) (viii) (viii) (viii) (i) (i) c u rr en t 1 0 7 (a ) potential (v) c u rr en t 1 0 7 (a ) potential (v) (a) (b) (i) (i) (viii) (viii) (viii) (viii) (i) (i) c u rr en t /1 0 -5 a c u rr en t /1 0 -5 a potential / v potential / vpotential, v potential, v c u rr e n t, 1 0 -5 a c u rr e n t, 1 0 -5 a j. electrochem. sci. eng. 7(2) (2017) 77-88 analysis of olivetol at gold electrode 84 to check the precision of the method, five experiments were carried out on the same day at the same standard conditions which were authenticated by plotting the graphs of peak current values (ipa) versus concentration of olv, as is shown in fig. 5. the graphs gave linear calibration curves in the concentration range of 0.11.3 µm (fig 5(a)) and 0.1-1.5 µm (fig 5(b)), respectively. a slight deviation in the calibration curve was observed as the [olv] was increased, which is due to adsorption behaviour of olv or its oxidation products on the electrode surface. characteristics of the calibration plot are given in table 1. the limit of detection (lod) and the limit of quantification (loq) values shown in the table 1 were calculated using the following two equations: lod = 3 s/m and loq = 10 s/m. (6) in eq. (6), s is standard deviation of the intercept and m is the slope of calibration curve, respectively. fig. 5. (a) calibration plot (ipa vs. concentration of olv) obtained using swv; (b) calibration plot (ipa vs. concentration of olv) obtained using dpv table 1. characteristics of olv calibration plot obtained using swv and dpv techniques at gold electrode. swv dpv linearity, µm slope intercept correlation coefficient rsd, % (slope) rsd, % (intercept) lod, m loq, m 0.1-1.3 0.307 3.105 0.990 0.569 0.357 4.754×10-9 1.585×10-8 0.1-1.5 0.408 0.701 0.986 0.856 0.599 1.936×10-9 6.453×10-9 effect of excipients for probable analytical applications, the effect of certain common excipients used in pharmaceutical preparations on olv determination has to be been checked. here, effects of different and potential excipients were studied using the dpv technique and 1.0 µm olv in the presence of 0.1 mm of the chosen excipient. the results are listed in table 2. the results shown in table 2 suggest that presence of even hundred-fold excess concentration of an excipient like glucose, citric acid etc., along with certain ions like mg2+, k+, cu2+ etc., does not interfere much. thus, the present technique can be used to assay olv in the presence of various excipients. colv / 10-6 m colv / 10-6 m i p a / 1 0 -7 a 1.6 1.2 0.8 0.4 0.0 9 7 5 3 2 i p a / 1 0 -7 a 0 0.5 1.0 1.5 2.0 0 0.5 1.0 1.5 2.0 j. t. bagalkoti j. electrochem. sci. eng. 7(2) (2017) 77-88 doi:10.5599/jese.382 85 table 2. influence of various potential excipients on the dpv response for 1.0 µm olv. interferents, 0.1 mm epa / v rsd, % oxalic acid 0.7028 -0.26 gum acacia 0.7001 0.01 lactose 0.6969 0.33 glucose 0.6938 0.64 starch 0.6880 1.22 citric acid 0.6854 1.48 sucrose 0.6798 2.04 mgso4 0.6850 1.52 kcl 0.6806 1.96 cucl2 0.6519 4.83 sodium citrate 0.6850 1.52 effect of surfactants it is well known that surfactants affect strongly the electrode processes even if they are present in trace quantities [17]. surfactants prove to be effective in the electroanalysis of biological compounds and drugs [18]. it was also shown that anionic surfactants could be used to improve the accumulation of some electroactive organic molecules [19]. cationic and anionic surfactants with varying hydrocarbon chains affect the redox behaviour of electroactive species and bring some irregularities in the voltammograms [20,21]. in present study, the dpv technique has been adapted to study the effect of surfactants. addition of the cationic surfactant, cetyltrimethyl ammonium bromide (ctab), to the solution did not have any effect on the peak potential value, but the peak current decreased with increase of the concentration of ctab that is accumulated at the interface. the repulsion between the hydrophobic end of the ctab and water phase reduces the peak current as the concentration of ctab is increased [22]. the anionic surfactant, sodium dodecyl sulfate (sds), influenced the voltammetric behaviour in an opposite way i.e. the peak potentials remained the same but the peak current increased with the increase in concentration of sds. in both cases only the peak current of olv showed the surfactant concentration-dependent behaviour. detection of olv in urine samples the most striking feature of the method applied to urine samples is that not any prior extraction step is needed. urine samples from a healthy human volunteer were obtained for the study and the dpv method for the determination of olv was applied. the urine samples were diluted 100 times using buffer solution before the analysis and not any pre-treatment was made. the recoveries were measured by spiking urine samples with known concentration of drug. the recovery results for three samples are listed in table 3, together with standard deviations (sd) and relative standard deviations (rsd) values. it is shown in table 3 that the percentage recovery was in the range of 95101 % table 3: determination of olv in urine sample olv added, µm olv found(a), µm recovery, % sd ± rsd, % bias, % 0.2 0.19 95.00 0.0016+0.82 0.050 0.6 0.61 101.66 0.0017+0.27 0.017 0.8 0.81 101.25 0.0019+0.24 -0.012 (a) average of five determinations j. electrochem. sci. eng. 7(2) (2017) 77-88 analysis of olivetol at gold electrode 86 application of pharmacokinetics studies clinical applications of pharmacokinetics have resulted in significant improvement in the drug therapy. pharmacokinetic properties of a particular drug is important to determine the route of administration, dose, onset of action, peak action time, duration of action and frequency of dosing. pharmacokinetics studies help primarily in enhancing efficacy and decreasing toxicity during a drug therapy. an investigation of effects of various concentrations of drug at the site of action has always been of importance. direct measurement of drug concentrations at the receptor sites is hardly practicable as the receptor sites might be distributed all over the body. concentration of drug in blood or plasma, urine, saliva and other easily sampled fluids can, however, be measured. the results suggest that the disposition of olv was comfortable to a one compartment open model [23]. the rate constant of elimination (ke) in a one compartment system is obtained from the plot of concentration of olv versus time (fig.6). using this ‘ke’ value, half life of the drug was calculated which helps in understanding the time interval between the doses and is very important in design of infusion system. time. min fig 6. concentration of olv vs. time function in spiked urine sample table 4 presents the peak current values of 10 m olv recorded at different times (025 minutes), and the values of the elimination rate constant and half-life of the drug. the analytical data obtained suggest that the method is sensitive, reproducible and precise for the determination of olv. table 4. response of peak current of 10µm olv in urine sample at different time intervals time, min peak current, 10-7 a concentration of olv, 10-6 m 0 9.58 10.00 5 7.59 7.91 10 5.59 5.82 15 4.82 5.03 20 4.06 4.23 25 4.02 4.19 elimination rate constant (ke), h-1 1.12 half-life of drug, h 0.67 10 8.0 6.0 4.0 2.0 0.0 c o n ce n tr a ti o n / 1 0 -6 m j. t. bagalkoti j. electrochem. sci. eng. 7(2) (2017) 77-88 doi:10.5599/jese.382 87 detection of olv in serum samples dpv was also applied for the determination of olv in spiked human serum plasma sample. the recoveries were measured by spiking drug free serum plasma with known amounts of olv. a quantitative study was made by adding known concentration of standard drug solution in the system of plasma sample which was determined successfully. the results of determination obtained for three plasma samples with recovery and rsd are listed in table 5. table 5: determination of olv in human blood serum added, µm found(a), µm recovery, % sd± rsd, % bias, % 0.35 0.34 97.14 0.0019+0.72 0.028 0.55 0.56 101.18 0.0011+0.32 -0.018 0.75 0.73 97.33 0.0021+0.18 0.026 (a) average of five determinations conclusion the voltammetric oxidation of olv at gold electrode was successfully studied using phosphate buffer, ph 5.0 solution. the oxidation reaction is adsorption controlled and irreversible process. it was found that olv undergoes a transfer of two electrons and two protons and a plausible mechanism was proposed. a linear relation between peak current and concentration of drug was obtained within a certain range and even 4.7 nm can be used to assay the drug in human urine and blood serum samples. this work has also provided a sensitive and selective method for investigation of the pharmacokinetics of olv and its possible utilization in the clinical practice. acknowledgement: the authors thank university grants commission, new delhi for the award of ugc-bsr faculty fellowship to dr. s.t.nandibewoor, and bsr fellowship to jyoti t.bagalkoti. references [1] p. lopez, g. ferrao, phytotherapy research 25 (2011) 271–276 [2] s. h. baek, y. o. kim, j. s. kwag, k. e. choi, w. yjung, d. s. han, archives of pharmacal research 21 (1998) 353-356. 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[21] j. herovsky, j. kuta, principles of polarography; academic press: new york, 1966. [22] j. abbar, s. t. nandibewoor, journal of critical reviews in analytical chemistry 42 (2012) 272-281 [23] j. i. gowda, a. t. buddanavar, s. t. nandibewoor, journal of pharmaceutical analysis 5 (2015) 231-238. © 2017 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/) an overview of recent advances in the detection of ascorbic acid by electrochemical techniques: http://dx.doi.org/10.5599/jese.1561 1081 j. electrochem. sci. eng. 12(6) (2022) 1081-1098; http://dx.doi.org/10.5599/jese.1561 open access : : issn 1847-9286 www.jese-online.org review paper an overview of recent advances in the detection of ascorbic acid by electrochemical techniques raad muslim muhiebes1,, farzaneh fazeli2, issa amini3 and vahid azizkhani3 1department of chemistry, college of science, university of baghdad, jadiriya, baghdad, iraq 2department of biology, payame noor university, po box 19395-4697 tehran, iran 3department of chemistry, payame noor university, po box 19395-4697, tehran, iran corresponding author: raadmuslim7@gmail.com received: october 24, 2022; accepted: november 7, 2022; published: november 30, 2022 abstract ascorbic acid is a water-soluble vitamin essential in human nutrition, an antioxidant, a scavenger of free radicals in biological systems, and a cofactor of several enzymes. the reference range for ascorbic acid in healthy people is 6 20 mg l-1. the variable concentration of ascorbic acid within biology fluids was found in clinical investigations to be a metric for assessing the exact amount of oxidative stress in the body's metabolism. electroanalytical techniques are a group of methods in analytical chemistry, especially with extensive application in pharmaceutical industries. these techniques attracted further attention due to their unique characteristics, such as reduced sample or solvent consumption, high analysis speed, low operating cost, and high sensitivity, which made them suitable candidates for replacement or supplementation for spectrophotometry and separation approaches. the purpose of this article is to scrutinize the mechanisms and applications of current electroanalytical methods, including amperometric techniques, square wave voltammetry, differential pulse voltammetry and cyclic voltammetry, in their applications in pharmaceutical analysis for the detection of ascorbic acid. related examples have been cited in the form of selected studies. keywords electrochemical sensors; cyclic voltammetry; modified electrode; differential pulse voltammetry introduction vitamin c (vit c) or l-ascorbic acid (aa) is a ubiquitous water-soluble antioxidant found in numerous foodstuffs and biosystems. the physiologically and biochemically active vit c is an lenantiomer of ascorbic acid with γ-lactone structure, which is a pivotal factor in the formation of collagen as a key structural protein in the various body tissues. evidence shows the direct role of aa in many bioprocesses, including neuropeptide amidation, synthesis of adrenal cortical hormones, http://dx.doi.org/10.5599/jese.1561 http://dx.doi.org/10.5599/jese.1561 http://www.jese-online.org/ mailto:raadmuslim7@gmail.com j. electrochem. sci. eng. 12(6) (2022) 1081-1098 detection of ascorbic acid 1082 wound healing, amino acid metabolism, iron adsorption, free-radical or singlet oxygen scavenging and chelation. this potent antioxidant is a donor of two electrons by transferring a hydrogen atom to form the ascorbate radical ion, called semidehydroascorbic acid and dehydroscorbic acid. aa also reportedly conserves oxidizable components such as flavor and phenolic compounds. aa can develop relatively unreactive radicals that are difficult to amplify because it reacts rapidly with hydroxyl radicals. aa can control diseases caused by free radicals due to its strong antioxidant activity. hence, the level of aa in the body is regulated through precise and programmed mechanisms [1-3]. the allowable level of aa in the body is estimated at 0.6 to 2 mg/dl. any change in the level of this compound directly leads to the development of many diseases [4]. for example, aa deficiency results in conditions such as cardiovascular disease (cvd), parkinson's disease (pd), alzheimer's disease (ad), scurvy, rheumatoid arthritis and even cancer [5-7], as well as a higher level of aa is associated with gastric irritation, diarrhea and urinary stones. high concentrations of aa in the body make it a powerful antioxidant only in aqueous solutions and in the absence of heavy metal cations because the heavy metal cations force it to play the role of a pro-oxidant that either forms reactive oxygen species (ros) or blocks the antioxidant activity, resulting in the induction of oxidative stress. accordingly, it can be claimed that it is vital to detect the presence of aa in biological fluids to diagnose multiple disorders, which can help its appropriate application in the food industry, pharmaceutical formulations and cosmetics [8-11]. therefore, the detection of aa is one of the fields of interest of research considering the importance of aa in industrial applications and human life. there are currently diverse techniques to detect the aa, including potassium iodate [12], fluorimetry [13], ultraviolet-visible spectroscopy [14], reaction with hexacyanoferrate(iii) and oxidation with cu(ii)-neocuproine complex [16], chemiluminescence and high-performance liquid chromatography (hplc) [18]. however, these techniques have some drawbacks, including the need for a pre-concentration step of the sample, prolonged duration of analysis, the need for skillful personnel, special equipment and expensiveness. the electrochemical detection of aa is based on the fact that it is the most common biologically electroactive compound that is easily oxidized [19,20]. in comparison with conventional instrumental techniques, there are some unique advantages for the electrochemical methods, including high analysis speed, simplicity, potent selectivity and sensitivity, portability, relatively low operating cost and no need for sample pre-treatment [21-84]. some problems with aa detection exist in physiological status, involving low aa concentration as well as interference with uric acid and dopamine in pharmaceuticals, food and biological media due to similar oxidation activities, thereby highlighting the necessity for the development of novel, easy and fast approach with great sensitivity and selectivity to detect the aa in various media [85,86]. to this end, an appropriate electrocatalyst can be modified on the bare electrode surface. for example, the direct electron transfer of analyte on the electrode surface can be achieved through chemically modified electrodes (cmes) reportedly [87-100]. in the continuation of the discussion, it should be mentioned that nanomaterials currently act as common transducers for the improvement of electrode surface in terms of electro-conductive and electrocatalytic profiles, providing more rapid electron-transfer and analyte-transform at the sensing interface. the nanomaterials, with their wide specific surface area (ssa) cause an interaction with dissolved materials. accordingly, the precise selection of nanomaterials is the basis for the development of highly selective and sensitive supramolecular equipment with affinity to the analyte of interest [101-114]. recent biological and chemical sensing electrodes have benefited from core/shell nanoparticles [115-118], metal nanostructures [119-126], oxides [127-135], bimetallic r. m. muhiebes j. electrochem. sci. eng. 12(6) (2022) 1081-1098 http://dx.doi.org/10.5599/jese.1561 1083 compounds [136,137], carbon-based structures [138-140] and other mediators [141,142]. chemical modification is possible for all electrodes used in electrochemical measurements, including carbon paste electrode gold electrodes, screen-printed electrodes and glassy carbon electrodes [143-145]. the purpose of this review article was to scrutinize different electrochemical methods previously used for aa detection, as well as recent developmental advances in different interface materials to modify the electrodes used in these methods. electrochemical methods to detect ascorbic acid electrochemical approaches refer to the transfer of charge between the electrode surface and the solid or liquid phase. this charge transfer procedure is amplified by electrical conductivity and interfacial reactions in the main solution. these are user-friendly methods because of no need for reagents, simplicity and cost-effectiveness, which can be used for in situ measurements through miniaturization and automation. other advantages of such methods are the need for the least changes of samples, no reagent-caused contamination, or minimal loss by adsorption on containers compared to other analytical methods [146-150]. scientists have recently made advances in improving sensitivity and limit of detection (lod), so there are now several electrochemical methods with current or potential modulation, including amperometric techniques, square wave voltammetry, differential pulse voltammetry and cyclic voltammetry. in addition, surface functionalization enhanced selectivity, especially for aa detection. cyclic voltammetry method to detect ascorbic acid in a study by tashkhourian et al. [151], mof mil-101(cr) was fabricated hydrothermally with a large pore volume, which played a role as an electrocatalyst in electrochemical processes for aa detection. to determine the electrocatalytic process of a mil-101(cr)-modified carbon paste electrode, the cv method was used to evaluate the electrooxidation of aa. empirically optimized electrode exhibited a linear relationship of oxidation peak current with aa levels (0.01 to 10.0 mm) having the limit of detection (lod) of 0.006 mm (3sb/m). the method was tested to detect aa levels in two real samples of vitamin c effervescent tablet and vitamin c tablet, which showed a recovery rate of 96.0 and 97.0 %, respectively [151]. chu et al. [152] replaced the enzymes with platinum nanoparticles (pt nps) as a novel nonenzymatic sensor electrocatalyst for aa detection. the graphene modified by poly(dimethyl diallyl ammonium chloride) (pdda) elevated the modification capacity of nanomaterials as well as provided great solubility and conductivity. a potent catalytic impact on aa was reported for the pt nps. the integration of both materials and the subsequent modification on glassy carbon electrodes (gce) led to the synthesis of a non-enzymatic sensor in accordance with a nanocomposite of a pddafunctionalized reduced graphene oxide-pt nps (pdda-rgo/pt nps/gce). in their study, the cv method was employed to evaluate the electrochemical behavior of pdda-rgo/pt nps, the results of which confirmed the capacity of the sensor for electrocatalytic aa detection with a linear range of 0.001 to 10.0 mm at 0 v and a low lod of 0.0005 μm (s/n=3) [152]. ahmed et al. [153] electrodeposited zinc oxide (zno) nps from the aqueous solution of zinc nitrate, as electrochemical detectors, at 70 °c onto a glassy carbon electrode (zno/gce). the cv method was applied to investigate the electrocatalytic oxidation of aa in the presence of 0.1 m phosphate buffer solution (pbs) at a ph value of 6.8. according to the response for aa oxidation on the bare electrode compared with the modified electrode, co-elevation in surface area and oxidation rate shifted the peak potential towards − 0.45 v on zno/gce, having a greater transfer http://dx.doi.org/10.5599/jese.1561 j. electrochem. sci. eng. 12(6) (2022) 1081-1098 detection of ascorbic acid 1084 coefficient and current density. the anodic peak current had a linear relationship with different aa levels (between 0.1 and 5.0 mm) [153]. wu et al. [154] fabricated a porous g-c3n4(pcn)/poly (3,4-ethylenedioxythiophene) compositemodified gce (pcn/pedot/gce) for the detection of aa using co-deposited strategy. they used cv to characterize the electrochemical properties of aa. the empirically optimal oxidation peak current was linearly related to different aa levels (between 10.0 and 1500.0 μm). a large electroactive area was obtained for the pcn/pedot composite with high electron transfer rate, making the composite a proper candidate for modified material to produce a sensor for the detection of aa [154]. the 3-chloropropyl silica gel (sg)-functionalized with imidazole ligand (sgi) was fabricated by da silveira et al. [155]. following the adsorption of cadmium ions, the sgi was combined with potassium hexacyanoferrate to produce cdhsgi, subsequently incorporated into a graphite paste electrode (cdhsgi/gpe) for the electrocatalytic detection of aa using cv method. the cv findings confirmed a redox couple with the mean potential of e ′=0.25 v (versus ag/agcl, nano3 1.0 mol l-1; v = 20 mv s-1), which was due to the process of fe2+(cn)6/ fe3+(cn)6. the voltammograms from the modified electrode for the aa detection showed a linear range between 0.10 and 0.90 mm (lod = 0.79 mm) [231551]. table 1 tabulates information about cv method based-electrochemical sensors, which have been reported by various works. table 1. cv method based-electrochemical sensors to detect aa electrochemical sensors lod, μm linear range, mm ref. mil-101(cr)-modified carbon paste electrode 6.0 0.01 to 10.0 [151] pdda-rgo/pt nps/gce 0.0005 0.001 to 10.0 [152] zno/gce 0.1 to 5.0 [153] pcn/pedot/gce 9.3 0.010 to 1.500 [154] cdhsgi/gpe 0.79 10 to 90 [155] dpv method to detect ascorbic acid murugan et al. [156] fabricated a catalytic material of mixed-phase 2d mxene for the codetection of uric acid (ua), dopamine (da) and aa biomolecules. they produced a ti-c-tx-modified gce (ti-c-tx/gce) sensor, whose electrochemical behavior was evaluated by dpv and cv, the results of which displayed potent electrocatalytic properties and individual oxidation peaks at 0.33, 0.2 and 0.01 v for ua, da and aa, respectively. the synthesized sensor could co-detect the biomolecules in physiological ph values of 0.5 4.0 μm and 100.0 1500.0 μm for ua, 5.050.0 μm for da and 100.01000.0 μm for aa, with the lod of 0.075, 0.06 and 4.6 μm for ua, da and aa, respectively [156]. mos2/acid-exposed multi-walled carbon nanotubes (mwcnts) composite (ms-atcnts) was first produced by kumar et al. [157]. subsequently, they applied this composite for the modification of the carbon paste electrode (cpe) and electropolymerized the alanine using naoh. the modified electrode (p-aln/ms-atcntcpe) produced was analyzed by cv and dpv methods to co-detect guanine (gu), serotonin (5-ht), aa and da in pbs. the gu, 5-ht, aa and da were co-detected electrochemically by the p-aln/ms-atcntcpe. moreover, the variation in concentration of the biomolecules was also cleared, with the lod of 0.1, 0.1, 3.9 and 0.08 µm for gu, 5-ht, aa and da, respectively [157]. in a study, a screen-printed carbon electrode covalently modified with self-assembled golddecorated-polydopamine nanospheres (au-pdns/spce) as a novel sensor was fabricated by arroquia et al. [158]. the sensor was utilized to co-detect the biomolecules of tryptophan (tr), ua, r. m. muhiebes j. electrochem. sci. eng. 12(6) (2022) 1081-1098 http://dx.doi.org/10.5599/jese.1561 1085 da and aa. the au-pdns were loaded on au-nps electrodeposited onto bare electrodes using cysteamine-glutaraldehyde bridges (figure 1). the novel tool responded ph-dependently to these analytes, choosing the optimal working conditions as a function of features for the sample. the codetection of tr, ua and aa in exposed to da, and tr, ua and da in exposed to aa was possible at ph of 3.0 and 8.0, having high sensitivity and broad linear range. tr, ua, da and aa were codetected at the ph value of 6.0 with competitive sensitivities in two consecutive linear ranges of 1.0 to 160.0 and 160.0 to 280.0 µm; 10.0 to 120.0 and 120.0 to 350.0 µm; 1.0 to 160.0 and 160.0 to 350.0 µm and 10.0 to 80.0 and 80.0 to 240.0 µm, with the lod of 0.2, 0.1, 0.1 and 0.1 nm, respectively [158]. figure 1. schematic process of electrode preparation [158] shalini et al. [159] applied a cellulose template for modification of polypyrrole grafted cellulose (ppy@c) surface through in-situ oxidation polymerization using ammonium peroxydisulfide (aps) as an oxidant under optimal state. high sensitivity and stability were reported for the modified electrode of ppy@c/gce during aa detection in the real sample of commercial fruits. the ppy@c/gce exhibited an acceptable regression coefficient and low lod (10.0 to 150.0 μm) in the optimal condition of dpv [159]. li et al. [160] fabricated thin flakes of hexagonal boron nitride (h-bn) by a low-temperature combustion synthesis (lcs) method using nitric acid and carbothermal reduction. dpv and cv techniques were employed to evaluate electrochemically the glassy carbon electrode modified with flake bn (bn/gce), the results of which introduced a new electrode with high electrocatalytic potential and potent selectivity for electrochemically aa, da and ua detection, with the liner relationships between current intensities and concentrations of 30.0 to 1000.0; 0.5 to 150.0 and 1.0 to 300.0 m, and lod of 3.77, 0.02 and 0.15 m, respectively [160]. in a study by selvarajan et al. [161], the gce modified with sno2/chitosan (sno2/chit/gce) as a new nanocomposite to co-detect aa, da, and ua using cv and dpv methods (figure 2). the modified gce showed high electrocatalytic performance compared to the bare electrode. the ternary mixture containing aa, da and ua could be well separated from each other at a scan rate of 0.050 v with a potential difference of 0.168, 0.326 and 0.612 v in the cv, as well as 0.178, 0.337 and 0.592 v in the dpv between aa and da, da and ua, ua and aa respectively. according to dpv findings, a linear relationship was found between concentration and peak current at different concentrations of 1.0 to 100.0 µm for ua, 1.0 to 18.0 µm for da and 20.0 to 220.0 µm for aa, with the lod (s/n=3) of 0.89, 0.77 and 6.45 µm for ua, da and aa, respectively [161]. http://dx.doi.org/10.5599/jese.1561 j. electrochem. sci. eng. 12(6) (2022) 1081-1098 detection of ascorbic acid 1086 figure 2. schematic process of glassy carbon electrode modified with sno2/chitosan (chit) nanocomposite to co-detect aa, da and ua [161] table 2 tabulates information about dpv method based-electrochemical sensors, which have been reported by various works. table 2. dpv method based-electrochemical sensors to detect aa electrochemical sensors lod, μm linear range, m ref. ti-c-tx/gce 4.6 100.0 to 1000.0 [156] p-aln/ms-atcntcpe 3.9 22.0 to 200.0 [157] au-pdns/spce 0.1 10-3 1.0 to 350.0 [158] ppy@c/gce 0.75 10.0 to 150.0 [159] bn/gce 3.77 106 0.030 10-3 to 1.000 10-3 [160] sno2/chit/gce 6.45 20.0 to 220.0 [161] square wave voltammetry method to detect ascorbic acid in a study by mohan et al. [162], bare toray paper was recruited as a working electrode whose electrocatalytic potential was analyzed by ag/agcl and platinum as a reference and counter electrodes, respectively. the oxidation of xanthine (x), da, aa and ua via toray was tested by cv method. the sharpest oxidation peak appeared at certain non-interfering oxidation potentials. the cv was performed to test the principle of electron transfer at various scan rates, the results of which confirmed the presence of a surface-confined reaction. swv and cv techniques in the absence of interference were applied to determine the electroactivity of these compounds. the obtained linear ranges included 10.0-1000.0 μm for ua, 7.0-300.0 μm for aa, 10.0-1000.0 μm for x and 30.0-1000.0 μm for da, with the lod values of 28.74 μm for ua, 97.12 μm for aa, 6.54 μm for x and 9.67 μm for da [162]. in a study by vedenyapina et al. [163], swv and cv were carried out to analyze the electrochemical properties of aa on a boron-doped diamond (bdd) electrode. they found that the voltammetric r. m. muhiebes j. electrochem. sci. eng. 12(6) (2022) 1081-1098 http://dx.doi.org/10.5599/jese.1561 1087 response signal can be used to quantify aa content in the aqueous solution. a function of analytically direct correlation was estimated on bdd for swva, and the lod was 1.87 μm for aa [163]. in a study, ni nps/poly (1,2-diaminoanthraquinone) modified gce (ni/pdaaq@gce) was constructed by hassan et al. [164] as a highly sensitive sensor using cv. the ni nps were incurporated by anodic polarization. the produced ni/pdaaq@gce was applied to co-detect ua, da and aa using swv. a strong electrocatalytic potential was found for the modified electrode relative to the electrooxidation of ua, da and aa in single, binary and ternary complexes in 0.1 m naoh solution. according to empirical data, the lod values for ua, da and aa were estimated at 1.2, 0.072 and 0.11 μm in a single complex and 0.12, 0.29 and 0.069 μm in a ternary complex, respectively [164]. fu et al. [165] aimed to regulate the electrocatalytic potential of commercial graphene ink using a novel water immersion treatment, which can discard the additives present in graphene ink and thus clear the defects on the surface. a glass coated by graphene ink (g-30) was produced to co-detect the presence of ua, da and aa using the cv method, the results of which exhibited the onset of electrocatalytic reaction following the penetration of additives within water immersing treatment. the optimized linear calibration curves for ua, da and aa were estimated at 0.5 to 150.0, 3.0 to 140.0 and 50.0 to 1000.0 μm, with the lod values of 0.29, 1.44 and 17.8 μm, respectively [165]. maouche et al. [166] aimed to detect the aa by constructing a polyterthiophene (p3t)-modified agnps-doped platinum electrode (p3t/agnps-pt electrode) as a new sensor. high sensitivity was seen for the agnps-doped p3t film in comparison with the films doped with other metallic particles (pd, au, co and cu). as well, the oxidation signals were enhanced in detection by swv than by cv. successful detection of aa can be achieved by optimizing multiple factors like film polymerization time and film immersion time in agno3 solution, which were 20 minutes and 60 seconds, respectively. the lod value estimated at s/n =3 was 0.517 nmol l-1 by swv. the produced sensors exhibited a potent selectivity for aa detection, and the reusability was up to 6 times with the best recovery of about 95 % [166]. table 3 tabulates information about swv method based-electrochemical sensors, which have been reported by various works. table 3. swv method based-electrochemical sensors to detect aa electrochemical sensors lod, μm linear range, μm ref. bare toray paper 97.12 7.0 to 300.0 [162] boron-doped diamond electrode 1.87 20.0 to 200.0 [163] ni/pdaaq@gce 0.11 100.0 to 700.0 [164] g-30 17.8 50.0 to 1000.0 [165] p3t/agnps-pt electrode 5.17 10-4 [166] amperometric method to detect ascorbic acid an analyte is detected using the amperometric method through the measurement of the current at a constant applied potential of the working electrode relative to the reference electrode. the potential in this method is directly stepped to the value of interest, followed by measuring current. the amperometric sensors provide extra selectivity due to redox potential in the analysis of analyte species [167]. in the study by xiao et al. [168], a new analytical approach with high simplicity, rapidity and stability was introduced to detect aa with the aid of a non-enzymatic amperometric sensor of gce loaded by mesoporous cuco2o4 rods (cuco2o4/ gce). figure 3 presents a mechanism of electrocatalytic oxidation of the electrode modified with porous cuco2o4 for the detection of aa. the produced structure acted as an excellent electrocatalyst to detect aa according to plotted i-t curve. http://dx.doi.org/10.5599/jese.1561 j. electrochem. sci. eng. 12(6) (2022) 1081-1098 detection of ascorbic acid 1088 the synthesized sensor sensitivity was estimated at 9.482ma mm-1cm-2 at different aa concentrations between 1.0 and 100.0 mm and 2.474 ma mm-1 cm-2 at aa concentrations between 100.0 and 1000.0 mm. the lod was decreased to 0.21 mm (r2= 0.99) [168]. figure 3. the possible pathway of electrocatalytic oxidation of electrodes modified with porous cuco2o4 for detection of aa [168] one of the simple approaches to developing electrochemical sensors is the optimization of the electrocatalytic behavior of porous metallic structures with large surface areas (such as nanoporous gold, npg) through structural manipulation. for example, a selective, sensitive and robust electroanalytical structure was developed by kumar et al. [169] based on npg to detect aa present in acidic extracts of arabidopsis thaliana and aspergillus. to this end, potentiostatic electrodeposition was used to electrodeposit the npg films on a gold microelectrode (npg-modified gold microelectrodes). amperometric parameters were measured at 0.3 v vs. ag/agcl (sat. kcl) in the acidic electrolyte to detect aa in biological samples and minimize autoxidation. modification of usual microelectrodes with npg film increased the sensitivity by about 1000-fold, which reached 2.0 na µmol-1 l-1. the lod of aa was calculated on the basis of a calibration curve of 2.0-μm flow concentration [169]. in a study by yu et al. [170], molybdenum oxide (moox) plus prussian blue (pb) were loaded on graphite felt (gf) as a sensor (moox@pb/gf) for aa detection. moo2 nanorods produced by moprecursor annealing enhance the pb fabrication from [fe(cn)6]3and fe3+. an excellent electrocatalytic oxidation was seen for aa by the modified electrode with a high sensitivity of 0.37 a/m, a strong selectivity, significant reproducibility, an acceptable linear response at concentrations of 0.0125-293.0 mm, and a very low lod of 0.0119 mm (s/n= 3) [170]. in a study by hei et al. [171], 3d hierarchical n-doped nano-scaled carbons (3d-nncshas) were produced with unique properties, including eco-friendly, cost-effectiveness, great precursor, large surface area, abundant defective sites, and wide distribution of pore size by sea-tangle (laminaria japonica) pyrolysis. as seen in figure 4, the produced 3d-nncshas were applied to fabricate a sensor for aa detection with high selectivity and sensitivity. the 3d-nncshas-modified gce (3d-nncshas/gce), when comparing with gce and carbon nanotubes-modified gce (cnts/gce), could better electrocatalytically detect aa, with smaller lod of 1.0 µm, broader linear range between 10.0 and 4410.0 µm and shorter electrooxidation peak potential (-0.02 v vs. ag/agcl). moreover, greater anti-fouling and anti-interference activities were reported for aa detection by the 3d-nncshas/gce [171]. r. m. muhiebes j. electrochem. sci. eng. 12(6) (2022) 1081-1098 http://dx.doi.org/10.5599/jese.1561 1089 figure 4. schematic process of inexpensive production of an amperometric sensor of nitrogen-doped nanocarbons using sea-tangle to detect ascorbic acid [171] scremin et al. [172] introduced tio2-au np integrated with mwcntmodified gce (tio2-au npmwcnt-dhp/gce) in a dihexadecyl phosphate film for aa detection, the results of which indicated rapid charge transfer and irreversible anodic property using cv. the analytical curve in optimized conditions and amperometry of 0.4 v exhibited linear aa concentration between 5.0 and 51.0 μmol l-1, having lod of 1.2 μmol/l [172]. the production of an organic electrochemical transistor sensor (oect) was reported by zhang et al. [173] using a gate electrode modified with molecularly imprinted polymer (mip) film for aa detection. a strongly selective and sensitive oect sensor was built by integrating the oect amplification function and mip selectivity. the formula of the mip film-modified oect sensor is shown in figure 5a, and the principle of action of the mip film-modified electrode is seen in figure 5b. all stages of producing modified electrodes and the adsorption ability of mip/au electrodes were analyzed by the cv method and electrochemical impedance spectroscopy (eis). a sensitivity of 75.3 μa channel current change per decade and a small lod of 10.0 nm (s/n > 3) were reported for the mip-oect sensor after altering relative aa concentration from 1.0 to 100.0 μm under optimal conditions [173]. figure 5. (a) schematic process of mip-modified oect sensor for aa detection. (b) schematic process of aa removing and rebinding on surface of mip film and of aa oxidation on surface of modified gate electrode [173] http://dx.doi.org/10.5599/jese.1561 j. electrochem. sci. eng. 12(6) (2022) 1081-1098 detection of ascorbic acid 1090 table 4 tabulates information about amperometric method based-electrochemical sensors, which have been reported by various works. table 4. amperometric method based-electrochemical sensors to detect aa electrochemical sensors lod, µm linear range, mm ref. cuco2o4/ gce 210 mm 1.0 to 1000.0 [168] npg modified gold microelectrodes 2.0 0.010 to 1.100 [169] moox@pb/gf 11.9 0.0125 to 293.0 [170] 3d-nncshas/gce 1.0 0.010 to 4.410 [171] tio2-au np-mwcnt-dhp/gce 1.2 0.005 to 0.051 [172] mip-oect 0.010 0.001 to 0.100 [173] conclusion in recent years, biomedical and pharmaceutical investigations and procedures have mainly focused on diverse electrochemical techniques based on voltammetric (swv, dpv and cv) and amperometric methods to a lesser extent. the need for near-patient testing or in vivo real-time analysis significantly enhances the functional parameters of the electrochemical method. there is still a need for further studies to increase the sensitivity and selectivity of the desired techniques. accordingly, a variety of new materials and surface modification methods have been proposed so far to fabricate the diagnostic electrodes for the detection of ascorbic acid, including screen-printed electrodes, carbonbased electrodes, nanoparticles, application of carbon-based nanotubes, enzymes, graphene, mediators and polymer films alone or in combination, to prevent the use of additives in the sample solution and minimize the sample preparation process. preferred and highly applied methods in this field include screen-printed and miniaturized electrodes due to potent flexibility, cost-effectiveness, minimal sample concentration, high reproducibility for measurements, the ability of in vivo analysis and capacity for testing 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the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.1134/s0036024419060335 https://doi.org/10.1007/s13738-018-1297-z https://doi.org/10.1016/j.talanta.2017.12.058 https://doi.org/10.1016/j.arabjc.2015.04.029 https://doi.org/10.1007/s10404-014-1385-z https://doi.org/10.1016/j.jallcom.2020.157045 https://doi.org/10.1016/j.aca.2019.10.022 https://doi.org/10.1016/j.electacta.2019.134712 https://doi.org/10.1016/j.aca.2018.05.041 https://doi.org/10.1007/s00604-018-2785-7 https://doi.org/10.1016/j.bios.2017.09.006 https://creativecommons.org/licenses/by/4.0/) @article{muhiebes2022, author = {muhiebes, raad muslim and fazeli, farzaneh and amini, issa and azizkhani, vahid}, journal = {journal of electrochemical science and engineering}, title = {{an overview of recent advances in the detection of ascorbic acid by electrochemical techniques:}}, year = {2022}, issn = {1847-9286}, month = {nov}, number = {6}, pages = {1081--1098}, volume = {12}, abstract = {ascorbic acid is a water-soluble vitamin essential in human nutrition, an antioxidant, a scavenger of free radicals in biological systems, and a cofactor of several enzymes. the reference range for ascorbic acid in healthy people is 6 20 mg l-1. the variable concentration of ascorbic acid within biology fluids was found in clinical investigations to be a metric for assessing the exact amount of oxidative stress in the body's metabolism. electroanalytical techniques are a group of methods in analytical chemistry, especially with extensive application in pharmaceutical industries. these techniques attracted further attention due to their unique characteristics, such as reduced sample or solvent con­sumption, high analysis speed, low operating cost, and high sensitivity, which made them suitable candidates for replacement or supplementation for spectrophotometry and separation approaches. the purpose of this article is to scrutinize the mechanisms and applications of current electroanalytical methods, including amperometric techniques, square wave voltammetry, differential pulse voltammetry and cyclic voltammetry, in their applications in pharmaceutical analysis for the detection of ascorbic acid. related examples have been cited in the form of selected studies.}, doi = {10.5599/jese.1561}, file = {:d\:/onedrive/mendeley desktop/muhiebes et al. 2022 an overview of recent advances in the detection of ascorbic acid by electrochemical techniques.pdf:pdf;:www/jese_v12_no6_1081-1098.pdf:pdf;:03_jese_1561_1081-1098.docx:word_new;:03_jese_1561_1081-1098.docx:word_new;:www/jese_v12_no6_1081-1098.pdf:pdf}, keywords = {electrochemical sensors, cyclic voltammetry, differential pulse voltammetry, modified electrode}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1561}, } synthesis of ammonia from water and nitrogen using a compo-site cathode based on la0.6ba0.4fe0.8cu0.2o3-δ-ce0.8gd0.18ca0.02o2-δ http://dx.doi.org/10.5599/jese.1535 s1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1507 open access : : issn 1847-9286 www.jese-online.org review paper electrochemical sensors for the safety and quality control of cosmetics: an overview of achievements and challenges totka dodevska, dobrin hadzhiev and ivan shterev department of organic chemistry and inorganic chemistry, university of food technologies, plovdiv, bulgaria corresponding author: dodevska@mail.bg; tel.: +359-32-603-679; fax: +359-32-644-102 received: september 5, 2022; accepted: november 19, 2022; published: november 22, 2022 abstract due to the rapid growth of the cosmetic industry in recent years, the development of new, reliable, cost-effective, ease of use and rapid methods to assay cosmetics’ quality is of particular importance. modern electrochemistry provides powerful analytical techniques with excellent sensitivity, instrumental simplicity and portability, providing reliable alternatives to conventional analytical methods. this review aims to give readers a clear view of advances in areas of electrode modification, successful strategies for signal amplification, and miniaturization techniques used in electroanalytical devices for cosmetics control and safety. we have summarized recent trends in the nonenzymatic electrochemical sensor systems applied in the analysis of cosmetic products revealing that there are a variety of efficient sensors for whitening agents, preservatives, uv filters, heavy metals, etc. in conclusion, current challenges related to the sensors design and future perspectives are outlined. keywords personal care products; ingredients; analytical methods; electroanalysis abbreviations adsasv adsorptive anodic stripping voltammetry ar arbutin (4-hydroxyphenyl-β-d-glucopyranoside) bp butylparaben bz3 benzophenone-3 cc catechol (1,2-dihydroxybenzene) cpe carbon paste electrode dpadsv differential pulse adsorptive stripping voltammetry dpv differential pulse voltammetry ep ethylparaben ergo electrochemically reduced graphene oxide fc ferrocene fia flow injection analysis gce glassy carbon electrode hq hydroquinone (1,4-dihydroxybenzene) http://dx.doi.org/10.5599/jese.1535 http://dx.doi.org/10.5599/jese.1507 http://www.jese-online.org/ mailto:dodevska@mail.bg j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 2 8-hq 8-hydroxyquinoline il-go ionic liquid functionalized graphene oxide lod limit of detection loq limit of quantification lsv linear sweep voltammetry mb methylene blue 4-mbc 4-methylbenzylidene camphor mips molecularly imprinted polymers mp methylparaben mwcnts multi-wall carbon nanotubes nps nanoparticles ocr octocrylene omc оctyl-methoxycinammate phba p-hydroxybenzoic acid pp propylparaben ppd p-phenylenediamine rs resorcinol (1,3-dihydroxybenzene) spe screen-printed electrode swadsv square wave adsorptive stripping voltammetry swv square wave voltammetry tcs triclosan (5-chloro-2-(2,4-dichlorophenoxy) phenol tga thioglycolic acid zpt zinc pyrithione (bis[(2-pyridyl-1-oxo)-thio]zinc) introduction in general, cosmetics (body and personal care products) are mixtures of substances intended to be applied to the external parts of the human body for the purposes of cleaning, moisturizing, beautifying, keeping them in good condition or correcting body odors. as cosmetic preparations are repeatedly applied directly to the human body (epidermis, hair, nails, lips, external genital organs, teeth), they should be efficient and safe for health. to ensure the safety and quality of cosmetics, all the ingredients used in cosmetic products meet certain regulatory requirements. variety substances are allowed within certain limits as they may produce pharmacological or toxic effects at higher concentrations. in terms of safety, some other important aspects should be considered, which include the possibility of long-term effects. recently, it was recognized that some topically applied substances penetrate through the skin and produce human systemic exposure. unfortunately, using cosmetic products in some cases is related to the occurrence of unfavorable effects resulting from the intentional or accidental presence of toxic chemical ingredients [1]. such substances may be a major component of the raw materials used in cosmetic manufacture or are deliberately added in cosmetics. it is considered that a small part of substandard cosmetics is falsified. the rest result from poor manufacturing practices, incorrect storage or inappropriate packaging. comprehensive coverage of the specific analytical procedure for various analytes and cosmetics samples, as well as the most recent developments in global legalization governing the cosmetics industry, were provided by salvador and chisvert [2]. a wide range of analytical techniques is used to accurately characterize and quantify ingredients in cosmetics. conventional methods such as high-performance liquid chromatography (hplc), gas chromatography, spectrophotometry, chemiluminescence, and fluorescence have been established for the analysis of cosmetic products. however, most of these methods have some disadvantages, such as specific expensive equipment, the need for complex pretreatment resulting in large time consumption and use of organic solvents in excess volumes, complicated operation, or a narrow linear range. therefore, the development of a precise and easy-to-handle alternative for quick and efficient quantitative detection of various ingredients in cosmetic products is necessary for consumer safety [3]. new analytical methodologies t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 3 are expected to use modern and efficient techniques with highly selective detectors, as well as rapid and simple sample preparation procedures with a high level of automation [4]. electrochemical techniques have attracted significant interest due to their high sensitivity, low detection limit, simple operation, extremely low sample consumption, and rapid analysis. electrochemical detection is also advantageous in terms of sensor fabrication cost, power consumption and miniaturization capability [5]. the following part will provide an overview of how advances in electroanalytical technology have contributed to developing sensor devices to screen for safety and quality markers associated with cosmetics and personal care products. current stage of electrochemical sensors for the analysis of cosmetics ingredients whitening agents hydroquinone, catechol, and resorcinol the popularity of skin-lightening (whitening/bleaching) products has grown markedly in recent years. the motivation for skin whitening may be to treat a pigmentary disorder, but it is often simply for cosmetic enhancement, as skin complexion is considered one of the most important beauty features. nowadays, skin lightening is a global phenomenon, with the highest rates in africa, asia, and the middle east. one of the most frequent targets for the development of hypopigmenting agents is tyrosinase a copper-containing enzyme that catalyzes the rate-limiting step of melanin biosynthesis. the skinlightening agents act by inhibiting of tyrosinase activity, thereby reducing the conversion of dihydroxybenzoic acid to melanin. the isomers of dihydroxybenzene hydroquinone (1,4-dihydroxybenzene, hq), catechol (1,2-dihydroxybenzene, cc), and resorcinol (1,3-dihydroxybenzene, rs) specifically inhibit the melanogenesis in cells and are used in hair dyes and medical whitening creams [6]. the most commonly used treatment of hyperpigmentation for over 50 years is topical hq. although very effective, its long-term application causes a number of adverse reactions, including skin irritation, burning sensations, leukoderma, ochronosis, and degenerative changes followed by the skin’s loss of elasticity and may have a cytotoxic effect on melanocytes. rs is an endocrine disruptor that can affect thyroid function by inhibiting thyroxin peroxidase. after prolonged exposure to rs, suppression of thyroid hormone synthesis in humans, hematological abnormalities, carcinogenesis, and fatal cases of human fetus poisoning have been observed [7]. hq, cc, and rs are included among the substances prohibited for use as skin whitening agents in cosmetic products (annex ii and annex iii) to regulation (ec) no 1223/2009 [8]. in the usa, the food and drug administration has proposed concentrations of hq between 1.5 and 2 % in skin lighteners [9]. according to china's hygienic standard for cosmetics, hq is limited to oxidation colorants for hair dyeing, with a maximum allowable concentration of 2 wt.% [6]. however, in developing countries, whitening cosmetics containing hq are circulating illegally, not to mention that the concentration level is relatively high [10]. hq, cc and rs are electroactive substances that can be detected using various electrochemical methods. conventional working electrodes, such as glassy carbon, graphite, carbon paste electrodes, and boron-doped diamond electrodes, have low reproducibility and stability for phenolic compounds detection because the redox reaction that occurs usually produces a polymer layer that adsorbs on the surface of the electrode, resulting in a fouling effect. this phenomenon severely affects the analytical performance in terms of sensitivity and linearity of the electrode signal. in order to reduce this effect and improve the electrode efficiency, modification of the working electrodes with nanomaterials (metal/metal oxide nanoparticles, cnts), graphene or polymers has been extensively investigated. http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 4 cotchim et al. reported a simple electrochemical sensor for the determination of hq at trace levels using a carboxylic acid-functionalized graphene-modified glassy carbon electrode (gr-cooh/gce) for adsorptive anodic stripping voltammetry (adsasv) [9]. the electroanalytical procedure involves two steps: i) accumulation or preconcentration of the analyte at the electrode surface, and ii) stripping of the accumulated analyte from the electrode surface by using a potential sweep. gr-cooh acts as an adsorbent, resulting in hq being closely adsorbed on the electrode surface. as a result, the detection efficiency increases, leading to high sensitivity and a low limit of detection (lod). the gr-cooh/gce exhibited good electrochemical oxidation behavior of hq with sensitivity ten times higher than the bare gce. the authors point out that after each experiment, the modified electrode was cleaned in the supporting electrolyte at a potential of +1.0 v (vs. ag/agcl) for 60 s to minimize electrode fouling and improve the electrochemical response. under the optimized conditions, the analytical performance of the proposed method was validated and exhibited a wide linear range (0.1 to 40.0 μm), high sensitivity (19.86 μa μm−1 cm−2), lod of 0.04 μm, good selectivity, high accuracy and precision. the modified electrode provides rapid response and good repeatability, and it successfully detects trace hq in cosmetic and skin-lightening products. concentrations of hq determined by the proposed sensor and by uv-derivative spectrophotometry showed no significant differences, indicating that the developed method can be successfully applied for the determination of hq in various cosmetic products. a simple electrochemical sensor based on nano-sepiolite clay-modified carbon paste electrode was developed for the analysis of hq in cosmetic samples by using differential pulse adsorptive stripping voltammetry (dpadsv) and square wave adsorptive stripping voltammetry (swadsv) [11]. the electrode has the advantages of high sensitivity, large linear range, low limit of detection, ease of preparation, practical surface renewal, and low cost. the authors reported that in order to prevent electrode fouling before all assays, surface cleaning of carbon paste sensors was carried out by washing with a water-ethanol mixture (1:1). the proposed sensor was applied to the analysis of hq in the cosmetic sample (expigment cream) with satisfying results. according to the presented results, the proposed methods have definite precision and accuracy. electrochemical techniques in combination with screen-printed electrodes (spes) have been proven as capable sensors to accelerate the change from conventional benchtop techniques/equipment to low-cost, robust and quick sensing devices [12]. with respect to the most used glassy carbon electrodes, spes show numerous advantages such as easy customization, cost-effectiveness, miniaturization, measurements that consume only a few microliters of sample, suitability for in-field analysis, and the absence of surface pre-treatment or polishing. it is therefore considered a potential tool for on-site applications [13]. a small and portable graphene-modified carbon paste sensor for hq was developed using the screen-printing technique [14]. the electrochemical detection is based on a one-drop analysis (60 μl of the sample). a linear calibration was achieved in the range of 10–4 to 510–3 m hq (lod = 710–5 m). the authors stated that the sensor provided good precision and accuracy with an analysis time of less than a minute. this device was applied to determine the presence of hq in whitening cosmetic products. the results obtained from the developed sensor satisfactorily agreed with hplc, indicating the reliability of the electrochemical method. an anodically pretreated screen-printed ring disk carbon electrode (sprdce*) coupling with a flow injection analysis (fia) system was developed as a simple, rapid, sensitive, and self-validated hq sensor [15]. using carbon-based electrode preanodized in phosphate buffer (a nontoxic solution), the sensitivity of electrochemical detection of hq was significantly improved owing to the increase of the polar oxygen-containing functional group during the anodization. the developed anodized sprdce* t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 5 presented high ability and feasibility for hq detection. under the optimized conditions, a linear range of 0.25 to 160 ppm and lod = 0.024 ppm for hq were defined. using this method, hq in cosmetic products was successfully quantified without any pre-treatment. sprdce* combined with the fia system offers important practical features for routine analysis of hq in cosmetic products (figure 1). a carbon paste electrode spiked with ferrocene (fc/cpe) was constructed by incorporation of ferrocene in a graphite powder–paraffin matrix [16]. ferrocene (fc) is a stable organometallic complex of iron. fast and reversible redox reactions involving the fc/fc+ couple make it a suitable mediator that accelerates electron transfer in the electrochemical systems. linear sweep voltammetry (lsv) data confirm that fc/cpe possesses good analytical performance working range of 0.20 to 10 µm, a sensitivity of 10.436 µa µm–1, and a detection limit of 0.06 µm hq. an efficient and economically viable electrochemical sensor for the quantitative determination of rs based on 77maghemite/multi-wall carbon nanotubes (m/mwcnts) modified carbon paste electrode was developed by manasa et al. [17]. quantitative analysis of rs was performed in pbs (ph 7.0) using differential pulse voltammetry (dpv) and two linear ranges (0.5 to 10 and 10 to 100 μm) were obtained as a result of the kinetic limitation at the electrode/solution interface; lod was found to be 0.02 μm. furthermore, to demonstrate the feasibility of the m/mwcnts/mcpe, the amounts of rs in hair dye samples were tested by dpv, which showed good recovery and practical applicability. figure 1. schematic representation for monitoring of hq at the sprdce* using fia. reproduced from ref. [15] with permission from the royal society of chemistry nowadays, the effective simultaneous detection of phenolic compounds that have analogous chemical features is a subject of major study interest in the field of analytical chemistry [18]. because of the similar electrochemical behaviour of hq, rs, and cc, their redox peaks tend to overlap and cannot be effectively distinguished using conventional glassy carbon electrodes [19]. therefore, to improve the detection sensitivity and selectivity of electrochemical sensors, it is necessary to search for new nanomaterials that are eco-friendly, economical, and show good catalytic performance. electrodes modified with magnetic nanomaterials present stable conductivity and accelerate the redox process. feng et al. studied the electrochemical behavior of dihydroxybenzene isomers on ndoped nickel carbide spheres (n-nicss/gce) in 0.1 m pbs (ph 7.0) using cyclic voltammetry (cv) and http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 6 dpv. n-nicss/gce has been applied to the simultaneous determination of hq, cc and rs with a wide linear range, good stability, reproducibility and high sensitivity, which fully meet the requirements of cosmetics testing [6]. under the experimental conditions, the three analytes are distinguishable by their peak potentials. the detection limits of hq, cc, and rs were 0.00152, 0.015 and 0.24 mm, respectively. the n-nicss/gce sensor has been successfully used to detect simultaneously hq, cc, and rs in local tap water, hair dye, and whitening cream samples with satisfactory recoveries demonstrating its practicability and reliability. the authors have concluded that their future research will focus on appropriate reverse electrode passivation materials to reduce the effect of fouling. ascorbic acid (aa), known as vitamin c, is a naturally occurring water-soluble organic compound with antioxidant properties. ascorbic acid plays a key role in maintaining skin health. it provides protection against uv-induced photodamage and participates in the formation of skin barrier lipids and collagen in the dermis. although the antipigmentary and skin-protective mechanisms of aa still need to be clarified, aa has been used widely as a skin-lightening, anti-aging, anti-oxidant and antiinflammatory agent in commercially available cosmetics such as creams and lotions, designed to protect and rejuvenate photoaged skin [20]. a new sensor based on copper oxide (cuo) nanostructures modified glassy carbon electrode (cuonps/gce) was designed for the simultaneous electrochemical determination of aa and hq [21]. cuonps with nanoflakes morphology were synthesized through an aqueous chemical growth method using copper acetate as precursor salt and sodium hydroxide as a reducing agent. cuonps were deposited on the gce surface by drop casting method. under the optimal conditions, the prepared electrode cuonps/gce reveals excellent sensitivity, selectivity and stability with a wide linear dynamic range (0.0001 to 0.30 mm for aa, and 0.0003 to 0.355 mm for hq) and limit of detection (0.01 μm for aa, and 0.009 μm for hq). the proposed sensor has shown a potential for the simultaneous determination of aa and hq and can be effectively used in real samples of skin-lightening creams. here it should be pointed out that the drop-casting approach could be characterized by some drawbacks, such as low homogeneity and stability of the resulting modified surface. a phenomenon known as the “coffee ring” effect was observed due to capillary forces present as a result of solvent evaporation which can push the modifier to the edges of the underlying electrode [13]. agglomeration of particles upon electrode surface modification during the drop-casting process affects many physical and chemical properties, thus affecting sensing performance by decreasing the fraction of electroactive nanoparticles. due to nps agglomeration, these modified electrodes may lack reproducibility. additionally, the modifier that has been physically adsorbed onto the electrode surface may be gradually stripped off in long-term operations. a sensor based on electrochemically reduced graphene oxide (ergo), carboxylated multi-wall carbon nanotubes (cmwcnt), and gold nanoparticles (aunps) was developed by domínguezaragón et al. for the simultaneous detection of dihydroxybenzene isomers [22]. тhe authors emphasize that carbon-based nanocomposites of ergo and mwcnt can be prepared within a single step, avoiding common drawbacks such as agglomeration and restacking of layers. carboxylated mwcnt shows good mechanical properties, porous structure, high surface area, enhanced hydrophilicity, excellent chemical stability, ability to promote electron transfer reactions and higher attachment properties due to the functional groups on their surface. therefore, cmwcnt can help to generate a more homogeneous surface with the better electrochemical activity of the material compared with mwcnt. the ergo-cmwcnt nanocomposite can be used as an excellent scaffold for catalytic nanomaterials deposition. on the other hand, aunps are of great significance for electrode modification because they provide additional features such as excellent conductivity, high t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 7 surface area and catalytic properties that make them excellent materials for the electrochemical detection of a wide range of analytes [23]. under optimized conditions, using differential pulse voltammetry, the gce/ergo-cmwcnt/aunps sensor exhibited a linear concentration range of 1.2 to 170 µm for hq and cc, and 2.4 to 400 µm for rs with detection limits of 0.39, 0.54 and 0.61 µm, respectively. the proposed sensor showed a great potential for the simultaneous, highly sensitive, precise, and easy-to-handle detection of hq, cc and rs in complex samples such as tap water and commercial skin-lightening cream (figure 2). edris and sulaiman presented a novel highly sensitive composite of electrochemically reduced graphene oxide-poly(procion red mx-5b)/gold nanoparticles modified glassy carbon electrode (gce/ergo-poly(pr)/aunps) for voltammetric detection of hq, cc, and rs [18]. the involvement of azo dye (pr) with multi-functional groups as a mediator makes the ergo more soluble as well as stabilizes the aunps in the composite. the synergistic effect makes the composite have better electrochemical catalytic activity for dihydroxybenzene isomers. the composite material showed the feature of high specific surface area and high conductivity that enhanced the electrocatalysis of hq, cc, and rc. the proposed sensor displays a linear range of 0.1 to 90 µm for hq, 0.4 to 90 µm for cc, and 4 to 350 µm for rs. the limits of detection are extremely low: 53, 53, and 79 nm for hq, cc, and rc, respectively. the gce/ergopoly(pr)/aunps was used for simultaneous voltammetric detection of hq, cc, and rs in the cosmetic sample (melashine cream 4 %) with satisfactory recoveries. the nanocomposite possesses adequate reproducibility, good stability and acceptable recoveries for wastewater and cosmetic samples analyses. figure 2. schematic representation for the simultaneous detection of hq, cc, and rs at the gce/ergocmwcnt/aunps sensor. reproduced from ref. [22]. licensee mdpi, basel, switzerland (2021) butwong et al. [19] described a simple strategy for the fabrication of a new modified electrode exploiting the synergetic effect of cnts and agnps capped by cysteamine (cst) for simultaneous detection of hq and cc [19]. cysteamine stabilized agnps and binds strongly to cnts to form a stable and sensing layer on a gce surface. the resulting agcst-cnts/gce possesses a large active surface area, high conductivity and excellent catalysis, with a detection limit of 10 and 40 nm for hq and cc, respectively. the sensor was demonstrated for the analysis of river water and a topical cream, as manifested by high accuracy and reproducibility. arbutin arbutin (4-hydroxyphenyl-β-d-glucopyranoside, ar), is a naturally occurring skin-lightening agent and has been found in the species of various plant families such as marjoram, cranberry, http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 8 blueberry, and several pear species. ar is often used in skin care products as it is an efficient agent for the treatment of hyperpigmentation disorders and shows less melanocyte cytotoxicity than hq. the practical applicability of hydroxyapatite-zno-pdnps modified carbon paste electrode (hap-zno-pdnps/cpe) [24], and a sepiolite clay-modified carbon paste electrode (clay/cpe) [25] to detect ar electively in cosmetics was tested and satisfactory results were obtained. hap-znopdnps/cpe for simultaneous quantitative detection of ar and aa (vitamin c) was developed by shahamirifard et al. [24] dpv data reveal that the modified electrode has exhibited excellent electrocatalytic activity towards the oxidations of ar and aa in buffer solution (ph 7.0). the corresponding electrochemical signals have appeared as two well-resolved oxidation peaks with a potential difference of 0.23 v (vs. ag/agcl, 3.0 m kcl) which is high enough for the simultaneous determination of the concentration of ar and aa. the linear response ranges were 0.12 to 56 μm for ar and 0.12 to 55.36 μm for vitamin c, with detection limits of 85.7 and 19.4 nm, respectively. the practical applicability of the fabricated sensor was confirmed in commercial lightening seagull cream. online derivatization followed by a disposable electrochemical sensor was also used to determine ar in cosmetic products [26]. the ar was chemically oxidized by mno2 and subsequently reduced at a screen-printed carbon electrode using a low detection potential which improved the selectivity of the method. the so-called “green synthesis”, using mild reaction conditions and natural resources such as plant extracts, has received more attention as a cost-effective and valuable alternative for environmentally safe and energy-efficient production of metal nanoparticles (mnps) [20]. unlike chemical and physical processes, bio-inspired synthetic methods restrict the use of toxic chemicals, energy and sophisticated instruments. nowadays, there is convincing evidence that green synthesis of mnps has the potential to provide a new direction in the fabrication of cheap and effective electrocatalysts applicable in pharmaceutical, environmental, and food analysis. khatoon et al. [27] reported for the first time utilization of bell pepper (bp) extract as a capping, stabilizing, and reducing agent for the cost-effective and environmentally safe synthesis of agnps and applicability of the nanoparticles for the development of an electrochemical sensor for ar detection [27]. the fabricated electrochemical platform based on bp-agnps-gce exhibits a linear current response in the concentration range from 10 to 350 µm, lod of 0.03 µm, and loq of 0.09 µm, respectively. the applicability of this sensor for sensitive detection of ar in real samples was confirmed in commercially available skin whitening creams. here we must note that the experimental data published so far show that the surfaces modified with biosynthesized mnps remain challenging as they are often not as stable and reproducible as one would hope. it is important to ensure that metal nanoparticles produced by the plants or plant extracts remain stable during their storage without changes in their morphology and size before using them in practical applications. further research needs to be done to address these issues and improve the electrode performance in terms of higher operational and storage stability of the electrocatalysts modified with nanoparticles synthesized in a green way. hydrogen peroxide hydrogen peroxide (h2o2) is classified as an antimicrobial agent, cosmetic biocide, oral health care agent, and oxidizing agent. hydrogen peroxide can often be found in many day-to-day life cosmetic products such as hair dyes, hair bleaches (diluted h2o2 mixed with ammonium hydroxide solution is used as a constituent), tooth whitening products and sold as an antiseptic. the ec restricts t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 9 the amount of h2o2 that may be present in cosmetic products. these restrictions included a maximum concentration of 4 % in products applied to the skin and 12 % in products applied to the hair; dyes intended to be used on eyelashes (professional use only) are safe when they contain up to 2 % h2o2 [28]. since h2o2 is an aggressive oxidizer that damages human skin, a higher concentration of h2o2 (> 20 %) is considered to be highly hazardous. the determination of h2o2 plays an important role in clinical research, medical diagnostics as well as in many industrial applications (food processing, paper, textile, pharmaceuticals, cosmetics, cleaning and disinfection products) [29]. the electrochemical nonenzymatic detection of h2o2 can provide many attractive characteristics, such as simple fabrication, ultrahigh sensitivity, and excellent stability. considering the rapid expansion of nonenzymatic h2o2 detection using advanced nanomaterials, several review papers were devoted to the design strategies for electrocatalysts used in h2o2 sensors by focusing on the sensing performance, electrocatalytic mechanism, and morphology of electrode materials [30-33]. here we refine the search and present exclusively the electrochemical sensors applicable in the h2o2 analysis of cosmetics, medical and cleaning products. copper oxide/graphitic carbon nitride composite (cuo/g-c3n4) dropped onto gce [34], nanosized bismuth particles modified electrode (spage-binano) [35], carbon paste electrode modified with nano-composite of reduced graphene oxide and cufe2o4 nanoparticles (rgo/cufe2o4/cpe) [36], cuo/cu wire [37], 3d printed graphene electrode with prussian blue (3dgre/pb) (figure 3) [38], as well as carbon electrodes modified with perovskites-type oxides [39-42] were developed, characterized and successfully used as working electrodes for the electrochemical detection of h2o2 in a wide range of cosmetics and antiseptic products (table 1). table 1. comparison of the analytical parameters of electrochemical nonenzymetic sensors for the determination of h2o2 in cosmetics, medical and cleaning products electrode material method linear range lod real sample ref. cuo/g-c3n4/gce1 dpv4 0.5 – 50 μm 0.31 μm makeup remover [34] spage-binano 2 cv5 100 µm – 5 mm 56.59 µm hair dyes [35] rgo/cufe2o4/cpe amp.6 2 – 200 µm 0.52 µm hair dye, mouthwash solution [36] dpv4 2 – 1000 µm 0.064 µm cuo/cu wire amp.6 10 – 1800 µm 1.34 µm listerine mouthwash [37] 3dgre/pb3 amp.6 1 – 700 µm 0.11 µm mouthwash [38] lani0.5ti0.5o3/cofe2o4/gce amp.6 0.1 µm – 8.2 mm 23 nm toothpaste [39] la0.66sr0.33mno3/cpe amp.6 – – cleaning product [40] lani0.6co0.4o3/cpe amp.6 10 nm – 100 µm 1 nm toothpaste [41] la0.7sr0.3mn0.75co0.25o3/cpe amp.6 0.5 – 1000 µm 0.17 µm toothpaste, medical solution [42] 1cuo/g-c3n4/gce: copper oxide/graphitic carbon nitride composite/glassy carbon electrode; 2spage-binano: three-in-one screenprinted electrode assembly containing nano bismuth species deposited silver as working, pre-oxidized silver as a reference and unmodified silver as counter electrodes; 33dgre/pb: 3d printed graphene electrode with prussian blue; 4dpv: differential pulse voltammetry; 5cv: cyclic voltammetry; 6amp.: amperometry. figure 3. schematic representation for the detection of h2o2 using 3d printed graphene electrode with prussian blue (3dgre/pb). reprinted with permission from ref. [38]. copyright (2019) american chemical society http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 10 preservatives preservatives are added to cosmetics in order to prevent microbial spoilage, and therefore prolonging products' shelflife. parabens, aldehydes, phenoxyethanol, glycol ether, isothiazolinones, quarternary ammonium compounds and organic acids (benzoic acid, hydroxybenzoic acid, dehydroacetic acid, sorbic acid, etc.) are commonly added as preservatives to a wide range of cosmetics. michalkiewicz et al. [43] reviewed the research achievements on modified carbon materials in the electroanalysis of preservatives most frequently used in food, cosmetic, and pharmaceutical preparations [43]. p-hydroxybenzoic acid p-hydroxybenzoic acid (4-hydroxybenzoic acid, phba) is a frequently used preservative substance in cosmetics. moreover, phba is the hydrolysis product of parabens, a popular preservative in cosmetics and foodstuffs. a few published works have reported the detection of phba using an electrochemical technique. electrochemical determination of phba is required for efficient surface modification. nickel titanate (nitio3) nanoceramics, synthesized by the sol-gel method, were used to fabricate a modified carbon paste electrode (nitio3/cpe) for quantitative detection of phba [44]. dpv results suggest that the modified electrode exhibits an electrocatalytic effect on the oxidation of phba, resulting in a marked enhancement of the peak current response at 1.0 v (vs. ag/agcl, 3 m kcl). under the optimized conditions in britton-robinson buffer solution (ph 2.0), the oxidation peak current was linearly dependent on the concentration of phba in the ranges of 0.7 – 80 μm and 80 – 1000 μm. the detection limit was estimated to be 62 nm. common interfering ions, such as na+, k+, nh4+, cl−, co32−, no3− and i−, do not influence the dpv response of phba with deviations below 5 %. however, the authors reported that methylparaben and propylparaben have the same oxidation potential and show interference effects. the ability of the nanostructured modified electrode was examined in the detection of phba in an oil-free liquid cream foundation. the satisfactory results from the analysis proved the practical applicability of nitio3/cpe towards the determination of phba in real samples. the same electrode was tested for the simultaneous voltammetric determination of o-hydroxybenzoic acid (ohba) and p-hydroxybenzoic acid (phba) [45]. peak overlap in voltammetry is a challenge for the quantitative analysis of electroactive isomers. the experimental results show that the optimum ph 2.0 can be used for the determination of ohba and phba individually. but, when ohba and phba determine simultaneously at ph 2.0, only one peak in dpv was registered at about 1.05 v. in order to obtain improved voltammetric peak separation, the authors used a higher ph value. in electrolyte britton-robinson buffer solution (ph 5.0), dpv shows an oxidation peak of phba at about 0.8 v, whereas an oxidation peak of ohba was observed at about 0.9 v. however, from the presented voltammogram, it is clear that the signals are not completely resolved. therefore, it is doubtful that the peak current can be measured accurately. the results would be more convincing if the authors presented the dpv records for various concentrations of phba and оhba, respectively, in the presence of a constant concentration of the counterpart. recently, charoenkitamorn et al. proposed the anodized spge (spge*) for phba detection [46]. the facile electrode preparation for phba detection and the utilization of phosphate buffer solution is environmentally friendly that could be counted as a green chemistry process. under optimal conditions, the detection limit for phba was achieved at 0.073 μm. the results from the interference tests revealed that the ionic substances have no effects on phba detection, whereas uric and t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 11 ascorbic acid can interfere after adding a concentration 50 times higher than phba. the oxidation potentials of both species were found to be 0.30 and 0.40 v (vs. ag/agcl), respectively, which is closed to the oxidation potential of phba and caused a lower tolerance limit than those obtained from other studied species. the established feature would limit the use of this sensor for phba analysis in cosmetics with high vitamin c content. here it should also be noted that the interference of parabens has not been investigated. five samples of skin lotions were determined to verify the practical performance of the proposed spge*. the hplc, a standard method for phba analysis, was performed in parallel with the same samples. the obtained results reveal that the proposed spge* has a highly effective feasibility for the determination of phba in cosmetics. not least, the spge* can be applied using a portable potentiostat, which is accessible and eligible for the end-users to operate onsite. parabens parabens (alkyl esters of p-hydroxybenzoic acid) have been used for over 70 years in cosmetics, foods, and pharmaceuticals as preservatives due to their broad antimicrobial spectrum, effectiveness and low price. methylparaben (mp), ethylparaben (ep), propylparaben (pp) and butylparaben (bp) are most frequently used in commercial applications. the antimicrobial activity of parabens increases with the increasing length of the ester group, but water solubility decreases. in practice, shorter esters (mp and ep) are commonly used because of their high solubility in water. product ingredient labels typically list more than one paraben in a cosmetic product. parabens are often used in combination since they have synergetic effects in a wide variety of products such as cosmetics, ointments and suspensions, which allows the use of lower levels while increasing preservative activity [47,48]. for the last few years, parabens have been the topic of controversies among scientists and consumers. long-term studies indicated that the use of these compounds could result in potential health risks [49-54]. oishi reported that pp adversely affects hormonal secretion and decreases male reproductive potential [51]. mp has a pronounced estrogenic effect and its constant use is associated with causes of breast cancer, ovarian cancer and endometriosis. according to the cosmetic directive in europe, the maximum allowable concentration of such compounds in cosmetic formulations in the final product is 0.4 wt.% for a single ester and 0.8 wt.% for ester mixtures, expressed as phba [55]. a few electroanalytical methods have been reported for the determination of parabens in cosmetics [48,56-61]. electrochemical quantification of mp was successfully performed using sensors based on: gce modified with graphene oxide and ruthenium nanoparticles [56], mwcnts coupled with nafion-modified gce [60], gold electrodes modified with layer-by-layer (lbl) films of magnetite nanoparticles and polypyrrole [57], pencil graphite electrode modified by molecularly imprinted polymer [61]. electroanalytical detection of pp using gold nanoparticles-electrodeposited onto indium-tinoxide electrode (aunp-ito) functionalized with pei (poly(ethylene imine)) and nitspc (nickel(ii) phthalocyanine tetrasulfonate) in the film (ito-aunps-(pei/nitspc)3 was described by de lima et al. [58]. a new approach to the study and simultaneous determination of mp and pp based on electrochemical and chemometric methods has been reported by behpour et al. [48]. the selective and reliable electrochemical method based on molecularly imprinted polymers (mips) with dual templates was developed by wang et al. in order to determine the total content of parabens in cosmetics [59]. rapid response of the mips sensor was obtained within 1 min. mips sensor was used to determine the total content of parabens in cosmetic samples with recoveries between 98.7 and 101.8 %. http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 12 8-hydroxyquinoline 8-hydroxyquinoline (8-hq), also known as oxine or 8-quinolinol, is an excellent antiseptic, preservative, bactericide and disinfector. 8-hq is widely used in the production and preservation of cosmetic products due to its strong antimicrobial properties. guo et al., 2011 fabricated a modified electrode by dropping a supernatant of mwcnts/nafion on a gce and, for the first time, reported the determination of trace 8-hq in cosmetics by electroanalytical method [62]. as a voltammetric sensor, mwcnts/nafion/gce shows excellent electrocatalytic activity towards the oxidation of 8-hq because of the synergistic effect of mwcnts and nafion – extremely low detection limit (9.0 nm) and high electrode sensitivity in the concentration range from 2×10−8 to 10−5 m. the results from the interference experiments showed that general concomitant substance in cosmetic products, such as 300-fold coconut oil propylbetaine, sodium lauryl sulfacid, glycerol, citric acid, vitamin c, and vitamin e, did not interfere with the determination of 8-hq. these data demonstrated that the mwnt/nafion/gce voltammetric sensor possessed high selectivity for 8-hq in cosmetic formulations. the analytical results obtained in the shampoo sample proved the practical applicability of the proposed method. a modified electrode, obtained by the electrodeposition of 1-amino-2-naphthol-4-sulfonic acid (ansa) on the surface of gce, for the rapid and inexpensive determination of 8-hq in cosmetic samples, was reported by calam et al. [63]. ansa/gce is a reliable sensor for the swv detection of 8-hq in a wide linear range of 5×10−7 to 1.4×10−5 m with a detection limit of 1.6×10−7 m. the voltammetric analysis using the ansa/gce provided excellent reproducibility (rsd = 4.45 %, n = 10), repeatability (rsd = 2.26 %, n = 10) and selectivity: the results showed that 100-fold excess concentrations of nacl, naoh, koh, and ascorbic acid; 30-fold excess concentrations of phosphoric acid, citric acid, alanine, and lysine; and 10-fold concentration of al(oh)3, glycerin, and benzyl alcohol did not significantly influence the peak current height, with variations of less than 5 %. the long-term storage stability of the electrode was also reasonable – a reduction of 9.56 % in the peak current was observed after 15 days. the real sample analysis reveals the applicability of the sensor for 8-hq detection in blush cosmetic products. recently, gao and co-authors [64] have established an electrochemical senor for the determination of 8-hq developed by electropolymerization of tannic acid (ta) on gce [64]. the modified electrode (pta/gce) showed a strong electrocatalytic activity toward the oxidation of 8-hq due to the enhanced surface area and abundant functional groups that exhibit affinity to 8-hq. using dpv, pta/gce shows two linear plots with 8-hq concentrations from 0.5 to 5 μm and 5 to 50 μm (lod = 36 nm). the electrochemical senor was applied in the quantitative determination of 8-hq in hair conditioner samples with a satisfactory result. triclosan triclosan (5-chloro-2-(2,4-dichlorophenoxy) phenol, tcs) as an antibacterial and an antifungal agent has been used in various cosmetic, healthcare, and personal care products, such as mouthwashes, toothpaste, soaps, dish-washing liquids, and antibacterial creams [65]. the european community cosmetic directive approved tcs usage as a preservative in cosmetics and toiletries up to 0.3 wt.% [66]. literature and related studies have shown inflammatory skin conditions and endocrine disruption after exposure to tcs. it was also reported that tcs is toxic to aquatic living organisms. once in the aquatic environment, tcs can undergo a series of transformation reactions to produce chlorophenols, chloroform and dioxin-type derivatives classified as human carcinogens [65,67]. therefore, the safety of tcs has been questioned in regard to environmental and human t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 13 health. a review article presented by sri et al. describes the progress made in the use of nanostructured particles as active electrode materials for the electrochemical sensing of tcs in environmental samples [68]. to the best of our knowledge, there are only a few reports on electrochemical sensors for tcs successfully applied in cosmetic samples [65,67,69-73]. table 2 presents a comparison of the analytical parameters of these sensors used for the determination of tcs in cosmetics and cleaning products. table 2. comparison of the analytical parameters of electrochemical sensors for the determination of triclosan (tcs) in cosmetics and cleaning products electrode material method linear range lod real sample ref. gce1 dpv7 10 – 600 µg l–1 1 – 8 mg l–1 5 µg l–1 toothpaste, hand gel, liquid soap [65] fe3o4@auppy/go/gce2 dpv7 10 nm – 1 μm 2.5 nm toothpaste, shampoo, liquid soap [67] spce3 dpv7 1.2 μm – 1 mm – toothpaste, mouthwash [69] mwcnts/gce dpv7 50 µg l–1 – 1.75 mg l–1 16.5 µg l–1 toothpaste [70] cnds-cs/gce4 lsv8 10 nm – 1 mm 9.2 nm toothpaste, mouthwash, hand sanitizer [71] ls11-oda-aunps/gce5 dpv7 0.01 – 0.5 μm 0.005 μm toothpaste, liquid soap [72] zif-11/hrac/gce6 dpv7 0.1 – 8 μm 0.076 μm lipstick, skin cream, cleansers [73] 1gce: glassy carbon electrode; 2ppy: polypyrrole, go: graphene oxide; 3spce: screen-printed carbon electrode; 4cnds-cs: composite film based on chitosan and carbon nanodots; 5ls11-oda-aunps: core-shell structure of bacteriochlorinsupported aunps; 6zif-11/hrac/gce: composite of zeolite imidazolate framework-11 and activated carbon derived from rice husks; 7dpv: differential pulse voltammetry; 8lsv: linear sweep voltammetry. the extremely low detection limit of 2.5 nm was reported recently by saljooqi et al. [67]. the electroanalysis of tcs was demonstrated by the use of fe3o4@auppy/go-nanocomposite modified gce. nanocomposites consisting of conducting polymers and nanomaterials have received great attention. the appropriate combination of different nanoscaled materials with the conductive polymer may open a new vista for utilizing novel enhanced electrochemical sensing platforms with high performance. the combination of carbon nanomaterials with metal nanoparticles and/or polymers to form composites could increase surface active sites to readily accumulate tcs into the hybrid films. iron oxide magnetic nanoparticles (fe3o4nps) were frequently applied in electrochemical sensors because of their great surface-to-volume ratio, fast electron transfer, and strong adsorption ability. in particular, if fe3o4 surface is loaded or coated with noble metal nanoparticles like au, the transfer of electrons and conductivity increase significantly. to evaluate the applicability of the present methodology on real samples, the authors performed experiments in toothpaste, shampoo, liquid soap, and urine. the results showed that the fe3o4@au-ppy/go nanocomposite displays an acceptable recovery and can be utilized for tcs sensing with the purpose of pharmacokinetics investigations and quality control. zinc pyrithione zinc pyrithione (bis[(2-pyridyl-1-oxo)-thio]zinc) is a broad-spectrum antimicrobial agent that has been used for more than 60 years to treat dandruff and seborrheic dermatitis. its effective bactericide, fungicide and algaecide functions are utilized in most daily hair care products. zinc pyrithione (zpt) has been established as one of the most effective antidandruff ingredients for use in shampoo, conditioner, rinse and hairdressing formulations. being subject to several safety evaluations, zpt was previously found safe as an antidandruff agent in rinse-off hair care products at a maximum concentration of 2.0 %. on the 4th march 2020, the scientific committee on http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 14 consumer safety concluded that zpt is safe when used in a concentration of maximum 1 % [74]. zpt is currently regulated as a preservative in rinse-off products (with the exception of oral hygiene products) in a concentration of up to 0.5 % in general products and up to 1.0 % in hair products [75]. zpt is substantially insoluble in water and is present in aqueous-based products as a dispersion of fine solid particles. the electrochemical oxidation of zpt using a metal oxide-modified carbon paste electrode (mxoy/cpe) has been investigated in various alkaline solutions and the optimum experimental conditions for the determination of zpt containing cosmetic samples were describeed [76]. the higher catalytic activities among ten metal oxides dispersed on cpe are shown at sno2, zro2 and bi2o3. the authors also have presented the results of a comparative study of various ph and supporting electrolytes. the height of the zpt wave in solutions of 0.1 m tetrabutylammonium hydroxide (bu4noh) was found to be higher. the calibration plot of sno2/cpe obtained by plotting the peak current at 0.620 v (vs. sce) against the concentration of zpt shows linearity over the range of 1.6 to 32 mg l–1. the direct determination of the concentration of zpt in commercial products (shampoo, conditioner, rinse and hairdressing products) was successfully accomplished by means of dpv using the same modified electrode. a disposable type of cobalt phthalocyanine-modified screen-printed carbon electrode (copc/spe) in a couple with fia was developed for the analysis of zpt in commercial hair care products [77]. the presented method shows several advantages for zpt detection over the sno2/cpe-based dpv assay, such as lower over-potential, the limit of detection and working volume. under the optimized hydrodynamic flow injection parameters, the copc/spe yielded a linear calibration plot in the window of 6 to 576 μm with a detection limit of 0.9 μm in 0.1 m koh solution at an applied potential of 0.3 v (vs. ag/agcl). the authors demonstrated that the copc/spe is effective and selective for the zpt analysis in commercial shampoos. both commented studies offer rapid, easy, selective and inexpensive approaches for direct routine zpt analysis in hair care products. the electrochemical procedures described above do not have the drawbacks of classical methods, namely trans-chelation at hplc analysis (the need for preceding formation of the cu(ii) complex), photodegradation of zpt at thin-layer chromatography, a lack of selectivity of titrimetric methods, etc. uv filters in sunscreen products sunscreens have been recommended by dermatologists for a long time as a protective measure against excessive amounts of sunlight to prevent uv-induced erythema. moreover, numerous studies show that the regular use of sunscreens contributes to the prevention of skin cancer and photoaging [78,79]. uv filters can be classified into two groups according to their nature: i) the inorganic uv filters (physical uv filters) principally work by reflecting and scattering the uv radiation; ii) the organic uv filters (chemical uv filters) absorb ultraviolet radiation. chemical uv filters are organic compounds with high molar absorptivity in the uv range and usually possess single or multiple aromatic structures, sometimes conjugated with c=c double bonds and/or carbonyl moieties [80]. cosmetics containing organic uv filters are most commonly used, despite causing different side effects. the adverse reactions to sunscreens include subjective irritation (stinging, burning), contact dermatitis and comedogenicity. the potential adverse effects induced by uv filters in experimental animals include reproductive/developmental toxicity and disturbance of hypothalamic–pituitary– thyroid axis (hpt) [81,82]. t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 15 organic chemicals that absorb uv radiation are added to sunscreen products in concentrations of up to 10 % for skin protection. some of these compounds are also included in other cosmetics, such as skin lotions, beauty creams, lipsticks, hair sprays, etc. the level and quality of uv protection provided by sunscreen products has improved significantly during the last three decades. nowadays, the trend toward cosmetic products with higher protective effects and screening efficiency against both uvb (290 – 320 nm) and uva (320 – 400 nm) wavelengths has led to the extensive development of formulations containing combinations of various organic uv filters at different concentrations. modern sunscreens should provide broad-spectrum uv protection because this ensures that the natural spectrum of sunlight is attenuated without compromising its quality. for example, оctyl-methoxycinammate (omc), benzophenone-3 (bz3) and 4-methylbenzylidene camphor (4-mbc) are uv filters usually combined in sunscreen formulations because their uv spectra have overlapping bands and the mixture allows a good sun protection factor (spf) in the whole range of uvb and uva radiation [83]. there are positive lists for sunscreen agents in the three main legislations regarding cosmetic products, that is, those in force in eu, usa and japan (in the usa they are considered as otc drugs) where the maximum authorized contents are stipulated [80]. however, until today there are no official analytical methods. methods based on hplc are the most widely employed for the separation and determination of sunscreen agents [84]. to the best of our knowledge, there are a limited number of published articles dealing with the determination of sunscreen agents in cosmetics by means of electrochemical techniques. differential pulse voltammetry using a carbon-epoxy composite electrode in non-aqueous solvents [85] or mercury film electrode in strongly alkaline media [86] have been successfully applied to determine different uv filters in sunscreen products. square wave voltammetry using gce is fast and sufficiently sensitive to ensure octocrylene (ocr) quantification at low concentrations [87]. ocr was reduced at a potential of -0.97 v (vs. ag/agcl) on a gce using a mixture of 0.04 m britton-robinson buffer and ethanol (7:3, v/v) as a supporting electrolyte. the results reveal satisfactory precision and accuracy, demonstrating that the proposed method is an acceptable alternative for the analytical determination of ocr in cosmetic preparations. cardoso et al., 2007 studied the voltammetric behavior of 4-mbc by square wave voltammetry (swv) using mercury electrode in britton-robinson buffer and cationic surfactant, cetyltrimethylammoniun bromide (ctab) [83]. a single peak of 4-mbc reduction was observed at -1.21 v (vs. ag/agcl), the calculated lod was 2.99 nm and the loq was 9.98 nm. authors applied the developed methodology to determine 4-mbc in commercial sunscreen spf 15, 20 and 30 and for the simultaneous determination when other protection agents were associated, such as bz3 or omc. subsequently, the research group published the first study dealing with the simultaneous determination of the three uv filters using surfactant and polarography [88]. a method based on electrochemical reduction for the simultaneous determination of three sunscreen agents (4-mbc, bz3 and omc) by differential-pulse polarography (dpp) was proposed. the authors validated the methodology using four commercial sunscreen preparations and the results showed high recovery rates. lopes neves et al. [89] developed a new analytical methodology for the determination of bz3, 4mbc and omc in cosmetics samples by swv using a mercury film modified gold electrode as an alternative to dropping mercury electrode, contributing to the generation of less toxic residues, a safer process for the analyst, and a low cost in terms of analysis. http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 16 hair-waiving agents thioglycolic acid thioglycolic acid (tga) is an organic compound containing both a thiol and a carboxylic acid functional groups. tga and its salts are frequently used in hair-waving and depilatory products. the chemistry of hair-waving is based on the cleavage of hair disulfide bonds by the thiol groups of the tga. here, it should be noted that skin contact with low-molecular-weight organic acids can lead to severe pain and burns, which can heal slowly with the formation of scar tissue. tga, even in dilute solutions, may cause conjunctival hyperemia and corneal injury. in this regard, the eu cosmetics directive has limited the maximum permissible concentration of tga and its salts, calculated as tga, to 11.0 % at ph 7.0 to 9.5 [90]. the voltammetric behavior of tga at a carbon paste electrode modified with cobalt phthalocyanine (copc) was studied by shahrokhian et al. [91]. the copc/cpe shows high electrocatalytic activity toward oxidation of tga, substantially lowering the overpotential of an anodic reaction. the results of these studies were used to develop a potentiometric method for determining tga and its salts in hair-treatment products. however, the authors note that оther compounds with reducing properties and redox potentials close to tga, such as hydrogen sulfide, sulfite and mercaptoethanol, may interfere if they are present in the sample. тhis issue has been resolved by zen and co-authors, who developed а sensitive and selective method combining hplc with electrochemical detection for the quantitative determination of tga [92]. in this study, hplc was used to eliminate interference from the matrix of real samples, and preanodized screen-printed carbon electrodes (spces*) were used to determine tga. spces* have the advantages of being inexpensive, easy to handle, disposable, flexible and stable. under optimized conditions, the linear range for tga is up to 20 ppm with a detection limit of 0.042 ppm. the practical application of the proposed method was demonstrated by the determination of tga in commercial hair-waving products. the authors stated that the new method offers a viable alternative to previous approaches used in the routine determination of tga. sulfites sulfites are commonly used as preservatives/antioxidants in the food and cosmetic industry. sodium sulfite is a reducing agent that alters the structure of hair; including sulfite in hair setting products, hair curvature or straightening can be regulated. in addition, to use as a hairwaiving/straightening agent, it can also function as a preservative in various cosmetic formulations [93]. exposure to sulfites has been reported to induce a range of adverse clinical effects in sensitive individuals, ranging from allergic contact dermatitis, urticaria, flushing, and life-threatening anaphylactic and asthmatic reactions [94]. scientific committee on cosmetic products and nonfood products (sccnfp) has suggested that the sulfite concentration added to cosmetic products has to be lower than 6.7 % to maintain the safety of hair settings and skin care products [95]. conventional methods applied in industry for sulfite analysis are monier–williams and iodometric methods. at present, only one publication is available on the development of an electrochemical method for the analysis of sulfites in cosmetics. chen et al. developed an oxygen-incorporated gold electrode by depositing gold particles on a screen-printed electrode in an aqueous solution [96]. the detection sensitivity of dissolved sulfite at the oxygen-rich gold electrode (auospe) and that at oxygen-poor gold electrodes was studied by an fia in a 0.1 m pbs (ph 6.0) at an applied potential of 0.3 v (vs. ag/agcl). bisulfite is the active species in this system according to its various forms at different ph solutions. the results showed that oxygen-incorporated atoms play an important role in improving the sensitivity of the new gold electrode. the practical applicability of developed electrode t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 17 material coupled with fia (auospe/fia) to detect selectively sulfite content in cosmetics was tested in two hair-waving products and satisfactory results were obtained. coloring agents in hair dyes commercial oxidative dye products form the permanent color in situ by a coupling reaction and oxidation. permanent hair dyes are generally composed of three constituents: a precursor agent (pphenylenediamine, ppd), a coupling agent (electron-donating group substituted aromatic compounds, for example, resorcin) and an alkaline medium-based oxidizer (e.g., h2o2 – ammonia mixture) [97]. black henna, used to color the hair or skin, also contains ppd as a chromophoric constituent. at the same time, ppd is the most frequent contact sensitizer, and thus a primary patch test is recommended before using any hair dyes/henna containing ppd. the clinical findings of hair dye allergy vary and include contact dermatitis, lichen simplex chronicus, non-specific eczema and dermographism on the hair dye-exposed area or/and extended area [98]. several experiments have shown a significant association between the use of products containing ppd and mutagenicity, according to the scientific committee on consumer products (sccp) [99]. a safe limit for ppd in hair dye products that can provide desirable coloring with minimum body exposure to ppd needs to be carefully defined [100]. european union cosmetic directive regulation has banned ppd in topical products intended for superficial purposes while allowing ppd inclusion in cosmetic products marketed as oxidizing coloring agents with a maximum concentration of 4 % (free base). specifically, the permitted concentration is 2 % (free base) when mixing ppd with h2o2 following the preparation protocol of hair dyeing products [101,102]. the risk associated with using ppd has motivated researchers to develop new strategies and reliable analytical techniques to detect and quantify this chemical in cosmetic products to guarantee legislative requirements and ensure their safety. due to their similar structural and chemical characteristics for phenylenediamine isomers and dihydroxybenzenes isomers, there are few reports on detection using the electrochemical method alone. a review article that discusses the sources and toxic effects of ppd, and makes a critical comparison between conventional and electrochemical methods (reported from 1935 to 2021) to detect and quantify ppd in various samples, was presented by singh et al. [97]. table 3 summarizes the electrochemical sensors applicable in the analysis of ppd in hair dyes. table 3. comparison of the analytical parameters of electrochemical sensors for analysis of pphenylenediamine (ppd) in hair dyes electrode material method linear range lod ref. β-mno2/cs/gce1 amp.5 0.2 – 150 μm 50 nm [103] psc82/gce2 amp.5 0.5 – 2900 μm 2900 – 10400 μm 0.17 μm [104] pani/zno-starch-rgo/gce3 dpv6 1 – 180 μm 0.1 μm [105] il-go@cu-ag/gce4 dpv6 0.018 – 22 μm 3.96 nm [106] 1β-mno2/cs/gce: glassy carbon electrode modified with β-mno2 nanowires/chitosan hydrogel; 2psc82/gce: sr-doped perovskite oxide with the composition of pr1-xsrxcoo3-δ(x = 0.2); 3pani/zno-starch-rgo/gce: glassy carbon electrode modified with nanocomposite „polyaniline/zno-anchored bio-reduced graphene oxide“; 4il-go@cu-ag/gce: glassy carbon electrode modified with bimetallic nanocomposite „ionic liquid functionalized graphene oxide wrapped cu-ag nanoalloy particles”; 5amp.: amperometry; 6dpv: differential pulse voltammetry. recently, singh and co-authors [106] reported an efficient electrochemical sensor based on a novel bimetallic nanocomposite „ionic liquid functionalized graphene oxide wrapped cu-ag nanoalloy particles” modified glassy carbon electrode (il-go@cu-ag/gce). the synergism between ionic liquid functionalized graphene oxide and cu-ag nanoalloy facilitated the electron transfer and http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 18 increased surface area, and therefore, excellent performance was explored with good storage stability, high sensitivity, the negligible influence of common interferents and the best limit of detection as compared to other reported works. the sensor demonstrated a high electrocatalytic effect on the oxidation of ppd at a potential of 0.21 v (vs. ag/agcl) in a linear concentration range of 0.018 to 22 μm and lod of 3.96 nm. the fabricated electrode showed good selectivity towards ppd in the presence of o-phenylenediamine, m-phenylenediamine, hydroquinone, resorcinol, ammonia and h2о2. real sample analysis in spiked dye and water samples exhibited good recovery results, which established the prepared nanocomposite as an alternative for the development of an efficient electrochemical tool for ppd detection. aluminium aluminium compounds are often used in personal care products, including antiperspirants, lipsticks and toothpastes. in particular, the most extensively used aluminium compound in cosmetic products as an antiperspirant is aluminium chlorohydrate (al2(oh)5cl×2h2o) in various cosmetic application forms, such as aerosols, liquid/powder roll-ons, solid sticks, gels, and creams. here, it should be noted that aluminium chlorohydrate is a cosmetic ingredient not regulated in cosmetic regulation 1223/2009. other aluminium salts, such as aluminium zirconium chloride hydroxide complexes and aluminium zirconium chloride hydroxide glycine complexes, are covered by entry 50 in annex iii of the cosmetic regulation for use as antiperspirants with specific conditions of use. there is convincing evidence that the use of aluminium salts-based antiperspirant products continues to increase worldwide. recent reports indicated that continuous usage of the aluminium chlorohydrate-containing cosmetic products allows transdermal absorption of cutaneous generated hormones and pheromones and as a link to breast and prostate cancers [107]. frequent application of aluminium salts to the underarm as an antiperspirant adds a high additional exposure directly to the local area of the human breast. coincidentally the upper outer quadrant of the breast is where there is also a disproportionately high incidence of breast cysts and breast cancer. aluminium has been measured in human breast tissues/fluids at higher levels than in blood, and experimental evidence suggests that at physiologically relevant concentrations, aluminium can adversely impact human breast epithelial cell biology [108]. the examination revealed that the number of published electrochemical sensors for the analysis of aluminium in cosmetics is limited. a new flow injection analysis (fia) based electrochemical measurement using built-in three-in-one screen-printed silver electrode (spage) suitable for rapid, low cost and low working volume detection of aluminium chlorohydrate has been introduced by chiu et al. [109]. spage configuration of ag-working, ag-counter and ag/agxo (silver oxides) pseudoreference electrodes was developed for sensitive and selective electroanalysis of aluminum chlorohydrate present in antiperspirants, through the free cl– ion liberated from aluminum chlorohydrate in aqueous medium, as a redox signal at ag-working electrode. the calibration graph was linear in the concentration range of 1 to 200 ppm and the limit of detection was 295 ppb aluminum chlorohydrate. the detection limit obtained is about ten times lower than that of previous work on aluminum chlorohydrate detection by linear sweep voltammetry approach (3.03 ppm) [110]. finally, four real antiperspirant samples (in the form of roll-on liquid and lotion) assays were successfully demonstrated, with recovery values lying in the potential window of 98 to 106 %. heavy metals bocca et al. [111] reviewed the concentration of metals in different types of cosmetics manufactured and sold worldwide and the data on metals’ dermal penetration and systemic t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 19 toxicology. next year, a review article focused on the problems related to the presence of heavy metals in cosmetics, including their sources, concentrations and law regulations, as well as the danger to the health of these products users was presented by borowska et al. [1]. studies have shown that low-dose but long-term exposure to heavy metals in the human body can cause chronic poisoning, causing irreversible adverse effects on human organs and seriously threatening human health. consequently, developing effective methods that are sensitive, highly selective and affordable for heavy metal ions detection is indispensable in the quality control of cosmetics. mercury (hg) has a long history of use as a whitening agent in cosmetic products. mercury suppresses melanin production and eliminates dead skin cells, resulting in a stereotyped slate-grey skin color [112]. inorganic mercury can be absorbed via the sweat glands, sebaceous glands, and hair follicles, and after absorption, it is distributed to all tissues. repeated topical applications can result in systemic toxicity, neurological disorders and kidney dysfunction. recently, a case of mercury intoxication caused by daily-use whitening cosmetics was reported by wang et al. [113]. japan strictly prohibits the use of mercury in cosmetics. in europe and china, the maximum concentration of mercury in cosmetic products for eyes and lips is 0.07 ppm. the food and drug administration (fda) limits the amount of mercury in cosmetics to trace amounts under 1 ppm. nevertheless, many cosmetics contain mercury above 1000 ppm to increase the whitening effect [114]. many studies were carried out to identify and quantify mercury in cosmetic products and detected too high mercury levels [115]. additionally, mercury is not always listed as an ingredient in mercury-containing products; in many cosmetics, the concentration of mercury is specifically hidden in their product labels [116,117]. monitoring the mercury ion levels in cosmetic products is therefore of fundamental importance. suherman et al. [118] presented a review article focused on recent advances in voltammetric techniques to detect ultra-trace levels of inorganic hg2+ in an aqueous solution, providing an overview and a critical evaluation. it is well known that the preconcentration time required to detect trace or ultra-trace levels of hg2+, can affect analysis sensitivity and sampling frequency. thus, an important improvement in the voltammetric analysis is decreasing the accumulation time without compromising the sensitivity, selectivity and linear range of the response. zen et al. [116] reported the determination of mercury in cosmetics using an in-built screen-printed three electrodes containing partially crosslinked poly(4-vinlylpyridine) (pcpvp) modified carbon working, carbon-counter, and ag+-quasireference electrodes (spe/pcpvp) by means of square wave anodic stripping voltammetric (swasv) technique. the preconcentration was performed at a fixed applied potential of -0.5 v (vs. ag/agcl) for 60 s. the calibration graph was linear in the window of 100 to 1000 ppb with a detection limit of 69.5 ppb. two unbranded skin-lightening lotions were tested in parallel with inductively coupled plasma-optical emission spectroscopic (icp-oes) measurements. calculated recovery values were 94.29 and 98.07 %, respectively. the authors concluded that the analytical methodology is suitable for rapid and easy single-use mercury detection in cosmetic products. a new method for selective determination of trace mercury by linear scan voltammetry using silver ink screen-printed electrode (agspe) in the presence of ki dissolved 0.05 m h2so4 solution has been demonstrated [117]. electrooxidation of iodide at agspe showed a systematic increase in inhibitory current against the mercury concentration at 0 v (vs. ag/agcl). the linear dynamic range of 500 to 4500 ppb was established; lod and loq were found to be 98 and 318 ppb, respectively. the method was considered appropriate for constructing a new detection technique without any preconcentration. such an approach is suitable for constructing a new detection technique without any interference from arsenic, cadmium, and lead ions. http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 20 chemically modified mwcnts paste electrode with chloroplatinum(ii) complex for the determination of mercury using square wave stripping voltammetry was presented [119]. under optimal conditions, the linear range was from 5.0 μm to 0.1 mm with a lod of 3.7 μm. the recovery values were between 98.9 and 101.1 %, indicating that the modified electrode was capable of the quantification of hg2+ in the skin-lightening cosmetics. pani/mwcnts/aunp-modified ito electrode was developed via direct electrodeposition of pani, mwcnts and aunps on a film-coated ito electrode and applied to analyze mercury in cosmetic products [112]. the optimum conditions for mercury detection using the modified ito electrode were of tris-hcl buffer (ph 7.0) in the presence of 1 mm methylene blue (mb) as a redox indicator. the sensor response was linear from 0.01 to 10.0 ppm (lod = 0.03 ppm), which gave a platform for the sensitive detection of mercury in real samples. however, the interference studies are inconclusive. тhe concentrations of mercury and the tested common substances in cosmetics are not specified. the presented graph clearly shows that some electrochemically active substances have significant interference. so the selectivity of this sensing platform remains questionable. lead (pb) and cadmium (cd) are two potentially harmful heavy metals that cause considerable concern. lead can impair almost all the organs of the human body and causes physical and mental impairments. lead affects the neurological, reproductive, and renal systems. symptoms may include headaches, memory problems, reduced fertility, etc. [120]. lead and its compounds are banned in the european union according to regulation (ec) no 1123/2009 of the european parliament and of the council of 30 november 2009 on cosmetics products under annex ii: list of substances prohibited in cosmetic products [121]. a compound of lead, lead acetate, is allowed as a colorant only in hair dyeing at concentrations lower than 0.6 % in asean countries under asean cosmetics directory (acd) annex iii [122]. cadmium is classified as a group i carcinogen; it can accumulate in various organs and tissues, but mostly in the kidney cortex [123]. cadmium and its compounds are listed as substances that must not form part of the composition of cosmetic products [124]. among cosmetic products, lipstick and eye cosmetics are most commonly used. they are produced by adding pigments that may contain heavy metals as impurities in the pigment formulation. an important non-dietary pb exposure pathway for women is the mouthing or ingesting lip products such as lipsticks and lip glosses. zhao and co-authors identified pbcro4 as the dominant pb species in lip products with extremely high pb concentrations [125]. feizi et al. [123] analyzed by icp-oes 60 samples of lipstick and eye pencil of different brands (produced by china, france, turkey, germany, and korea). mean concentrations of pb and cd in all the brands studied were 41.86 and 53.42 µg g-1, respectively. the overall results showed that in all brands and colors of lipsticks, only 33 % had pb content less than the fda limit (20 µg g-1) [126], and 44 % of all samples among lipsticks had cd content less than the fda limit of 3 µg g-1. in eye pencils, 100 % of samples had cd content above the recommended value of 3 µg g-1. in particular, kohl (also known as surma or kajal) is extensively used as traditional eye cosmetics in different countries of africa and south asia [127]. kohl is applied to the conjunctival surfaces rather than to the outside of the eyelids and can cause eye damage or long-term health problems because it contains lead (galena stone, pbs) and antimony (stibnite, sb2s3). lead is mainly responsible for kohl’s toxic effects, being associated with high concentrations of this element in blood samples from regular cosmetic users. several studies have demonstrated that pb levels are extremely high in this product and its use has been associated with the development of plumbism. although pb is the main component of kohl, sb also promotes pathological dysfunctions, and dermal and dna strand lesions [128]. t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 21 swetha et al. [129] reported a novel electrochemical sensor approach based on a high-index facets (hif)-silver nanoflower modified gce (agnf@gce) for anodic stripping voltammetric detection of pb2+ in cosmetics and human blood serum. unlike the conventional agnps, this platform displayed remarkable activity for sensitive and selective electrochemical analysis of pb2+ because it provides a highly crystalline and large surface area with hifs {422} and {111}. the newly developed agnf@ gce system can effectively determine pb2+ in the presence of other common interfering metal ions, such as cu2+, fe2+, mg2+, ni2+, k+, and na+. the method is defined as a promising approach for the rapid, sensitive detection of pb2+ without any complicated offline preparation of real samples. coated wire ion selective electrode (cw-ise) [130], composite cork–graphite sensor (figure 4) [131], disposable screen-printed non-single crystal silver electrode (agspe) [132], gold nanoparticle (aunp)/hexaammineruthenium(iii) ([ru(nh3)6]3+)/nafion modified gce [133], and carbon blackmodified carbon paste (cb-cp) electrode [134], were fabricated and used as working electrodes in the determination of pb2+ in various commercial cosmetics. figure 4. dpsv responses for the determination of pb(ii) using the cork–graphite sensor. (a) dpsv curves recorded for different concentrations of pb(ii) in 0.1m acetate buffer (ph 4.5): (1) 0, supporting electrolyte; (2) 3.19; (3) 6.35; (4) 9.46; (5) 12.53; (6) 15.57; (7) 18.56; (8) 21.51; (9) 24.43; (10) 27.31; (11) 30.16; (12) 32.92; (13) 35.75; (14) 38.49; (15) 41.19; (16) 43.87; (17) 46.51; (18) 49.13; (19) 51.70 and (20) 54.26 µm. inset: plot of the electrochemical response, in terms of current, as a function of lead concentration. (b) graphical representation of the residuals behavior, which confirms the linearity of the calibration curve. reproduced from ref. [131]. licensee mdpi, basel, switzerland (2022) a comprehensive elucidation of the recent developments in the electrochemical detection of cd2+ was presented by yi et al. [135]. the determination of cd2+ at trace and ultra-trace levels in environmental water samples by means of electrochemical methods was discussed in details. however, there hasn't yet been a published electroanalytical method for cd2+ alone in cosmetics. it is a challenging task to simultaneously, reliably and conveniently measure heavy metal ions in complex samples, such as cosmetics. new strategies were presented by de furtado et al. [136] for the simultaneous voltammetric quantification of pb2+ and zn2+ in hair cosmetics (color wash lotion and anti-dandruff shampoo) employing chemically modified composite electrodes. two methods for simultaneous determination of pb2+, zn2+ and cd2+, with a cheap and easy to fabricate graphite-epoxy composite electrode, were developed. one method uses an in situ bismuth film modification and the other uses an ex situ organic film modification via diazonium salt reduction. determinations were carried out in acetate buffer (ph 6.0) with a deposition potential of -1.4 v (applied during 120 s), followed by swv measurements and subsequent cleaning with an application of 0.3 v potential. detection limits of 0.07, 0.05 and 0.06 µm were achieved for pb2+, cd2+ and zn2+, respectively. the data were compared to those found by applying flame atomic absorption spectrometry (faas), being http://dx.doi.org/10.5599/jese.1535 j. electrochem. sci. eng. 00(0) (2022) 000-000 electrochemical sensors in cosmetics 22 considered statistically equivalent. тhe results are remarkable, considering that the cosmetic samples tested are quite complex, especially due to the greasy content and high concentration of surfactants. in summary, a comparison of various sensors applied to the electroanalysis of heavy metals in cosmetics is shown in table 4. table 4. comparison of the analytical parameters of the electrochemical sensors for the determination of hg2+ and pb2+ in cosmetics analyte electrode material method linear range lod real sample ref. hg2+ spe/pcpvp1 swasv7 100 – 1000 ppb 69.5 ppb skinlightening cosmetics [116] agspe2 lsv8 500 – 4500 ppb 98 ppb [117] cpc/mwcnts/pe3 swasv7 5.0 μm – 0.1 mm 3.7 μm [119] pb2+ agnf@gce4 asv9 10 – 700 ppb 0.74 ppb eyeliner, lipstick [129] cw-ise5 pot.10 10 – 100 μm 8.0 μm lipstick [130] cork-graphite dpsv11 3.19 – 54.26 μm 1.06 μm hair dyes [131] agspe swv12 0.06 – 0.79 μm 0.31 nm [132] aunps([ru(nh3)6]3+)nafion/gce asv9 0.3 – 0.75 ppm 0.045 ppm [133] cb-cp6 dpv13 0.04 – 3.2 μg 4 ng [134] 1spe/pcpvp: an in-built screen-printed three electrodes containing partially crosslinked poly(4-vinlylpyridine) modified carbon working, carbon-counter, and ag+-quasireference electrodes; 2agspe: silver ink screen-printed electrode; 3cpc/mwcnts/pe: chloroplatinum (ii) complex-modified mwcnts paste electrode; 4agnf@gce: silver nanoflower modified glassy carbon electrode; 5cw-ise: coated wire ion selective electrode; 6cb-cp: carbon black-modified carbon paste; 7swasv: square wave anodic stripping voltammetry; 8lsv: linear scan voltammetry; 9asv: anodic stripping voltammetry; 10pot.: potentiometry; 11dpsv: differential pulse stripping voltammetry; 12swv: square wave voltammetry; 13dpv: differential pulse voltammetry. wang et al. [137]reported the first simultaneous analysis of mercury, lead, and arsenic (as) in cosmetics based on the electrochemical method. а disposable cost-effective gold-sputtered plastic electrode was used for the simultaneous detection of pb2+, as3+, and hg2+ in cosmetics based on differential pulse stripping voltammetry (dpsv). under the optimized conditions, the dpsv peaks of these three metal ions could be well and reproducibly separated with satisfactory linear range and low detection limits of 2, 5 and 0.5 μg l−1 for pb2+, as3+, and hg2+, respectively. the method was successfully applied for the simultaneous determination of three target ions in real cosmetic samples (eye shadow, skin lotion and talcum powder) with satisfactory results. noting the low cost and convenient fabrication of the plastic-based sensors, the authors concluded that this approach might serve as an attractive alternative to conventional methods in routine analysis of those trace heavy metal ions in cosmetics. conclusion remarks considering that the use of cosmetic and personal care products in daily life has increased globally, it is important for researchers to keep on improving the performance of existing analytical methodologies, as well as developing innovative solutions. as the demand for cost-effective, rapid, highly selective and ultrasensitive quantification of various cosmetics ingredients rapidly increases, the electroanalytical methods provide a feasible path toward the next generation of reliable sensing devices (figure 5). t. dodevska et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1535 23 figure 5. cosmetic ingredients considered for electrochemical analysis the miniaturization potential of electrochemical systems attracts increasing research attention because electronic measurements can be integrated in hand-held devices and smartphones, which would allow fast, accurate, and on-site detection. perspectives the large number of applications discussed in the present article clearly demonstrates the feasibility and utility of electrochemical sensors for analyzing ingredients in real cosmetics samples. after a thorough critical review of previous research studies, the following challenges were identified as priorities for the development of new advanced electroanalytical devices for the quality control of cosmetics. • considering the high matrix complexity of cosmetic samples, effective approaches should be developed to suppress the non-specific adsorption of interfering species. • currently, nanotechnology in the field of electrochemical sensors has been a crucial strategy for constructing reliable monitoring systems for cosmetic products safety. future developments in electrochemical design will inevitably focus on the technology of new nanomaterials to improve the selectivity, reproducibility and operational stability of the sensor systems. • using easy and low-cost fabrication methods for mass production while 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https://doi.org/10.1246/bcsj.77.311 https://doi.org/10.1039/c7ay01114h https://doi.org/10.1016/j.talanta.2021.122881 https://doi.org/10.1016/j.teac.2021.e00152 https://doi.org/10.1016/j.measurement.2018.05.042 https://doi.org/10.1016/j.microc.2019.05.011 https://creativecommons.org/licenses/by/4.0/) electrochemical detection of sulfite in food samples: http://dx.doi.org/10.5599/jese.1555 1061 j. electrochem. sci. eng. 12(6) (2022) 1061-1079; http://dx.doi.org/10.5599/jese.1555 open access : : issn 1847-9286 www.jese-online.org review paper electrochemical detection of sulfite in food samples yasser fakri mustafa1,, gholamabbas chehardoli2, sepideh habibzadeh3 and zeinab arzehgar3 1department of pharmaceutical chemistry, college of pharmacy, university of mosul, mosul, iraq 2department of medicinal chemistry, school of pharmacy, hamadan university of medical science, hamadan, iran 3department of chemistry, payame noor university, p.o. box 19395-4697 tehran, iran corresponding author: dr.yassermustafa@uomosul.edu.iq received: october 21, 2022; accepted: november 8, 2022; published: november 29, 2022 abstract in various pharmaceutical and food industries, sulfite is utilized for the inhibition of nonenzymatic and enzymatic browning. also, in brewing industries, it acts as an antioxidizing and antibacterial agent. several toxic and adverse reactions, including vitamin deficiency, hypersensitivity, and allergic diseases, have been attributed to sulfite ingestion that may cause dysbiotic oral and gut microbiota events. thus, the content of sulfite in foods must be controlled and monitored, and it is essential to find a specific, reproducible, and sensitive method to detect sulfite. some analytical solutions are being tested to quantify sulfite. however, due to their advantage over traditional techniques, electroanalytical techniques are attracting much attention because they are simple, fast, affordable, and sensitive to implement. in addition, by the electrode modification, the morphology and size can be controlled, resulting in the miniaturization to be used in portable electrochemical devices. therefore, the present review addressed some articles on the electrooxidation of sulfite from real samples using various electrochemical sensors. keywords sulfite; modified electrode; voltammetry; electrooxidation; amperometry introduction experts in the field have largely considered the sulfite contribution and its respective alter ego, that is, sulfur dioxide, into agri-food products due to its impact on some medical conditions. the presence of sulfite produces different technical impacts on various kinds of food products like inhibiting the enzymatic browning in fresh products and shrimp, inhibiting the nonenzymatic browning in the dried or dehydrated vegetables and fruits, anti-microbial actions in wine-making and wet-corn milling, anti-oxidant impact, bleaching in the maraschino cherries and hominy and dough conditioning [1-4]. additionally, sulfite is utilized for preserving the effectiveness and stability http://dx.doi.org/10.5599/jese.1555 http://dx.doi.org/10.5599/jese.1555 http://www.jese-online.org/ mailto:dr.yassermustafa@uomosul.edu.iq j. electrochem. sci. eng. 12(6) (2022) 1061-1079 electrochemical detection of sulfite in food 1062 of specific medicines. put differently, the increased concentration of sulfite causes an allergic reaction in sensitive consumers. in general, human serum shows a sulfite concentration in ranges between 0 and 10 μm. moreover, the us food and drug administration (fda) suggested the existence of warning labels on all food products consisting of more than 10 mg/kg sulfite or beverages consisting of >10 mg/l sulfites. for this reason, sulfite content must be cautiously controlled in foodstuffs [5-8]. researchers have presented various methods to determine sulfite in the beverages and food like titrimetry, flow injection analysis (fia), iodimetry, ion chromatography, spectro-photometry, chemiluminescence, fluorometry, high-performance liquid chromatography (hplc) as well as gas chromatography. nonetheless, a number of them do not have accuracy and sensitivity (for example, spectrophotometry and iodimetry) and other methods need laborious sample preparation, costly instrument and professional experts [9-14]. experts in the field utilized electrochemical systems as one of the solutions to the decentralized testing for numerous specimens because they required cheap equipment and inexpensive operations, could be miniaturized, and have been accessible by simple-dipstick sampling [15-35]. in addition, electrochemical methods have been considered to be very selective, quantitative, less time-consuming, and strongly sensitive, and enjoy broad dynamic ranges and fast responses. moreover, sulfite's redox features have been defined so that it is possible to reduce or oxidize the analyte to easily modify sulfite for electrochemical determination [36-70]. nevertheless, electrochemical analysis of the analyte has been restricted by electrode surface fouling, time, and reagent consuming process for electrode surface regeneration. notably, while developing electroanalytical sensing systems to detect an analyte, sensing electrodes would be frequently modified with proper substances to achieve favorable outcomes in selectivity and sensitivity [71-97]. as nanoscience and nanotechnology have been developed, researchers have remarkably considered nanomaterials. thus, they contribute importantly to cases like catalysis, micro-electronics, as well as electrochemical sensors because of the respective thermal, optical, catalytic, and electrical features [98-113]. therefore, nanomaterials would be highly feasible in enhancing the sensors' performance due to the increased surface-to-volume ratio, higher electrical conductivity, suitable biocompatibility, particularly good catalytic abilities, and surface reaction activities [114-120]. furthermore, electroanalytical chemistry has been developed in presenting screen-printed electrodes (spe), modified carbon-based electrodes and other electrodes. therefore, our review is a summary of the current achievements of distinct chemically-modified electrodes for the electrochemical sensing of sulfite. design of the mentioned sensor-enabled researchers to improve the analytical performances of the available electroanalytical sensing systems with regard to selectivity, sensitivity, multiplexed detection capability as well as field portability. electrochemical detection of sulfite according to the research, sensing electrodes should be used for electrochemical sensing of sulfite to pass current to an aqueous solution and create a number of measurable and beneficial electrical signals relative to the electrochemical reactions in the solution of the presence of sulfite ions. since the electrodes can be miniaturized and easily modified, the electrochemical setup would usually be shorter, simplified and portable, resulting in their effectiveness and usefulness for sulfite determination. it is possible to modify the working electrode with various substances for the specific detection of sulfite [121-123]. y. f. mustafa et al. j. electrochem. sci. eng. 12(6) (2022) 1061-1079 http://dx.doi.org/10.5599/jese.1555 1063 the diverse carbon electrodes (from the traditional large-scale carbon-based materials (e.g., glassy carbon electrodes (gces), carbon paste electrodes (cpes) and graphite) to the well-known nano-sized carbon materials (e.g., carbon nanotubes (cnts))), the screen-printed electrodes (spes), gold (au) electrodes and pt electrodes, they have a widespread utilization in the electrochemical sensing systems. these electrodes can be modified with various materials and used for the determination of the sulfite. glassy carbon electrodes for sulfite detection it is widely accepted that glassy carbon (gc) artifacts have been created by a robust controlled heating plan of a pre-modelled polymeric (phenol-formaldehyde) resin body in an inert atmosphere so that the carbonization procedure initiates at a temperature >300 °c. gc has been extensively employed in voltammetric electrodes during the last twenty years. since they exhibit lower residual current in the ranges of close to 1 v in aqueous media as well as more extended ranges in organic solvents and aqueous micellar solution, they are usually utilized as one of the indicator electrodes. moreover, numerous papers of the gces exhibited higher sensitivity of the apparent rates of redox reactions toward the electrode surface state [124-131]. a lot of modified gces have been made to detect sulfite from real samples (table 1). in their study, manusha and senthilkumar [132] introduced a new redox mediator-based ionic liquid in order to selectively and sensitively detect sulfite. therefore, they devised phenothiazine imidazolium ionic liquid with hexafluorophosphate counter anion (ptz-il), synthesized it as a molten salt, and employed it as a redox mediator in preparing the modified electrode. then, ptz-il was immobilized on the multiwalled carbon nanotubes (mwcnts) deposited gce to fabricate the ptz il/mwcnt/gce modified electrode. figures 1a and 1b present the synthetic process of ptz-il formation and a schema of the electrode modification. in addition, researchers applied cyclic voltammetry (cv) to examine the electrochemical functions of the ptz-il/mwcnt/gce. it has been found that this new sensor has a redox couple with the cathodic and anodic peak potentials at 0.594 and 0.707 v, which have been attributed to the phenothiazine/phenothiazine radical cation redox couple. moreover, ptzil/mwcnt/gce exhibited very good electrocatalytic activities for sulfite oxidation, according to which an amperometric sensor has been presented to detect sulfite. furthermore, this nonenzymatic sensor displayed a wider linear range between 30.0 and 1177.0 μm with sensitivity 282.2 μa mm−1 cm−2 and a limit of detection (lod) equal to 9.3 μ m for detecting sulfite [132]. figure 1. (a) synthetic process of ptz-il (3) and (b) schema of the construction process of ptzil/mwcnt/gce [132] http://dx.doi.org/10.5599/jese.1555 j. electrochem. sci. eng. 12(6) (2022) 1061-1079 electrochemical detection of sulfite in food 1064 in this regard, zhu et al. [133] addressed the synthesis of lafeo3 using the sol-gel method and thus procured the lafeo3/graphene composite materials by ultra-sonic dispersion. then, they utilized the composite-modified electrode as the electrochemical sensor for detecting the sulfite content. according to the electrochemical test outputs, the lafeo3/graphene composite modified electrode had a suitable response signal for sulfite and the response current has been proportional to the sulfite concentration in ranges between 1.0 and 200.0 μm. moreover, the linear relationship equaled i = 0.119c +0.135, correlation coefficient (r2) equaled 0.994, and lod (s/n=3) equaled 0.21 μm. finally, this new technique has been substantially utilized in the white wine samples, and the recovery rate ranged between 97.63 and 103.02 % (n = 5) [133]. moreover, yang et al. [134] constructed a sulfite electrochemical sensor by nafion and molybdenum disulfide (mos2). according to differential pulse voltammetry (dpv) and cv, mos2 showed very good catalytic activities for the redox of so32-. upon the optimization of acidity of the scan rate, supporting electrolyte, and other variables for the electrochemical response of so32-, this new sensor exhibited a broad dynamic linear range between 5.0×10-3 and 0.5 mm (r2 =0.997, n=15) to detect so32with a lod equal to 3.3 µm that may be employed for so32content detection in water, which enjoys benefits like acceptable reproducibility, prolonged stability, reasonable recovery and anti-interference [134]. in addition, adeosun et al. [135] showed the electrochemical synthesis of the conductive polypyrrole chitosan (ppychi) thin film for sensitively detecting sulfite in real specimens. figure 2 is a schema of this synthetic process. as seen, this co-polymeric ppy-chi film has acceptable electrocatalytic behaviors for oxidizing sulfite. therefore, it has been employed to detect sulfite with dpv and a lod, linearity, and sensitivity equal to 0.21 µm (s/n = 3), 50.0 to 1100.0 µm and 15.28 µa µm cm-2, respectively [135]. figure 2. schema of the synthetic process for electro-polymerization of poly-pyrrolechitosan [135] furthermore, pandi et al. [136] electrodeposited the completely organized lutetium(iii) hexacyanoferrate micro-particles on the poly(taurine) modified gce; that is, (luhcf/poly(taurine)-gce). it has been found that this luhcf/poly(taurine)-gce enjoys very good electrochemical activity as well as higher sensitivity for sulfite sensing. they also examined the electrochemical oxidation of sulfite on the luhcf/poly(taurine)-gce using dpv and cv. results have shown that this modified electrode had higher sensitivity equal to 448.0 μa mm−1 cm−2 and 1.33 µm lod for electrochemical oxidation of sulfite with more acceptable functionality and selectivity [136]. moreover, yu et al. [137] study dealt with the synthesis of au nps-reduced graphene oxide composite (aunps-rgo) using the chemical coreduction method with ethylene glycol (eg) as the reducing agent. therefore, they modified the gce with an aunps-rgo composite through the dropping method. in addition, researchers employed cv and electrochemical impedance spectroscopy (eis) to investigate electrochemical behavior and electrocatalytic activities of the aunps-rgo/gce y. f. mustafa et al. j. electrochem. sci. eng. 12(6) (2022) 1061-1079 http://dx.doi.org/10.5599/jese.1555 1065 electrode for sulfite oxidation. with regard to the outputs, aunps-rgo/gce has higher electrochemical activities to oxidize sulfite through the synergistic effect of the rgo and aunps. based on the results in 0.10 m h2so4, the linear range for sulfite detection via amperometry at 0.40 v versus sce equaled 20.0 to 2300 µm (r2=0.995 6) with the lod equal to 45 nm (s/n = 3) [137]. in their study, devadas et al. [138] reported a shape-controlled preparation of praseodymium hexacyanoferrate (prhcf) with a simple electrochemical procedure. therefore, the electro-chemically fabricated prhcf-modified gce showed very good electrocatalytic activities for sulfite oxidation. electrochemical oxidation of sulfite on the prhcf-modified gce has been examined by the linear sweep voltammetry (lsv) and cv. results have shown this sulfite sensor sensitivity equal to 0.036 ma mm-1 cm-2 and lower lod as 2.15 mm. finally, researchers detected sulfite from the specimens obtained from the tap water and red wine to confirm the real-time utilization of the prhcf-modified gce [138]. wang and xu [139] presented one of the new methods for selectively extracting the free (ph 8.4) and total sulfite (ph 11.0) from the muscle foods and the following detection by a voltammetric sensor. their technique has been on the basis of electrocatalytic oxidation of sulfite at the modified gce constructed by immobilization of 9 μg of acetyl-ferrocene (afc) over the gce surface along with 35 μg of carbon black (cb) for improving the electron transfer into the poly(vinyl butyral) (pvb) membrane matrix. results indicated linearity of the external standard calibration curve in ranges between 0.03 and 4.0 mm with 15 μm lod. ultimately, this new technique could be utilized for determining free and total sulfite in the shrimp muscle-fortified specimens [139]. table 1. modified gces to detect sulfite ref. real samples linear ranges, μm lod, μm method gce modified with [132] vinegar and pickle 30.0 1177.0 9.3 amperometric ptz il/mwcnt [133] white wine 1.0 200.0 0.21 dpv lafeo3/graphene [134] water 5.000 0.500 3.3 dpv mos2¬/nf [135] orange juice, malt drink and human serum 50.0 1100.0 0.21 dpv ppy-chi [136] water 1.33 dpv luhcf/poly(taurine) [137] red wine 20 2300 450 amperometric aunps-rgo [138] red wine and water 600 8000 2.15 lsv prhcf [139] shrimp muscle 30 4000 15.0 cv afc/cb/pvb cpes for sulfite detection it is widely accepted that carbon paste, as one of the mixtures of graphite powder and an appropriate liquid binder, is the most flexible substrate for biological and chemical modifications. it is notable that the heterogeneous features of the cpes, specific effects of the liquid binder, variability in the employed carbon material, and diverse proportional compositions need extensive and reliable characterization procedures [140-145]. because of multiple beneficial features, cpes have widespread utilization in electrochemical measurements. [146-152]. table 2 tabulates information about modified cpes, reported by various works. in this regard, zabihpour et al. [153] dealt with the incorporation of the mgo/swcnts nanocomposite and 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide [bmim][tf2n] into the cp matrix for obtaining a modified cpe (mgo/swcnts-[bmim][tf2n]-cpe) as the electroanalytical instrument. according to the results, mgo/swcnts-[bmim][tf2n]-cpe created a better analytical oxidation signal for ferulic acid (fa) that was 3.17 times higher than the un-modified sensors. moreover, this electrode produced distinctive electrooxidation signals for fa with δep=300 mv in the http://dx.doi.org/10.5599/jese.1555 j. electrochem. sci. eng. 12(6) (2022) 1061-1079 electrochemical detection of sulfite in food 1066 presence of sulfite, verifying its utility for fa detection in the presence of sulfite ions that are usually found in the food samples such as red wine. additionally, mgo/swcnts-[bmim][tf2n]-cpe has been shown as one of the robust tools for fa and sulfite ions detection with the recovery data of 97.84 to 103.1 % in the concentration range between 0.009 and 450.0 μm as well as 0.1 and 450.0 μm with a lod equal to 3.0 nm and 50.0 nm [153]. in addition, norouzi and parsa created a modified electrode in the course of the electropolymerization of 4-aminobenzoic acid in the presence of sodium dodecyl sulfate (sds) [154]. therefore, they incorporated the ni(ii) ions into the polymer by immersing the modified electrode into a 0.1 m ni(ii) ions solution. then, researchers employed cv to investigate the electrochemical behaviors of the ni/poly(4 aminobenzoic acid)/sodium dodecyl sulfate/cpe (ni/poly(4ab)/sds/cpe). results have shown the acceptable activities of the polymer-modified electrode for sulfite electrooxidation in a phosphate buffer solution (pbs) at a ph of 11, and finally, lod equaled 0.063 mm. based on the optimum experimental condition, the peak current response linearly enhanced as the sulfite concentration elevated in ranges between 0.1 1.0 and 1.0 10.0 mm [154]. furthermore, miraki et al. [155] presented one of the electrochemical platforms based on the cpe modified with the nio nps and acetyl-ferrocene (af) (cpe/nio-nps/af). according to the findings, cpe/nio-nps/af had acceptable electrocatalytic activities to detect sulfite in the concentration range of 0.005 to 500.0 μm with a lod of 0.001 μm. it has been found that the electrocatalytic interactions of sulfite with af at the cpe/nio-nps/af surface may resolve the overlapping single of nitrite and sulfite for their simultaneous detection. finally, researchers found a higher performance of cpe/nionps/af to detect nitrite and sulfite in wastewater samples [155]. in another study by winiarski et al. [156] water-soluble form of 3-propyl(4-methyl-pyridinium) silsesquioxane chloride (si4pic+cl-) has been synthesized and characterized. therefore, researchers demonstrated its potency to be utilized as one of the stabilizing agents for aunp. in fact, cpes modified with the si4pic+cland aunp has been fabricated and employed in detecting and quantifying sulfite in the acidic medium. with regard to the voltammetric signal for sulfite, the electrode modified with the aunps (cpe/au-si4pic+cl-) exhibited an electrocatalytic impact of ca. -0.20 v in comparison with the bare electrode. according to the square wave voltammetry analysis (swva) and based on the optimum experimental condition, the reduction peak enhanced as the sulfite concentration elevated in ranges between 2.54 and 48.6 mg l-1. finally, loq and lod equaled 2.68 and 0.88 mg l-1 for sulfite [156]. another study conducted by sroysee et al. [157] described an amperometric sulfite bio-sensor, which contained a cpe (fe3o4@aucys-fa/cpe) modified with the immobilized sulfite oxidase (sox) on an au-coated magnetite np core encased within a conjugated folic acid (fa) cysteine (cys) shell. therefore, a poly-dimethylsiloxane (pdms), as well as a mineral oil mixture as the binder, was used to manufacture the bio-sensor electrode, which enhanced the sensitivity and physical stability of the electrode. figure 3 is an illustration of the preparation process of the sulfite bio-sensor. the fe3o4@au-cys-fa electrode exhibited acceptable electrocatalytic activities with reasonable retention of the chemisorbed sox on the electrode due to the respective larger surface areas. then, researchers employed amperometric measurements from the fe3o4@au cys-fa/cpe biosensor to quantify sulfite and used an in-house assembled flow cell at +0.35 v(versus ag/agcl) with the pb carrier of 0.10 m at a ph of 7.0 and the flow rate equal to 0.8 ml min-1. ultimately, this system detected sulfite between 0.1 and 200.0 mg l-1 with 10 mg l-1 lod (3 s of blank) [157]. y. f. mustafa et al. j. electrochem. sci. eng. 12(6) (2022) 1061-1079 http://dx.doi.org/10.5599/jese.1555 1067 figure 3. preparation process of the sulfite bio-sensors (a) fe3o4@au-cys-fa nano-composite with the immobilized sox; (b) fe3o4@au-cys-fa modified cpe [157] it is notable that silva et al. [158] also devised a square-wave voltammetric technique with regard to the sulfite electrochemical reduction to quantify this preservative in commercial beverages. therefore, they used cpe chemically modified with the mwcnts as the working electrode. it was found that at the optimal experimental condition, the linear response to the sulfite concentration ranges between 1.6.0 and 32.0 mg so2 l-1 (25.0 to 500.0 µm of sulfite) and lod equals 1.0 mg so2 l-1 (16 µm of sulfite) [158]. table 2. modified cpes to detect sulfite ref. real samples linear ranges lod method cpe modified with [153] red wine, white rice 0.1-450.0 μm 50.0 nm dpv mgo/swcnts[bmim][tf2n] [154] weak liquor 0.1 10.0 mm 0.063 mm amperometric ni/poly(4-ab)/sds [155] wastewater 0.005 500.0 μm 0.001 μm dpv nio-nps/af [156] white wine a, white wine b and coconut water 2.54 48.6 mg l-1 0.88 mg l-1 swv au-si4pic+cl [157] 0.1 – 200 mg l-1 10 mg l-1 amperometric fe3o4@au cys-fa [158] coconut water, fruit juices and white wine 25.0 500.0 µm 16 µm swv mwcnts spes for sulfite detection screen-printed technology involves the layer-by-layer deposition of ink on a solid substrate by a screen or mesh, which defines the sensor geometry. such a new technology enjoys design flexibility, acceptable reproducibility, process automation, and a selection of diverse substances. however, spes do not possess problems encountered with the classical solid electrodes, like laborious cleaning procedures and memory effects. finally, analyses have provided a new window due to the broad ranges of the form of the spes modification [159-181]. table 3 tabulates information about modified spes, which have been reported by various works. aflatoonian et al. [182] reported synthesizing the mows2 nano-composite and employed dpv, cv, and chrono-amperometry (cha) to examine the electrochemical behaviours of sulfite on the http://dx.doi.org/10.5599/jese.1555 j. electrochem. sci. eng. 12(6) (2022) 1061-1079 electrochemical detection of sulfite in food 1068 spes modified with mows2 nano-composite. according to the electrochemical specification, researchers observed acceptable electrocatalytic activity and surface area impact of the mows2 nano-composite. they also found a considerable enhancement of oxidation signals of sulfite on the mows2/spe compared to the bare spes. ultimately, with regard to the optimal condition, sulfite quantification ranged from 0.08-700.0 μm with a lower lod of 0.02 μm (s/n=3) [182]. furthermore, hemmati et al. [183] dealt with synthesizing the fe3o4 nps via a solvothermal approach using ethylene glycol (eg). then, the dropping procedure was used to modify the gse with the fe3o4 nps. researchers also employed dpv, cha, and cv for investigating electrocatalytic activities as well as the electrochemical behavior of fe3o4/spe electrodes for sulfite oxidation. according to the dpv analysis and based on the optimal experimental condition, the oxidation peak enhanced as the sulfite concentration elevates in ranges between 0.5 and 100.0 μm. finally, lod equaled 0.1 μm for sulfite [183]. moreover, maaref et al. [184] designed their electrochemical technique to detect sulfite using schiff-base modified graphite spes as a disposable chemo-sensor. therefore, they used differential pulse voltammograms of the modified spe in the presence of sulfite and observed a typical peak current at 350 mv. moreover, they found the linear sensor responses in the concentration ranges between 0.5 and 300.0 µm of the analyte. finally, lod equaled 0.3 μm for sulfite, and thus, this method could be used for sulfite detection in natural water samples [184]. in their study, molinero-abad et al. [185] modified the screen-printed carbon electrodes (spces) with tetrathiafulvalene (ttf) and sulfite oxidase enzyme in order to selectively and sensitively detect sulfite. therefore, they optimized the amperometric experimental condition regarding the significance of the sulfite quantification in the wine samples and the innate complicatedness of the samples, specifically red wine. researchers observed the biosensor response to sulfite with a cathodic current (at 200 mv versus the sp ag/agcl electrode and ph 6) in the broad concentration ranges with 6.0 µm lod at 60 °c. therefore, this method could be employed for sulfite detection in red and white samples, with the average recovery equal to 101.8 and 101.5 % [185]. consequently, molinero-abad et al. [186] reported an amperometric detection of sulfite with the use of the spces modified with au nps. it has been found that the electrode is relatively selective and shows responses to sulfite with the oxidation current at 300 mv and ph of 6 in the concentration ranges between 9.80 and 83.33 μm, with the recoveries in ranges between 96 and 104 %. hence, this method could be employed for sulfite detection in pickle juice, vinegar, and drinking [186]. table 3. modified spes to detect sulfite ref. real samples linear ranges, μm lod, μm method spe modified with [182] river and well water 0.08 700.0 0.02 dpv mows2 [183] water samples 0.5 100.0 0.10 dpv fe3o4 [184] well and river water 0.5 300.0 0.30 dpv schiff-base [185] white and red wine 9.9 82.6 6.00 amperometric tetra-thiafulvalene and sulfite oxidase enzyme [186] pickle juice, vinegar and drinking water 9.80 83.33 9.80 amperometric au nps other kinds of sensors for sulfite detection a composite has been prepared by do carmo et al. [187] from titanium (iv) silsesquioxane and phosphoric acid (ttip) and then occluded into hfau zeolite (zttip). in the next step, the material was modified chemically with nickel and potassium hexacyanoferrate(iii) (zttipnih). afterward, a y. f. mustafa et al. j. electrochem. sci. eng. 12(6) (2022) 1061-1079 http://dx.doi.org/10.5599/jese.1555 1069 modified gpe was used to obtain the voltammetric behavior of zttipnih, which indicated a completely organized redox couple with kcl (3 m) (20 wt.%; v = 20 mv s−1; kcl; 1.00 m) and a formal potential of eθ'=0.51 v (versus ag/agcl (sat.)) that corresponded to the niiifeii(cn)6/niiifeiii(cn)6 redox process. upon the precise voltammetric examinations, the gpe modified with zttipnih has been employed for easy and fast determination of sulfite. then, according to the analytical curve, a linear response has been achieved in the concentration range between 0.05 and 0.80 mm, the lod (3σ) equal to 0.05 mm, amperometric sensitivity equal to 14.42 ma m−1 and the relative standard deviation (rsd) equal to 4.21 % (n = 3) for sulfite [187]. in addition, preecharueangrit et al. [188] made their amperometric sulfite sensor on the basis of the nickel hexacyanoferrate (nihcf) electrodeposited on a layer of the ordered mesoporous carbon, which has been covered on an au electrode surface (nihcf/omc/au). this nihcf/omc/au had acceptable electrocatalytic activities for sulfite oxidation, and at the optimized condition, their sensor linearly responded in the concentration ranges between 2.5 μm to 50 mm and a 2.5 μm lod (with the s/n ratio equal to 3). finally, the electrode showed higher operational stability so that it could be applied up to 104 times (with an rsd equal to 5.9 %) and acceptable electrode-to-electrode repeatability (with an rsd of less than 7.0 %, for n = 6) [188]. furthermore, devaramani et al. [189] presented one of the simplified strategies for covalent anchoring of the cobalt hexacyanoferrate (cohcf) particles on the graphitic carbon substrate with the use of p-phenylenediamine as the linker molecule. therefore, they made a pellet electrode by the cohcf particles modified graphitic carbon and employed it in the electrocatalytic oxidation of sulfite. their sensor exhibited a linear range of concentration of 4.0 to 128.0 μm of sulfite with a limit of quantification (loq) of 5.8 μm and 1.7 μm lod [189]. moreover, adeloju and hussain [190] addressed the modification of a platinum electrode surface with platinum nps (ptnps). therefore, they entrapped the enzyme sulfite oxidase (sox) on the respective surface in an ultra-thin poly-pyrrole (ppy) film, and thus, those ptnps with 30 to 40 nm diameter have been deposited on the pt electrode (ptnps/ppy-sox/pt electrode) via cycling the electrode potential 20 times from -200 to 200 mv at the sweep rate equal to 50 mv s-1. additionally, researchers used cv, eis, potentiometry, and chrono-potentiometry to examine the electrochemical behaviors of the ptnps/ppy-sox film. based on the optimal condition, this biosensor showed 57.5 mv decade-1 sensitivity, a linear response between 0.75 and 65 μm of sulfite, 12.4 nm lod, and a response time between 3 and 5 s [190]. in another study, rawal and pundir [191] immobilized the sulfite oxidase (sox) (ec 1.8.3.1) purified from syzygium cumini leaves over the prussian blue nps/poly-pyrrole (pbnps/ppy) nanocomposite film that was electrodeposited on the au electrode surface. therefore, they built their electrochemical sulfite bio-sensor with the use of a sox/pbnps/ppy/au electrode as the working electrode, pt wire as the auxiliary electrode linked by a potentiostat as well as ag/agcl as the standard electrode. their bio-sensor has shown optimal response within two seconds while working at 20 mv s−1 in 0.1 m tris hcl buffer at a ph of 8.0 and a temperature of 30 °c. researchers showed the minimum lod and linear range to be 0.1 µm (s/n=3) and 0.5-1000.0 µm. this sensor has been assessed with 95.0 % recovery of the added sulfite in the samples of red wine and 1.9 and 3.3 % within and between the batch coefficients of variations [191]. table 4 tabulates information about other kinds of sensors, which have been reported by various works. http://dx.doi.org/10.5599/jese.1555 j. electrochem. sci. eng. 12(6) (2022) 1061-1079 electrochemical detection of sulfite in food 1070 table 4. other kinds of sensors to detect sulfite ref. real samples linear ranges, μm lod, μm method sensor [187] 50 800 50 cv zttipnih/gpe [188] instant noodles, noodles and macaroni 2.5 5000 25 amperometric nihcf/omc/au electrode [189] tomato ketchup, wine, jam, sugar and dry grapes 4.0 128.0 1.7 amperometric cohcf particles /graphitic carbon electrode [190] wine and beer 0.75 65.5 0.0124 potential time ptnps/ppy-sox/pt electrode [191] red, rose and white wine 0.5 1000.0 0.1 eis sox/pbnps/ppy/au electrode conclusions researchers have provided diverse electrochemical sensors to detect sulfite in different specimens due to the necessity of the application of a rapid procedure for determining the content of sulfite in food samples. moreover, modifying the electrochemical sensors referred to the enhanced current responses for sulfite electrooxidation, anti-fouling impact, and higher selectivity, stability, and sensitivity. in general, we overviewed 3 types of electrochemical sensors, including the modified gces, the modified cpes, and the modified spes. since gces were proposed to be the most applied substrates for modifications, they have larger diameters, whereas the spes and cpes are miniaturized instruments with very few samples. in fact, cpes are inexpensive and can be easily prepared. in addition, they exhibited rapid regeneration of the active surface areas with the ability to eliminate the fouling effects. while preparing the cpes, an oily binder would be employed to create the paste, and this mineral oil has been considered one of the insulating materials and a reason for lower current ranges while utilizing spe or gce. in this way, the problem has been resolved and the use of ionic liquids as the binder materials for cpe formation has shown that ils have higher viscosity and conductivity. therefore, the kind of real specimens employed should influence choosing the electrochemical sensor to detect sulfite. a number of utilizations require lower lod; others demand wider concentration ranges, and a number of them should use strongly stable and selective methods, whereas others should entail concentration ranges greater than micro-molar sulfite levels. we hope to provide a bridge to connect the gap between food and environmental science and nanotechnology. thus with collaborative endeavors of scientists/engineers in both fields, more novel nanomaterialsbased sensors will be designed, and new solutions will spring up. references [1] t. 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by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.5599/jese.1555 https://doi.org/10.1016/j.jelechem.2017.11.070 https://doi.org/10.1016/j.electacta.2017.02.085 https://doi.org/10.1007/s00604-016-1748-0 https://doi.org/10.1016/j.bej.2012.11.013 https://creativecommons.org/licenses/by/4.0/) @article{mustafa2022, author = {mustafa, yasser fakri and chehardoli, gholamabbas and habibzadeh, sepideh and arzehgar, zeinab}, journal = {journal of electrochemical science and engineering}, title = {{electrochemical detection of sulfite in food samples:}}, year = {2022}, issn = {1847-9286}, month = {nov}, number = {6}, pages = {1061--1079}, volume = {12}, abstract = {in various pharmaceutical and food industries, sulfite is utilized for the inhibition of nonenzymatic and enzymatic browning. also, in brewing industries, it acts as an antioxidizing and antibacterial agent. several toxic and adverse reactions, including vitamin deficiency, hypersensitivity, and allergic diseases, have been attributed to sulfite ingestion that may cause dysbiotic oral and gut microbiota events. thus, the content of sulfite in foods must be controlled and monitored, and it is essential to find a specific, reproducible, and sensitive method to detect sulfite. some analytical solutions are being tested to quantify sulfite. however, due to their advantage over traditional techniques, electroanalytical techniques are attracting much attention because they are simple, fast, affordable, and sensitive to implement. in addition, by the electrode modification, the morphology and size can be controlled, resulting in the miniaturization to be used in portable electrochemical devices. therefore, the present review addressed some articles on the electrooxidation of sulfite from real samples using various electrochemical sensors.}, doi = {10.5599/jese.1555}, file = {:d\:/onedrive/mendeley desktop/mustafa et al. 2022 electrochemical detection of sulfite in food samples(2).pdf:pdf;:02_jese_1555_1061-1079.docx:word_new;:www/jese_v12_no6_1061-1079.pdf:pdf}, keywords = {sulfite, amperometry, electrooxidation, modified electrode, voltammetry}, url = {https://pub.iapchem.org/ojs/index.php/jese/article/view/1555}, } determination of ascorbic acid at solid electrodes modified with l-cysteine http://dx.doi.org/10.5599/jese.1365 287 j. electrochem. sci. eng. 13(2) (2023) 287-296; http://dx.doi.org/10.5599/jese.1365 open access : : issn 1847-9286 www.jese-online.org original scientific paper determination of ascorbic acid at solid electrodes modified with l-cysteine nur i̇zi1, tuğçe göver2 and zafer yazıcıgil1, 1department of chemistry, faculty of science, selcuk university,42130, konya, turkey 2department of analytical chemistry, faculty of pharmacy, selcuk university, 42250 konya, turkey corresponding author: zyazicigil@selcuk.edu.tr received: april 28, 2022; accepted: january 27, 2023; published: february 19, 2023 abstract gold and glassy carbon electrode surfaces were modified with l-cysteine, and the electrochemical behavior of ascorbic acid (aa) was investigated on these new surfaces. to improve the efficiency of electrodes, the electrode surfaces were modified and optimum conditions for aa determination were established. electrochemical experiments were performed at different potential ranges, the concentration of aa, scan rates, number of polymerization cycles and ph values. using cyclic voltammetry (cv) technique, optimum conditions were determined as the potential scanning range of 0.2 to 1.5 v vs. ag/agcl in 0.1 m phosphate buffer solution (ph 7.02) for the l-cysteine/au electrode, and -1.95 to 1.9 v vs. ag/agcl in 0.1 m phosphate buffer solution (ph 2.7) for the l-cysteine/gc electrode. for the characterization of both modified electrode surfaces, a series of physicochemical techniques was also applied. the usability and selectivity of these two proposed modified electrodes for the determination of aa were investigated using square wave voltammetry (swv) in the presence of possible interferents, i.e., glycine, l-glutamic acid and uric acid. keywords modified electrodes; electropolymerization; poly(l-cysteine); voltametric sensor; vitamin c introduction ascorbic acid, an important water-soluble vitamin, has many names, such as antiscorbutic vitamin, l-ascorbic acid, and vitamin c [1,2]. ascorbic acid is necessary for the production of collagen, an important protein for the structure of muscles, bones, blood vessels and cartilage in the body [3]. the concentration of ascorbic acid in foods and pharmaceuticals is extremely important for determining the quality in the production and storage steps [4]. different techniques are usable for aa determination, such as chromatography [5], potentiometric titration [6], voltammetry [7], conductometry [8], titrimetry [9], amperometry [10], fluorometry [11], flow-injection analysis (fia) [12] and chemiluminescence [2]. the various voltammetric techniques, however, showed some advantages such as a very large linear concentration range for both inorganic and http://dx.doi.org/10.5599/jese.1365 http://dx.doi.org/10.5599/jese.1365 http://www.jese-online.org/ mailto:zyazicigil@selcuk.edu.tr j. electrochem. sci. eng. 13(2) (2023) 287-296 determination of ascorbic acid 288 organic species, high sensitivity, ability to work with a large number of solvents and electrolytes, ability to work in a wide temperature range, rapid analysis times, determination of several analytes at the same time, and ability to determine kinetic and mechanistic parameters [13]. at standard solid electrodes, the determination of ascorbic acid is not very reliable due to low sensitivity and poor reproducibility. therefore, it has become important to modify the electrode surface with different materials, such as gold nanoparticles [14], ruthenium oxide [15], chitosan [16] and poly(l-cysteine) film [3]. amino acids, the most basic substances of the organism, contain amino and carboxyl functional groups. because of its versatility and ease of preparation, poly(l-cysteine) is widely used to prepare voltammetric sensors [17]. the main objective of this work was to manufacture modified glassy carbon (gc) and gold (au) electrodes for the determination of ascorbic acid using poly(l-cysteine) film. both electrochemical performance and surface characterization of au and gc electrodes modified with poly(l-cysteine) are performed by voltammetry and sem and tem techniques. experimental chemicals and reagents in this study, the chemicals obtained from the related companies were used directly without a purification process. sodium dihydrogen phosphate (nah2po4), sodium dihydrogen phosphate dihydrate (nah2po4·2h2o), and sodium acetate trihydrate (c2h3nao2·3h2o) were supplied from vwr. sodium chloride (nacl), sulfuric acid (h2so4), ortho-phosphoric acid (85 %), boric acid, citric acid monohydrate, and ferrocene (c10h10fe) were supplied from merck. tetrabutylammonium tetrafluoroborate (tbatfb) (99 %) was supplied from sigma-aldrich. acetonitrile (ch3cn) (≥ 99.9 %) was supplied from isolab. glycine (99 %), uric acid (99 %), l-glutamic acid (99 %), potassium hexacyanoferrate (ii) trihydrate (k4[fe(cn)6]3h2o), and potassium hexacyanoferrate (iii) (k3[fe(cn)]6) were supplied from alfa aesar. sodium hydrogen phosphate (na2hpo4) and l (+) ascorbic acid were supplied from (john townsend baker, gliwice, poland). potassium chloride (kcl) and acetic acid (ch3cooh) were supplied from riedel-de haén. l-cysteine was supplied from acros organics and stored at 4 °c. aluminum oxide (al2o3) was supplied from nanografi company. ultrapure water and freshly prepared solutions were used throughout the experiment. all solutions were stored in the refrigerator. the solution of l-cysteine used for surface coating was prepared in the phosphate buffer solution. instrumentation and other equipment gamry reference 600 and series g 750 potentiostat/galvonastat/zra devices were used in the electrochemical examination of experiments. bioanalytical system (bas) c3 cell system was used, which includes the working (bas model mf-2012 gc and bas model mf-2013 au), reference ag/agcl/(sat. kcl)) used in aqueous media, or ag/ag+ (in 10 mm agno3), used in non-aqueous media) and counter (platinum wire) electrodes. scanning electron microscopy and transmission electron microscopy analyses were carried out at selcuk university advanced technology research and application center. during the study, special care was taken that the calibration of all used devices was done at certain time periods. bandelin rk 100 model ultrasonic bath was used for cleaning electrode surfaces. jenway 3010 model ph meter was used at room temperature for ph adjustments of prepared solutions. argon gas was passed through the solutions for at least 3 minutes prior to experiments. n. i̇zi et al. j. electrochem. sci. eng. 13(2) (2023) 287-296 http://dx.doi.org/10.5599/jese.1365 289 preparation of modified working electrode firstly, bare electrode surfaces were electrochemically cleaned by cyclic voltammetry at -0.2 / +1.5 v (10 cycles) in 0.1 mol dm-3 h2so4 solution. after that, electrodes were washed with pure water and cleaned in a circular direction with the suspension of alumina polishing powder of 0.3 and 0.05 μm dimensions on the velvet surface. at the end of the alumina powder process, electrodes were sonicated in pure water and acetonitrile for three minutes, respectively [18,19]. modified solid contact electrodes with poly(l-cysteine) were prepared by potentiodynamic polymerization. electropolymerization of l-cysteine on the gce surface was performed by 25 potential cycles at the scan rate of 150 mv/s between -1.95 and 1.9 v in ph 2.7 phosphate buffer containing 30 mmol dm3 l-cysteine (figure 1). electropolymerization of l-cysteine on the au electrode surface was performed by 25 potential cycles at 150 mv/s between 0.2 and 1.5 v in ph 7.02 phosphate buffer solution containing 1.0 mmol dm-3 l-cysteine (figure 2). prepared electrodes were named lcysteine/au and l-cysteine/gc, respectively. figure 1. electropolymerization of 30 mmol dm-3 l-cysteine on the gce surface with cyclic voltammetry between -1.95 and 1.9 v at 25 cycles in ph 2.7 phosphate buffer solution (scan rate: 150 mv/s) figure 2. electropolymerization of 1 mmol dm-3 l-cysteine on the au electrode with cyclic voltammetry between 0.2 and 1.5 v at 25 cycles in ph 7.02 phosphate buffer solution (scan rate: 150 mv/s) surface characterization of prepared electrodes l-cysteine/au, l-cysteine/gc and clean (bare) electrode surfaces were characterized by cv in the presence of redox probes. investigation of surface properties of electrodes was carried out with the cv technique in the presence of 1.0 mmol dm-3 of ferrocene as redox-active species in ch3cn solution containing 0.1 mol dm-3 of tbatfb. surface images and analysis of l-cysteine/au, lcysteine/gc and clean (bare) electrodes were investigated with sem and tem at selçuk university advanced technology research and application center. results and discussion cyclic voltammetry figures 1 and 2 show that the peaks in the first cycle decreased in the subsequent cycles of lcysteine polymerization. as a reason, it was thought that the film layer of poly(l-cysteine) formed on the electrode surfaces does not allow electron exchange. electrochemical probes were used in the characterization process of modified electrode surfaces [20,21]. voltammograms of bare and modified electrodes were taken using the cyclic voltammetry http://dx.doi.org/10.5599/jese.1365 j. electrochem. sci. eng. 13(2) (2023) 287-296 determination of ascorbic acid 290 technique in 1.0 mmol dm-3 ferrocene solution as a redox probe in ch3cn containing 0.1 mol dm-3 of tbatfb. the recorded voltammograms can be compared in figures 3 and 4. figure 3. comparison of voltammograms of (a) bare and (b) l-cysteine/gc electrodes in 1.0 mmol dm-3 ferrocene solution (in ch3cn containing 0.1 mol dm-3 of tbatfb) figure 4. comparison of voltammograms of (a) bare and (b) l-cysteine/au electrodes in 1.0 mmol dm-3 ferrocene solution (in ch3cn containing 0.1 mol dm-3 of tbatfb) it is seen in figures 3 and 4 that almost reversible oxidation and reduction peaks of the ferrocene/ferrocenium (fc/fc+) redox couple, characteristic for bare gc and au electrodes, cannot be observed for both l-cysteine/gc and l-cysteine/au electrode surfaces. these suggest that modified electrode surfaces do not allow electron transfer, while bare gc and au electrode surfaces allow electron transfer. it seems that rather different surfaces were obtained after 25 cycles of polymerization of l-cysteine, which resulted in poly(l-cysteine) molecules attached to the surface of the electrodes [22,23]. scanning electron microscopy the surface images of electrode surfaces were taken with the scanning electron microscopy technique in order to see the surface morphology of electrodes and observe physical changes on the electrode surfaces [24] (figure 5). figure 5. scanning electron microscopy images of (a) l-cysteine/gc and (b) l-cysteine/au electrodes when the sem images of the electrode surfaces were compared, it was seen that the surface images were different, but both images proved that electrode surfaces are modified with l-cysteine. n. i̇zi et al. j. electrochem. sci. eng. 13(2) (2023) 287-296 http://dx.doi.org/10.5599/jese.1365 291 transmission electron microscopy the morphology of electrode surfaces modified with l-cysteine was also investigated by tem studies. as seen in figure 6, l-cysteine is dispersed over the surfaces of both electrodes [25,26]. figure 6. transmission electron microscopy images of (a) l-cysteine/gc and (b) l-cysteine/au electrodes electrochemical behavior of ascorbic acid scheme 1 shows that the l-cysteine is attached to the electrode surface via sulfur atoms. it is also known from the literature that the sulfur atom is very well attached to the voids on the gold surface [27]. modification of the au electrode by l-cysteine and the predicted binding pattern of ascorbic acid (aa) to the modified electrode surface are shown in scheme 1. scheme 1. modification of au electrode surface by l-cysteine and the estimated bonding pattern of aa to the modified electrode surface scheme 2 shows that l-cysteine is attached to the surface of the gce with sulfur and nitrogen atom. since the sulfur atom did not provide good adhesion to the c atoms of the electrode, the nitrogen atom also affects the attachment. the modification of the gce surface by l-cysteine and the estimated binding pattern of ascorbic acid (aa) to the modified electrode surface are shown in scheme 2. scheme 2. modification of gce surface by l-cysteine and the estimated bonding pattern of aa to the modified electrode surface http://dx.doi.org/10.5599/jese.1365 j. electrochem. sci. eng. 13(2) (2023) 287-296 determination of ascorbic acid 292 figure 7 and figure 8 show cyclic voltammograms of 3.0 mmol dm-3 ascorbic acid (aa) prepared in phosphate buffer solution using modified gc and au electrodes in comparison with bare electrodes. figure 7. cyclic voltammograms of 3.0 mmol dm-3 aa on bare gc and l-cysteine/gc electrode surfaces in phosphate buffer medium (ph 5.0) figure 8. cyclic voltammograms of 3.0 mmol dm-3 aa on bare au and l-cysteine/au electrode surfaces in phosphate buffer medium (ph 7.02) from voltammograms presented in figures 7 and 8, much higher peak current values for ascorbic acid can be observed on the modified electrodes compared to bare electrodes. this suggests that, in this way, the amount of ascorbic acid can be determined using modified electrodes. in addition, the peak potential of ascorbic acid is shifted to a lower value, indicating the electrocatalytic effect of l-cysteine [28]. in a set of experiments (not shown here), a scan rate study was performed to determine whether the reaction of ascorbic acid on l-cysteine-modified electrode surfaces is diffusion-controlled or adsorption-controlled. for this purpose, cyclic voltammograms of 3.0 mmol dm-3 ascorbic acid at different scanning rates between 5 and 500 mv/s were taken. the slope value of the log ip vs. log v graphs gives information on either diffusion or adsorption control of the redox processes. it indicates that the reaction on the modified surface is not diffusion-controlled if the slope values are different from 0.5 [22,29]. in this study, it was observed that the reaction on the modified electrode surfaces was not diffusion-controlled since the slope values were different from 0.5. the fact that the reaction of aa at both modified electrode surfaces is not diffusion-controlled approves mechanisms drawn in schemes 1 and 2. the analytical performance of modified electrodes has also been studied. the effect of ascorbic acid concentration on electrode surfaces modified with l-cysteine using square wave voltammetry (swv) is shown in figures 9 and 10. both sw voltammograms show that the peak currents of ascorbic acid increased with the increase of the ascorbic acid concentration, meaning that both modified electrodes could serve for the evaluation of aa. interference study for determination of electrode selectivity, the modified electrode surfaces under optimum conditions were tested in solutions containing ascorbic acid (aa), glycine (gly), l-glutamic acid (ga) and uric acid (ua). for a selectable and objective comparison, all solutions were prepared at the concentration of 3.0 mmol dm-3 and tested by the cyclic voltammetry technique in the scanning range of -1.0 to 0.8 v at the scanning speed of 100 mv/s (figures 11 and 12). the most important feature of the prepared high-performance sensors is their selectivity. at the bare gc electrode (figure 11a), ascorbic acid (aa), glycine (gly), l-glutamic acid (ga) and uric acid (ua) showed oxidation peaks. since oxidation peak potentials are close to each other, it is difficult to distinguish them from each other. n. i̇zi et al. j. electrochem. sci. eng. 13(2) (2023) 287-296 http://dx.doi.org/10.5599/jese.1365 293 figure 9. effect of concentration of ascorbic acid using square wave voltammetry recorded on l-cysteine/gc electrode figure 10. effect of concentration of ascorbic acid using square wave voltammetry recorded on l-cysteine/au electrode figure 11. cyclic voltammograms recorded at: (a) bare gc electrode, (b) l-cysteine/gc electrode in 3.0 mmol dm-3 ascorbic acid (aa), 3.0 mmol dm-3 glycine (gly), 3.0 mmol dm-3 l-glutamic acid (ga) and 3.0 mmol dm-3 uric acid (ua) solutions figure 12. cyclic voltammograms recorded at: (a) bare au electrode, (b) l-cysteine/au electrode in 3.0 mmol dm-3 ascorbic acid (aa), 3.0 mmol dm-3 glycine (gly), 3.0 mmol dm-3 l-glutamic acid (ga) and 3.0 mmol dm-3 uric acid (ua) solutions in the case of the gc electrode modified with the l-cysteine (figure 11b), the potentials of ascorbic acid and uric acid oxidation were different from those monitored at the bare gc electrode. http://dx.doi.org/10.5599/jese.1365 j. electrochem. sci. eng. 13(2) (2023) 287-296 determination of ascorbic acid 294 also, an increase in the peak current of ascorbic acid was observed when compared to the bare gc electrode. at the bare au electrode (figure 12a), ascorbic acid, glycine, l-glutamic acid and uric acid showed oxidation peaks. in the case of the au electrode modified with the l-cysteine (figure 12b), the potentials of ascorbic acid and glycine oxidation were different from those monitored at the bare au electrode. also, an increase in the peak current of ascorbic acid was observed when compared to the bare gold electrode. the modified electrodes could also inhibit redox currents of glycine and l-glutamic acid, significantly reducing the current responses. due to the difference in oxidation peak potentials for ascorbic acid and uric acid in the modified electrodes, these electrodes can be expected to separate the oxidation peak potentials in the same solution with coexisting ascorbic acid and uric acid. conclusions in this study, glassy carbon and gold electrodes modified with electropolymerized l-cysteine were used for the determination of ascorbic acid. as the concentration of ascorbic acid in foods and drugs is extremely important for the determination of quality in the production and storage stages, it is important to develop a method for the determination of ascorbic acid. electrode modification was performed in an extremely simple and fast method. the proposed sensors (l-cysteine/gc and l-cysteine/au) provided sensitivity, selectivity, short measurement time, ease of preparation and good analytical performance. morphological changes on the electrode surfaces and the existence of poly (l-cysteine) were proven by sem and tem analyses. it was determined by the swv 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https://dergipark.org.tr/tr/download/article-file/498653 [29] d. he, p. zhang, s. li, h. luo, a novel free-standing cvd graphene platform electrode modified with aupt hybrid nanoparticles and l-cysteine for the selective determination of epinephrine, journal of electroanalytical chemistry 823 (2018) 678-687. https://doi.org/10.1016/j.jelechem.2018.07.022 ©2023 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://www.researchgate.net/publication/332211482 https://doi.org/10.1002/sia.6607 https://doi.org/10.29109/gujsc.623660 https://doi.org/10.1016/j.electacta.2009.07.050 https://doi.org/10.1038/s41598-019-42108-x https://doi.org/10.1016/j.snb.2007.06.028 https://tez.yok.gov.tr/ulusaltezmerkezi/tezdetay.jsp?id=g7li0hy0ijhopwy3icfbgq&no=xl1ixq0dt_z5uog4jbjpiw https://tez.yok.gov.tr/ulusaltezmerkezi/tezdetay.jsp?id=g7li0hy0ijhopwy3icfbgq&no=xl1ixq0dt_z5uog4jbjpiw https://doi.org/10.1016/j.apsusc.2019.05.167 https://dergipark.org.tr/tr/download/article-file/498653 https://doi.org/10.1016/j.jelechem.2018.07.022 https://creativecommons.org/licenses/by/4.0/) conduction mechanism in passive films on austenitic stainless steels in sulphate solutions doi:10.5599/jese.153 93 j. electrochem. sci. eng. 5(2) (2015)93-114; doi: 10.5599/jese.153 open access : : issn 1847-9286 www.jese-online.org original scientific paper conduction mechanism in passive films on austenitic stainless steels in sulphate solutions iva betova department of chemistry, technical university of sofia, kliment ohridski boulevard 8, 1000 sofia, bulgaria corresponding author: iva_betova@tu-sofia.bg; tel.: +359 889 295523 received: december 7, 2014; revised: march 15, 2015; published: august 26, 2015 abstract the passive state of three commercial highly alloyed austenitic stainless steels is studied by voltammetric, contact electric resistance and impedance measurements in 0.5 m sulphate solutions (ph 2-7). the three materials self-passivate and are susceptible to transpassive dissolution. the electric properties of the passive films formed are only slightly dependent on alloy composition. the impedance response can be interpreted as due to both the electronic properties of a thin semiconductor film of variable stoichiometry and the ionic defect migration through that film limiting the metal dissolution rate in the passive state. a range of kinetic, transport and structural parameters characterising the passive film and its interfaces with the underlying alloys and the electrolyte solution are determined by a quantitative comparison of the mixedconduction model to the experimental steady-state current and impedance data in a wide potential range. the relevance of the parameter values and the prospects of using the proposed approach to predict the steady-state metal dissolution rate and thus the general corrosion behaviour of stainless steels are discussed. keywords stainless steel, metal dissolution, passive film, electrochemical impedance spectroscopy, kinetic model introduction the high resistance of highly alloyed stainless steels to both general and localised corrosion in a variety of electrolytic media is determined by the protective ability of thin passive films formed on their surfaces [1-5]. a quantitative correlation between the composition and structure of such films, as a function of the composition of the alloy substrate, their electrical and electrochemical http://www.jese-online.org/ mailto:iva_betova@tu-sofia.bg j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 94 properties and the extent of their interaction with the corrosive medium is yet to be established [6,7]. such a correlation would make it possible to determine the extent of service life of a particular engineering material in a range of environments. a comprehensive kinetic model of the passive film has to include electrochemical reactions of defect generation and consumption at the film interfaces with the underlying alloy and the electrolyte solution, as well as a description of the defect transport resulting in film growth and dissolution of alloy constituents through the oxide. so far, a substantial part of these requirements have been met by the point defect model (pdm) developed extensively by macdonald and coworkers [8-11] during the past couple of decades. a further development of the pdm has been carried out [12-15] to quantify the relationship between ionic point defect structure and electronic properties of the passive film. a view of that film in terms of the mixed-conduction model (mcm), is considered a likely approach that will be able to ascertain a quantitative structure-property relationship and thus would allow a deterministic prediction of corrosion rates in a wide variety of conditions. according to the pdm and the mcm, both lattice-destructive (film growth and dissolution) and lattice-preserving (dissolution of the substrate metal through the film) processes are treated as sequences of generation, transport and consumption of point defects (vacancies and interstitials). in turn, these defects play the role of electron donors or acceptors, ensuring the electronic conductivity of the film. with the assumption of high-field migration as the dominating mode of ion transport, usual electrochemical kinetics of the interfacial reactions and band-to-band tunnelling for the electron conduction, the steady-state current, conductivity vs. potential and small amplitude alternating current response of passive films forming on model ferrous alloys in acidic and slightly alkaline environments have been predicted [12-15]. in the present paper, the conduction in the passive films on three highly-alloyed austenitic stainless steels aisi 904l (uns n08904), 254 smo (uns s31254) and 654 smo (uns s32654) in 0.5 m na2so4 solutions (ph 2, 4 and 7) is investigated by voltammetry, contact electric resistance (cer) and electrochemical impedance techniques. quantitative compositional data for passive films on comparable austenitic steels in similar environments indicate that the film is based on hydrated chromium-iron-molybdenum oxide [16-18]. thus a modified version of the mixedconduction model featuring iron dissolution through the film mediated by interstitial cations and transpassive dissolution of chromium mediated by cation vacancies is proposed to account for the experimental data. a range of kinetic and transport parameters are estimated from a simultaneous fit of the steady-state current and impedance vs. potential data to the model equations. on the basis of the model predictions, the relative importance of iron and chromium dissolution as depending on ph, electrode potential and alloy composition is assessed. experimental electrodes and electrolytes three types of commercial stainless steels, corresponding to aisi 904l (uns n08904, outokumpu polarit oy, finland), 254smo (uns s31254, avesta sheffield ab, sweden) and 654smo (uns s32654, avesta sheffield ab, sweden) have been investigated. their composition is given in table 1. for the voltammetric and electrochemical impedance spectroscopic measurements, discs of the studied materials were pressed with a screw against rubber o-rings and embedded in ptfe holders to expose an area of 0.2 cm 2 to the electrolyte. for the contact electric resistance (cer) measurements, electrode tips with a diameter of 2 mm (contact area ca. 0.015 cm 2 ) made of the investigated materials and insulated by a multi-layered ptfe tape were used. the pre-treatment of i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 95 the working electrodes consisted of polishing on 4000 grade emery paper, rinsing with milli-q  purified water and drying with hot air. furthermore, the working electrodes were polarized in the hydrogen evolution region to establish reproducible initial electrochemical conditions. a threeelectrode cell featuring a pt wire counter electrode and a saturated kcl|agcl|ag reference electrode (e = 0.20 v vs. she) has been employed. all the potentials in the paper are given in the standard hydrogen scale. the measurements were performed at room temperature (25 o c) in 0.5 m sulphate solutions (ph 2, 4 and 7) prepared from p.a. h2so4, na2so4 and milli-q  purified water and subsequently deaerated with n2 (99.999%). apparatus and procedures a wenking lb 81m potentiostat and a hitek ppr1 sweep generator were used for potential control in the cer measurements, the cer equipment being supplied by cormet ltd. a comprehensive description of the cer technique is given e.g. in ref. 19. voltammetric measurements have been carried out with an autolab pgstat 20 controlled by gpes software (eco chemie, the netherlands). to construct steady state current vs. potential curves, the current vs. time curve was recorded at each potential until a steady state current was reached (criterion: current variation with time  2 %). impedance spectra were measured at this steady state with the autolab pgstat 20 equipped with a fra2 module and controlled by fra software (eco chemie, the netherlands). a frequency range of 0.01 30,000 hz at an ac amplitude of 20 mv (rms) have been found to be appropriate. the reproducibility of the impedance spectra was ±1.5% by magnitude and ±2.5° by phase angle. the validation of the impedance spectra was performed by checking the linearity condition, i.e. measuring spectra at ac signal amplitudes between 2 and 20 mv (rms), and the causality by a kramers-kronig test embedded in the fra 4.9  software. the fitting of impedance spectra to selected transfer functions was based on microcal origin 6.0 software. table 1. composition of the stainless steels used in the present investigation alloy cr mo ni fe cu c mn s n amount, wt % 654 smo 24 7.3 22 balance 0.43 0.01 2.0 0.001 0.5 254 smo 20 6.1 18 balance 0.73 0.02 0.68 0.001 0.2 aisi 904l 20 4.5 25 balance 1.50 0.01 1.43 0.001 0.06 results current potential and contact resistance potential curves the current vs. potential curves of the studied materials measured using a linear sweep of 1 m vs -1 in the potential range from hydrogen evolution to transpassive dissolution are shown in fig. 1a (0.5 m so4 2, ph 2 solution), fig. 2a (0.5 m so4 2, ph 4 solution) and fig. 3a (0.5 m so4 2, ph 7 solution). the corresponding contact resistance vs. potential curves measured by the cer technique using the same sweep rate are presented in figs 1b-3b, respectively. the current vs. potential curves exhibit typical features of self-passivating materials (fig.1a-3a). namely, a passivation range (-0.2...0.3 v at ph 2, -0.4...0.2 v at ph 4 and -0.8...0.0 v at ph 7) is followed by a passive region in which the current is quasi-independent on potential (0.4...1.0 v at ph 2, 0...1.0 v at ph 4 and ph 7). the passive-to-transpassive transition is marked as a steep j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 96 increase in the current density at around 1.0 v. the current densities in the passivation range are the lowest for 654smo, i.e. this alloy passivates the most efficiently, in contrast to the transpassive range, in which the currents for 654smo are the highest [20,21]. the obtained results are logical in view of the fact that 654smo contains the highest amount of cr and mo among the investigated materials (table 1) which enhance both the ability of self-passivation and the stability of the passive film. the ph of the solution influences the zero-current potential on the alloys, namely it is situated at ca.0.2 v at ph 2 for all three materials, whereas at ph 4 and 7 the zerocurrent potential is considerably for aisi 904l than for the other two materials. this could be due to different mechanisms of the oxygen reduction reaction on the studied alloys, however, a more detailed study of this mechanism is beyond the scope of the paper. figure 1. current vs. potential curves (a) and resistance vs. potential curves (b) for the studied materials in 0.5 m na2so4, ph 2. sweep rate 1 mv s -1 figure 2. current vs. potential curves (a) and resistance vs. potential curves (b) for the studied materials in 0.5 m na2so4, ph 4. sweep rate 1 mv s -1 the contact resistance vs. potential curves are broadly analogous to those measured previously for ferritic steels in a na2so4 solution of ph 5 [12], as well as recent measurements on a wide range of austenitic stainless steels in a na2so4 solution of ph 4.8 reported by kim et al. [22]. that is, a low and constant resistance characteristic of a thin conductive film is observed around the open circuit potential (figs. 1b-3b). it is worth mentioning that the resistance stays low and constant also in the passivation region, which can be due to the very small thickness of the surface film allowing for direct tunnelling to take place through it. in the passive region, a quasi-0.4 0.0 0.4 0.8 1.2 1.6 10 -3 10 -2 10 -1 10 0 10 1 a 0.5 m so 4 2, ph 2 i / m a c m -2 e / v vs. she aisi 904l 254 smo 654 smo -0.4 0.0 0.4 0.8 1.2 1.6 10 -4 10 -3 10 -2 10 -1 10 0 10 1 b 0.5 m so4 2, ph 2 e / v vs. she r /  c m 2 aisi 904l 254 smo 654 smo -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 10 -4 10 -3 10 -2 10 -1 10 0 10 1 0.5 m so4 2-, ph 4 i / m a c m -2 e / v vs. she aisi 904l 254 smo 654 smo a -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 b 0.5 m so4 2-, ph 4 r /  c m 2 e / v vs. she aisi 904l 254 smo 654 smo i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 97 exponential increase of the resistance by 4-6 orders of magnitude is observed followed by a resistance maximum slightly below the passive-to-transpassive transition potential (figs 1b-3b). figure 3. current vs. potential curves (a) and resistance vs. potential curves (b) for the studied materials in 0.5 m na2so4, ph 7. sweep rate 1 mv s -1 . this feature is indicative of film growth and depletion of the film of current carriers [12, 14]. such a depletion can be due to an overall decrease of the degree of non-stoichiometry of the oxide by its dehydration or the solid-state oxidation of divalent fe in the film to trivalent [12-14, 23], i.e. to a redox reaction in the film accompanying film growth. it is worth mentioning that the maximum value of the resistance in the passive region obtained in the present work is ca. an order of magnitude lower than that measured previously for ferritic steels [12]. this may be related to the presence of ni(ii) in the film [16,18,23] increasing the overall degree of non-stoichiometry. in this connection, it has been demonstrated that the maximum resistance of passive films on ni-cr alloys is considerably lower than the corresponding resistance of the films on fe-cr alloys in sulphate solutions [25]. the resistance vs. potential curves in the ph 4 and ph 7 solutions are very close to each other regardless of the material (figs. 2b, 3b) and seem to demonstrate that the electrical properties of the passive films on all the materials are nearly identical in the present experimental conditions. as a conclusion, it can be stated that the voltammetric and contact resistance measurements allows to define a passive region in which the steady-state ionic conductivity and the electronic properties of the surface film are only slightly affected by potential. for potentials below that region, the degree of the non-stoichiometry of the film is increased by the generation of lowvalence or high-valence ionic defects, and thus both the ionic and electronic conduction through the oxide are enhanced. electrochemical impedance spectroscopy to adequately measure impedance spectra, the system under study needs to be in a steadystate. in the present work, impedance spectra were measured at a range of potentials after a steady-state current density was reached. examples of current vs. time curves obtained for 254 smo steel in the ph 2 solution are shown in figure 4. these curves are typical for passive film growth and are characterized by a quasi-linear decrease in log-log coordinates, the slope of the log-log plot being of the order of -0.8 in the passive region. steady-state current densities are reached typically after 1 h of polarization. -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 10 -3 10 -2 10 -1 10 0 10 1 a 0.5 m so 4 2, ph 7 i / m a c m -2 e / v vs. she aisi 904l 254 smo 654 smo -0.8 -0.4 0.0 0.4 0.8 1.2 1.6 10 -5 10 -4 10 -3 10 -2 10 -1 10 0 10 1 10 2 0.5 m so4 2, ph 7 r /  c m 2 e / v vs. she aisi 904l 254 smo 654 smo b j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 98 figure 4. current vs. time curves for 254 smo in a ph 2 solution for potentials between 0.5 and 1.0 v. impedance spectra in bode coordinates for 254 smo in 0.5 m so4 2, ph 2 solution at a number of potentials ranging from the passivation region to the beginning of the transpassive region are shown in fig. 5. the impedance response of this material in the transpassive region itself has been thoroughly discussed in previous papers [20,21] and is out of the scope of the present investigation. figure 5. impedance spectra of 254 smo in 0.5 m na2so4, ph 2 as a function of potential. left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. the spectra are qualitatively analogous to those obtained earlier for ferritic steels in sulphate solutions [12,14,25-27]. namely, in the potential region corresponding to the high and almost stable resistance (figs 1b-3b), the impedance spectra are quasi-independent of potential, showing a predominantly capacitive response, and the impedance magnitude at low frequencies is very high. the phase angle vs. frequency curves are asymmetrical with respect to the maximum phase 1 10 100 1000 0.01 0.1 1 10 100 1.0 0.9 0.8 0.7 0.5 i /  a c m -2  / s 254smo / ph 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 10 6 frequency / hz |z | / o h m c m 2 254 smo / 0.5 m na2so4, ph 2 1.2 v 1.1 v 1 v 0.8 v 0.4 v 0 v -0.1 v -0.2 v f / hz  / 0 f / hz |z | /  c m 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 254 smo / 0.5 m na2so4, ph 2 1.2 v 1.1 v 1 v 0.8 v 0.4 v 0 v -0.1 v -0.2 v i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 99 angle, and the phase angle decreases smoothly with decreasing frequency (fig.5). this shows that the spectra cannot be adequately interpreted using a single time constant involving a constant phase element. at potentials higher and lower than the high-resistance potential region, a decrease of the slope of the magnitude of the impedance vs. frequency dependence is observed (fig. 5). simultaneously, the low frequency impedance magnitude decreases and the frequency range in which the spectra have predominantly capacitive character becomes progressively narrower. in other words, the blocking character of the films is diminished and its quasi-steady state conductivity increases. in order to assess more comprehensively the differences in the behaviour of the studied steels, their impedance spectra are compared in at two characteristic potentials: -0.2 v (in the passivation region)and 0.8 v (at the upper end of the passive region) the spectra at these potentials in the ph 2 solution are shown in figures 6-7, those measured in the ph 4 solution in figures 8-9, and the spectra recorded in the ph 7 solution in figs 10-11, respectively. figure 6. comparison of the impedance spectra of the studied steels in 0.5 m na2so4, ph 2 at 0.2 v (in the passivation region). left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. in the passivation region (at -0.2 v), the values of the impedance magnitude at low frequencies (e.g. 0.01 hz) are the smallest for 254 smo, the difference between that steel and the two remaining materials being more pronounced at higher ph (figs 6, 8 and 10, left columns). in addition, the frequency range in which the phase angle has high and constant values is the narrowest on 254 smo when compared to the other two steels (figs 6, 8 and 10, right columns). these observations could be explained by the fact that 254 smo has the highest amount of fe (table 1) which probably dissolves through the film forming in the passive region at the highest rate, which is tantamount of a highest ionic conductivity of the film on this material. on the other hand, the higher amounts of mo and ni in 654smo and aisi 904l, respectively, could explain the fact that the films formed on these materials in the passivation region are less conductive. indeed, the differences between the impedances at 0.01 hz for 654smo and aisi 904l at -0.2 v are small regardless of the solution ph (figs 6, 8 and 10). 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 0.5 m na2so4, ph 2, -0.2 v aisi 904l 254 smo 654 smo 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 frequency / hz |z | / o h m c m 2 0.5 m na2so4, ph 2,-0.2 v aisi 904l 254 smo 654 smo |z | /  c m 2 f / hz f / hz  / 0 j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 100 figure 7. comparison of the impedance spectra of the studied steels in 0.5 m na2so4, ph 2 at 0.8 v (at the upper end of the passive region). left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. figure 8. comparison of the impedance spectra of the studied steels in 0.5 m na2so4, ph 4 at 0.2 v (in the passivation region). left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. in the passive region (at 0.8 v), the values of the impedance magnitude at low frequencies (e.g. 0.01 hz) are the smallest for 654 smo and the greatest for aisi 904l, although the differences are not very significant (figs 7, 9 and 11, left columns). further, the impedance magnitude at 0.01 hz for 254 smo and especially for aisi 904l increases slightly with ph, whereas no such effect is noticeable for 654 smo. regarding the phase angle curves, it can be stated that in the passive region, the range of frequencies of predominantly capacitive response is the largest for aisi 904l and the smallest for 654 smo (figs 7, 9 and 11, right columns). moreover, for 254 smo and aisi 904l, the frequency width of the quasi-capacitive impedance increases with ph (figs 7, 9 and 11, right columns). these observations correlate to a certain extent with the ranking of the impedance magnitude in terms of the blocking character of the passive films.       / 0 f / hz f / hz |z | /  c m 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 0.5 m na2so4, ph 2, 0.8 v aisi 904l 254 smo 654 smo 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 10 6 frequency / hz |z | / o h m c m 2 0.5 m na2so4, ph 2, 0.8 v aisi 904l 254 smo 654 smo 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 0.5 m na2so4, ph 4, -0.2 v aisi 904l 254 smo 654 smo       / 0 f / hz f / hz |z | /  c m 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 frequency / hz |z | / o h m c m 2 0.5 m na2so4, ph 4, -0.2 v aisi 904l 254 smo 654 smo i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 101 figure 9. comparison of the impedance spectra of the studied steels in 0.5 m na2so4, ph 4 at 0.8 v (at the upper end of the passive region). left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. figure 10. comparison of the impedance spectra of the studied steels in 0.5 m na2so4, ph 7 at -0.2 v (in the passivation region). left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. summarising, the impedance spectroscopic results indicate that the small amplitude alternating current response of the studied austenitic steels is qualitatively similar to that of ferritic steels as reported previously [12,14]. that is, a quasi-capacitive response only slightly affected by potential is found in the passive region, whereas for potentials more negative than this region, enhanced conductivity of the passive films is observed and accordingly their capacitive properties become less perfect. these observations are in accordance to the voltammetric and cer results and point to the occurrence of solid-state redox reactions in the oxide in the active-to-passive transition region. the choice of a proper transfer function to describe the alternating current response based on the mcm and the determination of kinetic, transport and structural parameters for the studied materials is described below.       / 0 f / hz f / hz |z | /  c m 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 0.5 m na2so4, ph 4, 0.8 v aisi 904l 254 smo 654 smo 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 10 6 frequency / hz |z | / o h m c m 2 0.5 m na2so4, ph 4, 0.8 v aisi 904l 254 smo 654 smo       / 0 f / hz f / hz |z | /  c m 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 frequency / hz |z | / o h m c m 2 0.5 m na2so4, ph 7, -0.2 v aisi 904l 254 smo 654 smo 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 0.5 m na2so4, ph 7, -0.2 v aisi 904l 254 smo 654 smo j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 102 figure 11. comparison of the impedance spectra of the studied steels in 0.5 m na2so4, ph 7 at 0.8 v (at the upper end of the passive region). left impedance magnitude vs. frequency, right phase angle vs. frequency. points experimental values, solid lines best-fit calculation. discussion kinetic model both in-situ and ex-situ surface analytical results [4,16-18,23,28,29] have clearly demonstrated that the thickness of the passive films on compatible highly alloyed austenitic steels are in the range of 2 7 nm and increase linearly with potential in na2so4 solutions (ph 2 to 7). the passive films have been shown to be based on chromium(iii) oxide-hydroxide with a certain amount of cr(vi), fe(ii), fe(iii), ni(ii) and some mo-containing species (probably a mixture of mo(iv) and mo(vi)) also present [16-18]. the amount of fe in the film increases with potential in na2so4 solutions (ph 2 to 7), which demonstrates that the extent of iron dissolution through the film decreases. this could be correlated to the fact that the higher the potential, the lower the amount of fe(ii) in the passive film. in addition, it has been shown that the higher the content of mo in the steel, the lower the iron content in the passive film at constant potential [17]. in order to simplify the modelling procedure, it is assumed that the passive film is a single phase oxide based on cr2o3 in which fe, ni and mo are treated as substitution defects. the reaction of passive film growth and dissolution is mediated by the generation and consumption of oxygen vacancies     •• m cr o •• •• o m/f o f/s •• + o f/s 2 o + 3+ 2 3 aq 2 cr cr +1.5v +3e´ v v 1.5v +1.5h o 1.5o +3h 0.5cr o +3h cr +1.5h o (1) this reaction does play a comparatively minor role in determining the steady-state and small amplitude alternating current response. this assumption is supported by electrochemical quartz microbalance measurements on comparable steels of olsson and landolt [30,31]. it has been shown by these authors that during a potential step, the fraction of cations formed at the metal|oxide interface that dissolve at the film|solution interface is between 0.66 and 0.84, i.e. the       / 0 |z | /  c m 2 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 -90 -75 -60 -45 -30 -15 0 frequency / hz p h a s e / d e g 0.5 m na2so4, ph 7, 0.8 v aisi 904l 254 smo 654 smo 10 -2 10 -1 10 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 10 4 10 5 10 6 frequency / hz |z | / o h m c m 2 0.5 m na2so4, ph 7, 0.8 v aisi 904l 254 smo 654 smo f / hz f / hz i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 103 main processes are related to the dissolution of cations through the oxide and not to film formation. in other words, at the steady state, both the current density and the impedance response are assumed to be mainly determined by the reactions of metal dissolution through the film ("corrosion") mediated by the generation, transport and consumption of point defects in the cation sub-lattice (interstitial cations and cation vacancies). the dissolution of fe through the passive film is expressed as the combination of the generation of interstitial fe cations of formal valence  (between 2 and 3) at the metal|film (m|f) interface, their transport through the film and their oxidative dissolution at the film|solution (f|s) interface    1i i 3i χ• m i m|f ,χ• χ• i m|f i f|s χ• y+ i f|s aq fe fe +χe´ fe fe fe fe +(y-χ)e´ k d e k (2) the fact that passive films on stainless steels contain iron in an oxidation state lower than iii has been demonstrated by xps [16-18, 29]. on the other hand, the dissolution of cr through the film to generate soluble cr(vi) is mediated by the generation of cr cation vacancies at the f|s interface via oxidative dissolution of cr(iii), their transport through the film and their consumption at the m|f interface    3 v 1 iii 6+ ´´´ cr aq cr f|s , e´´´ ´´´ cr f|s cr m|f ´´´ iii m cr m|f cr cr cr +v +3e´ v v cr +v cr +3e´ k d k (3) it can be in principle postulated that the dissolution of mo through the film follows the same route as that of cr, although at low potentials in the passivation region, the dissolution of mo via the interstitial route probably cannot be neglected. at difference to the film formation-dissolution sequence (1), the sequences of reactions (2) and (3) leading to cation dissolution through the film contain charge transfer steps at both the alloy|film and film|solution interfaces. thus it is expected that a discrimination between the contributions of the film growth-dissolution, on one hand, and the metal dissolution through the film, on the other, can be achieved by analysing the dependence of the steady-state current and the impedance response on the applied potential. the rate constants are thought to obey an exponential dependence on the potential drops at the appropriate interfaces: kj = kj 0 exp (bj n), where j=1, 1i, 3, 3i, n = m|f, f|s, kj 0 is a standard rate constant and bj = j f/rt is a tafel coefficient. strictly speaking, the potential needs to be expressed vs. the equilibrium potential of the cr2o3 / cr redox couple at each ph. thus in what follows, the values of the standard rate constants will be expressed vs. this equilibrium potential. the transport of point defects is proposed to obey a high-field conduction equation of the type (written for interstitial cations as an example)                i i i i i 2 ,( , ) , 0.5 ( , ) exp x fa x tc x t d j x t d c x t a rt e (4) where  i ,j x t is the instantaneous flux of interstitial cations, i( , )c x t is their instantaneous concentration, id their diffusion coefficient, a is the half-jump distance for hopping transport and  ,x te is the instantaneous field strength. j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 104 the applied potential is assumed to be distributed as potential drops at the interfaces ( |m f and  |f s ) and in the bulk film according to the pdm and mcm [8-14]    | |m f f se le (5) as demonstrated for compatible stainless steels in comparable media [16,18,28,30] the steadystate film thickness l depends linearly on potential e (i.e. the field strength in the passive film is independent on potential and distance)      0 1 e e l l e (6) in the above equation,  is the part of the potential consumed as drop at the f|s interface and e is the field strength. it follows from (5) and (6) that the steady-state potential drop at the m|f interface is independent of the applied potential. in such a case, only the dependences of the reaction rate constants at the film|solution interface on potential need to be considered [12,14,24]. the expression for the steady-current density vs. potential dependence can be obtained in terms of the pdm and mcm [8-14] and also related approaches [32]. according to the above assumptions, the steady-state current density ( i ) is the sum of the partial current densities due to the reaction sequences (2) and (3) i i v v( ) 3 ( )i fd c l fd c l    (7) where              1 i 1i 3i i ( ) exp l a c l k k a d (8) 3 1 ( )v k c l k  (9) and     3 i i v v 1 1 , 2 2 fa fa rt rtd d e d d e e e (10) are the apparent (field-dependent) diffusion coefficients of interstitial cations and cation vacancies, and vd the vacancy diffusion coefficient. inserting (8) and (9) in (7) the following expression for the steady-state current density is obtained 3 3 00 0 3i 1i v 10 0 3i 1 ´ 3 ´ 2i l b e b ea i kfd k fd i e e fk e a k a k       (11) it is worth mentioning that the steady-state current density is a function of both the apparent diffusion coefficients and the rate constants of the interfacial rreactions, i.e. the processes in the metal|film|solution system are both reaction and transport controlled. the impedance response of the film can be given as the sum of the impedances due to electronic properties of the film (ze) and ionic transport through it (rion) in parallel:   1 -1 -1 f e ionz z r    (12) i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 105 the validity of this equation for a film exhibiting both ionic and electronic conduction, with the principal ionic current carriers playing the role of electron donors or acceptors, could in principle be questioned because of the anticipated coupling between the ionic and electronic fluxes. however, during impedance measurements in electrolytes that do not contain redox couples, as in the present investigation, the electronic current carriers are blocked at the film|solution interface and in fact contribute to the capacitive charging current of the depletion layer in the semiconducting oxide, whereas the faradaic current is due to the transport of ionic point defects alone [15]. this justifies the use of equation (12) in the present experimental conditions. the transfer function (equation (12)) can account for the small amplitude alternating current response of the passive alloys provided that the interfacial impedances are negligible with respect to the impedance of the film. considering the ability of the ionic point defects to act as electron donors or acceptors [8-15] gives the possibility to express the impedance due to the electronic properties of the film in the form [12, 14, 33] 1/ d 0 e d 0 1 j e ln 1 j p p z j c         (13) where 1 lp a   (14) 3 3 2 0 0 -1 e 3 1i d 0 0 1 3i ib e b e f d k k e e rt k k            (15) c is the capacitance of the space charge layer, ed is the diffusion coefficient of electronic current carriers, is the dielectric constant of the oxide and the dielectric permittivity of free space. this type of impedance can be regarded as the impedance of a semiconductor film in which the defect densities are a function of both the potential and the distance within the film. it is worth noticing that a frequency dispersion of the impedance components consistent with such an expression has been already reported for similar steels in comparable media [34]. as no warburg impedance has been identified in the experimental spectra (figs 2-10), the impedance of ionic transport is proposed to reduce to a migration resistance rion [12,14,32]:                   3 3 02 0 22 1 0 31i v 1i0 0 3i 1 ( ) ´ 9 ´( ) 1 1 ´ 1i l b e b ei a ion kf d k f df r e e k e rta k rt rta k (17) once again, it is stressed that the ionic transport resistance depends not only on the diffusion coefficients and field strength in the oxide, but also on the rate constants of the interfacial reactions. summarising, we intend to describe both the steady state and the impedance response of the studied passive alloys in terms of four interfacial reactions (k1, k1i, k3, k3i), the rates of two of which (at the film|solution interface) are dependent on the applied potential (exponential coefficients b3 and b3i multiplied by the part of the potential consumed as a drop at the film|solution interface ), three diffusion coefficients (di, dv and de for interstitial cations, cation vacancies and electronic carriers), the half-jump distance for high-field transport, a, the field strength in the film, e, and its dielectric permittivity, . j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 106 comparison with experimental data as a first step in such a comparison, the extensive set of impedance data for the three materials in the three electrolyte solutions were fitted using the transfer function defined by equations (12), (13) and (17). in other words, every individual impedance spectrum is described by four parameters: c, p, d and rion in addition to the resistance of the electrolyte. the calculated spectra are shown in figs 5-11 with solid lines and demonstrate the ability of such a transfer function to account for the experimental data. the values of the parameters c, p, d and rion together with the steady-state current density are plotted in figs 12-14 as a function of potential. figure 12. dependence of the steady state current density and the parameters of the transfer function on potential and ph for 254 smo in 0.5 m na2so4: (a) film capacitance, c; (b) relative depth of penetration of the conductivity profile, p; (c) product of the defect induced interfacial resistivity and the dielectric constant of the film, (d) -1 ; (d) resistance of migration of ionic point defects, rion; (e) steady-state current density, iss. (f) steady-state thickness, l. the solid lines in (b)-(e) are predicted by the corresponding equations of the mcm. -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 3.0 3.5 4.0 4.5 5.0 254 smo / 0.5 m so 4 2l / n m e / v vs. she ph 2 ph 4 ph 7 f -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 10 -2 10 -1 10 0 10 1 254 smo / 0.5 m so 4 2i s s /  a c m -2 e / v vs. she ph 2 ph 4 ph 7 e -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.1 1 10 100 1000 254 smo / 0.5 m so 4 2r i o n 1 / ( m  c m 2 ) -1 e / v vs. she ph 2 ph 4 ph 7 d -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1e-4 1e-3 0.01 0.1 1 10 100 254 smo / 0.5 m so 4 2( d  ) -1 /  s c m -1 e / v vs. she ph 2 ph 4 ph 7 c -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 14 15 16 17 18 19 20 21 22 254 smo / 0.5 m so 4 2p 1 e / v vs. she ph 2 ph 4 ph 7 b -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.000 0.004 0.008 0.012 254 smo / 0.5 m so 4 2c -2 / (  f c m -2 )2 e / v vs. she ph 2 ph 4 ph 7 a i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 107 the capacitance of the passive film plotted in mott-schottky coordinates (c -2 vs. applied potential) in general decreases with potential in the passive region (figs 12a, 13a, 14a), demonstrating an apparent n-type semiconductor behaviour in accordance to previous capacitance and photo-electrochemical measurements on compatible steels [35-38]. such a behaviour of the capacitance is consistent with interstitial cations being the dominant ionic current carriers acting as electron donors [9]. the c -2 vs. e curves are not linear, however, indicating that the donor density is dependent on potential and/or distance within the film. it is worth noting that the capacitance values are the smaller, the higher the ph of the solution. this can be associated with a decrease of the dielectric constant of the films with increasing ph and correlated to the variation of the oxide composition and/or thickness with ph. figure 13. dependence of the steady state current density and the parameters of the transfer function used to fit the impedance data on potential and ph for aisi 904l in 0.5 m na2so4: (a) film capacitance, c; (b) relative depth of penetration of the conductivity profile, p; (c) product of the defect induced interfacial resistivity and the dielectric constant of the film, (d) -1 ; (d) resistance of migration of ionic point defects, rion; (e) steady-state current density, iss (f) steady-state thickness, l. the solid lines in (b)-(e) are predicted by the corresponding equations of the mcm. -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8 aisi 904l / 0.5 m so 4 2l / n m e / v vs. she ph 2 ph 4 ph 7 f -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.01 0.1 1 10 aisi 904l / 0.5 m so 4 2i s s /  a c m -2 e / v vs. she ph 2 ph 4 ph 7 e -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.1 1 10 100 aisi 904l / 0.5 m so4 2r io n -1 /  s c m -2 e / v vs. she ph 2 ph 4 ph 7 d -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1e-4 1e-3 0.01 0.1 1 10 100 aisi 904l / 0.5 m so 4 2( d  )1 /  s c m -1 e / v vs. she ph 2 ph 4 ph 7 c -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 13 14 15 16 17 18 19 20 21 22 23 aisi 904l / 0.5 m so 4 2p -1 e / v vs. she ph 2 ph 4 ph 7 b -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 0.000 0.002 0.004 0.006 0.008 0.010 aisi 904l / 0.5 m so 4 2c -2 / (  f c m -2 )2 e / v vs. she ph 2 ph 4 ph 7 a j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 108 figure 14. dependence of the steady state current density and the parameters of the transfer function used to fit the impedance data on potential and ph for 654 smo in 0.5 m na2so4: (a) film capacitance, c; (b) relative depth of penetration of the conductivity profile, p; (c) product of the defect induced interfacial resistivity and the dielectric constant of the film, (d) -1 ; (d) resistance of migration of ionic point defects, rion; (e) steady-state current density, iss (f) steady-state thickness, l. the solid lines in (b)-(e) are predicted by the corresponding equations of the mcm. as a next step in the comparison of the model to the experimental data, the p -1 , (d) -1 and rion -1 vs. potential dependences together with the dependence of the steady-state current density on potential for a particular material-electrolyte combination have been simultaneously fitted to the model equations (11), (14), (15) and (17). statistical weighting was used for the experimental data set and the errors of parameter estimation were multiplied by the square root of the reduced chisquare value resulting from the fit. the fitting was performed so that kinetic parameters that -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 3.0 3.5 4.0 4.5 5.0 5.5 654 smo / 0.5 m so 4 2l / n m e / v vs. she ph 2 ph 4 ph 7 f -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 10 -1 10 0 10 1 654 smo / 0.5 m so 4 2i s s /  a c m -2 e / v vs. she ph 2 ph 4 ph 7 e -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1 10 100 r i o n 1 /  s c m -2 654 smo / 0.5 m so4 2e / v vs. she ph 2 ph 4 ph 7 d -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1e-3 0.01 0.1 1 10 100 (  d  ) -1 /  s c m -1 654 smo / 0.5 m so 4 2e / v vs. she ph 2 ph 4 ph 7 c -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 14 16 18 20 22 24 654 smo / 0.5 m so 4 2p -1 e / v vs. she ph 2 ph 7 ph 4 b -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0.000 0.002 0.004 0.006 0.008 654 smo / 0.5 m so 4 2c -2 / (  f c m -2 )2 e / v vs. she ph 2 ph 4 ph 7 a i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 109 showed a mutual dependency higher than 0.7 have never been set free simultaneously. the goodness of fit as judged by the r 2 and chi-square values was deemed acceptable, taking into account the large difference in magnitude of both the fitted functions and the kinetic parameters. of course, the large amount of kinetic parameters does not in principle exclude the possibility of the fitting procedure to converge to a local minimum. however, taking into account the statistical validity of the fitting results, such a possibility is considered remote. the best-fit results are shown in figs 12(b-e) 14(b-e) with solid lines and demonstrate the ability of the model to reproduce both the general shape of the dependencies of the parameters on potential and the absolute values of the parameters. this can be taken as a proof for the ability of the model to predict both the steady-state and small amplitude alternating current response of the studied materials in a wide potential and ph range. the estimates of the kinetic parameters for the three steels in the three electrolyte solutions are collected in table 2 (254 smo), table 3 (aisi 904l) and table 4 (654 smo), respectively. as mentioned above, the values of the standard rate constants of the reactions at the film|solution interface are given vs. the equilibrium potential of the cr2o3/cr couple taken from the respective e-ph diagram (0.058 v at ph 2, -0.300 v at ph 4 and -0.83 v at ph 7, respectively). the overall set of parameters can be classified as homogeneous, especially concerning the exponential coefficients of the kinetic steps at the film|solution interface b3 and b3i and the transport parameters (diffusion coefficients and half-jump distances). this is perhaps not surprising taking into account the relatively small differences in the impedance spectra of the studied alloys, but demonstrates once again the stability of the calculation procedure. table 2. kinetic parameters for the passive film on 254 smo formed in 0.5 m na2so4 solutions of ph 2, 4 and 7. parameter ph 2 ph 4 ph 7 k1 0 / mol cm -2 s -1 2.5 × 10 -9 2.1 × 10 -9 7.0 × 10 -9 k1i 0 / mol cm -2 s -1 2.0× 10 -13 1.5× 10 -13 0.5× 10 -13 k3 0 / mol cm -2 s -1 3.3× 10 -19 1.1× 10 -21 3.7 × 10 -26 k3i 0 / mol cm -2 s -1 6.0× 10 -9 3.3× 10 -10 7.7× 10 -11 b3 / v -1 17.7 15.5 15.3 b3i / v -1 6.5 6 5 de / cm 2 s -1 2.5× 10 -9 3.0× 10 -9 5.0× 10 -9 di / cm 2 s -1 1.0× 10 -14 1.4× 10 -14 0.9× 10 -14 dv / cm 2 s -1 0.67× 10 -17 3.0× 10 -17 6.0× 10 -17 a / cm 2.0× 10 -8 2.2× 10 -8 2.2× 10 -8  50 30 15  0.78 0.78 0.78 e / v cm -1 3.0× 10 6 2.5× 10 6 3.0× 10 6 the following conclusions can be drawn from a comparison of the parameter values for the studied steels in the corresponding electrolyte solutions: the rate constants of the reactions at the alloy|film interface in general decrease with increasing ph, the effect being altogether not very significant (within an order of magnitude). it can be stated that the rate constant of consumption of cation vacancies is far higher than that of generation of interstitial cations, the difference being the highest for 654 smo. the values are in fair agreement with those obtained earlier for ferritic steels in a borate solution [15]. as the fe j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 110 content in the inner layers of the passive film is much smaller than the cr content, and bearing in mind that there is probably ni enrichment at the alloy|film interface beneath the passive film [16,18], such a difference can be understood by regarding the alloy|film interface as a contact between a metallic phase enriched in ni and a film based mainly on cr2o3 (which is in principle expected to exchange mainly cation vacancies with the underlying metal). as the rate of transpassive dissolution of cr in the most part of the passive range is very low, and the concentration of divalent fe is rather small [23], it follows from equation (6) that the steady-state current density, or the metal dissolution rate, is controlled by the k1i step. the rate constant of this step is the smallest for aisi904l (tables 2-4), which correlates well with the smallest passive current densities and steady-state ionic conductivities (rion -1 ) for this alloy (figs 14(d-e)-16(d-e)). it is tempting to relate this feature to the fact that aisi 904l has the highest ni content of all the studied materials (table 1). the somewhat surprising fact that the highest values of k1i (and accordingly, the highest passive current densities and steady-state ionic conductivities) are measured for 654 smo can be tentatively related to the highest amount of mo in this alloy when compared to the other two materials. in that connection, it has been reported that the dissolution rates of mo-containing ferritic and austenitic stainless steels in the passive state are higher than those for the steels without mo [30, 40].the values of the rate constant of oxidative dissolution of interstitial cations at the film|solution interface, k3i 0 , is many orders of magnitude higher than that of the oxidative dissolution of cr (k3 step) which is also in accordance to the suggestion that the main path for dissolution in the passive state is the interstitial path. on the other hand, the exponential coefficient of this step, b3i is ca. 3 times smaller than that of b3, i.e. at sufficiently high potentials the transpassive dissolution is expected to start dominating, as observed experimentally. the values of the standard rate constants of both reactions decrease significantly with increasing ph, indicating a decrease of the rate of ejection of cations at that interface and hence the dissolution rate of fe through the passive film. the rate constant k3i 0 is the smallest for aisi 904l and the largest for 654 smo (tables 2-4), whereas the values of k3 0 are the highest for 654 smo, i.e. correlate with the highest concentration of cr and mo in the alloy which should render it the most susceptible to transpasssive dissolution. table 3. kinetic parameters for the passive film on aisi 904l formed in 0.5 m na2so4 solutions of ph 2, 4 and 7. parameter ph 2 ph 4 ph 7 k1 0 / mol cm -2 s -1 1.0× 10 -8 4.0× 10 -9 3.9× 10 -9 k1i 0 / mol cm -2 s -1 0.8× 10 -13 0.5× 10 -13 0.3× 10 -13 k3 0 / mol cm -2 s -1 1.5× 10 -19 3.1× 10 -22 3.4× 10 -25 k3i 0 / mol cm -2 s -1 0.86× 10 -9 6.1× 10 -11 1.5× 10 -11 b3 / v -1 18.7 16.5 16.1 b3i / v -1 9 5.5 4.0 de / cm 2 s -1 1.0× 10 -9 1.0× 10 -9 1.0× 10 -9 di / cm 2 s -1 0.9× 10 -14 2.0× 10 -14 1.9× 10 -14 dv / cm 2 s -1 0.49× 10 -17 2.3× 10 -17 28× 10 -17 a / cm 2.0× 10 -8 2.0× 10 -8 2.3× 10 -8  45 25 20  0.75 0.75 0.75 e / v cm -1 2.5× 10 6 2.0× 10 6 2.0× 10 6 i. betova j. electrochem. sci. eng. 5(2) (2015) 93-114 doi:10.5599/jese.153 111 table 4. kinetic parameters for the passive film on 654 smo formed in 0.5 m na2so4 solutions of ph 2, 4 and 7. parameter ph 2 ph 4 ph 7 k1 0 / mol cm -2 s -1 32× 10 -9 8.0× 10 -9 7.9× 10 -9 k1i 0 / mol cm -2 s -1 3.5× 10 -13 3.5× 10 -13 1.9× 10 -13 k3 0 / mol cm -2 s -1 31× 10 -19 3.6× 10 -21 1.97×10 -25 k3i 0 / mol cm -2 s -1 4.9× 10 -9 0.81× 10 -9 0.48× 10 -9 b3 / v -1 18 18 17.5 b3i / v -1 7.4 6 3.6 de / cm 2 s -1 3.0× 10 -9 3.5× 10 -9 4.5× 10 -9 di / cm 2 s -1 0.38× 10 -14 0.57× 10 -14 0.40× 10 -14 dv / cm 2 s -1 0.36× 10 -17 0.26× 10 -17 0.90× 10 -17 a / cm 2.2× 10 -8 2.0× 10 -8 2.5× 10 -8  30 25 15  0.75 0.78 0.78 e / v cm -1 2.5× 10 6 2.5× 10 6 2.0× 10 6  the estimated values for diffusion coefficients of point defects are in broad agreement with those quoted by several authors for passive films [14,25,32,41-44]. it is worth noting that the diffusion coefficient of interstitial cations is much larger than that of cation vacancies, which once more points to the interstitial transport as mainly responsible for the metal dissolution through the film. the values of both diffusion coefficients are the highest for the film formed on 254smo and the lowest for and the film formed on 654smo. bearing in mind that the concentration of fe in the oxide is a function of its content in the underlying alloy [16,45], such a ranking can be tentatively correlated to the fact that the fe content in 654smo is considerably lower than that of 254 smo. the diffusion coefficient of cation vacancies seems to increase with ph (tables 2-4) whereas that of interstitial cations is quasi-independent of ph. in order to explain this trend, more detailed data on the structure of the films formed in solutions with different ph are obviously needed.  the values of the electric field strength are close to those estimated by olsson and landolt for films formed on comparable stainless steels [31] and also to our previous estimates for films formed on ferritic steels and nickel-based alloys [14,15,25]. the values of the polarizability of the film|solution interface are rather high, indicating that the major part of the potential drop is actually consumed at the film|solution interface. this calculation result is in accordance with the conclusion of olsson and landolt [31] that a better description of very thin passive films is obtained by assuming interfacial control of the processes instead of bulk control.  the values of the half-jump distance are of the order of those typically assumed for hopping motion processes. the values of the dielectric constant of the films are within the range usually assumed for passive films [34-37] and in general decrease with increasing ph, which is in accordance with the trend in the capacitance values calculated from the impedance spectra (figs 11a-13a). such a trend can be tentatively explained by a change in the composition and thickness of the passive film with increasing ph, as demonstrated by xps analysis [18].  the values given in tables 2-4 for the diffusion coefficient of electronic current carriers de are rather low, albeit considerably higher than those of the ionic point defects and also significantly higher than previous estimates for the passive films on ferritic steels [14,15,24]. in that context, the values of de reported in tables 2-4 are probably not a direct measure for the j. electrochem. sci. eng. 5(2) (2015) 93-114 passive films on austenitic stainless steels 112 mobility of electronic current carriers in the film as a whole. they most probably represent the minimum value of the mobility in the depletion layer of the semiconductor oxide, which is the primary barrier for the transport of electronic current carriers. an alternative explanation of the low values of the diffusion coefficient of electronic current carriers is that the mobile ionic defects in passive films can act as electron traps, immobilising and later releasing electrons. in that connection, the low value of de in nanocrystalline semiconductor oxide films has been recently attributed to electrons spending a large fraction of their transit time in traps [46,47]. in addition, the fast relaxation of photo-excited electrons in semiconductor nanoparticles has been associated with a high density of trap states or a strong coupling between intrinsic midbandgap states and trap states [48]. conclusions this paper demonstrates that the mixed-conduction model is able to reproduce quantitatively the electrochemical and transport properties of anodic films on several commercial highly alloyed stainless steels in sulphate solutions with a ph in the range 2 to 7. notably, it is evidenced that the metal dissolution through the film proceeds via two parallel paths involving the generation, transport and consumption of interstitial cations and cation vacancies. the first path is predominant in most part of the passive region and corresponds mainly to the dissolution of fe through the oxide. thus enhanced dissolution through the film via interstitial cations causes activation of the steels at low enough potentials in the active-to-passive transition region. the second path, mediated by cation vacancies, is responsible for transpassive dissolution of cr through the oxide at high enough potentials. the progress of this reaction increases significantly the ionic conductivity of the passive film and in the transpassive region itself the dissolution processes become totally controlled by the reactions at the film|solution interface. as the kinetic and transport properties of the passive film determine the protective ability of that film towards metal corrosion, it can be concluded that 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[48] n. j. cherepy, d.b. liston, j. a. lovejoy, h. m. deng, j. z. zhang, j. phys. chem. b 102(1998) 770-776. © 2015 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://creativecommons.org/licenses/by/4.0/ machinability behavior of human implant materials http://dx.doi.org/10.5599/jese.1514 1 j. electrochem. sci. eng. 00(0) (2022) 000-000; http://dx.doi.org/10.5599/jese.1514 open access : : issn 1847-9286 www.jese-online.org original scientific paper machinability behavior of human implant materials gurbhej singh, rajbir singh and jameel gul amritsar group of colleges amritsar, 143001, punjab, india corresponding authorс: gurbhejsingh612@gmail.com received: september 11, 2022; accepted: october 16, 2022; published: october 29, 2022 abstract in the present day and age, engineered materials are making significant strides in their application in the biomedical sector. human implants have piqued the interest of material and metallurgy researchers due to their unique properties. machining the materials into implants customized for individual patients has been the standard practice nowadays. to contribute further to the same area, the current research work aims to investigate the machinability of t-l107.12 (a titanium-based human implant) using a non-traditional method called wire electrical discharge machining (wedm). the machinability of the machining process has been evaluated based on the material removal rate using a roughness meter and the atomic force microscope. the further impact of the machining parameters on the output responses was analyzed based on the statistical analysis. keywords titanium; roughness; material removal rate; wear introduction the use of biocompatible metals in medical implants at the time of the industrial revolution in the 19th century led to a period that saw the beginning aspects of the process of the metal industry [1]. the key element that spurred the rise of the metal implant business was the growing need for surgical treatments to repair broken bones [2]. these operations often include the internal healing of long bone fractures. however, until the aseptic method of lister's surgery was used in the 1860s, there were so few successes in attempts to implant metal appliances like bone pins and spinal wires composed of gold, silver, or iron [3]. these attempts were met with extremely limited levels of success. up until recently, there were relatively few obstacles that needed to be overcome in order to implant metal appliances. after that, biocompatible metals led to the development of orthopedic applications that currently play an important role in orthopedic appliances [4-6]. these orthopedic appliances include temporary appliances like bone plates and pins and screws and permanent implants such as overall joint replacement [7]. meanwhile, biocompatible metals can also be found in orthodontic and dental applications, like dental roots and fillings [8]. this is because these metals can be used without causing any adverse reactions. this is due to the fact that there is no chance of http://dx.doi.org/10.5599/jese.1514 http://dx.doi.org/10.5599/jese.1514 http://www.jese-online.org/ mailto:gurbhejsingh612@gmail.com j. electrochem. sci. eng. 00(0) (2022) 000-000 machinability behavior of human implant materials 2 an allergic response while using these metals. applications such as vascular stents use shapememory alloys made of nitinol [9], while magnesium-based improvements on more contemporary alloys are employed for tissue engineering and bone regeneration [10]. recently, increased research on biocompatible metals has progressed for the non-conventional reconstructive surgery of organs and hard tissue application [11,12]. this study is being done in preparation for the use of these metals. these metals include magnesium-based alloys that increase current alloys' performance, which may be used for tissue engineering and bone regeneration [13,14]. only a tiny percentage of the alloys and metals that are produced during the manufacturing process are biocompatible and have the potential to be realized in a permanent form as implant materials [15,16]. this is despite the fact that the manufacturing process may create a large range of alloys and metals. the mechanisms used in the vast majority of orthopedic medical equipment can be bought and sold in the commercial market. the most often used kinds of biocompatible metal alloys are stainless steels, cobalt-chromium alloys, titanium and its alloys and titanium. stainless steel has higher erosion and corrosion resistance [17-27]. co-cr alloys have high wear and corrosion resistance as well as have ease of machinability [28-38]. ti and its alloys hold good wear and corrosion-resistant properties, but their machinability is a quite tough task [4,5,7,39-45]. y2o3, yttria-stabilized-zirconia (ysz), hap and zro2 are the most widely used for bio-implant applications [35,46-51]. this research refers to the function of the biocompatible metals used most often in biomedical applications. these metals are considered to be biocompatible. these metals consist of stainless steels, alloys based on cobalt and chromium, titanium and its alloys, and other alloys based on titanium. in addition, the behavior of biocompatible metal alloys, includingtheir biocompatibility and resistance to corrosion, will be researched. implant materials the medical industry's use of biocompatible materials has grown dramatically during the previous two decades. gold, silver and iron are some of the metals used most often in producing long bone fracture pins and spinal cables. joint/replacement fractured/damaged bone is the primary use of biocompatible materials in orthopedics. through examination, these materials are put to use as long-term implants, screws, and pins [52]. it is anticipated that alternative biomaterials will affect major areas of burden, including spinal components, tumor bones, orthopedic joints, and dental components. hence, it is essential to consider both the mechanical strength and performance. because the bone is so sensitive to changes in its mechanical properties, even at the microscopic level, implants have the potential to influence cellular responses such as differentiation and mineralization. during bone repair, the mechanical characteristics of the implant materials played a major role and had a significant effect. combining a material's mechanical strength with its ability to withstand fractures is ideal for transplant materials. in comparison to the other types of materials, the tensile strength of metals is the greatest, followed by polymers and ceramics (except for zirconia). in comparison to ceramics, tensile strength, ductility, and resistance to corrosion are all areas in which metals excel. the metallic biomaterial has several deficiencies that need to be addressed. the most important one is the release of toxic elements from metals during metallic corrosion. among the most common examples are cobalt alloys, stainless steel alloys, titanium alloys, and several other kinds of metallic biomaterials. since the 1930s, austenitic stainless steel has been used in around ninety percent of osteosynthesis devices. both the biocompatibility and the mechanical strength of the material were outstanding attributes. as a result of the outstanding strength-to-weight ratio that co-based alloys g. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1514 3 possess, they are often used in the study of human anatomy. the inclusion of chromium at a concentration of more than 18 percent contributes significantly to both the inherent strength and wear resistance of these materials. titanium's earliest use was in the 1950s when it was utilized in the aerospace industry. since then, the material has been modified for human medical implants. t krupp invented implant-grade stainless steel in 1926. it had 18 % cr and 8 % ni and became 18cr-8ni (aisi 302). the mechanical properties of some biomaterials are shown in table 1. table 1. mechanical properties of biomaterials material tensile strength, mpa compressive strength, mpa fracture toughness, mpa m−1/2 ref. bio-glass 42 500 2 [53-57] ti 345 250-600 58-68 [7,39,43,44] stainless steel 465-950 1000 55-95 [20,51,58,59] ti alloys 596-1100 450-1850 40-92 [6,7,39,40,43-45,60] alumina 270-500 3000-5000 5-6 [33,34,48,61,62] in biomedical sectors, commercial-grade stainless steel is utilized to make surgical and dentistry tools. manufacturing techniques that are specific to stainless steel implants, such as vacuum melting (vm), vacuum arc re-melting (var), or electroslag refining (esr), provide resistance to pitting and crevice corrosion and a reduction in the amount of non-metallic additives. most people choose austenitic stainless steel because of its versatility. low cost, high strength and ductility, machinability, and mechanical qualities can be controlled easily. implants made of vacuum-melted 316l stainless steel are available on the commercial market. afterward, biomedical industries began employing titanium implants in the 1930s, the titanium alloy plays a great role due to its thermo mechanical properties and its low density. vacuum processing necessitated a high temperature to avoid the formation of a reaction with oxygen. pure titanium (cp-ti) is the only commercially available form of the four grades. the present work focuses on these two materials for testing and evaluating their machinability and biocompatibility being an implant material using wire-cut electrical discharge machining (wedm )approach. wedm technique machine tools, control systems, power supply units, and dielectric supply units are all included in the wedm. in addition to the primary worktable, the tool control section includes an auxiliary work table, a wire feed mechanism, and a wire cutter. there is a large worktable on which the pieces of work are placed. 1 m increments are used in servo motor control of axial movement. the u and v axes of the table are parallel to the x and y axes, resulting in a flat configuration. continuous feed is ensured by two guides on either side of the wire feed system (in and out). guides aid in maintaining the wire's tension. due to the inactive u-v axis and lower guide, there is a transverse displacement of the top wire guide. to remove material from a workpiece, an electrical spark is generated by the pulse generator unit. as a dielectric liquid, deionized water was used to keep the top and bottom work pieces hydrated. the low viscosity and rapid cooling speeds of deionized water are its primary advantages. the controller stores the route of the work item and shifts it transversely. tilting the tapper as the x-y table moves along a specified route and the u-v table remains stationary is required for a straight cut. to carry out the operations, the route information is completely controlled by the cnc [8]. figure 1 given demonstrates the basic principle of wedm. based on its principle of working, wedm produced responses in terms of surface roughness and material removal rate. http://dx.doi.org/10.5599/jese.1514 j. electrochem. sci. eng. 00(0) (2022) 000-000 machinability behavior of human implant materials 4 figure 1. schematic diagram of the basic principle of the wedm process. surface roughness over a given length, it may be summed up as a deviation from the nominal surface in the vertical axis. the micro-level profile of a machined surface’s key nomenclature is: • roughness distance from the nominal surface to the peak of the waviness. • the roughness width is the distance between the peaks. • the maximum spacing of irregularities included in the measurement is represented by the cut-off distance/width. • in machining parlance, the term "lay" refers to the direction in which a surface pattern is generated as a reflection of the machining process. the roughness average (ra), sometimes referred to as the arithmetic mean of the precise quantity of each height slightly above the nominal surface, is a metric that may be used to highlight the surface imperfections. ra is frequently referred to be the arithmetic mean of each height slightly above the nominal surface. roughness may also be addressed in terms of roughness maximal (rz), which is the distance between the highest point as well as the lowest point on the surface. this distance is measured from one point on the surface to the next. similar to how this may be stated in terms of roughness, the roughness could also be described in terms of roughness. roughness peak (rp), also known as the maximum height value from the nominal surface, is an additional method that may be used to replicate the surface's roughness. this method is one of several alternative methods [63]. particularly in the case of the wedm process, the undulation that is present on the surface of the machined component runs parallel to the direction in which the wire is moving. because the energy applied to melt the specimen results in the volume lost in the metal, the surface roughness is dependent on the electrical parameters of the machining process, such as the voltage applied between the gaps, the pulse-on time, etc. material removal rate material removal rate(mrr) sometimes referred to as is the amount of material removed in a certain amount of time, with the objective of achieving higher levels of productivity overall. it is recommended that the mrr machining process continue its upward trajectory toward increased productivity without compromising surface quality (i.e. decrease in surface quality). high mrr combined with a high-quality surface finish should lead to increased productivity in the component production process. calculating the mrr of the machining process may be done using either the g. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1514 5 length of the machining time or the weight loss approach. both methods are available. eqs. (1) and (2) provides a representation of the empirical equation utilized for the calculation of mrr by measuring the amount of time spent cutting throughout the process of machining [64]. mrr = cutting rate  width  depth of work (1) cutting rate = total machining length / time of machining (2) where, mrr is measured in mm3 min-1, the cutting rate is in mm/min, and width and depth are in mm. the mrr of the machining process (g min-1) can also be calculated using the weight loss of the specimen, eqs. (3) and (4): mrr = weight loss / machining time (3) weight loss = weight before machining weight after machining (4) material and method for the purpose of this examination, orthopedic human implant materials that are available for purchase on the market were acquired for the research project. the technical grade of implants that were acquired from literature who served as the primary supplier of the implant material, 48 for orthopedics. for the purpose of this investigation, titanium-based human implant grade t-l107.12 (hole diameter: 4.9 mm; 12 holes) materials were used. table 1 details the material's nominal chemical composition, which may be obtained by utilizing the data from table 2. table 2. nominal composition of ti-based human implant material of grade t-l107.12 implant alloying elements al v c o n fe ti content, wt. % 5.5 4.5 0.08 0.13 0.05 0.25 balance figure 2. elektra wire edm machine wire electrical discharge machining the wedm machine, which is computer-aided and numerically controlled, is used in the performance of the experimental research (model: elektra). the wedm machines are made up of the power supply unit, the flushing unit, and the industrial machinery process speed. the definition of the method makes use of deionized water as the dielectric medium, and the electrode wire material is a half-hard edm brass wire with a diameter of 0.25 millimeters. figure 2 given shows the edm machine used in the present work. http://dx.doi.org/10.5599/jese.1514 j. electrochem. sci. eng. 00(0) (2022) 000-000 machinability behavior of human implant materials 6 specifications of the equipment the machine, the control cabinet, and the worktable are the three primary elements of the elektra wire edm. in table 3, the representation of the machine's specifications has been illustrated. table 3 provides an overview of the input process parameters that may be modified for implant machining. a problem has been constructed using the input variables and the minitab program to design twenty-one sets of input parameter combinations for experiments. this problem was built using the input variables. table 3. elektra wire edm technical specification no. parameter value 1 main table traverse (xy), mm 300400 2 aux. table traverse (xy), mm 8080 3 table size, mm 440650 4 maximum taper angle, ° ±30° / 50 mm 5 maximum workpiece height. mm 200 6 maximum workpiece weight, kg 300 7 resolution, m 0.5 8 max. jog speed, mm min-1 900 9 maximum wire spool capacity, kg 5 10 wire electrode diameter. mm 0.25 (standard) 0.15, 0.20 (optional) 11 least input increment, m 1 12 interpolation linear and circular 13 connected load, kva 10 14 wire tension, g 1600 experiment design for machining implant material in table 4, the data for the experiment design has been given. all conceivable permutations of the machinability assessment are tested in terms of surface roughness (ra) and material removal rate (mrr). the mitutoyo roughness meter is used to determine the surface roughness of each machined surface individually (model: surftest sj 410, japan). in order to compute the material removal rate, use the following mathematical relationship, eq. (5): mrr, mm3/min = cutting rate  width  depth of work (5) as a consequence of the investigation, the impact of each input process parameter is examined using minitab software to do the necessary mathematical calculations for the analysis of variance (anova). based on the data, conclusions were drawn that revealed the best process condition for machining stainless steel implantation materials in terms of surface quality and machinability. surface roughness the values of the surface's two-dimensional roughness may be assessed using a probe-style tester with an mitutoyo roughness meter (model number: surface test sj 410; manufactured in japan) which is configured in the manner seen in figure 3. the ra values show where the peaks and troughs are located in a certain direction parallel to the path taken by the probe. the measures of the surface roughness are as follows: they are manufactured with a cut-off value of 0.8 mm and thus are manufactured in the opposite direction from that in which the wire traverses, which ultimately results in increased values for the surface's roughness. vertical moments were able to measure motions to an accuracy of 0.01 mm. g. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1514 7 table 4. experiment design for machining implant material run pulse-on, µs pulse-on, µs voltage, v i 5 8 65 2 5 3 65 3 5 8 65 4 6 5 80 5 6 10 50 6 3 10 80 7 3 5 80 8 3 10 50 9 5 8 65 10 5 8 40 11 5 8 65 12 2 8 65 13 5 8 90 14 6 5 50 15 3 5 50 16 5 8 65 17 5 8 65 18 5 8 65 19 7 8 65 20 5 i2 65 21 6 10 80 figure 3. surface roughness tester analysis of variance the analysis of changes is a computationally based method that quantifies the impact of every input parameter on the overall response. the variation in the data caused by interactions is referred to as the variance within the control factors used in the experiment. these components are what make up the experiment. the f-ratio, also known as fisher's ratio, is used in order to determine which parameters are the most important and which are the least significant. the results of the anova test reveal which of the factors had the greatest impact on the overall response. http://dx.doi.org/10.5599/jese.1514 j. electrochem. sci. eng. 00(0) (2022) 000-000 machinability behavior of human implant materials 8 results and discussion machinability of implants surface roughness of the wedm machined implants experiments were done using a statistical model that had already been set up, and the measured surface roughness is shown in table 5. table 5. surface roughness with reference to individual process parameters involved in machining titaniumbased implant material run pulse-on, µs pulse-off, µs voltage, v ra / µm 1 6 7 65 3.284 2 6 4 65 2.883 3 6 7 65 2.524 4 5 6 80 2.274 5 5 9 50 3.613 6 4 9 80 2.103 7 4 6 80 2.131 8 4 9 50 2.608 9 6 7 65 2.781 10 6 7 50 3.385 11 6 7 65 2.385 12 3 7 65 1.739 13 6 7 90 1.883 14 5 6 50 3.378 15 4 6 50 3.237 16 7 7 65 2.429 17 7 7 65 2.374 18 7 7 65 2.247 19 9 7 65 2.782 20 7 11 65 2.222 21 7 9 80 2.112 when it comes to surface roughness, the lowest voltage combined with either a pulse-on (ton) or a pulse-on (toff) has the most impact. the linearity goodness of fit (r2) for the experimental data is 84.61 %, confirming all of this. a linear regression equation for such a suggested model is created utilizing the applied voltage (v) and the ton as the most essential parameters to analyze the theoretical difference in ra. the regression line's equation (6) is as follows: ra = 8.6487 v 0.052418 ton -0.30245 (6) surface roughness with reference to the applied voltage at three different ton is shown in figure 4. you may compute the theoretical changes in surface roughness by adjusting the applied voltage and pulse-on time. surface roughness has a higher order response to diverse applied voltages than pulse-on time. material removal rate of the wedm machined implants this condition existed, just like the roughness of the surface. the goodness of fit in linearity (r2) reveals that the experimental data for mrr are as high as 92.5 % based on all of the aforementioned. g. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1514 9 voltage, v figure 4. theoretical variations in surface roughness with reference to the applied voltage at three different pulses on time a linear regression equation for such a suggested model was created by taking into consideration the applied voltage and the pulse-on time to analyze the theoretical difference in mrr (ton). the regression equation is shown in the equation (7): mrr = 0.01164 v 0.236211 ton 1.0997 (7) the empirical calculations of the theoretical changes in surface roughness and the rate of material removal are accomplished by adjusting the applied voltage and the pulse-on time. material removing rate with reference to the applied voltage at three different pulses on time is shown in figure 5. convergences are seen in the material removal rate when the voltage is increased to its maximum for a variety of pulse-on times. voltage, v figure 5. theoretical variations in mrr1with reference to the applied voltage at three different ton / µs r a /  m m r r , m m 3 m in -1 http://dx.doi.org/10.5599/jese.1514 j. electrochem. sci. eng. 00(0) (2022) 000-000 machinability behavior of human implant materials 10 table 6. surface roughness with reference to individual process parameters involved in machining titaniumbased implant material run ton / µs toff / µs voltage, v ra / µm 1 5 8 65 3.193 2 5 3 65 2.992 3 5 8 65 2.412 4 6 5 80 2.180 5 6 10 50 3.504 6 3 10 80 2.003 7 3 5 80 2.232 8 3 10 50 2.507 9 5 8 65 2.981 10 5 8 50 3.475 11 5 8 65 2.175 12 2 8 65 1.929 13 5 8 90 1.994 14 6 5 50 3.269 15 3 5 50 3.126 16 5 8 65 2.318 17 5 8 65 2.264 18 5 8 65 2.136 19 7 8 65 2.692 20 5 12 65 2.111 21 6 10 80 2.023 influence of process parameters on surface roughness and material removal rate experiments conducted using a preset statistical model are submitted for a thorough examination. following the conclusion of the inquiry, the experimental data are statistically analyzed to establish the contribution of each individual process parameter and their effect on machining. table 7 depicts the findings of this investigation. table 7. level of process parameters contribution towards surface roughness and material removal rate contribution, % ton toff voltage error ra 9.12 0.41 75.11 15.36 mrr 2.36 1.20 88.94 7.50 calculating the contributions of the process parameters based on the results of an analysis of variance is an empirical approach (anova). surface roughness and material removal rate are shown in table 7. the three critical machining parameters for processing titanium-based human implant materials throughout the machining process are applied voltage, pulse-on time, and pulse-on time. t on is another name for pulse-on time. toff stands for "pulse-on time”. given this data, it is clear that the applied voltage is the most important variable in operation, as it has a significant impact on both the surface polish (75.11 %) and the rate of material removal (88.9 %). a wire-edm machining process very similar to the one described above was carried out. statistical analysis revealed that the voltage provided had a substantial influence on the surface finish produced. when machining is done, the voltage being applied will, in the majority of instances, be directly involved in the formation of resistance as well as plasma arcs (between the wire and the contact zone) between the dielectric medium and the wire. the other two parameters are the management of the g. singh et al. j. electrochem. sci. eng. 00(0) (2022) 000-000 http://dx.doi.org/10.5599/jese.1514 11 pulse-on time for plasma spark creation and ton's surface finish and material removal contributions, respectively. the surface finish contribution of ton is 9 %, and the material removal contribution is 2.3 %. the inaccuracy in the design may be described as a percentage. when that percentage is determined, it comes out to 15 % for the surface finish and 1.5 % for the material removal rate appropriately. the variation in inaccuracy for ra and mrr is due to the fact that the surface polish does not remain constant in relation to the quantity of material that has been removed. the contribution of process parameters will be directly influenced by the marginal variance obtained while taking the average of ra for three different sites. an sem surface investigation using micrographs taken by that instrument is carried out so that supporting evidence may be presented. the materials and different processes can be used to process the surface properties [65,66]. surface qualities may be modified via the use of various materials and techniques [67-72]. conclusions the ra value is measured in micrometers over the machined surface treated with wedm. when the largest pulse-on setting is utilized and the 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https://doi.org/10.1016/j.surfcoat.2022.128450 ©2022 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (https://creativecommons.org/licenses/by/4.0/) https://doi.org/10.4018/ijseims.2021070102 https://doi.org/10.1016/j.matpr.2020.02.226 https://doi.org/10.1016/j.matpr.2020.02.271 https://doi.org/10.1016/j.matpr.2020.02.493 https://doi.org/10.1016/j.matpr.2020.01.156 https://doi.org/10.1007/978-981-15-5151-2_2 https://doi.org/10.1016/j.surfcoat.2022.128450 https://creativecommons.org/licenses/by/4.0/) {electrochemical evaluation of corrosion resistance of chromium plated nickel and copper tin alloys. a comparative study} doi:10.5599/jese.429 227 j. electrochem. sci. eng. 8(3) (2018) 227-239; doi: http://dx.doi.org/10.5599/jese.429 open access : : issn 1847-9286 www.jese-online.org original scientific paper electrochemical evaluation of corrosion resistance of chromium plated nickel and copper tin alloys. a comparative study chike f. oduoza, stacey hingley faculty of science and engineering, university of wolverhampton, wv11ly, uk corresponding author e-mail: c.f.oduoza@wlv.ac.uk; tel.: +44-(0) 1902 323944; fax: +44-(0) 190232754 received: august 30, 2017; revised: december 1, 2017; accepted: december 19, 2017 abstract engineering materials and composites are frequently exposed to aggressive and chemically toxic environments with high probability for rapid corrosion and consequent deterioration and catastrophic degradation. occasionally, a new legislation regulates against the use of existing materials, necessitating the development of new methodologies or new materials in order to reduce material corrosion rates. the aim of this study is to test the corrosion resistance of the copper tin alloy, a material that could replace nickel compounds, recently reclassified as suspected carcinogens. standard sizes of precut nickel and brass panels were produced and then plated with different thicknesses of copper tin alloy and chromium for additional protection. evaluation of plated materials for corrosion rate and resistance was carried out using linear polarisation, electrochemical impedance and accelerated destructive testing. corrosion testing of the materials assessed qualitatively and quantitatively, showed that corrosion resistance was dependent on a combination of factors, including the thickness of chromium plating, type of material, type of testing and duration in a corrosion chamber. while linear polarisation experiment was useful in establishing corrosion rate of sample, electrochemical impedance and accelerated destructive testing experiments assessed corrosion resistance of the materials. keywords nickel, copper-tin alloy deposit; chromium deposit; corrosion resistance; corrosion testing introduction electrodeposited nickel is widely used in many applications. a typical application of plated nickel is in corrosion protection, particularly in conjunction with chromium plating. the reclassification of nickel and its salts to a more toxic chemical status has led the electroplating industry to research in order to find a replacement for the nickel layer [1]. in this study, corrosion rate and resistance of http://dx.doi.org/10.5599/jese.429 http://www.jese-online.org/ mailto:c.f.oduoza@wlv.ac.uk j. electrochem. sci. eng. 8(3) (2018) 227-239 corrosion resistance of cr plated ni and cu-sn alloys 228 the chromium plated copper tin alloy has been evaluated as an alternative to the chromium plated nickel. the colour and surface imaging of copper tin alloy with chromium deposits compared to the nickel deposit have already been studied by hingley and oduoza [1]. the eu risk assessment committee was set up in 1966 to investigate nickel and its compounds with regard to its effect on human health and environmental impact. danish environmental protection agency (d-epa) as a rapporteur was delegated to evaluate nickel and soluble nickel compounds (nickel sulphate, nickel chloride, nickel carbonate, nickel nitrate). in february 2007, there was a reclassification of the five nickel substances as category 1human carcinogens by inhalation (carcinogenic to man) and category 2 reproductive toxicants (may cause harm to the unborn child) and chronic toxicant. nickel metal has been classified as category 3carcinogen (limited evidence of a carcinogenic effect) and chronic toxicity [1]. the aim of this study is to test the corrosion resistance of the copper tin alloy that could replace nickel compounds reclassified as suspected carcinogens. corrosion resistance of a deposit is important to manufacturers and consumers since it determines the level of corrosion protection and life of the product. since an evaluation of a material corroding in real time can be very time consuming, electrochemical techniques can be used to accelerate the corrosion process and collate data rapidly. the principle behind this mode of testing is to apply the potential (voltage) across an electrochemical cell to cause current to flow in order to mimic the rate of electrochemical reaction taking place at electrodes. the electrode reaction is transfer of electrons from the electrode material to form ions (the corrosion process), triggered by the applied potential resulting in the flow of an electric current [2,3]. other corrosion tests used within the electroplating industry for measuring corrosion rates include neutral salt spray (nss) and copper accelerated acetic acid salt spray (cass) methods [4]. specifications used throughout this study make an important part of the electroplating industry and allow an accurate comparison of deposits used [4-6]. cass is a methodology used to enhance corrosion rate/resistance of a material in order to monitor the resilience of the material to corrosion. experimental procedure for preparation of sample for electrodeposition brass panels (size 10 × 7.5 × 0.05 cm) of 70:30 copper to zinc ratio were used as the substrate for all experiments carried out in this study. figure 1 shows the sequence for electrodeposition of nickel and chromium on brass panels. nickel was plated to a thickness of 20 μm followed by 0.3 μm of chromium, if required. the formulations and electroplating conditions for nickel and chromium depositions have been reported previously [7]. the electroplating sequence for deposition of white cu-sn alloy is outlined in figure 2. after pretreatment, copper was electrodeposited to a thickness of 20 μm, followed by deposition of white cu-sn alloy at thicknesses of either 0.5 μm or 2 μm and then chromium to a thickness of 0.3 μm, if applicable. all operating conditions for deposition of copper and white cu-sn alloy were described previously [7]. white cu-sn alloy deposit is not normally plated above a specific thickness due to a possible change in appearance from a silvery bright to dull grey colour. the sequence for electrodeposition of yellow cu-sn alloy is identical to those of white cu-sn alloy, followed by deposition of varying thicknesses of yellow cu-sn alloy, topped up with 0.5 μm of white cu-sn alloy, if applicable and finally 0.3 μm of chromium deposited, if specified. the formulation and operating parameters for the yellow cu-sn alloy were described by hingley [7]. c. f. oduoza and s. hingley j. electrochem. sci. eng. 8(3) (2018) 227-239 doi:10.5599/jese.429 229 figure 1. flow chart for electrodeposition of nickel and trivalent chromium on brass substrate details of the appearance, testing and deposits structure analysis for the panels have been reported elsewhere [1]. the corrosion resistance of each deposit was assessed electrochemically by linear polarisation [3,8-13], electrochemical impedance spectroscopy experiments and also by accelerated salt spray testing using copper accelerated acetic acid salt spray. all electroplated panels were analysed thrice on the same panel for corrosion resistance, appearance and deposit structure. the nyquist and bode plots for the electrochemical impedance study were taken from one set of results to characterise the selected deposits. figure 2. flow chart for electrodeposition of white cu-sn and trivalent chromium on brass substrate. testing of plated panels for corrosion rate and resistance evaluation of the deposits was also carried out by visual examination and guided by the specification astm b 537-70 [14]. it is determined by the percentage of corrosion that covers both alkali cathodic cleaner at 65 oc for 3 min (metex pe emphax) water rinse 50 % hydrochloric acid dip at 20 oc for 3 min water rinse electrodeposition of bright nickel at 60 oc, ph 4.2, 4 a/dm2 to a thickness of 30 µm (lumax) electrodeposition of trivalent chromium at 600c, ph 3.6, 10 a/dm2 to a thickness of 0.3 µm (trimaciii) dry water rinse water rinse alkali cathodic cleaner at 65 oc for 3 min (metex pe emphax) water rinse 50 % hydrochloric acid dip at 20 oc for 3 min water rinse electrodeposition of acid copper at 25 oc, 3 a/dm2 to a thickness of 30 µm (cumac crystal) electrodeposition of trivalent chromium at 60 oc, ph 3.6, 10 a/dm2 to a thickness of 0.3 µm (trimaciii) dry water rinse water rinse electrodeposition of cu-sn alloy at 60 oc, 0.5 a/dm2 to a thickness of 0.2-2.0 µm (starvet) water rinse j. electrochem. sci. eng. 8(3) (2018) 227-239 corrosion resistance of cr plated ni and cu-sn alloys 230 the deposit and substrate. the corrosion rating is given in two values x/y, with ‘x’ referring to the coverage and appearance of the substrate corrosion (known as the protection rating) and ‘y’ referring to the coverage and appearance of the coating itself and is known as the appearance rating. all ratings are given as a value of 110 with 10 being the best (no corrosion) and 1 being the worst (corrosion seen all over the specimen). instrumentation for linear polarisation and electrochemical impedance spectroscopy potentiostat, eg+g instruments, model 263a, was used to determine the corrosion current density (icorr). these experiments were carried out in 5 % neutral sodium chloride solution using an ag/agcl reference electrode over a potential range of +/20 mv versus open circuit potential. the equilibrium time for which 1 cm2 surface of panel was in contact with the nacl varied from 30 to 60 minutes. figure 3 shows the corrosion cell used for the linear polarisation and electrochemical impedance spectroscopy testing [6]. the working electrode is the electroplated brass panel of 1 cm2 area immersed in 5 % neutral nacl solution, the auxiliary electrode is platinum mesh, while the reference electrode is ag/agcl. figure 3. corrosion cell used for linear polarisation and electrochemical impedance spectroscopy experiments electrochemical impedance spectroscopy this study was carried out with a solartron 1252a frequency response analyser (fra) with princeton applied research potentiostat/galvanostat, model 263a, and zplot software. experiments were carried out using the test panel in 5 % neutral nacl solution, with equilibrium time prior measurements varied from 30 to 60 minutes. all electrochemical impedance experiments were carried out at the open circuit potential (ocp), with signal amplitude of 15 mv, within a frequency range of 106 to 10-1 hz, and 10 points measured frequency decade. figure 4 shows a simple electrical equivalent circuit with one-time constant that has usually been used for measured impedance data fittings and determination of all components, i.e. corrosion resistance (rcorr), solution resistance (rs) and double-layer capacitance (cedl) of a corrosive electrode. c. f. oduoza and s. hingley j. electrochem. sci. eng. 8(3) (2018) 227-239 doi:10.5599/jese.429 231 figure 4. one time constant-electrical equivalent circuit showing electrical behaviour of the cell with corrosive electrode [2] copper accelerated acetic acid salt spray test cass testing was carried out in salt solution comprising sodium chloride 45-55 g/l, copper chloride 0.25 g/l and made up to 1 litre. the ph was adjusted with glacial acetic acid. air pressure was 0.080.12 mpa and humidifier temperature was 60-65 °c. the chamber temperature was maintained within the range of 48-51 °c. the sample collection rate was 1.0-2.0 ml/hour [15]. experiments were carried out in a fog chamber with a provision for heating, salt reservoir, supply of compressed air, at least one atomising nozzle, specimen support, and means of control. all equipment was corrosion resistant to the fog [6,14]. the test specimen was thoroughly cleaned to avoid any side influence on the corrosion resistance of the deposit. to clean the sample, 10 grams of magnesium oxide powder in 100 ml of distilled water was used to wipe over the sample’s surface with cotton wool and rinsed with warm water before drying. in order to avoid localised corrosion on the non-protected areas, any cut edge or jig point was masked with either pressure sensitive tape, or stopping off by lacquer or wax [14]. positioning of test specimen to expose the test area was carried out at a 15o parallel to the principal direction of horizontal flow of fog through the chamber, based on the dominant surface under test. test specimens were not allowed to touch each other or with any other metal to avoid contamination [14]. each chamber, containing at least two fog collectors was used to evaluate the collection rate and also to analyse the salt concentration and ph [14]. the standard test method used for cass testing was astm b 368-97 [15] using liebisch, model 33649 corrosion cabinet. this method was used to evaluate performance of the decorative process for copper/nickel/chromium, or nickel/chromium plating on substrates. cass was used throughout for corrosion testing and appears to be the preferred method in the electroplating industry compared to nss which is relatively slow to corrode the nickel/chromium deposits. atomic force microscopy (afm) surface imaging technique was used to evaluate the surface properties of the plated samples at the nanometer scale. the technique produced a three-dimensional image that was used to characterise roughness, depth, and particle size of the surface [1]. results and discussion electrochemical corrosion rate and resistance results linear polarisation data was used to calculate corrosion current values, which were used to estimate corrosion rate of each electroplated deposit. polarization resistance (rp) is defined as the slope of the potential (e) vs. current density (j) plot measured close to the corrosion potential according to equation (1): p e r j    (1) in equation (1), rp is polarisation (corrosion) resistance in  cm2, δe is change in potential in v and δj is change in current density in a/cm2. rs rcorr cedl j. electrochem. sci. eng. 8(3) (2018) 227-239 corrosion resistance of cr plated ni and cu-sn alloys 232 there are lot of papers in the literature reporting application of the linear polarisation technique, including that of kouril et al. [16] who calculated the polarisation (corrosion) resistance of corroding reinforcements used in concrete. for a practical determination on how fast the deposit corrodes and to estimate how long the service life will be, equation (2), demonstrating the mathematical relationship between jcorr and rp, is used [2]: a c corr p a c 1 2.303 b b j r     (2) in equation (2) jcorr is corrosion current density in a/cm2, rp is corrosion resistance in  cm2, a is anodic tafel slope, while c is cathodic tafel slope, referred to as the tafel constant. calculation of the corrosion rate was carried out according to weast [17] using equation (3) showing how the corrosion current can be converted to a corrosion rate in millimetres per year (mpy) [2]: corrosion rate, mm year-1 = jcorr  (1/ρ)ε (3) in equation (3), jcorr is corrosion current density in a/cm2,  is a combination of several conversion terms equal to 1.2866×105 [equivalentsmm]/[ccmyear],  is metal density (g/cm3), while ε is equivalent weight of the metal (molecular weight of the metal divided by the number of electrons in the anodic half reaction) in g/equivalent. table 1 shows the results of the linear polarisation experiments highlighting data for polarisation resistance, corrosion current and an estimate of corrosion rate for the electroplated panels. the polarisation resistance measures resistance to corrosion in k cm2 while average corrosion current quantifies the current associated with corrosion of the deposit expressed in na/cm2. corrosion rate describes the rate of corrosion measured in mm/year. for nickel deposit, corrosion rate is 0.20 mm per year, but with chromium as topcoat, the corrosion rate decreased to 0.03 mm per year. this shows clearly that chromium deposit can significantly reduce the corrosion rate, providing an additional protection to nickel deposit. data in table 1 calculated on the basis of linear polarisation graphs presented in figure 5 show that polarization (corrosion) resistance increases with coating thickness for white cu-sn alloy, however, when top is plated with chromium the resistance to corrosion levels off (compare 0.5 µm white cu-sn alloy with 2.0 µm white cu-sn alloy both plated with chromium). figure 5. linear polarisation graphs for the white cu-sn alloy deposit with and without chromium as a topcoat -280 -260 -240 -220 -200 -180 -160 -140 -120 -100 -150 -100 -50 0 50 100 150 200 250 p o la ri sa ti o n , m v current density, na/cm2 white cu-sn 0.5um white cu-sn 2.0um white cu-sn 0.5um + chromium white cu-sn 2.0um + chromium white cu-sn 0.5 µm white cu-sn 2.0 µm white cu-sn 2.0 µm + chromium white cu-sn 0.5 µm + chromium c. f. oduoza and s. hingley j. electrochem. sci. eng. 8(3) (2018) 227-239 doi:10.5599/jese.429 233 table 1. linear polarisation data (polarisation resistance, corrosion current density and calculated corrosion rate) for electroplated deposits. panel identification rp / k cm2 average jcorr / na cm -2 corrosion rate, mm per year nickel 20 µm 47 482 0.20 nickel 20 µm + chromium 328 77 0.03 white cu-sn alloy 0.5 µm 156 197 0.096 white cu-sn alloy 2.0 µm 337 98 0.05 white cu-sn alloy 0.5 µm + chromium 207 108 0.05 white cu-sn alloy 2.0 µm + chromium 171 132 0.06 yellow cu-sn alloy 4.0 µm + white cu-sn alloy 0.5 µm 17 1848 0.91 yellow cu-sn alloy 12.0 µm + white cu-sn alloy 0.5 µm 63 672 0.33 yellow cu-sn alloy 24.0 µm + white cu-sn alloy 0.5 µm 104 273 0.13 yellow cu-sn alloy 4.0 µm + white cu-sn alloy 0.5 µm + chromium 22 1232 0.61 yellow cu-sn alloy 12.0µm + white cu-sn alloy 0.5 µm + chromium 41 903 0.44 yellow cu-sn alloy 24.0 µm + white cu-sn alloy 0.5 µm + chromium 14 1490 0.73 comparing copper tin alloy with nickel samples, it appears that 20 µm of nickel plated with chromium is more corrosion resistant than 0.5 µm or even 2.0 µm white cu-sn alloy. the corrosion rate is equal for 0.5 µm white cu-sn alloy and 20 µm nickel, both plated with chromium and showing a corrosion rate of 0.05 mm and 0.03 mm per year, respectively. the corrosion rate results showed that while increasing the thickness of the white cu-sn alloy improves corrosion resistance, additional plating with chromium does not make any difference in corrosion resistance, irrespective of the thickness of white cu-sn alloy plating. this set of linear polarisation results suggest that white cusn alloy has certain potential to be more corrosion resistant than nickel if the deposit could be plated thicker. this is due to the fact that polarisation resistance increases with thickness of the white cusn alloy. yellow cu-sn alloy was plated to a thickness of 4 µm, 12 µm and 24 µm and additionally with 0.5 µm of white cu-sn alloy plated on top of each variant. compared with white cu-sn alloy deposits only (table 1), yellow cu-sn alloy deposits showed much higher corrosion rates of 0.91, 0.33, and 0.13 mm per year, respectively. however, the results showed that thicker was the deposit, its corrosion resistance would be better. the polarisation resistance values for the yellow cu-sn alloy deposits are slightly higher than for those of nickel deposit, which shows that yellow cu-sn alloy deposits demonstrate improved corrosion resistance with increased deposit thickness. the yellow cu-sn alloy, plated at varying thicknesses with 0.5 µm white cu-sn alloy and a finish of chromium deposit, have higher corrosion resistance compared to the deposits without chromium deposit (table 1). this suggests that the chromium topcoat provided additional corrosion protection, but corrosion rates are still much higher than those of nickel with chromium finish. table 2 shows the electrochemical impedance spectroscopy data highlighting two constant phase elements (cpe1 and cpe2) values in farad (-1 sn ) for the electroplated deposits. although cpe1 and cpe2 increased in values from the pre-deposit (without chromium) to post deposit (with chromium) for each panel of both nickel and white copper tin alloy, this pattern was not observed for yellow copper tin alloy at all deposit concentrations. the constant phase element (cpe1) j. electrochem. sci. eng. 8(3) (2018) 227-239 corrosion resistance of cr plated ni and cu-sn alloys 234 represents the surface of the metallic deposit with polarisation resistance represented as r2, while the constant phase element (cpe2) represents the surface of the oxide layer with polarisation resistance r3. here, the combined r2 and r3 would give the overall polarisation resistance (rp). the corresponding two time constant-electrical equivalent circuit used for impedance data fitting is presented in figure 6. figure 6. two time constant-electrical equivalent circuit used to fit the eis data while the oxide layer response appears in the high frequency area of impedance spectrum, the low frequency section shows the response of the metallic layer with the metal/solution interface at the bottom of the pores [18]. a study by girault et al. [18] used this equivalent circuit for the interpretation of a passivated nickel deposit and although not directly related to this study, it does confirm that chromium oxidizes to give a protective chromium oxide layer. application of electrical equivalent circuit from figure 6 is also supported by the study of arslan et al. [19] with an oxide layer on a titanium substrate. in measured electrochemical impedance spectra, two-time constants can be clearly identified in the nyquist and bode plots presented in figures 79. figures 7a9a show nyquist, while figures 7b9b show bode plots of the nickel with and without chromium, the white cu-sn alloy with and without chromium and the yellow cu-sn alloy with and without chromium deposits, respectively. electrical equivalent circuit shown in figure 6, however, did not always clearly fit high frequency data of impedance spectra, which could be due to a third-time constant that was not identified in this study. however, the two-time constant-electrical equivalent circuit was still the best for fitting the data collected. here, cpe1 could be ascribed to the chromium and cpe2 to the nickel with a suspected oxide layer which was not identifiable from the data. the constant phase element cpe1 with a value of 1710-5 f is achieved in this study with the exception of yellow cu-sn alloy deposit with cpe1 value of about 210-4 f [20]. the topography of the yellow cu-sn alloys shows that the surface is rougher than the other deposits [1]. an increase in surface roughness is an indication that the overall surface area is larger than the nominated area of 1 cm2 and as a consequence, the cpe value could be higher than expected. the cpe2 value is found to be about 110-4 110-3 f, thus indicating neither metallic corrosion, nor the presence of an oxide layer. table 2. eis results for all electroplated deposits panel identification rp / kω cm2 cpe1×105, f cpe2×104, f nickel 20µm 36.1 1.2752 0.6421 nickel 20µm + cr 22.1 7.0763 1.7562 white cu-sn alloy 0.5 µm 17.3 2.6074 1.5468 white cu-sn alloy 2.0 µm 17.3 3.6444 3.9618 white cu-sn alloy 0.5 µm + cr 8.7 4.5903 6.8690 white cu-sn alloy 2.0 µm + cr 13.5 4.9812 6.7480 yellow cu-sn alloy 4.0 µm 0.178 6.7232 2.0780 yellow cu-sn alloy 12.0 µm 2.6 17.670 16.185 yellow cu-sn alloy 24.0 µm 3.3 1.3094 1.8789 yellow cu-sn alloy 4.0 µm + cr 8.8 4.1168 9.8390 yellow cu-sn alloy 12.0 µm + cr 14.1 1.4041 0.4123 yellow cu-sn alloy 24.0 µm + cr 14.0 1.2311 8.4956 r1 r2 r3 c. f. oduoza and s. hingley j. electrochem. sci. eng. 8(3) (2018) 227-239 doi:10.5599/jese.429 235 a z /  cm2 b frequency, hz c frequency, hz figure 7. a nyquist plots for the nickel deposit with and without chromium, and bode plots for the nickel deposit b with and cwithout chromium a z’ /  cm2 b frequency, hz c frequency, hz figure 8. a nyquist plot for the white cu-sn alloy deposit with and without chromium and bode plot for the white cu-sn alloy deposit b with and c without chromium copper accelerated acetic acid salt spray (cass) corrosion test results cass corrosion testing was carried out continuously over a period of 16 hours with corrosion rating taken every 8 hours since it is an aggressive test. the test was stopped after 16 hours due to the deterioration of the nickel deposit with no chromium as a top coat. table 3 shows data for cass testing of the various panels with and without chromium deposition, broken down into two chunks of 8 hours testing durations. data is shown as x/y (already described in the experimental section) where x stands for protection rating (extent of corrosion undergone by the substrate) and y stands for appearance rating of the plating. x or y is a value on a scale of 1 to 10, where 10 is perfect (no corrosion). for instance 20 m nickel after 8 hours of cass testing was 2/2, indicating high corrosion for both substrate and plating and this got worse after 16 hours deteriorating to 1/1. for 20 m nickel plus chromium plating, corrosion performance at after 8 hours of testing was excellent 10/10 without any evidence of corrosion on both substrate and plating. z ''/  c m 2  / o  / o  / o  / o nickel 20 µm .z nickel +chromium .z nickel 20 µm .z nickel +chromium .z white cu-sn alloy 0.5µm.z white cu-sn alloy 2µm.z white cu-sn alloy 0.5µm.+cr.z white cu-sn alloy 2µm+cr.z white cu-sn alloy 0.5µm.z white cu-sn alloy 2µm.z white cu-sn alloy 0.5µm.+cr.z white cu-sn alloy 2µm+cr.z z ''/  c m 2 j. electrochem. sci. eng. 8(3) (2018) 227-239 corrosion resistance of cr plated ni and cu-sn alloys 236 after 16 hours, however, there was a minor deterioration of the plating. comparatively, white copper tin alloy (0.5 m) was 4/4 after 8 hours and 2/2 after 16 hours. the corrosion performance was better than nickel. however, corrosion was worse when plated with chromium; 7/7 after 8 hours and 6/6 after 16 hours. a b frequency, hz c frequency, hz figure 9. a nyquist plot for the yellow cu-sn alloy deposit with and without chromium and bode plots for the yellow cu-sn alloy deposit b with and c without chromium for yellow copper tin alloy plus 0.5 m white copper tin alloy, the corrosion performances were all better than for white copper tin alloy but identical when plated with chromium. overall, corrosion performance for yellow copper tin alloy improved with coating thickness but unfortunately the surface finish deteriorated with coating thickness and therefore loss of lustre effect. z ’ /  cm 2 ѳ / o ѳ / o yellow cu-sn alloy 4 µm.z yellow cu-sn alloy 12 µm.z yellow cu-sn alloy 24 µm.z yellow cu-sn alloy 4 µm+cr.z yellow cu-sn alloy 12 µm+cr.z yellow cu-sn alloy 24 µm+cr.z yellow cu-sn alloy 4µm.z yellow cu-sn alloy 12µm.z yellow cu-sn alloy 24µm.z yellow cu-sn alloy 4µm+cr.z yellow cu-sn alloy 12µm+cr.z yellow cu-sn alloy 24µm+cr.z z ''/  c m 2 c. f. oduoza and s. hingley j. electrochem. sci. eng. 8(3) (2018) 227-239 doi:10.5599/jese.429 237 table 3. cass results for all electroplated deposits panel identification cass after 8 hours cass after 16 hours nickel 20 µm 2/2 1/1 nickel 20 µm + cr 10/10 10/8 white cu-sn alloy 0.5 µm 4/4 2/2 white cu-sn alloy 2.0 µm 4/4 4/4 white cu-sn alloy 0.5 µm + cr 7/7 6/6 white cu-sn alloy 2.0 µm + cr 6/6 5/5 yellow cu-sn alloy 4.0 µm + white cu-sn alloy 0.5 µm 5/5 2/2 yellow cu-sn alloy 12.0 µm + white cu-sn alloy 0.5 µm 5/5 3/3 yellow cu-sn alloy 24.0 µm + white cu-sn alloy 0.5 µm 3/3 3/3 yellow cu-sn alloy 4.0 µm + white cu-sn alloy 0.5 µm + cr 8/6 6/5 yellow cu-sn alloy 12.0 µm + white cu-sn alloy 0.5 µm + cr 8/6 5/5 yellow cu-sn alloy 24.0 µm + white cu-sn alloy 0.5 µm + cr 9/6 6/6 all these results suggest that chromium plays a significant role in protecting the substrate regardless of the underlying deposits but could not match the corrosion resistance of the nickel with chromium top layer. therefore, yellow copper tin alloy (plus white copper tin alloy) is not a suitable alternative to the current nickel/chromium plating. overall, the cass results summarized in table 3 showed that none of the alternative deposits with chromium top layer could be a potential replacement for the nickel/chromium deposit although the white and yellow cu-sn alloy deposits were promising to be more corrosion resistant than the nickel deposit. the results suggest that while chromium helps to provide corrosion protection, the underlying deposit has a larger influence on the corrosion protection of the substrate holistically. the white and yellow cu-sn alloys provide corrosion properties that have an advantage over the nickel deposit, but when used in the process of plating chromium as a top coat the corrosion properties diminish. this leaves the standard nickel with chromium plating by far the best in terms of providing corrosion protection to the substrate. if corrosion protection was not the primary objective, then 0.5 µm white cu-sn alloy plated with chromium on top has the potential to replace nickel with chromium in terms of appearance but this would not satisfy the requirement, overall. atomic force microscopy (afm) data atomic force microscope (afm) was used to illustrate the surface topography of each electroplated deposit and to calculate the rms (root mean square, average height deviations), ra (average roughness) and rmax (maximum roughness between the lowest and highest points). these roughness results are for all deposits collected in table 4. table 4. afm roughness results for all electroplated deposits panel identification rms, nm ra / nm rmax / nm nickel 20µm 3.08 2.46 18.6 nickel 20µm + cr 3.91 3.1 36.6 white cu-sn alloy 0.5µm 8.3 6.63 73 white cu-sn alloy 2.0µm 12 9.42 166 white cu-sn alloy 0.5µm + cr 5.61 4.48 33.7 white cu-sn alloy 2.0µm + cr 9.84 7.84 80 yellow cu-sn alloy 4.0µm 41.2 33.6 276 yellow cu-sn alloy 12.0µm 71.4 57.9 452 yellow cu-sn alloy 24.0µm 60.8 49.5 408 yellow cu-sn alloy 4.0µm + cr 22.9 18.4 150 yellow cu-sn alloy 12.0µm + cr 66.4 53.1 464 yellow cu-sn alloy 24.0µm + cr 107 84.4 639 copper 20µm 5.92 4.69 58.5 j. electrochem. sci. eng. 8(3) (2018) 227-239 corrosion resistance of cr plated ni and cu-sn alloys 238 figures 10a and 10b show afm images for the nickel deposit with and without chromium, respectively [1]. the nickel deposit appears bright and reflective, showing a relatively smooth finish. the rmax value of 18.6 nm supports this view with a small distance between the maximum and minimum points in the deposit. the rms value of 3.08 nm also indicates little height deviation across the deposit. chromium used as topcoat has the rmax value of 36.6 nm with rms value of 3.91 nm, the chromium is still a visually bright and reflective finish but with increased roughness in the topography in relation to the nickel deposit. a b figure 10. atomic force microscopy data for deposition of a 20 µm nickel on substrate; b 20 µm nickel and 0.3 µm chromium on substrate figures 11a and 11b show the afm image of the yellow cu-sn alloy plated to a thickness of 4.0 µm. the deposit is dull and has increased roughness. the rmax value is 276 nm and the rms value is 41.2 nm, which is considerably rougher compared to the nickel deposit. the chromium as a topcoat reduces the roughness of the deposit giving rmax value of 150 nm and rms value of 22.9 nm, still rougher than the standard nickel and chromium deposit. a b figure 11. atomic force microscopy data for deposition of a 4.0 µm yellow cu-sn on substrate; b 4.0 µm yellow cu-sn and 0.3 µm chromium on substrate c. f. oduoza and s. hingley j. electrochem. sci. eng. 8(3) (2018) 227-239 doi:10.5599/jese.429 239 conclusion corrosion testing for various plated copper tin alloys, with and without plated chromium, showed that resistance to corrosion was dependent on a combination of different factors, including the thickness of chromium plating, type of material, type of testing and duration of testing in a corrosion chamber. while linear polarisation experiment was useful to establish corrosion rate of sample, electrochemical impedance and accelerated destructive testing experiments assessed corrosion resistance of the materials. the results showed that the standard nickel with chromium top coat deposit could be replaced by 0.5 μm white cu-sn alloy with chromium top coat deposit for applications where appearance is of primary importance and little corrosion protection is necessary. however, for applications that require corrosion protection, no suitable alternative was found to replace nickel with chromium deposit. consequently, for majority of applications that currently use nickel with chromium deposit for corrosion protection, there is no suitable alternative evaluated in the course of this study. acknowledgements: the authors wish to thank engineering and physical sciences research council(epsrc) for sponsorship of this industry case award references [1] s. l. hingley, c. f. oduoza, journal of materials science and engineering b 1 (2011) 410-420. 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[20] e. radlein, g. h. frischat, journal of non-crystalline solids 222 (1997) 69-82. ©2018 by the authors; licensee iapc, zagreb, croatia. this article is an open-access article distributed under the terms and conditions of the creative commons attribution license (http://creativecommons.org/licenses/by/4.0/) http://www.gamry.com/assets/application-notes/basics-of-eis.pdf http://www.astm.org/standards/b537.htm http://www.astm.org/standards/b117.htm http://www.sciencedirect.com/science/journal/15726657 http://creativecommons.org/licenses/by/4.0/) reduced graphene oxide as efficient carbon support for http://dx.doi.org/10.5599/jese.1563 673 j. electrochem. sci. eng. 13(4) (2023) 673-686; http://dx.doi.org/10.5599/jese.1563 open access : : issn 1847-9286 www.jese-online.org original scientific paper annealing effect on structural and electrochemical performance of ti-doped lini1/3mn1/3co1/3o2 cathode materials kelimah elong1,2, muhd firdaus kasim1,2, , nurhanna badar3, norashikin kamarudin1, norlida kamarulzaman1 and zurina osman4 1centre for functional materials and nanotechnology, institute of science, universiti teknologi mara, 40450 shah alam, malaysia 2school of chemistry and environment, faculty of applied sciences, universiti teknologi mara, 40450 shah alam, malaysia 3faculty of science and marine environment, universiti malaysia terengganu, 21030 kuala nerus terengganu, malaysia 4physics department, universiti malaya, 50603, kuala lumpur, malaysia corresponding author: muhdfir@uitm.edu.my received: october 23, 2022; accepted: january 24, 2023; published: february 13, 2023 abstract nmc 111 cathode materials exhibit engaging properties in high energy density and low cost, making it great potential for the next generation of high-energy lithium-ion batteries. however, it still faces challenges such as fast capacity fade, especially at high c rates. herein, we implement the novel ti-doped cathode material, lini0.3mn0.3co0.3ti0.1o2 (nmct) synthesized via the combustion method. it was discovered that nmct can effectively improve capacity delivery at high c rates. the t80 material demonstrated superior electrochemical annealed at 800 ˚c for 72 h, with an exceptional specific discharge capacity of 148.6 mah g-1 and excellent cycle stability (capacity retention 96.8 %) after 30th cycles at 3 c. the results demonstrated that ti-doped nmc had superior advantages for lini1/3mn1/3co1/3o2 (nmc 111) material at the optimum temperature of 800 °c for 72 h. it is one of the potential cathode materials for li-ion batteries. keywords cathode material; ti doping; combustion method; rietveld refinement, li-ion battery; nmc 111 introduction the world’s fastest-growing battery technology is li-ion batteries (libs), which are widely used in the application of portable electronics, mobile phones, power tools, and electric vehicles (evs). in libs, it is important to secure stable cathode materials as they govern the capacity of the whole cell. lini1-x-ymnxcoyo2 (nmc), lini1-x-ycoxalyo2 (nca) and lifepo4 (lfp) are currently three cathode active http://dx.doi.org/10.5599/jese.1563 http://dx.doi.org/10.5599/jese.1563 http://www.jese-online.org/ mailto:muhdfir@uitm.edu.my j. electrochem. sci. eng. 13(4) (2023) 673-686 ti-doped lini1/3mn1/3co1/3o2 cathode materials 674 materials that constitute a majority use in libs [1]. among the most widely investigated cathode materials, nmc is well-investigated, showing improved structural/thermal stability and cycling performance [2]. nmc is an attractive cathode material, as it combines the advantages of licoo2 (high energy density), linio2 (high reversible capacity), and limno2 (high thermal stability) [3]. this material has better characteristics such as high capacity, less toxicity, and low cost compared to commercial licoo2 material [4]. no wonder that two nmc compositions which are lini1/3mn1/3co1/3o2 (nmc 111) and lini0.5mn0.3co0.2o2 (nmc 532), already used in the commercialization of lithium batteries [5]. although there are a large number of reports covering the properties of specific nmc composition, the challenges remain. it still has limitations, especially for evs applications, due to poor rate capability and higher degradation at high voltage [6], which are its main impediments to comercialization. therefore, batteries researchers have proposed some methods for the modification of nmc cathode, such as morphology control, use of electrolyte additives, ion doping, surface modification, etc., to remedy this drawback so that the properties, including high power and high energy densities, can be improved. as is widely known, different methods and process parameters play a vital role in the crystal structure, morphology, crystallite size, and electrochemical performance of the cathode materials [7]. many alternative methods have been used to prepare the nmc powders, such as the solid-state method [8], sol-gel method [9], co-precipitation method [10], spray pyrolysis [11], and so on. it is difficult to synthesize homogenous nmc materials, so selecting a suitable preparation method is important to obtain pure and single-phase final products that will give the optimum electrochemical performance. one of the effective methods that can be used is the combustion method. this method is simple, low cost eco-friendly, giving high crystalline phases and better control of the microstructures, which will influence the electrochemical performance [12]. zhao et al. [13] reported that nmc111 synthesized via combustion can achieve a high discharge capacity of 179.2 mah g-1 due to the high crystallinity and small particle sizes. element doping plays a critical role in stabilizing the structure and each transition metal added to the crystal structure of nmc has a distinct role. changes in the oxidation state of ni from ni2+ to ni3+ and to ni4+ help in improving the capacity [14]. after the ni2+ oxidation, co will oxidize from co2+ to co3+ to prevent the ni from drifting to the li sites, which causes less cation mixing between li+/ni2+ ions [15]. mn4+ ions are responsible for maintaining structural integrity and reducing production costs by replacing cobalt with cheaper manganese [16]. there are several elements used as dopants, such as aluminium (al), iron (fe), tin (sn), chromium (cr), zirconium (zr), copper (cu), magnesium (mg), and titanium (ti). eilers-rethwisch et al. [17] reported that capacity fading is reduced when al, fe, and sn were used as dopants. zheng et al. [18] proved that doping with al shows a higher specific discharge capacity of nmc 111, which is 154.9 mah g-1. this is due to the good crystallinity and homogeneous distributions of smaller particles size that provide higher surface area. however, samarasingha et al. [19] reported that the x-ray diffraction pattern of al dopant showed the existence of secondary phase impurity with lower specific discharge capacities of 120 mah g-1. thus, it is very important to choose the right dopant to improve the structural integrity, leading to excellent electrochemical performance. wang et al. reported ti doping improved the cycling performance of the li1.2mn0.54-xtixni0.13co0.13o2 cathode material by enhancing the framework between electrode/electrolyte interface stability [20]. ti used as a dopant in nmc 811 also showed much better cycling and rate property with a cut-off potential of 4.3 v [21]. table 1 summarizes the electrochemical properties of nmc from the previous k. elong et al. j. electrochem. sci. eng. 13(4) (2023) 673-686 http://dx.doi.org/10.5599/jese.1563 675 works. to the best of our knowledge, no study of ti dopant in nmc 111 prepared using the combustion method. therefore, in this paper, we aim to understand the impact of novel partial substitution of ti in lini1/3mn1/3co1/3o2 material on the phase, structure, morphology, size of the crystallite, element distribution, and electrochemical performances of the materials with increasing the annealing temperature that synthesizes using combustion method. table 1. a comparison of electrochemical performance of previously reported nmc materials materials synthesis method current density, ma g-1 voltage range, v initial discharge capacity, mah g-1 cycle number final capacity, mah g-1 capacity retention, % ref. nmc 111 sol-gel 2.0 2.54.5 187.7 50 80.9 43.1 [22] nmc 111 solvothermal 1.0 2.5 4.5 154.1 500 73.3 52.43 [23] nmc 111 wet-chemical 1.0 3.0 4.3 143.0 100 130.4 91.2 [24] nmc 111 solid state 1.0 2.8 4.3 157.7 100 151.1 95.8 [25] ti-doped nmc 111 co-precipitation 0.2 3.0 4.3 153.6 50 152.7 99.41 [26] ti-doped nmc 811 solid state reaction 0.1 2.8 4.3 180.7 100 160.3 88.71 [21] ti-doped nmc 532 solid state reaction 1.0 2.8 4.6 184.3 200 172.7 93.4 [27] experimental synthesis of cathode materials in a typical process, raw materials employed in the experiment were lithium nitrate, lino3, nickel(ii) nitrate hexahydrate (ni(no3)26h2o), manganese(ii) nitrate hexahydrate, mn(no3)2h2o, cobalt(ii) nitrate hexahydrate, co(no3)26h2o and titanium(iv) methoxide, ti(och3)4. all starting materials were weight according to the stoichiometric proportion of lini0.3mn0.3co0.3ti0.1o2 (nmct). lino3, ni(no3)26h2o, mn(no3)2h2o, and co(no3)26h2o were first dissolved in an appropriate amount of deionized water and ti(och3)4, was dissolved in nitric acid, hno3 to form titanium nitrate. the equation for the reaction is given by the following expression, equation (1): ti(och3)4 + 4hno3 → ti(no3)4 + 4ch3oh (1) after that, all starting raw materials were mixed in a 1 l beaker. the citric acid, c6h8o7, was added to the solution as a fuel and the mixture was uniformly mixed by stirring for 1 hour. the solution was heated slowly at 200 °c on a hot plate to give auto-combusted powders. finally, the powders were calcined under various temperatures for 72 hours to obtain the target nmct cathode material and then subjected to further analyses. finally, nmct materials annealed at 650, 700, 750, 800, 850 and 900 °c for 72 hours were denoted as t65, t70, t75, t80, t85, and t90, respectively. the synthesis process is illustrated in figure 1. characterization the phase composition and structural characteristics of all samples were studied with x-ray diffraction (xrd, panalytical xpert pro diffractometer) with cu kα radiation and x’celerator detector. the measurement data were collected over the range of 10° < 2 < 90° with a step size of 0.02°. the crystal structure was analyzed by the rietveld refinement using x’pert highscore plus software. the xrd pattern for rietveld refinement was carried out at a higher count up to 10,000 at 2  range 10 to 151°. field emission scanning electron microscope (fesem, jeol jsm-7600f) equipped with an energy dispersive x-ray spectroscopy (edx, oxford inca x-max 0021) was used for analyzing the morphology, element distribution, and composition mapping. http://dx.doi.org/10.5599/jese.1563 j. electrochem. sci. eng. 13(4) (2023) 673-686 ti-doped lini1/3mn1/3co1/3o2 cathode materials 676 figure 1. summary of combustion method the microstructural characteristics of the material were assessed by high-resolution transmission electron microscopy (hrtem, tem jeol 2100) at 200 kv. for the preparation, the sample was mixed with methanol and treated with ultrasound for 1 hour. the suspension was deposited onto standard c/cu grids and heated in the oven for 1 hour at 80 °c. electrochemical measurements active lini0.3mn0.3co0.3ti0.1o2 (nmct) material, polytetrafluoroethylene (ptfe) binder, and super p with the mass ratio of 8:1:1 were well mixed to prepare the working electrode. the mixture was pressed onto a stainless-steel grid and dried in an oven at 200 °c for 24 hours. the cell was assembled in a dry glove box (unilab mbraun) under an ar atmosphere (o2 < 0.1 ppm, h2o < 0.1 ppm) with pure lithium foil as the counter electrode and celgard 2400 microporous as the separator. the electrolytes are 1 m lipf6 dissolved in a 1:1:1 (v/v/v) mixture of dimethyl carbonate (dmc), ethyl methyl carbonate (emc), and ethylene carbonate (ec). the cyclic voltammetry (cv) was performed between 0.5 5.6 v with a scan rate of 3 mv s-1. galvanostatic charge-discharge tests were conducted using wonatech (wbcs 3000) battery tester at 43.5 ma g-1 current density in the voltage range of 2.5 4.2 v. results and discussion phase and structural studies the effect of calcination temperature on the formation of nmct is investigated using xrd patterns shown in figure 2. the main peaks of all samples are well indexed and isostructural with a typical hexagonal layered α-nafeo2 structure (icdd pattern number 01-070-2685 for lithium cobalt oxide) except for t85 and t90 materials. the presence of cobalt oxide (co3o4) peak at 30.9° made both samples cannot be used for further other characterization and also electrochemical measurement [28]. t65 material shows low-intensity peaks, which indicate low crystallinity, but with the increase in the calcination temperature, the intensities of all peaks increase and the peaks become sharper, especially for t80 material that reflects the increase of the crystallinity that expected will give good electrochemical performance [29]. k. elong et al. j. electrochem. sci. eng. 13(4) (2023) 673-686 http://dx.doi.org/10.5599/jese.1563 677 to further investigate the effect of temperatures on the crystal structure, we calculated the ratio of (003) and (104) peaks called rir in table 2, which indicates the degree of “layeredness”. all samples show an rir value over 1.2 and slightly increase with increasing calcination temperature, indicating a good crystallinity and lower cation mixing in nmct samples. rir > 1.2 is important for layered cathode material with a well-ordered structure with limited li/ni mixing [2]. t80 material gives the positive sign of good cation ordering and crystallinity when demonstrated with the highest i(003)/i(104) ratio [30]. it also causes a clear split between (006) and (102) peaks and good symmetry of the (108) and (110) peaks (in the circle zone of figure 2(d)), which indicates this material has a well-defined layered structure. the r-factor, which is defined as (i(006) + i(102))/ i(101), should be at a minimum in a system with good hexagonal ordering [31]. these results prove that the crystal structure of nmc 111 did not change with ti incorporation and increased calcination temperature promotes the formation of a well-defined layered structure. to further analyze the effect of temperature on nmct materials, all materials except t85 and t90 were analyzed by rietveld refinement. the xrd pattern for rietveld refinement, as shown in figure 3, was carried out at a higher count up to 10,000 at 2  range 10 to 151˚. the summary of crystallographic values is listed in table 2. the confidence profile factor, rwp shows the refinement results of nmct were credible (less than 10 %), indicating the results of the rietveld refinements have a good fitting degree [32]. all the materials show the ratios of the lattice constants, c/a values are larger than 4.9, representing that the nmct materials have a good hexagonal layered structure [33]. t65 material shows a c/a value of 4.9743, as the temperature increase, the crystallographic values of c/a ratio and volume (v) gradually enhance, corroborating that ti was incorporated into the nmc crystal lattice [21]. it was found that t65 has the largest c-axis compared to other samples, which supposedly gives an advantage for li ions to diffuse easier [34], but unfortunately, this material has the highest cation mixing, which about 10 % of ni2+ takes place at the li+ 3a site. refinement reveals that the c-axis was decreased when temperature increased except for the t80 sample. it was also found that, as the temperature increased, the cation mixing decreased. interestingly, the t80 sample possesses a larger c-axis and the lowest cation mixing of 6.8 % as compared to other samples. by having good structural integrity, it is believed that this material will have excellent electrochemical performance. refinements results prove that increasing annealing temperature can help promote the formation of a well-defined layered structure that allows li+ ions to move randomly between the layered cathode materials, subsequently improving the cathode's electrochemical performance materials [35]. the atomic distance of li-o, li-tm, and tm-o obtained from refinements is shown in table 2, where tm indicates transition metal. based on the outcome, increasing temperature shows the materials had a shorter bond length of li-o, a larger atomic distance of li-tm, and a longer bond length of tm-o. it can see that t65 has a larger atomic distance and longer bond length, but the material has a higher level of cation mixing than shown by the rir value. this may be one of the reasons why t65 does not show good electrochemical charge-discharge, as seen later. interestingly, the t80 sample has a larger atomic distance of li-o and tm-o, and in addition to that, this material has the smallest cation mixing as compared to other samples. by having a larger atomic distance of li-o, less energy is needed for li-ions to break free from their position in the crystal structure, which could promote more li-ions diffusion, which later gives a higher specific capacity for the material to hold [36]. therefore, t80 is estimated to exhibit better electrochemical performance due to atomic distance and rir value. http://dx.doi.org/10.5599/jese.1563 j. electrochem. sci. eng. 13(4) (2023) 673-686 ti-doped lini1/3mn1/3co1/3o2 cathode materials 678 2 / ° 2 / ° figure 2. xrd patterns of (a) t65 (b) t70 and (c) t75 (d) t80 (e) t85 and (f) t90 table 2. crystallographic parameters of nmct materials annealed at different temperatures (sof site occupancy factor) sample a = b / å c / å v / å 3 c / a rw sof bond length, å rir* co 3b ni 3b ni 3a ti 3b mn 3b o 16c li-o li-tm tm-o t65 2.8774 14.3130 102.8434 4.9743 2.0354 0.3218 0.2270 0.1063 0.0925 0.3252 1.0000 2.139 2.911 1.962 1.34 t70 2.8651 14.2428 101.2548 4.9711 1.8968 0.3225 0.2406 0.0927 0.0924 0.3253 1.0000 2.127 2.893 1.952 1.44 t75 2.8621 14.2324 101.8540 4.9727 2.2172 0.3238 0.2532 0.0801 0.0930 0.3261 1.0000 2.125 2.890 1.950 1.52 t80 2.8690 14.2883 101.8920 4.9802 2.2468 0.3226 0.2653 0.0680 0.0921 0.3252 1.0000 2.132 2.901 1.955 1.80 *i(003) / i(104) k. elong et al. j. electrochem. sci. eng. 13(4) (2023) 673-686 http://dx.doi.org/10.5599/jese.1563 679 figure 3. rietveld refinements of xrd patterns of (a) t65, (b) t70, (c) t75 and (d) t80 morphology morphology and particle size are one of the main effects that can affect the electrochemical performance of cathode materials. the sem images of the nmct materials at 30000 magnification are shown in figure 4(a-d). the materials have a spherical shape with some polyhedral-type and smooth crystals. as the calcination increase, the average crystallite sizes become larger, significant to the crystal growth, and the distribution of particle size more uniform when increasing the temperature. the average crystallite size for all the samples was measured via fesem images by randomly measuring 80 crystallites, as shown in table 3. from figure 4(e), we can see that the crystallite size distribution of the t80 sample approximates to the normal distribution and agrees with the average crystallite size of 155.57 nm shown in table 3. the edx spectra in figure s-1 (supplementary material) indicate the presence of all elements in the materials, and the chemical quantification of ni, mn, co, and ti are given in the spectra. 2 / ° 2 / ° 2 / ° 2 / ° http://dx.doi.org/10.5599/jese.1563 j. electrochem. sci. eng. 13(4) (2023) 673-686 ti-doped lini1/3mn1/3co1/3o2 cathode materials 680 figure 4. sem image of (a) t65 (b) t70 (c) t75 (d) t80 and (e) average crystallite size of t80 due to the excellent structural integrity possessed by the t80 sample (table 2), this material was then subjected to the tem studies and the results are shown in figure 5. in figure 5(a), it is clear that the morphology and crystallite size of the t80 sample from the tem micrograph agree with the sem results. a high-resolution image of the t80 nanocrystals is shown in figure 5(b) for lattice studies, where the selected spot used can be seen from the marked circle in figure 5(a). the higher magnification image clearly shows the t80 nano crystal's lattice, demonstrating that the t80 sample is crystalline in nature with a single crystalline structure, indicating atomic distribution with high ordering. figure 5. tem image of (a) t80 (b) hrtem image of t80 in the circle zone of and corresponding element mapping of a t80 material good crystal structure and orderly atomic distribution are speculated to facilitate the electrochemical performance of the nmct [23]. the hrtem image of the t80 nano crystal marked by the rectangle zone in figure 5(b) displays an interplanar distance of 0.47 nm, which corresponds k. elong et al. j. electrochem. sci. eng. 13(4) (2023) 673-686 http://dx.doi.org/10.5599/jese.1563 681 to the (003) plane spacing of nmc [37]. the (003) crystal plane is identified using the selected area fft results (inset image in figure 5(a)). the existence of (003) crystal plane also confirms the hexagonal layered α-nafeo2 structure of the t80 nanocrystal as revealed by the xrd data. eds mapping of t80 material manifested in figure 5 reveals the presence of nickel (blue), cobalt (green), manganese (red), and titanium (grey), respectively. overall, the area distribution of all elements was fully uniform on the surface of the material and the effective incorporation in the particles. table 3. the crystallite size and cyclic voltammetry of lini0.3mn0.3co0.3ti0.1o2 cathode materials cathode material crystallite size range, nm average crystallite size nm eoxidation / v ereduction / v δe / v t65 47 113 80.34 5.59 3.11 2.48 t70 56 143 100.68 5.24 2.61 2.63 t75 53 263 134.8 5.59 2.71 2.88 t80 103 268 155.57 4.78 3.26 1.52 electrochemical performance of cathode materials cyclic voltammetry (cv) was a measure to study the redox potential of the transition metal ions in nmct. figure 6(a) shows the cv curves in which lithium metal is used as the counter and reference electrodes. the broad peak between 4.78 v to 5.60 v on the charge curve has been ascribed to the oxidation of co ions (co3+/co4+) and ni ions (ni2+/ ni4+), accounting for the li+ extraction from the cathode material. the reduction peak between 2.71 v to 3.26 v on the discharge curve illustrates the reduction of co ions (co4+/co3+) and ni ions (ni4+/ ni2+), which means that li+ insert back into the layered material [38]. figure 6. (a) cyclic voltammetry, (b) 1st charge discharge capacity (c) cyclability and (d) comparison of cycling performance vs. c-rates for t80 material http://dx.doi.org/10.5599/jese.1563 j. electrochem. sci. eng. 13(4) (2023) 673-686 ti-doped lini1/3mn1/3co1/3o2 cathode materials 682 the potential difference, δe (eoxidation – ereduction) was calculated to explain the cycle reversibility of the electrode of nmct. δe value was increased with the increasing annealing temperature but not for t80 and the details are revealed in table 3. this material shows smaller δe and higher peak currents indicating better electrode reaction kinetics and rate performance [39]. this may be one of the reasons why t80 material shows good electrochemical performance, as seen later. within the voltage range, it is worth pointing out that the performance of nmct materials is comparable to the reported in the literature [40,41]. as shown in figure 6(b), the best material, t80 exhibited the highest 1st discharge specific capacity of about 148.6 mah g-1. this seemed to be consistent with the good cation ordering shown by xrd analysis. overall, nmct materials demonstrated superior electrochemical after 30th cycles shown in table 4 and figure 6(c), especially t80 material with a higher discharge capacity of 143.7 mah g-1 with 96.8 % capacity retention. this is probably due to the well-formed layered structure, as confirmed by the xrd result (table 2), low cation mixing, and also uniform particle distribution. in addition, to improve the rate capability of t80 (figure 6(d)), a current density corresponding to a rate from 1 c to 15 c was applied for the 5th cycles. with increasing current, the capacity was decreased progressively and the capacity of t80 at 15 c was only 23 mah g-1 after the 5th cycle. however, almost no capacity decay was observed when the current rate returned to 3 c, suggesting the capacity decrease at high rates was mainly due to the kinetic barrier [42]. therefore, in this study, we used the 3c rate as the official scan rate for the electrochemical measurements. table 4. electrochemical performance of lini0.3mn0.3co0.3ti0.1o2 cathode materials cycles capacity, mah g-1 t65 t70 t75 t80 charge discharge charge discharge charge discharge charge discharge 1st cycle 134.1 125.4 146.1 142.5 154.6 142.5 159.4 148.6 2nd cycle 125.8 125.3 142.5 141.3 142.5 142.5 147.3 147.3 10th cycle 123.2 121.9 141.3 134.1 140.1 140.1 144.9 146.1 20th cycle 120.4 118.4 140.1 140.1 137.7 137.7 143.7 144.93 30th cycle 103.2 103.4 136.5 136.5 136.5 136.5 142.5 143.7 capacity retention after 2nd cycle, % 99.9 99.2 100 99.1 capacity retention after 30th cycle, % 82.5 95.8 95.8 96.8 efficiency, % 93.5 97.5 92.2 93.2 conclusion in summary, nmct materials calcined at different temperatures were successfully prepared via the combustion method. rietveld refinements reveal that annealing temperature has a significant effect on the structural parameters of cathode materials. refinements revealed that t80 material has the best structural integrity by having larger parameters of the c-axis, the lowest cation mixing, and a larger atomic distance of li-o, which makes this material capable of possessing excellent electrochemical performance behavior compared to the other samples. the sem result shows the crystallite size increasing with increasing calcination temperatures, indicating that high temperatures may promote the formation of more ordered layered structures, and we can see the distribution of the particles is more uniform for t80 material. furthermore, t80 material shows a smaller δe value, indicating better electrode reaction kinetics and rate performance, promising superior electrochemical performance. so, in this case, this material demonstrated excellent cycling stability, with a higher initial discharge capacity of 148.6 mha g-1 and capacity retention of 96.8 % k. elong et al. j. electrochem. sci. eng. 13(4) (2023) 673-686 http://dx.doi.org/10.5599/jese.1563 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license (https://creativecommons.org/licenses/by/4.0/) http://dx.doi.org/10.1016/j.jallcom.2015.02.114 https://doi.org/10.1016/j.jpowsour.2021.230067 http://dx.doi.org/10.1016/j.jpowsour.2015.01.173 https://doi.org/10.1016/j.electacta.2018.01.119 https://doi.org/10.1021/jacs.5b04040 https://creativecommons.org/licenses/by/4.0/) a sensitive and simple electrochemical technique for detecting ascorbic acid content in pharmaceutical and biological compounds: https://dx.doi.org/10.5599/jese.1366 1133 j. electrochem. sci. eng. 12(6) (2022) 1133-1142; https://dx.doi.org/10.5599/jese.1366 open access : : issn 1847-9286 www.jese-online.org original scientific paper a sensitive and simple electrochemical technique for detecting ascorbic acid content in pharmaceutical and biological compounds farideh mousazadeh1 and sayed zia mohammadi2, 1school of medicine, bam university of medical sciences, bam, iran 2department of chemistry, payame nour university, tehran, iran corresponding author: szmohammadi@yahoo.com received: april 29, 2022; accepted: june 4, 2022; published: august 10, 2022 abstract in the current study, a glassy carbon electrode (gce) modified with graphene-cos2 nanocomposite was investigated for electrochemical sensing of ascorbic acid. the electrochemical performance of the modified electrode was examined using differential pulse voltammetry (dpv), linear sweep voltammetry (lsv) and chronoamperometry (cha) techniques. the electrochemical behavior of ascorbic acid at the graphene-cos2/gce displayed a higher oxidation current and lower oxidation potential than bare gce. under the optimal experimental conditions, the sensor presented a good linear response between the current and the ascorbic acid concentration range of 0.15–245.0 μm, with a low detection limit of 0.05 μm. finally, the graphene-cos2 nanocomposite-modified gce was applied for the determination of ascorbic acid in real samples and displayed excellent recoveries. keywords glassy carbon electrode; graphene-cos2 nanocomposite; sensors introduction ascorbic acid, also known as vitamin c, is a natural water-soluble vitamin broadly existing in a variety of fruits and vegetables [1]. ascorbic acid is one of the most common small biological molecules found in human blood, and it plays an integral part in the body’s general physiological functioning [2]. ascorbic acid acts as a neuro-modulator and critical nutrient for the human body, with a daily dose of 100 mg necessary for good health [3]. ascorbic acid plays crucial role in many physiological reactions and biochemical processes, including the synthesis of collagen and wound healing as well as its redox functions [4-6]. it promotes cellular immunity by increasing the number of immune cells, including lymphocytes and neutrophils. ascorbic acid guards against any oxidative damage to dna, membrane lipids, and proteins, which has been implicated as a major factor in the development of chronic diseases such as cataracts, cancer, and cardiovascular diseases [7]. this vitamin is also considered an indirect antioxidant by regenerating other biologically important antioxidants such as vitamin e and glutathione to their active state [8]. ascorbic acid is included in https://dx.doi.org/10.5599/jese.1366 https://dx.doi.org/10.5599/jese.1366 http://www.jese-online.org/ http://www.jese-online.org/ mailto:szmohammadi@yahoo.com j. electrochem. sci. eng. 12(6) (2022) 1133-1142 detecting ascorbic acid content 1134 the biosynthesis of collagen and acts as a co-factor in the biosynthesis of cholesterol, l-carnitine, catecholamines, amino acids, and some peptide hormones [9,10]. ascorbic acid deficiency reduces resistance to bacterial, viral and fungal infections and is associated with symptoms of scurvy such as muscle weakness, tooth loss, rash, tiredness, and joint pain [11]. therefore, developing an approach for the detection of ascorbic acid is an essential step. until now, several techniques, including electrophoresis [12], spectrophotometry [13,14], chemiluminescence [15], fluorescence and chromatography [17,18] have been investigated and applied. although these techniques are superior in accuracy and selectivity, they require sample purification steps and skilled technicians and are expensive. the alternative methods developed for the detection of ascorbic acid are required to be sensitive and precise. at this point, the electrochemical techniques that provide highly sensitive data with precision, as well as ease of experimental setup and rapid response, attracted great attention [19-27]. glassy carbon electrode (gce) is a relatively pure material, highly inert to chemical attack, gas impermeable, electrically conductive, and easily modified [28]. the electron transfer efficiency and detection sensitivity of electrochemical sensors will be improved with the selection of suitable sensor surface modification materials [29-38]. advances in nanotechnology and nanomaterials bring new perspectives to various fields [39-44]. their unique properties, such as physical, chemical, biological, optical, and magnetic properties, nanoscale materials have high surface/volume ratios that facilitate the recognition of molecules and contribute to the enhanced signal transmission between the interface and the target molecule. hence, many research efforts have been devoted to the modification of electrodes by nanomaterials to improve analytical performance [45-53]. carbon-based compounds are commonly used because of their biocompatibility, good chemical stability, high surface area, unique electrical properties, good electron transfer kinetics, and low cost. graphene is one kind of two-dimensional carbon-based nanomaterial, and it has received considerable interest since it was discovered [54]. because of extraordinary physicochemical properties, including large surface area, excellent conductivity, and good biocompatibility, inexpensive graphene-based materials have been widely employed as electrode materials for constructing electrochemical sensors [55-60]. furthermore, electrodes modified with integrated graphene-based materials and transition metal sulfides nanoparticles can generally retain the properties of each material, increase surface area and improve the analytical performances for analyte determination [61]. transition metal sulfides such as cobalt sulfide and their composites have attracted considerable attention in electrochemical due to their fascinating properties, including high specific capacity, improved electric conductivity, and desirable electrochemical activity compared to their corresponding oxides [62-64]. these marvelous properties endow them with more outstanding electrochemical performance, providing the tremendous potential for a modified electrode. this paper illustrates the development of an electrochemical sensor to measure ascorbic acid. the modification of gce with graphene-cos2 nanocomposite provided a sensitive platform for the quantitative analysis of ascorbic acid by dpv technique. experimental apparatus and chemicals all the electrochemical measurements were carried out on a pgstat302n potentiostat/galvanostat autolab consisting of a traditional three-electrode system: a bare or modified gce as the working electrode, an ag/agcl as the reference electrode and a pt wire as a counter electrode. f. mousazadeh and s. z. mohammadi j. electrochem. sci. eng. 12(6) (2022) 1133-1142 http://dx.doi.org/10.5599/jese.1366 1135 solution ph values were determined using a 713 ph meter combined with a glass electrode (metrohm, switzerland). ascorbic acid and other chemicals used were analytical grade and were purchased from merck. synthesis of graphene-cos2 nanocomposite in the synthesis of the graphene-cos2 nanocomposite, 80 mg of graphene oxide powder was ultrasonically dispersed in 100 ml of deionized water to form a clear suspension. 0.2022 g cocl2·6h2o and 0.1294 g thiourea were then dissolved into the above suspension. subsequently, the prepared suspension was transferred into a teflon-lined stainless-steel autoclave with heat at 160 °c for 12 h. eventually, the product was obtained by centrifuging and washing with deionized water and then dried at 70 °c in vacuum. the fe-sem image of the graphene-cos2 nanocomposite is shown in fig. 1. figure 1. fe-sem image of the graphene-cos2 nanocomposite preparation of graphene-cos2/gce prior to modification of gce with graphene-cos2 nanocomposite, a gce was polished with alumina slurries and rinsed with acetone, ethanol, and deionized water under sonication for 2 min. then, 1 mg graphene-cos2 nanocomposite was dispersed in 1 ml aqueous solution within 20 min ultrasonication. then, 4 µl of the prepared suspension was dropped on the surface of the working electrode. it remains at room temperature until it becomes dry. the surface area of graphene-cos2/gce and the bare gce were obtained by cv using 1 mm k3fe(cn)6 at different scan rates. using the randles-sevcik formula for graphene-cos2/gce, the electrode surface was found 0.13 cm2 which was about 4.1 times greater than bare cpe. http://dx.doi.org/10.5599/jese.1366 j. electrochem. sci. eng. 12(6) (2022) 1133-1142 detecting ascorbic acid content 1136 results and discussion electrochemical behavior of ascorbic acid at the surface of various electrodes the effect of the electrolyte ph on the oxidation of 35.0 μm ascorbic acid was investigated at graphene-cos2/gce using dpv measurements in the pbs in the ph range from 2.0 to 9.0. according to the results, the oxidation peak current of ascorbic acid depends on the ph value and increases with increasing ph until it reaches the maximum at ph 7.0, and then decreases with higher ph values. the optimized ph corresponding to the higher peak current was 7.0, indicating that protons are involved in the reaction of ascorbic acid oxidation. the electrochemical behavior of ascorbic acid was investigated by dpv. the differential pulse voltammograms obtained using the bare gce, and graphene-cos2/gce in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm ascorbic acid are shown in figure 2. at the bare gce, a weak oxidation peak current (ipa = 3.9 μa) could be seen at 0.43 v. in contrast, graphene-cos2/gce exhibited an enhanced sharp anodic peak current (ipa = 17 μa) at a much lower overpotential ep = 0.32 v. these results confirmed that the graphene-cos2 nanocomposite improved the sensitivity of the modified electrode by enhancing peak current and decreasing the overpotential of the oxidation of ascorbic acid. figure 2. differential pulse voltammograms of (a) bare gce and (b) graphene-cos2/gce in 0.1 m pbs (ph 7.0) in the presence of 100.0 μm ascorbic acid at the scan rate 50 mv s-1 f. mousazadeh and s. z. mohammadi j. electrochem. sci. eng. 12(6) (2022) 1133-1142 http://dx.doi.org/10.5599/jese.1366 1137 effect of scan rate on the determination of ascorbic acid at graphene-cos2/gce the influence of the scan rate (ʋ) on the peak currents (ipa) of ascorbic acid at graphene-cos2/gce was investigated by lsv. figure 3 shows the voltammetric response of 70.0 μm ascorbic acid at graphene-cos2/gce at different scan rates in the range of 10 to 600 mv s-1. the oxidation peak current of ascorbic acid increases linearly with increasing scan rate. a linear regression equation was obtained from the plot ipa vs. 1/2 as follows; ipa = 1.7128  1/2 – 2.0173 (r2 = 0.9991) for the oxidation process, which indicates the reaction of ascorbic acid at graphene-cos2/gce is diffusion controlled. figure 3. linear sweep voltammograms of graphene-cos2/gce in 0.1 m pbs (ph 7.0) containing 70.0 μm ascorbic acid at various scan rates; a-i correspond to 10, 30, 70, 100, 200, 300, 400, 500, and 600 mv s-1, respectively. inset: variation of anodic peak current vs. 1/2 chronoamperometric analysis the analysis of chronoamperometry for ascorbic acid samples was performed by use of graphene-cos2/gce vs. ag/agcl/kcl (3.0 m) at 0.37 v. the chronoamperometric results of different concentrations of ascorbic acid in pbs (ph 7.0) are demonstrated in figure 4. the cottrell equation for the chronoamperometric analysis of electroactive moieties under mass transfer limited conditions is as in equation (1): i = nfad1/2cbπ-1/2t-1/2 (1) 1/2 / mv1/2 s-1/2 http://dx.doi.org/10.5599/jese.1366 j. electrochem. sci. eng. 12(6) (2022) 1133-1142 detecting ascorbic acid content 1138 where d represents the diffusion coefficient (cm2 s-1), and cb is the applied bulk concentration (mol cm-3). experimental results of i vs. t-1/2 were plotted in figure 4a, with the best fits for different concentrations of ascorbic acid. the resulting slopes corresponding to straight lines in figure 5a, were then plotted against the concentration of ascorbic acid (figure 4b). the mean value of d was determined to be 9.3×10-5 cm2/s according to the resulting slope and cottrell equation. t / s a figure 4. chronoamperograms obtained at graphene-cos2/gce in 0.1 m pbs (ph 7.0) for different concentrations of ascorbic acid; a-d correspond to 0.1, 0.4, 1.0, and 1.5 mm of ascorbic acid. insets: (a) plots of i vs. t-1/2 obtained from chronoamperograms a-d. (b) plot of the slope of the straight lines against ascorbic acid concentration calibration curve because dpv commonly has a higher sensitivity than the cv technology, the dpv technique was applied for the quantitative detection of ascorbic acid. figure 5 shows the differential pulse voltammograms of ascorbic acid at various concentrations using graphene-cos2/gce (step potential=0.01 v and pulse amplitude=0.025 v). as seen, the oxidation peak currents of ascorbic acid enhance gradually by increasing its concentration. the oxidation peak currents (ipa) show a good linear relationship with the concentrations of ascorbic acid ranging from 0.15 m to 245.0 μm. the linear equation is ipa = 0.1552cascorbic acid + 1.1919 (r2 = 0.9994) (figure 5 (inset)). also, the detection limit, cm, of ascorbic acid was obtained using the equation (2): cm = 3sb / m (2) in the above equation, m is the slope of the calibration plot (0.1552 μa μm-1) and sb is the standard deviation of the blank response obtained from 20 replicate measurements of the blank solution. the detection limit is 0.05 μm. t-1/2 / s-1/2 cascorbic acid / mm y = 32.966x + 8.5256 r2 = 0.9976 f. mousazadeh and s. z. mohammadi j. electrochem. sci. eng. 12(6) (2022) 1133-1142 http://dx.doi.org/10.5599/jese.1366 1139 figure 5. dpvs of graphene-cos2/gce in 0.1 m (ph 7.0) containing different concentrations of ascorbic acid. numbers 1–9 correspond to 0.15, 2.0, 7.0, 15.0, 45.0, 70.0, 100.0, 150.0, 200.0, and 245.0 µm of ascorbic acid. inset: plot of the electrocatalytic peak current as a function of ascorbic acid concentration in the range of 0.15-245.0 µm analysis of real samples the real samples for the analysis were prepared and quantified by dpv method. the developed sensor was applied to detect ascorbic acid in effervescent tablets, vitamin c ampoules and multivitamin syrup samples. the results are summarized in table 1. each measurement was repeated five times. the recovery and relative standard deviation (rsd) values confirmed that the graphene-cos2/gce sensor has a great potential for analytical application. table 1. application of graphene-cos2/gce for the determination of ascorbic acid in real samples (n=5 sample concentration, μm recovery, % rsd, % spiked found effervescent tablet 0 2.5 3.2 2.5 4.9 98.0 1.9 3.5 6.1 101.7 2.5 vitamin c ampoule 0 5.0 1.7 1.0 6.2 103.3 2.9 2.0 6.9 98.6 3.3 multivitamin syrup 0 2.0 2.6 2.5 4.6 102.2 3.0 3.5 5.4 98.2 2.1 conclusion in this work, the modification of a gce with graphene-cos2 nanocomposite and its use as a sensor for ascorbic acid in a pbs (ph 7.0) were reported. the modified electrode exhibited good cascorbic acid / m http://dx.doi.org/10.5599/jese.1366 j. electrochem. sci. eng. 12(6) (2022) 1133-1142 detecting ascorbic acid content 1140 electrochemical performance in the determination of ascorbic acid. the dpv demonstrated a linear range of ascorbic acid concentration from 0.15 to 245.0 μm, with a calculated detection limit of 0.05 µm. also, the diffusion coefficient of ascorbic acid was obtained 9.3×10-5 cm2/s. moreover, the practical analytical application of the graphene-cos2/gce was assessed by measurement of ascorbic acid in the real samples with satisfactory results. references [1] n. smirnoff, g. l. wheeler, critical reviews in plant sciences 19 (2000) 267-290. https://doi.org/10.1080/07352680091139231 [2] s. chambial, s. dwivedi, k. k. shukla, p. j. john, p. sharma, indian journal of clinical biochemistry 28 (2013) 314-328. https://doi.org/10.1007/s12291-013-0375-3 [3] a. c. carr, b. frei, the american journal of clinical nutrition 69 (1999) 1086-1107. https://doi.org/10.1093/ajcn/69.6.1086 [4] j. mandl, a. szarka, g. banhegyi, british journal of pharmacology 157 (2009) 1097-1110. https://doi.org/10.1111/j.1476-5381.2009.00282.x [5] j. du, j. j. cullen, g. r. buettner, biochimica et biophysica acta reviews on cancer 1826 (2012) 443-457. https://doi.org/10.1016/j.bbcan.2012.06.003 [6] s. j. padayatty, m. levine, oral diseases 22 (2016) 463-493. https://doi.org/10.1111/odi.12446 [7] s. j. duthie, a. ma, m. a. ross, a. r. collins, cancer research 56 (1996) 1291-1295. 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